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Polymer Processing Instabilities Control and
Understanding 1st Edition Savvas G. Hatzikiriakos
Digital Instant Download
Author(s): Savvas G. Hatzikiriakos, Kalman B. Migler
ISBN(s): 9780824753863, 0824753860
Edition: 1
File Details: PDF, 6.56 MB
Year: 2004
Language: english

Polymer Processing
Instabilities
Control and Understanding
DK1205_half-series-title 10/18/04 10:46 AM Page ACopyright 2005 by Marcel Dekker. All Rights Reserved.

CHEMICAL INDUSTRIES
A Series of Reference Books and Textbooks
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1.Fluid Catalytic Cracking with Zeolite Catalysts,
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Ethylene: Keystone to the Petrochemical Industry,
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Compressors and Expanders: Selection and
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Deactivation and Poisoning of Catalysts,edited by
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Shape Selective Catalysis in Industrial Applications,
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Alpha Olefins Applications Handbook,edited by
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Process Modeling and Control in Chemical Industries,
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Clathrate Hydrates of Natural Gases,
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Catalysis of Organic Reactions,edited by
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Fuel Science and Technology Handbook,
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Octane-Enhancing Zeolitic FCC Catalysts,
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Industrial Drying Equipment: Selection
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Novel Production Methods for Ethylene, Light
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Catalysis of Organic Reactions, edited by
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Catalyst Manufacture,Alvin B. Stiles
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Handbook of Grignard Reagents, edited by
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Shape Selective Catalysis in Industrial Applications:
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Industrial Gases in Petrochemical Processing,
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Clathrate Hydrates of Natural Gases: Second Edition,
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Fluid Cracking Catalysts, edited by Mario L. Occelli
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The Desulfurization of Heavy Oils and Residua,
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103.Polymer Processing Instabilities: Control
and Understanding
, edited by Savvas Hatzikiriakos
and Kalman B . Migler
104.
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Gasification Technologies: A Primer for Engineers
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Polymer Processing
Instabilities
Control and Understanding
edited by
Savvas G. Hatzikiriakos
The University of British Columbia
Vancouver, British Columbia, Canada
Kalman B. Migler
National Institute of Standards and Technology
Gaithersburg, Maryland, U.S.A.
Marcel Dekker New York
DK1205_half-series-title 10/18/04 10:46 AM Page iCopyright 2005 by Marcel Dekker. All Rights Reserved.

5396-0_Hatzikiriakos_Prelims_R2_100604
Although great care has been taken to provide accurate and current information, neither the
author(s) nor the publisher, nor anyone else associated with this publication, shall be liable for
any loss, damage, or liability directly or indirectly caused or alleged to be caused by this book. The
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are used only for identiﬁcation and explanation without intent to infringe.
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Preface
Polymer processing has grown in the past 50 years into a multi-billion dollar
industry with production facilities and development labs all over the world.
The primary reason for this phenomenal growth compared to other materials
is the relative ease of manufacture and processing. Numerous methods have
been developed to process polymeric materials at high volume and at rela-
tively low temperatures.
But despite this success, manufacturing is limited by the occurrence of
polymer processing instabilities. These limitations manifest themselves in two
ways; ﬁrst as the rate limiting step in the optimization of existing operations,
and second in the introduction of new materials to the marketplace. For
example, it is natural to ask what is the rate limiting step for processing
operations; why not run a given operation 20% faster? Quite frequently, the
answer to this question concerns ﬂow instabilities. As the polymeric material
is processed in the molten state; it retains characteristics of both liquids and
solids. The faster one processes it, the more solid like its response becomes;
seemingly simple processing operations become intractably diﬃcult and the
polymer ﬂow becomes‘‘chaotic’’and uncontrollable.
A second problem concerns the development of new materials; this is
particularly pressing because new materials with enhanced properties oﬀer
relief from the commodities nature of the polymer processing industry. But in
order to gain market acceptance, they must also enjoy ease of processability
MD: HATZIKIRIAKOS, JOB: 04351, PAGE: iii
5396-0_Hatzikiriakos_Preface_R2_092204Copyright 2005 by Marcel Dekker. All Rights Reserved.

and there are numerous examples of new materials which processed poorly
and suﬀered in the market.
The past decade has seen great progress in our eﬀorts to understand and
control polymer processing instabilities; however much of the success is
scattered throughout the scientiﬁc and technical literature. The intention of
this book is to coherently collect these recent triumphs and present them to a
wide audience. This book is intended for polymer rheologists, scientists,
engineers, technologists and graduate students who are engaged in the ﬁeld of
polymer processing operations and need to understand the impact of ﬂow
instabilities. It is also intended for those who are already active in ﬁelds such
as instabilities in polymer rheology and processing and wish to widen their
knowledge and understanding further.
The chapters in this book seek to impart both fundamental and practical
understanding on various ﬂows that occur during processing. Processes where
instabilities pose serious limitations in the rate of production include
extrusion and co-extrusion, blow molding, ﬁlm blowing, ﬁlm casting, and
injection molding. Methods to cure and eliminate such instabilities is also of
concern in this book. For example, conventional polymer processing addi-
tives that eliminate ﬂow instabilities such as sharkskin melt fracture, and non-
conventional polymer processing additives that eliminate ﬂow instabilities
such as gross melt fracture are also integral parts of this book. Materials of
interest that are covered in this book include most of the commodity polymers
that are processed as melts at temperatures above their melting point (poly-
ethylenes, polypropylenes, ﬂuoropolymers, and others) or as concentrated
solutions at lower temperatures.
Greater emphasis has been given to the ﬂow instability of melt fracture
since such phenomena have drawn the attention of many researchers in
recent years. Moreover, these phenomena take place in a variety of processes
such as ﬁlm blowing, ﬁlm casting, blow molding, extrusion and various
coating ﬂows. Equally important, however, instabilities take place in other
processing operations such as draw resonance and the‘‘dog-bone eﬀect’’in
ﬁlm casting, tiger-skin instabilities in injection molding, interfacial instabil-
ities in co-extrusion and several secondary ﬂows in various contraction ﬂows.
An overall overview of these instabilities can be found in the introduction
(Chapter 1).
It is hoped that this book ﬁlls the gap in the polymer processing lite-
rature where polymer ﬂow instabilities are not treated in-depth in any book.
Research in this ﬁeld, in particular over the last ten years, has produced a
signiﬁcant amount of data. An attempt is made to distil these data and to
deﬁne the state-of-the art in the ﬁeld. It is hoped that this will be useful to
researchers active in the ﬁeld as a starting point as well as a guide to obtain
5396-0_Hatzikiriakos_Preface_R2_092204
MD: HATZIKIRIAKOS, JOB: 04351, PAGE: ivCopyrightﬁ)MMCﬁbyﬁMarcelﬁDekkerjﬁAllﬁRightsﬁReservedj

helpful direction in their research. It would be almost impossible to include all
the knowledge generated over the past ﬁfty-to-sixty years in a single book. As
such, we would like to apologize for not citing several important reports and
contributions to the ﬁeld.
Savvas G. Hatzikiriakos
Kalman B. Migler
5396-0_Hatzikiriakos_Preface_R2_092204
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Contents
Preface
Contributors
INTRODUCTION
1. Overview of Processing Instabilities
Savvas G. Hatzikiriakos and Kalman B. Migler
PART A: VISCOELASTICITY AND BASIC FLOWS
2. Elements of Rheology
John M. Dealy
3. Secondary Flow Instabilities
Evan Mitsoulis
4. Wall Slip: Measurement and Modeling Issues
Lynden A. Archer
MD: HATZIKIRIAKOS, JOB: 04351, PAGE: vii
5396-0_Hatzikiriakos_Contents_R2_092104Copyright 2005 by Marcel Dekker. All Rights Reserved.

PART B: MELT FRACTURE AND RELATED
PHENOMENA
5. Sharkskin Instability in Extrusion
Kalman B. Migler
6. Stick–Slip Instability
Georgios Georgiou
7. Gross Melt Fracture in Extrusion
John M. Dealy and Seungoh Kim
8. Conventional Polymer Processing Additives
Semen B. Kharchenko, Kalman B. Migler,
and Savvas G. Hatzikiriakos
9. Boron Nitride Based Polymer Processing Aids
Savvas G. Hatzikiriakos
PART C: APPLICATIONS
10. Draw Resonance in Film Casting
Albert Co
11. Fiber Spinning and Film Blowing Instabilities
Hyun Wook Jung and Jae Chun Hyun
12. Coextrusion Instabilities
Joseph Dooley
13. Tiger Stripes: Instabilities in Injection Molding
A.C.B. Bogaerds, G.W.M. Peters, and F.P.T. Baaijens
5396-0_Hatzikiriakos_Contents_R2_092104
MD: HATZIKIRIAKOS, JOB: 04351, PAGE: viiiCopyright 2005 by Marcel Dekker. All Rights Reserved.

Contributors
Lynden A. ArcherCornell University, Ithaca, New York, U.S.A.
F.P.T. BaaijensEindhoven University of Technology, Eindhoven, The
Netherlands
A.C.B. BogaerdsEindhoven University of Technology, Eindhoven, and
DSM Research, Geleen, The Netherlands
Albert CoUniversity of Maine, Orono, Maine, U.S.A.
John M. DealyMcGill University, Montreal, Quebec, Canada
Joseph DooleyThe Dow Chemical Company, Midland, Michigan, U.S.A.
Georgios GeorgiouUniversity of Cyprus, Nicosia, Cyprus
Savvas G. HatzikiriakosThe University of British Columbia, Vancouver,
British Columbia, Canada
Jae Chun HyunKorea University, Seoul, South Korea
Hyun Wook JungKorea University, Seoul, South Korea
MD: HATZIKIRIAKOS, JOB: 04351, PAGE: ix
5396-0_Hatzikiriakos_Contributors_R2_092104Copyright 2005 by Marcel Dekker. All Rights Reserved.

Semen B. KharchenkoNational Institute of Standards and Technology,
Gaithersburg, Maryland, U.S.A.
Seungoh KimVerdun, Quebec, Canada
Kalman B. MiglerNational Institute of Standards and Technology,
Gaithersburg, Maryland, U.S.A.
Evan MitsoulisNational Technical University of Athens, Athens, Greece
G.W.M. PetersEindhoven University of Technology, Eindhoven, The
Netherlands
5396-0_Hatzikiriakos_Contributors_R2_092104
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1
Overview of Processing Instabilities
Savvas G. Hatzikiriakos
The University of British Columbia, Vancouver,
British Columbia, Canada
Kalman B. Migler
National Institute of Standards and Technology
Gaithersburg, Maryland, U.S.A.
1.1 POLYMER FLOW INSTABILITIES
Hydrodynamic stability is one of the central problems of ﬂuid dynamics. It is
concerned with the breakdown of laminar ﬂow and its subsequent develop-
ment and transition to turbulent ﬂow (1). The ﬂow of polymeric liquids diﬀers
signiﬁcantly from that of their low-viscosity counterparts in several ways and,
consequently, the nature of ﬂow instabilities is completely diﬀerent. Most
notably, whereas for low-viscosity ﬂuids it is the inertial forces on the ﬂuid
that cause turbulence (as measured by theReynolds number), for high-
viscosity polymers, it is the elasticity of the ﬂuid that causes a breakdown in
laminar ﬂuid ﬂow (as measured by theWeissenberg number). Additionally, the
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Oﬃcial contribution of the National Institute of Standards and Technology; not subject to
copyright in the United States.
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high viscosity and the propensity for the molten ﬂuid to slip against solid
surfaces contribute to a rich and diverse set of phenomena, which this book
aims to review.
In this introductory chapter, an overview of most ﬂow instabilities that
are discussed and examined in detail in this book is presented. Both
experimental observations as well as modeling of ﬂows with the purpose of
predicting ﬂow instabilities are of concern in subsequent chapters. In addi-
tion, ways of overcoming these instabilities with the aim of increasing the rate
of production of polymer processes are also of central importance [e.g., use of
processing aids to eliminate surface defects (melt fracture), adjustment of
molecular parameters, and rational adjustment of operating procedures and
geometries to obtain better ﬂow properties].
1.2 PART A: THE NATURE OF POLYMERIC FLOW
Polymeric liquids exhibit many idiosyncracies that their Newtonian counter-
parts lack. Most notably are: 1) the normal stress and elasticity eﬀects; 2)
strong extensional viscosity eﬀects; and 3) wall slip eﬀects. These properties of
polymeric liquids change dramatically the nature and structure of the ﬂow
compared to the corresponding ﬂow of Newtonian liquids.
2 presents a comprehensive overview of such striking diﬀerences in New-
tonian ﬂow compared to their non-Newtonian counterparts. Part A of this
book (Chapters 2–4)
ﬂow from the perspective of processing regime, where the above eﬀects are
most manifest.
Chapter 2 by Dealy introduces the reader to the concepts of polymer
rheology that are most important to polymer processing and deﬁnes a large
number of the terms needed to read the rest of the book. It describes how to
relate the stress on a ﬂuid to an imposed strain, ﬁrst for the simple linear case
and then for the cases more relevant to processing, such as nonlinear ﬂows
(such as shear thinning) and also extensional ﬂows (which are prevalent in
most processing operations.) It carefully deﬁnes theWeissenbergnumber,
which characterizes the degree of nonlinearity or anisotropy exhibited by the
ﬂuid in a particular deformation, as well as theDeborahnumber, which is a
measure of the degree to which a material exhibits elastic behavior. It cautions
the reader that our knowledge and measurement ability in this area are still in
their infancy.
Chapter 3
ations, in particular those in which vortices or helices appear. This is a classic
case in which a ﬂow phenomenon (vortices), which occurs for Newtonian
ﬂows, also occurs for polymeric ﬂows, but the nature of the ﬂow is quite
distinct. For polymeric ﬂuids, the transition to vortices is governed largely by
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the Deborah number. Depending on the response of the ﬂuid to extensional
ﬂows, the occurrence and magnitude of the vortices can be quite diﬀerent.
This chapter describes the occurrence of these ﬂows in a number of processing
ﬂows including extrusion, calendaring, roll/wire coating, and coextrusion.
Chapter 4
ﬂuid velocity does not go to zero at a solid wall. Again, although this
phenomenon has been reported for Newtonian ﬂuids, it is relatively weak
and diﬃcult to observe. However, for polymeric liquids, wall slip can be quite
large; it signiﬁcantly impacts on the ﬂow behavior (and necessary modeling)
of processing operations. This chapter describes the signiﬁcant advances
made recently on the measurement of wall slip, on the molecular theory
describing it, and on the relationship at the interface between a polymer and a
solid substrate. Although wall slip has been studied extensively in regard to its
eﬀects on extrusion instabilities, it plays a critical role in other processing
operations as well.
Unlike Newtonian ﬂuids, polymer melts slip over solid surfaces when
the wall shear stress exceeds a critical value. For example, Fig. 1.1 illustrates
the well-known Haagen–Poiseuille steady-state ﬂow of a Newtonian ﬂuid
(Fig. 1.1a) and two typical corresponding proﬁles for the case of a molten
polymer (Fig. 1.1b and c), indicating a small deviation from the no-slip
boundary condition and plug ﬂow. Although the classical no-slip boundary
condition applied in Newtonian ﬂuid mechanics (perhaps with the exception
of rareﬁed gas dynamics where the continuum hypothesis does not apply),
wall slip is typical in the case of molten polymers.
For the case of a passive polymer–wall interface where there is no
interaction between the polymer and the solid surface, de Gennes (4)
proposed an interfacial rheological law suggesting that a melt would slip at
all shear rates. This theory was extended by Brochard-Wyart and de Gennes
(5) to distinguish the case of a passive interface (no polymer adsorption) from
that of an adsorbing one. It has been predicted that there exists a critical wall
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FIGURE1.1Typical velocity proﬁles of polymer melt ﬂow in capillaries: (a) no-
slip; (b) partial slip; and (c) plug ﬂow.9opyrightﬀ:]]CﬀbyﬀQarcelﬀ0ekkerBﬀ7llﬀRightsﬀReservedB

shear stress value at which a transition from a weak to a strong slip takes
place. These predictions have been suggested to be true through experimen-
tation (6,7), and
Archer. The various mechanisms of wall slip in the case of ﬂow of molten
polymers are still under debate and many observations need better and
complete explanation.
1.3 PART B: MELT FRACTURE IN EXTRUSION
Part B is devoted to describing extrusion—one of the simpler processing
operations but is critically important because it is ubiquitous in polymer
manufacturing and exhibits a full range of instabilities (Chapters 5–9). Here
the instabilities revolve around the phenomena ofmelt fracture, wall slip, and
polymer elasticity. The phenomena of melt fracture and wall slip have been
studied for the past 50 years but have not yet been explained (3). Not only are
these phenomena of academic interest, but they are also industrially relevant
as they may limit the rate of production in processing operations. For
example, in the continuous extrusion of a typical linear polyethylene at some
speciﬁc output rate, the extrudate starts losing its glossy appearance; instead,
a matte surface ﬁnish is evident and, at slightly higher output rates, small-
amplitude periodic distortions appear on its surface (see Fig. 1.2). This
phenomenon, known assharkskinorsurface melt fracture, is described in
Chapter 5
At higher values of the output rate, the ﬂow ceases to be constant;
instead, the pressure oscillates between two limiting values and the extrudate
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FIGURE1.2Typical extrudates showing: (a) smooth surface; (b) shark skin melt
fracture; (c) stick–slip melt fracture; and (d) gross melt fracture.9opyrightﬀ:]]CﬀbyﬀQarcelﬀ0ekkerBﬀ7llﬀRightsﬀReservedB

surface alternates between relatively smooth and distorted portions
(Fig. 1.2c). oscillating,stick–stick,orcyclic
melt fracture, and are discussed in
focuses on the critical stresses for the instability as well as the eﬀects of
operating conditions and molecular structure. A one-dimensional phenom-
enological model successfully describes much of the data.
At even higher values of the output rate, a new instability known as
gross melt fracture(GMF) occurs, which is described in
and Kim. Whereas sharkskin and stick–slip instability are associated with the
capillary tube, GMF originates in the upstream region where the polymer is
accelerated from a wide-diameter barrel to a narrow-diameter capillary or
oriﬁce. The extensional stress on the polymer associated with such a ﬂow can
rupture it, leading to a chaotic appearance when it ﬁnally emerges from the
capillary. This chapter presents a number of observations and contains a
critical review of our understanding of GMF from the perspective of our
limited understanding of what causes the rupture of molten polymers.
To increase the rate of production by eliminating or postponing the melt
fracture phenomena to higher shear rates, processing additives/aids must be
used. These are mainly ﬂuoropolymers and stearates, which are widely used in
the processing of polyoleﬁns such as high-density polyethylene (HDPE) and
linear low-density polyethylene (LLDPE). They are added to the base
polymer at low concentrations (approximately 0.1 %), and they eﬀectively
act as die lubricants, modifying the properties of the polymer–wall interface
(increasing slip of the molten polymers).
Hatzikiriakos, and Migler discusses processing additives with particular
emphasis on ﬂuoropolymer additives. These ﬂuoropolymer additives have
been known for a long time, but until recently, one could only speculate as to
precisely how they function. Recently, through visualization methods, the
precise nature of the coating and how it is created through the ﬂow have
become clear. Although these ﬂuoropolymer additives are eﬀective for shark-
skin and stick–slip instability, they remain ineﬀective for the case of GMF.
It has been recently discovered that compositions containing boron
nitride can be successfully used as processing aids to not only eliminate
surface melt fracture, but also to postpone gross melt fracture and thereby
permit the use of signiﬁcantly higher shear rates. These processing aids can be
used for a variety of important extrusion processes, namely, tubing extrusion,
ﬁlm blowing, blow molding, and wire coating. The mechanisms by which
boron nitride aﬀects the processability of molten polymers and other impor-
tant experimental observations related to the eﬀects of boron nitride-based
processing aids on the rheological behavior and processability of polymers
can be found in
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1.4 PART C: APPLICATIONS
1.4.1 Draw Resonance in Film Casting
In ﬁlm casting as well as in extrusion coating of polymeric sheet, a polymeric
melt curtain is extruded through a narrow die slot, across an air gap or a liquid
bath, and then onto a pair of (or just a single) take-up or chill rolls (see
Fig. 1.3). Eﬀorts to increase production speed and/or reduce ﬁlm thickness by
going to higher draw ratio (take-up speed/extrudate speed) are hampered by
edge neck-inandbead formation, but mainly by process instabilities (draw
resonance and edge weave) (7), which give rise to spontaneous thickness and
width oscillations (Fig. 1.4).
is usually pulled downstream by the drum, resulting in a long eﬀective casting
span. As a result, the curtain necks-in at the edges giving rise to a nonuniform
gauge proﬁle with a characteristic‘‘dog bone’’shape (Fig. 1.4). On the other
hand, draw resonance is accompanied by spontaneous thickness and width
oscillations—eﬀects that are undesirable in ﬁlm production.
Chapter 10
instabilities that might occur in ﬁlm casting and extrusion coating of
polymeric materials. In addition, modeling and stability analysis of draw
resonance with the aim of predicting such instabilities are also thoroughly
discussed. As will be seen in this chapter, computational ﬂuid mechanics
modeling can help predict such instabilities and extend the parameter range of
stable and defect-free operation (so-called‘‘process operability window’’).
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FIGURE1.3Schematic of a typical ﬁlm casting process.Copyright 2005 by Marcel Dekker. All Rights Reserved.

1.4.2 Fiber Spinning Instabilities
A simple schematic of the ﬁber spinning process is depicted in
polymer melt is pumped by an extruder and ﬂows through a plate containing
many small holes—the spinneret. Fig. 1.5 shows the ﬂow of the ﬁber through
one such hole. The extruded ﬁlament is air-cooled by exposure to ambient air,
and it is stretched by a rotating take-up roll at a point lower to its solidiﬁcation
point.
As in all other polymer processing operations discussed before, the rate
of production is limited by the onset of instabilities, and these are discussed in
Chapter 11
spinning (8). The ﬁrst instability is calledspinnability, which is deﬁned as the
ability of the polymer melt to stretch without breaking (8).Neckingmight
occur due to capillary waves or abrittletype of fracture due to crystallization
induced by stretching. The second type of instability is referred to asdraw
resonance, which manifests itself as periodic ﬂuctuation of the cross-sectional
area in the take-up area. This latter instability is similar to draw resonance
occurring in the ﬁlm casting process of polymers (see Fig. 1.4). Finally,melt
fracturephenomenon, as with other types of instabilities that may limit the
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FIGURE1.4Instabilities associated with the ﬁlm casting process of polymers.Copyright 2005 by Marcel Dekker. All Rights Reserved.

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rate of production such as ﬁber spinning, involves extrusion through dies at
high shear rates.
Experimental observations in ﬁber spinning application for various
resins are discussed in conjunction with modeling techniques of the process
with a focus on draw resonance. Attempts to predict such instabilities with the
aim of predicting operability windows for increasing the rate of production
are also discussed. The rheological properties of the melts that play a role in
stabilizing/destabilizing the process are also relevant in such discussions.
1.4.3 Film Blowing Instabilities
A simple schematic of the ﬁlm blowing process is depicted in
extruder melts the resin and forces it to ﬂow through an annular die. The melt
extruded in the form of a tube is stretched in the machine direction by means
of nip roles above the die, as shown in Fig. 1.6. Air ﬂows inside the ﬁlm bubble
to cool down the hot melt. Thus, a frost line is established. The ratio of the ﬁlm
velocity to the average velocity of the melt in the die is referred to as thedraw
down ratio(DDR).
The ﬁlm blowing instabilities are discussed in great detail in
by Jung and Hyun. As in all polymer processes discussed so far, several
FIGURE1.5Schematic of a typical ﬁber spinning process.Copyright 2005 by Marcel Dekker. All Rights Reserved.

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instabilities might result as the extrusion rate (production rate) and DDR are
increased. First, as the extrusion rate is increased (shear rate in the die), melt
fracture might result depending on the molecular characteristics of the resin.
These are manifested as loss of gloss of the ﬁlm, small-amplitude periodic
distortions, or more severe form of gross distortions. These instabilities
inﬂuence seriously the optical and mechanical properties of the ﬁnal product.
Melt fracture is discussed in detail in
postponing these instabilities to higher shear rates by means of using
processing aids are discussed in
Draw resonance also occurs in this process as the DDR ratio increases
(9). In the present process, draw resonance appears as periodic ﬂuctuations of
the bubble diameter known also as bubble instability shapes (10). Another
type of draw resonance that might occur in ﬁlm blowing is in the form of ﬁlm
gauge (ﬁlm thickness) nonuniformity. This typically occurs at high produc-
tion rates when the cooling rate requirements are increased (11). Molecular
orientation during stretching of the ﬁlm and the extensional properties of the
resin in the melt state are important factors inﬂuencing the quality of the ﬁnal
products (11).
1.4.4 Coextrusion and Interfacial Instabilities
Coextrusion instabilities are discussed in depth in
Coextrusion refers to the process when two or more polymer liquid layers are
extruded from a die to produce either a multilayer ﬁlm or a ﬁber. There are
three main problems associated with coextrusion. First, depending on the
FIGURE1.6Schematic of a typical ﬁlm blowing process.Copyright 2005 by Marcel Dekker. All Rights Reserved.

materials to be processed,melt fracturephenomenon might appear. This has
already been discussed as these phenomena occur in most polymer processes
that produce extrudates at high production rates.
More importantly are the instabilities caused by diﬀerences in the
viscous and elastic properties of the components (see
signiﬁcant viscosity diﬀerence between two liquids (i.e., two-layer ﬂow), then
the ﬂuid having the lower viscosity will tend to encapsulate the ﬂuid having
the higher viscosity (9). For this to occur, the length-to-gap ratios should be
relatively high, which gives enough time for rearrangement. Finally, even in
the absence of a viscosity mismatch, the interface can become wavy. These
instabilities are collectively known in coextrusion asconvective interfacial
instabilities, or simplyinterfacial instabilities. These instabilities are demon-
strated in
ﬂuids coextruded, together with the geometry used for the process (11–14).
For example, the point of layer merging or the point on the interface at the exit
might cause interfacial waviness. The phenomena are complicated and not
completely well understood as there are multiple factors involved.
1.4.5 Injection Molding and Tiger Stripes Instability
In injection molding, the polymer melt is softened ﬁrst in an extruder and
pumped forward through a runner to ﬁll in a mold that is in the shape of the
product article. The challenge is to produce a product that is free of voids, has
a smooth and glossy surface, exhibits no warpage, and has suﬃcient mechan-
ical strength and stiﬀness for its end use. The latter is signiﬁcantly inﬂuenced
by residual stresses due to the viscoelastic nature of ﬂow, as well as due to
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FIGURE1.7Instabilities in coextrusion: (a) gradual encapsulation of a viscous
ﬂuid (2) by a less viscous ﬂuid (1) as both ﬂow in a circular tube; and (b)
interfacial instability in the form of a wavy interface.9opyrightﬀ:]]CﬀbyﬀQarcelﬀ0ekkerBﬀ7llﬀRightsﬀReservedB

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shrinkage during cooling of the mold. As in most polymer processing
operations, several problems (instabilities) might appear.
First, if the cavity to be ﬁlled has an insert, then the melt ﬂow splits and
ﬂows around the obstacle. Consequently, these moving fronts meet again to
result aweld lineorknit line. In other cases, multiple runners are used to ﬁll in
the mold and, therefore, multipleweld linesresult (see Fig. 1.8a). Lack of
suﬃcient reentanglement of molecules across this line would lead to poor
mechanical strength of the ﬁnal product.
Jetting is another problem that might occur in injection molding. When
the size (gap or diameter) is much smaller than the mold gap, the melt does not
properly wet the entrance to the mold in order to ﬁll it gradually. Instead, it
‘‘snakes’’its way into the gap. This is shown in Fig. 1.8b.
Flow instabilities during injection molding (mold ﬁlling) also result in
surface defects on polymer parts. For example, in ﬁlled polypropylene
systems, the regular dull part of ﬁnished parts is broken by periodic shiny
bands perpendicular to the ﬂow direction (15). The appearance of these
striped surface defects is known astiger stripes. At the leading edge of the ﬂow
during mold ﬁlling, the polymer undergoes what is known as‘‘fountain ﬂow.’’
Great recent progress in this area is summarized in
Peters, and Baaijens, in which they demonstrate that there is a viscoelastic
ﬂow instability at this air–polymer–wall juncture that leads to the tiger stripe
phenomenon.
REFERENCES
1. Drazin, P.G.; Reid, W.H.Hydrodynamic Stability; Cambridge University Press:
New York, 1981.
FIGURE1.8Instabilities in injection molding: (a) formation of a weld line; and
(b) the phenomenon of jetting.Copyright 2005 by Marcel Dekker. All Rights Reserved.

2. Bird, R.B.; Armstrong, R.C.; Hassager, O.Dynamics of Polymeric Liquids: Vol.
1. Fluid Mechanics; John Wiley and Sons: New York, 1987.
3. Pearson, J.R.A.Mechanics of Polymer Processing; Elsevier: New York, 1985.
4. de Gennes, P.G. Viscometric ﬂows of tangled polymers. C. R. Acad. Sci. Paris,
Ser. B 1979,288, 219–222.
5. Brochard-Wyart, F.; de Gennes, P.G. Shear-dependent slippage at a polymer/
solid interface. Langmuir 1992,8, 3033–3037.
6. Migler, K.B.; Hervet, H.; Leger, L. Slip transition of a polymer melt under
shear stress. Phys. Rev. Lett. 1993,70, 287–290.
7. Wang, S.-Q.; Drda, P.A. Stick–slip transition in capillary ﬂow of polyethylene.
2. Molecular weight dependence and low-temperature anomaly. Macro-
molecules 1996,29, 4115–4119.
8. Petrie, C.J.S.; Denn, M.M. Instabilities in Polymer Processing. AIChE 1976,
22, 209.
9. Baird, G.B.; Kolias, D.I.Polymer Processing; Butterworth-Heinemann:
Toronto, 1995.
10. Kanai, T.; White, J.L. Kinematics, dynamics and stability of the tubular ﬁlm
extrusion of various polyethylenes. Polym. Eng. Sci. 1984,24(15): 1185–1201.
11. Rincon, A.; Hrymak, A.N.; Vlachopoulos, J.; Dooley, J. Transient simulation
of coextrusion ﬂows in coat-hanger dies. Society of Plastics Engineers. Proc.
Annu. Tech. Conf. 1997,55, 335–350.
12. Tzoganakis, C.; Perdikoulias, J. Interfacial instabilities in coextrusion ﬂows of
low-density polyethylenes: experimental dies. Polym. Eng. Sci. 2000,40, 1056–
1064.
13. Martyn, M.T.; Gough, T.; Spares, R.; Coates, P.D. Visualization of melt
interface in a co-extrusion geometry. Proceedings of Polymer Process Engineer-
ing Conference; Bradford, 2001, 37–45.
14. Dooley, J.Viscoelastic Flow Eﬀects in Multilayer Polymer Coextrusion. Ph.D.
Thesis, Technical University of Eindhoven, 2002.
15. Grillet, A.M.; Bogaerds, A.C.B.; Bulters, M.; Peters, G.W.M.; Baaijens, F.P.T.
An investigation of ﬂow mark surface defects in injection molding of polymer
melts. Proceedings of the XIIIth International Congress on Rheology: Cam-
bridge, 2000; Vol. 3, 122.
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2
Elements of Rheology
John M. Dealy
McGill University, Montreal, Quebec, Canada
2.1 RHEOLOGICAL BEHAVIOR OF MOLTEN POLYMERS
Flow instabilities that occur in melt processing arise from a combination of
polymer viscoelasticity and the large stresses that occur in large, rapid
deformations. This is in contrast to ﬂows of Newtonian ﬂuids, where inertia
and surface tension are usually the driving forces for ﬂow instabilities. Both
the elasticity and the high stresses that occur in the ﬂow of molten polymers
arise from their high molecular weight (i.e., from the enormous length of their
molecules). The high stresses are associated with the high viscosity of molten
commercial thermoplastics and elastomers. Typical values range from 10
3
to
10
6
Pa s, whereas the viscosity of water is about 10
F3
Pa sec. An easily
observed manifestation of melt elasticity is the large swell in cross section that
occurs when a melt exits a die.
The way melts behave in very small or very slow deformations is
described quite adequately by the theory oflinear viscoelasticity(LVE), and
one material function, the linear relaxation modulusG(t), is suﬃcient to
describe the response of a viscoelastic material to any type of deformation
history, as long as the deformation is very small or very slow. This informa-
tion is useful in polymer characterization and determination of the relaxation
spectrum. However, there is no general theoretical framework that describes
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viscoelastic behavior in the large, rapid deformations that arise in polymer
processing (i.e., there exists no general theory ofnonlinear viscoelasticity).
Nonlinear phenomena that arise in large, rapid ﬂows include the strong
dependence of viscosity on shear rate and the very large tensile stresses that
arise when long-chain branched polymers are subjected to extensional
deformations.
Flow instabilities nearly always occur in response to large, rapid
deformations. This means that a ﬂow simulation able to predict when a ﬂow
will become unstable must be based on a reliable model of nonlinear
viscoelastic behavior. The lack of such a model at the present time greatly
limits our ability to model instabilities. Also lacking are generally accepted,
quantitative criteria for the occurrence of instabilities, for example, in terms
of a dimensionless group.
Although there remain major diﬃculties in the modeling of melt ﬂow
instabilities, many experimental observations have been reported, and em-
pirical correlations with various rheological properties have been proposed.
In order to understand these, some knowledge of the rheological behavior of
molten polymers is needed. It is the purpose of the present chapter to provide
a brief overview to prepare the uninitiated reader to understand the later
chapters of this book and the current stability literature.
2.2 VISCOELASTICITY—BASIC CONCEPTS
Molten polymers are elastic liquids, and it is their elasticity that is the root
cause of most ﬂow instabilities. Thus, these are usuallyhydroelasticrather
thanhydrodynamicinstabilities. Melts of high polymers can store elastic
energy and, as a result, they retract when a stress that has been applied to them
is suddenly released. However, they do not recover all the strains undergone
as a result of this stress, as they are liquids, and their deformation always
entails some viscous dissipation. These materials are thus said to have afading
memory.
The elasticity of polymers is intimately associated with the tendency of
their molecules to become oriented when subjected to large, rapid deforma-
tions. This molecular orientation gives rise to anisotropy in the bulk polymer,
and manifestations of this anisotropy include ﬂow birefringence and normal
stress diﬀerences.
A variable of central importance in the rheological behavior of poly-
mers is time. Any system whose response to a change in its boundary
conditions involves both energy storage and energy dissipation must have
at least one material property that has units of time. Examples of such systems
are resistance–capacitance electrical circuits and the suspension systems of
automobiles, which include springs (to store energy) and shock absorbers (to
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dissipate energy). A system often used to demonstrate this fact consists of a
linear (Hookean) spring and a linear dashpot, connected in series as shown in
Fig. 2.1. This system is called aMaxwell element, and we will see that its
behavior has proven very useful in describing the viscoelastic behavior of
polymers.
The force in the spring is proportional to the distance through which it is
stretched (F
s=K eDXs), and the force in the dashpot is proportional to the
velocity with which its ends are separated [F
d=Kv(dXd/dt)]. In the absence of
inertia, the forces in the two elements are equal, and the displacement of one
end relative to the other isDX=DX
s+DX
d. From this, it is easy to
demonstrate that if the assembly is subjected to sudden stretching in the
amount ofDX
0, the force will rise instantaneously toK eDX0and then decay
exponentially, as shown by Eq. (2.1):
FðtÞ¼K
eDX0exp
F
FtðK e=KvÞ
l
ð2:1Þ
We see that the ratio (K
v/Ke) is a parameter of the system and has units of
time. It is thus therelaxation timeof the mechanical assembly.
2.3 LINEAR VISCOELASTICITY
Linear viscoelasticity is a type of rheological behavior exhibited by polymeric
materials in the limit of very small or very slow deformations. Although not
directly applicable to the deformations that give rise to gross melt fracture,
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FIGURE2.1The simplest mechanical analog of a viscoelastic liquid; the single
Maxwell element consisting of a linear spring in series with a linear dashpot.9opyright0ﬀII=0by0¨arcel0Cekkerk07ll0Rights0Reservedk

LVE behavior is important for two reasons. First, information about linear
viscoelastic behavior is very useful in the characterization of a polymer (i.e., in
determining its molecular structure). Second, it provides information about
the linear relaxation spectrum, which is an essential element of a model of
nonlinear viscoelastic behavior. A comprehensive discussion of the linear
viscoelastic properties of polymers can be found in the book by Ferry (1).
The most basic experiment in viscoelasticity is the measurement of the
transient stress following a sudden‘‘step’’deformation. In the case of molten
polymers, this is nearly always a shearing deformation, and the measured
stress is reported in terms of the relaxation modulusG(t). This function of
time is deﬁned as the stress divided by the amount of shear straincimposed on
the sample att=0:
GðtÞurðtÞ=c ð2:2Þ
A function such as this, which is a characteristic of a particular material, is
called amaterial function.
The basic axiom of linear viscoelasticity, the Boltzmann superposition
principle, tells us that this function is independent ofcand contains all the
information needed to predict how a viscoelastic material will respond to any
type of deformation, as long as this deformation is very small or very slow.
This principle can be stated in terms of an integral equation as follows for
simple shear deformation:
rðtÞ¼
ð
t
tVwFl
GðtFtVÞdcðtVÞð 2:3Þ
whereris the shear stress at timet,anddc(tV) is the shear strain that occurs
during the time interval dtV. This can easily be generalized for any kinematics
by use of theextra stress tensorand theinﬁnitesimal strain tensor, whose
components are represented byr
ijandc ij, respectively:
r
ijðtÞ¼
ð
t
tVwFl
GðtFtVÞdc
ijðtVÞð 2:4Þ
and, in terms of the rate-of-strain tensor, this is:
r
ijðtÞ¼
ð
t
tVwFl
GðtFtVÞc
: ijðtVÞdtV ð2:5Þ
The extra stress is that portion of the total stress that is related to
deformation. We recall that for in an incompressible ﬂuid, an isotropic stress
(i.e., one that has no shear components and whose normal components are the
same in all directions) does not generate any deformation. For example, the
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Earth’s atmosphere at sea level, when completely still, is under a compressive
stress of one atmosphere. Thus, there is an isotropic component of the total
stress tensor that is not related to deformation in an incompressible ﬂuid. And
the inﬁnitesimal strain tensor is a very simple measure of deformation that is
valid only for very small deformations. The rate-of-deformation tensor is
related to the velocity components by Eq. (2.6):
˙c
iju
Bvi
Bxj
þ
Bvj
Bxi
wi
ð2:6Þ
Eqs. (2.4) and (2.5) are alternative, concise statements of the Boltzmann
superposition principle. A very simple model of linear viscoelastic behavior can be obtained by inserting into Eq. (2.5) the relaxation modulus that is analogous to a single Maxwell element (i.e., a single exponential):
GðtÞ¼Ge
Ft=s r
ð2:7Þ
whereGis the instantaneous modulus ands
ris arelaxation time. This is called
theMaxwell modelfor the relaxation modulus. Inserting this into Eq. (2.4), we
obtain the simplest model for linear viscoelasticity:
r
ijðtÞ¼G
ð
t
tVwFl
e
FltFtVÞ=s r
dc
ijðtVÞð 2:8Þ
The stress relaxation in an actual polymer can only very rarely be
approximated by a single exponential, andG(t) is usually represented either
by a discrete or a continuousrelaxation spectrum. In the case of a discrete
spectrum, the relaxation modulus is represented as a sum of weighted
exponentials, as shown by Eq. (2.9):
GðtÞ¼
X
N
i¼1
Gie
Ft=s i
ð2:9Þ
Comparing Eqs. (2.1) and (2.9), we see that this is analogous to the response of
a mechanical assembly consisting of a series of Maxwell elements connected in
parallel, so that the displacements of all the elements are the same, and the
total force is the sum of the forces in all the elements. Thus, Eq. (2.9) is called
thegeneralized Maxwell modelfor the relaxation modulus.
The continuous spectrumF(s) is deﬁned in terms of a continuous series
of exponential decays, as shown by Eq. (2.10):
GðtÞ¼
ð
l
0
FðsÞe
Ft=s
ds ð2:10Þ
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It turns out to be more convenient to work with the time-weighted spectrum
functionH(s), which is deﬁned assF(s), as shown by Eq. (2.11):
GðtÞ¼
ð
l
Fl
HðsÞe
Ft=s
dlns ð2:11Þ
It has been found that frequency (oscillatory shear) domain experiments
are much easier to perform than time domain (step strain) experiments, and
these provide an alternative means of establishing the linear behavior of a
polymer. In oscillatory shear, the input strain is sinusoidal in time:
cðtÞ¼c
osinðxtÞð 2:12Þ
As long as the response is linear (i.e., ifc
ois suﬃciently small), the shear stress
is also sinusoidal, but with a phase shiftd, as shown by Eq. (2.13):
rðtÞ¼r
osinðxtþdÞð 2:13Þ
The response could be characterized by the amplitude ratio (r
o/co) and the
phase shiftdas functions of frequency, but it is more informative to represent
the response in terms of frequency-dependent in-phase (GV) and out-of-phase
(G) components, as shown by Eq. (2.14):
rðtÞ¼r
oðGVsinxtþGVVcosxtÞð 2:14Þ
The two material functions of this relationship areGV(x), thestorage modulus,
andGU(x), theloss modulus. The most common way of reporting linear visco-
elastic behavior is to show plots of these two functions. Ferry (1) provides
formulas for converting one LVE material function into another.
An alternative approach to viscoelastic characterization is to impose a
prescribed stress on the sample and monitor the strain. If a stressr
ois imposed
instantaneously at time zero, and the resulting shear strain is divided by this
stress, the resulting ratio is the creep complianceJ(t):
JðtÞucðtÞ=r
o ð2:15Þ
If the imposed stress is suﬃciently small, the result will be governed by the
Boltzmann superposition principle, and the creep compliance can, in princi-
ple, be calculated if the relaxation modulusG(t) is known.
One can also impose a sinusoidal stress and monitor the time-dependent
deformation as an alternative technique to determine the storage and loss
moduli.
2.4 VISCOSITY
Viscosity is deﬁned as the steady-state shear stress divided by the shear rate in
a steady simple shear experiment. For a Newtonian ﬂuid, the viscosity is
independent of shear rate, and this is also the prediction of the theory of linear
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viscoelasticity. This means that a molten polymer will exhibit a shear rate-
independent viscosity at a suﬃciently small shear rate, although such a low-
shear rate may be diﬃcult to access experimentally, especially in the case of
polymers with broad molecular weight distribution or long-chain branching.
Outside this range, the viscosity is a strong function of shear rate, and the
dependency of viscosity on shear rateg(˙c) is an example of a nonlinear
material function.
2.4.1 Dependence of Viscosity on Shear Rate, Temperature,
and Pressure
At suﬃciently high-shear rates, the viscosity often approaches a power–law
relationship with the shear rate. Figure. 2.2 is a plot of viscosity vs. shear rate
for a molten polymer, and it shows both a low-shear rate Newtonian region
and a high-shear rate power–law region. These data were reported by
Meissner (2) some years ago and represent the ultimate in rheometrical
technique. The variation ofgwith˙cimplies the existence of at least one
material property with units of time. For example, the reciprocal of the shear
rate at which the extrapolation of the power–law line reaches the value ofg
0is
a characteristic time that is related to the departure of the viscosity from its
zero-shear value. Let us call such a nonlinearity parameters
n.
The viscosity falls sharply as the temperature increases, as shown in Fig.
2.2. Two models are widely used to describe this dependency: the Arrhenius
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FIGURE2.2Viscosity vs. shear rate at several temperatures for a low-density
polyethylene. (From Ref. 2.)9opyright0ﬀII=0by0¨arcel0Cekkerk07ll0Rights0Reservedk

equation and the Williams-Landel-Ferry (WLF) equation. These are given in
standard rheology references (1,3,4).
2.4.2 Dependence on Molecular Structure
The limiting low-shear-rate viscosityg
0, thezero-shear viscosity, increases
linearly with weight-average molecular weight when this is belowan entan-
glement molecular weight M
C, whereas above this value, the viscosity increases
in proportion to (M
w)
a
, whereais usually around 3.5. This is one of several
dramatic rheological manifestations of the phenomenon calledentanglement
coupling, which has a very strong eﬀect on the ﬂow of high-molecular-weight
polymers. Although it was once thought that these eﬀects were caused by
actual physical entanglements between molecules, it is now recognized that
they are not the result of localized restraints at speciﬁc points along the chain.
Instead, it is understood that they result from the severe impediment to lateral
motion that is imposed on a long molecule by all the neighboring molecules,
although the termentanglementcontinues to be used to describe this eﬀect. At
higher shear rates, the eﬀect of molecular weight on viscosity decreases, so it is
the zero-shear valueg
0that is most sensitive to molecular weight.
2.5 NORMAL STRESS DIFFERENCES
The shear rate-dependent viscosity is one manifestation of nonlinear visco-
elasticity, and there are two additional steady-state material functions
associated with steady simple shear when the shear rate is not close to zero.
These are theﬁrst and second normal stress diﬀerences N
1(˙c) andN 2(˙c). The
three material functions of steady simple shearg(˙c),N
1(˙c), andN 2(˙c) are
known collectively as theviscometric functions.
These are deﬁned using the standard frame of reference for simple shear
shown in Fig. 2.3. The shear stressrisr
21(equal tor
12), and the three normal
stresses are:r
11, in the direction of ﬂow (x
1);r
22, in the direction of the
gradient (x
2); andr 33,intheneutral(x 3) direction. As this is, by deﬁnition, a
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FIGURE2.3Standard coordinate deﬁnitions for simple shear.9opyright0ﬀII=0by0¨arcel0Cekkerk07ll0Rights0Reservedk

two-dimensional ﬂow, there is no velocity and no velocity gradient in thex
3
direction.
In an incompressible material, normal stresses are themselves of no
rheological signiﬁcance because as long as they are equal in all directions, they
cause no deformation. However, diﬀerences between normal stress compo-
nents are signiﬁcant, because they do cause deformation. For steady simple
shear, the two rheologically signiﬁcant diﬀerences are deﬁned by Eq.
(2.16a,b):
N
1ð˙c
:
Þur 11Fr22 ð2:16aÞ
N
2ð˙c
:
Þur 22Fr33 ð2:16bÞ
In the limit of zero-shear rate (i.e., for linear viscoelastic behavior), these two
material functions approach zero, and as the shear rate increases, they are at
ﬁrst proportional to the square of the shear rate. Thus, although the shear
stress becomes linear with shear rate as the shear rate approaches zero,N
1and
N
2are second order in˙cin this limit. This dependency inspired the deﬁnition
of the two alternative material functions deﬁned by Eq. (2.17a,b):
W
1ðc
:
ÞuN 1=c
:
2
ð2:17aÞ
W
2ðc
:
ÞuN 2=c
:
2
ð2:17bÞ
In other chapters of this book, it will be shown that these functions are related
to several types of ﬂow instability that occur in ﬂows of viscoelastic melts.
2.6 TRANSIENT SHEAR FLOWS USED TO STUDY
NONLINEAR VISCOELASTICITY
The response of a molten polymer to any transient shear ﬂow that involves a
large or rapid deformation is a manifestation of nonlinear viscoelasticity.
Some examples of ﬂows used to characterize nonlinear melt behavior are
described in this section.
For large-step stress relaxation, the relaxation modulus is a function of
the imposed strain as well as time:G(t,c). Except at very short times, the
nonlinear relaxation modulus is often found to exhibittime–strain separabil-
ity, which means that it can be represented as the product of the linear
relaxation modulus and a function of strainh(c) called thedamping function,
as shown by Eq. (2.18):
Gðt;cÞ¼GðtÞhðcÞð 2:18Þ
The damping function is thus a material function that is wholly related to
nonlinearity in a step strain test. As the strain approaches zero,h(c) obviously
approaches one, and as the strain increases, it decreases.
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The damping eﬀect can be understood in terms of thetube modelof the
dynamics of polymeric molecules. In this model, the constraints imposed on a
given polymer chain by the surrounding molecules and that give rise to
entanglement eﬀects are modeled as a tube. In response to a sudden
deformation, the tube is deformed, and the relaxation of the molecule of
interest is constrained by its containment in its tube. When the imposed
deformation is very small, two mechanisms of relaxation occur:equilibration
andreptation. Equilibration involves the redistribution of stress along the
chain within the tube. Further relaxation can only occur as a result of the
molecule escaping the constraints of the tube, and this requires it to slither
along orreptateout the tube. This is a much delayed mechanism, and this is
the cause of the plateau in the relaxation modulus for polymers with narrow
molecular weight distributions. If the molecular weight is not narrow, the
shorter molecules making up the tube will be able to relax fast enough to cause
a blurring of the tube. This phenomenon is calledconstraint releaseand speeds
up the relaxation of a long molecule in its tube.
The relaxation processes described above apply to linear viscoelastic
behavior. If the deformation is not small, there is an additional relaxation
mechanism—retraction within the tube. This is a fast relaxation, and once it is
completed, the remainder of the relaxation process occurs as in the case of a
linear response. It thus results in a relaxation modulus curve that has an early,
rapid decrease due to retraction, followed by a curve that has the same shape
as that for linear behavior. Thus, except for the very short-term relaxation, the
relaxation modulus can be described as the linear modulus multiplied by a
factor that accounts for the relaxation by retraction. This factor is the
damping function.
Instart-up of steady simple shear, the measured stress is divided by the
imposed constant shear rate to obtain theshear stress growth coeﬃcient,
which is deﬁned as follows:
g
þ
ðtÞurðtÞ=˙c ð2:19Þ
And the similarly deﬁnedshear stress decay coeﬃcientg
F
(t) describes stress
relaxation following the cessation of steady simple shear.
2.7 EXTENSIONAL FLOWS
Most experimental studies of melt behavior involve shearing ﬂows, but we
know that no matter how many material functions we determine in shear,
outside the regime of linear viscoelasticity, they cannot be used to predict the
behavior of a melt in any other type of ﬂow (i.e., for any other ﬂow
kinematics). A type of ﬂow that is of particular interest in commercial
processing and the instabilities that arise therein is extensional ﬂow. In this
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type of ﬂow, material points are stretched very rapidly along streamlines. An
important example is the ﬂow of a melt from a large tube into a much smaller
one. In order to satisfy mass continuity, the velocity of a ﬂuid element must
increase markedly as it ﬂows into the smaller tube, and this implies rapid
stretching along streamlines. This is shown schematically in Fig. 2.4.
Although entrance ﬂow subjects some ﬂuid elements to large rates of
elongation, the rate of elongation is not uniform in space, so this ﬂow ﬁeld is
not useful in determining a well-deﬁned material function that describes the
response of a material to extensional ﬂow. It turns out to be quite diﬃcult to
subject a melt to a uniform stretching deformation, and this is why reports of
such measurements are much rarer than those of shear ﬂow studies.
The experiment usually carried out to study the response of a melt to
uniaxial extension is start-up of steady simple extension at a constantHencky
strain rate˙e. The Hencky strain rate is deﬁned as follows, in terms of the length
Lof a sample:
˙e¼dlnL=dt ð2:20Þ
Note that the length of a sample subjected to a constant Hencky strain rate
increases exponentially with time. This strain rate is a measure of the speed
with which material particles are separated from each other. The nonlinear
material function most often reported is thetensile stress growth coeﬃcient,
which is deﬁned as the ratio of the tensile stress to the strain rate, as shown by
Eq. (2.21):
g
þ
E
ðt;e
:
Þur Eðt;e
:
Þ=e
:
ð2:21Þ
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FIGURE2.4Sketch of entrance ﬂow showing stretching along streamlines: (a)
without corner vortex; (b) with corner vortex.9opyright0ﬀII=0by0¨arcel0Cekkerk07ll0Rights0Reservedk

2.8 DIMENSIONLESS GROUPS GOVERNING THE BEHAVIOR
OF VISCOELASTIC FLUIDS
When making a general statement about the ﬂow behavior of polymers, it is
useful to represent the results of theoretical treatments and experiments in
terms of dimensionless variables. Two dimensionless groups are often used to
describe the rate or duration of an experiment. One of these, the Weissenberg
number (Wi), is a measure of the degree of nonlinearity or anisotropy
exhibited by the ﬂuid in a particular deformation, and the second, the
Deborah number (De), is a measure of the degree to which a material exhibits
elastic behavior. We will see in later chapters that these groups are useful for
describing ﬂow instabilities.
TheDeborah number(De) is a measure of the degree to which the ﬂuid
will exhibit elasticity in a given type of deformation. More speciﬁcally, it
reﬂects the degree to which stored elastic energy either increases or decreases
during a ﬂow. In steady simple shear, at steady state, when all stresses are
constant with time, the amount of stored elastic energy is constant with time,
so the Deborah number is zero. Thus, it is only in transient ﬂows that the
Deborah number has a nonzero value. Here‘‘transient’’means that the state
of stress in a ﬂuid element changes with time. This can arise as a result of a
time-varying boundary condition, in a rheometer, or of the ﬂow from one
channel into a smaller one so that acceleration is involved. This dimensionless
group is the ratio of a time arising from the ﬂuid’s viscoelasticity (i.e., its
relaxation time) to a time that is a measure of the duration of the deformation.
For example, consider the response of a ﬂuid to the start-up of steady
simple shear in which a shear rate of˙cis applied to a ﬂuid initially in its rest
state. In the initial stages of the deformation, the shear stress will increase with
time and will eventually reach a steady value. At timestafter the shearing is
begun, the stress will continue to change with time untiltis long compared to
the relaxation time of the ﬂuid. Thus, the Deborah number for this defor-
mation iss
r/t; when this ratio is large (short times), the amount of stored
elastic energy is increasing, but when it is small (long times), it becomes
steady, andDeapproaches zero. In this example, the Deborah number varies
with time.
The Deborah number is only zero in deformations with constant stretch
history (steady from the point of view of a material element). It is diﬃcult to
write a concise deﬁnition of a time constant that governs the rate at which
stored elastic energy changes in a given deformation without reference to a
speciﬁc rheological model, and we therefore give a deﬁnition of the Deborah
number in general terms:
Deu
sr
characteristic time of transient deformation
ð2:22Þ
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Turning to the Weissenberg number, its use can be demonstrated by
reference to steady simple shear ﬂow. We have seen that any ﬂuid that has a
shear rate-dependent viscosity or is viscoelastic must have at least one
material constant that has units of time and is characteristic of rheological
nonlinearity, and we have called this times
n. Furthermore, the type of
behavior exhibited by a particular material depends on how this time constant
compares with the reciprocal of the rate of the deformation. For example, let
us say that a non-Newtonian ﬂuid has a nonlinearity time constants
nof 1 sec.
This implies that if the shear rate˙cin steady simple shear is much smaller than
the reciprocal of this time, its viscosity will be independent of shear rate. This
suggests the deﬁnition of theWeissenberg number(Wi) as follows:
Wiuc
:
s
n ð2:23Þ
(The symbolsWeandWsare also used for the Weissenberg number.) Thus,
this dimensionless group is a measure of the degree to which the behavior of a
ﬂuid deviates from Newtonian behavior. For single-phase, low-molecular-
weight ﬂuids, the time constant of the material is extremely short, so that the
Weissenberg number is always very small for the ﬂows that occur under
normal circumstances. But for molten, high-molecular-weight polymers,s
n
can be quite large. For polymeric liquids, the Weissenberg number also
indicates the degree of anisotropy generated by the deformation. And in
the case of steady simple shear, the normal stress diﬀerences are manifes-
tations of anisotropy and thus of nonlinear viscoelasticity. Therefore, the
Weissenberg number also governs the degree to which the normal stress
diﬀerences will diﬀer from zero.
To summarize, when the Weissenberg number is very small in a simple
shear ﬂow, the viscosity will be independent of shear rate, and the normal
stress diﬀerences will be negligible. These phenomena reﬂect the fact that in
the limit of vanishing shear rate, the shear stress is ﬁrst order in the shear rate,
whereas the normal stress diﬀerences increase with the square of the shear
rate. The Weissenberg number is easily deﬁned for any ﬂow with constant
stretch history. For example, for steady uniaxial extension, we need only
replace the shear rate by the Hencky strain rate˙e.
In general, the degree to which behavior is nonlinear depends on the
Weissenberg number. At very small deformation rates,Wiis much less than
one, and the stress is governed by the Boltzmann superposition principle. At
higher deformation rates,Wiincreases and nonlinearity appears, as reﬂected,
for example, in the dependence of viscosity on shear rate and the appearance
of normal stress diﬀerences. The two characteristic times deﬁned above (s
n
ands
r) are closely related, and in recognition of this, the subscripts will be
dropped in the rest of this discussion.
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There are several pitfalls in the use of the two dimensionless groups
deﬁned above to characterize the response of a material to a given type of
deformation. First, there are hardly any materials whose viscoelastic behavior
can be described using a single relaxation time. More typically, a spectrum of
times is required, and this causes diﬃculty in the choice of a time for use in
deﬁning the Deborah number. The‘‘longest relaxation time’’is often
identiﬁed as the appropriate one for deﬁning these groups, but for highly
polydispersed or branched polymers, it may be impossible to identify a
‘‘longest time.’’
Another problem in the use of dimensionless groups to characterize
deformations is that for a melt consisting of a single polymer, in several ﬂows
of practical interest,WiandDeare directly related to each other. This causes
confusion, as authors of books and research papers tend to use the two groups
interchangeably in all circumstances. Some examples will help to illustrate the
correct use of the Weissenberg and Deborah numbers.
A ﬂow that has been of great interest to both experimental and
theoretical polymer scientists is the entrance ﬂow from a circular reservoir
into a much smaller tube or capillary. A Weissenberg number can readily be
deﬁned for this ﬂow as the product of the characteristic time of the ﬂuid and
the shear rate at the wall of the capillary. However, entrance ﬂow is clearly not
a ﬂow with constant stretch history, and the Deborah number is thus nonzero
as well. Furthermore, as demonstrated by Rothstein and McKinley (5), in
such ﬂows, the two groups are directly related. This can lead to confusion
when experimental data and ﬂow simulation results are compared (6). Note
that for fully developed capillary ﬂow,De=0.
On the other hand, a ﬂow in which the two numbers can be varied
independently is oscillatory shear. As used for the determination of the linear
viscoelastic behavior of polymers, this deformation is carried out at very small
strain amplitudes, so that the Weissenberg number will be much less than one.
As the frequency is increased from zero, the Deborah number, deﬁned here as
xs, is at ﬁrst very small, and the response is purely viscous. BecauseWiis also
very small, the melt behaves like a Newtonian ﬂuid. If now the frequency is
increased, the Deborah number increases and the importance of elasticity
grows, and at very highDe, the behavior becomes almost purely elastic.
If, however, the strain amplitudec
ois increased, then the strain rate
amplitudec
.
ouxco, and the Weissenberg number, which is equal toc
. os, also
increases. By changing the frequency or the amplitude,WiandDecan be
varied independently. A convenient way of representing the parameter space
of large-amplitude oscillatory shear is a Pipkin diagram, which is a graph of
Weissenberg number vs. Deborah number (4,7). In such a diagram, the
behavior in the lower left-hand corner (Wib1;Deb1) is that of a
Newtonian ﬂuid. Near the vertical axis (Deb1), the behavior is nonlinear
but inelastic and is governed by the viscometric functions. Near the hori-Copyright 2005 by Marcel Dekker. All Rights Reserved.

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zontal axis (Wib1), linear viscoelastic behavior is expected, and far to the
right (DeH1), the behavior becomes indistinguishable from that of an elas-
tic solid.
2.9 EXPERIMENTAL METHODS IN RHEOLOGY—RHEOMETRY
The objective of rheometry is to make measurements whose data can be
interpreted in terms of well-deﬁned material functions, without making any
assumptions about the rheological behavior of the material. Examples of
material functions are the viscosity as a function of shear rate and the
relaxation modulus as a function of time. These functions are physical
properties of a material, and the detailed method by which they are deter-
mined need not be reported in order for them to be properly interpreted. In
contrast to this type of measurement are empirical industry tests that yield
numbers that can be used to compare materials but are not directly related to
any one physical property. Such test data are widely used as speciﬁcations for
commercial products and for quality control. An important example that
involves the ﬂow of molten plastics is themelt ﬂow rate, often called themelt
index(8).
In an experiment designed to determine a material function, it is
necessary to generate a deformation in which the streamlines are known a
priori (i.e., that are independent of the rheological properties of the material).
Such a deformation is called acontrollableﬂow, and the number of such
deformations is very limited. In fact, the only practically realizable control-
lable ﬂows are simple shear, simple (uniaxial) extension, biaxial extension,
and planar extension. And the last two of these are suﬃciently diﬃcult to
generate that they are rarely used. Additional limitations on our ability to
determine rheological material functions in the laboratory are imposed by
various instabilities that occur even in these very simple ﬂows.
There are two ways of generating a shear deformation in a rheometer:
drag ﬂow and pressure ﬂow. In drag ﬂow, one surface in contact with the
sample moves relative to another to generate shearing. In pressure ﬂow,
pressure is used to force the ﬂuid to ﬂow through a straight channel, which
may be a capillary or a slit. Drag ﬂow can be used to determine a variety of
material functions, including the storage and loss moduli as functions of
frequency, the creep compliance as a function of time, the viscosity and
normal stress diﬀerences as functions of shear rate, and various nonlinear
transient material functions. Pressure-driven rheometers are useful primarily
for the measurement of viscosity at high-shear rates.
Extensional rheometers are most often designed to generate uniaxial
(tensile) extension in which either the tensile stress or the strain rate is
maintained constant.Copyright 2005 by Marcel Dekker. All Rights Reserved.

Below are presented brief overviews of the way melt rheometers are
used. More detailed information about experimental rheology can be found in
various rheology books (3,4).
2.9.1 Rotational Rheometers
Rotational rheometers can be classiﬁed according to the type of ﬁxture used
and by the variable that is controlled (i.e., the independent variable). Two
types of ﬁxture are commonly used with molten polymers: cone-plate and
plate-plate. The ﬂow between a cone and a plate, one of which is rotating with
respect to the other at an angular velocityX, closely approximates uniform
simple shear, as the shear rate at a radiusris the local rotational speedXrof
the rotating ﬁxture atr, divided by the gap between the ﬁxtures at this value of
r, which we callh. In the cone and plate geometry, this distance is linear withr,
with the result thatrX/h(i.e., the local shear rate) is uniform. This feature of
cone-plate ﬂow makes it useful for studies of nonlinear viscoelasticity. Cone-
plate ﬁxtures are used to determine the viscosity and ﬁrst normal stress
diﬀerence as functions of shear rate at low-shear rates, as well as the response
of the shear and normal stresses to various transient shearing deformations.
The equations for calculating the strain rate and stresses of interest are
as follows for the case of a small cone angleH
o:
˙c
:
¼X=H
o ð2:24Þ
r¼3M=2pR
3
ð2:25aÞ
N
1¼2F=pR
2
ð2:26Þ
whereXis the angular velocity of the rotating ﬁxture;ris the shear stress;Mis
the torque measured on either the rotating or the stationary shaft;Fis the
total normal thrust on the ﬁxtures; andRis the radius of the ﬁxtures.
Uniform simple shear is very well approximated by the ﬂow between a
cone and a plate; if the cone angle is small, and the shear rate is not very high.
Starting at moderate shear rates, however, various types of instability render
the ﬂow unsuitable for rheological measurements. Thus, cone-plate ﬁxtures
are useful for determining moderate departures from linear viscoelasticity.
The major concerns in using this technique are calibrating the sensors,
avoiding degradation of the sample, and recognizing when an instability
has occurred.
Although cone-plate ﬁxtures can be used to determine the material
functions of linear viscoelasticity, it is more convenient to use plate-plate
ﬁxtures for this application because the preparation and loading of samples,
as well as the setting of the gap between the two ﬁxtures, are much simpliﬁed.
Although the local shear strain between the parallel plates varies linearly with
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De voedingswaarde van de
paddenstoelen.
Wanneer ik met de menschen over de paddenstoelen als voedsel spreek, dan
krijg ik geregeld te hooren: “ze bestaan immers voor 90% uit water, hoe
kunnen ze nu als voedsel eenige waarde hebben!”
Ik begin dan altijd met ze te vragen, of ze wel weten dat “vleesch” en eieren
ook eventjes voor 80% uit water bestaan en dan doe ik natuurlijk het
bekende verhaal van de Polen, die, door hongersnood gedreven, er toe
kwamen gedurende eenigen tijd uitsluitend van paddenstoelen te leven en er
sinds dien tijd groote liefhebbers van gebleven zijn.
Maar ook zonder hongersnoodtijd zijn er heel wat menschen in Rusland,
Frankrijk, Duitschland, Japan en Indië, die in den herfst en in andere
paddenstoelrijke tijden, vnl. van paddenstoelen leven.
Het verhaal gaat van een bergbewoner in Thüringen, die op 100-jarigen
leeftijd stierf en die zich de laatste 30 jaren uitsluitend met paddenstoelen
gevoed had.
Een fransch mycoloog leefde gedurende eenigen tijd alleen van een portie
paddenstoelen van 3 ons per dag en hij bevond er zich uitstekend bij.
De groote “Persoon” vertelt in zijn “Traité sur les champignons”, van een
professor in de Botanie uit Leipzig, die, op een botanische reis in de
omstreken van Neurenberg, zich gedurende vele weken op voorbeeld van
de bevolking aldaar, alleen voedde met, door anijs en karwij gekruid, zwart-
brood en met rauwe paddenstoelen. In plaats van door deze manier van
voeden te verzwakken, voelde hij er zich in tegendeel krachtiger door
worden.

En zóó zou ik nog vele verhalen kunnen doen; zóó o.a., vertelt de Russische
Professor Socoloff (1873), dat een groot gedeelte van de bevolking uit zijn
landstreek zich gedurende de vasten met paddenstoelen voedde. Hij ook
zegt er bij, dat naar zijn meening de paddenstoelen het dierlijk voedsel zeer
nabij komen en zeer goed kunnen vervangen.
Men werpt mij, als ik over “paddenstoelen als voedsel” spreek, niet alleen
hun groot watergehalte, maar ook de “onverteerbaarheid” voor de voeten.
Zelfs mijn collega’s “mycophagen” hebben mij soms verzekerd, dat je er ’s
nachts zoo geweldig zwaar van slaapt en droomt.
Ik voor mij, geloof dat dit meer ligt aan het niet maat kennen van mijn
collega’s of het niet goed toebereiden, want al 7 jaar lang heb ik ze gegeten,
zonder ooit last van die onverteerbaarheid te hebben gehad. (Zie over de
boleten ook blz. 67).
Laat ik nu echter eens vertellen, welke voedende stoffen de paddenstoelen
wel bevatten.
Er zijn in de laatste 50 jaren al heel wat onderzoekingen over de
scheikundige stoffen der paddenstoelen gedaan. Deze zijn vnl. van:
Loesecke (1876), Kohlrausch (1867), König (1903), Margewicz (1890),
Strohmer, Zega (1902), Lafayette en Mendel. Volgens Dr. Julius Zellner,
wiens onderzoekingen over de scheikundige stoffen der paddenstoelen wel
een der nieuwste zijn en aan wiens boek “Chemie der Höheren Pilze”
(1907) ik de volgende gegevens ontleen, hebben deze heeren allen, de
voedingswaarde, die er in de paddenstoelen zit, sterk overdreven. Zij gaven
een groot stikstofgehalte op, terwijl volgens Dr. Zellner de voedende stof,
de z.g.n. proteïnstof, het verteerbare eiwit, slechts zeer gering is en lang
geen 6½% bedraagt, zooals zij opgeven.
De scheikundige bestanddeelen van de paddenstoelen staan in nauw
verband met het substraat, waar zij op voorkomen. Zoo zullen die van de op
de aarde of humus levende soorten, weer anders zijn dan die der
boomzwammen. Toch gaan de stoffen van het substraat niet onverwerkt in

het zwammenlichaam over, maar worden door assimilatieprocessen in
andere verbindingen omgezet.
De paddenstoelen zijn, als alle levende wezens, opgebouwd uit water,
anorganische (minerale) stoffen, vetten, koolhydraten en
stikstofverbindingen (eiwit).
Het watergehalte wisselt natuurlijk zeer af naarmate de exemplaren jong of
oud zijn, naar den vochtigheidstoestand van grond en lucht, en is ook bij de
onderlinge soorten zeer verschillend. Gemiddeld bevatten volwassen,
versche paddenstoelen 90% water. Een der waterrijkste soorten is wel
Coprínus comátus (94%). Psallióta arvénsis bevat er slechts 56% van.
Truffels en boomzwammen gemiddeld 65–80%. Over het algemeen schijnt
het watergehalte in de stelen grooter te zijn dan in de hoeden.
Om de scheikundige stoffen te onderzoeken, die den paddenstoelen eigen
zijn, maakt men gebruik van de zoogenaamde asch-analyses. Het
aschgehalte van de paddenstoelen bedraagt bij versch materiaal 0.48–2%,
bij droog meest 4–10%. De grootste helft bestaat hiervan uit
kaliumverbindingen, vnl. potasch (K
2
O) voor 19–57% en voor 18–39% uit
phoshorzuur (P
2
O
5
), verder een weinig, 4%, natrium en zeer geringe
hoeveelheden ijzer, (vrij veel bij Merúlius lácrymans) magnesium, kalk,
zwavelzuur, kiezelzuur en chloor.
Het vetgehalte der paddenstoelen wisselt bij versche exemplaren van 0.12%
(Fistúlina hepática) tot 67% (Lactárius deliciósus), bij droge van 1.3–8%,
een gehalte zooals meest alle groenten hebben.
De, in het eiwit van kippeneieren en in visch voorkomende, zoo
zenuwsterkende stof, het Lecithin, een phosphorzure vetverbinding, wordt
ook vrij algemeen in de paddenstoelen aangetroffen. Psallióta campéstris
schijnt hiervan in verschen staat 0.32%, Bolétus edúlis 1.94% te bevatten.
Wat de koolhydraten betreft, zijn ten eerste de cellen van de paddenstoelen
opgebouwd uit een chitinachtige stof fungicellulose of fungin genaamd,
welke 3% van het aschgehalte uitmaakt. Deze fungi-cellulose wijkt af van

de gewone cellulose, waarvan de planten zijn opgebouwd en is eenig in het
plantenrijk. Zij wordt ’t meest gevonden in den steel en ’t minst in de
sporen voortbrengende deelen. Hare taaie substantie veroorzaakt een deel
van de onverteerbaarheid der paddenstoelen (stelen zijn daardoor dan
meestal ook voor de consumptie onbruikbaar). Door toevoeging echter van
een weinig dubbelkoolzure natron wordt bij soorten, die een hoog
fungingehalte hebben, de verteerbaarheid van deze fungi-cellulose
bevorderd. Behalve deze in het plantenrijk niet voorkomende stof schijnen
de boleten nog een aparte stof te produceeren in hun cellen, nl. viskosin en
mycétide, die de, bij het koken optredende, bekende slijmerige massa
geven, welke de boleten voor sommige personen tot een onverteerbaar
voedsel maakt. Tot de verteerbare koolhydraten in de paddenstoelen
aanwezig, behoort verder het in alle soorten voorkomende glycogeen, een
aan zetmeel verwante stof, welke wel in ’t dierlijk organisme, echter nooit
in andere planten voorkomt. Het echte zetmeel ontbreekt geheel bij
zwammen. Vooral in jonge exemplaren is het glycogeen aanwezig; bij
oudere, volwassen soorten is het meestal vervangen door het manniet, een
stof waaraan v.n.l. gedroogde zwammen en ook de stelen van versche
exemplaren zeer rijk zijn.
Verder vond Bourquelot (1893–1896) in wel 200 verschillende
paddenstoelensoorten suikers, vooral een suikerstof threhalose of mycose
geheeten, die nog maar zelden in het plantenrijk is aangetroffen. Vooral
jonge exemplaren en in ’t bijzonder de soorten van het geslacht Cortinárius
schijnen aan die threhalose rijk te zijn. Ook de glucose is algemeen in de
zwammen vertegenwoordigd, echter in geringe mate en slechts bij
volwassen exemplaren.
Ten slotte de stikstofverbindingen, het eiwit. De paddenstoelen bevatten
over ’t algemeen 2–3% verteerbaar eiwit, wat ze in voedingswaarde gelijk
maakt, b.v. aan kool, iets minder dan brood, veel minder dan erwten en
boonen (de truffel heeft een eiwitgehalte gelijk aan deze laatste). Het
meeste eiwit huist in de sporen voortbrengende deelen, vooral in de buisjes
der boleten, zoodat men deze vooral zoo min mogelijk weg moet snijden bij
de bereiding. Tot de meest eiwitbevattende eetbare paddenstoelen behooren
wel: Psallióta campéstris en Lycopérdon bovísta.

Hieronder volgt een tabel volgens Villiers, Collin en Lehman ter
vergelijking van de voedingswaarde van versche paddenstoelen met brood
en vleesch:
Versche paddenstoelen.Brood. Ossenvleesch.
Water 900 300 à 400 800
Minerale zouten 8 5 à 7 30
Weefsel 30 2 à 4 150 à 170
(fungocellulose)(cellulose)(spieren, peezen enz.)
38
Koolhydraten (thréhalose)500 à 600 15 à 25
9
(manniet) (zetmeel) (vet)
Verteerbaar eiwit 15 75 20 à 30
Maken wij uit het voorgaande eenige besluiten, dan zien wij, dat, ofschoon
de paddenstoelen door hun klein gehalte aan verteerbaar eiwit, het in
voedingswaarde tegen brood en vleesch moeten afleggen, we ze door een
vrij groot gehalte aan voedende zouten en bovenal door de aan het lichaam
zooveel warmte-energie gevende suikers, werkelijk niet als een
voedingsmiddel moeten onderschatten. Men vergete daarenboven niet, dat
zij bij het koken meer water verliezen dan vleesch en men dus, bij een
gelijk volume van gekookt materiaal, in vergelijking meer procenten
voedingsstoffen krijgt, terwijl men er bovendien meer van eten kan dan van
vleesch. In elk geval zijn zij door grooter gehalte aan minerale zouten en
suikers, voedzamer dan de bladgroenten waarmee ze gewoonlijk als spijzen
vergeleken worden. Deze missen ook het in de paddenstoelen voorkomende
glucogeen, lecithin en het chitinachtige celweefsel, allen stoffen, die in het
dierlijk organisme thuis behooren. Naar mijn meening zijn dan ook
diegenen, die de paddenstoelen “plantaardig vleesch” of “vleesch van het
woud” noemen, dichter bij de waarheid, dan de hygiënisten, die ze onder de
groenten rangschikken.

Zoo beschouwd, (en wie, die reeds eenige malen paddenstoelen gegeten
heeft, vindt de gelijkenis met vleesch en vooral kalfvleesch, niet frappant),
is het zeker waar, dat ofschoon dan iets minder voedzaam, een voortdurend
en niet overmatig gebruik van deze natuurproducten den mensch minder
zullen schaden dan een dergelijk gebruik van dierlijk voedsel, ten minste:
als wij alleen die soorten eten, die we als goede ongevaarlijke soorten
kennen en nogmaals kan ik er niet genoeg op wijzen om zonder kennis der
soorten ze niet te zoeken of te gebruiken.

Het verzamelen en het toebereiden van
eetbare paddenstoelen.
De “jacht” op paddenstoelen is voor mij maar heel kort gesloten, ten minste
in vorstvrije winters, hoogstens maar een maand, van half Februari tot
April, want in ’t laatst van Maart, dan komen de morieljes (fig. 20) No. 12,
alweer te voorschijn en als die afgeloopen zijn vallen er al inktzwammen
(fig. 114) No. 266 en 267 en de voorjaars Tricholóma gambósum No. 26, te
plukken voor den mycophaag.
Dan volgen, in parken en weilanden, in ’t laatst van Mei de overheerlijke
Marásmius oréades (fig. 25 en 89) No. 211, in Juni de Reuzenbovist No.
271 (fig. 116) en dan komen langzamerhand de champignons, Psallióta-
soorten, (fig. 18 en P1. 2 fig. VII) No. 263 en 264 haar smakelijke hoeden
boven ’t gras der weilanden uit steken en wordt de boschgrond weldra
oranje gekleurd door de cantharel of dooierzwam (fig. 19) No. 206. In den
na-zomer en de herfstmaanden, heb ik tot in ’t laatst van November volop
keus van allerlei smakelijke soorten en als die hun aardsch bestaan
geëindigd hebben, is het smakelijke “fluweelpootje” Collýbia velútipes (fig.
61) No. 117, zoo vriendelijk mij tot half Februari een smakelijke soep en
groente te verschaffen.
De meeste kans op goeden buit bij de jacht heeft men bij eerst
overvloedigen regenval, gevolgd door warmte, ook na sterken dauw en op
zwoele warme herfstdagen. Ik trek er dan op uit met mijn mandje en
verschillende zakjes gewapend, want ik houd bij het plukken graag elke
soort apart, omdat ik ze zelden dooréén gemengd, toebereid.
Als regel pluk ik de hoeden af en laat de stelen staan, die bij de meeste
soorten onbruikbaar zijn. Dat afplukken geeft het voordeel niet alleen van
meer te kunnen bergen maar ook houdt men op die manier zijn voorraad
reiner, daar de stelen meest met aarde en vuil bedekt zijn. Een uitzondering
hierop maak ik met het plukken van de champignons Psallióta-soorten (fig.

17), om reden dat de stelen van jonge exemplaren ten eerste zeer goed
eetbaar zijn, doch v.n.l. om de bekende, op blz. 85 zeer uitvoerig,
beschreven reden tot mogelijke vergissing met de op hen gelijkende Groene
Knolzwam: Amaníta phalloídes (fig. 16).
Ik pluk bij voorkeur jonge exemplaren (bij de Bolétus-soorten (fig. 21 en
26) ook met de stelen), daar deze natuurlijk het smakelijkste zijn. Echter
frissche, volwassen exemplaren, die goed zijn voor de paddenstoelensoep,
worden niet versmaad, doch verwaterde, slappe, oude paddenstoelen
worden nimmer door mij medegenomen.
Het gebruik van dergelijke oude zwammen heeft reeds dikwijls aanleiding
gegeven tot het optreden van ernstige ongesteldheden. Ik voor mij geloof,
dat een gedeelte van de gevallen van niet ernstige
paddenstoelvergiftigingen, in de kranten altijd met zooveel ophef vermeld,
te wijten zijn niet aan het eten van vergiftige soorten (want de menschen
zijn in ’t algemeen niet zoo roekeloos), doch aan het consumeeren van te
lang bewaarde of te oude zwammen. Er schijnen dan in het
zwammenweefsel z.g.n. secondaire zwammen op te treden die een geweldig
toxineerende werking hebben. Men wachte er zich dus ook voor, te zuinig
te zijn en verwijdere wel degelijk uit de verzamelde massa, de niet meer
frissche exemplaren.
Men leest verder ook dikwijls, dat de, door de maden van zwammuggen of -
vliegen aangetaste, paddenstoelen giftig zouden zijn. Ze zijn dan echter
meestal minderwaardiger dan de onaangetasten en vanzelf worden ze
daarom verwijderd. Echter menig jong exemplaar waar ik het aangetaste
deel uitsneed, is door mij als uitstekend voedsel bevonden en me altijd best
bekomen.
Met den buit thuis gekomen, maak ik ze zoo spoedig mogelijk “panklaar”,
want het is verwonderlijk hoe spoedig de paddenstoelen, eenmaal afgeplukt
zijnde, tot rotting overgaan.
Als dan de jacht nog al lang geduurd heeft, valt dat “panklaar” maken nog
wel eens leelijk tegen, want het schoonmaken van de paddenstoelen is een

werkje, dat niet meevalt en veel zorg eischt. Is het huidje van den hoed
afneembaar, dan wordt dat er zorgvuldig afgehaald, liefst met een houten
vruchtenmesje (metaal kleurt het paddenstoelen-vleesch bruin). Jonge
exemplaren, ook jonge champignons reinig ik met een borsteltje.
De schubben op sommige hoeden, b.v. bij de Lepióta-soorten (fig. 24)
worden er afgesneden. Vele soorten echter, zooals Marásmius oréades (fig.
25) en Collýbia velútipes (fig. 61) laten geen aftrekken van ’t huidje toe
door de dunvleezigheid en deze worden daarom alléén zorgvuldig
afgewasschen.
Bij de soorten, die geen taaie maar eenigszins vleezige stelen hebben, welke
daarom mede afgeplukt zijn (zooals het geval is bij de Psallióta-, Lepióta-,
Coprínus- en jonge Bolétus-soorten), snijd ik den steel van den hoed af,
daar zij vrij wat langer moeten koken dan de hoeden.
Hun, die kippen houden of een visch-vijver hebben, kan ik aanraden, den
afval, verkregen bij het paddenstoelenschoonmaken, aan die dieren te
voederen. Ook kan men dien bij wijze van mest gebruiken.
Wat de plaatjes der plaat- en de buisjes der buisjeszwammen betreft, die,
zooals in ’t vorige hoofdstuk vermeld, eigenlijk de meeste voedende stoffen
bevatten, men snijde ze niet te veel af, hoewel het niet valt te ontkennen, dat
die buisjes een onaangename slijmerigheid geven en de zwarte plaatjes het
maal niet smakelijk kleuren.
Na het schoonmaken komt het wasschen, wat eveneens een geduldwerkje
is, daar gewoonlijk een 10–12 maal afwasschen voor elke soort noodig is,
wil men niet “tandenknarsend” van het zand, straks zijn maaltje nuttigen.
Dat afwasschen moet vlug gaan, daar anders het paddenstoelen-aroma in
het water achter blijft. Voor de Psallióta’s (champignons) en boleten doe
men een weinig citroensap in het afwaschwater voor het blank blijven van
het vleesch. Voor de morieljes geeft men altijd op: eerst 24 uur in water
laten staan, opdat het giftige helvella-zuur er uittrekke. Aangezien echter dit
zuur er onmiddellijk bij ’t koken uittrekt, laat ik dat na, omdat ik bij
ervaring ondervonden heb, dat ze dus behandeld, veel van hun geur

verliezen. Ze eischen echter eventjes een 20–24 keeren afwasschen in
water, waarin wat zout is gedaan.
Zijn de paddenstoelen éénmaal gewasschen, in de pan (steen of emaille)
gedaan en met een weinig zout bestrooid, zoo kunnen ze bij gebrek aan tijd,
gerust tot den volgenden morgen ter verdere toebereiding in den kelder
gezet worden.
Men late de paddenstoelen, vóór men ze in de pan doet vooral niet op een
vergiet uitdruipen, daar het daaraan hangen blijvende water juist voldoende
is voor de gaarkokerij. Bijvoeging van water voor ’t koken is slechts voor
enkele soorten, die niet zeer waterrijk zijn zoo o.a. bij Cantharéllus cibárius
(fig. 19) het geval is, noodig en ook wanneer groote hoeveelheden te gelijk
gekookt worden. Het smakelijkste worden ze, wanneer men er bij ’t koken
een flink stuk boter aan toevoegt, maar noodig is dit niet.
Merkwaardig is het, dat bijna alle paddenstoelensoorten denzelfden tijd tot
gaarkoken hebben. Moeilijk kan ik dien tijd voor de verschillende
verwarmingstoestellen opgeven. Ik kook ze zelf altijd op een matig vuur,
n.l. op een petroleumstel met 3 pitten, vol aan, en dan reken ik van het
opzetten af altijd 20 minuten (voor groote hoeveelheden van 20–30
minuten) waarna ze een heerlijk, malsch en gaar stadium bereikt hebben.
Uitzonderingen hierop maken voor minder dan 20 min.: Marásmius oréades
(fig. 25) met 15 min., Lycopérdon Bovísta (fig. 116) met 10 min., terwijl de
Morieljes (fig. 20) 30 min. noodig hebben. Ook de Cantharel (fig. 19) kreeg
ik bij 20 min. koken gaar, terwijl de boeken daarvoor den tijd van een uur
opgeven. Overschrijdt men die tijden, dan krijgen ze de bekende taaie,
onverteerbare consistentie en verdwijnt het “aroma” geheel.
De Duitsche manier om de paddenstoelen te bakken is mij nog steeds maar
matig bevallen; ze deden mij zoo behandeld, meer aan gebraden leeren
lappen dan aan een lekkernij denken. De biefstukzwam, Fistulína hepática
No. 170 (fig. 79) op deze wijze toebereid, voldeed goed en geeft plus een
eenigszins zuur smaakje, een gerecht, dat eenige overeenkomst vertoont
met echte biefstuk.

En is het nu nog noodig hierbij neer te schrijven, dat de beruchte zilveren
lepel (er zijn altijd nog menschen, die er aan gelooven) er bij het koken
heusch niet bij hoeft, want dat het een totaal onbetrouwbaar middel is. Het
zwart worden van den lepel is een gevolg van zwavelverbindingen in de
spijzen en in zooverre zou het nog van eenig nut kunnen zijn den lepel er bij
te voegen, omdat hij zwart geworden, zou aangeven, dat de gekookte
zwammen niet frisch meer waren; doch de giftige zwamstoffen maken den
lepel in ’t geheel niet zwart. Ik heb ze zelf gekookt met een der giftigste
soorten en de lepel bleef zoo blank als zilver. Evenzoo is zeer af te raden
een middel, dat nog door sommige boeken wordt aangegeven om tegen
zwammenvergiftigingen gevrijwaard te worden, nl. door de paddenstoelen
eens of meer keeren in water met azijn af te kooken en dan dit water weg te
gooien. De paddenstoelen, zeggen ze dan, kunnen vervolgens met een
gerust hart gegeten worden. Doch dit is zeer zeker niet waar; met sommige
weinig giftige soorten schijnt inderdaad op deze manier die stof er uit te
trekken, doch met de giftigste der giftigen de Amaníta phalloídes, (fig. 16)
No. 24, zou dit pas het geval zijn na een keer of 8 afkoken. En wie zal, na al
dien schoonmaak en afwaschpartijen ten eerste nog eens lust hebben 8 keer
iets te koken en dan.... welk een smakeloos gerecht zal men op die manier
nog over houden! Neen, men bereidt ze als boven gezegd is (en gooie
vooral het nat dat wel het meeste aroma bevat niet weg) als men zeker is
een goede soort te hebben en is men dat niet, dan doe men al die moeite
liever niet en brenge ze niet op zijn tafel. Daar sommige soorten, zooals de
Morieljes (fig. 20) de Helvélla’s (fig. 40 en 41) en Marásmius oréades (fig.
25) een zuur bevatten, de eerste twee het helvella—de derde, blauwzuur, dat
vluchtig is en eerst pas bij ’t koken voorgoed het zwammenweefsel verlaat,
moet men zorg dragen, dat deze soorten vooral goed doorgekookt hebben.
Zijn de paddenstoelen eenmaal gekookt, dan kunnen ze gerust op een koele
plaats, een of twee dagen (langer vooral niet) bewaard worden om er
desverlangd smakelijke gerechten van te bereiden.
De nu volgende recepten, berusten op eigen praktijk en zijn zoo eenvoudig
mogelijk.
Naar mijn meening toch, geniet men het meeste van het paddenstoelen-
aroma als men er al dien poespas van kruiderijen enz. die de Duitschers er

o.a. zoo graag bij gebruiken, er uit laat.
Voor hen die meer gecompliceerde recepten wenschen noem ik het boekje:
de “Champignonkeuken” paddenstoelen-recepten, bijééngebracht door
Lucullus, uitgegeven te Haarlem bij J. L. E. J. Kleinenberg 1910, kostende
40 cts. en naar de werkjes van Michaël en Dumée en anderen, zie blz. 101.

Recepten voor paddenstoelengerechten.
Paddenstoelensoep.
Hiertoe leenen zich alle eetbare soorten, Marásmius oréades (fig. 25) wel in ’t
bijzonder. Voor de bereiding gebruik ik ook een gefiltreerd aftreksel van
stukken en afsnijdsels, welke b.v. niet mooi genoeg zijn voor het steriliseeren,
ook stukken met plaatjes en buisjes, enz.
De paddenstoelen worden op de gewone wijze gekookt, het nat afgeschonken
en vermengd met het voor de hoeveelheid van de soep benoodigde water,
waarin een of eenige fijn gesneden uien zijn gekookt. Een flink stuk boter
wordt met bloem (½ eetlepel per persoon) opgesmolten en dan met het nat
van de paddenstoelen en uien aangemengd. Even voor het opdoen worden de
paddenstoelen er in gedaan en ook eenige lepels soya, terwijl men het geheel
nog met eierdooiers kan binden.
In plaats van het uienwater, kan ook bouillon, het nat van asperges of van
andere groenten gebruikt worden.
Extract van paddenstoelen.
Hiervoor kan eveneens weer het aftreksel van afsnijdsels, plaatjes, buisjes
enz. gebruikt worden en wel van alle eetbare soorten. Men late het nat
zoolang koken, totdat het de dikte van stroop heeft verkregen, waarna het in

Wecks-flesschen gesteriliseerd wordt. In paddenstoel-arme tijden is dit
extract te gebruiken v.n.l. voor soep; 1 eetlepel van dit extract is voldoende
voor 5 personen.
Paddenstoelen als groente.
Hiertoe leenen zich alle eetbare soorten.
De gekookte paddenstoelen worden gestoofd in een sausje, gemaakt van het
nat van de paddenstoelen met wat boter en meel. In plaats van het
paddenstoelennat kan men ook bouillon of room nemen. Dit laatste is zeer
aan te bevelen bij morieljes.
Paddenstoelensla.
Men koke de in stukjes gesneden paddenstoelen niet te gaar, laat ze bekoelen
en maakt ze dan op dezelfde manier aan als kropsalade. Vooral de
boletussoorten leenen zich hier uitstekend voor.
Paddenstoelen met tomaten.
Men bereide de paddenstoelen als boven aangegeven voor paddenstoelen als
groente en vermenge ze met de in stukjes gesneden, gekookte tomaten.

De soorten: Lycopérdon bovísta (jong) No. 27, de boleten en champignons
(Psallióta’s) leenen zich hier bij uitstek voor.
Paddenstoelen met roereieren.
De eieren worden geklopt en daarna met wat zout, een stukje boter en wat
melk op een matig vuur in een pannetje verhit en geroerd tot het een dikke
gelijke massa is geworden. Hierdoorheen roert men de in stukjes gesneden,
reeds gekookte paddenstoelen (kleine exemplaren ook heel) en een weinig
soya.
Lycopérdon bovísta, Coprínus comátus (No. 266) en de champignons leenen
zich hier bij uitstek voor.
Paddenstoelen ragoût.
Hiervoor kan men alle goede soorten gebruiken.
Men bereide de paddenstoelen zooals voor paddenstoelen als groente is
aangegeven en voege daarbij gekookte gehaktballetjes en ragoût-sausijsjes.
(Ook restanten van allerlei vleesch kunnen hiervoor gebruikt worden).
Toevoeging van eenige lepels soya verhoogt den smaak.
Paddenstoelen op gebakken broodjes.

Men make een dik sausje van het paddenstoelennat met boter en meel en wat
kerry; daarin de in kleine stukjes gesneden paddenstoelen en het geheel
opgediend op in vet gebakken vierkante stukjes brood.
De Champignons, Lycopérdon bovísta en de boleten leenen zich hiervoor het
beste.
Omelet met paddenstoelen.
De omelet wordt op de gewone manier bereid en vóór het dichtslaan, met de
gekookte paddenstoelen gevuld.
Paddenstoelen in ’t zuur.
Hiertoe leenen zich alle vleezige soorten, in ’t bijzonder jonge, kleine
exemplaren van Cantharéllus cibárius (fig. 19 No. 206) en Clavária-soorten
(fig. 93 en 94 No. 227, 228, 230).
Zij worden daarvoor (ongekookt) met wijn-azijn en de noodige kruiderijen,
als: peperkorrels, kruidnagelen, dragon, laurier, chalotjes, Spaansche peper,
enz. in de Weck-flesschen gedaan en gesteriliseerd.

Fig. 27. Clitócybe nebuláris.
Fig. 28. Clitócybe (Laccária) laccáta.

Bijzonder goed leent zich Canth. cib. voor zoet-zuur. Het wecken gebeurt dan
met wijn-azijn, bruine suiker en kruidnagelen (evenals meloenschillen).
Maakt men niet in volgens Weck-systeem, dan kan men de paddenstoelen in
’t zuur inmaken op de manier waarop men augurken enz. inmaakt. Ze zijn
dan echter spoediger aan bederf onderhevig en een herhaald opkoken van de
azijn is aan te raden.

Het conserveeren van eetbare
paddenstoelen.
In de tijden, dat de paddenstoelen in groote hoeveelheden verzameld
kunnen worden, is het zaak aan “magere tijden” te denken en ze te
conserveeren.
Dit conserveeren kan op verschillende manieren geschieden. De
opdrogende soorten, zooals: Marásmius oréades (fig. 25), de Morieljes (fig.
20) en ook de jonge boletus-soorten (fig. 21 en 26) kunnen na eerst (droog)
zooveel mogelijk van ’t aanhangende vuil bevrijd te zijn, in hun geheel
gelaten of in stukken gesneden, in de zon of in een matig verwarmden oven
gedroogd worden en dan in luchtdichte glazen stopflesschen of blikken
bussen met wat peperkorrels op een koele plaats bewaard worden.
Vóór het gebruik moeten de op deze manier geconserveerde zwammen,
eenige uren in lauw water worden opgeweekt en als versche exemplaren
worden afgewasschen.
Deze manier van conserveeren heeft mij steeds matig voldaan, daar het
altijd een heele toer is om ze goed (schimmelvrij) droog te krijgen. Toch
wordt deze drogerij veel toegepast, vooral in Italië, b.v. met de boleten, die
daar dan tegen zeer matigen prijs aan de bevolking verkocht en zoo des
winters algemeen gegeten worden. In Amsterdam is een sigarenwinkel,
waar men deze gedroogde boleten uit Italië koopen kan.
Dat drogen van paddenstoelen strekt men ook wel zóóver uit, dat men ze tot
poeder maalt en dit poeder eveneens in stopflesschen of bussen bewaart.
Als kruiderij voor vele spijzen schijnt dit poeder goed te voldoen. Een
andere manier van conserveeren is in sla-olie of boter. De paddenstoelen
worden daarvoor eerst gekookt en dan in de olie of in de gesmolten en half
bekoelde boter geconserveerd in flesschen die luchtdicht zijn afgesloten. In

Frankrijk worden de boletus-soorten, in schijven gesneden, aldus in blikken
geconserveerd.
De meest practische en goedkoopste inmakerij, n.l. die in blikken, zooals
we de gekweekte champignons meestal koopen, is natuurlijk voor een
particulier niet te bereiken, maar dat het “wecken” van alle soorten
uitmuntend gaat, kan ik uit eigen ervaring mededeelen.
De gereinigde paddenstoelen worden daarvoor met wat zout bestrooid, even
op het vuur gezet en z.g.n. “opgesmolten”, omdat ze anders te veel plaats
zouden beslaan in de glazen. Met een gaatjeslepel worden ze nu in de
glazen geschept en dan het gefiltreerde afgekookte nat daarbij gegoten. Is
de temperatuur tot 100° gestegen, dan steriliseere men nog een uur.
(Voor paddenstoelen in het zuur steriliseere men slechts ½ uur).

Het kweeken van paddenstoelen.
Ook het kweeken van de paddenstoelen dateert al van de oudste tijden en werd reeds
door de Romeinen uitgeoefend.
Van Dioscorides is bekend, dat hij ze kweekte op tot poeder gemalen schors van
populieren, gestrooid op een in een kuil gebrachten hoop van mest en aarde. De Grieken
schenen ze te kweeken op vijgenboomen. De Japanners en Chineezen volgen heden ten
dage nog de methode om boomzwammen op boomen en stronken te kweeken en met
zoo’n gunstig gevolg, dat er aldaar in die boom-paddenstoelen een levendige handel
gedreven wordt. Zij wrijven daarvoor de schors van takken en stammen duchtig met de
sporendragende deelen van de gewenschte rijpe zwam in.
De voornaamste soort op deze manier daar gekweekt, is de Pholióta aegeríta. In
Duitschland heeft men hetzelfde succes gehad met de smakelijk eetbare soort: Pholióta
mutábilis (No. 132) en mijzelve is het gelukt met de smakelijke winterzwam: Collýbia
velútipes (fig. 61 No. 117). Ik zaaide daarvoor in Januari op een vorstvrijen dag de in
water geschudde sporen dezer zwam uit over een in een kuil gelegden boomstronk en in
November van hetzelfde jaar, ten tijde dat ze buiten ook verschenen, kwamen de eerste
vruchtlichamen te voorschijn en kon ik er bij voortduring hoeveelheden voor een
bescheiden maaltje afplukken.

Fig. 29. Reinkultuur uit sporen van Collýbia velútipes, het “fluweelpootje” op takken van
eschdoorn.
Ook reeds van de tijden der Romeinen dateert de kweekerij van de champignon:
Psallióta compéstris (fig. 17) en alle tijden door is deze paddenstoel in kultuur gebleven,
terwijl zij in de laatste 50 jaren een enorme vlucht heeft genomen; vooral Frankrijk staat
hierin bovenaan. Oorspronkelijk alleen in de catacomben gekweekt, verrijzen er
tegenwoordig reusachtige champignon-kweekerijen en als dagelijksche oogst aan
gekweekte champignons wordt alleen al voor Parijs ± 27000 K.G. gerekend. Berekent
men het K.G. op 50 cents waarde, dan geeft deze kweekerij een belangrijke bron van
inkomsten. Professor Dufour in Parijs schat de waarde der in geheel Frankrijk gekweekte
champignons per jaar op 18 millioen gulden.

Ook in Amerika en de andere Europeesche landen wordt de champignon reeds algemeen
gekweekt en de laatste jaren ook in verschillende streken van ons eigen land zooals in
Putten, in Arnhem bij den Heer J. M. Hulsken (fig. 37) en in Utrecht bij den Heer W.
Ruurds.
Ook sinds eeuwen reeds heeft men geprobeerd de morielje (fig. 20) te kweeken, doch al
is het sommigen gelukt gedurende eenigen tijd een kleine oogst daarvan te verkrijgen,
toch kan het niet gezegd worden, dat het iemand nog gelukt is deze zoo smakelijke en
geliefde eet-zwam in ’t groot te kweeken. Die “goudmijn”, zooals de verschillende
auteurs er altijd van zeggen, is nog door niemand ontdekt.
Op een plaats ergens in Frankrijk, waar oude documenten verbrand waren, verrezen in ’t
volgend voorjaar morieljes tot een gezamenlijk gewicht van 2 kilogram. Over ’t
algemeen trouwens schijnen paddenstoelen een voorkeur te hebben voor een dergelijken
grond, rijk aan asch. Na boschbranden heeft men later op de ruïnen veelal een rijke
paddenstoelen-flora gevonden.
Fig. 30. Reinkultuur uit weefsel van Pleurótus ulmárius op hout.
(Ten einde beter gephotographeerd te kunnen worden, is het stuk hout uit de kolf genomen).

Photo A. v. Luók.
Fig. 31. Reinkultuur uit weefsel van Pholióta squarrósa, op kersen-agar in
reageerbuisje.
Voor het kweeken van morieljes wordt nog de volgende methode opgegeven: In een
schaduwrijken hoek van zijn tuin brengt men een mengsel van tuinaarde, zand en vette
compostaarde of koeien- en paardenmest en spit alles diep onder. Hierop wordt gelegd
compostaarde, vermengd met oude eikenschors en om den grond kali-rijk te maken,
strooie men er zuivere houtasch op, liefst tijdens een regenbui. Na eenige dagen, met nu
en dan begieten, is deze kweekplaats gereed en kunnen de morieljes uitgezaaid worden.
Men legt ze daartoe evenals voor ’t schoonmaken, in water, waarbij de sporen zich in dat
water verspreiden en giete dan dit water over het kweekbed uit. Tot het volgende jaar
behoeft men niets meer aan dit kweekbed te doen dan het nu en dan eens te begieten en
het met nog een laag van eikenschors of dennennaalden te bedekken, opdat er geen
onkruid op groeie. In het volgende voorjaar zullen zich de morieljes dan (wellicht)
vertoonen.
Evenals met de morielje heeft men al sinds eeuwen getobd met de truffelkweekerij, doch
van deze kunnen de beijveraars daarvan nu zeggen dat ze de zaak sinds de laatste 50 jaar
onder den knie hebben. De “truffel”, die aan een ieder haast bekende en zéér gezochte,
duurbetaalde lekkernij, is een paddenstoel die bij ons tot nog toe niet gevonden is, wat
niet zegt dat hij er werkelijk ook niet zal zijn. Tot nu toe zijn bij ons wel z.g.
schijntruffels (fig. 103, No. 241) en ook wel hertentruffels gevonden, maar die zijn voor
de consumptie waardeloos.
Het land van de truffels is Frankrijk, terwijl ze ook in België en in Duitschland
voorkomen. Voor hen die het nog niet weten, worde nog even verteld, dat deze zwam

uitsluitend groeit op en in de nabijheid van de wortels van eiken en dat men ze opspoort
en verzamelt met behulp van zwijnen of daarop afgerichte honden, die er dol op zijn en
den reuk er van reeds op vrij verre (40 M.) afstand kunnen bemerken. Wie weet, als men
die Fransche zwijnen eens in ons land lieten werken, of zij dan op sommige plaatsen ook
niet door een hevig wroeten de aanwezigheid van de echte truffels zouden aangeven. Of
iemand al eens dergelijke proeven genomen heeft in ons land om zoo’n truffel-goudmijn
te ontdekken, is mij niet bekend.
Als de fijnste truffel geldt de “Truffe de Perigord”, (zwarte truffel: Tuber
melanosporum). Zij wordt geoogst van September tot half April. Reeds een tiental jaren
geleden werden van uit die streek (Dordogne) 420.000 K.G. truffels verhandeld.

Fig. 33. Kiemende sporen van Rússula
nígricans uit reeds rottende exempl.
Fig. 34. Kiemende spoor van het Judasoor: Hirneóla
Aurícula Júdae.
Bij b is de spoor zelf geheel door ’t groeiende mycelium,
dat zich spoedig gaat vertakken (c), uitgezogen.
Fig. 32. Reinkultuur uit sporen van Armillária múcida, de
“porseleinzwam”, op kersen-agar.
Photo A. v. LUYK.
Hoe de kweekerij precies plaats heeft, heb ik nergens vermeld kunnen vinden, doch wel,
dat zij pas gelukt is, nadat men de eik bij uitnemendheid geschikt voor ’t kweeken van
truffels, de “Quercus pubescens”, bij groote hoeveelheden heeft aangeplant en dan op
kalkhoudenden grond. Voor wie nog wat meer van de truffelkultuur wil weten, zij
gewezen op het volgende werk: “Guide practique du trufficulteur”, par Charles Laval.
Verder heeft men ook nog enkele andere zwammen
in het groot gekweekt, zoo o.a. in Engeland de
Paarlzwam: Amaníta rubéscens No. 21, die met de
Panterzwam No. 23 verwisseld kan worden. Uit
deze zwam bereiden zij een paddenstoelen-extract,
dat ze Ketchup noemen en dat daar zeer gezocht
schijnt te zijn. Ook de bij ons in eikenbosschen zeer
veelvuldig voorkomende, naar mijn smaak niet zeer
lekkere, paarse ridderzwam, Tricholóma núdum No.
29, is ergens in kelders gekweekt geworden en ook
de naar mijn meening nog minder smakelijke
Fopzwam: Laccária laccáta en amethýstina (fig. 28).
Er zijn nog heel veel andere
zwammen, wier kultuur, als ze
gelukte, zoude blijken niet minder
een “goudmijn” op te leveren dan die
van de morielje, zooals o.a. het geval
zou zijn met de soorten: Bolétus
edúlis, het eekhoorntjesbrood (fig.
21) en van de inktzwam: Coprínus
comátus (fig. 114).
De Amerikaan Prof. B. M. Duggar,
heeft met veel zorg reeds getracht
deze soorten te kweeken, maar
zonder succes. De groote
moeilijkheid in de kultuur van de
meeste en ook deze zwammen, die
tot de z.g. aard- of humuszwammen
behooren, zit hierin, dat men hare

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