Maturation, Drying and vulcanization.pdf

Dry rubber

2. Latex
Compounding ingredients
Latex maturation/ Pre vulcanization
Dipping
Drying
Vulcanization
Stabilizers, curing agents, viscosity
modifiers, dispersions and solutions
Removal of moisture before curing
Improve mechanical and chemical
performances through crosslinking

3. Depending on the processing requirements, the compounded latex kept for the certain
period of time with slow stirring before dipping process take place. This stage is often
called as maturation

4. During maturation period,
1.The stabilizer particles adsorb on to the latex particle surface and achieve the
maximum stabilization effect
2.Inter-particle crosslinking (crosslinks are created within the individual rubber
particles)

6. With proper maturation period,
1.The latex will maintain its stability throughout the dipping process and improve the
properties of latex compound
2.Increase the compound pick up
3.Reduce the quality defects
4.Improve the physical properties (specially NR products)

7. The maturation time depend on,
•the latex type
•the formulation
•the application
•the production requirements
Therefore, a compromise between acceptable maturation time and properties will need to
be established

8. Under maturation
when the compounded latex is not yet reach to the required maturation time
Over maturation
when the compounded latex exceed the defined maturation time

9. Coagulant dipping Latex dipping

11. The process of forming a film from an aqueous dispersion of latex particles involves the
removal of water from the latex dispersion surface and compaction of latex particles into a
close pack array.
Basic steps include in this process are,
I.loss of water from the wet gel film by means of evaporation
II.particle deformation
III.coalescence of particles forming a coherent film

13. Vulcanizationisacrosslinkingprocessinwhichindividualmoleculesofrubber(polymer)
areconvertedintoathreedimensionalnetworkofinterconnected(polymer)chains
throughchemicalcrosslinks(ofsulphur).Thisprocessisalsoknownascrosslinkingor
curingprocess
The vulcanization process was discovered in 1839
Charles Goodyear in USA
Thomas Hancock in England.
The discovery was a major technological breakthrough for the advancement of the world
economy

15. •Vulcanization of rubbers by sulphuralone is an extremely slow and inefficient process
•By inventing accelerators and activators for the vulcanization process has increased
the effectiveness of the process (early 20
th
century)
The main requirement to obtain sulphurevulcanization is,
the elastomers/ latex must contain chemical unsaturation (C=C double bonds) for
sulphurcross linking

16. Natural rubber Nitrile butadiene latex (NBR)

17. Activators Latex compound Accelerators Sulphure

18. •sulphurMonochloride
•Metallic Oxides
•Organic Peroxides
Examples:
•Carboxylatednitrile butadiene (NBR) through ZnO
•Polychloroprenelatex through ZnO

19. •Sulphure
•Accelerators
•Activators
An accelerator is defined as the chemical added into a rubber compound to increase
the speed of vulcanization and to permit vulcanization to proceed at lower temperature
and with greater efficiency

20. Depending on the role they play in a given compound, accelerators are categories as,
•Primary or Secondary accelerators
•Slow, semi Ultra fast and ultrafast accelerators

21. •Solubility in rubber (to avoid blooming issue & improve dispersibility),
•Processing conditions such as maximum oven temperatures
•The rate of curing of the product
•Vulcanization method to be used
•Mechanical and chemical properties of the final product
•No adverse effects for the end-user of the rubber product (e.g. accelerators used in the
manufacture of rubber articles intended for food contact / surgical use),
•Stability of the accelerator as a chemical (e.g. problems with the use of decomposed
sulphenamideaccelerators),

22. Often primary accelerators are combined with secondary accelerators enhance
the properties by synergistic effects
Examples:
•Dithiocarbamateswith thiazoles
•Dithiocarbamteswith amines
•Thioureaswith DPG
•Thioureaswith Thiuramsulfides and polysulfides
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An activator is improve both the efficiency of the vulcanization reaction and the physical
properties of the finished product
Ex: ZnO

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25. .
•Crosslinking density depends on,
•sulphurdosage
•Accelerator type
•Accelerator / sulphurratio
•Cure time & temperature
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Cross link density is number of molecules of cross linked units per unit weight of the cross
linked polymer.
VulcanizateProperties of a rubber vulcanizatestronghlydepend on the cross link density

26. •Conventional Cure (CV),
•Efficient Vulcanization Cure (EV)
•Semi Efficient Vulcanization Cure (SEV)
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27. •contains high proportions of sulphur(2.0-3.5 phr).
•low proportions of accelerator (0.4-1.2 phr)
•accelerator to sulphurratio is 0.1-0.6
•sulphurto accelerator ratio > 1.0
•low accelerator to sulphurratio
•Network will be mainly polysulphidic(above 65%), which are thermally unstable
•Fair degree of wasted sulphidesand main chain modification
•Excellent mechanical strength
•Poor heat & ageing resistance
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28. •Contains very little sulphur(0.4-0.8 phr)
•High proportion of accelerator (2-5 phr)
•Accelerator to sulphurratio is 2.5-12
•Efficient use of sulphur
•Mainly monosulphidicbonds (75% mono & 25% disulphidic)
•low set and slightly lower mechanical strength
•Excellent oxidative ageing resistance
•Excellent heat and reversion resistance
•Poor flex-fatigue properties –not suitable for dynamic applications
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•Sulphur levels are intermediate between conventional system and EV system.
•Used fora compromise in cost and/or performance.
•low set and slightly lower mechanical strength
•Excellent oxidative ageing resistance
•Excellent heat and reversion resistance
•Poor flex-fatigue properties –not suitable for dynamic applications

30. High Accelerator / sulphurratio and longer cure time
•increase the number of monosulfidecross link formation at the expense of polysulfide cross
links.
•Such vulcanizatesexhibit better heat stability, lower compression set and longer reversion time
Low accelerator /sulphurratio
•Predominantly polysulfide network due to better stability of C-S bonds as compared to S-S
bonds.
•Such vulcanisatesoffer higher tensile strength, tear strength & flex-fatigue resistance due to the
ability of S-S bonds to break reversibly and there by locally releasing high stresses which could
initiate failure
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33. Practical Test Methods for Monitoring Vulcanization
•Mechanical properties
•Stress-Strain Properties
•Hardness
•Curometers( useful for dry rubber compounds ; not normally used for latex
compounds)
•Oscillating Disc Rheometer(ODR)
•Chemical Method:
•(Swelling in solvent)
•For latex compounds : chloroform Number
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34. Mechanical props
•Stress-Strain Properties
Need a tensometer
•Tensile strength
•Modulus
•Elongation at break
•Hardness
•Convenient pocket models available for quick assessment
•Shore A / Shore D
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35. Curometers
Oscillating Disc Rheometer
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36. Typical information from Rheometer
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37. Swelling
•Measurement of amount of swelling after specified time of
immersion in a solvent
•eg Toluene
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