Polymers

polymers in dentistry

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2. Polymers & polymerisation Presented By Dr. Abbasi Begum M Pg.Department Prosthodontics 5/1/2015 60

3. contents INTRODUCTION CLASSIFICATION REQUISITES FOR DENTAL RESINS FUNDAMENTAL NATURE OF POLYMERS Physical Properties of Polymers Chemistry of Polymerization Copolymerization Acrylic Dental Resins Applications of Resins in Dentistry RECENT ADVANCES 5/1/2015 59

4. INTRODUCTION Polymers are finding increased application in almost every sphere of life. In almost every field of dentistry these are being used successfully. However as the field is dynamic & new types of resins are being developed on regular basis, it is important for us to know the basic chemistry of polymers so that new developments can be critically evaluated. 5/1/2015 4

5. g Polymers & polymerisation 5/1/2015 58

6. DEFINITION MONOMER A chemical that can undergo polymerization; any molecule that can bound to a similar molecule to form a polymer. (GPT-8) POLYMER A chemical compound consisting of large organic molecules built by repetition of smaller monomeric units. (GPT-8) DENTURE BASE : A denture base may be defined as the part of the denture that rests on the foundation and to which teeth are attached. Gpt-8 ( 2005) 5/1/2015 57

7. POLYMERIZATION : The forming of a compound by the joining together of molecules of small molecular weights into a compound of large molecular weight. (GPT-8). polymerization Monomer + Monomer + Monomer + Monomer Mcr - Mcr - Mcr –Mcr Polymers have had an enormous impact on dentistry used as sealants, bonding materials, restorative materials, Veneering materials, denture bases, denture teeth, and impression materials. polymers sealants Impression materials Bonding material veneering Denture bases 5/1/2015 5 6

8. BASIC NATURE OF POLYMERS Poly – Many Mer – Parts Derived from the Greek language It consist of large no. of repeating units known as monomers The no. of repeating units in a chain of polymer is known as degree of polymerization Naturally occurring polymers are agar ,cellulose D.N.A proteins , natural rubber, collagen & silk. Synthetic polymers are rubber & nylon. 5/1/2015 55

9. Structure of a polymer 5/1/2015 54

10. classification Based on their 1) thermal behavior , they can be divided into Thermoplastic Thermosetting Based on 2 ) Classification based on polymerization a) Addition polymers formed by the repeated addition of monomer molecules possessing double or triple bonds b) Condensation polymers formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units. 3 ) Source [origin] Natural Synthetic 5/1/2015 53

11. 4) Technological Use Adhesives Fibers Plastics Elastomers Paint & Coatings 5 ) Architecture Linear Branched Cross-linked Linear Branched   5/1/2015 52

12. 6 ) Microstructure Crystalline –chains align themselves to form highly ordered ,or crystalline structure Amorphous-chains are randomly coiled & entangled in a very distorted pattern 7 ) Chemistry of Composition Homopolymer Copolymer AAAAAAAAAAA Block Graft Alternating Homopolymer Consist of only one type of constitutional repeating unit (A) AAAAAAAAAAAAAAA 5/1/2015 51

13. copolymerization Two or more chemically different monomers, each with a desirable property can be combined to yield specific physical properties ,such a polymer is called as copolymer The process of formation is called as copolymerization RANDOM CO-POLYMER BLOCK CO-POLYMER 5/1/2015 50

14. RANDOM CO-POLYMER If a monomer A is mixed with monomer B prior to polymerization they will co-polymerize to form polymer chains containing both A & B monomer units. The sequence of the original monomer s in the polymer may be random producing a ‘ RANDOM CO-POLYMER’ A-B-A-B-A-B-B-A-B-B-A-A-B-B-A BLOCK CO-POLYMER If the monomers self polymerize more readily then they copolymerize that they will result in a “block copolymer” where segments' of each homo polymer are linked. - A-A-A-B-B-B-A-A-A-B-B-B- 5/1/2015 49

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16. Several classes of copolymer are possible Statistical copolymer (Random) ABAABABBBAABAABB two or more different repeating unit are distributed randomly Alternating copolymer ABABABABABABABAB are made of alternating sequences of the different monomers Block copolymer AAAAAAAAABBBBBBBBB long sequences of a monomer are followed by long sequences of another monomer Graft copolymer AAAAAAAAAAAAAAAAAA B B B B B B 5/1/2015 47

17. REQUISITES FOR DENTAL RESINS Methacrylate polymers are capable of providing an excellent balance of performance features and characteristics needed for use in the oral cavity. Ideally, these characteristics include (1) biological compatibility ( 2) physical properties (3) ease of manipulation (4 ) aesthetic qualities ( 5) relatively low cost (6) chemical stability in the mouth 5/1/2015 46

18. 1)Biological Compatibility 5/1/2015 45

19. 2)Physical Properties 3)Aesthetic Properties 5/1/2015 44

20. 4)Economic Considerations should not require complex and expensive equipment . 5)Chemical Stability PHYSICAL PROPERTIES OF POLYMERS Deformation and Recovery Applied forces produce stresses within polyrners that can cause elastic strain, plastic strain, or combination of elastic plus plastic strain . plastic deformation is irreversible and results in a new permanent shape. Elastic deformation is reversible and will be completely recovered when the stress is eliminated. Viscoelastic deformation is a combination of elastic and plastic strain, but recovery of only elastic strain occurs as the stress is decreased. ' 5/1/2015 43

21. 2.Rheometric Properties Flow behavior of solid polymers involves combination of elastic & plastic deformation & the elastic recovery when the stresses are eliminated This combination of elastic and plastic changes is termed viscoelasticity Actual dental polymeric materials are deformed by a combination of elastic plus plastic strain processes . chain length, no. of cross links, temperature , & rate of force application determines which type of behavior dominates elastomers do not always recover fully and retain a small degree of plastic deformation , Where as plastics exhibits high level of plastic deformation ,but they also have at least some small degree of elastic recovery . This phenomenon is called as viscoelastic recovery Stretching only No additional stretching Full recovery 5/1/2015 42

22. . 3. Solvation Properties The longer the chains (the higher the molecular weight), the more slowly a polymer dissolves. Polymers tend to absorb a solvent, swell, and soften, rather than dissolve. Cross-linking prevents complete chain separation and retards dissolution. highly cross-linked polymers cannot be dissolved. Elastomers swell more than plastics. 5/1/2015 22 Stretching only Loading continued Unloaded Partial recovery with permanent deformation loaded Slipping occurred (Quantity of slippage depends on the duration of loading)

23. A small amount of swelling of dental polymeric devices can have undesirable results on the fit of prostheses. Absorbed m olecules (e.g., water- ) spread polymer chains apart and facilitate slip between chains. This lubricating effect is called plasticization . 4.Thermal Properties The physical properties of , polymer are influenced by changes in temperature and environment by the composition, structure, and molecular weight of the polymer. 5/1/2015 41

24. CHAIN CONFIGURATIONS POLYMER CHAINS ARE HELD TOGETHER BY 1) weak secondary bonds( Vander wall bonds) 2) entanglement of chains if they are sufficient long. The higher the molecular weight ,the more entanglements there will be , giving a stiffer & stronger polymer . GLASS TRANSITON TEMPERATURE In a polymer ,such as polyethylene which has a linear chain configuration ,the weak bonds between chains can be easily be broken by increasing the temperature of the polymer. When this happens the chain can flow past one another so that the polymer softens and readily deforms. 5/1/2015 40

25. On cooling the bonds are re-established and the polymer becomes hard again and retains the shape it was in at the higher temperature . The temperature at which a plastic softens such that the molecules can begin to flow is defined as glass transition temperature . They are similar to those of glasses except that the temperatures involved are much lower in case of plastics. 5/1/2015 39

26. Thermoplastic polymers made of linear & or branched chains They may be softened by heating & solidify on cooling …….the process being repetable E.g . –poly (methyl methacrylate) , polystyrene etc. when heated above the glass (transition temperature (Ig), at which molecular motion begins to force the chains apart The resin can then be shaped and molded , upon cooling, they soften again and can be reshaped if required before hardening as the temperature decreases- t his cycle can be carried out repeatedly 5/1/2015 38

27. Thermosetting polymers undergo a chemical change and become permanently hard when heated above the temperature at which they begin to polymerize, and do not soften again on reheating . Thermosetting plastics have superior abrasion resistance and dimensional stability compared to thermoplastic which have better flexural & impact properties. Eg . cross linked poly methyl methacrylate.silicones 5/1/2015 37

28. Thus heat plays an important role in affecting the physical properties of resins When temp.↑ it results In thermal expansion, break polar bonds & facilitate disentanglement of chains This causes slippage of chains & results in thermoplastic behavior When crosslinking is present it is difficult to soften resin ,as slippage cannot occur………. 5/1/2015 36

29. CHEMISTRY OF POLYMERIZATION The process of polymerization were divided by Flory (1953) and Carothers (Mark 1940) into two groups, two types of reaction: addition polymerization……. monomers are activated one at a time and add together in sequence to form a growing chain. and step-growth or condensation polymerization……………. POLYMERIZATION IS ACCOMPLISHED BY repeated elimination of small molecules 5/1/2015 35

30. Addition polymerization Monomers are activated one at time and together in a sequence to form growing or smaller chains Types Free Radical Ring opening Ionic polymerization Eg.heat cured or autopolymerizing poly methyl methacrylate etc. Fast process Step Growth polymerization / Condensation Polymerization Chemical reaction takes place with the release of by-product or smaller units and there is change in the final composition. Ex . Proteins, carbohydrates Slow process 5/1/2015 34

31. Addition Polymerization Addition polymerization occurs among the molecules containing double or triple bonds No small molecules are liberated and reaction is rapid chain type reaction . Monomers add sequentially to the end of a growing chain Is very fast and exothermic Produces high molecular weight polymers The macromolecules are formed from smaller units, or monomers, without change in composition, because the monomer and the polymer have the same empirical formulas . In other words the structure of the monomer is repeated many times in the polymer 5/1/2015 33

32. Types of addition polymerizations Free-radical polymerization Active center is a radical (contains unpaired electron) and the propagating site of reactivity is a carbon radical. Cationic polymerization The active center is an acid, and the propagating site of reactivity is positively charged Anionic polymerization The active center is a nucleophile, and the propagating site of reactivity is negatively charged 5/1/2015 32

33. Few common polymers prepared via addition polymerization 5/1/2015 Name(s) Formula Monomer Properties Polyethylene low density (LDPE) –(CH2-CH2)n– ethylene CH2=CH2 soft, waxy solid Polyethylene high density (HDPE) –(CH2-CH2)n– ethylene CH2=CH2 rigid, translucent solid Polypropylene (PP) different grades –[CH2-CH(CH3)]n– propylene soft, elastic solid isotactic: hard, strong solid Poly(vinyl chloride) (PVC) –(CH2-CHCl)n– vinyl chloride CH2= CHCl strong rigid solid Polytetrafluoroethylene (PTFE, CF2=CF2 –(CF2-CF2)n– Teflontetrafluoroethylene resistant, smooth solid Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas) –[CH2- C(CH3)CO2CH3]n– methyl methacrylate CH2=C(CH3)CO2CH3 hard, transparent solid

34. STAGES The process of free radial additional polymerization to produce the polymers involves 4 stages Activation 1. Induction Initiation 2. Propagation 3. Chain Transfer 4. Termination 5/1/2015 31

35. Two process control the Induction stage. They are a)Activation b)Initiation To begin additional polymerization process. A source of free radicals “R” is required. Free radical is an atom or group of atoms possessing an unpaired electron. 5/1/2015 30

36. The process of producing free radicals -Activation . Free radicals can be generated by activation of radical producing molecule using a)Chemical agents b)Heat c)Visible light d)Ultra Violet Light e)Energy transferred from another compound which acts as a free radical . Of these : Chemical agents, Heat & Visible light are most often used in dentistry. Benzoyl peroxide-Peroxide commonly used in dentistry. 5/1/2015 29 Induction by heat or chemical

37. The Activation is done by the decomposition of the benzoyl peroxide , due to the use of an activator such as Heat : When heated above 50 -100 o C , benzoyl peroxide decomposes to form a free radical. Heat cure acrylic resins B . Chemical Compounds : Benzoyl peroxide can also be activated when brought into contact with a tertiary amine. This method is employed in cold cure acrylic resins , also used in chemically cured composite restorative materials , which consists of a base paste ( tertiary amine activator) & a catalyst paste (containing benzoyl peroxide initiator) 5/1/2015 2 8

38. C)Light : Other forms of free radical production include the use of 1) ultra violet light in conjunction with a benzoic methyl ether. 2) visible light with an alpha- diketone and an amine. 5/1/2015 27

39. INITIATION The free radicals will react with a monomer such as ethylene and initiate the polymerization process. When this occurs ,the remaining unpaired electron makes the new molecule & a free radical 5/1/2015 26 H H H H R* + c = c -  R- c - c* H H H H

40. PROPAGATION: The free radicals are transferred to the monomer which in turn reacts with other monomer Repeating this process again and again generates the polymer chain until the growing chains collide or all the free radicals have reacted . As the initiated molecule approaches other methyl methacrylate molecule,the free electron interacts with the double bond of the methymethacylate molecule, & a new, longer free radical is formed……. 5/1/2015 25

41. CHAIN TRANSFER: In this process, the active free radical of a growing chain is transferred to another molecule and a new free radical for further growth is created. Chain transfer occurs when a free radical approaches a methyl methacrylate molecule and donates a hydrogen atom to methyl methacrylate molecule . This causes the free radical to rearrange to form a double bond & become unreactive, & the MMA monomer to form a free radical that can participate in the chain propagation reaction………….. 5/1/2015 24

42. TERMINATION The chain reactions can be terminated by : Direct coupling .of two free radical chain ends or by Exchange of a hydrogen atom from one growing chain to another. Collision of a growing chain with an initiation radical. Collision of a growing chain with inhibitor . Inhibition Any impurity if it is present ,it reacts with the monomer and inhibit polymerization . eg . Hydroquinone 0.006% Eugenol , Oxygen. Main functions: aids in prevention of polymerization during storage in ca se of two parts(self cure) system….provides adequate time for mixing & placement 5/1/2015 23

43. STEP-GROWTH POLYMERIZATION When two molecules react to form a large molecule with the elimination of a smaller molecule- such as water, alcohol, halogen acids & ammonia….. Polymers are formed more slowly than by addition polymerization because the reaction proceeds in a stepwise fashion from monomer to dimer to trimer …….until large polymer molecule containing many monomer molecule are eventually formed Polymers are generally of lower molecular weight E.g used in biological tissues to produce proteins, carbohydrate, DNA etc… 5/1/2015 22

44. prepolymers A monomer or system of monomers that have been reacted to an intermediate molecular mass state or a substance which represents an intermediate stage in polymerization, & can be usefully manipulated before polymerization is completed This material is capable of further polymerization by reactive groups to a fully cured high molecular weight state As, such mixture of reactive polymers with unreacted monomers may also be referred to as pre-polymers They are also called as polymer precursor 5/1/2015 21

45. Typical applications of polymers in dentistry Dentures (bases, artificial teeth, relining materials) Filling materials (composites, cements, adhesives ) Impression materials Obturation materials Equipment ( spatulas) 5/1/2015 20

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47. Acrylic resins Mostly used in fabrication of all complete dentures. Thermal energy – required for polymerization- water bath or microwave . Dr. Walter Wright (1937) introduced Polymethyl methacrylate as a denture base material …. COMPOSITION : HEAT CURE ACRYLIC DENTURES POWDER LIQUID Polymethyl meth acrylate (Prepolymerized spheres) Benzoyl Peroxide( Initiator) Pigment ( 1%) Methyl meth acrylate Hydroquinone ( <1% Inhibitor) Glycol methacrylate ( Crosslinking agent 1-2%) 5/1/2015 18

48. COMPOSITION – SELF CURE ACRYLIN RESIN Inhibitor- prevents undesirable polymerization during storage 5/1/2015 17

49. Glycol methacrylate – Crosslinking agent. Chemically and structurally similar to Methyl meth acrylate. Cross linking agents are added into liquid component in concentration of 1- 2% by volume. The interconnections – netlike structure- increased resistance to deformation . POLYMER –TO- MONOMER RATIO The polymer to monomer ratio is 3:1 by volume. Using this ratio the volumetric shrinkage can be limited to 7% 5/1/2015 16

50. Polymer – monomer interaction When mixed in proper proportions, the resultant mass passes through five distinct stages 1.Sandy 2. stringy 3. Dough like 4. rubbery 5. Stiff 5/1/2015 15

51. SANDY STAGE : - Little or no interaction at molecular level. - polymer beads remain unaltered ( Corse or grainy) STRINGY STAGE: - Monomer attacks the individual polymer beads and is absorbed into the beads. - Some polymer chains are dispersed into monomer. - characterized by Stringiness or Stickiness- touched DOUGH STAGE: - A t the molecular level, an increased number of polymer chains enter the solution, thus monomer and dissolved polymer- formed. - It is no longer tacky or and will no adhere to the mixing vessel or spatula . -The later phase of this stage are ideal for compression molding. 5/1/2015 14

52. RUBBERY STAGE: - Because the monomer is dissipated by evaporation and by further penetration into remaining polymer beads. - Mass rebounds when compressed or stretched. STIFF STAGE : - Continued evaporation of unreacted monomer. - Material looks dry and resistant to mechanical deformation 5/1/2015 13

53. DOUGH FORMING TIME The time required for the resin mixture to reach a dough like stage. In clinical use, majority of the denture base products reach dough stage consistency in less than 10 min. WORKING TIME Defined as the time a denture base material remains in the dough like stage. This period is critical to the compressive molding process. Ambient temperature affects the working time. Working time 5minutes. 5/1/2015 12

54. Residual Monomer Despite of various polymeriszation techniques used to initiate polymerization reaction ,the conversion of monomer to polymer is not complete…………. some unreacted monomer persists--------- residual monomer During polymerization it decreases first rapidly & later more slowly after most polymerization is over Residual monomer content in heat cure resins is around 0.3-0.5% Where as in self cure it is around 3-5% 5/1/2015 11

55. EFFECT: This residual monomer might leach out & contact oral mucosa ,specially denture bearing mucosa Irritant or allergic reaction by MMA monomer The amount of MMA released was related with the processing condition & composition of material Light cure resins released comparatively less residual monomer than heat or self cured For minimizing monomer released heat cure & chemical cured dental resins must be immersed for 1 day in water before wearing Huang.F.M , Chin Hu.C , Chang.Y.C , Chou.M.Y: Residual monomer releasing from acrylic denture base in water , Chin Dent J 19(1) 17-22 March 2000 5/1/2015 10

56. Polymerization shrinkage It is the volumetric shrinkage that occurs during the polymerization of MMA to PMMA polymer Density of the mass changes from 0.94 to 1.19gm/cm3 Volumetric shrinkage of 21% Along with volumetric shrinkage linear shrinkage occurs during the cooling process It can be minimized by a correct proportioning of the monomer & the polymer Packing in the dough stage 5/1/2015 09

57. Water Sorption & Solubility The dimensions of acrylics vary with water absorption This is reversible & denture contracts when dried Heat cure dentures immersed in water exhibits a linear expansion of 0.1%to 0.2% This compensates partially for polymerization shrinkage of 0.3% -0.5 % Reason : water absorption results in a slight expansion of denture due to 2 reasons Water enters the mass & occupies the space in between the polymeric chains & causes a slight expansion of polymeric mass Water also interferes with entanglement of polymeric chains there by acts as plasticizers. 5/1/2015 08

58. Recent Advances Flexible Thermoplastic Denture Base Materials - Removable Partial Denture Framework . The resin is a biocompatible nylon thermoplastic unique physical and aesthetic properties that provides unlimited design versatility eliminates the concern about acrylic allergies . allows the restoration to adapt to the constant movement and is flexible in the mouth . VALPLAST   Flexible Dentures Maxillary esthetic valplast denture 5/1/2015 07

59. FLEXIBLE DENTURE BASE MATERIAL Free movement is allowed by the overall flexibility and can, therefore, be referred to as “a built in stress breaker”. Sunitha N ,Shamnur, Jagadeesh KN, Kalavathi SD, Kashinath KR :“Flexible dentures” – An alternate for rigid dentures. Journal of Dental Sciences & Research 1:1: Pages 74 - 79 5/1/2015 06

60. FIBER-REINFORCED DENTURE BASE RESINS Reinforced denture base with glass fillers: (Ref: JOP1999, 18-26, vol-8, no.1) Mona K.Marie has conducted a study to evaluate the effect of short glass fibers on the transverse strength of a heat polymerized denture base material . Optimal adhesion between the fibers and the polymer matrix can be obtained by mixing with silane coupling agents Incorporation of glass fibers in a continuous roving form increases the strength of dentures and enhances the fracture resistance disadvantage of this system is difficulty in handling the fibers and inadequate degree of impregnation of fibers with the resin. 5/1/2015 05

61. C ONCLUSIO N Polymers are very rarely used in their pure form Modifications are carried out to improve the properties of the polymers . Depending upon the use modifications are done in order to meet the requisites of the material . Some of the fairly good properties of the polymers make, the use of the polymers, in dentistry very wide . In almost every field of dentistry they can be used successfully. 5/1/2015 04

62. REFERENCES PHILLIPS SCIENCE OF DENTAL MATERALS- ANUSAVICE .11 th edition Craig’s restorative dental materials- powers.J.M , Sakaguchi.R.L-12 th edition William J .O’Brien-Dental materials & selection-4t edition Singh.H , Aeran.H, Kumar ,Gupta.N ;Flexible Thermoplastic Denture Base Materials - Removable Partial Denture Framework . Kutsch VK, Whitehouse J, Schermerhorn K, Bowers R. The evolution and advancement of dental thermoplastic. Dental Town Magazine . 2003 . 5/1/2015 03

63. Negrutiu M, Sinescu C, Romanu M, Pop D, Lakatos S. Thermoplastic resin for removable partial dentures. TMJ. 5 (3). Uzun.G ,Hersek.N, Teoman .T. Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. J Prosthet Dent 1999;81:616-20 Marie.M.K- FIBER-REINFORCED DENTURE BASE RESINS . JOP1999 , 18-26, vol-8, no. Sunitha N ,Shamnur, Jagadeesh KN, Kalavathi SD, Kashinath KR :“Flexible dentures” – An alternate for rigid dentures. Journal of Dental Sciences & Research 1:1: Pages 74 - 79 Huang.F.M , Chin Hu.C , Chang.Y.C , Chou.M.Y: Residual monomer releasing from acrylic denture base in water , Chin Dent J 19(1) 17-22 March 2000 5/1/2015 02

64. Next seminar Dr. NAVEEN REDDY TOPIC: IMPLANT FAILURES date:06/01/2015 5/1/2015 1

65. THANK YOU 5/1/2015