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Addition Polymerisation of Dental Material

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 Addition polymerizations involve the addition of a reactive species with a monomer to form a larger reactive species which is capable of further addition with monomer  Addition pol ymerisation  John F . McCabe, Applied Dental Materials Ninth E
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Addition polymerisation

Addition polymerizations involve the addition ofa reactive species with a monomer to form a largerreactive species which is capable of further addition with monomerAddition polymerisationJohn F. McCabe, Applied Dental Materials Ninth Edition

ActivationThis involves decomposition of the peroxide initiator using either thermal activation (heat), chemical activators or radiation of a suitable wavelength .For benzoyl peroxide the activation reaction is represented by the equation R O O R 2RO where R represents any organic molecular grouping.John F. McCabe, Applied Dental Materials Ninth Edition

InitiationThe polymerisation reaction is initiated when the radical, formed on activation, reacts with a monomer molecule. This is illustrated for the specific case of the benzoyl peroxide radical and the methacrylate monomerRO + M RO M where the symbol M represents one molecule of monomer. It can be seen from the above equation and from Fig. 12.4 that the initiation reaction is an addition reaction producing another active free radical species which is capable of further reaction.John F. McCabe, Applied Dental Materials Ninth Edition

PropagationFollowing initiation, the new free radical is capable of reacting with further monomer molecules. Each stage of the reaction produces a new reactive species capable of further reaction, as illustrated in the following equations:RO M + M RO M M RO M M + M RO M M M RO M M M + M RO M M M M John F. McCabe, Applied Dental Materials Ninth Edition

TerminationIt is possible for the propagation reaction to continue until the supply of monomer molecules is exhausted. In practice however, other reactions, which may result in the termination of a polymer chain. These reactions produce dead polymer chains which are not capable of further additions.One example of termination is the combination of two growing chains to form one dead chain as follows:RO (M)n M + RO (M)x M RO (M)n M M (M)x OR

Other examples of termination involve the reactions of growing chains with molecules of initiator, dead polymer, impurity or solvent, if present.John F. McCabe, Applied Dental Materials Ninth Edition

Chemically Activated (Self-Cure) ResinsTwo pastes, one of which contains the benzoyl peroxide initiatorthe other an aromatic tertiary amine activator (e.g., N, N-dimethyl-p-toluidine),When the two pastes are mixed together, the amine reacts with the benzoyl peroxide to form free radicals, and addition polymerization is initiated.

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

PROBLEMS:Almost impossible to avoid incorporating air into the mix, thereby forming pores that weaken the structure and trap oxygen, which inhibits polymerization during curing. No control over the working time after the two components have been mixed. Therefore both insertion and contouring must be completed quickly once the resin components are mixed. Thus, today they are mainly used for restorations and large foundation structures (buildups) that are not readily cured with a light source. Formation of an oxygen inhibited layer. This reaction forms an unpolymerized surface layer. The thickness of the unpolymerized film on the surface is dependent on the viscosity of the resin, the solubility of oxygen in the monomer, and the initiating system used.

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

PHILLIPS SCIENCE OF DENTAL MATERIALS

Dual-Cured Resins

Able to overcome limits on curing depth and some of the other problems associated with light. Consist of two light-curable pastesone containing benzoyl peroxide one containing an aromatic tertiary amine accelerator. Cure slowly when mixed via the self-cure mechanism. The cure is then accelerated on command via light-curing promoted by the amine/ photoinitiator combination. PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

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The major advantage of this system is assurance of completion of cure throughout, even if photocure is inadequate.The disadvantages porosity caused by the required mixing. But this has been greatly alleviated by the use of mixing syringes. Less color stability than with the photocure resins owing to the aromatic amine accelerators, but this is still better than for self-cure systems, since the concentration of the accelerators is reduced compared with self-cure resins. Intended for any situation that does not allow sufficient light penetration to produce adequate monomer conversionfor example, cementation of bulky ceramic inlays.

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

Polymerization shrinkage

Shrinkage can compromise marginal seal and rupture adhesive bonds created at the tooth-restorative interface. It has been claimed that the slight expansion, due to absorption of water, over a period of several weeks following placement can help to partially off-set the effects of shrinkage.Another potentially serious effect of shrinkage - stress placed on tooth substanceparticularly on the residual cusps of posterior teeth in relatively large cavities. Such stresses, caused by the composite material pulling-in cusps to which it may adhere, is thought to be responsible for some cases of post-operative pain experienced after placement of so-called posterior composites. In extreme cases the stress on the tooth may be great enough to cause cuspal fracture.John F. McCabe, Applied Dental Materials Ninth Edition

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

Craigs RESTORATIVE DENTAL MATERIALSTHIRTEENTH EDITION

Reduction of Shrinkage Stresses

Two general approaches have been followed in seeking to overcome the problem of stress concentration and marginal failure experienced with light-activated resins: (1) reduction in volume contraction by altering the chemistry and/or composition of the resin system(2) clinical techniques designed to offset the effects of polymerization shrinkage. Incremental Buildup and Cavity ConfigurationOne technique attempts to reduce the C-factor, which is related to the geometry of the cavity preparation and represented by the ratio of bonded to nonbonded surface areas

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

PHILLIPS SCIENCE OF DENTAL MATERIALS 12TH Ed

REFERENCESJohn F. McCabe, Applied Dental Materials Ninth EditionPHILLIPS SCIENCE OF DENTAL MATERIALS 12TH EdCraigs RESTORATIVE DENTAL MATERIALS THIRTEENTH EDITION


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