Polymerization shrinkage

Acrylic Resins. The first synthetic polymer denture material, used throughout much of the 20th century, was based on the discovery of vulcanised mbber in 1839. Other polymers explored for denture and other dental uses have included ceUuloid, phenolformaldehyde resins, and vinyl chloride copolymers. Polystyrene, polycarbonates, polyurethanes, and acryHc resins have also been used for dental polymers. Because of the unique combination of properties, eg, aesthetics and ease of fabrication, acryHc resins based on methyl methacrylate and its polymer and/or copolymers have received the most attention since their introduction in 1937. However, deficiencies include excessive polymerization shrinkage and poor abrasion resistance. Polymers used in dental appHcation should have minimal dimensional changes during and subsequent to polymerization exceUent chemical, physical, and color stabiHty processabiHty and biocompatibiHty and the abiHty to blend with contiguous tissues. 

These two are used either separately or as a mixture. Because of the presence of the somewhat flexible ethylene oxide and related units and use of appropriate fillers, these materials give composite fillings with lower polymerization shrinkage, enhanced mechanical properties, lower solubility and water adsorption, better thermal expansion characteristics, and good biocompatibility with aesthetic properties closely matching those of the tooth itself. 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

Lack of adhesion of a dental restoration to tooth structure results in microleakage at tooth-restoration interface. This occurrence can result in discoloration at the margin of the restoration, or in the formation of caries. Occlusal forces on the restoration and differences between the coeffidents of thermal expansion of the cement and tooth material can lead to leakage. In addition, oral fluids and moisture may affect the adhesion. Microleakage of composite resin restorations has been reviewed by Ben-Amar [233]. Microleakage is not as serious a problem with glass-ionomer cements as it is with resin-based restorative materials, due to reduced polymerization shrinkage [234]. 

Though these may provide a standard for screening production quality, they are merely representative. The flexural properties will be a consistent test of the many possible mechanical property testing modalities. Other areas of physical properties that are important to the success of a composite dental restorative are radiopacity, polymerization shrinkage and thermal interactions, e.g., thermal expansion and thermal diffusivity. 

Another type of thermoset polymers is based on epoxy monomers. These thermosets are cured by use of a two-component system or by photo initiators. Disadvantages of epoxies are high water uptake in service and polymerization shrinkage (22). 

A typical example of frontal polymerization is the polymerization of methyl methacrylate (or an oligomer), placed inside a long aluminum tube 249 these tubes continuously dip into a bath with a liquid heated up to temperature of 70 - 80°C. The part at the tubes above the bath are cooled so that the reactive material does not polymerize. Polymerization shrinkage is compensated by continuous injection of a monomer or oligomer into the reaction zone. The appropriate combination of injection rate, velocity of tube movement through the reaction zone, and tube diameter are chosen according to experimental studies of the process. 

Inorganic fillers may be added to the adhesive composition to reduce the concentration of epoxy resin in the formulation as the degree of polymeric shrinkage is dependent on the volume of epoxy in the joint (Chap. 9). 

Diacrylate monomers have been prepared that are photocurable in visible light and that have small polymerization shrinkage and high X-ray contrast properties. When polymerized with 0.01 to 0.04 pm glass powder, these dental composites were easily machined into artificial teeth. 

For our purposes, reaction injection molding is defined as a process in which two or more reactive streams are combined and mixed with an impingement mixing device and injected into a closed mold where they polymerize to rapidly form a plastic part. To compensate for polymerization shrinkage, RIM parts are foamed to some degree. 

In low-pressure casting, static or dynamic mixers are employed and the pumping systems are set up to compensate for polymerization shrinkage by packing the mold with additional material. Thus, a solid part can be produced. 

Molds with open-topped risers and vents, like those used for casting of metals, may be used for certain thick parts. Makeup for polymerization shrinkage comes from the volume of material in the vents and risers which are eventually machined off. 

C.J. Kleverlaan, A.J. Feilzer, Polymerization shrinkage and contraction stress of dental resin composites, Dent. Mater. 21 (2005) 1150-1157. 

The phenomenon of polymerization shrinkage in composite resins based on either bisGMA or UDMA is considered to be their most important deficiency... 

D. Truffier-Boutry, S. Demoustier-Champagne, J. Devaux, J.J. Biebuyck, M. Mestdagh, P. Larbanois, G. Lehoup, A physico-chemical explanation of the post-polymerization shrinkage in dental resins, Dent. Mater. 22 (2006) 405 12. 

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