Influence of carboxylic anhydrides on selected mechanical properties of heat-cured resin composites

Resin composites in dentistry: the monomer systems

The present review outlines the history of monomers used in resin composites, motivates further development, and highlights recent and ongoing research reported in the field of dental monomer systems. The monomer systems of most present-day resin composites are based on BisGMA, developed some 40 years ago, or derivatives of BisGMA. In the remaining resin composites, urethane monomers or oligomers are used as the basis of the monomer system. The main deficiencies of current resin composites are polymerization shrinkage and insufficient wear resistance under high masticatory forces. Both factors are highly influenced by the monomer system, and considerable efforts are being made around the world to reduce or eliminate these undesirable properties. The use of fluoride-releasing monomer systems, some of which are under investigation, has been suggested to mitigate the negative effects of marginal gaps formed in consequence of polymerization shrinkage. The very crux of the problem has also been approached with the synthesis of potentially low-shrinking/non-shrinking resin composites involving ring opening or cyclopolymerizable monomers. By the use of additives with a supposed chain transfer agent function, monomer systems have been formulated that improve the degree of conversion of methacrylate double bonds and mechanical properties. Many promising monomer systems have been devised, the implementation of which may be expected to improve the longevity of resin composite fillings and expand the indications for resin composites.

Current trends in dental composites

The clinical performance of dental composites has been significantly improved over the past decade through modifications in formulation that include: using more stable polymerization promoters for greater color stability; incorporating high concentrations of finely ground fillers to produce adequate strength and excellent wear resistance while retaining translucency; adding radiopacifying agents for improved diagnostics; and utilizing dentin adhesives. However, there are problems which limit the use of composites, especially in posterior teeth. The materials remain very technique-sensitive, due to the extensive contraction which accompanies polymerization and negatively influences marginal sealing. In addition, the materials are generally considered to have inadequate mechanical properties and wear resistance in contact areas to serve as total replacements for amalgams. Current efforts are focusing on several areas, including the development of non- or minimally-shrinking dental composites containing spiro-orthocarbonates as additives to dimethacrylates or epoxy-base resins, and the production of alternative filler materials for ideal wear resistance and esthetics. This paper reviews the composition and characteristics of current dental composites, as well as recent areas of study.

Effect of temperature and duration of post-cure on in vitro wear and quantity of remaining double bonds of resins containing carboxylic anhydride

The present study determined the effect of post-cure temperature and duration on in vitro wear resistance and quantity of remaining double bonds of anhydride-containing resins. Temperatures were varied between 37 degrees C and 225 degrees C, and durations were varied between 0 and 24 h. The quantity of remaining double bonds could not be established for post-cure temperatures of 200 degrees C or more due to melting of the polymer. A temperature of approximately 120 degrees C had optimal effect on wear resistance. Quantity of remaining double bonds and wear were found to decrease with increasing duration of post-cure. Low quantities of remaining double bonds were generally associated with low in vitro wear.

Influence of ketones on selected mechanical properties of resin composites

The present study investigated a concept for additional cross-linking of dental polymers, by which resistance to wear of resin composites might be increased. Bifunctional ketones were added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and ketone. For measurement of possible effects of the cross-linking agents added, four mechanical properties of the experimental resin composites were determined. Addition of the bifunctional ketone diacetyl resulted in the following increases in mechanical properties: diametral tensile strength, 11%; flexural strength, 29%; modulus of elasticity, 19%; and modulus of resilience, 50%.

Effect of temperature and duration of post-cure on selected mechanical properties of resin composites containing carboxylic anhydrides

Resin composites containing carboxylic anhydrides have recently been formulated for use as inlay materials. To optimize the post-cure, the present study measured the effect of post-cure temperature and duration on four mechanical properties of anhydride-containing resin composites. Temperatures were varied between 37 degrees C and 225 degrees C, and durations were varied between 0 and 24 h. An optimum in strength and stiffness was found when post-cure temperatures of 125-150 degrees C were used. Strength and stiffness were found to increase with increasing duration of post-cure. Due to discoloration of the resin composites when post-curing for 6 or 24 h, a post-cure period of 1 h was preferred.

Influence of aldehydes on selected mechanical properties of resin composites

The present study investigated whether propanol, a monofunctional aldehyde, was able to improve the mechanical properties of dental polymers. The underlying hypothesis was that a cross-linking reaction is possible between various functional groups of different polymers. Propanol was added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to monomer composition and content of aldehyde. Four mechanical properties of the experimental resin composites were determined. Addition of propanol gave rise to significant improvements in mechanical properties, which may be indicative of a cross-linking ability of monofunctional aldehydes. With the exception of modulus of elasticity, the mechanical properties of resin composites based on UEDMA/HEMA were superior to those of BISGMA/TEGDMA-based materials, even though the improvements in flexural strength and modulus of resilience were most pronounced for the BISGMA/TEGDMA-based resin composites.