Лако-красочные материалы — производство

Basically composed of poly(methyl methacrylate) upon setting, the acrylic resin cements in the unfilled state are simpler, linear organic polymers.

Although known in dentistry for several decades, they have not as such enjoyed much acceptance because of considerable volume shrinkage on polymerization (21 to 22%) [6] and consequent microleakage, a high coefficient of thermal expansion (about 10 times greater than observed for tooth substance), high exothermicity of the polymerization reaction, and other shortcomings. Strong points, on the other hand, include low solubility in oral fluids, good thermal insulation, outstanding transparency, and ease of manipula­tion in the virgin (uncured) state. The commercial products generally are two-part systems made up, first, of a powder that contains fine (<50 pm) beads of poly(methyl methacry­late) and a peroxide-type initiator, and second, a liquid composed of methyl methacrylate monomer and a chemical activator, usually a tertiary amine, such as N, N-dimethyl-p — toluidine. More recent products use peroxide/alkylborane, peroxide/sulfinic acid, and other initiator/activator systems. The set cements bond mechanically to the tooth structure.

The development of filled resin cements (composites) has helped overcome some of the shortcomings of the unfilled resins. The two-part materials comprise a powder com­ponent, such as silanized silica of small (10 to 15 pm) particle size, combined with peroxide initiator, and a liquid component consisting of a bisacrylate monomer, such as 2,2-bis[4- (2-hydroxy-3-(methacryloyloxy)propoxy)phenyl]propane (bis-GMA), a diluent comono­mer, usually triethylene glycol dimethacrylate (TEGDMA), and some 0.5% of a tertiary amine activator. Paste-paste and paste-liquid systems comprising the aforementioned reactants and activators are also on the market. The recent advent of light-activated composite materials for restorative applications has prompted the development of simi­larly composed, single-paste composite cements for luting purposes. Such light-cured cements contain certain diketones (0.03 to 0.09 wt %), the most popular being camphor — quinone, which in the presence of amines generate free radicals upon irradiation with visible light. Halogen lamps (400 to 500 nm) are the standard light sources, although argon ion lasers (476.5 nm) are being used in current experimental studies with variable results. The free radicals so generated then initiate methacrylate polymerization. Also in use are dual-cure composites, in which a primary polymerization phase is photoinitiated, to be followed by chemically initiated secondary polymerization. Composite hardening pro­ceeds with comparatively low polymerization shrinkage (1.2 to 2.7%) and low exothermi — city, and the set cements feature low solubility and coefficients of thermal expansion significantly lower than observed with the unfilled resin materials. Both chemically and photochemically initiated low-viscosity resin systems of the bis-GMA-TEGDMA type as presented in the foregoing, yet containing little or no filler reinforcement, are now widely employed as pit and fissure sealing materials. Such sealants serve to protect natural enamel faults from becoming carious and thus represent a vital tool in preventive dentistry. Irrespective of the specifics of application, use of methacrylate-type resins requires brief
(20 to 60 s) acid-etching pre-treatment of enamel surfaces, generally with aqueous phos­phoric acid as detailed in Section V. B.3. The surface roughening so achieved will allow resin tags to anchor to the enamel adherend; hence the mode of bonding to tooth structure is of the micromechanical kind. In view of the predominant part played by the composite resins as cavity-filling restoratives, the topic will be discussed in more detail in Section V. B.2.