Fillers composite resins

To optimize the lesin system foi a given process and part, consideration should be given to fillers that can gready affect the cost and performance of the composite. Because of their low viscosity, fillers can often be added to polyesters. Fillers are often much cheaper than the resin they displace, and they can improve the heat resistance, stiffness, and hardness of the composite. Certain fillers, such as fumed siUca, impart thixotropy to the resin, increasing its resistance to drainage. 

Composite Resins. Many composite restorative resins have incorporated fluoride into the filler particles. One commonly used material, yttrium trifluoride [13709-49-4] is incorporated as a radiopaque filler to aid in radiographic diagnosis, and is also responsible for slow release of fluoride from the composites (280). This same effect is achieved with a barium—alumina—fluoro-siUcate glass filler in composite filling and lining materials. Sodium fluoride [7681-49-4] has also been used in composites by incorporating it into the resin matrix material where it provides long-term low level release (281-283). 

For the manufacture of medium-shock-resisting grades the preblend of resin, filler and other ingredients does not readily form a hide on the mill rolls. In this case the composition is preblended in an internal mixer before passing on to the mills. 

Resin with an accelerator added but not catalyst. According to ASTM, those plastics having superior properties over those consisting of the base resin, due to the presence of high-strength fillers embedding in the composition. Reinforcing fillers are fibers, fabrics or mats made of fibers. 

Sheet molding compounds (SMCs) and bulk molding compounds (BMCs) are the dominant materials used in automotive applications. These composites of unsaturated polyester resin, fillers and fiberglass have advantages of high stiffness, heat resistance and low coefficient of expansion. Coupled with low creep resistance, which is a distinct advantage over thermoplastic competition, and low-profile additives, which can yield Class A surfaces, these materials are well suited for applications from exterior body panels to under the hood components. 

Thermal expansion differences exist between the tooth and the polymer as well as between the polymer and the filler. The tooth has a thermal expansion coefficient of 11 x 10-6/°C while conventional filled composites are 2-4 times greater [63, 252], Stresses arise as a result of these differences, and a breakdown between the junction of the restoration and the cavity margin may result. The breakdown leads to subsequent leakage of oral fluids down the resulting marginal gap and the potential for further decay. Ideal materials would have nearly identical thermal expansion of resin, filler, and tooth structure. Presently, the coefficients of thermal expansion in dental restorative resins are controlled and reduced by the amount and size of the ceramic filler particles in the resin. The microfilled composites with the lower filler loading have greater coefficient of thermal expansions that can be 5-7 times that of tooth structure. Acrylic resin systems without ceramic filler have coefficients of thermal expansion that are 9 times that of tooth structure [202-204, 253],... 

With filled, re-inforced, and composite products the separation of unwanted material can present special problems, particularly if fibres or fibrous substances are incorporated there may be a tendency for the fibres to hold unwanted portions, while attempts at cutting or smoothing with abrasives can take away resin, filler, and re-inforcement selectively. Because of this it is difficult with such products to obtain finish of high quality unless a suitable film or some other coating is applied later. 

Westlane Plastics (USA) [45] uses an extrusion method to produce rods of up to 31.75 mm diameter, stri K, pipes with 9.52-12.7 mm wall thickness and other profile artides from compositions based on phenol, melamine, carbamide, and alkyd resins. Fillers used incluse wood crush, cellulose, chalk, talc, clay, mica, silica, coke, graphite, and carbon fibers. 

Glasses are used in a wide variety of dental products. Glass powders are used as fillers in composite resin materials. These glasses must have very fine particle sizes, and typically contain strontium or barium to aid in... 

Any additive more rigid than the base resin produces a more rigid composite. Particulate fillers severely degrade impact strength. 

Composite resins consist of blends of large monomer molecules, filled with unre-active reinforcing filler. As such, they are hydrophobic, which means that they are unable to bond to the hydrophilic prepared tooth surface [1]. Glass-ionomer cements, by contrast, consist of aqueous solutions of polymeric acid, typically poly(acrylic add) and powdered reactive glass. These two components react together in an acid-base reaction, and thus cause the cement to set. These materials are hydrophilic, and therefore capable of wetting the prepared tooth surface and forming tme adhesive bonds. 

In addition to the blend of monomers, composite resins contain fillers. These are typically finely divided quartz or barium silicate glasses, and their function is to provide strength for the fully formulated composite [2]. These fillers are linked to the polymer phase by coupling agents, which are typically silane-based substances [2]. Composite reins are characterized by the absence of a chemical reaction between the filler and the monomer or polymer phase. Also, they show no inherent adhesion to the tooth but instead they have to be bonded to the tooth with bespoke bonding agents. These are discussed in detail in Chapter 5. 

Polyacid-modified composite resins were developed in an attempt to make a composite resin with the sort of ion-release capability of glass-ionomer cements, especially of fluoride [38]. They are similar to conventional composites in that they are mainly based on the hydrophobic monomers bis-GMA or urethane dimethaaylate, and their setting is typically initiated by light. In addition, they contain inert fillers of appropriate particle size. 

Composite resins have continued to undergo improvements, notably through modifications to the filler type and content, and filler particle size and distribution [1,30]. In addition, there have been considerable developments in bonding systems, as discussed in detail in Chapter 5. 

Another thing that follows from this is that occasional recent claims from manufacturers of the invention of a completely new type of materials are incorrect. Thus, both giomers and ormocers are types of composite resin, albeit with novel fillers and, in the case of ormocers, novel monomers, though they still set by the same type of chemistry, ie, addition polymerization [45]. They are also fundamentally hydrophobic, and do not form inherent bonds to the tooth surface. These materials are discussed later in the book, in Chapter 3, where their principal features are described and related to their essential chemistry as types of composite. 

The role of the fillers in composite resins is to reinforce their mechanical properties and provide a blended material whose overall properties make it suitable for the clinical repair of teeth. A limited range of materials has been used, with greater emphasis on variations in the particle size and size distribution than on chemical composition. Early materials were filed with powdered quartz, whereas modem composites are more likely to be filled with finely divided barium silicate or a radio-opaque silicate glass [8]. Filler loadings are typically of the order of 55% by volume, as they were in Bowen s original formulation [9]. 

The size range of the filler particles was the basis of an early classification of composite resins [102], Though the range of particle sizes has now been extended to include nano-particles [103], this is still a useful approach to classifying these materials. Table 3.1 shows the order of development of composite resins based on the particle size of their fillers. 

As well as conventional composites of the type based on bisGMA and/or UDMA and filled with silicate-based filler, there are now materials available that are essentially composites in that they comprise a polymeric matrix reinforced with finely divided filler. However, either the polymer system or the filler phase is of a different chemical composition from that of conventional composite resins. Three such materials are currently available, and these are the ormocers, the siloranes and the giomers. Their details are given in Table 3.3, and their characteristics are described in the following subsections. 

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