Fillers toughness considerations

The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

Most commercial WPCs are considerably more flexible than solid wood. WPCs creep more than solid wood, are less tough, and can handle less fatigue before failure [5, 38]. The use of wood fibers as reinforcing agents rather than just as fillers increases MOE, MOR, and the ultimate tensile strength (UTS) [39] as well as the unnotched energy... 

Attempts have been made to improve the mechanical properties of polymer-based materials, by adding a percentage of selected filler particles. There has been considerable improvement of properties such as elastic modulus, fracture toughness, flexural strength and hardness with the increase of the filler volume. 

Another considerable disadvantage of established members of the glass-ionomer family is their brittleness. Including extra filler in the glass carbomer type in and of itself does not help with this drawback indeed, it makes the situation worse. However, in its final formulation, the glass carbomer also contains a novel component, a silicone oil, which improves the toughness of the set cement and makes the potential durabifity better. 

The trend toward the lowering of fracture toughness by particulate fillers is consistent with considerable experience [see property tables in, e.g.. Modern Plastics Encyclopedia (1974-1975)] and with results of several other studies of the effects of filler on the area under a stress-strain curve... 

Aishima et al. used a variety of unsaturated acids as pretreatments for silicate fillers in PE and PP [19] and incorporated a polymerization inhibitor to prevent loss of reactivity during the filler coating process along with a small amount of peroxide. Improvements in both strength and toughness were reported (Table 6.6). They also found improvements in dynamic fatigue resistance. Fatigue is a property that is not often mentioned but is of considerable practical importance in some applications. 

Fillers can modify the properties of the cured polymer in a wide variety of ways. Some examples of properties readily modified by incorporation of fillers are mechanical properties such as stiffness, hardness, strength and toughness, and thermal properties such as expansion, conductivity or thermal decomposition. The latter can have a considerable effect on flammability and smoke generation. Fillers are also an important way of varying and enhancing aesthetics. 

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