RIGID AND BRITTLE

FIGURE 4-27 Stress-strain graph of a rigid and brittle material 

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For reasons explained below, the effect of increasing the vinyl alcohol content in EVOH is quite different to that of increasing the vinyl acetate content in EVA. In the case of ethylene-vinyl acetate (EVA) copolymers, increasing the vinyl acetate content up to about 50% makes the materials less crystalline and progressively more flexible and then rubbery. In the range 40-70% vinyl acetate content the materials are amorphous and rubbery, whilst above 70% the copolymers become increasingly rigid and brittle. 

The linear polystyrene exists as a hard, rigid and brittle material. It is having a relatively low softening point. Polystyrene Organisation and Qualities... 

The inherent thermal stability of the phenol formaldehyde chemistry is preserved but with the crosslinking characteristics of the epoxy groups. However, epoxy novolacs also form very rigid and brittle polymers when fully cured because of their high crosslink density. For this reason, they are often used as modifiers in epoxy adhesive systems rather than as the base polymer. 

A plasticizer is a compatible low-melting-point solid or a high-boiling point liquid which when added to a rigid and brittle material in relatively... 

Other amorphous solids such as polymers, being rigid and brittle below. Tg, and elastic above it, also exhibit this behavior. Table 2.1 lists the glass transition temperatures of common solid materials. Although most solid-state textbooks deal almost exclusively with crystalline materials, this text will attempt to address both the crystalline and amorphous states, describing the structure/property relationships of major amorphous classes such as polymers and glasses. 

Poly(vinyl acetate) (PVAc), -[CH2-CH(OCOCH3) , is also prepared as an emulsion for adhesive applications, and is familiar to users as white glue. As mentioned already, the Tg of PVAc is above room temperature, which makes the polymer rigid and brittle at room temperature. For adhesive application, therefore, an external plasticizer, such as dibutyl phthalate, is added to lower the Tg below room temperature and to facilitate film formation from emulsions. 

Compression-molded soy protein plastics are rigid and brittle without plasticizers present in the formulation (Paetau et al., 1994). Water absorption of early protein plastics could not be reduced to the very low levels that were obtained with synthetic, petroleum-derived resin products however, color, dyeing, and strength properties were good and production costs were relatively low (Johnson Myers, 1995). 

Polymers show a similar response to temperature and strain rate (time), as might be expected from the time-temperature superposition principle (compare Figures 13.31 and 13.32). Specifically, the effect of decreasing temperature is equivalent to that of increasing the strain rate. As has become evident from our previous discussions, low temperature restricts molecular movement of polymers, and consequently they become rigid and brittle. Materials deform to relieve imposed stress. High strain rates preclude such deformation and therefore result in brittle failure. 

Because of their characteristic rigidity and brittleness in their cured state, when MF resins are used for impregnated paper overlays, small amounts (typically 3 to 5%) of modifying compounds are often copolymerized with the MF resin during its preparation to give better flexibility to the finished product and better viscoelastic dissipation of stress in the joint. Most commonly used are acetoguanamine, s-caprolactam, and p-toluene-sulfonamide (Formula 2). 

Calcium is relatively immobile in plants and deficiency symptoms appear first in the younger leaves. Meristematic regions of stem, leaf and root tips are affected and eventually die. Roots become short, stubby and brown (60).. Cell walls become rigid and brittle. 

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