Toughening additives

Improve both impact strength and rigidity of thermoplastics by using up the energy of crack propagation. Elastomers are prototypical toughening additives. Examples of high-polymeric impact modifier/thermoplastic matrix systems are EVA, CPE and MBS in PVC, EP(D)M and SBS in PA, and acrylic rubbers in polyesters. 

Another demonstrated application is the use of epoxidized hyperbranched polyesters as toughening additives in composites [120-122]. 

Over the past several decades, significant advances have been made in developing epoxy-based adhesives having improved performance over these early adhesive systems. These improvements were made possible by (1) the incorporation of toughening additives into epoxy resin formulations and (2) the use of multifunctional epoxy resins primarily for high-temperature applications. These innovations are discussed in later chapters. 

The use of epoxidized hyperbranched polyesters as toughening additives in carbon-fiber reinforced epoxy composites has been demonstrated (Boogh et al., 1995). Since a hyperbranched resin has a substantially lower viscosity and much shorter drying time than a conventional (less branched) resin of comparable molecular weight, hyperbranched polymers have been used as the base for various coating resins (Pettersen and Sorensen, 1994). 

These blends are immiscible, thus should be compatibilized and toughened. Addition of POM to PC improves the solvent and chemical resistance [Miller, 1972], PC blends with POM and TPU were easy to mold into articles having high impact... 

Epoxy toughening additives initially were based on rubbery inclusions or functionalized oligomers (carboxy or amine terminated butadiene/ acrylonitrile copolymers). More recently, impact modifiers (core-shell type) similar to that commonly employed with PVC have been proposed. For composites, tougher epoxy matrix candidates... 

Figure 1.2 Typical schematic tensile stress-strain curves for polymers (a) brittle, amorphous thermoplastic, (b) same polymer with toughening additive, (c) intrinsically tough, semi-crystalline thermoplastic. The curves should be taken only to convey trends and not relative breaking stresses, which vary with the precise materials...

Unmodified Epoxide adhesives are versatile materials, but like many polymers below their Glass transition temperatures, they suffer from low resistance to impact and peel forces. In Toughened adhesives, some means of addressing such problems were discussed in general terms. This article considers the basis of toughening in epoxies, and related articles (cross-referenced herein) discuss the types of toughening additives employed. 

Rubbers, often based on poly diene rubbers or else copolymers of dienes like 1,3-butadiene, were the first successful toughening additives, and they are effective partly because they have a low modulus, 100 to 500 times lower than that of most thermoplastic polymers. Unfortunately polydienes introduce chemical double bonds which are susceptible to UV, thermal and oxidative degradation. Hydrogenation removes some of them. Acrylic compounds and ethylene copolymers are also popular impact modifiers, and they do not necessarily introduce double bonds. 

One drawback with addition-type polymers is that the cured adhesive tends to be brittle. Attempts to toughen addition polyimides have met with some success, though improvements in this area are needed. The development of systems exhibiting the same level of toughness as today s state-of-the-art epoxy adhesives is a goal for the future. 

The improvements in performance have been largely due to the addition of fillers, often wollastonite and talc in combination with rubber toughening additions. This has produced a family of adhesives which have excellent toughness up to 50 °C and down to -40 °C. 

Apart from the usually reported morphological analysis (electron microscopy and rheological analysis) and evaluation of the mechanical properties of the prepared nanocomposites, some authors have published more specific papers on the use of SBS-montmorillonite nanocomposites as toughening additives for polypropylene [65], as rheological modifiers for asphalts [66], and as irradiation (UV-light, y-rays, and electron beams) stabilizers [67]. 

It is desirable for plastics articles to be able to withstand cracking when subject to minor impact. Failure to resist cracking is a common feature of plastics and this entry discusses the way in which polymers can be used as toughening additives to overcome the problem. 

Research is also needed to broaden the applications of polymeric materials. Materials with tailored properties based on blends, high-strength fibers, new matrices for composites, and improved stability of toughening additives are finding new uses as materials substitutes and in unique applications. This trend will accelerate as failure mechanisms become better understood. The areas for substitution of materials span automobiles, aircraft, boats, construction, machinery, and many other specialty items. While military applications are growing (e.g., body armor, uniforms, and aircraft), the field is ripe for rapid growth and penetration by polymeric materials. 

N MN 93D. The graph shows the improvement in notched Izod performance versus temperature with differing levels of toughener additive. As shown in this hgure, the performance can he dramatically improved. 

Considering next the toughened structural adhesives, then a more complex picture emerges since the effect of the toughening additives is to invoke crack-tip micromechanisms which increase the extent of energy dissipation. Thus, the energy-dissipation zone at the crack tip is larger, which increases the... 

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