Toughened polymers

The incorporation of rubber particles into a brittle polymer has a profound effect upon the mechanical properties as shown from the stress-strain curves in Fig. 5.66. This can be seen in Fig. 5.66(a) for high-impact polystyrene (HIPS) which is a blend of polystyrene and polybutadiene. The stress-strain curve for polystyrene shows brittle behaviour, whereas the inclusion of the rubbery phase causes the material to undergo yield and the sample to deform plastically to about 40% strain before eventually fracturing. The plastic deformation is accompanied by stress-whitening whereby the necked region becomes white in appearance during deformation. As will be explained later, this is due to the formation of a large number of crazes around the rubber particles in the material. 

35 is because at high values of Wp the particles become so close that they touch and interact with each other and thereby reduce the efficiency of toughening. 

Both crazing and shear yielding involve the absorption of energy and most methods of toughening brittle polymers involve modifying the polymer 

The behaviour of HIPS can be contrasted with that of rubber-toughened poly(methyl methacrylate) (RTPMMA) shown in Fig. 5.68(b). In this case 

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Glass-fibre-filled grades of these toughened polymers are also available but these do not show the same improvement in toughness over normal glass-fibre-filled nylons. 

Figure 14.12 Notched Izod impact strength data (on crystallized PET) for samples of toughened polymer as a function of the ratio of interparticle distance O, amorphous x, crystalline [28]. Reprinted with permission from Pecorini, T. J. and Calvert, D., in Toughening of Plastics - Advances in Modelling and Experiments, Pearson, R. A., Sue, H.-J. and Yee, A. F. (Eds), ACS Symposium Series, 759, American Chemical Society, Washington, DC, 2000, Ch. 9, pp. 141-158. Copyright (2000) American Chemical Society...

The fracture behavior of toughened polymers, containing rubber or inorganic fillers, may involve several mechanisms, as schematically illustrated in Fig. 8.1 (Garg and Mai, 1988a). These include ... 

The technical and commercial success of high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) has led to a widespread research program on the use of rubbers as toughening agents for plastics. There is now an impressive 11st of rubber-toughened polymers including both amorphous and... 

Bucknall CB (2000) Deformation mechanisms in rubber-toughened polymers. In Paul DR and Bucknall CB (eds) Polymer Blends, Vol 2. Wiley, New York p 83... 

Partridge IK (1992) Rubber Toughened Polymers. In Rostami S (ed) Multicomponent Polymer Systems. Longman Scientific Technical Ltd., Essex, England, p 149... 

Bucknall, C. B. Toughened Polymers , London, Applied Science Publ. 1977... 

Distance between particles in rubber toughened polymers... 

A very small air bubble (microvoid) created to toughen polymer... 

The question of toughening polymers (especially against Impact loading) by the judicious Incorporation of rubbery phases has also received much attention in recent years (see, for example, references and 22). Depending on the concentra-... 

Bucknall, Proceedings, International Conference on Toughened Polymers(July 1978), the Plastics and Rubber Institute, London, paper 24. 

Summary of Different Toughening Mechanisms. In toughened polymers with a dispersed modifier phase (i.e., in the dispersed systems), the three mechanisms sketched in Figure 19 may, in general, be distinguished. The characteristics of these different mechanisms are as follows. 

The different basic mechanisms shown in Figure 19 are valid for other toughened polymers as well. For example, the mechanism in Figure 19 is decisive for rubber-toughened poly(vinyl chloride) (37), polycarbonate, 38, 39), poly(methyl methacrylate) 40, 41), and rubber-toughened epoxies 42, 43). 

To overcome the above drawbacks, a new method based on essential work of fracture concept was introduced [Broberg, 1971, 1975]. In this method, it is proposed that when a cracked ductile solid, such as a toughened polymer blend is loaded, the fracture process and the plastic deformation take place in two different regions, viz. the inner process zone and the outer plastic zone. Much of the fracture work during crack propagation, dissipated in the plastic zone, is not directly associated with the fracture process. Only that work that goes into the fracture process zone is a material constant. Hence, the total fracture work, Wp should be separated into two parts, i.e., the essential work of fracture (i.e., the work required to create two new fracture surfaces, W),... 

A. Lazzeri, The kinetics of dilatational bands and the interparticle distance effect in rubber toughened polymers, in Proc. 10th International Conference on Deformation, Yield and Fracture of Polymers, Cambridge, UK, The Institute of Materials, London, 1997, p. 442. 

The well-known examples of blends are impact modified, toughened polymers, where polymers with different glass transition temperatures are blended, such as a rubber with a thermoplastic. Many other blends are known, such as barrier polymers for packaging, where specific polar or nonpolar polymers improve the properties of polymer blends, combined to increase the resistance against transport of water and a certain gas (oxygen, carbon dioxide, etc.), such as PA (barrier to oxygen) with a polyolefin (barrier to water vapor). 

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