Amorphous polymers impact strength

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

Polycarbonates are an unusual and extremely useful class of polymers. The vast majority of polycarbonates are based on bisphenol A [80-05-7] (BPA) and sold under the trade names Lexan (GE), Makrolon (Bayer), CaUbre (Dow), and Panlite (Idemitsu). BPA polycarbonates [25037-45-0] having glass-transition temperatures in the range of 145—155°C, are widely regarded for optical clarity and exceptional impact resistance and ductiUty at room temperature and below. Other properties, such as modulus, dielectric strength, or tensile strength are comparable to other amorphous thermoplastics at similar temperatures below their respective glass-transition temperatures, T. Whereas below their Ts most amorphous polymers are stiff and britde, polycarbonates retain their ductiUty. 

Polystyrene. Polystyrene [9003-53-6] is a thermoplastic prepared by the polymerization of styrene, primarily the suspension or bulk processes. Polystyrene is a linear polymer that is atactic, amorphous, inert to acids and alkahes, but attacked by aromatic solvents and chlorinated hydrocarbons such as dry cleaning fluids. It is clear but yellows and crazes on outdoor exposure when attacked by uv light. It is britde and does not accept plasticizers, though mbber can be compounded with it to raise the impact strength, ie, high impact polystyrene (HIPS). Its principal use in building products is as a foamed plastic (see Eoamed plastics). The foams are used for interior trim, door and window frames, cabinetry, and, in the low density expanded form, for insulation (see Styrene plastics). 

Polymers with differing morphologies respond differentiy to fillers (qv) and reinforcements. In crystalline resins, heat distortion temperature (HDT) increases as the aspect ratio and amount of filler and reinforcement are increased. In fact, glass reinforcement can result in the HDT approaching the melting point. Amorphous polymers are much less affected. Addition of fillers, however, intermpts amorphous polymer molecules physical interactions, and certain properties, such as impact strength, are reduced. 

Rubber-modified amorphous polymers The brittleness of amorphous polymers has been a hindrance in their commercial development. Fortunately, for reasons still not fully understood, the addition of rubbery polymers as dispersed droplets, or sometimes in a network form, into the glassy polymer can often lead to substantial increases in impact strength, albeit usually at... 

The strong sub-rg relaxation in PCT also contributes to increased toughness of this polymer in the amorphous state. When measured on amorphous specimens, the notched Izod impact strength of PCT is greater than 1000 J/m, while that of PET is less than 100 J/m [22],... 

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...

PET can be blended with PBT or polycarbonate (PC) to make blends or alloys. Polycarbonate is a high Tg (150 °C) ductile, high-impact-strength polymer but has rather poor solvent resistance because of its amorphous nature. PET/PC blends have therefore proved advantageous because they combine the solvent-resistance... 

Between Tg and T the polymer contains a crystalline and an amorphous, rubbery phase. The rubbery phase is responsible for the high impact strength. 

Mechanical Properties. The room temperature modulus and tensile strength are similar to those of other amorphous thermoplastics, but the impact strength and ductility are unusually high. Whereas most amorphous polymers arc glass-like and brittle below their glass-transition temperatures, polycarbonate remains ductile to about — 10°C. The stress-strain curve for polycarbonate typical of ductile materials, places it in an ideal position for use as a metal replacement. Weight savings as a metal replacement are substantial, because polycarbonate is only 44% as dense as aluminum and one-sixth as dense as steel. 

Polyvinylchloride (PVC) is a hard, amorphous polymer which softens at about 85 °C. Also in PVC rubbers are sometimes added in order to improve the impact strength. The main applications of PVC are pipes, gutters, front panels of buildings, cables, bottles, floor tiles. A much softer and more flexible material is obtained by blending with plasticizers soft or plasticized PVC is being used in artificial leather, tubes and hoses, footwear, films, etc. 

Polystyrene (PS) is an amorphous, very brittle, hard polymer with a softening temperature of about 90 °C. Improvement of its impact strength is again obtained by... 

A crystalline polymer is a two-phase system for T < Tg it contains a glassy phase and a crystalline phase, with, in general, a low impact strength. For T > Tg the amorphous phase is in the rubbery condition, which results in a much higher impact strength. 

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