Polystyrene, high-impact

Polystyrene is extensively used in a range of applications, such as car headlamp lenses, where its mechanical properties are important. Polystyrene has a Tg value of 100 °C and can withstand a reasonable impact. However, improvement of its impact properties is commercially desirable and requires incorporation of a 

The main deficiency of polystyrene is its brittleness this may be improved by a number of techniques, which are generally applicable in polymer technology  

Although there is limited scope for improving impact performance by increasing the molecular weight, (iv) and (v) provide the main methods by which the toughness of polystyrene is improved. 

The addition of rubbers to PS is widely practised the secret of obtaining good impact behaviour without compromising rigidity is that the blend should be heterogeneous. Butadiene-styrene rubber (SBR) may be blended with PS, but this can lead to molecular mixing, resulting in plasticization and 

The modification of PS with rubber results in a considerable increase in impact toughness, by an order of magnitude, but with deterioration in other properties 5-15°C in softening temperature and 20-40% in rigidity and strength (see Table 3.1). The ageing behaviour is also worsened by inclusion of rubber, but incorporation of antioxidant largely restores the situation. 

There have been several examples of the unsatisfactory use of PS, because of the brittle nature of the material the application in toys is typical. Children frequently abuse their toys and thus a good impact performance is essential. General-purpose polystyrene (GPPS) breaks and exposes a sharp edge as an additional hazard. The advent of HIPS allowed the good features of PS to be retained, with the additional benefit of toughness at room temperature (the toughness falls dramatically below — 20°C), for only a small increase in 

For many applications the homopolymer of styrene is too brittle. To overcome that, many different approaches were originally tried. These included use of high molecular weight polymers. 

Addition of rubbery materials, however, does improve the impact resistance of polystyrene. This is therefore done extensively. The most common rubbers used for this purpose are butadiene-styrene copolymers. Some butadiene homopolymers are also used, but to a lesser extent. The high-impact polystyrene is presently prepared by dissolving the rubber in a styrene monomer and then polymerizing the styrene. This polymerization is either done in bulk or in suspension. The product contains styrene-butadiene rubber, styrene homopolymer, and a considerable portion of styrene-graft copolymer that forms when polystyrene radicals attack the rubber molecules. The product has very enhanced impact resistance. 

In high-impact polystyrene, the rubber exists in discrete droplets, less than 50 //m in diameter. In effect, the polymerization serves to form an oil-in-oil emulsion, where the polystyrene is the continuous phase and the rubber is in a dispersed phase. The graft copolymer that forms serves to emulsify this heterogeneous polymer solution.  

Commercial high-impact polystyrene usually contains 5-20% styrene-butadiene rubber. The particle size ranges from 1-10 fim. High-impact polystyrene may have as much as seven times the impact strength of polystyrene, but it has only half its tensile strength, lower hardness, and a lower softening point. 

Styrene-acrylonitrile copolymers are produced commercially for use as structural plastics. The typical acrylonitrile content in such resins is between 20-30%. These materials have better solvent and oil resistance than polystyrene and a higher softening point. In addition, they exhibit better resistance to cracking and crazing and an enhanced impact strength. Although the acrylonitrile copolymers have enhanced properties over polystyrene, they are still inadequate for many applications. Acrylonitrile-butadiene-styrene polymers, known as ABS resins, were therefore developed. 

The photo-oxidation rates of polystyrene and polyacrylonitrile are considerably accelerated when they are grafted into polybutadiene. The rate of photooxidation of polystyrene becomes linear after 250 h whereas high impact polystyrene (polystyrene-butadiene) reaches the same stage after 25 h (Fig. 3.63). The rubber segment thus behaves as a photopro-oxidant for the polystyrene matrix and is itself destroyed in the process with the disappearance of the impact resistance of the polymer [761, 1922]. Thermal processing of high impact polystyrene increases the initial rate of photo-oxidation of the polymer 

A similar behaviour has been observed with polybutadiene-containing polyblends acrylonitrile-butadiene-styrene (ABS) (cf. section 3.12.13), methyl methacrylate-butadiene-styrene (MBS) and methyl methacrylate-acrylonitrile-butadiene-styrene (MABS), and the accelerated photo-oxidative effect has been found to be related to the amount of unsaturated groups present [1936]. 

The rapid loss in such elastomeric properties of the impact polystyrene (5.150) as impact strength and elongation at breakage has been explained in terms of scission reactions of the graft of polybutadiene with the polymer matrix. The separation of a graft in two entities, polystyrene and polybutadiene is produced by chain scission of the impact polystyrene graft [1497]. 

It has been suggested that hydroperoxides introduced into high impact polystyrene (polybutadiene modified polystyrene) by a priori heating in the presence of oxygen are primarily responsible for the initiation process which leads to the photo-oxidative destruction of the useful properties of the polymer [761-763, 1497]  

The chain scission is however accomplished by crosslinking reactions, and these two processes account for the loss of useful properties of the impact polystyrene. 

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Triphenyl phosphate [115-86-6] C gH O P, is a colorless soHd, mp 48—49°C, usually produced in the form of flakes or shipped in heated vessels as a hquid. An early appHcation was as a flame retardant for cellulose acetate safety film. It is also used in cellulose nitrate, various coatings, triacetate film and sheet, and rigid urethane foam. It has been used as a flame-retardant additive for engineering thermoplastics such as polyphenylene oxide—high impact polystyrene and ABS—polycarbonate blends. 

Property AST M test ABS Noryl a Nylon b P< Polyeste d r HD PE Polypropyle ne High impact polystyrene Polyurethane PV C... 

Structural Components. In most appHcations stmctural foam parts are used as direct replacements for wood, metals, or soHd plastics and find wide acceptance in appHances, automobUes, furniture, materials-handling equipment, and in constmction. Use in the huil ding and constmction industry account for more than one-half of the total volume of stmctural foam appHcations. High impact polystyrene is the most widely used stmctural foam, foUowed by polypropylene, high density polyethylene, and poly(vinyl chloride). The constmction industry offers the greatest growth potential for ceUular plastics. 

The oxidative coupling of 2,6-dimethylphenol to yield poly(phenylene oxide) represents 90—95% of the consumption of 2,6-dimethylphenol (68). The oxidation with air is catalyzed by a copper—amine complex. The poly(phenylene oxide) derived from 2,6-dimethylphenol is blended with other polymers, primarily high impact polystyrene, and the resulting alloy is widely used in housings for business machines, electronic equipment and in the manufacture of automobiles (see Polyethers, aromatic). A minor use of 2,6-dimethylphenol involves its oxidative coupling to... 

To complete the assembly of a cell, the interleaved electrode groups are bolted to a cov er and the cover is sealed to a container. Originally, nickel-plated steel was the predominant material for cell containers but, more recently plastic containers have been used for a considerable proportion of pocket nickel-cadmium cells. Polyethylene, high impact polystyrene, and a copolymer of propylene and ethylene have been the most widely used plastics. 

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). 

Foams have limited use for these purposes. Rigid cellular PVC is good as a thermal barrier but aot for stmctural parts. Doors and frames of stmctural molded foam, eg, foamed high impact polystyrene, can be made by iajection mol ding, with recesses for hinges, striker plates, and miter corners. Sohd polystyrene and stmctural foam-molded polyurethane have been molded for door frames. 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Cheap moulded objects. Toughened with butadiene to moke high-impact polystyrene (FIIPS). Foamed with CO2 to moke common packaging. 

In the period 1945-1955, while there was a noticeable improvement in the quality of existing plastics materials and an increase in the range of grades of such materials, few new plastics were introduced commercially. The only important newcomer was high-impact polystyrene and, at the time of its introduction, this was a much inferior material to the variants available today. 

In the mid-1950s a number of new thermoplastics with some very valuable properties beeame available. High-density polyethylenes produced by the Phillips process and the Ziegler process were marketed and these were shortly followed by the discovery and rapid exploitation of polypropylene. These polyolefins soon became large tonnage thermoplastics. Somewhat more specialised materials were the acetal resins, first introduced by Du Pont, and the polycarbonates, developed simultaneously but independently in the United States and Germany. Further developments in high-impact polystyrenes led to the development of ABS polymers. 

The rubber particles should not be so small that they are completely embedded in a craze. It is interesting to note that in high-impact polystyrene crazes tend to be about 2 p.m thick and the optimum particle sizes observed as a result of experience are quoted in the range 1-10 p.m. For ABS the figures are about 0.5 p.m and 0.1-l.Op.m respectively. 

By blending with semi-compatible materials which have a well below the expected service temperature range (e.g. high-impact polystyrene—as described in Chapter 3). 

Around Izod notch Low-density polyethylene Ethylene-propylene block copolymers Cellulose nitrate and propionate ABS and high-impact polystyrene Bis-phenol A polycarbonate... 

This lower has a number of ramifications on the properties of polybutadiene. For example, at room temperature polybutadiene compounds generally have a higher resilience than similar natural rubber compounds. In turn this means that the polybutadiene rubbers have a lower heat build-up and this is important in tyre applications. On the other hand, these rubbers have poor tear resistance, poor tack and poor tensile strength. For this reason, the polybutadiene rubbers are seldom used on their own but more commonly in conjunction with other materials. For example, they are blended with natural rubber in the manufacture of truck tyres and, widely, with SBR in the manufacture of passenger car tyres. The rubbers are also widely used in the manufacture of high-impact polystyrene. 

Because of such desirable characteristics as low cost, good mouldability, excellent colour range, transparency, rigidity and low water absorption, polystyrene became rapidly developed. For many purposes, however, it was considered to be unacceptably brittle and this led to the development of the rubber-modified high-impact polystyrene (HIPS) and to the complex ABS, AMBS and... 

In addition to polystyrene and high-impact polystyrene there are other important styrene-based plastics. Most important of these is ABS, with a global capacity of about 5 X 10 t.p.a. and production of about 3 X 10 t.p.a. The styrenic PPO materials reviewed in Chapter 21 have capaeity and production figures about one-tenth those for ABS. Data for the more specialised styrene-acrylonitrile copolymers are difficult to obtain but consumption estimates for Western Europe in the early 1990s were a little over 60000 t.p.a. 

High-impact Polystyrenes (HIPS) (Toughened Polystyrenes (TPS)) 437... 

HIGH-IMPACT POLYSTYRENE (HIPS) (TOUGHENED POLYSTYRENES (TPS))... 

A high-impact polystyrene (polystyrene SBR blend) may have seven times the impact strength of ordinary polystyrene, but about half the tensile strength, a lower hardness and a softening point some 15°C lower. Because of the rubber content there may be a reduction in light and heat stability and stabilisers are normally incorporated. 

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

As mentioned earlier, unmodified polystyrene first found application where rigidity and low cost were important prerequisites. Other useful properties were the transparency and high refractive index, freedom from taste, odour and toxicity, good electrical insulation characteristics, low water absorption and comparatively easy processability. Carefully designed and well-made articles from polystyrene were often found to be perfectly suitable for the end-use intended. On the other hand the extensive use of the polymers in badly designed and badly made products which broke only too easily caused a reaction away from the homopolymer. This resulted, first of all, in the development of the high-impact polystyrene and today this is more important than the unmodified polymer (60% of Western European market). 

In recent years general purpose polystyrene and high-impact polystyrenes have had to face intensive competition from other materials, particularly polypropylene, which has been available in recent years at what may best be described as an abnormally low price. Whilst polystyrene has lost some of it markets it has generally enjoyed increasing consumption and the more pessimistic predictions of a decline have as yet failed to materialise. Today about 75% of these materials are injection moulded whilst the rest is extruded and/or thermoformed. 

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