Plasticisers rubbers

Reaction of polyhydroxy compounds with polybasic acids gives rise to condensation polymers containing ester (—COO—) groups. Because of the presence of these groups such polycondensates are known as polyesters and find use in such diverse applications as fibres, surface coatings, plasticisers, rubbers and laminating resins. These materials are discussed in detail in Chapter 25. 

The hydrohalide is liable to dehydrochlorination, particularly when moist acid is used in its preparation, so that hydrochloric acid acceptors such as lead carbonate are useful stabilisers. Dibutyl phthalate and tritolyl phosphate are effective plasticisers. Rubber hydrochloride is used as a packaging film (Pliofilm) and as a rubber-to-metal bonding agent (e.g. Typly). 

Modem designs fitted with non-retum valves through which the plasticised rubber compound passes into the injection cylinder portion are capable of giving precise shot volume. The non-retum valve is activated by the injection pressure. The injection ram/non-retum valve unit ensures complete purging of the injection chamber. 

Lichenin, cellulose rayon, intensely plasticised rubber. 

Mesocolloidal (pol. degree 100—500) Normally plasticised rubber, native balata, and gutta percha i -cellulose, rayon, cellophane-foil Pepsin Insulin i ditto, in the solid state sometimes rubbery Shellac (probably), somewhat polymerised damar and copal. Phenol- or urea-formaldehyde between the A-and B-stages. 

Pre-plasticising is the term given to the conversion of uncured rubber to a hot, soft (relative), and homogeneous, plastic mass. This is achieved by the rotation of a metal screw within a heater-jacketed barrel. The relationship between the temperature of the pre-plasticised rubber and that of the jacketed barrel is dynamic, as the hot rubber exchanges frictional heat to the barrel walls or, depending on the heater setting, gains more heat. Rubber strip is fed to the throat, or inlet to the screw where it is sheared... 

The time delay, between the end of the injection phase and the automatic refilling of the screw and barrel with rubber should be set to minimise the time that plasticised rubber is held in the barrel before injection. If there is any undue delay before the start of the next cycle, this rubber should be purged from the barrel and the process restarted. 

Epoxies can be modified with various additives, fillers, plasticisers, rubber tougheners and other polymers and so there are a huge number of epoxy adhesives available and numerous specialist grades. 

Comparison of Table 5.4 and 5.7 allows the prediction that aromatic oils will be plasticisers for natural rubber, that dibutyl phthalate will plasticise poly(methyl methacrylate), that tritolyl phosphate will plasticise nitrile rubbers, that dibenzyl ether will plasticise poly(vinylidene chloride) and that dimethyl phthalate will plasticise cellulose diacetate. These predictions are found to be correct. What is not predictable is that camphor should be an effective plasticiser for cellulose nitrate. It would seem that this crystalline material, which has to be dispersed into the polymer with the aid of liquids such as ethyl alcohol, is only compatible with the polymer because of some specific interaction between the carbonyl group present in the camphor with some group in the cellulose nitrate. 

Before providing such an explanation it should first be noted that progressive addition of a plasticiser causes a reduction in the glass transition temperature of the polymer-plasticiser blend which eventually will be rubbery at room temperature. This suggests that plasticiser molecules insert themselves between polymer molecules, reducing but not eliminating polymer-polymer contacts and generating additional free volume. With traditional hydrocarbon softeners as used in diene rubbers this is probably almost all that happens. However, in the... 

In the rubber industry hydrocarbon oils are often used to reduce the softness and facilitate the processing of hydrocarbon rubbers. These appear to have a small interaction with the polymer but spacing effects predominate. Such materials are generally referred to as softeners. The rubber industry, like the plastics industry, commonly uses the term plasticisers to describe the phthalates, phosphates and sebacates which are more commonly used with the more polar rubbers. 

A substantial part of the market for the ethylene-vinyl acetate copolymer is for hot melt adhesives. In injection moulding the material has largely been used in place of plasticised PVC or vulcanised rubber. Amongst applications are turntable mats, base pads for small items of office equipment and power tools, buttons, car door protector strips and for other parts where a soft product of good appearance is required. Cellular cross-linked EVA is used in shoe parts. 

The price of these rubbers has become such that for many applications they have been replaced by less expensive alternatives. These include the use of EPDM rubbers for automotive parts not requiring oil resistance and plasticised PVC for applications where flexibility rather than high elasticity is required. 

Blending of polymer, plasticiser and filler may be carried out using two-roll mills or internal mixers as commonly used in the rubber industry. Alternatively, since the raw polymer is supplied as a free-flowing powder a dry blending process similar to that now widely used with PVC (see Chapter 12) is also used. 

Plasticised PVC, referred to below as PPVC, is used in a wide variety of applications. Originally a substitute for natural rubber when the latter material became difficult to obtain during World War II, it is frequently the first material to consider where a flexible, even moderately rubbery, material is desired. This arises from the low cost of the compounds, their extreme processing versatility, their toughness and their durability. 

The resins act as a plasticiser during processing but they cross-link while the rubber is vulcanising to give a harder product with improved oxidation resistance, oil resistance and tensile strength. The addition of sufficient resin will lead to an ebonite-like product. 

Polyesters are eneountered in many forms. They are important as laminating resins, moulding compositions, fibres, films, surface coating resins, rubbers and plasticisers. The common factor in these widely different materials is that they all contain a number of ester linkages in the main chain. (There are also a number of polymers such as poly(vinyl acetate) which contain a number of ester groups in side chains but these are not generally considered within the term polyester resins.)... 

A wide range of polyurethane-type products has become available in recent years for coating applications. These include simple solutions of linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate systems and a variety of prepolymer and adduct systems. The coatings can vary considerably in hardness and flexibility and find use mainly because of their toughness, abrasion resistance and flexibility. Uses include metal finishes in chemical plant, wood finishes for boats and sports equipment, finishes for rubber goods and rain-erosion-resistant coatings for aircraft. One type of coating is potentially competitive with PVC leathercloth. Both alkyd-di-isocyanate and adduct-diisocyanate compositions may be coated on to fabrics from solutions of controlled viscosity and solids content. Such coated fabrics are soft, flexible and, unlike PVC leathercloth, free from plasticisers. 

The hydrohalide is usually prepared by passing hydrogen chloride into a solution of masticated high-grade raw rubber in benzene at 10°C for about six hours. Excess acid is then neutralised and plasticisers and stabilisers are added. The benzene is removed by steam distillation and the product washed and dried. Alternatively the solution is cast on to a polychloroprene rubber belt, leaving a tough film after evaporation of the solvent. 

Chlorinated rubber is extensively employed in industrial corrosion-resistant surface coatings, for which purpose it is marketed by ICI under the trade name Alloprene. Although thermoplastic moulding compositions have been made by plasticising with the common ester plasticisers such as tritolyl phosphate they are of no commercial importance. 

The minimum service temperature is determined primarily by the Tg of the soft phase component. Thus the SBS materials ctm be used down towards the Tg of the polybutadiene phase, approaching -100°C. Where polyethers have been used as the soft phase in polyurethane, polyamide or polyester, the soft phase Tg is about -60°C, whilst the polyester polyurethanes will typically be limited to a minimum temperature of about 0°C. The thermoplastic polyolefin rubbers, using ethylene-propylene materials for the soft phase, have similar minimum temperatures to the polyether-based polymers. Such minimum temperatures can also be affected by the presence of plasticisers, including mineral oils, and by resins if these become incorporated into the soft phase. It should, perhaps, be added that if the polymer component of the soft phase was crystallisable, then the higher would also affect the minimum service temperature, this depending on the level of crystallinity. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

Chlorinated rubber is soluble in aromatic solvents, and paints made from it dry by solvent evaporation alone. In contrast to the vinyls, there is less difficulty in formulating systems that are suitable for brush application. It has excellent resistance to a wide range of chemicals and to water, but as it is extremely brittle it needs to be plasticised. To preserve chemical resistance it is necessary to use inert plasticisers such as chlorinated paraffin wax. Due to the presence of ozone depleting solvents, chlorinated rubber coatings are being phased out and largely replaced by vinyl acrylic coatings which have very similar performance and can be formulated from lower aromatic or aliphatic solvents. 

Thermoplastics may themselves be considered in four sub-classes (a) amorphous thermoplastics, (b) rubber-modified amorphous thermoplastics, (c) plasticised amorphous thermoplastics and (d) crystalline thermoplastics. 

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