Plasticisers and Softeners

Another interesting innovation is that developed by the Malaysian Rubber Producers Research Association. In this case the coupling agent is first joined to a natural rubber molecule involving an ene molecular reaction. The complex group added contains a silane portion which subsequently couples to filler particles when these are mixed into the rubber. 

It has been common practice to blend plasticisers with certain polymers since the early days of the plastics industry when Alexander Parkes introduced Parkesine. When they were first used their function was primarily to act as spacers between the polymer molecules. Less energy was therefore required for molecular bond rotation and polymers became capable of flow at temperatures below their decomposition temperature. It was subsequently found that plasticisers could serve two additional purposes, to lower the melt viscosity and to change physical properties of the product such as to increase softness and flexibility and decrease the cold flex temperature (a measure of the temperature below which the polymer compound loses its flexibility). 

Today plasticisers are used in a variety of polymers such as polyvinyl acetate, acrylic polymers, cellulose acetate and, most important of all, poly(vinyl chloride). 

It was pointed out in Chapter 5 that plasticisers were essentially non-volatile solvents. Consequently they were required to have solubility parameters close to that of the polymer and a molecular weight of at least 300. If the polymer or the plasticisers had a tendency to crystallise then there would need to be some sort of specific interaction between the polymer and the plasticiser. Tables 5.4 and 5.6 gave some figures for the solubility parameters of polymers and plasticisers. 

Of particular interest is the fact that two plasticisers of similar molecular weight and solubility parameter can, when blended with polymers, lead to compounds of greatly differing properties. Many explanations have been offered of which the most widely quoted are the polar theory and the hydrogen bonding theory. 

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Vinyl chloride can be polymerised to form polyvinyl chloride (PVC) which is fairly brittle and unsuitable for food contact applications, so it is mixed with plasticisers to soften the polymer and impart flexibility. Plasticised PVC may contain about 30% of plasticisers and is used to make stretch films and flexible PVC. Flexible PVC used for tubing and gaskets may contain di(2-ethylhexyl)phthalate, and stretch films will probably contain di(2-ethylhexyl)adipate and a polymeric adipate plasticiser. Rigid PVC may... 

Pure PVC is thermally and photochemically unstable and has a tendency to lose hydrogen chloride when heated, hence a stabiliser (based on tin, lead, or other heavy metals) is commonly used. Pure PVC is also brittle and needs a plasticiser to soften and flexibilise the system. 

The amoimt of plasticiser required can sometimes be reduced by replacing part of it with a different, cheaper softening agent, sometimes known as an extender. Extenders have similar effects to plasticisers, but are not capable of completely replacing them because of their inadequate compatibility. Examples include the chlorinated hydrocarbons, epoxidised soya bean oil and epoxidised linseed oil. These last two substances have already been described as plasticisers the boimdaiy between plasticisers and extenders is not sharp, becanse compatibiUty depends on the formulation. Note that the term extender is sometimes used for other low cost additives designed to bulk out the volume of a mix, such as cheap fillers. 

Plasticisers. They soften and make hair supple and easy to work with (lanolin derivatives, silicones, fatty acid esters, glycerine andpolyethylenglycols). 

Effect of additives (e.g. plasticiser and fillers) on the softening point of polymers. 

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. 

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA. Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point. 

Soften and render products more flexible by reduction of the brittleness of the end-product. Also used to modify viscosity and improve the flow characteristics and processability of a polymer. Are designed to space out the polymer molecules, facilitating their movements and leading to enhanced flexibility (lower modulus) and ductility. Plasticisers may play a dual role as stabilisers or cross-linkers. Performance criteria are compatibility, plasticising efficiency, processability and permanence. 

A machine for masticating rubber. The principle is that of an extruder, the rubber being softened by working between the threads of the screw and the surface of the barrel. The best known type is the Gordon Plasticator . Now obsolete. Plasticiser... 

The use of softeners and plasticisers presents a problem. TP90B, thioethers and certain adipates can give low temperature flexibility to -45 °C, but these are volatile at post cure temperatures, and hence their effect is easily lost. 

Fitted to an extruder a gear pump is particularly useful for difficult compounds and process conditions. The compound is softened and plasticised in the extruder and fed into the gear pump under low intake pressure. The gear pump intake pressure regulates the speed of the extruder so that the gear pump is always filled. The gear pump builds up the pressure required at the die. A screen can be fitted if desired. 

A report by a scientific advisory panel to Health Canada is one of the most extensive to analyse the possible health risks from di-2-ethylhexyl phthalate (DEHP), a plasticiser used to soften PVC medical products such as bags and tubing. This article gives details of the report, which has concluded that DEHP does pose health risks for some critical groups. 

The starting material in these processes is not necessarily in the liquid state also pastes of PVC with a plasticiser can be used, and even a well-flowing powder of a thermoplast. The particles will melt when in touch with the heated wall and will form a compact layer of material. In these cases the centrifugal force does not necessarily play a role the softened powder sticks to the wall and other particles gradually complete the shaping process. Only the outer surface of an article made this way will be smooth, which is no objection for many applications. 

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