Plastics, engineering

Many thermoplastics are now accepted as engineering materials and some are distinguished by the loose description engineering plastics. The term probably originated as a classiflcation distinguishing those that could be substituted satisfactorily for metals such as aluminium in small devices and structures from those with inadequate mechanical properties. This demarcation is clearly artificial because the properties on which it is based are very sensitive to the ambient temperature, so that a thermoplastic might be a satisfactory substitute for a metal at a particular temperature and an unsatisfactory substitute at a different one. 

In recent years a whole new generation of high performance engineering plastics have become commercially available. These offer properties far superior to anything available so far, particularly in regard to high temperature performance, and they open the door to completely new types of application for plastics. 

A number of these materials offer service temperatures in excess of 200°C and fibre-filled grades can be used above 300 C. 

About 90wt% of plastics can be classified as commodity plastics (CPs), the others being engineering plastics (EPs). The EPs such as polycarbonate (PC) representing at least 50wt% of all EPs, nylon, acetal, etc. are characterized by improved performance in higher mechanical properties, better heat resistance, and so forth (Table 1.4). 

Best property balance Good impact resistance 

Toughest thermoplastic Absorbs moisture Polyester (glass-reinforced) High stiffness Lowest creep Excellent electrical properties Good electrical properties 

The EPs demand a higher price. About a half century ago the price per pound was at 204 at the turn of the century it went to 1.00, and now higher. When CPs with certain reinforcements and/or alloys with other plastics are prepared they become EPs. Many TSs and RPs are EPs. 

In the past rubber meant a natural thermoset elastomeric (TSE) material obtained from a rubber tree, hevea braziliensis. The term elastomer developed with the advent of rubber-like synthetic materials. Elastomers identify natural or synthetic TS elastomers (TSEs) and thermoplastic elastomers (TPEs). At room temperature all elastomers basically stretch under low stress to at least twice in length and snaps back to approximately the original length on release of the stress, pull, within a specified time period. 

Commonly accepted practice restricts the term to plastics that serve engineering purposes and can be processed and reprocessed by injection and extrusion methods. This excludes the so-called specialty plastics, eg, fluorocarbon polymers and infusible film products such as Kapton and Upilex polyimide film, and thermosets including phenolics, epoxies, urea—formaldehydes, and silicones, some of which have been termed engineering plastics by other authors (4) (see Elastomers, synthetic-fluorocarbon elastomers Fluorine compounds, organic-tetrafluoroethylene copolymers with ethylene Phenolic resins Epoxy resins Amino resins and plastics). 

Lubricity of crystalline polymers is usually higher than that of amorphous polymers. Excellent machinery parts are made from crystalline nylon-6,6 resins, eg, gears, cams, wedges, and other components not requiring lubrication. Gears made of amorphous polyimide resin, on the other hand, do not exhibit this feature. 

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

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

Amoco Performance Polymers Xydar liquid crystal polymer 

With a high degree of crystallinity and no water absorption. 

It will be helpful to remark that the principal difference between these two groups is in the manner of forming. A thermoplastic is heated and then constrained by a mould or die (see Chapter 7) so that it flows into the required shape. It is then cooled and removed from the mould. The forming of thermosets is somewhat more complicated. The precursors are normally injected into a mould which is then heated so that the resulting polymeric mass, when set, has the required shape. It is then cooled and removed from the mould. 

The increase in the mass of plastics in cars has occurred in the luxurious, as well as in the cheaper models. In BMW cars, for instance, the proportion of plastics has been double that of the average for German cars. Three specific mechanical examples of the use of plastics in BMW cars are (1) the universal joint which is used to operate the water valve in the heating system (2) the heating air intake grill and (3) centrifugal fans for the heating system. 

The above results indicate that the molecular mobility of lignin is affected by rigid aromatic ring in the main chain and that molecular flexibility is controlled by paraffinic chains. 

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AMNES - AMINES,AROMATIC - PHENYLENEDIAMINES] (Vol 2) -as engineering resins [ENGINEERING PLASTICS] (Vol 9)... 

Plastics carboys Plastics compounding Plastics, engineering Plastic sheet Plastics processing Plastics testing Plastic tapes Plastic working Plastiform Plastisols... 

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