外文资料翻译
资料来源：书籍
文章名：Chapter 2 The Properties of Plastics 书刊名：《English for Die & Mould Design and Manufacturing》
作者：刘建雄王家惠廖丕博主编
出版社：北京大学出版社，2002
章节：2.2 The Properties of Plastics
页码：P24~P31
文章译名：塑料的性能
The Properties of Plastics
Plastics are organic materials made from large molecules that are constructed by a chain-like attachment of certain building-block molecules. The properties of the plastic depend heavily on the size
of the molecule and on the arrangement of the atoms within the molecule. For example, polyethylene is made from the ethylene building block that is initially a gas.Through    a  process called polymerization,    a chain of ethylene molecules is formed by valence bonding of the carbon atoms within the ethyle ne molecule. The high molecular weight product which results iscal led a polymer. Hence, the designation polyethylene is used todisti nguish the high- molecular-weight plastic from its gaseous counterp art, ethylene, which is the monomer that becomes polymerized.The “poly”refers to the “many”ethylene building block molecules
or monomers, which join to form the polyethylene plastic molecule. Frequently, the term “resin”is used, interchangeably with “poly
mer”to describe the backbone molecule of    a plastic material. Ho wever, “resin”is sometimes used to describe a syrupy liquid of both natural and synthetic resin.
Plastics, in the finished product form, are seldom comprised e xclusively of polymer but also include other ingredients such as fillers, pigments, stabilizers, and processing aids. However, design ation of the plastic material or molding compound is always taken from the polymer designation.
Broadly speaking plastics may be divided into two categories: thermoplastics and thermoset plastics.
The classes of materials are so named because of the effect of temperature on their properties .
2.2.1Thermosets
Thermoset plastics are polymers which are relatively useless in their raw states. Upon heating to    a certain temperatureaa chemical reaction takes place which causes the molecules to bond together or cross-link. After vulcanization and polymerization, or curing,the thermoset material remains stable and cannot return to its origi nal state. Thus, ├thermo-setâ” identifies those materials that become set in their useable state resulting from the addition o f heat. Normally,    a thermoset polymer is mixed with fillers and reinforcing agents to obtain the properties of    a molding compound. Thermosets are the hardest and stiffest of all plastics, are chem ically insoluble after curing, and their properties are less affec ted by changes in temperature than are the heat-sensitive thermopl astics. The closest non-plastic counterparts to thermosets in prope rties are ceramics. Common examples of thermoset plastics are: phe nolics, melamine, urea, alkyds, and epoxies. Molding compounds made from these polymeric resins always contain additional fillers and reinforcing agents to obtain optimum properties.
design翻译2.2.2Thermoplastics
Thermoplastic polymers are heat-sensitive materials which are solids at room temperature,like most metals. Upon heating, the thermopla stics begin to soften and eventually reach a melting point and b ecome liquid. Allowing    a thermoplastic to cool below its melting
point causes resolidification or freezing of the plastic. Successiv e heating and cooling cycles cause repetition of the melting-freez ing cycle just as it does for metals.
The fact that thermoplastics melt is the basis for their proc essing into finished parts. Thermoplastics may be processed by any method which causes softening or melting of the material. Exampl es of thermoplastic fabrication techniques using melting are: injec tion molding, extrusion, rotational casting, and calendering. Fabric ation methods which take advantage of softening below the melting point are: thermoforming (vacuum or pressure), blow molding, and forging. Of course, normal metal-cutting techniques can also be applied to thermoplastics in the solid state. Common examples of thermoplastics are: polyethylene, polystyrene, polyvinyl chloride (PV
C), and nylon (polyamide).
2.2.3Fillers
Plastics often contain other added materials called fillers. Fi llers are employed to increase bulk and to help impart desired p roperties. Plastics containing fillers will cure faster and hold c loser to established finished dimensions, since the plastic shrinka ge will be reduced. Wood flour is the general-purpose and most c ommonly used filler. Cotton frock, produced from cotton linters, i ncreases mechanical strength. For higher strength and resistance to impact, cotton cloth chopped into sections about 1/2-inch square c an be processed with the plastic. Asbestos fiber may be used as a filler for increased heat and fire resistance, and mica is u sed for molding plastic parts with superior dielectric characterist ics. Glass fibers, silicon, cellulose, clay, or nutshell flour may also be used. Nutshell flour is used instead of wood flour where a better finish is desired. Plastic parts using short fiber fil lers will result in lower costs, while those with long fiber fil lers having greater impact strengths are more expensive. Other m aterials, not defined as fillers, such as dyes, pigments, lubrican ts, accelerators, and plasticizers may also be added. Plasticizers are added to soften and improve the moldability of plastics. Fi ller and modifying agents are added and mixed with the raw plast ic before it is molded or formed.
2.2.4Properties of Plastics
1.General Properties
The problem of selecting plastic materials is that of finding the material with suitable properties from the standpoint of inten ded service, methods of forming and fabricating, and cost.
New and improved plastic materials possessing almost any desired c haracteristic are being introduced continually. There are plastics that do not require plasticizers that have greater flexibility und er lower temperatures, and are stable under higher temperatures. S ome resist water, acids, oils, and other destructive matter. The wide use of plastics testifies to their value; however, fundamenta l limitations should be considered when applying    a new material o r adapting an old material to new applications.
2.Effects of Temperature
Plastics are inclined toward rigidity and brittleness at low t emperatures, and softness and flexibility at high temperatures. The y are fundamentally unstable dimensionally with respect to temperat ure, and are susceptible to distortion and flow when subjected to elevated temperatures. The thermoplastics are particularly suscepti ble, while the thermosetting plastics are much more resistant, dif fering, however, only in degree. The distinction between the therm al stability of the thermosetting and thermoplastic resins is not well defined.    A true distinction can be drawn only between indi vidu
al plastics, rather than between classes of plastics. High tem peratures not only seriously reduce the mechanical properties of p lastics, but also accelerate the destructive action of external ag ents to which they are sensitive. Continuous heating also may ind uce brittleness and shrinkage in heavily plasticized materials by volatilization of plasticizers. The use of one plastic in contact with    a dissimilar plastic in    a proposed application should be c hecked first in the light of possible ├migration of plasticizer â”
, sometimes resulting in discoloration or hardening of one of the plastics.
In general, moderate temperatures are required for storage of plas
tics over long periods; low temperatures are to be avoided becaus e of the low-temperature brittleness of most of the plastics, and high temperatures should be avoided because of the rapid loss o f mechanical properties, volatilization of plasticizers, and the su sceptibility of    a large number to distortion.
Plastics, with only    a few exceptions, are extremely sensitive to the effects of water. High- humidity atmospheres induce water absorption and varied resulting effects, depending upon the composi tion and formulation of the plastics. Increased water content plas ticizes some materials, and there is    a
general lowering of the m echanical properties. Water absorption is responsible for swelling in certain plastics and the ultimate decomposition of    a few. Mois t or wet atmospheres may extract plasticizers from heavily plastic ized materials and also provide conditions favorable to fungal gro wth. In recent years, however, new plastics have come into use t hat have first-class moisture resistance and may contain water ind efinitely while resisting other influences at the same time.
Extremely dry environments may cause brittleness in certain pla stics as    a result of loss of water that normally contributes to their plasticity. Cyclic wet and dry atmospheres are more destruct ive to plastics than continuous exposure at constant humidity beca use of the mechanical stresses induced in the plastics by swellin g and shrinking with moisture absorption and moisture emission. Re latively constant, moderate to low humidities are preferred for pl astic storage because of the adverse effects of water on the str ucture and properties of these materials, and the possibility of plasticizer loss by extraction and fungal attack in moist atmosphe res
3.Effects of Light
Prolonged exposure to sunlight will affect adversely all plastics with exception of tetrafluoroethylene (Teflon). The change induced by the ultraviolet components may vary in kind and severity from slight
yellowing to complete disintegration as    a result of the chemical degradation of the polymeric compound or plasticizers. Los s of strength, reduced ductility, and increased fragility usually accompany such action. Many plastics are offered in special formul