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Properties of Plastics

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Properties of Plastics --- Aheadmold

1.    General Properties
The problem of selecting plastic materials is that of finding the material with suitable properties from the standpoint of intended service, methods of forming and fabricating, and cost.New and improved plastic materials possessing almost any desired characteristic are being introduced continually. There are plastics that do not require plasticizers that have greater flexibility under lower temperatures, and are stable under higher temperatures. Some resist water, acids, oils, and other destructive matter. The wide use of plastics testifies to their value; however, fundamental limitations should be considered when applying a new material or adapting an old material to new applications.

2.    Effects of Temperature
Plastics are inclined toward rigidity and brittleness at low temperatures, and softness and flexibility at high temperatures. They are fundamentally unstable dimensionally with respect to temperature, and are susceptible to distortion and flow when subjected to elevated temperatures. The thermoplastics are particularly susceptible, while the thermosetting plastics are much more resistant, differing, however, only in degree. The distinction between the thermal stability of the thermosetting and thermoplastic resins is not well defined. A true distinction can be drawn only between individual plastics, rather than between classes of plastics. High temperatures not only seriously reduce the mechanical properties of plastics, but also accelerate the destructive action of external agents to which they are sensitive. Continuous heating also may induce 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 checked first in the light of possible “migration of plasticizer”, sometimes resulting in discoloration or hardening of one of the plastics.
In general, moderate temperatures are required for storage of plastics over long periods; low temperatures are to be avoided because of the low-temperature brittleness of most of the plastics, and high temperatures should be avoided because of the rapid loss of mechanical properties, volatilization of plasticizers, and the susceptibility of a large number to distortion.

3.    Effects of Humidity
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 composition and formulation of the plastics. Increased water content plasticizes some materials, and there is a general lowering of the mechanical properties. Water absorption is responsible for swelling in certain plastics and the ultimate decomposition of a few. Moist or wet atmospheres may extract plasticizers from heavily plasticized materials and also provide conditions favorable to fungal growth. In recent years, however, new plastics have come into use that have first-class moisture resistance and may contain water indefinitely while resisting other influences at the same time.
Extremely dry environments may cause brittleness in certain plastics as a result of loss of water that normally contributes to their plasticity. Cyclic wet and dry atmospheres are more destructive to plastics than continuous exposure at constant humidity because of the mechanical stresses induced in the plastics by swelling and shrinking with moisture absorption and moisture emission. Relatively constant, moderate to low humidities are preferred for plastic  storage because of the adverse effects of water on the structure and properties of these materials, and the possibility of plasticizer loss by extraction and fungal attack in moist atmospheres.

4.    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. Loss of strength, reduced ductility, and increased fragility usually accompany such action. Many plastics are offered in special formulations containing “ultraviolet inhibitors” which should be utilized when this influence is present. Exposure of plastics to sunlight during storage should be avoided, especially when the transparency of clear materials is to be preserved.

5.    Weight
As a family, plastics are light when compared to metals. Most plastics have a specific gravity between 1.35 and 1.45, which is less than that of magnesium.

6.    Electrical Resistivity
Plastics have excellent electrical resistivity making them have wide application as an insulating material. In the high-frequency applications, plastics are particularly advantageous and, consequently, are being used to a large extent in the fields of radar and television.

7.    Heat Insulation
Plastics have low heat conduction and, consequently, have application as an insulating material. In particular, they are used as handles for appliances and tools subjected to heat.

8.    Fabrication
The principal characteristic of plastics from fabrication standpoint is case of molding. Both thermosetting and thermoplastic materials lend themselves to molding irregular and complex shapes with relatively short curing cycles.
Plastics may be joined by using various cements, chemical solvents, and mechanical fasteners. Heat-sealing, which parallels somewhat the welding process of metals, is used extensively in joining light thermoplastic films. In such cases, dielectric heating is the technique usually used. Friction adhesion has had moderate application also in the joining of small thermoplastic parts.
Plastics can be machined with conventional machine tools. However, certain  cautions should be exercised. In order to maintain a good finish, a heavy flow of coolant should be used so as to avoid temperatures that will distort the work. In some thermosetting laminates (glass, for example), the customary high-speed steel tool will not stand up in view of the abrasive action of the laminating material. Here, either tungsten carbide or ceramic cutting tools must be used.

9.    Effects of Oxygen
Organic plastics are nearly all subject to oxidation when exposed to the atmosphere. The process is accelerated by high temperatures and light; but, over long periods of time, oxidative deterioration may take place at room temperature. Oxidation susceptibility depends largely upon the chemical nature of the plastic and its compounding. Materials with the greatest number of double bonds in their molecular structure will generally be the most sensitive to oxidation. Yellowing and a gradual loss of strength and ductility are the principal results of oxidative processes.
Oxidation is not a problem of great magnitude in storage, since the rigid plastics are rather resistant to oxidative deterioration under moderate conditions.

10.    Effects of Loading
Under moderate conditions the common thermoplastic materials are subject to distortion and flow when significantly loaded. Such plastics cannot be expected to maintain a high degree of mechanical stability over extended periods when subjected to stress; especially is this true when they are also exposed to relatively high temperatures. The thermoplastics should, however, ma- intain themselves fairly well when not subject to load or when subjected to only moderate load. Recently, fillers, such as glass wool, have been added to thermoplastics to further improve this property.
The thermosetting plastics are much more load-stable than the thermoplastics because of their structure and the inclusion of fillers in their formulation. In the laminated form they provide a rather high order of distortion and creep resistance. When not subjected to mechanical stress they may be considered to be highly stable. These materials, however, may suffer creep over long periods, especially when maintained at elevated temperatures.
The thermoplastic types should not be subjected to load when stored; and, whenever possible, the loading of stress-bearing thermosetting moldings or laminates should be removed or reduced.

11.    Chemical Stability
Plastics, in general, possess a high degree of inherent stability with respect to chemical deterioration. In many instances, this stability may be fortified by the addition of the proper stabilizers during compounding. While there is vast difference from one plastic to another, the general statement may be made that there is a plastic available to resist virtually any commercial chemical.

Article come from AHEADMOLD,website is WWW.AHEADMOLD.COM.

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