Selection of Plastics

The previous section has given an indication of the range of plastics available to the design engineer. The important question then arises How do we decide which plastic, if any, is best for a particular application Material selection is not as difficult as it might appear but it does require an awareness of the general behaviour of plastics as a group, as well as a familiarity with the special characteristics of individual plastics. 

The first and most important steps in the design process are to define clearly the purpose and function of the proposed product and to identify the service environment. Then one has to assess the suitability of a range of candidate materials. The following are generally regarded as the most important characteristics requiring consideration for most engineering components. 

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Increasingly, plastics are being used as parenteral packaging (qv) materials. Plastics such as poly(vinyl chloride), polyethylene, and polypropylene are employed. However, plastics may contain various additives that could leach into the product, such as plasticizers (qv) and antioxidants. PermeabiUty of plastics to oxygen, carbon dioxide, and water vapor must be tested in the selection of plastic containers. Furthermore, the plastic should withstand sterilization. Flaking of plastic particles should not occur and clarity necessary for inspection should be present. 

Optical Properties. The optical properties of a plastic which are important are refraction, transparency, gloss and light transfer. The reader is referred to BS 4618 1972 for precise details on these terms. Table 1.9 gives data on the optical properties of a selection of plastics. Some plastics may be optically clear (e.g. acrylic, cellulosics and ionomers) whereas others may be made transparent. These include epoxy, polycarbonate, polyethylene, polypropylene, polystyrene, polysulphone and PVC. 

SOFINE Grade Specific Selection of Plastics Materials... 

Proper selection of plastic matrix fire retardants and property enhancers offers acceptable combinations of impact properties and heat-distortion temperature (HDT) values for fire retardant plastics. This can be demonstrated by fire retardant styrenics. Fire retardant enhancers have special interest as property enhancers for example the addition of a highly flammable material such as ethylene propylene diene terpolyer (EPDM), dramatically improves the bromine efficiency of octabromodiphenyl oxide in ABS by increasing char-forming without changing the Sb-Br reaction. 

Figure 2i. A selection of plasticizers used in organic photorefractive materials...

Novel PVC foams are produced with wood fibers. Untreated and silane treated wood fibers were used. It was determined that by the selection of plasticizer level, foaming time, foaming temperature, and silane treatment of wood fiber, the foam of expected properties can be obtained. 

Selection of plastic type—general physical and chemical properties. Ideally food grade approved. 

Selection of plastic grade—detailed physical and chemical properties, plus knowledge of constituents including any toxicity and irritancy aspects. 

The World Health Organisation has proposed international requirements for plastic containers for pharmaceutical preparations. These include guidelines on selection of plastics/code of practice, covering the following. 

Let us consider shear rate for a wider selection of plastics, HDPE-composite materials, and regrinds (Table 17.30). 

Permeation can be defined as the passage of gases and liquids through a second material such as a solid. It is a significant consideration in the selection of plastic material for the construction of chemical processing equipment because process fluids may travel across the thickness of the polymer by permeation. Permeated species in sufficient quantities could cause... 

An alternative to the use of additives is selection of plastics that are inherently static-dissipative. While such materials are still largely experimental, one promising alternative is polyamide ethylene oxide block copolymers. 

Carmen, R., The Selection of Plastics Materials for Blood Bags. Transfusion Med. Rev., 7,1, 1993. 

Selection of plasticizer should be based on main characteristics of plasticizer and polymer. Plasticizer characteristics determine its amoimt which can be added into composition and the thermodynamic stability of mixture. 

The above information shows that there is a high potential in material modification by educated selection of plasticizers but substantially more fundamental studies are needed before this happens. 

Commercial literature usually contains some data on mechanical properties of test formulations. This information is frequently not very useful because it refers to a simple formulation (very different from real industrial formulations). The commercial data are determined for formulations freely selected by manufacturers and therefore cannot be compared between different manufacturers. Finally, these results are not presented in fundamental form which may guide the user in the selection of plasticizers for Ms needs. Open literature usually offers information " aiming at analysis of reasons for the observed behavior of materials but the number of pubhshed studies is limited by interest and resources. 

The selection of plasticizers for polyurethane compositions depends on satisfying various tasks such as lowering viscosity, increase in potlife, lowering modulus and hardness of polyurethane, an increase in the low temperature resistartee (lowering the glass transition temperature of soft chains), or dissolution of components in reaction mixture. 

Plasticizers have influence on all listed properties. Heat stability can be improved by selection of plasticizers. Plasticizers decrease gelation temperatnre and time. Melt viscosity depends on both the amonnt and the type of plasticizer as does the rate of the fusion process. Excessive volatiles generated from plasticizers and stabilizers may cause smface marring of the product. 

Selection of plasticizer or plasticizing system goes beyond lowering the viscosity. The amount of form coating, its nniformity, and appearance depend on rheological properties of plastisols. Forms are subjected to a controlled movement which has several steps ... 

Selection of plasticizers depends on destination such as primary insulation or sheat-ing. It also depends on required electrical volatihty and low temperature performance in which plasticizers play a decisive role. Plasticizers mustprodrree thermally stable nonvolatile products. ... 

Flame resistance is an important property of electric conductors which can be improved by plasticizers. Typical goal of selection of plasticizers is to improve flame resistance, reduce smoke and decrease acceptable temperatine of service. Tetra-bromo- and tetrachlorophthalates are suitable for such proposes. 

It should be pointed out that properties of plasticizers make their selection very easy. It was shown above that selection of plasticizers with a higher molecular weight of alcohol, improved the performance of material. Higher alcohols have little effect on the viscosity of the plasticizer (Figure 2.4). This effect is even less pronounced in PVC plastisols because adipates of higher alcohols have lower solvating strength and thus decrease the viscosity of plastisols as compared with esters with lower alcohols, which are more compatible. 

The most eflicient prevention of plastics fires is a careful selection of plastics for the particular applications on the basis of relevant fire-safety regulations. 

The effect of temperature on the mechanical properties of plastic materials has a fundamental role in the selection of materials. Unlike metals and ceramics, plastics are extremely sensitive to the slightest changes in temperature. The selection of plastics for applications under different temperatures is a complex task. The plastic material must be able to support a stress under operating conditions without getting distorted. The effect of temperature on geometrical stability and mechanical properties in general can be studied following different procedures and methods like at constant temperature or with a temperamre ramp. 

Table 8.6 Selection of plastic with excelle elongation pro nt UV resistance and tensile and perties ...

The first step starts with determining the types and quantities of discarded materials. The selection of plastics recycling streams involve ... 

Selection of Plastic with Excellent UV Resistance and Tensile and Elongation Properties... 

In the selection of plastics materials, invariably compliant with UL 94 VO, for use in relays, low degassing properties are required. This is necessary because some polymers are inherently hygroscopic and so must be dried thoroughly before use. Other polymers may absorb gases during... 

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