Comparison

The following informadon provides examples of guidelines on performance comparisons of different plasdcs. As an example, if the product requires flexibility, examples of the choices include polyethylene, vinyl, polypropylene, EVA, ionomer, urethane-polyester, fluorocarbon, silicone, polyurethane, plasdsols, acetal, nylon, or some of the rigid plasdcs that have limited flexibility in thin secdons.

The subject of strength can be complex since so many different types exist: short or long term, stadc or dynamic, etc. Some strength aspects are interrelated with those of toughness. The crystallinity of TPs is important for their short-term yield strength. Unless the crystallinity is impeded, increased molecular weight generally also increases the yield strength. However, the crosslinking of TSs increases their yield strength substantially but has an adverse effect upon toughness (Chapter 1).

Increasing the secondary bonds' strength and crystallinity than by increasing the primary bond strength increases long-term rupture strengths in TPs much more readily. Fatigue strength is similarly influenced, and all factors that influence thermal dimensional stability also affect fatigue strength. This is a result of the substantial heating that is often encountered with fadgue, particularly in TPs.

Polystyrene, styrene-acrylonitrile, polyethylene, acrylic, ABS, polysulfone, EVA, polyphenylene oxide, and many other TPs are satisfactorily odor-free. FDA approvals are available for many of these plastics. There are food packaging and refrigerating conditions that will eliminate certain plastics. Melamine and urea compounds are examples of suitable plastics for this service.

Thermal considerations will eliminate many materials. Examples for products operating above 450°F (232°C) include the silicones, fluoro-plastics, polyirnides, hydrocarbon resins, methylpentene cold mold, or glass-bonded mica plastics may be required. A few of the organic plastic-bonded inorganic fibers such as bonded ceramic wool, perform well in this field. Epoxy, diallyl phthalate, and phenolic-bonded glass fibers may be satisfactory in the 450 to 550°F (232 to 288°C) ranges. A limited group of ablation material is made for outer space reentry use.

Between 250 and 450°F (121 and 232°C) glass or mineral-filled phenolics, melamine, alkyd, silicone, nylon, polyphenylene oxide, poly-sulfone, polycarbonate, methylpentene, fluorocarbon, polypropylene, and diallyl phthalate can be considered. The addidon of glass fillers to the thermoplastics can raise the useful temperature range as much as 100°F (212°C) and at the same time shorten the fabricadng cycle.

In the 0 to 212°F range, a broad selecdon of materials is available. Low temperature consideradons may eliminate many of the thermoplastics. Polyphenylene oxide can be used at temperatures as low as -275°F. Thermosetting materials exhibit minimum embritdement at low temperature.

Underwriters' Laboratory (UL) ruling on the use of self-extinguishing plastics for contact-carrying members and many other components introduces critical material selection problems. All thermosets are self-extinguishing. Nylon, polyphenylene oxide, polysulfone, polycarbonate, vinyl, chlorinated polyether, chlorotrifluoroethylene, vinylidene fluoride, and fluorocarbon are thermoplastics that may be suitable for applications requiring self-extinguishing properties. Cellulose acetate and ABS are also available with these properties. Glass reinforcement improves these materials considerably.

Many TPs will craze or crack under certain environmental conditions, and products that are highly stressed mechanically must be checked very carefully. Polypropylene, ionomer, chlorinated polyether, phenoxy, EVA, and linear polyethylene offer greater freedom from stress crazing than some other TPs. Solvents may crack products held under stress.

Toughness behaviors and evaluation can be rather complex. A definition of toughness is simply the energy required to break the plastic. This energy is equal to the area under the tensile stress-strain curve. The toughest plastics should be those with very great elongations to break, accompanied by high tensile strengths; these materials nearly always have yield points. One major exception to this rule is RPs that use reinforcing fibers such as glass and graphite that have low elongation. For high toughness a plastic needs both the ability to withstand load and the ability to elongate substantially without failing except in the case of RPs (Fig. 6.2).

It may appear that factors contributing to high stiffness are required. This is not true because there is an inverse relationship between flaw sensitivity and toughness; the higher the stiffness and the yield strength of a TP, the more flaw sensitive it becomes. However, because some load-bearing capacity is required for toughness, high toughness can be achieved by a high trade-off of certain factors.

Toughness behaviors (courtesy of Plastics FALLO)

Toughness behaviors (courtesy of Plastics FALLO)

Elastic Limit Percent

Crystallinity increases both stiffness and yield strength, resulting usually in decreased toughness. This is true below its glass transition temperature (Tg) in most noncrystalline (amorphous) plastics, and below or above the Tg in a substantially crystalline plastic (Chapter 1). However, above the Tg in a plastic having only moderate crystallinity, increased crystallinity improves its toughness. Furthermore, an increase in molecular weight from low values increases toughness, but with continued increases, the toughness begins to drop.

Deformation is an important attribute in most plastics, so much so that it is the very factor that has led them to be called plastic. For designs requiring such traits as toughness or elasticity this characteristic has its advantages, but for other designs it is a disadvantage. However, there are plastics, in particular the RPs, that have relatively no deformation or elasticity and yet are extremely tough where (a) toughness is related to heat deflection or rigidity and (b) toughness or impact is related to temperature for polystyrene (PS) and high impact polystyrene (HIPS).

This type of behavior characterizes the many different plastics available. Some are tough at room temperature and brittle at low temperatures. Others are tough and flexible at temperatures far below freezing but become soft and limp at moderately high temperatures. Still others are hard and rigid at normal temperatures but may be made flexible by copolymerization or adding plasticizers.

By roughness is meant resistance to fracture. However, there are those materials that are nominally tough but may become embrittled due to processing conditions, chemical attack, prolonged exposure to constant stress, and so on. A high modulus and high strength with ductility is the desired combinadon of attributes. However, the inherent nature of plasdcs is such that their having a high modulus tends to associate them with low ductility, and the steps taken to improve the one will cause the other to deteriorate.

Soft, weak materials have a low modulus, low tensile strength, and only moderate elongation to break. According to ASTM standards, die elastic modulus or the modulus or elasticity is the slope of the initial straight-line portion of the curve. Hard, brittle materials have high moduli and quite high tensile strengths, but they break at small elongations and have no yield point. Hard, strong plastics have high moduli, high tensile strengths, and elongations of about 5% before breaking. Their curves often look as though the material broke about where a yield point might have been expected.

Soft, tough plastics are characterized by low moduli, yield values or plateaus, high elongations of 20 to 1,000%, and moderately high breaking strengths. The hard, tough plastics have high moduli, yield points, high tensile strengths, and large elongations. Most plastics in this category show cold drawing or necking during the stretching operation. The RPs will have at least one modulus but some materials can have two or three.

Although impact strength of plastics is widely reported, the properties have no particular design values and can be used only to compare relative response of materials (toughness, etc.). Even this comparison is not completely valid because it does not solely reflect the capacity of the material to withstand shock loading, but can pick up discriminatory response to notch sensitivity.

A better value is impact tensile, but unfortunately this property is not generally reported. The impact value can broadly separate those that can withstand shock loading vs. those that are poor in this response. Therefore, only broad generalizations can be obtained on these values. Comparative tests on sections of similar size which are fabricated in accordance with the proposed product must be tested to determine the impact performance of a plastics material. The laminated plastics, glass-filled epoxy, melamine, and phenolic are outstanding in impact strength. Polycarbonate and ultrahigh molecular weight PE are also outstanding in impact strength.

In general, rigid plastics are superior to elastomers in radiation resistance but are inferior to metals and ceramics. The materials that will respond satisfactorily in the range of 1010 and 1011 erg per gram are glass and asbestos-filled phenolics, certain epoxies, polyurethane, polystyrene, mineral-filled polyesters, silicone, and furane. The next group of plastics in order of radiation resistance includes polyethylene, melamine, urea formaldehyde, unfilled phenolic, and silicone plastics. Those materials that have poor radiation resistance include methyl methacrylate, unfilled polyesters, cellulosics, polyamides, and fluoro-carbons.

Maximum transparency is available in acrylic, polycarbonate, polyethylene, ionomer, and styrene compounds. Many other thermoplastics may have adequate transparency.

Urea, melamine, polycarbonate, polyphenylene oxide, polysulfone, polypropylene, diallyl phthalate, and the phenolics are needed in the temperature range above 200°F (93°C) for good color stability. Most TPs will be suitable below this range.

Deteriorating effects of moisture are well known. For high moisture applications, polyphenylene oxide, polysulfone, acrylic, butyrate, diallyl phthalate, glass-bonded mica, mineral-filled phenolic, chlorotrifluoro-ethylene, vinylidene, chlorinated polyether chloride, vinylidene fluoride, and the fluorocarbons should be satisfactory. Diallyl phthalate, polysulfone, and polyphenylene oxide have performed well with moisture/steam on one side and air on the other (a troublesome combination), and they also will withstand repeated steam autoclaving. Long-term studies of the effect of water have disclosed that chlorinated polyether gives outstanding performance. Impact styrene plus 25wt% graphite and high density polyethylene with 15% graphite give long-term performance in water.

Depending on what is required, the different plastics can provide different rates of permeability properties. As an example certain polyethylenes will pass wintergreen, hydrocarbons, and many other chemicals. It is used in certain cases for the separation of gases since it will pass one and block another. Chlorotrifluoroethylene and vinylidene fluoride, vinylidene chloride, polypropylene, EVA, and phenoxy merit evaluation.

There are materials with low or no permeability to different environments or products. Barrier plastics are used with their technology not becoming more complex but more precise. Different factors influence performance such as being pinhole-free; chemical composition, crosslinking, modification, molecular orientation; density, and thickness. The coextrusion and coinjection processes are used to reduce permeability while retaining other desirable properties. Total protection against vapor transmission by a single barrier material increases linearly with increasing thickness, but it usually is not economical. Thus extensive use is made of muldple layer construcdons. This composite would include low cost as well as recycled plastics that provide mechanical support, etc. while an expensive barrier material thickness is significandy reduced.

With crystalline plasdcs, the crystallites can be considered impermeable. Thus, the higher the degree of crystallinity, the lower the permeability to gases and vapors. The permeability in an amorphous plasdc below or not too far above its glass transition temperature (Tg) is dependent on the degree of molecular orientation. It is normally reduced when compared to higher temperatures, although small strains somedmes increases the permeability of certain plasdcs. The orientation of elastomers well above their Tg has relatively less effect on the overall transport property. Crosslinking thermoplastics will decrease permeability due to the decrease in their diffusion coefficient. The effect of crosslinking is more pronounced for large molecule size vapors. The addition of a plasticizer usually increases the rates of vapor diffusion and permeation (Chapter 1).

The permeation of vapors includes two basic processes: the sorption and diffusion of vapors in the plastic. As an example in the packaging industry, the resistance of moisture is essential for the preservation of many products. The loss of moisture, flavor, etc. through packaging materials may damage foodstuff. The prevention of the ingress of moisture by a barrier is essential for the storage of dry foods and other products. In other applications, the degree of resistance to water and oxygen is important for the development of corrosion resistance coatings, electrical and electronic parts, etc.

Fluorination is the process of chemically reacting a material with a fluorine-containing compound to produce a desired product. As an example it can improve the gasoline barrier of PE to nonpolar solvents. A barrier is created by the chemical reaction of the fluorine and the PE, which form a thin (20 to 40 mm) fluorocarbon layer on the surface. Two systems can be used to apply the treatment depending on the results desired. With the "in-process" system, such as that used during blow molding PE gasoline tanks, fluorine is used as a part of the parison expanding gas in the blowing operation (the result is no gasoline leakage). The barrier layer is created only on the inside. In a post-treatment system, botdes and other products are placed in an enclosed chamber filled with fluorine gas. This method forms barrier layers on both the inside and outside surfaces.

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