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Screw fitments
Self-tapping screws are the most commonly used form of mechanical fastener for polymers. Thermoplastics can employ thread-forming screws, where elastic relaxation processes ensure a tight fit. Thermosets are too brittle for this technique and they tend to crack in use, so recourse has to be made to thread- cutting screws for these materials.
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energy to break covalent bonds in organic compounds, but ultraviolet wavelengths will selectively excite electrons in the polymer chain. This increases the vibrational and rotational energy of the covalent bonds, leading to degradation by bond cleavage. Fortunately, the intensity of light in the 290–320 nm wavelength region is estimated to account for no more than 0.5% of the radiant energy of the sunlight at noon in southern regions. If this were not the case, few plastics would be of use outside.
Typical types of degradation include yellowing, chalking, surface embrittlement, loss of tensile or impact strength, and cracking. The process usually proceeds from the surface layers and the weakened surface can act as a site for crack nucleation. Such cracks may then propagate into the undegraded material beneath, causing failure.
No photochemical changes occur when the light is dissipated harmlessly as heat, and pigments such as carbon black are commonly added to absorb ultraviolet radiation and then re-emit it as thermal energy. Hydroxy- benzophenones and benzotriazoles are also widely used as ultraviolet absorbers.
Aliphatic polymers such as PE are degraded by gamma radiation, although aromatic polymers, such as PS, are relatively resistant to high-energy radiation.
Utensils made from PP may be embrittled by radiation damage if they are subject to sterilization treatment by doses of gamma radiation of 2.5 Mrad.
Equipment should be designed to last 40 years when subjected to a dose of 20 Mrad.
5.6.3 Mechanical degradation
Mechanical degradation may occur when stress is imposed on a polymer through machining, stretching or ultrasonics. Bond cleavage may occur, forming macroradicals which can add oxygen and produce compounds which will undergo degradative reactions.
5.6.4 Microbiological degradation
Most widely used plastic materials are inert in the presence of microbes and this stability is important in many applications. Only short-term performance is required in certain situations before the material is discarded, for example in fast food packaging. It is considered an advantage if the discarded plastic degrades when exposed to microbes and it is a challenge to polymer scientists to develop plastics that possess the requisite properties for their anticipated service life, but which are eventually capable of degrading in a safe manner.
In 1987, 6 × 109 kg of plastics were used for short-term packaging applications and today this figure exceeds 60 × 109 kg.
We may define biodegradable plastics as those whose physical integrity is
Organic polymeric materials 181 lost upon contact with microbial and/or invertebrate activity in a natural environment within a limited period of time. At the limit, this represents conversion of the material to carbon dioxide, water, inorganic salts, microbial cellular components and miscellaneous by-products.
Most plastics at present used for packaging consist of high- and low- density PE, which do not degrade by microbiological action. Due to their large chain lengths and high molecular weight, most widely used alkane- derived plastics may have lifetimes of hundreds of years when buried in typical solid-waste disposal sites. Low molecular weight hydrocarbons can be degraded by microbes, but the rate of degradation becomes very slow when the length of the polymer chain exceeds 24 to 30 carbon atoms. Decreasing typical polymer molecules to biologically acceptable dimensions requires extensive destruction of the PE matrix. This destruction can be partly accomplished in blends of PE and biodegradable natural polymers by the action of macroorganisms such as arthropods, millipedes, slugs and snails.
Starch–polyethylene complexes have been manufactured which exhibit physical properties approaching those of low density PE. The starch is present as a separate phase and is attacked by fungi and bacteria. This weakens the polymer matrix and the eventual breakdown of the polymer chains reduces the molecular weight which enhances microbial attack.
5.6.5 Chemical attack
Plastics are susceptible to environmental failure when exposed to certain organic chemicals and this limits their use in many applications. Even aqueous media can cause degradation, however, although the processes involved differ from the corrosive attack of metals in such environments.
Diffusion of species into plastics is common and adverse effects can arise which are not chemical in nature. In most interactions of water with structural plastics no chemical bonds are altered, but damage known as ‘physical corrosion’ may occur. Absorbed moisture has been shown to act as a plasticizer, reducing the glass transition temperature and the strength of the polymer.
These effects are essentially reversible, although other, irreversible, effects may be encountered, such as microcracking or crazing, as well as chemical degradation of the polymer structure.
Organic liquids, such as cleaning fluids, detergents, petrol and lubricants may seriously reduce the mechanical properties of plastics. As already discussed, the most serious problems arise when a material is exposed to aggressive fluids when it is under stress. Organic liquids may interact both chemically and physically with a polymer. Chemical interactions may involve a decrease in the molecular weight by chain breakage; this in turn may cause a reduction in mechanical properties such as tensile strength, stiffness and fracture toughness. Figure 5.11 illustrates the decrease in tensile strength of
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polyester-base polyurethane (PUR) as a function of exposure time in methanol.
The methanol is believed to swell the PUR and also causes molecular weight reduction through chain breakage.
As an example of physical interaction when a plastic component is exposed to an organic chemical, aggressive molecules may diffuse into the material leading to plasticization. Swelling of the polymer results in high stresses which can cause crazing or cracking. Fracture arising from physical effects has been observed in many glassy plastics such as PMMA, PS and PC because of anisotropic swelling.