Creep Engineering polymers

Creep appears in the 7-t plane (iso stress), while the stress-strain relationship (at a fixed time) is described by the S-7 plane. Stress relaxation (at constant strain) is described by the S-t plane. Premium engineering polymers or reinforced thermosets illustrate a relatively low creep. Fiber reinforcement usually decreases the creep. 

Thermoplastic polymers have links of intermolecular interactions or van der Waals forces in the form of linear or branched structures. They can melt easily when they are heated, are soluble in certain solvents, and have good resistance to creep. Two main groups of thermoplastic polymers are commodity polymers [eg, polyethylene (PE), polypropylene (PP), and polystyrene (PS)] and engineering polymers [polyoxymethy-lene (POM), polyamides (PA), and polycarbonate (PC)] [1]. 

Engineering polymers are polymer materials with exceptional mechanical properties such as toughness, stiffiiess, and low creep. Therefore, they are used in products like gears, bearings, electronic devices, automotive devices. About 10 % of the produced polymer mass accounts for engineering polymers. 

Polycarbonate was the first amorphous engineering polymer to be commercialized, and it possesses an enviable combination of performance attributes. Clearly, PC has the most advanced combination of properties such as impact resistance, ductility, clarity, dimensional stability, inherent ignition resistance, high-temperature resistance, rigidity, and creep resistance. On the other hand, the weaknesses of PC lie in its solvent resistance (organics), abrasion resistance, ultraviolet (UV) resistance (limited), notch sensitivity, and hydrolytic stability (limited). 

Fluoropolymers have outstanding chemical resistance, low coefficient of friction, low dielectric constant, high purity, and broad use temperatures. Most of these properties are enhanced with an increase in the fluorine content of the polymers. For example, polytetrafluoroethylene, which contains four fluorine atoms per repeat unit, has superior properties compared to polyvinylidene fluoride, which has two fluorine atoms for each repeat unit. Generally, these plastics are mechanically weaker than engineering polymers. Their relatively low values of tensile strength, deformation under load or creep, and wear rate require the use of fillers and special design strategies. 

The acetal resins show superior creep resistance to the nylons but are inferior in this respect, to the polycarbonates. It is to be noted, however, that limitations in the load-bearing properties of the polycarbonates restrict their use in engineering applications (see Chapter 20). Another property of importance in engineering is abrasion resistance—a property that is extremely difficult to assess. Results obtained from various tests indicate that the acetal polymers are superior to most plastics and die cast aluminium, but inferior to nylon 66 (see also Section 19.3.6 and Chapter 18). 

To overcome the disadvantages of nylon as an engineering material-high water absorption and poor creep strength at elevated temperatures—many newer polymers were developed. Table 3.47 lists polymers that are among the most commercially important acetal, polycarbonate, polyphenylene oxide and polysulfone. 

Polyester, thermoplastic TP polyesters have different grades. Polybutylene tereph-thalate (PBT) a crystalline polymer and an excellent engineering material. It has marginal chemical resistance but resists moisture, creep, fire, fats, and oils. Molded items are hard, bright colored, and retain their impact strength at temperatures as low as — 40°F (-40°C). Uses include auto louvers, under-the-hood electricals, and mechanical parts. 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

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