Effects on mechanical properties

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

H. J. Hoffman, Moisture Effects on Mechanical Properties of Solid Propellants, CPTR 84-29, CPIA Pubhcations, Johns Hopkins University, Laurel, Md., Feb. 1984. 

E = no adverse reaction, Htde or no absorption, htde or no effect on mechanical properties G = some effect, some absorption causing slight sweUing, and reduction in mechanical properties and NR = not recommended, material adversely affected in a short time. 

Copolymerisation also affects morphology under other crystallisation conditions. Copolymers ia the form of cast or molded sheets are much more transparent because of the small spheruHte size. In extreme cases, crystallinity cannot be detected optically, but its effect on mechanical properties is pronounced. Before crystallisation, films are soft and mbbery, with low modulus and high elongation. After crystallisation, they are leathery and tough, with higher modulus and lower elongation. 

Wood preservatives ate appHed either from an oil system, such as creosote, petroleum solutions of pentachlorophenol, or copper naphthanate, or a water system. Oil treatments ate relatively inert with wood material, and thus, have Htde effect on mechanical properties. However, most oil treatments require simultaneous thermal treatments, which ate specifically limited in treating standards to preclude strength losses (24). 

C. C. Gerhards andj. M. McMiUen, eds.. Proceedings of the Kesearch Conference on High-Temperature Dying Effects on Mechanical Properties of Softwood Dumber, U.S. Department of Agriculture, Eorest Service, Eorest Products Laboratory, Madison, Wise., 1976. 

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

It will be observed that molecular weight has little effect on mechanical properties but does influence the flow temperature. 

The type of reinforeement. In the case of fabric reinforcement, factors such as cloth weight and crimp will have a large effect on mechanical properties. 

For partially crystalline ionomers, such as those based on copolymers of ethylene and methacrylic acid, even time or aging at room temperature can have an effect on mechanical properties. For example, upon aging at 23°C, the modulus of the acid form of the copolymer increased 28%, while in the ionomer form, the increase ranged up to 130%, with the specific gain in modulus depending on the degree of conversion and on the counterion that was present [17]. 

Reference No. (BS 3468 1974) Typical composition (%) Heat resistance Effect on mechanical properties Overall... 

Wang S, Huang Y, Cong G (1997) Study on nitrile-butadiene rubber/poly(propylene carbonate) elastomer as coupling agent of poly(vinyl chloride)/poly(propylene carbonate) blends I. Effect on mechanical properties of blends. J Appl Polym Sci 63 1107-1 111... 

K. Simino and I. Yonenaga, Oxygen Effect on Mechanical Properties W. Bergholz, Grown-in and Process-Induced Effects... 

It has been shown that the spacial location of the rubber can have a profound effect on mechanical properties [80] and may be influenced by the relative chemical affinity of rubber and plastic matrix towards the filler, the imposed shear during blending and the procedure adopted to combine the component phases, i.e. sequential or simultaneous. 

In rubber-plastic blends, clay reportedly disrupted the ordered crystallization of isotactic polypropylene (iPP) and had a key role in shaping the distribution of iPP and ethylene propylene rubber (EPR) phases larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains [22]. Clays, by virtue of their selective residence in the continuous phase and not in the rubber domains, exhibited a significant effect on mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This resulted in nanocomposites with increased stiffness, impact strength, and thermal stability. 

In order to prepare ENR/silica nanoscale organic-inorganic hybrid composites, nanosilica has been generated by the sol-gel technique using TEOS as a precursor. Their effect on mechanical properties of the resultant nanocomposites have been... 

Processing Stability. As with elastomers or other rubber modified polymers, the presence of double bonds in the elastomeric phase increases sensitivity to thermal oxidation either during processing or end use. Antioxidants are generally added at the compounding step to ensure retention of physical properties. Physical effects can also have marked effects on mechanical properties due to orientation, molded-in stress, and the agglomeration of dispersed rubber particles under very severe conditions. Proper drying conditions are essential to prevent... 

---