Crazing stress

Applied stress There are TPs that will craze or crack under certain environmental condition. Products that are highly stressed mechanically must be checked very carefully. Polypropylene, ionomer, chlorinated polyether, phenoxy, EVA, and linear polyethylene are examples that offer greater freedom from stress crazing than some other TPs. Solvents may crack products held under stress. TSs is generally preferable for products under continuous loads. 

High reflectance, pipes light, excellent low temperature properties, brittle, machines poorly stress-crazes. 

EP 323142 (European) 1989 Ternary polyether ketone blend wire insulations Pirelli General PLC, UK CK Alesbury, RJ Murphy Formulation shows excellent stress craze resistance, flexibility and flame resistance Blends of polyarylene ether ketones, polyether imides and polyfimide siloxanes) were coated onto wire for solvent resistance, O index and abrasion resistance. Blends without siloxane-imide copolymer did not meet these properties... 

Bucknall and Smith s theory has been further confirmed by recent work. Matsuo (42) published electron micrographs of stress-crazed rubber-reinforced polymers and found his results to be in good agreement with those of Bucknall and Smith. Recently, Arends (3) related the cold-flow of thermoplastics to Eyring s theory of viscous flow and enlarged the scope of their theory. 

Bernier GA, Kambour RP (1968) The role of organic agents in the stress crazing and cracking of poly(2,6-dimethyl-l,4-phenylene oxide). Macromolecules l(5) 393-400... 

White SA, Weissman SR, Kambour RP (1982) Resistance of a polyetherimide to environmental stress crazing and cracking. J Appl Polym Sci 27(7) 2675-2682... 

Glass transition temperature Secondary transition Extension ratio Maximum extension ratio Craze intensification stress Craze initiation stress Tensile strength Compressive yield stress Drop in after yielding A measure of strain softening Test frequency... 

Fig. 25. Variation of craze initiation-stress, craze intensification stress, and tensile fracture stress, Oj, with AN content...

The influence of solvents has been touched on in Sect. 2.3.2. In fact, the influence of environment on crazing and fracture properties of polymers is of major importance in the practical uses of these materials. There are many ways for the environment to induce fracture by means of stress cracking, stress crazing, chain scission, chain crosslinking, etc. Therefore, environmental fracture has been widely studied, specially from the experimental point of view. Reference is a review of environmental cracking of polymers. Most work on environmental crazing has been done in liquid environments - (solvents and non solvents of the material), or high pressure gas environment, near condensation pressure (liquid nitro-... 

Decohesion and void growth are controlled by the debonding stress at the particle/matrix interface it only occurs when the local debonding stress is lower than the fracture stress (crazing stress) of the matrix itself G is correlated with the local toughness at the interface it is dependent on the particle/matrix adhesion energy. 

Low values have also been recorded in the solvent stress-crazing of glassy plastics, where in appropriate circumstances S 0.1 Jm . In this case, however, the surface energy in question is that of voids and cavities in the craze. 

Chemical properties The bulky shape of the MMA group ensures an amorphous structure because it prevents close packing of polymer chains. PMMA has extremely good weathering resistance. Acrylics are dissolved or stress crazed on contact with ketones, esters, hydrocarbons, acids and alkalis. ... 

Other desirable changes in properties may be produced by orientation, for example polystyrene sheet that is biaxially oriented (see section 10.2.2) can be highly flexible, whereas unoriented sheet is brittle oriented sheet exhibits substantial increases in impact strength, in tensile-yield strength and in resistance to stress crazing. It should not, however, be assumed that such improvements will always result from orientation. 

Physical ageing in thermoplastics can lead to crazing, a failure process normally produced simply by high tensile stress. Crazing involves the prior development of regions called crazes, resembling cracks bridged by several strands of oriented polymer. 

Crazing requires a stress field that must have at least one tensile component where in unoriented homo-polymers crazes form and grow normal to the maximum principal tensile stress. Craze microstructures in homo-polymers that have been widely studied consist of drawn polymer fibrils a few nanometers in diameter and have extension ratios of 2-4 that bridge the two faces of a craze and result in a density reduction of up to 0.75 locally. 

This immediately establishes the critical particle diameter Dc below which under an applied stress crazes cannot form. This critical particle diameter is... 

Cationic surfactants can also be used as functional additives in traditional chain lubricant formulations, including biocides, stress-craze inhibitors and coupling agents. As mentioned above, the first use of alkylamines in chain lubricants was as an additive in a traditional, soap-based formulation to improve the stability of the system. Ethoxylated amines have also been employed as coupling agents in an alkaline diamine track lubricant, while tertiary amines have found utility as saponifying agents in traditional, soap-based chain formulations. 

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