Plastics, stress cracking

FIGURE 3.92 Test for environmental stress cracking of ethylene plastics. Stress cracking resistance (F50) = time taken for 50% of the specimens to fail Qi). Standard test method ASTM D1693. 

The effect of liquid surfactants can powerfully accelerate stress crack formation. Nevertheless, stress crack formation in plastics must be distinguished from stress crack corrosion as known in particular in metallic materials. Corrosion is understood as the erosion of atoms from the material by chemical processes and in metals particularly by electro-chemical reactions. Additional influence by stresses leads to crack formation and brittle fracture which often resembles of the failure of stress cracks in plastics. Stress crack formation in thermoplastics is, however, a purely physical process. No chemical changes take place in the material even under the influence of surfactants. The terminology is nevertheless not completely uniform. The accelerating effect of liquids on stress crack formation in plastics is occasionally described as stress crack corrosion although no real corrosion process is connected with it. 

Not recommended under any conditions plasticizer stress crack agent ... 

The principal advantage of plastic dmms and liners is their resistance to corrosion. This aspect of their performance requires the lading to be investigated in terms of capacity for chemical attack on the dmm. Stress-cracking tests should be performed in all instances where the compatibiUty of jading and dmm material has not been estabUshed (6). 

MSTM D1693, ed. Test Methodfor Environmental Stress-Cracking of Ethylene Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1988. 

ISO 4599, Plastics Determination of Resistance to Environmental Stress-Cracking, Pent Strip Method, ISO, Geneva, Swit2edand, 1986. 

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA. Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point. 

Stress crack Internal or external defect in a plastic caused by tensile... 

Fig. 8.83 Stressed crack tip with associated plastic zone...

It has been found with many rigid plastics materials that under stresses well below the normal yield stress, cracking occurs in environments where when free of stress the polymer will be unaffected. The mechanism for this stresscracking phenomenon is not well understood and indeed there may well be... 

Environmental stress cracking is the cracking of certain plastic products that becomes exposed to a chemical agent while it is under stress. This effect may be caused by exposure to such agents as cleaners or solvents. The susceptibility of affected plastics to stress cracking by a particular chemical agent varies considerably among plastics, particularly the TPs. 

The other effect of having a stretched area is a reduction in resistance to stress cracking. Crazing is a possibility in such areas such as in polystyrenes, and environmental stress cracking caused by solvent substances will occur in the stretched areas. This is a particularly important consideration in vacuum formed products used for packaging food that frequently has some solvent action on the plastics. 

Published by the Plastics Design Library, PDLCOM is an exhaustive reference source of how exposure environments influence the physical characteristics of plastics. Data include resistance to thousands of chemicals, weathering and UV exposure (i.e. color change after accelerated weathering or outdoor exposure) sterilization (radiation, ethylene oxide, steam, etc.) thermal air and water aging environmental stress cracking and much more. 

In the polymers and plastics industry ether carboxylates are used to improve antistatic properties [214] and stress-cracking resistance of olefin polymers by... 

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