Environmental stress cracking of polypropylene

Environmental stress cracking (ESC) is the premature cracking of a polymer due to the combined action of a stress and a fluid. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. The embrittlement by oxidative or other chemical degradation is not included under ESC, but is classed as corrosion stress cracking (CSC). 

Data collected from commercial extrusion grades of iPP shows that at ambient temperature the principal plastic deformation is crazing [2]. The plastic deformation mode is, however, dependent upon temperature. A transition is observed whereby the mode of plastic deformation changes 

Crazing can be considered as a type of inhomogenous localized yielding. A major factor affecting ESC is yield strength CTj, and this can be expressed in terms of thermally activated shear flow, as described by Eyring s equation  

The localization of yield can be explained in terms of density fluctuations. On cooling from the melt, low density regions are frozen in, the volume of these regions increasing with cooling rate. Also, in semicrystalline polymers, such as iPP, the relative amounts of different crystal phases and the stresses between phases need to be considered when calculating bulk properties. 

From the thermal activation and density fluctuation treatments, it can be seen that yield stress increases with decreasing temperature, increasing strain rate, increasing hydrostatic pressure and annealing below Tg. An accurate predictive model for pelding requires an integrated approach using both flow and density considerations. 

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Furthermore, the crazes in PP show other similar characteristics to those of amorphous polymers. They grow apparently normal to the direction of major tensile stress which somewhat deviates from the tensile direction because of spherulitic structure. There are similar environmental effects on craze initiation (see also Environmental stress cracking of polypropylene in this book). Crazing is also an important source of toughness in toughened PP alloy systems such as propylene-ethylene block copolymers. 

Standard test method for environmental stress-cracking of ethylene plastics Insulating and sheathing materials of electric and optical cables - Common test methods - Part 4-1 Methods specific to polyethylene and polypropylene compounds - Resistance to enviromnental stress cracking - Measurement of the melt flow index - Carbon black and/or mineral filler content measurement in polyethy-lene by direct combustion - Measurement of carbon black content by thermogravimetric analysis (TGA) - Assessment of carbon black dispersion in polyethylene using a microscope... 

The solubility characteristics of polypropylene are similar to those of polyethylene. Thus polypropylene is insoluble at room temperature but is soluble in hydrocarbons and chlorinated hydrocarbons at temperatures above about 80°C. Polypropylene does not suffer environmental stress cracking of the kind shown by polyethylene. 

The launch of new resins being rare, progress on materials intended for medical applications is by way of formulation, alloys and modifications of existing resins. Thus, new radiation-stable polypropylene grades are available for the manufacture of cups, boxes, baskets, mixers, etc. that are radiation sterilized. And polyesters with better environmental stress cracking resistance are extending their potenti in the area of medical testing. 

Blends of polyolefins (e.g., HPDE/LDPE, LDPE/ ethylene copolymers, PP/EPDM, PP/HDPE/EPDM, HDPE/butyl rubber) have been commercial since the late 1960 s and early 1970 s. Specific film formulations were commonly based on polyolefin blends to achieve the proper balance of processing, environmental stress crack resistance, modulus, toughness, cling, transparency, filler acceptance, printability, tear resistance, shrinkage characteristics, and permeability. Ethylene-propylene mbber (EPR, EPDM) was commonly incorporated into polypropylene as an impact modifier at moderate levels and as a flexibilizer at high levels. One of... 

Polypropylene One of the vinyl polymers with good flex life and good environmental stress crack resistance. 

An important application for polybntylene is plumbing pipe for both coimnercial and residential use. The exceUent creep resistance of polybntylene aUows for the manufacture of thiimer wall pipes compared to pipes made from polyethylene or polypropylene. Polybutylene pipe can also be used for the transport of abrasive fluids. Other applications for polybutylene include hot melt adhesives and additives for other plastics. The addition of polybutylene improves the environmental stress cracking resistance of polyethylene and the impact and weld line strength of polypropylene. Polybntylene is also used in pack aging applications. 

Other applications include hot-melt adhesives and additives for other plastics. The addition of polybutylene improves the environmental stress cracking resistance of polyethylene and the impact and weld line strength of polypropylene. Polybutylene is also used in packaging applications. 

Polypropylene may be subject to environmental stress cracking (see Chapter 1). The occurrence is difficult to predict. It is dependent upon the process chemistry, operating conditions, and quality of fabrication. In table 3.9, which gives the compatibility of polypropylene with selected corrodents, certain chemical/liner ratings are identified by an e, indicating that the liner in contact with that specific chemical may be susceptible to ESC. Reference 2 provides a more comprehensive compatibility listing. 

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