Stress cracking of polyolefins

Another example is stress cracking of polyolefins, such as high-density polyethylene, in the presence of surfactants. When polyethylene is held under stress in the presence of some detergents, its behavior changes from short time ductile failure at high stresses to brittle fracture at low stresses after relatively longer times with very small break elongations. 

With the exception of the polyolefins and other low surface energy plastics, they cope with almost all common engineering alloys and many plastics. The stress cracking of some plastics is a hazard for some versions of these adhesives. None of them bonds rubbers satisfactorily and polyurethane plastics may prove difficult. 

Oridation. This is caused by contact with oxidising acids, exposure to u-v, prolonged application of excessive heat, or exposure to weathering. It results in a deterioration of mechanical properties (embrittlement and possibly stress cracking), increase in power factor, and loss of clarity. It affects most thermoplastics to varying degrees, in particular polyolefins, PVC, nylons, and cellulose derivatives. 

PP/PA (polypropylene/polyallomer). These plastics are similar to polyethylene, but each unit of their chains has a methyl group attached. They are translucent, autoclavable, and have no known solvents at room temperature. They are slightly more susceptible than polyethylene to strong oxidizing agents. They offer the best stress-cracking resistance of the polyolefins. Products made of polypropylene are brittle at ambient temperature and may crack or break if dropped from benchtop height. 

Some of these plastics often compete for the same applications. Strength, modulus of elasticity, impact strength, and other properties vary greatly with type, degree of crystallinity, and their preparations that result in different densities. Their stress-crack resistance and useful service temperature ranges may also vary with type of polyolefin, their crystalline structure, etc. 

Long-Term Permeability in Polyethylene-Nylon Laminar Walled Containers. Polyolefin containers often are subject to loss of properties with aging. These result from plasticization, crystallization, and stress cracking. Figure 8 shows the permeability properties of 66/6 copolyamide based "laminar" blends. The permeability performance was found to be unchanged during over 5 years of storage in the two separate samples shown here. 

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... 

Polybutene-1, PB-1, or polybut-l-ene is another stereospecific (isotactic) polyolefin polymer, discovered by Prof. Giulio Natta in 1954. It is a linear high molecular weight crystalline thermoplastic polymer, with low density (0.91). The ethyl side groups create entanglement, which provides for the very good creep resistance of this polymer, which also has an abrasion resistance comparable to UHMWPE, and an excellent resistance to chemicals and environmental stress-cracking. 

ASTM D 5397-99 Standard Test Method for Evaluation of Stress Crack Resistance of Polyolefin Geomembranes Using Notched Constant Tensile Load Test... 

As a result of the controlled long-chain branching in otherwise linear polymers, the processability of these resins is claimed to be significantly enhanced compared to the standard LLDPE and EP rubber materials. Envircmmental stress crack resistance of metallocene polyolefin is also claimed to be significantly better. A wide range of densities (0.86-0.93), flexural modulus (10-100 MPa), melting points (60-120 °C), and melt flow index (0.5-125) is available in the commercial metallocene-based ethylene copolymers. 

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