Polyolefin crosslinked polyethylene

Denning s three papers in the late 1960s (a.2-a.4) reviewed the development of closed-cell polyolefin foams, and their mechanical properties. Some of his predictions on materials development turned out to be true. In Part I he explains that non-crosslinked polyethylene (PE) foams have inferior creep properties to crosslinked foams this appeared to be the tensile creep of the melt, rather than compressive creep of the... 

No.4, July 2001, p.333-52 INVESTIGATION OF THE RADIATION CROSSLINKED FOAMS PRODUCED FROM METALLOCENE POLYOLEFIN ELASTOMERS/ POLYETHYLENE BLEND Kim D W Kim K S... 

New theories and findings on the formation of ionic species and radicals, which become trapped in the polymers, are discussed in the first two chapters, written by the two European authorities, Chapiro and Charlesby. The kinetics of crosslinking polyethylene is the subject of the American authority, Dole. A higher yield of crosslinking polyolefins was observed in the presence of nitrous oxide by the Japanese scientist, Okada. 

Within the family of polyolefins there are many individual families that include low density polyethylenes, linear low density polyethylenes, very low polyethylenes, ultra low polyethylenes, high molecular weight polyethylenes, ultra high molecular weight polyethylenes, polyethylene terephthalates, ethylene-vinyl acetate polyethylenes, chlorinated polyethylenes, crosslinked polyethylenes, polypropylenes, polybutylenes, polyisobutylene, ionomers, polymethylpentene, thermoplastic polyolefin elastomers (polyolefin elastomers, TP), and many others. 

In spite of these drawbacks, the described method represents a prospective procedure for polyolefin crosslinking which may be applied not only to polyethylene and ethylene copolymers but with s ne modifications to polypropylene and to other polyol ins with a branched main chain. 

Polyolefin floor coverings (PP and PE), power cables with PE coverings and HDPE pipes and wall covering materials, halogen free linear low-density PE and thermoset crosslinked polyethylene are all suggested as alternatives to PVC by Greenpeace. 

The entire class of olefin polymers tends to have a linear structure. Some branching is possible, but highly branched polymers are not usually formed. Crosslinkages between chains may also be formed, but such materials are a small part of the total production. Crosslinked polyolefins, especially polyethylene, have been developed to improve on the standard polymer s tendency to creep or cold flow under the influence of temperature, load, and time. However, these polymers are usually thermoplastics. The general linear structure also indicates suitability for fiber and film applications, which are common. 

Figure 5.258 shows the creep behavior of various polyolefins at 95 °C. Crosslinked polyethylene with 80% gel content exhibits only Stage 1 failure up to 100,000 h, which reflects this material s good fracture strength. The fracture strength of PE-MD, PB, and PP Is comparatively lower For PP and PB, Stage 2 failure occurs after 1,000 h and for PE-MD after 10,000 h. The creep behavior exhibited by these plastic materials can be regarded as typical however, stabilizer and filler content must always be taken into consideration [191]. 

By replacing water as the exterior medium with air, embrittlement occurs after just 20,000 h. Fracture is initiated at degraded, oxidized areas in the pipe wall. These oxidized areas (that occur only in polyolefin pipe) are discolored and covered by cracks, illustrating the degree of embrittlement. The brittle failure mode of crosslinked polyethylene pipe with less than 68% gel content is Stage 2 failure. 

There are many applications for polyolefin compounds in the processing of pipes, tubings, and fittings from polyethylene (PE), crosslinked polyethylene (XPE), biaxially oriented polyethylene (PEO), polypropylene, and polybutylene. 

For example, Tables 4 and 5 give the values of two basic properties for two types of polyolefin pipes crosslinked polyethylene XPE SDR 9 (degree of cross-linking, 64- 89%) and polybutylene PB 21-SDR 11. 

Keywords Polyolefin foams Polyethylene foams Crosslinking Foaming agents Physical properties of foams... 

The FR characteristics of PCP and Anorin-38 were improved substantially by introducing bromine. When five bronine atoms were introduced by controlled bromi-nation, the resin (the product obtained was still fluid in nature and could be crosslinked by hexamine to get hard partially brominated PCP-PBPCP) showed excellent FR characteristics exhibiting self-extinguishing property and UL 94 V-0 grade when blended with polyethylene, NR, etc. There was excellent compatibility with polyolefins as well as with other polymers such as cellulose, polyurethane, etc. Table 13 shows the FR properties of NR-PBPCP. 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

Harden s (27) market survey of the growth of polyolefin foams production and sales shows that 114 x 10 kg of PE was used to make PE foam in 2001. The growth rate for the next 6 years was predicted as 5-6% per year, due to recovery in the US economy and to penetration of the automotive sector. In North America, 50% of the demand was for uncrosslinked foam, 24% for crosslinked PE foams, 15% for EPP, 6% for PP foams, 3% for EVA foams and 2% for polyethylene bead (EPE) foam. As protective packaging is the largest PE foam use sector, PE foam competes with a number of other packaging materials. Substitution of bead foam products (EPP, EPE, ARCEL copolymer) by extruded non-crosslinked PE foams, produced by the metallocene process was expected on the grounds of reduced costs. Compared with EPS foams the polyolefin foams have a lower yield stress for a given density. Compared with PU foams, the upper use temperature of polyolefin foams tends to be lower. Eor both these reasons, these foams are likely to coexist. 

CROSSLINKING OF POLYOLEFIN FOAMS. I. EFFECT OF TRIALLYL CYANURATE ON DICUMYL PEROXIDE CROSSLINKING OF LOW-DENSITY POLYETHYLENE... 

Polyolefins can be chemically modified with the purpose of changing some of their properties. For example, polyethylene can be chlorinated or chlorosulfonated. Chlorosulfonation can be done with a mixture of chlorine and sulfur dioxide, leading to a material partially chlorinated and partially containing sulfonyl chloride groups. This treatment imparts elastomer character and the capability to be crosslinked, for example, with metallic oxides. 

Additives in polyolefins which improve their application properties affect the crosslinking whenever they take part in the generation of free radicals or enter the propagation reactions.With the presence of fillers with acid sites, peroxides are, e.g., decomposed by an ionic medmnism and formation of a crosslinked structure in polyethylene is suppressed. To remove such an eff t, acid centres should be neutralized by basic substances. 

Crosslinking reduces the polyolefin crystallinity. Crosslinks play the role of defect centres which impede the folding of macromolecules and thus decrease the sizes of the lamellar crystals. Even a small number of crosslinks reduces the crystallinity of low density polyethylene provided that it is performed above the melting tonperature of the polymer crystallites. The linkage of polymer chains impedes their independoit displacement and the degree of crystallinity is distinctly Iowa even when only one macromolecule involves one crosslink [160]. 

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