Extrusion crosslinked polyethylene

Which of these two principal types is the better is not clear-cut. The plastics insulation is relatively easy to apply by cross-head extrusion. PVC, polyethylene, crosslinked polyethylene, ethylene propylene rubbers are commonly used. This is a relatively cheap and efficient process. Paper cable manufacture, by comparison, is a complex and labour-intensive... 

The term gel has different meanings in the polymer extrusion industry. The term gel is used as gel content in crosslinked polymers. Determination of gel content is described in ASTM D2765, standard test methods for determination of gel content and swell ratio of crosslinked polyethylene plastics [163]. The gel content (insoluble fraction) produced in ethylene plastics by crosslinking is determined by extracting with solvents such as decahydronaphthalene or xylenes. 

Maleic anhydride may be grafted to polyolefins, such as polyethylene, by mechanochemical, mechanochemical with free-radical initiators, free radical,ionic,and radiation-initiation techniques. The solid-phase mechanochemical modification studies were run by a low-temperature mechanical dispersion, extruder,die extruder,and grindingtechniques. Modification of polyethylene with <20% MA leads to crosslinking during extrusion of the polymer-anhydride blend in the extruders. This crosslinked material generally retains its form during heating to >300°C. 

High Density Polyethylene - [Both regular and high molecular weight (HMW)]. The time to increase in torque at 220°C. by the Brabender Plasticorder was determined as a measure of crosslinking. Yellowness Index after the Brabender test at 220°C. and after the first extrusion at 260°C. were also performed. 

Cross-linking of the outer casing is achieved by grafting a vinyl silane to polyethylene by means of a peroxide as part of the cable extrusion process. The chemistry is shown in Scheme 2.1. When the cable is subsequently soaked in water, the alkoxy silanes hydrolyse and crosslink within the matrix. It is an unresolved question whether polymers that have been modified by a free radical mechanism will survive for the expected lifetime of underground cables (ca. 75 years). 

The wire is a low voltage wire for telecommunications equipment. It has a total diameter of 1 mm. The conductor is around 0.6 mm diameter. The outer layer compound is manufactured by conventional processes, for example, dry blending, pellet manufacture, extrusion, and then irradiation up to 10 MRad dose level. The irradiation crosslinked outer layer offers sufficient abrasion resistance to protect the inner layer (polyethylene) and the conductor in usage for a wire in switchboard or main frame wiring. It also gives the required degree of non-flammability. 

Uses Coactivator, crosslinker, coagent for wire and cable, hard rubber rolls, polybutadiene, polyethylene, moisture barrier films and coatings, plastisols and vinyl acetate latexes, adhesives, molding compds., textile prods. comonomer for polymerizable/softened PVC plastisols reactive diluent for radiation-cured inks/lacquers processing aid for extrusion and molding of plastisols and rubber compds. Improves scorch resist. 

However, their safe working temperature can be raised to meet the demands of both products by crosslinking the polyethylene after the extrusion process has been completed. Although the same techniques are used for water pipes as for the longer established crosslinked cables, application of cable technology to pipes needs changes to satisfy the different service conditions. 

Following the extrusion process, insulated wire and cable can be heated to dissociate the peroxide into free radicals and crosslink the polyethylene. Pipe initially at its melting point and with no wire support will need special supporting facilities (which are normally proprietary) for post extrusion crosslinking. 

The basic chemistry of this process is that a silane such as tri-methoxy vinyl silane is grafted on to polyethylene and following extrusion the insulated wire or pipe is exposed to hot water or steam. The silane groups react with the water to form cross links with the elimination of methanol (Figure 12.13) [11]. A tin catalyst such as dibutyl tin dilaurate speeds up the crosslinking reaction. 

Kneading a mixture of the polyethylene, free-radical initiator and MA below the decomposition temperature (T) of the radical initiator, subjecting it to shear forces, and heating to a temperature above T is reported to give uniform grafting with substantial reduction of crosslinking. Extrusion of... 

There are many properties to consider when developing a new UHMWPE material for implant applications. The characteristics of the UHMWPE powder may be measured to verify that there are not variations in different manufacturing lots. After consolidation, when the polyethylene powder is compressed into a solid slab of material via ram extrusion, compression molding, or other techniques including isostatic pressing, manufacturers will test the solid polyethylene slab to determine the quality of the consolidation and to determine if any deleterious effects have occurred. Highly crosslinked UHMWPE is commonly used now, whereby the polyethylene slab is exposed to radiation, which forms chemical bonds between polymer chains. Several analytical techniques have been developed to assess if the material has been exposed to enough radiation to improve the desirable properties of the UHMWPE,... 

Shen and co-workers [29] used a twin-screw extrnder to devnlcanise waste tyre rubber and then blended the resnlting DR into high-density polyethylene (HDPE) to produce thermoplastic elastomers (TPE). They found that the high shear conditions during the extrusion process induced chain scission and oxidative degradation of the GTR. The reduction in crosslink density was apparent in a decrease in the gel content of the rubber/HDPE blends and an increase in the melt flow of the materials. 

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