Extruded polyethylene coatings

Extruded polyethylene. Extruded polyethylene has been in general use for 25 y. The polyethylene is applied by either a forward extrusion or a side extrusion process. The forward extrusion is presently limited to 24 in. (610 mm) outside diameter pipe. The side extrusion can be used for sizes up to -120 in. (3,050 mm). Extrusions are usually shop-applied but can be applied at the railhead if economics permit. In addition to excellent temperature (-40 to 150°F (-40 to 66° CJ) and water resistance, the extruded polyethylene coatings exhibit excellent resistance to disbonding and soil stresses. The commonly used 50 mil (1.3 mm) thickness offers excellent resistance to handling damage. The bonding problems with early applications of the side extrusion appear to be solved. Field joints are often made with heat-shrinkable sleeves. 

Chlorinated polyether used in temperatures up to 120°C in forms of coatings, extruded pipe and sheet linings is an intermediate between the low-cost commodity polymers such as PVC and polyethylene, and fluorinated polymers [61 ]. 

Extruded/sintered polyethylene DIN 30670 DIN 30674 By extrusion or sintering in factory 1-8-3-5 Rugged, heavy coating. Limited track record. 

Heat-shrink crosslinked polyethylene DIN 30672 Flame or heat gun in fieid 1-25-2-25 Utiiised for coating of field weld joints and repairs on extruded polyethylene coated pipes. Careful application required to achieve consistent bond. 

Polyethylene coating on ferrous pipes may be applied by means of one of the following processes circular or ring-type head extrusion, side extrusion and wrapping or powder sintering. The commercially available coating systems also differ further in that the extruded polyethylene may be applied in conjunction with various primer/adhesive systems. 

Formation of carbonyl groups which may occur when polyethylene is overheated, such as in an extrusion coating operation. Here a proper balance between extrudate temperature and exposure time to air is required. 

In the manufacture of fast cord the first process is to coat paper or textile yarns with a blackpowder/nitrocellulose dope by a dipping process. These yarns are thoroughly dried and a number of them passed through an extruder and given a thick covering of plastic incendiary composition. The cord thus produced is covered with a protective layer of plastic, usually polyethylene. The overall diameter of the cord is about 2-5 mm. 

Miscellaneous Extrusion-Applied Polymers. As mentioned earlier, there is a tendency to develop solventless magnet wire enamel formulations, and extrudable polymer systems would fulfill that requirement. There have been reports about extrusion of thin coatings of polyesters over copper wire. At this point, the state of the art allows extrusion of thin insulating films only with thermoplastic materials. The reliable extrusion of uniform and concentric insulating films of approximately 0.001-0.002 in. wall thickness is already an improvement over the more traditional extrusions of polyethylene, poly(vinyl chloride), and several fluoropolymers in much greater wall thicknesses. Because cross-linked insulation is ultimately required for most magnet wire applications, further materials development needs to be done to provide polymer compositions that are both extrudable as thin films and can be cross-linked in an economical process suitable for large-scale industrial application. 

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