Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes, and containers including bottles, etc.)
They account for more than 70% of total plastics market. Polyethylene is easily available, at relatively low cost and easily processable. It finds applications in household items, packaging, insulation and medical applications, etc. Polyethylene is processed at temperature in the range 150-250°C. Most polyethylene compounds contain reasonably good number of fillers. Polyethylenes are thermoplastic in nature and therefore they can be reprocessed repeatedly. Polyethylene, however, will soften and flow, and lose critical physical properties at elevated temperature thereby limiting its applications.
Therefore, crosslinking of polyethylene is carried out to retain desirable properties at high temperature. Crosslinking will change the nature of polymer from thermoplastic to thermoset to yield a non-melting, more durable polymer matrix.
All types of important polyethylenes are crosslinked, like Linear low density polyethylene (LLDPE), Low density polyethylene (LDPE), High density polyethylene (HDPE) and Ethyl vinyl acetate copolymer (EVA) and Polyolefinic elastomer (POE). Branched structure is more suitable for crosslinking. Therefore, crosslinking of LLDPE and HDPE requires more attention.
Crosslinking leads to the formation of insoluble and infusible polymers in which polymer chains are joined together to form three-dimensional network structure. Crosslinked polyethylene forms a dense network of high molecular weight, which improves impact strength, environmental stress crack resistance (ESCR), creep and abrasion resistance without influencing tensile strength and density to any appreciable extent, but on contrary, it reduces the melt index and elongation at break. Figure 1 shows the polyethylene chains before and after crosslinking.
In this method, crosslinking is obtained by free radical mechanism. The free radical is generated in polymer chain by using high energy radiations. This process is shown schematically in Fig.2.
1) crosslinking reaction takes place at room temperature.
2) reaction is completed in fraction of seconds, hence high output is obtained.
3) reaction can take place without any additives.
4) crosslinking takes place in only one step.
1) high capital cost
2) difficult to cross-link article with irregular shapes.
3) Safety precautions are needed to protect operators from radiation.
Degree of crosslinking in thermoplastic resin varies according to crosslinking process. Chemical crosslinking by using peroxide gives highest and uniform degree of crosslinking as compared to physical crosslinking method. radiation crosslinking yields between 34-75% degree of crosslinking. In chemical crosslinking method, peroxide gives much high degree of crosslinking (up to 90%), while silane-based crosslinking can be 45-70% degree of crosslinking.
1) excellent heat stability
2) simple compounding
3) good shelf-life stability.
1) higher cost
2) surface tackiness in presence of oxygen
silane molecules are grafted on polyethylene chain. For grafting, the peroxide such as dicumyl peroxide is mixed with the polyethylene in small percentage. The peroxide initially generates free radical on polyethylene chain and grafting of silane takes place at the site where free radical is generated and finally Extruded or injection moulded article is crosslinked with the help of water. The silane crosslinking process is schematically represented in Fig. 4.
1) curing time is very high as compared to peroxide crosslinking.
2) extra downstream equipment is required (for condensation)
3) bond strength of crosslinking is weaker than bond strength in peroxide crosslinking system.
The most common application of cross-linked polyethylene is in the electrical cable industry. Crosslinking, while having no effect on the dielectric properties of polyethylene, causes resistance to flow and permanent deformation at temperatures above the softening point. Also, polyethylene mesh is widely used in hot and cold water pipes. The use of polyethylene cross-linked (PEX) plastic water pipes, replacing copper pipes, is increasing widely in construction and construction. In addition, PEX pipes are widely used in underground water lines, indoor water supply systems and underfloor heating systems.1-Flexibility
2– Less pressure drops.
3– Lower materials cost
4– Easier installation
5– No fire risk during installation
6 Ability to merge new PEX with existing copper and PVC systems.
7– Less likely to burst from freezing.
8– Pipe insulation possible
1-Degradation from sunlight
2-Odors, chemical taste, and possible health effects.
3-Perforation by insects
References:
1. Oral E, Muratoglu OK. Crosslinked polyethylene. Materials For Total Joint Arthroplasty: Biotribology Of Potential Bearings. 2015 Oct 14:133.
2. https://en.wikipedia.org/wiki/Main_Page.