Cross-linked polyethylene (XLPE) is a polyethylene thermoplastic material that has been chemically or electronically cross-linked using irradiation, peroxide agents or silanes. This cross-linking introduces branches into the material's molecular structure. The resulting molecular structure is a three-dimensional rather than linear chain molecular structure. The cross-linking process enhances many key properties of the material including density, tensile strength, improved environmental stress crack resistance and its resistance to degradation from heat and UV radiation.
Properties
Cross-linking changes polyethylene from a linear-chain thermoplastic to a three-dimensional network polymer. Some key properties of XLPE that make it useful for various applications include:
Density
Cross-linking increases the density of polyethylene by introducing branches into the molecular structure. The density can be modified during the manufacturing process to suit different applications. Typical densities range from 0.930 to 0.955 g/cm3. This increased density provides strength and durability.
Strength and Toughness
The three-dimensional network structure formed during cross-linking greatly enhances the tensile strength and impact resistance of polyethylene. XLPE can withstand stresses and pressures that linear polyethylene cannot. Impact strength in particular is much higher than linear polyethylene.
Improved Temperature Resistance
Cross-linking polyethylene gives it better heat resistance than linear polyethylene. It can withstand higher continuous use temperatures in the range of 80-105°C compared to 60-80°C for linear polyethylene. This makes it suitable for applications requiring higher temperature performance.
Environmental Stress Crack Resistance
XLPE has much higher resistance to environmental stress cracking than linear polyethylene. Stress cracking occurs when polyethylene is exposed to chemicals, heat, stress or a combination of these factors. Cross-linking prevents crack propagation through the polymer structure. This makes it suitable for applications with chemical exposure.
Radiation and UV Resistance
The cross-linked structure protects polyethylene from degradation caused by heat, radiation or UV light exposure over long periods. This translates to improved durability and longer lifespan when exposed to the outdoor environment.
Applications
Due to the versatile properties offered by cross-linking, XLPE finds wide use in many applications:
Wire and Cable Insulation
Given the improved mechanical strength and heat resistance, XLPE has largely replaced PVC as the insulation material for high voltage underground power cables. It has enabled increased ampacity and operating temperatures.
Pipe Systems
Cross linked polyethylene (PEX) tubing is commonly used for potable water delivery and residential plumbing systems. The increased temperature and pressure resistance of PEX allows its use in both hot and cold water pipes.
Geomembranes
XLPE sheets are used as containment membranes in landfills, mining projects and agriculture applications due to high puncture and tear resistance along with resistance to chemicals in soil/waste.
Medical Devices and Tubing
The biocompatibility, flexibility and durability of cross linked polyethylene make it suitable for medical tubing, blood bags and implantable devices.
Automotive
Due to its impact resistance and durability, XLPE components are increasingly replacing metal components in vehicles. Common usages are for exterior body panels, front-end modules, and under-hood shrouds. The isotropic strength profile also means complex 3D molded shapes can be created.
Future Developments
While cross linked polyethylene is a well-established and widely used material, ongoing research continues to enhance its properties and extend its applications. Some potential future developments include:
Higher Performance Cross Linking: New cross linking techniques are being developed to provide even higher strength, temperature and chemical resistance specifications tailored to extreme conditions.
Reinforced Composites: Adding nanotube, fiber or particle fillers during cross linking is seeing use to create XLPE composite materials with tailored stiffness, strength or functional properties for specific structural uses.
Lightweighting: Cross linked polyethylene with very high densities approaching 1 g/cm3 offers weight savings potential compared to metals for under-hood and body applications in the automotive industry.
New Catalyst Systems: Silane grafting and new organic peroxide systems aim to reduce manufacturing costs and enhance processability of high density grades of XLPE.
Biobased Feedstocks: With global drives towards sustainability, research explores using biobased alcohols and olefins as raw materials to produce bio-derived, compostable cross linked polyethylene formulations.
The improved properties imparted by cross linking make polyethylene a versatile and essential plastic material for varied industries. Continuing innovation is further expanding its capabilities for high performance applications. Cross linked polyethylene will remain an important engineered thermoplastic into the future.
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