Every day, new materials obtained by artificial means are introduced into the sphere of human activity. One of these is high molecular weight polyethylene, which has already become a commercial product since the 50s of the last century, but it is gaining real popularity only now.
Description
High molecular weight polyethylene is one of the varieties of thermoplastic polymerized ethylene. Its main feature is very long molecular chains. They are better perceived and transmit loads, “compensating” for their intermolecular interaction.
The material does not differ in appearance from other varieties of plastic. It has no smell, taste, and toxic properties. It can be given any color by the introduction of tinting. At the same time, it has a set of properties that distinguish it favorably with other polymers.
High molecular weight polyethylene is a tough material with high impact strength. It is able to withstand significant voltages. It does not absorb moisture, which is why it does not interact with human skin and seems slippery. In addition, it has high abrasion resistance (more than steel) and a low coefficient of friction.
Properties of high molecular weight polyethylene
As already noted, the main feature of this high molecular weight polymer are long molecular chains. Moreover, they are oriented in the same direction. They are located almost parallel to each other. This explains the strength of the material.
Despite the fact that high molecular weight polyethylene forms long chains, the bond between the individual molecules is weak. This figure is an order of magnitude lower than that of Kevlar no less durable material. This feature makes the polymer not heat-resistant - it melts at a temperature of 144 degrees Celsius.
This polyethylene does not contain esters, amines or hydroxyl groups that make materials susceptible to chemically active and heavy media. Due to this, the substance is not subject to water, moisture, aggressive reagents, microorganisms and UV rays.
Basic processing methods
The requirements according to which high molecular weight polyethylene is to be made, GOST 16338-85 contains. According to them, the synthesis of the material is carried out by the action of metallocene catalysts on the monomer ethylene. The following main types of processing are used for the production of products directly:
- Gel spinning. The feed is mixed with a solvent. The resulting mass is forced through holes in the water. The formed filaments are fired in furnaces with their simultaneous drawing and removal of the solvent.
- Hot pressing and sintering. The powder mass is squeezed with great force, thereby obtaining a homogeneous material. Then it is subjected to heat treatment - sintering at a temperature of 150-200 degrees.
- Plunger extrusion. The raw material is melted to a homogeneous, rubber-like mass, and then squeezed out through special nozzles.
The most common was gel spinning. Indeed, it is in this way that high molecular weight polyethylene fibers with high strength are obtained.
Use in the military industry
Fibers, for the production of which high molecular weight polymer is used, are widely used to create personal protective equipment, in particular body armor. Due to such properties of threads as low specific gravity and high conditional yield strength (the ratio of these indicators is 7-8 times higher than that of steel), the armor is light and resistant to bullets, splinters and other damaging elements.
Bulletproof vests are made of sheets obtained by applying fibers to each other at different angles. Thanks to this technology, multiaxial fabrics appear - a special kind of glassy canvases that can withstand stresses in any direction. Used to protect the torso and limbs of high molecular weight polyethylene. The properties of multi-axial fabric can protect armored vehicles as well as use fibers to create cut-resistant gloves.
Medical use
In medicine, high molecular weight polyethylene is mainly used to create implants for the hip joint and spinal column, knee joints. It was first used with a similar purpose in the distant 1962. And since then it has become dominant.
Widely used and modified material - mesh or cross-linked polymer. It is obtained by shelling fibers of high molecular weight polyethylene with gamma quanta or electrons, which sew the threads together. After that, it is exposed to heat, which reduces its redox ability.
Fibers based on this raw material are also used for suturing. The leader in the production of these products is DSM, which supplies Dyneema Purity suture threads to the medical market.
Industrial use
The greatest application in industry has been found in high molecular weight polyethylene sheet, which is supplied to the production market with flat blanks 2 cm thick. It is used to make prestigious class plastic windows, PVC panels and PVC profiles of various configurations.
In mechanical engineering, for the production of o-rings, bearings, the creation of parts operating in a hydraulic or oil environment, as well as in pneumatic units with a high working pressure, high molecular weight polyethylene 1000 is most often used - one of the main varieties of polymer.