What is a cross linked polymer: Crosslinked polymers are obtained by cross-linking macromolecules in polymerization or polycondensation processes, under the action of chemical agents (vulcanization, curing) or ionizing radiation and other effects on pre-synthesized linear or branched polymers, or oligomers, or on the corresponding monomers, if they contain more of two functional groups (see p. With the development of the structuring process, an increasing number of chains are involved in it and at a certain stage the line between the macromolecule and the macroscopic body disappears.
What is a cross linked polymer?
Crosslinked polymers or polymers as a branched chain, in contrast to conventional polymers and vinnltoluola copolymers are opaque and substantially insoluble in solvents such as benzene, toluene, xylenes, ethylbenzene, ethyl toluene, chloro -. And dihloraameschennye benzenes, carbon tetrachloride, etc. There is only a small swelling of the crosslinked polyether in some of the above solvents.
The properties of cross-linked polyethylene come from the capabilities of its polymer base and molecular features.
Conventional polyethylene consists of large molecules with many free branches that freely “float” in space. That is why, with many positive properties, it is still a rather soft material with a relatively low melting point.
The creators of cross-linked polyethylene were able to eliminate this drawback, strengthening the structure of the material while maintaining its positive characteristics. Crosslinked polyethylene has a wide-cellular network of molecular bonds.
It is formed by the appearance in the molecular structure of the polymer, along with the longitudinal compounds, also transverse ones in the form of chains of hydrogen atoms uniting the molecules into a three-dimensional network.
The individual strands of polyethylene produced by the polymerization reaction are tightly connected here. Such a “fabric” has a much higher molecular density and a larger specific gravity, and is also much stronger than a “fibrous” counterpart in both the mechanical and physicochemical sense.
In addition to high density and strength, cross-linked polyethylene has a number of original properties, due to which polyethylene products have been introduced into almost all areas of modern human activity. Crosslinking molecules gave him:
- The main thing is to increase the melting point. The modified polymer softens when the temperature rises above 150 ° C, melts at 200 ° C and burns at 400 ° C with decomposition into water and carbon dioxide.
- With crosslinking, stiffness and tensile strength increased with a simultaneous decrease in tensile elongation.
- This material does not change properties with a sharp change in environmental conditions, which is similar even to such strong metals as steel.
- Its resistance to chemical reagents and biological destroyers is very high,
- Compared with plain polyethylene, cross-linked has a higher hydro and vapor barrier,
- There was a possibility of “shape memory”, in which the polymer changed the property of plasticity to elasticity.
Significant disadvantages of cross-linked polyethylene are the following properties:
- As with other ethylene polymers, it begins to slowly degrade under the influence of sunlight,
- The negative effect of oxygen upon penetration into the structure of the material.
Both disadvantages are eliminated by coating products with protective shells from other materials or by applying a layer of paint.
One of the most notable manufacturers of cross-linked polyethylene pipes today is the STOUT brand . All products are manufactured using modern equipment in Europe at the same factories where premium brands order their goods.
STOUT pipes are significantly cheaper, but they are not inferior in quality to more expensive pipes: it makes no sense for the buyer to overpay for a big brand name. The client pays for quality and reliability, getting all this in full. Products are adapted for the operating conditions in our country, installation is easy and does not take much time.
Crosslinking Technology Polyethylene is crosslinked chemically or physically using one of the following technologies:
- The chemical peroxide method (PEx a) produces very high-quality, but quite expensive products. As a reagent, hydrogen peroxide is used here. The process proceeds at a temperature of about 200 0C. Crosslinking is most uniform, since the number of crosslinked molecules in the total amount will be up to 85%.
- The chemical silane method (PEx b) produces crosslinked polyethylene in the presence of silane, catalysts and water. This method is the most common, although the percentage of crosslinking here is only 65-70%.
- Physical Radiation (PEx c). This crosslinking is carried out by sweeping the polyethylene mass through an electron accelerator, where it is exposed to x-ray or gamma radiation. In this case, free atoms enter into the reaction, but not of carbon with hydrogen, but of the same name with each other, forming new bonds. The degree of crosslinking is approximately 60%.
- Chemical nitric (PEx d), using nitrogen radicals, get a crosslinking quality of up to 70%. This method is rarely used, since it requires sufficient time and certain reaction conditions.
Comparison of properties by type of crosslinking
Crosslinked polyethylene that has passed any of the aforementioned crosslinking technologies obtains an ordered mesh structure similar in properties to the crystalline lattice of solids. However, in each case, the resulting material has its own slight differences:
- As already noted, the most uniform crosslinking is peroxide, although less productive and more expensive,
- The peroxide method is not applicable to the manufacture of multilayer pipes,
- The fastest way to get finished products is with the silane method
- The simplest process and cheap raw materials are used in the radiation method,
- The silane method gives the densest, but also the least flexible material.