Classification of High Density Polyethylene
For the classification of the different types of high density polyethylene, there are criteria that intervene, such as: Density, Monomer Content, Molecular Weight, Molecular Weight Distribution, Fluidity Index and Modification:
Density, Monomer Content, Molecular Weight, Molecular Weight Distribution, Fluidity Index and Modification.The most commonly used classification is density, as this parameter is a good indication of the qualities among Polyethylenes, where in general, with higher densities, higher mechanical properties. According to density, high density Polyethylene can be divided into:
1. High density polyethylene .
2. High molecular mass high density polyethylene (HDPE-HDPE-HDW-HDPE).
3. Ultra high molecular mass polyethylene .
4. Polyethylene modified by crosslinking.
The monomer content largely determines the structure of the ethylene polymer, in particular the degree of branching. In this respect, the qualities of the plastic depend on this.
Ethylene polymers can be classified according to their monomer content as follows:
Homopolymers
Copolymers
The copolymer type has small methyl, ethyl, butyl, etc. branches. Depending on the comonomer (propylene, butene, hexene, etc.); the amount of these branches depends on the comonomer incorporated. In fact, the addition of the comonomer is to control the degree of branching that the polymer will present and, therefore, the qualities of the polymer. Homopolymers develop a higher degree of crystallization than copolymers because they have less branching. For this reason the homopolymer has a higher melting point than the copolymer.
Molecular weight or degree of polymerization is a measure of the length of the molecule. Mechanical qualities tend to improve as this parameter increases, as well as its resistance to flow in processing machinery.
Molecular weight distribution refers to the statistical study of the size of the chains, describing a generally normal distribution curve, i.e. "Gaussian bell". Two types of average molecular weights are defined; average in weight Mw and average in number Mn. The relative difference between the two averages depends on how wide or narrow the molecular weight distribution is. Their Mw/Mn ratio is an index of the width of the distribution or dispersion. Mw/Mn is always greater than unity, and moves further away from 1 the wider the dispersion.
If the chains composing the polymer all have not very different lengths, the distribution is narrow and Mw/Mn does not differ much from unity. Molecular weight averages and their distribution are determined by different experimental techniques, such as osmometry, viscosimetry, light scattering, exclusion chromatography.
Both the molecular weight and the molecular weight distribution depend on the type of synthesis the polymer has as well as the probabilities in which the HDPE polymerization reaction ends, either by coupling or disproportionation.
The flow index is a parameter indicative of the flow capacity of the plastic in the molten state. This information is obtained under standardized laboratory conditions, so that comparisons can be made under similar conditions. The flow index or flow capacity of polyethylene does not depend only on the molecular weight, but also on the molecular weight distribution and the degree of branching, among other factors, affect the flow capacity of the polymer.
Polyethylene modification refers to blends, fillers, reinforcements and crosslinking agents, which change the intrinsic properties of the virgin polymer. Crosslinking agents change, as already mentioned, the behavior of the polymer; improving its temperature performance and wear resistance, among many others.
Source: UNIVERSIDAD DE SAN CARLOS DE GUATEMALA FACULTY OF ENGINEERING ERNESTO ROCA GIRÓN SCHOOL OF CHEMICAL ENGINEERING