Classification of High-Density Polyethylene
Several criteria are used to classify the different types of high-density polyethylene, including:
Density, monomer content, molecular weight, molecular weight distribution, melt flow rate, and modification.The most commonly used classification is based on density, as this parameter is a good indicator of the qualities of polyethylenes; generally, higher densities correspond to better mechanical properties. Based on density, high-density polyethylene can be divided into:
1. High-density polyethylene.
2. High-molecular-weight high-density polyethylene (HMW-HDPE).
3. Ultra-high-molecular-weight polyethylene.
4. Cross-linked polyethylene.
The monomer content largely determines the structure of the ethylene polymer, particularly its degree of branching. The properties of the plastic depend on this.
Ethylene polymers can be classified based on their monomer content as follows:
Homopolymers
Copolymers
Copolymers have small methyl, ethyl, butyl, etc., side chains. Depending on the comonomer (propylene, butene, hexene, etc.), the number of these side chains depends on the comonomer incorporated. In fact, the comonomer is added to control the degree of branching in the polymer and, therefore, its properties. Homopolymers exhibit a higher degree of crystallization than copolymers because they have fewer branches. For this reason, homopolymers have a higher melting point than copolymers.
Molecular weight, or degree of polymerization, is a measure of the molecule's length. Mechanical properties tend to improve as this parameter increases, as does the material's resistance to flow in processing equipment.
Molecular weight distribution refers to the statistical study of chain lengths, which typically follows a normal distribution curve—that is, the “Gaussian bell curve.” Two types of average molecular weights are defined: weight-average molecular weight (Mw) and number-average molecular weight (Mn). The relative difference between these two averages depends on how broad or narrow the molecular weight distribution is. Their ratio, Mw/Mn, is an index of the width of the distribution or dispersion. Mw/Mn is always greater than one, and deviates further from 1 as the dispersion increases.
If the chains that make up the polymer are all of similar length, the distribution is narrow, and Mw/Mn does not differ greatly from one. The average molecular weights and their distribution are determined using various experimental techniques, such as osmometry, viscometry, light scattering, and exclusion chromatography.
Both the molecular weight and the molecular weight distribution depend on the type of synthesis used to produce the polymer, as well as on the likelihood of the high-density polyethylene polymerization reaction ending through either coupling or disproportionation.
The melt flow index is a parameter that indicates the flow capacity of plastic in its molten state. This data is obtained under standardized laboratory conditions, making it possible to make comparisons under similar conditions. The melt flow index or flow capacity of polyethylenes does not depend solely on molecular weight; molecular weight distribution and the degree of branching, among other factors, also affect the polymer’s flow capacity.
The modification of polyethylenes involves blends, fillers, reinforcing agents, and crosslinking agents, all of which alter the intrinsic properties of the virgin polymer. As mentioned earlier, crosslinking agents change the polymer’s behavior, improving its performance in terms of temperature resistance and wear resistance, among many other properties.
Source: UNIVERSITY OF SAN CARLOS DE GUATEMALA, FACULTY OF ENGINEERING, SCHOOL OF CHEMICAL ENGINEERING—ERNESTO ROCA GIRÓN