Polyethylene belongs to the polyolefin group of polymers. Other polymers in this group are polystyrene and polypropylene. Polyethylene is the most well-used class of polymers in the industry today. This is owing to its being low cost and is available. It is also one of the most versatile plastics that gets used in a wide range of applications. Because in fact when one mentions the term polyethylene, it is very general. The term polyethylene can refer to different types of plastics. Not even all are thermoplastics. Of course in the common language. It refers to low-density or high-density polyethylene. But polyethylene exists in various forms. These can be different physical forms. Like films, sheets, molded parts, pipes, and others. This can also be the different chemical forms of polyethylene. The fundamental structure of polyethylene is a repeating unit of -C2H4-. But there are several different ways these repeating units can form a polymer. Thus the basis of the difference between LDPE, LLDPE, MDPE, HDPE, XLPE, and UHMW. LDPE refers to low-density polyethylene, LLDPE refers to linear low-density polyethylene. MDPE refers to medium density polyethylene, HDPE to high-density polyethylene. XLPE refers to crosslinked polyethylene. While UHMW refers to ultra-high molecular weight polyethylene. Each of these has its unique property and range of applications. In this article, I will explain the difference between them. I’ll also explain how they get distinguished from one another. Using physical means of testing.
Low-Density Polyethylene (LDPE)
This is the most basic form of polyethylene. It gets formed from polyethylene that has a high level of branching. The process of polymerization occurs randomly when left to run unhindered. How the monomers attach to the growing chain is a random process. It depends on factors like the type of catalyst used and the conditions of the reaction. Where there isn’t much control over the arrangement of the monomers in the polymer chain. This results in a high branched form of polyethylene. The more branched the polyethylene is, the less tightly packed the polymer chains are. This makes for a lower density material. LDPE has a density of around 0.94g/cm3. LDPE is a safer option for low-temperature microwaves than plastics like polycarbonate. Although the use of plastics in the microwaving of food is not highly recommended. It finds use in low strength applications where its flexibility is useful. Examples are inflexible films in packaging and mulch in agriculture.
High-Density Polyethylene (HDPE)
This form of polyethylene has gained popularity over the years. It is the more rigid form of polyethylene. It has higher tensile and impacts strength. HDPE has a much lower level of branching than LDPE. The level of branching gets reduced by having a more controlled polymerization process. The use of catalysts allows for a more coordinated arrangement. Of monomers in the polymer chain. The reduced branching means that the polymer chains are more able to pack close. Thus resulting in a higher density compared to LDPE. Other than the higher density. HDPE also has superior properties like melting point and impact strength. It also has a higher resistance to radiation and chemicals. Common applications of HDPE include. Bottle caps in drinking water bottles. It also finds use as shampoo bottles, pipes, and vegetable oil containers.
Example of HDPE products
Medium Density Polyethylene (MDPE)
MDPE has properties intermediate between low-density polyethylene and high-density polyethylene. Its production was borne out of the need for a plastic that is a little stronger than LDPE. But it does not need to be as strong as HDPE. It has a moderate level of branching. The level of branching gets controlled by regulating the polymerization condition. And the catalyst area available. Although not as common in use as LDPE and HDPE. MDPE finds use in applications like pond liners. And other low to moderate performance uses. In such applications, it is useful as a physical barrier and moisture resistant.
Linear Low-Density Polyethylene (LLDPE)
Linear low-density polyethylene is a little advancement of low-density polyethylene. Whereby the chains have shorter branches. These shorter branches are possible by the use of catalysts. And in some cases copolymerization. Copolymerization with other monomers like butane and propane. Which results in shorter branches, For example, propane only has the -CH3- pendant group. Whereas LDPE polymer chains can extend to branches with several carbon units. The shorter branches make for closer packing of the polymer chain. The idea is that it does not need the level of control of the polymerization process as HDPE. So rather than preventing branching. The branches are shorter but still at the same level of branching. Although not as high a density as HDPE. It offers a lightweight plastic with similar density to LDPE but with more strength. LLDPE can achieve thinner films than LDPE. These films will also have improved toughness. The commercial advantage is being able to get material with superior properties. But without the expense required to make the polymer less branched. Applications of LDPE include plastic bags, pouches, and molded parts like toys.
Example LLDPE products
Cross-Linked Polyethylene (XLDPE)
This form of polyethylene is a polymer with high flexibility. Its properties are like those of common elastomers. It is a cross-linked form of polyethylene. The crosslinking can be with different crosslinking agents. These can be with peroxide, silane, or radiation. Once crosslinked polyethylene loses its thermoplastic property. Crosslinking makes the plastic thermosetting. Although it has no melting point. XLPE can withstand temperatures of around 120oC without decomposition. This means XLPE has a higher service temperature than thermoplastic polyethylene. They keep their solid form at the upper end of their service temperature. Even HDPE begins to soften as it approaches temperatures close to 100oC. The cross-linking improves the chemical and heat resistance of the plastic. To recycle XLPE, it has to get heated at specific conditions. For example heating in presence of subcritical water at around 340oC. This breaks the cross-links and makes the plastic thermoplastic again. Crosslinking refers to the covalent bond formation between functional groups. These exist on the polymer chain. The result is that more than one polymer chain joins together. This forms a network of one giant molecule. The cross-link is in a more sponge-like way rather than an end to end chain connection. This alters the physical properties of the plastic. Applications of XLPE include heated water pipes and footwear. XLPE has come to replace PVC. This in applications like pipings, roofing sheets, and electrical fittings. It offers the mechanical strength of PVC without the hazard. Most often HDPE is the polyethylene form crosslinked to get XLPE. Such that much of the properties of XLPE once can get described as an upgraded version of HDPE.
XLPE insulated cables
Ultra High Molecular weight Polyethylene (UHMW)
s the name implies. This form of polyethylene gets characterized by its very high molecular weight. In special reaction conditions, long-chain polymers can get formed. There are correlations between molecular weight and some polymer properties. For example, the higher the molecular weight the higher the melt temperature. Mechanical properties like tensile strength are also affected by molecular weight. In the polymerization reaction. The length of the polymer chain is often limited by several factors. Ultra-high molecular weight polyethylene gets achieved by preventing early chain termination. This is through well-controlled reactions and the use of catalysts. UHMW brings polyethylene into the league of high performing plastics. Applications of UHMW include bulletproof wearables to medical devices. It is much stronger and has a higher melting point. Higher than the other thermoplastic polyethylenes. It can withstand even higher temperatures. Then the degradation temperature of cross-linked polyethylene.
Differentiating between LDPE, LLDPE, HDPE, MDPE, UHMW, and XLPE
One of the first obvious differences is that of all the polyethylenes XLPE is a thermoset. The rest are thermoplastics. So when heated XLPE will not melt. Rather begin to degrade or char above its degradation temperature. All the polyethylenes have a higher density than water so they will float on water. Some plastics are denser than polyolefins. For example, PVC has a density of 1.38 g/cm3 which is higher than that of water. So PVC will sink in water. The tear test can also distinguish between the plastic. Yet there are such similarities between the flexibility of some like LDPE and LLDPE. So this might not get distinguished but still useful. XLPE would be more difficult to tear. It would have high flexibility like elastomers. UHMW would feel more rigid due to its high molecular weight. Flame tests on LDPE and HDPE have similar characteristic observations. The flame is blue with a yellow tip. Plastic burns slow and drips as it burns giving out a paraffin-like smell. Although the flame test should not get done without the utmost caution. The burning of plastics releases toxins into the air. This should occur in controlled environments. HDPE has plastic number 2 while LDPE has plastic number 4. The other forms of polyethylene get placed under plastic number 7. This signifies “other” that doesn’t belong to the range 1 to 6. So the plastic types can get identified according to their labels. When not labeled and other tests are not decisive. Then chemical tests become necessary.
Below is a comparison of some of the properties of LDPE, LLDPE, MDPE, HDPE, XLPE, and HMW.
| Polyethylene type | Melting pointoC | Elongation at break % | Modulus of elasticity GPa | Tensile strength MPa | Density (g/cm3) |
| LDPE | 105 | 500 | 0.29 | 17 | 0.94 |
| LLDPE | 148 | 900 | 1.5 | 45.5 | 0.925 |
| MDPE | 126 | 150 | 0.3 | 19 | 0.94 |
| HDPE | 130 | 700 | 1.5 | 33 | 0.93 |
| XLPE | — | 3500 | 0.8 | 18 | 0.93 |
| UHMW | 155 | 200 | 0.9 | 40 | 0.93 |
Conclusion
Polyethylene is a polymer that has had a long reign in industry. Its different forms make it a very versatile polymer. Its properties can range from the more flexible soft low-density polyethylene. To stronger ultra-high molecular weight polyethylene. The crosslinked form of polyethylene is the thermoset form. XLPE has come to replace PVC in many applications. With the increasing focus of the industry on recyclable plastics. And these plastics need to meet diverse demands. Polyethylene plays an important role in the present and future industry. Polyethylenes are also injection moldable polymers. For professional services in injection molding of polyethylene contact www.cavitymold.com
