Polyolefins : A Backbone of Modern Packaging

Written by Dr. Bhargav Raval | Updated: September 24, 2025

Polyolefins (Polyalkenes)

PROPERTIES

The most common type of thermoplastic used today is polyolefin, commonly known as polyalkene. Polymers and copolymers of simple alkenes such as ethylene, propylene, butenes, and pentenes Polyethylene (PE) and polypropylene (PP) are the two most widely used types of polyolefins. Standard thermoplastic machinery can be used to manufacture them by injection molding, blow molding, extrusion, or thermoforming. They are the most widely used resins and frequently the best option for a wide variety of plastic applications due to their low cost, ease of processing, and high-quality chemical and physical qualities.

Polyethylene, also known as polymethylene by its IUPAC designation, is the most widely produced polyolefin. Several quality levels are readily accessible for purchase. The molecular weight, crystallinity (density), and branching of these are all different. Only certain exceptional grades can be crosslinked. Density ranges from 0.87 g/cm3 to 0.97 g/cm3, depending on factors such as the degree of branching and branch length. The degree of crystallinity, stiffness, and hardness of a polymer increase with its density.

The mechanical characteristics and melt flow behavior of polyethylene depend on both the degree of branching and the molecular weight and distribution of the polymer. Without branching, high-molecular-weight polyethylene can be quite fragile. When ethylene is copolymerized with low-molecular-weight alkenes like butene-1, hexene-1, or octene-2, small chain branches are introduced to the otherwise linear polymer chain, increasing its flexibility. Commercially accessible grades range in branching degree and molecular weight. Here are the top ten most prevalent academic levels:

Polyethylene with an extremely high molecular weight (HMWPE)
Polyethylene with a high molecular weight (HMWPE)
Polyethylene terephthalate (PT)
HDXLPE stands for high-density cross-linked polyethylene. Materials: MDPE (Medium Density Polyethylene) and
Polyethylene (Low, Medium, and High Density)
Polyethylene (PEX or XLPE) that has been cross-linked
LDPE, or low-density polyethylene,
LLDPE stands for linear low-density polyethylene.
Polyethylene with a very low density (VLDPE)
Polyethylene wax (PE-WAX) has a very low molecular weight.
HDPE, LLDPE, and LDPE are the most commonly produced types.

The polyethylene used to make LDPE is both malleable and sturdy. It has more short and long side-chain branching than HDPE. Lower density and crystallinity, as well as increased flexibility and toughness, are the effects of branching because it lessens the tendency of the molecules to cluster closely together in hard, stiff, crystalline domains. In contrast to HDPE, LDPE has a much lower melting point, heat deflection temperature, and tensile strength. Its longer side and subside branching result in smaller crystallites than LLDPE. Depending on the branching and temperature history, the degree of crystallinity is typically between 40% and 55%.

LLDPE is significantly more flexible than HDPE while maintaining equivalent strength. There are many more and shorter branches in the polymer chains than there are in LDPE. This prevents the chains from getting tangled up under pressure because they may readily glide against each other. The molecular weight distribution (polydispersity) of LLDPE grades is typically more narrow than that of LDPE grades. Higher tensile and impact strength as well as increased puncture resistance are achieved in comparison to LDPE due to the lack of lengthy side chains and the narrower dispersion. This type of low-density polyethylene has higher environmental stress crack resistance than HDPE and LDPE and may be manufactured into thinner films as a result.

Compared to HDPE, MDPE sees far less widespread use. Similarly to linear low-density polyethylene (LLDPE), it is made by low-pressure polymerization methods with transition metal catalysts. It is less rigid and hard than HDPE but more so than LDPE, and it has superior impact and environmental crack resistance than HDPE. It is often more durable than LDPE but less elastic.

HDPE’s crystallinity is significantly higher than LDPE’s, often falling between 70% and 80% depending on the polymer’s molecular weight and its thermal history. Compared to LDPE, the crystals are bigger and more homogeneous. With the exception of HMWPE and UHMWPE, HDPE is the densest, most rigid, and least permeable of the polyethylenes. Compared to low-density polyethylene (LDPE), it is significantly more robust and rigid, but it is also less tough and flexible and has less resistance to stress cracks. HDPE is typically put to use in situations calling for extreme toughness, stiffness, and/or chemical resistance.

Very-low-density polyethylenes (VLDPE) have a density between 0.89 and 0.914 gm/cc, whereas ultra-low-density polyethylenes (ULDPE) have a density of about 0.888 gm/cc. Compared to regular polyethylene, they contain more alpha-olefin. The alkyl branches of the alpha-olefins reduce the packing density and crystallinity, which results in a material that is extremely tough and elastic but has a very low tensile strength.

Polyethylene that can be cross-linked is called XLPE. It’s a dense polyethylene that’s been chemically altered to include reactive side groups. These grades can undergo crosslinking via moisture, radiation, catalysts, or chemical means (Polidan® PEX/XLPE). The impact strength, creep, abrasion, and stress fracture resistance of polyolefins are all enhanced once they have been crosslinked to make them insoluble and infusible polymers.

Among commodity thermoplastics, polypropylene (PP) has the second highest sales volume. In general, isotactic polypropylenes (i-PP) predominate. They typically fall somewhere in the middle between LDPE and HDPE in terms of crystallinity. PP has an elongation at break that is comparable to LDPE, but a tensile modulus and impact strength more akin to HDPE. The semicrystalline structure actually makes it quite malleable.

Polypropylene’s greater melting temperature (160°C to 180°C) and lower density (0.9 g/cc) than most polyethylenes make it well-suited for high-temperature applications like retortable plastic products. However, i-PP isn’t as resistant to impacts as regular PP. It is sometimes copolymerized with ethylene to make it tougher and more flexible, a process that also reduces its brittleness.

Smaller factories create syndiotactic polypropylene. Less crystalline, but often clearer, more elastic, and more resistant to impact

Higher n-olefins, cyclic olefins, and polar monomers can all be copolymerized with ethene and propene. Since these copolymers can exhibit a wide range of characteristics, their potential uses have been broadened.

Polybutene-1 (PB-1) and polyisobutylene (PIB) are two other notable polyolefins. These polymers have a considerably lower production volume. The higher molecular weight grades are rubbery, while the lower molecular weight grades are sticky liquids or solids. Small amounts of isoprene are frequently used to copolymerize with isobutylene. Butyl rubber (IIR) is the name given to this random copolymer. It has a stellar reputation for impermeability and outstanding flexibility. The polymer of 1-butene is called polybutylene (PB-1). It is highly resistant to stress cracking and corrosion and can be welded. In order to provide the desired thermal bonding qualities and peel strength, it is frequently combined with other polyolefins.