Polyolefin films: their history, properties, additives, and main types

Brief history of film materials

A film is a flexible polymer material with a thickness below 300 µm. It was invented relatively recently, and its history began just in the middle of the 19th century. The first films were made from celluloid – a plastic material derived from cellulose nitrate, discovered in 1855 by the British metallurgist Alexander Parkes. Celluloid has several flaws, therefore the search for other materials for films continued. Science was quick to explore new opportunities, so with time more types of plastics suitable to be used in films emerged.

The History of Films

Films have multiple usages:

• Packaging for industrial products and semi-finished products.
• Technical films for industry and agriculture.
• Packaging for various types of non-edible products sold in retail.
• Packaging for food and edible raw materials.
• Packaging for medical, hygienic, and pharmaceutical products.

The vast majority of films are used in packaging materials. Different goods require different types of packaging with specific properties. The properties of a film are determined by what it’s made of – the polymer material. Let’s take a closer look at polymers.

A few facts about polymers

Polymers are macromolecular compounds with large molecular masses (ranging from several thousand to several million) consisting of many repeating similar or different subunits – monomers, which are chemically bound into lengthy macromolecules.  

The number of monomers in a polymer macromolecule is called the degree of polymerization. The majority of polymers used in industry have a degree of polymerization in the range between 100 and 10000. The properties of a polymer are determined primarily by its molecular mass and degree of crystallinity.

Polymers are semi-crystalline compounds, part of their molecules is ordered and form a crystalline phase, whereas the other part is unordered and amorphous. The mass fraction of a compound’s crystalline phase is called the degree of crystallinity. In polymers, the degree of crystallinity never reaches 100% and depends on several factors which include the type of polymer (and its co-polymers, if there are any) and the catalysts which were used in its manufacturing process.

What are polyolefins and how they are used in films?

Polyolefins are polymers derived from olefins – unsaturated hydrocarbons with double bonds. The most common polyolefins used in the manufacturing of modern films are polyethylene and polypropylene. They are used in the production of the main types of films:

• LDPE (low-density polyethylene).
• LLDPE (linear low-density polyethylene).
• HDPE (high-density polyethylene).
• VLDPE, или POP (very low-density polyethylene).
• ULDPE или POE (ultra low-density polyethylene).
• PP (polypropylene).
• r-PP (polypropylene random copolymer).
• Block-PP (polypropylene block-copolymer).

Polyolefins’ popularity as a raw material for films can be explained by the following facts: these films are easily processed, they have low mass, high strength and tear resistance, they are flexible even in low-temperature conditions, they have high chemical resistance and cost relatively cheap when compared to other types of polymers. It’s also important to point out that ethylene and propylene are some of the most widespread and processible monomers in the world.

How characteristics of polymers impact the properties of films

Film properties depend on the characteristics of the polymers from which they are made of. Let’s examine the key characteristics of polymers.

Molecular weight

The average molecular weight of a polymer is determined by the melt flow index (MFI). MFI measures the ease of flow of the melt of a thermoplastic polymer. It is defined as the weight of the polymer in grams flowing in 10 min through a die of specific diameter and length by a pressure applied by a given weight at a given temperature. Low MFI means high viscosity and higher molecular weight. High MFI, on the contrary, is observed for polymers with low viscosity and lower molecular weight.

Density, branching, and melting point

The density of a polymer is related to its degree of crystallinity and determines its ability to crystallize. If a polymer is linear and doesn’t have branches (like HDPE), its density and degree of crystallinity are high. When the density of a polymer increases, so do the following characteristics:

• rigidity
• tensile strength
• softening point
• fold retention
• curling
• chemical resistance
• heat resistance
• barrier properties

If a polymer contains many short and/ or long branches (like LDPE), its density and degree of crystallinity are low. When the density of a polymer decreases, the following characteristics increase:

• tear strength
• puncture resistance
• impact viscosity
• friction coefficient
• optical properties

Molecular weight distribution (MWD)

Synthetic polymers consist of molecules with different masses, which is explained by the statistical nature of polymerization. The ratio of molecules with different molecular masses in a polymer is defined as molecular weight distribution. Gel-penetrating chromatography or fractioning can be used to determine MWD, but it’s possible to estimate it by determining a polymer’s MFI under different loads: the more the ratio of MFI, the wider is MWD.

Wider MWD is associated with the following positive effects:

• increased ability to process the polymer (the pressure of the melt is lower, which decreases the gear load, since the molecules with lower molecular weight act like lubricants)
• the durability and stability of melt increase
• fewer surface defects of the film (sharkskin)
• orientation of the polymer is increased

Negative effects attributed to wider MWD:

• less durable welding joints
• worse optical properties
• lower ripping resistance
• low-weight molecules can form residues on the surface of the equipment

Why are additives used in films?

Additives are intended to improve various properties of polymers. The following requirements must be adhered to regardless of additive function:

• good compatibility with the polymer
• no effects which could contribute to polymer destruction
• resistance to water exposure
• lack of smell
• safety for health and environment
• convenient dosing and usage

Additives used in polyolefin films can be divided into two broad categories:

• additives which increase the stability and processability of the polymer
• functional additives which improve certain properties of the polymer

The following types of additives are used to increase stability:

The common goal of these additives is to prevent molecular mass and structural changes in the polymer. The consistency of the polymer’s properties enables the use of it as intended for as long as possible. It expands the recycling window – the ability to reuse the polymer after the initial product has lost its consumer properties.

Functional additives used in polyolefin films include:

Anti-blocking and slip agents are often used together. Anti-block agents prevent film surfaces from sticking together in a roll, creating a rough surface that forms an air gap between film layers. Slip agents migrate to the film surface and form thin layers that reduce the coefficient of friction between film layers.

Polyolefins for film manufacturing produced by SIBUR’s enterprises

SIBUR’s enterprises manufacture both polymers (raw materials for film manufacturing) as well as finished film materials. The assortment of polyolefins produced by SIBUR includes LDPE, LLDPE, mLLDPE, HDPE, EVA (ethylene-vinyl acetate) copolymers, HPP, RPP, and IPP. SIBUR BIAXPLEN manufactures biaxially oriented polypropylene (BOPP) films which are highly durable, flexible and have good optical properties.

Conclusion

Flexible polyolefin packaging became popular in the middle of the last century, and it is still irreplaceable in many applications. In this article, we have just begun examining the properties of polyolefins and the ways of using them to manufacture films. Later on, we plan to look at technologies for processing polyolefins into films, the main types of films as well as various details of their manufacturing processes and uses.