Polyethylene for Food and Technical Films: Properties

Polyethylene for Film - Complete Overview

What is Film-Grade Polyethylene?

Film-grade polyethylene is a special type of polymer designed for the production of blown and cast films by extrusion. Unlike general-purpose polyethylene, film grades have strictly defined rheological properties that ensure stable bubble formation and high quality of the finished web. Polyethylene films are one of the most widely produced polymer products, used in the food industry, agriculture, construction, medicine, and many other sectors.

Several main types of polyethylene are used for film production: low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and their blends. Each of these materials has its own characteristics that determine the properties of the final film - transparency, strength, elasticity, heat-shrink properties, and other features.

Classification of Film-Grade Polyethylene

Film grades of polyethylene are classified according to several criteria.

By polymer type: LDPE film grade - classic low-density polyethylene produced at high pressure. It features good transparency, flexibility, and easy processability. LLDPE film grade - a copolymer of ethylene with alpha-olefins, offering higher strength and puncture resistance. Blends and compounds of LDPE and LLDPE combine the advantages of both materials.

By application: grades for food films that meet strict safety requirements and are approved for food contact; grades for technical films intended for packaging industrial goods, construction, agriculture, and other non-food applications.

By processing method: grades for blown (tubular) films - the most common method, and grades for cast (flat) films.

How is Film-Grade Polyethylene Produced?

Film-grade polyethylene is produced on the same plants as general-purpose polyethylene, but using special formulations and polymerization modes that ensure the required properties.

LDPE for films is produced by radical polymerization of ethylene at high pressure (1500-3500 bar) and temperature around 200 °C in tubular or autoclave reactors. For film grades, precise control of molecular weight distribution and melt flow rate (MFR) is essential. For thin films, grades with MFR of 0.3-2.0 g/10 min are typically used; for thicker films, grades with MFR up to 7 g/10 min.

LLDPE for films is produced by copolymerization of ethylene with 1-butene, 1-hexene, or 1-octene in the presence of Ziegler-Natta or metallocene catalysts. The process is carried out at low or medium pressure. The choice of comonomer affects the properties of the final film: octene and hexene provide higher strength and impact toughness.

Film grades contain special additives: antioxidants to protect against thermo-oxidative degradation; processing aids to improve processability; slip agents to reduce the coefficient of friction; antistatic agents to prevent static electricity buildup; antiblocking agents to prevent film layers from sticking together.

Key Physical and Mechanical Properties

Polyethylene films have a number of unique properties that determine their wide application.

Density depends on the polymer type. For LDPE, density is 0.910-0.925 g/cm³; for LLDPE, 0.915-0.935 g/cm³.

Melt Flow Rate (MFR) for film grades is typically 0.2-7.0 g/10 min (at 190 °C, 2.16 kg). Low MFR grades (0.2-1.0) provide high strength but require more extrusion effort. Medium MFR grades (1.0-3.0) are the most versatile. High MFR grades (3.0-7.0) are used for thin films.

Tensile strength for LDPE films is 11-15 MPa; for LLDPE films it is significantly higher, up to 25-35 MPa. Elongation at break reaches 600 % and more.

Transparency and gloss are important optical properties. LDPE films have better transparency and gloss compared to LLDPE. Special additives or blending with LDPE are used to improve the optical properties of LLDPE.

Chemical resistance of polyethylene films is very high: they are resistant to acids, alkalis, salts, and many organic solvents. Films do not absorb moisture and are an excellent barrier to water vapor.

Heat-shrink properties - the ability of the film to shrink when heated - are important for shrink packaging.

Food Films: Features and Requirements

Food films are one of the most critical segments of polyethylene application. They are used for packaging fresh and frozen products, baked goods, meat, fish, vegetables, fruits, dairy products, confectionery, and many other food items.

Key requirements for food films: safety for health - the material must not release toxic substances upon contact with food; no odor or taste that could be transferred to products; high transparency for visual inspection of contents; good weldability for hermetic packaging; resistance to punctures and tears; stability of properties at low temperatures (for frozen products); ability to print labels and markings.

Special polyethylene grades that meet strict regulatory requirements are used for food films. In Russia, these are GOST 16337-2022 for LDPE and GOST 10354-82 for finished film. In Europe - Regulation (EU) 10/2011, in the USA - FDA 21 CFR 177.1520, in China - GB 4806.6-2016.

Food film grades undergo strict migration testing. Migration levels must be below established limits to ensure safety.

Technical Films: Types and Applications

Technical films cover all non-food applications of polyethylene films.

Stretch film - elastic film for palletizing and securing loads. Produced mainly from LLDPE, which provides high elasticity and strength.

Heat-shrink film - film that shrinks when heated, used for group packaging of goods.

Agricultural films - greenhouse films, mulching films, silage films, protective covers.

Construction films - waterproofing, vapor barrier, windproof films, as well as films for temporary covering of construction sites.

Industrial films - liners for containers and big bags, protective films for equipment, packaging for industrial goods.

Medical films - packaging for sterile instruments, disposable medical products.

Garbage bags and sacks - one of the largest segments of technical films.

Features of Polyethylene Film Processing

Polyethylene film is produced by extrusion through an annular die followed by pneumatic blowing (blown films) or through a flat die (casting, flat films).

For blown films, the process includes extrusion of the melt through an annular die, inflation with compressed air to the required bubble diameter, cooling, and winding into rolls. Processing temperature is typically 170-220 °C depending on the polymer type.

Critical process parameters: temperature profile along extruder zones; draw ratio; blow-up ratio (bubble diameter to die diameter ratio); film thickness, which can range from 5 to 300 microns and more.

Major Producing Countries of Film-Grade Polyethylene

Film-grade polyethylene is produced in all countries with petrochemical facilities. The largest producers are the United States, China, Saudi Arabia, South Korea, and Western European countries. China is the world's largest producer and consumer of polyethylene films.

Major companies producing film grades include ExxonMobil Chemical (USA), SABIC (Saudi Arabia), LyondellBasell (Netherlands/USA), Borealis (Austria), INEOS (UK), Dow Chemical (USA), as well as Chinese Sinopec and CNPC.

In Russia, film grades are produced by Kazanorgsintez, Tomskneftekhim, Nizhnekamskneftekhim, Stavrolen, and other enterprises.

Conclusion

Film-grade polyethylene is one of the most important segments of the polymer industry. Film grades of LDPE, LLDPE, and their blends provide the production of a wide range of products - from the thinnest food packaging to thick technical and agricultural films. The combination of affordable price, high performance properties, and recyclability makes polyethylene films an indispensable material in the modern economy. Understanding the features of different grades, their properties, and applications helps manufacturers and consumers choose optimal solutions for their tasks.