As a special film material, heat-shrinkable films have seen increasingly widespread application in recent years, with their usage growing annually. Introducing new monomers to modify traditional PET chains endows them with excellent shrink properties and gradually replaces previously used shrinkable film materials such as PP, PE, and PVC. The modified PET polyester resin effectively addresses several drawbacks of traditional shrinkable film materials, such as low shrinkage rates, difficulty recycling, and environmental pollution. As a result, they have become increasingly favored by customers. They are gradually becoming the primary raw material for heat-shrinkable films on the market, with promising prospects for future development.

1. Overview

Heat shrink films are a new type of packaging material widely used in labels, food and beverage packaging, and electronics and electrical appliances. According to statistics, the total annual usage of heat shrink films in Europe, the USA, and Japan exceeds 100,000 tons. In Sweden, approximately 30% of packaging has shifted from corrugated boxes to shrink combo packaging, and in Western Europe, about 15% of the distribution sector has adopted shrink packaging. Domestic usage is also at a certain scale.

Currently, heat shrink films are typically made from amorphous polymers such as polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC). Among them, the heat-shrinkable films made from polyethylene and polypropylene have lower strength and poor impact resistance, limiting their applications. Polyvinyl chloride, though widely used, is toxic and difficult to recycle. In particular, its use in food packaging is not environmentally friendly, which has led to its ban on packaging in Europe, the United States, and Japan. Therefore, developing eco-friendly PET heat shrinkable films and their raw materials has become an inevitable trend.

As a semi-crystalline material, conventional PET polyester films, after specific processing, typically have a shrinkage rate of less than 30%. To achieve a heat shrinkage rate of more than 70%, PET must be modified. By introducing a third, fourth, or even fifth component into PET, high-performance, environmentally friendly, and economically feasible PET resin for heat shrink films has been developed. If these resins result in films with a shrinkage rate of over 70%, they will likely replace expensive PETG heat shrinkable films and non-environmentally friendly PVC shrink films.

2. Feasibility Analysis

Polyethylene terephthalate (PET) is a linear polyester synthesized from dibasic acid and dibasic alcohol, with a molecular structure that gives it excellent crystallization properties. PET’s high rigidity in the molecular chain and the strong intermolecular forces endow it with good rigidity and strength, while also providing excellent barrier properties against non-polar gases. Additionally, PET has a low coefficient of thermal expansion, high dimensional stability, and good wear and creep resistance, allowing it to withstand weak acids, alkalis, and common solvents. Its colorless, tasteless, and non-toxic properties make it suitable for direct contact with food and pharmaceuticals.

PET is a typical semi-crystalline polymer with a density of 1.3-1.38 g/cm³ and the melting point ranges from 225°C to 260°C. Highly crystalline PET has a melting point of 261°C, and its glass transition temperature ranges from 70°C to 80°C, with a long-term use temperature of up to 120°C. Furthermore, PET has good thermal stability and can be recycled.

Due to its eco-friendly characteristics and recyclability, PET has been widely used in the packaging industry, especially in food packaging, with its usage increasing year by year. By introducing co-monomers such as flexible chains and asymmetrical structures into PET, its heat shrinkage properties can be improved, making it superior to conventional PET films. Thus, the development of heat shrink films PET has become an inevitable industry trend.

3. Research Content

3.1 Catalyst Selection

In the synthesis of PET polyesters, various catalysts can be used, but each catalyst or catalyst combination is suited to specific systems. This research focuses on three catalyst systems to verify their impact on PET coloration. The selected catalyst systems are titanium-based, germanium-based, and antimony-based catalysts.

3.2 Synthesis Method Selection

PET can be synthesized by two methods: the DMT method and the PTA method.

DMT Method: This involves the ester exchange reaction of dimethyl terephthalate (DMT) with ethylene glycol (EG), followed by polycondensation to produce the required polyester chips. This method is suitable for the introduction of a single monomer, especially modified alcohols, but is less applicable when introducing multiple monomers. Moreover, methanol is produced during the synthesis process, which is difficult to handle and has an environmental impact, making it unsuitable for large-scale production.

PTA Method: This involves the esterification reaction of terephthalic acid (PTA) with ethylene glycol (EG), followed by polycondensation to produce the required polyester chips. This method allows for multiple monomers to directly participate in the esterification reaction, and reaction conditions can be controlled to achieve the desired intrinsic viscosity. This method is more feasible for large-scale production.

Given the modification goals of this study, the PTA method offers greater advantages in achieving PET polyester modification.

3.3 Monomer Introduction Experiments

Currently, a common modification approach is the introduction of flexible chain segments and asymmetrical structures. Commonly used modifying monomers include isophthalic acid (IPA), neopentyl glycol (NPG), and 1,4-cyclohexanedimethanol (CHDM).

PETG is an amorphous copolyester partially modified by replacing ethylene glycol with 1,4-cyclohexanedimethanol. This product was first introduced by Eastman Chemical Company in 1977 and is mainly used in markets for sheet, film, shrink films, heat-sealing films, bottle applications, and profiles. PETG products have high transparency, excellent impact resistance, and good processing performance, and can be processed using traditional extrusion, injection molding, and thermoforming methods to meet various application needs. Copolyester films modified with 1,4-cyclohexanedimethanol exhibit the best heat shrinkability, with a shrinkage rate of over 60%, but their products and monomers are expensive. Research has shown that polyester films modified with 20% IPA can achieve heat shrinkage rates close to those of PETG, but require a higher heat treatment temperature (125°C) and longer time (5 minutes). NPG is the lowest price of the three monomers, but the thermal shrinkage of polyester film modified with NPG is generally difficult to exceed 50%.

Thus, reducing production costs while ensuring high heat shrinkage rates for modified polyesters is a pressing issue in large-scale industrial production. A few domestic manufacturers have begun producing polyester shrink films, but they typically use imported PETG raw materials, resulting in high product costs, and limiting the widespread application of PET shrink films.

4. Research Results

By adjusting the synthesis process and ratios, our company ultimately achieved a modified PET chip with a film shrinkage rate of ≥75%, which was successfully mass-produced. And the monomer we use is the cheapest NPG, which makes our product very competitive in the market. For more information about the high-shrinkage PET resin, please refer to https://www.polyestermfg.com/petg-chip/.

5. Conclusion

Through the industrial-scale production of modified PET polyester chips, not only was the production cost of heat shrinkable films reduced by more than 30%, but the environmental issues of PVC and the durability limitations of PP and PE shrink films were also addressed. This technology fills a gap in the domestic market and offers a potential substitute for imported PETG products, holding significant importance for future market expansion.