How was a finished PET film created?

2026-06-23

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The bright film on food packaging, the protective film substrate for mobile phone screens, the surface material for printed labels, the insulation sheet for electronic circuits, the base film for photovoltaic backplates... We come into contact with PET film every day, but few people can explain what material it is. As one of the most widely used and well-balanced polymer films, polyester film (PET film) has already penetrated into almost all industrial fields such as packaging, electronics, optics, energy, and building materials, and is truly the "industrial panacea".

1、 PET film from petroleum particles to amorphous castings

The birth of a finished PET film can be traced back to the origin of petroleum refining. It is the process of polymer polymerization and casting, and also the key stage that determines the purity and quality limit of the film foundation.

Step 1: Raw material polymerization - from small molecules to polyester chips

The p-xylene (PX) obtained from petroleum refining is oxidized to produce terephthalic acid (PTA), which then undergoes esterification and condensation reaction with ethylene glycol (EG) under the action of a catalyst to produce high molecular weight polyethylene terephthalate. Finally, it is cut into PET primary slices of rice grain size.

This step determines the purity, molecular weight distribution, and intrinsic viscosity of PET resin. High end optical grade and electronic grade PET films must use high-purity native slices and prohibit the addition of recycled materials, otherwise problems such as crystal dots, impurities, and uneven performance may occur.

Step 2: Ingredient drying - precise control of formula

According to the functional requirements of the final film, the glossy PET slices are mixed with functional masterbatch in proportion, such as adding silicon-based open masterbatch to improve roll adhesion, adding anti-static masterbatch to achieve anti-static effect, and adding UV absorbers to enhance weather resistance. The mixed raw materials need to undergo crystallization drying to remove moisture and avoid bubbles and hydrolysis degradation during melt extrusion.

Step 3: Melt extrusion casting - forming amorphous thick sheets

The dried mixture enters the extruder and is melted and plasticized at a high temperature of 260-290 ℃. After removing impurities through a filter, it is uniformly extruded from the narrow slit die and cast onto a low-temperature cooling roller for rapid quenching and cooling.

The purpose of rapid cooling is to keep PET in an amorphous and non crystalline state, obtaining transparent and uniform thick sheets (castings), preparing for subsequent biaxial stretching. At this time, the cast sheet has low strength, is prone to breakage, and has ordinary performance, so it cannot be called a true PET film.

2、 PET film biaxially stretched

Biaxially oriented stretching is the core process of PET film manufacturing, and it is also a crucial step in the transformation of "cast sheets" into "high-performance films". By stretching vertically and horizontally twice, PET molecular chains are oriented bidirectionally in the plane, combined with heat setting fixed structures, ultimately achieving excellent comprehensive performance. The entire process can be clearly broken down into four steps:

Step 1: Longitudinal stretching (MDO) - molecules oriented along the machine direction

The casting sheet first enters the longitudinal stretching machine, and is heated to above the glass transition temperature (about 80-90 ℃) by the preheating roller. Then, it is stretched by a speed difference between the fast and slow rollers along the direction of the film's advance by 3-3.5 times. After stretching, the molecular chains are arranged in an orderly manner along the longitudinal direction, and the longitudinal strength and stiffness of the film are significantly improved. However, at this time, the transverse direction is still prone to tearing, and the overall performance is uneven.

Step 2: Transverse Stretching (TDO) - molecules oriented along the width direction

After longitudinal stretching, the film enters the stretching machine, and the edge of the film is clamped by the clamps on both sides. It gradually widens along the transverse track in the oven, and the film is stretched horizontally by 3.5-4 times while the temperature gradually increases. After transverse stretching, the molecular chains are oriented in both longitudinal and transverse directions, forming a uniform two-dimensional network structure, and the strength, transparency, and barrier properties of the film are comprehensively improved.

Step 3: Heat setting - fixing the structure and eliminating internal stress

The stretched film is still in a high elastic state, with a large amount of internal stress inside, and will shrink severely when exposed to heat. Therefore, it is necessary to perform heat setting treatment at a high temperature of over 200 ℃ under tension to promote crystallization, fix molecular orientation, and eliminate internal stress. This step directly determines the thermal shrinkage rate and dimensional stability of PET film, which are key indicators of industrial grade films.

Step 4: Post processing and winding slitting - delivery of finished products

After shaping, the film undergoes processes such as cooling, thickness measurement, corona treatment, and surface coating, and is finally rolled into large mother rolls, which are then divided into finished rolls of different widths according to customer needs. Corona treatment can increase the surface tension of the film, making the adhesion between the ink and the adhesive layer stronger during printing and lamination. It is a standard post-processing process for packaging printing grade PET film.

At this point, from petroleum to polyester chips, and then from cast sheets to biaxially stretched finished products, a PET film is considered complete.

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How was a finished PET film created?

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