What is the difference between PET and PETG?

PET, a polycondensate, is used primarily in food packaging films and textile fibers, among other applications. It is now increasingly used as a packaging material, not only in soda bottles, but also in amorphous PET (APET), crystalline PET (CPET) cans and plates. In the past five years, engineering-grade PET and copolyesters, as new polymer products, have been used in engineering and specialty packaging materials, respectively.
The success of PET in soda packaging is due to its toughness and transparency, orientation capabilities, excellent economic value, and the development of high-speed bottle processing technology. PET beverage cans are lightweight, shatterproof, reusable, and have good airtightness. A filled 2-liter PET beverage bottle is 24% lighter than a similar glass bottle; an empty bottle weighs 1/10 of a glass bottle of the same size. This saves labor, energy, and costs in all links from manufacturer to consumer.
Chemistry and Properties

PET for beverage bottles is produced from terephthalic acid (TPA) obtained by oxidation of p-xylene. Terephthalic acid is purified or reacted with methanol to form dimethyl terephthalate (DMT), or further oxidized to form pure terephthalic acid (PTA). Another basic raw material for PET is ethane, which is converted to ethylene glycol (EG) by reaction. PET is a condensation polymer formed by the continuous polymerization of DMT (or PTA) and EG in the molten state, and then a solid-state polymerization process to obtain large crystals and the final molecular weight and intrinsic viscosity. The solid-state process also makes the ethanol content of the polymer sufficiently low.

General commercial PET resin melts at about 480F (), but the melting point of high-crystalline PET is about 520F ().

Oriented crystallized PET has excellent strength. Toughness and transparency, and is resistant to weak acids, bases and many solvents.

Special grades

Stretch blow molding grade PET can provide solid color, green and light yellow PET. The colored polymer in the reactor does not need to be compounded with adverse effects on physical properties, and improves color uniformity. Pure resins of various intrinsic viscosities are available. PET copolymers crystallize slowly, which allows them to produce high-quality soda bottles over a wide range of processing conditions. An extrusion blow moldable polymer is also available. This material combines the advantages of good melt strength and slow crystallization, and can be easily processed on suitable extrusion blow molding equipment. Various reinforced, flame retardant and other specialty polymers are constantly being introduced or modified to meet new applications.

PETG copolyester is another example of a large number of copolyesters. Unlike PCTA, which is modified with acid, PETG is a diol-modified polymer made by combining CHDM diol with TPA (terephthalic acid) and ethylene glycol. PETG copolymers can be molded or extruded and usually remain amorphous, transparent and virtually colorless, even in large cross-sections.

It has high stiffness, hardness and good toughness, even at low temperatures. The combination of transparency, toughness and melt strength makes it useful for injection molding, blow molding and extrusion of profiles, pipes, films and sheets. PETG is available in unmodified form or with various additives, including release agents, masterbatches, and impact modifiers for injection molding.

PETG should be dried at 120-160F for about 4-6 hours before molding or extrusion. In both processes, the melt temperature ranges from 420F to 510F. The holding time of processing equipment at higher temperatures should be as short as possible to prevent excessive degradation. Injection molding should be performed on an injection molding machine, requiring each shot to be 50% to 80% of its capacity.

PETG can be extruded and blow molded at melt temperatures between 400-450F to make transparent bottles for shampoo, liquid detergents, hygiene products, mineral oils, and food packaging. This material meets FDA standards for contact with food.

Extrusion can produce a wide range of profiles, as well as packaging tubes, films, and sheets, including medical device packaging. PETG and PCTA can be sterilized with ethylene oxide and Y rays.

When used for injection molding, PETG is usually processed at a melt temperature range of 450-510F, with a mold temperature of about 70-130F. Current applications include instrument covers, machine shields, cosmetic containers, lever device pointers, display components and toys.

PET is mainly used in soda and soft drink packaging. PET has nearly 100% of the 2-liter packaging non-recyclable container market, and 1.5-liter, 1-liter, 0.5-liter and smaller PET bottles have also been widely recognized.

PET is used in food, alcohol, detergents. Demand for PET is expected to continue to grow for uncarbonated beverages and industrial product packaging. Packaged foods include mustard, rubber products, peanut butter, condiments, cooking oils, cocktails and concentrated juices. New colors, especially Weber colors, are popular in the packaging of medicines, vitamins and detergents.

One of the newest and fastest growing applications for PET containers is food or beverage packaging, which requires filling at high temperatures. Many foods, especially fruits or foods or beverages with a high fruit content, must be packaged at 180F or higher. This provides pasteurization (sterilization) of the product and container at the time of filling. Conventional oriented containers, such as bags for soda and soft drinks, tend to shrink and deform when subjected to temperatures above 160F, which is due to a certain stress relaxation. Stress concentration is generated during the stretch blow molding of the container. Technology has been developed to improve heat resistance during processing, usually referred to as "heat setting" technology. Depending on the specific processing technology, there are several processing technology details, which are highly proprietary, based on which containers suitable for filling at 190-195F can be produced. Products that require packaging with this characteristic include pure fruit juices. High-juice beverages, teas, certain isotonic and sports drinks, condiments, concentrated juices and certain mineral waters.

Other end uses of PET are widely used in extrusion coating and extrusion film and sheet. PET is used as an extrusion coating material for ovenable paperboard packaging. In addition, crystalline PET (CPET) can be used as the basic material to make oven trays.

PET film is usually biaxially oriented and used as X-ray and other photographic films, meat and cheese packaging, magnetic tapes, electrical insulation, printing plates and bottle packaging bags. PET is also used as industrial tape material. Non-crystalline, unoriented PET film and sheet are beginning to be used for forming containers, trays, foam products and beverage cups.

Summary: PETG is an upgraded version of PET, with higher transparency, higher toughness, better impact resistance, and of course a higher price.