Ethan Clarke
Polyethylene terephthalate (PET) is a common thermoplastic polymer resin of the polyester family, used in fibres for clothing, containers for liquids and foods, and thermoforming for manufacturing. PET has a range of desirable characteristics, including heat resistance, flexibility, and strength, which make it a popular choice for product packaging. However, it is important to understand the temperature limits of PET to avoid shrinkage and cracking during storage and shipping. This article will explore the temperature thresholds at which PET plastic begins to shrink and the factors that influence its thermal behaviour.
| Characteristics | Values |
|---|---|
| Temperature at which PET bottles should be shipped | 80°F/27°C or lower |
| Temperature at which PET bottles can further shrink | Above 100°F/38°C |
| Temperature causing severe distortion of the amorphous areas of the bottle | Above 131°F/55°C |
| Glass transition temperature | Around 80°C |
| Cold crystallization of the amorphous parts | Around 148°C |
| Melting peak temperature | Between 225 °C and 255 °C |
| Melting peak | 230°C |
| Maximum fill temperature | 160°F |
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What You'll Learn
PET plastic shrinks at temperatures above 38°C
Polyethylene terephthalate (PET) is a thermoplastic polymer resin commonly used in fibres for clothing, containers for liquids and foods, and manufacturing processes. It is known for its strength, flexibility, and resistance to heat, making it a popular choice for packaging and insulation.
However, PET plastic does have its limitations when it comes to temperature. While it can withstand moderate temperature changes without becoming brittle or losing flexibility, extreme temperatures can cause the plastic to shrink and distort.
Elevated storage temperatures above 38°C (100°F) can cause empty PET bottles to shrink due to the relaxation of the oriented and partially oriented regions of the bottle. This shrinkage is more pronounced in amorphous (transparent) PET, where the molecules have not been given enough time to arrange themselves in a crystalline structure during the cooling process.
When temperatures exceed 55°C (131°F), severe distortion can occur in the amorphous areas of the bottle, including the finish and neck. This is because PET has a glass transition temperature (Tg) of around 80°C, above which the molecules can move and crystals can grow, leading to cold crystallization.
To avoid shrinkage and distortion, it is recommended to store and transport PET bottles in temperature-controlled containers or trailers that can maintain a temperature of 27°C (80°F) or lower.
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Extreme temperatures (above 55°C) cause severe distortion
Polyethylene terephthalate (PET) is a thermoplastic polymer resin commonly used in fibres for clothing, containers for liquids and foods, and engineering resins. It has a range of applications, from a stable, solid form to a supple and flexible nature. PET is known for its strength, transparency, and shape stability.
While PET is generally resistant to heat and can withstand temperature fluctuations without becoming brittle or losing flexibility, extreme temperatures can still cause distortion. Specifically, when exposed to temperatures above 55°C (131°F), the amorphous areas of PET bottles, such as the finish and neck, can undergo severe distortion. This is due to the relaxation of the oriented and partially oriented regions of the bottle at elevated temperatures.
To prevent this distortion, it is recommended to use temperature-controlled containers or trailers capable of maintaining temperatures of 80°F (27°C) or lower when shipping empty PET bottles through areas where outdoor temperatures may exceed 90°F (32°C). Additionally, long-term storage of PET bottles should be done using sealed polyethylene plastic bags or lined containers to limit exposure to high temperatures and humidity, which can also impact the shrinkage and stress crack resistance of the bottles.
The distortion of PET at extreme temperatures is an important consideration for its various applications. For example, in the electronics industry, PET is used for its electrical insulation properties and shape stability. In the automotive industry, PET is used for window films to protect occupants from UV radiation and reduce interior temperatures. In these applications, maintaining the structural integrity of PET at high temperatures is crucial for optimal performance.
Overall, while PET is known for its thermal stability and ability to withstand high temperatures without deforming, extreme temperatures above 55°C can cause severe distortion, particularly in the amorphous regions of the material. Therefore, it is important to consider the storage and shipping conditions of PET products to prevent undesirable changes in their physical properties.
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PET has a maximum fill temperature of 160°F
Polyethylene terephthalate, commonly known as PET, is a thermoplastic polymer resin of the polyester family. It is used in fibres for clothing, containers for liquids and foods, and thermoforming for manufacturing. It is also combined with glass fibre for engineering resins. PET is a clear, tough plastic that is a good barrier to gas and moisture. It also has good resistance to heat and can be recycled.
However, it is important to note that extreme temperature conditions above 131°F/55°C can cause severe distortion of the amorphous areas of the bottle, including the finish and neck. Therefore, when shipping PET bottles to or through areas where outdoor temperatures may exceed 90°F/32°C, it is recommended to use a temperature-controlled container or trailer capable of maintaining a temperature of 80°F/27°C or lower.
The flexibility in the structural composition of PET allows its use across a broad spectrum of product categories. It is often copolymerized with other diols or diacids to optimize its properties for particular applications. For example, cyclohexanedimethanol (CHDM) can be added to the polymer backbone to interfere with crystallization and lower the polymer's melting temperature. This type of PET is known as PETG or PET-G and is a clear amorphous thermoplastic that can be injection-molded, sheet-extruded, or extruded as filament for 3D printing.
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PET demonstrates impressive thermal stability
Polyethylene terephthalate (PET) is a thermoplastic polymer resin of the polyester family. It is commonly used in fibres for clothing, containers for liquids and foods, and thermoforming for manufacturing. The thermal stability of PET allows its use in numerous products and applications.
PET films, for example, maintain their shape and transparency across varying temperatures, making them ideal for food packaging. They can be used in both cool and warm environments without losing clarity or shrinking. This temperature resistance also makes them suitable for use in the electronics industry as insulating materials or substrates for flexible circuits.
The glass transition temperature of PET typically ranges between 70°C and 80°C, and it can remain stable over a broad temperature range. This stability is advantageous in packaging applications, where PET films can withstand temperature fluctuations without becoming brittle or losing their flexibility.
The impressive thermal stability of PET is further demonstrated in its ability to withstand elevated storage temperatures. Empty PET bottles can be stored at temperatures above 38°C without significant shrinkage. However, it is important to note that extreme temperature conditions above 55°C can cause severe distortion in the amorphous areas of the bottle.
The thermal stability of PET is also evident in its recycling process. Recycled PET (rPET) has been shown to have comparable thermal stability and lifetime performance to virgin, unrecycled material. This indicates that products made with recycled PET feedstock will have similar thermal stability over their lifetime as those made with virgin material.
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PET's crystallinity determines its characteristics
Polyethylene terephthalate, commonly referred to as PET, is a semi-crystalline plastic polyester material with a global production volume of 83 Mt per year. PET is mainly used in textiles, but it is also widely used for packaging materials, notably plastic bottles, and is a major contributor to environmental plastic waste accumulation. The degree of crystallinity in PET can be determined by comparing the enthalpy of the cold crystallization peak with that of pure crystalline PET. The crystallinity of PET can be influenced by its processing conditions, thermal history, and the presence of additives.
Depending on its processing and thermal history, PET may exist as an amorphous (transparent) or semi-crystalline polymer. Amorphous PET is formed when the molecules are not given enough time to arrange themselves in an orderly, crystalline fashion during cooling. This can be achieved by rapidly cooling molten polymer below the glass transition temperature (Tg), which typically ranges between 70°C and 80°C. Amorphous PET is valued for its shape stability and electrical insulation properties, making it suitable for use in electronic components.
On the other hand, crystalline PET is known for its transparency and strength, and it is commonly used in bottles and films. Crystalline PET can be produced by allowing the molten polymer to cool slowly, giving the molecules time to arrange themselves in a crystalline structure. The degree of crystallinity in PET can also be increased by thermal or mechanical means, such as heating or stretching. However, it is important to note that the enzymatic degradation of PET becomes more challenging as the crystallinity increases.
The crystallinity of PET can be fine-tuned by modifying the processing conditions and adding different comonomers. For example, the addition of cyclohexanedimethanol (CHDM) interferes with crystallization and lowers the polymer's melting temperature, resulting in a clear amorphous thermoplastic known as PETG or PET-G. This modification makes PETG suitable for injection moulding, sheet extrusion, and 3D printing filament extrusion. In contrast, the use of small amounts of comonomers like isophthalic acid, CHDM, or diethylene glycol (DEG) can slow down but not prevent crystallization, making it possible to produce clear and crystalline bottles through stretch blow moulding.
Overall, the crystallinity of PET plays a crucial role in determining its characteristics and applications. By adjusting the degree of crystallinity, manufacturers can optimize the mechanical and optical properties of the final product, as well as its degradation behaviour. This makes PET a versatile material for various industrial and commercial applications.
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Frequently asked questions
PET plastic can withstand temperature fluctuations without becoming brittle or losing flexibility. However, elevated storage temperatures above 38°C (100°F) can cause empty PET bottles to shrink due to the relaxation of oriented regions. Extreme temperatures above 55°C (131°F) can severely distort the amorphous areas of the bottle.
When shipping empty PET bottles through areas with outdoor temperatures exceeding 32°C (90°F), it is recommended to use a temperature-controlled container capable of maintaining a temperature of 27°C (80°F) or lower.
The glass transition temperature of PET plastic is around 80°C, followed by a cold crystallization of the amorphous parts starting at approximately 148°C.
The melting point of PET plastic ranges from 225°C to 255°C, depending on its processing and thermal history.
Crystallinity plays a significant role in determining the characteristics of PET plastic. Amorphous PET is valued for its shape stability and electrical insulation properties, while crystalline PET is known for its transparency and strength.
