Thermal Properties of Plastics | The Ultimate Guide

 What are the Thermal Properties of Plastics?

Thermal properties in plastics mean their response to temperature changes and how they react when applied to heat. As a solid material absorbs heat, its temperature rises with a slight increase in dimensions.

There are generally 4 thermal properties in plastic materials – heat deflection temperature, glass transition temperature, Continuous Service Temperature, and Coefficient of Thermal Expansion.

Here is a table explaining the plastic thermal properties.

Materials	Max Continus Servoce temprature in air	Heat Deflection Temperature	Coefficient of Linear Thermal Expansion
		°F, 264/psi
ASTM Test	°F	D648	in/in/°F x 10-5, D696
ABS (Acrylonitrile-Butadiene-Styrene)	160	177	5.6
Acetal	212	257	6.8
Acrylic	160	195	4
CAB	–	181	–
ECTFE (Ethylene Chlorotrifluoroethylene)	302	160	5
ETFE (Ethylene Tetrafluoroethylene)	311	–	7.4
HDPE	170	–	9
High Impact Polystyrene	–	169	4.5
LDPE	–	–	–
Nylon	210	194	4.5
PAI (Polyamide-imide)	500`	532	1.7
PBT	245	130	–
PEEK	480	306	2.6
PET	230	175	3.9
PETG	–	157	3.8
Polycarbonate	240	270	3.8
Polypropylene	180	–	5
PTFE	500	–	8.9
PVC	140	158	3.2
PVDF	302	235	7.1
TPE	–	–	–
UHMW	180	–	11.1
Ultem®	338	392	3.1

Plastic Thermal Properties

• Glass Transition Temprature 
• Continues Service Temprature 
• Heat Deflection Temperature
• Coefficient of Linear Thermal Expansion 

Glass Transition Temprature

When an amorphous material is heated, the temperature at which the material transforms into a viscous liquid is called the glass transition temperature.

Crystalline Polymers

Crystalline materials don’t have a specific glass transition temperature. Instead, they have a melting point. Their melting point is a temperature where an ordered molecular structure becomes disordered liquid.

Crystalline materials have a highly rated and defined molecular structure.

It includes materials like PEEK, PPS, PFA, etc. These polymers have regular chain structures and are more likely to form crystalline regions.

More crystallinity means more strength and less flexibility for the materials. High crystallinity also means that less light will pass through these materials.

Crystallinity will provide benefits like chemical resistance, strength, stability, and stiffness.

Amorphous Polymers

Amorphous polymers are also known for not having a melting point but a glass transition temperature. These polymers soften upon heating over a wide range of temperatures. Their molecular structure comprises unsynchronized molecular chains not arranged in ordered crystals.; however, they have scattered around even though they are not solid-state.

Semi-crystalline Polymers

There are polymers with a partially crystalline structure, with the crystalline regions spread within the amorphous material. The crystalline molecules have a melting point, and the amorphous molecules have a glass transition temperature.

Interesting Read – How is Plastic Made? A Simple and Detailed Explanation.

Continues Service Temprature

Continuous service temperature is when a material can perform assuredly in a long-term application. Over the temperature limit, the mechanical and electrical properties of the material will start degrading over time.

The crucial factors affecting the continuous service temperature of many materials are – time and loading levels and additives and reinforcements used in the formations.

The most common method for comparing different materials in terms of continuous service temperatures is the Underwriter Laboratory (UL) Relative Thermal Index or RTI.

The process of RTI is utilized to figure out the loss of properties of plastic versus time. Generally speaking, when plastic exhibits maximum continuous temperature – robust and long-term performance is observed.

In a nutshell, continuous service temperature backs up the materials’ integrity for their intended application period.

Let’s see the use of continuous service temperature in commonly used polymers:

Amorphous Polymers

Material	Max. Value (°C)	Min. Value (°C)	Glass transition temp. (°C)
PC	140	100	147
PEI	170	170	217
PMMA	90	70	105
PESU	185	175	230
PSU	180	150	190

The table above shows continuous temperatures’ maximum and minimum values and glass transition temperatures.

The glass transition temperature is directly proportional to the continuous use temperature for all the above amorphous polymers. A significant change in the mechanical and electrical properties of the polymer at Tg can be seen.

Semi-crystalline Polymers

Material	Max. Value (°C)	Min. Value (°C)	Glass transition temp. (°C)
PEEK	260	154	143
PET	140	80	69
PPS	220	200	126
PBT	140	80	40
PA6	120	80	50

The above table shows that the continuous service temperatures for all the polymers are more than the Tg value.

Heat Deflaction Temprature

Heat deflection temperature or heat distortion temperature is a way to measure the polymer’s resistance or withstanding capacity towards distortion at a given temperature.

It can be any particular temperature at which the test bar will be warped by 0.35 mm under a given load (0.35 is a random value and has no significance).

The factors making HDT significant are

1. Used in many characteristics products design and manufacturing of parts using thermoplastic components.
2. Provides a value to be compared within different materials
3. The greater the heat deflection, the higher the chances of a faster molding process in the injection molding method.

Engaging Read – Mechanical Properties of Plastic Materials | The Definitive Guide

Coefficient of Linear Thermal Expansion

The coefficient of linear thermal expansion is a polymer attribute that comprises the ability of a plastic to expand under temperature elevation. It shows us the dimensional stability of a developed part under temperature variation.

The linear coefficiency is measured using the following formula:

α = ΔL / (L0 * ΔT)

Thermal expansion and differences can be detrimental and develop internal stresses and unusual warping in the material, which can hurt the integrity of the final part, thus making CLTE crucial for the unit economies of production and the functioning and aesthetics of the product. Below are some applications:

1. It assists in determining the dimensional characteristics of parts subject to temperature changes.
2. It helps maintain a product’s aesthetic by predicting shrinkage in injection molded components.
3. It also comes in handy for predicting the thermal stresses that can occur while bonding plastic material with metals.

Below is the table with CLTE value for all mainstream plastics

Material	Max. Value (10-5 /°C)	Min. value (10-5 /°C)
ABS	15	7
CA	18	8
CAB	17	10
ECTFE	9	6
EVA	20	16
HDPE	11	6
HIPS	20	5
LDPE	20	10
PA 6	12	5
PBT	10	6
PC	4	2
PE	5	5
PEEK	10.8	4.7
PEI	6	5
PET	8	6
PETG	8	8
PMMA	9	5
PP	17	7
PS – Crystal	8	5
PTFE	20	7
PVC	4	2
UHMWPE	20	13
XLPE	10	10

All the values shown in all the tables in the post are derived after extensive research, but they are for information purposes only. Do consult your manufacturer for accurate values.

Suggested Read – 

• What is Compressive Strength of Plastics | The Complete Guide
• Physical Properties of Plastic Materials | The Complete Guide
• What is Hardness in Plastics? An In-Depth Guide
• What is Shore Hardness Scale? | The Complete Guide
• What is Processing and Drying Temperatures of Plastics

FAQs

Below are the frequently asked questions on thermal plastics. Let’s dig deep to know more.

Is plastic a good thermal insulator?

Plastics possess outstanding insulating properties, allowing them to effectively retain heat – a feature that can benefit objects like a coffee cup sleeve.

What plastic is the best thermal insulator?

Polybenzimidazole (PBI) is the leading engineering thermoplastic with exceptional heat and wear resistance, strength, and mechanical property stability. PBI fibers are also noteworthy for their lack of a known melting point, non-flammable nature, and inability to adhere to other plastics.

Does plastic hold heat better than metal?

Metals typically have high thermal conductivity and are more responsive to temperature changes in their environment than plastics or foams. In contrast, plastics are considered insulators and respond very slowly to changes in the surrounding temperature.

Final Words 

To sum up, comprehending plastic thermal properties is vital to their successful implementation in various industries. These materials can display diverse thermal behaviors, such as thermal conductivity, specific heat, and coefficient of thermal expansion, which are shaped by factors like their chemical composition, structure, and processing techniques.

Precisely anticipating and controlling these thermal properties is indispensable to guaranteeing plastic products’ optimal functionality and robustness.

Thanks to continuous research and progress in plastic technology, we can look forward to observing even more ingenious uses for these multipurpose materials. Nonetheless, it is also critical to acknowledge plastic manufacturing and disposal’s ecological impact and strive for more sustainable substitutes.

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