Thermal Properties of Plastics | The Ultimate Guide

What are the Thermal Properties of Plastics?

Thermal properties in plastics mean their response to changes in their temperatures and how they react when applied with 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 thermal properties of all mainstream plastic materials.

Thermal Properties in Plastics Materials- 

Glass Transition Temprature 

Continues Service Temprature 

Heat Deflection Temperature

Coefficient of Linear Thermal Expansion 

1. 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 is ordered molecular structure becomes disordered liquid. Crystalline materials have a highly ordered 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.

2. Continues Service Temprature:

The temperature at which a material can perform assuredly in a long-term application is called continuous service temperature. Over the temperature limit, the mechanical and electrical properties of the material will start degrading over a period of time.

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

The most common method used for comparing different materials in terms of continuous service temperatures is called 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 – strong, long-term performance is observed.

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

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

Amorphous Polymers:

The above table shows the maximum and minimum values of continuous temperatures along with their glass transition temperature.

For all the above-mentioned amorphous polymers, the glass transition temperature is directly proportional to the continuous use temperature. There can be seen a significant change in the mechanical and electrical properties of the polymer at Tg.

Semi-crystalline Polymers:

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

3. Heat Deflaction Temprature:

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

In simple words, it can be any particular temperature at which the test bar will be warped of 0.35 mm under a given load (0.35 is a random value and doesn’t have any 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 withing different materials
3. The greater the heat deflection higher the chances of a faster molding process in the injection molding method.

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

4. 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 the production as well as 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 in maintaining the aesthetic of a product 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

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.

Final Words – 

That was my take on the thermal properties of plastics. Ensuring accurate thermal properties for your plastic materials can do wonders as far a the production economics is concerned.

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