Influence of crystallinity on the material properties of plastics

Materials are now indispensable in almost every branch of industry – from automotive engineering and medical technology to the packaging industry. Their versatility is based on the targeted control of their internal structure. An important parameter here is the crystallinity of plastics. It describes the proportion or ratio of ordered crystalline to disordered amorphous regions within the polymer structure.

Even small differences in the degree of crystallinity of a plastic can influence mechanical stability, heat deflection resistance, density, transparency, dimensional accuracy and chemical resistance. Anyone who knows these relationships can select materials in a more targeted way and partly influence the material properties through processing.

What is meant by crystallinity?

Polymers consist of long chain molecules that can organise themselves differently at the microscopic level. In crystalline regions, parts of the polymer chains are arranged regularly. In amorphous regions, by contrast, the molecular chains are largely disordered.

Technically relevant semi-crystalline plastics contain both structures at the same time: crystalline and amorphous regions. You can find a detailed comparison of the two material groups under amorphous and semi-crystalline plastics.

What does degree of crystallinity mean?

The degree of crystallinity, often also called the degree of crystallisation, describes what proportion of a semi-crystalline plastic is present in a crystalline structure.

The value is not an invariable material constant. It is influenced by, among other things:

• the chemical and molecular structure of the polymer
• the cooling rate from the melt
• mould and processing temperatures
• additives and nucleating agents
• orientation of the polymer chains during processing
• thermal post-treatments

As a result, components made from the same base polymer can have different degrees of crystallinity, and therefore different properties, depending on processing and thermal history.

As crystallinity increases, the density of the material rises in many cases, because the molecular chains are packed more densely. At the same time, strength, stiffness, dimensional stability, chemical resistance and optical properties change, among other things.

Influence on mechanical properties

The mechanical properties of a plastic are directly related to its degree of crystallinity. As the crystalline proportion increases, strength and stiffness can rise, because the polymer chains in the crystalline regions are arranged more densely and regularly.

In this way, components made from highly semi-crystalline plastics can withstand high mechanical loads and exhibit a low tendency to deform. Hardness can also increase with a rising degree of crystallinity, which can be advantageous for example in gears, plain bearings or technical guide elements.

At the same time, a high degree of crystallinity can reduce deformability and impact strength. The properties of a component therefore always result from the interaction of crystalline and amorphous regions.

Influence on thermal properties

The thermal properties of a plastic are closely linked to its structure. Semi-crystalline plastics possess, in addition to a glass transition temperature, a pronounced melting temperature of the crystalline regions.

A higher degree of crystallinity can help a plastic to remain dimensionally stable for longer as the temperature rises. For technical applications, therefore, not only the type of polymer plays a role, but also the crystal structure formed during production.

Particularly in the case of thermally loaded components, the actual state of the material should be taken into account.

Influence on optical properties

The arrangement of the molecular chains has a direct effect on light transmission. Interfaces between crystalline and amorphous regions can scatter visible light. This is why many semi-crystalline plastics appear milky, translucent or opaque.

As the degree of crystallinity increases, transparency can decrease further. However, the optical properties also depend on crystallite size, polymer structure, wall thickness and processing.

Amorphous plastics such as polycarbonate or PMMA, by contrast, can achieve high transparency, since pronounced crystalline structures are absent.

Influence on density and chemical resistance

As crystallinity increases, the polymer chains in the crystalline regions are packed more densely. As a result, the density of the material typically rises.

The denser structure can also make it more difficult for certain media to penetrate. A higher degree of crystallinity can therefore contribute to improved chemical resistance and lower permeability in many materials.

However, the actual resistance of a plastic also depends substantially on the type of polymer, the medium, concentration, temperature and duration of exposure.

Influence on barrier properties

Crystalline regions can hinder the transport of small molecules through a plastic. Gases or liquids must thereby predominantly travel along paths through the amorphous regions of the material.

A higher degree of crystallinity can therefore reduce permeability and improve the barrier effect. This relationship plays a role, among other things, in packaging and in technical applications with requirements for gas or liquid tightness.

Processing and factors influencing crystallisation

The crystallinity of a plastic is not determined exclusively by its chemical structure. Processing also has a considerable influence on the formation of the crystalline structure.

• Cooling rate: Slow cooling gives the polymer chains more time to arrange themselves in an ordered way. Very rapid cooling, by contrast, can limit crystallisation.
• Temperature control: Mould, melt and component temperature influence nucleation and crystal growth.
• Nucleating agents and fillers: Certain additives can provide additional crystallisation nuclei and alter the crystallisation process.
• Flow and orientation: In extrusion or injection moulding, shear and orientation of the polymer chains can influence the crystal structure.
• Thermal post-treatment: Through targeted annealing, further crystallisation can take place in suitable semi-crystalline plastics.

Particularly in thick-walled semi-finished products or components, different cooling rates between the edge and core regions can lead to local differences in the degree of crystallinity.

Post-crystallisation in plastics

Post-crystallisation in plastics refers to the formation of further crystalline regions after the original shaping and cooling process.

This can occur when a semi-crystalline plastic has not reached the degree of crystallinity possible under the given conditions during production. If the polymer chains later become sufficiently mobile again through elevated temperatures, additional ordered structures can form.

Targeted post-crystallisation is often supported by thermal post-treatment or annealing.

Depending on the plastic, the following properties can change, among others:

• degree of crystallinity
• stiffness and strength
• density
• heat deflection resistance
• chemical resistance
• permeability
• dimensional stability
• shrinkage and warpage

The last point in particular is relevant for precise technical components. A change in the crystal structure is accompanied by volume changes and can therefore cause dimensional changes or warpage. You can find further background in the article warpage in plastic components – causes and solutions.

Whether post-crystallisation is worthwhile and what temperature control is required depends on the respective polymer, the initial state of the semi-finished product and the required component properties.

Crystallisation in polyolefins

Technically important polyolefins include in particular polyethylene (PE) and polypropylene (PP). Both material groups can form semi-crystalline structures.

How strongly a polyolefin crystallises depends, among other things, on its molecular structure and the processing conditions. In polyethylene, for example, the degree of branching of the polymer chains influences how densely these can arrange themselves. In polypropylene, the spatial arrangement of the side groups also plays an important role.

In polyolefins too, the degree of crystallinity and the cooling conditions affect, among other things, stiffness, density, shrinkage and optical properties.

Examples of semi-crystalline plastics

Typical semi-crystalline engineering plastics are:

• polyethylene (PE)
• polypropylene (PP)
• polyamide (PA)
• polyoxymethylene (POM)
• polyethylene terephthalate (PET)
• polybutylene terephthalate (PBT)
• PEEK

How high the actual degree of crystallinity turns out to be depends on the material and processing. Therefore, a distinction should not be made only between amorphous and semi-crystalline. For the behaviour of a specific component, it is also relevant how strongly the material has actually crystallised.

In conclusion

The degree of crystallinity influences numerous properties of engineering plastics. As crystallinity increases, stiffness, strength, density, heat deflection resistance, chemical resistance and dimensional stability can increase, among other things. At the same time, transparency and deformability can decrease.

Alongside the polymer itself, it is above all the thermal and processing history that determines the actual structure. Cooling, temperature control, additives and possible post-crystallisation should therefore be considered together in demanding applications.

For material selection, this means: it is not the name of the plastic alone that decides its later properties. The degree of crystallinity and processing can also be relevant to the performance of a component.

Frequently asked questions

What does crystallinity mean in plastics?

Crystallinity describes the proportion of ordered crystalline regions within a plastic. Semi-crystalline plastics possess crystalline and amorphous structures at the same time.

What is the degree of crystallinity?

The degree of crystallinity, or degree of crystallisation, indicates how large the crystalline proportion of a semi-crystalline plastic is. It depends both on the polymer structure and on the processing conditions.

What are crystalline or semi-crystalline plastics?

In semi-crystalline plastics, parts of the polymer chains are arranged regularly, while other regions remain amorphous. Fully crystalline engineering polymers are not the norm under normal conditions.

What does post-crystallisation mean in plastics?

In post-crystallisation, additional crystalline regions form after the original processing. The cause can be, for example, later thermal loading or a targeted heat treatment.

How does cooling influence crystallinity?

Slow cooling gives the polymer chains more time for ordered arrangement and can promote crystallisation. Rapid cooling can reduce the achievable degree of crystallinity.

Which properties change as crystallinity increases?

Depending on the material, stiffness, strength, density, heat deflection resistance and chemical resistance can increase, among other things. Transparency, elongation and impact strength, by contrast, can decrease.

Which plastics are semi-crystalline?

Typical examples are PE, PP, PA, POM, PET, PBT and PEEK. The actual degree of crystallinity differs depending on the material and processing.

Can annealing increase crystallinity?

In suitable semi-crystalline plastics, a controlled thermal post-treatment can enable further crystallisation. Possible dimensional changes, shrinkage and warpage must be taken into account.