Degree of crosslinking in plastics: Optimizing material properties
Lightweight and high-performance materials are essential in many industries in order to meet the ever-growing demands for efficiency, resource conservation, and sustainability in production and product design. Whether in the automotive industry, electrical engineering, mechanical engineering, or medical technology, plastics offer enormous potential and enable innovative design approaches. However, they are still only suitable for many applications to a limited extent, as they have to withstand extreme conditions such as high temperatures, chemical exposure, or mechanical wear.
BGS uses the radiation crosslinking process on thermoplastics to transform conventional plastics and give them properties of high-performance plastics: a true upgrade through radiation crosslinking. This process specifically enhances key material properties:
- increased temperature and heat resistance,
- improved mechanical strength and creep resistance,
- optimized friction and wear behavior (tribology),
- increased chemical and media resistance, and
- functionality without lubricants.
The aim of the refinement process is to specifically optimize the degree of crosslinking of the plastics in order to achieve an optimal property profile at economically acceptable costs.
Definition: What does degree of crosslinking mean?
The degree of crosslinking of plastics describes the relative proportion of polymer chains that are crosslinked to each other by covalent chemical bonds. A high degree of crosslinking means that a large number of reactive sites in the polymer chains have formed chemical bonds, resulting in the formation of a crosslinked three-dimensional network. With a low degree of crosslinking, there are fewer bonds and thus a less tightly knit polymer network.
The degree of crosslinking has a direct influence on the material’s mechanical, elastic, thermal, and chemical properties: materials with a higher degree of crosslinking are stiffer, more heat-resistant, and less deformable; those with a lower degree of crosslinking are more flexible and elastic.
How does the degree of crosslinking of polymers influence the material properties?
The degree of crosslinking significantly determines how a plastic behaves under mechanical, thermal, and chemical stress. Depending on how strongly the polymer chains are linked together, properties such as modulus of elasticity, temperature resistance, and melting behavior change.
Low degree of crosslinking
With a low degree of crosslinking, only a few polymer chains are chemically bonded to each other. The molecular chains can move relatively freely against each other, which means that the material remains flexible and easily deformable. Plastics with a low degree of crosslinking can usually be melted and reprocessed – typical polymers are thermoplastics. They are suitable wherever good formability and easy processing are more important than high temperature or chemical resistance.
Medium degree of crosslinking
With a medium degree of crosslinking, the chains are already partially linked together. The material remains elastic but exhibits significantly higher dimensional stability than not crosslinked plastics. This structure is characteristic of elastomers: they can be stretched but return to their original shape when the load is removed. At the same time, resistance to heat and mechanical stress increases without completely losing flexibility.
High degree of crosslinking
A high degree of crosslinking results in a dense, three-dimensional polymer network. The material is very strong, hard, and retains its shape even at high temperatures. Plastics with a high degree of crosslinking, such as thermosets, often exhibit high heat resistance and chemical stability, but are also often very brittle, which has a negative effect on their mechanical properties. They are used when components have to withstand extreme conditions, such as in automotive or electrical engineering.
Low degree of crosslinking | High degree of crosslinking |
|---|---|
Polymer chains can slide past each other, melting possible | Melting no longer possible, material degrades at high temperatures |
More flexible deformation, often reversible | Less elastic, stiffer, brittle or hard in some materials |
Higher creep under load at temperature | Less creep, better long-term stability under load and heat |
Faster wear, especially in amorphous areas, higher abrasion | Improved surface, less wear, better sliding properties |
Higher chemical solubility, stronger swelling | Reduced solubility and swelling behavior, better resistance to aggressive media |
Radiation crosslinking can be used to specifically optimize the degree of crosslinking of many thermoplastics and elastomers (overview of radiation crosslinkable polymers). Since radiation crosslinking is carried out after molding as the final process step, all the advantages of processing thermoplastics can still be exploited.
Although the achievable degree of crosslinking is lower than that of thermosets and their high dimensional and heat stability is not fully achieved, radiation crosslinking leads to a significant improvement in material properties. This results in materials with increased temperature resistance, improved creep resistance, and increased chemical resistance. The resulting property profile is thus between thermoplastics and thermosets and combines their advantages without taking on the typical disadvantages of brittle thermosets.
Classification of plastics according to degree of crosslinking
Based on the degree of crosslinking, polymers can be divided into three main classes:
Thermoplastics
Thermoplastics consist of long, linear or only slightly branched polymer chains that are not crosslinked with each other. This also results in their special properties. Thermoplastics are characterized by the fact that they can be reversibly melted and thus processed in a variety of thermal forming processes.
Thermoplastics have only loose cross-links or none at all. The polymer chains are physically intertwined, but can flow viscously and deform at higher temperatures and are meltable at high temperatures.
Radiation crosslinked thermoplastics
Radiation crosslinked thermoplastics are a special type of modified thermoplastics. Treatment with ionizing radiation (e.g., electron or gamma radiation) creates additional chemical bonds between the polymer chains. These bonds lead to partial crosslinking, which makes the material more thermally stable and mechanically resilient without completely losing the basic structure of a thermoplastic. Radiation crosslinking usually takes place after molding, so that the advantages in processing can be combined with the property upgrade achieved by radiation crosslinking. Radiation crosslinked thermoplastics can no longer be melted.
Elastomers
Elastomers are loosely crosslinked. This means that there are many cross-links, but they are not so dense that the material becomes hard and brittle. Elastomers exhibit high reversible deformability and react elastically under load. Certain thermoplastic elastomers can also be radiation crosslinked and thus specifically modified in their property profile.
Thermosets
Thermosets have a very high degree of crosslinking. All chains are connected, creating a single hard and non-meltable polymer network.
Degree of crosslinking in production: What matters when it comes to crosslinking
There is no general answer to the question of what degree of crosslinking is right, as it depends on several factors. These include, for example:
- Raw material: Not all polymers are equally suitable for crosslinking. The degree of crosslinking depends, among other things, on the availability of reactive groups, the crystalline and amorphous components, and thus crucially on the base material (polyethylene, polyamide, etc.).
- Radiation cross-linkability: Some polymers require monomeric additives, known as crosslinking additives, to enable refinement by radiation.
- Desired properties: Which material properties need to be improved? Temperature resistance, abrasion, friction, strength, creep behavior, chemical resistance? The optimum degree of crosslinking can vary depending on the intended use.
“Radiation crosslinking allows the limits of conventional engineering plastics to be specifically expanded. The process improves their resistance to heat, chemicals, and mechanical stress, thus not only creating an economical alternative to expensive high-performance plastics, but also opening up entirely new fields of application.”