Crosslinking

Crosslinking takes place by introducing bonds between polymer chains. This is done through chemical reactions activated by heat, pressure or radiation. There are two main types of crosslinking:

1. Chemical crosslinking

Stable chemical bonds are formed between the polymer chains. Examples are:

• Sulphur crosslinking: typical of rubber vulcanisation, in which sulphur bridges are formed between the chains.

• Peroxide crosslinking: use of peroxides to trigger radical reactions.

• Epoxy crosslinking: use of hardeners to crosslink epoxy resins.

2. Physical crosslinking

This process is based on physical interactions without chemical bonds being formed. Examples are:

• Crystalline crosslinking: formation of crystalline structures in semi-crystalline plastics.

• Ionic bonding: forces of attraction between charged molecules.

Depending on the method and material, crosslinking can be divided into different categories:

Type	Process	Typical applications
Sulphur crosslinking	Vulcanisation with sulphur or sulphur compounds	Sheet material, moulded parts, extrusions, seals
Peroxide crosslinking	Radical reactions with peroxides	Heat-resistant rubber products
Siloxane crosslinking	Condensation reactions of silicone resins	Medical technology, sealants
UV crosslinking	Initiation by UV light	Paints, coatings
Electron beam crosslinking	Radiation-induced crosslinking	Cable insulation, engineering plastics

Crosslinking changes the properties of a material considerably. Typical characteristics are:

• Higher strength: crosslinked materials are mechanically more stable.

• Elasticity: elastomers retain their shape even after stretching.

• Thermal stability: better resistance to high temperatures.

• Chemical resistance: resistance to solvents and chemicals.

• Ageing resistance: less susceptible to environmental influences such as UV radiation.

Crosslinking is a central process in many industries:

Rubber industry

• Tyre manufacturing: vulcanisation to increase abrasion resistance and elasticity.

• Seals: crosslinked elastomers for durable, flexible sealing systems.

Plastics industry

• Cable insulation: crosslinked plastics such as polyethylene offer excellent electrical insulation.

• Engineering plastics: production of heavy-duty components.

Coatings and paints

• UV-curing coatings: fast-curing surfaces for industrial applications.

• Corrosion protection: crosslinked epoxy coatings for metals.

Medical technology

• Silicone products: crosslinked silicones for implants and medical devices.

• Prostheses: highly flexible, durable materials.

Crosslinking offers numerous advantages:

• Higher load-bearing capacity: suitable for demanding mechanical and thermal applications.

• Optimised material properties: combination of flexibility and stability.

• Longer service life: high resistance to environmental influences and wear.

Crosslinking also brings challenges:

• Process control: temperature, pressure and time must be controlled precisely.

• Material compatibility: not all polymers are suitable for every crosslinking method.

• Costs: high-quality crosslinking processes and additives can be expensive.

• Which materials can be crosslinked? Elastomers, plastics, silicones and resins are typical materials for crosslinking.

• How does crosslinking affect the properties of rubber? Vulcanisation improves elasticity, strength and resistance to heat and chemicals.

• What is the difference between chemical and physical crosslinking? Chemical crosslinking forms stable bonds, while physical crosslinking is based on intermolecular interactions.

• Is crosslinking reversible? Chemical crosslinking is usually irreversible, but physical crosslinking can be reversed by heat or solvents.

Crosslinking is an essential process for improving the mechanical, thermal and chemical properties of materials. It enables high-performance products to be manufactured in numerous industries, from rubber and plastics to coatings and medical applications. By choosing the right crosslinking method, material properties can be optimised to meet specific requirements.