Precise material refinement: Crosslinking methods for plastic upgrades

Optimization of thermoplastic materials

Unlike crosslinked plastics, non-crosslinked plastics, also known as thermoplastics, are meltable and soluble and therefore cannot withstand high temperatures.
To make thermoplastics usable under precisely these conditions and to optimize their properties, they are crosslinked. This gives technical or standard plastics mechanical, thermal, and chemical properties that are comparable of high-performance plastics. The three most common processes for this type of plastic refinement are peroxide crosslinking, silane crosslinking, and radiation crosslinking. In this article, we provide a structured overview of the types of crosslinking, their advantages and disadvantages, and their areas of application in the field of cable, tube, and pipe crosslinking.

The three relevant types of crosslinking at a glance

The crosslinking methods used in thermoplastic finishing for extruded products differ primarily in terms of their degree of crosslinking, durability, and production rate. Among crosslinked thermoplastics, crosslinked polyethylene, designated PE-X in accordance with DIN 16893, occupies a special position. The crosslinking of PE leads to a significant improvement in important material properties such as creep resistance, stress crack resistance, and impact strength at low temperatures. In addition, PE-X can withstand temperatures of up to 250 °C for short periods.

The most common types of crosslinking for PE and other thermoplastics are:

• PE-Xa: peroxide-crosslinked polyethylene

• PE-Xb: silane-crosslinked polyethylene

• PE-Xc: radiation-crosslinked polyethylene

Below, we present the crosslinking methods and their influence on the development of durable and high-performance materials in detail.

Peroxide crosslinking (PE-Xa)

In peroxide crosslinking, polyethylene is crosslinked in a targeted manner using peroxides such as dicumyl peroxide. The peroxides decompose at high temperatures, forming radicals and triggering thermally initiated radical crosslinking. This takes place in the melt, i.e. in the completely amorphous state of the polyethylene. This results in lower crystallinity and density compared to other processes, as crystallization is impeded by the crosslinking sites. Peroxide-crosslinked PE is defined as PE-Xa.

The advantages of this process lie in its high thermal stability (up to 250 °C) and high degree of crosslinking. In addition, stable C-C bonds are formed between the polymer chains, significantly improving the mechanical properties of the material. Peroxide crosslinking is currently the only way to make even very large diameter pipes (90–550 mm) resistant.

The disadvantages include high pressure and energy requirements as well as limitations in terms of crystallinity and density, which make peroxide crosslinking unsuitable for certain applications.

Silane crosslinking (PE-Xb)

Silane crosslinking is a two-step process: First, peroxides are used to create reactive sites in the polymer chain to which silane molecules attach. In the second step, the actual crosslinking takes place through a polycondensation reaction, which occurs under the influence of water and a catalyst through the formation of multifunctional Si-O-Si bonds. This process does not take place in the melting phase, but after shaping, typically in hot water or steam at 80–95 °C.

A major advantage of silane crosslinking is that the crystallinity and density of the starting material are largely retained, as crystallization is already complete before crosslinking. The process is versatile and is also used for other polymers such as PVC, PP, or polyamide.
Disadvantages compared to other crosslinking methods are the longer process times, which can sometimes last several hours, and the need for moisture and catalysts for crosslinking. In addition, the chemical resistance of silane crosslinking is lower than that of PE-Xa or PE-Xc.

Radiation crosslinking (PE-Xc)

Radiation crosslinking can be used to crosslink conventional engineering plastics and enhance their properties. In this process, crosslinking is triggered after shaping by high-energy electron beams (1–10 MeV). The radiation breaks chemical bonds in the polymer chains, creating reactive radicals that combine to form three-dimensional networks. Crosslinking takes place below the crystallite melting temperature, typically at room temperature, so that the crystallinity and density of the material are retained. Typical thermoplastics that can be enhanced by radiation crosslinking include PE, TPE, PBT, PA, and many others. Depending on the material selected, a crosslinking additive may be required for the process.

The advantages of radiation crosslinking result primarily from its unique physical principle of operation: the energy required to trigger chemical reactions is introduced via ionizing electron beams that can penetrate deep into the material. This creates highly reactive radicals inside the plastic without exposing the product to high temperatures or pressures. This enables particularly gentle, precise, and controllable crosslinking, even with temperature-sensitive components.

At the same time, the crystallinity and density of the starting material are largely retained, as crosslinking only takes place after the material has been completely shaped. The refined materials exhibit improved heat resistance, increased stress crack and chemical resistance, and higher mechanical strength.

The process is limited by the product diameter or cross-section of the cables, pipes and tubes, which can restrict its range of applications.

What are the properties of radiation crosslinked thermoplastics?

Radiation crosslinking gives thermoplastics a targeted structural upgrade. The formation of a three-dimensional network within the plastic leads to a significant improvement in key material properties such as mechanical strength, thermal stability, and chemical resistance.

Improvement of mechanical properties

Radiation crosslinked thermoplastics show a significant improvement in their mechanical strength. This offers advantages particularly with regard to long-term use and mechanical wear:

• Increased modules
• Improved stress crack resistance
• Increased strength, especially long-term strength
• Improved abrasion resistance and tear resistance
• Increased hardness
• Reduction of cold flow (creep)
• Reduction in elongation at break
• Optimized resilience (memory effect)

These improvements make crosslinked thermoplastics particularly suitable for mechanically demanding applications.

Improvement of thermal properties

A key feature of radiation crosslinked materials is their significantly increased heat resistance, which means they can also be exposed to higher temperatures:

• Increased heat resistance
• Increased thermal stability under pressure and temperature

Compared to non-crosslinked plastics, radiation crosslinked materials are less flammable, which is a clear advantage in terms of fire behavior. At the same time, they offer a significant increase in performance under thermal stress.

Improvement of chemical properties

The chemical structural change caused by radiation crosslinking leads to a gradual increase in resistance to aggressive media:

• Improved resistance to chemicals and solvents
• Reduced solubility
• Reduced swelling behavior

The properties resulting from the modification enable crosslinked thermoplastics to be used in a wide range of applications. These primarily include the Automotive and E-mobility, Infrastructure and building technology, Electrical and electronics, and Mechanical engineering industries.

High-voltage cables play a central role in modern vehicles: they transport electrical energy at voltages ranging from 400 to 1500 volts and are exposed to mechanical, thermal, and chemical stresses. High temperatures, vibrations, mechanical stress, and resistance to various media place extreme demands on the material used. In addition, high flexibility, flame resistance, and good aging properties are required.
Radiation crosslinked thermoplastics provide a solution here. Targeted crosslinking at the molecular level makes the cable insulation significantly more robust. Specifically, the optimized performance of the materials is reflected in the following properties:

1. Increased thermal resistance: Heat resistance is increased, ensuring reliable operation even at high operating temperatures.
2. Optimized aging behavior: Aging behavior is improved, which extends the service life of the components.
3. Greater durability and strength: Resistance to chemicals, moisture, stress cracks, and mechanical abrasion is optimized.

Thanks to these properties, radiation crosslinked high-voltage cables distribute the required energy safely and reliably throughout the vehicle.

Conclusion: Crosslinked plastics as the key to high-performance materials

Crosslinked thermoplastics open up new possibilities for adapting the properties of engineering plastics to demanding application conditions. Whether through peroxide crosslinking, silane crosslinking, or radiation crosslinking, these processes enable significant improvements in thermal resistance, chemical resistance, and mechanical robustness, giving standard plastics a real upgrade. Networked materials are becoming increasingly important, especially in light of current market developments :

“In the industry, we are seeing a clear trend towards cables with larger cross-sections and thicker-walled tubes. Higher performance requirements, the focus on energy efficiency, and the expansion of electrified infrastructures will require even more robust cables in the future. New safety standards are also driving this development. BGS is well prepared for these changes: With state-of-the-art plant technology, we can efficiently radiation crosslink even large-dimension products.”