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Irradiation of plastics
MATERIALS : REACTION TO RADIATION
The energy of the radiation triggers changes at a molecular level that cannot usually be generated in the materials in any other way. This opens up numerous possibilities for optimizing products in a wide range of industries. Materials in general can react very differently to radiation. Metals and most inorganic materials are not changed by radiation, although color changes can occur in transparent materials. Polymers undergo two fundamentally different reactions that occur in parallel and whose course is influenced by the chemical properties.
Crosslinking
The radiation generates free radicals in the polymer matrix, which are created by the breaking of chemical bonds. These radicals can recombine and form new chemical bonds. This leads to a three-dimensional polymer network with improved properties in terms of heat resistance as well as chemical and mechanical resistance. Some polymers are so inert that special crosslinking accelerators are required to achieve a sufficient level of crosslinking. They must be added to the polymer compound in small quantities.
Chain scission (degradation)
Some materials react in the form of chain scission (degradation). In this case, the molecular weight is reduced in relation to the radiation dose applied. As a result, the material loses its strength and becomes brittle. For some applications, this is an undesirable effect as it negatively influences the function. For other materials, this may be desirable, as the molecular weight can be adjusted very precisely so that melting flow and processing properties (rheology) can be specifically modified. The modification of starch, cellulose or polypropylene are important examples of the application of this effect.
MATERIALs: REFINEMENT OF POLYMER RAW MATERIALS
In principle, the changes caused by irradiation, such as crosslinking and molecular weight reduction, can also be used for polymer raw materials – for example plastic granulates. Applications include the targeted molecular weight build-up of ethylene (co)polymers or the introduction of long-chain branches to achieve higher processing viscosities or melt strengths.
POLYMERS AND BIO-POLYMERS : SUITABLE MATERIALS FOR RADIATION CROSSLINKING
In general, all plastics that can be crosslinked with radical initiators such as peroxides can be optimized by radiation crosslinking. In contrast to chemical crosslinking methods, however, radiation crosslinking takes place at low temperatures. The most frequently crosslinked plastics are those with the widest range of applications: polyethylene (PE) and its copolymers, polyamide (PA), thermoplastic elastomers (TPE) and polyvinyl chloride (PVC). Biopolymers (e.g. PA410, PA610, PA11, PA12) can also be radiation crosslinked and are increasingly being used. For some materials with low reactivity, a special crosslinking additive is required (see table below). These additives can either be added directly before molding, added as a master batch together with the raw granulate or used directly as a finished compound.
RADIATION CROSSLINKABLE POLYMERS: CROSSLINKING WITH AND WITHOUT ADDITIVES
Description | Crosslinking additive (Yes) | Crosslinking additive (No) | |
|---|---|---|---|
Thermoplastics | Polyethylene PE (LLDPE/LDPE/MDPE/HDPE/UHMWPE) | x | |
Polypropylene PP (homopolymers/ copolymers) | x | ||
Polyamides (PA6/PA66/PA11/PA12) | x | ||
Polybutylene terephthalate (PBT) | x | ||
Polyvinylidene fluoride (PVDF) | x | ||
Ethylene-tetrafluoroethylene (ETFE) | x | ||
Polyvinyl chloride PVC (plasticized PVC only) | x | ||
Ethylene vinyl acetate (EVA) | x | ||
Chlorinated polyethylene (PE-C) | x | ||
Thermoplastic elastomers | Polyether-ester block copolymer (TPE-E) | x | |
Polyurethane block copolymer (TPE-U) | x | ||
Polyether block amide (TPE-A) | x | ||
Elastomers | Styrene butadiene rubber (SBR) | x | |
Silicone rubber | x | ||
Biopolyamides | Biopolyamides (PA11/PA12/PA410/PA610) | x |
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Overview: RADIATION CROSSLINKABLE POLYMERS
When polyamides are radiation crosslinked, they can withstand significantly higher temperatures of up to 350°C and exhibit significantly improved wear behavior. Dimensional stability under thermal stress is also improved. Radiation crosslinked polyamide can often replace thermosets or more expensive high-performance plastics such as PPS, PEI, LCP, etc. Long-proven applications include radiation crosslinked components for the electrical and automotive industries – such as switching components or engine interior components – as well as components for mechanical engineering. The currently preferred types include PA 6, PA 66, PA 11 and PA 12. Polyamides must contain a special additive (crosslinking additive) to enable crosslinking.
The crosslinking of polyethylene (PE) extends the range of applications for this plastic for use at elevated temperatures or with high mechanical and chemical requirements. All types of polyethylene (PE-HD, PE-LD, PE-UHMW, etc.) and their copolymers (EPDM, EVA) can be radiation crosslinked. As a semi-crystalline material, PE is essentially crosslinked in the amorphous areas – the degree of crystallization and density remain virtually unchanged. Radiation crosslinked PE-Xc is a proven material for pipes and hoses used, for example, in underfloor heating systems and gas and water supply systems. However, other areas of application, such as transport crates and rotating components, also benefit from the significantly improved mechanical properties of irradiated PE.
The advantage of radiation crosslinking with PBT is a considerably higher heat resistance: this allows short-term temperature loads of up to 400 °C. An important area of application is the electrical industry, for example. Thanks to radiation crosslinking, thermosets can be replaced by thermoplastics, resulting in significant processing advantages.
The crosslinking of thermoplastic elastomers (TPO, TPC and TPA) is becoming increasingly important. The advantages are improved compression set and hot set values. In combination, this results in the advantage of easy processing of a TPE with the properties of an elastomer.
Due to their chemical structure, standard PU types cannot be crosslinked by irradiation. However, modified grades are available that can be easily crosslinked by irradiation. Please speak to your raw materials supplier about this.
Many new materials and copolymers have come onto the market in recent years. It is not possible to list all materials suitable for radiation crosslinking. Development in the field of biopolymers is also progressing steadily. In various R&D projects, we have already been able to prove that bio-based plastics, especially “drop-ins” such as Bio-PA and Bio-PE, can be radiation crosslinked. In principle, radiation crosslinking always works if chemical crosslinking with radical initiators (such as peroxides) is possible. Please contact our experts!
PLASTIC UPGRADE : PROPERTY IMPROVEMENT THROUGH RADIATION CROSSLINKING
Thermal property improvements
- Improvement of heat resistance
- Targeted adjustment of the thermal expansion (hot set/hot modulus)
- Improvement of the compression/tension set
- Increase in glow wire strength
- Increased resistance to ageing
Tribological property improvements
- Greater wear resistance
- More resistance to frictional heat: no melting of the sliding surface
- Increased dimensional accuracy
- Wide range of lubrication options
- Higher mechanical load capacity (transmitted torques/gears)
Chemical property improvements
- Reduction in solubility
- Improvement of the swelling behavior
- Increased resistance to stress cracking
- Improved hydrolysis and oil resistance
Mechanical property improvements
- Increase in modules
- Reduction of cold flow (creep)
- Improvement of the alternating bending strength
- Increased the weld line strength
- Improvement of creep behavior under internal pressure
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