Radiation
crosslinking
Plastic refinement through irradiation
Radiation
Plastic upgrade : Improving the properties of plastics
Radiation crosslinking is an established process for optimizing the range of properties of polymer materials using ionizing electron beam (E‑Beam) and gamma rays. For this purpose, the plastic is exposed to a defined dose of radiation in order to precisely control the crosslinking of the plastic molecules. Radiation crosslinking is similar to the well-known vulcanization of rubbers. As a physical process, irradiation offers the advantage that the crosslinking effects are already achieved at room temperature. The results can also be achieved precisely and without qualitative fluctuations compared to chemical processes. Another advantage of crosslinking with radiation is the speed of the process.
During radiation crosslinking, the material absorbs the energy introduced. As a result, chemical bonds are broken and free radicals are formed. In the next step, these form new molecular structures, creating new chemical bonds in the polymer matrix (radical crosslinking) , which form a three-dimensional irreversible network with improved properties.
The desired and predefined material properties can be set via the radiation dose that is introduced into the material. After irradiation, the plastic parts exhibit significantly improved properties in terms of
- heat resistance,
- stability,
- creep behavior and
- abrasion resistance.
The mechanical, thermal and chemical properties of standard plastics and engineering plastics are upgraded, so to speak, by radiation crosslinking: Their properties are optimized by radiation treatment to such an extent that they approach the property profile of high-performance plastics and can replace them in numerous applications.
radiation crosslinking: APPLICATION AREAS
Automotive & e-mobility
In numerous applications in combustion vehicles and electric cars, radiation crosslinking is an economical solution for expanding the range of applications of more cost-effective and already established materials (e.g. PA). Whether it is lightweight construction or dealing with high electrical currents, radiation crosslinking even allows the materials to meet complex requirements.
Infrastructure & building technology
Plastic pipes have been radiation crosslinked for decades to ensure their performance characteristics over very long periods of time. The areas of application range from district and local heating networks to surface and industrial heating systems, water supply and various indoor and outdoor applications. Shrink technology is also an important area of application for radiation crosslinked polyolefins.
Electrical & electronics
Plastics have long been an important basic material in the electrical industry due to their good insulating properties and almost unlimited shaping possibilities. Radiation crosslinking, for example, allows the thermal short-term application limits of PA or PBT to be significantly extended, so that the use of high-performance plastics is not necessary.
Mechanical engineering
The changed material profile and improved durability of radiationcrosslinked products is also expanding the fields of application in mechanical engineering: for example, metals in functional components can be replaced by radiationcrosslinked, injection-molded components made of plastic (e.g. PA or PBT). Among other things, this offers enormous potential for reducing the overall weight of components.
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CROSSLINKING OF PLASTICS : beta (e-beam) & GAMMA RAYS
Higher irradiation intensities (50 to 250 kGy) are required for the crosslinking of plastics than for sterilization (usually between 25 and 45 kGy). Therefore, the high dose rates of electron accelerators are mainly used to apply the required dose in economical irradiation times. In the case of radiation-sensitive materials, the irradiation time can also have a negative influence on the property spectrum depending on the dose rate – another reason for using E-Beam with a high dose rate and therefore short irradiation times. For compact components with high wall thickness gamma irradiation can also be used for crosslinking due to the greater penetration depth required.
PLASTIC UPGRADE : Material-related requirements
Radiation crosslinking is generally suitable for selected amorphous and semi-crystalline thermoplastics. Depending on the molecular structure, these can be radiation crosslinked without or with the use of crosslinking agents, whereby the amorphous and not the crystalline areas are usually crosslinked. In terms of quantity, the crosslinking of polyethylene (PE), polyamides (PA), thermoplastic elastomers (TPE) and polyvinyl chloride (PVC) is the most important. The use of biopolymers is also increasing. Bio-polyamides in particular have great potential for radiation crosslinking.
The range of properties that can be achieved depends on the material selection of the base polymer. If crosslinking additives are required, they can be added during the production of the plastic granulate, in the compounding process or as a master batch before molding. These additives enable or improve crosslinkability and can further optimize the property profiles of the plastic. More about the material selection.
At BGS, we are constantly working on the further development of crosslinkable polymers and crosslinking technology – both on the material side and on the process side. To this end, we work together with universities, institutes and various research networks. The current focus is on the development of new crosslinkable materials and alternative crosslinking additives.
PLASTIC PYRAMID : RADIATION CROSSLINKABLE POLYMERS
The pyramid represents technically relevant plastics – all polymers marked in bold are suitable for radiation crosslinking. They are based on inexpensive standard or engineering plastics that can achieve the mechanical, thermal and chemical properties of high-performance plastics with the help of crosslinking – in the illustration, these are gathered at the tip of the pyramid.
PROPERTY IMPROVEMENT : UPGRADING FOR PLASTICS
Radiation crosslinking gives inexpensive standard plastics or engineering plastics the mechanical, thermal and chemical properties that can come close to high-performance plastics. After radiation crosslinking, the plastics can be used under conditions that they would otherwise not be able to withstand. In short: radiation crosslinking is an upgrade of the material.
MATERIAL OPTIMIZATION : THERMAL PROPERTY
IMPROVEMENT
Thermoplastic materials become thermoelastic during radiation crosslinking at higher temperatures. The crosslinking reaction creates a molecular network that prevents the plastic from flowing. The improved temperature resistance and the significantly improved mechanical properties at higher temperatures are key characteristics of radiation crosslinking. For example, the moduli of non-crosslinked reinforced PA66 drop to practically zero above the melting temperature. In contrast, the significantly higher moduli of a crosslinked plastic guarantee sufficiently high strength even at temperatures of more than 350 °C. In addition, crosslinking leads to a reduced coefficient of thermal expansion and thus to greater dimensional stability. Furthermore, the creep resistance improves at higher temperatures.
MATERIAL OPTIMIZATION : MECHANICAL PROPERTY
IMPROVEMENT
Radiation crosslinking improves the mechanical strength of plastics, particularly at higher temperatures. The better adhesion of the fillers to the polymer matrix, caused by activation of the interfaces, contributes to this. The weld seam strength on vibration-welded components and the bond strength between material combinations (e.g. polymer/polymer and polymer/metal) are also increased through radiation crosslinking. Radiation crosslinked components can be an economical alternative to metallic materials or expensive polymers (PEEK, PAI, etc.).
MATERIAL OPTIMIZATION : TRIBOLOGICAL PROPERTY
IMPROVEMENT
An important selection criterion for machine elements made of plastic is their friction and wear behavior. At ever-higher operating temperatures, friction and wear shorten the service life of plain bearings and gears. As a rule, the increased amorphous parts of the material on the surface of plastic components due to the manufacturing process exhibit unfavorable wear behavior. However, it is precisely these amorphous areas that are particularly good for radiation crosslinking, which means that significantly higher sliding speeds can be achieved with a simultaneously reduced wear coefficient. For example, non-crosslinked polyamide has an application limit of 120°C when subjected to friction. Radiation crosslinking prevents the material from melting and increases the continuous operating temperature by up to 100°C while simultaneously reducing the wear rate. Gear wheels made of radiation crosslinked plastic can therefore replace metal components and are considerably lighter.
MATERIAL OPTIMIZATION : CHEMICAL PROPERTY
IMPROVEMENT
Radiation crosslinking increases resistance to aggressive media and hydrolysis. This is reflected, for example, in improved resistance to stress cracking and significantly reduced loss of strength after exposure to solvents. The crosslinking also significantly reduces solubility through solvents. This is used, for example, to determine the degree of crosslinking by means of the extraction test. The gel value determined in this way correlates directly with the degree of crosslinking.
Brochure radiation crosslinking
Our technical brochure answers central questions on technology, procedures and process integration.
OVERVIEW: ADVANTAGES OF radiation CROSSLINKING
Above the melting point of the base material, radiation crosslinked materials remain highly dimensionally stable when exposed to heat.
Through radiation crosslinking, higher mechanical load capacity and lower creep under load is achieved (if the working temperature is below the former melting temperature of the source material).
Radiation crosslinking improves resistance to aggressive media and many chemicals.
The wear behavior and tribological properties can be positively influenced by radiation crosslinking.
The shaping process (extrusion, injection molding) and the crosslinking process are decoupled and can therefore be optimized independently for the best possible process performance and quality.
Radiation crosslinkable standard and technical plastics enable the substitution of high-performance plastics – this simplifies the processing/shaping process, which has a positive effect on the overall economic efficiency (crosslinkable raw material, shaping, crosslinking, logistics).
Crosslinked plastics can replace metals in certain applications. This creates design freedom and makes shaping more flexible, cheaper and lighter.
“Cold” crosslinking = low temperature development during radiation crosslinking. The shaping state is virtually frozen and has a positive influence on dimensional stability during temperature change processes in the application.
Reduction or elimination of flame retardants, as radiation crosslinking improves flame retardancy.
Optimization of compound systems (multi-layer systems, including metal composites) due to the radiation capability at high energies.
By irradiating plastic granulate and other starting materials (e.g. fibers), the flow behavior can be specifically adjusted for the manufacturing process.
Radiation crosslinking promotes thermoforming and creates a memory effect or shape memory in shrink products such as hoses, films and molded parts.
RECYCLING: SUSTAINABILITY OF RADIATION
CROSSLINKING
Due to the improvement in material properties through radiation crosslinking, the corresponding plastic components are extremely resistant and can be used over a very long time. Radiation crosslinked pipes, for example, last 30 years and longer, and the same applies to components in cars – this reduces the use of resources and is an active contribution to greater sustainability.
Once one of the durable plastic parts has reached the end of its useful life, there are three processing options for radiation crosslinked materials, as there are for plastics in general: raw material (chemical), energy (thermal) and, within certain limits, material (physical or mechanical) recycling.
The material recycling of radiation crosslinked polyamides was recently the subject of a two-stage research project carried out by BGS together with partners. The project team’s approach was to obtain a valuable filler by grinding the radiation crosslinked, unmixed polymer materials, which is added to the virgin material together with a crosslinking additive in the compounding process. The results not only show the technical feasibility of recycling radiation crosslinked polyamides, but also demonstrate material savings and an improved CO2 balance by reducing virgin material. A further joint effort will be to identify and establish suitable material cycles in a circular economy model.
PRODUCTION PROCESS : RADIATION CROSSLINKING IN THE LOGISTICS CHAIN
A particular advantage of radiation crosslinking is that the finishing process takes place on the finished component after shaping (e.g. injection molding, extrusion and blow molding). As crosslinking at BGS is an external step in the production process, the optimum process speed is not affected by production requirements. Accordingly, there are no costs for the purchase of new tools or machines. The downstream irradiation of finished injection molded parts also has the advantage that sprue pieces, for example, can be easily fed back into the production process.
FAQ: RADIATION CROSSLINKING: FREQUENTLY ASKED QUESTIONS
Depending on the radiation dose applied, there is no significant heating of the products during irradiation. When higher doses are applied, the process can be optimized accordingly to avoid heating the material.
The energy of the rays is expressed in millions of electron volts (MeV), which describes the strength of the electric field used to accelerate the electrons. It is directly related to the penetration depth of the beams. It is limited to a maximum of 10 MeV by the design of our systems in order to prevent activation of the products. The radiation dose is expressed in the unit Gray (Gy), which determines the desired effect of the irradiation. A dose of 25 kGy is often sufficient for sterilization, while a dose of over 100 kGy is typical for crosslinking.
Products or raw materials can be delivered as packaged units, as bulk goods or as coiled continuous strands, profiles or tubes. We accept industrial packaging such as cartons on standard pallets, pallets, bulk goods in recyclable containers or coiled products on standard drums. We can also accept special shapes such as pipes on transport racks or other shapes – please speak to our experts. For safety reasons, we do not accept flammable liquids or pressurized containers at our facilities.
The usual form of delivery is Euro pallets (80 cm x 120 cm), although it is also possible to handle dimensions up to a maximum of 12.0 m x 1.6 m, depending on weight and composition.
BGS has developed equipment and maintenance systems that ensure high reliability. In most cases, redundant production facilities are available to ensure continuous high availability of our production capacity. The reproducibility of the E-Beam process is extremely high as all parameters are physical properties that are electrically controlled and documented.
We assure you that we will fulfill the dose and processing conditions for your product with the greatest reproducibility. However, we do not guarantee final product properties as these are dependent on a variety of parameters outside of our control. These include, for example, processing variations during manufacture and variations in raw material quality or composition over which we have no control.
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