Cross-links in Silicone
Silicone rubber is an inorganic polymer, it consists of silicon (Si), oxygen(O), carbon(C) and hydrogen(H). The main chemical chain is silicone and oxygen(-O-Si-O-), so we also call this compound as siloxane. Carbon and hydrogen form into lateral chains as methyl(-CH3), vinyl(-CH=CH2), phenyl(-C6H5) or other organic groups. Similar to all polymers, silicone rubber is created in a polymerization process from Si-OH monomers, which react with metal catalysts (Me3Si-O-SiMe3) to increase chain length, and end these chain with metal catalysts. After this polymerization process, the uncured silicone rubber contains different length backbone chains.
However, in order to obtain the final properties of silicone rubber. It is necessary to create chemical bonds between the backbones of -Si-O-Si-, which are cross-links. These crosslinks are from exothermic chemical reactions between the backbones and chemical crosslinker compounds. In silicone rubber, there are two general crosslinker compounds: peroxides, platinum or tin based catalysts.
Peroxide Catalytic Process
In peroxides catalytic process, peroxide is decomposed into highly reactive radicals at elevated temperature. Meanwhile, organic chains (carbon-hydrogen chains or lateral groups) have energy to create free space in backbone. Then radicals (decompressed peroxide) apply the free space and binds different backbones to form the cross-link.
Tin or Platinum Based Catalytic Process
In tin-based or platinum-based catalytic process, we activate this cross-link reaction by mixing. The metal catalyst reacts with the unsaturated double bond of the vinyl group, and facilitates active sites formation (free spaces) in the backbones. Therefore, this kind of cross-linking method is only useful for certain types of silicone. In addition, the curing time with tin-catalyst is long, it will reduce with higher catalyst amount.
Depending on various silicone materials, we should select the most effective cross-linker in curing process. These two common crosslinkers are high effective to cure liquid silicone rubber.
In LSR products manufacturing, we should not only consider the feasibility of manufacturing process, but also the curing time. This is directly impact by the product thickness. In fact, the curing time will increase exponentially with each additional millimeter in thickness. In the specific case, the thickness is over design rules and higher than 25.4 mm. We should apply different methods to increase curing speed, optimize productivity with degradation generation in silicone. Otherwise, excessive curing speed could cause thermal degradation, which decrease product’s lifetime exponentially.
So, different methods, which are used to modify the curing speed, should depend on the natural of silicone, type of process and final applications. Firstly, we should understand the variations involved in curing process: silicone composition, crosslinker and catalyst, processing temperatures.
Platinum Catalyst Curing
For platinum catalytic LSR curing process, the increase of processing temperature has similar effect as increase catalyst amount in curing process. To optimize the curing speed, the cure rate will increase approximately 20-25% with every 10℃ increase. Otherwise, the degradation probability will increase without well controlling of built-up temperature. So we recommend that once processing temperature is increased, curing agent amount shouldn’t increase at the same time. It is necessary to analyze the effect of specific temperature on curing speed at first, then adjust the curing agent amount.
Platinum-cured Silicone
There is no type of silicone material right for every process. The main benefit of platinum-cured silicone is that the platinum curing process won’t production peroxide residues, and reduce the risk of mold fouling. This reduces downtime and improve overall productivity in LSR production. Other advantages of platinum curing include:
- LSR parts clarity and consistency improvement,
- Better elongation, tensile strength and tear strength comparing to peroxide-cured LSR materials.
However, platinum-cured silicone also has some downsides. Firstly, platinum curing LSR is more expensive, this will drive up initial operation cost. Secondly, platinum catalyst is sensitive to contamination, its curing efficiency will be inhibited by exposure to latex, tine-cure silicone, sulfur. So we need to avoid these issues in platinum curing process.
Applications
Platinum-cured silicone has unique properties, this encourages its wide applications. Such as:
- Any process with resistance of extreme temperature, radiation, corona, moisture, steam, ozone. High purity platinum-cured silicone is suitable for pharmaceutical, injectables, biotechnology, food and beverage products.
- Any process with requirement of long-term heat or tensile exposure, extreme hardness, detailed customization. Platinum-cured silicone provides high tear and tensile strength, low shrinkage, fine feature reproduction and extensive range of harnesses.
Platinum-cured silicone molding and tubing are widely in fabrication of automotive parts, electronics, clothing and footwear, food and beverage storing products.
UV Curing
Ultraviolet (UV) curing is a new technology to increase silicone curing speed. In this curing process, the silicone rubber structure has slight modifications in backbones to accelerate bond breakage. This cure system is platinum-based catalyst addition, photosensitive catalyst has function to absorb the UV radiation. The UV area is between 320 and 400 nm and generated by gas discharge lamps. It fabricates products with high purity and low extractable lelves. Its cross-linking reaction promotes curing speed at room temperature without substantial heat input, while eliminate high reactive peroxides.
Comparing with conventional curing silicone rubber. The UV curing silicone rubber has only 5% variation of final properties, its curing time is as short as 0.5 seconds. Although final curing time depends on the speed of production line, process temperature, and UV intensity. UV curing speed is independent of thickness, it has uniform cure degree from skin to core. This allows high speed production with less cost. Such as, for thickness of 100 mm silicone parts, conventional process will take 30 minutes or more, while UV curing only take 5 minutes.
UV Curing Silicone
This type of silicone must be relatively clear, it also can accommodate with bright and vivid transparent colors, once the pigments are compatible with platinum catalyst. UV curing silicone has advantages as following:
- Low tooling cost, in reason of no need of water-cooled barrel or cold desk on molds.
- No risk of scorching in room temperature process.
- Low post-mold shrinkage due to no pressure buildup in the mod.
- High quality for innovative production design, as porosity is reduced and eliminated.
- Perfect for thick parts and temperature sensitive pharmaceutical additives.
Contact our engineering team for more information about Liquid Silicone Rubber Curing.
