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Painting and gluing is almost impossible because of the low surface energy and the sticky and static surface.
Nanon has developed two technologies, Softplasma and Cohancement, that create further possibilities using silicone rubber. Softplasma is a surface-treatment process that polymerizes a nano-scale layer with hydrophilic functional groups on the silicone rubber surface. The permanent layer bonds strongly to many types of glue or paint. In Cohancement, undesired residues from the silicone rubber are removed in an environment-friendly process. This process is a substitution for the conventional heat tempering process used to post-cure silicone rubber.
This article will describe different applications of these technologies.
Softplasma What is plasma?
Plasma is an ionized gas. While, e.g., the sun or stars are totally ionized, artificial plasmas used for surface treatment contain only about 0.1-10 ppm ions and electrons. The rest are neutral species.
Artificial plasma can be realized by exposing a gas to an energy field at low pressure.
Etching When initiating a plasma, electrons are accelerated in the electrical field. When hitting an atom or molecule of the gas, ionization or fragmentation can occur. Positively charged ions from the gas phase hit the substrate surface and crack chemical bonds of the outer layer. This leads to etching of the surface because atoms and small fragments evaporate. The etching cleans the surface and creates active sites where the plasma-polymerized layer can be bonded later on.
Plasma polymerization A monomer is applied to the plasma, activated and polymerized on the substrate surface. The power input of the plasma has a direct influence on the character of the plasma reaction. At high energies, the monomer that is led into the plasma is completely fragmentized and radical formation occurs in the gas phase, leading to a highly crosslinked polymer. At low energies, the lnonomer structure is preserved and radical polymerization occurs on the substrate surface.
Influence of different energy levels on the plasma polymerization Ethyl-2-cyanoacrylate was used as monomer. It was evaporated and fed into the plasma chamber, where it was activated in argon-plasma at different energy levels. As substrate, NaCl-crystals were used. After the process, the crystals were analyzed in a Thermo FTIR spectrometer by transmission measurements (figure 1).
[FIGURE 1 OMITTED] The typical peaks at 1,745 [cm.sup.- and 1,249 [cm.sup.- are decreasing when the power level is raised. That means that the ester group cannot withstand high energy levels without degrading Softplasma Softplasrna is the trade name for a low pressure plasma treatment and polymerization technology developed by NKT Research. Softplasma allows plasma polymerization at very low energies without serious damage to either the substrate or functional groups of the monomer. To obtain this low energy plasma, a three-phase AC plasma with 50 Hz is used. The shape of the electrodes gives the opportunity to create homogenous, 3D-plasma that oilers the possibility to treat items with various geometries.
In theory, all kinds of monomers could be used for plasma polymerizing to obtain desired surface properties. However, the monomer has to fulfill the condition of being evaporable in vacuum at room temperature.
It depends on the application which kind of functional group is required on the surface to create better bonding to a top coating. Vinyl or acrylic compounds with suitable functional groups are often used as monomers in order to obtain the requested surface properties.
Applications Coating of silicone rubber with PU Coating of silicone robber switches with polyurethane coating becomes possible when a plasma pretreatment is made (figure 2). The plasma treatment creates NH--and CN--groups on the surface that give a higher surface energy and a good bonding to the PU top-coating.
[FIGURE 2 OMITTED] Over-molding of polyamide with silicone rubber When over-molding polyamide with silicone rubber, it is difficult to obtain good adhesion between the materials without using primer chemistry and special silicone rubber (figure 3).
[FIGURE 3 OMITTED] With Softplasma, we apply a silicone-like surface with a low surface energy onto the polyamide that afterwards is able to create adhesion to the over-molded silicone robber (figure 3).
Cold tempering (Cohancement) Problems from plasma When starting plasma treatment of silicone, we soon had to cope with problems concerning the volatiles that evaporate from silicone rubber. These silicone oligomers migrate to the material's surface during vacuum treatment, and the plasma polymerization is made on an oily film instead of the silicone surface. A further coating is not possible. When looking for solutions to remove the volatiles, we investigated different kinds of methods and found that carbon dioxide (C[O.sub.) under pressure is a good solvent for silicone oligomers.
When C[O.sub. is pressurized, it becomes liquid or supercritical (figure 4). In these states the surface tension is 0-5 mN/m.
[FIGURE 4 OMITTED] Because of its low surface tension, C[O.sub. wets most materials completely, and especially silicone rubber. The liquid goes through the silicone rubber, swells it and dissolves silicone oligomers and by-products from catalysts and peroxides. Additionally, other compounds can be impregnated into the silicone material, leading to new properties.
Cold tempering Today, the majority of silicone rubber is post cured in heat for several hours to get rid of by-products from the vulcanizing step and to remove low molecular silicone compounds from the rubber. The process often takes a long time, and sometimes silica-dust from decomposed silicone is formed on the silicone robber surface, which causes troubles in subsequent surface treatments. Another problem is the healing (re-polymerization) of cuts and holes caused by heat from the post-curing step that makes it difficult to make the final design of the silicone rubber item in the mold already. To remove these drawbacks, a good alternative is to use supercritical or liquid C[O.sub. to post-cure silicone rubber. We developed the cold tempering process that uses liquid C[O.sub. (at 40 bars, 10[degrees]C), which makes the process much cheaper and easier to handle than a process under supercritical conditions (at least 31 [degrees]C, 74 bars).
The following analyses were made: Baby soothers made of silicone rubber (LSR) were post-cured by common heat tempering (4 hours, 200[degrees]C, air ventilated) and by cold tempering (washed in liquid C[O.sub., 40 bars, 10[degrees]C, 1 hour). Afterwards, the remaining content of volatiles was analyzed by EN 14350-2 and by Nanon's acetone soxhlet method. Mechanical properties were measured by customers' standards. GC and GPS analyses of the residues were made by the customers' standards. Cut healing was visually investigated.
The effect of removal of volatiles and low molecular silicone oligomers is shown in table 1.
In comparison to the EN 14350-2, the acetone soxhlet method can be used to determine other residues besides volatiles. In acetone, low molecular residues, which would not evaporate at atmospheric pressure at 200[degrees]C, are dissolved, leading to a higher value of weight loss.
Gas chromatography analysis showed that harmful cyclic siloxanes in the range of D4 to D20 are removed. But also in the range up to D30 (20 cST), GPC analysis shows significant decrease of tree silicone oligomers.
The mechanical properties are shown in table 2.
The C[O.sub. treatment has no negative impact on the robber-like properties of the silicone. Especially the values of modula 100% and 300% show that there is no further cure reaction in the C[O.sub.
process. However, there is a difference in comparison to the heat cure values of the compression set, tear resistance and hardness. These values do not change in the C[O.sub. treatment but normally change in heat curing.
Prevention of cut-healing The cuts did not heal during the C[O.sub. process and we could state that the cut edges were less sticky than before the treatment. The final dimensions and details of the silicone rubber being unaltered during the treatment with liquid C[O.sub. means that no re-polymerization takes place.
Sterilizing effect in cold tempering From literature, it is known that supercritical C[O.sub. is an unfriendly environment for bacteria.
Initial trials have been started to determine if the liquid C[O.sub. process can give the same effect. E. coli bacteria have been applied to a silicone rubber surface and the samples have been treated in liquid C[O.sub. for two hours. Afterwards, the numbers of bacteria have been analyzed by measurement of the pH value. The pictures (figure 5) show that there are no bacteria on the C[O.sub. treated sample.
Because of the conditions in liquid C[O.sub. (10[degrees]C, 40 bars), one can state that the bacteria were not killed by extreme temperature, but by an effect of the pressure or the C[O.sub..
[FIGURE 5 OMITTED] This article is based on a paper presented at Silicone Elastomers 2006, a Rapra Technology conference. (www.rapra.net/ conferences).
by Maike Benter, Anne-Marie Jensen and Thomas Emil Andersen, Nanon A/S, Denmark (www.nanon.dk)
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