Plasma Treatment Enables Strong Bonding Without Adhesives

Polymers containing plastics are critical to modern life. Because they are lightweight, strong and inert, a wide range of technologies depend on them. However, most polymers do not naturally adhere to other materials, so they require adhesives or abrasive chemical treatments to bond to other materials. This is a problem in areas such as food and pharmaceuticals, where contamination must be avoided at all costs. There is an urgent need for a clean path to making industrial polymer adhesive. Now, a team at Osaka University has done exactly that. Researchers have developed a series of plasma processes to make vulcanized rubber and plastic PTFE (polytetrafluoroethylene) adhere to each other or to other materials. As described in a study in Scientific Reports, the method activates the surface chemistry of polymers. Lead author Yuji Ohkubo says, "If you spray PTFE with a helium plasma at 200 degrees, it can stick to unvulcanized rubber - this is a technique we previously developed in our lab." "But vulcanized rubber presents a bigger challenge. In our latest study, we customized a new plasma process that enabled vulcanized silicone rubber to adhere strongly to PTFE for the first time." The silicone in question is PDMS (polydimethylsiloxane), a well-known resin. While the most important advance in PTFE adhesion was heat-assisted plasma treatment, the key to PDMS was forcing nitrogen/air plasma through a small orifice to bombard the surface with a plasma jet. The jet breaks silicon-carbon bonds on the surface and converts them to silanol (Si-OH). These silanol groups, more reactive than the original silicon surface, can bond with PTFE. Under high pressure, hydrogen bonds form between silanols and oxygen-containing functional groups on the PTFE. Strong covalent bonds (C–O–Si, where C comes from PTFE and Si from silicone) bind the two polymers together without adhesive. Combining the two materials allows each to benefit from the other's advantages - that is, PTFE's chemical resistance, dirt repellency and slipperiness, and silicone's flexibility. Opaque PTFE can also be replaced with PFA (perfluoroalkoxy alkane) if transparency is needed. And it doesn't stop there - when the back of PDMS is also plasma-sprayed, it can stick to copper and even glass. Like an extremely strong double-sided tape, this three-layer sandwich enables fluoropolymers to adhere cleanly to other useful materials. Co-author Katsuyoshi Endo explained, "PDMS is widely used in medicine, for example in microfluidic chips." "There could be significant benefits to making both PTFE and PDMS more versatile for medical and food technologies through adhesive-free bonding. Combined with the fact that volatile chemicals are not needed, we hope our method will expand the horizons of high-tech polymers." [caption id="attachment_143974" align="aligncenter"] Figure 1. Photographs of three-layer assemblies of (a) PTFE/PDMS/Cu, (b) PTFE/PDMS/SUS430 and (c) PTFE/PDMS/glass. When the PTFE or PDMS sheets were shaken and agitated, there was no peeling at the PTFE/PDMS, PDMS/Cu, PDMS/SUS430 and PDMS/glass interfaces. This demonstrates that PDMS treated with PJ can be used as a strong adhesive alternative for bonding fluoropolymers to other types of materials. (Source: Osaka University)[/caption]   [caption id="attachment_143975" align="aligncenter"] Figure 2. Preparation procedure for a two-layer assembly such as PTFE/PDMS (steps 1-3) and a three-layer assembly such as PTFE/PDMS/Cu (steps 1-6). PFA/PDMS, PFA/PDMS/glass, PTFE/PDMS/glass and PTFE/PDMS/SUS430 assemblies were prepared similarly. (Source: Osaka University)[/caption]     Source More information: The article, "Adhesive-free adhesion between heat-assisted plasma-treated fluoropolymers (PTFE, PFA) and plasma-jet-treated polydimethylsiloxane (PDMS) and its application," was published in Scientific Reports at DOI: https://doi.org/10.1038/s41598-018-36469-y . / https://resou.osaka-u.ac.jp/en/research/2018/20181225_1