Adhering without adhesives

Materials World magazine
,
31 Jan 2019
Variations of three-layer structures. Image credit: Osaka University

A method to adhere vulcanised rubber to plastic or other materials without an adhesive has been developed by Japanese researchers. Idha Valeur reports. 

A process to enable vulcanised rubber to adhere with plastic without adding an adhesive layer and or using corrosive solutions has been developed. This low-chemical approach could be beneficial for sectors such as the food industry and medical industry. 

Although polymers containing plastics have a variety of properties, being able to adhere naturally is not one of them. This is due to their low surface energy and weak boundary layer. However, researchers from Osaka University, Japan, have now produced plasma treatments that allow vulcanised rubber to adhere to PTFE plastic. 

Sticking with it 

Formerly, chemical etching using corrosive solutions containing sodium was the preferred technique of making polytetrafluoroethylene (PTFE) adhere. According to the study, this method has numerous disadvantages such as malodour, unfortunate colouring of the plastic surface and that it is toxic to humans. Due to these downfalls, there was a need for an alternative method that eliminated the corrosive solutions. 

Yuji Ohkubo, the study leader, said the technique of spraying PTFE with a plasma of helium at 200°C to unvulcanised rubber was previously developed in their lab, but that the vulcanised rubber proved to be more difficult. ‘Vulcanised rubber presents a greater challenge. In our latest study, we customised a new plasma treatment for vulcanised silicone rubber, making it adhere strongly to PTFE for the first time,’ he said. 

The key to making silicone PDSM (polydimethylsiloxane) adhere is to blast the surface using a plasma jet. During this process, nitrogen/air plasma is driven through a small hole, breaking the silicon-carbon bonds on the surface before converting them to silanol (Si-OH). As these are more reactive than the silicone surface, the silanol groups will bond with PTFE. Covalent bonds, C-O-Si, where C is from PTFE and Si from silicone, provide further stitching of the two polymers, without any adhesives. 

Sum of parts 

Joining the materials gives each the desirable characteristics of the other, such as  chemical resistance, dirt-repellence, the ability to slide off PTFE, and the elasticity of silicone. If a transparent look is wanted, the PTFE, which is opaque, could be swapped for PFA (perfluoroalkoxyalkane). If the reverse side of the silicone is also treated with the plasma jet, it can bond to other materials, including copper and glass. Acting as an incredibly strong double-sided tape, the three-layer stack allows the fluoropolymers to adhere cleanly to other convenient materials. 

Co-author of the study, Katsuyoshi Endo, said ‘PDSM is widely used in medicine, for example in microfluidic chips. There could be huge benefits in making both PTFE and PDSM more versatile for medical and food technologies through adhesive-free adhesion. Combined with the lack of any need for volatile chemicals, we hope our method will broaden the horizons for polymers in high technology.’

The study, 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. Read the study here: http://go.nature.com/2T5JuyL