Adhesive-free Adhesion Research Articles

Surface modification and adhesive-free adhesion of polytetrafluoroethylene (PTFE) and silicone gel containing oleophilic SiO2 powder by plasma treatment.

Polytetrafluoroethylene (PTFE) has high-frequency characteristics and low transmission loss, and is expected to be used as a substrate material of printed wiring board for high-frequency applications. Meanwhile, silicone gel has superior properties such as attaching/detaching, weather resistance, and human safety. If the PTFE and silicone gel can be strongly adhered to, they can be applied to internet of things (IoT) devices that can be attached and detached freely. However, adhesion between PTFE, which has poor adhesion, and silicone gel, which has low mechanical strength, is difficult and has not been reported. In this study, PTFE was modified with heat-assisted plasma treatment, and silicone gel was treated with oleophilic SiO2 powder to improve elastic modulus and modified with plasma jet treatment, and then bonded without adhesive. The adhesion strength of PTFE/silicone gel assembly was 1.13 N mm-1 when treated moderately, but only 0.01 N mm-1 when untreated and treated excessively. To investigate the factors causing the difference in the adhesion strength, the surface of silicone gel was evaluated by water contact angle measurement, Fourier transform infrared spectroscopy, and confocal laser scanning microscopy. When treated moderately, hydrophilic functional groups and cross-linking were most frequently increased. Furthermore, when treated excessively, surface degradation was observed, which was expected to lower the adhesion strength. The adhesive-free bonding between PTFE and silicone gel can open a new path for developing IoT devices that can be freely attached and detached.

Adhesive-Free Adhesion between Plasma-Treated Glass-Cloth-Containing Polytetrafluoroethylene (GC-PTFE) and Stainless Steel: Comparison between GC-PTFE and Pure PTFE.

In this study, the effect of plasma treatment on glass-cloth-containing polytetrafluoroethylene (GC–PTFE) was investigated. Previous plasma studies investigated pure PTFE (which does not contain glass cloth) but not GC–PTFE. The effect of Ar + H2O plasma treatment on GC–PTFE was investigated. The Ar + H2O plasma-treated GC–PTFE sheets were thermally compressed to stainless steel (SUS304) foils without using adhesive, and the GC–PTFE/SUS304 adhesion strengths were measured using a 90° peel test. The adhesion strength increased with the increase in the plasma treatment time (0.8 and 1.0 N/mm at 20 s and 300 s, respectively). Thus, strong adhesion between GC–PTFE/SUS304 was achieved without adhesive. This improvement in the adhesion properties of GC–PTFE can be attributed to the generation of oxygen-containing functional groups and the decrease in the surface roughness of the samples. Thereafter, the adhesion properties of GC–PTFE and pure PTFE were compared. Because, unlike pure PTFE, GC–PTFE has no weak boundary layer, GC–PTFE exhibited better adhesion properties than pure PTFE under short plasma treatment times.

Effects of He and Ar Heat-Assisted Plasma Treatments on the Adhesion Properties of Polytetrafluoroethylene (PTFE).

Heat-assisted plasma (HAP) treatment using He gas is known to improve the adhesive-bonding and adhesive-free adhesion properties of polytetrafluoroethylene (PTFE). In this study, we investigated the effects of He and Ar gaseous species on the HAP-treated PTFE surface. Epoxy (EP) adhesive-coated stainless steel (SUS304) and isobutylene–isoprene rubber (IIR) were used as adherents for the evaluation of the adhesive-bonding and adhesive-free adhesion properties of PTFE. In the case of adhesive bonding, the PTFE/EP-adhesive/SUS304 adhesion strength of the Ar-HAP-treated PTFE was the same as that of the He-HAP-treated PTFE. In the case of adhesive-free adhesion, the PTFE/IIR adhesion strength of the Ar-HAP-treated PTFE was seven times lower than that of the He-HAP-treated PTFE. The relation among gaseous species used in HAP treatment, adhesion properties, peroxy radical density ratio, surface chemical composition, surface modification depth, surface morphology, surface hardness, and the effect of irradiation with vacuum ultraviolet (VUV) and UV photons were investigated. The different adhesive-free adhesion properties obtained by the two treatments resulted from the changes in surface chemical composition, especially the ratios of oxygen-containing functional groups and C–C crosslinks.

Adhesive-free adhesion between heat-assisted plasma-treated fluoropolymers (PTFE, PFA) and plasma-jet-treated polydimethylsiloxane (PDMS) and its application

Conventional low-temperature plasma treatment was reported to minimally improve the adhesion property of polytetrafluoroethylene (PTFE), whereas heat-assisted plasma (HAP) treatment significantly improved the same. An unvulcanized rubber was previously used as an adherent for PTFE. This study aimed to achieve strong adhesive-free adhesion between PTFE and vulcanized polydimethylsiloxane (PDMS) rubber. As-received vulcanized PDMS rubber did not adhere to HAP-treated PTFE, and as-received PTFE did not adhere to vulcanized rubber of plasma-jet (PJ) treated PDMS rubber; however, HAP-treated PTFE strongly adhered to vulcanized PJ-treated PDMS rubber, and both PTFE and PDMS exhibited cohesion failure in the T-peel test. The surface chemical compositions of the PTFE and PDMS sides were determined using X-ray photoelectron spectroscopy. The strong PTFE/PDMS adhesion was explained via hydrogen and covalent bond formation (C–O–Si and/or C(=O)–O–Si) between hydroxyl (C–OH) or carboxyl (C(=O)–OH) groups of the HAP-treated PTFE. This process was also applied to adhesive-free adhesion between a tetrafluoroethylene–perfluoroalkylvinylether copolymer (PFA) and PDMS; subsequently, a translucent PFA/PDMS assembly with strong adhesion was realized together with the PTFE/PDMS assembly. Strong adhesive-free adhesion between fluoropolymers (PTFE, PFA) and vulcanized PDMS rubber without using any adhesives and graft polymer was successfully realized upon plasma treatment of both the fluoropolymer and PDMS sides. Additionally, a PDMS sheet, which was PJ-treated on both sides, was applied to strongly adhere fluoropolymers (PTFE, PFA) to materials such as metal and glass. PJ-treated PDMS was used as an intermediate layer rather than a strong adhesive, achieving PTFE/PDMS/metal and PTFE/PDMS/glass assemblies. The PTFE/PDMS, PDMS/metal, and PDMS/glass adhesion strengths exceeded 2 N/mm.

Comparison between adhesion properties of adhesive bonding and adhesive-free adhesion for heat-assisted plasma-treated polytetrafluoroethylene (PTFE)

ABSTRACTHeating during plasma treatment, known as heat-assisted plasma treatment, has recently reported to positively affect the adhesion properties of polytetrafluoroethylene (PTFE). In the present study, the adhesion properties of adhesive bonding and adhesive-free adhesion were compared for plasma-treated PTFE with different plasma treatment times and with or without heating during the plasma treatment. The relations among adhesion strength, plasma treatment time, radical density ratio, surface morphology, and surface hardness were investigated. No correlation was found between the adhesion strength and the radical density ratio or between the adhesion strength and the oxygen-containing-functional-group ratio. In contrast, correlation was observed between the adhesion strength and the surface hardness. In addition, the heat-assisted plasma treatment time affected the recovery of the weak boundary layer on the PTFE surface. Adhesive-free adhesion was found to require a longer heat-assisted plasma treatment time than adhesive bonding in order to achieve a high adhesion strength such as 1 N/mm.

Correction: Adhesive-free adhesion between polytetrafluoroethylene (PTFE) and isobutylene–isoprene rubber (IIR) via heat-assisted plasma treatment

Correction for ‘Adhesive-free adhesion between polytetrafluoroethylene (PTFE) and isobutylene–isoprene rubber (IIR) via heat-assisted plasma treatment’ by Y. Ohkubo et al., RSC Adv., 2017, 7, 6432–6438.

Adhesive-free adhesion between polytetrafluoroethylene (PTFE) and isobutylene–isoprene rubber (IIR) via heat-assisted plasma treatment

A polytetrafluoroethylene (PTFE) surface was modified using atmospheric pressure plasma treatment under heating (heat-assisted plasma treatment) to promote its direct adhesion to isobutylene–isoprene rubber (IIR) without any adhesives.

Surface modification of polymers for adhesion enhancement

Surface modification of low-density polyethylene (LDPE) and polytetrafluoroethylene (PTFE) films via graft copolymerization with acrylic acid (AAc), Na salt of styrenesulfonic acid (NaSS), N,N-dimethylacrylamide (DMAA), 2-(dimethylamino)ethyl methacrylate (DMAEMA), glycidyl methacrylate (GMA), acrylamide (AAm) and 3-dimethyl(methacryloylethyl) ammonium propanesulfonate (DMAPS) was carried out for adhesion improvements. The surface microstructure and composition of the modified film surfaces were studied by angle-resolved X-ray photoelectron spectroscopy (XPS). It was demonstrated that the graft-modified LDPE and PTFE films are capable of exhibiting adhesive-free adhesion or auto-adhesion. The GMA graft copolymerized PTFE surfaces adhere strongly to one another when brought into direct contact and cured (i) in the presence of an epoxy adhesive and (ii) in the presence of a diamine curing agent alone. It was further demonstrated that the ozone-pretreated LDPE films and the Ar plasma pretreated PTFE films readily undergo photografting or thermal grafting with concurrent lamination when lapped together in the presence of a small quantity of the monomer solution. The simultaneous graft copolymerization and lamination process can be carried out under atmospheric conditions and in the complete absence of an added initiator or system degassing. Copyright © 1999 John Wiley & Sons, Ltd.

Surface modification of polytetrafluoroethylene filmsvia graft copolymerization for auto-adhesion

The surfaces of Ar plasma-pretreated polytetrafluoroethylene (PTFE) films are further functionalized via UV-induced graft copolymerization with amphoteric N,N′-dimethyl(methacryloylethyl)ammonium propansulfonate (DMAPS) either in Ar atmosphere, or under atmospheric conditions and in the absence of a polymerization initiator. The so-modified PTFE films from either process are capable of exhibiting adhesive-free adhesion or auto-adhesion with one another when brought into intimate contact in the presence of a small quantity of water. The lap shear adhesion strength increases with increasing graft concentration and can readily exceed the yield strength of the PTFE substrate. Two plasma-pretreated PTFE films also readily undergo thermal graft copolymerization with concurrent lamination when lapped together in the presence of a small quantity of the DMAPS monomer solution at elevated temperature in the atmosphere. The surface compositions of the graft-copolymerized PTFE films and the delaminated surfaces were characterized by X-ray photoelectron spectroscopy (XPS). In most cases, adhesional failure occurred near the graft-substrate interphase. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3107–3114, 1998

Surface graft copolymerization of low density polyethylene films and its relevance to auto-adhesion

The surfaces of low density polyethylene (LDPE) films were modified first by ozone pretreatment, followed by thermally-induced graft copolymerization with the amphoteric monomer, 3-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS). The DMAPS graft copolymerized LDPE films were capable of exhibiting adhesive-free adhesion when brought into contact in the presence of water and subsequently dried. Lap shear adhesion strengths in excess of 100 N/cm 2 were achieved for lamination carried out at elevated temperature (60°C). It was also demonstrated that when two ozone-pretreated LDPE films were brought into contact in the presence of a small quantity of the aqueous DMAPS solution at an elevated temperature (70°C and above), surface graft copolymerization with concurrent lamination were achieved under atmospheric conditions and in the complete absence of an added polymerization initiator or oxygen scavenger. The adhesion strengths in both cases were governed by the concentration-dependent chain diffusion and entanglement, as well as by the electrostatic interaction, at the lapped interface. The graft copolymerized surfaces in both cases were characterized by X-ray photoelectron spectroscopy (XPS).

Surface modification and adhesion characteristics of polycarbonate films after graft copolymerization

The surfaces of ozone-pretreated polycarbonate films were subjected to further modification by thermally induced graft copolymerization with acrylic acid (AAc), sodium salt of styrene sulfonic acid (NaSS), N,N-dimethylacrylamide (DMAA), N,N-(dimethylamino)ethyl methacrylate (DMAEMA) and 3-dimethyl(methacryloyl ethyl)-ammonium propanesulfonate (DMAPS) monomers. The structure and composition at the copolymer interface were studied by angle-resolved X-ray photoelectron spectroscopy (XPS). For polycarbonate films with a substantial amount of grafted polymer, the hydrophilic graft penetrates or becomes partially submerged beneath a thin surface layer of dense substrate chains. This microstructure was further supported by the water contact angle measurements. Adhesive-free adhesion studies revealed that the AAc, DMAA or DMAPS graft copolymerized polycarbonate film surface adhered strongly to another similarly modified surface (homo-interface) when brought into direct contact in the presence of water and subsequently dried. The development of the lap shear strength is dependent on the concentration of the surface graft, the microstructure of the grafted surface, the adhesion (drying) time, and the nature of the interfacial interaction. The simultaneous presence of chain entanglement and electrostatic interaction readily results in substantially enhanced adhesion strengths between two DMAPS graft copolymerized surfaces or between an AAc and a DMAA graft copolymerized surface (hetero-interface). XPS analyses of the delaminated surfaces suggest that failure occurred cohesively below the graft-substrate interface. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 357–366, 1998

Surface modification of polymer films by graft copolymerization for adhesive-free adhesion

The surfaces of low density polyethylene (LDPE), poly(carbonate) (PC), polytetrafluoroethylene (PTFE) and emeraldine (EM) base films of poly(aniline) are modified by graft copolymerization with N,N-dimethylacrylamide (DMAA), Na salt of styrenesulfonic acid (NaSS), 3-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS) and acrylic acid (AAc). Two surface graft copolymerized films are capable of exhibiting ‘adhesive-free’ adhesion or auto-adhesion when brought into contact in the presence of water and subsequently dried. The development of the lap shear adhesion strength depends upon the concentration of the surface graft, the contact adhesion (drying) time, the nature of the substrate, the microstructure of the graft copolymerized surfaces and the extent of molecular interaction (dispersive, electrostatic, etc.) at the lapped junction. Lap shear adhesion strengths in excess of 90 N cm −2 were readily achieved between films graft copolymerized with the amphoteric DMAPS, as well as between a DMAPS graft copolymerized EM film and an AAc graft copolymerized PTFE film. X-ray photoelectron spectroscopy (x.p.s.) analysis of the delaminated surfaces suggests that the failure mode involves the fracture of the graft chains at the substrate-graft interface.

Surface-Functionalized Polyaniline Films

The surfaces of emeraldine (EM) base films of polyaniline (PANi) have been functionalized by thermal graft copolymerization with acrylic acid (AAc) and N,N-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS). The AAc graft copolymerized EM films were further functionalized via covalent immobilization of glucose oxidase (GOD) and collagen. The so-immobilized GOD still retains a substantial amount of activity. The amphoteric DMAPS graft copolymerized EM base films are capable of exhibiting strong adhesive-free adhesion to one another and to another similarly modified dielectric film, such as a surface graft copolymerized poly(tetrafluoroethylene) (PTFE) film. Lap shear adhesion strengths on the order of 300 and 100 N/cm2 were obtained for the respective cases. Pristine PANi films of various intrinsic oxidation states are capable of undergoing spontaneous surface metallization through reduction of precious metal ions, such as gold and palladium, in acid solutions to their elemental forms. The rate o...

Surface modification and functionalization of polytetrafluoroethylene films via graft copolymerization

Argon plasma pretreated polytetrafluoroethylene (PTFE) films were subjected to further surface modification by near-UV light induced graft copolymerization with hydrophilic and epoxide-containing monomers. Thus, the functional monomers used include acrylic acid (AAc), sodium salt of styrenesulfonic acid, N,N-dimethylacrylamide (DMAA) and glycidyl methacrylate (GMA). The surface composition and microstructure of the modified films were characterized by angle-resolved X-ray photoelectron spectroscopy. A stratified surface microstructure with a significantly higher substrate-to-graft chain ratio in the top surface layer than in the subsurface layer is always obtained for the PTFE surface with a substantial amount of graft. In all cases, the graft yield increases with plasma pretreatment time and monomer concentration. The PTFE films with molecularly redesigned surfaces are capable of exhibiting a number of new functionalities. These new functionalities include (i) covalent immobilization of an enzyme, such as trypsin (for AAc graft copolymerized surface), (ii) charge transfer induced coating of an electroactive polymer, such as polyaniline (for AAc and styrenesulfonic acid graft copolymerized surfaces), (iii) adhesive-free adhesion between two PTFE surfaces (for AAc, styrenesulfonic acid and DMAA graft copolymerized surfaces), and (iv) improved adhesive bonding via interfacial crosslinking of the grafted chains (for GMA graft copolymerized surfaces). © 1997 John Wiley & Sons, Ltd.

Surface Structures and Adhesive-Free Adhesion Characteristics of Polyaniline Films after Modification by Graft Copolymerization

Surface modifications of emeraldine base (EB) films have been carried out via thermally induced graft copolymerization with acrylic acid (AAc), sodium salt of styrenesulfonic acid (NaSS), N,N‘-dimethyl(methacryloylethyl)ammonium propanesulfonate (DMAPS), and acrylamide (AAm). The structure and chemical composition of each graft-copolymerized surface were studied by angle-resolved X-ray photoelectron spectroscopy (XPS). In all cases, the concentration of grafting increases with monomer concentration. Surface grafting with the four functional monomers also leads to a more hydrophilic EB film surface. Two graft-copolymerized EB film surfaces are capable of exhibiting adhesive-free adhesion when brought into direct contact in the presence of water and subsequently dried. This adhesive-free adhesion also exists between a surface-modified EB film and a similarly modified polytetrafluoroethylene (PTFE) film from graft copolymerization. The development of the lap shear adhesion strength depends on the concentrati...

Surface structures and adhesion characteristics of poly(tetrafluoroethylene) films after modification by graft copolymerization

Surface modifications of Ar plasma-pretreated poly(tetrafluoroethylene) (PTFE) film were carried out via near-UV light-induced graft copolymerization with acrylic acid (AAc), Na salt of styrenesulfonic acid (NaSS) and N,N-dimethylacrylamide (DMAA). The structure and chemical composition of each copolymer surface and interface were studied by angle-resolved X-ray photoelectron spectroscopy (XPS). For each PTFE substrate with a substantial amount of grafting, the hydrophilic graft penetrated or became partially submerged beneath a thin surface layer of dense substrate chains to form a stratified microstructure. This surface microstructure was further confirmed by the water contact angle results. In all cases, the concentration of surface grafted polymer increased with the plasma pretreatment time and monomer concentration. Adhesive-free adhesion studies revealed that the AAc graft copolymerized PTFE surface adhered strongly to another similarly modified surface (homo-interface) or to a DMAA graft copolymeri...

Adhesive-free adhesion of grafted surfaces with different wettabilities

A polyester film surface was graft-polymerized with various water-soluble monomers by a combination of plasma pretreatment and photoirradiation. The grafted surfaces showed strong adhesion to another non-grafted or grafted film when brought into direct contact in the presence of water and subsequently dried. The adhesion force depended on the hydrophilicity of the adhering surfaces and the graft density. The film having a larger graft density generally showed stronger adhesion in the final stage of drying, but it took longer to achieve high adhesion because of the larger amount of water existing in the interfacial region between the two surfaces. On the other hand, substantial adhesion was obtained almost instantaneously upon contact when one was grafted with an anionic polymer and the other grafted with a cationic polymer. Adhesion between similarly charged surfaces was very weak at the beginning of drying, probably because of the electrostatic repulsion between the charged groups.