Effect of rubber compounding agent on adhesion strength between rubber and heat-assisted plasma-treated polytetrafluoroethylene (PTFE)

Although heat-assisted plasma treatment enables drastic improvement of the adhesion property of polytetrafluoroethylene (PTFE), plasma-treated PTFE does not strongly adhere to any adherend. To clarify which rubber compounding agents positively affect the adhesion strength of a plasma-treated PTFE/rubber assembly, six types of unvulcanised rubbers were prepared and thermally compressed to a plasma-treated PTFE sheet. Thus, it was found that SiO₂ addition to rubber drastically increased the adhesion strength of a plasma-treated PTFE/rubber assembly and cohesion failure of rubber occurred with large fractions of SiO₂ although no adhesives were used. To confirm the reaction between plasma-treated PTFE and SiO₂ powder, X-ray photoelectron spectroscopy (XPS) measurements were performed for the thermally compressed SiO₂/PTFE assembly after repeated washing. The XPS results indicated that hydrophilic SiO₂ powder strongly adhered to the plasma-treated PTFE, whereas hydrophobic SiO₂ powder did not adhere to the PTFE. In this paper, a model was proposed for a possible mechanism of strong adhesion of a PTFE/rubber assembly through both hydrogen and covalent bonds between silanol groups of the SiO₂ powder surface in the rubber and hydroxyl or carboxyl groups on the plasma-treated PTFE.     

The role of chemical surface modification for structural adhesive bonding on polymers - Washability of chemical functionalization without reducing adhesion

This study investigates the role of surface functionalization to increase adhesion forces on polymers. The effects of two different physical pre-treatment methods (oxygen low-pressure plasma - LPP, vacuum-UV - VUV) are investigated on four different polymer matrices (polyetheretherketone - PEEK, polyetherimide - PEI, polyethersulfone - PES and the epoxy resin RTM6). Polymer surfaces were additionally washed after surface treatment with different polar solvents. Surface chemistry, wettability, and topography were investigated before pre-treatment, after pre-treatment and after washing of the samples using x-ray photoelectron spectroscopy (XPS), contact angle measurements and atomic force microscopy (AFM).The results show, that washing of the samples after pre-treatment lead to a chemical surface condition similar to the initial surface. Interestingly, the tensile bond strength of centrifugal adhesion tensile test (CATT) specimens however remained high. In consequence, the thus far widely accepted understanding of surface functionalization as the dominating factor for adhesion promotion on polymers has to be re-evaluated.     

Atmospheric pressure plasma surface modification of titanium for high temperature adhesive bonding

In this investigation surface treatment of titanium is carried out by plasma ion implantation under atmospheric pressure plasma in order to increase the adhesive bond strength. Prior to the plasma treatment, titanium surfaces were mechanically treated by sand blasting. It is observed that the contact angle of de-ionized water decreases with the grit blast treatment time. Optical microscopy and scanning electron microscopic (SEM) analysis of untreated and atmospheric plasma treated titanium are carried out to examine the surface characteristics. A substantial improvement in the surface energy of titanium is observed after the atmospheric pressure plasma treatment. The surface energy increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength of the titanium. Unmodified and surface modified titanium sheets by atmospheric pressure plasma were adhesively bonded by high temperature resistant polyimide adhesive. The glass transition temperature of this adhesive is 310 °C and these adhesively bonded joints were cured at high temperature. A substantial improvement in adhesive bond strength was observed after atmospheric pressure plasma treatment.     

Effect of silane treatment on adhesion of adhesive-bonded carbon fiber reinforced nylon 6 composite

In this study, a silane coating, KH560, along with an abrasion pretreatment procedure for carbon fiber reinforced PA 6 composite with 30% mass fiber (C_f/PA6) and AA6061-T4 was developed. Composition and structure of this silane coating and effect of this coating on the strength and water resistance of the adhesive-bonded C_f/PA6 and AA6061 were investigated. Test results revealed that silane pretreatment improved the static strengths of the adhesive-bonded C_f/PA6-C_f/PA6 and C_f/PA6-AA6061 by 23% and 21%, respectively. X-ray photoelectron spectroscopy (XPS) analysis of the adhesive-bonded treated C_f/PA6 indicated that the improvement in strength was primarily attributed to the formation of Si-O-Si and Si-N covalent bonds on the surface of C_f/PA6, which enhanced the bond adhesion between the silane coating, adhesive, and adherends. In addition, abrasion of adherends prior to silane pretreatment further improved the strengths of the adhesive-bonded C_f/PA6-C_f/PA6 by another 20%. While silane and abrasion-silane treatment improved the strength of the adhesive-bonded C_f/PA6-C_f/PA6 and C_f/PA6-AA6061 in ambient conditions, the strength of water immersed silane coated joints with and without abrasion was decreased by about 50% (i.e., immersed in water at 54 °C for one week). All tested bare and treated joints with and without water immersion exposure were fractured in adhesive mode. Analyses of the test results revealed that to improve the failure mode and bond strength it is necessary to form a Si-N functional group in the silane coating by thoroughly cleaning and removing the contaminants and weak layers on the surface of C_f/PA6.     

Impact of adhesive ingredients on adhesion between rubber and brass-plated steel wire in tire

Proficiency on underlying mechanism of rubber-metal adhesion has been increased significantly in the last few decades. Researchers have investigated the effect of various ingredients, such as hexamethoxymethyl melamine, resorcinol, cobalt stearate, and silica, on rubber-metal interface. The role of each ingredient on rubber-metal interfacial adhesion is still a subject of scrutiny. In this article, a typical belt skim compound of truck radial tire is selected and the effect of each adhesive ingredient on adhesion strength is explored. Out of these ingredients, the effect of cobalt stearate is found noteworthy. It has improved adhesion strength by 12% (without aging) and by 11% (humid-aged), respectively, over control compound. For detailed understanding of the effect of cobalt stearate on adhesion, scanning electron microscopy and energy dispersive spectroscopy are utilized to ascertain the rubber coverage and distribution of elements. X-ray photoelectron spectroscopy results helped us to understand the impact of Cu_XS layer depth on rubber-metal adhesion. The depth profile of the Cu_XS layer was found to be one of the dominant factors of rubber-metal adhesion retention. Thus, this study has made an attempt to find the impact of different adhesive ingredients on the formation of Cu_XS layer depth at rubber-metal interface and establish a correlation with adhesion strength simultaneously.     

Bonding adhesive strength for plasma spray with cathode spots of low-pressure arc after grit blasting and removal of oxide layer on metal surface

Cathode spots of a low-pressure arc can remove oxide layers and evaporate impurities on metal surfaces. Removal of the oxide layer using cathode spots is expected to solve recent obstacles due to chemical and mechanical cleaning methods. Various phenomena of cathode spots have been investigated for pre-treatment of atmospheric pressure plasma spray (APPS). This study treated the surface shapes of oxide and non-oxide samples using a composite pre-treating method: cathode spots after grit blasting. In addition, the samples are compared with conventionally treated cathode spots and mechanical blasted surfaces. Results show that roughness on the sample surfaces becomes higher in concomitance with the initial oxide layer thickness. This fact reveals the factors those dominant bonding strength on the pre-treated surface using cathode spots of a low-pressure arc. Bonding strength becomes higher in relation to the arithmetical mean height of the surface roughness caused by the initial oxide layer thickness. Bonding strength is higher when the mean spacing of profile irregularities is narrower. The bonding adhesive strength after treatment using cathode spots after grit blasting is greater than 90 MPa. However, cathode spot treatment must be limited not to destroy the projection formed by melting after grit blasting in an arc time up to 5 s. And basically the removal characteristics of oxide layer using the cathode spots were discussed on the trace, roughness, number of cathode spots in time and pressure, moving speed on the metal surface, etc.     

Surface modification of natural leather using diffuse ambient air plasma

The aim of this work was to activate the surface of dyed natural leather using a diffuse ambient air plasma treatment. The plasma was generated by the so-called diffuse coplanar surface barrier discharge (DCSBD). It was observed that a 10 s plasma treatment time is enough to decrease the water contact angle from 85° to 45°. Improvement of wettability is important for gluing of dyed leather and better adhesion of glue to leather parts. XPS study indicated that the percentage of oxygen-containing bonds responsible for hydrophilicity is significantly increased by the plasma treatment. Influence of the plasma treatment on mechanical properties of dyed leather was evaluated using tensile strength measurement and no significant changes in the surface morphology and mechanical properties were observed. The results indicate that the DCSBD technique can provide high throughput, technical simplicity and economy required by the leather industry.     

Filler-elastomer interactions: influence of oxygen plasma treatment on surface and mechanical properties of carbon black/rubber composites

In this work, the influence of oxygen plasma treatment on the surface and adsorption properties of carbon blacks was investigated using X-ray photoelectron spectroscopy (XPS), ζ-potential, and BET isotherms. Then the mechanical properties [tensile strength and tearing energy (G_IIIC)] of carbon black/acrylonitrile butadiene rubber (NBR) composites were measured. As a result, it was found that oxygen plasma treatment generated oxygen-containing functional groups, such as, carboxyl, hydroxyl, lactone, and carbonyl groups, on the carbon black surfaces, resulting in a decrease in the equilibrium spreading pressure or London dispersive component of surface free energy. The tearing energy of the carbon black/NBR composites improved as the oxygen-containing functional groups on the carbon black surfaces increased. This revealed that there is a relatively high degree of interaction between the polar NBR and the oxygen-functional groups of carbon blacks.     

Molecular structure of interfaces between plasma-polymerized acetylene films and steel substrates

Plasma-polymerized films of acetylene were deposited onto steel substrates in an inductively coupled reactor by exciting the plasma in an argon carrier gas and then injecting the monomer into the afterglow region. The molecular structure of the film/substrate interface was determined using reflection–absorption infrared spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS) to characterize the films as a function of thickness. RAIR showed that thick (∼ 900 Å) as-deposited plasma-polymerized acetylene films had a complicated molecular structure and contained methyl and methylene, mono- and disubstituted acetylene, vinyl, and cis- and trans-disubstituted olefin groups. Evidence of oxidation resulting from the reaction of trapped radicals with atmospheric oxygen and moisture to form O—H and C=O groups was also obtained. The molecular structure of thin films (∼ 60 Å) was similar although evidence was obtained to indicate that acetylide groups (H—C≡C⁻) were present at the film/substrate interface. Results obtained using angle-resolved XPS analysis showed that carbonaceous contamination was removed from the substrate and that oxides and hydroxides on the substrate surface, especially FeOOH, were chemically reduced during deposition of the films. XPS also confirmed that plasma-polymerized acetylene films deposited on steel substrates contained groups. Preliminary results also showed that films deposited in an inductively coupled reactor were good primers for rubber-to-metal bonding, whereas films deposited in a capacitively coupled reactor were not. The differences may be due to the wide variety of functional groups found in the former type of films but not in the latter. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1283–1298, 1998     

Oxygen plasma modification of pitch-based isotropic carbon fibres

An isotropic carbon fibre was surface-treated by microwave oxygen plasma at different conditions and characterised by scanning electron microscopy (SEM), scanning tunneling microscopy (STM), N₂/CO₂ adsorption, Raman spectrometry, X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is shown that the structure of the fibre suffers only limited alterations upon plasma treatment in such a way that the local disorder on its surface, which was already large in the fresh material, barely increases after the plasma exposure, as detected by Raman measurements. At the nanometre scale, STM images revealed a moderate increase in surface roughness. Evidence for chemical changes undergone by the fibre following the etching was provided by XPS and TPD, showing that stable oxygen functionalities were introduced by the plasma exposure, a result of practical importance for the application of this treatment not only to this type of carbon fibre, but to carbon materials in general. It was also observed that very gentle plasma exposures were generally sufficient to provide the fibre surface with a large amount of oxygen functional groups and that more intense treatments had a negative effect in this respect (i.e. they were not able to supply oxygen to the surface in larger amounts than the softer treatments did).     

Surface modification of polypropylene fiber for hydrophilicity enhancement aided by DBD plasma

Low-wetting low-hydrophilicity polipropylene fiber has been treated by means of non-thermal dielectric barrier discharge plasma. The fiber has been characterized before and after the treatment by scanning electron microscopy (SEM), X-ray diffraction, atomic force microscopy (AFM) and industrial tests of submersion in water. The cold plasma was generated in a helium–air mixture at a 2 kHz frequency and 12–18 kV potential. The treatment periods ranged from 1 min up to 3 min. The results present an increase in the oxygen content which rose from 2.5% to 4.8%. SEM analysis reveals a surface change of the fiber with the formation of nodules, which is confirmed by AFM. Water submersion experiments show an enhancement of the hydrophilic capability of the samples.     

Corona-discharge treatment of polyetheretherketone (PEEK) for adhesive bonding

Poly(etheretherketone) (PEEK) has been treated by corona-discharge in air with added oxygen, argon, ammonia or sulfur dioxide. The observed effect is to almost double the strength of lap joints and increase surface polarity. Treated surfaces have been stored for 90 days without significant loss of joint strength. The improvements brought about by corona-treatments are not removed by wiping the surface with solvents.     

Analysis of hydrothermally-treated and weathered coals by X-ray photoelectron spectroscopy (XPS)

Analysis of hydrothermally-treated and weathered coals by X-ray photoelectron spectroscopy (XPS) was carried out, and the XPS C(1s) and N(1s) spectra obtained were curve-resolved into four peaks (C-C/CC/C-H, C-O, CO, and O-CO) and three peaks (pyridinic-N, pyrrolic-N, and quaternary-N), respectively. Upon hydrothermal treatment, the amount of carbon-oxygen forms decreased; while the ratio of pyridinic-N increased and quaternary-N decreased. On the other hand, some bituminous coals were subjected to natural weathering and laboratory oxidation, which gave opposite results compared to the hydrothermal treatment. The changes in the carbon-oxygen and organic nitrogen forms were discussed in terms of the effect of hydrothermal treatment and weathering (oxidation). Also, the XPS analysis of various kinds of coals (43 SS coals) was carried out, and the amounts of carbon-oxygen and organic nitrogen forms were discussed in terms of coal rank (carbon content).     

Controlled reduced-strength epoxy-aluminium joints validated by ultrasonic and mechanical measurements

In this study a series of joint systems, consisting of aluminium substrates bonded using an epoxy adhesive, were produced. Several levels of adhesion were achieved by altering the substrate surface treatment and the curing cycle of the adhesive. The goal of this study was to produce reduced-strength epoxy-aluminium joints that could be used as reference samples for ultrasonic non-destructive testing (NDT) studies. There is clearly a continuing challenge to improve the quality of the adhesively-bonded joint inspection to ensure the durability of the bonds, to monitor repairs, and to evaluate the strength of the bonds. However, developing and qualifying innovative or advanced non-destructive testing requires an essential preliminary step: a method for repeatedly producing reduced-strength bonded test specimens must be developed. In this study, in addition to a rigorous protocol to produce bonded joints, complementary ultrasonic CSCAN were realised to validate the homogeneity of the joints and to ensure that samples met all requirements so as to be considered as reference samples. Mechanical tests were performed to evaluate the mechanical strength of each joint and Acoustic Emission (AE) was used during the tests in order to confirm the expected fracture mechanisms.     

Improvement of the properties of ground tire rubber (GTR)‐filled nitrile rubber vulcanizates through plasma surface modification of GTR powder

The surface modification of ground tire rubber (GTR) powder to enhance its adhesion to nitrile rubber (NBR) vulcanizates was investigated. The hydrophobic surface of GTR powder has been transformed to a hydrophilic one through atmospheric pressure dielectric barrier discharge (DBD). The water contact angle dropped markedly from 116 to 0° after being treated for more than 10 s. Attenuated total reflectance Fourier transform infrared spectral (ATR‐FTIR) studies revealed the increase in peak intensity at 3298 and 1640 cm^?1 that correspond to O? H and C?C, respectively, on the surface of the GTR powder. The X‐ray photoelectron spectroscopic (XPS) analysis further confirmed the presence of oxygen containing polar functional groups on the surface of the GTR powder after atmospheric plasma treatment. The improvement in tensile strength and tear strength was observed for the modified GTR‐filled NBR vulcanizates, which is attributed to the enhanced interfacial interaction between modified GTR and NBR matrix. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Hydrophobic recovery of repeatedly plasma-treated silicone rubber. Part 2. A comparison of the hydrophobic recovery in air,water, or liquid nitrogen

Surfaces of medical grade silicone rubber (Q7-4750, Dow Corning) were modified by repeated (six times) RF plasma treatments using various discharge gases: oxygen, argon, carbon dioxide, and ammonia. The treated samples were stored for a period of 3 months in ambient air, water, or liquid nitrogen. Subsequently, the temporal behavior of the effects of the plasma treatment on the physicochemical surface properties of the silicone rubber was investigated using water contact angle measurements and X-ray photoelectron spectroscopy (XPS). Hydrophobic recovery during 3 months storage in ambient air was considerable and nearly complete for all four plasmas used. Hydrophobic recovery was almost completely suppressed during storage in liquid nitrogen, and only a minor increase of around 10° in advancing water contact angle was observed for all four plasma treatments. Also during storage of treated samples in water, hydrophobic recovery was minimal and initiated again by returning the treated samples to ambient air. XPS analyses showed that argon, carbon dioxide, and ammonia plasma-treated silicone rubber all had increased carbon percentages at the expense of oxygen and silicon after storage in water, or in liquid nitrogen, compared with after storage in ambient air. Interestingly, the carbon content of oxygen plasma-treated silicone rubber decreased during storage in water, or in liquid nitrogen, compared with storage in ambient air, while its oxygen and silicon percentages increased.     

The effects of Al surface treatment,adhesive thickness and microcapsule inclusion on the shear strength of bonded joints

The influences of various Al surface treatments, adhesive thicknesses as well as the incorporation of synthesized microcapsules into epoxy adhesive on the shear strength of adhesive/ Al joints have been investigated using lap-shear tensile tests. First, the influence of adhesive thickness on the shear strength of joints has been presented. Then, the effects of various Al surface treatments on the surface roughness of Al and shear strength of joint have been researched. Atomic force microscopy was used to study the Al surface morphologies and textures. Finally the few micron-sized polymeric microcapsules were synthesized and the shear performances of microcapsule filled epoxy adhesives were inspected. It was observed that the HCl acid based etching increased both micro-roughness and nano-texture of the Al surface and led to the peak shear strength. Moreover, HCl-nitric acid treatment offered the maximum value for the cohesive failure. Capsule inclusions into the adhesive displayed different influences on the joint shear performances depending on the capsule morphology and the surface treatment of Al.     

Hydrophobic recovery of repeatedly plasma-treated silicone rubber. Part 1. Storage in air

Silicone rubber is used for a wide variety of biomedical and industrial applications due to its good mechanical properties, combined with a hydrophobic surface. Frequently, however, it is desirable to alter the surface hydrophobicity of silicone rubber. Often this is done by plasma treatments but the effects are usually transient. In this study, surfaces of medical grade silicone rubber have been repeatedly modified by means of oxygen, argon, carbon dioxide, and ammonia RF plasma treatments with a 24 h time interval in between treatments. Treated samples were stored in air prior to surface characterization by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), streaming potential measurements, and profilometry for surface roughness. The carbon percentage of the surfaces decreased after plasma treatment, while the silicon and oxygen percentages increased irrespective of the plasma used. The formation of Si-O-Si bridges between siloxane chains after plasma treatment was demonstrated by the appearance of a new component in the Si_2p peak but the degree to which this occurred differed per gas. Streaming potential measurements in a 10 mM potassium phosphate buffer indicated a more negatively charged surface for treated samples compared to untreated samples (-23.3 mV at pH 7.0). Surface roughness increased slightly for repeatedly plasma-treated samples from R_A = 0.35 μm to R_A = 0.46 μm, while scanning electron microscopy showed the presence of several 'cracks' spanning the surface after repeated treatment. Argon, carbon dioxide, and ammonia plasmas significantly reduced the advancing water contact angle from 115° to 58°, 72°, and 85°, respectively, on a more permanent basis (especially when the treatments were repeated after recovery). Oxygen plasma effects on water contact angles generally disappeared within 5 h, also after repeated treatment.     

Field emission properties of N2 and Ar plasma-treated multi-wall carbon nanotubes

We modify multi-wall carbon nanotubes (MWCNTs) by plasma treatment with N₂ and Ar for varying durations and measure their field emission characteristics. The N₂ treated MWCNTs showed significant improvement in field emission properties, while the Ar treated MWCNTs displayed poorer field emission characteristics compared to untreated MWCNTs. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman spectroscopy and work function measurements are used to investigate the field emission mechanisms after plasma treatments.