Tantalum and chromium adhesion to polyimide. Part 2. Peel and locus of failure analyses

Ta and Cr adhesion to 3,3'-4,4'-biphenyl tetracarboxylic acid dianhydride-p-phenylenediamine derived (BPDA-PDA) polyimide (PI) surfaces has been studied before treatment, and after CF₄ reactive ion etching (RIE), and Ar sputtering. The initial peel adhesion results for both metals on the BPDA-PDA surfaces are comparable and show increased peel adhesion as a function of the surface treatment in the following order: virgin (no treatment) < Ar sputter < CF₄ RIE ~ CF₄ RIE followed by Ar sputter. The surface roughness effect on metal/PI adhesion has also been investigated. The data suggest that the surface roughness does not primarily affect peel adhesion. In this case, it is the removal of the weak boundary layer and the cracking of the residual PI on the metal peel interface surface during the peeling process which cause the increase in the peel strength. It is also proposed that the changes observed in the peel strength as a function of the surface treatment are due to differences in the fracture toughness of the modified PI layers rather than differences in the surface roughness.     

Surface Characterization of Chlorinated Synthetic Vulcanized Styrene-Butadiene Rubber Using Contact Angle Measurements,Infra-Red Spectroscopy and XPS

The surface of a sulfur-vulcanized synthetic styrene-butadiene rubber (SBR) was treated with ethyl acetate solutions containing different amounts of trichloroisocyanuric acid (TCI). The chlorinated SBR surfaces were characterized using contact angle measurements (water, ethane diol, n-hexadecane), infra-red (IR) spectroscopy and XPS. Chlorination produced an increase of surface free energy which was mainly due to the enhancement of the acid-base component of the surface free energy, which remained almost unchanged when the amount of TCI was increased. Depending on the amount of chlorination agent, several chemical species were present on the SBR surface: i) For low amounts of TCI (up to 2 wt%), mainly chlorinated hydrocarbon and C – O species were present on the surface; ii) For medium amounts of TCI (between 2 and 5 wt%), an excess of unreacted TCI remained on the surface and a relatively small amount of isocyanuric acid was deposited; iii) For high amounts of TCI (larger than 5 wt%), a weak boundary layer (mainly composed of isocyanuric acid) was formed on the surface and thus the effects due to chlorination were decreased. There was good agreement between the experimental results obtained with contact angle measurements and XPS because both provided information on a surface region close to 100 Å, whereas IR spectroscopy results showed deeper penetration of the chlorinating agent into the SBR surface. The estimated thickness of the chlorinated layer was near 5000 Å as estimated from XPS measurements of SBR surfaces modified by argon ion bombardment.     

Surface modification of ultra-high strength polyethylene fibers for enhanced adhesion to epoxy resins using intense pulsed high-power ion beam

The effects of intense pulsed high power ion beam (HPIB) treatment of ultra-high strength polyethylene (UHSPE) fibers on the fiber/epoxy resin interface strength were studied. For this study, argon ions were used to treat Spectra? 1000 (UHSPE) fibers in vacuum. Chemical and topographical changes of the fiber surfaces were characterized using Fourier transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR), X-ray photoelectron spectroscopy (XPS), dynamic wettability measurements, and scanning electron microscopy (SEM). The fiber/epoxy resin interfacial shear strength (IFSS) was evaluated by the single fiber pull-out test. The FTIR-ATR and XPS data indicate that oxygen was incorporated onto the fiber surface as a result of the HPIB treatment. The wettability data indicate that the fibers became more polar after HPIB treatment and also more wettable. Although the total surface energy increased only slightly after treatment, the dispersive component decreased significantly while the acid-base component increased by a similar amount. SEM photomicrographs revealed that the surface roughness of the fibers increased following the HPIB treatment. The single fiber pull-out test results indicate that HPIB treatment significantly improved the IFSS of UHSPE fibers with epoxy resin. This enhancement in IFSS is attributed to increased roughness of the fiber surface resulting in mechanical bonding and in increased interface area, increased polar nature and wettability, and an improvement in the acid-base component of the surface energy after the HPIB treatment.     

Effects of argon plasma treatment on surface characteristic of photopolymerization PEGDA–HEMA hydrogels

The aim of this research was to determine the influence of argon plasma treatment condition on the surface properties of poly(ethylene glycol) diacrylate (PEGDA)–hydroxyethly methacrylate hydrogel films, a kind of scaffold materials for tissue engineering. The changes of surface properties have been evaluated by contact angles, X‐ray photoelectron spectra (XPS), and scanning electron microscopy (SEM). From the contact angle measurements of different liquids, the surface free energy of the hydrogel was calculated according to approaches by Owens–Wendt–Kaelble. Results showed that the contact angle of the hydrogel to water decreased remarkably after argon plasma treatment, which was caused by the changes in morphology (SEM images) and chemical composition (XPS results) of the argon plasma‐treated surface. The surface free energy increased with the increase of the argon plasma treated time and power, and these increasing was mainly due to the increase of polar component. The XPS results confirmed that plasma oxidation reaction produced oxygen‐containing functional groups onto the surface. This functional group played an important role in increasing the hydrophilic properties of the hydrogel. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Improvement of adhesion of PET fibers to rubber by argon-oxygen plasma treatment

Polyethylene terephthalate fibers cords were modified with argon, oxygen, and successive argon/oxygen cold plasmas as a function of treatment time. Plasma treated cords were coated with resorcinol formaldehyde latex, then tested as rubber reinforcing materials. The peel strength was discussed with respect to the polar component of the surface energy and the etching of the fibers. An increased adhesion of ∼ 280% was obtained with 30 min argon plasma followed by 30 min oxygen plasma, at 75 W power and 40 Pa pressure without altering the traction strength of the fibers cords. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2321–2330, 1998     

Characterization of TiO2 surfaces active for novel organic syntheses

Quantitative distributions were determined for the oxidation states of titanium cations on TiO₂(001) surfaces reduced by argon ion bombardment and reoxidized by thermal treatment. Information about the inhomogeneity of these distributions within the near-surface region sampled by XPS was obtained by angle-resolved measurements in which the position of the analyzer axis was varied with respect to the surface normal. These experiments demonstrated that (1) sputtering with 2 keV ions produced a surface containing Ti cations in the 1+, 2+, 3+, and 4+, but not 0 oxidation states; (2) these were reoxidized in a roughly sequential fashion as the surface was annealed to 750 K, at which temperature reoxidation to Ti⁴⁺ was complete; and (3) the average oxidation state of the topmost layers of the ion-bombarded surface was slightly higher than that determined from XPS spectra collected normal to the surface. Since the surfaces reduced by argon ion bombardment are active for organic assembly processes, including reductive coupling of aldehydes to form symmetric olefins, these surface characterization results demonstrate that Ti⁰ sites arenot required to effect reductive coupling, in contrast to conclusions from previous slurry-phase studies. These observations enhance the possibility that reductive carbonyl coupling might be rendered catalytic, since a smaller cycle of oxidation states is required than previously recognized.     

Adhesion to Plasma-Modified LaRC-TPI II. Effect of Plasma Treatment on Peel Strength

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. A 180[ddot] peel test with an acrylate-based pressure sensitive adhesive tape as an adherend was utilized to study the interactions of the plasma-treated polyimide surface with another polymeric material. The peel strengths of the pressure sensitive adhesive tape on the plasma-treated LaRC-TPI fell below the level of the non-treated controls, regardless of the plasma treatment used. Failure surface analysis by XPS revealed the presence of polyimide on the pressure sensitive adhesive failure surface, indicating failure in the plane of a weak boundary layer created by plasma treatment. The removal of the weak boundary layer by a solvent rinse restored the peel strength to the level of the control. Comparison with tape adhesion peel strengths of oxygen plasma-treated high density polyethylene showed that the physical condition of a polymer surface following plasma treatment plays an important role in determining the level of adhesion which can be achieved.     

Effect of woody biomass surface free energy on the mechanical properties and interface of wood/polypropylene composites

The aim of this study is to investigate the effect of surface free energy of wood flour (WF) and silanized WF on the mechanical properties and interface of wood/polypropylene (PP) composites. The contact angles of three probe liquids against unmodified and modified spruce WF were tested by capillary rise method based on the Washburn equation. Then the surface free energy and its corresponding dispersion and polar components were calculated according to the method developed by Owens–Wendt–Kaelble. The tensile strength and flexural strength of the wood/PP composite samples made with unmodified and modified WF were tested and the flexural fracture surfaces were analyzed by scanning electron microscopy (SEM). The results showed that the surface free energy of WF increased from 26.0 to 36.1?mJ/m², which was higher than that of PP (29.4?mJ/m²), and its corresponding polar component decreased from 13.1 to 4.4?mJ/m², and the dispersion component increased from 12.9 to 31.7?mJ/m² after the modification with 4 wt.% vinyltriethoxy silane, which makes it possible for spreading of PP on the surface of WF, the tensile strength and flexural strength of wood/PP composites made with modified WF were obviously improved. In addition, the improved compatibility between WF and PP was well confirmed by SEM.     

X-ray photoelectron spectroscopic study of the ability to monofunctionalize polymer surfaces by low energy atomic bombardment

Low energy atomic bombardment is designed to limit the potentially reactive species to the atoms which are present in a radiofrequency (r.f.) plasma discharge. This might give the ability to monofunctionalize surfaces for specific applications such as biocompatibility. Bombardment of polystyrene and poly(trans)isoprene with nitrogen atoms does not result in the incorporation of any heteroatoms at the polymer surface. Other attempts were made on poly(acrylic acid) and poly(methyl methacrylate) without any results, even though the samples had been activated by argon ion bombardment; the thermalized atoms seem to have insufficient energy to functionalize these polymers. In contrast, ion bombardment performed with low energy nitrogen ions (500–1500eV) is able to incorporate a majority of amine functions. © 1998 Society of Chemical Industry.     

Surface modification of polyimide films by argon plasma for copper metallization on microelectronic flex substrates

Surface modification of polyimide films such as Kapton E(N) and Upilex S by argon plasma was investigated because of the enhanced adhesive strength with sputtered copper. Peel tests demonstrated this improvement, with a peel strength of 0.7 and 1.2 g/mm for unmodified Kapton E(N) and Upilex S, respectively, and 110.3 and 98 g/mm for argon plasma–modified Kapton E(N) and Upilex S, respectively, in certain plasma conditions. This study showed that the enhanced adhesive strength of polyimide films with sputtered copper by argon plasma was strongly affected by the surface characteristics such as surface morphology and surface energy of polyimide films. Atomic force microscopy and the sessile drop method indicated that the surface roughness and surface energy of the polyimide films were greatly increased by argon plasma, resulting in highly increased peel strength of the polyimide films with sputtered copper. It was observed in electron spectroscopy for chemical analysis (ESCA) that the increased surface energy of the polyimide films from argon plasma was a result of more of the surface being composed of O and N and of the increased number of C? O, C?O, and C? N chemical bonds. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 744–755, 2006     

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.     

The surface treatment and adhesive bonding of polyetheretherketone. Part II. Surface characterization

A range of surface treatments including chromate and plasma etching, which are capable of producing both chemical modification and surface topography modification of polymer surfaces, were applied to polyetheretherketone. The resulting surfaces were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and contact angle studies. An enhanced oxygen concentration measured by XPS corresponded to a high polar surface energy contribution and formed the main requirement for a high adhesive joint strength.     

Plasma surface modification of polyimide for improving adhesion to electroless copper coatings

The influences of oxygen plasma treatment of polyimide (PI) films on the adhesion of electroless copper coatings as well as on the chemical composition of the film surface and the PI surface morphology were investigated. The plasma operating parameters were 1800 W forward power with O₂ flowing at a rate of 300 cm³/min at a pressure of 200 mTorr. The peel strength increased with decreasing plasma treatment temperature. However, extension of the treatment time at higher temperatures had a positive effect on adhesion. A correlation between the enhancement in peel strength and the content of oxygen-containing groups at the PI surface (investigated using XPS) was observed. A change in the morphology as a result of plasma etching was also observed, in the formation of pits in the film surface. The pits ranged from 3 to 6 μm in depth and the diameter varied from 10 to 200 μm. Comparison of the data obtained after plasma treatment with the results of chemical etching in alkaline solutions of permanganate showed approximately the same adhesion increase (to 0.6 kN/m) in both cases. However, chemical etching did not affect the surface morphology and increased the oxygen content at the PI surface less than the plasma treatment.     

Fluoroalkylation of polyester by end-capped fluoroalkyl-functional silanes

The surface modification of polyester was examined using both monomeric and oligomeric silanes having end-capped fluoroalkyl groups. From contact angle measurements, the surface free energies of polyester were reduced to 15-20 mJ/m² for the dispersive component and 1 -3 mJ/m² for the polar component, respectively, and all the surfaces were shown to be both highly water- and oil-repellent. By XPS (X-ray photoelectron spectroscopy) measurements, using the C l.s peak attributable to the C=O of polyester, the thickness of the siloxane layer on the surface was shown to be less than 5 nm. The solvent durability (resistance) of the modified surfaces was evaluated using contact angle and XPS measurements. Although all the modified surfaces showed durability against dodecane, xylene, ethyl acetate, tetrachloroethylene, and hydrochloric acid, long-time immersion in fluorine-containing solvents reduced the oil repellency of some of the surfaces modified with monomeric or oligomeric silanes having short fluoroalkyls. In particular, immersion in alkaline solution destroyed the siloxane network and thus reduced their water repellency, while interestingly their oil repellency remained unchanged. The modification mechanism is also discussed in terms of simultaneous thermal anchoring and polymerization of silanes.     

Calendar of events

This review reports the successful synthesis of novel oligomeric silanes having end-capped fluoroalkyl groups. Glass surface was effectively modified by these oligomeric silanes. In particular, oligomeric silanes were more reactive and effective in the surface fluoroalkylation than monomeric silanes. From contact angle measurements, surface free energies were reduced to 15–20 and 1–3 mJ/m² for the dispersive and the polar components, respectively, and the surfaces were shown to be both highly water- and oil-repellent. Modified glass surface was analyzed using XPS. A linear correlation was observed between the dispersive component of surface free energy γ_S ^d and the area ratio of the F1s peak to the Si2p peak. The structure of the siloxane layer on the modified glass surface is discussed in terms of a network interphase model.     

Surface analysis and adhesion properties of coalesced latex films

The surface composition of films obtained from acrylic latexes (45 wt% MMA, 55 wt% BuA) stabilized by two anionic surfactants (either sodium dodecyl sulphate (SDS) or sodium dodecyl diphenyl ether disulphonate (SDED) has been studied by attenuated total reflection with Fourier transformed infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The results of this surface analysis are summarized and used to interpret adhesion properties of the films on glass substrate. Adhesion properties were derived from peeling tests. In the case of SDED, the peel strength increases with surfactant surface concentration. This is due to strong polar interactions between the hydrophilic part of the surfactant and the polar groups of the glass surface and to the anchorage of the hydrophobic part of the surfactant in the copolymer matrix. In the case of SDS, the peel strength decreases with increasing surfactant surface concentration. The same polar interactions as with SDED exist, but the SDS layer at the interface is very thick. It forms what is called a ‘weak boundary layer’ in which rupture propagates easily.     

Inhibition of aging in plasma-treated high-density polyethylene

The effects of cross-linking and crystallinity on the aging of plasma-treated high-density polyethylene (HDPE) have been investigated. In the case of mixed argon and oxygen, aging has been found to be reduced with an increased amount of argon in the mixture owing to an increased degree of cross-linking. A similar decrease in hydrophobic recovery has been achieved by increasing the crystallinity of HDPE. Diffusion of polar functional groups from the surface into the bulk has been observed to be lowered by both increasing the degree of cross-linking and crystallinity. The samples were analyzed by angle-resolved XPS, contact angle measurements and SEM investigations.     

Adhesion Properties of Acrylic Copolymers Modified with Si-Containing Copolymer

We made clear the cause of the increase in peel strength of pressure sensitive (PS) adhesives as a function of contact time, and investigated how to modify PS adhesives to maintain a low and constant peel strength for a long time. It was found that polar groups in the adhesive orient to the interface between the adhesive and the (stainless steel) metal substrate (SUS 304) so as to minimize interfacial free energy during adhesion, and the orientation increased affinity between the adhesive and the metal material and increased the peel strength as a result. The use of modifier which contained both P(MMA-co-SiMA) and PDMS showed an excellent modification effect, although modification with only PDMS or P(MMA-co-SiMA) was not sufficient. It was suggested that PDMS which migrated to the surface was extended uniformly over the surface by PDMS segments of P(MMA-co-SiMA) and that the enriched layer of PDMS on the adhesive surface worked as a barrier to prevent the orientation of polar groups in bulk. Therefore, low and constant peel strength could be achieved.     

Adhesion Properties of Acrylic Copolymers Modified with Si-Containing Copolymer

We made clear the cause of the increase in peel strength of pressure sensitive (PS) adhesives as a function of contact time, and investigated how to modify PS adhesives to maintain a low and constant peel strength for a long time. It was found that polar groups in the adhesive orient to the interface between the adhesive and the (stainless steel) metal substrate (SUS 304) so as to minimize interfacial free energy during adhesion, and the orientation increased affinity between the adhesive and the metal material and increased the peel strength as a result. The use of modifier which contained both P(MMA-co-SiMA) and PDMS showed an excellent modification effect, although modification with only PDMS or P(MMA-co-SiMA) was not sufficient. It was suggested that PDMS which migrated to the surface was extended uniformly over the surface by PDMS segments of P(MMA-co-SiMA) and that the enriched layer of PDMS on the adhesive surface worked as a barrier to prevent the orientation of polar groups in bulk. Therefore, low and constant peel strength could be achieved.     

End-capped fluoroalkyl-functional silanes. Part II: Modification of polymers and possibility of multifunctional silanes

This work reports the fluoroalkylation of polymer surfaces using novel oligomeric silanes having end-capped fluoroalkyl groups. Polymer surfaces such as cellulose, poly(ethylene terephthalate) (PET), polyethylene, and poly(methyl methacrylate) (PMMA) etc. were effectively modified by these oligomeric silanes as well as the glass surface. From the contact angle measurements, the dispersive and polar components of surface free energies were reduced to 15–20 and 1–3 mJ/m², respectively, and the surfaces were shown to be both highly water- and oil-repellent. Modified cellulose and PET surfaces were analyzed using XPS measurements. In the case of cellulose, a linear correlation was observed between the dispersive component of surface free energy γ_S ^d and the area ratio of the F1s peak to the Si2p peak. In the case of PET, the hickness of siloxane layer on the surface was shown to be less than 8 nm. The modified PET surface showed a high solvent durability against common organic and inorganic solvents except fluorochemicals and alkalis. The structure of the siloxane layer on the modified surface is discussed in terms of a network interphase model. It was also shown to be quite easy to add another function such as hydrophilicity (flip-flop character) and/or antibacterial property in addition to the water- and oil-repellency imparted by fluoroalkyl groups.