Effects of surface modification by oxygen plasma on peel adhesion of pressure‐sensitive adhesive tapes

Surfaces of poly(isobutylene) (PIB) and poly(butylacrylate) (PBA) pressure‐sensitive adhesive tapes were treated by oxygen plasma, and effects of surface modification on their adhesive behavior were investigated from the viewpoint of peel adhesion. The peel adhesion between PIB and PBA pressure‐sensitive adhesive tapes and stainless steel has been improved by the oxygen plasma treatment. The surface‐modification layer was formed on PIB and PBA pressure‐sensitive adhesive surfaces by the oxygen plasma treatment. The oxygen plasma treatment led to the formation of functional groups such as various carbonyl groups. The treated layer was restricted to the topmost layer (50–300 nm) from the surface. The GPC curves of the oxygen plasma‐treated PBA adhesive were less changed. Although a degradation product of 1–3% was formed in the process of the oxygen plasma treatment of the PIB adhesive. There are differences in the oxygen plasma treatment between the PIB and PBA adhesives. A close relationship was recognized between the amount of carbonyl groups and peel adhesion. Therefore, the carbonyl groups formed on the PIB and PBA adhesive surfaces may be a main factor to improve the peel adhesion between the PIB and PBA adhesive and stainless steel. The peel adhesion could be controlled by changing the carbonyl concentration on the PIB and PBA adhesive surfaces. We speculate that the carbonyl groups on the PIB and PBA adhesive surface might provide an interaction with a stainless steel surface. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1392–1401, 2000     

Adhesion to Plasma-Modified LaRC-TPI I. Surface Characterization

LaRC-TPI, an aromatic thermoplastic polyimide, was exposed to oxygen, argon and ammonia plasmas as pretreatments for adhesive bonding. Chemical changes which occurred in the surface as a result of the plasma treatments were investigated using x-ray photoelectron spectroscopy (XPS) and infrared reflection-absorption spectroscopy (IR-RAS). Water contact angle analysis was utilized to characterize the changes in surface wettability, and the ablative effects of the plasmas were monitored using ellipsometry. Both XPS and IR-RAS results indicated the formation of polar functional groups at the surface. Contact angle analysis showed enhanced water wettability of the plasma-treated surface. Oxygen and argon plasmas were highly ablative, whereas ammonia plasma was only moderately so. Oxygen and argon plasmas appear to react with the LaRC-TPI via a fragmentation/oxidation mechanism; the effect of ammonia plasma is postulated to be imide ring-opening resulting in the formation of amide functional groups.     

Viscoelastic analysis on peel adhesion of adhesive tape by matrix method

Analysis by means of matrix method is presented on the phenomenon of peel adhesion for 90° peeling of adhesive tape. A model of framed structure was assumed to duplicate the viscoelastic behavior of the tape: The adhesive layer is composed of a network structure made by elastic members for lattice elements and viscous members for diagonal elements. Calculated force distribution near the bond boundary showed good agreement with the experimental results of Kaelble. It was also found that the curve of peel rate versus peel force for the cohesive failure occurred in the adhesive layer was S-shaped; the change of peel force was affected severely by particular range of peel rate. For the interfacial failure at the bound boundary, on the other hand, the peel force possessed a maximum value for medium peel rate. Predicted failure mode for the adhesive tape changed from cohesive failure to interfacial failure with increasing rate of separation. Analytical results for the dependences of thickness of flexible members and adhesive layers on peel forces showed qualitative correlation with the experimental results.     

Adhesion Between Plasma-Treated Polypropylene Films and Thin Aluminum Films

Polypropylene (PP) film was treated with radio-frequency-induced oxygen plasma, followed by the vacuum deposition of aluminum (Al) thin film, and the peel strength of the Al deposited PP film (Al/PP) was examined. The peel strength of plasma-treated PP film varied widely in the range of 6.7 to 157 N/m depending upon the plasma treatment conditions, whereas that of the untreated PP was 5.2 N/m. The peel strength was minimized at oxygen pressure near 13.3 Pa (0.1 Torr), and decreased with increasing discharge power. The peel strength rapidly increased at the initial stage of plasma treatment (∼ several seconds), decreased at the second stage, and slightly increased again at the third stage. A good agreement was found between the peel strength of Al/PP and the amounts of oxygen introduced onto the PP surface at the initial stage. A short-time treatment was very effective to improve the adhesion of Al/PP. At the end of the second stage, a large amount of carbon was detected by XPS on the Al layer of the peeled interface of Al/PP, which gave a minimum peel strength. Cohesive failure of PP film might have occurred. SEM photograph showed that PP surface was etched by oxygen plasma at the thrid stage. These peel behaviors of Al/PP were explained by the chemical and physical changes of the PP surface caused by oxygen plasma treatment: (1) introduction of O-functional groups onto the PP surface at the initial stage, (2) formation of weak booundary layers resulting from the partial scission of PP molecules at the second stage, and (3) plasma etching of the PP surface at the third stage.     

The Peeling Behavior of Pressure Sensitive Adhesives from Uncoated Papers

When a tape based on pressure sensitive adhesive (PSA) is peeled from paper, either the tape comes off leaving a clean paper surface or the paper undergoes catastrophic cohesive failure which is the delamination of fiber layers in the paper sheet. The objectives of this work were to determine the links between paper properties and peel characteristics. Peel tests and microscopic analysis of a variety of handmade and commercial papers yielded the following conclusions. The tendency for paper failure is not very sensitive to surface energy. Paper roughness and density seems to be the dominant factors. A relatively smooth but weak paper will give a clean peel whereas a strong but rough handsheet or filter paper will always give paper failure. Lamination pressure is also important. Peel force increases with lamination pressure up to a limit where paper failure begins and the peel force plummets. The peel response of common paper types is mapped onto a 2-D surface whose axis reflects paper surface chemical and structural properties. It is proposed that the initiation of paper failure in peel occurs at fiber ends.     

The Peeling Behavior of Pressure Sensitive Adhesives from Uncoated Papers

When a tape based on pressure sensitive adhesive (PSA) is peeled from paper, either the tape comes off leaving a clean paper surface or the paper undergoes catastrophic cohesive failure which is the delamination of fiber layers in the paper sheet. The objectives of this work were to determine the links between paper properties and peel characteristics. Peel tests and microscopic analysis of a variety of handmade and commercial papers yielded the following conclusions. The tendency for paper failure is not very sensitive to surface energy. Paper roughness and density seems to be the dominant factors. A relatively smooth but weak paper will give a clean peel whereas a strong but rough handsheet or filter paper will always give paper failure. Lamination pressure is also important. Peel force increases with lamination pressure up to a limit where paper failure begins and the peel force plummets. The peel response of common paper types is mapped onto a 2-D surface whose axis reflects paper surface chemical and structural properties. It is proposed that the initiation of paper failure in peel occurs at fiber ends.     

Effects of moisture and temperature ageing on reliability of interfacial adhesion with black copper oxide substrate

The reliability of adhesion performance of bare Cu, as-deposited and surface-hardened black oxide coatings on Cu substrates was studied. The interfacial adhesion with a polyimide adhesive tape and an epoxy moulding compound was measured using the button shear and tape peel tests after hygrothermal ageing in an autoclave, high temperature ageing and thermal cycles. Moisture adsorption and desorption studies at different aging times suggested that the black oxide coating was effective in reducing the moisture adsorption. The bond strengths for all substrates remained almost unchanged after thermal ageing at 150°C for 8 h. Thermal cycling between ?50°C and 150°C for 500 cycles reduced by about 20% the button shear strength of the as-deposited black oxide substrate, but it did change much the bonding performance of the bare Cu substrate. Hygrothermal ageing at 121°C/100% RH in an autoclave was most detrimental to adhesion performance because of the combined effect of elevated temperature and high humidity. The reduction in button shear strength after the initial ageing for 48 h was 50–67%, depending on the type of coating. In all accelerated ageing tests, the residual interfacial bond strengths were consistently much higher for the black-oxide-coated substrates than the bare Cu surface, confirming a higher reliability of black oxide coating. Fracture surfaces analysis of tape-peeled bare copper substrates after 500 cycles of thermal loading revealed a transition in failure mechanism from interfacial to cohesive failure. In contrast, the failure mechanism remained unchanged for black-oxide-coated substrates. The observations made from the button shear and tape peel tests were generally different because of the different fracture modes involved.     

Atmospheric pressure plasma activation as a surface pre-treatment for the adhesive bonding of aluminum 2024

A low-temperature, atmospheric pressure helium and oxygen plasma has been used for the surface preparation of aluminum 2024 prior to adhesive bonding. The plasma converted the aluminum from a water contact angle (WCA) of 79° to down to 38° within 5 s of exposure, while sanding reduced the WCA to only 51°. Characterization of the aluminum surface by X-ray photoelectron spectroscopy revealed a decrease in carbon contamination from 70 to 36% and an increase in the oxygen content from 22 to 50% following plasma treatment. Similar trends were observed for sanded surfaces. Lap shear results demonstrated bond strengths of 30?±?2?MPa for the sanded aluminum vs. 33?±?1?MPa for plasma-treated aluminum, where sol–gel and primer coatings were added to the surface preparation. Following seven days of aging, wedge crack extension tests revealed cohesive failure percentages of 86, 92, and 96% for sanded, plasma-treated, and sanded/plasma-treated aluminum, respectively. These results indicate that atmospheric pressure plasmas are an attractive alternative to acid treatment or abrasion techniques for surface preparation prior to bonding.     

Improvement in the adhesion of polyimide/epoxy joints using various curing agents

An improvement in the adhesion strength of polyimide/epoxy joints was obtained by (1) introducing a functional group on the polyimide surface, (2) improving the mechanical properties of the epoxy adhesive, (3) increasing the curing temperature, and (4) using polyamic acid as an adhesion‐promoting layer. The functional group on polyimide was introduced via treatment with aqueous KOH. An adhesion‐promoting layer was formed by spin coating polyamic acid onto a modified polyimide surface. The maximum adhesion strength of the polyimide/epoxy joint was obtained using polyamic acid as both the adhesion‐promoting layer and as the curing agent. The surface energy of the modified polyimide was examined using contact angle measurements and Fourier transform infrared spectroscopy, and the peel strength was determined by the T‐peel method. The peeled surfaces were analyzed using scanning electron microscopy and X‐ray photoelectron spectroscopy.© 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 812–820, 2002     

Effects of environment and heat treatment on an oxygen plasma-treated polyimide surface and its adhesion to a chromium overcoat

—The effects of oxygen plasma treatment time, duration of storage, and heat treatment on the surface chemistry of and Cr adhesion to Dupont RC5878 and Kapton polyimides were investigated using X-ray photoelectron spectroscopy (XPS), and contact angle and peel strength measurements. The XPS results indicate that the initial stage of plasma treatment involves oxygen adsorption with insignificant modification of the surface chemistry. After 5 s of plasma treatment the surface chemistry is modified, as suggested by the changes in the carbonyl and partially oxidized carbon functional groups' contributions to the C(1s) line shape. These modifications resulted in an increase in the peel strength and a decrease in the contact angle of water. Over the first month of storage, the intensity of the carbonyl functional group peak decreased, while the contact angle increased and reached a steady-state value of 30° after 20 days of storage. These changes are mainly attributed to moisture absorption. Importantly, the metal adhesion to polyimide remained fairly constant over the storage period. The aged plasma-treated surface experienced loss of moisture when baked at 150°C for less than 5 min. This was followed by an increase of the partially oxidized carbon at the expense of the plasma-induced carbon-oxygen bonds at higher baking temperatures or longer times.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Corona-discharge Treatment of Polypropylene films-Effects of Process Parameters

Corona treatment of films, mainly polypropylene (PP)-copolymers, was studied at commercial levels in a 2.7 kVA treater. The films were produced on a flat-film extruder with chill rolls. Degree of treatment was characterized by power of the generator divided by web speed and width of film (m Ws/cm²).

The effectiveness of the treatment was measured in terms of the polar and dispersion components of surface-energy, the peel adhesion of pressure sensitive tape (similar to ASTM Adhesion Ratio) and the peel adhesion of polyurethane adhesives.

The polar component of surface energy is a measure of the effectiveness of corona pretreatment. For a given degree of treatment, the polar surface energy component becomes greater as the film cooling rate increases (and the degree of crystallization falls).

A comparison of homopolymers and copolymers does, however, reveal that even where these have the same density or the same degree of crystallization one cannot count on them having equally-sized polar components.

Peel strengths of pressure-sensitive tapes and polyurethane-bonded patches confirm the influence of cooling conditions on wetting properties.

Contrary to the case for tape adhesion, the polyurethane adhesive strengths reach their maximum value at much lower treatment intensities, i.e. with much lower polar surface energy components, and thus question the validity of the ASTM tests for adhesion properties.     

Investigation of a new reactant for fluorinated polymer surface treatments with atmospheric pressure glow plasma to improve the adhesive strength

Poly(tetrafluoroethylene-co-perfluoro [alkyl vinyl ether]) (PFA) and polytetrafluoroethylene (PTFE) films were treated by three kinds of atmospheric pressure glow plasmas: an untreated sample was treated by He plasma or trimethoxyborane (TMB)/H₂/He plasma, and a TMB-absorbed sample was treated by H₂/He plasma. TMB was a new reactant for the treatment, to increase the films’ adhesive strength with an epoxy glue. These films were also treated by a wet method using a sodium solution (Tetra-Etch compound) and such films were used as the control samples. The peel strength values of the controls of PFA and PTFE were 3.5 and 9.5 N cm⁻¹, respectively. The adhesive strengths of all plasma-treated PFAs were stronger than those of untreated one. Especially, the peel strength of the TMB/H₂/He plasma-treated PFA showed the maximum value of 4.5 N cm⁻¹, which was bigger than that of the control one. The adhesive strength of the TMB/H₂/He plasma-treated PTFE films also showed the maximum peel strength, 7.9 N cm⁻¹, but this value did not exceed that of the control PTFE. Such results suggested that the TMB/H₂/He plasma had the advantage of providing better adhesive improvement of those polymers, especially PFA than the wet method could provide. The results of XPS and SEM indicated that TMB actively removed fluorine atoms from the polymer surface. Therefore, boron compounds are effective for the improvement of the adhesive strength between the fluorinated polymer and the epoxy glue.     

Plasma-induced adhesion improvement of cotton/polypropylene-laminated fabrics

In this study, improvement in the adhesion strength of plasma-pretreated and laminated cotton/polypropylene (PP) fabrics using acrylic-based adhesive was investigated. Low-temperature, low-pressure oxygen plasma was utilized for surface modification of cotton/PP-laminated fabrics. Water absorption time was measured on plasma-treated cotton fabrics at different plasma power and treatment time conditions. The plasma conditions providing the fastest liquid absorption on the surface were selected and applied during plasma pretreatments. Surface wettability increased with increasing plasma power and plasma exposure time. Plasma-induced surface morphology changes were observed via Scanning Electron Microscope (SEM) images. X-ray Photoelectron Spectroscopy (XPS) analysis showed that oxygen content on the surface increased with plasma treatment, which contributed to the surface polarity and hydrophilicity. Peel bond strength results of untreated and plasma-treated samples were analyzed to determine the effect of plasma pretreatment process. Adhesion strength values of laminated samples, before washing and after 40 wash cycles, were determined by peel bond strength tests. Before washing, adhesion strength of plasma pre-treated, laminated samples was 28–60% higher than that of untreated laminated fabrics. After 40 wash cycles, adhesion strength of plasma pre-treated and laminated samples was about 40–69% higher than the untreated laminated fabrics. Peel bond strength values decreased with the increased number of wash cycles. Plasma pretreatment enhanced both the adhesion strength and washing resistance of laminated samples.     

An Investigation of the Effect of Plasma Treatment on the Surface Properties and Adhesion in Sheet Molded Composite (SMC) Material

Sheet molded composite was treated with different plasmas (oxygen, dry air, nitrogen, and argon). Plasma treatment of SMC alters the surface properties in a manner dependent on the type of plasma used and the time of treatment. The surface properties were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Fourier transform infrared spectros-copy (FTIR). A two-part urethane adhesive was used to prepare lap shear specimens. Untreated SMC, plasma-treated SMC, and primer-treated SMC were prepared, bonded and tested. The surface properties of the failed specimens were measured. The adhesion characteristics of SMC and the surface properties of the failed specimens were correlated with the type of treatment and the surface properties of treated SMC. Comparison of the surface and adhesive properties of plasma-treated samples with those for untreated samples indicates a) an increase in roughness, b) an increase in the level of SMC surface oxidation, and c) an increase in the failure force for lap shear tests.     

An Investigation of the Effect of Plasma Treatment on the Surface Properties and Adhesion in Sheet Molded Composite (SMC) Material

Sheet molded composite was treated with different plasmas (oxygen, dry air, nitrogen, and argon). Plasma treatment of SMC alters the surface properties in a manner dependent on the type of plasma used and the time of treatment. The surface properties were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Fourier transform infrared spectros-copy (FTIR). A two-part urethane adhesive was used to prepare lap shear specimens. Untreated SMC, plasma-treated SMC, and primer-treated SMC were prepared, bonded and tested. The surface properties of the failed specimens were measured. The adhesion characteristics of SMC and the surface properties of the failed specimens were correlated with the type of treatment and the surface properties of treated SMC. Comparison of the surface and adhesive properties of plasma-treated samples with those for untreated samples indicates a) an increase in roughness, b) an increase in the level of SMC surface oxidation, and c) an increase in the failure force for lap shear tests.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T , where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T , i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T, where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T, i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

Ta and TaN adhesion to high-temperature fluorinated polyimides. Surface and interface chemistry

The practical adhesion of Cu/Ta to high-temperature fluorinated polyimides (FPIs) was initially good but failed after the reliability test involving treatment under the FPI curing condition five times (T₅). But a thin layer (40 nm) of TaN greatly improved the reliability of the Cu/Ta-to-FPI adhesion. Both CF₄ and in situ Ar plasma treatments of FPIs prior to metal deposition enhanced the metal-to-FPI adhesion strength. CF₄ plasma enriches the FPI surface with fluorine atoms and most of fluorine is bound to carbon as CF₃, CF₂, and CF. Ar plasma first destroys CF₃ and then C=O groups of the FPIs to yield a polar surface. The locus of failure by a 90° peel test was found to be within the Ar-plasma-modified FPI layer but it moved toward the bulk of FPI, i.e. away from the metal-polymer interface, after the T₅ reliability test. The locus of failure in the case of weak adhesion where no plasma treatment was done on FPI films was in the near-interface region within the FPI layer, and the failure seemed to occur in the weak boundary layers of FPI surfaces. Plasma treatment removes weak boundary layers and also increases FPI surface roughness. These two effects combined improved the metal-to-FPI practical adhesion.     

The aging effects of repeated oxygen plasma treatment on the surface rearrangement and adhesion of LDPE to aluminum

The effects of aging temperature and time on the adhesion properties of oxygen plasmatreated low-density polyethylene (LDPE) were investigated. As the aging temperature and time increased, surface rearrangement and the migration of molecules containing polar functional groups into the bulk were accelerated to the surface to form a hydrophobic surface. The adhesion strength of oxygen plasma-treated LDPE/aluminum joints was measured using a 90° peel test by varying the plasma treatment time and aging temperature. The adhesion strength was constant, regardless of the plasma treatment time. As the aging temperature increased, the adhesion strength of the LDPE/aluminum joints decreased and the locus of failure changed from cohesive to interfacial failure. It was also found that the polar functional groups buried in the bulk could be reoriented to the surface in a polar environment. This study also investigated whether repeated oxygen plasma treatment would increase the concentration of polar functional groups at the surface and reduce the surface rearrangement and the migration of molecules containing polar functional groups during aging. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) showed that repeated oxygen plasma treatments increased the concentration of polar functional groups at the surface. However, the aging time between plasma treatments had a negligible effect on the concentration of polar functional groups at the surface.