Effect of Cross-Linking on Tack and Peel Strength of Polymers

In this work the influence of cross-linking on the adhesive fracture energy and the peel strength is studied choosing polydimethylsiloxane (PDMS) as a model polymer. A series of samples was prepared by electron-beam irradiation which covers the whole range from a viscoelastic liquid to a cross-linked rubber. The mechanical behaviour of these PDMS samples was characterized by mechanical spectroscopy. Tack measurements with an instrument described elsewhere⁵ and peel measurements show that the adhesive fracture energy after short contact times as a measure of tack and the peel strength have a pronounced maximum in the range above the gel point, where the PDMS consists of a very loose and imperfect network and a high fraction of soluble polymer. In this range debonding is connected with the formation of fibrillar structures within the polymer.     

The Influence of Surfactants on the Peel Strength of Water-based Pressure Sensitive Adhesives

In order to study the effect of surfactants on the adhesive properties, peel measurements were performed with two series of model polymers of ethylhexylmethacrylate (PEHMA), the first prepared by emulsion polymerization with four anionic surfactants, and the second by post-adding the same surfactants to a surfactant-free latex. Cohesive fracture is observed at low peel rates; the peel strength depends on the bulk mechanical properties and is independent of the emulsifier. A transition to another type of separation occurs at higher peel rates, which seems to be an interfacial failure by visual inspection. Surface analytical studies, however, give evidence that this “interfacial” failure is, in fact, a mixed failure, leaving traces of the polymer on the substrate surface. The peel rate at this transition as well as the peel strength at mixed fracture are influenced by the surfactants. Large differences were observed between the four surfactants as well as between both series of polymers, leading to the conclusion that the surfactants have a different mobility within the film. This is also reflected by a different aging behaviour of the films.     

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.     

Comparison of Peel Tests for Metal–Polymer Laminates for Aerospace Applications

Standard peel tests for aerospace laminates based on metal–polymer systems, namely floating-roller and climbing-drum peel methods, have been accommodated in a unified theory of peeling. This theory also accommodates more basic peel tests such as T-peel and fixed-arm peel and also newer methods such as mandrel peel. These five methods have been applied to two aerospace laminate systems to critically examine their use in the determination of adhesive strength. The theory has been used to unify the outputs from the tests in terms of adhesive fracture toughness. In this way, the comparative merits of the methods can be commented on.

The validity of the standard methods has been put in doubt because of the absence of a correction for plastic bending energy and also because of the poor conformance of the peel arm to the roller system used in these methods. The unified theory and some measurements of peel-arm curvature help but not completely overcome some of these difficulties.

A further complication that arises in peel is a change in the plane of fracture. This reflects a transition from cohesive fracture in the adhesive to an adhesive fracture at the interfaces among adhesive, primer, and substrate. It is likely that such plane-of-fracture phenomena are intrinsic to evaluation of the laminate and that contemplation of cohesive fracture toughness for the adhesive cannot accommodate such events.     

Contents Lists and Abstracts from the China Journal “Chemistry and Adhesion”

In order to study the effect of surfactants on the adhesive properties, peel measurements were performed with two series of model polymers of ethylhexylmethacrylate (PEHMA), the first prepared by emulsion polymerization with four anionic surfactants, and the second by post-adding the same surfactants to a surfactant-free latex. Cohesive fracture is observed at low peel rates; the peel strength depends on the bulk mechanical properties and is independent of the emulsifier. A transition to another type of separation occurs at higher peel rates, which seems to be an interfacial failure by visual inspection. Surface analytical studies, however, give evidence that this “interfacial” failure is, in fact, a mixed failure, leaving traces of the polymer on the substrate surface. The peel rate at this transition as well as the peel strength at mixed fracture are influenced by the surfactants. Large differences were observed between the four surfactants as well as between both series of polymers, leading to the conclusion that the surfactants have a different mobility within the film. This is also reflected by a different aging behaviour of the films.     

The Use of Peel Ply as a Method to Create Reproduceable But Contaminated Surfaces for Structural Adhesive Bonding of Carbon Fiber Reinforced Plastics

In the fabrication of fiber-reinforced plastics materials peel plies are commonly used as an additional layer on top of the laminates to sponge up the surplus resin and to create an activated surface for adhesive bonding or coating by peel ply removal. In theory, the peel ply removal results in a new and uncontaminated fracture surface that is activated by polymer chain scission. The peel ply method is often presented as being a good surface treatment for structural bonding.

In this study carbon fiber-reinforced plastics (Hexcel® 8552/ IM7) were produced by the use of five different peel plies and a release foil made of polytetrafluorethylene (PTFE). The peel plies themselves and the surfaces on the CFRP created by peeling were examined by scanning electron microscopy (SEM), x-ray photo electron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), infrared (IR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements to characterize the surfaces produced. Furthermore, the bond strength of lap shear and floating roller peel samples was determined with and without additional plasma treatment. For bonding, a room temperature-curing two-component-epoxy adhesive (Hysol® 9395) was used to prove the applicability of different peel plies for structural adhesive bonding under repair conditions.     

On the effect of pulsed laser ablation on shear strength and mode I fracture toughness of Al/epoxy adhesive joints

The present work describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with substrates pre-treated by pulsed laser ablation. An ytterbium-doped pulsed fiber laser was employed to perform laser irradiation on AA6082-T4 alloy. Morphological and chemical modifications were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Thick adherend shear tests were carried out in order to assess the shear strength while the mode I fracture toughness was determined using the double cantilever beam. For comparison, control samples were prepared using classical surface degreasing. The results indicated that laser ablation has a favorable effect on the mechanical behavior of epoxy bonded joints; however, while a + 20% increase was recorded for shear strength, a remarkable threefold enhancement of fracture toughness was observed with respect to control samples. XPS analyses of treated substrates and SEM observations of the fracture surfaces indicated that laser pre-treatment promoted chemical and morphological modifications able to sustain energy dissipation through mechanical interlocking. As a result cohesive failure within the adhesive bond-line was enabled under predominant peel loading.     

Tailoring adhesive forces between poly(dimethylsiloxane) and glass substrates using poly(vinyl alcohol) primers

A thin poly(vinyl alcohol) (PVA) layer has been found to control adhesive forces between poly(dimethylsiloxane) (PDMS) and a glass substrate. Various PVAs were coated on glass substrates on top of which PDMS pre‐polymer was cast. After thermal curing, the peel strength was tested. It was found that the fundamental adhesive forces are attributed to the degree of hydrolysis (or saponification value) of the PVAs. For a PVA modified with a silanol group, strong adhesive force resulted. The range of tailoring the force with the PVAs was 16 kgf/m. The production of thin interlaminated PVA layers as primers was demonstrated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39927.     

Mechanical Properties of Acrylic Pressure Sensitive Adhesives and Their Relationships to Industry Standard Testing

Dynamic mechanical and tensile stress-strain properties were measured for four sets of acrylic pressure sensitive adhesives, and were compared to industry standard “applications” peel and shear properties. Correlations were established showing that more than half of the range of performance shown by commercial PSA's is controlled by the bulk mechanical properties of the adhesive polymer. A few exceptions stand out clearly. Also, room temperature performance properties were found to correlate better with DMA at higher temperatures than with room temperature DMA. The contribution of tensile properties to peel strength and failure mode is discussed. The results can be used to relate PSA performance to well-known concepts in other areas of materials science, e.g. fracture toughness, rubber elasticity, and rheology, and to key variables in the adhesive formulation or selection process.     

The effect of polymer molecular weight and crosslinking reactions on the adhesion properties of microsphere water-based acrylic pressure-sensitive adhesives

In this work, the effect of polymer molecular weight and crosslinking reactions on the end-use properties of the microsphere water-based acrylic pressure-sensitive adhesives (PSA) is presented. Polymer molecular weight and polymer microstructure were regulated using different chain transfer agent (CTA) concentrations and by addition of a diacrylic monomer (MM). The adhesion properties of the synthesized PSAs were characterized via measurements of tack, peel adhesion and shear strength. The results of experiments have shown that the kinetics of suspension polymerization is relatively independent on the amount of CTA and MM. The amount of gel phase in the adhesive was reduced with increasing amount of CTA agent, and gel phase amount may be considered as a function of polymer molecular weight. With a combination of CTA and MM was possible to regulate the amount of formed gel phase in the adhesive, as well sol phase molecular weight. All of the measured adhesion properties strongly depend on molecular weight of the synthesized polymer and on the amount of gel phase. For adhesives synthesized solely with addition of CTA, tack decreased with lower polymer molecular weight and consecutively also with lower amount of gel phase. The same trend was also observed for peel strength measurements, whereas a cohesive failure was observed for adhesives with low amount of gel phase. A maximum value for tack and shear strength was observed at 80 wt% of gel phase. In case of syntheses with a combination of CTA and MM (amount of gel phase in range from 70 to 80 wt%), tack values were distributed in quite narrow range. On the other hand, peel strength values decreased in comparison with adhesives synthesized only with CTA, regardless to the equal amount of gel phase. Poor shear strength was observed for all adhesives synthesized by combination of CTA and MM.     

Patterning of worm-like soft polydimethylsiloxane structures using a TiO2 nanotubular array

Patterned polydimethylsiloxane (PDMS) is an important structure for soft lithography. Various materials have been deployed as mold for patterning PDMS. Anodized nanotubular array has been sought after as cost-effective alternative for textured silicon. An array of TiO₂ nanotubes with characteristic diameter ≈140 nm and the length of ≈1.5 microns, created by anodic oxidation of a titanium substrate, was used here as a template for soft PDMS molding. The optimal molding process was developed by a combination of silanization, use of solvent, application of a vacuum, and hydraulic pressing. The silanization was confirmed by Fourier transform infrared spectroscopy and contact angle measurements while the PDMS structure was examined by scanning electron microscope and energy dispersive X-ray spectroscopy. Hydraulic pressing significantly improved the infiltration of PDMS into the pores of nanotubular array resulting in formation of PDMS nanobumps after separation of the polymer from the template. Complete infiltration of PDMS precursor into the cavity of nanotubes was observed on the hydraulic-pressed sample without toluene impurities. The hydraulic-pressed samples exhibited higher adhesion strength than nonpressed ones. The adhesive strength was measured by a simple experimental arrangement, in which the PDMS layer was stuck on a vertical glass surface followed by pulling it downwards.     

Comparison of Peel Tests for Metal-Polymer Laminates for Aerospace Applications

Standard peel tests for aerospace laminates based on metal-polymer systems, namely floating-roller and climbing-drum peel methods, have been accommodated in a unified theory of peeling. This theory also accommodates more basic peel tests such as T-peel and fixed-arm peel and also newer methods such as mandrel peel. These five methods have been applied to two aerospace laminate systems to critically examine their use in the determination of adhesive strength. The theory has been used to unify the outputs from the tests in terms of adhesive fracture toughness. In this way, the comparative merits of the methods can be commented on.

The validity of the standard methods has been put in doubt because of the absence of a correction for plastic bending energy and also because of the poor conformance of the peel arm to the roller system used in these methods. The unified theory and some measurements of peel-arm curvature help but not completely overcome some of these difficulties.

A further complication that arises in peel is a change in the plane of fracture. This reflects a transition from cohesive fracture in the adhesive to an adhesive fracture at the interfaces among adhesive, primer, and substrate. It is likely that such plane-of-fracture phenomena are intrinsic to evaluation of the laminate and that contemplation of cohesive fracture toughness for the adhesive cannot accommodate such events.     

The brittle fracture of amorphous thermoplastic polymers

The brittle fracture properties of polyphenylene oxide, polysulfone, polycarbonate, and poly(methyl methacrylate) thermoplastic polymers were investigated over a wide range of temperatures. Fracture energy measurements were made using double edge-notched tensile samples. Tensile strength, tensile strain, and initial elastic modulus were measured for calculation of the fracture energy and further analysis of the polymer behavior. It was found that mechanical transitions in the tensile properties corresponded reasonably well with transitions in the fracture energy in the temperature range investigated. Fracture surface photographs permitted visual analysis of the fracture process. It was found that the roughest fracture surface corresponded to the maximum in the fracture energy for a given polymer. A theory for prediction of polymer tensile yield strain is presented, based on the volume dilation concept. The implications of this theory are discussed in terms of the crack tip flow process leading to brittle fracture.     

Influence of Layer Thickness on Cohesive Properties of an Epoxy-Based Adhesive—An Experimental Study

Cohesive laws are determined for different layer thicknesses of an engineering adhesive. The shape of the cohesive law depends on the adhesive layer thickness. Of the two parameters of the cohesive law—the fracture energy and the strength—the fracture energy is more sensitive to thickness variation than the strength. The fracture energy in peel mode (Mode I) increases monotonically as the thickness is increased from 0.1 to about 1.0 mm. At an adhesive thickness of 1.5 mm, the fracture energy is slightly lower than for a 1.0 mm adhesive thickness, indicating a maximum between 1.0 and 1.5 mm. In shear mode (Mode II), the thickness dependence is not as strong, but an increasing trend in fracture energy with increasing adhesive thickness is evident. A slight decrease in strength with increasing adhesive thickness is found in both loading modes.     

Effect of Pressure Cycling on Fracture Energy of Polyurethane/Aluminum Adhesive Bonds

An experimental study was conducted to investigate the effect of pressure-cycling on adhesive bond fracture energy of polyurethane/aluminum adhesive bond joints. Initially, two types of peel tests were conducted to characterize adhesive bond strength and challenges associated with pre-mature polyurethane cracking and failure during these tests are discussed. A modified double cantilever beam (MDCB) specimen configuration was specially designed and opening-mode loading conditions were employed to determine the interfacial adhesive bond energy (G_C). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The G_C of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased G_C values to those of the control and water-soaked samples: hence, prolonged pressure-cycling could be problematic to polymer/metal adhesive bonds of hardware installed outboard of submarine pressure hulls.     

Effect of the type of plasma on the polydimethylsiloxane/collagen composites adhesive properties

Polydimethylsiloxane (PDMS) films were treated with either oxygen (O₂), nitrogen (N₂) or argon (Ar) plasma between 40 W and 120 W for 5–15 min and their surface properties studied by contact angle measurements, infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Lower contact angles and increases in surface roughness, assessed by SEM and AFM, were observed for all used gases when plasma power and time increased, with argon treatment being the one that showed the most significant change in roughness.PDMS/collagen type I composites obtained after treating PDMS with oxygen at 80 W for 13 min or nitrogen and argon at 80 W for 14 min showed a peel strength of 0.1N/mm (oxygen plasma), 0.08 N/mm (nitrogen plasma) and 0.09 N/mm (argon plasma). In all cases, peel strength was higher than that measured for the untreated bilayer composite. An increase in adhesion strength, after oxygen and nitrogen plasma, was mostly attributed to chemical interaction between functional groups introduced on the PDMS surface and the functional groups on collagen as detected by FTIR. In contrast, the high peel strength observed on PDMS treated with argon plasma was attributed to its increased roughness which in turn increased mechanical interlocking. The properties of these composites render them suitable for adhesive free skin substitutes.     

Adhesion behavior of PDMS-containing polyimide to glass

The effect of the incorporation of poly(dimethyl siloxane) (PDMS) segments into a poly[N,N'-(p,p'-oxydiphenylene) pyromellitimide] (PMDA-ODA) polyimide backbone on the adhesion between PMDA-ODA polyimide and glass was investigated using X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, contact angle measurements, and the peel test. The peel energy of PMDA-ODA polyimide to glass was significantly improved when low molecular weight PDMS (248.5 g/mol) was incorporated, while little improvement was observed for the incorporation of high molecular weight PDMS (900 or 1680 g/mol). Exposure to air resulted in a considerable deterioration in the peel energy for the pure PMDA-ODA polyimide, while no deterioration was observed for the PDMS-containing polyimides. The improvement in peel strength was successfully achieved by the incorporation of very small quantities of PDMS such as 2 wt%. Based on XPS, IR spectroscopy, and contact angle measurements, it is suggested that the incorporated PDMS segments migrated from the bulk polyimide to the polyimide/glass interface and chemically bonded to the glass surface, which resulted in enhancement of the peel energy. However, a weak boundary layer was formed between the bulk polyimide and glass when a high molecular weight PDMS (900 or 1680 g/mol) was incorporated and thus the peel energy deteriorated.     

Influence of Nanoclay on the Adhesive and Physico-Mechanical Properties of Liquid Polysulfide Elastomer

Addition of Cloisite 30B nanoclay particles to a typical ammonium dichromate cured liquid polysulfide elastomeric adhesive leads to very dramatic increase in the aluminum–aluminum joint strength measured by 180° peel test. The increase in adhesive strength could be explained in terms of higher cohesive strength of the nanocomposite adhesives, which has been derived from good interaction between the nanoclay and the polysulfide elastomer. The addition of nanoclay also facilitates the adhesive to dissipate greater amount of energy (by fibrils formation) during the debonding process of the peel test. In addition, the nanoclay aids the low polarity polysulfide elastomer to achieve better molecular contact with the aluminum substrate. The nanoclay particles are very well dispersed (mostly exfoliated) in the polymer matrix even at 8 wt% of nanoclay concentration. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies provide evidences for the excellent dispersion of the nanoclay platelets in the polymer matrix. The results of the mechanical and dynamic mechanical studies confirm the excellent interaction between the nanoclay and the polysulfide elastomer.     

Effects of peel angle on peel force of adhesive tape from soft adherend

In the case of the peeling of adhesive tapes from soft adherends, the contributions of the compressive force at the adhered portion as well as the larger deformation of adherend have essential roles in determining the peeling properties. In this paper, the peel force of an adhesive tape from a soft adherend has been measured to understand the peeling mechanism, which is greatly affected by the peel angle. A commercially available pressure-sensitive adhesive was used as the tape, and a cross-linked polydimethylsiloxane (PDMS) was used as the soft adherend. The purpose of this study is to clarify the effects of the peel angle on the peel behavior of this system at room temperature under different material specifications and different experimental conditions. The factors that affect the peel force of the PDMS adherend included the degree of cross-linking in PDMS, the thickness of PDMS, peel angle, and peel velocity. Two characteristic peel patterns were observed, which depended on the material specifications and different experimental conditions. The peel mechanism was discussed in terms of the deformation of the adherend.