Peeling of PSAs on Viscoelastic Substrates: A Failure Criterion

This work deals with the study of the viscoelastic and adherence properties of pressure-sensitive adhesive (PSA) formulations dedicated to medical applications. We have developed a specific viscoelastic substrate to measure the adherence properties of PSAs that mimics adhesion on human skin. In the present article, we describe several experiments dedicated to a better understanding of adhesion on viscoelastic substrates without discussing specifically the case of human skin. In this way, we have studied different model adhesive formulations based on real medical formulations, and we have related the rheological behavior to the adherence properties obtained on different substrates to study the various specific effects due to the viscoelasticity of soft substrates. We propose from this study a failure criterion that allows one to derive a reasonable estimate of the peeling transition rate from cohesive to interfacial or stick–slip failure.     

Rheology and Adherence of Pressure-Sensitive Adhesives

We have studied the relationship between rheological and peeling properties for hot-melt pressure-sensitive adhesives based on homopolymers or copolymers blended with tackifying resins. In this article, we particularly try to demonstrate that it is possible to define a quantitative link between rheology and adherence when the model formulations are deposited on substrates with strong (thermodynamic) adhesion. We describe the experimental results obtained on these model formulations and discuss the quantitative relationships obtained. In the case of “adhesion modulation” (derived from different treatments of the substrates), we show that the relationships become much more complicated, even with the same model adhesives. At the end, we discuss on the competition between adhesion and dissipation in the case of poor adhesion.     

Tailoring the adhesion properties of polyacrylamide-based hydrogels. Application for skin contact

In the present study, we investigated the adhesive performances of polyacrylamide-based hydrogels intended to be used as dermatological patches. Accordingly, we have prepared conventional copolymer poly(acrylamide-co-hydroxyethyl methacrylate) hydrogels and nanocomposite copolymer poly(acrylamide-co-hydroxyethyl methacrylate) hydrogels filled with poly(butyl acrylate) nanoparticles. We evaluated their adhesive properties when they were applied to different substrates (equivalent human skin, stainless steel) using a probe tack test. The adhesion energy was found to be related to the chemical composition and the rheological properties of the hydrogels which were also measured.     

Effect of the rheological properties of industrial hot-melt and pressure-sensitive adhesives on the peel behavior

Three factors govern the adhesion properties of hot-melt and pressure-sensitive adhesives: (1) thermodynamic interfacial properties (Dupré adhesion energy); (2) interfacial losses due to specific interactions; and (3) viscoelastic losses in the bulk related to the rheological properties of the adhesive. In the present paper, we focus on the main factor in the adhesion properties, which is the viscoelastic factor. We extend in this paper the results obtained on a series of model adhesives to the case of industrial formulations: one SIS triblock copolymer-based PSA formulation and one EVA copolymer-based hot-melt formulation. After studying the rheological properties of these adhesives over a wide frequency range using time-temperature equivalence, we present data obtained on peel tests at various temperatures. As with model adhesives, the peel rate-temperature equivalence leads to the same shift factors as rheology. The experiments demonstrate that there is a one-to-one relationship between the cohesive fracture domain and the terminal region of relaxation exhibited in rheological testing. The first interfacial fracture mode is related to the rubbery plateau, and the brittle interfacial fracture mode observed at high peel rates to the glassy behavior exhibited at very high frequencies in rheological measurements.     

Skin-customized wearable device adhesive without skin damage

Wearable medical devices are gaining popularity owing to their potential for seamless integration with the human body and long-term monitoring of physiological activity. However, conventional adhesives were developed based on the assumption of healthy adult skin and may not account for variations in skin characteristics across different species, environments, and body parts. Consequently, the adhesive strength of wearable devices may significantly differ depending on the skin surface to which they are attached, potentially causing skin damage. In this study, we developed a customized wearable-device adhesive without skin damage by analyzing the characteristics of the skin surface based on oil and water content and roughness according to different species and parts. Our findings demonstrated that increased root-mean-square roughness of the skin surface led to reduced contact area and decreased adhesion force between the polydimethylsiloxane (PDMS) pad and skin surface. Surprisingly, hairless skin exhibited 1.5 times higher adhesion strength than hairy skin due to stronger molecular forces resulting from the higher surface energy of the skin. Additionally, the hole-patterned PDMS pad on sweaty skin displayed improved adhesion properties compared to the cylinder-patterned PDMS pad. Therefore, customized wearable adhesives provide an effective strategy for developing skin-damage-free wearable devices.     

Medical-grade acrylic adhesives for skin contact

Pressure-sensitive acrylic adhesives for application to skin are made from 2-ethylhexyl acrylate, isooctyl acrylate or n-butyl acrylate copolymerized with polar functional monomers such as acrylic acid, methacrylic acid, vinyl acetate, methyl acrylate, N-vinylcaprolactam, or hydroxyethyl methacrylate. Functional comonomers increase cohesive strength, provide surface polarity, and enhance wear performance. Tack, adhesion to skin, adhesive transfer to skin, and wear performance of the adhesive are governed by the molecular weight, glass transition temperature, and the viscoelastic behavior of the adhesive. Viscoelastic properties of the adhesive as measured by the Williams plasticity number (WPN), dynamic storage modulus (G′), dynamic loss modulus (G″), and tan δ are important polymer properties for good wear performance. Sweating skin, a moist environment, and physical activity are the most important factors influencing the failure of an adhesive tape during wear. A medicalgrade adhesive for application to human skin should be hypoallergenic. Medical-grade adhesives are utilized in making surgical tapes for holding dressings in place, adhesive bandages, adhesive dressings to cover wounds, and surgical operating drapes.     

The effect of cataphoretic and powder coatings on the strength and failure modes of EN AW-5754 aluminium alloy adhesive joints

The aim of this study is to determine the effect of cataphoretic and powder coatings and also the method of application the primer on the adherends surface on the strength and failure modes of EN AW-5754 aluminium alloy adhesive joints. The study is performed on lap joints made of EN AW-5754 aluminium alloy, subjected to three different types of surface treatment; namely a) polyurethane cataphoretic coating, b) powder coating based on black mat RAL 9005 UL polyester resin and c) no coating. The tested adhesive joints were made using a one-component polyurethane adhesive Terostat 8596, which was dedicated for automotive and cured under a constant load of 0.018 MPa at 20 ± 2 °C. In addition, this study investigates the effect of the application of Terostat 8519P adhesion promoter which is a liquid polyurethane-based primer containing solvents and which is corresponding to Terostat 8596 polyurethane adhesive. Terostat 8519P adhesion promoter was applied in two different ways: a) to one substrate and b) to both substrates. The produced adhesive joints were subjected to strength tests using the Zwick/Roell Z150 testing machine. The examination of fracture in the tested adhesive joints was performed in accordance with the EN ISO 10365 standard. The shear strength results have demonstrated that both the method of application of the adhesion promoter (Terostat 8519 P) and the presence of cataphoretic coating had an influence on adhesive joints strength. The use of the adhesion promoter significantly affects the strength of both uncoated EN AW-5754 aluminium alloy adhesive joints and the adhesive joints subjected to powder coating. The use of the adhesion promoter has a less significant effect on the cataphoretic-coated samples.     

Bio-adhesion evaluation of a chitosan-based bone bio-adhesive

Conventional treatment of complex fractures includes the use of plates and nails, which may compromise the affected limb's functionality. Previous studies have demonstrated promising results through chemical, mechanical, and cytotoxicity tests of a chitosan-based adhesive—proposed as a new method to bond high energy fractures—in dry environments with adequate adhesion, malleability, and biocompatibility. In this study, we focused on performing an evaluation of bio-adhesives’ mechanical properties and bone-adhesive joint using two chitosan-based formulations (with and without a cross-linking agent). The texture profile analysis determined adhesive properties, such as cohesiveness, adhesiveness, hardness, and resilience at different cure times. Bone-adhesive joint was evaluated according to the tensile bond strength test and shear bond strength test. Fracture toughness and cohesive strength were calculated through a rigid double cantilever beam test at mode I failure. Bone-adhesive joints were tested in two environments: dry and submersed in water at 37 °C for 1, 6, and 24 h (curing time), an approximation of surgery conditions. The experimental results showed an incremental of adhesiveness and hardness of the cross-linked adhesive during the first 15 min, which was determined as the usage time to spread on the bone fracture. The joint interaction between the adhesive and bone surfaces was studied; chitosan-based formulations showed an adhesive joint failure under dry conditions in most of the cases. However, this behavior changed under aqueous conditions, presenting cohesive failures. Under aqueous conditions, cross-linked bone-adhesive presented an augmented tensile bond strength up to 0.024 ± 0.0036 MPa, a shear bond strength up to 0.031 ± 0.0069 MPa, and fracture toughness of 2.38 ± 0.54 J/m² was observed with a cure time of 24 h. Finally, the presence of the cross-linking agent in the cross-linked bio-adhesive reduced the sensitivity of the adhesive to water; a promising finding that should be explored in future studies.     

Experimental and statistical study of three adherence tests for an epoxy-amine/aluminum alloy system: Pull-Off,Single Lap Joint and Three-Point Bending tests

The mechanical resistance of a bonded joint depends on the adhesive interaction onto the substrate and the mechanical properties of the adhesive itself. Many existing tests can be useful for measuring the adherence or evaluating mechanical adhesive response. All these tests do not provide the same information: in particular, adherence measurements can be split into initiation tests and propagation ones. In this paper, three adherence tests have been considered for the evaluation of the fracture initiation between a poly-epoxide adhesive (a mixture of pure epoxy and amine) and an aluminum surface (AA 2024-T3), namely the Pull-Off, Single Lap Joint (SLJ) and Three-Point Bending tests. Various surface preparation protocols before bonding have been tested and optimized for aluminum substrates, including mechanical and chemical surface treatments, followed by the application of an appropriate primer before bonding. This study paves the way for the future development of adhesive systems as it provides reliable surface preparation protocols for aluminum surfaces and gives an insight into the choice of an adequate adherence test dedicated to high-performance adhesives. The load at break (F_Max), the experimental error, the failure mode and statistical studies according to the Weibull model and Principal Component Analysis (PCA) were studied on each surface preparation configuration. It has been shown that the application of a primer, especially a sol-gel product increases the load at break and provides more reliable results. Then, this paper shows that the two tests can quantify the failure initiation and distinguish the different surface preparation efficiency, are the Single Lap Joint test (mode II or mode I + II) and the Three-Point Bending test (mode I), with an increase of the results reliability with the latter one. The Pull-Off test (mode I) is useful as a routine checking, and particularly interesting because its response does not depend on the substrate thickness, even though it cannot highlight the difference between all surface preparations.     

Fracture of metal/polymer/metal assemblies: Viscoelastic effects

The purpose of this work is to link the polymer viscoelastic properties (especially its relaxation time) and the adhesive behaviour of steel/polymer/steel assemblies. A wedge test device developed in the laboratory allows one to introduce the wedge into the assembly at a controlled speed and to follow the crack propagation with a camera-equipped microscope. The adherence energy (calculated from the equilibrium crack length) and the crack propagation rate are measured for different wedge introduction rates. Polymer equivalent relaxation time is determined for each introduction rate according to the time-temperature superposition principle. Relations between adherence energy, crack propagation rate, and calculated equivalent relaxation time values are proposed. These quantitative relations confirm the major influence of polymer viscoelastic properties on the rate sensitivity of adhesive behaviour.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Effect of adhesive characteristics on static strength of adhesive-bonded aluminum alloys

The use of adhesive is posed to increase dramatically for application to the next generation of vehicle structures as is the use of aluminum. In this study, the effect of adhesive characteristics on the strength of adhesive-bonded lap shear aluminum was investigated. It was found that the joint strength depended on not only the adhesive properties but the bond adhesion between the adhesive and adherend. For the given selected aluminum substrates, to ensure the cohesive failure mode and consistent joint strength it is necessary to select an adhesive which had a weaker than or comparable strength to the bond adhesion. To improve the failure mode from adhesive to cohesive, atmospheric pressure plasma surface treatment of X610-T4PD and X626-T4P aluminum was performed and results showed that it improved not only the joint strength but degree of cohesive failure mode.     

Comparative study of zein- and gluten-based wood adhesives containing cellulose nanofibers and crosslinking agent for improved bond strength

Many of the currently used wood adhesives contain chemicals that are harmful to human health and the environment. Increasing environmental and human health concerns have made the development of safe biobased adhesives a priority. In this study, two plant proteins, i.e., zein and wheat gluten, were used to develop wood adhesives and their performance was compared through simple lap shear tests and plywood flexural/internal bond tests in dry and wet conditions. To increase their bond strength, cellulose nanofibers were added to create nanocomposite adhesives and glutaraldehyde was also used to crosslink the proteins. Single-lap shear test was performed to measure the bond strength of different adhesive formulations and determine the optimal formulations and processing conditions. Fractured bond surfaces were studied using optical observation and scanning electron microscopy to determine bond failure mechanisms. Thermal and chemical properties of the adhesives were evaluated using thermogravimetric analysis and Fourier transform infrared spectroscopy, respectively. The bond strength of both zein and gluten adhesives was significantly increased by the addition of the cellulose nanofibers and/or glutaraldehyde, although the two adhesives responded differently to the two reinforcement materials due to the different solvents used to prepare the adhesives. The bond failure mode changed from cohesive failure of the adhesive to structural failure of the adherent for the gluten adhesive containing CNFs and glutaraldehyde. Potential zein and gluten adhesive formulations were used to produce plywood samples and their performance was assessed under different conditions. The formulations with industrial potential were discovered through this study.     

Adhesion and interfacial healing between supramolecular hybrid networks and glass substrates

Adhesion towards glass and interfacial healing of partially supramolecular hybrid polymer networks featuring a range of H-bonds content were investigated through two dedicated adhesion test methods. In a first series of tests, adhesion strength was measured by separating two substrates containing a cured inner resin layer, and shown to decrease with increasing H-bonds content in the polymer network (from 0 to 50%) as the mechanical strength of the polymer also decreased while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with glass substrates. In a second step, the test was used to evaluate interface restoration through healing of the polymer matrices and results showed an increased from none to a tensile strength recovery up to 70% after 1 h healing time for the 50% H-bond polymer. Then, self-adhesion of freshly cut polymer surfaces to glass substrates was investigated, showing increasing tack with increasing H-bonds content. The influence of glass surface treatments on adhesion and interfacial recovery properties was also explored: while aminosilanes did not influence the interfacial behavior of partially supramolecular self-healing polymers towards glass, trimethoxy (octadecyl)silane (ODS) modification strongly hindered their adhesion abilities, further highlighting the fundamental role of hydrogen bonds interaction with the substrates.     

一次性医疗设备组装用紫外光固化胶粘剂

研究了一种可用于各种不同基材的一次性医疗设备组装的紫外光固化胶粘剂。考查了单体和偶联剂对粘接强度和耐老化性能的影响。研制的胶粘剂对PVC、PC的粘接强度可达6MPa以上，其耐老化性能也很优异：可用于呼吸面罩、喉罩、导管等医疗设备的粘接。     

Hydrophobic or Hydrophilic Fumed Silica as Filler of Polyurethane Adhesives

Two hydrophilic and two hydrophobic fumed silicas of different characteristics were added to solvent-based polyurethane adhesives (PU). IR spectroscopy, contact angle measurements and rheology (viscosity measurements, determination of viscoelastic properties) were used to monitor the variation of properties of PU adhesives produced by addition of silica. Immediate (green) adhesion was determined by T-peel testing of halogenated synthetic rubber/PU adhesive/halogenated synthetic rubber joints. Silica addition produced a noticeable increase in the PU adhesive viscosity which can be related to the variation of viscoelastic properties. Viscosity of PU adhesives containing hydrophilic silica slightly increased with time after preparation; the increase was less significant in PU adhesives with hydrophilic silica. In the rheological studies, silica imparted shear thinning and negative thixotropy to PU adhesives due to a better dispersion of the silica in the polyurethane during shearing. The addition of silica produces an increase in the storage modulus (G') of PU adhesives, the values obtained being independent of the hydrophilic or hydrophobic nature of the fumed silica. The increase of G' and the changes in tan δ of PU adhesives containing silica corresponded to an improvement in the green adhesion properties of chlorinated rubber/PU adhesive/chlorinated rubber joints. In general, in disagreement with previous results,¹ the presence of silica did affect the properties of solvent-based PU adhesives, but these properties were not dependent on the type of silica (hydrophobic or hydrophilic) used in this study.