Relationship between Viscoelastic and Peeling Properties of Model Adhesives. Part 1. Cohesive Fracture

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(ω), as a function of frequency, ω and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

After showing the objective character of the peeling curves obtained, the variations of the peeling force and peeling geometry have been studied as a function of volume fraction of the tackifying resin.

In this first paper, the analysis is focused on the first domain of the peeling curves, i.e. the cohesive fracture region. In this region, the peeling properties have been related to the viscoelastic properties in the terminal region of relaxation. It is shown that the longest relaxation time, τ_o, is a reducing parameter of the peeling curves, so a peeling master curve-which is independent of temperature, resin volume fraction and polymer molecular weight-may be defined. Furthermore, the variations of the test geometry as a function of peeling rate have been investigated: the variations of the radius of curvature of the aluminium foil have been analyzed with respect to the viscoelastic behavior of the adhesive, which in fact governs the test geometry.

A detailed analysis of all these features leads to a model which allows one to calculate the peeling curves in the cohesive domain from the adhesive formulation.     

Relationship Between Viscoelastic and Peeling Properties of Model Adhesives. Part 2. The Interfacial Fracture Domains

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(w), as a function of frequency, W, and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

The first paper of this series presented the cohesive fracture domain and the present paper explores the interfacial fracture domain: (i) rubbery interfacial (interfacial 1); (ii) stick-slip; (iii) glassy interfacial (interfacial 2). After a general survey of the properties in the three domains we present a quantitative relationship between the peeling and linear viscoelastic properties as a function of the adhesive formulation, discussing the use of time-temperature equivalence for adhesive properties. The third part of the paper presents the trumpet model of de Gennes describing the crack shape and propagation: starting from a mechanical analysis of the peeling test, it is shown how one may calculate the variations of the peeling force as a function of peeling rate in the various interfacial fracture domains: this model defines a single interfacial fracture criterion which coexists with the cohesive fracture criterion defined earlier, whatever the fracture location.

We present as a conclusion a critical discussion of the relevance and physical meaning of such a criterion and present a new outlook for the modeling and improvement of adhesive formulations.     

Relationship between Viscoelastic and Peeling Properties of Model Adhesives. Part 1. Cohesive Fracture

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(ω), as a function of frequency, ω and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

After showing the objective character of the peeling curves obtained, the variations of the peeling force and peeling geometry have been studied as a function of volume fraction of the tackifying resin.

In this first paper, the analysis is focused on the first domain of the peeling curves, i.e. the cohesive fracture region. In this region, the peeling properties have been related to the viscoelastic properties in the terminal region of relaxation. It is shown that the longest relaxation time, τ_o, is a reducing parameter of the peeling curves, so a peeling master curve-which is independent of temperature, resin volume fraction and polymer molecular weight-may be defined. Furthermore, the variations of the test geometry as a function of peeling rate have been investigated: the variations of the radius of curvature of the aluminium foil have been analyzed with respect to the viscoelastic behavior of the adhesive, which in fact governs the test geometry.

A detailed analysis of all these features leads to a model which allows one to calculate the peeling curves in the cohesive domain from the adhesive formulation.     

Effect of Peel Load on Stringiness Phenomena and Peel Speed of Pressure-Sensitive Adhesive Tape

The factors governing interfacial separation in lightly cross-linked polymer adhesives at low pulling rates as demonstrated by their stringiness phenomenon are investigated.

Cohesive failure and adhesive/substrate interfacial separation of uncross-linked polymer adhesives have been adequately explained. However, in lightly cross-linked polymer adhesives, where cohesive failure cannot occur because there is no viscous flow, there are two regions of interfacial separation at low rate and this phemonenon cannot be readily explained by present viscoelastic theories.

Investigation of the stringiness phenomenon of peeling pressure-sensitive adhesive tapes at constant loads shows that two peeling speeds exist for any peeling load up to the vicinity of 200 g/25 mm. Also it is clear that stringiness structure differs greatly at each peeling speed. The stringiness phenomenon of each of these two regions is analyzed using Miyagi's observation apparatus. These two measurements are then reversed and a comparison shows that the two peeling speeds correspond to each steady peeling region.

This field of investigation, when added to the present viscoelastic property studies, should lead to a new peeling adhesive theory which, in turn, may lead to the development of new high peel force pressure-sensitive adhesives.     

The Mechanisms of Peeling of Uncross-Linked Pressure Sensitive Adhesives

We analyze the peeling properties of an uncross-linked pressure sensitive adhesive. 90° peeling master curves on Pyrex^TM and PMMA (polymethylmetacrylate) are constructed The shift coefficients a_T are compared with the ones obtained from rheometrical shear tests.

With our machine, the peeling front is kept fixed, enabling us to observe the mechanisms of deformation of the adhesive. We count four different mechanisms of peeling in cohesive failure, and three in interfacial peeling (the last being unstable); they correspond to various slopes that we identify. The flow patterns at slow reduced velocities are two-dimensional. Then they undergo transitions to three-dimensional periodic complex flows, due to instabilities in the flow of thin adhesives. Interpretation of these peeling master curves are discussed in terms of rheology and physico-chemistry. It appears necessary to take into account the elongational properties of the adhesive, as well as the surface energy properties, to predict adhesion.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

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.     

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.     

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.     

Physical Considerations in the Development of Pressure-Sensitive Adhesive Sheets

A pressure-sensitive adhesive sheet is a special kind of paper used in non-impact printers which use a heating process to apply toner to paper. As a result, it needs special characteristics that general pressure-sensitive adhesive paper for labels do not require.

One of these characteristics is that the edge of the folded paper used in non-impact printers must not incline after printing. This was done by making the degree of orientation of the fibers in the face stocks and the release liners low.

The other characteristics are that adhesive must not ooze out from the edges during the slitting or guillotining process and that the labels must not come off of the release liner by themselves during the printing process. Ooze characteristics were found to be related to the adhesive coat weight. An adhesive paper with both a high peel strength and lower adhesive coat weight was developed by studying the dynamic viscoelastic properties of adhesives and release layers. The storage modulus of the release layer concerned with the release force was also found to be related to the self-peeling tendency of the labels.

These points were considered during the development of pressure-sensitive adhesive paper used in non-impact printers which use a heating process to apply toner to paper.     

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.     

Interfacial and Bulk Contributions to Peeling Energy

Thin polyurethane films, having low adhesion to dried protein, were developed as candidate materials for non-adhesive surgical dressings. In order to model wound-adhesion, gelatine was cast from solution on to the film and allowed to dry. The film was peeled from the gelatine at 180° peel angle, and the peel force measured as a function of the temperature of test. The dynamic mechanical properties of the films were measured over the range -90°C to 110°C and values of tan δ were determined at the temperatures employed for peeling. Thus, a correlation was obtained between peeling energy and tan δ for each of eight films.

The generalised theory of fracture mechanics states that the adhesive failure energy is given by the product of an interfacial energy term and a “loss function” involving the hysteresis ratio of the material. If the strains are small the hysteresis ratio is proportional to tan δ. The experimental results show excellent agreement with the theory, but the interfacial term turns out to be much greater than the true interfacial energy (or thermo-dynamic work of adhesion). The reason for this result is discussed.     

Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

Peel Strength of Aluminium-Aluminium Joints Bonded by Adhesive Based on Carboxylated Nitrile Rubber-Chlorobutyl Rubber Blend

The peel strength of aluminium-aluminium joints bonded by an adhesive based on carboxylated nitrile rubber and chlorobutyl rubber was found to depend on surface topography and use of a silane primer. Anodization causes a marginal increase in bond strength while the silane primer improves the adhesive joint strength remarkably.

The peel strength was also found to be dependent on test conditions (test rate and temperature). The threshold peel strength value obtained by measurements at low peel rate and high test temperature was found to depend on the type of failure during peeling (cohesive or interfacial) which, in turn, is controlled by the presence of silica filler in the adhesive. Two different threshold values of peel strength were obtained: 60 N/m for interfacial failure (in silica-filled adhesive), 140 N/m for cohesive failure (in unfilled adhesive).     

Adhesive Joint Durability Assessed Using Open-faced Peel Specimens

This paper presents an investigation of the durability of two aluminum-epoxy adhesive systems by means of open-faced peel specimens. A peel analysis model was used to determine the fracture energy from the peel data. Both wet and dry peel tests were conducted in order to distinguish between the reversible and the permanent effects of water. The effects of water on the cohesive properties of the adhesives were also assessed by tension tests. It was found that, for the two-part epoxy adhesive, which plasticized to a large extent, the peel testing should be carried out in a dry state to assess the interfacial weakening. It was also observed that the two-part adhesive was much stiffer in the dry, degraded state, and it was important to take account of such permanent changes in the cohesive properties associated with water uptake when determining the fracture energy from the peel data. In contrast, the one-part epoxy system did not suffer from appreciable cohesive changes, either reversible or permanent. In this case, both wet and dry failure loci were interfacial, and some of the interfacial damage was found to be reversible. Finally, surface analyses of the peel failure surfaces were carried out, and the formation of micro-debonds was identified as a possible mechanism of degradation for the two-part system.     

Hot Melt Adhesive Model: Interfacial Adhesion and Polymer/Tackifier/Wax Interactions

This work continues our study of the hot melt adhesive (HMA) model published earlier [1]. This HMA model was developed based on the pressure sensitive adhesive (PSA) tack model established previously [2]:

P = P₀BD (1)

where P is the adhesive bond strength, P₀ is the interfacial (intrinsic) adhesion term, B is the bonding term and D is the debonding term. The previous paper [1] describes the B and D terms in detail. However, only a brief discussion of the P₀ term was given. The present paper will provide a more in-depth but still rather qualitative study of the P₀ term within the framework of the adhesion model described in Eq. (1). HMAs studied are ethylene/vinyl acetate copolymer (EVA)/tackifier/wax blends. Substrates studied are untreated and corona-discharge-treated polyolefins such as polypropylene (PP) and polyethylene (PE). First, it has been found that the tackifier surface tension could be roughly correlated with one of its thermodynamic parameters: the solubility parameter dispersion component. Secondly, except for EVA/tackifier binary blends, the compatibility of any two of these three components, the EVA polymer, the tackifier and the wax, in a HMA can be estimated from surface tension and X-ray photoelectron spectroscopy (XPS) measurements. Thirdly, based on the study of the EVA/mixed aliphatic-aromatic tackifier/wax model HMA system, it has been observed that the HMA/polyolefin substrate interfacial composition depends on the wax/substrate compatibility. The cause of an inferior peel strength of a HMA containing a high wax content to a polyolefin substrate is possibly due to the formation of a weak boundary layer (WBL) of wax at the interface and/or low dissipative properties of the HMA.

Also, the relationship between EVA/tackifier/wax interactions and HMA peel strength will be discussed. A correlation between the EVA/tackifier compatibility measured by cloud point and viscoelastic experiments to the debonding term, D, in Eq. (1) has been found.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90-100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Time Dependent Crack Growth and Loading Rate Effects on Interfacial and Cohesive Fracture of Adhesive Joints

Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.     

The Effect of Moisture Content in Epoxy Film Adhesives on their Performance. III: Bulk Properties

Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The consequences of vacuum drying are manifested in enhancement of both the peel and shear properties of bonded joints (Part I and Part II of this series of papers) and the thermal, physical and mechanical properties of the bulk adhesive, characterized in the present study.

Experimental results have shown that the bulk properties of structural epoxy based adhesives are highly correlated with the aging processes caused by water absorption in the prepolymerized adhesive. Applying the vacuum process is harmful to fresh unaged adhesive due to devolatization of low molecular species of the film adhesive.

The characterization of bulk properties for the purpose of following the aging and recovery processes is advantageous, since the bulk is independent of geometrical and interfacial effects which dominate in the case of property evaluation of the adhesive in a bonded joint.