The effects of substrate surface properties on tack performance of acrylic Pressure-Sensitive Adhesives (PSAs)

The effects of substrate surface free energy (SFE) and substrate roughness on tack performance of adhesive tapes containing synthesized model acrylic pressure-sensitive adhesive (PSA) have been investigated. In order to study the influence of substrate SFE on tack the adherents with the same surface roughness (expressed by selected amplitude parameters) were used: PTFE, PP, PE, ABS, PC, PMMA, stainless steel and glass. The relationship between substrate roughness and tack was investigated using two polypropylene plates (PP and PP_rough) characterized as having the same wettability (SFE). For tack determination the most common method in the PSA tapes industry was employed (loop tack test). The conducted experiments showed that substrate SFE is a crucial factor governing tack properties of acrylic PSAs. In general, a larger difference between the SFE values of the substrate and adhesive were correlated with greater tack values. The dependence of tack and SFE was significantly influenced by crosslinking degree and layer coat weight of model acrylic PSA. The experiments carried out in the second part of the study revealed that the adhesive׳s viscoelastic properties control the tack properties on rough substrates, however, the final tack performance was found to be strongly affected by the level of substrate roughness and PSA thickness.     

Surface energy, surface topography and adhesion

In this paper are discussed some of the fundamental principles which are relevant to an understanding of the influence that interfacial roughness may have on adhesion. The surface energies of the adhesive, substrate and of the interface between them determine the extent of wetting or spreading at equilibrium. Numerical values for surface energies may be obtained either from contact angle measurements or from analysing force–displacement curves obtained from the surface forces apparatus. The extent to which the relationships, appropriate for plane surfaces, may be modified to take into account interfacial roughness are discussed. For modest extents of roughness, the application of a simple roughness factor may be satisfactory, but this is unrealistic for many of the practical surfaces of relevance to adhesive technology which are very rough, and is ultimately meaningless, if the surface is fractal in nature. Some examples are discussed of published work involving polymer–metal and polymer–polymer adhesion, where the roughness of the interface exerts a significant influence on the adhesion obtained. Roughness over a range of scales from microns to nanometres may strengthen an interface, increasing fracture energy by allowing bulk energy dissipating processes to be activated when the bond is stressed.     

Effect of adhesive thickness on the wettability and deformability of polyacrylic pressure‐sensitive adhesives during probe tack test

Effect of adhesive thickness on the wetting and deformation behaviors during probe tack test of pressure‐sensitive adhesive (PSA) was investigated. For this purpose, cross‐linked poly(n‐butyl acrylate‐acrylic acid) [P(BA‐AA)] and poly(2‐ethylhexyl acrylate‐acrylic acid) [P(2EHA‐AA)] random copolymers with an acrylic acid content of 5 wt % and thicknesses in the range of ～15–60 μm were used. Tack was measured using the probe tack test and the fracture energy was calculated from the areas under force–displacement curve recorded during debonding process. From contact time dependence of fracture energy, the rising rate of fracture energy with contact time increased with increasing of adhesive thickness and was P(2EHA‐AA) > P(BA‐AA). The fracture energy was P(BA‐AA) > P(2EHA‐AA) at shorter contact time, whereas it reversed at longer contact time. This was caused by two different interfacial adhesions: the physical wetting of PSA molecules to the adherend surface with contact time and the chemical interaction between the acrylic acid units and the adherend surface. From the force–displacement curve measured under the condition of sufficient interfacial adhesion, both maximum force and displacement—namely, the deformability of PSA during debonding process—increased with adhesive thickness. The degree of increase of deformability was P(2EHA‐AA) > P(BA‐AA). The fracture energy was found to depend on the development of interfacial adhesion during contacting process and the deformability of PSA during debonding process. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43639.     

The Adhesion‐Enhanced Contact Electrification and Efficiency of Triboelectric Nanogenerators

In the present work, the contact electrification of polymers that differ in adhesion strength is studied. Electrical current is measured along with adhesion in macroscale contacting‐separation experiments. Additionally, local adhesion and roughness are studied with atomic force microscopy to get deeper insight into relations between surface properties and electrification. Measurements reveal that higher surface charge is formed on more adhesive surfaces, thus confirming covalent bond cleavage as a mechanism for contact electrification of polymers. Investigated materials possess enhanced contact electrification making them attractive candidates for the conversion of mechanical energy to electrical in triboelectric nanogenerator devices.     

Studies on tack of pressure–sensitive adhesive tapes

It has been found that tack values of pressure-sensitive adhesives are dependent on the surface energies for adherends. The parabolic curves with maxima are obtained from the plots of tack values versus critical surface tensions for the substrates. The maximum occurs at the region where the two surface tensions are almost similar. On the other hand, tack is measured as a dynamic value indicated as the force necessary to deform the adhesive mass. To explain why the tack values is controlled by the surface character of the adherend, a mechanism is proposed for adhesive bond breaking. Primarily, bonding occurs by wetting the surface with adhesive. When the adhesive bond breaks by external force, unbonding proceeds from the viscoelastic deformation of the adhesive mass around the wetted spots on the surface of the adherend. As the total area of wetted spots is determined by surface energy, the tack value is dependent on the critical surface tension of the adherend.     

Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy

In this work aluminium alloy surfaces have been subjected to three different methods of surface pre-treatments such as solvent degreasing, FPL (Forest Products Laboratory) etching and priming using an epoxy based primer. The treated surfaces were evaluated for surface energy, contact angle, surface topography, surface roughness and adhesive strength characteristics. The influence of surface pre-treatments on the variation of polar, dispersive and total surface energy of the surfaces is addressed. A wettability test was performed on the surfaces using an epoxy adhesive in order to assess the influence of the pre-treatment techniques on substrate/adhesive interaction. Theoretical work of adhesion values for the various pre-treated surfaces were calculated using the contact angle data and further tested experimentally by adhesive bond strength evaluation by tensile testing of a single lap aluminium-epoxy-aluminium assembly. The method of surface pre-treatment showed a profound effect on the surface topography and roughness by AFM. This study reveals that a combination of high surface energy and high surface roughness of the substrate along with good wettability of the adhesive contributed to the highest joint strength for the aluminium alloy through the FPL etching pre-treatment.     

Semi-Structural Hot Melt Adhesives Based on Crosslinkable Functionalized Polyolefins

A structural or semi-structural adhesive is usually applied to the substrates as monomers, oligomers, or melts of polymers with reactive groups and is then polymerized or crosslinked in situ in the joint between the substrates. We have been studying a number of crosslinked functionalized polyolefins blended with tackifier used as semi-structural adhesives for bonding to oily galvanized steel surfaces. The functions of takifier, surface properties of adhesive and substrate, geometry effects of lap joints, adhesive T_β, chain end defects, network chain length, and cure kinetics of these systems will be discussed. Our experimental results indicate that lap shear strengths of galvanized steel joints depend on adhesive storage modulus to the power of roughly 1/2. A rough estimate of the fracture energy of the adhesive bond, G_a could be obtained from this relation. Although some estimated G_a values are too low while the others are too high, they seem to be in rough accord with the degree of interfacial bonding and the locus of failure of the lap shear bonds.     

Adhesive Failure and Deformation Behaviour of Polymers

An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress-strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

How Compliance Compensates for Surface Roughness in Fibrillar Adhesion

Fibrillar interfaces play an important role in the ability of many small animals to adhere to surfaces. Surface roughness is generally deleterious to adhesion because it hinders the ability of mating surfaces to make contact, but fibrillar surfaces compensate for surface roughness by virtue of their enhanced compliance. We examine the relationship between roughness and compliance by analyzing the mechanics of detaching an array of fibrils from a substrate. The theory of Johnson, Kendall, and Roberts is used to describe the interfacial adhesion of each fibril, and roughness is modeled by making the fibril length a random variable subject to a probability distribution. We solve for the mean force response of a fibrillar array as a function of the displacement of the entire array. From these results we extract the mean fibrillar pull-off force and work to separate the fibrillar array and substrate. We show how the mean fibrillar pull-off force decreases with increasing roughness-height standard deviation: the relationship is linear for small height standard deviation, and the pull-off force trails off to zero for very rough surfaces. Conversely, the work of separation is shown to be unaffected by small roughness-height standard deviation, although it decreases toward zero for rougher surfaces. The effects of roughness may be offset by increasing fibrillar compliance; for small roughness-height standard deviation, we show that the reduction in pull-off force is inversely proportional to the normalized compliance. We also show that the work of separation increases linearly with the compliance when the compliance is large compared with the roughness-height standard deviation.     

Adhesive Failure and Deformation Behaviour of Polymers

An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress–strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.     

Effect of different skin permeation enhancers on tack of a pressure sensitive adhesive

Solutions of an acrylic copolymer pressure sensitive adhesive with different concentrations of propylene glycol (PG) and oleic acid (OA) were cast on a PET film. A rolling ball tack test was carried out on the adhesive coated tapes with different thicknesses. The results were explained on the basis of the surface (energy and roughness) and viscoelastic properties of the copolymer, which were related to the glass‐transition temperature. The 60‐μm PG samples with an approximately equal glass‐transition temperature and surface energy did not have a significantly different tack value. The tack value of the 30‐μm tapes decreased with PG concentrations above 15% (w/w), which was related to an increase in the surface roughness with a more prominent effect at the lower thickness. OA, which improved both the surface and viscoelastic properties, increased the tack value up to 15% (w/w). However, the tack value decreased above 15% (w/w). This was explained on the basis of OA large crystals, which can decrease viscoelastic energy dissipations and form a mechanically weak surface layer. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1287–1291, 2005     

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.     

Adhesion properties of polyurethane pressure-sensitive adhesive

In this study, the adhesion properties of polyurethane (PUR) pressure-sensitive adhesive (PSA) were investigated. The PUR-PSA was prepared by the cross-linking reaction of a urethane polymer consisting of toluene-2,4-diisocyanate and poly(propylene glycol) components using polyisocyanate as a cross-linking agent. The peel strength increased with the cross-linking agent content and exhibited cohesive failure until the maximum value, after which it decreased with interfacial failure. The PUR-PSA exhibited frequency dependence of the storage modulus obtained from dynamic viscoelastic measurements, but did not show dependence of the tack on the rolling rate measured using a rolling cylinder tack test under the experimental conditions used, which is quite different from the acrylic block copolymer/tackifier system. The PUR-PSA showed strong contact time dependence of tack measured by a probe tack test. The tendency was significantly larger than for the acrylic block copolymer/tackifier system. Therefore, the storage modulus increased, whereas the interfacial adhesion seems to be decreased with increase in the rolling rate for this PUR-PSA system. It was estimated that the influence of rolling rate on the interfacial adhesion and the storage modulus was offset, and, as a result, the rolling cylinder tack did not exhibit rate dependency.     

Molecular simulations of deformation and failure in bonds formed by glassy polymer adhesives

The mechanical behavior of glassy polymer bonds is examined with molecular dynamics simulations. We show that the interfacial strength of the bond in mode I (tensile) and mode II (shear) fracture is strongly influenced by the coupling between the adhesive and adherends as well as by the roughness of the substrate surface. Failure occurs at the substrate (interfacial failure) when the interaction is weak, and in the bulk (cohesive failure) when the interaction is strong. The transition from interfacial to cohesive failure under mode I loading is nearly unaffected by roughness, while roughness leads to a dramatic increase in interfacial strength under mode II loading. Stress mixity is another crucial parameter that determines whether the polymer fails through shear deformation or through cavitation and crazing. By varying the geometry of the adhesive bond, we illustrate different limiting behaviors of a rupturing film.     

Relaxation Properties and Chemical Nature of Polymer Surface Layers as Related to the Strength of Adhesive Joints with Metals

A new approach based on the theory of elasticity is proposed to study relaxation properties of adhesive transition layers. It involves experimental evaluation of the rate dependence of the fracture energy of the bulk polymer and its adhesive joint.

The investigation of the interaction of a polymer surface layer with electroplated copper by XPS using the effect of differential charging (the latter produced by potential shift of the sample by 10V), makes it possible to identify the functional groups of adhesive brought into contact with substrate surface. For ABS copolymers a bond of -O … Cu type was formed.

The mechanism of adhesive contact formation and factors affecting the strength of adhesive joints could be understood better by determining the properties of surface and transition layers.     

Adhesive properties of acrylate copolymers: Effect of the nature of the substrate and copolymer functionality

The adhesion behavior of statistical, uncrosslinked butyl acrylate-methyl acrylate copolymer on different surfaces (stainless steel, polyethylene, glass and Si-wafer) has been investigated using a combination of probe tack test and simultaneous video-optical imaging. Tack and stress peak values increase and the final number of cavities as well as cavity growth rate decreases with increasing surface energy of the substrate due to better wetting.The influence of the incorporation of an additional comonomer, namely, hydroxyethyl acrylate, methyl methacrylate and acrylic acid, on the adhesion of statistical, uncrosslinked butyl acrylate-methyl acrylate copolymer has been studied. Steel probes with different average surface roughness (R_a=2.9 and 291.7 nm) have been used for tack tests. The increasing polarity of the incorporated comonomer has no measurable effect on the surface tension but leads to an increase of shear modulus and consequently, to an increase in the stress peak, deformation at break, tack values, as well as the total number of cavities. The latter is a consequence of worse wetting. Cavity growth rate on the smooth surface is insensitive to copolymer composition, on the rough surface, the increase in the modulus associated with the additional monomers, leads to a decrease in the cavity growth rate. This indicates different cavity growth mechanisms: predominately lateral growth on the smooth surface and omnidirectional growth on the rough surface.The adhesion performance of uncrosslinked and crosslinked butyl acrylate-methyl acrylate copolymers is compared. The latter exhibit adhesive, and the former cohesive failure. The total number of cavities and cavity growth rate is found to be controlled by viscoelastic properties of PSA independent of the debonding mechanism and the latter decreases significantly with increasing shear modulus.     

Role of acid-base interfacial bonding in adhesion

The strength of macroscopic adhesive bonds of polymers is known to be directly proportional to the microscopic exothermic interfacial energy changes of bond formation, as measured by Dupre's 'work of adhesion'. Since the work of adhesion can be very appreciably increased by interfacial acid-base bonding with concomitant increases in adhesive bond strength, it is important to understand the acid-base character of polymers and of the surface sites of substrates or of the reinforcing fillers of polymer composites. The best known acid-base bonds are the hydrogen bonds; these are typical of acid-base bonds, with interaction energies dependent on the acidity of the hydrogen donor and on the basicity of the hydrogen acceptor. The strengths of the acidic or basic sites of polymers and of inorganic substrates can be easily determined by spectroscopic or calorimetric methods, and from this information one can start to predict the strengths of adhesive bonds. An important application of the new knowledge of interfacial acid-base bonding is the predictable enhancement of interfacial bonding accomplished by surface modification of inorganic surfaces to enhance the interfacial acid-base interactions.     

Influence of Aging on Autohesive Tack of Brominated Isobutylene-co-p-methylstyrene (BIMS) Rubber in the Presence of Phenolic Resin Tackifier

The role of phenolic resin tackifier on autohesive tack of brominated isobutylene-co-p-methylstyrene (BIMS) rubber was studied by a 180° peel test with particular reference to aging. Phenolic resin showed very little effect on the unaged tack of BIMS rubber. The tack strength of the rubber/resin mixture marginally increased at 1 phr resin concentration, beyond which it decreased. Based on the data on the compression creep, maximum tensile stress, and viscoelastic properties of the rubber/resin mixtures, phenolic resin did not enhance the interfacial viscous flow behavior of the rubber/resin mixtures. The results from dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM) confirmed the existence of a phase-separated morphology in the rubber/resin blends even at low resin concentration. Upon aging at 100°C for 36 h, the rubber/resin blend containing 1 phr of phenolic resin showed further increase in tack strength which was attributed to migration of the tackifier to the rubber surface and the changes in the compression creep, viscoelastic behavior, and maximum tensile stress of the rubber/resin mixtures. This is also a function of aging time. Surface energy analysis by contact angle measurement, Fourier Transform Infrared Spectroscopy (FT-IR/ATR) studies, and surface roughness measurement by atomic force microscopy (AFM) elucidate the enrichment of the phenolic resin on the rubber surface upon aging and the mechanism of enhanced tack strength.     

London-van der Waals adhesiveness of rough particles

Van der Waals forces often dominate interactions and adhesion between fine particles and, in turn, decisively influence the bulk behaviour of powders. However, so far there is no effective means to characterize the adhesive behaviour of such particles. A complication is that most powder particles have rough surfaces, and it is the asperities on the surfaces that touch, confounding the actual surface that is in contact. Conventional approaches using surface energy provide limited information regarding adhesion, and pull-off forces measured through atomic force microscope (AFM) are highly variable and difficult to interpret. In this paper we develop a model which combines the Rumpf-Rabinovich and the JKR-DMT theories to account simultaneously for the effects of surface roughness and deformation on adhesion. This is applied to a ‘characteristic asperity’ which may be easily obtained from AFM measurements. The concept of adhesiveness, a material property reflecting the influences of elastic deformability, surface roughness, and interfacial surface energy, is introduced as an efficient and quantitative measure of the adhering tendency of a powder. Furthermore, a novel concept of specific adhesiveness is proposed as a convenient tool for characterizing and benchmarking solid materials. This paper provides an example to illustrate the use of the proposed theories.