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.     

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.     

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.     

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.     

Biocompatible Catechol-Functionalized Cellulose-Based Adhesives with Strong Water Resistance

Numerous traditional adhesives have good adhesion in dry environments. However, non-environmental-friendliness and poor water resistance largely limit their practical applications. To prepare biocompatible adhesives with strong water resistance and adhesion strength, in this paper, catechol-functionalized cellulose-based adhesive polymers are synthesized by grafting N-(3,4-dihydroxyphenethyl)methacrylamide and methyl methacrylate onto cellulose chain through atom transfer radical polymerization (ATRP). The successful synthesis of the catechol-functionalized cellulose-based adhesive polymers is confirmed by FTIR and ¹H NMR. The different characteristics of the adhesive polymers, such as thermal stability, swelling ratio, biocompatibility, and adhesion strength are investigated. Strong water resistance on various substrates is realized in underwater environment for the catechol-functionalized cellulose-based adhesive with addition of Fe³⁺. The adhesion strength and thermal stability are enhanced when the catechol content is increased. The adhesive with catechol content of 25.4% shows the adhesion strength of 0.45 MPa for iron substrate in underwater environment. In addition, the adhesive with addition of Fe³⁺ exhibits excellent adhesion in dry environment, with maximum adhesion strength of 3.50 MPa for iron substrate. The cell culture test shows that the adhesive polymers have excellent biocompatibility. The biocompatible adhesives with strong water resistance have potential application in electronic, wood, and building fields.     

Effect of citric acid,PVOH, and starch ratio on the properties of cross-linked poly(vinyl alcohol)/starch adhesives

The properties of tapioca starch adhesives were improved by cross-linking and the cross-linked adhesive compared with pure tapioca starch and poly(vinyl alcohol) adhesives. The effect of starch ratio, type of PVOH, and adding citric acid were important factors on the cross-linked adhesives. Wood adhesives made from cross-linked PVOH/starch were prepared by PVOH and tapioca starch, using hexamethoxymethyl melamine (HMMM) and citric acid (CA) as a cross-linking agent and catalyst, respectively. The effect of CA, PVOH/starch ratio, and type of PVOH such as medium (M PVOH) and high molecular weight (H PVOH) were investigated. The condition of the cross-linking reaction was 175?°C for 15?min. The structural properties of cross-linked adhesive were investigated by FT-IR spectroscopy. The results were confirmed in terms of thermal properties with a differential scanning calorimeter (DSC) and the shear strength of the adhesive. The cross-linked adhesive resulted in the increase of T _g and showed good blend compatibility with all of the cross-linked adhesives. The adhesive strength significantly increased when using CA as a catalyst in the cross-linking reaction. The optimum contents of the cross-linked PVOH/starch adhesives were 1:1.8 for M PVOH and 1:0.5 for H PVOH.     

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.     

Eco-Friendly Adhesives Based on Tannin and N,N-Bis(2-Hydroxy-Ethyl) Fatty Amides (HEFAs) from Non-Traditional Oils for Wood Bonding

Abstract

Wood adhesives were formulated using tannin and N,N-bis(2-hydroxyethyl) fatty amides (HEFAs). The natural tannin-based adhesives can be used to replace formaldehyde-based adhesive systems and thereby reduce formaldehyde and volatile organic compound (VOC) emissions from adhesives used for plywoods. Performance properties of the adhesively bonded wood joints viz., tensile strength, impact strength and chemical resistance were measured. N,N-bis(2-hydroxyethyl) fatty amides (HEFAs) from non-traditional oils were mixed with a pure tannin-based adhesive as a crosslinker, and this increased the tensile strength, impact strength and chemical resistance of wood joints. The results revealed that a high performance and eco-friendly adhesive system for wood can be successfully formulated using tannin and HEFA.     

Preparation and Properties of Styrene-Ethylene-Buthylene-Styrene Hot Melts

Abstract

Hot melt-type adhesives were prepared by mixing Styrene-Ethylene-Buthylene-Styrene (SEBS) rubber with various tackifying resins. Thermal and peeling properties measured on the resulting blends show that it is indeed possible to produce a working SEBS-based solvent-free adhesive by properly combining the properties of the corresponding components in the formulation.     

Study of Properties of One-Component Moisture-Curable Polyurethane and Silane Modified Polyurethane Adhesives

Abstract

In this paper, the curing behavior and adhesion properties of five kinds of polyurethane (PU) and silane modified polyurethane (SPU) adhesives were studied. Fourier transform infrared spectra showed that the adhesive Sikaflex-555 had an SPU structure and that the other four adhesives were one-component polyurethanes. The experimental results of curing behavior show that the tack-free times and curing rates of these adhesives were significantly affected by the relative humidity and the temperature. The adhesion of the SPU adhesive to glass sheet, zinc galvanized steel sheet and steel sheet was much better than that of PU adhesives. The tack-free times and curing rates of all the adhesives was measured during the 12 months of storage.     

Evaluation of Some Factors Affecting the Performance of Polyester Hot-Melt Adhesives

Abstract

A study was carried out to establish the importance of Theological, thermal, and chemical variables in the performance of polyester hot-melt adhesives. The performance criteria are based on the shear strength of Al-Al lap shear joint formed under optimal conditions.

The results are presented showing the effects of: application and testing temperatures, chemical composition of the adhesive, linear vs. branched polymer, crystallinity, rate of crystallization and molecular weight.     

Dependence of Adhesion Properties of Cross-linked Epoxidized Natural Rubber (ENR 25)-Based Pressure-Sensitive Adhesives on Benzoyl Peroxide Loading in the Presence of Gum Rosin and Petroresin Tackifiers

The effect of benzoyl peroxide loading on the adhesion properties of cross-linked epoxidized natural rubber (ENR 25)-based adhesives was studied using gum rosin and petroresin as tackifiers. Toluene and polyethylene terephthalate (PET) were used as solvent and coating substrate, respectively. The adhesion properties were determined by a Lloyd adhesion tester operating at 30 cm min^?1. Results indicate that the loop tack and peel strength of gum rosin and petroresin pass through a maximum value at 2 parts per hundred parts of rubber (phr) and 3 phr benzoyl peroxide concentration, respectively, an observation which is attributed to the optimum cross-linking of ENR 25 where optimum, cohesive and adhesive strength is obtained. The shear strength, however, increases steadily with increasing benzoyl peroxide loading due to the steady increase in the cohesive strength. At the optimum benzoyl peroxide concentration, the petroresin-based adhesive consistently exhibits higher adhesion properties compared to that of gum rosin-based adhesives. The adhesion properties of both adhesive systems increase with increasing coating thickness.     

Preparation and Characterization of Electroconductive Adhesives of NanoG/Polyurethane-Epoxy IPNs

Polyurethanes (PU) based on toluene diisocyanate (TDI) and polypropylene glycol 2000 (PPG) were reacted with an epoxy resin (EP) to prepare interpenetrating polymer networks (IPNs). Three kinds of electroconductive adhesives were prepared by dispersing nano-graphite (NanoG) into different matrices, i.e., pure PU, crosslinked PU/EP, and pure EP. The effects of epoxy content on morphological structure, conducting properties, thermal stability, and adhesive properties of the electroconductive adhesives were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, standard digital multimeter, dynamic mechanical thermal analysis, and lapshear tests. The results indicate that epoxy in the polyurethane-epoxy IPN adhesives plays an important role in clanging the morphological structure and improving conductivity properties, thermal stability, and adhesive properties of the electroconductive adhesives of PU.     

Preparation and water resistance of crosslinked waterborne polyurethaneurea

Abstract

Two series of cross-linked polyurethaneurea (PUU) aqueous dispersions with polyoxypropylene glycerol and pentaerythritol as internal cross-linking agents were prepared and characterised. The results revealed that in comparison with the uncross-linked one, the cross-linked PUU films exhibited excellent waterproof performance and mechanical properties. The amount of water absorption was as low as 2?5 wt-%, the contact angle of water on the surface of this kind of film was as high as 96°, and the tensile strength was as high as 42?8 MPa. The cross-linked PUU films with polyoxypropylene glycerol and pentaerythritol as cross-linked agents showed different properties at the same cross-linking agent content. The prepared triol-cross-linked or tetra-cross-linked PUUs had great potential application in meeting the highly diversified demands in modern technologies such as coatings, leather finishing, adhesives, sealants, plastic coatings and wood finishes, where high water resistance and durability were required.     

Plasticity in peeling

Contrary to classical theory, a high proportion of bond failures by peeling involve progressive plastic adherend flexural yield. Such yield occurs with adherend thicknesses below a critical value, T_c, which is shown calculable by combining elastic peel mechanics with plastic bending criteria. The geometry of such “peel with yield,” and thence the moment-controlled peel forces, can be accounted for only if the adhesive is also recognized as behaving essentially plastically. Subsequent plastic adherend unbending is important with highly extensible adhesives. The geometry of “legging” peel in such cases is best described by fully plastic mechanics. These are derived and shown to account for literature data on dependencies of peel force upon peel rate and adhesive thickness. “Stick-slip” peel phenomena are indicated to be controlled by recurring interacting plastic–elastic transitions, in both adhesive and adherend: adhesive strain rate is critical in such phenomena. Four regimes of peel behavior can therefore apply as adherend thickness (T) increases, with peel forces proportional respectively to T⁰, T^2/3, T^3/2 (above T_c) and finally controlled by moment limitations due to joint configurational constraints (“cleavage”).     

Cleavage and Static Toughness

The cleavage of adhesive joints allows the experimental study of the process of fracture in the low speed range. The value of the fracture energy deduced from the fracture length is the static toughness of the adhesive. This value, which determines the endurance limit of the joint, is much larger than can be explained by the current theories. It depends on the surface treatment of the substrate and results from the damage of the adhesive bonds. To take into account these results, the equation describing the fracture of adhesive joints as it was proposed by A. N. Gent and J. Schultz has to be extended. When that is done, it applies to viscoelastic adhesives, whether pressure sensitive or hot melts, and probably also to cross-linked adhesives.

If G is the fracture energy of the joint, the equation G = G ₀ + α K ₂ ·v^a accounts for most experimental results and even for the fatigue of adhesive joints.     

Rheological Study on Tack of Pressure-Sensitive Adhesives

It is pointed out that the tack of pressure-sensitive adhesives should be expressed in terms of rolling friction coefficient f of the adhesives. Values of f were determined by both rolling ball and pulling cylinder experiments, and the dependence of f upon viscoelastic properties and thickness of the adhesives was studied. The experimental results were interpreted by the model theory previously proposed. It is also shown that the tack of pressure-sensitive adhesives by the conventional ball tack tests corresponds to f measured at the velocity ranging from v ～ 10 to v ～ 10² cm/sec.     

Rolling Motion of a Ball on Pressure Sensitive Adhesives

Rolling motion of a ball on pressure sensitive adhesives was carefully observed under well-controlled conditions. Rolling distance was measured as a function of time by means of stroboscopic photography, and rollout distance was measured as a function of initial height of a ball. Both rolling distances and rollout distances were analysed according to a unified theory, where rolling friction coefficient (f) of a pressure sensitive adhesive is involved. It is suggested that f depends on viscoelastic properties of the adhesives.     

Elastoplastic Fracture Behavior of Structural Adhesives Under Monotonic Loading

Elastic-plastic fracture behavior of a structural adhesive in the bulk and bonded forms is discussed. The model adhesive chosen, Metlbond 1113 (with scrim carrier cloth) and 1113-2 (neat resin) solid film adhesives exhibit a relatively brittle material behavior to justify the use of LEFM methods.

The solid film adhesives are first cast in the form of tensile coupons to determine the bulk fracture properties with the use of single-edge-cracked specimen geometry. K_Ic evaluation is done using the procedure suggested by the ASTM standard. A K-calibration method based on application of boundary collocation procedure to the William's stress function is utilized to relate the measured critical loads to the K_Ic values. The yield stresses and elastic moduli values in the bulk tensile mode are also evaluated. The availability of K_Ic à _y E and v (Poisson's ratio) values makes the calculation of crack tip plastic zone radii (r_yc ) and fracture energy (G_Ic ) values possible on the basis of Irwin's theory. The bulk casting procedure is done under different cure (temperature, time and cool-down) conditions to determine optimum properties.

The fracture behavior of the same adhesives in the bonded form is studied with the use of Independently Loaded Mixed Mode Specimen (ILMMS) geometry. This specimen allows independent measurement of P_I and P_II (and consequently G_I and G_II ) values. Since the fracture energy values are affected by the thickness of the adherend and the bondline, an experimental program is executed first by varying these geometrical parameters to determine the plane strain conditions. The relationship between the bondline thickness and the crack tip plastic zone radius values calculated earlier is also studied. Expressions developed on the basis of LEFM assumptions are utilized to calculate G_Ic and G_IIC values in the bonded form. The G_IC values obtained in this manner are compared to the bulk G_IC values obtained earlier.

With the availability of P_I and P_II (G_I and G_II ) values that result in failure in the bonded form, the fracture condition (i.e. the fracture failure criterion) in mixed mode (modes I and II) loading is determined for adhesively bonded joints. The use of both 1113 and 1113-2 adhesives also reveals the effects of the carrier cloth on the mechanical phenomena cited above.