Bonding of Natural Rubber to Steel: Surface Roughness and Interlayer Structure

This paper is concerned with two aspects of the adhesion produced by the vulcanisation bonding of a simple natural rubber (N.R.) compound to mild steel. Adhesion was measured using a 45° peel test.

When the N.R. was bonded, using a proprietary bonding agent (Chemlok 205/220), to ‘smooth’ steel (acid etched) or to ‘rough’ steel (phosphated) high values of peel energy (≥ 4.5 kJm^?2), and good environmental resistance to water were obtained, with failure cohesive largely within the rubber. The highest values of peel energy (≈ 7.5 kJm^?2) were associated with a phosphated surface which consisted of plate-like crystals which directed the stresses away from the substrate in a way which produced a failure surface within the rubber which showed extensive tearing and cracking.

The nature of the layer formed in the interfacial region by interaction between bonding system and rubber was investigated using a chlorinated rubber as a ‘model compound’ representing the adhesive and uncompounded N.R. to represent the rubber. When a blend of the two was heated in air at 150°C, evidence was found of a solid state chemical reaction in which carbonyl groups were incorporated into the blend which became visually homogeneous. Further evidence points to the relevance of this change to adhesion in rubber-to-metal bonding.     

A Technique for the Measurement of Adhesive Fracture Energy by the Blister Method

The adhesive fracture energy, G_1c, of a model adhesive/adherend system, consisting of poly(methyl-methacrylate) plates bonded with a cyanoacrylate adhesive, has been evaluated using the Tapered Double Cantilever Beam and Blister test geometries. A refined Blister testing technique is described which, using relatively large diameter test plates [200mm], is capable of arresting the initial propagation of the-invariably less than naturally sharp-starter crack. This allows us to three subsequent G_c determinations for the same specimen from starter cracks of natural sharpness.

Adhesive fracture energy values determined for the model system using TDCB test pieces, 0.110±0.017 kJm^-2, were in good agreement with those obtained for Blister specimens in which arrested cracks had been repropagated, 0.119±0.013 kJm^-2. As is generally observed, values calculated from the initial propagation of starter cracks were somewhat higher for the TDCB specimens, 0.140±0.045 kJm^-2. Corresponding values for the Blister tests were significantly higher with more scatter, 0.194±0.074 kJm^-2.

It is concluded that meaningful G_1C data are only obtained from the Blister test if values obtained from the propagation of the initial starter crack are discarded. No matter how carefully prepared, these flaws will be less than naturally sharp. The precracking technique described here detects crack growth and, by releasing the hydraulic pressure driving it, arrests a propagating crack before catastrophic failure can take place.     

Adhesion of Natural Rubber Compounds to Plasma-Polymerized Acetylene Films

Plasma-polymerized acetylene films were shown to be novel, highly effective primers for rubber-to-steel bonding. However, the performance of the primers depended strongly on processing variables such as the substrate pretreatment and the carrier gas. Miniature lap joints were prepared by using natural rubber as an “adhesive” to bond together pairs of pretreated steel adherends primed with plasma-polymerized acetylene films which were deposited using various carrier gases. The initial strength of joints prepared from substrates which were mechanically polished and then coated with plasma-polymerized acetylene films deposited using an argon or nitrogen carrier gas was 2000 N for a bonded area of 64 mm² and failure was 100% cohesive in the rubber. Similar results were obtained for joints prepared from mechanically-polished brass substrates. However, the initial strength of joints prepared from polished substrates which were coated with plasma-polymerized films deposited using oxygen as a carrier gas was lower by a factor of two and there was only 30% rubber coverage on the substrate failure surfaces. demonstrating the importance of the carrier gas.

The initial strength of joints prepared from substrates which were pretreated by alkaline cleaning, acid etching, or mechanical polishing and then coated with plasma polymers using argon as the carrier gas was also approximately 2000 N/64 mm² and failure was again 100% cohesive in the rubber. However, the strength of joints prepared from substrates which were pretreated by ultrasonic cleaning in acetone and then coated with plasma polymers using argon as the carrier gas was lower by a factor of almost two, demonstrating the significance of substrate pretreatment.

During exposure to steam at 121°C, the durability of miniature lap joints prepared from polished steel substrates primed with plasma-polymerized acetylene films using argon as a carrier gas was excellent. After exposure for 3 days, the breaking strength of the joints decreased slightly, from 1740 to 1410 N/64 mm², but the locus of failure remained cohesive in the rubber, implying that effect of steam was mostly to reduce the cohesive strength of the rubber. Similar results were obtained from joints prepared from polished brass substrates. However, the durability of joints prepared from polished brass substrates and from polished steel substrates primed with plasma-polymerized acetylene was poor during exposure to aqueous salt solutions for three days. Although all of the joints decreased significantly in breaking strength, the strength of the joints prepared from brass substrates was about 400 N/64 mm² higher than that of joints prepared from steel primed with plasma-polymers. Most of the joints prepared from steel primed with plasma-polymerized acetylene films failed near the interface between the primer and the steel substrate although some specimens had 20-40% rubber coverage on the failure surfaces.     

Studies on Adhesion Between Natural Rubber and Polyethylene and the Role of Adhesion Promoters

Studies on adhesion between natural rubber (NR) and polyethylene (PE) with different levels of interaction (physical and chemical) have been carried out. Ethylene propylene diene rubber (EPDM) and chlorinated polyethylene (CPE) were used as physical promoters and epoxidised natural rubber/modified polyethylene (ENR/PEm) and sulfonated ethylene propylene diene rubber/modified polyethylene (S-EPDM/PEm) were used as chemical adhesion promoters. The failure surfaces were examined with the help of scanning electron microscopy (SEM), optical photography and electron spectroscopy for chemical analysis (ESCA) techniques.

The peel strength between natural rubber and polyethylene as measured in this study is 140 J/m². With the incorporation of physical promoters such as EPDM, the peel strength increases twenty fold because of structural similarity of EPDM with PE and the rubbery nature of EPDM. Similarly, the other promoters show significant improvement in peel strength. At high temperature and low rate of peeling, the nature of failure is mainly “stick-slip” for joints with interaction promoters. The average peel strength increases with increase in test rate and decrease in test temperature for most of the joints. All the data could be shifted onto a master curve indicating that the increase in strength is a result of viscoelastic dissipation. NR/EPDM/PE and NR/CPE/PE systems, however, behave in a different way probably because they alter the nature of crack propagation at or near the interface. ESCA results of the peeled PE surface show a chemical shift of C_1S peak. SEM photographs also indicate interaction at the interface when modifiers are used. An increase in crystallinity of PE from 30% to 64% and modulus increase the peel strength of NR/PE joints by a factor of four. The results of peel strength measurement at 90° are lower than those at 180°. Lap shear results are in line with peel strength.     

Brass Powder in Rubber Vulcanizates. The Effect on Adhesion

Brass plating is presently used for promoting the adhesion of rubber to steel cords used in tires. It is generally accepted that the adhesion is due to chemical bonds that are formed between the rubber and brass,¹ although evidence has been obtained indicating that the adhesion is the result of a physical phenomenon.² In addition, a catalytic oxidation theory also has been advanced.¹

Natural rubber vulcanizates loaded with brass powder showed an increase in crosslinking (by swelling measurements), indicating an interaction or bonding between the rubber and brass. Techniques previously employed with reinforcing fillers such as carbon black were used to evaluate the effect of brass on adhesion. The addition of brass powder decreased the scorch time and optimum cure time indicating that the rubber immediately surrounding the brass was being cured at a faster rate. Copper powder, on the other hand, inhibited the cure completely. When exposed to air or oxygen, even small amounts of brass catalyzed oxidation of the rubber.     

Applied Research and Development of Adhesives for Bonding Filled Carboxyl Terminated Polybutadienes to Various Substrates

A one-year applied research and development program was conducted on the bonding of carboxy-terminated polybutadiene (CTPB) propellant to various substrate materials encountered in solid propellant rocket motors. Under this program, in addition to CTPB liners, liners were also prepared from polyesters, polyethers, polyurethanes, polyacetal polymers, and epoxy resins. The use of various crosslinkers, emulsifiers, wetting agents, fillers, and stabilizers was also evaluated.

Four optimized liner formuations with the best all-round properties were fully characterized. The optimized formulations represented an HC liner formulation with two Ievels of glycerol additive, an HC formulation with a sorbitol additive, and a Butvar polyacetal-type liner. A standard HC-polymer liner formulation, designated as TL-H-304, was used as a control.

Unaged liner peel and shear properties were measured at -65°F, 77°F, and 160°F. Samples, aged for 30 days at 160°F, were tested at 77°F only.

The liners were tested against propellant, steel, aluminum, magnesium, titanium, epoxy-fiberglass, phenolics, polyisoprene, and butadiene-acrylonitrile as substrate materials. The steel, titanium, and polyisoprene rubber substrates gave the best adhesive results.

The substitution of asbestos and Cab-O-Sil for the Thermax filler in the liner gave comparable adhesive results while the substitution of clay fillers gave poor results.

This program was performed while at the Elkton Division of the Thiokol Chemical Corporation in fulfillment of the requirement of Contract N123 (60530-53329A) U.S. NAVAL ORDNANCE TEST STATION, China Lake, California, reported previously in U.S. Naval Report NOTS-TP4283.     

Determination of Quasiequilibrium Work of Adhesion of Elastic Coatings

Present methods for the determination of adhesion bonding of elastic polymeric materials entail certain experimental difficulties. In particular, the necessity of strict centering of the test specimen, and the difficulty associated with application of a homogeneously distributed stress over the whole cross-sectional area (homogeneous detachment or shear), or the excessive expenditure of work resulting from polymer deformation (peel).^1,2

We are suggesting a method to determine the quasi-equilibrium work of adhesion during the peeling process for elastic polymeric coatings, the value of which, as was demonstrated experimentally, does not depend on the coating thickness, deformation or rate of peeling.     

STEAM CONDENSATION IN A VERTICAL CORRUGATED DUCT

An experimental investigation was made on the condensation of water steam in a vertical corrugated duct. The data have been correlated as follows

Co=5.11Re^-0.431 150 ≤ Re ≤ 350 Co = 0.034311Re^-0.425350 ≤Re ≤l000

The vertical corrugated duct is constructed of two corrugated plates with corrugation inclination angles of β = 0 and β = 45° respectively (relative to the overall flow direction).

The condensation heat transfer coefficient in the corrugated duct is more than two times higher than that of bulk condensation on a vertical plate. A physical model was proposed to explain the heat transfer enhancement. Attention was also paid to the effect of exit steam velocity on the heat transfer during partial condensation. It was demonstrated that the heat transfer in the corrugated duct was strongly affected even at a low exit velocity, which was different from the case of bulk condensation on a vertical plate. Experimental apparatuses and the method for examining their reliability are described in detail.     

Hydrogen Bonding and the Interfacial Component of Adhesion: Acid/Base Interactions of Corona-Treated Polypropylene

The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γ^LW, γ^⊕ and γ^⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed.

The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ^⊕ and γ^⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ^⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m². The acidic component, γ^⊕, rose from zero to as high as 1.9 mJ/m². Its decay with time was only qualitatively the same as that of γ^⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data.

The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.     

Adhesion of Plasticized Polyvinyl Butyral) to Glass

A study of safety glass provides a good example of the interplay among the many physical properties involved in “adhesion”, and the relationship between adhesion and performance. This work demonstrates the value of applying known fundamentals to practical problems.

An idealized model of a windshield fracture event is described in terms of interactions among mechanical responses of the interlayer, the fracture characteristics of the glass and the high speed, low angle peel behavior.

Data on the surface energies of glass, polyvinyl butyral) and water show that at thermodynamic equilibrium a stable system comprising glass, water and polyvinyl butyral) phases, an aqueous phase must lie between the glass and PVB.

The potassium salts are shown to be effective because they are deliquescent and give solutions at equilibrium with the water in the PVB at water contents of ∼0.40% or higher. The greater the amount of salt at the interface and the higher the water content of the sheeting during lamination, the thicker the interfacial layer and delamination occurs more readily. This relationship is quantified using a modified form of the Stefan equation.

Data on diffusion of water and salt are shown to be consistent with the amount of salt at the interface required for the observed performance (∼ 3 mg KAc/m²).

Data on electrical resistivity of the interface correlate with peel force and provide convincing support for the hypothesis.     

Corona-discharge Treatment of Polypropylene films-Effects of Process Parameters

Corona treatment of films, mainly polypropylene (PP)-copolymers, was studied at commercial levels in a 2.7 kVA treater. The films were produced on a flat-film extruder with chill rolls. Degree of treatment was characterized by power of the generator divided by web speed and width of film (m Ws/cm²).

The effectiveness of the treatment was measured in terms of the polar and dispersion components of surface-energy, the peel adhesion of pressure sensitive tape (similar to ASTM Adhesion Ratio) and the peel adhesion of polyurethane adhesives.

The polar component of surface energy is a measure of the effectiveness of corona pretreatment. For a given degree of treatment, the polar surface energy component becomes greater as the film cooling rate increases (and the degree of crystallization falls).

A comparison of homopolymers and copolymers does, however, reveal that even where these have the same density or the same degree of crystallization one cannot count on them having equally-sized polar components.

Peel strengths of pressure-sensitive tapes and polyurethane-bonded patches confirm the influence of cooling conditions on wetting properties.

Contrary to the case for tape adhesion, the polyurethane adhesive strengths reach their maximum value at much lower treatment intensities, i.e. with much lower polar surface energy components, and thus question the validity of the ASTM tests for adhesion properties.     

The adhesion of natural rubber to steel and the use of the peel test to study its nature

The peel test has been used to study the adhesion achieved between a simple natural rubber compound and mild steel by injection moulding in the presence of a commercial bonding system. Peel energies were calculated from the peel force results by allowing for the elastic energy stored in the detached rubber strip. The peel energy was found to be insensitive to the changes of peel rate evaluated in this work. Peel energy increased by a factor of about five as the peel angle increased from 30° to 90°. The locus of failure was cohesive within the rubber, but moved more deeply within the rubber and produced a severely torn fracture surface.     

Interlaminar Bond Strength and Failure Mechanisms in Commercial Flexible Polymer-Metal Laminates

An alternative to the 180° “T” peel test (called simply the “T-peel test” in the USA) was developed by Cropper and Young for the measurement of interlaminar bonding in three-ply polypropylene-aluminium-polyester laminates used in food packaging applications. The effect of temperature on the interlaminar bond strength of three laminate systems has since been studied. In particular, the effect of temperature on both the failure mode and on the adhesive's appearance after testing has been determined. It is shown that as the temperature is raised about 23°C, the laminating adhesive begins to soften and the failure mode changes from almost exclusively adhesive failure at the polyurethane adhesive-aluminium interface to cohesive failure of the polyurethane adhesive itself. The change in the failure mode is accompanied by the appearance of a meniscus instability. The temperature at which the meniscus instability patterns become more prominent correspond to the temperature at which the maximum interlaminar bond strength is attained.

It is thought that this new test can be used to characterise the behaviour of laminating adhesives more fully, both in their change in appearance with temperature, and in their effectiveness in bonding layers together as temperatures are increased above ambient conditions.     

MASS TRANSFER AND DIFFUSION COEFFICIENT DETERMINATION IN THE WET AND DRY SALTING OF MEAT

Dehydrated salted meat is widely used in Brazil as a very important source of animal protein. The main objective of this kind of processing is water removal. initially by osmotic pressure changes and then by drying, resulting in a product with intermediate moisture levels.

In this work, mass transfer and salt diffusion in pieces of meat submitted to wet and dry salting were studied. Slabs of beef m. trapezius with an infinite plate geometry were salted in a NaCl saturated solution or in a dry salt bed, at two temperatures (10 and 20°C) and different time exposures (120 min and 96 hours). Equilibration studies were extended up to six days.

It was observed that water loss increased with salt uptake, for increasing periods of times. At 20°C the moisture loss was higher than it was at 10°C in both salting processes. On the other hand, the kinetics of salt uptake and moisture loss were of greater importance in the process of dry salting than in that of wet salting.

The salt diffusion coefficient for wet salting was 0.26 × 10^-10m²/s at20°C and 0.25 × 10^-10 m²/s at 10°C and for the dry salting the values were 19.37 × 10^-10 m²/s at 20°C and 17.21 × 10^-10 m²/s at 10°C.     

Silane Coupling Agents as Adhesion Promoters for Aerospace Structural Film Adhesives

The performance of eight organofunctional silane coupling agents as adhesion promoters for the bonding of aluminium with two 121°C and two 177°C curing structural film adhesives was investigated and compared to the chromic acid (FPL) etch pre-treatment process and two non-chemical pretreatments. Aspects considered were shear strength of joints at ambient and elevated temperatures and durability, as judged by the wedge test.

The epoxy silane, γ-glycidoxypropyltrimethoxy silane, was found to be a very efficient adhesion promoter with all film adhesives evaluated. The cationic styryl silane, a neutral diamine monohydrochloride, showed promise with two adhesive systems. Four other neutral silanes were less effective.

Performance of amine functional silanes was mixed. Although the shear strength of joints with the primary amine silane at its natural pH of ∼10.3 was relatively good, durability was poor. However, good durability was obtained if the primer was first adjusted to pH 8 with hydrochloric acid, but not if acetic or phosphoric acids were used. Diamine silane was not an effective adhesion promoter at either its natural pH or when acidified with hydrochloric acid.     

Physico-Chemical Criteria for Maximum Adhesion. Part II: A New Comprehensive Thermodynamic Analysis

In this paper, two parameters defined as the relative work of adhesion [W_A/γ_L] and the relative interfacial energy [γ_SL/γ_L] have been examined for their assumed usefulness in correlating the thermodynamic properties of the components of the system substrate/ adhesive with its practical performance (strength). It is shown that the minimum value of [γ_SL/γ_L] relevant to conditions for the maximum adhesion becomes zero only for those systems (relatively rare) for which interaction factor Φ₀ is equal to 1.0.

Several transition points were identified for boundary conditions acquired at θ = 0° and θ = 90° which can be used to predict the properties and performance of an adhesive joint. These transition points are: a_MIN—energy modulus of the system (E. M. S.), relevant to the minimum interfacial energy; a_S—E. M. S. where self-spreading of adhesive occurs; a_CRIT—E. M. S. relevant to conditions under which the thermodynamic work of adhesion becomes negative and the system exhibits a tendency for self-delaminating or has “zero-strength”; a_CF—E. M. S. beyond which the geometry of the interface at any interfacial void or boundary of the joint may be regarded as a crack tip.

It is shown that only in those systems for which Φ₀ = 1.0 can a minimum contact angle of 0° indicate a condition for the maximum strength. If Φ₀ is known, the optimum contact angle can be estimated and hence the optimum surface energy of the substrate (adjusted by surface treatment, etc.) for the maximum adhesion.     

Particle Adhesion in Model Systems: 16. Barium Sulfate Particles on Glass and Protein Surfaces

The adhesion phenomena of monodispersed barium sulfate (BaSO₄) particles on gelatin-coated glass beads were evaluated using the packed column technique and compared with the same system in the absence of the protein.

Multilayer deposition was observed with the uncoated glass beads at pH 4, 5 and 6, while at pH 9, which is above the isoelectric point (pH ∼ 6) of BaSO₄ particles, monolayer deposition took place, even though the BaSO₄ particles and glass beads bore the same sign of charge. At pH = 10, no uptake was observed on the glass beads, but the addition of 10^-4 mol dm^-3 BaCl₂ induced multilayer deposition due to the adsorption of the Ba²⁺ cation on BaSO₄ particles, which causes a reversal of their charge to positive.

The formation of multilayers was found to occur over a much wider pH range on the gelatin coated glass beads.

BaSO₄ particles deposited in multilayers could not be removed from either glass beads or gelatin-coated glass beads by rinsing the loaded column with solutions of pH 11.5, but could be detached from monolayers on glass beads only.     

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.     

Behaviour of Urethane Adhesives on Rubber Surfaces

The paper presents the results of chemical investigations into some of the bonding problems of the footwear industry and pays particular attention to the importance of the substrate surfaces.

The importance of the presence of metal soaps on rubber surfaces and their detrimental effect on adhesion is pointed out together with the concomitant beneficial effects of solvent soap dispersing treatments on subsequent adhesion.

The special needs of the footwear industry for a single adhesive system capable of adhering strongly to a wide range of substrates led to the requirement of improving the bond of urethane adhesives to rubber surfaces. The role of free isocyanates in promoting this bond is outlined.

The practical advantages of halogenating rubber surfaces in conferring excellent adhesion properties on moulded rubber surfaces when used with solvent urethane adhesives are pointed out. The observed phenomena associated with halogenation are discussed qualitatively in relation to proposed theories of adhesion.     

Strengthening interfaces between biaxial oriented PET and PSMA: Effects of nitrogen plasma and bonding treatments

The fracture toughness, G_c, of the interface between a nitrogen plasma-treated poly(ethylene terephthalate) (PET) film and a poly(styrene-co-maleic anhydride) (PSMA) substrate was measured by using asymmetric double cantilever beam method. The effects of plasma treatment condition on PET films and post-plasma bonding treatment of the bi-material on the adhesion and the failure mechanism were investigated. For a given plasma pressure and energy, the amount of incorporated nitrogen on the PET surface as determined from X-ray photoelectron spectrometry (XPS) increased with increasing plasma treatment time and reached a plateau value of 7.7 at.%. XPS measurement showed that the incorporated nitrogen was primarily in the form of amine and amide. For bonding temperatures between 130 °C and 160 °C, the fracture toughness increased with increasing nitrogen incorporation on PET surface and reached a saturation G_c which significantly depended on the bonding temperature. The saturation G_c increased from 10 J/m² at 130 °C to 40 J/m² at 140 °C, reached a maximum of 120 J/m² at 150 °C, and then decreased to 60 J/m² at 160 °C. The location of failure also changed drastically with the bonding temperature. SEM and XPS measurements showed that for bonding temperature < 140 °C, failure occurred at the PET/PSMA interface. For bonding temperature = 150 °C, the interfacial adhesion exceeded that of the cohesive strength of PET film and failure occurred within the PET film. At the bonding temperature of 160 °C, failure occurred within PSMA bulk material. XPS measurement was used to measure the areal joint density, Σ_cross of PSMA chains pinned on the functionalized PET film surface. A transition in areal joint density below which G_c scales linear with Σ_cross and above which G_c scales with was found. The transition was identified as the transition from the pure chain scission of in situ formed copolymers to plastic deformation of the interface.