Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

Application of the Island Blister Test for Thin Film Adhesion Measurement

The island blister test has recently been proposed as an adhesion test which allows the peel of thin, well-adhered films without exceeding the tensile strength of the film. The island blister test site is a modification of the standard blister test site, consisting of a suspended membrane of film with an “island” of substrate at the film center. The membrane support and island are secured to a rigid plate and the film is pressurized, peeling the film inward off the island. A model for this inward or “annular” peel indicates that even for systems of good adhesion, peel can be initiated at low enough pressures to prevent film failure by making the center island sufficiently small relative to the size of the film.

We have fabricated island blister test sites using micromachining techniques and have used them to measure the debond energy of polymer films on various substrates. The peel data obtained from these island sites match well to the behavior predicted by a simple fracture mechanics analysis. This paper reports the fabrication of the island test sites, the experimental verification of the test, and the results of application of the test to polyimide films on metallic and polymeric substrates.     

Book Reviews

“lssledovaniya v oblasti poverkhnostnykh sil” (“Research on surface forces”) B. V. Deryagin, editor. Nauka Publ., Moscow 1967.544 pp. Rubles 3.00.

G. D. Andreevskaya, “Vysokoprochnye Orientirovannye Steklo-plastiki” (“High-strength Oriented Glass-Plastics”). Nauka Publ., Moscow 1966. 370 pp., Rubles 2.12.

A. D. Zimon, “Adgeziya pyli i poroshkov” (“Adhesion of dust and powders”) Khimiya Publ., Moscow 1967.372 pp. Rubles. 1.51.

J. J. Bikerman, “The Science of Adhesive Joints”. Academic Press, New York 1968.349 pages. $16.00.     

MULTI-STAGE SPRAY DRYING METHOD IN MANUFACTURING HEAVY DUTY DETERGENTS

Collision of droplets; counter-current spray dryer; drying rate; heat transfer; nonphosphated detergent; spray drying

The spray drying method of non-phosphated granular detergents is studied to decrease the amount of agglomerate particles. The formation of agglomerates is mainly influenced by the concentration of droplets in spray cloud and the water content of droplets at the time of collision. The overlaps of different spray clouds should be de- creased.

The drying rate near the nozzle zone is considerably faster than that calculated by Ranz-Marshal's equation. According to these phenomena, “Multi-stage spray drying” is developed, which is characterized by in stalling plural spraying stages in a spray dryer.

Consequently, non-phosphated detergents are manufactured with the same powder properties and productivity as phosphated detergents.     

ESTIMATION OF FUNCTIONS IN DRYING EQUATIONS

In drying problem, particularly for drying foodstuff, modelling is very difficult. Many physical effects have to be taken into account for mass transfer ; then mass transfer coefficient varies

In different models unknown functions must be estimated. It is particularly the case in simple models of drying using average values of water content, where the mass transfer varies versus mean water content in falling rate period. On the other hand in the “diffusion model” we have the same problem concerning the diffusion coefficient which must be also estimated

The method we propose in this paper for these two models : simple and “diffusion model” of drying consists from measurements of temperature and water content of the product to search a numerical approach of the unknown function. This method uses optimization techniques on computer and least squares criterion between model values and experimental data

Results are given for the “diffusion model” applied to shelled corn drying to find the diffusion coefficient and for a simple 11107 del applied to plum drying to find the mass transfer coefficient.     

Aging Process of Metal/Epoxy Laminates Investigated with X-ray Photoelectron Microscopy and Spectroscopy

In this article we describe the application of X-ray photoelectron spectroscopy to epoxy/dicyandiamide laminates on zinc galvanized steel which were aged under different environmental conditions involving high humidity and temperatures.

X-ray photoelectron microscopy allows us to identify the distribution of chemical elements with a lateral resolution of 10μm. Areas selected in the microscopy mode were then analyzed in the spectroscopy mode in order to get information on the local chemical composition.

We compared the spectroscopic features of the aged but freshly delaminated surfaces of samples stored under ambient conditions at room temperature with samples exposed to the “Kataplasmann” and the “KWT” test, respectively. Furthermore, a comparison was made with a model sample which was prepared in vacuum and on which the curing process was investigated.

Though there is no substantial loss in the lap-shear strength of the samples, we find drastic spectroscopic changes in the Kataplasma and KWT treated samples compared with the sample kept at room temperature. We conclude that the chemical changes induced by these tests cause an internal interphase boundary between the epoxy/metal interface and the bulk adhesive along which delamination occurs. Comparison with the behavior of the water-vapor-treated model sample gives evidence that hydrolysis is the main reaction in these tests.

The results described here complement our former study.¹     

Numerical analysis of the peel test for characterisation of interfacial debonding in laminated glass

Laminated safety glass is widely used in construction and as automotive windshield. When the glass plies break under dynamic loading, the adhesion between the glass plies and the interlayer is key to achieving the required safety performance. However, direct measurement of the interfacial adhesive properties is not possible with the existing test methods. In corresponding calculations, material behaviour is often simplified, which leads to inaccurate results. In this article, a finite element model for the 90° peel testing of laminated glass is studied. Hydrogen bonding at the interface between poly-vinyl butyral (PVB) interlayer and glass is represented in the model by a cohesive zone. It is seen that the experimentally measured peel force can successfully be matched by the simulations, but several combinations of variables can give the same result. Therefore, a parameter study is performed to establish the influence of each variable. It is found that the peel arm, consisting of the PVB and an aluminium backing foil, cannot be regarded as a thin film. Furthermore, the exact shape of the traction-separation law governing the cohesive zone has negligible influence on the simulation results, whereas the combination of interfacial strength and fracture energy fully characterises the delamination. The simulation results show that small-strain material behaviour can no longer be assumed for the PVB material in the vicinity of the crack tip.     

RIBBING INSTABILITY OF A TWO-ROLL COATER: NEWTONIAN FLUIDS

Data are presented for the critical conditions at which the ribbing instability appears on the surface of a Newtonian liquid film emerging from the nip between two rotating rolls. Data are obtained over two decades in Capillary number (Ca = μU/σ) for values of H₀/R in the range 3 × 10^-3 to 5 × 10^-2. The critical Capillary number is found to be given by the expression

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Data are compared to the predictions that follow from a theory due to Savage, and agreement is found to be poor.     

Synthesis and application of dual-curable PVB based adhesive formulations for cord/rubber applications

In this research, formaldehyde-free dual-curable adhesive formulations containing polyvinyl butyral (PVB) were prepared with the reaction of 2,4-toluene diisocyanate (TDI) and 2-hydroxyethyl methacrylate (HEMA) and then applied on cord fabrics upon adhere onto the rubber surfaces. The effects of PVB ratio on peel strength value between the cord and rubber were studied. The structure of the oligomer was characterized by FTIR and ¹H NMR spectroscopy. Thermal properties of coated and UV-cured fabrics were investigated by TGA and DSC. Surface wettability properties of the fabrics after coating were observed with contact angle measurement. The peel strength between cord/rubber surfaces was determined by T-peel test after thermal curing stage under heat and pressure. Results showed that peel strength value increases with increasing PVB amount in the formulation. The highest peel strength of 94.7 N/cm was observed when 5% PVB was included in the formulation. This study leads to a new type of promising adhesives with superior peel strength for cord/rubber applications as it is being a totally formaldehyde-free process.     

聚乙烯醇缩丁醛的合成新工艺

通过对聚乙烯醇缩丁醛(PVB)的合成工艺进行改进,并在不加任何乳化剂的条件下,制得了完全粉末状的PVB产品;将其用于夹层安全玻璃的制备,则雾度值为0.41%,完全符合夹层安全玻璃的使用要求;并讨论了各因素对PVB产量的影响。实验结果表明,当m(PVA)∶m(正丁醛)∶V(HCl)=100 g∶56 g∶15 mL时,PVB的产量最高,其乙烯醇缩丁醛含量可达77.7%。     

ABSORPTION AND DESORPTION TO AND FROM LIQUID FILMS ON INCLINED PLANES

Two published theoretical models are examined and applied to experimental results for absorption and desorption. The system used was CO₂/H₂O and studies were made for liquid film flow down inclined planes. Experimental results give “Reduced” values of mass ransfer rates.

Interferometric studies give interfacial concentration, penetration and film depths, and take-up of carbon dioxide. In the case of desorption the interferograms are distorted by “deflections.”

All the experimental values for absorption and desorption differ from those calculated from theoretical models.

Desorption is not a mirror image of absorption, and it is approximately 75% of the transfer rate of absorption over a wide operating range.

A comparison is made of the behaviour of static pools and flowing liquid films.     

UV-curable epoxy varnishes modified with polyvinyl resins

Epoxy varnishes based on 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexanecarboxylate (EEC) and polyvinyl components, i.e., two polyvinyl butyral resins (PVB) and one polyvinyl butyral-stat-acetal resin (PVBA) were prepared and cured using a commercial cationic iodonium photoinitiator and medium-pressure mercury lamp. An influence of a type and content of polyvinyl resin (5, 10, and 20 wt.%) on thermal, optical, mechanical, and barrier features of varnish coatings was investigated. Coats containing the highest amount of PVB resins exhibited higher epoxy group conversion, higher gloss, and smoother surface in comparison with EEC/PVBA samples. Moreover, cured coats and films with PVB reached markedly higher barrier features, i.e., limited distilled water absorption and low water vapor transmission rate. An influence of reactivity and UV transmittance value of the photocurable varnishes on coat wrinkling process was presented and discussed.     

The Fracture Mechanics of the Pin and Collar Test for High Temperature Anaerobic Adhesives

A mechanical test method for the studies of high-temperature anaerobic adhesives has been established, based on fracture mechanics, by modifying the standard test method of collar and pin test. Linear Elastic Fracture Mechanics approach was applied to the establishment of the relationship between adhesive fracture surface energy “R”, fracture load and crack length. Hence, from the joints containing a given artificial flaw the adhesive fracture surface energy can be determined; alternatively, from the strength of the joints without artificial flaws the inherent flaw size “a_i” can be calculated to account for the decrease of joint strength.

The experimental techniques were applied to examine the mechanical behaviour of the joint system based on high temperature anaerobic adhesives. It was found that the joints cured at room-temperature had higher adhesive fracture surface energy but lower joint strength than the joints postcured at high temperatures. The “a_i” data explained this interesting phenomenon. The joints cured at room-temperature had extraordinarily large “a_i”, which was found to be formed by the uncured adhesive near the edges of the joints and the adhesive further cured in the postcure processes to reduce the “a_i”. Also, the growth of intrinsic flaw was found to be responsible for the deterioration of the joints in a short-term, high-temperature ageing process.     

ON CHARACTERIZATION AND MITIGATION O F COMBUSTION HAZARDS INVOLVED IN THE HANDLING OF PARTICULATE MATERIALS: A REVIEW OF THREE BOOKS

The three publications reviewed herein deal with the complex of hazards which may derive from the agricultural and commercial handling of combustible particulate and gaseous materials. The principal thrusts of each book differ substantially from those of the other two. This review first attempts to consider the essential combustion processes which underlie the hazards of common concern in all three works. Each of the three books is then considered on an individual basis. The three publications are designated Books (1)- (3) respectively and are listed below:

(1) “User Guide to Fire and Explosion Hazards in the Drying of Particulate Materials.” 1977. Institution of Chemical Engineers. Warwick Printing Company Limited, England.

(2) “Prevention of Grain Elevator and Mill Explosions.” 1982. NAS-NRC Publication NUB 367-2. National Academy Press, Washington, D. C.

(3) “Guidelines for the Investigation of Grain Dust Explosions.” 1983. NAS-NRC Publication NMAB 367-4, National Academy Press, Washington, D. C.     

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.     

COHESIVE ZONE MODELS AND THE PLASTICALLY DEFORMING PEEL TEST

The peel test is a popular test method for measuring the peeling energy between flexible laminates. However, when plastic deformation occurs in the peel arm(s) the determination of the true adhesive fracture energy, G c , from the measured peel load is far from straightforward. Two different methods of approaching this problem have been reported in recently published papers, namely: (a) a simple linear-elastic stiffness approach, and (b) a critical, limiting maximum stress, σmax , approach. In the present article, these approaches will be explored and contrasted. Our aims include trying to identify the physical meaning, if any, of the parameter σmax and deciding which is the better approach for defining fracture when suitable definitive experiments are undertaken.

Cohesive zone models Fracture mechanics Laminates Peel tests Plastic deformation     

ESCA Studies of Laminated Safety Glass and Correlations with Measured Adhesive Forces

Electron Spectroscopy for Chemical Analyses (ESCA) has been used to examine surfaces of safety glass specimens. These specimens consisted of tin float glass laminated with plasticized polyvinyl butyral. ESCA was used to characterize the glass surface after the plasticized polyvinyl butyral had been peeled away. The results show clearly that the specimens fail cohesively and a layer of the plasticized polyvinyl butyral is left on the glass surface. The thickness of this layer was measured and was found to be directly proportional to the force of adhesion, measured as 90° peel strength.     

ADDITION OF PORE DIFFUSION LIMITATION TO THE “UCKRON-I” KINETIC RATE OF METHANOL SYNTHESIS

Kinetic data with pore diffusion limitation on methanol synthesis were generated by extending the “UCKRON-I” kinetic rate expression. The best fit model and the extended “true” model were compared using their respective rates to simulate temperature profiles in a non-isothermal plug flow tubular reactor.

The objective of this work was to add pore diffusion resistance to the UCKRON-1 kinetic rate for methanol synthesis (Berty, et al. 1983). Kinetic modeling of the data with 5% experimental error added, showed the best model to be that developed from a previous kinetic model (Shalabi, et al. 1983) with apparent activation energy approximately one-half the activation energy at no pore diffusion.

Methods used in this work to determine and evaluate pore diffusion parameters can be utilized for other reaction systems where pore diffusion may play a role in reaction rate.

Temperature profiles estimated from reactor simulation studies showed good argeement between ideal and predicted models for temperature.     

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.