Surface chemistry of galvanized steel sheets relevant to adhesion performance

A review is presented on the recent development of surface treatment technologies for hot-dip galvanized steels relevant to adhesion of organic coatings. Applications of surface analytical techniques have elucidated that the surface layers of the nanometer scale dramatically govern the adhesion performance of painting or adhesive bonding. Surface enrichment of aluminium in the zinc layer deteriorates paint adhesion due to the reduction in phosphatability on the galvanized steel sheets and decreases the adhesive strength of the epoxy/dicyandiamide-bonded sheets due to the loss of acid-base interaction at the adhesive-substrate interface. In addition, the co-segregation of Al and Pb into the surface layer is responsible for the intergranular corrosion of zinc and facilitates the formation of a weak boundary layer, resulting in poor bond durability in a wet atmosphere. Improved adhesion performance has been established by developing new technologies that reduce the surface enrichment of minor elements or impurities in the zinc layer on the galvanizing line or that adopt a surface conditioning process prior to pretreatment in subsequent coil coating lines.     

Epoxidation of Ethylene by Silver Oxide (Ag<Subscript>2</Subscript>O) Cluster: A Density Functional Theory Study

Abstract  

Density functional theory (DFT) calculations were employed to study epoxidation of ethylene on a [Ag₁₄O₉] cluster model representing silver oxide (001) surface. Theoretical results obtained in this study showed that formation paths of acetaldehyde and vinyl alcohol have higher activation barriers than that of ethylene oxide formation path on silver oxide (35 and 35 vs. 20 kcal/mol). Formation of the ethylene oxometallocycle intermediate is found to have a low probability on Ag₂O(001) surface. The essential reason for this may be lower basicity of surface oxygen atom on silver oxide surface and the absence of a surface vacancy position to activate ethylene. Adsorbed EO is formed on Ag₂O surface cluster without an activation barrier. The activation barriers of the rate-limiting steps for the production of EO mechanisms (via ethyleneoxy and non-activated paths, 20 vs. 14 kcal/mol) are in relatively good agreement with the experimental activation energy values (14, 17 and 22 kcal/mol) reported for EO formation on silver catalyst.     

Preparation of metallic surfaces with microfibrous topography and their effect on adhesion of hot-melt and thermoset adhesives

Various chemical and electrochemical methods have been developed which produce acicular oxide or metal growths on the metal surface, and the adhesion of various polymers to such microfibrous surfaces has been studied. Polyethylene, which normally adheres poorly when applied to metals as a hot-melt coating unless conditions permit its oxidation, gives good adhesion to microfibrous surfaces in the presence of antioxidants. Similar surfaces give good adhesion with ethylene vinyl acetate. Comparative examination of the fracture surfaces enables the phenomenon to be related to theories of adhesion.

The effect of microfibrous surfaces on steel, copper and zinc on the adhesion of epoxy resins has been studied using fracture mechanics tests giving values for G_c fracture energy. In some cases these strong engineering adhesives give problems with the strength of bond between the metal and microfibrous surface layer. However, in many cases changing from a smooth to a microfibrous surface alters the mechanical response of the adhesive from a brittle to a more ductile one, thus increasing the value of joint toughness. With a rubber-toughened epoxy, the microfibrous surface enables much more of the potential toughness of the resin to be realized.     

Viscoelastic properties of sulfonated ethylene–propylene terpolymer neutralized with zinc cation

Stress–relaxation measurements in uniaxial extension, in the terminal zone, were made of sulfonated ethylene–propylene terpolymer (EPDM), neutralized with zinc cation. The sulfonation levels were 15 to 20 meq/100 g polymer (0.47 and 0.62 mol %), and for each level the effect of the ionic plasticizer, zinc stearate, was investigated. For all the polymers, the decay of the modulus with elapsed time was gradual and featureless, reflecting a broad distribution of relaxation times. Sensitivity of the ionomers' structures to thermal history before an experiment was suggested by irreproducibility in the magnitudes, but not the distribution, of relaxation times at a given temperature. For polymers containing zinc stearate measured at 80°C, the amount of permanent set was low for relaxation up to about 20h, but increased rapidly with additional hours of stretch. Time–temperature superposition did not apply. Composite curves were constructed by matching the stress–relaxation response at short times; moduli measured at long times fell above the composite curve. The shapes of the composite curves were similar, regardless of the level of EPDM sulfonation or the presence of zinc stearate. The apparent Arrhenius activation energy for short-time relaxation at similar levels of the modulus was about 35 kcal/mol at 140°C for the unplasticized polymers and about 70 kcal/mol at 90°C for the polymers containing zinc stearate, in contrast to the usual effect of plasticizer on the rate of relaxation of amorphous polymers except near the glass transition temperature.     

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.     

Thin ZrO2 coatings on polymers: role of interfacial adhesion

The interfacial characteristics between thin layers of zirconium dioxide and different thermostable polymers - polyimide, poly(ether ether ketone), and poly(ethylene terephthalate) - were studied. These layers, with a thickness ranging from a few nanometers to a few micrometers, were deposited by means of magnetron sputtering from a ZrO₂ target. In some cases, the polymers were subjected to plasma treatment (in the presence of different gases: air, Ar, and CO₂) prior to ZrO₂ deposition. X-ray photoelectron spectroscopy (XPS) analyses indicated clearly that zirconia coatings can be considered uniform when their thickness exceeds c. 10 nm. No interfacial covalent bonds between zirconia and polymers, such as Zr-O-C, were demonstrated. The level of adhesion at the coating-substrate interface was determined by a fragmentation test. The interfacial shear strength, related to the thermodynamic aspects of the polymer surfaces, i.e. their surface energy, was estimated. The result was in good agreement with the existence of physical interactions only, like van der Waals interactions, as shown by the XPS investigation.     

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.     

Engineering cohesion and adhesion through dynamic bonds for advanced adhesive materials

Adhesive materials have been widely used in almost every industrial domain. The art of the design principles of high-performance adhesives lies in the regulation of adhesion and cohesion strengths. Compared with permanent covalent bonds, dynamic bonds, including dynamic covalent and noncovalent bonds, are vulnerable under mechanical stimuli but rebuildable, making them promising candidates to manipulate the internal and interfacial energy. In this review, we mainly discuss how dynamic bonds can be applied to adhesive materials to regulate adhesion and cohesion, simultaneously. Representative samples are rationally selected to elaborate on the engineering designs for improving the adhesive performance via incorporating dynamic bonds. We also offer a perspective on future research directions on the engineered construction of advanced adhesive materials.     

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.     

Molybdenum metal catalyzed reaction of ethylene

The reaction of ethylene catalyzed by clean molybdenum foil is examined between 800 and 880 K. Products up to C₇ are detected in measurable quantities and hydrocarbons up to C₈ are observed. The product distribution is well described by a Schulz-Flory function where a plot of ln (W_n/n) versus n, where W_n is the yield of each C_n hydrocarbon, is a good straight line. Analysis of the slope and intercept of the distribution shows that the slope has a zero-order ethylene pressure dependence and varies with temperature with an activation energy of 4.6±0.6 kcal/mol, while the intercept has a pressure dependence of 1.3±0.3 and an activation energy of 46±4 kcal/mol. A kinetic model is proposed that successfully accounts for these pressure dependences. These data suggest that the activation energy for degenerate ethylene metathesis should be 55±5 kcal/mol and that the reaction should be first order in ethylene. These kinetic parameters are in good agreement with those found experimentally for the non-degenerate meta thesis of propylene.     

Flame Treatment and Surface Characterisation of Rubber-Modified Polypropylene

The flame treatment of a rubber-modified polypropylene has been studied using XPS, contact angle measurement, SEM, vapour-phase derivatisation, and a composite butt adhesion test. Optimum air-to-gas ratio and the distance from the inner cone tip to the polymer surface were found to be ∼ 11:1 and ∼ 0.5 cm, respectively. The lack of correspondence between contact angle and surface oxygen concentration was proposed to be due to the reorientation/migration of surface functional groups that had been incorporated during flame treatment. SEM shows changes in surface topography induced by intense flame treatment. Trifluoroacetic anhydride (TFAA) was found to derivatise-OH groups selectively by using model polymers. About 30% of the incorporated oxygen on flame-treated polypropylene surfaces was found to be present as-OH. Under most flame conditions studied, the interfacial adhesion with an epoxy adhesive or a polyurethane paint was found in excess of the polymer's cohesive strength. The results obtained are compared with those from a propylene homopolymer and an ethylene/propylene copolymer.     

Acid-base effects in polymer adhesion at metal surfaces

The Lewis acid-Lewis base properties of various polymers have been determined by measuring the contributions γ_s ⁺ and γ_S ^- to the solid surface free energy using the contact angle approach of van Oss, Chaudhury, and Good. A new linear method to solve for γ_S ⁺ and γ_S ^- is employed in addition to the usual approach which uses three simultaneous equations. The set of liquid surface tension parameters developed by van Oss, Chaudhury, and Good, and the recent set of values developed by Della Volpe and Siboni are both useful in distinguishing between acidic and basic polymers. The adhesion (peel force) of an acidic pressure-sensitive adhesive is greatest on a basic oxide film. In addition, the adhesion (pull-off force) of the basic polymer poly(methyl methacrylate) is greatest for acidic oxide films. Thus, direct experimental evidence is provided as to the importance of Lewis acid-Lewis base effects in the adhesion of polymers on oxide-covered metals.     

Epoxysilane as adhesion promoter in duplex system

Coatings are one of the most used protection methods for metals. Metallic coatings, such as zinc and its alloys, are used to protect steel in mild corrosive environments. In aggressive environments, on the other hand, organic coatings must be employed in the so-called duplex systems. However, the galvanized steel/organic coating adhesion is a problem and many attempts had been done to solve it with the incorporation of a chromate-based or phosphate-based interlayer. Nowadays, the use of these compounds is questioned due to their environmental impact and new adhesion promoters, like silanes, are being investigated. The aim of this paper was to study the adhesion and the anticorrosive behavior of a duplex system with a layer of glycidoxypropyltrimethoxysilane (γ-GPS) between the zinc and the coating. Polarization tests and corrosion potential measurements were done on the γ-GPS/galvanized steel to select the better anticorrosive pretreatment conditions for the application of an organic traditional paint. Dried and wet adhesion of the coating to the pretreated substrate was studied by the standard tape test. Salt spray test and electrochemical noise technique were employed to study the corrosion behavior of the duplex systems. Results showed that the films of γ-GPS formed on galvanized steel diminished the corrosion current of the metal, but they do not protect the substrate by a barrier effect. The incorporation of the pretreatment in the duplex system increased the adhesion of the paint, especially when the pretreated substrate was cured 1?h at 200?°C.     

Strength Loss Mechanisms for Adhesive Bonds to Electroplated Zinc and Cold Rolled Steel Substrates Subjected to Moist Environments

Mechanisms of strength toss which affect the durability of epoxy adhesive bonds in moist environments were investigated for electroplated zinc and cold rolled steel substrates. Activation energies for adhesion loss, formation of corrosion product on the substrate surface, and moisture diffusion in the adhesive were determined experimentally. For cold rolled steel substrates, the activation energy for adhesion loss was identical, within experimental error, to the measured activation energy for moisture diffusion in the adhesive. Both of these values were substantially less (=40%) than the activation energy for formation of corrosion product. This confirms the previous results of Gledhill and Kinloch (J. Adhesion 6, 315 (1974)), who attributed strength loss to thermodynamic instability of the adhesive/substrate interface due to the presence of moisture. In contrast, for electroplated zinc substrates, activation energies for adhesion loss and corrosion product formation were essentially equal, and were both significantly higher than that for moisture diffusion. Consequently, it was concluded that corrosion of the electroplated zinc layer was responsible for bond strength loss. Formation of corrosion product in the bond was not, therefore, a post-failure phenomenon as was the case for cold rolled steel.     

The Study of Reactive Functional Groups in Adhesive Bonding at the Aramid-Epoxy Interface

In polymeric composites, reactive functional groups on the fiber surface are assumed to enhance the mechanical strength of the fiber-matrix interface greatly by forming covalent bonds with the matrix. To test this assumption, we sought to promote covalent bonding at the aramid fiber-epoxy matrix interface by attaching flexible reactive pendent groups to the fiber surface. Other factors that could affect interfacial adhesion were kept constant, i.e., surface energy and surface topography. Quantitative analysis showed a pendent group attachment level of 1.5 to 4.5 groups per 100 Ų of fiber surface, a level that agrees well with the theoretical amount. Surprisingly, in adhesive performance tests, the presence of these reactive pendent groups did not improve the fiber-matrix interface strength. Specific chemical tests for covalent bond formation between the terminal amine of the pendent group and the epoxy molecule showed that covalent bonding did not occur, thus explaining the unexpected lack of improvement in adhesive performance.     

Strength Loss Mechanisms for Adhesive Bonds to Electroplated Zinc and Cold Rolled Steel Substrates Subjected to Moist Environments

Mechanisms of strength toss which affect the durability of epoxy adhesive bonds in moist environments were investigated for electroplated zinc and cold rolled steel substrates. Activation energies for adhesion loss, formation of corrosion product on the substrate surface, and moisture diffusion in the adhesive were determined experimentally. For cold rolled steel substrates, the activation energy for adhesion loss was identical, within experimental error, to the measured activation energy for moisture diffusion in the adhesive. Both of these values were substantially less (=40%) than the activation energy for formation of corrosion product. This confirms the previous results of Gledhill and Kinloch (J. Adhesion 6, 315 (1974)), who attributed strength loss to thermodynamic instability of the adhesive/substrate interface due to the presence of moisture. In contrast, for electroplated zinc substrates, activation energies for adhesion loss and corrosion product formation were essentially equal, and were both significantly higher than that for moisture diffusion. Consequently, it was concluded that corrosion of the electroplated zinc layer was responsible for bond strength loss. Formation of corrosion product in the bond was not, therefore, a post-failure phenomenon as was the case for cold rolled steel.     

Influence of Covalent Bonds on the Adhesion Energy at Elastomer-Glass Interfaces

The aim of this study was to analyze the effect of interfacial covalent bonds on the adhesive behavior of an elastomer, a crosslinked polydimethylsiloxane, and a glass substrate. These covalent bonds were created by applying to both materials an appropriate surface treatment by means of plasma polymerization. Adhesion measurements were carried out by analyzing the contact area between a rubber hemisphere and a flat rigid glass plate. The contact was forced under a given compressive loading for different times t_c, then the load was removed and the fracture propagation at the interface was recorded as a function of relaxation time t_r. Finally, adhesion energies were also determined by means of a probe test using a tensile testing machine.     

An ab initio study on properties of cationic chalcogen bonds in XF2Y+⋯NCZ (X═H,CN, F; Y═S,Se; Z═H,Cl, Br) complexes

The geometries, interaction energies, and bonding properties of cationic chalcogen bonds are studied in binary complexes XF₂Y⁺?NCZ (X═H, CN, F; Y═S, Se; Z═H, Cl, Br). The nature of these interactions is studied by a vast number of methods, including molecular electrostatic potential (MEP), Noncovalent Interaction Index (NCI), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analyses. The interaction energies of these complexes vary between ?20.94?kcal/mol in HF₂S⁺?NCH and ?33.72?kcal/mol in F₃Se⁺?NCBr. According to the QTAIM analysis, all these cationic chalcogen bonds are classified as a closed-shell interaction with a partial covalent character. Moreover, cooperative effects between cationic chalcogen bond and hydrogen or halogen bond interactions are studied in ternary XF₂Y⁺?NCZ?NH₃ complexes. These cooperative effects are analyzed in terms of the parameters derived from the QTAIM and NBO analyses, and electron density difference plots.     

Poly(ether-ester)s modified with different amounts of fumaric moieties

A series of novel poly(ether-ester)s modified with fumaric moieties is synthesized by transesterification in the melt of dimethyl terephthalate (DMT), dimethyl fumarate (DMF), 1,4-butanediol (BD) and poly(tetramethylene oxide) (PTMO, M?_n=1000 g/mol). The effect of the introduction of double bonds into both the hard and soft segments and their content on the structure and the thermal and rheological properties of the synthesized polymers are investigated. The introduction of double bonds into the polymer main chains increases the molecular weight of the copolyesters, but reduces the crystallinity of the hard segments and related properties such as modulus and hardness. The presence of double bonds improves the high temperature properties and thermal stability, especially the thermo-oxidative one, lowers the melting and crystallization temperature and increases the glass transition temperature. The incorporation of fumaric moieties into the macrochains improves the adhesive strength of the polymer to a metal surface.     

A study of paint adhesion to polymeric substrates

In order to explore the fundamental mechanism of paint adhesion to polymer substrates the surface of polypropylene- ethylene propylene rubber (PP-EPR) blends was modified by flame or plasma treatments. The changes in surface composition and properties were investigated and discussed in light of the results of simple adhesion tests. The topography and surface properties of the PP-EPR samples were studied by employing various surface sensitive techniques. Additionally, the surface properties of the pre-treated PP-EPR were compared with the model polymers poly(methyl methacrylate) (PMMA) and polycarbonate (PC) displaying a poor and an excellent paint adhesion, respectively. Differential scanning calorimetry (DSC) measurements showed that the miscibility of the polymer substrate with paint components was an essential factor for the understanding of the adhesion mechanism. A general model of paint adhesion to polymer surfaces is proposed, where the degree of interdiffusion of the polymer chains of the substrate and paint in the interphase determines the adhesion strength.