Progress in Organic Coatings (v.53, #1)

Contents (v).

Mechanism of wrinkle formation in curing coatings by Soumendra K. Basu; L.E. Scriven; L.F. Francis; A.V. McCormick (1-16).
Five wrinkling coating systems – ranging from liquid-applied, thermally cured acrylic-melamine and alkyd systems to powder-applied, thermally cured epoxy and polyester systems to a liquid-applied, UV-cured acrylate system – were investigated by optical microscopy and mechanical profilometry. Each system had multi-functional reactants and cross-linkers, and produced a highly cross-linked coating. Upon curing, each appeared to produce a depth-wise gradient in degree of solidification and thereby developed a mechanical skin. The presence of a mechanical skin was demonstrated by physically probing the top surface of the semi-cured coating. Under externally applied compressive stresses, the skin showed wrinkled patterns. It is hypothesized that during cure compressive elastic stress developed in the skin when unreacted low-molecular weight oligomer below the skin diffuses up into the oligomer-depleted cross-linking skin and tends to swell it. This compressive stress, once above a critical value, could be relieved by out-of-plane deformation or buckling that wrinkles the skin. Experimental support for the hypothesis is gathered by wrinkling a homogeneous skin solely by absorption of unreacted material, and by noting the similarity of patterns between curing wrinkled coatings and those produced by compressed elastic films on elastic or viscous sub-layers. Moreover, experiments agree with buckling analysis of a skin-sublayer system, which predicts a linear relationship between the critical wrinkle wavelength and skin thickness.
Keywords: Cross-linking; Depth-wise gradient; Mechanical skin; In-plane stress; Swelling; Buckling; Surface texture;

Effect of a mixed aqueous solution of HCl and CaCl2 on adhesion of coating to evaporated indium layer by Kentaro Watanabe; Masahiko Yamanaka; Toshiyuki Mozawa; Norihiko Kobayashi (17-22).
The effect of a mixed aqueous solution of HCl and CaCl2 on the adhesion of a coating with an evaporated indium layer was evaluated. Peeling was observed between the evaporated indium layer and the clear-coat after dipping a coated panel in the mixed aqueous solution, and the coating films were seen to lose their metal gloss. Observation of the evaporated indium layer with scanning electron microscope (SEM) revealed various particle sizes of indium. A higher area ratio of indium particles in the indium layer led to faster peeling after dipping the coated panels in the mixed aqueous solution.The effect of the cross-linking density of clear coating film on the peeling between the indium layer and the clear-coat was investigated. A clear coating film with a lower cross-linking density resulted in a shorter peeling time in the mixed aqueous solution.It was considered that the peeling was caused by condensation of water in the spaces where indium was dissolved by penetration of the HCl from the HCl/CaCl2 aqueous solution into the evaporated indium layer.
Keywords: Indium; Vacuum evaporation; Adhesion; Urethane coating; Viscoelasticity; Moisture permeability;

Evaluation of scratch resistance of an acrylic-melamine clear coat based on its fracture energy by M. Sadati; N. Mohammadi; N. Taheri Qazvini; N. Tahmasebi; S. Koopahi (23-28).
Tensile strength and scratch resistance of an acrylic-melamine clear coat were evaluated at room temperature and various test speeds. The basic fracture property, tensile strength, was measured on free films while the applied property, scratch resistance, was characterized on glass-supported thin film by a homemade instrument under different applied loads. The results showed similar trends for the reciprocal of scratch width and fracture energy as a function of test speeds. In other words, both properties decreased linearly and approached a plateau by raising the rate of the experiments. Therefore, it appeared that scratch resistance of the acrylic-melamine sample could be superimposed on its fracture energy by applying a suitable shift factor. On the other hand, by varying the applied load the characteristics scratch resistance dependence on rate would not change while its location of occurrence would shift. Dynamic mechanical analysis of the free film showed a broad tan  δ between −20 up to 80 °C, which could be a representative of a conterminous network of acrylic chains connected by melamine joints. Apparently, the loss capability of the coating at high temperature and low strain (tan  δ) could be shifted and observed at lower temperatures (e.g. room temperature) and high strains (tensile measurements). Therefore, the superimposable correlation of scratch resistance and fracture energy as a function of test speed can be attributed to the same fundamental controlling molecular motions which are activated by various combinations of stimulus.
Keywords: Scratch resistance; Tensile strength; Fracture energy; Acrylic-melamine;

Flame-retardant nano-coatings were prepared by adding flame-retardant nano-concentrates to APP/PER/EN coating. Dispersion morphology and stability principle of nano-size magnesium aluminum-layered double hydroxides (nano-LDHs) have been studied by using transmission electron microscopy (TEM). Relation of added amount of nano-concentrates in flame-retardant coating to flame-retardant properties for APP/PER/EN system has been studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), thermogravimetry (TG) and fire protection test. It was indicated that nano-LDHs could catalyze the esterification reaction between ammonium polyphosphate and pentaerythritol, and IPN network formed by nano-size thermal-decomposed products of LDH could efficiently enhance char formation and structure of char layer. Only specific content (1.5%) of nano-LDHs in flame-retardant coating could efficiently improve its char layer structure and fire-resistant properties. Nano-LDHs (1.5%) greatly improve mechanical properties (bonding strength, bending resistance and resistance to freeze–thaw cycle) of flame-retardant coating.
Keywords: APP/PER/EN coating; Char formation; Intumescent flame-retardant coating; Nano-size magnesium aluminum-layered double hydroxides (nano-LDHs); Flame-retardant nano-concentrates;

Polypyrrole/polythiophene coating for copper protection by T. Tüken; B. Yazıcı; M. Erbil (38-45).
The electrochemical synthesis of polythiophene (PTh) has been investigated on copper electrode, in LiClO4 containing acetonitrile medium (ACN–LiClO4). It was shown that a corrosion protective PTh film cannot be obtained on copper surface, by direct electro oxidation of thiophene, because the surface could not be passivated sufficiently and copper dissolution continued in the monomer oxidation potential region. This prevented the deposition of homogeneous polymer film on the surface. The synthesis of polythiophene (PTh) was achieved on copper electrode, which had been previously coated with very thin polypyrrole (PPy) film. The PPy film was obtained in 0.1 M monomer containing aqueous oxalic acid solution. Then, the synthesis of PTh film was achieved on this PPy-coated copper electrode, in 0.1 M thiophene containing ACN–LiClO4. The corrosion behaviour of PPy/PTh-coated copper was investigated in 3.5% NaCl solution. For this aim, electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential–time (E ocpt) diagrams were utilized. It was shown that the PPy/PTh coating could provide important protection against copper corrosion for considerable immersion periods.
Keywords: Copper; Polythiophene; Polypyrrole; Corrosion;

UV curing of photoinitiator-free systems containing a bismaleimide and different diacrylates was investigated. Firstly we investigated the copolymerization of a bismaleimide and a polysiloxane diacrylate resin using different monomer ratios. Moreover, a typical diacrylate (bisphenolAdiglycydyletherdiacrylate) (BHEDA) was employed and the bulk properties of the networks were modulated varying the amount of bismaleimide in the photocurable mixture. The curing process was monitored by FTIR and the thermal properties by DSC analyses. Surface properties were changed by addition of small amounts (less than 1 wt.%) of a polysiloxane diacrylate to the bismaleimide/BHEDA 50–50 mol% mixture: notwithstanding the low concentration of the siloxane monomer, highly hydrophobic surfaces were obtained.
Keywords: Photoinitiator-free photopolymerization; Bismaleimide; Polysiloxane acrylates; Bulk and surface properties;

The maleinization of liquid polybutadienes with different microstructures was studied in two solvents: o-xylene and decahydronaphtalene. The concentration of maleic anhydride versus time was determined by gas chromatographic analysis. The results obtained in o-xylene were processed according to a first-order model, hence the kinetic constants and estimated activation energy values showed a solvent effect with respect to the bulk reaction. Moreover, the model was improved by taking into account that at 140 °C, the kinetic curves reached after some time a final constant yield depending on microstructure and maleic anhydride concentration. On the other hand in decahydronaphtalene at 130 °C, the reaction was very fast with very high yields. Nevertheless, in the range from 150 to 180 °C, the amount of linked anhydride in the final products did not exceed 11% (w/w) probably because of a decarboxylation of the maleinized products favoured by decahydronaphtalene.
Keywords: Polybutadiene; Maleinization; Grafting; Water-borne; Rubber;

Electrostatically and electrosterically stabilized emulsion copolymers based on poly(butyl acrylate-co-vinyl acetate-co-acrylic acid) and poly(2-ethylhexyl acrylate-co-vinyl acetate-co-acrylic acid) were prepared by semicontinuous emulsion polymerization in order to obtain pressure-sensitive adhesives. The prepared dispersions were characterized by their concentration, intrinsic viscosity, rheological behavior, glass transition temperature, and adhesive performances on the standard test panel, i.e. stainless steel. The surfactants have a great influence on latices viscosities, which become smaller when the content of nonionic surfactant increases, and affect also the shear and peel strength of the final adhesives. The viscosity and rheological behavior are also influenced by the presence of 2-ethylhexyl acrylate instead of butyl acrylate.
Keywords: Emulsion polymerization; Mixed surfactants; Pressure-sensitive adhesives; Rheology;

The work aims at investigating the effect of the nature of the intermediate zinc-containing layer in coil coatings has on their corrosion resistance against cathodic delamination. Over a 3 months period, cathodic disbonding has been measured for three types of coil-coated steel whose difference was in the zinc-based metallic intermediate coating, namely galvanised, galfan and aluzinc. The test specimens were exposed in 3.5 wt.% NaCl aquaeus solution and polarised to either −1050 mV SCE with aluminium anodes or to −1300 mV SCE with magnesium anodes. For the sake of comparison, immersed specimens at their spontaneous open circuit potential were also studied. The corrosion performance was determined on the basis of coating degradation and interface delamination of the tested samples. It was demonstrated that the nature of the zinc-based metallic layer had a large effect on cathodic disbonding. Under all the conditions explored, coil coatings containing an aluzinc (Zn–55Al) or a galfan (Zn–5Al) intermediate layer show higher resistance against disbonding that those with galvanised (Zn).
Keywords: Coil coatings; Cathodic disbanding; Zinc-based metallic coating; Corrosion;

Cathodic polarisation has been used as an accelerated test method to assess the degradation of coil-coated architectural cladding. Data were recorded via electrochemical impedance spectroscopy (EIS) measurements. Coil-coated steel (CCS) sheet with three different intermediate zinc-based metallic coatings were investigated, namely galvanised, galfan and aluzinc. Corrosion tests were performed by polarizing the painted panels to two cathodic potentials corresponding to either aluminium or magnesium sacrificial anodes while immersed in 3% NaCl aqueous solution in a horizontal flat-cell. The cathodic potential values considered were −1.082 and −1.522 V versus saturated calomel electrode (SCE). Cathodic polarisation conditions were found for which differences in the corrosion resistance of the three systems were detected from the analysis of the EIS data at significantly shorter exposure times than in the case of EIS data measured at their corresponding open circuit potential, thus providing an accelerated test method for determining the quality of the CCS system.
Keywords: Coil coating; EIS; Corrosion; Cathodic polarisation; Zinc-based metallic coating; Failure modes;

Synthesis of polyamide-silicone polyacrylate and its UV curing properties by L.H. Li; W.J. Xu; Y.M. Wu; Y.Q. Xiong; W.H. Lu; Q.J. Tang (77-80).
A novel polyamide-silicone polyacrylate was prepared by reacting hexamethylene-1,6-diisocynate trimer (HDT) with γ-aminopropyl triethoxysilane and modification by β-hydroxyethyl acrylate. The structure of the oligomer was characterized by FT-IR. The physical properties of UV curing films, such as hardness, traction, flexibility, max curing speed and heat resistance were examined. Researches showed that this kind of polyacrylate has some superior properties and can be used for UV curing coating.
Keywords: Polyamide-silicone polyacrylate; UV curing coating;

by W. Funke (81).