Effect of physical aging on thermal stress development in powder coatings

Thermal stress and physical aging are inherent to thennosetting systems (such as powder coatings) and may affect the coating durability leading to damage such as detachment and cracking. Both phenomena occur principally below the glass transition temperature (T_g) of the coating and affect each other. It is shown that the measurement of stress, as a function of temperature of coatings aged at different temperatures and during various times, represents a simple and interesting way to study these phenomena. The results obtained which show changes in the stress magnitude with aging are explained in terms of stress relaxation and structural recovery. The latter process is especially evident in the T_g region and can prevent the correct determination of the T_g by means of thermal stress measurements. The thermal expansion coefficient and the elastic modulus, two properties directly affecting the thermal stress magnitude were determined separately, and agree well with the proposed interpretation of experimental data. The linear dependence of thermal (compressive) stress on the logarithm of time indicates the possibility of predicting the effect of physical aging.     

Effect of physical aging and thermal stress on the behavior of polyester/TGIC powder coatings

Physical aging, thermal stress, dynamic mechanical characteristics and thermal expansion of powder coatings from model terephthalic acid (TPA) and isophthalic acid (IPA)-based polyesters (PEs) cross-linked with triglycidyl isocyanurate (TGIC) were studied. Enthalpy relaxation and maximum compressive thermal stress were the properties used to follow the physical aging. The better mechanical properties of TPA-based PE/TGIC powder coatings seem to be due to a combined effect of several properties, i.e., cross-link density (ν_e), thermal expansion coefficient (_F^T) and glass transition temperature (T_g). The higher the values of ν_e, (_F^T) and T_g, the tighter is the coating network, the greater is the coating ability to expand thermally and the slower the physical aging process, respectively, all factors favoring the TPA polyester-based powder coatings.     

Water uptake of epoxy coatings modified with γ-APS silane monomer

Epoxy resin was modified by a silane monomer, γ-aminopropyltrimethoxy silane (γ-APS), and was used as the protective coatings for LY12 aluminum alloys. The aim of the modification is to reduce the water uptake of polymeric coatings. The water absorption of coatings was measured by coating capacitance method in 3.5 wt.% NaCl aqueous solution. The result indicates that water uptake of epoxy coatings modified with 1.0 wt.% γ-APS decreases compared with pure epoxy coatings, whereas larger amounts of sliane result in the deterioration in performance against water permeation, due to the excessive consumption of epoxide group in epoxy resin by amino-group in silane agent, thus reduce the cross-linking of epxoy coating as a result of presence of excessive curing agent (polyamide). T_g of silane-modified coatings increases slightly after immersion, extremely contrasting with that of pure epoxy coating, which was observed to decrease significantly after water permeation. The formation of Si–O–Si structure resulting from the hydrolysis and condensation of silane components during the immersion in aqueous media may be a reasonable explanation for the abnormal change in T_g of silane-modified coatings. In addition, all silane-modified coatings display better protection performance, which is characterized by higher charge transfer resistances (R_ct) and lower double layer capacitance (C_dl) at substrate/electrolyte interface.     

高生物基含量光固化涂料的制备及其性能研究

环氧化大豆油丙烯酸酯(AESO)已大规模用于UV固化涂料中,但目前同时获得高生物基含量以及优异机械性能的大豆油基光固化涂料依然是一个很大的挑战。文中设计并合成了异山梨醇甲基丙烯酸酯(ISDMA)作为光固化活性稀释剂。使用流变仪研究了ISDMA对AESO稀释性的影响。将ISDMA与AESO混合制备了一系列生物基UV固化涂料,并对这些涂料的热机械性能、力学性能和涂层基本性能进行了评估。结果表明:ISDMA对AESO表现出良好稀释性的同时,可以有效地提高固化涂层的玻璃化转变温度(T_g)、储能模量和硬度。     

Diffusion controlled kinetics of crosslinking

Reactions of polymer formation and crosslinking become diffusion controlled when, during the reaction, the increasing glass transition temperature T_g comes close to the reaction temperature, the reaction still goes on below T_g but the reaction rate decreases steeply. A theory is presented relating the apparent rate constant to the difference between the reaction temperature and T_g based on the free volume or the Adam-Gibbs theory of glass transition. The theory is correlated with experiments on curing of diglycidyl ether of bisphenol A with 1,3-propanediamine. The implications for formation of protective films chemically crosslinked are discussed. The presence of a solvent and its evaporation affects the reaction rate through a change in concentrations of reactants as well as in T_g.     

Rosin derivatives: novel film forming materials for controlled drug delivery

Two new rosin derivatives (RD-1 and RD-2) were synthesized in the laboratory and evaluated for physicochemical properties, molecular weight (M_w), polydispersity (M_w/M_n) and glass transition temperature (T_g). Plasticizer free films of the derivatives were produced by casting/solvent evaporation method. The surface morphology (SEM), water vapour transmission and mechanical properties (tensile strength, percent elongation and modulus of elasticity) of the films were investigated. The derivatives were further evaluated for pharmaceutical film coating by characterizing the release of a model drug (diclofenac sodium) from non-pariel seeds (pellets) coated with the derivatives. Pellet film coating could be achieved without agglomeration of the pellets within a reasonable operation time. Drug release from the coated pellets was sustained up to 10 h with the two rosin derivatives. These findings suggest the possible application of rosin derivatives (RD-1 and RD-2) for film coating.     

Inorganic–organic hybrid materials with zirconium oxoclusters as protective coatings on aluminium alloys

Inorganic–organic hybrid materials are attracting a strong scientific interest mainly for their outstanding inherent mechanical and thermal properties, which can be traced back to the intimate coupling of both inorganic and organic components. By carefully choosing the experimental parameters used for their synthesis, chemically and thermally stable acrylate-based hybrid material embedding the zirconium oxocluster Zr₄O₄(OMc)₁₂, where OMcCH₂C(CH₃)C(O)O, can be deposited as UV-cured films on aluminium alloys.

In particular, the molar ratios between the oxocluster and the monomer, the polymerisation time, the amount of photo-initiator and the deposition conditions, by using an home-made spray-coating equipment, were optimised in order to obtain the best performing layers in terms of transparency and hardness to coat aluminium alloy (AA1050, AA6060 and AA2024) sheets. Furthermore, it was also evaluated whether the hybrid coatings behave as barrier to corrosion.

Several coated samples were prepared and characterised. Environmental scanning electronic microscopy (ESEM) and scratch test were used to investigate the morphology of the films and to evaluate their scratch resistance, respectively. Electrochemical impedance spectroscopy (EIS) was performed in order to evaluate if the coatings actually protect the metallic substrate from corrosion.

In order to measure shear storage modulus (G′) and loss modulus (G″) of the materials used for coatings, bulk samples were also obtained by UV-curing of the precursors solution. Dynamical mechanical thermal analysis (DMTA) was performed in shear mode on cured disks of both the hybrid materials and pristine polymer for comparison. The values of T_g were read off as the temperatures of peak of loss modulus. The length and mass of all the samples were measured before and after the DMTA analysis, so that the shrinkage of the materials in that temperature range was exactly evaluated.     

蓖麻油基聚氨酯-丙烯酸酯复合乳胶膜的水白化研究

采用蓖麻油基聚氨酯作为大分子乳化剂制备聚氨酯-丙烯酸酯(PUA)复合乳液。采用红外光谱表征了PUA乳胶膜的结构;用紫外分光光度计、动态热机械分析仪和差示扫描量热仪对PUA乳胶膜的水白化进行了表征。结果表明:在实验的时间范围内,乳胶膜的水白化程度随其浸水时间和浸水温度的增加而增加;当浸水温度(4 ℃)远低于干膜的玻璃化转变温度(T_g=57.8 ℃)时,乳胶膜浸水24 h不会水白化,湿膜的T_g及其储能模量与干膜相比几乎没有变化;在28 ℃的水中浸泡24 h时,湿膜的T_g小于干膜的T_g,其储能模量也降低,乳胶膜出现轻微水白化,将乳胶膜再次烘干后,胶膜可恢复原有清晰度。当乳胶膜在高于干膜T_g的水中(70 ℃)浸泡24 h时,其储能模量下降,湿膜的T_g大于干膜的T_g,再次烘干乳胶膜不会恢复原有的清晰度,其原因是膜内出现了与PUA分子依靠氢键紧密相连的冻结键合水。     

Tetrafunctional acrylates based on β-hydroxy alkyl amides as crosslinkers for UV curable coatings

Ultraviolet (UV) curable coating formulations were prepared from novel crosslinking acrylates synthesized from β-hydroxy alkyl amides. Variations were done with respect to the nature of the monomer backbone by introducing aliphatic, aromatic and cycloaliphatic structures in between the amide groups. The synthesized multifunctional alcohols and acrylates were subjected to IR and NMR analysis for structure confirmation. The acrylates were dissolved in chloroform and the UV–vis spectrum was recorded to substantiate the presence of carbonyl group conjugated to the vinyl group. The new acrylates were added to basic formulation and exposed to UV radiation to get transparent films. The dynamic mechanical properties of the films cast from the formulations were assessed and studies on the effect of the synthesized crosslinking acrylate content and their nature on the viscoelastic properties of the films were carried out. Improvements in glass transition temperature (T_g) were observed with increasing percentage of crosslinker. Acrylates based on aromatic acid amides showed higher T_g than aliphatic acid amides. The storage modulus (E′) increased up to 10% and further increases in the crosslinker percentage led to a decrease in the modulus. The rubbery modulus depended on percentage of crosslinker and not on crosslinker structure.     

Film formation from cationic electropaint systems containing resins with different glass transition temperature

Some phenomena in the deposition process of electropainting have not been well elucidated till now. In this paper, to investigate an influence of glass transition temperature (T_g) on film formation, the deposited film was observed with an atomic force microscope and the electrochemistry was investigated, using two kinds of cationic acrylate resin with different T_g (methyl methacrylate system (MMAs): T_g=70°C, and methyl acrylate system (MAs): T_g=5°C). Electrodeposition was performed under constant voltage or current condition.

At constant voltage, the deposition behavior in the two resin systems differed extremely. The MMAs, the resin with high T_g, produced a high resistance film. The MAs, the resin with low T_g, was deposited forming a film at a voltage lower than 20 V. At constant current, the film formation did not result in a rise in voltage. It behaved like a conductive film. When the resin with high T_g was used, particulate deposits were observed by AFM even in the induction period. The resin with low T_g formed flat deposits. These results suggest that paint deposition is initiated once electrolysis of water starts. In addition, there are two types of film formation on the cationic electropainting: high resistance film formation for the resin with high T_g, and ion-permeable film formation for the resin with low T_g. In both cases, film growth occurs at the film/bulk solution interface.     

Ultraviolet light absorber mobility in crosslinked coatings: Experiments and modeling

The diffusion of an ultraviolet absorber (UVA) across a concentration step change in an acrylic melamine clearcoat was measured via micro-ultraviolet spectroscopy. Concentration profiles were obtained after diffusion times of 4–200 h at steady temperatures of 70, 80, and 90 °C. The glass transition temperature of the coating was measured via dynamic mechanical analysis for each time–temperature pair, and T_g was observed to increase with thermal aging, with a faster rate of increase at higher aging temperatures. The change in T_g was modeled using a first-order dependence on the deviation from a long-time T_g asymptote. The diffusion coefficient was then represented by a free volume expression, and the combination provided an accurate model of the measured concentration profiles. The time-dependent increases in glass transition temperature helped to explain why UVA diffusion does not alleviate concentration gradients created by photooxidation under natural weathering conditions.     

Chemical approaches toward the definition of new high-solid and high-performance fluorocoatings

Hydroxy bifunctional fluoropolyether oligomers (ZDOL) are presented suitable for use as resins or for the preparation of polyurethane, polyester and polyether higher OH functionality resins. The synthesis procedures are described and discussed, and the effect of fluorine content and resin structure on glass transition temperature T_g and viscosity of final resin is stressed. It appears that the highly fluorinated polyether type resins, like the tetraolic derivative (TOL) and its mixtures with ZDOL, are characterized by only one, extremely low T_g and minimum viscosity values. Solvent–resin interactions and criteria for obtaining compatibility with polyisocyanate crosslinkers are discussed, pointing out the key role of hydrogen bonding. High solid coatings (solvent content less than 30%) are obtained by curing with blocked aliphatic polyisocyanates, and a preliminary characterization of final materials based on their thermal transitions and mechanical properties is reported.     

Small angle neutron scattering studies of composite latex film structure

The mechanical properties and structure of composite films made of high T_g polystyrene (PS) nodules dispersed in a low T_g polybutylacrylate (PBuA) matrix were studied by means of dynamic mechanical spectrometry and small angle neutron scattering. For films cast from mixtures of PS and PBuA latexes, film mechanical reinforcement was obtained above a percolation threshold of about 30% PS volume fraction. A segregation of PS particles into dense clusters in the PBuA continuous matrix, reminiscent of a phase separation, was observed. For films cast from core-shell particles, this segregation phenomenon may be prevented, depending on the coverage of the PS core by the PBuA shell. An efficient core encapsulation in the core-shell morphology leads to poor contact between PS cores, and the elastic moduli are then close to that of the PBuA matrix. Upon annealing the films above the T_g of PS, extensive coalescence of PS particles occurred when large contacts were already present in the dry film at room temperature, and a percolating network of coalesced PS domains provides large elastic moduli at temperatures between the T_g of PBuA and PS. The coalescence was prevented when PS particles were taken apart by the PBuA shell.     

Mechanical and lamination properties of alumina green tapes obtained by aqueous tape-casting

Mechanical characterisation and lamination were carried out on alumina green tapes prepared by aqueous tape casting using two acrylic emulsions having different glass transition temperatures (T_g) as binders. The tensile strength and strain were strongly dependent on the binder nature and content. Namely, the mechanical properties of the green tapes reflected those of the binders at room temperature: the green tapes obtained with the higher T_g binder showed a brittle behaviour, whereas those obtained with the lower T_g binder showed an elastoplastic behaviour. The mechanical properties of the green tapes prepared by mixing the two acrylic binders lies in between, giving the possibility of tailoring the flexibility and strength in the range of the values obtained for pure binders. Lamination gave rise to an increase of both green and sintered densities, compared with monolayer specimens, whatever the composition of the binder system. Such improvements significantly depended on lamination pressure, but were insensitive to lamination temperature for the two temperatures tested higher than the T_g of the two binders. ©     

Effects of freeze-drying on the glass temperature of cyclic polystyrenes

The calorimetric glass temperature was measured for three cyclic polystyrenes with apparent molecular weights ranging from 4.0×10³ to 195.5×10³ g/mol for both bulk material and for samples freeze-dried from dilute solution. Freeze-drying from dilute solution was found to reduce the glass temperature by 7–14 K depending on the sample. These T_g depressions are 5–12 K greater than those found previously for freeze-dried linear polystyrene. Annealing at 403.2 and 443.2 K (130 and 170 °C) resulted in recovery of the T_g back to the bulk value with the time scales depending on both temperature and the magnitude of the T_g reduction; the low apparent activation energy dependence of the recovery of T_g precludes its being due to viscous flow.     

The glass temperature of polymer blends

The entropy theory of glasses is used to derive the glass temperature, T_g, of a binary polymer blend in terms of the glass temperatures of the two substituents. The formula is T_g = B₁T_g1 + B₂T_g2, where B_i is the fraction of flexible bonds of substituent i. A bond is flexible if rotation about it changes the shape of the molecule. Bonds in side groups as well as in the backbone are to be counted. This formula assumes that the free volume, taken here to be the volume fraction of empty lattice sites, is the same for each of the three materials. It has no parameters. The above equation expressed in weight fractions, W_i, is (T_g − T_g1)W₁(γ₁/ω₁) + (T_g − T_g2)W₂(γ₂/ω₂) = 0, where ω_i is the weight of a monomer unit and gg_i is the number of flexible bonds per monomer unit. A more general treatment is given. One variation of the more general treatment which expresses the properties of the blend in purely additive terms gives T_g = B₁T_g1 + B₂T_g2 + KB₁B₂(T_g1 − T_g2)(V₀₁ − V₀₂), where V_0i are the free volume fractions of the homopolymers at their glass temperatures and K is a constant. The added term is usually small. The most general form of the equation requires the energy of interaction between the two unlike molecules, which can be estimated by volume measurements on the blend.     

FTi.r.-rheo-optical characterization of the molecular orientation behaviour of amine cured epoxy resins during cyclic deformation

Rheo-optical FTi.r. spectroscopy was used to study molecular orientation phenomena in highly crosslinked epoxies on the basis of the diglycidyl ether of bisphenol-A and a polyetherdiamine. In previous publications, their orientation behaviour during continuous uniaxial deformation was described. The present paper reports on the rheo-optical characterization of these resins during specific cyclic deformation experiments below the glass transition temperature (T_g). Epoxy films were subject to various successive loading—unloading cycles including elongation, recovery, annealing, and stress relaxation in order to study the reversibility of the orientation during relaxation processes. The investigations show that the orientation is only in part reversible upon unloading or stress relaxation below T_g. However, it can be annealed by heating the epoxy resin to above T_g. Furthermore, it was found that no significant fatigue due to chain scission occurs until failure of the sample. The results were discussed with respect to the mechanism of plastic deformation.     

Towards anti-corrosion coatings from surfactant-free latexes based on maleic anhydride containing polymers

We report on the film formation of surfactant-free, artificial latexes based on copolymers containing maleic anhydride. Different metallic substrates, such as aluminum, steel and magnesium alloys, were coated with three different latexes. A commercial polyester based coating was used as a comparative sample. Two of the latexes were based on polymer with a high T_g (resp. 100 and 130 °C), and one on a polymer with a T_g of −70 °C. The wetting of the substrates could be optimized by etching the metal substrate, acidic or alkaline, leading to homogenous film formation and improved adhesion. For aluminum substrates an alkaline pretreatment improved the adhesion with the polymer films, whereas for magnesium the acidic pretreatment improved the adhesion. Furthermore, acid pretreatment increased the corrosion resistance of the coated magnesium substrate, when compared to an alkaline pretreatment. The films formed from latex displayed comparable or better anti-corrosive properties when compared to the commercial polyester.     

Preadhesion laser surface treatment of polycarbonate and polyetherimide

The effects of carbon dioxide (10.6 μm) and argon fluoride excimer (193 nm) lasers as preadhesion surface treatments for polycarbonate and polyetherimide were studied. Single lap shear specimens bonded with a flexible polyurethane adhesive were utilized to investigate the effect of the various laser treatments on final shear properties and mode of failure. Experimental results based on mechanical properties, scanning electron microscopy and Fourier transform infra-red have indicated that the excimer laser is more effective than the carbon dioxide laser with respect to shear properties and the level of adhesion obtained with the thermoplastic adherends and polyurethane adhesive studied. Polycarbonate with its lower glass transition temperature T_g was affected by the carbon dioxide laser to a higher extent than the higher T_g polyetherimide. Furthermore, polycarbonate exhibited enhanced shear properties and a higher level of chemical modification following excimer laser irradiation compared to polyetherimide.