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.     

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.     

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. ©     

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.     

木器高光面漆用丙烯酸酯树脂的制备与性能研究

以甲基丙烯酸甲酯(MMA)和丙烯酸丁酯(BA)为主单体,多种交联体系为功能单体,采用水性固体丙烯酸树脂(SR-675、QZ-7001、QZ-7002)和聚合型乳化剂NRS-10搭配作为体系的表面活性剂,通过预乳化半连续乳液聚合工艺合成了木器高光面漆用丙烯酸酯树脂。探究了水性固体丙烯酸树脂酸值、T_g、相对分子质量以及用量;NRS-10用量、引发剂(APS)用量和树脂T_g对树脂及涂膜性能的影响。结果表明:当采用水性固体丙烯酸树脂SR-675且用量占单体总量的20%(下同)、NRS-10占0.5%、APS占0.75%,树脂T_g设计为40 ℃时,合成的丙烯酸酯树脂制备成木器面漆,涂膜光泽高、附着力好、耐水性好,铅笔硬度可达2H。     

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.     

UV crosslinkable microsphere pressure sensitive adhesives—influence on adhesive properties

In the following study, a synthesis and characterization of UV crosslinkable acrylic pressure sensitive adhesives are presented. Different amounts of unsaturated photoinitiator 4-acryloyloxy benzophenone (4-ABF) were added in t-butyl acrylate/2-ethylhexyl acrylate monomer mixture and then polymerized using a suspension polymerization technique. The adhesive suspension was coated on a pilot coating machine, dried by application of IR and subsequently crosslinked under UV light. The copolymerized 4-ABF photoinitiator will produce reactive radicals upon absorption of UV light, which are capable of initiating a rapid chain reaction with neighboring C-H positions of polymer side chains, what leads to formation of crosslinked polymer structures. UV crosslinking process was monitored by ATR-FTIR spectroscopic technique. Adhesion properties of the synthesized materials were determined using standard measurements of tack, peel and shear strength. Results have shown that all adhesive properties are strongly influenced by the degree of crosslinking of the microspheres, which increased with higher amounts of added 4-ABF photoinitiator. All the three measured adhesive properties showed a substantial decrease even at small amounts of added 4-ABF. The decrease in adhesion may be correlated with higher crosslinking density, what also resulted in higher gel phase amounts. Determination of glass transition temperature showed minor difference between adhesive coatings.     

Some observations on the copolymerization of styrene with furfuryl methacrylate

The tendency for crosslinking to occur either during free radical polymerization in solution or to develop subsequently in air after precipitation of linear polymer has been observed for both polymerization of furfuryl methacrylate (FM) and copolymerization of FM with styrene (ST). Reactivity ratios r_FM=0.33±0.08 and r_ST=0.44±0.02 have been determined. Thermo-oxidative stability is lowered by incorporation of FM moieties into poly-ST. Glass transition temperatures (T_g) of copolymers accord with the Fox equation and extrapolation affords a value of 64°C for the T_g of linear poly-FM. After admixture of solutions of poly(FM-co-ST) and bismaleimide (BM) at ambient temperature crosslinked gel is produced slowly by intermolecular Diels–Alder reaction between diene units (FM) and the dienophile (BM). The retro Diels–Alder reaction occurs rapidly on heating, regenerating linear copolymer and BM. The crosslinked gels exhibit considerable swelling in chloroform and toluene.     

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.     

Synthesis of a latex with bimodal particle size distribution for coating applications using acrylic monomers

Multipart emulsion polymerization of methyl methacrylate (MMA) and butyl acrylate (BA) was carried out in this work. The target was to achieve stability during the polymerization and to determine the proper hydrophilic–lipophilic balance (HLB) value for the stable system, using different types of non-ionic emulsifiers, sodium lauryl sulphate and their combinations. After determination of proper value of HLB (36.2), the best emulsifier combination on the basis of minimization of coagulum level was determined. This combination was 20 wt.% of KENON30 to SLS. The effect of monomer feed composition on the dry latex film properties was investigated to approach a monomer feed composition dealt with a proper T_g. The prepared latex showed a bimodal particle size distribution, due to the proper feeding policy in semibatch emulsion copolymerization process. The monomer feed composition of 45 wt.% MMA and 55 wt.% BA with a proper T_g was selected for the final improvement of coating properties such as UV resistance and adhesion. Adding acrylic acid (AA) and N-methylol acrylamide (NMA) to the reaction mixture improved the UV resistance and adhesion property of dry latex film. The flow and leveling, gloss, adhesion, UV resistance and water resistance of the produced bimodal latex showed good quality in comparison with the similar commercial resins used in coating applications.     

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.     

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.     

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.     

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.     

Influence of co-solvent content on electro-deposition behavior of acrylic lattices of different glass transition temperatures

The influence of co-solvent content on the constant-voltage electro-deposition behavior of three acrylic lattices was investigated. It was found that co-solvent effect is system-specific and the type and the extent of the influence depend on the specific features of the latex itself, specially its glass transition temperature, T_g. For a low-T_g high molecular weight latex, the addition of co-solvent makes the electro-coagulates more sticky but it does not show a quick influence on the deposition behavior. In contrast for higher T_g lattices the influence is more pronounced. For medium and high-T_g lattices, the co-solvent was found to have a significant influence on the electro-deposition behavior and the film morphology.     

Efficient homogenous catalysis of CO2 to generate cyclic carbonates by heterogenous and recyclable polypyrazoles

The cycloaddition between CO₂ and epoxides to produce cyclic carbonate is an attractive and efficiency pathway for the utilization of CO₂ as C1 source. The development of catalyst to mediate cycloaddition between CO₂ and epoxides at low temperature and pressure is still a challenge. Herein, a series of polypyrazoles with glass transition temperature (T_g) in the range of 42.3–52.5 ℃ were synthesized and served as catalyst to mediate the cycloaddition of CO₂ and epoxides by the assistant of tetrabutylammonium bromide. The catalytic behaviors of polypyrazole on the model cycloaddition of CO₂ to epichlorohydrin, including the reaction parameters optimization and versatility were investigated in detail, and excellent yield (99.9%) and selectivity (99%) were obtained under the optimized reaction conditions of 70 ℃ and 1.0 MPa for 6.0 h. Noteworthily, the polypyrazole acts as homogeneous catalyst during reaction (higher than T_g). And under room temperature, polypyrazoles can be easily separated and recovered, which is a promising feature of a heterogeneous catalyst. Furthermore, the reaction mechanism was proposed. The DFT calculation suggested that the formation of hydrogen bond between pyrazole and epoxide greatly reduced the energy barrier, which play an important role in promoting CO₂ cycloaddition.     

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.     

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.     

十一烯酸/马来酸酐共聚物的制备与表征

以十一烯酸(UA)和马来酸酐(MAH)为原料,偶氮二异丁腈(AIBN)为引发剂,通过沉淀聚合的方法制备了十一烯酸/马来酸酐共聚物(UMA),研究了单体配比、引发剂用量及反应温度对共聚反应的影响,通过FT-IR、¹³C NMR、DSC分析和酸酐质量分数的测定对共聚物进行了表征。FT-IR和¹³C NMR结果表明：在实验条件下,十一烯酸(UA)与马来酸酐(MAH)发生了共聚反应。当反应温度为75 ℃、AIBN用量为0.75%时,随着MAH用量的增加,共聚物相对分子质量减小,得率和酸酐质量分数呈现先增大后略有减小的趋势,当UA与MAH的物质的量之比为40:60时共聚物的得率及酸酐质量分数均达到最大值,分别为61.78%和20.05%,与DSC曲线中玻璃化转变温度(T_g)的变化趋势基本一致,即当n(UA):n(MAH)为40:60,T_g达到最大值71.98 ℃。提高引发剂用量和反应温度有利于共聚反应的进行,但相对分子质量有所下降,因此可根据所需聚合物的性质来选择合适的反应条件。     

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。