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.     

Investigation on the structure-property relationships for films obtained from UV curable coatings

A number of poly(ethyleneglycol)- and poly(propyleneglycol)- , ω-diacrylates, with different MW in the range 200–2000, were subjected to UV curing as thin films and the properties of the obtained crosslinked polymers were investigated. The total double bond conversion was determined by Fourier transform infrared spectroscopy (FTIR). Direct scanning calorimetry (DSC), thermal mechanical analysis (TMA) and dynamic-mechanical thermal analysis (DMTA) were used to evaluate the thermal and dynamic-mechanical behaviour of the networks obtained. The double bond conversion was found to be always complete, with the exception of the lowest MW oligomers. T_g values were found to decrease with increasing the MW of the oligomers used for both series. The equilibrium swelling in water was found to increase with increasing MW that is with decreasing the crosslinking density of the network. Mixtures of different oligomers with a typical epoxyacrylate resin were also examined. A sharp increase of the total double bond conversion in the presence of the oligomers was observed. The T_g values of the obtained films were found to decrease with increasing oligomer MW in agreement with the data obtained by using the pure acrylate oligomers. The results obtained are discussed in terms of the crosslinking density of the networks formed in the curing process.     

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。     

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。     

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.     

Crosslink density of oligourethane diols cured with amino resin at different temperatures

Oligourethane diols were crosslinked with melamine formaldehyde (N,N,N′,N′,N″,N″-hexakis (butoxymethyl)-2,4,6-triamino-1,3,5-triazine). Their films were wetted with dichloromethane and the increase in dimensions of the swollen film under equilibrium swelling conditions was used to calculate crosslink density by using the Flory–Rehner equation. The molar mass between two crosslinks M_c was also determined. The crosslink density of the coating is found to be influenced by a trans-etherification reaction as well as by self-condensation of the crosslinker. In formulations containing primary hydroxylated diol a major part of the crosslinker is consumed in a trans-etherification reaction, whereas a part of the crosslinker undergoes self-condensation in formulations containing secondary hydroxylated diols. The aromatic nature of 4-methyl-m-phenylene diisocyanate (TDI) imparts rigidity while the aliphatic nature of 5-isocyanato-1-(isocyanatomethyl)-1,3,3-trimethylcyclohexane (IPDI) results in flexibility of the backbone. The formulations containing 30% crosslinker and cured at 140°C for 20min are found to undergo completion of the major part of the reaction, indicating ideal conditions for their crosslinking from a commercial point of view. ©1997 SCI     

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. ©     

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.     

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.     

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.     

Crosslinkable poly(phenylene oxide) containing pendent 1,2-diphenylcyclopropane groups

Crosslinkable poly(phenylene oxide)s containing pendent 1,2-diphenylcyclopropane groups have been synthesized by oxidative polymerization from four new phenols containing the 1,2-diphenylcyclopropane group. The monomers were prepared by base-catalysed decomposition of the corresponding hydroxy-substituted 3,5-diphenyl-2-pyrazolines, which were the products of the reaction between hydroxy-substituted chalcones and hydrazine monohydrate. Characterization of and crosslinking studies on these polymers were carried out utilizing differential scanning calorimetry, thermogravimetric analysis, gel permeation chromatography and nuclear magnetic resonance. The polymers can be thermally cross-linked when heated to 350°C and the glass transition temperatures (T_g) of the polymers increase after crosslinking. The resulting crosslinked networks are insoluble in all solvents examined. Thermogravimetric analysis shows that no significant weight loss accompanies the crosslinking reaction.     

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.     

Preparation and properties of disyndiotactic poly(alkyl crotonate)s

Ethyl, n-propyl, iso-propyl and n-butyl crotonates were polymerized by group transfer polymerization (GTP) using ketene trialkylsilyl acetals as initiators in the presence of mercury (II) iodide as catalyst and iodotriethylsilane as co-catalyst. Predominantly disyndiotactic polymers with number-average molecular weights ranging from 56,000 to 90,000 were obtained almost quantitatively. Thermal and mechanical properties of poly(alkyl crotonate)s, such as glass transition temperature (T_g), refractive index, transmissivity, Izod impact strength, Young's modulus, hardness, tensile strength, tensile elongation at break, gas permeability coefficient and density were measured. All the polycrotonates have 65–90 °C higher T_g than the corresponding polymethacrylates.     

Soy-based,high biorenewable content UV curable coatings

Utilization of biorenewable materials in UV curing coatings technology is a “green + green” solution to sustainable development. In this work, acrylated epoxidized soybean oil based UV curable coating performance was significantly enhanced by incorporating synthesized acrylated sucrose and hyperbranched acrylates. Acrylated sucrose monomers were synthesized using solventless bulk reactions and characterized by mass spectroscopy and infra-red spectroscopy. Synthesized acrylated sucrose comprised of acrylated monomers with varying degree of acrylation, and the degree of acrylation of the acrylated sucrose increased with reaction time. Acrylated sucrose monomers obtained after 6 and 12 h reactions were formulated into acrylated soybean oil based UV curable coatings with commercial hyperbranched acrylates to produce biorenewable materials based UV curable coatings. Acrylated sucrose monomers increased the toughness but reduced the water resistivity and thermal stability of the coatings, the toughening effect was explained by the soft and hard micro phase separation in the cured coating films. Hyperbranched acrylates increased the coating hardness, adhesion, modulus, solvent resistance and glass transition temperature. Coating formulations containing both the acrylated sucrose monomers and the hyperbranched acrylates exhibited balanced coating performance with reasonably high biorenewable content.     

Imide-aryl ether benzoxazole random copolymers

Novel imide-aryl ether benzoxazole copolymers were prepared and their morphology and mechanical properties investigated. A key feature of these copolymers is the incorporation of a benzoxazole moiety by the use of 2,2′-bis[4-(3-aminophenoxy)phenyl]-6,6′-bibenzoxazole or 2,2′-bis[4-(4-aminophenoxy)-phenyl]-6,6′-bibenzoxazole as co-diamines in polyimide syntheses. The preparation of these diamines involved the nucleophilic aromatic substitution of 2,2′-bis(4-fluorophenyl)-6,6′-bibenzoxazole with either 3- or 4-aminophenol in the presence of K₂CO₃. The diamines were co-reacted with various compositions of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) to synthesize the desired poly(amic acids). Films were cast and cured (350°C) to effect the imide formation, affording films with elongations between 40 and 110% and moduli in the 2000–2750 MPa range. The copolymers exhibited good dimensional (T_g in excess of 300°C) and thermal stability. Wide-angle X-ray diffraction measurements on the copolymers showed that the ordered morphology characteristic of PMDA/ODA polyimide was retained. Improvements in the auto-adhesion were observed, particularly in those copolymers which displayed a T_g.     

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.     

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.     

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.     

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.     

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.