Thermoplastic polymers as modifiers for urea–formaldehyde wood adhesives. III. In situ thermoplastic‐modified wood composites

Acrylic monomers and free‐radical initiators were dispersed in an aqueous urea–formaldehyde (UF) suspension and polymerized in situ to afford a suspension containing 5 wt % thermoplastic (5 g of thermoplastic/100 mL of suspension). The viscosity of the thermoplastic‐modified UF suspension (65 wt % solids at 25°C) ranged from 240 to 437 cP versus 121 cP for the unmodified UF control. Wood‐flour composites (sugar maple and 50 wt % adhesive) were prepared with thermoplastic‐modified UF suspensions and cured with the same cycle used for the composites prepared with the unmodified UF adhesive (control). The effect of the thermoplastic‐modified UF adhesive was evaluated on the notched Izod impact strength and equilibrium moisture uptake of the wood‐flour composites. The notched Izod impact strength of the composites prepared with modified UF adhesives increased by as much as 94% above that of the control. The increase depended on the initiator and the monomer composition. The modification affected the equilibrium moisture uptake and rate of moisture uptake in the wood‐flour composites. Preliminary results for particleboard prepared with 10 wt % modified UF adhesive (5% thermoplastic in the UF resin) and unoptimized cure conditions confirmed a significant effect of the thermoplastic modification on both the internal‐bond strength and thickness swelling of the particleboard. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Differential scanning calorimetry and dynamic mechanical analysis of amine-modified urea–formaldehyde adhesives

Urea–formaldehyde (UF) resins are susceptible to stress rupture and hydrolytic degradation, particularly under cyclic moisture or warm, humid conditions. Modification of UF resins with flexible di- and trifunctional amines reduces this problem. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the thermal behavior of modified and unmodified adhesives to identify the physical and morphological factors responsible for the improved performance. A UF resin modified by incorporating urea–capped poly(propyleneoxidetriamine) during resin synthesis exhibited a higher cure rate and greater cure exotherm than the unmodified resin. Resins cured with a hexamethylenediamine hydrochloride curing agent had slower cure rates than those cured with NH₄Cl. DMA behavior indicated that modified adhesives were more fully cured and had a more homogeneous crosslink density than unmodified adhesives. DMA behavior changed with storage of specimens at 23°C and 50% relative humidity, after previous heating for approximately 20 min at 105°C to 110°C. The initial changes were postulated to occur because of physical aging (increase in density) and continued cure. These were followed by physical breakdown (microcracking) and possibly cure reversion. © 1995 John Wiley & Sons, Inc.     

Thermoplastic polymers as modifiers for urea‐formaldehyde (UF) wood adhesives. II. Procedures for the preparation and characterization of thermoplastic‐modified UF wood composites

Wood composites were prepared by using wood flour (sugar maple, Acer saccharum March) and thermoplastic‐modified urea‐formaldehyde (UF) suspensions. Thermoplastic (5–10% w/v) was introduced into the UF suspension as an aqueous solution, a self‐stabilized dispersion in water, or as a surfactant‐stabilized latex. The modified suspension was blended with wood flour, and the blend was cured by using a cure cycle that was suitable for all the thermoplastic‐modified UF formulations and unmodified UF controls. The wood flour composites were tested by using a notched Izod impact strength test. All formulations containing surfactant decreased the impact strength by ～ 30–40% relative to the unmodified UF control, whereas the water‐soluble thermoplastic had no effect on the impact strength. The formulations with self‐dispersed thermoplastics all increased the notched Izod impact strength, with the greatest increase being 69% more than the UF control, except in a single instance when the molecular weight of the thermoplastic was very high, which decreased resin flow. Increasing the thermoplastic content from 5 to 10% w/v did not further improve the impact test results. Scanning electron microscopy of the fracture surfaces showed morphological differences in the systems that varied with the thermoplastic and method of thermoplastic addition to the UF suspension.© 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 898–907, 2003     

Hydrolytic stability of cured urea‐formaldehyde resins modified by additives

Urea‐formaldehyde (UF) resins are prone to hydrolysis that results in low‐moisture resistance and subsequent formaldehyde emission from UF resin‐bonded wood panels. This study was conducted to investigate hydrolytic stability of modified UF resins as a way of lowering the formaldehyde emission of cured UF resin. Neat UF resins with three different formaldehyde/urea (F/U) mole ratios (1.4, 1.2, and 1.0) were modified, after resin synthesis, by adding four additives such as sodium hydrosulfite, sodium bisulfite, acrylamide, and polymeric 4,4′‐diphenylmethane diisocyanate (pMDI). All additives were added to UF resins with three different F/U mole ratios before curing the resin. The hydrolytic stability of UF resins was determined by measuring the mass loss and liberated formaldehyde concentration of cured and modified UF resins after acid hydrolysis. Modified UF resins of lower F/U mole ratios of 1.0 and 1.2 showed better hydrolytic stability than the one of higher F/U mole ratio of 1.4, except the modified UF resins with pMDI. The hydrolytic stability of modified UF resins by sulfur compounds (sodium bisulfate and sodium hydrosulfite) decreased with an increase in their level. However, both acrylamide and pMDI were much more effective than two sulfur compounds in terms of hydrolytic stability of modified UF resins. These results indicated that modified UF resin of the F/U mole ratio of 1.2 by adding acrylamide was the most effective in improving the hydrolytic stability of UF resin. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Melt viscoelasticity of polyethylene terephthalate resins for low density extrusion foaming

The rheological properties of conventional polyethylene terephthalate (PET) resins are not particularly suitable for low density extrusion foaming with physical blowing agents; as a result, chemically modified resins through chain extension/branching reactions are often used. Such resins have overall higher melt viscosity and higher melt strength/melt “elasticity” than unmodified materials. In this work, following a review of the prior art on PET chemical modification, an unmodified and a chemically modified resin were selected and characterized for their melt viscoelastic properties including shear and dynamic complex viscosity over a broad shear rate/frequency range, storage and loss modulus, and die swell. Certain rheological models were found to provide better fits of the entire viscosity curve for the unmodified vs. the modified resin. Foamed extrudates having variable densities (from about 1.2 to 0.2 g/cc), were prepared by carbon dioxide injection in monolayer flat sheet extrusion equipment. Foams with increasingly lower density, below 0.5 g/cc, were obtained by increasing gas pressure only in the case of the chemically modified resin. The effects of variables such as concentration of the physical blowing agent, resin rheology, resin thermal properties and choice of process conditions are related to product characteristics including density, cell size and crystallinity.     

UV curable glycidyl carbamate based resins

The synthesis and characterization of UV curable resins based on glycidyl carbamate chemistry have been explored. Glycidyl carbamate (GC) functional resins have been used to obtain crosslinked coatings with a wide range of properties using several crosslinking techniques such as epoxy-amine, self-crosslinking, and sol-gel. GC resin technology was further expanded to UV curable coatings by reacting polyfunctional GC resins with acrylic acid to yield acrylated glycidyl carbamate (AGC) resins. Alcohol-modified UV curable GC resins were also prepared to obtain lower viscosity. Commonly used reactive diluents were used to prepare a UV curable GC coating formulations. The coatings were cured in air using a Fusion LC6B Benchtop Conveyer with an F300 UV lamp. The degree of conversion of acrylic double bonds during UV curing was determined using real time FTIR and showed that the resins had fast cure rates and high extents of conversion of acrylate groups. Coating properties such as hardness, impact strength, methyl ethyl ketone double rubs, flexibility, and adhesion were studied. Dynamic mechanical analysis was used to determine crosslink density of the coatings. Differential scanning calorimetry and thermogravimetric analysis were used to study the thermal properties of the coatings. The type of polyisocyanates and the extent of modification in GC resins influenced the degree of conversion, crosslink density, and coating performance.     

The effect of soda bagasse lignin modified by ionic liquids on properties of the urea–formaldehyde resin as a wood adhesive

The aim of this research was to investigate the influence of lignin modified by ionic liquids on physical and mechanical properties of plywood panels bonded with the urea–formaldehyde (UF) resin. For this purpose, soda bagasse lignin was modified by the 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) ionic liquid and then the various contents of unmodified and modified lignins (10, 15, and 20%) were added at pH=7 instead of second urea during the UF resin synthesis. The physicochemical properties of the prepared resins as well as the water absorption, shear strength, and formaldehyde emission of the plywood panels made with these adhesives were measured according to standard methods. According to Fourier Transform Infrared (FTIR) Spectrometry, by treatment of lignin, the C=O, C–C, and C–H bonds decrease while the content of the C–N bond dramatically increases. Based on the finding of this research, the performance of soda bagasse lignin in UF resins dramatically improves by modification by ILs; as the resins with modified lignin yielded lower formaldehyde emission and water absorption when compared to those made from unmodified lignin and commercial UF adhesives, respectively. The shear strength as well as wood failure percentages are lower for the panels produced with modified lignin than for the panels produced with UF resins alone.     

Synthesis and properties of vinyl siloxane modified cresol novolac epoxy for electronic encapsulation

Vinyl siloxane (VS) modified cresol novolac epoxy (CNE) and cresol novolac hardener (CNH) resins are synthesized and both components are capable of further crosslinking. The reaction kinetics for both components are studied so that they can crosslink simultaneously in a designed synthesis procedure. Through careful adjustment of a triphenylphosphine dosage, the glass‐transition temperature (T_g) of CNE/CNH resins can be effectively controlled. Phenomena characteristic of the existence of a diffusion‐controlled reaction are also observed. The relationships between the T_g and crosslinking density for the CNE/CNH resin are explicitly revealed through gel content and swell ratio experiments. CNE/CNH resins with a higher T_g have lower equilibrium moisture uptake because of the higher fraction of free volume. The coefficient of diffusion also shows a similar but less apparent trend. The incorporation of VS incurs a 35% reduction in the equilibrium moisture uptake and a 20% reduction in the coefficient of diffusion for the modified resin. The VS‐modified CNE/CNH resin possesses a lower Young's modulus and a higher strain at break than its unmodified counterpart does. This modified resin can help to alleviate the popcorning problems in integrated circuit packages, which results from hygrothermal stresses. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 652–661, 2001     

有机硅改性松香基环氧树脂的制备及阻燃性能

制备了聚甲基苯基硅氧烷（PMPS）改性松香基乙二醇二缩水甘油醚AR-EGDE。红外光谱(IR)、核磁共振（¹³C NMR）和环氧值测试结果表明有机硅成功接枝至环氧树脂。同时,将PMPS与AR-EGDE充分混合得到物理改性树脂。通过力学性能和极限氧指数测试探讨了改性方法对改性树脂力学及阻燃性能的影响：化学改性优于物理改性及未改性的AR-EGDE。热失重、炭层分析表明,PMPS改性的树脂在受热和燃烧过程中,都能形成含硅炭层,该炭层可延缓内部材料热分解,同时阻止可燃裂解气体的释放和熔滴发生,从而提高材料的耐热和阻燃性能。物理改性松香基环氧,燃烧时无法形成有效富硅炭层覆盖于底部材料,从而使其阻燃性劣于化学改性。     

环氧/双氰胺衍生物/促进剂体系性能的研究

本文通过对双氰胺的改性,得到了一种在环氧树脂和低极性的混合溶剂中有良的溶解性的固化剂ＴＨ－１１。     

环氧大豆油低聚物增韧环氧树脂胶粘剂

合成了三种环氧大豆油低聚物作为室温和高温固化环氧树脂增韧剂,对其增韧环氧体系的粘接性能和力学性能进行了考察。试验结果表明,环氧树脂低聚物对固化体系的初期粘度等性能没有影响,对固化体系粘接性能和力学性能等有较大影响。与未改性的环氧树脂相比,由顺丁烯二酸酐扩链的环氧大豆油低聚物改性的环氧树脂剪切强度提高了56.64%。     

Thermoplastic polymers as modifiers for urea‐formaldehyde (UF) wood adhesives. I. Procedures for the preparation and characterization of thermoplastic‐modified UF suspensions

Acrylic thermoplastic copolymers with different degrees of hydrophilicity were prepared and introduced into a commercial aqueous urea‐formaldehyde (UF) suspension at 5–10% w/v. The most hydrophilic acrylic thermoplastic was introduced into the UF suspension as an aqueous solution, whereas the most hydrophobic acrylic was introduced as a surfactant‐stabilized suspension. Acrylics with intermediate hydrophilicity were introduced into the UF suspension as a self‐dispersed aqueous suspension. The thermoplastic‐modified UF suspensions with 5% thermoplastic (58% solids) had a viscosity at 30°C of ～ 114 cP, compared with a viscosity of ～112 cP for the original UF suspension (60% UF solids). At 10% thermoplastic (63% solids), all the thermoplastic‐modified UF suspensions exceeded 200 cP. The viscosity of the UF suspension modified with self‐dispersed thermoplastic was reduced by ～ 50% by reducing the thermoplastic molecular weight. SEM micrographs of cured thermoplastic‐modified UF showed phase‐separated thermoplastic domains in a continuous UF phase for the UF modified with self‐dispersed and surfactant‐stabilized thermoplastic, but UF modified with the water‐soluble thermoplastic showed a single phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 890–897, 2003     

Modification of urea-formaldehyde resin adhesives with blocked isocyanates using sodium bisulfite

Polymeric 4-4 diphenyl methane diisocyanate (pMDI) was blocked with an aqueous sodium bisulfite solution to obtain water-dispersible blocked pMDI (B-pMDI) resin with different HSO₃/–NCO mole ratios for the modification of urea-formaldehyde (UF) resin. Fourier transform infrared (FTIR) spectra of the B-pMDI resin clearly showed that all isocyanate groups of the pMDI resin were successfully blocked by sodium bisulfite. As the HSO₃/–NCO mole ratio increased, the de-blocking temperature of the B-pMDI resin also increased. Two addition levels (1% and 3%) of the B-pMDI resin with different HSO₃/–NCO mole ratios were mixed with UF resins and used as an adhesive for plywood. The gel time of the UF/B-pMDI resins decreased to a minimum at a mole ratio of 0.9 and then increased with the HSO₃/–NCO mole ratio, and was consistent with the peak temperature (Tp). However, as the HSO₃/–NCO mole ratio increased, the viscosity of the modified UF resins by 1% B-pMDI resin addition slightly increased, whereas those of modified resins with 3% B-pMDI resin addition rapidly increased. The adhesion strengths of plywood bonded with the hybrid resins were greater for 1% B-pMDI resin addition than for 3% B-pMDI resin addition. Formaldehyde emission of plywood bonded with the UF/B-pMDI resins significantly decreased up to 34% by the addition of B-pMDI resin at a mole ratio of 1.8. These results suggest that the modification of UF resins by mixing with water-dispersible B-pMDI resin can be a method for improving the water resistance and formaldehyde emission of UF resins for wood-based composites.     

Hydrolytic stability and crystallinity of cured urea-formaldehyde resin adhesives with different formaldehyde/urea mole ratios

This study investigated the relationship between the hydrolytic stability and the crystalline regions of cured UF resins with different formaldehyde/urea (F/U) mole ratios to better understand the hydrolysis of cured urea-formaldehyde (UF) resin adhesives responsible for its formaldehyde emission in service. As the F/U mole ratio decreased, the hydrolytic stability of cured UF resins improved, but decreased when the particle size of the resin was reduced. To further understand the improved hydrolytic stability of cured UF resin with lower F/U mole ratios, X-ray diffraction (XRD) was extensively used to examine the crystalline part of cured UF resins, depending on F/U mole ratios, cure temperature and time, hardener type and level. Cured UF resins with higher F/U mole ratios (1.6 and 1.4) showed amorphous structure, while those with lower F/U mole ratios (1.2 and 1.0) showed crystalline regions, which could partially explain the improved hydrolytic stability of the cured UF resin. The crystalline part intensity increased as cure temperature, cure time and hardener content increased. But the 2θ angles of these crystalline regions did not change, depending on cure temperature and time, hardener type and level, suggesting that the crystalline regions of the cured UF resin were inherent. This study indicates that the crystalline regions of cured UF resins with lower F/U mole ratio contribute partially to the improved hydrolytic stability of the cured resin.     

二烯丙基双酚A对低黏度氰酸酯树脂的改性研究

为改善低黏度氰酸酯树脂基体的综合性能,研究了二烯丙基双酚A(DBA)对其的改性作用。通过粘温试验、DSC、力学试验、TGA、DMA、吸水率和介电测试等分析手段对DBA改性前后的树脂进行性能表征。结果表明:改性后树脂的粘度仍然满足直接热熔法的工艺性要求;DBA对氰酸酯树脂具有显著的催化作用,可以提高转化率;加入适量的DBA对氰酸酯树脂有一定的增韧作用,其中10%DBA的树脂冲击强度和弯曲韧性分别是未加DBA树脂的179%和165%;加入DBA后树脂基体的耐热性能、耐湿热性能和介电性能均有降低,但含量较少时仍满足应用要求。综合试验结果,当DBA含量为5%~10%时,树脂综合性能较好。     

Reduction of Formaldehyde Emission from Particleboard by Phenolated Kraft Lignin

The aim of this study was the reduction of formaldehyde emission from particleboard by phenolated Kraft lignin. For this purpose, the lignin was extracted from black liquor and then modified by phenolation. During the urea formaldehyde (UF) resin synthesis different proportions of unmodified and phenolated Kraft lignins (10%, 15%, and 20%) were added at pH = 7 instead of the second urea. Physicochemical properties and structural changes of resins so prepared, as well as the internal bond (IB) strength and formaldehyde emission associated with the panels bonded with them were measured according to standard methods. The Fourier transform infrared (FTIR) analysis of lignin indicated that the content of O–H bonds increased in phenolated lignin while the aliphatic ethers C–O bonds decreased markedly in the modified lignin. Since both synthesis of UF resins and lignin phenolation are carried out under acid conditions, phenolation is an interesting way of modifying lignin for use in wood adhesive. The panels bonded with these resins showed significantly lower formaldehyde emission compared to commercial UF adhesives. The UF resin with 20% phenolated lignin exhibited less formaldehyde release without significant differences in internal bond strength and physicochemical properties compared to an unmodified UF resin. XRD analysis results indicated that addition of phenolated lignin decreased the crystallinity of the hardened UF resins.     

Water uptake effects in resins based on alkenyl‐modified cyanate ester–bismaleimide blends

The analysis of cured resin blends comprising a commercial dicyanate, bismaleimide and a range of novel alkenyl‐substituted cyanates, to determine the chemical effects of long‐term exposure to water, is reported. The cured resin blends underwent accelerated water uptake by immersion at temperatures of 50 °C and 70 °C, for a period of 14–17 months. The presence of about 10–15% (by weight) of alkenyl‐substituted cyanate in the blend leads to a marked reduction in moisture absorption in comparison with the unmodified bismaleimide/cyanate blend containing a comparable amount of bismaleimide. All samples display non‐Fickian diffusion behaviour at both immersion temperatures, although this is most marked at the higher temperature. Thermogravimetric analysis was performed on alkenyl‐modified neat resin samples before and after the immersion period. The modified samples display thermal stabilities that are indistinguishable from cured resins that have not undergone immersion. Spectroscopy (near infrared Raman and mid‐infrared attenuated total reflectance) was performed on cured resin plaques to determine the sample composition as a function of modifier content. The elemental composition of the samples was also determined before and after the immersion period, and no significant variation in elemental composition was recorded for the modified samples. © British Crown Copyright 2001/DERA. Reproduced with the permission of Her Majesty's Stationery Office. Published for the SCI by John Wiley & Sons, Ltd.     

Correlations between dynamic mechanical properties and nodular morphology of cured epoxy resins

Various epoxy resin formulations, based on the diglycidyl ether of bisphenol A (DGEBA) and cured with diethylene triamine (DETA) were studied. Dynamic mechanical measurements were used to characterize changes in mechanical properties as a function of temperature. The morphology of the cured resins was investigated by transmission electron microscopy. Correlations between dynamic mechanical properties and morphology were described and discussed by applying the concept of inhomogeneous (nodular) thermoset morphology. The elastic storage modulus in the glassy state was determined primarily by the internodular matrix, whereas the glass transition of cured resins depended upon the intranodular crosslink density.     

酚醛树脂/古马隆树脂对再生橡胶性能的影响

研究了204酚醛树脂(204树脂)和古马隆树脂在再生橡胶中的应用。测定了2种树脂对再生橡胶的交联密度、硫化特性、门尼粘度、力学性能、动态力学性能的影响。结果表明,2种树脂均可以有效地降低再生橡胶的门尼粘度;204树脂的固化作用可以提高硫化胶的整体交联密度,有利于再生橡胶力学性能的提高,当其用量为3phr时达到最佳效果;古马隆树脂有利于再生橡胶断裂伸长率的提高,当其用量为4.5phr时达到最佳效果。