Investigation of epoxy-based wood adhesives

Cold-set epoxy-based wood adhesives were investigated for production of exterior plywood. Effective adhesives were composed of bisphenol A diglycidyl ether (BPADGE), polyamidoamine (PAA), and polyethylenimine (PEI). Three-ply plywood panels were prepared with BPADGE–PAA–PEI adhesives and evaluated for their strengths and water resistance in accordance with a standard for exterior plywood. The effect of BPADGE/(PAA + PEI) weight ratio, PAA/PEI weight ratio, the mixing time for preparing the adhesive, and the pressing time for making plywood panels on the water resistance and the shear strengths of the plywood panels was investigated. The pot life of the adhesive was also measured. Plywood panels made with the BPADGE–PAA–PEI adhesives met the industrial requirements for exterior applications. Adhesion mechanisms are discussed in detail. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47741.     

Shear strength and water resistance of modified soy protein adhesives

Soy protein polymers recently have been considered as alternatives to petroleum polymers to ease environmental pollution. The use of soy proteins as adhesives for plywood has been limited because of their low water resistance. The objective of this research was to test the water resistance of adhesives containing modified soy proteins in walnut, maple, poplar, and pine plywood applications. Gluing strength and water resistance of wood were tested by using two ASTM standard methods. Glues with modified soy proteins had stronger bond strength than those containing unmodified soy proteins. Plywood made with glue containing urea-modified proteins had higher water resistance than those bonded with glues containing alkali-modified and heat-treated proteins. After three 48-h cycles of water-soaking, followed by 48 h of air-drying, no delamination was observed for either walnut or pine specimens glued with the urea-modified soy protein adhesives. Gluing strength for wood species with smooth and oriented surface structure was lower than for those with rough, randomly oriented, surface structures. Wood species with greater expansion of dimensions during water-soaking had a higher delamination rate than those showing less expansion.     

An All‐Natural Adhesive for Bonding Wood

A novel adhesive that is solely based on natural materials of defatted soy flour (SF) and magnesium oxide (MgO) has been investigated for preparation of five‐ply plywood panels. The resulting plywood panels met the industrial water‐resistant requirement for interior plywood. In this study, mechanisms by which an aqueous mixture of SF and MgO served as a strong and water‐resistant adhesive for bonding wood were investigated. SF was first fractionated into soy protein isolates (SPI), a water‐soluble fraction, and insoluble carbohydrates (ICs) that were mixed with MgO, respectively, for preparation of maple laminates. The water resistance of the resulting maple laminates was evaluated by a three‐cycle water‐soaking‐and‐drying (WSAD) test and a two‐cycle boiling‐water test (BWT). The mixture of MgO and the soluble fraction was not able to bond maple veneers together. The shear strengths of the resulting maple laminates before and after WSAD and BWT all had the following order: MgO–SPI > MgO–SF > SF only > MgO–IC. The water solubility of SF in the heat‐cured SF–MgO mixture was much lower than that of the heat‐cured SF. We believe that the low water solubility of SF–MgO and close interactions between MgO and soy proteins instead of soy carbohydrates were responsible for the superior strengths and high water resistance of the soy‐MgO adhesive.     

The chemistry and development of tannin-based adhesives for exterior plywood

Condensed tannins are phenolic in nature and undergo reaction with formaldehyde to form resins. Tannin/formaldehyde condensates tend to have a low degree of condensation resulting in bonds lacking strength and water resistance. Small amounts of short-length phenol/resorcinol/formaldehyde, phenol/formaldehyde, and urea/formaldehyde polymers are used to increase the degree of polymerization of tannin-formaldehyde resins, decreasing brittleness and increasing water resistance. Furfural is used to plasticize tannin/formaldehyde resins to improve the distribution of stress forces on the glueline, overcoming weakness due to stress concentration. Exterior plywood panels were prepared using these modified tannin/formaldehyde resins as adhesives. The modified tannin/formaldehyde adhesives perform well, have similar characteristics to and are as easy to use as phenol/formaldehyde adhesives, and also have a few advantages, especially tolerance to faster pressing times.     

Reduction of formaldehyde emission from plywood

The aim of this work is to evaluate performances of tannin-based resins designed as adhesive in the plywood production. For this purpose, a part of phenol formaldehyde (PF) and melamine formaldehyde (MF) in the classic adhesive formulation was replaced by tannin. The physical properties of the formulated resins (rheological characterization, etc.) were measured. In order to analyze the mechanical performance of tannin-based resins, plywood panels were produced and the mechanical properties including tensile strength wood failure and three-point bending strength were investigated. The performance of these panels is comparable to those of plywood panels made by commercial PF and MF. The results showed that the plywood panels bonded with tannin–PF (PFT) and tannin–MF (MFT) resins exhibited better mechanical properties in comparison to the plywood panels made of commercials PF and MF. The introduction of small properties of tannin in PF and MF resins contribute to the improvement of the water performance of these adhesives. The formaldehyde emission levels obtained from panels bonded with tannin-based resins were lower than those obtained from panels bonded with control PF and MF. Although there are no actual reaction at all between PF, MF, and tannin, addition of tannin significantly improves the water resistance of PF and MF resins. This is a novel finding that manifests the possibility of replacing a convention PF and MF resins by tannin. Modified adhesive is one of the goals in the plywood production without changing any of their production conditions with improvement to their overall properties.     

Effects of typical soybean meal type on the properties of soybean-based adhesive

In order to effectively apply soybean meal for the preparation of water-resistant soybean-based adhesives for plywood, the effects of three typical soybean meal products, namely, low-temperature soybean meal (LM), high-temperature soybean meal (HM), and physical soybean meal (PM), on the properties of soybean-based adhesive were investigated. The results indicated that the number of reactive groups in the three soybean meals followed the order LM > HM > PM, which in turn led to various crosslinking densities when these soybean meals were crosslinked by epichlorohydrin-modified polyamide (EMPA) during the curing process. The LM soybean adhesive had 6.6% higher soaking bond strength and 16.5% higher boiling-dry-boiling bond strength than the HM soybean adhesive, and 19% higher soaking bond strength and 33% higher boiling-dry-boiling bond strength than the PM soybean adhesive, respectively. These three soybean meals could be used to prepare soybean adhesives for interior-use plywood because all plywood panels bonded with their adhesives passed a water-soaking test at 63 °C for 3 h, but only the LM soybean adhesive achieved the desired water resistance for floor-base plywood. Among the three evaluated soybean meals, LM was the most promising raw material for the preparation of soybean-based adhesive because of a greater number of reactive groups, higher crosslinking density, and superior bond strength. Plywood panel bonded with HM soybean adhesive had a water resistance lower than, but very close to, the standard required value (>0.8 MPa) for floor-base plywood.     

A New Soy Flour-Based Adhesive for Making Interior Type II Plywood

In this study, we developed a formaldehyde-free adhesive from abundant, renewable, and inexpensive soy flour (SF). The main ingredients of this adhesive included SF, polyethylenimine (PEI), and maleic anhydride (MA). The optimum formulation of this adhesive and the optimum hot-press conditions for making plywood were investigated. A three-cycle soak test and a boiling water test (BWT) were employed for evaluating the strength and water-resistance of plywood bonded with this adhesive. Results showed that SF, PEI, MA and sodium hydroxide were all essential components for the adhesive and the SF/PEI/MA weight ratio of 7/1.0/0.32 resulted in the highest water-resistance. When the hot-press temperature was in the range of 140–170 °C, both water-resistance and shear strength of plywood bonded with the adhesive remained statistically the same, except that the dry shear strength of plywood at 170 °C was statistically lower than that at 160 °C. When the hot-press time ranged from 2 to 6 min, the plywood panels at 5 min had the highest boiling water test/wet (BWT/w) shear strength. The plywood panels made at 5 min had a higher dry shear strength than those made at 3 min. Plywood panels bonded with this SF/PEI/MA adhesive exceeded the requirements for interior applications.     

Perception of Aspen and Sun/Shade Sugar Maple Leaf Soluble Extracts by Larvae of Malacosoma disstria

We investigated the behavioral feeding preference and the chemoreception of leaf polar extracts from trembling aspen, Populus tremuloides, and from sun and shade sugar maple, Acer saccharum, by larvae of the polyphagous forest tent caterpillar, Malacosoma disstria, a defoliator of deciduous forests in the Northern Hemisphere. Three polar extracts were obtained from each tree species: a total extract, a water fraction, and a methanol fraction. M. disstria larvae were allowed ad libitum access to an artificial diet from eclosion to the fifth instar. Two-choice cafeteria tests were performed comparing the mean (±SE) surface area eaten of the total extracts, and the following order of preference was obtained: aspen > sun maple < shade maple. Tests with the other fractions showed that M. disstria larvae preferred the total aspen extract to its water fraction, and the latter to its methanol fraction. The response to sun maple was similar to aspen. However, for the shade maple experiment, there was no difference between the total extract and its water fraction. Electrophysiological recordings for aspen showed that the sugar-sensitive cell elicited more spikes to the water fraction, followed by the total extract, and finally the methanol fraction. Spike activity to stimulations of sun and shade maple extracts revealed a similar trend, where methanol fraction > water fraction > total extract. Our findings are discussed in light of previously known information about this insect's performance on these host plants.     

Adhesive properties of water-washed cottonseed meal on four types of wood

The interest in natural product-based wood adhesives has been steadily increasing due to the environmental and sustainable concerns of petroleum-based adhesives. In this work, we reported our research on the utilization of water-washed cottonseed meal (WCM) as wood adhesives. The adhesive strength and water resistance of WCM adhesive preparations on poplar, Douglas fir, walnut, and white oak wood veneers were tested with press temperatures of 80, 100, and 130 °C. Our data indicated that raising the hot press temperature from 80 to 100–130 °C greatly increased the bonding strength and water resistance of the WCM adhesives. The general trend of the adhesive strength of WCM on the four wood species was Douglas fir > poplar ≈ white oak > walnut. The rough surface of Douglas fir with tipping features could enhance the mechanical interlocking between the wood fibers and adhesive slurry, contributing to the high adhesive strength. The dimensional swelling of the bonded wood pairs due to water soaking was in the order: thickness > width (i.e. perpendicular to the wood grain) > length (i.e. parallel to the wood grain). The greatest dimensional changes were observed in Douglas fir specimens. However, the highest decrease in adhesive strength by water soaking was with poplar wood specimens. These observations suggested that the wood dimensional changes were not dominant factors on water weakening the bonding strength of these wood pairs.     

Preparation of a New Type of Polyamidoamine and Its Application for Soy Flour-Based Adhesives

The most commonly used curing agents for soy-based adhesives are polyamines, which have the problem of low solid content and/or high viscosity. To overcome this problem, a new type of polyamidoamine (PADA) resin was synthesized and applied to soy flour-based adhesives to improve their water resistance. The PADA solution obtained had a high solid content of 50 wt% and low viscosity of 270 cP. The optimum weight ratio of soy flour/PADA/maleic anhydride to prepare adhesive was 40/7/1.68. The wet strength of plywood prepared at the optimum weight ratio was 0.82 MPa, which meant the plywood could be used as type II plywood according to the Chinese National Standard GB/T 9846.7-2004. The results of water-insoluble solid content measurement and SEM observation demonstrated that cured soy flour–PADA–maleic anhydride adhesive had a 16 % greater water-insoluble solid content than soy flour–NaOH adhesive. The cross-linking network formed by the reactions of PADA and MA would increase the water-insoluble solid contents and improve water resistance of cured soy flour-based adhesives.     

双酚芴环氧树脂的合成研究

以双酚芴(BHPF)和环氧氯丙烷(ECH)为原料制备了双酚芴环氧树脂。研究了ECH与BHPF物质的量比、NaOH用量、催化剂十六烷基三甲基溴化铵(CTBA)用量、反应温度、反应时间等对产品质量的影响,并采用红外、质谱等对产物结构进行了表征。结果表明:较优的工艺条件为:ECH与BHPF物质的量比10∶1,CTBA质量分数3%(基于BHPF质量),NaOH与BHPF物质的量比2.4∶1;醚化反应温度54℃,加碱反应温度65℃,精制反应温度80℃,上述各阶段反应时间分别为60 min,120 min,120 min,产物环氧值可达到0.408。     

Soy-based,tannin-modified plywood adhesives

In this research, two different types of commercial tannins, namely a hydrolysable tannin (chestnut) and a condensed flavonoid tannin (mimosa), were used to prepare two types of soy-based (soy flour (SF) and soy protein isolate) adhesives for making plywood. Thermogravimetric properties (TGA) and its derivative as function of temperature (DTG) of different soy-based adhesive were measured in the range 40°C–300°C. Thermomechanical analysis (TMA) from 25°C to 250°C was done for the different resin formulations. Duplicate three-ply laboratory plywood panels were prepared by adding 300 g/m² of the adhesives’ total resin solid content composed of SF or isolated soy protein (ISP), urea, chestnut, and mimosa tannin extracts with hexamine as hardener. Based on the results obtained, tannins can improve SF adhesion properties. The TMA showed that chestnut tannin extract appeared to react well with SF, while mimosa tannin extract appeared to react well with ISP. Matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry also showed that among other reactions, the soy protein amino acids reacted with the tannins. Furthermore, delamination and shear strength test results showed the good water resistance of plywood bonded with soy-based tannin modified adhesive.     

The preparation and application of a new formaldehyde-free adhesive for plywood

In this study, we developed a new formaldehyde-free adhesive prepared by in-situ chlorinating graft copolymerization for plywood. The main ingredients of this adhesive include maleic anhydride (MAH) and high density polyethylene (HDPE) that is MAH grafted onto HDPE (PE-cg-MAH). The reaction between this adhesive and veneer, the optimum formulation to bond veneer and the optimum hot-press conditions to prepare the plywood were investigated. A boiling water test was employed to evaluate the strength and water resistance of plywood bonded with this adhesive. The results showed that the properties of the resulting plywood using PE-cg-MAH as an adhesive can meet the standard of Type I plywood and the optimum hot-press conditions were 160-165 °C and 5 min. When the chlorine contents of PE-cg-MAH was about 3% (wt%), the plywood panels had a higher shear strength after boiling water test above the hot-press conditions.     

Combination of lignin polyol–tannin adhesives and polyethylenimine for the preparation of green water‐resistant adhesives

In this study, a green adhesive from renewable lignin and tannin was developed with polyethylenimine (PEI) with a method to improve the water resistance of the lignin/tannin adhesive. Lignin polyols were prepared through the liquefaction of oil‐palm empty fruit bunches. The characteristics of the adhesive samples were compared with those of a commercial phenol–formaldehyde resin. Three plywood specimens bonded with the new adhesive showed a very high tensile strength (63.04 MPa) and were very water resistant. The effect of the solid content of the adhesives on the tensile strength and gel time and various weight ratios of PEI on the tensile strength and water resistance of the plywood specimens were evaluated. Thermal stability tests revealed that the lignin polyol–tannin/PEI adhesives had a high heat resistance (360 °C). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43437.     

Soybean meal-based adhesive enhanced by MUF resin

Soybean meal flour, polyethylene glycol (PEG), sodium hydroxide (NaOH), and a melamine-urea-formaldehyde (MUF) resin were used to formulate soybean meal/MUF resin adhesive. Effects of the adhesive components on the water resistance and formaldehyde emission were measured on three-ply plywood. The viscosity and solid content of the different adhesive formulations were measured. The functional groups of the cured adhesives were evaluated. The results showed that the wet shear strength of plywood bonded by soybean meal/NaOH adhesive increased by 33% to 0.61 MPa after adding NaOH into the adhesive formulation. Addition of PEG reduced the viscosity of the soybean meal/NaOH/PEG adhesive by 91% to 34,489 cP. By using the MUF resin, the solid content of the soybean meal/MUF resin adhesive was improved to 39.2%, the viscosity of the adhesive was further reduced by 37% to 21,727 cP, and the wet shear strength of plywood bonded by the adhesive was increased to 0.95 MPa, which met the interior plywood requirements (≥0.7 MPa). The formaldehyde emission of plywood bonded by the soybean meal/MUF resin adhesive was obtained at 0.28 mg/L, which met the strictest requirement of the China National Standard (≤0.5 mg/L). FTIR showed using the MUF resin formed more  CH₂ group in the cured adhesive. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Influence of acrylamide copolymerization of urea–formaldehyde resin adhesives to their chemical structure and performance

To lower the formaldehyde emission of wood‐based composite panels bonded with urea–formaldehyde (UF) resin adhesive, this study investigated the influence of acrylamide copolymerization of UF resin adhesives to their chemical structure and performance such as formaldehyde emission, adhesion strength, and mechanical properties of plywood. The acrylamide‐copolymerized UF resin adhesives dramatically reduced the formaldehyde emission of plywood. The ¹³C‐NMR spectra indicated that the acrylamide has been copolymerized by reacting with either methylene glycol remained or methylol group of UF resin, which subsequently contributed in lowering the formaldehyde emission. In addition, an optimum level for the acrylamide for the copolymerization of UF resin adhesives was determined as 1%, when the formaldehyde emission and adhesion strength of plywood were taken into consideration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

耐水胶合板用环保乳清蛋白基水性胶粘剂

乳清蛋白是干酪加工过程中所产生的一种副产品,除被用作食品添加剂外,仍有相当数量乳清蛋白并未得到有效利用。为了实现乳清蛋白的高附加值利用,本文以乳清蛋白为原料,研制了具有良好耐久性和环保的乳清蛋白基胶合板用水性胶粘剂,并评价了变性处理、改性剂种类及其用量对乳清胶粘剂的胶合性能及游离甲醛释放量的影响。结果表明,热变性使乳清蛋白胶粘剂的胶接耐久性提高;不同改性剂对乳清蛋白胶粘剂的性能影响不同。采用1％多异氰酸酯改性胶接耐久性最好;采用0．15％戊二醛／1％乙二醛改性胶接强度最高。中试结果表明,所研制的耐水性环保乳清蛋白基胶合板胶粘剂的干胶接强度达到1．98MPa,煮－烘－煮28h后湿胶接强度为1．14MPa,游离甲醛释放量仅为0035mg／L（干燥器法）,达到了1ISK6806--2003环保结构胶合板用胶粘剂要求。     

Plant Polyphenol-Inspired Crosslinking Strategy toward High Bonding Strength and Mildew Resistance for Soy Protein Adhesives

Soybean protein adhesives are environmentally friendly biomass-based aldehyde-free adhesives that have good economic value for the wood industry; however, it remains challenging to produce soybean protein adhesives with excellent water resistance, toughness, and mildew resistance through a simple modification method. In this work, inspired by plant polyphenols, a novel crosslinked soybean meal adhesive (SMPT) is obtained using a facile economic method. Polyamidoamine-epichlorohydrin (PAE) and tannic acid (TA) are combined with a soybean meal matrix to form a tough co-crosslinked network through strong intermolecular forces (covalent bonds, ionic bonds, and hydrogen bonds) in adhesive system. The results show that the wet bonding strength of SMPT adhesives for plywood is 134.1% higher than the unmodified soybean meal adhesive. The adhesion properties met the standard requirements for interior-use plywood. And the compact cross-linking network structure is accelerated the greater energy dissipation, which improves the toughness of adhesive. Moreover, cationic azetidinium groups in PAE and phenol hydroxyl groups in TA synergistically not only exhibit the good antibacterial activities but also improve mildew resistance for SMPT adhesives. This facile strategy provides an economic sustainable method to prepare high-performance environmentally friendly wood adhesives.