Wood/adhesive interactions and the phase morphology of moisture-cure polyurethane wood adhesives

Two moisture-cure polyurethane adhesives (PURs) were studied: a model (MPUR) prepared from a symmetrical diisocyanate that gave rise to a continuous soft phase, and a commercially relevant adhesive (CPUR) prepared from an asymmetrical polyisocyanate that resulted in a continuous hard phase. Atomic force microscopy revealed that the size and size distributions of the respective dispersed phases were altered by wood, as observed in localized specimen regions. Bulk effects were observed with dynamic mechanical analysis; both soft and hard phase MPUR relaxations were altered by wood (increased transition temperatures), whereas only the continuous CPUR hard phase revealed wood-induced changes. Furthermore, infrared analysis showed that wood promoted the formation of hydrogen-bonded urea structures in CPUR bondlines. Significant wood/PUR interactions were detected, however it was not possible to determine if the effects arose from direct intermolecular associations, or from indirect effects arising from the mere presence of wood and how this impacts forces acting upon the liquid adhesive (i.e. moisture transport, differing interfacial energies through which transport occurs, and CO₂ bubble nucleation). Regardless, it is clear that PUR studies should be conducted under conditions that simulate real wood/PUR bondlines.     

Characterization of canola oil based polyurethane wood adhesives

A novel bio-based poly (ether ester) polyol containing both primary and secondary functional groups was synthesized from canola oil using a low cost and efficient procedure. In this work, use of the new canola oil derived polyol for the production of polyurethane (PU) adhesives was demonstrated. The canola oil based PU adhesives had similar or better adhesive properties in terms of lap shear strength than three commercial PU adhesives. The effect of NCO/OH ratio and temperature on adhesive characteristics on wood bonding was also evaluated by lap shear tests. It was found that the use of an elevated curing temperature (i.e. 100 °C), as well as optimized NCO/OH molar ratio (higher than 1.5/1.0), improved the wood adhesive properties. The overall chemical resistance of bio-based PU adhesives to cold water, acid and alkali was comparable to that of commercial PU adhesives whilst its resistance to hot water was superior.     

单组分聚氨酯反应型热熔胶的研制

以异氰酸酯和多元醇为主要原料合成了系列单组分反应型湿固化聚氨酯热熔胶(PUR),考察了不同的软段结构、-NCO含量、固化时间、催化剂和增粘树脂等对其性能的影响。研究结果表明,采用自制的多元醇,设定w(-NCO)≈3.5%,当w(催化剂)=0.1%、w(增粘树脂)=20%时,所得PUR的综合性能较好,开放时间为7min,固化时间为24h,拉伸强度达到9.3MPa,初粘强度达到0.44MPa,剪切强度达到7.9MPa。     

Influence of wood extractives on two-component polyurethane adhesive for structural hardwood bonding

ABSTRACT

When bonding wood for structural applications, the wood–adhesive bond is influenced by a variety of factors. Besides the physical and mechanical properties of wood species, their chemical composition, e.g. wood extractives, can play a role in bonding wooden surfaces. A two-component polyurethane system (2C PUR) was chosen to better adapt to the current adhesion problem. The influence of extractives on crosslinking was determined by Attenuated Total Reflection-Fourier Transform Infrared Spectrometer (ATR-FTIR) and on the rheological behavior in terms of gel point and storage modulus. Therefore, 2C PUR was mixed with 10% of eight common wood extractives separately. Furthermore, the mechanical properties of beech wood (Fagus sylvatica L.) bonded with extractive enriched adhesive were tested by means of tensile shear strength tests and evaluation of wood failure. These results of ATR-FTIR clearly show that the majority of crosslinking was terminated after 12 hr. Acetic acid and linoleic acid expedited the isocyanate conversion during the first 2.5 hr. The curing in terms of gel point and storage modulus of 2C PUR was accelerated by starch, gallic acid, linoleic acid, and acetic acid. Heptanal, pentanal, 3-carene, and limonene decelerated the curing. All extractives lowered the storage modulus determined after 12 hr. The bonding of beech wood with extractive–adhesive blends showed a slight decrease of the mechanical properties, with the exception of a marginal increase in the case of linoleic acid and pentanal.

In summary, it can be said that 2C PUR is sensitive to the influence of wood extractives and can therefore be partly held responsible for adhesion problems occurring when extractives in surface-wide and higher contents are available.     

The role of polyol reaction catalysts in the cure kinetics and mechanical behavior of polyurethane adhesives

ABSTRACT

Polyurethane is one of the most versatile and commonly applied polymers in the world. Its properties are strongly dependent on the polyol/isocyanate structures and NCO:OH ratio, among other parameters. Additionally, the polyol structure is affected by the catalyst used for the reaction and production. The catalyst may indirectly affect the curing kinetics and mechanical strength of the PU, due to changes of polyol properties. This work evaluates the effects of two catalysts on the polyol structure and thus the effect of these polyol changes on the curing kinetics and mechanical strength of a PU adhesive. Two polyols were synthetized using the same hydroxyl-containing molecules derived from renewable natural sources, but with different types of catalysts (based on Sn or Li/Ti). The synthesized polyols were separately mixed with diphenylmethane diisocyanate in a 1.1:1 ratio of NCO to OH to form two bio-based PUs. The polyols were investigated by Fourier transform infrared (FTIR) spectroscopy, which showed the occurrence of some changes in the chemical groups of the polyol structure for both catalysts, as well as an influence on the curing kinetics. The mechanical behavior of the PU adhesives was characterized by dynamic mechanical analysis (DMA) and peeling tests. The results show a higher curing rate and higher peeling strength for the PU adhesives obtained using the Sn-based catalyst; both adhesives showed cohesive failure.     

Influence of the chemical structure of PUR prepolymers on thermal stability

The thermal stability of adhesives for load-bearing construction has been one of their key parameters since engineered wood products were introduced in timber construction. In the case of one-component moisture-curing polyurethane (1C PUR) adhesives, knowledge about relationships between their chemical structure and the resulting bonding properties is limited, especially under high-temperature conditions. In this study the structure-property relationships of 1C PUR prepolymers were analyzed in the temperature range from 20 to 200 °C by means of mechanical and rheological tests. NCO-terminated urethane prepolymers were prepared from systematically varied MDI and polyether mixtures. The structural parameters investigated were the urea and urethane group content, cross-link density, ethylene oxide content and the adjustment of functionality via NCO or polyether component. Bonded wood joints were tested for their tensile shear strength and polymer films were analyzed by means of DMA and DSC. The results revealed a significant influence of hard segment content and cross-link density on the thermal stability of the prepolymers. Not all parameters that affect the film properties significantly influence bonding.     

Liquid‐type nucleating agent for improving thermal insulating properties of rigid polyurethane foams by HFC‐365mfc as a blowing agent

The effects of liquid‐type additives on the morphology, thermal conductivity, and mechanical strength of polyurethane (PUR) foams were investigated. The PUR foams synthesized with perfluoroalkane showed a smaller average cell diameter and a lower thermal conductivity than PUR foams prepared with propylenecarbonate or acetone. The average cell diameter of the PUR foams decreased from 228 to 155 μm and the thermal conductivity decreased from 0.0227 to 0.0196 kcal/mh °C when the perfluoroalkane content was 0.0 to 2.0 php (parts per hundred polyol by weight). The perfluoroalkane likely acted as a nucleating agent during the formation of the PUR foams. The addition of perfluoroalkane induced the smaller cells size of the PUR foams probably due to lower surface tension of the polyol and perfluoroalkane mixture, resulting in high nucleation rate. The smaller cell size appears to be the main reason for the improvement in the thermal insulating and the mechanical properties of these PUR foams. The compressive strength of the PUR foams prepared with perfluoroalkane was higher than the PUR foams prepared with the propylenecarbonate and acetone. Based on the morphology, thermal conductivity, and compressive strength, it is suggested that the perfluoroalkane is an efficient liquid‐type additive for the improving the thermal insulation of PUR foams. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43557.     

Polyurethane wood adhesive from palm oil-based polyester polyol

Polyurethane (PU) adhesives for wood bonding were prepared from palm oil-based polyester polyol in a solventless condition that reduces the risk of volatile organic compounds to human health and the environment. The polyester polyol was synthesized from epoxidized palm olein prior to reacting with polymeric 4,4′-methylene diphenyl diisocyanate (pMDI) and toluene 2,4-diisocyanate (TDI) to produce wood-bonding PU adhesives. The effect of glycerol cross-linker, dibutyltin dilaurate catalyst and NCO/OH ratio on lap shear strength and pot life of the PU adhesives were studied. The green strength of the PU adhesives was achieved on day 4 for TDI-based adhesives and day 5 for pMDI-based adhesives. The newly formulated PU adhesives have superior chemical resistance in cold water, hot water, acidic medium and alkaline medium by only showing light deterioration (2–8%) in lap shear strength. The PU adhesives prepared from pMDI exhibited higher lap shear strength and thermal stability as compared to adhesives prepared from TDI adduct. Both adhesives have improved mechanical performance (two folds higher in lap shear strength) as compared to commercial wood bonding adhesives.     

Influence of humidity and frequency on the energy dissipation in wood adhesives

In this paper, the frequency dependent energy dissipation of typical wood adhesive under cyclic stress was studied on film adhesive samples. Three moisture-curing one component polyurethane (1C-PUR) adhesives with relative ductile behavior, one melamine formaldehyde (MF) and one phenol formaldehyde resorcinol (PRF) adhesives both with a more brittle behavior were prepared to study the viscoelastic properties at different relative air humidities (RH). Dynamic Mechanical Analysis (DMA) in tensile mode was used to determine loss modulus, storage modulus and loss factor Tan Delta on free standing adhesive films. It has been shown that 1C-PUR adhesives dissipate proportional more of the stored energy than MF and PRF adhesives. Humidity increased the dissipative processes in all PUR adhesives, especially in the polyamide fiber filled adhesive. PRF adhesive is less influenced by humidity. While for all other tested adhesives the dissipative processes generally increased with higher humidity, humidity decreased the damping of the investigated MF adhesive. The influence of the frequency on the energy dissipation is low for all tested adhesives in the investigated frequency range. Further fatigue tests with glued wood samples are needed to confirm the results observed on the free standing adhesive films.     

Eco-economical polyurethane wood adhesives from cellulosic waste: Synthesis, characterization and adhesion study

This study reports the preparation of polyurethane adhesives using polyols obtained from cellulosic waste and detailed study on its adhesive strength in wood joints. Keeping in view the environmental hazards related to the huge paper-waste generation across the world, low-viscosity polyols have been prepared using magazine paper waste and vegetable oils with different physicochemical properties and were used to prepare two-component polyurethane adhesives for wood bonding. Polyurethane was analyzed by FTIR spectroscopy and TGA was used for the analysis of thermal properties. The adhesive strength was measured and compared with commercially available adhesives under different environmental conditions. The synthesized adhesive with NCO/OH ratio of 1.2 and curing time of 5 days was found to be superior to the commercial adhesives Fevicol™ and Araldite™ when compared simultaneously for the single-lap shear strength in different environmental conditions.     

High‐build alkyd urethane coating materials with a partially solvolyzed waste polyurethane foam

Solvent‐borne alkyd urethane coating compositions containing 20 or 40 wt% (on solids) of partially solvolyzed waste polyurethane foam (PUR) were prepared and analyzed. Partial solvolysis of the soft polyurethane foam was carried out via extrusion with either 5 or 10 wt parts of solid polyol, that is, trimethylolpropane (TMP) or tris(hydroxymethyl)aminomethane. The addition of a partially solvolyzed PUR significantly affected the properties of cured coats, such as adhesion to a steel substrate, hardness, abrasion resistance, indentation depth, water absorption, tensile strength, and thermal stability. Moreover, the viscosity of ready‐to‐use coating compositions (containing a diisocyanate hardener) depended on a type and content of the processed PUR. Generally, a set of the most positively upgraded properties (viscosity, hardness, tensile strength, and thermal stability) was observed for coating compositions containing 20 wt% of PUR extruded with a higher dose of TMP (i.e., 10 wt parts of TMP/100 wt parts of waste polyurethane foam). POLYM. ENG. SCI., 55:2174–2183, 2015. © 2015 Society of Plastics Engineers     

New compounds for production of polyurethane foams

A method of boroorganic compound preparation with boric acid, 1,3‐propanediol, 2,3‐butanediol, and 1,4‐butanediol is described in this article. The obtained compounds were characterized with respect to their usability as polyol components for the production of polyurethane (PUR) foams. New boroorganic compounds were applied as polyol components for the foaming of rigid PUR–polyisocyanurate (PIR) foams. The method of preparation, foaming parameters, and physicochemical properties of the PUR–PIR foams and their results are presented. Application of the prepared borates as polyol components in the production of foams had a favorable effect on the properties of the foams. The obtained rigid foams were characterized by lower brittleness, higher compressive strength, content of closed cells, and considerably reduced flammability in comparison with standard foams. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5918–5926, 2006     

硅烷Y9669改性湿固化聚氨酯热熔胶的研制

以4,4′-二苯基甲烷二异氰酸酯(MDI)、聚醚多元醇(N-210)和聚己二酸己二醇酯(PHA)为主要原料,控制R=n(-NCO)/n(-OH)=2.0,制得PUR(湿固化聚氨酯热熔胶)的预聚体;然后以不同的硅烷偶联剂[如Y9669(N-苯基-γ-氨丙基三甲氧基硅烷)、KH-550和KH-560等]作为PUR中部分端-NCO基的封端剂,制备SPUR(硅烷化PUR)。结果表明:Y9669是较理想的封端剂,能使SPUR的剪切强度增加37.23%;SPUR的硬度、热稳定性能随Y9669封端率增加而增大;SPUR的剪切强度随Y9669封端率增加呈先升后降态势,并且在Y9669封端率为20%时相对最大(17.73 MPa)。     

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde and polyurethane adhesives

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde (MUF) and polyurethane (PUR) adhesives was monitored by measuring their rheological properties by means of a rheometer. Instead of the standard aluminium discs, wooden discs, made from heat-treated wood with different degrees of thermal modification and conditioned in different climates, were used. The wooden discs provided more realistic curing of the adhesives compared to the real-life bonding of wood, because of solvent absorption. The results of the rheological measurements suggested that the modified wood inhibited the curing of MUF and PUR adhesives. The curing of the MUF adhesive was slower because of the reduced absorption of water from the adhesive. The curing of the one-component PUR adhesive was affected by the lower moisture content (MC) of the modified wood.     

书刊装订联动线用反应型聚氨酯热熔胶的研制

以聚酯多元醇、聚醚多元醇为软段,二异氰酸酯为硬段,辅以多种添加剂,制得一种性能优异的高结晶性单组分反应型聚氨酯(PUR)热熔胶。探讨了二异氰酸酯与多元醇的组成、投料比、扩链剂和添加剂等对产品性能的影响。结果表明:选用聚己二酸丁二醇酯二醇(PBA)为软段、1,4-丁二醇(1,4-BDO)为扩链剂以及4,4′-二苯基甲烷二异氰酸酯(MDI)为硬段,当R=n(-NCO):n(-OH)=(1.5～2.0):1.0、w(-NCO)=1%时,PUR热熔胶的软化点为85～95℃,开放时间为3～9 min,符合书刊装订联动线使用的技术要求,具有很好的应用前景。     

Influence of the filler material on the thermal stability of one‐component moisture‐curing polyurethane adhesives

Filler materials are part and parcel for the adjustment of adhesives, in particular, their rheological and mechanical properties. Furthermore, the thermal stability of adhesives can be positively influenced by the addition of an expedient filler, with inorganic types common practice in most cases. In this study, one‐component moisture‐curing polyurethane adhesives for engineered wood products based on isocyanate prepolymers with different polymer‐filled polyether polyols were investigated with regard to the filler's potential to increase the thermal stability of bonded wood joints. The property changes due to the addition of fillers were determined by means of mechanical tests on bonded wood joints and on pure adhesive films at different temperatures up to 200°C. Additional analyses by atomic force and environmental scanning electron microscopy advanced the understanding of the effects of the filler. The tested organic fillers, styrene acrylonitrile, a polyurea dispersion, and polyamide, caused increases in the cohesive strength and stiffness over the whole temperature range. However, the selected filler type was hardly important with regard to the tensile shear strength of the bonded wood joints at high temperatures, although the tensile strength and Young's modulus of the adhesive films differed over a wide range. Prepolymers with a lower initial strength and stiffness resulted in worse cohesion, in particular, at high temperatures. This disadvantage, however, could be compensated by means of the filler material. Ultimately, the addition of filler material resulted in optimized adhesive properties only in a well‐balanced combination with the prepolymer used. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Rubber solution adhesives for wood-to-wood bonding

Rubber solutions were prepared and used for bonding wood pieces. The effect of the variation of chlorinated natural rubber (CNR) and phenolformaldehyde (PF) resin in the adhesive solutions on lap shear strength was determined. Natural rubber and neoprene-based adhesive solutions were compared for their lap shear strength. The storage stability of the adhesive prepared was determined. The change in lap shear strength before and after being placed in cold water, hot water, acid, and alkali was tested. The bonding character of these adhesives was compared with different commercially available solution adhesives. The room-temperature aging resistance of wood joints was also determined. In all the studies, the adhesive prepared in the laboratory was found to be superior compared to the commercial adhesives. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1185–1189, 1998     

车灯用反应型聚氨酯热熔胶的研制

以多元醇、二苯基甲烷4,4′-二异氰酸酯(MDI)为原料,合成了湿固化反应型聚氨酯热熔胶黏剂用于汽车车灯粘接,研究了胶黏剂的黏度、黏度稳定性和初黏力等性能。结果表明,当高结晶性聚酯和热塑性聚酯用量在10%和15%时,胶黏剂剥离强度达到980N/25mm,耐热性达到180℃,满足了汽车灯具密封与装配的要求。     

夹层玻璃用高透明性聚氨酯热熔胶的研制

选用不同结构多异氰酸酯、低聚物多元醇、1,4-丁二醇为主要原料,用双螺杆挤出机法制备了可用于夹层玻璃的高透明性聚氨酯热熔胶.讨论了多元醇种类及其相对分子质量、二异氰酸酯等原料对聚氨酯热熔胶的光学透明性、力学性能、粘接强度的影响.结果表明:由IPDI、1,4-丁二醇、相对分子质量为1000的PTMG所制备的聚氨酯热熔胶具...     

Characterization of the ageing process of one-component polyurethane moisture curing wood adhesive

The aging resistance of five different one-component polyurethane (1C-PUR) adhesives with different mechanical properties has been investigated. The glue layer has been modeled as a superposition of three different layers representing the pure adhesive layer, the interaction layer between the wood and the adhesive and the plain wood layer. The modification of the mechanical properties of each layer was studied with a specific specimen for each layer. Also, a comparative analysis between natural and artificial weathering was conducted. The artificial weathering consists of a cyclic hydro-thermic treatment (treatment A5 according to DIN EN 302-1). The influence of a long time (5 months) mid-range temperature (70 °C) treatment was tested. Additionally, Fourier transform infrared spectroscopy (FTIR) analyses were conducted to determine the hypothetical chemical modification of the bond line. Generally, the strength properties of the samples decreased with the duration of the artificial weathering. However, significant differences were observed between the adhesive of the same system, especially during the temperature treatment. Furthermore, no relevant chemical degradation of the bond line was measured after 5 years of natural weathering (tested from 10 mm thick glued samples). The chosen method found that in the majority of cases, the wood is the weak layer in the bond line. Therefore, for moisture and temperature solicitation, there are no reasons to doubt the durability of the 1C-PUR glue layer for long term use. Nevertheless, further investigations are needed to better characterize the life expectancy of the wood and adhesive interface layer and to establish the influence of parameters, such as UV-radiation, wood extractives and moisture-induced stress, on the life expectancy of the bond line.