Physico-mechanical properties of particleboards bonded with pine and wattle tannin-based adhesives

This study investigated the physical and mechanical properties of particleboards made using two types of tannin-based adhesives, wattle and pine, with three hardeners, paraformaldehyde, hexamethylenetetramine (hexamine) and TN (tris(hydroxyl)nitromethane), by measuring the physical (thickness swelling, linear expansion and water absorption) and mechanical properties (bending strength and internal bond strength). The performance of the particleboards made using tannin-based adhesives was influenced by physical conditions such as press time and temperature as well as by chemical conditions, such as the chemical structure of the tannin and the hardener. Wattle tannin-based adhesive being a thermoset, the wattle tannin-based particleboards were more influenced by physical conditions, while the pine tannin-based particleboards were influenced by the chemical structure of the pine tannin nuclei, which include phloroglucinolic A-rings. The reactivity of the hardener toward the tannin was in the order: paraformaldehyde > hexamine > TN for wattle tannin, while for pine tannin the order was hexamine > paraformaldehyde > TN.     

A new class of tannin adhesives for exterior particleboard

A new class of exterior and semi-exterior grade wattle tannin adhesives for particleboard is presented. Commercial formaldehyde concentrates containing uncondensed methylol ureas are used as hardeners-fortifiers of tannin-based adhesives. Moderate sulphitation of wattle extracts is also used to produce low cost tannin-based adhesives for exterior and semi-exterior particleboard.     

The chemical,kinetic and mechanical characterization of tannin-based adhesives with different crosslinking systems

Network formation, cure characteristics and bonding performance of tannin-based resins were investigated in order to establish structure–property relationships between the stage B and stage C. Tannin–aldehyde and base-catalyzed autocondensed tannin resins were synthesized and characterized for molecular weight distribution, cure kinetics and cure chemistry by means of GPC, DMA and ¹³C CP/MAS NMR spectroscopy and solvent stability tests. The resins performance as wood adhesives was further established from lap-shear tests and microscopic observation of the bondline. Resins prepared with highly reactive aldehydes, such as formaldehyde or glyoxal, exhibited a significant extent of hetero-condensation reactions, fast cure kinetics, a high storage modulus and good solvent stability of the stage C-resin. In contrast, resins prepared with bulky aldehydes of low reactivity, such as citral, were dominated by autocondensation reactions, and exhibited slower cure kinetics, a lower storage modulus and solvent-stability of the stage C-resin, alike those neat autocondensed tannin resins. However, all resin systems fulfilled the standard requirements for wood adhesive bonding for interior applications. Additionally, storage modulus increase during cure was found to be a good predictor of the stiffness of the wood-bonded assembly, useful to discriminate between the autocondensation and heterocondensation cure chemistries.     

Eco-Friendly Adhesives Based on Tannin and N,N-Bis(2-Hydroxy-Ethyl) Fatty Amides (HEFAs) from Non-Traditional Oils for Wood Bonding

Abstract

Wood adhesives were formulated using tannin and N,N-bis(2-hydroxyethyl) fatty amides (HEFAs). The natural tannin-based adhesives can be used to replace formaldehyde-based adhesive systems and thereby reduce formaldehyde and volatile organic compound (VOC) emissions from adhesives used for plywoods. Performance properties of the adhesively bonded wood joints viz., tensile strength, impact strength and chemical resistance were measured. N,N-bis(2-hydroxyethyl) fatty amides (HEFAs) from non-traditional oils were mixed with a pure tannin-based adhesive as a crosslinker, and this increased the tensile strength, impact strength and chemical resistance of wood joints. The results revealed that a high performance and eco-friendly adhesive system for wood can be successfully formulated using tannin and HEFA.     

Laminating wood adhesives by generation of resorcinol from tannin extracts

Resorcinol has been generated in situ by high levels of sulfitation of the tannin extract of the black wattle tree (Acacia mearnsii, formely mollissima). Consequently, weather and boil-proof tannin-based cold-setting adhesives for wood, satisfying the requirements of international standards, have been prepared and optimized by halving the amount of resorcinol chemical added to the tannin. Sulfitation afforded also considerable improvement of the “drying out” time characteristics of these adhesives.     

Effects of Pinus pinaster bark extracts content on the cure properties of tannin-modified adhesives and on bonding of exterior grade MDF

Phenol-urea-formaldehyde-tannin (PUFT) adhesives with different degrees of phenol substitution by Pinus pinaster bark tannins were thermally characterized by dynamic mechanical thermal analysis (DMTA) and by differential scanning calorimetry (DSC). They were employed to prepare exterior grade Medium Density Fiberboards (MDF) according to European Standards. DMTA and DSC experiments showed, first, that the tannin-modified adhesives hardened in the low temperature range (30-110°C) and, second, that increasing the tannin content of the adhesives reduced the curing temperature, obtaining at least the same mechanical strength (stiffness) and higher curing enthalpies (ΔH) than the commercial phenolic resin. Although only the MDF boards made using the lowest viscosity tannin-modified adhesive (PUFT-10), with a 44% phenol replacement by tannin, met the outdoor requirements, all the other tannin-modified adhesives boards met the interior grade specifications. Among the board properties evaluated, the low value of thickness swelling after 24-h water immersion of MDF boards prepared using the PUFT-10 (6.6%) is particularly noticeable, which means an improvement of almost 50% compared to that of the commercial PF (12.6%).     

Tannin-based adhesive cross-linked by furfuryl alcohol-glyoxal and epoxy resins

To prepare a natural tannin-based adhesive with good water resistance, an environment friendly furfuryl alcohol-glyoxal resin (FG) synthesized in the laboratory was developed as a cross-linker for tannin-based adhesives. ¹³C Nuclear Magnetic Resonance (NMR) and Matrix-Assisted Laser Desorption-Ionization Time-of-Flight (MALDI-TOF) mass spectroscopy results indicated that furfuryl alcohol and glyoxal reacted under acidic conditions and that the -CH-(OH)- groups could be shown to be the ones involved in the cross-linking of the tannin-furfuryl-glyoxal adhesive (TFG). The results for the wet shear strength of TFG-bonded plywood showed that the cured TFG was improved and better than that bonded with a tannin-furfuryl alcohol (TF) adhesive. Moreover, the TFG adhesive cross-linked with 12% epoxy resin (EPR) presented a good water resistance. It had a modulus of elasticity (MOE) higher than that of tannin-furfuryl alcohol-formaldehyde (TFF), TF and phenol-formaldehyde (PF) adhesives.     

Mechanism of viscosity variations during treatment of wattle tannins with hot NaOH

The mechanism causing viscosity decrease and subsequent viscosity increase of wattle tannin extract solutions under hot caustic soda treatment during the preparation of tannin-based adhesives for particleboard has been investigated. While the decrease of viscosity appears to be due, according to accepted theory, to hydrolysis of the hydrocolloid gums present in the extract, experiments with (+)-catechin as a tannin model compound indicate that the subsequent slow increase of viscosity of the solution, slow as related to the relatively small amount of alkali used, appears to be due to a tannin autocondensation reaction caused by cleavage of the etherocyclic rings of a certain of the tannins' flavonoid units.     

Shear Refinement of Formaldehyde-Free Corn Starch and Mimosa Tannin (Acacia mearnsii) Wood Adhesives

The aim of this work was to reduce the viscosity of formaldehyde-free corn starch–mimosa tannin wood adhesives, without adversely affecting the mechanical properties of the product. The reduction of viscosity was achieved using shear refinement. The study focused on the physical phenomena before cross-linking of the wood adhesive. The physical (rheological characterization) and mechanical (bond strength) properties of formaldehyde-free corn starch and mimosa tannin wood adhesives were measured. The results showed that the shear refinement (290 rpm and 5 min, optimal conditions) reduced the viscosity of the corn starch–mimosa tannin wood adhesives (from 100 000 to 458 Pa s) with the advantage of being stable over time. Mechanical tests showed that the shear refinement did not influence the mechanical properties of corn starch–mimosa tannin wood adhesives.     

Tannin-based adhesives: new theoretical aspects

New theoretical aspects relating to the manufacture, application and ease of handling of tannin-based phenolic adhesives are discussed. These are: (1) the effect of bivalent and trivalent metallic ions catalysts in the accelerating and/or retarding of phenols-aldehydes and tannins-aldehydes reactions; and (2) the generation in situ of resorcinol from the tannin skeletal structure which leads to reduced levels of resorcinol chemical additions in tannin-based glulam and fingerjointing adhesives. It was found that both these approaches had a pronounced effect on tannin adhesives and can be beneficially introduced in industrial tannin adhesives applications.     

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.     

Preparation and characterization of UV/thermal dual-curable polyurethane acrylate adhesive for inertial confinement fusion experiment

A new type of polyurethane acrylate (APUE) oligomer, which contains both acrylic and epoxy group in its molecular structure, was synthesized. UV/thermal dual-curable adhesives were prepared using various contents of APUE oligomers, reactive diluent, photoinitiators and thermal-curing agent. The curing behaviors of the dual-curable adhesives were investigated using photo-DSC, on-line FT-IR spectroscopy, the determination of gel fraction and dynamic mechanical thermal analysis (DMTA). The reaction rate and extent of UV curing were studied and found to be strongly dependent on the concentration of C=C bonds in the APUE oligomers. The gel fraction was also evaluated as a function of C=C bond concentration, demonstrating that gel fraction increased with increasing C=C bond contents in the APUE oligomers. Moreover, when the dual-curable adhesive was thermally-cured, the gel fraction was largely improved, due to the thermal-curing agent initiating the unreacted epoxy groups to allow the formation of further crosslinking. The adhesion strength at −196 °C increased with increasing C=C bond content. The latent thermal-curing agent contributed to the enhancement of the adhesion strength.     

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.     

Study of Properties of One-Component Moisture-Curable Polyurethane and Silane Modified Polyurethane Adhesives

Abstract

In this paper, the curing behavior and adhesion properties of five kinds of polyurethane (PU) and silane modified polyurethane (SPU) adhesives were studied. Fourier transform infrared spectra showed that the adhesive Sikaflex-555 had an SPU structure and that the other four adhesives were one-component polyurethanes. The experimental results of curing behavior show that the tack-free times and curing rates of these adhesives were significantly affected by the relative humidity and the temperature. The adhesion of the SPU adhesive to glass sheet, zinc galvanized steel sheet and steel sheet was much better than that of PU adhesives. The tack-free times and curing rates of all the adhesives was measured during the 12 months of storage.     

Polycondensation and autocondensation networks in polyflavonoid tannins. I. Final networks

A thermomechanical analysis (TMA) study, directly on a bonded joint during the process of adhesive hardening, on the sinergy and interference between polycondensation and autocondensation on procyanidin, and profisetinidin–prorobinetinidin-type polyflavonoid tannins network formation and hardening, confirmed that also at the higher curing temperatures characteristic of the hardening of tannin-based wood adhesives, hardening by polycondensation can be coupled with simultaneous hardening of tannins by autocondensation. Some coreactants appear to depress the tannin autocondensation while still leaving a small contribution of this reaction to the formation of the final crosslinked network. Other coreactants instead appear to enhance formation of the final network by sinergy between the 2 condensation mechanisms, while still others do not show any interference between the 2 types of reaction. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1083–1091, 1998     

UV curable pressure‐sensitive adhesives for fabricating semiconductors. II. The effect of functionality of acrylate monomers on the adhesive properties

In an attempt to control the adhesive properties of acrylic copolymer‐based pressure‐sensitive adhesives, a series of multifunctional acrylate monomers were added and UV cured. The adhesive compound with a difunctional monomer had increased peel strength after UV curing. On the other hand, the compound with a tri‐ or more functional (polyfunctional) monomer had markedly decreased strength after UV curing. Those adhesives containing any polyfunctional monomer also showed much higher storage modulus than an adhesive containing a difunctional monomer. The greater volume contraction of UV‐cured polyfunctional monomer suggested microvoids at the interface between the adhesive layer and the adherent, resulting in poor strength. Estimated values of the peel strength of UV‐cured adhesives according to the theoretical equations proved that the strength is approximately inversely proportional to the elastic moduli. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2889–2895, 2004     

CHT and TTT curing diagrams of polyflavonoid tannin resins

TTT and CHT curing diagrams for tannin-based adhesives were built by thermomechanical analysis (TMA) by following the in situ hardening directly in a wood joint, and the curve trends observed were similar to those previously observed for synthetic polycondensation resins on lignocellulosic substrates. Of the parameters that most influence the relative position of vitrification and gel curves on the diagrams (i.e., where the influence has been quantified), chief among them is the reactivity of the tannin with formaldehyde and any factor influencing it: thus, the inherent higher reactivity of the A-ring of the tannin (such as in procyanidins versus prorobinetinidins) and the pH of the tannin solution. The percentage formaldehyde hardener has some influence in CHT diagrams, especially for the slower-reacting tannins, but practically no influence in TTT diagrams within the 4–10% formaldehyde range used. As in the case of synthetic polycondensation adhesive resins, regression equations relating the internal bond strength of a wood particleboard, prepared under controlled conditions, with the inverse of the minimum deflection, obtained by constant heating rate TMA of a wood joint during resin cure, have been obtained for the two types of tannins of lower reactivity (profisetinidins/prorobinetinidins) but not for the faster-reacting procyanidin and prodelphinidin tannins. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3220–3230, 2001     

UV curable pressure‐sensitive adhesives for fabricating semiconductors. I. Development of easily peelable dicing tapes

In order to develop easily peelable dicing tapes from diced wafers, UV curing of various pressure‐sensitive adhesives (PSAs) was studied. After UV irradiation, the adhesive strength of a PSA composition including a diacrylourethane oligomer (UDA) decreased drastically compared with other compositions. Because of network formation via UV irradiation, this composition had a greater volume contraction that might yield microvoids at the interface between the adhesive and the wafer, resulting in the loose adhesion. Its storage modulus increased up to about 1000 times that before UV curing, which was due to the crosslinking of the UDA component. It was suggested that the increased crosslinking density and the high internal coagulant energy of the UDA backbone structure caused a remarkable decrease of the adhesive strength. Furthermore, it was ascertained that the UV‐irradiated UDA adhesives left few residual deposits on the wafer released from the tape. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 436–441, 2003     

Studying the effects of chain extenders chemical structures on the adhesion and mechanical properties of a polyurethane adhesive

Chain extenders including diethanol amine (DEA) and 2-cyano-N,N-bis-hydroxyethyl acetamide (CNBA) were utilized to enhance the adhesion and mechanical properties of a polyurethane (PU) adhesive. The adhesion and mechanical properties of the adhesives were studied by peel test, tensile test and dynamic thermal analysis (DMTA) respectively. Results revealed that both chain extenders (DEA and CNBA) increased peel test modulus. The cross-linking density and toughness of the adhesive were significantly increased using chain extenders. Results showed that chain extender without side chain (DEA) could improve the adhesion and mechanical properties of the adhesive greater than chain extender with aliphatic side group (CNBA).     

协同交联型大豆胶黏剂的流变与固化性能研究

利用饲料级大豆粉为原料,通过水性聚酰胺和异氰酸酯对其进行协同复合改性制备出具有良好流变行为和固化性能的大豆胶黏剂。采用旋转流变仪研究了复合改性剂用量对大豆胶黏剂流变行为和固化性能的影响,结果表明：改性后的大豆胶黏剂属于假塑性流体,当水性聚酰胺添加量为10%时,所得大豆胶黏剂的流变性能较优,而异氰酸酯添加量对大豆胶黏剂的流变行为几乎没有影响;运用动态温度扫描模式研究大豆胶黏剂的黏弹性能,异氰酸酯添加量对大豆胶黏剂的储能模量和损耗模量影响较大。水性聚酰胺/异氰酸酯协同交联体系可与大豆蛋白分子间发生交联,当10%水性聚酰胺和4%异氰酸酯协同改性时,所得胶合板的胶合强度可达0.74 MPa,满足国家Ⅱ类胶合板使用要求（≥0.70 MPa）。