Wood adhesives based on tannin extracts from barks of some pine and spruce species

Barks available in commercial quantities in Australia and overseas have been examined for their efficacy as raw materials for conversion to high quality adhesives for reconstituted wood products. Previously bark from maturePinus radiata was found to be suitable. This paper examines the suitability of barks from four mature pine species (Pinus caribaea, Pinus elliottii, Pinus pinaster andPinus sylvestris), one young pine species (Pinus radiata) and one spruce species (Picea abies). Only the bark extracts ofPinus caribaea andPinus pinaster gave high quality (Type A bond, WBP) wood adhesives. The gluing properties of the adhesives derived from the extracts appeared to be dependent on their contents of formaldehyde-reactive polyflavanoids as indicated by their Stiasny values, with a value of 65% being the minimum for producing a high quality adhesive by the methods used.     

Low formaldehyde emission,fast pressing,pine and pecan tannin adhesives for exterior particleboard

Exterior grade particleboard satisfying the requirements of the V100 test, of formaldehyde emission well within the emission standard E1 class, and of pressing times as fast as 8.5 sec/mm can be produced at higher moisture contents using fast-reacting non-modified and non-fortified procyanidin/prodelphinidin tannin extracts such as pine bark tannin extract and pecan nut pith tannin extract. Such results, under the conditions reported are not achieved with the slower reacting tannins traditionally associated with the practice of tannin adhesives. The results reported establish that the faster is the reaction of the tannin with formaldehyde, the lower is the amount of formaldehyde needed to achieve V100 standard, the lower is the formaldehyde emission, the faster is the pressing time achievable and the higher is the tolerance of the adhesive to high moisture content of the resinated wood particles.     

Lignin-based cold setting wood adhesives structural fingerjoints and glulam

A traditional-type cold-setting wood adhesive as well as separate applications fast-set wood adhesives for exterior-grade structural fingerjoints and glulam based on soda bagasse lignin were developed and tested. Their results satisfy the requirements of the relevant international specifications. The soda bagasse lignin used composed approximately 75% of the total adhesive solids while resorcinol content as low as 13.6% on liquid resin was used, with good results. On top of their novelty and of their excellent performance these adhesives appear to be economically very attractive.     

Honeymoon fast-set adhesives for glulam and fingerjoints of higher natural materials content

Addition of flavonoid tannin extract to the PRF component A of a PRF/Tannin honeymoon fast set adhesive system for glulam and fingerjointing yields (PRF?+?Tannin)/Tannin adhesive systems where the total percentage of natural material can be as high as 65% (2/3) of total adhesive resin solids without any loss of either long term performance or fast curing rate.     

Factory trials of Chinese wattle tannin adhesives for antislip plywood

As the final step in the series of studies on the development of Chinese wattle tannin adhesives, factory trials were carried out in order to evaluate the feasibility of this technology for producing exterior grade plywood in China. More than 1 tonne of wattle tannin adhesives were formulated using PF and PUF resins developed as cross-linking agents. A total of 460 sheets of plywood (1830 × 915 × 9 mm) were produced during four and a quarter working shifts totalling 34 working hours in the plywood factory. The results of gluebond assessments strongly indicate that the technology developed would be extremely feasible for application in the Chinese plywood manufacturing industry.     

Potential of tannin extracts as resorcinol replacements in cold cure thermoset adhesives

Blends of phenol resorcinol formaldehyde (PRF) resin with tannin obtained from bark offers the potential to minimise the quantity of resorcinol required in conventional two pot, cold set PRF wood adhesives. In the presence of ammonia, both quebracho and radiata pine bark extracts, resorcinolic and phloroglucinolic type tannins respectively, showed accelerated adhesive cure behaviour with formaldehyde, in a similar manner to PRF adhesive Greenweld technology for modifying glued timber processing rates. Segregation of pine bark components established that only tannin-containing fractions exhibited accelerated adhesive cure behaviour. Results from adhesive testing indicated adhesive blends of quebracho extract and PRF resin failed to attain satisfactory bond strength at full cure. Of the pine bark components, only a pure tannin fraction produced acceptable bond strength when blended with PRF (3:1). The study indicated pine bark extract can be substituted into PRF adhesives and adhesive cure rate accelerated using ammonia. However, extracts containing crude tannin or carbohydrate materials in adhesive blends led to inferior adhesives.     

Chinese wattle tannin adhesives suitable for producing exterior grade plywood in China

Wattle tannin adhesives were successfully formulated from tannin extracts, which were commercially produced from black wattle (Acacia mearnsii) bark in China. Using the Chinese wattle tannin adhesives a total of 153 plywood panels were prepared in order to study effects of pH, paraformaldehyde, glue spread, hot pressing temperature and time, and closed assembly time on bonding strength and their bonding quality was assessed according to the Chinese Standard for exterior grade plywood. Statistical analysis results revealed that Chinese wattle tannin can be used for producing exterior grade plywood under Chinese plywood factory conditions. One of the specific characteristics of these Chinese wattle tannin adhesives was that their use resulted in plywood of high quality bonding over the wide range of closed assembly time from 30 minutes to 16 hours. These Chinese wattle tannin adhesives have a great potential for commercialization in China.     

Synthetic-resin-free wood panel adhesives from mixed low molecular mass lignin and tannin

Mixed interior wood panel tannin adhesive formulations were developed in which lignin is in considerable proportion, 50%, of the wood panel binder and in which no “fortification” with synthetic resins, such as isocyanates and phenol-formaldehyde resins as used in the past, was necessary to obtain results satisfying relevant standards. A low molecular mass lignin obtained industrially by formic acid/acetic acid pulping of wheat straw was used. Environment-friendly, non-toxic polymeric materials of natural origin constitute up to 94% of the total panel binder. The wood panel itself is constituted of 99.5% natural materials, the 0.5% balance being composed of glyoxal, a non-toxic and non-volatile aldehyde, and of hexamine already accepted as a non-formaldehyde-yielding compound when in presence of condensed tannin. Both particleboard and two types of plywood were shown to pass the relevant interior standards with such adhesive formulations. Moreover, the much cheaper non-purified organosolv lignin showed the same level of results as the more expensive purified type.     

Hexamine hardener behaviour: effects on wood glueing, tannin and other wood adhesives

13 C NMR evidence is presented confirming that the main decomposition (and recomposition) mechanism of hexamine is not directly due to formaldehyde but rather proceeds through now-identified intermediates, i.e. mainly through the formation of reactive imines rather than methylene bases, possibly also forming a very slight amount of iminomethylene bases. This also confirms that any species with strong real or nominal negative charge under alkaline conditions, be it a tannin, resorcinol or other highly reactive phenols, be it melamine or another highly reactive amine or amide, or an organic or inorganic anion, it is capable of reacting with the intermediate species formed by decomposition (or recomposition) of hexamine far more readily than formaldehyde explaining the capability of wood adhesives formulations based on hexamine to give bonded panels of extremely low formaldehyde emission. If no highly reactive species with strong real or nominal negative charge is present, then decomposition of hexamine proceeds rapidly to formaldehyde formation as reported in previous literature. The elucidation of the hexamine decomposition mechanism, which is presented, and a scanning electron microscopy (SEM) investigation also allowed to advance a reason for the without-curing-formation of ambient temperature stiff gels in tannin/hexamine glue mixes and to propose chemical structures for the ionic coordination linear polymers formed.

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13 C NMR-Spektren machten deutlich, da? der haupts?chliche Zersetzungs- und Wiederaufbaumechanismus des Hexamins nicht unmittelbar über Formaldehyd, sondern über jetzt identifizierte Zwischenverbindungen verl?uft, d.h. überwiegend über die Bildung von reaktiven Iminen anstatt über Methylenbasen; nur ein sehr geringer Anteil an Iminomethylenbasen kann dabei ebenfalls entstehen. Die Untersuchungen zeigten auch, da? jede Komponente mit starker realer oder nomineller negativer Ladung unter alkalischen Bedingungen in der Lage ist, mit den Zwischenprodukten beim Abbau und Wiederaufbau des Hexamins viel schneller zu reagieren als Formaldehyd. Das gilt sowohl für Tanrin, Resorcin oder andere hochreaktive Phenole, als auch für Melanin oder andere hochreaktive Amine oder Amide, und allgemein für organische oder anorganische Anionen. Dies erkl?rt die Besonderheit, da? mit Holzklebern unter Hexaminzusatz Platten mit extrem niedriger Formaldehydemission hergestellt werden k?nnen. Wenn keine Komponente mit starker negativer Ladung im Harzgemisch vorhanden ist, dann führt der Hexaminabbau rasch zu Formaldehyd wie es bisher in der Literatur beschrieben wurde. Die Aufkl?rung des Mechanismus des Hexaminabbaus sowie Rasterelektronenmikroskopische Untersuchungen lieferten zudem eine Begründung für die Bildung fester Gele (ohne Aush?rtung) bei Umgebungstemperatur. Daraufhin konnte auch eine chemische Struktur für die gebildeten ionischen linearen Polymere vorschlagen werden.

     

Tannin-based adhesives for bonding high-moisture Eucalyptus veneers: Influence of tannin extraction and press conditions

The alkaline extraction of tannins from Pinus pinaster bark was studied as regards the influence of solid/liquid ratio, extraction time and NaOH concentration on the yield and Stiasny number of the extracts precipitated at pH 2. The extract with the highest Stiasny number(97) was copolymerized with a previously prepared resol to afford a resin with a 1∶1 phenol/tannins ratio. Plywood boards of Eucalyptus globulus bonded with this resin at board humidities of 10 or 16%, temperatures of 130 or 185°C and press times of 5 or 8 min all passed the WBP knife test (with one exception for the longer press time). We conclude that this resin is superior to commercial phenol-formaldehyde resins as regards its tolerance to moisture in plies and adhesive, its press time requirement, and its ability to bond veneers of woods known for their high levels of adhesive-repellent substances.     

Increased pine tannins extraction and wood adhesives development by phlobaphenes minimization

The phlobaphenes formation and precipation during sulphite/water extraction of pine tannins from pine bark was minimized by blocking tannin self-condensation by the addition of small amounts of a strong nucleophile such as phloroglucinol, m-phenylenediamine and urea, the latter due to its low cost for industrial application. These inhibitors reacted preferentially with the reaction intermediates formed in the tannins self-condensation mechanism, stopping the reaction reaching the molecular rearranged precipates known as phlobaphenes. The principle of inhibition appears to be effective at both the laboratory and industrial level. The results of laboratory and industrial extractions and the results of the adhesives from the modified pine tannin extract/urea adducts obtained are presented. The pine tannin/urea extracts proved to yield good thermosetting wood adhesives for panel products. The minimization of phlobaphenes precipitation increased the yield from 19% to 25% industrial scale.     

Evaluation of formaldehyde emission of pine and wattle tannin-based adhesives by gas chromatography

Formaldehyde emissions from wood particleboards bonded with pine and wattle tannin-based adhesives, using hardeners paraformaldehyde, hexamethylenetetramine and TN (tris(hydroxyl)nitromethan), were measured by the perforator method (DIN EN 120—European Committee For Standardization 1991). All particleboards made using the wattle tannin systems with the three different hardeners were satisfied grade E₁, while in the case of pine tannin only the use of the hexamine hardener led to grade E₁ being satisfied. This tendency was attributed to the curing mechanism of the hardener, the reactivity of the tannin molecule toward formaldehyde and the fast reactivity toward formaldehyde of pine tannin, due to the inclusion of phloroglucinolic A-rings within its molecular structure. Hexamine was effective at reducing formaldehyde emission in tannin-based adhesives when used as the hardener. Being a more sensitive and advanced method, it was also used. The samples used for gas chromatography were gathered during the perforator method. The formaldehyde emission levels obtained from gas chromatography were similar to those obtained from the perforator method.     

Deactivation of tannin in high tannin milo by treatment with urea

Experiments were conducted to determine the effectiveness of urea in deactivating tannin in high tannin milo. High tannin milo (Pioneer B 815, 3.4 +/- .3% tannin) was reconstituted with aqueous urea solutions to give combinations of 26, 30, and 34% moisture with 2, 3, and 4% urea (percentage of urea per dry weight of milo). All treatments were maintained at 25 degrees C and were effective in deactivating tannin with no differences among moisture or urea content. The average rate of tannin deactivation was 68 +/- 2% d-1. Temperature affected rate of tannin deactivation in milo reconstituted to 30% moisture and 3% urea when stored at 25 or 60 degrees C. Rate of tannin deactivation was 44 +/- 5 and 89 +/- 18% d-1 at 25 and 60 degrees C, respectively. Tannin in high tannin milo can be deactivated rapidly and completely by reconstitution with aqueous urea. These studies demonstrate that tannin is deactivated completely under conditions where urea is an effective preservative of high moisture milo.