Tannin Adhesives Research Articles

Autocondensation-based, zero-emission, tannin adhesives for particleboard

Interior grade particleboard prepared with tannin adhesives to which no aldehyde hardener was added were prepared. The hardening of the tannin adhesive was induced by the reaction of autocondensation tannins undergo when catalyzed by a lignocellulosic substrate or by a weak Lewis acid as alkali dissolved silica. The more alkaline the tannin extract solution the higher the ceiling internal bond strength that the particleboard bonded with it can reach. Differences between four commercial tannins are discussed. Pecan nut tannin and pine tannin need only lignocellulosic induced autocondensation to give excellent interior grade particleboard. Slower reacting tannins such as mimosa need instead both the catalytic effect of the substrate as well as that of a weak Lewis acid such as silica. Only pecan nut tannin appears at this stage to be able to give boards of acceptable dry internal bond strength at industrially significant pressing times.

Chinese wattle tannin adhesives for exterior grade plywood

Chinese wattle tannin adhesives were prepared in conjunction with the preparation of phenol-formaldehyde (PF) and phenolurea-formaldehyde (PUF) resins as crosslinking agents which can be substitutes for expensive paraformaldehyde. Adhesive formulations and gluing conditions were examined by preparing laboratory plywoods (300×300 mm). Based on the results obtained, plywoods (600×600 mm) ofTilia species and ofPinus massoniana were prepared using wattle tannin adhesives in pilot plant trials. The gluing results revealed that the tensile strength of the plywood can meet the requirements of the Chinese Standard GB9846.4-88 for exterior grade plywood.

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.

Ambient temperature gelling of Tannin adhesives with hexamine

Ambient temperature gelling of Tannin adhesives with hexamine

Fast vs. slow-reacting non-modified tannin extracts for exterior particleboard adhesives

Non-fortified, non-modified tannin extracts, rather than chemically-modified tannin adhesives intermediates, can be used as effectively to produce excellent exterior grade particleoboard at fast pressing times of industrial singnificance, with considerable advantages in both handling and cost. This can be achieved by simple pH-controlled reactivity adjustments of the tannin extract in the glue mix. This new concept is shown to be applicable to all types of polyflavanoid tannin extracts: Pine, mimosa, pecan, quebracho, and gambier tannin extracts are compared as regards their performance and capabilities. The new concept applied to the faster-reacting tannins, such as pine tannin and pecan tannin extracts, is shown to produce fast hot-pressing rates and excellent high moisture content tolerances which are undreamed of for the slower-reacting tannins, modified or non-modified, which are traditionally associated with thermosetting wood adhesives. Industrially used techniques to control well viscosity and pot-life of the glue-mixes of accelerated tannin adhesives are also presented.

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.

A 13C NMR study of polyflavonoid tannin adhesive intermediates. I. Noncolloidal performance determining rearrangements

AbstractChemical and heat treatments introduce noticeable modifications on the structure of four different polyflavonoid tannin extracts during their modification for use as tannin‐form aldehyde polycondensates. Such modifications are performance determining and account for the profound differences in behaviour of the four tannin extracts when used as tanninformaldehyde adhesives. Such modifications are directly followed by 13C NMR on the tannin extracts. Although some of the modifications observed by 13C NMR appear to correlate well with previous findings by model compounds studies, others could only be observed by 13C NMR of the modified tannin extracts themselves. Structural rearrangements to phlobatannins, depolymerization, recyclation, rearrangement of the type of interflavonoid monomer links, and autocondensation have been followed by 13C NMR and found to occur to different extents in the four tannins accounting for many of their differences in adhesive performance and characteristics. A novel reaction of autocondensation was observed for pecan nut pith tannins, which is different in applied characteristics from other polyflavonoid tannin adhesives. © 1994 John Wiley & Sons, Inc.

The Chemistry and Development of Pine Tannin Adhesives for Exterior Particleboard

The Chemistry and Development of Pine Tannin Adhesives for Exterior Particleboard

The formulation and commercialization of glulam pine tannin adhesives in Chile

The results of fingerjointing and glulam industrial trials in Chile using a “honeymoon” fast-set adhesive system manufactured using commercially-produced mildly-sulphited pine tannin extract are reported. The results obtained satisfy the relevant international standards specifications and constitute the first occasion that a commercially produced pine tannin adhesive has been used succesfully in industrial productions runs. This pine tannin adhesive is now available commercially in Chile for fully-exterior, structural-grade, solid timber applications. A new “honeymoon” type adhesive formulation had to be developed due to the peculiar gelling of the pine tannin extract used. The Chilean pine tannin extracted from a mixture of Pinus insignis and Pinus radiata shows gel times longer than what reported for other conifer tannins with the exception of Pinus brutia. Such differences in curing rate are tentatively reported to be related to the relative branching of the different conifer tannins.

Development of wood-products adhesives from mangrove bark

Mangrove bark was extracted using either water, ethanol, acetone, or various binary mixtures of these, as solvent. With plywood panels as substrates, joints prepared with adhesives derived from these extracts were tested for strength properties. In terms of the quantity of extracts from mangrove bark, water was the most effective solvent, followed sequentially by ethanol and acetone. However, in terms of the tannin content of these extracts, the order of extraction effectiveness of these solvents was reversed. The use of solvent mixtures in tannin extraction did not produce an observable synergistic effect. The quantity of extract and its tannin content were dictated primarily by the predominant solvent in the binary mixture. The reactivity of mangrove bark extracts is influenced significantly by formaldehyde concentration, cure temperature, type, and pH of the adhesive medium. Adhesive joints of significant bond strength were obtained from mangrove tannin adhesives. Prolonged cure periods, particularly at elevated temperatures, have deleterious effects on the strength properties of these adhesive joints.

Decrease of pressing temperature and adhesive content by metallic ion catalysis in tannin-bonded particleboard

The use of bivalent metallic ion catalysts such as zinc acetate in wattle tannin adhesives for exterior grade particleboard results in the attainment of better board properties at lower pressing temperatures without any lengthening of the pressing time of the board. Lower pressing temperatures achieved in this manner also allow a reduction in the percentage resin solids in the board. There are indications that the same effect is present when using synthetic phenol-formaldehyde resins.

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.

Tannin-based adhesives: new theoretical aspects

Abstract 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.

Wattle tannin adhesives for radio-frequency curing

Wattle tannin adhesives for radio-frequency curing