brief history
In 19 18, J.P.McCoy put forward the viewpoint that tannin is a component of phenolic condensate. 1932 ~ 1942, G.J. Morgan and R. Philips have successively developed various modeling materials containing tannin. 1950 dalton (L.K.Dalton) The viscosity of black wattle tannin and eucalyptus tannin was reduced by sulfite, and the resin prepared was better than urea-formaldehyde resin. In 1957, Ploom K.F.Plomley and hillis used black wattle tannin and red bark tannin to make adhesives for outdoor plywood. 196 1 year, A.B.Anderson and others developed pine bark tannin adhesive for particleboard. 197 1, boxer & Shucks Limited (Boxes &; Shooks ltd) obtained the patent right of black wattle tannin adhesive and began to be used in particleboard production. Since 1975, H.M.Saayman and A.Pizzi have studied the quality problems of tannin adhesive successively, and put forward many improvement schemes, which have made great achievements in the popularization and application of tannin adhesive.
Before China 1949, Xu Biao and others used gallnut tannin to make thermoplastic materials. During 1976, a wood factory in Hunan tried to use red root tannin extract as urea-formaldehyde resin filler, phenolic resin curing accelerator and wet fiberboard sizing agent. From 65438 to 0977, Zhang Yingbo and others developed larch tannin adhesive and carried out the production and application test of particleboard, and achieved good results.
In 1970s and 1980s, tannin adhesive, especially black wattle tannin adhesive, made great achievements in replacing phenolic adhesive. In South Africa, black wattle tannin glue has been used to produce plywood for ships, outdoor particleboard, corrugated board and finger-jointed wood. Australia, New Zealand, Finland, Brazil, India, Malaysia, etc. Tannin gum is also widely used as woodworking glue.
nature
Tannin gum takes tannin extract containing condensed tannin as the main raw material. Condensed tannins are divided into resorcinol type (such as black wattle tannin and 6745 lignans) and phloroglucinol type (such as larch tannin) in chemical structure (see figure). Condensed tannins are condensed from flavonoids. Flavonoids include flavan -3- alcohol, flavan -3,4-diol and hydroxystilbene. The two aromatic nuclei are connected by a straight carbon chain. The straight chain of flavanol molecule has three carbons (C6 C3 C6 type). The straight chain of hydroxystilbene has two carbons (C6 C2 C6 type). The main flavonoids in black wattle tannin are (-)-fisetin and (-)-Robinia pseudoacacia. Black wattle tannin is a tannin with different polymerization degrees condensed from these compounds, with a molecular weight of over 600 and an average molecular weight of 1250.
When tannin reacts with formaldehyde, it bonds with the reaction group (4,6 or 4,8) of ring A through methylene bridge (—CH2—), which is the basic principle of making glue.
The physical and chemical properties of tannin gum include the following contents:
stickiness
It varies with the solid content of tannin extract, the gum content of polymer in tannin extract, the temperature and pH value of gum. The main disadvantage of tannin glue is its high viscosity, which affects sizing. It can be improved by chemical treatment, reducing the solid content of glue or adding low viscosity resin.
Gelation time
The reactivity of different tannins varies greatly at different pH values. The gelation time of tannin from 35% black wattle, mangrove and Pinus radiata at pH 6 was 56, 65438 0 3 and 3 minutes, respectively, indicating that the reaction rate of tannin was Pinus radiata > mangrove > black wattle.
Formaldehyde dosage
The amount of formaldehyde required by tannin is very small. Black wattle tannin needs 3 ~ 4% (particleboard glue needs 6 ~ 8%), mangrove tannin needs 4%, and radiation pine tannin needs 6% formaldehyde as crosslinking agent.
In terms of performance, tannin glue should have the same performance as synthetic resin. Such as moderate viscosity, long life, enough aging time, and bonding quality reaching the standard of synthetic resin.
Formula application
Tannin gum can be divided into thermosetting (fixed above 100℃) and cold curing (fixed at 20 ~ 30℃).
Thermosetting adhesive formula
Thermosetting is divided into indoor and outdoor levels. Glue made of tannin instead of phenol is mainly used to make outdoor particleboard, plywood, laminated board and corrugated board. Cold-set adhesives are used to bond finger-jointed wood and handicrafts (such as musical instruments).
Formula of tannin adhesive for plywood
① Indoor grade adhesive. Black wattle tannin (partial weight) 100, water 100, sodium hydroxide (1. 15), coconut shell powder (10 ~ 15), paraformaldehyde (/. The quality of this adhesive is better than that of urea-formaldehyde resin, but the cost is higher. ② Outdoor grade adhesive. The formula is as follows:
Tannin gum
100 part of tannin-phenolic resin (50% of which is tannin).
Flour (200 mesh)
20 copies
Wood flour (200 mesh)
5 copies
25 parts water
2.5 parts of formaldehyde (37%)
Bonding condition
Veneer (pine)
The moisture content of veneer is 6-8%
The sizing amount is 200g/m3.
Pressurization temperature 120℃
Pressure 9.87× 105 Pa
Pressurization time 1 min/mm thickness
Plywood properties
The wood breakage rate (dry) is 90 ~ 100%.
The wet strength (cooking12h) is 59.22×105 ~ 69.09×105 Pa (60 ~ 70 kg/cm2).
Wood breakage rate (wet) 40 ~ 70%
The commercial black wattle tannin adhesive for outdoor use has been greatly improved. The content of tannin instead of phenol reaches about 90%, and the quality of adhesive meets the requirements. The formula is as follows:
Modified wattle tannin extract 36.4 (parts by weight)
Urea-formaldehyde enhancer 6.78
Paraformaldehyde 4. 10
Water 4 1.22
Defoamer 0. 15
Mould inhibitor 0.40
Coconut shell powder 6.75
The viscosity of the glue is 0.23 pasec.
PH 4.8 ~ 5. 1 (when used)
Make a living for 5 hours
Tannin adhesive formula for particleboard (outdoor grade) The outdoor grade adhesive formula is as follows:
Black wattle tannin extract 95.0 (parts by weight)
Water 138.0
Sodium hydroxide 1.6
Reinforced resin 5.0 (dry weight)
Paraformaldehyde 15.0
The viscosity (25℃) of the adhesive is 0.065438 0.5 ~ 0.02 Pa sec.
Rubber pressing condition
Temperature 160℃
Pressure 13.82× 105 Pa
Resin solid (surface layer) 14% (core layer) 10%
Plate quality
The bulk density is 700 kg/cm3.
The pressure immersion thickness is increased by < 12%, which meets the requirements of ASO115-1968.
Cold curing formula
Cold setting adhesive with high activity and high crosslinking degree made of tannin was developed only in 1980s. In 1978, A.Pizzi recommended a quick-setting tannin adhesive. Firstly, 90 parts of black wattle tannin extract with a solid content of 66.7%, 9.6 parts of 38% formaldehyde, 30 parts of methanol and 0.3 parts of defoamer were mixed at room temperature. Heat reflux 10 ~ 15 minutes at 76℃ and keep reflux 120 minutes at pH4.5, then add 30 parts of 99% resorcinol and 30 parts of water, cool the temperature to 62℃, add 2.4 parts of 25% sodium hydroxide solution, and reflux the mixture for 60 minutes to evaporate excess methanol.
When in use, 16 parts of 96-97% powdered paraformaldehyde, 7 parts of coconut shell powder (200 mesh), 7 parts of wood powder (200 mesh) and 10 part of industrial wetting agent are added to the resin, and a proper amount of water is added to make the viscosity reach 0.26-0.30 seconds.
The dry shear strength of wood chips bonded with cold-cured tannin adhesive is 5508.9 kPa (799 psi), and the wood breakage rate is 100%. It meets the standard of commercial phenol-resorcinol-formaldehyde cold setting adhesive.
standard
Tannin glue should meet the standards of phenolic glue (outdoor grade) and urea-formaldehyde glue (indoor grade). The standards of plywood can be divided into grade A (equivalent to 72 hours boiling), grade B (6 hours boiling), grade C (hot water resistance) and grade D (cold water resistance).
trend
Condensed tannin (especially black wattle tannin) can replace phenol as binder. The advantages of glue are: short curing time, good bonding performance, short hot pressing time and good water resistance; The adhesive has little toxicity and no corrosive effect; Easy to manufacture and low in cost. This kind of glue has achieved great success, and its use value and economic effect have attracted the attention and attention of wood processing industries all over the world.
Due to the increasing cost of petroleum products worldwide, the high price of phenol and the increasing consumption of adhesives for wood-based panels, people are urged to develop high-quality and low-cost adhesive materials (such as tannin) from renewable resources. Using renewable resources to produce adhesives is of great significance to developing countries that lack petroleum resources and petrochemical products and need to develop wood processing industry. In the development and utilization, we should attach great importance to black wattle tannin with fast growth, high content, good performance and low cost.
China needs to vigorously develop the wood-based panel industry, and the corresponding adhesives should also be accelerated. Therefore, in the future, it will play an important role in ensuring the development of wood-based panel industry by making use of domestic natural conditions and processing capacity to vigorously develop black wattle and provide a large number of black wattle tannin for gum making every year.