Tannin extraction and processing

A method for producing a relatively low viscosity tannin extract from bark and/or wood and suitable for use in formaldehyde-condensation adhesives comprises providing tannin extract obtained from bark and/or wood, separating from the tannin extract the high molecular weight material, subjecting the high molecular weight material to treatment with one or more sulphite compounds to reduce the molecular weight and recombining the sulphited material with the tannin extract. The tannin extract may be recovered by subjecting bark and/or wood to a first stage hot aqueous tannin extraction process, thereby obtaining a first stage tannin extract, subjecting the extracted bark and/or wood to a second stage hot aqueous extraction process at a higher pH than the first stage, thereby obtaining a second stage tannin extract, and optionally recombining the first and second stage tannin extracts.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to tannins (predominantly natural 
polyphenolics), especially but not only for use in adhesives for wood and 
other products, and is particularly concerned with methods of processing 
tannins, including tannin extraction. 
2. Description of the Related Arts 
The bark and wood extracts of various commercially important trees contain 
polyphenolics in the form of tannins which can form condensation products 
with formaldehyde to thereby act as bases for wood adhesives. Such 
condensation products have been widely studied particularly with a view to 
obtaining suitable adhesives for plywood and particle-board. These tannin 
bark and wood extracts are well known in the art and may be obtained, for 
example, by extraction from comminuted wood and bark in water in which the 
tannins tend to dissolve. However, the molecular weights of the 
polyhydroxy phenols may range from 3.times.10.sup.2 to 3.times.10.sup.8 
and difficulties have been encountered primarily due to excessive 
viscosity of the extracts and the difficulty in obtaining uniformity in 
product quality. 
The lack of uniformity in product quality may arise because sugars and 
other carbohydrates, as well as other contaminants, may also dissolve in 
the water while the high molecular weight polyphenolics may not dissolve 
if the water is cold. The low molecular weight tannin extract may 
accordingly have a substantial amount of contamination and a low viscosity 
leading to excessive penetration of the resin into the material being 
bonded. 
There have been various proposals for alleviating these difficulties, the 
main one of which is to conduct the extraction in hot water, preferably at 
about 100.degree. C. This increases the solubility in water of the high 
molecular weight polyphenolics and increases the ratio of polyphenolic 
material to contaminants such as sugar. 
It is important where the tannin is used in adhesives to include the highly 
viscous extract material in the resin since this contains the higher 
molecular weight polyphenolics which provide better bonding 
characteristics of the tannin extract due to their lower contaminant ratio 
and since excess penetration of the adhesive into the material being 
bonded may otherwise occur. On the other hand, the tannin extract may 
become excessively viscous and be difficult to mix and spread. 
For example, the yields of tannin from aqueous single stage extraction of 
one sample of radiata pine (Pinus radiata) bark, carried out at 
20.degree., 60.degree. and 100.degree. C., were 2%, 5% and 10% 
respectively, and viscosities of 40% aqueous solutions of the 20.degree., 
60.degree. and 100.degree. C. aqueous extracts were 300 cP, 800 cP and 
5000 cP at 25.degree. C. The yields are accordingly low and the known 
process is not economically favourable for commercialization. Of more 
importance, the highest yield product is of poor quality because it is 
very difficult to formulate adhesives from the 100.degree. C. aqueous 
extracts; the viscosities of the resultant adhesives are too high for 
either plywood or particleboard bonding processes. For particleboard 
manufacture a viscosity of approximately 1500 cP is considered to be 
maximum and viscosities less than 1200 cP are desirable. It is very 
difficult to produce such adhesives from the 100.degree. C. tannin 
extract. 
One proposal for reducing the viscosity of the bark and wood extracts has 
been to treat the extracts with a sulphite compound such as sodium 
sulphite which tends to cleave the polyphenolic chains and also to 
sulphonate polyphenolics to produce compounds with greater water 
solubility. However, we have found that the sulphite reaction is 
non-selective for the higher molecular weight materials so that the low 
molecular weight polyphenolics are more readily cleaved. For example, when 
a sample of 100.degree. C. aqueous extract of radiata pine bark is 
sulphited (25 parts dried extracts, 1.25 parts sodium metabisulphite, 75 
parts water, refluxed for 2 hours) the changes in the percentage of the 
extract falling within various molecular weight ranges are shown in Table 
1. 
TABLE 1 
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Molecular size distribution 
100.degree. C. aqueous extracts 
Fraction by Unsulphited 
Sulphited 
molecular weight 
(%) (%) 
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More than 10.sup.6 
22.4 19.1 
10.sup.5 .about. 10.sup.6 
5.5 2.4 
10.sup.4 .about. 10.sup.5 
29.2 26.0 
10.sup.3 .about. 10.sup.4 
11.5 1.5 
Less than 10.sup.3 
31.4 51.0 
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It is evident that sulphitation results in a substantial increase in the 
fraction of lowest molecular weight (&lt;10.sup.3) as a result of selective 
cleavage of the chains in the fractions having molecular weights of up to 
about 10.sup.6, whereas the percentage of the fraction having the highest 
molecular weight is little affected. Adhesives made from such sulphited 
extracts may spread more readily but are frequently of poor quality 
because their high content of low molecular weight components leads to 
excessive penetration of the adhesive into the substrate to be bonded. For 
example, when total aqueous extracts are sulphited, converted to adhesives 
and used in the production of plywood the quality of the bonds is only B 
as described in Australian Standard 2754.1-1985 Adhesives for Timber and 
Timber Products, Part 1 Adhesives for Plywood Manufacture:Standards 
Association of Australia, North Sydney N.S.W., 1985, which calls up 
Australian Standard 2098.2-1977 Methods of Test for Veneer Plywood:Bond 
Quality of Plywood (Chisel Test) Standard Association of Australia, North 
Sydney, N.S.W., 1977 as the method for assessing quality. 
A further proposal for providing relatively low viscosity tannin extracts 
from bark and wood is made in our Australian patent specification 533791 
in which it is suggested that following aqueous extraction of the 
polyphenolics, any high viscosity material is separated by ultrafiltration 
and discarded. This process has been found to provide satisfactory 
extracts for the formulation of adhesives even though the high viscosity 
extracts have the best bonding characteristics. However, in commerical 
applications the provision of membranes to achieve the necessary 
fractionation is costly and the fractionation itself may be difficult to 
control. 
SUMMARY OF THE INVENTION 
It is an object of a first aspect of the present invention to 
satisfactorily reduce the viscosity of wood and/or bark extracts which are 
suitable for the formulation of adhesives. 
According to the present invention there is provided a method for producing 
a relatively low viscosity tannin extract from bark and/or wood and 
suitable for use in formaldehyde-condensation adhesives, which method 
comprises providing a tannin extract obtained from bark and/or wood, 
separating from the tannin extract the high molecular weight material, 
subjecting the high molecular weight material to treatment with one or 
more sulphite compounds to reduce the molecular weight and recombining the 
sulphited material with the remainder of the tannin extract. 
Further according to a first aspect of the present invention there is 
provided a tannin extract having a relatively low viscosity which is 
produced by the method proposed in the immediately preceding paragraph. 
DESCRIPTION OF THE INVENTION 
By the first aspect of the present invention the high molecular weight 
material is cleaved and therefore its viscosity reduced without reducing 
the viscosity of the lower molecular weight material and the sulphited 
material is recombined with the lower molecular weight material to provide 
a tannin extract of desirable viscosity, for example of 1500 cP or less, 
preferably 1200 cP or less which is especially suitable for formulation as 
an adhesive. The pH of the extract may be adjusted following 
recombination. 
Separation of the high molecular weight fraction of the extract may be 
performed in any convenient manner. Included in the possible separation 
processes is ultrafiltration, for example in the manner proposed in our 
Australian patent specification 533791. Alternatively, the separation may 
be performed for example by adjusting the pH to precipitate higher 
molecular weight material from solution or by use of a centrifuge to 
remove fine particles of suspended high molecular weight material which 
may be of the order of 0.45 microns in size. 
The high molecular weight material may have an equivalent molecular weight 
of from 10.sup.5 to 10.sup.6 and higher. The term "equivalent molecular 
weight" as used herein means the material has a molecular size equivalent 
to that of dextran of the stated molecular weight. 
The sulphite treatment of the high molecular weight extract may be 
equivalent to that previously proposed for use on the full hot water 
extract, that is the extract containing both high and low molecular weight 
material. 
Various sulphite compounds may be used as well known in the art, such as 
sulphur dioxide and sodium sulphite. Preference is given to using sodium 
metabisulphite. In one preferred embodiment, various extracts were 
adjusted to pH6 and the insoluble portions thereof collected by 
centrifugation and freeze-dried. A 25% aqueous suspension of the 
freeze-dried insolubles was then heated in a 5% solution of sodium 
metabisulphite for 2 hours at 100.degree. C. under reflux conditions, 
although other conditions may be suitable for carrying out the sulphite 
treatment of the high molecular weight material. The sulphited extracts 
may then be dried, for example by freeze-drying. 
The preferred barks for use with the first aspect of the present invention 
are those of radiata pine, maritime pine (Pinus pinaster) and caribbean 
pine (Pinus caribaea) but other products such as Quebracho wood and wattle 
bark may also be used. 
Generally the tannin extract will be obtained from the bark and/or wood by 
a hot aqueous tannin extraction process. Such a process may be 
conventional and be carried out up to about 100.degree. C. but the 
relatively low yield may still present difficulties in commerical 
applications. 
Yield may be increased by squeezing the hot water extract from the 
wood/bark. It may also be advantageous to use a higher temperature, for 
example 105.degree. C. or more, as this may result in an increased yield 
of high molecular weight tannin. Alkalis such as carbonates and hydroxides 
may be added to increase the pH of the aqueous solution and this has been 
found to improve the yield. By way of example, the pH of the aqueous 
extract of Pinus radiata bark is approximately 3.5 and alkali (eg. NaOH) 
may be added to increase this to about 6 which is generally the 
approximate pH for curing formaldehyde-condensation adhesives. It is also 
possible to extract the polyphenolics at up to about pH 9 to increase the 
yield. 
Such an alkaline extraction has been proposed previously but provides 
inferior extract for uses including adhesive production because 
neutralization by mineral acids to pH 6 for the formaldehyde condensation 
reaction causes the formation of salts which contribute to poor adhesive 
performance. 
It is an object of a second aspect of the present invention to provide a 
tannin extraction process which may result in increased yield while 
alleviating disadvantages associated with other such processes. 
According to the second aspect of the present invention there is provided a 
method for obtaining tannin extract from bark and/or wood, which comprises 
subjecting the bark and/or wood to a first stage hot aqueous tannin 
extraction process, thereby obtaining a first stage tannin extract, 
subjecting the extracted bark and/or wood to a second stage hot aqueous 
extraction process at a higher pH than the first stage, thereby obtaining 
a second stage tannin extract, and recombining the first and second stage 
tannin extracts. Thus we have found that alkaline extraction may be used 
without seriously affecting the bonding characteristics of the extract by 
carrying out the extraction process in two stages, using a higher pH, for 
example pH 8-9 in the second stage after removing the first stage tannin 
extract from the extraction bath. The alkaline tannin extract is adjusted 
as desired by combining it with the first stage extract and may be 
subsequently further adjusted. Two stage extraction may increase the yield 
by 50-100%. 
Further according to the second aspect of the present invention there is 
provided a tannin extract when recovered by the method described in the 
immediately preceding paragraph. 
For the purposes of the first aspect of the present invention, the first 
and second stage tannin extracts may be recombined prior to separating out 
and sulphiting the high molecular weight material or the second stage 
tannin extract may comprise the high molecular weight material which is 
sulphited and then recombined with the first stage tannin extract. 
However, it is to be understood that the second aspect of the present 
invention may be used independently of the first aspect. Thus, for 
example, the second aspect of the present invention may additionally 
comprise separating from the recombined tannin extracts the high molecular 
weight material and subjecting the high molecular weight material to 
treatment with one or more sulphite compounds to reduce the molecular 
weight of said material. Accordingly, the sulphited material need not be 
recombined with the low molecular weight material from which the high 
molecular weight material is separated, although the separating and 
sulphiting steps may be as discussed with reference to the first aspect of 
the present invention. 
The tannin extracts of the present invention may be particularly suited to 
bonding plywood in their unrecombined condition, that is where the 
sulphited material is used alone, and in recombined condition may be more 
suitable for particleboard. The extracts may also be used to advantage in 
bonding other reconstituted wood products such as laminated timber. It is 
known in the art that the assessment of bond quality of reconstituted wood 
products is best carried out by tests on plywood produced with the 
adhesive because the conditions of production can be controlled and the 
type of bond failure more readily assessed than for the other products.

EXAMPLES 
In the Examples, in which the tannin extract was sulphited as proposed in 
the preferred embodiment above, the sulphited tannin extract was 
recombined with the remaining tannin extract to produce a tannin extract 
which is less prone to separation of the molecular weight species in the 
glueline of bonded plywood because the individual flow rates of the 
molecular components are more uniform, resulting in higher quality 
bonding. Assessment of bond performance was conducted on plywood using the 
standard chisel technique and showed that using the process of the 
Examples there is a surprising increase in bonding capability and a 
reduction in viscosity to levels that can be readily handled by 
industry--eg., viscosity of 1200 cp at total solids content of 40% and at 
25.degree. C. A further observation was that the resins produced were more 
tolerant of the moisture content of the veneers used in the Examples. 
The following Examples illustrate the preparation of various tannin 
extracts in accordance with the invention and subsequent processing for 
use as wood adhesives. In all cases there was no selection of bark for 
processing. 
Example 1 
Tannin extract (1 part) obtained by hot (100.degree. C.) water extraction 
from radiata pine (Pinus radiata) bark was dissolved in water (9 parts) 
and the pH was adjusted to pH 6 with NaOH. The soluble and insoluble 
portions were separated by centrifugation at 1000 g for 10 minutes and 
freeze-dried. The ratio of solubles to insolubles was about 3:1. The dried 
insolubles (25 parts), sodium metabisulphite (1.25 parts of 5% solution) 
and water (75 parts) were mixed and heated at 100.degree. C. under reflux 
for 2 hours. The dried solubles (3 parts) and the sulphited insolubles (1 
part) were recombined so that the ratio of solubles and insolubles was the 
same as that of the original tannin extract. The viscosity of the 
recombined tannin extract at 40% solids content was 1200 cP at 25.degree. 
C., whereas prior to this treatment the viscosity at the same solids 
content and temperature was 5000 cP. 
Example 2 
Commercially available radiata pine (Pinus radiata) bark (1 part) was 
extracted with water (3 parts) at 100.degree. C. for 10 minutes. This 
first stage extract at pH 3.5, was filtered and stored until required 
later for recombination with the second stage extract. In the second stage 
of extraction the extracted bark from the first stage extraction was 
further extracted in the same manner as described with reference to the 
first stage except that the pH of the bark was adjusted with 10% NaOH 
aqueous solution so that the pH of this filtered extract was approximately 
pH 8.5. The tannin extracts from the first and second stage extractions 
were combined to give a solution of pH approximately 6.8. The total yield 
of tannin extract was 30%. The pH of the combined extracts was adjusted to 
pH 6 and the insoluble portion (25% by solids of total extract) was 
separated by centrifugation, sulphited and recombined with the soluble 
portion as described in Example 1. 
Example 3 
Commercially available maritime pine (Pinus pinaster) bark was sequentially 
extracted with hot water and then with aqueous NaOH solution at pH 8.5 as 
described in Example 2. The tannin extracts from the first and second 
stage extractions were combined to give a solution of pH 6.2. The total 
yield of tannin extracts was 25% of dried bark. The insoluble portion 
(25%) was separated, sulphited and recombined with the soluble portion as 
described in Example 1. 
Example 4 
Commercially available carribean pine (Pinus caribaea) bark was extracted 
in the same manner as described in Example 2. The total yield of tannin 
extracts was 21% of dried bark. The pH of the combined tannin extracts was 
6.5, this was then adjusted to pH 6. The insoluble portion (24%) was 
separated, sulphited and recombined with the soluble portion as described 
in Example 1. 
Example 5 
First stage (aqueous) tannin extracts from barks of (a) radiata pine (Pinus 
radiata) yield 15%, (b) maritime pine (Pinus pinaster) yield 14%, and (c) 
caribbean pine (Pinus caribaea) yield 11%, were obtained in the manner of 
Example 2 and held. The bark residues were then further extracted with 
alkali (pH 8.5) in the manner of Example 2 and these second stage 
extracts, yields 15, 10, and 10% respectively, were sulphited with sodium 
metabisulphite in the same manner as described in Example 1 and then 
combined with the respective first stage tannin extracts. 
Example 6 
In Examples 3, 4 and 5 the viscosities of non-sulphited extracts were 
greater than 5000 cP and the viscosities of recombined sulphited extracts 
less than 1500 cP when measured at 40% solids content and 25.degree. C. 
Plywood samples were prepared by bonding coachwood (Ceratopetalum 
apetalum) veneers with the recombined extracts from Examples 1 to 5. All 
adhesive formulations consisted of the aqueous solution of the tannin 
extracts, a formaldehyde source as a condensing agent and a nut shell 
flour as a filler. All the formulations utilized the same tannin 
solids/formaldehyde source/filler ratio. No attempt was made to optimize 
individual adhesive formulations. Resulting plywood specimens were then 
tested according to Australian Standard AS 2754.1-1985 Adhesives for 
Plywood Manufacture. Test results for examples 1-5 are recorded in Table 
2. 
TABLE 2 
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Gluebond quality of plywood 
Average gluebond quality 
Wet 
after 72 hours 
Examples Bark species 
Dry boiling 
______________________________________ 
1 P.radiata 9.0 9.0 
2 P.radiata 9.5 7.0 
3 P.pinaster 6.0 5.5 
4 P.caribaea 6.5 6.5 
5(a) P.radiata 6.5 6.5 
5(b) P.pinaster 6.5 6.5 
5(c) P.caribaea 8.5 7.0 
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The tested Examples all satisfied the requirements for type A bond 
adhesives (fully weather and boil proof) as classified in Australian 
Standards. By comparison, none of the total aqueous extracts from the same 
barks when sulphited under the same conditions and similarly converted to 
adhesives and used to produce similar plywood achieved A Bond 
classifications. It would be expected that the adhesive properties of some 
Examples could be improved by further adjustments to adhesive 
formulations.