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(11) Patent Number: KE 37
(45) Date of grant: 21/08/1997
(12) PATENT
(51) Int.Cl.6: C 08K 11/00
(21) Application Number: 1994/000152
(22) Filing Date: 03/10/1994
(30) Priority data: 4334962.513/10/1993 DE
(54) Title: TANNIN-BASED BINDING AGENTS
(73) Owner:
RUTGERSWERKE AKTIENGESELLSCHAFT of , MAINZER LANDSTRASSE 217, D-60326 FRANKFURT/MAIN, GERMANY, Germany
 
(72) Inventor: ANTONIO PIZZI; WILLI ROLL and BERTHOLD DOMBO
(74) Agent/address for correspondence: Kaplan & Stratton Advocates„ P.O. Box 40111-00100, Nairobi
 
(57) Abstract:
The invention relates to new binding agents which harden at elevated temperatures and are compatible with cellulose-containing materials, such as, for example, wood ,and are well suited for the manufacture of derived timber products, such as chipboards. They are composed of tannins and compound with a weak acid reaction
 
TANNIN-BASED BINDING AGENTS
The invention relates to new binding agents which are thermosetting at elevated temperatures and are compatible with cellulose-containing materials, such as, for example, wood, and are suitable for the manufacture of derived timber products, such as chip boards.
In the search for natural and, in particular, regrowing raw materials for the production of binding agents for derived timber products, the use of tannins is also known (J. Macromol. SCI.-Chem. A 16 (7), 1243-1250 (1981)).
Despite their ready availability, tannins have not been widely used, since the tensile strength of tannin-bound derived timber products is not satisfactory, in particular after exposure to moisture.
Indeed, the Applicant in the past developed a tannin-containing thermosetting binding agent (DE 43 28 220.2) which hardens as a result of the addition of a substance which, under the effect of heat, releases formaldehyde, and produces derived timber products having considerable strengths and low swelling values under wet storage conditions, but these good values are achieved solely when the tannin of the pecan nut is used. In addition, if the apportioning of the formaldehyde-releasing substance is incorrectly carried out, the separation or release of formaldehyde after pressing cannot be excluded entirely.
Accordingly, it is the object of the invention to provide agents which will broaden the basis of raw materials for use as binding agents which will produce derived timber products which have improved mechanical properties and, simultaneously, release less or no formaldehyde.
According to the invention, there is provided a thermosetting, formaldehyde-free binding agent which is based on tannin, the binding agent containing tannin of the polyflavonoid type and a compound with a weak acid reaction as a hardening catalyst. The binding agent may contain Si02 or boric acid as the hardening catalyst, in a quantity of up to 10 % by mass, or in a quantity of 1 to 6 % by mass.
When the binding agent contains Si02 as the hardening catalyst, it may have a pH value of 0 to 2, or a pH value of 7.5 to 14.
The thermosetting binding agents according to the invention may contain the tannins of the pecan nut, of pinus radiata, acacia mearnsii (wattle) or schinopsis balansae (quebracho) on their own or in mixtures thereof.
The invention extends to the use of thermosetting formaldehyde-free binding agents as described above for the manufacture of derived timber products or materials based on cellulose-containing products.
 
The invention still further extends to a method of manufacturing materials, in which a binding agent as described above is mixed with a cellulose-containing product, the mixture is placed in a mould and is treated there at a temperature ranging from 150 to 210° C and at a pressure in the region of 0.1 to 4 mPa/mm2.
Compounds with a weak acid reaction are inorganic acids or substances with an acid reaction in an aqueous medium which substances have a pka value of more than 7.5.
Examples of such compounds with a weak acid reaction are boric acid, aluminium trichloride, zinc dichloride, stannic tetrachloride or silicon dioxide.
Thus, for example the tannin extracts of the pecan nut (pH 9.55) gel at room temperature after the addition of, in each case, 6 % by mass of A1CI3 (pka = 8.6) within 780 s H3B03 (pka = 9.2) within 360 s SiO2  (pka = 10) within 49 s

Examples of tannins of the polyflavonoid type are the tannins of the pecan nut, of pinus radiata (pine), of acacia mearnsii (wattle) or schinopsis balansae (quebracho), on their own or in mixtures with one another.
These binding agents can be used to manufacture derived timber products or materials based on cellulose-containing products, in that they are mixed with a cellulose-containing product or are sprayed on to cellulose-containing products, such as wood chips or shavings, the mixture or the wetted cellulose-containing products are placed in a mould and are treated there at a temperature in the range from 150 to 210° C, at a pressure in the region of 0.1 to 4 mPa/mm2.

With the aid of such formaldehyde-free binding agents, it is possible to manufacture materials, the tensile strength of which corresponds, for example, to phenolic resin-bound materials but the moisture resistance of which is greater, such that they are particularly well suited for exterior use.
It is of particular advantage that the tannins can also be used without any decomposition treatment, such as is frequently described in the relevant literature.
For example, the tannin of the pecan nut (Carya illinoensis) can be used in the form of the commercially available product which has heretofore been used predominantly as a tanning agent.
It was found that it was possible, for the production of the binding agent according to the invention, for the adjusted pH value to vary. Hardening was achieved both in the acid region {pH < 2) and also in the alkaline region (pH > 7.5). The adjusting of the pH value in turn influences the quantity of compound with an acid reaction, in particular the quantity of Si02, that must be added as the hardening catalyst to the binding agent in order to achieve an optimal degree of hardening.
If, for example, 6 % by mass of SiO2, is added to a pecan-nut tannin solution having a pH value of 8.2, chipboards manufactured from this mixture reach a maximum of the tensile strength of 0.55 mPa after a relatively long pressing time of 7.5 minutes. If, however, the same quantity of SiO2 is added when the pH value is 10.2, chipboard production is no longer possible, since such binding agents harden too rapidly already at low temperatures. This means that, in the case of reactive tannins, the higher the adjusted pH value, the less SiO2, must be added as hardening catalyst. This is so pronounced that a V 20 chipboard (DIN), which is manufactured using pecan-nut tannin at a pH value of 10.2 but with an addition of only 0.1 to 0.2 % by mass of Si02, has a tensile strength of 0.71 mPa after a pressing time of 7.5 minutes. Even if, in this case, the pressing time were to be reduced to 2 minutes (10 sec/mm), the chipboards still have a tensile strength of 0.41 mPa, i.e. still satisfactory values for V 20 boards.
The picture looks different when tannins which are less reactive are used. For example, chipboards, in which tannin from the wattle bark (acacia mearnsii) is used as the sole binding agent, reach only about 39 % of the strength of a pecan-nut-tannin board. Yet, as a result of the addition of about 3 % by mass of S102, a strength is achieved which meets the requirements in respect of V 20 boards.
Tannins from pine bark (pinus radiate) are, as such, rather unreactive, but when they are mixed with pecan-nut tannin, in the presence of the SiO2 hardening catalyst according to the invention, they also have an adequate reactivity for the use thereof as binding agents for cellulose-containing products. Already the addition of 10 % by mass of pecan-nut tannin is adequate for industrial application.
Mixtures such as those in which the mass ratio of pecan-nut tannin to pine tannin is 30: 70 to 50 : 50 are, however, particularly well suited. Using such mixtures, pressing rates of 10 sec/mm can be achieved.
Satisfactory results in the manufacture of V 100 boards can also be achieved with a mixing ratio of 30: 70 to 35: 75 parts by mass of pecan-nut tannin to pine tannin. Reduced pressing times can, however, be achieved in the case of mixing ratios of between 35: 75 to 40: 60. Similar results were achieved with the rather unreactive quebracho tannin.
All in all, tests using binding agents according to the invention showed that, in comparison to urea-formaldehyde binding agents, it was possible to use considerably smaller binding agent concentrations, and that the pressing times are considerably reduced when using higher concentrations of binding agents.
For the manufacture of cellulose-containing bound products, the tannins, or the tannin mixture, are/is adjusted to the desired pH value, then mixed with the compound with the weak acid reaction, in particular the SiO2, which can be used both in crystalline form or in a micro dispersed form, and with the cellulose-containing products, before said products are pressed to produce the corresponding materials.
Usually, up to 10 % by mass, in particular 1 to 6 % by mass, of the compound with the weak acid reaction is used for binding agents based on less reactive tannins and to achieve short pressing times. Larger additions of hardening catalyst than 10 % by mass have not been found to be effective since no improvement is achieved thereby.
Corresponding cellulose-containing products are, for example, wood veneer, wood shavings, and cellulose-based fibres or even straw, from which chipboards, plywood, or sound and heat insulating boards are manufactured.
The materials are manufactured in a manner such that the binding agents according to the invention are mixed with cellulose-containing products; the mixture is placed in a mould and is hardened, under pressure, at a temperature below the decomposition temperature of the tannin, preferably at 150 to 210 ° C. In this regard, depending on the starting material and the desired thickness, the pressure is in the region of 0.1to4 mPa/mm2.
Thus, for example, the manufacture of three-ply chipboards is also possible, applying a pressure of 2 to 3.5 mPa/mm2.
Depending on the desired material and the strength desired, the quantity of binding agent is in the region of 4 to 20 % by mass, relative to the cellulose-containing product. The binding agent can be prepared as a solution of the components in water, alcohol or in a water/alcohol mixture.

The following Tables set out the binding agents according to the invention when used to manufacture chipboards, using microdispersed Si02 (Aerosil®200) as hardening accelerator.
Wood shavings are sprayed with a water/alcohol solution which contains 11 % by mass of binding agent, relative to the mass of the wood shavings treated, and then dried. In a manner known per se (2.5 N/mm2; 195° C), they are then pressed and hardened after having been moulded to form boards having the dimensions 400 x 350 x 12 mm.
 
TABLES

Table 1
Influence of the quantity of Si02 added to pecan-nut tannin as the binding agent for 12 mm thick chipboards at a pH of 8.2 and a pressing time of 7.5 minutes.
                                                            Transverse tensile strength

% of SiO2                         (dry)                  density                    moisture

                                           [mPa]                 [g/cm3]                         %
0                                         0.230                  0.706                           13
3                                         0.329                  0.702                           14
6                                         0.547                  0.702                           15
9                                         0.356                  0.703                           14
18                                       0.343                  0.700                           22

Table 2
Influence of the quantity of $102 added to wattle tannin as the binding agent for 12 mm thick chipboards at a pH of about 10 and a pressing time of 7.5 minutes.
                                                         Transverse tensile strength
% of SiO2                            (dry)                     density                   moisture
                                            [mPa]                    [g/cm]                      [%]
0                                          0.160                     0.699                       20
3                                          0.475                     0.698                       18
6                                          0.449                     0.699                       20
9                                          0.385                     0.701                       21
 

Table 3
Influence of the proportion of components of pecan-nut tannin and pine tannin at a pH of 10.2 and a pressing time of 7.5 minutes on the properties of 12 mm thick chipboards.
                                            Transverse tensile strength

Pecan-nut            Pine                   (dry)                 Density             Moisture
tannin [%]          tannin [%]          [mPa]               [g/cm3]              [%]
100                     0                         0.710               0.705                  21
50                      50                        0.530               0.704                  22
40                      60                        0.555               0.705                  22
30                      70                        0.590              0.699                  22
20                      80                        0.535              0.704                  22
10                      90                        0.450              0.704                  22
0                       100                       0.185              0.698                  17
 

CLAIMS
1.Thermosetting, formaldehyde-free binding agent which is based on tannin, the binding agent containing tannin of the polyflavonoid type and a compound with a weak acid reaction as a hardening catalyst.
2. Binding agent according to claim 1, which contains Si02 as the hardening catalyst.
3. Binding agent according to claim 1, which contains boric acid as the hardening catalyst.
4. Binding agent according to one of claims 1 to 3, which contains the hardening catalyst in a quantity of up to 10 % by mass.
5. Binding agent according to one of claims 1 to 3, which contains the hardening catalyst in a quantity of 1 to 6 % by mass.
6. Binding agent according to claim 2, which has a pH value of 0 to 2.
7. Binding agent according to claim 2, which has a pH value of 7.5 to 14.
 
8. Thermosetting binding agents according to one or more of claims 1 to 7, the binding agents containing the tannins of the pecan nut, of pinus radiate, acacia mearnsii (wattle) or schinopsis balansae (guebracho) on their own or in mixtures thereof.
9. Use of thermosetting formaldehyde-free binding agents according to one or more of claims 1 to 8 for the manufacture of derived timber products or materials based on cellulose containing products.
10. Method of manufacturing materials, in which a binding agent according to one or more of claims 1 to 8 is mixed with a cellulose-containing product, the mixture is placed in a mould and is treated there at a temperature ranging from 150 to 210° C and at a pressure in the region of 0.1 to 4 mPa/mm?.
11. A new thermosetting, formaldehyde-free binding agent which is based on tannin, substantially as described herein.
12. A new use of thermosetting formaldehyde-free binding agents for the manufacture of derived timber products or materials based on cellulose-containing products, substantially as described herein.
13. A new method of manufacturing materials, substantially as described herein.
 
ABSTRACT
The invention relates to new binding agents which harden at elevated temperatures and are compatible with cellulose-containing materials, such as, for example, wood, and are well suited for the manufacture of derived timber products, such as chipboards. They are composed of tannins and compounds with a weak acid reaction.

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