GB1601751A

GB1601751A - Adhesives based on phenol-formaldehyde resin and alkali lignins

Info

Publication number: GB1601751A
Application number: GB19/78A
Authority: GB
Original assignee: Metsaliiton Teollisuus Oy
Current assignee: Metsaliiton Teollisuus Oy
Priority date: 1977-10-18
Filing date: 1978-01-03
Publication date: 1981-11-04
Also published as: JPS5477643A; FI58788B; PL114593B3; PL210358A1; JPS61166875A; NO147722C; NO783487L; FI773094A; SE446987B; NO147722B; FR2406658B2; FI58788C; BR7806894A; SE7810829L; CH622026A5; DE2758572A1; JPS6256193B2; FR2406658A2

Abstract

The adhesive for producing plywood boards, chipboards and fibreboards and the like, which contains, as main components, phenol formaldehyde resin and lignin derivatives fractionated according to molecular weight, does not, as in the main patent, need to contain only those lignin derivatives of which at least 50% by weight have a molecular weight of over 5000. It is rather sufficient, in order to obtain weather-resistant adhesives of the aforementioned kind, if at least 35% by weight have a molecular weight of over 5000. This simplifies the production considerably.

Description

(54) ADHESIVES BASED ON PHENOL-FORMALDEHYDE RESIN AND ALKALI LIGNINS (71) We METSALIITON TEOLLISUUS OY a Finnish Corporation of SF-02100 Espoo 10, Finland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention concerns a binding agent for the manufacturing of plywood, chipboard and fibreboard and like products, said binding agent being based on phenol-formaldehyde resin and alkali lignins. The invention also concerns a process for preparing the binding agent.

Binding agents which are made of lignin derivatives and phenol-formaldehyde resin and intended for use in the production of plywood, chipboard and fibreboard are known e.g.

Danish Patent No. 100984, Canadian Patent No. 735,389, U.S. Patents No. 2,786,008 and 3,185,654 and the articles "Thermosetting adhesives from electrodialyzed lignosulfonates," TAPPI 50 (1967), 92-94 A and "Reactive Lignin-Derived Products in Phenolic High Pressure Laminates", TAPPI 44 (1961), 823-830. Furthermore, weather-resistant binding agents made from fractionated lignin derivatives and phenol-formaldehyde resin are known through the same applicant's earlier Finnish Patents No. 51105 and 51946. Of the lignin derivatives, lignosulfonates or alkali lignins respectively, used in the preparation of binding agent according to the teachings of the last mentioned references, more than 50% have a molecular weight higher than 5,000.Such binding agents made of fractionated lignin derivatives have proved significantly superior to all binding agents made before from lignin derivatives in particular as regards their weather resistance properties, while their strength characteristics are fully equivalent to those of commercial phenol-formaldehyde glues.

The above-mentioned binding agents of the constituent lignin derivatives of which more than 50% have a molecular weight higher than 5,000, are made of lignin derivatives obtained from cellulose digestion and fractionated by their molecular weight. Fractionation is usually effected by ultrafiltration of spent liquor from the digestion with the aid of a semipermeable diaphragm.

In attempts to develop the above-mentioned binding agents made of lignin derivatives appropriate for use in industrial large scale production, the fraction containing a great amount of high molecular lignin derivatives which is indispensable in the making of the binding agent constitutes a comparatively minor fraction of the dry substance in the liquor obtained from cellulose digestion, since e.g. of the alkali lignins obtained from sulphate digestion only about 25 to 30% by weight have a molecular weight higher than 5,000. If the aim is that of the lignin derivatives used towards a binding agent more than 50% by weight should have a molecular weight higher than 5,000, only a small fraction of the original dry matter of the spent liquor will be utilized, which naturally detracts from the significance of the proposal in view of the total utilization of spent liquor and of wood.

The binding agent is made by mixing together fractionated lignin derivatives and phenol-formaldehyde resin. The phenol-formaldehyde resin is usually made of phenol and formaldehyde. Abundant use of formaldehyde promotes the formation of methylol groups in the phenol-formaldehyde resin. The abundant presence of methylol groups in the phenol-formaldehyde resin is to the advantage of the binding agent made from the fractionated lignin derivatives because it is likely that these are the particular groups which react with the lignin derivatives. It should be noted however, that abundant use of formaldehyde in the preparation of phenol-formaldehyde has the consequence that the phenol-formaldehyde thus produced contains much free formaldehyde, and this is a working safety risk owing to the volatility and toxicity of formaldehyde.Therefore the use of such phenol-formaldehyde containing free formaldehyde, as a component in a binding agent, is not recommended and it has been prohibited in several countries.

According to the invention there is provided a binding agent for the manufacture of plywood, chipboard and fibreboard and like products, said binding agent comprising a phenol-formaldehyde resin and alkali lignins fractionated by molecular weight, not less than 35% by weight and less than 50% by weight of the alkali lignins having a molecular weight higher than 5.000 and an aqueous solution of the binding agent having a pH between 8 and 14.

The invention is based on the unexpected fact observed in most recent research, that it is possible of fractionated alkali lignins of which even as little as at least 35% by weight preferably at least 40cue by weight, and less than 50% by weight have a molecular weight higher than 5,000. to prepare a weather-resistant binding agent with excellent strength, for use in the manufacturing of plywood, chipboard and fibreboard and like products. In the preparation of the phenol-formaldehyde resin used in the binding agent, phenol and formaldehyde are advantageously used in a molar proportion between 1:1.8 and 1:3.

By the invention, it is only necessary to carry the fractionation so far that at least 35% O/o by weight, preferably at least 40% by weight, and less than 50% by weight of the alkali lignins have a molecular weight higher than 5,000. The making of the binding agent of the invention will then be considerably more advantageous, and cheaper, compared, e.g. with that of the binding agents disclosed in the Finnish Patents, No. 51105 and 51946, in the case of which it has been necessary to fractionate the lignin derivatives used for them, much farther, so that more than 50% by weight of said lignin derivatives have a molecular weight higher than 5,000.On the strength of the facts presented, the manufacture of the binding agent of the invention is considerably superior in simplicity and low price to that of binding agents of the same type known in the prior art.

The molecular weight distributions of lignosulfonates and of alkali lignin may be determined by a gel chromatography method, as has been presented e.g. by Whitaker, J.R., Anal. Chem. 35 (1963):12, 1950-1953, Forss, K.G. and Stenlund, B.G. Paperir ja Puu 48 (1966):9, 565-574 and 11, 673-676. and by Forss, K.G., Stenlund, B.G. and Sagfors, P.-E., Applied Polymer Symposium No. 28 (1976), 1185-1194, John Wiley & Sons, Inc. In these procedures, the samples are eluted through a gel chromatography column. The molecular weight distributions are determined on the basis of the correlation between molecular weights and corresponding retention volumes. This correlation may be established by determining molecular weights of various fractions by the light scattering method or by applying osmometry or ultracentrifuging technique.However, these methods are exceedingly laborious and it is therefore more expedient from the practical point of view to calibrate the gel chromatography column by the aid of readily obtainable substances having known molecular weights. Such a substance is, for instance, glucagon, which has the molecular weight 3483. It is thus also possible to compare the molecular weights of the alkali lignin used in the binding agent of the invention, with glucagon. In such terms, preferably more than 45% by weight, of the alkali lignin derivatives used towards the binding agent of the invention have a molecular weight higher than that of glucagon.

An aqueous solution of the binding agent of the invention has a pH between 8 and 14.

When preparing the binding agent from acid lignin derivatives one may advantageously to the binding agent add alkali metal or alkaline earth metal hydroxide, for instance, in order to increase the pH to within the range from 8 to 14. The lignin derivatives are most appropriately used in the form of alkali metal or alkaline earth metal salts, possibly containing an excess quantity of the respective alkali. Alkalinity of the binding agent is also favourable because an alkaline binding agent causes less corrosion than an acid binding agent. Furthermore, when for instance sodium hydroxide is added to the binding agent the sodium hydroxide will lower the viscosity of the binding agent, and this facilitates the preparation and further handling of the substance.

In an advantageous embodiment of the invention, the binding agent contains between 1 and 15, preferably between 5 and 10%, by weight urea calculated on the amount of dry matter in the phenol-formaldehyde resin. On admixture of urea to phenol-formaldehyde resin, the urea reacts with the formaldehyde excess, thus binding the said formaldehyde excess present in the mixture. Thus, the use of urea in the binding agent improves the working safety and enables a formaldehyde excess improving the strength characteristics of the binding agent to be used in connection with the manufacturing of the phenolformaldehyde resin. Together with formaldehyde, urea forms a permanent polymer, thereby increasing the strength of the binding agent in this manner, too.

The lignin derivatives for use in preparing the glue of the invention are alkali lignins obtained from the alkaline digestion of raw material containing lignocellulose, such as the soda process (the digesting liquor containing sodium hydroxide), from the sulphate process (the digesting liquor containing sodium hydroxide, sodium sulphide and hydrosulphide) or from the oxygen-alkali process (digestion being carried out with sodium hydroxide in the presence of oxygen.

Fractionation of the alkali lignin derivatives may be effected by any fractionating method known per se, such as precipitation, extraction method or ultrafiltration. Such fractionation methods have been described, for instance, in the U.S. Patent No. 3,825,526.

The invention is described in detail in the following, with the aid of examples.

The molecular weight distributions of alkali lignins were determined by the following procedure in the examples.

The alkali lignin samples were analysed by gel chromatography, using "Sephadex" (Registered Trade Mark) columns (150 cm long, 1 cm diameter). The alkali lignins, solved in a sodium hydroxide/water solution, were eluted through a "Sephadex" G-50 column with NaOH (0.5 M) used as eluting agent. The rate of elution was 20 ml per hour.

The lignin concentration in the eluent fraction was determined by- absorption measurements (280 nm.). The retention volume was determined by weighing the eluent fractions.

In order to obtain results independent of the gel filling density, a relative retention volume scale was introduced, using two calibration substances as internal standards. The retention peak produced by Blue Dextran (M = 2.106) was adopted as first reference point, giving the value 0. The second reference point, equivalent to the value 1, was obtained by determining the retention peak obtained with sulfosalicylic acid (M = 218) Figure 1, Sephadex G-50).

Calibration of the columns was carried out by simultaneous determination of the ratio of the logarithm of molecular weight and the relative retention volume for substances of known molecular weight and which were easily obtainable. The Sephadex G-50 column was calibrated, using as reference Cytochrom C, Glucagon and Bacitracin (M = 1,423) and as eluting agent, NaOH (0.5 M) (Figures 1 and 2).

Hereby those lignin derivatives which are eluted through the gel chromatography column faster than glucagon (which have a lower relative retention volume) consist of molecules having a molecular weight higher than 3,483 (glucagon).

Example I (Comparison Example) Spent sulphate liquor from pine sulphate digestion (Pinus silvestris) was inspissated to 33% by weight dry matter content. The pH of the solution was 12.7. The molecular weight distribution was determined, using the Sephadex G-50 column according to the procedure described above. The chromatogram which was obtained is seen in Figure 3 (A). The molecular weight distribution, Figure 4 (A) was calculated by the aid of the calibration graph shown in Figure 2. Accordingly, 25.3% by weight of the alkali lignins in the sulphate spent liquor has a molecular weight higher than 5,000 and 32.9% by weight had one higher than 3,483 (glucagon).

In the preparation of phenol-formaldehyde resin the molar proportion 1:2.5 of phenol and formaldehyde was used. The resin had a dry matter content of 46% by weight and pH 11.1. The binding agent was prepared by admixing the phenol resin prepared (450 g) to the evaporated sulphate spent liquor (418 g). The mixture was agitated for 10 min. and thereafter there was added 132 g of a filler mixture consisting of wheat flour (13 g), quebracho (32 g), chalk (61 g) and wood flour (26 g). The binding agent had viscosity 240 mPa.s and pH 12.1.

The binding agent was used to manufacture 3-ply birch veneer sheets. The application was 150 g/cm', prepressing pressure 0.7 MPa and prepressure time 6 min. The sheets were hot-pressed at 135"C, under pressure 1.7 MPa, the pressure periods used were 2, 3 and 4 min. The properties of the sheets were determined in dry condition and after boiling, according to the Finnish plywood standard SFS 2416. The properties of the sheets are shown in Table 1, wherein each figure is the mean of five specimens.

Example 2 (Comparison Example) The spent sulphate liquor of Example 1 was ultrafiltrated, whereby an alkali lignin product was obtained the molecular weight distribution of which is shown in Figure 4, Graph B (the gel chromatogram, in Figure 3, B). Accordingly, 30.9% by weight of the lignin derivatives had a molecular weight higher than 5,000 and 40.0% by weight had one higher than 3,483 (glucagon). A binding agent was prepared as in Example 1, using the same phenolic resin. The viscosity, after filler addition, was 320 mPa.s.

Using this binding agent, 3-ply veneer sheets were made under identical conditions as in Example 1. The properties of the sheets are presented in Table 1.

Example 3 The alkali lignin product was isolated by ultrafiltration of a spent sulphate liquor. Its molecular weight distribution was determined by gel chromatography, the chromatogram being shown in Figure 3, C, and the molecular weight distribution in Figure 4, C.

Accordingly, 36.0% by weight of the sulphate lignins had a molecular weight higher than 5,000 and 46.5% by weight had one higher than 3,483 (glucagon).

The binding agent was prepared in Example 1. The binding agent had viscosity 420 mPa.s and pH 12.0.

Using this binding agent, 3-ply birch veneer sheets were made as in Example 1, and the properties of the sheets can be read from Table 1.

Example 4 By ultrafiltration, an alkali lignin fraction was prepared of which the gel chromatogram is shown in Figure 3, D, and the molecular weight distribution in Figure 4, D. Accordingly, 43.2% by weight of the alkalilignins had a molecular weight higher than 5,000, and 541.1% by weight had one higher than 3,483 (glucagon).

The binding agent was prepared as in Example 1; it had viscosity 470 mPa.s and pH 12.0.

Using this binding agent, 3-ply birch veneer sheets were prepared as in Example 1. The properties of the sheets can be read from Table 1.

Example 5 The spent sulphate liquor used in Example 1 was ultrafiltrated, whereby an alkali lignin fraction was obtained the gel chromatogram of which is shown in Figure 3, E. Its molecular weight distribution is shown in Figure 4, E. Accordingly, 46.9% by weight of the lignin derivatives had a molecular weight higher than 5,000, and 57.7% by weight had one higher than 3,483 (glucagon).

The alkali lignin fraction was used towards the preparation of a binding agent as in Example 1. The viscosity, after filler addition, was 560 mPa.s and pH was 11.9.

Using this binding agent, 3-ply veneer sheets were made as in Example 1. The properties of the sheets are shown in Table 1.

Example 6 (Comparison Example) Ultrafiltration was applied to isolate from the spent sulphate liquor used in Example 1, a high-molecular alkali lignin fraction of which the gel chromatogram is shown in Figure 3, F, and the molecular weight distribution in Figure 4, F, according to which 53.48to by weight of the alkali lignins had a molecular weight higher than 5,000 and 64.7% by weight had one higher than 3,483 (glucagon).

The binding agent was prepared as in Example 1. Its viscosity was 680 mPa. s at 25"C and its pH, 11.8.

This binding agent was used in the manufacturing of 3-ply birch veneer sheets applying the same conditions as in Example 1. The properties of the sheets have been shown in Table 1.

TABLE 1 Comparison of the gluing properties of spent sulphate liquor and different alkali lignin fractions.

Example Pressing In dry condition After boiling No. time Shearing Failure Shearing Failure min. strength through strength through N/mm2 wood, % N/mm2 wood, % 1 2 1.80 57 0.55 8 3 1.84 70 0.63 14 4 1.98 73 0.71 12 2 2 2.00 63 1.37 38 3 2.10 80 1.38 39 4 2.08 90 1.50 82 3 2 2.20 88 1.43 71 3 2.42 93 1.55 89 4 2.39 98 1.71 94 4 2 2.35 90 1.49 88 3 2.40 92 1.68 97 4 2.42 92 1.65 98 5 2 2.46 98 1.58 90 3 2.55 96 1.63 93 4 2.43 99 1.70 96 6 2 2.57 100 1.65 99 3 2.60 100 1.76 100 4 2.58 98 1.72 92 As specified in the Finnish plywood standard SFS 2415, the shearing strength in dry condition shall not be less than 2.10 N/mm, after boiling not less than 1.40 N/mm2, and if the values fall below this, the failure through wood shall not be less than 50%.In practice, however, considerably much higher failures through wood are required, i.e., higher than 80% after boiling as well.

Scrutiny of the characteristics of the sheets, presented in Table 1, reveals that the requirements were met in Experiments 3 to 6 already with pressing times 2 and 3 min. The binding agent of Example 2 required a longer pressing time for meeting the specifications.

In Example 1, prolonged pressing time was of no avail: the glue joint could not harden to become waterproof.

Example 7 As raw material for the binding agent, sulfate lignin was used in which the weight proportion of high molecular constituents (Mw > 5,000) was 40% of all lignin derivatives and 50% by weight had a molecular weight higher than that of glucagon.

As phenol-formaldehyde resin a product was used which had been prepared from phenol and formaldehyde in molar proportion 1:2.5 and contained 2.1% by weight free formaldehyde. To the solution urea was added in an amount of 3% by weight per resin dry matter. The mixture was heated to 300C and agitated for 3 hrs. After that the formaldehyde content decreased to 0.4% by weight.

The treated 46% by weight phenol-formaldehyde resin (520 g) was admixed with the sulfate lignin solution (340 g), which had 30% dry matter content. The pH of the glue was 11.8 and its viscosity, 160 mPa.s. Subsequently, 140 g of filler mixture were added, containing wheat flour 14g, chalk 56 g, quebracho 35g and wood flour 35 g; the viscosity was 640 mPa.s after 30 min. mixing.

The glue mixture was used to make 3-ply birch veneer sheets. The application was 150 g/m2, prepressing time 6 min. and prepressing pressure 0.8 MPa. The sheets were hot-pressed at 1350C under pressure 1.8 MPa. The pressing times were 2,2 1/2, 3 and 4 min.

The properties of the sheets were determined in dry state and after boiling. In the table below (Table 2) each figure is the mean from five sheets, that is from 25 specimens.

TABLE 2 Characteristics of 3-ply birch veneer sheets Pressing time In dry condition After boiling (min). Shearing Failure Shearing Failure strength through strength through N/mm2 wood % N/mm wood % 2 3.31 93 1.58 81 2 1/2 2.95 100 1.62 92 3 3.06 98 1.88 94 4 3.21 97 1.66 91 With pressing times shorter than normal, too, (2 and 2 1/2 min.) good results were obtained with this binding agent.

Example 8 For the preparation of binding agent alkali lignin was used in which the high molecular proportion (MW > 5,000) by weight was 48% of all lignin derivatives, and 59% by weight had a molecular weight higher than that of glucagon.

The phenol-formaldehyde resin had been prepared with molar proportion 1:2.2 and it contained 1.7% by weight free formaldehyde. To the resin 5% by weight urea per resin dry matter were added. The mixture was agitated at room temperature during 5 hrs. The free formaldehyde quantity had then gone down to 0.08% by weight. The resin-urea mixture was added to the alkalilignin solution, which had 42% by weight dry matter content (432 g of 51% by weight resin to 524 g of lignin solution) 50% sodium hydroxide solution was added, 44g. The mixture was agitated during 30 min. and pH was 11.8. The viscosity at 25"C was 320 mPa.s.

Chipboards were made using the binding agent prepared. Thereafter to it was added 10% of paraffin emulsion (dry weight referred to glue dry matter).

Using this binding agent, chipboards of 15, 20 and 30 mm thickness were made which had nominal weight 750 g/m3. The binding agent quantity sprayed onto the chips was 10% by weight, referred to dry matter. In the hot pressing of the chipboard combined contact and high frequency heating was applied. The press plate temperatures were 1800C and the pressure, 2.65 N/mm2. The pressing times and the characteristics of the boards can be read from the table below.

TABLE 3 Characteristics of chipboards, using combined contact and high frequency heating.

Board Pressing time used Bending Tensile Thickness V 100* thick- min. sec/mm strength strength swelling ness, N/mm2 N/mm2 2 h 24 h N/mm2 mm % 15 4 1/2 18 19.8 0.47 2.3 11.5 0.21 20 5 15 19.2 0.52 1.8 10.8 0.19 30 5 1/2 11 19.2 0.49 2.2 11.1 0.23 DIN 68761 17.7 > 0.34 46.0 412.0 > 0.15 specification *V100 means boiled in water at 100"C The requirements imposed by the German standard DIN 68761 on phenol-glued chipboards were fully met by the boards. The boards were completely free of formaldehyde after hot pressing.

Example 9 For the binding agent a phenol-formaldehyde resin was prepared having phenol:formal- dehyde molar proportion 1:3. After preparation, it contained 4.8% by weight free formaldehyde. 10% by weight urea were added to the resin and the mixture was agitated at 25 C for about 4 hrs.

The mixture was then left to stand another 24 hours, and thereafter the free formaldehyde was determined. It had gone down to 0.1% by weight.

The alkali lignin fraction the characteristics of which have been presented in Example 8 was used for the binding agent preparation. To 640 g of the alkali lignin solution, dry matter content 42% by weight 360 g of the above-mentioned phenol-formaldehyde-urea resin were added, the latter having 50% dry matter content. The proportion of lignin and pheno-formaldehyde resin was 60:40. The binding agent had pH 11.0.

The binding agent was used to make formaldehyde-free chipboards.

The binding agent quantity was 8% by weight as dry matter calculated on the weight of dry centre chips and 10% of the weight of surface chips. The chips were furthermore sprayed with paraffin emulsion at 1% dry matter per dry chip weight. Using this binding agent, 15 mm 3- chipboard with 750 kg/m3 volume weight were made. The press plate temperature was 215"C, pressing pressure 2.96 MPa and pressing time, 20 sec/mm. After hot pressing the boards were after-hardened at 1800C.

The characteristics of the boards are shown in Table 4.

TABLE 4 Characteristics of chipboards, employing after-hardening.

Board Thick- Volume Bending Tensile Thickness V 100 No. ness weight strength strength swelling mm kg/m3 N/mm2 N/mm2 2 h 24 h N/mm2 % % 1. 15.08 760 20.6 0.47 2.4 9.8 0.18 2 15.10 758 20.3 0.46 2.5 10.2 0.19 3 15.08 756 20.5 0.55 1.9 10.5 0.21 4 15.09 762 20.9 0.54 2.8 11.0 0.20 5 16.10 755 19.4 0.46 2.5 10.8 0.15 The boards met the specification of DIN 68761. No - smell of formaldehyde was observable during the manufacturing and the formaldehyde was not released from the boards afterward.

The viscosities stated in the embodiment examples were measured with a Brookfield viscometer at 25"C, 50 r.p.m.

The invention is naturally not confined to the embodiment examples presented, and its embodiments may vary within the scope of the attached claims.

Hereby the dry weight proportion of the lignin derivatives and the phenol-formaldehyde resin contained in the binding agent may vary e.g. within the range from 90:10 to 20:10.

Further the molar proportion of phenol and formaldehyde in the phenolic resin may vary, for instance, in the range from 1:1.4 to 1:3. Further, it is possible to the glue prepared from the binding agent to add fillers known in themselves in prior art, such as chalk, wood flour, quebracho and wheat flour.

WHAT WE CLAIM IS: 1. A binding agent for the manufacture of plywood, chipboard and fibreboard and like products, said binding agent comprising a phenol-formaldehyde resin and alkali lignins fractionated by molecular weight, not less than 35% by weight and less than 50% by weight of the alkali lignins having a molecular weight higher than 5,000 and an aqueous solution of the binding agent having a pH between 8 and 14.

2. A binding agent according to claim 1, wherein over 45% by weight of the alkali lignins have a molecular weight higher than that of glucagon.

3. A binding agent according to any preceding claim, wherein the alkali lignins are in the form of an alkaline alkali metal salt or alkaline earth metal salt.

4. A binding agent according to any preceding claim, wherein the binding agent contains urea in an amount from 1 to 15% by weight, calculated on the dry matter quantity of the phenol formaldehyde resin.

5. A binding agent according to claim 4, wherein urea is present in an amount of from 5 to 10% by weight calculated on the dry matter quantity of the phenol-formaldehyde resin.

6. A process for the preparation of a binding agent according to any preceding claim, said process comprising mixing phenol-formaldehyde resin with an aqueous solution of alkali lignins fractionated by molecular weight, not less than 35% by weight and less than 50% by weight of the alkali lignins having a molecular weight higher than 5,000, the pH of the resultant mixture being adjusted, if necessary to be in the range of from 8 to 14.

7. A process according to claim 6, wherein a filler is mixed with the resin and alkali lignins.

8. A process according to claim 6 or claim 7 wherein more than 45% by weight of the alkali lignins have a molecular weight higher than that of glucagon.

9. A process according to any of claims 6 to 8 wherein the phenol-formaldehyde resin is made from phenol and formaldehyde in a molar proportion between 1:1.4 and 1:3.

10. A process according to claim 9 wherein the phenol-formaldehyde resin is made from phenol and formaldehyde in a molar proportion between 1:1.8 and 1:3.

11. A process according to any one of claims 6 to 10 wherein urea is added to the phenol-formaldehyde resin.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. TABLE 4 Characteristics of chipboards, employing after-hardening. Board Thick- Volume Bending Tensile Thickness V 100 No. ness weight strength strength swelling mm kg/m3 N/mm2 N/mm2 2 h 24 h N/mm2 % % 1. 15.08 760 20.6 0.47 2.4 9.8 0.18 2 15.10 758 20.3 0.46 2.5 10.2 0.19 3 15.08 756 20.5 0.55 1.9 10.5 0.21 4 15.09 762 20.9 0.54 2.8 11.0 0.20 5 16.10 755 19.4 0.46 2.5 10.8 0.15 The boards met the specification of DIN 68761. No - smell of formaldehyde was observable during the manufacturing and the formaldehyde was not released from the boards afterward. The viscosities stated in the embodiment examples were measured with a Brookfield viscometer at 25"C, 50 r.p.m. The invention is naturally not confined to the embodiment examples presented, and its embodiments may vary within the scope of the attached claims. Hereby the dry weight proportion of the lignin derivatives and the phenol-formaldehyde resin contained in the binding agent may vary e.g. within the range from 90:10 to 20:10. Further the molar proportion of phenol and formaldehyde in the phenolic resin may vary, for instance, in the range from 1:1.4 to 1:3. Further, it is possible to the glue prepared from the binding agent to add fillers known in themselves in prior art, such as chalk, wood flour, quebracho and wheat flour. WHAT WE CLAIM IS:

1. A binding agent for the manufacture of plywood, chipboard and fibreboard and like products, said binding agent comprising a phenol-formaldehyde resin and alkali lignins fractionated by molecular weight, not less than 35% by weight and less than 50% by weight of the alkali lignins having a molecular weight higher than 5,000 and an aqueous solution of the binding agent having a pH between 8 and 14.

12. A process according to claim 11 wherein urea is added to the phenol-formaldehyde

resin in a quantity of 1 to 15 % by weight, calculated on the dry matter quantity of the resign.

13. A process according to claim 12, wherein urea is added to the phenol-formaldehyde resin in a quantity of from 5 to 10% by weight calculated on the dry matter quantity of the phenol formaldehyde resin.

14. A binding agent according to claim 1 substantially as described herein with reference to any one of the Examples.

15. A process for the preparation of a binding agent according to claim 6 substantially as described herein with reference to any one of Examples 3, 4, 5, 7, 8 and 9.