WO1996015169A1

WO1996015169A1 - Resins

Info

Publication number: WO1996015169A1
Application number: PCT/AU1994/000687
Authority: WO
Inventors: Peter James Collins; Yoshikazu Yazaki
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1993-05-12
Filing date: 1994-11-09
Publication date: 1996-05-23

Abstract

A method for preparing a phenolic resin, the method including a first stage in which an aldehyde compound is reacted with a phenol compound in a molar ratio of aldehyde to phenol of less than 1 in the presence of a catalyst such that the pH does not rise above about 10.5 to form an intermediate product; and a second stage in which the intermediate product is reacted with additional aldehyde compound such that the molar ratio of aldehyde to phenol is at least 1 in the presence of a strongly alkaline catalyst.

Description

RESINS

The present invention relates to phenolic resins and to adhesive compositions containing these resins. More particularly it relates to phenolic resins suitable for the manufacture of adhesives for the bonding of wood and wood-based products, especially adhesives for use in the manufacture of plywood, particleboard, medium density fibreboard (MDF), laminated veneer lumber (LVL) and similar wood-based products. The invention is also concerned with a method for making such phenolic resins.

Conventional phenolic resins for use in manufacturing such adhesives are known as resoles and are usually prepared by reacting phenol with formaldehyde in the presence of a strongly alkaline catalyst such as sodium or potassium hydroxide. The molar ratio of formaldehyde to phenol (the A/P ratio) used is typically about 1.5 to about 3.5 and the reaction, especially in its initial stages, is highly exothermic and difficult to control so that the provision of adequate cooling is essential to obtain precise control over the reaction temperature.

Some or all of the formaldehyde and/or the alkaline catalyst may be added to the phenol at the beginning of the reaction, with the remainder being introduced portionwise or at a controlled rate as the reaction proceeds to facilitate keeping the reaction temperature within predetermined limits. The viscosity of the reaction mixture rises as the reaction proceeds, due to the formation of phenol- formaldehyde polymers whose molecular weight increases as the reaction progresses. This increase in viscosity must be carefully monitored and is frequently used to determine when further additions of formaldehyde or catalyst, or both, should be made.

A common procedure is to initially add some or all of the alkaline catalyst and to meter in the formaldehyde as the reaction proceeds, either continuously or portionwise until predetermined viscosity points are reached, at which points further additions of catalyst and/or formaldehyde may be made. The reaction is then continued until the desired end-point viscosity is achieved.

If only a portion of the alkaline catalyst is added at the beginning, the reaction mixture may have an initial pH of about 9, but as further catalyst is introduced the pH rises and may attain a final value of about 12 or even as high as 13.

Regardless of which of the many variants known in the art are employed in their manufacture, the aforementioned resole-type phenolic resins all exhibit certain common characteristics as a result of the chemical reactions that occur during their formation. Essentially these reactions result in the linking together of phenolic rings through bridging methylene groups with a concomitant increase in molecular weight, together with the introduction of methylol substituents into the aromatic nuclei. As the reaction proceeds some of the methylol groups further react to create additional methylene bridges, thus increasing the molecular weight still further and raising the viscosity of the reaction mixture. If the reaction is allowed to proceed too far or in an uncontrolled fashion the molecular weight and viscosity rise very rapidly, with the onset of crosslinking and eventual gelation. Such a situation must be avoided if resins suitable for the manufacture of wood adhesives are to be obtained.

Overall it is found that the reaction between phenol and formaldehyde catalysed by strong alkalis results at any given time in the formation of a mixture of methylene and methylol compounds, and that the reaction rate and the balance between methylene and methylol compounds i.e. the composition of the resin are strongly temperature dependent. Additionally, it has been found that viscosity is not a good indicator of the composition of the resin, in that it is possible to obtain resins having the same viscosity but widely different compositions. The aforementioned phenolic resins possess a number of disadvantages that mitigate against their use in the manufacture and application of wood adhesives. Firstly, they are unstable and must be cooled or refrigerated during transport to and storage at the point of use if an acceptable and economic service life is to be obtained. Storage life of the resin may be extended by adding extra sodium hydroxide but in many instances this is unacceptable as the final cure time may become excessive or the end product unacceptably hygroscopic.

In the formulation of wood adhesives from conventional phenolic resins it is customary to add one or more extenders in order to reduce the cost of the adhesive. Formulation of the adhesive is normally carried out at the point of use. The extender also serves to thicken the resin and may typically comprise wheat flour which is often digested with sodium hydroxide. Where it is required, digestion of the flour and blending with the resin not only introduces an extra step into adhesive preparation but is in itself a hazardous operation requiring the use of protective clothing. Whilst digestion with alkali would pose no special problems if carried out in a chemical manufacturing plant, it is not well suited to being performed in the environment of a wood products factory. There is thus a need to provide a phenolic resin which can be extended and formulated as an adhesive by simpler, less hazardous procedures.

We have found, surprisingly, that phenolic resins suitable for the formulation of wood adhesives and which overcome many of the aforementioned disadvantages of conventional phenolic resoles may be prepared by a two-stage reaction of a phenol with an aldehyde in which the first stage employs a catalyst such that the pH does not rise above about 10.5 and the second stage employs a strongly alkaline catalyst. The resins of the invention have been found to be faster curing yet to possess a surprisingly enhanced storage life as compared with conventional resole-type resins. They are readily formulated to produce adhesives for the manufacture of plywood and LVL, though they are also suitable as the basis for the adhesives used in manufacturing other wood composites such as particleboard and MDF. In general, the resins can be formulated for a wide range of applications without the need to resort to special additives. Moreover, they can be readily extended by the direct addition of wheat flour without the latter having to be pre-digested with sodium hydroxide.

Adhesives formulated from the resins of the invention exhibit enhanced tolerance to the moisture content of the wood substrate in comparison to conventional phenolic-based wood adhesives which, in general, are very use-specific and only perform satisfactorily over a narrow range of wood moisture contents. For example, adhesives based on the resins of the invention will successfully bond plywood veneers having moisture contents of about 12%.

It is believed that the superior properties of the resins of the invention result from their differing substantially in composition and structure from conventional phenolic resoles. The composition of the resins will be described in more detail later.

According to the present invention there is provided a method for preparing a phenolic resin said method including: 1) a first stage including reacting an aldehyde compound with a phenol compound in a molar ratio of aldehyde to phenol (A to P ratio) of less than 1 in the presence of a catalyst such that the pH does not rise above about 10.5 to form an intermediate product; and

2) a second stage including reacting the intermediate product with additional aldehyde compound such that the molar ratio of aldehyde to phenql (A to P ratio) is at least 1 in the presence of a strongly alkaline catalyst.

A feature of the method of the invention is that the two stages of the reaction are easier to control than the production of conventional resoles and the conditions required are not narrowly critical. The first stage is conveniently carried out under reflux conditions and typically is completed within about 2 to 5 hours. The reaction mixture may then be cooled for the second stage which is only mildly exothermic and may be conducted at temperatures up to about 80°C. After the exotherm has subsided the mixture may be heated to a predetermined temperature and held at that temperature until the desired end-point viscosity, typically 300-400 cP, is reached.

Any suitable phenol compound may be used in the practice of the invention. Suitable phenol compounds include phenol, substituted phenols such as the cresols, and polyhydric phenols such as resorcinol, or mixtures of two or more thereof. The use of particular phenols may be desirable in special circumstances or in order to impart specific properties to the end-product resin. For reasons of cost and availability phenol is generally preferred.

The aldehyde to be used may be any aldehyde capable of reacting to form methylene bridges between phenolic nuclei. Formaldehyde is preferred on account of its low cost, reactivity and ready availability in the form of an aqueous solution typically containing about 50% of the aldehyde.

The catalyst in the first stage is preferably a mildly alkaline catalyst.

In accordance with a further aspect the invention provides novel phenolic resins suitable for use as adhesives. Accordingly, the present invention provides a phenolic resin said resin having a polydispersivity in the range of about 15.0 to about 22.0.

In yet a further aspect the present invention provides a phenolic resin adhesive composition including a phenolic resin in accordance with the invention.

The phenolic resin adhesive composition of the invention may further include one or more extenders. The extender may be starch, wheat flour or other extenders known in the art.

The phenolic resin adhesive composition may include a surfactant for particular pruposes, for example, to bond difficult to glue wood. The surfactant may be one or more surfactants, for example, the hydrophilic anionic surfactants and fiuorochemical surfactants described in Australian patent application 77851/91 , the disclosure of which is incorporated herein by reference.

The two stages of the reaction will now be described in more detail, for convenience with respect to the use of phenol and formaldehyde though it is to be understood that the description is not limited to the use of these compounds.

Stage 1

In Stage 1 phenol is reacted with an aqueous solution of formaldehyde in the presence of a water-soluble alkaline catalyst under conditions such that essentially all of the formaldehyde is consumed in the formation of methylene linkages between the phenol nuclei. It is preferred that the molar ratio of formaldehyde to phenol compound (A/P ratio) lie in the range from about 0.2 to 0.999. For the manufacture of resins suitable for use in wood adhesives an A/P ratio of about 0.6 to about 0.8 is preferred and when the resin is intended for use in adhesives for the manufacture of plywood it is preferred that the ratio lie in the range of about 0.7 to about 0.75.

During Stage 1 it is essential that the pH not be allowed to exceed about 10.5 and this governs the choice of the catalyst that may be used. Suitable catalysts include alkaline phosphate or other buffers having pH values in the range from about 7 to 10.5 and alkali metal carbonates, though other water-soluble alkaline catalysts may be used. Sodium and potassium carbonates are particularly preferred. If stronger bases than the alkali metal carbonates are used, such as sodium hydroxide, extreme care is necessary to ensure that the abovementioned pH limit is not exceeded and this in turn requires very precise control of the small amount of catalyst that can be tolerated. A further disadvantage of using catalysts more strongly basic than sodium carbonate is that the necessarily low concentration thereof imposed by the aforementioned pH limitation may result in an unacceptably slow reaction.

When the catalyst used is sodium carbonate, the molar ratio of sodium carbonate to phenol compound should lie in the range from about 0.03 to 0.2. Below this range the rate of reaction is unacceptably slow whilst at ratios above 0.2 the sodium carbonate may not dissolve completely in the limited amount of water present. The optimal amount of sodium carbonate to be used must be determined by experiment, taking account of the amount of water present and the level of sodium carbonate that can be tolerated in the final adhesive composition since it is well known that the presence of excessive amounts of sodium carbonate in a phenolic wood adhesive decreases the stability of the adhesive and promotes glueline dryout. Generally we prefer to use about 0.05 to 0.08 mole of sodium carbonate per mole of phenol, especially when the final Stage 2 resin is intended for use in plywood production. The amount of water to be added in Stage 1 , additional to that introduced with the formaldehyde, is determined by the amount of sodium carbonate used and the percentage resin solids content desired at the completion of Stage 2 and may be calculated by methods well known in the art.

In the practice of Stage 1, it is convenient for ease and speed of operation to charge the phenol, formaldehyde solution, catalyst and additional water (if any) to a reactor and allow the temperature of the mixture to rise to reflux. Alternatively, and preferably, the phenol, water and catalyst may be charged initially and the formaldehyde then introduced, either as a single charge or by metering it in.

After the initial reaction reflux conditions are maintained until substantially all methylol compounds have been converted to methylene bridged compounds. The time required for the disappearance of methylol compounds is generally 2 to 5 hours. If desired, reflux conditions may be maintained beyond this point without adversely affecting the suitability of the product for use in Stage 2.

In general, the above reaction conditions are not narrowly critical and other conditions may be suitable provided the reaction is progressed to the point where essentially no free methylol compounds are present.

The product of stage 1 comprises a mixture of unreacted phenol, low molecular weight phenolic compounds and highly polymerised novolac-type compounds, depending on the A/P ratio chosen, having a wide range of molecular weights and molecular weight distribution. The polydispersivity of the product i.e. the ratio of its weight average molecular weight (M_w) to its number average molecular weight (M_n) is also high. For example, a Stage 1 resin made as described above from phenol (1 mole), formaldehyde (0.73 mole) and sodium carbonate (0.07 mole) was found to have an M_w of 40449 and an M_n of 2044, corresponding to a polydispersivity of 19.8.

_____n2 For Stage 2 the product from Stage 1 is preferably cooled to a temperature of 80°C or less. Water and a strongly alkaline catalyst are added, followed by additional formaldehyde. The reaction mixture is held at a predetermined temperature, of about 80°C, until the mildly exothermic reaction has subsided, whereupon the temperature is adjusted to about 75°C and held at that level until the desired end-point viscosity, typically 300-400 cP at 25°C, is reached.

The preferred catalyst for Stage 2 is sodium hydroxide but other alkali metal hydroxides, ammonium hydroxide, quaternary ammonium hydroxides, and amines may be used. The amount of catalyst may range from about 0.05 to 1.0 mole per mole of phenol compound. In general, the level of catalyst used in Stage 2 should be substantially greater than that used in Stage 1. The optimal level of catalyst must be determined by experiment, having regard also to the overall solids content and the end use to which the resin is to be put. As noted above, excessive levels of sodium hydroxide, for example, may adversely affect curing of the final adhesive or render the wood product unacceptably hygroscopic. When the resin is intended for use in the manufacture of plywood we prefer to use about 0.6 to about 0.8 mole sodium hydroxide per mole of phenol compound, but lower ratios are desirable in resins for particleboard manufacture.

The A/P ratio should also be higher than that used in Stage 1 and may lie in the range from about 1.0 to 2.2 or even higher. A ratio of 1.0 to 1.6 has been found satisfactory for resins intended for plywood manufacture. The temperature at which the exothermic phase of the Stage 2 reaction is. conducted is not narrowly critical, however, it is preferred that the indicated upper limit of about 80°C is not exceeded. We have found no substantial difference in the molecular weight, composition and properties of the final resin when the exothermic phase is controlled at temperatures in the range 40 to 80°C. For convenience we prefer a temperature of about 75°C, which allows completion of the exothermic phase in about 30 minutes.

Addition of the water and alkaline catalyst prior to the formaldehyde, as indicated above, assists in cooling the Stage 1 product to the desired temperature. The introduction of the catalyst prior to the formaldehyde is believed to be important in determining the course of the reaction when the formaldehyde is subsequently added, either as a single charge or gradually. The conditions for the first part of Stage 2 are essentially those of a conventional methylolation reaction which is well known to be favoured by lower temperatures and high levels of strong alkali.

The resins produced in Stage 2 contain both a highly polymerised phenol- formaldehyde resin portion and low molecular weight phenol-formaldehyde condensates comprising essentially mono-, di- and trimethylol phenols and methylolated diphenylmethanes and trinuclear phenols. They are characterised by having very wide molecular weight distributions and polydispersivities which, it is believed, contribute to their enhanced stability and tolerance to wood moisture content as compared with conventional resole-type adhesives.

Typically, in a Stage 2 resin M_n, M_w and the polydispersivity, D, are found to have increased only slightly over the values for the corresponding Stage 1 resin, as evidenced in the following table for a Stage 2 resin prepared at an A/P ratio of 1.27 from the resin mentioned in the description of Stage 1 above. The Stage 2 resin had a viscosity of 380 cP at 41.5% solids. The polydispersivity (D) of the intermediate product produced in Stage 1 may be in the range of about 14,0 to 20.0. The polydispersivity (D) of the product produced in Stage 2 may be in the range of about 15.0 to 22.0.

a o

Stage 1 0.73 2044 40449 19.8

Stage 2 1.27 2264 48806 21.6

Total 2.0

We have found that the polydispersivity of Stage 2 resins is significantly higher than that of phenolic resoles conventionally used as wood adhesives, as shown in the following table for three such commercial phenol formaldehyde resoles A, B and C, A and B having been made at a A/P ratio of 1.8. Resole C is a commercial fast cure resin made at a A P ratio in excess of 2.2 and having a very short storage life.

Resole M_n M_w D

A 2803 24585 8.8

B 2857 29849 10.5

C 3441 41646 12.1

The invention is further illustrated by reference to the following non-limiting examples. Example 1

This example demonstrates the two-stage preparation of a resin according to the invention and having enhanced storage stability compared with conventional resoles used in the manufacture of wood adhesives. For Stage 1 the following ingredients were charged to a reaction vessel, the mixture stirred and allowed to reach reflux temperature.

Ingredient Parts Molar ratio

5 By weight %

Phenol 94.0 55.52 1.0 0 Water 24.0 14.18

Sodium carbonate 7.7 4.55 0.07

Formaldehyde (50% solution) 43.6 25.75 0.73 *5

169.3 100.00

The mixture was held at reflux temperature (approximately 100°C) for 3 hours, at 0 which time NMR analysis indicated substantially no methylol groups remained. The mixture was then cooled to approximately 55°C by addition of the water and sodium hydroxide solution required for Stage 2, as shown in the table below, and by external cooling.

5 Inqredient Parts Molar ratio

By weight %

Water 30.2 14.10 0 Sodium hydroxide (25% solution) 107.3 50.12 0.67

Formaldehyde (50% solution) 76.6 35.78 1.27 5

214.1 100.00

Total (Stages 1 and 2) 383.4 0

The formaldehyde solution for Stage 2 was then added and the mixture stirred and kept at a temperature of 55°C for 30 minutes, by which time the methylolation exotherm had subsided. The temperature was then raised to 75°C and kept at that level until the desired end-point viscosity of 380 cP at 25°.C was reached. The solids content of the finished resin was 41.5%.

Storage stability was assessed by measuring the time taken for the viscosity to double when the resin was stored at 25°C. For the above resin this period was 33 days, as compared with only 4 to 5 days for a commercial fast curing resole suitable for plywood manufacture.

Example 2 This example demonstrates the superior performance of the resin of Example 1 as an adhesive for LVL, and as an extended phenol-formaldehyde adhesive for plywood manufacture that can tolerate a wide range of assembly times without loss of gluebond quality. It further shows that the superior results in bonding plywood were obtained without the extender having to be predigested with sodium hydroxide.

The resin of Example 1 was evaluated by making plywood and LVL and testing the products according to Australian Standards 2098.2 (1977) and 2754.1 (1985).

Plywood and LVL samples were prepared by bonding veneers from radiata pine (Pinus radiata). The LVL samples were made from 21 veneers each 3.2 mm thick, and the plywood from 7 veneers each 2.5 mm thick. The moisture content of the LVL veneers was 4.4% and that of the plywood veneers 4.0%

The overall adhesive composition for LVL comprised 76.6% resin, 13.8% filler and extender, and 9.6% water. The adhesive composition for plywood was that given in the footnote to the table below. The glue spread for LVL was 480 grams per m² of double glueline and 390 grams per m² for plywood. The test results were as follows: Sample Resin Total Hot Press Gluebond

Assembly Quality

Time Temp Time (Average)

(min) (PC) (min) Dry V.P.S.^*

LVL Example 1 60 185 34.15 8.8

Conventional 60 185 47.00 >5.0

72 h Boil

Plywood A 30 140 8.00 8.0 8.3

A 180 140 8.00 8.3 8.5

B 120 140 10.00 >5.0

(^*) Tested after vaccum pressure impregnation with water Plywood samples

(A) Example 1 resin plus 16 parts per 100 parts resin of extender and 36 parts per 100 parts resin of water; no NaOH.

(B) Conventional resin plus 16 parts per 100 parts resin of predigested extender; extender predigested in 30 parts per 100 parts resin of water with 3.25 parts per 100 parts resin of 46% NaOH.

Although the invention has been described in reference to particular embodiments it will be clear to the reader that the present invention is susceptible to modification or variation without departing from the spirit or scope of the invention.

Claims

CLAIMS:

1. A method for preparing a phenolic resin said method including:

(i) a first stage including reacting an aldehyde compound with a phenol compound in a molar ratio of aldehyde to phenol (A to P ratio) of less than 1 in the presence of a catalyst such that the pH does not rise above about 10.5 to form an intermediate product; and

(ii) a second stage including reacting the intermediate product with additional aldehyde compound such that the molar ratio of aldehyde to phenol (A to P ratio) is at least 1 in the presence of a strongly alkaline catalyst.

2. A method according to claim 1 wherein the A to P ratio in the first stage is in the range of about 0.2 to 0.999.

3. A method according to claim 2 wherein the A to P ratio in the first stage is in the range of 0.6 to 0.8.

4. A method according to claim 3 wherein the A to P ratio in the first stage is in the range of about 0.7 to 0.75.

5. A method according to any one of the preceding claims wherein the catalyst used in the first stage is a mildly alkaline catalyst.

6. A method according to any one of the preceding claims wherein the catalyst used in the first stage is selected from a buffer having a pH in the range of from about 7 to 10.5, an alkali metal carbonate and a water soluble alkaline catalyst.

7. A method according to claim 6 wherein the buffer is an alkaline phosphate buffer.

8. A method according to claim 6 wherein the buffer is sodium carbonate or potassium carbonate.

9. A method according to claim 8 wherein the catalyst is present in a molar ratio of catalyst to phenol compound in the range of from about 0.03 to 0.2, preferably 0.05 to 0.08.

10. A method according to any one of the preceding claims wherein the first stage is carried out under reflux conditions until substantially all methylol compounds have been converted to methylene bridged compounds.

11. A method according to any one of the preceding claims wherein the phenol compound is selected from phenol, a substituted phenol, a polyhydric phenol, or mixtures of two or more thereof.

12. A method according to claim 11 wherein the phenol compound is selected from phenol, resorcinol, a cresol or mixtures of two or more thereof.

13. A method according to any one of the preceding claims wherein the aldehyde compound is formaldehyde.

14. A method according to any one of the preceding claims wherein the intermediate product from the first stage is cooled to a temperature of 80°C or less before carrying out the second stage.

15. A method according to any one of the preceding claims wherein the strongly alkaline catalyst is selected from alkali metal hydroxides, ammonium hydroxide, quaternary ammonium hydroxides or amines.

16. A method according to claim 15 wherein the strongly alkaline catalyst is sodium hydroxide.

17. A method according to any one of the preceding claims wherein the strongly alkaline catalyst is present in an amount in the range of about 0.05 to 1.0 mole per mole of phenol compound, preferably about 0.6 tc about 0.8 mole per mole of phenol compound.

18. A method according to any one of the preceding claims wherein the

A to P ratio in the second stage is in the range from about 1.0 to 2.2.

19. A method according to claim 18 wherein the A to P ratio is in the range of about 1.0 to 1.6.

20. A method according to any one of the preceding claims wherein the second stage is carried out at a temperature not exceeding 80°C.

21. A method according to claim 20 wherein the second stage temperature is in the range of 40 to 80°C.

22. A method according to claim 21 wherein the temperature is about 75°C.

23. A phenolic resin produced by a process in accordance with any one of the preceding claims.

24. A pheonlic resin suitable for use in a wood adhesive said resin having a polydispersivity in the range of about 15.0 to about 22.0.

25. A resin according to claim 24 wherein the resin is formed from the reaction of phenol with formaldehyde.

26. A resin according to any one of claims 23 to 25 wherein the viscosity of the resin is in the range of 300-400 cP at 25°C.

27. A phenolic resin adhesive composition including a phenolic resin according to claims 23 or 26.

28. An adhesive composition according to claim 27 wherein the composition includes an extender.

29. An adhesive composition according to claim 27 or claim 28 wherein the adhesive composition contains a surfactant.

30. An adhesive composition according to claim 29 wherein the surfactant is one or more surfactants selected from fiuorochemical surfactants or hydrophilic anionic surfactants.

31. An adhesive composition according to any one of claims 27 to 30 including a filler.

32. A method for bonding a wood-based product wherein the adhesive is an adhesive composition in accordance with any one of claims 27 to 31.

33. A method according to claim 32 wherein the wood-based product is selected from plywood, particle board, medium density fibreboard or laminated veneer lumber.