GB2089358A - Liquid phenolic resin composition, a process for its preparation and a process for waferboard manufacture using the resin composition - Google Patents

Abstract

A pre-cure resistant liquid phenol-formaldehyde resin binder composition is disclosed having low viscosity and low surface tension and comprising a highly condensed and cross-linkable phenol-formaldehyde resin of relatively high average molecular weight e.g. in the range of from 2000 to 6000, and a viscosity of 100 to 450 cps at 25 DEG C and a non- resinous methylolated phenol condensate having an average molecular weight of 200-300. The composition is formed by reacting phenol and formaldehyde with an alkaline catalyst at above 80 DEG C, reducing the temperature of the resulting resin to about 60 DEG C, adding further phenol, formaldehyde and alkaline catalyst and reacting the resin further at 45-70 DEG C, then chilling to below room temperature. Uses:- as a binder in waferboard manufacture.

Description

SPECIFICATION Liquid phenolic resin composition, a process for its preparation and a process for waferboard manufacture using the resin composition This invention relates to a pre-cure resistant liquid phenol-aldehyde condensation resin composition, a process for its preparation and a process for waferboard manufacture using the resin composition.

In the prior art manufacture of waferboard, green wood wafers, which have been dried to 36% moisture content, are sprayed with about 2% of molten wax based on the dry wood weight, and 2-3 weight percent of powdered phenolic resin is blown into a rotating blender containing the wood wafers.

The resulting wax- and resin-coated wafers are felted on a warm caul plate which is recycled through a hot-press. In mill practice, the warm caul plates are recycled directly without cooling at a temperature of about 70-11 1 OOC. The resulting loosely formed wafer mats are then hot-pressed to consolidate the board and cure the powdered resin binder. The maximum pressure of the hot press is about 450-500 pounds per square inch (31.5-35 kg/cm2) and the hot press closing time is usually 1-2 minutes, depending upon the board thickness desired, due to the high rigidity of the dry wood wafer mat. The surface of the mats are subjected to high temperatures, i.e., 190--2100C, during the loading and hotpress closing stage.

As stated above, powdered phenolic resins, such as novolac, resole or combinations thereof, are generally used in prior art waferboard manufacture since they easily blend into the wafer to ensure uniform resin distribution, they are pre-cure resistant and have a long storage life. In one prior art process, a spray-dried powdered phenol-formaldehyde resin, modified with added non-phenolic polyhydroxy compound, is used for waferboard manufacture. On the other hand, powdered resin has the disadvantages of being (1) expensive to produce, (2) requiring a high wax content (about 2%) to improve affinity of resin powder.on the wafer surface and (3) contributing to mill dust which in turn causes air pollution and health and safety (e.g. explosion) hazards.

Liquid phenolic resins have been used in particleboard and plywood manufacture, but their application to waferboard, by the airless spray system described above, has been found to be ineffective due to problerns of pre-cure and large resin droplet size. Pre-cure occurs before the waferboard is completely solidified, so that the board surface layers are undesirably weak. The geometrical shape of the wood wafers also does not allow transfer from resin-excessive wavers to resin-deficient wafers by the rubbing effect experienced between wood particles.

An efficient waferboard liquid resin must therefore be pre-cure resistant and be able to be atomized in fine droplets to maximize wafer surface coverage. Thus, low viscosity and low surface tension are essential properties of the waferboard resin.

It has also been proposed to form a composite board using post heat treatment to completely cure a binary resin binder consisting of a green phenolic resin and an advanced phenolic resin. The green phenolic resin is used to impregnate the wet wood particles and the high molecular resin is retained on the wood particle surface to serve as a binder after the resin-treated wood is oven dried. The dissimilar character and geometrical shapes of dry wood wafers preclude proper impregnation of a green phenolic resin and it is not suggested that the resin be applied by spraying in fine droplets on a dry wood wafer.

Other aqueous alkaline phenol-formaldehyde resins have been proposed as adhesive formulations for plywood and hardboard, such as a resin formed by first producing a highly methylolated phenol under conditions which "prevent the formation of condensation but reduce free-formaldehyde" level and then further reacting the methylolated phenol at reflux temperatures to produce a highly reactive resin which, still has a "low enough molecular weight to provide for penetration of the wooden adherends". The resin, though of lower viscosity, lacks the pre-cure resistance necessary for waferboard application.

It has also been proposed to use in hardboard a liquid phenol-formaldehyde resin which has a low molecular weight by combining a low viscosity resin with a resin which has been advanced to a high viscosity. This resin also lacks pre-cure resistance and has a viscosity which is still too high, i.e., 600-800 cps at 700F (21 OC), to be suitable for waferboard application.

A further prior art low viscosity advanced phenolic resin for particleboard is prepared by a two stage condensation reaction. The resin lacks pre-cure resistant properties and has a viscosity and surface tension considerably higher than that required for waferboard application.

Further prior art proposals include an aldehyde condensation copolymer formed by co-condensing linear and non-crosslinkable aldehyde prepolymer with a highly thermosettable and cross-linkable aldehyde prepolymer and on improved resin for flakeboard made from mixed hardwoods by formulating phenolic resin with a second formaldehyde additive near the end of a conventional phenolic resin cook.

These resins do not contain significant amounts of non-resinous phenol-formaldehyde condensates to impart pre-cure resistant properties and have viscosities and surface tensions significantly higher than that required for atomization of waferboard.

Thus, hitherto there has been no liquid phenolic-resin composition suitable for use in the manufacture of waferboard. The present invention seeks to provide such a resin.

Accordingly, the invention provides a liquid phenolic resin binder composition characterized by being pre-cure resistant, having low viscosity and low surface tension, the composition comprising: (a) 20-80 by weight of a highly condensed and cross-linkable phenol-formaldehyde resin having a relatively high average molecular weight and a viscosity of 100450cps at 250 C; and (b) 80-20% by weight of a non-resinous phenol-formaldehyde condensate comprising methylolated phenol having an average molecular weight of 200-300.

The invention also provides a process for preparing a pre-cure resistant liquid phenolic resin binder composition characterized by: (a) reacting formaldehyde and phenol in a molar ratio of 1:1 to 3 :1 in the presence of an alkaline catalyst at a temperature between 800C and reflux to produce a highly-condensed and cross-linkable phenol-formaldehyde resin having a relatively high molecular weight and a viscosity of 1 00-450 cps at 25no; (b) reducing the temperature of the resulting resin to 60--700C; (c) adding to the cooled resin further quantities of formaldehyde and phenol in a molar ratio of 1.5:1 to 3::1 and an alkaline catalyst and reacting the resultant mixture at 45-700C to product a non resinous phenol-formaldehyde concentrate comprising methylolated phenols having an average molecular weight of 200-300; and (d) cooling the resultant liquid phenolic resin composition to a temperature below room temperature.

Finally, the invention provides a process for manufacturing waferboard comprising: (a) treating wood wafers having a moisture content of 36% with a phenolic resin composition so as to deposit on the wood wafers an amount of resin solids effective to bind the wafers; (b) felting the resulting the resin-treated wafers on a heated caul plate to produce loosely-formed wafer mats; and (c) consolidating the loosely-formed mats under elevated temperature to cure the resin and produce waferboard, characterized in that in step (a) of the process the phenolic resin composition used is a pheno;ic resin composition of the invention or produced by the aforementioned process of the invention, this phenolic resin composition being sprayed onto the wood wafers in the form of fine droplets, thereby producing in stop (c) of the process a waferboard having substantially equal internal bond strength on its face and core.

In the phenolic-resin composition of the invention, the non-resinous condensate component preferably substantially consists of mono-, di- and tri-methylol phenols and methylolated di- and tri nuclear phenols. Also, preferably the average molecular weight of the highly condensed resin component is in the range of 2000-6000. The presence of both the highly condensed resin component and the non-resinous condensate component is essential since neither component alone can achieve the good quality bond of waferboard produced by using the liquid phenolic resin binder composition of the invention.

The desired molecular weight distribution of the liquid resin is produced by controlling the reaction temperature, time and additions of phenol, formaldehyde and alkaline catalysts in the process for preparing the liquid phenolic resin composition of the invention already described. This process produces a very wide range of resin molecular weight distribution. The poly-dispersity (the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn)) is typically about 2.5-6.5 for the composition of the invention.

The presence of the considerable proportion of low-molecuiar-weight non-resinous condensate component in the composition of the invention gives the composition a low overall viscosity and a low surface tension, thereby allowing it to be atomized efficiently by conventional air and airless spray nozzles or centrifugal atomizers. The resultant very fine resin mist allows maximum coverage of the wood water surface to be treated.

When the composition of the invention is used in the manufacture of waferboard, the nonresinous condensate component and the highly condensed resin component exhibit different curing behaviors during the curing process. The non-resinous condensate component melts under the heat and pressure of curing and continues to wet the wood surface of the wafers. This results in over-penetration and long curing time. Thus, use of the condensate component alone would result in problems of undercure and low bond strength. The highly-condensed resin component, on the other hand, becomes rubbery instead of meiting and thus has poor wood surface wettiny ability and rapid cure under heat.

Use of the highly condensed resin component alone would therefore result in fast curing in the core and pre-cure on the face of the waferboard.

In the waferboard manufacturing process of the invention, when water in the highly condensed resin component has been evaporated under heat and pressure during the curing process, the highlycondensed resin component swells in the presence of the non-resinous condensate component. This prevents over-penetration of the non-resinous condensate component into the wood tissue, provides optimum resin flow for wetting the wood surface and accommodates wood wafer movement during the hot-press closing stage. The resin composition of the invention is thus highly resistant to pre-cure in the surface layers of the waferboard and to fast-curing in the core of the board.

Although in the process for the preparation of the composition of the invention phenol is the preferred phenolic compound and formaldehyde (either in the form of the simple monomer or in the form of paraformaldehyde) is the preferred aldehyde, other phenolic and aldehyde components may be used. Thus, for example, part of the phenol may be replaced by resorcinol, xylenols, cresols or catechol.

Similarly, aldehydes such as acetaldehyde and furfuraldehyde may be used. Accordingly, the terms "phenol" and "formaldehyde" are to be construed accordingly.

The preferred alkaline catalyst for preparing the phenolic resin composition of the invention is sodium hydroxide, bu-t other alkali metal hydroxides, carbonates, ammonium hydroxide and amines can be used either alone or as a co-catalyst with the sodium hydroxide.

The alkaline catalyst used to prepare the composition of the invention may all be added during stage (a) of the process, or part of the alkaline catalyst may be added during step (a) and part during step (c). When sodium hydroxide is used as the alkaline catalyst, it is desirable that it be used overall in an amount of from 0.1 to 0.6 moles per mole of phenol, and preferably from 0.17 to 0.35 moles per mole of phenol. The amount of alkaline catalyst used affects the storage life of the resin composition.

In step (a) of the process for preparing the composition of the invention, it is preferred to use a formaldehyde: phenol molar ratio of 2.0:1 to 2.5 :1. Similarly, in step (c) of the process to prepare the non-resinous condensate component, the formaldehyde: phenol molar ratio is preferably from 1.8 :1 to 2.3:1 to minimize residual formaldehyde in the resin and to obtain a desirable curing rate. In the composition of the invention as a whole, it is preferred to use a formaldehyde : phenol molar ratio of 1.5:1 to 2.6:1, with a range of 1.7 :1 to 2.4:1 being especially preferred.

Normally, the resin composition of the invention is prepared in an aqueous medium and has a resin solids content of 3570%, preferably 50-60Y0, This resin solid content is dependent upon the ratio between the highly condensed resin and non-resinous condensate components. Thus, to obtain a low viscosity resign, a resin which contains high proportion of the highly condensed resin component should have a low resins solids content. On the other hand, a composition having a high resins solids content and low viscosity can be obtained by increasing the proportion of the non-resinous condensate componen..

In step (a) of the process for preparing the composition of the invention, the condensation is carried out to a viscosity of 100-450 centipoises, as measured on a Brookfield viscosimeter Model RVF100 at 250C. This corresponds to an average molecular weight (Mw) of 2000-6000 or a viscosity of D to Son the Gardner-Holdt viscosity scaie at 250C.The reaction temperature used in step (c) of the process is 45--700C, preferably 55--65'C and this step is desirably continued until the residual formaldehyde content of the resin is 0-4%. In order to ensure complete methylolation of the phenol in step (c) the reaction time is desirably 60-180 minutes; this reaction time depends, of course, both the molar ratio of phenol to formaldehyde and sodium hydroxide and the reaction temperature.

The resin residual formaldehyde content can be determined by the hydroxylamine hydrochloride method described at pp. 493-494 of J. F. Walker, FomaIdehyde, Third Edition, Reinhold Publishing Corporation, New York, 1964.

At the end of step (c) of the process, the residual formaldehyde content of the resin is desirably reduced by the addition of the formaldehyde scavenger. Suitable formaldehyde scavengers include ammonium hydroxide, resorcinol, 3,5-xylenol, 3-cresol, urea and sulfides, ammonium hydroxide being the preferred scavenger.

Although the resin composition of the invention comprises 20-80% by weight of the highly condensed resin component and 8020% by weight of the non-resinous condensate component, these proportions are not critical in and oF themselves. The relative proportions of these two components can be adjusted to suit the waferboard manufacturing conditions, having regard to such factors as press temperature, press closing time, press time and caul plate temperature during felting.A high press temperature, a slow closing of the hot press (i.e. a low press temperature) and/or a high caul plate temperature during felting require a resin with a higher proportion of the non-resinous condensate component, whereas a low press temperature, a fast closing of the hot press (i.e. a high press pressure) and/or a low caul plate temperature during felting require a composition having a higher proportion of a highly condensed component.Using conventional waferboard manufacturing conditions involving a press temperature of I 90-21 00C, a maxirrium pressure of 450-500 psi (31.5-35.0 kg/cm2) and a caul plate temperature during felting below 1200C, it has been found that best results obtained with resin compositions containing 40---600 of the highly condensed resin component and 6040% of the non-resinous condensate component.

As the following examples illustrate, the liquid resin composition of the invention gives entirely satisfactory results when used alone in the manufacture of waferboard; however although this is not recommended because of the probiems described above when using powdered resin in waferboard manufacture), the liquid resin composition of t;ie invention may be used in combination with a minor proportion of powdered resin for the manufacture of waferboard.

The following Examples are now given, though by way of illustration only to show details of particuiarly preferred compositions and processes of the invention.

EXAMPLE 1 This example shows a normal-cook process of making pre-cure resistant waferboard liquid phenol-formaldehyde resin composition of the invention.

A reactor vessel was charged with the following ingredients: Ingredients Parts By Weight Molar Ratio First Phenol (90% Concentration) 15.65 1.00 First Formaldehyde (46.556 Concentration) 24.14 2.50 First Water 10.81 First Sodium Hydroxide (500/0 Concentration) 4.17 0.35 (54.77) Second Phenol (90% Concentration) 15.99 1.02 -Second Formaldehyde (46.5% Concentration) 21.76 2.25 Second Water 1.76 Second Sodium Hydroxide (50% Concentration) 3.83 0.32 Ammonium Hydroxide (28-30% Concentration) 1.89 (45.23) The reactor vessel was charged with first phenol, first formaldehyde and first water. Then the first sodium hydroxide was slowly added over a 10--15 minute period and the temperature was allowed to rise to 950C in 50 minutes.The resin was cooked at 950C to a Gardner-Holdt viscosity (250 C) of AB, e.g., approximate 42 minutes at 950C and then cooled over approximately 9 minutes to 800 C. The temperature was held at 800C until the viscosity (250C) was KL. Ten (10) minutes after KL viscosity, the resin was cooled to 700 C. Second phenol, second formaldehyde and second water were then added to the resin. The temperature was adjusted to 650C and it was held at 650C until Gardner-Holdt viscosity (250C) was A1A. Ten minutes after A,A viscosity (250C), or 65 minutes after second phenol and formaldehyde were added, the resin was cooled to room temperature. When the temperature was cooled to 40--300C, second sodium hydroxide and ammonium hydroxide were added.

EXAMPLE 2 (Control) This example is a first-cook resin of a two-cook method in Example 1. The resin contains only the highly condensed and cross-linkable phenol-formaldehyde resin. The resin formulation and cooking schedule are identical with the first-cook resin of Example 1.

Ingredients Parts By Weight Molar Ratio Phenol (90% Concentration) 15.65 1.00 Formaldehyde (46.5% Concentration) 24.14 2.50 Water 10.81 First Sodium Hydroxide (50% Concentration) 4.17 0.35 Second Sodium Hydroxide (50% Concentration) 0.31 0.03 Ammonium Hydroxide (28--3056 Concentration) 0.94 As described in Example 1, the reactor vessel was charged with phenol, formaldehyde and water Then the first sodium hydroxide was slowly added over a 1 0-1 5 minute period and the temperature was allowed to rise to 950C in 50 minutes. The resin was cooked at 950C to a Gardner-Holdt viscosity (250C) of AB and then cooled to 800 C. The temperature was held at 800C until the viscosity (250C) was KL. Ten minutes after KL viscosity, the resin was cooled to 650C and held for 65 minutes.The resin was then cooled to room temperature. When the temperature was cooled to 40--300C, the second sodium hydroxide and ammonium hydroxide were added slowly.

EXAMPLE 3 (Control) This example is equivalent to the second-cook resin of Example 1. It contains only the nonresinous phenol-formaldehyde condensates. The formulation and cooking schedule are identical with the second-cook resin of Example 1.

Ingredients Parts By Weight Molar Ratio Phenol (90% Concentration) 15.99 1.00 Formaldehyde (46.5% Concentration) 21.76 2.20 Water 1.72 First Sodium Hydroxide (50% Concentration) 2.10 0.17 Second Sodium Hydroxide (50% Concentration) 1.42 0.12 Ammonium Hydroxide (2830% Concentration) 0.94 The reactor vessel was charged with phenol, formaldehyde and water. The first sodium hydroxide was then added slowly over a 1 minute period. In the meantime, the temperature was allowed to rise to 650C in 10 minutes. The temperature was held at 650C for 65 minutes. The phenol formaldehyde condensates were then cooled to room temperature. The second sodium hydroxide and ammonium hydroxide were added when the temperature was cooled to 40--300C.

RESIN ANALYSIS OF EXAMPLES 1-3 The resins of Examples 1 to 3 where analyzed for Gardner-Holdt viscosity (250C), refractive index (250C), sodium hydroxide content, non-volatile content and resin molecular weight distribution by high pressure gel permeation chromatograph. The resin weight average molecular weight ( w) and number average molecular weight (Mn) were calculated from gel permeation chromatograms.

The high pressure gel permeation chromatograph was a Waters Association Model ALC/GPC-201 equipped with a series of different pore size gel permeation columns. The column combination used for this analysis was 104, 103, 500 and 100 A, y-styragel.

Resin samples for gel permeation chromatograph were prepared according to the following procedure: 1. A liquid resin sample of about 0.30-0.35 grams was dissolved into 10 grams of tetrahydrofuran solvent.

2. The pH of the resin solution was adjusted to pH 4.0 + 0.2 with 1 N sulfuric acid solution.

3. The resin solution was then dehydrated by adding 10 grams of sodium sulfate.

4. The resin solution was filtered using a sample clarification kit.

5. About 200 iul of the resin solution was injected into the chromatograph.

The high pressure gel permeation chromatograph was operated under the following conditions: Solvent: Tetrahydrofuran Temperature: 250C Detector: Refractive Index, 8X Flow Rate: 1.0 ml/minute The results of the gel permeation chromatograms of the resin made in Examples 1 to 3 are shown in Table 1.

As discussed previously, a desired molecular weight distribution of the waferboard liquid resin can be made by controlling temperature, cooking time, and the steps of phenol and formaldehyde addition.

The origin of the molecular weight distribution of the Example 1 resin was demonstrated by separating the normal cooking method into two cooks (Examples 2 and 3). It is readily seen that the non-resinous phenol-formaldehyde condensate (low molecular weight portion) of the resin result primarily from the second cook (Example 3) analogous to the second addition of phenol and formaldehyde in the method of the invention (Example 1); while the high molecular weight portion results from the first cook (Example 2) -- analogous to the first charge of phenol and formaldehyde in the process of the invention (Example 1). Only a small amount of overlap exists in the 400-700 molecular weight range.

Table 1 shows the results of resin analyses including the weight average molecular weight (Mw) and number average molecular weight (Mn) calculated from the gel permeation chromatogram.

The resin characteristics (viscosity, surface tension, refractive index, sodium hydroxide content and non-volatile content) of the physical mixture of Example 2 and Example 3 at a 56 to 44 weight ratio were very close to those of the Example 1 resin.

TABLE 1 Summary of Resin Analysis Example 2/Example 3 Example 1 Example 2 Example 3 56/44 By Wt. Mix Viscosity (25 C) Gardner-Holdt AB XX-Y A2-A1 AB SurfaceTension*(250C)dynes/cm 56.5 71.5 54.0 59.0 Refractive Index (25cm) 1.4660 1.4543 1.4758 1.4650 Sodium Hydroxide Content, % 4.14 4.18 4.40 4.28 Non-volatile Content, % 45.38 40.83 49.99 44.86 Non-voiatile Phenol-Formaidehyde 41.24 36.65 45.59 40.58 Content, % 2410 2410 4121 264 Mn 381 1314 175 irw/Mn 6.33 3.14 11.51 * Determined by Fisher Model 215 Surface Tension Analyzer. Elevator speed of ring was 0.05 inch (0.13 cm)/minute.

In summary, Examples 1 to 3 demonstrate that the resin of the invention consisting of the highly condensed and cross-linkable resin and the non-resinous phenol-formaldehyde condensates can be made by controlling cooking temperature, cooking time and the steps of phenol and formaldehyde addition.

IEVALUATION OF RESINS PREPARED FROM EXAMPLES 1 TO 3 The resins prepared in Examples 1 to 3 were evaluated by making waferboard and testing for internal bond and the accelerated aging modulus of rupture (MOR). The Canadian Standard Association (CSA) Standard CAN3-0188.0-M78 was followed.

Laboratory size (lOx 10 x 1/2 inch) (254 x 254 x 13 mm) boards were made according to the conventional waferboard mill conditions. Thus a commercial aspen wood waferboard furnish with 4.5% moisture content was sprayed with 2% resin (non-volatile phencl-formaldehyde) based on dry wood weight. A laboratory type air-sprayer and blender were used for the liquid resin application.

The resin sprayed wood furnish was formed into a mat and pre-pressed in a cold press. In simulating the waferboard mill conditions, the mat was topped with a 1!8 inch thick stainless steel caul plate which was pre-heated to 930C. To ensure good contact between the hot caul and the mat, a weight to give 0.1 psi was placed on the hot caul. The hot caul on the mat was allowed to stand for 10 minutes prior to hot-pressing. The half inch (13 mm) thick waferboard was then made by hotpressing at 2000C press temperature for 6 minutes with maximum pressure of 450 psi. The average specific gravity of the board was 0.65.

Three boards were made for each resin prepared in Examples 1, 2, 3 and the physical blend of Examples 2 and 3 at 56/44 ratio by weight. Six internal bond samples were prepared from each board, and tested according to CSA Standard CAN3-O1 88.O-M78. The rate of face failure (%) was determined from the ratio of the number of face failure (breaks) samples to the total internal bond tested specimens.

The results are summarized in Table 2.

TABLE 2 Internal Bond Face Failure Resin psi n SD Example 1 64.5 18 11.5 66.7 Example 2 30.7 18 10.5 100 Example 3 24.4 18 7.7 0 Mixture* of Examples 2 and 3 59.1 18 10.6 66.7 * The resin was prepared by mixing Example 2 and Example 3 resins at a 56/44 ratio by weight.

n = number of specimens.

SD = standard deviation.

Face Failure, % = the ratio of the face failure specimen to the total internal bond tested specimen.

The following findings were made from the results shown in Table 2: (1) The highly condensed and cross-linkable resin, prepared in Example 2 showed low internal bond strength, and all the tested internal bond samples failed on face layers of the waferboard. This indicates the pre-cure of the resin.

(2) The non-resinous phenol-formaldehyde condensates prepared in Example 3, also produced very poor internal bond strength, and all the internal bond samples failed in the core layer of the waferboard. This indicates the undar-cure of the binder.

(3) The resins of the invention which consisted of the highly condensed and cross-linkable phenolformaldehyde resin and the non-resinous phenol-formaldehyde condensates, prepared in Example 1 as well as the physical blend of Examples 2 and 3 at a 56/44 ratio by weight, produced very high internal bond strength waferboard. The rates of the face failure are 66.7% for both Example 1 resin and the resin of the physical blend of Examples 2 and 3.

Furthermore, in order to simulate the various caul plate conditions in the commercial waferboard production line, a wide range of the caul plate temperatures (250C to 1 490C) were tested for the resin of the invention prepared in Example 1. For comparison with the Example 1 resin, one commercial particleboard resin (W135) and plywood resin (W838LV) of Borden Chemical Western were also tested.

As in the previously described procedure, laboratory size (10 x 10 x 1/2 inch) waferboards were made. Therefore, a commercial aspen wood waferboard furnish with 4.5% moisture content was sprayed with 2% resin (non-volatile phenol-formaldehyde resin) based on dry wood weight.

The resin sprayed furnish was formed into a mat and pre-pressed in a cold press. Five 1/8 inch thick stainless steel cual plates were preheated to 800C, 930C, 121 0C, 1 350C and 1 490C respective!y and the pre-pressed mat was topped with the hot caul. To ensure good contact between hot caul plate and mat, a weight to give 0.1 psi pressure was placed on the hot caul. Again, the hot caul on the mat was allowed to stand for 10 minutes to simulate mill conditions. A cold (250C) caul plate on a mat was used for a control. Then the waferboards were made by hot-pressing at 2000C for 6 minutes with maximum pressure of 450 psi for half inch (13 mm) thick board.Under the same pressing condition, waferboards were also made with W1 35 and W838LV of Borden Chemical Western, with a cold (250C) caul plate on the mat for 10 minutes before hot-pressing. The specific gravity of the waferboards was 0.65 + 0.02.

The waferboards were tested for internal bond and accelerated aging modulus of rupture according to the aforementioned CSA standard. The results are shown in Table 3.

TABLE 3 Example 1 Resin Caul Temperature, OC 25 80 93 121 135 149 Internal Bonda, psi 63.9 68.8 63.9 74.2 44.0 39.0 Standard Deviation 9.7 12.9 13.4 12.0 10.9 12.0 Face Failureb, % 0 33 22 33 89 100 AcceleratedAgingMOR,psi 1332 1539 1215 1359 CaulTemperature,0C 25 80 93 121 135 149 Commercial Particleboard Resin (Borden W135) Internal Bondd, psi 42.3 Face Failure, % 100 Commercial Plywood Resin (Borden W838LV) Internal Bondd, psi 36.0 Face Failure, % 100 a. Average of 9 specimens.

b. The ratio of the face failure specimen to the total internal bond tested specimen.

c. Two-hour boil modulus of rupture according to CSA standard CAN3-0188.0M78.

d. Average of 6 specimens.

By evaluating the internal bond and the rate of face failure shown in Table 3, the waferboard resin of the invention prepared in Example 1 was significantly better in pre-cure resistance than the commercial particleboard resin (W135) and plywood resin (W838LV). The resin prepared in Example 1 could endure up to a 1 200C caul plate temperature, whereas the commercial particleboard resin and plywood resin showed pre-cure even at 250C caul plate temperature. In addition, the accelerated aging modulus of rupture results in Table 3 showed that the Example 1 resin passed the CSA standard.

Moreover, reducing the press time for manufacturing waferboard is vitally important in order to achieve high productivity. Therefore, the curing rate of the invention resin (Example 1) was examined by comparing it with a commercial novolac type powdered phenolic resin which was being used in a waferboard production line.

As in the previously described procedure, laboratory size (10 x 10 x 1/2 inch) (254 x 254 x 13 mm) waferboards were made with different press times at constant press temperature. The cure of the resin was evaluated from the internal bond strength and the accelerated aging modulus of rupture.

A commercial aspen waferboard furnish with 4.0% moisture content was sprayed with the resin of the invention to a 2% resin solids level (non-volatile phenol-formaldehyde resin) based on the dry wood weight. For powder resin, the aspen wafer was first sprayed with 2% molten wax and then blended with 2% powder resin based on dry wood weight.

In order to simulate waferboard mill conditions, the pre-pressed mats were topped with a 1/8 inch thick stainless steel hot caul plate at 930C initial temperature for 10 minutes prior to hot-pressing.

Again, to ensure good contact of the mat and hot caul, a weight to give 0.1 psi pressure to the mat, was placed in the hot caul. The half inch (13 mm) thick waferboards were made by the following conditions: Press Temperature: 2100C Maximum Press Pressure: 450 psi Press Time: 4.5, 5.0, 5.5, 6.0 and 7.0 minutes Three boards were made for each pressing condition. The average board's specific gravity was 0.65.

The effect of press time on the internal bond strength and the accelerated aging modulus of rupture (MOR) of the waferboards are shown in Table 4.

TABLE 4 Press Time, Min 4.5 5.0 5.5 6.0 7.0 Example 1 Internal Bond*, psi 61.2 65.4 57.3 64.1 - Standard Deviation 12.8 13.1 11.4 11.0 Face Failure, % 1 6.7 16.7 72.2 55.0 Accelerated Aging MOR**, psi 1278 1452 1 704 1614 Commercial Novolac-Type Powder Resin Internal Bond*, psi --- 18.6 28.7 34.2 56.0 Standard Deviation - 4.5 5.5 5.0 Face Failure, - 0 0 0 0 Accelerated Aging MOR**, psi - Delam. Delam. 1000 1179 * Average of 1 8 specimens.

** Average of 3 specimens.

Face Failure, % = The ratio of the face failure specimens to the total internal bond tested specimens.

The results shown in Table 4 indicate that the liquid resin of the invention (Example 1) cured considerably faster than the commercial powder resin. To cure the resin, the press time for 0.5 inch thick waferboard can be only 4.5 miniites for the resin of the invention; whereas, for the powder resin, the press time has to extend to more than 6.0 minutes. Therefore, according to the preceding investigations, the following conclusions were made: (1) The commercial particleboard and plywood liquid phenolic resins showed pre-cure for waferboard manufacture.

(2) High quality waferboard can be manufactured with the liquid phenolic resins which consist of a mixture of highly condensed and crosslinkable phenol-formaldehyde resin and non-resinous phenolformaldehyde condensates.

(3) A method has been invented for making a high efficiency liquid waferboard resin by controlling resin cooking temperature, time and the steps of phenol and formaldehyde addition as shown in Example 1.

(4) The resin of the invention (Example 1) is pre-cure resistant for waferboard surface and fastcure in the core, and it satisfied the wide range of caul temperatures present in commercial waferboard production lines.

EXAMPLE 4 This example demonstrates the method of manufacturing the waferboard resin of the invention.

The resin is evaluated for pre-cure resistance and curing reaction rate by making waferboard under simulated mill conditions.

A reactor vessel was charged with the following ingredients: Ingredients Parts By Weight Molar Ratio First Phenol (90% Concentration) 15.97 1.00 First Formaldehyde (46.5% Concentration) 24.64 2.50 First Water 11.02 First Sodium Hydroxide (50% Concentration) 4.26 0.35 (55.89) Second Phenol (90% Concentration) 16.33 1.02 Second Formaldehyde (46.5% Concentration) 22.21 2.25 Second Water 1.90 Second Sodium Hydroxide (50% Concentration) 1.74 0.02 Ammonium Hydroxide (28-30% Concentration) 1.93 (44.11) The resin manufacturing process is similar to Example 1. The reactor vessel was charged with first phenol, first formaldehyde and first water. The first sodium hydroxide solution was slowly added over a 10-15 minute period, and the temperature was allowed to rise to 950C in 50 minutes.The resin was advanced at 950C to a Gardner-Holdt viscosity (250 C) of AB and then cooled to 800 C. The temperature was held at 800C until the resin viscosity (250C) was FG. The resin was cooled to 700C immediately.

Second phenol, second formaldehyde and second water were then added to the resin. The temperature was adjusted to 650C and it was held at 650C for 100 minutes. The resin viscosity (250C) was A,A,??? and the residual formaldehyde was 4%. The resin was cooled to room temperature. When the temperature was cooled to 4030 C, second sodium hydroxide and ammonium hydroxide were added slowly to the resin.

The results of the resin analysis are shown in Table 5: TABLE 5 Viscosity (250C) Gardner-Holdt A,A Surface Tension* (250C) dynes/cm 57.0 Non-Volatile Content, % 46.0 Sodium Hydroxide Content, % 3.0 Non-Volatile Phenol-Formaldehyde Content, % 43.0 * Determined by Fisher Model 215 Surface Tension Analyzer.

Elevator speed of ring was 0.05 in/minute.

As shown in Table 5, the resin has low viscosity and low surface tension. The resin can be efficiently atomized by using a conventional spray system.

The resin was evaluated for pre-cure resistance and the rate of curing reaction.

The previously described pre-cure resistant test method was followed. A commercial aspen waferboard furnish with 4.5% moisture content was sprayed with the resin prepared in Example 4. The resin.add-on was 2% resin solids (non-volatile phenol-formaldehyde resin) based on dry wood weight.

The resin sprayed furnish was formed into a mat and prepressed in a cold press. Three 1/8 inch thick stainless steel plates were preheated to 93, 121 and 1 350C, and the pre-pressed mat was topped with the hot caul. To ensure good contact between hot caul plate and mat, a weight giving 0.1 psi pressure was placed on the hot caul. The hot caul on the mat was allowed to stand for 10 minutes before hotpressing. A cold (250C) caul plate on a mat was used for a control. Three boards were made for each caul temperature pre-treatment.

The half inch (13 mm) thick waferboards were made by hot-pressing at 21 00C for 6 minutes with the maximum pressure of 450 psi.

The waferboards were tested for internal bond and the accelerated aging modulus of rupture (MOR) according to the aforementioned CSA standard. The results are shown in Table 6: TABLE 6 Caul Temperature, OC 25 93 121 135 Internal Bond*, psi 72.0 70.5 73.1 66.9 Face Failure*,% 0 55.6 50.0 88.9 Accelerated Aging MOR**, psi 1 830 1 854 1 950 1944 * Average of 1 8 specimens.

** Average of 3 specimens.

The results shown in Table 6 indicate that the resin of the invention is pre-cure resistant, because the waferboard internal bond is not deteriorated with the initial caul temperature pre-treatment up to 1200C.

Furthermore, the effect of press time on waferboard properties was evaluated. As a previously described procedure, the aspen waferboard furnish with 4.0% moisture content was sprayed with 2% resin solids (non-volatile phenol-formaldehyde resin) based on dry wood weight. Again, in simulating mill conditions, the pre-pressed mat was topped with a 1/8 inch (3 mm) thick stainless steel caul plate with an initial temperature of 93"C. To ensure good contact between hot caul and mat, a weight was placed on the caul plate to give 0.1 psi pressure on the mat. The hot caul on the mat was allowed to stand for 10 minutes before hot-pressing.The 1/2 inch thick waferboard was made under the following conditions: P-ress Temperature: 21 00C Maximum Press Pressure: 450 psi Press Time: 4.5, 5.0,5.5 and 6.0 minutes Three boards were made for each pressing condition, and the average board's specific gravity was 0.65. The effect of press times on the waferboard internal bond and the accelerated aging modulus of rupture (MOR) is shown in Table 7.

TABLE 7 Press Time, Minutes 4.5 5.0 5.5 6.0 Internal Bond*, psi 57.9 62.6 62.4 66.4 Standard Deviation 8.2 12.2 10.8 11.6 Face Failure, % 0 1 7.6 22.2 1 6.7 Accelerated Again MOR**, psi 1332 1386 1 626 1 788 * Average of 18 specimens.

* Average of 3 specimens.

Face Failure = The ratio of the face failure specimens to the total internal bond tested specimens.

With 4.5 minutes press time for 1/2 inch thick waferboards, the internal bond strength and the accelerated aging modulus of rupture are acceptable by the Canadian Standard Association (CSA) Standard CAN-0188.2-M78.

In summary, the example demonstrates that the resin made by the invented process of the invention has low viscosity and low surface tension. When it is used for waferboard manufacture, the resin is pre-cure resistant on face layers and fast cure resistant in the core of the waferboard.

EXAMPLE 5 This Example also demonstrates the method of manufacturing the resin of the invention. The resin is evaluated for pre-cure resistant property for waferboard manufacture.

A reactor vessel was charged with the following ingredients: Ingredients Parts By Weight Molar Ratio First Phenol (90% Concentration) 16.54 1.00 First Formaldehyde (46.5% Concentration) 25.53 2.50 First Water 9.38 First Sodium Hydroxide (50% Concentration) 4.42 0.35 (55.87) Second Phenol (90% Concentration) 18.66 1.13 Second Formaldehyde (46.5% Concentration) 23.41 2.29 Second Water 0.06 Second Sodium Hydroxide (50% Concentration) 1.00 0.08 Ammonium Hydroxide (2830% Concentration) 1.00 (44.13) The reactor vessel was charged with first phenol, first formaldehyde and first water. The first sodium hydroxide was slowly added over a 10-1 5 minute period and the temperature was allowed to rise to 900C in 50 minutes. The temperature was held at 900C until the Gardner-Holdt viscosity (250C) of the resin was AB.Then the resin was cooled to 800 C. The resin was advanced at 800C until the Gardner-Holdt viscosity (250C) was D. The resin was cooled to 650C. Second phenol, second formaldehyde and second water were then added to the cooked resin. The sodium hydroxide was added slowly (approximately 10-20 minutes). The temperature was adjusted to 650C by cooling; it was held at 650C for 140 minutes. The residual formaldehyde of the resin was 1.8% and the resin viscosity (250C) was A of the Gardner-Holdt scale. The resin was then cooled to room temperature. When the temperature was cooled to 40--300C, ammonium hydroxide was added to the resin.

The results of the resin analysis are shown in Table 8: TABLE 8 Viscosity (250C) Gardner-Holdt A Surface Tension* (25"C) dynes/cm 59.0 Sodium Hydroxide Content, % 2.7 Non-Volatile Content, % 48.3 Non-Volatile Phenol-Formaldehyde Content, % 45.6 * Determined by Fisher Model 215 Surface Tension Analyzer.

Elevator speed of ring was 0.05 in/minute.

The resin was evaluated for making waferboards with different moisture contents of aspen wood furnish.

The commercial aspen wafer samples of 3.0% and 4.7% moisture contents were sprayed with 2% resin solids (non-volatile phenol-formaldehyde) based on dry wood weight. The resin-sprayed wood wafers were formed into a mat and pre-pressed in a cold press.

The resin pre-cure resistance was tested by pre-heating three 1/8 inch (3 mm) thick stainless steel caul plates to temperatures of 65, 93 and 121 0C. The pressed mats were then transferred onto the hot caul plates and allowed to stand for 10 minutes prior to hot-press into waferboard. A mat on a cold (250C) caul plate was used for control.

The 1/2 inch (13 mm) thick waferboard was made by hot-pressing at 21O0Cfor 6 minutes with a maximum pressure of 450 psi (31.5 kg/cm2). The waferboard specific gravity and the effect of caul plate temperatures on board internal bond strength and the rate of face failure are shown in Table 9:: TABLE 9 Caul Temperature, OC 25 65 93 121 3.0% Moisture Content Wood Furnish Board Specific Gravity 0.62 0.64 0.63 0.63 Internal Bond*, psi 66.6 64.8 59.7 58.2 Standard Deviation 13.8 10.9 10.8 7.0 Face Failure, % 22.2 38.9 52.9 77.8 4.7 ,0 Moisture Content Wood Furnish Board Specific Gravity 0.63 0.64 0.64 0.63 Internal Bond*, psi 62.2 67.3 54.2 55.4 Standard Deviation 7.9 8.9 8.2 12.2 Face Failure, % 11.8 17.7 55.6 55.8 * Average of 18 specimens.

Face Failure, % = The ratio of the face failure specimens to the total internal bond tested specimens.

Although the rate of face failure increased as the caul plate temperature was increased, the resin of the invention was able to endure up to 121 OC initial caul temperature without substantial deterioration of internal bond strength.

Claims (23)

1. A liquid phenolic resin binder composition characterized by being pre-cure resistant, having low viscosity and low surface tension, the composition comprising: (a) 20-80 by weight of a highly condensed and cross-linkable phenol-formaldehyde resin having a relatively high average molecular weight and a viscosity of 100-450 cps at 250C; and (b) 80-20% by weight of a non-resinous phenol-formaldehyde condensate comprising methylolated phenol having an average molecular weight of 200-300.

2. A liquid phenolic resin as claimed in claim 1 characterized in that the non-resinous phenolformaldehyde condensate substantially consists of mono-, di- and tri-methyiol phenols and methylolated di- and tri-nuclear phenols.

3. A liquid phenolic resin as claimed in claim 1 or 2 characterized in that the highly condensed resin has an average molecular weight in the range o-F from 2,000 to 6,000.

4. A liquid phenolic resin as claimed in any of the preceding claims characterized in that the molar ratio of formaldehyde to phenol is in the range of from 1.5 l to 2.6 :1 for the resin as a whole, of 1:1 to 3:1 in the highly condensed resin component and from 1.5:1 to 3:1 in the non-resinous condensate component.

5. A liquid phenolic resin as claimed in claim 4, wherein the aforesaid molar ratio is in the range of from 1.7:1 to 2.4:1 for the resin as a whole, from 2.0:1 to 2.5:1 in the highly condensed resin component and from 1.8:1 to 2.3 :1 in the non-resinous condensate component.

6. A liquid phenolic resin as claimed in any of the preceding claims characterized in that it comprises 35-70% by weight of resin solids in an aqueous solution.

7. A liquid phenolic resin as claimed in claim 6 characterized in that it contains from 5060% by weight of resin solids in the aqueous solution.

8. A liquid phenol resin as claimed in any of the preceding claims characterized by comprising 50-60 by weight of the highly condensed resin component and 6040% by weight of the nonresinous condensate.

9. A process for preparing a liquid phenolic resin binder composition characterized by: (a) reacting formaldehyde and phenol in a molar ratio of 1:1 to 3:1 in the presence of an alkaline catalyst at a temperature between 800C and reflux to produce a highly-condensed and cross-linkable phenol-formaldehyde resin having a relatively high molecular weight and a viscosity of 100-450 cps at 250C; (b) reducing the temperature of the resulting resin to 60--700C; (c) adding to the cooled resin further quantities of formaldehyde and phenol in a molar ratio of 1.5:1 to 3::1 and an alkaline catalyst and reacting the resultant mixture at 45-700C to product a nonresinous phenol-formaldehyde concentrate comprising methylolated phenols having an average molecular weight of 200-300; and (d) cooling the resultant liquid phenolic resin composition to a temperature below room temperature. \

10. A process as claimed in claim 9 characterized in that the resultant liquid phenolic resin composition comprises 2080% by weight of the highly-condensed resin component and 8020% by weight of the non-resinous condensate and in that the overall formaldehyde: phenol molar ratio in the composition is in the range of from 1.5:1 to 2.6:1.

11. A process as claimed in claim 9 or 10 characterized in that the non-resinous condensate component substantially consists of mono-, di-, and tri-methylol phenols and methylolated di- and trinuclear phenols.

12. A process as claimed in any of claims 9 to 11 characterized in that the highly condensed resin component has an average molecular weight of from 2,000--6,000.

13. A process as claimed in any of claims of 9 to 12 characterized in that the alkaline catalyst in stages (a) and (c) is sodium hydroxide present in an amount of from 0.1 to 0.6 moles of phenol.

14. A process as claimed in claim 1 3 characterized in that the sodium hydroxide is present in an amount of from 0.17 to 0.35 moles per mole of phenol.

1 5. A method according to any of claims 9 to 14 characterized in that the molar ratio of formaldehyde to phenol is in the range of from 1.7 :1 to 2.4:1 in the composition as a whole, in the range of 2.0:1 to 2.5:1 in step (a) and in the range of from 1.8 :1 to 2.3:1 in step (c).

1 6. A process according to any of claims 9 to 1 5 characterized in that the residual formaldehyde content of the resin at the end of step (d) is not more than 4%.

17. A process to any of claims 9 to 1 6 characterized by adding to the composition at the end of step (c) to reduce residual forr,laldehyde any one or more of ammonium hydroxide, resorcinol, 3,5-xylenol, 3-cresol and a sulphite.

1 8. A process as claimed in any of claims 9 to 17 characterized in that the resin composition produced comprises 4060% by weight of the highly condensed resin component and 6040% by weight of the non-resinous condensate component.

1 9. A process as claimed in claim 1 8 characterized in that the liquid phenolic resin composition comprises 3570% resin solids in an aqueous solution.

20. A process according to claim 1 9 characterized in that the liquid phenolic resin composition comprises 5060% by weight of resin solids in an aqueous solution.

21. A process for manufacturing waferboard comprising: (a) treating wood wafers having a moisture content of 36% with a phenolic resin composition so as to deposit on the wood wafers an amount of resin solids effective to bind the wafers; (b) felting the resulting the resin-treated wafers on a heated caul plate to produce loosely-formed wafer mats; and (c) consolidating the loosely-formed mats under elevated temperature to cure the resin and produce waferboard, characterized n that in step (a) of the process the phenolic resin composition used is a phenolic resin composition as claimed in any one of claims 1 to 8 or a phenolic resin composition produced by a process as claimed in any one of claims 9 to 18, this phenolic resin composition being sprayed onto the wood wafers in the form of fine droplets, thereby producing step (c) of the process a waferboard having substantially equal internal bond strength on its face and core.

22. A process as claimed in claim 21 characterized in that the amount of phenolic resin composition sprayed onto the wood wafers in step (a) is substantially 2% of the dry wood weight of the wafers.

23. A process as claimed in claim 21 or 22 characterized in that the resin treated wafers are felted on the core plate at a temperature below 1 200C and the loosely formed mats are consolidated in a hot press at a temperature of 190-21 00C and a maximum pressure of 450-500 psi (31.5-35 kg/cm2).