This invention relates to a cellulose building substrate incorporating a formaldehyde based resin wherein said substrate has improved formaldehyde emission properties and a method of manufacturing such a product.

Building substrates manufactured from lignocellulose material have been in wide spread use for a number of years. Composite boards including medium density fibre boards, particle board, chip board and plywood are well known examples of such boards. Conventionally/ composite boards are manufactured from lignocellulose material derived from trees. This material is chipped or made into fibres and then formed into a board by combining the material with a resin and forming the combined material into a board in a press. A number of different resins may be used as is known in the art but conventionally a formaldehyde based resin such as an aminoresin is used and the present invention relates to boards using formaldehyde based resins.

Particularly useful and popular resin systems are urea/formaldehyde, urea/melamine/formaldehyde, phenol/ formaldehyde and formaldehyde based resins comprising natural tannins. Whilst formaldehyde based resin systems are particularly effective resins for use in manufacturing such boards, the use of formaldehyde has in recent times become the subject of much closer scrutiny. In particular, at exposures over approximately 2 ppm, formaldehyde can cause eye, nose and throat irritation and in recent times, the American Environmental Protection Agency has classified formaldehyde as a probable human carcinogen. As such, various countries throughout the world are beginning to implement standards for maximum formaldehyde emission from products containing resins based on formaldehyde. The test method most popularly employed to measure emissions is the "WKI" test which was developed in Germany. This is a standard test procedure known to those in the art which expresses the formaldehyde

emission in milligrams per 100 grams a panel. The European standard is an E classification wherein El 10 mg/100 gms E2 30 mg/100 gms E3 60 mg/100 gms

In the United States, a standard has been introduced known as the Hudd standard and its value is 17 mg per 100 grams by the WKI method.

Composite boards including particle boards, chip boards and fibre boards and also plywoods conventionally utilize formaldehyde based resins. The most popular of these resin systems are the aminoresins such as urea formaldehyde and melamine formaldehyde. Conventionally, these aminoresins have comprised formaldehyde in a mole ratio to the amine moiety of greater than 1:1. In fact, it is well known that to obtain the most effective resin system an excess of formaldehyde over the reactive amino compound is required. Typical molar ratios for a strong urea/formaldehyde resin are 1 molar ratio of amino compound to at least 1.2 mols formaldehyde. Melamine/urea formaldehyde resins conventionally comprise formaldehyde on a molar ratio to the amino compound of 1.6-2.0:1. Such boards emit quite high levels of formaldehyde for an extensive period following manufacture. Various different proposals have been made to reduce formaldehyde levels from such building substrates. The common approach has been to use one or more of the following four alternatives:-

1. Lower the mole ratio of the resin (effectively reducing the amount of formaldehyde in the resin) .

2. Reduce the mole ratio of the resin with melamine fortification.

3. Add to or replace resin solids with scavengers.

4. Substitute the resin used for an alternative resin which does not incorporate formaldehyde.

Each of these various alternatives has disadvantages.

Primarily, any of the changes foreshadowed above, tend to reduce the strength and therefore effectiveness of the

resin leading to lower performance characteristics of the final product. This can be overcome by using higher resin loadings but of course this raises the cost.

It is an object of the present invention to provide a substrate bound with a formaldehyde based resin which has reduced formaldehyde emission levels.

We have now found a method whereby a resin system containing formaldehyde maybe used to give a product having reduced formaldehyde emissions over the product made using the same resin in the same portions but not using our process. The invention is of particular interest for the manufacture of substrates containing strong conventional resins having a molar excess of formaldehyde. According to the present invention there is provided a method of manufacturing a building substrate comprising a formaldehyde based resin wherein said board has reduced formaldehyde emissions, said method comprising:- (a) combining lignocellulose material with a formaldehyde based resin in a quantity of greater than 5.0%; and (b) compressing said combined material to form a building substrate, such compression being conducted at a temperature and pressure such that the core temperature of the substrate exceeds about

105°C during the course of such compression.

(Throughout this specification, the expressed percentage is a weight percentage ratio based on the dry weight of the cellulose fibre. Thus reference to 5% aminoresin is equivalent to 5 gms resin to 100 gms dry cellulose fibres.)

The nature of the formaldehyde based resin is not narrowly critical and any resin made using formaldehyde may be used. Catalysed resin systems such as phenol formaldehyde resin may be used. However, the problem of high formaldehyde emission is greatest when a heat cured system is used. Common heat cured resin systems are amino formaldehyde resins. The amino moiety of the resin system

aay he any multifunctional amino compound. Well known amino compounds are for example urea, melamine and phenol urea. The preferred formaldehyde based resins used are formaldehyde based aminoresins. Preferably, the resin used is made using formaldehyde in a molar ratio or more than 1:1 to the compound comprising the amino group or groups.

The present invention also embraces a lignocellulose building substrate containing a formaldehyde based resin made in accordance with this method.

According to another aspect of the invention, there is provided a building substrate manufactured from a lignocellulose material and a formaldehyde based resin wherein said resin has been made using formaldehyde in a molar ratio (to the compound comprising the other functional group) of more than 1:1 and is present in an amount of 5.0% or greater and wherein the formaldehyde emission from said board is less than 30 mgs/100 gms panel weight when tested according to the WKI method.

Preferably, the formaldehyde emission from the board is less than 10 mgs/100 gms panel weight using the WKI method.

The formaldehyde based resin is preferably present in an amount ranging from between 5.0% to 20.0% and most preferably in the range between 7.0% to 17.0%.

In conventional processes for the manufacture of composite boards (including medium density fibre board), the core temperature of the substrate does not exceed about 105°C even if the externally applied temperature in the press is significantly higher than this temperature. This occurs as a result of the moisture in the processed product. This moisture or water once heated to a temperature exceeding 100°C converts into steam and escapes from or through the edges of the building substrate. Through this process, the heat applied to the panel is dissipated. In accordance with the present process, the temperature of the core is increased to a temperature greater than 105°C. This may be achieved by

a number of different methods. For instance, the external pressure around the press may be increased to restrict the egress of moisture from the panel. This may be done by means of an appropriately designed autoclave. Alternatively, the sides or edges of the press within which the panel is being compressed may be sealed. In a still further arrangement, the building substrate may be compressed to form a high density board in which the average pore size is reduced so to restrict the passage of steam out of the board when heated to temperatures above

100°C.

It has been found that composite boards having

₃ densities in excess of about 1100 kg/m have sufficiently small pores to enable the temperature of the core to be raised to the levels contemplated by the present invention.

Preferably, for the purposes of this invention, the temperature of the core of the substrate is raised to a temperature greater than 120°C. Most preferably, the temperature ranges between 135°C to 145°C.

As previously indicated, the preferred formaldehyde based resins for use in this invention are formaldehyde based aminoresins. These include melamine urea phenol formaldehyde (MUPF), melamine phenol formaldehyde (MPF) , tannin formaldehyde (TF) , phenol urea formaldehyde (PUF), recorcinol urea formaldehyde (RUF) and urea formaldehyde (UF) . The most preferred resins are melamine urea formaldehyde and urea formaldehyde.

When using any of the above resins, the level of formaldehyde emission is largely dictated by the molar ratio of the formaldehyde to the urea, melamine or other compound used in the resin system. However, in general whilst the lowering of the formaldehyde content of the resin reduces overall emissions of the final product, reduction of formaldehyde content of the resin also reduces the adhesive effectiveness of the resin. The method of the present invention does not involve the reduction of the formaldehyde content of the resin below conventional levels. This eliminates the need to increase

the resin content of the board to achieve conventional performance characteristics. The formaldehyde molar ratio depends on the nature of the resin system but it is generally most preferably in the order of 1 part urea/melamine to about 1.2 to 2.0 parts formaldehyde.

The formaldehyde based resin content is dictated to some extent by the type of board being made. The present invention relates to those boards comprising greater than 5.0% of such resin. For medium density fibre boards, it is preferred that the resin be present in an amount between approximately 7.5% to 10.0%. High density fibre boards require higher resin content and preferably, the resin is present in high density fibre boards in a range between 10.0 to 20.0%. A particularly suitable amount is between 12.0 to 15.0%.

Preferably, particulate lignocellulose material is used in the practice of this invention wherein following combination with the aminoresin the combined material is formed into a low density mat prior to compression. The lignocellulose material may be chosen from a range of natural fibrous materials known in the art. Preferably, pinus radiata wood chips are utilized. However, fibres from other varieties of pine, cedar, beech, oak, Cyprus and similar such materials may be used. Vegetable fibre such as bagasse may also be utilized.

This raw cellulose material is preferably refined into small particles or fibres prior to combination with the formaldehyde based resin. In the manufacture of fibre board in accordance with the present invention, this method preferably involves at least the following steps:- (a) Debarking of the logs.

In this step debranched tree logs are stripped of external barking and hard external materials. (b) Chipping the debarked logs into chips.

Preferably the logs are chipped into wood chips approximately 25mm square and about 10mm thick, (c) Washing the wood chips.

The wood chips are then washed and screened to get

rid of sand, grit and other contaminants. Preferably this washing is conducted in warm water. The chips are screened to eliminate wood fines or chips which are too large. (d) "Digestion" of the washed chips.

The wood chips are transferred to a steam digester unit. Preferably this is in the form of a large chip bin through which steam is passed. The digestion step is conducted to soften the wood chips and to extract water soluble chemicals out of the chips e.g. tannins in the wood. For fibre board manufacture this digestion is a brief process conducted usually at about 140°C and about lOOkPa. This differs from conventional hard board manufacture where this step is conducted under more severe conditions for a longer period of time and used to convert the wood carbohydrates to resinous materials. It is important to remove a major portion of the water soluble resins and saps in the wood chips and this is effected by squeezing the softened chips. If these materials are not removed the finished product would be dark and have areas of discolouration. Further the resins added at a later stage bind better to the chip once these water soluble chemicals have been removed. Once the chips have been subjected to this initial softening and water soluble chemical extraction step they are then preferably "cooked" in a vertical refiner column at a steam pressure of approximately 900-1000kPa at a temperature of about 180-185°C to further soften the chips. The chips are cooked under these conditions for a short time of about 5 minutes.

(e) Refining softened chips.

After the chips have been softened they are then separated into small fibres typically of 5-8mm length and 0.1-0.2mm thickness. This is normally conducted using a rotating disc defibrator which rubs the wood apart into separate fibres. To the separated fibres molten paraffin wax is preferably added. The wax can be added at a later stage but is preferably added to the fibre/steam matrix at the refining step. The paraffin wax is added in an amount

from 0.5 to 2.0%. Preferably it is added in a quantity of about 1.0% (again on a dry basis - i.e. dry wax to dry wood). The wax is added to enhance the water resistance of the end product. Further its presence in the end product reduces swelling and water absorption, (f) Resin addition.

The refined chips are then preferably coated with the desired resin. Most desirably this is achieved in a short blow line (e.g. 8-10 metres) into which the steam/fibre/wax matrix is introduced. Flow through this tube is achieved by maintaining a pressure differential between the inlet and outlet of the defibrator. Resin may be injected into the side of the blow line to coat the wood fibres. The resin is preferably introduced in an aqueous dispersion with about 40-50% solids to water and sprayed into the blow line by resin injectors in the form of an atomised spray. The turbulence within the blow line causes the resin to be distributed evenly over the fibres. This has the advantage of reducing the prominence of glue patches on the finished board. The resin is added in a amount greater than 5.0%. For MDF it is preferred to add 7.5 to 10% resin and in high density fibre boards the resin content is preferably 10.0 to 20.0%. More preferably it ranges between 12.0% to 15.0% (again all on a dry weight basis)

(g) Water content reduction.

After wood chip refining and resin addition the water content of the matrix is reduced. It is preferably reduced so that it is 15% or less prior to the board product being compressed. In the case of pinus radiata wood chips, the moisture content before refinement is usually in the order of 100 to 140% by weight. The moisture content may be reduced in accordance with any of a number of methods known in the art. Preferably, a long (e.g. 70 metres) flash dryer tube is used into which the moist fibres are introduced and blown using hot air. Preferably, the temperature of the dryer tube and the residency time are such that the moisture content of the fibre matrix is reduced to between 8 to 15% and most

preferably, between 11 to 14%.

(h) Mat formation.

The dried and resin coated fibres are then laid into a mat preferably by way of a vacuum system which draws the fibres onto a conveyor belt in substantially random orientation. The low density mat may be precompressed through rollers prior to final compression and resin curing.

In the case of an end product high density fibre

₃ board (e.g. 1100 kg/m of 18mm thickness) the initial mat thickness will be about 0.5 metres. Precompression reduces this thickness to about 120mm by applied pressure of about 1000 kPa.

(i) Mat Compression. The fibrous mat is then compressed under conditions such that the core temperature of the mat exceeds 105°C during the course of such compression as described above. The actual temperature and pressure at which this step is carried out depends largely on the nature of the desired end product. To manufacture a high density product, ram pressures of up to 12,500 kPa or 1800 psi (platen pressure up to 6,000 kPa) may be required. However, for a medium density product, pressures of about 6,200 kPa or 900 psi are conventional. The pressure, temperature and time the product is retained in the press are all variables which are dependant on the thickness and type of finished product desired and the desired density profile through the finished board. For high density fibre board the platen pressure applied to the mat is preferably between 4,000 to 6,000 kPa and the product is maintained under such pressure conditions for between about 3 minutes for a 5mm board to about 18 minutes for a 22mm board. A standard 18mm would be maintained in the press for between 9 to 15 minutes. Most preferably about 12 minutes. The moisture content of the board is obviously reduced by the application of heat and pressure in the press. The final moisture content of the board is important for providing dimensional stability in the finished product. For high density board the moisture content following compression

is designed to fall within the range of 4 to 11%. Preferably between 7 to 11% and most preferably between 7 to 8%.

For the purposes of the present invention, it is the core temperature of the board which has been found to be critical. It has been discovered that when the core temperature of the product exceeds approximately 130°C during this step, the formaldehyde emission level of the finished product is dramatically reduced from that level expected for boards comprising such high levels of formaldehyde based resins. To achieve a core temperature of 130 C usually requires an applied temperature of about 165°C to 185°C for a high density fibre board, (j) Product finishing. After the compressed board is removed from the press it is allowed to post cure preferably in a suitable rack. Post cure for a 18mm board usually takes between 7 to 20 days. After this period the pre-cure layers of the board are removed by sanding. On both sides of the board a low density layer of between 0.5 to 1.5mm is formed by the hot platens when they come into contact with the surface of the board prior to the application of the total pressue required to form the desired board. These layers are undesirable as they do not exhibit the same characteristics of density and strength as the rest of the board.

In addition to low formaldehyde emission it is desirable to manufacture a board with minimal water swelling characteristics when exposed to moisture and which is dimensionally stable under changing climatic conditions of use. Fibre boards characteristically equilibrate with time to a moisture content proportional to the ambient conditions. Thus the parameters of moisture content as recited above are critical in the production of a board which is not just low formaldehyde emitting but also dimensionally stable.

Uptakes of water is normally uneven and can lead to buckling of the substrate and other instability. In most areas of the world the average relative humidity is around

50 - 60%. The applicants have found that the equilibriu moisture content of high density fibre board (ie. ove

3 1100 kg/m ) is approximately 7 - 8%. However in ver dry areas the equilibrium moisture content can be as lo as 4% and in very wet areas the equilibrium moisture content may be as high as 11%.

To make sheeting which has a moisture content of 4

- 11% as indicated above it is necessary to start with a mat having a moisture content from about 8 - 15%. The applicants have found that 4 - 6% moisture is normally lost during the press cycle. Hence to prepare a high density substrate having a content of 7 - 8%, the moisture content of the fibre mat should be about 11 - 14%, preferably about 12 - 13%. To avoid blow out of the sheeting due to build up of steam pressure during the press cycle, it is preferred that the temperature of the press and press time be kept as low as possible consistent with obtaining a proper cure of the board. Blow out refers to the characteristic of rapid steam escape after the substrate has been compressed which results in board damage.

The higher the density of the sheeting the more prone it is to blow outs during manufacture and therefore it is preferred that the density of the sheeting is the lowest consistent with obtaining the required low water swell for the required resin content.

The present invention may be employed for the manufacture of a range of different boards and substrates. As described above it has particular application in the manufacture of medium and particularly high density fibre boards. A high density fibre board made in accordance with the above mentioned methodology has the following preferred characteristics after pressing and cure:- Broad Range Preferred Range

Resin content : 10 to 20% 12 to 15%

Moisture content : 8 to 15% 11 to 14%

(Pre-Compression)

Moisture content : 4 to 11% 7 to 8% (Post-Compression)

Wax content : 0.5% to 1.5% 1% Density : 1100 to 1300 kg/m ³ 1150 to 1200 kg/m ³ Water Swelling

(British Standard BS5669) Less 1.0%

Formaldehyde emission Less than 10 mgs/lOOgm.

The present invention has the additional benefit of making the use of phenol/formaldehyde resins more commercially acceptable. In the past, phenol/formaldehyde resins have been avoided due to the prolonged periods required to cure such resins at temperatures slightly over

100oC. As the core temperature of' the board is raised above this level for the purposes of the present invention, it allows the use of phenol/formaldehyde resins with more rapid curing.

The reason for the reduction in formaldehyde levels in the boards of the present invention is not clearly understood and the applicant does not wish to bind itself to any one theory or explanation. However, it is believed that the higher core temperature leads to resin polymerisation reactions which are limited in their occurence at lower core temperatures. When an aminoresin polymerises, two bond types are formed, namely:

1. an ether linkage;

2. a methylene linkage.

For example, urea/formaldehyde resins polymerise in the following ways:

ldehyde

H

methylene bridge e Tther bridge (linkage) (linkgage) Melamine formaldehyde polymerises as follows:-

Ether linkages are weak and easily hydrolised to form water and formaldehyde which is emitted. Methylene linkages are stronger and more stable and not prone to hydrolysis. It is believed that the formation of a higher proportion of methylene linkages in the urea/formaldehyde resin is enhanced by polymerising the resin under higher temperature conditions.

The following examples illustrate the significantly reduced formaldehyde levels from boards produced in accordance with the present invention compared with boards manufactured in accordance with conventional processes.

Example 1

A medium density fibreboard manufactured from pinus radiata wood fibres and melamine formaldehyde resin was manufactured in accordance with the following method:-

Washed wood chips were digested in a steam digester for a period of approximately 5 minutes at a temperature of 140°C and a pressure of 101 kPa and then refined using a rotating disc defibrator. The separated fibres were hen combined and coated with the resin in a blow line dryer. The resin used was a commercially available

MUF resin manufactured by ICI Australia Limited called DPL

560. The resin was added in an amount equal to 8.5% of the dry weight of the wood fibre. In the dryer, the moisture content of the wood fibre was reduced to about

13%. After pre-compression the moisture content was about

12%. The dried and resin coated fibre was then applied to a belt by way of a vacuum system which drew several layers of fibre on top of one another on the belt to form a low density mat approximately 500 mm thick. This mat was brought to a more manageable thickness of 180 mm by passing through a precompressor.

The mat was then passed into a hot press and compressed at approximately 170°C and a pressure of 800 psi (ram pressure). The core temperature of the substrate did not rise above approximately 105 C. The resulting product had the following characteristics:

3 Density : 740 kg/m

Moisture Content : 7.6%

Board thickness : 18mm

Water swelling : Greater than 1.0% (BS5669) Resin content : 8.5%

Formaldehyde Emission: 45-52 mgs/100 gms (The formaldehyde emission was measured using the aforementioned WKI methodology)

The board produced was then ground to produce wood panel dust and the formaldehyde emission level was again tested.

The level was higher: 65-78 mgs/100 gms. Example 2

A high density fibreboard product was manufactured substantially in accordance with the method described in example 1. The same resin, namely DPL 560 was used but in an amount equal to 12.5% of the dry weight of the wood fibre. In the dryer, the moisture content of the wood fibre was reduced to about 13%. After precompression it was about 12%. The mat was compressed at 170°C and at a pressure of approximately 1800 psi (ram pressure). This pressure enabled the production of a board having a high enough density to reduce the internal pore size such that the core temperature could be increased significantly above the temperature of steam. The core temperature of the substrate was measured to reach 145°C. The resulting product had the following characteristics:

Density 1250 kg/m ³

Moisture content 7.0% Board thickness 18 mm Resin content 12.5% Water Swelling Less than 1.0% (BS5669)

Formaldehyde Emission: 4-6 mgs/100 gm The board was then ground to produce wood panel dust and the formaldehyde emission was again tested and found to be 6 - 8 mgs/100 mgs.

Surprisingly, the formaldehyde emission from this product was substantially reduced from the product

described in example 1 even though the resin content of the board was increased.

Example 3

A medium density fibre board product was manufactured substantially in accordance with the method described in example 1. However, a resin containing a higher molar ratio of formaldehyde was used. This resin was an MUF resin manufactured by ICI Australia Limited called DPL 527. This resin was added in an amount equal to 8.5% of the dry weight of the wood fibre. In the dryer the moisture content of the wood fibre was reduced to about 13%. After precompression it was about 12%.

Identical pressure and temperature conditions were used for the compression of the board as in example 1 and the resulting product had the following characteristics:

Density 740 kg/m 3'

Moisture content 7.8%

Resin content 8.5%

Board thickness 18mm Water swelling Greater than 1.0%

(BS 5669)

Formaldehyde Emission: 110-125 mg/100 gm

(note the WKI method begins to become inaccurate above levels of 100 mg/100 gm) Dust from this board was measured as having a formaldehyde emisssion level of 120-130 mg/100 gm.

Example 4

A high density fibre board product was manufactured substantially in accordance with the method described in example 2 but using the MUF resin DPL 527. The resin was present in an amount equal to 14% of the dry weight of the wood fibre. After drying and precompression the moisture content of the wood fibre was reduced to about 12%. The core temperature of the substrate during compression was rraaiisseedd ttoo 114400°°CC.. TThhe resulting product had the following characteristics;

Density ^" 1"2 ^ΛΛ 0 ^Λ 0 k ⁴ 'g ^~ / ' ^~ m'

Moisture content 7.0% Resin content 14%

Board thickness : 18mm

Water swelling : Less than 1.0%

(BS 5669)

Formaldehyde Emission: 6-9 mg/100 gm Dust from this product was also measured and the formaldehyde emission was found to be between 7-9 mg/100 gms.

The above examples indicate the surprising benefits of raising the core temperature of the substrate during the resin curing and compression step in conventional processes. The product produced using the method of the invention had substantially reduced formaldehyde emission levels which meet even the most stringent international standards. The products described in examples 2 and 4 both had emission levels well below the El classification of the relevant European standard. This level of formaldehyde emission has additionally been possible without involving significant changes in processing apparatus and methods and enables one to continue to use conventional resins without reducing the molar ratio of the formaldehyde. This means that the tensile strength and other performance characteristics of the end product are not significantly effected yet the formaldehyde emissions of boards made in accordance with the invention are substantially reduced.

Finally, it is to be understood that various modification and alterations may be made to the methods previously described without departing from the ambit of the present invention as defined in the following claims.