WO2001064408A1

WO2001064408A1 - Impact resistant substrate particleboard and composite material using same

Info

Publication number: WO2001064408A1
Application number: PCT/AU2001/000227
Authority: WO
Inventors: Anthea Louise Airey; Tony Franco; Trevor John Bryant
Original assignee: Wesfi Manufacturing Pty Ltd
Current assignee: Wesfi Manufacturing Pty Ltd
Priority date: 2000-03-03
Filing date: 2001-03-02
Publication date: 2001-09-07
Anticipated expiration: 2002-09-03
Also published as: AUPQ603900A0

Abstract

An impact resistant substrate particleboard, composite material using same, and method of forming the composite material, the substrate particleboard comprising at least a core layer and a surface layer, each layer in turn comprising at least wood chip/flake and resin, the resin of the core layer having a melamine content of at least 7 % melamine solids on resin solids, wherein each layer has a resin content of 10 - 15 % therein.

Description

"Impact Resistant Substrate Particleboard And Composite Material Using Same"

Field Of The Invention

The present invention relates to an impact resistant substrate particleboard and composite material using same. More particularly, the impact resistant substrate particleboard and composite material of the present invention have particular application in the commercial furniture market.

Background Art

High pressure laminate technology, involves the bonding together of multiple phenolic resin-impregnated kraft (brown) papers at high pressures (7MPa) to form sheets, which may later be glued using polyvinyl acetate adhesive (to the surface of a particleboard or medium density fibreboard benchtop, for example). The result is a product known for its very high impact qualities.

Low pressure technology differs from high pressure in that single or few sheets (rather than multiples) are impregnated with thermosetting resins of high clarity (often melamine or melamine-urea-formaldehyde) and directly bonded to a substrate which might be a wood composite such as particleboard, or medium density fibreboard. The impregnation process requires the papers to absorb ca 1-3 times their own weight in resin. The process involves laying up the impregnated papers onto the board then pressing in a hydraulic press at 2-3 MPa and 160-180 °C for 12-50 seconds, causing the resin to flow, and bond to the surface of the substrate, and also form a smooth plastic surface. The resultant product is widely used in cabinetry such as kitchen "carcasses", shelving, and a variety of furniture applications.

Ideas from HPL technology have been extended into the low pressure melamine arena by Laminex. "Structural Board" products, developed in the early 1990's, sought to provide an enhanced impact-resistant low pressure melamine for the commercial furniture market by utilising a single layer of phenolic-impregnated kraft paper as underlay to the decorative low pressure melamine paper to impart additional impact resistant properties to the product. The specification (drop height 45 cm) suggested the product consistently achieved above/the top end of ordinary high moisture resistant particleboard range. To bring the product up to near the minimum wear requirement of a high pressure laminate (HPL), a clear overlay was employed as a final layer, in the case of woodgrains and prints.

The impact resistant particleboard of the present invention has as one object thereof to overcome the above problems associated with the prior art.

Solid colour (e.g. blue, green) low pressure melamine finishes have traditionally been used in the office furniture market since the wear factor is higher than that offered by the woodgrain LPM products. The impact resistant particleboard of the present invention offers a means by which woodgrains can be used for commercial furniture.

The present invention overcomes the need for costly high pressure laminate where low-pressure melamine quality finish is required, high pressure laminate being too thick and plastic-looking. High pressure laminate, suitable for benchtops, usually exceeds the market need for less impact-prone areas such as

• desks. It is therefore envisaged to be a more suitable product for this market.

The composite material present invention overcomes problems found in the applicant's Impact Armacote product wherein the aluminium oxide content of the overlay presented problems for customers blunting their saws.

The composite material of the present invention exhibits superior wear resistance when compared with low pressure or high pressure laminates. The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure Of The Invention

In accordance with the present invention there is provided an impact resistant substrate particleboard comprising at least a core layer and a surface layer, each layer in turn comprising at least wood chip/flake and resin, the resin of the core layer having a melamine content of at least 7% melamine solids on resin solids, wherein each layer has a resin content of 10 - 15% therein.

Preferably, the resin of the core layer has a melamine content of about 31 % melamine solids on resin solids.

Preferably, the core layer has a resin content of about 11.5%.

Preferably, the surface layer has a resin content of about 13%.

Preferably, the core layer comprises chip/flake of the size about 0.35 - 0.45mm, and the surface layer predominantly comprises chip/flake of the size about 0.125 - 1.0mm.

In one form of the present invention the surface layer of the substrate particleboard has a chip/flake composition of about:

< 0.1 - 0.125 mm 6%

0.125 - 0.25mm 18%

0.25 - 0.5 mm 33%

0.5 - 0.71 21 %

0.71 - 1.0 18%

1 - 2 mm 3%

Preferably, the ratio of the density of the surface layer or layers to that of the core layer is at least 37%. The ratio of the density of the surface layer or layers to that of the core layer may be at least 37%. for particleboard of 33mm thickness or less. The ratio of the density of the surface layer or layers to that of the core layer may be at least 42% for particleboard of 16-18mm thickness. The ratio of the density of the surface layer or layers to that of the core layer may be at least 5 60%) for particleboard of 9-12mm thickness.

The ratio of the density of the surface layer or layers to that of the core layer will preferably be at least 70% for particleboard of less than 9mm thickness.

Preferably, the substrate particleboard has a density of at least 680-720 kg/m³.

In accordance with the present invention there is further provided an impact 10 resistant composite material comprising a substrate particleboard and at least one layer of melamine or melamine-urea-formaldehyde paper on each side thereof, the substrate particleboard having at least a core layer and a surface layer, each layer comprising at least wood chip/flake and resin, the resin of the core layer having a melamine content of at least 7% melamine solids on resin 15 solids, wherein each layer has a resin content of 10 - 15% therein, the paper layers increasing the wear resistance of the composite material.

Preferably, there is at least one additional clear surface layer provided over the or each paper layer.

Still preferably, the resin of the core layer has a melamine content of about 31 % 20 melamine solids on resin solids.

Still further preferably, the ratio of the density of the surface layer or layers to that of the core layer is at least 37%.

In accordance with the present invention there is still further provided a method

25. for the manufacture of an impact resistant composite material, the method comprising: a first stage in which a resin is used to saturate the pores of one or more papers and resin delivered into the paper core, the resin percentage adjusted, and the water content reduced to about 10 - 14%; a second stage to coat the paper; and a third stage in which the or each paper is applied to a substrate particleboard, as defined hereinabove, by way of pressing to liquefy the surface for a smooth finish prior to gelation and vitrification.

A two bath system may be used to impregnate the or each paper with resin, the first stage utilising a first bath, and the second stage utilising a second bath.

The or each paper may preferably be coated over a path length of at least 2 metres.

Preferably, the third stage utilises press platen temperatures of about 160-220°C, cycle time of about 30-60s, and pressures of 2-3MPa.

Brief Description Of The Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:-

Figure 1 is a schematic perspective view of a composite material in accordance with the present invention.

Best Mode(s) For Carrying Out The Invention

The following example is that of a preferred embodiment of the invention and should not be considered to limit the above generality of the invention.

Substrate Manufacture

Cellulosic fibre or chip, is bonded using a thermosetting resin such as urea- formaldehyde, melamine-formaldehyde, melamine-urea-formaldehyde, phenol- formaldehyde, tannin resin system or the like. When the composite mixture is pressed to shape under heat and pressure, the thermosetting resin is cured, and a composite wood panel is formed. A wide variety of manufacturing arrangements exist to effect this process. Chip or flake may be manufactured from logs by debarking then use of a knife or drum flaker. These are dried by passage through gas, or reject-material fired burners to reduce moisture content of the chip to 2-5%. Particles are screened through mechanical screens then wind-sifters to sort the furnish into fractions. In the process described herein, a three layer board is produced where a core layer and a surface layer exist. The flake size in the core fraction is 0.35-0.45 mm and the chip in the surface layer is of the following composition:

< 0.1 - 0.125 mm 6%

0.125 - 0.25mm 18%

0.25 - 0.5 mm 33%

0.5 - 0.71 21 %

0.71 - 1.0 18%

1 - 2 mm 3%

Flakes of the respective core or surface layers are passed through separate blenders, where resin is injected as a fine spray; the volume of resin injected is controlled by means of PLC feedback control to the "glue kitchen" or other dosing system. At the glue kitchen, resin, wax, water are mixed. Water is added to bring the surface layer furnish to the desired moisture content (11-14 %). Wax (1-2%, wax solids on resin solids) may be added as an emulsion in the resin mix or as slack (hot, molten) wax to the blenders. Additives such as urea & hardeners (1-2% mass/resin solids in core layer), or treatments (such as permethrin, a termite treatment) may also be added either through the glue kitchen mixture or into the blenders at this point. In order to achieve the impact qualities described herein (>45 cm), 10-15% resin must be applied to the surface layer and 10-15% resin applied to the core layer with melamine content of at least 7% (melamine solids on resin solids) required in the core layer.

The flakes are distributed onto a moving conveyor belt and formed into a mat by a wind or mechanical former, which might be in one or more layers. In the case of this example a three layered mat is produced. The two outside layers are finer and provide a dense surface, the larger particles are laid as the core and provide strength. However, a graded-density former, where one furnish source is used, and particles are graded in size by wind-sifting nozzles and screens, could also be used; or, any other means of laying up a mat where particle sizing to form a density gradient is possible. The overall size of the mat formed is controlled by means of a weighing device in the blender infeed (the weighfeeder) in a PLC- controlled feedback loop. In order to achieve desired Impact qualities a target density of 680-720 kg/m³ or greater is required. The surface:core ratios (mass/mass) required for different thicknesses, for example, are:

33 mm >37%

16-18 mm >37%

9-12 mm >60%

< 9 mm >70%

The mats progress by means of a mat transfer belt or caul plates into the press where the resin component is reacted. The mats may be pre-pressed to a controllable height before entering the press. Heat (press temps 140-220 °C), pressure (2-3 MPa) and time (6-15 seconds/mm board) must be controlled. In a batch-continuous process a single-daylight (one mat pressed) or multi-daylight

(many mats pressed) may be used. The thick mats are compressed in the hydraulic press to a certain distance controlled by bars, hydraulics and the like.

Full pressure (2-3 MPa) at the stops is held for a short time (30-50s) until the board hardens then pressure is slowly released (9-12mm; 16-18mm 3 min;

25-33 mm 5 min). These airing cycles are required to release built-up steam to prevent "blowing".

Another method of pressing in common use is continuous pressing (Conti-Press) whereby the mats are carried onto heated calendar rollers by steel belts, to produce a continuous sheet.

Hot boards are mechanically removed from the press, and cooled by means of a star-wheel cooler. The boards are then sanded (1.3-1.7 mm sandoff) to fine tolerances (± 0.2 mm), cut-to-size for lamination, and equilibrated for 3 - 7 days to stabilise moisture gradients and fully cure the resin. Low Pressure Melamine Paper Manufacture and Application

In low pressure melamine application, papers which have been impregnated with urea-formaldehyde, melamine-urea-formaldehyde or melamine are used. As discussed hereinafter, one, two, three or more papers, on each side of the board, may be used (2-6 or more in total). The papers provide a decorative, wear resistant surface for the product.

The production process involves three phases of reaction of thermosetting polymers. During paper treatment, paper is impregnated with resin in a two bath system with first and second stage drying. A specially developed resin loading and mixture program is used. In the first stage, the aim is to saturate the pores of the paper and deliver resin into the core. The paper is wet by means of the pre- wetting roller in the first bath, where rapid absorption of resin takes place. After a short distance the paper fibres swell, and absorption slows down. A relatively long path length, for example 2m, over an elevation roller is therefore required to saturate the paper. The web is then completely submerged. The squeeze rollers following this impregnation are used to bring the resin percentage into appropriate ranges prior to drying through the first stage dryers. - Settings will depend on the length of the oven, temperature, speed of the line, and grammage of the paper.

During first stage drying, the water content is reduced and the resin is dried without gelation, by means of controlling volatile content. Melamine or melamine-urea-formaldehyde resins are generally dried in the range of 10-14% during the first drying cycle.

Most papers, for example overlays, woodgrains and prints, will require only a single application of resin, then drying to target volatile contents (5-7%) through second stage ovens. If a second application is required, for example with solid colour papers, the paper is then impregnated through a second bath. The papers are coated according to a specially developed resin loading and mixture program. The rollers may be of gravure type with scrapers, a similar arrangement to the first bath, or other arrangement enabling adjustment of the coat weight. The paper is dried to a volatile content of 5-7%.

The final step is to apply the papers to the board (lamination). Press platen temperatures of 160-220 °C, cycle time 30 - 60 s and pressures of 2-3 MPa provide the best conditions for liquefying the surface for a smooth finish prior to gelation and vitrification. Cycle times will depend on mechanical conditions of the press, such as whether single or multiple paper layup is possible, infeed and outfeed rates which may be limiting, and the pressure exerted by the hydraulics. Other factors such as the ambient temperature and humidity can affect results if the factory is not conditioned.

The pressed sheets are held in packs for 3-5 days to effect cooling, moisture equilibration and curing of the low pressure melamine surface. Testing of panels in this example was carried out after 72 hours conditioning (on the fourth day). Prior to this period lower Impact and Taber results could be observed. Impact and Taber values appear to plateau after equilibration of the pack to room temperature.

In Figure 1 there is shown an impact resistant composite material 10 in accordance with the present invention, the material 10 comprising a substrate particleboard 12, and a first paper layer 14, a second paper layer 16 and a clear surface layer 18, provided on each side 20 thereof. It is to be understood that the clear surface layer 18 is necessary only for circumstances in which the paper layer 16 is provided in the form of a print, woodgrain or pattern.

Test Methodology The impact resistant composite material of the present invention notionally addresses the market niche between low pressure and high pressure melamine products. A national or international impact or wear standard does not currently exist for such products. However BS-EN 438-1 : 1991 , "Decorative high-pressure laminates (HPL) - Sheets based on Thermosetting Resin" specifies a range of wear and impact limits for high pressure laminates; the most relevant set for comparison are set out in Table 1 following:

Table 1 : Wear and Impact Requirements according to BSEN 438-1 for Laminates

Impact Resistance Method

The method involves dropping a large 42.8 + 0.2 mm diameter stainless steel ball weighing approximately 324 grams from a height onto the sample mounted in a test jig, five times over the surface. The impact diameter is recorded on a piece of carbon paper on the surface of the sample. The method tests to the greatest height at which failure does not occur; therefore, it is necessary to induce failure and non-failure at adjacent levels to establish this. The method models fairly severe & repeated knocks, as might be experienced by objects falling onto benches during building construction and wear/tear, knocks from pedestrians or trolleys.

A kitchen standard which utilises a variation of the large ball method, to assess suitability for benchtops, is BS 6222 part 3:1999. In this standard the ball is dropped from a constant height of 45 cm. It the sample cracks or the diameter of indentation exceeds 10 mm, a fail is recorded. Otherwise, the sample passes the standard for Impact.

Another impact test exists in the European standard, BSEN 438-2, where a smaller stainless steel ball is fired at the sample. Due to the ability to define the force above that provided by gravitational acceleration used in the Large Ball method, the small ball test is a more severe test of puncture: This test models the force that might be exerted on a floor, for example, by a pedestrian with a stiletto heel. A combination of the two tests are used in for the High Pressure Laminate Standard EN-438-1 and the provisional standard for Laminated floating floors, prEN13329, to assess suitability for various classes of use.

Wear Resistance Method

For the purposes of this disclosure, the appropriate test is BSEN-438-2,6, also sourced from the European High Pressure Laminate test methods standard EN 438-2. Sandpaper is mounted onto a calibrated wheeled device (known as a Taber machine), and used to progressively wear through the melamine surface. Changes of the calibrated sandpaper are made every 500 cycles.

Taber wear may be thought of as the extent of pattern removal. Low pressure melamine patterns/woodgrains offer a basic wear resistance since the print is on the top of the paper, whereas solid colours are inked throughout. A Taber test for woodgrains/prints might therefore be expected to complete midway through the low pressure melamine paper, whereas the Taber test would continue almost through to the substrate in low pressure melamine with a solid colour.

The formula for Taber wear value according to BSEN-438-2,6 is:

IP + FP 2 where

IP = initial point; first recognisable wear-through of print exposing sub-layer in four quadrants. The sub-layer for prints and woodgrains is the background on which it is printed; for solid colours it is the first sub-layer of different colour.

FP = final point of 95% wear. This occurs in a patterned laminate when 95% of the pattern is removed or in the case of a plain colour laminate, when the underlayer is exposed over 90% of the abraded area.

The Taber method requires a reasonably skilled practitioner and strict adherence to controlled conditions. The estimation of initial and final points can be subjective. For low Taber numbers (< 500) a reliable result can usually be obtained. The method requires three replicates. The above discussion, however, demonstrates that the significance of the result should be carefully considered before product claims can be made. Products with Taber of 40, 350 and 6000 cycles are clearly of different performance levels whereas for products of 300 and 370 Taber cycles the difference will clearly not be as significant. Results and Discussion

Five variants were prepared using primarily UF resin (variants 1-3) and a melamine-containing resin (variants 4, 5). In the case of variants 1-3, the density and surface:core layer ratios were varied using a primarily UF-only resin base, to examine the effect of a changed density profile. In the case of variants 4-5, the density and surface: core layer ratios were varied using a melamine based resin holding resin loading constant. The results were evaluated by laminating the test surface and carrying out the large-ball Impact test EN-438-2,12. Wear tests were carried out on the most successful impact variants according to BS-EN438- 2,6. The products were also evaluated according to the requirements of AS/NZS 1859.3 which includes stain, steam, porosity and craze tests; all variants passed and were in the top category. The substrate tests revealed all exceeded the requirements for HMR particleboard in Australian and New Zealand standard AS/NZS 1859.1.

Impact Resistance

The results suggest both higher melamine content resin and high surface:core ratios in the substrate must be present to obtain impact values above ordinary performance with a high moisture resistant substrate. Surface:core ratio and overall density alone are not sufficient for impact resistance.

From Table 2, it can be seen that Variant 3 or 2 (with 40% surface:core ratio) compared with variant 1 (of 33% surface:core ratio) show little or no improvement, whereas variant 5 (of 42% surface:core ratio and melamine resin) shows significant improvement from those of similar surface:core ratio. Likewise, melamine content alone is a necessary but not sufficient condition for good impact resistance. The high melamine content, variant 4, shows little improvement over 1-3, but when surface:core ratios are improved, an enhanced effect is demonstrated for variant 5.

Table 2 also demonstrates that the diameter of impact increases with increasing impact height. To verify the accuracy and precision of the applicant's laboratory results, samples were sent to an external UK-based laboratory at FIRA (Furniture Industries Research Association). For method BSEN 438-2, FIRA results are of higher drop level, larger radius, and exhibit less spread than that of the applicant's laboratory. It is possible that the panels reached a more final state of cure over the period in transit (3 weeks) and testing (3 months); however, the interpretation of cracks in product failure could be a significant contributor to this variation. Over the course of the research, the applicant's laboratories became quite practised at observing the fine hairline cracks which might occur in only one of five drops required for each test piece.

A second impact test, BS 6222 part 3: 1999 recently developed at FIRA, was carried out on the product. This confirmed product types 4 and 5 would be suitable for kitchen benchtops, whereas 1 ,2 and 3 would not.

Table 2. Impact and Taber Results for Prototype boards.

CO c

CD CO

m

CO

I m m

73 c \— m ro σ>

*J O

C

Figures in parentheses indicate standard deviation.

Where available, at least six test pieces were tested on front and back to establish the standard deviation. When considered in different colour groupings/number of paper layers (Table 3), some differences in Impact result were observed. For 5 variant 5, the three-layer structure with a woodgrain achieved an average impact result of 55 cm (standard deviation 6 cm) whereas on the same substrate with a solid colour the result was 64 with a much larger range. The solid colour distribution was skewed, with range 500-1000, skewness -0.7. As the test method is only calibrated to a height of 100 cm, products outperforming this 10 maximum would have truncated impact height values to 1 m. A similar distinction was seen for variant 4 suggesting different paper types and their treatment could contribute up to 10 cm to the final impact result, however, the presence of additional layers as seen in the woodgrain examples is not necessarily favourable to the impact resistance.

15

A source of contributing variation in the test pieces is the top-to-bottom variation seen in the solid colour product where superior results were obtained on the bottom compared with the top. Too much surface sandoff from the top of the raw board is thought to be the reason.

20

Table 3. Contribution of Paper Type to Impact Resistance

25 Wear Resistance

A key feature of the product is the high wear resistance compared with traditional high pressure laminates and low pressure melamine products.

For solid colours, this is primarily achieved by using doubled papers of the same colour. Once pressed, the inter-paper bonding is such that the layers are indistinguishable to the eye or in the Taber wear test. The necessity to use a double layer comes about due to the light grammages (70-80 grams/square meter) in the low pressure melamine papers commonly available to low pressure melamine manufacturers. The Taber wear value obtained (ca 1400 cycles) is surprisingly, more than double that of one low- pressure laminate layer (500 Taber cycles), suggesting some synergy of common-coloured melamine or melamine- urea impregnated papers. Possible causes might be prevention of resin penetrating the board by the first paper, some structural advantage in the resulting polymer, or contributing non-linearity of the Taber method. FIRA Taber results were even higher on the solid colours with the experiment discontinued after 2000 cycles, suggesting that the durability is approximately four times that of the woodgrains (WESFI ratio 3). The longer cure time available to the panels, inter-laboratory differences and variation inherent in the method, described hereinafter, and smaller statistical sample sent to FIRA, may account for this difference.

Prior art (Laminex) utilised a brown kraft paper as underlay, which shows through on the edges of the cut product in furniture; also, this means that wear and tear will expose the brown underlay sooner than the present invention. Wear values achievable in competitor products were therefore not better than ordinary low pressure melamine result.

For prints and woodgrains, a different technology is required due to the print lying only on the surface of the paper. A clear overlay is used so that nearly four hundred Taber cycles are required before wear occurs on the print. Reasonable agreement was achieved between the FIRA and the applicant's Taber results (FIRA: 458 as compared with Applicant: 394). However, the applicant's test results would appear more repeatable as evidenced by the low standard deviation (Table 2). To place an overlay onto low pressure melamine to improve its wear is known art.

In general terms, the inventors believe the substrate conveys a substantial portion of any impact resistance, and the arrangement of the decor papers confers the wear resistant finish.

It is envisaged that non-matched papers (solid colours) may be used and not effect the impact result. It is expected that this would affect the Taber outcome (results 200-300 would occur, rather than 1400 or so).

It is further envisaged that good impact results may be achieved with only one or two layers of melamine-urea-treated paper. The paper itself does not add to the impact resistance, but may detract from it; for example, if the paper has the incorrect resin loading or lacks plasticity, it may crack.

It is still further envisaged that rather than melamine, it may be possible to use a plasticiser in the paper to carry out the same function.

It is yet still further envisaged that it may be possible to develop an Impact- resistant MDF product using the same structural optimisation principles.

In addition to the properties of impact and wear resistance of the composite material of the present invention, the substrate particleboard of the present invention exhibits a number of properties that compare favorably with standard, high moisture resistant or flooring grade products produced either by the present applicants or other manufacturers (Australian Wood Panels Association Test Centre Report And Statistics for Year 2000), as can be seen with reference to Table 4 below: Table 4. Comparative Board Strength Properties -Applicant's Particleboard

Products (Typical Values when tested to AS/NZS 1859.1 and methods cited therein)

NT = not tested

These features are further enhanced by the addition of the melamine layers to provide the composite material of the present invention. The melamine layers restrict the intake of water through the surface during testing and add surface stability, thus enhancing screwholding results and other structural results. The results are shown to be similar or improved upon the published values by Laminex (Laminex 1994 Catalogue) for their structural board product, see Table 5 below: Table 5. Comparative Board Strength Properties

(Typical Values when tested to AS/NZS 1859.1 and methods cited therein)

NR = not reported.

The values obtained for these and additional tests, obtained independently at FIRA for the composite material of the present invention are shown in Table 6 0 below. These results are presented in a separate table since slight method differences exist between AS/NZS 4266 and BS EN 300-series tests. In particular, 1 N/mm² is equal to 0.001 MPa. Whilst MOR MOE tests are very similar, screws for BSEN screwholding tests are different from those specified in AS/NZS standards. The V313 durability tests are virtually identical. BS EN 5 Dimensional stability testing occurs at 20 degrees between 35 and 85 % relative humidity whereas for the AS/NZS, the test occurs at 25 degrees between 35 and 85 % relative humidity. Surface soundness (BS EN) is carried out with a round, rather than square (AS/NZS) jig. In wet cyclic durability testing, also known as the V313 test, the composite material product of the present invention is subjected to three iterations of soaking, drying and freezing. This methodology and has been found to correlate to external weathering (K.W. Maun, An Assessment of Exterior Medium Density Fibreboard (MDF), BRE Information Paper April 1996) and is generally accepted as an accelerated aging test. The composite material of the present invention performs well at this test with a retained internal bond of 0.48 N/mm² (480 kPa) by comparison with ordinary HMR particleboard (typically 0.2 N/mm², 200 kPa). A low residual swell value of the laminated product (1.4%) after cyclic testing also compares well with unlaminated HMR products (which typically give a final swell value of 8 %).

Table 6. Results of Impact Board Tests from FIRA (UK).

In performance the substrate particleboard of the present invention is superior in its capacity to hold screws than the other particleboard products referred to. Notably, the edge screw holding results are much greater than the corresponding HMR particleboard or Laminex Structural Board results, suggesting the gain is due to the substrate integrity, not purely the low pressure melamine surface present in the composite material. Thus, other surfaces could be applied and still gain benefit from the substrate strength.

The Wet cyclic durability test results show the composite material of the present invention to be highly durable. This test, modelling the longer term performance, in addition to the impact results, reported in Table 2, suggest the product will resist structural defects in high use commercial furniture installations.

The high surface soundness of the particleboard product, compared with other particleboard types improves routability.

Often, in the marketplace customers are less concerned with durability and more with appearance. For example, a customer may claim on their warranty to remove benches or doors which have developed swelling, even if the product is still structurally sound. The extremely low swell characteristics (during both short term and durability testing) means the product is not only resistant to water incursions into the substrate particleboard, but responds with a barely perceptible swell. Thus it will perform better cosmetically than HMR particleboard or MDF products currently on the market, leading to greater customer satisfaction, and less frequent claims against benchtop postforming specialists, installers and manufacturers.

The relatively high modulus of rupture (breaking strength) of the substrate particleboard indicates that the product may bear heavier loads than standard or high moisture resistant particleboard products.

As the substrate particleboard exceeds the requirements for Class 1 Flooring (AS/NZS 1859.1 ) it is envisaged that it may be used as a flooring panel according to the standard. Further, with appropriate design, it may be used for structural or decorative walls. The finish of these walls might be with the composite material described hereinabove, or with paint, low pressure melamine, veneers, lacquers or other decorative surface finish.

The substrate particleboard may be used as a component of a finished flooring system, such as access flooring (computer flooring). A variety of surface finishes could be applied. The applicant has low pressure melamine (LPM) flooring surface technology available to manufacture a product, which would entail a low- pressure melamine backing paper, surface melamine impregnated decor paper and a resin-impregnated aluminium oxide-containing clear overlay. The wear rating of the overlay could be readily varied. Such a product might be cut to size. For other applications it could feature a tongue and groove. A finished flooring product could alternately feature a vinyl, high pressure laminate, or continuously processable laminate surface which could be applied to the substrate with glue.

It is envisaged that a further application of the substrate particleboard of the present invention is its substitution for hardboard or ultra-high density fibreboard, such as in toilet partitions in public buildings. The surface may be the low pressure melamine finish described for the composite material hereinabove, or may be a high pressure laminate or a CPL surface.

Further, in the cabinetry industry, the weakening over time of screwed joints leads to problems, such as carcass joint failure. Such problems lead to the use of alternate materials in the frame, specialised hinges, and a limit to the warranty available. Use of the substrate particleboard or composite material of the present invention could appreciably extend cabinetry lifespans. In particular, as white low pressure melamine product (kitchen carcasses) the product could extend kitchen lifespans, therefore providing added value to the community.

It is envisaged that the surface low pressure melamine layers of Impact Board may be substituted with a high pressure laminate or CPL surface of impact and wear resistant quality. Such a product will not presently be as cost effective as the low pressure melamine surface described hereinabove. However, such a product would provide the same or an even further enhancement of impact and wear resistance at a certain cost. Further, an overlay containing aluminium oxide might be used. However, such a product would have disadvantages overcome by the composite material of the present invention.

Surfaces which are not inherently wear or impact resistant may be attached to the substrate particleboard of the present invention. This would deliver a lesser product in terms of surface properties but still deliver the advantage of the impact qualities, moisture resistance, durability, screwholding and strength properties. Such a surface could be, for example, a single layer of white or coloured/woodgrain low pressure melamine-impregnated papers on one or both sides, or a veneer, or a glued paper, paint or lacquer.

It can be seen from the foregoing description that the present invention provides an improved impact resistant particleboard substrate and a method of manufacturing a product with a high Taber value, without the need to use aluminium oxide or formally wear-resistant materials which have detrimental (and prohibitively costly) effects on non-diamond tooth saws (compared to the applicant's own product Impact Armacote), improved impact resistance compared to standard low pressure melamine products and competitor benchmarks, and high wear resistance compared to standard low pressure melamine products and high pressure laminates.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

1. An impact resistant substrate particleboard comprising at least a core layer and a surface layer, each layer in turn comprising at least wood chip/flake and resin, the resin of the core layer having a melamine content of at least 7% melamine solids on resin solids, wherein each layer has a resin content of 10 - 15% therein.

2. A substrate particleboard according to claim 1 , wherein the resin of the core layer has a melamine content of about 31 % melamine solids on resin solids.

3. A substrate particleboard according to claim 1 or 2, wherein the core layer has a resin content of about 11.5%.

4. A substrate particleboard according to any one of claims 1 to 3, wherein the surface layer has a resin content of about 13%.

5. A substrate particleboard according to any one of the preceding claims, wherein the core layer comprises chip/flake of the size about 0.35 -

0.45mm, and the surface layer predominantly comprises chip/flake of the size about 0.125 - 1.0mm.

6. A substrate particleboard according to any one of the preceding claims, wherein the surface layer has a chip/flake composition of about:

< 0.1 - 0.125 mm 6%

0.125 - 0.25mm 18%

0.25 - 0.5 mm 33%

0.5 - 0.71 21 %

0.71 - 1.0 18%

1 - 2 mm 3%

7. A substrate particleboard according to any one of the preceding claims, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37%.

8. A substrate particleboard according to any one of the preceding claims, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37%. for particleboard of 33mm thickness or less.

9. A substrate particleboard according to any one of the preceding claims, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37% for particleboard of 16-18mm thickness.

10. A substrate particleboard according to any one of claims 1 to 7, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 60% for particleboard of 9-12mm thickness.

11. A substrate particleboard according to any one of claims 1 to 7, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 70% for particleboard of less than 9mm thickness.

12. A substrate particleboard according to any one of the preceding claims, having a density of at least 680-720 kg/m³.

13. An impact resistant composite material comprising a substrate particleboard and at least one layer of melamine or melamine-urea- formaldehyde paper on each side thereof, the substrate particleboard having at least a core layer and a surface layer, each layer comprising at least wood chip/flake and resin, the resin of the core layer having a melamine content of at least 7% melamine solids on resin solids, wherein each layer has a resin content of 10 - 15% therein, the paper layers increasing the wear resistance of the composite material.

14. An impact resistant composite material according to claim 13, wherein at least one additional clear surface layer is provided over the or each paper layer.

15. An impact resistant composite material according to claim 13 or 14, wherein the resin of the core layer has a melamine content of about 31 % melamine solids on resin solids.

16. An impact resistant composite material according to any one of claims 13 to 15, wherein the core layer has a resin content of about 11.5%.

17. An impact resistant composite material according to any one of claims 13 to 15, wherein the surface layer has a resin content of about 13%.

18. An impact resistant composite material according to any one of claims 13 to 15, wherein the core layer comprises chip/flake of the size about 0.35 - 0.45mm, and the surface layer predominantly comprises chip/flake of the size about 0.125 - 1.0mm.

19. An impact resistant composite material according to any one of claims 13 to 18, wherein the surface layer has a chip/flake composition of about:

< 0.1 - 0.125 mm 6%

0.125 - 0.25mm 18%

0.25 - 0.5 mm 33%

0.5 - 0.71 21 %

0.71 - 1.0 18%

1 - 2 mm 3%

20. An impact resistant composite material according to any one of claims 13 to 19, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37%.

21. An impact resistant composite material according to any one of claims 13 to 20, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37% for particleboard of 33mm thickness or less.

22. An impact resistant composite material according to any one of claims 13 to 20, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 37% for particleboard of 16-18mm thickness.

23. An impact resistant composite material according to any one of claims 13 to 20, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 60% for particleboard of 9-12mm thickness.

24. An impact resistant composite material according to any one of claims 13 to 20, wherein the ratio of the density of the surface layer or layers to that of the core layer is at least 70% for particleboard of less than 9mm thickness.

25. An impact resistant composite material according to any one of claims 13 to 24, wherein the moisture content of the surface layer is about 11-14%.

26. An impact resistant composite material according to any one of claims 13 to 25, wherein the substrate particleboard has a density of at least 680-720 kg/m³.

27. A method for the manufacture of an impact resistant composite material, the method comprising: a first stage in which a resin is used to saturate the pores of one or more papers and resin delivered into the paper core, the resin percentage adjusted, and the water content reduced to about 10 - 14%; a second stage to coat the paper; and a third stage in which the or each paper is applied to a substrate particleboard, as defined in any one of claims 1 to 12, by way of pressing to liquefy the surface for a smooth finish prior to gelation and vitrification.

28. A method according to claim 27, wherein a two bath system is used to impregnate the or each paper with resin, the first stage utilising a first bath, and the second stage utilising a second bath.

29. A method according to claim 27 or 28, wherein the or each paper is coated over a path length of at least 2 metres.

30. A method according to any one of claims 29 to 31 , wherein the third stage utilises press platen temperatures of about 160-220°C, cycle time of about 30-60s, and pressures of 2-3MPa.

31. An impact resistant composite material substantially as hereinbefore described with reference to Figure 1.

32. A method for the manufacture of an impact resistant composite material substantially as hereinbefore described with reference to the example.