Manufacture of Decorative and Industrial Laminates

Field of the invention

The invention relates to the manufacture of composite woodpanels and to decorative or industrial laminates. A composite woodpanel includes particleboard, fibreboard and oriented strand board. A laminate can be produced by various methods known as high pressure (HPL), continuous pressure (CPL) or low pressure (LPL) lamination. These various methods are well known and described for example by reference to EN norms such as EN14322, EN438-2 & EN13329.

These various methods all use heat, pressure and time to combine the individual layers to form the finished laminate.

Any reference herein to a prior document or other prior disclosure is not to be taken as an admission that the contents of the document or the disclosure is common general knowledge, either in Australia or elsewhere.

Background of the invention Typically one or more papers or non-woven materials are saturated with a thermosetting resin and later combined with a substrate to form a laminate. Herein for convenience, the term "paper" also embraces non-woven materials suitable for the respective embodiments under discussion.

In the case of HPL, typically 1 or more sheets of kraft paper impregnated with a thermosetting resin, usually phenol formaldehyde resin (PF), are used as the substrate and combined with a melamine formaldehyde resin (MF) impregnated decorative paper and optionally with a melamine formaldehyde resin impregnated clear overlay paper.

Optionally the decor paper is not impregnated and the overlay paper provides the surface resin, a technique known as dry pressing. This set of impregnated papers is placed in a press and using pressure, heat and time a HPL is produced. Typically a high pressure press will be a multi-daylight loaded press with more than one set of impregnated papers.

In the case of CPL, typically 1 or more layers of kraft paper impregnated with a thermosetting resin, usually PF resin, are combined with a MF resin impregnated decorative paper and fed on a continuous basis between 2 endless belts to produce the CPL. Optionally a clear overlay paper impregnated with MF resin is included in the lay- up as the outermost layer.

An alternative use of a continuous press is to fuse one or more impregnated papers directly to one or both sides of a wood based panel substrate.

In the case of LPL, typically 1 or more impregnated papers are placed on both sides of a substrate, usually a wood based panel such as particleboard, fibreboard or oriented strand board, and the combined woodpanel and impregnated papers are placed in a press and using pressure, heat and time a LPL is produced.

All of the aforementioned examples are well known to one skilled in the art of laminate manufacture, with varying settings being used for pressure, heat and time to reach the desired properties of the laminate. Furthermore, it is common knowledge that impregnated barrier papers can be used below the decorative layer, and that additives such as corundum and or anti-static are included in or on the surface layers to achieve enhanced properties such as abrasion resistance or a reduction in static build up.

It is also well known that to maintain a reasonably useful shelf life and ability to handle impregnated papers prior to pressing, the impregnated papers typically have a residual volatile content in the range 4 - 10%.

It is an object of the invention to at least in part gain an economic advantage by reducing the time in the press under pressure, and where convenient by reducing the press temperature, in manufacturing any of the aforementioned examples of an industrial or decorative laminate.

In one particular aspect described herein, the invention achieves a reduction of emissions of formaldehyde and other volatile organic compounds (generally referred to as VOCs) from woodpanels.

Summary of the invention

It has been discovered, in accordance with the first aspect of the invention, that by directing heat at the b-stage impregnated paper before the laid up paper or papers enter the press, the remaining volatiles can be further reduced thus enabling a shorter pressing time and avoiding sticking of the laminate in the press. It is generally thought that sticking occurs when short chain length polymers migrate to the surface of the laminate and cause the laminate surface to stick to the press plate or press endless belt.

More generally, according to the first aspect of the invention, the invention provides a method of manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised in that, prior to said combining in the press zone, said one or more resin-impregnated papers are pre- treated by being heated to reduce residual volatile content in the paper(s).

In its first aspect, the invention further provides apparatus for manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised by means disposed upstream of said press zone for pre-treating said one or more resin-impregnated papers by application of heat to reduce residual volatile content in the paper(s).

The invention accordingly provides, in its first aspect, a method and apparatus entailing reduction of the volatile content in the b-stage impregnated papers or non-woven material and further advancing the degree of cure just prior to entering the press apparatus.

Additionally, it is known to one skilled in the art of lamination that when a woodpanel is employed as the substrate, whether the woodpanel is a particleboard, fibreboard, oriented strand board or similar panel based on cellulosic material, and to prevent cracking after lamination, the woodpanel should contain a degree of moisture. The degree of moisture is generally thought to be beneficial around 5%, however not all

substrates are kept in an ideal environment prior to lamination. Therefore a method of measuring and controlling the surface moisture of the woodpanel prior to lamination would be beneficial, and heating the surface of the woodpanel an additional beneficial step. It has been discovered, in accordance with a second aspect of the invention, that by applying heat, preferably by means of electromagnetic radiation, and most preferably by near infra-red (NIR) radiation, to the surface of the substrate prior to lay-up and laminating of the impregnated papers or non-wovens in the press zone, the press time can be reduced and the laminate suffers less delamination as measured by the simple cross-hatching technique.

Furthermore it has been discovered that by applying heat to the woodpanel, preferably by NIR radiation, the degree of cure of the resin in the surface of the woodpanel is advanced. The woodpanel can be irradiated on one or both sides, whether symmetric radiation wherein both sides receive a similar dosage of radiation, or asymmetric radiation wherein one side receives a higher dosage of radiation.

It has also been discovered that single side or asymmetric radiation causes the woodpanel to bend in a concave manner in respect to the face of the woodpanel receiving the higher amount of radiation, in the case of asymmetric irradiation in respect to the face of the woodpanel receiving the higher amount of radiation. In respect of asymmetric radiation this may be both woodpanel faces simultaneously or as separate process steps. The ability to recover the woodpanel to a lay-flat position is achieved in one example through preferential resin loading, wherein the convex surface has a higher resin quantity in the impregnated papers than the concave surface, for example the impregnated balancing paper on the reverse side, in this case the concave side, of a laminate flooring woodpanel. This pre-stressing of the reverse side of a woodpanel being used for laminate flooring enables a lower grammage impregnated paper to be employed instead of having to more closely match the resin loading on both sides of the woodpanel to ensure a flat panel, especially important to achieving a good fit of the engaging system that locks the laminate flooring panels together. The electromagnetic radiation used to cure the panel surface or surfaces can also be used beneficially to further cure the shaped edges of the laminate flooring woodpanel .

According to its second aspect, the invention provides a method of manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised in that, prior to said combining step, said substrate is pre-treated by being heated, on one or plural sides, whereby to enhance the bonding and the flatness of the laminate.

In its second aspect, the invention further provides apparatus for manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised by means disposed upstream of said press zone for pre-treating said substrate by application of heat, to one or plural sides of the substrate, whereby to enhance the bonding and the flatness of the laminate.

In addition it has been discovered that by applying heat to the surfaces of a woodpanel, emissions from the woodpanel are reduced, in particular formaldehyde and other VOC emissions. It is common practice in manufacturing woodpanels that melamine is added to the bonding resin to improve resistance to water ingress, and at the same time to act as a scavenger for free formaldehyde to enable the panel to be rated for formaldehyde emission - E1, EO or Super EO (also known as F**, F*** and F**** respectively) - according to established norms such as EN 120 or JIS 5908. However, addition of melamine is an economic burden due to the fact that melamine (usually found in the form of melamine urea formaldehyde (MUF resin) is more expensive than urea (usually found in the form of urea formaldehyde resin), and that the cure rate of resins containing melamine is slower than standard UF resins, causing a reduction in the output of the press. Woodpanel resins containing urea and or melamine are not limited to the examples aforementioned; those skilled in the art will be aware that variations of resins are numerous with regard to actual formulation and components.

The irradiation of woodpanels preferably takes place after the board exits the woodpanel press, whether the press is multi daylight or a continuous press. If it is intended that the woodpanel will be sanded to final thickness and surface quality, the irradiation preferably takes place after the sanding procedure.

According to its third aspect, the invention provides a method of reducing longer term formaldehyde and other VOC emissions from a substrate, e.g. a woodpanel, made by a process in which the bonding system results in formaldehyde within the substrate, characterised in that the substrate is treated by application of heat to one or more surface zones of the substrate to further advance the cure of formaldehyde-based resin in the substrate, and to volatilise and drive off contained free formaldehyde and other VOCs.

In its third aspect, the invention also provides apparatus for reducing longer term emissions of formaldehyde and other VOCs from a substrate, e.g. a woodpanel, made by a process in which the bonding system results in formaldehyde within the substrate, characterised by means to treat the substrate, by application of heat to one or more surface zones of the substrate, to further advance the cure of formaldehyde based resin in the substrate, and to volatilise and drive off contained free formaldehyde and other VOCs. It is common practice to test the quality of a laminate surface, for example by applying shoe polish. If the shoe polish cannot be cleaned from the surface, it is assumed that the degree of cure required to achieve a closed surface has not been reached, even though there has not been any sticking of the laminate in the press. To reach the desired degree of cure a combination of time under pressure and press temperature is calculated. To reach the desired degree of cure within a shorter time period it is usual to employ a higher temperature, alternatively if the temperature is not increased then the time in the press and under pressure has to be extended.

It has been discovered in accordance with the invention in its fourth aspect, that a final curing of the laminate surface while not under pressure, that is to say outside of the press subsequent to the pressing step conducted under pressure, is possible. By exposing the surface of the laminate after pressing to heat radiation, a fully cured surface can be achieved with shorter press time, without higher temperature having to be employed in the press zone.

In its fourth aspect, the invention therefore provides a method of manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers

and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised in that the resin in said resin-impregnated papers is partially cured in the press zone and, after said combining step, the laminate is post- treated downstream of said press zone to cure residual resin in the laminate surface or surfaces.

In its fourth aspect, the invention also provides apparatus for manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised by means for post-treating the laminate downstream of said press zone, to cure residual resin in the laminate surface or surfaces.

The invention accordingly provides, in this aspect, a method for reducing the press cycle time and or the press temperature, and achieving final cure outside of the press.

The invention also extends to methods and apparatus combining two or more of the three aspects of the invention.

In each aspect of the invention, the heat or treatment is preferably applied by directing electromagnetic radiation onto the respective surface or body to be treated. The radiation is more preferably near infra-red [NIR] radiation. By "near infra-red" is herein meant the wavelength range between the visible region and 2.5um, i.e. about 0.7 to 2.5um.

The NIR radiation is advantageously applied so as to heat or treat a pre-determined region in the surface of the body, typically a region at or close to the surface being treated. In the second aspect, for example, significant heating of the core of the substrate is advantageously minimised or avoided. The fact that the energy source is to a significant degree preferably only applied to the outermost thermosetting resin layer avoids building in thermal stresses which require the laminate to be stored for several days before being further processed. Obvious productivity gains of shorter press heat cycle times, and reduction in inventory and storage space, provide an economic advantage by application of the invention technology.

It is known per se that NIR irradiation, e.g. applied from halogen tube radiators, is an effective mechanism for drying or curing a variety of systems. It has been appreciated by the inventor that NIR radiation can be usefully applied to the production of industrial or decorative laminates with significant process advantages without incurring problems in the press due to sticking, and that a significant reduction in press heat cycle time can be achieved by applying the final curing step on the laminate surface after the initial pressing step.

A significant element of the fourth aspect of the invention is thus the uniform application of an energy source onto the surface of a pressed laminate to complete the cure of the thermosetting resin surface.

The amount of NIR electromagnetic radiation will vary depending on the properties of the impregnated papers or non-wovens and the pressed laminate surface. A black laminate will absorb energy more readily than a white surface. Options to manage the NIR process include the speed at which the impregnated papers or pressed laminate pass through the NIR unit or units, and or controlling the power output of the NIR unit or units.

In a fifth aspect, the invention generally also provides a method of manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterized by employing one or more NIR radiation units before and or after the press zone to treat said substrate and or said papers.

In its fifth aspect, the invention provides apparatus for manufacturing a decorative or industrial laminate by combining one or more resin-impregnated papers and a substrate in a press zone in which heat and pressure are applied, whereby to form said laminate, characterised by NIR irradiation units disposed upstream and or downstream of the press-zone, to treat said substrate and or said papers.

Preferably, in the fifth aspect of the invention, there are NIR radiation units upstream and downstream of said press zone.

Brief description of the drawings

The invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic depiction of an integrated plant for processing composite woodpanel substrates and partially cured resin-impregnated papers, known as b-stage pre-pregs, into decorative or industrial laminates, incorporating NIR irradiation stations according to various aspects of the invention; and

Figure 2 is a diagrammatic plan view of a typical NIR irradiator unit suitable for any of the NIR irradiation stations shown in the plant of Figure 1.

Embodiments of the Invention

For convenience, Figure 1 illustrates an integrated plant in which composite woodpanels 5 are produced and finished and then utilised as substrates in the manufacture of decorative or industrial laminates 10. More typically, the woodpanels might instead be made and finished at a separate plant remote from the laminate production line to which they would be delivered in packs 6, but an integrated plant is presented for purposes of exemplification.

The composite woodpanels 5, which might, for example, comprise particle board, fibre board or oriented strand board, emerge from a special purpose hot press 20 of known construction, as raw woodpanels 5a. The raw panels are conveyed through a finishing station 22 in which they are subjected to fine sanding and/or other treatment, to thereby form a final product 5 of adequate specification for use as a substrate in laminate production.

Panels 5 are individually laid up with the other laminate plies at a collation station 24 and the assembly 9 is then cured, stabilised and bonded in a flat bed press 26 to form laminate 10. As discussed earlier, there are a wide variety of known laminate structures and a number of different pressing processes are in widespread use, according to the end applications and desired properties of the laminates. In the simple case depicted in Figure 1 , respective resin-impregnated b-stage decor papers 8a, 8b are placed on both

sides of substrate panel 5, and the press 26 operates as a single daylight press utilising relatively lower pressure, heat and an appropriate dwell time to produce a low pressure laminate (LPL) product. This is a batch process, but the resin-impregnated papers may alternatively be applied in roll format to a sequence of woodpanels and finished as a continuous laminate product in a continuous laminate hot press.

In the alternative process of high pressure lamination (HPL), one or more sheets of kraft paper impregnated with a suitable thermosetting resin, usually phenol formaldehyde (PF) resin, form a substrate for a melamine formaldehyde resin-impregnated decor paper and optionally a resin-impregnated clear overlay paper. In this case, the press is operated at relatively higher pressure and longer dwell times and will typically be a multi-daylight hot press. The product is known as a high pressure laminate (HPL). Continuous laminate high pressure processes are also known.

The woodpanels 5a, 5, the collated assembly 9 of laminate plies 5, 8a, 8b, and the end product laminate 10 are moved through the successive stations of the plant by conveyors of suitable form. This is not illustrated in detail but components are depicted diagrammatically as rolls 13. It will be understood however that any of a variety of handling systems, including air flotation systems, may well be used rather than contact conveyors implied by rolls 13.

In accordance with respective aspects of the invention, novel treatment processes are applied at various stages of the plant, utilising near infra-red (NlR) irradiation as the preferred mode of treatment.

According to a first aspect of the invention, an NIR irradiation station 30 is provided immediately upstream of hot press 26 to direct NIR radiation at the b-stage impregnated papers 8a, 8b before the laid up papers enter press 26. It is believed that the volatiles remaining in the papers can be thereby further reduced, so enabling a shorter pressing time and avoiding sticking of the laminate assembly in the press. As discussed earlier, it is generally thought that sticking occurs when short chain polymers migrate to the surface of the laminate and cause the laminate surface to stick to the press plate 27 or endless press belt.

Treatment station 30 includes respective NIR irradiator units 34, 35 directed at the respective sides of the collated laminate assembly 9. The period of irradiation may typically be in the range 0.1 to 2 seconds.

Each NIR irradiator unit 34, 35 conveniently comprises a bank of elongate NIR emitters 36 (Figure 2) preferably arranged in functional blocks 38 of six emitters each. The bank extends as an upper or lower bridge across the panel or laminate path with the emitters aligned in the direction of travel. Each block may, for example, extend 120mm laterally of the web, with each emitter of a length, e.g. 250 mm. The distance from the treated elements to the emitters 26 is adjustable according to parameters such as the type of emitter, web or laminate speed.

According to a second aspect of the invention, heat is applied to the woodpanel substrates 5, preferably by way of a near infra-red irradiation station 40, prior to lay up with the papers 8a, 8b at the collation station 24. It has been found that by so treating the woodpanels prior to collation and lamination, dwell time in press 26 can be reduced and delamination (as measured by the simple cross hatching technique) occurs less often. Moreover, as discussed earlier, the degree of cure of the resin in the surface of the woodpanel is further advanced by the heating by NIR radiation. Overall, the bonding and flatness of the laminate is enhanced.

In a fully integrated plant where woodpanels 5 are transferred direct into the lamination plant after finishing, NIR treatment station 40 serves a further purpose, in accordance with the invention, of reducing subsequent emissions of formaldehyde and other VOCs.

However, in a more general plant, a specific NIR irradiation treatment plant 50 is positioned to irradiate woodpanels 5 immediately downstream of the finishing sanding station 22 or, if this finishing process is not required, immediately downstream of the woodpanel press 20. It is found that this treatment of the woodpanels further advances the cure of formaldehyde based resin, and is also effective to volatilise and drive-off contained free formaldehyde and other VOCs.

It will be understood that the layout of NIR treatment stations 40 and 50 may be similar to that illustrated in Figure 2.

In the fourth aspect of the invention, a final curing of the surfaces of laminate 10 is provided while the laminate is not under pressure, that is to say downstream of and outside of press 26 subsequent to the pressing step conducted under pressure. This station 60 again preferably uses NIR irradiators for this purpose. In this approach, the resin-impregnated papers are partially cured in the hot press 26 and then post-treated downstream of and outside of the hot press zone to cure residual resin in the laminate surface or surfaces. In this way, it is found that a fully cured surface can be achieved with shorter press time, at lower temperatures than conventionally required in the hot press zone. In other words, the invention provides in its fourth aspect a method of reducing the press cycle time and the press temperature, in which final cure is achieved outside the press.

Again, the NIR irradiation station 60 may be of the form illustrated in Figure 2.

Examples (Third Aspect of Invention)

Example 1

An A4 sized sample from a 19mm thick particleboard (Eurospan from the firm Egger) was tested for formaldehyde emission according to the standard JIS 5908 (Desiccator method), the result being 1.15 mg/l. From the same master panel another A4 sample was irradiated with NIR emitters. The NlR emitters were 4400 watt operating at 100% power applied to both sides of the particleboard at a distance of 50mm. The surface of the particleboard reached approximately 146 - 150C measured with a handheld pyrometer. The formaldehyde emission of the NIR irradiated sample was tested according to the standard JIS 5908 (Desiccator method), the result being 0.917 mg/l, a reduction of approximately 20%.

Example 2

Eight A4 sized samples of a 16mm thick particleboard were tested for formaldehyde emission according to the Chamber method (1 square metre of surface in a 1 cubic metre chamber), the result being 0.156 ppm. From the same batch of particleboard another sixteen A4 samples were prepared and then irradiated with NIR emitters. The

NIR emitters were 4400 watt operating at 100% power applied to both sides of the particleboard at a distance 50mm. The surface of the particleboard reached approximately 145 - 153C measured with a handheld pyrometer. The formaldehyde emission of the NIR irradiated sample was tested twice, each test with eight A4 samples, according to the Chamber method (1 square metre surface in a 1 cubic metre chamber), the results being 0.129 ppm and 0.063 ppm, a reduction of approximately 20% and 60% respectively.