Method of preparing a layered wood veneer product, a layered wood veneer product and use of polycaprolactone

FIELD

[0001] The present invention relates to binder compositions that are suitable for the production of layered wood veneer products. Particularly, the present invention relates to binders comprising biodegradable polymers.

BACKGROUND

[0002] It is previously known to manufacture laminated wood-veneer products by coating the wood veneers with resin, stacking the wood veneers on top of each other, and heating and pressing the wood veneers, so that the resin hardens and binds the veneers to each other. [0003] A typical manufacturing process involves the following steps: Wood veneer plates, or wood veneers, are prepared by cutting or rotary-cutting wood into veneer. Before the cutting or rotary-cutting, sawn and barked wood is steamed or soaked to increase the temperature of wood in order for it to plasticise. The normal veneer thickness is about 0.5 to 5 mm. The veneers are dried, sorted and possibly plugged. Then, the veneers are coated with resin and assembled into plates. Typically, veneers are coated with resin and assembled such that the resin is applied on both sides of every second veneer and, except for one outer surface, on one side of each veneer. The veneers are then placed on top of each other and the resin is cured by pressing and heating, whereby the resin cures and binds the veneers together. Finally, the plates are trimmed, polished and sorted. [0004] Most common resins are phenol- formaldehyde resin and urea- formaldehyde resin. Melamine-formaldehyde resin is also used, particularly when strength and clearness are required of the glued joint.

[0005] A major disadvantage of the traditional synthetic resins is their non biodegradability. [0006] Another disadvantage is the release of formaldehyde, which is a carcinogen, from the final product. Therefore there is a need for developing formaldehyde-free adhesives.

[0007] Alternative resins or binders comprising biodegradable polymers have been described in prior art to some extent.

[000S] WO 2010006252 A2 describes a laminate that includes a cover layer and a substrate layer. The substrate layer includes a natural fiber composite where natural fibers are bonded together by a polymer binder.

[0009] In WO 2014078720 Al adhesives and methods for preparing them are disclosed that include polylactic acid irradiated with gamma radiation.

[0010] DE 102016205194 describes structural elements made of veneer layers and thermoplastic films.

[0011] There is a need for providing improved binders that are biodegradable and capable of producing high strength.

SUMMARY OF THE INVENTION

[0012] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0013] According to a first aspect of the present invention, there is provided a method of preparing a layered wood veneer product, the method comprising: piling wood veneers on top of each other and applying a binder between the wood veneers; and pressing the piled wood veneers together, whereby the wood veneers are glued to each other to form the layered wood veneer product, wherein the binder comprises polycapro lactone or a co-polymer comprising caprolactone monomers. [0014] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

The binder is applied on each wood veneer and between the wood veneers, to glue all the wood veneers to each other. • The binder is substantially biodegradable.

• The binder substantially consists of polycapro lactone, optionally combined with a solvent.

• The binder substantially consists of polycapro lactone combined with other biodegradable polymers and/or biodegradable co-polymers, optionally combined with a solvent.

• During the pressing step the temperature is in the range from 30 to 160 °C, preferably from 50 to 90 °C, more preferably from 60 to 80 °C.

• During the pressing step the pressure is in the range from 10 to 200 bar, preferably from 50 to 100 bar.

• The molar mass of the polycapro lactone or the co-polymer comprising capro lactone monomers is in the range from 40 000 to 150 000 g/mol.

• The method comprises applying 100-300 g/m ² , advantageously 130-200 g/m ² of the binder on a surface or surfaces of each wood veneer. · The thickness of the wood veneers is 0.1-10 mm, advantageously 1.0-3.5 mm.

The number of the wood veneers to be pressed together is 3-70, advantageously 12-24.

• The wood veneers are of a material which is chosen from birch, pine, spruce, poplar, beech, maritime pine, aspen, thuja, teak, balsa or other wood species suitable for the method, advantageously birch.

• The binder is applied on a surface or surfaces of each wood veneer by spraying, casting, foaming, using liquid extrusion and/or by using rolls, advantageously by using rolls.

• The binder is in the form of an aqueous solution the solids content of which is 45- 70% by weight, advantageously 55-65% by weight.

The binder is applied in the form of a film, which preferably is a multilayer film that comprises or consists of at least two layers. • The film comprises a first layer comprising or consisting of polycapro lactone having a first melting point and a second layer comprising or consisting of polycapro lactone having a second melting point.

[0015] According to a second aspect of the present invention, there is provided a layered wood veneer product, comprising at least two wood veneers piled on top of each other and glued to each other by a binder applied between the wood veneers, wherein the binder comprises polycaprolactone or a co-polymer comprising caprolactone monomers.

[0016] Various embodiments of the second aspect may comprise at least one feature from the following bulleted list: · The layered wood veneer product is plywood.

[0017] According to a third aspect of the present invention, there is provided use of polycaprolactone as the main component of a binder in the manufacturing of a layered wood veneer product.

[0018] Various embodiments of the third aspect may comprise at least one feature from the following bulleted list:

• The binder is biodegradable.

[0019] At least some embodiments of the present invention provide advantages.

[0020] The present binder can be recycled and remelted. Waste is not produced because all of the binder can be recycled to the gluing process. [0021] The present binder provides good adhesion and strength. These properties enable the preparation of various composite and layered wood veneer structures.

[0022] The present binder provides flexibility to the final product. Stretch can have values up to 800%. Upon bending the product, a small radius can be achieved.

[0023] The structure prepared according to the present invention is self-healing and can be reshaped multiple times.

[0024] The processing temperature in the present method can be low, which saves energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIGURE 1 is a graph showing gluing results in accordance with Example 1.

EMBODIMENTS

[0026] DEFINITIONS

[0027] In the present context, the term”wood-veneer product” refers to a product, for example a laminated board, plywood, or wood board, formed of wood veneers comprising a wood material or wood-fibre-based material. The wood material is typically coniferous or deciduous wood, such as birch, aspen, pine, spruce, or some other suitable tree species, preferably birch. The term“layered wood veneer product” refers to a wood veneer product comprising at least two layers attached, for example glued, to each other.

[0028] In the present context, the term“biodegradability” comprises ability to decay naturally and in a way that is not harmful. For the purposes of the present application, the term“biodegradable” also encompasses“compostable” and“environmentally degradable”. For example, a biodegradable material is disintegrated by the action of by bacteria, fungi, or other biological means.

[0029] In the present context, the term“binder” comprises compositions, mixtures, adhesives, glues and resins that are used for binding materials together. [0030] We have surprisingly observed that particular biodegradable polymers provide excellent binders for bonding wood veneers into layered structures or composite structures.

[0031] All components of the binder according to the present invention are preferably entirely biodegradable. [0032] In a preferred embodiment, the binder comprises polycaprolactone (PCF).

PCF is a biodegradable polymer and a thermoplastic. In some embodiments, the binder further comprises other biodegradable polymers. Preferably, the other biodegradable polymers are selected from the following group: polylactic acid, polyhydroxyalkanoates. [0033] Preferably, the binder comprises or consists of biodegradable plastic material.

[0034] In a particularly preferred embodiment, the binder comprises polycapro lactone and/or a co-polymer comprising caprolactone monomers. Polycapro lactone can be in a network or linear form. Polycapro lactone may be in an amorphous, crystalline or semi-crystalline form. The film may be either oriented or non- oriented.

[0035] The melting point of the PCL is preferably in the range 30 to 160 °C, more preferably in the range 50 to 100 °C, such as 55 to 65 °C.

[0036] The molar mass of the PCL is preferably in the range from 40 000 to 150 000 g/mol, more preferably in the range from 60 000 to 120 000 g/mol.

[0037] The binder is applied or spread on a surface or surfaces of wood veneers in the form of granulates, fibers, melt, emulsion, liquid, pasta, film or wax, preferably in the form of granulates or film.

[0038] In a preferred embodiment, the binder is applied in the form of a film. An advantage of this embodiment is that the applied binder amount can be adjusted accurately and uniformly across the surface.

[0039] In one embodiment, the film is a multilayer film comprising or consisting of at least two layers, for example 2 to 5 layers. The layers may be different with regard to their properties, structure and/or function. For example, the layers may have different melting points of softening points.

[0040] In one embodiment, the binder is applied in the form of a film, and said film comprises a first layer comprising or consisting of polycaprolactone having a first melting point and a second layer comprising or consisting of polycaprolactone having a second melting point. The first melting point may be in the range 40 to 60 °C. The second melting point may be larger than 60 °C and up to 80 °C.

[0041] The temperature in the application or spreading phase is preferably room temperature, for example in the range from 20 to 30 °C.

[0042] In some embodiments, the binder in the form of a film is placed between wood veneers to form a stack in which wood veneers and binder films alternate, the outermost layers of the stack being wood veneers. The stack is pressed in a hot press to join the wood veneers to each other. Preferably, the pressed stack is cooled to room temperature under pressure to ensure formation of good-quality seams.

[0043] The advantages of using polymers comprising caprolactone monomers as a component of the binder include good adhesion and strength properties, biodegradability, recyclability and improved self-healing properties.

[0044] In some embodiments, the binder comprises polycapro lactone which has been copolymerized or mixed with other polymers or monomers. The other polymers or monomers can be selected from the following group: polylactic acid (PLA), polyglycolic acid (PGA), polyhydroxyalkanoates (PHAs) such as polyhydroxybutyrate (PHB), polyolefins, polyvinyl chloride (PVC), styrene acrylonitrile resin (SAN), acrylonitrile butadiene styrene (ABS) and polycarbonate (PC).

[0045] In some embodiments, the binder comprises polycapro lactone which has been copolymerized or mixed with another polymer, preferably a biodegradable polymer. [0046] In one embodiment, polycapro lactone is the main component of the binder.

Preferably, the binder comprises at least 50 wt-% of polycaprolactone. Additionally the binder may comprise a solvent, for example water, and/or other biodegradable polymers.

[0047] In one embodiment, the binder comprises polycaprolactone and water. Advantageously the binder is in the form of an aqueous solution the solids content of which is 45-70% by weight, advantageously 55-65% by weight.

[0048] The present binder is preferably used for binding wood veneers into a layered wood veneer product, for example plywood, laminated board or wood board.

[0049] The plywood can have a normal crossply construction or an oriented construction. [0050] The layered wood veneer product may be formed of one or more wood species. Optionally it is reinforced by other materials, for example by reinforcing fibres, and has a composite structure. The reinforcing material may be added to the binder or incorporated into the wood material. [0051] Generally the thickness of the veneers is in the range of 0.1-10 mm. Typically, the thickness of the veneers is about 0.4-5 mm, particularly about 0.5-4 mm, most suitably about 0.7-3.8 mm, and usually 1.0-3.0 mm, e.g. 1.3-2.0 mm.

[0052] The thicknesses of the individual veneers can vary, so that in one embodiment at least some of the veneers are thicker or thinner than the rest of the veneers. In another embodiment all the veneers of the present wood veneer product are of the same thickness.

[0053] The thickness of the resulting layered wood veneer product is advantageously 3 to 70 mm, most advantageously 5 to 50 mm. [0054] The number of wood veneers is advantageously 3 to 70, most advantageously

12 to 24, and they are advantageously chosen of birch, pine, spruce, poplar, beech, maritime pine, aspen, thuja, teak, balsa or other wood species suitable for the method. The most advantageous wood species is birch.

[0055] In some embodiments, the binder is applied on one or both surfaces of each veneer. In one embodiment, the binder is applied on top of a ready wood veneer product, on one of its outer surfaces.

[0056] In the present context, the binder refers to an unmixed binder or a mixture substantially containing the binder.

[0057] The binder is typically applied on wood veneer surfaces by spraying, casting, foaming, using liquid extrusion and/or by using rolls. The most advantageous coating method is application by means of rolls.

[0058] Preferably, the dosage of the binder is 80 to 250 g/m ² , more preferably 100 to 200 g/m ² , for example 120 to 180 g/m ² .

[0059] After the application of the binder, one or more of the following steps are performed: spreading without pressure, piling, prepressing, hot pressing, cooling under pressure, trimming, polishing, and sorting.

[0060] The method described above can contain other steps in addition to the aforementioned steps. Thus after the spreading of the binder, the veneers can be allowed to stand without pressure. After stacking and before hot pressing the veneers may be pre- pressed.

[0061] The hot pressing of the wood veneers stacked on top of each other is performed preferably for at least 10 minutes and, according to thickness, for up to 60 minutes.

[0062] The temperature during the hot pressing is in the range 30-160°C, preferably 50 to 150 °C, more preferably 60 to 120 °C, for example 60 to 80 °C. During the hot pressing, the binder melts and becomes spread to its final form.

[0063] The pressure during the hot pressing is typically 10 to 200 bar, preferably 50 to 100 bar. The pressing pressure can, however, be raised higher than this, if it is wished to increase the density of the final product. A cooling step, optionally under pressure, may follow. Finally, the sheets can be trimmed, sanded, and sorted.

[0064] In some embodiments, effective agents, especially effective agents that alter the chemical, mechanical, or microbiological properties of the veneers, such as their rot resistance, insect resistance, fire resistance, or deflection, impact, compressive and/or tensile strength, can be brought between the veneer layers along with the binder.

[0065] The layered wood-veneer product manufactured according to the invention is particularly suitable for applications in which bio degradability together with strength is of importance.

[0066] One possible application area of the present product is boards in wood furniture manufacturing in which the bending properties of the boards are important.

[0067] The layered wood veneer products according to some embodiments of the present invention can be reheated and shaped to a different form several times.

[0068] Examples

[0069] Example 1

[0070] In this example, wood veneers were glued into plates by using a binder consisting of substantially pure polycaprolactone (PCL) without any solvent. The melting point of the PCL was 60 °C.

[0071] Birch veneers having a thickness of 1.5 mm were used. The binder was applied on the surfaces of the veneers in a granulate form with a dosage of 100 to 200 g/m ² . After applying the binder, the veneers were piled on top of each other and pressed to a plate. The pressing time was 10 to 20 minutes in the temperature of 60 to 120 °C and a pressure of about 70 bar.

[0072] Two plates (Plate 1 and Plate 2) were prepared by using the PCL binder and one reference plate (ref) by using a phenol- formaldehyde binder. Similar processing conditions were used.

[0073] The results are shown in FIG. 1 for 20 parallel measurements for each plate. It was observed that shear strengths exceeding 2.5 N/iran ² could be achieved by using the PCL binder.

[0074] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0075] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or“in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0076] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0077] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0078] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0079] The verbs“to comprise” and“to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of“a” or“an”, i.e. a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0080] The binders according to the present invention are industrially applicable in the manufacturing of layered wood veneer products, such as plywood.

ACRONYMS LIST PLA polylactic acid

PHA polyhydroxyalkanoate

PGA polygly colic acid

PHB polyhydroxybutyrate PVC polyvinyl chloride

SAN styrene acrylonitrile resin,

ABS acrylonitrile butadiene styrene

PC polycarbonate

CITATION LIST

Patent Literature

WO 2010006252 A2

WO 2014078720 Al

DE 102016205194