FIELD OF THE INVENTION

The invention relates to a process and composition for binding of a particulate or fibrous substrate, in particular natural particles or fibres such as for example wood particles and fibers, but also other natural fibers such as straw, flax, grass, hemp, bagasse, bamboo and agricultural waste. The current invention describes the composition of a cheap binder that can bind particles and fibers in a fast way, resulting in substrates that have the required mechanical properties. The binder has a good sustainability so that the end product also has an improved sustainability profile. Furthermore, the new binder composition will make the resulting substrates (composite materials) flame retardant.

BACKGROUND OF THE INVENTION

There is a need for a cheap binder system that can bind particles or fibers to each other and this in a more sustainable way than with current binder systems. Currently, mostly binder products based on formaldehyde resins are being used. These formaldehyde resins are condensation products mainly between formaldehyde and urea. However, this addition reaction can be reversible leading to the release of formaldehyde. Formaldehyde is known for its potential carcinogenic effects and therefore finding substitutes for this type of binders is highly desirable.

Formaldehyde resin binders are often used to bind natural particles or fibers. The best known example is the binding of wood fibers and particles leading to reconstituted wood panels. Such panels are well known under the names of e.g. medium dense fiber board, high density fiber board, oriented strand board, chipboard, particle board, hard board. These products are predominantly based on products with a high sustainability profile. However, the use of formaldehyde resins have a dramatic effect on its sustainability. Often melamine is added to the condensation reaction to make the addition reaction less reversible. As a result less formaldehyde can be released. However, there is still some release of formaldehyde.

Furthermore, the use of formaldehyde resins works well for wood, but not for e.g. other fibers such as straw. Isocyanates have being introduced as alternatives for the formaldehyde resins. These products are much more expensive and they need to be handled with great precautions because of the toxicity of the isocyanates. Therefore, a lot of investments are needed to be able to handle these isocyanates products in a safe way. Other companies have started with the commercialization of polyacrylate dispersions as binder materials. Preferably, a crosslinker needs to be added to the polyacrylate so that the binding properties are enhanced. However, these polyacrylates are not only quite expensive, but their chemical curing reaction is often slow. High curing temperatures or times are needed to give the resultant substrate sufficient strength.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a binder for binding of a particulate or fibrous substrate, in particular natural particles or fibres, said binder comprising a water borne binder and a thermoplastic material

In a preferred embodiment, the binder according to the present invention is characterized in that the thermoplastic material consists of solid thermoplastic particles whereby 90% wt of the particles have a particle size below 500 micrometer, in particular between and about 0.5 to 500 micrometer, more in particular, below 200 micrometer, in particular between and about 1 .0 to 200 micrometer.

In another preferred embodiment, the thermoplastic material has a melting point or a glass transition temperature of between and about 40°C and 250°C.

The thermoplastic material as used in the present invention preferably comprises a polymer selected from the group consisting of polyesters, polyamides, polyvinylacetate, polyolefins, polyvinyls, polyacrylates, polyurethanes, and combinations/ copolymers thereof, more in particular an ester-containing polymer, such as a polyester, polyvinylacetate or a polyacrylate.

In another preferred embodinemnt, the binder may further contain one or more other materials such as fillers, pigments, opacifiers, UV-stabilizers, cross-linkers and catalysts. In yet another preferred embodiment, the thermoplastic material is present as a water borne dispersion of the thermoplastic particles.

In a preferred embodiment, the water borne binder is a binder such as water glass, starch polyvinylalcohol, styrene-butadiene, or polyvinylacetate; that is water borne by solution, emulsion, dispersion or combinations thereof; in particular the water borne binder is the combination of a water soluble binder and a water borne emulsion; more in particular the water borne binder is a water soluble binder; even more particular the water borne binder is water glass.

The water borne binder as used in the present invention is preferably water glass having a ratio of Si0 ₂ to Na ₂ 0 ranging from about 1 to about 3.5; in particular from about 1 .5 to about 3.5.

The present invention also provides a binder according to the present invention whereby the ratio between the water borne binder and the thermoplastic material amounts to between about 1 : 3 and 50 : 1. In a further aspect, the present invention provides a composition comprising a binder according to the present invention and a particulate or fibrous substrate, in particular natural particles or fiber substrates such as for example consisting of wood, flax, grass, hemp, bamboo, bagasse, agricultural waste, or combinations thereof. In another preferred embodiment, the particulate or fibrous substrate is a man-made (synthetic) substrate such as glass- or rock wool.

The present invention further provides a composition according to the present invention wherein the weight ratio between the binder and the substrate amounts between 1 : 200 to 1 : 4.

The composition according to the present invention may further contain additives such as fillers, dyes, crosslinkers, pigments, UV-stabilizers, waxes.

In yet a further aspect, the present invention provides substrates (composite materials) made by heating the composition according to the present invention at a temperature above the melting point or glass transition temperature of the thermoplastic material. In particular, a pressure of at least 2 bar is used during the curing process. DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the finding that the presence of a thermoplastic material as solidifying agent in a binder composition for binding of a particulate or fibrous substrate, results in a composite material with previously unknown characteristics. As will be evident from the examples hereinafter, using a binder according to the present invention, the resulting material has a very homogenous structure, a high mechanical strength, an improved sustainability profile, an improved water resistance, and a excellent dimensional stability. It is accordingly a first aspect of the present invention to provide a binder for binding a particulate or fibrous substrate, in particular natural particles or fibers, said binder comprising a water borne binder and a thermoplastic material.

Within a first embodiment the thermoplastic material is characterized in that it consists of a thermoplastic material that is solid at room temperature and hence its melting point (m.p.) or its glass transition temperature (Tg) should preferably be above 40°C. A m.p or Tg higher than 250°C makes hardly sense because a too high temperature will be needed to let the thermoplastic material flow. Consequently in a further embodiment the present invention provides the aforementioned binder, further characterized in that the thermoplastic material consists of thermoplastic particles that are solid at room temperature; more in particular wherein the thermoplastic material has a melting point or a glass transition temperature of between and about 40°C and 250°C.

In as far there are no particular requirements regarding the dimensions of said solid thermoplastic particles, best results are obtained when the thermoplastic material is evenly distributed over the fibers or the particles to be solidified. Consequently, in a further aspect of the present invention, the thermoplastic particles have a small particle size. In one embodiment 90% wt of the thermoplastic particles have a particle size below 500 micrometer, in particular between and about 0.5 to 500 micrometer; in a further embodiment the thermoplastic material is characterized in that it consists of solid thermoplastic particles whereby 90% of the particles have a particle size below 200 micrometer, in particular between and about 1.0 to 200 micrometer.

Examples of suitable thermoplastic materials are polyesters, polyamides, polyolefins such as polyethylene and polypropylene, polyvinylacetates, polyvinylchloride, polyacrylates. A preferred thermoplastic binder is one that contains ester groups such as a polyester, a polyvinylacetate or a polyacrylate. Thus in another aspect the thermoplastic material comprises a polymer selected from the group consisting of polyesters, polyamides, polyolefins, polyvinyls, polyvinylacetate, polyacrylates, polyurethanes, and combinations/ copolymers thereof. More in particular the thermoplastic material comprises an ester-containing polymer, such as a polyester, a polyvinylacetate or a polyacrylate.

In a preferred embodiment of this invention the thermoplastic material stems from a waste stream. Such a waste stream is for instance the powder coming from the production and the use of powder paints. Usually, such a powder is contaminated with other products and can not longer be used as a powder paint. This waste stream is usually burnt and the generated energy can be used for other purposes. By using the waste stream as a binder to make new substrates it gets a higher added value.

This thermoplastic material may contain other materials such as fillers, pigments, opacifiers, UV-stabilizers, crosslinkers and catalysts. Thus in another embodiment of the present invention, the thermoplastic material further contains one or more other materials such as fillers, pigments, opacifiers, UV-stabilizers, cross-linkers and catalysts.

The water borne binder and the thermoplastic material can be mixed together before it is added to the fibers or the particles. In case more than one water borne binder is used, the thermoplastic material can be added to one of the water borne binders or to both. It is also possible to add the water borne binder and the thermoplastic material separately. In a preferred form the water borne binder is added first to the fibers or the particles, subsequently followed by the addition of the thermoplastic binder. The water borne binder will as a liquid be well distributed over the fiber or particles. Due to the presence of the water borne binder the thermoplastic binder will as well be better distributed over the fibers or particles.

Evidently from the foregoing, and given the presence of a water borne binder, depending on the ratio of water borne binder to thermoplastic material, the consistency of binder according to the present invention will change, from a more liquid (higher amount of water borne binder) to a more solid state (higher amount of thermoplastic particles). Within the context of the present invention, the ratio between the water borne binder and the thermoplastic material typically amounts to between about 1 : 3 and 50 : 1 ; in particular to between about 1 : 2 and 40 : 1 . Irrespective of the aforementioned ratio, a proper distribution of the thermoplastic material in the binder is clearly enhanced when present as a water borne dispersion. Due to fine particle size of the thermoplastic material it can just be mixed with water to form a dispersion. Upon stirring the thermoplastic material will be kept homogeneously distributed in the water phase. Consequently, in a further aspect the thermoplastic material is present as a water borne dispersion of the thermoplastic particles.

The water borne binder can be a binder that is water borne by solution, emulsion or dispersion. A solution of a water borne binder can be starch, polyvinylalcohol, water glass. A water borne emulsion can be a styrene butadiene latex, polyvinylacetate or alkyd emulsion.

Accordingly, in an embodiment of the present invention, the water borne binder is a binder such as water glass, starch, polyvinylalcohol, styrene-butadiene, or polyvinylacetate; that is water borne by solution, emulsion, dispersion or combinations thereof; in particular the water borne binder is the combination of a water soluble binder and a water borne emulsion (in particular water glass and polyvinylacetate); more in particular the water borne binder is a water soluble binder; even more particular the water borne binder is water glass.

Preferably, the binder is water glass. Water glass is derived from sand and sodium hydroxide and as such does not contain products derived from fossil fuel. The type of water glass is described by its ratio Si0 ₂ to Na ₂ 0. A higher ratio results in a product with lower water sensitivity. Principally, all types of water glass can be used in this application, ranging from a ratio of 1 to 3.5, but preference is given to a ratio of 1.5 to 3.5.

As already explained hereinbefore, the binder of the present invention is of particular interest to bind particulate or fibrous substrate. Examples of the fibers or particles that can be bound are wood, flax, grass, hemp, bamboo, bagasse and agricultural waste. Preferably, these products have a natural origin. It is also quite possible to use this binder concept for man-made fibers such as glass- or rock wool or their combinations with natural fibers and particles.. Thus, in a second embodiment, the present invention provides a composition comprising a binder as defined herein and particulate or fibrous substrate, in particular natural particles or fiber substrates such as for example ground wood, flax or bamboo, waste streams from sawing mills, rest material from sugar cane, or grass that has been cut in the desired fiber length. In a preferred embodiment the natural particles or fiber substrates are selected from the group consisting of wood, flax, grass, hemp, bamboo, bagasse and agricultural waste.

In the solidification of the particulate or fibrous substrate using the binder according to the present invention, it is important that the binder and the substrate are good admixed with one another and that there is an evenly distribution of either component in the compositions thus obtained. Especially, when the amount of binder is below 10% of the total mass a very well mixing is crucial. Within the context of the present invention the weight ratio between the binder and the substrate typically amounts between 1 : 200 to 1 : 4.

As will be evident from the examples hereinafter, any one of the aforementioned compositions may further contains additives such as fillers, dyes, crosslinkers, pigments, UV-stabilizers, waxes.

In a third aspect the present invention provides substrates (composite materials) made using the binder as described herein, i.e. using any one of the aforementioned compositions. In the manufacture of said substrates, the aforementioned compositions comprising the natural fibers and particles, and the binder are brought in a recipient and cured under high temperature and pressure. The curing temperature will be above the melting point or glass transition temperature of the thermoplastic material, alternatively even above 100°C to remove the present water. A pressure of at least 2 bar is used during the curing process.

In the particular embodiment of the invention, wherein the binders are a combination of a water glass and an ester containing thermoplastic material. While the curing reaction takes place some hydrolysis of the ester containing thermoplastic material will take place, leading to the formation of the sodium salt of the carboxylic acid. As a result the ratio Si0 ₂ to Na ₂ 0 of the water glass will increase. Therefore the water resistance of the cured material will become higher.

The water resistance of the substrate can be further increased by adding hydrophobizing agents such as e.g. oil or waxes. This invention will be better understood by reference to the Experimental Details that follow, but those skilled in the art will readily appreciate that these are only illustrative of the invention as described more fully in the claims that follow thereafter. Particular embodiments and examples are not in any way intended to limit the scope of the invention as claimed. Additionally, throughout this application, various publications are cited. The disclosure of these publications is hereby incorporated by reference into this application to describe more fully the state of the art to which this invention pertains.

EXAMPLES

Example 1

To 1000g of dried wood fibers, with an average fiber length of 2 mm, is added 137,6 g of water glass (from Silmaco Belgium) and 16g of a 50% polyvinylacetate dispersion (Mowilith DC from Celanese, Germany). The water glass has an average Si0 ₂ to Na ₂ 0 ratio of 3.2. This mixture is well stirred during 1 minute. Subsequently, 30 g of a recycled powder paint is added to the mixture. The powder paint has a particle size distribution of which 90 % lies between 1 and 100 micrometer. The chemical composition of the powder paint is for about 50% polyester with a glass transition temperature of 55°C. Other main components in the powder paint are Ti0 ₂ , pigments and fillers. The resulting mixture is further mixed for 20 seconds.

The content of the mixture is brought into a mould with a surface of 30 cm by 30 cm and a height of 50 cm. The mixture is pressed at room temperature until the height of the content is 30 cm. The pressed material is now removed from the mould and brought between heated plates of 200°C that are pressed together with a pressure of 100 tons. The thickness of the wood mixture is brought to a thickness of 15 mm and the pressure and the temperature of the plates at 200°C is maintained during 3 minutes. Thereafter the pressure is released and the pressed wood is removed from the heated plates. After cooling down the mechanical properties of the wood panel are measured.

Density: 680 kg\m ³

Tensile strength: 0.8 N\mm ²

Bent strength: 25 N\mm ²

Surface strength: 2.4 N\mm ² Example 2

Example 1 was repeated but now the 30 g of recycled powder paint was substituted by 15 g of a finely ground polyester. The polyester has a Tg of 58°C and is mainly built up by terephthalic acid, isophthalic acid and neopentyl glycol. The polyester has a particle size distribution of which 90 % lies between 1 and 150 micrometer.

The obtained panel has the following properties:

Density: 680 kg\m ³

Tensile strength: 0.8 N\mm ²

Bent strength: 24 N\mm ²

Surface strength: 2.3 N\mm ²

All the mechanical strength values of the two panels from the examples are higher than a panel with a similar density and made with an equal amount of binder based on formaldehyde resins. Furthermore, the panels made in these examples will not burn when brought in a flame of 700°C for 10 minutes