METHOD FOR MANUFACTURING A DRY-LAID MAT FOR THERMOFORMING

Abstract

The present invention is directed to a method for manufacturing a drylaid mat suitable for thermoforming. The present invention is directed to a dry forming process, wherein cellulosic or lignocellulosic fibers have been impregnated, but not cross linked, with a cross linking agent prior to forming in a dry forming method. The invention is also directed to dry-laid mats manufactured according to the method as well as to thermoformed products manufactured from such dry-laid mats.

Claims

1. A method for manufacturing a dry-laid mat suitable for thermoforming, the method comprising the steps of: a) mixing or impregnating cellulosic or lignocellulosic fibers with a cross linking agent, followed by drying the mixture of cellulosic or lignocellulosic fibers and cross linking agent at such conditions that a temperature of the fibers does not exceed 150 C.; followed by b) forming a mat comprising the product of step a), said product of step a) having a moisture content of less than 10 wt-%, by a dry forming process carried out at such conditions that the temperature of the fibers does not exceed 150 C.

2. The method according to claim 1, wherein the mat formed in step b) further comprises at least one thermoplastic polymer.

3. The method according to claim 1, wherein the mat formed in step b) further comprises at least one coupling agent.

4. The method according to claim 1, wherein the dry forming in step b) is carried out by air-laying.

5. The method according to claim 1, wherein the cross linking agent is an organic carboxylic acid having at least two carboxyl groups.

6. The method according to claim 5, wherein the cross linking agent is citric acid.

7. The method according to claim 1, wherein the cellulosic or lignocellulosic fibers are provided in a form of chemical pulp, thermomechanical pulp (TMP), mechanical fiber intended for medium density fiberboard (MDF-fiber), or chemo-thermomechanical pulp (CTMP).

8. The method according to claim 1, wherein the mixing or impregnation in step a) is carried out by spraying the cross linking agent onto the fibers while the fibers are carried in an air stream in a conduit.

9. The method according to claim 8, wherein the conduit is a blowline of a mechanical pulp refiner or a tube reactor or a fluidized bed coater.

10. The method according to claim 1, wherein the mixing or impregnation is carried out in a drum blender or a drum mixer.

11. The method according to claim 1, wherein the mixing or impregnation is carried out before introducing cellulosic or lignocellulosic material into a mechanical pulp refiner or the cross linking agent is provided in a dilution water of the mechanical pulp refiner.

12. The method according to claim 1, followed by thermoforming the mat from step b), wherein the thermoforming is carried out at a temperature of from 150 C. to 220 C. and at a pressure of from 1 to 100 MPa during a time sufficient to cure the crosslinking agent.

13. A dry-laid mat suitable for thermoforming, obtainable obtained by the method of claim 1.

14. A thermoformed product obtained by the method according to claim 12.

Description

DETAILED DESCRIPTION

[0017] In the method according to the present invention, a dry-laid mat suitable for thermoforming is manufactured, wherein the fibrous material of the mat is not cross linked until at the time of thermoforming.

[0018] In the context of the present invention, the term dry-laid refers to a web formation process in which a web is formed by mixing the components to be used in the mat, such as fibers, with air to form a uniform air-fiber mixture which is then deposited on a moving air-permeable belt or wire.

[0019] During the cross linking reaction, the natural cellulose or lignocellulose fibers are chemically cross linked by a reaction at, at least, two sites with a cross linking agent which contains at least two chemical groups able to react with groups on these fibers. By this cross linking, the ability of the fiber wall to swell in contact with moisture will decrease radically and the fiber will be much less sensitive to contact with moisture in air or water. With the method according to the present invention, both inter-fiber and intra-fiber cross linking can be achieved, wherein the inter-fiber cross linking particularly contributes to strength of the final product. The mat produced according to the present invention may also comprise thermoplastic binders or matrixes which may contain compatibilizing substances and other additives. The mat produced is dry-laid, such as a dry-laid fiber mat, suitable for thermoforming.

[0020] The natural fibers used in accordance with the present invention are natural fibers that contain cellulose and, in many cases, lignin and/or hemicelluloses. They are, typically, wood fibers produced by chemical, mechanical or chemo-mechanical pulping of softwood or hardwood. Examples of such pulps are chemical pulp such as sulfate or sulfite pulp, thermomechanical pulp (TMP), mechanical fiber intended for medium density fiberboard (MDF-fiber) or chemo-thermomechanical pulp (CTMP). The fibers can also be produced by other pulping methods such as steam explosion pulping and from other cellulosic or lignocellulosic raw materials such as flax, jute, hemp, kenaf, bagasse, cotton, bamboo, straw or rice husk.

[0021] The cross linking agent used in accordance with the present invention is a substance which contains chemical groups that may react to form at least two covalent bonds with groups in the cellulose or lignin. Suitable cross linking agents include organic carboxylic acids having at least two carboxyl groups, glyoxal (oxalaldehyde), reaction products of glyoxal with dimethyl urea or reaction products of glyoxal with urea and formaldehyde and possibly with alcohols such as 1,3-bis(hydroxymethyl)-4,5-dihydroxyimidazolidine-2 or its reaction products, reaction products of urea and formaldehyde with possible alcohols or amines such as dimethylol urea or bis(methoxymethyl) urea and reaction products of melamine and formaldehyde. Most these require the presence of a catalyst.

[0022] A preferred cross linking agent is citric acid. This cross linking agent is cheap, non-toxic and environmentally friendly and does not require a catalyst.

[0023] The weight ratio of cellulosic or lignocellulosic fibers to cross linking agent is typically between 50:1 to 1.5:1.

[0024] In the process of impregnating the cellulose or lignocellulose fibers with the cross linking agent, the cross linking agent must be adsorbed onto the fiber surfaces and for maximum efficiency also absorbed into the pores of the fiber structures. One way to do this is to dissolve the cross linking agent in a solvent which is able to penetrate into these. For citric acid this solvent is, preferably, water. Impregnation of the fibers with the cross linking agent solution can be accomplished by the spraying of this onto the fibers while these are carried in an air stream in a conduit, such as the blowline of a mechanical pulp refiner or a tube reactor designed especially for the purpose or in a fluidized bed coater (Wurster coater or top-sprayed fluidized bed coater). After this operation the fibers can be carried further by the air-stream into a drier, such as a flash drier, for drying. During drying, the temperature of the fibers is kept below 150 C., to ensure that essentially no cross linking reaction takes place.

[0025] To accomplish impregnation of the fibers with the cross linking agent, there is also a possibility to spray the solution of the cross linking agent onto the fibers while these are agitated or tossed around in a drum blender or a drum mixer, such as a rotating drum wherein fibers are exposed to spraying of the cross linking agent.

[0026] The cross linking agent can also be impregnated into the fibers from a solution in which the fibers are suspended. After this operation, the excess solution has to be pressed out of the fibers to be cycled back to the process. After this the fibers can be dried to be prepared for inclusion into a dry-laid mat. In this case, it is especially favorable to dry the fibers by the method used for fluff pulp drying which provides the pulp in the form of sheets which are of a looser structure than those of normal commodity pulps and therefore easier to disintegrate in the following process steps. During the drying, the conditions are such that the temperature of the fibers does not exceed 150 C., to ensure that essentially no cross linking reaction takes place. Preferably, drying is carried out at a temperature of from 30 C. to 110 C., more preferably from 50 C. to 110 C., most preferably from 70 C. to 110 C.

[0027] In another embodiment especially pertaining to mechanical pulps, the cross linking agent can be added in the dilution water of the pulp refiner, alternatively the cross linking agent can be added before introducing cellulosic or lignocellulosic material into a mechanical pulp refiner. A prerequisite for this is that the cross linking agent is water soluble, such as citric acid. With this form of addition the impregnation will take place simultaneously with the disintegration of the raw material into pulp and there will be no need for a separate impregnation step. The impregnated fiber can be led directly from the blowline of the refiner to drying before they are applied into a dry-laid mat.

[0028] According to the present invention, the dry-laid mat is formed by dry forming. The mat may be manufactured in the form of porous webs, sheets or mats by what is commonly denoted air-laying technology, of which there are several different varieties available to the skilled person. The fibrous material can be provided to the air laying line in the form of loose material or in the form of sheets. If the fibrous material is provided to the air laying line in the form of a sheet, this sheet normally needs to be disintegrated before feeding into the line. This is most conveniently done in an appropriate device installed in-line with the air-layer, usually a hammer mill. The air-laid mats can be made solely out of the fibers mixed or impregnated with the cross linking agent or these may be combined with a suitable amount of thermoplastic polymer fibers, which function as a binder to hold the mats or sheets together. If a larger amount of thermoplastic polymer fibers are used, these will also melt and form a matrix around the natural fibers after consolidation. The binder/matrix polymer can also be applied to the fiber mat in powder or liquid form according to methods known to the skilled person.

[0029] If the matrix polymer is of a non-polar and hydrophobic nature such as a polyolefin, it is preferable that at least one additive in the form of a coupling agent is incorporated into the mat. A coupling agent is a polymer of similar chemistry as the matrix polymer, which has been co-polymerized or grafted with entities that can form covalent bonds with groups in cellulose or lignin, usually maleic anhydride or silanes. These bonds will attach polymer chains to the, originally often polar and hydrophilic, fiber surface and thus compatibilize it to the matrix polymer. In many cases, this coupling agent is included in the formulation of the polymer binder fiber.

[0030] If the matrix polymer is a thermoplastic polymer which is a condensation product such as a polyester or polyamide, it is possible that the presence of the cross linking agent, especially if this is an acid such as citric acid, will bring about the hydrolysis of bonds in the polymer and self form a bond with part of the polymer chain and thus couple this to the fiber surface. In this case the cross linking agent will also act as a coupling agent and compatibilize the fiber to the polymer with the advantages explained above.

[0031] Examples of matrix polymers include polyethylene (PE), polypropylene (PP), high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear-low density polyethylene (LLDPE), polybutene, polybutadiene, other polyolefins, polyvinyl chloride (PVC), polyamide (PA), acrylonitrile butadiene styrene (ABS), polystyrene (PS), polylactic acid (PLA), polycaprolactone, polyglycolide (PGA), ethylene vinyl acetate (EVA). The matrix polymer may be a recycled material. The matrix polymer may be partly or entirely bio-based.

[0032] The mats formed in accordance with the present invention are subsequently thermoformed into 3D-structures and also consolidated in this operation. The mats typically become dense composites, with a maximum amount of contact surface and a minimum amount of voids, after the thermoforming. The cross linking reaction, i.e. the curing, is taking place in this thermoforming step.

[0033] Therefore, the duration, time and temperature used in the thermoforming is such that the cross linking reaction occurs. The thermoforming step is carried out according to methods known in the art.

EXAMPLES

[0034] For the composite sheets, MDF (medium density fiberboard) type wood fibers were refined from 100% Norway spruce chips in a one stage mechanical refining process. In the refining, the cooking temperature was 195 C., steam flow 200 I/min and refiner pressure 8 bars. After the refining, the fibers were dried at ambient conditions to a moisture content of 6-8%.

[0035] Part of the fiber batch were subsequently impregnated with aqueous citric acid solution by spraying in a drum blender until the amount of citric acid had reached 5% dry citric acid on dry fiber weight. They were dried at ambient temperature to a moisture content of approx 8%.

[0036] Mixed wood fiber-polymer mats were formed by the process commonly known as air-laying on a Spike air-laying line of 60 cm width. The mats contained 90% of the untreated or treated MDF fiber and 10% of a PP/PE bi-component binder fiber, AL Adhesion II (ES Fibervisions, Denmark) of 6 mm length. The fiber mixtures were passed through the separating parts of the line twice to ensure sufficient mixing. The mats were passed through a single zone bonding oven twice to make sure the major part of the binder fibers had been activated.

[0037] In the following examples these mats were pressed into flat plates to produce specimens for mechanical and water absorption testing but these mats may also be pressed into complex 3D structures with double curvatures in similar pressing operations or by matched rigid moulds.

Example 1

[0038] Mats of untreated and citric acid treated fibers were preheated to 150-155 C. in a laboratory oven. One or two layers, depending on initial grammage, were put on a flat steel plate heated to approximately 180 C. and covered with baking paper and pressed at 20 MPa. Pressing time at full pressure was 3 s which gave a total cycle time, i.e. the time the mat was in contact with the heated mold, of approximately 30 s. The temperatures of the pressed composite plates were measured upon unloading with an IR thermometer and found to be 165-170 C. Resulting grammages were in the 2200 to 2500 g/m.sup.2 range and these and the thicknesses differed somewhat at different lateral positions on the plates, probably due to variations in the air-laying process.

[0039] Test specimens for mechanical and water absorption testing were cut from the plates by laser cutting.

[0040] Tensile testing conformed to ISO 527 with the exception that the test specimens, type A, had a thickness of 2.7-3.7 mm instead of 4 mm. The thickness of each specimen was measured individually before the testing to be used in the calculation of the tensile strength and modulus for the specimen.

[0041] Flexural testing conformed to ISO 178 with the exception that a few of the test specimens had a thickness slightly below the 3-5 mm interval specified in the standard for specimens of 10 mm width. The thickness of each specimen was measured individually before the testing to be used in the calculation of the flexural strength and modulus for the specimen.

[0042] The results of the mechanical testing of the 20 MPa samples are presented in Table 1 where a comparison is made of the mechanical properties of composite materials from untreated fibers and fibers treated with citric acid.

TABLE-US-00001 TABLE 1 Mechanical properties of composite plates from untreated and citric acid treated fibers pressed at 20 MPa. Citric acid treatment of Tensile Tensile Flexural Flexural fibers strength modulus strength modulus (Y/N) (MPa) (GPa) (MPa) (GPa) N 12 2.1 15 1.4 Y 21 3.1 28 2.5

[0043] Water absorption was measured in partial accordance with SS EN 15534-1. The deviations were that there was only one test specimen of each kind, these were not dried before the immersion but conditioned at 23 C. and 50% Rh for at least 48 h, that the water temperature was 23 C. and the periodicity of the measurement was different as is shown in Table 2 where the weight increase upon immersion for different time periods is presented.

TABLE-US-00002 TABLE 2 Water absorption of composite plates from untreated and citric acid treated fibers pressed at 20 MPa. Citric acid treatment of Weight increase due to water absorption (%) fibers (Y/N) 0 h 2 h 24 h 72 h 168 h N 0.0 94 106 119 124 Y 0.0 16 47 68 76

Example 2

[0044] Mats of untreated and citric acid treated fibers were heated as above, put in two layers and pressed at 100 MPa. Pressing time at full pressure was 3 s which gave a total cycle time, i.e. the time the mat was in contact with the heated mold, of approximately 40 s. The temperatures of the pressed composite plates were measured upon unloading with an IR thermometer and found to be 170-172 C. Resulting gram mages were around 2100 g/m.sup.2 but these and the thicknesses differed somewhat at different lateral positions on the plates, probably due to variations in the air-laying process. Test specimens for mechanical and water absorption testing were cut from the plates by laser cutting.

[0045] Tensile testing conformed to ISO 527 with the exception that the test specimens, type A, had a thickness of 2.2-2.6 mm instead of 4 mm. The thickness of each specimen was measured individually before the testing to be used in the calculation of the tensile strength and modulus for the specimen.

[0046] Flexural testing conformed to ISO 178 with the exception that the test specimens had a thickness below the 3-5 mm interval specified in the standard for specimens of 10 mm width. The thickness of each specimen was measured individually before the testing to be used in the calculation of the flexural strength and modulus for the specimen.

[0047] The results of the mechanical testing of the 100 MPa samples are presented in Table 3 where a comparison is made of the mechanical properties of composite materials from untreated fibers and fibers treated with citric acid

TABLE-US-00003 TABLE 3 Mechanical properties of composite plates from untreated and citric acid treated fibers pressed at 100 MPa. Citric acid treatment of Tensile Tensile Flexural Flexural fibers strength modulus strength modulus (Y/N) (MPa) (GPa) (MPa) (GPa) N 19 4.0 23 3.0 Y 27 4.1 35 3.1

[0048] Water absorption was measured in partial accordance with SS EN 15534-1. The deviations were that there was only one test specimen of each kind, these were not dried before the immersion but conditioned at 23 C. and 50% Rh for at least 48 h, that the water temperature was 23 C. and the periodicity of the measurement was different as is shown in Table 4 where the weight increase upon immersion for different time periods is presented.

TABLE-US-00004 TABLE 4 Water absorption of composite plates from untreated and citric acid treated fibers pressed at 100 MPa. Citric acid treatment of Weight increase due to water absorption (%) fibers (Y/N) 0 h 2 h 24 h 72 h 168 h N 0.0 16 48 63 69 Y 0.0 14 41 48 52

[0049] It is to be noted in the results presented above that there are significant differences in both strengths and water absorption despite that the times the samples have resided in temperatures where crosslinking is thought to occur is very short and it may be suggested that longer residence times would have increased the effect of the citric acid.

[0050] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.