Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C1/00—Pretreatment of the finely-divided materials before digesting
  • D21C1/02—Pretreatment of the finely-divided materials before digesting with water or steam
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
  • B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
  • B27N1/00—Pretreatment of moulding material
• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01C—CHEMICAL OR BIOLOGICAL TREATMENT OF NATURAL FILAMENTARY OR FIBROUS MATERIAL TO OBTAIN FILAMENTS OR FIBRES FOR SPINNING; CARBONISING RAGS TO RECOVER ANIMAL FIBRES
  • D01C1/00—Treatment of vegetable material

Definitions

• the present invention relates to a process for the preparation of vegetable fibres having an improved moisture resistance, and more in particular to the preparation of long multicelled vegetable fibres having an improved moisture resistance; to the moisture resistant vegetable fibres prepared by said process and to their use.
• Vegetable fibres form a class of very desirable products for a wide range of applications, for reasons of cost and performance.
• the mechanical performance properties of flax which is a well-known example of a long multicelled vegetable fibre, are comparable to those of glass and metal fibres, when taken on weight basis.
• the vegetable fibres not only have a much lower density than glass and metal, but are moreover also considerably cheaper. Due to their lower rigidity or higher flexibility it can be expected that the vegetable fibres will result in a more favourable processability compared to glass, especially with regard to abrasion of equipment and to fibre-breakage, e.g. during fibre dispersion.
• a field of application wherein the hereinbefore mentioned characteristics of vegetable fibres may play an important part, is their use as reinforcing fibres, for example in polymer composites.
• the vegetable fibres belong to the class of lignocellulosic materials, which materials have three major components in common, i.e. cellulose, hemicellulose and lignin.
• Cellulose is a high molecular weight linear polysaccharide, which as result of its high molecular weight is insoluble at room temperature in water and dilute acids and alkali. It is a substantially crystalline material and the major structural component of the cell wall of said lignocellulosic materials, such as the vegetable fibres, and is primarily responsible for the strength of these fibrous materials.
• Hemicellulose is a poorly ordered, non-crystalline, relatively low chain length non-cellulosic polysaccharide, and occurs in close association with the cellulose in the cell wall as well as with lignin between the cells.
• Lignin is chemically speaking a totally different type of compound, being a phenol-based aromatic polymer comprising phenyl-propane units. It occurs mainly as an encrusting agent between the fibres and on the outer wall of the cell.
• hemicellulose is not only the most hygroscopic of the three components mentioned hereinbefore but moreover also the most accessible. Hence the hemicellulose component present in the lignocellulosic materials, is generally considered to be the cause of their dimension instability due to moisture absorption.
• Cellulose is also hygroscopic, but contrary to hemicellulose it hardly swells, if at all, as a result of moisture absorption and hence does not contribute to the dimension instability of vegetable fibres.
• Lignin does not play a part in the process of moisture absorption.
• a process for the preparation of composite products from ligno-cellulosic materials, which comprises treating the lignocellulosic material in divided form with steam to heat the lignocellulosic material to a temperature high enough to release hemicellulose and for a time sufficient to decompose and hydrolyse hemicellulose.
• the thus treated lignocellulosic material is subsequently formed into a mat and compressed at a temperature not exceeding the temperature at which the mat would char, and at a pressure and for a time sufficient to transfer the hemicellulose decomposition products into a polymeric substance, which adhesively bonds together the lignocellulosic material.
• the cellulose cell walls of the vegetable fibres are irreversibly deformed, i.e. flattened, and due to a combination of compression and reaction of the hemicellulose decomposition products, said vegetable fibres have permanently lost the characteristics of individual vegetable fibres.
• the treated materials are almost completely free from hemicellulose and hence have a better dimension stability.
• the steam pressure is suddenly released upon completion of the steam treatment, which treatment is known to explode and shred the treated material into a fibrous lump.
• the vegetable fibres which have been treated by anyone of the hereinbefore mentioned processes have in common that at least part of their hemicellulose has been removed by means of hydrolysis and hence it can be expected that this will result in an improvement of their moisture resistance.
• the problem underlying the present invention is to develop a suitable process for the production of these fibres, i.e. fibres having an improved dimension stability as a result of reduced moisture absorption combined with good mechanical performance properties, i.e. fibres wherein the original cellulose cell wall has essentially been maintained.
• the invention provides a process for upgrading long, multicelled vegetable fibres or assemblies thereof, which comprises the steps of a submitting long multicelled vegetable fibres or an assembly based thereon, to a water and/or steam treatment at a temperature in the range of from 130 to 250°C and at a pressure which is at least the equilibrium pressure of the operating temperature and for a time sufficient to decompose at least part of the hemicellulose present in said fibres, b) a controlled decompression/cooling of the reactor contents, c) isolating the fibres, and d) submitting the isolated fibres in a heating chamber to a temperature in the range of from 140 to 200°C.
• the nature of the vegetable fibres which may be employed in the process of the present invention is not critical and may include long, multicelled vegetable fibres which originate from the leaves, stem and bark of plants. They may be employed as harvested or after having been submitted to a subsequent treatment which leaves the cellulosic cell wall essentially unaffected, such as drying, retting, hackling, stripping, scraping and carding.
• the vegetable fibres may be employed in the form of individual fibres, i.e. the fibres as harvested or as obtained by anyone of the treatments as described hereinbefore, but also in the form of a fibre assembly, i.e. individual fibres which have for example been converted to woven and non-woven fabrics, yarns, cords and paper sheet.
• fibres or individual fibres refers to bundles of single fibrous cells, wherein each cell contains a variety of layers which together form a basically cylindrical arrangement, hence the term multicelled fibre.
• leaf fibres examples include abaca, bowstring hemp, pineapple and sisal; examples of bast fibres include jute, flax, hemp, kenaf and ramie, while suitable stem fibres include common bamboo, banana stalk and coconut.
• the fibres may be employed as harvested but also in a dried form.
• the vegetable fibres are presoaked in water to obtain fibres which are saturated with water, before being introduced into the reactor.
• the fibres are contacted with water in a water/fibre weight ratio which generally is in the order of at least 1 when working batchwise, whereas the water/fibre ratio will typically be >10 and preferably >20 when operating continuously under slurry conditions.
• the reactor contents are treated at a temperature in the range of from 130 to 250°C and a pressure which is at least equal to the equilibrium vapour pressure at the temperature of operation, and for a time sufficient to decompose at least part of the hemicellulose present in said vegetable fibres.
• lignin can also be partially decomposed, which results in the formation of fenol type or fenol derivative type of decomposition products.
• a complete decomposition of the lignin would require a temperature which is considerably higher than the temperature at which the process of the present invention is conducted.
• the temperature for treating the fibres is in the range of 160-190°C.
• the temperature for treating the fibres will be in the range of from 180-190°C
• low-lignin fibres such as abaca, flax and linen
• said temperature will be in the range of from 160-170°C.
• very good results were obtained when the vegetable fibres had been submitted to a heat treatment at a time/temperature in the range of from 60 min/160°C to 15 min/180°C, although shorter and longer exposure times at the indicated temperatures should not be excluded.
• One of the hemicellulose decomposition products which may be formed in addition to sugars and aldehydes when exposing vegetable fibres to water/steam as described hereinbefore, is acetic acid, and will result in a drop in the pH of the reaction medium.
• acetic acid may accelerate the hemicellulose decomposition, simultaneously in a partial decomposition of the cellulose.
• Suitable buffering agents have a pH in the range of from 4-7 and more preferably from 4.5 - 6.5.
• the buffering agent is suitably a mixture of a base or an acid, and a salt of an organic acid.
• the buffering agent is preferably a mixture of acetic acid and an ammonium or alkali metal salt thereof; alkaline earth metal salts such as magnesium and calcium salts may also be used.
• alkali metal salts are sodium and potassium salts.
• the concentration of the buffering agent in water is suitably between 0.01 and 5 mol/litre and preferably between 0.05 and 2 mol/litre, and wherein the concentration of the buffer is considered to be the joint-concentration of the salt and acid or base. Should an aqueous buffering solution be used in the process of the present invention, it may, when appropriate, already be present during the presoaking of the vegetable fibres as described hereinbefore.
• the complete pH buffer e.g. the acid and alkali metal salt thereof
• the alkali metal salt e.g. sodium acetale
• the use of a pH buffer in the process of the present invention is especially advantageous when conducting said process on a large scale.
• the cooling of the reactor contents can be accomplished by means of external and/or internal cooling.
• a further possibility to lower the temperature of the reactor contents is via decompression of the reactor, which will result in "adiabatic" evaporation of the liquid phase and hence in a reduction of the temperature thereof.
• the decompression mode of cooling is applied, care should be taken that it is a very gradual and well controlled decompression which allows evaporation and diffusion of moisture within the fibres, but does not result in rupture of the cell walls due to an explosive evaporation of the liquid phase within the fibres.
• the pressure within the reactor as a result of the decompression should not be very different to that of the equilibrium vapour pressure at the prevailing temperature.
• Advantageously cooling and decompression are applied simultaneously to reduce the temperature of the reactor contents.
• the vegetable fibres, of which at least part of the hemicellulose has been decomposed may be isolated from the aqueous reaction medium via known techniques such as filtration and decantation.
• the isolated fibres are heated at a temperature in the range of from 140 - 200°C.
• the curing step it is believed that a reaction will occur between the various decomposition products, which will increase the moisture resistance and the dimension stability of the treated vegetable fibres.
• This curing step can conveniently be conducted in a heating chamber. In order to achieve the highest possible degree of reaction during said curing step, care should be taken to reduce the loss of reactive decomposition products. In this context it is considered advantageous to dewater and dry the isolated fibres, preferably at ambient temperature, with the aid of a drying agent, prior to the curing step.
• Suitable drying agents include silicagel, magnesium sulfate and calcium chloride.
• the time during which the fibres are submitted to the curing step is largely determined by the actual cure temperature. Conveniently the cure time may vary from 0.25 - 10 hours at a temperature in the range of from 200 - 145°C respectively.
• green fibres i.e. fibres which have not undergone any treatment
• the ultimate products are coloured and/or soiled. Should this be unacceptable for certain applications, then the steam/water treated fibres may be submitted to an aqueous washing procedure for which generally water having a temperature in the range of from room temperature to 65°C is used, prior to being cured.
• the resulting cured fibres will generally have a considerably lighter colour.
• the long, multicelled vegetable fibres as prepared according to the process of the present invention were indeed found to have improved moisture resistance and dimensionstability when compared with the corresponding untreated fibres.
• These improved fibre characteristics can be determined by measuring the moisture take up of the fibres after having been exposed to moisture under varying conditions or after having been soaked in water, and subsequently measuring the corresponding degree of swell of the exposed fibres. Said degree of swell being a useful yardstick for the dimensionstability of the vegetable fibres. Moisture take up can conveniently be determined by measuring the weight increase of the exposed fibres.
• a suitable method for measuring the degree swell of the exposed fibres is comparing the diameter of the fibres before and after exposure to moisture with the aid of a microscope having sufficiently large magnification, e.g. 52 x.
• a further method for measuring the degree of swell is with the aid of a so-called dedicated image analyzer (Vidas, ex Kontron, Germany). With this technique, which is considerably faster than the microscopic route, the volume of the fibre is projected on a screen.
• Confocal Laser Scanning Microscopy is a novel form of optical microscopy having an advantage over the conventional light- and electronmicroscopy in that it possesses a large depth of focus, and moreover rejects all out of focus information.
• the images of the fibre cell structures and walls produced via the hereinbefore described confocal laser technique can be made visible by projection on e.g. a screen or on photographic paper.
• the latter mode of operation having the advantage in that it makes comparing the different images a lot easier, and moreover also allows said results to be saved.
• the long, multicelled vegetable fibres having improved moisture resistance and dimension stability and wherein moreover the original cellulose cell structure has been essentially maintained, as described hereinbefore, are novel products and form another aspect of the present invention.
• said fibres will also have maintained their mechanical performance properties, thus making them valuable materials e.g. for use as fibre reinforcement in polymer matrices.
• the method or process used for the preparation of the fibre-reinforced polymer matrices or composites as they will be referred to hereinafter, is not critical, but will of course be governed by the nature of the polymer(s) used, i.e. be it a thermoplastic or thermosetting polymer.
• any process used for the preparation of fibre reinforced composites employing conventional fibrous reinforcements, e.g. glass fibres or green vegetable fibres, can also be used when the fibrous reinforcement comprises the vegetable fibres having improved moisture resistance and dimension stability as described hereinbefore.
• Well-known examples of such processes include melt-mixing, laminating and pultrusion.
• melt-mixing will generally be conducted by mixing the loose vegetable fibres and polymer at a temperature at which the polymer is in the molten form. Conveniently melt-mixing can be conducted in an extruder wherein the polymer and fibrous reinforcement can be fed separately or jointly. In the latter case the joint polymer and fibre addition may be preceded by a dry blending step.
• the ultimate fibre containing polymer melt or extrudate can be employed in e.g. moulding operations for the preparation of shaped articles.
• both loose fibres and fibre assemblies can be used.
• the general procedure for the preparation of such laminates comprises stacking alternating layers of polymer and reinforcement, e.g. in a mould; heating the contents of the mould to melt the polymer, evacuating the air from the mould followed by cooling and compressing the mould contents to obtain the laminate.
• the polymer may be used in powder form but advantageously the polymer is employed in the form of a film or sheet.
• Suitable fibre assemblies which may be employed include woven and non-woven cloth based on vegetable fibres as described hereinbefore.
• the fibre reinforcement With pultrusion the fibre reinforcement, is conveniently is contacted with the molten polymer by drawing the fibre reinforcement with the molten polymer through an orifice or die, followed by cooling of the resulting product.
• the resulting coated yarn can then be cut to the desired fibre length, and the resulting granules, each containing the correct blend of fibre and polymer, can be applied in further processing to provide the final fibre-reinforced article.
• the fibre reinforcement used in the preparation of the fibre-reinforced composites as described hereinbefore will be based on a single type of vegetable fibre. It is of course also possible or sometimes even advantageous to use blends of two or more types of vegetable fibres or use fibre assemblies based on more than a single type of vegetable fibre. In general the fibres will comprise in the range of from 10 to 90%W of the total reinforced composite, and preferably from 20 to 80%W.
• the polymer matrix of the fibre reinforced composites may be based on both thermoplastic and on thermosetting polymers.
• suitable thermoplastic polymers include polyethylene, polypropylene, polybutylene, polystyrene, polyamides, polyethyleneteraphthalate, polycarbonate, polyketones, polyphenyleneoxides, polyesters such as polymethacrylates, functionalized, polyolefins such as those which have been modified to carry one or more polar groups via grafting or copolymerization; preferred polar groups being acid groups and especially carboxylic acid groups or derivatives thereof.
• the composites may be based on a single thermoplastic polymer or blends of two or more thermoplastic polymers. It is moreover feasible that when employing a laminating technique that the composites may combine polymer sheets or films from different polymers.
• thermosetting polymer systems examples include polyepoxides in combination with a wide range of curing agents, unsaturated polyesters, phenolic resins and isocyanate curable systems.
• the fibre reinforced composites based on the vegetable fibres or assemblies thereof as described hereinbefore, are novel.
• said vegetable fibres as fibre-reinforcement in the polymer composites as described hereinbefore allows said composites to be prepared via a fully integrated process, i.e. a process wherein the heattreatment or cure step of the vegetable fibres takes place during the moulding operation of the composites. Under said circumstances it is required that the isolated non-cured vegetable fibres should be dry, i.e. water free.
• Water absorption was determined by measuring the weight increase of fibres after having been exposed to moisture.
• Degree of swell was determined with the aid of a so-called dedicated image analyzer (Vidas, ex Kontron, Germany) by projecting the volume of a fibre on a screen and measuring the dimensions of the projection before and after exposure to moisture.
• Reinforcement and polymer film were cut in the dimensions of the cavity of a positive mould and 14 layers of polymer film and 13 layers fibre based mats were stacked in alternating layers.
• the stacked layers were compression moulded to a sheet of 4 mm thickness using the following moulding conditions: preheating for 1 minute at 200°C and 4 bar, subsequently evacuating the air from the mould followed by heating for 2.5 min at 200°C and 80 bar and cooling to room temperature at 80 bar.
• the dried sheet was further converted to composites via the compression moulding technique described hereinbefore.
• the water contained 0.05 mol/l of sodium acetate.
• a further variable in the process condition was submitting the fibres to a water wash before cure, to which end the fibres were stirred in an excess of demi water at approximately 50°C. The thus obtained fibres were tested for water absorption after having been placed in an environment of 98% RH at room temperature.
• example 1 The procedure of example 1 was repeated using green flax and abaca but restricting the steam treatment to 165°C for 1 hour, followed by a wash step at 50°C and cure at 140°C for 2 hours.
• the abaca "paper” sheet, the treated non-woven flax, the woven jute and linen were each used in combination with polypropylene film for the preparation of fibre-reinforced polypropylene composites having a fibre content of 30% w (20% V), via the stacking method described hereinbefore.
• Reference composites based on the corresponding non-treated fibres were prepared likewise.
• the flexural properties (Modulus and Strength) of the thus prepared composites were determined on a fully computerized Instron testing machine using standard test specimens having a thickness of 4 ⁇ 0.2 mm and a width of 10 ⁇ 0.5 mm. Testing conditions: span 64 mm, crosshead speed 20 mm/min and straining rate 0.01 min ⁇ 1.

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• 1994
  • 1994-01-24 EP EP19940200164 patent/EP0608949B1/fr not_active Expired - Lifetime

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