GB2616691A - Method of extraction of fibres - Google Patents

Abstract

Method comprising step a) treating palm tree stems to delignify by application of alkali solution and b: scraping treated stems to mechanically remove non-cellulosic material and fibrillate the fibre bundles by application of a blunt scraper blade. The palms may be date palms; the alkali may be sodium hydroxide. Initial steps may include separating leaflets from palm fronds, peeling the outer layer from the stems and/or slicing stems to reduce thickness. Subsequent steps may include washing/neutralising the product from step b and may further comprise the step of drying the washed product and brushing the dried product. Also included are claims to the fibre obtained by the method and clothing, sheets, rope and items for use in boards or insulation comprising such fibres.

Description

METHOD OF EXTRACTION OF FIBRES

Field of the Invention

This disclosure relates to a method of extraction of fibres and more particularly, but not exclusively, the invention relates to extraction of long textile fibres from palm midribs and spadix stems, such as the fibres from date palms.

Background

Date palms are mainly cultivated in the Middle East and North African regions. Typically, global harvests exceeding 1 million hectares. The estimated annual byproducts of pruning of date palms globally are around 5 million tons (air dry weight), in the form of midribs, spadix stems, leaflets and leaf sheath. These are often treated as agricultural waste.

Previous attempts to extract fibres from date palm midribs and spadix stems were limited to milling them into small particles with a high percentage of non-cellulosic impurities. These tended to be used in manufacture of worktop and building panels such as particle boards or (MDF) or as fillers in synthetic plastics material.

Some of these are described in the documents mentioned below.

Prior Art

US patent US 10 767 267 (United Arab Emirates University).

International patent application WO 2020/139088 (Sultan Qaboos University).

Canadian patent application CA 2 504 227 (Soil Sub Technologies PTY Ltd).

US patent US 10 655 009 (United Arab Emirates University).

None of these attempts were capable of extracting long textile grade fibres.

An object of the present invention is to valorize large quantities of agricultural residues generated by palm, particularly date palm, plantations and to extract long textile fibres from the midribs and spadix stems.

Summary of Invention

According to a first aspect of the present invention there is provided a method of obtaining fibres from species of palm trees, the method including the steps of a) treating stems to delignify them by application of an alkali solution and b) scraping the treated stems to mechanically remove non-cellulosic matrix.

Ideally fibre vascular bundles are also fibrillated, for example by application of a blunt scraper blade.

Ideally the species of palm is date palm. However, other types of palms may be used. In some embodiments the alkali solution is a sodium hydroxide solution.

Preferably the method includes the steps of soaking stems in an alkali solution at a temperature t where t is in the range of t = -4.82 (c x d) + 92.3 °C +/-12.5°C, where c is the concentration of sodium hydroxide in %, or pH equivalent, and d is the duration of treatment in hours. Followed by the steps of scraping the stems by the application of a blunt scraper blade.

In some embodiments the method step comprises soaking stem material in an alkali solution at a temperature t where t is in the range of t = -4.82(c x d) + 92.3 °C +/-6°C.

Optionally the method includes an initial step c) of separating the leaflets from palm fronds.

In some embodiments the method includes a prior step d) peeling the outer layer of the palm stems to remove the waterproof wax and pectin layer that prevent the treatment.

The method may include a further prior step of e) of slicing the stems longitudinally to reduce their thickness and ensure uniform treatment across the stems thickness.

In the claims the term "stem" includes palm midribs and spadix stems.

According to a second aspect of the invention there is provided a fibrous product derived using the aforementioned method.

Fibrous products produced according to the method may be used to make or incorporated in, a myriad different items or products, including: an item of clothing or footwear: a sheet of flexible material; items for use in automotive or construction, (such as boards or insulation panels) or rope or twine.

The invention will now be described, with reference to examples, and the following Figures in which:

Brief Description of Drawings

Figures la and lb show date palm midribs and spadix stems respectively; Figure I is a SEM micrograph showing the building blocks of the date palm midrib; Figure 3a shows damage and breakage to fibrils due to nonoptimized treatment; Figure 3b shows pure undamaged midrib and spadix fibrils resulting from optimization of process steps; Figures 4 and 5 show the resulting fibre at the end of the extraction method; and Figure 6 shows a representation of a method steps according to one example. Detailed Description of Preferred Embodiments Figures la and lb shows date palm midribs and spadix stems respectively.

Date palm midrib and spadix fibres are considered leaf fibres similar to sisal, abaca, pineapple, and banana fibres. Generally, leaf fibres are monocotyledons which consist of numerous fibre vascular bundles in varying sizes. Most fibre bundles are circular or oval with single or multiple vascular voids and they are composed of elemental fibrils which can have multiple cell walls and a central lumen as shown in Figure 2, which shows SEM micrographs showing the building blocks of the date palm midrib.

Conventional fibre extraction routes used with other leaf fibres (sisal, banana, abaca or pineapple), such as scraping-washing-brushing, cannot be applied in the case of extracting fibres from date palm midrib and spadix stems. This is because, the midrib and spadix stem are thicker and have lower moisture content and they are covered with a hard layer of wax and pectin. Therefore, any attempt to directly scrape them will result in crushing rather than extraction.

Moreover, there are two major challenges associated with extracting long textile fibres from the date palm midribs and spadix stems. Firstly, the vascular fibre bundles are embedded inside the midrib or the spadix stem and surrounded by a matrix of complex natural binder made up of lignin and hemicellulose, which makes it difficult to extract the pure fibres without breaking or damaging them. Secondly, the vascular fibre bundles are coarse and hollow, and they do not have enough flexibility to be processed into any textile form. The fibres therefore tend to break whenever they are bent or twisted.

In exemplar methodology is provided a process to delignify and fibrillate the fibre vascular bundles into pure long and flexible elemental fibrils or smaller bundles, without causing any damage to the fibrils during extraction.

Methodology The method is shown diagrammatically as a series of steps in Figure 6. Initial Preparation In an initial step the leaflets are separated from the green freshly cut date palm fronds either manually by hand, knife, or by using a mechanized system with blades to peel off the leaflets to obtain the midribs, which are then bundled for further processing.

In case of the spadix stems, top twigs are cut from the green fresh spadices using knife, manual saw or mechanized saw.

Peeling.

Peeling of outer wax and pectin layer surrounding the midrib and spadix stem, is an important step to ensure effective extraction. Because the wax and pectin layers act as a waterproof membrane that prevents any downstream wet treatments penetrating and reacting with the inner core of the midrib and spadix.

The peeling may be performed mechanically by scraping or stripping blades, or by abrasive grinding rollers or papers. Peeling may also be performed thermally using singeing in which a direct flame source is used to burn off the wax and pectin layer. Alternatively, a high intensity heat source, such as laser, may be used.

Finally, the peeling can be achieved chemically using solvent, including methanol, hydrochloric acid and other commercial solvents used in wax and pectin dissolution.

Slicing Slicing is the process of reducing the thickness of the midrib or spadix stem and splitting them into smaller longitudinal sticks with regular or irregular cross-sectional areas, ideally which do not exceed 3 cm2. The slicing process is very important to ensure that downstream wet treatment will have a uniform effect and to avoid shell-treatment issues, in which the outer shell of the sticks is treated, while the inner core remains untreated.

Slicing can be performed manually using a knife or can be performed using a mechanized system, in which the midrib or spadix stem is fed into a machine with conveying rollers that force the stems to emerge through the blades of a slicing die with the predefined cross-sectional shape and size. Slicing can also be performed by using mechanized pressure rolling, in which the midrib and spadix stems split open into irregular longitudinal strips.

Main Treatment The treatment of the midrib or spadix slices is a critical step in the fibre extraction process. The main purpose of the treatment is to weaken or break the binding effect of the non-cellulosic matrix of lignin and hemicellulose.

The treatment may be achieved by several methods under varying conditions. Generally, the main step is to treat the midribs or spadix slices with alkali.

In one example the methodology is to treat the midribs/spadix slices (for example after peeling and slicing) with mild alkaline treatment, for example using a low concentration sodium hydroxide solution <5%. Here the midrib or spadix slices may be immersed into a tank containing sodium hydroxide solution with a bath (liquor) ratio which is ideally not less than 10:1.

To ensure uniform treatment, there must be continuous agitation or circulation of the sodium hydroxide solution by means of agitator or stirrer or circulation pump. Optionally a wetting agent can also be added to the bath to ensure uniform treatment. The treatment can be performed at various combinations of duration, temperature, and sodium hydroxide concentration.

The treatment results in the removal of the surface impurities and fibrillation of the vascular bundles, which may be achieved by varying for example, increasing any of the three treatment conditions: concentration, temperature, and duration. However, increasing, the sodium hydroxide concentration and/or treatment temperature has been found to increase the purity of the extracted fibres and resulted in more fibrillation. Moreover, it was noticed that increasing the temperature had more pronounced effect than increasing the sodium hydroxide concentration; since, increasing the temperature tends to make the reaction more aggressive.

On the other hand, increasing the treatment duration had less pronounced effect on the removal of surface impurities, yet it had more pronounced effect on the fibrillation. Any further increase in the severity of the treatment conditions may result in over-treatment of the midrib and spadix fibres and so risk causing damage and breakage to the fibrils as shown for example in Figure 3a.

Hence, in preferred examples there is optimization of the treatment conditions to extract pure undamaged midrib and spadix fibrils as shown in Figure 3b.

In one example this is achieved by treating the midrib and spadix slices at between 65 °C and 110 °C, preferably further between 75 °C to 100 °C, while using low sodium hydroxide concentration of 0.5% to 1.5%, for a duration of preferably 0.8 hour to 1.2 hour.

More preferably the concentration is between 0.9% to 1.1% for short durations (for example) 30 minutes to 2 hours and preferably for around 1 hour. However, it will be appreciated that longer processing periods may be selected, and concomitantly the concentration of sodium hydroxide may be reduced and/or the treatment temperature may be reduced accordingly in order to save energy.

Alternatively, the method can be achieved by treating the midrib and spadix slices at room temperature, with higher sodium hydroxide concentrations (5%) for longer duration (3 hours).

Thus, any of the parameters of time and/or alkali strength and/or temperature may be maintained.

Generally, the temperature of treatment in degrees Celsius is preferably in the following range: -4.82(ed) + 92.3°C +/-12.5°C, or preferably, -4.82(ed) + 92.3°C +/-6°C, where "c" is the concentration of sodium hydroxide in % (or equivalent) and "d" the duration of treatment in hours.

Other alkalis may be used and the term "equivalent" should be understood as a solution which has the same pH as stated the sodium hydroxide (NaOH) solution.

Scraping After treatment of the midrib and spadix stem slices, the slices become swollen with partial dissolution of the non-cellulosic matrix. However, the matrix in this case is still surrounding the vascular fibre bundles and elemental fibrils. Hence, the scraping process is an essential step to mechanically separate the non-cellulosic matrix (delignification) without causing any damage to the fibre.

Furthermore, the scraping action causes the coarse fibre vascular bundles to split (fibrillate) into finer bundles or fibrils and eliminates the hollow content, resulting in long fine flexible textile fibres.

The scraping action can be performed using a reciprocating blunt scraper along the length of the midrib and spadix slices. It can also be performed using a rotating drum (decorticator) with multiple blunt scraping blades rotating against a stationary roller. In this case the midrib and spadix slices are fed into the clearance (<2 mm) between the rotating scraping blades and the stationary roller, in one or multiple strokes from one or both ends. It is important that the scraping is performed while the treated slices are still moist.

Washing and/or Neutralizing The fibres have to be neutralized in a 5% solution of acetic acid after extraction to ensure that the fibre has reached a pH of around 7. Following the neutralization, the fibres are washed with water to remove any residual impurities.

Drying After washing, the fibres are dried to a moisture content of less than 20%, preferably less than 15%. The first step in the drying is either roll squeezing to remove as much water as possible or centrifugal dewatering using a mechanized drying system.

The second drying step involves hang drying the fibres in open air for several days depending on the ambient conditions or oven drying at temperature 50°C until the moisture content drops below 15%.

Brushing After washing and drying the fibres they stick to each other by hydrogen bonding. To open the fibres, they have to be beaten or brushed. Beating and brushing is performed by a rotating drum with multiple blunt blades or short metal combs, in which the fibres are held from one end and fed into the clearance between the rotating drum and a flat metal surface in one or multiple strokes from one or both ends.

The resulting fibre at the end of the extraction method are shown in Figures 4 and 5.

Reference to Figure 6 has been made and below is a Table which shows properties of a material obtained using the abovementioned method steps.

Technical Data Table 1 Average properties of long fibre extracted from date palm midrib and spadix stem using the above method.

Average fibre properties Midrib Spadix Standard Cellulose wt. (%) 52.52±0.162 58.91±0.24 T 203 cm-09 Hemicellulose wt. (%) 20.41±0.06 19.18±0.4 T 223 cm-10 Lignin wt. (%) 8.91±0.23 4.96±0.113 T 222 om-11 Ash wt. (%) 2.55±0.04 0.56±0.05 Extractives wt. (%) 15.61 16.39 Cross sectional area SEM (mm2) 0.0136 0.0119 Equivalent diameter (pm) 131.7±9.7 123±8.5 SEM 1.178- 1.178- ASTM D8171 -18 Density (g/cm3) 1.324 1.324 Thermogravimetric Thermal stability (CC) 250 250 Analysis Crystallinity index (%) 58.5 59.93 X-Ray Diffraction Tensile strength (MPa) 583. 5±280 601.7±167 ASTM D 3822 -01 Tensile modulus (GPa) 22.5±8.83 11.67±1.95 ASTM D 3822 -01 Failure strain (%) 2.58±0.32 5.09±0.66 ASTM D 3822 -01 It will be appreciated that variation may be made to the aforementioned embodiments, without departing from the scope of protection as defined by the claims.

For example, the method can also be used to extract long textile fibres from the midribs of other palm species, including but not limited to Washingtonia palm, oil palm, coconut palm, and sugar palm. It may also be applied to other agriculture residues in form of long stalks, including but not limited to sorghum.

Claims (5)

1. Claims 1. A method of obtaining fibres from species of palm trees, the method including the step (a) of treating stems to delignify them by application of an alkali solution and b) scraping the treated stems to mechanically remove the non-cellulosic matrix and fibrillate the fibre vascular bundles by application of a blunt scraper blade.
2. 2. A method as claimed in claim 1 wherein the species is date palm.
3. 3. A method as claimed in either claim 1 or 2 wherein the alkali solution is a sodium hydroxide solution.
4. 4. A method as claimed in any of claims 1 to 3 wherein step (a) comprises soaking stems in an alkali solution at a temperature t where t is in the range of: t = -4.82(ed) + 92.3 °C +/-12.5°C, where c is the concentration of sodium hydroxide in %, or pH equivalent, and d the duration of treatment in hours.
5. 5. A method as claimed in claim 4 wherein the step comprises soaking the stem in an alkali solution at a temperature t where t is in the range of: t = -4.82(c*t) + 92.3 °C +/6°C.;6. A method as claimed in any preceding claim including an initial step (c) of separating the leaflets from palm fronds.;7. A method as claimed in any preceding claim including a prior step (d) of peeling the outer layer of the palm stems.;8. A method as claimed in any preceding claim including a prior step (e) of slicing the stems longitudinally to reduce their thickness.;9. A method as claimed in any preceding claim including a subsequent step of (f) which washes and/or neutralizes the product obtained using step (b).;10. A method as claimed in any preceding claim including a subsequent step of (g) which is drying the product obtained from step (f).;11. A method as claimed in any previous claim including a step (h) of brushing the fibres/product obtained from step (g).;12. A fibrous product derived from the method according to any preceding claim.;13. An item of clothing or footwear including the fibrous product according to claim 12.;14. A sheet of flexible material including the fibrous product according to claim 12.;15. An item for use in automotive or construction, such as an boards or insulation panel, including the fibrous product according to claim 12.;16. Rope including the fibrous product according to claim 12.