EP0931862A1

EP0931862A1 - Process for the production of elementary vegetable bast fibres

Info

Publication number: EP0931862A1
Application number: EP98200195A
Authority: EP
Inventors: Yijing Zhang; Edserd De Jong
Original assignee: Instituut Voor Agrotechnologisch Onderzoek ATO DLO
Current assignee: Instituut Voor Agrotechnologisch Onderzoek ATO DLO
Priority date: 1998-01-23
Filing date: 1998-01-23
Publication date: 1999-07-28
Also published as: AU2441399A; WO1999037834A1

Abstract

A process for the production of elementary vegetable bast fibres is provided, which process comprises the steps of

a) cutting raw vegetable bast fibres to a fibre length of 8 to 400 mm,

b) chemically treating the cut fibres to remove lignin and pectin compounds,

c) drying the fibres obtained in step b),

d) mechanically opening the dried fibres using a mechanical opening device.

The process allows the production of elementary, largely cellulosic vegetable bast fibres having the following chemical properties:

a lignin content of 0 to 5%, a uronic acid content of 0 to 2.5%, a rhamnose content of 0 to 0.5%, based on the oven-dry weight of the fibres,
and the following physical properties:

a single elementary fibre tensile strength of 100 to 600 MPa and an elasticity modulus of 35 to 100 GPa.

Description

The present invention relates to a process for the production of elementary vegetable bast fibres.
Vegetable fibres are used more and more as a substitute for synthetic fibres in the non-food industry because of their advantageous technical properties, price and performance, but also for environmental and agricultural considerations.
Non-wood vegetable fibres are generally used in pulp production. Examples of such fibres are hemp and flax. These crops consist of two major, very distinct lignocellulosic fractions. The woody core is a lignin rich, hardwood resembling material. These cores can be delignified to produce a pulp for such paper grades as printing and copying paper.
A process for the production of pulp is described in JP-A-63-264991 relating to the production of bast fibre pulp by cooking bast fibres with a mixture of oxalic acid or its salt, H₂O₂ or H₂O₂-generating agents, alkali metal carbonates, chelating agents, and anthraquinone compounds at 60 to 130 °C.
Although these pulps can be used as such for the production of paper, they do not result in elementary fibres which are suitable for other applications.
It has now been found that elementary fibres which are particularly suitable for the production of non-woven materials, can be obtained from a specific pulp of bast fibres by a combination of mechanical and chemical treatments.
The present invention relates to a process for the production of elementary vegetable bast fibres comprising the steps of
a) cutting raw vegetable bast fibres to a fibre length of 8 to 400 mm,
b) chemically treating the cut fibres to remove lignin and pectin compounds,
c) drying the fibres obtained in step b), e.g. at a temperature of 20 to 90°C, and
d) mechanically opening the dried fibres using a mechanical opening device.
Fibre materials which may be used are vegetable bast fibres of lignocellulosic nature which have a lignin content between 0.3 and 10% by weight and which contain elementary fibres with a fibre length of at least 10 mm. Examples of such fibres are flax, oil seed flax, hemp, ramie, milkweed and hop bast fibres. These are the principal commercial vegetable fibres. Preferably flax and hemp are used because of their high cellulose content and their long ultimate fibre length. Especially preferred is flax because flax can be dew-retted more easily than hemp.
Preferably the raw vegetable bast fibres are subjected to a retting process before being cut. Retting (controlled rotting) is a process which promotes the removal of the bast fibres from bark and woody stem parts. It is an enzymatic or bacterial action on the pectinous matter of the stem. The retting can be carried out in the field after harvesting (so called dew-retting for flax) or in warm water (so called warm water retting) or with enzymes.
As mentioned above step a) of the process of the invention consists of cutting the fibres to a length of 8 to 400 mm, preferably between 10 and 100 mm, depending on the intended use. The purpose is to produce fibres of uniform length. An example of a cutting device is a "Rotary Cutter", type 231, available from Laroche Machines Textiles, 69470 Cours, France. Any machine of similar construction may also be used.
Step b) of the process of the invention consists of a chemical treatment of the, optionally, retted and cut vegetable bast fibres to remove pectin and lignin compounds. Step b) advantageously comprises:
i) impregnating the fibres with a chelating agent followed by draining the mixture,
ii) cooking the fibres obtained in step i) with an oxidising agent and
iii) washing the fibres obtained in step ii) with water.
Such a treatment is known for making pulps for paper and has been optimised for the specific objects of the present invention.
It is also possible to eliminate step i) and to add the chelating agent to the fibres together with the oxidising agent.
Preferably step b) of the process of the present invention comprises:
i) mixing the fibres with water in a fibre to liquor weight ratio of 1:5 to 1:99, adding a chelating agent in an amount of 0.1 to 1.0% by weight based on the oven-dry weight of the fibres, allowing the mixture to stand for 1 to 12 hours and then draining the mixture,
ii) mixing the impregnated fibres with an chemical mixture containing 2 to 8% by weight H₂O₂, 0.5 to 12% by weight NaOH, and 1 to 8% by weight of a stabiliser for H₂O₂, based on the oven-dry weight of the fibres, and a suitable amount of water to reach a fibre to liquor ratio of 1:5 to 1:20 in this fibre-chemical mixture, heating to a temperature of 30 to 130°C and allowing the mixture to stand at said temperature for 0.1 to 10 hours and
iii) washing the fibres obtained in step ii) with water.
Suitable chelating agents include polyamine-polycarboxylic acids, such as EDTA (ethylenediaminetetraacetic acid), N-(hydroxyethyl)ethylenediaminetriacetic acid, piperazinediacetic acid and especially DTPA (diethylenetriaminepentaacetic acid). The oxidising agent is preferably H₂O₂, or a stabilised form thereof, but may also be a peracid, hydroperoxide, perborate etc. Suitable stabilisers for H₂O₂ include alkaline metal and alkaline earth metal silicates, phosphates and sulphates and mixtures thereof, such as a mixture of sodium silicate and magnesium sulphate. These substances can be added e.g. in an amount of 1 to 8% by weight of sodium silicate and 0 to 2% by weight of MgSO₄ based on the oven-dry weight of the fibres. The oven-dry weight of the fibres after cutting is measured after drying the fibres e.g. for 12 hours at 105 °C.
Step c) of the present process, drying the cooked fibres, may be carried out in any conventional oven. The fibres are left in the oven until 60-95% dryness, preferably at a temperature of 20 to 90 °C.
Step d) of the process consists of mechanically opening the fibres obtained in step c). The device used for this may be any carding or opening device known in the art. These devices are used for cleaning cotton fibres from trashes. The aim of the use of such a mechanical opening device is to loosen the fibres obtained in the cooking step so as to form elementary fibres which have a fluffy, cotton-like appearance.
An example of a mechanical opening device is a "Shirley Analyser", type SOL 102, available from Shirley International Development Ltd., Stockport, UK. Any machine of similar construction may also be used.
To ascertain whether elementary fibres are obtained, the fibres are analysed by light and electron microscopy.
The present application also relates to the elementary vegetable bast fibres which can be obtained by the process described above. These fibres have the following chemical properties:
a lignin content of 0 to 5.0%, especially 1 to 4%,
an uronic acid content of 0 to 2.5%, especially 0.1-1.5%, the uronic acid content being calculated on the basis of hexuronic acid,
a rhamnose content of 0 to 0.5%;
a cellulose content of 75 to 95% and
a hemicellulose content 5 to 15%,
based on the oven-dry weight of the fibres,
and the following physical properties:
a single elementary fibre tensile strength of 200 to 500 MPa and
an elasticity modulus of 35 to 100 GPa.
The present invention also relates to the use of these elementary vegetable bast fibres for the production of wet or dry-laid non-woven materials as a substitute for synthetic fibres which are normally used in such materials. The elementary vegetable bast fibres of the invention may also be combined with synthetic fibres in any ratio, e.g. a weight ratio of 20:80 up to 100:0
Industrial, hygiene or clinical non-wovens are all produced with different kinds of synthetic fibres such as polypropylene ad polyester. These non-woven products are often used as disposables and therefore their biodegradability is important. Thus there is a strong interest, especially in Europe, to replace the synthetic fibres with vegetable fibres. Besides the advantage of being biodegradable, these fibres can also bring unique properties, e.g. better water absorbency, to the final non-woven product.
Due to the long elementary fibre length and high cellulose content in the fibres according to the invention, these vegetable fibres serve as excellent candidates for replacing synthetic fibres in non-wovens. The key factors in using vegetable bast fibres are good fibre length control and high degree of fibre separation. Such properties can be obtained with the process according to the invention and therefore the elementary fibres obtained are very suitable for non-wovens.
The fibres of the invention also show good non-woven sheet formation and comparable sheet properties as those made with synthetic fibres. The non-woven materials may be prepared by any available technique.

Example 1

Dew-retted flax, which had gone through a regular breaking and scutching process, was cut to 9.5 mm. These fibres were first impregnated with 0.5% DTPA in hot water in a fibre to water ratio of 1:20 overnight and then drained out to about 20% consistency. The impregnated fibres were treated with a mixture containing the following compounds:

Hydrogen peroxide: 4%

MgSO₄.7H₂O: 0.1%

Sodium silicate: 4%

NaOH: 7.8%
The percentages are weight to weight percentages based on the oven-dry weight of the fibres. Water was added to adjust the fibre to liquor ratio to 1:5.67. These fibres were heated up to 90-95°C for 1 hour. Then the residual liquor was drained out and the cooked fibres were washed with a sufficient amount of water. These fibres were dried with hot air of 50°C. The dried fibres were then fed into a mechanical opening device, "Shirley Analyser", type SOL 102, obtainable from Shirley International Development Ltd., Stockport, UK, where the fibres were opened up. This resulted in well-separated, fluffy elementary fibres.

Chemical characteristics:

Acid insoluble lignin:	1.07%
Acid soluble lignin:	0.05%
Uronic acid:	1.1%
Rhamnose:	0%
Ethanol extractives:	1.36%
Cellulose content:	87.3%
Hemicellulose content:	6.1%

Physical characteristics of single elementary fibres:

Tensile strength: 250 MPa

Elasticity modulus: 56 GPa

Example 2

Dew-retted flax, which had gone through a regular breaking and scutching process, was cut to 15.8 mm. These fibres were first impregnated with 0.5% DTPA in hot water in a fibre to water ratio of 1:20 overnight and then drained out to about 20% consistency. The impregnated fibres were treated with a mixture of the following compounds:

Hydrogen peroxide: 2%

MgSO₄.7H₂O: 0.1%

Sodium silicate: 4%

NaOH: 1.22%
The percentages are weight to weight percentages based on the oven-dry weight of the fibres. Water was added to adjust the fibre to liquor ratio to 1:5.67. These fibres were heated to 85°C for 0.5 hour. Then the residual liquor was drained out and the cooked fibres were washed with a sufficient amount of water. These fibres were dried with hot air of 40 °C. The dried fibres were fed into a mechanical opening device, "Shirley Analyser", type SOL 102, available from Shirley International Development Ltd., Stockport, UK, where the fibres were opened up. This resulted in well-separated, fluffy elementary fibres.

Chemical characteristics:

Acid insoluble lignin:	1.3%
Acid soluble lignin:	0.1%
Uronic acid:	1.7%
Rhamnose:	0.2%
Ethanol extractives:	1.4%
Cellulose content:	71.9%
Hemicellulose content:	10.1%

Physical characteristics of single elementary fibres:

Tensile strength: 330 MPa

Elasticity modulus: 68 GPa

Example 3

Dew-retted flax, which had gone through a regular breaking and scutching process, was cut to 15.8 mm. These fibres were first impregnated with 0.5% DTPA in hot water in a fibre to water ratio of 1:20 overnight and then drained out to about 20% consistency. The impregnated fibres were treated with a mixture of the following compounds:

Hydrogen peroxide: 2.5%

MgSO₄.7H₂O: 0.1%

Sodium silicate: 4%

NaOH: 1.8%
The percentages are weight to weight percentages based on the oven-dry weight of the fibres. Water was added to adjust the fibre to liquor ratio to 1:5.67. These fibres were heated up to 100 °C for 75 minutes. Then the residual liquor was drained out and the cooked fibres were washed with a sufficient amount of water. These fibres were dried with hot air of 40 °C. The dried fibres were fed into a mechanical opening device, "Shirley Analyser", type SOL 102, obtainable from Shirley International Development Ltd., Stockport, UK, where the fibres were opened up. This resulted in well-separated, fluffy elementary fibres.

Chemical characteristics:

Acid insoluble lignin:	0.5%
Acid soluble lignin:	0.1%
Uronic acid:	1.1%
Rhamnose:	0.3%
Ethanol extractives:	0.9%
Cellulose content:	72.7%
Hemicellulose content:	9.9%

Physical characteristics of single elementary fibres:

Tensile strength: 280 MPa

Elasticity modulus: 60 GPa

Claims

A process for the production of elementary vegetable bast fibres comprising the steps of

a) cutting raw vegetable bast fibres to a fibre length of 8 to 400 mm,

b) chemically treating the cut fibres to remove lignin and pectin compounds,

c) drying the fibres obtained in step b),

d) mechanically opening the dried fibres using a mechanical opening device.
A process according to claim 1, wherein the raw vegetable bast fibres are flax, oil seed flax, hemp, ramie, milkweed or hop bast fibres.
A process according to claim 2, wherein the raw vegetable bast fibres are flax fibres.
A process according to any one of claims 1 to 3, wherein step b) comprises

i) impregnating of the fibres with a chelating agent followed by draining the mixture,

ii) cooking the fibres obtained in step i) with an oxidising agent and

iii) washing the fibres obtained in step ii) with water.
A process according to claim 4, wherein step b) comprises:

i) mixing the fibres with water of 20-90°C in a fibre to liquor weight ratio of 1:5 to 1:99, adding DTPA as a chelating agent in an amount of 0.1 to 1.0% by weight based on the oven-dry weight of the fibres, leaving the mixture to stand for 1 to 12 hours and thereafter draining the mixture,

ii) mixing the impregnated fibres with a chemical mixture containing 2 to 8% by weight H₂O₂, 0.5 to 12% by weight NaOH, and 1 to 8% by weight of a stabiliser for H₂O₂, based on the oven-dry weight of the fibres, and a suitable amount of water which is added to reach a fibre to liquor ratio of 1:5 to 1:20 in this fibre-chemical mixture, heating to a temperature of 30 to 130 °C and leaving the mixture to stand at said temperature for 0.1 to 10 hours and

iii) washing the fibres obtained in step ii) with water.
A process according to claim 5, wherein in step ii) 1 to 8% by weight of sodium silicate and 0 to 2% by weight of magnesium sulphate based on oven-dry weight of fibres are added as a stabiliser for H₂O₂.
A process according to any one of claims 1 to 6, wherein the mechanical opening of the fibres in step d) is accomplished by means of a carding or opening device.
Elementary vegetable bast fibres, obtainable by the process of any one of claims 1 to 7, having the following chemical properties:

a lignin content of 0 to 5%, a uronic acid content of 0 to 2.5%, a rhamnose content of 0 to 0.5%, based on the oven-dry weight of the fibres,
and the following physical properties:

a single elementary fibre tensile strength of 100 to 600 MPa and an elasticity modulus of 35 to 100 GPa
Elementary vegetable bast fibres according to claim 8, having a cellulose content of 75 to 95% and a hemicellulose content of 5 to 15%, based on the oven-dry weight of the fibres.
Use of elementary vegetable bast fibres according to claim 8 or 9 for the production of wet or dry-laid non-woven materials.