DESC:
FIELD OF THE INVENTION
The present invention relates to a high tenacity cellulosic fibre. The invention also relates to a process for preparing a high tenacity cellulosic fibre, i.e. cellulosic fibre having a tenacity higher than 5 grams per denier.
BACKGROUND OF THE INVENTION
Cellulosic fibres are versatile and high-quality fibres obtained from a solution of natural cellulose in N-Methylmorpholine-N-Oxide (NMMO) by solvent spinning process. Cellulosic fibres produced using this organic solvent spinning process is also known as Lyocell fibres.
Lyocell fibres have application in everyday fabrics including apparels and home products such as bath towels, sheets, pillowcases, etc. Such fibres also have industrial applications in making wipes, medical swabs and gauzes, filters, bicomposites, battery separators, etc. Lyocell fabrics have the unique characteristics of soft and silky handle, lustre, high strength and excellent drape. Interestingly, lyocell has high dry tenacity and modulus. It is the strongest cellulosic fibre when dry, even stronger than cotton or linen. It also retains much of its strength when wet. Compared to viscose, it is two times stronger when dry and three times when wet. In general, lyocell fibre are produced from cellulose pulp having degree of polymerization (DP) in the range of 700-1000. The fibre has tenacity in the range of 3.9 – 4.3 gpd.
However, for industrial applications such as in tire cord, high-speed carding machines, high strength composites etc., fibres with higher tenacity are desired. Known methods to improve tenacity of cellulosic fibres include utilizing pulp with higher DP or addition of modifiers such as ammonium chloride to the dope. Use of alternate solvents such as ionic liquid for preparation of cellulose dope is also known to yield high tenacity fibres. However, all such methods add to the complexity of the manufacturing process for cellulosic fibres including pulping and doping, consequently lowering the yields. There is, therefore, a need for an alternative, yet simple method of obtaining cellulose fibres with even higher tenacity.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows the molecular weight distribution of Pulp A versus Pulp B
Fig. 2 is a graphical representation of the stress-strain curve of six cellulose fibres numbered I, II, III, IV, V and VI illustrating changes in tenacity with percentage increase in elongation of the respective fibres
SUMMARY OF THE INVENTION
According to an embodiment of the invention, there is provided a high tenacity cellulosic fibre having a tenacity higher than 5 grams per denier comprising 87% to 90% by weight of cellulosic raw material based on the total weight of the fibre wherein the cellulose fibre is obtained from cellulosic raw material wherein at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by said cellulose having polydispersity index less than 5 and up to 30% cellulosic raw material comprises cellulose obtained from standard sulphite process.
According to another embodiment of the invention there is provided a process for preparing a cellulosic fibre having a tenacity higher than 5 grams per denier, the process comprising the steps of:
a) dissolving a cellulose raw material at a temperature of 90 to 120 degree Celsius in an aqueous solution of 60% to 80% N-methylmorpholine-N-oxide using high shear mixing under vacuum to remove excess water to obtain a cellulose dope; and
b) spinning the cellulose dope obtained in step (a) by discharging the cellulose dope into air through a spinneret at the temperature of 90 to 125 degree Celsius and pressure of 4 to 10 bar, submerging the discharged dope into a liquid coagulation bath to draw a fibre at temperature of 10 to 40 degree Celsius followed by washing and drying at temperature of 90 to 110 degree Celsius to form a high tenacity cellulosic fibre;
Provided that the cellulose dope spun in step (b) comprises 10% to 20% by weight of cellulosic raw material based on the total weight of the cellulose dope and at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by polydispersity index less than 5.
According to yet another embodiment of the invention there is provided a process for preparing cellulose dope comprising dissolving a cellulose raw material at a temperature of 90 to 120 degree Celsius in an aqueous solution of 60% to 80% N-methylmorpholine-N-oxide using high shear mixing under vacuum to remove excess water to obtain cellulose dope,
Provided that the cellulose dope comprises 10% to 20% by weight of cellulosic raw material based on the total weight of the cellulose dope and at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by polydispersity index less than 5.
DETAILED DESCRIPTION OF THE INVENTION
The present disclosure relates to a high tenacity cellulosic fibre and a process for preparing the same, the fibre having a tenacity higher than 5 grams per denier.
The dope comprises cellulose raw material dissolved in an aqueous solution of N-methylmorpholine-N-oxide (NMMO). The cellulose has a DP of less than 650 and weight average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by said cellulose having polydispersity index (PDI) less than 5.
The cellulose dope comprises 10% to 20% by weight of cellulosic raw material to the total weight of the cellulose dope.
In accordance with an embodiment, preparation of the cellulose dope comprises dispersing and dissolving cellulosic raw material or cellulose pulp or mixture of pulps in an aqueous solution of NMMO followed by swelling and dissolution using high temperature i.e. at 90-120 °C and high shear. The process may further comprise evaporating the excess water to obtain the dope.
Using a cellulose pulp having DP less than 650 and weight average molecular weight less than 300000 according to the present disclosure provides a dope having a viscosity that makes it is suitable for spinning into fibres. Thus, using cellulose pulp with lower molecular weight and narrow molecular weight distribution provides better spinning ability to the dope and higher tenacity.
Using cellulose pulp with DP less than 650, narrow molecular weight distribution and average molecular weight less than 300000 reduces the swelling time of cellulose while at the same time allows complete and uniform penetration of solvent within the cellulose. Thus, using cellulose pulp with above characteristics contributes towards achieving a homogenized dope which in turn results in cellulosic fibres with excellent physical and chemical properties.
In accordance with an embodiment, cellulose pulp having DP less than 650, narrow molecular weight distribution and weight average molecular weight less than 300000 may be obtained through any conventional pulping method. The cellulose pulp may be obtained through Prehydrolyzed Kraft (PHK) process to obtain cellulose pulp having a PDI of less than 5 or through sulphite process with broad molecular weight distribution to obtain cellulose pulp having PDI greater than 6.5.
In accordance with an embodiment, the dope may be prepared using two or more cellulose pulps from different sources or using 100% cellulose pulp obtained by PHK process. PHK pulp is ordinarily used in the viscose process but due to impurities present in the pulp, PHK pulp was not used in any other processes. Cellulose pulp obtained from PHK process having a narrow molecular weight distribution of polymer molecules may be blended with cellulose pulp having a broad molecular weight distribution of polymer molecules to prepare the dope, i.e. the dope may be prepared by blending cellulose pulp obtained from PHK process (PHK pulp) with cellulose pulp obtained from sulphite process (sulphite pulp). According to an embodiment of the invention, the dope may be prepared by blending at least 70% PHK pulp with sulphite pulp. Preferably, the at least 70% cellulose raw material is prepared by Prehydrolyzed Kraft process and up to 30% cellulosic raw material is prepared by standard sulphite process.
In accordance with an embodiment, the molecular weight distribution of the cellulose pulp represents a normal distribution i.e., the majority of the polymer molecules of the cellulose pulp have a molecular weight close to a central value.
Spinning the dope comprises discharging the dope into air through a spinneret and submerging said discharged dope into a liquid coagulation bath to form fibres. The fibres obtained are washed and dried.
In accordance with an embodiment, dope is spun at a spinning temperature of about 90-125 °C at about 4 -10 bar pressure depending on flow rates and number of holes in spinnerets.
Preferably, dope is spun using air gap length of about 10-50 mm with air temperature of about 5 – 50 °C and relative humidity of about 20-80%.
Preferably, the coagulation bath has a temperature of about 10-40 °C. Also, the NMMO in the coagulation bath should preferably be in a concentration of 10 to 50%.
In accordance with an embodiment, fibre is drawn 3-15 times. In accordance with an embodiment, the fibres are washed at 90-100 °C and dried at 100-110 °C.
Tenacity is the customary measure of strength of a fibre. Typically, it is defined as the ultimate (breaking) force of the fibre (in gram-force units) divided by the denier. The unit of tenacity is cN/tex (centi Newton per tex) or gram/denier (g/d or gpd).
1 Grams-force/Denier (g/d) = centiNewton/tex (cN/tex) x 0.113
Denier is a unit of measure for the linear mass density of a fibre and is defined as the mass in grams per 9000 meters. The definition of denier is based on a natural reference: a single strand of silk is approximately one denier; a 9000-meter strand of silk weighs about one gram. Tex is another unit for measurement of linear mass density and is defined as the mass in grams per 1000 meters.
Degree of Polymerization (DP) is usually defined as the number of monomeric units in a macromolecule or polymer. For a homopolymer, there is only one type of monomeric unit and the number-average degree of polymerization is given by
,
where Mn is the number-average molecular weight and Mo is the molecular weight of the monomer unit.
Polydispersity Index (PDI) or heterogeneity index, or simply dispersity, is a measure of the distribution of molecular weight in a given polymer sample. It is calculated as the weight average molecular weight (Mw) divided by the number average molecular weight (Mn).

PDI can also be calculated according to degree of polymerization, as the weight average degree of polymerization (Xw) divided by the number average degree of polymerization (Xn)
Molecular Weight Distribution (MWD) of a polymer refers to the relationship between the number of molecules of each polymer species (Ni) and the molecular weight (Mi) of that species.
Weight average molecular weight (Mw) is a way of describing the molecular weight of a polymer. It is based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do. The weight average molecular weight is calculated as

where Ni is the number of molecules of molecular weight Mi.
Number average molecular weight (Mn) is another way of determining the molecular weight of a polymer. It is just the total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample. It is determined as:

An exemplary method of determination of molecular weight distribution of a polymer is the Gel Permeation Chromatography (GPC). It involves forcing a polymer solution through a matrix of cross-linked polymer particles at a pressure of up to several hundred bar. The limited accessibility of stationary phase pore volume for the polymer molecules results in shorter elution times for high-molecular-weight species.

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:
Example 1: Characterisation of Cellulose raw material
Cellulose raw material was prepared from PHK process (Pulp A) and from a sulphite process (Pulp B). Pulp C was prepared by blending Pulp A and Pulp B in a ratio of 80:20 and Pulp D was prepared by blending Pulp A and Pulp B in the ratio of 90:10 respectively. The pulps were then characterised as below.
Molecular weights (weight average and number average), PDI and DP for the four cellulose pulps were determined. Table 1 gives the characterisation values for the four pulps.
Sample Mn Mw PDI DP
Pulp A 67783 288321 4.254 641.4
Pulp B 41807 363449 8.694 684.9
Pulp C (A:B =80:20) 64525
313762
4.86 658.0
Pulp D (A:B =90:10) 66276 299224 4.51 641.4
Table 1
Molecular Weight Distribution (MWD) was determined using Gel permeation chromatography (GPC). GPC was performed by dissolving the pulp samples in Lithium chloride (LiCl) in Dimethylacetamide (DMAc) solution. Figure 1 illustrates the molecular weight distribution (MWD) of Pulp A and Pulp B. Pulp B shows a broader molecular weight distribution as compared to Pulp A. At the higher and lower Mw values the difference is clearly visible. This difference leads to differences in molecular weights of pulp reported in Table 1 above.

Example 2: Preparation of Fibre
Pulp A, B, C and D were used for dope preparation and spinning to prepare cellulosic fibres. The process for dope preparation and spinning is provided below.
Cellulose pulp was swelled and dissolved in NMMO- water mixture (76% NMMO and 34 % water) at 120 °C using high shear mixture. The excess water was removed using a vacuum system to get a spinnable dope of 10-20% cellulose. Six dopes with different concentrations of cellulose prepared from the four pulps (Pulp A, Pulp B, Pulp C and Pulp D) were spun to obtain six different fibres I to VI. The details of the six fibres are provide in Table 2 below.
The dope was spun at a spinning temperature of 100-125 °C at 4-bar pressure using air gap length of 10- 50 mm with air temperature of 5 - 50 °C and relative humidity of 20-70 %. The bath temperature was kept at 10 - 40 °C with 10 - 50% NMMO concentration. Fibre was drawn from 3- 10 times and then washed at 95 °C and dried at 105 °C. The fibre denier was fixed to 1.2- 1.4 denier.
It was observed that the dope prepared from the pulp having higher Mw (Pulp B) was more viscous and required more pressure during spinning. Pulp A with low molecular weight and the narrower PDI was observed to have a better spinning ability.
The denier and tenacity of the dried fibres obtained from different dope concentrations of pulp A, B,C and D were measured by vibroscope/ vibrodyne using ASTM standard. The results are given in Table 2. A stress strain curve (plot of tenacity vs. percent elongation) for each of the six fibres (I to VI) obtained is given in Figure 2.

Code in Graph Description Tenacity ( gpd) Elongation (%)
I 13% Pulp A present in final dope spun 5.13 11.4
II 14% Pulp A present in final dope spun 5.84 12
III 13% Pulp B present in final dope spun 4.79 9.66
IV 13% Pulp C (80% Pulp A and 20% Pulp B) present in final dope spun 5.42 8.76
V 13% Pulp D(90% Pulp A and 10% Pulp B) present in final dope spun 5.03 11.6
VI 12% Pulp B present in final dope spun 4.35 9.07
Table 2
Figure 2 illustrates that fibres prepared solely from Pulp A or fibres derived from pulp comprising over 70% Pulp A had significantly higher tenacity than fibres prepared from Pulp B alone.
It is clear from the foregoing examples that the cellulose raw material selected in the process of the invention, and the proportions in which the cellulose pulps having different PDI are used in the process of the invention are a key factor in controlling the tenacity of the spun cellulose fibres. Cellulose raw material primarily comprising lower DP cellulose pulp provides dope having a viscosity which aids in better spinning ability of fibres. Further, using cellulose pulp with lower DP and smaller Mw distribution contributes towards achieving a fully homogenized dope, which in turn confers cellulosic fibres of the present disclosure with excellent physical and chemical properties. The homogenized dope prepared from cellulosic raw material with lower DP (less than 650) and PDI (less than 5) after regeneration presumably leads to better packing of cellulosic chains which could result in higher order structures having lower tie chains and high crystallinity conferring the property of high tenacity to the cellulosic fibres.
The high tenacity cellulosic fibres obtained have a number of industrial applications. Said fibre can run through high speed-carding machines with a speed more than 200 m/min. Further, the fibres obtained by using the disclosed dope and the disclosed method can be used for tire chord application and also find application in composites where strength is the main criteria.

The above examples are non-limiting. The invention is defined by the claims that follow.
,CLAIMS:We Claim:

1) A high tenacity cellulosic fibre having a tenacity higher than 5 grams per denier comprising 87% to 90% by weight of cellulosic raw material based on the total weight of the fibre wherein the cellulose fibre is obtained from cellulosic raw material wherein at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by said cellulose having polydispersity index less than 5 and up to 30% cellulosic raw material comprises cellulose obtained from standard sulphite process.

2) A process for preparing a cellulosic fibre having a tenacity higher than 5 grams per denier, the process comprising the steps of:
c) dissolving a cellulose raw material at a temperature of 90 to 120 degree Celsius in an aqueous solution of 60% to 80% N-methylmorpholine-N-oxide using high shear mixing under vacuum to remove excess water to obtain a cellulose dope; and
d) spinning the cellulose dope obtained in step (a) by discharging the cellulose dope into air through a spinneret at the temperature of 90 to 125 degree Celsius and pressure of 4 to 10 bar, submerging the discharged dope into a liquid coagulation bath to draw a fibre at temperature of 10 to 40 degree Celsius followed by washing and drying at temperature of 90 to 110 degree Celsius to form high tenacity cellulosic fibres;
Provided that the cellulose dope spun in step (b) comprises 10% to 20% by weight of cellulosic raw material based on the total weight of the cellulose dope and at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by polydispersity index less than 5.

3) The process as claimed in claim 2, wherein the at least 70% cellulose raw material is prepared by Prehydrolyzed Kraft process and up to 30% cellulosic raw material is prepared by standard sulphite process.

4) The process as claimed in claim 2, wherein the N-methylmorpholine-N-oxide in the coagulation bath has a concentration of 10 to 50%.

5) A process for preparing cellulose dope comprising dissolving a cellulose raw material at a temperature of 90 to 120 degree Celsius in an aqueous solution of 60% to 80% N-methylmorpholine-N-oxide using high shear mixing under vacuum to remove excess water to obtain cellulose dope,
Provided that the cellulose dope comprises 10% to 20% by weight of cellulosic raw material based on the total weight of the cellulose dope and at least 70% of the cellulosic raw material has a degree of polymerization less than 650, average molecular weight less than 300000 and a narrow molecular weight distribution as expressed by polydispersity index less than 5.

6) The process as claimed in claim 5, wherein the at least 70% cellulose raw material is prepared by Prehydrolyzed Kraft process and up to 30% cellulosic raw material is prepared by standard sulphite process.

Dated this 21st day of June 2016

Essenese Obhan
Of Obhan & Associates
Agent for the Applicant
Patent Agent No. 864