Technical field

The present invention relates to the field of dissolving pulps. In particular it relates to a novel sulfite dissolving pulp suitable as starting material for the production of high tenacity rayon and technical filaments.

Background

Dissolving pulp is a cellulose pulp, with low contents of lignin, hemicellulose and resin which makes it suitable as a raw material for the production of regenerated cellulose. In the present disclosure the term "dissolving pulp" refers to a pulp having a cellulose content of at least 90% (w/w, of bone dry pulp). Dissolving cellulose can be produced by the sulfite process or the prehydrolysis Kraft processes. One disadvantage with the sulfite process, compared to prehydrolysis Kraft processes, is that it results in pulps with a broad molecular weight distribution of cellulose (1 , 2). Furthermore, compared to prehydrolysed Kraft pulp, sulfite dissolving pulps generally have a higher content of hemicellulose. Both these features have been considered as negative for the tenacity of regenerated cellulose produced from the pulp. Therefore, sulfite dissolving pulp has generally been considered unsuitable for production of regenerated cellulose with high tenacity applications.

In the production of regenerated cellulose, the cellulose is dissolved in a solvent to form a cellulose dope which is processed to regenerate the cellulose fibers in different forms. There are a number of different methods of producing regenerated cellulose known in the art, including the viscose process, the lyocell process, the cellulose acetate process and the cellulose carbamate process.

The viscose process has been used industrially for many years for the production of rayon fibers, used in the textile industry. One advantage with the method is that it is possible to use wood as a starting material whereas several other methods need lignin-free cellulose as starting material. The viscose process can be divided into the following steps: -Mercerization

-Pre-ripening

-Xanthation

-Dissolving

-Ripening and

-Regeneration

In the Mercerization step the cellulose is typically soaked in 17-20% NaOH solution at room temperature for a few hours such that the cellulose is converted into alkali-cellulose. During this process the cellulose fibers are activated by the swelling of the fibers leading to an increased accessibility of the cellulose fibers to the chemicals in the solvent (e.g. carbon disulfide) such that the reactivity of the cellulose fibers with the chemical are increased. In addition to the increased accessibility, the breakage of intra- and inter molecular hydrogen bounds in and between the cellulose chains caused by the alkaline conditions also contributes to the increased reactivity of the swelled fibers. In the pre-ripening step, the alkali-cellulose will be degraded into desired degree of polymerization through depolymerization reaction with the oxygen in the air. The reaction can be catalyzed by cobalt or manganese. In the xanthation step, the pre-riped alkali-cellulose is mixed with carbon disulfide (CS2) resulting in a cellulose xanthate derivative which is dissolved in a dilute sodium hydroxide solution to from a cellulose xanthate solution (viscose solution). This is followed by a ripening step where some of the alkali xanthate groups are removed from the cellulose. In the regeneration process the cellulose xanthate is applied to a coagulation bath. Typically, this involves pumping of the cellulose xanthate through spinnerets, into an acid bath where cellulose is regenerated in the form of long filaments. In this process the CS ₂ will be released which enables recycling of the chemical. After this, the cellulose is stretched, washed and then undergoes further processing depending on the intended use of the fibers.

Viscose fibers produced in the regeneration of cellulose can either be used for the production of staple fibers or filaments, such as filament yarn. In the production of staple fibers, the viscose fibers are cut into short pieces after the spinning bath. These short fibers, which are each about 4 cm long, can be spun into textile yarns or processed into non-woven products. In contrast, filament yarns are winded up into very long continuous fibers which due to their strength is highly suitable as technical filaments e.g. in tire cords and in other applications where high tenacity fibers are requested.

The production of high tenacity fibers differs in some aspects from the production of viscose fibers used for production of staple fibers. For example, a higher concentration of carbon disulfide can be used in the xanthation step and the stretching of the fibers can be performed differently.

Summary of the present disclosure

It is known in the art that the characteristics of pulp can influence the tenacity of regenerated cellulose fibers and filaments produced from the pulp. Thus, production of high tenacity fibers and technical filaments generally needs a pulp having these desired characteristics. In particular pre-hydrolysed Kraft pulp with low hemicellulose content (i.e. high R18) values have been described as suitable starting material for production of high tenacity fibers and technical filaments. Sulfite pulps have been regarded as less suitable for production of high tenacity fibers and technical filaments, partly due to its high content of hemicellulose and to a broad molecular weight distribution of cellulose. It is generally accepted in the art that, in order to get optimum strength for the regenerated fibers, the polydispersity (PD) shall be as low as possible, i.e. as close to 1 as possible.

The present inventors have surprisingly discovered that a novel sulfite pulp can be produced, which despite its high content of hemicellulose and relatively broad molecular weight distribution gives rise to regenerated cellulose fibers and filaments of remarkably high strength. The PD of this pulp can be as high as 15; the R18 value as low as 93% and the R10 value as low as 92%. Compared to known sulfite dissolving pulps, this pulp has a relatively high viscosity and a lower proportion of cellulose chains having a molecular weight below 25 KDa. It also have a high content of cellulose chains having a molecular weight above 100 KDa compared to other known sulfite dissolving pulps. Thus, a first aspect of the present invention therefore relates to a sulfite dissolving pulp having:

a) a viscosity of 700-1000 ml/g;

b) a R18 value of 93-95.5%;

c) a R10 value in of 92-94%;

d) a PD of 9-15, such as 12-15;

e) a proportion of cellulose chains having a molecular weight below 25 KDa of less than 8% (w/w), such as less than 7% (w/w);

f) a proportion of cellulose chains having a molecular weight above 100 KDa of above 75% (w/w), preferably above 78% (w/w), most preferably above 80% (w/w). A second aspect of the present invention relates to use of a sulfite dissolving pulp according to the first aspect in a viscose process for the production of filaments or viscose staple fibers having a wet tenacity of at least 1 .9 cN/dtex.

A third aspect of the present invention relates to a method for production of a sulfite dissolving pulp according the first aspect comprising the following steps:

i) pulping wood chips from soft wood in a sulfite cook

such that a pulp having a viscosity of 700-900 ml/g is obtained. ii) hydrolyzing the hemicellulose present in the pulp obtained in step i) by exposing the pulp to NaOH such that a pulp having an R18 value of 93-95.5% is obtained Definitions:

Dissolving pulp: A pulp having a cellulose content of at least 90% (w/w, of bone dry pulp).

Sulfite pulp: A pulp obtained by the sulfite pulping process.

Pulp viscosity; The pulp viscosity is a measure of the average chain length (degree of polymerization) of cellulose. The viscosity is determined after dissolving the pulp in a suitable solvent such as a cupriethylene diamine solution. Methods for measuring pulp viscosity are well known for the skilled person. One such method is the SCAN-CM 15:88 method (3).

Degree of polymerization (DP): Average number of glucose units that make up the cellulose polymers in the pulp.

R18 value: The percentage of a pulp which is not soluble in a 18% sodium hydroxide solution at 20 ^Q C for 60 minutes reaction time (ISO 699:1982). R10 value: The percentage of a pulp which is not soluble in a 10% sodium hydroxide solution at 20 ^Q C for 60 minutes reaction time (ISO 699:1982).

M _n value: Number average molar mass is the arithmetic mean or average of the molecular masses of the individual macromolecules. It can be determined by measuring the molecular mass of n polymer molecules, summing the masses, and dividing by n:

M _n =∑Ni ^* Mi/∑Ni

M _w value : Weight average molecular weight. The mass average molar mass is calculated by

M _w =∑iNi ^* Mi ^* Mi/∑iNi ^* Mi where N, is the number of molecules of molecular mass M,. Polydispersity (PD): The polydispersity (PD) is a measure of the distribution of molecular mass in a given polymer sample. The PD is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual molecular masses in a batch of polymers. The PD has a value equal to or greater than 1 . As the polymer chains approach uniform chain length, the PD approaches 1 . In the art the polydispersity can also be referred to as Polydispersity index (PDI)

Detailed description

Compared to for example prehydrolyzed Kraft pulp, sulfite dissolving pulp have been regarded as less suitable for production of high tenacity fibers and technical filaments, partly due to its high content of hemicellulose and to a broad molecular weight distribution of cellulose. The present inventors have discovered that a novel dissolving sulfite pulp can be obtained which, despite a relatively high PD and high content of hemicellulose, is suitable for high tenacity applications.

As can be seen in table 1 , compared to a prehydrolyzed Kraft pulp ("high alpha pulp"), the novel pulp ("high viscosity sulfite pulp") has a relatively high PD and low R10 and R18 values. In fact, these values are similar to the values observed for the standard sulfite pulp, "standard viscosity sulfite pulp". Thus one would expect that the novel "high viscosity sulfite pulp" would be less suitable for production of high tenacity rayon and technical filaments. However, the present inventors surprisingly demonstrate that the dry tenacity of filaments produced from the novel "high viscosity sulfite pulp" is much higher than for filaments produced from the reference standard viscosity sulfite pulp. In fact it is even higher than for filaments produced from a high alpha pulp which is widely used for high tenacity applications. Also the wet tenacity of filaments produced from the novel pulp is higher than for the ones produced from the standard viscosity sulfite pulp and very close to the ones produced from high alpha pulp, see table 2. Thus, by changing a few parameters in the production process, such as cooking to a higher viscosity and modifying the E-step, a dissolving pulp suitable for high tenacity applications can be obtained from a sulfite pulp mill. This can be achieved without the need of reducing the hemicellulose content to R18 values above 93% or by changing the process such that a low PD, such as below 9, can be obtained. Thereby a pulp suitable for high tenacity applications can be obtained at a comparably low cost.

Thus a first aspect of the present invention relates to a sulfite dissolving pulp having

a) a viscosity of 700-1000 ml/g;

b) a R18 value of 93-95.5%;

c) a R10 value in of 92-94%;

d) a PD of 9-15, such as 12-15;

e) a proportion of cellulose chains having a molecular weight below 25 KDa of less than 8% (w/w), such as less than 7% (w/w);

f) a proportion of cellulose chains having a molecular weight above 100 KDa of above 75% (w/w), preferably above 78% (w/w), most preferably above 80% (w/w).

In one embodiment the proportion of cellulose chains having a molecular weight below 25 KDa is 5.0 to 8.0% (w/w) such as 6.5 to 7.5% (w/w). In one embodiment the sulfite dissolving pulp is further having:

g) a proportion of cellulose chains having a molecular weight below 10 KDa of less than 4% (w/w). In one embodiment the sulfite dissolving pulp is having a proportion of cellulose chains having a molecular weight below 10 KDa of 2 % (w/w) to 4% (w/w). In one embodiment the proportion of cellulose chains having a molecular weight above 100 KDa is 78% (w/w) to 82 % (w/w). In one embodiment the sulfite dissolving pulp has:

h) a proportion of cellulose chains having a molecular weight between 25 KDa and 100 KDa of 10-16% (w/w), preferably 12-14% (w/w). The viscosity of the pulp can be measured using the SCAN-CM 15:88 method and thus in one embodiment the viscosity is measured using the SCAN-CM 15:88 method. In one embodiment the R18 and R10 values are measured using the standard method ISO 699:1982.

Surprisingly the inventors have demonstrated that the novel pulp, despite a relatively high PD and high content of hemicellulose, is suitable for high tenacity applications. In particular the inventors demonstrate that the novel pulp is suitable for the production of technical filaments. However the pulp is also suitable for production of high tenacity staple fiber. Thus a second aspect of the present invention relates to use of a sulfite dissolving pulp according to the first aspect in a viscose process for the production of filaments or viscose staple fibers having a wet tenacity of at least 1 .9 cN/dtex. In embodiment the filaments or viscose staple fibers have a dry tenacity of at least 3.0 cN/dtex.

The novel sulfite dissolving pulp disclosed in the present application can be produced without any major changes in the process. Importantly, the process does not need to be adapted to reduce the hemicellulose content to such low levels that the R18 value is above 96% as in known pulps suitable for high tenacity applications. Neither does the process need to be adapted to reduce the PD value to below 5 as in known pulps suitable for high tenacity

applications. The novel sulfite dissolving pulp according to the present application can be produced by a method which involves pulping of wood chips from soft wood in a sulfite cook to a viscosity of 700-900 ml/g and thereafter hydrolyzing the hemicellulose present in the pulp by exposing the pulp to NaOH such that the R18 value of the obtained pulp is 93-95.5%.

Therefore, a third aspect of the present invention relates to a method for production of a sulfite dissolving pulp according the first aspect comprising the following steps:

i) pulping wood chips from soft wood in a sulfite cook such that a pulp having a viscosity of 700-900 ml/g is obtained. ii) hydrolyzing the hemicellulose present in the pulp obtained in step i) by exposing the pulp to NaOH such that a having an R18 value of 93-95.5% is obtained In one embodiment the method further comprises a step

iii) bleaching the pulp obtained in step ii) in the presence of hydrogen peroxide such that a bleached pulp is obtained.

In one embodiment the method further comprises a step of

iv) dewatering the bleached pulp obtained in step iii) such that a dewatered pulp is obtained.

In one embodiment the method further comprises a step of:

v) drying the dewatered pulp obtained in step iv) such that a dried pulp is obtained.

In one embodiment step ii) is performed using 60-90 kg NaOH per tonne of bone dry pulp, at a temperature of 85-95 °C for 80-100 minutes.

In one embodiment the method further comprises a washing step where the pulp obtained in step i) is washed with water prior to step ii).

In one embodiment the method is further comprising a washing step where the bleached pulp obtained in step iii) is washed with water prior to step iv).

Examples

Example 1

Softwood chips where cooked in a sulfite cook to viscosity of 800 ml/g. The pulping step was followed by an E-Step where the obtained pulp was treated with NaOH at a temperature of 85-95 °C for 90 minutes. 75 kg of NaOH per tonne of bone dry pulp was used in the E-step. The other steps in the process were standard processes for sulfite pulping well known to the skilled person. For example the pulp was bleached in the presence of hydrogen peroxide in a P-step and dewatered and dried by methods well known in the art.

R10 and R18 of the obtained pulp were measured by the standard method ISO 699:1982. The molecular weight distribution of the obtained pulp was analyzed using the chromatographic system PL-GPC 210 {Polymer

Laboratories), with refractive index detector. Colums: Guard column Mixed-A 20 μηπ, 7.5 ^* 50 mm and 2 Mixed-A 20 μητι, 300 ^* 7.5 mm from Polymer

Laboratories, connected in series.

Mobile phase: 0.5% (w/v) LiCI/DMAc(N,N-Dimethylacetamide).

Flow rate: 1 ml/min.

Temperature: 70 °C.

Injection volume: 100 μΙ.

Calibration standards: Pullulan polysaccharide standards from Polymer Lab with molecular masses of 708000, 344000, 200000, 107000, 47100, 21 100, 9600, and 6100 dalton.

Calculation program: Cirrus GPC software from Polymer Lab.

Sample dissolution: 25 g of pulp is solvent exchanged three times with five ml of methanol for 30 minutes followed by three times for 30 minutes with DMAc. The excess of DMAc is then removed and 5 ml of 8% (w/v) LiCI/DMAc is added, and left overnight at room temperature with mild magnetic stirring. Before analysis this sample is diluted with 20 ml of DMAc.

M _w , M _n and PD was calculated by the software. The proportion of cellulose chains having a molecular weight below 20 KDa (w/w) was calculated using the program Cirrus GPC, Polymer lab. R10, R18, PD, M _w , M _n and the proportion of cellulose chains having a molecular weight below 25 KDa (w/w) of the obtained pulp is shown in table 1 ; "high viscosity sulfite pulp". The same values for two reference pulps; a standard sulfite pulp, "Standard viscosity sulfite pulp", (Domsjo Fabriker) and a prehydrolyzed Kraft pulp, "High alpha pulp" are also shown.

Table 1

Example 2

Filaments were produced from the three pulps described in example 1 by a commercial standard method for technical fibre production. Wet tenacity and dry tenacity of three types of filament were investigated by the standardized method SSE-ISO 5079. As can be seen in table 2, the dry tenacity of filaments produced from the novel pulp "High viscosity sulfite pulp" is much higher than for the filaments produced from the standard viscosity sulfite pulp and even higher than for the filaments produced from high alpha pulp. Also the wet tenacity of the filaments produced from the novel pulp is higher than for the filaments produced from the standard viscosity sulfite pulp and very close to the ones produced from the high alpha pulp. Also wet elongation are on same level in filaments from high viscosity pulp as from high alpha pulp; 24.2 respectively 24.5. These results are surprising since the R18, R10 and PD values of the novel pulp is much higher compared to pulps that previously have been used for high tenacity applications, see table 1 .

Table 2

REFERENCES

1 . Christoffersson (2005) "Dissolving pulp : Multivariate Characterisation and Analysis of Reactivity and Spectroscopic Properties" Ph.D thesis Umea

University.

2. Sixta, et al "Evaluation of new organosolv dissolving pulps. Part I: Preparation, analytical characterization and viscose processability". (2004) Cellulose 11 (1 ): 73-83

3. SCAN Test Methods, Scandinavian Pulp, Paper and Board Committee, Sweden, (1988).