GB2081175A - Production of Crimped High Tenacity Rayon Fibres - Google Patents

Abstract

Rayon fibres having a wet modulus of at least 0.5 grams per denier are prepared by spinning a viscose solution having a salt index between 2.5 and 6 into a coagulating spin bath containing 0.8 to 3.9% sulphuric acid 1-6 wt% ZnSO4 and 10-20% Na2SO4, stretching the resulting incompletely regenerated filaments and completing the regeneration of the cellulosic filaments in an acid bath at above 80 DEG C. The viscose solution is prepared from an unbalanced ratio of sodium hydroxide to cellulose of from 0.60 to 0.87:1. The ratio of acid in the spin bath to sodium hydroxide in the viscose solution ranges from 0.20 to 0.66:1.

Description

SPECIFICATION Production of High Performance Rayon Fibres This invention relates to a process for the production of high performance, high wet modulus rayon filaments and fibres.

Regular low wet modulus rayons normally are prepared from a viscose containing 8 or 9% cellulose and 5 to 6% caustic soda: the acid in the spin bath usually being much in excess of that required to neutralize the alkaline caustic soda - 6.5% to as high as 9% or more of acid. Low modulus rayons when converted to fabrics have a tendency to shrink when laundered, the principal cause of shrinkage being their low resistance to stretching when wet (i.e. low modulus). Rayon researches have found that when the wet modulus of rayon is sufficiently high (over 0.5 g/d), progressive shrinkage is essentially controlled and a more stable fabric and garment result.

On the other hand, in the production of high wet modulus (HWM) rayon fibres, that is fibres having a wet strength of at least about 0.5 grams per denier, it is customary to use a "rich" viscose composition in which the amount of caustic soda is essentially equal to the amount of cellulose. For example, our U.S. Patent 3,720,743 discloses a process for the production of HWM rayon fibres having a number of desirable properties in addition to high wet strength. The patent indicates that a balanced ratio, for example, of 7.5% cellulose and 7.5% Caustic soda should be used. U.S. Patents 3,632,468 and 4,121,012 are similarly directed to HWM rayon fibres prepared from balanced ratios of caustic and cellulose.

So-called poiynosic rayons, having high strength and modulus, are prepared from relatively smaller (and unbalanced) amounts of cellulose and caustic. However, the polynosic rayons differ from other HWM rayons in both their properties and their preparation. Because of the high degree of polymerization of the cellulose required for these rayons, high viscosity viscoses are used. High salt indices (about 20) are very large amounts of CS2 are necessary to effect solubility and good spinnability. Low concentrations of acid at low temperatures in the spin bath are also necessary to form the fibrillar polynosic structure. Such fibres, while they have high wet modulus properties, have low elongations, in the range of 7 to 10%, which result in difficulties in conversion to yarn and fabric.

Polynosic fibres made by such processes are usually brittle and have accordingly achieved only limited commercial acceptance.

Thus, it would be desirable to produce dimensionally stable, strong high performance rayons with high wet modulus, using, however, more economical conditions than have heretofore been thought possible.

According to the present invention there is provided a process for the production of crimped high tenacity rayon filaments and fibres having a wet modulus of at least 0.5 g/d and a wet elongation of about 11-1 7% comprising preparing a viscose solution containing a mixture of viscose modifiers which substantially retard regeneration, said modifier viscose solution having a salt index between about 2.5 and 6, spinning said viscose solution into a coagulating spin bath containing sulphuric acid, from 1 to 6% by weight zinc sulphate and from 10 to 20% by weight sodium sulphate, immediately stretching the resulting incompletely regenerated filaments and completing the regeneration in an acid bath maintained above 80"C and relaxing the tension of said cellulosic filaments to permit crimp development, wherein said viscose solution is prepared from an unbalanced ratio by weight of sodium hydroxide to cellulose ranging from 0.60 to 0.87 and said spin bath contains from 0.8 to 3.9% by weight of sulphuric acid, the ratio by weight of acid in said spin bath to sodium hydroxide in said viscose solution ranging from 0.20 to 0.66.

It has been found quite unexpectedly that high performance, HWM rayon fibres having a desirable level of elongation may be produced by the use of an unbalanced ratio of cellulose and caustic in the viscose if it is accompanied by a reduced amount of acid in the primary spin bath.

It should be noted that the reduction of only a percentage or two in the amount of caustic soda and acid used in the viscose process has considerable economic significance. These are two of the major raw materials used, which when neutralized, produce water and salts. Evaporation of the water and recovery of the salt are energy intensive operations. By reducing the quantities of the raw materials used in the process, significant energy and cost savings are effected.

It was quite surprising to find that HWM fibres of outstanding quality could be spun from leaner viscose compositions, i.e. viscose compositions containing less caustic by weight than cellulose. The key to HWM properties was thought to be high caustic. Ordinarily, when the caustic content is lowered and all other spinning conditions are held constant, the wet elongation increases and the wet modulus properties of the resulting fibre deteriorate dramatically. Most commercially produced HWM fibres are spun at acid levels which cannot be lowered without encountering spinnability problems. Often poor spinning, under normal HWM process conditions, can be improved simply by increasing the primary bath acid concentration and this is frequently done in commercial spinning operations.We have now found that for high wet modulus production, the reduced caustic requires less acid not only less acid resulting from less caustic requiring neutralization, but less acid so that the fibres set up more slowly, and stretchability is improved. With balanced ratios of caustic and cellulose, the acid concentration cannot be lowered without adversely impacting fibre spinning, resulting in a condition known to the art as slubbing. The present invention involves the discovery that the caustic level may be reduced in the viscose only if the level of acid in the spin bath is also reduced. Not only does this prevent adverse impact on spinnability and fibre properties, but the fibre properties, principally wet modulus, are actually improved.

Generally, the process of the invention involves the preparation of a viscose solution from cellulose xanthate. Purified chemical cellulose, such as bleached sulphite and prehydrolyzed kraft wood pulps as well as cotton linters having a relatively high uniform degree of polymerization are converted into alkali cellulose by steeping in sodium hydroxide, aged to a cuene l.V. (intrinsic viscosity) of from 2.0 to 3.6 dl/g (decaliters/gram) and xanthated with 26 to 40% by weight of carbon disulphide, based on oven dried cellulose weight, at approximately ambient temperatures (e.g. 200--300C). The amount of carbon disulphide is not critical as long as the salt index is correct and viscose filterability is satisfactory. The viscose solution is modified with a regenerating retardant of the type shown, for example, in U.S.Patent 2,942,931 and which preferably comprises from 0.5 to 2.5% each of dimethylamine and polyethylene glycol. Alternatively, modifiers such as ethoxylated amines sold under the trademarks Ethomeen C-25 and Leomin-AC80 can be used in place of dimethylamine, and branched chain polyglycols such as those sold under the trademark Berol Visco 399 may be substituted for polyethylene glycol. The salt index of the viscose spinning solution should be between 2.5 and 6 and the gamma number between about 20 and about 45 when spun with ripening selected to attain this level. The specific salt index and gamma number depend upon the amount of carbon disulphide used in xanthation and the temperature and time of ripening used.The viscosity of the spinning solution is not particularly critical and can range between about 50 and 1 50 ball fall seconds, or between 75 and 225 poise, measured at 200C. The viscose solution is prepared from an unbalanced ratio by weight of sodium hydroxide to cellulose ranging from 0.60 to 0.87 and preferably from 0.65 to 0.80. The amount of cellulose should be from 59%, preferably 68%, by weight of the solution. The amount of sodium hydroxide should be from 91 7%, preferably from 56%, also by weight of the solution.

The coagulating spin bath contains a sulphuric acid concentration of from 0.80 to 3.9%, preferably 2.5 to 3.5% by weight of the bath. The ratio by weight of acid in the spin bath to sodium hydroxide in the viscose spinning solution should range from 0.20 to 0.66, preferably from 0.40 to 0.60. The spin bath should also contain about 1 to 6% by weight of zinc sulphate and from 1020% by weight of sodium sulphate. It may also contain from 0.01 to 0.1 percent by weight of a surface active agent or lubricant such as lauryl pyridinium chloride, and a modicum of regeneration retardants carried in with the tow.

The deaerated viscose is spun through a spinnerette into a coagulating spin bath at about 30C to 450C. Travel of the filament though the primary spin bath should be limited to that required to develop sufficient strength for stretching, in order to avoid unnecessary regeneration, with the greater percentage of stretch achieved prior to substantial regeneration. Immediately after leaving the spin bath, the filaments as a group or tow, and while they are substantially soluble in dilute alkali, are stretched from about 100 to 300 percent. To effect this stretch, the tow is drawn from the bath the desired distance, passed several times around a driven godet to prevent slippage and then several times around one or more stretch rolls driven at a sufficiently greater speed to provide the desired continuous stretching.

Since filaments made by the procedures described above are highly plastic and relatively strong immediately upon extrusion into the coagulation bath, it is important to stretch as quickly and as much as possible prior to regeneration in order to obtain the desired high wet modulus. The strongest filaments with the highest wet modulus are produced when the stretching takes place immediately after the onset of coagulation in a gradual fashion. For example, when only one godet and one stretch roll are used, the stretching occurs on, or as the tow leaves, the first roll and is then completed as the tow of filaments passes through the regeneration bath or baths.

To facilitate stretching and to regenerate the coagulated filament tow, it is conducted through one or more hot regeneration baths of sufficient length which contain hot dilute acid. We prefer dilute acid baths for our purpose, maintained at 80C to 1000C (preferably about 950--980C), in any event, sufficiently hot to substantially regenerate the newly formed filaments in the tow. The stretch bath or baths contain from about 0.5 to 3.0 percent sulphuric acid and a stabilized modicum of salts carried over from the preceding coagulation bath.

After the filaments are substantially regenerated, tension is reduced or removed to permit crimp development. Following relaxation, the filaments are treated with hot dilute acid, desulphurized, neutralized, washed, finished and dried by conventional techniques.

Alternatively, the filaments can be cut into staple fibres which develop a high degree of crimp on relaxation. These highly crimped staple fibres are acidified, desulphurized, neutralized, washed, finished and dried by conventional techniques. Cutting of the two into staple fibres is usually performed in the acid state.

The finishing of the crimped filaments and staple fibres should be balanced to preserve crimp, high modulus and high strength, while building adequate elongation for good conversion properties and good wear and abrasion-resistance in end-products. Use of a commercially available staple fibre finishing agent is advantageous to insure processability for efficient conversion to yarn and fabric.

Fibres produced in accordance with the invention may have a variety of cross-sectional configurations and degrees of crimp, primarily dependent on the amount of acid used in the spin bath for a given caustic to cellulose ratio in the viscose. The cross-sectional configuration may vary from essentially circular and with the least amount of crimp, spun at acid concentrations within the lower portion of the claimed range, to bilobal filaments at intermediate acid levels having the most crimp. At the higher end of the acid range, multilobal cross-sections are obtained. The properties of the fibres produced in accordance with the invention are at least the equivalent of high performance fibres produced from prior art, rich caustic, viscose solutions and higher acid spin baths, while the wet modulus properties are in many instances higher.Properties of the fibres of the invention will have the following general ranges: Wet modulus 0.5-1.2 g/d maximum 12% Conditioned Tenacity 3 4 g/d Conditioned Elongation 10-1 5% Wet Tenacity 2-3 g/d Wet Elongation 1117% Wet modulus as measured herein is the wet tenacity in grams per denier at 5% elongation. The minimum and maximum wet modulus values have been rounded off to the first decimal place. Thus, a wet modulus valve of 0.46 is considered to fall within the above range. "Sa 5" is a measure of the fibre's resistance to laundering and specifically the solubility of the fibre in 6.5% NaOH at 200C. The tenacity values are in accordance with ASTM test number D-1577-66, using 1/2 inch gauge length.

The elongation values are in accordance with ASTM test number D-540-64.

The following examples illustrates the practice of the invention. Unless otherwise indicated, all parts and percentages are by weight.

Example 1 This example shows the preparation of a prior art HWM fibre produced by a process similar to that described in the aforesaid U.S. Patent 3,720,743. A modified viscose spinning solution was produced from chemical cellulose which was prepared by steeping a high purity wood pulp, having an S10 of 2.2 percent and an 818 of 1.4 percent. (S10 and S18 are a measure of the solubility of the fibre in 10% and 18% NaOH, respectively, at 250C). The thus formed alkali cellulose contained, after being pressed, 34% cellulose and 1 5% sodium hydroxide. The alkali cellulose was shredded and aged to a cuene l.V.

of about 3.0 dl/g. It was then reacted with 32% carbon disulphide, based on the weight of cellulose, at 300C to form cellulose xanthate which was dissolved in sodium hydroxide at 1 00C and mixed for two hours to provide a balanced viscose solution containing 7.5% cellulose and 7.5% sodium hydroxide. To this viscose solution, 1.3% dimethylamine and 1.3% polyethylene glycol of M.W. 1540 (both based on the weight of cellulose) were added. The viscose solution was ripened to a salt (NaCI) index of 5.0.

The well-deaerated viscose then was extruded through a cluster of spinnerettes with 24,200 holes -- 0.0025 inch diameter each, into a primary acid coagulating-type spin bath containing 5.0% sulphuric acid, 1 5% sodium sulphate, and 2.8% zinc sulphate at a temperature of 400C. The coagulated filament tow was wrapped around a godet and led through a hot secondary acid bath to a wash reel on which it was wrapped several times to prevent slippage.

The secondary acid bath contained 3.0% sulphuric acid and residues of salts carried over from the primary bath. It was maintained at about 95 to 980C. The tow was spun at 30 meters per minute and stretched through the secondary bath at 120%.

The tow was collected wet, cut into staple fibre lengths, washed, desulphurized, and finished in the usual manner with a staple fibre finish. After drying and conditioning, single filament test were run under standard procedures. The single filament fibre physical property test results are shown in Table I.

The ratio of NaOH/cellulose in this Example was 1:1 and the ratio of H2SO4 in the spin bath to NaOH in the viscose was 0.67.

Example 2 The process of Example 1 was substantially repeated except that the primary bath sulphuric acid concentration was reduced to 3.5%. The lower acid level resulted in unacceptable extrusion occurring at the spinnerette face in the primary bath (i.e. slubbing) which resulted in unregenerated, uncoagulated viscose in the tow which prevented it from being cut into staple or processed properly.

Example 3 The rayon fibre of this example is typical of commercially produced regular rayon staple.

A viscose spinning solution was produced by steeping a chemical cellulose wood pulp, having an S10 of 8.8 and an S,8 of 4.4. The thus formed alkali cellulose was pressed, shredded, and aged to a cuene l.V. of 2.2 dl/g. it was then reacted with 28% carbon disulphide, based on the weight of cellulose, to form cellulose xanthate which was dissolved in sodium hydroxide and mixed to produce a viscose solution containing 9.0 cellulose and 5.0% sodium hydroxide. The viscose was ripened to a salt (NaCI) index of 4.0 and deaerated.

It was then extruded through a spinnerette into a primary acid coagulating-type spin bath containing 6.8% sulphuric acid, 21% sodium sulphate, and 1.0% zinc sulphate at a temperature of 550C. The coagulated tow was wrapped around a godet several times to prevent slippage and stretched 50% while spinning at 100 meters per minute.

The tow was collected wet, cut into staple fibre lengths, washed, desulphurized, and finished in the usual manner with a staple fibre finish, After drying and conditioning, single filament fibre physicial tests were run under standard procedures. The results are given in Table I.

The ratio of NaOH to cellulose in this Example was 0.56 and the ratio of H2SO4 in the spin bath to NaOH in the viscose was 1.36.

Example 4 This example illustrates the practice of the present invention. The HWM rayon of this example was produced by the process of Example 1 except that an unbalanced viscose composition of 7.5% cellulose and 4.5% sodium hydroxide was used.

It was spun under the same conditions except that 1.0% sulphuric acid was contained in the primary spin bath. All other process variables were the same. Fibre physical test results are listed in Table I.

The ratio of NaOH to cellulose was 0.6 and the ratio of H2SO4 in the spin bath to NaOH in the viscose was 0.22.

Example 5 The HWM rayon of this example was produced by a process similar to that in Example 4 except that an unbalanced viscose composition of 9.0% cellulose and 6.0% sodium hydroxide was used and was prepared at a cellulose l.V. of 2.3 dl/g and ripened to a 4.9 salt (NaCI) index.

It was spun under the same conditions except that the primary bath contained 3.5% sulphuric acid. All other process variables were the same except that stretch was 130%. Fibre physical test results are given below in Table I with results from fibres produced as described in Examples 1-5.

Table I Ex. I Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ratio: 1.0 1.0 0.56 0.6 0.67 NaOH/Cellulose in viscose Ratio: 0.67 0.47 1.36 0.22 0.42 H2SO4 in spin bath NaOH in viscose Tenacity g/d Conditioned 3.5 would 2.8 3.5 3.7 Wet 2.2 1.5 2.2 2.4 Elongation % Conditioned 13 not 20 13 12 Wet 15 25 15 13 Wet Modulus g/d 0.5 0.2 0.5 0.8 Full Wave spin Crimps/in. 15 10 20 18 Example 6 A process similar to that of Example 4 was used to produce the fibre of this example except that an unbalanced viscose composition of 8.5% cellulose and 6.0% sodium hydroxide was used to prepare the viscose. The cellulose in viscose was at l.V. 2.2 dl/g: it was ripened to a 5.1 salt (NaCI) index, and 1.3% Ethomeen C-25 (an ethoxylated amine) and 1.3% polyethylene glycol were added (based on the weight of cellulose) as regeneration retardants.

The viscose was spun into a primary bath containing 2.9% sulphuric acid. All other process variables were the same. Fibre physical test results are given in Table II. The ratio of acid in the spin bath to NaOH in the viscose was 0.48.

Example 7 The HWM rayon produced in this example was prepared similar to that of Example 6, using similar viscose composition, cellulose cuene l.V., and other parameters, except that the regeneration retardants used were dimethylamine and a branched polygycol, Berol Visco 399 (1.3% of each). All other viscose and spinning conditions were the same. The fibre physicial test results are listed in Table II together with those of Example 7.

Table II Example 6 Example 7 Ratio: 0.71 0.71 NaOH/Cellulose in viscose Ratio: 0.48 0.48 H2SO4 in spin bath/ NaOH in viscose Modifiers Ethomeen C-25 dimethylamine polyethylene Berol Visco 399 glycol Tenacity g/d Conditioned 3.5 3.6 Wet 2.2 2.3 Elongation, % Conditioned 1 5 13 Wet 17 15 Wet Modulus, g/d 0.5 0.7 Full Wave 22 18 Crimps/in.

Example 8 In this example, fibres were prepared with progressively decreasing amounts of acid and acid to caustic ratios.

A modified viscose spinning solution was produced in much the same manner as in Example 1 including steeping, pressing, shredding, aging, xanthating, and mixing operations. However, an unbalanced viscose composition of 7.5% cellulose and 6.0% sodium hydroxide was used. The viscose was ripened to a salt (NaCI) index of 5.2 with a cuene l.V. of 2.8 dl/g resulting in a viscosity of 100 ball fall seconds at 200C. The modifiers, dimethylamine and polyethylene glycol were added to the viscose at 1.3% each based on the weight of cellulose. 30% carbon disulphide was injected during xanthation.

The deaerated viscose was extruded through a cluster of spinnerettes with 24,200 holes (0.0020 inch diameter each) into a primary acid coagulating-type spin bath containing 2.9% sulphuric acid, 1 5% sodium sulphate, and 2.8% zinc sulphate at a temperature of 400 C. The coagulated tow was wrapped around a godet and led through a hot secondary acid bath to a wash reel on which it was wrapped several times to prevent slippage.

The secondary acid bath contained about 2.0% sulphuric acid and residues of salts carried over from the primary bath. It was maintained at about 950 to 980C, and the tow was spun at 30 meters per minute and stretched 130% through the secondary bath.

The tow was collected wet, cut into staple fibre lengths, washed, desulphurized, and finished in the usual manner with a staple fibre finish. After drying and conditioning, single filament tests were run under standard procedures. The single filament fibre physical property test results are shown in Table Ill as a family of fibres with different primary bath acid concentrations. The acid level had a direct effect on secondary bath stretch and fibre wet elongation and modulus; i.e., the lower the acid concentration with respect to caustic in viscose, the higher the modulus.

In Table Ill, Sample 1 is outside the scope of the invention. At acid levels of 4% and higher, the wet elongation is too high and there is some sacrifice of wet modulus. It will be noted from this table that wet modulus and wet elongation properties are directly related to acid level. The lower the acid, the lower the wet elongation and the higher the wet modulus. The relationship is essentially linear with respect to both of these properties. The invention thus makes possible the ability to control wet elongation and modulus by control of acid level. Insofar as is known, this has never previously been possible.

Table 111 H2SO4 %H2SO4 in Prim. - Stretch Conditioned Wet Wet Crimp, Full Sample Bath% %NaOH % Denier Ten., gld Elong., % Ten., gld Elong., gld Mod., gld Waveslin.

1 4.6 0.76 110 1.48 3.50 13.7 2.20 18.8 0.37 14 2 3.9 0.66 119 1.43 3.28 12.3 2.35 15.6 0.46 17 3 3.5 0.58 121 1.40 3.59 11.8 2.37 14.6 0.53 18 4 3.2 0.54 125 1.47 3.30 11.2 2.32 14.3 0.60 16 5 2.9 0.49 132 1.46 3.35 11.3 2.34 13.4 0.64 16 6 2.6 0.44 139 1.51 3.31 10.7 2.28 13.3 0.68 15 7 2.1 0.25 148 1.58 2.93 9.5 2.10 12.8 0.80 13 8 1.7 0.28 211 1.62 2.99 9.5 2.01 11.4 0.82 12

Claims (10)

Claims

1. A process for the production of crimped high tenacity rayon filaments and fibres having a wet modulus of at least 0.5 g/d and a wet elongation of about 1117% comprising preparing a viscose solution containing a mixture of viscose modifiers which substantially retard regeneration, said modified viscose solution having a salt index between about 2.5 and 6, spinning said viscose solution into a coagulating spin bath containing sulphuric acid, from 1 to 6% by weight zinc sulphate and from 10 to 20% by weight sodium sulphate, immediately stretching the resulting incompletely regenerated filaments and completing the regeneration in an acid bath maintained above 800C and relaxing the tension of said cellulosic filaments to permit crimp development, wherein said viscose solution is prepared from an unbalanced ratio by weight of sodium hydroxide to cellulose ranging from 0.60 to 0.87 and said spin bath contains from 0.8 to 3.9% by weight of sulphuric acid, the ratio by weight of acid in said spin bath to sodium hydroxide in said viscose solution ranging from 0.20 to 0.66.

2. A process as claimed in Claim 1, wherein the ratio by weight of sodium hydroxide to cellulose is from 0.65 to 0.80.

3. A process as claimed in Claim 1, wherein from 5 to 9% by weight of cellulose is present in the viscose solution.

4. A process as claimed in Claim 3, wherein from 4 to 7% by weight of sodium hydroxide is present in the viscose solution.

5. A process as claimed in Claim 1, wherein from 2.5 to 3.5% by weight of acid is present in the spin bath.

6. A process as claimed in Claim 1, wherein the acid to sodium hydroxide ratio is from 0.40 to 0.60.

7. A process as claimed in Claim 1, wherein the viscose spinning solution has a cuene l.V. of from 2.0 to 3.6 dl/g and is prepared from 26 to 40% by weight of CS2, based on cellulose weight.

8. A process as claimed in Claim 1, wherein the filaments are cut into fibres which fibres have at least 12 full-wave crimps per inch.

9. A process for the production of high tenacity rayon filaments and fibres having a wet modulus of at least 0.5 g/d substantially as herein described with reference to Examples 4 to 7 or Example 8 (excluding sample 1).

10. High tenacity rayon filaments or fibres produced by a process according to any one of the preceding claims. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~