Wear ..-- Else-&r Sequoia S.A., Lausanne - Printed in the Netherlands 149

FIBRE LUBRICATION IN YARN SPINNING*

H P. STOUT

.Sc.dtish Textile Research Associatim., Dundee (Gt. Britain)

(Received November I, 1969; in revised form January 15, 1970)

SUMMARS

The role of lubrication and friction in yarn spinning is briefly reviewed. The types of lubricants and friction-increasing additives used are described and their mech- anism of action in controlling the important ratio of the fibre-fibre coefficient of friction to the fibre-pin coefficient of friction is discussed.

INTRODUCTION

Friction plays an important part in many aspects of textile processing, but nowhere is it more important than in the production of yarn, or thread, from staple fibres.

The basic stages of yarn production involve, first, the conversion of a bulk of fibres into a continuous, ribbon-like, strand of overlapping fibres. This strand may contain thousands of fibres in its cross-section and it must next be drawn down, or drafted, to reduce its thickness to the size of the desired yarn. The drawing is done in several stages, and, as the final yarn is very thin, twist is inserted in the last stage to hold the fibres together and to give the yarn strength.

In spinning the jute yarns which form the underside of carpets, for example, the linear density of the initial fibre strand may be 300 kg/km and that of the yarn 300 g/km so that a total attenuation in the region of 1,000 is required. This means that, ide- ally, I km of the initial fibre strand becomes 1,000 km of yarn, and IOO,OOO fibres in the initial cross-section are reduced to IOO in that of the yarn.

The attenuation is affected by feeding the strand into a drawing frame at a fixed speed and then arranging that, as the leading ends of the fibres enter a par- ticular zone of the machine, they are accelerated and delivered from the drawing frame at a higher speed. The ratio of output to input speeds is the draft, or degree of attenu- ation.

Ideally, each fibre would be accelerated individually, but, in practice, one fibre tends to drag other fibres with it so that groups of fibres move together. This produces thick places in the emergent strand which will, necessarily, be followed by thin places and the final yarn is thus irregular in thickness. To prevent too much group movement of fibres, restraining forces are introduced by roller nips or by running the fibres through rows of pins like a comb.

* Paper presented at the Symposium on “The Effect of Temperature on Lubrication Systems”. Paislelr College of Technology, Paisley, Scotland, October x4-17, 1969.

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A high fibre-pin coefficient of friction will help to prevent fibrcls t)c,ing dragged forward out of turn, as also will a low fibre--fibre coefficient. Too lligll ;I valuc~ of fihrt

pin friction must be avoided, however, or other difficulties will arise, whilst too low a value of fibre-fibre friction will make the fibre strands bulky and not sufficientI>. coherent, as well as increasing the slippage between fibres in thrl twisted yarn, thus lowering its strength.

The interplay of the different coefficients of friction, which are altered to different extents by different lubricants, is complicated and not all are of equal im- portance. By suitable choice of lubricants, considerable improvements c-an be made, Ilowever, although it is impossible to produce staple yarns which are uniform in thick- ness, like wire, and some irregularity is inevitable. The strength of a yarn depends greatly on this irregularity, because, in general, breakages occur at the thinnest places and thus, for any purpose in which strength is important, tllc best !.arns arc’ usually those which have the most uniform thickness.

X useful practical measure of irregularity is the Percentage Mean Deviation (P.M.D.) of a mass of short lengths, say I in. obtained either by cutting and weighing, or by some method which scans the yarn continuously. The measure of strength is commonly taken as the breaking load of single threads of yarn, each 24 in. long.

The foregoing discussion applies to all types of staple fibre, but for purpose of illustration some effects obtained in jute yarn spinning will now be described.

LITRIII(‘ATION WITH MINEKAL OIL

Cnlike some other natural fibres, jute has only a low content of fats and waxes, and some kind of lubrication is essential. In the early days of the industry, whale oil was in common use, but nowadays the usual lubricant is mineral oil applied as an emulsion in water. The amount added ranges from about 0.75-5 wt,“b in particular cases, with an addition of I-I .5 Tb p roviding optimum results. Less than I yh results in a marked fall-off in yarn strength, whilst above z ‘$4 the yarn strength falls steadill but more slowly.

The mineral oil used is of low viscosity, around zoo set Redwood at 70°F, and, whilst small variations in this figure are of little consequence, more substantial varia- tions in viscosity can produce a marked effect on the yarn strength.

Mineral oils are absorbed to some extent into the body of the fibre through crevices in the surface and internal holes. Low-viscosity oils are absorbed quickly leaving the surface relatively oil-free, corresponding to boundary lubrication condi- tions. High-viscosity oils have a slower rate of absorption into the fibre and tend to remain on the surface giving rise to hydrodynamic conditions. There is, in fact, an optimum viscosity in the region of 250-300 set Redwood which gives the maximum yarn regularity. By the time the viscosity has increased to 5,000 set the yarn reg- ularity and strength have deteriorated substantially.

It is not common commercial practise to scour jute yarns to remove the added oil before use, but if this is done, the strength of the yarn is generally increased by 5 or b:i. This effect is due to increased friction between the fibres on removal of the oil, which helps to prevent slippage of the fibres over one another before they break and thus allows the fibres to sustain a greater load. A similar effect can be demonstrated 1,). adding friction-increasing substances.

FIBRE LUBRICATION IN YARN SPINNING 151

~Ii~CTI~~-INCREASING ADDITIVES

If a friction-increasing additive such as colloidal silica is added to the fibres before processing, the regularity of the yarn is considerably reduced, as might be expected from the earlier discussion, and this results in a corresponding fall in strength. By adding colloidal silica, not to the fibres before processing, but to the yarn as it is being twisted on the spinning frame, however, a very different effect is obtained. The regularity of the yarn is now unchanged, because all the drafting has been done prior to the application, but the strength may be increased by IO;/, or more depending on the amount added. In this case also, the fibres are prevented from slipping over one another and are thus able to sustain a higher load. When the fibres do break, it is generally found that the elongation of the yarn has been increased slightly.

Similar effects of colloidal silica may be demonstrated with viscose rayon fibres, and in this case the effects seem to be more marked with fine-denier fibres rather than coarse ones. This is presumably due to an increased number of fibre-fibre con- tacts in yarns containing low-denier fibres.

ORGANIC LUBRICANTS

Although mineral oils are in common use as inexpensive lubricants, many chemical firms supply proprietary additives of an organic chemical nature which confer particular properties on fibres. These additives are often of a long-chain nature in which fatty acids or alcohols are condensed with varying numbers of ethylene oxide groups.

The basic frictional effects of these additives are summarised in an early paper by R&xR’, in which it is shown that, with viscose rayon fibres, the coefficient of static friction between fibres increased from 0.175 to 0.198 as the number of ethylene oxide groups attached to stearyl alcohol increased from 4 to 15. A similar effect was observed with stearic amide except that the coefficients of friction were about 0.01

lower in each case. Increasing carbon chain length in the fatty part of the additive reduced the coefficient of friction, although the double bond of oleic acid caused an increase. There was little difference to be found between the effects of carboxyl and alcohol groups, but an amide group gave a marked decrease in friction.

To examine the effects of similar compounds in jute spinning, experiments were made with nonyl phenol, oleyl cetyl alcohol, and stearic acid, each condensed with varying numbers of molecules of ethylene oxide. Stearic acid has a much longer carbon chain than nonyl phenol, whilst oleyl cetyl alcohol, although having much the same number of C atoms as stearic acid, is a branched chain and also contains the oleic acid double bond.

Without detailing the results, the P.M.D. values, averaged over all the ethy- lene oxide chain lengths, were 22.5% for nonyl phenol compared with 17.5% for stearic acid and 18.6~/~ for oleyl cetyl alcohol. The longer chain lengths thus produce more regular yarns, whilst the double bond and branched chain is not as effective as the straight chain.

Average P.M.D. values corresponding to different numbers of ethylene oxide molecules were 15.7% for z-4 molecules, 19.4:/o for 8 molecules, and 20.3% for 12-14 molecules, showing that the lower the ethylene oxide content, the more regular the yarn.

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These results are in general agreement with Kor)ei:‘s frictictn Illtl;i’;llr~‘nlCIlt~, and demonstrate that increasing ethylene oxide content increases fibre fibre frictictu and diminishes the degree of regularity of the yarn. Similarly, increasing the C‘ ciiain length of the fatty acid or alcohol reduces friction and improves the J-arn regularity. A double bond appears to be less effective in reducing friction than a single bond.

It is tempting to relate the frictional effects direct]\; to the c-hemical nature ot the additive. Unfortunately, the physical state of the chemicals is not alwa!~ the same and whilst the nonyl phenol condensates are liquid, the stearic acid condensates var\ from soft to hard solids.

Some effect of physical state is to be expected, in view of the viscosity effects already mentioned. In the case of some paraffin wax dispersions made from waxes of varying degrees of hardness, it was found that, although the regularity of tlie yarn was much the same for all the waxes, the strength of the varn was reduced as thtx hardness of the wax increased.

In conclusion, reference should be made to a recent article b>. SPENCXR- SMITH ANI) Tor)b” dealing with the irregularity of staple yarns spun on flax-type machinery. They conclude that the ratio of the fibre -fibre coefficient of friction to the fibre-yin coefficient is all important, and regularity of thickness seems likely to require the ratio to be kept as small as possible. The best value depends on othci features of the machinery such as density of pinning and linear density of fi bre strands. There is no doubt, however, that suitable choice of a lubricant, perhaps better termed a “surface finish”, is an itnportant feature of staple-fibre spinning.

The organic con~l?t)unds used were obtained through the courtesy of Imperial Chemical Industries Ltd., and the author is indebted to J. EARNSHAW for his ready cooperation.

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