|INDEX 93 CONGRESS
SESSION 5c - NONWOVENS MANUFACTURING
HIGH PRESSURE HYDROENTANGLEMENT OF CELLULOSIC FIBRES
Calvin R Woodings
Courtaulds, United Kingdom
IntroductionThe hydroentanglement process is unique among nonwovens bonding systems because it allows the production of lightweight fabrics which accurately reflect the characteristics of the fibres used. Latex bonding and thermal bonding both weld fibres together, restricting the interfibre movement which is fundamentally important to fabric texture and drape. In addition, latex bonding covers the fibre surface with a polymer film which, in the case of cellulosic materials, completely masks the soft feel of the fibres.
Hydroentanglement (HE) is therefore important in the development of fibres for nonwovens because it allows the effects of changes in fibre to be evaluated in fabric form without the dilution of fibre effects which accompany the other bonding systems. Furthermore, it is particularly suited to the development of durable materials because the entanglement mechanism is a two-dimensional analogue of the fibre twisting mechanism used to make yarns. This gives it the potential to make, in a single operation, nonwovens which look and feel like woven or knitted textiles.
In 1987, recognising the relevance of HE to the long term future of textile materials and fibres, Courtaulds Research installed a small HE pilot line. Early work involved the treatment of webs with water pressures up to 100 bar (1400 psi) and demonstrated the "durables potential" of the approach. At the same time early trials indicated that some fibres could be split or fibrillated by the water jets. Further work on a higher pressure system at Perfojet (up to 140 bar - 2030psi) confirmed these observations and led to the development and installation of a new high pressure pilot line for fibre and fabric research at Courtaulds in Coventry.
This paper presents more of the data generated on this new machine in the course of the ongoing programme aimed at developing better cellulosic fibres for the nonwovens industry. The data chosen compares the two HE systems now available in Courtaulds and illustrates the effects of water pressure in the 100 - 220 bar range (1400 to 3080 psi) on the properties of various cellulosic nonwovens.
ExperimentalFigure 1 illustrates the bonding section of the machine. It is set-up to receive webs directly from an inclined wire wet lay pilot line (Neue Bruderhaus) or indirectly from an Automatex card and cross-folder.
Figure 2 is a diagram of the water circuit which provides water pressures at the nozzles of up to 240 bar (3360 psi).
In a typical experimental run, a 4 meter length of carded web of the desired basis weight is rolled up interleaved with tissue paper. This is then unwound onto the conveyor of the HE machine while removing the tissue for reuse. The first pass through the water jets attaches the web to the conveyor, and allows single-sided treatment at the rate of three nozzles per revolution of the conveyor. If lengths longer than the conveyor loop are required the web must be rolled up after each pass, this procedure reducing the precision of any patterning which may be required. The machine can be fitted with various conveyor patterns, although at the very high pressures, best results are obtained with fine mesh stainless steel belts.
Many hundreds of fabrics have been produced since the new machine was commissioned in April 1991. The results presented below are inevitably a small selection of the total and are oriented towards illustrating the effects of HE on viscose rayon and the new solvent spun cellulosic fibre, "Tencel" .
The fabrics described here were produced using a "standard" 4-pass procedure to treat the web with 8 nozzles using gradually increasing nozzle pressure. For instance a sequence of 40, 60, 80, 100, 120, 160, 180, 220 bar was used for fabrics labelled "220 bar", and a sequence of 40, 40, 60, 60, 80, 80, 80, 100 was used for the "100 bar" fabrics. Two-sided bonding was achieved by turning the web over after the first 6 nozzles. Conveyor speeds have to be kept low to allow man-handling of the short lengths of web used for this sort of experimentation. In this series 2.5 metres/minute was used for the first and last passes, and 7 metres/minute for the middle two.
Results and Discussion
The effect of HE systemAs the basic data on the hydroentanglement of cellulosics had been generated on an early Honeycomb Systems Pilot Line, the first trials with the new high pressure Perfojet line were designed to repeat earlier work. This would indicate whether conclusions reached over the years on the old system were applicable to the new one when using similar webs and processing conditions. The comparison which follows is only relevant to these pilot lines and in no way indicates the performance of commercial lines.
Table 1 shows the main effects using results pooled from a balanced experiment using numerous fibres and blends:
The results indicate reasonable comparability between the two pilot systems, with the Perfojet machine giving better M.D. properties. Honeycomb gives better C.D. and absorbent capacity results. The differences are explicable in terms of fibre orientation leaving the entanglement conveyor, the steel Perfojet conveyor being more retentive than the plastic on the Honeycomb machine. The higher absorbency arising from the Honeycomb machine could be due to the larger pore volume which accompanies the better CD orientation of the Honeycomb fabrics.
The Effect of Water Pressure on Fabric Strength
Early work had shown that the strength and appearence of both 80 gsm viscose and 60 gsm polyester fabrics deteriorated as water pressures were raised from 110 to 160 bar.
Figure 3 illustrates the relationship between water pressure and strength for both 80 gsm and 60 gsm "Tencel" webs. The key points to note are:
100 bar appears to be quite a sharp optimum for the "Tencel" fabric MD strength while the cross directional strength reaches a more gentle optimum at about 150 bar.
The upward turn of the "Tencel" breaking load above 200 bar corresponds to increasing fibrillation reducing the effective filament denier.
While the tensile properties go through a maximum at around 100 bar, the fabric extensibility (Figure 4) shows a linear decrease with pressure. This suggests that continued consolidation of the fabric is occuring as treatment proceeds above the breaking load optimum. (The actual optimum pressures identified in this series will only be applicable at the basis weights and speeds used.)
Figures 5 and 6 illustrate how blending the two cellulosics affects the strength properties at the higher bonding pressures. The Machine Direction results (Fig 5) suggest that the blend fabric strengths are roughly as expected from the average of the 100% fabric strengths. However, the Cross Direction results (Fig 6) suggest that the 50/50 blend achieves better than 90% of the "Tencel" dry strength, and better than 80% of the "Tencel" wet strength.
The Effect of Water Pressure on Fabric AbsorbencyFigure 7 shows the GATS rate of absorbency curves for "Tencel", Viscose, "Fibre ML" and blends when 40 gsm fabrics are processed under the standard 100 bar conditions. Figure 8 shows how moving to extreme bonding pressures changes the performance of 80 gsm webs of "Tencel" and Viscose.
"Fibre ML" and its blends with "Tencel" and viscose give the best rates of absorbency and the highest totals. There is little to choose between "Tencel" and viscose on this test. Polyester blends start absorbing much more slowly, but have total capacities similar to the cellulosics.
The "Tencel" rate of uptake is reduced by increasing HE pressure, whereas the viscose rate is increased. The viscose performs as expected, while the "Tencel" results are harder to explain. Fabric thickness (Figure 9) shows that the change from 150 to 200 bar has no effect on the already fully collapsed viscose web, but makes the "Tencel" web substantially thinner. Perhaps the pore volume in the 200 bar "Tencel" fabric is too low to transport significant volumes of fluid. It is also possible that finish removal in HE, which profoundly affects the polyester fabrics, is also affecting the "Tencel". (The 100% "Tencel" fabrics may have benefited from finish being incompletely removed at low pressure. The non-wicking nature of the 100% polyester fabrics was presumably a result of the hydrophilic finish being completely stripped off even at low pressures.)
Figure 10 illustrates the effect of HE pressure changes on Total Free Absorbency of the fabrics arranged in order of increasing capacity (standard treatment) on the X-axis. On this test Fibre ML and its "Tencel" blend give the highest capacities. The use of the higher pressures giving lower capacities for each of the fabric types evaluated. Again the PET fabrics proved unwettable in this test.
The Effect of Water Pressure on Fabric Thickness.Considering the Thickness data (Fig 9) in more detail:
Polyester and its blends give bulkier fabrics than the cellulosics.
At 150 bar "Tencel" retains thickness better than viscose to the extent that a 100% "Tencel" sheet is equivalent to a 50/50 PET/viscose blend.
Increasing HE pressure from 150 to 200 bar collapses both "Tencel" and Polyester fabrics to give thicknesses in line with the viscose figures. This change of thickness suggests that the fabric structure is continuing to be consolidated despite the reductions in MD and CD strengths.
Miscellaneous Effects of High PressureThe fact that "Tencel" begins to fibrillate at pressures above 100 bar leads to some unexpected effects.
Fabric opacity, which generally decreases as bonding levels increase and thickness decreases, tends to increase with "Tencel". In fact at 200 bar, bright (undelustred) fibres take on a fully delustred "dull" appearance. The average overall opacity coefficients (ISO 2471-1977) of "Tencel" increase from 72.8 to 78.2 as the pressure is increased from 150 to 200 bar. For comparison a similar polyester fabrics opacity decreased from 67.9 to 65.8.
80 gsm "Tencel" Fabric Porosity as measured by the Frazier Differential Air Permeability test (Basically EDANA method 140.1-81) drops from 105 to 75 mls/sq.cm/sec as the HE pressure is increased from 150 bar to 200 bar. For comparison a similar polyester fabric dropped from 149 to 140.
Fabric stiffness, which generally increases with degree of bonding increases very sharply with "Tencel" due to hydrogen bonding of the fibrils locking the structure together. With 150 bar treatment, 80 gsm "Tencel" had an MD flexural rigidity of 556 mg.cm compared with the equivalent viscose at 189 mg.cm. However, as with any cellulosic which develops a harsh handle after vigourous washing, the fabrics are easily softened with the usual commercial fabric softners.
The Importance of Fibre Denier and Web Forming MethodReducing fibre diameter reduces fibre stiffness and if grammage is held constant, it also increases the number of fibres per unit area. So, finer fibres should yield more efficient entanglement, and stronger fabrics. This trend is illustrated by the work on viscose (Figure 11).
One of the problems of this type of work is the impossibility of keeping web quality constant as the fibre denier is reduced. Very fine fibres have a high cohesion and are hard to card, especially if the card clothing and settings cannot be changed. Wet-laying is much easier for deniers below 1.0, but here the fibre length has to shortened if flocculation of the fine fibres is to be avoided.
Nevertheless the wet method is probably the only way currently available to make highly uniform fabrics out of 0.7 denier rayon. At this denier and at a length of 8mm a very attractive soft, almost suede-like fabric can be made with similar average strengths to 1.5 denier 38mm carded fabrics.
A very limited amount of work has been done with fully randomised card webs and probably the best fabric from the work on HE in Courtaulds was obtained from this system. Some results are given in the next section.
Comparisons with Woven FabricsFigure 12 illustrates some basic strength comparisons between wovens and nonwovens using the same fibres. It indicates that HE gets close to the properties of some conventional textiles in strength as well as aesthetics. The air-laid "Tencel" webs chosen for their anisotropic character have, at 220 bar, given HE fabrics which are stronger than the equivalent weight of plain woven cotton or viscose. Only the equivalent woven "Tencel" fabric is stronger. While stronger wovens could be made by using finer yarns and premium grades of cotton, further strength improvements in HE fabrics can also be expected to follow the development of finer "Tencel" fibres, and more efficient entanglement systems.
However, while fabric strengths obtainable from the HE process can be of the same order as those of wovens, other properties such as abrasion resistance and dimensional stability need to be improved substantially before they would be good enough to replace woven or knitted textiles in apparel markets. Combinations of techniques already used in either the nonwovens industry or the textile industry could perhaps be used to overcome these difficulties. The resulting durable nonwovens could then enjoy significant growth at the expense of woven and knitted textiles in non-apparel markets.
AcknowledgementsThanks are due to David Bertram (Focus Nonwovens) who organised fabric making and testing, and to Pam Bertram ("Tencel"), Mohammed Chowdhury (Courtaulds Engineering Ltd.), Neil Gallagher (Viscose), Jim James ("Tencel") and Andy Wilkes (Viscose) for the experimental designs and analysis.
"Tencel" is the registered trade mark for Courtaulds solvent spun cellulose fibre.
"The hydroentanglement of a range of staple fibres", C R Woodings, Impact Conference, March 1989.
See Also "Cellulosic Fibres in Hydroentanglement", D. Bertram, INDA Fundamental Research Conference. 1992
"Latest Advances in Hydroentanglement Technology", Andre Vuillaume, (Perfojet) Impact Conference, March 1989.
"A New Viscose Rayon Fibre for Nonwovens", A G Wilkes, INDA-TEC 89 (June)