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CSIRO Textile and Fibre Technology CHARACTERISING THE STABILITY OFTHE SURFACE FINISH ON WOOL FABRICS C. Le, N. Ly, D. Phillips, A. De Boos Textile and Fibre Technology, Belmont, Victoria, Australia Many operations in finishing contribute to what is termed the ‘surface finish’; a feature of wool fabrics that is of considerable commercial significance to the finisher, the tailor and the consumer. Such operations normally include high-speed scouring and milling, cropping, pressing and decatising. For wool fabric, where any ‘finish’ imposed in processing can be held by either temporary or permanent set (or both), the stability of the finish imparted is also an important issue. A technique is described in this paper, in which the ‘finish’ and the stability of that ‘finish’ are characterised in terms of two properties: Effective Flat Set and Stable Flat Set. Both properties are determined from the surface thickness and the relaxed surface thickness of the fabric measured before and after any pressing or decatising operation. The data is normally graphically represented and the position of the fabric on the graph gives information on the type and stability of the surface finish on the fabric. The technique is particularly useful in evaluating the finish imparted by final pressing and decatising operations. The technique can also be used to optimise the operation of pressing and decatising machines.

‘Surface finish’ is a particularly important feature of the handle of wool fabrics and an attribute that is prized by processors and consumers. For this reason, a variety of machines and processes are used to control the surface finish of high quality wool fabrics. An understanding of the actions of those finishing processes which affect surface finish, and the use of test methods to characterise that finish, are of considerable commercial significance to the finisher, the tailor and the consumer. The finishing operations that contribute to the surface characteristics of wool fabrics, include: • • • •

high-speed scouring and milling, which create a fibrous surface as well as modifying other properties of the fabric, notably shear rigidity and specific volume (1,2) cropping and singeing, which are designed to remove fibrous protrusions from the body of the fabric and thereby create a smooth surface, raising and teaselling, which draw fibres to the surface of a fabric to create a pile, pressing and decatising, which are designed to flatten the fabric and create a smooth surface.

Many techniques have been developed to characterise the differing aspects of the surface of fabric. The behaviour in lateral compression has been studied using measurements of thickness, compressibility, the work to compress and energy released in recovery (3). Resiliency or recovery from this deformation can be described in terms of residual thickness, loss of energy in the compress-release cycle (3) or as some measure of stress relaxation under load. The measurement of surface thickness, as described in the SiroFAST system (4) for fabric objective measurement, is an alternative measure of compressibility and is derived from more fundamental studies of fabric lateral compression. These studies described two components of the thickness of fabrics; the ‘core’ and ‘surface’ thickness (5). Other more complex mathematical models have also been described (6). Frictional measurements are also used to characterise the surface of fabrics. Several devices have been proposed to measure the surface frictional properties of fabric. These instruments measure friction against a plastic measuring head, against another surface of the same or a different fabric or against a series of wires designed to look like a human fingerprint (3). Both contact and non-contact techniques (eg laser) have also been developed to describe the geometry of the fabric surface (7-9). Any ‘finish’ imposed on wool fabric during dry finishing processes can be held by either temporary or permanent set (or both). The stability of the surface characteristics imparted is an important issue for both finishers and their customers. When the finish is held only by temporary set, then the handle and flat appearance of the fabric, especially in jacket sleeves, can be lost during the steaming operations in garment manufacture, dry cleaning or during wear. An important feature of high temperature or pressure decatising is that the process imparts permanent set to the wool fibres so that the finish is more stable to garment manufacture, wetting, and the conditions found in normal consumer use. There are a limited number of techniques that can be used to determine the permanence of the finish on wool fabrics. The surface properties described above can be re-measured after a relaxation process and any change measured. Alternatively, permanent set can be measured directly using yarn snippets taken from creases deliberately held in the fabric during the finishing process (10). Permanent set is normally determined from the angle formed by the yarn snippets after their release in water at 70oC for 30mins. This paper describes a technique for characterising the ‘finish’ on wool fabrics (including blends) and the stability of that ‘finish’ in terms of two new properties: Effective Flat Set (EFS) and Stable Flat Set (SFS). These properties are determined from the surface thickness and the relaxed surface thickness of the fabric measured before and after any pressing or decatising operation. Information on the repeatability of the data obtained using various methods to relax the fabric specimens is also presented. Experimental Methods A wide range of fabrics and finishing conditions were used in this work. Details are given in the tables and text of the paper. Permanent set was calculated from the mean angle of yarn snippets (A) taken from a crease deliberately imposed in the fabric during the pressing or setting process (10). The angles were measured after relaxation of the yarn snippets in water at 70oC for 30min. Permanent set (%) = (180 – A) x 100 180 2

All measurements of fabric thickness were conducted after conditioning the specimens at 65%rh, 20oC for in excess of 16hrs. Fabric thickness and surface thickness were measured using the SiroFAST-1 thickness meter using the methods described in the SiroFAST manual. Surface Thickness: ST = T2 – T100 where, T2 is the fabric thickness at 2 gf/cm2 (0.196 kPa) and, T100 is the fabric thickness at 100 gf/cm2 (9.81 kPa) Finish Stability Ratio (FSR - called ‘Finish Stability’ in the SiroFAST User’s manual) was determined as follows; FSR (%) = ST x 100 RST where RST is the surface thickness of the fabric after it had been relaxed in water at 20oC for 30 min and air dried or in steam from an open trouser press while enclosed, but not sealed, with a perspex lid. Effective Flat Set(EFS) is a new parameter developed to characterise the surface finish on fabrics. This parameter was determined from the relaxed surface thickness of the fabric before a pressing or decatising operation and its surface thickness after that process. The pressed fabrics were also then relaxed in water or steam and the thickness re-measured. The results from the relaxed fabrics were used to calculate the Stable Flat Set(SFS) as follows: EFS (%) = RSTo - STp RSTo

x 100

SFS (%) = RSTo - RSTp RSTo

x 100

where RSTo is the relaxed surface thickness of the fabric before a pressing operation, STp is the surface thickness after the pressing or decatising operation and RSTp is the relaxed surface thickness of the pressed fabric Temporary Flat Set may also be calculated (TFS = EFS-SFS). The measurements of Effective and Stable Flat Set may also be plotted on a chart as shown in Figure 1. The lines of constant Finish Stability Ratio are also shown on the diagram. Included on the diagram are the data obtained using a number of commercial fabrics and as well as those obtained using a number of commercial processes on a single fabric. Results and Discussion An example of the change in the surface thickness of wool fabrics during finishing is shown in Figure 2. As has been previously published, there is a significant increase in thickness after piece dyeing and a reduction in fabric thickness after pressing and decatising operations (11), (12). The ‘finish’ imparted to a fabric in pressing and decatising operations depends on the extent to which the fabric is compressed during the operation.

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100 C om m erc ial P roc es s es

C om m erc ial F abric s

STABLE FLAT SET (%)

80

K D -hi

K D -low

60

C D (P ) P oorly piec e-dyed

40

O pen-blow 20

CD CD C alendering FS R = 8 0 %

FS R = 6 0 %

FS R = 4 0 %

FS R = 2 0 %

20

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0 0

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EFFE C TIVE FLAT SE T (% )

Figure 1

The use of Effective Flat Set(EFS) and Stable Flat Set(SFS) to characterise Surface Finish (CD-Continuous decatiser, CD(P)-continuous decatiser under pressure)

Surface Thickness (mm)

0.3

0.2 before relax after relax 0.1

0 Crab

Scour

Dye

KD

Process

Figure 2

Change in Surface Thickness during Finishing (wool plain weave185 g/m2)

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Fabrics that are heavily pressed have low thickness (and surface thickness) and develop a smooth firm handle. Depending on the conditions used, this handle can also be characterised as stiff. Where the compression of the fabric is smaller, such as when rotary pressing is avoided or when a ‘molleton’ wrapper is used in decatising, the fabric is normally less smooth but more supple. The change in surface thickness after a subsequent wet relaxation is also shown in Figure 2 and was dependent on the stability of the finish. This, in turn, depended on the conditions of processing, particularly in pressing and decatising. Measurement of Effective and Stable Flat Set. Although measurements of thickness (or surface thickness) can be used to evaluate some aspects of the finish imposed on individual fabrics (13), they are less useful when fabrics of widely differing thicknesses are compared. Moreover thickness cannot be used to obtain a direct measure of the permanence of the finish on a fabric. The use of EFS, which has a range of 0-100% for all fabrics, gives information on the amount by which the thickness of the fabric has changed in the pressing operation and allows the assessment of surface finish, independent of the initial thickness of the fabric. In combination with SFS, the EFS measurement gives, on a scale common to all fabrics, a measure of the finish imposed and the stability of that finish. Included in Figure 1 are data from one fabric finished using a variety of machines as well as data from a number of commercial fabrics, which were sampled prior to and after pressing and decatising operations. Fabrics with a flat smooth surface (well pressed) are characterised by a high EFS. If the finish is stable, the fabrics are plotted close to the diagonal line where SFS = EFS. The advantage of EFS/SFS measurements over the use of Finish Stability Ratio (FSR – described in Expt section) can also be seen in Figure 1. The Finish Stability Ratio of fabric treated in one of the conventional continuous decatisers (CD) is very similar to that of fabric treated in the pressure decatiser under mild conditions (KD-low). However, the finishes imparted by the alternative machines are considerably different. Table I

Effect of Relaxing Conditions on Effective and Stable Flat Set

Conditions used to relax fabric

Setting conditions Mild Severe EFS SFS EFS SFS Open steam (30 sec) 63 51 81 79 Enclosed steam (30 sec) 66 43 81 75 Wet out (20C, 30 min), air dry 61 43 82 75 Wet out (20C, 30 min), oven dry 67 46 84 76 • Average value taken from a range of fabrics • All fabrics preconditioned in a standard atmosphere (20oC, 65%rh) for 24hrs • All testing carried out using SiroFAST-1 thickness meter The numerical value of both Effective and Stable Flat Set depended on the conditions used to relax the fabric, ie to release temporary set (Table I). These conditions affected both RSTo and RSTp. Unless the unset fabric was already fully relaxed, the more severe the relaxation conditions that were used, the higher the apparent value of EFS. The value of SFS depended 5

on the relative change of RSTo and RSTp but, in most cases, the more severe the relaxation conditions, the lower the SFS. SFS cannot exceed EFS. The repeatability of these measurements was established in an inter-laboratory trial involving 11 companies in 8 countries. “Repeatability” is the value below which the absolute difference between two single test results obtained on the same sample by the same operator in the same laboratory may be expected to lie (95% probability). The repeatability of the tests (shown in Table II) using steam as the relaxation medium was marginally better than that obtained when water was used to relax the fabric. Table II

Repeatability of EFS/SFS Measurements

Relaxation method

Effective FS Mean Repeat. Water (20oC, 30 min), air dry 71.8 4.3 Steam (30 sec) with cover 74.6 4.7 • Six wool fabrics used: Analysis using ISO5725

Stable FS Mean Repeat 56.7 7.3 62 6.2

The use of Effective and Stable Flat Set to characterise the stability of surface finish on a fabric offers considerable advantages over the use of yarn snippets taken from creases deliberately imposed in the fabric to determine the permanent set imparted in pressing and decatising processes. Using EFS and SFS : • the measurement is simpler to carry out, • there is no need to sew a crease into the fabric prior to the pressing-decatising process, • there is no marking off of this crease on adjacent layers of the fabric or the wrapper. There was some correlation between permanent set measured using a crease angle and SFS. However, the correlation was not high (see Figure 3). Classically, permanent set is calculated from dimensions of the fabric before the setting operation (B), during the setting operation (S) and after the fabric has been relaxed (R): Permanent set (%) = (B – R) x 100 (B – S) In the measurement of permanent set using yarn snippets B=180 deg, S=0 deg and R is the mean angle of the measured snippets after relaxation in water at 70oC. For a similar calculation of permanent set based on surface thickness measurements, B=RSTo, R=RSTp and S would be the surface thickness during the setting operation. The thickness of a fabric can only be measured after the pressing deformation has been removed and is not necessarily the same as that during the setting process. Consequently, it is not possible to determine permanent set (using the formula above) from fabric thickness measurements. On the other hand, thickness during the pressing operation is not required for the calculation of SFS. Thus, while there is some similarity between the calculations for permanent set and SFS with both measurements being affected by similar changes within the wool fibres, they measure different aspects of fabric setting so that, a perfect correlation would not be expected. Moreover the conditions normally used to relax the samples are also different.

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100

Line of best fit r² = 0.75

PERMANENT SET (%)

80

60

40

20

0 0

20

40

60

80

100

STABLE FLAT SET (% )

Figure 3

Relationship between Stable Flat Set and Permanent Set measured using Crease Angle

Alternative Measurements of Surface Thickness. The surface thickness of fabrics may also be determined using other thickness gauges such as the KES-FB3 instrument. However, to calculate EFS and SFS from thickness measurements at pressures other than those used by the SiroFAST system, certain assumptions must be made. It has been observed (5) that, at pressures at or below 100gf/cm2, there is a linear relationship between the measured thickness and the reciprocal cube root of the applied pressure. To a first approximation in which stress relaxation effects are ignored, surface thickness, as defined in the experimental section of this paper, can be calculated from the thickness measurements made at any two pressures below 100 gf/cm2. ST =

(T1 – T2) x 1.254 (F1 – F2)

where F1 and F2 are the reciprocal cube roots of the pressures (in kPa) applied to measure thicknesses (T1 and T2). For the KES-FB3 instruments the recommended loads of 0.5 gf/cm2 and 50 gf/cm2 can be used, and the surface thickness is calculated as follows: ST = 0.585 (T1 – T2) where T1 and T2 are the thicknesses corresponding to the two loads specified. Application of EFS and SFS in Finishing. The measurements of EFS and SFS can be used to evaluate the effectiveness of machinery and/or finishing routines. Figure 1 shows the effect of a number of pressing operations on the EFS and SFS of a single fabric. Calendering was characterised by a high value of EFS but, because this process imparted only temporary set, there was a low value of SFS. On the other hand, pressure decatising (KD) produced a finish 7

with high EFS and high SFS. Ideally, where a very stable finish is required (eg washable fabrics) the fabric should be close to the diagonal line where SFS = EFS Figure 4 shows the effect of fabric pH on the properties of fabrics after decatising and on the effectiveness of the decatising process. As previously reported(10), the pH of the fabric had little effect on the pressing action of the decatiser (EFS), but the low SFS at low pH indicated that the permanent setting action was inhibited. This effect was confirmed by the measurements of permanent set. The results demonstrated the usefulness of the SFS measurement in measuring the effect of fabric pH on the type and stability of the finish imparted.

PERMANENT, STABLE AND EFFECTIVE SET (%)

100

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0 0

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P erm anent S et

Figure 4

4 p H O F A F AB R IC S ta b le F la t S e t

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E f fe c tive F la t S e t

Effect of Fabric pH on Measured Properties after Decatising

Measurements of EFS and SFS have also been found to be particularly useful in optimising the operation of individual pressing and decatising machines. By comparing the properties of test fabrics after processing under a range of conditions, the operation or finishing sequence can be optimised to meet the requirements of individual customers. Furthermore, by assessing the finishes applied by alternative machines to samples of the same fabric, the technique can be used to compare these machines. Examples can be seen in Figure 1 where it is possible to compare two conventional continuous decatising machines or assess their action against conventional pressure decatising. This gives quantitative information that can assist the finisher in making any decision to purchase new equipment.

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Conclusions A technique is described, in which the ‘finish’ and the stability of that ‘finish’ on woolcontaining fabrics are characterised in terms of two properties: Effective Flat Set and the Stable Flat Set. These parameters are determined from the surface thickness and the relaxed surface thickness of the fabric measured before and after any pressing or decatising operation. The data are normally represented graphically and typical results taken from the large number of commercial and experimental fabrics that have been evaluated are shown. The ‘position’ of the fabric on the graph gives information on the type and stability of the surface finish on the fabric. The technique has been shown to be particularly useful in evaluating the finish imparted by final pressing and decatising operations. It can also be used to compare or optimise the operation of individual pressing and decatising machines. Acknowledgements The authors would like to acknowledge the contributions of Ms Yvonne Douglas and Ms Irene Slota to this work. The financial support of the Australian Government (CSIRO) and Australian Woolgrowers in cooperation with the Australian Wool Research and Promotion Organisation is gratefully acknowledged. References 1 2 3 4 5 6 7 8 9 10 11 12 13

Stewart B.F., Postle R. (1974) Textile Research Journal, 44, 192. De Boos A.G. (1988) J. Soc. Dyers Colourists, 104, 339. Kawabata S. (1980) in The Standardisation and Analysis of Hand Evaluation The Textile Machinery Society of Japan, Osaka, Japan, Ly N.G., Tester D.H., Buckenham P., Roczniok A.F., Adriaansen A.L., Scaysbrook F., De Jong. S. (1991) Textile Research Journal, 61, 402. De Jong S., Snaith J.W., Michie N.A. (1986) Textile Research Journal, 56, 759. Hu J., Newtown A. (1997) J. Textile Institute, 88, 242. Ramgulam R.B., Amirbayat J., Porat I. (1993) J. Textile Institute, 84, 99. Xu B., Cuminato D.F., Keyes N.M. (1998) Textile Research Journal, 68, 900. Blankenburg G., Phillipen H., (1973). in Proc. IWTO Conference, Vol. Report #2,, Paris, France. Kopke V. (1970) J. Textile Institute, 61, 361. Le C.V., Tester D.H., Ly N.G., De Jong S. (1994) Textile Research Journal, 64, 61. De Boos A.G., Harrigan F.J., White M.A. (1983).in Objective Evaluation of Apparel Fabrics (Postle R., Kawabata S. and Niwa M., eds) 311, Textile Machinery Society of Japan, Osaka, Japan, Blankenburg G., Breuers M., (1975). in Proc. Int. Wool Textile Research Conference, Vol. 5, 139, Aachen, Germany.

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