Analyzing the thermal and hygral behavior of wool and its impact on fabric dimensional stability for wool processing and garment manufacturing

Fiber samples

Australian Merino top with an average fiber diameter of 21.0 µm was used. Fiber samples were first cleaned by washing twice with Hydrapol 1 g/l TN450 (Huntsman, Australia) and 0.5 g/l sodium carbonate at 60℃ for 10 min and afterward rinsed well with warm water, dried and conditioned at 20 ± 2℃ and 65 ± 2% relative humidity.

Measurement of fiber diameter

The average diameters of wool fibers were measured with an OFDA 2000 (BSC Electronics) fiber diameter analyzer. ¹⁸ For each measurement, the whole plate was scanned at least five times and a minimum of 8000 fibers were measured in each scan.

Fabric samples

Unset fabrics were loomstate, undyed, plain weave and twill pure wool fabric samples taken from ordinary mill production, designated A1 and A2 respectively. The samples were prepared for measurement after being scoured in aqueous solutions containing 2 g/l Hydrapol TN450 and 0.5 g/l sodium carbonate, at room temperature for 1 h.

Permanently set fabrics were prepared from the scoured unset plain (A1) and twill (A2) fabrics described above, and were designated B1 and B2. Fabrics were permanently set by stretching them while wet on a pin-frame by 2–3% in both warp and weft directions and then boiling them in water (pH 7.2) for 30 min. The level of permanent set (PS) of the fabrics was calculated from the crease angle according to Equation (1). The crease angle test method was described in the FAST (Fabric Assurance by Simple Testing) test procedure ¹⁹

PS ( % ) = 1 - θ 180 × 100

(1)

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Figure 2.

Crease angle test procedure for the determination of the level of permanent set.

Table 1 shows the structural details of the fabrics tested.

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Table 1.

Structural details of the pure wool fabric samples

Fabric	Weave structure	Finishing treatment	Weight (SD) (g/m2)	Ends & picks (/cm)
A1	Plain	Scoured	140 (1.03)	27 × 23
A2	2/1 twill	Scoured	157 (0.98)	33 × 27
B1	Plain	Permanently set	144 (0.87)	29 × 27
B2	2/1 twill	Permanently set	166 (1.01)	35 × 29

Hygral expansion

HE was determined by Equation (2), according to the FAST procedure ¹⁹

HE ( % ) = Dw - Dd Dd × 100

(2)

Measurement of fabric dry and conditioned dimensions

Samples were dried in two stages to prevent the introduction of relaxation shrinkage. ²⁰ Firstly, the samples were gently pressed between layers of absorbent paper and left to dry under ambient conditions on a non-stick horizontal surface, for 12 h. The samples were then placed on a non-stick horizontal surface in an oven at 105^oC for 60 min. The dimensions were measured immediately after removal of the samples from the oven.

Dimension of fabrics were measured after samples were conditioned at 20 ± 2℃ and 65 ± 2% relative humidity for 24 h. At least three fabric samples were used for the testing. Two sets of measurements were made on each sample of fabric and the results for the warp and weft were averaged.

Measurement of the fabric wet dimension

Fabric was wet-relaxed by soaking, without restraint, in aqueous solution contained in a shallow trough, for 2 h. Dimensions of the wet fabric were measured directly in the solution. Fabric samples were subjected to two repeated FAST cycles to check that the measurement was reproducible before the thermal experiment.

Procedure for the measurement of dimensional properties

Fabric samples were cut in appropriate sizes and then marked with dark sewn-in threads, about 20 cm apart, in both the warp and weft directions. At least three fabric samples were used for the testing. Two sets of measurements were made on each sample of fabric and the results for the warp and weft were averaged. The treatment sequence for measurement followed the FAST test method procedures ¹⁹ such that condition → dry → wet → dry → condition.

Heating bath

It is known that the pH value affects the degree of PS obtained in wet finishing processes. The higher the pH, the greater the degree of set. ²¹ Experiments for measuring temperature-induced dimensional changes of fiber and fabric were carried out in buffer solution at pH 2.1, obtained by mixing 0.2 M sodium di-hydrogen phosphate and 0.2 M phosphoric acid. For making comparisons, a heating bath of deionized water at pH 7.2 was also used for the measurement of temperature-induced dimensional changes of fabrics.

Measurement of thermally induced dimensional changes

Fabric samples were heated in a bath while laid flat without restraint. The temperature in the heating bath was increased at approximately 30℃/h and raised from room temperature of 20℃ to 95℃. The dimensions of the fabrics were recorded directly when the fabric was wet at 20℃, 40℃, 70℃ and 90℃. After heating, the heating bath was allowed to cool down to room temperature of 20℃, and the dimensions were recorded again for the calculation of unrecovered dimensional changes (UD), using Equation (3)

UD ( % ) = Da - Db Db × 100 %

(3)

Fiber swelling as a result of changes in moisture regain and temperature

Results of measured fiber mean diameter and calculated values of the percentage of fiber radial swelling relative to the dry diameter, as a function of moisture regain and temperature, are shown in Figure 3. It can be seen that there is an increasing fiber diameter as the environment changed from dry to wet (saturated regain). A significant increase occurred when regain increased from conditioned (65% relative humidity) to saturation. Upon reaching saturation, an approximately linear increase in average wool fiber diameter is shown as a function of temperature. It can also be seen that the mean diameters increased by around 15% as the temperature rose from ambient to 70^oC. OPEN IN VIEWER

It can also be seen from Figure 3 that the percentage value for fiber radial swelling at saturated regain (20^oC) relative to the dry state is around 21%, which is greater than the value of 16% known for wool. ¹ This is because the measurement was conducted at a buffer solution of pH 2.1, which was designed to keep the pH value inside the fibers low and to give a consistent experimental condition for the subsequent measurements of fabric dimension. In water at around pH 7, the polypeptide chains of wool bear a weak negative charge. However, at pH 2.1, keratin assumes a high net positive charge and the consequent repulsion between the polypeptide chains is apparently responsible for the increased fiber swelling. ⁷

Level of permanent set and fabric hygral expansion

The results of crease angle measurements described in the Fabric samples section showed that 85% PS was introduced into fabric samples B1 and B2 after permanent setting. This permanent setting treatment produced permanent changes in the dry and wet dimensions of the fabrics at room temperature and significantly increased the HE values, as shown in Figure 4. It is also seen from Figure 4 that for both unset and permanently set fabric samples, fabrics with a twill structure presented higher HE values than the fabrics with a plain weave structure. It followed, therefore, that fabric with less yarn crimp produced greater HE. OPEN IN VIEWER

Fabric dimensional changes as a function of temperature at saturated regain

As mentioned previously, a pH 2.1 buffer solution was used as the heating bath in the experiments to keep the pH value inside the fibers low, in order to minimize the permanent setting of wool fabrics while they were being heated and thereby minimize the possibility that any dimensional changes observed were due to the permanent setting of fabric rather than to thermal effects. Deionized water at pH 7.2 was used for comparison purposes. The results of temperature-induced fabric dimensional changes are shown in Figure 5. It can be seen that as the temperature increased, contraction of all fabrics was observed. The contraction was approximately linear within experimental error in each case. The contraction of the plain weave fabric samples (A1 and B1) was greater than that of the twill fabric samples (A2 and B2). It followed, therefore, that fabric structure must modify thermal dimensional behavior to some extent. OPEN IN VIEWER

When the mean HE values of the samples, shown in Figure 4, are compared with the slopes of the lines in Figure 5, it appears that there is a general trend for fabrics with low values of HE to also show relatively lower contraction with increasing temperature. In particular, this can be seen by comparing the properties of the plain weave and twill fabric samples before and after the preliminary permanent setting treatment. The unset samples (A1 and A2) gave lower values of both HE and thermal contraction than the samples that had been permanently set under tension (B1 and B2).

Fabric unrecovered dimensional changes resulting from heating are shown in Figure 6. As can be seen when measured in pH 2.1, unset samples A1 and A2 recovered to their original dimensions to a large extent (within experimental error of ±0.2%) when they were cooled to ambient temperature at the end of the heating cycle. The samples that had received a PS nearly, but not totally, returned to the initial values. It appears also that fabrics heated in water at pH 7.2 tended to contract to a greater extent than fabrics that received treatment in pH 2.1 buffer solution. This observation could be due to the fact that fabric had been permanently set to a greater degree when it was heated in water at pH 7.2 than at pH 2.1. OPEN IN VIEWER

The relationship between hygral behavior and temperature-induced shrinkage

It is known from fabric hygral response that fabric dimensions initially increase with increasing regain but eventually reach a maximum, after which the fabric begins to contract. ²² This is referred to as fabric “swelling shrinkage” at saturated regain.

If further swelling of fibers occurred in water as the temperature was increased beyond ambient, then the observed fabric contraction could be readily explained by the swelling shrinkage of an already jammed fabric structure. Evidence for increased fiber swelling in water with increasing temperature was shown in Figure 3. It appeared, therefore, that fiber thermal swelling at saturated regain was the cause of fabric shrinkage. This is represented in Figure 7. OPEN IN VIEWER

Figure 8 shows the variations in fabric dimensions (relative to the dimensions in the dry state) with fiber swelling. The curves are third-order polynomial regression lines derived from the data. It is seen that the shape of the curves is similar to that of hygral curves, with a pronounced maximum between the dry and saturated states. OPEN IN VIEWER

It is seen from Figure 8 that the onset of swelling shrinkage in all the fabrics occurred after the fibers were swollen by at least 8%, although the amount of swelling required to initiate swelling shrinkage seems to have varied to some extent. The effect of permanent setting of fabrics B1 and B2 under tension was to increase the value of fiber swelling at which swelling shrinkage became established, relative to the unset fabrics A1 and A2, respectively. The shapes of the curves of fabrics B1 and B2 also indicated that the level of inter-yarn forces remained relatively high, despite the fact that some stress-relaxation of the fibers has taken place during the permanent setting process. This was due to the actual construction of the fabric and the fact that the fabrics were permanently set under tension. ²²

None of the fabrics examined in this work showed thermal expansion. The thermal expansion observed by other researchers could be because of the use of piece dyed fabrics under relaxed conditions. These fabrics generally have a low level of yarn–yarn interaction and their hygral curves would not reveal the presence of swelling shrinkage at high regains.

A further factor that may have influenced the magnitude of the temperature-induced fabric contraction is the progressive reduction in fiber bending rigidity that is known to take place with increasing temperature in water. Figure 9 shows the Hookean modulus of wool fibers measured in water at various temperatures. It is seen that as the temperature increased from ambient to 100℃, there was a 25% decrease in the Hookean modulus of wool fibers. OPEN IN VIEWER

The consequence of losing rigidity would be to make the fibers more flexible and less stiff. In this case, swelling shrinkage is enhanced in fabrics with well-defined weave structures because the fabrics have become more pliable.

In closely woven unset fabrics, inter-yarn forces at yarn crossover points are high and, consequently, the fabric is stiff and unyielding, with high values of bending and shear modulus. As already described, all setting media (e.g. steam, reducing agents, hot water, etc.) cause the fibers and yarns to swell to some degree, and in order to accommodate this swelling, the yarn crimp has to increase, and fabric becomes set in this configuration. On drying after setting, de-swelling occurs and results in a fabric structure with lower inter-yarn forces. Consequently, the permanently set fabric is less stiff in bending and shear, and is more extensible. ²⁴ This could be the other reason why permanently set fabric exhibited more thermal contraction than unset fabric for a given fabric structure.

Industrial implications

This work has important implications for the pressure decatizing process. It is clear that the usual practice of applying warp tension to fabric before steaming, as it is being interleaved with the wrapper during formation of a batch, can have the effect of reducing, or eliminating, the shrinkage that would otherwise occur as a result of the permanent setting process.

The pressure steaming setting method used in these experiments had the advantages that the fabric dimensions and regain were under more accurate control. In decatizing, the use of tension rails to set warp tension is not very precise and there is little control over weft tension. In addition, dimensional changes in fabrics during steaming are only inhibited by lateral pressure exerted by a wrapper blanket. Also, the regain of the wool during pressure decatizing is much more problematical because of the influence of the blanket. ²¹ Under industrial conditions, incomplete cooling of batches is likely to result in less steam being condensed in the wrapper, so that the wrapper may take moisture from incoming wool reducing its regain before the steaming process commences. On the other hand, inefficient drying of the wrapper could result in more moisture being transferred to the wool so that setting would take place at higher than expected regains.

It is clear that even if temperature and time of steaming are controlled accurately, reproducible results in pressure decatizing will not be possible unless the amounts of stretch in both the warp and weft directions also are accurately controlled during batching up. At present, decatizers are not fitted with suitable devices to achieve this. However, width and overfeed control is mandatory on stenters and is also used in compressive shrinkage machines, particularly for cotton fabrics. Further work is needed to quantify optimum fabric tensions in association with the hygral behavior of the fabric and other setting variables during pressure decatizing.

Although it is clear that an amount of pre-tensioning is able to reduce setting shrinkage, HE and relaxation shrinkage that result from pressure decatizing, it is not suggested that fabric be stretched too much as an excessive strain would decrease extensibility due to reductions in HE. This must be taken into account when assessing the required properties of the finished fabric. There will always be a trade-off between HE and extensibility.