Understanding the difference in softness of Australian Soft Rolling Skin wool and conventional Merino wool

Abstract

Softness is one of the key elements of textile comfort and is one of the main considerations when consumers make purchasing decisions. In the wool industry, softness can reflect the quality and value of wool fibers. There is verifiable difference in subjective softness between Australian Soft Rolling Skin (SRS) wool and conventional Merino (CM) wool, yet the key factors responsible for this difference are not yet well understood. Fiber attributes, such as crimp (curvature), scale morphology, ortho-to-cortex (OtC) ratio and moisture regain, may have a significant influence on softness performance. This study has examined these key factors for both SRS and CM wool and systematically compared the difference in these factors. There was no significant difference in the crimp frequency between these two wools; however, the curvature of SRS wool was lower than that of CM wool within the same fiber diameter ranges (below 14.5 micron, 16.5–18.5 micron). This difference might be caused by the lower OtC ratio for SRS wool (approximately 0.60) than for CM wool (approximately 0.66). The crystallinity of the two wools was similar and not affected by the change in OtC ratio. SRS wool has higher moisture regain than CM wool by approximately 2.5%, which could reduce the stiffness of wool fibers. The surface morphology for SRS wool was also different from that of CM wool. The lower cuticle scale height for SRS wool resulted in its smoother surface than CM wool. This cuticle height difference was present even when they both had similar cuticle scale frequency.

Keywords

Wool softness curvature ortho-to-cortex ratio moisture regain cuticle scale

Wool is an important natural fiber used in the textile industry. The functionality of thermal regulation enables wool to isolate skin from outside conditions and keep people warm in cold weather and regulate body moisture in warm weather. It is therefore generally used as the raw material for suits, coats and sweaters. In fact, the thermal regulation of wool can create a cozy environment around skin where the temperature and moisture is neutral, benefiting sleep quality.^1,2 Wool can be a good material for active wear due to moisture wicking and odor resistant traits.³ Recent research has demonstrated that wool can offer both warmth and a cooling sensation, depending on the outside environment, through a moisture management effect, reflecting the potential of wool as an all-weather apparel material.⁴ However, wool fibers have a historic reputation of being itchy, which is caused by fiber prickle (associated with softness). This adversely affects the application of wool products in the next-to-skin apparel market. Many consumers have this rigid perception about wool, leading to softness being a determinant for consumers to purchase wool products. This is the reason why many researchers have devoted their time to the investigation of fiber properties contributing to the softness of wool.

The relevant literature revealed that a positive relationship could be found between the mean fiber diameter (MFD) and mechanical properties of knitted fabrics, such as bending and shear rigidity.⁵ Finer fibers are able to be spun into finer and more even yarns, and these yarns can be made into light weight fabrics that are softer and smoother.^6,7 Therefore, the MFD may predominantly determine the handle and softness of woven and knitted wool products. The MFD is also the key factor responsible for the softness and handle of loose wool fibers, influencing the mechanical and physical properties of fibers.⁸ A remarkable change in flexibility can be created as the MFD changes slightly, because both the shearing and bending rigidity vary with fiber radius to the fourth power.⁹ Meanwhile, a lower MFD also positively influences the handle and softness of next-to-skin textile products by improving the skin comfort and reducing the prickle sensation.^10,11

A resistance force will be generated when the crimp structure of wool fibers is compressed by an applied force. Because of this significant effect of fiber crimp on the compressional properties, a relationship was developed between the crimp characteristics with the softness of loose wool fibers.¹² Wool fleeces with a given MFD are still likely to have different levels of softness or handle if they differ in mean fiber curvature (MFC) (positively related to crimp height and frequency).¹³ Lower and higher MFC correspond to soft and less soft samples, respectively, in the case of the same diameter, since it becomes increasingly difficult for wool to be compressed with the increase in MFC or crimp height. A higher crimp frequency can also lead to a harsh handle due to the difficulty for the fiber to be compressed. Crimp frequency and MFC may be the dominate factors for compression properties of wool fiber.^14,15

Fiber surface friction is also regarded as a determining factor for the softness of wool fabrics,^16,17 which is inspired by the understanding of tactile perception by touching and rubbing fabrics. The directional effect of surface friction of wool fibers determines that the against-scale friction force should represent the influence of the surface morphology on the softness of wool fibers.^18,19 A higher cuticle scale height causes a rough surface, which is responsible for larger friction force on the surface of wool fibers. Similarly, the surface frictional coefficient increases as the scale frequency becomes lower at the same level of the cuticle scale height, since this leads to the formation of an irregular and rough surface.

The resistance to compression (RtC) test is a commercial assessment method to quantify the softness of loose wool samples in the textile industry and it could create a correlation between the softness and the MFD and crimp.¹² The primary principle or mechanism of RtC for loose fibers is to determine the applied force that reflects the level of deformation of wool samples after being compressed within a fixed volume.²⁰ RtC values are negatively related to softness, since wool fibers with lower RtC can be easier to compress and bend. Some studies reported that the RtC value was not a good indicator of the softness of wool fibers with low crimp or varying MFD,^21,22 the reason for which may be that the RtC test failed to reflect the significant influence of surface friction on the softness of wool fibers. This problem can be solved by using a pulling force test, another objective method for softness assessment, revealing the important effect of surface morphology and friction on the softness of wool fibers or fabrics.^23–25 The pulling force measures the force required to pull a wool bundle through parallel pins, reflecting the combined effect of bending stiffness, surface friction and smoothness,²⁶ which can be a good indicator of the softness of loose wool fibers.

Intermediate filaments (IFs) are the basic component units in the cortex of wool,²⁷ previously known as microfibrils. These IFs that are oriented parallel to the fiber axis and form the para-cortex, while other IFs are mainly twisted and aggregate into macrofibrils, which are inclined to the fiber axis, forming the ortho-cortex. The ortho-cortex and para-cortex are bilaterally arranged in the cross-section of wool fiber. To relieve internal strain energy, the wool fiber will curl and the ortho-cortex is always on the outer side, forming the curvature structure.²⁸ Therefore, the distribution of the ortho-cortex and para-cortex in wool fibers (ortho-to-cortex ratio, referred as to OtC ratio) is believed to lead to the difference in curvature, so as to influence the softness of wool.

The difference in OtC ratios between Soft Rolling Skin (SRS) and conventional Merino (CM) wool can reflect the change of distribution of the ortho-cortex and para-cortex. An increase in the para-cortex may be measurable as an increase in the crystal structure of wool fibers,²⁹ which has an influence on the mechanical properties (e.g. stiffness). In this case, a comparative study on the crystallinity of SRS and CM wool should be conducted to explore if there is a relationship between the crystallinity and OtC ratio, as well as softness.

In addition, the comfort of wool fibers can also be affected by moisture regain in two aspects. One of them is that the ability to absorb water can regulate humidity and temperature in the microenvironment between woolen products and a wearer’s skin, while the other is to influence the mechanical properties (the initial Young’s modulus and torsional rigidity) that are related to the stiffness of wool fibers. Higher moisture regain enables fibers to keep skin in a comfortable environment where the moisture and heat are stable. Meanwhile, the moisture regain is negatively related to the Young’s modulus,³⁰ as well as to torsional rigidity³¹; wool fibers become softer because fiber stiffness reduces with the increase of moisture regain.

In the first part of this work, Australian SRS wool was systematically compared with CM wool in terms of softness. Testing was conducted using RtC and pulling force testing. Results showed that SRS wool had a softer handle (lower RtC values and pulling forces, corresponding to the better subjective softness) at the same level of the MFD.³² The reason why SRS wool is softer than CM wool is still unknown. The comprehensive studies on the key factors affecting the softness performance of wool fibers discussed above are necessary to reveal the differences between the two fibers. This can contribute to better understanding of the difference between SRS and CM wool. This paper examines the contribution that fiber curvature, surface morphology, OtC ratio, crystallinity and moisture regain have on wool fiber softness.

Experimental details

Materials

All wool samples, namely 34 groups of SRS wool and 31 groups of CM wool, were collected from the same farm in New South Wales, Australia, with the same shearing timing. Both types of wool samples were collected from the mid-side of the sheep in March 2018. They were sorted into five sets according to the wool grade (Table 1).

Table 1.
Merino wool micron grade³³

MFD (micron) Grade

14.5 and finer Extrafine

14.6–16.5 Ultrafine

16.6–18.5 Superfine

18.6–20.5 Fine

20.6–22.5 Medium

MFD: mean fiber diameter.

Methodology

Wool scouring

All raw wool samples were scoured in a FSS 40DM ultrasonic cleaning bath (Unisonics, Australia) using an ultrasonic irradiation assisted method. Ultrasonic irradiation was used instead of mechanical agitation as it provided very efficient cleaning whilst avoiding fiber entanglement.^34,35 The scouring recipe is listed in Table 2.

Table 2.
Recipe for the wool scouring process

Condition Recipe

Detergent 0.08% Hydrapol® BD40

Liquor ratio 47:1

Temperature 55°C

Ultrasound frequency 50 kHz

Time of irradiation 3 min, repeated three times

Scoured samples were then lightly squeezed by hand in gloves to remove water before moving them to a rinsing bath (pure water), in which wool samples were slightly agitated manually for 8 minutes. The rinsing process was repeated three times before squeezing the rinsed samples to remove excess water and moving them to a fume hood for air-drying. After the drying process, they were rewashed in pure ethanol at room temperature to remove residual grease. All samples were then dried in the running fume hood before moving them to the standard conditioned laboratory (temperature: 20 ± 1°C, humidity: 65 ± 3%) for 24-hour conditioning.

Measurement of MFC and crimp frequency

An OFDA 2000 (BSC electronics, Australia) was used for measuring wool fiber MFC and MFD according to the International Wool Textile Organisation (IWTO) method TM 47. During the measurement, a bundle of wool fibers was randomly selected from the scoured samples and cut into snippets of approximately 2 mm with a microtome guillotine. These short fibers were then spread on a 70 mm glass slide using the OFDA snippet randomizer and then scanned over 8000 fiber snippets using an optical camera with a light-emitting diode (LED) light source to determine the MFC and MFD (used in the Comparison of the MFC and crimp frequency of SRS and CM wool, Study of the crystallinity of SRS and CM wool and Exploring the influence of moisture regain on the softness of SRS and CM wool sections). Three subsamples from each group of wool samples were measured.

Ten wool bundles (each from a staple) were randomly collected from each group of the remaining raw wool samples. After relaxation of the wool bundles, the crimps within 25 mm length were counted using a ruler (Staples, China). The number of crimps per 10 mm of length was then calculated and used as the crimp frequency.

Ortho-to-cortex ratio determination

Staining the ortho-cortex and para-cortex

Staining the cross-section of a wool fiber with the correct dye can be used to distinguish the ortho-cortex and para-cortex ratios. This enables the calculation of the area proportion of ortho cortical cells within the entire cortex region (OtC ratio). The flowchart of the staining process used is shown in Figure 1. Clean wool fibers were selected from the ethanol-treated samples and weighed at 0.267 g using an analytical balance (Mettler Toledo Ltd, Australia). The clean sample was then put into the dyeing solution (bath ratio at 1:30) constituted by 2.4 ml disodium hydrogen phosphate-sodium dihydrogen phosphate buffer (pH = 7.4), 5.6 ml deionized (DI) water and 3.6% owf methylene blue. A SWB20D vibration dyeing machine (Ratek, Australia) was used to stain wool fibers in a 90°C dyeing bath for 3 hours, after which stained samples were clamped in bundles to a rack in a running fume hood and dried under atmospheric laboratory conditions.

Figure 1.
Schematic progress of staining the ortho-cortex and para-cortex. DI: deionized.

Observing the distribution of the ortho-cortex and para-cortex

Before making an ultra-thin slice of wool cross-section for observation, stained wool fibers were combed into bundles and embedded in hydrophilic resin using the Technovit 7100 embedding system (Kulzer, Germany) based on HEMA* (2-hydroxyethyl methacrylate). Fiber sample bundles were placed into a silicone mold that had been sliced at either end to form a vertical slit. The fibers were placed into the vertical slit under slight tension as this held the fibers parallel while they were mounted in resin. The resin solution was prepared following the step-by-step infiltration and polymerization process before being pipetted into the mold until it was filled. After 5 days hardening in a fume hood, the resin mounted fibers were resin fixed in a parallel orientation. The embedded fibers were then taken out from the silicone mold and cross-sectioned perpendicular to the direction of the fiber into 8 μm slices with a 5062 microtome (SLEE CUT, Germany). The slices were unfolded onto a glass slide using DI water and dried on a heat platform, which assisted in flattening and fixing the slices onto the glass slides. A type F immersion liquid (Wetzlar, Germany) was used to assist imaging by increasing the resolving power and aperture of the objective lens. This was spread evenly on the stained cross-section by using a cover glass. Images were captured using a BX51 optical microscope (Olympus, Japan) in transmission mode.

Some 150–200 fiber cross-sections were randomly selected and examined for each sample group. Cross-sections cut obliquely or buckled were excluded from the analysis. ImageJ 1.5j8 software (Wayne Rasband, USA) was used to draw cross-sections to record the areas of the lightly stained (para-cortex) and heavily stained cells (ortho-cortex) and then measure these areas. The mean diameters of the cortex (used in the Comparative study of the OtC ratios of SRS and CM wool section) were also determined using the drawn round area of the fiber cross-section.

Measurement of crystallinity

Randomly selected wool fibers from each group were integrated into a bundle of around 2000 tex after a manual combing process. Each of these wool bundles for all groups was tightly twisted and firmly adhered to a low noise background holder before it was scanned by an X-pert Pro MRD XL X-ray diffraction (XRD) instrument (Panalytical, Netherlands) with CuKα radiation at a wavelength of 1.542 Å, a generator voltage of 40 kV and a tube current of 30 mA. The XRD pattern was recorded with a Bragg angle 2θ from 5° to 80° at a scanning speed of 0.02° per second. Each sample of 65 groups was scanned only once. A crystallinity index, which reflected the relative crystallinity of the fiber, was introduced and calculated using the following equation³⁶
$C I = \frac{I_{9} - I_{14}}{I_{14}} \times 100 %$
(1)
where CI is the crystallinity index; I₉ represents the maximum intensity of crystal lattice diffraction with Bragg angle 2θ at approximately 9°; and I₁₄ refers to the minimum diffraction intensity in the same unit with 2θ at about 14°. In general, higher CI values corresponded to higher crystallinity of a wool fiber sample.

Moisture regain measurement

Three randomly selected wool samples from each group were weighed at 1.000 ± 0.001 g using an XS204 analytical balance (Mettler Toledo Ltd, Switzerland) and then wrapped in numbered tin foils (Confoil, Australia) before being dried in an FP 115 oven (Binder, Germany) with a temperature of 105 ± 3°C for 4 hours. Bone dry samples were sealed in the tin foil to isolate samples from moisture in the air before they were weighed together, noted as M₀. The tin foil mass was measured after wool sample removal and noted as M₁. All weighed samples were moved to the standard laboratory (temperature: 20 ± 1°C, humidity: 65 ± 3%) to condition for 48 hours, then weighed and noted as M₂. The moisture regain was calculated with the equation below
$Moisture regain = \frac{M_{2} {- (M}_{0} - M_{1})}{{(M}_{0} - M_{1})} \times 100 %$
(2)

Surface morphology

In this study, a Supra 55VP (Carl Zeiss AG, Germany) was used to take scanning electron microscope (SEM) images of the wool fibers. Wool fiber bundles were randomly selected from each conditioned sample (65 groups of samples in total) and mounted under a small tension on a specialized holder to enable parallel fibers for imaging. Fibers were coated with 5-nm gold powder by an ACE600 sputter coater (Lecia EM, Australia) in the tilted mode. Image magnification varied according to measurement requirement: 5000× for the measurement of wool scale height; 1000× for the measurement of scale frequency and MFD (used in the Influence of surface morphology on the softness of SRS and CM wool section). During measurement, the fiber should be straight and horizontal along the length direction to facilitate measurement for cuticle scale height and frequency. For a single wool fiber, five positions along the fiber length were randomly selected and every corresponding SEM image containing two top ends of scales were taken for scale height measurement. The SEM images for the measurement of scale frequency and MFD were also randomly taken from five positions along the fiber length. For each group of wool samples, 10 fibers were randomly selected, and their morphologies were recorded as images of two different magnifications. ImageJ 1.5j8 (Wayne Rasband, USA) was then utilized to extract data of the scale height, scale frequency and MFD from the SEM images. Two scale heights were determined from each image and this measurement was repeated for five images for each single fiber. Every single fiber was measured five times for scale frequency according to the images of different segments, while the fiber diameters in 10 randomly selected positions in each of these images were measured.

Results and discussion

Comparison of the MFC and crimp frequency of SRS and CM wool

The MFC results of SRS and CM wool staples are compared in Figure 2; they decreased as the MFD increased for both SRS and CM wool. The rate of fall of the regression line for SRS wool was steeper than that of CM wool, while the overlapping area could indicate that there may not be a significant overall difference in the MFC for the two wools with MFD finer than 16 microns. Further comparison was conducted using the analysis of variance (ANOVA) test method (IBM SPSS Statistics). Analysis of the data (as shown in Figure 2) revealed that the MFC of SRS wool was significantly lower than that of CM in the Extrafine set (approximately by 7.9%) and Superfine set (approximately by 12.6%), respectively, while the figures of these two types of wool were similar in the Ultrafine set. This was the reason why the difference in the RtC values (softness) of SRS and CM wool is larger in the Extrafine and Superfine sets than in the Ultrafine set in Table 3 (the Fine and Medium sets were excluded from this comparison due to an insufficient number of specimens).

Figure 2.
Relationship between the mean fiber curvature and mean fiber diameter of each staple. SRS: Soft Rolling Skin; CM: conventional Merino.

Table 3.
Resistance to compression (RtC) for Soft Rolling Skin (SRS) and conventional Merino (CM) wool staples in different wool grades³²

Wool grade n Mean RtC (kPa) Mean difference (SRS – CM) Std. deviation Std. error

Extrafine SRS [CM] 24 [9] 5.18 [6.19] –1.01* 0.55 [0.25] 0.11 [0.08]

Ultrafine SRS [CM] 37 [28] 5.16 [5.86] –0.70* 0.36 [0.55] 0.06 [0.10]

Superfine SRS [CM] 36 [23] 4.96 [6.65] –1.7* 0.65 [0.76] 0.11 [0.16]

Fine SRS [CM] 5 [30] 5.16 [6.19] –1.03* 0.45 [0.52] 0.20 [0.09]

Medium [CM] [3] [5.61] – [0.18] [0.11]

n: the number of samples involved.

[] The content in this bracket is for CM wool.

* This means the difference is statistically significant at the level of 5% (P = 0.05).

During sample preparation for the pulling force test, the MFC of wool bundles would be affected by the strain caused by separating these bundles from the clean wool sample mass, which might lead to the different distributions of MFC in each wool grade set between SRS and CM wool (Figure 3). There was a similar drop trend in the MFC for both SRS and CM wool as the MFD increased. Even though the regression line for SRS wool was below that of CM wool, the overlapping area also indicated that there may not be a significant overall difference in the MFC for the two wools. Results from the multiple comparison of MFC based on wool grade sets have shown that the MFC of SRS wool is also lower than that of CM wool in the Extrafine set and the Superfine set (approximately lower 9.7% and 10.0%, respectively). The significant mean difference in the pulling force (softness) between SRS and CM wool in the Extrafine set (–14.44) was similar to that in the Ultrafine set (–14.96), and this similarity was also found between the Superfine set (–20.20) and the Fine set (–19.01) (Table 4). A lower MFC did not always correspond to a lower pulling force, suggesting that MFC had a small influence on the pulling force.²⁶ It is likely that there are other factors that affect the pulling force (softness) of SRS and CM wool.

Figure 3.
Relationship between the mean fiber curvature and mean fiber diameter for each bundle. SRS: Soft Rolling Skin; CM: conventional Merino.

Table 4.
Pulling force for Soft Rolling Skin (SRS) and conventional Merino (CM) wool staples in different wool grades³²

Wool grade n Mean pulling force (cN/ktex) Mean difference (SRS – CM) Std. deviation Std. error

Extrafine SRS [CM] 47 [23] 97.87 [112.31] –14.44* 8.70 [10.56] 1.27 [2.20]

Ultrafine SRS [CM] 141 [85] 102.82 [117.78] –14.96* 11.43 [12.94] 0.96 [1.40]

Superfine SRS [CM] 109 [62] 103.24 [123.44] –20.20* 13.18 [16.19] 1.26 [2.06]

Fine SRS [CM] 43 [105] 104.24 [123.25] –19.01* 15.29 [14.14] 2.33 [1.38]

Medium [CM] [35] [130.37] – [11.97] [2.02]

n: the number of samples involved.

[] The content in this bracket is for CM wool.

* This means the difference is statistically significant at the level of 5% (P = 0.05).

One of the factors affecting softness might be the crimp frequency; a comparison of this between SRS and CM wool is shown in Figure 4. The crimp frequency decreased as the MFD increased, but increased with the increase in MFC. Finer wool fiber had a higher crimp frequency. There was no significant difference in the crimp frequency between SRS wool (7.8/cm) and CM wool (7.9/cm) (Figures 4(a) and (b)). In each set, the difference in the crimp frequency between the two wools was statistically non-significant.

Figure 4.
Relationship of crimp frequency between Soft Rolling Skin (SRS) and conventional Merino (CM) wool against (a) mean fiber diameter and (b) mean fiber curvature.

Comparative study of the OtC ratios of SRS and CM wool

Although no overall differences were found in the MFC and crimp frequency between SRS and CM wool, the MFC of SRS wool was lower than CM wool in the Extrafine and Superfine sets. The underlying reason needed to be explored. As discussed in the introduction, the OtC ratio might affect the fiber curvature structure, so staining of the ortho-cortex and para-cortex was conducted.

The cross-sections of wool fibers are observed in Figure 5. All cross-sections were completely stained by methylene blue, even if the ortho-cortex and para-cortex could not be clearly distinguished before the immersion liquid was used (Figure 5(a)). In contrast, Figure 5(b) shows two stained areas with different color depths.

Figure 5.
Stained cross-sections of wool fibers (a) before and (b) after using immersion liquid.

According to the clear difference, the OtC ratio of SRS and CM wool was measured using image analysis. The results from Figure 6(a) demonstrate that the OtC ratio randomly changed with the increase of MFD, while the ratio for SRS wool was partially lower than that of CM wool over the whole MFD range. Statistical analysis of the OtC ratio was conducted and the means of each grade set for the two types of wool are compared in Figure 6(b). They both revealed that SRS wool had a lower OtC ratio (approximately 0.60) than CM wool (approximately 0.66) in every set except for the Broad set. The difference in the Broad set needs more specimens involved in further study to eliminate the high standard error. The lower OtC ratios reflected more balanced bilateral distribution of the ortho-cortex and para-cortex, which could reduce the internal strain energy. This may be a reason for the lower MFC of SRS wool in the Extrafine and Superfine sets, but it is not the only one, because there was no significant difference in the MFC between SRS and CM wool in other wool grade sets in which the OtC ratios were lower for SRS wool.

Figure 6.
(a) Change of ortho-to-cortex (OtC) ratio against mean fiber diameter. (b) Comparison of OtC ratio between Soft Rolling Skin (SRS) and conventional Merino (CM) wool in wool grade sets.

Study of the crystallinity of SRS and CM wool

Typical XRD spectra for each of the two wool types are shown in Figure 7(a). There were two strong peaks at Bragg angle 2θ = 9° and 21° for an individual wool sample. Relevant research demonstrated that these two peaks were attributed to the α-helix and β-sheet structure of peptide chains in wool,^37,38 respectively, while a diffraction trough observed at 2θ° = 14° between these two characteristic diffraction peaks represented the amorphous region of wool fiber. The XRD patterns of SRS and CM wool were similar, as well as the intensity of both characteristic peaks and troughs, meaning that the difference in the crystal structure of these two wools was very low. The results from Figure 7(b) illustrate that there was no correlation between the CI value and MFD. The CI value remained relatively stable as the MFD increased. The CI values of SRS wool were the same as those of CM wool and all of them fluctuated around 45.9%.

Figure 7.
(a) X-ray diffraction patterns and (b) CI of Soft Rolling Skin (SRS) and conventional Merino (CM) wool.

Measuring the difference in crystallinity of the wool fiber was envisaged to be a simple rapid softness evaluation method for SRS wool because of the influence of crystallinity on fiber softness. The lack of observable difference between the two wools means that this may not be an option. The low difference may be due to the small OtC ratio difference (0.06) between ortho- and para-cortical cells being reduced by the measured difference of crystallinity between the two cells.

Exploring the influence of moisture regain on the softness of SRS and CM wool

Figure 8(a) illustrates that the change of moisture regain was independent of the increase of MFD, and there was a difference in the distribution of moisture regain between SRS and CM wool. The mean moisture regain of these two wools is compared in Figure 8(b). The moisture regain was higher for SRS wool (14.13%) than CM wool (13.79%). Even though the moisture regain difference was small, it was significant (P < 0.05). This meant SRS wool may have a slightly lower Young’s modulus or torsional rigidity than CM wool as they have a negative relationship with moisture regain,³⁹ which contributed to the lower pulling force and RtC values, reflecting the softer handle.

Figure 8.
(a) Moisture regain against mean fiber diameter. (b) Mean moisture regain of Soft Rolling Skin (SRS) and conventional Merino (CM) wool.

It is not clear if the change in the OtC ratio of SRS wool compared to CM wool led to the difference in moisture regain. Further research is needed to explore the relationship between the OtC ratio and moisture regain.

Influence of surface morphology on the softness of SRS and CM wool

The surface morphologies of SRS and CM wool with similar diameter are shown in Figure 9. These two fibers were selected to best represent the differences seen over the 650 fibers measured in this work. SRS wool had lower cuticle scale height when comparing Figures 9(a) and (c), which led to a smoother surface than CM wool, displayed in Figures 9(b) and (d). According to these images, there was no significant difference in the cuticle scale frequency between SRS and CM wool.

Figure 9.
Micromorphology of Soft Rolling Skin (a), (b) and conventional Merino wool (c), (d) with similar fiber diameter at magnifications of 5000× and 1000×.

The quantified results of cuticle scale height for the two wools are compared in Figure 10(a). The cuticle scale height increased as the MFD increased for both SRS and CM wool, and the figure for SRS wool was approximately 0.11 μm lower than that for CM wool in the total mean (P < 0.05), which that revealed the difference in the cuticle scale height between the two wools was approximately 0.07 μm for Extrafine, Ultrafine and Superfine sets and 0.09 μm for Fine and Medium sets. The parallel regression line revealed that the cuticle scale height of SRS wool was approximately 0.08 μm (average 13.5%) lower than CM wool, meaning that SRS wool had a smoother surface (corresponding to lower pulling force and RtC values in Tables 3 and 4). This is another significant reason for the softer handle of SRS wool. The cuticle scale frequency of wool fiber had a negative relationship with MFD (Figure 10(b)), decreasing with the drop in MFD. No significant difference in the cuticle scale frequency was found between SRS and CM wool, and this explained why the cuticle scale frequency was not a factor in the different softness performance of these two wools.

Figure 10.
Changes of (a) scale height and (b) scale frequency with the increase in mean fiber diameter for Soft Rolling Skin (SRS) and conventional Merino (CM) wool.

Conclusion

The curvatures of SRS wool in the Extrafine and Superfine sets were lower than those of CM wool and were similar in other sets (Ultrafine, Fine and Medium). The difference in the curvature contributes to the softer handle for SRS wool. There is a difference in the distribution of the ortho-cortex and para-cortex, reflecting in the different OtC ratios for the two types of wool fibers. The lower OtC ratio for SRS wool may correspond to the lower curvature compared with CM wool. The small change in OtC ratio does not lead to a significant difference in crystallinity between SRS and CM wool, meaning that comparing softness using the observed difference in crystallinity may not be suitable for these two wools. The moisture regain is higher for SRS wool, contributing to its better softness compared with CM wool. More research is necessary to confirm whether the OtC ratio is correlated with moisture regain. SRS wool has different surface morphology compared with CM wool, while the scale frequency of these two wools is similar. The scale height of SRS wool is lower, meaning a smoother fiber surface and softer handle.

The softness of wool fibers is associated with various factors, and SRS wool is softer than CM wool. Understanding these key factors contributing to the softer handle of SRS wool adds new knowledge to wool science and benefits future studies on the performance of this fiber at the yarn and fabric stages.

MFD (micron)	Grade
14.5 and finer	Extrafine
14.6–16.5	Ultrafine
16.6–18.5	Superfine
18.6–20.5	Fine
20.6–22.5	Medium

Condition	Recipe
Detergent	0.08% Hydrapol® BD40
Liquor ratio	47:1
Temperature	55°C
Ultrasound frequency	50 kHz
Time of irradiation	3 min, repeated three times

Wool grade	n	Mean RtC (kPa)	Mean difference (SRS – CM)	Std. deviation	Std. error
Extrafine SRS [CM]	24 [9]	5.18 [6.19]	–1.01*	0.55 [0.25]	0.11 [0.08]
Ultrafine SRS [CM]	37 [28]	5.16 [5.86]	–0.70*	0.36 [0.55]	0.06 [0.10]
Superfine SRS [CM]	36 [23]	4.96 [6.65]	–1.7*	0.65 [0.76]	0.11 [0.16]
Fine SRS [CM]	5 [30]	5.16 [6.19]	–1.03*	0.45 [0.52]	0.20 [0.09]
Medium [CM]	[3]	[5.61]	–	[0.18]	[0.11]

Wool grade	n	Mean pulling force (cN/ktex)	Mean difference (SRS – CM)	Std. deviation	Std. error
Extrafine SRS [CM]	47 [23]	97.87 [112.31]	–14.44*	8.70 [10.56]	1.27 [2.20]
Ultrafine SRS [CM]	141 [85]	102.82 [117.78]	–14.96*	11.43 [12.94]	0.96 [1.40]
Superfine SRS [CM]	109 [62]	103.24 [123.44]	–20.20*	13.18 [16.19]	1.26 [2.06]
Fine SRS [CM]	43 [105]	104.24 [123.25]	–19.01*	15.29 [14.14]	2.33 [1.38]
Medium [CM]	[35]	[130.37]	–	[11.97]	[2.02]

Footnotes

Acknowledgements

The authors wish to acknowledge Graham Walters, Errol Brumpton, Jim Gordon, Sally Watts and the late Dr Jim Watts for their great assistance with wool sampling for this work. The authors are also grateful to the Deakin Advanced Characterisation Facility (Dr Adam Taylor and Dr Andrew Sullivan) for technical support.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported in part by the ARC Research Hub for Future Fibers (IH140100018).

ORCID iDs

Xin Liu

Xungai Wang

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