Determining the Effect of Pigmentation on Some Physical and Mechanical Properties of Yak and Cashmere Down Fibers

ABSTRACT In this study, it was aimed to investigate the effect of pigmentation on the physical and mechanical properties of white and brown colored cashmere and yak down fibers in comparison with wool. According to experimental results, it was found that the pigmented fibers have higher diameter and length compared to their white equivalents. However, tenacity and elongation values were lower in pigmented fibers. On the other hand, it can be said that brown fibers had lower felting tendency compared to white fibers.


Introduction
Cashmere goats have two fibrous coats: a coarse guard hair (outer coat) and a fine down coat (undercoat) (Hunter 2012). Cashmere fibers have colors such as light gray, dark gray, and brown tones as well as white (Dalton and Robert 2001). There is also black cashmere (Hunter 2012). On the other hand, the fleece of yak contains down fiber, mid-type hair, and coarse hair (Jie and Yangxi 2004). The hair color of yaks is strongly influenced by the local climate and can be divided into the colored and colorless types, in which black yak makes up 80% of the total survey, followed by blurred black and brown (Li et al. 2016). Although many animals are black, a small number of animals have brown colored coat. On the other hand, white is the most valuable due to its dyeing potential. The approximate proportions of the different natural colors of yak fibers can be summarized as; white: ~10%, fawn: ~20%, dark gray: ~10%, dark brown: ~60% (Das et al. 2017). There are limited number of studies examining physical and/or mechanical properties of yak and cashmere down fibers. Cashmere, alpaca, and mohair and other rare animal fibers from different origins were objectively measured by McGregor (2014) for diameter, crimp, and resistance to compression (softness). The softest fibers were alpaca, mohair, and cashgora, and all fibers tested were softer than most Merino wool. In the study carried out by McGregor and Schlink (2014), the feltability of cashmere down fiber from different origins and yak down fiber showed large variation between and within these fiber types. Liu, Xie, and Liu (2018) found the fineness of white cashmere, gray yak, and black yak down fibers as 16.5 µm, 18.8 µm, and 20.0 µm, respectively. Cashmere was a little longer than yak fibers, the length of cashmere was 40.3 mm, and the length of yak fibers was about 35.0 mm. On the other hand, it was stated that the strength and elongation of yak fibers were better than cashmere. Wang and Qu (2021) studied the properties of the yak and cashmere down fibers. It is shown that the compositions of yak and cashmere are similar, and this makes the physical properties of yak and cashmere fibers similar.
Color is very common in luxury fibers, and white fibers are generally preferred since they can be dyed to the desired color. However, the use of pigmented fibers in their natural colors is a very environmental-friendly approach since it does not require a dyeing process. At this point, it is necessary to examine the advantages or disadvantages of colored cashmere and yak fibers in terms of fiber properties compared to white ones. Therefore, in this study, it was aimed to investigate the effect of pigmentation on some physical and mechanical properties of white-and brown-colored cashmere and yak down fibers in comparison with wool. Pigmentation is a common feature in luxury fibers, and it has not been revealed what kind of disadvantages, apart from not being able to be dyed to the desired color, or advantages, if any, pigmented fibers have compared to white ones. Only in some studies mentioned above, some fiber properties were investigated for white and pigmented fibers. However, to the best of the authors' knowledge, there is no comprehensive study on how pigmentation affects various properties of the cashmere and yak down fibers in comparison with wool.

Material and method
In this study, sheep wool fibers, yak down fibers and cashmere down fibers in two different colors (white and brown) were used. Cashmere and wool fibers were kindly supplied from Yünsa Inc.-Turkey. Yak fibers were obtained from local farms in Mongolia. Photos of fiber samples were given in Table 1.
Fiber samples were selected randomly from the main stack for testing and each sample was conditioned under standard laboratory conditions (20 ± 2°C, 65 ± 2% relative humidity). Measurements of fiber fineness were carried out with optical fiber diameter analyzer (OFDA) according to IWTO 47 standard (IWTO 47 2013). The diameters were converted to dtex values by the formula given in Equation (1), where d is the diameter, Ndt is the dtex, and γ (taken as 1.32 g/cm 3 ) is the density of fiber (Liu, Xie, and Liu 2018). Felting propensity of each fiber was examined according to Aachen felting test method. 1 g of each type of fiber was soaked in 50 mL of distilled water and agitated in a Termal HT type dyeing machine for 60 minutes to form felt balls. Then, the formed balls were taken out and dried. The diameter of each ball was measured in nine directions as described in previous studies (Gürkan Ünal and Atav 2018;Liu and Wang 2007) which was used to calculate the ball volume. After obtaining felt ball volumes, densities of each felting ball (g/cm 3 ) were calculated. From the individual values of diameter after rotation, 3 means in three dimensions (d X , d Y and d Z ) were obtained. Thus, the mean felt ball diameter "d" in centimeters was obtained from 9 individual results. The mean felt density δ for a felt ball was calculated by: where; δ is mean felt density in g/cm 3 , d is mean felt ball diameter from the nine measurements on one felt ball in cm, and G is the weight of the felt ball. As specimen weight of 1 g was prescribed for the felting test, the above formula could be simplified to (IWTO-20-04 2004): The felt ball density (δ) is used as an indication of the feltability. The smaller the ball, the greater the felting. In other words, the higher the ball density value, the greater the feltability of fibers.
For the statistical analysis of the fiber properties, Minitab 20 program was used. Energy diffractive X-ray (SEM-EDX) analysis of the fibers were carried out with the FEI brand QUANTA FEG 250 model device to determine the elemental composition of fibers.

Results and discussions
The results of some physical and mechanical properties of fibers were presented in Table 2 and variance analysis results were summarized in Table 3.
Fiber type, fiber color and interaction of these two factors had a statistically significant (p < .05) effect on fiber fineness as can be seen in Table 3.
In terms of fiber fineness (µm) it was found that fibers in ascending order were white cashmere (14.64) → brown cashmere (19.09) → white wool (20.39) → white yak (22.06) → brown yak (27.81) → brown wool (28.04). It is important to note that the mean fiber diameter is mainly determined by the genotype of the animal, but it is also considerably affected by external factors (e.g., nutrition). Another important result was that the pigmented fibers (in this study brown fibers) had higher diameters compared to their white equivalents, which meant pigmented fibers were coarser. As it is known, the color of pigmented fibers are derived from colored pigments in the cortical cells (Tarakçıoğlu 1983). Discrete granules of colored pigments are 2 μm in size (Anton and Lawrance 2010). Therefore, follicles producing pigmented fibers are larger and they produce coarser fibers. Based on multiple comparison results, the effect of fiber color (brown and white) and fiber type (wool, yak and cashmere) on fiber fineness were statistically significant (p < 0:05). The diameter of yak fiber is around 15-30 μm for down hair and around 35-80 μm for coarse hair (Markova 2019). In the previous studies (Li et al. 2016;Liu, Xie, and Liu 2018;McGregor 2014;Phan et al. 1987;Wang and Qu 2021;Xiaoxuan et al. 2019;Zoccola et al. 2013) average fiber diameter was found as 15-20 µm. However, Wang and Qu (2021) found that fiber diameter of yak down was 16.25-23.73 μm. On the other hand, in this study average fineness value was found to be 22.06 and 27.81 for white and brown yak down fibers, respectively. Furthermore, both yak fibers were coarser than white wool used in this study which was 20.39 µm. It could be said that average fiber diameter of yak fibers was found to be higher, especially for pigmented yak fiber, compared to literature. The reason of this is that the yak fibers used in this study were dehaired by hand and for this reason coarse hair content could be higher and this increased the average value.
As it is known, the diameters of the cashmere down fibers vary between 13 and 19 µm (Vineis, Aluigi, and Tonin 2008). Phan et al. (1987) stated that the diameters of cashmere fibers run on an average between 13 µm and at most 15-16 µm. Zoccola et al. (2013) found the average fineness value of white and pigmented cashmere fibers as 15.02 and 16.54, respectively. Liu, Xie, and Liu (2018) stated that the fineness of white cashmere was 16.5 um. Wang and Qu (2021) found the average fineness value of white, purple, and cyan cashmere fibers as 15.87, 16.45 and 16.49, respectively. From this point of view, it can be said that the values obtained in this study were within normal limits. First quality cashmere, on the other hand, should have an average fiber diameter of less than 15.5 μm. Cashmere with an average fiber diameter greater than 15.5 μm is second grade (Hunter 2020). This situation reveals that the pigmentation in cashmere fibers leads to poor quality. The comments made regarding the fineness values of the fibers in microns were also valid for the values in dtex given in Table 2 since dtex values were transformed from micron values.
Results of mean fiber length according to single fiber length measurement method were presented in Table 2, and analysis of variance results of fiber type and color factors on the fiber length were summarized in Table 3. Fiber type had a statistically significant effect on the fiber length (p < 0:05). However, fiber color and interaction of two factors did not have a statistically significant effect (p > 0:05) on fiber length.
Concerning the fiber length (cm) it was found that fibers in ascending order were white yak (6.74) → brown yak (7.53) → white cashmere (7.76) → brown cashmere (8.10) → white wool (9.34) → brown wool (9.45). Based on multiple comparison results, the difference between the length of wool and other fibers (yak and cashmere) were statistically significant (p < 0:05). Even if the difference between the length of cashmere and yak fibers are statistically insignificant, it can be said that comparing with the cashmere fiber, the average length of the yak fiber was lesser. Another important result was that the pigmented fibers had higher fiber length compared to their white equivalents. As known, sheep breeds giving coarse fibers, generally have longer fiber length because coarser fibers can reach higher length on the animal. The numbers in bold means that the factor is important in terms of that fiber property.
The fiber length mainly influences the spinning performance and the fabric quality (Liu, Xie, and Liu 2018). Besides, it influences the type of manufacturing process (worsted, semi-worsted or woolen) used (Khan et al. 2012). For very high-quality knitting and weaving yarns, it is desired that the cashmere is 40 mm or even longer (Hunter 2020).
As stated in the literature the average length of yak down was 3.5-5 cm ). Liu, Xie, and Liu (2018) found the average length (cm) of gray and black yak fibers as 3.56 and 3.51, respectively. Xiaoxuan et al. (2019) found average fiber length of brown yak fiber as 3.05 cm. Wang and Qu (2021) found the average length (cm) of brown and cyan yak fibers as 2.77 and 2.67, respectively. However, the results obtained in this study was higher compared to literature. As explained previously, the yak down fibers used in this study were dehaired by hand and for this reason coarse hair content could be higher and this might have increased the average value.
As it was stated, the length of the fine cashmere down fibers varied between 2 and 5 cm on average (Vineis, Aluigi, and Tonin 2008). Wang and Qu (2021) found the average length (cm) of white, purple, and cyan cashmere fibers as 3.67, 3.60 and 3.36, respectively. Liu, Xie, and Liu (2018) found average fiber length as 4.03 cm. The average fiber length of the down fibers of Chinese cashmere in the raw state ranged from about 21 to 40 mm, and the single fiber lengths ranged from about 5 to 80/90 mm. The length of Asian cashmere was between 21 and 40 mm, and single fiber lengths were between 10and 90-mm. Cashmere grown in Australia and New Zealand was longer than that produced in other countries (Hunter 2020). In the study by McGregor (2007)the fine down fiber length for Australian cashmere was found to be 75.3 ± 20.4 mm. For this reason, it could be said that results obtained in this study were consistent with literature.
Mean results of fiber strength, and analysis of variance results of fiber type and color factors on the fiber strength and elongation were presented in Tables 2 and 3, respectively. Fiber type and color were found to have a statistically significant effect on the fiber strength (p < 0:05). However, interaction of fiber type and color did not have a statistically significant effect (p > 0:05) on fiber strength.
As one of the basic properties of fiber, strength determines the fiber application directly and influences the spinning process (Wang and Qu 2021). Fiber strength plays an important role in worsted processing performance, in terms of fiber breakage during processing, particularly during carding, and in the strength of the yarn and fabric (Anton and Lawrance 2010). Concerning the fiber strength (cN) it was found that fibers in descending order were brown wool (8.78) → brown yak (7.56) → white yak (5.66) → white wool (5.54) → brown cashmere (5.42) → white cashmere (4.10). Liu, Xie, and Liu (2018) found the average strength (cN) of gray and black yak fibers as 4.5 and 7.5, respectively. Xiaoxuan et al. (2019) found average strength of brown yak fiber as 8.32 cN. Wang and Qu (2021) found the average strength (cN) of brown and cyan yak fibers as 9.01 and 7.46, respectively. On the other hand, strength of white cashmere fibers was found as 4.3 cN by Liu, Xie, and Liu (2018). Wang and Qu (2021) found the average strength (cN) of white, purple, and cyan cashmere fibers as 5.46, 5.22 and 5.78, respectively. It could be stated that strength values of yak and cashmere fibers obtained in this study were consistent with literature.
Tenacity values in Table 2 were obtained by dividing the strength by the fiber diameter. It can be clearly seen from Figure 1 that pigmented fibers had lower tenacity values compared to their white equivalents. Also, it can be said that cashmere fibers had higher tenacity values compared to yak and wool fibers of same color. The structure that played an important role on the strength-elongation behavior of protein fibers was disulfide bridges between macromolecules. Cystine amino acids were the most important among the 22 types of alpha-amino acids forming wool keratin due to the formation of covalent bonds (disulfide bridges) between fiber macromolecules. The distribution within the macromolecule, the distribution within the fibers, and the amount in the various fibers could be different for cystine amino acid. It is possible to determine the amount of cystine by determining the amount of S in the fibers. Because a very large part of the sulfur belongs to the cystine monomer. The remaining few belongs to other building blocks such as cysteine, methionine, lanthionine and cysteine acid (Seventekin 2004). SEM-EDX analyses were performed to determine the amount of S in the fibers, and the results were given in Table 4.
When Table 4 was examined, it was seen that the sulfur content (%) of pigmented fibers obtained from each fiber types was lower. This explained the strength-elongation differences between pigmented fibers and white ones. The sulfur content compared here is the S content in % of the fiber structure. It was therefore normal that the sulfur content in the pigmented fibers, which were coarser and containing the medulla layer, was lower than the white fibers. This situation also caused the tenacity values of pigmented fibers to be lower compared to their white equivalents.
Mean results of fiber elongation and analysis of variance results of fiber type and color factors on the fiber elongation were presented in Tables 2 and 3, respectively. It was found that neither fiber type and color nor interaction of these two factors had a statistically significant effect on fiber elongation. Liu, Xie, and Liu (2018) found the average elongation (%) of gray and black yak fibers as 40.5 and 36.3, respectively. Xiaoxuan et al. (2019) found average fiber elongation of brown yak fiber as 32.78%. Wang and Qu (2021) found the average elongation (%) of brown, and cyan yak fibers as 49.59 and 36.41, respectively. On the other hand, elongation of white cashmere fibers was found as 37.9 by Liu, Xie, and Liu (2018). Wang and Qu (2021) found the average elongation (%) of white, purple and cyan cashmere fibers as 44.17, 40.69 and 49.79, respectively. It could be said that elongation at break (%) values of yak fibers found in this study were consistent with literature, however elongation values of cashmere fibers were lower than the previous studies.
Results of felt ball densities of fibers, which was used as an indicative for fiber felting, were presented in Table 2 and analysis of variance results of fiber type and color factors were presented in Table 3.
Fiber type and fiber color had a statistically significant effect on the fiber felting (p < 0:05). However, interaction of these two factors did not have a statistically significant effect on fiber felting.
Felting is a unique property of many animal fibers. Scales of wool and other animal fibers are believed to be the major contributor to the felting shrinkage of products made from these fibers (Liu and Wang 2007). When the felt ball density values given in Figure 2 were examined in detail, it could be said that the fibers having the highest and lowest felting tendency were white wool and brown cashmere, respectively. Concerning the fiber felt ball density values, it was found that fibers in descending order were white wool (0.093651) → white yak (0.088693) → brown yak (0.069114) → brown wool (0.066698) → white cashmere (0.064314) →brown cashmere (0.060014). Based on multiple comparison results, the difference between the white and brown fibers were statistically significant (p < 0:05). It can be said that white fibers had higher felting tendency compared to brown fibers. Fiber diameter on felting shrinkage has been studied by many workers and it is generally accepted that fine wools felt more than coarse wools (Liu and Wang 2007). That is coarser fibers have a less felting propensity due to high bending rigidity of fibers. From this point of view, the finer white fibers felted more than the coarser pigmented fibers, which is compatible with the literature. But in terms of fiber type, cashmere fibers felted the least despite being the finest. The reason of this phenomenon has been well explained by Gürkan Ünal and Atav (2018). They stated that since the felting tendencies of fibers from various origins were compared with each other, it was difficult to find a direct relation between felting tendency and fiber properties such as fineness, length etc. When the effects of the parameters from literature on the felting tendency were considered, it is supposed that while any fiber shows a low felting tendency regarding one of fiber properties such as fineness, length, the other fiber property of the same fiber can affect its felting tendency reversely. For instance, in this study cashmere fiber is supposed to felt more due to the lower fiber diameter, on the other hand from the literature (Hunter 2012;Wortmann and Arns 1988) it is known that the scale height of cashmere fiber is weaker than wool which will cause less felting. For this reason, cashmere fiber felted less than wool in despite of lower diameter.
Liu and Wang showed that fiber length had a significant influence on fiber feltability, with longer fibers felting more than shorter fibers (Liu and Wang 2007). Also, in this study the same tendency was observed. White wool, which is longer than white cashmere and white yak, felted more. Similarly brown wool, which is longer than brown cashmere and brown yak, felted more. However, even though pigmented fibers are longer than white ones, they are less felted because the difference in length between them is small, but the difference in fineness is significant, so the effect of fineness on the felting tendency outweighs.

Conclusions
In this study, it was aimed to investigate the effect of pigmentation on some physical and mechanical properties of white and brown colored cashmere and yak down fibers in comparison with wool. According to these results it can be concluded that the pigmented fibers (in this study brown fibers) are coarser and longer compared to their white equivalents However, tenacity and elongation values were lower and the variance of fiber strength was larger in pigmented fibers. This will make the yarn spinning more difficult for pigmented fibers compared to white ones even if the pigmented fibers are longer which is an advantage in terms of spinning. Although the tenacity and elongation of pigmented fibers are lower compared to white fibers, they are still at a very good level, and it should be noted that if the correct process conditions are set, it will be possible to spin yarn from them. On the other hand, it can be said that brown fibers had lower felting tendency compared to white fibers. This is very important issue for wool fabrics in terms of consumer expectations. These results indicate that pigmented fibers can also be used in textile production and this will be environmentally friendly approach since they do not require a dyeing process. However, at this point, it would useful to remind the fact that the variation in natural animal fibers is very high. For this reason, it should be underlined that for each fiber type, more precise judgments can be made by analyzing controlled samples from a large population representing white and colored fibers.

Highlights
• Article represents the results of a comprehensive study on how pigmentation affects various properties of the fibers in cashmere and yak fibers in comparison with sheep wool. • It was found that the pigmented fibers (in this study brown fibers) have higher micron values and average fiber length compared to their white equivalents. • Tenacity and elongation values were lower, but the variance of fiber strength was larger in pigmented fibers. • Brown fibers had lower felting tendency compared to white fibers, which is very important issue for wool fabrics in terms of consumer expectations. • These results indicate that pigmented fibers can also be used in textile production and this will be environmentally friendly approach since they do not require a dyeing process.