Optimization and performance evaluation of silk fabrics

dried in the domestic dryerYuhui Wei1,2,3,5, R. Hugh Gong3, Zhaowei Su2,4,Xu Wang2, and Xuemei Ding1,5

1College of Fashion and Design,Donghua University, Shanghai 200051, P.R. China; 2Henan University of Engineering, Zhengzhou 451191,P.R. China;

3Textiles, School of Materials, University of Manchester, Manchester, M13 9PL, U.K.;4Hangzhou technology college, Hangzhou 310018,P.R. China;

5Key Laboratory of Clothing Design & Technology (Donghua University), Ministry of Education, Shanghai 200051

ABSTRACTDespite the silk fabrics were widely used as luxury textile products, little information is available for dryer manufacturer setting drying parameters that consumers can use for silk production care. The purpose of this study is to determine the optimal drying procedure for large load and non-ironing procedure for small load when silk fabrics were used in daily life. Change in appearance, mechanical properties and microstructure of silk fabrics before and after drying were investigated. Drying experiments with various drying parameters (heater power, air flow rate and drum rotating speed) were performed on a self-modified domestic hot-air drum dryer. Experimental results reveal that the performance of silk fabric after drying is more sensitive to air flow rate (relative humidity of drying air) rather than heater power (temperature). The minimum drying damage is caused by lower heater power, absence of mechanical agitation and above moderate air flow rate. The total drying time is strongly dependent on heater power and air flow rate. Increasing these parameters shortens the drying time considerably. A drying procedure of heater power of 3000W, air flow rate of 8.5m/s and drum rotating speed of 45-50rpm (No.2) is proved to be optimal for drying silk fabrics due to its ideal smoothness appearance, dimensional stability and drying efficiency. When consumers dry a small amount of clothes, a drying procedure of heater power of 3000W, air flow rate of 8.5m/s and drum rotating speed of 0rpm (No.6) is more reasonable procedure because it reduces or eliminates ironing. And results shows that drum drying in dryer can be an effective alternative to indoor-drying or sunlight-drying if a suitable procedure is used. This finding can help the drying machine manufacturers to design and improve of dryers, especially for drying procedures optimization of delicate textiles. And it also provides consumers a feasible care method that does not reduce the quality of the dried silk fabric and the drying efficiency.

KEYWORDS: silk fabrics; domestic drying care; damage; performance optimization

IntroductionSilk fabrics were widely used for luxury textile products for thousands of years, owing to their natural and gentle luster, smooth and soft texture, superior drapability, good hygroscopicity, excellent wearing comfort and environment friendliness (Hipparagi, Srinivasa, Das, Naik, & Purushotham, 2016; Zhang, Yan, & Li, 2009). However, little research provides feasible evidence to consumers how to care silk fabric products. Thus sunlight-drying was extensively used by consumers when silk fabrics

*Corresponding author: Tel.: +86 15821882692; Fax: +86 02162193067;E-mail address: fddingxm@dhu.edu.cn(X. Ding)

needed drying care (Yao, Tang, Chen, Sun, & Wang, 2016). Nevertheless, due to delicate nature of silk fiber, during conventional home sunlight-drying, silk fabrics suffer easily from several problems such as shrinkage, yellowing, wrinkling, mechanical property deterioration, etc. (Helliker et al., 2014; Hipparagi et al., 2016; Ma, You, Chen, & Zhou, 2014; Yao et al., 2016) . Moreover, conventional sunlight-drying has many other disadvantages such as long drying time, bacterial contamination, mildew (especially the rainy season) growth, vulnerability to weather conditions and ultraviolet radiation damage (Hino, Tanimoto, & Shimabayashi, 2003; Shao, Zheng, Liu, & Carr, 2005; Yao et al., 2016). Therefore, silk fabric care (laundering and drying) became a major challenge during daily usage (Hino et al., 2003; Yao et al., 2016).

In contrast, dryer is widely utilized by consumers as it is not subject to weather conditions, offers sterilization (killing bacteria by high temperature) and high drying efficiency (Holst & Payne, 1994; Ng & Deng, 2008; Piccagli, Visioli, & Colombo, 2009; Yadav & Moon, 2008). This drying method also suits the trend of improving living standard and increasing pace of life (Varesano, Dall'Acqua, & Tonin, 2005). However, commercial dryers do not have specific drying procedure for silk fabrics (Deans, 2001; Wei, Hua, & Ding, 2016). If drying procedures for other fabrics are used, fiber fractures in the form of fibrillation and degradation will result (Buisson, Rajasekaran, French, Conrad, & Roy, 2000). This is because drying using automatic drying machines is a process that involves continuous mechanical actions such as beating, falling, entanglement and rubbing of fabric itself, aggressive agitator actions, repeated actions by high temperature and humidity air flow (Buisson et al., 2000; Gotoh, Nakatani, & Tsujisaka, 2015; Helliker et al., 2014; Ma et al., 2014; Wu et al., 2016) . In other words, if inappropriate drying parameters are used, drying becomes one of the most aggressive degradative agents during daily usage, resulting in changes in physical and mechanical properties. For example, Buisson pointed out that the desized and scoured cotton fabric suffered server abrasion and heat damage during drying in a household tumble dryer (Buisson et al., 2000). He also proposed that moisture loss patterns during drying was a function of time or temperature, which indicated that it is possible to minimize dryer damage by adjusting drying parameters, reducing tumble-drying time and energy input (Buisson et al., 2000). But Buisson did not study the effect of drying parameters of dryer on properties of silk fabrics. Rollins and Goynes published a transmission electron-microscope survey of replicas of wet and dry abrasion patterns of cottons, including a limited number of samples abraded by drying, but no machine-washed and dried samples of silk fabrics were studied (DeGruy, Carra, Tripp, & Rollins, 1962; Goynes & Rollins, 1971). Many researchers have proposed that the main objective of drying process is to produce a dried product of desired quality at minimum energy consumption (Akyol, Kahveci, & Cihan, 2013; Daghigh, Ruslan, Zaharim, & Sopian, 2010; Yahya et al., 2009). Weiss reported that the major source of degradation of clothes is the drying process instead of everyday wear (Orr, Weiss, Humphreys, Mares, & Grant, 1954). This implies that in the drying process, beside the energy requirement, the quality of dried product must be taken into consideration. It is especially necessary to develop appropriate and effective drying procedure for drying delicate fabrics such as silk fabrics.

In addition, previous studies were mainly focused on the change in the surface and mechanical properties of silk fabric due to dyeing process and ultrasonic or plasma finish in silk fabric instead of drying treatment (Hino et al., 2003; Su & Zuo, 2011; Tsukada, 1986; Wang et al., 2012). There are few references on the influence on silk fabrics by domestic drum drying which is an important part of clothing care and daily usage. In this work, we evaluate the dimensional stability, smoothness appearance, chemical structure, crystallinity, microstructure, tensile extension (EMT), tensile energy

(WT) and tensile resilience (RT) of silk fabric during the drying processes and investigate how to optimizing the drying parameters to minimize the deterioration of mechanical properties of silk fabric during the drying process.

The current study is important because woven fabrics made from specialty protein fibers such as silk are becoming increasingly popular for products intended for everyday consumer use. This study not only provides a feasible approach of adjusting drying parameters to minimize damage in the structure and surface properties of silk fabrics during daily usage involves no chemical reagent, but also is helpful to dryer manufacturer in setting the drying parameters for delicate clothes especially silk fabrics. This study is therefore practical and environment friendly.Experiments MaterialsPure silk fabrics (plain woven, 55.82 g/m2) made of degummed silk fibers (3.9/5.6 Tex) without any treatment or purification were purchased from our partner company--High fashion Silk (Dali) Co., Ltd. (Hangzhou, Zhejiang Province, China) and was chosen as test sample for the present study. The detailed properties of silk fabrics used are listed in Table 1. Samples of 38×38cm were prepared from the fabric. All samples were washed in deionized water of 27°C, then hang-dried indoors and conditioned at constant temperature and humidity conditions (according to ISO 139: 2005 for a minimum of 24 hours according to ISO 139: 2005 (20±2°C; 65% relative humidity (RH) ± 2% RH)), and finally dried under different drying conditions.

Drying proceduresTo understand the impact of different drying procedures on the appearance and physical properties of silk fabrics, the experimental study of silk fabric drying was carried out using a drying platform modified from a commercial electric heated clothes dryer (Haier GDZ10-977) as shown in Figure 1. The dryer’s tumbler outlet was modified to be integrated with a programmable constant temperature and humidity system (FTS64-2011-MD with a accuracy±0.4% for temperature scale 0-100 and℃ ±2% for relative humidity in the interval 0-90%, Eyc, Masuda Technology Co., Ltd, Taiwan) to monitor temperature and humidity of the drying air in the drum. Two digital voltage regulators were connected respectively to an electric heating element of 4000W and to a fan for adjusting the heater power and air flow rate according to the drying conditions. Instead of AC motor, a DC motor was utilized for driving the drum for the purpose of understanding the effect of drum movement on damage behavior of silk fabric.

During the drying process, based on the recommendations of dryer specifications, our pre-experiments of drying parameters and related literature, heater powers was set to 1500, 3000, and 4000W, the effective rotating speed of drying drum of 0 (stationary) and 45-50rpm, and flow rates of 3.5, 5.5 and 8.5m/s, respectively. And the details of experiments are presented in Table 2. Before drying, samples were rinsed in deionized water (27 ± 3 °C) for 8 min with a liquor ratio of 100:1 and then dewatered in the machine at 800r/min for 2min to remove excess water (namely ensure that moisture content of fabric is 70±5%). After drying in the domestic dryer, the silk fabrics were conditioned under standard atmospheric pressure at 65±2% relative humidity and 20± 2°C for at least 24 h before further evaluation. To compare the differences of each procedure, samples obtained from each procedure were repeatedly dried 20 times under corresponding procedures.

Dimensional stability

According to ISO 3759: 2007: Textiles-preparation, marking and measuring of fabric specimens and garments in test for determination of dimensional change, the dimensional change in the sample (with three benchmarks marking out 15 cm × 15 cm in each direction using indelible ink) after being tumble dried 20 cycles under different drying conditions was evaluated according to Equation (1). The measurements were repeated at least three times for the same specimen to get the average values. Additionally, samples are conditioned for 24 hours at 20±2°C and a relative humidity of 65±2% before taking the measurements.

DC %= (B−A )A

× 100 % (1)

Where DC is dimensional change; A is original dimensions; and B is dimensions after drying.

Smoothness appearance ratingSmoothness appearances (SA) of the samples were rated in comparison with standard replicas according to AATCC Test Method 143-2006. Six graduate students were involved in this experiment, and the average grade for each sample was calculated from the individual rating values. In the above investigations, all the samples were prepared in quintuplicate.

Fourier transform infrared spectroscopyTo characterize the dried fabrics and determine whether the drying in dryer leads to chemical changes, silk fabrics after drying were further analyzed using FTIR with advanced attenuated total reflectance (Nicolet iS50 FT-IR Spectrometer). Scans were taken using a 400 to 4,000 cm-1 spectrum, 16 co-addition scans, and 4 cm-1 resolution. Data were generated and analyzed using OMNIC™ Spectra software (Thermo Scientific, USA).

X-ray diffractionIn order to study the effects on the secondary structure of silk fabrics, XRD analysis of silk fabrics was carried out on X-ray diffractometer (D/MAX-1200; Rigaku Denki, Tokyo, Japan) with monochromatic Cu Ka radiation source (λ = 1.54181 Å) at a voltage of 40 k V and a current of 30 m A. XRD data were collected with 2θ range of 3°–80° and a scanning rate of 2°/min at a step of 0.01°. The integrated peak intensity for each high crystal reflection was selected and the amorphous background was extracted by a curve-fitting program with the multiple peak separation method. The crystallinity (vc) was obtained from the ratio of the integrated area of all crystal peaks to the total integrated area (including the amorphous area) according to the peak fit results.

SEM evaluating Potential changes in the fabric surface morphology due to drying were measured by scanning electron microscopy (SEM) (Model S-4800, Hitachi, Tokyo, Japan) at an acceleration voltage of 3 k V under vacuum condition. Prior to scanning, the specimens were coated with a thin layer of gold to limit the charging effect.

Mechanical properties analysis To quantify any changes in fabric mechanical properties during drying, the Kawabata Evaluation System Fabric (KES-F) was used for measuring the low-stress mechanical parameters of the silk fabrics before and after drying treatment (0 and 20 drying cycle). These included tensile extension

(EMT), tensile energy (WT) and tensile resilience (RT). Specimens of 20 cm ×20 cm were used. Each value is the average of four test results. All tests were carried out at standard laboratory conditions (20±2°C and 65± 2% relative humidity (RH) for 24h.

Results and discussion Dimensional stabilityThe dimensional change of silk fabric after different procedures was measured. As shown in Figure 2, regardless of the drying procedure, the dimensional change of the fabrics in the weft direction varied from 0.2% to 1.7% with standard deviations less than 2%, while the dimensional change of the fabrics in the warp direction varied from 2.9% to 7.9%, indicating that the shrinkage to drying for silk fabrics in the weft direction is not as significant as that of the silk fabrics in the warp direction. In other words, fabric shrinkage takes place mainly in the warp direction. The significant warp-wise shrinkage was due to the stress relaxation and the swelling of silk fabrics during drying. This is also consistent to our general view that fabric shrinkage takes place mainly in the warp direction(Ma et al., 2014).

Moreover, Figure 2 also illustrates that under the same air velocity (8.5m/s) and rotating speed of drum (45-50rpm) conditions, shrinkage of fabric increased with the increase in heater power; under the same heater power (3000W) and rotating speed of drum (45-50rpm) conditions, shrinkage of fabric reduced with the increase in air velocity; under the same heater power (3000W) and air velocity (8.5m/s) conditions, shrinkage of fabric after rotating drying (45-50rpm) significantly increased comparing to drum-static (0rpm). And indicated that the increases in heater power and decreases in air flow rate are associated with increases in the dimensional shrinkage of both warp and weft directions.

Specially, comparing procedures No.1, No.2 and No.3, shrinkage at No.2 is lower than that at No.1 and No.3. Because excess heater power (4000W) of No.1 leads to lower final moisture content of fabric affinity and then fabrics becoming more compact when drying ending, comparing to lower heater power (3000W). Whereas shrinkage at No.3 was larger than that of No.2 due to low heater power (1500W) suffering more mechanical agitation and longer drying time. This is because silk fabrics placed in a high temperature environment for prolonged periods, hydrolysis occurs owing to protein (in silk) having strong hydrogen bonding networks. While it is known that hydrogen bonding is perhaps the most important bond between and within fibers that obviously influences shrinkage stabilization. Hydrogen bonds that contribute to the transverse structure of the fibers are more or less readily broken and reformed with water molecules, and then affect shrinkage stabilization. It takes longer to reach dry requirements at medium and low heater power settings than at higher heater power under, and so there would be more abrasion damage with the lower heater power settings. Accordingly, heater power of 3000W is the most potential within our research.

Comparing procedures No.2, No.4 and No.5, it is clear that the effect of air flow rate on shrinkage is significant. Dimensional shrinkage of procedure No.4 (air flow rate of 3.5m/s) is more significant than that of No.5 (air flow rate of 5.5m/s) as shown in Figure 2. Dimensional shrinkage of procedure No.2 (air flow rate of 8.5m/s) is the smallest among the three procedures. This is because that lower air flow rate leads to higher relative humidity of drying air. Silk fabrics are susceptible to performance degradation under high humidity conditions (Ma et al., 2014). Additionally, change in dimensional shrinkage of the drying fabric is much higher at the high humidity than at the low humidity, which indicated that the high humidity has a greater degrading effect than a low humidity. Based on the above analysis, procedure No.2 (heater power of 3000W, air flow rate of 8.5m/s and drum rotating speed of 45-50rpm) is the best procedure for normal home drying of the silk fabrics considered in this study.

Comparing procedure No.6 and No.2, the relatively lesser shrinkage of fabrics dried at drum speed of 0rpm (procedure No.6) may be attributed to the absence of mechanical agitation. Mechanical agitation helps in overcoming the inter-fiber and inter-yarn frictional resistance and, in turn, leads to swollen fibers and yarns easily return to its original position and fewer wrinkles or creases of fabrics, and therefore resulted in less dimensional shrinkage. Accordingly, procedure No.6 can potentially be a non-ironing procedure to meet consumer easy-care requirement (Gocek, Sahin, Erdem, Namal, & Acikgoz, 2013).

In addition, dimensional shrinkage (Figure 2) of procedures No.0 (hang-drying indoors), No.2 and No.7* (normal procedure of commercial dryer) is 4.9%, 5.9% and 7.9%, respectively. This is because that dimensional shrinkage can be attributed to two competing effects: hygral expansion tends to increase of fabric dimension due to reducing yarn interaction and increasing gaps between yarns, while the drying reduces the moisture content of silk fabrics and leads to shrinkage of fabric dimension (Cookson, 1992; Urs, Prakash, Ananda, & Somashekar, 2016). And the final moisture content of fabric drying after different drying procedures is different, suggesting the higher final moisture content of fabric, the more dominate hygral expansion, and thus the smaller dimensional shrinkage value. These results also correspond with what is reported in literature. It is therefore possible to minimize dimensional changes of silk fabrics during drying in dryers by adjusting the drying parameters.

Smoothness appearance ratingSA level of silk fabrics after drying with different procedures are given in Figure 3. It is observed that there is a drop in the SA level of silk fabrics after drying regardless of drying procedures. This may be because that the relatively high swelling and inherently poor wet resiliency of silk fiber make it easier to deform and difficult to recover during drying (Cheriaa, Marzoug, & Sakli, 2016). Additionally, the intense movement and friction in the drying machine lead to additional deformation of silk fabrics, and much more mechanical damage and greater surface wrinkles.

As shown in Figure 3, the mean SA grade of fabrics after hang-drying indoor (No.0) is highest (4.7) among this study. However, after drying with procedures No.1, No.2 and No.3, the SA grade decreased to 2.4, 2.8 and 2.2, respectively. This is because amorphous region in silk fiber accounts for approximately 47.1-50.7% where large amount of hydrophilic groups exists, such as hydroxyl and carboxyl. Once absorbed in the non-crystalline region, water molecules bond with hydrophilic groups in the form of hydrogen bonds, and then the molecular chains become loose and easy to slip, and thus wrinkle occurred. Additionally, with the increase in heater power, the number of molecular motion increased, leading to increasing in the binding opportunity of water molecules and silk fiber, and caused to fabric wrinkle resistance reduce, and then resulting in wrinkles formation increasing. In addition, in drying process, volume expansion of fabric caused by absorb moisture was not necessarily to fully recover to its original volume when fabrics were dried. Pressure difference were caused by the temperature difference of between fabric inside and outside, which led to the shrinkage difference of inner and outer fabric, uneven bubbles wrinkles were formed on the fabric surface. The higher temperature, the more obvious the uneven recovery, and thus leaded to wrinkles formation.

Additionally, Figure 3 also illustrated the SA grade of fabric after drying with procedure No.2, No.4 and No.5 is respectively 2.8, 1.2 and 1.8, and indicated the SA level increases significantly (from 1.4 to 2.8) with increasing air flow rate. This is because lower air flow rate (3.5m/s and 5.5m/s) leaded to relatively higher swelling and more abrasion of silk fibers and thus resulted in lower smoothness appearance level, compared with higher air flow rate (8.5m/s). And found that without drum rotation

(procedure No.6), the SA level of about 3.2 is achieved because of the absence of mechanical action. Dried fabrics with a SA level above 3.0 are considered to have a durable press appearance, thus drying without drum rotation meets the requirements of drying degree of silk fabrics (Hua, Tao, Cheng, Xu, & Huang, 2013; Ma et al., 2014). For large load, the SA level of procedure No.2 (SA=2.8) is significantly higher than No.7* (normal procedure for commercial dryer GDZ10-977) (SA=1.4), indicating that the normal drying procedure is not effective for delicate fabric drying. Procedure No.2 is optimal for practical apparel drying, especially for silk fabrics, whereas procedure No.6 is optimal when consumers want ideal smoothness without ironing, but this procedure is only suitable for small load becoming of fabric being fixed on shelf to hang-drying in drum, especially a shirt.

Chemical Composition AnalysisThe FTIR spectra of silk fabrics drying with different drying procedures are shown in Figure 4. The FTIR spectra of silk fabrics after drying with procedures of No.0, No.1, No.2, No.3, and No.6 had similar profiles , indicating that the drying in dryer have no obvious influence on the chemical structure of silk fiber. Characteristic peaks were at 3456.9, 2917.8, 1659.6, 1164.6, 898.7, and 608.2 cm−1 for the silk fiber (Figure 4). These peaks are those for N–H stretching bands, C-H stretching, β-sheet, C-O-C,

C-H stretching out-of-plane, C=O stretch, respectively. However, for procedures No.4, No.5 and No. 7*(normal procedure of available dryer), some peaks at 898.7, and 608.2 cm−1 were observed, and this agreed well with latter X-ray photoelectron spectroscopy analysis (Figure 5). The shifts arise because of cleavage of the hydrogen bonds and mechanical abrasion, which indicates the change in the secondary structure of silk fibroin molecules from β -sheets to irregular structures with certain settings of drying parameters.

Structural analysisXRD analysis is used to test the change in the crystal structure of silk before and after drying treatment. As seen in Figure 5, there was a strong peak for silk II at 20.8°, and two weaker peaks at 14.7° and 24.5°. However, there is a spiral feature diffraction peak of Silk I. The degree of crystallisation were calculated by using Peak Fit software; the crystallisation degrees of the eight curves are 43.16%, 41.94%, 42.17%, 41.56%, 40.76%, 41.87%, 43.34% and 40.88%, respectively. The results indicated that different drying processes have no obvious influence on the crystal structure of silk fiber. Slight reduction in crystallisation may be suggestive of damage to the silk fibril and the secondary structure of fibroin altered to the β -sheet conformation as a result of the friction of the fabric with the machine, the friction among the different fabrics and absorbed water during drying atmosphere. This was critical to both manufacturers and consumers, as consumers may not always follow recommended drying procedures. Combined with infrared spectroscopy (Figure 4), the procedure of heater power of 3000W, air velocity of 8.5m/s and drum rotating speed of 45-50rpm (No.2) is shown to be the optimum drying parameters for lower damage of the surface properties and structure of silk fabrics.

Morphology of silk fabricTo further clarify the difference of drying damage between the different procedures, the surface morphology of the fabrics was analyzed by SEM. Figure 6 displayed the SEM images of silk fabrics in three magnifications 50, 150 and 1000 after drying 20 cycles. The following fiber damages can be observed: longitudinal splitting of the fiber; fragmentation and fraying of broken fibers to produce frazzled brushes: mashing or bruising of the fiber. Silk fabrics withstood 20 drying cycles without

development of holes or yarn breakage, but surface fibers had been abraded and extensive fibrillation in the fibers in the form of stringy and peels(Figure. 6), especially for procedures No.4 and No.7*. It is also observed that the original silk fibers displayed a very smooth surface (Figure 6(a, b and c)).

Comparing Figure 6 (d, e and f) with Figure 6 (j, k and i), it is found that damage of silk fabrics after drying with procedure No.1 is more serious than that of drying with procedure No.3, indicating more damages with greater heater power. However, there is no significant change except a few floating fibrils with procedure No.2. Fibrillation frequently occurs during wet processing and home drying due to the high swelling property of the fiber and the inherently weak non-covalent bonding between silk molecules. In other words, the high swelling weakens the inter-micro fibril interactions that make the fiber more susceptible to fibrillation under conditions of dry abrasion especially over-drying.

In the case of No.4 and No.5 procedures, some silk fibers are pulled out leaving frayed febrile bundles, indicating that the secondary wall of fiber was peeled from the outer layers and sloughed off in long tangles of fibrils as shown in Figure 6 (m-v). Less damage is observed after drying with procedure No.5 (Figure 6 (p, q and s)) compared with procedure No.4 (Figure 6(m, n and o). This indicates lower air flow rate accelerates clothing damage because of longer drying time, higher air humidity and more abrasion.

As seen from Figure 6(t, u and v), when fabrics are subjected to machine drying without drum rotation, there is no sign of damage to the surface of the fabric, indicating that damage of silk fabric depends largely on abrasion and mechanical impact during the drying process. More specifically, fiber damages in the form of fibrillation and degradation usually occur due to fabric rubbing during drying. Therefore, it can be concluded that controlling the speed of drum can effectively reduce damage. On the other hand, as shown in Figure 6(w, x and y), fiber fibrillation and mechanical damage on the surface of the silk fibers with the normal procedure (procedure No.7*) can be seen clearly. Wet and excessively swollen fibers are rather easily separated by physical forces and dry abrasion during the machine drying process. This can result in a dull fabric appearance and impair fabric tensile properties. Moreover, in the present research, the micro-cracking induced by drum drying is not found , in contrast to those found in the scale structure of drum dried cotton fibers, and only a slightly rough surface is observed (Figure 6) (Buisson et al., 2000; Goynes & Rollins, 1971). Accordingly, drum-static drying shows a significantly better effect in reducing drying damage of silk fabrics.

These observations from the SEM images are consistent with the measured mass loss, thermal shrinkage, and mechanical property deterioration of the test fabrics after drying 20 cycles.

Physical and mechanical propertiesThe mechanical properties of samples treated by various methods were tested with KES-F, the results are shown in Table 3. The tensile energy (WT), tensile extensibility (EMT) and tensile resilience (RT) of silk fabrics after drying with different procedures drying presented slight difference, and indicating drying procedure (heater power, air flow rate and drum rotating speed) slightly affected mechanical properties of fabrics.

As shown in Table 3, tensile resilience (RT) of the silk fabrics dried with procedures No.1, No.2, No.3, No.4, No.5, No.6 and No.7* were 59.71%, 60.21%, 59.84%, 57.92%, 60.12%, 59.93% and 58.17%, respectively (Table 3). RT of fabric un-dried (No.0) was 59.48%, which was slightly higher than that of the silk fabrics dried in dryer (from No.1 to No.7*). Table 3 also showed that tensile energy (WT) of fabric drying with procedures No.1, No.2, No.3, No.4, No.5, No.6 and No.7* were 19.35 gf.cm/cm2, 19.29 gf.cm/cm2, 19.72 gf.cm/cm2, 20.13 gf.cm/cm2, 19.64 gf.cm/cm2, 19.66 gf.cm/cm2 and

19.93 gf.cm/cm2, respectively, while that of the fabric un-drying in dryer was 19.72 gf.cm/cm2. This suggested that with the increase of heater power and the decrease of air flow rate, tensile energy (WT) and tensile extensibility (EMT) of dried silk fabrics tend to decrease. But the degree of decrease was insignificant and belonged to the scope of the standard, indicating the drum-drying did not change mechanical properties of fabric. And this slight change can be attributed to irreversible slipping between molecular chains caused by the hydrolysis bonds between molecules in high humidity drum. Additionally, when external force continuously stretch one position of the fabrics in during drying process, it was easy to produce stress concentration, and then results in the decrease of tensile properties. This also indicated that drying degradation weakens the fabrics by creating additional weak points along the fabric length. Additionally, these data coincide with that in the literature on hot-wet aging properties of silk fabrics (Van Amber, Niven, & Wilson, 2010). The drying process is a process that includes continuous beating, falling, rubbing, impacting, sliding and rotating. These actions contribute to the mechanical breakdown of textiles during drying. In other words, loss of fabric tensile properties due to mechanical abrasion may take place because of breakdown of cohesion between fibers in the yarn or breakdown of internal cohesion within the fiber.

Additionally, Table 3 indicated that RT (tensile resilience) in both warp and weft directions decreases as the heater power slightly increases. Because some friction was created on the surface of silk fibers during the drying process, and then leads to RT (tensile resilience) declining. This can also be attributed to the high temperature and humidity environment of the drying drum damage of silk fibers. Additionally, we have found that the absence of agitation causes less fiber migration than adjusting heater power and air flow rate. The reduction in the elongation at break indicated that drum drying weaken the fabrics by creating additional weak points along the fabric length.

Therefore, the optimal drying procedure is with heater power of 3000W, air velocity of 8.5m/s and drum speed of 0rpm. This was because that appearance, physical and mechanical properties of fabrics only slightly modified under such conditions.

Drying efficiency To investigate the impact of drying procedures on drying efficiency, drying energy consumption, drying time, and the final moisture of fabric are also analyzed. Each experiment was repeated 3 times and the average results are shown in Table 4.

Comparing procedures No. 1, No.2 and No.3, it is clear that drying time decreases approximately 20.8% accompanied by an increase in energy consumption of approximately 12.6% for each 1000W increase in the heater power (Table 4). The reduction in drying time and the increase in drying energy are both related to the increase of heater power. With higher heater power, the temperature of the drying air increases considerably at the inlet of the drying drum, and therefore a large amount of energy is transferred to the wet fabric, so the drying rate is increased and drying process is shorted.

Comparing procedures No. 2, No.4 and No.5, it can be concluded that increase in air flow rate leads to a considerable decrease in drying time. This is expected due to the increase in moisture removal capacity of the drying air. Drying takes longer time at lower drying air flow rates and energy consumption gets higher values because of too long residence time of drying air reduces the ability of absorb moisture. As shown in experimental results given in Table 4,the air flow rate is increased from 3.5 to 5.5 m/s, drying time decreases 25% with a decrease of 6.9% at the energy consumption and when the air flow rate is increased from 5.5 to 8.5 m/s, the drying time decreases 8.3% with a decrease of 4.4% at the energy consumption for the drying condition of heater power of 3000W and drum rotating

speed of 45-50rpm, and indicated energy consumption decreases with an increase in air flow rate due to the decrease at drying time. However, energy consumption may show an decrease after an increase with increasing air flow rate as the amount of the drying air exceeds the moisture removal rate from the silk fabrics and high amounts of energy is transferred to the wet fabric in the drum (see Table 4). These results show that the drying air flow rate has a strong effect on silk fabric drying process. Drying should be operated at higher air flow rates for shorter drying time and at mild air flow rates for lower energy consumption depending on the drying load.

Additionally, as can be seen in Table 4, dehumidification capacity per unit time and dehumidification capacity per unit energy consumption of the drying procedure with drum rotation (procedure of No.2) is respectively 8.0 times and 4.56 times that of drum-static drying (procedure of No.6). This is because that moisture removal may be increased by the use of more mechanical action. Because more mechanical action is helpful to spread the wet fabric and increases the contact surface between the fabric and the air, subsequently heat exchange efficiency of drying air and fabric. Specifically, the rotating drying method developed a complex movement pattern involving sliding, falling, swinging and rotating. In the case of a complex movement pattern, the movement of fabrics was diverse and the position of the fabric that received drying air also changed continuously, thus making drying efficiency higher. On the contrary, under the drying of drum-static condition, fabric was fixed on a meta shelf, and was easy to make fabric overlapping and twining, implied increasing the thickness of requiring drying and considerably reduced exchange area of drying air and fabric, resulting into drying efficiency reducing. Accordingly, it can be deduced that the drying of static drum requires more energy consumption and drying time compared with the model of rotating-drying in dryer (procedures of No. 1, No.2, No.3, No.4, No.5 and No.7*). However, based on favorable appearance of fabric after drying with procedure of No.6, it can still be chosen as a non-ironing procedure when requiring better appearance.

However, in normal home drying, machine-dried load is 1.5-3.0 Kg instead of only a shirt. This test condition of static drum drying was therefore used only to completely isolate dryer damage, it is suitable for condition when consumers want to dry a small amount of load (for example a shirt) and don’t consider the drying time instead of only considering optimal appearance and minimum damage. In contrast, comparing with other drying procedures listed in Table 2, procedure of No.2 (heater power of 3000W, air flow rate of 8.5m/s and drum rotating speed of 45-50rpm) has the potential to become optimal procedure for delicate fabric drying (especially silk fabrics) based on higher drying efficiency and more excellent drying quality (Table 4).ConclusionsDrying procedure significantly affect the appearance and mechanical properties of silk fabrics, and indicated that using dryer to drying delicate fabrics is possible and effective if setting reasonable drying parameters (heater power, air flow rate and rotating speed of drum). And experimental results reveal that heater power of 3000W, air flow rate of 8.5m/s and drum rotating speed of 45-50rpm is the optimal procedure for silk fabric drying in dryer because of improving 1.4 grade of the appearance smoothness, reducing 2.0% of dimensional shrinkage, saving 16.5% of energy consumption, 10% of the drying time, as well as significantly reduce the microscopic damage of fiber compare to the normal procedure of dryer (NO.7*). Additionally, the results also demonstrate that drying procedure with satisfactory smoothness appearance were obtained for silk fabrics by drying using procedure No.6 (heater power of 3000W, air flow rate of 8.5m/s and drum speed of 0rpm), and thus achieved the purpose of saving ironing after conventional drying. Finally, also found that the extent of damage is governed by the air

flow rate, drum rotating speed and heater power, with air flow rate and drum rotating speed more influential than heater power. Drying in dryer causes surface damage but has no overall effect on the molecular structure and chemical composition of the silk fabrics.

In conclusion, the finding obtained in this study not only provide a piece of information that dryer manufacturers can utilize in their decision making for drying parameters setting, but also provide references for further studies on the effect of different drying cycles on the performances of fabrics in terms of appearance and mechanical properties.

Acknowledgement

This research is funded by Donghua University Institute for Nonlinear Sciences（15D110926）and funded by Doctoral Program of Higher Education of China (CUSF-DH-D-2016067).

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Figure Captions

Figure 1. Experimental setup for silk fabrics drying.Figure 2. Change in dimensional stability of silk fabrics after drying with different procedures.Figure 3. Change in smoothness appearance of silk fabrics after drying with different procedures (SA is abbreviation of smoothness appearance).Figure 4. FTIR spectra of silk fiber after drying with different procedures.Figure 5. X-ray diffraction patterns of silk fiber after drying with different procedures.Figure 6. SEM photographs of silk fabric drying under different drying conditions: (a-c) silk fabrics with procedure of No.0 (un-drying in dryer), (d-f) silk fabrics with procedure of No.1, (g-i) silk fabrics with procedure of No.2, (j-l) silk fabrics with procedure of No.3, (m-o) silk fabrics with procedure of No.4, (p-s) silk fabrics with procedure of No.5, (t-v) silk fabrics with procedure of No.6, (w-y) silk fabrics with procedure of No.7*. Magnification ratio of each vertical column is 50×, 150× and1000×,

respectively.

Table Caption

Table 1. Details of silk samples used in this study.

Test sample

Fabric design

Yarn linear density (Tex)

Fabric density (picks/10cm)

Mass per unit area

(g/m2)

Thickness(mm)

Warp Weft Warp Weft

Pure silk Woven fabric 3.9 5.6 625 405 55.82(±0.03) 0.13(±0.03)

Table 2. Experimental details for silk fabric drying.

TestDrying parameters

Heater power (W) Air flow rate (m/s) Drum rotating speed (rpm)0 - - -1 1500 8.5 45-502 3000 8.5 45-503 4000 8.5 45-504 3000 3.5 45-505 3000 5.5 45-506 3000 8.5 07* 4000 5.5 45-50

Drying load of NO.6 is 1.0±0.01kg; Drying load of other procedures is both 3.0±0.01kg; Initial moisture content of all fabric is 70±5%. NO.0 means hang-drying indoor instead of being dried in

dryer, and control sample for comparing with other procedures. No.7* is normal procedure for the dryer (Haier GDZ10-977) and used for comparing optimal procedure (No.6) to prove the practical value of development of optimal procedure (No.6). Additionally, it should be pointed out that N0.1, N0.2 and N0.3 are used for investigating the effect of heater power on various properties of silk fabric; N0.2, N0.4 and N0.5 are used for investigating the effect of air velocity on various properties of silk fabric; N0.2 and N0.6 are used for investigating the effect of rotating state of drum on various properties of silk fabric to develop the drying procedure without ironing.

Table 3. KES tensile properties of silk fabrics.

Drying procedures

Tensile properties

WT (gf.cm/cm2) EMT (%) RT (%)

Warp Weft Warp Weft Warp Weft

0 19.72 11.79 17.02 12.31 59.48 52.461 19.35 11.68 16.62 11.73 59.71 53.012 19.29 11.73 16.34 11.89 60.21 52.713 19.72 11.58 16.93 11.75 59.84 53.234 20.13 11.59 17.81 11.96 57.92 53.315 19.64 11.62 16.78 11.93 60.12 52.126 19.66 11.75 16.82 11.95 59.93 53.72

7* 19.93 11.56 17.53 11.96 58.17 53.36

Table 4. Drying efficiency of different procedures.

Drying testDrying efficiency index

Energy consumption (KW﹒h) Drying time (min) Final moisture of fabric (%)

0 0 480 5.671 3.26 80 3.562 3.07 45 2.973 2.78 30 -1.784 3.43 75 1.985 3.21 60 1.636 4.67 120 4.987* 3.68 50 -2.78