Study on the Aging Mechanism of Down Fiber Under the Daily Washing and Drying Conditions

ABSTRACT In order to clarify the aging mechanism of down fiber under the daily washing and drying conditions, fluffiness, whiteness, microscopic morphology, crystallinity and molecular structure of down fiber after different washing and drying treatments were observed. Results showed that the daily washing conditions of weak acid or alkali (pH = 5–9) and drying conditions of low temperature drying (55°C and sunlight) only slightly affected fluffiness, whiteness, and microscopic morphology of down fiber. However, the addition of short-term mechanical agitation reduced damage and even improved fluffiness, whiteness. Additionally, down fiber did not experience chemical damage in the above environment. However, the strong alkali (pH = 10, pH = 12) washing environment and high temperature (105°C,155°C) drying treatment not only caused the decline of fluffiness, whiteness, microscopic morphology but also caused the change of crystallinity and molecular structure. Additionally, the stronger the alkalinity, the longer the time, the higher the drying temperature, the more obvious the performance decline. Moreover, regardless of the washing and drying conditions, mechanical agitation for a long time easily led to performance degradation. Therefore, down fibers were suitable for the daily washing conditions of weak acid and alkali (pH = 5–9) and drying conditions of low temperature drying (55°C or sunlight) with slightly mechanical agitation.


Introduction
Owing to excellent thermal insulation, lightweight, soft touch, and warm keeping, down fiber has long been widely used as superior and luxurious filling material for clothes and quilts against cold climates (Chen et al. 2016;Gao, Pan, and Yu 2010;Yang, Li, and Shen 2018).Additionally, it was essential to wash and dry for clothes and quilts with filling down fiber because of easily absorbing dust, grease, and other stains during the process of daily reusage (Gao, Pan, and Yu 2010;Muthusamy et al. 2022;Sun et al. 2019).In addition, reports on daily washing care for fabric or clothing showed that daily washing environment belongs to an interactive process with detergent, mechanical force, and water flow (Nam et al. 2022;Qiang, Pu, and Zhang 2022).Daily drying environment belongs to an interactive process with high temperature and humidity, hot air flow, and mechanical force (Bao, Shen, and Ding 2020;Kim, Kim, and Park 2018;Wei et al. 2019).Unfortunately, down fiber is a kind of protein fiber with weak strength, acid, and alkali intolerance (Gao, Pan, and Yu 2010;Jin et al. 2021;Yang, Li, and Shen 2018).Therefore, performance degradation of down fiber (such as, yellowing, weak in strength, fluffiness declining and aging) was easily observed, especaily in the strong acid and alkali, high temperature, mechnical agitation environment (Kim, Kim, and Park 2018;Muthusamy et al. 2022;Sun et al. 2019).However, current studies mainly focused on improving the fluffiness and thermal insulation of down fiber and down assembly by surface chemical modification, green-cleaning technology for down fiber, and the structure analysis of down fiber (Fang, Zhang, and Meng 2022;Gao, Pan, and Yu 2010;Qiang, Zhang, and Chen 2019).Documented knowledge on the aging mechanism of down fiber is scarce, fragmented, and superficial; especially, the scientific researches focused on the daily washing and drying process of down fibers or down fiber assemblies have been surprisingly rare.Meanwhile, with the improvement of people's environmental awareness and the change of daily washing-drying care requirements, people are more and more pursuing the daily washing and drying methods with comprehensively balancing of washing efficiency and performance deterioration (Cho, Yun, and Park 2017;Gotoh, Nakatani, and Tsujisaka 2015;Wei et al. 2019).Therefore, it is necessary to clarify the aging mechanism of down fiber under the daily washing and drying conditions for better utilization and even developing an optimal daily washing and drying methods for down fiber assemblies.
In this study, the relationship between the daily washing and drying conditions and the physical chemistry properties of down fiber including fluffiness, whiteness, microscopic morphology, crystallinity and molecular structure was systematically investigated with the help of fluffiness test, data color 650, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR).This work may provide the possibility to further improve the service life of textiles by filled down fiber and reduce resource waste.Moreover, the finding not only assists understanding of the aging mechanisms of down fiber under the daily washing and drying conditions but also provided reference for washing and drying equipment manufacturers to accurately develop the daily washing and drying procedures for down product.Additionally, it is also helpful to guide consumers to reasonably care for down product in daily usage.

Experimental procedure and sample preparation
In order to study the effect of daily-care conditions on the physical and chemistry performance of down products (clothes and quilts), the common washing and drying conditions in daily life were simulated according to the textile household washing and drying standards (GB/T 8629-2001, HG/T 5953-2021, GB/T 23,343-2009, GB/T 26,398-2017) and the experimental design of previous studies about textiles aging, the daily washing and drying, and the daily-use life cycle theory of textiles.Detailed conditions are shown as follows:

Washing-care simulation experiment
As the commonly used laundry detergents in daily life are generally acid, alkaline, or neutral, the acid and alkali water solution of PH = 5, 7, 9,10 ,12 was prepared to simulate the daily washing environment.The acidic solution was obtained using HCl (molar concentration was 11.64 mol/L), and the alkaline solution was prepared using NaOH (relative molecular weight was 40.01).White duck down was immersed in the prepared acid and alkali water solution according to the bath ratio of 50:1 at 25°C, respectively.Twenty grams of white duck down was taken out every 24 h, and for taking 4 times, as aged down sample of daily washing care.Then the sample was thoroughly washed with deionized water, and then soaked in deionized water until the soaking solution was neutral.Finally, the samples were put into nylon bag, and then dried at room temperature and ready for tests.

Drying-care simulation experiment
In order to simulate the daily drying environment of clothes by the sunlight and dryer, white duck down was treated at 55°C, 105°C, 155°C, and sunlight, respectively.Under the conditions of 55°C, 105°C, and sunlight, 20 g of white duck down was taken out every 24 h, and for taking 4 times, and then separately put into the sample bag as an experimental sample for later testing.White duck down was completely carbonized after 155°C treatment for 24 h, so only one sample was prepared at 155°C for 24 h.
In addition to simulating mechanical washing and drum drying environments, the above experimental conditions were done with and without mechanical agitation.The stirring speed of the mechanical washing environment was set at 25 rpm, and that of the drum-drying environment was set at 20 rpm based on the results of light mechanical forces given in previous washing and drying studies (Liu et al. 2018;Wei et al. 2019;Yun and Park 2015).

Fleeciness analysis
In order to evaluate the relationship between the different daily-care (washing and drying) conditions and the fleeciness of down, the fleeciness of white duck down with different daily-care conditions was tested according to GB/T 10288-2016 (methods for examination of down and feather).Graduated barrel of antistatic plexiglass (height 500 mm, diameter 288 ± 1 mm) and pressure plate of poly(methyl methacrylate) (diameter 284 ± 1 mm, mass 94.25 ± 0.5 g) were used to measure fleeciness.Prior to tests, all the specimens were conditioned in a standard atmosphere of temperature 20°C ±2°C and relative humidity 65 ± 2% for 24 h.The rate of change in fleeciness was calculated by the following formula (Eq.( 1)).In addition to ensuring the stability of the test, the fleeciness of each sample was evaluated at least three times under each experimental condition.
where ΔH is change rate of fleeciness, %; h 0 is fleeciness of down before washing or drying treatment (down fiber that was not been washed or dried), mm; and h 1 is the fleeciness of down after washing or drying treatment, mm.

Whiteness analysis
In order to assess the effect of daily washing and drying environment on whiteness of down fiber, the whiteness of down after different treatment were tested with the help of Data color 650.Moreover, because of down belonging to dispersive material (different from fabric), samples of down fiber should be preparation by loading 100 bunch down onto a transparent slide glass (76 mm � 26 mm) to form a layer sample of down fiber for whiteness test before testing.Additionally, in this study, the average whiteness of sample tested for 3 times was taken as the whiteness of the tested sample.The rate of change in whiteness was calculated by the following formula: where ΔW is change rate of whiteness, %; w 0 is whiteness of down before processing (down fiber that was not been washed or dried); and w 1 is the whiteness of down after processing.

Microscopic morphology analysis
In order to investigate the change in morphology of down after different daily-care treatment, microscopic morphology of down fiber treated by different daily washing and drying condition was conducted on S-4800 scanning electron microscope (Hitachi, Japan) at room temperature.Samples for SEM test were observed at 10 keV acceleration voltage, after gold coating under reduced argon atmosphere with a Med 020 Coating System (BAL-TEC).In addition, an ordinary camera and optical microscope were used to observe appearance changes of down fiber.

XRD analysis
To identify whether the decrease of crystallinity after different daily-care conditions occurs or not, XRD experiments were conducted using a diffractometer (Model D/Max 2500PC Rigaku, Japan) with Cu Ka radiation (λ = 0.154 nm), scanning speed of 2°/min, scanning range of 5°−40° operating at an accelerating voltage of 40 kV, and the current of 20 mA.Moreover, before testing, all test samples for XRD were prepared by cutting fragments.

FTIR analysis
In order to analyze the effect of the daily washing and drying environment on molecular structure of down, infrared spectrum of down after different aging conditions was carried out using a spectrum Two Fourier transform infrared spectrometer (PerkinElmer, USA).The samples were cut into powder, and then mixed with KBr and ground, pressed into thin disc, directly placed in the optical path for testing.Wavenumber scanning range was 4000-400 cm −1 , wavenumber as horizontal coordinate, percent transmittance as vertical coordinate, resolution 4 cm −1 , scanning times 32.

Results and discussion
In order to investigate the relationship between the daily-care environment and aging of down fiber, whiteness, fleeciness, crystallinity, molecular structure, and morphology under the commons daily washing and drying conditions were analyzed and compared.

Fluffiness analysis
Figure 1 clearly shows that the change of the fluffiness of down fiber after different daily washing and drying condition.As shown in Figure 1a,b, no matter how the washing environment, the fluffiness of down fiber assemblies after washing treatment declined.Moreover, under the same acidic or alkaline washing conditions, the decrease of fluffiness also showed an increasing trend with the increase of washing time.Additionally, compared with the same washing time, different acidic or alkaline washing conditions showed that the decrease in the fluffiness of down fiber assemblies was pH = 7 < pH = 5 < pH = 8 < pH = 9 < pH = 10 < pH = 12, which indicated that down fiber was acid-resistant instead of alkali-resistant; neutral or acid detergent should be developed for the daily washing-care of down fibers.This was because the down fiber belonged to the protein fiber, the adsorption isoelectric point was 3.5-5.2, it was the acid material, and the alkali sensitivity was greater than that of acid sensitivity.Therefore, in the alkaline washing environment, polar amino acid sol stripping and sub valence hydrogen bond breaking occurred easily, which led to down fiber occurring the aging phenomenon of shedding, dissolution, and other macro-damage, and thus the fluffiness of down fiber assemblies under the alkaline washing environment decreased more than that of acid washing environment (Yang, Li, and Shen 2018;Yang, Yu, and Pan 2011).In addition, comparing with or without mechanical agitation environment of neutral or weak acidic or alkaline conditions, it was found that the addition of mechanical agitation reduced the change rate of decline in the fluffiness of down fiber assemblies due to the more fully contact of down fiber with acidic or basic ions caused by the addition of mechanical agitation (see Figures 2 and 3).Moreover, it was also found that the decline of fluffiness of down fiber assemblies slightly reduced under the environment of neutral condition owing to the shake-off effect from mechanical agitation.However, under other washing conditions (same washing time, washing acid, and alkali environment), the decrease degree of fluffiness of down fiber assemblies increased due to the addition of mechanical agitation, which indicated that the mechanical agitation had a great influence on the fluffiness of down fiber assemblies, mechanical agitation should be minimized during the daily washing process of down products.This was because the mechanical agitation provided more opportunity for down fibers to contact with acid, alkali and water solution, and then more likely to cause performance degradation.Down fibers were fragile protein fibers with weak strength, mechanical agitation easily caused fiber breaking and even powder in alkaline washing environment (see Figures 2 and 3).In addition, the stronger the alkalinity, the longer the washing time, the more obvious the decrease of fluffiness caused by the increase of firnification, fracture, embrittlement, and dissolution.Compared with Figure 1c,d, it was found that the fluffiness of down fiber assemblies increased slowly with the increase of drying time in the environment of sunlight and 55°C low temperature drying without mechanical movement.The addition of mechanical movement further improved fluffiness of down fiber assemblies in this low temperature environment.However, under the drying condition of 105°C for 24 h, the fluffiness of down fiber assemblies remained unchanged or even slightly improved.Additionally, beyond this range, the fluffiness of down fiber assemblies decreased rapidly, with the addition of mechanical movement and the increase of drying time.The longer the drying time, the more significant the fluffiness decreased.Under the drying condition of 155°C for 24 h, down fiber showed serious embrittlement, fracture, yellowing, and even carbonization, the fluffiness of down fiber assemblies decreased significantly.Furthermore, down fiber appeared serious embrittlement, fracture, yellowing and even carbonization phenomenon (as shown in Figures 2 and 3).This indicated that drying time and drying temperature significantly correlated with the fluffiness of down.The longer the drying time and the higher the drying temperature, the more significant the fluffiness decreased.This was because the sunlight and 55°C drying treatment belong to the low temperature environment, did not exceed the pyrolysis temperature of down fiber, and thus damage was not caused.Additionally, the loss of quality was the water molecules or surface oil that was not conducive the fluffiness of down fiber assemblies.Therefore, with the extension of drying time, the fluffiness of down fiber assemblies showed an upward trend.However, the drying condition of 105°C and 155°C caused peptide chain of down fiber breaking, intermolecular hydrogen bond breaking, basic amino acid deamination, acidic amino acid decarboxylation, hydroxyl amino acid oxidation, ammonia emission, and even scorching; therefore, the assemblies decreased significantly.In addition, the down fiber was protein fiber, the strength of the fiber was weak, severe mechanical agitation easily led to the branch, sub-branch node, crotches, cluster of down fiber breaking, even the fracture of the feather shaft; therefore, in the daily drying care, high temperature and mechanical agitation should be avoided as far as possible.It was further confirmed that the down products were suitable for drying at lower temperature (55°C) or sunlight during the daily usage.Moreover, this also explained that the increase in the fluffiness of down fiber assemblies after drying under sunlight and at lower temperature for a short time.In addition, compared with the environment of washing and drying, it was found that the decrease of fluffiness of down fiber assemblies after washing was obviously higher than that of drying treatment.This may be the reason that the down fiber was a kind of flowers fiber with each down filament stretching with a certain repulsion, and being bonded together by water adsorption in the washing environment, and the strong alkali washing environment was more likely to cause down fiber fracturing, or even powdering (see Figures 2 and 3); therefore, the fluffiness decreased significantly.Under drying environment, down fiber avoided damage form mechanical shock of water flow, chemical corrosion of acid or alkali water solution, so fluffiness slightly decreases.In addition, surface lipids and volatility of down fiber were removed in drying environment, leading to the whiteness and fluffiness of down fiber decreased slightly.

Whiteness
Figures 4 and 5 show the effects of daily washing and drying conditions on the whiteness of down fiber.As shown in Figures 4a,b and 5, in the weak acid washing environment of pH = 5, the whiteness of down fiber increased after a short time of washing treatment, but it decreased slightly after longer time of washing treatment, and the whiteness of down fiber decreased slowly with the increase of washing time.Under the neutral washing condition of pH = 7, the whiteness of down fiber decreased slightly with the increase of washing time, but the change was not significant.Under pH = 8, 9, 10, 12 alkaline environment, the whiteness of down fiber slightly increased after short time (24 h) washing treatment, but after 24 h, the whiteness of down fiber decreased and the yellowing showed an increasing trend with the prolonging of washing time, and the more alkaline, the longer the treatment time.Namely, the whiteness declined more significant, yellowing phenomenon was more obvious.This indicated that the effect of acidic or neutral treatment on the whiteness of down fiber was slight.The short-term weak acid and weak alkali washing care for down fiber was appropriate because of the whiteness of down fiber slightly being improved by the weak acid and weak alkali washing care.However, the alkali environment was unfriendly to the whiteness of down fiber.This was because the down fiber belongs to the protein fiber, the adsorption isoelectric point was 3.5-5.2, it was a acidic substance, the sensitivity to alkali was greater than acid, and acid-resistant instead of alkali-resistant.Thus in an alkaline washing environment, the phenomenon of polar amino acid sol stripping and hydrogen bond sub-valence bond breaking was more likely to occur.Additionally, it was also found that the whiteness of down fibers decreased and the yellowing increased more evenly in the mechanical agitation environment compared with non-mechanical agitation environment.The results implied that the mechanical agitation reduced the uneven color difference caused by insufficient dithering in the process of washing care.However, in the alkaline environment, mechanical agitation provided the more frequent and full contact opportunity for down fibers with acid-base ions, the break of peptide chain in amino acid molecule of down fiber increased and the damage was more obvious, especially in strong alkali environment.Figures 4c,d and 6 display the effect of daily drying-care conditions on the whiteness of down fiber.As shown in Figures 4c,d and 6), the whiteness of down fiber decreased slowly and the yellowing increased slowly with the increase of drying time in sunlight.This was because down fiber belongs to protein fiber that contains tyrosine, L-phenylalanine and other aromatic amino acid with benzene ring structure of large π bond, easily absorbing ultraviolet light, and thus yellowing easily occurred when being exposed to the sunlight.Additionally, the longer the irradiation time, the yellowing more significant.Moreover, it was also found that the whiteness of down fiber remained stable regardless of drying time at 55°C, but the whiteness decreased significantly under the drying condition of 105°C and 155°C, and indicated that high temperature drying caused down material yellowing, and the higher the temperature, the more prone to yellowing and even carbonization (155°C for 24 h) in Figure 6.This was because in the high temperature environment, the down protein in the hydroxyl amino acids (such as tyrosine, tryptophan, etc.) occurred oxidation, the formation of colored material, and then down fiber occurred yellowing.In addition, at high temperatures, basic amino acids were susceptible to deamination, while hydroxyl amino acids and acidic amino acid also experienced oxidation and decarboxylation, resulting in the decrease in amino acid content and a continuous break and shortening of the down fiber molecular chain.Therefore, the whiteness at 105°C decreased almost linearly with drying time.In addition, compared with the same drying temperature, drying time, the drying environment without mechanical agitation was prone to yellowing and yellowing uneven.Because the mechanical agitation made down fiber fulling shaking, more uniformly heating, avoiding heat accumulation phenomenon, some fibers overheated, so the overall less heat damage.In addition, the results of washing and drying conditions showed that the decline of whiteness of down fiber after daily washing treatment was lower than that of the drying environment, which indicated that the yellowing of down fiber was mainly from the drying environment.

Microscopic morphology
Figures 7-14 clearly show the effect of different daily washing and drying treatments on down morphology.Figure 7 was the mircroscopic morphology of down fiber without washing and drying treatment.As shown in Figure 7, down fiber belongs to the flower shape, scatter-like distribution, the center has a down nucleus, to its center, the growth of a root of fine soft down branches.Down branches constitute the main body of down fiber.There were a lot of twigs arranged at a certain angle (30°−90°) on the twigs, and the arrangement angle decreased gradually from the root of the twigs to the tip of the twigs.At the same time, there were triangular nodes and forked nodes on the twigs, which were related to the growth and position of the twigs and the position of the twigs on the twigs.Compared with down fibers before and after washing treatment , it was found that after washing, the shape of down fibers decreased, down fibers were tangled together, and some feather branches were broken, the number of down branchlets on the pinnacle decreased obviously, and the down filament became shorter and sparse.At the same time, compared with the untreated down, the shedding of down floss was smaller in the neutral washing environment, followed by the acidic washing environment, and the most significant shedding occurred in the alkaline environment, the stronger the alkalinity, the more obvious the shedding, indicating that down fiber was more suitable for neutral or weak acid washing environment.In addition, it was found that obvious radial grooves appeared on the surface of down fiber after washing in alkaline environment, and the longer the treatment time, the stronger the alkalinity, and the deeper and more obvious the grooves, the results showed that the down fiber was damaged seriously in alkaline environment.This also explained the reason that the fluffiness of down fiber after washing in the alkaline environment was more obvious than that of the acid or neutral environment.
Figures 11-14 clearly show that the relationship between daily drying conditions and aging of down fiber.As seen Figures 11-14, compared with the untreated down fibers before and after drying, branches and twigs of down fiber were elongated at the first 96 h sunlight drying environment, and extend more than a certain time (96 h), extension state of fiber tended to be stable, and the down yellowing occurred; therefore, shorter period of sunlight drying for down fiber was acceptable.In low temperature environment of 55°C, the change in morphology of down fiber was not obvious, indicating that low temperature drying care has no effect on down fiber morphology, and thus it was conformed that the drying condition    of low temperature without mechanical was suitable for down fiber drying.After 155°C high temperature treatment, there were a lot of fiber fragments, fracture and crack on the surface of down fiber.This was because cohesion of down fiber declined, fiber chain fractured, brittle strengthened after high temperature treatment, thermal degradation obvious, so the higher the temperature, the longer the time, the more obvious damage.In addition, comparing the presence or absence of a mechanical environment at a low temperature of 55°C (Figures 11 and 12), we found that at the same drying temperature and time, the mechanical effects would aggravate the break of down twigs and the shedding of down twigs of down fibers.In other words, the branchlets peeling from down fiber in the non-mechanical environment was less that of the mechanical environment.This indicated that long-term mechanical action aggravated the damage of down fiber.Because the down fiber was the protein fiber with weak mechanical strength, in the fall, tumbling, throwing and other complex mechanical environment with greater strength, it was easy to break and peel off the floss, branches and twigs, so the damage was more obvious than that of environment without mechanical treatment.The longer the time, the higher the temperature, the more obvious the damage.At the same time, it was also found that: short time mechanical treatment, not only    did not cause down fiber damage, but also enhanced down fiber stretch, fluffiness of fiber increased.This was because the mechanical force on the down fiber for short time mechanical action did not reach the fracture threshold of down fiber, therefore, the fluffiness of down fiber after low temperature drying for a short time was increased.

XRD analysis
Figure 15 shows the relationship between the crystallinity of down fibers and the daily washing and drying conditions.As shown in Figure 15, compared with the untreated down fiber, the intensity of diffraction peak and 2θangle of down fibers after sunlight, low-temperature-drying (55°C), weak acid and alkali washing conditions for 120 h, hardly changed, and indicated that the effect of sunlight, low temperature (55°C) drying, and weak acid and alkali (pH = 5 and pH = 8) washing treatment on the crystallinity and crystal structure were not obvious.However, the position and strength of the diffraction peaks of the treated down fibers after the high temperature drying treatment (105°C, 96 h and 155°C, 24 h) and strong alkali washing treatment (pH = 13, 24 h) were changed compared with the untreated down fibers.This implied that the crystal structure of down fiber was changed significantly under the condition of the high-drying temperature treatment (105°C, 96 h and 155°C, 24 h) and strong acid and alkali treatment (pH = 13, 24 h), and indicated that down fiber should avoid too high drying temperature and strong alkali detergent during the process of daily washing and drying care.This change may be the reason that in the strong alkali washing environment, the orderly arrangement of down fiber molecules was destroyed, the intermolecular force decreased, the structure of the main valence bond inside the fiber was broken, the crystal area with close structure was destroyed.Therefore, the crystallinity of down fiber decreased.However, the α-helix structure and βfold structure slightly changed, this implied that the molecular crystal form of down fiber has no change.Additionally, the changes in degree of crystal structure increased with the increase of the time and intensity of washing and drying treatment.This was because that the stronger the alkalinity, the longer the action time, the more chance of contact between alkali ion and down fiber, the more complete the reaction, the greater the damage.Under the drying condition of 155°C for 24 h and 105°C for 96 h or longer, the amorphous region was destroyed and the crystalline region was retained intact, the crystallinity of down fiber decreased, and then the compactness of the whole structure decreased.Moreover, under the condition of 105°C drying treatment, it also found that the crystallinity of down fibers was unchanged (24 h), but increased (after 48 h), and then decreased (after 48 h) with the increase of treatment time.This was because only water molecules were removed in the first 24 h of drying treatment, and did not cause the crystal region and amorphous region broken, so the crystallinity almost no changed.However, as the treatment time was further prolonged, the amorphous region was destroyed and the crystalline region was retained intact, the crystallinity of down fiber decreased, and then the compactness of the whole structure decreased.

FTIR analysis
The effects of different daily washing and drying conditions on the aggregation structure of down fibers were observed respectively, the result was shown in Figure 16.As can be seen from Figure 16, under of the conditions of the low temperature drying (55°C and sunlight) and weak acid and alkali (pH = 5-9) for 120 h, and 105°C for less than 48 h, the infrared spectrum curves of down fiber were almost the same as those of the untreated down fiber, but the infrared spectrum curves of down fiber of being treated under the condition of 105°C for more than 48 h, 155°C for 24 h, pH = 10 h for more than 96 h and pH = 12 for 24 h are significantly different.This was because that surface temperature of down fiber did not exceed pyrolysis temperature of down protein under the drying conditions of 55°C and sunlight.Down fiber is a kind of acidic material, its isoelectric point was 3.5-5.2;therefore, it was not sensitive to the washing environment of weak acid, neutral, weak alkali (pH = 5-9).However, under the strong alkali washing condition, the internal polar amino acids dissolved and stripped, the intermolecular hydrogen bond broken, the molecular structure of β-type changed to α-type, the aggregation structure of down fiber was deteriorated.The Microscopic SEM results (Figures 7-14) are strong evidences for this.Therefore, it was conformed that the drying environment of lowtemperature (55°C and sunlight) and washing environment of weak acid, neutral, weak alkali (pH = 5-9) did not change the crystalline structure and functional groups.In other words, daily washing and drying conditions of the low temperature drying (55°C and sunlight) and weak acid and alkali (pH = 5-9) did not cause chemical damage of down fiber.Therefore, the low-temperature or neutral washing environment was applicable for down fiber during the daily washing and drying care.Additionally, compared the different daily washing and drying condition in Figure 16, it also clearly found that the aggregation structure of the down fibers was damaged gradually with the increasing of alkalinity of the washing environment, drying temperature and time.The stronger the alkalinity, the higher the temperature, the longer the time, the greater the difference.This further proves that the lowtemperature or neutral washing environment was suitable for down fiber during the daily washing and drying care, and thus the low-temperature or neutral washing environment should be chosen during the daily washing and drying care.

Conclusions
A thorough experimental investigation regarding the impact of the daily washing and drying conditions on performance of down fibers was conducted to explore the aging mechanisms behind the performance for better washing and drying care and even develop specific daily-care procedures for down fiber.Results showed that in the relatively neutral (pH = 5, 7, 8) daily washing environment, the aging of down fiber was not obvious because of the physical chemistry properties of down fiber suffering a slight change; Moreover, it was also found that the addition of slight short-time mechanical agitation was beneficial to the improvement of the fluffiness and whiteness of down fiber in this washing environment, which implied that the weak acid or neutral and slight mechanical agitation washing environment was more suitable for down fiber in the daily washing-care process.However, in strong alkali (pH = 9, 10, 12) daily washing environment, the aging of down fiber was obvious because of the physical chemistry properties of down fiber decreased significantly and even dissolving; the aging of down fiber increases rapidly with the increase of alkali strength and time; Moreover, the addition of mechanical agitation further aggravated the deterioration of performance due to the more fully contact of the alkaline ions and down fiber in the strong alkali (pH = 9, 10, 12) washing environment.At low temperature (55°C and sunlight) daily drying conditions, the whiteness, fluffiness, crystallinity and molecular structure of down fibers were almost unchanged, and then the aging of down fiber did not occurred regardless of time; the addition of slight mechanical agitation also improved the fluffiness and whiteness of down fiber at low temperature drying conditions.On the contrary, these physical chemistry properties of down fiber undergone degradation in different degrees under the high temperature (105°Cand 155°C) drying conditions, the higher the temperature, the longer the time for drying, the worse the performance.And this performance degradation was exacerbated owing to the addition of mechanical agitation.Additionally, it also found that no matter what the drying temperature, the addition of mechanical agitation increased the down fiber fluffiness, but reduced the color difference caused by uneven-drying when the drying temperature and drying time was same.This indicated that down fiber was more suitable for the daily drying treatment with low temperature and slight mechanical agitation.Moreover, this finding was not only helpful to understand of aging mechanisms of down fiber under the daily washing and drying conditions, but also provided scientific care guidance and optimization ideas for subsequent research and applications.

Disclosure statement
No potential conflict of interest was reported by the authors.

Figure 1 .
Figure 1.The change of fluffiness under different daily washing and drying conditions.(a) The daily washing condition without mechanical agitation, (b) the daily washing condition with mechanical agitation, (c) the daily drying condition without mechanical agitation, and (d) the daily drying condition with mechanical agitation.

Figure 2 .
Figure 2. The morphology of down flower under different daily washing and drying conditions.

Figure 3 .
Figure 3.The morphology of down branch under different daily washing and drying conditions.

Figure 4 .
Figure 4.The change of the whiteness of down fiber under different daily washing and drying conditions.(a) The daily washing condition without mechanical agitation, (b) the daily washing condition with mechanical agitation, (c) the daily drying condition without mechanical agitation, and (d) the daily drying condition with mechanical agitation.

Figure 5 .
Figure 5.The morphology of down fiber assemblies under different daily washing conditions.

Figure 6 .
Figure 6.The morphology of down fiber assemblies under different daily drying conditions.

Figure 7 .
Figure 7.The microscopic morphology of down fiber without washing and drying treatment.

Figure 8 .
Figure 8. microscopic morphology of down fiber after treatment in neutral washing environment.

Figure 9 .
Figure 9.The microscopic morphology of down fiber after treatment in acid washing environment.

Figure 10 .
Figure 10.The microscopic morphology of down fiber after treatment in alkaline washing environment.

Figure 11 .
Figure 11.The microscopic morphology of down fiber after treatment in sunlight drying environment.

Figure 12 .
Figure 12.The microscopic morphology of down fiber after treatment in the drying environment of 55°C without mechanical agitation.

Figure 13 .
Figure 13.The microscopic morphology of down fiber after treatment in the drying environment of 55°C with mechanical agitation.

Figure 14 .
Figure 14.The microscopic morphology of down fiber after treatment in the drying environment of 155°C.

Figure 15 .
Figure 15.XRD picture of down fiber after different washing and drying treatment.

Figure 16 .
Figure 16.The Infrared spectrogram of down fiber after different washing and drying treatment.