Stain removal performance of moisture-wicking fabric during home laundry

2.1 Materials

Fabric: 100% polyester plain weave, weight: (154–160) g/m², longitude and latitude density: 32 × 21, yarn thickness: (0.48–0.56) mm. Textile fabrics for sweating sportswear were used for the cleaning performance test, which was purchased from Quanzhou Haitian Material Technology Co., Ltd. The information on fabric is shown in Table 1. Materials for the preparation of stains of sebum and carbon black oil were purchased from Hangzhou Lvze Yuhe Biotechnology Co., Ltd.

2.2 Preparation of the stained fabrics

The stains were placed on the printing screen equipment (LY-1600 Multicolor Printing Machine, Dongyuan Experiment Equipment Factory, Jiangsu, China) to prepare the stained fabric with the stain evenly distributed. After printing, the stained fabric was dried in an oven at 60 °C. After 30 min, the stained fabric was cured at 120 °C for 120 min.

2.3 Preparation of the standard detergent and surfactant

To prepare a standard detergent, 8% alkylbenzene sulfonic acid (calculated by active ingredient content), 4% polyethoxylated fatty alcohol (average EO addition number is 9), 2% sodium ethoxyl alkyl sulphate (2 EO, calculated by active ingredient content), 0.5% triethanolamine, 0.6% trisodium citrate dihydrate and 0.1% preservative were weighed in successively. Then this mixture was added to a certain amount of water, dissolved and stirred while heating. The pH of the solution was adjusted to 8.5–9.0 with sodium hydroxide, and this solution was made up to 100% with further water. The prepared standard detergent was stored at room temperature for 24 h. All the above materials were purchased from Guangzhou Blue Moon Co. Information on other surfactants used in the experiment can be found in Table 2.

2.4 Laundry conditions

Wash load fabric = 15 g; washing machine: wash fastness tester from Ningbo Textile Instrument Factory (Zhejiang, China), washing bath: 300 mL water with a hardness of 250 ppm [16]; washing detergent: 0.6 g liquid detergent (Tide), washing mechanics: 5 steel balls; washing temperature: 40 °C; duration of one wash cycle: 30 min; rinsing bath: 1× with water, drying: oven.

2.5 Stain removal index (SRI)

The K/S value is broadly used in the textile industry as an indication of fabric color depth. The higher the K/S value, the deeper the fabric color. The difference in the K/S value can be used as the fabric color change before and after laundering [17]. A Datacolor SF650-PLUS (Datacolor, United States) was employed for K/S value measurement. The following equation was used to calculate the stain removal index (SRI).

In Eq. (1), K/S_BS is the K/S value of the stained area before washing, while K/S_AS is the K/S value of the stained area after washing. K/S_BU is the K/S value of the unstained area before washing while K/S_AU is the K/S value of the unstained area after washing.

2.6 Characterization

3.1 Physical morphology of fabrics

The physical morphology of moisture-wicking fabric is very different from ordinary polyester fabric. The main physical parameters of the fabrics used in this investigation are listed in Table 3.

Table 3:

Physical parameters of the fabrics.

Fabric		Thickness (mm)	Weight (g/m2)	Fiber morphology (SEM)
Pure polyester plain weave fabric	Original cloth	0.48	160
	Degreasing	0.56	165
	Degreasing & thermosetting	0.48	154

After degreasing, the thickness and weight of the moisture-wicking fabric increased, indicating that the fabric had shrunk. The reason for this could be that the macromolecules of the fibres formed hydrogen bonds after degreasing and the distance between the fibres decreased. After stereotype finishing process, the fabric was stretched in width and its weight and thickness were reduced.

The fabric was examined using SEM. The results showed that the cross-section of the fabric was hexagonal and that there were small particles on the fibre surface. These small particles were caused by the spinning process of the polyester, due to the poor compatibility of polyethylene glycol terephthalate with a polyvalent hydrophilic finishing agent (N,N-di-(hydroxyethyl) coconut oil amide) and a cationic quaternary ammonium surfactant [19], [20], [21]. Therefore, some of the hydrophilic finishing agent and surfactant was deposited on the fibre surface during the melt spinning of the polyester fibres.

3.2 Influence of washing on the air permeability of fabrics

The diagram in Figure 1 shows the change in air permeability of moisture-wicking fabrics before and after washing. After the degreasing & thermosetting process, the air permeability of the moisture-wicking fabric increases from 1013 mm/s to 2911 mm/s. The air permeability of the fabric is greatly improved after the thermosetting process, proving that the thermosetting finish is particularly critical for improving the gas permeability of the fabric. All three fabrics show a slight decrease in air permeability after washing. After a single washing cycle of the fabric (fabric treatment: degreasing & thermosetting), the air permeability decreases by 11.40%. After the second and third wash, the air permeability of the fabric changes less than after the first wash. These results indicate that the air permeability of moisture-wicking garments decreases slightly after the first wash, but does not change significantly with subsequent washes.

Figure 1:

Influence of washing cycles on the air permeability of fabrics.

3.3 Influence of different finishing processes on fabric stain removal performance

The stain removal performance of moisture-wicking fabric was evaluated by the SRI index. As shown in Figure 2, the SRI of sebum on the fabric (degreasing and degreasing & thermosetting) increases from 37.85% to 75.19% and 66.67%, respectively, indicating that the decontamination performance of sebum is significantly improved. In the case of carbon black oil, the SRI index of the fabrics after degreasing and after degreasing & thermosetting increases from 26.90% to 56.32% (1.09-fold) and 61.03% (1.27-fold), respectively.

Figure 2:

Carbon black oil and sebum removal indices on moisture-wicking fabrics.

If the fabrics had excellent hydrophilicity, the spaces between the fibers and the stains are more easily wetted by water molecules, resulting in a loose binding force between the stains and the fibers. In addition, since the surfactant can effectively reduce the surface tension of water during washing, the fabric is more easily wet by water, and the stain is more easily removed during the washing process [22], [23], [24]. The hydrophilic performance parameters of the three fabrics are shown in Table 4. The wetting time of the original cloth was about 31 s, and the contact angle between the water droplet and the fabric was large (about 140°) after wetting 1 s, which indicated that the original cloth was hydrophobic and had poor wettability. After degreasing or degreasing & thermosetting, the hydrophilicity of the fabric had been significantly improved, and the water droplets penetrated the fiber within 0.5 s, which indicated that the hydrophobic oil on the surface of the fabric was removed, and the wettability of the fabric was improved. This also explained the significant improvement in the SRI of carbon black oil.

More interestingly, the cleaning power of sebum stains was decreased after thermosetting compared to the degreasing treatment. The reason might that the stains of sebum had more saturated and unsaturated oils and the fabrics were saturated with oils during the aging process. The standard detergent used in washing, anionic surfactants account (alkylbenzene sulfonic acid) for the main component, and the content of nonionic surfactants (polyethoxylated fatty alcohol) is less. The removal of grease stains was not ideal, which also made the results more obvious. On the other hand, there was a layer of hydrophilic silicone oil on the surface of the fabric after thermosetting [25, 26]. These silicone oils were segmented hydroxyl silicone oils, and the branched-chain contained EO groups, which increased the intermolecular force between the stain of sebum and the fabric (hydrogen bond, van der Waals force, etc.), resulting in the physical bonding force between the stain and the finishing agent was strengthened, the stain removal performance was decreased.

3.4 Influence of washing on decontamination performance of fabrics

The fabric was washed before the dyed fabric was made. The washed fabric was then printed with stains. Figure 3a shows that the SRI of the carbon black oil on the original fabric hardly changed after several washing cycles, while the SRI of the carbon black oil on the degreased & thermosetted fabric was significantly improved. For the sebum stains, the SRI of the three fabrics is increased after washing (Figure 3b). The decontamination performance of moisture-wicking fabric after washing was affected. With the extension of washing cycles, the decontamination performance of stains on the fabric was improved, but not significantly changed.

Figure 3:

The SRI of stained fabrics: (a) carbon black oil, (b) sebum.

3.5 Surface morphology of sebum and carbon black oil on fabrics

The morphology of the stains on the fabric before and after washing was examined with the SEM. The images in Figure 4a and b shows that there are many lumpy particles on the fabric and that the sebum stains aggregated on the fibre after ageing. At the same time, the fibre surface is covered with sebum stains and there is a lot of sebum between the fibres and in the spaces between the fibres. After washing, most of the lumpy particles on the fibre surface are removed and the fibre surface is smooth, but a small amount of sebum can still be observed in the gaps between the fibres (Figures 4c and d).

Figure 4:

The topography of the fabric (degreased) to sebum before (a and b) and after washing (c and d).

The topography of the carbon black oil on the fabric (degreased) is shown in Figure 5. As shown in Figure 5a and b, the carbon black oil is evenly distributed in the fibre interstices and on the fibre surfaces. After washing, the large carbon black oil particles on the fabric have been removed (Figure 5c and d). However, when magnified, small particles can still be seen on the fibre surface, indicating that the carbon black oil has hardly been removed.

Figure 5:

The topography of the fabric degreasing process to carbon black oil before (a and b) and after washing (c and d).

3.6 Element analysis of sebum and carbon black oil on fabrics

Three different positions on the moisture-wicking fabric before and after washing were selected for elemental analysis, and the elements found in sebum are shown in Table 5.

Table 5:

Element analysis of sebum on the fabric (degreased).

Element	Atomic ratio (%)
	Before washing			After washing
	Fiber	Stain on bright	Stain on dark	Fiber	Stain on bright	Stain on dark
C	61.43	42.71	86.91	77.03	45.81	74.37
O	30.73	43.11	12.55	22.97	44.76	17.06
Si	6.15	13.45	0.33		6.79	8.47
Al	1.17	0.55	0.12		1.74	0.1
Fe		0.11			0.29
K	0.42	0.06			0.9
S		0.02	0.07
Ca			0.03
Mg	0.1				0.4

In addition to C and O, Si and some metal elements such as Al, K, Mg, etc. were also found on the fiber surface before washing. Silicone was mainly derived from the hydrophilic silicone finishing agent on the fabric, and the sludge component in the stains of sebum had many metal elements. After washing, other elements except C and O were hardly detected, which proved that most of the sludge on the surface of fabric was removed, and the hydrophilic silicone oil was also washed away. The oxygen content in the stain on the bright spots was significantly increased, indicating that a lot of oil was in the stains on the bright spots. After washing, the carbon and oxygen ratio of the stain was changed little, and the oil components of the stains at the bright spots were almost changed little during washing. The carbon content of the stain on the dark (fiber and fiber gap) was significantly increased and contained a small amount of Al, S, and Ca, indicating that there were large amounts of carbon black and a small amount of sludge in the dark spot. After washing, the carbon content of the dark spot was decreased, the Al content was decreased, and the element of S and Ca disappeared. Therefore, some carbon black and most of the sludge were washed off during the washing process.

As shown in Table 6, a simple element analysis of carbon black oil is also performed on the stains before and after washing. The results show that most of the carbon black in the stained area was also removed after washing, and most of the oil substances are washed off. In summary, sludge and carbon black of the stains of sebum were mainly removed after washing, while oil components were hardly removed.

Table 6:

Element analysis of carbon black oil on the fabric (degreased).

Element	Atomic ratio (%)
	Before washing		After washing
	Fiber	Stain	Fiber	Stain
C	74.2	75.42	74.29	46.79
O	25.8	24.51	25.71	52.29
Si		0.08		0.93

3.7 Influence of different surfactants on stain removal performance of fabrics

Different types of surfactants were used to improve the stain removal performance of moisture-wicking fabrics. The decontamination effect of different surfactants on two types of stains on the fabric subjected to the degreasing & thermosetting finish treatment was verified by comparing them with the standard detergent. The results are shown in Figure 6. It can be seen that TO-8, FMEE, AEO9, and CAPAO have a better decontamination effect on sebum than the standard detergent (66.50%). Their SRI is 68.44%, 70.72%, 72.01%, and 68.41%, respectively. Only FMEE (71.03%) has a better removal performance of carbon black oil than the standard detergent. These results indicate that the soil removal performance of anionic surfactants is not as good as that of standard detergent, and the non-ionic surfactants, especially FMEE, have better decontamination effects. Most interestingly, the removal performance of XO-30 is very poor, but the effect of TO-8 with a longer carbon chain is much better, indicating that the decontamination effect is better when the carbon chain is longer. Non-ionic surfactants with long carbon chains have a better removal effect.

Figure 6:

SRI of different surfactants: (a) stains of sebum, (b) carbon black oil; S.D. = standard detergent.

In a detergent formulation, surfactants play a key role in washing as they reduce the surface tension of water and emulsify, disperse, etc. soil components [15]. As shown in Figure 7, due to the strong repulsive force between the hydrophobic chain of the surfactant and the water molecules and the strong force between the hydrophilic chain of the surfactant and the water molecules, the surfactant diffuses to the gas-liquid interface when added to an aqueous solution and forms micelles surrounded by water molecules [27]. As a result, the surface tension of the water and the wettability of the fabric are affected during the washing process.

Figure 7:

The functions of surfactants in aqueous solution.

To investigate the function of the surfactant in aqueous solution, the surface tension of different surfactants was measured, and the wetting time of the water-permeable fabric was also measured after adding the surfactant. As shown in Table 7, the surface tension of the pure water solution is 72.83 mN/m at 25 °C. After adding detergent or surfactant to the water solution, the surface tension of the water solution is reduced. The capillary action of the fabric is 24.70 cm when there was no surfactant in the water solution. After adding 2 g/L of standard detergent or surfactant, the capillary action of the water-permeable fabric is improved, indicating that the water molecules could easily spread on the surface of the water-permeable fabric and the surface of the water-permeable fabric could be easily wetted. Therefore, water molecules can diffuse well between fibres and stains with the help of surfactants, weakening the binding force between stains and fabric and improving the decontamination performance of the fabric.

Table 7 shows that the surface tension of water is reduced from 72.83 mN/m to 33.61 mN/m after adding 2 g/L of standard detergent to the water solution. The comparison of the different surfactants shows that the surface tension of water is lowered by TO-8 (isotridecanol polyoxyethylene ether) and FMEE (fatty acid methyl ester ethoxylates) from 72.83 mN/m to 31.79 mN/m and 30.51 mN/m, respectively. However, the surface tension of water is lowered to 38.79 mN/m by XO-30. The hydrophobic chain of TO-8 molecular structure contains 13 carbon atoms while the molecular structure of FMEE contains 18 carbon atoms. However, the molecular structure of XO-30 contains only 10 carbon atoms. Therefore, the surface tension of water is reduced more significantly by the two surfactants with the longer hydrocarbon chains TO-8 and FMEE.

In addition, the capillary action of substances can be significantly improved by TO-8 and FMEE. LAS and AES are anionic surfactants with good water solubility, which would affect their adsorption on the surface of the water solution. Therefore, the surface tension of water is reduced by LAS and AES from 72.83 mN/m to 36.16 mN/m and 34.32 mN/m, respectively. CAPAO (alkylamidopropyldimethylamine oxide, amphoteric surfactant) can lower the surface tension of water to 35.04 mN/m, and the capillary action of the fabric is only 27.21 cm. Regarding the influence of surfactants on stain removal performance, it can be found that C₁₈ fatty acid methyl ester ethoxylates and isotridecanol polyoxyethylene ether effectively reduce the surface tension of water and improve the wettability of water-permeable fabric. Therefore, these surfactants are suitable for use in detergent formulations.