Effects of washing on the mechanical properties of medical compression stockings

Abstract

The performance of medical compression stockings can undergo notable changes after washing. This study aimed comprehensively to evaluate how the mechanical properties of medical compression stockings would be changed after undergoing washing under different conditions. We further investigated the effect of washing parameters on medical compression stockings. By subjecting medical compression stockings to washing under varying conditions, and measuring the mechanical properties (including the tensile properties, bending property, friction property, and bagging property) of medical compression stockings from the knee to the thigh, distinct alterations were observed. Notably, it was concluded that the elongation, the tension, and the bending rigidity of medical compression stockings increased. In addition, the residual rate of bagging was reduced after washing. In addition, the washing temperature was the most significant factor affecting the mechanical properties of medical compression stockings. Meanwhile, the mechanical actions of the washing machine, such as drum speed and washing time, also exerted varying degrees of influence on the mechanical properties of different medical compression stockings. Furthermore, this study not only provides essential guidance for consumers regarding effective washing practices for medical compression stockings, but also contributes valuable insights towards optimizing the mechanical properties of medical compression stockings to maintain their key properties.

Keywords

Medical compression stockings machine washing mechanical properties washing parameters

The principle behind compression stockings (CSs) lies in their ability to generate a controlled pressure gradient from the ankle to the thigh, exerting the highest pressure at the ankle, and gradually reducing it toward the thigh. These stockings find extensive application across numerous sectors, including medicine, healthcare, sports, and body shaping.^1
–6 To maintain the functionality and applicability of CSs, the original pressure performance of CSs should be maintained.⁷

While CSs are commonly worn in daily life, their original performance diminishes over time due to repeated wearing and washing. Harpa et al.⁸ investigated the ability of CSs to maintain their designated gradual compression pressure after repeated 15 and 30 wearing–washing cycles. Their findings revealed a consistent decrease in compression pressure of CSs following repeated wearing–washing cycles. In a separate study, Gohar and Mazari⁹ examined the impacts of washing on the compression pressure and the dimensional change of CSs, which resulted in there being an insignificant change in the pressure and the dimensional deformation. In particular, no fabric extension was observed, but shrinkage occurred for CSs. In addition, the washing action was also controlled by washing temperature. Siddique et al.¹⁰ evaluated the performance of CSs by washing them at different temperatures (30, 50 and 75°C). Their findings indicated a significant increase in compression pressure with higher washing temperatures, attributed to a gradual increase in CSs shrinkage. Notably, these products were washed without being worn between washes. Similarly, Akçagün¹¹ discovered that pressure values increased as the washing temperature (30–40–50°C) rises, with the compression pressure showing the highest increase at 50°C. However, Siddique et al.¹² noted limitations in the temperature level scale used in the research of Akçagün, considering it too narrow with a lower high-temperature level. Thus, Siddique et al. extended the study by machine washing CSs at 30, 50, 75, and 100°C. They reported that the increase in the temperature from 30 to 100°C resulted in a consistent increase in compression pressure. Furthermore, Siddique et al.¹³ investigated the impacts of multiple machine washes on CSs compression pressure. Their findings indicated that the influence on compression pressure increased as the washing cycle rises.

Evidently, the impact of washing on the performance of CSs primarily centers on their compression pressure. However, the compression pressure of CSs is determined by their mechanical properties. In addition, several studies have demonstrated a significant correlation between the mechanical properties and compression pressure of CSs.^14
–20 Furthermore, the mechanical properties of fabrics are correlated with the hand feel of the fabric as well as wearing comfort performance (i.e. friction property and bagging property).^14,21,22 Therefore, analyzing the mechanical properties of CSs forms the basis for understanding their performance. However, there are few works on the research of the change in mechanical properties of CSs after washing.

The CSs belong to knitted fabrics. To date, many scholars have paid attention to the effects of washing on the mechanical properties of knitted fabrics. For instance, Kim and Chung²³ investigated the effects of washing on the mechanical properties by putting the knitted fabrics into the drum-type washer for 30 laundering cycles, and they found that the properties (including the tensile, bending, shearing, and surface properties) of the knitted fabrics changed continuously throughout all the washing cycles. Moreover, Roh and Kim²⁴ studied the changes in the mechanical properties of knitted fabrics associated with washing conditions. The results showed that the washed knit fabrics were extended, stiff, rough, and had a smaller volume. After washing the properties of knitted fabrics would be changed.

From the above literature, it can be concluded that: (a) Washing parameters not only include the temperature and washing cycles, but also have other factors, such as drum speed and washing time. In addition, different factors exert mutual influence on each other. Consequently, examining the influence of a single washing factor on CSs performance may result in limitations, potentially compromising the accuracy of the outcomes. (b) There is a scarcity of studies regarding the impact of washing parameters on the mechanical properties of CSs. Even though the CS is a kind of knitted fabric, and washing can make a difference to the knitted fabric, the CS has the special region division from the ankle to the thigh part, which makes it functional. Therefore, the main objectives of this study were to investigate how the mechanical properties of CSs change after washing, including different regions from the ankle to the thigh. In addition, it seeks to ascertain the impact of various washing parameters of the washing machine (including washing time, drum speed, temperature, and their interactions) on the mechanical properties of CSs.

Experiment and materials

The objective of this study is to analyze the effects of washing parameters, including the main effects (washing time/washing temperature/drum speed) and their interactions on the mechanical properties of medical compression stockings (MCSs), which requires the experimental design to include the following two characteristics: (a) it is suitable for multi-factor analysis; (b) it is able to explore the impact of interactions on responses. Therefore, the first step is to determine the design of the experiment by comparing the following commonly used experimental design methods (as shown in Figure 1).

Comparison and determination of design of experiment

In this study, various experimental design methods have been compared including single-factor experimental design, full factorial experimental design, orthogonal experimental design, and custom experimental design (Figure 1). Ultimately, custom experimental design was determined for the design of the experiment. Because single-factor experimental design focuses on the effects of a single independent variable,²⁵ it is not applicable for studying the impact of multiple factors on the response. Although orthogonal experimental design can address this limitation of simultaneously investigating multiple factors on the response variable,²⁶ it is unable to investigate the interactions among multi-factors. Full-factorial experimental design allows for the estimation of main effects and interaction effects of the factors,²⁷ which can meet the experimental objective of this study. However, testing all possible combinations of factors and levels can require a large number of experimental runs, making the design resource-intensive in terms of time, cost, and effort. Unlike a full-factorial design in which all possible combinations are tested, custom design of the experiment (custom design of the experiment refers to a tailored experimental design in which specific factors, levels, and their combinations are selected based on the research objectives of the study) allows for a more efficient use of resources by strategically choosing the most informative combinations to achieve the research goals. Meanwhile, it can explore the effects of interactions, which can meet the experimental objective of this study. Therefore, custom design of the experiment was identified as the experimental design method for this research (Figure 1).

Process of custom design of washing experiment

The custom design of the experiment was constructed utilizing JMP 14.0 (JMP software, SAS, USA). Besides, the process of the experimental design is listed below and shown in Figure 2:

Setting of main factors: The main factors of this study were washing time, washing temperature, and drum speed. In addition, the levels of three main factors were set according to the actual washing conditions and the adjustable range of the washing machine, as depicted in Figure 2.

Selecting the model: The response surface methodology (RSM) model was used to approximate the effects of interactions on the responses. The interactions of these factors are listed in Figure 2(a).

Number of the center point: Setting the center point can test the curvature effects of factors on the response, helping to determine whether quadratic effects or interaction effects need to be introduced. Therefore, one center point (30 min, 45°C, 40 rpm) was set (Figure 2(a)).

Number of replicate runs: By conducting a partial repetition experiment, the overall scale and complexity of the experiment can be reduced while maintaining the effectiveness of the experimental design, leading to improved efficiency in experimental execution. The number of replicated runs in this experiment was three runs (i.e. group i-1 and i-11, i-8 and i-13, i-6 and i-15 are three replicate runs).

Optimal design: ‘I-Optimality’ was chosen to determine the optimal experimental points, which could minimize the total variance of parameter estimates.

After setting all the conditions and making the design, all the experimental groups were designed for each compression level of the medical compression stockings, including three different compression levels. The experimental groups are listed in Figure 2(b). (a) process of custom design of washing experiment, (b) experimental groups designed utilizing JMP software.

Materials

Procurement of MCSs

In this study, all MCSs were purchased from the manufacturer (L size; Zhejiang Tuozhen Medical Technology Co., Ltd., China). Unlike usual CSs (i.e. sports compression stockings), MCSs were designed to generate a more accurate graduated pressure and distribution of structural parameters from the ankle to the thigh (i.e. to form a degressive gradient pressure profile, the knitting densities of the laid-in structures from the ankle to the thigh region presented gradient variations along the MCSs tubes (Figure 3(a)). Besides, the MCSs used were thigh MCSs (i.e. including ankle B, ankle brachial B1, calf C, knee E and thigh F) exhibiting three different compression levels designated as class I (Ccl1), class II (Ccl2), and class III (Ccl3) (Figure 3(a)). Moreover, all MCSc were made up only of polyamide and spandex contents; the structure (1 × 1 laid in structure) and the microscopic structures of MCSs are shown below in Figure 3(a).

Determination of basic physical properties of MCSs

All MCSs were analyzed with great precision and accuracy under standard atmospheric conditions for preliminary characterization of MCSs from the ankle to the thigh region. Basic physical properties of all MCSs included the wale density (loop numbers of 5 cm length in wale), circumference (the girth of each cross-section), and weight per unit area of fabrics (the weight of the cut strips of dimension 5 × 5 cm² was obtained from each cross-section). The samples were conditioned for 24 h in the desecrator before testing to keep them free from moisture. The detailed measurement values are shown in Figure 3(b).

Marking MCSs from ankle B to thigh F region

As depicted in Figure 4, five distinct cross-sections were designated following the Chinese textile standard (FZ/T 73031-2009) for MCSs: B: ankle; B1: area wherein Achilles tendon transitions into the calf muscles; C: maximum calf circumference; D: just below the tibial tuberosity; E: mid-thigh. Each MCSs level was positioned on a standard wooden leg mannequin (W01, Shanghai Qirui Electric Co., Ltd., China) to be marked with a line at these cross-sections. Subsequently, the MCS was removed from the wooden leg and left for 24 h to recover its original shape. Then, all samples were marked with lines referring to the first one, with the length of each line denoting the circumference of each cross-section, using a textile marker pen, as illustrated in Figure 4.

Experimental details of machine washing

All MCSs (including Ccl1 (16 groups), Ccl2 (16 groups), and Ccl3 (16 groups)) were washed employing a front-loading, automatic washing machine (Wascator FOM71 CLS, Electrolux, Sweden) according to the washing programme listed in Figure 2(b). Other default washing programme settings in the experiment included a washing load of 2 kg (2 kg ± 0.1) by adding the ballast fabrics, the water level was 13.5 L; the rinsing runs were repeated three times, and the spinning time was 5 min. Each MCS sample was washed three times. After washing, all the MCSs were offloaded for flat drying for 24 h in still air under ambient conditions, ensuring no wrinkles without stretching or the existence of induced forces (Figure 5).

Measurement of the mechanical properties

To analyze the effect of washing on the mechanical properties of MCSs (including Ccl1, Ccl2, and Ccl3), all MCSs before and after washing (51 groups) were examined to record mechanical property measurements such as the tensile property, bending property, friction property, and bagging property. Every cross-section (including five regions from the ankle to the thigh) of each MCS was measured with the mechanical properties; that is, every type of mechanical property testing included 255 groups to clarify the mechanical property changes in every region (Figure 6). Each experiment was repeated three times, and their average was calculated.

Tensile properties

Preparation of the sample: take the marking line on every cross-section of the MCS as the midline, and then take 2.5 cm from the top and bottom to set a width of 5 cm, and mark it successively from the B region to E region (255 groups in total, and each group repeated three times); cut out the marked partitions on the MCS (Figure 7(a) and (b)). Experimental process: an AGS-X Series Universal Testing Machine (AGS-X (vertical), Shimadzu Co., Ltd., Japan) was used to take the tensile properties of the fabric according to the experimental procedures of the EU 3392 standard²⁸ (Figure 7(c)), and stretch the samples from 0 to 80% at a constant speed of 300 mm/min.

The formula for calculating the elongation of medical compression stockings is

Elongation = \frac{L_{1} - L_{0}}{L_{0}} \times 100 %

(1)

where L₁ is the circumference of the wooden leg mannequin (FZ/T 73031-2009); and L₀ is the circumference of the MCS.

The tension is the value at the corresponding elongation of every cross-section.

Bending property

Preparation of the sample: take the marking line on every cross-section of the MCS as the midline, and then take a 50 × 55 mm sample (255 groups in total, and each group repeated three times) (Figure 8(a) and (b)). Experimental process: the experiment was carried out according to the FZ/T01054.1∼6-1999 standard, and the bending property of every cross-section of the MCS was measured by the fabric handle tester (YG821, Laizhou Electron Instrument Co., Ltd., China).

Friction property

Preparation of the sample: take the positioning line of every cross-section marked before washing as the midline, and then take a sample (255 groups in total, and each group repeated three times) with a width of 30 mm and a length of 105 mm (Figure 9(a) and (b)). Experimental process: it was carried out according to the operation steps of the friction experiment in the YG821 Fabric Handle Tester. During the experiment, it is necessary to keep it in a flat state for the experiment because the curl of the knitted fabric is severe (Figure 9(c)).

Bagging property

The preparation of samples: take the positioning line of every cross-section marked before washing as the center line, and then take a circular sample with a diameter of 76 mm at every region (255 groups in total, and each group repeated three times) (Figure 10(a) and (b)). Experimental process: it was carried out according to the operation steps of the bagging deformation experiment in the fabric handle tester (Figure 10(c)).

Statistical analysis

In this study, JMP 14.0 (JMP software, SAS, USA) was applied to analyze the relationship between washing parameters and the mechanical properties of MCSs, including the significance analysis of the influencing factors. Boxplot and line charts were generated using the Origin software framework (OringinPro 2021, OriginLab Corporation, USA).

Results and discussion

Effects of washing on the tensile properties of MCSs

Changes in elongation

As shown in Figure 11, before washing, the elongation of MCSs at every cross-section in Ccl1 and Ccl2 increased gradually from B to C; the elongation of part D was smaller than that of C and E. In addition, the elongation trend from part B to E of Ccl3 was the same as that of Ccl1 and Ccl2, except that part B was larger than B1 in Ccl3.

After washing under different conditions, the elongation of MCSs increased, which was caused by the circumference of MCSs at every cross-section becoming smaller, leading to an increase in the elongation of MCSs. Meanwhile, the trend of elongation at every cross-section was maintained.

As shown in Figure 12, for different levels of MCSs, the elongation of Ccl1 was close to Ccl2 and greater than that of Ccl3 after washing. Because the circumference in Ccl1 and Ccl2 at every cross-section was more closed and larger than that in Ccl3, caused by the fact that the content of spandex in Ccl3 (32%) was more than that in Ccl1 and Ccl2 (28%) (shown in Figure 3). In addition, the original circumference of Ccl3 before washing was relatively smaller, leading to the elongation of Ccl3 being significantly smaller than that of Ccl1 and Ccl2 as a whole.

Significance analysis

To analyze the effects of washing parameters and their interactions on the MCSs, the least squares analysis was performed using JMP. In addition, the significance factor (P > |t|) is less than 0.05, which means that the factor is significant; and the significance factor (P > |t|) is less than 0.0001, meaning that the factor is highly significant and it is the main significant factor in all factors; when the t ratio is greater than 0, the factor has a positive effect; when the t ratio is less than 0, the factor has a negative effect.

All the main factors and their interactions, sorted by the significance from highest to lowest, are listed in Table 1. It was evident that all the factors were significant. Notably, the washing temperature, washing temperature × drum speed (the interaction between washing temperature and drum speed), and washing time × drum speed (the interaction between washing time and drum speed) were highly significant. This indicated that these three factors were the primary influencers. Therefore, the elongation of MCSs can be changed by adjusting the washing temperature, drum speed, and washing time. With the washing temperature increasing, the elongation became greater. The heat caused the molecules within the fibers to align more closely together, which can further lead to the shrinkage of the fabric of MCSs.²⁴ The higher the temperature, the greater the shrinkage of the circumference, which led to the elongation increase. In addition, within a specific range, as the drum speed increased, the frequency of MCSs sliding and slapping also increased while following the movement of the drum. Furthermore, moisture penetrated the interior of the fiber during the sliding and slapping process, causing the yarn crimp to increase and finally the fabric shrunk.²⁹ The faster the drum speed, the greater the shrinkage of the circumference, which can increase the elongation. When the drum speed reaches its peak, the centrifugal force exerted on the MCSs intensified, and the times of sliding down and beating decreased accordingly. This diminished the interaction between the MCSs and the water, thereby reducing the elongation change of the MCSs. Over time, the cumulative effects of mechanics and temperature led to an increase in elongation with a more significant shrinkage of the MCSs. Thus, the interaction between washing time and drum speed, as well as washing time and washing temperature, significantly affected the elongation of MCSs. Furthermore, an increase in elongation will make the leg tight and have more compression on the leg, as elucidated in the research of Siddique.¹² Therefore, MCSs should be washed at a lower temperature to maintain their performance. Simultaneously, the mechanical actions of the machine, such as drum speed, should be slower because drum speed significantly influenced the elongation change of MCSs, particularly for MCSs in Ccl1 and Ccl2.

Table 1.

Ranking of factors influencing the elongation of MCSs from high to low

Item	t Ratio	P > \|t\|
Washing temperature (30, 60)	20.11752	<0.0001∗
Washing temperature × drum speed	−11.2482	<0.0001∗
Washing time × drum speed	10.96061	<0.0001∗
Drum speed (20, 60)	8.393816	0.0002∗
Washing temperature ×washing temperature	7.499465	0.0003∗
Drum speed × drum speed	−6.82489	0.0005∗
Washing time × washing time	5.959033	0.0010∗
Washing time (15, 45)	5.024746	0.0024∗
Washing time × washing temperature	3.542382	0.0122∗

MCS, medical compression stocking.

*: the factor is significant.

Changes in tension

Figure 13 illustrated the trend of the tension at every cross-section in Ccl1 and Ccl2 before washing: the tension gradually increased from B to C, then decreased from C to D; the tension at E was slightly greater than D. The trend for Ccl3 was consistent with that of Ccl1 and Ccl2, except for the parts B and B1, that is, the tension in part B of Ccl3 was higher than in part B1. In addition, the trend of tension across the three levels at every cross-section correlated with the elongation, indicating a positive relationship between the tension of MCSs and their elongation. Because greater elongation of MCSs leads to more significant lateral shrinkage, it requires a higher tensile force to achieve the corresponding elongation during the stretching process. After washing, the tension at every cross-section of different levels increased, caused by the fact that the circumference of MCSs shrunk after washing. Furthermore, the increase in elongation also contributed to the increase in the tension. After washing, the tension at every cross-section across different levels increased due to the shrinkage of the circumference of MCSs post-washing. Furthermore, the increase in elongation also contributed to the increase in tension. However, the trend from B to E remained unchanged and was the same as before washing.

Figure 14 revealed that the tension at every cross-section in Ccl1 and Ccl2 was relatively close, and the tension in Ccl3 was significantly higher than that in Ccl1 and Ccl2 after washing. This difference was attributed to the smaller circumference of Ccl3 compared with that of Ccl1 and Ccl2, and the elongation of Ccl3 was larger, resulting in higher tension in Ccl3.

Significance analysis

With the exception of the washing time × washing temperature, and the washing time ×drum speed, all other factors were significant (as presented in Table 2). Furthermore, the washing temperature and washing time had a highly significant impact on the tension of MCSs, indicating that these two factors were the primary influencing factors. Consequently, an increase in washing temperature led to a rise in the tension of MCSs. This is because the increase in temperature can alter the structure of the fabric, such as the shrinkage of the circumference; the elongation also increased with the increase of temperature, which in turn increased the tensile force. Meanwhile, as the temperature rose, the initial modulus of MCSs also increased, making the MCSs more resistant to deformation. Thus, the tension of stretching the MCSs also increased. Moreover, increasing the drum speed up to a certain extent can enhance the tensile force of MCSs, and when it continued to increase to the highest level, the degree of change in the tensile force started to diminish. Over time, the temperature and mechanical action effects accumulated; namely, with the washing time increasing, the tension likewise increased.

Table 2.

Ranking of factors influencing the tension of MCSs from high to low

Item	t Ratio	P > \|t\|
Washing temperature (30, 60)	20.32756	<0.0001*
Washing time (15, 45)	10.62815	<0.0001*
Drum speed (20, 60)	4.847035	0.0029*
Drum speed × drum speed	−4.42324	0.0045*
Washing time × drum speed	3.733908	0.0097*
Washing time × washing time	−3.41034	0.0143*
Washing temperature ×washing temperature	3.311808	0.0162*
Washing time × washing temperature	1.900423	0.1061
Washing temperature × drum speed	1.023797	0.3454

MCS, medical compression stocking.

*: the factor is significant.

Effect of washing on the bending properties of MCSs

Changes in bending properties

As shown in Figure 15, before washing, the trend in bending rigidity at every cross-section decreased gradually from B to E, attributed to the decreasing fabric density of MCSs from B to E. The greater the density, the greater the bending rigidity. In addition, at different compression levels, the bending rigidity ranked as follows: Ccl3 > Ccl2 > Ccl1, indicating that the higher the compression levels, the greater the bending rigidity of the MCSs.

After washing under different conditions, the overall bending rigidity increased, including different compression levels and cross-sections. However, the trend of bending rigidity from B to E had not changed, and maintained a gradually decreasing trend from B to E. The degree of change in bending rigidity in Ccl3 was less significant than that for Ccl1 and Ccl2, due to the fact that the spandex content of MCSs in Ccl3 was relatively higher than that in Ccl1 and Ccl2, which conferred greater stability and resulted in smaller alterations in bending rigidity.²⁷

Moreover, Figure 16 shows that after washing, the hierarchy of bending rigidity among different compression levels remained the same as before washing: Ccl3 > Ccl2 > Ccl1.

Significance analysis

The bending rigidity of MCSs was affected by the washing temperature, the washing time ×washing temperature, the washing time × drum speed, as well as the drum speed × drum speed (as presented in Table 3). The most significant factor was the washing temperature; an increase in temperature led to more significant changes in bending rigidity. Prolonged thermal stress can cause an increase in fabric stiffness.²⁴ In addition, when the drum speed increased to the highest level, the change of bending rigidity was gradually restrained. Moreover, the increase in washing time was the accumulation of mechanical action from the washing machine, thereby increasing the bending rigidity of MCSs over time.

Table 3.

Ranking of factors influencing the bending rigidity of MCSs from high to low

Item	t Ratio	P > \|t\|
Washing temperature (30, 60)	3.85673	0.0084*
Washing time × washing temperature	−3.54518	0.0121*
Washing time × drum speed	3.02910	0.0231*
Drum speed × drum speed	−3.02874	0.0231*
Washing time (15, 45)	−2.33111	0.0586
Washing temperature ×washing temperature	−2.15555	0.0745
Drum speed (20, 60)	1.86156	0.1120
Washing time × washing time	1.67221	0.1455
Washing temperature × drum speed	1.65813	0.1484

MCS, medical compression stocking.

*: the factor is significant.

Effects of washing on the friction property of MCSs

Changes in friction property

As depicted in Figure 17, before washing the lowest coefficient of friction (COF) was observed at the ankle part (B), and the highest COF was at the thigh part (E). This variation was attributed to the density at B being the highest; the greater the density, the smaller the loops and the smoother the fabric surface. Conversely, the density at E was the smallest, resulting in a relatively looser fabric. Larger fabric loops lead to more pronounced raised positions on the fabric surface, making it rougher.³⁰ After washing under different conditions, the COF of MCSs exhibited diverse changes. The COF increased under some washing conditions and partially decreased. Nonetheless, the trend of COF from B to E remained after washing.

It can be seen from Figure 18 that after washing, the COF of Ccl3 was larger than that of Ccl1 and Ccl2, with the COF of Ccl1 and Ccl2 being relatively similar. Besides, the COF among different compression levels remained consistent with that before washing.

Significance analysis

The COF of MCSs was influenced by all washing parameters and their interactions (as presented in Table 4). Notably, the most significant factor was the interaction between washing time and washing time, indicating that an increase in washing time led to an increase in the COF of MCSs. In addition, higher washing temperature and drum speed resulted in a lower COF of MCSs. The increase in washing temperature and drum speed gave rise to an increase in the shrinkage of MCSs, and the number of loops per unit area would increase, making the fabric tighter and closer. Consequently, the loops raised on the surface would decrease, resulting in a lower COF. Conversely, the COF of the fabric became greater.

Table 4.

Ranking of factors influencing the COF of MCSs from high to low

Item	t Ratio	P > \|t\|
Washing time × washing time	8.94978	<0.0001*
Washing temperature (30, 60)	−4.53824	0.0039*
Washing time (15, 45)	3.63616	0.0109*
Drum speed (20, 60)	−3.54350	0.0122*
Washing time × drum speed	−3.31948	0.0160*
Washing temperature ×washing temperature	−3.20481	0.0185*
Drum speed × drum speed	−2.85741	0.0289*
Washing time × drum speed	−2.82172	0.0303*
Washing time × washing temperature	−2.77843	0.0321*

COF, coefficient of friction; MCS, medical compression stocking.

*: the factor is significant.

Effect of washing on the bagging property of MCSs

Changes in bagging property

As illustrated in Figure 19, before washing the trend of the residual rate of bagging at every cross-section in Ccl3 showed a gradual increase from B to E; the residual rate of bagging in Ccl1 and Ccl2 was relatively close, with a consistent increase from B to E. Moreover, this trend correlated with the trend of the fabric density; higher density resulted in a tighter fabric with greater elasticity.

Furthermore, the trend of bending rigidity from B to E was inversely related to the residual rate of bagging. This indicated that regions of MCSs with higher rigidity exhibited a lower residual rate of bagging, suggesting that the increasing of rigidity made the fabric less prone to bagging. Conversely, areas with lower stiffness experienced larger bagging deformations, leading to poorer recovery and stability in MCSs.

After washing of MCSs, the residual rate of bagging in Ccl1 decreased from B to D, and the rate of bagging of E increased, which was consistent with that in Ccl2 apart from B and B1. In Ccl3, the residual rate of bagging decreased at every cross-section. Overall, the residual rate of bagging tended to decrease after washing because the tightness and rigidity of MCSs increased, making elastic deformation more challenging. Therefore, the difficulty of bagging deformation also increased, and the residual rate of bagging decreased.

Furthermore, after washing the bagging residual rate of Ccl1 and Ccl2 was relatively similar, while Ccl3 exhibited a lower rate than both Ccl1 and Ccl2. This difference was attributed to Ccl3 having a higher spandex content, which confers greater elasticity, fabric rigidity, and stability, enhancing recovery after bagging and resulting in a lower residual rate of bagging (Figure 20).

Significance analysis

As presented in Table 5, all factors affecting the residual rate of bagging of MCSs were listed from high to low. Except for the washing time × washing time, and the washing temperature × washing temperature, other factors were significant. After all, the washing temperature, drum speed, and washing time all had a significant impact on the residual rate of bagging. As the drum speed increased, the change in the residual rate of bagging initially increased and then slowed down. This occurred because higher drum speed could increase the shrinkage of MCSs, making them tighter. Once the rigidity of the fabric increased, the tension also increased. That is, the fabric required more force for stretching and deformation, making bagging deformation more difficult, and decreasing the residual rate of bagging. Similarly, an increase in the washing temperature led to the decrease of shrinkage and rigidity, resulting in a decreased residual rate of bagging. With the prolongation of the washing time, thermal and mechanical effects accumulated, making MCSs tighter and stiffer, thus more resistant to deformation, and reducing the residual rate of bagging.

Table 5.

Ranking of factors influencing the residual rate of bagging of MCSs from high to low

Item	t Ratio	P > \|t\|
Drum speed × drum speed	–5.00301	0.0024*
Washing time (15, 45)	–4.06186	0.0066*
Washing time × washing temperature	3.61471	0.0112*
Drum speed (20, 60)	3.60333	0.0113*
Washing temperature × drum speed	3.06579	0.0221*
Washing time × drum speed	2.93966	0.0260*
Washing time (15, 45)	–2.61922	0.0396*
Washing time × washing time	–2.38688	0.0543
Washing temperature ×washing temperature	2.31246	0.0601

MCS, medical compression stocking.

*: the factor is significant.

Conclusions

This study aimed to determine the effects of machine washing on the mechanical properties of MCSs by examining how the mechanical properties of MCSs, including three compression levels and five cross-sections from the ankle part to the thigh part, changed after washing under different conditions. Furthermore, the effects of washing parameters on the MCSs were investigated. The following conclusions are presented:

After washing under different conditions, the elongation at every cross-section was observed to increase compared with that before washing. Therefore, the tension correspondingly increased, and the changing trend of the tensile force in each part aligned with the elongation. Meanwhile, the bending rigidity of MCSs gradually increased, although the trend of different parts remained unchanged. In addition, the COF of MCSs after washing either increased or decreased, depending on the extent of fabric shrinkage. Moreover, the residual rate of bagging inversely related to the bending rigidity, decreased after washing. The greater the rigidity, the lesser the bagging deformation of MCSs.

In addition, the influence of various washing parameters and their interactions on the mechanical properties was evaluated. The washing temperature significantly impacted the mechanical properties of MCSs. An increase in washing temperature led to an increase in both the elongation and tension of MCSs; concurrently, the bending rigidity of MCSs also increased, indicating a reduction in the softness of MCSs, which narrowed the residual rate of bagging. The COF of MCSs also decreased. Furthermore, the mechanical action of the washing machine (drum speed), washing time and their interaction also affected the mechanical properties of MCSs.

Increasing the drum speed to a certain extent resulted in the increase of the elongation, tension and bending rigidity of MCSs, while both the COF and residual rate of bagging decreased. The accumulation of mechanical action over time also cumulatively affected all mechanical properties of MCSs, promoting their changes. Therefore, some qualitative conclusions can be drawn from our work, and further research can focus on the specific mechanism between the mechanical action of the washing machine and the fabric. In addition, this study offers consumers suggestions on washing MCSs: by lowering the washing temperature, reducing the drum speed, and shortening the washing time, the changes in MCSs caused by washing can be decreased.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The author(s) would like to thank the following ﬁnancial support for the research, authorship and/or publication of this article: This work was supported by the National Key R&D Program of China (project 2018YFC2000905).

ORCID iD

Guangwu Sun

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