The influence of mechanical action on felting shrinkage of wool fabric in the tumble dryer

Abstract

Felting shrinkage of untreated wool fabric occurs easily during tumble drying. Mechanical action applied on fabrics plays a significant part in felting shrinkage of wool fabric. In general, the more severe the mechanical action of a washing or drying machine, the more rapid the felting shrinkage. However, both the degree and type of mechanical action applied on the fabric can influence felting shrinkage of untreated wool fabric.

In the current study, fabric movement and felting shrinkage of untreated wool fabric at different rotation speeds of the drum in a tumble dryer without heating were studied. Based on the different fabric movements at different rotation speeds of the tumble drum, the extents of impact and rubbing forces at different rotation speeds were assessed by ranking. The total mechanical action applied on the fabric was expressed as the percentage of thread removal of “thread removal fabric” during the drying process. The results showed that lowest mechanical force on fabrics could be achieved when higher rotation speeds of the drum were used to dry wool fabrics in the tumble dryer, and could prevent wool felting shrinkage. It was also found that falling of the fabric followed by impact on the drum wall caused less felting shrinkage than sliding with rubbing between fabrics. Therefore, maintaining the falling movement could be a potential method to dry wool fabric in drying machines without causing severe felting shrinkage.

Keywords

wool fabric felting shrinkage mechanical force fabric movement impact force tumble dryer

Domestic tumble dryers are popular for drying garments, especially during winter or bad weather conditions.¹ However, felting shrinkage of wool fabric occurs easily during laundering and drying in tumble machines. Felting is the process of progressive entanglement of fibers in an assembly, which occurs as a result of persistent rootward migration of fibers due to the differential frictional effect (DFE) of scale-configured wool fibers.²

Many theories have been proposed to explain the mechanism of wool felting.³ Shorter’s popular explanation describes felting shrinkage as being due to the existence of differing degrees of constraint (“partial entanglement” and “complete entanglement”) along the length of any one fiber and DFE.^4–6 As described:

“… a casual disturbance could cause fiber creep to diminish the length between the entanglements so that there would be a tightening up. In the other case, casual disturbances could increase the length of fiber between the two entanglements causing loops to form in the fiber (because a fiber is not rigid like a rod)” (Shorter, [1923]).

The essential conditions for felting to take place are: (a) possession by the fiber of a scale structure, (b) moisture, and (c) mechanical action.³

During tumble drying, mechanical action caused by the rotating drum plays a significant part in felting shrinkage of wool fabric. Many researchers have explored the relationship between the severity of mechanical action and felting shrinkage of wool fabrics in washing machines, and have found that the more severe the mechanical action acting on wet wool, the more rapid the felting shrinkage. Within certain limits, the felting shrinkage of wool fabric increases with increasing agitator speed for the agitator washing machine.⁶ An increase in speed of the agitation paddle in the washing machine has two effects: an increase in the mechanical force upon samples at each impact and an increase in the frequency of impact.⁷ The rate of felting also varies considerably in different types of washing machines, and this was usually contributed to the different degrees of mechanical action.^8–10 For example, it is believed that impellers may be “severe” in their action but that rotating drums are “gentle”; thus, the rate of felting of untreated wool fabric may be much higher in impeller-type washing machines than that in drum-type machines.¹⁰ However, the mechanical forces applied on fabrics during tumble rotation of drums or during rotation of impellers are very complicated, and the different influences caused by the different types of mechanical force are usually overlooked.

In addition, in most previous studies of the effects of mechanical action on felting shrinkage of wool fabric, the severity of mechanical action in washing machines was not measured. In other words, variation in rotation speeds of machines producing more severe washing actions have not been adequately confirmed to be related to increased mechanical force on fabrics.¹¹ Wei et al. characterized mechanical action at different rotation speeds of drums in a tumble dryer.¹² It was found that with an increasing rotation speed of the drying drum, the mechanical action decreased.¹² However, the range of rotation speed of the tumble dryer studied was limited.

The current study focuses on the influence of mechanical action on the felting shrinkage of untreated wool fabric. The movement of fabric at different rotation speeds of the rotating drum was investigated. Furthermore, mechanical action applied on the fabric due to different movements was compared. The total mechanical action at different rotation speeds was determined by measuring the extent of threads removed at the outer edges of thread assemblies in woven fabric during tumble drying. Finally, felting shrinkage of wool fabric due to different mechanical actions in the tumble dryer was assessed and analyzed. The present study contributes to our knowledge of suitable drying programs for wool fabric in tumble dryers to prevent felting shrinkage.

Experimental details

Wool sample preparation

100% wool fabric used¹³ was made from 19.5 µm merino wool fibers without shrink-resistance treatment. Plain fabric was weft-knitted using a 12-gauge flat-knitting machine. The samples of wool fabric were prepared as double layers sized 300 × 400 mm. The marked size for the measurement of the fabric’s dimension was 220 × 300 mm. According to Test Method – TWC - TM309 Performance of Domestic Tumble Driers for “Hand Wash” Wool Products (Woolmark Test Method), all fabric samples were relaxed by the relaxation procedure, i.e. they were soaked once in water at 40℃ for 30 min and then twice at 20℃ for 2 min, followed by flat drying. After flat drying, the fabric samples were conditioned at a standard atmosphere of 65% (±3) relative humidity and 20℃ (±2) for at least 24 h.

Two wool fabric samples with wool fabric ballast, sufficient to make up a total fabric weight of 2.00 kg (±0.01), were soaked in water in the same way as used for the relaxation process. This simulated the hand-washing procedure. After soaking, the moisture content of the fabric load was maintained at 60.0% (±2.5) by spinning in a Haier top-load washer.¹³ The moisture content of fabric was determined to be the ratio of the mass of water to the mass of wool fabric that was conditioned at the standard atmospheric conditions.

Controlled parameters of the drying procedures

In order to investigate the effect of mechanical action on the felting shrinkage of wool fabrics, tumble rotation of fabric samples at varying drum rotation speeds was carried out at room temperature (around 20℃). The drying programs for wool fabrics in the tumble dryer at room temperature and their corresponding parameters are shown in Table 1. The tumble dryer used in the study was a modified domestic thermoelectric air-vented dryer (GDZ10-977, Haier Co. Ltd, China) with adjustable parameters.^13,14 The drum radius of the tumble dryer was around 32.5 cm. At different drum rotation speeds, fabric movements varied. In order to compare the mechanical actions applied to fabric samples at each cycle of fabric movement, the number of fabric movement cycles was kept the same. Therefore, fabric drying at room temperature (19 ± 3℃) was stopped when the number of fabric movement cycles reached around 3800 under different drying programs. The moisture contents of all wool fabric samples after the completion of drying programs were reduced from 60% to around 55%.

Table 1.

The drying programs of wool fabrics in the tumble dryer at room temperature and their corresponding parameters

Drying programs	Heater power (W)	Rotation speed of drum (±2 r/min)	Rotation speed of fabric movement (±2 r/min)	Total drying time (min)	Number of fabric movements	Initial moisture content (%)	Air flow velocity (m/s)
1	0	17	50	77	3800 ± 50	60.0 ± 2.5	0
2		34	54	70
3		50	55	70
4		66	66	58

Initial moisture content refers to the moisture contents of the fabric samples before the drying programs. The exhaust fan was switched off.

Assessment of wool fabrics

In order to understand the relationship between mechanical action and felting shrinkage in a tumble dryer, fabric movements were observed at different rotation speeds of the drum in the tumble dryer. The degree of total mechanical action during tumble rotation and the dimensional changes of wool fabrics were determined.

Video capture of different movements of wool fabrics at varying rotation speeds of a tumble dryer drum

During the drying process of wool fabric at varying rotation speeds in a tumble dryer drum, a camera (Fujifilm, X-A10) was placed in front of the transparent door of the tumble dryer to capture the movement of fabric during tumble rotation. The fabric sample to be traced was dyed yellow to differentiate it from others during the monitoring of fabric movement (Figure 1). Two-dimensional tracks of fabric movement during tumble rotation were captured. Three key positions in each cycle of fabric motion were identified by analyzing recorded tracks of fabric movement following the observation of five cycles of fabric movement. The three key positions were: (a) the starting position at which the fabric started to be projected after being impacted by the lifter, (b) the highest position to which the fabric was projected, and (c) the impact position at which the fabric hit the drum wall. Based on a rectangular coordinate system with the position of the drum’s center as the origin point, medians of coordinates x and y of the observed five cycles of fabric movement were identified as the three key positions of the tracer fabric in the tumble drum.

Figure 1.

Fabric movement captured during the process of drum rotation in the tumble dryer.

The degree of total mechanical action applied on fabrics during tumble rotation

Mechanical actions on fabrics during the tumble-drying process are influenced by the speed of tumble rotation. The degree of total mechanical action on fabrics was determined by the percentage of thread removal of a specific polyester woven fabric,¹⁵ which is called “thread removal fabric” in the current work. The threads at the fringes of the thread removal fabric were loose and thus were easily removed as mechanical action was imparted on the fabric during tumble drying. The measuring method for thread removal from the polyester fabric (Figure 2) was carried out based on EMPA 304 (International Electrotechnical Commission, IEC PAS 62473-2007). The thread removal fabric was fixed to the center of the wool fabric sample by stitching. Because the number of threads of the fabric was hard to count, the lengths of the original and final woven structures of the fabric were substituted for measurement to calculate the percentage of thread removal. In each test, five pieces of thread removal fabric were used. The percentage of thread removal of the fabric was calculated using equations (1) to (3) with reference to IEC PAS 62473-2007

A_{warp} = \frac{T_{warp, o} - T_{warp, r}}{T_{warp, o}} \cdot 100 %

(1)

A_{weft} = \frac{T_{weft, o} - T_{weft, r}}{T_{weft, o}}

(2)

A = \frac{A_{warp} + A_{weft}}{2}

(3)

where A_warp is the percentage of warp thread removal from the whole thread removal fabric, A_weft is the percentage of weft thread removal, A is the percentage of thread removal, T_warp,o is the average length across the original warp woven structure, T_warp,r is average length across the remaining warp woven structure, T_weft,o is average length across the original weft woven structure, and T_weft,r is average length across the remaining weft woven structure.

Figure 2.

The thread removal fabric that was used as a measure of total mechanical action on the fabric in the tumble dryer.

The thread removal fabric method has been used to measure the severity of the mechanical action of washing machines in previous studies.^15,16 In order to check the suitability of the thread removal fabric to be used as a measure of the total mechanical action on the fabric in the tumble dryer, the percentage of thread removal at different drying times (10, 20, 30, and 40 min) was measured under the drying program, as shown in Table 2.

Table 2.

Tumble drying programs of experiments used to measure the percentage of thread removal of fabric

Total time of drying (min)	Heater power (±10 W)	Rotation speed of drum (±2 r/min)	Air flow velocity (±0.5 m/s)	Initial moisture content (±2.5%)
10, 20, 30, or 40	3000	48	5.5	60

Dimensional change of wool fabric

The changes in length of wool fabric samples in the warp direction, called the “length change,” were used to characterize the dimensional changes of wool fabrics. They were calculated using equation (4) based on AATCC (American Association of Textile Chemists and Colorists) test method 135–2014 “Dimensional changes in automatic home laundering of woven or knitted Fabrics”

LC = \frac{B - A}{A} \cdot 100 %

(4)

where LC is the percentage length change of the fabric sample, A is the original length of the fabric sample before washing and drying, and B is the length after washing and drying. All of the specimens were measured after being conditioned in a standard atmosphere of 65% (±3) relative humidity and 20℃ (±2) for at least 16 h.

Results and discussion

In order to investigate the relationship between felting shrinkage of wool fabric and mechanical action caused by the rotating drum of a tumble dryer, movements of fabric samples and mechanical actions at different rotation speeds of the drum were examined, and length changes of wool fabrics due to different mechanical actions in the tumble dryer were studied.

Movements of fabrics at varying rotation speeds of the drum

Rotation speeds of drums affect fabric movement in tumble machines. Conceptual diagrams of movement of a tracer fabric with three key positions are shown in Figure 3(a) to (d), according to our video capture of fabric movement in a tumble dryer. The three key positions recorded at drum rotation speeds of 17, 34, and 50 r/min were: (a) the starting position at which the fabric started to be projected after being impacted by the lifter, (b) the highest position to which the fabric was projected, and (c) the impact position at which the fabric hit the drum wall. Although this method of tracking fabric motion during tumble rotating is not wholly accurate, it could be used to compare different fabric motions at different rotation speeds in this study.

Figure 3.

Conceptual diagrams showing fabric movement at different rotation speeds.

Figure 3 shows that when the rotation speed of the drum was increased from 17 to 66 r/min, the circulating area of fabric movement increased. When the rotation speed of the drum was between 17 and 50 r/min, the starting position at which the fabric started to be projected after being impacted by the lifter became higher with increasing rotation speed. It was also observed that the fabric sample moved around one corner of the drum when the drum’s rotation speed was 17 r/min (Figure 3(a)), while at 66 r/min the fabric sample adhered to the drum wall and rotated with the drum.

In order to analyze the effect of the rotation speed of the drum on fabric movement in a simplified way, the fabric sample could be regarded as a particle. Thus, the effect of rotation speed of the drum on the movement of the fabric sample when only a single fabric is in the tumble dryer can be theoretically analyzed. If the mass of the fabric sample is m, gravitational acceleration is g, rotation speed per second of the drum is n, and the circular radius of the drum is r, then the gravity of the fabric sample would be mg and the centripetal force required for circular motion would be m·(2πn)²·r. When mg ≤ m·(2πn)²·r, (i.e. the rotation speed per second of the drum n ≥ $\sqrt{\frac{g}{r}} / (2 π)$ , the fabric would rotate with the drum.¹⁷ This could explain the fabric rotation movement when the rotation speed was 66 r/min. It should be noted that in reality, the required rotation speed to make the fabric rotate with the drum was higher than that in the theoretical calculation $\sqrt{\frac{g}{r}} / (2 π)$ because the fabric sample was larger than an actual particle, which means that the effective radius was smaller than the radius of the drum.

We following with an analysis of fabric movement when n< $\sqrt{\frac{g}{r}} / (2 π)$ . A rectangular coordinate system was generated based on the position of the center of the drum. Regardless of air resistance, the forces acting on the fabric at the fourth quadrant (IV) are shown in Figure 4. According to Newton’s second law, the movement of the fabric is determined by the balance of the component of gravity along the tangential direction (mgsinθ) and the frictional force (µmgcosθ + µm(2πn)²r).¹⁸ When a fabric moves anticlockwise, the angle θ will increase, while the frictional force will decrease. If the rotation speed of the drum (n) is small, the fabric will first accelerate and then decelerate. When the speed of the fabric decreases to 0, the fabric may slide backward against the anticlockwise rotation direction of the drum. Then, the fabric will be impacted by the lifter, will be projected into air, and will finally fall down and impact the drum wall. This could explain the fabric movement in the drum when the rotation speed was 17, 34, or 50 r/min.

Figure 4.

Analysis of forces on the fabric during fabric movement in the drum.

The mechanical actions at different rotation speeds

Total mechanical actions at varying rotation speeds

The intensity of total mechanical action on fabrics was determined by measuring the degree of thread removal of a thread removal fabric in washing machines in previous studies.^16,17 The percentage of thread removal of the thread removal fabric was tested at various drying times (10, 20, 30, and 40 min) to check whether this method could be used in the tumble dryer to assess the extent of mechanical action. Figure 5 shows that with extension of the drying time, the mechanical action applied on the fabric increased. The percentage of thread removal increased linearly with increasing drying time in the tumble dryer with less than 2% of the SD. Therefore, the percentage of thread removal was used as a measure of the total mechanical action applied on the wool fabric in the tumble dryer.

Figure 5.

Percentage of thread removal over different drying times at a rotation speed of 48 r/min in the tumble dryer.

Figure 6 shows the relationship between the percentage of thread removal and the rotation speed of the drum. The corresponding parameters for the drying programs are shown in Table 1. The results show that the total mechanical action experienced by the thread removal fabric increased when the rotation speed of the drum increased up to 34 r/min, but decreased when the rotation speed was increased further. Mechanical action applied on the fabric at varying rotation speeds depended on the different movements of fabrics. This is analyzed in the following sections.

Figure 6.

Relationship between the percentage of thread removal and the rotation speed of the drum.

Mechanical actions during tumble rotation

At the high drum rotation speed (66 r/min), fabrics adhered to the wall of the drum and rotated with the drum. Therefore, fabrics were subjected to low levels of impact forces, and rubbing forces between fabrics or between the fabric and the drum wall. Compared with lower rotation speeds of the tumble drum, the mechanical forces applied on the wool fabric samples at a drum rotation speed of 66 r/min may have been the weakest. Mechanical forces applied on fabrics at 17, 34, and 50 r/min during tumble rotation are very complicated. However, the main mechanical forces in the tumble machine considered in this study were the impact force between fabric and the drum wall and the rubbing force between wool fabrics.¹⁹ Fabric experiences the greatest mechanical action upon impact after having been projected into the air in the motion of the rotating drum at a normal rotation speed.¹⁹ It is difficult to measure or calculate accurate values of impact and rubbing forces applied on fabrics; therefore, the intensities of impact forces applied on fabrics and rubbing forces between fabrics in the cycles of fabric movement at different rotation speeds of the drum were qualitatively analyzed.

Impact forces between fabrics and the drum wall at different rotation speeds

The impact force applied on the fabric in one cycle of fabric movement at different rotation speeds of the drum were qualitatively analyzed using the empirical formula of the momentum theorem. The tracer fabric was regarded as a particle. Ignoring air resistance and the influence of other fabrics on the tracer fabric, and assuming that the velocity of the fabric after impact was approximately zero, the amount of impact force can be calculated from equation (5) as follows

F = \frac{mv}{t}

(5)

where F is the impact force applied on the wool fabric sample, m is the mass of the fabric sample, t is the time of impact, and v is the velocity of the fabric just prior to its impact on the drum wall.

If the impact time (t) at rotation speeds of 17, 34, and 50 r/min and the mass of the fabric (m) of different movements are same, the intensity of the impact force depends on the velocity of the fabric sample just prior to its impact on the drum wall (v). According to the kinetic energy theorem and the law of oblique projectile motion, the velocity of the fabric just prior to its impact on the drum wall (v) can be described using equation (6)

v = \sqrt{2 gh + v_{o}^{2}}

(6)

in which h is the height between the position where the fabric sample started to be projected after being impacted by the lifter of the tumble drum (i.e. position a₁ in Figure 3(b') or position a₂ in Figure 3(c')) and the position where the fabric impacted to the drum wall (i.e. the position c₁ in Figure 3(b') or position c₂ in Figure 3(c')). As shown in Figure 3(b') and (c'), the height at a drum rotation speed of 34 r/min (h₁) was similar to that at 50 r/min (h₂), which was higher than the height at 17 r/min. That is to say, the height at different rotation speeds of the drum increased in the following order: the height h at 17 r/min < 34 r/min ≈ 50 r/min. v_o is the velocity of the fabric sample when it started to be projected. v_o at different rotation speeds of the drum increased in the following order: v_o at 17 r/min < 34 r/min < 50 r/min. Therefore, according to equation (6), the velocity of the fabric just prior to impact on the drum wall (v) increased in the following order: v at 17 r/min < 34 r/min < 50 r/min. Based on equation (5), the impact force at different rotation speeds of the drum increased in the same order.

Rubbing mechanical forces between wool fabrics at different rotation speeds

Rubbing intensity between fabrics in one cycle of fabric movement was related to the size of the circular track area of fabric movement during tumble rotation. With the drum rotation speed decreased, the circular area of fabric movement became smaller, as shown in Figure 3. Within small areas for all fabric movement, the pressure between fabrics and the frequency of fabrics contacting each other could increase, resulting in increased rubbing intensity between fabrics at lower rotation speeds of the drum at the following order: 66 r/min < 50 r/min < 34 r/min < 17 r/min (ranked from 1 to 4). Different mechanical actions in the cycle of fabric movement at different rotation speeds of the drum are ranked in Table 3. Based on the rankings of mechanical actions at various rotation speeds, the influence of mechanical action on the felting shrinkage of wool fabric is discussed in the next section.

Table 3.

Ranking of the magnitudes of different mechanical actions in one cycle of fabric movement at different rotation speeds of the drum

Rotation speed of the drum (r/min)	Impact	Rubbing between fabrics	Degree of total mechanical action
17	2	4	2
34	3	3	4
50	4	2	3
66	1	1	1

Rankings are in ascending order from 1 to 4, i.e. 1 is the lowest and 4 is the highest; The ranking of the degree of total mechanical action is based on the percentage of thread removal of the thread removal fabric.

The dimensional change of wool fabric due to different mechanical actions

The mechanical action caused by a rotating drum plays an important role in felting shrinkage of wool fabric. In general, the more severe the mechanical action of a washing machine, the more rapid the felting shrinkage, when other factors are held constant.⁷

In order to investigate the relationship between felting shrinkage of wool fabric and the total mechanical action applied on a fabric in the tumble dryer, the percentage of thread removal from the thread removal fabric was used as a measure of total mechanical action applied on the fabric. The relationship between felting shrinkage and the total mechanical action experienced by the thread removal fabric is shown in Figure 7. Roughly, increased mechanical action caused increased felting shrinkage of wool fabric. The shrinkage of wool fabric at a drum rotation speed of 66 r/min was the lowest due to the lowest level of mechanical action. However, comparison of the mechanical action impacting on fabrics at 50 and 17 r/min showed that the intensity of total mechanical action on fabric at 17 r/min was slightly lower, but felting shrinkage of the fabric was increased. This might have been because the felting shrinkage of wool fabrics can be more severe when caused by rubbing mechanical forces rather than impact forces. The similar results from the previous study demonstrated that rubbing can cause severe felting shrinkage upon hand scrubbing,⁶ while impact force that is perpendicular to the fabric does not cause significant felting shrinkage.^3,10 Another possible reason is that within the cycle of fabric movement, impacts are applied to wool fabric instantaneously, while constant rubbing between fabrics can occur.²⁰

Figure 7.

Relationship between the length shrinkage of wool fabric and total mechanical action.

Conclusions

Mechanical action applied on fabrics plays a significant part in the felting shrinkage of wool fabrics during tumble drying. The current study investigated the influence of mechanical action on the felting shrinkage of wool fabric in a tumble dryer.

Mechanical actions applied on fabrics at varying rotation speeds depend on fabric movements in a tumble dryer. When the drum rotation speed increases from 17 to 66 r/min, the circular area of fabric movement increases. When the rotation speed of the drum reaches 66 r/min, fabric samples adhere to the drum wall and rotate with the drum, so little impact or rubbing occurs. With the rotation speed of the drum is reduced within certain limits during the tumble drying of fabrics, the total mechanical action applied on fabrics is increased. At a low drum rotation speed (17 r/min), fabrics move in the small area around one corner of the drum and the main mechanical actions applied on fabrics are rubbing forces. However, the main mechanical actions on fabrics at 50 r/min are impact forces. At 34 r/min, both rubbing and impact forces act on fabrics.

From analysis of the relationship between mechanical action and felting shrinkage, it is suggested that both the severity and the type of mechanical action influence the felting shrinkage of wool fabric. It is understandable that the more severe the mechanical action, the greater the felting shrinkage of wool fabric. Impacts of fabrics on the drum wall appear to make smaller contributions to felting shrinkage than rubbing between fabrics. To prevent wool felting shrinkage, high rotation speeds of drums could be used to make fabrics adhere to the wall of the drum and rotate with the drum in the tumble dryer. However, in order to achieve both the prevention of wool felting shrinkage and efficient energy use during the drying of wool fabrics, fabric movements at high speeds of rotation and fabric falling movements should be further investigated to develop new methods for drying wool fabrics in drying machines.

Footnotes

Declaration of conflicting interests

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

Funding

The authors received the following financial support for the research, authorship, and/or publication of this article. This research was supported by funding from The Fundamental Research Funds for the Central Universities (project number 2232020G-08), the National Key R&D Program of China (project number 2019YFB1706304), the Shanghai Science and Technology Committee (project number 17DZ2202900), Shanghai Summit Discipline in Design (project number DD18005), the Institute for Nonlinear Sciences of Donghua University (project number INS1901) and the China Scholarship Council.

ORCID iD

Wei Bao

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