Exploitation of Up–Down Tapping Washing Method for Removing Different Soil types from Cotton Fabric

Abstract

More gentle washing methods are required in order to maintain the satisfactory appearance of delicate or luxury garments. In the current study, a new washing method with an up–down tapping action was introduced for delicate garments. The influence of tapping washing parameters used in the new washing method on the removal of five different types of IEC soiling from cotton fabrics was investigated. The cleaning performance of the tapping washing method mainly relies on the turbulent flow of the washing liquor and a gentle mechanical tapping action without a friction force being applied to fabrics during laundry. Tapping washing could maintain the good appearance of the fabrics without fibre damage and remove water-soluble soil but it had difficulty removing water-insoluble soil. However, the washing efficiency of water-insoluble soil could be improved by adjusting the levels of washing parameters. Further development and optimisation of the up–down tapping washing method could make a good balance between sufficient washing of soiled fabrics and the maintenance of fabric appearance without causing fibre damage.

Introduction

The washing efficiency of garments mainly relies on the combined effect of the physicochemical contribution of the detergent action and the mechanical contribution of washing agitation.^1,2 In 1959, Sinner first summarised the temperature, contact time, detergent and mechanical agitation as the main factors that influence the efficiency of the washing process. This is the called ‘Sinner’s Circle’ and is commonly accepted. Temperature is crucial for stain removal, probably owing to the effect of temperature on the solubility of detergent ingredients and contaminants. The kinetics of diffusion of surfactants to the contaminated surface and emulsification of the washed off contaminants in a detergent solution are also temperature-dependent. However, elevated temperatures can exert adverse effects, such as decreasing the surface activity of certain detergents and reducing the stability of emulsions. ³ Overall, the washing temperature should comprehensively consider the individual properties (solubility, viscosity, surface tension, stability of critical micelles, etc.) of surfactants, soil, fabric and other components in laundry. Researchers^4,5 found that the percentage of soil removal increased with the logarithmic increase of cumulative washing time. Generally, most of the soil on the fabric was removed in the early washing stage (about 15 min). ⁶ Chemical action with detergent is shown to have a significant effect on soil loosening but its effect on soil removal is less significant when compared to mechanical action. ⁶

Mechanical agitation plays an important role in the washing process, not only for separating the soil from the fabric, ⁴ but also for maintaining a suspension of the detached soil in detergent solution. In general, the soil droplets detached from the fabrics could be too large to be stably suspended without mechanical agitation. Adequate mechanical agitation could break down the suspended soil droplets to one-tenth or one-hundredth of the original sizes, enabling stability of the suspended emulsion for preventing the redeposition of soil back to the garments. ⁷

According to Kissa, ⁴ the mechanical action can be classified into three types: the turbulent flow of the detergent washing liquor, the fabric flexing action and the abrasion action on the fabric surface. Van der Donck ⁸ found that the extent of soil removal increased with the extent of fabric movement and washing liquor flow. The abrasion action on the fabric surface was most effective for soil removal among three mechanical actions, ⁶ while it is known that abrasion is a major contributing factor to wear or gradual damage to textiles. ⁹ Hence, controlling or inhibiting the friction action on fabrics in laundry is a way to reduce damage to fabrics.

In the washing process, the affinity of soil to fabrics or their bonding strength has a direct influence on the soil removal from the fabrics under the mechanical force applied to the soiled fabric. ⁶ It was found that washing efficiency for the removal of particulate composite soil such as carbon black/mineral oil, chocolate/milk and pig’s blood is more highly dependent on the type of mechanical action than that of water-soluble soil such as the red wine. ⁶

There are three main types of household washing machines worldwide: the front-load drum machine, the top-load agitator and the impeller machine. ¹⁰ In these washing machines, severe mechanical actions or agitations are involved in the washing process, although gentle washing programmes were widely designed and added to front-load or top-load drum washing machines for consumers to choose for different types of delicate garments, especially those made of wool and silk fibres. In fact, these gentle washing programmes neither provide sufficient washing efficiency nor maintain a good appearance ¹¹ of delicate garments. For dedicate or luxury garments, traditional washing machines are not suitable for garment washing. Wool garments are still carefully hand-washed in order to avoid the felting shrinkage of wool garments or dimensional change.

There are increasingly needs for diversified laundry and care methods that provide not only sufficient detergency but also maintenance of good appearance. Recently, tapping washing ¹² and ultrasonic washing ¹³ have been designed as gentle washing methods to control friction force or abrasion as much as possible for preventing fibre or fabric damage during the laundry process. Ultrasonic washing has been found to increase the convective diffusion of soil particles in laundry processes owing to acoustic cavitation.^14–18 However, ultrasonic wave could cause surface destruction of fibres.^18,19 Recently washing methods based on a tapping action on the fabrics during laundry have been introduced and investigated. Yun et al. ²⁰ confirmed that beating actions in the washing process resulted in less damage to fabrics compared with mechanical rubbing and tumble actions in a front-loading washer. Horibata et al.^21–24 invented a washing machine for delicate garments by using two air bags that are inflated and deflated through an air pump, in which a gentle reciprocating force was applied vertically on the wet garment by the air bags. Zhao et al. ¹¹ and Luo et al. ¹² have compared the washing performance of wool sweaters and silk dresses in the air-bag tapping washing, hand washing, gentle and normal machine washes. The results showed that the airbag tapping washing always gave wool and silk samples the best appearance and mechanical properties owing to the prevention of felting shrinkage and surface degradation or minimum fibre damage for up to 10 washing cycles. Zhao et al. ¹¹ inferred that the air-bag tapping washing can give the lowest felting rate to wool samples, mainly because the mechanical forces in the air-bag tapping washing were applied on all parts of the fabric in a similar direction. This unidirectional and up–down tapping action during the washing process effectively reduced the friction between the garments to avoid severe distortion damage to fabric. The up–down tapping washing method could be a potential method for delicate or luxury garments to maintain good appearance and avoid damage.

Zhao et al. ²⁵ built a tapping washing platform with adjustable washing parameters, and found that it could maintain the sample’s good appearance compared with traditional drum rotation washing on wool and silk fabrics, while the washing efficiency for the removal of soil from garments needs to be explored. The current research aims to explore the tapping washing method and to determine the washing capability towards different types of soil on the garments. For preventing the felt shrinkage of fabrics affecting the assessment of soil removal and the use of all conventional soil types on fabrics, standard soiled cotton fabrics rather than delicate wool fabrics were used in the study. A series of experiments on tapping washing with detergent has been carried out for understanding the effect of tapping washing’s parameters on soil removal, detergency and damage. Scanning electron microscopy (SEM) was performed to detect any minor damage to the cotton fabric. The results of this work could provide the implications of the tapping laundry method for sufficient detergency of delicate garments with the maintenance of good appearance.

Materials and Methods

Apparatus of Up–Down Tapping Washing

The tapping washing platform with adjustable washing parameters was built as shown in Figure 1. ²⁵ The tapping washing platform consists of control panel for controlling the washing parameters including washing time and speed of up–down tapping, flywheel linked with the motor, tapping mesh plate and perforated loading plate in the washing tank. As shown in Figure 1(b), the platform frame, the guide rail, tapping mesh plate and perforated loading plate are all made of stainless steel to prevent corrosion due to the high level of humidity during washing process. A lifting platform was used to support the wash tank up to a load of 500 kg. The plastic washing tank (45 × 32 × 22 cm) was covered with a layer of heat-insulating material to maintain the washing temperature during washing process.

Figure 1.

Prototype of the tapping washing platform with adjustable washing parameters. ²⁵ (1) Centric slider-crank mechanism structure, (2) washing tank, (3) motor control (switch/frequency/timer), (4) tapping mesh plate, (5) tagging paddle fastener, (6) wash load, (7) perforated loading plate, (8) spring, (9) washing solution, (10) lifting platform, (11) motor, (12) flywheel, (13) connecting rod, (14) slide bar and (15) guide rail. (a) Specific structure design of tapping washing platform and, (b) prototype of the designed tapping washing platform.

The main controllable parameters of the tapping washing platform are the magnitude and frequency (F) of the up–down tapping force, washing temperature (T) and duration (D). The influence of these parameters on the washing performance and surface appearance of the fabrics was investigated. The frequency of tapping (F) is controlled by the rotation of flywheel through the centric slider-crank mechanism to convert the rotation action to slide up–down movement. Therefore, each cycle of up and down movement of the tapping mesh plate corresponds to one rotation of the flywheel controlled by the motor. The magnitude of the tapping force could be adjusted through the rigidity coefficient of the spring (K) and the compressed height (H) of the spring underneath of the perforated washing plate by adjusting the lifting platform to lift or low the washing tank. The temperature and volume of detergent washing solution were controlled manually.

Washing Programme of Tapping Washing Platform

The washing programme of the tapping washing platform is designed similarly to the washing steps of a traditional gentle drum washing machine. The detailed washing programme of the tapping washing platform comprised eight steps as shown in Scheme 1. In detail, prior to washing process, the six springs with rigidity coefficient 9.3, 14.9 or 18.4 N/mm were attached beneath the perforated loading plate. The wash load of garment or fabric was neatly folded and laid on the perforated loading plate and secured with tagging paddle fasteners using tagging gun. The washing tank was lifted to the starting position by adjusting the lifting platform. After the fabric sample had soaked for 5 min, the main wash proceeded. The tapping mesh plate moved up–down to tap regularly the soaked sample according to selected parameters as shown in Table 1. When the main wash was completed, the wash tank was lifted to the maximum compressed height of the spring (20 mm in this study) by adjusting the lifting platform, and the tapping mesh plate was moved down to keep pressing the fabric sample for 1 min to squeeze excessive water out of the fabric. The washing liquor was drained out from the washing tank but replaced with 6 L of tap water at room temperature for the first rinse for 3 min under tapping mesh plate up–down movement using the same levels of the washing parameters as in main wash stage. The repeat steps of press, drain and replacement of fresh water were carried out for further second and third rinses. Finally, the fabric was pressed for the removal of excessive water and released to lie flat to dry at room temperature.

Soaking→Main wash→1st Rinse→2nd Rinse→3rd Rinse→Final Press and Dry

Scheme 1. The washing programme of the tapping washing platform

Table 1.

Multiple parameter used in tapping washing and Electrolux drum washing.

Washingparameterlevel	Tapping washing platform setting			Main washing
	Rigidity coefficient of spring (K) (N/mm)	Motor frequency(F) (rpm)	Compressed max height of spring(H) (mm)	Volume(V) (L)	Temperature(T) (°C)	Duration(D) (min)
Tapping washing
Level 1 (low)	9.3	20	7	5	20 (±3)	10
Level 2 (medium)	14.9	40	16	7.5	40 (±3)	20
Level 3 (high)	18.4	60	20	10	60 (±3)	30
Drum washing
7A gentle	–	–	–	20	40	(15+5)
5A normal	–	–	–	15	40	(15+15)

Washing Parameters of Tapping Washing and Drum Washing

The tapping washing experiment with multiple parameters at multiple levels was designed to explore the influence of these parameters on the washing efficiency in tapping washing. Three levels of low, medium and high intensity for each parameter were set up as shown in Table 1. The sets of experiments was carried out to investigate the effect of low, medium and high levels of each parameter on the washing performance while all other parameters used were kept at low level. The tapping washing platform was designed to be a gentle washing method for delicate garments, therefore washing temperature was controlled in the range of 20–60°C within the deviation of ±3°C.

Washing under the wash programmes 7A (gentle) and 5A (normal) using an Electrolux drum washing machine (Wascator FOM71 CLS, Electrolux, Sweden) was also carried out for comparison. Prior to the main wash stage of the gentle (7A) and normal (5A) drum wash, the washing procedure took nearly 15 min to increase the temperature of the fed tap water to the designed temperature, whereas the drum stood still for fabric sample soaking for 15 min, then the drum started rotation for 5 min for 7A washing or 15 min for 5A washing. The liquor ratio in the contrast tapping, 7A and 5A drum washing experiments was 1:10, and the temperature of the 7A and 5A drum washing was 40°C.

Materials

Artificially soiled cotton fabrics supplied by EMPA (Swiss Federal Laboratories for Materials Science and Technology, Switzerland) were used for testing. These fabrics are woven cotton fabrics soiled with red wine, blood, carbon black/mineral oil, chocolate/milk, sebum/pigment, according to International Standard: IEC 60456. ²⁶ The cleaning performance of the tapping washing platform towards the various types of IEC soil on the fabrics was investigated. The ingredients of the five different soil types for IEC soiled fabrics is shown in Table 2. These soiled fabrics are named the IEC soiled fabrics in this paper.

Table 2.

The ingredients of the five kinds of soil used for corresponding IEC soiled fabrics.

Soil type	Ingredients
Red wine	‘Alicante’ red wine
Blood	Pig’s blood, 10 g/L ammonium citrate
Sebum/pigment	Synthetic sebum (32.8% cows fat, 18.3% wool fat, 18.0% free fatty acid, 3.7% cholesterol,8.9% squalen, 3.6% coconut oil, 3.1% hard paraffin), pigment (carbon black, kaoline, yellow and black iron oxide)
Carbon black/mineral oil	Pigment (carbon black, kaoline, yellow and black iron oxide), carbon black, mineral oil, paraffin oil
Chocolate/milk	Unsweetened cocoa (20–22% fat, not alkalised), full-cream cow’s milk

A 100% cotton knitted jersey vest (Uniqlo, China) was used as a base fabric for 15 pieces of different IEC soiled fabrics to be attached to in the arrangement as shown in Figure 2. Firstly, the 100% cotton knitted jersey vest was folded from its waist into a rectangle size of 30 × 40 cm. Then, 15 pieces of IEC soiled fabrics with five different soil types in the size of 5 × 5 cm were sewn onto the surface of the folded jersey vest in a diagonal arrangement of the same soiled swatches. In three repeated experiments, the position of each soiled fabric was adjusted to ensure diversity of position of each soiled fabric on the perforated loading plate. Finally, the soiled fabric-attached jersey vest was placed and fixed on the perforated loading plate with tagging paddle fasteners.

Figure 2.

The arrangement of the IEC soiled fabrics on the 100% cotton jersey fabric.

OMO laundry gel (1 kg, Unilever, China) was used at a concentration of 2 g/L in the all washing experiments according to detergent manufacturer’s recommendation. The major components of OMO laundry gel are surfactants, phosphate-free water softener, enzymes, fragrance, isothiazolinone (C₈H₉ClN₂O₂S₂) and colourant.^27–30

Repeat of Washing and Post Conditioning

For matching with the scenario of real uses, Shanghai urban residential tap water with an approximately 50 ppm hardness was used for all washing experiments in this study. Three replications of each test were carried out. The detergent solution was agitated for enough time to be homogenous. All washed samples were dried naturally by laying them flat in a conditioning room without noticeable wind and at a temperature of 25(±2)°C. Before washing and prior to measuring, all test samples were conditioned for at least 24 h in the standard atmosphere of 65(±2)% RH and temperature of 20(±2)°C.

Evaluation of Washing Capability to Remove Soil from the Fabrics

For assessing the soil removal from the fabric, colour measurements of the stained fabrics before and after washing were performed using a spectrophotometer (X-rite, Ci7800, USA) under CIE standard illuminant D65/10° standard observer. The washing efficiency (D) was calculated using equation (1) according to GB/T 4288-2018, ³¹ European Standard of EN 60456 ³² and International Standard of IEC 60456 ²⁶ :

$$\mathrm{D}(\%)=\frac{R_{\mathrm{w}}-R_{\mathrm{s}}}{R_{\mathrm{o}}-R_{\mathrm{s}}}\times 100(\%)$$

(1)

where $R_{\mathrm{o}}$ , $R_{\mathrm{s}}$ and $R_{\mathrm{w}}$ are the reflectance of the unsoiled fabric sample and the soiled fabric samples before and after washing, respectively.

According to the European Standard of EN 60456 ³² and International Standard of IEC 60456, ²⁶ the tristimulus Y scale can be used as a simple whiteness index and closely reflects human eye perceptions. Therefore, the tristimulus Y of sample is set as the reflection (R) in equation (1).

SEM

To observe the extent of soiling and alteration on the surface of fabric samples before and after washing, scanning electron microscopy (SEM) (Phenom XL, Netherlands) was undertaken at magnifications of 3000 and 8000.

Results and Discussions

Washing Efficiency of Tapping and Drum Washing

The washing efficiency of tapping washing process could be influenced by multiple parameters: rigidity coefficient of spring (K), motor frequency (F), compressed max height of spring (H), volume of washing liquor (V), washing temperature (T) and duration (D). Multiple levels of parameters were investigated to find out the effect of each parameter in tapping washing process on soil removal from standard soiled cotton fabrics, or a comparison of each parameter at three different levels, low, medium and high, whereas all other parameters were kept at the low level. Figure 3 shows the washing efficiency of the tapping washing process under three different levels of parameters. The washing efficiencies from the drum washing under 7A and 5A washing programmes are also shown for referencing.

Figure 3.

The washing efficiency of tapping washing process under three different levels of parameters: rigidity coefficient of spring (K), motor frequency (F), compressed max height of spring (H), volume of washing liquor (V), washing temperature (T) and duration (D), and of the drum washing process under 7A and 5A programmes for comparison: (a) red wine, (b) blood, (c) sebum/pigment, (d) carbon black/mineral oil, (e) chocolate/milk.

Among different soil types on the cotton fabrics, the tapping washing can achieve higher washing efficiency towards the soil types of red wine and blood than sebum/pigment, carbon black/mineral oil and chocolate/milk. The washing efficiency of tapping washing on red wine and blood soiled cotton fabrics can reach to the similar level as that of front-load drum washing machine, especially under tapping washing parameter: high temperature (T) or long washing duration (D), although the mechanical agitation of fabric samples at rotation drum washing machine is much higher than that of up–down tapping washing platform. This is due to the composition of the soil. The red wine soil used for soiling on cotton fabric contains a small proportion of alcohol, sugar, phenol and aromatic compounds in a large amount of water. Therefore, most components of red wine soil on the fabric are water-soluble and can be spontaneously dissolved in detergent solution and removed in the flow of washing liquor during laundry. Similarity, blood soil consisting of protein, fat and carbohydrate is well water-soluble and can be easily dissolved, especially with the aid of enzyme-added detergent. Therefore, turbulent flow of detergent washing liquor caused by the up–down tapping action can achieve the removal of red wine and blood soil even without additional rubbing friction between fabrics during laundry. In addition, with increasing duration of the wash process, the washing efficiency increased towards blood soil but not red wine soil. Most components of red wine soil (alcohol, sugar) have a better water solubility than the materials (protein, fat, carbohydrate) in blood soil. Therefore, the washing duration has more significant effect on the washing efficiency of blood soil than that of red wine soil. Temperature (T) parameter had a positive influence on the washing efficiency of red wine soil, because a higher temperature promotes dissolution of red wine, but temperature had a negative effect significantly on the washing efficiency of blood soil. This could be due to the coagulation of protein when the temperature exceeded over 40°C.

In Figure 3(c)–(e), tapping washing had a much lower washing efficiency towards the soil of sebum/pigment, carbon black/mineral oil and chocolate/milk than drum washing. In Lee et al.’s study, ⁶ abrasion action could have a significant effect on the washing efficiency of carbon black/mineral oil and chocolate/milk soil comparing the turbulent flow action of the washing liquor on soiled fabric. Therefore, the lack of friction force between fabrics in tapping washing might be the reason for the low washing efficiency. IEC sebum/pigment soil is made from synthetic sebum (32.8% cows fat, 18.3% wool fat, 18% free fatty acid, 3.7% cholesterol, 8.9% squalene, 3.6% coconut oil, 3.1% hard paraffin) and pigment (carbon black, kaolin and iron oxide). The melting point of cow fat, wool fat and hard paraffin is higher than 40°C, so these wax-based soil types would be spontaneously removed to a large extent in the washing liquor at a temperature above 40°C. Therefore, the temperature is an important factor for the washing efficiency of sebum/pigment soil in laundry. The IEC carbon black/mineral oil soil is composed of carbon black in nano-sizes, pigment (made of black carbon, kaolin and iron oxide) and paraffin oil. Carbon black particles ³³ in the pigment have very low surface energy and are difficult to wet by water or common surfactants for dispersing, and the detached carbon black particles also tend to aggregate and redeposit back to the fabric. In addition, the fabrics have a rough surface structure with pores and gaps, so these particles of carbon black in nano-sizes, kaoline and iron oxide could be trapped in the rough surface, and not sufficiently removed by the flow action of washing liquor only. Mechanical rubbing friction action on fabrics is needed to remove semi-solid fats and particles. ⁶

In Figure 3(e), the washing efficiency of tapping washing under different washing parameters towards the chocolate/milk soil on cotton fabric was very low, near to 0. Only the temperature (40°C) at the washing parameter of medium level shows a positive effect on the washing efficiency. This is because that the soil composition of IEC chocolate/milk soil contains unsweetened cocoa (20–22% fat, not alkalised) with sugar, full-cream cow’s milk and water. The natural cocoa contains protein, various amino acids, cocoa fat, various vitamins, mineral elements and biologically active alkaloid. Therefore, cocoa fat cannot be effectively liquified under 40°C, while protein could become coagulated at a washing temperature over 40°C, causing difficulty in removing it. Two ingredients in chocolate/milk soil are contrary on washing temperature. Lee et al. ⁶ proposed two reasons to explain negative washing efficiency values on chocolate/milk soil. First, a large amount of the dark coloured and particulate ingredients such as cocoa in the chocolate/milk soil remained on the sample, although the other light colour ingredients, such as sugar, were dissolved and removed after washing. Another reason is due to the transfer of the soil from the inner part of the yarn to the outer layer of the yarn. Thereby, the surface colour of the washed chocolate/milk soil fabric became to dark brown.

From the washing results towards standard soiled cotton fabric samples, up–down tapping washing provides a turbulent flow action of detergent washing liquor and tapping action which could remove the soluble soil such as the ingredients of the red wine and blood soil. However, the gentle mechanical forces such as turbulent flow action of washing liquor or up-down tapping force but without rubbing abrasion mechanical action on fabric are not sufficient to remove the partially water-insoluble sebum/pigment, carbon black/mineral oil and chocolate/milk soil.

In addition, there is an obvious difference in the washing efficiency between tapping washing and conventional drum rotation washing because they provide different mechanical actions during the laundry. In up–down tapping washing, the fabric sample was tapped in the tapping action by the tapping mesh plate during up and down vertical movement, so that the unidirectional pressure on the fabric produces lower abrasion between fabrics. In drum washing, the rotation of the drum causes severe rubbing friction between fabrics which could be the key factor to influence the removal of the soil. The study of Lee et al. ⁶ also showed that washing efficiency is highly influenced by the mechanical actions including the hydrodynamic flow action, the fabric flexing action and the rubbing abrasion action. It was found that the abrasion action is the most effective mechanical action for the removal of water-insoluble soil from the stained fabrics.

Optimised Tapping Washing

In order to improve the washing efficiency of up–down tapping washing method on IEC soiled fabrics, different levels of wash parameters were selected and optimised based on the analysis of the results shown in Figure 3. The tapping washing method would not cause noticeable damage to fibres or fabrics; therefore, if the different levels of washing parameters didn’t make much difference in the washing efficiency, the high level of the parameters (but not exceeding the extreme level) was selected to provide sufficient mechanical and thermal action for soil removal. Further tapping washing processes under the selected levels of each parameter as shown in Table 3 were carried out.

Table 3.

Selected washing parameters for optimisation of up–down tapping washing and their washing efficiency (D) on IEC soiled fabrics.

Soil types	Optimised tapping washing parameters						Washing efficiency (D, %)
	K	F	H	V	T	D
	(N/mm)	(rpm)	(mm)	(L)	(°C)	(min)
Red wine	4.0	40	7	5.0	60	10	49.0
Blood	14.9	40	20	10.0	20	30	42.5
Sebum/pigment	14.9	60	20	10.0	60	30	20.0
Carbon black/mineral oil	14.9	60	20	10.0	60	30	24.5
Chocolate/milk	9.3	60	16	10.0	40	30	14.4

From the tapping washing under the selected parameters, the washing efficiencies towards the stains of red wine, blood, sebum/pigment, carbon black/mineral oil and chocolate/milk fabric were 49.0%, 42.5%, 20.0%, 24.5% and 14.4%, respectively. The tapping washing at the selected and optimised washing parameters improved the removal of red wine, blood, sebum/pigment, carbon black/mineral oil and chocolate/milks soil types from the soiled fabrics to achieve similar or even higher levels of washing efficiency to the 7A gentle rotation-drum washing, shown in Figure 3. However, the tapping washing for the soil of red wine, sebum/pigment and carbon black/mineral oil used the washing temperature of 60°C which is higher than the temperature of 40°C used in 7A gentle rotation-drum washing, although the tapping washing method kept the low washing temperature at 20°C to achieve the efficient removal of blood soil. The increase in the washing temperature could compensate for the gentle mechanical tapping action to achieve a certain level of washing efficiency towards difficult soiling. In order to avoid the use of high temperature in the tapping washing, appropriate enzymes could be incorporated in the detergent to achieve satisfied washing efficiency on the difficult soil types.

Figure 4 shows the images of soiled fabrics after different tapping washing procedures, 7A gentle and 5A normal drum washing under standard light sources with an illumination of D65. The fabric appearance in terms of crease and smoothness was much better from up–down tapping process than with rotation drum washing. Based on the reduction of staining colour of soiling, the tapping washing under the optimised parameters achieved better soil removal than the tapping washing under minimum (or low) washing parameters or the drum washing under 7A gentle washing programme. It is noticed that although the washing programmes of the 7A gentle washing and optimised tapping washing methods were similar, their washing parameters or conditions were not exactly matched, especially at the washing temperature. However, this could reflect on the level of the washing performance of the tapping washing method towards different soiling types. Gentle drum washing maintained the sample appearance by reducing its clean performance, but it is difficult to balance between the appearance maintenance and cleaning performance. To some extent, the up–down movement tapping washing method could achieve a good cleaning performance on lightly soiled fabrics and maintain the appearance of the fabrics. This could be a suitable method for the laundry of delicate garments.

Figure 4.

Photos of soiled fabrics before and after washing under standard light sources with an illumination of D65.

Microscopic Observation of Soil Removal from Cotton Fabrics

Scanning electron microscopy was used to observe the soiling on the surface of IEC soiled fabric samples before washing, after rotation drum washing under 7A and 5A washing programmes, and after optimised tapping washing under the selected parameters as shown in Figure 5.

Figure 5.

Microscopic view of five kinds of IEC soil samples before washing, after gentle (7A) and normal (5A) drum washing, and optimised tapping washing at magnifications of 3000 and 8000.

The SEM images clearly show soil particles or soil materials attached on the surface of the IEC soiled cotton fibres before washing under a magnification of 3000 or 8000. Aggregated particals were found on the surface of blood and chocolate/milk soiled cotton fibres, but nanosize particles appeared on the surface of sebum/pigment and carbon black/mineral oil soiled fibres. Particles of the chocolate/milk soil were trapped between the yarns in the woven structure of the fabric.

After gentle drum washing (7A), it was found that the blood and chocolate/milk soil particles were still remained on the surface of fibres. Few fibre splits were noticed on the surface of the red wine and sebum/pigment soiled fabrics. This means that gentle drum washing (7A) could not achieve satifactory soil removal, but after normal drum washing (5A), almost all soil particles were removed from the surface of cotton fibres. After optimisation of tapping washing using the selective parameters, there were no soil particles or materials visible on the surface of tapping washed fabric samples under SEM. Overall, it is found the optimised tapping washing could make good balance between sufficient washing and maintaining of fabric appearance without causing fibre damage.

Tapping washing generally cause much less damage to the washed fabrics than traditional rotation drum washing due to the different mechanical action applied on the fabrics. Previous researchers^6,20 verified that rubbing friction force is the most effective force on the removal of soil from the fabrics when compared with mechanical tapping force, fabric flexing force or liquor flow force in the washing process. However, the friction force in drum washing is likely to cause damage to delicate fabrics, such as silk, after repeat washing cycles. Panasonic tapping washing ¹² and ultrasonic washing ¹³ are gentle washing methods to control friction force or abrasion movement as much as possible for preventing fibre or fabric damage during the laundry process. Therefore, compared with a gentle drum wash, the up–down movement of the tapping washing method could suppress the firction force during laudary and make a good balance between sufficient washing and maintaining of fabric appearance without causing fibre damage.

Conclusions

In order to meet the requirement for delicate garment washing, a novel tapping washing platform was set up to investigate the washing efficiency towards different types of standard soiling on cotton fabric. The washing parameters for the tapping washing programme were tested to explore its washing performance in comparison with conventional rotation drum washing.

The results show that up–down tapping washing could remove water-soluble based soil types such as red wine and blood, but this washing method was not sufficient to remove standard sebum/pigment, carbon black/mineral oil and chocolate/milk artificial soil types due to their insolubility. It was found that the removal of the insoluble soils from the fabrics could be improved by increasing the washing temperature to 60°C and mechnical tapping force. In order to avoid the use of high temperature in the tapping washing, the appropriate enzymes could be incorporated in the detergent to achieve satisfactory washing efficiency on the difficult soil types. For real practice, the tapping washing method is unlikely to be used for washing of heavily soiled fabrics like IEC soiled fabrics but for care and maintenance washing of lightly soilded delicate garments. Therefore this up–down tapping washing method needs to be further investigated and tested on lightly soiled luxury fabrics or garment. Further improvement and optimisation of the tapping washing platform and washing programme could give this novel washing method potential for the care and washing of delicate garments or the conservation of heritage and valuable textiles without causing damage of materials or change in their appearnce.