Development of Natural Rubber-Based Elasto Ball as an Alternative Material to Substitute Pumice in the Garment Washing Process

Abstract

Distressed fabric is a popular fashion trend that adds a distinct visual appeal to garments. Distressing involves acid washing with pumice stones containing potassium permanganate. This approach is inappropriate for knitted textiles, which can generate holes and reduce quality. This project seeks to create an Elasto Ball (EB) as an alternative to pumice stones in the acid-washing procedure of knitted materials. The Elasto Ball consists of natural rubber foam filled with silica and a silica–lignin hybrid derived from rice husks. The efficacy of the filler is enhanced during the manufacturing of Elasto Ball by employing the NXT silane coupling agent throughout the silanization process. The silanized elasto ball exhibits thermal stability up to 400 °C and a porosity of up to 5%. In garment washing assessments, the Elasto Ball can diminish the fabric’s color by 40–50% without causing damage. The findings of this study indicate that Elasto Ball can function as an efficient, eco-friendly substitute for washing balls in garment washing procedures.

1. Introduction

Garment washing is an essential phase in the manufacturing process to satisfy consumer demand for visually appealing clothing. This method, originally utilized for woven fabrics such as denim, has now extended to knitted fabrics, creating a worn and faded appearance that enhances product marketability [1,2]. Various methods can be employed in the fabric distressing process, such as stone washing, enzyme treatment, and acid treatment [3]. Pumice stone is commonly utilized as a medium in these processes. The pumice stone features a porous structure, exhibits a high silica content, has a hardness of 6–7 on the Mohs scale, and has a density ranging from 0.5 to 0.75 g/cm³ [4,5]. Its distinctive properties enable it to float and provide effective abrasiveness during washing, resulting in a sought-after uneven color fade. The stone wash and acid wash processes in the textile industry commonly employ pumice stones as an abrasive medium to erode the fabric’s surface, yielding a more worn-out appearance and visually fading hues. The acid wash method involves initially immersing pumice stones in a potassium permanganate (KMnO₄) solution within an acidic environment, subsequently followed by a neutralization process [6]. Potassium permanganate, as a strong oxidizing agent, reacts with the fabric to induce a color-fading effect [7]. The abrasion of the pumice stones during the washing process facilitates the elimination of color particles from the fabric fibers, resulting in contrasting color patterns and a softer fabric surface [8].

Although utilizing pumice stone for washing might provide an attractive color gradient, it presents several considerable drawbacks in the finishing process of knitted fabrics. The pumice stone can harm the thread seams and jeopardize the fabric’s structure, leading to the formation of holes and ultimately diminishing the quality of the fabric [9,10]. Furthermore, pumice stones may generate residue at the end of the washing process, which can lead to increased labor and energy expenses relative to conventional washing methods. Several substitutes have been investigated to address these issues, such as thermocol balls, rubber soles, or other polymer-based foams. However, these alternatives, while partially mitigating pumice stone drawbacks, often fall short of achieving a balanced performance in abrasiveness, durability, fabric protection, and environmental compatibility, thus underscoring the need for a more comprehensive solution [6].

Several studies have investigated alternative materials to replace pumice stone in garment distress. Khalil et al. employed thermocol balls composed of polystyrene, with diameters ranging from 0.75 to 1 cm, to distress knitted fabric. This method resulted in a color fastness rating of 4–5 [11]. This rating is deemed insufficient, as a lower color fastness rating correlates with increased color fading. Alam et al. attained a color fastness value of 2 in denim fabric with post-use rubber soles. Nonetheless, the fabric’s physical strength, as measured by bursting tests, was exceptionally high, which could potentially harm the fibers [11]. This study aims to propose a novel design for a washing ball, considering the limited research on alternative washing balls for garment washing. The characteristics of the proposed washing ball nearly correspond with those of pumice stone, encompassing dimensions, weight, porosity, thermal resistance, and chemical resistance.

Natural rubber is a sustainable polymer obtained from Hevea brasiliensis trees, recognised for its superior mechanical qualities, such as high elasticity, substantial tensile and tear strength, hydrophobicity, and abrasion resistance [12,13]. The attributes of natural rubber provide it with an optimal matrix for washing ball applications, since it retains its form while in use, while reducing fabric damage during the washing process. Rice husk, a plentiful agricultural residue, is rich in silica and possesses significant promise as a filler material [14]. The amalgamation of silica and lignin in rice husk produces a silica–lignin hybrid material characterised by an increased surface area and a greater number of active sites compared to pure silica [14,15]. Lignin inherently adheres to silica via hydroxyl groups, augmenting sorption capacity and acid–base interactions, while also boosting abrasiveness, porosity, and heat and chemical resistance [15]. The use of natural rubber with silica–lignin filler offers a viable method for creating an eco-friendly, effective washing ball for use in the textile industry.

Building on these material advantages, this study suggests the creation of a new washing ball material, the Elasto Ball, which combines natural rubber with rice husk-based additives to improve thermal stability, porosity, and abrasive performance. The flexibility of natural rubber enables its shaping into spherical forms, while the use of silica or silica–lignin hybrids from rice husk enhances the ball’s functional qualities. The principal aim of this research is to develop Elasto Ball as a sustainable alternative to pumice stones in the garment washing process, providing enhanced durability, efficiency, and environmental compatibility. Unlike previous substitutes such as thermocol balls or recycled rubber, the Elasto Ball is specifically engineered to balance abrasiveness and eco-functionality, particularly through its capacity to absorb residual Mn²⁺ from potassium permanganate. This dual role positions the Elasto Ball not merely as a physical substitute but as an environmentally responsible innovation tailored for the textile sector.

2. Materials and Methods

2.1. Material

Rice husk (RH) was collected from local rice fields in Bandung, Indonesia. An analytical grade of hydrochloric acid (HCl, 37%), sulfuric acid (H₂SO₄, 96%), and sodium hydroxide (NaOH, 98%), as well as a technical grade of toluene and ethanol, were purchased from Sopyan Java Cemerlang Inc., Bandung, Indonesia. Ribbed Smoked Sheet (RSS1) was purchased from Perkebunan Nusantara VIII, Bandung, Indonesia. Zinc oxide, an anhydrous mixture of fatty alcohol and fatty acid ester (Affux 42 M, RheinChemie), and N-cyclohexyl-2-benzothiazolesulfenamide (CBS) 98.50% were obtained from Multi Citra Chemindo Inc., Central Jakarta, Indonesia. The silane coupling agent, 3-Octanoylthio-1-propyltriethoxysilane (NXT), was obtained from Rena Haniem Mulia Inc., South Tangerang, Indonesia. The blowing agent azodicarbonamide was provided by Nata Kimindo Pratama Inc., West Jakarta, Indonesia, and sulphur was obtained from Teja Rubber Compounding Inc., Bandung, Indonesia.

2.2. Extraction of Silica and Silica–Lignin Hybrid from Rice Husk

A silica–lignin hybrid was extracted from rice husk (RH) by soaking it in a 1 M HCl solution, using a solid-to-solution ratio of 1:16 (w/v). This mixture was then refluxed at 90 °C for 90 min. After refluxing, the mixture was filtered and washed with water until a neutral pH was achieved. Finally, it was dried overnight at room temperature. The HCl-treated RH was treated with 2 M NaOH solution with a solid-to-solution ratio of 1:16 (w/v) for 4 h at 90 °C under reflux conditions. Afterward, the mixture was filtered, and the filtrate was set aside for the next precipitation process. The filtered solution was subsequently treated by adding 2 M H₂SO₄ dropwise at room temperature until a pH of 3–4 was reached. The resulting precipitate was allowed to stand at room temperature for 8 h, and the final residue was washed with water until neutral pH and dried at 80 °C for 12 h.

Silica was obtained from RH by direct burning in a furnace at 700 °C for 8 h. Rice husk ash (RHA) was soaked in a 1 M HCl solution at a ratio of 1:16 (w/v). It has been reported that 1 M HCl is the most effective agent for removing metal impurities from rice husks. The mixture was then heated and stirred at 90 °C for 90 min. The solution was filtered, and the residue was washed with water until neutral, then dried overnight at room temperature. Next, the HCl-treated RHA was mixed with a 2 M NaOH solution at a ratio of 1:16 (w/v). The solution was heated and stirred at 90 °C for 4 h, then filtered. The filtrate was used for the precipitation process. The filtered solution was subsequently treated by adding 2 M H₂SO₄ dropwise at room temperature until a pH of 3–4 was reached. The resulting precipitate was allowed to stand at room temperature for 8 h, and the final residue was washed with water until neutral pH and dried at 80 °C for 12 h.

2.3. Treatment Silanization of Silica and Silica–Lignin Hybrid

The silica and silica–lignin hybrid were modified through ex situ silanization using 3-octanoylthio-1-propyltriethoxysilane (NXT). Dried silica or silica–lignin hybrid was dispersed in toluene at a ratio of 1:10 (w/v). Then, 6 wt% of NXT was added (based on the weight of powder) to the system, and it was refluxed at 90 °C for 2 h. Afterwards, the mixture was washed with ethanol and dried at 80 °C for 12 h.

2.4. Compounding Process

The rubber compounds were prepared in a two-roll mill XSK-160 machine at a friction ratio of 1:1.35 and a rotational speed of 14.29 rpm. The compounding was carried out at room temperature in the range of 25–30 °C. The formulation used in this study is presented in

Table 1. Natural rubber and filler formula.

	Formulation (phr)
	BL-0 (a)	Si (b)	Si-NXT (c)	Si-Lig (d)	Si-Lig-NXT (e)
RSS1	100	100	100	100	100
Afflux 42M	2	2	2	2	2
Silica	0	5	0	0	0
Silica silane NXT	0	0	5	0	0
Silica–lignin hybrid	0	0	0	5	0
Silica–lignin hybrid silane NXT	0	0	0	0	5
ZnO	5	5	5	5	5
Azodicarbonamide	4	4	4	4	4
CBS	0.4	0.4	0.4	0.4	0.4
Sulfur	2	2	2	2	2

Note for symbol: (BL) is a control no filler (a); (Si) is NR with silica filler (b); (Si-NXT) is NR with silica and silane NXT filler (c); (Si-Lig-NXT) is NR with silica–ligin filler (d); and (Si-Lig-NXT) is NR with silica–lignin hybrid and silane NXT filler (e).

. NR was initially masticated for two minutes, after which Afflux 42 M, fillers, ZnO, and a blowing agent were added. The curing packages were added in the last step, followed by two more minutes of homogenization. The final compounds were rested overnight before being taken for the curing process.

2.5. Manufacturing of Elasto Ball for Garment Washing Applications

The fabrication of the Elasto Ball was conducted using a compression molding machine. Using a 2 g rubber compound, each variation was prepared and placed into a pre-heated spherical mold with a diameter of 2 cm. The compound was subsequently vulcanized at 150 °C for a duration determined by the t₉₀ value acquired from the rheometer test. Following this process, five different sample variations were obtained, each designated a specific sample code, namely: EB (Elasto Ball without filler), EB/Si (Elasto Ball with silica filler), EB/Si-NXT (Elasto Ball with silica and silane NXT filler), EB/Si-Lig (Elasto Ball with silica–lignin filler), and EB/Si-Lig-NXT (Elasto Ball with silica–lignin and silane NXT filler).

2.6. Characterization

2.6.1. Characterization of Silica and Silica–Lignin Hybrid

The functional groups of the extracted filler silica, silica–lignin hybrid, and silane with NXT were identified using FTIR-ATR spectroscopy, employing FTIR-ATR Shimadzu Prestige-21, Shimadzu, Kyoto, Japan in the range of 400–4500 cm⁻¹.

The thermal stability of the extracted silica, silica–lignin hybrid, and silane with NXT was determined using TG/DTA characterization, wherein the samples were placed into a sample holder with a heating rate of 10 °C/min from 30 to 1000 °C.

The particle size distribution of a filler was assessed using a Particle Size Analyzer (PSA) Horiba SZ-1000, Horiba, Kyoto, Japan. Filler materials such as silica, silica–lignin hybrid, and silane with NXT were examined using a Horiba SZ-100. The sample was prepared with the powder being dispersed in analytical-grade ethanol. The measurements were conducted using the principles of light scattering, specifically through the Scattering Light Intensity method.

The morphology of the extracted silica and silica–lignin hybrid was observed using a Hitachi SU3500 SEM, Hitachi, Tokyo, Japan. The samples were prepared in an aluminum stub using adhesive tape and then coated with a layer of gold. The samples were placed into a sample holder and used for imaging.

2.6.2. Characterization of Rubber Elasto Ball

The rubber compounds and fillers undergo a vulcanization process. The vulcanization time was evaluated using the MDR 2000 Alpha, Alpha Technologies, Hudson, OH, USA. The testing temperature was set at 150 °C for 30 min.

Porosity Measurement

Porosity measurement of the produced Elasto ball was carried out. This testing was conducted using a modified gravimetric method. The porosity of the washing balls (ε) was measured by determining the percentage of the weight of water that fills the pores of the washing balls relative to their dry weight at specific dimensions. Samples with a diameter of 2 cm were soaked in demineralized water for 1 h, after which their surfaces were dried with tissue paper and weighed (Ww). The samples were then dried at 60 °C for 24 h and weighed again to obtain the dry weight (Wd). Porosity (ε) was determined using Equation (1) [16].

$$\mathsf{\epsilon }=\left({\displaystyle \frac{\mathrm{W}\mathrm{w}-\mathrm{W}\mathrm{d}}{\frac{4}{3}\mathsf{\pi }{\mathrm{r}}^3\mathsf{\rho }}}\right)\times 100\mathrm{\%}$$

(1)

where Ww is the wet weight, Wd is the dry weight, 4/3 πr³ is the volume of the washing ball, and ρ is the density of water.

The density of the natural rubber filler (NRF) was determined using geometric methods. The relative density (RD) of the NRF is calculated by dividing the density of the NRF by that of natural rubber solid (0.93 g/cm³), as shown in Equation (2). Additionally, the volumetric expansion ratio (ER) is calculated as the reciprocal of the relative density, as indicated in Equation (3) [14].

$$\mathrm{Relative} \mathrm{Density} (\mathrm{RD})={\displaystyle \frac{\mathsf{\rho } \mathrm{N}\mathrm{R}\mathrm{F}}{\mathsf{\rho } \mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d} \mathrm{n}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{l} \mathrm{r}\mathrm{u}\mathrm{b}\mathrm{b}\mathrm{e}\mathrm{r}}}$$

(2)

$$\mathrm{Expansion} \mathrm{ratio} (\mathrm{ER})={\displaystyle \frac{1}{\frac{\mathsf{\rho } \mathrm{f}\mathrm{o}\mathrm{a}\mathrm{m}}{\mathsf{\rho } \mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}}}}$$

(3)

Cell Size Measurement

The cell size of the natural rubber filler (NRF) was measured from SEM images and then analysed using ImageJ software version 1.54d. Due to the anisotropic shape of the cells, their sizes were measured along two different axes: x (horizontal) and y (vertical). Twenty cells were measured from each sample, and the average values were recorded as Φx and Φy. The surface morphology and cross-section were observed using a desktop scanning electron microscope (SEM).

Characterization of Hardness

The hardness of rubber foam was measured using a Shore A durometer (Bareiss HPE-III, Bareiss, Oberdischingen, Germany), according to ASTM D2240 [17]. Test specimens were prepared in the form of flat plaques with dimensions of approximately 25 × 25 × 10 mm³. For each formulation, hardness was measured at five different locations on each plaque, and the reported value is the average of these measurements. This approach ensures reproducibility and comparability across samples.

2.7. Performance of Elasto Ball in the Garment Washing Process Using the Acid Washing Method

The acid wash process was performed using potassium permanganate (KMnO₄). The Elasto Ball was first immersed in a KMnO₄ solution at a concentration of 20 g/L for 30 min, after which the Elasto Ball was dried at 100 °C for 5 min. The Elasto Ball that had absorbed potassium permanganate was then used in the washing process. The washing process was conducted using a laboratory-scale gryowash machine at 40 °C for 30 min, with Elasto Ball 14 pcs (size 2 cm ± 200 g) and for cotton fabric 15 × 15 cm. The washed fabric was neutralized using metabisulfite at 1 g/L and oxalic acid at 1 g/L at a temperature of 50 °C for 10 min.

2.7.1. Characterization for Fading Color with Spectrophotometer

The fabric produced from the garment washing with the Elasto Ball was evaluated for color fading using a benchtop spectrophotometer. The fabric was measured from a wavelength (λ) of 400 nm to 700 nm, and the color strength (K/S) was determined. The percentage of color decrease and color uniformity was calculated compared to the fabric that did not undergo the process.

2.7.2. Characterization for Cotton Fiber After Washing

The morphology of the cotton fiber after the acid washing process was observed using a Hitachi SU3500 SEM, Japan. The samples were prepared in an aluminum stub using adhesive tape and then coated with a layer of gold. The samples were placed into a sample holder and used for imaging. Then, the samples underwent EDX analysis and chemical analysis by exposing them to an X-ray light beam. The physical properties of the processed fabric were tested using the Textile Bursting Strength Test according to ISO 13938-1:2019 [18].

2.8. Statistical Analysis

All experimental data were expressed as mean values, and statistical analyses were conducted using one-way ANOVA to determine the significance of treatment effects. Where applicable, Tukey’s post hoc test was applied for pairwise comparisons. A significance level of p < 0.05 was considered statistically significant.

3. Results and Discussion

3.1. Characterization of Silica Filler and Silica–Lignin Hybrid Fillers

3.1.1. Characterization of Silica Filler and Silica–Lignin Hybrid Fourier Transform Infrared (FTIR) Spectrophotometer

Fourier Transform Infrared (FTIR) Spectroscopy is employed to examine the existence of functional groups and to verify the chemical interaction between the silica/silica–lignin hybrid filler and the silane coupling agent (NXT). The FTIR spectra patterns for silica and NXT-silanized silica, as well as for silica–lignin hybrid and silanized silica–lignin hybrid, are shown in Figure 1. The FTIR spectra patterns are derived from the final composition of silica and silica–lignin hybrid after the extraction process, which eliminates lignocellulose and other trace components from the rice husk precursor.

In Figure 1, the FTIR spectrum and

Table 2. FTIR data of functional groups in silica, silica–lignin hybrid, and silanized products with NXT.

Functional Group	Wavenumber (cm−1)				References [19,20,21,22]
	Experiment
	Before Silane NXT		After Silane NXT
	Silica (Si)	Silica–Lignin Hybrid (Si-Lig)	Silica Silane NXT (Si-NXT)	Silica–Lignin Hybrid-NXT (Si-Lig-NXT)
O-H stretching (silica–lignin)	3318	3327	3333	3340	3450
C-H asym bending in CH3 in Si-O-CH2-CH3	2627	2728	2856	2849	2844
C=O stretch	1634	1681	1645	1692	1693
C-H stretching of methyl and methylene groups Aromatic skeletal vibration of lignin	1508	1510	1513	1571	1641
C-O stretching peak of the Si-O-CH2-CH3 group of silanes	1065	1062	1086	1097	1075
Si-C-stretching vibration	795	794	799	797	798
Si-O bending vibration	611	613	612	616	568
Si-OH	965	974	966	987	964

present data for the FTIR spectrum of silica filler, hybrid lignin silica, and silanized silica using the silane coupling agent NXT. The addition of 6% NXT silane (6 wt% based on filler silica or silica–lignin hybrid) indicates that the FTIR spectrum of NXT exhibits bending with the silica filler and hybrid lignin silica that have been silanized with NXT. The FTIR absorption peaks at 3318 cm⁻¹, 3327 cm⁻¹, 3333 cm⁻¹, and 3340 cm⁻¹ are attributed to O–H stretching vibrations, which are characteristic of hydroxyl groups in silica and lignin structures, while the C–H stretching vibrations of the CH₂ group are observed separately at 2856 cm⁻¹ and 2849 cm⁻¹ and correspond to the –CH₂ groups of 3-Octanoylthio-1-propyltriethoxysilane (NXT). Peaks due to C= stretching appear (from C=O stretching vibrations of silane) at 1645 cm⁻¹ (Si-NXT) and 1692 cm⁻¹ (Si-Lig-NXT) in the 3-Octanoylthio-1-propyltriethoxysilane (NXT) monomer [19]. The peaks at 1086 cm⁻¹ and 1097 cm⁻¹ for silanized silica and hybrid lignin silica correspond to the alkoxy C-O stretching peak from the silane group Si-O-CH₂CH₃. The newly formed Si-O-Si bond at 1100 cm⁻¹ is associated with the interaction of the silane coupling agent with the silica/silica–lignin hybrid, merging with the peak at 1097 cm⁻¹ and broadening. The peak at 1097 cm⁻¹ is caused by the asymmetric Si-O-C or Si-O-Si bond stretching, while the Si-O-Si bending vibration appears in the spectrum at 798 cm⁻¹ due to Si-O-Si bending. The interaction between the silica/silica–lignin hybrid and silane NXT was evidenced not solely by peak intensity, but also by characteristic spectral changes in the FTIR spectrum. These include relative variations in intensity, broadening, and slight wavenumber shifts in the O–H and Si–O–Si/Si–O–C regions when compared to the unsilanized samples as described in Table 2. Such modifications indicate the formation of hydrogen bonding and possible covalent bonding between the filler and silane, suggesting genuine chemical interactions rather than simple concentration effects [20].

3.1.2. Thermogravimetry–Derivative Thermogravimetry (TG–DTA)

The thermal properties of the modified fillers were assessed using thermogravimetric analysis to ascertain their stability and the degree of silane grafting. This analysis is essential for comprehending the breakdown profile and the interaction between the silane coupling agent and the silica/silica–lignin hybrid surfaces.

The TGA data, shown in Figure 2 and

Table 3. Weight loss (%) data of silica, silica–lignin hybrid, and silanized products with NXT.

Sample	Wt Loss % from TGA (%)
	25–200 °C	200–700 °C
Si	7.6	-
Si-NXT	3.8	6.7
Si-Lig	3.6	6.2
Si-Lig-NXT	2.5	4

, indicate an initial weight reduction occurring between 80 and 100 °C, followed by a reduced weight loss after 200 °C. The initial weight reduction zone is mainly due to the desorption of physically adsorbed water and the loss of silane that is either weakly adsorbed or covalently attached to the silica/silica–lignin hybrid filler. The reduction in the second zone, ranging from 200 °C to 700 °C, corresponds with the breakdown of silane molecules adsorbed on the surface through hydrogen bonding, indicating possible interactions between the silane and the silica/silica–lignin hybrid structure. This heating process, together with solvents and energy, may affect the modifications and diffusion of the silane coupling agent NXT into the silica/silica–lignin hybrid filler [23].

The initial weight loss up to 200 °C indicates that the weight loss percentage post-silanization process is 3.8% for silica (Si-NXT) and 2.5% for the silica–lignin hybrid (Si-Lig-NXT), as shown in Table 2. This results from the adsorbed silane being connected by bonds to hydroxyl groups and/or covalent bonds [24]. The weight loss from 300 °C to 400 °C is ascribed to the diffusion of silane within the layers of the silica/silica–lignin hybrid filler, whereas the weight loss from 400 °C to 700 °C is related to the chemical fixation of silane bonded to the silica/silica–lignin hybrid filler. The percentages for silica-NXT (6.7%) and silica–lignin hybrid-NXT (4%) indicate that the combustion of grafted silane results in the production of volatile carbon dioxide, water, and solid SiO₂. This final compound interacts with the existing silica to form the residual solid phase. The grafting degree expressed in wt% refers to the attachment of the alkylsiloxy residual group to the silica surface [25].

3.1.3. Particle Size Analysis (PSA)

The particle size distribution of silica, silica-lignin hybrid, and silanized fillers are presented in Figure 3. The particle size distribution for pure silica was 113.33 nm, whereas for the silica–lignin hybrid it decreased to 79.4 nm. Following NXT silanization, the particle size for silica decreased to 68.8 nm, while the silica–lignin hybrid showed a slightly higher distribution size of 77.53 nm. To confirm the statistical significance of these differences, a one-way ANOVA was conducted, which revealed a significant effect of treatment on particle size (F(3,8) = 14.43, p = 0.0014). This indicates that the observed variations in particle size distributions are not due to random error but result from the different treatments applied. The changes can be attributed to the sol–gel process during particle formation, encompassing physical and chemical processes such as hydrolysis, polymerization, gelation, condensation, drying, and densification [26].

3.2. Characterization of Natural Rubber Filler (NRF), Silica, and Silica–Lignin Hybrid Fillers

3.2.1. Cure Characteristic of Natural Rubber Compound

The characteristics of natural rubber compounds, including the curing curve and the curing characteristics of natural rubber compounds at 150 °C, are presented in Figure 4 and

Table 4. Cure characteristics of natural rubber compounds with various filler compositions at 150 °C.

Sample Code	ML (dNm)	MH (dNm)	$∆{\mathbf{S}}^{\mathbf{\prime }}$ (dNm)	ts2 (min)	t90 (min)
BL (blanko)	0.19	7.34	7.15	6.39	14.22
S (Silica no silane)	0.60	7.28	6.68	8.42	16.53
Si-NXT (silica silane NXT)	0.46	6.92	6.46	9.12	19.29
Si-Lig (silica–lignin hybrid)	0.45	7.19	6.74	8.37	18.15
Si-Lig-NXT (silica–lignin hybrid silane NXT	0.56	6.49	5.93	8.29	18.03

. The addition of fillers to natural rubber compounds can reduce the maximum torque in the silica–lignin hybrid and silica samples, as well as with the addition of silanization. The maximum torque of natural rubber compounds decreases with the addition of silane and filler materials [15]. This effect may be due to a reduction in crosslink density, as indicated by the value of ∆S’ [27]. The decrease in crosslink density is believed to be caused by the acidic nature of both filler materials. Additionally, FTIR analysis shows that the silica–lignin hybrid and silica contain -OH functional groups. The acidic nature of these functional groups tends to delay vulcanization [22].

3.2.2. Density Dan Cellular Structure

The density ratio and expansion ratio (ER) of natural rubber with filler (NRF) are shown in

Table 5. Density and expansion of natural rubber compound and fillers at various compositions.

Parameters	BL	Si	Si-NXT	Si-Lig	Si-Lig-NXT
${\displaystyle \frac{\rho f}{\rho s}}$	0.8616	0.6728	0.6058	0.4266	0.5252
ER	1.16	1.49	1.32	1.69	1.58
$\Phi$ x (mm)	0.62 ± 0.19	0.99 ± 0.12	0.82 ± 0.21	0.87 ± 0.26	0.66 ± 0.14
$\Phi$ y (mm)	0.55 ± 0.18	0.86 ± 0.14	0.71 ± 0.23	0.72 ±0.23	0.57 ± 0.22

. Generally, most samples indicate that the relative density decreases with the addition of silane into the filler material. The relative density of NRF containing the silica–lignin hybrid slightly decreases with the presence of silane [28]. This pattern is similarly observed in the NRF containing silica. The increase in density is attributed to the influence of the silica–lignin hybrid and the silica filler during the vulcanization process [29]. Therefore, gas decomposition utilizing a foaming agent in conjunction with a filler cannot be confined within the rubber [30]. The ER findings corroborate that the incorporation of silane into the filler material yields reduced density values. This is observed by the SEM morphology results in Figure 4, where all NRF samples display anisotropic cells of various sizes. The silanization of filler materials, including the silica–lignin hybrid and silica, produces relatively smaller cell sizes and fewer cells formed [31].

The incorporation of silica and silica–lignin hybrid into natural rubber leads to enhanced foam density and reduced cell size [32]. This indicates that the addition of silane, at a curing temperature of 150 °C, results in increased cell sizes and an irregular cell size distribution [29]. It is estimated that the types of filler and silanization restrict bubble growth due to a faster curing reaction. Lower viscosity and reduced rubber strength at high temperatures enhance cell incorporation, leading to bigger cell sizes and an irregular cell distribution [30].

3.2.3. Porosity

Porosity denotes a polymer’s capacity to develop cavities or holes inside a component due to the incorporation of various additives [16]. The measurement findings shown in Figure 5 indicate that the incorporation of silica filler and silica–lignin modified with the organosilane NXT, coupled with various additives during the rubber compounding process, enhances the porosity of the Elasto Ball (EB). The porosity of the EB with added NXT silane reaches 5% in comparison to other filler types. Meanwhile, the EB treated with silica–lignin hybrid filler and silanization reaches 40–50%. Measurement results indicate an increase in porosity, indicating that silica filler derived from rice husk possesses Si-OH groups, leading to the formation of porous particles, which can influence water absorption [33]. Moreover, the presence of a foaming agent during the rubber compounding process leads to the formation of an open-cell structure [34].

To confirm the statistical significance of these differences, a one-way ANOVA was conducted, revealing a highly significant effect of treatment on porosity (F(4,20) = 850.20, p < 0.0001). This demonstrates that the observed increases in porosity are not due to random variation but are strongly associated with the different filler modifications. Post hoc Tukey’s test further confirmed significant pairwise differences among nearly all treatments, with EB-Si-Lig-NXT showing the highest porosity levels compared to other groups. These statistical findings provide strong evidence that silanization and lignin hybridization substantially influence the porosity of Elasto Ball composites.

3.2.4. Hardness Analysis of Elasto Ball

The Elasto Balls were subjected to shore A hardness testing specimens, as shown in Figure 6. The Elasto Balls treated with silanization for silica filler and silica–lignin hybrid exhibit higher hardness, reaching 20–25 Shore A, in contrast to 15–20 Shore A for those without silanization [35]. Silica and silica–lignin hybrid fillers exhibit polar characteristics, while natural rubber is non-polar. Therefore, the non-polar characteristics of both substances, combined with the addition of a silane coupling agent, lead to crosslinking [35]. Silane coupling agents are additives used to improve the adhesion of fillers to their matrix. The adherence of fillers to the polymer matrix can enhance or diminish the composite’s strength. Composites with coupling agents show higher hardness compared to those without coupling agents when using silica filler at 5 phr. Coupling agents alter the reinforcement’s surface to make it hydrophobic, hence enhancing its adhesion with the polymer [35]. The formation of bonds between the hydroxyl groups on the silica filler and the silane groups in the matrix, as well as the alkoxy groups of the coupling agent, enhances bonding at both the surface and within the matrix, resulting in improved hardness [36].

To statistically validate these differences, a one-way ANOVA was performed and revealed a highly significant effect of treatment on hardness (F(4,45) = 423.35, p < 0.0001). This confirms that the variations in hardness values are not random but are strongly influenced by the type of filler modification applied. Furthermore, Tukey’s post hoc test showed that all pairwise comparisons among the treatments were statistically significant, indicating that each modification (silica, silica-NXT, silica–lignin, and silica–lignin-NXT) resulted in distinct hardness levels. These results provide strong statistical evidence that silanization and lignin hybridization substantially enhance the hardness properties of the Elasto Ball composites.

3.3. Application of Rubber Washing Balls/Elasto Balls in Garment Washing Process

The following image of the Elasto Ball is depicted in Figure 7A, while an illustration of the garment washing process utilizing the acid wash method is presented in Figure 7B. In this process, the Elasto Ball immersed in potassium permanganate (KMnO₄) is utilized in a laboratory-scale washing operation as described in Section 2.7.

3.3.1. Color Degradation Analysis

The garment washing process utilized for assessment indicates that a greater degree of color fading correlates with superior outcomes. The Elasto Ball containing silanized silica filler (EB/Si-NXT) exhibits a color reduction of up to 50%, as shown in Figure 8.

This indicates that the Elasto Balls containing silica filler can provide abrasive properties during washing, generating friction between the fabric and the Elasto Ball, while also possessing the capacity to absorb the oxidizing agent KMnO₄. Furthermore, KMnO₄ can reduce the color of the fabric, resulting in a more faded appearance [37]. Natural rubber treated with silanization using the silane coupling agent NXT can mitigate the non-polarity of both materials, hence imparting abrasive properties and enhancing resistance to the oxidizing agent potassium permanganate.

To validate these findings, a one-way ANOVA was conducted, which revealed a highly significant effect of treatment on the degree of color fading (F(5,12) = 91.98, p < 0.0001). Post hoc Tukey’s test further confirmed that most pairwise comparisons between treatments were statistically different (p < 0.05). In particular, EB/Si-NXT showed a significantly higher fading effect compared to EB, EB/Si, and EB/Si-Lig, demonstrating that silanization with NXT markedly enhances the performance of the Elasto Balls. On the other hand, no significant differences were found between EB/Si and EB/Si-Lig, or between EB/Si and pumice stone, suggesting that these treatments provide comparable fading performance. Taken together, these results provide strong statistical evidence that silanization and filler modification play a decisive role in improving the abrasive and fading properties of Elasto Ball composites during washing tests.

KMnO₄ initially interacts with the dyed thread, gradually degrading some fibers from the thread chain, which causes the threads to become finer and reduces the weight of the fabric. It progressively infiltrates the fabric. Therefore, the chemical bonds of the primary wall (outer layer) are disrupted by the breakdown of the KMnO₄ solution in water. Subsequently, it attacks the secondary walls of the cotton fibers [38]. The outcomes of this reaction are that the primary wall (outer layer) of the cotton fibers deteriorates and loosens more rapidly due to the friction (mechanical force) applied by the rotating cylinder of the washing machine and the surface formed by the Elasto Balls filled with abrasive pumice during the washing process.

In addition, reusability tests of the Elasto Balls were conducted up to 10 washing cycles to evaluate their durability and color degradation performance. The results showed that Elasto Balls maintained significantly better fading efficiency compared to pumice stones, underscoring their superior reusability. However, the detailed cycle-by-cycle data will be presented in a subsequent publication.

3.3.2. Surface Analysis of Washed Samples Using SEM

Surface analysis of the washed sample was conducted on cotton fabric using the Elasto Ball, with various fillers and pumice as a comparison. The analysis of the washing results on cotton fabric subjected to an acid wash treatment indicates that many surface fibers are released, resulting in a more faded color in Figure 9 (left side). This results from the friction between the elastomeric balls and the fabric surface, which has abrasive properties.

SEM morphological analysis was conducted to observe the impact of fillers on acid-washed cotton fabric. Figure 9 (right side) shows that the fibers subjected to the acid washing process demonstrate surface degradation. The surface of the cotton fabric in the section without filler seems twisted and smooth. The addition of filler during the use of elastomeric balls influences this effect, as indicated by the silanized filler, which shows that the twisted fibers are slightly opened [39]. This is due to the friction or abrasive properties occurring between the silica filler and the cotton. The presence of the oxidizing agent also enhances the fading of color on the surface of the cotton fabric [37]. Cotton is a cellulose polymer composed of glucose units linked by β-1,4-glycosidic bonds, while potassium permanganate, which consists of manganese ions in the form of Mn(VII), has the ability to attract electrons from organic compounds (such as cellulose) and oxidize certain functional groups in the cellulose structure [40]. The oxidation process by KMnO₄ begins with the transfer of electrons from the hydroxyl (OH) groups present in the cellulose molecule to potassium permanganate, resulting in changes to the cellulose molecular structure [41]. Thus, KMnO₄ not only interacts with the internal structure of cotton fibers but also modifies the fiber surface, thereby increasing its ability to interact with other chemicals, such as dyes. This oxidation reaction can lead to a reduction in the color intensity of cotton, resulting in a bleaching or brightening effect on cotton fabrics [42]. This process is known as oxidation. Each glucose unit in cellulose contains a hydroxyl group (-OH) at position 6 on the glucose ring, and potassium permanganate can oxidize this alcohol group into an aldehyde or carboxylic acid. Aldehydes and carboxylic acids increase the polarity of cotton fibers and enhance their ability to interact with water, dyes, or other chemicals [43]. The cleavage of glycosidic bonds during this process can also lead to the breaking of the glycosidic bonds between glucose units in the cellulose molecule [44]. This cleavage reduces the cellulose chain length and decreases its molecular weight, which in turn weakens the structural integrity of the cotton fibers [45]. Element percentage after washing with potassium permanganate (KMNO₄) for cotton is shown in

Table 6. Element (%) cotton after acid washing with potassium permanganate (KMNO₄).

Sample Code	Element (%) After Acid Washing
	C	O	S	Si	Mn
EB/Si	64.03	32.4	3.24	0.26	0.12
EB/Si-NXT	63.45	32.01	4.05	0.09	0.39
EB/Si-Lig	72.04	20.63	5.36	1.76	0.09
EB/Si-Lig-NXT	72.66	23.55	3.18	0.51	0.21
Pumice stone	66.45	29.44	3.6	0.38	0.14

.

Table 6 shows that the addition of fillers increases the absorption capacity of Mn²⁺ for both silica and silica–lignin hybrid, which is influenced by the porosity and open-cell structure of the elastomeric balls. The Mn absorption capacity in silanized silica (Si-NXT) tends to be greater compared to that of silanized silica–lignin hybrid (Si-Lig-NXT). This is influenced by the interaction of the filler with the non-polar rubber; however, the silanization process enables this non-polar interaction to be overcome, as evidenced by the formation of Si–O–Si groups in the silica–lignin hybrid filler, confirmed by the FTIR spectra. Furthermore, the analysis indicates that the Elasto Ball exhibits a higher Mn²⁺ absorption capacity compared to pumice stone, particularly in the samples with silanized silica fillers (Si-NXT). This ability is related to the porous and open-cell structure of the Elasto Ball, which allows for better uptake of residual potassium permanganate (KMnO₄) oxidizing agents. Thus, the Elasto Ball not only functions as an abrasive medium but also contributes to reducing harmful chemical residues remaining in the washing process. This dual functionality makes the Elasto Ball more environmentally friendly than pumice stone, which mainly provides abrasive effects without contributing to the mitigation of chemical waste.

In addition, reusability tests were conducted up to 10 washing cycles to evaluate durability and color degradation performance. The results demonstrated that Elasto Balls consistently maintained superior fading efficiency compared to pumice stones, confirming their potential for repeated industrial application. However, the detailed cycle-by-cycle data will be presented in a subsequent publication. Regarding environmental aspects, while this work provides preliminary evidence of eco-friendliness through agricultural waste valorization and reduced chemical residues, comprehensive life cycle assessment (LCA) and biodegradability studies are planned for future research to strengthen the sustainability claims.

3.3.3. Analysis for Bursting Testing for Cotton Fabric

The acid washing process, particularly when using chemical agents like potassium permanganate (KMnO₄), can significantly impact the structural integrity and strength of cotton fibers. Through oxidation, KMnO₄ breaks down certain functional groups in the cellulose structure, weakening the fibers by cleaving glycosidic bonds and reducing the cellulose chain length [40]. This leads to a decrease in both tensile and bursting strength.

Based on the graph of breaking strength values, it can be seen that fillers have an impact on the breaking strength of knitted cotton fabric in Figure 10. The mechanical stress during the washing process can further compromise the fabric when combined with abrasive materials such as Elasto ball (made from natural rubber filled with silica or silica–lignin hybrids [41]. Abrasive particles in the Elasto ball cause surface abrasion, wearing down the cotton fibers and leading to a reduction in overall fabric strength. Additionally, acid washing alters the surface properties of cotton fabric, increasing its porosity and roughness. These changes reduce the fabric’s ability to withstand internal pressure, making it more susceptible to bursting. As the fabric becomes more porous and less cohesive, its structural integrity weakens, resulting in a lower bursting strength. The elastomeric balls treated with fillers and silanization show a significant reduction, with a decrease of up to ±30% for silica treated with NXT (3-Octanoylthio-1-propyltriethoxysilane). This is due to the interaction between natural rubber and silanized silica filler, as the silanol groups interact with the oxidizing agent KMnO₄ during the acid wash process, which can dissolve the color of the fabric [46]. Additionally, the use of this oxidizer can also damage the cotton fibers [47].

To statistically verify these results, a one-way ANOVA was performed and revealed a highly significant effect of filler treatment on the fabric’s breaking strength (F(6,21) = 97.74, p < 0.0001). Tukey’s post hoc test further confirmed that most groups differed significantly (p < 0.05). Specifically, fabrics treated with EB/Si-NXT, EB/Si-Lig, EB/Si-Lig-NXT, and pumice stone exhibited significantly lower breaking strength compared to unprocessed and blanko groups. The comparison between unprocessed and EB alone, however, was not statistically significant, suggesting that the addition of rubber without fillers does not markedly affect fabric strength. Meanwhile, pumice stone and silanized filler groups showed the most pronounced weakening effect, consistent with their stronger abrasive properties.

Therefore, the longer the contact time between the oxidizer in the elastoball and the fabric, the greater the reduction in strength, although it does not cause the fabric to develop holes. In this research, a washing ball/Elasto ball material has been successfully developed based on natural rubber and rice husk filler, which has pores and abrasive properties to substitute pumice in the garment washing process. The material is designed to be used in the textile industry, especially in the garment washing process, by offering advantages in terms of efficiency, durability, and environmental impact compared to existing conventional materials.

4. Conclusions

This study successfully developed Elasto Ball, a washing ball made from natural rubber reinforced with silica and silica–lignin hybrid fillers derived from rice husks, as a sustainable alternative to conventional pumice in the acid wash process of knitted cotton garments. The incorporation of a silane coupling agent improved filler–rubber compatibility, while the composite exhibited thermal stability up to 400 °C, moderate porosity (~5%), and achieved 40–50% fabric color reduction without causing hole defects.

This research also highlights the potential of agricultural waste (rice husk) valorization in developing eco-friendly washing materials, supporting UN Sustainable Development Goals (SDGs) such as Responsible Consumption and Production (SDG 12), Climate Action (SDG 13), and Industry, Innovation, and Infrastructure (SDG 9).

Performance tests of the Elasto Ball have been conducted up to 10 washing cycles, including the evaluation of reusability, bursting force, and color degradation in each cycle. However, the detailed results of these tests will be presented in a subsequent publication. Future work will focus on scaling up production, refining filler loading and silanization strategies, and assessing biodegradability to strengthen the sustainability profile and broaden applicability in the garment industry.