Enhancing antibacterial performance while maintaining natural rubber foam specifications: Is it possible?

Abstract

Using an antibacterial agent in rubber normally affects the material’s physical properties. This study assesses the possibility of improving the antibacterial activity of natural rubber (NR) foam while maintaining the physical characteristics of the sample. The physical properties were controlled based on the classifications stated in ASTM D1056. The antibacterial agent used in this work was Triclosan where it was varied from 0–0.8 phr in NR foams. The results found that increasing the content of Triclosan increased the inhibition zone of S. aureus and E. coli. Upon inclusion of Triclosan, there was little effect on the foam classification. At low content of Triclosan, i.e., 0.2 and 0.4 phr, the grade number was maintained as 2A3 grade. However, the grade was changed from 2A3 to 2A4 when adding Triclosan at 0.6 and 0.8 phr. It is clear that adding an antibacterial agent does influence the characteristics of the foam.

Keywords

natural rubber foam antibacterial Triclosan foam classification

Introduction

Rubber foam is a type of product that has a porous structure, which can be processed in both latex and dry rubber forms. Dry rubber foam is produced by using substances that generate bubbles or gases in the rubber, known as blowing agents.^1–3 The gas trapped in the rubber foam results in low density, light weight, flexibility, heat and sound insulation, and good impact resistance. Therefore, rubber foam is widely used in various industries. The cellular structure and porosity of the rubber foam make it easier for bacteria to thrive compared to non-porous materials, which is a significant reason why porous materials become a source of pathogens after some period of use.^4–6 Therefore, preparing rubber foam with antibacterial properties is an interesting area of research. The contamination of pathogens can impact human health These pathogens can spread from infected individuals directly or through contamination in the environment, such as soil, water, air, as well as adhering to floors, walls, surfaces, and various utensils.

The bacteria can be classified into two main groups, which are gram-positive and gram-negative bacteria. Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) are examples of bacteria that represent the gram-positive and gram-negative, respectively. These bacteria can be found easily in daily life. Both S. aureus and E. coli are bacteria that can cause serious health issues, particularly when they infect the human body. Proper hygiene, food safety, and antibiotic management are crucial in preventing infections caused by these harmful bacteria. To stop the bacterial colonization on the porous materials, an antibacterial agent must frequently be added to the material formulations. Besides, the primary considerations of using antibiotics in rubbers are compatibility, long-term stability, and tolerance under processing circumstances. Triclosan is one of the choices. Triclosan is a single-hydroxyl group trichlorinated diphenyl ether antibacterial agent.^7–9 Gram-positive and gram-negative bacteria, as well as numerous viruses and fungi, can be inhibited by Triclosan at low concentrations due to its broad-spectrum activity.¹⁰ Also, Triclosan has special properties enabling it to interact with a polar polymer in certain ways, with secondary bonding at the ether oxygen or the phenyl hydroxyl functional groups.

The use of Triclosan in polymeric materials is widespread. Prapruddivongs and Sombatsompop¹¹ found that using Triclosan in Polylactic acid (PLA) resulted in antimicrobial activity against E. coli, with inhibition rates of up to 83.4%. Zhang et al.¹² modified the surface of Polyvinyl chloride (PVC) with plasma treatment before coating it with Triclosan. This method also enhanced the antibacterial effectiveness of the PVC. Similarly, Asadinezhad et al.¹³ used Triclosan in PVC and chemically modified the polymer surface. The resulting product showed significant antibacterial efficacy. Based on the previous findings, Triclosan offers many advantages in polymeric materials. This has brought this interest in using Triclosan in formulating the antibacterial NR foam.

In fact, the use of an antibacterial agent does not only affect the antibacterial activity of the NR foam. It may influence the physical and mechanical characteristics of the foam. This would ruin the product design as the preparation of the NR foam must meet the criteria stated in the recognized standards. This study aimed to develop the antibacterial NR foam while maintaining the classification of foam properties based on ASTM D1056 requirements stated in ASTM D1056. Table 1 presents the fundamental properties required by ASTM D1056. This standard specifies key characteristics for classifying foam grades, including compression-deflection before and after thermal aging, as well as water absorption capacity. These criteria help differentiate between open and closed-cell foams, providing rubber compounders with a recognized framework for foam grade classification.

Table 1.

Basic requirement of cellular rubber (Closed-cell Sponge) according to ASTM D1056.

Grade number	Compression-deflection at 25% deflection (kPa)	Change in compression-deflection after aging at 70°C for 168 h (%)	Water absorption (%)
Grade number	Compression-deflection at 25% deflection (kPa)		Density over 160 kg/m³	Density of 160 kg/m³ or less
2A0	Less than 15	±30	5	10
2A1	15–35	±30	5	10
2A2	35–65	±30	5	10
2A3	65–90	±30	5	10
2A4	90–120	±30	5	10
2A5	120–170	±30	5	10

Remark: Type 2 is for Closed-cell rubber and Class A is for cellular rubber made from synthetic rubber, natural rubber, reclaimed rubber, or rubber-like materials where a number from 0–5 is based on a specific range of firmness as expressed by compression-deflection.

In this study, NR foams were produced by varying the concentration of Triclosan (0.2–0.8 phr) while maintaining a constant processing time and temperature. The effects of Triclosan content on the physical and mechanical properties were examined. The objective was to develop NRF that meets the fundamental requirements of ASTM D1056, the standard specification for flexible cellular materials such as sponge or expanded rubber. The evaluated physical properties included curing characteristics, relative foam density, compression-deflection before and after aging, and morphology. To date, no studies have extensively focused on balancing antibacterial agents with the physical properties of foams based on ASTM D1056. Additionally, this study introduces a new formulation for foam preparation, specifically incorporating processing oil and calcium carbonate to enhance the uniform distribution of the cellular structure. Processing oil helps reduce viscosity, facilitating gas diffusion, while calcium carbonate is widely recognized as a nucleating agent in foam production.¹⁴ The findings from this research will provide valuable insights for rubber foam manufacturers in optimizing the preparation of antibacterial NR foam.

Experimental methods

Materials

The NR used in this study was STR 5L, sourced from Yala Latex Industry Co., Ltd., Yala, Thailand. Triclosan was obtained from Chemipan Corporation Co., Ltd., Bangkok, Thailand. Azodicarbonamide (ADC), used as the blowing agent, was purchased from A.F. Supercell Co., Ltd., Rayong, Thailand. Treated distillate aromatics extract (TDAE oil) was supplied by H&R ChemPharm (Thailand) Co., Ltd. Calcium carbonate (CaCO₃) was acquired from Krungthepchemi Co., Ltd., Bangkok, Thailand, while stearic acid was sourced from Imperial Chemical Co., Ltd., Bangkok, Thailand. Zinc oxide (ZnO) was obtained from Global Chemical Co., Ltd., Samut Prakan, Thailand. N-Cyclohexyl-2-benzothiazole sulfenamide (CBS) was supplied by Flexsys America L.P., West Virginia, USA, and sulfur was procured from Siam Chemical Co., Ltd., Samut Prakan, Thailand.

Preparation of NR foam

Table 2 provides a list of materials used in the initial stage of NRF compounding. According to the Food and Drug Administration (FDA) guidelines, Triclosan is suggested to be used at a concentration of up to 0.3 wt%, which is equivalent to 0.47 phr. This has made this study to vary its content from 0–0.8 phr. The entire amounts of NR, Triclosan, ADC, TDAE oil, and other additives were mixed using a two-roll mill. The compounds were then free-blown into specific shapes with a thickness of 12.5 mm and compression-molded at 150°C. The processing time was determined based on the curing times measured by a moving-die rheometer (MDR), as detailed in the following section.

Table 2.

Formulation of NR foams prepared by various Triclosan content.

Rubber ingredients	Amount (phr)
STR 5L	100
Stearic acid	1
Zinc oxide	5
TMQ	1
ADC	6
TDAE oil	10
CBS	1
CaCO₃	30
Triclosan	0–0.8
Sulfur	2.5

Measurement of the curing characteristics

The curing characteristics of NR compounds were analyzed using a moving-die rheometer (MDR) (Rheoline, Mini MDR Lite) at 150°C. This analysis measured key parameters, including torque, scorch time (ts₁), curing time (tc₉₀) and cure rate index (CRI), in accordance with ASTM D5289 standards. The CRI is calculated according to equation (1).

C R I = \frac{100}{({t c}_{90} - {t s}_{1})}

(1)

Measurement of Mooney viscosity

The viscosity of NR compounds were evaluated using a Mooney viscometer, MV 3000 Basic (MonTech). The tests were conducted at 100°C with a large rotor, following the ASTM D1646 standard.

Measurement of relative foam density and expansion ratio

The relative foam density and expansion ratio were among the physical characteristics that were examined. Equation (1), as shown below, was used to determine the relative foam density following ASTM D3575.

R e l a t i v e f o a m d e n s i t y = \frac{D_{f}}{D_{c}}

(2)

where D_f is foam density (g/cm³) and D_c (g/cm³) is compound density.

Equation (2) displays the expansion ratio, which was computed as the density of the NR compound divided by the density of the NR foam.

E x p a n s i o n r a t i o = \frac{D_{c}}{D_{f}}

(3)

Measurement of hardness

The hardness of the NRF was measured using an indentation durometer Shore OO, following ASTM D2240, with readings taken after a 10-s indentation.

Measurement of compression-deflection before and after thermal aging

The compression-deflection test was conducted in accordance with ASTM D575. Specimens with a minimum area of 161 cm² and an approximate thickness of 12.5 mm were used for testing. A universal testing machine (Tinius Olsen, H10KS) compressed the samples between parallel metal plates until their thickness was reduced by 25%, at which point the load was immediately recorded. Each specimen was tested repeatedly until the load variation did not exceed 5%. To evaluate compression-deflection changes due to thermal aging, comparable test specimens were cut and subjected to 168 hours in air-circulating oven at 70°C. After aging, the samples were cooled for at least 24 hours before their compression-deflection properties were reassessed. The change in compression-deflection was calculated using equation (4).

P = (\frac{A - O}{O}) \times 100

(4)

where P is the change in compression-deflection (%), O is the original compression-deflection and A is the final compression-deflection after oven ageing.

Compression set

The compression set was measured in accordance with ASTM D395. A standard sample with a minimum area of 161 cm² and an approximate thickness of 12.5 mm was used for testing. The specimens were compressed to 50% of their original thickness. After 22 h, the load was removed, and the final thickness was measured at room temperature after 24 h. The compression set calculation was as follows;

C o m p r e s s i o n s e t (%) = [\frac{(t_{0} - t_{1})}{(t_{0} - t_{s})}] \times 100

(5)

where t₀ is the original thickness, t₁ is the thickness of the specimens after the specified recovery period and t_s is the thickness of the spacer bar used.

Water absorption

For this test, circular test specimens were selected, each approximately 12.5 mm thick with an area of 2500 mm². The samples were fully submerged in distilled water at room temperature for 3 min. Afterward, they were removed and blot-dried. The water absorption was then calculated using equation (6).

W a t e r a b s o r p t i o n = (\frac{A - B}{B}) \times 100

(6)

where A and B are the mass after and before immersing in water.

Scanning electron microscopy

A FEI Quanta™ 400 FEG scanning electron microscope (SEM) was used to analyze the materials’ morphology. A thin coating of gold/palladium was applied to each specimen to remove any charge accumulation during imaging,

Antibacterial performance

Study of the antibacterial efficacy of NR Foam incorporated with Triclosan using the Halo Test method. The bacteria used for testing include Gram-positive bacteria Staphylococcus aureus (S. aureus, TISTR 512) and Gram-negative bacteria Escherichia coli (E. coli, TISTR 746). The halo test method is a qualitative evaluation of antibacterial activity. This test depends on the ability of the antimicrobial agents to diffuse on an agar medium. The testing procedure begins by introducing a bacterial concentration of 1 × 10⁸ colonies per milliliter (CFU/mL), applying 100 μL uniformly onto the culture medium. Then, a foam sample with a diameter of approximately 6 mm is placed at a designated position and incubated at 37 ± 0.5°C for 24 h. The antibacterial radius is then measured.

Results and discussion

Mooney viscosity

The relationship between Mooney viscosity (ML 1 + 4 at 100^oC) over the Triclosan content is shown in Figure 1. It was found that the Mooney viscosity of the rubber compound increased slightly with the addition of Triclosan, with values ranging from 9.8 to 10.5, respectively. This is because Triclosan acts as a filler in the rubber. However, the use of Triclosan is considered quite low, so it did not significantly alter the Mooney viscosity of rubber compounds.

Figure 1.

Mooney viscosity as a function of Triclosan content of NR compounds.

Curing characteristics

Figure 2 shows the rheometric curves of the NR compounds prepared by various Triclosan content. While the raw outputs obtained from rheometric curves are listed in Table 3. Increasing the Triclosan content increased the scorch time (ts₁) and curing time (tc₉₀), with ts₁ and tc₉₀ ranging from 1.57 to 2.23 min and 12.35 to 13.68 min, respectively. This is likely due to the hydroxyl group (-OH) in the structure of Triclosan, which leads to the absorption of the accelerator. Triclosan is considered to be non-polar. However, the structure of triclosan can switch to a more polar alternative by rotation of the ether bond in triclosan.

Figure 2.

Rheometric curves of NR foam prepared by various Triclosan content.

Table 3.

Cure characteristics of NR foam prepared by various Triclosan content.

Triclosan content (phr)	M_H (dN.m)	M_L (dN.m)	M_H-M_L (dN.m)	t_S1 (min)	t_C90 (min)	CRI (min⁻¹)
0	28.70	1.58	27.12	2.35	15.48	7.62
0.2	28.15	0.91	27.24	1.57	12.35	9.28
0.4	28.68	1.11	27.57	2.02	12.33	9.70
0.6	28.94	1.24	27.70	2.10	13.66	8.65
0.8	29.81	1.03	28.78	2.23	13.68	8.73

Moreover, both the ts₁ and tc₉₀ were found to be the lowest with the addition of 0.2 phr Triclosan. This may be attributed to the competitive interaction between the curing and foaming processes observed during the rheometer test. At this concentration, the minimum torque (M_L) was also the lowest, indicating reduced initial viscosity of the rubber compound. The lower viscosity likely facilitated gas expansion, enhanced heat transfer, and consequently contributed to the reduction in both ts₁ and tc₉₀ values.

Peterson et al.⁷ have reported that triclosan can be compatible with a polar polymer due to steric hindrance. The secondary bonding made possible by the ether oxygen and phenyl hydroxyl functional groups accounts for most chemical interactions. Triclosan thus possesses the mechanomolecular ability to rapidly adjust to various polarity conditions. Therefore, it is predicted that the absorption of the accelerator may occur over the addition of Triclosan, leading to longer vulcanization times. The cure rate index (CRI) is calculated based on the ts₁ and tc₉₀, where a higher CRI indicates a faster curing rate. Based on the results, the CRI agreed well with the tc_90, where longer curing rates were found over the addition of Triclosan. When considering the maximum torque (M_H), minimum torque (M_L), and torque difference (M_H – M_L), it was found that the torque did not change. The main influences that can change the torque value are the crosslink density, rubber-filler interaction and hydrodynamic effect. In this case, Triclosan was added at a very small amount which did not affect the rheometric torques.

Physical properties

The physical properties of the foam are the key factors considering the processability of the porous materials. Table 4 lists the compound density, relative foam density, and expansion ratio of NR foams prepared with various Triclosan content. It was observed that Triclosan resulted in a slight increase in relative foam density. As mentioned previously, the Triclosan slightly increased the Mooney viscosity of the rubber compounds, and the slight increase in viscosity reduced the expansion of gas bubbles in the rubber matrix. As a result, the relative foam density increased when using Triclosan. Triclosan reduced the gas phase, causing the thicker cell walls. Therefore, the relative foam density increased. Furthermore, the relative foam density of the foam rubber is directly related to the expansion ratio of the foam. As the amount of Triclosan is increased, it resulted in a reduction in the expansion ratio of the foam rubber. This is responsible for the difficulty of gas bubbling in high-viscosity rubber compounds.

Table 4.

Foam density, compound density, relative foam density, and expansion ratio, hardness, and compression set of NR foams prepared by various Triclosan content.

Triclosan content (phr)	Compound density (g/cm³)	Foam density (g/cm³)	Relative foam density	Expansion ratio
0	1.04 ± 0.01	0.60 ± 0.04	0.58	1.74
0.2	1.04 ± 0.01	0.60 ± 0.02	0.58	1.72
0.4	1.04 ± 0.00	0.61 ± 0.02	0.58	1.71
0.6	1.04 ± 0.00	0.62 ± 0.01	0.59	1.68
0.8	1.05 ± 0.00	0.63 ± 0.02	0.60	1.66

Figure 3 shows the SEM images of NR foams prepared with various Triclosan content. It is seen that Triclosan has a little bit of effect on the cellular structure of the foam. The morphology of the NR foams is explained together with the average cell size and cell density shown in Table 5. The cell morphology of the sample is seen in relatively spherical cells, with cell sizes ranging from 0.232 to 0.247 mm and cell densities between 1.29 × 10⁴ and 1.27 × 10⁴ cells/cm³ when Triclosan was added at 0.2 and 0.8 phr, respectively. This finding agreed well with the previous Mooney viscosity and relative foam density. Triclosan reduced the gas phase, causing the thicker cell walls. Therefore, the cell size reduced over the addition of Triclosan.

Figure 3.

SEM images at 50x magnification of NR foams prepared by various Triclosan content.

Table 5.

Cell density, average cell size and hardness of NR foams prepared by various Triclosan content.

Triclosan content (phr)	Average cell size (mm)	Cell density (cell/cm³)	Hardness (shore OO)
0	0.258 ± 0.10	1.27 × 10⁴	60 ± 1.51
0.2	0.235 ± 0.09	1.33 $\times$ 10⁴	60 ± 0.50
0.4	0.236 ± 0.09	1.34 $\times$ 10⁴	60 ± 0.46
0.6	0.235 ± 0.08	1.30 $\times$ 10⁴	61 ± 0.12
0.8	0.238 ± 0.07	1.33 $\times$ 10⁴	60 ± 0.35

The hardness indicates how the NR foam responds to the indentation test. The result shows that Triclosan did not modify the hardness of the sample (see Table 5). This is because the amount of Triclosan used in this study was very small, the amount of Triclosan did not affect the response of indentation while testing, making the hardness unchanged over the addition of Triclosan.

Water absorption

Water absorption is one of the key properties of the NR foam, as the cellular material is likely to absorb water. Figure 4 shows the effect of Triclosan content on the water absorption of NR foam. The water absorption values remained unchanged over the addition of Triclosan. The water absorption of the cellular foam depends on the cellular structure of the foam. The foam should have more open-cell structures to acquire water diffusibility. In this case, there was no significant change in the cellular structure of the NR foams. Hence, the use of Triclosan did not alter the water absorption of the NR foam. Apart from that, the water absorption still does not exceed the limit value as referred to in ASTM D1056. In this first stage of evaluation, Triclosan does not affect the classification of the foam based on the ASTM D1056.

Figure 4.

Water absorption of NR foams prepared by various Triclosan content.

Compression set, compression-deflection, and foam classification

The compression set of the NR foams prepared by various Triclosan content is shown in Figure 5. Triclosan has caused to a slight increment of the compression set, i.e., from 4.01%–4.24%, respectively. This suggests that the addition of Triclosan to the NR foam does not significantly affect its compression set. The set property correlates well with the crosslink density of the rubber vulcanizates. Higher crosslink density indicates good recovery after a certain strain is applied to the sample. It can be said that Triclosan did not alter the cross-linking of the foam, and thus, it did not influence the set property of the NR foam. This can be correlated to the torque difference (M_H–M_L) that was earlier reported in Table 3. The torque difference is indirectly related to the degree of crosslink density. Higher torque difference can relate to a higher degree of crosslinking. Here, there is no difference in this value. It can be assumed that Triclosan did not alter the cross-linking of the foam.

Figure 5.

Compression set of NR foam prepared by various Triclosan content.

The compression-deflection before and after thermal aging is listed in Table 6. The compression-deflection of the NR foams increased over the addition of Triclosan. According to previous results, Triclosan has little effect on most results, but not for the compression-deflection. This is quite surprising. The reasons can be explained in two different aspects. Firstly, it is likely due to differences in testing methods. The compression-deflection was tested by placing the sample between two metal plates and applying compressive force. The sample was subjected to force along its entire surface. This is unlike other tests where there is no full compressive contact with the sample. This may be responsible to the increment of compression-deflection. Secondly, Triclosan may play an important role in the significant change of compression-deflection of NR foams. According to previous research by Petersen,⁷ Triclosan has specific properties that allow it to interact with both polar and non-polar polymers. This is due to the structure of Triclosan, which can arrange itself in different configurations depending on the rotation angle or after dispersion in the polymer. This may increase the compression-deflection.

Table 6.

Compression-deflection, change in property, and the grade number of NR foam prepared by various Triclosan content.

Triclosan content (phr)	Compression deflection (kPa)		Change in property (%)	Grade number
Triclosan content (phr)	Before ageing	After ageing	Change in property (%)	Grade number
0	68.11 ± 4.05	85.33 ± 1.88	25.28	2A3
0.2	86.44 ± 3.60	108.80 ± 0.46	25.87	2A3
0.4	88.39 ± 2.43	111.48 ± 0.84	26.12	2A3
0.6	90.39 ± 1.79	114.12 ± 3.64	26.25	2A4
0.8	93.84 ± 2.17	118.62 ± 3.93	26.42	2A4

Additionally, Peterson⁸ also simulated the behavior of Triclosan from their computational simulation. It was found that the rotation of the Triclosan structure increased the bending force required in proportion to the amount of Triclosan. After rotating the chemical structure of Triclosan at various angles, it was found that Triclosan exhibited specific characteristics in both non-polar and polar segments, showing distinct behaviors at different rotating angles. This could be the possible reason that Triclosan plays an important character in promoting the compression-deflection values. Moreover, the values of compression-deflection before and after thermal aging remained the same trend over the addition of Triclosan. The values after thermal aging are higher than those of the samples before thermal aging. This is because the polymer backbones experienced the chain scissions, where the obtained radicals recombined with each other, making it increase the stiffness of the samples.

According to the foam specifications outlined in ASTM D1056 (see Table 1), the grading of foam rubber is also based on the compression-deflection before and after thermal aging. As been mentioned, Triclosan affected the compression-deflection of the NR foam, so, there are two grades were classified for such foams. The control sample and the NR foams containing 0.2 and 0.4 phr are classified as grade 2A3 whereas 2A4 grade was for the NR foams containing 0.6 and 0.8 phr of Triclosan. Adding Triclosan to NR foam obviously altered the foam classification, where two grades were classified. In addition to this, the change in compression-deflection value is also reported in Table 6. The change in such value slightly increased over the addition of Triclosan. The thicker cell walls in the case of NR foams containing Triclosan may be responsible for enhancing the degradation process, as more rubber phase is presented in such foams. As a result, the values after thermal aging were higher for NR foams containing more Triclosan.

Antibacterial performance

This section studied the effect of Triclosan content on the antibacterial performance of the NR foams. S. aureus and E. coli represent gram-positive and gram-negative bacteria, respectively. Table 7 lists the inhibition zone of NR foams against S. aureus and E. coli. The images captured from their clear zone are also shown in Figure 6. The use of Triclosan clearly exhibited antibacterial activity against S. aureus and E. coli, where the result is clear when compared to the samples without Triclosan. Triclosan can inhibit S. aureus rather than E. coli. This finding agreed well with the report of McDonnell and Russell,¹⁵ who stated that Triclosan is more effective against S. aureus than E. coli. This is because Triclosan is more effective against Gram-positive bacteria.

Table 7.

The average diameter of inhibition zone of NR foam prepared by various Triclosan content.

Triclosan content (phr)	Average diameter of inhibition zone (mm)
Triclosan content (phr)	S. aureus	E. coli
0	No inhibition	No inhibition
0.2	17.0	4.0
0.4	19.0	9.5
0.6	21.0	12.5
0.8	23.0	13.0

Figure 6.

Images captured from the inhibition zone of NR foams against S. aureus and E. coli.

Additionally, gram-positive and gram-negative bacteria have different cell wall structures and compositions, which affect the permeability of Triclosan into the bacterial cells. For S. aureus, the cell wall primarily consists of peptidoglycan, which is 20–80 nm in thickness and makes up about 90% of the dry weight of the bacteria.^16,17 Peptidoglycan forms a layered network that strengthens the cell but does not prevent the passage of substances into or out of the cell. Therefore, Triclosan can effectively inhibit S. aureus. In the case of E. coli, it has a complex outer membrane (OM) made up of phospholipids and lipopolysaccharides (LPS),^18,19 which serve as a protective barrier preventing foreign substances from entering the cell. The LPS component, which is unique to Gram-negative bacteria, consists of three parts, as shown in Figure 7: the O-antigen (or polysaccharide), which is composed of repeating units of several types of sugars and coats the outer membrane; the core oligosaccharide (core-OS), which carries a highly negative charge; and lipid A, which is located on the outside of the outer membrane and is hydrophobic due to its fatty coating. This structure of LPS causes the hydrophobic Triclosan molecules to spread throughout the outer membrane layer, preventing them from passing into the E. coli bacterial cell. As a result, the antibacterial effectiveness of Triclosan is not sufficient to effectively inhibit E. coli as it does with S. aureus.

Figure 7.

Schematic illustration representing the killing efficiency of Triclosan against S. aureus and E. coli.

The antibacterial mechanism of Triclosan varies with its concentration. At low concentrations, Triclosan primarily affects the cytoplasmic membrane of the bacteria.¹⁵ Due to its hydrophobic nature, Triclosan accumulates in the head group of the phospholipid membrane. This accumulation destabilizes the bacterial cell membrane, affecting bacterial cell division. In addition to this, the chemical structure of Triclosan can directly interact with Enoyl-acyl, an enzyme crucial for fatty acid synthesis, which is a major energy source for bacteria. This interaction makes fatty acid synthesis more difficult.^20,21 Furthermore, the initial antimicrobial action of Triclosan targets ribonucleic acid (RNA) and protein synthesis in bacteria.²² Inhibiting RNA synthesis disrupts the transcription process, preventing the formation of messenger RNA (mRNA), which is essential for protein synthesis. Without mRNA, protein synthesis is halted.²³ Therefore, the use of Triclosan at low concentrations limits bacterial growth. At high concentrations, Triclosan penetrates the bacterial cell wall, inhibiting protein synthesis.²² This results in bacterial membrane leakage and the loss of intracellular components, which halts metabolic processes within the cell, ultimately leading to bacterial cell death.^7,8

Antibacterial performance against different sample surfaces

The sample prepared in this experiment was in the cellular structure where the main objective is to perform the test only for the porous sample. However, the process of rubber foam requires compression molding to vulcanize and generate gas in the foam. Therefore, there is a smooth surface on the surface compressed by the compression molding. In this experiment, both smooth and porous surfaces were also taken into consideration. Table 8 lists the inhibition zone of NR foams sampling from smooth and porous surfaces. The images captured from the clear zone are also shown in Figure 8. The results show no significant difference in antibacterial ability between the smooth and porous surfaces of the NR foam. This is a clear indication that Triclosan can effectively inhibit the bacteria regardless of the sample’s surfaces. It can be applicable to NR foams on any form of surface.

Table 8.

The inhibition zone of NR foams sampling from smooth and porous surfaces.

Antibacterial agent	Average diameter of inhibition zone (mm)
Antibacterial agent	S. aureus	E. coli
Smooth surface	21.0	13.0
Porous surface	23.0	13.0

Figure 8.

Images captured from the inhibition zone of NR foams sampling from smooth and porous surfaces against S. aureus and E. coli.

Conclusion

This study aimed to develop the antibacterial NR foam while considering the foam classifications based on ASTM D1056. Surprisingly, Triclosan did affect the antibacterial performance of the foam but provided significant changes in the foam grades. Increasing the Triclosan content increased the inhibition zone of bacterial growth against S. aureus and E. coli. The clear zone was seen to enhance from 0–23.0 mm and 0–13.0 mm against S. aureus and E. coli, respectively. The compression deflection of the NR foams increased over the addition of Triclosan. Triclosan affected the compression deflection of the NR foam by offering two grade numbers, namely 2A3 and 2A4. The control sample and the NR foams containing 0.2 and 0.4 phr are classified as grade 2A3, whereas 2A4 grade was for the NR foams containing 0.6 and 0.8 phr of Triclosan. This clearly answers the question asked in this manuscript’s title where Triclosan affected the classification of the NR foam. When considering the amount of Triclosan highly suggested by the Food and Drug Administration (FDA) guidelines, Triclosan is allowed to be used at a concentration of up to 0.3 wt%, equivalent to 0.47 phr. Therefore, it can be concluded that Triclosan is suggested to be used no more than 0.4 phr to balance the mechanical properties, foam classification, and antibacterial activity as well as to meet the requirements of the FDA. With consideration of both our research findings and FDA input, this antibacterial NR foam can be prepared in accordance with the 2A3 grade requirements.

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 disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: We gratefully acknowledge the financial support by Natural Rubber Innovation Research Institute, Prince of Songkla University through the Grant No. SAT6201175S.

ORCID iD

Nabil Hayeemasae

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