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

Foaming temperature and grade of dry natural rubber were varied to evaluate their effects on the morphology and mechanical properties of natural rubber (NR) foams. Three different grades of NR were used; namely ENR‐25, SMR‐L, and SMR‐10. NR foams from these grades were produced at three different foaming temperatures, i.e. 140, 150, and 160°C. The study was carried out using formulated compositions containing sodium bicarbonate as the chemical blowing agent and were expanded using conventional compression molding technique via a heat transfer foaming process. The NR foams were characterized with respect to their relative foam density, density of crosslinking, cell size, compression stress, and compression set. Increase in foaming temperature resulted in lower relative density and larger cell size. It was also discovered that the crosslink density slightly decrease with increasing foaming temperature. For mechanical properties, the highest foam density resulted in the highest compression stress. Compression stress at 50% strain increased with increasing foaming temperature and ENR‐25 foam has the highest compression stress among the produced foams. The results showed that the morphology, physical, and mechanical properties of the rubber foams can be controlled closely by the foaming temperature and rubber grades. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Natural rubber foams were made with a one-step foaming process, the effect of density, expansion ratio, and closed-cell properties via microscope were investigated. This research aims to investigate novel half vulcanization and full vulcanization process with three various blowing agents by compression moulding to fabricate natural rubber and carbon black foams. Closed cells of natural rubber used as shoe soles were formed by azodicarbonamide (ADC) and organic blowing agents. The experimental design used three blowing agent of Azodicarbonamide (ADC) A, B C, and control. The data analysis for images was performed in ImageJ software to get more quantitative results. Experimental data were created to perform a full (AF; BF; and CF) and half vulcanization (AH; BH; and CH) process. The different structure model based on digital microscope images was shown. The AH, BH, and CH with half vulcanisation process revealed a higher expansion ratio and cell number value. The microscope showed that rubber foam with the highest expansion ratio and cell number has a more homogenous structure with more cell connection of AH, causing a lower density value of 0.6480. In addition, AH and CH with ADC showed higher values than BH with an organic blowing agent, respectively.

Natural rubber (NR) foam can be prepared by the Dunlop method using concentrated natural latex with chemical agents. Most previous studies have focused on the thermodynamic parameters of solid rubber in extension. The main objective of this study is to investigate the effect of the NR matrix concentration on the static and dynamic properties of NR foams, especially the new approach of considering the thermodynamic aspects of NR foam in compression. We found that the density and compression strength of NR foams increased with increasing NR matrix concentration. The mechanical properties of NR foam were in agreement with computational modelling. Moreover, thermodynamic aspects showed that the ratio of internal energy force to the compression force, Fu/F, and the entropy, S, increased with increasing matrix concentration. The activation enthalpy, ∆Ha, also increased with increasing matrix concentration in the NR foam, indicating the greater relaxation time of the backbone of the rubber molecules. New scientific concepts of thermodynamic parameters of the crosslinked NR foam in compression mode are proposed and discussed. Our results will improve both the knowledge and the development of rubber foams based on the structure–properties relationship, especially the new scientific concept of the thermodynamical parameters under compression.

The sound absorbing efficiency of natural rubber (NR) foam is affected by the cell morphology of foam. Potassium oleate (K-oleate) and sodium bicarbonate (NaHCO3) were used as blowing agents to create open-cell foam. Amounts of the blowing agent were varied from 0.5 to 8.0 part per hundred of rubber (phr) to evaluate cell size and number of foam cell as well as sound adsorption coefficient of NR foam. The NR foam specimens were prepared using mould and air-circulating oven for vulcanizing and foaming processes. The results indicated that K-oleate at 2.0 phr and NaHCO3 at 0.5 phr led to form NR foam with the smallest cell size and the largest number of foam cell. At low frequencies, the optimum sound adsorption coefficient of NR foam was caused by filling K-oleate 2 phr. However, that of NR foam at high frequencies was provided by 0.5 phr-NaHCO3 addition.

In the past decades, natural rubber (NR) foams became popular in the automotive, construction and aerospace industries because of their lightweight, flexibility and shock-absorbing properties. The selection of optimal formulation and processing parameters is critical to produce foam with specific properties depending on the application. In this study, the effect of foaming agent concentration, foaming temperature and time on the morphological and mechanical properties of NR foams was investigated. First, increasing the foaming agent content from 5 to 9 phr (parts per hundred rubber) increased the cell size (16%), while decreasing the compression modulus (28%). In the second part, increasing the foaming temperature (145 to 155°C) resulted in larger cell size (163%); while decreasing the cell density (28%), compression modulus (2%), and hardness (1%). In the third part, increasing the foaming time (25 to 45min) led to smaller cell size (63%) combined with higher cell density (100%), compression modulus (16%), and hardness (3%). Based on all the results obtained, the best NR foam was obtained with 7 phr of foaming agent and produced at 150°C for 35min leading to superior morphological and mechanical performance: the smallest cell size (25µm) and the most uniform cell size distribution (Đ = 1.03) generating the highest compression modulus (3.36MPa). Finally, the experimental compression results were combined to build a nonlinear regression model to optimize the formulation and processing conditions leading to 6.5 phr of OBSH molded at 150°C for 36min. The model showed good agreement with a validation test with less than 2% deviation observed for both compression modulus and strength.

Natural rubber foam (NRF) can be prepared from concentrated natural latex, providing specific characteristics such as density, compression strength, compression set, and so on, suitable for making shape-memory products. However, many customers require NRF products with a low compression set. This study aims to develop and prepare NRF to investigate its recoverability and other related characteristics by the addition of charcoal and silica fillers. The results showed that increasing filler loading increases physical and mechanical properties. The recoverability of NRF improves as silica increases, contrary to charcoal loading, due to the higher specific surface area of silica. Thermodynamic aspects showed that increasing filler loading increases the compression force (F) as well as the proportion of internal energy to the compression force (Fu/F). The entropy (S) also increases with increasing filler loading, which is favorable for thermodynamic systems. The activation enthalpy (∆Ha) of the NRF with silica is higher than the control NRF, which is due to rubber–filler interactions created within the NRF. A thermodynamic concept of crosslinked rubber foam with filler is proposed. From theory to application, in this study, the NRF has better recoverability with silica loading.

This study explores the effect of internal gas pressure (P) on closed-cell natural rubber (NR) foams. Three key factors are analyzed using a 3D model during uniaxial compression: (1) the initial gas pressure (P0 = 1, 2, and 3 atm) inside the cells, (2) different cell sizes (D = 0.1, 0.2, 0.3, and 0.4 mm in diameter), and (3) the presence of defects (holes in the cell walls) in terms of their sizes (d = 0.07 to 0.1 mm). The findings reveal a negative relationship between the initial gas pressure and the relative internal gas pressure (α = P/P0) and a direct correlation with stress during compression. For instance, a change from 1 to 3 atm of the initial internal gas pressure results in a 158% decrease in α with only a 3% increase in stress. Larger cell sizes contribute to higher α but lower stress levels during compression. Changing the cell size from 0.1 to 0.4 mm generates a 27% increase in α but a 45% drop in stress. An analysis of hole sizes (cell connection) indicates that larger holes result in higher relative internal gas pressure, while smaller holes lead to higher stress levels because of more flow restriction. For example, increasing the hole size from 0.07 to 0.1 mm leads to an 8% higher α but a 32% stress reduction. These findings highlight the significant effect of the internal gas pressure inside the cells in determining the mechanical properties of rubber foams, which are generally neglected. The results also provide useful insights for better material design and different industrial applications. This study also generates predictive models to understand the relationships between stress, strain, initial gas pressure, cell size, and defects (holes/connections), enabling the production of tailor-made rubber foams by controlling their mechanical behavior.

Deformation Behaviour of Single and Gradient Density Natural Rubber Foams

Natural rubber composite foam with carbon such as carbon black (CB), carbon synthesized from durian bark (CDB), graphite (GPT), graphene oxide (GO), graphene (GPE) and multi-walled carbon nanotubes (MWCNT) was studied in this work to investigate the relationship between foam formation during decomposition of chemical blowing agent mechanism and crosslink reaction of rubber molecules by sulphur. Natural rubber composite foam with carbon particle was set at 3 parts per hundred of rubber (phr) to observe the effect of carbon allotropes on foam formation with different microstructure and properties of natural rubber composite foam. The balancing of crosslink reaction by sulphur molecules during foam formation by the decomposition of chemical blowing agent affects the different morphology of natural rubber foam/carbon composites leading to the different mechanical and thermal properties. The result showed the fastest cure characteristics of natural rubber foam with 3 phr of graphene (NRF-GPE3) which was completely cure within 6.55 minutes (tc90) measured by moving die rheometer resulting in the smallest bubble diameter among other formulas. Moreover, natural rubber foam with 3 phr of MWCNT (NRF-MWCNT3) had the highest modulus (0.0035 ± 0.0005 N/m2) due to the small bubble size with high bulk density. In addition, natural rubber foam with 3 phr of GPT (NRF-GPT3) had the highest thermal expansion coefficient (282.12 ± 69 ppm/K) due to high amount of gas bubbles inside natural rubber foam matrix and natural rubber foam with 3 phr of GO (NRF-GO3) displayed the lowest thermal conductivity (0.0798 ± 0.0003 W/m.K) which was lower value than natural rubber foam without carbon filler (NRF). This might be caused by the effect of bubble diameter and bulk density as well as the defect on surface of graphene oxide compared to others carbon filler.

In this work, self‐healing natural rubber (SHNR) foam incorporating an intrinsic zinc thiolate ionic network was successfully prepared. The materials exhibited the ability to autonomously repair damage at room temperature without the need for external triggers. The investigation focused on the effect of sodium bicarbonate, employed as a blowing agent, on the self‐healing performance, as well as the physical and mechanical properties of the foam. Various concentrations of sodium bicarbonate (0, 1, 4, 8, and 10 phr) were employed. The conventional two‐roller mill was used for mixing and compounding, while compression molding was utilized for the vulcanization process. With increasing sodium bicarbonate concentration, the density, tensile strength, elongation at break, and compression set of the self‐healing NR foam were found to decreased. Conversely, the porosity, shrinkage, compressive strength, and water uptake of the SHNR foam increased as the concentration of sodium bicarbonate increased. Scanning electron microscopy analysis revealed that the optimal concentration of sodium bicarbonate (8 phr) resulted in smaller, finer, and more uniform porous structures. The self‐healing rubber foam incorporating 8 phr sodium bicarbonate exhibited improved properties in terms of tensile modulus, elongation at break, and tear strength, with healing efficiencies of 91.27%, 69.39%, and 83.99%, respectively.

Natural rubber foam (NRF) was produced by chemical blowing technique with compression molding process. This process still has a problem with oversized dimension after removing specimen from the mold. The effect of azodicarbonamide which is the chemical blowing agent, sulfenamide accelerators and surface treated graphene with cyclohexyl diamine via diazonium reaction were investigated to solve this problem and improve properties of NRF specimen. Although the presence of azodicarbonamide can accelerate sulfur vulcanization due to amine derivatives from thermal decomposition of chemical blowing agent, NRF with high content of azodicarbonamide reduces the m order of autocatalytic reaction which means low crosslink of rubber molecules. Moreover, NRF at 4 phr of azodicarbonamide shows the smallest bubble diameter with good properties. The different chemical structures of sulfenamide accelerators are also studied. NRF with N-cyclohexyl benzothiazole-2-sulfenamide (CBS) system reveals the fastest sulfur vulcanization rate resulting in the smallest bubble diameter with narrow size distribution and lowest thermal expansion coefficient. This system also has the lowest activation energy (Ea) among other rubber foams with sulfenamide accelerators owing to high basicity from high stability of amine species after this accelerator forms as a complex species with other ingredients in sulfur vulcanization system. The last factor that should be concerned is the presence of graphene and surface treated graphene with cyclohexyl diamine via diazonium reaction. The higher graphene content in rubber matix, the faster sulfur vulcanization rate is obtained as a result of the remaining oxygen functional group on surface of graphene. Furthermore, surface treatment of graphene improved dispersion in rubber matrix. It also shows the slowest sulfur vulcanization rate due to the reaction between cyclohexyl diamine and oxygen functional groups on the graphene surface. NRF with 3 phr of treated graphene has the highest tensile strength at break due to good dispersion of graphene particle and low cell density in natural rubber foam product.

Natural rubber foam is used in industries as a gasket and insulation product. Dimensional expansion of specimen is the crucial problem of rubber foam specimen, especially in the automotive parts assembly. This work aims to understand the behavior of natural rubber foam (NRF)/carbon composites on microstructure and properties of NRF before being used. Carbon was synthesized from durian bark which was the agricultural waste that had the potential for using as reinforcing filler. The result showed the fast sulfur vulcanization rate with small bubble size at high amount of carbon. In addition, the young modulus of NRF/carbon composites increased with increasing carbon content.

This study investigated the properties of the natural rubber (NR) foam filled with azodicarbonamide (ADC) blowing agents by combination to various ratios of epoxidized NR (ENR) for flexible foam applications.Compound operation was prepared with an open two-roll mill and the production was fabricated by compression molding.The study elucidated properties related to crosslinking behaviors, mechanical and dynamic properties, elasticity, abrasion, weathering resistance, and sound absorption efficiency.The ENR and ADC concentrations affected the tensile testing and also the durability properties of the NR/ENR.The NR and ENR foam of 60/40 filled with 10 phr of ADC demonstrated good properties across various parameters, showing acceptable tensile properties, abrasion resistance, and QVA light resistance.Additionally, the presence of a closed-cell structure in the blends reduced crack propagation in the NR matrix during aging, improving weathering resistance.The absorption coefficient increased with higher ADC content, being optimal at 15 phr, due to the lower density and higher porosity of the opened-cell material, which enhances its ability to interact more effectively with incoming energy at 1600 and 6400 Hz.The findings encourage the use of ENR for blending in NR for improved ENR and ADC concentrations since dipole-dipole interaction from ENR-ADC caused ADC dispersability, providing complexed foam structures for force expansion and aslo sound wave absorption.

Oil removal from water has become more important for environment sustainability since there are many cases of oil leakage accidents. Oil spills, whether caused by accidents or other factors, can have devastating effects on aquatic ecosystems and wildlife. To clean up by utilizing green materials, it can mitigate the environmental impact of oil spills and align with the principles of conservation science. Therefore, in this work, an attempt was made to clean oil contamination using rubber foam attached with cotton fabric. The enhancement of hydrophobicity would increase the efficiency of oil removal. The cotton fabric was coated by natural rubber (NR) foam and subsequently treated with hexadecyltrimethoxysilane (HDTMS) as hydrophobic agent. The results show the treated NR-coated fabric exhibited excellent oil absorption, oil selectivity and completely removed the oily layer from water. It is because hydrophobic enhancement of cotton fabric and rubber foam surfaces were achieved at 157° and 140°, respectively. The formation of hydrophobic agent with rough surface was revealed by SEM micrographs. From the results, the prepared NR-coated fabric presented an attractive hydrophobic property with simple preparation and high efficiency of oil removal. It could be said that NR-coated fabric is a green material potentially used as a perfect oil-separator to relieve the pollution from oil contamination. Conservationists and researchers can work together to raise awareness about the benefits of using green materials for oil spill cleanup, fostering a sense of environmental stewardship.

Natural rubber (NR) foams reinforced by a physical hybrid of nanographene/carbon nanotubes were fabricated using a two-roll mill and compression molding process. The effects of nanographene (GNS) and carbon nanotubes (CNT) were investigated on the curing behavior, foam morphology, and mechanical and thermal properties of the NR nanocomposite foams. Microscope investigations showed that the GNS/CNT hybrid fillers acted as nucleation agents and increased the cell density and decreased the cell size and wall thickness. Simultaneously, the cell size distribution became narrower, containing more uniform multiple closed-cell pores. The rheometric results showed that the GNS/CNT hybrids accelerated the curing process and decreased the scorch time from 6.81 to 5.08 min and the curing time from 14.3 to 11.12 min. Other results showed that the GNS/CNT hybrid improved the foam’s curing behavior. The degradation temperature of the nanocomposites at 5 wt.% and 50 wt.% weight loss increased from 407 °C to 414 °C and from 339 °C to 346 °C, respectively, and the residual ash increased from 5.7 wt.% to 12.23 wt.% with increasing hybrid nanofiller content. As the amount of the GNS/CNT hybrids increased in the rubber matrix, the modulus also increased, and the Tg increased slightly from −45.77 °C to −38.69 °C. The mechanical properties of the NR nanocomposite foams, including the hardness, resilience, and compression, were also improved by incorporating GNS/CNT hybrid fillers. Overall, the incorporation of the nano hybrid fillers elevated the desirable properties of the rubber foam.