The Effect of Internal Gas Pressure on the Compression Properties of Natural Rubber Foams

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.

Structure-Property Correlation in Natural Rubber Nanocomposite Foams: A Comparison between Nanoclay and Cellulose Nanofiber Used as Nanofillers

Nanocomposite foams of natural rubber (NR) with 5 phr of two kinds of nanofillers, nanoclay (NC) and cellulose nanofiber (CNF), were produced using the latex mixing method and foaming with azodicarbonamide. The effect of the nanofiller on the structure and mechanical properties of NR foams was investigated through SEM, TEM, tensile tests, WAXD, and compression set measurements. Smaller cells with a narrower distribution were attained in the NC/NR foam when compared to the NR and CNF/NR foams, and the expansion ratio was larger due to the suppression of the shrinkage in the NC/NR foam. The foaming of the NR nanocomposites reduced the size of the filler aggregates and improved the dispersion and alignment of nanofillers in the cell walls. The addition of NC and CNF enhanced the tensile strength of the NR foam by 139% and 62%, respectively, without sacrificing the excellent strain of the NR, due to the acceleration of the strain-induced crystallization and small size of the filler aggregates. The compression set of the NR foam could also be reduced in the NC/NR foam compared with the NR and CNF/NR foams.

Experimental and Finite Element Simulation of Polyolefin Elastomer Foams Using Real 3D Structures: Effect of Foaming Agent Content

In this study, polyolefin elastomer (POE) foams were prepared without any curing agent using a single-step foaming technique. The effect of azodicarbonamide (ADC) content as a chemical foaming agent on the foams’ morphology and mechanical properties was studied using scanning electron microscopy (SEM), mechanical properties (tension and compression) and hardness. The results showed that increasing the ADC content from 2 to 3, 4 and 5 phr (parts per hundred rubber) decreased the foam density from 0.75 to 0.71, 0.65 and 0.61 g/cm3, respectively. The morphological analysis revealed that increasing the ADC content from 2 to 4 phr produced smaller cell sizes from 153 to 109 µm (29% lower), but a higher cell density from 103 to 591 cells/mm3 (470% higher). However, using 5 phr of ADC led to a larger cell size (148 µm) and lower cell density (483 cells/mm3) due to cell coalescence. The tensile modulus, strength at break, elongation and hardness properties continuously decreased by 28%, 21%, 16% and 14%, respectively, with increasing ADC content (2 to 5 phr). On the other hand, the compressive properties, including elastic modulus and compressive strength, increased by 20% and 64%, respectively, with increasing ADC content (2 to 5 phr). The tensile and compression tests revealed that the former is more dependent on foam density (foaming ratio), while the latter is mainly controlled by the cellular structure (cell size, cell density and internal gas pressure). In addition, 2D SEM images were used to simulate the foams’ real 3D structure, which was used in finite element methods (FEM) to simulate the stress–strain behavior of the samples at two levels: micro-scale and macro-scale. Finally, the FEM results were compared to the experimental data. Based on the information obtained, a good agreement between the macro-scale stress–strain behavior generated by the FEM simulations and experimental data was obtained. While the FEM results showed that the sample with 3 phr of ADC had the lowest micro-scale stress, the sample with 5 phr had the highest micro-scale stress due to smaller and larger cell sizes, respectively.

Electromagnetic Absorption and Mechanical Properties of Natural Rubber Composites Based on Conductive Carbon Black and Fe3O4

In contemporary civilization, the electromagnetic radiation from electronic devices and communication systems has become a substantial pollutant. High-performance electromagnetic absorbers have become a solution for absorbing unwanted electromagnetic waves. This research proposed a lightweight and flexible electromagnetic absorber produced from natural rubber filled with conductive carbon black (CCB) and Fe₃O₄. The effect of CCB, Fe₃O₄, and a combination of CCB and Fe₃O₄ as a hybrid filler on foam morpholog, electromagnetic reflectivity, tensile strength, and compression set properties were investigated. In addition, the effect of the alternating layered structure of CCB and Fe₃O₄ on electromagnetic absorption was investigated. The results indicated that the composite foam exhibited an interconnected network structure that enhanced the electromagnetic attenuation in the absorber. CCB increased the electromagnetic absorption of the foam, whereas Fe₃O₄ had less of an effect. The foam filled with the hybrid filler at the CCB/Fe₃O₄ ratio of 8/2 exhibited excellent electromagnetic absorption. The composite foam had a higher tensile modulus and higher strength compared to neat foam. The addition of CCB decreased the compression set; however, the compression set was improved by the incorporation of Fe₃O₄. Composite foams filled with hybrid filler can serve as highly efficient electromagnetic absorbing materials.

Physical and mechanical properties of hybridized elastomeric foam based on ethylene-propylene-diene-monomer, multiwall carbon nanotube, and barium titanate

The use of hybrid fillers in rubbers can provide additional benefits to rubber foams compared to individual micro- or nano-scale particles due to an optimum packaging and synergic effects. The present work reports the development of vulcanized ethylene-propylene-diene-monomer nanocomposite hybrid foams filled with barium titanate and multiwall carbon nanotube (BT/MWCNT), prepared via a scalable protocol. The developed foams presented a high shear-thinning behavior, suggesting the formation of a 3D interconnected physical network of MWCNT within the polymer matrix. This network resulted in a notable improvement of the mechanical properties under tension and compression with increasing of MWCNT content. Also, the incorporation of MWCNT and BT enhanced thermal stability and thermal conductivity. Meanwhile, BT did not show any influence on the measured physical properties, due to the lack of interaction between BT and the EPDM matrix.

Physical Hybrid of Nanographene/Carbon Nanotubes as Reinforcing Agents of NR-Based Rubber Foam

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 T_g 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.

Chemistry, Processing, Properties, and Applications of Rubber Foams

With the ever-increasing development in science and technology, as well as social awareness, more requirements are imposed on the production and property of all materials, especially polymeric foams. In particular, rubber foams, compared to thermoplastic foams in general, have higher flexibility, resistance to abrasion, energy absorption capabilities, strength-to-weight ratio and tensile strength leading to their widespread use in several applications such as thermal insulation, energy absorption, pressure sensors, absorbents, etc. To control the rubber foams microstructure leading to excellent physical and mechanical properties, two types of parameters play important roles. The first category is related to formulation including the rubber (type and grade), as well as the type and content of accelerators, fillers, and foaming agents. The second category is associated to processing parameters such as the processing method (injection, extrusion, compression, etc.), as well as different conditions related to foaming (temperature, pressure and number of stage) and curing (temperature, time and precuring time). This review presents the different parameters involved and discusses their effect on the morphological, physical, and mechanical properties of rubber foams. Although several studies have been published on rubber foams, very few papers reviewed the subject and compared the results available. In this review, the most recent works on rubber foams have been collected to provide a general overview on different types of rubber foams from their preparation to their final application. Detailed information on formulation, curing and foaming chemistry, production methods, morphology, properties, and applications is presented and discussed.

Enhancing the EMI shielding of natural rubber-based supercritical CO2 foams by exploiting their porous morphology and CNT segregated networks

Natural rubber/carbon nanotubes composite foams (F-NR/CNTs) with high electrical conductivity and excellent electromagnetic interference (EMI) performance were developed through a multi-step process including: (a) CNTs assembled on natural rubber latex particles, (b) pre-crosslinking of natural rubber, (c) supercritical carbon dioxide foaming of pre-crosslinked composite samples and (d) post-crosslinking of foamed composite samples. A closed-cell porous structure and a segregated CNT network are clearly observed in the resulting foams. Due to this morphology, F-NR/CNTs exhibit low density, good mechanical properties, and high electrical conductivity. Owing to the multiple radiation reflections and scattering between the cell-matrix interfaces, the composite foams presented an excellent specific shielding effectiveness (SSE) of 312.69 dB cm2 g-1 for F-NR/CNTs containing 6.4 wt% of CNTs, which is significantly higher than those already published for rubber composites containing comparable filler content. Furthermore, the analysis of EMI SE highlights that absorption efficiency is more significant than reflection efficiency, implying that most of the incident electromagnetic radiation is dissipated in the form of heat. This work provides the fundamentals for the design of innovative light weight and efficient EMI shielding foams characterized by a three-dimensional segregated CNT network with huge potential for use in the electronics and aerospace industries.

Development of high-porosity resorcinol formaldehyde aerogels with enhanced mechanical properties through improved particle necking under CO2 supercritical conditions

A new high porosity resorcinol-formaldehyde (RF) aerogel with improved particle necking is presented in this work. This RF aerogel was developed under CO₂ supercritical drying conditions without any structural shrinkage. The water content and the catalyst percentage were varied to modify the particles' nucleation and growth mechanisms and to control particle-particle connections. The nucleation mechanism solely dependent on the initial catalyst percentage; the number of nuclei increased with the catalyst percentage. However, the growth and connection of the particles dependent on both the water content and the catalyst percentage through their effect on the pH value. As the water content increased to have a larger void fraction, the pH value decreased. Consequently, the spherical growth of the particles became dominant and, thereby, the connection of the particles became more difficult. But as the catalyst percentage increased, the pH value increased, and the connection of the particles became facilitated with the formation of necks around the particles. As a result, the semi-fibril-like structure was developed with a high void fraction. A 30% increase in the structural elasticity and a very low thermal conductivity of 0.0249W/mK were obtained.