The effect of azodicarbonamide as chemical blowing agent on the morphology, cure kinetics and physical properties of natural rubber foam is investigated. From the morphology, when the amount of chemical blowing agent increases from 3 to 4 phr, the bubble size in the rubber matrix slightly decreases due to the increase of vulcanization reaction rate from the presence of amine fragment species as by-products from the decomposition of azodicarbonamide. The coalescence between bubbles is observed in the specimen with 5 and 6 phr of azodicarbonamide owing to high gas content in the rubber matrix. Moreover, the scorch time slightly reduces and cure rate increases as a function of azodicarbonamide content. The autocatalytic model can be used to explain the curing reaction and mechanism of this natural rubber foam. Furthermore, the activation energy (Ea) directly relates to the bubble size and microvoid structure of natural rubber foam. When compared with the vulcanized natural rubber without adding chemical blowing agent, it is found that the bulk density of natural rubber foam significantly decreases and the volumetric expansion ratio of natural rubber foam increases at high content of chemical blowing agent. Moreover, natural rubber foam at 4 phr of azodicarbonamide exhibits the lowest thermal expansion coefficient due to the smallest bubble size with less coalescence.
The effect of types of sulfenamide accelerator, i.e., 2-morpholinothiobenzotiazole (MBS), N-t-butylbenzothiazole-2-sulfenamide (TBBS), and N-cyclohexyl benzothiazole-2-sulfenamide (CBS) on the cure kinetics and properties of natural rubber foam was studied. It has been found that the natural rubber compound with CBS accelerator shows the fastest sulfur vulcanization rate and the lowest activation energy (E a ) because CBS accelerator produces higher level of basicity of amine species than other sulfenamide accelerators, further forming a complex structure with zinc ion as ligand in sulfur vulcanization. Because of the fastest cure rate of CBS accelerator, natural rubber foam with CBS accelerator shows the smallest bubble size and narrowest bubble size distribution. Moreover, it exhibits the lowest cell density, thermal conductivity and thermal expansion coefficient, as well as the highest compression set as a result of fast crosslink reaction.
The influence of functionalization of cyclohexyl diamine onto the graphene surface via diazonium reaction on the cure kinetics as well as morphology, mechanical, thermal and physical properties of natural rubber (NR)/graphene nanocomposite foam was investigated. The surface functionalization of graphene was confirmed by Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetric analysis (TGA). Without an addition of blowing agent, the vulcanized NR composites containing cyclohexyl amine‐treated graphene showed not obvious different dispersion of graphene nanoplatelets than those containing untreated graphene as confirmed by transmission electron microscopy micrographs. Although the addition of untreated graphene could accelerate the sulfur vulcanization rate of NR nanocomposite foam due to the high thermal conductivity of graphene particle, and the remaining oxygen functional groups on the surface of graphene, the amines from cyclohexyl diamine on the treated graphene could react with carboxylic and epoxide group on the surface of graphene leading to a reduction of reactive oxygen functional groups on graphene surface and in turn slowed vulcanization rate. The fast vulcanization rate in untreated graphene system led to small bubble size in the NR nanocomposite foams. In contrast, the treated graphene system showed high tensile strength and low thermal expansion coefficient owing to low cell density. Finally, the defect on the graphene surface after functionalization was detected by Raman spectroscopy; this resulted in poor thermal conductivity for the treated graphene composite system. POLYM. COMPOS., 40:E1766–E1776, 2019. © 2018 Society of Plastics Engineers
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.
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