This article discusses the effect of introducing nano-silica on the thermo-mechanical properties of jute/thermoplastic composites.
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Background
Depleting petroleum resources and growing environmental concerns due to the non-biodegradability of plastic products have shifted the focus to novel alternative materials. Bio-composites are being developed to overcome these challenges as they are eco-friendly, non-toxic, and biodegradable.
Thermoplastics (TPS) can be used as bio-resin owing to their low production cost, biodegradable nature, reusability, renewable origin, and abundance compared to other bio-resins. Jute fiber is often used as a natural fiber to increase the TPS matrix strength owing to its low specific cost and high strength among natural fibers.
Although inorganic fiber-based thermoset composites were typically used in the aerospace industry, these composites release toxic gases when exposed to fire, which limits the availability of fresh air and increases the breathing difficulties for the survivors.
Bio-composites can be used in such applications to reduce environmental risks and toxicity. However, previous studies have demonstrated that any improvement in thermal stability and flame retardancy of bio-composites led to a decline in their mechanical properties, which restricted their use in these applications.
For instance, the use of double-layer hydroxides to improve the thermal stability and flame retardancy of polypropylene adversely affected the mechanical properties of the TPS. Thus, a simultaneous improvement of mechanical properties, thermal stability, and flammability of bio-composites is necessary to effectively use them in practical applications.
However, both jute and TPS are not flame retardants, which necessitates the use of a bio-based flame retardant filler to achieve a fully bio-based sustainable jute/TPS composite. The filler must be able to improve flame retardancy without affecting the mechanical properties of the composite.
Importance of Nano-silica to Change the Thermo-Mechanical Properties of Jute/TPS Composites
Silica nanoparticle (SNP), a halogen-free flame retardant, can be used as a flame retardant filler owing to its high versatility and surface area, low toxicity, and good biocompatibility. Moreover, SNP adds fertility to the soil after disposal, which eliminates any adverse impact on the environment.
In a study published in the journal Materials Chemistry and Physics, researchers incorporated SNPs as a single nano-filler to fabricate biodegradable high-strength flame-retardant jute/TPS composites and investigated their biodegradability, mechanical performance, thermal stability, and flame retardancy.
Overall, samples with 4%, 3%, 2%, 1%, and 0% SNPs by weight of TPS were fabricated and designated as JS4, JS3, JS2, JS1, and JS, respectively, and the weight of jute fibers in the total composition of all composites was kept at 40%.
The findings demonstrated a strong bonding between the silica and jute/TPS in the fabricated SNP-based composites due to the interaction of silica with a hydroxyl group of jute/starch. For instance, the SNPs in the JS3 sample facilitated the formation of longer hydrocarbon chains that increased the homogeneity of the composite during the plasticizing process.
The SNPs filled several pores during TPS synthesis due to the interaction between starch and silica, which reduced the crack initiation sites and improved the composite mechanical performance significantly.
The tensile modulus (TM) and tensile strength (TS) increased in all jute/TPS composites after the addition of SNPs, with the highest TS of 34.40±0.53 MPa observed in the JS3 sample. The tensile strength of the JS3 composite was 12 times higher than the TS of the TPS matrix and 23% higher than that of the JS composite.
However, the TS was reduced in the JS4 sample due to SNP agglomeration that affected uniform SNP dispersion and decreased homogeneity. The TM of the composites also increased after SNP incorporation due to the higher stiffness of SNPs, with the maximum TM of 890.50±85.41MPa observed in the JS2 composite, which is 157% higher than that of the JS composite.
The JS3 composite displayed the least elongation at a break of 6.4% among all samples as the addition of SNPs restructured the plasticized TPS by increasing rigidity and reducing the mobility of the composites.
Flexural properties increased significantly in the jute/TPS composites after SNP incorporation, with the highest flexural strength (FS) of 16.25± 1.32 MPa observed in the JS3 composite, which was 97% higher than that of the JS composite, and the maximum flexural modulus (FM) of 930.13±36.96 MPa was observed in JS4 sample, which was 80% higher than JS sample.
The increase in FM was attributed to the nano-scale size of silica that covered a higher surface area, which increased its absorption in polymer chains. Maximum impact strength (IS) of 14.30 ± 0.99 kJ/m2 was observed in the JS3 composite, which was 1.7 times the impact strength of the JS composite. However, the IS and FS were reduced in the JS4 sample due to SNP agglomeration.
The addition of higher weight percentages of SNPs improved the thermal stability of the composites, with the temperature for 75% weight loss increased from 370.47 oC in the JS composite to 386.29 oC in the JS4 composite.
The char residues increased at higher silica concentrations owing to the high thermal stability of SNPs. Silica-based composites demonstrated a lesser weight loss compared to JS composites, with the least weight loss observed in the JS3 composite due to uniform silica particle distribution.
The flammability of all JS composite samples changed significantly after SNP addition as the silica loadings promoted fire-resistant behavior by restricting free radicals that support combustion and heat penetration.
Delayed burning was observed in composites with higher silica loadings, including JS4, JS3, and JS2 samples, and the burning rate decreased proportionally with the increasing SNP concentration.
JS4 sample showed the least burning rate of 21.8 mm/min, which was 62% lower than the JS sample owing to the formation of a condensed clear layer of excess silica. Thus, excess silica imparted flame-resistant properties and restricted flammability. Carbon monoxide and carbon dioxide emissions were also reduced due to silica loadings, demonstrating the sustainable and eco-friendly nature of the composites.
Although all composites were biodegradable, the rate of degradation of composites with higher silica concentrations was reduced due to the rigid nature of SNPs that hindered the microbial attack on the fibers and resin, which indicated better durability of such composites. Maximum degradation was observed in the JS1 sample in the first four weeks of the soil burial test.
In another study published in the journal Cellulose, researchers introduced nano-silica and nano-silica modified by coupling agents into the polylactic acid (PLA) matrix to effectively improve the toughness and strength of the jute fiber-reinforced PLA composites.
The thermal, stretching, and bending properties of the fabricated jute/PLA composites were investigated. The findings demonstrated that nano-silica particles improved the interface performance between the jute and the PLA matrix, leading to an improvement in the thermal stability and mechanical properties of the jute/PLA composites. Moreover, the addition of silica also improved the jute/PLA interfacial adhesion and increased the glass transition temperature of the material.
To summarize, the addition of SNPs in the TPS/jute composites could help in realizing bio-based composites with improved fire-retardant, thermal, and mechanical properties.
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References and Further Reading
Azad, M. M., Ejaz, M., Shah, A. R., Afaq, S. K., Song, J. (2022). A bio-based approach to simultaneously improve flame retardancy, thermal stability and mechanical properties of nano-silica filled jute/thermoplastic starch composite. Materials Chemistry and Physics, 289. https://doi.org/10.1016/j.matchemphys.2022.126485
Song, X., Fang, C., Li, Y., Wang, P., Zhang, Y., Xu, Y. (2022) Characterization of mechanical properties of jute/PLA composites containing nano SiO2 modified by coupling agents. Cellulose 29, 835–848. https://doi.org/10.1007/s10570-021-04300-z
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