The necessity to overcome petroleum-based polymers toward a more environmentally natural-sourced materials has been applied during the last decade to common use linear polyamides (nylons) [1]. In this field, commercially available polyamide 5,6 (PA 5,6) is one of the very few cases of biobased nylon containing an odd-unit monomer. Both 1,5-diaminopentane and adipic acid, which constitutes this polymer, can be produced and obtained from natural resources like glucose, vegetable oils, hemicellulose or lysine [2,3]. Moreover, PA 5,6 has been studied in the past for the crystalline structure, which is characterized by the presence of hydrogen bonds formed in two directions [4]. Typical of nylons, the high resistance to chemicals and high temperatures is couple with a rigidity and low elasticity of the processed material in the film or plate forms. Rubber (NR) is a naturally occurring material, based on poly-isoprene chains with the presence of minor organic molecules. For its low Young’s modulus, elasticity and easy applicability, it has been wide-world used since centuries ago.

Considering the necessity to enhance the poor impact strength behavior of nylon, mixtures of PA 5,6 and NR were produced. Rubber content in the mixture was fixed at 15% and 30% in weight with respect to the nylon. Samples were prepared by mixing the two components into a Brabender mixer at 280 °C for 5 minutes. The solid cooled material was then grinded into flocks and melt-pressed under a pression of 10 bar and a temperature of 260 °C. Blends were chemically characterized by FT-IR and thermally by DSC and TGA. Mechanical tensile strength analysis over the obtained samples showed a decrease in the Young’s modulus values from 0.64 GPa of the plain PA5,6 to the 0.39 and 0.26 GPa for the 15% and 30% mixtures, respectively.

The combination of nylon and natural rubber offers greater deformability, which can be advantageous in applications requiring flexibility. Crystallization kinetics were studied by means of polarized-light optical microscopy (POM) in a Linkam controlled heating stage. Thin films of the materials were sandwiched between microscopy cover glasses and heated to 10 °C above the melting temperature. After a rapid cooling, samples were crystallized isothermally at the selected temperature, while the growth of the crystals spherulites were followed by digital camera. Indeed, the amorphous, disordered structure of natural rubber can interfere with the formation of the crystalline structure when incorporated into a polymer that can crystallize by disrupting the molecular order necessary for crystallization. Mechanical tensile strength analysis over the obtained samples showed a decrease in the Young’s modulus values from 0.64 GPa of the plain PA5,6 to the 0.39 and 0.26 GPa for the 15% and 30% mixtures, respectively. The combination of nylon and natural rubber offers greater deformability, which can be advantageous in applications requiring flexibility.

References

1. M. Winnacker, Biomat. Sci. 2017, 5, 1230.
2. N. Buschke, J. Becker, R. Schäfer, P. Kiefer, R. Biedendieck, C. Wittmann Biotechnol. J. 2013, 8, 557.
3. W. Niu, K.M. Draths, J.W. Frost Biotechnol. Progr., 2002, 18, 201.
4. L. Morales-Gámez, D. Soto, L. Franco, J. Puiggalí Polymer, 2010, 51, 5788.

Acknowledgments

All the authors acknowledge financial support from Spanish Ministry of Science and Innovation through PID2022-140302OB-I00 and “Ayudas para contratos predoctorales para la formación de doctors/as 2019” and to the Generalitat de Catalunya under the project 2021-SGR-01042.