Mechanical and degradation properties of molded straw flowerpot prepared by modified urea-formaldehyde adhesive with feather keratin

Abstract: Discarded Straw flowerpots are usually difficult to degrade in the soil environment, due mainly to the non-degradable urea-formaldehyde adhesive during molding. In this study, keratin was extracted from the wastes of poultry feathers using reduction, instead of part of urea and the extracted keratin polycondenses with formaldehyde, further to synthesize a modified urea-formaldehyde adhesive for the straw flowerpots. The modified urea-formaldehyde adhesive was then mixed with the rice straw of biological modification to mold a green and environment-friendly degradable flowerpot. An investigation was made to explore the effect of different contents of keratin on the physical and chemical properties of modified urea-formaldehyde as well as the influence of modified urea-formaldehyde on the mechanical and biodegradable properties. Fourier infrared spectroscopy and thermogravimetric analysis were used to characterize the basic adhesive properties of urea-formaldehyde adhesive, thereby analyzing the changes of functional group and thermodynamic properties of the modified urea-formaldehyde adhesive. At the same time, three-point bending tests, microbial degradation ability tests, and degradation in soil tests were also carried out to determine the variation in the mechanical strength and biodegradability of molded straw flowerpots. The results showed that keratin significantly reduced the content of free formaldehyde in the urea-formaldehyde adhesive with a higher viscosity. The -NH and -COOH groups in keratin were favorable for the copolymerization with UF adhesive, where a cross-linking network was formed. The temperature of the third pyrolysis peak was 285.8 ℃ representing the adhesive polycondensation cross-linked structure when the keratin content was 3%. The modified urea-formaldehyde adhesive increased by 5.0%, compared with non-free formaldehyde, indicating the best thermal stability. Furthermore, the final carbon residuals of modified urea-formaldehyde adhesive were all reduced with different contents of keratin, compared with UF, indicating suitable for natural degradation of composites prepared by the modified urea-formaldehyde and straw fibers. Meanwhile, the best elastic modulus and flexural strength were achieved in 2 552 and 47.7 MPa for the straw flowerpots with 3% keratin-modified urea-formaldehyde adhesive, indicating the increases of 8.97% and 85.59%, respectively, compared with the unmodified. NH and - COOH groups of keratin normally participated in the adhesive body structure, thereby achieving better crosslinking adhesive in the substrate surface infiltration formed on the solid bonding interface. Since too much keratin cannot participate in the shape of the structure during the crosslinking reaction, the reduction in the conduction process made a great contribution to the decrease in the strength of bonding interface stress of composites, thus determining the macroscopic mechanical properties of the material. Additionally, the best biodegradability was also achieved in the straw molded flowerpot with 5% keratin-modified urea-formaldehyde adhesive. The microbial growth area was 91.2% on the straw flowerpot surface in 28 days, while the residual mass percentage of straw flowerpot in 6 months was 64.10%. Consequently, the degradation muss loss increased by 80.95%, respectively, compared with the unmodified. This finding can provide strong theoretical support to create the degradable straw flowerpots. In this case, crops straws can widely be expected for waste disposal and utilization in sustainable agriculture.Key words: mechanical strength; thermal stability; keratin; straw flowerpot; degradation