Improvement of Water Resistance of Vegetable Proteins by the Use of Synthetic Origin Additives: Trials with Resins and Metal Ions

Benedetto Pizzo,Ilaria Santoni

doi:10.3390/coatings11070859

Abstract

The adhesives industry is increasingly interested in products coming from natural and renewable resources. The aim of the present work was to improve the water resistance of soy-based proteins by using synthetic and formaldehyde-free additives. These include polyamide-amine epichlorohydrin (PAE), different types of isocyanates, and combinations of these cross-linkers between them and with other agents, including metal ions. In addition, the effect of both curing temperature and maturation time was assessed. Performances were evaluated by means of shear strength tests, solubility tests, and spectroscopic analysis. The obtained results showed that while isocyanates reacted completely but with water instead of proteins, tests with PAE were generally successful. In fact, the insoluble residue as well as the shear strength in wet conditions dramatically increased after PAE addition. Moreover, the wet performances of protein/PAE formulations appreciably increased gluing at 60 °C instead of room temperature. Furthermore, the maturation time had a positive effect on the formulations where metal ions were added, both for solubility and wet shear strength. Actually, for the very long conditioning time of 3 months, a significant and substantial increase of wet shear strength was observed for the series protein/PAE/aluminum.

Highlights

Research and market trends in the wood adhesives sector have led to an increasing search for alternatives to formulations containing formaldehyde due to their negative impact on the environment and human health
Analysis carried out on soy proteinbased adhesives evidenced that strengths are mainly based on their renewability and absence of formaldehyde in formulations; weaknesses include high viscosity with low solid content and long pressing time due to water excess; opportunities are the crop availability and stringent legislation on volatile organic compounds (VOCs) for wood products; threats are related to cost and climatic factors [11]
As in the case of isocyanates, basing on IR spectra, it is not possible to establish whether this is a homo-crosslinking between terminal carboxylic groups of polyamide-amine epichlorohydrin (PAE) or between PAE and carboxyl groups of amino acids of the protein

Summary

Introduction

Research and market trends in the wood adhesives sector have led to an increasing search for alternatives to formulations containing formaldehyde due to their negative impact on the environment and human health. As a matter of fact, this product has been recently classified as a carcinogenic agent [1] Within this framework, protein-based adhesives have received much interest due to their availability, excellent performances in dry conditions, and provenance from renewable sources [2,3,4,5,6,7,8,9,10]. A SWOT analysis (strengths, weaknesses, opportunities, and threats) carried out on soy proteinbased adhesives evidenced that strengths are mainly based on their renewability and absence of formaldehyde in formulations; weaknesses include high viscosity with low solid content and long pressing time due to water excess; opportunities are the crop availability and stringent legislation on volatile organic compounds (VOCs) for wood products; threats are related to cost (compared with UF resins, which are relatively inexpensive) and climatic factors (such as droughts or floods) [11].

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Methods

Discussion

Conclusion