The influence of protein–polyphenolic interactions on macromolecular behaviour associated with wood composite adhesion

Abstract

There is increasing concern regarding the environmental impact and sustainability of manufacturing processes and products, bio-based polymers are currently receiving strong interest as alternatives to petrochemical polymers for adhesive application. Bio-based protein and polyphenolic by-products from the agricultural and forestry industries are widely recognised as suitable substitutes for synthetic materials in adhesive development. Within the adhesive industry, soy and corn proteins, as well as tannin and lignin phenolics are well published as successful examples of partial substitutes and fillers. In 2014, research institute Scion filed the patent Adhesives for lignocellulosic materials, identifying novel approaches to a wholly bio-based lignin adhesive technology, without the reliance on petrochemicals. In this research, pure and crude samples of soy and zein protein as well as tannin and lignin samples are characterised as starting materials for wholly bio-based adhesive formulations. Binary conjugates and ternary mixtures of protein-polyphenolics are achieved by conjugation of the materials at different ratios. This work examines the chemical and physical relationships of protein-phenolic interactions as a function of ratio, temperature, moisture and shear. The samples are analysed and macromolecular behaviour such as glass transition and cross-linking temperature is observed. Thermal analysis such as differential scanning calorimetry (DSC) of the individual materials reveal sample moisture loss on heating, but are able to retain moisture at temperatures >100°C, related to the initial moisture content. The thermograms exhibit the temperatures where moisture is dynamic and could correlate to observed rheological behaviours. Rheological assessment of the materials show that above 100°C, samples are still dehydrating , the residual moisture delaying the onset of cross- linking to occur at higher temperatures. For example soy flour with sample moisture 30-70% presented full cross-linking achieved at elevated temperatures (125-150°C) compared to maximum cross-linking of soy protein (125°C). Thermogravimetric analysis (TGA) of the protein-phenolic binary material presents good thermal stability and degradative behaviour to temperatures >200°C. Rheological assessment of the binary conjugates reveal that the influence of the phenolic component on protein increases stability and was shown to extend the linear viscoelastic region. Generally, the stiffness of the binary conjugates is greater than that of the individual material. Temperature sweeps of binary conjugates also showed the brittle nature of the material suggesting that cross-linking may initiate during manufacture of the material. Moreover, it appears that moisture as low as ca. <5% facilitates glass transition (Tg). Once the binary cross-linking reaction occur, moisture content is irrelevant and the phenolic component determines behaviour most likely acting as a plastercizer (e.g. as lignin increases, the Tg decreases). Results concerning the protein-polyphenolic ternary mixtures confirm the influence of polyphenolics, notably lignin in regards to greater viscoelastic stability and stiffness. Rheological assessments of the ternary mixtures show the Tg occurs between 105-120°C. The assessment show that Tg was influenced by the protein- polyphenolic ratio (particularly the low tannin ratios). The ternary material was also characterised by dynamic mechanical thermal analysis (DMTA) and correlates to Tg observed in rheology. Contrary to rheological results, DMTA exhibits ternary mixture cross-linking behaviour initiating at 185°C, with only the high protein high polyphenolic ternary sample achieving full cure at 195°C. If considering these materials for wholly bio-based wood composite adhesive, the results confirm that thermoset conditions would need to employ a high water content in order to manufacture any potential adhesive formulation. Current industry press schedules for commerical wood composite adhesives are <180°C, with a press time of <12 minutes. The results show that if employing a bio-based adhesive formulation, fully cured adhesives are feasible so long as higher temperatures and longer cure times are implemented into the schedule, which when compared to current commerical adhesives could present some issue and would required more investigation.