Abstract
The aim of this research was to prepare an antifungal soybean protein concentrate (SPC) adhesive containing carvacrol (CRV) as a bioactive agent able to delay the attack of molds and yeast during storage of SPC adhesive at 4 °C as water-based systems. CRV was incorporated in SPC slurry at 0.5% v/v (~10 times its minimum inhibitory concentration against Aspergillus terreus, used as model fungus), to ensure its long-term action. CRV scarcely altered the thermal properties, structure and apparent viscosity of SPC adhesive. Active SPC aqueous dispersion was microbiologically stable for at least 30 days at 4 °C where the colonization begins, while control SPC was visually colonized from the second day. Rice husk (RH) particleboards of density ~900 kg/m3 were manufactured using the active SPC stored for 0, 10, 20, and 30 days as a binder. Modulus of elasticity, modulus of rupture and internal bond of RH–control SPC (without CRV) panels were 12.3 MPa, 2.65 GPa and 0.27 MPa, respectively, and were statistically unaltered compared with those obtained with fresh SPC, regardless of the presence of CRV or the storage time. This last implies that active SPC should not necessarily have to be prepared daily and/or be used immediately after its preparation. Since it is microbiologically stabilized, it can be store at least for 30 days, ensuring the stability of the protein. The quality of the adhesive was evidenced by the consistent properties of the adhesive, expanding its potential use and commercialization.
Highlights
Synthetic adhesives such as amino-based, phenolic and isocyanate have dominated the wood composites and particleboards industry due to their low cost, low curing temperature, short pressing time, aqueous solubility, and high dry bond strength, among other important characteristics [1,2,3]
SDS consists of a polar sulfate head that strongly binds to the positively charged protein groups, and a hydrophobic 12-carbon chain that interacts with hydrophobic regions of proteins [33], keeping them in solution, and facilitating the interaction with hydrophobic additives [12,17]
Results revealed that the amide I band was composed by the superposition of at least 6 absorption bands related to different secondary structures: two bands at 1626 and 1683 cm−1 are associated with the amide groups involved in extended beta sheets, while that at 1651 cm−1 is related to α-helix and at 1643 cm−1 to random structures
Summary
Synthetic adhesives such as amino-based, phenolic and isocyanate have dominated the wood composites and particleboards industry due to their low cost, low curing temperature, short pressing time, aqueous solubility, and high dry bond strength, among other important characteristics [1,2,3]. There are three main driving forces leading the gradual replacement of such synthetic systems with formaldehyde-free bio-sourced substitutes: the fluctuating price of the oil, the awareness over the limited fossil resources and the concern for lowering indoor air pollution associated with formaldehyde emission [4]. Soybean proteins (SPs) have been the subject of extensive research as raw materials of wood adhesives [6,7,8,9,10,11,12,13,14,15,16,17], because they are renewably sourced, processed and available worldwide at reasonable cost. SP adhesives are both non-toxic and economical, but certain obstacles are associated with SP adhesives such as their poor moisture and microbiological resistance and reduced shelf life [12]
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