The protective coatings of the bio-based composite boards formed using the drying and semi-drying oils

The bio-degradation of bio-based composite materials is an unwanted process. Excess humidity accelerates it, therefore hydrophobic protective coatings are used. In this work, the hydrophobic protective coatings were formed by the autooxidation and polycondensation mechanisms using drying oils such as a tung tree, linseed, and semi-drying hempseed oil. The bio-based composite boards were made of hemp shives, corn starch binder and flame retardants such as expanded graphite or multifunctional aqueous mixture based on phosphorus and nitrogen organic compounds. The hydrophobic coatings were formed by covering the bio-based composite boards (BcBs) with selected oils in the vacuum desiccator. After this procedure, BcBs covered with oil were maintained at 40^oC, 90^oC, or 120^oC temperature, respectively, until the protective film was formed. It was demonstrated that the compressive strength (at 10% of relative deformation) of the BcBs with the oil protective coatings compared to untreated BcB increased up to 4.5 fold and could reach up to 14 MPa, bending strength increased by 2.78 fold (up to 19 MPa), the water absorption decreased around 4-fold (from 1.34 kg/m2 to 0.37 kg/m2), swelling in thickness decreased from 7.51% to 4.59%, while the thermal conductivity remained unchanged and was around 0.085 W/(m·K) [1]. It was concluded that the most desired properties of such materials were obtained when the tung tree oil was used for coating.

References:

1. Vasiliauskienė, D.; Balčiūnas, G.; Boris, R.; Kairytė, A.; Kremensas, A.; Urbonavičius, J. The effect of different plant oil impregnation and hardening temperatures on physical-mechanical properties of modified biocomposite boards made of hemp shives and corn starch. Materials (Basel). 2020