The Impact of Microorganisms on the Performance of Linseed Oil and Tung Tree Oil Impregnated Composites Made of Hemp Shives and Corn Starch

Abstract

In this study, the performance characteristics of hemp shives impregnated with linseed oil and tung tree oil (HS)- and corn starch (CS)-based biocomposites containing flame retardants were evaluated before and after treatment with the mixture of bacterium Pseudomonas putida and fungus Rhizopus oryzae. Enzymatic activities and physical-mechanical properties such as water absorption, thickness swelling, compressive strength, and thermal conductivity were tested to evaluate the suitability of selected composites for thermal insulation purposes. In addition, electron microscopy was used to investigate the impact of microorganisms on the microstructure of the material. It was determined that the type of oil used for impregnation significantly affects the properties of biocomposites after 6 months of incubation with mixture of bacterium P. putida and fungus Rh. oryzae. Biocomposites impregnated with linseed oil and after treatment with a mixture of microorganisms had cellulase activity of 25 U/mL, endo β-1-4-glucanase activity of 26 U/mL, lipase activity of 101 U/mL, only a 10% decrease in compressive strength, 50% higher short-term water absorption, unchanged swelling in thickness, and slightly decreased thermal conductivity compared to control biocomposites. At the same time, biocomposites with tung tree oil had a much more pronounced deterioration of the properties tested, cellulase activity of 28 U/mL, endo β-1-4-glucanase activity of 37 U/mL, lipase activity of 91 U/mL, two times lower compressive strength and two times higher short-term water absorption, 2.5 times greater thickness swelling, and a slightly increased thermal conductivity. We conclude that linseed oil provides better protection against the action of microorganisms compared to impregnation with tung tree oil.

Keywords: biocomposites; biodegradation; biostability; enzymatic activity; microorganisms; performance characteristics.

Figure 1

Conjugated and unconjugated double bonds of some components of drying oils: (a) linolenic acid in linseed oil; (b) α-eleostearic acid in tung tree oil.

Figure 2

Visual assessment of cellulase activity in CMC medium by flooding Petri dishes with Congo red: (a) P. putida after growing for 13 days; (b) Rh. oryzae after growing for 12 days.

Figure 3

The surface appearance of HS- and CS-based biocomposites impregnated with different oils after 2 weeks, 5 weeks, 9 weeks, and 6 months: (a) control samples without linseed oil; (b) samples with linseed oil; (c) control samples without tung tree oil; (d) samples with tung tree oil.

Figure 4

Enzymatic activities of HS- and CS-based biocomposites impregnated with different oils after incubation with microorganisms for 6 months: (a) cellulase and endo β-1-4-glucanase activities; (b) lipase activity.

Figure 5

Microstructure of HS- and CS-based biocomposites impregnated with different oils after incubation for 6 months (1—fungus Rh. oryzae, 2—fungus A. fumigatus, 3—bacterium P. putida): (a) surface of biocomposite with linseed oil (magnification ×100); (b) surface of biocomposite with linseed oil (magnification ×1000); (c) 5 mm depth of biocomposite with linseed oil (magnification ×3000); (d) surface of biocomposite with tung tree oil (magnification ×100); (e) surface of biocomposite with tung tree oil (magnification ×1000); (f) 5 mm depth of biocomposite with tung tree oil (magnification ×1500).

Figure 6

The impact of microorganisms on the selected properties of HS- and CS-based biocomposites impregnated with different oils before and after incubation for 6 months: (a) density; (b) thermal conductivity.

Figure 7

Moisture-related properties of HS- and CS-based biocomposites impregnated with different oils before and after incubation for 6 months: (a) short-term water absorption after immersion in water for 24 h; (b) swelling in thickness after immersion in water for 24 h.

Figure 8

Compressive strength of biocomposites based on HS and CS and impregnated with different oils before and after 6 months of incubation.