© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] from: High Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6

Mechanical properties of heat-treated natural

S.Ochi’, H. Takagi’ & R.Niki2 I Department of Ecosystem Engineering, The University of Tokushima, Japan. 2 Tokwhima PrefectureGeneralForestry Technological Centel; Japan

Abstract

Tensile properties of heat-treated and fibers were examined. The average tensile strengths of hemp and bamboo fibers without heat treatment are 863 h4Pa and 516 hPa, respectively. After heat treatment at 200 “c for 7.2 ks, both fibers show a 60 percent decrease in tensile strength. The tensile strength of hemp is almost unchanged below 160 “c, however it decreases drastically above tis temperature.The bamboo fiber also has thesame temperature dependence,but its degradationtemperature decreases to 140 “c. Young’s moduli for both fibers are almost constantand independent of tieheat treatment condition.

1 Introduction h recent years, the life cycle of goods is getting shorter, thus the problem of waste managementhas been looked at more closely. There are various problems on waste management, such as increase in quantity and diversity of wastes and depletion of disposal sites, thus urgent provision against such problems is needed. This is especially important for plastic materials used in broad fields that are usually disposedoff m the ground. Smce they are hardly decomposedm the soil, theyhave a lasting bad effecton the environment. The utilization of biodegradable plastics, instead of conventional plastics that do not decompose naturally, may become one of the most effective provisions against such waste problems [l, 21. When the biodegradable plastics are buried in the soil, they are decomposedby the action of microbes, and the biodegradableresin fmdly becomes water and carbon dioxide, which are absorbed by . However, © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). 1 l8ISBNffigh 1-85312-904-6h-jiortnance Structures and C‘omposites biodegradableplastics have low strengthin comparison with conventional plastics. Therefore it is necessary to increase their strength by combining with a suitable strengthening material. This research wasperformed to examine the effect of temperature on the strength of natural fibers to determine the proper molding temperaturem preparing biodegradable composite materials.

2 Methods

2.1 Preparation of fibers h tis research, fiber bundles were used, because it was difficult to separate single fibers of bamboo and (hereafter hemp) [3]. Steam explosion method was wed to tieout the bamboo fiber, and then the parenchyma attached around the fibers was removed by wiping with a wet cloth. Hemp fibers were extractedfiom stems by boiling h an alkaline liquid.

2.2 Heat treatment

Heat treatment for natural fibers was performed in an electric furnace at 140 “c, 160 “c, 180 “c,and 200 “c for 0.9 ks, 1.8 ks, 3.6 ks, 5.4 ks, and 7.2 h in air.

2.3 Tensiletest h order to minimize the damages introduced mto the fiber during its handling, the paper sheet shown in Fig. 1 was used. The cross-sectional area of fibers was calculated fiom tie diameter measured using m optical microscope.A fibex was glued on a paper sheet. This sheet was carefully gripped, and then cut With heated ti metal weat the cuttingpoint as indicated in Fig. 1. Tensile test was performed at strain rate of 6.67 X 10“ C’. Ten specimens of bamboo and hemp fibers each were tested for thecurrentstudy and analyzed.

3 Results and discussion

3.1 Screeningof fibers

The reinforcing fiberswed for biodegradable composites must have high strength and be replenished m a short time. Firstly, m order to select the strong natural fiber, tensile tests for some natural fibers were performed. The typical stress-strain curves of hemp, bamboo, , and fibers are shown in Fig. 2. Tensile strength andYoung’s modulus decrease in orderof hemp, bamboo, jute, and straw. The growth cycle of bamboo and hemp fibers is 1 or 2 years. Thus, these two natural fibers are considered environment-tiendly and the use of thesefibers does notlead to global deforestation. Therefore, we decided to exmethe mechanical properties of heat-treated bamboo and hemp fibers. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6 High Performance Structwe.s and Composites 1 19 Natural fiber

Figure 1: Dimensions of sheet and specimen for natural fibers.

0 0.01 0.02 0.03 0.04 Strain E

Figure 2: Tjqicd stress-strain curves for some natural fibers.

3.2 Mechanical propertiesof heat-treated fibers

The relationship between the tensile strength of bamboo and hemp fibers and heating temperature is shown m Figs. 3 and 4, respectively. A 95% confidence © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). 120ISBNHigh 1-85312-904-6Per;for*nzanceStrucftwes and Composites interval has been plotted dong with the average value at various temperatures. The average tensile strengths of hemp and bamboo fibers without heat treatment are 863 MPa and 5 16 MFa, respectively. The tensile strength of bamboo fiber does not decrease below 140 “c, however it rapidly decreases around 160 “c. On the other hand, the tensile strength values of hemp fiber do not change until 160 “c. But it significantly decreases &er treatment at 180 “c for 3.6 ks or more. Figures 5 and 6 show the effect of heating time on the tensile strength of bamboo and hemp fibers, respectively. We can see that the tensile strength of bamboo fiber rapidly decreases at heating temperatures of 180 “c and 200 “c, however it shows a tendency to decrease gradually at 160 “c. On the other hand, there is no significant effect of heating time on the tensile strength of bamboo fiber heat-treated at 140 “c. The tensile strength of hemp fiber decreases with increasing heating time in the case of 200 “c. The tensile strength of hemp fiber heat-treated at 180 “c for 1.8 ks is almost same as that of non-heat-treated fiber. At160 “c, even when heatingtime is mcreased, the tensile strength of heat-treatedhemp fiber does not decrease. According to these results,the processing temperature for fabricating natural fiber composites should be below 140 “c and160 “c forbamboo and hemp fibers, respectively in orderto minimizethe strength loss dueto thermal degradation.

- 1500

EW 1000- 5 W E Ll c,m 500

0 I I I I I Non 10050150200 Heating temperature T (“C) Figure 3: Relation between heating temperatureand tensile strength of bamboo fiber. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6 ftighPerjiomzance Stmctwes and Composites 121

1500 I I I I -e :/Hemp1 v5s b &

a S m 5 F I I I I 1 nv Non 50 100 150200 Heating temperature T ('C)

Figure 4: Relation between heating temperatureand tensile strength of hemp fiber.

Heating time t (h) Figure 5: Relation between heating timeand tensile strengthof bamboo fiber. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6 122 Iligh f’erfomunce Sfmctwesund Composites 1500 n“-

WE l000

500

0 0 1.8 3.6 5.4 7.2 Heating time t (ks)

Figure 6: Relation between heating time and tensile strength of hemp fiber.

3.3 Young’s modulus

A Young’s modulus was calculated from the stress-strain curve. Before heat treatment, the elastic modulus of bamboo and hemp fibers was 17.7 GPa and 28.1 GPa, respectively. We found that various heat treatments to bamboo and hemp fibers have no effect on their elasticmodulus.

3.4 Fracture behavior of fiber

SEM photomicrographs of fractured bamboo and hemp fibers without any heat treatment are shown in Fig. 7. Figure 8 shows the SEM photomicrographs of fiactured natural fibers with heat treatment at 200 @Cfor 7.2 ks. As observed m Fig. 7, thenon-heat-treated bamboo andhemp fibers fracture by defibrillation. It seems that adhesive points of each single fiber are the sting point of hcture. However, as clearly seen m Fig. 8, the fractured surface of the specimen is relatively flat with no evidence of splitting of . Thus, after heat treatment the cellulose [4], which constitutes most fibers is damaged and tiscauses sucha fracture. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6 High Performance Stmctut-es and Composites 123

Figure 7: SEM photomicrographs of fracturednatural fibers without heat treatment; a) bamboo and b) hemp fibers. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). 124 HighISBN 1-85312-904-6Performance Structwes and Conzposites

Figure 8: SEM photomicrographsof fkactured ~tidfibers afterheat treatment at 200 @Cfor 7.2 ks ;a) bamboo and b) hemp fibers. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: High Performance Structures and Composites, CA Brebbia and WP de Wilde (Editors). ISBN 1-85312-904-6 H+1 Per/oumme .9mctwes mdComposites 125 4 Conclusions

This research was performed to examine the effect of heating temperature on the strengthof bamboo and hemp fibers. The results obtained are as follows: 1) Tensile strengthsof bamboo and hemp fibers decreased after heat treatment. 2) For bamboo and hemp fibers, heat treatment at temperature up to 140 “c and 160 “c,respectively,does not decrease the tensile strength. So, 140 “c and 160 “C are the highest fabrication temperatures that do not effect the strength. 3) Manila hemp fiber has higherstrength andbetter heat resistance than bamboo fiber.

References

[l] Yoshiharu, D, The story of biodegradable plastics, Japanesestandards association, Japan, 1991. [2]Siraishi, N, The biodegradable plastics, Mokuzai Gakktishi, 41-7, pp. 621-630, 1995. [3] The society of fiber science and technology Japan, The illustrationoffiber‘s morphology,Asakura-Syoten, Japan, 1986. [4] The cellulose society of Japan, The dictionary of cellulose, The cellulose society of Japan, 188-193,2000.

Acknowledgements

The authors would like to express special thanks to Dr. Katsunobu Konishi (The University of Tokushima), Mr. Yoichi Fujmori (Fujiseishi Co., LTD), and Mr. Toshihide Sugahara (MamhachiCo., LID) forsupplying the experimental materials.