POLLACK PERIODICA An International Journal for Engineering and Information Sciences DOI: 10.1556/Pollack.8.2013.2.5 Vol. 8, No. 2, pp. 41–46 (2013) www.akademiai.com
PROPERTIES MONITORING OF FIBROUS COMPOSITES BASED ON HEMP HURDS WITH DIFFERENT MEAN PARTICLE SIZE Julia CIGASOVA, 2Nadezda STEVULOVA, 3Jozef JUNAK
1
Institute of Environmental Engineering, Faculty of Civil Engineering Technical University of Košice, Vysokoskolska 4, 042 00 Kosice, Slovakia e-mail: 1
[email protected], 2
[email protected],
[email protected]
Received 27 October 2012; accepted 19 February 2013
Abstract: In this paper, the effect of mean particle length of hemp hurds on compressive strength and other parameters of fibrous-composites is studied. Hemp hurds of various origin (Hungarohemp LTd, Nagylak, Hungary; Hempflax, Netherlands) with a wide particle size distribution were used in the experiments. Six samples of hemp hurds with various mean particle length (from 7.3 mm to 39.9 mm) were used for the preparation of fibrous composites based on MgO-cement as a binder. Bulk density, compressive strength, thermal conductivity and water absorbability of fibrous composites after 28 days of hardening were tested. The impact of men particle length of hemp hurds slices on values of bulk density, compressive strength and water absorbability of hardened composites was confirmed. Keywords: Composite, Hemp hurds, Mean particle size, Compressive strength
1. Introduction In recent years, there has been a growing interest for the use of natural fibers in composite applications. Nonwood or agro-based fibers are a potential source of material as reinforcing agents in composite materials. However, these fibers have tremendous variations in chemical and physical properties compared to wood fibers. Hemp (Cannabis sativa L.) is a particularly interesting plant fiber material for reinforcing lightweight composites [1]. The use of natural fibers and in particular hemp hurds as reinforcing agents in composite materials offers many advantages, e.g. a low density and an enhanced biodegradability, over glass fibers. The properties of natural fibers at the use in lightweight composites vary considerably depending on the fiber diameter,
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structure (e.g. type of cellulose, nature of defects, orientation of the chains of noncrystalline cellulose and crystalline fibrils, proportion of crystalline fibrils and noncrystalline regions, spiral angle), supramolecular structure (degree of crystallinity), degree of polymerisation, void structure (pore volume, specific interface, size of pores) and whether the fibers come from the plant stem, leaf or seed. Climatic conditions, age and the digestion process influence the chemical composition of the fibers and their chemical characteristics. Plant fibers include bast fibers, leaf or hard fibers, seed, fruit, wood, cereal straw, and other grass fibers. Bast (flax, hemp, jute) fibers and hard fibers (sisal, coir) are commonly used in composites [2], [3]. Hemp contains bast fibers and a woody material called hurds. Fibers and hurds are used in a range of products like textiles, paper and building materials. Hemp hurds consist of mainly crystalline cellulose (55-72%) as well as hemicelluloses (8-19%), lignin (2-5%) and waxy substances [4]. The inherent difficulty of using natural fibers is due to the fact that their chemical and structural characteristics are complex. Specifically, hemp fibers are very heterogeneous and it can be considered as a composite system. Fibers architecture is depicted in Fig. 1. The basic unit consists of cellulose polymeric chains aligned and gathered in microfibrils. They are linked to each other by lignin, pectin and hemicellulose. The strength and stiffness of the fibers are provided mostly by hydrogen bonds between the different chemical components. Other characteristics like thermal stability, resistance to UV attack or biodegradation depend on the concentration of each component characterized by its individual properties. Hemicellulose is responsible for the biodegradation, moisture absorption and thermal degradation of the fiber. Lignin and pectin are thermally stable but are responsible for the UV degradation of the fiber [2], [5].
Fig. 1. Hemp fiber architecture
However, the major disadvantage of cellulosic fibers is a high moisture sensitivity, which can cause the chemical degradation of the structure of fibers as well as dimensional variations of the fibers according to the percentage of moisture. This last effect has an impact on the quality of the mechanical interaction between hemp fibers and matrix. Thus, their incorporation in a polymer or mineral matrix implies the
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overcoming of interface incompatibilities by means of fibers chemical pretreatments [6]. The effect of using different binding agents: hydrated lime, cement, zeolite and MgO-cement in combination with hemp hurds on composites properties was examined in previous papers [6-9]. The study [7] shows the mechanical and thermal degradation behavior of hemp composites after different thermal loading. In this paper, the effect of mean particle size (based on the length) of hemp hurds on compressive strength and other parameters of fibrous-composites is studied in dependence on the origin of the hemp hurds.
2. Materials Hemp hurds used in the experiments (Hungarohemp LTd, Nagylak, Hungary and Netherlands company Hempflax had a wide particle length distribution. Six samples of hemp hurds (2 original samples and 4 fractions) of various origins were used. Mean particle length values (calculated from granulometric analysis) of all samples are given in Table I. Table I Mean particle size (dm) values of hemp hurds samples Sample
Granularity [mm]
dm [mm]
Hungarohemp Fraction 1 Fraction 2 Hempflax Fraction 3 Fraction 4
8- 0.063 8- 4 4- 0.063 8- 0.063 8- 2 2- 0.063
33.7 27.7 7.3 24.96 39.9 10.1
Chemical analysis of hemp hurds showed that content of polysaccharide component (holocellulose) is 71.5% in the case of Hungarian hurds and 74.5% in the case of Netherlands hurds. Chemical characteristic of components of holocellulose (cellulose and hemicellulose) and other components are shown in Table II. Table II Chemical analysis of initial hemp hurds Sample
Hungarohemp
Hempflax
toluene ethanol extract lignin cellulose hemicellulose ash
6.2 % 22.0 % 44.3 % 27.2 % 1.6 %
3.5 % 24.4 % 44.2 % 30.3 % 1.4 %
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Milled magnesium oxide (MgO), silica sand (SiO2) and sodium hydrogen carbonate (NaHCO3) are components of binder (MgO-cement) in mixture. In experiments, mixture consisted of 40 vol.% of hemp hurds, 29 vol.% of MgO-cement and 31 vol.% of water. The effect of MgO milling has been investigated in order to reduce its particle sizes and details of milling procedure are described in the paper [10]. The fresh mixtures were prepared in the laboratory mixer and the standard steel cube forms with dimensions 100mmx100mmx100mm were used to preparation of specimens. Next two days the composites were taken out of the forms and cured under laboratory conditions according to standard rules during 28 days.
3. Methods The bulk density, thermal conductivity coefficient, compressive strength and water absorbability were measured on hardened composites based on hemp hurds. Bulk density was determined in accordance with standard STN EN 12390-7 [11]. The thermal conductivity coefficient of samples, as the main parameter of heat transport, was measured by the commercial device ISOMET 104 (AP Germany). The measurement is based on the analysis of the temperature response of the studied material to heat flow impulses. The heat flow is induced by electrical heating using a resistor heater having direct thermal contact with the surface of the sample. Compressive strength of all composites was determined using the instrument ADR ELE 2000 (Ele International Limited, United Kingdom). Water absorbability was specified in accordance with the standard STN EN 12087/A1 (727056) [12].
4. Results and discussion During the laboratory study, physical and mechanical properties of composites based on hemp hurds with different mean particle length and various origins were compared to referential composite with polydispersive hemp hurds. In Table III, bulk density and compressive strength values of 28 days hardened composites are given. Changes in thermal conductivity coefficient and water absorbability values prepared composites are shown in Table IV. The measured values of compressive strength show that the mechanical properties of prepared lightweight composites from hemp hurds depend on mean particle length of used filler materials and also undoubtedly the origin has a great effect on the properties of composites (Fig. 2). Changes in behavior of prepared composites are observed, with decreasing particle length increases compressive strength of composites. This behavior depends on origin of used hemp hurds for preparation of lightweight composites also. Higher compressive strength values were reached for hardened composites based on Hungarian hemp hurds. The values of the water absorbability increase in the dependence on decreasing mean particle size of hemp hurds slices, whereas the effect of mean length of hemp hurds slices on thermal conductivity coefficient was not confirmed.
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Table III Bulk density and compressive strength of composites based on hemp hurds with different mean particle size and various origins after 28 days of hardening Sample
Bulk density [kg. m-3] 1040±5 1070±5 1150±5 1113±5 1120±5 1230±5
Hungarohemp Fraction 1 Fraction 2 Hempflax Fraction 3 Fraction 4
Compressive strength [MPa] 2.73 4.20 5.20 1.863 1.605 2.19
Table IV Thermal conductivity coefficient and water absorbability of composites based on hemp hurds with different mean particle length and various origins after 28 days of hardening Sample
Thermal conductivity coefficient [W/m.K] 0.111 0.115 0.110 0.082 0.069 0.074
Hungarohemp Fraction 1 Fraction 2 Hempflax Fraction 3 Fraction 4
Water absorbability [%] 21.38 25.81 11.88 14.33 7.59 6.30
Compressive strength [MPa]
6 5 4 Hungarohemp
3
Hempflax
2 1 0 0
10
20
30
40
50
mean particle size [mm]
Fig. 2. Dependency of compressive strength on mean particle size of hemp hurds
5. Conclusions The properties of composites based on hemp hurds of various origins with different mean particle length are studied. The results of these experiments confirmed the
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influence of mean particle size of hemp hurds slices depending on their origin on physical and mechanical properties of fibrous composites based on MgO-cement. Increase in values of bulk density, compressive strength and water absorbability of composites after 28 days of hardening with decreasing mean length of hemp hurds slices was found. The change in the density determines the compressive strength. The impact of mean length of hemp hurds slices on thermal conductivity parameter was not confirmed.
Acknowledgements The Authors are grateful to the Slovak Grant Agency for Science for financial support of the VEGA project 1/0231/12.
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