MECHANICAL PROPERTIES OF HEMP FIBRES AND INSULATION MATERIALS

Liga Freivalde
Riga Tehnical University, Latvia

Silvija Kukle, Dr.habil.sc.ing.
Riga Tehnical University, Latvia

Introduction

Hemp fibres are a renewable natural resource combining the benefits of a sustainable development promotion. Naturally derived fibres occupy an increasing proportion among renewable resources, replacing man-made fibres in a wide range of technical applications including nonwoven products. Hemp fibres are hygroscopic, with a low density and generate a limited number of toxic substances during combustion. All these properties are essential for industrial thermal insulation products. There is a positive hemp impact on the environment, as this crop consumes significantly less synthetic fertilizers and pesticides than other crops. There are thousands of products made from hemp, and some of them, undoubtedly, are being used as building insulation. Thus, hemp fibers are used in the production of insulating materials to minimize the damage to the environment, as they contribute to the reduction of CO2 emissions (Kozłowski et al,2008), have a low energy demand in production, high potential for recycling, and positive effect on indoor (Kymäläinen, 2004 ).

Latvian hemp cultivation is slowly being resumed. The revival of hemp fiber production in Latvia dates back to 2008. High hemp straw yields are being obtained with excellent physical and mechanical fibre properties (Baltina et al, 2009; Baltina, Zamuska, 2010; Freivalde et al, 2010 a, b).This study is concerned with determining hemp fibre bundle strength and development of nonwoven insulation materials using local hemp varieties based on appropriate specifications and comparative analysis with the EU introduced varieties, which are applicable to the climatic and soil conditions in Latvia.

People have always tried to improve their living place, to make it warmer, cosier and also good for health. Insulation materials made from renewable, natural and eco-friendly raw materials have a broad perspective for the construction industry. The development of this sector can ensure a high quality ecological, environmental and human-friendly Latvian production of a high added value.

1. Materials and methods

Hemp fibres used in the present study were obtained from hemp stems harvested from a trial plot – a local dioecious  genotype “Purini” and an ES registered monoecious industrial hemp variety “Bialobrzeskie” in the Vilani district of Latvia.

Fibres for the fibre break test were obtained in 2009. The break force test of hemp bast fibres was performed on fibre pull equipment PM-3-1 on fibre bundle level trim at various clamping distance lengths: 3 mm, 10 mm and 20 mm. During the sample preparation process bast fibres were separated from a woody core mechanically with the following bast fibre combing to separate large fibre bundles into single fibres and small fibre bundles, and remove secondary fibres. Samples for testing with the mass of 2 grams, length - 40 mm were prepared. To prevent fibre bundles from sliding out of the jaws, their ends were pasted into millimeter  graduated paper frames.

The fibres used in the development of nonwoven insulation materials were obtained from hemp stems grown in the year 2010. In their preparation for carding, fibres were pre-cut to 6 - 10 mm in length. Parallel-laid webs were prepared by carding where fibres are disentangled and mixed to create a homogeneous web. Nonwoven specimens were made from these webs with the help of hydroentangling, which is the process of  bonding fibres in a web by means of high-velocity water jets (Russell, 2007). Hydro-entanglement was performed on a seven injector 0.5 m wide machine. Webs were pre-wetted and hydroentangled using jet strips with a nozzle diameter of 150 μm jet strips at 100 bar (10 MPa) applied in an alternating face and back profile. The conveyor speed was fixed at 5 m/min. The fibres were  entangled, intertwined and interlaced with each other to produce a coherent structure. Normally,  immediately after hydroentangling a large fraction of interstitially held water in the fabric is mechanically removed by suction; but as  hemp is cellulosic, the water content remains very high even after mechanical extraction, and through-air drying is required (Russell, 2007). In this article four hydroentangled (H) samples for each variety are denoted with reference to the original web basis weights (g/m²) used to produce the fabrics: H100, H120, H140 and H160. The samples were also denoted according to the origin of the constituent hemp fibre, i.e. Purini (P) or Bialobzeskie (B).

The mechanical and physical properties of the nonwoven fabrics obtained were determined according to the internationally accepted standards: 1) Fabric thickness, mm (BS EN ISO 9073-2:1997), where method B was adopted with slight modifications: uniform pressure 0,02 kPa, i.e., 10 grams on 50.2 cm²;  2) Fabric mass per unitarea, g/m² (BS EN 29073-1:1992, ISO 9073-1:1989), modification: test area of the sample was 10 000mm²  instead of 50 000 mm².3) Maximum pore size mean value, µm . 4) Water vapour transmission, % (BS EN ISO 12572:2001), used glass dishes. 5) Thermal resistance, m²·K/W, (BS 4745: 2005, ISO 5085-1:1989, ISO 5085-2:1990), used two-plate method: fixed pressure procedure; 6) Thermal conductivity (k), W/(m·K), k=d/R, where d – thickness, R - thermal resistance, (BS 4745: 2005, ISO 5085-1:1989, ISO 5085-2:1990), 7) web weight, grams.

2. Results and discussion

Hemp fibre bundle strength averages depending on the clamping distance length (test span length) and corresponding confidence levels are included in Table 1. Experiments were performed to determine the hemp fibre bundle strength and the results are  shown in Fig. 1. By increasing the clamping distance length for Bialobrezskie variety, the fibre strength decreases. For Purini variety the fibres are strongest at the lowest clamping distance length - 3 mm and 20 mm, but weakest - at 10 mm test clamping distance length (test span length) (Table 1).

The fibre bundle tensile strength tests show slightly higher values at clamping distance length  3 mm and 20 mm, and lower  values for the test span length 10 mm.

Comparing the fibres organoleptically, it was found out that fibres of cultivar Bialobrezskie are more difficult to remove from the wooden core and they have much higher stiffness than Purini. It means that Bialobrezskie could be used as a typical  technical fibre resource for insulation and composite materials. Purini fibers as much softer fibres are more suitable for yarn spinning.

A summary of the hydroentangled sample test results is given in Table 2, which compares values for both types of the hemp variety.

When speaking about thermal insulation materials of textile fabrics, which also include nonwovens, the thermal insulation depends on several properties such as thermal conductivity, density, thickness and thermal emission characteristics (Zeinab Abdel-Rehim, et al 2006).

The original fibre amounts (in grams) used to produce the webs by carding for both varieties were the same: 100, 120, 140 and 160 grams.  But the web weight after carding was higher for variety Purini, as there was a higher weight loss during the carding process for variety Bialobrzeskie. The web weight after carding was in the range from 56,5 to 98,6 g for P and 41,7 to 82,4 for B (Tab.2. and Fig.1). Also the following fabric mass per unit area for webs after hydroentangling, shows the same coherence between the varieties – the overall tendency is that B samples are lighter than P samples. The fabric mass per unit area for P fabrics was in the range of 206 – 378 g/m²in contrast to B fabrics it was 185 – 305 g/m², which is apparent from Table 2 and Fig.2.

It can be seen from Table 2 and Fig. 3. that the fabric thickness for variety P was in the range of 2,1 till 3,32 mm, where for the variety of B fabrics it varied from 2,68 – 3,74 mm. Sample H120 thicknesses for both varieties are the same: 2,71 mm, but in all the other cases, P fabrics are thinner than B fabrics.

The density of the nonwoven fabrics depends directly on the material weight, thickness and area, as density can be obtained by dividing the fabric mass per unit area by the material volume. The density for samples of variety P varied from 98,1-116,2 kg/m³, where for B samples 69 – 89,3 kg/m³ (see Table 2 and Fig.4) and is nonlinear for both varieties. The relationship between density and thermal properties is not linear and it varies in different studies (Kymalainen, 2008).

The thermal conductivity of insulations made of bast fibres is compatible with conventional insulations. However, the variation between the conductivity values of all insulations varies, for example, in relation to bulk density and to thickness.

From Table 2. and Fig.5 it can be seen that the thermal resistance of the Purini fabrics during the experiment is changing relatively slowly - increasing by 25.4%, while the fabric thickness increased by 58.1%. The overall thermal resistance of the Bialobrzeskie fabrics was more sensitive to increases in fabric thickness than the Purini fabric - the thermal resistance overall increased by 34.3% in relation to the fabric thickness increase only by 39.6%.

Heat transfer through the material is known as conduction. The thermal conductivity, fabric bulk density, porosity and fabric architecture are the material structural parameters affecting heat transfer (Mao, et. al. 2007).

Thermal conductivity (k) was obtained from the results about the material thickness (t) and thermal resistance (R) from equation k = t/R. The thermal conductivity of the hydroentangled fabrics containing Purini fibres increased linearly, whereas no such trend was evident for the Bialobrzeskie fabrics (Fig.6). For the Purini group, an overall increase of 29% in the thermal conductivity was observed by increasing the fabric thickness from 2,1 - 3,32 mm; for the Bialobrzeskie it increased only by 12 % if thickness was in the range from 2,68 – 3,74 mm. But, all in all, for each type of samples when comparing the varieties, it can be seen that the results are very similar or even the same. The fabric samples of hemp variety P show the mean values of thermal conductivity in the range 0,031- 0,040 W/(m·K), but of B fabric samples it is  from 0,035 - 0,040 W/(m·K). Where a typical mineral wool has thermal conductivity in the range 0.035-0.040 W/mK, wood 0.21 W/mK, air 0.026 W/mK.

It is apparent from Table 2 and Fig. 7 that water vapour transmission is nonlinear for both varieties. Different disposition of the test results could be explained by distinctions in elementary fibre/ small fibre bundle properties such as  their diameter, elasticity and flexibility: if B is a source of highly technical and hard fibers, P fibers are softer, more flexible, and, as a result, better at hydro-entanglement exposure by creating a capillary structure through which water vapor moves quite well, even through the thicker layers, too.

The porous structure of the bast fibres, small diameter and the low bulk density, leading to trapping of a large amount of air between the fibres in the material, makes them suitable for thermal insulations. Table 2. and Fig. 8 report the pore dimensions of samples produced at H120 and H160. The pore sizes of the Purini fabric samples were found to be up to 17% higher than those composed of Bialobzeskie variety. As expected, an increase of water pressure which increases the specific energy consumed by the web, decreased the pore size by up to 30% due to the increased compactness  and fibre  entanglement.

Conclusions

This was an exploratory study intended for utilizing hemp fibers to determine their strength properties and to develop thermal insulation material samples for the construction industry.

Purini variety showed slightly better fibre bundle strength results at the lowest and highest clambing distance length - 3 mm and at 20 mm while Bialobrzeskie - at the clambing distance length 10 mm.

The experimental results of creating and testing nonwoven insulation materials from hemp fibre show that hydro-entanglement is a good technology for hemp fibre processing, enabling to obtain nonwoven materials with predefined properties according to the intended use. As the fibres of fabric B are coarser, more rigid, with less elasticity than P fibres, there was a higher weight loss during the carding process; as a result, intangled fabric thickness and mass per unit area are lower with the same raw fiber volume. Good water vapour permeability of a relatively large thickness range allows to apply hydroentangled nonwoven hemp fiber materials in natural breathing packet formation for   household, industrial and construction uses.

In the construction industry a material is defined as insulating if its thermal conductivity is less than 0.065 W/mK. A typical mineral wool has thermal conductivity in the range 0.035-0.040 W/mK, wood 0.21 W/mK, air 0.026 W/mK. The developed hemp fibre hydroentangled materials have an excellent insulation performance due to optimal thermal insulation properties, where thermal conductivity is from  0,031 to 0,040 W/mK for the tested variety P and 0,035 to  0,040 for variety B, due to the energy needed to heat the building is reduced. These results are consistent with the requirements relating to the natural insulating materials, where thermal conductivity is equal to 0.040 - 0.045 W/m· K. P fibers hydroentangled fabrics (average thickness 2,1 mm) with a good water vapour transmission and low thermal conductivity, could be of great interest for usages in specific garments and as  specific light weight insulation materials in building industry. The obtained results highlight the opportunity to develop local Latvian hemp genotype Purini as a  fiber hemp variety with a wide range of potential applications, processed not only by conventional, but also highly yielding, advanced technologies, thus, obtaining products with a high added value.

Acknowledgment

This work has been supported by the European Social Fund within the project “Establishment of interdisciplinary research groups for a new functional properties of smart textiles development and integrating in innovative products" (ESF Nr. 2009/0198/1DP/1.1.1.2.0./09/APIA/VIAA/148).

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