Analysis of thermal insulation materials based on plant fibers with Altami Studio software
Abstract – The paper presents results of analysis of thermal insulation materials based on plant fibers. It is suggested to use secondary raw materials of flax processing plants – flax noils – as a fiberfill in production of insulating slabs.
The paper presents results of research on the microstructure of flax noils and flax fibers conducted using light microscopy and electron microscopy, respectively. A set of experiments aimed at determining physical and mechanical characteristics of thermal insulation materials has been conducted. Test results attest higher efficiency of the insulating slabs made from flax noils in comparison with flax fiber-based thermal insulation material. The main factors facilitating reduction of thermal conductivity coefficient of thermal insulation material made from flax noils have been determined.
INTRODUCTION
In the conditions of global energy crisis, development of manufacturing of efficient building materials and economy of fuel and energy resources, including minimization of heat loss through building, construction and technological equipment envelopes, remain one of the priority tasks. Improvement of thermal resistance of building envelopes with the help of thermal insulation materials is the main solution in reduction of heating energy costs.
Apart from significant saving of heating energy resources, plant-based insulating materials may help improve ecological conditions, including reduction of СО2emissions. Therefore, development of new efficient thermal insulation materials based on plant fibers meeting the above-mentioned criteria is very topical in the field of insulation material manufacturing.
Insulating slabs Ecoteplin produced in Russia can be considered a successful development in this area. The slabs are produced from flax fiber and a binder – starch. Borates are used as fire and bioresistance agents [3], [4].
Research was conducted at Riga Technical University with an aim to develop thermal insulation materials based on hempshives, hydraulic lime and various additives. As a result, thermal insulation materials with the density of 312337 kg/m3 were obtained, with thermal conductivity ranging from0.101 W/(m°С)
MATERIALS AND METHODS
Analysis of the microstructure of flax noils and the surface of flax fibers was conducted using light microscopy. Analysis of the microstructure on the optical microscope Altami MЕТ 5 С allows obtaining images of the surface structure of the analyzed objects. The microscope has a special lamp installed at the lens that is based on light reflection. The obtained images were displayed on the computer screen and recorded on the hard disk. Application of Altami Studio software allows combining the obtained consequent images of sample fragments to enlarge the area of an image of the analyzed material.
The main set of experiments aimed at determining physical and mechanical properties of the samples was conducted separately for the fiberfill made of flax noils and flax fibers, and also for the fiberfill made of the mixture of flax noils and flax fibers at the ratio of 80:20.
DETERMINING THERMAL CONDUCTIVITY OF PLANT FIBER-BASED INSULATION MATERIALS
At the initial stage of research, the following plant fibers were considered as potential raw materials for obtaining efficient fiberfill for thermal insulation slabs: nettle, reed, flax, flax noils, oil palm bark. Sodium silicate was used as binder. Thermal conductivity was determined for fiber fill of various origin in thermal insulation materials with density of 50 kg/m3.
Among the obtained materials made from plant fibers, the lowest rate of thermal conductivity was demonstrated by the samples made from nettle and flax noils, it equals 0.041 W/(m°С), which is by 0.0060.014 W/(m°С) lower than the rates of materials based on other types of fiberfill analyzed.
APPLICATION OF LIGHT AND ELECTRONMICROSCOPY IN ANALYSIS OF MICROSTRUCTURE OF FLAX NOILS AND FLAX FIBERS
The images of a flax fiber (Fig. 1) and a flax noil (Fig. 2) were obtained with light microscopy combining the pictures of the consequently placed fragments of the analyzed sample. For example, Fig. 2 presents an image of a 6 cm long flax noil. The fragment highlighted by the frame in the figure is presented in the enlarged form in Fig. 2 b.
The obtained images attest that a flax fiber consists of a conglomeration of thinner fibers bundles of elementary fibers firmly attached to each other through elementary fibers, as a result firm longitudinal connection of the fibrous system of a flax stem is ensured. At the same time, a flax noil consists from ragged bundles of elementary fibers (Fig. 2 a). Elementary fibers in a noil periodically interconnect with each other due to chaotic contact connections. As a result, mesh fiber frame is formed that ensures firm longitudinal connection of the entire structure of the flax noil. The noils get interconnected due to side branches in the form of elementary fibers forming spatial mesh fibrous system. Elementary fiber is a spindle shaped plant cell. In the micro-image (Fig. 2 b) in the reflected light it can be seen that elementary fibers have narrow inner channels from 4 μm to 6 μmin diameter. The length of elementary fibers varies from 10 mm to 40 mm at diameter from 8 μm to 12 μm.
Several concentrically arranged layers differing by various degree of refraction are distinguished in the structure of an elementary fiber [14]. The first region of the cover is relatively thin, it mainly consists of pectic substances that glue cells among each other. The primary wall consisting of cellulose with significant hemicellulose content, pectins and often lignin form the next region. The secondary wall is also formed from cellulose and is characterized by various rates of refraction due to smaller amount of additions of the above-mentioned substances. At the initial stage of development, elementary fibers are essentially round cells filled with plasma. As the respective zone grows, these cells elongate, their cover considerably thickens from the inside and reaches such degree of thickness that the inner cavity with plasma can be spotted only as a very narrow channel.
Thus, the obtained results of light microcopy aimed at the analysis of the fiber structure (Fig. 2 b) fully attest the presence of a void channel in an elementary fiber.
Application of a scanning electron microscope allowed visually attesting that the flax fiber consists of bundles of elementary fibers (Fig. 3 а). In the image of the fiber, the frame highlights the fragment that is enlarged in Fig. 3 b. White formations marked by the arrow are micro fibrilles forming due to presence of non-cellulose polysaccharides and pectin [15]. The conducted electron microscopy analysis attests morphometric parameters of elementary fibers determined while studying flax noils with light microscopy, it also allows determining that the size of bundles is from 50 μm to 70 μm in diameter, given there are from 10 to 20 elementary fibers in the structure of the bundle. The data of microscopic analysis demonstrate that a less “coarse” and more efficient thermal insulation micro-mesh structure can be formed from flax noils in comparison with the materials based on flax fibers.
CONCLUSIONS
Among the considered thermal insulation materials based on plant fibers, the insulation materials based on flax noils or nettle fibers display the best thermal conductivity indicators.
The conducted light microscopy tests allowed determining that the microstructure of the flax noil is made from a conglomeration of chaotically interconnected elementary fibers, which conditions formation of the mesh fibrous frame of the flax noil. In the process of contact, flax noils form a spatial micro-mesh fiber system. It has been determined that elementary fiber is a micro-tube from 8 μm to 12 μm in diameter, with void channel from 4 μm to 6 μm in diameter, which is compatible with the size of solid fibers of rock wool, ensuring formation of efficient isolating structure.
The obtained insulation materials made from flax noils have thermal conductivity of 0.036-0.041 W/(m°С) and compressive strength at 10 % deformation from 0.11 ∙ 10‒2 MPа to 0.33 ∙ 10‒2 MPа at the density from 50 kg/m3 to 110 kg/m3.
The factors that have a significant impact on the thermal conductivity rate of fiberfill have been determined: presence of the fibers less than 20 μm in diameter; presence of void channels in the fibers; chaotic directionality of fibers in space that ensures formation of mesh structural framework; reduction of the total fiber contact area; reduction of the size and localization of the micro-voids in the structure of thermal insulation material.
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