A material characterization and embodied energy study of novel clay-alginate composite aerogels
Introduction
Since two thirds of the heat generated in a building can be lost through the building fabric [12], the use of appropriate thermal insulation is critical in helping to minimise energy losses and reduce fuel costs for the occupants. It also decreases the reliance on mechanical heating systems. Effective insulation materials are generally those which have a cellular or porous structure, a low density and a low thermal conductivity. Common products therefore include the likes of mineral wool which typically achieves a thermal conductivity of 0.03–0.05 W/m-K [16]. Polymer based products such as polyurethane (PUR) and expanded polystyrene (EPS) are also particularly good insulators, exhibiting thermal conductivities as low as 0.02–0.03 W/m-K [35]. However these petrochemical derived insulation materials also exhibit poor environmental credentials due to energy-intensive processing techniques and the use of fluorocarbon gases [60]. Furthermore, PUR based products also perform poorly in fire scenarios and can emit toxic substances such as hydrogen cyanide [80]. When selecting insulation materials, it is therefore important to consider both the environmental impacts associated with production and the potential to reduce operational energy demand over the building's lifetime by reducing heat loss [47].
The use of natural, renewable materials has been identified as a potential means of reducing the embodied energy and carbon footprint of buildings [24]. Natural insulation products which include organic fibres such as those obtained from plant or animal sources have been commercialized in recent years and continue to be investigated with academic research [40], [49], [62], [73], [89]. The thermal properties of these materials are typically within the 0.04–0.08 W/mK range [75], [82]. This makes them generally inferior to polymer based products meaning that greater thicknesses are required in order to achieve comparable thermal performance. Natural materials are also disadvantaged in terms of their durability, moisture sensitivity and cost and therefore form only a small part of the UK market [82]. On the other hand, LCA studies [75] have shown that the embodied energy and global warming potential values of natural fibre products are generally lower than polymer based products, although some natural products like cork can in fact be worse than the likes of EPS and PUR. Indeed, mineral wool products also perform well in LCA terms despite being produced from non-renewable resources. As a result, when selecting appropriate insulation materials, there is often a trade-off made between technical performance, cost and environmental impact. Whilst mineral wool products remain the most popular choice for standard building insulation [36], offering the best balance between cost and technical performance, alternatives which offer lower thermal conductivities are now being investigated for high-performance applications. One such development is the introduction of aerogels into the building insulation market. These materials originated from early research by Kistler [37], [38] and are prepared by removing the liquid phase from a hydrated gel using supercritical drying. The result is a highly porous, low density material which can be formed into aerogel monoliths and granules or combined with another material to form a composite product [56]. The majority of commercially available aerogels are silica-based aerogels and these can be used as high performance insulation products for buildings in the form of boards, blankets or loose-fill granules [7], [16], [35], [61]. They can also be incorporated into vacuum insulation panels (VIPs), a composite product consisting of a cellular core which is then vacuum sealed within a layer of foil faced plastic [1], [44]. More recent developments have included insulated plasters which incorporate silica aerogels [11], [79] and aerogel/glass fibre composites [84]. Aerogel products therefore offer very low thermal conductivities for relatively small thicknesses, making them particularly useful within retrofit projects where space is often restricted [51]. For example Lolli and Hestnes [47] demonstrated that an aerogel insulation product of 45 mm would achieve the same thermal performance as 100 mm of mineral wool. Furthermore, for a VIP, the equivalent thickness was only 25 mm. Indeed high performance VIPs with an aerogel core can reportedly achieve thermal conductivities as low as 0.012 W/mK [43] whilst Buratti et al. [11] described aerogel granules with a thermal conductivity of 0.019–0.023 W/(mK). Commercial silica aerogel blankets offer values of 0.018 W/mK (Aspen [5]) but silica-based aerogels are however disadvantaged with respect to their environmental performance due to the energy-intensive production processes and hazardous solvents used in their production [21]. In addition, high production costs are a limiting factor on their widespread use [16], [71], particularly in the case of monolithic aerogels which have been more difficult to commercialize than granules and composite products [56]. Some authors have therefore proposed alternatives to silica based aerogels which are derived from more environmentally friendly precursors. Kistler [38] for example experimented with various natural substances such as cellulose, gelatine and agar during his early work and van Olphen [85] also studied various water-soluble polymers in combination with clay minerals. Aerogels produced from other natural polymers such as starch [23] are also being investigated as a means of producing thermal insulation materials. More recently, Schiraldi et al. [76] have developed a product known as Aeroclay™ using clay aerogels modified by a range of polymeric sustances: epoxy [3]; PVOH and various natural fibres [14], [25]; casein [28], natural rubber [63] and alginate [13]. Reportedly, these clay-based composites can be manufactured at a competitive price utilising a relatively simple freeze-drying process, making them potential alternatives to silica aerogels [17], [77]. As discussed by Madyan et al. [50], the physical properties of clay aerogels, including the density, thermal conductivity and combustion behavior, can also be tailored by modifying the processing conditions and through the use of various additives or coatings. There are however limited details of the embodied energy of clay aerogels and to what extent the inclusion of additives, whether synthetic or bio-based, influences their overall environmental impact.
Given that the ideal product would be one which offers thermal properties comparable to high performance insulations combined with minimal environmental impact, it was postulated by the authors that a clay-polymer aerogel consisting of natural raw materials may offer a potential solution. For the purposes of this study, a natural bentonite clay and one of the aforementioned biopolymers, alginate, were therefore used to create a series of composite aerogel materials which could be studied in relation to both their physical properties and production. Whilst a few studies have demonstrated that aerogels with high porosity and low bulk density can be created using layered silicates and alginate [13], [59], the role of the alginate, which is a biopolymer obtained from seaweed, is not discussed in great detail. This is an important aspect to consider given that alginate is a natural material which can vary widely in its composition and functionality depending of the specific seaweed from which it is sourced. There has therefore been no comprehensive study to date which discusses the role of alginate variables (source, M/G ratio, viscosity and concentration) on the structural properties of composite alginate-clay aerogels such as density, mechanical strength and morphology. A total of four different alginate products were therefore tested in order to assess the feasibility of producing such a composite and to determine the relative importance of different alginate variables on the final properties of the aerogel. The final objective was to assess both the commercial viability and environmental impact of the alginate-clay composite in comparison with other aerogel materials.
Section snippets
Alginate
Alginate is a biopolymer obtained from brown seaweeds - also referred to as macro-algae. More specifically, alginate is the collective term for the salts of alginic acid which are obtained from the cell walls of the macro-algae; these salts, usually in the form of sodium or potassium, contribute to 20–60% of the algae's dry matter. [69]. Alginate is obtained by firstly washing the milled seaweed in acid in order to eradicate the cross-linking ions and solubilize the alginate salts [54]. The
General properties
In general the quality and homogeneity of the samples was found to improve upon the addition of alginate. Indeed the clay only samples (E) were very friable and crumbled into a powder upon removal from the vial meaning that suitable monoliths for further tests could not be produced. The alginate containing samples were much more stable and easier to handle although in some cases (AC-A, AC-D, PR24-A and PR24-B) visible air voids and defects were observed (Fig. 1). These defects appeared to be
Conclusions
In the past decade, high performance insulations such as aerogels have emerged as alternatives to conventional insulating materials and have the potential to reduce heat losses in buildings, particularly in retrofit scenarios where minimum product thicknesses are desirable. Although the high costs of aerogel insulations still hinder their widespread use, strategies to reduce their processing costs and make use of lower cost raw materials, will likely make aerogels more affordable in the future
Acknowledgments
The author wishes to thank the funding providers for the project including the University of Strathclyde, the Energy Technology Partnership and Marine Biopolymers Ltd. Acknowledgement is also made to the Advanced Materials Research Lab and the Chemical Processing and Engineering Department at the University of Strathclyde where the experimental work was conducted.
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