One of the most important challenges for the construction industry is to reduce the energy demands of buildings throughout their entire life cycle. During the use of a building, its thermal demands are undoubtedly influenced by its insulation. Commonly used thermal insulation materials for building insulation are produced from petrochemical products or from natural sources, but their production is highly energy intensive (glass, rock, wool) [1
]. From this perspective, the use of recycled and plant materials is very promising for the production of thermal insulation. In the case of plant materials, rice husks [2
], sunflower stalks [3
], wheat straw [4
], wheat husks [5
] flax fibers [6
], hemp fibers [7
], larch bark [8
], and many others can be considered for thermal insulation production. Recycling synthetic materials or using agricultural or industrial residues can be an effective way to reduce virgin materials consumption [9
]. Products from recycled plastics such as polyethylene terephthalate [10
] and recycled textile fibers [11
] provide very good thermal insulation properties.
However, a significant disadvantage of plant materials consisting mainly of cellulose, hemicelluloses, lignin, and pectins is their flammability [12
]. In terms of building materials, their resistance to burning by geopolymer applications [13
] can be improved significantly. Geopolymers are materials usually synthesized using an aluminosilicate raw material and an activating solution mainly composed of alkalis of sodium or potassium and water glass [15
]. Due to the properties of geopolymers in the form of high strength, resistance to chemicals and, in particular, thermal stability and fire resistance, applications of these geopolymers in the form of protective coatings or coatings on structures [17
] have been studied in recent years. In the past, the fire resistance of particleboards based on winter rapeseed stalks [13
] has been successfully increased by the geopolymer layer. Even better geopolymer properties can be achieved, for example, via the implementation of carbon fibers, which result in better mechanical properties of the entire composite [22
An important property of cellulose-based plant fibers is hygroscopicity. This property may be an advantage in some applications and a disadvantage in other applications. However, in terms of thermal insulation of structures, high humidity in the insulation is undesirable, as water reduces the thermal insulation properties of the material [23
]. On the other hand, we require, from natural fiber, thermal insulation interior vapor permeability through the building envelope to the exterior [24
]. Preventing the permeability of liquid water from the exterior into the building envelope and, at the same time ensuring the transport of water vapor from the interior through the building envelope to the exterior, is ensured by a suitably-selected wall structure [26
]. One of the elements that can be used in the wall structure for this purpose can be a nanofibrous membrane, which provides water vapor permeability, but prevents the permeability of water in the liquid state [27
]. In addition, a suitably-designed nanofibrous membrane can withstand a very high water column, which can affect the building, for example during floods [27
This paper deals with the use of post-harvest residues of winter wheat and recycled polyurethane foam in combination with geopolymer foam and a nanofibrous membrane for the production of composite materials with properties for the given purpose of use. The aim of this work is to determine the influence of winter wheat husk and the implementation of a nanofibrous membrane and a geopolymer layer into the sandwich panel structure on its mechanical and physical properties. This paper contributes to finding material utilization of wheat husks, which provides storage of CO2
in comparison with energetic utilization of this raw material. Moreover, addition of husks into the heat insulation panel may bring additional benefits during manufacturing of these panels. Since wheat husks contain 12.7% moisture [5
], no steam injection would be necessary for hardening of polyurethane adhesive.
3. Results and Discussion
and Figure 4
show the distribution of the PUR foam crushed fraction and the winter wheat husk. While the predominant part of the crushed PUR foam particles is in the range of 5 to 15 mm, the predominant part of the husk can be characterized by dimensions of 1.5 to 3 mm. The particle size has a major influence on the mechanical properties of the boards [32
]; however, in the case of the sandwich panels, where one layer is significantly stronger than the other, the geopolymer layer takes over all the flexural strength [33
]. In this research, the particle size affected tensile strength perpendicular to the level of the board.
shows the effect of the weight ratio of husk in the insulation board on the thermal conductivity coefficient. The picture shows that in both cases the measured thermal insulation cores achieved very good thermal conductivity values in the range from 0.0427 to 0.0452 W/(m·K). The addition of the husk to the crushed PUR foam resulted in a slight deterioration of 0.0025 W/(m·K) (a statistically significant difference); nevertheless, these are still very good values compared to other alternative raw materials. The achieved thermal conductivity values are slightly lower than in the case of thermal insulation boards made from reeds [1
], bagasse [34
], or cotton stalks [35
]. However, it should be noted that, in the above competing products, the manufactured boards had a higher density. For example, 30 kg/m3
recycled polyethylene terephthalate boards achieved a thermal conductivity coefficient of 0.0355 W/(m·K) [10
shows the effect of the weight proportion of husk in the insulation board on the volumetric heat capacity. The difference between the individual variants is statistically significant at a level of 0.05. As with the thermal conductivity coefficient, the addition of husk increased this characteristic. However, in this case, this is an improvement in the characteristic that can compensate for the increase in the thermal conductivity coefficient, in the form of a higher accumulation capability of the material and the retention of heat in the walls at a slight decrease in exterior temperature [36
]. However, panel cores containing husks achieved, still, a much lower volumetric heat capacity than another agriculture by-product—corn husks [37
shows the calculated thermal conductivity coefficient values of the entire sandwich composite panels and the measured density values of the individual materials. There were slight deviations in the actual thermal insulation board densities from their nominal values. The influence of nanofiber membranes on thermal insulation properties or fire resistance was not evaluated. The geopolymer layer only slightly worsened the thermal insulation properties of the sandwich composite. The total thermal conductivity coefficient is around 0.05 W/(m·K), which is a fully adequate value for thermal insulation materials [9
], and produced panels are comparable to other commonly used materials [38
]. The reached thermal conductivity coefficients are higher than the thermal conductivity coefficients of commercially-produced heat insulation panels from PUR or PIR (polyisocyanurate) panels; however, the developed panels are from recycled materials and from recycled PUR that was initially not produced for thermal insulation.
There was a statistically significant effect of the proportion of husk in the thermal insulation core on its internal bonding (Figure 7
). With an increase in the proportion of husk in the material, internal bonding was reduced to 0.64 kPa, which is already insufficient for thermal insulation materials according to standard EN 13162+A1 [39
]. For the production of industrially-useable thermal insulation panels with winter wheat husk admixtures, it would then be necessary either to select a higher proportion of adhesive [40
] or to include pre-treatment of wheat husks in the production process, in order increase their surface energy and thus reach higher bonding [5
The graphs in Figure 8
show the behavior of the entire panel under fire load. The samples were exposed to a flame with rapid (Figure 8
a) and gradual (Figure 8
b) temperature increases. No effect of the wheat husk additive on fire resistance was observed. However, the fire resistance of the boards was affected by the rate of temperature increase. In the case of a fast onset, the boards withstood the effect of flame for approximately 500 s, and more than 800 s in the case of gradual onset. Regardless of the steepness of the onset temperature curve, it was observed that when the temperature inside the furnace rises to around 400 °C, the temperature on the outer surface of thermal insulation boards increases to around 60 °C, which is then held constant until the material burns. These results correspond with results for sandwich-structured composites made from rapeseed stalks [13
], and, because of the flammable insulation core, the panel withstood lower temperatures than in [16
], where geopolymer composites were filled only by basalt microfibrils.
The developed sandwich panels were able to withstand fairly long-term exposure to a water column with a height of 80 cm. In 24 h, only 486 g of water flowed through the 154 cm2
area (Figure 9
). There was no difference found between the sandwich panel with the addition of husk and no husk. All of the resistance of the sandwich composite to the long-term effect of the water column is due to the used nanofibrous membrane and the interface between the nanofibrous membrane and the geopolymer. With regard to the thermal insulation sandwich panel without a nanofibrous membrane, this sandwich is virtually unable to prevent water flow. When the sandwich without a nanofibrous membrane was encumbered with a water column with a height of 80 cm, 3700 g of water flowed through the 154 cm2
area over 4 min.
The results show that the geopolymer layer in the entire sandwich panel suitably complements the thermal insulation core. The geopolymer layer provided the material with fire resistance, and it can be assumed that it would increase flexural strength [41
], while only slightly worsening the overall thermal conductivity coefficient. The geopolymer layer was thoroughly bonded to the thermal insulation core, and in the tensile strength test perpendicular to the plane of the board, there was no breach between these layers, but in the insulation core. The nanofibrous membrane also contributed to improving the properties of the entire sandwich composite. It gave the material resistance to long-term exposure to the water column, while not negatively affecting any other material properties.