6962
Cellulose (2020) 27:6961–6976
Graphic abstract
Keywords Foam forming Cellulose nanofibril Wood fines Fibre Softwood Hemp Polyvinyl alcohol Locust bean gum Compression test Stress Strain Model
idealized fibre systems (Alimadadi et al. 2018; Hos- sain et al. 2019). The material characterization is challenging as strength can depend on the structural features in several size scales. These include the overall material network geometry, fibre properties (size distribution and flexibility) and the bonding ability of the fibres (total bond area and bond strength) (Alava and Niskanen 2006). Moreover, strength is a physical property that is not necessarily determined by the average structure but rather by structural hetero- geneity affecting both stress concentration and local load-carrying capacity. For that reason, the deforma- tion behaviour can be sensitive to the size distribution of fibres and fine components. With optimal combi- nation and distribution of raw materials, it is possible to increase compression strength through local load- bearing regions without deteriorating large-scale material homogeneity. Current paper investigates this question applying a selected set of raw materials that have an exceptionally wide variety of characteristic sizes. There is vast literature (Alava and Niskanen 2006; Picu 2011) on the strength properties of fibre networks in the medium density range of 300–1000 kg/m 3 , where the number of inter-fibre joints per fibre is large. The number of joints is proportional to the square of total fibre length in unit volume (Komori and Mak- ishima 1977). Thus, with decreasing density, the number of joints per fibre and the relative bonded area
Introduction
New low-density cellulose-based fibre materials made with the foam forming technique have recently been introduced as potential solutions to replace oil-based fibrous or foamed materials (Poranen et al. 2013; Madani et al. 2014). Feasible applications can be found in air filtration (Jahangiri et al. 2014), thermal insulation (Jahangiri et al. 2014; Po¨hler et al. 2017), in acoustic control as sound absorption material (Ja- hangiri et al. 2016; Po¨hler et al. 2016), and as cushioning material in packaging (Luo et al. 2017; Paunonen et al. 2018). These new lightweight mate- rials are made by mixing fibres with aqueous wet foam generated with the help of a foaming agent (Al- Qararah et al. 2015a). The bubbles in the foam support fibres until drying so that a highly porous, deformable fibre network is formed. Rather little is known about the structure-strength relationship of these ‘‘fibre foams’’, despite several recent experimental studies (Alimadadi and Uesaka 2016; Burke et al. 2019; Ketoja et al. 2019) and model simulations using
123
Made with FlippingBook Online newsletter maker