# Fabricating Sustainable All-Cellulose Composites
## 5 Production of ACC
### Factors Influencing Composite Properties  
The mechanical properties of composites are influenced by several factors (Figure 7). The source of the raw materials is a crucial element. Amongst the cellulose-based natural fibres, flax, hemp and ramie fibres are having the highest tensile strength (345-1830 MPa, 550-1110 MPa, 400-938 MPa respectively) and the highest specific Young's moduli (18-53 GPa/g cm\({}^{-3}\), 39-47 GPa/g cm\({}^{-3}\), 29-85 GPa/g cm\({}^{-3}\) respectively) [174]. However, there are much variability within literature. The average modulus of tensile strength of regenerated cellulose fibres has been measured to have lower values than flax fibres, but fracture toughness of regenerated cellulose fibres was superior to flax [175]. In comparison with natural fibres, the man-made cellulose fibres also show better homogeneity in terms of their structure and mechanical properties [176]. Nanocellulose as the raw material can improve tensile modulus of the composites and can be used in the production of high-performance composites. Methods and values regarding the properties of nanocellulose varies in the literature, but generally smaller specimens tend to have higher density and higher strength [177]. Therefore, it is not unexpected that a nanofiber-based ACC sheet has been proven to be 11 times stronger than a similar sheet from microfiber, when same raw material (canola straw) and dissolution method (DMAc/LiCl) was used [178]. Besides the fibre size, also the source of the raw material has a huge influence on the ACC properties. Comparative research, where two different forms of cellulose nano-whiskers were used to produce the ACC, concluded that tunicate cellulose enhanced the mechanical properties of the composites to a greater degree than cotton derived nanocellulose [179]. Volume fraction, aspect ratio and adhesion between reinforcement and matrix are highly relevant for the creation of the composites [39; 138]. High cellulose content in reinforcement and good fibre dispersion will promote better mechanical performance and interfacial bonding in natural fibre composites [174; 180]. In general, the strength of an ACC increases up to a certain limit proportionately with increasing filler fraction [102]. After certain limit the increase of filler content will not enhance properties, but rather decrease them. When the maximum content is achieved, the strain to failure of the composite decreases, leading to an embrittlement of the material [179]. In addition, when the  
Figure 7: Physical and chemical properties of ACC depend on raw materials (and their compatibility), architecture of the composite, chosen solvent system, processing method and applied experimental conditions. reinforcement content is very high, a large number of voids might appear, which is seen as low density [173]. Better compatibility between matrix and reinforcement has been researched for the two-step processed ACC when there was higher cellulose content in matrix constituent. According to [PERSON] [181], it would be beneficial to have a high cellulose content in the solution of dissolved cellulose to minimize matrix shrinkage during regeneration, which helps to maintain good bonding between the matrix and the reinforcement. On the other hand, solutions with high cellulose content have high viscosity, which makes their impregnation challenging. Very low cellulose concentrations, in turn, lead to inconsistent cellulose network formation. The possible formation of pores or voids after regeneration leaves the ACC structurally heterogeneous and mechanically weak. Similarly, the use of cellulose reinforcement with a high degree of crystallinity makes the interfacial bonding poor, as the particles are less accessible to the hydroxyl groups of the more open-structured matrix cellulose. Therefore, the degree of shrinkage for dissolved and undissolved cellulose will vary during the drying process, which can also create unwanted interfacial voids.