# Fabricating Sustainable All-Cellulose Composites
## 4 All-Cellulose Composites  
### Raw Materials for the ACC  
Natural fibres are divided into three categories: plant fibres, animal fibres and mineral fibres (Figure 5). Plant fibres are the most used in conventional composite processing only as a reinforcement. All plant fibres contain cellulose, while animal fibres are made from proteins [136].  
Plant-based cellulose is considered one of the most promising substitutes for petroleum-based raw materials [137]. The fibre structure and characteristics depend on and vary with the plant species, age, growth location, climate and part of the plant. Therefore, one of the main challenges with plant fibres is the large variation in their properties [42; 138]. Fibres are extracted from trees using either chemical or mechanical methods. The characteristics of wood fibre depend somewhat on the processing method used, but mostly on the anatomy of the wood material in the particular tree species. Wood fibres can be divided into softwoods, such as spruce and pine, and hardwoods, such as birch and eucalyptus. Both can be used to create pulp or dissolving pulp. Dissolving pulp has a higher cellulose content and lower hemicellulose content compared to paper-grade pulp, and therefore it is used more commonly in upgraded and high-quality products. Two widely used commercial production methods for dissolving pulp are pre-hydrolysis kraft (PHK) and acid sulphite (AS) [98]. Pulp can also be produced from cellulosic waste with new innovative technologies [139; 140].  
Man-made cellulose fibres, such as viscose or lycoll, are developed from pulp or dissolving pulp. They are mainly used in the textile industry [141] but are also important materials used in composite manufacturing [142]. Textile-reinforced composites typically have high strength-to-weight and stiffness-to-weight ratios, which can improve the mechanical properties of the composites [143]. Cellulose-based textiles are renewable, but at the same time, textile industry as a whole (including the unsustainable cotton production) are one of the largest contributors to harmful environmental effects on Earth, and it also generates large amounts of textile waste [144; 145]. Therefore, the industry is searching for  
Figure 5: Classification of natural fibres with examples of their origin. This overview excludes nanosized cellulose fibrils and cellulose crystals that are found in bacteria and tunicates.  
more sustainable plant-based fibres to meet the demands of current standards, where the focus is on waste reduction, production efficiency, durability, water saving, recycling and closed-loop production [145].  
The frontrunner in the field of sustainable fibre production, Lensing AG, produces wood pulp-based fibres called EcoVeroTM, the production of which consumes less water and releases less emissions than generic viscose production, and their TencelTM x RefibraTM fibre contributes to the circular economy by using an efficient closed-loop process [146, 147]. Kuura-fiber is another new pulp-based wood fibre, which shares similar sustainable values, and it is currently on pilot scale production by Metsa Fibre in Aanekoski bioproduct mill, Finland [148]. Recycled textiles and agriculture waste are used in a fibre technology called InfinnaTM, created by the Infinitely Fiber company [149], and recycled cotton fibres have been transformed into sustainable yarn by Haksa Textiles [150]. More recently, new types of wood fibre have been mechanically produced without using harmful chemicals or solvent systems. In Spinnova's innovative fibre process, the pulp is mechanically refined and transformed into spinning-ready fibre suspension without dissolution or regeneration [151].  
These novel textile fibres offer versatile reinforcement choices for a more sustainable composite industry. Recently, biocomposites [152] and all-cellulose composites [153] have also been created using denim waste.