# Fabricating Sustainable All-Cellulose Composites
## 5 Production of ACC
### Factors Influencing Composite Properties
The negative impact of internal cracking can be avoided and controlled to some degree by applying light pressure during drying [182]. One way to improve mechanical performance is to apply additional chemical modification. For instance, alkali treatment can fill the cracks and voids between reinforcement and matrix, which contributes to improving the interface and the quality of the composite [183]. The engineered architecture and alignment of the reinforcing fibres have a major impact on the mechanical properties of the composite. Fibres can be continuous or discontinuous, aligned randomly or unidirectionally, and fabric architecture can be nonwoven, woven or knitted [2]. ACC with uniaxial reinforcement offer generally higher stiffness and strength along the fibre direction, compared to randomly reinforced composites [102; 184]. Lyocell fabric, with modified structure, had massive improvement in microstructure of ACC. Here, the fabric was modified by removing weft yarn to create unidirectional effect. Compared to regular fabric, this modified structure helped to bind the undissolved cellulose fraction properly, leaving almost no voids. This significant reduction of internal void content generated ACC with better tensile and flexural properties than those most conventional biocomposites [185]. ACC can be produced by using multiple different solvent systems and manufacturing methods. Comparative studies on how different solvent systems affect the properties of ACC can be challenging to carry out reliably, when each solvent system still need to be optimized individually. Notwithstanding, [PERSON] et al. (2014) found out that LiCl based matrix performed better than NaOH/urea-based system for ACNC production regarding tensile strength and modulus [179]. Many of the process variables in ACC production, such as temperature, humidity, pressure and dissolution time depend on chosen solvent system. Generally, the strength of the fibres increases with increasing moisture content and decreases as temperature increases [186]. When producing ACC-based aerogels, a major influential factor is the drying method--freeze-drying, CO2 drying or vacuum drying--since it determines the resulting morphology of the ACC [187]. [PERSON] and [PERSON] [185] have researched the effect of applied pressure on the structure and properties of ACCs. According to the study application of pressure during composite manufacturing, controlled shrinkage and void formation and improved adhesion. This significantly contributed to better tensile and flexural properties of ACC laminates. In another study related to pressure, the elongation at break of cotton fabricated ACC decreased and the tensile modulus increased with increasing pressure of hot press. In the same study on hinoki lumber based ACC, a diverse outcome was obtained: applying 10 MPa and 5 MPa pressure gave higher tensile modulus than when applying a pressure of 15 MPa. The decrease in tensile modulus at 15 MPa can be related to observed cracks in formed composite [188]. The optimal immersion and dissolution time depends on the characteristics and volume of the cellulose and chosen solvent system; therefore, the dissolution time needs to be tailored for each composite. Ideally, in partial dissolution, when the immersion time in the solvent increases, the non-crystalline regions form as long as the dissolution process is active and re-solidified parts turn into amorphous regions [6]. When higher amount of cellulose is dissolved, the formation of internal voids will be reduced, due to improvement of intra and inter-laminar adhesion. The phenomenon has been reported in multiple studies [6; 185; 189]. The speed of regeneration also affects the properties of the composite, since slow precipitation develops a more ordered packing structure compared to fast generation [10]. [PERSON] et al. [17] showed that the optimal dissolution time with NaOH-urea solvent system was five minutes for rayon-reinforced laminate.