# Towards Self-Reinforced PLA Composites for Fused Filament Fabrication
## 2 Materials  
Herein, we used composite materials as bio-plastic materials, consisting of (i) the matrix and (ii) a filler/reinforcement phase. The matrix material surrounded the filler phase. Based on the individual material properties of the fibre reinforcements used in the study, two different base matrix materials were utilised as mentioned in Table 1.  
Since PLA is a hygroscopic material, it is recommended to pre-dry (dehumidify) the material before processing. This step not only prevents unwanted foaming during the filament processing but also prevents possible changes of chain structure because of the chemical reactions in the presence of water [34]. The two fibre types--stereocomplex (Sc) and bi-component (bi-co) fibres--used in the study are hereafter referred to as material types A and type B, respectively. The base matrix material for type A and type B were pre-dried at 60 \({}^{\circ}\)C and 40 \({}^{\circ}\)C, respectively, in a vacuum oven. PLA is a crystalline thermoplastic polymer capable of being spun into numerous fibre types such as monofilaments, multifilaments, bulked continuous filaments, staple fibres, short-fibres and spun bound fabrics [35]. However, there are still difficulties in using PLA to its full potential in a variety of applications, which is constrained by sub-optimal material properties such as weak thermal and hydrolytic stability [36]. Figure 2 shows the two optically active states of the monomer (lactic acid). The ratio of L\(-\) and D\(-\)lactic acid in the PLA polymer determines the crystallinity resulting in an increase in the mechanical, thermal and electrical properties of the polymer. Stereocomplex crystallisation occurs when the enantiomeric pair of PLA, i.e., L\(-\)lactide (PLLA) and D\(-\)Lactide (PDLA) are taken in racemic mixture (1:1). As a result of multicentre hydrogen bonding interactions, an alternative arrangement of helical chains is formed between PLLA and PDLA units possessing an opposite chiral formation [36]. The stereocomplex method of synthesising highly ordered structures was reported as early as the 1980s, stressing several challenges [37]. The conventional method of synthesising Sc-PLA involves utilising high temperatures of 220 \({}^{\circ}\)C to 260 \({}^{\circ}\)C, usually yielding low Sc content (low molecular weight) [38; 39; 40]. Many studies have been conducted to enhance the formation of high molecular weight polymers within a PLLA/PDLA blend [41]. However, for industrial applications, it is more relevant that the enhancement offers advantages of scalability and the low cost of raw materials as compared to molecular level engineering [36]. PLA-Sc's wide application is limited as there are still some challenges that need to be resolved:  
* One major obstacle is the high cost of D\(-\)lactide (PDLA). As stated before, the crystal formation in the Sc-PLA decreases with an increase of PDLA homo crystal. Therefore, for industries, the development of high-performance PLA-Sc with low PDLA content is beneficial;
* The spinning of the stereocomplex fibres needs further research, especially the parameters (pre-treatment, dyeing, clearing, post-processing/ finishing treatment) that impart the physical and chemical effects of the Sc-PLA fibre [35; 36];
* So far, the literature is focused on the controlled structure and properties of Sc-PLA blends, but not enough studies have been conducted to understand the biodegradation mechanism and the kinetics [36]; \begin{table}
\begin{tabular}{c c c c} \hline  & **Matrix** & **Reinforcement** & **Composite** \\ \hline Matrix Material A & PLA Luminy L130 & Stereocomplex PLA & Unreinforced \\ Matrix Material B & PLA Ingeo 6302D & Bi-component PLA & Unreinforced \\ \hline \end{tabular}
\end{table} 
Table 1: Materials used in the study. The base matrices and the corresponding reinforcement for the bio-based polymer composite production. Both matrix materials are procured from Total Corbion [32] and NatureWorks [33], respectively.