# Lactic Acid Modified Natural Rubber-Bacterial Cellulose Composites
## 1 Introduction  
Natural rubber (NR) is a raw material for a variety of elastomer industries, such as tires, gloves, automotive interior parts and packaging, because of its excellent elastic properties. However, NR has some less desirable properties, such as low strength and abrasion resistance. NR's mechanical properties vary with temperature; the softness of NR increases with increasing temperature, whereas its brittleness increases at low temperatures. NR has poor chemical resistance to non-polar solvents such as acetone and benzene. Reinforcement is used as a method for increasing NR's mechanical properties, for instance its modulus and strength. Carbon black and silica nanoparticles have been widely used as reinforcing agents [1; 2; 3]. Bacterial cellulose (BC) is a nanocellulose that has been studied for use in reinforcing NR [4]. BC is produced by _Acetobacter xylinium_ in the form of nanofibers. Eventually, the nanofibrils bundle to form microbial cellulose ribbons. BC fibers have a significantly smaller diameter compared with plant fibers. Specifically, BC has a more crystalline structure and has a short-time synthesis process when compared with plant celluloses [5; 6; 7]. The main obstruction to reinforcing NR with BC is that the two materials are incompatible because of their polarity differences. NR is a non-polar material, whereas BC is a polar material.  
Cellulose has a strong affinity toward materials containing hydroxyls groups. Cellulose swells in a high-polarity solvent and its internal hydrogen bonds weaken in the presence of a large number of hydroxyl groups. Previously, for the fabrication of a BC and NR composite, BC was treated to decrease its polarity and NR was grafted with high polarity functional groups on its structure. The composite fabrication via the solution blending method also helped reduce the hydrophilicity and polarity of the nanofibers; hence, this method improves the adhesion of cellulose fibers to the NR matrix [4; 7]. The addition of BC into the NR matrix affects the properties of the NR composites, such as reduced toughness and increased polymer swelling in the polar solvents [4]. In addition, crosslinking hasbeen applied as a method for improving properties of rubber. The network structure formed under crosslinking conditions is a key leading to modified properties. One of the earliest examples of the crosslinking process is the vulcanization of rubber by adding sulfur under high temperature, which creates links between the latex molecules [8]. Vulcanization gives the rubber its strength over temperature ranges in which non-vulcanized rubber could not perform. The cross links increase the toughness, strength, and hardness of rubber composites. In particular, cross-linked rubber is resistant to attack from oxidizing agents. However, because of the side-effects of sulfur, such as undesirable colors and toxicity, organic acids (e.g., formic, maleic, stearic, citric, and dicarboxylic acid) have been applied and evaluated in cross-linking studies of NR composites [9; 10; 11; 12].  
Previously, NRBC composite films had been successfully prepared via a latex aqueous microdispersion process by thoroughly mixing BC slurry with natural rubber latex (NRL) [4]. In this work, lactic acid cross-linking was applied to improve the properties of NRBC composites. The goal of this research is to develop biodegradable films of NRBC composites with improved mechanical properties, good resistance in water and non-polar solvents, together with high thermal stability and non-toxicity in order to be further applied in the fields of packaging and biomaterials.