# Lactic Acid Modified Natural Rubber-Bacterial Cellulose Composites
## 4 Discussion
Similarly to the swelling of the composite films in toluene, the improved molecular packing and interfacial interactions between the BC fiber and NR chains by lactic acid modification reduced the degree of swelling in water. The acid modification promoted water resistance in the ANRBC films when BC loading was \(\geq\) 50 wt.%. The improved NR-BC interfacial interaction and their compatibility should decrease void volume in the composites and, accordingly, limit water absorption in the composite films. Among all the composite films, ANRBC20 had the highest structural stability against a non-polar solvent (toluene) and water; consequently, its degrees of swelling in toluene and in water were the lowest among the composites. The WVTR is an important property for packaging applications. Generally, WVTR varies in factors such as the viscosity of forming solution, film formation procedure, composition, and film thickness [30; 31]. The WVTR of the composite films was enhanced with increasing BC fiber content. The increase in the WVTR is possibly based on the increase in the degree swelling in water and the hydrophilicity of the films because of the high hydrophilic nature of the BC fibers. Previously, poor interfacial attraction between cellulose fibers in agar-based bionanocomposites increased the diffusion of water vapor through the void between the fibers and agar [32]. The crystallinity and the rod shape of BC whiskers also affected the mechanism of water transmission [33]. The WVTR of ANRBC at 20 wt.% BC loading was relatively lower than that of NRBC, which can be explained by the physicochemical characteristics of the acid modified surfaces and the effect of hydrogen bond formation leading to decreased porosity. However, at \(\geq\)50 wt.% BC loading, no difference in the WVTR between NRBC and ANRBC was observed because acid modification might have only a minor effect on the WVTR as compared with the effect of the hydrophilicity of composites with a high BC concentration. Biodegradable properties are an important requirement for green packaging materials. Many studies have been carried out on the degradation of rubber [34]. NR slowly degrades in nature because the specific bacteria that use rubber as a sole carbon source grow quite slowly [33]. Many bacterial strains that use rubber as the sole source of carbon and energy have been previously reported [35; 36; 37]. Thus, the biodegradability of the composites was increased with the BC loading content because of the much higher biodegradation rate of cellulose in soil as compared with that of NR [38]. Nanocellulose fibers in the composite films were consumed by soil microorganisms much faster than was NR, which led to increased porosity, void formation, and a loss of integrity of the NR matrix. The rubber matrix was thus broken down into smaller particles. Previously, disintegration of nanocomposite films containing cellulose whiskers was reported to be faster than that of pure NR films [33]. Previously, the average degradation rate of NR/cellulose composites with 10 wt.% cellulose loading was reported, and when cross linked with KOH solution, the average degradation was approximately 0.15 times that of the non-cross linked counterparts [39]. However, in this study, the acid modification had only a slight effect on the reduction of the biodegradation of composites with \(\leq\)50 wt.% BC loading, and there was no significant reduction effect of the acid modification on the biodegradation of ANRBC80 (80 wt.% BC loading) as compared with NRBC80. NRBC and ANRBC films did not show toxicity against the HEK 293 or HaCaT cell lines. Furthermore, by the disc diffusion method, no antibacterial effect of NRBC and ANRBC against _E. coli_ and _S. aureus_ was detected, which implied no significant release of lactic acid from the acid modification into the agar. ANRBC composites showed significant improvement in mechanical properties, melting temperature, and high resistance to polar and non-polar solvents.