Natural rubber nanocomposites with cellulose nanofiber reinforcements produced by latex mixing: effects of fiber morphology and modification

Introduction: The natural biocomposites should favour from renewable resources including both matrix materials and fillers. The natural rubbers are an interesting class of biopolymers because of their extremely high molecular weight providing elastic nature and high extensibility. Therefore, the low glass transition temperature is favourable for the homogeneous film-forming properties. The amorphous structure of the polymer with low moduli, however, requires adaptation of mechanical properties through reinforcement with fillers (1). The nanocellulose provides good potential because of its fine fibrillar network causing high mechanical strength. However, the polar nature and hydrophilic properties need to be tuned in combination with the more apolar character of the natural rubber. The nanocomposites may be applied in future applications of novel packaging materials replacing oil-based plastics, with improved barrier properties and anti-tackiness properties.

Methods: The fibrillated cellulose nanofibers were produced by mechanical homogenization of native kraft pulp fibers, through mechanical processing in a microfluidizer towards different degree of fibrillation, resulting in micro- to nanofibrillated fiber morphologies (MFC/NFC) (2). A chemical surface modification of the fibers was subsequently performed by the deposition of hydrophobic nanoparticles at the fiber surface, including vegetable oils and styrene-maleimide (3). The effect of fiber morphologies and content (0 to 15 wt.-%) on mechanical reinforcement and structural variations of natural rubbers were investigated by adding the fibers during a rubber latex mixing process. The natural rubber latex (Vytex, Vystar Company) was mixed in the non pre-vulcanized state and further cured at 120°C with sulpher as vulcanizing agent.

Results & Discussions: Depending on the degree of fibrilation, the native cellulose fibers were prone to pull-out from the natural rubber matrix resulting from weak interphase compatibility. The medium fibrillated fibers (e.g., MFC) provided enhanced modulus with rather low strength as the finer branched fibers cause more interaction points. However, the interphase   remains weak and is mainly due to mechanical interlocking. The finest fibers (e.g., NFC) have higher intrinsic strength and elongation, but the natural rubber latex could not function as a favorable dispersant as the agglomeration of fibers was observed. Alternatively, the surface-modified fibers had better compatibility and were added until high concentrations of 15 wt.-%. The mechanical properties of the natural rubber/nanocellulose composites indicated that the NFC provides higher strength and elongation, while mechanical properties for MFC indicated worse quality. The high strength of composites with modified fibers indicated a reactive interface with high compatibility and homogeneous dispersion.

Conclusions:  The mechanical properties of rubber latex is improved through mixing of fibrillated cellulose in the latex phase, but the effect of strength and elongation strongly depends on the fiber morphology. The good compatibility between fibrillated cellulose and the rubber matrix was obtained after unique surface hydrphobization with deposition of oil-containing nanoparticles at the cellulose surface, resulting in superior strength of the natural rubber nanocomposite. The in-situ vulcanization is induced in presence of the fibrillated cellulose.  

Keywords: Natural rubber, Nanoellulose, Latex, Vulcanization, Mechanical properties

References

1. Low DYS, Supramaniam J, Soottitantawat A, Charinpanitkul T, Tanthapanichakoon W, Tan KW, Tang SY. Recent developments in nanocellulose-reinforced rubber matrix composites: a review. Polymers 2021; 13:550.

2. Taheri H, Sampyn P. Effect of homogenization (microfluidization) process parameters in mechanical production of micro- and nanofibrillated cellulose on its rheological and morphological properties. Cellulose 2016; 23(2):1221-1238.

3. Rastogi VK, Samyn P. Reaction efficiency and retention of poly(styrene-co-maleimide) nanoparticles deposited on fibrillated cellulose surfaces. Carboydr Polym 2016;141:244-252.