Spray Coated Nanocellulose Films Productions, Characterization and Application

Nanocellulose (NC) is a biodegradable, renewable and sustainable material. It has strong potential to use as a functional material in various applications such as barriers, coatings, electronics and biomedical applications. Key limitation for its commercial application is the lack of an efficient method to roll to roll preparation of nanocellulose films. Therefore, development of a rapid method for the fabrication of NC films is required to meet their high demand. Vacuum filtration is a commonly used method to prepare NC films. However, the film preparation is not time efficient because of the high drainage time and difficulty in separation of wet film from filter after drainage. In this thesis, a novel method to prepare a smooth NC film using spray coating was developed, which is delineated in the first chapter. The NC fibre suspension is sprayed on a metal surface, imitating the spraying of a paint. This work highlights that film preparation time is unaffected by the target basis weight. NC films with the basis weight (weight per unit area) and thickness ranging from 50 to 200 g/m2 and 84 μm to 243 μm, respectively were prepared just by varying the NC suspension concentration. One side of this film, which is in contact with the metal surface (stainless-steel plate) was smooth, having the RMS roughness of 389 nm evaluated over a 1 cm2 inspection area with an optical profiler and 81.1 nm with an inspection zone of 100 μm2 from AFM. Lastly, film was prepared with high manufacturing speeds, with each film only taking a minute of operator time to form the sheet, independent of the mass per unit area of the film. However, the evaporation of water from the spray coated wet film took more than 24 hours in air drying and 1 hour in an oven at 105 °C. The removal of water from the wet films severely limits the production rate in the spraying process. In the second chapter, the optimization of spraying process parameters is explained and also the range of mechanical properties and bulk properties of the spray coated NC film is listed. Additionally, the experimental set up configurtions to enhance the uniformity of spray coated NC films and their properties is explained. In the first set of experiments, conveyor velocity varied while keeping NC suspension concentration fixed at 1.5 wt. %. In the next two set of experiments, NC suspension was varied, while keeping conveyor speed fixed at either 0.32 or 1.05 cm/s. By varying speed and solid concentration, spray-coating was found to allow efficient production of films with basis weights ranging from 38 to 187 g/m2, with the film thicknesses ranging from 59 μm to 243 μm, respectively. It was found that, there is a universal linear relationship between the thickness and basis weight, independent of the process conditions. Film uniformity is also noticeably dependent on the spraying process. The uniformity index of spray coated NC films, relative to vacuum filtered films, increased with conveyor speed at 1.5 wt. % solid concentration and was independent of solid concentration at low speed. Forming at a higher speed of 1.04 cm/s produced a maximum in uniformity in the concentration range from 1.5 to 1.75%, with these films being more uniform than conventional films produced through vacuum filtration. The most uniform films produced by spraying also had a tensile index (TI) of 79 Nm/g comparable to (TI) 72 Nm/g of films produced via vacuum filtration. With an understanding of these parameters and effects, this method was demonstrated to be a more time efficient alternative method to produce NC films, than vacuum filtration. The research shows that the properties of the films produced by spraying can be much more easily tailored to the required application. In the third chapter, the recyclability of spray coated NC films and the barrier performance of recycled NC films were investigated. The strength of recycled films retained 70 % of the virgin films. Due to poor hydrogen bonding between nanofibrils and hornification, a drop in strength of recycled fibres was observed. The water vapour permeability (WVP) of recycled NC film approximately doubled and increasing to 1.29x10-10 g.m-1. s-1 Pa-1 which is still comparable to synthetic plastics such as Polyethylene (PE), Plasticized PVC, Oriented Nylon 6 and Polystyrene (PS). The formation of agglomerates during recycling due to the poor breakdown of the fibre could be the probable reason for the decrease in barrier performance. The optical uniformity measurements of recycled NC film confirm a drop in uniformity of the film at smaller length scales and an increase in uniformity at larger length scales, compared with virgin NC film. The retained strength and barrier properties and facile re-processability of the spray coated NC films promise a sustainable and recyclable alternative to conventional polyolefin packaging. Commonly, Commercial Sodium Montmorillonite (MMT) is added to NC films to enhance the barrier properties of the composite. However, operation time to prepare NC/MMT composites increases enormously. In fourth chapter, a time efficient method to prepare NC/MMT composites is delianated. It was found that the operation time to form composite film is less than 1 minutes independent of MMT concentration in NC suspension. However, the removal of water from the wet composite film consumed more than 24 hours via air drying. Water vapour permeability (WVP) of spray coated composites from the raw NC decreased initially with MMT loading and then increased with MMT addition above 5 wt. %. This could be due to the aggregation of MMT when the loading higher than 5 wt. %. The sodium from of the MMT allows to swell and exfoliate in the composite and is another reason for elevation of WVP. The water vapour permeability of the spray coated homogenized composites decreased with MMT loading from 5 wt. % to 20 wt. % and then increased due to the aggregation and swelling characteristic of Sodium MMT. At the optimal addition level, lowest WVP achieved with spraying method is 8.3 x 10-12 g/m.s.pa when sprayed at 2.4 wt. %, which included 20 wt. % MMT and 2 wt. % NC. The XRD measurements showed that the MMT were aligned in the plane of the film and were strongly interacting with the cellulose nanofiber matrix, as the interlayer spacing increased from 11.78 to 14.16 Å. The tensile index results showed the composites had good strength, stiffness and were significantly flexible. Considering the barrier performance and strength of the nanocomposites, spray coated nanocomposites can perform as an effective barrier material, and spraying has the potential for the scalable process. The spray coated NC film has two unique surfaces with a shiny and glossy smooth surface contacting the stainless-steel surface and a rough side exposed to air. In this chapter, methods to enhance the smoothness of NC films was delineated. The three parameters controlling the roughness of the resultant NC films were the roughness of the substrate surface, the fibril diameter of the NC and Carboxymethylcellulose (CMC) addition. The aerial average roughness (Sa) on the smooth side of NC film was evaluated and varied between 1055±8 nm and 402±23 nm. The aerial RMS roughness (Sq) on the smooth side of NC film varied between 1400±269 nm and 550±36 nm at an inspection area of 0.07 mm2. High-pressure homogenization reduces the nanofibril diameter of NC depending on the number of passes. When homogenized NC was used to spray NC films, the surface roughness decreased. The Sa of homogenized NC film varied from 1125nm to 362±14 nm and Sq varied from 1517 nm to 470± 20 nm. This compares with the Sa and Sq of 4 wt. % CMC and 1.75 wt. % NC composite film which varies from 396±28 nm to 262±26 nm and 507±34 nm to 335±117 nm, respectively. The controlling order of roughness of NC films is CMC addition>fibril diameter of NC> base substrates. The addition of CMC in the NC suspension decreases the surface roughness of the NC film. As a result, the fibril network formed a smoother film compared to a film without the presence of CMC. This study has shown that spraying of NC on base surfaces produces a smooth NC film and its smoothness was engineered via varying base surface and reducing fibre diameter of cellulose fibrils or adding CMC in NC suspension. These films could be potentially used in electronic applications. To sum up, the spraying method on stainless steel surface is a rapid process to form film. The time taken for drying the spray coated wet film is a limitation in this method. This method is a potential alternative for roll-to-roll preparation of NC films and composites with smooth and glossy surface. It is not limited with only this NC material and could also make any films or composites from fibrous biopolymers.