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Development of Cellulose Nanofibre (CNF) Coating on (1) Metal Surface for Free Standing CNF Film and (2) Paper Substrates for CNF Barrier Laminates
Paper is widely used in packaging applications and is biodegradable and therefore perfectly safe as green packaging wrap for the environment. The hydrophilic nature of cellulose fibrils limits the water vapour and oxygen barrier properties of paper. To mitigate these limitations, paper is often associated with other materials, such as plastics, wax and aluminum, for achieving their good barrier properties. However, these materials suffer from serious environmental issues, as difficult and inefficient to recycle. Recently, cellulose nanofibre (CNF) based materials has been considered as an alternative to produce eco-friendly barrier materials. Existing techniques to prepare cellulose nanofibre films/sheets/composites/ laminates on the paper substrates are commercially not feasible and expensive. Therefore, other cost effective and readily implementable methodologies are required to achieve cellulose nanofibre barrier layers. In the present report, a novel approach is developed using spray coating technique to produce CNF materials with excellent barrier properties. Among many coating techniques, the spray coating has many advantages such as the production of even coating surface on the base sheet and contactless coating with the substrate. A laboratory scale spray coating of cellulose nanofibre suspension on a paper substrate was developed. When the cellulose nanofibre suspension concentration was varied from 0.5 to 1.5 wt. %, coat weight is increased from 2.9±0.7 to 29.3±6.9 g/m<sup>2</sup>. As a result, the air permeability of composite was decreased 0.78±0.17 to <0.0030 µm/Pa.s. Scanning electron microscopy studies of spray coated CNF laminates on the paper confirms that the surface pores in the paper substrates are filled with sprayed cellulose nanofibre and forms a continuous film on the surface of the substrate. These are the probable reasons for the reduction of air permeability of composites. A rapid preparation technique to prepare free standing cellulose nanofibre films/sheets was also developed using a bench scale spray coating system. Cellulose nanofibre suspension with concentration ranging from 1 to 2 wt% was sprayed onto a stainless steel plate, which is moving on a conveyor at a velocity of 0.32 cm/sec and then air dried. The basis weight of produced cellulose nanofibre films is varied from 52.8±7.4 to 193.1±3.4 g/m<sup>2</sup>. Processing time taken to prepare films was approximately 1.0 min, which is much less than processing times reported in the previous literature. Thus, the significant reduction in preparation time for producing the cellulose nanofibre sheet recommends that this spray coating technique can be utilized for the development of a scalable process for the fabrication of various cellulose based nanocomposite. Therefore, the laboratory scale spray coating confirms that the spraying could provide a platform for development of films/sheets/nanocomposite and also a CNF barrier layer on the base sheet. The future work is the development of a continuous spray coating of cellulose nanofibre on the base sheet and evaluation of mechanical and barrier properties spray coated barrier layers on the base sheet.
Paper is widely used in packaging applications and is biodegradable and therefore perfectly safe as green packaging wrap for the environment. The hydrophilic nature of cellulose fibrils limits the wat... Read more
Figure 1. Different coating processes for paper applications, (a) extrusion coating; (b) curtain coating; (c) Size press coating; (d) bar coating; and (e) dip coating. (Vibhore Kumar Rastogi <i>et al, </i>Coatings, 2015, 5, 887-930)
Figure 2. Concept of Spray coating and surface finishing. (Vilho Nissonen, 2002) [48].
Figure 3. Experimental Set up for Lab Scale Spray Coating System for Preparation of Nano cellulose Film.
Figure 4. Mapping of thickness of the micro-fibrillated sheet. The thickness is measured in the centre region of the sheet. The square section of centre part of the sheet is used for contour plotting. The grey point of thickness used for mapping to confirm the uniformity of the sheet.
Figure 5. Viscosity of cellulose nanofibre suspension using dip cup method
Figure 6. Effect of suspension concentration on the basis weight of the nano cellulose film prepared using spray coating technique at a constant velocity of 0.32 cm/sec.
Figure 7. Thickness Distribution of the cellulose nanofibre sheets –Spray coated at Conveyor velocity of 0.32cm/sec and Vacuum Filtration
Figure 8. SEM image of the cellulose nanofibre sheet – Rough and Smooth surface and AFM Image of the both sides of the CNF Film.
Figure 9. Effect of the velocity on the basis weight of the CNF sheet and Figure 3– Effect of Velocity of the conveyor on thickness of the spray coated shee
Figure 10. Uniformity of the spray coated sheets prepared at various velocity of the conveyor
Figure 11. Formation test of Spray Coated sheet prepared at various of velocity of the conveyor
Figure 12. Uniformity test of spray coated paper prepared different suspension of CNF
Figure 13. Formation Test of Spray Coated sheet made by different suspension of CNF
Figure 14. Effect of suspension concentration on apparent density of cellulose nanofibre sheet.
Figure 15. Effect of thickness of the cellulose nanofibre sheet on the basis weight
Figure 16. Tensile Index of the spray coated sheet.
Figure 17. Young’s modulus of the spray coated sheet
Figure 18. Influence of conveyor velocity on Tensile Index of the spray coated sheet.
Figure 19. Influence of Conveyor Velocity on Young’s Modulus of the spray coated sheet
Figure 20. Zero span test of spray coated sheet prepared by different concentration of CNF.
Figure 21. Influence of velocity of conveyor on the spray coated sheet.
Figure 22. Stress and strain curve for CNF Film
Figure 23. Stress and strain curve for CNF Film
Figure 24. Plot of coat weight on the base sheet against suspension consistency
Figure 25. Plot between air permeance vs coat weight
Figure 26. Micrograph of coated and uncoated paper
Figure 27. Components of Dow Web Coater integrated with Spray System
Cellulose Nanofiber Lamination of the Paper Substrates via Spray Coating – Proof of Concept and Barrier Performance
Cellulose nanofibre (CNF) is a biorenewable and biodegradable nanomaterial and belongs to fibrous based carbohydrate polymers applied in the fabrication of various functional materials such as coating, nanocomposite, flexible electronics substrates and biomedical devices. Recently, CNF can be used as coating material for papers and paperboards to replace synthetic plastics, wax and aluminum foil which is not recyclable and also a threat to environment. The coating of CNF on the paper substrates enhances their barrier and mechanical properties. Spray coating is a newly proposed technique to deposit CNF on the paper and produce CNF laminates on the surface of paper to block their surface pores and allowing improve their barrier performance against water vapor, air and oxygen. Various concentration of CNF was sprayed on various paper substrates such as newsprint papers, packaging paper (brown paper) and blotting papers. The air permeability of CNF laminated paper substrates is completely impermeable against air. The SEM micrograph reveals that the surface pores in the paper substrates are filled with sprayed CNF and formed a barrier film as a laminate on the paper substrates. As a result, a considerable drop in the air permeability of the paper substrates was observed. Given this correspondence, spraying of cellulose nanofiber on the paper substrates allows the improvement of barrier performance and proof of concept for coating CNF on the paper and paperboard.
Cellulose nanofibre (CNF) is a biorenewable and biodegradable nanomaterial and belongs to fibrous based carbohydrate polymers applied in the fabrication of various functional materials such as coating... Read more
Figure 1. Necessity of CNF coating on the paper substrates [2]
Figure 2. Different coating processes for paper applications, (a) extrusion coating; (b) curtain coating; (c) size press coating; (d) bar coating; and (e) dip coating. (36)
Figure 3. Spray coating experimental system for coating CNF on the paper substrates
Figure 4. Viscosity of CNF suspension using Brookfield dip cup Method
Figure 5. Viscosity of CNF suspension using dip cup Method
Figure 6. Spray Coated Wet Laminates of Cellulose nanofibre on the paper
Figure 7. Dried Spraycoated CNF laminates on the paper
Figure 8. Plot between CNF suspension consistencies versus Air permeability of the CNF coated sheet.
Figure 9. Plot between CNF coat weights versus Air permeability of the CNF coated sheet.
Figure 10. Plot between CNF coat weight and CNF coat thickness
Figure 11. Plot between surface roughness (ml/min) and CNF suspension consistency
Figure 12. Mechanical Strength of CNF laminated paper sheet
Figure 13. Sample Breakage during Mechanical Testing. Sometime the breakage of CNF layer will be first and then paper substrate.
Figure 14. SEM </b><b>Micrographs of Packaging Paper (Uncoated).</b> The high magnification image shows the particles of cellulose clumped with the micro fibres in the surface of the uncoated sheet. The low magnification confirms the presence of the cellulose interconnected fibres with various size of surface pores.
Figure 15. SEM micrographs of CNF laminated Packaging Papers.</b> The SEI image of the peeled cellulose film from the base sheet at High and Low magnification. The cellulose film has surface roughness due to the presence of difference size in fibers.
Figure 16. SEI Micrograph of the spray coated paper. These are rough images of spray coated paper and confirm the presence of cellulose fibrils and various size of the pores in the surface of spray coated sheet.
Figure 17. SEI Micrographs of the spray coated paper at High and Low magnification. The high magnification image shows the coating coverage of the surface of the paper substrates. The surface of the coated base sheet shows different size of the cellulose fibers. SEM micrographs reveal the closure of surface pores on the paper by cellulose nanofibres.
Figure 18. Effect of Homogenized cellulose nanofibre on the barrier performance of brown paper. The air permeance of the brown paper before coating, wetting with water via spraying, coating with raw cellulose nanofibre and coated with homogenized cellulose nanofibre via spraying were evaluated.
Figure 19. Dowweb coater for Scale up of spray coating process.