Structurally Coloured Cellulose Nanocrystal Films via Controlled Drying
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Solution Overview
Problem
Current methods for producing structurally coloured cellulose nanocrystal films lack scalability and result in films that are not industrially viable, with issues such as anisotropy, limited optical quality, and sensitivity to water, preventing their use as effective pigments.
Innovation Solution
A method involving depositing a cellulose nanocrystal suspension onto a substrate, spreading it, allowing for structure recovery through an ageing step, and then drying to form a structurally coloured film, with optional annealing for increased water resistance, enabling the production of highly reflective and saturated films and particles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If roll-to-roll coating is used to deposit cellulose nanocrystal suspension, then continuous manufacturing is achieved, but the cellulose nanocrystals align parallel to the substrate resulting in lack of structural colour
Solution Approach 1:
The patent applies a preliminary action by depositing the cellulose nanocrystal suspension onto the substrate before the drying process begins. This allows the nanocrystals to self-assemble into chiral nematic structures during the drying phase, rather than being forced into parallel alignment by shear forces during deposition. The preliminary deposition step sets up the conditions for subsequent structural colour formation without the harmful shear alignment that would occur if structuring attempts were made during active coating.
2Manufacturing precision
If drop-casting method is used to produce structurally coloured films, then optical quality is improved, but the method is not industrially scalable
Solution Approach 1:
The patent makes the roll-to-roll coating apparatus perform multiple functions: it both deposits the suspension continuously (original function) and enables structural colour formation through controlled drying conditions (new function). By adjusting the drying rate and controlling substrate movement, the same continuous manufacturing equipment produces optically quality structurally coloured films, eliminating the need for separate batch processing equipment.
Solution Approach 2:
The patent changes the drying rate parameter from rapid (typical of roll-to-roll) to controlled/slow drying. This parameter change allows sufficient time for cellulose nanocrystals to self-assemble into chiral nematic structures during the drying phase, producing structural colour. The controlled drying rate is maintained while preserving continuous manufacturing by coordinating substrate movement through the drying zone.
3Manufacturing precision
If high shear is applied during R2R deposition to achieve good coverage, then coating quality is improved, but chiral nematic order is lost resulting in no colour
Solution Approach 1:
The patent segments the coating process into two distinct phases: (1) a rapid deposition phase that applies suspension with good coverage, and (2) a controlled drying phase that allows chiral nematic structure formation. By separating these functions in time and space, the harmful shear forces are applied only during deposition, while the structure formation occurs during the subsequent low-shear drying phase.
Solution Approach 2:
The substrate acts as an intermediary that carries the deposited suspension through a controlled drying zone. This intermediary function allows the suspension to be deposited with good coverage initially, then gradually dried under controlled conditions that enable structural organisation without the continuous presence of high shear forces.
4Manufacturing precision
If long evaporation times are used to enable optimum self-assembly, then optical quality is improved, but this conflicts with the limited time window in continuous processing
Solution Approach 1:
The patent maintains continuous useful action by implementing continuous controlled drying throughout the substrate's passage through the drying zone. Rather than interrupting production for batch drying, the substrate moves continuously through a controlled atmosphere that maintains appropriate humidity and temperature gradients, allowing self-assembly to proceed during the entire transit time without stopping the manufacturing process.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method allows for the reliable and reproducible production of structurally coloured films and particles with high reflectivity and water resistance, suitable for large-scale industrial applications and use as pigments, maintaining colouration even when suspended in water.
Implementation Method 1
colloidal cellulose nanocrystals can self-assemble in suspension upon solvent evaporation into periodic chiral nematic structures that can reflect vivid and durable photonic colour
Implementation Method 2
The interference colour follows Bragg's law when the characteristic size of the periodic arrangement of cellulose nanocrystal known as the pitch is of the same size range as the wavelengths of the interfering light
Implementation Method 3
colloidal cellulose nanocrystals can self-assemble in suspension upon solvent evaporation into periodic chiral nematic structures
Implementation Method 4
annealing the structurally coloured film to increase the water resistance of the film
Data Source
AI summary
The invention relates to a method for producing structurally coloured films, particles and interference pigments comprising cellulose nanocrystals, such as neutralised cellulose nanocrystals. The films and particles can be used as interference pigments or coloured particles such as glitters for various applications. The method comprises steps of depositing a nanocrystal suspension comprising cellulose nanocrystals onto a substrate: spreading the nanocrystal suspension across the substrate using a spreader: ageing the nanocrystal suspension to partially or completely recover the cholesteric structures lost during deposition and spreading: drying the deposited nanocrystal suspension so that the nanocrystals self-assemble to form a structurally coloured film: and annealing the structurally coloured film to increase the water resistance of the film. The structurally coloured film comprises nanocrystals which are organized into chiral nematic structures to provide the structural colour.