modelling the 3d printing of nanocellulose hydrogels - vtt of the future/3d printing... ·...
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Modelling the 3D printing of
nanocellulose hydrogels
Tatu Pinomaa VTT Technical Research Centre of Finland Ltd
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Contents
• Motivation
• Nanocellulose-based hydrogels
• Material characterisation
• CFD models
– Printer head
– Deposition process
• Validation experiments
• Conclusions
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Motivation
• Bio-based dispersions for direct-write-printable structures with embedded sensing for health care applications (BioDisp3D)
– Academy of Finland, VTT and the University of Tampere; years 2014–2017
– Goal: to study the potential of nanocellulose-based pastes for the fabrication of sheets or 3D structures with special functionality for medical applications
• For example: Custom wound pads with the possibility to monitor healing without removing the pad. For large, hard-to-treat wounds.
• Modelling goals
– Predict the 3D printability of the material candidates
– Predict the sensitivity of the outcome to the process parameters
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Direct write paste -technology
4
• Direct write technique for printing pastes
• Micro-dispensing environment based on nScrypt technology
• Prints 3D structures on 2D and 3D surfaces
• Post-treatments are usually needed (drying, sintering, UV-curing, laser, etc.)
• Compatible with all kinds of Nordson EFD tips and needles
• Line widths from 20 to 3000 μm, thicknesses from 5 μm upwards
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Nanocellulose hydrogels
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• Three-dimensional networks that consist of nanostructured forms of cellulose in a water matrix
• Reference material (TCNF)
– TEMPO-oxidised cellulose nanofibrils in water
– Produced from never-dried bleached hardwood kraft pulp
– Dry matter content of 1.06 wt-%
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Rheometry
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• Material models based on rheological characterisation
– Dynamic viscosity as a function of shearing conditions (incl. transient)
– Yield stress behaviour
• Anton Paar MCR-301 rheometer
– vane spindle and concentric cylinder geometries
• Measuring procedure
– Pre-shear at 100 s-1 for 60 seconds
– Rest period at 0.01% strain and 1 Hz for 300 seconds
– Gel strength: shear strain sweep from 0.01% to 100% at 1 Hz frequency
– Viscosity: shear rate sweep from 0.0001 s-1 to 3160 s-1
• Dispersing with an impeller, Ultra-Turrax and a sonifier
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Viscosity: steady-state behaviour
η = 𝑘𝛾 𝑛−1 How far does the power-law hold?
How does it level?
How does the paste settle?
𝑘cc, 𝑛cc
𝑘va, 𝑛va
Apparent power-law fluid in the measured range
Dependence on measurement geometry
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Goniometry
Contact angle in a sessile drop experiment
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CFD models for the printer heads
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• One-phase system of hydrogel
• Driven by a constant velocity boundary condition
• Volume of fluid (VOF) method used for compatibility with the deposition model
• No turbulence modelling applied
• Used to predict the dependence of hydrogel mass flux on the operating pressure
• Employs the open-source software OpenFOAM 2.3.x
Cylindrical steel tip Conical plastic tip
Axisymmetric mesh
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Analytical solution for power-law fluids
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ds dt
ls lt
P2 P1
𝑄s =𝜋𝑟s
3
1𝑛 + 3
Δ𝑃2→1𝑟s2𝑙s𝑘
1𝑛
P0
vs, Qs
vt, Qt
𝑄t =𝜋𝑟t
3
1𝑛 + 3
Δ𝑃1→0𝑟t2𝑙t𝑘
1𝑛
𝑄s = 𝑄t
Δ𝑃2→1 =
1𝑛 + 3
𝜋𝑟s3
𝑛
2𝑙s𝑘
𝑟s𝑄𝑠𝑛
Δ𝑃1→0 =
1𝑛 + 3
𝜋𝑟t3
𝑛
2𝑙t𝑘
𝑟t𝑄𝑡𝑛
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CFD model for the deposition process
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• Two-phase system of hydrogel and air
• Volume of fluid (VOF) method & Reynolds- averaged stress (RAS) model for turbulence
• Mobile substrate and air flow boundary conditions used to simulate printer head movement
• Wettability of the plastic substrate determined by an experimental contact angle
• Used to establish a connection between the paste rheology, printing parameters and the line profile
Printer head boundary condition
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Validation experiments
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• Step 1: pressure-massflux dependency
Hydrogel printed on the substrate for a fixed time at various operating pressures, then weighed
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Validation results
13
• Step 1: pressure-massflux dependency (analytical predictions based on rheometry)
200 μm steel tip
510 μm steel tip
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Validation results
14
• Step 1: pressure-massflux dependency (analytical predictions based on rheometry)
200 μm steel tip
510 μm steel tip
Pressure loss is dictated by the asymptotic behaviour of viscosity at high shear rates
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Validation results
15
• Step 1: pressure-massflux dependency (numerical predictions based on rheometry & the massflux data)
200 μm steel tip
510 μm steel tip
Behaviour explained by an effective power-law model with viscosity cut-offs and slightly slower shear-thinning
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Validation results
16
• Step 1: pressure-massflux dependency
Viscosity curves of CNF suspensions: rheometry with the parallel plate geometry
Kumar et al. (2016) Ind Eng Chem Res 55:3603
Effective power-law fluid model with viscosity cut-offs at both low and high shear rates
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Validation results
17
• Step 2: printing parameters-line profile dependency
Option A: 2D image analysis (for the TCNF gel)
Option B: 3D profilometry (for more rigid gels)
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Validation results
18
• Step 2: printing parameters-line profile dependency
Sensitive to the contact angle value Roughly 100 μm resolution feasible with successful massflux prediction
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Conclusions & further work
19
• Modelling the 3D printing of nanocellulose hydrogels is feasible with currently available CFD tools
• Conventional rheometry is not necessarily a sufficient basis for the needed rheology models
– High shear rate behaviour dominates the flow within the printer head
– Low shear rate behaviour dominates the deposition process
• To be considered
– Rheometry (or capillary viscosimetry) at higher shear rates
– Thixotropic effects
– Boundary conditions at solid surfaces (slip etc.)
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Goniometry
Predicts hydrogel mass flow for a given operating pressure
Predicts deposition profile for given printing conditions
Summary
Rheometry
Rheology model
CFD simulations
Something
Experiments
Deposition model
Printer head model