Modelling the 3D printing of

nanocellulose hydrogels

Tatu Pinomaa VTT Technical Research Centre of Finland Ltd

www.nafems.org

Contents

• Motivation

• Nanocellulose-based hydrogels

• Material characterisation

• CFD models

– Printer head

– Deposition process

• Validation experiments

• Conclusions

2

www.nafems.org

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

3

www.nafems.org

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

www.nafems.org

Nanocellulose hydrogels

5

• 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-%

www.nafems.org

Rheometry

6

• 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

www.nafems.org

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

www.nafems.org

Goniometry

Contact angle in a sessile drop experiment

www.nafems.org

CFD models for the printer heads

9

• 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

www.nafems.org

Analytical solution for power-law fluids

10

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

www.nafems.org

CFD model for the deposition process

11

• 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

www.nafems.org

Validation experiments

12

• Step 1: pressure-massflux dependency

Hydrogel printed on the substrate for a fixed time at various operating pressures, then weighed

www.nafems.org

Validation results

13

• Step 1: pressure-massflux dependency (analytical predictions based on rheometry)

200 μm steel tip

510 μm steel tip

www.nafems.org

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

www.nafems.org

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

www.nafems.org

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

www.nafems.org

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)

www.nafems.org

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

www.nafems.org

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.)

www.nafems.org

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