Inkjet-printed Polyvinyl Alcohol Multilayers

Summary

An inkjet printer was used to manufacture polyvinyl alcohol multilayers. Polyvinyl alcohol water-based ink was formulated, and the main physical properties were investigated.

Abstract

Inkjet printing is a modern method for polymer processing, and in this work, we demonstrate that this technology is capable of producing polyvinyl alcohol (PVOH) multilayer structures. A polyvinyl alcohol aqueous solution was formulated. The intrinsic properties of the ink, such as surface tension, viscosity, pH, and time stability, were investigated. The PVOH-based ink was a neutral solution (pH 6.7) with a surface tension of 39.3 mN/m and a viscosity of 7.5 cP. The ink displayed pseudoplastic (non-Newtonian shear thinning) behavior at low shear rates, and overall, it demonstrated good time stability. The wettability of the ink on different substrates was investigated, and glass was identified as the most suitable substrate in this particular case. A proprietary 3D inkjet printer was employed to manufacture polymer multilayer structures. The morphology, surface profile, and thickness uniformity of inkjet-printed multilayers were evaluated via optical microscopy.

Introduction

Polyvinyl alcohol is semicrystalline, artificial, non-toxic, water-soluble, insoluble in most organic solvents, biodegradable, and biocompatible in human tissues and has excellent gas-barrier properties¹. Furthermore, due to its many useful properties, PVOH is widely used in a large number of applications. Nowadays, PVOH is used in: the manufacture of cleaning and detergent products, the food packaging industry, water treatment, textile, agriculture, and construction (as additives)¹. However, PVOH has recently attracted an increased amount of attention for pharmaceutical uses² (i.e., drug delivery) and in medical applications^3,4 (e.g., wound dressing, soft contact lenses, eye drops, and soft implants for cartilage replacement). PVOH films are produced either through a melt or solution form. Melt processing is compatible only with PVOH with low hydrolysis levels or heavily plasticized PVOH. Thus, when using this pathway, some properties can be sacrificed¹. On the other hand, a PVOH layer can be deposited via the solution form by drop casting⁵, spin coating⁶, or electrospinning⁷. However, these methods have a number of limitations in terms of the waste of unwanted material. For example, in the case of spin coating, it has been reported⁸ that 95% of the material is wasted. Additionally, these methods are quite rigid in term of design/features (no patterning capability) and have high overall processing costs. In order to overcome the limitation of the conventional solution processing, we here explore the potential of inkjet printing technology to provide a novel platform to produce polyvinyl alcohol (PVOH) multi-layer structures that have a strong impact on both the material and application perspectives.

Recent developments in the manufacturing sector have focused on cheap, simple, eco-friendly, and energy-saving processes. Inkjet printing (IJP) is a modern fabrication process that fits perfectly within this framework. The major advantages of IJP technology are the efficiency of material use, the digital (mask-free) and additive patterning, the large area capability, the compatibility with rigid/flexible substrates, and the low cost.

IJP is a deposition method that uses polymeric materials dispersed in a solvent. To date, functional polymer-⁹, ceramic-¹⁰, conductive nanomaterial-¹¹, 2D-¹², biologically-, and pharmaceutically-based¹³ materials have been successfully deposited. Recently, it has been reported that IJP was involved in the deposition of components as part of electronic devices, such as transistors¹⁴, sensors¹⁵, solar cells¹⁶, and memory devices¹⁷, as well in electronic packaging¹⁸.

The ink, cartridge, and substrate are equally important components that are employed in the printing process. First, the physical properties of the ink, such as the surface tension and the rheological properties (i.e., shear viscosity), have a significant impact on the printability behavior. Also, the pH plays an important role on both the solution (e.g., drying, foaming, and viscosity) and on the lifetime of the IJP print cartridge. Second, for the cartridge (piezoelectric), the driving voltage waveform actually defines the drop formation and both the directionality and the uniformity of the liquid jet. Finally, it is imperative that the ink/substrate interaction is very well understood, as the resolution and accuracy of the printed object are strongly dependent upon this interface. Solvent evaporation, phase changes from liquid to solid, and chemical reactions are the main processes that occur between the fluid drop and the substrate. All aspects involved in the IJP, from ink properties to drop/substrate mechanisms, are highlighted in review papers by Hutchings¹⁹ and by Derby²⁰.

In this study, we explore the capabilities of IJP to manufacture polyvinyl alcohol multilayers. First, a PVOH water-based ink was formulated, and the main physical properties, such as rheological behavior, surface tension, and pH, were investigated. In this work, a piezoelectric inkjet printer was employed, and the appropriate waveform parameters were then identified. PVOH multilayers were printed, and the quality and surface/thickness profiles were assessed by optical microscopy.

Protocol

1. Ink Formulation

1. Prepare the solution for IJP by dissolving polyvinyl alcohol (8 wt.% PVOH in water) in purified water heated to 60 °C.
2. Add 10 g of mono-propylene glycol (MPG) (10 wt.% mono-propylene glycol in water), as a humectant, to the solution.
   NOTE: The role of the humectant is to prevent blockages in the printhead.
3. Stir the solution for several hours to ensure homogeneity and then filter it through a 5 µm filter to remove any particulates that might block the nozzles.
4. Visually assess the ink for homogeneity, especially for any incidence of sedimentation. If sedimentation is observed, then either stir/ultra-sonicate the solution for a long time (days) or formulate a new water-based solution with a PVOH with low molecular weight.
   NOTE: Store all fluids in sealed beakers at room temperature.

2. Ink Characterization

1. Perform all the ink characterization tests at room temperature in a clean room environment.
2. Measure the viscosity of the solution using a viscometer.
   NOTE: This test is required in order to ensure that the formulated ink is compatible with the IJP hardware. The inkjet printing process requires a low-viscosity solution of 4-20 cP. Measure the viscosity of the ink as a function of the shear rate using a rotational viscometer.
3. Test the surface tension of the ink at room temperature using the pendant drop method. Use an appropriate measurement tool such as a tensiometer. Use the manufacturer's protocol.
   NOTE: A typical solution for inkjet printing has a surface tension of 30-40 mN/m.
4. Test the pH using a pH meter. Use the manufacturer's protocol.
   NOTE: The pH is an essential parameter in water-based inks, as it provides essential information about both the properties and the stability of the formulated solutions. A neutral solution pH of 7 guarantees a stable process and a good lifetime for the printhead.
5. Assess the wettability of the ink on different substrates by measuring the contact angle via a sessile drop experiment. Use a tensiometer to measure the surface energy of the possible substrates (e.g., glass, plastic, and paper). Measure the surface energy using the protocol supplied by the tensiometer manufacturer.
   NOTE: The interaction between the drop and the substrate has a strong impact on the printing quality. To ensure the good adhesion of the ink to the substrate, the surface energy of the substrate should exceed the surface tension of the ink by 10-15 mN/m.

3. Inkjet Printing

NOTE: All the inkjet printing depositions were carried out at room temperature. The PVOH multilayers were deposited using a piezoelectric hybrid ink jet printing machine. A printhead with 512 nozzles (256 x 2 rows), a 30 µm nozzle diameter, and a 42-pL drop size was used in this work.

1. Prior to printing, thoroughly clean the glass substrates with acetone/methanol/isopropanol and Di water. Dry the substrates with an N₂ gun.
2. Load the substrate onto the print-bed and secure it firmly.
3. Prepare the cartridge by flushing the ink through the print head. Remove any air or cleaning solution from the reservoir and nozzles.
4. Imbed the cartridge into the printer. Connect the printhead to the global inkjet systems (GIS) print manager via the head personality board.
5. Load the solution into the 150 mL syringe situated above the cartridge and seal the syringe with an airtight cap.
6. Purge the ink through the nozzle by pressing the purge button.
   NOTE: The nozzle-substrate distance has a strong influence on the jet trajectory and hence on the quality of the printed pattern. Therefore, adjust the nozzle-substrate distance using the software of the printer to reduce jet spreading.
7. Set up the waveform and printing parameters using the GIS print software and Table 2.
   NOTE: The GIS software interface allows control over both the draw and release amplitude and the width.
8. Load the desired image file for printing using the GIS print manager software.
9. Start the digital process and print the image pattern onto the substrate.

4. Analysis of the Printed Pattern

1. Investigate the quality of the printed patterns using an optical microscope. Check for the presence of defects within printed features and assess the improvement in the quality when more layers were printed.
2. Evaluate the surface topology and thickness profile of the inkjet-printed multilayers using a non-contact 3D surface profilometer (based on white light interferometry) via a 3D optical microscope.
   NOTE: More details about measurements and the instruments that were used to formulate/print and characterize the printed patterns are presented in reference²¹.

Results

The physical properties of PVOH water-based ink, such as surface tension, viscosity/rheological behavior, pH, wetting, and time stability, were investigated. The viscosity of the ink used in this work was 7.5 cP, and the surface tension was 39.3 mN/m. Additionally, the formulated ink was neutral (pH 7), with the results summarized in Table 1.

Ink

Discussion

In this work, we successfully demonstrated the ability of inkjet printing technology to deposit polymer multilayers. The rheological behavior was investigated, and the experimental results demonstrate that the formulated ink displays pseudoplastic shear thinning behavior. Also, the PVOH ink is a neutral solution (pH 7) and shows good stability over time. Notably, it was successfully demonstrated that IJP technology is capable of producing polyvinyl alcohol multilayer structures, but further improvements in printing cover...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Polyvinyl alcohol	PVOH Polymers Ltd, UK	Poval 4-88
Mono-propylene glycol	Sigma Aldrich, UK	W29004
DV2T viscometer	Brookfield, UK
Attension Theta Optical Tensiometer	Biolin Scientific, Sweden
HANNA pH meter	HANNA Instruments, UK
industrial Inkjet XYPrint100Z	Industrial Inkjet Ltd, UK
ContourGT-K 3D optical microscope	Bruker Corp, USA