PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Reactive inkjet printing of polyurethanes Peter Kr€ ober,a Joseph T. Delaney,abc Jolke Perelaerab and Ulrich S. Schubert*abc Received 6th January 2009, Accepted 11th May 2009 First published as an Advance Article on the web 12th June 2009 DOI: 10.1039/b823135d Reactive inkjet printing technology was used to create micron-scale polyurethane structures, such as dots, lines and pyramids. These structures were fabricated in situ and cured within five minutes by inkjet printing two separate inks successively from two separate print heads, with one ink containing isophorone diisocyanate, and the other consisting of an oligomer of poly(propylene glycol), a catalyst, and a cross-linking agent. The fast polymerization reaction that forms polyurethane at the surface opens a new route for rapid prototyping, as well as the use of inkjet printing as a technique for handling moisture-sensitive reactions. By the addition of fluorescent dyes to the polyol ink, confocal laser scanning fluorescence microscopy was used to investigate the miscibility behavior of both solutions on the substrate.
Introduction In recent years, many methods have been applied to build up 3D structures in successive layers. The so-called rapid prototyping manufacturing approach was developed to produce arbitrary shapes quickly at lower costs than previous prototyping techniques, such as injection molding, die casting, or machining parts. This highly flexible approach to device fabrication has gained considerable use in applications involving customized parts for value-added products. The most important applications can be found in the automotive industry and for medical devices.1–5 This approach has been used to fabricate concept models used for testing aerodynamic behavior and mechanical characteristics of designs, as well as for custom-fitted medical devices for patients, and even entire aeroplane bodies.6 Another important application of 3D structures is the manufacturing of visual aids that allow evaluation of the ergonomics of prototypes. Computer-aided-design (CAD) is used to design the physical layout of the models,4 and the generated CAD files can then be automatically fed into the rapid prototyping fabrication processes by separating the CAD design into a series of thin layers to be built up, one-at-a-time. Many different techniques exist in rapid prototyping for the fabrication of solid structures. Additive, successive layer deposition of certain materials has been done, for example, by stereolithography,7 fused deposition modeling, selective laser sintering and 3D powder bed printing.8–10 These techniques are a few of the most widely studied ones. Other techniques, such as electrospinning combined with the use of electrostatic lenses,11 are also the focus of intense academic investigation, particularly towards life science applications.12,13
a Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Humboldtstr. 10, 07743 Jena, Germany. E-mail:
[email protected] b Dutch Polymer Institute (DPI), P. O. Box 902, 5600 AX Eindhoven, The Netherlands c Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands
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One of the main drawbacks of rapid prototyping technologies is that only a selected range of materials can be processed directly. Some materials, like bio-ceramics, biodegradable polymers, and silicones are accessible only by post-treatment processes like pre-molding of the samples. From a practical standpoint, direct printing methods, e.g. fused deposition modeling, 3D printing or stereolithography are preferred. In such cases, biopolymers like poly(caprolactones) are currently handled using this approach.14 As an applied technology, inkjet printing has been used in a wide array of different disciplines.15,16 Beyond its ubiquitous application as a means of dispensing ink for office document printing, inkjet printing is utilized to dose many different kinds of materials, such as conductive polymers and nanoparticles,17–19 sol–gel materials,20 structural polymers,21 ceramics22,23 and even molten metals.24 Polyurethanes (PU) are interesting and widely-used materials, because of their tunable mechanical and biocompatible properties. They are applied in everyday materials, such as in components in shoes and other sporting goods, in medical devices, and as insulation for electric cables. Furthermore, an enormous demand for PU exists in the automotive industry, where PU is used for dashboards or coatings for abrasion protection. Piccin et al. studied the potential of PU on rapid prototyping, and produced capillary electrophoresis microchips from that material in less than one hour.25 In this case, photolithography served as a method for micro-patterning. In order to achieve the multi-micron feature resolution typically associated with rapid prototyping techniques, inkjet printing was considered as a synthesis tool. This technique is also called reactive inkjet printing, and is a precise method for building up small structures using certain reactive materials, like PU. We here demonstrate that defined micron-scale PU-based structures can be fabricated via reactive inkjet printing starting from the corresponding monomers in a reactive, in situ manner on glass substrates. This approach yields unique, cross-linked thermoset PU materials with spatial resolution in the range of tens of microns. Another aim of this research is to explore the use of inkjet printing as a technique for undertaking moisture-sensitive This journal is ª The Royal Society of Chemistry 2009
Fig. 1 (a) Schematic representation of the reaction scheme for the formation of PU from the reactants PPG 400 and IPDI. (b) In situ reaction kinetics for the formation of PU.
chemical reactions. While reactive inkjet printing has been demonstrated as a successful technique for material handling involved in such diverse fields as free-radical polymerizations,26 DNA synthesis,27 and peptide synthesis,28 reported usage of inkjet as a microfluidics tool for handling moisture-sensitive reagents remains limited. The classical synthesis of AABB-type polyurethane polymers by inkjet, starting from the corresponding diols and diisocyanates, represents an opportunity to evaluate the utility of inkjet as a means of processing solutions with special considerations. Towards this aim, the synthesis of polyurethane-based materials was seen as a particularly illustrative example. The chemistry of this involves the preparation of two separate inks, one containing a diol, and the second containing a diisocyanate (Fig. 1a). The two inks are printed as successive layers on a surface, and are allowed to react to form urethane bonds. The addition of a cross-linking agent, such as trimethylolpropane (TMP), results in a solvent-resistant, thermoset film with micronscale structural detail. In selecting an appropriate diol, two wellstudied candidates were chosen, namely hydroxyl-terminated telechelic poly(ethylene glycol) (PEG), and hydroxyl-terminated telechelic poly(propylene glycol) (PPG), respectively. The intent was to evaluate what effect the role of a side-chain would have on the quality of the final film. In order to prepare these as printable (i.e. low-viscosity) solutions, low molar mass fractions of both were selected, with a molar mass of 400 Da, and an absolute This journal is ª The Royal Society of Chemistry 2009
viscosity at room temperature of 100 mPa s, as reported by the vendor. To further decrease the viscosities into a more manageable range, the aprotic, polar solvent N,N-dimethylformamide (DMF) was added, yielding inks that were easily printable. The second set of inks, containing diisocyanate compounds, were formulated using an analogous strategy. The diisocyanates selected, namely isophorone diisocyanate (IPDI) and 2,4-toluene diisocyanate (TDI) respectively, were made more printable by the addition of a small aliquot of DMF, and the reaction was promoted using the catalyst bismuth neodecanoate (BiNeo). A key factor to many rapid prototyping applications is an understanding of how the materials may be built up threedimensionally. It is important then, to study how the subsequent layers of material behave as they are deposited on previously deposited layers. Feature sizes, surface spreading, and related properties are important issues to keep in mind, and are best measured (preliminarily at least) with the simplest geometry possible. To examine such properties more readily, it was decided that printing droplets onto surfaces of the same PU composition with the flattest geometry possible, these issues could be studies more directly. To accomplish this, substrates were spin-coated with the appropriate PU solution prior to printing. To monitor the progress of the reaction in situ, a literature search demonstrated that the most facile, well-developed approach to studying the polymerization kinetics in cross-linked urethane films was by FT-IR spectroscopy.29–32 The reagents and products each contain polar groups that are all strong IRabsorbers, with peaks that are distinct enough to measure them easily. As the isocyanate and diol functional groups are consumed, the result is an attenuation of the isocyanate peak with a maximum around 2260 cm1. By using an unchanging reference peak (e.g. the alkane peak at 2960 cm1), the degree of conversion can be calculated as a function of the change in the intensity of the isocyanate peak relative to the alkane peak at a particular time point, compared to the initial isocyanate– alkane peak height ratio.
Experimental Materials Poly(propylene glycol) 400 (PPG, Mn ¼ 400 Da), poly(ethylene glycol) 400 (PEG, Mn ¼ 400 Da), isophorone diisocyanate (IPDI), 2,4-toluene diisocyanate (TDI), trimethylolpropane (TMP), and bismuth neodecanoate (BiNeo) were purchased from Sigma Aldrich (St. Louis, USA), and were used without further purification. N,N-Dimethylformamide (DMF, 99.8%;
