Water-Based Polyurethanes: Synthesis and Overview

1. Introduction

Polyurethanes are polymeric materials with versatile applications that encompass different mechanical properties[1]. The ability to prepare polyurethane materials with desired properties and formulations in industry enables their use across a broad range of applications, from the production of flexible foams to rigid foams, from durable elastomers to high-performance adhesives, from the production of synthetic fibers to hard plastics [2,3]. However, the fact that commercially applied polyurethane products contain significant amounts of organic solvent and residual isocyanate monomers has led to a decline in their share of use in industry [4]. Nevertheless, environmental regulations such as clean air laws are directed towards the development of environmentally friendly products based on global restrictions of volatile organic compounds released into the atmosphere [5]. In particular, the toxic properties of organic solvents used in traditional polymer synthesis have led to the development of emulsion and suspension polymerization methods in the synthesis of addition polymers. The same situation applies to polyurethane synthesis.

In the traditional synthesis method for polyurethane structures, organic solvents such as toluene, xylene, and tetrahydrofuran are used. However, the use and recycling of such organic solvents in continuous and large-scale production poses a problem.

This situation has made water-based (waterborne) polyurethane systems one of the actively and rapidly developing branches due to both their environmental friendliness and technological advantages in polyurethane technology. Water-based polyurethane dispersions are defined as a binary system in which polyurethane particles are dispersed in a continuous phase of water [6]. Water-based polyurethane dispersions possess important superior properties such as containing either very low volatile organic compound emissions or no volatile organic compound emissions at all, ease of application, and low viscosity at high molecular weight. Over time, due to these advantages, water-based polyurethane substances have replaced solvent-based polyurethane substances. Water-based polyurethane materials are used particularly in the adhesive, coating, paper and textile industries [7]. In this context, although water-based polyurethane substances that can be prepared with desired properties and formulations have existed, new materials continue to be developed.

2. Basic Properties of Water-Based Polyurethanes

  Water, used as a tool for the synthesis and production of polyurethane products, is preferred because it shows no toxic effects, is inexpensive and safe [8]. The most important technical advantage of water-based polyurethane systems is that the dispersion viscosity operates independently of the polymer's molecular weight. This property enables the formation of polyurethane films with high molecular weight in the desired structure [9]. When water-based polyurethanes are compared with polyurethanes prepared using solvents, they possess superior properties such as suitable viscosity, flexibility, non-toxicity, flame resistance, high mechanical strength, and stability over a wide temperature range. Furthermore, they attract considerable interest due to their high adhesion and rheological properties. Water-based polyurethanes, which are quite compatible with the green chemistry approach of today, possess these advantages alongside some disadvantages such as low chemical resistance, poor wettability, and limited electrostatic stability. Water-based polyurethane materials have a wide range of applications in the adhesive, coating, paper and textile industries [10, 11, 12]. Water-based polyurethane structures entering commercial applications are predominantly linear in character and can be formulated in conditions containing minimal solvent or no solvent at all. Improving the water and solvent resistance of these linear water-based polyurethane structures is quite important. Current developments are directed towards improving these properties, particularly involving the use of cross-linkers or work on grafting other polymers [13].

The desired structural properties of a water-based polyurethane material depend on the properties of two main components. Changes in these two main components during synthesis (isocyanate (hard segment) and polyol (soft segment)) affect the polymer's physical strength and elasticity.

In water-based polyurethane synthesis, polyether, polyester, polycaprolactone and polycarbonate are particularly preferred as polyols. Additionally, many researchers hold the view that the use of polyols derived from renewable sources in water-based polyurethane synthesis has great research value [14]. Research has shown that the use of polyester in water-based polyurethane synthesis increases the polymer's strength and solvent resistance, while polyether use increases the polymer's flexibility and hydrolysis resistance [9]. Another important approach in water-based polyurethane synthesis is isocyanate selection. Although research has involved the use of almost every type of isocyanate with aliphatic and aromatic character, aliphatic isocyanates are preferred in commercial applications. The main reason for preferring aliphatic isocyanates is that reaction control can be achieved as a result of their showing low reactivity towards water [13]. In addition, water-based polyurethanes obtained as a result of using aromatic isocyanates show easy degradation against light, which limits their presence in commercial applications [15].

In general water-based polyurethane production, 4,4-dicyclohexylmethane diisocyanate, 1,6-hexamethylene diisocyanate, and isophorone diisocyanate are the most commonly used aliphatic diisocyanate monomers.

In water-based polyurethane systems, in addition to the main components, the chain extenders and emulsifying agents used are quite important. The emulsifying agent used to provide dispersion of the synthesized polymer in the aqueous environment can be a diol, or it can be an ionic or non-ionic group [16]. The most important compounds used as emulsifying agents are sulfonate diamines, diols and dihydroxy carboxylic acids. DMPA, which is both a glycol and a carboxylic acid, is the most frequently used emulsifying agent. The main reason for using this compound is that the reactivity with isocyanate is minimized as a result of steric hindrance of the COOH group [17]. Chain extenders, which play an important role in water-based polyurethane morphology, are low molecular weight compounds terminated with hydroxyl or amine groups. Ethylene glycol (EG), propylene glycol (PG), 1,4-butanediol (BD), 1,6-hexanediol (HD), cyclohexane dimethanol (CHD) and hydroquinone bis (2-hydroxyethyl) ether (HQEE) are important chain extenders used in water-based polyurethane synthesis. While the timing of the chain extension step in water-based polyurethane production is important, it can be performed at the desired yield depending on the reactivity of the NCO-terminated prepolymer, the prepolymer temperature and the temperature of the dispersion medium. Furthermore, the accumulation of high molecular weight polyurethane is controlled by ionic groups acting as an emulsifying agent and the chain extension step [13].

3. Synthesis Methods for Water-Based Polyurethanes

The basis of water-based polyurethane production lies in the polymerization reaction that diisocyanates or polyisocyanates in molar excess form with suitable diols or polyols in the presence of an emulsifier. In this context, in the first stage of synthesis conducted in two different steps, hard segments containing urethane or urea bonds with hydrophobic properties are prepared as an ionomer containing hydrophilic centers and ionomers. In the second stage of the work, deionized water is added through chain extension to the emulsified and dispersed hydrophilic polyurethane prepolymer. The acetone process, prepolymer mixing method, hot melt method and ketimine/ketazine method are the most important known methods in the synthesis of water-based polyurethanes [18,19].

3.1. Acetone Process

The acetone process, the most popular method in the preparation of water-based polyurethane systems, is based on viscosity control in the chain extension stage being carried out using an organic solvent (Figure 1). The method consists of two separate steps: the inclusion of hydrophilic and potentially charged groups into the polymeric structure in the presence of an organic solvent such as acetone, and the addition of water to the polyurethane/acetone system [17, 20]. In this context, chain extension of the prepolymer terminated with NCO groups is carried out through the use of bifunctional chain extenders in an organic solvent such as acetone, methyl ethyl ketone or tetrahydrofuran. Following the chain extension phase, the solvent is removed using simple distillation methods to obtain a solvent-free dispersion. Acetone, which gives the method its name, is preferred because it has a low boiling point, is miscible with water and shows inert properties. In addition to these, the method, which enables polymer formation in a homogeneous solution, possesses superior properties such as enabling the formation of polyurethane structures of different molecular weights, obtaining high-quality final products and reliable reproducibility [9].                                    

3.2. Prepolymer Mixing Method

The prepolymer mixing method is based on emulsion formation by mixing hydrophobically modified NCO-terminated polyurethane prepolymers with deionized water (Figure 2). The method prevents the use of large amounts of solvent [6, 22]. For this reason, prepolymer viscosity is quite important, and the method is used for low viscosity prepolymers. In the prepolymer mixing method, aliphatic isocyanates, which are known to show low reactivity towards water, are preferred and chain extension is carried out at low temperatures.                                  

3.3. Melt Dispersion Method

The melt dispersion method, one of the synthesis methods for water-based polyurethanes, is based on the production of polyurethane structure through the dispersion of ionic and/or non-ionic hydrophobically modified NCO prepolymers in water [23]. In the melt dispersion method, the amine compound used as a chain extender can be added to the water dispersion or to the prepolymer/water dispersion, and the desired final product is obtained.                    

3.4. Ketimine/Ketazine Method

In the ketimine/ketazine process, another method used in the production of water-based polyurethanes, blocked diamine (ketimine) or (ketimine) hydrazine is homogeneously mixed with the NCO prepolymer. Following transition to the aqueous phase, in the chain extension stage diamine or hydrazine is released through hydrolysis and the NCO-prepolymer reacts spontaneously [9].

4. Conclusion

The prominence of environmental concerns in recent years has increased interest in work involving the transition from organic solvent-based systems to water-based synthesis systems. In this context, water-based polyurethanes have become quite important recently. Taking into account the advantages and disadvantages of this synthesis system, new materials continue to be developed and are being applied to many systems. For this reason, considering the developments in polyurethane technology, it is clear that in the future, work directed towards the production of polyurethane systems using greener processes will gain even more importance and many technologically important polyurethanes containing these systems will be created. Dr. Merve Gökşin Karaaslan Department of Chemistry / Molecular Biology and Genetics Faculty of Arts and Sciences / İnönü University Prof. Dr. Burhan Ateş Department of Chemistry Faculty of Arts and Sciences / İnönü University Melike Kantarcıoğlu M.Sc. / Graduate / Department of Chemistry Faculty of Arts and Sciences / İnönü University Assoc. Prof. Dr. Süleyman Köytepe Department of Chemistry Faculty of Arts and Sciences / İnönü University

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