Acrylic Hybrid Waterborne Dispersions: Synthesis

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Polyurethane/Acrylic Hybrid Waterborne Dispersions: Synthesis, Properties and Applications Samane Mehravar,† Nicholas Ballard,†,‡ Radmila Tomovska,†,‡ and Jose ́ M. Asua*,† †

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POLYMAT and Departamento de Química Aplicada, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Hiribidea 72, Donostia-San Sebastian 20018, Spain ‡ IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain ABSTRACT: Polyurethane (PU)/acrylic hybrid dispersions are an important class of polymeric dispersions that are produced on an industrial scale. In this review, we detail the design and synthesis of PU/acrylic dispersions, with a particular focus on how the structure of the hybrid can be tuned toward commercial applications. After a brief introduction, we describe the building blocks used in each polymer and their role in the synthesis/application of the final dispersion. Subsequently, we discuss the range of procedures that have been developed for the synthesis of such hybrids and how the structure of the hybrids influences the physical properties of films cast from the dispersion. Finally, we highlight commercial applications of PU/acrylic hybrids, with a focus on potential future developments.

1. INTRODUCTION Much of modern materials science relies on the synergetic combination of different components for the production of hybrid materials with superior performance.1−5 The combination of polyurethanes (PUs) and poly(meth)acrylates in aqueous polymer dispersions is one such example in which the two polymers are mixed at the nanoscale in order to produce a composite that has positive aspects of both components. In the case of PU/acrylic hybrid dispersions, as is the case for many synthetic polymers, the nature of the interaction between the two polymers and the properties of each polymer phase are determined almost entirely during the polymerization process itself, and therefore control over the properties of the polymer hybrid can only be properly achieved through an understanding of the chemistry involved. PU chemistry was first discovered by Dr. Otto Bayer and coworkers in the 1930s6,7 during the search to develop polymers capable of competing with the polyesters being developed by Carothers at DuPont. The unique mechanical properties of PUs brought about their rapid development and subsequently led to PUs taking a significant market share of the worldwide production of polymers. In an effort to minimize the use of volatile organic compounds (VOCs) that are involved in the synthesis and application of traditional solvent based PUs, waterborne polyurethane dispersions (PUDs) were subsequently developed.8−12 As a result of the continuing drive toward waterborne systems, PUDs are now an important industrial product and the global PUD market size exceeded 290 kilo tons in 2014 with a projected worth in 2020 of USD 2.04 Billion by 2020.13 © XXXX American Chemical Society

Today, PUDs are used in a wide variety of applications, including insulating materials, adhesives, elastomers and perhaps most notably coatings. What highlights them among alternative waterborne polymer dispersions is their superior physical properties such as toughness and flexibility.14−16 However, although PUDs have successfully been developed into practical systems, their high raw material costs and low water and alkali resistance are a drawback for some applications.8,17,18 Therefore, to optimize the cost/performance ratio, PUDs are often used in hybrid dispersed systems with polyesters,19 alkyds11 and acrylics,19,20 of which the combination of PU/acrylic is the most common.21−26 In these hybrid dispersions, the acrylic component provides the outdoor and alkali resistance, as well as good pigment compatibility and low cost, while the PU enhances the toughness, flexibility and film forming performance.20,27,28 In order to ensure that such hybrid materials offer a true synergy between the PU and acrylic components, control of the polymer microstructure is of great importance.29 The aim of this review is to provide a comprehensive stateof-the-art study covering, on the one hand, the various synthetic pathways toward PU/acrylic waterborne dispersions, and on the other, their structure−property relationships. Although there exists some previous reviews in this area,27,29 Special Issue: Mohamed El-Aasser Festschrift Received: Revised: Accepted: Published: A

April 30, 2019 June 17, 2019 June 21, 2019 June 21, 2019 DOI: 10.1021/acs.iecr.9b02324 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Typical Synthetic Procedure for a Polyurethane Dispersion

Figure 1. Common diisocyanates used in PU synthesis.

used in their synthesis, which determine the final properties of the product. In a typical PUD synthesis, an isocyanate (NCO) terminated prepolymer is first synthesized by step-growth polymerization of a macrodiol, a diol/diamine that contains a stabilizing hydrophilic moiety, and an excess of diisocyanate (NCO/OH > 1) in a solvent such as acetone or methyl ethyl ketone (see Scheme 1). Following this, the PU prepolymer is dispersed in water to form spontaneously self-assembled micelle-like structures.30 The dispersed prepolymer is then chain extended using a diamine to give a high molecular weight polymer and the solvent removed.10 The chain extension step leads to formation of urea bonds as opposed to urethane bonds and as such, although commonly referred to as polyurethane dispersions, PUDs contain significant amount of urea groups as well. The distinction is particularly important given that the urea groups contribute significantly to the final mechanical strength of the material.

the link between synthesis, structure and properties, which is crucial for industrial applications, has not been well documented. After an explanation of the chemistry of both constituents, the various techniques for preparing of PU/ acrylic hybrids are presented. Different approaches for preparation of chemically bonded hybrids are also discussed, and the performance of grafted and nongrafted hybrids is compared. Subsequently, following a review of the most important structure−property relationships, an overview of the main applications of PU/acrylic hybrids is presented. Finally, we provide an outlook for the future of this interesting class of polymer dispersions along with the open questions regarding the synthesis and application of PU/acrylic hybrids.

2. CHEMISTRY OF PUDS AND (METH)ACRYLIC DISPERSIONS 2.1. Polyurethane Dispersions. The versatility of PUDs can be directly correlated to the high variety of raw materials B

DOI: 10.1021/acs.iecr.9b02324 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. Examples of commonly used polyols in PU synthesis.

between 500 and 5000 g/mol. In practice, molecular weights of 1000 and 2000 g/mol are usually employed. Short chain diols, such as 1,4-butane diol or 1,6-hexane diol, can also be used to control the hard segment content.10,27 Figure 2 presents some examples of common polyols. Given the importance of the polyol on polymer properties, the influence of the type (polyether, polycarbonates or polyester) and molar mass of the polyol on the characteristics of PUDs have been studied extensively.30,39−50 Polyether polyols have the advantage of imparting flexibility,39,45 hydrolytic stability and low cost. However, they are susceptible to light and oxygen under heat, and the films tend to have poor exterior durability and poor solvent resistance. The increased polarity of the ester carbonyl group in polyesters leads to stronger hydrogen bond and thus, polyesters are tougher and more durable.10,31 Similarly, polycarbonates offer good mechanical properties as well as providing good resistance to hydrolysis, oil, weathering and fungi,41,51−53 but at higher cost. Recently, because of sustainability and environmental concerns, special attention has been given to the use of polyols originating from renewable resources, particularly from vegetable oils.54−58 However, in spite of their promise, polyols from vegetable oils and their derivatives are yet to be used significantly in commercial products. 2.1.3. Colloidal Stabilizer. Conventional PUs are hydrophobic materials immiscible with water. In order to impart water dispersibility, a diol/diamine containing a hydrophilic

Modifications to the synthesis of PUDs in which the chain extension is done prior to dispersion and in which either the isocyanate or the amine is initially in a “blocked” unreactive form are also widely used, in all cases making use of similar building blocks for the PU. Each component in the synthesis influences the final properties of the material, both for conventional PUDs and PU/acrylic hybrids, and therefore in this section an overview of the most common materials utilized in synthesis of PUDs is provided and their influence on the final polymer properties is highlighted. 2.1.1. Isocyanates. In Figure 1 some examples of the most common diisocyanates used for PU synthesis are shown.31 The isocyanates can be broadly classified into aromatic or aliphatic compounds. Aliphatic diisocyanates are more widely used in PUDs as they are less reactive toward water,27,32,33 offer ultraviolet stability and are more resistant to hydrolytic degradation. Materials made from aromatic isocyanates are less expensive and have improved mechanical strength, but, as they undergo photodegradation, they are inappropriate for many coatings applications and are less used in PUDs.8,31,34 Due to the high toxicity of isocyanates, some research is currently being devoted to the developed of so-called nonisocyanate polyurethanes (NIPUs),35−38 but this has yet to be translated to substantial commercial activity. 2.1.2. Polyol (Macrodiol). Long-chain diols or macrodiols, which make up the soft segment in PUs, are typically polyester, polyether or polycarbonate polyols, with molecular weights C


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Figure 3. Examples of some ionic internal emulsifiers used in PUD synthesis.

Figure 4. Preparation process for nonionic dispersing center. Reproduced from ref 97 with permission from John Wiley and Sons.

side group is commonly added to the formulation.59−61 Figure 3 presents some examples of stabilizers used based on ionic side groups, also termed ionomers. There are three types of PU ionomers:62 anionomer,63−65 cationomer66−68 and zwitterionomer.69,70 Cationic PUDs provide good adhesion to various substrates and they have been proven to be useful in applications such as adhesives, coagulants and also in membranes and synthetic leathers with micropores,66 but anionic PUDs are of greater commercial importance. Sulfonic71,72 phosphoric73 or carboxylic acids74,75 are the most used functions in preparation of PU anionomers. Dimethylol propionic acid (DMPA), which possesses sterically hindered carboxylic acid groups preventing reaction with isocyanates, is the most commonly used stabilizing monomer.76−80 Apart from the role of ionic group in the stability of PUDs, the introduction of pendant ionic groups to the polymer has been found to influence the physical properties of the films due to the formation of intermolecular Coulombic forces between ionic centers.53,81,82 Thus, the type and concentration of ionic groups not only affect the particle size,83−86 but also influence the dispersion viscosity,87,88 the glass transition temperature (Tg),53,89,90 thermal and mechanical53,90,91 and adhesive88,89,92 properties of the polymer films. PUDs can also be stabilized by nonionic hydrophilic pendant groups (predominantly poly(ethylene oxide)).59,93−95 In this case, the nonionic dispersing center is

usually prepared by reacting poly(ethylene oxide) units with a diisocyanate at 35−50 °C, followed by reaction with a dialkanolamine (such as diethanol amine) (Figure 4). In order to have an efficient stabilization, the dispersing center should be long. Therefore, it is necessary to build a high number of hydrophilic polyether segments into the PU chains.10,59,96 However, this hydrophilic segment may have a negative effect on the polymer film water sensitivity. 2.1.4. Catalyst. Catalysis plays an important role in the reactions of isocyanates.98 Catalysts are especially necessary when less reactive aliphatic diisocyanates are employed in the formulation. Aromatic and aliphatic amines (e.g., diaminobicyclooctane), organometallic compounds (e.g., dibutyltin dialaurate) and alkali metal salts of carboxylic acids are commonly used as catalysts.98,99 In waterborne systems, the catalytic influence is particularly important as different catalysts have different effects on the relative rates of reaction with alcohols, giving urethane bonds, and water, leading urea bonds (Table 1). Tin based catalysts are particularly useful as they strongly accelerate the rate of urethane formation,24 although regulation of tin based compounds will likely require them to be replaced in the near future. It is worth mentioning that catalyst removal from PUs is often exceedingly difficult and cost prohibitive, which is an important drawback for most applications.100,101 With this in mind, some attempts have been made in order to replace them with less toxic metal based catalysts,102−104 as well as organocatalysts.105,106 D


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that the chain extension step is done before emulsifying of the prepolymer in water. However, this makes the dispersion more difficult due to the increase in the viscosity of the PU prepolymer. Diamines typically react with the isocyanate groups several orders of magnitude faster than water does. Therefore, using diamines it is possible to extend the NCOterminated prepolymer in dispersed form.10 The type of chain extender strongly influences the mechanical performance of the PUs.116,117 When a diamine is used as a chain extender, better physical properties are obtained compared to diol chain extenders, due to stronger interchain hydrogen bonding interactions arising from the urea linkage.118−120 The N−H groups present in urea groups interact not only with carbonyl groups of the urea groups but also weakly with ether oxygens that are commonly found in the macrodiol of the soft segment. The presence of strong hydrogen bonding interactions allows films cast from PUDs to display high strength despite the relatively low molecular weights. It has been found that the PU properties depend on whether the chain extender has an even or odd number of methylene (CH2) groups.31,116,121−123 With an even number of carbon atoms in the chain extender the polymer adopts a fully extended conformation that allows hydrogen bonding in both directions perpendicular to the chain axis, which is not possible for a chain extender with odd numbers of carbon atoms. This leads to a polymer film with higher tensile strength in the case of an even number of carbon atoms in the chain extender.123 In addition to diols and diamines, low molecular weight multifunctional polyols and polyamines can also be used as chain extender. PUs produced with polyamines such as diethylene triamine (DETA) and triethylene tetramine (TETA) lead to internal cross-linking and results in an increase in modulus, strength and thermal stability as well as the water and solvent resistance of the dispersion cast films.124 2.2. (Meth)acrylic Polymers. (Meth)acrylic dispersions are most commonly produced by free radical polymerization of (meth)acrylic monomers via emulsion polymerization.125,126 Emulsion polymerization is a complex, multiphase synthetic process that is used to produce millions of tonnes of vinyl based polymers annually. The versatility and low cost of both reactants and the production process has led to the implementation of acrylic polymers in a wide range of commercial applications, from coatings, textiles, plastics and adhesives, to superabsorbents, detergents, dispersants, flocculants and thickeners. A selection of the most often used (meth)acrylic monomers is shown in Figure 6. Unlike PU formulations, drastically different copolymer properties can be obtained simply by changing copolymer composition, which allows for control over the glass transition temperature of the copolymer. Thus, a butyl acrylate (BA) homopolymer (Tg ≈ −54 °C) makes a good adhesive while a butyl acrylate/methyl methacrylate (MMA) copolymer with equal weight proportions of the two monomers has a Tg of approximately 15 °C and can be used in coatings applications. Other important features of the polymer such as molecular weight and network structure can similarly be tuned by, for example, addition of a chain transfer agent127,128 and cross-linker, respectively.129

Table 1. Rate Constants of the Different Reactions Using Various Catalystsa catalyst triethylamine (TEA) 2-2′-diazabicyclooctane (DABCO) trimethylamino-ethylethanolamine dibutyltin dilaurate (DBTDL)

urethane formation k (L2/(g mol·h)) 11 109 28.9 144

urea formationb k (L2/(g mol·h)) 6.0 14.5 43.3 4.8

a

Adapted with permission from ref 107. Copyright 2013 American Chemical Society. bReaction with water in the presence of CO2 as blowing molecule.

2.1.5. Neutralization of the Ionic Groups in the PU Chains. In the case of charge stabilized PUDs, after the prepolymer synthesis step, neutralization of ionic groups is often necessary before dispersing the PU prepolymer in water. In anionomers, the acid group is neutralized with a base, typically triethylamine. As this process strongly influences the stability and properties of the PUDs it has been widely investigated, and it has been found that the neutralization degree influences the particle size,85,108,109 the molar mass of PUs and mechanical properties of the films.108 In commercial formulations full neutralization is not common as any presence of free amine produces undesirable odor. 2.1.6. Chain Extender. PU prepolymers are chain extended either in solution or in the dispersed state with low molecular weight (Mw) < 400 g/mol difunctional compounds in order to increase polymer molecular weight and improve the mechanical properties.31 Diamines53,83,92,108,110 or diols44,46,48,49,111−115 are suitable examples of chain extenders leading to produce urea and urethane segments, respectively. Figure 5 shows some examples of diamines and diols used as chain extenders. In the case of diols, they react with NCO at roughly the same rate as water and it is usually recommended

3. PREPARATION OF PU/ACRYLIC HYBRID DISPERSIONS Knowing the basic building blocks available for the preparation of PU and acrylic dispersions, we can now turn to the synthesis

Figure 5. Examples of commonly used diol and diamine extenders. E


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as well as significant problems with regards to the toxicity of the product. A different approach was used by Okamoto et al.20 who blended an acrylic emulsion polymer containing ketone groups and a PUD containing hydrazine group. The carbonyl groups (present from using diacetone acrylamide in the acrylic polymerization) reacted with the hydrazine at the chain ends of the PU. The authors reported that the cross-linked blends exhibited synergistic effects in film properties, such as good solvent resistance, flexibility at low temperature, toughness at high temperature and good abrasion resistance. Although in this synthetic method the phase separation between the PU and the acrylic polymer decreased, the control of the final film nanostructure is not guaranteed and other approaches are necessary to avoid phase separation in many cases.132,134 3.2. Hybrid Particles. An alternative to physical mixing of dispersions of acrylic polymers and PUDs is the direct generation of a particle dispersion in which both phases are contained in the same particle. The high reactivity of isocyanates with water and the insolubility of macrodiols makes translating PU synthesis into conventional emulsion polymerization processes challenging. Therefore, there are basically two routes that can be used to generate a PU/acrylic hybrid particle (see Scheme 2). In the first case an NCO terminated prepolymer is dissolved in acrylic monomer and subsequently dispersed with the aid of shear forces and external emulsifier to generate a miniemulsion. The acrylic phase can then be polymerized as in a conventional miniemulsion polymerization and the PU chain extended.133,138−141 A second possibility is the use of a seeded emulsion polymerization process in which a polyurethane dispersion is first formed (sometimes in the presence of acrylic monomer) and subsequently the PU dispersion is used as a seed for the acrylic polymerization process.21,26,131,132,142−144 In order to avoid possible phase separation between PU and acrylic polymers, in both synthetic routes covalent bonds between PU and acrylic polymers can be formed by means of grafting or cross-linking mechanisms. In this section, we will first describe the procedure for synthesizing the particles and

Figure 6. Examples of commonly used (meth)acrylic monomers in the synthesis of acrylic dispersions.

of hybrids. In the following section, the different synthetic methods to produce PU/acrylic hybrid dispersions are outlined. 3.1. Dispersion Blends. Blending of PUDs and acrylic dispersions is a simple approach to combine the beneficial properties of both polymers. However, in many cases the performance of blended dispersions is compromised because of the incompatibility between the two polymers and the fact that both polymers are present in separate particles,22,130−133 which typically leads to a high degree of phase separation in the final material.130,133,134 To overcome this drawback, the acrylic and/ or the PU particles can be functionalized in order to generate interpolymer linkages.20,135−137 A common way to achieve this is through reaction of hydroxyl groups on the acrylic chains with isocyanates in the PU.135−137 For example, Nienhaus et al. patented a procedure in which an acrylic dispersion containing hydroxyl groups in the side chain was blended with a water dispersible polyisocyanate. The resulting blend was deposited on a surface and following drying and curing; the hybrid was shown to be suitable as a topcoat in automotive applications.136 Although such processes are used commercially, blends containing reactive isocyanates have limited open time

Scheme 2. Synthetic Procedures for Polyurethane/Acrylic Hybrid Waterborne Dispersions Using Either (left) Seeded Emulsion Polymerization or (right) Miniemulsion Polymerization

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due to the reactivity of the isocyanates and challenges associated with aqueous phase transport of the other components of the PU formulation, it is not possible to generate the hybrid starting from an acrylic seed. However, using a conventional polyurethane dispersion as a seed, it is possible to subsequently add the acrylic phase to give a PU/ acrylic hybrid. There are several examples in both open21,131,132,144,163 and patent135,164−169 literature of seeded emulsion polymerization involving PU/acrylic dispersions. Generally, this process involves preparing the PU prepolymer containing a DMPA moiety in a solvent followed by neutralization of the carboxylic acid group as shown in Scheme 1. After dispersing in water and chain extension, the (meth)acrylic monomers are added to the PUD and polymerized in batch21,142 or semibatch22 mode. For example, Hirose et al.21 prepared a PU/Acrylic hybrid dispersion by seeded emulsion polymerization in a two step process. First, an NCO-terminated PU prepolymer was synthesized using a combination of ethyl acetate and N-methyl pyrrolidone as the solvent. The solvent was necessary to reduce the viscosity to such an extent that it could later be easily dispersed. The prepolymer was neutralized, dispersed in water and chain extended before subsequent removal of the solvents at elevated temperature (95 °C). To this PU dispersion was added a mixture of vinylic monomers, which were then polymerized by use of a H2O2/Na2S2O3 redox initiating system. An obvious downfall of such synthesis is the need to remove solvent following synthesis of the PUD, which not only increases process time but also decreases the sustainability of the process. 3.2.2.1. Solvent-Free Processes for PU/Acrylic Hybrids from PUDs. Due to a combination of environmental and economic reasons, solvent-free PU/acrylic hybrids have attracted increasing attention from academia and industry.137,170−192 In this process, the acrylic monomers are used as solvents in the synthesis of the PU prepolymer, which allows the viscosity to be lowered sufficiently to allow for easy mechanical agitation during the dispersion step (see Scheme 2). It should be noted most conventional PUDs utilize water miscible solvents such as n-methyl pyrrolidone, acetone and methyl ethyl ketone; therefore, the dispersion process using water insoluble acrylic monomers as solvents has significant differences that are yet to be fully understood. As an alternative to the use of a solvent, for low molecular weight macrodiols and a higher NCO/OH ratio, where viscosity is kept minimal, it is also possible to synthesize PUDs directly without addition of a diluent and subsequently add the acrylic monomers.175,193,194 In comparison to conventional PUDs that are synthesized with the aid of water miscible solvents, the use of acrylic monomers as a diluent appears to result often in dispersion with poor colloidal stability. Mehravar et al. demonstrated that the PU prepolymers are highly heterogeneous in composition, resulting in an uneven distribution of the hydrophobic macrodiol (PPG) and the hydrophilic stabilizing diol.195 Following dispersion, when chain extended the chains rich in stabilizing diol polymerize primarily in the aqueous phase, thus limiting their ability to stabilize the particles. It was shown that the hydrophobicity of the chain extender controls the particle size and the colloidal stability, as more hydrophobic chain extenders led to a reduced amount of water-soluble species in the hybrid dispersions. This also had consequences for the water sensitivity of the resulting polymers.

will follow with the possible options with regards to grafting of the two phases. 3.2.1. Miniemulsion Polymerization. Invented by Ugelstad, El-Aasser and Vanderhoff, miniemulsion polymerization is the most versatile technique to produce complex dispersed materials.145 A “typical” miniemulsion polymerization of an acrylic monomer consists of monomer, costabilizer, water and surfactant.140,146 In the preparation of a PU/acrylic hybrid the formulation can be simply adapted by dissolving a preformed PU in a mixture of acrylic monomers and subsequent dispersion in an aqueous solution of emulsifier using a high energy dispersion device (see Scheme 2). In some cases,138 the PU even can act as a costabilizer against Ostwald ripening, avoiding use of an additional costabilizer. The polyurethane is typically a prepolymer that is subsequently chain extended following dispersion,25,147−149 but there are also reports of the use of higher molecular weight PUs synthesized in bulk and dissolved directly in the acrylic monomers.138,150 The use of preformed, high molecular weight PUs allows for control over the polyurethane structure and avoids complications arising from the reaction between isocyanates in the formulation and the aqueous phase. However, even at moderate fractions of the PU the viscosity of the organic phase can increase such that the generation of the miniemulsion becomes a challenge.151,152 This is particularly important as it is well recognized that it is the generation of the initial dispersion that limits the commercial potential of miniemulsion procedures.153 Therefore, it is common to use a low molecular weight PU prepolymer that can be polymerized either independently or in parallel with the radical polymerization of the acrylic monomers. In this way the final particle size can be largely controlled at the dispersion stage by controlling the emulsifier concentration and the shear forces used in the generation of the miniemulsion.154 As an example of the use of independent and parallel polymerization processes using hybrid miniemulsion polymerization, Asua and co-workers have worked extensively with miniemulsion polymerization of adhesive formulations using Incorez 701, an aliphatic NCO terminated PU prepolymer, and a 2-ethylhexyl acrylate rich acrylic monomer mixture using in addition 2-hydroxyethyl methacrylate (HEMA) as a comonomer.147,148,155−162 In all cases an initial miniemulsion consisting of the PU prepolymer and the acrylic monomers dispersed in water was obtained by high shear forces. When the radical reactions were conducted at elevated temperatures the PU reacted at a similar rate and thus the two processes occurred concurrently.147,155,156 By reducing the temperature, it was shown that the radical polymerization can be conducted largely independently of the chain extension of the PU, which occurred over a period of days at lower temperature during storage.160 By using the same initial miniemulsions in a tubular photoreactor setup, the radical polymerization could be completed almost instantaneously at room temperature while most of the PU remained unreacted and polymerized independent of the acrylic phase.148,161,162 3.2.2. Preparation from PUDs. Unlike preparation of PU/ acrylic hybrids by miniemulsion polymerization, seeded polymerizations do not require the high energy emulsification step and are therefore more favorable industrially, despite the fact that control of the dispersion step is significantly harder than in a miniemulsion polymerization.25 In addition, as seeded emulsion reactions tend to avoid the use of a surfactant, it is possible to obtain high gloss coatings. As discussed before, G


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One of the most common ways to achieve grafting in PU/ acrylic systems is to incorporate a vinylic moiety into the PU that can subsequently react during the radical polymerization of the acrylic phase to generate grafting points.23,26,28,138,147,155−157,197 This is typically achieved by the use of alcohol functionalized vinyl monomers (see Scheme 3). For example, Mehravar et al. explored the use of a variety of monomers in the seeded batch emulsion polymerization of a PU/acrylic hybrid aimed at coating applications.198 Using vinyl monomers containing one alcohol group, the functional monomer served to terminate the PU chain, leading to a vinyl terminated PU, while monomers with two alcohol groups extend the PU, introducing vinyl functionality along the PU backbone (see Scheme 3). The use of monofunctional monomers such as HEMA led to a lower average number of double bonds per PU chain and therefore the extent of grafting and the absolute molecular weight of the grafted polymer was reduced when compared to the diol functionalized monomers. Mehravar et al.also explored the influence of the reactivity of the vinylic group on the extent of grafting.199 In batch systems using (meth)acrylic functionalized monomers, rapid incorporation into the PU was achieved and high gel content was observed while vinyl ethers were poorly incorporated due to the reduced reactivity with the monomers used in the synthesis. Grafting can also be achieved through alternative functional groups in the two polymer phases postpolymerization. Thus, Okamoto et al. reported the use of diacetone acrylamide incorporated into the acrylic chain to react with a hydrazine terminated PU.20 Systems containing the reactive functional groups exhibited behavior consistent with a cross-linked material such as increased solvent resistance and the presence of an insoluble fraction of material.

Although the solvent-free method is a step forward with respect to sustainability, the PU/acrylic ratio is limited by the need to use a certain amount of acrylic monomer to reduce with the viscosity of the initial PU sufficiently for the dispersion step and therefore decreases the flexibility of the synthesis. Furthermore, the dispersion is sensitive to the monomer used and there is some difficulty in generating the initial dispersion in the case of significantly more hydrophobic acrylic monomers (such as 2-ethylhexyl acrylate) that are commonly used in PU/acrylic adhesive formulations. These observations are yet to be explained and significant efforts will need to be made to understand the dispersion process itself before any conclusions can be drawn. 3.2.2.2. Solvent-Free Processes for PU/Acrylic Hybrids from Acrylic Dispersions. As an alternative to using PU dispersions as a seed in emulsion polymerization, it has also been reported that PU/acrylic dispersions can be formed by performing the dispersion of an initial PU prepolymer directly into an acrylic dispersion.173,196 Yuan et al.196 prepared an acrylic dispersion by conventional emulsion polymerization and subsequently added this to a water dispersible PU prepolymer at elevated temperatures in order to disperse the PU. The structure of the particles appeared to be core−shell with the acrylic phase encapsulated by the more hydrophilic PU (see Figure 7). This route offers slightly more flexibility to

4. STRUCTURE−PROPERTY RELATIONSHIPS OF WATERBORNE PU/ACRYLIC HYBRIDS Water-borne solvent-free PU/acrylic hybrids are versatile materials that can easily be varied though changing the chemical composition of each component, the ratio between the two components, the extent of grafting and cross-linking and the particle morphology. Therefore, given the synthetic protocols discussed above, a wide variety of PU/acrylic hybrids with mechanical properties tailored to a specific application may be synthesized. In this section, we will detail the established structure−property relationships that can be exploited in order to control the physical properties of films cast from the hybrid polymers. 4.1. PU/Acrylic Ratio. With the aim of finding the optimum mechanical properties for coating applications, the effect of the weight ratio of PU/acrylics has been studied by different authors.26,134,138,143,200,201 Son et al.143 observed that the yield point of the tensile stress−strain curve, hardness and water resistance of hybrid films increased with the acrylic monomer content, whereas the abrasion resistance and elongation at break decreased. They concluded that the optimum (meth)acrylic monomer content to have a balance of good mechanical properties and abrasion resistance as well as an acceptable water sensitivity was about 30 wt %. In another study,26 it was observed that by increasing the acrylic content to 50 wt %, the hardness, alkali resistance and solvent resistance increased. However, by further increase in the acrylic content, all the mentioned properties decreased. It was claimed that this was due to the incompatibility of PU and acrylic

Figure 7. Transmission electron microscopy (TEM) micrographs of with varying PU/acrylate ratio (a) PUA-0 (0% acrylic), (b) PA (100% acrylic), (c) PUA-20 (20% acrylic) and (d) PUA-30 (30% acrylic) emulsion particles. Reproduced from ref 196 with permission from John Wiley and Sons.

with regards to the acrylic/PU ratio, but it remains unclear the extent to which particle morphology can be controlled by this method. 3.2.3. Grafting in PU/Acrylics. A significant issue in the synthesis and application of hybrid polymers is phase separation between the two polymers. This can occur within the polymer particles, resulting in nanosized domains, as well as in the film state, where phase migration results in decreased transparency and changes in mechanical properties. Grafting of the PU and acrylic phases offers a route to reduce the extent of phase separation and improve the compatibility of the polymer. In addition to this, through controlled grafting of the two polymer phases a certain degree of control over polymer microstructure can be achieved that has subsequent effects for the physical properties of the hybrids. H


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Industrial & Engineering Chemistry Research Scheme 3. Monomers Commonly Used for Grafting in PU/Acrylic Hybrid Dispersions

phases that at higher acrylic content resulted in phase separation during film formation. Therefore, it was concluded that the optimum performance properties for coating applications were provided by a hybrid synthesized with a 50:50 wt ratio of PU and polyacrylic components. Peruzzo et al. studied the effect of acrylic content in PU/acrylic hybrids formed by a seeded emulsion polymerization process using acrylic terminated PU. They found that up to 50 wt % of the acrylic polymer could be used before a significant negative impact became clear in the tensile test (see Figure 8).200 The acrylic content at which mechanical properties begin to show a significant decline is particularly important as acrylic polymer is often used to reduce the overall price of the dispersion. Therefore, the maximum value of acrylic polymer that does not influence the final properties tends to be targeted. In adhesive formulations, the PU component is typically responsible for increasing the shear strength of the polymer, which is notably poor in most acrylic-only adhesives. The effect of PU weight fraction (5−50 wt % relative to acrylic monomers) and the degree of grafting on the mechanical and adhesive properties were studied in PU/acrylic dispersions using a commercially available PU prepolymer and a butyl acrylate rich acrylic phase.202 It was observed that PSAs with PU fractions lower than 25 wt % had low fibrillation stress and a high maximum strain (strain at debonding), which is characteristic of PSAs with a low level of elasticity due to the low level of cross-linking. In this case, the cross-linking was

Figure 8. Effect of PU/acrylic ratio on tensile stress strain properties. PUpol (100% PU), H90:10 (90% PU), H70:30 (70% PU), H50:50 (50% PU), H30:70 (30% PU), H10:90 (10% PU), AC (0% PU). Reproduced from ref 200 with permission from John Wiley and Sons.

present both in the form of chemical cross-links from grafting of the PU to the acrylic and also physical cross-links due to the more entangled nature of the PU. For PSAs with more than 35 wt % PU fraction, probe-tack curves showed almost no fibrillation plateau and the maximum strain was low (see I


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Industrial & Engineering Chemistry Research Figure 9). The PSA with 25 wt % of PU showed the best adhesive properties as it formed a fibrillar structure and debonded at a maximum strain.

Figure 10. Effect of acrylic composition varying from 100% MMA to 100% BA on tensile stress−strain properties. Samples grafted are shown with bold lines while nongrafted are shown with dashed lines. Reproduced from ref 205 with permission from John Wiley and Sons.

Figure 9. Probe tack test of PU/acrylic PSAs with different PU weight fractions as highlighted in the figure. Reproduced with permission from ref 202. Copyright 2011 American Chemical Society.

almost 100% elongation and still forming a film at room temperature. These results are particularly interesting as despite the Tg being significantly higher than room temperature in many cases; all films had a low MFFT. This is in stark contrast to conventional acrylic latexes where the MFFT is limited by the Tg of the polymer. The cause of this is related to the typical particle morphologies observed in PU/acrylic dispersions for which an acrylic core-PU shell is usually observed (see section 4.5).205 As a result, films cast from PU/acrylic dispersions do not have the typical requirements of acrylic dispersions and polyacrylates with significantly higher Tg values can be used, which act as fillers in the final film and reinforce the mechanical properties. In adhesive formulations, there is little difference between the composition of the acrylic phase in conventional acrylic dispersions and PU/acrylics; in both cases, low Tg polymers are utilized. However, the performance of the adhesive is strongly affected by the molecular weight of the soluble part of the polymer and the gel content. Thus, changes in the synthesis procedure or recipe that affect the molecular weight of the acrylic phase have a strong influence on the adhesive properties. For example, addition of chain transfer agents results in reduced gel content and leads to more liquid-like behavior (high tack, low shear strength).156 On the other hand, lowering the rate of radical generation leads to higher gel content and improves the shear strength of the adhesive.161 4.3. PU Composition. The polyurethane component is a critical component in determining the mechanical strength of PU/acrylic hybrids aimed at coating applications. Mehravar et al. studied the influence of the diol used in the PU phase on the tensile properties of PU/acrylics using the same acrylic component and in the presence and absence of grafting (see Figure 11).205 In each case, diols of 2000 g/mol were used. Similar to conventional PUDs, it was observed that polyether based diols (poly(tetrahydrofuran), PTHF and poly(propylene glycol), PPG) had reduced strength compared to polyesters and polycarbonate based PUs. Sebenik and Krajnc have demonstrated that use of lower molecular weight polyols in the PU synthesis results in films of higher hardness due to the increased chain rigidity arising from hydrogen bonding interactions of the urethane groups as well as the decreased soft segment content in the PU.144

4.2. Acrylic Composition. In conventional acrylic dispersions, a copolymer composition that targets a glass transition temperature around room temperature is generally used for coatings applications. The reason for this is the need for particle deformation during the film formation process itself.203 As such, in the absence of added plasticizers, there is an upper limit of the Tg of the polymer that can be used in order to give an acceptable minimum film formation temperature (MFFT). This upper limit of the Tg is a significant drawback in acrylic polymers and limits the mechanical properties of acrylic coatings. For adhesive applications, significantly lower Tgs are targeted in order to have low modulus at the application temperature.204 In PU/acrylics, these limitations on the acrylic composition are not necessarily true. For example, the effect of different ratios of methyl methacrylate (MMA) and butyl acrylate (BA) was studied by Š ebenik et al.144 PU/acrylic (50/50 wt/wt) hybrids were prepared using semibatch emulsion polymerization of MMA and BA. The results revealed that the hybrid prepared with a highest MMA/BA weight ratio of tested (65/ 35) showed the best properties for the production of a coating for hard substrates. Shi et al. studied the synthesis of TDI based PU/acrylic composites using different proportions of MMA/BA (from 0/100 to 100/0 wt/wt) as diluents for application as binders for coatings.189 As the MMA/BA ratio increased, the tensile strength and the pendulum hardness of the hybrids increased due to the high Tg of poly(methyl methacrylate) (PMMA) while still forming a cohesive film at room temperature. Similarly, Ryu et al. used a mixture of monomers (MMA/BA/glycidyl methacrylate, GMA) and acrylonitrile, AN) as a solvent to obtain PU/acrylic hybrids for binders.186 By increasing the GMA/AN ratio, polymer films with higher modulus and higher hardness were obtained, but the elongation at break and water uptake decreased. Mehravar et al. synthesized a series of hybrids with varying MMA/BA ratios.205 Again, it was observed that higher quantities of MMA results in higher Young’s modulus and tensile strength but with decreased elongation at break (see Figure 10). Given a typical upper limit of tensile strength for film-forming acrylic dispersions in the region of 5 MPa, Figure 10 demonstrates just how strong PU/acrylic dispersions can be, achieving tensile strength of greater than 30 MPa while maintaining J


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method, stabilization was provided by the DMPA contained in the PU. It was observed that the DMPA has a reinforcement effect on the mechanical properties of the polymer film. However, no information regarding water sensitivity was given and it would be interesting to compare the water resistance of PU/acrylics containing DMPA with that of the conventional surfactants. Mehravar et al. have shown that the chain extender also influences the behavior of PU/acrylics with short chain extenders such as ethylene diamine giving higher Young’s modulus than longer chain extenders such as diaminododecane.195 It was, however, noted that the water sensitivity was much improved for the longer alkyl chain extenders. 4.4. Grafting. The effect of grafting is one of the most frequently studied aspects of the PU/acrylic system, but despite this, the overall effect of grafting remains challenging to pinpoint and is highly system dependent. In a large number of cases, grafting of the PU and the acrylic phase has been reported to result in a significant increase in the mechanical strength and the Young’s modulus. For example, Wang et al. synthesized a PU/acrylic hybrid by miniemulsion polymerization using a preformed PU of molecular weight ca. 6000 g/ mol end-capped with either 2-hydroxyethyl methacrylate or methanol.138,208 In the case where the methacrylic monomer was used to terminate the PU, the resulting PU/acrylic had high gel fraction, indicating that the PU acted as a cross-linker during the acrylic monomer synthesis. The grafted polymers typically showed a higher Young’s modulus and more strain hardening but reduced elongation at break. A similar effect has also be reported by Zhang et al., who prepared hybrids containing varying amounts of hydroxypropyl acrylate in order to graft the PU and acrylic phases. In their case, a significant improvement in tensile strength was observed with increasing vinyl functionality in the PU, although at high values the film became brittle and had significantly reduced elongation at break. However, in some instances grafting has been reported to have a negligible effect. Mehravar et al. have investigated a number of PU/acrylic systems with different functional monomers to induce grafting.198,199,205 Although huge differences could be observed in the polymer architecture, notably the gel content and the molecular weight of the polymer (including gel fraction), only a very slight increase was observed in terms of Young’s modulus and mechanical strength.198,199,205 It is notable that only in the case where the molecular weight of the PU was low was an effect of grafting observed.198 This suggests that when the PU is above the entanglement molecular weight, there is little to be gained in terms of mechanical properties by grafting. It was speculated that one reason for the lack of increase in mechanical strength with grafting is due to the improved compatibility, which reduces the extent of H-bonding between PU chains such that grafting effectively replaces physical cross-links (H-bonding between urethane groups) with chemical ones (grafting points). Fang et al. have presented data that suggests that the extent of H-bonding in PU/acrylic hybrids is indeed reduced due to disruption by the acrylic phase.209 Similarly, Alvarez et al. reported that the Tg of the hard PU segment disappeared in the cross-linked polymer coatings, which the authors associated with phase mixing of the grafted polymers.188 The influence of grafting cannot just be observed in terms of mechanical strength but also has a significant influence on other properties of the material. Cross-linking the material by

Figure 11. Effect of macrodiol PPG, PTHF, polyester diol (Polyester) and polycarbonate diol (PCD, ETERNACOLL UH200)) on tensile stress−strain properties. Reproduced from ref 205 with permission from John Wiley and Sons.

Tian et al. studied the influence of the molecular weight of the PU on the properties of the hybrid polymer (see Figure 12).150 PUs of varying molecular weights (Mn = 2800−12 000

Figure 12. Tensile stress−strain properties for varying the molecular weight of the PU. Mn = 2800, 4600, 6000 and 11 900 g/mol for PU1PBMA, PU2-PBMA, PU3-PBMA and PU4-PBMA, respectively. Reproduced from ref 150 with permission from John Wiley and Sons.

g/mol) were synthesized in bulk by using ethanol as a chain terminator, and dissolved in butyl methacrylate then dispersed, and the BMA was polymerized. They observed an increase in mechanical strength with increasing molecular weight of the PU with a major difference above ca. 6000 g/mol. This is likely because below this value, the PU is shorter than the entanglement molecular weight and therefore has a relatively low modulus. Peruzzo et al. compared the mechanical properties of PU/ acrylic hybrids synthesized using different diisocyanates.206 They observed higher Young’s modulus and tensile strength values for 4,4-dicyclohexylmethane diisocyanate based polymers than for tetramethylxylene diisocyanate, which was related to a higher degree of phase separation and increased hydrogen bonding within the PU part of the polymer. Wang et al.201,207 compared PU/acrylic hybrids synthesized via miniemulsion polymerization and seeded emulsion polymerization. An external emulsifier was used in the miniemulsion polymerization while in the seeded emulsion polymerization K


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Industrial & Engineering Chemistry Research grafting was reported by Mehravar et al. to reduce the water uptake.198 Similarly, Mestach et al. developed a grafted polyurethane/polyacrylate hybrid dispersion with (meth)acrylic monomer as diluent for coating or adhesive applications.191 The cross-linked polymer binder improved the film properties such as chemical resistance and mechanical strength. In adhesive formulations, grafting has a huge influence on performance as linking the PU and acrylic phases leads to a formation of a gel fraction that strongly influences the shear strength, tack and peel of the polymer.157 Degrandi-Contraires et al. synthesized a number of PU/acrylics designed for pressure sensitive adhesives containing different amounts of hydroxyethyl methacrylate to react with the terminal NCO groups of the PU prepolymer used.202 As the amount of grafting was increased, the hybrid displayed increasing gel fraction149 and an improved shear resistance, as expected. In the probe-tack test, the transition toward a highly cohesive elastomeric material can be observed with the increase of the fraction of HEMA used (see Figure 13). On the other hand, at

Figure 14. Probe-tack test of latexes with identical formulation but varying irraditation intensity in a tubular photopolymerization process. Reprinted with permission from ref 161. Copyright 2014 Elsevier.

distribution of the polymer it was possible to obtain a system that had both good shear strength and peel strength. It is worth mentioning that in these adhesive systems, there is often no evidence of phase separation between PU and acrylic polymer and the polymer particles usually present a homogeneous morphology.155 4.5. Particle Morphology. Particle morphology plays an important role in hybrid systems because during film formation, the morphology can often be retained.210 Thus, a core−shell type particle will tend to form a film in which the shell makes up a continuous phase with a dispersed phase of the core polymer. In the case of PU/acrylic hybrids, a core/ shell morphology with the acrylic in the core and the PU in the shell is usually obtained, leading to a film with the PU as the continuous phase (see Figure 15).193,199,200,206 In coating

Figure 13. Effect of extent of grafting on probe tack test. The samples are synthesized varying the relative amount of HEMA to bisphenol A, leading to various degrees of acrylic functionality on the PU. Reproduced with permission from ref 202. Copyright 2011 American Chemical Society.

low grafting levels it was observed that both adhesion energy and shear resistance could be improved as compared to an acrylic blank, a particularly impressive result given that these two properties generally work antagonistically. Although variation of the amount of functional monomer is the obvious way to influence grafting, the polymer microstructure and the extent of grafting can also be readily modified by (i) varying the PU chains (changing the chain extender and its concentration),155 (ii) altering the acrylic chains (varying concentrations of chain transfer agent, reaction temperature, radical flux etc.)156,160,161 and (iii) altering the PU−acrylic ratio.149 For example, Daniloska et al. synthesized a series of adhesive PU/acrylic hybrids using a continuous tubular reactor to photopolymerize a preformed miniemulsion.161 They observed that although the formulation was identical in each case, by changing the intensity of the UV light irraditation, and thus the radical flux and rates of termination, different adhesive properties were observed (see Figure 14). By analysis of the gel content and the molecular weight of the polymer by asymmetric flow field flow fractionation, it was clearly seen that at low irradiation intensities the gel fraction and the absolute molecular weight of the gel increased. This led to increasing shear strength. By optimizing the molecular weight

Figure 15. Structure of a 50:50 by weight PU/acrylic dispersion and cross section of a film cast from the same dispersion. The acrylic phase is that with darker contrast. Reproduced with permission from ref 205. Copyright 2018 American Chemical Society.

applications, this allows the PU, which has good film forming properties, to impart low MFFT, while the a high Tg acrylic phase can be used to reinforce the mechanical strength.21 The reason for the tendency toward PU-shell structures has been analyzed by Li et al.139 They considered the morphology of a series of PU/poly(methyl methacrylate) hybrids on the basis of the minimization of interfacial energy, a common method to estimate the equilibrium particle morphology in hybrid latexes.211−214 In all the cases that were explored, due to the lower interfacial tension of the PU-water phase, the thermodynamic minimum was one that consisted of a polyurethane shell and an acrylic core with an interphase consisting of a mixture of the two polymers. It may be noted that in this case, PMMA was used for the acrylic (around the L


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Industrial & Engineering Chemistry Research lowest acrylic−water interfacial tension expected) and the PU was produced by miniemulsion and no stabilizing groups were used (around the highest PU−water interfacial tension expected), and thus the PU-shell/acrylic-core morphology can be assumed to be the thermodynamic equilibrium in most practical cases. In a large number of systems, this thermodynamic equilibrium has been achieved and the observation of a core−shell structure is common. However, kinetically frozen states have also been observed.21,23,215,216 As described above, due to the importance of particle morphology on the film formation process and final mechanical strength of the film, the existence of nonequilibrium morphologies may have significant implications in commercial applications and needs to be seriously considered. Mykhaylyk et al. observed a heterogeneous surface structure on PU/PMMA hybrids by atomic force microscopy (AFM) (see Figure 16). Subsequent analysis of the

Figure 17. Particle morphology of nongrafted and grafted PU/acrylic hybrids. Reprinted with permission from ref 199. Copyright 2014 Elsevier.

Figure 16. Observation of nonequilibrium morphology by AFM. Reproduced from ref 215 with permission of the International Union of Crystallography.

As previously mentioned, adhesive formulations, which often contain low volume fractions of the PU, have been observed to be fully homogeneous with no phase separation.156 However, in many cases, TEM images clearly indicate a significant degree of phase separation at high PU contents.217 Thus, DegrandiContraires et al. showed that while at 5 and 25 wt % PU, the particles were predominantly homogeneous, at 50 wt % PU particles with core−shell morphology were observed, with PU rich domains preferentially located in the outer shell (Figure 18). It was observed that the core−shell morphology persisted in the adhesive films, resulting in a percolating network of stiffer PU rich shells. This in turn led to highly stiff adhesive films with low extension, demonstrating the importance of understanding particle morphology when considering the physical properties of the materials.

scattering pattern of such hybrids by small-angle X-ray scattering (SAXS) confirmed that the morphology consisted of a distribution of kinetically frozen morphologies.215 Jiang et al. have recently explored potential kinetically frozen structures during seeded emulsion polymerization in the synthesis of PU/ acrylic hybrids.216 By comparison of differential scanning calorimetry analyses of blends of latexes or annealed samples to the as-synthesized hybrids, they demonstrated that in all the latexes tested, a significant amount of phase mixing was present that did not represent a thermodynamically favorable state. This observation was linked to the hydrogen bonding between urethane/urea groups and the carbonyl groups of the acrylic phase restricting phase separation. Interestingly, their study also highlighted that the hybrids were highly swollen with water, which has likely implications for the water sensitivity of the latex as well as its film formation properties. Grafting of the PU and acrylic phases leads to increased compatibility and a tendency toward a more homogeneous particle structure (see Figure 17).198,199 Phase mixing can often result in changes in the physical properties of the film. For example, Mehravar et al. prepared PU/poly(methyl methacrylate) hybrids made in the presence and absence of glycerol monomethacrylate (a methacrylic containing diol).199 The sample with no grafting presented a core−shell structure and formed a coherent film at ambient temperature due to the ability of the PU in the shell to deform. However, in the grafted sample, the particle was more homogeneous and the modulus of the material was too high for particle deformation leading to poor film formation and a highly cracked film.

5. APPLICATIONS OF WATERBORNE PU/ACRYLIC HYBRIDS 5.1. Application in Coatings. The excellent physical properties of waterborne PU/acrylic hybrids such as toughness, chemical, water and abrasion resistance, hardness to flexibility balance and substrate adhesion, in combination with the high versatility in terms of chemical structure make them suitable for a wide range of coating applications. However, due to the higher cost of the PU component, PU/acrylics tend to be used only in applications where cheaper materials, such as acrylics, do not perform to a sufficient level. As a result, PU/ acrylics are often found in applications in hard coatings, such as for protection of wooden floors or furniture164 or metal surfaces in the automotive industry.135,136 PU/acrylics are also commonly used for carpet backing, leather and textile or paper finishing in addition to some specialty applications such as binders for inks218 and ceramic powders.219 In the following, the progress in coating applications will be presented, covering the patent and open literature. 5.1.1. Coating Applications. Coatings should be designed to provide high durability, hardness, wear and chemical resistant and low friction simultaneously with good wetting and adhesion to the substrate. While hybrid PU/acrylics provide optimum performance, even simple blends of acrylic and PU dispersions provide sufficiently good properties in many instances. The blending process has been shown to be practical for two-component hard coating application in automotive finishing,135,136 assuring good leveling, good mechanical properties and weathering resistance, as well as in surface finishing coatings for application in constructions.137 M


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Figure 18. TEM images of latex particles stained with PTA for three different PU weight fractions. Reprinted with permission from ref 217. Copyright 2014 Elsevier.

As highlighted in section 3, using acrylic monomers as solvents for synthesis of PUs it is possible to produce low VOC PU/ acrylic hybrids, and the potential of such hybrids for use in coatings has been explored. Mestach et al. reported the solvent-free synthesis of a PU/ acrylic hybrid by seeded emulsion polymerization.191 In a first step, a 2-hydroxyethyl methacrylate terminated PU macromer was obtained by bulk polymerization and dissolved in the acrylic monomer. Following dispersion, the polymerization of the acrylic phase led to a grafted PU/acrylic. The acrylic phase also contained some diacetone diacrylamide for further crosslinking with adipic dihydrazides in the dry state, leading to hard films. A similar process using a sulfonate based stabilizer in place of DMPA has been reported by Liao et al.183 The major advantage of the sulfonate sytem as compared to DMPA is that the neutralization step is not necessary, and therefore the use of triethylamine, which contributes to the VOC content, can be reduced. While the solvent-free process represents a substantially more environmentally friendly synthesis, in most commercial synthesis nonrenewable monomers are used. Killilea patented a process for the production of PU/acrylics formed from a plant derived alkyd as a diol in the formation of the PU prepolymer.174 Following drying and curing of the film, mechanical properties suitable for hard floor coatings were obtained. Similarly, Zhang et al.178 have patented the use of polyols derived from renewable feedstock, such as natural or genetically modified plant vegetable seed oils or animal fats to synthesize PU/acrylic hybrid binders for coatings with incerased weather durability, UV resistance and water whitening resistance. 5.2. Application in Adhesives. Adhesives are very important products that find use in numerous applications. Among the various types of polymeric adhesives, PU/acrylic hybrids are distinguished by very strong adhesion, isolation, flexibility, color stability and UV degradation resistance. Therefore, these hybrids find application as adhesives between others, in construction, woodworking, shoe production, and lamination.

PU/acrylic hybrids synthesized by emulsifier-free seeded semicontinuous emulsion polymerization have been shown to be more promising than blends for hard coatings applications,26,142,144,163 especially if cross-linking is introduced between both phases.142 The versatility of the chemistry of these hybrids in combination with modifications in the synthesis procedures permits the production of a range of different hybrids with a huge variation of physical characteristics, such as glass transition temperature, flexibility, hardness, toughness, gloss and durability. For example, Carlsen et al. patented the synthesis of a PU/acrylic for use in floor coatings by the seeded emulsion polymerization of (meth)acrylic monomers mixture in the presence of an acrylic support resin and a commercial PUD. It was demonstrated that by variation of the acrylic copolymer composition and the PU/ acrylic ratio, polymer films suitable for a diverse range of applications ranging from floor coatings to blisterpack adhesives could be obtained.164 There are numerous modifications to the general seeded emulsion process that may be undertaken to improve performance of the polymer. In order to avoid the loss of adhesion that can be observed for many PU/acrylics when exposed to humid atmospheres, Swarup et al. patented a process that gave films with improved humidity resistance by using trifunctional amines in the chain extension process to give a branched/cross-linked structure.165 Hartig et al. reported a multistep synthesis of the acrylic phase to produce a low Tg as well as a high Tg acrylic copolymer.167 While no details were provided on the structure of such polymers the blocking resistance and resistance to water whitening were significantly improved against blank experiments. Similar polymers containing multiple acrylic phases were reported by Mehravar et al. to be useful in hardwood coatings.199 5.1.2. Coatings Produced in Solvent-Free Procedures. In many of the systems highlighted above, the PUDs were produced in the presence of toxic solvents as diluents to limit the viscosity. As controls over limits on VOCs, and use of solvents in general, are becoming increasingly strict, such hybrids will need to be replaced by more sustainable processes. N


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Industrial & Engineering Chemistry Research Recently, Che et al.220 have synthesized solvent-free and emulsifier-free waterborne PU/acrylic hybrids that were used to prepare adhesive formulations for footwear applications. The molar ratio of cross-linkable monomer trimethylol propane (TMP)/chain extender ethylene diamine (EDA) was varied to synthesize a range of waterborne hybrids, leading to high performance adhesives for a synthetic leather/ethylene vinyl acetate sole at both dry state and wet state. The adhesive strength of formulated adhesives in the dry state was not dependent on the TMP/EDA molar ratio. However, the adhesive strength in the wet state increased significantly with increasing TMP content, and for the highest ratio, the adhesive strengths were higher than the footwear adhesion requirements. Gruber and Derby patented a process for the production of PU/acrylic hybrids for laminating adhesive in flexible packaging.171 They used an otherwise conventional PU/acrylic, but in the chain extension step employed a mixture of difunctional and monofunctional amines. When the monofunctional amine was included in the formulation, the PU/acrylic system was found to offer good peel strength, even after soaking in water. Furthermore, it made an excellent laminating adhesive, with peel strength as high as that from solvent borne systems. Pressure sensitive adhesives (PSAs) are a special type of adhesive that exhibit excellent tackiness when exposed to light pressure and ideally leave little/no residue if removed from the substrate. To achieve these specifications, the PSA has to accomplish a set of requirements regarding the polymer microstructure that will ensure balanced adhesive−cohesive behavior. Specifically, the polymer chains have to be crosslinked to provide cohesiveness to the adhesive film, but not to such an extent so as to compromise the adhesiveness. PU/acrylic hybrids have been shown to be excellent for this application as they offer the possibility of cross-linking between the both polymer types to control the gel content while still allowing control over the low molar mass polymer chains. Degrandi-Contraires et al.202,217 observed an improvement in the shear resistance while maintaining good tack adhesion in PU/acrylic hybrid polymers synthesized by miniemulsion polymerization. This was explained by the heterogeneous structure within the hybrid film due to phase separation at higher PU loads within the hybrids. A comprehensive study of PU/acrylic hybrid PSAs was performed by Lopez et al.155−157 based on the variation of the polymer microstructures achieved by varying PU and/or acrylic chains or cross-linking between both. This led toward a better understanding of the requirements for balanced PU/acrylic PSAs; the polymer should contain sufficient gel content to give good shear strength, sol molar mass within appropriate limits to give rise of entanglements while maintaining a reasonable degree of viscous flow giving good tack and peel properties. It is interesting to note that the development of hydrogen bonding networks in the PU/acrylic hybrids also appears to contribute to a shift in the adhesive properties. Thus, Daniloska et al. noted that PU/acrylic latexes undergo spontaneous formation of nanogels within the latex particles during storage at room temperature, first due to NCO reactions (with the OH groups of both HEMA and water) and afterward due to supramolecular interactions and Hbonding.162 Therefore, the polymer microstructure and hence the adhesive properties evolved during storage and the final configuration was reached in periods as high as 7 months.

Along similar lines to the above academic studies, Hartig et al. have developed a PU/acrylic based PSA made via a seeded emulsion polymerization process. Through variation of the acrylic comonomer composition and the PU composition were synthesized a number of PSAs covering a wide spectrum of adhesive properties.168 With the right combination of the two components, it was possible to obtain a good balance between the shear adhesion force and the tack of the polymer.

6. CONCLUSIONS AND OUTLOOK In this review the different synthetic routes and the structure− property relationships of PU/acrylic hybrid dispersions have been summarized. Although there are numerous methods described in the literature for the synthesis of PU/acrylic dispersions, seeded emulsion polymerization is the most suitable for large-scale production of surfactant-free waterborne PU/acrylic hybrids. However, within this one technique lies an endless combination of building blocks that allow for the tailored design of functional hybrids assuming that the structure−property relationships are properly understood. In most PU/acrylic dispersions that undergo phase separation, a PU shell−acrylic core type of structure is obtained. Since this structure is often retained in the film, the film structure tends to consist of dispersed acrylic domains within a PU matrix. As a result, the PU part mainly controls the film formation. Hence, acrylic compositions leading to Tg values significantly higher than in conventional acrylic dispersions can be used without large effects on the film formation process. Grafting of PU/acrylic hybrids can lead to significant improvements in solvent/water resistance, but it does not necessarily lead to an increase in the mechanical properties of the material. This seems to be particularly the case when relatively high molecular weight PUs are used such that they are above the entanglement molecular weight. In adhesive formulations, the systems are much more sensitive to grafting as the gel content is a critical parameter in the adhesive properties. Knowing these fundamentals of how polymer structure influences the properties, a number of commercial systems have been developed, particularly in the area of hard coatings. PU/acrylics are a well developed class of materials, but there remains much to learn and more importantly, with future concern over the environmental impact of the synthetic process, it is clear that many aspects that are already well researched will need significant rethinking. First, although there is now a general trend toward low VOC synthetic routes that avoid solvents, the dispersion step is not well understood. This makes the task of complete removal of VOCs very challenging without resorting to simple trial-and-error. Second, the chemicals used in the synthesis will need to be adapted to new legislation that will undoubtedly limit their use. Diisocyanates in particular are likely to become increasingly restricted, and therefore nonisocyanate synthetic routes will need to be considered. Aside from the NCO groups, at present almost all of the reagents are ultimately sourced from petrochemicals, and there is currently limited work directed toward the use of alternative sources. Even with these drawbacks, it is clear that the unique blend of strength and flexibility displayed by many PU/acrylic hybrids will continue to allow them to dominate markets in which mechanical properties of polymer films are of primary importance. O


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ethanes. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA, 2005. (17) Ramesh, S.; Tharanikkarasu, K.; Mahesh, G. N.; Radhakrishnan, G. Synthesis, Physicochemical Characterization, and Applications of Polyurethane Ionomers: A Review. J. Macromol. Sci., Polym. Rev. 1998, 38, 481−509. (18) Lee, S. W.; Lee, Y. H.; Park, H.; Kim, H. Do. Effect of total acrylic/fluorinated acrylic monomer contents on the properties of waterborne polyurethane/acrylic hybrid emulsions. Macromol. Res. 2013, 21, 709−718. (19) Schick, M. F.; Hickman, W. C.; Clark, G. T.; Stockl, R. R. Aqueous dispersion blends of polyesters and polyurethane materials and printing inks therefrom. Patent US4847316, 1989. (20) Okamoto, Y.; Hasegawa, Y.; Yoshino, F. Urethane/acrylic composite polymer emulsions. Prog. Org. Coat. 1996, 29, 175−182. (21) Hirose, M.; Kadowaki, F.; Zhou, J. The structure and properties of core-shell type acrylic-polyurethane hybrid aqueous emulsions. Prog. Org. Coat. 1997, 31, 157−169. (22) Kukanja, D.; Golob, J.; Zupancic-valant, A.; Krajnc, M. The structure and properties of acrylic-polyurethane hybrid emulsions and comparison with physical blends. J. Appl. Polym. Sci. 2000, 78, 67−80. (23) Hirose, M.; Zhou, J.; Nagai, K. Structure and properties of acrylic-polyurethane hybrid emulsions. Prog. Org. Coat. 2000, 38, 27− 34. (24) Barrere, M.; Landfester, K. High Molecular Weight Polyurethane and Polymer Hybrid Particles in Aqueous Miniemulsion. Macromolecules 2003, 36, 5119−5125. (25) Li, M.; Daniels, E. S.; Dimonie, V.; Sudol, E. D.; El-Aasser, M. S. Preparation of Polyurethane/Acrylic Hybrid Nanoparticles via a Miniemulsion Polymerization Process. Macromolecules 2005, 38, 4183−4192. (26) Athawale, V. D.; Kulkarni, M. A. Preparation and properties of urethane/acrylate composite by emulsion polymerization technique. Prog. Org. Coat. 2009, 65, 392−400. (27) Manock, H. L. New developments in polyurethane and PU/ acrylic dispersions. Pigm. Resin Technol. 2000, 29, 143−151. (28) Kim, B. K.; Lee, J. C. Modification of waterborne polyurethanes by acrylate incorporations. J. Appl. Polym. Sci. 1995, 58, 1117−1124. (29) Kozakiewicz, J. Developments in aqueous polyurethane and polyurethane-acrylic dispersion technology Part II. Polyurethaneacrylic dispersions and modification of polyurethane and polyurethane-acrylic dispersions *). Polimery 2016, 61, 81−91. (30) Li, F.; Tuinier, R.; van Casteren, I.; Tennebroek, R.; Overbeek, A.; Leermakers, F. A. M. Self-Organization of Polyurethane PrePolymers as Studied by Self-Consistent Field Theory. Macromol. Theory Simul. 2016, 25, 16−27. (31) Prisacariu, C. Polyurethane Elastomers; Springer Vienna: Vienna, 2011. (32) Noble, K.-L. Waterborne Polyurethanes. Prog. Org. Coat. 1997, 32, 131−136. (33) Wicks, Z. W.; Wicks, D. A.; Rosthauser, J. W. Two package waterborne urethane systems. Prog. Org. Coat. 2002, 44, 161−183. (34) Madbouly, S. A.; Otaigbe, J. U. Recent advances in synthesis, characterization and rheological properties of polyurethanes and POSS/polyurethane nanocomposites dispersions and films. Prog. Polym. Sci. 2009, 34, 1283−1332. (35) Guan, J.; Song, Y.; Lin, Y.; Yin, X.; Zuo, M.; Zhao, Y.; Tao, X.; Zheng, Q. Progress in Study of Non-Isocyanate Polyurethane. Ind. Eng. Chem. Res. 2011, 50, 6517−6527. (36) Cornille, A.; Auvergne, R.; Figovsky, O.; Boutevin, B.; Caillol, S. A perspective approach to sustainable routes for non-isocyanate polyurethanes. Eur. Polym. J. 2017, 87, 535−552. (37) Kathalewar, M. S.; Joshi, P. B.; Sabnis, A. S.; Malshe, V. C. Non-isocyanate polyurethanes: from chemistry to applications. RSC Adv. 2013, 3, 4110−4129. (38) Maisonneuve, L.; Lamarzelle, O.; Rix, E.; Grau, E.; Cramail, H. Isocyanate-Free Routes to Polyurethanes and Poly(hydroxy Urethane)s. Chem. Rev. 2015, 115, 12407−12439.

AUTHOR INFORMATION

Corresponding Author

*J. M. Asua. E-mail: [email protected]. ORCID

Radmila Tomovska: 0000-0003-1076-7988 José M. Asua: 0000-0002-7771-1543 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the Industrial Liaison Program in Polymerization in Dispersed Media (3M, Akzo Nobel, Allnex, Arkema, Asian Paints, BASF, DSM, Inovyn, Stahl, Solvay, Synthomer, Vinavil and Wacker). N.B. acknowledges the financial support obtained through the Post-Doctoral fellowship Juan de la Cierva−Incorporación (IJCI-2016-28442), from the Ministry of Economy and Competitiveness of Spain.

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