Structure-property relationships of waterborne polyurethane (WPU) in aqueous formulations

This study provides an overview of the rheological properties of aqueous polyurethanes (WPU), as the main component, or as a thickening additive in aqueous formulations. Waterborne polyurethanes (WPU) have been proposed as an environmentally friendly alternative to conventional solvent-based solutions in a variety of industrial applications such as coatings, adhesives, inks. In all these fields, the control of rheological properties became an important prerogative to determine the quality of the dispersion and its potential applicability. First, the effect of parameters such as components, particle size and content, temperature, and interactions on dispersion viscosity was reported. Then, the effect of two additives, i.e. thickeners and nanomaterials, on structure – property relationships of WPU-base systems, was described. Thickeners are rheological modifiers, commonly used to stabilize the dispersion and prevent flocculation and sedimenta-tion of the particles, or to change the flow behavior of dispersions from Newtonian to pseudoplastic. These species can interact with water and polymer particles to create a network structure that alters the flow resistance, and thus viscosity. The use of hyperbranched aqueous polyurethane as thickening agent in WPU formulations was also presented. On the other hand, nanostructured fillers (0D/1D/2D) or a combination thereof in waterborne polyurethane led to the formation of specific microstructures that prevented the penetration of water, oxygen, and corrosive substances, also improved mechanical and thermal properties, allowing the development of high-performance WPU-based products.


| INTRODUCTION
Polyurethane (PU) has attracted scientific and industrial attention, due to the versatility of pristine constituents, [1] that allowed an adaptable behavior for a large variety of requirements and customized products (i.e. foams, [2] composites, [3] coatings and adhesives, [4] inks, [5] etc.). This polymer is commonly used in advanced coating technology given improvements in the quality, appearance, and lifespan of treated substrates. Traditionally, polyurethane formulations are made from harmful organic solvents, such as toluene, xylene, formaldehyde. During the evaporation, large quantities of volatile organic compounds (VOCs) and free isocyanates are released to the air by damaging air quality and environment. [6] Due to stringent environmental guidelines, promulgated by the Environmental Protection Agency (EPA) and other air quality regulators, industries are forced to develop eco-friendly solutions by replacing noxious organic solvents with water. In this way, the traditional solvent-based dispersions were quickly replaced by water-based dispersions (WPU) with low volatile organic compounds (VOCs) and no co-solvents. [7] Afterwards, a wide range of additives were introduced into the formulations, as reinforcement for the PU matrix, or to improve the final performance of WPU-based products, such as clay, [8] silica (SiO 2 ) nanoparticles, [9] calcium carbonate (CaCO 3 ), [10] UV light stabilizers (i.e., organic UV absorbers, hindered amine light stabilizers (Hals) or functional particles), [11] crosslinking agent, [12] thickener [13] or a combination of more than one components, [14] and recently, water-dispersible entities (i.e. nanocellulose, starch, chitosan) to reinforce the environmental-friendly character of PU dispersions. [15] The rheological properties of dispersions are crucial in a variety of industrial applications. For instance, in the paper coating industry, the shear thickening behavior of suspensions should be avoided as materials are subjected to high shear rates. [16] Rheology is an important property for checking the quality of coating formulations in the storage, efficient handling, and application of paints. Rheology monitors dispersion properties under different shear regimes so to verify overall performance in storage, processing, handling, transport, and application properties. [17] In paints, cosmetics (emulsions or suspensions), and pharmaceutical formulations (emulsions, suspensions, creams, and gels), the knowledge of rheological behavior provides useful information about the long-term stability of products. In food industry, the control of rheological parameters allows to check the good texture and mouthfeel of several foods, such as mayonnaise, salad creams, and desserts. [16] Adhesives should flow through the substrate and form a strong bond to the substrate. These coatings and extruding systems demand high performance at high shear rates and over a wide temperature range. Therefore, understanding viscosity in relation to shear rate or temperature is essential to provide valuable information on the nature of adhesives. In the case of direct ink writing, a colloidal suspension is deposited into controlled continuous filament through a needle. This process requires a controlled rheological behavior of inks. In particular, the material should move very easily through the needle, and withstand deformation immediately after printing. This flow behavior can be achieved thanks to shear-thinning characteristics which facilitate deposition and passage through the nozzle. Inks, which have a low viscosity and form a gel after the printing process, could be developed. In this way, a yield stress sufficiently high to withstand deformation is induced by gelation. Another method is to make viscoelastic suspensions with a sufficient yield strength to support the stacking of layers. [18] A wide variety of industrial applications involved WPU-based formulations: footwear [19] ; coatings, paints, inks, adhesives [20] ; and leather industries [21] ; or more specific applications to improve the bacterial resistance in food-contact, water purification systems, prosthetic devices, and hospital equipment surfaces [22] ; to improve the mechanical and water resistance in textiles [23] ; to reduce the electromagnetic reflections in vehicles such as airplanes and ships, and decrease the susceptibility to radar [24] ; to improve the wear and corrosion resistance, [25] also regarding metal surfaces for oil pipeline, coastal transmission tower, ships; to develop high temperatureresistant adhesives in advanced aircraft, space vehicles, missiles, ground vehicles and electronics. [26] The stability, film forming capacity, and viscoelastic properties of the WPU are expected to be strongly influenced by their colloidal state, structure, and composition. Understanding the rheological behavior of dispersions is vital for controlled and repeatable preparation and use. [27] Several recent reviews illustrate the synthesis and properties of WPU and WPU-based products. [6,15,28,29] However, to our knowledge, no cases have reported aspects related to rheology and changes in properties based on the microstructure of derived products.
The purpose of this work has been to provide an overview of the rheological characteristics of waterborne polyurethanes, and of the effect of specific additives, i.e. thickeners and nanomaterials, able to change the structure-property relationships. The properties of specific synthesized waterborne polyurethanes, which act as a thickening agent in aqueous formulations, were also discussed. The study was carried out using a combination of main keywords such as waterborne or water-based polyurethane, rheological properties or viscosity, thickeners, nanoparticles or nanomaterials on academic databases such as Scholar, Scopus, and ScienceDirect. Possibly accessible articles were selected on the basis of the title and summary that seemed more essential, not too specific, and better form a general overview of the topic. A table of contents was developed based on: (i) definition of waterborne polyurethane; (ii) effects of polymer particle size and shape, polydispersity, and interactions, on the rheological properties of aqueous polyurethane; (iii) the importance of rheological characteristics of WPU-products depending on the application methods, and changes in flow behavior of WPU formulations following the introduction of common rheology-modifiers (i.e. thickening agents); (iv) WPU-based formulations incorporating nanoparticles for specific needs and products, as well as the effect of fillers on the microstructure and rheological properties of aqueous dispersions. The review incorporates over than 80 contributions in total. The number of documents on Scopus depending on the different used keywords, and the number of total contributions that make up each paragraph are summarized in Table 1.

| WATERBORNE POLYURETHANE (WPU)
PU macromolecules are block copolymers consisting of polar crystallizable units, the so-called hard segments (HS) and non-polar amorphous units, the so-called soft segments (SS). [30] The hard segments are constituted by isocyanates (mostly aliphatic or aromatic) whereas the soft segments are composed of macrodiols (usually polyester-, polyether-, or polycarbonate-based). Chain extenders (CE) are reactive species of low molecular weight (hydroxyl amines, glycols, or diamines) that can react with isocyanate in the final stage of polyurethane synthesis, by influencing the mechanical behavior of PU through interactions between hard and soft segments ( Figure 1).
Depending on the type of dispersive phase and dispersive medium, different types of systems can be distinguished: suspension (solid in liquid), emulsion (liquid in liquid), gel (liquid in solid), aerosol (liquid in gas), foam (gas in solid), composite (solid in solid). If the size of the particles or droplets of the internal phase is between 1 nm and 1 μm, the system is called a "colloidal system". [16] Waterborne polyurethane is an aqueous colloidal system of PU particles, containing hydrophilic groups in the polymer backbone. This system is endowed with high surface energy which is the driving force for coalescence T A B L E 1 Number of total documents on Scopus starting from a specific year and number of selected contributions used in the creation of this overview.

Topic
Main keywords after water evaporation. [6] Compared to its solvent-based counterparts, the components of WPU are always polyols, isocyanates, and a few additives. The use of water as a solvent also ensures the elimination of toxic unreactive isocyanate groups in the final products. [28] Three reactions occur during the preparation of WPU dispersions: (i) reaction between the excess isocyanate and the polyols; (ii) reaction between isocyanate and amine (as chain extender) to form the urea group; (iii) reaction between water and residual isocyanate. [28,31] A schematic representation of the chemical structure of polyurethane, and the main reactions involved in WPU synthesis, can be seen in Figure 2. Stabilization of polyurethane, which is generally hydrophobic and insoluble in water, is usually achieved by adding an external or internal emulsifier to the polymer backbone. If the internal emulsifier contains an ionic center, Àsuch as anionic species (including sulfonic and carboxylic acids), cationic species (including quaternary ammonium or sulfonium ions)-, the stabilization is achieved by the formation of an electrical double layer and repulsive electrostatic interactions. On the other hand, if the emulsifier contains a non-ionic center poly(ethylene oxide), stabilization is achieved by the steric hindrance. [15] Depending on the particle size, and appearance, different types of waterborne polyurethane can be distinguished [32] : (i) transparent aqueous solutions, when the particle size is below than 1 nm; (ii) slightly turbid dispersions, when the particle size is in the range between 1 and 100 nm; (iii) white and turbid emulsions, when the particle size is above 100 nm.
A distinction can then be made between onecomponent and two-component systems. The one-component dispersion can be used without a crosslinker, since the reaction takes place under the influence of temperature, atmospheric oxygen, or radiation to form highly crosslinked films. The two-component dispersion usually requires a crosslinker: the reaction occurs immediately after mixing the components at room temperature, and the crosslinker is usually added immediately before use. [32] The different characteristics of the original components allow thermoplastic, elastomeric, and thermoset behavior to be exhibited. [33] 3 | RHEOLOGICAL PROPERTIES OF WPU The viscosity of the dispersion depends on various parameters, such as particle size and content, polydispersity of size, and interactions. [34] The rheological behavior of waterborne polyurethane was studied by Madbouly et al. [27] largely as a function of solid content, chain extension, pre/post-neutralization, and temperature.
A sharp increase in complex viscosity of about four orders of magnitude was observed when the concentration of PU was changed from 24 to 46 wt% ( Figure 2). The rheological behavior was determined by two different forces: the repulsive force between the similar charges surrounding the particles and the hydrodynamic interactions ( Figure 3). At low concentrations, the particles were far enough apart, and the interactions were mainly by repulsive forces between similar charges surrounding the particles. In contrast, at high concentrations, the system became increasingly crowded with particles, the distance between them was reduced, and hydrodynamic interactions were considered to be the main contributors to the increase in viscosity.
No significant effect of the chain extender on the complex viscosity of WPU systems was demonstrated, although the molecular weight of the PU polymer increased by four orders of magnitude by changing the chain extender from 0% to 90%. However, regardless of the molecular weight, the particle size and its distribution remained unchanged. This resulted in similar interactions between particles in each dispersion, and consequently similar viscosity. [27] In contrast, temperature was found to be an important parameter affecting the rheological properties of WPU-based systems. Indeed, the complex viscosity of PU dispersion was greatly increased by increasing the temperature from 70 C to 90 C. This increment was not attributed to the water evaporation but to a thermally induced gelation process. However, the thermal energy used in heating led to increase F I G U R E 2 Complex viscosity as a function of shear rate for WPU at different PU content. Reproduced from. [27] Copyright (2005), with permission from American Chemical Society.
interactions and gelation of the system only when PU particles were of a large size. [27] The same considerations were made from the newly synthesized bio-renewable waterborne polyurethane based on castor oil. Recently, under the pressure of green chemistry and sustainability, much emphasis has been placed on replacing petroleum-based polymers with more environmentally friendly biopolymers derived from renewable resources. [35] Abundant, low-cost and ecological vegetable oils can be considered as raw material source for the production a range of polyols with different chemical structures and functionalities and thus for bio-based WPU. Castor oil is 90% ricinoleic, 4% linoleic, 3% oleic, 1% stearic, and less than 1% linolenic fatty acids. The high content of ricinoleic oil ensures the presence of hydroxyl groups, i.e. suitable functionalities for a variety of chemical reactions such as halogenation, dehydration, alkoxylation, esterification, and sulfation, and allows a wide range of applications in the chemical industry (paints, coatings, inks, and lubricants). [36,37] An overview of the possible chemical modifications with different reactive groups of castor oil is shown in Figure 4. [38] Hydrogenated castor oil has improved thermal stability, melting point and thermal properties compared to base oil, and is used in grease formulations, leather polishes, paint additives, wax and in rubber and plastic manufacturing. Epoxidized castor oil is used in the formulation of paints, coatings, and lubricants, or as plasticizer and heat stabilizer. Due to the high reactivity of the epoxy ring, epoxidized castor oil is used in the production of alcohols, glycols, polyols, and various other polymers. [39] In, [40] three types of castor oil were produced by epoxidation and ring opening reactions in the presence of two catalysts (γ-alumina and formic acid). Then, the products were combined with saponified or basic castor oil to prepare polyols, and the synthesis of water-based polyurethane was successfully carried out. The number of hydroxyl groups in saponified castor oil affected the development of hydrogen bonds, the molecular weight of polyols, and the viscosity of WPU. [40] The effects of solid content of PU particles and temperature on the viscoelastic behavior of castor oil based WPU were presented in. [41] Increases in PU content from 16 and 32 wt% resulted in an increase of the complex viscosity, and in changes in the flow behavior of the F I G U R E 3 Schematic of (A) repulsive forces and (B) hydrodynamic forces among the PU particles in the aqueous medium.
F I G U R E 4 Main chemical modifications and reactive groups in castor oil. Reproduced from. [38] Copyright (2015), with permission from American Chemical Society. dispersion from Newtonian to non-Newtonian. These changes were attributed to a reduction in free water of dispersion as the loading of PU particles was increased. Each particle picked up a thin layer of bound water, and the amount of unbound water was reduced. The effect of temperature was demonstrated using a dispersion containing 32 wt% of PU. Initially, the complex viscosity decreased with increasing temperature from 10 C to 60 C. At a temperature above 60 C, the complex viscosity increased again. This trend was attributed to the occurrence of gelation, which led to a discrepancy with time-temperature superposition (TTS) principle and the WilliamsÀLandelÀFerry (WLF) model. The occurrence of the sol-gel transition in the dispersion at 32 wt% of the PU polymer was illustrated in Figure 5 in terms of zero-shear (η 0 ) versus temperature. The evolution of thermally induced gelation was shown by TEM images obtained on a dispersion containing 27 wt% of PU at different annealing times (50, 100, 150 min) and a fixed temperature of 60 C ( Figure 6).
Castor oil, polypropylene glycol (PPG), toluene diisocyanate (TDI), and (2 E)-4-(2,3-dihy-droxypropoxy)-4-oxobut-2-enoic acid [glycol semi-ester (GSE)] were used as raw materials for the preparation of novel WPU. [42] The presence of castor oil was expected to improve the crosslinking ability and lead to excellent mechanical properties in WPU-based products. Since this species is endowed with long aliphatic moieties, it could also improve the water resistance. By providing ionic groups, addition of GSE could improve stability and homogeneity of PU dispersion in water. The influence on the final properties of components such as: GSE, castor oil, and polymer, was studied. Complex viscosity increased with the concentration of carboxyl groups from 2.5 to 3.2 wt% (corresponding to GSE concentrations of 11 to 12.78 wt%, respectively). This was attributed to the presence of an increasing number of particles in dispersion. In fact, the more the ionic groups present in dispersion, the smaller the particles, and hence, the larger number of particles. [43] Increasing the castor oil concentration resulted in particles with larger diameter and wider distribution, and thus increasing the complex viscosity of the overall system. The large number of hydroxyl groups on castor oil increased the urethane and hydrogen bonds and determined more connections in the system.

| RHEOLOGICAL PROPERTIES OF WPU INCORPORATING THICKENERS
Adequate control of rheology was required to ensure good thin film formation in waterborne coatings. [44] In each step of the coating process, certain viscosity conditions as a function of shear rate should be recommended [44] : (i) storage requires a shear rate of 1 s À1 and viscosity value of less than 500 poise; (ii) shear rate of 20 s À1 and viscosities lower than 25 poise are required for transfer to dripless brush; (iii) substrate transfer with good film build-up and without excessive brush drag necessitates shear rate of 10 4 s À1 and viscosities in the range between 1-3 poise; (iv) drying with good leveling and minimal sagging necessitates viscosity in the range between 50-100 poise for shear rate of 1 s À1 . In other words, high viscosities are required for waterborne coatings in the low shear region to avoid settling and instability during storage and transport, whereas low viscosities are preferred in the high shear rate region during applications ( Figure 7A). [34] In the case of water-based adhesives, the viscosity should be high enough to prevent settling during the storage, but low enough to allow application or proper atomisation during brushing or spraying. Shear-thinning or pseudo-plasticity is the term used to describe this flow behavior. The dispersion state itself is responsible for settlement and instability. Aggregates should be set apart to avoid flocculation and sedimentation, resulting from increasing size and weight. Aggregates should thus be stabilized through steric and/or electrostatic repulsion. After application, during drying and solvent evaporation, the volume fraction of polymer molecules increases by leading to an increase in viscosity. [34] The stabilization of particles in dispersion and the prevention of flocculation are characteristics that can be achieved in a variety of ways, including charges in polymer backbones or the inclusion of ionic surfactants, or an increase in repulsive forces between particles. Thickeners improve the stability of dispersions by increasing the volume fraction. [34] Thickeners are made up of water-soluble polymers with low or medium molecular weight (10000-50000), that contain trace amounts of hydrophobic species. [45] The presence of hydrophilic F I G U R E 5 Zero-shear viscosity evaluated vs changes in temperature for a dispersion containing 32 wt% of PU particles. Reproduced from. [35] Copyright (2013), with permission from American Chemical Society. groups promotes hydration, swelling and entanglements, whereas the presence of hydrophobic groups promotes micellization process and prevents contact with water. [34] Traditional thickeners were frequently used in waterbased adhesive formulations, but because of their poor flow properties, associative thickeners were mostly preferred. [46] Associative polymers are macromolecules with attractive groups, including charged polymers, block copolymers in strongly selective solvents, and polymers with hydrogen bonding. [47] F I G U R E 6 TEM micrographs of WPU dispersion at 27 wt% of PU content during 50, 100, and 150 min of annealing process performed at a temperature of 60 C. As the annealing time increases, the particles aggregate and combine into large cluster, and then into large aggregates. Reproduced from. [35] Copyright (2013), with permission from American Chemical Society.
F I G U R E 7 Viscosity trend as function of shear rate (A) and time (B). Reproduced from. [34] Copyright (2001) from Elsevier.
When the three-dimensional network structures are generated by simple entanglements or electrostatic repulsion among aggregation points, no differences between solving and reconstruction of the aggregation points are observed. The process of solving the network through shear stress takes the same amount of time as reconstructing the network. On the contrary, in presence of associative polymers, differences in solving and reconstruction of aggregation points can be noted. This behavior can be considered a time-dependent process ( Figure 7B) with slower (i.e., thixotropic behavior) or faster (i.e., rheopectic behavior) recovery of structures as the shear rate decreases. The rheopectic properties are thought to be useful for holding and freezing structures formed during the application phase until film formation. Because the structures take longer to recreate, thixotropic properties are thought to help with storage stability, better flow and leveling. [34,48] Thickeners are rheological additives that increase viscosity. They form structures in a solution by interacting with water or latex particles. The proper thickener selection is defined by the formulation and application method, and it results in proper rheology control. Thickening agents, depending on their type, can influence the viscosity at high shear rate to improve pick-up and transfer properties or the viscosity at low shear rate, the flow and leveling properties. [49] It should be noted that while all thickeners are rheological modifiers, not all rheological modifiers are thickeners. This means that the rheological modifiers are additives that change the viscosity at low (<10 s À1 ) or high shear rates (>1000 s À1 ) but have a minor effect on consistency (small thickening effect). [50] Cellulosic and associative thickeners are the two primary categories of thickeners. [17,51,52] The mechanism of action for cellulosic thickeners is the development of three-dimensional network structures through physical interactions between thickening molecules and dispersive medium. [53] For the associative thickeners, intra-or intermolecular linkages are created between the hydrophobic groups on their molecules and other hydrophobic species in the formulation, including those found on the surface of the particles. In more detail, in an aqueous phase, strong hydrogen bonds are established between water molecules, and associative thickener's hydrophobic groups force the polymer to form clusters to lessen the disconnection of water molecules. The hydrophobic portions may then be connected to additional hydrophobic portions that are located on the surfaces of polyurethane particles. The creation of pseudo-polymer network can therefore be facilitated by intramolecular or intermolecular hydrophobic clusters, functioning as connection points, and rising the system's overall viscosity.
The chemical potential of hydrophobes was considered a sign of the durability and strength of clusters. Clusters associative strength increases as chemical potential decreases.
The chemical potential (ΔμÞ is described in Equation (1): In Equation (1), R is the ideal gas constant, T is the temperature, X is the hydrophobe volume fraction, δ s À δ p À Á is the difference between the solubility parameter of solvent(δ s Þ and hydrophobe δ p À Á , V P þ V s ð Þ is the sum between the molar volume of solvent (V s Þ and hydrophobe (V P Þ. The greater the difference between solubility parameter and hydrophobe volume fraction, the greater the chemical potential (more negative). By changing the solvent' solubility parameter the chemical potential, and thus the viscosity of the system, can be altered. [44] Examples of cellulosic thickeners include methyl cellulose, hydroxyl ethyl cellulose (HEC), carboxy methyl cellulose, hydroxy propyl cellulose, and hydrophobically modified HEC. [54] Associative thickeners have a hydrophilic polymer backbone and hydrophobic groups that are pendent or terminal positions. [54] Examples of associative thickeners are: hydrophobically modified hydroxy-ethyl cellulose (HMHEC), hydrophobically modified alkali swellable emulsion (HASE), hydrophobically modified ethoxylated polyurethane (HEUR), hydrophobically modified poly(acetal-or ketal-polyether) (HMPE), hydrophobically modified aminoplast (HEAT), and hydrophobically modified polyurea (HMPU), etc. [17,51] Fang [55] combined commercial aqueous polyurethane with various additives to prepare appropriate emulsions for water-based ink and replace the traditional solventbased binders. The authors concluded that 2 wt% of commercial thickener in the waterborne emulsion promoted the network structure formation. The change in microstructure was responsible for the increase in viscosity, and in the corresponding shear stress, which was found to be very useful in the grinding process.
To modify the rheology of waterborne polyurethane adhesives, different percentages (0.5-3 wt%) of commercially available urethane-based thickener were incorporated into commercial waterborne polyurethane, and solution viscosity measurements were performed. [13] A Newtonian trend of viscosity as a function of shear rate was displayed for the base polyurethane adhesive. Subsequent addition of a thickener increased the viscosity compared to the pure formulation. When thickener content exceeded 2 wt%, shear thinning behavior as a function of shear rate and thixotropic properties as a function of time were reported ( Figure 8A). For the solutions containing 3 wt% of thickener, two runs were performed, highlighted by arrows in Figure 8(A). One arrow corresponds to decreasing shear rate and the other arrow corresponds to increasing shear rate. The presence of pseudoplasticity and thixotropy in adhesive solutions has been attributed to interactions between thickeners and polyurethanes. The chemical properties of the polyurethane remained the same in the presence of the thickener, as verified by infrared (IR) spectroscopy. Therefore, the interactions between polyurethanes and thickeners were thought to be primarily of physical in nature (ionic adsorption, hydrogen bonding). Morphological changes occurring in polyurethane solutions with different amounts of thickener were shown by confocal laser scanning microscopy ( Figure 8B).
The effectiveness of hydrophobically modified ethoxylated urethane-based thickener (HEUR) was also demonstrated in various water-based polyurethanes with different hard-to-soft segment ratios (NCO/OH = 1.2 and 1.4) and ionic groups. [19] Dimethylolpropionic acid (DMPA) was used at levels of 4 and 6 wt% (based on prepolymer weight) to modify the ionic groups in the polyurethane. A level of 5 wt% of thickener was used. This thickener's percentage was found to increase WPU viscosity by increasing the ionic group content and decreasing the hard-to-soft segments ratio. This result was due to physical interactions between the polyurethane particles and the thickener. At a given DMPA content, as the NCO/OH ratio decreased in the polyurethane (the hard segment content decreased, or the soft segment content increased) the physical interactions between the soft segments were facilitated and smaller particles during the synthesis of the dispersion were obtained. Second, for a given NCO/OH ratio, by increasing the DMPA, the proportion of ionic groups and the hydrophilicity of the dispersion increased. In this situation, the average particle size of the polyurethane dispersion was thereby reduced. Third, hydrophilic polyoxyethylene (POE) units within the chemical structure of the positively charged thickener interacted electrostatically with the ionic groups on the surface of the polyurethane particles (negatively charged), increasing the pH value of the polyurethane dispersion. Finally, a stronger polyurethane/thickener interaction accelerated the crystallization rate, led to improved mechanical resistance and adhesive strength in leather/thickened polyurethane adhesive/SBR rubber.
During film-forming process, water evaporation can affect the degree of crosslinking, and the adhesion properties of WPU-based products. Thermal annealing is a F I G U R E 8 (A) Viscosity as function of shear rate and (B) morphological aspects of waterborne polyurethane (DPU) containing up to 3 wt% of thickener (DPU/3 e). Reproduced from. [13] Copyright (2008) with the permission of Elsevier Ltd. method to completely remove water and properly cure waterborne polyurethanes. The effects of annealing of thickened waterborne polyurethane adhesives on their rheological, thermal and adhesive properties were discussed in. [56] The results suggested that annealing did not significantly affect the rheological and thermal properties of the thickened polyurethane. After annealing in an oven at 80 C for 24 h to completely remove water from the films, the glass transition temperatures of all polyurethanes were slightly lower than those of the unannealed products, but kinetics of crystallization and decomposition remained the same. Mechanical properties in terms of adhesive strength proved to be optimal for nonannealed polyurethanes.
A series of hyperbranched polyurethane-based thickeners (HWPU) including hydrophobic end groups were prepared through hyperbranched core and polyurethane prepolymer. [57] The hyperbranched core consisted of a polyol incorporating pentaerythritol, hyperbranched polyester with six hydroxyl end groups (HPE-6) and hyperbranched polyester with 12 hydroxyl end groups (HPE-12). The thickening effect of HWPU on waterbased polyurethane dispersion was shown in terms of viscosity changes by increasing the HWPU content. This result was correlated to the length of hydrophilic chain and the relative content of hydrophobic end groups. When HWPU was dispersed in the aqueous medium, the hydrophilic groups could interact with water and form hydrogen bonding, whereas the hydrophobic groups could be combined with emulsion particles ( Figure 9). These two types of interactions led to the formation of a spatial network structure that hindered the flow and increased the viscosity of polyurethane emulsion.
Further investigations enabled the detection of different structures formed during the thickening process of polyurethane dispersions from hyperbranched polyurethane-based thickeners (HWPU): Duckweed state, 3D network structure or Bead-like texture. Such structures were dependent on concentration and flow activation. The duckweed state occurred when the hydrophobic groups of HWPU adsorbed onto the particle surface and the hydrophilic chains interacted with water. As a result, the viscosity increased, and the particles approached to be uniform in size. Once the hydrophobic end-groups with the latex particles achieved a critical combination, the particles were well-dispersed, the size distribution was narrow, and a three-dimensional structure between HWPU, polymer particles, and water was realized. Under these conditions, the flow viscosity and activation energy reached maximum values. The hydrophobic terminal groups of HWPU in free movement could collide and rearrange in mono-or multi-molecular micelles. In this situation bead-like texture was created. Micelles were network-independent and exerted a lubricating effect that reduced the flow viscosity and activation. [58] In water-based coatings, organic solvents are often used in order to improve the film-forming ability. In any case, they may affect hydrophobic interactions by reducing the associative thickening effect of waterborne polyurethanes. Silicon-based species were introduced to limit the influence of organic solvents on the thickening process. Water-based silicon-modified associative polyurethane thickeners were prepared from PDMS with different molecular weight (300, 600, 900). [59] Branched (B-HEUR) and linear (L-HEUR) hydrophobically modified ethoxylated polyurethanes (HEUR) were used for comparison. The viscosity of emulsions containing silicon-based molecules was found to be higher than that of B-HEUR (1.26 times) and L-HEUR (2.96 times) under low shear conditions. PDSMs with the lowest silicon content were more effective than others with higher molecular weights. If on the one hand the presence of silicon reduced the impact of organic solvent on the hydrophobic ends, on the other hand, the silicon-based species also contained the hydrophobic ends. The higher the silicon content, the greater the impact on organic solvents, but the greater the negative impact of siliconcontaining hydrophobic molecules. Table 2 shows examples of WPU formulations containing thickeners for specific applications.

| RHEOLOGICAL PROPERTIES OF WPU INCORPORATING NANOPARTICLES
Over the past two decades, environmental concerns have prompted industry to develop environmentally friendly alternatives, such as low-content or solvent free coatings at high solid content, highly branched, waterborne or UV curable coatings. Nevertheless, the presence of hydrophilic groups in aqueous solutions resulted in poor film forming ability, poor water and corrosion resistance, and poor mechanical durability. [29,67] The use of nanomaterials as fillers in waterborne coatings has become a key point for the development of highperformance waterborne coating. [29] The presence of nanomaterials in waterborne coatings resulted in the formation of specific microstructures that prevented the penetration of water, oxygen, and corrosives, and improved the mechanical properties and thermal stability. [67] Different types of nanostructures can be distinguished according to their dimensions. Zero-dimensional (0D) nanoparticles have all the dimensions of the nanoscale. They are uniform particle arrays (quantum dots), heterogeneous particle arrays, core-shell quantum dots, onions, hollow spheres, and nanolenses. [68] During the shrinkage of the curing process, these species can occupy voids and prevent the formation of microcracks in the coatings. [67] Onedimensional (1D) particles have one dimension outside the nanoscale. They can be nanowires, nanorods, nanotubes, nanobelts, nanoribbons, and hierarchical nanostructures. [68] When added to waterborne coatings, 1D nanomaterials can be laterally arranged by forming dense layers that prevent the penetration of corrosive species. [67] Similarly, twodimensional (2D) nanomaterials have two off-nanoscale dimensions. They are represented by nanoprisms, nanoplates, nanosheets, nanowalls, and nanodiscs. [68] In the waterborne coatings, such particles can occupy the spaces of microcracks formed during the curing process, making the coating more compact. [67] The dispersion of nanofillers in the polymer resins is an essential aspect to control the quality of nanomaterial-based WPU coatings, and achieve superior properties in downstream products. Dispersion and interfacial compatibility between the two phases (filler and matrix) were typically improved by covalent or non-covalent modification of nanomaterial surface. Such a general approach made it possible to tune the performance of organic coatings and add specific functionality. [26] The viscoelastic properties of materials are strongly correlated with their microstructure and can be used as a powerful tool to monitor polymer phase transitions. For example, in thermosetting resins during the curing process, or in polymers at critical concentrations during gelation (from liquid-like to solid-like structures). [69] In suspensions, particles tend to settle over time under the influence of gravity, whereas particles in dispersions are relatively stable and do not tent to agglomerate. The stability of colloidal systems depends on the balance of forces such as Brownian motion, gravity, electricity, heat, and magnetic fields. The rheological behavior of nanoparticles in an aqueous media is usually associated with complex fluid behavior and is explained by the typical pseudoplastic tendencies. Depending on the particle content, flow instabilities with particle agglomeration and flocculation can occur in the system under uncontrolled surface interactions. Aggregation reduces the effective surface area and should be avoided to preserve the beneficial properties of nanoparticles. [70] Clays are suitable for incorporation into aqueous polyurethane-based nanocomposites because of their excellent thermal stability, mechanical strength, and excellent barrier properties. [71] However, these particles are unstable in various solvents and readily aggregate in nanocomposite films. For best performance in final system, clay agglomeration should be avoided during both in solvent and in composite stages. Various WPU formulations were prepared according to the carboxyl acid salt content (i.e. DMPA, dimethylol propionic acid) and clay content, and the dispersion stability was verified. [72] It was found that increasing carboxylic acid salt content to 23.58 mol% made the dispersion more stable when the  [20] clay had OH groups. Dispersions were considered unstable if the segmentation (precipitation) was observed immediately after preparation, or within 2 days after preparation. The modified organoclays had extensive anionic layers balanced by quaternary ammonium cations. This feature favored the interaction between the organoclay and polyurethane particles and modified the electrostatic forces on the particles. This interaction directly affected the stability and shelf life of WPU/clay nanocomposite dispersions. Viscosity trends of WPU/nanoclay dispersions as a function of different shear rate (rpm) values were investigated by changing particle content and carboxylic acid salt content. At a DMPA level of 12.54 mol%, increasing the clay loading to 1 wt% increased the viscosity. This result was attributed to the formation of an internal network structure due to the interaction between the clay particles and the polyurethane particles (carboxylic acid salt groups) as the clay content increased. As the shear rate increased, the breakdown of the internal structures was considered responsible of the pseudoplastic behavior. At fixed percentage of clay in WPU (1 wt%), the viscosity was increased by changing the DMPA content up to 23.58 mol%. The presence of high amounts of DMPA in the dispersion increased the hydrogen bonds and coulombic forces, increasing the final viscosity. Hydrogen bonding between hydrogen atoms of N-H and -C=O groups was detected using FTIR measurements. [71] A deconvolution study of the carboxylic group (-C=O) showed two overlapping bands corresponding to the wavelength number ranges between 1700-1720 cm À1 and 1640-1650 cm À1 . The first was usually due to the presence of free urethanes and the second was related to bonded ureas. The peak area of the hydrogen bonded C=O region started at a value of 314 for the basic WPU and reached 1428 for WPU with 3 wt% of clay nanoparticles. In contrast, the peak area associated with the free C=O peak was equal to 1289 for the basic WPU and decreased to 298 for dispersion containing 3 wt% of clay nanoparticles.
The viscosity of WPU colloidal silica hybrid solutions with different silica contents (up to 50 wt%) is shown as a function of the mixing time in Figure 10.
The rheological parameters of the WPU-silica hybrid solution increased with increasing mixing time and colloidal silica. At low silica percentages, the viscosity quickly reached a certain value meaning that interaction between silica and polyurethane occurred easily. In contrast, at higher particle loadings, the viscosity increased with increasing silica content. This implied a stronger interaction between the silanol groups (of the silica particles) and the carboxyl groups (C = 0 of the polymer), leading to the formation of an internal network structure. A pure WPU solution exhibited Newtonian behavior. Addition of colloidal silica to WPU changed the rheological behavior from Newtonian to pseudoplastic.
In the work by Chen et al., [74] rheological measurements were used to aid printability and adjust the F I G U R E 1 0 Viscosity of silica/ WPU systems up to 50 wt% of silica nanoparticles as a function of time. Reproduced from. [73] Copyright (2006), with permission from Elsevier Inc. amount of neutralizing agent (TEA) during the in-situ composite fabrication. A water-based biodegradable polyurethane was synthesized with poly(ε-polycaprolactone) diol (soft segment), isophorone diisocyanate (hard segment) and two chain extenders. Cellulose nanofibers (CNF) were added to the dispersions to modify the rheology, biocompatibility, and degradation. However, the PU/CNF suspension was not viscous enough for 3D printing. A small amount of TEA was added to promote the interaction between CNF and PU and increase the viscosity of the suspension. Figure 11 shows the rheological properties of PU/CNC nanocomposite. The viscosity of the base mixture without extra addition of TEA (PU3I) was greatly increased by adding 0.7 (0.7 T) and 0.8 (0.8 T) equivalent of extra TEA to the in-situ synthesis. However, when additional TEA (0.8 T*) was introduced 15 min after mixing the CNF, the viscosity remained similar to the base mixture (PU3I). Both PU nanoparticles and CNFs had negative COOÀ groups that were mutually excluded in hindering interactions. Extra TEA added to PU/CNF formulation could react with COO-groups and reduce the repulsive forces.
Developed nanocomposite inks have been directly proposed in 3D printing for a variety of applications including medical devices and scaffolds for tissue engineering.
Nanomaterials are aften incorporated into multicomponent hybrid materials. Various combinations of hybrid nanostructures such as graphene oxide sheet and silica nanoparticles (2D/0D), [75] nano cellulose crystalline and silver nanoparticles (1D/0D), [76] zinc oxide nanorods and graphene oxide (GO) nanosheets (1D/2D), [77] have also been explored in coatings. These systems combine the unique properties of each component to produce an effect greater than the sum of the individual components ("synergism"). The synergistic efficacy was attributed to interactions between dispersed particles that led to interconnected structures.
For example, nanocellulose crystals and graphene oxide have been used simultaneously in waterborne polyurethane to improve the UV-resistance, electrical conductivity, and mechanical strength and water resistance of the final material. [78] Nanocellulose (1D) crystals have been commonly used as environmentally friendly fillers in composites due to their high aspect ratio, excellent UV scattering/adsorption, and excellent mechanical properties. Graphene oxide (2D) is another most-commonly used filler in polymer composites due to its high specific surface area, excellent UV absorption, strong mechanical strength, and electrical conductivity. A composite made of functionalized graphene oxide and functionalized nanocellulose (FGO@FNC) was prepared by a sol-gel method and incorporated into waterborne polyurethane dispersion. Infrared spectroscopy allowed to confirm the covalent grafting of the urethane chains onto the surface of the combined fillers, and the siloxane bonds formed between the nanocellulose crystals and graphene oxide. The highest efficiency was achieved when the concentration of FGO@FNC in the WPU matrix was equal to 3 wt %. This formulation had a tensile strength of 28.7 MPa, a water contact angle of 96.5 , and an electrical conductivity of 0.252 S/m. The physicochemical parameters reached almost 78% or more after 5 days of UV irradiation.
Some examples of common nanofillers that have been introduced into waterborne polyurethane dispersions to improve specific properties and with a view of specific applications are shown in Table 3.
F I G U R E 1 1 Images of different PU/CNC formulations (on the left) and corresponding viscosity trend as a function of the shear rate. PU3I identifies the dispersion containing no extra TEA during in-situ process; 0.7 and 0.8 T are the dispersions made by adding 0.7 and 0.8 equivalent of extra TEA to the in-situ synthesis; and 0.8 T* is a formulation equal to 0.8 T but the TEA addition occurred 15 min later the CNF mixing. Reproduced from. [74] Copyright (2019), with permission from Elsevier Ldt. Wen, 2019 [26] Titanium dioxide 40 wt% 1D No Improvement of mechanical properties and self-healing in paints and coatings.

| CONCLUSIONS
In this review, the rheological properties of waterborne polyurethane, which also contained two specific additives, were described, mainly in terms of the change in viscosity as a function of various parameters. Water-based polyurethanes are a colloidal system of PU particles in an aqueous medium, stabilized mainly by the presence of internal emulsifiers in the chemical structures. As usually with suspensions, the rheological properties were influenced by various parameters such as the type of components, particle size and content, and possible interactions between species.
A sharp increase in complex viscosity with increasing PU content was observed. This was attributed to the effect of two different forces acting between the particles. Repulsive forces between the similar charges surrounding the particles were assumed to dominate at low PU content, while hydrodynamic interactions dominated at high PU content. A strong effect of temperature on the viscosity of WPU was also found, especially for large particles. This result was attributed to the occurrence of gelation phenomena. Increasing the proportion of chain extenders in the backbones of PU led to an increase in the molecular weight of PU, but had no effects on particle size and distribution. This resulted in a negligible effect of chain extension on viscosity dispersion. On the other hand, the presence of components with ionic groups (such as, castor oil) in the PU chemical structure resulted in particles, with smaller size and wider distribution, leading to increased bonds in the polymer system, and thus in an increased viscosity.
A method of stabilizing PU particles in aqueous media or altering the Newtonian in pseudoplastic behavior was the addition of water-soluble polymer, with low or medium molecular weight containing hydrophilic and hydrophobic groups (i.e., thickeners). The mechanism of action of such species in WPU was realized by interactions between the hydrophilic groups and water, and between hydrophobic groups and PU particles. This resulted in a three-dimensional network structure that resisted the flow of dispersion, impeding movements and increasing overall viscosity. It turned out that the thickening effect depended on the chemical structure of the polyurethane (i.e., hardto-soft segment ratios and ionic groups). A water-based hyperbranched polyurethane was also used as a thickener for the WPU itself. In these cases, four different network structures (Duckweed state, 3D network structure, or Bead-like texture with monomolecular or multi-molecular micelles) could be identified, depending on WPU content.
On the other hand, the addition of nanoparticles to WPU has become essential to improve the final performance of WPU-based products and achieve the same standards achieved with their solvent-based counterparts. Zero-, one-, or two-dimensional nanoparticles, often chemically or physically modified, such as calcium carbonate, silver, zinc oxide, halloysite, clay, magnetite, or a combination of nanostructures such as graphene oxide and silica (2D/0D), cellulose and silver (1D/0D), and so on, have been found to be effective in improving thermal and mechanical properties, UV resistance, electrical resistance, and wear properties of WPU-based products. These advantages have been attributed to the ability of the nanoparticles to interact with the polymer in the dispersion, to create specific microstructures, also resulting in changes in the rheology of the dispersion.    [78] Aluminum oxide and graphene 5 wt% 0D/2D No Corrosion and wear resistance Chang, 2022 [25] FUNDING INFORMATION This research received no external funding.

DATA AVAILABILITY STATEMENT
The data presented in this study are available on request from the corresponding author.