Chapter 1: Development, Characterization, and Application of Novel High Temperature Thermoplastic and Thermosetting Dispersions

Dow's HIPE (High Internal Phase Emulsion) process, also known as BLUEWAVE™ technology, has been practiced at Dow for over 20 years. It is a process and formulation approach for creating aqueous emulsions or dispersions of polymers, which cannot otherwise be made via polymerization of monomers in aqueous systems (i.e. by emulsion or suspension polymerization). Examples of such polymers include polyurethanes, epoxies, polyolefins, silicones, polyesters, and alkyds. Advantages of this process include:

• Controlled internal phase concentration (very dilute to >95% internal phase)

• Controlled particle or droplet size (200 nm–50 μm, although 500 nm–5 μm is most typical)

• Minimal surfactant requirements (typically 2–6% based on internal phase)

• Ability to process high viscosity materials (>100 000 cP)

• Solvent free

Compared to emulsion polymerization, BLUEWAVE™ technology generally requires slightly higher surfactant amounts and generates slightly larger particle size because it is a direct emulsification technique, as opposed to a polymer synthesis technique. The use of appropriate process equipment and surface active ingredients also allows for the creation of emulsions of polymer melts at temperatures above the boiling point of water. For example, an emulsion of high density polyethylene (HDPE, melting temperature=135 °C) in water can be generated at a process temperature of 150 °C and then cooled down to room temperature to give a stable HDPE in water dispersion.

Once the polymer is present in the form of a water-borne dispersion, it can be processed using standard emulsion application tools, such as printing or coating processes, dipping, spraying, and froth foaming. This provides a very different set of physical properties, such as the high crystallinity and melting temperature of a polyolefin, with the application convenience of an emulsion polymer. Compared with an extruder applied coating of the same polymer chemistry, water-borne submicron particles allow for significant down gauging of coating thickness.

There are a number of commercial products made with BLUEWAVE™ technology, including:

• CANVERA™ polyolefin dispersions for metal beverage packaging

• ECOSMOOTH™ polyolefin dispersions for skin and hair care

• HYPOD™ polyolefin dispersions for paper coating

• ProSperse™ epoxy dispersions

• ACCENT™ polyolefin co-polymer dispersions for oil and gas

Small particle size emulsions can be generated by a range of mechanical equipment or non-mechanical processes. Examples of mechanical emulsification techniques include rotor–stator mixing systems,^1,2  ultrasound,³  high pressure impingement systems,^4–6  and membrane emulsification.^7,8  The non-mechanical processes for emulsion formation include phase inversion,⁹  either by composition or temperature, as well as precipitation and solvent exchange.¹⁰  However, the ability of these standard processes to create very small (sub-micron) particle dispersed systems of very high viscosity materials, such as polyolefin elastomers, is limited. BLUEWAVE™ technology combines mechanical (process) and non-mechanical (formulation) approaches to generate challenging dispersions.

A general schematic of the continuous process used in BLUEWAVE™ technology is shown in Figure 1.1. The particular equipment that is used for the primary mixer and dilution mixer unit operations depends upon the material properties of the polymer being fed into the process. For example, a thermoplastic polyolefin may have both mixing operations performed in series in a single twin screw extruder. For an amorphous polymer feed, such as an alkyd or high molecular weight polydimethylsiloxane, it may be more effective to use separate rotor stator mixers as the primary and secondary mixers.

In the BLUEWAVE™ technology, a polymer melt phase, a surfactant, and a small amount of initial water are combined in a primary mixing device at a temperature above the glass transition temperature (T_g) and melting temperature (T_m) of the polymer to create a polymer melt in water High Internal Phase Emulsion (HIPE). The HIPE can be thought of as a liquid/liquid foam, and is, by definition, an emulsion where the internal phase is greater than 74.5% of the total volume, which is the limit for close packed mono-dispersed spheres. Figure 1.2 shows a scanning electron micrograph of a polyolefin high internal phase ratio emulsion that has been allowed to cool below the polymer T_m without dilution. The polyhedral nature of the solid internal phase particles, as well as their high volume fraction, is clear.

With the BLUEWAVE™ technology, the particle size of the internal phase droplet is set with the creation of this HIPE, which is then combined with additional dilution water to yield the final dispersion product at the desired internal phase volume concentration. For some applications, such as cosmetic emollient concentrates,¹¹  the final product may itself be a HIPE, as it is desirable that it contains as little water as possible. In other applications where a low viscosity dispersion is desired for spray coating, the final solids level may be in the range of 50% by volume. Internal phase polymers that solidify above room temperature must be diluted down below ∼60% by volume before the polymers cool and solidify to avoid mechanical interlocking of the HIPE, which makes further dilution of the dispersion impossible.

The rest of this chapter is separated into two sections. In Section 1.2, we will discuss the advantages, in the context of droplet breakup theory, of passing through this HIPE phase to produce small, mono-disperse emulsion particles. The mechanism of droplet breakup in the concentrated (concentrated internal phase) system differs from the droplet breakup in the conventional (dilute internal phase) system. These differences result in the production of the small, monodisperse droplets generated by our process. We will also discuss the complications of finding a stabilizing agent that is effective at helping to form this polymer melt/water interface at high temperatures. In Section 1.3, we will discuss the applications that are enabled by the BLUEWAVE™ technology.

In the following sections we will discuss classical droplet breakup theory, and how it has been extended to more concentrated systems in order to gain insight into the mechanisms that allow for the formation of small, monodisperse particles with our BLUEWAVE™ mechanical dispersion process technology.

The particle or droplet size of an aqueous emulsion depends upon how the internal oil phase breaks up during mixing. Promoting drop deformation and breakup is the shear stress, τ, caused by the flow field within the mixer, which is generally defined as τ=η_c where η_c is the continuous phase viscosity and  is the shear rate. Counteracting that force is the interfacial stress σ/R, where σ is the oil–water interfacial tension and R is the drop radius. The ratio of these values is the dimensionless capillary number Ca:

Taylor¹²  was the first to provide a theoretical analysis of droplet deformation and breakup. Within the constraints of his system (simple steady-state shear flow, no droplet–droplet interactions, small drop deformation, and zero inertial effects), Taylor showed that the drop behavior depends on only the capillary number and the viscosity ratio, λ, defined as follows, where η_i is the internal phase viscosity:

For a given flow field and viscosity ratio, there exists a critical capillary number, Ca_crit, above which a drop of radius R is unstable and breaks up into smaller drops. The maximum droplet radius, R_max, which can exist at a given critical capillary number is expressed in eqn (1.3). Drops with a radius below R_max are stable; drops with a radius above R_max will break up into smaller drops.

Numerous theoretical and experimental studies on droplet breakup have been reviewed by Rallison,¹³  Bentley and Leal,¹⁴  Stone,¹⁵  Leng and Calabrese,²  Cristini and Renardy,¹⁶  and others. One of the most widely cited works is the extensive experimental study conducted by Grace.¹⁷  Grace determined the critical capillary numbers, Ca_crit, below which a drop in dilute conditions would remain stable and not break for both simple shear and elongational flows under steady-state conditions. His work showed that for steady-state conditions, the smallest droplet size is always attainable at a viscosity ratio of 1 and that above a viscosity ratio of about 4, droplet breakup in simple shear is impossible.

In BLUEWAVE™ technology, small droplet sizes are achievable across a very broad viscosity ratio range. There are several reasons why this technology is effective even when the viscosity ratio is far from unity. One is that a HIPE, because of the close-packed nature of the droplets, is more effective at transferring shear energy to the small droplet than would be the case for a single droplet in a dilute system. The use of a HIPE to control particle size in a batch process has been known for some time.¹⁸  Aronson¹⁹  created emulsions of controlled droplet size by preparing surfactant solutions of 20–60 wt% and then slowly adding oil while mixing to create a concentrate. He showed an inverse relationship between the viscosity of the surfactant solution and the particle size of the droplets and proposed that instead of the true continuous viscosity, an effective continuous viscosity should be used that takes into effect the increased viscosity due to the surfactant and the packing of the system. By using the effective continuous phase viscosity, he argued that his system had a droplet breakup similar to that observed by Taylor.

Jansen, Agterof, and Mellema²⁰  also showed that in concentrated systems under steady-state simple shear flow, the experimental Grace curve will shift downward and to the right. They looked at emulsion concentrations from about 0% up to 70% oil phase. At 70% oil, the critical capillary number was reduced by a factor of about 10 and the optimum viscosity ratio increased by close to two orders of magnitude. The authors found that if they replaced the continuous phase viscosity in Ca_crit and λ with the emulsion viscosity at the critical shear rate (a necessary correction since the emulsion is shear thinning), the data fell very close to that reported by Grace. Finally, Tcholakova et al. experimentally demonstrated that by increasing the drop volume fraction up to HIPE concentrations (Φ>75%) they could dramatically improve the efficiency of emulsification for viscous oils in turbulent flow. They were then able to describe the behavior of the concentrated emulsions using a simple scaling expression.²¹ 

Another reason for the insensitivity to the viscosity ratio is the nature of the shear stress experienced by the droplet. Up to this point, only steady-state shearing conditions or systems with changes in shear rate so small that they can be considered steady state have been reviewed. Under real mixing conditions, shear rates will be far from equilibrium. Torza, Cox and Mason²²  observed that the rate of increasing shear rate (d/dt) influences both when and how the droplet will burst. In particular, they observed that large and viscous drops were more easily pulled into liquid threads than smaller, inviscid drops. This is explained by a relaxation time in which large, viscous drops readjust their shapes very slowly so that high rates of increasing shear stretch the drops directly into threads which have no time to relax but break up into many drops via Rayleigh instabilities. This capillary breakup mode is quite different from the splitting of a drop into two daughter droplets (binary breakup) which is characteristic of most of the steady-state work. A schematic comparing the mechanisms for breakup of different size drops is shown in Figure 1.3.

Elemans, Bos, Janssen, and Meijer²³  observed that under transient simple shear flow conditions where a shear stress is suddenly applied such that the Ca≫Ca_crit, a Newtonian droplet within another Newtonian fluid will extend into a thread. After a certain time, the thread will exhibit sinusoidal distortions and break up into several droplets. Also, Janssen and Meijer^24,25  examined the transient breakup mechanisms for Newtonian fluids in both simple shear and elongational flow. The extending thread formed under flow undergoes capillary instabilities at the interface. In quiescent conditions, one wavelength will dominate at the thread diameter and lead to breakup. With an extending thread, the dominant wavelength is continuously changing as the thread thins and thus the breakup of the thread is delayed compared to the fixed width thread. Eventually, the amplitude of a wavelength will exceed the diameter of the continuously decreasing thread and breakup will occur. The droplets formed will be independent of the original droplet radius. The conclusion of their work is that the optimum viscosity ratio for transient breakup is much larger than unity. This is partly because capillary disturbances develop more slowly on a highly viscous thread and thus it has more time to thin before breakup.

Janssen and Meijer's theoretical and experimental work was only carried out with Newtonian fluids. The presence of a viscoelastic continuous phase may help further improve the droplet breakup. Zhao and Goveas²⁶  experimentally observed that a viscoelastic continuous phase greatly narrowed the distribution of thread widths at breakup, as well as the resulting droplet size distribution compared to a Newtonian continuous phase.

Similar to Aronson, Mason and Bibette^27,28  also created stable, mono-dispersed emulsions by shearing a pre-emulsion within a mixing apparatus containing a well-defined, narrow shear gap and by creating viscoelasticity within the emulsion either through increasing the emulsion volume fraction to create a HIPE or varying the surfactant concentration to create a viscoelastic continuous phase. A similar thread breakup mechanism has been proposed to explain their work²⁹  and their observations further suggest that partial elasticity in the emulsion is necessary to achieve the monodispersity.³⁰ 

The surfactant added to reduce the interfacial tension and stabilize the final emulsion against agglomeration will also affect the interfacial rheology. It has been recognized as early as Rumscheidt and Mason³¹  that droplet breakup changed when emulsifiers were present, most likely because of the formation of viscoelastic interfaces. Flumerfelt^32,33  modeled droplet deformation in steady-state simple shear and elongational flows that accounted for interfacial properties such as surface shear viscosity and surface dilational viscosity. When interfacial properties are significant, they can dominate over the viscosity ratio. Davies and co-workers³⁴  compared emulsion drop size to rates of turbulent energy dissipation for a series of high shear emulsifiers and noted the critical role of the non-continuous phase viscosity in final droplet size as well as the role in adsorbed surfactant in droplet breakup. The influence of surfactant on interfacial rheology may also amplify the transient breakup effects discussed above by increasing the effective continuous viscosity and dampening out the disturbance formed at the interface, allowing the thread to extend and thin even more before breakup. The viscoelastic nature of the interfacial film may also help to narrow the size distribution similar to that observed for viscoelastic fluids by Zhao and Goveas. Mabille and co-workers,³⁰  in extending the work of Mason and Bibette, argue that the elasticity of the emulsion is absolutely necessary to get a monodisperse emulsion and that this elasticity can be controlled either by controlling the rheology of the continuous phase using surfactant and polymers or by concentrating the emulsion into the HIPE regime.

A final factor that can affect drop breakup is the nature of the boundaries within the shear field. Migler and co-workers^35–37  have shown that drops in confined shear flow will form strings or threads more readily than in the bulk. In tight gaps, the walls will act to stabilize the formation of the threads and prevent them from rupturing during flow. The threads will remain stable until they thin down to a point where they no longer experience the stabilizing effect of the walls, at which point they break up into droplets by Rayleigh instabilities. Thus, smaller drops can be generated within small gaps as long as the drop size is not significantly smaller than half of the gap width. More recently, Squires and co-workers³⁸  characterized droplet breakup of closely spaced fluid threads using a microfluidic multi-inlet co-flow system. Their experiments indicated that the Rayleigh–Plateau instability of adjacent, closely spaced threads (HIPE-like conditions) were collective and the cooperative breakup led to reduced polydispersity in the emulsion.

A simple way of expressing the effects of different variables on the droplet size is shown in eqn (1.4), where the droplet radius (R) is directly proportional to the oil–water interfacial tension (σ), inversely proportional to the shear rate (), and a function of the viscosity ratio, temperature (T) and time (t).

It is quickly apparent that reducing interfacial tension is an easy way to reduce droplet size with the same energy input. The interfacial tension is dependent not only on the identity and concentration of the surfactant, but on the concentration of all of the phases: surfactant, oil, and water. In a batch process, the local concentrations of the phases will vary widely as the surfactant, water, and oil are mixed together. In BLUEWAVE™ technology, the ratio of oil phase, water phase, and surfactant can be precisely controlled at the point where they first meet and mix together, allowing for precise optimization of the interfacial tension for the creation of emulsion droplets of controlled size. By concentrating the surfactant in the water phase, the effects of dynamic interfacial tension, or the increase of the interfacial tension due to the time required for surfactant to diffuse to the newly created interface, can also be reduced. Of course, the viscosity of the continuous phase can also be influenced by the presence of the surfactant, especially in the concentrated (low amount of dispersed phase) system, where a HIPE is formed.

A unique obstacle with many polymer systems is that they require high temperatures, typically over 100 °C, in order to flow. Besides the complications of maintaining pressure in the equipment above the boiling point of water, there is the additional problem of finding a surfactant that will be effective above 100 °C. The use of a surfactant is required because even at higher temperatures, the interfacial tension of a typical polymer in water system without surfactant will still be too high for good emulsification. For example, the interfacial tension of a styrene-isoprene (SI) oligomer (a free flowing room temperature liquid) with water in the absence of surfactant is shown in Figure 1.4. These measurements are made with a high temperature pendant drop tensiometer (Tracker, from IT Concept-Teclis), similar to the work done by Chaverot and co-workers.³⁹ 

Also, most surfactants lose their effectiveness at high temperatures. For example, non-ionic surfactants become less soluble at high temperatures and become ineffective above their cloud point, at which point the solubility drops enough that the surfactant precipitates into separate phase.⁴⁰  Anionic surfactants also change in both solubility and effectiveness at elevated temperatures, sometimes in counterintuitive fashion. As temperature increases, the hydrophobicity of the surfactant hydrocarbon tail can decrease rendering the molecule more hydrophilic, and therefore less interfacially active and less effective at reducing interfacial tension. The anionic surfactant head group will also increase in solubility with higher temperature, further reducing the interfacial activity of shorter hydrocarbon chain surfactants.

Figure 1.5 shows the interfacial tension of the same styrene-isoprene (SI) oligomer as in Figure 1.4 with water in the presence of a series of carboxylic acid potassium soaps, as a function of carbon chain length. As temperature increases, the size of the surfactant hydrocarbon tail required for a minimum interfacial tension also increases. The much higher molecular weight C50 fatty acid has a much lower interfacial activity at these temperatures.

By measuring interfacial tension of materials at elevated temperatures, we have identified materials that are not particularly interfacially active, or water soluble, at room temperature, but are effective surfactants at elevated temperature. Use of these materials by themselves can cause issues in room temperature storage stability, since they lose their interfacial activity as the dispersions cool. This is usually remedied by blending them with surfactants that are more effective at room temperature.

Some conventional ionic and non-ionic surfactants are effective at elevated temperatures. For example, some EO-PO type surfactants have cloud points over 100 °C, as do some alkyl polyglucosides where the –OH functionality loses its hydrophilic nature slowly with increasing temperature because of strong hydrogen bonding interactions.⁴¹ 

In conclusion, the BLUEWAVE™ dispersion technology can be used to produce sub-micron sized, low polydispersity dispersions for many challenging high viscosity resin systems. We do this by taking advantage of high concentration emulsion conditions to better enable more efficient mechanical mixing of the oil and water phases as well as to promote a thread breakup mechanism within our dispersion devices. Through careful selection of surfactants we are able to maintain low interfacial tension during particle formation and stabilize the resultant dispersion formulations to ensure good shelf stability.

Classical droplet breakup theory can provide many useful insights into high concentration emulsification approaches such as the BLUEWAVE™ mechanical dispersion process. However, a rigorous description is more challenging because of the difficulty in directly characterizing interfacial tension and internal and external phase rheology under normal BLUEWAVE™ technology process conditions, which can be significantly above the boiling point of water. In addition, selecting the most effective high temperature interfacial stabilizer for a particular resin can be challenging for similar reasons. Continued research in the area of high temperature emulsification as well as the development of new in-situ characterization methods would greatly assist the improvement of fundamental understanding of these interesting and useful colloidal systems.

As mentioned previously, a wide range of polymer types can be converted into a water-borne form factor through the BLUEWAVE™ mechanical dispersion technology. This includes materials such as high molecular weight and/or high viscosity polyurethanes, epoxies, polyolefins, silicones, polyesters, and alkyds, which cannot be synthesized by emulsion polymerization. Once these chemistries are incorporated into a water-borne dispersion, they can be processed using the standard emulsion polymer application tools such as rotogravure coating, dipping, spray application, and even frothed foam. This range of chemistries and application techniques allows for a wide variety of uses including coatings on substrates such as metal, paper, or plastic, as well as additives for oil and gas applications, or use as personal care ingredients in products such as skin creams or lotions, shampoos, and body wash.

Other form factors for the polymer chemistries can be generated with water-borne dispersions as the starting point. With proper additives, many of the higher T_g/T_m materials such as polyolefins and polyesters can be spray dried into a powder, such as that shown in Figure 1.6. This powder may or may not be water re-dispersible, depending upon the application. For example, a spray dried polyolefin elastomer powder could be used in rotomolding applications, where it would have benefits in both cost to manufacture and flowability, compared to a powder of the same elastomer generated by cryo-grinding.⁴²  Powder coating with these dispersion based powders is also possible, as well as the generation of interesting combinations of chemistries not possible with traditional techniques.⁴³ 

An aqueous dispersion can also be used in place of a viscous polymer melt/resin in the creation of a composite part.⁴⁴  Typically, a viscous resin is forced into a dense fiber weave under pressure and it is challenging to fully coat all the fiber with the resin. When the resin phase is emulsified into an aqueous dispersion the viscosity of the resin is effectively decoupled from the viscosity of the material being infused into the fiber. This allows the fiber to be completely wetted without the use of high pressure infusion techniques and can result in the creation of composite parts with the same strength as the conventionally manufactured piece utilizing less resin, reducing both weight and cost.

In the next sections, we will discuss additional application areas for some specific polyolefin dispersions in more detail. However, first we will describe why polyolefins are a particularly useful polymer type to use from a water-borne form factor. There are a wide range of polyolefin chemistries available, with widely differing physical properties such as flexibility and melting temperature. In general, as the amount of crystallinity in a polyolefin increases, both its melting temperature and its modulus increase, but it becomes more brittle. For example in Figure 1.7 the low crystallinity elastomer would have a low melting temperature (perhaps 65 °C) and be very flexible. However, it would be a poor barrier to the diffusion of water or grease because of its low level of crystallinity. The highly crystalline HDPE would provide good barrier properties, but would require a high melting temperature to generate a cohesive film (T>135 °C), and would be inflexible and more prone to cracking upon large deformations.

In all cases, a temperature above the melting temperature of the polyolefin base resin is required to generate a coalesced film from the dispersion particles. This is in contrast to materials such as the emulsion polymers used for architectural paints, which are amorphous and have a T_g below room temperature, and are therefore able to coalesce into cohesive films without additional heat. Because of this required “cure temperature” for the polyolefin dispersions to form a continuous film, they are best suited for factory applied or OEM (original equipment manufacturer) applications, as opposed to “field applied” applications; however, the polyolefin dispersions can be used as additives in another film forming polymer.

After it is generated with the BLUEWAVE™ technology, the polyolefin dispersion has the general morphology of a core of non-functional “base resin,” such as the highly crystalline HDPE discussed above, and a shell of the dispersant, which is used to lower the interfacial tension during the manufacturing process and provide colloidal stability at room temperature, as seen in Figure 1.8.

Figure 1.9 shows the formation of a coalesced film from a collection of polyolefin dispersion particles. As the temperature increases, the morphology of the film changes from being a collection of individual particles continuous in the stabilizing agent (upper left), to become a phase inverted film continuous in the base resin. At this point, the individual particles are no longer observed in the coating morphology (lower right).

When developing a dispersion for a specific application, we need to select the components of the dispersion to take into account both the needs of the BLUEWAVE™ process technology, as well as the performance requirements of the application. For example, in a paper coating application, it may be important to have a low cure temperature to allow for fast coating line speeds, and a high barrier may not be as important, as the coated article is intended only for a single, short term use (e.g. paper beverage cup). For a metal coating application, it will likely be more important to have the barrier from the higher crystallinity polyolefin, even if it requires a higher curing temperature.

A dispersion for metal coating may also require some functional olefin to help with the adhesion of the coating to the metal substrate. Fortunately, in many cases this functional olefin can double as the dispersant for the BLUEWAVE™ technology. This dispersant may not be melt miscible with the base resin in certain cases, such as a functional polyethylene dispersant and a polypropylene base resin. The representative coating morphologies that result from “compatible” and “incompatible” base resin and stabilizing agent are shown in Figure 1.10.

These large domains of stabilizing agent in the coating can act as defects for coating failure and are undesirable. Even in the system with the “compatible” stabilizing agent, the coating morphology may not be entirely single phase, as can be seen in the cross sectional micrograph of the “compatible” system shown in Figure 1.11. The stabilizing agent is visible as very small domains scattered throughout the coating thickness.

In addition to control of coating morphology through the use of specific base resin and stabilizing agent combinations, it can also be controlled through the blends of different dispersions. This concept is well known in the emulsion polymer field,⁴⁵  but we propose that the BLUEWAVE™ technology provides an advantageous way of creating specifically designed dispersions of other types of polymers for the design of exotic coating morphologies. For example, a water vapor permeable hydrophobic coating can be generated from a bi-modal aqueous polyolefin dispersion.⁴⁶ 

A population of large particle size, high melting point (T_mH) particles of a hydrophobic polymer, such as HDPE, is combined with a population of small particle size, lower melting point (T_mL) polymer particles in an appropriate ratio to create a coating that is porous, but mechanically robust, when cured at a temperature T_mL<T<T_mH. Appropriate selection of composition, formulation, and cure conditions result in a coating that assembles into a water vapor permeable morphology, but is resistant to liquid water (contact angle ∼130°). Key to achieving this useful combination of properties is both the very hydrophobic nature of the large particle population and its presence above the critical pigment volume concentration, such that the cured coating is porous with a rough surface. However, a sufficient population of lower melting point binder particles is still required to provide mechanical integrity. A surface micrograph of such a coating is shown in Figure 1.12, as well as the image of a water droplet beading up with a high contact angle on the same coating.

In addition to the morphology of a stabilizing agent shell around a base resin core, shown in Figure 1.8, it is easy to imagine other possible particle morphologies⁴⁷  that could be prepared by the BLUEWAVE™ mechanical dispersion process technology. Illustrations of some possible morphologies are shown in Figure 1.13, with the morphology of Figure 1.8 represented by the upper left drawing. The illustrations of the lower middle and lower left of Figure 1.13 show a more substantial shell than that from the stabilizing agent alone. This type of shell may be generated by interfacial polymerization, for example.⁴⁸  This type of morphology would have the benefits of the controlled particle size distribution generated by the BLUEWAVE™ technology, combined with the protective barrier of an interfacially polymerized shell, perhaps for the encapsulation of actives in a cosmetic application.

In the two middle illustrations of Figure 1.13, the added element of an individual inorganic particle, on the order of size of the dispersion particle itself, is included within the dispersion particle. A micrograph of a polyester+TiO₂ dispersion system, with the upper middle morphology of Figure 1.13, is shown in Figure 1.14. Because of the small volume % loading of the TiO₂ in this particular example, not every dispersion particle contains a TiO₂ particle.

This “encapsulated pigment” morphology is particularly useful in enhancing the efficiency of a pigment in an optical hiding application.⁴⁹  Figure 1.15 shows opacity data for three different architectural paint formulations, where a TiO₂ pigment is added alone (control), added with an ethylene acrylic (EAA) co-polymer to help act as a pigment dispersant, or added encapsulated in a linear low density polyethylene (LLDPE) dispersion. As the pigment goes from unmodified, to well dispersed by the EAA, to fully encapsulated in the LLDPE, the pigment efficiency (opacity at a given volume % loading) increases until it hits a limiting value of ∼95%. The pigment volume % at this limiting value is lowest for the fully encapsulated pigment, at ∼23 vol%, compared to ∼27 vol% for the EAA dispersed pigment, and ∼32 vol% for the TiO₂ control.

The advantage of the encapsulated pigment comes from keeping each individual pigment particle separate, and therefore able to scatter at the maximum efficiency. The more efficient hiding of the encapsulated pigment greatly reduces the total amount of pigment required in the formulation to reach the desired level of opacity. There may also be the advantage of another refractive index mismatch between the LLDPE encapsulating polymer and the coating matrix.

Polyolefins have excellent mechanical properties, recyclability, and chemical resistance^50,51  at a reasonable cost. However, because of their low surface energy and lack of polar functional groups, polyolefins have poor adhesion to paints requiring modification of both the substrate and paint for practical use in applications where paintability is required. Various technologies have been developed to improve the paintability of polyolefins. These include bulk modification by blending with polar polymers or additives like polyurethane,^52,53  surface pretreatment such as flame, corona, or plasma treatments,^54,55  and application of primers such as solvent-borne or aqueous-based chlorinated polymers as adhesion promoters.^56,57  However, there is still a need for better performing, more environmentally friendly solutions such as a chlorine-free and solvent-free aqueous adhesion promoter system.

In a recent study by Wan and co-workers,⁵⁸  the BLUEWAVE™ mechanical dispersion technology was used to produce water-borne dispersions of functional polyolefins. These materials were proposed as an adhesion promoter to improve the paintability of treated thermal plastic olefins (TPO). The functional polyolefins contain a polar functional group that increases the surface energy relative to that of the TPO substrate, which allows for good adhesion with a conventional polyurethane or acrylic top coat.

Three different polyolefin dispersions were evaluated as adhesion promoters: a dispersion of an unfunctional polyolefin base resin (control), a dispersion of an –OH functional polyolefin base resin, and a blend of the –OH functional dispersion with a dispersion of a maleic anhydride (MAH) functional polyolefin base resin. These dispersions were all low viscosity (<500 cP), and high solids (44%–52%), similar to a conventional water-borne latex. They were coated onto a TPO substrate with a Meyer rod to form a 20 μm wet film thickness and dried at 90 °C for 15 minutes. The primed substrates were then coated with 50 μm wet film thickness layer of either a water-borne acrylic topcoat, or a solvent-borne polyurethane topcoat (PU) and dried at 60 °C for 15 minutes.

Crosshatch adhesion was used to evaluate the adhesion strength of primer+topcoat system to the TPO substrate. In the crosshatch adhesion test, a crosshatch scribe is used to make parallel linear cuts through the coating. A similar set of linear cuts is then made perpendicular to the original cuts in order to obtain a checker board pattern. Adhesive tape (3M #810) is then applied to the scribed surface and rubbed down with fingers in order to apply adequate pressure to ensure good contact between the tape and coating. The loose end of the tape is then pulled smoothly at an angle of 135° to remove the tape from the surface. After the tape is removed, the coating is visually evaluated for adhesion and ASTM D3359 is followed to rank the adhesion strength. The adhesion is ranked from 0B to 5B with 0B being the worst (>65% squares fail) and 5B being the best (0% squares fail). Representative photographs of a “0B” and a “5B” crosshatch adhesion rank are shown in Figure 1.16.

Table 1.1 summarizes the crosshatch adhesion ranking of three primers with polyurethane (PU) and acrylate top coating on the thermoplastic polyolefin substrate (TPO). The unmodified polyolefin dispersion primer has poor adhesion for both PU and acrylate topcoats, with crosshatch adhesion ranking of 0B, where the majority of the squares would peel off with the ASTM tape, as illustrated in Figure 1.16 (left image). The MAH-g-polyolefin and OH-g-polyolefin blend dispersion has excellent crosshatch adhesion to both PU and acrylate topcoats, with crosshatch adhesion ranking of 5B, where none of the scribed squares peeled off, as illustrated in Figure 1.16 (right image). Maleic anhydride modified polyolefin dispersion has excellent adhesion to the PU topcoat (5B), and slightly inferior adhesion (4B) to the acrylic topcoat.

Both PU and acrylic topcoats are more polar than unmodified polyolefin, and therefore do not adhere well to the unfunctional polyolefin primer layer or to the unprimed TPO substrate (data not shown), and easily delaminate in the crosshatch adhesion test. Maleic anhydride or hydroxyl functionalized polyolefins increase the polarity, and thus, the surface energy, of the primed surface. These functional groups can also provide some chemical bonding, polar–polar surface interaction, or hydrogen bonding of primed surfaces with PU or acrylic topcoats. Therefore, the overall adhesion performance is greatly improved, and there is little, to no, adhesive failure in the top coated systems with these primers.

Pressure sensitive labels represent a large and growing market segment within the packaging industry.⁵⁹  A typical pressure sensitive label consists of a facestock (either paper or plastic), a pressure sensitive adhesive (PSA), and a siliconized release liner, which serves the purpose of protecting the label during manufacture and storage, and which must be disposed of at the time of application. The siliconized release liner is not currently recyclable and represents a large amount of waste. Labels which do not require a release liner, or “linerless” labels, are gaining popularity in order to reduce waste and overall cost (Figure 1.17). The most widely adopted solution for linerless labels is to top-coat the label surface with a silicone release coating. This eliminates the liner stock, but not the cost associated with the silicone itself. The top coating approach also does nothing to address the problems associated with the adverse impact on the appearance of the label or difficulty printing, which a topcoated silicone release layer creates. An alternative approach is to use an activatable adhesive, which can be transformed from hard to tacky with heat, UV, or some other activation method.

There are several potential approaches to a heat-activated adhesive, such as the encapsulation of the adhesive,^60,61  or blending a dispersed polymer with a solid plasticizer (a dispersed tackifier is also commonly included in the formulations), which then combine upon heating. The requirement of heterogeneity on the micro scale precludes solvent-borne or hot melt polymers from being used in this application. Therefore, a water-borne adhesive must be used in order to prevent premature mixing of the polymer and the plasticizer.

The water-borne formulation has the advantage of being highly tunable with a variety of additives such as rheology modifiers, wetting agents, and other surfactants. It can be applied by a broad range of coating methods such as curtain, gravure, reverse gravure, and pattern coating techniques that are not accessible with hot melt polymers, which must be coated by extrusion methods.

The materials used as tackifiers and plasticizers in the deconstructed water-borne pressure sensitive adhesive (PSA) are low molecular weight, low melting temperature molecules that can be dispersed by conventional phase inversion or direct emulsification¹⁷  processes. The polymers typically used in this water-borne approach are those readily available in a water-borne form factor such as acrylics, styrene-acrylics, polyurethanes, and natural rubber derivatives.^62–64  It would be advantageous to be able to use styrene-isoprene-styrene (SIS) type block co-polymers in these deconstructed pressure sensitive adhesive formulations, but it is not possible to synthesize this type of polymer via emulsion polymerization. However, through the use of BLUEWAVE™ mechanical dispersion technology, it is possible to generate SIS dispersions to combine with water-borne tackifier, and plasticizer to form the novel pressure sensitive adhesive.⁶⁵  Alternative methods to generate SIS dispersions including grinding⁶⁶  and using solvent to aid the dispersion process, are not desirable from a process economics standpoint because of the elastomeric nature of the polymer, and the need to remove the solvent from the dispersion before use, respectively.

Neat SIS polymers are of such high modulus that they have very low room temperature tack. There are many tackifiers for SIS that have glass transitions above room temperature, are available as dispersions, and also have limited tack at room temperature. At higher temperatures, above the T_g of the tackifier, a precipitous drop in modulus is measured as the phases mix and the adhesive activates. The wide range of tackifier molecular weight, glass transition temperature, and hydrophobicity that are readily available offer many additional formulating options. These changes can markedly affect rheology, tackifier partitioning, and can be used to engineer the performance targets for a given adhesive application. The plasticizer for the SIS-based PSA is selected based on its compatibility with the styrene domains.

Scanning electron microscopy (SEM) was used to visually observe the morphological changes within the SIS-based PSA coating during the heat activation step. A formulation containing SIS (tri-block co-polymers with 16% polymerized styrene units and 56% diblock) dispersion, benzyl-2-napthyl ether plasticizer dispersion, and Snowtack 100G rosin ester tackifier dispersion is shown in the SEM image of Figure 1.18, both before and after activation at 110 °C. In the image of the unactivated adhesive film (Figure 1.18, left), the large benzyl-2-napthyl ether crystals are dominant at the surface. After heat activation at 110 °C (Figure 1.18, right), the film is much more uniform.

Pressure sensitive adhesives (PSAs), suitable for use in label applications without a silicone-coated release liner (so-called “linerless” labels), were developed utilizing heat-activated aqueous adhesive formulations comprised of dispersed styrenic block copolymers (SIS), dispersed plasticizers, and dispersed tackifiers. A water-borne system of an SIS polymer with 19% diblock dispersed with a long-chain primary carboxylic acid combined with a sucrose benzoate plasticizer resulted in the best combination of high peel and low blocking in the end-use application. Peel forces upwards of 10 N/in were obtained for the heat-activated adhesive, indicating their applicability for many label applications.

The previous sections have provided an introduction to the types of particles that can be generated and applications that can be addressed by thermoplastic polyolefin dispersions generated by the BLUEWAVE™ mechanical dispersion technology. We have also pointed out the limitations of these systems, such as the requirement of heat above the polymer T_g/T_m to coalesce the particles to form a defect-free film for protective coating applications. The application sections are not intended to be exhaustive, but to provide examples of the types of problems that can be addressed with BLUEWAVE™ mechanical dispersion technology. Some important application areas, such as polyurethane dispersions, and reactive systems such as dispersion enabled thermosetting composites,⁴⁴  are not discussed here because of their complexity.

The authors would like to gratefully acknowledge the assistance and technical insights of Rick Lundgard, Manesh Sekharan, Jodi Mecca, Grace Wan, Bobby Moglia, Jinghang Wu, Mike Hus, Grace Wan, Ralph Even, Dan Himmelberger, Ray Drumright, and Jay Romick in the writing of this chapter.