New Additive Chemistry for Improved Pigment Suspension Control
This paper provides insight into THIXATROL® UV1104, a new polyester-based, rheology-modifying additive chemistry, to help eliminate some common rheological problems and optimize pigmented UV coatings. Coatings formulators will get a full picture regarding a new developmental composition, its base chemistry and incorporation procedures to enhance coating properties and performance.
When the end user or applicator opens the container, the last thing they would like to experience is the pigment hard settled at the bottom, which brings the product's storage stability into account. From a product stability standpoint, the issue of pigment settling is not simply a matter of aesthetics. Performance too is an issue. When pigments hard settle they tend to pack to the point where they become dry (Figure 1). The dispersants and solvating fluids that initially dispersed the pigments into suspension have been forced off of the pigment particles. One symptom of this phenomenon is a loss of the fineness of grind after attempting to reincorporate the settled cake. In most cases the pigments cannot be re-incorporated back into suspension. These dry particles are applied with the coating onto the substrate. They will tend to form surface defects and inhibit the coating from forming a uniform film. Premature coating failure is likely, along with poor appearance.
To a degree, these same forces involved in pigment settling affect the sag resistance and leveling of coatings. To better understand these interactions and forces and how to mitigate them, a basic knowledge of rheology and how to measure it is required.
Basic Practical RheologyWhen we think of the word viscosity we relate it to how "thick" something is when it is mixed or shaken. Viscosity is the resistance of a material to flow when we apply a force (or stress) such as mixing.1 Shear stress (t pronounced "tau") is an external force (mixing, shaking).2 Shear rate (g· pronounced "gamma dot") measured in reciprocal seconds (s-1) is how fast we move the liquid layers in a material of given thickness when we stir with an applied force.3 Shear rate is a velocity gradient to liquid thickness relationship. Viscosity (h, pronounced "eta") is then the ratio of shear stress to shear rate and can be measured and reported as a Pascal second (Pa•s) or in Poise where 1 Pa•s = 10 Poise = 1000 centi-Poise (cP) or 1000 mPa•s.
Viscosity measurement is relative to the shear rate at which it is tested by use of various viscometers that have become common to us. A Stormer viscometer will measure viscosity at one point in the medium shear rate range equivalent to normal stirring or hand mixing, ~200 s-1. Brookfield viscometers can measure multiple-point shear rates using different spindle sizes and rotational speeds to determine viscosities in the range of 1.0 - 100 s-1. Because of the expanded measurement ranges in shear rate, albeit a small portion of the overall rheology profile, a Brookfield will help provide a better picture of the coating's flow. Pigment settling, leveling and resistance to sag cannot be measured efficiently with these methods, as they can only be measured at ultra-low shear rates. Therefore, a rotor stator type rheometer, which can provide more precise measurements of ultra-low, low, medium and high shear rates is the preferred method when studying the rheology and flow of coatings.
Knowing the viscosity at the different shear rates during production, application, transportation and storage can be invaluable. By using a rheometer with a series of viscosity measurements at these shear rates, a viscosity profile or flow curve can be generated for a coating. Such a profile will provide reliable information as to how a coating will perform at a given point of its life prior to application and cure.
Types of FlowWhen the viscosity is measured at increasing or decreasing shear rates, the result is a flow diagram or viscosity profile that can be compared against that of other coatings to help judge their overall performance. A Newtonian flow profile (Figure 2, lowest flat line) maintains constant viscosity regardless of the applied shear rate. This type of flow is typical of common materials such as honey, solvent and resins, but is undesirable in most finished coatings.4
The goal of many formulators is to provide a coating with a pseudoplastic or thixotropic flow profile (Figure 2, top (down arrow) line - initial; lower (up arrow) line - recovery) that is ‘shear thinning' to some degree.
In a shear thinning system the cohesive bonds or attractive forces between molecules and particles are broken as shear rate increases, resulting in a lower viscosity. Once the shear is removed, a pseudoplastic system will recover immediately (e.g., latex emulsion) while a thixotropic system will recover with time (e.g., oil based dispersion paint). The area between the initial viscosity curve and the recovery curve, the hysteresis loop, is a measure of the system's thixotropy.
Pigment suspension is determined by how well the coating resists the pigments tendency to settle through the coating to the bottom of the container when affected by shear stresses, in the ultra-low shear rate ranges, exerted by gravity, post-production packaging and transportation.5 Sag resistance and leveling are also influenced and controlled in these low shear regions.
As a shear thinning system recovers its low shear viscosity after application, yield values (the minimum shear stress needed to induce flow (measured in Newton/m2 or Pascal's (Pa)) must be high enough to exceed the effect of gravity, which causes settling in the paint or the applied coating to sag.
For a shear thinning system, fast recovery equals good sag control but causes poor leveling as evidenced by ‘brush strokes'. If the time to recover is too long, then the coating will sag. The need for a coatings system to exhibit a proper balance between flow and leveling becomes readily apparent, as does the need for rheology modification to obtain these properties.
Formulation Concerns and LimitationsWhen the form, function and cost of a coating system have been determined between the manufacturer, customer and applicator, the balancing act for the formulator begins.
It is a common practice to alter a coating's viscosity until it has achieved the desired application viscosity "spray viscosity" or good "in-can" or "feel". It is beneficial for the coating to be at the application viscosity when the customer opens the can to retard the desire to improperly thin the coating and cause other application difficulties. However, as previously mentioned, if a low viscosity is prevalent in the ultra-low shear range, pigment suspension and sag control suffer. Obtaining a suitable application viscosity also presents additional concerns for the formulator when the coating contains little or no solvent. The oligomer-to-monomer ratio in many cases is the first area subject to adjustment. Reduction of a viscous high-molecular-weight oligomer with a monomer may diminish desired properties over the reality of needing a lower viscosity, lower average molecular weight oligomer/monomer blend. This may adversely affect the performance of the coating and simultaneously hinder pigment suspension as the nature of the flow will lean toward Newtonian.
This balancing act also affects the types of pigment used and their loading. If a low-viscosity system is desired, using pigments and extenders with a low oil absorption such as calcium carbonate or barium sulfate (barytes) must be used. Benefits include improved sanding capabilities, hiding power and a lower overall cost. One drawback in the use of these pigments is their high density combined with their low oil absorption properties, which greatly increases their tendency to hard-pack settle. Thus, their use is limited. Silicas or other higher oil absorption pigments may be needed to achieve the desired gloss or hiding, which typically drive up the viscosity as well as raw material cost. While silicas can improve pigment settling to a degree, many times their handling during manufacture becomes an issue.
The result of manipulating the viscosity by using these methods will result in a primarily Newtonian flow that remains still be plagued with the common problems - lack of rheology control, pigment settling and poor sag resistance. Only by the use of efficient rheology modifiers can practical solutions be found.
Formulation Solutions and Additional BenefitsThe availability of rheology additives to the energy-cured coatings market has been rather limited. Solvent content, incompatibilities and products that provide too great an increase in the application viscosity have failed to bridge the gap between necessity and practicality. A new, polyester-based rheology modifier chemistry has been developed in response to offer a practical solution to these vexing problems. This new rheological additive (THIXATROL UV1104) was designed as a 100% non-volatile, easily incorporated, liquid thixotrope for use in pigmented radiation-cured and conventional high-solids coatings. It can effectively replace conventional thixotropes or be used as a synergist in combination with other rheological additives. Rheology control is now possible as it imparts high viscosity and good yield value in the ultra-low shear rate regions with a highly desirable shear thinning effect and excellent recovery to establish a thixotropic flow profile. A lower application viscosity than traditional thixotropes is now realized while maintaining the rheological performance properties necessary to control sag and pigment suspension in the final coating formulation.
Functionality and MechanismUV1104 functions by use of an associative mechanism with pigments or extenders. It is specifically designed to provide superior rheological characteristics based on its two-part structure (Figure 3). This structure includes an anchoring functional group attached to a solvatable chain and hydroxyl groups. Multiple anchoring groups associate with various particle surfaces. The structure resulting from the use of the hydroxyl groups is an artifact of polar bonding such as hydrogen bonding. This same phenomenon is observed between polar species with other components present in the coating formulation. This provides a three-dimensional structure, which increases the molecular volume and chain entanglement, subsequently increasing the viscosity in the ultra-low shear ranges.
These structures are weakly associated with their attractive forces and are easily disrupted under applied shear during mixing, spraying, etc. This results in a desirable shear thinning viscosity reduction for easy handling and application properties while providing good pigment suspension during storage.
Additive Use/Order of AdditionWhen formulating with this associative thixotrope, order of addition is important. UV1104 requires intimate contact with dispersed particulates and should always be added after the pigment. Figure 4 relates the best possible scenario for obtaining the desired results. Temperature ranges will vary due to the overall solvency and shear imparted into the system during coating manufacture.
Supporting DataTwo case studies have been provided to show the efficiency of the rheology modifier.
Study 1 - Wood Primer/Sealer
The first study concerns a 100%-solids UV-curable primer/sealer wood coating with a polyester/polyether acrylate oligomer blend, various common monomers, and talc and barium sulfate as fillers. The application is designed for room-temperature HVLP spray application; the viscosity is measured using a # 4 FORD cup at 25 °C.
The following rheology profiles (Figure 5) exhibit the changes produced by the addition of this thixotrope at 0.6% by weight opposed to using none. The flow profile for the blank and the sample with the rheology modifier show a similar response to shear. However, further examination reveals that the sample with UV1104 provides a higher ultra-low shear viscosity and the development of more thixotropy (as denoted by the area inside the hysteresis loop) than does the blank. The blank has hard pigment settling and no measurable resistance to sagging (Table 1), which relates to the lack of ultra-low shear viscosity and how it will impact a coating's performance. The sample with UV1104 is very efficient in eliminating hard-packed settling with a slight improvement in sag resistance without a significant effect on viscosity.
Also notice the improved cleanliness in the fineness of grind and the stable viscosity upon oven aging. The sample using UV1104 shows no change in these values, as the additive has a stabilizing effect on the pigments in the coating. Shelf life and performance have been markedly improved by use of this product. As previously discussed, using fillers such as barytes can improve the coating's performance during sanding operations and reduce the raw materials cost while having a low impact on the viscosity because of its low oil absorption. Unfortunately, the low oil absorption increases the pigments' settling tendency, as evidenced in the blank sample. It was the goal of this evaluation to have coatings at a "spray viscosity" with a variety of dense pigments to show the usefulness of the rheological additive and how it prevents hard pack settling. This has been accomplished.
Study 2 - White Plastic TopcoatThe second case study is about a 100%-solids UV-curable white topcoat for plastic substrates. The oligomer is a polyester acrylate. Other constituents include an alkoxylated triacrylate adhesion promoter, common monomers and titanium dioxide pigment. This system was designed for application using a roll coater, heated spray or as a reducible concentrate.
Performance issues regarding the coating included pigment settling and unstable viscosity upon oven aging. Aesthetically, the coating had exhibited orange peeling as a surface defect (Table 2). When UV1104 was incorporated, the pigment settling and viscosity stability of the sample were considerably improved without an unacceptable increase in viscosity. Surface defects were eliminated and sag resistance had also increased. The blank sample exhibits a far less shear thinning flow profile (Figure 6). Increases in the low-shear viscosities as evidenced by the rheological profiles provide a valid illustration that many issues regarding product stability and appearance will benefit when the proper thixotropic additive is utilized.
When using UV1104 to control the viscosity recovery (shown by the back (lower curve)) we can attain the proper balance between sag control and leveling needed to prevent surface defects while maintaining the maximum sag control. This case study is regarded as a success as well.
ConclusionsA novel polyester rheological modifier, THIXATROL UV1104, has been developed in response to the needs of the energy-cured coatings industry to assist the formulator in rheology control. This product provides lower viscosity than traditional thixotropes while maintaining rheological performance properties necessary to improve sag and pigment suspension in the final coating. It will help the formulator to meet and exceed performance and storage stability expectations over current methods. Additional potential benefits include improved performance, appearance, and cost reduction in raw materials and in coating application.
For further information, contact Scott Murray or William A. Reynolds, email@example.com, or Detlef van Peij, firstname.lastname@example.org.