The polyurethane industry has faced a challenge in offering a highly versatile, UV-stable coating system that can meet the standard properties of the paint and powder coatings market. A newly developed aliphatic system has been designed to satisfy this need in all related markets. LINE-X has introduced a new development in this field by engineering a thin-film, two-component, 100% solid aliphatic polyurea system. LINE-X ULTRA® offers economic value and exceptional physical properties including weather stability, toughness, flexibility, along with an alluring semi-gloss look in both texture and smooth finish.
This paper discusses details of thin-film aliphatic polyureas including different chemistries and process parameters. ULTRA® is the result of polyaspartic esters blended with a hexamethylene diisocyanate trimer utilized as a hardener. The exotherm reaction achieves a 45-second gel time along with a tack-free time of less than 3 minutes at room temperature with 50% Relative Humidity (RH), yielding an appealing smooth or textured semi-gloss application. In this study, ULTRA® coating characteristics and test results are discussed. The details of the testing are presented to explain the uniqueness of this chemistry. Every step and modification over the course of the product’s development is referenced. In addition, ULTRA® test results are compared to similar coatings such as a powder coat and a paint. The feasibility of the newly-developed thin-film system is also examined.
The wide applicability of PU coatings is due to the versatility in selecting monomeric materials from a huge list of macrodiols, diisocyanates and chain extenders (CEs). The PU coating industry has entered a stage of stable progress and advanced technological exploitation.1
Development of polyurea films made from polyaspartic esters were first started in 1990. Zwiener, et.al., first showed the applicability of polyaspartic esters as co-reactants for polyisocyanates.2,3 Polyaspartic esters are excellent reactive diluents for high-solids polyurethane coatings. They can be blended with hydroxyl-functional polyester and polyacrylate co-reactants thus allowing for reduction of VOCs in relatively high-solvent-containing coatings systems.4,5 Because of the moderately fast curing feature of those esters with aliphatic polyisocyanates, these coatings can provide productivity improvements, along with high film build, low-temperature curing, and abrasion and corrosion resistance.
Polyaspartic technology allows for the formulation of coatings which exhibit fast cure/high productivity, high film build (up to 15 mils), bubble-free film surface, formulation flexibility, controlled cure, non-yellowing, high gloss retention, less waste and high solids from 70% to 100%. These features allow the applicator to rethink the painting process. Thicker films can mean less coats. Fast cure can mean fast return to service. Low temperature cure can extend painting seasons. Conventional application can mean little investment.
Polyaspartics also allow for formulations tailored to pot life and cure speeds. In many instances a manufacturing line which uses heating ovens to cure paint can simply be turned off because of the fast-ambient cure as well as the ability to time the cure characteristics of polyaspartics.6
As mentioned, a unique feature of polyaspartic esters is the ability to tailor the dry time of a coating. In ULTRA® thin-film coatings, different blends of polyaspartics with HDI trimers have been tried to achieve right dry times and individual film properties. Dry time changed from hours to minutes in different blends. Proper additives were added to the resin to attain a smooth or semi-gloss finished look based on application.
Experimental and Results Coating Chemistry
Polyaspartic esters have a unique reactivity with aliphatic polyisocyanates because of their chemical structure. These secondary aliphatic diamines can be prepared from a variety of different primary amines and dialkyl maleates via the Michael addition reaction. Thus, it is possible to create an entire family of aspartates with structural variations which result in varying degrees of reactivity and film properties when reacted with polyisocyanates.6 In Figure 1 polyaspartic esters were reacted with iminooxadiazine dione to evaluate and compare the speed of reactions and tack free time. Common diamines are all aliphatic with cyclic or linear characteristics.
The unique structural feature of polyaspartic ester is a sterically crowded environment around the nitrogen. Additionally, the ester portion of the structure provides inductive effects. These features both act to slow down the reaction of the amino group of the polyaspartic ester compound and the isocyanate group of the polyisocyanate. Practically speaking, slower reaction speed between the isocyanate and an aliphatic diamine results in longer gel times and, thus, a longer application window. The ability to change the amount of crowding around the nitrogen allows the reactivity to be tailored to suit the needs of the applicator. In Table 1 the change in drying characteristics of aspartate coatings including LINE-X ULTRA® has been illustrated. The nine different resin blends have been evaluated for dry time in reacting with low-viscosity aliphatic isocyanate. Those resins have different ratios of blending polyaspartic esters and additives. All formulations are at 100% solids.
When reacting with a low viscosity polyisocyanate, ULTRA® has a 45-second gel time and is dry to touch in 3 minutes. The film has a smooth semi-gloss look. This chemistry builds up a 15-20 mil film which can have a smooth or fine texture look. While the chemistry was in the range of ideal gel time and tack free, the study of the fluid dynamics of the resin and isocyanate along with fine tuning the chemistry helped engineer the development of ULTRA®.
High-pressure impingement mixing is particularly useful in preparing coatings and elastomers using polymeric systems that have very fast reaction kinetics.7 ULTRA® is 100% solids with stoichiometry of the polyisocyanate and resin to provide a volumetric ratio of 1:1. This ensures an easy-to-use process for the applicator. A plural component, high-pressure, high-temperature spray machine is used to spray the ULTRA® system. Different combinations of pressure, temperature, mixing chambers and atomizing tips were tried. Results of each combination were tested following ASTM D412 to evaluate the mixing quality. Data details are reported in Table 2.
To find the minimum thickness that would have the fastest cure and best physical properties, different coating film thicknesses were sprayed. Also, the effects of different weathering conditions on the curing process was evaluated following ASTM D412. Results of those tests are reported in Tables 3-5. Curing process versus weathering conditions where the other study which was done and the results reported as ASTM D412 Tensile Strength in Table 5.
Based on the results in Table 5, we found weathering conditions would affect the curing process. So, sample plates were sprayed in eight different locations in the United States and the curing process evaluated following ASTM D412 Tensile strength. Results, (Figure 2) showed 75% of locations had a tensile strength of more than 1800 psi between 9-15 days. Cleaning the gun, the right range of thickness and keeping the machine set up in the right range were the other factors which we found would affect the result for the remaining 25%. For those locations, the minimum tensile strength was 1652 psi which was still remarkable compared to other thin-film UV-stable applications.
To evaluate ULTRA® performance per industry requirements, other tests were run after finalizing thickness, machine set-up, weathering conditions and surface preparations. Results proved that ULTRA® can compete with other paint and powder coatings in the same applications. Test results are reported in Table 6.
Comparing ULTRA® performance with other coatings confirmed that this thin-film coating would perform as good or better for the same application. Tests such as ASTM D2794 for impact resistance, ASTM D3359 for cross-hatch adhesion and ASTM G154 for light stability were performed on ULTRA®, a powder coating and another paint. Results are reported in Tables 7-9 and Figure 3.
Tests results and actual case studies showed that ULTRA® is a permanent solution for any application in which return to service and capital investment are crucial business parameters. Features such as fast cure, weather stability and bubble-free film allow the applicator to rethink the coating process. Also with superior weather stability and physical properties, ULTRA® gives applicators a long-term solution to the ongoing repair process of today’s thin-film coatings. Unique, durable, armor-like, scratch and dent resistance, and light weight characteristics of ULTRA® are advantages of this custom coating for rocker panels, fender flares, Jeeps, complete exteriors and other automotive uses. Applications in marine, automotive exterior and interior, and amusement parks indicate the wide range of use for this coating chemistry.
We would like to thank COVESTRO for providing raw materials and continued testing to finalize the product and Exova/OCM Laboratories and Technical Micronics Control Inc., for their assistance with the ASTM tests performed for this study.
1. Chattopadhyay, D.K.; K.V.S.N. Raju, K.V.S.N. Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 2007, 32, 352–418.
2. Zwiener, C.; Schmalstieg, L.; Sonntag, M.; Nachtkamp, K.; Pedain, J.; Buechel, K.H. New Concepts for Two-Pack Polyurethane Coatings.Farbe & Lack. 1991, Nr. 12, 1052.
3. Zwiener, C.; Sonntag, M.; Kahl, L. Aspartic Acid Esters – “A New Line of Reactive Diluents for High Solids Two-Pack Polyurethane Coatings,” Proceedings of the Twentieth FATIPEC Congress, (1990) p. 267
4. Jorissen, S.A.; Rumer, R.W.; Wicks, D.A. “Proceedings of the Nineteenth Waterborne Higher Solids and Powder Coatings Symposium”, February 1992, p.182.
5. Joriseen, S.A. “Polyurethane Coatings for Automotive Plastics”, Finishing Automotive Plastics, April 1992, EM92-157.
6. Angeloff, C.; Squiller, E.P.; Best, K. “Bayer Polymers.“. Two-Component Aliphatic Polyurea Coatings for High Productivity Applications”, PCL 20.11 (2003).
7. Patent N0.: US 6,403,752 B1
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