Peter Schreiber, Albemarle Corporation, Baton Rouge, LA 70806, USASam Lane, Albemarle Corporation, Baton Rouge, LA 70801, USA
MOCA (methylene bis-ortho-chloroaniline) is the curative-of-choice for waterproof roof coating in Japan. These coatings are typically hand applied and require long pot-lives. Basic formulations use 3-6% NCO TDI prepolymers and are highly plasticized. However, due to the carcinogenic nature of MOCA, many customers want to move away from MOCA. The major technical challenge is to determine a curative system that gives a prolonged pot-life yet a relatively short tack-free time, at the same time delivers appropriate physical properties. Presented here are our efforts to develop a curative formulation to be used in the Japanese market as a MOCA replacement.
The goal of this project was to develop a curative formulation for use in the Japanese market as a MOCA replacement. Due to the carcinogenic nature of MOCA, customers want to move away from MOCA if a suitable replacement was found. The replacement will be a curative package, likely a blend of curatives to achieve the desired properties. Albemarle has been successful at this endeavor previously. The development of Ethacure 300, a liquid aromatic diamine curative from Albemarle Corporation, (E-300) circa 1990 was in effort to gain market share of the cast applications dominated by MOCA. While E-300 was found to be a less toxic alternative to MOCA, two major deficiencies of E-300 were discovered: odor and poor weatherability. The odor problem has been mitigated; however, no economical solution was ever found to prevent the degradation of parts made by E-300 when exposed to UV light. Other aromatic diamines, such as Ethacure 100 from Albemarle (E-100), do not have the weatherability problem that Ethacure 300 does; however, E-100 reacts too quickly to provide useful formulation working times.
The current state of the art includes Hodogaya US Patent No. 5,688,892, which claims a process that uses a plasticizer (dioctyl phthalate) to slow down E-100. The major technical challenge is to determine a curative blend that gives a prolonged pot-life yet a relatively short tack-free time, at the same time delivers appropriate physical properties. Each customer will have its own formulation and product specifications, so open communication with the customers will be of utmost importance.
The formulation techniques used in this work were based on those described in Hodogaya's US Patent No. 5,688,862. The prepolymer and resin were formulated as one-to-one by mass. Calcium carbonate was added to the resin blend at half the mass of the prepolymer. The amines were used at a level that would give a 1.20 index and the remaining mass of the resin was made up with dioctyl phthalate plasticizer. An example formulation is provided in the Table 1.
Table 1: Example roof coating formulation
All components were added to a 200 mL polypropylene cup and mixed using a dual asymmetric axis centrifugal mixer (Speedmixer DAC 600). Complete mixing, solids dispersal, and bubble elimination were complete in less than a minute when mixed at 1800 rpm. The formulation was poured onto a 6" x 9" x 1/8" open mold coated with silicone mold release. Excess formulation was drawn down using a metal bar to produce a smooth, uniform coating. The coatings were allowed to cure overnight then demolded. Samples were cured for 7 days testing their physical property testing.
The pot-life was defined as the time required for the formulation to reach a viscosity of 100,000 cPs. The viscosity rise was measured using a Brookfield viscometer (DV-I, RV-7 spindle). Before pouring the formulation onto the mold, it was poured into a 40 mL vial. The vial was secured and the viscometer spindle was lowered into the curing formulation. The viscometer was connected to a data logging A/D device and software, from which viscosity curves were generated.
For the results of this work to be accessible to all potential customers, it was necessary to generate data for coatings prepared with a general commercial prepolymer. Air Product's Airthane PPT-80a prepolymer is a 3.52 %NCO, TDI based prepolymer that can be poured at room temperature. This prepolymer is PPG D-2000 based and is used mostly for cast elastomers with a hardness of 80A, but it is similar enough this application's prepolymers for comparable work. The TDI prepolymer was purged with nitrogen after use and stored in a nitrogen purge box.
Table 2: Source of material
Results and Discussion
The coatings in Table 3 contained 25% CaCO3 and ~20% DOP, and were prepared with Air Product's PPT-80a at a 1.20 index. Blends A, B, and C contained DETDA and a slow reacting aromatic diamine. The government standard for this application is the Japanese Industrial Standard for Roof Coatings for Waterproofing (JIS A 6021).
Table 3: Coatings made with Air Products Airthane PPT-80a
The viscosity rise curves (Figure 1) show the time for the viscosity to reach 100,000 cPs as well as the viscosity profile during the cure process. For this application, the ideal curve would remain flat for about an hour, and then sharply increase, building viscosity and physical properties quickly. The coatings prepared with E-100 alone increase in viscosity exponentially. The coating made with E-300 shows a long, flat section followed by a relatively sharp increase.
Figure 1: Viscosity rise of CP-series roof coatings
To increase the attractiveness of the CP series curatives, the physical properties of the coatings produced must be improved while keeping acceptable pot-lives and end-cures. The variables chosen to investigate were % slow amine, amount of plasticizer and index.
Effect of Plasticizer
To determine the effect plasticizer, formulations with an increasing amount of DOP were prepared (Table 4). Air Products PPT-80a was used at a 1.20 index and CP-14 was used as the curative.
Table 4: Coatings made by varying amounts of plasticizer
Figure 2: Physical property data of formulations with varying amounts
The plasticizer played an important role in these formulations; however, it is necessary to understand its effect on physical properties. Hodogaya successfully practices their patent, which describes the use of dioctyl phthalate to slow down the isocyanate reaction with Ethacure 100. As the amount of plasticizer increased, the viscosity rise vs. time became more linear (Figure 3).
Figure 3: Plot of viscosity rise
DOP not only increases the pot-life of formulations, it makes the formulation less viscous, which allows trapped air to escape before the gel time. The coating prepared with no DOP was opaque due to trapped air bubbles. Adding 10 g DOP alleviated the problem, but it took 25 g DOP to produce a clear coating (Figure 4).
Abb. 4: Beschichtungen mit unterschiedlichen DOP-Anteilen
Effect of Index
The use of a secondary amine to increase pot-life may require the optimization of the index (equivalents NCO/equivalents active amine groups). Higher indices typically yield greater cross-linking within the polymer, which may compensate for the disruption of the hard-segment by the secondary amine. The index was varied from 0.90 to 1.30; the results are summarized in Figure 5. The tensile strength, M-100, and M-300 have maximum values at a 1.00 index. The % elongation had a maximum value at an index of 0.95. An index of 1.00 is optimal of tensile strength, whereas an index of 0.95 gave the greatest elongation-at-break. As the index increased, elongation-at-break decreased, which is consistent with a more-crosslinked polymer.
Figure 5: Physical properties of formulations varying index
While decreasing the index boosts physical properties, but it also shortens pot-life (Figure 6). Changing from a 1.2 index to a 1.0 index decreases the pot-life by 82%.
Figure 6: Pot-life vs. index
The variables described above (index and plasticizer) can be easily optimized separately; however, optimizing the entire system is more complex. To determine how the variables interact, a designed experiment was performed. The Minitab 15 statistical software program was used to set up the experiment, evaluate the results, and generate the graphs. The three variables probed were index, amount of plasticizer, and % slow aromatic amine; the contour plots of the variable interactions are provided below. If the lines on the contour plots are straight, there are no interaction effects between the two variables.
The more the lines are curved, more interaction exists between the variables that affects the specific property. The following contour plots give a qualitative understanding of the magnitude of the interactions and responses.
In the contour plots for pot-life (Figure 7), the contour lines are curved for all three variable combinations. It has been shown above that all three pots affect pot-life significantly and it is no surprise that two variables interact to affect the potlife.
For tensile strength, the contour plots (Figure 8) the plasticizer*index plot shows slight curvature, indicating slight interaction. The straight lines for the two other interactions suggest that the slow amine*index and slow amine*plasticizer do not interact to affect the tensile strength.
Figure 7: Contour plots of pot-life
Figure 8: Contour plots of tensile strength
In contrast to the contour plots of tensile strength, the contour plot lines for % elongation (Figure 9) are curved, indicating strong interactions between the variables. The effects plasticizer and slow amine loading have on % elongation are stronger at a 1.0 index than they are at a 1.3 index. At lower plasticizer loadings, the slow amine has a greater impact on the % elongation than at high plasticizer loadings.
For Shore A hardness, the lines are generally straight, except for slight curvature for the % slow amine*plasticizer interaction.
Figure 9: Contour plots of % elongation
Figure 10: Contour plots for Shore A hardness
The Japan roof coating market uses MOCA to cure waterproof coatings. This work demonstrates that blends of aromatic diamines can produce coatings with similar pot-lives and physical properties as coatings cured with MOCA. These blends are liquid and less toxic than MOCA and are thus a viable alternative.
Peter Schreiber was born and raised outside of Chicago, IL. He obtained his B.S. degree from DePaul University in Chicago, IL, in 2000. He completed his Ph.D. degree in 2007 at the University of Colorado, Boulder under the direction of Professor Josef Michl. His thesis dealt with the synthesis and physical properties of the CB11 anion and its derivatives. After graduation, he joined the Curatives group at Albemarle Corporation in Baton Rouge, LA, where he works on the development of flame-retardants and chain extenders for polyurethane and polyurea applications.
Sam received his BS in chemical engineering from Stanford University (1988) and his PhD in chemical engineering from the University of Houston (1993). He was with Dupont/Invista from 1993-2008 where he worked in various roles in R&D, technology, marketing and business development. Currently, he is Business Development Manager with Albemarle's Polymer Solutions business unit, and is based at their corporate headquarters in Baton Rouge, LA.
*This technical paper (presented at CPI's Polyurethanes 2011 Technical Conference) is published with the permission of CPI.