SELF-CROSSLINKING WATER BORNE CHEMISTRY FOR EXCELLENT SHELF STABILITY AND ROOM TEMPERATURE CURE Presenter Author: Dr. Brian Hanrahan KH Neochem Americas, Inc. INTRODUCTION In recent years, the usage of water borne resins has been growing in response to the requirement to improve local air quality by reducing volatile organic emissions (VOC). Concern about VOC is also driving the trend of reducing the solvent content in water borne coatings. This has led to the challenge to provide good coating performance without the use of solvents to enhance film formation and coalescence. Resin chemists and formulators use a number of tools to maintain coating performance at ever lower VOC levels. One option available to resin chemists is to use a Self Crosslinking resin. Self crosslinking in this paper refers to a chemistry that is stable in the can for a long time, hopefully years, but will crosslink the resin binder at room temperature after film formation to improve the final coating properties. There are a few chemistries that fit this definition of self crosslinking, including oxidative cure. However, this paper will focus on the reaction of a ketone group from the monomer diacetone acrylamide (DAAM), with the crosslinking agent adipic dihydrazide (ADH). The reaction between these groups eliminates water which is part of the reason the system is stable during storage as wet emulsion or paint. Since only water is produced in the crosslinking reaction, no additional VOC or troublesome reaction products are created by this chemistry. The mechanism for the crosslinking reaction is illustrated in Figure 1. The rate of reaction is much higher at low ph and is almost nonexistent at a ph above 7 (1). Good in can stability can also be attributed to the high ph normally found in wet acrylic emulsions. One would expect a combination of both intraparticle and interparticle crosslinks as the ADH reacts with the DAAM monomer units. Interparticle crosslinks will build solvent resistance and gel fraction more efficiently, but both types of crosslinks should improve performance in general. While this paper will focus on emulsion resins using the crosslinking between DAAM and ADH, other work has covered the potential to use this chemistry in other types of resins such as polyurethane dispersions (2).
Diacetone Acrylamide (Monomer) Diacetone acrylamide, as a monomer, is provided as solid flakes. However, the monomer is easily dissolved into warm water and many other monomers. Diacetone acrylamide has unique polarity in that it is 100% soluble in water and 100% soluble in Styrene. The monomer is fairly hard with a Tg of around 85 C (3). Table 1: Physical Properties of Diacetone Acrylamide (3). Boiling Point (C/8mmHG) 120 Melting Point (C) 56.0-57.0 Flash Point (C, open cup) 126 Viscosity (cps, 60 C) 17.9 (molten) Specific Gravity (60 C) Solubility (g/100g solv. 25 C) 0.998 (molten) H20 >100 Methanol >100 Ethanol >100 Acetone >100 THF >100 Benzene >100 Acetonitrile >100 Styrene 98 n-heptane 1.0 The reactivity ratios of diacetone acrylamide with other standard monomers are provided in Table 2. Table 2: Reactivity Ratios of diacetone acrylamide with other monomers (3). Monomer M1 r1 r2 Acrylic acid 0.49 0.92 Methyl Acrylate 1.13 1.55 Ethyl Acrylate 2.93 0.98 Methyl methacrylate 1.68 0.57 Experimental Styrene 1.77 0.49 Acrylnitrile 3.38 0.86 Acrylamide 0.67 1.13 Emulsion Polymerization The emulsion polymerization was conducted in glassware under a nitrogen atmosphere. A pre-emulsion prepared in a feed tank of the monomers and surfactants to be used were stirred at room temperature. A separate catalyst solution was also prepared. The emulsion reactor was heated to 80 C and a small amount of the pre-emulsion was pumped into the reactor (1.5 wt %) followed by 35% of the initiator solution. This was stirred for 15 minutes. Then, both the remaining pre-emulsion and initiator solution were pumped into the reactor over the next 3 hours at 80 C with continuous stirring and nitrogen purge. After all of the ingredients were added, the reaction was continued for another 2 hours to ensure good reaction of the monomers. The emulsion was then cooled to 30 C and neutralized with ammonia to a ph of between 8 and 9. Other neutralizing agents have been used in other work, but this paper will focus on ammonia. A solution of adipic dihydrazide was then added to resins that contained DAAM. All of the emulsion resins were filtered before storage. An example of an emulsion containing DAAM is provided in Table 3.
Table 3. Emulsion resin composition. Composition Emulsion / Polymer Properties Reactor Deinonized water 90.0 g Non-Volatile ca. 50% Feed tank A (Preemulsion) Feed tank B (Catalyst solution) Deinonized water 372.0 g Acid Value 7.2 mgkoh/g Rhodapex CO-436* 8.2 g (1 wt% / Particle Size 0.22 µm DAAM 9.4 g (1.9 wt% / ph 9.0 (NH 3 aq.) AA (Acrylic acid) 4.6 g (0.9 wt% / Viscosity 99 mpa s St (Styrene) 74.6 g (15.1 wt% / Tg (calculated.) -14 C MMA (Methyl 80.3 g (16.3 wt% / Tg -12 C methacrylate) (measured.) BA (Butyl acrylate) 195.2 g (39.5 wt% / 2EHA (2-Ethylhexyl acrylate) 129.6 g (26.3 wt% / (Total) 873.9 g Deinonized water 34.6 g Ammonium persulfate 1.46 g (0.3 wt% / (Total) 36.1 g Total 1000.0 g Different resin compositions were used to adjust the resin Tg over a range of calculated Tgs from -30 C to +30 C. Most of the data was collected with resins having a 0 Tg. The monomer compositions for the different resins used in this work are provided in Table 4 Table 4: Monomer compositions and calculated Tg for resins evaluated in this paper. Monomer wt% BA MMA Styrene 2-EHA MAA AA DAAM Surfact. Acid # Calc. Tg 1 53.1 45.4 1.5 0 3 10 0 2 53 43.6 1.5 1.9 3 10 0 3 52.7 40.8 1.5 5 3 10 0 4 34.2 64.3 1.5 0 3 10 30 5 33.8 59.7 1.5 5 3 10 30 6 40.2 17.9 15.1 25.9 1.5 0.9 0 1 7.2-30 7 39.5 16.3 15.1 26.3 0.9 1.9 1 7.2-30 8 40.2 13.2 15.1 25.6 0.9 5 1 7.2-30 Adipic dihydrazide was added to the emulsion as a separate solution at 0.8 Equivalents to the diacetone acrylamide ketone groups for resins 2,3,5,7 and 8. For each 1 weight percent of DAAM used in the resin, around 0.4 weight percent of ADH would be added. All of the resins used 0.3 wt% of ammonium persulfate as the initiator. Film Preparation Clear emulsion films were drawn down onto zinc phoshpated steel panels to a dry film thickness of 30 um. The films were allowed to dry at a standard condition of 23 C at 50% relative humidity. Unless noted in the data, all films were allowed to dry for 1 week before testing. Other conditions for film preparation: Scrub resistance, stress/strain data and some water resistance. Scrub resistance included TiO2 pigment at a PVC of 22% to make the test more realistic.
Stress/Strain Test Specimen: Dumb-bell #3 (JIS A 6021). Dumb-bell specimens were 1 mm thick and involved heating to 40 C to remove water from the films. Films were allowed to sit for 1 day at 23 C and then 2 days at 40 C demolded and another day at 40 C. Block resistance and Hot-cold check testing was run on cedar planks. Film Testing Pencil hardness: JIS5400, evaluated by break of paint film. This is not scratching the film, but film removal. Chemical Resistance: Solvent rubbing, Load of 500g, the number of double rubs before the appearance of paint film changes. Gel fraction: 19 samples of paint film were extracted by Tetrahydrofuran under reflux. The weight ratio of the residue is measured as the gel fraction. Adhesive property: JIS5400, Cross-cut test Scrub Resistance: ASTM D2486-06 Block resistance on wood panels was measured by stacking coated panels and heating them at 55 C for 10 days. The tendency of the panels to stick together was rated. Hot-cold cycle test had coated cedar panels cycle between 60 C for 2 hours and then -20 C for 2 hours through 5 cycles. The panels were then visually rated for cracks. Mechanical properties used test specimen: Dumb-bell #3 (JIS A 6021). The tests were run at 23 C with a crosshead speed of 200 mm/min. Results Emulsion Stability and Film Cure Emulsion resins that contain a backbone of diacetone acrylamide with adipic dihydrazide as a crosslinking component have good in-can stability. We compared the emulsion resin properties of a self crosslinking and standard thermoplastic emulsion just after production and after aging for 4 weeks at 40 C. The main difference is in a viscosity change that seems similar between the 2 types of resins as shown in Table 5. Table 5: In can stability comparison of a thermoplastic and self crosslinking resin. DAAM Content (wt%) 0 wt% 1.9 wt% Hydrazide (ADH) None 1.0 eq. Initial Viscosity(mPas) 71 67 ph 9.2 9.2 Particle size(µm) 0.106 0.106 4 week after at 40 C Viscosity(mPas) 55 57 ph 8.9 8.9 Particle size(µm) 0.106 0.106 Film Drying and Crosslinking The rate of crosslinking is affected by both the presence of water and the ph of the film. As the film dries and the ph decreases crosslinking will start. Anything that would speed up the loss of water and create a faster rate ph decrease would be expected to increase the rate of crosslinking. One approach to faster crosslinking is increasing the acid value of the emulsion resin. This was evaluated by comparing the change in pencil hardness with time for resins that have a range of acid values. Pencil hardness increased faster in films with a higher acid value as illustrated in Figure 2. The pencil hardness listed is total failure of the film. Other work has also shown that this chemistry will react more quickly at lower ph (1). The film should coalesce before crosslinking in order to have good film properties. One way to slow down the crosslinking reaction, allowing more time for film formation, is to use larger amines that evaporate more slowly. Heat would be expected to affect the rate of crosslinking, but we have not studied this in detail.
Dried Film Properties Diacetone acrylamide and adipic dihydrazide will react under ambient conditions in the dried film. Gel fraction increases from 0% to 95% with the use of 2wt% DAAM. In Figure 3, the solvent rubs using both ethanol and xylene are compared for emulsion resins 1, 2 and 3 in Table 2. The solvent rubs observed increases for both solvents with the DAAM content or the amount of crosslinking in the film. Scrub resistance also increases with crosslinking as illustrated in Figure 4. The results reported in this study are generally for films dried under ambient conditions of 23 C and 50% humidity. The influence of DAAM/ADH crosslinking was evaluated on films force dried at 80 C for 3 hours. Gel fraction increased from 0 to 99% with the use of 2wt% DAAM and 1 equivalent of ADH. Resins 4 and 5 from Table 2 were evaluated for application in wood coatings. Some coalescing solvent was added to the emulsion to help film formation at 23 C. Figure 5 provides a comparison between the film properties of the 2 resins. Solvent resistance, block resistance and impact strength were all improved by crosslinking the resin. In addition, the crosslinked resin had improved toughness as demonstrated by much better hot/cold check cycle resistance on a cedar plank.
A soft resin was made with a Tg of -30 C to check the potential for this chemistry in elastomeric coatings for concrete. We compared the resins 6, 7 and 8 from Table 2 for their mechanical properties. The films become dramatically harder as the DAAM concentration increases as seen in Figure 6. One can easily control the tensile properties of a dried emulsion film by incorporating some DAAM into the resin and adding ADH to the emulsion. Crosslinking also increases the durometer hardness of the films, Figure 7. Harder films are less tacky and have better dirt pickup resistance. Another study made resins containing DAAM and then compared the mechanical properties of the resins with and without ADH. Adding ADH to crosslink the resins lead to improved mechanical properties (4).
Conclusions The use of Diacetone acrylamide and adipic dihydrazide will improve the performance of emulsion and other types of water borne resins. The crosslinking chemistry provides good in-can stability, but will crosslink dried films at room temperature to improve solvent, scrub and dirt pickup resistance. It has also been observed, that film hardness has improved when incorporating this chemistry into water borne resins. Force dry can also be used with this crosslinking system. Acknowledgments I would like to thank Mr. Akihiro Gonno of our Yokkaichi research laboratory for his extensive work and help on this project. References 1. N. Kessel, D. Illsley and J. Keddie, Journal of Coatings Technology Research, 5, 285-297 (2008). 2. X. Zhu, Q. Zhang, L. Liu, X. Kong and S. Feng, Progress in Organic Coatings, 59, 324-330 (2007) 3. Technical information sheet, Diacetone Acrylamide, N-(1,1-Dimethyl-3-Oxobutyl )- Acrylamide, KH Neochem Co., LTD. pages 3, 9 4. C. Koukiotis, M. Karabela and I.Sieridou, Progress in Organic Coatings, 75, 106-115 (2012)