NON-ISOCYANATE 2K COATINGS

Transcription

1 O-ISOCYAATE 2K COATIGS Colin Brogan, Kuang-Jong Wu, Bud Equi, Sarah Quinn Allnex ITRODUCTIO Coatings, regardless of the specific application or substrate (industrial wood, automotive refinish, industrial plastic, direct-to-metal, etc.), serve to protect and beautify. Coating films enhance the aesthetics of objects as well as prevent damage resulting from exposure to weather, chemicals, dirt and abrasive materials, and impacts. Thus, they play a critical role in meeting customer expectations for products that are attractive and long-lasting. At the same time, coating manufacturers must meet the growing demands for products that have a minimal impact on worker safety and the environment. Thus, both raw material suppliers and coating formulators are challenged to design new technologies that meet these evolving requirements while still maintaining performance and ease-of-use. With its expertise in crosslinkers, Allnex has, as one approach, focused on the development of new crosslinking systems that can meet the expectations of the marketplace. Even though crosslinkers account for only 10 to 30 percent of the resin solids in a coating formulation, they play an important role in determining the final properties of the coating and contribute to its overall environmental and safety profile. Herein is presented initial results obtained for a novel isocyanate-free, low-formaldehyde crosslinker for ambient and low temperature cure applications. The performance of the very fast-curing melamine resin in industrial wood, auto refinish, and pigmented direct-to-metal coatings is reviewed, and key learnings to date are highlighted. Further advancements in these prototypes and technology platforms to address remaining performance gaps will lead to the introduction of a new family of products that will expand the options available to formulators. ISSUES WITH ISOCYAATES, FORMALDEHYDE, AD HIGH-TEMPERATURE BAKES As hazardous materials, isocyanates and formaldehyde both present environmental and safety management issues. It is thus not surprising that many coatings companies and end users are looking for alternative solutions that do not contain these compounds. Separately, the use of high-temperature bakes requires higher energy consumption and results in greater emissions of carbon dioxide and a larger carbon footprint. Addressing these issues while maintaining performance is important for enhancing the viability of future coating technologies. LOW TEMPERATURE CURE CROSSLIKER DESIG AD SCOPE The Low Temperature Cure (LTC) crosslinker consists of a highly alkylated melamineformaldehyde (MF) resin (Figure 1) with ultra-low free formaldehyde content (< 0.1 wt%). It is compatible with typical acrylic, alkyd, and polyester polyol resins, and exhibits the highest reactivity with primary hydroxyl groups. The chemistry of the crosslinker is effective in solventborne, two-component (2K) systems designed for ambient and low-bake applications. It should be noted that the current resin is not meant for use in water-based or high bake systems. Typical properties are listed in Table 1. Despite the fact that the LTC crosslinker is an MF resin, it has several unique characteristics that make it suitable as an alternative for the isocyanate-based crosslinkers that are currently used in high-performance, ambient and low-bake applications 1

2 Figure 1 Generic structure of the LTC crosslinker CH 3 OCH 2 CH 3 OCH 2 CH 2 OCH 3 CH 2 OCH 3 CH 3 OCH 2 CH 2 OCH 3 Table 1. Typical properties of the Low Temperature Cure (LTC) crosslinker Solids Content (Foil), wt % 98 min Free Formaldehyde, wt % < 0.1 Viscosity, mpas, 25 C Equivalent weight, g/eq 75 FORMULATIG GUIDELIES The first stage of the investigation of this LTC crosslinker resin was focused on determining clear guidelines for optimum formulation. The resin is considered to be trifunctional, and thus it was found that a constant alkoxy/hydroxyl ratio of 2.0 was the most effective. Therefore, this ratio was used to prepare all of the formulas discussed below. Evaluation of the reactivity of the LTC crosslinker revealed that the type and concentration of added alcohol in the formulation has a significant impact on its stability and on the hardness development of the applied coatings. Specifically, the addition of primary alcohols (methanol, ethanol, or butanol) is required for package stability, and higher concentrations of alcohol lead to an increase in the catalyzed pot life. However, the use of higher chain alcohols, such as n-butanol, can negatively influence the hardness development and dry times of the final coatings. SUCCESS I IDUSTRIAL WOOD APPLICATIOS Customers formulating industrial wood coatings are seeking technologies with both lower free formaldehyde and lower evolved formaldehyde, but that offer the same or an improved level of productivity, performance and appearance compared to that of existing industrial wood finishes. Thus, this market provided an ideal opportunity for the LTC MF crosslinker resin. To test the performance of the LTC crosslinker, a urea-formaldehyde/melamineformaldehyde control system and the LTC crosslinker were used to prepare alkyd-based conversion varnishes, and the several properties of the two varnishes were evaluated. The formulations are presented in Table 2, and the properties of the two coatings are listed in Table 3. 2

3 Table 2. Formulations of industrial alkyd-based wood varnishes Part A LTC Crosslinker UF/MF Control Alkyd Resin 1 (60% V) LTC Crosslinker 30.0 Butylated UF Resin, (62% V) 51.6 Butylated MF Resin 8.0 Butyl Acetate Flow Control Agent, 10% solution Part B ptsa, 16% in Ethanol Total A+B % onvolatile Crosslinker Loading, % on TRS 30 32/8 p-tsa, % on TRS Coconut-based Short Oil Alkyd (OH#: 160) Table 3. Properties of alkyd-based varnishes prepared using the LTC crosslinker and a UF/MF control Varnish prepared using the LTC crosslinker Varnish prepared using the UF/MF Control 1 Hour Print Resistance (0-5; 0=best) Sandable in: 30 minutes 30 minutes Konig Film Hardness, seconds: 1 day days Chemical/Stain Resistance: Covered 24 hours (5 = no effect) Coffee 5 5 Grape Juice % Ethanol 5 5 Vinegar % Ammonia % Sodium Hydroxide Coconut-based Short Oil Alkyd (OH#: 160) 3

4 Viscosity, secs #2 Zahn Cup As can be seen in Table 3, the LTC crosslinker provided a fast cure response and equivalent performance in terms of print resistance, early sanding, hardness development, and chemical resistance. otably, the alkyd varnish formulated with the LTC crosslinker exhibited improved catalyzed viscosity stability (Figure 2). However, it should be noted that a loss of reactivity was observed with time, and thus the addition of fresh material was required after 24 hours. Figure 2. Superior catalyzed viscosity stability of the alkyd varnish prepared with the LTC crosslinker Catalyzed Pot Life Hours UF/MF Control LTC crosslinker IITIAL PERFORMACE I AUTO REFIISH COATIGS The automotive refinish market is a key segment for ambient and low-temperature bake isocyanate-based coatings. This market includes auto body repair/paint shops, production auto body paint shops, new car dealer repair/paint shops, fleet operator repair/paint shops, and custom- made car fabrication facilities, Unlike automotive OEM coatings, which can be applied at high temperature, refinish coatings must provide similar performance in terms of appearance and durability, but in a system that cures at either room temperature or up to 66 ºC (150 ºF). While acrylic-based coatings are used to some extent, acrylic urethane and polyurethane-based systems are most widely employed in the highest-performing coating products. There is an opportunity for coating technologies that can provide similar performance to isocyanate based systems, with the desire to reduce the potential exposure of workers in this industry to isocyanates. The potential for the LTC crosslinker to meet the performance requirements for automotive refinish clear coats was thus evaluated. The first step was to determine the influence of the acrylic resin on the coating properties. A total of 11 different commercially available acrylic resins, including several that are designed to be used with isocyanates, were selected for this study and varied with respect to their glass transition temperature (Tg), hydroxyl number (OH#) and molecular weight (Mw). The coatings were applied to primed metal substrates, and their performance was evaluated based on the following criteria: dry time, film hardness development, solvent resistance, and humidity resistance. Of these 11 resins, five showed promise (Table 4). 4

5 Table 4 Suitable Acrylic Resins for LTC Crosslinker in Auto Refinish Clear Coats Acrylic A B C D E Tg, C Mw 8,500 12,000 2,200 20,000 9,450 OH# Acrylic/Crosslinker Ratio: 75/25 82/18 77/23 81/19 76/24 DDBSA 2 Catalyst, % on TRS Ethanol, % on TRS Through drying time on glass strips, hrs Coating Properties: applied on ED primed CRS Gloss, 20 /60 87/92 94/96 88/92 87/93 89/94 Konig film hardness, 7 days MEK double rubs, 7 days Film Haze on Humidity Exposure 3 (5 = best) Mixture of 1 and 2 OH; Formulated using 1 OH only. 2 Dodecylbenzene sulfonic acid 3 Tested at 60 C for 10 days after 7 days of post cure As to why the remaining six acrylic polyols performed poorly is unknown without further compositional information. Factors which would negatively influence the cure of the LTC crosslinker include: 1. Excessive Tg: Resins with Tg s >40 C can lead to incomplete crosslinking because the rate of crosslinking becomes mobility controlled. 2. Lack of 1 hydroxyl functionality: melamine resins are slow to react with 2 hydroxyl functionality particularly at ambient or low temperatures. 3. The presence of an auxiliary amine catalyst added to enhance the cure with polyisocyanates which interfere with the cure of the LTC crosslinker. Regardless of the reasons, the results obtained for the coatings prepared using the LTC crosslinker and these five acrylic resins, when applied over primed metal, demonstrated the necessary coating properties. ext, these same five acrylic resins were individually formulated into coatings using the LTC MF crosslinker and a trimer of hexamethylene diisocyanate (HDI), and the performance of these coatings over a solventborne refinish base coat was compared. The different formulations are presented in the top half of Table 5, while the results of various coating performance tests are shown in the bottom half of the table. As can be seen in the table, in this initial set of trial tests, the coatings prepared using the LTC crosslinker, when applied over a solventborne automotive refinish base coat, exhibited poor hardness development and solvent resistance relative to the coatings prepared using the HDI trimer. 5

6 Table 5 LTC Crosslinker vs Isocyanate in Auto Refinish Clear Coat Acrylic A B C D E Crosslinker LTC HDI LTC HDI LTC HDI LTC HDI LTC Binder ratio 75/25 70/30 82/18 66/34 77/23 72/28 81/19 77/23 76/24 Ethanol, % on TRS DDBSA, % on TRS DBTL 1, % on TRS Viscosity at 50% V, #4 Ford Cup 21.1" " " " " Substrate: Primer coated ED primed CRS with freshly applied White Refinish Base Coat Konig film hardness MEK double rubs After 7 days After 30 days After 7 days After 30 days Cleveland Humidity Resistance - Results after 7 days at 40 C 1 Dibutyltin dilaurate Blistering Side: 8M Main: Appearance Slight swelling o Slight swelling swelling Portion: 2M Main: 10 o Slight swelling swelling 10 6M o Slight swelling swelling o Slight swelling swelling Of the five acrylic resins evaluated, Acrylic E showed the best results. Interestingly, changing the catalyst from dodecylbenzene sulfonic acid (DDBSA) to para-toluene sulfonic acid (ptsa) and increasing its concentration resulted in better hardness development and solvent resistance, although the performance of the coating prepared with the LTC crosslinker still lagged behind that of the best CO system. While these initial results have identified a clear performance gap between the LTC crosslinker and HDI trimer, they are still encouraging. There are many variables that can impact a coating formulation, and further work is ongoing to optimize the chemistry of the crosslinker, identify the most effective structures and functionality in acrylic resins, and determine critical formulation parameters for boosting the performance of coating systems that contain the LTC crosslinker to meet the expectations of the demanding automotive refinish market. The results obtained during this investigation of the potential use of the LTC crosslinker in automotive refinish coatings revealed the potential for its use in other ambient and lowtemperature cure coating applications including pre-coated metal and plastics. Formulations made with Acrylic A compare the dry time, hardness development, solvent resistance and humidity resistance of the LTC crosslinker, formulated with either methanol or n-butanol, to a polyisocyanate (HDI trimer). Films containing the LTC crosslinker exhibit faster dry times relative to the polyisocyanate sample formulation and the results of performance testing on this system are presented in Table 6. The impact of alcohol type on dry time and film hardness is also highlighted. 6

7 Viscosity, secs #4 Ford Cup Table 6 Performance of LTC Crosslinker vs. Isocyanate in Acrylic A Formulation Parameters LTC Crosslinker/ n-butanol LTC Crosslinker/ methanol HDI Trimer Acrylic A/Crosslinker (solids basis) 77/23 77/23 70/30 Catalyst, % on TRS Butanol, % on TRS (DDBSA) 2.0 (DDBSA) 0.03 (DBTDL) Methanol, % on TRS 7.4 % onvolatile 54% 54% 54% Viscosity, seconds #4 Ford Cup Dry Through Time, hours Konig Film Hardness at 50 micron film thickness on primed CRS, seconds MEK Resistance, double rubs 1 day days days day days Cleveland Humidity Resistance, 40 C (Determined 10 days after application) Blistering, 12 days As noted in the comparison to UF/MF resins in industrial wood applications, the systems formulated with the LTC crosslinker deliver superior catalyzed pot life relative to the polyisocyanate system (Figure 3). Additionally, viscosity stability is found to be superior with methanol versus n-butanol, but the reason for this is unknown. Figure 3. Superior catalyzed viscosity stability of the acrylic system prepared with the LTC crosslinker Catalyzed Pot Life Time, hours HDI Trimer LTC Crosslinker (BuOH) LTC Crosslinker (MeOH) REAL POTETIAL I PIGMETED DIRECT-TO-METAL APPLICATIOS End users in the industrial coatings market are looking for coating solutions that provide durable protection and an attractive appearance and meet environmental requirements, all at the lowest possible cost-in-use. As a result, interest in direct-to-metal (DTM) coatings as 7

8 alternatives to systems that require a primer and top coat is growing. The LTC crosslinker is anticipated to have numerous applications in these DTM coatings. To test the performance of the LTC crosslinker in DTM coatings, white (titanium dioxide, TiO 2 ) pigmented solventborne systems based on Acrylic E were prepared with both the LTC crosslinker and a polyisocyanate (biuret of HDI). The ambient cure pigmented formulations and their in-can properties are listed in Table 7, while their applied coating properties on Bonderite 1000 CRS are shown in Table 8. Table 7: Formulation Parameters for Pigmented DTM Coatings Isocyanate LTC Crosslinker Acrylic E/Crosslinker (solids basis) 70/30 76/24 Pigment/Binder ratio Catalyst, % on TRS 0.05 (DBTDL) 2.0 (p-tsa) n-butanol, % on TRS 0 15 % onvolatile VOC, gram/liter It should be noted that the stability of the formulation using the LTC crosslinker was better than that of the formulation prepared with the polyisocyanate resin. For example, the #4 Ford cup viscosity for the LTC crosslinker formulation increased from 48 to 223 seconds in 4 hours, while the polyisocyanate formulation has a #4 Ford cup viscosity of > 500 seconds after just 1 hour. However, the performance of the applied coating cured with the LTC resin at ambient temperature was very poor compared to the coating cured with the polyisocyanate with respect to gloss, hardness development, and chemical resistance. Table 8 Cure Response of LTC Crosslinker vs. Isocyanate in Pigmented DTM Coating Substrate: Bonderite 1000 CRS Dry film thickness: 40 microns Isocyanate LTC Crosslinker Gloss, 20 /60 82/93 64/67 Adhesion 5B 5B Pencil Hardness: MEK double rubs: 1 day H 5B 2 days H - 2H B 7 days 2H B 1 day days days Much more promising were the results for the LTC crosslinker formulation when cured at 80 C for 30 minutes. In this case, good film hardness and solvent resistance were observed, while the gloss was still lower than desired. This problem was addressed with the addition of a blocking amine (pyridine), which improved both the gloss and the catalyzed pot life without impacting the cure response (Table 9). 8

9 Table 9: Impact of Heat and Blocking Amine on Cure Response and Gloss of LTC Crosslinker in DTM Coating Substrate: Bonderite 1000 CRS Cure Schedule: 80 C for 30 minutes Film thickness: 40 microns Free ptsa Blocked ptsa Gloss, 20 /60 58/82 76/89 Pencil hardness F H MEK double rubs COCLUSIOS AD FUTURE WORK The coatings market is dynamic, with varying needs across different industry sectors, end use applications, and even regionally. An over-arching theme, however, is the need to reduce the use of hazardous materials and consequently reduce the environmental impact and increase worker safety with respect to coatings manufacture and application. At the same time, any LTC raw materials that offer these advantages must also provide a very high level of performance and achieve all of these attributes at a reasonable cost-in-use. Allnex is committed to developing LTC technologies that meet these criteria. One of its latest product developments addresses the need in the coatings industry to reduce the formaldehyde and isocyanate content in coating formulations. The LTC crosslinker technology presented here demonstrates that LTC solutions are possible for the future. The novel melamine-formaldehyde crosslinking resin can be formulated into solvent-based, very low free-formaldehyde, isocyanate-free ambient and low energy cure coatings with the potential to meet expectations for high performance and appearance in a number of clear coating applications. The LTC crosslinker provides excellent performance in clear coats for industrial wood and pre-coated metal with respect to such properties as dry time, early sanding, print resistance, hardness development, and chemical resistance. In addition, initial results in automotive refinish and direct-to-metal clear coatings show promise, and work is ongoing to address the identified performance gaps. With the LTC isocyanate-free, low-free formaldehyde crosslinking resin for ambient and low-bake applications, worker and consumer safety issues associated with exposure to formaldehyde and isocyanates have the potential to be eliminated while still providing the expected level of coating performance. The development of this LTC technology is just one example of the ongoing innovation that the coatings industry is recognized for. The LTC system also compliments other recent work on low-temperature cure crosslinkers and water-based UV-cure resin technology at Allnex. We will continue to bring such solutions that address the needs of formulators and end users for improved coating performance in terms of environmental, safety, durability and appearance properties. otice: Trademarks indicated with the, or * are registered, unregistered or pending trademarks of Allnex Belgium SA and its direct and indirect affiliates. Disclaimer: Allnex Belgium SA in its own name and on behalf of its directly or indirectly affiliated companies (collectively, "Allnex") decline any liability with respect to the use made by anyone of the information contained herein. The information contained herein represents Allnex's best knowledge thereon without constituting any express or implied guarantee or warranty of any kind (including, but not limited to, regarding the accuracy, the completeness or relevance of the data set out herein). othing contained herein shall be construed as conferring any license or right under any patent or other intellectual property rights of Allnex or of any third party. The information relating to the products is given for information purposes only. o guarantee or warranty is provided that the product and/or information is adapted for any specific use, performance or result and 9

10 that product and/or information do not infringe any Allnex and/or third party intellectual property rights. The user should perform its own tests to determine the suitability for a particular purpose. The final choice of use of a product and/or information as well as the investigation of any possible violation of intellectual property rights of Allnex and/or third parties remains the sole responsibility of the user Allnex. All Rights Reserved. 10