Waterbased Coatings with Improved Resistance Properties Robert Harrer, Anton Arzt, Markus Schafheutle, Cytec, Austria Bud Equi, Cytec, USA Introduction Interest in sustainable paint and coatings formulations, driven by both growing consumer demand for more environmentally friendly products and increasingly strict regulations, is driving the switch from solvent-based to waterborne products. For many applications, however, performance of waterborne coatings falls short of expectations. Cytec has followed a systematic design of experiment methodology approach to identify critical parameters for improving key properties of polyurethane dispersions. Using this carefully designed strategy, initial resins with improved durability in terms of hardness and chemical resistance have been produced. These early results confirm that high performing waterborne coatings can be developed for even the most challenging applications. Shift to Waterborne Technologies Throughout the world, formulators are being challenged to move to lower VOC coating systems. Regulatory compliance requirements and consumer desire for lower odor, more environmentally friendly products are the drivers for this shift. Even with such strong pressures, the replacement of solvent-based products, which have been the standard for decades, introduces perceived market risk for formulators and end users. European car manufacturers are already extensively using waterborne coatings. There is pressure to expand the use of water-based coatings for plastics used in electronics as well as wood and concrete flooring. Low odor systems are increasingly in demand for wood coatings in homes and schools. Concrete surfaces pose their own set of challenges. The variation in surface preparation, cure temperatures and performance expectations in garage/warehouse applications combine to make development efforts very difficult. At the same time, end users expect equivalent performance to traditional solventborne systems and are not satisfied with slow hardness development or a loss of durability in a lower VOC coating. Plastics Pose Ever Changing Challenges Plastic substrates present yet another set of challenges as formulators strive to accommodate the needs of the constantly changing collection of plastic materials. The wetting properties of solvent based, higher VOC coatings enable strong adhesion to plastic surfaces. In addition to excellent adhesion, coatings on plastics must also provide excellent initial appearance, wear or scratch resistance and resistance to household and commercial chemicals, preferably without any pretreatment or priming. Early water based coatings provided many of the target properties, but were inevitably lacking the full slate of performance characteristics associated with standard solventborne formulations. Newer systems have continued to improve, but formulators are still striving to deliver the total package of benefits. One of the most challenging combinations, for example, is to deliver early hardness development and resistance properties in a low VOC system. Resin Design Strategy It is apparent that one resin technology cannot be expected to provide all of the properties desired for low VOC coatings. Surface wetting and adhesion are often the strength of one technology, while hardness development and resistance properties are the benefits delivered by another. Therefore, the best strategy for meeting all of these requirements is to develop a resin combining the best characteristics of different technologies. Recognizing this reality, we are pursuing a multi-technology, modular strategy in our product design work. With access to numerous technology platforms, Cytec Industries has the luxury of being able to integrate the most attractive properties of acrylic, epoxy, phenolic, polyurethane
dispersion (PUD) and alkyd technologies to develop new resins and resin blends. Additionally, we have the ability to fine tune the performance of these new coating systems using a selection of crosslinkers and additives. Initial Goals We elected to develop a two component PUD for use in plastic coatings that would meet the performance requirements of a broad range of end-use markets including automotive OEM, ACE, general industry and teletronics. The ideal waterborne resin would be completely solvent free and provide excellent direct adhesion on various substrates, very high water and chemical resistance, high hardness, high gloss and outdoor durability. It was also determined that the production process used to manufacture the new resin should be as energy efficient as possible and result in minimal side or waste products, thus also minimizing CO 2 emissions to the environment. This approach is based on the premise that real green products can only come from green processes. Evaluating Parameters Using Multidimensional DOE We determined that the use of a multidimensional design of experiment (DOE) approach would be most effective in developing a new resin with a multitude of specific characteristics. Therefore, this methodology was employed for the product design and development of our new 2K high performance PUD. In a polyurethane resin, hardness is affected by the choice of the macro-diol and the ratio of hard to soft segments, which is also influenced by monomer composition. Hardness in turn is the key property that determines the toughness of the film and protects the substrate from mechanical attack. Molecular weight also affects the toughness of the film and contributes to chemical resistance. Molecular weight distribution affects the viscosity of the polymer melt, which drives the leveling performance of the coating. Molecular weight is determined by the stoichiometry of the resin monomers, while process conditions control the molecular weight distribution. Figure 1. Multidimensional DOE approach for developing new 2K PUD with multiple desirable performance properties Within the resin there are reactive groups that can be activated to allow polymer chains to be interconnected or crosslinked. The number and type of reactive groups are determined by the choice of monomers. Crosslinking agents are selected based on this reactivity. Coating properties influenced by crosslinking in a binder include chemical resistance, mechanical performance, flow (viscosity) and leveling. Generally, the higher the degree of crosslinking, the higher the hardness and the higher the chemical resistance. The choice of crosslinking agent also affects the hardness and hydrophobicity of the coating.
Polymer morphology, or the physical structure of the resin, is determined by monomer composition. This characteristic can affect the viscosity and rheological behavior of the polymer melt and also plays some role in determining chemical resistance. With blending of resins, it is possible to bring together various performance characteristics in a synergistic way. There is also the potential to have destructive interactions between different classes of resins, so selection of co-resins must be done very carefully. The creation of a 6-dimensional synthesis plan (Figure 1) and the early identification of key parameters helped us to find structure-property relationships leading to development of a product fulfilling the above mentioned requirements. What we hoped to determine was if these correlations, which are well understood in solventborne systems, are also applicable to waterbased formulations. In addition, we hoped to identify the most important interactions and discover how significant their influence on performance can be. Another aspect of this strategy was to identify process conditions that would minimize environmental impact. Throughout the development program, production process issues were always considered as different resins were evaluated and tested. These efforts have yielded an efficient and environmentally friendly process. Specifically, the heat generated in the exothermic reaction of the isocyanate with the functionality of the polymer component is channeled into the reaction, reducing the need for an external energy source and shortening the reaction time. In addition, unlike common polyurethane production methods that utilize acetone, our process is solvent free. By eliminating the use of a solvent, we do not consume energy to strip away and then further purify or dispose of that solvent. The overall result is significantly reduced energy consumption and thus minimal CO 2 production. Initial Results and Revisions At the outset of the project we understood that it might not be possible to develop a single resin that could provide all of the desirable attributes for a coating system with applications in a broad range of different end markets. Therefore, we not only investigated the performance of our newly developed resins alone, but also with blends of other binders such as acrylics and polyesters. We evaluated performance of the various systems to determine water and chemical resistance, hardness (development), water uptake, hydrolysis resistance, sun cream resistance, mechanical properties, curing speed, gloss, adhesion, UV resistance and other characteristics. According to our DOE strategy, we first synthesized a series of resins with varied properties matching each of the six dimensions of our DOE in order to determine the most relevant structure/ property relationships. After evaluation of the first series of lab batches, we were able to identify key parameters such as raw material composition, molecular weight and others. With this information we then were able to fine tune our initial DOE in order to develop further specific lab batches that fullfilled the earlier mentioned requirements in terms of adhesion, resistance properties, etc. Successful Waterborne PUD Our most successful developmental PUD to date is prepared in a solvent free process without using acetone as a processing aid. The developmental binder has a relatively low molecular weight, but high branching, which leads to a low particle size distribution. Table 1 shows the basic properties of our development resin Experimental PUD-1.
Table 1: Basic properties of Experimental PUD-1 Physical Property Non volatile matter DIN EN ISO 3251 (125 C, 1 g, 1 h) Dynamic viscosity DIN EN ISO 3219 (23 C, 25 /sec) ph-value DIN ISO 976 (10%) Hydroxyl value DIN 53240 (solids) Approximate Values 41 % 500 mpa*s 7.5 8.5 170 mg KOH/ g Solvent content 0 % For various application tests, Experimental PUD-1 was formulated both as a clearcoat and in pigmented systems in order to test its applicability for primer, basecoat or clearcoat/ topcoat systems. Curing was accomplished using a hexamethylene-dipolyisocyanate (HDI) grade polyisocyanate. With this type of curing agent, it was possible to achieve monocoats with an excellent degree of gloss, hardness and chemical resistance. Table 2: Formulation of Experimental PUD-1 for hardness and chemical resistance testing Ingredients Experimental PUD-1 250.00 Wetting agent 0.50 Defoamer 0.50 Deionized water 50.00 Parts HDI grade polyisocyanate 0-140% When used on its own and crosslinked with an HDI grade polyisocyanate, Experimental PUD-1 adheres to a wide variety of plastics. While it exhibits very high hardness, it nevertheless retains a desirable degree of flexibility. For example, when melamine cured on polyvinyl chloride (PVC), it can be bent at a 180 angle without cracking or los s of adhesion. Investigation of Crosslinking Because the degree of crosslinking can affect many key properties of a formulation, we first examined the degree of crosslinking (no crosslinking, under- and overcuring of the binder) on both hardness and chemical resistance of a coating prepared with Experimental PUD-1 (see Table 2 for coating formulation). The coating was cured for 30 minutes with an HDI grade polyisocyanate at 80 C and the pendulum hardness wa s measured after 1 day and 1 week at room temperature (Table 3). Not surprisingly, in the control coating with no crosslinking, hardness improved dramatically with the longer room temperature cure compared to the values measured after 24 hours. A small increase in hardness was also noted after the longer cure time at all levels of crosslinking. Most importantly, at both cure times, hardness was dramatically improved at a crosslinking rate of 60% with good hardness achieved at approximately 80-90% crosslinker level and excellent hardness at 100-110% (stoichiometric NCO:OH). For conventional OH functional waterborne resins, typically a 140% level of crosslinker is required to achieve excellent hardness performance. Experimental PUD-1 therefore offers a real cost advantage because significantly less of the expensive polyisocyanate crosslinking agent is required. Chemical resistance was evaluated using acetone solvent and measured directly after curing for 30 minutes with an HDI grade polyisocyanate at 80 C. The cured film was then exposed to acetone and every 30 seconds a scratch test was performed in order to determine if the film experienced any degradation. With no crosslinking, the coating formulated with Experimental PUD-1 was, not surprisingly, readily dissolved by acetone. As the degree of crosslinking increased, acetone resistance improved, with the greatest difference noted when going from 60%
to 80% crosslinking. After that, acetone resistance continued to increase, but at a slower rate. Again, one can see that with this new developmental resin, the formulator can easily achieve high performance properties even at lower degrees of crosslinking. Table 3. Influence of the rate of crosslinking in Experimental PUD-1* on hardness and chemical resistance Crosslinking (stoichiometric ratio) Pendulum hardness** 30 80 C + 1 day RT Pendulum hardness** 30 80 C + 1 week RT Acetone-resistance*** (30 80 C) 0 % 87 sec 134 sec 0 min 60 % 161 sec 178 sec 2 min 80 % 168 sec 178 sec 8 min 100 % 168 sec 181 sec 9 min 120 % 166 sec 179 sec 10 min 140 % 165 sec 177 sec >10 min *Formulation described in Table 2. **DIN EN ISO 1522. ***Lab norm Because crosslinking so dramatically affects critical coatings performance characteristics, and there is a recognized loss of polyisocyanate due to the side reaction with water, the use of excess ratios of crosslinker has become state of the art in the industry. Therefore, further lab tests were conducted at approximately 140-150% crosslinking. Additional studies to further evaluate the effects of different degrees of crosslinking on the performance of developmental Experimental PUD-1 are in progress. Adhesion Studies Adhesion is one of the most critical properties of a coating. As mentioned above, many end users would like to be able to purchase one water-based coating that will perform well on many different plastic substrates. The adhesion of Experimental PUD-1, formulated as described above, was tested on several common plastics was evaluated using a cross hatch test with tape pull off (Table 4). This is a short list simply to demonstrate the potential while recognizing performance will vary depending on the plastic producer and surface treatments. Table 4. Adhesion of Experimental PUD-1 to various plastics* Crosshatch Adhesion** Plastic Substrate GT 5 = Excellent adhesion GT 0 = Poor adhesion Acrylonitrile butadiene styrene (ABS) 5 Polycarbonate (PC) 5 Polymethyl methacrylate (PMMA) 0 Polypropylene (PP flamed) 5 Polyvinyl chloride (PVC-h) 5 Polystyrene (PS) 0 Polyphenylsulfone (PPSU) 5 Cured 140% with HDI grade polyisocyanate for 30 minutes @ 80 C followed by a post cure for 12 hours @ 70 C. ** DIN EN ISO 2409 Hardness and chemical resistance were investigated using the same formulation after curing at a 150% index level with an HDI grade polyisocyanate for 30 minutes at 80 C followed by a post cure for 12 hours at 70 C (Table 5). These conditio ns were chosen because they are considered to be state of the art in various industrial segments, and are particularly used by European automotive plastic coaters.
The hardness was similar to that obtained when using 60% crosslinking with a 1 day room temperature post cure, while acetone resistance was one and a half times that measured at the same degree of crosslinking with no post cure. No water uptake was observed, and adhesion was maintained on ABS before and after hydrolysis. On polycarbonate, however, while the resin showed good adhesion initially, it was lost after hydrolysis. The automotive OEM market has been seeking for many years a coating which will provide good adhesion to polycarbonate surfaces even after hydrolysis. While Experimental PUD-1 failed when formulated as the sole binder into a coating, we have identified a resin blend containing the developmental PUD that does pass the adhesion test after hydrolysis on both ABS and PC. Further evaluation of this resin blend is ongoing. Table 5. Hardness and water/chemical resistance of pure Experimental PUD-1 after curing Property Pendulum hardness (König)* Results 160 sec Water test (16 h, 40 C) No effect (no water uptake ) Hydrolysis test** ABS PC Solvent resistance (Acetone)*** 0/0 (adhesion before and after hydrolysis) 0/5 (loss in adhesion after hydrolysis) >15 minutes *DIN EN ISO 1522. **According to VW-Norm TL 226 (Abschn. 3.12.1, 72 h, 90 C, 95 % humidity). ***Lab norm UV-Resistance Evaluation In addition to targeting the automotive OEM market, our initial goals included the development of a binder for other sectors such as the ACE market. Coatings intended for outdoor applications must exhibit excellent UV-resistance and good leveling and gloss. Tests on Experimental PUD-1 formulated in a white pigmented coating without any added UV stabilizers revealed its excellent gloss stability and UV-resistance. In a standard UVCON-test, after 1500 hours of exposure, no change in gloss of the coating (81 measured at 20 angle) was measured, while yellowing of only b* = 0.38 was observed. These results suggest that this new developmental PUD may perform very well as the sole binder or in combination with waterborne acrylic or polyester resins for outdoor applications. Seeing is Believing Experimental PUD-1 can be formulated into very high gloss coatings with excellent image quality. Table 6: Guiding formulation for a black high gloss monocoat based on Experimental PUD-1 Ingredients Experimental PUD -1 36.00 Water based Polyester 12.00 Deionized water 24.00 Surface wetting additive 0.25 Leveling additive 0.25 Defoamer 0.70 Co-solvent 5.00 Water based black pigment dispersion 2.50 Hydrophilised HDI grade polyisocyanate 20.00 Parts Figure 2 shows an example of a high gloss black monocoat, where the XP-PUD has been formulated in a blend with a waterborne polyester resin.
Figure 2. High Gloss Black Monocoat Formulated with Experimental PUD-1 blended with a waterborne polyester resin Conclusion Using a structured experimental approach we have developed a unique new resin that has significantly improved properties for coating plastic substrates. The resin imparts high hardness yet retains a good level of flexibility. It adheres to a wide variety of plastics and has good chemical resistance. In addition, the process for manufacture of the resin has been designed to be green, and thus we have minimized its environmental footprint. Further experimentation and optimization efforts are underway to make improvements to its performance. Included is an investigation of 1K systems, hybrid PUDs, as well as blends with acrylic emulsions and polyester dispersions. Modification of these binders for use in other applications such as wood and concrete floor coatings is also planned. We are pioneering sustainable change with the development of low-voc coatings, resins and additives that allow our customers to create sustainable change for the industries they serve. Based on the initial results presented here, we believe we are on the right path to provide new waterborne resin systems to deliver a solution for the challenges faced by products currently on the market.