How to Choose PU crosslinker?
Crosslinking Polyurethane Dispersions | -09-06 - PCI Magazine
Polyurethane dispersions (PUDs) consist of polyurethane particles dispersed in water with the aid of an organic solvent or a dispersing agent. These dispersions have good film formation and provide superior appearance and resistance properties.1 They are also free of isocyanate residues and can be formulated at low VOC levels, both of which make them safer to use than their solvent-based counterparts. While these advantages are desired by formulators, the usage of PUDs has been limited by their cost, which is sometimes higher than conventional acrylic emulsions. To offset the cost, PUDs can be combined with acrylic emulsions through a physical blend2 or a chemical reaction, resulting in a PUD-acrylic hybrid.3 In this study, we have evaluated the performance of a PUD-acrylic hybrid, Joncryl® HYB , as a high-performance wood coating using added crosslinkers to improve the crosslink density in the film. The objective of the study is to evaluate how crosslinkers affect properties and which crosslinker would provide the best performance in a PUD-based wood coating. In addition, the mechanism of crosslinking will be investigated.4
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Crosslinking is the process of chemically joining two or more polymer chains by covalent bonds and is usually initiated by heat, pressure or change in pH. Increasing the crosslink density of a coating forms an impenetrable barrier that prevents water and chemicals from reaching the substrate. This results in improved chemical resistance. Crosslinking also augments adhesion and scratch resistance, thereby providing a tougher coating with overall better performance properties.5 Crosslinking is usually done by using external crosslinkers such as isocyanates, aziridines, melamines, polycarbodiimide (PCDI), etc. Table 1 shows the different functional groups on a polymer backbone that could react with a crosslinker.
Functional groups like amines or alcohols are nucleophilic6 and react readily with crosslinkers.7 Carboxylic acids are not as nucleophilic and react much slower.8 The Joncryl HYB is not OH or amine functional. We are therefore relying upon two different mechanisms for the resin to react with external crosslinkers, shown in Figure 1.
Since Joncryl HYB is not OH functional, the crosslinker may not react with it at all. Instead, there is a reaction between the crosslinker and water to form an interpenetrating network.9 This network of crosslinker formed within the framework of the PUD increases the crosslink density and thereby improves performance. In the second potential mechanism, the crosslinker could react with the carboxylic acid functionality on the PUD to form a crosslinked system. Either of the mechanisms is possible depending upon the crosslinker used.
There are certain factors that must be considered while choosing a crosslinking agent. Cost is the main driver, but other features like ease and safety of use, performance properties expected and pot life requirements also affect the choice of the crosslinker. In our study we have evaluated four different crosslinkers, as shown in Table 2.
Testing Procedure
A clear formula was prepared for Joncryl HYB and the crosslinkers added to the formulated material at 3% and 6% based on total formula weight under agitation. Ten different combinations of PUD and crosslinker, F1-F10, were evaluated, as depicted in Table 3.
The blends of PUD and crosslinker were used immediately after mixing and cured according to the timelines in Table 4. Formulations were drawn down on Leneta 2A cards for assessing gloss, on aluminum for hardness, and on wood for chemical resistance and adhesion. The dry film thickness (DFT) for a single coat was 1.6-1.8 mils. Two coats were applied on wood with the total DFT varying from 3.2-3.6 mils.
Results
Gloss and Hardness
Gloss for all the PUD-crosslinker blends was evaluated on Leneta 2A cards, while hardness was tested on aluminum panels using a König Pendulum Hardness tester. Results for gloss are shown in Table 5, while Figure 2 represents the hardness at day 1 and after baking at 50 °C overnight.
From Table 5, it is evident that Joncryl HYB is a high-gloss resin. Addition of the crosslinkers did not negatively impact the gloss of the coatings. In fact, a slight increase in gloss was observed in all the systems when compared to F10. Figure 2 demonstrates that hardness of the crosslinked coatings was slightly improved in systems with isocyanate or PCDI, but since Joncryl HYB is a PUD-acrylic hybrid, it has good final hardness even without using a crosslinker.
Adhesion to Wood
Adhesion was tested on maple wood following ASTM D, Method A.10 Joncryl HYB has good wet and dry adhesion to wood, and the crosslinkers did not adversely impact this property.
Stain and Chemical Resistance
Both stain and chemical resistance were tested according to ASTM D- where the coating was exposed to the chemical for a specified amount of time (1 hour if not noted). After removing the chemical, its effect on the coating was observed and the resistance was rated on a scale of 0 to 5, where 0 indicated failure and 5 indicated no effect. Stain resistance is shown in Table 6, while Table 7 has data on chemical resistance.
Stain resistance of the resin as represented by F10 is good, but addition of isocyanate and PCDI in formulations F1-F5 helps to reduce degradation by stain agents, while the melamine crosslinker in F8 and F9 did not help with stain resistance. A similar trend was observed for chemical resistance where formulations F1-F5 have significantly better chemical resistance than F10. Aziridine and melamine crosslinkers did improve the resistance to chemicals but not as much as the isocyanate and PCDI.
MEK Double Rubs
Solvent resistance of the coatings was analyzed using MEK double rubs according to ASTM D- where a cheesecloth soaked in MEK was rubbed on a coated Leneta 2A card using a 1-kg weighted hammer. The number of double rubs was measured until the coating degraded. The results for the MEK double rubs are shown in Figure 3.
As seen in the graph, F10 has 20 MEK rubs but in formulations F1-F4, the number of rubs is nearly doubled when an isocyanate or PCDI (at 6%) is used as a crosslinker. The addition of the melamine formaldehyde crosslinker has the opposite effect where the number of double rubs is drastically reduced. This could be because of poor crosslinking, which affects the integrity of the film.
Impact Resistance and Flexibility
Addition of crosslinkers has been known to make coatings more brittle and susceptible to cracking. We therefore evaluated the impact resistance and flexibility of the crosslinked systems in our study. Impact resistance was measured for direct and reverse impacts, and the data is depicted in Table 8. For most of the crosslinkers, the increased crosslink density does not affect the impact resistance except with the melamine formaldehyde in F8 and F9, which makes the coating brittle and reduces the resistance for both direct and reverse impacts.
Flexibility was evaluated using a ¼&#; conical mandrel bend tester following ASTM D522.13 All the crosslinked systems passed the test for flexibility with no cracking observed for any of the formulations F1-F10, as seen in Figure 4. Joncryl HYB has excellent flexibility, and crosslinking does not affect the flexibility of the resin.
Taber Abrasion
Taber abrasion was measured using CS-10 stones on a Taber abrader. The amount of coating lost was measured every 250 cycles for 1,000 cycles and is presented in Figure 5. Joncryl HYB in F10 has good abrasion resistance, losing only 26 mg after 1,000 cycles. Amongst all the crosslinkers tested, addition of aziridine in F6 and F7 improves the resistance significantly. Abrasion resistance is important for applications like floor coatings, which are subjected to a lot of traffic, and this data is crucial to choose the crosslinker with the best performance properties.
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Mechanism and Properties of the Crosslinkers
To try to understand the mechanism of crosslinking, IR analysis was performed using a Thermo Scientific Nicolet iS50R FT-IR. Figure 6 shows the IR spectrum of the Joncryl HYB and the main functional groups seen in the spectrum.
The two main peaks in the spectrum are 1) a broad peak at 3,349 cm-1 that indicates an OH group in the resin and water from the formulation; 2) the carbonyl group that is the peak at 1,723 cm-1 and represents the carboxylic acid functionality in the polymer backbone.14 To understand the crosslinking mechanism, these peaks were compared to those in the crosslinked systems. Figure 7 shows the IR spectrum for the resin crosslinked with isocyanate as well as the proposed mechanism for the crosslinking.
In the IR spectrum for the system crosslinked with isocyanate, the peak at 2,100 cm-1 is absent; implying complete consumption of the isocyanate. A new carbonyl peak at 1,690 cm-1 is formed, which could represent polyurea. The carbonyl peak from the PUD is unchanged, signifying that this group was not involved in the reaction with the isocyanate. All these factors lead us to propose the mechanism shown in Figure 7, where the isocyanate reacts with water to produce polyurea, forming an interpenetrating network within the framework of the PUD.
When PCDI is used as the crosslinker, there is a shift in the OH peak of the resin. This indicates that the COOH groups on the resin have reacted with the PCDI to form a polyurea. Since the carbonyl group remains mostly unchanged, we concluded that the polyurea peaks overlap with that of the PUD C=O group. The IR spectrum and the proposed mechanism of the reaction with PCDI are shown in Figure 8.
A mechanism similar to that with the PCDI was proposed for both the aziridine and melamine formaldehyde crosslinkers based on the shifted OH peak observed in IR. Figures 9 and 10 represent the IRs and the projected mechanisms.
Conclusion
Four different crosslinkers were evaluated along with a PUD-acrylic hybrid to determine their performance as a crosslinker in a wood coating. Several properties were studied, and IR analysis was done to determine the crosslinking mechanism. The results for the application testing can be summarized in Figure 11. The isocyanate and PCDI provided the most significant improvement in chemical and stain resistance without affecting the flexibility of the resin. While isocyanates can be skin sensitizers, PCDI is much safer to use. PCDI as a crosslinker has a longer pot life when compared to that of the isocyanate, however, PCDI needs a higher temperature for complete cure. Excellent abrasion resistance can be achieved by aziridine, though these tend to yellow more rapidly under artificial weathering. Melamine formaldehyde is the cheapest, but in our system was a poor crosslinker and in some cases negatively affected the performance properties of the resin. It is therefore important to evaluate different crosslinkers and understand their benefits and drawbacks in a given system to enable the formulation of a high-performance wood coating.
References
1 Noble, K-L. Waterborne Polyurethanes, Progress in Organic Coatings 32, , 131-136, doi: 10./S-(97)-4.
2 Petschke, G.; Yang, S. Urethane-Acrylic Hybrid Polymer Dispersion, Patent US B1.
3 Hirosea, M.; Zhoua, J.; Nagaib, K. The Structure and Properties of Acrylic-Polyurethane Hybrid Emulsions, Progress in Organic Coatings 38, , 27-34, doi: 10./S-(99)-8.
4 Harmsen, A.S. et.al. European Coatings Journal, , 14.
5 Making a Difference with Crosslinking Technologies, Coatings Tech https://www.paint.org/article/making-difference-crosslinking-technologies/
6 Henderson, W.A.; Schultz, C.J. Journal of Organic Chemistry , 27, .
7 Vandenabeele-Trambouze, O.; Mion, L.; Garrelly, L.; Commeyras, A. Advances in Environmental Research , 6, 45.
8 Bender, M.L. Chemical Reviews , 60, 53.
9 Lucas, H.R. Interpenetrating Polymer Network Compositions&#; USA.
10 ASTM D-17, Standard Test Methods for Rating Adhesion by Tape Test, ASTM International, West Conshohocken, PA, , DOI: 10./D-17, www.astm.org.
11 ASTM D-02, Standard Test Method for Effect of Household Chemicals on Clear and Pigmented Organic Finishes, ASTM International, West Conshohocken, PA, , DOI: 10./D-02, www.astm.org.
12 ASTM D-15, Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs, ASTM International, West Conshohocken, PA, , DOI: 10./D-15, www.astm.org.
13 ASTM D522 / D522M-17, Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings, ASTM International, West Conshohocken, PA, , DOI: 10./D_DM-17, www.astm.org.
14 Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. Spectrometric Identification of Organic Compounds. 4th ed. New York: John Wiley and Sons, . QD272.S6 S55.
High-Performance PU Crosslinkers for Two-Component Waterborne ...
Two-component solventborne polyurethanes are the benchmark standard for high-performance coatings in the majority of coatings markets. For the formulator the advantages include:
- wide formulation latitude due to the variety of co-reactants and crosslinkers available;
- high-quality appearance;
- UV stability and weatherability using aliphatic polyisocyanates;
- chemical and solvent resistance; and
- hardness, flexibility, and toughness due to the urethane and urea linkages.
One way to improve this situation was to use a polyisocyanate crosslinking agent. The first polyisocyanate crosslinkers were introduced in the early s to much skepticism. Then and today, the challenge in waterborne systems is to effectively disperse a reactive system in water and retain high-performance properties. Due to ongoing research and development, properties of 2K-waterborne PUR coatings have constantly been improving. So today&#;s waterborne PUR systems show equal, or sometime even better, overall performance than comparable solventborne systems.
Market/Performance Drivers
From its introduction the market drivers for two-component waterborne PU systems have undergone a transformation. Initially the drivers were achieving low VOC, low odor, easy application and clean up. The uniqueness of this chemistry and the ability to meet these demands resulted in Bayer MaterialScience being awarded an EPA Green Chemistry Award in . As the markets have matured higher demands have been placed on these systems and the challenges have been met by ever-improving systems. Most recently the dominant driver has been to match 2K-solventborne characteristics wherever possible. This has been seen most in areas requiring chemical and stain resistance, applications requiring fast drying characteristics, unique film properties like soft touch, and good adhesion to difficult substrates such as various plastics and wood.Resin Design - Co-Reactants and Crosslinkers
Extensive resin design has taken place to allow 2K-waterborne polyurethane systems to reach their current state of performance. Work had to be done on both the resin or Bayhydrol® polyol portion, as well as the Bayhydur® polyisocyanate crosslinkers. Either research thrust would make for interesting reading. However, the focus of this report is to discuss the chemistry and development of the polyisocyanate crosslinkers that have been introduced from the early s until today.A number of properties are important to coatings formulators. One of the important properties of coatings is their clarity and overall appearance. The importance of effectively dispersing an isocyanate crosslinker can be easily demonstrated simply by observing the appearance of two similar systems.
Dispersibility Importance
In this first example it can be seen that a standard hydrophobic polyisocyanate is not properly dispersed in the polyol dispersion when using a hand mix application. In the electron microscope picture on the right in Figure 1 the undispersed domains of polyisocyanate in the polyol matrix are plainly visible. On the left in Figure 1 an actual film is laid over the top half of the system label. The poor dispersion of the polyisocyanate is demonstrated by the opacity of the film.In the second system a hydrophilic polyisocyanate is hand mixed with the same polyol dispersion as the first example. In this system, the electron microscope picture shows a uniform surface and even dispersion in the film (Figure 2). Again, on the left an actual film is laid over the system label. In contrast to the first example, this film has high clarity and good appearance due to the excellent dispersibility of the hydrophilic polyisocyanate chosen. Gloss readings as high as 95 are now routinely available. This clearly demonstrates the importance of a good dispersion on coating properties.
Water-Dispersible Development
The development of water-dispersible polyisocyanates can take a couple of different conceptual approaches. One of the first is whether an internal or an external emulsifying agent should be used. A wide range of surfactants are available commercially. However, at Bayer MaterialScience the decision was made very early on to focus on internal emulsifying agents. It was felt that using external surfactants could lead to problems such as blistering, decreased water resistance and blushing. This is mainly due to the ability of an external surfactant to migrate through the coating to the surface.Generation 1 Products
Since an internal surfactant is reacted into the backbone of the polyisocyanate resin it does not have the ability to migrate through the coating. Conceptually, either an ionic or non-ionic emulsifying agent could be used. The initial development work was focused on modifying HDI polyisocyanates with monofunctional hydrophilic polyethers as the emulsifying agent. These were incorporated into the standard hydrophobic polyisocyanate crosslinker through a urethane linkage. This was the first generation of hydrophilic products (Figure 3). The generic structure shown is based on an HDI isocyanurate trimer.Generation 1 products based on HDI have a good overall blend of properties. They are relatively easy to disperse and they form stable emulsions. They have good reactivity and can be used in a wide range of formulations. By varying the MW and amount of polyether it is possible to achieve higher chemical resistance at the expense of ease of dispersibility. It is also possible to use lower viscosity starting polyisocyanates to get a lower-viscosity, water-dispersible crosslinker. This improves dispersibility, but can also lead to a somewhat lower functionality. Another feature of this product line is the ability to tailor the products for adhesive applications, having higher functionality and less water sensitivity. Even today the majority of development effort has been done with HDI-based products.
Finally, the Generation 1 products have been produced using IPDI-based polyisocyanates. This crosslinker is a high Tg material that can be used in automotive refinish applications, for instance, to yield fast-drying coatings having high hardness. This type of crosslinker is supplied in solvent to keep it liquid and has a lower reactivity than the HDI-based materials.
Generation 2 Products
Generation 2 products were developed with an eye toward making a step change increase in the overall properties of the final coating. The monofunctional hydrophilic polyether used to modify the starting polyisocyanate is incorporated into the water-dispersible crosslinker through an allophanate linkage instead of a urethane (Figure 4).This allows the use of less polyether, while obtaining a higher level of dispersibility. Correspondingly, this reduces the water sensitivity of the final film because there is less polyether incorporated. Finally, the crosslinker has a higher functionality resulting in better chemical resistance and hardness, with faster property development.
Generation 3 Products
In the most recent developments, a new approach has been taken. Instead of using non-ionic internal emulsifying agents, the use of ionic emulsifiers has been pioneered. These can be reacted into the resin backbone similar to the approach taken with the Generation 1 and 2 products. Using a unique sulfonic acid these are reacted in using a urea linkage. The urea linkage provides additional hydrogen bonding contributing to the overall properties of the system. This unique combination results in improved dispersibility combined with higher hardness and comparable, or even improved, chemical resistance relative to the Generation 2 products. In addition to a higher NCO content, these products also give lower water sensitivity relative to the non-ionic emulsified crosslinkers. Similar to a polyurethane dispersion, the neutralization amine shown in Figure 5 evaporates, leaving a lower residual hydrophilicity in the final coating. Therefore, long-term water sensitivity of Generation 3 products is reduced relative to Generation 1 and 2 products.A final area of development has been the use of low-viscosity HDI polyisocyanates as blending partners for water-dispersible and hydrophobic systems. By reducing the viscosity of the final crosslinker mixture, the ease of dispersing any system is improved. This is true whether you need to hand mix your ingredients or whether you are mixing a hydrophobic crosslinker into a waterborne system to achieve the highest level of performance available.
Formulation Considerations
The generic formulation found in Figure 6 has wide utility. By adjusting the ingredients it can be tailored to meet a wide variety of performance targets. The main ingredients are the polyol and the polyisocyanate crosslinker. The greatest ingredients by weight are the polyol emulsions. These come in a wide variety of chemical compositions. By careful selection, a formulator can vary hardness, chemical and corrosion resistance, film forming temperature, ease of pigmentation, weatherability, feel, VOC, and cost.The second ingredient by weight is the polyisocyanate crosslinker. This is the enabling technology that allows the formulation of 2K WB PU systems. Careful selection of the crosslinker has a strong effect on ease of dispersibility, property development, chemical resistance, water sensitivity, cost, potlife and VOC.
Of equal importance to the selection of resin systems is the choice of various additives and formulation variables. These include, but aren&#;t limited to, index ratio, catalysts, flow aids, thickeners, defoamers, etc. For instance, the isocyanate index ratio not only has an impact on cost, but it also has a dramatic influence on such properties as chemical resistance, dry time, potlife and property development. Figure 7 contains an overview of the polyisocyanates.
Application Examples/Performance Advantages
A wide variety of formulating latitude is available to a formulator when working with 2K-waterborne polyurethane coatings. This allows tailoring the formulations to meet vastly different target markets and performance criteria.As discussed at the beginning, one of the current market drivers is getting waterborne systems to meet the performance standards set by 2K PU solventborne coatings. Some of the recent areas where 2K WB PU coatings are having success and their performance benefits are shown in Figure 8.
Conclusions
It is clear that we have come a long way from the early skepticism that this technology was greeted with when it was first introduced. There has been a great deal of developmental effort put forth into optimizing both the co-reactants and polyisocyanates necessary to make this technology feasible and allow it to reach the high standards of performance that we have come to expect from polyurethane coatings.In this work, a quick overview of WB polyisocyanate crosslinker development and how each generation has furthered the performance window of 2K WB polyurethane coatings has been presented. Obviously this is complimented by the ongoing development of waterborne coreactants. However, the enabling technology for this area is the performance of the variety of hydrophilic polyisocyanate crosslinkers available. There is a wide range of formulating expertise that allows the tailoring of properties for a multitude of applications for coatings, adhesives and sealants. As we expand upon this, it will allow us to continue to meet ever more stringent requirements from both legislation regarding VOC and HAPS, and market needs for improved performance, user friendliness and &#;green&#; characteristics.
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