## Novel Super High Solids Polyurethane Topcoat for Protective Coatings with Increased Productivity

### Novel Super High Solids Poly urethane Topcoat for Protective Coatings with Increased Productivity
Dr. Malte Homann, Dr. Ralf Lemmerz, Thomas Bäker (Bayer AG, D-51368 Leverkusen)

Introduction Current developments for anti-corrosion coati ngs are driven both by the productivity of the coating and by demands for reduced environmental impact of such coatings. The productivity of an anti-corrosion coating is calculated as the co st of the corrosion protection system per square meter and y ear. Service life has thus become a determining factor in productivity calculations . In light of this, a three-coat coating system comprising primer, intermediate c oat and topcoat, each based on chemically crosslinking binders, has established itself as the standard solution for heavy-duty corrosion protection. Epoxy resins and – much less commonly – polyurethanes are used in the primer containing active pigm ent and the intermediate coat on top of it. The topcoat is almost always a polyure thane coating based on hydroxyfunctional acrylates and aliphatic polyisocyanates as these provide good corrosion protection and superior weather stability. Demands for reduced environmental impac t caused by organic solvents in the coatings are another driving force for resear ch activities. New developments in anticorrosion coatings are oriented primarily to ward restrictions on solvents already in place or restrictions that are expected in the medium te rm. A solvent content of 250g VOC/L or less (corresponds to a solids cont ent of 75 % by volume) is currently themedium-term objective within the EU for the reduction of solvent emissions from anticorrosion coatings. Near-zero emission c oatings are being pursued as a long-term goal. In principle, water-reducible coating system s and powder coatings make it possible to significantly reduce or completely elim inate solvent emissi ons. However, both approaches also bring with them some i nherent properties which complicate the application of the coatings on site. High solid coatings are formulated with very little solvent content One method for formulating low-solvent (high solid) coatings wh ich in practice has proved easier is the use of coating raw materials which require very little solvent. One of the greatest difficulti es associated with low-solv ent coating raw materials is the requirement that the pot life and drying speed of low- solvent coatings must at least equal those of the coat ings currently in use. The processes employed today in corrosion protection, beginning with the pr etreatment of the steel components and coating application, fo llowed in the case of new cons truction by intermediate storage, transport and installation, have been tailored to the pot life and drying speed of the coatings used. The applicatio n characteristics of the coatings often determine the rate of the overall process, which has been optimized for produc tivity. Disruption of these processes due to adverse changes to the pot life and drying time of the coatings results in significant cost incr eases, thus every advancement in the coatings sector is also intended to increase t he productivity of the overall process.Novel super high solid polyurethane t opcoat meets low solvent requirements and complies with established application processes This paper describes a super high soli d anti-corrosion topcoat based on newlydeveloped polyurethane raw materials and offering significant productivity advantages over the conventional two-co mponent polyurethane (2K PUR) topcoats currently used for corrosion protection. T he designation “super high solid” is intended to convey the fact that the solvent content is so low as to equal the solvent content of water-reducible coatings. The super high so lid coating described satisfies the most stringent requirements for corrosion resistanc e. Its application properties facilitate productivity-optimized processes in corro sion protection applications. With its excellent tolerance to high film thicknesses, it also offers economic advantages over the coatings in use today without sacrificing quality.
Capabilities and limits of today’s conventional topcoat binders The conventional two-component polyure thane anti-corrosion topcoats used today are based on hydroxyfunctional polyacryl ates that react with aliphatic polyisocyanates – generally HDI biurets or isocyanates – to form polyurethanes. The hydroxyfunctional polyacrylates have high molecula r weights, e.g. between 10,000 and 20,000 g/mol. In their pure polymerform, these polyacrylates are solids. The formulated coating can be adjusted to the required viscosity for application in airless spray equipment through the use of significant amounts of organic solvent. Typical 2K PUR anti-corrosion topcoats have a solids content between 50 – 65 % by volume.Figure 1: Application characteristics of 2K PUR topc oats based on hydroxyfunctional acrylates and aliphatic polyisocyanates

Hours

Vol.-% solids Status quo: 50 - 65 % vol.- solids Higher vol .- solids Pot life: > 3 h Sand dry: 1-2 h Slower drying Shorter pot life

An increase in the solids content of 2K PUR anti-corrosion topcoats based on hydroxyfunctional polyacrylates and alipha tic polyisocyanates is theoretically possible. 2K PUR topcoats with significant ly higher solids contents can be achieved by using viscosity-optimized hydroxyfuncti onal acrylates of low molecular weight. However, these are associated with severe degradation of the application characteristics of the coati ngs. Instead of a pot life of at least 3 hours and a sand-dry time of 1 to 2 hours to suit applicati on requirements, the pot life is decreased significantly and drying retarded. The pr ice for the ecological advantage of low solvent content is disadvantages in terms ofoverall process efficiency as a result of longer drying times. The lowest achievabl e VOC content in a 2K PUR topcoat that suits application requirements is approxim ately 250 g VOC/L. It should be possible to achieve the medium-term goal of reducing solvent emissions with hydroxyfunctional polyacrylates and aliphatic polyisocyanates. Further significant reductions of the solvent content require a fundamental c hange in the chemical approach to the binders.A novel chemical approach to binders with very low solvent requirements We therefore based our approac h to super high solid 2K PUR anti-corrosion coatings on a fundamentally new orient ation for the resin component s that are reacted with the low-viscosity aliphatic polyisocyanates. We did this by combining two different chemical approaches: - a low-viscosity, sterically hindered aliphatic diamine t hat has only moderate reactivity with isocyanates
- an aldimine reactive thinner to further reduce viscosity. Aliphatic polyamines have long been used as reaction partners for polyisocyanates in the formulation of weather-stable PUR coat ings. The reaction products of polyamines and polyisocyanates are polyureas and yield hi ghly resistant coat ings which exhibit pronounced elasticity with high mechanical st rength. The reaction of a polyamine and polyisocyanate is significantly faster t han that of a polyol and polyisocyanate. It is slowed less at low temperatures than is the polyol/polyisocyanate reaction and at the same time is less sensitive to undesired blistering caused by the water/polyisocyanate reaction. However, until now no aliphatic polyamines were available whose reactivity with polyisocyanates was low enough for the formulation of coatings for manual application. The extremely high reactivityof commercially available aliphatic polyamines with polyisocyanates requir ed the use of multi-component spray equipment with very effective mixing devices. This significantly reduces the versatility of the coatings. Aliphatic polyamines with reduced r eactivity ensure a reasonable potlifeOnly after aliphatic polyamines were modifi ed by the addition of st erically demanding side chains on the aminofunctional groups was it possible to reduce reactivity sufficiently to formulate a coating with a pot life comparable to that of conventional 2K PUR anti-corrosion coatings. Figure 2: Aspartic acid modification

O O

O O R'
R' NH 2 R NH 2 O O

O O R'
R' O O

O O R'
R' N H R N H

di-aspartic acid ester + 2

R‘ = short chain alkyl R = HMDA derivative
The side chains are inserted via the Mi chael addition of amine nitrogen atoms to maleic acid diester to create a structure similar to aspartic acid. Polyamine A is an example of such a sterically modified al iphatic polyamine. Vi scosity is 150 mPa s (23 °C, 90 % in butyl acetate). The ster ic hindrance reduces the reactivity of polyamine A such that the viscosity increas es only slowly after mixing with aliphatic polyisocyanates. After curing, the films are hard, weather-stable and resistant to abrasion. Figure 3: Reaction of aspartic acid ester and isocyanate

O O

O O N Et
Et R + R' N C O R' O O

O O Et
Et N C N H R

O H

aspartic acid ester isocyanate compound urea structure A peculiarity of aliphatic polyamines modified with aspartic acid ester is the fact that water accelerates their reaction with aliphati c isocyanates. This means that the mixedcoating remains easy to apply for a long ti me but quickly hardens once applied to the substrate and exposed to humidity.
Reactive thinners are low-viscosity coreactants. Like solvents, they serve to reduce viscosity but unlike solvents are incorporat ed into the binder matrix during curing. Polyaldimines are extremely well-suited as reactive th inners because of their low viscosity and their reactivity with polyisocyanates. Figure 4: Aldimine formation

NH 2 R NH 2 N R N R' H H R' +

diamine R = cyclohexane derivative - 2 H 2 O

aldimine O R' H 2
aldehyde R' = alkyl Polyaldimines are formed from the reaction of aldehydes and polyamines. Polyaldimine B is the reaction product of an aliphatic diamine and short-chain aldehyde. Its viscosity is only 25 mPa s (23 °C ). In contrast to polyisocyanates, the reaction of polyaldimine B prim arily results directly in the formation of an unsaturated urea (en-urea). This reac tion can be considered a cycloaddition reaction between aldimine and isocyanate.
The reaction is much faster than the undesired hydrolytic reverse cleavage of polyaldimine B, which produces polyamine and aldehyde. It also prevents the release of aldehydes when the coating is cured in contact with atmospheric moisture, thus avoiding unpleasant odors. Instead, the low solvent content means that the resulting coating is atypically odor-free for a coating.Figure 5: Reaction of aldimine and isocyanate to produce en-urea

H C R'' R''' H N R' N C R
O N H O N R'

H R R'' R''' unsaturated urea electrocyclic reaction

Formulation of the super high solid coating Combining polyamine A modified with aspar tic acid ester and polyaldimine reactive thinner B with low-viscosity polyisocyanat e C (HDI isocyanate) yields an anticorrosion coating with a solids content of 93 % by volume, which corresponds to a VOC content of 90 g/L. The coating formulation is essentially as follows: Table 1: Coating formulation and composition A component Polyaspartic amine 21.85 binder Polyaldimine reactive thinner 6.21 binder Zeolite paste, 50 % in castor oil 5.27 dewatering agent Multiple 45.34 pigment, charge, additives B component Low-viscosity HDI isocyanurate 21.33 binder 100% Coating composition Crosslink ratio (NH/NCO) 1 : 1.1 Solvent content 7 % (90 g/L) Solids content ( calculated) 93 % Pigment volume concentration 23 %

Characteristics of the super high solid coating Novel super high solid coating is even superior to conventional 2K PUR topcoat The high gloss of the coatings is preserved if a hydroxyfunctional acrylate is replaced with an amine/aldimine combination. The high gloss values of e.g. 90 units (60°) typical for conventional 2K PUR topcoats can also be easily achieved in the novel super high solid coating. T he novel super high solid coating formulation is actuallysuperior to conventional 2K PUR topcoats with respect to gloss retention after weathering. Another benchmar k can be seen in Epoxisilo xane coatings which are marketed as being superior to standard polyu rethane coatings in their weathering performance and in terms of solvent content of the formulations. However, with the new super high solid polyurethane similar or even better weathering results are obtained which is shown here. Figure 6: Change in gloss after weathering of Epoxisiloxane topcoat and the super high solid polyurethane coating (test according to QUV A)

Application characteristics: long pot life, fastdrying The super high solid coating has a pot life of 3 hours (time until viscosity doubles). It is therefore equally suited for applicati on using airless spray equipment or manual application using a roller or brush. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 h 5 00 h 1

00 0h 1

50 0h 2

00 0h 2

50 0h 3

00 0h 3

50 0h 4

00 0h 4

50 0h 5

00 0h 5

50 0h 6

00 0h 6

50 0h 7

00 0h 7

50 0h 8

00 0h Time / hLoss of gloss / % Super High Solid EpoxisiloxanTable 2: Comparison of characteristics of the supe r high solid topcoat and a conventional 2K PUR anti-corrosion topcoat (Binder = polyisocyanate D, 75 % in solvent; NCO content = 16.5 %; and hydroxyfunctional polyacrylate E, 60 % insolvent, OH content = 1.6 %. Both are commercial binders for 2K PUR topcoats)

*: Time until viscosity doubl es under standard conditions (23 °C, 50 % rel. humidity) **: After storage for 7 days under standard conditions Super high solid coating Conventional 2K PUR topcoat Pot life* 3 h 5 h 15 min Sand dry time T1 (120 μm, dry) 70 min 90 min Through drying T6 (120 μm, dry) 3 h approx. 12 h Pendulum hardness (König)** 140 s. 115 s.

With respect to film drying, the super high solid coating is superior to the conventional 2K PUR topcoats used in co rrosion protection applications. The sanddry time (T1) is only 70 minutes under st andard conditions. T6 drying, i.e. the time until the applied topcoat is capable of withstanding moder ate mechanical loads, is only 3 hours and thus significant ly shorter than the corresponding drying times of the 2K PUR topcoats used today. Because of the accelerated T1 and T6 drying, steel components coated with the novel super high solid coati ng are sooner resistant to contamination of the paint film by dirt, e.g. due to blasting operations being performed nearby, and to trans portation damage. This means a significant increase in productivity for the coating applicator. This advantage over today's 2K PUR t opcoats becomes even more pronounced with decreasing temperatures, as has been dem onstrated and quantified on the basis of measurements with an ultrasonic film r heometer. This met hod of measurement determines the storage modulus of a paint film via the re flection of an ultrasonic wave from the interface between the coating film and the measuring cell. It is thus possible to track the increase in f ilm hardness during curing under realistic conditions. Thetransition from low-viscosity liquid after mixing of the component s to cured coating can be quantitatively documented usi ng the change in storage modulus.

Figure 7: Change in normalized storage moduli at various temperatures (Film thickness in each case approx. 200 μ m; Normalization = current value/final value, both in [Pa])

0.0 0.2 0.4 0.6 0.8 1.0

0 10 20 30 40 50 60 70 80 90 100 Time [ h]Normalizedstorage mo dulus 1 2 3
4

1: Super high solid coating, 23 °C, 50 % rel. humidity 2: 2K PUR standard coating, 23 °C, 50 % rel. humidity 3: Super high solid coating, 5 °C, 50 % rel. humidity 4: 2K PUR standard coating, 5 °C, 50 % rel. humidity Figure 7 shows the change in the normaliz ed storage modulus fo r the super high solid coating and a standard 2KPUR coating (hydroxyf unctional polyacrylate E and polyisocyanate D), both at 23 °C, 50 % re l. humidity and 5 °C, approx. 50 % rel. humidity. It is clear that curing of the super high solid coating (amine/isocyanate reaction) is slowed less by reducing t he temperature than is curing of the conventional 2K PUR t opcoat (alcohol/isocyanate reacti on). At 5 °C (Curve 2), the super high solid coating cures almost just as fast as the conventional coating at 23 ° C (Curve 3). This means that the produc tivity gained through the us e of the super high solid coating either outdoors or in partially unheated shops is particularly apparent duringthe winter. That work at t he job site can proceed faster or that stationary coating equipment can be used more effectively is a true cost advantage.
High chemical resistance, good over coatability, excellent mechanical properties
The high pendulum hardness of t he super high solid coating (cf. Table 2) reflects a higher density of crosslinker groups (urea structures) than in the standard 2K PUR topcoat (polyisocyanate D and hydroxyfunc tional polyacrylate E). These urea structures are capable of forming hydr ogen bridge bonds and thus contribute to a greater resistance to or ganic solvents and aqueous acids and bases. Nevertheless, the super high solid topcoat can be ov ercoated easily. In a peeling test, a panel weathered outdoors in an indu strial climate for 6 months before rinsing and overcoating exhibited adhesion of more than 11 Mpa between the old and new topcoat layers (xyz fracture). This suggests that it should be possible to refinish the super high solid coating even years later with no significant pretreatment of the surface to be recoated. Measurements yielded by a mandrel bend te st reveal that the super high solid coating is no less flexible despite it s greater pendulum hardne ss compared to the standard coating. The super high solid coat ing is also at least on par with the standard 2K PUR coating with respect to impact toughness (Impact Test), chipping and abrasion resistance (Taber Abraser, CS 10 roller).

Similar film thickness achievab le with two-coat systems Another substantial advantage of am ine-based PUR anti-corrosion coatings containing reactive thinner is the high film thickness in which the coatings can beapplied without blistering. For the formul ation described above, this is approx. 250 μ m when applied with airless spray equipm ent and significantly higher if applied by hand. Susceptibility to blistering is also significantly reduced compared to 2K PUR topcoats crosslinked in a conventional isocyanate/pol yol reaction. The desired crosslinking reaction in the isocyanate amine/aldimine system is faster than the isocyanate/polyol reaction and thus the undesired isocya nate/water reaction which can lead to blistering is more strongly suppressed.
Total film thicknesses of 220 μ m can be reliably obtained in two steps by combining a zinc-rich 1K PUR primer (film thickness 100 μ m) with a 120 μ m thick film of the super high solid topcoat. Such a two-coat syst em effectively protects steel against corrosion. This was demonstrated in a salt spray test (ISO 7523) on low-alloy structural steel (pretreated by blasting to Sa. 2 1/2), which the described coating system withstood for over 2,000 hours. It is thus possible to reduce the number of coating steps from three to two while yielding the same film thickness as achi eved with the conventional three-coat system comprising primer, intermediate and topcoat. If small amounts of the aldimine polyald imine B are added to the zinc-bearing 1K PUR primer prior to application, drying of the prim er is so strongly accelerated that it can be overcoated with the super high solid topcoat after only 1 hour. Because the topcoat dries within only 3 hours to t he point where it c an withstand moderate mechanical loads, steel components for new construction can be completely coated and transported (dry to handle) only 5 hours after pretreatm ent of the substrate. In contrast, a conventional coating system of epoxy resin primer and intermediate coat with a 2K PUR topcoat requires approxim ately 36 hours to achieve this state.In the case of refinishing operations, the fast overcoatability of the primer combined with the short sand-dry time s of the topcoat means that a surface can be completely coated and resistant to soiling resulting from work on adjacent surfaces only 3 hours after pretreatment of the substrate. The comparison with the conventional coating system described above, which requires appr oximately 25 hours to achieve the same state, is another impressive illustration of the producti vity advantages gained by the combination of an accelerated 1K PUR primer and a super high solid topcoat. Table 3: Productivity of super high solid coating on accelerated 1K PUR primer compared with conventional coating system. All times from completion of substrate pretreatment (assumes shop working one shift only)

5 h 36 h 25 h

3 h Time until the coated parts can be transported Time until the topcoat is resistant to soiling

Front: Super high solid topcoat, two-coat system; Rear: Conventional three-coat system Economic coating process Although the costs of raw materials for su ch a super high solid coating are higher than those for the conventi onal PUR topcoat crosslink ed in an isocyanate/polyol reaction, the elimination ofone coating step more than o ffsets the added cost of the coating raw materials. If one also consider s the increased productivity of the coating process due to faster drying, the cost balance is improved even further. Taking into consideration the entire co rrosion protection process, the novel super high solid coating achieves the high per formance of a conventional 2K PUR topcoat with more reliable application and at a lower overall cost. Thus, in terms of productivity, thenovel super high solid coating is in no wa y inferior to the conventional PUR topcoats used in corrosion protection applications. Table 4: Economic comparison of the technologies Economic Comparison
2-coat system 1K- PUR / 2K- PUR super high solid Conventional 3-coat system
Cost of raw materials higher lower Application cost lower higher Productivity higher lower Overall cost lower higher

Summary The use of low-viscosity, sterically-hinder ed aliphatic polyamines in combination with aldimine reactive thinners and low-viscosityaliphatic polyisocya nates permits the formulation of a super high solid anti-corro sion topcoat with low VOC values of the same order as water-reducible coatings. The coating is characterized by its high quality in terms of its ve ry good weather stability sim ilar to epoxisiloxane and low soiling.
The elimination of one coat for anti-co rrosion coatings per ISO 12944 C5 and the further increase in productivity due to r apid drying even at lo w temperatures more than offset the increased cost of the binder for the superhigh solid coating compared to the coating systems used today. The newly developed PUR raw materials for anticorrosion coatings thus unite the adv antages of reduced solvent emissions and greater productivity.The manner in which you use and the purpose to which you put and utilize our products, technical assistance and information (wh ether verbal, written or by way of production evaluations), including any suggested formulations and recommendations are beyond our control. Therefore, it is imperative that you test our products, technical assistance and information to determine to your own satisfaction whether they are suitabl e for your intended uses and applications. This application-specific analysis must at least include testing to determine suitability from a technical as well as health, safety, and environmental standpoint. Such testing has not necessarily been done by us. Unless we otherwise agree in writing, all products are sold strictly pursuant to the terms of our standard conditions of sale. All information and technical assistance is given without warranty or guarantee and is subject to change without notice. It is expressly understood and agreed that you assume and hereby expressly release us from all liabil ity, in tort, contract or otherwise, incurred in connection with the use of our products, technical assistance, and information. Any statement or recommendation not contained herein is unauthorized and shall not bind us. Nothing herein shall be construed as a recommendation to use any product in confl ict with patents covering any material or its use. No license is implied or in fact granted under the claims of any patent. Outlook In addition to polyamine A, a range of aspar tic acid ester-modified aliphatic amines with even lower viscosity and greater reacti vity is already available. These can be used to formulate lightfast, solvent-fr ee and extremely fast-drying coatings.
Recent tests show that although replacing the HDI isocyanurate with an IPDI isocyanurate results in a slightly reduced solids content due to the higher viscosity, it also results in even longer pot life and si gnificantly accelerated drying. This can further increase productivity, even at low temperatures. A more detailed study must still be conducted. The anti-corrosion effect of single-coat s uper high solid coating system is also the subject of intensive study.
Bibliography 1. 'Two-Component Aliphatic Polyurea Coatings for High Productivity Applications', C. Angeloff, E. Squiller, Kurt Best; Jour nal of Protective Coatings & Linings; August 2002, pp. 42ff 2. 'New Very High Solids Protective Topcoats Reduce Costs and VOC Levels', Dr. R. Lemmerz; Protective Coat ings Europe, July 2002, pp. 17ff 3. 'Aliphatic Polyurea Coatings Based on Polyaspartic Esters', E. Squiller, Polyurea Development Association Conference, December 2000 4. 'Imine-Isocyanate Chemistry: New Technology for High Solid Coatings', L. Venham, D. Wicks, P. Yeske, present ed at the Division of Environmental Chemistry - American Chemical Society, August 1994