case study Benefits of oxazolidine moisture scavengers in polyurethane protective top coats for wind turbines

Transcription

1 case study Benefits of oxazolidine moisture scavengers in polyurethane protective top coats for wind turbines

2 Benefits of oxazolidine moisture scavengers in polyurethane protective top coats for wind turbines case study 2

3 Introduction Wind turbines, particularly offshore installations, are exposed to hostile environments which demand that the coatings systems must be able to withstand these conditions for the lifetime of the turbine, typically 2 to 25 years. Difficulties related to access makes coating maintenance expensive as well as technically challenging for offshore wind turbines and lifetime should therefore be of primary concern when specifying coatings. Key characteristics of the final coatings are: They must be able to withstand UV and heat radiation They must be able to withstand a high degree of weathering erosion including salt water resistance They must be tough and durable yet flexible enough to withstand impacts without chipping The ideal coating to achieve these aims also contains minimum solvent for low VOC compliance and has fast cure characteristics. The principal chemistries used in a wind turbine coating system are a solventless 2K epoxy primer overcoated with a stateof-the-art 2K PU Acrylic topcoat. Typically, the topcoat comprises a formulated high solids acrylic polyol combined with an aliphatic isocyanate (usually HDI) solventless or high solids hardener. The key to eliminating this problem is to stop the moisture reaction with the hardener during the application of the topcoat. This is simply achieved by adding Incozol 2, a very effective moisture scavenger, to the formulation. O Incozol 2 C 4 H 9 C 2 H 5 N C 4 H 9 Incozol 2 preferentially reacts with water over isocyanate. It serves to eliminate carbon dioxide generation leading to loss of coating integrity. One major disadvantage of using isocyanate trimer/biuret hardeners is their preferential reactivity with moisture. During the application of the topcoat, the isocyanate can preferentially react with moisture to produce micro-bubbles of carbon dioxide within the coating. Once the coating has fully cured, the bubbles remain as defects which will seriously compromise the integrity of the coating to protect the wind turbine from moisture penetration and thereby limiting the lifetime durability of the coating. 3

4 Benefits of oxazolidine moisture scavengers in polyurethane protective top coats for wind turbines case study Study The study investigated the impact on the cure and film properties of incorporating Incozol 2 into 2K wind turbine topcoats. Steel panels coated with a 2K epoxy primer were overcoated with 2K acrylic PU paint with and without Incozol 2 in the formulation. On application, the reaction with moisture produces visible pinhole defects (below left) in comparison to the formulation incorporating Incozol 2 (below right). The consequences of corrosion and weathering in a defective system are shown below. The test panels were subject to a 2-hour anti corrosion prohesion test. Corrosion through pin-holes was clearly shown in the coating system without Incozol 2 whilst the system incorporating Incozol 2 remained intact. This demonstrates the poor durability caused by film gassing under harsh corrosive environments such as those experienced offshore. The use of Incozol 2 can prevent such scenarios from occurring. Panels after 2 hours prohesion test. The panel without Incozol 2 shows rust through pin-holes (left). 4

5 The test panels were subjected to an accelerated weathering test. Each panel was coated with the same 2K PU paint formulations, one with Incozol 2 in the formulation and one without. Both panels were cured under 9% humidity conditions. The panels were then subjected to a 1-hour QUV-A (ASTM G154-4 Cycle 3) test. The figures below (left) show the gloss retention levels of the panels, with and without Incozol 2 in the paint formulation, after exposure to the above cycle. The panels incorporating Incozol 2 obtained better gloss retention results than those without. From the colour retention figures below (right) the film incorporating Incozol 2 showed lower levels of yellowing than the control formulation (without Incozol 2). These results can also confirm the better durability of the coating brought about by the inclusion of Incozol 2. Gloss retention after 1-hour QUV-A Colour change after 1-hour QUV-A Gloss retention (%) Delta E Unit Incozol 2 little impact on coating cure and film properties Whilst having the benefit of preventing gassing, the addition of Incozol 2 moisture scavenger has no detrimental impact on the film properties during the application process and in the final cured film. Due to the low viscosity of Incozol 2, including it in the formulation can reduce the overall viscosity of the coating and also the VOC level. Formulation viscosity VOCs at same viscosity (25 s) 3 45 Din 4 Cup (seconds) VOC (g/l)

6 Benefits of oxazolidine moisture scavengers in polyurethane protective top coats for wind turbines case study From the figures shown below (left), it can be seen that the addition of Incozol 2 extends the pot life slightly whereas the through cure rate results (below right) were similar. Incozol 2 should increase the cure rate once the film is applied as it is a tertiary amine catalyst itself before ring opening and releases faster curing amine after hydrolysis. This benefit would be more exaggerated as the level of Incozol 2 in the formulation is increased. Pot life (minutes) BK Dryer tests Pot life (minutes) Through cure (hours) The rate of hardness development shown below (left) suggests the impact on overall hardness development is insignificant. Likewise, the initial gloss levels (below right) are similar when Incozol 2 is included. Hardness development Initial gloss Persoz hardness (s) Reflection % hr 1d 3d 7d 14 Gloss 2 Gloss 6 Gloss 85 Cure time Angles 6

7 Conclusion The data demonstrates that the inclusion of Incozol 2 in 2K polyurethane protective topcoat formulations has clear benefits. In its capacity as a moisture scavenger, Incozol 2 will eliminate the generation of carbon dioxide gas by preferentially reacting with moisture over isocyanate. This moisture may originate in the atmosphere or be present in the raw materials, such as the pigments, extenders, solvents, etc. Preventing this reaction from occurring prevents pinhole defects from appearing in the coating surface which will affect the coating integrity and ultimately lead to corrosion of the substrate. Whilst the data clearly shows the benefits of Incozol 2 as a moisture scavenger in the coating system, it also shows that its inclusion in the formulation will not detrimentally affect film performance during application or in its final cured state. 2K PU/Acrylic guide formulation incorporating Incozol 2 FORMULATION PART A PART B Total Materials Synocure 852 BA 8 DBTDL (.2%) Tinuvin 113 on binder (.5%) Tinuvin 292 on binder (.5%) Incozol 2 (2% replacement of polyol) Gemsperse White UX 6 (25% on solids) Butyl acetate (35% w) Tolonate HDT LV2 Amount (g) (1% solution) Supplier Cray Valley Akcros BYK Incorez Ltd Gemini Samuel Banner Perstorp Properties % Solids VOC (g/l) Specific Gravity (SG) Viscosity - Flow Cup (secs)

8 Manufacturers of specialist resins & polymers: Oxazolidines Waterborne epoxy curing agents Polyurethane dispersions Polyurethane/acrylic hybrid dispersions Polyurethane prepolymers Incorez Limited Iotech House Miller Street Preston Lancashire PR1 1EA England Incorez Corporation 79 Bradley Street Middleton Connecticut 6457 USA t: +44 () f: +44 () e: w: t: +1 (86) f: +1 (86) e: w: ISSUE 1-3/11