NEW SOLUTIONS FOR INDUSTRIAL COATINGS USING HIGH PERFORMANCE MINERAL FILLERS

Transcription

1 NEW SOLUTIONS FOR INDUSTRIAL COATINGS USING HIGH PERFORMANCE MINERAL FILLERS Paper 140 ABRAFATI Topic #1 - Resistance to rain erosion - innovative filler concepts for coating systems for offshore rotor blades Introduction: Extreme and opposing environmental loads cause wear to the huge rotor blades of wind turbine generator systems (WTGSs). Rain erosion is one of the biggest loads. The resistance to rain erosion of the coating systems can be improved by using high performance fillers and it can be increased yet again by specific surface treatment of the functional fillers. The simulation of the rain erosion test in a newly developed miniature simulator is based on real conditions. The rotor blades used on WTGSs in the offshore sector are supposed to survive a service life of 20 years without any impairment. They are exposed to an extremely wide range of environmental influences, such as snow, rain, salty sea water, hail, heat and UV radiation. Wind speeds of up to 500 km/h act upon the blade tips. This area is also one of the weak points of the rotor blade. The coating is subject at this point to particularly high wear and destruction by rain erosion. The influencing factors are the impact speed and the size of the raindrops. A change in the aerodynamics of the blade surface takes place and as a result deterioration in the output yields. The basis of a rotor blade is formed by a composite material, consisting of bonded glass- or carbon-fibre mats, that is injected under vacuum with epoxy resin. A multi-stage coating, as shown in Fig. 1, is then applied. Structure of the coating system, source: BASF The objective of the present study, besides finding a suitable binder system, was, to contribute to the longevity of such coating systems and of the rotor blades through the use of special high performance fillers. This purpose was served by a coating structure consisting of a gel coat, the pore filler and the top coat, which provided the basis for the tests. Polyaspartic-based binders were used following the recommendation of Bayer Material Science (BMS as they have already proven their value for the coating of WTGS rotor blades). Polyaspartics are characterized among other things by a low VOC content. They are fast curing and high layer thicknesses are achieved.

2 Corresponding starting formulations (Table 1-3) were made available by BMS and the pore filler was optimized with regard to reactivity. The fillers (talcum and synthetic barium sulphate) included in the starting formulation served as reference fillers in the pore filler. Raw material Wt% Function Component A Desmophen NH ,50 Polyaspartic, binder AEROSIL 720 TS 2,90 Silicic acid, thickener Component B Desmodur N ,00 Aliphatic polyisocyanate, hardener Desmodur N ,60 Aliphatic polyisocyanate, hardener 100,00 Table 1: 2-component-PU Gelcoat; starting formulation MMT-E-221/3 Raw material Wt% Function Component A Desmophen NH ,13 Polyaspartic, binder SYLOSIV 3A 1,71 Molecular sieve Butyl acetate 9,33 Solvent BYK-P 104 S 0,50 Wetting and dispersing additive BYK-354 1,43 Surface additive Titanium dioxide 17,56 White pigment Talcum AT 1 4,45 Filler Bentone MP 100 1,86 Thickener Tinuvin 292 0,57 Light stabilizer ACEMATT OK 412 5,57 Matting agent DOWANOL MPA 9,33 Solvent Component B Desmodur N ,39 Aliphatic polyisocyanate, hardener Desmodur N ,17 Aliphatic polyisocyanate, hardener 100 Table 2: 2-component-PU topcoat; Starting formulation MMT-B-228/A Raw material Wt% Function Component A Desmophen NH ,76 Polyaspartic binder BYK-P 104 S 0,55 Wetting and dispersing additive BYK-066 N 0,55 Defoamer AEROSIL 720 TS 0,55 Silicic acid SYLOSIV 3A 1,84 Molecular sieve LUVOTIX HT 0,92 Castor oil derivative TiO2 3,68 White pigment Talcum 27,57 Filler synth. barium sulphate 27,57 Filler 100,00 Component B Desmodur N ,50 Hardener Table 3: 2-component PU pore filler; starting formulation in conformity with MMT-E-215/1

3 The corresponding high performance fillers were substituted on equal volume of the reference fillers. The corresponding volumes were calculated via the density as a percentage by weight (wt%) (Tab. 4). Filler designation Proportion [wt%] reference filler system: talcum + barium sulphate 55,14 TREMIN 47,29 SIKRON 43,97 SILBOND 43,97 TREMICA 47,29 MICROSPAR 43,14 Table 4: - filler parameters Coating structure: 1. Adhesive primer; 1-2 µm 2. Gelcoat; approx. 250 µm 3. Pore filler; approx. 500 µm 4. Top coat; approx. 120 µm After coating the GRP rods, they were cured for 7 days at room temperature. The tests were then performed. Rain erosion test set-up The company 'DAS Lack GmbH', based in Essen/Germany, in cooperation with the company 'Quarzwerke GmbH', based in Frechen/Germany, has developed a method for quick determination of the rain erosion incurred by the rotor blades of WTGSs (Fig. 2). The objective of the test method was to simulate on a laboratory scale the effect of raindrops impinging on a coated glass-fibre reinforced plastic (GRP) test specimen, on the basis of an epoxy resin, rotating at high speed. To do so, the functional principle of a rotor blade for WTGSs was considered and transposed to a small scale. Diagram of the rain erosion test set-up A centrifuge (1), electronically controlled for rotating speed and temperature, served as a simulation chamber. The rotor (2) was modified to provide a mounting for the test rod (2). In addition, a watertight chamber (3) was constructed around the rotor to avoid any damage being caused to the unit by splashing water. A hole with a diameter of 4 mm was drilled into the outer edge of the cover of the centrifuge. An aluminium tube (4) with an internal diameter of 3 mm was introduced through this hole and positioned over the outer edge of the test rod (5). Water was pumped through this tube at a rate of 0.5 l/min via a pump (6) that was connected in the circuit. The water jet resulting from this impinged directly onto the tip of the test rod. The purpose of this procedure was to simulate rain.

4 The GRP rods with an overall length of 22 cm were rotated about their own axis in a centrifuge at a horizontal rotational speed of 10,000 rpm. A maximum speed of the rotor tips of ca. 415 km/h was achieved. Fig. 3: Damage patterns on the coated GRP epoxy rods Filler 5' rpm Reference filler Fintalc M30/EWO MICROSPAR AST TREMICA AST rpm rpm rpm rpm SILBOND 600 AST TREMIN AST Table 5: evaluation damage pattern The surface-treated variants increase resistance against rain erosion Evaluation of the damage patterns (Fig. 3) was performed purely visually. The photos clearly show the influence of the high performance fillers on the resistance to rain erosion as a function of their morphology, hardness and specific surface treatment. These results were confirmed by double determination. The substitution by volume of the reference filler combination of talcum and synthetic barium sulphate by high performance fillers results in significantly improving resistance to impinging raindrops. Already after 5 min at 10,000 rpm a serious damage pattern was created with the reference fillers. This manifests itself in that at the corners of the rod the complete coating is worn off and, what is even more critical, the substrate is worn (Fig. 3, top left photo). An even more resistant coating is achieved with a specific surface modification of the filler. A damage pattern does not set in with surface-modified TREMIN and SILBOND until a double load time for the test rods of 10 min at 10,000 rpm has passed. Apart from the surface modification, the greater hardness of 4.5 and 7 and the grain morphologies (rectangular of TREMIN and angular of SIKRON ) contributes to the better results of these high performance fillers in comparison to the reference fillers. In order to complete this investigation with regard to our portfolio, a MICROSPAR -type in treated version was included. It was found that the version of this high performance filler was outperforming any of the test candidates - please see figure 3, line 2. The platelet-shaped MICA does not have a positive influence on the coating. After the lowest load of 5 min at 10,000 rpm (not shown) the damage is just as big as with the reference fillers. Even a surface modification of the MICA (TREMICA ) does not lead to an improvement in the wear resistance of the coating. The damage pattern remains unchanged.

5 All filler systems have been tested for their viscosities in the coating system, showing no evidence of a relation between filler type and anti-wear performance. Surface modification Surface treatment of the mineral filler with silane or silane-based compounds ensures optimal compatibility at the boundary surface of the polymer matrix and the filler system. System-improving properties of the inorganic filler are achieved and fully exploited by this means. Silanes are bi-functional compounds that consist of stable organo-functional and hydrolizable reactive terminal groups. The hydrolizable group bonds with the filler surface, while the organo-functional groups harmonizes with the polymer (Fig. 4). Diagram of the silanization reaction on the mineral surface LANIZATIONThe test results show that the resistance of the rotor blade coatings of a wind turbine generator system depends considerably on the fillers used. A substitution of the reference filler combination (talcum/synth. barium sulphate) by surface-modified high performance fillers based on MICROSPAR, TREMIN and SIKRON decisively increases the resistance of the rotor blade coatings. Results The resistance of rotor blade coatings to rain erosion is distinctly increased by using rectangular MICROSPAR, TREMIN and angular SIKRON in the pore filler. Surface treatment of high performance fillers increases the resistance yet again. The performance of a result-relevant comparison test has been successful through the development of a suitable test method for the simulation of rain erosion on laboratory scale. There is no evidence of the viscosity showing adverse effects, so we can assume the compatibility for the tested fillers with other binder systems. Topic #2 - Surface treatment for effective corrosion protection Zinc phosphate is an established anticorrosive pigment. In 2004 zinc phosphat was classified environment endangering. Since then, alternative pigments for corrosion protection for EP anticorrosive coatings have been tested. Best results in aqueous 2 components EP-one layer covering varnishes could be achieved with a combination of surface treated granular TREMIN AST and plately MICA SG. components with nat. barium sulphate without zinc phosphate without zinc phosphate with 2,5 wt.% zinc phospate

6 Epilink Hardener* Water, de-ionised Tego Dispers Tego Airex zinc phosphate Talcum Hombitan R Pigment nat. BaSO Tremin AST MICA SG Inhibitor L Byk Water, de-ionised Silan DOW Z component B Epoxy-Emulsion 3:2 3:2 3:2 3:2 solvent-free Table 6: Tested Recipes with different fillers combinations and concentrations => Pic - salt water - rust The atomized salt-spray-test according to DIN SS was performed over 1,000 hours. In comparison with the basic formulation with BaSO 4 and zinc phosphate the use of MICA and TREMIN AST reduces blistering on the surface and rusting along the cross Condensed water - rust The test by condensation water was performed according to DIN EN ISO at 40 C and 100 % air moisture over 500 hours. The best result shows the formulation, which is only filled with wollastonite TREMIN AST. It is characterized by marginal blistering on the surface andrusting along the cross. In EP-one-layer-varnishes the combination of TREMIN AST and MICA SG effects: no or only partial need of a functional pigments. an improved anticorrosive protection. This is demonstrated convincingly by salt-spray- and condensation water test. an enhanced filling degree and covering capacitiy. In spite of a high filling degree the varnish systems are characterized bygood levelling and surface properties

7 Topic #3 - Reduction of VOC content in industrial coatings, and improved ecology Considering the background of the VOC Directive to reduce the emission of volatile organic compounds in paints, High Solid systems are an excellent way for a VOCcompliant coating. The maximum VOC content of such topcoats is 300 g/l (valid from 1st January 2010 according to ChemVOCFarbV). With formulations containing TREFIL and its surface-treated type, based on a natural calcium sulfate (anhydrite), it is possible to improve optical, mechanical and anticorrosion properties and at the same time reduce volatile organic compounds. In addition cost-efficiency can be achieved. High solid systems High solid systems are formulations that are rich in solids, low in solvents and have reduced volatile compounds. Two-component PU high solid paints specifically are used as coating paints for high-quality industrial products on account of their good corrosion protection effect, optical properties and insensitivity to mechanical loads. Typical applications are, e.g. commercial vehicles and steel components. Certain requirements are made of the fillers used in a high solid system in order to obtain a processing viscosity of high solid systems that is comparable with conventional paint systems. A low density and a low oil number are therefore advantageous in order to achieve a low overall density with a simultaneous reduction in the viscosity. TREFIL SEM pic Anhydrite (anhydrous calcium sulphate) Formula: CaSO 4 Density of 3.0 g/cm 3 Hardness of 3.5 (Mohs) Whiteness Y > 89 Tabular structure With TREFIL and a silanized variant, based on a natural calcium sulphate (anhydrite), possibilities arise of achieving a formulation with good optical, mechanical and corrosion protection properties and with a higher yield, thereby reducing the VOC. Filler parameters Anhydrite, anhydrous calcium Synthetic barium sulphate sulphate Morphology Tabular structure Rhombic Hardness (Mohs) *

8 Density [g/cm³] * Whiteness (Y) >89 98* Grain size d50 [µm] * Oil number [g/100 g] 19 13* BET [m²/g] * Table 7- Comparison of the different fillers * Literature values Table 8 Two-component PU high solid coating paint, basic recipe with synthetic barium Raw material Function Supplier TREFIL EST/2 Desmodur N 3390 sulphate Batch 1 [% by weight] Batch 2 [% by weight] Batch 3 [% by weight] Synthalate A- 149 HS Binding agent Synthopol Chemicals Solvent naphtha Solvent Brenntag Tixogel MP 100 Rheology additive Rockwood Additives BYK AT 203 Wetting additive Byk Chemicals Titanium dioxide Pigment Kronos Titanium BYK 057 Defoaming agent Byk Chemicals BYK S 706 Levelling agent Byk Chemicals Synthetic barium sulphate Filler Diverse TREFIL anhydrite Alternative filler Alternative filler Hardener Quarzwerke - HPF Minerals Quarzwerke - HPF Minerals Bayer Material Science :1 4:1 4:1 The maximum limit value of the VOC content for such coating paints is 300 g/l (valid as of according to ChemVOCFarbV). Paint properties / processability / yield Batch 1 Batch 2 Batch 3 Density of mixture [g/cm³] Solids [% by weight] Solids [% by volume] Consumption [m²] at 80 µm, dry [g] VOC [g/l] VOC [g/l] with a set efflux time of 30 s Table 9: Paint properties / processability / yield

9 Manufacture / application Manufacture of the paints by way of dissolver grinding with subsequent sieving over an 80 µm fast screen. Maturation of the produced batches over 24 hours. Application by means of brush, approx. 100 µm, dry, control with BYK micro TRI gloss µ Substrate test plates made of steel DC 04 B, 190 x 150 x 0.8 mm, Krüppel Co. (degreased with ethanol) Drying conditions: at room temperature (20 C) 3 days, then 24 hours at 40 C Batch 1 Synthetic BaSO 4 Batch 2 TREFIL Batch 3 TREFIL EST/2 Optical parameters Colour values against white (BYK colour guide gloss) L* a* b* Gloss 20 av. [%], 7d (BYK micro haze plus) Haze Hlin av. (BYK micro haze plus) Haze Hlog av. (BYK micro haze plus) Mechanical parameters Pendulum hardness according to König [s] (based on DIN 53157) Cupping [mm] (based on ISO1520:1973) Impact test [drop height mm] (based on DIN 55669) Corrosion protection test < Condensation test (based on DIN EN ISO 11997) 500 h 0 (S0) / Ri0 0 (S0) / Ri0 0 (S0) / Ri h 4 (S2) / Ri0 2-3 (S2) / 2 (S2) / Ri0 Ri0 Salt spray test

10 (based on DIN 50021) 500 h 0 (S0) / Ri0 0 (S0) / Ri0 0 (S0) / Ri h 0 (S0) / Ri0 0 (S0) / Ri0 0 (S0) / Ri0 Table 10: Test results The results of our investigations at a glance: The following results of our investigation show the performance data of TREFIL in comparison to a normally used synthetic barium sulphate in a two-component PU high solid coating paint. The replacement by weight of synthetic barium sulphate with TREFIL and TREFIL EST/2 in a two-component PU high solid system leads to a clear reduction in the VOC contents. By thinning the paint systems to a normal application viscosity the VOC content remains clearly below the maximum limit of 300 g/l through the use of TREFIL TREFIL EST/2 reduces the VOC content again. The paint viscosities are lowered by anhydrite and create very good surface properties. The colour values are comparable with the system with synthetic barium sulphate. The highest gloss with the lowest haze results with TREFIL EST/2. Excellent adhesion to the substrate was able to be observed through the use of anhydrite. Through the use of silane treatment of the anhydrite the elasticity is compatible with the synthetic barium sulphate. After 1000 hours of condensation water test the formation of blisters was able to be clearly reduced through the use of anhydrite products in comparison to barium sulphate. Very good corrosion protection after the salt spray test. Increase in the yield by more than 3% through the use of anhydrite.