Matrix system with enhanced mechanical performance: new infusion system for wind energy applications enables lighter, longer, lower cost rotor blades

Transcription

1 White Paper Matrix system with enhanced mechanical performance: new infusion system for wind energy applications enables lighter, longer, lower cost rotor blades Malte Wichmann Momentive Specialty Chemicals GmbH, Esslingen, Germany

2 Matrix system. Enhanced performance. 1 Introduction The need to make wind a more competitive energy source is driving rotor blade design toward larger turbines and longer rotor blades. In 12, the m plus generation of rotor blades is in concrete planning, with several leading manufacturers announcing blade lengths around 8m. In a recent white paper, Momentive Specialty Chemicals showed the potential of carbon fiber reinforced epoxy to reduce the structural weight of rotor blades. The new EPIKOTE Resin MGS RIM145 infusion system for carbon fiber infusion was presented, eliminating the need for prepreg technology [1]. With this paper, we are presenting an epoxy resin system tailored especially for rotor blade shell infusion. This newly developed matrix resin system, EPIKOTE Resin MGS RIM 245, exhibits enhanced transverse tensile properties, enabling higher strain levels in the whole blade structure and thus providing the potential for significant weight and cost savings. 2 Technical Background In many rotor blade designs made from glass fiber reinforced epoxy, the transverse tensile (i.e. interfiber failure, IFF) properties are a design-limiting factor. The structural stiffness of a rotor blade is provided by the spar caps, which incorporate unidirectional (UD) glass fiber fabrics. The shell structure provides torsional stiffness as well as the aerodynamic contour and is typically constructed of a ±45 biaxial layup. The interfiber failure properties of the shell laminate determine the maximum allowable strain level in the main load direction (flapwise). A matrix resin system that provides enhanced transverse tensile properties, such as, allows for higher tolerable strain levels and thus a reduction of the spar cap dimensions. This volume decrease results in significant weight savings, enabling the construction of longer blades, as well as raw material cost reductions. A reduced blade weight results in significantly decreased fatigue loads and can positively influence the overall turbine design as well, i.e reduction of pitch bearing dimensions etc. In a recent study, the blade design and manufacturing company EUROS calculated the weight saving potential of and reported a possible reduction of approximately 1 ton for a 59 m rotor blade design [2]. The results of this study will be made available in a subsequent white paper. White Paper: Matrix Infusion System. 2

3 3 Transverse properties - mechanical performance of EPIKOTE Resin MGS RIM 245 with state-of-the-art glass fiber reinforcement The transverse tensile performance of the new system was tested on various roving types and fabrics. Two glass fiber roving types commonly used in the wind energy industry the PPG2 and the 3B SE type were employed as glass fiber reinforcements. The well established Momentive infusion resin system RIMR135 (with curing agent RIMH137) served as a reference. Transverse tensile tests were performed according to DIN EN ISO 527-5B/1 (fiber direction 9 ), at a crosshead speed of 1mm/min, after curing h at 7 C. In Figure 1, the transverse tensile strain (strain to first crack) values, as well as the transverse tensile strength (stress at first crack) values of the RIM135 and systems are displayed for a Kuempers HPT fabric with the standard PPG2 roving. It should be noted that the fabric contained 8% 9 fibers (stabilizers) oriented in the testing direction. The appearance of the first crack in the respective stressstrain curves was evaluated (see the red dotted lines in Figure 2). A +53% increase in transverse tensile strain was found (from.42% to.64%) in comparing the industry benchmark system, the RIM 135 with the new system RIM 245. The transverse strength value increased from.6 to 65 MPa (+28%). a),8,7 Kümpers HPT, PPG 2, V f =52% TTStrain [MPa],,,, +53%,,, 8 7 Kümpers HPT, PPG 2, V f =52% TTStrength [MPa] +28% Figure 1: a) Mean transverse tensile strain and mean transverse tensile strength values of the RIM135 and systems, tested on a PPG2 roving (Kuempers fabric HPT ) at a fiber volume content of 52% 3 White Paper: Matrix Infusion System.

4 a) 9 8 7,5 1 1,5 2 2, ,5 1 1,5 2 2,5 Figure 2: Stress strain curves for the a) RIM135 and transverse tensile tests on the Kuempers HPT fabric In Figure 3, the respective system performances were tested on an SE sizing (3B fiberglass), which is also widely used in the wind energy industry. The fabric was a Saertex UD (type V518), with a 9 fiber weight proportion of only 3%. Because of the lower 9 fiber content, these laminates show a sudden failure, as can be seen in Figure 4. White Paper: Matrix Infusion System. 4

5 The strain to failure is improved by 29% (.42% vs..54%) and transverse tensile strength is improved by 35% (36.7 MPa vs MPa) in the sample versus the RIM135 reference. a),8,7 Saertex UD, 3B SE, V f =51,5%, TTStrain [%],,, +29%,,, 7 Saertex UD, 3B SE, V f =51,5% TTStrength [MPa] +35% Figure 3: a) Mean transverse tensile strain and mean transverse tensile strength values of the RIM135 and systems, tested on a 3B SE sizing (Saertex UD V518) at a fiber volume content of 51.5% 5 White Paper: Matrix Infusion System.

6 a),2,4,6,8 1,2,4,6,8 1 Figure 4: Stress strain curves for the a) RIM135 and transverse tensile tests on the Saertex UD V518 fabric (3B SE sizing) 4 Transverse properties - mechanical performance with epoxy-optimized glass fiber reinforcement Recently, the fiberglass manufacturer 3B brought newly developed glass fiber sizing to the market. This sizing, SE, was specifically developed to promote a good interfacial interaction between glass fibers and epoxy based matrix systems. 3B has shown that with standard epoxy matrix systems, the SE roving significantly enhances transverse tensile properties as compared with the SE sizing. In Figure 5, the transverse tensile properties with the new system compared against the RIM135 on an SE sizing are displayed. The fabric type was the same as used to obtain the test results shown in Figures 3 and 4, namely a Saertex UD (type V518), with a 9 fiber weight proportion of 3%. The strain to failure is improved by 44% (.63% vs..91%) and transverse tensile strength is improved by 11% (58.7 MPa vs MPa) with versus the RIM135 reference. In Figure 6, the respective stress/strain curves are shown. Again, the samples fail at the first crack, because of the low proportion of 9 fibers in this fabric. White Paper: Matrix Infusion System. 6

7 It should be noted that the combination of a very high strain to failure of.91% and a strength value of 65.1 MPa is an extraordinary and unique result. The relative improvements to the current market standard, e.g. the combination of the RIM135 system and an SE roving are +121% in strain to failure (.42% vs..91%) and +77% in transverse tensile strength (36.7% vs. 65.1%). a) 1, 1, Saertex UD, 3B SE, V f =51,5% TTStrain [%],8,, +44%,, 8 7 Saertex UD, 3B SE, V f =51,5% TTStrength [MPa] +11% Figure 5: a) Mean transverse tensile strain and mean transverse tensile strength values of the RIM135 and systems, tested on a 3B SE sizing (Saertex UD V518)at a fiber volume content of 51.5% 7 White Paper: Matrix Infusion System.

8 a) 8 7,1,2,3,4,5,6,7,8,9 1 1,1 1,2 8 7,1,2,3,4,5,6,7,8,9 1 1,1 1,2 Figure 6: Stress strain curves for the a) RIM135 and transverse tensile tests on the Saertex UD V518 fabric (3B SE sizing) White Paper: Matrix Infusion System. 8

9 5 Processing properties The system is a two component system, consisting of EPIKOTE Resin MGS RIMR 245 and EPIKURE Curing Agent MGS RIMH 247. It is tailored to perform well in common wind energy infusion processes and features a low system viscosity. The processing properties of the resin system are given in Table 1. Additional mechanical performance data of the neat resin material is presented in Table 2, as well as in-plane shear data on GFRP laminates, presented in Table 3. The relatively low shear modulus of ca 3 GPa is worth mentioning, as it is -15% lower than that of a standard system such as RIM135. As the strain level in the rotor blade shell is determined by the spar cap design, a lower matrix system shear modulus results in significant stress reduction in the shell laminate. Table 1: Processing properties of the system System viscosity at 25 C 29 mpa s System viscosity at C 5 mpa s Tg mp at 6h 7 C cure 72 C Time to mpa s at C 5 min Potential glass transition Tg ult 84 C Table 2: Mechanical properties of the RIMR245/ RIMH247 system, 6h 7 C cure, all values in MPa Young s modulus 87 ± 38 Ultimate tensile strength 59.2 ±.2 Bending modulus 2899 ± 116 Bending strength 3 ± 2.4 Compressive strength 83.4 ± 1.9 Table 3: In-plane shear performance of RIM 245 on a ±45 biax layup (Kuempers HPT fabric, Vf =49.5%), all values in MPa Standard RIM 245 DIN EN ISO 527-4/2/2 DIN EN ASTM D 778 Tensile modulus 7 Modulus std. dev. 289 Tensile strength 9 Strength std. dev. 1.8 Shear modulus Modulus std. dev. 68 In-plane shear strength 36.7 Strength std. dev..6 Shear modulus 41 Modulus std. dev. 4.5 Shear strength 45.4 Strength std. dev..6 9 White Paper: Matrix Infusion System.

10 6 Conclusions With the new wind energy infusion system EPIKOTE Resin MGS RIM 245, Momentive offers a solution for lighter and longer blades, as well as the potential for substantial cost savings through decreased volume requirements for both fiber and resin. With this system in use, transverse tensile strain and strength values were shown to improve by -% over the market benchmark. Furthermore, compared with standard composites, RIM 245 in combination with state-of-the-art glass fiber reinforcements epoxy-optimized by special sizing such as 3B s SE, achieved improvements of up to 1%. In a rotor blade design study by EUROS [2] (results forthcoming in another Momentive white paper), the calculated weight saving potential was ca. 1 metric ton for a 59 m blade. This translates to a reduction of 19 kg m of static moment or a % reduction in fatigue loads. It is also noteworthy that the calculation only covers the first rotor blade design iteration and does not take into account further weight and cost savings that may be realized by such means as reducing the bolt circle diameter, the pitch bearing dimensions, and/or the rotor hub size. Smaller blade bolts, as well as a modified layup of the root section, could contribute to additional savings. References 1 C. Scheuer, Efficiency breakthrough in rotor blades: new epoxy systems enable direct infusion of thick carbon fiber structures eliminating the need for prepreg. Momentive Specialty Chemicals, 11 2 S. Löser, A. Krimmer, Comparison of the potential for light weight construction from different epoxy resin systems and construction methods. R&D report, EUROS Entwicklungsgesellschaft für Windkraftanlagen mbh, 12. White Paper: Matrix Infusion System.

11 Momentive: The Science Behind What Lies Ahead At Momentive, our global team is focused on delivering value by helping our customers make their products and processes better. We do this by bringing our deep technical expertise, market experience and technology portfolio to bear on their specific challenges. We are the science behind thousands of innovations that enhance both our customers business results and everyday life. Reach our Global Customer Service network at: U.S. and Canada Europe All Others Please refer to the literature code MSC-147 when contacting us. Momentive Specialty Chemicals Inc. World Headquarters 18 East Broad Street Columbus, OH Momentive Specialty Chemicals Inc. All rights reserved. and denote trademarks owned by or licensed to Momentive Specialty Chemicals Inc. MSC-179 /12 Printed in U.S.A. The information provided herein was believed by Momentive Specialty Chemicals ( Momentive ) to be accurate at the time of preparation or prepared from sources believed to be reliable, but it is the responsibility of the user to investigate and understand other pertinent sources of information, to comply with all laws and procedures applicable to the safe handling and use of the product and to determine the suitability of the product for its intended use. All products supplied by Momentive are subject to Momentive s terms and conditions of sale. MOMENTIVE MAKES NO WARRANTY, EXPRESS OR IMPLIED, CONCERNING THE PRODUCT OR THE MERCHANTABILITY OR FITNESS THEREOF FOR ANY PURPOSE OR CONCERNING THE ACCURACY OF ANY INFORMATION PROVIDED BY MOMENTIVE, except that the product shall conform to Momentive s specifications. Nothing contained herein constitutes an offer for the sale of any product.