Polyurethanes build a better bond
October 5, 2011 by Windpower Engineering
Filed under Construction, Materials, Turbine Blades
The trend toward longer turbine blades means greater energy capture at lower wind speeds. However, with bigger blades comes increased weight and stress on the adhesive bond line. Polyurethane adhesives can improve the long-term performance of future blades while reducing total manufacturing cost.
Made essentially of laminated glass-reinforced plastic, today’s rotor blades can be almost 61-m long and weigh up to 15 metric tons. This challenges adhesive strength and durability.
To boost energy yield, rotor blades are becoming larger and heavier. The longest production blades can span about 61-m and weigh 15 metric tons. Future designs call for lengths of 80 to 100 m and beyond. They are primarily polymer composite structures (e.g. glass-reinforced plastic) expected to last at least 20 years in a challenging service environment. When rotor blade tips slice through the air at speeds up to 300 km per hour, it’s not hard to imagine the extreme stresses they must withstand.
The trend toward larger, stronger, heavier blades is expected to continue because such designs will deliver more energy. In addition, the area available to capture wind is greater, making the turbine more efficient at lower wind speeds.
But as blade length increases, stresses on the blade are magnified. Larger blades experience more deflection across the length of the blade, increasing the dynamic load and stress on the adhesive bond line. Adhesives that offer superior long-term dynamic fatigue strength will provide better performance over the blade’s life and lower the risk of damage that requires repair or replacement.
Along with the need to improve performance, wind-turbine manufacturers must also lower their total production cost. Wind-equipment manufacturers work diligently to shorten production cycles and lower overall time and turbine production costs. Also, manufacturers are evaluating options to make reductions in lengthy production cycle times. Shorter cycle times translate into capital, labor, utility, and overhead savings, while also allowing greater flexibility in meeting unplanned demands.
Henkel Macroplast UK 1340, represented in blue, bonds the spar in place and the leading and trailing edges of the blade assembly.
Traditional blade assembly
There are two fundamentally different approaches to rotor-blade construction. Non-self supporting structures involve a box spar with an aerodynamic profile. Self-supporting structures consist of two blade halves with bonded spars that transmit all forces. The structural bonding of self-supporting blades requires adhesives with exceptional mechanical properties.
Regardless of size or construction, the quality of a rotor blade depends upon the reliability of its adhesive bonds. Glass-reinforced plastic rotor blades are fabricated in a simple sandwich design. Two blade halves are created in a composite mold. After demolding, the two halves are mated and bonded together to form the rotor blade.
The bonded blade construction is exposed to long-term direct loading. If the bond fails under stress, the blade may be damaged or even ruptured. For this reason, Germanischer Lloyd (GL) approval for blade bonding adhesives is only awarded after an adhesive passes numerous physical parameter checks.
A technician applies Macroplast UK 1340, a polyurethane adhesive that speeds rotor blade production and reduces costs by 15 to 30%. Offering breakthrough Tg better than typical polyurethanes, it is the only adhesive in its class to satisfy GL’s requirements.
Until recently, self-supporting structures such as rotor blades have been typically bonded with GL-certified two-component reactive epoxy resins. Widely accepted in the aerospace industry for structural bonding applications, epoxies offer excellent adhesion, tensile strength, and chemical and heat resistance. Limitations include extended cure times, high exotherm, and higher process costs.
Two-part epoxy adhesives are thermoset materials that cure using heat, or heat with pressure. Used for high-load assemblies and in severe service conditions, cured thermoset adhesives may soften when heated, but do not melt or flow.
Epoxies begin curing once the two parts are mixed, but require substantial time to cure completely. The total cure involves several heat-cure cycles and can last in excess of 24 hours. Longer cycle times limit a manufacturer’s throughput and result in elevated work-in-progress inventories, decreased mold use, and increased energy costs associated with cure ovens.
Epoxies also generate extreme heat, greater than 120 to 150ºC, when curing. This naturally-occurring heat can cause the bond line to swell during cure, and then shrink as it cools, which leads to stress building in the blade assembly. This stress can translate into a concentration that causes cracks under dynamic loading. Furthermore, epoxies can be inherently brittle, which limits the blades’ resistance to crack propagation. Also, epoxies can become more brittle over time, so the likelihood of cracking, especially under dynamic loads, may increase.
These limitations increase the risk for warranty claims, loss of production, and finally increased maintenance cost and blade failure. With the new manufacturing requirements for larger blades, epoxyies are reaching their limits for bonding blades.
An advanced polyurethane
A recent polyurethane (PUR) adhesive satisfies specific mechanical requirements for use in the wind industry while improving the long-term reliability of rotor blades and making rotor-blade production faster and less expensive. The material, Macroplast UK 1340 from Henkel Corp., contributes to the assembly of highly efficient wind turbines.
Because of a large spectrum of possible polymer architectures, polyurethane adhesives have been employed as a bonding agent in many different industrial sectors for more than 30 years. Construction, automotive, transportation, and shipping vessels have all benefited from the use of polyurethanes.
The recent polyurethane adhesive provides superior dynamic fatigue strength and increased resistance to crack propagation, while making the production of rotor blades more efficient than with epoxy technology. For instance, the PUR adhesive requires fewer curing steps than epoxies, resulting in reduced production costs and production cycles that are 15 to 30% shorter.
In addition to blade bonding, the two-component polyurethane adhesive is also used in other structural bonding applications on turbines including bonding components to the rotor blade, performing field repairs of blades, and securing various components inside the tower assembly.
GL’s requirements for the adhesive primarily relate to its tensile shear strength, long-term durability, creep behavior, and glass transition. The adhesive’s physical properties are temperature-dependent. Within a temperature range known as glass transition (Tg), the change in the adhesive’s mechanical properties is considerable. The glass-transition temperature separates the lower, brittle, or glass range from the upper, flexible, or rubbery-elastic range.
Extensive tests demonstrate a tensile shear strength for Macroplast UK 1340 exceeding 20 MPa in the -40 to +80°C temperature range and a Tg of 65°C and higher.
For turbine rotor blades, a Tg of at least 65°C is required to prevent bond creep or relative movement of the substrates, and achieve a certain degree of rigidity at higher ambient temperatures. However, for typical polyurethane adhesives this is usually within the range of only -30 to 45°C, depending on the required elasticity.
Tests have demonstrated a tensile shear strength exceeding 20 MPa in the -40 to +80°C temperature range for the material and a Tg of 65°C and higher. This improved tensile fatigue strength lets wind blades handle the deflection across the length of the blade and the dynamic load and stress on the adhesive bond line better than epoxies, thereby reducing the risk of damage that may require repair or replacement.
The two-component polyurethane adhesive consists of a resin and a hardener. After mixing, pot life ranges from 60 to 80 minutes at the optimal ambient temperature of 20°C. The reaction speed of polyurethane-based adhesives can be altered, for considerably decreased time spent to produce rotor blades.
The adhesive’s pot life can adapt as required for a specific manufacturer’s production without the disadvantage of partially overheating the adhesive joint. Manufacturers can maximize throughput without the risk of elevated stress in the part or the potential cracking associated with high exotherm.
The recent PUR adhesive cures at a much lower reaction temperature of up to 75°C maximum when compared to epoxies that cure at 120 to150ºC. Reducing the exothermic reaction benefits the process in two ways. First, when bonding composite materials, stress cracking caused by excessive thermal loading may weaken the rotor blade. This risk is significantly reduced when the reaction temperature is lower.
Second, polymerization that takes place at high temperatures is always accompanied by changes in volume. Low-heat emission and lower thermal loading during chemical crosslinking reduces heat-related shrinkage of the adhesive. Shrinkage in the bond line can increase internal stresses, hence, a controlled shrinkage results in a more durable bond over time. The heat generated from an exothermic reaction will vary depending on the quantity of adhesive undergoing cure. In areas where larger amounts of adhesive are applied, temperatures get much hotter than in areas with less adhesive volume.
On a single rotor blade, there are considerable differences in the quantity of adhesive applied and, therefore, considerable differences in stress. At interfaces between such stress fields, mechanical flaws will occur unless the stress is relieved through elaborate tempering. This is where polyurethane systems with low-temperature exothermic reactions have big advantages, particularly when applied in thick films.
Adhesives that address the higher dynamic load and stress associated with larger blades will provide better performance over the life of the blade and lower the risk of damage that may require repair or replacement.
Adhesives that cure at room temperature and those which cure rapidly at elevated temperatures can shorten production cycles, and reduce the overall time and cost of turbine production. Room temperature curing adhesives eliminate the need for ovens, while faster curing adhesives improve throughput during the blade bonding assembly process.
Compared to conventional epoxy resin systems, the latest polyurethane delivers a number of improvements for wind-turbine manufacturers. The adhesive’s shorter cycle times boost productivity while reducing energy costs. It provides superior tensile and fatigue strength, and resists crack propagation, meeting high standards of reliability and quality.
Dr. Michael Gansow, Corp. Dir. PUR Development
Jason Spencer, Business Manager
Thomas Buckley, Market Application Engineer
Henkel Corp.
www.henkelna.com/windpower
WPE
Pneumatic servo dispenser controls flow and precise material volumes
September 26, 2011 by Paul Dvorak
Filed under Manufacturing, Turbine Blades, Wind Power News
A complete PSD meter system includes supply pumps and a dispense valve available in Tip-Seal, No-Drip and Snuf-Bak models for accurate start-stop of material flow for dispensing beads, shots or dots in extruding, streaming, spray, and high-speed stitching applications.
A positive rod-displacement dispenser combines the cost efficiency of a pneumatic-drive motor with the controlling accuracy of a servo-drive motor. The Pneumatic Servo Dispensing (PSD) system uses a positive rod displacement metering principle for maximum dispensing precision and eliminates need for a servo-drive motor or material-flow meters in dispensing equipment.
The PSD works with a pneumatic drive motor and servo package to provide precise metered volumes, accurate flow control, and consistent bead profiles. The PSD’s pneumatic cylinder and positive-displacement meter are lightweight and assembled into a single compact metering module for floor, robot or pedestal mounting. A robot integrated PSD system provides precision dispensing of fluids with excellent response time for dispensing extruded beads.
The PSD is intended for long life and maximum uptime. Components are readily accessed and easy to maintain. Users operate the PSD by its own control panel or by automation integrated controls with dispensing software for a smaller footprint, lower system cost, and one-stop operator control of the entire robotic dispensing system. The unit is well suited for dispensing adhesives, filling, gasketing, molding, sealing, other operations.
Sealant Equipment
sealantequipment.com
Vibration control for turbines
July 12, 2011 by Paul Dvorak
Filed under Maintenance & operations, Manufacturing, Wind Power News
Proper vibration control and structural adhesives can keep a wind farm from shaking its turbines into early retirement.
Andrew B. Swoyer Jr., Manager, Marketing and Sales
Carlos Cruz, Market Manager Product Assembly Adhesives and Coatings, Americas Region
Lord Corp., Cary, N.C., www.lord.com
The components around the nacelle show a few mounting and vibration control devices from Lord Corp..
Ground-based power generation equipment has the advantage of mounting to foundations with large masses that dissipate vibration so it is not passed to nearby equipment. Wind turbines have no such advantage. Their equipment is bolted to a large frame but vibration control is minimal. The good news for turbine OEMs and O&M crews is that there are a range of devices for controlling vibration.
Mounting devices
Untamed vibration shortens major component life, loosens bolts, causes weld failures, and contributes to gear and bearing failures. Because nacelles are perched 200 to 300-ft up and at the end of a swaying tower, equipment mounts, often vibration isolators for the equipment in a nacelle, must work reliably for long periods to minimize maintenance. Noise and vibration attenuation are key considerations driving a need for isolation systems that are easy to install, provide a long service life, and reduce the transmission of noise and vibration.
Gearboxes and generators inside the nacelle are often mounted on vibration isolators. These should reduce vibration amplitude, lower structure-borne noise levels, and extend equipment life – all important to the wind industry.
Meshing gears, rotating components, and generators on the turbines also produce excess vibration and noise signatures in large, small, new, and old wind turbines. Several manufacturers are focusing on ways to improve the operation of 1.5-MW versions, a size frequently found on today’s wind farms.
Large isolators provide a ‘soft’ attachment between the gearbox, nacelle, and tower structures to interrupt the noise path, reducing stress on large, expensive components, and eliminating critical vibration modes that could cause structural damage. Torque-restraining mounts that react to torque and provide isolation have also been considered.
Isolating torsional vibration
Rotating components in turbine drivetrains also present challenging reliability issues. One solution to drivetrain component-life issues in similar equipment is to apply torsionally soft couplings to reduce variable torque and tune away harmful resonances. Side benefits to this solution include reduced drive-train noise and improved life of rotating component
Structural adhesives and passive motion control devices from Lord Corp. can be used in a variety of applications in the wind-power industry.
Noise and vibration in smaller turbines are equally troublesome, but solutions differ. For instance, isolators for small turbines are readily available, cost-effective, durable, and maintenance-free. Center-bonded mounts have been used in turbines with generating capacities of 100 kW and smaller to isolate most equipment. One application used standard company products and a center-bonded mount.
The isolation mounts were field-tested and installed in a particular 100-kW unit, resulting in a unit that ran quieter and lasted longer. Peripheral equipment in the nacelle, such as electrical cabinets, enclosures, and wire harnesses can also benefit from vibration isolation. Wiring-connector failures are just one example of common failures due to excessive vibration.
Standard, readily available isolation devices are commercially available for these components. Center-bonded mounts, platforms, grommets, and bushings are just a few of the possible isolation solutions. All have been field-proven in a wide variety of similar industrial and aerospace applications.
Selecting isolators
Generally, selecting a vibration isolator requires collecting structural information and parameters about the system or unit to be isolated. Key information includes:
1.) Dimensions of the unit
2.) Static weight, center-of-gravity location, and maximum torque reaction forces
3.) Mounting locations
4.) Vibration excitation characteristics
5.) Environmental resistance requirements such as ozone, chemical, or temperatures
Rubber-to-metal mounts are widely used on helicopters, aircraft, locomotives, and large construction equipment to support equipment and isolate intermittent or continuous vibration. Easy-to-install anti-vibration mounts are resistant to oils and weather-related stresses making them appropriate for work in turbine nacelles. Furthermore, the devices are rugged, offer long service life, and require no maintenance – especially important for structures that are located up to 300-ft high.
Most companies that design vibration isolators also provide assistance selecting them. For that purpose, most vibration control company’s like ours maintain a staff of applications engineers and a comprehensive database of available components, ready to analyze needs and recommend isolators.
The company also provides design testing and validation services to help demonstrate that the mounting device meets the customer’s exact requirements. And where conventional, passive rubber-to-metal isolation solutions are limited in performance, semi-active devices may be useful. Semi-active devices react in real-time to mitigate displacement, velocity, and acceleration events that can cause damage to mounted equipment or discomfort to equipment operators. Although these devices have not been applied thus far to wind turbines, they serve as an example of possible next-generation isolators for the wind market.
Structural adhesives
Structural adhesives in wind-power applications are used to join the two halves of a turbine blade after they are removed from their molds. Such adhesives can also affix smaller items. Modern structural adhesives offer the benefits of design flexibility, high performance, corrosion resistance, and durability, along with an ability to join dissimilar materials. The adhesives, designed to replace traditional mechanical fastening methods, are available in three chemistries: acrylics, epoxies, and urethanes.
Recent structural adhesives are applicable to a variety of turbine structure components including:
- Blade assembly (blade bonding and component bonding, void filling, along with bolt, stud, and metal-insert fixing)
- Nacelle assembly (panel and stringer structural bonding)
- Hub, bearing, and gearbox assembly (bearing-to-hub assembly)
- Electrical components
- Tower assembly (component bonding)
- Installation and maintenance, such as blade repair.
Engineering assistance is available for joint design, fixturing recommendations, adhesive selection, and MMD (bulk dispensing) equipment guidance. Engineers looking to tame vibrations can pick-and-choose what they need, such as design assistance, fixturing help, adhesive selection, and joint design – most any advice a vibration engineer would need.
Curing times are a crucial aspect of bonding. Some applications require adhesives that cure quickly, such as repair operations, while other applications need a slower cure time, as in manufacturing.
Large wind blades, for one, need a slower cure time. When bonding the two blade halves and laying down beads of adhesives, it’s good to have a curing process begin at the same time. An adhesive should not start curing before all of it is applied to the entire blade. When the dispensing finishes, the two halves are placed together, and the curing can commence. Curing at room temperature lets the adhesives provide crack resistance for a variety of composite, metal, and plastic assemblies.
As global demand for energy increases, wind power and its advantages are garnering interest as an economical source of energy. The Global Wind Energy Assn says demand “will require significant investment in new power generation capacity and grid infrastructure.” Wind-turbine manufacturers are ready to make investments, so they are looking for methods to increase production, decrease costs, and provide reliable solutions on the production line and in the field.
Adhesives, sealants, and more for wind-turbine manufacturing
July 7, 2009 by Paul Dvorak
Filed under Maintenance, Materials, Turbine Blades
Henkel’s Frekote (red areas) is used to seal blades and nacelles against the weather.
To support growth in the wind energy market, Henkel Corp., Rocky Hill, Conn., has developed a range of products for manufacturing and maintaining wind turbines. These include structural adhesives, mold-release agents, machinery adhesives, sealants, grouts, cleaners, surface treatments, tapes, and metalworking materials. Company wind-turbine products, such as Loctite, Teroson, Bonderite, Frekote, Multan, and P3, are used for blade and nacelle manufacturing, tower assembly, hub, bearing and gearbox assembly, field installation, and maintenance.
The company formulated the first Germanischer Lloyd-certified two-component structural polyurethane adhesive, as well as MMA adhesives for blade bonding and repair. These adhesives cure at room temperature and provide crack-resistant, long-lasting bonds on composites and metals. Loctite machinery adhesives such as threadlockers, thread sealants, retaining compounds, and gasketing materials, prevent fasteners from loosening, seal pipes and flanges, and bond cylindrical components. Applications include blades, gearboxes, hubs, brakes, and pitch and yaw bearings.
In addition, says the company, butyl tapes and sealants are used to vacuum form composite laminates, while Frekote, semi-permanent mold release agents, are used when molding large-scale composite components such as blades and nacelles. For waterproofing blades, nacelles and towers, and general turbine maintenance, Henkel sealants offer flexibility, primerless adhesion, short cure periods, and temperature resistance. For tower installation, Loctite high-strength grouts withstand high-torque loading and are available in grades for deep pours.
Furthermore, says Henkel, its Multan and P3 metalworking products are used during machining, forging and general forming processes and for short-term corrosion protection. Bonderite and Alodine metal surface pretreatment materials protect towers, hubs, and gearbox components from corrosion. Henkel cleaners are environmentally responsible formulations for cleaning during manufacturing and on-site maintenance of blades, nacelles, and towers.
