ISMR June 2022 - Page 22

FOCUS ON BONDING

Wind turbines ( image : Shutterstock . com ).
Material issues
The type of material to be bonded also has an impact . For example , stainless steel may need to be cleaned to remove dirt , dust , fingerprints and machine oil .
Steel surfaces are typically coated before or after bonding to prevent corrosion , either with zinc ( galvanized steel ) or some type of paint . If coated before , the adhesive bonds to any coating rather than the steel itself , which affects surface energy and how well the adhesive wets out . Other adhesives and tapes are designed to survive the coating or painting process .
Aluminium resists corrosion because a thin layer of aluminium oxide forms almost immediately when the bare metal is exposed to air . Aluminium often needs to be cleaned or abraded to remove dirt , dust , fingerprints and machine oil .
Metals that can corrode over long periods of time , such as copper and its alloys brass and bronze , require special consideration when selecting an adhesive .
Metal surface preparation , before the bonding process , is essential . Surface pretreatment improves wetting with adhesive .
“ Oxidations as well as oils and greases , form separating layers on which adhesives can bond well . However , this layer in turn does not adhere sufficiently firmly to the metal , so oxidations / oil / greases must be carefully removed . First clean or degrease the metal , then grind or blast , otherwise impurities can be ground ( blasted ) into the metal during the grinding process and continue to have a separating effect on the adhesive . Metals that are stored at cold or lower temperatures should be sufficiently tempered before bonding to avoid condensation ,” advised 3M .
Mechanical roughening ( abrasion ) of the surfaces to be bonded is also a simple , highly effective pre-treatment , even for difficult-tobond metal surfaces like titanium .
Epoxy applications in wind power
Bonding specialist , MasterBond , outlined some of the benefits of epoxy applications in wind power .
“ Epoxies offer high mechanical strength properties , superior dimensional stability and excellent adhesion to similar and dissimilar substrates . Offering better chemical and temperature resistance , epoxies usually exhibit a very wide working temperature range ,” it explained .
Wind turbine blade assemblies are made of many bonded and some fastened elements , and must also accommodate pieces including rain deflectors , flaps and other aerodynamic and environmental assists . Epoxies can be very successfully employed for bonding , void filling , metal insert attachment , thread locking and joining of external items .
“ Structural elements , such as stringers , are often added to improve the strength and dimensional stability of the nacelle . Mechanical fastening in such applications is problematic . Drilled holes can become weak points and must be sealed from the elements . Fasteners driven into the nacelle itself can later cause stress cracking . In nut-and-boltstyle arrangements , assembly efforts are considerable since two-sided
Image : Shutterstock . com . access is often troublesome
if not impossible . Epoxies eliminate those problems since the nacelle needn ’ t be drilled , which also eliminates sealing concerns ,” continued MasterBond .
“ Epoxy cure times can be varied so that stringers can be attached easily and permanently . Environmental and temperature extremes cause problematic expansion and contraction ; special grades of epoxies will generally expand and contract with the substrate , so stress cracking and delamination are eliminated . Should a repair be required later , epoxies are easy to use , set quickly and can be applied with single-sided access ,” it added .
All the elements within a turbine ’ s nacelle — from generators to gearboxes and even hatches and steps — require strong mounts . Epoxies can serve as a solution to affix such mounts completely and effectively to the nacelle , with no maintenance required . Some of those elements ( such as hatches ) require cut-outs in the nacelle that need to be sealed . Epoxy can be used for mounting weather-
Roof covering in layers . Cross section and technical details of house roof . Metal sheets , different layers of insulation and wooden planks . Image : Shutterstock . com .
22 | sheetmetalplus . com | ISMR June 2022