ISMR June 2022 - Page 21

FOCUS ON BONDING

Below and right : HENKEL bonding technology . options . Each chemistry has its own unique benefits . These include :
■ Structural adhesives , such as structural acrylic adhesives and epoxies , are suitable metal adhesives for forming strong bonds that resist stress , temperature and chemicals .
■ Anaerobic adhesives / sealants are used as threadlockers , pipe thread sealants , retaining compounds or FIP gaskets . Anaerobic adhesives are only used as metal adhesives and sealants . They do not cure on other substrates such as glass and plastic .
A sealant is a soft , pliable material that is used to seal cracks or joints where structural strength is not required . The sealant , initially a fluid or semi-fluid , or alternatively hot applied , placed between two opposing solid materials , becomes solid itself ( by solvent evaporation , chemical reaction or both ), and bonds to the surfaces to which it is applied . It therefore accommodates joint movement without adhesion loss . The sealant purpose is to prevent excessive absorption of water , penetration of other liquids , gaseous substances or airborne particulates . A sealant has the adhesive and cohesive properties to form a permanent seal .
“ Adhesives and sealants share similar characteristics and perform similar roles . The main difference between the two is that sealants typically have lower strength and higher elongation than adhesives ,” added FEICA .
Image : Shutterstock . com .
Today ’ s adhesives and sealants are based on a wide variety of natural and synthetic technologies . These numerous formulating options allow products to be tailored that match the requirements of a huge range of disparate substrates and applications .
Choosing the adhesive
High-precision bonding was expected to become a top requirement in electronics , luxury packaging , healthcare and jewellery applications by analyst Frost and Sullivan . It advised adhesive manufacturers to invest in product development and plans to provide exclusive technical support to their “ extremeprecision adhesive customers .”
“ There is no such thing as one adhesive for metals . The properties of the adhesive ( such as composition , viscosity or temperature resistance ) must be matched to the surface to be bonded . For example , precisely fitting parts where small surfaces are joined together place different demands on the adhesive than , for example , in the case of surface laminations ,” continued 3M .
This means choosing an adhesive with low surface tension , because the lower the viscosity of the adhesive , the easier it is to wet the workpiece surface . Most adhesives for metal are therefore two-component adhesives . Acrylic , epoxy and polyurethane adhesives are predominantly used .
It can be challenging to choose the best metal adhesive , given the vast number of
■ Cyanoacrylate adhesives develop strength very quickly . These bonds perform well in shear and resist non-polar solvents . Consider structural adhesives if you require an adhesive for metal that provides impact resistance or resistance to polar solvents .
■ UV Curable adhesives are suitable for invisible bonds between glass and metal . These are used extensively to bond metal hinges , knobs and fixtures to glass doors .
■ MS Polymer adhesives are suitable for thin sheets of metal , such as truck panel bonding . The low shrinkage provides no read-through ( you can ’ t see where the adhesive is placed through the panel ), providing a good aesthetic finish .
Hot-melt adhesives are also common . These are applied in a molten state approx . 150 ° C - 190 ° C and the parts must be joined before the “ melt ” solidifies . An example of this is the hot glue gun . Adhesive tapes are suitable for bonding similar and different materials such as metal ( e . g . steel , stainless steel and aluminium , also with powder-coated surfaces ) with plastic , wood or glass .
However , before deciding on a suitable adhesive , it is worth asking the questions below :
■ What kind of metal are you bonding ?
■ What is the metal surface ( oxidised , oily , surface-treated , painted )?
■ What is the geometric bonding surface ( flat , angular or punctual )?
■ Are there any processing specifications or constraints ?
■ What is the type and extent of load on a bond ( dynamic , heavy-duty , heatresistant )?
■ Where will the bonded metal be used later ?
ISMR June 2022 | sheetmetalplus . com | 21