Frictionless rigid joining of two similar or dissimilar substrates using an adhesive is called structural bonding. Typically, structural bonding joints exceed shear strengths of 10N/mm2 and are load bearing joints that behave like a single unitized part when subjected to external forces.
Adhesives that Produce Structural Joints
Traditionally single part and two-part epoxies have been used as the most common structural adhesives in industries such as aerospace, automotive, wind and general engineering for bonding and maintenance repair applications. In recent decades Methyl Metha Acrylates, also commonly known as MMAs or simply acrylic adhesives are being widely preferred due to their versatility and ability to bond a variety of substrates rapidly. In addition to Epoxy and MMAs, two-part Polyurethane adhesives can also be used for structural bonding applications.
Welding in Powder Coated Parts
There are numerous powder coated parts in industries that are welded prior to powder coating process. Applications such as stiffener joining on panels, bracket joining, fixtures, channels etc are but only a few examples where parts are welded. Welded joints when properly pre-treated adhere well to fused powder and form a long lasting and reliable coating.
Although generally very reliable, the challenges related to welding process can create complexities in part design and drive the cost up which is why in many applications manufacturers are looking to adopt adhesive bonding as a preferred method. Some of the common difficulties associated with welded joints are:
Welding demands certain minimum part thickness leading to thicker and heavier parts.
Welding requires skilled manpower and if not performed right can create porosities, warpage and slag.
Often additional sealant may be needed to prevent moisture and dust from settling into crevices.
All this drives the application time and cost high.
Bonding Before Or After Powder Coating?
The important question in this application is weather one must bond parts before powder coating or after. The answer depends on a variety of factors and many aspects need to be considered to arrive at the answer. Let’s first look at the key challenges in both the bonding approaches.
Bonding prior to powder coating
Bonding before powder coating process is simpler and easier in terms of complying with existing assembly procedures but it can pose multiple challenges in the form of:
- Resistance of adhesive to the pre-treatment processes such as pickling, rinsing, phosphating among others. All adhesives will have an eventual drop in strength owing to this process.
- Because in real life environment a powder coating oven can exceed 200°C, the thermal performance of the adhesive needs to be carefully evaluated.
- Water wash out of the uncured one part adhesives during rinsing and cleaning process may be possible leading to uneven finishing or additional finishing process prior to powder coating.
- Depending upon the type of adhesive selected, additional fixturing mechanism in the form of weld spots along with adhesive bonding may be necessary.
- The glue must be compatible with powder coating to allow for a uniform coating bond strength throughout the part. If the adhesive does not allow for strong bonding with powder, it may lead to weak coating strength in the bond edge.
The graph below depicts how a high performing Methyl Metha Acrylate structural adhesive performs when exposed to high temperature applications such as powder coating. While it exhibits excellent strength retention upto 180°C, the strength drops significantly at temperatures beyond 180°C under ageing.
The graph below depicts how a single part structural epoxy adhesive performs when exposed to high temperature applications. Although Single part epoxy exhibit better strength retention beyond 180°C, strength drops nevertheless.
The above data and graph indicates that on account of thermal performance alone, structural bonding prior to powder coating has many variables that need to be looked into which makes this option difficult to adopt.
Bonding after powder coating
As per Elixir’s research, bonding parts after powder coating process can be more reliable and predictable while creating fewer variables in achieving solid results. In general Elixir’s adhesives exhibit strong bond with surfaces that are coated and treated with both thermoplastic and thermoset powder coatings. Yet an important consideration in this case is bonding strength of powder coating to the metal body in case of cantilever design where continuous peel forces act upon powder coating.
The graph below indicates progressive strength developed on a powder coated surface by a 5min cure structural methacrylate adhesive
Generally speaking, powder coated surfaces are well suited for adhesive bonding and if the bonded joints are not subjected to very high external forces, powder coated surfaces can be directly bonded together.Another approach is to “mask” the surface to be bonded while powder coating to allow for true metal to metal contact. This approach can significantly up the reliability and durability of the joint.
The graph below depicts how a Crestabond M7-05, a MMA structural adhesive rapidly develops strength on bare aluminum to aluminum joint at room temperature.
Elixir-India has executed multiple structural bonding applications where life span of a bonded joint is expected to be in decades in outdoor and dynamic environment. Solar energy, wind turbines, appliances are a few industries that benefit from our adhesive solutions.
We recommend that to achieve a reliable bond strength and better results bonded area be masked or powder can be wiped off during the coating process exposing the bare surface for metal-to-metal joints. Alternatively, light sanding be undertaken to bring metal to metal contact. Some of the benefits of this recommendation will be:
Eliminating exposure of bondline to harsh pre-treatment process that will impact the performance.
Eliminating exposure of bondline to high temperature and thus retaining room temperature strength.
Weld spots necessary for additional fixturing for bonding before powder coating process is completely avoided.
Ruling out guess work in the entire bonding process and achieving all the advantages of bonding.