Adhesives Help Auto Manufacturers Eliminate Bond Line Read Through (BLRT)
As auto manufacturers continue to move towards thinner composites in order to meet lightweighting targets, bond-line read-through (BLRT) has become an increasingly important issue. Ideally, bond lines should be invisible to the naked eye, and when they can be seen, consumers may reject the finished product. As a result, quality controllers may reject components that exhibit BLRT, resulting in rework and higher costs.
The use of dissimilar substrates that expand at different rates when heated or contract to a greater or lesser degree when cooled contributes to BLRT. This can cause deformation of the adhesive, particularly when large volumes are used. Differences in the bond gap have a similar effect.
To solve the problem, experts recommend design, process, and adhesive choices that will limit BLRT. Thus, the properties of the adhesive, the design of parts, and production processes will all have a role to play in mitigating BLRT.
The issue has been thoroughly researched by Ashland, and the result was a selection of epoxy and polyurethane structural adhesives, and hybrids using both adhesive technologies, that would also cure quickly and be heat resistant. These adhesives had to pass ecoat – a 40-minute exposure to a temperature of204°Cfollowed by a lap shear strength of 0.3MPa at the same temperature.
Once the testing had been concluded, Ashland’s research team selected four two-part polyurethane/epoxy adhesives and one epoxy which showed promise as a means of reducing BLRT while still passing ecoat. In addition, the chosen adhesives had to meet rapid curing targets of eight to sixty minutes. Of the selected adhesives, the epoxy demonstrated the highest tensile strength and modulus.
The next step was to measure BLRT with the help of a Zeiss surface analyzer. Adhesive bead width, the relationship between bond gap and curvature, and cure time were among the variables taken into account in Ashland’s study. The researchers were able to make the following conclusions:
Shorter cure times resulted in lower curvature
As bond gaps increased, so did curvature
Lower curvature was associated with lower modulus
Adhesive bead width did not affect curvature
They therefore concluded that high-elongation, low-modulus adhesives allow engineers to achieve better results when using composite substrates.
Composite manufacturers will need to bear adhesive properties in mind when designing parts. Uniform bond gaps, and uniform bond heat across the bond-line must be taken into account. Process engineers will have a role to play. They must allow for the coefficient of thermal expansion and design bond fixtures in such a way that pressure and heat transfer will be uniform across the assembly. In this way, they will be able to reduce variations in bond gap and therefore also minimize BLRT.
In addition, using a thicker gauge on panel edges will allow for thinner interior gauges without increasing BLRT. However, the team noted that the usefulness of this strategy may be limited by engineering constraints.