Structural Adhesives Tailored to Meet Growing Automotive Manufacturing Needs

Posted on 7/7/2016 11:48:07 AM By ASC

There are many advantages that can be gained by replacing fasteners with structural adhesives, particularly in the auto industry. Much lighter bonded composites can replace metal parts helping to improve performance and fuel economy while reducing emissions. Composites also allow materials to perform better by eliminating the weak points created by fastener holes, and help to reduce costs by speeding up assembly, even when the adhesive is more costly than the fasteners it replaces.

Although aerospace structures could theoretically use adhesives only, redundant fasteners are mandated, but in other industrial sectors including wind energy and marine fabrication, adhesives-only bonding has been embraced. The automotive industry is using adhesives on a larger scale than ever before, and so is the construction industry.

A growth industry

As a result, the market for structural adhesives is experiencing sustained growth, while an increasing number of new structural adhesives are being developed. Research indicates anywhere from 4.5% - 8% growth in the worldwide adhesives market.

Technical Challenges Overcome

Manufacturers are eager to adopt structural adhesives to simplify assembly, but adhesives chemistry is complex. In recent years, the adhesives industry has found a variety of ways to overcome or mitigate technical and formulating challenges, allowing its clients increased freedom in their materials choices and designs. Design engineers have enjoyed expanded capabilities as a result.

Recent Advances in Automotive Adhesives

The ability to bond composites to metals has become necessary for the auto industry, and these dissimilar substrates have widely differing CTEs. Automotive adhesives have to be able to accommodate this while retaining the integrity of bonds, despite movement and vibration. Auto makers also want adhesives that cure quickly and can be used with little or no surface preparation.

BMW is among the auto manufacturers to use adhesives extensively, and BETAFORCE urethane composite bonding adhesive from Dow Automotive Systems is being used to bond the composites used on BMW’s new electric car, the BMWi3. The adhesive matches BMW’s requirement for strength, flexibility, and the capacity to bond dissimilar materials. The adhesive uses a one-of-a-kind pre-polymer chemistry that imparts greater mixability and stability to the adhesive. Dow reports that the adhesive has superior energy absorption under dynamic and fatigue loading thanks to its up to 400% elongation, and a modulus varying from 20 MPa to 300 MPa depending on the specific formulation chosen.

The new grade of BETAFORCE can be used without the application of primers or activators. The adhesive is applied by a metering and mixing machine and cures in 1 – 3 minutes when exposed to temperatures higher than 80°C. Dow automotive currently seeks to develop its adhesive still further to allow for longer open times and even faster curing. Improved application methods are also being explored.

L&L Products also supplies BMW, this time with adhesives that reduce noise, vibration and harshness (NVH), but it is also supplying the auto manufacturer with an adhesive film that is used to bond a composite stiffener to the steel B-pillar of the BMW 7-series. High temperature curing produces a cure time of less than 1 minute, but for the BMW, cure had to match the profile of the carbon prepreg in a compression molding process. Nevertheless, cure time is still a rapid 2 minutes. Once again, elongation properties allows for bonding of composites to steel, and the adhesive film has the additional benefit of separating the two substrates to eliminate galvanic effects and corrosion.

SCIGRIP Smarter Adhesive Solutions supplies several clients in the automotive sector, including GRP HighTech, a Thailand-based firm that supplies a number of OEMs, including Toyota. It uses SG5000 1:1 high-performance methyl methacrylate (MMA) to bond fiber-reinforced acrylonitrile butadiene styrene (ABS) to metal. The acrylics acrylics form strong covalent bonds that exhibit high tensile strength while also having up to 150% elongation. Surface preparation can often be eliminated altogether and the adhesive cures rapidly at room temperature.

SciGrip adhesives can be formulated as a 1:1 mix paste formulation for most transport and industrial applications, or it can be formulated as a 10:1 mix ratio that is used with a peroxide activator for even stronger bonds across a wide variety of substrates. The company reports that the adhesive is up to 90% as strong as epoxies. The SG230HV (high viscosity) and SG300 adhesives are both 10:1 mix ratio products designed for use in automotive assembly lines.

3M produces epoxies based on the Scotch-Weld chemistry it pioneered 30 years ago. The company reports that its Scotch-Weld two-part 420NS epoxy has the advantage of being able to tolerate the high temperatures used in automotive E-coat and powder-coating processes. In response to its customers’ needs, it has developed the 3M Scotch-Weld urethane adhesive 620, a low dispensing viscosity product with non-sagging properties and the flexibility to bond composites to metals.

Ashland's polyurethane, epoxy and acrylic Pliogrip structural adhesives have been used in the manufacture of cars and trucks for the last 40 years. Ashland reports that its Pliogrip 9100 two-component heat cure urethane offers manufacturers the benefits of a fast cure time and doesn’t require any surface preparation prior to bonding. Pliogrip 8500, on the other hand, was formulated for room temperature curing while Pliogrip 5500 two-component epoxy tolerates the high temperatures used when E-coat or powder-coating is applied. Open time is 30 minutes and the epoxy heat cures in 2 minutes, withstanding temperatures of up to 205°C. Most Class 8 truck hood and raised roof assemblies are bonded with Pliogrip. With lightweighting gaining momentum in the auto industry, Ashland is working on adhesives that will have even faster curing speeds to allow for high-volume production.

LORD Corporation supplies bus and commercial truck OEMs with epoxy, urethane and acrylic adhesives. The company reports that its 600-series allows for longer working times that help in the assembly of large parts such as truck fairings. It is possible to form large beads of adhesive when the bond gap is large, and is able to bond composites to metals. The new 800-series acrylics will reduce or eliminate bondline read-through. Urethane adhesives from LORD include sag resistant low-viscosity7542 as well as the higher viscosity 7545 adhesive.

Permabond  has an extensive presence in the adhesives market, and its MMA product TA4810 is currently finding a novel use – in the construction of the all-composite chassis of the Sunseeker solar challenge car currently being created at Western Michigan University. The adhesive is easing the process thanks to its ability to form thick bondlines without sagging, and its ability to cure in just 15 minutes.