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Modern technologies for joining of similar and dissimilar materials

( 02/23/2018 )


Fiber Reinforced Plastics (FRP) are well known for use in industrial applications, but can also be viewed as recently “re-discovered” materials. A wide range of FRP is available on the global market, with various specifications for multiple applications in general industry, automotive, marine, railway and aerospace. The amount of FRP being used in the world has been growing significantly in recent years, and it is projected that these materials will be used more and more in the future. The most common types of FRP are Sheet Molding Compounds (SMC) or Bulk Molding Compounds (BMC), Glass Fiber Reinforced Plastics (GFRP) and Carbon Fiber Reinforced Plastics (CFRP). All of these types of FRP are well known for their excellent strength and rigidity. This is made possible by the specific mix of fibers and matrix materials (resins) used to make up the composite substrate. Depending upon the fiber material, length, amount, and direction of the fibers, custom tailored characteristics can be designed into the matrix. After the resin is cured, a light-weight high-performance material is created.


JOINING TECHNOLOGY FOR FRP? In many applications, FRP is matched with FRP in a structural assembly. However, there are also assemblies where FRP is bonded with other materials such as aluminum or steel. These types of hybrid joints are often difficult to handle.

Mechanical fastening such as welding, riveting, or use of screws requires a lot of preparation and a specific material thickness, and in many cases these fastening methods simply cannot be used for assemblies with FRP. For light-weight constructions, in particular, material thickness is essential, and causes a further challenge towards the use of mechanical fastening. For the majority of applications where FRP is used, adhesives are the best choice for the process of assembly. Adhesives can simplify the assembly process, help in reducing material thickness through distribution of stress, and bond both similar and dissimilar materials. With the wide choice of high performance structural adhesives available in the market today, the right adhesive solution can usually be found for even the high number of various FRP-to-substrate structure combinations that may be considered. BONDING SHEET MOLDING COMPOUNDS (SMC) – WHICH ADHESIVES ARE RECOMMENDED? Epoxy or Polyurethane adhesives are usually good choices for bonding SMC to SMC. Epoxies are generally chosen if high temperature performance or resistance is needed, and Polyurethanes are the preferred choice for thinner materials where substrate surface distortion in the bonded area (termed Bond Line Read-Through) may be a concern. Common Applications using SMC include bonding of spoiler, trunk lids, hoods, or battery boxes in Automotive. One challenge with bonding SMC can be to find the right method of substrate surface preparation. Most SMC and BMC formulations contain quantities of release agents that could bloom to the surface and affect the bond. Cleaning processes can be designed to remove these release agents, typically with solvents or solvent-free systems. Mechanical pre-treatment (like sanding or scuffing) may be necessary, but is generally not preferred. Bonding SMC to bare metal is possible with Epoxy or Methyl Methacrylate-based (acrylic) adhesives. Bonding “big head” fasteners to SMC for the hood (Automotive) or attaching hinges to SMC panels (General Industry) might be examples where bare metal would be bonded to SMC.


GFRP can be bonded to itself using Methyl Methacrylate-based (Acrylic) or Polyurethane adhesives. Examples of typical GFRP bonded assemblies can be found in the railway industry, where adhesives are used not only in production but also as a repair solution. One challenge in this industry is to find an effective adhesive working process for the large number of GFRPs available, plus all of the gelcoats, fire retardants coatings, and types of paints that may be used. With the right choice of adhesive, such as a structural Acrylic adhesive, aggressive substrate surface treatments such as abrasion or plasma can be replaced by simple solvent cleaning. In hybrid bonding of GFRP to bare metals, use of Acrylic adhesives can also allow for minimum surface preparation of the metal parts prior to assembly.


Many CFRPs can be bonded readily with Epoxy adhesives. CFRP is usually a material of choice if high strength, light-weight, and very good resistance against mechanical, physical, and/or chemical impact is needed. Epoxy adhesives can usually bond to CFRP with a minimum of substrate surface treatment. Adhesives are an excellent alternative to mechanical fasteners for joining a wide range of CFRP assemblies. Effectively joining dissimilar materials is also possible through the use of adhesives. Durability of adhesive bonded assemblies, such as weathering and ageing resistance, is well established and tested for many applications. Simplifying the assembly process, reducing costs, and saving material and weight are other major benefits that can be obtained, making adhesives a good choice for a large number of industrial markets and applications.