Joining and bonding of composite parts

Manuela Compostella, Portfolio & Activation Marketer,
Patrizia Capogreco, Application Engineer,
Daniele Trinchero, Application Engineer
from 3M Industrial Adhesives & Tapes

This project has been awarded by Compositi’s editorial staff and technical-scientifical Commitee!

The structural adhesive advantage

Recent trends and advancements in the manufacturing and cost reduction of composite materials have increased their usage in the transportation, industrial, and many other markets in addition to their traditional use in the aerospace field. Driven by increased government regulations on vehicle emissions, the need for light weighting, and increased end consumer demand for higher performance products, composite materials, and parts are increasingly becoming part of an engineer’s day to day design specification. Composites are used in a wide variety of applications to reduce weight, provide improved environmental resistance, improved aesthetics, greater design options and increased stiffness to weight ratio.

ADVANTAGES OF ADHESIVES FOR COMPOSITE BONDING

Composites require new methods of bonding or joining (beyond traditional mechanical and thermal methods) to allow for design and performance optimization. Fortunately, advances in structural adhesives (such as epoxies, acrylics, and urethanes) have enabled designers to create products meeting structural integrity requirements without the use of mechanical fasteners, rivets, or welding. Additionally, these structural adhesives work well with multiple substrates including plastics, metals, and composites without sacrificing performance properties.

Even Low Surface Energy (LSE) plastics, such as thermoplastic polyolefin (TPO), polypropylene (PP), and polyethylene (e.g. HDPE), which in the past had to be mechanically attached or heat welded, can now be bonded with specialty structural adhesives. To join composites or mixed materials, mechanical attachments (such as clips, screws, etc.) can be used with virtually any surface, but they require additional steps to mold or create features for the attachment. This can lead to stress concentrations, which may result in plastic cracking and premature failures. Also, drilling holes into composite materials will result in reduced strength due to the introduction of discontinuities in the matrix and reinforcing fibers.

Heat and friction welding is a common alternative for certain composites. However, these welding techniques are energy and tooling-intensive and limited in the geometries and substrate combinations that can be addressed. In addition to forming strong bonds, structural adhesives can lower overall costs while increasing the durability of products; and are typically lighter weight than mechanical fasteners. Durability is improved because adhesives distribute stress across the entire bonded area, whereas mechanical fasteners, rivets, and spot welding can create stress concentrations leading to weak points across the substrates.

Another advantage for adhesive bonding is the ease in which it allows different materials to be combined compared to conventional mechanical methods. For example, structural adhesives prevent galvanic corrosion between dissimilar metals. Finally, the cleaner look of bonded joints versus mechanical fasteners allows for better looking, more efficient product builds without additional finishing work.

To be continued…