Tougher and stronger glue

Scott D. Anderson,
Lead Market Application Engineer,
Loctite Industrial Adhesives
Henkel Corp.
Rocky Hill, Conn.

Edited by Jean M. Hoffman

You will probably never live to see the day when cars and trucks are completely glued together, with no mechanical fasteners to be found. But more and more connections are going with adhesives. The reason: Recent advances in structural adhesives deliver the same strength as welds, or go through paint baking unscathed. Other new adhesives require no surface cleaning, a process that was once a “given” before any bonding could commence.

Today’s structural adhesives are formulated to withstand severe shock, peel, and impact. They bear heavy loads, withstand chemicals, endure extreme temperatures, absorb energy, and can take large deformations without rupturing.

Specialty-vehicle manufacturers, for example, now use structural adhesives to replace or augment rivets, bolts, welding, and other traditional fastening methods. Adhesives rather than fasteners assemble frames, panels, booms, and cabs. Better product aesthetics result from adhesive assembly and, thus, can greatly boost vehicle value. Structural adhesives also simplify production by eliminating the need to precisely bore holes for installation of rivets or other mechanical fasteners.

Likewise, in moist environments such as tubs and spas, structural adhesives attach and bond galvanized-steel frames to fiberglass and ABS. And commercial furniture manufacturers bond plain, painted, or powder-coated metals and plastics in chairs, desks, and cabinets.

Structural-assembly options
There are three traditional fastening methods thermal, mechanical, and chemical. Thermal methods weld, braze, or solder two homogenous materials with similar melting points. Mechanical fastening secures dissimilar substrates with bolts, screws, or rivets. And chemical assembly bonds similar or dissimilar substrates using adhesives.

Cost is the most noted limitation of thermal joining. It usually takes specialized labor for thermal joining. Welded joints are often nonuniform, lack the clean aesthetics for high-end applications, and aren’t easy to disassemble. Although it does not take skilled workers to drill holes and insert fasteners for mechanical fastening, fasteners can loosen and require a large inventory of parts. It is not a process that can be easily automated. Holes for fasteners can create leak paths, a starting point for corrosion, and may detract from visual aesthetics. Fasteners and thermal joints concentrate stress at a single point. Under flexing and vibrations, the assembly may prematurely fail at the joint.

In contrast, adhesive bonding takes fewer skilled workers and can be 20 faster than welding. Adhesives distribute the stress load evenly over a broad area and resist flex and vibration stresses. They also form a seal as well as a bond which helps protect joints from corrosion. They are applied inside the joint and are nearly invisible. Some adhesives fill large gaps between irregularly shaped surfaces more easily than mechanical and thermal fastening. They weigh little, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates as well as heat-sensitive materials.

Assembly is easily automated because adhesives are liquids prior to curing. It is easy to meter just the right amount of adhesive to ensure strong bonds with good aesthetics. Excess adhesive easily removes before painting. This is a significant advantage over welded joints that need grinding and abrading to get smooth surface finishes.

Adhesives do have several limitations. It can take seconds to hours before a joint gets to handling strength. This can slow an assembly process. One way around the problem is with temporary mechanical fasteners that hold the assembly as it moves to the next processing step. Adhesives are also chemicals and, thus, handling and disposal is subject to government regulations. They also can’t be easily disassembled unless high heat is applied to breakdown the adhesive.

Structural adhesives
Structural-adhesive technologies include epoxies, acrylics, methylmethacrylates (MMAs), modified silanes, and polyurethanes. Adhesives can be tailored to deliver processing and performance benefits. For example, adhesives come as onepart no-mix or two-part mix systems. They can cure under ambient room temperature or at elevated temperatures in cure ovens. For high-speed assembly, there are fastcuring adhesives as well as those with long work lives for parts that need alignment once they are in place. High, medium, and low-viscosity formulations can fill large gaps or provide thin, virtually invisible bond lines.

It is usual practice to add dispensing and curing equipment to a specific assembly process. This lets manufacturers optimize process efficiency, keep down waste, and make dispensing and curing repeatable and consistent.

Gauging performance
There are various ways to analyze adhesive bond strength. The usual goal is to prove that adhesive bonding provides an economic advantage while delivering final assemblies that are stronger and more reliable.

Tests typically compare an existing method of assembly to one with adhesives by pulling joints apart and analyzing tensile shear strength, impact strength, cyclic loading/fatigue strength, fracture toughness, and peel strength. Strength under environmental conditioning is also an important testing parameter that involves analyzing humidity, salt fog, heat aging, chemical resistance, vibration, and other conditions an assembly will see. Testing can be simulated in a lab or conducted in the actual end-use environment.

A key to performance for any adhesive is its ability to withstand force. In one test, researchers compared the ultimate strength of a cold-rolled steel assembly joined using adhesives (MMA, epoxy, and two-part acrylic), mechanical fasteners (≥/8-in. bolt and three POP rivets), and welding (butt weld and two spot welds). All three adhesive technologies were stronger than the mechanical methods and similar in strength to the thermal assembly methods.

Advances in structural-adhesive technology have dramatically expanded the scope of potential bonding applications. Over time, traditional structural adhesives lose strength on substrates such as galvanized steel. Here, the zinc coatings used on the galvanized steel eventually delaminates the bond.

New structural adhesives are providing long-term durability on such hard-tobond substrates. Prefabricated building components including galvanized trusses, joists, shear walls, headers, studs, and cantilevered beams are also adhesively bonded. Traditional structural adhesives can also have trouble maintaining longterm strength when exposed to elevated temperatures. This data is significant for a manufacturer of electric motors trying to decide whether to attach magnets using mechanical clips or adhesives. Electric motors continuously operate at elevated temperatures. Adhesives joints in the magnet assembly will gain strength over time and will withstand vibration better than mechanical fasteners.

Adhesives are also making inroads in loudspeakers that are getting smaller with each new generation. Speaker manufacturers are demanding impact and temperature- resistant structural adhesives that cure rapidly. As speakers get smaller, adhesives must also withstand high temperatures that result from tighter operating spaces. The demand for more portable speakers also means adhesives must have higher resistance to impacts. And to improve the bottom line, speaker manufacturers demand fast-fixturing adhesives to speed production, reduce work-in-process, and eliminate mechanical-fastener inventories.

Environmental considerations can also greatly affect the long-term performance of an assembly. A two-part MMA, for example, was exposed to salt fog and humidity for 1,000 hr. Here the adhesive maintains more than 95% of its strength when compared to the control sample.

Cyclic loading or fatigue testing assesses an adhesive’s ability to perform over the expected life of a device. Tests have shown that advanced structural adhesives can withstand 10 million cycles of exposure to extreme tensile and shear forces before failing. The key here is to design the joints with minimal gaps.

An often-tested criteria is fracture toughness. Fracture toughness is the adhesive’s ability to resist further cracking after a flaw is induced. This data helps the engineer understand how the adhesive performs once compromised.

Recently introduced structural adhesives can withstand welding, phosphate pickling, and powder-coating processes while maintaining strength. And some can replace costly MIG or TIG welds by bonding two substrates and supporting the bond with low-cost spot welds that hold the assembly together while the adhesive cures. These fast-fixturing, weld-tolerant adhesives let assemblies go in pickling and paint-bake processes within 20 min.

MMA adhesives can bite through surface oils and contaminants, eliminating the need for surface preparation and activators. For traditional adhesives, surface preparation has always been critical to ensure a long-lasting, reliable bond. Surface-treatment requirements depend on the level of contamination, the substrates, the initial and long-term bond performance, and the financial practicality of the treatment process. For difficultto- bond substrates such as polyolefin plastics, new adhesives bond exceptionally well without the need for primers or surface treatments.

For flexible joints, the latest polyurethanes and modified silanes withstand vibration and provide long-term durability. They are used for window glazing and roofpanel bonding on heavy-duty construction equipment, trailers, utility trucks, agricultural equipment, and school buses.

And recently developed structural adhesives seal out moisture and gases in applications where substrates have different coefficients of thermal expansion (CTE). For example, paintable, clear, UV-resistant modified silanes are being used for bonding appliance assemblies, truck bodies, and in HVAC applications.

Make contact
Henkel Corp., (860) 571-5100, loctite.com