Better bonds

Lawrence W. Fisher
Manager, Mechanical Systems Dept.
Measurement Analysis Corp.
Torrance, Calif.

Adhesives are unlike other fastening systems: It's difficult to determine whether the bonded joint has the required strength and integrity without destroying it. It's easy to judge the quality of other fastening methods, such as threaded bolts with torque wrenches. The direct measurement of applied torque confirms the joint preload while joint-qualification tests provide a rational basis for inferring the bolted joint's strength and integrity.

In contrast, adhesive joints don't allow for this type of direct measurement of strength and integrity. It's difficult to inspect or evaluate the integrity of bonded assemblies without simultaneously destroying them. Trained assemblers can inspect a joint for obvious flaws as well as subtle ones. The latter include lack of bond line uniformity or a soft, improperly cured adhesive. Although this information is useful, it by no means guarantees an effective adhesive bond. Designers rely on five key factors to help ensure adhesive-bonded assemblies have strong reliable joints:

• Strength/stiffness of the substrates to be bonded
• Adhesive bond strength to the substrates
• Adhesive strength/modulus
• Geometry of bonded area relative to applied load
• Adhesive thickness

These important joint properties are best considered early in the design process so they can be appropriately included in the joint design. Another key element in achieving maximum joint strength is good preparation of the substrate surface. Designers must include this preparation process when selecting the substrate and adhesive that provide desired joint properties. Poor surface preparation often results in reduced joint strength. This region on the bond depends on the substrate's chemical composition and processing methods that may produce undesirable surface conditions. For example, metal oxides often form on metallic substrates during processing or storage. Plastics can have impurities on their surface related to processing or as a result of the bonding process.

SURFACE PREPARATION
The primary objective of preparing the substrate surfaces is to remove contaminants such as oils, grease, rust, oxides, films, and dust. Surface preparation is also performed to improve the adhesion properties above that which would normally be obtained through cleaning. For example, abrading the surface improves mechanical interlocking while flame treatment of some plastics significantly increases the surface energy, both resulting in improved adhesion.

The mechanical and physical properties imposed by the application often dictate the substrate composition. But all metal, plastic, elastomer, and ceramic substrates require cleaning or surface modification. This preparation boosts the surface energy that promotes wetting of the adhesive and subsequent adhesion. A surface that produces the best bond is clean and free of contamination, uniform and continuous in finish, and stable with high surface energy.

Surface energy defines the ability of adhesives and pressure-sensitive adhesive tapes to "wet out" on to the substrate surfaces and promote adhesion. Materials with a low surface energy — including olefin-based thermoplastics and polypropylene — may need priming or flame or corona treating before they can take an adhesive. These pretreatments convert low-surface-energy substrates to a higher surface energy better suited for strong adhesive bonds. Likewise, anodizing aluminum surfaces is one method that will boost reliability of adhesive-bonded joints incorporating those materials. Anodizing increases surface roughness, resulting in a significant improvement in mechanical adhesion behavior of the substrate.

The simplest way to prepare a substrate for bonding is to just clean it. Generally, the quality of the surface prep is proportional to both the complexity and cost of the assembly. All surfaces benefit from proper preparation, but not all applications require extensive cleaning or modification. Solvent cleaning followed by mechanical abrasion is the simplest and most effective preparation process.

This technique will improve bonding when applied to almost any substrate. It is easy to confirm the surface is clean without complex equipment or procedures. Verification by wiping with a white cloth, for example, is often enough.

Solvent on a white cloth is the first line of attack for removing oils, grease, and mold-release agents. Surfaces should get a thorough cleaning until the cloth no longer picks up dirt or oils. Otherwise residual contamination can be forced into the surface, reducing the effectiveness of treatments to follow.

Special surface pretreatments enhance the bond strength of many substrates. In any case, designers should factor in the cost of cleaning and surface preparation when they design the joint. Adhesive manufacturers have specific requirements that must be spec'd as early in the design as possible. A few common techniques include:

Mechanical abrasion with abrasive blasting, sanding (medium grit, 180 to 325), and wire brushes must abrade surfaces with features small enough to promote capillary action of the adhesive into the substrate microstructure.

Degreasing does not increase surface energy of the substrate. There are several ways to degrease parts: pressure washing, mechanical agitation, vapor degreasing, mechanical abrasion in solvents, and ultrasonic agitation. Substrate qualities are generally what determine the degreasing method.

Acid etching removes organic contamination and generates a clean, microetched surface. The three most common etches are dilute solutions of chromic, sulfuric, and phosphoric acids.

Gas-plasma treatments take place in low-pressure chambers under high frequency and high voltages in the presence of different gases. This costly process produces an ionized gas that oxidizes the substrate surface to improve adhesion.

Primers are typically low viscosity liquids applied directly to the substrate to promote adhesion and prevent oxidation.

Conversion coatings take the form of acidic solutions applied to metal surfaces. They produce smooth, uniform organic coatings that promote adhesion and prevent corrosion. They are typically used on nonstructural aluminum joints.

Flame treatments oxidize surfaces and make them more easily wetted by the adhesive. The process employs a blue flame from an oxyacetylene or propane torch to turn substrate surfaces glossy or shiny. Care must be taken to not overheat and distort the substrate. Following treatment, parts need to be washed with soap, rinsed, and dried. Flame treatment is commonly used with polypropylene and polyethylene polymers.

Ionizing treatments are used on nylons to alter surface crystallinity and thus promote adhesion.

Anodizing with chromic and phosphoric solutions produces a protective layer and microporosity that gives aluminum and titanium alloys stronger adhesive bonds.

UV exposure oxidizes substrate surfaces and promotes adhesion.

Laser treatments remove surface contamination and roughen substrate surfaces. Excimer lasers can also oxidize substrate surfaces to promote adhesion.

MAKE CONTACT
Ellsworth Adhesives, (800) 888-0698, ellsworth.com
Measurement Analysis Corp., (310) 378-5261, macorp.net
Lord Corp., (877) 275-5673, lord.com
Scelzi Enterprises Inc., (800) 858-2883, seinc.com