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

Scelzi Enterprises Inc., Fresno, Calif., originally used 100 rivets to attach aluminum diamond plates to the steel bodies of their custom service trucks. A switch to an engineered adhesive from Lord Corp., Cary, N.C., let them reduce assembly costs: The adhesive system bonds through oily layers so minimal surface prep is needed before parts are joined.

Scelzi Enterprises Inc., Fresno, Calif., originally used 100 rivets to attach aluminum diamond plates to the steel bodies of their custom service trucks. A switch to an engineered adhesive from Lord Corp., Cary, N.C., let them reduce assembly costs: The adhesive system bonds through oily layers so minimal surface prep is needed before parts are joined.

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.

ADHESIVE SELECTION

 
Acrylic
Anaerobic
Cyanoacrylate
Epoxy
Hot melt
Polyurethane
Polysulfide
Silicone
Solvent base
Water base
UV
Viscosity
Medium
Low
Low
Medium to thick
Thick
Medium
Thick
Thick
Low to medium
Low to medium
Low to medium
Void filling
Good
Poor to fair
Poor to fair
Very good
Very good
Good
Very good
Very good
Fair
Poor
Low to medium
Heat resistance
Good
Good
Fair
Good
Poor to fair
Fair
Good
Very good
Good
Fair
Fair
Cold resistance
Good
Good
Fair
Fair
Fair
Good
Good
Very good
Good
Fair
Good
Flexibility
Good
Good
Poor to fair
Fair
Fair to good
Good
Good
Very good
Good
Poor
Good
Chemical resistance
Good
Good
Good
Good
Fair
Good
Very good
Very good
Good
Poor
Fair
Humidity resistance
Good
Good
Fair
Good
Good
Fair
Good
Very good
Good
Poor
Good
Work time
Medium to fast
Medium
Fair
Slow to medium
Fast
Slow to medium
Medium
Slow to medium
Slow to medium
Medium
Slow
Cure time
Medium to fast
Medium
Fair
Slow
Fast
Medium
Medium
Medium
Medium
Medium
Fast
Metal bond (steel, aluminum)
Good
Fair
Good
Good
Fair
Good
Good
Fair
Good
Poor
Good
Plastic bonding (ABS, styrene)
Very Good
Fair
Very good
Fair
Fair
Very good
Fair
Fair
Fair
Poor
Good
Polyolefin bonding
Fair
Not suggested
Good
Poor
Poor
Good
Not suggested
Fair
Fair
Poor
Fair
Wood
Not suggested
Not suggested
Not suggested
Good
Very good
Not suggested
Not suggested
Not suggested
Good
Very good
Fair
Paper cardboard
Not suggested
Not suggested
Not suggested
Not suggested
Very good
Not suggested
Not suggested
Not suggested
Good
Very good
Fair
Ellsworth Adhesives, Germantown, Wis.

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.

Surface Pretreatments Of Common Metals And Ceramics For Improved Adhesive Bonds

Substrate
Solvent cleaning*
Intermediate cleaning
Chemical treatment or “other”
Aluminum
Chlorinated solvent, ketone, or mineral spirits**
Detergent scrub
Sulfuric (96%) acid /sodium dichromate (77.8/22.2-pbw) solution at 25°C for 20 min; rinse with tap water followed by distilled water, dry for 30 min at 70°C
Beryllium/copper
Chlorinated solvent, ketone, or mineral spirits**
Wet-abrasive blast
Copper
Chlorinated solvent, ketone, or mineral spirits**
Dry abrasion or wire brushing
Nitric (69%) acid/ferric chloride/distilled water (12.4/6.2/81.4-pbw) solution at 21 to 32°C for 1 to 2 min; rinse in tap water followed by distilled water; dry at 65°C maximum
Steel (stainless)
Chlorinated or aromatic solvent**
Heavy-duty alkaline cleaner
Nitric (69%) acid/distilled water (20/80-pbw) solution at 21 to 32°C for 25 to 35 min; rinse with tap water followed by distilled water; dry at 65°C maximum
Steel (mild)
Same as stainless
Same as stainless
Ethyl alcohol (denatured)/orthophosphoric (85%) acid (66.7/33.3-pbw) solution at 60°C for 10 min; rinse in tap water followed by distilled water; dry for 60 min at 120°C
Titanium, titanium alloys
Ketone or aromatic solvent
Mild alkaline cleaner or wet- abrasive scour
Nitric (69%) acid/hydrofluoric (60%) acid/distilled water (28.8/3.4/67.8 pbw) solution at 38 to 52°C for 10 to 15 min; rinse with tap water followed by distilled water ; dry 15 min at 71 to 82°C; brush off carbon residue with nylon brush while rinsing
Ceramic
Ketone solvent
Sulfuric (96%) acid/sodium dichromate/distilled water (96.6/1.7/1.7 pbw) at 20°C for 15 min; rinse with tap water followed by distilled water; dry at 65°C maximum
* Immerse, spray, or wipe
** Or vapor degrease with chlorinated solvents
Source: Ellsworth Adhesives, Germantown, Wis.

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.

Surface Pretreatments Of Common Plastics For Improved Adhesive Bonds

Substrate
Solvent cleaning*
Intermediate cleaning
Chemical treatment or “other”
Acetal (Delrin)
Ketone solvent
Dry abrasion or wet or dry-abrasive blast
Sulfuric (96%) acid/potassium dichromate/distilled water (88.5/4.4/7.1-pbw) solution at 25°C for 10 sec; rinse with tap water followed by distilled water and dry at room temperature
ABS
Ketone solvent
Dry abrasion or wet or dry-abrasive blast
Sulfuric (96%) acid/potassium chromate/distilled water (65/7.5/27.5-pbw) solution at 60°C for 20 min; rinse with tap water followed by distilled water; dry with warm air
Polycarbonate (PC)
Alcohol
Dry abrasion or wet or dry-abrasive blast
Polyethylene (PE) and polypropylene (PP)
Ketone solvent
Sulfuric (96%) acid/sodium dichromate/distilled water (88.5/4.4/7.1 pbw) at 70°C for 60 sec; expose surface to gas-burner flame until the substrate is glossy. Can also be treated with corona discharge or flame.
*Immerse, spray, or wipeSource: Ellsworth Adhesives, Germantown, Wis.

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