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