Surface Prep Guide for Adhesives

Contents

• Overview of Surface Preparation
• Specific Instructions for Individual Substrates

Overview of Surface Preparation

After selecting the proper adhesive, surface preparation is the most important thing you can do for making a quality bond. Why? Because adhesives adhere to surfaces, and surfaces are often covered with dust, dirt, grease, oil, oxide films, rust inhibitors, and in the case of plastics, mold release agents. All these contaminants, even at a microscopic level, interfere with the adhesive doing its job. In fact, poor surface prep is the #1 reason for bond failure.

Below is a cross section diagram of a typical substrate surface. For the strongest and most durable bonds, the goal is to apply adhesive to the substrate itself. However, often there are layers of surface contaminants that obstruct the surface. The loose layer is all dust, dirt, and other loose particles. Beneath the loose layer lies surface oils and grease. Finally, there is sometimes an embedded layer clinging firmly to the surface such as rust, other oxide films, rust inhibitors, additives that have migrated to the surface such as mold release agents, and surface coatings like paint and sealants. All of the above get in the way of making a strong bond, which is why surface prep is so important.

Cross-section of surface layers on a substrate

Below are our recommendations for prepping surfaces for adhesive bonding. Please note: all manufactured materials vary significantly, even within the same type, as a result of different additives and methods used in the manufacturing process. This is especially true for plastics.

Always test first on samples to determine suitability.

How much surface prep do I need?

As a general rule, the more surface prep you do, the stronger and more durable a bond. Minimal surface prep, such as wiping the area clean, might be fine for simple bonds. For structural strength bonds, more treatment is required.

Here are the types of surface prep you can do, in order of greater effectiveness:

1. Clean the surface
2. Degrease
3. Abrade
4. Surface Alteration

1. Clean the surface

At a minimum, you need to have a CLEAN, dry surface on which to apply your adhesive. Remove as many surface contaminants as possible from the bond area, including dust, dirt, and grease. For delicate surfaces, clean off as much as practical, especially any loose particles or surface films. If you use detergents or liquids to clean the surface, make sure the surface is dry and void of trace cleaners before applying adhesive.

2. Degrease

The next step you can do is degreasing the surface. Degreasing is removing all traces of oil and grease (even relatively clean surfaces have oils from fingerprints). If possible, use a solvent such as methyl ethyl ketone (MEK), acetone, or isopropyl alcohol*. Wipe or bathe the surface clean. We recommend degreasing whenever possible, because surface oil or grease will get in the way of almost all adhesives and weaken adhesion (with the exception of methacrylates).

How to test if your surface is clean and degreased: For many surfaces, especially metals, a simple test to determine if your surface is clean and degreased is to drip a few drops of water on the cleaned area. If the water spreads to cover the area with a continuous film, the bond area is clean. If the water beads, that means the surface needs to be degreased further. Remember: all surfaces need to be dry before bonding. Also, bond the surfaces as soon as practical after cleaning and degreasing to avoid having a recontaminated surface. Note: This test will not work well for certain plastics, because the water might bead, even though the surface is not clean enough for bonding.

3. Abrade

For the strongest and most durable bonds possible, we also recommend abrading. Abrading a surface gets rid of surface films, mill scale, paint, and oxides and also gives a much better area for the adhesive to "grip." We recommend abrading if practical for the material you are bonding. Use fine grain sand paper (120-200 grit), emery cloth, steel wool, or grit blasting. Abrade enough just to clean and roughen the surface. After an abrasion treatment, remember to remove all loose particles before applying adhesive. Brush the surface or blow with compressed air, and then degrease to clear away all loose particles and residual oils. Abrading and degreasing is enough surface prep for all but the most demanding industrial applications.

4. Surface Alteration

Finally, some materials are simply difficult to bond. These include EPDM rubber, and "polyolefin" plastics (such as polyethylene, polypropylene, acetal, and hard nylon). Also, some metals such as copper, stainless steel, and aluminum are slightly more challenging to bond than other metals. If you are bonding these materials, you might need to change the surface properties to get good adhesion. Specifically, these are considered low surface energy surfaces, and you need to alter the surface to increase the surface energy. This involves using chemical primers, scorching the surface, grit blasting, or acid etching the surface. Because all materials are slightly different, consult your material supplier for the most effective way to treat the material for best adhesion.

* NOTE: Always first determine if a solvent can be used without harming your material, especially in the case of plastics. Also, organic solvents are highly flammable. Follow all safety instructions and local regulations when using such solvents.

Specific Instructions for Individual Substrates

Below are some of our recommendations for prepping specific surfaces for structural strength bonding. As mentioned above, all materials are slightly different, so use these as guidelines and always test on samples to determine suitability and efficacy.