Master Bond manufactures a wide range of adhesives for bonding plastics, as well as plastics to other substrates. Surface preparation plays a key role in ensuring that good bond strength is achieved. The following techniques are typically used as practices for good surface preparation:
Degreasing
Degreasing is carried out in order to remove any loosely held dirt or other contaminants from the surface. Surfaces can be degreased using volatile solvents such as toluene, acetone, methyl ethyl ketone, methyl alcohol, isopropyl alcohol and trichloroethylene. However, kindly make sure that all the environmental, health and safety regulations are met prior to selecting a solvent. The most common methods used, typically include the three main steps:
- Vapor degrease/ clean/rinse the parts with the appropriate solvents
- Immerse the substrates in a fresh bath of solvent for the wash and follow by an immersion in the second tank for a rinse
- Cleaning and drying the substrate post the degreasing
Abrasion
The surfaces need to be degreased and cleaned before as well as after abrasion to remove any pre-existing contaminants on the surface. Once the surfaces have been abraded, they need to be degreased to remove the debris from abrasion.
Depending on the exact plastic being used, the abrasion technique that is eventually employed might vary. Please note caution must be exercised based on the exact material used regarding the feasibility of this technique.
Roughening the surface with a sandpaper can be successfully used for many plastics, such as PEEK, nylon, polycarbonate and others. Below are the photos of PEEK coupons prepared for bonding.
PEEK coupons before and after surface roughening with 60 grit sandpaper accompanied by a magnified image of the partially roughened surface.
Physical Methods
These techniques make use of the surface reactivity of the plastics and modify the surface chemistry to achieve better adhesion, rather than making use of hazardous chemicals. Some the most common physical methods include:
- Corona Discharge: The ionized air generated by a corona discharge, reacts with the surface of the substrate to form free radicals. They react with oxygen in the atmosphere and increase the surface energy of the substrate to be bonded.
- Flame Treatment: Flame treatment involves exposing the surface to be bonded to a gas flame for a few seconds. The flame oxidizes the surface to be bonded and increases the surface energy by forming higher surface energy functional groups. Warping might be a potential hindrance during processing with this method.
- Plasma Treatment: Plasma treatment differs from corona discharge and flame treatments in a sense that plasma treatment is typically carried out under partial vacuum. In plasma treatment, gas plasma is activated by the appropriate techniques to produce excited species that react with the plastic substrate. Often plasma treatment tends to provide substrates with better stability as compared to corona discharge, chemical treatment or flame treatment.
Chemical Treatments
Specific chemical techniques have been developed for treating different substrates. These treatments change the physical as well as the chemical properties of the surface in order to improve the adhesion. A wide range of acids and alkalis are used for this purpose. Typically, the specific chemical or a mixture of chemicals is placed in a chemically resistant container. The chemical bath is then heated to the appropriate temperature and the plastic is immersed in the chemical bath for the required amount of time. After chemically treating the surfaces, it is important to rinse the surface with DI water and thoroughly dry the surface before it is subjected to further use. Extreme care should be taken while handling chemicals. Good laboratory skills should be used while handling these chemicals. The personnel should be wearing the appropriate personal protective equipment and should be well trained in handling of these chemicals.
The table below lists the chemical treatments typically used for some most commonly used plastic substrates:
| Substrate | Etching Solution | Composition (Wt%) | Pretreatment Conditions |
|---|---|---|---|
| Polyphenylene Sulfide (PPS) |
Conc. Chromic Acid | 100 | Immersion: 2 min. @ 71°C Rinse: Tap water followed by DI water Dry by evaporation |
| Acetal (Delrin) | Conc. Chromic Acid (69%) Sodium dichromate Distilled Water |
84.2 0.6 15.2 |
Immersion: 5 min. @ 20°C Rinse: Tap water followed by DI water Dry in warm air |
| Nylon | Aq. Phenol (80%) | 100 | Immersion: 15 - 20 min. @ 70°C - 90°C Rinse: Tap water followed by DI water Air Dry @ 60°C - 70°C |
| Acrylonitrile Butadiene Styrene (ABS) |
Conc. Sulfuric Acid (69%) Sodium dichromate Distilled Water |
84.2 0.6 15.2 |
Immersion: 20 min. @ 60°C Rinse: Tap water followed by DI water Dry in warm air |
| Polytetra- fluoroethylene (PTFE) |
Tetrahydrofuran Naphthalene Sodium |
85.5 12.3 2.2 |
Immersion: 1 - 2 min. @ 70°C - 90°C Rinse: Ketone followed by DI water Dry in warm air@ 65°C |
| Polyethylene (PE) or Polypropylene (PP) |
Plasma or Corona treatment Alternatively, use X21 primer |
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A combination of degreasing, abrasion, and chemical treatment techniques can be employed for preparing plastic surfaces.
