After the same treatments, poly-propylene showed a 200-fold improvement with plasma treatment but a 600-fold improvement in bonding with chromic acid treatment. Why would chromic acid oxidation be so much more effective on polypropylene than on polyethylene and why are the improvements so great?
Polypropylene has a pendant -CH3 (methyl) group on every other carbon. Those groups are easily oxidized to carboxylic acid by oxygen plasma or chromic acid. Polyethylene has no such groups. Even if only a few methyl groups are oxidized on the polypropylene, the presence of added carboxylic acid functionality would be expected to increase polarity and bonding significantly. In this case,
the chemical attack appears more efficient than the phys-ical oxidative attack. Thus, the chemical structure of the polymer has an effect on the relative usefulness of different types of surface preparation, and all methods do not give good results on all polymers.
A copolymer of fluoropolymer
and polyethylene shows a 10-fold improvement in bond strength using either plasma treatment or sodium naphthanate etch. The plasma treatment attacks the polyethylene portion while the naphthanate etch attacks the fluorinated portion. There is a choice as to which part of
the copolymer chain to treat, depending on the convenience of the
method. So the composition of a copolymer can have an impact on treatment effectiveness.
Many structural polymers have some polarity. These include polyesters, epoxies, urethanes and polyamides. Polyesters and epoxies are the most polar and can bond well with only a scuff sanding. Rigid urethanes are a bit less polar and can be bonded with urethane adhesives fairly easily, but may require treatment for bonding with an epoxy. Polyamides are the least polar of this group and can be bonded without treatment, but benefit from priming. As polymer polarity decreases, the requirement for treatment increases.
Solvent bonding and solventborne adhesives are decreasing in favor because of environmental and workplace concerns. The next approach is to look at a primer system. When using a solventborne primer as opposed to a solventborne adhesive, there is an overall reduction in solvent emissions and hazardous waste because much less primer is used than adhesive. Those polymers having very high solvent or chemical resistance are good candidates for priming because they are difficult to treat otherwise. Elastomers like rubber or EPDM are frequently primed before bonding, replacing traditional chemical etching from hot bleach or sulfuric acid baths.
Given the choice, many people prefer priming to etching or oxidation. Etching is messy and involves hazardous materials while oxidation by other means is process intensive. It is easy to apply a primer and move on.
On the smaller parts often found in critical medical or electronics applications, plasma treatment is preferred, even with the investment cost, because there is no danger of contaminating the parts from additional chemical interactions, such as primers or etching baths. Corona treatment is also used instead of plasma treatments, if effective.
Another important factor is the time lag between surface treatment and bonding. Cleaned surfaces and especially oxidized surfaces should be bonded quickly. If long lag times are involved, such as shipping from one plant to another, a primer might be a good idea to protect the surface from contamination. Plastics that have been plasma-treated, corona-treated or chemically oxidized are susceptible to picking up anything from their surroundings, causing the surface to degrade. Reactive primers may need to be bonded quickly to prevent loss of activity.
The best way to evaluate the value of a given surface treatment is to bond parts having little or minimum treatment, such as a cleaning only, and compare results against treated parts in a test program. Normally, with plastics bonding, the issue is bond strength, although temperature resistance is frequently a consideration, and chemical or humidity resistance may be important for electronic or medical applications. These can be included in a test protocol as step variables or as "torture" variables where the parts must survive some exposure level for a predetermined time.
There is an art to selecting and evaluating surface-preparation methods for plastics. "State of the art" is whatever works for your application. Keep in mind the best surface treatment will be determined by polymer surface chemistry, polymer chain structure and the specific needs of your application. High-reliability bonding usually requires high-reliability surface preparation. More importantly, it requires the right surface preparation for the intended end use.