Surface Preparation IV: Treatments for Plastics II

The last article¹ reviewed the basic methods for surface treatment of plastics. The high-energy methods include plasma, flame and corona treatments. Chemical oxidative processes include chromic acid, nitric acid and peroxide attack. Some plastics can be etched with strong base such as sodium or potassium hydroxide. In this article, we examine some specific methods for certain types of plastics.

Structural Influences

Polyolefins, such as polyethylene and polypropylene, have low-energy surfaces, typically falling in the range of the 30- to 34-dyne level. Their surface energies should be raised to at least the 40-dyne level to get good bonding. In bonding experiments involving different types of surface treatments, polyethylene showed about a 10-fold improvement in bond strength by plasma treatment and about a five-fold increase in bonding after a chromic acid treatment.

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.

Composition Influences

Another example of treatment selectivity can be found in bonding to fluoropolymers and their copolymers. These materials have lower surface energy than polyolefins, typically 18- to 26-dyne level. Highly fluorinated species, such as PTFE, show about a 10-fold increase in bonding after sodium naphthanate etch but only about a three-fold improvement after oxygen or argon plasma treatment. Polyethylene shows just about the opposite trend, with the plasma treatment producing roughly a 10-fold increase in bonding.

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.

Which Method to Choose?

Since the structure or the chemical composition of a polymer can have an effect on the choice of surface treatment, there are guidelines for choosing a method. For the low-energy plastics (< 35-dyne level), the selection process becomes more empirical. For higher-energy plastics, it is easier to use a method based on convenience because the inherently higher surface energy provides better bonding capability in the beginning. Almost any method will be somewhat useful on a polymer having higher surface energy.

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.

Lower-Energy Plastics

As we move lower in energy, treatment requirements increase. Even though surface energies are lower, some plastics are susceptible to solvent attack. ABS, polycarbonates, styrenics, acrylics and vinyls (PVCs) can be solvent-bonded. Barring this, solventborne adhesives inherently etch the surface, improving adhesion. Acrylic adhesives lend themselves to bonding of these materials because many acrylic adhesives have an inherent solvating action. Solvent-resistant materials, such as acetals, polyphenylene oxide, polyphenylene sulfide and allied structural aromatic polymers, often require surface oxidation or priming. Acid or base etching becomes necessary on more difficult materials such as polyimides and polyetherimides.

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.

How Much?

For any bonding application, it is only necessary to use the amount of surface preparation you need. If the end use does not call for a destruct bond (substrate failure), it is not necessary to provide treatment to that extent. If the bond survives its intended use, time and money are wasted on excessive surface preparation. The treatment method is often determined empirically based on testing. Once the treatment has been devised, a production protocol can be written and used to define critical parameters such as etchant concentrations, time and temperature of treatment, exposure levels, drying conditions, and so on.

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.