Gas-Phase Surface Pretreatments for Plastics Adhesion
by Scott Sabreen
The Sabreen Group, Inc.
Surface pretreatments are used to promote adhesion between difficult-to-bond plastic substrates and adhesives, coatings and inks. Among pretreatments, "gas-phase surface oxidation" processes are proven highly effective, economical and environmentally safe. The selection of which method is best for any given application can be challenging, in part, due to misconceptions and confusing terminology. Let’s examine unbiased facts and debunk common myths.
Why is pretreatment necessary?
It’s important to understand why pretreatments are needed and the mechanisms through which they improve adhesion bond strength. The underlying reasons why many plastics are difficult to bond are because they are hydrophobic non-polar materials, chemically inert and possess poor surface wettability (i.e., low surface energy). While these performance properties are ideal for designers, they are the nemesis for manufacturers needing to bond these materials. As a general rule, acceptable adhesion is achieved when the surface energy of the plastic substrate is approximately 8-10 dynes/cm greater than the surface tension of the liquid adhesive, coating or ink. In this situation, the liquid is said to "wet out" or adhere to the surface. A method for measuring surface energy "wetting" is the use of calibrated dyne solutions in accordance with ASTM D2578.
Gas-phase pretreatment methods
Widely used "gas-phase surface oxidation" pretreatments include electrical corona discharge, electrical atmospheric plasma, electrical air plasma, flame plasma and low-pressure RF cold gas. These processes are characterized by their ability to generate "gas plasma," an extremely reactive gas consisting of free electrons, positive ions and other species. Chemical surface functionalization also occurs. In the science of physics, the mechanisms in which these plasmas are generated are different, but their effects on surface wettability are similar.
Classical Electrical Corona Discharge is obtained using a generator and electrode(s) connected to a high-voltage source, a counter electrode at potential zero and a dielectric used as a barrier. That is, a high-frequency, high-voltage discharge (step up transformer) creating a potential difference between two points requiring earth ground 35+kV and 20-25kHz. Custom electrode configurations allow for treating many different surface geometries – flat, contoured, recessed, isolated, etc. One application example is a corona discharge treating system for electrical connectors in which a combination of pin and ball electrodes concomitantly treats 3D small diameter holes and flat exterior surfaces, US Patent US5051586 (1991). Ozone is produced in the plasma region as a result of the electrical discharge. Corona discharge has virtually no cleaning capabilities.
Myth: Atmospheric plasma is a low-cost replacement technology for corona discharge.
Fact: Corona discharge often is more effective for treating larger surface areas and greater depth.
Atmospheric Plasma or Electrical Blown Ion Plasma (also termed Focused Corona Plasma) utilizes a single narrow nozzle electrode, powered by an electrical generator and step-up transformer, and high pressurized air in which intense focused plasma is generated within the treatment head and streams outward. This pretreatment process can clean dirt, debris and some hydrocarbons from the substrate, but not most silicones and slip agents. New research indicates that fine etching of the surface can create new topographies for increased mechanical bonding. Ozone is not a byproduct, but nitrogen oxides (NOx) are produced, which may have deceivingly similar odor.
Myth: Atmospheric plasma is electrically potential-free.
Fact: Atmospheric plasma is better characterized as "low potential" unless definitively proven to be zero on any specific application. There are performance differences between equipment manufacturers.
Electrical "Air Plasma" is a corona discharge spot treatment (also termed blown air plasma/forced air corona/blown arc). This treatment head consists of two hook electrodes in close proximity to each other connected to a high-voltage transformer generating an electric arc of approximately 7-12 kV, lower frequency 50-60 cycles/sec (relative to electrical corona discharge). Then using forced air, a continuous electric arc produces a corona discharge, "plasma." No positive ground is needed. This pretreatment process has virtually no cleaning capabilities. Ozone is produced.
Myth: Corona discharge spot treatment yields longer shelf life than flaming.
Fact: Documented scholarly studies show flame plasma on polyolefins produces longer post-treatment shelf life, due to its relatively shallower depth of treatment.
Flame Plasma Treatment uses the highly reactive species present in the combustion of air and hydrocarbon gas (to create the plasma). While flame treatment is exothermic, heat does not create the chemical functionality and improved surface wetting. Flaming will clean dirt, debris and some hydrocarbons from the substrate. Flaming will not remove silicones, mold releases and slip agents. Flame treatment can impart higher wetting, oxidation and shelf-life than electrical pretreatments due to its relative shallower depth of treatment from the surface, 5-10nm. Ozone is not produced. When procuring flame treatment burners, compare ribbon versus drilled port and the benefits of zero balanced regulators.
Myth: Flame treating is unsafe.
Fact: For decades, flame treating has been used as a safe and effective technique across many industries. Criticism arises from competitive equipment manufacturers.
Cold Gas Plasma, also termed "Low Pressure Cold Gas Plasma," is conducted in an enclosed evacuated chamber, in comparison to atmospheric (air) surface pretreatment methods. Industrial-grade 100-percent Oxygen gas (O2) commonly is used. Gas is released into the chamber under a partial vacuum and subjected to an RF electrical field. It is the response of the highly reactive species generated with the polymers placed in the plasma field, on inner conductive electrode aluminum shelves or cage, breaking molecular bonds that result in cleaning and chemical/physical modifications (including an increase in surface roughness, which improves mechanical bonding). A significant benefit of cold gas plasma processes is the removal of hydrocarbons, thereby eliminating solvent cleaning. Atmospheric pretreatments do not remove/clean all poly-aromatic hydrocarbons, so solvent cleaning (prior to pretreatment) may be necessary.
Myth: Cold gas plasma batch processing is too slow compared to inline treatment methods.
Fact: Large volumes of parts often can be pretreated (batch) and fed into automated assembly operations; thereby, no additional processing is needed. Cold gas plasma-treated parts tend to demonstrate the highest quality treatment and longer shelf-life. Criticism arises from competitive equipment manufacturers.
Selecting a pretreatment method
Recognize that each method is application-specific and possesses unique advantages and potential limitations. Consider the following factors:
- Polymers react differently to oxidation processes. The type of polymeric substrate and its end use performance requirements are critical in determining the selection of pretreatment method.
- Is the substrate (product to be treated) conductive? For example, unassembled plastic electronic connector bodies – without metal contact pins – can be treated electrically; whereas, assembled connectors may experience electrical arcing problems.
- Part geometry – Flat surfaces easily are treated compared to deep recesses, extreme tapers and other irregularities. Wetting tests are difficult to conduct in small areas and on heavily textured surfaces.
- Material handling automation – Conductive belts and chains may cause electrical arcing with classical electrical corona discharge and spot treaters. Alternatively, consider using flame, cold gas plasma or low potential atmospheric plasma.
- Avoid overtreatment. Excessive plasma-oxidized surfaces may deleteriously affect downstream assembly processes such as poor heat sealing/welding.
- All pretreatment equipment is NOT created equal! Examine the quality of constructed systems in action. For electrical treatment processes, observe the uniformity of the plasma discharge; for flame treatment, consider the differences between ribbon versus drilled port burners and combustion system components; for cold gas plasma, examine the quality of the chamber construction, electrode shelves and particularly the manufacturer of the vacuum pump.
Plasma-treated surfaces age at different rates and to varying extents relative to factors with the surrounding environment. Variation in temperature and humidity can affect the quality and uniformity of treatment.