by Brittany Willes, editor, Plastics Decorating
Low surface energy can result in poor adhesion quality, which can lead to further issues with printability, metallization, adhesive assembly and more. Fortunately, manufacturers have several options when it comes to activating surface energy for processing, such as corona, flame, plasma and reactive gas technologies. The trick is to determine which surface treatment option is most appropriate for a manufacturer’s needs.
Many plastic materials, such as polypropylene and polyethylene, possess highly beneficial attributes that include puncture and tear resistance, chemical inertness, high wear and low friction coefficient. Unfortunately, the surface tension of these materials also lends itself to poor wettability, making them inadequate for bonding or printing. Corona surface treatment applies a corona discharge to alter surface adhesion properties of polymer-based materials.
“Corona treatment is a safe, cost-effective option compared to other methods, such as flame and chemical treatment, and there are no consumables, as required for these and gas plasma systems,” stated Dennis Damon, regional sales representative for Corotec Corporation based in Farmington, Connecticut. According to Damon, corona treatment also “offers the advantages of accurate control and repeatable results, and corona is far less sensitive to individual operator setup and technique than flame systems generally tend to be.” Using corona, treatment results can be tied directly to energy density that is completely controllable.
Corona surface treatments generally are suited to promote the adhesion of inks, coatings and adhesives on film, foil, foam, sheet and #D products, along with many others. As Damon explained, corona treatment systems are “designed for either inline or offline treatment for printing, decorating, laminating and bonding applications. These applications are virtually limitless and run the gamut from industrial to aerospace to medical to automotive industries.”
In more recent years, corona surface treatment has become prominent in pre- and post-improvement of digital printing and secondary finishing processes. However, there are instances in which corona treatment would not be suitable. “Corona discharge generally is not well suited to materials with a thickness greater than approximately half an inch, Damon stated. “Certain product structures or additives may not lend themselves well to the process; however, this is determined early based on lab evaluation,” he explained. For instance, Corotec systems are custom designed and manufactured to meet product and process parameters, and laboratory testing typically is performed to validate system sizing and performance.
Reactive gas treatment
Introducing reactive gases is another way to effectively raise surface energy before processing. Inhance Technologies LLC, Houston, Texas, applies its Reactive Gas Technology™ (RGT) to a range of plastic and elastomeric materials, including polyolefins, polyesters, polyamides, thermoplastic elastomers, thermoplastic polyolefins, rubber materials and other surfaces. “Inhance’s proprietary RGT offers permanent surface activation of plastics for post-processing applications, such as printing, decorating and bonding,” said Dr. Prakash Iyer, senior vice president of business development and technology.
Iyer went on to explain how RGT allows for treatment of products of all shapes and sizes. With reactive gas, individual parts or continuous surfaces are uniformly treated, allowing 360° coverage of all areas. The treatment is not line-of-sight dependent, due to the process configuration. Films, molded goods and raw materials, such as powders and pellets, can be activated using reactive gas treatments. “There are many diverse markets and applications utilizing RGT to activate plastics surfaces,” Iyer stated. These applications include, but are not limited to, the following.
- The transportation industry, which uses RGT for interior and exterior plastic components such as air bags, steering wheels, instrument panels, door locks, soft trim, fascia, and gaskets and seals.
- Consumer products, such as cosmetic applicators and packaging, container decoration and food storage.
- Outdoor recreation markets, which employ RGT for sports equipment, power equipment and textiles.
- Industrial applications, where RGT is used for mining and agricultural equipment, as well as seals and gaskets for a variety of environments.
- Health care, with RGT applications in medical disposables, devices, and pharma and nutraceutical packaging.
For example, Inhance was approached by a sports equipment manufacturer to provide increased surface energy on protective helmets made from thermoplastic polylefin (TPO). These helmets had been decorated using digital inkjet printing and hydrographics to produce a highly detailed product.
“The existing approach for surface activation gave inconsistent and low surface activation due to the curved helmet surface,” Iyer explained. Inhance worked with the customer to provide parts that were uniformly and permanently activated to high surface energies (greater than 64 dynes/cm) using its proprietary RGT process. The parts then were decorated using the existing digital inkjet and hydrographic printing processes. ” of the parts that were activated by Inhance’s RGT technology passed the cross-hatch quality control tests,” Iyer affirmed. Because the parts were permanently and uniformly treated, activated helmets could be stored. According to Iyer, RGT-activated parts can be stored for months with no deterioration of surface energy. Furthermore, gloss and other aesthetic or mechanical properties are not affected by the RGT process.
As with corona and other surface treatments, RGT is not suitable for all surfaces. While almost any plastic surface can be activated using RGT, the technology is not effective for glass, metal or wooden surfaces. “The best utilization of the technology can be determined by working with Inhance and its engineering team,” asserted Iyer. “A typical project begins with sample testing and process definition, followed by scale up and customization of the final solution.”
When dwell time becomes a concern, flame surface treatment is particularly useful for large surfaces requiring high production rates with limited treatment dwell time. “Dwell time always should be limited to the least amount required to obtain the surface activation desired,” stated Mark Plantier, vice president of marketing for Enercon Industries, Menomonee Falls, Wisconsin. Given that heat is a natural byproduct of each process, surfaces that are particularly sensitive to heat need to be tested to ensure optimal results.
While flame treatment can be applied to many of the same plastics, glass and metals as plasma, flame treatment is more effective at volatizing and cleaning the surfaces of materials with high degrees of organic contaminants. “Because flame burners may be designed to various widths and are better able to reach contours,” stated Plantier, “flame is a good choice for applications with complex contours, as more surface area can be covered in a single pass.”
Available in many different widths, flame burners are able to accommodate the treatment areas of a specific application. For instance, according to Plantier, Enercon offers a PowerFlame™ burner design, which yields a high-velocity flame capable of treating flat surfaces as well as those with detailed contours. Enercon has, in fact, supplied flame treaters for numerous applications, including decorating, labeling and structural bonding for the automotive, aerospace, assembly and packaging industries. Today, flame treaters incorporate advanced combustion control architecture, which ensures consistent flame over the length of the burner and air/gas control for repeatable results over time. Additionally, the advanced safety features of todays flame systems ensure safe and reliable operation.
Since Enercon offers both plasma and flame surface treatment technology, the company can provide comparisons of the technologies and share insights into which is appropriate. For instance, flame treatment is not as effective as plasma in treating rubber and other materials with high amounts of plasticizers. However, as Plantier said, “Neither plasma nor flame should be used in hazardous locations where solvent vapors are present.”
Like other surface treatment options, plasma is popular in many automotive, medical, electronics assembly and aerospace applications. For example, automotive headlight and taillight assemblies are plasma-treated to improve the sealant performance in preventing moisture from getting inside the assembly. Audi instrument panels are plasma-treated prior to being bonded to polyurethane foam. Plasma allows the part to be treated without having to use masking, as with conventional flame treatment. Due to its ability to provide a safe, sterile means of facilitating the printing and bonding of new material combinations for implant and for applying a functional coating to medical instruments, clothing and equipment, plasma is becoming widely adopted by medical device manufacturers.
“Plasma treatment is best suited for applications where the performance of a coating, ink or adhesive is important and where the costs of part failure are high,” stated Andy Stecher, president of Plasmatreat North America, headquartered in Elgin, Illinois.
Plasma surface treatment results in three kinds of surface conditioning in a single step. “For plastic part manufacturers, plasma provides destatification. This means that the part surface is much less likely to attract airborne contaminants like dust, which can cause imperfections in a finish,” Stecher said. Next, plasma cleans organic contaminants from the surface, turning hydrocarbons into carbon dioxide and water vapor. Finally, plasma activates the clean surface by replacing inert hydrocarbon bonds with more reactive hydroxyl and carbonyl groups. This raises the surface energy of the plastic, creating stronger bonding of the plastic to the adhesive, ink or coating – which means better adhesion.
With no dangerous flames, chemicals or electrical hazards for workers to contend with, plasma treatment works especially well for manufacturers who desire a particularly safe and environmentally compliant process, either because of regulations or concerns for worker safety. Additionally, plasma is being used more and more by plastic part producers that are turning to regrind or recycled plastics, which might vary from batch to batch. “In those cases,” Stecher remarked, “plasma provides a measure of insurance against unknown additives or contaminants by offering a more robust process.”
Of course, plasma has its limits. For instance, in applications with low performance requirements, there are less costly ways to clean a surface that may be more appropriate. “Plasma is really ideal where the cost of failure and the added value of an ink, coating, or adhesive are high,” stated Stecher. Plasma tends to be used more in manufacturing on medium- to high-volume, automated production lines. It is not particularly well suited to lower volume or manual processes. “As with many of the more sophisticated pieces of process equipment on a plastics decorating line,” Stecher explained, “plasma equipment varies from one manufacturer to another. Plasmatreat’s customers tend to be those where the cost of making a bad part is high. Failure can mean scrap, rework, warranty returns, unhappy customers and damage to a manufacturer’s reputation.”
Challenges with adhesion and printability are easily overcome with the right technology. Knowing which method is best for a given product goes a long way toward reducing production costs while maintaining quality and reliability. By comparing the different surface treatment options available, manufacturers are able to better serve the needs of their customers while maintaining high standards of quality and safety.