Plasma is a state of matter, like a solid, liquid or gas, created by combining energy and gas, which causes ionization. Injection and blow molders can control the collective plasma properties (i.e.-ions, electrons and reactive species) to clean, activate, chemically graft and deposit a wide range of chemistries onto a material.
In plastics, the most common plasma application is improving the bonding power of chemical adhesives; this can involve bonding metal to plastic, silicon to glass, polymers to other polymers, biological content to microtiter plates and even bonding to polytetrafluoroethylene (PTFE).
When manufacturing plastic parts for industries such as consumer products, automotive, military and medical devices, plasma treatments are utilized to solve difficult challenges. Typically, this relates to raw plastic material applications with incompatibility issues.
“Plasma can transform the surface properties of plastic to achieve aims that normally would not be feasible [without treatment],” said Ryan Blaik, sales manager for PVA TePla America, Inc., which designs and manufactures plasma systems for surface activation, functionalization, coating, ultra-fine cleaning and etching. “This can include cleaning surfaces, resolving difficulties applying printing inks to plastics, improving the adhesion of plastics to dissimilar materials and applying protective coatings that repel or attract fluids.”
According to Blaik, plasma is being used to treat everything from syringes to bumpers on trucks and automobiles.
“Plastic parts manufacturers are always looking for unique ways to gain a technology edge to become a market leader,” Blaik said. “To achieve this today, top-tier products incorporate some form of advanced coating to functionalize the surface.”
He added, “In the plastics industry, more specialized offerings can create a competitive advantage and drive up the value of each part or product. When you treat plastic with plasma, it can transform a $2 item into a $50 product.”
Blaik outlined some of the essential areas of plasma treatment in the industry, including printing on plastics, microfluidic devices, bonding plastic with dissimilar materials, treating plastic labware, coating plastics to prevent leaching and facilitating R&D for injection and blow molders.
Printing on Plastics
When printing on plastics is required, binding the ink to the surface can sometimes be challenging; the ink beads up on the surface or does not sufficiently adhere to it. Greater print durability may be needed, including fade resistance under high heat or repeated washings.
To resolve the beading issue, plasma treatment can make the surface hydrophilic (attracted to water). The treatment facilitates spreading out the ink on the surface, so it does not bead up.
For many applications, plasma treatments are utilized to increase the surface energy of the material. Surface energy is the sum of all intermolecular forces on a material, the degree of attraction or repulsion force a material surface exerts on another material.
When a substrate has high surface energy, it tends to attract, so adhesives and other liquids often spread more easily across the surface. This “wettability” promotes superior adhesion using chemical adhesives.
On the other hand, substrates with low surface energy — such as silicone or PTFE — are difficult to adhere to other materials without first altering the surface to increase the free energy.
According to Blaik, depending on what is required, organic silicones can also be used to create intermediate bonding surfaces with either polar or dispersive surface energy to help printing inks adhere to the surface of the plastic.
“This approach can facilitate the durable printing of a logo on the surface of bottles when the logo cannot fade after the first wash,” Blaik said. He notes that another application is printing on plastics used for syringes, which do not bond easily with biodegradable inks that are friendly to the human body.
Microfluidic Devices
Typically, microfluidic systems used for medical or industrial applications transport, mix, separate or otherwise process small amounts of fluids using channels made of plastics, which measure from tens to hundreds of microns.
Microfluidic devices usually have various wells containing different chemicals either mixed or kept separate. So it is imperative to either maintain flow through the channel or prevent any residual liquid flow in the channel after the chemistry has passed through it.
“With microfluidics, plasma treatment is used to disperse liquid on the surface to allow it to flow through easily,” Blaik said. “Or it can make the surface more hydrophobic (water repellent) to prevent the fluids from clumping together in unintended areas. When the fluids are ‘pushed away,’ this minimizes the chance of any sticking or getting left behind.”
In such cases, plasma treatment of plastic surfaces can facilitate the smooth, precise flow of liquids in the narrow channels. This can be critical not only for safety in medical procedures, but also for quality for industrial processes.
Bonding Plastic with Dissimilar Materials
In the automotive industry, there is a push to use different plastic materials to reduce the vehicles' weight and make them safer. However, getting plastic to adhere to metal, rubber or other types of plastic can sometimes be exceedingly difficult.
When traditional chemical adhesives fail to sufficiently bond dissimilar types of materials, or if companies are looking to reduce the amount of chemical waste produced, engineers often turn to plasma treatments to solve complex adhesion problems.
Plasma treatment can assist the bonding of dissimilar materials. While treating the plastic alone can improve its binding, treating both materials enhances the binding of both by improving adhesive wicking across the surface.
“Whether bonding metal to plastic, silicon to glass, polymers to other polymers [of different durometers], biological content to [polymeric] microtiter plates or even bonding to PTFE, plasma can be used to promote adhesion,” Blaik said.
As with printing, adhesion promotion is achieved by increasing the surface free energy through several mechanisms. This includes precision cleaning, chemically or physically modifying the surface, increasing surface area by roughening and using primer coatings, Blaik explained.
“The net effect is a dramatic improvement in bonding. In some cases, up to a 50x increase in bond strength can be achieved,” he said.
Although there are many applications, he points to one common but overlooked example: the rubber soles of shoes. Good adhesion is necessary between the shoe insole and its rubber sole, and plasma treatment can promote the binding of the adhesive used.
Plasma Treatment of Plastic Labware
Each year, billions of multi-well plates, pipettes, bottles, flasks, vials, Eppendorf tubes, culture plates and other polymer labware items are manufactured for research, drug discovery and diagnostics testing.
Although many are simple, inexpensive consumables, an increasing percentage are now being surface treated using gas plasma or have functional coatings specifically designed to improve the quality of research and increase the sophistication of diagnostics. Among the goals of surface modification is improved adhesion and proliferation of antibodies, proteins, cells and tissue.
Most of the plasma applications for plastic labware can be categorized as “simple” treatments, such as Oxygen or Argon plasma for cleaning the substrate at the molecular level. The use of plasma is also well established for surface conditioning to make polymers more hydrophobic or hydrophilic.
Potential plasma treatment applications include coating polypropylene or polystyrene plates with alcohol, or to facilitate protein binding to the surface.
“Gas plasma can provide surface conditioning of in vitro diagnostic platforms before the adsorption of biological molecules (protein/antibody, cells, carbohydrate, etc.) or biomimetic polymers,” Blaik said.
Multi-well, or microtiter, plates are a standard tool in analytical research and clinical diagnostic testing laboratories. The most common material used to manufacture microtiter plates is polystyrene, because it is biologically inert, has excellent optical clarity and is tough enough to withstand daily use.
Most disposable cell culture dishes and plates are made of polystyrene. Other polymers such as polypropylene and polycarbonate are also used for applications that must withstand a broad range of temperatures, such as for polymerase chain reaction (PCR) for DNA amplification.
However, untreated synthetic polymers are highly hydrophobic and provide inadequate binding sites for cells to anchor effectively to their surfaces.
To improve biomolecule attachment, survivability and proliferation, the material must be surface modified using plasma to become more hydrophilic.
“If you treat polystyrene with oxygen plasma, it will become very hydrophilic, so water spreads everywhere. This allows aqueous solutions containing biological content to spread and deliver biomolecules to the surface while providing a hydrogen bonding platform to adhere to them,” Blaik said.
Treating the surface in this manner has many benefits, including improved analyte wetting of wells, greater proliferation of cells without clumping, reduced amount of serum, urine or reagents required for testing and lower risk of overflow and cross-well contamination.
Coating Plastics to Prevent Leaching
Using plastic labware can raise concerns about leaching. Since plastic labware is susceptible to leaching from plasticizers, stabilizers and polymerization residues, plasma is used to coat the inside of containers with a quartz-like barrier material. These flexible quartz-like coatings are polymerized onto the plastic by plasma-enhanced chemical vapor deposition.
The resulting coating can be a very thin (100-500 nanometers), non-crystalline, highly conformal and highly flexible (180o ASTM D522) coating.
Similarly, there can be concerns about potential leaching from plastics in contact with the product in the food and beverage industry.
To prevent plastic leaching, industry producers can coat the plastic using plasma treatment. The two options are a PTFE-type coating, or on the opposite side of the spectrum, a silicone quartz coating to create a near-glass-like surface.
Blaik points to sports water bottles with a different interior surface, typically due to plasma treatment or application of a coating.
R&D Assistance
When injection and blow molders are developing a new product or process that could require plasma treatment to ensure production quality and efficiency, the two options are purchasing in-house tools and developing the necessary expertise, or using toll processing services.
If R&D assistance is required, plasma treatment is standard enough that leading equipment providers can modify existing, mature tools and technology, complete with fixturing, to deliver what are essentially drop-in solutions, according to Blaik,
Like PVA TePla, some providers provide access to on-site research and development equipment and engineering expertise.
For injection molders that may be doing various work for different manufacturers in a range of industries, similar to a contract shop, purchasing a plasma treatment system is flexible and not just specific to one part.
“You can [plasma] treat multiple parts and have multiple recipes with a system. You can use it on multiple product lines. It is not fixed to one usage,” Blaik said.
However, for those who want plasma-treated parts or components without investing in in-house equipment, the solution is to utilize a contract processor. With this approach, the parts are shipped, treated and returned within a mutually agreed timeframe. For small or infrequent batches, this can significantly lower the price per part.
Working with a contract processor has advantages in tapping into the years of technical expertise applying various plasma treatments; this can often speed R&D efforts.
As applications and production volumes continue to evolve, collaborating with a partner with deep plasma treatment expertise can provide a quicker time to market for a customer’s product.
Either way manufacturers choose, by altering the surface properties of plastics, executives in charge of R&D and production improve the quality of test results while increasing the value of their products.
For more information, contact Ray Chen or Ryan Blaik at PVA TePla America via phone 951-371-2500, toll free 800-527-5667, email at [email protected], or visit www.pvateplaamerica.com.
Jeff Elliott is a Torrance, Calif.-based technical writer. He has researched and written about industrial technologies and issues for the past 20 years.
