by Ray Chen and Ryan Blaik, PVA TePla America
For manufacturers and injection and blow molders that work with different kinds of plastics (i.e., polycarbonate, polyethylene and polypropylene), utilizing plasma treatments can create competitive advantages and transform specific parts into specialized, engineered components with 10 times the value.
Plasma is a state of matter – like a solid, liquid or gas – created by combining energy and gas, which causes ionization. Then injection and blow molders, for instance, 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 or 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 that exist.
Plasma can transform the surface properties of plastic to achieve aims that normally would not be feasible without treatment. 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. Plasma is being used to treat everything from syringes to bumpers on trucks and automobiles. In the plastics industry, more specialized offerings can create a competitive advantage and drive up the value of each part or product. When plastic is treated with plasma, it can transform a $2 item into a $50 product.
Some of the essential areas of plasma treatment in the industry are outlined in this article, including printing on plastics, bonding plastic with dissimilar materials, treating plastic labware coating plastics to prevent leaching and facilitating R&D.
Printing on plastics
When printing on plastics is required, binding the ink to the surface sometimes can be challenging, particularly when the print beads up on the surface or does not sufficiently adhere to the surface. 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 defined as 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. For this reason, 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.
Depending on what is required, organic silicones also can 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. Another application includes the printing on plastics used for syringes, which do not bond easily with biodegradable inks that are friendly to the human body.
Typically, microfluidic systems used for medical or industrial applications transport, mix, separate or otherwise process small amounts of fluids using channels made of plastics, measuring from tens to hundreds of micrometers. These devices usually have various wells containing different chemistries, 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.
Plasma treatment is used to disperse liquid on the surface to allow it to flow through easily, or it can make the surface more hydrophobic 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, which is 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 other types of plastic can be 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.
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 by increasing the surface free energy through several mechanisms. These include precision cleaning, chemically or physically modifying the surface, increasing surface area by roughening or using primer coatings. The net effect is an increase in bonding, which can be up to a 50-times increase in bond strength in some cases.
Although there are many applications, one common but overlooked example is in adhering to 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 now are being surface treated using gas plasma or have functional coatings specifically designed to improve the quality of research and increase the sophistication of diagnostics.
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 also is well established for surface conditioning to make polymers more hydrophobic or hydrophilic. Potential applications include coating polypropylene or polystyrene plates with alcohol.
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. 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.
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 (100-500nm), non-crystalline, highly conformal and highly flexible (180° 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. An example would be sports water bottles with a different interior surface, typically due to plasma treatment or application of a coating.
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 equipment providers can modify existing, mature tools and technology, complete with fixturing, to deliver what are essentially drop-in solutions. Some providers provide access to onsite research and development equipment and engineering expertise.
For injection molders that may be doing work for different manufacturers in a range of industries, purchasing a plasma treatment system is flexible – it can be used on multiple product lines and is not fixed to one usage. However, for those that 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 lower the price per part and often speeds R&D efforts due to the technical expertise of the contract processor.
Either way manufacturers choose to alter 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.
Ray Chen is a sales representative and Ryan Blaik is a sales manager at PVA TePla America. The company designs and manufactures plasma systems for surface activation, functionalization and coating, as well as ultra-fine cleaning and etching. Chen and Blaik can be reached at 951.371.2500 or 800.527.5667, [email protected] or www.pvateplaamerica.com.