There is a trend today in manufacturing to use materials that are lighter, more functional, aesthetically more pleasing and “green”. Plastics and composites are replacing metals where possible and consequently there is a need for coatings for plastics to have superior adhesion, to cure faster and be environmentally friendly. This is not new, but certainly the advances each year are new and technically innovative. Hence, the need for a symposium on Coatings for Plastics.
Over 120 attendees and 25 exhibitors set their sights on Chicago for the 10th Annual Coatings for Plastics Symposium. The two-day event was rather intense, as it was scheduled with 22 papers covering aspects of coatings for plastics with a focus on: nanotechnology; adhesion technology; application techniques; UV/EB cure, powder and waterborne technology; film laminates; innovations in substrate technology; paint flow and rheology; regulatory issues; test methods and characterization; and advances in the appliance, electronics and architectural markets. Attendees and exhibitors alike were impressed with the range and variation in technical papers. Many attendees commented on the value of networking and the contacts they made. They also appreciated the venue that was located within easy and free shuttle access to O’Hare. The following represents a brief synopsis of presentations.
The award-winning paper discussed new chemistry to improve surface performance – a hydrosilylation cure-enabled fluoropolymer coating. The physico-chemical properties of fluorine are highly desirable for a coating so a silicone-cure-enabled alternating copolymer of TFE copolymer would be highly desirable and provide weatherability, anti-corrosion and chemical resistance due to the fluorine functionality, while the silicone-cure site provides fast and low temperature cure. In short we have the synergistic effects of a fluorine-based polymer and silicone cure chemistry working together to provide fast and low temperature cure, hardness and flexibility, good adhesion to many substrates without the need for a primer, high gloss and gloss retention and weatherability. The introduction of hydrosilylation-cure fluorine resin technology for coating applications can significantly broaden the range of applications where this technology could be applied. This paper was published in the September 2007 issue of PCI.
Atmospheric plasma treating technology was also discussed and provides long-lasting treatment for both conductive and non-conductive substrates. There is growing interest in a clear barrier for flexible packaging of goods. Clear barrier means ceramic – typically inorganic oxides or nitrides of metals. The deposition of inorganic oxides is fairly routine at reduced pressure, but the equipment is expensive. Therefore adding value by depositing an inorganic oxide barrier layer using inline, inexpensive atmospheric pressure is attractive. Organo-silicon compounds are a logical choice and oxidation yields a thin functional layer of oxide on a substrate such as a plastic film or sheet. Developing methodology enables the production of plasma-enhanced coated substrates at atmospheric pressure with properties equal to, or better than, those previously obtained under vacuum plasma conditions. It is possible to tailor the treatment and application to specific desired results.
There was also a very interesting talk on open-air plasma technology breakthrough for painting fascias and also for nanotechnology applications. Plasma is a state of matter – some describe it as the fourth state of matter. It is like a gas but with much higher energy. Atmospheric pressure plasma used on high-speed production lines modifies the surface of materials to increase their functionality. This improves bonding and eliminates solvent emissions. The plasma beam, directed at the surface, cleans and activates and thereby makes the surface more wettable. Openair® plasma is particularly valuable in functionalizing modern materials which have low surface energies and are challenging to handle, including: cyclic olefins, polypropylene (PP), polyethylene (PE), nylon (PA), polystyrene (PS), thermoplastic olefins (TPO), thermoplastic elastomers (TPE), synthetic rubbers (EPDM), silicones, polyether ketone (PEEK) and liquid crystal polymers (LCP).
Openair plasma can be used for two different coating applications. The first is to prepare surfaces so that both liquid and powder coatings adhere better and the resulting finish has a higher quality. The second is to use the plasma jet to apply nanoscale-thin coatings that polymerize on the surface; a process called PlasmaPlusTM.
PlasmaPlus is an exciting new technology for functionalizing surfaces with coatings that are nanometers thick. It is different from traditional coatings that are primarily needed for protection and aesthetics. Functional coatings can change the surface characteristics to increase strength, reduce friction, repel or attract liquids, provide a gas barrier and many other potential uses. PlasmaPlus works by vaporizing a liquid precursor or by using a specialty gas that is then injected directly into the plasma stream creating smaller molecules that are deposited onto the substrate where they bond and re-polymerize. The thickness of the coating can be controlled by regulating the flow rate of the precursor, the residence time over the surface and the energy of the plasma. Films of between 10 nanometers and 700 nanometers can be applied in a single pass.
This technology is new and was announced in 2006. The potential for this new approach to surface functionalization provides customers with new opportunities and significant competitive advantages.
Several papers covered various aspects of testing. A discussion on inverse gas chromatography (IGC) for measuring crosslink densities for UV-cured coatings at different depth profiles provided valuable insight. Other typical techniques for measuring degree of crosslink are: DMA, swelling experiments, vapor sorption techniques, vapor pressure osmometry and NMR. IGC is a gas phase technique for characterizing powders, fibers and thin films. For IGC the roles of the stationary and vapor phase are inverted as compared to normal analytical GC. The stationary phase is the component with the unknown physico-chemical properties and the vapor phase is the probe molecule with known physico-chemical properties. The concept is based on solubility parameters and the findings on an automotive clearcoat study indicate that the solubility parameter decreases further away from the surface as it is harder for solvents to penetrate; that the degree of crosslinking increases further away from the surface (stronger film); and that oxygen present during UV cure acts as a scavenger for free radicals and thereby decreases polymerization at the surface.
Artificial weathering test methods were also discussed along with test method variables and the importance of standard reference materials. The importance of standards cannot be overemphasized, as they have undergone extensive characterization, have known degradation mechanisms and have undergone round robin studies. They can then be used to verify testing parameters, evaluate uniformity of conditions, serve as a troubleshooting tool, etc.
Assessing surface damage on coated plastics is important, particularly in determining degree of wear, scratch or marring. Recreating surface damage in a laboratory is not only a complex process but it involves evaluating multiple influences that impact the rate of surface damage. This is particularly true for evaluating normal wear of a coating. An excellent review of these laboratory techniques is included in the full paper of this discussion, which was published in the September 2007 issue of PCI.
In automotive, the trends are: (1) a definite increase in the use of PP and TPO, which impacts on the use of CPO (chlorinated polyolefin) or CFPO (chlorine-free modified polyolefin) as an adhesion promoter; (2) increase in waterborne technology due to environmental concerns; (3) lower baking temperatures of 80 °C and below; and (4) a decrease in the amount of rubber in TPO to save cost. Unfortunately these trends have a negative impact on adhesion. Due to the high chemical stability, low price, excellent balance of physical properties, possible recycling, etc., the amount of PP and TPO consumed by automotive parts, household electrical appliances and the molded general goods businesses continues to increase. However, PP and TPO are plastics with low surface energies that make painting and adhesion problematic, hence chlorinated polyolefin (CPO) or chlorine-free modified polyolefin (CFPO) resins have found wide use as adhesion promoters.
In recent years, waterborne adhesion promoters applied to TPO substrates have been widely studied for use as both interior and exterior automobile coatings due to increasing environmental awareness by industry. Furthermore, the automotive coating industry desires waterborne coatings for TPO substrates that coalesce and adhere well at low baking temperatures, such as 80 °C and below, in order to save energy costs and to avoid thermal deformation of TPO substrates caused by higher temperatures.
The study that investigates propylene-base polyolefins with relatively low crystallinity and Tm, which have different propylene content, homogeneity and tacticity, as the starting materials of the WCFPOs is presented in a paper which will be published in this (October, 2007) issue of PCI. The relationship between the physical properties of the WCFPO and the resulting coatings performance such as the adhesive strength to TPO substrates with different rubber content, gasoline resistance and water resistance is discussed.
Another paper discussed the effects of TPO composition on the performance of CPO adhesion promoters and stressed the need for coatings suppliers and TPO suppliers to work together. Findings of this investigation included the following: physical interactions, flex modules, and temperature are still important factors; the co-monomer (C8) in the TPO is an important factor for adhesion; and ester formation and dehydrohalogenation of CPO is possible and would enhance adhesion of systems.
Because of the nature of the substrate, wetting is always a very important discussion. The basic rule of thumb is that the coating (solvent) has to be lower in surface tension than the substrate. Unfortunately water has a very high surface tension due to the hydrogen bonds in the water molecule. So not only is the measurement of surface tension important but the use of proper additives to lower the surface tension of the coating and enable good wetting becomes a critical parameter in good formulating. There are well-characterized substrate wetting additives that make substrate wetting on low-surface-energy substrates possible. Synthetic and adhesion resins significantly improve adhesion to plastics.
Because of environmental concerns and energy costs there is a continued push for waterborne coatings and trends are toward eliminating primer steps when possible, replacing paint with molded-in color and having quicker bake cycles. Consequently a 1K-waterborne coating for TPO/PP is highly desirable. The formulating strategy is as follows: balance properties, cost and usability; identify target substrates; identify OEM requirements; work collaboratively with suppliers; screen with most difficult tests first; start weathering concurrently; use DOE methodology; verify resin compatibility, film formation and additional properties. Most importantly recognize that one size does NOT fit all. It is important to note that the chemical resistance of waterbornes is rapidly approaching that of solventborne.
Plastics for powder-coated application focused on four main areas: the challenges and methods of powder coating plastics; new and innovative alloy for powder coating; benefits and performance of powder-coated plastics; and a comparison of GE’s Noryl GTX to various substrates. The Noryl resin is a blend of two immiscible polymers that forms a high-performance alloy to capitalize on the strengths of both resins. Conductive Noryl has optimized conductivity and mechanical properties for electrostatic painting.
There is an obvious challenge for powder coating plastics as the typical cure temperature range of 140 – 200 °C, and the electrostatic application of powder is actually a better fit for high-temperature conductive materials and not the typical thermoplastic profile. The powder coating application methods are usually flame spray, fluidized bed, electrostatic spray and flocking.
For auto refinish, compliance is a problem, and VOC limits exist in the European Union and in some parts of North America. There is a new additive being tested now for clearcoats that has a positive impact on the drying profiles of VOC-compliant clearcoat refinish formulations. Mechanical properties are improved, sanding and polishing much easier and the system seems to dry fairly quickly.
There was also an update on the VOC exemption of TBAC and an overview of its uses in low-VOC, solventborne coatings. TBAC was added to the EPA list of VOC-exempt solvents at the end of 2004. EPA considers all organic chemicals VOCs except a few specifically exempted because they do not contribute significantly to ground level ozone. Canada is enacting stringent new VOC limits based on OTC, CARB and EPA rules.
Another paper discussed various control technologies in a practical case-history, real-life example. Technologies considered were catalytic oxidizers, thermal oxidizers, regenerative thermal/catalytic oxidizers, rotor concentrators coupled with a thermal oxidizer, microwave VOC reduction technologies and greencell biofilter VOC reduction technologies. In this case a company whose business was rapidly growing found itself in need of dealing with these issues based on growing state regulations. For this particular application a regenerative thermal oxidizer solved the company problems. This paper was also published in the September 2007 issue of PCI.
Because polyolefin substrates, particularly TPO, continue to increase in usage for automotive exterior use, techniques that can reduce the pretreatment process and cost are advantageous. Basecoats (1K) with direct adhesion to polyolefin substrates accomplish this by eliminating the pretreatment process. The obvious advantages are reduced process time, reduced solvent/HAPs emissions, low bake temperatures, waste elimination and ease of handling.
Decorative film laminates are rapidly increasing in use and are a cost-effective and environmentally friendly alternative to traditional plastic coating methods. Film laminates are composed of three layers; a substrate layer (such as ABS, TPO, PC), a decorative layer (gravure printed wood grain patterns or technical finishes; or continuous coat of paint film or chrome), and a cap or top layer (acrylic, urethane or fluoropolymer-based). This paper is scheduled for publication in the November 2007 issue of PCI.
Robotic UV curing of large, complex, 3-D plastic parts was the subject of another interesting paper. Robots and UV technology are not new but making them work well is new. Some lamps do not work as well as robotically. Robotic curing is well suited to large or complex parts, flexible lines and chemistry requiring high peak irradiance. If you can have robotic application of coatings, there seems to be the logical step forward that says you can cure efficiently in the same way.
One paper on UV coatings technology discussed the economic values that should be assessed. Specifically the discussion focused on the return on investment and the environment. The paper discussed all aspects and stressed the importance of understanding the true costs of various aspects of coatings application. This paper may also be read in its entirety in the October issue of PCI.
The UV process was also covered in a review of current equipment available, components available, and the testing of formulations and designing LED-curing systems. UV LED (light emitting diode) technology provides a wide range of wavelengths and formats and has the greatest flexibility for new development. Some advantages of UV LED cure are: instant on/instant off; no heat emission; lamp life over 50,000 hrs; uses low voltage and it expands the market potential to include areas previously unavailable to UV technology.
This amazing new lamp can simply perform in ways that current lamps cannot. In order to truly maximize the potential of this lamp technology, you need to forget all of your current assumptions in regards to how UV curing works. This lamp technology defies accepted UV curing limitations. It is simply doing what it should not be allowed to do. It’s curing faster. Its power consumption is minimal. The mechanism of cure is operating beyond current standards, due to the fact that this lamp is 100% efficient.
TPO and PP plastics are desirable because they are lower in cost and flexible but they also have low surface tension and therefore it is difficult to obtain good adhesion. UV curables have some intrinsic advantages compared to thermal curing in a shorter cycle time, increased throughput, lower energy use, less floor space required and reduced scrap. Other advantages include high to 100% solids, which means little to no VOC. There can also be improved properties such as crosslink density, scratch and mar resistance, and chemical resistance. Increased transfer efficiency allows for less waste and reclaimability.
Exhibitors and attendees alike enjoyed this meeting, and the fellowship and networking were wonderful. In general, people were pleased with the location, the length of the symposium and found the papers and exhibits very helpful. Reserve the dates now for the 11th annual Coatings for Plastics Symposium.