Different types of plasma coating processes
Depending on the chemistry, different types of processes are performed for PECVD (Plasma enhanced chemical vapor deposition).
When pure gases are used, materials are cleaned and/or activated, and PCBs can be etched.
When using monomers, they will polymerize on the substrate into a nanocoating. The properties of the nanocoating depend on the monomer used.
Process gas from a precursor
The process gas is obtained from a precursor. This precursor may be gaseous or evaporated from a liquid or solid monomer. A monomer supply unit ensures that controlled amounts of process gas are entered into the chamber.
Required pressure
In order to effect the plasma treatment in sufficiently pure process gas conditions, a base pressure in the 5 to 50mTorr range is required prior to processing. Once the base pressure is reached, the process gas is introduced into the vacuum chamber. The plasma process is typically performed at a working pressure range of 25 to 250mTorr. The vacuum chamber is connected to a pumping group that will maintain the vacuum during processing and evacuate the chamber after the process, to remove residues of the process gas.
Nanocoatings
In recent years low pressure plasma technology has been improved to achieve specific functionalities: permanent hydrophilic, hydrophobic (water repellent) or oleophobic (oil repellent). Liquid repellent nanocoatings exist already for quite some years, but recent improvements in process and machine design allow to deposit super-hydrophobic and super-oleophobic nanocoatings. These find a growing number of applications on new high tech materials, such as microporous membranes and nanofibres.
In the late nineties the breakthrough of low pressure plasma technology came with the development of plasma coating processes. These processes use plasma to polymerize monomers on the surface of the material. Nano plasma coatings are deposited on the surface, adding new and permanent functionalities to the material.
Continuous research and development lead to processes to deposit nano plasma coatings with high end hydrophobic and oleophobic properties, up to the highest levels measurable. Coatings can be deposited on rigid and flexible materials, which can be planar (2D) or complex shaped (3D). Other plasma processes have been developed to obtain the opposite effect by making substrates hydrophilic.
The newest developments in nano plasma coatings are super-hydrophobic and super-oleophobic coatings free of PFOA- and PFOS or halogen free, deposited with innovative low pressure plasma processes.
Together with development and improvement of low pressure plasma processes, the equipment has also improved. Better uniformity and more stable processing are realised thanks to a better design of the building blocks. The size of the system is fully adaptable to the needs of the customer, and fits into a production environment.
Plasma coating on industrial scale for filter media
Producers of gas filter media were of the first to adopt low pressure plasma coating on industrial scale. They are looking for a permanent hydrophobic and/or oleophobic effect, which can be achieved by polymerizing fluorocarbon precursors.
Electrets
These coatings are used in, for example, the production of respiratory masks. Typical filter media consist of several layers of melt blown nonwoven PP, which are electrostatically charged (electrets). Filtration efficiency for oily particles can be greatly improved by applying a hydrophobic/oleophobic coating prior to electrostatic charging. Typical oil repellency levels of 3 to 4 according to the 3M procedure (AATCC 118-1997 and ISO 14419) are achieved.
Table 1 gives an overview of filtration efficiency as measured with a CERTITEST 8130 equipment using DOP particles. Initial filtration efficiency and evolution of filtration efficiency can be measured. During measurement, the filter (consisting of five layers of single ply nonwoven PP) is loaded with 200 mg of DOP particles. Penetration tests have been carried out on electrostatically charged filters.
Table 1 clearly illustrates that a thin oleophobic plasma coating increases both initial and final filtration efficiency. In this sense, it can convert, for instance an R95 filter into an R99 filter.
Filter medium | Conditioning | Initial penetration (%) | Penetration after |
Supplier 1 – 28 g/m2 | Uncoated | 1.20 | 6.40 (30) |
Supplier 1 – 28 g/m2 | Plasma coated | 0.48 | 1.08 (30) |
Supplier 1 – 22 g/m2 | Uncoated | 1.25 | 3.90 (10) |
Supplier 1 – 22 g/m2 | Plasma coated | 0.40 | 0.75 (10) |
Supplier 2 – 25 g/m2 | Uncoated | N.A. | N.A. |
Supplier 2 – 25 g/m2 | Plasma coated | 0.02 | 0.03 (10) |
Table 1: filtration measurement results obtained with a CERTITEST 8130 for different kinds of uncoated and plasma coated filter media (five layers)
HVAC applications
Plasma coatings are also interesting for charged filter media used in HVAC applications. According to changing regulations for air conditioning filters, it is recommended to measure the filter efficiency after discharging with isopropanol. The filter efficiency of a charged plasma treated filter is much higher than the efficiency of a charged untreated filter. The efficiency of the untreated filter drops off after isopropanol treatment. This is not the case for the plasma treated filter. The plasma treatment makes the filter resistant to alcohol. After discharging in isopropanol, the plasma treated filter therefore shows a much higher efficiency than the untreated filter. The difference between the two filters is much larger than before discharging in alcohol.
Diesel engines
Another application in the filtration industry is the improvement of filtration efficiency and water repellency of air filters for diesel engines by low pressure plasma coating. The effectiveness of plasma coating on the filter performance was measured by a water decantation test. The coating should not reduce the material properties of the air filter: the dust holding capacity and the air filter efficiency. Nonwoven PBT filter media were used for the test.
In table 2 the results are shown from the water decantation test of the untreated reference material and the filters with four different plasma coatings. The test measures the performance of a complete filter element. The water is decanted at the dirty air side and the time is measured until the first droplet appears on the clean air side. The target was to achieve a duration of the test of minimum 50 minutes. With the plasma coating a duration of the test up to 93 minutes is achieved.
The plasma nanocoatings are deposited with a mix of fluorocarbon precursors. Varying process parameters are for example the process time and the power put on the electrodes inside the plasma chamber.
Table 2 also presents the results after weathering of the filters. After weathering, the duration of the water decantation test was reduced, but it is still above the target of 50 minutes. The efficiency of the decantation is high for new and weathered filters that are coated.
| New filters | Weathered filters | ||
| Duration of test (min) | Efficiency of decantation (%) | Duration of test (min) | Efficiency of decantation (%) |
Reference | 2.3 | 40.7 | – | – |
Test 1 | 80 | 98.7 | 60 | 95.8 |
Test 2 | 90 | 98.4 | 60 | 84.8 |
Test 3 | 93 | 97.8 | 62 | 95.7 |
Test 4 | 90 | 99.1 | 60 | 97.6 |
Table 2: water decantation results on reference filter and four plasma coated nonwoven PBT filters
Further investigation has also shown that both the dust holding capacity and the filter efficiency are not altered by the plasma treatment.
Environmental friendly plasma coating processes
History of Nanoscale plasma coatings
Europlasma CEO Tim Beulens shares, “Europlasma has been developing, finetuning and delivering plasma surface treatment and coating solutions and the systems to apply these solutions for over 30 years.
These hydro- and/or oleophobic low pressure plasma nanocoatings were marketed by Europlasma under the brand name Nanofics®.”
Europlasma’s first developed nanocoatings were given the name Nanofics 120®. Throughout the years new regulations stimulated research into new chemistries and Nanofics 120 was phased out and evolved into a new wide portfolio of plasma nano coatings.
Nanofics 120® represented a group of precursor monomers which allow to obtain water contact angles of 120° and more on nonwoven membranes (PTFE, PET, PBT). Oleophobicity (oil-repellency) levels up to level 7 (3M scale) on woven fabrics (PET, cotton, PA) and up to level 8 on nonwoven membranes were obtained.
Nanofics 110® represents a group of precursor monomers which give coatings with water contact angles of 110° and more on nonwoven membranes. The oleophobicity level is up to 6.
Typically, the highest repellency for water and oil is obtained with long perfluoroalkyl chain precursor monomers, mostly C8 – indicating a perfluoroalkyl chain with 8 carbon atoms. These molecules contain PFOA and/or PFOS as by-products that are generated during monomer production.
Already for several years PFOA and PFOS are under severe environmental and health pressure. They are persistent in the environment, but also inside animals and humans. Several chemical producers, such as 3M and AGC, have stopped the production of C8 chemistries and of PFOA and PFOS. The production of products containing PFOA and PFOS were phased out.
For that reason, another development was started. In the Crosstexnet Era-Net Transnational call 2011, the project H/OPLA-3D had its origin. The super-hydrophobic and super-oleophobic nanocoatings, free of PFOA and PFOS, based on short perfluoroalkyl chain chemistry have been developed with our partners of the H/OPLA-3D project. Short chain chemistry are for example C6 – indicating a perfluoroalkyl chain with 6 carbon atoms.
As the degree of water- and oil-repellency (oleophobicity) depends on the length of the perfluoroalkyl chains, the repellency of shorter perfluoroalkyl chain coatings is expected to be lower than that of the C8-based chemistry. To boost the performance of the shorter chain coatings, additives and boosters, such as crosslinkers are added to the dispersions and emulsions used in wet chemical processes. The main disadvantage is that boosters and cross linkers may change the hand feel and look of the coated textile materials. They also may reduce the air permeability of the substrate material, which is a limiting factor for filtration applications.
Therefore a new, innovative low pressure plasma process was developed. It allows to obtain super-hydrophobic and super-oleophobic nanocoatings with C6-based chemistry, and gives performances as close as possible to C8-based chemistry.
In low pressure plasma, C8-based chemistry is deposited at very low average powers to avoid fragmentation of the long perfluoroalkyl chain. Very low average powers can be obtained by both continuous wave plasma and pulsed plasma. In continuous wave plasma, the applied power is very low and is maintained continuously during the total coating process duration, without falling back to zero Watt. With pulsed plasma, the power is applied as a peak power for short to very short on-times, and falls then back to zero Watt for a long off-time. By varying the peak power, the on-time and the off-time, it is possible to apply a very low average power with pulsed plasma. For C8-based chemistry, the oil repellency (oleophobicity) levels that are obtained with continuous wave plasma and with pulsed plasma are the same (Table 3).
Contrary to C8-based chemistry, the best levels of water repellency and oil-repellency using C6-based or shorter chemistries are obtained with higher average powers through continuous wave plasma only (Table 3). Further, we developed innovative continuous mode low pressure plasma processes to deposit these C6-based coatings. This process shows improved quality compared to the traditional continuous wave plasma processes (patent pending).
Continuous wave | Pulsed mode | Innovative continuous mode | |
C8-based chemistry | Level 8 | Level 8 | |
C6-based chemistry | Level 6 | Level 4 – 5 | Level 6 |
Table 3: Oil repellency levels for C8-based and C6-based nanocoatings, deposited via continuous wave and pulsed processes.
These innovative low pressure plasma processes are very well suited for small volume chambers, but offer advantages in large volume systems as well. During these continuous mode processes, the power applied to the electrodes is always higher than zero Watt. This ensures a good and constant ignition of the plasma inside the chamber, combined with a more stable plasma. This allows to obtain oil repellency (oleophobicity) level 6, which equals the oil repellency level obtained with continuous wave plasma (the power is not modulated but has a constant value), but at the same time, the coating deposition rate is increased. This allows to deposit thicker coatings in the same process time, or to reduce the coating time for a given coating thickness.
Plasmaguard
Recently introduced PlasmaGuard™ coatings provide hydrophobic (water repellency) splash-proof or waterproof moisture protection to devices such as wearables and consumer electronics. The ultra-thin conformal nanocoatings are halogen free, making them an ideal coating solution for the protection of sustainable electronics.
Hydrophilic coating
Over time industry has looked to achieve a more permanent hydrophilic effect. In response Europlasma has developed several gas mixtures for PECVD. The mixtures are for long term to even permanent hydrophilic functionalization, using a permanent chemical modification of the outer surface layers. These gas mixtures are typically composed of hydrocarbons and activation gases such as O2 or N2O. The process conditions for hydrophilic functionalization are however more demanding than for simple activation.
Blood filter media
A well-known application of hydrophilic plasma functionalization is found in the production of blood filter media. The PECVD process is typically performed roll-to-roll or reel-to-reel on nonwoven materials such as PP (polypropylene) or PBT (polybutylene terephthalate) .
Rechargeable batteries
Similar plasma treatments are applied to battery separator materials for NiMHydride rechargeable batteries. The base material, typically nonwoven PP or PBT, is hydrophobic in nature. In order to improve wetting of the material with the electrolyte, the surface energy must be increased and remain high during the lifetime of the battery.
A comparative study has been done on PP non-woven. After the low pressure plasma functionalization, the surface tension was increased to 60 – 72 mN/m. 7 days after the treatment, the rate of absorption has been tested with a 1 minute wicking ( to move moisture from the inside to the surface) test according to AATCC 197 on 25 mm by 200 mm samples in a 30% KOH + H2O solution. The untreated PP had a wicking of 0 mm. Commercially available solutions showed a wicking of 5 – 10 mm, where the plasma treated materials showed a significant improvement, with a wicking of 27 – 31 mm.
Plasmalex treatment portfolio
Plasmalex offers several hydrophilic coatings. Halogen free coatings Artifics10 and Nanofics10S are created in Europlasma,; the vacuum (low pressure) plasma division
CPI, the roll to roll atmospheric plasma division has a full portfolio for hydrophilic coating and activation. Underneath you can find the overview tables.
Vacuum plasma portfolio
Roll to troll atmospheric plasma portfolio
Benefits of plasma technology for surface treatment
Very consistent quality
On top of cleaning, activating and etching materials with very high precision, plasma nano coating guarantees a very consistent quality.
Dry technology
Low pressure plasma is a “dry” technology, especially when compared to traditional wet processes where chemicals are mixed into (often heated) water to form solutions and dispersions for application of the coatings. With low pressure plasma, there is no water effluent that needs to be recycled and purified, which means substantial cost savings as well given the actual price for water and for recycling of effluents.
Low energy consumption
A wet chemical process typically has a drying and curing line afterwards to evaporate the water and to cure the monomers so that they polymerize into a polymeric coating. Depending on the base material and the application, the energy consumption is 2 to 3 times less with low pressure plasma polymerization than with traditional wet chemical polymerization.
Low use of chemicals
The plasma coating process penetrates in the molecular level of material. As the process is performed on a molecular scale, relatively low amounts of chemicals are used.
For coating processes, the amount of chemicals used is 5 or more times less than in traditional wet coating processes. Less chemical consumption reduces the amount of chemical residues in the exhaust air as well. Using a scrubber and a filter, the concentration of components in the exhaust is easily brought below the legislation level.
Clean operation
As the main parameters of the controlled environment are flow, pressure, power and temperatures, the operator is not exposed to the reaction chemicals of the closed system, highly reducing the risk for accidents. Moreover less pressure is needed as the plasma process works under reduced pressure.
