Vacuum Plasma Deposition of Water and Oil Repellent Nano-coatings AIMCAL Web Coating & Handling Conference 2014 Europe Dr Nicholas Rimmer P2i 127 Olympic Avenue, Milton Park, Abingdon, Oxfordshire, OX14 4SA, U.K. Tel: +44 (0)1235 833 100, Fax: +44 (0)1235 861214, email: Nick.Rimmer@p2i.com 1. Introduction Several applications of web-based media require a high level of liquid repellency. This may be to provide protection from environmental conditions (for example outdoor clothing, protective coverings, and vent filters used to protect automotive electronics), or to provide protection from specific chemicals (for example in the oil and gas processing industry). In some cases it will be possible to select naturally water resistant materials such as PTFE or polypropylene. Where higher levels of repellency are required (for example to resist oils or alcohols), the choice of materials is much more limited, especially where there are other specifications to be achieved simultaneously such as mechanical strength requirements or the need to maintain airflow through a porous material. No materials have inherent repellency to lighter oils such as gasoline and other short chain hydrocarbons. 2. Coating Technologies The application of a surface coating provides an opportunity for materials engineers to introduce additional functionality to a material without changing the bulk properties. For example the appearance or surface roughness can be modified without affecting the overall density, strength, or thermal expansion properties of a material. Similarly, liquid repellency can be achieved by the use of a thin surface coating. In each case, the coating thickness should be the minimum possible to give the desired effect; any additional thickness will add unnecessary cost and could affect the bulk properties of the material. Traditional coating technologies add a liquid chemical to the surface, which then needs to be baked or cured to drive off solvents and dry the chemical on to the substrate. The liquid chemical may be applied by spray, screen, spreader, or dip coating and generally requires an oven or UV curing process immediately after coating. These approaches have several limitations: poor adhesion to the substrate, high volumes of chemical and solvent usage, and they are not suitable for highly porous or breathable materials due to blocking of the pore structure. Low pressure plasma deposition processes can achieve highly uniform, thin films with precisely controlled composition and molecular structure. In this presentation, we present results from a patented pulsed plasma deposition process that was originally developed to provide liquid repellency to specialist clothing for the UK military.
3. Pulsed Plasma Deposition A pulsed plasma deposition technique has been developed by P2i to provide exceptionally high levels of liquid repellency with a coating that is only a few nanometres thick (more than one thousand times thinner than a human hair). The technology is applied in high-volume manufacturing environments to products as diverse as mobile phones, hearing aids, pipette-tips, and clothing such as gloves and shoes. The process operates at low pressure in a vacuum-tight chamber. A typical processing system for web based materials (Figure 1), consists of a processing chamber connected to a vacuum pump, a set of wind/rewind spindles, and a mechanism for handling the material between a set of radio frequency (RF) electrodes which are used to generate the plasma. Tension control is provided by a series of load cells and servo-driven rollers. Process gases are introduced via a vaporiser system. Figure 1: Schematic and photographs of a low pressure plasma processing system
A roll of material is loaded into the chamber and threaded through the electrode zone. The system is then closed and pumped to low pressure to remove all the air from the system. The 2-step deposition process is then initiated as the web material is wound through the system. The first step is a plasma activation process that uses an inert gas plasma (typically Ar or He) to activate bonding sites on the surface of the material. The second step then bonds the fluorocarbon monomer to the surface of the material. This second step of the process utilises a patented pulsed plasma process which ensures that the molecular structure of the monomer is retained in the polymer structure deposited on the surface of the web, thus ensuring the highest levels of repellency. Once the deposition process is completed along the full length of the roll, any excess monomer and residual gases are pumped from the system before venting back to atmospheric pressure and unloading the roll of treated material. The advantage of utilising a pulsed plasma process, rather than a more conventional continuous plasma process can be seen by comparing Fourier Transform Infra-Red (FTIR) spectra of the precursor monomer and the deposited polymer (Figure 2). The molecular structure of the polymer coating created in a pulsed plasma process retains the same molecular structure as the original precursor monomer (as evidenced by the clear, distinct peaks in FTIR spectra a and b), whereas the polymer created in a continuous plasma process has a broader peak (FTIR spectrum c), suggesting a mix of many different molecular structures. Figure 2: FTIR Spectra of (a) Monomer (b) Deposition with pulsed plasma (c) Deposition with continuous plasma
4. Results Water repellency can be assessed by measuring the contact angle of a water droplet on the surface (a higher contact angle is achieved on a more repellent surface). Figure 3 and Table 1 compare the contact angles measured on untreated and treated surfaces using a VCA Optima system. It can be seen that the P2i process increases the contact angle dramatically in each case. Figure 3: Contact angles of a 3µl water droplet on an untreated (LHS) and treated (RHS) PEEK film Material Average contact angle (ᵒ) Untreated Average contact angle (ᵒ) P2i Treated PEEK film 82 122 Spunbond PP 125 142 PVDF film 120 139 Table 1: Contact angles measured on untreated and P2i treated materials An alternative method to measure repellency uses drops of various test liquids to determine which are repelled or bead-up on the surface and which are absorbed or wet-out. The 3M and AATCC118-2002 tests (see appendix 1) were used to measure the level of repellency on several types of air filter material before and after treatment with the P2i process. Figure 4 shows the water/alcohol and oil repellency levels measured. Figure 4: Water and oil repellency measured on various air filtration media It can be seen that for all material types, the level of repellency has been increased dramatically after treatment with the pulsed plasma process. The levels of oil repellency (oleophobicity) achieved are higher than have been reported for any other fluorocarbon based coating process.
The pulsed plasma deposition process is particularly beneficial for porous media since the nanoscale coating does not block the pore structure. Table 2 shows results from airflow measurements performed before and after treating various air filter materials with the oleophobic treatment (the measurements were performed at 125 MPa pressure using a TexTest FX3300 airflow measuring system). The largest change in airflow (22%) was measured on an expanded PTFE membrane material which has a pore size less than 100nm. For the other materials, the change in airflow was negligible. Material Airflow BEFORE Treatment Airflow AFTER Treatment % Change cm 3 cm -2 s -1 cm 3 cm -2 s -1 Polyester spunbond 9.30 9.60 + 3.0 % Cellulose/nanofibre filter media 12.10 12.08-0.5 % Polyether Sulfone filter media 2.58 2.50-3.1 % Microporous polyurethane 0.042 0.039-7.8 % PTFE spun nanofibre 2.86 2.77-3.0 % eptfe membrane 0.151 0.118-22 % Table 2: Airflow measurements through various porous media Figure 5 shows SEM images of untreated and treated eptfe filter media. There is very little difference between the structures and no evidence of blocking of the pore structure. Figure 5: SEM images of untreated (LHS) and treated (RHS) eptfe membrane Figure 6 shows a membrane that has been treated with a conventional pad coating technique. There is a clear difference between the front and rear faces of the material and the pore structure is almost completely blocked on the front-side.
Figure 6: SEM images of conventionally treated (pad coated) eptfe membrane. Note the difference in structure between the front and rear faces and the blocking of the pore structure on the front face. Figure 7, by contrast, shows the same eptfe material treated with the pulsed plasma treatment process. In this case, there is no difference between the front and rear faces of the material and no blocking of the pore structure. Figure 7: Plasma treated eptfe membrane. Note that the pore structure has not been blocked by the coating.
5. Discussion The oil repellency levels that can be achieved using the P2i process are higher than can be achieved using any other deposition technology. This exceptionally high level of repellency is achieved using a nanoscale coating so that other properties of the web material such as basis weight, breathability, and mechanical strength are unaffected. The use of a gas phase plasma process ensures that all surfaces of a porous media are treated without significantly affecting the pore size or structure (this is particularly important for materials to be used as air filtration media). The deposition process is run at low pressure so, with the current technology, it is not possible to run in a true continuous mode (the chamber must be vented at the end of each roll and a new roll loaded). This introduces both speed and cost limitations for high volume / low cost materials such as papers and standard packaging materials. For specialist applications where the highest levels of oil and water repellency are required, such as vent filters for automotive electronics, specialist industrial air filters, or high performance protective clothing and wraps, the process has been successfully deployed in manufacturing environments. Many more applications are anticipated as productivity and process efficiency improvements increase the processing speed and further reduce the costs of the process.
Appendix 1: Measurement of water and oil repellency Two scales are commonly used to measure liquid repellency (3M for water/alcohol, and AATCC118-2002 for hydrocarbon oils). Each uses a series of test liquids. A drop of each liquid is placed on the surface of the media and observed over a fixed time (10 seconds for the 3M test and 30 seconds for the AATCC test). If the drop is seen to wet-out or wick into the substrate material, then it has not achieved that level of repellency. Figure 8: 3M and AATCC water and oil repellency scales