Cold Plasma Covering: Applications and Experimental Investigation
Alonso-Montemayor, F. J.1*
1Programa de Posgrado en Ciencia y Tecnología de Materiales, Departamento de Materiales Cerámicos. Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila. Blvd. Venustiano Carranza, 25,000. Saltillo, Coahuila, México.
*Autor de correspondencia: firstname.lastname@example.org Tel: (844)3507038
JBCT Vol. 10, No. 19
El recubrimiento por plasma frío es una tecnología ecológica, económica y menos dañina para el material tratado, en comparación con la tecnología de recubrimiento químico. El recubrimiento por plasma frío es útil en varias áreas manufactureras industriales como los textiles, biochips, rodamientos de fricción, tanques de combustible, CDs (discos compactos) y DVDs (discos versátiles digitales), entre otras. La deposición por plasma frío permite depositar diferentes tipos de materiales, como películas poliméricas o partículas y nanopartículas (NPs) tipo películas, sobre una variedad de superficies de sustratos, porosos (como membranas) y no porosos, orgánicos e inorgánicos, el recubrimiento por plasma es una tecnología versátil e interesante. Por lo tanto, es importante conocer cómo trabaja la deposición por plasma frío, para desarrollar nuevas técnicas de plasma y mejorar las ya existentes. La presente revisión ofrece una comprensión concisa e introductoria acerca de la tecnología industrial del plasma frío y algunas investigaciones relacionadas.
Palabras clave: Deposito por plasma frío, plasma frío, polimerización por plasma frío, recubrimiento superficial, recubrimiento por plasma frío.
Cold plasma covering is an ecological, economically, and less harmful to treated material, technology in comparison with chemical covering technology. Cold plasma covering useful for various industrial manufacture of textiles, biochips, friction bearings, fuel tanks, CDs (compact discs), DVDs (digital versatile discs), among others. Cold plasma covering can deposit different kinds of material, as polymeric film or particles and nanoparticles (NPs) film, on a variety of different substrate surfaces, porous (as membranes) and no-porous, organics and inorganics, cold plasma covering is a versatile and interesting technology. So, is important to know how cold plasma covering works, to develop new plasma technics and improve existing ones. The present review offers a concise and introductory understanding about cold plasma covering industrial technology and some related research’s.
Keywords: Cold plasma, surface covering, cold plasma covering, cold plasma deposit, cold plasma polymerization
Interfacial adhesion between two or more material components is an important property to achieve its performance expected. Being necessary, in many cases, modify component surface, in such way that they transfer their properties to each other.
About this, cold plasma methods for surface treatment are gaining great acceptance, due plasma, being partial ionized gas by electromagnetic forces (Peratt, 2015), releases reactive species that interact with material surface, changing its chemistry and morphology. Cold plasma, as surface modification technology, promote interfacial adhesion in low range of temperature (40-120°C) working in high void, which avoids important damage in treated material, by shorts expositions (Wolf, 2016;Borjas-Ramos et al., 2014), in addition to being economically and environmentally benign technology, since it meets with some ¨green chemistry¨ principles (Anastas, 1998) such as preventing wastes, implies atomized economy, performed less hazardous synthesis, is designed for efficient energy expenditure, use renewable raw materials and is designed for degrade materials.
So, an ecological and economical way, in comparison with conventional chemical and thermal methods, to improve materials interfacial adhesion, and in consequence their physical properties, is to apply coatings by cold plasma on composed material components, so that these being chemically related. So, understand how this state-of-the-art technology can be developed not only in research level, but also industrial is important. Being in consequence, this review objective, offer a concise understanding about industrial technology and research of covering cold plasma.
Cold plasma concept and classification
Talking about plasma in general, can be defined as ionized gas, classified in two kinds according to its ionization grade: a) hot plasma is one whose free electrons and heavy particles are near of 100% ionization; b) cold plasma is one whose heavy particles (combination of ions and neutral atoms) are at low temperature in relation with plasma free electrons. This plasma is associated with ionization range of 10-4-10%(Denes et al., 2004). On the other hand, cold plasma can be classified in two kinds according with its density, which is a physical property related with pressure: a) atmospheric plasma is that obtained at atmospheric pressure, whose density goes from 1011-1016 cm-3; b)low pressure plasma whose density goes from 109-1013 cm-3 (Bárdos et al., 2010).
Cold plasma can be generated by electromagnetic field or high energy radiation induction, where particles that will make up plasma are excited by electromagnetic forces (field or energy) (Halliday et al., 1992), provoking electron-heavy particle and heavy particle-heavy particle energetic collisions, resulting in gas partial ionization (a process named breakdown), besides that photon emission occurs, due electron-heavy particle collisions transfer enough energy to atoms to make their electrons reach a ¨higher¨ orbital, but being an unstable state, the electrons return to their original orbital, emitting UV photons that could collide with other atoms causing the emission of visible light photons (Misra et al., 2016; Pankaj et al., 2014). This process is illustrated by Figure 1.
Atmospheric cold plasma often uses high electrical frequencies (in the order of GHz), but low pressure cold plasma usually is induced at three electrical frequencies: a)low frequencyis considered from 0.5 MHz to less than 13.56 MHz; b) radiofrequency from 13.56 MHz to less than 27 MHz; c) microwave is available at higher frequencies of 27 MHz. However, some authors consider other classification: a) audi ofrequency is considered from 100 Hz to 10 kHz; b) radiofrequency from 100 kHz to 100 MHz; c) microwave is available at higher frequencies of 1 GHz (Li et al., 1997).
Cold plasma industrial applications
Cold plasma surface modification is not only a phenomenon of scientific interest, because it is a pragmatic technology due it has a variety of industrial applications as automotive and electronics new material development, packing technology, among others. Cold plasma, as industrial technology, is used to modify materials selectively under normal conditions (room temperature and pressure) (Plasma treat Gmbh, 2018;Woedtke et al., 2013).
All cold plasma surface treatment is based on the surface interactions of partial ionized gas with treated material surface, where these interactions can be understood in four basic processes (Coates et al., 1996): covering, cleaning, activation and erosion (as Figure 2 illustrate). However, such comprehension is not deep enough to clarify cold plasma physicochemical mechanism, due it is complex phenomenon, so more basic research is required to obtain better cold plasma description (Woedtke et al., 2013;Thiry et al., 2016;Alam et al., 2017).
Cold plasma covering industrial applications
As for the coating cold plasma, is considered a versatile method to apply ultra-thin films on a wide materials range. Cold plasma coating can be performed by physical vapor deposition (PVD) or plasma enhanced chemical vapor deposition (PECVD). Some coating cold plasma industrial applications are (Thierry; Diener, 2007):
- Textile manufacture. Coating the cotton by plasma can make it hydrophobic and dry repellent.
- Biochips manufacture. Deposition of functional films in glass to fix organic molecules.
- Friction bearing manufacture. Coating of moving parts with carbonated diamond-like films to achieve lubricant-free systems.
- Fuel tanks manufacture. Coating of corrosion resistant films based in polytetrafluoroethylene (PTFE).
- CD and DVD manufacture. CD and DVD can be cover with thin anti-scratch polymeric films without affecting their performance.
Coating cold plasma experimental research
Cold plasma coating can occur in two modalities according with deposited material: a) of polymerization where plasma is composed by monomeric gas, whose reactivity tends to reside on its radicalization; b) of deposited particles where plasma is capable of condense particles on a substrate by its formation in situ or by substrate surface granulate growing.
Cold plasma polymerization
Compared to conventional polymer films, plasma polymerized films have high interlacing degree. Generally, plasma polymerization consists on organic precursor vaporization in a deposition chamber, where activation occurs when precursor molecules and free electrons begin to collide each other, monomers radicalization happening by dissociation, that lead to radical-radical and radical-monomer reactions, where film will come from. Later, radical recombination formed molecules can be reactivated by electronic impact, so a thin and solid film will grow by reactive species condensation (Thiry et al., 2016;BeMiller et al., 2015).
By plasma polymerization is possible to improve materials interfacial adhesion, and therefore other mechanical properties like tension resistance, as Aguilar-Rios et al. (2014)investigation ascertain. Study results and parameter are shown in Table 1 and can be explained it if is considered that henequen fibers are polar, while high density polyethylene (HDPE) is no-polar, so is would expect low interfacial adhesion between both. However, when polymerizing a no-polar polymer layer on henequen fibers is possible interfacial adhesion with HDPE. Also. Is necessary do not work with enough high electric frequencies, due plasma erosion could have adverse effects on the structural integrity of henequen fiber, reducing its tension resistance and therefore of composite too. This goes hand in hand with monomer flux magnitude, because enough high flows imply shorter residence times, so formation of a homogeneous polymer layer can be deplored.
Table1. Polymerized ethylene (C2H4) at atmospheric pressure on henequen fibers used as HDPE reinforcement
Polymeric film plasma deposition can be indirectly verified by its effect of polarity change on contact angle, as Barra et al. (2015)investigation ascertain, where methane (CH4) at low pressure was polymerized on sisal fibers for 10 min. Pristine fibers contact angle was 83±13°, while for treated fiber was 105±4°. Pristine fibers showed higher polar character than those treated because their hydrophilic surface was coating with non-polar thin film of polymerized CH4. Also, is important to mention that plasma polymerization increased fiber tension strength used as Portland cement reinforcement, from 70 MPa for pristine fibers composite to a range of 150 MPa-400 MPa for plasma treated fibers composite, because plasma coating could protect fibers from the abrasive effect induced by composite material processing. This effect has been observed in several compounds with different polymeric matrices and reinforcements, as Table 2 investigations revealed.
Table 2. Plasma polymerization effects on different composed material substrates.
Even plasma polymerized films polarity changes not only affect mechanical properties, it is also possible to modify properties of water absorption and reverse osmosis, as Tran et al. (2007) investigation showed, where ally lamine film were deposited on microporous polyamide membrane. Untreated membrane allowed water flux of 100 L/m2 h without apparent salt rejection despite presenting considerable contact angle (85.1°). After treatment with plasma, water flux was reduced to 75 L/m2 h with 73% of salt rejection presenting hydrophilicity increase (46.5°). Contact angle diminution could be due to polymeric film hydrophilic groups presence. On the other hand, membrane permeability alteration could be due to morphological changes like pore size reduction and membrane surface thickness increase, both effects due to polymeric film deposit.
Cold plasma particle deposition
Cold plasma not only can be also used to deposit polymeric films, it is also possible to deposit individual particles on substrates by plasma particle in situ synthesis or previous particle precursors decomposition, being these activated by plasma. Also, particle coating cold plasmas usually direct particles to substrate surface by carrier gases that can also be activated by plasma effect.
Deposit metallic NPs by cold plasma is possible as Chou et al. (1992) investigation showed, where iron (Fe) NPs and Fe-based NPs were deposited on glass substrate from ferrocene decomposition for 5 h using hydrogen (H2) and oxygen (O2) as carrier gases, both separately at 2.45 GHz of electrical frequency, 200 W of electrical power and 6.67 Pa of work pressure with gas carrier flux maintained at 400 K to avoid deposition on transport line walls. Only metallic Fe NPs with maximum diameter of 25 nm could be deposited by H2 carrier gas, while O2 carrier gas induced formation and deposition of both metallic Fe NPs and iron oxide (FexOy) NPs with maximum diameter of 40 nm. Deposited NPs diameter by cold plasma can be controlled by NPs deposit time manipulation (as Table 3 show) and its atomic concentration by carrier gas flux manipulation (as Table 4 show).
Table 3. Diameter control of silicon (Si)-based NPs deposited on a stainless-steel substrate by coating time manipulation (Kim et al., 2009 1st and 2nd).
Table 4. Diameter control of SiOx NPs by carrier gas (O2) flux manipulation, being NPs precursor flux of 2 cm3/min (Kim et al., 2010).
It is possible to deposit thin films composed of individual NPs distributed homogenously on a substrate as Jeong et al. (2009) investigation showed, where organometallic cobalt (Co) NPs film (with 10-30 nm diameter) was deposited on aluminum (Al) substrate. In the beginning organometallic Co was solubilized (1.6 ppm/cm3) then it was vaporized in a controlled manner by CVD equipment. Helium (He) was used like carrier gas for PECVD and deposition was realized at 40 Pa of pressure with organometallic precursor flux of 200 cm3/min. The objective of this research was to obtain a composed material with antimicrobial properties which was achieved successfully since Co-based films inhibited in 100% Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) microbial activity, that were incubated for 24 h on Al substrate previously plasma treated.
Even to combine plasma polymerization and plasma particle deposit is possible as Wallenhorst et al. (2018 2nd) investigation showed, where based-polymethylmethacrylate (PMMA) and based-aluminum hydroxide (Al(OH)3) films were deposited on beech wood for 10 s at 50 kHz of electrical frequency, 2000 W of electrical power at atmospheric pressure. The effect of different carrier gases (air and phenol formaldehyde) would have on composite material contact angle was studied. Beech wood are completely hydrophobic due its measured contact angle was 100°, but when exposed to plasma carrier O2 and phenol formaldehyde its contact angle was of 8° and 40° respectively. These results can be explained one consider that there should be induced surface roughness, as well as chemical changes caused by Al(OH)3 exposure, hydroxyl groups (-OH) presence and PMMA oxidation.
Cold plasma coating can be achieved by polymerization or particle deposition (with micrometric and nanometric diameters). Generally coating plasma allows condense thin layers that can act as homogenous or porous films, and this last mentioned can be used as permeable and semipermeable membranes.
Cold plasma is ecologically and economically important since it works at low pressure (equal to or less than atmospheric) reaction gases, electrical energy used, and waste result are considerably smaller compared to other treatments. Other advantage that cold plasma offers is that do not alter treated materialsinternal properties, because its effects are limited to their surface.
In addition, plasma coating is an innovative industrial technology that generally take place under atmospheric conditions, usually using plasma JET equipment. For all the above, it is expected that plasma technology will find more useful industrial applications for development of cheap and ecological materials.
Thanks to Coahuila’s Autonomous University (UAdeC) and its Faculty of Chemistry Sciences for provide installations and information access (UAdeC Digital Library, 2018) that allowed realize this short review.
JBCT Vol. 10, No. 19
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