Barrier properties of plasma-modified polypropylene and polyethyleneterephthalate

Plasma treatment changes the solvent absorption and permeation as well as the swelling properties of polymers. Enchanced solvent absorption and swelling are effects of an improved solvent compatibility. The plasma introduces a large number of different groups at the polymer surface depending on the nature of the plasma. Fluorine-containing plasmas can replace hydrogen atoms of the polymer molecule with fluorine atoms. Moreover, fluorine-containing plasma polymer layers can be formed. All these processes reduce the resulting surface free energy, reduce the diffusion length of solvent molecules, and produce a barrier layer. We have studied the formation of solvent barriers by plasma fluorination and by crosslinking by ultraviolet (UV) radiation. Thin foils of polypropylene (PP) and polyethyleneterephthalate (PET) were used as substrates. CF₄, SF₆, and SOF₂ were applied as sources of fluorine atoms. Hexafluoropropene, tetrafluorethylene, and perfluorohexylethylene form plasma polymer layers on the polymer substrates. Test solvents were n-pentane, tetrachloroethylene, dimethylsulfoxide, and mixtures of n-pentane and methanol. The permeation rate of solvents through plasma-modified polymers was measured gravimetrically. Mass spectrometry was applied to analyze the permeating components of the solvent mixtures. Fluorination of surface layers by plasma-chemical (CF₄, SF₆) means considerably reduces the permeation rate of PP (95% barrier effect) and PET (100%). The preferred permeation of one component of the pentane/methanol mixture is influenced by the polarity of plasma-introduced groups at the polymer surface.     

Plasma polymer membranes from hexafluoroethane/hydrogen mixtures for separation of oxygen and nitrogen

Thin plasma polymer layers were produced employing feed mixtures of hexafluoroethane and hydrogen in an rf parallel-plate reactor. The layers are intended for use in membrane-based separation of oxygen and nitrogen. The hexafluoroethane-to-hydrogen mixture ratio was varied over a wide range, whereas all other process parameters (power, pressure, substrate temperature, and total gas flow) were held constant. The plasma polymers were examined by scanning electron microscopy, X-ray analysis, quantitative elemental analysis, and X-ray photoelectron spectroscopy. Permeability coefficients of oxygen and nitrogen and selectivities of the pure gases were determined. Pinhole-free plasma polymer films containing different amounts of fluorine, carbon, and hydrogen were formed. The distributions of fluorine and hydrogen in the products reflect their distributions in the feed gas. Traces of oxygen in some of the polymers are explained by the reaction of trapped radicals with atmospheric oxygen on the samples' exposure to air. Fluorine-containing carbon moieties such as CF₃, CF₂, and CF, and carbon moieties with fluorine atoms exclusively in secondary positions are present. A method of calculating crosslink density using the analytical data is described. The oxygen permeability coefficient and the selectivity of the plasma polymers increase as the hexafluoroethane content of the feed gas is raised. This behavior is attributed to growing solubility selectivity as a result of the rising fluorine content of the polymers. Maximum selectivity amounts to 3.4 at an oxygen permeability coefficient of 21 Barrer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1517–1526, 1997     

A fluidized bed kinetic model for the fluorination of dissociated zircon

A reaction kinetic model for the fluorination of plasma dissociated zircon (PDZ, or ZrO₂.SiO₂) with hydrogen fluoride was developed. The model uses reaction as rate‐limiting step, with a shrinking core of PDZ in a porous matrix of zirconia (ZrO₂). This model was used to develop models for a multi‐stage fluidized bed reactor. These models facilitated the determination and design of an optimally configured multi‐stage fluidized bed reactor for the fluorination of PDZ. It was shown that a combination cross‐/countercurrent multi‐stage fluidized bed reactor could yield significant improvement over the conventional countercur‐rent multi‐stage fluidized bed reactor for certain reaction kinetic conditions.     

Plasma surface treatment of HDPE powder in a fluidized bed reactor

Summary A plasma surface treatment of HDPE has been carried out in a fluidized bed. The effects of operation parameters on the surface composition and hydrophilicity have been determined. The oxygen functionalities are formed at the outermost layer of HDPE powder. The hydrophobic surface of HDPE powder has been transformed to hydrophilic surface by the oxygen-plasma treatment in a fluidized bed. The contact angle of the plasma treated HDPE powder decreases linearly with radio frequency (rf) power but increases with O₂ flow rate. Also, the angle decreases with increasing the composite parameter as the total plasma energy down to 6,000 [(W/FM)t]. The contact angle has been correlated with the composite parameter as: =90–6.64×10^-3 [(W/FM)t].     

Surface modifications of polymers with fluorine-containing plasmas: Deposition versus replacement reactions

Surfaces of polyethylene; poly(vinyl fluoride), poly(vinylidene fluoride), poly(tetrafluoroethylene), cellulose acetate butyrate, and polyoxymethylene were modified in various cold plasma reactions; feed gases to the plasma reactor were trifluoromethane, hexafluoroethane, and tetrafluoromethane. Using X-ray photoelectron spectroscopy (ESCA) to characterize the surfaces, it was established that the plasma reactions lead to fluorinated surfaces containing ? CF₃, ? CF₂, and ? CF groups, All of these fluorinated surfaces exhibit advancing contact angles (with water) larger than 900. However, differences in the ESCA spectra, weight-gain/-loss measurements and scanning-electron-microscopy (SEM) photographs reveal that the mechanisms of fluorination in the various plasma environments are markedly different. The CF₃H gas polymerizes in the gas phase of the plasma and deposits a smooth, fluorinated film on polymers and other substrates. The C₂F₆ plasma simultaneously etches polymers and polymerizes onto polymer surfaces. The CF₄ plasma etches and reacts with the polymer surface but does not polymerize. For polyoxymethylene, the combined roughening (by etching) and fluorination of the surfaces lead to completely non-wettable surfaces (water contact angle approximately 180°). The highly non-wettable surfaces of these two polymers are believed to result from the physical etching and roughening at a very fine scale (approximately five micrometers) while the outermost surfaces are reacting to become highly fluorinated.     

Xenon difluoride plasma fluorination of polymer surfaces

Xenon difluoride (XeF₂) plasma treatment of a series of polymers containing different repeat units gives rise to varying levels of surface fluorination. Alkene and aromatic C-H bonds appear to be more susceptible towards reaction compared to their sp³ counterparts. The extent of fluorine incorporation can be accounted for in terms of a structure-behaviour relationship derived from extended Huckel molecular orbital calculations. Comparison with CF₄ plasma modification shows that XeF₂ electrical discharges are more effective at fluorinating polymer surfaces.     

Grafting fluorocarbons to polyethylene in glow discharge

A systematic surface fluorination of high-density polyethylene was carried out using CF₄, CF₃H, CF₃Cl, and CF₃Br, in a radio-frequency glow discharge. Based on ESCA and wettability measurements, all of these compounds provided a fluorocarbon layer on high-density polyethylene surface, but the fluorine to carbon ratio and extractability of the films were strongly dependent on the starting materials and the location of the sample specimen in the reactor chamber as well as the duration of the reaction. The results with vertically held, CF₃H-treated samples showed a high level of nonextractable surface fluorination and very little change in wetting properties before and after extraction with CF₂ClCFCl₂.     

Surface modification of HDPE powders by oxygen plasma in a circulating fluidized bed reactor

Summary Surface modification of HDPE powders by oxygen plasma has been carried out in a circulating fluidized bed (CFB) reactor. The effects of solid holdup, treatment time and radio frequency (rf) power of oxygen plasma on the surface composition and hydrophilicity of HDPE powders have been determined. The solid holdup in the plasma reaction zone mainly governs the stability of plasma glow and surface property of the powders. The hydrophobic surface can be transformed to hydrophilic of HDPE powder and the oxygen functionalities are formed including C=O and C(O)O-, which reach 12% and 8% respectively of the total carbon elements by the oxygen plasma treatment. The CFB reactor outperforms 3.4 times to obtain the similar level of hydrophilicity compared to that in a bubbling fluidized bed based on the composite parameter [(W/FM)t]. The present CFB reactor can be utilized to modify the polymeric surface from hydrophobic to hydrophilic with high-energy efficiency. Received: 21 June 2001/Revised version: 10 August 2001/Accepted: 20 August 2001     

Surface modification of low density polyethylene in a fluorine gas plasma

Low density polyethylene was fluorinated in a glow discharge generated from a dilute mixture of fluorine in helium. The effects of pressure, flow rate, power and time of treatment have been examined. The fluorinated polymers were characterized using e.s.c.a. to identify the species present in the surface layer and to estimate the depth of fluorination. For typical plasma treatment of polyethylene films the fluorination depth was about 40 Å; a competition of ablation and ion-assisted etching processes with chemical reaction was observed. The depth of fluorination was increased to values above 60 Å by carrying out the reaction within a metal screen. The presence of the screen also reduced the reaction rate by about a factor of two. Other features of the treated polyethylene were determined by contact angle measurements, surface infra-red spectroscopy, solvent sorption and differential scanning calorimetry. Some early results of treatment of polyethylene powders in a fluidized bed reactor are also presented.     

Plasma modification of cellulose derivatives as biomaterials

A film of cellulose acetate was submitted to a cold plasma of tetrafluoromethane or of sulfur hexafluoride. The interactions of these cold plasmas and cellulose acetate lead to a material whose surface has been modified by fluorination. Comparison of CF₄ or SF₆ plasma treatment shows that fluorine atoms provided by each kind of plasma induce degradation and grafting of fluorocarbon radicals on the surface. As a consequence, the surface energy decreases and offers the possibility of a better response of plasma-modified cellulose derivatives used as biomaterials (e.g. hemodialysis membrane).     

Surface characterization of nickel alloy plasma-treated by O2/CF4 mixture

The micro- or nano-structured mold used for polymer embossing typically must be coated with an anti-adhesion material to reduce its interaction with the embossing. The mold is typically made by nickel sulphamate electroforming. For the anti-adhesion coating to adhere to the mold, the nickel mold surface must be clean and preferably unoxidized or possess reactive groups suitable for covalent bonding with the anti-adhesion coating. The effectiveness of plasma cleaning using mixtures of oxygen (O₂) and tetrafluoromethane (CF₄) with varying ratios versus liquid-only cleaning was investigated. To simulate the nickel mold, Ni200 alloy was used. Plasma treatment using mixtures of O₂ and CF₄ was found to be more effective in cleaning the Ni200 surface than liquid-only cleaning or pure O₂ or pure CF₄ plasma treatment. Using a 1 : 1 O₂ /CF₄ mixture plasma, the contact angles of water, glycerol and diiodomethane on Ni200 were the lowest and the calculated surface energy was the highest among the investigated treatments. From X-ray photoelectron spectroscopy (XPS), the amount of organic contamination on Ni200 was significantly reduced with plasma treatment. For liquid-only cleaned samples, metallic nickel, NiO and Ni(OH)₂ are present on the surface. With pure O₂ or pure CF₄ or 1 : 1 O₂ /CF₄ mixture plasma, both oxidation and fluorination occur and the surface contains combinations of NiF₂, Ni(OH)₂, Ni(OH)F, Ni₂O₃ and NiO₁₅F instead (without metallic nickel and NiO). The proportions of these different compounds vary according to the O₂/CF₄ ratio; O/Ni ratio is highest for pure O₂ plasma treatment, whilst F/Ni is highest for pure CF₄ plasma treatment.     

Ta and TaN adhesion to high-temperature fluorinated polyimides. Surface and interface chemistry

The practical adhesion of Cu/Ta to high-temperature fluorinated polyimides (FPIs) was initially good but failed after the reliability test involving treatment under the FPI curing condition five times (T₅). But a thin layer (40 nm) of TaN greatly improved the reliability of the Cu/Ta-to-FPI adhesion. Both CF₄ and in situ Ar plasma treatments of FPIs prior to metal deposition enhanced the metal-to-FPI adhesion strength. CF₄ plasma enriches the FPI surface with fluorine atoms and most of fluorine is bound to carbon as CF₃, CF₂, and CF. Ar plasma first destroys CF₃ and then C=O groups of the FPIs to yield a polar surface. The locus of failure by a 90° peel test was found to be within the Ar-plasma-modified FPI layer but it moved toward the bulk of FPI, i.e. away from the metal-polymer interface, after the T₅ reliability test. The locus of failure in the case of weak adhesion where no plasma treatment was done on FPI films was in the near-interface region within the FPI layer, and the failure seemed to occur in the weak boundary layers of FPI surfaces. Plasma treatment removes weak boundary layers and also increases FPI surface roughness. These two effects combined improved the metal-to-FPI practical adhesion.     

Plasma polymerization of tetrafluoroethylene

The plasma polymerization of C₂F₄ was carried out in both continuous wave and pulsed rf discharges to establish the effects of reaction conditions on the kinetics of polymer deposition and the polymer structure. ESCA spectra of the polymer show evidence for ? CF₃, ? CF₂, and ? CH₂? groups. Under conditions favoring low deposition rates, the dominant functional group is ? CF₂? . At higher deposition rates the concentration of ? CF₂? groups is reduced and a more crosslinked polymer is produced. Both polymer deposition rates and polymer structures were essentially identical when using continuous wave and pulsed rf discharges.     

Thermal Reaction Analysis of Oxidative Coupling of Methane

For more than three decades, the oxidative coupling of methane (OCM) process has been investigated as a promising alternative approach for ethylene production. Simulations of different sets of surface mechanisms over the Na₂WO₄/Mn/SiO₂ catalyst and the gas phase reactions that come along with the OCM reaction were analyzed in a fixed‐bed, membrane, and fluidized‐bed reactor. The results were compared with the experimental data generated in an OCM mini‐plant. It was observed that the gas phase reactions are crucial in reducing the overall selectivity, especially in the fluidized‐bed reactor.     

Hydrophobic polymer films plasma-polymerized from CF4/hydrocarbon and hexafluroacetone/hydrocarbon mixtures

Polymer films were deposited from the plasma polymerization of the mixtures of hydrocarbons, ethane, ethylene, and acetylene, and tetrafluoromethane (CF₄) or hexafluoroacetone (HFA). The surface properties, the advancing contact angle of water, and surface energy of the films deposited and the chemical composition at the outermost layer of the films are discussed from the data of the angular XPS measurements. The plasma polymers deposited from the CF₄/hydrocarbon and HFA/hydrocarbon mixtures contained fluorine atoms whose content depended on the CF₄ or HFA concentration of the mixtures. The hydrophobicity of the films deposited could not be determined by the fluorine content of the films but by the chemical composition of the fluorine moieties at the outermost layer of the films. The CF₃ moieties rather than the CF₂ and CF moieties contribute largely to the hydrophobicity of the films. The plasma polymer films deposited from the HFA/acetylene (87.5 mol % HFA) showed higher hydrophobicity (the surface energy is 9.7 mJ/m²) than those from the CF₄/acetylene mixture (87.5 mol % CF₄) (the surface energy is 13 mJ/m²).     

Syngas methanation for substitute natural gas over Ni–Mg/Al2O3 catalyst in fixed and fluidized bed reactors

A comparative study was conducted for laboratory syngas methanation over a self-made Ni–Mg/Al₂O₃ catalyst to demonstrate the technical advantages of fluidized bed over fixed bed reactor. At different reaction temperatures, gas velocities and pressures, the CO conversion and selectivity to CH₄ in fluidized bed were shown to be higher than in fixed bed, and much closer to the thermodynamic equilibriums. The spent catalysts from fluidized bed methanation had distinctively low and easy-oxidizing deposited carbon in comparison with that from fixed bed. The results were attributed to the bigger effective catalytic surface, better heat and mass transfer in fluidized bed reactor.     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Surface Fluorination of Poly(fluorenyl ether ketone) Ionomers as Proton Exchange Membranes for Fuel Cell Application

A series of sulphonated poly(fluorenyl ether ketone) ionomers were successfully fluorinated by the means of direct surface fluorination. Polymer ionomer samples in two different states (membrane and powder) were treated with F₂ gas which is diluted in N₂ in a special reactor. X‐ray photoelectron spectroscopy (XPS) was used to examine the F/C ratios of the fluorinated materials. The results revealed that the fluorination only occurred on the membrane surface and the fluorination degree increased with increasing F₂ concentration in N₂. The membrane subjected to fluorination shows an obviously enhanced oxidative stability. The endurance in a Fenton's reagent of FSPFEK‐P‐28 is longer than 180 min which is two times longer than that of un‐fluorinated SPFEK. The PEM properties and single fuel cell performances were investigated by comparison of un‐ and fluorinated polymer ionomers. The fluorinated membranes demonstrated an enhanced hydrophobic surface property, increased proton conductivities and better single fuel cell performances. Surface fluorination provides a convenient and useful approach to prepare highly proton conductive membrane with long life‐time PEM fuel cell applications.     

Surface crosslinking of high‐density polyethylene beads in a modified plasma reactor

High‐density polyethylene (HDPE) beads were successfully surface‐crosslinked in a modified plasma reactor. The modified plasma reactor treats large amounts of beads, which are uniformly surface‐crosslinked. In this study, effects of the gas pressure, radio‐frequency (RF) power, and the treatment time on the degree of surface crosslinking were systematically investigated. Degree of surface crosslinking was measured by solvent extraction method (boiling xylene method, BXM). The gel content of plasma‐treated HDPE increases from 0.0 to 1.05% within 10 min at 100 mTorr, 200 W. FTIR and DSC analyses show that the crosslinked layer after plasma treatment is limited only at HDPE surface without changing the bulk thermal property of HDPE. Through the analysis of FTIR, it was confirmed that main peaks corresponding to CH₂ bands were decreased and two peaks corresponding to CF₂ and CF₃ were observed after plasma surface modification. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2921–2929, 2002; DOI 10.1002/app.10295     

Structural and chemical characterization of fluorinated amorphous carbon films (a-C:F) as a liquid crystal alignment layer

a-C:F thin films with varying fluorine content were prepared by plasma CVD and the sputtering method as inorganic alignment layers for overcoming the disadvantages of conventional liquid crystal (LC) alignment layers. The material and structural properties were investigated by X-ray photoelectron spectroscopy, Fourier transform infrared absorption, and contact angle measurement. For elucidation of the liquid crystal alignment layers, LC cells with a-C:F films were fabricated, followed by examination of the textures of the LC and electro-optical characteristics. The fluorine concentrations of a-C:F films were controlled by changing the mixture gas ratio (R_G) in CVD and applied power ratio (R_P) in the sputter system. An increase in R_G and R_P led to increase fluorine incorporation, and the film microstructure changed from a diamond-like to a polymer-like structure. In addition, the sputtered a-C:F films showed a higher fluorination than the CVD sample since the PTFE target was only composed of CF₂ functional groups. Surface composition influenced the surface energy of thin films and an extremely hydrophobic property was obtained in the case of fluorine-rich a-C:F films. LC orientations were observed in various compositions of a-C:F films, and the vertically self-aligned LC textures confirmed that a sputtered a-C:F film is a good candidate for an alignment layer without any post-treatment.