Barrier properties, surface treatments

Plasma processing technologies ate used for surface treatments and coatings for plastics, elastomers, glasses, metals, ceramics, etc. Such treatments provide better wear characteristics, thermal stability, color, controlled electrical properties, lubricity, abrasion resistance, barrier properties, adhesion promotion, wettability, blood compatibility, and controlled light transmissivity. 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Films of polyolefins, polyamides and poly(vinylidene dichloride) are made using this technique. As most of the films are used for flexible packaging, further down-stream surface treatments are usually applied to improve performance. For example, aqueous polymer emulsions, e.g., poly(vinylidene dichloride), or delaminated clay particles improve the barrier properties as will metallising with aluminium vapour. Corona discharge, causing slight surface oxidation, improves printability. 

The above results suggest a potential for utilizing surface fluorinated plastics for modified atmosphere and vacuum packaging applications. If needed, the barrier properties can be improved further by vacuum metalization. Excellent adhesion was found for vacuum-deposited aluminum on this new fluorinated layer, achieving an adhesion index of 10 using the classical cross-hatch method. With this treatment method it was also possible to heat-seal PP or PE onto PET. 

The function of the mucosal pellicle is to serve as a barrier between the oral epithelial surface and the external environment, and so it may also act as a barrier to drug delivery. However, there are limited studies assessing the role of the mucus layer in buccal permeability. In one study, treatment of the oral mucosa with anticholinergic agents resulted in an increased permeability of certain compounds, and it was suggested that the reduced salivary flow may have been responsible for the reduced barrier properties of the tissue [113]. In... 

Semiconductor electrodes can be used in galvanic cells like metal electrodes and a controlled electrode potential can be applied by means of a potentiostat, if the electrode can be contacted with a suitable metal without formation of a barrier layer (ohmic contact). Suitable techniques for ohmic contacts have been worked out in connection with semiconductor electronics. Surface treatment is important for the properties of semiconductor electrodes in all kind of charge transfer processes and especially in the photoresponse. Mechanical polishing generates a great number of new electronic states underneath the surface 29> which can act as quenchers for excited molecules at the interface. Therefore, sufficient etching is imperative for studying photocurrents caused by excited dyes. 

H.E. Wagner, R. Brandenburg, K.V. Kozlov, A. Sonnenfeld, P. Michel, J.F. Behnke, The barrier discharge Basic properties ans applications to surface treatment. Vacuum 71, 417-436 (2003)... 

Surface treatment of plastic containers is sometimes used to improve barrier properties, or to enhance printability or label adhesion. 

Sidel, a French company, has developed the Actis plasma process that coats the inside of PET bottles with a 0.15 micron thick layer of amorphous carbon to improve oxygen and carbon dioxide barrier. Actis stands for Amorphous Carbon Treatment on Internal Surface. The carbon is deposited from acetylene gas, using a microwave-assisted process to excite the gas into plasma. The bottles are clear, and the process is reported to increase the carbon dioxide barrier of beer bottles by up to seven times, while not interfering with recycling. Actis has been approved by FDA, and is reported to cost 20 to 25 percent less than multilayer PET bottles with comparable barrier properties [10], Sidel has also developed Actis Lite a lower level of treatment for carbonated soft drink bottles, sparkling waters, juices, teas, and sauces, which do not require as good a barrier as beers and ciders. 

For many industrial applications of plastics that are dependent on adhesive bonding, cold gas plasma surface treatment has rapidly become the preferred industrial process. Plasma surface treatment, which is conducted in a vacuum environment, affords an opportunity to minimize or eliminate the barriers to adhesion through three distinct effects (1) removal of surface contaminants and weakly bound polymer layers, (2) enhancement of wettability through incorporation of functional or polar groups that facilitate spontaneous spreading of the adhesive or matrix resin, and (3) formation of functional groups on the surface that permit covalent bonding between the substrate and the adhesive or matrix resin. Since plasma treatment is a process of surface modification, the bulk properties of the material are retained. The nature of the process also allows precise control of the process parameters and ensures repeatability of the process in industrial applications. Finally, several studies have demonstrated that these surface modifications can be achieved with minimum impact on the environment. 

PET, PEN, and PC exhibit different surface roughness and surface energy. These parameters are important factors that affect the growth of AlxOy. It was found that a smooth substrate surface is a prerequisite to get a good moisture barrier independent of the barrier property of the polymer substrate. The surface roughness decreases from PEN via PET to PC. Eor this reason, the treatment with aluminum oxide is not effective for PEN. 

Dreux et al. studied the effect of CF4 and CO2 plasma treatment on the barrier properties of polyamide 12 (PA12, ATOFINA, Serquigny, France) toward permeant molecules of opposing characters, i.e., water and toluene [44]. While CF4 treatment made the surface more hydrophobic, CO2 treatment made it more hydrophilic. The surfaces were studied by AFM and XPS. 

Dreux, F., Marais, S., Poncin-Epaillard, F., Metayer, M., Labbe, M. and Saiter, J.M. 2003. Water and toluene barrier properties of a polyamide 12 modified by a surface treatment using cold plasma. Mater. Res. Innovat. 1 183-190. 

A principle advantage of organic electronics is that large, flexible and low cost substrates can be used. Polymer films, such as polyester, are the most widely used today but paper, cardboard, thin glass and stainless steel are also potential candidates. Special surface treatments or barrier layers can be added if necessary. The material best suited for a specific application depends on the process conditions, surface roughness, thermal expansion, and barrier properties. 

Both treatments improve the water barrier properties of paper-board by decreasing the water vapor transmission rate by 77-78%. The results suggest that the surface coating by biodegradable polymers may be utilized for the manufacture of paperboard containers even in industrial applications (26). 

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