Plastics sodium etch

Surface preparation of fluoroplastics can be achieved by a number of methods.h i] The plastic surface can be roughened by blasting with water. Other methods include corona bombardment, cross-linking by activated inert gas species, molten metal alloy treatment, sodium hydride treatment, and sodium etching. The treated layer is at most 1 pm thick and does not alter the bulk properties of the fluoroplastic. Surface... 

Etch the plastic in the sections slightly in order to allow for penetration of reagents into the samples. Float the grids section side down on drops of a freshly prepared saturated solution of sodium meffl-periodate see Note 11). 

Etch Araldite- or Epon-embedded sections to expose the antigenic sites. Kuhlmann and Peschke (1982) found a treatment of 1% NaOH in 50% ethanol for 10 min easier to handle or more beneficial than sodium ethoxide or peroxide treatments. The sections are then jet washed by a spray of PBS from a plastic spray bottle and blotted by holding the grid edgewise on filter paper. 

Chemical surface treatments vary with the type of plastic being bonded. These processes can involve the use of corrosive and hazardous materials. The most common processes are sulfuric acid-sodium dichromate etch (polyolefins) and sodium-naphthalene etch (fluorocarbons). Both of these processes are described in ASTM D-2093. 

The fluorocarbon surface may be made more wettable by exposing it for a brief moment to a hot flame to oxidize the snrface. The most satisfactory surface treatment is achieved by immersing the plastic in a sodium-naphthalene dispersion in tetrahydrofuran. This process is believed to remove fluorine atoms, leaving a carbonized surface that can be wet easily. Fluorocarbon films treated for adhesive bonding are available from most suppliers. A formulation and description of the sodium-naphthalene process may be found in Table 7.10 (p. 7.37). Commercial chemical prodncts for etching fluorocarbons are also listed. 

Each fission that occurred in the thin film caused a damaged area in the plastic that resulted in a short track when etched in sodium hydroxide. The plastic was then photographed resulting in a 170 x blowup from which the tracks were counted visually. This count yielded the relative fission rate in the two thin films that had been calibrated previously by absolute alpha counting. 

The most difficult-to-bond plastics family is polytetrafluoroethylene and the other fluori-nated polymers which are the Teflon-like materials. The widespread use of these polymers as coatings for cookware is, of course, based on this property of nonsticking. The conventional methods of etching which can be useful with other difificult-to-bond plastics, like polyethylene and polypropylene, simply do not work on these fluorinated polymer surfaces. Instead the surfaces must be treated with an exotic mixture of metallic sodium and naphthalene in tetrahydrofuran.( 37,i38) More recently, ionized gases (plasma treatment) have also been used successfully. Once treated the surfaces become bondable using conventional two-part liquid... 

The combination of properties that makes fluorocarbons highly desirable engineering plastics also makes them nearly impossible to heat or solvent weld and very difficult to bond with adhesives without proper surface treatment. The most common surface preparation for fluorocarbons is a sodium naphthalene etch, which is believed to remove fluorine atoms from the surface to provide better wetting properties. A formulation and description of the sodium naphthalene process can be found in another chapter. Commercial chemical products for etching fluorocarbons are also listed. 

Epoxy and nitrile-phenolic adhesives have been used to bond these plastics after surface preparation. The surface can be etched with a sodium sulfuric-dichromate acid solution at elevated temperature. Flame treatment and corona discharge have also been used. However, plasma treatment has proven to be the optimal surface process for these materials. Shear strengths in excess of 3000 Ib/in have been reported on polyethylene treated for 10 min in an oxygen plasma and bonded with an epoxy adhesive. Polyolefin materials can also be thermally welded, but they cannot be solvent cemented. 

PTFE and other fluoropolymers have been treated using a solution of sodium in liquid ammonia and other etching solutions [3]. This method dramatically improves surface-wetting characteristics, and the plastic can then readily be bonded using a wide range of adhesives. 

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