Etching treatments polyolefins

Chromic or sulfo-chromic acid etching, for polyolefins, polystyrene, ABS, polyacetal, polyphenylene ether. .. These treatments have two effects ... 

Many chemical pretreatments have been reported for polyolefins but the only one of commercial importance is etching in chromic acid. It is typically used for the treatment of irregularly shaped or large objects when it would be difficult to use either flame or corona-discharge (Section 4.2.7) treatments. The chromic acid etches the polyolefin surfaces and results in loss of material into solution. 

Figure 14 Fabrication procedure for the pile-up microreactor. (1) Photolithography Conventional photolithography/wet etching methods were applied. The back side of the glass plate was covered with polyolefin tape during the HF treatment. (2) Drilling Penetrating holes were drilled at the inlet and outlet ports of the micro-channel circuit. (3) Thermal bonding The required number of glass plates with microchannels and one cover plate were laminated and bonded thermally at 650°C.

There are many different methods for modifying polymer surfaces to improve their adhesion and wetting properties. They include chemical etching and oxidation, ion bombardment, plasma treatments, flame treatment, mechanical abrasion and corona-discharge treatments (1.2). Especially flame and corona treatments are widely used for the modification of polyolefin surfaces to enhance, for instance, their printabilify. Despite the widespread use of such processes in industry, the understanding of the fundamental processes which occur at the polymer surface is very limited. This is undoubtedly due to the shallow depth to which the polymer is modified, typically 5 nm or less. 

The critical surface tensions of polymeric materials such as polyolefins can be increased by surface treatment such as corona treatment, chemical etching, flame treatment, and mechanical abrasion, in order to facilitate adhesive bonding. 

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. 

Epoxy and nitrile-phenolic adhesives have been used to bond these plastics after surface preparation. The surface can be etched with a sodirim sulfiiric-dichromate add solution at an elevated temperature. Flame treatment and corona discharge have also been used. However, plasma treatment has proven to be the optimum siuface process for these materi2ds. 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. 

Liquid etchants can be used for chenucal modification or dissolving surface contamination. Etchants effectively treat irregularly shaped objects that are difficult to treat by other adhesion-promoting processes such as corona or flame treatment. A number of etching solutions and procedures have been developed for specific polymeric surfaces. The choice of the liquid etchant depends on the polymers. Polyolefins are usually treated by oxidizing acids such as chromic, sulfuric, nitric, or mixtures of these. Fluorocarbons are usually treated by sodium-napthalene etching solution. 

Cyanoacrylate adhesives will bond most substrates to themselves and to each other. The few adherends which do not bond well with standard adhesives are polyethylene, polypropylene, EPDM rubber, plasticized PVC, teflon, and acidic surfaces. A few manufacturers sell modified adhesives which will bond some of these materials, such as EPDM and flexible PVC. Adhesion to low surface energy plastics like polyolefins and Teflon can be improved by an etching or oxidizing treatment. 

These non-stick materials can be bonded after surface treatment, which have the effect of introducing polar chemical groups to the surfaces. Methods for polyolefins include flame, corona discharge and chemical etching. Solutions of sodium naphthalenide in THF and sodium in liquid anunonia are used for FITE. 

---