The use of polymers to replace traditional materials in the automotive, aerospace and other industries is continuing at a high pace. Polymers offer distinct advantages over glass and metals, such as weight, impact strength, and the ability to be formed into complex shapes. To ensure long service life, however, in some applications the polymers must be coated to provide protection from damage due to mechanical abrasion or chemical attack.

 

One such protective coating currently being developed is a microwave based PECVD coating using tetramethyldisiloxane (TMDSO) and oxygen. This system can be used to deposit an amorphous siloxane coating several microns thick, at industrially relevant speeds. PECVD has high deposition rates with no cure required; it can coat complex parts and has the ability to vertically grade the composition of the coating. These siloxane coatings can provide the polymers with protection from in-service wear and chemical attack. However, traditionally, adhesion of these coatings to many polymers has been marginal.[1]

 

In this study it has been found that plasma pre-treatments using vapours from the alcohol family are effective at modifying the adhesion of these coatings to five commercial polymers, polycarbonate (PC), polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA) and acrylonitrile butadiene styrene (ABS). We have observed that the plasma pre-treatment both etches the polymer and deposits a carbonaceous film some 10 to 20 nm thick. It has also been established that both of these mechanisms are required to improve adhesion. Of further interest is the observation that the pre-treatment time must be tailored depending on the polymer substrate. Optimum adhesion is only achieved after some critical time. For ABS and PS, less than 15 secs is adequate, however for polymers, such as PMMA and PC, treatment times up to 120 and 180 secs are necessary. The duration of pre-treatment appears to be related to the etch rate of the particular polymer. Investigation has revealed that the alcohol plasma creates hydroxyl radicals.[2,3]These radicals are capable of initial hydrogen abstraction from the polymer; however, the subsequent etching proceeds depending on the structure of the polymer. As such there is a need to tailor the pre-treatment time depending on the polymer being treated.

 

1. B. W. Muir, H. Thissen, G. P. Simon, P. J. Murphy, H. J. Griesser, Thin Solid Films 2006, 500, 34.

2. C. Hall, P. Murphy, H. Griesser, Plasma Process. Polym. 2012, Accepted, In Press, DOI: 10.1002/ppap.201100159.

3. C. Hall, P. Murphy, H. Griesser, Plasma Process. Polym. 2012, Accepted, In Pres.