Crosslinking plasma treatment

In the presence of an inert gas, such as He or Ar, crosslinking can be introduced into the surface layer of material by plasma treatment. Hansen and Schon-horn [60] named this Crosslinking by Activated Species of Inert Gases (CASING). As a result, bond strength is enhanced because crosslinking strengthens the surface layer. 

In a previous section, the effect of plasma on PVA surface for pervaporation processes was also mentioned. In fact, plasma treatment is a surface-modification method to control the hydrophilicity-hydrophobicity balance of polymer materials in order to optimize their properties in various domains, such as adhesion, biocompatibility and membrane-separation techniques. Non-porous PVA membranes were prepared by the cast-evaporating method and covered with an allyl alcohol or acrylic acid plasma-polymerized layer the effect of plasma treatment on the increase of PVA membrane surface hydrophobicity was checked [37].The allyl alcohol plasma layer was weakly crosslinked, in contrast to the acrylic acid layer. The best results for the dehydration of ethanol were obtained using allyl alcohol treatment. The selectivity of treated membrane (H20 wt% in the pervaporate in the range 83-92 and a water selectivity, aH2o, of 250 at 25 °C) is higher than that of the non-treated one (aH2o = 19) as well as that of the acrylic acid treated membrane (aH2o = 22). 

The use of static SIMS for the characterization of surfaces of polypropylene (PP), PTFE and a PMDA-ODA type poly-imide is described. Interfaces between evaporated copper or chromium films onto PTFE and polyimide were also analyzed. Some of the polymer substrates were modified by ion beams, corona discharge in air or plasma treatments in air, At and H2. It is demonstrated that SIMS is highly complementary to XPS for the analysis of such modified surfaces, in that effects such as crosslinking, unsaturation and formation of low-molecular weight material at surfaces can be detected. 

SIMS spectra is an indication of increasea crosslinking, although the effect can as yet not be quantified. Some evidence for this postulation is that all three fragments are highly unsaturated, as can be expected when C-C crosslinks are created in the polymer. Very similar effects have been observed upon plasma treatments of polyethylene, i.e., in air, N2 and H2 111. 161. In such treatments the unsaturated secondary ions always increase, relative to the more saturated ones. In addition, the higher mass fragments, which are more branched, also increase in relative intensity. They are more stable than the linear ions of the same overall composition. 

Step 3 The PS particles are removed by an O2 plasma treatment (2 min at 250 W, and SOOmTorr). In Chemical Properties we present XPS results, which describe the effect of O2 plasma treatment regarding removal of the PS particles by O2 plasma and oxidation of Pt. It should be noted that it is not possible to dissolve the PS particles in acetone after the Ar+ etching process, which is believed to be due to ion-induced crosslinking of the polymer chains during ion etching (111), making them resistant to normal solvents for PS. The radial distribution function, g r), from the initial colloidal adsorption step is preserved throughout the nanofabrication procedure. 

With plasma treatment, surface wettability can be readily induced on a variety of normally non-wettable materials as shown in Table P. 5. Certain polymeric surfaces, such as the polyolefins, become crosslinked during plasma treatment. The surface skin of polyethylene, for example, will become crosslinked so that if the polymer were placed on a hot plate of sufficient heat, the interior would turn to a molten liquid while the crosslinked outer skin held a solid shape. Other polymers have their critical surface energy affected in different ways. Plasma-treated polymers usually form adhesive bonds that are two to four times the strength of nontreated polymers. Table P.5 presents bond strength of various plastic adherends pretreated with activated gas and bonded with an epoxy or urethane adhesives. 

The plasma surface treating process was originally developed by Bell Laboratories in the 1960s. It was then determined that the plasma treatment caused crosslinking of the surface of polyethylene. As a result this process was called CASING (crosslinking by species of inert gases) in its earliest days. 

Figure 7. Typical syringe fnction diagram measured while moving a stopper at a velocity of 100 mm/min in a syringe barrel. Shown is the frictional force (in kg) for a typical three-piece syringe (Figure 1) lubricated with a conventional silicone lubricant (curve labeled "Not plasma treated") and the reduced frictional force after plasma treatment (crosslinking) of the lubricate syringe barrel (curve label "Plasma treated").

---