By Dr. Eldridge M. Mount
In the case of metallization, it has been found that a hydroxylated HDPE surface (EVOH, flame-treated HDPE, in-chamber plasma, or combinations) gives the best metallization barrier [6]. HDPE films are also more easily corona-treated to give improved adhesion [7] than homopolymer PP due to the differences in surface chemistry of the skin polymer produced by the flame vs. corona.
Corona treatment of homopolymer PP results in chain-scission of the PP, producing acid and keto end groups [8] and a lower molecular weight surface while flame treatment produces a lower oxidation state with less chain scission. In addition, flame treatment has been found to give higher dyne levels for lower surface oxygen concentrations and to give predominantly a hydroxyl (-OH) or alcohol surface chemistry with HDPE [9] and perhaps for homopolymer PP.
In general, the EPcopolymers, BPcopolymers and EBPterpolymer are easier to corona- and flame-treat for enhanced adhesion than the homopolymer-PP surfaces, also giving higher dyne levels, similar surface chemistry and likely lower levels of chain-scission, resulting in stronger surfaces. The closeness of the slopes between the flame- and corona-treatment curves suggest a similarity of surface chemistry for EPcopolymer with both flame- and corona-treatment methods.
Overtreatment downsides
The greatest problem (danger!?) with surface treatment of any polymer surface is the tendency to overtreat to obtain high dyne levels. Higher dyne levels do not necessarily indicate an improved bonding surface, just improved wettability. In general, increased dyne levels in OPP films indicate an increase in wettability but also surface degradation for homopolymer-PP skins and, hence, the formation of a weak boundary layer which diminishes adhesion.
In conclusion, we can see that for OPP, the skin polymer and the surface-treatment type interact to give various dyne levels vs. surface-oxygen concentration and oxygen-functional groups. These differences all factor into improving surface wettability, adhesion and, for metallized films, improved barrier properties. These concepts also will hold true for other substrates based on HDPE, PET and Nylon, and the interrelationship between surface layers, treatment type and treatment level on surface properties and performance must be determined experimentally.
Having developed many metallized films, my personal preference is for flame treatment for all skin resins, first because it has no backside treatment, which is common and unavoidable in corona treatment. Backside treatment can lead to metal pickoff and, second, because it gives lower oxidation-state oxygen functions (hydroxyl groups) on the surface which are better for adhesion and metallized barrier properties.
References
6. Mount III & Wagner, US Patent 5,981,079, Nov, 9, 1999
7. Migliorini & Mount III, US Patent 5,194,318, March 16, 1993
8. Chi-Ming Chan, Polymer Surface Modification and Characterization, Hanser/Gardner Publications, Cincinnati, (1994), Chapter 7
9. Figures 7.9 and 7.10, Chi-Ming Chan, Polymer Surface Modification and Characterization, Hanser/Gardner Publications, Cincinnati, (1994)