Enhancement of the gas barrier property of polypropylene by introducing plasma-treated silane coating with SiOx-modified top-surface

• Amino silane was coated on polypropylene and treated by oxygen plasma.
• Gas barrier of a plasma-treated silane-coated polypropylene was greatly enhanced.
• Barrier effects were analyzed through free volume and chemical compositions.
• Free volume size was reduced on the silane surface after plasma treatment.
• Composition of the plasma-treated silane surface was changed into SiOx.

Polypropylene (PP) is a widely used packaging polymer due to its high transparency and high thermal durability. It is, however, also known to possess a relatively low gas barrier property. To solve the gas barrier problem of PP, a surface modification method by a silane coating was introduced: 3-aminopropyltrimethoxysilane (APTMS) was coated onto PP substrate (APTMS/PP) and treated by oxygen plasma. It was found that, after 60 s of the oxygen plasma treatment, the oxygen transmission rate (OTR) of the APTMS/PP was reduced by a factor of 15 as compared with that of pure PP. The free volume size analyzed by the monoenergetic positron beam significantly decreased at the surface of APTMS observed through the plasma-treatment time. The results indicated that the amount of the free volume of APTMS, through which the gas molecules permeated, was significantly decreased, eventually causing a considerable reduction in the OTR of the APTMS/PP. From the X-ray photoelectron spectroscopy (XPS), it was found that the plasma treatment of APTMS notably reduced both the carbon and the nitrogen atom fractions, simultaneously generating additional Si–O bonding to generate SiOx-like structures at the surface. The formation of the SiOx-like structure at the top surface of the APTMS/PP was considered as the major reason for the enhancement of the oxygen barrier property. In fact, the drastic decrease in the OTR was actually observed at a certain ratio of C/Si. The time of flight secondary ion mass spectroscopy (ToF-SIMS) also revealed that the side-chain scission from the APTMS molecules could generate amine and amide fragments, which could hinder the polymerization of the SiOx. Thus the removal of the fragments would promote the efficient formation of SiOx networks at the top surface of APTMS. Additional plasma treatment could introduce more polymerized SiOx networks in APTMS, resulting in the smaller size of the free volume and hence the higher gas barrier property of APTMS/PP.