Chlorobutyl rubber (CIIR) is widely adopted as elastic sealing materials for pharmaceutical packaging owing to its outstanding gas tightness and chemical stability. Nevertheless, CIIR still carries potential interfacial safety hazards during long-term storage of high-grade injections and sensitive pharmaceutical preparations. On the one hand, low-molecular-weight residues inside CIIR, such as incompletely reacted monomers and plasticizers, may diffuse and migrate into liquid medicines, raising risks of extractables and leachables (E&L). On the other hand, for strongly polar pharmaceutical systems represented by paclitaxel formulations, interfacial chemical reactions may occur between pharmaceuticals and chlorine functional groups on CIIR. Such reactions trigger dissociation of surface chlorine atoms to generate chloride ions (Cl⁻), which under certain conditions catalyze or accelerate the degradation of drug molecules and thereby compromise the stability and safety of pharmaceutical products. Constructing a stable, inert functional coating with excellent barrier performance between liquid drugs and CIIR has become an optimal strategy to mitigate the above risks. Among candidate coating materials, polydimethylsiloxane (PDMS) stands out as a promising material for fabricating functional barrier coatings on CIIR surfaces by virtue of favorable biosafety, chemical inertness and barrier capacity. However, vulcanized CIIR features a highly cross-linked network with scarce polar functional groups on its surface, presenting low intrinsic surface energy and poor wettability. Accordingly, PDMS coatings applied on CIIR are prone to insufficient wetting, interfacial defects and even coating delamination. Surface modification is capable of introducing polar groups or active sites onto CIIR surfaces to elevate surface energy and wettability, strengthening physical adsorption and chemical bonding between PDMS coatings and substrates, so as to achieve stable interfacial adhesion.

Plasma modification is a solvent-free, low-temperature and inline-applicable surface treatment technology. It only alters the chemical composition and micro-morphology within dozens of nanometers of the material surface without remarkably deteriorating the bulk intrinsic properties of the substrate. Existing studies have verified that oxygen plasma can graft oxygen-containing polar groups including –OH, C=O and C–O onto the surface of polymers such as rubber; meanwhile, mild etching increases surface roughness, which jointly raises surface energy and effective contact area and significantly reinforces the bonding strength between coatings/adhesives and substrates. Moderate introduction of surface oxygen-containing functional groups together with synchronous regulation of microscale roughness via oxygen plasma treatment (without damaging bulk CIIR properties) is expected to optimize the wetting behavior and interfacial bonding of PDMS/CIIR systems, improving both adhesion strength and barrier performance of PDMS-coated CIIR composite structures.

Effect of Oxygen Plasma Treatment Parameters on Surface Morphology of Chlorobutyl Rubber

With the oxygen plasma power fixed at 200 W, the influence of treatment duration on the surface topography of CIIR was investigated, and the results are displayed in Figure 1. The pristine untreated CIIR surface is relatively flat with only a small number of processing scratches observable. After 5 min of plasma treatment, slight surface undulations can be observed under low magnification, while scattered tiny particles and irregular micro-protrusions/indentations appear in high-resolution images. When the treatment time is prolonged to 10 min, surface roughness increases further accompanied by micrometer-scale cracks and cavities in local regions. After 20 min of treatment, these cracks interconnect and extend to form a relatively continuous reticulated cracking morphology. Overall, the CIIR surface evolves from a smooth state to a rough, defect-rich structure with prolonged treatment time, demonstrating the prominent modifying effect of oxygen plasma on CIIR surface morphology.

Figure 1 SEM images of chlorobutyl rubber surfaces treated by oxygen plasma at a fixed power of 200 W with different treatment durations

Effect of Oxygen Plasma Treatment Parameters on Surface Chemical Structure of Chlorobutyl Rubber

X-ray photoelectron spectroscopy (XPS) characterization was performed on samples subjected to various plasma treatment conditions to clarify variations in the surface chemical composition of CIIR, as illustrated in Figure 2. Figure 2(a) shows full-range XPS survey spectra of CIIR surfaces treated under different plasma power and duration combinations. The surface elemental composition mainly consists of carbon (C), oxygen (O), silicon (Si), magnesium (Mg) and calcium (Ca), with no new heteroatoms introduced before and after treatment. The surface oxygen content and O/C atomic ratio generally rise with elevated plasma power and extended treatment time. Deconvolution fitting of high-resolution O1s spectra (Figure 2(b)) reveals three constituents: inorganic oxygen originating from Mg₃Si₄O₁₀(OH)₂, C–O bonds and Ca–O bonds. The fraction of C–O species in untreated blank samples is the lowest (approximately 5.8%). The proportion of C–O groups rises to 6.3% after treatment at 100 W for 10 min, reaches a maximum value of 19.8% under 200 W & 10 min treatment, and remains at a relatively high level for the 20 min treatment group. For samples treated at 300 W for 10 min, the C–O content decreases slightly but is still higher than that of the untreated control sample. Meanwhile, the atomic percentages of Si and Ca increase noticeably in the 300 W/10 min specimen, indicating exposure of more inorganic fillers on the material surface.

Combined SEM and XPS analysis demonstrates that increased oxygen plasma power and treatment time induce plasma etching and roughening on CIIR surfaces, transforming smooth surfaces into rough, crack-containing morphologies. Synchronously, the content of surface polar oxygen-containing groups (C–O, C=O) increases along with partial exposure of inorganic filler phases. The synergistic effect of the two changes converts CIIR surfaces from "smooth/low-polarity" to "rough/high-polarity", which creates favorable conditions for the wetting, spreading and interfacial adhesion of subsequent PDMS coatings.

Figure 2 (a) Full XPS survey spectra of CIIR surfaces treated under various plasma power and time conditions;

 (b) High-resolution O1s XPS spectra of CIIR surfaces under different plasma treatment regimes

In response to the inherent drawbacks of CIIR sealing materials including potential pharmaceutical leaching risks and poor coating adhesion caused by low surface polarity, plasma treatment can graft oxygen-containing polar functional groups and construct rough microstructures favorable for mechanical interlocking on CIIR surfaces. On the premise of preserving the bulk material performance and pharmaceutical safety of CIIR, this approach achieves remarkable improvement in the interfacial adhesion capacity of chlorobutyl rubber substrates.