For perfume caps, “acrylic” normally refers to poly(methyl methacrylate) (PMMA). PMMA is preferable when luxury appearance is the priority. Forte et al. (2021) identify transparency, tensile strength, and processability as important advantages. PMMA can therefore create glass-like walls, optical depth, saturated colours, and sharply moulded decoration. Its feasibility is demonstrated by a patented three-dimensionally embossed PMMA cosmetic cap (Lu et al., 2021). Its rigid feel also communicates precision and perceived value.
However, PMMA has mechanical and chemical limitations. Conventional PMMA has poor impact resistance and ductility (Ishigami et al., 2020), so a dropped cap may chip or crack. Transparent polymers are also vulnerable to abrasion (Chouwatat et al., 2016). Moreover, PMMA can undergo environmental stress cracking. Bubmann et al. (2022) found that the contacting medium and wetting method affected fatigue-crack propagation; n-heptane caused more brittle behaviour under in-situ exposure. This does not mean every perfume attacks PMMA, but it supports testing the actual ethanol–fragrance formulation on moulded, internally stressed caps.
Several remedies are available. A PMMA/SMM/polyrotaxane blend produced 60% higher notched-Charpy impact strength and 2.5 times greater elongation at break than unmodified PMMA, although tensile strength and Young’s modulus decreased by 16% (Ishigami et al., 2020). A transparent hard coating over a compliant base layer can reduce mar and abrasion damage (Chouwatat et al., 2016). Rounded corners, controlled moulding stress, and preventing leakage onto PMMA provide additional protection.
Polypropylene (PP) is more suitable for a functional inner insert because it is inexpensive, lightweight, chemically stable, and readily moulded (Mandolfino et al., 2019). A perfume-cap patent demonstrates the feasibility of mounting decorative components on a separate insert with elastic interlocking (Thorez et al., 2022). Nevertheless, PP looks less glass-like and is harder to paint, print, or metallise. Its non-polar, low-energy surface can cause coating failure (Bisneto et al., 2022). Low-pressure plasma treatment improves PP wettability and adhesive-joint performance (Mandolfino et al., 2019), while clarifying additives can improve transparency when their concentration and neutraliser are correctly selected (Iwasaki et al., 2021).
PP is not automatically a complete fragrance barrier. It exhibited high eucalyptol permeability in a comparative packaging study (Leelaphiwat et al., 2016), although Bogdan et al. (2019) found a tested PP homopolymer compatible with a lavender-oil cosmetic. Accelerated ageing with the actual formula is therefore essential. Under severe exposure, barrier coatings or fluorination may reduce fragrance permeation (Kovara et al., 2011).
Overall, the best-balanced design is a mechanically separable PMMA decorative shell over a PP insert: PMMA provides optical luxury, whereas PP supplies resilient attachment and functional performance.