Aging of Industrial Polypropylene Surfaces in Detergent Solution and Its Consequences for Biofilm Formation

Abstract

The performance of plastic components in water-bearing parts of industrial and household appliances, often in the presence of harsh environments and elevated temperatures, critically relies on the mechanical and thermal polymer stability. In this light, the precise knowledge of aging properties of polymers formulated with dedicated antiaging additive packages as well as various fillers is crucial for long-time device warranty. We investigated and analysed the time-dependent, polymer-liquid interface aging of different industrial performance polypropylene samples in aqueous detergent solution at high temperatures (95 °C). Special emphasis was put on the disadvantageous process of consecutive biofilm formation that often follows surface transformation and degradation. Atomic force microscopy, scanning electron microscopy, and infrared spectroscopy were used to monitor and analyse the surface aging process. Additionally, bacterial adhesion and biofilm formation was characterised by colony forming unit assays. One of the key findings is the observation of crystalline, fibre-like growth of ethylene bis stearamide (EBS) on the surface during the aging process. EBS is a widely used process aid and lubricant enabling the proper demoulding of injection moulding plastic parts. The aging-induced surface-covering EBS layers changed the surface morphology and promoted bacterial adhesion as well as biofilm formation of Pseudomonas aeruginosa.

Keywords: Pseudomonas aeruginosa; biofilm; detergent; ethylene bis stearamide; filler; liquid aging; polypropylene.

Figure 1

SEM images in grey and AFM images in gold-brown show the structural changes during aging of samples PP, PP-T, and PP-G. The first column (A,D,G) depicts the untreated state of the samples with the typical injection moulding surface and the filling material. Two samples show fibre-like EBS on the surface after 168 h in the detergent at 95° (E,H). After 4000 h the crack formation (C,F,I) as well as the difference in degradation of the skin layer (C,F,I inlay) can be seen. Black arrows indicate the injection direction, white arrows the fillers.

Figure 2

ATR-FTIR spectra of pure PP, pure EBS, PP-G after 0 h and 168 h, PP-T after 0 h and 168 h, and PP after 0 h and 168 h in the detergent at 95 °C. EBS bands 3300, 1635, and 1560 cm⁻¹ are highlighted. The EBS structural formula is presented in the upper right corner.

Figure 3

DSC graphs of samples (A) PP, (B) PP-T and (C) PP-G after 0 h, 168 h and 3000 h, respectively, with corresponding crystallinity. Minimum of the heat flow corresponds to the melting temperature.

Figure 4

Bacterial attachment on the unaged as received polypropylene samples. SEM image of (A) P. aeruginosa attached to a reference glass sample, (B) unfilled PP, and (C) talc filled PP-T. (D) Quantification of the attached P. aeruginosa onto the surfaces by CFU Assay. Mean ± standard error of the mean was statistically analysed by Mann–Whitney U tests (* p ≤ 0.05).

Figure 5

Influence of aged polypropylene samples on bacterial attachment. SEM image of (A) P. aeruginosa attached to PP-G after 0 h with EPS indicated by white arrows, (B) after 168 h with visible EBS, (C) after 1000 h, (D) after 2000 h, and (E) after 3000 h. (F) CFU assay results quantifying the attached P. aeruginosa, compared to the reference glass sample. Mean ± standard error of the mean are shown.

Figure 6

Observation of the influence of pure EBS on P. aeruginosa attachment and biofilm formation. (A) SEM image of pure EBS micropearls, (B) frequency diameter distribution of EBS particles with Gaussian fit central peak at 17.25 µm. (C) P. aeruginosa attachment to a glass slide in LB medium supplemented with 1 g/Lpure EBS, after an incubation period of 24 h, P. aeruginosa forms filament-like network structures (black arrow) and (D) after 48 h, white arrow pointing on the EPS layer, black arrows pointing on the crystalline EBS fragments protruding from the particle. Results of microtiter plate method with crystal violet staining, after an incubation period of (E) 24 h and (F) 48 h. Mean ± standard deviation are shown.