In recent years, thermoplastic pipes have become one of the most widely used materials for culverts and other highway applications [1]. High-density polyethylene (HDPE) is one of the materials of interest for these applications due to its numerous advantages. HDPE pipes are generally lighter, more flexible, and easier to install than other types of pipes as well as involving entail low maintenance costs and greater resistance to chemical attack [2–6]. Once the manufacturing process has been completed, however, pipes are often stored outdoors for a period of time before being installed. During the storage, installation, and repair process, the combination of ultraviolet (UV) light, temperature, and moisture can produce weak spots and lead to pipe degradation. The presence of impurities during the manufacturing process causes the pipes to absorb UV radiation. Since PE materials only contain C–C and C–H bonds, they are considered to be easily susceptible to degradation when exposed to high temperatures and UV radiation [7]. Culverts used in road drainage systems incur damage to their end openings due to UV radiation [7–9]. UV degradation can alter the physicomechanical, chemical, and macromolecular structure of the polymer [10, 11]. This degradation can alter color, tensile strength, elastic modulus, and impact strength of the materials [9, 12]. The presence of free radicals leads to chain scission, which usually takes place in the amorphous phase and at the amorphous-lamellar interface [13], whereas the crystalline phase remains inert [14]. In fact, UV stabilizers such as carbon black are integrated into pipes to prevent UV-induced reactions. In general, BNQ 3624-120 (2016) [15] and ASTM D4218 (2015) [16] require a minimum of 2–4% carbon black in pipes. Carbon black helps protect pipes against UV-radiation damage by limiting penetration to the external surface of the pipe wall (corrugated parts).
Attwood et al. (2006) studied the effects of UV degradation on recycled polyolefin blends. Tests were performed using QUV accelerated weathering testers with an irradiance of 0.68 w/m2 at 340 nm. The process included 8 hours of UV radiation at 60°C and 4 hours of condensation at 50°C. Test exposure time was 1000 to 5000 hours. The results showed that UV radiation had little effect on tensile, impact, and chemical properties; melt flow tests; gloss; and color analyses. Some surface changes were, however, observed [17].
Maria et al. (2011) studied the impact of UV radiation on PE pipes under IR-microscopy. The samples tested contained a phenolic stabilizer (Irganox 1010) as a primary antioxidant, a processing stabilizer (Irgafos 168), carbon black, and UV stabilizers. The specimens were exposed to UV radiation for 144 to 2208 hours. IR-microscopy and OIT measurements revealed loss of the phenolic antioxidant Irganox 1010 in the outer pipe wall. No changes in the degree of crystallinity were noted [18].
Jassim et al. (2017) evaluated the tensile strength at break of medium-density polyethylene (MDPE) water pipes made with and without carbon black after exposure to UV radiation for 200 hours. The results showed that the tensile strength at break for MDPE pipe made with carbon black was higher that of than virgin MDPE pipe. The tensile strength at break after exposure to UV radiation was unchanged, however, due to the carbon black playing a key role as a UV stabilizer [19].
Jiang et al. (2019) investigated the influence of UV absorbers on the UV resistance of HDPE. They concluded that the HDPE samples maintained their thermal and mechanical properties after exposure to UV irradiation for 600 hours under the aging condition of 0.51 w/m2 and a wavelength of 350 nm. In addition, slight changes in crystallinity were noted before and after exposure to UV radiation [20].
The incorporation of UV stabilizers, carbon black, and antioxidants in the polymer matrix helps limit the impact of UV radiation on the properties of materials. Past studies, however, have focused on HDPE films or HDPE water pipes. In contrast, very little has been developed about the effect of UV radiation on corrugated HDPE pipes used in road drainage systems. In addition, the current use of recycled HDPE pipes is of interest due to their sustainability and cost-effectiveness as compared to virgin pipes [6, 21–24]. Recycling products can reduce the carbon footprint, the use of raw materials, energy requirements, water consumption, and greenhouse-gas emissions in the production process [25, 26]. The main objective of our study was to assess the effect of UV radiation on the properties of recycled and virgin corrugated HDPE pipes used in road drainage systems. The techniques FTIR (Fourier-transform infrared spectroscopy) and SEM (scanning electron microscopy) were used to investigate the formation of degradation and any changes in pipe-wall morphological parameters. The thermal properties and crystallinity of the HDPE pipes were assessed with DSC (differential scanning calorimetry). The OIT measurements were conducted to profile antioxidant concentrations. In addition, the mechanical properties of the recycled and virgin HDPE corrugated pipes—such as tensile strength and hardness—were investigated before and after accelerated UV exposure.
This study is a part of ongoing research between the University of Sherbrooke and Quebec’s Ministry of Transportation (MTQ) to jointly investigate the short- and long-term performance of new corrugated HDPE pipes made with or without recycled resins for use in road drainage systems under North American climate conditions. The study included six new corrugated HDPE pipes from three different North American manufacturers, including three HDPE pipes made with virgin resins and three HDPE pipes made with recycled resins.