Correlation of a Temperate UV-Weathering Cycle to Outdoor Exposure for the Determination of the Environmental Instability of Polyethylene Films Using HT-GPC Analysis
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polymers Communication Correlation of a Temperate UV-Weathering Cycle to Outdoor Exposure for the Determination of the Environmental Instability of Polyethylene Films Using HT-GPC Analysis Gavin Hill, Celine Moreira, Florence Huynh, Ana Trufasila, Faith Ly, Richard Lloyd, Hasan Sawal and Christopher J. Wallis * Polymateria Limited, i-Hub, Imperial College White City Campus, 84 Wood Lane, London W12 OBZ, UK; [email protected] (G.H.); [email protected] (C.M.); [email protected] (F.H.); [email protected] (A.T.); fl@polymateria.com (F.L.); [email protected] (R.L.); [email protected] (H.S.) * Correspondence: [email protected] Abstract: Accelerated UV-weathering cycles are predominately used for evaluating the durability of plastic materials, particularly polyethylene (PE) films. The point of failure for this testing is usually the loss of a physical property, such as the loss of tensile strength over time. For plastics designed to be instable under environmental conditions, the accelerated weathering cycles are yet to be defined and their correlation to outdoor exposure has yet to be made. This study demonstrates the utility of a newly defined temperate accelerated UV-weathering cycle, recently codified in the British Standard PAS 9017:2020. In addition, the effectiveness of the laboratory weathering cycle has been correlated to real-world outdoor exposure through simultaneous testing of the same samples at a specialist outdoor exposure site in Florida. The utility of the testing methodology and the performance of Citation: Hill, G.; Moreira, C.; the polyethylene samples was demonstrated through the use of High Temperature Gel Permeation Huynh, F.; Trufasila, A.; Ly, F.; Lloyd, Chromatography (HT-GPC) analysis. The data led to a detailed insight into the physico-chemical R.; Sawal, H.; Wallis, C.J. Correlation of a Temperate UV-Weathering Cycle changes occurring in the PE films upon exposure to environmental stimuli. By comparison, and to Outdoor Exposure for the surprisingly, the techniques employed appear to provide an insight into the processes in which Determination of the Environmental secondary micro-particles of PE are formed from macro-polyethylene samples. The temperate Instability of Polyethylene Films accelerated UV-weathering cycle over 14 days demonstrated an approximate correlation to 90 days Using HT-GPC Analysis. Polymers of outdoor exposure in Florida for the PE film studied. 2021, 13, 591. https://doi.org/ 10.3390/polym13040591 Keywords: accelerated UV-weathering; polyethylene; outdoor exposure; weathering correlation; HT-GPC; carbonyl index; molecular weight Academic Editor: Victor Tcherdyntsev Received: 26 January 2021 Accepted: 12 February 2021 1. Introduction Published: 16 February 2021 The stability of plastic materials is a key determining factor for many plastic products Publisher’s Note: MDPI stays neutral whose application is based upon outdoor use. Examples of this include, piping, automobile with regard to jurisdictional claims in parts and even packaging, to name just a few. Consequently, the Service Life Prediction published maps and institutional affil- of these polymeric materials has gained scientific enquiry since the large-scale adoption iations. of polymers as commercial materials [1]. It is equally well-noted that the determination of the Service Life Prediction of a polymer is dependent on the point of failure chosen, as determined by the testing methodology employed [1,2]. Hence the point of failure of a plastic bag is often chosen to be a decrease greater than 50% of its original elongation at break, as this would indicate the point at which it can no longer hold a sufficient quantity Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of load. This article is an open access article The weathering of polyolefinic materials has received significant study due to the distributed under the terms and ubiquitous use of polyolefinic materials in many outdoor applications. It therefore became conditions of the Creative Commons critical to correlate accelerated laboratory weathering techniques to outdoor exposure [3,4]. Attribution (CC BY) license (https:// Polyethylene and polypropylene are susceptible to degradation through oxidative-UV creativecommons.org/licenses/by/ irradiation, often resulting in the formation of secondary polyolefinic microplastics. Inter- 4.0/). national standards currently govern the use of UV-accelerated weathering, namely ASTM Polymers 2021, 13, 591. https://doi.org/10.3390/polym13040591 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 591 2 of 9 D4329-13 [5] and ISO 4892-3 [6]. Moreover, standards governing the outdoor exposure methodologies for plastics, such as ASTM G113 [7], state that indoor accelerated tests should match results obtained through outdoor exposure. To this end, recent studies have been conducted that have shown the correlation between the durability of polypropylene in laboratory accelerated weathering compared to outdoor exposure in different regions of China [8] and Japan [9]. Furthermore, similar correlated studies have been conducted on polymers used as high-voltage insulators on overhead electric power cables for trains in India [10]. The point of failure for these studies focused upon determining the durability of these materials and thusly, the weathering cycles used in the laboratory accelerated testing are often designed to simulate multi-years’ worth of UV irradiation. Interestingly, whilst the majority of studies into laboratory accelerated testing have focused on durability, little work has been done on cycles that are specifically designed to simulate a very short outdoor exposure time. The necessity for such cycles has been gathering due to the increased awareness of plastic pollution accumulating in the natural environment, combined with the need to evaluate the production rate of secondary microplastics from primary microplastic pollution [11]. In addition, technology solutions to such a problem need to be verified as having a real-world impact in real-world environments to determine their performance, whilst evidencing their effectiveness, as recently codified in BS PAS 9017:2020 [12]. To that end this communication presents a temperate accelerated weathering cycle for UV-aging of polyolefin materials. To demonstrate the correlation to outdoor exposure, the accelerated laboratory method is compared to outdoor exposure in Florida, where outdoor exposure was conducted at a specialist site. The correlation between the performance of standard polyethylene films and polyethylene films containing Polymateria’s BiotransformationTM technology is demonstrated by comparison of the loss of the polymer structure over time. 2. Materials and Methods 2.1. Materials The samples for the accelerated and outdoor weathering study were produced on Collin Film Blowing line BL 180/400 where additive dosage can be well controlled (Collin Lab & Pilot Solutions, Maitenbeth, Germany). Sample SF 01 was a monolayer polyethylene film (87.5 µm thick) composed of 75% low density polyethylene (LDPE) (ExxonMobil LD150 BW) and 25% linear low-density polyethylene (LLDPE) (Total Lotrene 1018H). Sample SF 02 contained the proprietary biotransformation additive [13] dosed at 2% by weight (87.5 µm thick). 2.2. Methods Outdoor weathering was carried out at Q Lab Corporation (Westlake, OH, USA) testing site close to Miami, Florida; following the specification of ASTM D1435 20 [14]. Samples were cut to 30 cm × 15 cm and held in high density polyethylene (HDPE) net to prevent sample cross contamination and loss due to wind during the aging process. The samples were mounted at 45◦ south facing, weathering data were recorded in vicinity of the test racks for every day of testing: average daily temperature, humidity, rain fall and irradiance (UV irradiance measured between 285 nm and 385 nm with Eppley TUVR 295–385 nm). Samples measured in Florida are suffixed _Fl. Accelerated laboratory weathering testing for polyethylene (PE) film was carried out by Polymateria Ltd. using a QUV A fluorescence tester from Q Lab Corporation (Westlake, OH, USA) where irradiance and black panel temperature were calibrated at 340 nm. An irradiance setpoint of 0.8 W/m2 and BPT of 60 ◦C for 1 h with 23 h dark at 60 ◦C in 24 h repeated over 14 days. Samples prepared by the QUV cycle are suffixed with _QUV Infra Red analysis was carried out on either a Nicolet iS10 or a Nicolet iS5 equipped iD7 Diamond ATR (Thermofisher Scientific, Waltham, MA, USA) between 4000 and 600 cm−1 at a resolution of 4 cm−1. The carbonyl index was calculated using the SAUB method [15] Polymers 2021, 13, 591 3 of 9 where CI = Area (1850–1650)/Area (1500–1420). This method was recommended as it accounted for the full range of carbonyl species generated during the weathering. Molecular weight analysis was carried out using an Agilent 1260 Infinity II High Temperature Gel Permeation Chromatography system equipped with Refractive Index detector (Agilent, Santa Clara, CA, USA), 2× Olexis PL Gel columns and calibrated using PS standards. Samples were measured at 160 ◦C at a flow rate of 1 ml/min and samples were dissolved and analyzed according to ASTM D6474 20 [16]. For most HT GPC experiments only a single measurement was recorded, this is due to time and cost in running samples and that the HT GPC has a high reproducibility when samples are prepared correctly. 3. Results and Discussion The key to correlating the performance of a polyolefin material relative to environ- mental stress is the identification of the point of