Abstract
Nonwoven geotextiles are geosynthetic products that are highly susceptible to ultraviolet degradation because light can reach a large area of the material due to its fiber arrangement. Even with additives, which delay the degradation process, material decomposition still occurs, and therefore the product’s long-term durability can be affected. In this paper, the mechanical and thermal behavior of a commercial nonwoven polyester geotextile subjected to accelerated ultraviolet aging tests were evaluated. The deterioration was evaluated by comparing the physical properties (mass per unit area, thickness, and tensile strength) and thermal behavior (thermogravimetry—TG, thermomechanical analysis—TMA, and differential scanning calorimetry—DSC) before and after exposure times of 500 h and 1000 h. The results showed that the ultraviolet aging tests induced some damage in the polyester fibers, leading to the deterioration of their tensile strength. For 1000 h of exposure, in which the reduction was larger, scanning electron microscopy (SEM) found some superficial disruption of the fibers, indicative of damage. TG and DSC could not capture the effects of UV radiation on polymer degradation, unlike TMA. This latter technique was effective in showing the differences between specimens before and after UV exposure.
1. Introduction
Geosynthetics is a relatively recent material for application in engineering work when compared to most construction materials. As a polymeric material, it is subjected to degradation that impacts its long-term durability, which raises the question of its suitability for use in permanent civil engineering systems designed with long service lifetimes. Aging is a major factor impacting geosynthetic service lifetime, acting even before the material installation in civil and environmental engineering works. For this reason, the success of the solution relies on the proper transport and storage of the geosynthetic material. According to specific guidelines, the product package should be intact and have minimal contact with the atmosphere. In the case of open-air storage, the material should be covered with a black polyethylene sheet to ensure adequate protection from ultraviolet (UV) radiation, moisture, and contaminants. However, when contact with these factors is inevitable, it is advisable to discard the outer roll material [1].
Particularly in the case of geotextiles, direct exposure to sunlight can cause severe degradation [2] due to UV radiation. Sunlight covers a wide range of wavelengths from infrared (>700 nm) to ultraviolet (<400 nm), reaching a lower value of around 300 nm related to atmospheric conditions. According to Suits and Hsuan [3], photons of similar or higher energy than the chemical bond strength of the geosynthetic material can lead to the degradation of the polymer properties (physical, mechanical, and chemical) due to the start of a series of reactions that can break the polymer chain. In the case of polyester (PET), the UV degradation causes polymer chain scission, forming a carboxyl group.
Therefore, as it is one of the major factors responsible for polymeric degradation, the influence of ultraviolet radiation (UV) on geosynthetics behavior, focused on in the present paper, is an important research subject in geotechnical engineering, considering that in many applications these materials are installed in an open environment, subject to sunlight exposure [4,5].
To protect the polymers against UV effects, different products are added during material manufacturing, the so-called UV stabilizers. The most common one is carbon black. This is a particulate added to the material’s surface to mechanically protect it by absorbing UV radiation. However, the absorption efficiency depends on the carbon black particle size. Smaller particles show a larger contact surface and thus offer better protection against UV until a limit of 20 nm size, below which no additional gain is obtained [6]. For geotextiles, for example, the particle size of carbon black is typically in the range of 22–25 nm diameter [3].
The study of UV radiation effects in polymer degradation can be conducted under natural sunlight exposure or under accelerated weathering by the emission of wavelengths in the UV spectrum. According to Suits and Hsuan [3], accelerated laboratory weathering methods can give more consistent results as the environment can be controlled. For these tests, UV weathering chambers equipped with lamps are used that irradiate UV light, exposing the geotextile samples to aggressive cycles of UV light, moisture, and temperature [7]. The currently used equipment differs in terms of the lamp that is used as follows: xenon-arc, carbon-arc, and fluorescent-UV lamps. They work similarly, with programmed cycles of condensation and radiation according to test requirements. However, according to Allen [7], the most used lamp worldwide is the fluorescent-UV lamp because it radiates light at the range of the wavelengths that are more damaging for polymers (300–400 nm) and still presents an economical operation.
Previous studies regarding the effects of UV radiation on geotextile behavior have mainly focused on comparing different weathering methods and the evaluation of the degradation by accessing the loss in mechanical properties of the material, such as the tensile strength and strain. Koerner et al. [2], for instance, evaluated seven nonwoven geotextiles under natural sunlight exposure for 12 months and accelerated laboratory tests for 500 h. The authors evaluated the aging effects by means of tensile tests and observed that the polyester geotextiles were less degraded than the polypropylene ones. Besides, their results indicated that none of the accelerated tests modeled field conditions exactly, and therefore, they should be considered an index test rather than a performance one. Carneiro et al. [8] also compared UV-aging behavior of geotextiles in the laboratory and outdoors. For their study, the authors used a polypropylene (PP) geotextile and evaluated the material changes based on physical and mechanical properties and also on scanning electron microscopy (SEM).
Guimarães et al. [9] studied the synergetic effects between creep and weathering for a woven polypropylene geotextile. The material was submitted to outdoor exposure, and the degradation was evaluated by comparing tensile test results, in which the synergy effects were evident. Carneiro and Lopes [10] also evaluated the changes in the mechanical properties (tensile and static puncture tests). They studied four nonwoven polypropylene geotextiles, with a varied amount of stabilizer in their composition, after natural exposure to sunlight during a total period of three years. They found relevant reductions in the mechanical properties of the material. The authors also used SEM photographs to evaluate the damage to the geotextile fibers due to the exposure. Similar studies were conducted by Carneiro et al. [11] with PP nonwoven geotextiles.
The use of thermal analyses to evaluate the durability of nonwoven geotextiles is still developing [4,5,12,13]. Thomas and Verschoor [12], for example, used thermal analyses (differential scanning calorimetry—DSC, thermomechanical analysis—TMA, and dynamical mechanical analysis—DMA) and physical property testing to study the chemical aging of nonwoven polyester geotextiles. Some studies assessed thermal analyses in different applications in other geosynthetics, such as geomembranes used in environmental works [14,15,16,17]. However, despite their potential, these techniques have still not been used to evaluate the effects of UV radiation on nonwoven PET geotextiles, which is the focus of this study.
Therefore, in order to contribute to the study of geotextile durability, the objective of this paper was to evaluate the suitability of three thermoanalytical methods to access the mechanical, thermal, and thermomechanical effects of UV radiation on nonwoven PET geotextiles. The thermoanalytical studies were carried out with thermogravimetric (TG), DSC, and TMA. The density (specific gravity method) and the tensile strength of the samples (dumbbell-shaped test specimens) were evaluated. Additionally, scanning confocal electron microscopy was used to make a qualitative visual analysis of the samples in an attempt to determine possible microstructural changes in the outside surfaces of the fibers of the geosynthetic material due to the degradation process.
2. Experimental
One commercial nonwoven polyester geotextile was used in the present study. This type of material was selected because nonwoven geotextiles have a particularly high susceptibility to photo-initiated degradation due to their large surface area [2].
A UV-weathering chamber from Equilam (model EQUV 003, São Paulo, Brazil) with fluorescent UVA-351 lamps was used, programmed to work in cycles of 8 h of UV light at 70 °C followed by 4 h of condensation at 50 °C (Figure 1). Two geotextile samples (20 × 30 cm2) were exposed during periods of 500 h and 1000 h, respectively. After exposure, sub-samples were taken from each sample to perform physical, mechanical, and thermal analyses of the aged material. The effects of aging were then evaluated by comparing the results between virgin (reference) and aged samples.
Last modified: 8 de abril de 2023
