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Exposure response function for a quantitative prediction of weathering caused aging of polyethylene
(2019)
The exposure response function of the carbonyl formation over the bulk has been determined for a high-density polyethylene of a thickness of 200 μm, which was used as a weathering reference material according to ISO TR 19032. To this end, spectral sensitivity was studied by local measurement of the effect of spectrally dispersed irradiation. Both the exposure device and the methodology of determination are described.
The temperature dependency of photooxidation was determined by UV exposure at various temperatures between 23 and 80 °C. Deviations from linearity and thus reciprocity below 40 °C are discussed and assumed to be related to diffusion limitations. An Arrhenius approach –based on data of linear carbonyl formation – has been incorporated into the exposure response function. Using this exposure response function, aging in terms of the distribution of a quantitative property change over a plastic component can be predicted for a specific outdoor location with real chronologic weather data as input for the exposure. Thus, artificial and natural weathering can be linked and compared. The established exposure response function has been validated by outdoor exposure results from the literature. If an estimated diffusion limitation is taken into consideration, calculations and published data are in good agreement.
The excitation energy transfer (EET) of a bichromophoric cross-shaped molecule was investigated by stationary polarized fluorescence spectroscopy in the solid state. For this purpose 2,2[prime],7,7[prime]-tetrakis(biphenyl-4-yl)-9,9[prime]-spirobifluorene was embedded in a polymeric bisphenol-A-polycarbonate (PC) matrix. The dependence of the fluorescence on concentration and wavelength was determined. The role of the intermolecular and intramolecular EET is dealt with separately and discussed by means of the degree of polarization. The intermolecular excitation energy transfer is described in terms of a Förster transfer mechanism. The intramolecular transfer is prevented for the zero-point vibrational levels by the molecular cross-shaped structure, but is found for a wide range of wavelength, presumably based on vibrationally excited states.
The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.
A phosphaphenanthrene and triazinetrione group containing flame retardant (TAD) is combined with organically modified montmorillonite (OMMT) in epoxy resin thermosets (EP) to improve the performance of the flame-retardant system. When only 1 wt% OMMT/4 wt% TAD is introduced into the EP, the limited oxygen index (LOI) rises from 26% to 36.9% and a V-0 rating is achieved in a UL 94 test. The decomposition and pyrolysis products in the gas phase and condensed phase were characterized using thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR). The influence on the decomposition of EP, such as the increase in char yield, is limited with the incorporation of OMMT; a large amount of the phosphorus is released into the gas phase. The flame-retardant effect evaluation based on cone calorimeter data testified that OMMT improves the protective-barrier effect of the fire residue of OMMT/TAD/EP on the macroscopic scale, while TAD mainly causes flame inhibition. The fire residues showed a corresponding macroscopic appearance (digital photo) and microstructure (scanning electron microscope [SEM] results). The protective barrier effect of OMMT and the flame-inhibition effect of TAD combined to exert a superior flame-retardant effect, resulting in sufficient flame-retardant performance of OMMT/TAD/EP
The effects of weathering exposure on unfilled and filled thermoplastic polyurethanes (TPU) materials are described as performed under different humidity conditions. For this purpose, a weathering device was used with UV-A 340 nm lamps at a constant temperature of 40 °C. The effects of environmental (UV and humidity condition) degradation on the frictional properties of TPU materials are presented along with surface analyses to characterize the chemistry of the degradative process. Photooxidative degradation of unfilled polymer leads to deterioration of physical and mechanical properties, which affects its tribological behavior significantly. Due to crosslinking, the stiffness of the material increases, reducing drastically the friction coefficient of unfilled TPUs. The frictional behavior of glass fiber reinforced TPU is less affected by radiation.
Scientific publications addressing the durability of the flame retardance of cables during their long-term application are rare and our understanding lacks. Three commercial flame retardants, aluminum hydroxide, aluminum diethyl phosphinate (AlPi-Et), and intumescent flame retardant based on ammonium polyphosphate, applied in ethylene-vinyl acetate copolymer (EVA) model cables, are investigated. Different artificial aging scenarios were applied: accelerated weathering (UV-irradiation/temperature/rain phases), humidity exposure (elevated temperature/humidity), and salt spray exposure. The deterioration of cables’ surface and flame retardancy were monitored through imaging, color measurements, attenuated total reflectance Fourier transform infrared spectroscopy, and cone calorimeter investigations. Significant degradation of the materials’ surface occurred. The flame retardant EVA cables are most sensitive to humidity exposure; the cable with AlPi-Et is the most sensitive to the artificial aging scenarios. Nevertheless, substantial flame retardance persisted after being subjected for 2000 h, which indicates that the equivalent influence of natural exposure is limited for several years, but less so for long-term use.
Discoloration Effects of High-Dose gamma-Irradiation and Long-Term Thermal Aging of (U) HMW-PE
(2017)
Two polyethylene types with ultra-high (UHMWPE) and high molecular weight (HMW-PE) used as neutron radiation shielding materials in casks for radioactive waste were irradiated with doses up to 600 kGy using a 60Co gammasource. Subsequently, thermal aging at 125∘C was applied for up to one year. Degradation effects in the materials were characterized using colorimetry, UV-Visspectroscopy, IR spectroscopy, and DSC. Both materials exhibited a yellowing upon irradiation.The discoloration of UHMW-PE disappeared again after thermal aging.Therefore, the yellowing is assumed to originate fromannealable color centers in the formof free radicals that are trapped in the crystalline regions of the polymer and recombine at elevated temperatures. For the antioxidantcontaining HMWPE, yellowing was observed after both irradiation and thermal aging. The color change was correlated mainly to decomposition products of the antioxidant in addition to trapped radicals as in UHMW-PE. Additionally, black spots appeared after thermal aging of HMW-PE.
Scrap tyres are widely reused in the form of granules as infill materials or component of sub-base layers in construction of sporting grounds such as synthetic turf areas and athletic track surfaces. The recycled rubber may contain organic and inorganic pollutants due to the presence of vulcanisation catalysts, stabilisers and other additives which can be released into soil and groundwater in the case of outdoor sports facilities. In order to evaluate the amount of potentially mobile pollutants, leaching tests can be employed. This study aimed particularly at the investigation of the feasibility of column tests using complete synthetic surface flooring or turf system assemblies. The release of zinc and polycyclic aromatic hydrocarbons (PAH) leached from components of sports surfaces containing recycled rubber granules was considered. Additionally, batch tests on coated and uncoated styrene butadiene rubber (SBR) granules aged by artificial weathering were performed to indicate its possible influence on the leaching behaviour of zinc and PAH. The chosen approach of column tests simulating typical complete synthetic sports surface installations has been proven to be feasible and robust. The time-dependent leaching behaviour and interactions between single components can be evaluated by column tests. Zinc was the most relevant contaminant in the eluates and its leachability was enhanced after accelerated ageing by artificial weathering. Leachable PAH were found in relatively low but well detectable amounts which increase by ageing as well.
For polypropylene of varied stabilization, spectral sensitivity as well as temperature dependence of irradiation caused crack formation was determined in artificial irradiation tests. UV radiant exposure HUV necessary to generate crack formation was measured both in spectrally dispersed irradiation and artificial irradiation in a Fluorescent UV lamp device. Dependencies were fitted to a plateau function and an Arrhenius function, respectively, to describe the action of irradiation by response functions. Applied to weather data from Phoenix, the results were compared with respective outdoor exposure results.
Size-exclusion chromatography (SEC) was used to monitor changes of the molecular masses of thermoplastic polyether – and polyester urethane (TPU) exposed to thermal, hydrolytic, and photo-oxidative (UV) Degradation conditions for several days. The thermal treatment was performed at elevated temperatures (100–200 °C) under oxidative (air) as well as non-oxidative (nitrogen) conditions to evaluate the specific influence of oxygen on the degradation. At higher temperatures (≥175 °C) a fast decrease of the molecular masses of both PU accompanied by a high degree of crosslinking was found. At lower temperatures (≤150 °C) the polymers remained widely unaffected by thermal degradation within the investigated degradation interval of up to two weeks. Surprisingly, the influence of oxygen (air) was found to be less distinct. In contrast to that, UV treatment at 25 °C at less than 10% rel. humidity (RH) resulted in a fast crosslinking, whereas the molecular masses of both PU decreased slower than for thermal treatments. The depth of penetration of the UV radiation was estimated using 3D printed PU samples with different thicknesses. Hydrolysis based degradation effects were less significant. Only slight molecular mass changes were detected at temperatures ≤80 °C within a time span of 14 days, while no crosslinking could be measured. Considering the degradation results at the investigated exposure parameters, it could be shown that esterbased PU in general exhibits a significant higher stability compared to ether-based materials.
The durability of flame retardancy is a challenge for cables over long lifetimes. The degradation of flame retardance is investigated in two kinds of exposures, artificial weathering and humidity. In this basic study, typical mineral flame retardants in two polymers frequently used in cable jackets are investigated to get the fundamental picture. Aluminum hydroxide (ATH) and magnesium hydroxide (MDH) are compared in ethylene‐vinyl acetate (EVA), and further in EVA and linear low‐density polyethylene (LLDPE) cables containing the same ATH. The changes in chemical structure at the surface are studied through attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), the formation of cracks, and changes in color are investigated. The cone calorimeter and a bench scale fire testing cable module are utilized to evaluate the fire behavior of the cables. Although the flame retardancy deteriorated slightly, it survived harsh exposure conditions for 2000 h. Compared to EVA/MDH and LLDPE/ATH, the fire behavior of EVA/ATH is the least sensitive. Taken together, all of the results converge to estimate that there will be no problem with flame retardancy performance, for materials subjected to natural exposure for several years; the durability of fire retardancy is questionable for longer periods, and thus requires further investigation.
Recycled plastic granules of high-density polyethylene, polyvinyl chloride and polystyrene the size of microplastics were exposed to artificial aging conditions (2000 h; photooxidative and thermo-oxidative) to simulate their fate outdoors. Their potential to leach into water during the aging process was investigated using column percolation tests.
Aging-related changes on the surface of the material were characterised by IR measurements indicating oxidation reactions with the formation of new adsorption bands (C=O, C–O and OH), especially in the case of photooxidative aging.
These findings were confirmed by the identification of leachable organic compounds. Leaching of total organic carbon, Cl, Ca, Cu and Zn is clearly affected by changes due to aging, and their release is increased after photooxidative aging.
In general, exposure to photooxidative conditions shows a greater influence on aging and thus on leaching and seems to be the more important mechanism for the aging of microplastic in the environment. Comparison with the total content of inorganic species revealed that, for most elements, less than 3% of the total content is released after 2000 h of photooxidative aging.
In a laboratory test, transparent high‐density polyethylene (HDPE) jerrycans have been exposed to both UV radiation and 55 wt‐% nitric acid solution at (41 ± 2)°C, for up to 20 days. For comparison, UV radiant exposure (21 days) and nitric acid exposure (up to 6 weeks) were performed separately, at nearly equal temperatures. The damages are compared with FTIR spectroscopy in ATR and HT‐gel permeation chromatography(GPC) on a molecular level and with hydraulic internal pressure testing as a component test. For the used jerrycans, relevant oxidation can only be found after the combined exposure. This is caused by the decomposition of nitric acid into nitrous gases by UV radiation, which is also observed at lower concentrations (28 wt‐%). After 6 days of laboratory exposure, this is rated as critical, which corresponds to about 1/10 year in Central Europe, according to the UV radiant exposure. The gradual increase in oxidative damage shows the reproducibility of the test.
Unpigmented HDPE jerrycans filled with nitric acid (55%) and water respectively had been exposed to outdoor conditions for one Berlin summer season. As both liquids underwent equal temperature progression, exposure effects of UV radiation and nitric acid as well as of their combination can be separated and compared. On the basis of various property changes after these exposures, synergistic action is evaluated and compared to a damaged Intermediate Bulk Container (IBC) from a transport accident.
It is found that carbonyl formation goes along with lightness increase in color measurement due to microcracking and with a worsening in mechanical behavior, all of them showing synergistic effects of UV and nitric acid exposure. In contrast, embedding nitrogen compounds goes along with yellowing of the material but cannot be correlated to oxidation.
The reason for intensified damaging is the decomposition of the 55-percent nitric acid and formation of nitrogen oxides even at ambient temperatures, caused by UV radiation. Thus, damaging effects become similar to those caused by fuming nitric acid exposure at temperatures above 60 °C, with the result of strong oxidative degradation of the polyethylene.
In contrast, exclusive exposure to the 55-percent nitric acid at 40 °C does not cause any failure.
It can therefore be assumed that also the damaged IBC had been exposed to both UV radiation and nitric acid, probably outdoors.
Combined impact of ultraviolet and chemical fluids on high-density polyethylene packaging material
(2012)
To investigate the ageing behaviour of filled plastic containers outdoors, square cuts of the wall of two high-density polyethylene (HDPE) types were exposed to ultraviolet (UV) radiation at their front side and to specific liquid chemicals (de-ionized water, surfactant or White Spirit) at their back. The UV radiant exposure at the front side was 80 MJ/m².
To compare the actions of the different exposures, separate dark backside fluid exposures were performed, in parallel. Besides, UV weathering was carried out until a UV radiant exposure of 325 MJ/m², being roughly comparable to outdoor exposure of one year in Northern Australia.
An unpigmented HDPE included in the investigation gave no sufficient protection for the White Spirit. In addition, it showed clear degradation after several of these exposures. In combination with the White Spirit, an increase of carbonyl bonds was measured, presumably assignable to degradation products of the White Spirit.
For a pigmented HDPE material, with the implemented combined exposures, no relevant damage was observed, within applied the exposure period.
The optical transmission stability was investigated for commercially available polymer optical fibers (POFs) which were exposed to a climate of 92°C and 95% relative humidity for about 3300 h. The optical transmission stability of POFs was correlated to their thermooxidative stability. POFs possessed identical core material, poly(methyl methacrylate), but they differed in the materials used for the claddings. The optical transmission was measured online using a prototype device called multiplexer. The chemiluminescence (CL) technique was applied to characterize the thermooxidative stability and degradation of POFs. CL analysis reveals the thermooxidative degradation of bare POFs (core and cladding), predominantly of the claddings, as a result of climatic exposure. Ultraviolet-visible transmittance measurements demonstrated more changes in the claddings as compared to the cores due to degradation. The CL and optical measurements data indicated that the optical transmission stability of POFs was dependent mainly on the thermooxidative stability of the claddings and their chemical compositions.
The thermo-oxidative degradation of a polymeric optical cable is investigated by chemiluminescence, The results are reliable and reproducible. Two distinct processes are reported marked by a peak and a plateau behavior versus the time, respectively. Both processes are ruled by thermally activated processes. Beside the dependencies of temperature and time, the influence of absorbed water is discussed. Chemiluminescence is proposed as a promising candidate for a suitable testing method assessing the thermo-oxidative stability of plastic optical fibers and cables. it requires not more than a simple one-day testing procedure and has the advantage that it can be carried out even within the lo cv temperature ranges of the cables' intended use.
Chemiluminescence for the early detection of weathering effects of coatings - Part I: Fundamentals
(2010)
The thermo-oxidative stability of commercially available polymer optical fibers (POFs) and their components (cores and claddings) was investigated. All the bare POFs (core and cladding only) studied here were based on poly(methyl methacrylate) (PMMA) core. The fibers were exposed to 100 °C/low humidity for about 4200 h. Chemiluminescence (CL) technique was applied to investigate the thermo-oxidative stability and for measuring the transmission loss during exposure a prototype device called multiplexer was used. POFs exhibited variation in thermo-oxidative stability although they possessed identical core material PMMA. This was due to difference in the chemical compositions of claddings. Claddings were more susceptible to the thermo-oxidative degradation compared to cores. The thermo-oxidative degradation of both the cladding and the core was found in POFs as a result of climatic exposure. POFs showed an early drop-off followed by a slow decline of transmission. The early drop-off of transmission was attributed to physical changes like thermal expansion and the slow decline of transmission to chemical changes like oxidative degradation of POFs. A good linear relationship between optical transmission stability and thermo-oxidative stability of POFs was established from these studies.