A macroscopically nondestructive method for characterizing surface mechanical properties of polymeric coatings under accelerated weathering

The surface of coatings and plastics is the first target in any degradation process initiated by ultraviolet (UV) radiation or mechanical stress (via scratch and abrasion). Surface damage can lead to changes in optical, morphological, and mechanical properties and can result in pathways for ingress of moisture and corrosive agents. Current test methods for monitoring performance of protective coatings focus on chemical properties and optical properties, such as color and gloss measurements, or invasive tests such as abrasion and cross-cut adhesion. In this study, a macroscopically nondestructive performance protocol using nanoindentation metrology via a well-controlled scratch test was applied to evaluate the scratch resistance and monitor the surface mechanical property changes in a protective coating under accelerated weathering. Polyurethane (PU) coatings with different polyol compositions were chosen for this study. Coating specimens were exposed to high-intensity UV radiation at 55°C and 75% RH conditions. Exposed specimens were removed at specified UV exposure times for surface modulus/hardness and scratch resistance characterization via nanoindentation and scratch test. The effect of polyol type and UV radiation dose on the scratch damage (scratch morphology) was investigated and correlated with the surface hardness and modulus of the materials.     

Effect of titania and zinc oxide particles on acrylic polyurethane coating performance

The outer environment, especially UV portion of solar radiation and water (in the form of moisture or rain) has an adverse effect on the surface appearance of heat-treated wood. Exposure to UV triggers the chain scission reactions which change the intrinsic properties of heat-treated wood and discoloration of wood surface. Repeated temperature and humidity variations cause swelling and shrinking of wood surface, which consequently create cracks and fissures exposing wood's sub superficial layers to atmospheric agents. Therefore, wood industries move towards the development of coatings in order to protect the heat-treated wood while retaining wood's natural look. Water based acrylic polyurethane coatings are highly efficient, non toxic and durable coatings with upgraded film properties. In this study, an attempt is made to improve the performance of these coatings by incorporating natural antioxidant (bark extract) and inorganic UV absorbers (nano and micro titania and nano zinc oxide) into the coatings. The main objectives of this study are to investigate the wetting and penetration characteristic of these new coatings on the wood surface and to study coating thickness variation with weathering time. The Sessile-drop method and fluorescence microscope are used for this investigation. The wettability of different coatings applied to heat-treated jack pine early wood and late wood is compared. The results show that there is a significant difference between the contact angle of early wood and late wood for acrylic polyurethane coating containing titania micro particles. The contact angle between water and coated wood surface reveals that the degree of orientation of the coating materials increases as the weathering time increases. The penetration characteristics of all the four coatings are found to be very poor. In addition, the relationship between the coating thickness and the UV exposure time is studied for four water based acrylic polyurethane coatings with different additives. It is found that the coating thickness decreases with increasing weathering time and a tissue deformation beneath the coating surface takes place during weathering.     

Effect of artificial weathering and temperature cycling on the performance of coating systems used for wooden windows

The effects of ultraviolet light irradiation and water spray and temperature cycling on the color stability of waterborne coating systems on spruce were investigated. The test samples were treated with three coating systems (white and brown pigmented acrylate waterborne coatings systems with different layering) from six producers. The artificial weathering was carried out based on standardized (504 h) and increased weathering parameters (504 h). Thirty cycles of temperature changes were performed. Discoloration suggesting the rate of degradation process and color stability was measured by a spectrophotometer in L*a*b* color space. The results showed greater color stability of white coating systems during both experiments. The producer of acrylate coatings with iodopropynyl butyl carbamate and permethrin as biocides and benzotriazole, silicon dioxide and methylsiloxane as additives was characterized by the best coatings performance. The most sensitive color parameter to describe the surface quality as a result of irradiation and water spray has proven to be a change of lightness. A tendency towards degradation with increasing lightness was observed during weathering. The effect of temperature itself on color change was minimal. Scanning electron microscopy and energy dispersive spectroscopy revealed microscopic changes of coatings caused by artificial weathering.     

Effect of Moisture and Temperature on the Mechanical Properties of an Epoxy Reinforced with Boron Carbide

The degree of degradation in a polymer composite is directly related to the amount of moisture it absorbs. Plasticization and swelling are among the undesirable consequences of absorbed water. This effect is rather important in materials under severe requirements. The use of these composites as coatings requires studying changes in their properties. For this reason, the aim of this work was to study the effect of moisture and temperature on the mechanical properties of an epoxy reinforced with boron carbide. Different B₄C particle sizes (7 and 23 μm) were studied, and the carbide used was 6 wt%. The specimens were exposed to two moisture environments (50 and 95% RH) at 60°C to quantify composite degradation level. Shore D hardness, three-point bending, and pin-on-disk wear tests were used to determine the effect of humid environments. Mechanical properties were determined at several exposure time intervals. Besides, the degradation process was analysed with differential scanning calorimetry (DSC) and infrared spectroscopy (FTIR–ATR). A general loss of properties was observed after water absorption. However, most cases showed recovery after the drying process, depending on the amount of water remaining in the material, which acts as a plasticiser, particularly improving strength.     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.     

老化试验中PE-HD的结构与力学性能研究

研究了高密度聚乙烯在海南自然曝露和氙灯人工老化后的结构和力学性能变化。结果表明：相比于自然曝露，高密度聚乙烯主要力学性能的氙灯人工老化加速倍率大约为4；老化期间，高密度聚乙烯分子间存在着交联和降解二种竞争反应，老化早期以交联为主，后期以降解为主；氙灯老化试验相比于自然曝露，老化主要集中在材料表层。     

Effect of Aging on the Mechanical Properties of UV Curable Optical Fiber Coatings

Abstract

A dual-layer urethane acrylate UV-cured coating is widely used to protect optical fibers because of its well-balanced mechanical properties, weathering resistance and rapid curing. The long-term mechanical behavior of fiber coatings is important for the reliability of optical fibers. Long-term exposure of UV-cured polyether urethane acrylate films was carried out in dry air and in water at elevated temperatures. Tensile testing was performed to reveal changes in mechanical properties and dynamic mechanical analysis to determine both the glass transition temperature and the crosslink density. The equilibrium swelling allowed assessment of the crosslink density. Tensile testing and strip force measurements were performed on virgin and aged optical fibers. Initially the fracture strengths of the secondary coatings increased under all aging conditions indicating post-curing reactions and the possible loss of uncrosslinked species. Aging under wet conditions led at a later stage to hydrolytic degradation of the network and to a decrease in the fracture stress. The equilibrium swelling and equilibrium modulus measurements showed good correlation with the changes in strength. The primary coatings showed a decrease in mechanical strength after only 2–4 weeks under all conditions.     

Recovery of surface defects on epoxy coatings and implications for the use of accelerated weathering

Surface roughness, arising from photodegradation, increases overall during weathering but may relax and diminish during episodes when exposure is limited. Different ambient temperatures will change the balance between photodegradation defect size and recovery, depending on the value of the glass transition temperature of the polymer. Epoxy coatings were exposed to periods of ultraviolet irradiation, after which the recovery of the surface roughness was monitored at several temperatures, above and below their glass transition temperatures. Atomic force microscopy, as well as following the increase in roughness with exposure, showed that increased exposure made phase separated domains more distinct. Recovery of nanoindentation on un-damaged coatings produced a similar value of the glass transition temperature to that deduced from the degradation roughness recovery. This was significantly lower at the surface of the epoxy coatings than was measured for the bulk. Confocal Raman spectroscopy was unable to detect any chemical difference between the surface of any films and deeper in their bulk. This evidence suggests that the low glass transition temperature is not due to different curing chemistry at the surface of the coating, but hints that the surface of these crosslinked coatings may relax differently to the bulk or have a different physical structure. These results lead to questions about how to change accelerated testing to better serve the needs of coatings’ technology and how to make progress in the overall goal of service lifetime prediction.     

Concurrent physical aging and degradation of crosslinked coating systems in accelerated weathering

Coating degradation is a combination of both chemical and physical processes; however, physical processes have not received much attention. Physical aging has a non-negligible effect on coatings’ mechanical properties and permeability etc. through the densification that continues as a polymer approaches its thermodynamic equilibrium below the glass transition temperature, T _g. Observations in recent work showed that physical aging affects coatings’ mechanical property response during accelerated weathering and is, itself, affected by the associated chemical degradation. Two crosslinked coating systems were studied in order to compare different chemical compositions, their T _g, and their thermal response in accelerated weathering. During thermal cycling, physical aging measured by enthalpy recovery exhibited different trends in the two coatings. A “rejuvenation” mechanism was observed in the coating with a T _g between the top and bottom limits of the exposure cycle; continued aging was observed for the coating with a high T _g. Stress relaxation tests detected aging and “memory” behavior over periods comparable with accelerated weathering cycles. Both thermal and mechanical responses changed in complicated and different ways as the coatings degraded. Different degrees of coating thickness reduction were observed in both isothermal relaxation and degradation. When various coatings are evaluated, simply judging their performance under the same weathering environment is not reliable since polymer relaxation behavior depends on the relationship between the exposure temperatures and the T _g of each polymer. This paper was awarded First Place in the 2007 Gordon Awards technical paper competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in Toronto, ON, Canada, on October 3–5, 2007.     

Statistical approaches for predicting weathering degradation and service life

Predicting the service lifetime of any material is very important. One of the major difficulties is relating individual, nanoscale, degradation events to the eventual deterioration in macroscopic properties. Monte Carlo simulations and the Central Limit theorem provide approaches to the kinetics of how general coating topology, or bulk morphology, changes during long term weathering. Results on structural changes, from these approaches, may then be translated, via well-known models, into the degradation of macroscopic, “use” properties. Thus service lifetime may be related to composition and degradation mechanisms. Results on pigmented, acrylic coatings demonstrate the applicability and utility of this statistical approach. Changes in gloss and contact angle were related, via these models and provide insights into the processes of degradation in pigmented coatings. Values of parameters for the models that relate surface roughening with exposure are consistent with results from atomic force microscopy. The models provide justification for using simple expressions for trends in non-destructive monitoring, e.g. gloss, or contact angle or for comparing different systems and estimating long-term changes in properties.     

Efficacy of hindered amines in woodflour‐polypropylene composites compatibilized with vinyltrimethoxysilane after accelerated weathering and moisture absorption

In this work, the aim was to analyze the efficacy of hindered amine light stabilizers (HALS) in woodflour‐polypropylene composites compatibilized with vinyltrimethoxysilane after moisture absorption and accelerated weathering. Moisture uptake of materials decreased with incorporation of silane due to diminished accessibility of water molecules to reactive regions. In dynamic mechanical experiments performed on wet samples, a marked reduction in the storage modulus in the glassy and rubbery zone was observed, since water has a plasticizing effect. After sample weathering, in a xenon‐arc apparatus, the changes in chemical structure and physical properties after exposure were analyzed by attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy, color measurement, flexural properties, and morphological analysis by scanning electron microscopy (SEM). The data showed that HALS maintain the brightness of the materials after aging and prevent sample whitening. They also reduced color loss after aging and the SEM micrographs revealed that they inhibit surface cracking during weathering. Although a slight decline in the mechanical properties was not completely avoided, the combination of the additives studied (UV absorbers and HALS) successfully prevented the deterioration of surface materials by UV radiation. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effects of weathering aging on mechanical and thermal properties of injection molded glass fiber reinforced polypropylene composites

The effect of weathering aging on the degradation behavior of injection molded short glass fiber reinforced polypropylene composites (GFPP) is studied. First, the effect of outdoor weathering on mechanical properties of GFPP composite was investigated by tensile, flexural, and impact tests. Furthermore, to clarify the degradation behavior under natural weathering environments, differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) measurements were carried out to analyze the structural and molecular changes during weathering aging. The results show that weathering aging has a significant influence on changes in mechanical properties, melting temperature and the degree of crystallinity of PG6N1 without added carbon black and UV absorbing agent. Those degradations not only occurred on the surface of GFPP but also proceeded to the inner matrix and interface. However, GFPP GWH42 with added carbon black and UV absorbing agent shows excellent weathering stability.     

Transport and related properties of paint films. II. Dynamic mechanical properties and humidity effects

A systematic four-stage investigation of eight unpigmented coating formulations, including three vinyl, two polyurethanes, and three epoxy systems was done to provide baseline structural information upon which an improved understanding and an optimization of protective coatings can be founded. First, the results from dynamic mechanical measurements are provided and discussed for the base polymer component in each coating system. Second, the effects of humidity on the dynamic mechanical properties of these base polymers were determined at room temperature. The extent of property degradation was monitored by calculating the T_g depression with increased humidity, assuming a temperature–humidity superposition. The extent of degradation, as monitored by the T_g, was found to correlate directly with the level of hydrogen bonding in these coatings. Third, the influence of typical coating additives (a TCP plasticizer and a rosin hardener) on the properties of two of the vinyl coating systems was investigated. In the final stage, the synergistic effects of absorbed moisture and these additives on the coatings properties were investigated at room temperature. Increases in the concentration of these additives was found to magnify the degradation effect of increased humidity. This magnified degradation has been assigned to increased water absorption with increases in the concentration of either of these additives.     

Effect of weathering cycle and manufacturing method on performance of wood flour and high‐density polyethylene composites

Wood–plastic lumber is promoted as a low‐maintenance high‐durability product. When exposed to accelerated weathering, however, wood–plastic composites may experience a color change and loss in mechanical properties. Differences in weathering cycle and composite surface characteristics can affect the rate and amount of change caused by weathering. In this study, 50% wood flour filled high‐density polyethylene composite samples were injection molded, extruded, or extruded and then planed to remove the manufacturing surface characteristics. Composites were exposed to two accelerated weathering cycles in a xenon arc weathering apparatus. This apparatus exposed the samples to xenon arc radiation, which is a combination of UV, visible, and IR radiation that is similar to solar radiation. Composites were exposed to radiation with or without water spray. After exposure to radiation and water spray, composites with more wood component at the surface (i.e., planed samples) experienced a larger percentage of total loss in flexural modulus of elasticity and strength after weathering compared with the other composites. Composites exposed to radiation only did not experience as much change in properties as those exposed to radiation with water spray. The results of this study demonstrate that exposing wood–plastic composites to water spray in combination with radiation is more severe than exposing wood–plastic composites to radiation only. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3131–3140, 2006     

Hygrothermal aging of an adhesive reinforced with microparticles of cork

In the present study, natural microparticles of cork are used with the objective to increase the toughness of a brittle epoxy adhesive. The cork particles act as a crack stopper, leading to more energy absorption. This fact occurs because cork presents a remarkable combination of properties (low density, low cost and sustainability of the raw material). Adhesives are susceptible to the presence of moisture in the environment. There are several studies that refer that moisture can degrade the molecular structure of the adhesive, and, therefore, its mechanical properties. The main objective of this research is to investigate the effect of moisture on the degradation of an adhesive reinforced with micro cork particles, knowing that cork presents a great capability to absorb water. The water absorption and desorption characteristics have been studied, for specimens without cork and with 1% cork, 125–250 μm. The moisture uptake behaviour in the adhesive was studied to obtain the coefficient of moisture diffusion. The effect of water exposure on the mechanical properties and glass transition temperature was also investigated. It was observed that the presence of water alters the mechanical properties of the adhesive (with and without cork), but these changes are not permanent.     

Experimental and numerical investigation of the static response of environmentally aged adhesively bonded joints

The aim of this research is to investigate the effect of moisture on the static response of adhesively bonded monolithic single lap joints and laminated doublers loaded in bending. All joints were made of aluminium alloy Al 2024-T3 bonded using epoxy film adhesive FM 73M OST. The joints were aged in deionised water at a temperature of 50 °C for up to 2 years exposure. The use of different widths of specimen (5 mm for monolithic single lap joints and 15 mm for laminated doublers) allowed both full and partial saturation of the adhesive layer. The bulk adhesive has been characterised to obtain the coefficient of moisture diffusion, the coefficient of thermal and moisture expansion and the moisture dependent mechanical properties. The testing results showed that the mechanical properties degraded in a linear way with the moisture content. The residual strength after exposure decreased with increasing moisture content (exposure time) and tended to level off towards saturation. The damage evolution and failure of the joint has been successfully monitored using the backface strain technique and in-situ video microscopy. Progressive damage finite element modelling using a moisture dependent, bilinear traction-separation law has been undertaken to predict the residual strength. Residual stresses due to thermal and swelling strains in the adhesive layer have been included; however their effect on the predicted static strength was not significant. Good agreement was found between the predicted residual strength and the experimental result.     

Effect of titanium dioxide particles on the surface morphology and the mechanical properties of PVC composites during QUV accelerated weathering

In this study, QUV accelerated weathering of polyvinyl chloride (PVC) composites with different amounts of titanium dioxide (TiO₂) particle was conducted to investigate the effect of TiO₂ particle on the surface morphology and the mechanical properties. The results indicate that the surface morphology of PVC without TiO₂ particle did not exhibit changes up to 960 h, but exhibited a rough and brittle surface after 1920 h of QUV accelerated weathering. In addition, the tan δ intensity, the elongation at break, and the mean failure energy (MFE) decreased significantly with increasing exposure time due to embrittlement. In contrast, for TiO₂ particle‐loaded PVCs, no significant influence on the tan δ intensity and the mechanical properties after accelerated weathering were observed, despite the appreciable degradation that occurred in the surface layer. The weatherability, as determined by the mechanical performance, was improved with increasing loading of TiO₂ particle in the PVC composites. Although the TiO₂ particle in the PVC matrix acts as a photocatalyst to enhance the surface degradation, it is also an effective radiation screener that inhibits embrittlement and retards the decrease in mechanical properties caused by the accelerated weathering process. POLYM. COMPOS., 37:3391–3397, 2016. © 2015 Society of Plastics Engineers     

The effect of absorbed moisture on dielectric behavior of silica (micro)-unsaturated polyester composites

It is well known that most of the dielectric failures occur in high voltage equipment due to absorption of moisture by the insulating material from the environment. Hence, the effect of absorbed moisture on electrical and mechanical properties of silica-unsaturated polyester resin (UPR) composites has been evaluated. The absorption of moisture in silica-UPR composites does not show any significant change in electrical and mechanical properties. The effect of acetone and water absorption on silica-UPR composites was determined and it was that found silica-UPR composite shows higher acetone absorption when compared with water. The differential scanning calorimetry and thermal gravimetric analysis studies of silica-UPR composites show no significant change in glass transition temperature using prehumidified (0–95% RH) silica filler. As there is no significant change in thermal and mechanical properties after exposure to humid conditions, it can be concluded that water does not penetrate inside the polymer matrix. Hence, the silica particles are the best choice to use as filler in UPR matrix for UPR composite used in electrical equipment. The developed silica-UPR composite was successfully used in the preparation of medium voltage inductive transformers. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Surface chemistry and mechanical property changes of wood‐flour/high‐density‐polyethylene composites after accelerated weathering

Although wood–plastic composites have become more accepted and used in recent years and are promoted as low‐maintenance, high‐durability building products, they do experience a color change and a loss in mechanical properties with accelerated weathering. In this study, we attempted to characterize the modulus‐of‐elasticity (MOE) loss of photostabilized high‐density polyethylene (HDPE) and composites of wood flour and high‐density polyethylene (WF/HDPE) with accelerated weathering. We then examined how weathering changed the surface chemistry of the composites and looked at whether or not the surface changes were related to the MOE loss. By examining surface chemistry changes, we hoped to begin to understand what caused the weathering changes. The materials were left unstabilized or were stabilized with either an ultraviolet absorber or pigment. After 1000 and 2000 h of accelerated weathering, the samples were tested for MOE loss. Fourier transform infrared (FTIR) spectroscopy was employed to monitor carbonyl and vinyl group formation at the surface. Changes in the HDPE crystallinity were also determined with FTIR techniques. It was determined that structural changes in the samples (carbonyl group formation, terminal vinyl group formation, and crystallinity changes) could not be reliably used to predict changes in MOE with a simple linear relationship. This indicated that the effects of crosslinking, chain scission, and crystallinity changes due to ultraviolet exposure and interfacial degradation due to moisture exposure were interrelated factors for the weathering of HDPE and WF/HDPE composites. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2263–2273, 2004     

Considerations for characterizing moisture effects in coatings weathering studies

This article investigates the effects of weathering factors on moisture absorption of coatings and presents results from several projects including real-time and accelerated weathering studies of coatings moisture absorption. Moisture variable characteristics, observed in outdoor exposure environments, are reviewed. Data are presented from the experiments performed in end-use and accelerated weathering environments to characterize moisture characteristics on coatings. The information from natural outdoor environment characterizations and design of experiments (DOEs) indicates the new approaches for characterizing coatings weathering with regard to moisture effects. Considerations and new approaches for performing coating weathering studies are discussed. Presented at the 2007 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, October 3–5, 2007, in Toronto, Ont., Canada.