Monte carlo approach to estimating the photodegradation of polymer coatings

The degradation of a polymer coating and predicting the coating lifetime based on physical properties and distribution within the coating of the polymer binder, pigments, and fillers are economically very important. As technologies advance and allow for control of coatings at the nanoscale level, methods such as Monte Carlo can be used not only to predict the behavior of a nanodesigned coating with time but also to design coatings, such as optimizing pigment particle distributions or optimum hard and soft phase distributions of the binders in multiphase systems for maintaining the desired property with time. Erosion of the coating surface was simulated using Monte Carlo techniques where terrestrial solar flux is the initiator for polymer segment cleavage and removal. The impact on the sensitivity of the polymer adjacent to the detached polymer segment can be increased or decreased in the model based on the chemistry and surface energy of the remaining polymer matrix. Multiple phases with varying sensitivity to degradation can be modeled. The Monte Carlo generates a statistically similar surface topography and chemistry of the coating. The results of the Monte Carlo model are compared to measurable properties such as gloss, fracture toughness, and wetting contact angle, using various published correlations of the property to the surface topology. The simulated properties change through the life-time of the coating in ways that are consistent with observed behavior. Apparently, complicated changes in many properties can be described by the repeated application of simple, random processes. Presented at the 79th Annual Meeting of the Federation of Societies for Coatings Technology, October 28–November 1, 2002, in New Orleans, LA.     

Fabrication and properties of 3D printed zirconia scaffold coated with calcium silicate/hydroxyapatite

The scaffold of bone repair needs a variety of material combinations to meet its intended performance; a typical single material such as zirconia has excellent mechanical properties, while hydroxyapatite and calcium silicate are bioactive materials with different degradation rates. In this paper, porous zirconia scaffolds were fabricated using 3D printing technology. The surface of the scaffold was coated by dipping with different contents of calcium silicate and hydroxyapatite to improve the biological activity and mechanical properties. Mechanical tests show that the coating material can effectively fill the pores of the porous scaffold, increasing its compressive strength by an average of 55%. The simulated body fluid (SBF) test showed that the higher calcium silicate in the coating increased the degradation rate. Cell experiments showed that the coated scaffolds exhibited good cytocompatibility and were beneficial to the proliferation and differentiation of cells. In conclusion, coated scaffolds have potential applications in the field of bone repair.     

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.     

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.     

Processing and properties of hydrophilic electrospun polylactic acid/beta‐tricalcium phosphate membrane for dental applications

Typical electrospun polylactic acid (PLA) membranes revealing potential hydrophobicity and inflammation from acid release during degradation are major drawbacks as an ideal guided tissue regeneration (GTR) barrier. This study investigated the in vitro degradation properties of electrospun PLA/beta‐tricalcium phosphate (β‐TCP) membranes treated by polyethylene oxide dip‐coating process. After surface modification, the membranes revealed good wettability in contact angle measurement. The addition of β‐TCP can render good pH buffering properties for electrospun PLA membranes during the in vitro degradation test. The mechanical properties of the hybrid membrane showed no significant difference in suture pullout force at a dried or wetted state. For cell adhesion and proliferation, the membranes with hydrophilicity can enhance the cell attachment at early stage. Overall, these results show that electrospinning combined with dip coating is a feasible processing technology for producing hydrophilic fibrous GTR membranes. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

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.     

The single fiber pull-out test analysis: influence of a compliant coating on the stresses at bonded interfaces

The mechanical properties at the fiber/matrix interface play a significant role in controlling the fracture resistance of fiber-reinforced composites. By coating the fiber with sizing and coupling agents, these interfacial properties can be modified. The aim of the present analysis was to examine the effects of the coating thickness and modulus on the stresses at the bonded interfaces between the fiber, coating, and matrix. Using the fiber pull-out test as the analytical model, the stresses are first obtained by minimizing the total complementary energy in the coated fiber/matrix composite. The analytical results show that the interfacial shear stress between the fiber and the coating is higher than that between the matrix and the coating. Also, a thin and compliant coating reduces substantially the peak interfacial shear stress but not the interfacial radial stress due to Poisson's effect on the fiber. Furthermore, the shear stress transfer from the fiber to the matrix across the coating layer is found to be more uniform. The implications of these findings are discussed.     

The influence of ageing of epoxy coatings on adhesion of polyurethane topcoats and protective properties of coating systems

The dependence of adhesion and protective properties of coating systems on surface properties of epoxy intermediate coatings, aged and non-aged before an application of polyurethane topcoats, were examined. The intermediate coatings were aged 500 h in UV chamber. The surface free energy and polar groups were estimated after ageing. After applying polyurethane topcoats on aged and non-aged epoxy coatings, resistance to salt spray and thermal shocks were tested as well as internal stresses were measured before and after corrosion tests.The results showed that adhesion in coating systems with polyurethane topcoats applied on aged epoxy coatings depends strongly on the degradation degree of epoxy intermediate coatings and the value of generated internal stresses. Coatings with good adhesion retention in corrosion environments have good protective properties even when temporary blistering has occurred.     

Effect of mechanical stress on kinetics of degradation of marine coatings

Water, UV and temperature are well-known factors for organic coating degradation. Mechanical stress can also affect long-term lifetime in marine environments and probably is conducive to synergistic effects with other ageing parameters. The present work proposes a method to estimate the role of a stress–strain state on the protective properties of two marine epoxy coatings onto mild steel (with and without VOC). Preliminary mechanical measurements on free films by Dynamic Mechanical Analysis (DMA) indicated that the two coatings have a visco-elastic behaviour for a stress level lower than 3.3 MPa. Consequently, a stress equal to 3 MPa was applied on coated substrates using bent samples which were immersed in 3% NaCl solution and in natural seawater (“Les Minimes” yachting harbour in La Rochelle). This test is innovative because a visco-elastic deformation implies that the chain motion remain unchanged after a time of recovery (total restoration of strain in unloading stage) so coatings are not disturbed without applied stress. Non-bent coated samples were also immersed in the same environments as references. The coating degradation was followed by Electrochemical Impedance Spectroscopy (EIS) on both sides of the sample (compressed and stretched sides). The results allowed evaluation of the influence of mechanical state on the coatings degradation in visco-elastic regime and also demonstrate that the mechanical effect depends on the direction of the stress loading.     

Protection of wood surfaces against photooxidation

Protection of wood surfaces against photoinduced oxidative degradation can be achieved by using effective coating agents. Several penetrating chemical agents, such as triol-G 400, PEGG 400, and 1-octadecanol, were shown to provide good protection for wood surfaces against discoloration during ultraviolet irradiation. In addition, wood surfaces coated with clear film-forming finishes, such as homo- and copolymers of 2-hydroxy-4 (3-methacryloxy-2-hydroxy-propoxy) benzophenone, exhibited outstanding performance into inhibiting discoloration and surface deterioration. Infrared and ultraviolet absorption spectra and scanning electron microscopy studies revealed that the clear polymeric coatings containing an internal UV absorber were very stable and resistant to photooxidation. Possible chemistry and mechanisms of protection of wood surfaces provided by these polymeric coating systems are discussed.     

Degradation study of polymer coating: Improvement in coating weatherability testing and coating failure prediction

Increased awareness of the environmental impact of solvent-borne anticorrosive coatings has increased the focus on long-term performance of a coating in order to make maintenance less frequent. In this work, three anticorrosive coating systems were tested. All samples were exposed to different artificial weathering tests: a neutral salt spray test (NSS ISO 9227) and four cycles (QUV ASTM G53, ASTM D5894, ISO 20340 and a new laboratory cycle). Besides, the same samples were exposed to a 4 years field test in locations with corrosivity classified as C3 and C5M. The degradation of the individual coats of the coating system (topcoat, basecoat and primer) were studied by FTIR spectroscopy and Dynamical Mechanical Analysis. The impact of these physico-chemical changes on the corrosion protection properties of the whole system was discussed based on electrochemical impedance spectroscopy (EIS) in 3 wt% NaCl solution, SEM observations, pull-off test and measurements of delaminated areas from a scribe. On the basis of these results, the controlling parameters in coating degradation mechanism were identified. The relevance of various accelerated test cycles is discussed based on correlation between test results and field exposure.     

Poly(maleic acid-co-propane-1,2-diol-co-adipic acid) for pH-triggered drug delivery

Poly(maleic acid-co-propane-1,2-diol-co-adipic acid) was synthesized using xylene as the reaction medium in Dean–Stark apparatus. The synthesized co-polyester was characterized by its FTIR-spectrum, elemental analysis, molecular weights, solubility in common organic solvents and hydrolytic degradation test. From the hydrolytic degradation study it was found that the polyester sample remained almost intact in the acid medium but gradually degraded in basic medium. Because of such pH-responsive degradation nature, this polyester was tried as an enteric coating material for non-steroidal anti-inflammatory drugs. Drug release from the polymer coated specimen in simulated physiological environments was investigated and British Pharmacopoeia standard enteric coating properties of the polymer were observed.     

Composite coatings of lanthanum-doped fluor-hydroxyapatite and a layer of strontium titanate nanotubes: fabrication,bio-corrosion resistance,cytocompatibility and osteogenic differentiation

Poor bio-corrosion resistance and undesirable incomplete osseointegration restrict the application of hydroxyapatite (HA) as an implant coating material. In this study, a novel F-and-La co-substituted hydroxyapatite (FLaHA) coating, which was reinforced with strontium titanate nanotubes (STNTs), was applied on Ti substrates using a combination method of anodization, electrochemical deposition and hydrothermal treatment. To the best of our knowledge, this is the first report on the development of FLaHA/STNT coatings for improving the chemical stability and the mechanical and biological properties of Ti substrates. The STNT exhibits an evenly-distributed porous and latticed structure on Ti substrates that favours the infiltration of FLaHA crystals. Different characterisation techniques, such as x-ray photoemission spectroscopy, x-ray diffraction, field-emission scanning electron microscopy and energy-dispersive spectroscopy, have clearly confirmed the successful synthesis of STNT-FLaHA coatings that constitute oriented nanorod arrays. Isolated hexagonal nanorod grains, with diameters of 200–300?nm, that stand on a substrate provide a uniform morphology to the surface of electrodeposited thin films at micro-scales. The survival of the coatings was prolonged because of their good degradation resistance. Owing to the anchoring effect of the STNT layer, the adhesion strength of the FLaHA/STNT coating was 15.9?±?5.4?MPa, which was two times higher than that of STNT-free HA coatings. The potentiodynamic polarisation curves and the Nyquist plot confirmed that the conversion coating significantly improved the bio-corrosion resistance of the Ti substrates in the SBF solution. Roughness and hydrophilicity of the control HA layer were even greater than those of the FLaHA/STNT coating. However, it provided better cell adhesion, spreading, proliferation and osteogenic differentiation for mouse pre-osteoblasts cells. That is, the FLaHA/STNT coating could enhance osteoconductivity by improving the cell-adhesion, proliferation and differentiation of osteoblast. Therefore, FLaHA/STNT nanocomposite coatings can be used as implant materials with multi-functional properties, such as good biocompatibility and high mechanical and corrosion-inhibiting properties.     

Electrochemical measurements to evaluate the damage due to abrasion on organic protective system

Organic coatings are the most commonly used system for protection of metals from corrosion. In several applications organic coatings have to show, in addition to the protection properties and a good aesthetic appearance, good resistance to impact and abrasion. In fact, the mechanical damage can remarkably decrease the protection properties, even in the case of a very protective organic coating.

To evaluate the abrasion resistance of organic coatings the Taber Abraser test is frequently used. The mechanical damage is determined by measuring the mass loss without considering the form of damage (morphology and influence on corrosion protection performances).

The aim of this work is to evaluate the degradation of protection properties, caused by abrasion in Taber tests, using electrochemical methods.

Polyester powder coated steels were studied. Several parameters were considered such as the number of cycles, hardness of abrasive grinders and imposed weight.

By considering the resistance and capacitance of the organic coatings, obtained by fitting the electrochemical impedance data, it is possible to evaluate the trend of damage evolution as a function of the number of cycles and of the test parameters.

These EIS technique can distinguish the slight difference in aggressiveness of the two types of grinders used. The CS10 grinders produce debris which show the tendency to stick to the grinder itself and to the paint, reducing the abrasive action. Nevertheless, in this case the abrasion of the coating appears more uniform. In contrast, CS17 grinders increase the tendency to produce localised defects, which reduces the performance of the paint.

A different degradation rate and morphology were observed: an initially high damage value, followed by a decrease was observed due to both the presence of debris and the progressive efficiency loss of the abrasive wheels. For this reason, polishing of the grinder after every 1000 cycles is necessary to maintain a constant abrasion efficiency.     

New coating materials prepared by radiation-induced polymerization. I. Mar-resistant coating composition and properties

A mar-resistant coating system was obtained by screening combinations of silicone and vinyl compounds. It was found that binary systems, such as the γ-glycidoxypropyltrimethoxysilane–glycidyl methacrylate system, were excellent not only in mar resistance but also in adhesion to base resin polymers. Haze values of these coatings, after the sand-falling test, reached less than 10%. The coating process consisted of three steps: the preparation of prepolymer, the coating to plastic surface, and the curing of the coating by heating. It was found that the prepolymerization step could be performed by irradiation very conveniently; the control of viscosity for coating application and thickness control could be possible without any gel formation. These coatings were also good in weather resistance due perhaps to the good adhesion to base resin and could be applicable to plastic spectacles, glazing materials, and a variety of other products.     

Erosion mechanism of ternary-phase SiC/ZrB2-MoSi2-SiC ultra-high temperature multilayer coating under supersonic flame at 90° angle with speed of 1400 m/s (Mach 4)

A ternary-phase SiC/ZrB₂-MoSi₂-SiC multilayer coating was prepared on graphite by two-step reactive melt infiltration (RMI) method. The formation mechanism of the coating was studied by HSC chemistry software 6.0. The erosion resistance of the coating was investigated by supersonic flame erosion test at 90° angle, temperature of 2173 K and speed of 1400 m/s (Mach 4) for 120 s. Erosion test results revealed that the SiC/ZrB₂-MoSi₂-SiC multilayer coating had very good erosion resistance. Weight change percentage, mass erosion rate and linear erosion rate of the coating were −0.18 %, −0.027 × 10⁻³g cm⁻² s⁻¹ and 0.33 μm s⁻¹, respectively. Microstructural characterization demonstrated that interesting structures such as rod-like, flake-like, spherical, worm-like and fibrous structures were formed during erosion test. The erosion mechanism of ZrB₂-MoSi₂-SiC coatings is controlled both chemically and mechanically. The reduction of chemical degradation can be attributed to the presence of MoSi₂ particles and the reduction of mechanical degradation can be related to the presence of ZrB₂ particles.     

Effect of five kinds of pores shape on thermal stress properties of thermal barrier coatings by finite element method

Thermal ablation is a very important technique to characterize the thermal properties of coating systems. On account of the concentration of thermal stress, thermal barrier coatings (TBCs) often break off from the substrate partly or completely during the thermal erosion. In this paper, the thermal erosion simulation of finite element geometric models based on the possible pore shapes were implemented, especially, the influence of pore shapes on the distribution of coating temperature, X component of stress, Y component of stress, XY-shear stress and von-Mises stress were focused on. The effects of the different porosity of square pore coatings on thermal insulation properties and thermal stresses were discussed in term of the simulation results. The simulation results indicate that different shape pores not only affect the thermal stress distribution above the contact area between the bond coating and top coating surface, but also affect the plastic deformation behavior of TBCs. The micromechanism was discussed in details in this study. The computed results verified that, the computational method can successfully predict thermal shear, crack initiation and normal failure mode of the studied TBCs. All the results are in good agreement with the corresponding experimental findings. The failure mechanism factors in this paper are of great importance to explain the failure micro-mechanism of TBCs.     

改性聚乙烯醇添加酒糟制备缓释肥包膜材料的表征及其性能

采用聚乙烯吡咯烷酮（PVP）,通过溶液共混的方式对聚乙烯醇（PVA）进行改性,添加酒糟（JZ）制备复合包膜材料,并对氮肥进行包膜制备缓释肥料。研究了JZ的添加量对复合包膜材料性能以及包膜肥料缓释性能的影响。结果表明：JZ与各组分之间通过氢键作用相互结合,相容性良好;JZ的添加使复合膜材料热稳定性得到了显著提高;120d PUPZ5复合包膜材料降解率相比未添加JZ提高了20.11%;随着JZ的加入,复合包膜材料的机械性能先增加后下降,当JZ与PVA的质量比为15∶100时复合膜材料的力学性能达到了20.75MPa,比未添加酒糟提高了528.79%,且缓释肥料具有良好的缓释性能,可以通过调节JZ在复合包膜材料中的含量来控制缓释肥料中N的释放速率。     

Cracking analysis of HVOF coatings under Vickers indentation

The fracture strength of five HVOF coatings, which are made of hard metals, Tribaloy alloy, and superalloys, respectively, coated on 1018 low carbon steel substrate, is studied under Vickers indentation, associated with FEA stress computation. The cross sections of the coating specimens are examined on a Hitachi Model S-570 scanning electron microscope (SEM), which investigates the quality and measures the geometry of the coatings. The mechanical properties of the coatings and the substrate are determined in the cross sections using the nano-indentation technique. The cracking behavior of the coatings under different indentation loads is investigated using a Vickers hardness tester. Three-dimensional finite element analysis (FEA) simulation of the Vickers indentation test is conducted to determine the stress fields in the coating/substrate systems in order to understand the fracture mechanisms of the coatings under the indentation loads using the ABAQUS software package. The FEA stress results are in good agreement with the experimental observation of Vickers indentation.     

Effect of mechanical stresses on marine organic coating ageing approached by EIS measurements

In this work, the role of a stress-strain state (visco-elastic domain) on the protective properties of two marine epoxy coatings (with and without VOC) applied onto mild steel was studied. Different stress values were applied on coated substrates and bent samples were immersed in 3 wt.% NaCl solution at different temperatures. Non-bent coated samples were also immersed in the same conditions as references. Electrochemical Impedance Spectroscopy was used to evaluate the organic coating degradation on the compressed and the stretched sides periodically.The degradation kinetics showed that the tensile mode was very damaging for one coating while a slight effect was observed on the other coating. In the first case, the water uptake was found to be more important in the tensile mode for higher stress values. A particular attention was focussed on the initial relative permittivity which appeared as a thermo-activated function of the absolute value of the applied stress, for both coatings. Using a thermodynamic approach, the influence of the enthalpic and entropic part of the permittivity was discussed. The diffusion coefficient of the solution into the coating was also measured. The results showed that the diffusion coefficient is strongly modified by the mechanical stress but different behaviours were obtained with both coatings. It is proposed that the entropic contribution plays a major role on the modification of this coefficient.