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.     

Mechanical property changes and degradation during accelerated weathering of polyester-urethane coatings

Chemical changes, measured using spectrocopy, and crosslink density, measured by mechanical thermal analysis, were determined during accelerated weathering on a model polyester-urethane coating of known composition. The tensile modulus, measured above the glass transition temperature, and thus the crosslink density, decreased with exposure, as expected from the chemical changes. However, the tensile modulus, measured at room temperature, increased with exposure. Physical aging of the polymer network was found to occur concurrently with photodegradation and accounts for much of the increase in room temperature modulus. Increased hydrogen bonding in the increasingly oxidized polyester-urethane may also contribute to the increase in modulus at room temperature. Both physical and chemical changes must be determined if changes, and rates of change, in performance due to weathering are to be understood. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Characterization of polymer-coated optical fibers using a torsion pendulum

A freely oscillating torsion pendulum has been used to characterize the dynamic mechanical behavior of single polymer-coated optical fibers. The dynamical mechanical spectra of the polymer coatings exhibit a glass transition temperature (T_g), a cryogenic glassy-state relaxation (T_sec), and another cryogenic relaxation that is attributed to water present in the coating (TH₂O). The shear modulus (G′) of the coating was computed from the shear moduli of the composite specimen and the core, assuming that the coating and core deform through the same angle on oscillation. The glassy-state modulus was the same for both thin and thick coatings, although the intensity of the damping peaks, as measured by the logarithmic decrement, increased with coating thickness. Comparison of the dynamic mechanical behavior of a coated optical fiber and of a free film cast from the same reactive components shows that the polymer itself can absorb water at ambient conditions and display a mechanical relaxation at cryogenic temperatures. The T., H₂O and T_sec relaxations are coupled with respect to their intensities. Latent chemical reactivity was found in one coating above its maximum temperature of cure. In this, the temperature of cure determines the glass transition temperature.     

Thermal-induced recovery of small deformations and degradation defects on epoxy coating surface

Surface defects, which are dependent on the surface properties, determine the appearance, toughness, and other properties of coatings; thus, changes to the morphology of a damaged epoxy coating surface were investigated on annealing. Changes in mechanically produced indentations and scratches with annealing indicate a surface transition temperature about 10 degree lower than the measured bulk glass transition temperature (T _g). Polymer molecules at a free surface, even in a crosslinked coating, may have higher mobility, and thus allow different relaxation activity from the bulk material. The eventual extent of deformation is a function of the annealing temperature and time. Results showed that the deformation profile diminished, driven by viscoelastic deformation and surface energy; the effect of structural relaxation will be further studied. When the surface features were generated by photodegradation, the roughness increased initially when temperature was increased, possibly due to phase coarsening, and then the roughness diminished during extended annealing.     

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.     

Easy one-pot method to control the morphology of polyethylene/carbon nanotube nanocomposites using metallocene catalysts

An easy one pot method is demonstrated for the controlled periodical surface coating of polyethylene over multiwalled carbon nanotubes (MWCNT) by insitu polymerization of ethylene using highly active metallocene catalysts (Cp₂ZrCl₂ and Cp₂TiCl₂) in combination with methylalumoxane. The crystallinity of the nanocomposite was increased and its morphology could be tuned from “sausage” like to “shish-kebab” in the presence of CNT depending on the experimental condition and choice of metal atom.     

Regulation of the structure of fibres from polymer solutions

The properties of fibres “wet” spun from solutions of polymers can be altered within wide limits by using information on the rheological properties of the spinning solutions, the phase equilibrium diagrams of polymer—solvent—precipitator systems, and information on the glass transition and crystallization in the system. The homogeneity of the structure and properties of the fibre increase in the order of spinning methods: diffusion, thermotropic, mechanotropic. __________ Translated from Khimicheskie Volokna, No. 4, Pp. 10–17, July–August, 2006.     

Exterior wood coatings. Part-2: Modeling correlation between coating properties and their weathering performance

Understanding how a coating’s properties are related to its performance characteristics and how measuring its properties can be used for predicting the performance of coating in service was the main focus of this study. A number of exterior penetrating wood stains were characterized in terms of glass transition temperature (T _g), solid content, viscosity, and surface tension. The contact angles of liquid coatings were measured on wood treated with chromated copper arsenate, alkaline copper quat, and copper azole as well as untreated wood. Also, the film thicknesses of the cured coatings on wood surfaces were measured by back-scattered electron imaging of osmium-treated samples. Partial least square-regression (PLS-R) modeling was used to correlate measured coating properties with their weathering performance on the preservative-treated and untreated wood substrates, based on water repellency, color retention, and visual ratings after 3 years of natural weathering. A PLS model was developed with 77% fitting accuracy and 69% prediction ability. This model indicated that preservative treatments significantly affected coating performance, and among coating properties, film thickness, viscosity, and glass transition temperature had the greatest effects on the coatings’ performance in exterior.     

Study of siloxane-modified epoxy resin using thermally stimulated current(II)

Thermally stimulated current (TSC) and relaxation map analysis (RMA) was used to characterize the low temperature relaxation of epoxy resin modified with siloxane oligomers. In aminopropyl-terminated siloxane oligomer (ATSO) the β-relaxation of epoxy resin and the glass transition temperature of siloxane oligomer were folded regardless of the concentration of diphenyl. The β-relaxation of epoxy resin and the glass transition temperature of oxiranylmethoxy-terminated siloxane oligomer (OTSO) were folded and shifted to higher temperature as the concentration of diphenyl in siloxane oligomer increased. In the systems containing of diphenyl in siloxane oligomer a new relaxation peak due to the space charge was observed in the range of − 80 °C to − 50 °C and − 30 °C to 5 °C. As the concentration of diphenyl increased the compensation temperature (T_c) and the degree-of-disorder (DOD) were increased while the compensation time, τ _c was decreased. Received: 26 May 1997/Accepted: 27 June 1997     

Enthalpy relaxation of photopolymerized multilayered thiol‐ene films

Multilayered thiol‐ene network films with two and three different components were fabricated by spin coating and photopolymerization. The distinctive glass transition temperatures of each layer component were observed at corresponding glass transition regions of each bulk sample. Sub‐T_g aging of 10‐, 21‐, and 32‐layered thiol‐ene films was investigated in terms of enthalpy relaxation. Enthalpy relaxation of each layer component occurred independently and presented the characteristic time and temperature dependency. Overlapped unsymmetrical bell‐shaped enthalpy relaxation distribution having peak maximum at T_g‐10°C of each layer component was observed, resulting in broad distribution of enthalpy relaxation over wide temperature range. In addition, enthalpy relaxation of each layer component in the multilayered thiol‐ene films was significantly accelerated comparing to that of bulk thiol‐ene samples. Dynamic mechanical thermal properties of multilayered thiol‐ene films also showed two and three separated glass transition temperature. However, for 32‐layered thiol‐ene film consisting of three different layer components, glass transition and damping region are overlapped and the width is extended more than 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Characterization of structural heterogeneity of polyurethane coatings

The thermal analysis techniques—Differential Scanning Calorimetry and ThermoStimulated Current—have been used to characterize a polyurethane high solid coating. The glass transition temperature, as determined by DSC, is 60°C. Below this glass transition temperature, an α_sub dielectric relaxation mode has been observed; it corresponds to cooperative movements precursor of the glass transition. The α_ss dielectric relaxation mode, located at ≈ T_g has been attributed to movements of soft sequences of the amorphous phase liberated at the glass transition temperature. The analysis of the fine structure shows that they are constituted of elementary processes characterized by relaxation times following a compensation law. Above T_g, the α_hs dielectric relaxation of hard sequences has been shown. It corresponds to hard sequences hydrogen bonded in polyurethane. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2786–2790, 2001     

Density and surface tension of Se(90?0.9 x )S(10?0.1 x )Me x and Se(60?0.6 x )S(40?0.4 x )Me x (Me = Sb), Ge glass-forming meltsMe x (Me = Sb), Ge glass-forming melts

The properties of melts in the Se_(90−0.9x)S_(10−0.1x)Me _x and Se_(60−0.6x)S_(40−0.4x)Me _x (Me = Sb, Ge) systems (wherex ranges from 2.5 to 15 mol % with a step of 2.5 mol %) have been investigated. The density and surface tension of melts and the wetting angles of the PbSe semiconductor by melts are measured. The work of adhesion and adhesion strength of the glass coating are calculated. The experimental data are processed using the least-squares method, and the corresponding regression equations are derived. From the materials of the paper reported at the International Conference “Glasses and Solid Electrolytes” (St. Petersburg, 1999, May 17–19).     

Density and surface tension of Se(90−0.9x )S(10−0.1x)Me x and Se(60−0.6x)S(40−0.4x)Me x (Me = Sb), Ge glass-forming meltsMe x (Me = Sb), Ge glass-forming melts

The properties of melts in the Se_(90−0.9x)S_(10−0.1x)Me _x and Se_(60−0.6x)S_(40−0.4x)Me _x (Me = Sb, Ge) systems (wherex ranges from 2.5 to 15 mol % with a step of 2.5 mol %) have been investigated. The density and surface tension of melts and the wetting angles of the PbSe semiconductor by melts are measured. The work of adhesion and adhesion strength of the glass coating are calculated. The experimental data are processed using the least-squares method, and the corresponding regression equations are derived. From the materials of the paper reported at the International Conference “Glasses and Solid Electrolytes” (St. Petersburg, 1999, May 17–19).     

Myuller’s concept of the viscous flow: Prospects for its evolution

The salient points of Myuller’s concept of viscous flow are set forth. The concept is analyzed in terms of the basic principles of quantum mechanics and statistical thermodynamics. Particular emphasis is placed on the activation entropy of the viscous flow S _η ^* and the glass transition entropy ΔS _g . It is demonstrated that, when the tunnel penetration gives way to over-the-barrier passage, the temperature dependences are not described by the Arrhenius equation. The information aspect of the discrete transformations of chemical bonds is described in detail. The publication of the Proceedings will be continued in the next issue of the journal. The paper “Rudolf Ludvigovich Myuller—A Man, Scientist, and Organizer” by S. V. Nemilov was published in the journalFiz. Khim. Stekla, 1999, vol. 25, no. 2, pp. 121–129 [Glass Phys. Chem. (Engl. transl.), 1999, vol. 25, no. 2, pp. 93–99].     

Flow of polymer systems in the viscoelastic state through round capillaries. Deformation analysis

Flow of polymer systems in the viscoelastic state through a capillary is considered a deformation process; the deformation characteristics used were the equilibrium shear modulus G_R, equilibrium relaxation time θ_R, and parameter m, and the product of the shear gradient by the equilibrium relaxation time S_o = θ_Rq_o was used as the shear deformation. __________ Translated from Khimicheskie Volokna, No. 2, pp. 39–42, March–April, 2006.     

Synthesis, characterization, thermal degradation, and comparative chain dynamics studies of weak-link polysulfide polymers

Synthesis, spectroscopic and thermal characterization of two new classes of polysulfide polymers: poly[1-(phenoxymethyl) ethylene polysulfide] (PPMEP), and poly[1-(phenoxy) ethylene polysulfide] (PPEP) is presented. The direct pyrolysis mass spectrometry (DP-MS) technique, used to study the thermal degradation behavior of these polysulfide polymers, indicated that the polymers underwent degradation through the weak-links scission. The thermal stability of the polysulfide polymers decreased as the “rank” (number of sulfur atoms in the polysulfide linkage; n = 1, 2, 4) increased. The main-chain flexibility of these polysulfide polymers in terms of their ¹³C NMR spin-lattice relaxation time (T ₁ ) measurements on the backbone methine (-CH-) and methylene (-CH₂-) carbons are reported here for the first time. A comparative study of the solution chain dynamics indicated that it increased as “rank” of the polysulfide linkages decreased as well as by introducing side chain spacers such as, ether (-O-) or methyleneoxy (-CH₂O-) groups.     

The thermal,mechanical and viscoelastic properties of poly(butylene succinate-<Emphasis Type="Italic">co</Emphasis>-terephthalate) (PBST) copolyesters with high content of BT units

Poly(butylene succinate-co-terephthalate) (PBST) copolyesters, with rigid butylene terephthalate (BT) units varying from 50 to 70 mol%, were synthesized via direct esterification route. The chemical structure and comonomer composition were characterized by ¹H NMR. The weight-average molecular weights (M _w ) of the prepared products measured by GPC spanned a range of 1.39 × 10⁵–1.93 × 10⁵ with corresponding M _w /M _n value of 2.23–2.42. Based on the WAXD analysis, PBST copolyesters were identified to have the same crystal structure as that of poly(butylene terephthalate) (PBT). The researches on the thermal properties showed that the melting temperature and decomposed temperature of PBST copolyesters increased with the increasing content of rigid BT units through DSC and TGA measurement. Furthermore, the tensile test results presented that the copolyester with higher content of BT units had higher initial modulus, higher breaking strength but lower elongation at break. Additionally, the viscoelastic properties of the prepared PBST films were analyzed by DMA measurement. It was found that both storage modulus (E′) and loss modulus (E″) corresponding to the peak tended to heighten with the increase of BT units, indicating the copolyester with higher BT units content had the more prominent viscoelasticity. The peak of loss factor (tan δ) curve shifted to higher temperature as the content of rigid BT units increased due to the increasing of the glass transition temperature (T _g).     

Accelerated weathering performance of Scots pine preimpregnated with copper‐based chemicals before varnish coating Part:1 coated with cellulosic and polyurethane varnishes

This study aimed to determine the effect of accelerated weathering on gloss, surface hardness and colour changes of Scots pine (Pinus sylvestris L.). Test samples were impregnated with Adolit KD‐5, Wolmanit CX‐8 and Celcure AC‐500 covered with cellulosic and polyurethane varnishes. The results showed that the values of surface hardness and gloss increased after accelerated weathering. While the surface hardness of Scots pine was increased for impregnated and polyurethane‐coated varnish, it decreased for impregnated and cellulosic varnish‐coated Scots pine after 1000 hours of accelerated weathering exposure. Copper‐based chemical impregnation and varnish coating developed the gloss of Scots pine specimens relative to the surface characteristics observed in single‐coated Scots pine specimens. While the most appropriate chemical was Celcure AC‐500 for surface hardness, it was Adolit KD‐5 for the gloss of Scots pine after 1000 hours of accelerated weathering exposure. Wood specimens impregnated prior to the application of varnish were more effective in stabilising the colour of Scots pine than Scots pine only coated with varnish. Polyurethane varnish‐treated Scots pine showed better colour stability for each partial and total accelerated weathering exposure period. The total colour changes were lowest for polyurethane varnish‐coated Scots pine impregnated with Celcure AC‐500 after 1000 hours of accelerated weathering exposure.     

Poly(imide-amide)-poly(ethylene glycol) hybrid networks: nanostructure,molecular dynamics and membrane properties

Summary Nanometer-scale structure, molecular dynamics (at 100-580 K) and membrane properties were studied in two series of poly(imide-amide) (PIA)-poly(ethylene glycol) (PEG) hybrid networks with regularly varied composition and different lengths of PEG crosslinks (M_n= 1000 or 3400). Combined WAXD/SAXS/polarized microscopy/DSC/DRS/TSDC/creep rate spectroscopy (CRS) analysis of these hybrids was performed. Depending on their composition, semicrystalline or mesomorphous, or amorphous state, and nanostructural heterogeneity were observed for these networks. They could be subdivided into (a) the PIA-rich hybrids with spatially isolated PEG domains, “suppressed” dynamics in the PEG glass transition, and PIA domains with T_g=520–570 K (group 1), and (b) the other hybrids with a continuous PEG phase and low-temperature glass transition only (group 2). Heterogeneity in segmental dynamics was revealed by CRS over the temperature range from T_g^PEG to T_g^PIA. In the second group of hybrids, the permeability coefficients were higher, by two or three orders of magnitude, for organic vapors than those for air gases.