Probing photodegradation beneath the surface: a depth profiling study of UV-degraded polymeric coatings with microchemical imaging and nanoindentation View Full Text


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Article Info

DATE

2007-09-09

AUTHORS

Xiaohong Gu, Chris A. Michaels, Peter L. Drzal, Joan Jasmin, David Martin, Tinh Nguyen, Jonathan W. Martin

ABSTRACT

Photodegradation of polymer coatings generally involves photooxidation, resulting in the formation of oxidized products, chain scission, and crosslinking. On severe exposure to ultraviolet (UV) light in the presence of air, chemical degradation transforms into substantial changes in the physical and mechanical properties, leading to failures of the coatings. Systematic research by NIST on service life prediction of polymeric coatings indicates that the degradation of polymer coatings starts from the sub-micrometer degradation-susceptible regions at the surface and then grows in width and depth. Additionally, due to the oxygen diffusion effect and the attenuation of the UV light passing through the polymer, the degradation can be spatially heterogeneous. In this study, the changes with depth of the mechanical and chemical properties of a UV-exposed epoxy/polyurethane system were measured by nanoindentation and Fourier transform infrared spectroscopy (FTIR) microscopy using cross-sectioned specimens. Multilayers of epoxy/polyurethane samples were prepared by a draw-down technique. After curing, samples were exposed to the outdoors in Gaithersburg, MD, for four months. Cross-sectioned slices of the exposed and unexposed samples, approximately 500 nm thick as-prepared by microtoming, were used for micro-FTIR imaging. Samples for nanoindentation were prepared by embedding the epoxy/polyurethane multilayers (both exposed and unexposed) in a molding compound, followed by microtoming and polishing the embedded films in the thickness direction. Micro-FTIR images clearly show that, for the outdoor exposed samples, substantial amounts of oxidation products are distributed in the 60 μm deep region from the surface to the epoxy bulk, decreasing in the center of epoxy region and increasing again toward the epoxy/urethane interface. Nanoindentation results also show that the modulus significantly increases in the first 60 μm region after UV degradation, and then decreases gradually with depth until a value slightly higher than the modulus of the undegraded epoxy is reached. The modulus rises again in the region near the epoxy/urethane interface. These similarities in the depth profiles of the properties indicate the linkage between the chemical degradation and the mechanical degradation. The study clearly shows that the spatial distribution of chemical species and mechanical properties is heterogeneous in the thickness direction for polymer coatings after UV degradation. It also demonstrates that cross-sectional analysis using nanoindentation and micro-FTIR imaging techniques is a useful method to characterize the mechanical and chemical depth profiles of polymer coating degradation. More... »

PAGES

389

References to SciGraph publications

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  • 2003-06. Relating laboratory and outdoor exposure of coatings: IV. Mode and mechanism for hydrolytic degradation of acrylic-melamine coatings exposed to water vapor in the absence of UV light in JOURNAL OF COATINGS TECHNOLOGY
  • 2004-07. Advanced techniques for nanocharacterization of polymeric coating surfaces in JOURNAL OF COATINGS TECHNOLOGY AND RESEARCH
  • 2005-07. Microstructure and morphology of amine-cured epoxy coatings before and after outdoor exposures—An AFM study in JOURNAL OF COATINGS TECHNOLOGY AND RESEARCH
  • 2004-10. Use of laser scanning confocal microscopy for characterizing changes in film thickness and local surface morphology of UV-exposed polymer coatings in JOURNAL OF COATINGS TECHNOLOGY AND RESEARCH
  • 1998-05. Microstructure of weathered paint and its relation to gloss loss: Computer simulation and modeling in JOURNAL OF COATINGS TECHNOLOGY
  • 1992-06. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments in JOURNAL OF MATERIALS RESEARCH
  • 2006-07. Relationship between chemical degradation and thickness loss of an amine-cured epoxy coating exposed to different UV environments in JOURNAL OF COATINGS TECHNOLOGY AND RESEARCH
  • 2004. Nanomechanical Properties of UV Degraded TiO2/Epoxy Nanocomposites in MRS ADVANCES
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    DIMENSIONS

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    34 UV light
    35 UV-degraded polymeric coatings
    36 UV-exposed epoxy/polyurethane system
    37 air
    38 amount
    39 analysis
    40 attenuation
    41 bulk
    42 center
    43 chain scission
    44 changes
    45 chemical degradation
    46 chemical degradation transforms
    47 chemical depth profiles
    48 chemical properties
    49 chemical species
    50 coating degradation
    51 coatings
    52 compounds
    53 cross-sectional analysis
    54 cross-sectioned specimens
    55 deeper regions
    56 degradation
    57 degradation transforms
    58 degradation-susceptible regions
    59 depth
    60 depth profiles
    61 depth profiling studies
    62 diffusion effects
    63 direction
    64 distribution
    65 effect
    66 embedded films
    67 epoxy
    68 epoxy bulk
    69 epoxy region
    70 epoxy/polyurethane multilayers
    71 epoxy/polyurethane samples
    72 epoxy/polyurethane system
    73 epoxy/urethane interface
    74 exposure
    75 failure
    76 films
    77 formation
    78 images
    79 imaging
    80 imaging techniques
    81 interface
    82 life prediction
    83 light
    84 linkage
    85 mechanical degradation
    86 mechanical properties
    87 method
    88 micro-FTIR imaging
    89 micro-FTIR imaging techniques
    90 microchemical imaging
    91 microscopy
    92 microtoming
    93 modulus
    94 molding compound
    95 months
    96 multilayers
    97 nanoindentation
    98 nanoindentation results
    99 outdoors
    100 oxidation products
    101 oxygen diffusion effects
    102 photodegradation
    103 photooxidation
    104 polymer coating
    105 polymer coating degradation
    106 polymeric coatings
    107 polymers
    108 polyurethane multilayers
    109 polyurethane samples
    110 polyurethane system
    111 prediction
    112 presence
    113 presence of air
    114 products
    115 profile
    116 profiling studies
    117 properties
    118 region
    119 research
    120 results
    121 samples
    122 scission
    123 service life prediction
    124 severe exposure
    125 similarity
    126 slices
    127 spatial distribution
    128 species
    129 specimens
    130 spectroscopy microscopy
    131 study
    132 sub-micrometer degradation-susceptible regions
    133 substantial amount
    134 substantial changes
    135 surface
    136 system
    137 systematic research
    138 technique
    139 thickness direction
    140 transform
    141 undegraded epoxy
    142 unexposed samples
    143 urethane interface
    144 useful method
    145 values
    146 width
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