Identification of plastic object

Mark Ritchie asked about the identification of clear rigid plastic
strips. Plastics and constituent plasticizers, fillers and colorants
can be differentiated and identified using infrared spectroscopy
(a.k.a. FTIR). Some methods provide surface information only;
microscope analyses are well suited to analysis of laminates and
minute samples.  Four methods used to identify plastics are
described.

    1.  ATR (attenuated total reflectance) is used to make in-situ
        contact analysis on flexible plastics that yield to the
        shape of the ATR crystal (flat or domed).  Analysis provides
        surface information only; consequently, ATR is ideal for
        analysis of homogeneous bulk plastics or very thin surface
        layers of laminates.  It also is preferred for analysis of
        plastics (and rubbers) bulked or colored with carbon black.

    2.  Transmission is used to make analysis of thin bulk films of
        homogeneous plastic without sampling, or analysis of
        microscopic samples and laminates with a microscope
        accessory.  The latter method is well suited to analysis of
        forensic evidence and required for analysis of individual
        layers in a laminate structure.  Microscope analyses also
        are used to study contaminants and degradation gradients
        within plastic materials.

    3.  Specular reflection is used to make in-situ remote analysis
        on flat, glossy plastics.  Analysis provides surface
        information only.  Specular reflection does not require
        contact or samples; consequently, it is the least invasive
        of the four methods.  Specular reflection is less sensitive
        than ATR and transmission, and provides less information
        about plasticizers, fillers, and colorants.

    4.  Diffuse reflectance is used to make analysis of particles
    abraded from a plastic surface onto silicon carbide abrasive
    paper; consequently, this method usually is restricted to rough
    surfaces of relatively large objects. Diffuse reflectance is
    less sensitive than ATR and transmission, and provides less
    information about plasticizers, fillers, and colorants.

James Martin
Orion Analytical, LLC


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                  Conservation DistList Instance 15:52
                Distributed: Thursday, January 24, 2002
                       Message Id: cdl-15-52-001
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