Glass transition temperature of cellulose nitrate and B-72

Rohm & Haas, manufacturer of B-72, lists the Tg of the co-polymer at 40
degrees C (104 F). In practice, the Tg can vary plus or minus a couple of
degrees, depending on application, addition of plasticizers and solvent
choice.  We have looked at the adhesive as supplied by HMG (the purple
tube) using GCMS and determined that it does not have a plasticizer in it.
We have not yet examined what solvents are present.

The HMG cellulose nitrate is more difficult.  Shashoua (see ref below),
et al, state cellulose nitrate "softens at approximately 80-90 degrees C",
but I don't know that anyone has determined the molecular weight of the
cellulose nitrate that HMG uses.  Plasticizer(s) are present in the HMG
cellulose nitrate, which will lower its glass transition temperature.
Schilling mentions "The existence of glass transition data in the
Conservation Information Network", so you might want to look there.

I have seen HMG cellulose nitrate yellow within 2-3 years, even in the
tube, and become brittle and lose adhesion in less than 10 years of
archaeological storage.  I have used B-72 often on very hot excavations,
even working outdoors, but I recommend you work in the shade!

See:

    Shashoua, Y., et al, "Degradation of cellulose nitrate
    adhesive", Studies in Conservation, Vol. 37, No. 2, pp 113-119.

    Schilling, M., "The glass transition of materials used in
    conservation", Studies in Conservation, Vol. 34, No. 3, pp.
    110-116.

Stephen Koob
Department of Conservation and Scientific Research
Freer Gallery of Art
Smithsonian Institution
Washington DC 20560
202-357-4880 x276

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                  Conservation DistList Instance 9:74
                   Distributed: Thursday, May 9, 1996
                        Message Id: cdl-9-74-004
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