Why modify the calculated PEG concentrations to avoid the eutectic?

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Why modify the calculated PEG concentrations to avoid the eutectic?

The authors recommend that wood that is to be freeze-dried not be treated in solutions of PEG with a total concentration greater than 45% and less than 65%. Our experience suggests that the results will be poor. The PEG will migrate to the wood surface during freeze-drying and the ASE will be reduced. The Union Carbide product literature for PEG states that solutions of PEG below 25°C supercool and do not completely freeze. The freezing curves for PEG 400 and 3350 are shown.

A theory explaining what might be occurring in the wood during freeze-drying is included in the paper by Cook and Grattan. There was also a discussion of the eutectic and freezing behaviour of PEG solutions after our paper on non-vacuum freeze-drying in the 1981 Ottawa proceedings of the ICOM-CC WOAM meeting. A discussion that occurred at the 2001 WOAM meeting in Stockholm revealed progress in the understanding of the physical states of the PEG solutions in the wood. Jensen, Jorgensen and Schnell suggested a separation of the ice and PEG 3350. The SEM images clearly showed large ice crystals with interstitial pockets of solid PEG 4000 (PEG mixtures were not studied).

The simplest solution is to allow the value for normal density to be reduced until the calculated PEG concentrations total less than 45%.