Polyester Wax Processing

Steedman introduced a ribboning embedding medium based on polyethylene glycol distearate mixed with 1-hexadecanol and named it Polyester wax. The degree of polymerization of the polyethylene glycol component of the stearic acid ester determines the melting point of the compound. Steedman originally used polyethylene glycol 400 distearate, which gives a melting point of 37°C, about the same as human body heat. The low melting point can cause some problems with sectioning if the ambient temperature is too high, so Sage suggested that laboratories with an ambient temperature above 24°C should use a polyester wax made with polyethylene glycol 600 distearate, which gives a melting point of about 43°C. The low melting point, particularly of the original polyester wax formulation, does mean that the blocks should be handled as little as possible since body heat can cause softening. As with ester wax, polyester wax is also susceptible to humidity.

1-Hexadecanol has several synonyms, including cetanol, cetyl alcohol and palmityl alcohol. Wikipedia has an article under the name cetyl alcohol. It also has articles on polyethylene glycol and stearic acid.

One of the major advantages of polyester wax is that its low melting point avoids many of the shrinkage artifacts caused by the temperatures necessary for paraffin wax infiltration. It is quite feasible to prepare tissues without exceeding the natural body temperature by more than a few degrees, from fixation to the final infiltrated block.

Polyester wax is miscible with 90% and higher grades of ethanol, and with methanol, esters, ethers, ketones, hydrocarbons, natural oils such as cedarwood oil, 2-ethoxy-ethanol and similar compounds, and most other fluids that are used as wax antemedia and clearing agents. Most of these can be used in processing as the fluid immediately preceding wax infiltration, although cedarwood oil and monochlorisothymol are specifically noted as being unsuitable because they are non-volatile and infiltration is by displacement. Their viscosity at 40°C, which is the recommended temperature for infiltration, makes displacement very slow.

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Tissues embedded in polyester wax can be sectioned at about 4µ to 20µ at room temperature after about 24-48 hours for block hardening. Sectioning quality improves on standing, so thinner sections may be possible if the block is allowed to sit at room temperature for a few days to allow the crystal structure to properly develop.

If a section adhesive is required, it is recommended that a 2% amylopectin solution be used. Sufficient is placed on a clean, grease free slide to wet the glass, and a section is floated onto it at room temperature. Excess amylopectin solution is drained off and the section baked on at a temperature not to exceed 30°C. Do not use this adhesive if a PAS reaction is to be employed as it will cause a background positive reaction.

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