Re: ORGLIST:Oleic acid and paraffin wax

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From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Fri May 14 2004 - 11:34:16 EDT


Hello Alan,

Various phenomena are possible and probably occur simultaneously.

1. Oleic acid exposed to the air slowly undergoes autoxidation,
yielding hydroperoxides at the beginning. Slowly means many days.
Rising the temperature accelerates this process.

2. When making the mixture it probably cools down rather quickly. This
leaves crystals with what is called microstrain. In a low m.p. mass
this strain is slowly released by attaining progressively better
crystallization. This process is accelerated at a slightly higher
temperature, still below the m.p.

3.Mixing oleic acid into the paraffin probably lowers its m.p.

4. Oleic acid is a pure compound, but the paraffin is a mixture.
Furthermore, the oleic acid molecules tend to form head-to-head dimers.
The geometry of these molecules cannot be linear because of a cis
double bond, whereas the paraffin molecules tend to cristallize in a
straight zigzag configuration. It is hard to predict with which
paraffin molecules would these dimers preferably be associated in a
crystalline lattice. In fact, the plasticizing effect of oleic acid is
that of disruption of the paraffin crystalline structure.

5. There should be a slow tendency of the mixture to expel the oleic
acid and attain better crystallization. So it is better to store the
material in compact chunks, and not as thin films or flakes, to prevent
extensive diffusion to the surface.

6. The described behavior is probably dependent on the temperature
regime used for making the mixture, say fast or slow cooling of the
melt. The method leading to the most stable mixture should be chosen.

7. There are several ways of investigating the described phenomena,
including X-ray powder diffraction and IR spectroscopy over a period
of several weeks.

All the best,

Jacob

On May 14, 2004, at 2:14, walker$##$netcon.net.au wrote:

>
> I have been working with a mixture of oleic acid and paraffin wax as a
> binder. The oleic acid is in the mix as a rheology modifier. I have
> noticed something strange about the physical properties of the
> material and I hope someone can explain it to me. I am not an organic
> chemist.
>
> I use a low melting (around 50 deg C) paraffin wax which is a mixture
> of C20 to C35 alkanes, mostly C23 - C26. I use lab grade (99.9%)
> oleic acid.
> I melt the wax at 100 deg C and mix in the oleic acid very thoroughly
> then cool it to solidify.
> I have tried 2 weight percent oleic acid and 5 weight percent oleic
> acid.
> The strange effect is that the physical properties of the resulting
> solid material changes over time. The low oleic acid mix becomes more
> brittle in 2 - 5 days and the high oleic acid becomes stronger and
> tougher in 2 - 5 days.
> Why is this? Is there some sort of cross-linking reacion with the
> double bond? Can the oleic acid oxidise a little after a few days
> exposed to air?
> Any comments?
> Thanks
> Alan Walker
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Prof. Jacob Zabicky
Institutes for Applied Researcfh
Ben-Gurion University of the Negev
P. O. Box 653
Beer-Sheva, 84105
Israel
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