Lactide

Lactides purification

 
High-volume consumable plastics continue to be dominated by petroleum-based materials. However, two factors recently have made biodegradable polymers economically more attractive. These are (i)rising costs of petroleum production resulting from the depletion of the most easily accessible reserves and (ii) environmental and economic concerns associated with waste disposal. These pressures have, increasingly, driven commercial ventures worldwide to develop biodegradable polymers.
Biodegradable polymers simply have to exhibit properties comparable to oil based plastics in order to win the battle. Due to their origin (such as sugar, starch, cassava, rye or corn) they readily degrade into environmentally acceptable by-products.

Feed stock

While the present applications for biodegradable plastics are mainly limited to predominately medical applications as sutures, stents, dialysis media and drug delivery devices, the strength against UV-radiation will make them very interesting for use as packaging material for foodstuff with a limited shelf life, compost bags, and disposable tableware. Furthermore, particularly as biodegradable fibers and non-woven textiles, they can be utilized as e.g. disposable garments and disposable diapers.
For producing a biodegradable polymer with properties comparable to oil-based plastics, the upstream process step has to provide monomer building blocks in the purest possible form. Any impurity present in the starting material can have a devastating influence on e.g. the mechanical properties or UV-radiation resistance of the built polymers.

The applied purification technology can be, amongst others, distillation or crystallization. However, in general elevated operating temperatures need to be avoided in order to prevent spontaneous, irreversible linking of the monomers into dimers, trimers, oligomers or even polymers. Distillation therefore would need to be executed at reduced pressures in order to lower the boiling points, significantly adding to the energy requirement of the distillation process.
Crystallization for now offers the lowest possible process temperatures at which an efficient separation of the desired monomer building blocks can be established.
Compared to the batch wise operated static or dynamic layer crystallization, the suspension based melt crystallization process features continuous operation at significant lower energy consumption. The continuous suspension based melt crystallization process of Niro P.T. has been successfully applied for purification of lactide. Starting with an impure feed an ultra-pure lactide product is produced in only one process step. The purified product is suitable for downstream processing into the full range of above indicated materials. Crystal growth rates are over a factor 1000 lower as with layer crystallization, ensuring gentle formation of ultra pure crystals.
As a solid liquid separator, Niro P.T. applies wash columns which can efficiently separate the pure crystals from the mother liquor. In the wash column, the pure crystals are rigorously washed with their own melt, eliminating the use and related recovery of a separate wash liquid. A wash column has low operating speeds which results in a proven long lifetime of the moving parts.