Two primary methods for the production of di-esters used as plasticizers are the following: esterification (employed by Grupa Azoty ZAK S.A. for production of Oxoviflex® (DEHT, DOTP) ) and transesterification (employed by some other manufacturers). The composition and properties of plasticizers may diﬀer based on the production technology.
Oxoviflex® is produced in a continuous process, based on a direct reaction of terephthalic acid with 2-ethylhexanol. It enables continious monitoring of the process and maintaining stable quality parameters of the product. In DEHT transesterification, the batch production method isdominant , which may result in the inconsistencies in the product batches, especially if the production is carried out in swing mode at the same production line with another plasticizer.
Another property that diﬀerentiates both processes are raw materials. The esterification process is carried out directly in the reaction of terephthalic acid with 2-ethylhexanol (PTA + 2EH). Therefore, the risk of undesirable impurities aﬀecting the quality of the final product is substantially lower. Additionally, conducting the process solely with PTA and 2EH reduces the risk of forming more volatile – methyl octyl esters.
In case of transesterification, the basic raw material, apart from 2-ethylhexanol, is dimethyl terephthalate (DMT). DMT may be obtained from terephthalic acid (PTA) or by alcoholysis of polyethylene terephthalate with methanol (recycling of PET products), which may result in the introduction of impurities into the raw material.
The esterification process employed in Oxoviflex® production is carried out in the presence of titanium mainly TnBT as catalyst. Titanium catalyst is an amphoteric catalyst that decomposes in water to TiO2, this ilitates its separation from the product during distillation, and reduces the quantity of media required to neutralise raw DEHT. Metalorganic catalysts are more selective, their use results in the production of few by-produc and they highly facilitate the separation of the catalyst residue by its precipitation.
Transesterification uses an acidic catalyst (sulphuric acid, methanesulfonic acid, p-Toluenesulfonic acid) or a Sn, Ti, Zr-based catalyst. In this case larger quantities of by-products are obtained and may aﬀect the quality of the final product, in particular, the colour.
Oxoviflex® esterification is carried at 180–200 °C and under reduced pressure. The process is carried in the terephthalic acid suspension in an excessive quantities of 2-ethylhexanol. It improves the separation of unreacted terephthalic acid from ester through adequate filters. The transesterification process requires increased pressure and higher temperature of 180–260 °C. Moreover, it is carried in a homogeneous mixture of reactants, which contributes to the occurrence of undesired by-products.
All of the above diﬀerences aﬀect the final product quality. Oxoviflex® is produced employing estryfication method and is chemically pure because of its manufacturing conditions that prevent occurrence of excessive by-products while DEHT manufactured in the transesterification method may contain even 3% of by-products. Furthermore, Oxoviflex® contains, among others, min. 98% of bis (2-ethylhexyl) terephthalate while the products obtained in transesterification process – 96%.
In the process of selective esterification, employing terephthalic acid and 2ethylhexanol only, DEHT produced is of good qualitative parameters, pure and with low content of by-products.
Oxoviflex® is a safe product that does not contain acidic catalyst residue, residual methanol or phthalates. As for the metal content, it is comparable to a plasticizer produced for medical purposes for medical purposes and meets top qualitative criteria, including those specified in the European Pharmacopoeia.
DEHT produced according to esterification method is characterised by much higher homogeneity of subsequent batches with respect to transparency (colour). It can turn out especially important in the production of coloured elements as the risk of colour inconsistencies of the products is minimised; thus, contributing to lower volume of pigments used in PVC processing.
It particularly pertains to bright colours (white through beige to light brown tones), where the changes in the plasticizer’s transparency significantly aﬀect the perception of the final product.
- US 7361779 B1 “Low-melting mixtures of di-n-butyl and diisobutyl terephthalate”
- US 7964658 B2 “Dialkyl terephthalates and their use“
- WO 2010/071717 “Polymer compositions comprising terephthalates”
- PL 216179 B1 “The method of obtaining dioctyl terephthalate”
- US 20150307435 A1 “Esterification Process”
An expert paper, author: Szymon Ustrzycki
Specialist Product Engineer / OXO Segment / Grupa Azoty ZAK S.A.