ІПДО НУХТ

KINETICS OF BASIC TOXIC COMPONENT MIGRATION
FROM PLASTIC PIPE LINE

Katayeva S.E., Prof, D. of Sci. (Biol.), Professor of the Biotechnology Department
National University of Food Engineering
Institute of the Advanced Studies
8a Estonska St., 03190 Kyiv, Ukraine
tel.: +38 449 21 04
fax: +38 449 12 33

Since practically all plastics, applied in the practice of public water supplies, are used for a long-term period (15-50 years and more), in order to substantiate hygienic regulations for their safety use, of great interest are the results of kinetics of basic toxic plastic component migration to main water within decreasing regime at exposure from 1 till 1.5-3 and more years.
No such data can be found in the literature.
Among the polymer materials applied in the practice of public water supplies, which are under study now, actual danger, when contacting with potable water, is presented by dimension products (pipe lines, water reservoirs, hoses, etc.).
In their production polyolefmes, PVC, polystyrene, polyamides and epoxy materials are widely used.
To identify main toxic substances in these materials, we have focused on toxicity, solubility in water, as well as the substance quantitative content in formulation. The study in kinetics of toxic substances migration from polymer materials was conducted within the decreasing regime at the temperature of 20, 45 and 70 °C, and exposure from 1 day till 2,5 - 3 years. Main water was used as the investigated medium. To carry out the study, there were taken laboratory and industrial samples of different thickness (and in case of pipe lines - of different diameter) and different qualitative and quantitative plastic formulation.
Since inflexible PVC materials stabilized with the lead compounds, which are used in pipe lines, are of great interest from a hygienic standpoint, most studies were concerned with these materials.
In formulations of the pipe line samples under study the stabilizer content varied from 1.6 to 3.7 micro particles. The material samples were stabilized with tribasic lead sulphate (TBLS), dibasic lead stearate (DBLS), lead stearate (LS) and lead phthalate (LP), as both an individual stabilizer and a mixture of stabilizers (more than 80 samples).
Studying kinetics of lead migration to water was carried out by the methods of atomic absorption spectrophotometry, scanning electron spectroscopy and electron probe analysis (focused on investigating structure and distribution of stabilizers through the PVC layer of the pipe line).
In all the samples of the PVC pipe lines and PVC tubes, lead release into main water was observed at 20 °C within 1-6 months (depending on quantitative content and chemical structure in the formulation), its concentration exceeding that set up in hygienic regulations.
Lead release rate was found to vary with time and to be divided into: maximum - within 1-6 months (apparently, prior to lead washing off the material surface) and minimum (practically constant) in the subsequent period of 2.5 years.
The studies performed with using the "Polyus-4" spectrometer by the ion probe method revealed that the lead content in the inner surface layer of the samples, which contacted with water under static and dynamic conditions, was 10-12 times lower than that compared with the initial content.
Amount of the lead released, in terms of that introduced into plastic, constituted 4.7% within 2.5 years and 5.2 % within 3 years.
Lead migration depends on quantitative material content in the formulation, as well as on water temperature and pH, pipe diameter and the stabilizer chemical nature.
Comparison between IR-spectra of the initial sample and those contacting with water at 20°C (exposure of 3 years and 14 months) showed the appearance of new characteristic absorption peaks in the range of wave distances of 870 and 690 nm. Based on the peak heights, one can conclude that the sample of the PVC-pipe, after the contact with water under dynamic conditions within 14 months was exposed to water action to a greater extent than the sample with exposure of 3 years under static conditions.
The obtained data on the kinetics of lead migration from PVC-materials made it possible to calculate effective diffusion coefficient and the process activation energy. The values of the diffusion coefficient depend on the diameter and the thickness of the PVC-tubes studied.
Based on the experimental data, there were also calculated the pair regression parameters and obtained mathematical models describing the process of lead release into water, that makes it possible to predict the lead release levels, depending on exposure, at any moment, as well as the time when the lead concentration reaches the value of the hygienic standard.
The structural and morphological methods of analysis (electron-microscopic analysis with using the scanning electron microscope "ISM-V3" and the X-ray microanalysis at the "Kevex" device) of the samples revealed that the stabilizer is irregularly distributed in the thickness of material. The lead concentration in the inner surface layer is an order and more of magnitude higher than the substance concentration in the volume. As it takes place, the lead concentration in the surface layer increases with an increase in quantitative content of the stabilizer in the formulation. In the process of lead migration to water, its concentration in the surface layer decreases; as a result, the lead maximum is observed in the centre of the sample.
The crucial factor in the lead release to water is the PVC destruction. High rate of the substance migration within 1-6 months is due to not only irregularity of the stabilizer distribution in the material but also to lead chloride formation as far back as at the stage of plastic processing into complete products.
The studies on choosing the way of washing PVC-tubes, containing 2 m.p. of the stabilizer, out of lead chloride with using the main water at 20 and 50 °C, as well as 2% acetic acid, within 35 days showed that it was the use of hot water (50 °C) within 3-5 days that had the greatest impact on washing out the near-surface layer of the product.
Change in color and formation of microdefects in the samples (cracks, interstices, etc.) contacted with water, according to the authors /1, 2, 3/, is due to complex influence of various factors (temperature, pressure, mechanical effects, etc.) and, first of all, water, and is explained by formation of hydrophilic products of stabilizer interchange with HC1, releasing from PVC-materials during their use.
The PVC-material destruction occurs with changes in the surface layer of 0.1 mm thick and more, depending on exposure (in our studies - 0.3 mm within 3 years), which bear no relation to the changes in deeper layers of the material, as proved by structural and morphological studies (Fig. 8-12), i.e. with degradation of its properties.

References:
1.    Minsker K.S., Fedoseyeva G.T. PVC Destruction and Stabilization, - M: Chemistry, 1979. - 271 p.
2.    Vorobyova G.Ya. Chemical Stability of the Polymer Materials. - L.: Chemistry, 1987. - p. 98-99.
3.    Pavlov N.N. Plastic Aging under Natural and Artificial Conditions.- M.: Chemistry, 1993. - 248 p.