CLEANING OF LOW MOLECULAR WEIGHT POLYETHYLENE WITH BENTONITE ADSORBENT

ABSTRACT

This study examined the efficiency of low-molecular-weight polyethylene purification using an adsorbent—PBG bentonite—produced in Uzbekistan. Experiments were conducted to determine the effects of temperature, time, and bentonite concentration on the degree of decolorization. Optimal process conditions were established: a temperature of 150°C, an adsorption time of 30 minutes, and a bentonite concentration of 15–20%. Under these conditions, the brightness of the low-molecular-weight polyethylene was increased to 70–75 ASTM E313 units without significant weight loss.

These results demonstrate that the use of bentonite is an effective and cost-effective method for purifying and decolorizing low-molecular-weight polyethylene, expanding its industrial application and improving the environmental performance of polymer waste recycling.

АННОТАЦИЯ

В настоящем исследовании изучена эффективность очистки низкомолекулярного полиэтилена с помощью адсорбента — бентонита марки PBG, произведённого в Узбекистане. Проведены эксперименты по определению влияния температуры, времени и концентрации бентонита на степень обесцвечивания. Оптимальными условиями процесса установлены температура 150 °С, время адсорбции 30 минут и концентрация бентонита 15–20 %. При таких условиях удалось повысить показатель белизны низкомолекулярного полиэтилена до 70–75 единиц ASTM E313 без значительных потерь массы продукта.

Полученные результаты демонстрируют, что применение бентонита является эффективным и экономически целесообразным способом очистки и обесцвечивания низкомолекулярного полиэтилена, что позволяет расширить его промышленное применение и повысить экологическую эффективность переработки полимерных отходов.

Keywords: low molecular weight polyethylene; by-products; decolorization; bentonite; adsorption; purification; temperature; processing time; concentration; wax.

Ключевые слова: низкомолекулярный полиэтилен; побочные продукты; обесцвечивание; бентонит; адсорбция; очистка; температура; время обработки; концентрация; воск.

Introduction

Polyethylene is one of the most widely produced polymers in the world and is used in various industrial applications. During its synthesis, several by-products are formed, among which low molecular weight polyethylene (LMP) is of particular interest. This by-product, unlike the main polymer fraction, does not undergo strict quality control. As a result, mixtures containing different grades of LMP are obtained, often contaminated with colored impurities formed due to high temperature and other technological factors during production. This contamination leads to dark coloration of the product, which limits its direct use, especially in the production of wax.

One of the most promising applications of LMP is in wax production, where color and purity are key quality parameters. To obtain white wax, the LMP must first be purified from coloring and other impurities.

Bentonite, a natural adsorbent, has shown good potential for removing impurities from organic substances such as natural waxes (e.g., beeswax, jojoba wax, montan wax, carnauba wax). [1,2,3] Currently, there is no experience in the industry or scientific literature of producing white low molecular weight polyethylene from black low molecular weight polyethylene. Inspired by these methods, this study investigates the effectiveness of using locally produced PBG bentonite for the purification and discoloration of low molecular weight polyethylene. The influence of temperature, contact time, and bentonite concentration on the bleaching efficiency was systematically studied to determine optimal process conditions.

Materials and Methods

The low molecular weight polyethylene (LMP) used in this study was obtained as a by-product from polyethylene synthesis at a local production plant. The material was divided into two classes according to its initial whiteness index:

Bentonite of the PBG brand, produced in Uzbekistan, was used as an adsorbent.

Whiteness index were conducted according to international ASTM E313 standards. For each property, the mean value (M), standard deviation (σ), and number of samples (n = 5) were determined.

Results

In our scientific work, we aimed to study the effectiveness of cleaning low molecular weight polyethylene using bentonite. [ 4,5]

PBG bentonite produced in Uzbekistan was used to clean low molecular weight polyethylene. The characteristics of PBG bentonite are shown in Table 1.

Table 1.

The characteristics of PBG bentonite

For ease of application in industrial processes, the low molecular weight polyethylene obtained from polyethylene synthesis was classified into two categories based on its initial degree of whiteness. The purpose of this classification is to optimize processing conditions: lighter-colored low molecular weight polyethylene requires lower temperatures and milder treatment to achieve the desired quality, while darker-colored polyethylene must be processed for a longer duration and under more intensive conditions.

Class 1 low molecular weight polyethylene has a whiteness index higher than –10, whereas Class 2 has a whiteness index lower than –10 (ASTM E313). Typically, low molecular weight polyethylene with an acceptable whiteness level exhibits a whiteness index of 70–75. Therefore, in the experiments conducted, the target whiteness was set within this range, and values above 75 were not pursued. Achieving higher whiteness would require either increasing the bentonite concentration or extending the treatment time. However, both of these measures would result in greater material loss and reduced yield.

The darkening of low molecular weight polyethylene formed during polyethylene synthesis is primarily caused by oxidized derivatives of olefins. Exposure to elevated temperatures further intensifies this discoloration, resulting in a darker product. Therefore, temperature is a critical factor in the purification process: lower temperatures decrease the efficiency of bleaching and impurity removal, while excessively high temperatures accelerate degradation reactions, leading to undesirable changes in the material’s properties. Therefore, in the first stage of the study, it is necessary to find the optimal indicator by cleaning low molecular weight polyethylene at different temperatures. The effectiveness of cleaning was determined by measuring the whiteness degree according to the ASTM E313 method. The first figure shows the dependence of the whiteness degree on temperature. In the initial phase of our experiments, we used the recommendations of the patent for the purification of beeswax with bentonite [3]. In these experiments, the bleaching time was assumed to be approximately 30 minutes and the bentonite concentration was assumed to be 20%.

A three-necked flask was used for the experiments. The flask was equipped with a reflux condenser, a thermometer, and a capillary tube for the introduction of inert gas. Mixing was performed using a magnetic stirrer. Initially, 300 g of low molecular weight polyethylene was placed into the flask. After the polyethylene was completely melted, the magnetic stirrer was set to a speed of 600 rpm. Subsequently, BPG bentonite equivalent to 20% of the total mass was gradually added to the molten polyethylene. The mixture was stirred continuously under an inert gas atmosphere for 30 minutes at temperatures of 130 °C (experiment 1), 140 °C (experiment 2), 150 °C (experiment 3), 160 °C (experiment 4), and 170 °C (experiment 5).

After the adsorption process was completed, the mixture was transferred to a filtration unit preheated to 150 °C and filtered under pressure. The purified low molecular weight polyethylene was then cooled on a flat surface and prepared for subsequent whiteness measurements.

Figure 1. Temperature dependence of the whiteness index of first-class (curve 1) and second-class (curve 2) low molecular weight polyethylene (T = 30 min, bentonite concentration = 20%)

As can be seen from the curves, the optimal temperature for the bleaching of low molecular weight polyethylene with bentonite is 150 °C. Increasing the temperature beyond this point results in a decrease in the whiteness index. For instance, when the temperature was raised from 150 °C to 170 °C for Class 1 low molecular weight polyethylene, the whiteness index decreased from 72 to 58.

In the second stage of the experiment, the effect of adsorption time on the bleaching process was investigated. The duration of adsorption varied from 20 minutes to 1 hour. The results showed that extending the adsorption time beyond 30 minutes did not lead to any significant improvement in the whitening of low molecular weight polyethylene. The experimental results are presented in the diagram in Figure 2.

Figure 2. Effect of adsorption time on the whiteness index of low molecular weight polyethylene (T = 150 °C, bentonite concentration = 20%)

In the next step, the optimal concentration of bentonite was selected for the bleaching process of low molecular weight polyethylene. The results of the experiments are shown in Tables 2 and 3.

Table 2.

Effect of bentonite concentration on the whiteness index and yield of first-class low molecular weight polyethylene

For first-class low molecular weight polyethylene, treatment with different bentonite concentrations showed that 15% bentonite was sufficient to achieve the target whiteness index of 70–75. Increasing the bentonite concentration to 25% did not significantly improve the whiteness (only up to 75), but resulted in considerably higher material losses. Similar trends were also observed when the bentonite concentration was increased to 30%

The second table presents the effect of bentonite concentration on second-class low molecular weight polyethylene.

Table 3.

Effect of bentonite concentration on the whiteness index and yield of second-class low molecular weight polyethylene.

For second-class low molecular weight polyethylene, treatment with different bentonite concentrations showed that 20% bentonite was sufficient to achieve the target whiteness index of 70. Increasing the bentonite concentration to 25% slightly improved the whiteness to 72, but resulted in significantly higher material losses. Similar trends were observed at a bentonite concentration of 30%. However, the results indicated that achieving a whiteness index of 75 with second-class low molecular weight polyethylene was not possible, and the maximum achievable value was 72.

A comparison of the results presented in Tables 1 and 2 shows that for Class 1 low molecular weight polyethylene, 15% bentonite is sufficient to achieve a whiteness index of 70, whereas for Class 2, the same level requires 20% bentonite.

Conclusion

The results of this study clearly demonstrate that bentonite can be effectively used as an adsorbent for the bleaching of low molecular weight polyethylene obtained as a by-product of polyethylene synthesis.  It was found that an optimal temperature of 150 °C ensures efficient bleaching without accelerating degradation processes. Lower temperatures reduce bleaching efficiency, while excessively high temperatures lead to structural deterioration and decreased whiteness.

Experimental data showed that the optimal conditions for achieving the target whiteness index of 70–75 include an adsorption time of 30 minutes and bentonite concentrations of 15% for Class 1 and 20% for Class 2 low molecular weight polyethylene. Increasing the bentonite concentration beyond these levels did not result in a significant improvement in whiteness, but it did cause increased material loss, which is undesirable for industrial applications.

Importantly, the bleaching process using bentonite did not negatively affect the other physical properties of the polyethylene samples, indicating the practicality of this method. These findings suggest that bentonite-assisted bleaching is a simple, cost-effective, and environmentally friendly method that can be integrated into existing industrial processes. Implementing this approach can increase the usability and economic value of dark-colored low molecular weight polyethylene, while also contributing to waste minimization and improved resource efficiency in polymer production.

References:

1. Bonvehi J.S., Orantes-Bermejo F.J. Discoloration and adsorption of acaricides from beeswax. Journal of Food Process Engineering, 2016.
2. Eur. J. Lipid Sci. Technol. Effect of the extraction and bleaching processes on jojoba (Simmondsia chinensis) wax quality. European Journal of Lipid Science and Technology, 2003, 105, pp. 749–753.
3. Mahler P. Process of bleaching beeswax. U.S. Patent No. 1,739,796, 1929. U.S. Patent Office.
4. Norqobilov A.E., Adilov R.I., Aykhodjayev B.B. Cleaning of low molecular weight polyethylene with bentonite adsorbent. International Scientific Forum on Sustainable Development and Green Economy, Conference Proceeding, Zenodo, 2025. https://doi.org/10.5281/zenodo.15647231
5. Norqobilov A.E., Adilov R.I. Коллоидная химия: инновации и решения для химической технологии, экологии и промышленности, 2025.