BIOBASED CHITOSAN–POLY(METHACRYLIC ACID-CO-STYRENE) COMPOSITE AS A POUR-POINT DEPRESSANT FOR HYDROTREATED DIESEL

УДК 66.01

ABSTRACT

This study reports on the development of a novel composite additive aimed at improving the low-temperature performance of hydrotreated diesel and biodiesel fuels, which are prone to operational degradation due to n-paraffin crystallization. The additive was synthesized via ionic complexation between chitosan, sourced locally from Apis mellifera, and a methacrylic acid-styrene copolymer. Optimal copolymerization conditions yielded a high conversion of 89.2%. The formation of the polyelectrolyte complex was confirmed by FTIR, ¹H NMR, and X-ray diffraction analyses.

At an optimal dosage of 0.30 wt.%, the developed additive significantly enhanced fuel performance: the pour point was lowered from -10°C to -26.4°C, the cold filter plugging point improved by more than 10°C, and the cloud point was considerably reduced. These results are in full compliance with the requirements of the State Standard of Uzbekistan, UzDSt 989:2010. Polarized light microscopy revealed a morphological transformation of wax crystals from elongated, needle-like structures into small, isodiametric particles.

Pilot-scale industrial trials demonstrated a significant reduction in the frequency of fuel filter replacements and a notable decrease in fuel consumption. Furthermore, the additive is eco-friendly, highly biodegradable, and cost-effective compared to imported commercial analogues. The research findings confirm the effectiveness of the proposed composite as a sustainable and economical agent for diesel fuel.

АННОТАЦИЯ

В настоящем исследовании представлена разработка новой композитной присадки, предназначенной для улучшения низкотемпературных свойств гидроочищенного дизельного и биодизельного топлива, которые склонны к ухудшению эксплуатационных характеристик из-за кристаллизации н-парафинов. Присадка была синтезирована методом ионного комплексообразования между хитозаном, полученным из местного сырья (Apis mellifera), и сополимером метакриловой кислоты и стирола. Оптимальные условия сополимеризации позволили достичь высокой конверсии — 89,2%. Образование полиэлектролитного комплекса было подтверждено методами ИК-фурье-спектроскопии, ¹Н ЯМР-спектроскопии и рентгенофазового анализа.

При оптимальной дозировке 0,30% мас. разработанная присадка значительно улучшила показатели топлива: температура застывания снизилась с -10°C до -26,4°C, предельная температура фильтруемости улучшилась более чем на 10°C, а температура помутнения была существенно снижена. Эти результаты полностью соответствуют требованиям государственного стандарта Республики Узбекистан UzDSt 989:2010. Поляризационная световая микроскопия выявила морфологическую трансформацию кристаллов парафина из удлиненных игольчатых структур в мелкие изодиаметрические частицы.

Опытно-промышленные испытания продемонстрировали значительное сокращение частоты замены топливных фильтров и заметное снижение расхода топлива. Кроме того, присадка является экологически безопасной, обладает высокой биоразлагаемостью и экономически выгодна по сравнению с импортными коммерческими аналогами. Результаты исследования подтверждают эффективность предложенного композита в качестве экологичного и экономичного модификатора для дизельного топлива.

Keywords: chitosan; Apis mellifera; methacrylic acid–styrene copolymer; polyelectrolyte complex; pour-point depressant; cold-filter plugging point; hydrotreated biodiesel; synergism.

Ключевые слова: хитозан; Apis mellifera; сополимер метакриловой кислоты и стирола; полиэлектролитный комплекс; депрессорная присадка; предельная температура фильтруемости; гидроочищенное биодизельное топливо; синергизм.

1. Introduction

Hydrotreated biodiesel and ultra-low-sulphur diesel (ULSD) fuels offer well-recognized environmental benefits, including reduced CO and particulate emissions and a closed carbon cycle when derived from renewable feedstocks. However, their high content of long-chain n-paraffins—particularly C₁₆–C₂₂ homologues—causes wax crystallization at sub-zero temperatures, leading to filter clogging, fuel-line obstruction and engine startability problems in cold-climate regions. Continental Uzbekistan, where winter temperatures routinely drop to −20 °C, is particularly affected: the Bukhara and Fergana refineries currently rely on imported pour-point depressants (PPDs) at a CIF-Tashkent cost of 25 000–45 000 USD t⁻¹, with annual demand of 1 200^–1 400 t.

Three main classes of PPDs dominate the global market: poly(alkyl methacrylates) (PAMA), ethylene–vinyl-acetate (EVA) copolymers and alkyl-aromatic comb polymers. All are derived from petrochemical feedstocks. Their depressant action is generally attributed to co-crystallization with paraffin lamellae, retarding crystal growth and reorienting the developing wax habit from anisotropic plates to compact aggregates. Yet the global pour-point-depressant market—valued at USD 2.21 billion in 2025 and projected to reach USD 3.03 billion by 2032—is increasingly seeking biobased alternatives that combine cold-flow efficacy with biodegradability and low ecotoxicity.

Chitosan, a partially deacetylated derivative of chitin, is the most abundant natural amino-polysaccharide and offers an attractive route to biobased PPDs because its protonated amino groups (–NH₃⁺) can form significant electrostatic interactions with carboxylate groups (–COO⁻) of synthetic acrylic copolymers. Such ionic polyelectrolyte complexes (PECs) have been studied extensively in pharmaceuticals and water-treatment applications, but their use as fuel additives is essentially unexplored. Moreover, the conventional crustacean source of chitin (shrimp, crab) is not available in landlocked Central Asia. The present work therefore exploits an alternative, locally renewable source: the exoskeletons of dead Apis mellifera honey-bees, which contain 15–20 % chitin and are produced as inevitable apicultural waste in Uzbekistan.

The objectives of this article are: (i) to identify the optimum copolymerization conditions for poly(methacrylic acid-co-styrene); (ii) to demonstrate, by spectroscopic and diffractometric evidence, that the chitosan–copolymer pair forms a true ionic PEC rather than a simple physical blend; (iii) to quantify the synergistic depression of pour point, CFPP and cloud point in real hydrotreated diesel; (iv) to validate the additive at pilot scale and assess its ecological and economic competitiveness against imported analogues.

2. Experimental section

2.1. Materials

Methacrylic acid (MAA, ≥ 99 %, GOST 20299-74) and styrene (ST, ≥ 99 %, GOST 10003-90) were freshly distilled under reduced pressure to remove the inhibitor. 2,2′-Azobisisobutyronitrile (AIBN, ≥ 98 %) served as the radical initiator. N,N-Dimethylformamide (DMF, ≥ 99.5 %) and acetone (≥ 99.5 %) were used as polymerization and precipitation solvents, respectively. Apis mellifera exoskeletons were collected from local apiaries (Tashkent and Bukhara regions) and processed in-house. Hydrotreated TDU-0.5-grade diesel fuel (sulphur ≤ 0.05 mass %) was supplied by the Fergana Oil Refinery and characterized prior to use (Section 2.4).

2.2. Chitin extraction and chitosan preparation from Apis mellifera

Dried bee exoskeletons (60 °C, 24 h) were milled to 0.5–2.0 mm and processed in three sequential steps: (i) demineralization in 2 M HCl (solid:liquid 1:10) at 60 °C for 6 h, removing carbonates and phosphates; (ii) deproteinization in 3.5 % NaOH (solid:liquid 1:15) at 65 °C for 8 h; (iii) decolourization with 0.5 % H₂O₂. The purified chitin was then deacetylated in 50 % NaOH at 100 °C for 4 h under nitrogen. After neutralization, the resulting chitosan was washed to pH 7.0, dried under vacuum and characterized (DD = 85–88 % by FTIR ratio A₁₆₅₅/A₁₄₂₀; viscosity-average molecular weight Mᵥ = 98–107 kDa by Mark–Houwink in 0.1 M CH₃COOH/0.2 M NaCl).

2.3. Free-radical copolymerization of MAA and ST

Copolymerizations were conducted in a 500-mL three-necked glass reactor equipped with a reflux condenser, mechanical stirrer and N₂ inlet. After de-aeration (30 min, N₂ flow), MAA and ST were charged at the desired molar ratio in DMF at a total monomer concentration of 0.8 mol L^-1 AIBN (0.3–1.0 mass % of monomers) was added; the reaction proceeded at 60–90 °C for 1–6 h. Conversion was monitored by Abbe refractometry. Copolymers were precipitated in cold methanol (3 vol), Soxhlet-purified with petroleum ether and dried under vacuum at 40 °C to constant mass. Reactivity ratios were determined by the Mayo–Lewis graphical procedure; the activation energy was extracted from Arrhenius plots in four solvents of increasing polarity (1,4-dioxane, benzene, DMF, DMSO).

2.4. Composite preparation and cold-flow testing

A 2 mass % solution of chitosan in 1 % aqueous acetic acid was added drop-wise to a 10 mass % copolymer solution in DMF at 50 °C under stirring (sopolimer:chitosan = 4:1 by mass, i.e. 80:20 in the final composite). After 1 h the gelled product was precipitated in acetone (3 vol), filtered and dried at 60 °C for 4 h under vacuum. Cold-flow properties of TDU-0.5 diesel doped with 0.05–0.50 mass % composite were measured according to GOST 20287-91 (pour point, T), GOST 5066-2018 (cloud point, T꜀) and EN 116 (CFPP) using an FPP-5Gs automated cold-filter analyser. Each datum is the mean of three independent measurements; standard deviations did not exceed ± 0.4 °C.

2.5. Instrumental characterization

FTIR spectra were recorded on a Shimadzu IRAffinity-1S spectrometer (KBr pellets, 4000–400 cm⁻¹, 32 scans, 4 cm⁻¹ resolution). ¹H and ¹³C NMR spectra were measured on a Bruker Avance 400 MHz instrument in CDCl₃ or DMSO-d₆. Powder XRD patterns were obtained on a Bruker D8 Advance (CuKα, 2θ = 5°–70°, step 0.02°). Thermal stability was assessed by TGA (PerkinElmer TGA 4000, 25–600 °C, 10 °C min⁻¹, N₂). Wax-crystal morphology was visualized on a polarized-light microscope (Olympus BX53P) equipped with a Linkam THMS600 cooling stage. Biodegradability was determined by OECD 301B (Closed Bottle Test), and ecotoxicity by OECD 202 on Daphnia magna.

3. Results and discussion

3.1. Optimization of MAA–ST copolymerization

Reactivity ratios obtained by the Mayo–Lewis procedure were r₁(MAA) = 2.5 and r₂(ST) = 0.66 (r₁·r₂ = 1.65), revealing a clear preference of the MAA-terminated macroradical for self-propagation while keeping copolymerization feasible without azeotropic conditions. The Alfrey–Price reactivity (Q) and polarity (e) parameters for MAA were Q₁ = 3.59 and e₁ = −0.118, indicating moderate resonance stabilization and weak electron-donating character. Kinetic measurements in four solvents (Table 1) gave activation energies of 43.91–56.31 kJ mol⁻¹, decreasing with solvent polarity in the order dioxane > benzene > DMF > DMSO; this trend is consistent with a charge-transfer–assisted propagation mechanism that is stabilized by polar media.

Table 1.

Solvent and temperature effects on the kinetics of MAA–ST copolymerization ([AIBN] = 3.79 × 10⁻³ mol L⁻¹; total monomer = 0.8 mol L⁻¹)

Systematic variation of MAA:ST molar ratio (10:90 → 90:10), temperature (60–90 °C), initiator concentration (0.3–1.0 mass %) and reaction time (1–6 h) identified the optimum operating window: T = 75 °C, MAA:ST = 70:30, [AIBN] = 0.5 mass %, t = 3 h, giving a copolymer yield of 89.2 % and a viscosity-average molecular weight of 8.7–13.2 kg mol⁻¹ (Mark–Houwink in benzene). Further increases in initiator loading reduced molecular weight by chain-transfer to monomer; longer times caused gelation through MAA–MAA self-condensation.

3.2. Spectroscopic evidence for polyelectrolyte complex formation

Figure 1. FTIR spectra of (a) Apis mellifera chitosan (DD = 88 %), (b) poly(MAA-co-ST) and (c) the Cs–co(MAA-ST) composite, evidencing PEC formation through the −20 cm⁻¹ shift of the C=O band and the new −NH₃⁺ band at 1530 cm⁻¹.

The pristine copolymer (Figure 1b) showed the expected –COOH carbonyl stretch at 1725 cm⁻¹, aromatic ring vibrations at 1602 and 1495 cm⁻¹, and out-of-plane =C–H deformations of the monosubstituted benzene at 760 and 700 cm⁻¹. Pure chitosan (Figure 1a) exhibited the broad O–H/N–H envelope centred at 3420 cm⁻¹, the amide-I band at 1655 cm⁻¹ and a strong free –NH₂ scissoring at 1590 cm⁻¹. In the composite spectrum (Figure 1c) two diagnostic changes were observed: (i) the carbonyl band shifted from 1725 to 1705 cm⁻¹ (Δν = −20 cm⁻¹), characteristic of carboxylate (–COO⁻) anchored by an ammonium counter-ion; (ii) a new, well-resolved band appeared at 1530 cm⁻¹, assigned to the asymmetric deformation of –NH₃⁺. Both features are textbook signatures of an ionic polyelectrolyte complex and rule out a simple H-bonded blend, in which the carbonyl shift would be smaller than 10 cm⁻¹ and no –NH₃⁺ peak would emerge.

¹H NMR spectra recorded in DMSO-d₆ supported this conclusion: in addition to the expected aromatic protons (δ = 6.8–7.4 ppm), aliphatic methyl/methylene protons (δ = 0.9–2.5 ppm) and chitosan H-2 to H-6 (δ = 3.0–4.0 ppm), a new low-field resonance appeared at δ = 8.2–8.5 ppm corresponding to the –NH₃⁺ proton. This signal disappeared on titration with KOH/D₂O, consistent with rapid proton exchange. Powder XRD revealed that the characteristic chitosan reflections at 2θ = 10.1° and 19.8° broadened (FWHM 2.1° → 2.8°) and decreased in intensity in the composite. Application of the Scherrer equation D = Kλ/β·cosθ gave a crystallite size of 4.2 nm, indicating that the chitosan crystallites are dispersed and partially de-crystallized within the amorphous copolymer matrix—an arrangement that maximizes interfacial contact between –NH₃⁺ and –COO⁻ groups and is critical for high colloidal stability in non-polar diesel media.

3.3. Cold-flow performance of the composite additive

Figure 2. Concentration dependence of pour point (Tₚ), cloud point (T꜀) and CFPP of TDU-0.5 hydrotreated diesel doped with the Cs–co(MAA-ST) composite. Optimum loading is 0.30 mass %; further addition gives diminishing returns (over-saturation regime).

Figure 2 shows the strong, monotonically improving cold-flow response with increasing additive concentration up to 0.30 mass %, after which a plateau (and slight worsening above 0.40 mass %) is observed. This behaviour is typical of nucleation-template additives: at low loading the additive provides additional nucleation sites that fragment the wax network; beyond saturation, additive self-aggregation reduces the effective number of active sites. At the optimum (0.30 mass %) the pour point falls from −10 °C to −26.4 °C (ΔTₚ = 16.4 °C), the CFPP from −7 °C to −17.3 °C (ΔCFPP = 10.3 °C) and the cloud point from −5 °C to −12.1 °C (Table 2). The treated fuel meets all 20 specifications of UzDSt 989:2010 (Z-1 grade); it also shows reduced coke residue (0.18 → 0.016 %, 10×) and acid number (0.25 → 0.18 mg KOH/100 mL).

Table 2.

Cold-flow performance of the Cs–co(MAA-ST) composite at 0.30 mass % in TDU-0.5 diesel, benchmarked against UzDSt 989:2010 limits and three commercial PPDs

3.4. Synergism: additive sum versus measured composite response

Figure 3. Mechanistic evidence for the depressant action: (a, b) polarized-light micrographs at −15 °C show the transition from needle-like wax crystals (50–120 μm, untreated) to compact isodiametric crystals (3–35 μm, +0.30 % composite); the right panel quantifies the synergism with respect to the algebraic sum of individual components (S = 1.07).

The synergistic action of the composite was quantified using the dimensionless coefficient S = ΔTₚ(composite) / [ΔTₚ(chitosan) + ΔTₚ(copolymer)]. Chitosan alone (0.06 mass %) gave only ΔTₚ = 3.5 °C; the copolymer alone (0.24 mass %) gave ΔTₚ = 11.8 °C; their algebraic sum is therefore 15.3 °C. The composite at the same total mass loading delivered 16.4 °C, hence S = 1.07. Although modest in absolute terms, this value is statistically significant (paired t-test, p < 0.05) and corresponds to a 7 % gain over a simple physical blend.

The mechanism we propose comprises two cooperating actions visible in the polarized-light micrographs: (i) the hydrophobic styrene segments of the copolymer co-crystallize with n-paraffins, terminating their lamellar growth axis; (ii) the protonated chitosan amine pendants adsorb on the freshly formed crystallite surface and electrostatically prevent inter-crystallite agglomeration, ultimately giving small isodiametric crystals (3–35 μm) instead of the large needle-like ones (50–120 μm) that would otherwise form a percolating network and arrest fuel flow. Although the synergism coefficient (S = 1.07) is moderate, the observed cooperative interaction between chitosan and the copolymer contributes to improved wax-crystal dispersion and enhanced low-temperature operability compared with the individual components.

3.5. Oxidation stability and storage behaviour

In addition to depressant action, the composite confers measurable oxidation stability. Accelerated tests according to ASTM D2274 (95 °C, 16 h, air bubbling) showed the sediment level decrease from 2.8 mg/100 mL (neat) to 1.4 mg/100 mL (+ 0.30 % composite, −50 %), and the increase of acid number was reduced from +0.18 to +0.06 mg KOH/100 mL (−67 %). Six-month real-time storage (20 ± 2 °C, 100 % RH) confirmed these findings: no sediment formed, viscosity increased by only 1.1 % and Hazen colour stayed below 20 (vs. 40 for untreated). The antioxidant action is attributed to the radical-scavenging capacity of the chitosan amino groups, similar to the mechanism described for crustacean chitosan in food matrices, and to the hydrogen-donor activity of the styrene-rich domains of the copolymer.

3.6. Pilot-scale validation and ecological profile

A 60-t pilot trial at SEG LLC (Fergana, February–September 2025) compared two 20-t tankers of TDU-0.5 diesel — one neat, one doped with 0.30 mass % composite — used in a mixed ZIL-131 / KAMAZ-5320 truck fleet over three winter months. Quantitative outcomes (SEG MCJ certificate 001/2089 of 6 September 2025) included: (i) no cold-start failures in the doped fleet, vs. 18 % failure rate below −15 °C in the control; (ii) 2.3-fold longer fuel-filter service interval; (iii) statistically significant 2.8 % reduction in fuel consumption (p < 0.01); (iv) the doped fuel could be pumped at −20 °C, whereas the control showed transfer problems already at −15 °C. The ecological profile (Table 3) is also markedly favourable: ready biodegradability of 87.3 ± 2.1 % at 28 d (OECD 301B classification "readily biodegradable"), an EC₅₀(Daphnia magna, 48 h) > 500 mg L⁻¹ (low toxicity, hazard class IV), and a cradle-to-gate CO₂-equivalent of 1240 ± 85 kg t⁻¹—about 2.1–2.6 times below the imported analogues, owing to local

Table 3.

Ecological and economic comparison of the Cs–co(MAA-ST) composite with two commercial pour-point depressants

Apis mellifera as a renewable feedstock and to a closed-loop solvent recovery (DMF, 97 % recycle; acetone, 97 %). On a four-stage pilot plant of 4 000 t a⁻¹ capacity the modelled break-even is 2 386 t a⁻¹ (60 % of nameplate capacity, safety margin 40 %), and even under a pessimistic scenario (raw-material prices +30 %) the local product remains 4.3-fold cheaper than its imported analogue. The replacement of the present 1 300 t a⁻¹ import volume would yield an annual saving of approximately USD 25.9 million (~ UZS 329 billion at the 2025 exchange rate).

4. Conclusions

A novel biobased pour-point depressant for hydrotreated diesel was successfully designed by ionic complexation of locally produced Apis mellifera chitosan (DD = 85–88 %) with a free-radically synthesized methacrylic-acid–styrene copolymer (MAA:ST = 70:30 mol %, yield 89.2 %). The polyelectrolyte complex nature of the composite was unequivocally established by FTIR (−20 cm⁻¹ shift of the C=O band; new −NH₃⁺ band at 1530 cm⁻¹), ¹H NMR (δ 8.2–8.5 ppm) and powder XRD (Scherrer crystallite size 4.2 nm). At 0.30 mass % loading the composite reduces the pour point from −10 °C to −26.4 °C, the CFPP from −7 °C to −17.3 °C and the cloud point from −5 °C to −12.1 °C, fully satisfying UzDSt 989:2010 Z-1 specifications.

A synergism coefficstient S = 1.07 quantifies the cooperative action of the two components, and polarized-light microscopy reveals the underlying morphological transition from needle-like (50–120 μm) to isodiametric (3–35 μm) wax crystals. Pilot trials at SEG LLC (Fergana, 2025) confirmed industrial viability — 2.3-fold longer filter intervals, 2.8 % lower fuel consumption, no cold-start failures down to −22 °C — and the composite is readily biodegradable (87.3 % at 28 d) and 4.9–5.9 times cheaper than imported analogues. Together, these findings establish a fully Uzbek, eco-friendly route to depressant additives that can replace the country’s 1 200^–1 400 t a⁻¹ import demand and yield an estimated USD 25.9 million in annual import substitution. Although the developed composite demonstrated acceptable stability during six-month storage tests, further long-term investigations under different climatic conditions are required to fully assess its industrial applicability.

Funding. This work was supported by the Tashkent State Technical University and was carried out within the framework of the cooperation agreement with UZBEKISTAN GTL LLC (2022–2024) and the Uzbekneftgaz JSC contract No. 30/13 (2025–2026).

Conflict of interest. The authors declare no competing interests.

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