INVESTIGATION OF PHYSICAL AND CHEMICAL PROPERTIES OF THERMOSETTING COPOLYMERS OF POLYETHYLENE GLYCOL MALEATE WITH ACRYLIC ACID DURING THEIR “COLD” CURING

Abstract. The study investigated the physicochemical properties of polyethylene glycol maleate (p-EGM) solutions in acrylic acid and their products after “cold” curing. The weight-average molecular weight of p-EGM was determined by GPC, while the degree of unsaturation of both the initial polyester and the obtained copolymers was evaluated using the bromide–bromate method. The viscosity of the polymer–monomer mixtures was measured by viscometry, and density values were obtained using pycnometric (solutions) and hydrostatic (cured products) methods, which further allowed calculation of the overall shrinkage. The copolymer composition was analyzed by HPLC, swelling degree was determined gravimetrically, and structural identification was carried out using IR and ¹H NMR spectroscopy. SEM was employed to study surface morphology. The findings demonstrate the possibility of controlling the properties of cured unsaturated polyesters by varying the initial composition of the polymer–monomer mixtures, enabling the design of polymer matrices for bulk products with favorable physicochemical characteristics.

Keywords: unsaturated polyester, “cold” curing, density, viscosity, total volumetric shrinkage, hydrophobi city, thermosetting polymers, bulk products.

Thermosetting polyester binders are widely applied across multiple industries due to their stability and resistance to environmental factors [3,9]. In the automotive sector, they are employed for producing high-strength composites, coatings, and durable adhesives [11]. In aerospace, polyester binders serve as lightweight yet strong structural materials capable of withstanding elevated temperatures and mechanical stresses [10,12]. In construction, they are used in concretes and mixtures to enhance strength and resistance to environmental impacts [14,15], as well as in advanced insulation materials for thermal and corrosion protection in construction and electrical applications.

A key direction in the development of thermosetting polyester binders is the design of polymers with tailored properties for specific technological applications. This is achieved by introducing additives and fillers that allow adjustment of mechanical, thermal, and chemical characteristics; for example, minerals and carbon fibers enhance strength and environmental resistance. Due to the presence of unsaturated bonds, polyesters undergo crosslinking to form three-dimensional networks, which improve mechanical performance, thermal stability, and chemical resistance. By varying monomer ratios and curing methods, the properties of unsaturated polyesters can be modified to meet modern requirements. Current research aims at improving resistance to UV radiation, mechanical strength, and adhesion, expanding their use in electronics, biomaterials, and nanocomposites [11,14].

The curing reactions of unsaturated polyesters with styrene [1] and methyl methacrylate [2] are well documented, but no studies report the use of unsaturated carboxylic acids as solvent-curing agents for polyester binders. Our earlier work examined polyethylene(propylene)glycol fumarate curing products as potential polymer matrices for hermetic materials [4,13]. The present study focuses on the physicochemical properties of polyethylene glycol maleate (p-EGM) solutions in acrylic acid (AA) and their cured products, evaluating the potential of the p-EGM–AA system as a thermosetting polyester binder for filler materials with minimal mineral additives. Thermosetting polyester binders have gained wide application in modern industry due to their outstanding performance properties, such as high strength, thermal and chemical resistance, and long-term durability under various environmental influences. These characteristics explain their active use in diverse industrial sectors. In the automotive industry, they serve as a basis for high-strength composites and protective coatings, while also being applied in adhesive formulations to ensure durable joining of different materials. In the aerospace sector, polyester binders are employed in the production of lightweight yet robust structural materials that are capable of withstanding extreme operating conditions, including elevated temperatures and mechanical stress. Their role is also highly significant in construction, where they are used as polymer binders for concretes and mixtures in order to enhance mechanical strength and environmental resistance. Furthermore, polyester-based systems are involved in the fabrication of advanced insulation materials, providing protection against high temperatures and corrosion in both the construction and electrical industries. While the curing of unsaturated polyesters with styrene and methyl methacrylate has been well studied, the use of unsaturated carboxylic acids as curing agents has not been reported. This work investigates the physicochemical properties of polyethylene glycol maleate (p-EGM) solutions in acrylic acid (AA) and their cured products, demonstrating the potential of the p-EGM–AA system as a binder for filled materials.

Polyethylene glycol maleate was synthesized by polycondensation of ethylene glycol and maleic anhydride in the presence of ZnCl₂ catalyst at 150–160 °C, yielding 98% product with an average molecular weight of ~1232 Da and unsaturation degree of 89% (bromide–bromate method). Solutions of p-EGM in AA at different mass ratios (60:40, 70:30, 80:20) were prepared, and viscosity and density were measured. Curing was performed at 293 K using a cold-initiating system based on benzoyl peroxide and dimethylaniline, followed by purification with dioxane. The composition of the copolymers was determined by high-performance liquid chromatography (HPLC), yields were measured gravimetrically, and functional group identification was carried out using IR and NMR spectroscopy[5,6]. Volumetric shrinkage was calculated from density data, and surface morphology of cured products was examined by SEM at nanometer resolution.

Solutions of p-EGM in acrylic acid were prepared in different mass ratios (60:40, 70:30, and 80:20). These mixtures were analyzed for viscosity and density, both of which increased systematically with higher polyester content: at 60% p-EGM, viscosity and density were the lowest (294.3 mPa·s; 1.2069 g/cm³), while at 80% p-EGM they reached maximum values (736.8 mPa·s; 1.2412 g/cm³). Such tunability of viscosity and density is of practical significance, since it allows tailoring of resin formulations for specific technological processes, including casting and molding of bulk products. The copolymers obtained were insoluble, indicating the formation of a crosslinked three-dimensional network. HPLC showed that the composition of the cured products closely corresponded to the initial ratios of p-EGM and AA, and the yield exceeded 89%, which demonstrates the efficiency of the curing process. Moreover, the degree of unsaturation of the copolymers was directly proportional to the content of p-EGM, ranging from 37.7% to 54.8%, confirming that polyester concentration has a defining role in the structural characteristics of the resulting networks. The chemical structure of the cured products was investigated using IR and NMR spectroscopy. IR spectra confirmed the presence of residual double bonds and carboxyl groups, while NMR analysis indicated a predominance of cis-isomerism in the polymer backbone. These findings prove that the cured copolymers retain a certain level of unsaturation, which may influence their reactivity and mechanical performance. Importantly, the degree of retained unsaturation provides a potential for further chemical modification of the cured materials.

The technological properties of the resulting polymers were also thoroughly evaluated. One of the key findings was the low volumetric shrinkage of the cured systems, which did not exceed 10%. For pure p-EGM, shrinkage was as low as 4.4%. This is an important characteristic for practical applications, since low shrinkage prevents the formation of internal stresses, cracks, and warping in bulk polymer products. The swelling degree was also strongly dependent on the p-EGM/AA ratio: it decreased significantly as polyester content increased, from 144.6% in AA-rich copolymers to only 12.8% for pure p-EGM.

Surface morphology studies showed that acrylic acid increases porosity and water absorption, while higher p-EGM content yields denser, less porous structures with improved hydrophobicity. These differences determine application: porous materials suit filtration and sorption, whereas dense copolymers are preferable for construction and protective coatings. The analysis of the obtained results concerning the physicochemical properties of polyethylene glycol maleate (p-EGM) solutions in acrylic acid (AA) of various compositions, as well as the properties of their cured copolymers, clearly demonstrates the potential of this binary system for practical applications in the production of filler products. What is especially noteworthy is that the proposed system requires only a minimal amount of mineral bulk fillers, which makes it both technologically and economically advantageous. The study revealed that the viscosity and density of the solutions increase with the concentration of p-EGM, indicating high sensitivity of their structure to polyester content. This allows tuning the rheological behavior to technological needs. Cured copolymers from solutions with higher p-EGM content show greater density and unsaturation, enhancing their stability and durability.

The analysis of volumetric shrinkage and swelling revealed that higher p-EGM content reduces water sorption, with the copolymers overall showing low moisture uptake and hydrophobic character, which enhances their resistance to environmental factors and durability. It is also worth emphasizing that in the studied binary system acrylic acid plays a dual role — serving both as a solvent and as a curing agent. Owing to this, the resulting cured products show relatively low values of volumetric shrinkage, not exceeding 10%. Such a parameter is highly desirable for industrial applications, since excessive shrinkage often leads to the formation of internal stresses, microcracks, and dimensional instability of the material. The obtained experimental values can be regarded as key indicators when evaluating the suitability of polymer solutions for the production of filler and construction materials. Among the most promising applications is the production of polymer tiles and other bulk construction products, where both mechanical reliability and chemical resistance are crucial. Based on the conducted studies, the most preferable composition, providing the optimal balance of physical and chemical properties (including high density, low swelling, and pronounced hydrophobicity), corresponds to the p-EGM–AA solution with a mass ratio of approximately 80:20. This composition, due to its improved set of characteristics, can be considered a promising candidate for use as a thermosetting polyester binder in the manufacture of durable filler products with reduced dependence on mineral additives, which opens the way for its application in modern construction and related industries.