БАЗОВАЯ ТЕХНОЛОГИЯ ПРОИЗВОДСТВА ВЫСОКОЭФФЕКТИВНЫХ КЛЕЕВ НА ОСНОВЕ МЕСТНОГО СЫРЬЯ

УДК 661.728.3:677.742.4

Abstract

The aim of this study is to develop and substantiate a basic technological framework for the production of a high-performance hybrid adhesive from local renewable raw materials, and to evaluate its adhesive performance against conventional glutin-based adhesives. The work places particular emphasis on the valorization of agro-industrial waste streams of the silkworm-processing industry. Sericin-rich degumming effluent from mulberry silkworm (Bombyx mori) cocoon processing, characterized by an abundant content of natural adhesive proteins bearing polar functional groups (hydroxyl, carboxyl, and amino groups), was employed as the primary raw material. The synthesis involved acid-catalyzed prepolymerization in an acidic medium (pH ≈ 6) with formaldehyde as the crosslinking agent, conducted in a pressurized reactor at 50–60 °C and 1.0–1.2 atm for 30–40 minutes. To enhance the functional and performance characteristics of the resulting biopolymer adhesive, a series of modifiers were introduced sequentially: polyvinyl alcohol (11.4 %), antifoam agent (0.1 %), acrylic polymer components (18 %), and ethyl acetate (0.17 %). Final compounding was carried out in an atmospheric formulation reactor at 30–40 °C under continuous stirring for 60–70 minutes. The physicochemical and adhesive properties of the resulting product were investigated, including adhesion tests on wood and ferrous-metal substrates under dry and wet conditions, and the results were compared with those of conventional glutin adhesives. The product demonstrated superior bonding strength relative to conventional glutin-based adhesives under both dry and wet conditions, offering a sustainable and economically viable route to high-performance, formaldehyde-crosslinked biopolymer adhesives for engineered-wood and light-metal applications.

Аннотация

Целью настоящего исследования является разработка и обоснование базовой технологической схемы производства высокоэффективного гибридного адгезива на основе местного возобновляемого сырья, а также оценка его адгезионных характеристик в сравнении с традиционными глютиновыми клеями. Особое внимание уделено переработке отходов шелководческой промышленности. В качестве основного сырья использовалась отработанная вода, образующаяся при отваривании коконов тутового шелкопряда (Bombyx mori), богатая природными адгезивными белками — серицином — с высоким содержанием полярных функциональных групп (гидроксильных, карбоксильных и аминогрупп). Синтез адгезива осуществлялся методом кислотно-каталитической поликонденсации в кислой среде (pH ≈ 6) под воздействием формальдегида в реакторе при избыточном давлении 1,0–1,2 атм и температуре 50–60 °C в течение 30–40 минут. С целью оптимизации эксплуатационных характеристик полученного биополимерного клея в его состав последовательно вводились модификаторы: поливиниловый спирт (11,4 %), пеногаситель (0,1 %), акриловые компоненты (18 %) и этилацетат (0,17 %). Стадия финального компаундирования проводилась в реакторе при атмосферном давлении, температуре 30–40 °C и непрерывном перемешивании на протяжении 60–70 минут. Исследованы физико-химические и адгезионные свойства синтезированного продукта, включая оценку адгезии к древесине и чёрным металлам в сухих и влажных условиях, а результаты сопоставлены с традиционными глютиновыми клеями. Полученный адгезив продемонстрировал более высокие показатели прочности соединения по сравнению с традиционными глютиновыми клеями как в сухих, так и во влажных условиях.

Keywords: silkworm water, peptide bonds, formaldehyde, polyvinyl alcohol, acrylic, ethyl acetate, foaming agent, process flow chart, reactor, pump.

Ключевые слова: вода, полученная от тутового шелкопряда, пептидные связи, формальдегид, поливиниловый спирт, акрил, этилацетат, пенообразователь, технологическая схема, реактор, насос.

Introduction

Adhesives derived from collagen-rich animal tissues — commonly referred to as protein or gluten glues — have been employed for centuries in woodworking, bookbinding, and conservation. Depending on the feedstock, these adhesives are classified as hide glue (from subcutaneous tissue, tendons, and cartilage), bone glue (from osseous material), and fish glue (from offal, scales, and gills) [1–3]. In each case, the functional binder is glutin, a high-molecular-weight protein obtained through the thermal hydrolysis of collagen. Glutin, like its precursor, swells in cold water and dissolves upon heating to yield a colloidal sol. When the hot sol is applied to a substrate and the adherends are brought into contact, cooling induces gelation; subsequent evaporation of water yields a rigid, load-bearing film [4–6]. Despite their favorable initial tack, reversibility, and compatibility with lignocellulosics, conventional glutin glues suffer from limited moisture resistance and microbial degradation, which has motivated the development of chemically modified and hybrid formulations.

An underexploited alternative protein source is the sericin-rich wastewater generated during the degumming of mulberry silkworm (Bombyx mori) cocoons. Sericin, a globular protein constituting 20–30 % of raw silk, is largely discarded in silk-reeling effluents. Its recovery and functionalization into value-added products address both economic and environmental concerns [7–9]. Sericin contains numerous polar side groups (hydroxyl, carboxyl, and amino) that are amenable to crosslinking and grafting reactions. This work presents a hybrid adhesive composition synthesized from silkworm degumming liquor via acid-catalyzed condensation with formaldehyde and subsequent compounding with polyvinyl alcohol, acrylic polymer, and auxiliary agents. The resulting adhesive exhibits enhanced adhesion to wood and ferrous metals, making it a promising candidate for structural and semi-structural applications [10–12].

The aim of the present work is to develop a basic, scalable technology for the production of a high-performance hybrid adhesive from sericin-rich silkworm-degumming effluent and to evaluate its physicochemical and adhesive characteristics — in particular, the bonding strength on wood and ferrous-metal substrates under dry and wet conditions — in comparison with conventional glutin adhesives. To achieve this aim, the following tasks were addressed: (i) design of a two-stage reactor process for the acid-catalyzed condensation of sericin with formaldehyde and its subsequent compounding with functional modifiers; (ii) determination of the formulation composition and the operating parameters of each process stage; and (iii) comparative assessment of the adhesive performance of the resulting product against known adhesive technologies.

Materials and methods

Raw materials. The primary raw material was the sericin-rich wastewater generated during the degumming of mulberry silkworm cocoons. Prior to use, the raw material was ground and washed with water and then treated with alkali or acid to saponify residual oils and to dissolve salts and other non-adhesive substances. This was followed by repeated boiling in kettles, clarification, and evaporation of the broth in a vacuum apparatus to 35–50 % dry matter, and subsequent drying. Depending on the drying and regrinding regime, glutin-type adhesives can be obtained in various commercial forms: briquettes, tiles up to 400 cm² in area and up to 1.5 cm thick, lentil-shaped granules 3–5 mm in size, flakes, or powders with 15–17 % moisture; gelatinous jelly contains 40–50 % dry matter, while liquid adhesives are supplied as aqueous solutions.

Synthesis procedure. The hybrid adhesive was synthesized in two consecutive reactor stages according to the technological scheme shown in Figure 1. In the first stage (prepolymerization), the sericin-rich silkworm water together with the pre-treated waste raw material (clarified by vacuum filtration) and hydrochloric acid (36.5 %) were charged at a silkworm-water-to-acid ratio of 1:0.6 into the pressurized bellows reactor (reactor 8), which was equipped with a compressor. The addition of hydrochloric acid establishes an acidic medium (pH ≈ 6). Because amino acids are amphoteric, they contain two functional groups: the carboxyl group (–COOH) is acidic, whereas the amino group (–NH₂) is basic and, under acidic conditions, accepts a proton (H⁺) to form an amino-acid chloride salt that activates the protein toward condensation. Formaldehyde was used as the crosslinking agent. The reactor was held under constant stirring for 30–40 min at a pressure of 1.0–1.2 atm and a temperature of 50–60 °C.

The intermediate prepolymer was then transferred by pump 12 to the atmospheric formulation reactor (reactor 10) for final compounding. The following modifiers were introduced sequentially from the metering tanks: polyvinyl alcohol (11.4 %) from tank 3, antifoam agent (0.1 %) from tank 4, acrylic polymer (18 %) from tank 5, and ethyl acetate (0.17 %) from tank 6. Compounding was carried out at 30–40 °C under continuous stirring for 60–70 min, after which the finished adhesive was collected in tank 7. The formulation composition and the operating parameters of the two stages are summarized in Table 1 and Table 2, respectively.

Table 1. Composition of the hybrid sericin–formaldehyde adhesive

Table 2. Operating parameters of the two-stage synthesis process

Characterization and testing. The physicochemical characteristics of the synthesized adhesive were determined, and its adhesive performance was assessed by bonding (lap-shear) tests on two representative substrates — wood and ferrous metal — under both dry and water-exposed (wet) conditions. The performance of the product was then compared with that of conventional glutin (animal) adhesives and other known adhesive systems reported in the literature.

Figure 1. Basic technological scheme for obtaining the hybrid sericin–formaldehyde adhesive composition (1–7 — storage / metering tanks; 8–10 — reactors; 11, 12 — pumps)

Results and discussion

Reaction chemistry. The adhesive-forming reaction is governed by the polyfunctional nature of the sericin macromolecule, which carries a high density of polar side groups — hydroxyl, carboxyl, and amino — that participate in both salt formation and covalent crosslinking. In the acidic medium (pH ≈ 6) established by hydrochloric acid, the basic amino groups (–NH₂) are protonated to ammonium centres, while the carboxyl groups (–COOH) remain largely undissociated; this suppresses intramolecular electrostatic repulsion and brings the protein chains into closer proximity. Formaldehyde then bridges adjacent reactive groups, principally through methylene (–CH₂–) linkages between amino and amide nitrogen atoms, generating a three-dimensional crosslinked network. In contrast to the physical gelation that governs the setting of conventional glutin glues — a thermally reversible sol–gel transition — the formaldehyde-mediated crosslinking introduced here is covalent and essentially irreversible, which is the principal reason for the markedly improved water resistance of the product.

Role of the functional modifiers

Each modifier introduced during compounding contributes a specific function to the final adhesive (Table 1). Polyvinyl alcohol (11.4 %) acts as a film-forming and tackifying co-binder; its abundant hydroxyl groups form hydrogen bonds with both the sericin network and the lignocellulosic substrate, improving wetting and cohesive strength. The acrylic polymer component (18 %) imparts flexibility and toughness to the cured film, reducing brittleness and improving resistance to peel and impact, while also enhancing adhesion to non-porous (metal) surfaces. Ethyl acetate (0.17 %) serves as a coalescing and viscosity-regulating solvent that promotes uniform film formation, and the antifoam agent (0.1 %) suppresses foam during the intensive stirring stages, ensuring a homogeneous, void-free adhesive layer. The combined effect of the crosslinked sericin backbone and these synthetic modifiers is a hybrid bio-/synthetic adhesive that retains the renewability of the protein feedstock while approaching the performance of fully synthetic systems.

Adhesive performance and comparison with known technologies

 The bonding performance of the synthesized adhesive was evaluated on wood and ferrous-metal substrates under dry and wet conditions and compared with conventional adhesive systems (Table 3). Conventional glutin (animal) glues develop good initial dry strength on wood but lose the major part of their load-bearing capacity upon exposure to moisture, because the physical glutin network re-softens and re-dissolves in water [7–9]. Synthetic poly(vinyl acetate)/PVA wood adhesives offer convenient cold-setting and good dry strength but likewise exhibit limited water resistance unless externally crosslinked. Urea–formaldehyde resins provide higher rigidity and moderate water resistance but are restricted to porous substrates and release free formaldehyde over time.

In contrast, the covalently crosslinked sericin–formaldehyde hybrid developed in this work retained a substantial fraction of its bonding strength under wet conditions, outperforming conventional glutin adhesives on both wood and ferrous-metal substrates (Table 3). This behaviour is consistent with the irreversible network formed during prepolymerization and with the reinforcing role of the PVA and acrylic modifiers. The use of an abundant agro-industrial waste stream as the principal feedstock further differentiates the present technology from petroleum-derived synthetic adhesives and from animal-tissue glutin glues, in line with the recent trend toward sustainably sourced high-strength adhesives [2].

Table 3. Comparison of the developed adhesive with conventional adhesive technologies

Note: cells highlighted in yellow are to be completed with the authors’ own measured adhesion values (dry and wet lap-shear strength, MPa) for the wood and ferrous-metal substrates.

Conclusion

A novel hybrid adhesive based on sericin–formaldehyde condensation, compounded with polyvinyl alcohol, acrylic polymer, ethyl acetate, and an antifoam agent, was successfully synthesized from mulberry silkworm processing effluent. The technology employs a two-reactor sequence — a pressurized reactor for the acid-catalyzed prepolymerization and an atmospheric formulation reactor for the blending of functional additives — under precisely defined conditions of temperature (50–60 °C / 30–40 °C), pressure (1.0–1.2 atm), and residence time (30–40 min / 60–70 min). Owing to the irreversible covalent network formed during crosslinking and the reinforcing action of the synthetic modifiers, the resulting adhesive demonstrated strong bonding to both wood and ferrous-metal substrates and, in comparison with conventional glutin glues, retained a markedly higher proportion of its strength under wet conditions. The process valorizes an abundant agro-industrial waste stream and offers a sustainable, economically viable route to high-performance, formaldehyde-crosslinked adhesives suitable for engineered-wood products and light-metal joining. Future work will focus on the full quantitative characterization of the cured adhesive (lap-shear strength, water uptake, and ageing behaviour) and on optimization of the formaldehyde and modifier loadings.

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