Lecturer Erdi Buluş
Istanbul Arel University ArelPOTKAM
erdibulus@arel.edu.tr
1. INTRODUCTION
Adhesives are substances that enable two or more surfaces to be bonded together, and this function is carried out through chemical or physical bonds that occur at the molecular level between the surfaces [1-3]. Historically, biologically sourced adhesives such as plant glues, animal glues and natural resins have been used, and with the development of modern industry, synthetic polymer-based adhesives have come to the fore [4,5]. Today, adhesives play a critical role in many sectors such as automotive, construction, electronics, medicine and packaging [6]. They can be classified according to their chemical structures and curing (hardening) mechanisms. While physical adhesives generally form a bond by solidifying as a result of solvent evaporation or temperature decrease, chemical adhesives cure through chemical reactions such as polymerization, polycondensation or polyaddition. These two basic mechanisms are important determinants in terms of the adhesive's area of use, strength requirement and suitability for environmental conditions [7,8].
The performance of adhesives is explained by the concepts of adhesion and cohesion. Adhesion refers to the contact force of the adhesive with the surface; cohesion describes the ability of the adhesive to maintain its internal integrity. Factors such as surface energy, roughness and cleanability directly affect the effectiveness of adhesion. In addition, appropriate surface preparation is critical for a successful bonding process, especially when working with materials with low surface energy such as metal, plastic and composites. Adhesives used today have different physical and chemical properties depending on their intended use [9,10].
Epoxy While adhesives based on epoxy resin have high mechanical strength and chemical resistance, polyurethane-based adhesives are preferred in terms of flexibility and impact resistance [11]. Acrylic adhesives are widely used in industrial assemblies due to their fast curing times and wide surface compatibility [12]. Silicone based adhesives stand out in sealing applications with their excellent temperature resistance and elasticity [13,14]. Environmental impacts are also taken into account in the development process of adhesives. Sustainability-oriented innovations such as the reduction of volatile organic compound (VOC) emissions, the use of biodegradable polymers and solvent -free formulations are among the main trends of the modern adhesive industry. In addition, the use of nanotechnological approaches in adhesive systems is aimed at developing stronger, lighter and multifunctional products [15].
Adhesive formulations do not only consist of the main binder polymers; they contain various additives to optimize the performance of the product, improve its processability and suitability for usage conditions [16,17]. These additives offer the opportunity to customize the physical, chemical and mechanical properties of the adhesive according to the targeted application. One of the most commonly used additives is fillers. Fillers are added to the formulation to increase the viscosity of the adhesive, reduce costs and improve the mechanical strength of the final product . Inorganic particles such as talc, calcium carbonate, silica and alumina are particularly effective in imparting hardness and wear resistance [18,19].
Plasticizers are used to give the adhesive flexibility and prevent brittleness at low temperatures. These additives increase the mobility between polymer chains, improving the impact resistance and extensibility of the adhesive . Phthalates , sebacates and small amounts of biobased Plasticizers are included in the formulations for this purpose [20,21]. Solvents and carrier liquids are used to facilitate the applicability of the adhesive and to ensure homogeneous spreading on the surface. Water, acetate derivatives, aromatic hydrocarbons and alcohols are widely preferred in solvent-based adhesive systems. However, the use of low-volatility solvents is increasing day by day due to environmental concerns [22].
Accelerators and retarders are added to the formulation in order to control the curing times. These additives increase the speed of the polymerization reaction and provide fast adhesion or reduce the reaction rate to offer processing flexibility in production processes [23, 24]. Such additives are of great importance especially in epoxy and polyurethane-based adhesive systems. Improvers (adhesion promoters ) increase the adhesive's ability to adhere to the substrate surface. Silane based compounds significantly improve the bond strength by forming chemical bonds with the surface on materials with low surface energy such as metal, glass and plastic [24,25]. Stabilizers and antioxidants are used to extend the shelf life of the adhesive and increase its resistance to environmental factors. These additives help protect polymers from degrading agents such as ultraviolet rays, heat and oxygen. In this way, the adhesive maintains its properties over time [25,26]. Thixotropic agents and rheology modifiers are added to the formulation to control the adhesive's flow behavior. Such additives prevent undesirable effects such as dripping or spreading during application, allowing the product to be applied in a more controlled and uniform manner [25-27]. Colorants and pigments are added to adhesives for both aesthetic purposes and to facilitate visual monitoring during production and quality control processes. In some cases, a color change can also be used as a functional indicator to show that the curing process is complete [26,27].
adhesive formulations, fillers play a critical role in both cost optimization and product performance improvement. In this context, inorganic compounds such as talc, calcium carbonate, silica and alumina are widely used to increase the mechanical properties, workability and durability of adhesives. These additives provide various advantages in different types of adhesives thanks to their physical and chemical properties [28].
Talc (Mg₃Si₄O₁₀(OH)₂) is a mineral known for its softness and laminar structure. The use of talc in adhesive systems provides significant contributions in terms of viscosity control, ease of application and surface smoothness of the final product. The low hardness of talc increases the elasticity of adhesives while also improving the workability of the product. In addition, talc reduces water vapor and gas permeability by creating a good barrier effect; thanks to this feature, it is especially preferred in applications with moisture sensitivity [29].
Calcium carbonate (CaCO₃) is a widely available, economical and effective filler. The use of calcium carbonate in adhesive formulations offers the opportunity to improve the mechanical strength of the product without increasing its density. It is preferred especially for low-cost adhesives to improve properties such as hardness, abrasion resistance and surface stability. In addition, calcium carbonate supports the formation of a homogeneous film during the application process by adjusting the viscosity [30].
Silica (SiO₂) is an additive that stands out with its high surface area and chemical inertness . Silica acts both as a structural strengthening agent and as a rheological modifier in adhesives. In particular, the amorphous form of silica increases thixotropic behavior in low- viscosity systems , preventing dripping and sagging during application. In addition, the chemical resistance of silica increases the resistance of the adhesive to environmental factors such as temperature, chemicals and UV radiation [31].
Alumina (Al₂O₃) is a ceramic material that stands out with its high hardness and thermal resistance properties. The use of alumina in adhesives is preferred to increase mechanical strength, especially at high temperatures and harsh environmental conditions. Thanks to its thermal conductivity properties, alumina is also used as an important additive in electronic adhesive systems where heat distribution is critical. In addition, since it has a low abrasive effect, it minimizes equipment wear during application [32]. These additives not only increase the mechanical performance of adhesives; they also allow the development of features such as ease of application, environmental resistance and long-term use. Each filler is formulated according to the target application and desired performance criteria in line with its unique physical and chemical properties. Therefore, the selection of additives requires a strategic engineering process according to the area of use of the adhesive and the expected functionality [33].
In order to achieve high performance and long life in adhesive technologies, it is critical that all components in the formulation are homogeneously distributed and stable [34]. The dispersion of additives, especially small particle size fillers and additives, without agglomeration within the adhesive matrix is decisive in terms of the mechanical strength, application quality and aesthetic appearance of the product. In this context, surface active agents (surfactants) play a fundamental role in ensuring the effective integration of additives into the adhesive system [35].
Tween 80 (Polysorbate 80) and Tween 20 (Polysorbate 20) are widely used nonionic surfactants. Thanks to their high hydrophilic-lipophilic balance (HLB) values, they prevent agglomeration of filler particles by providing effective dispersion, especially in water -based adhesive systems. Tween series surfactants reduce the surface tension between different phases in the adhesive, supporting the achievement of a homogeneous mixture. In addition, due to their low toxicity and chemical stability, they are frequently preferred in adhesive applications requiring both industrial and biological compatibility [36,37].
Span 60 (Sorbitan monostearate) and Span 20 (Sorbitan monolaurate) are lipophilic with lower HLB values These substances are surfactants , especially those containing organic phase or solvents. In adhesives based on additives, it prevents coagulation and sedimentation by forming a protective layer on the surfaces of the additives. Surfactants create a physical barrier on the particles, increasing both physical stability and improving workability by keeping the viscosity of the adhesive at the desired level [38].
Sodium dodecyl sulfate (SDS) is a powerful anionic surfactant, which is widely used in adhesives, especially in emulsion- based adhesives. SDS creates electrostatic repulsion between particles by forming negatively charged surface groups, thus effectively preventing agglomeration [39]. In addition, it supports better wetting and adhesion of the adhesive to the substrate surfaces due to its low surface tension. However, the use of SDS should be carefully formulated with foam control agents, as it can cause foaming in the system. In addition, other surfactants such as lecithin, pluronics (PEO-PPO-PEO block copolymers) and sorbitan esters are also used in adhesive formulations according to specific application needs . These substances are preferred to increase the stability of additives, especially in systems where thermal and mechanical stability is desired [40].
With the contribution of surfactants, filler and additive particles are distributed uniformly within the adhesive matrix, and negative effects such as sedimentation and agglomeration are minimized. Thus, both the ease of application and the mechanical and optical properties of the adhesive after curing are optimized [41]. In addition, the correct surfactant selection plays a critical role in the long-term storage stability and service life of the adhesive. Surfactants are an indispensable component group in ensuring the homogeneous distribution of additive particles and their compatibility with the adhesive matrix [42-44]. The selections made by considering the chemical nature, HLB value, ionic character and compatibility with the application environment of each surfactant are decisive in the final product performance. Therefore, surfactant selection should be considered as a fundamental part of formulation engineering in adhesive technologies [43-45].
In this study, calcium carbonate was added to polyurethane (PU) polymer as an additive at different rates and the agglomeration prevention performance was investigated with the addition of different surfactants. Within the scope of our study, the increasing problems in the adhesive sector will be eliminated and product quality and performance will be increased.
2. MATERIAL AND METHODS
Materiel
In our study, polyurethane (PU) polymer, dimethylformamide (DMF), calcium carbonate, tween 20, tween 80, span 60, span 20 and sodium dodecyl Sulphate materials were used from Istanbul Arel University Polymer Technologies and Composite Application and Research Center (ArelPOTKAM).
Method
10 grams of PU polymer was weighed on a precision scale and added to 100 ml of DMF solvent. It was homogeneously dissolved at 70 oC for 45 minutes. 10 ml of PU solution system was added to each beaker. 1% calcium carbonate by weight was added to the beakers containing 10 ml of PU solution, respectively. Again, tween 20, tween 80, span 60, span 20 and sodium dodecyl sulphate substances were added as 0.5 ml in liquid form and 0.5% by weight in solid form. The calcium carbonate particle diameters used were 80-150 micrometers thick. These mixtures were mixed at 50 oC for 30 minutes to ensure a homogeneous mixture .
3. RESULTS AND DISCUSSION
In terms of PU-Calcium carbonate mixture compatibility, agglomeration (clumping) occurred in the use of tween 20, tween 80, span 60 and span 20 surfactants , while particles rose to the surface and prevented homogeneous mixing. However, sodium dodecyl sulfate acted as the best surfactant . In the PU-Calcium carbonate mixture, calcium carbonate was homogeneously distributed in PU and no leaching occurred on the surfaces. In the use of nonionic surfactants such as T ween 20, Tween 80, Span 60 and Span 20, it was revealed that the expected homogeneous dispersion could not be achieved. In the presence of these surfactants, it was observed that calcium carbonate particles tended to agglomerate over time and rose to the surface, creating phase separation from the mixture.
As a result, the homogeneity in the PU matrix was disrupted, irregularities in particle densities occurred throughout the mixture and basic performance parameters such as adhesion were negatively affected. Nonionic The low surface charge of surfactants and their inability to establish sufficient interaction with the filler surface support this observation. In contrast, the use of sodium dodecyl sulfate (SDS) exhibited a significantly superior dispersion performance in the PU-calcium carbonate system. Thanks to its anionic character, SDS created negative surface charges on the calcium carbonate particles , creating strong electrostatic repulsion forces between the particles and effectively preventing agglomeration . In this way, calcium carbonate particles were homogeneously distributed in the PU matrix, and no phase separation or particle ejection to the surface was observed. In addition, the surface tension-reducing capacity of SDS allowed the filler to exhibit better wetting and physical bonding with the polymer matrix. This positively affected the application performance of the adhesive and the mechanical strength after curing [46,47].
4. CONCLUSIONS
In this study, it was shown that the use of additives in adhesive systems and the homogeneous distribution of these additives have a direct effect on the performance of the product. In particular, the uniform distribution of fillers and additives in the adhesive matrix plays a critical role in improving basic properties such as mechanical strength, bonding strength, workability and long-term stability. However, the natural agglomeration tendencies of additive particles make homogeneous distribution difficult, which can lead to undesirable differences in material performance. The use of surfactants stands out as an important strategy to ensure homogeneous distribution. In this context, surfactant selection should be made by considering the surface properties of the filler, the chemical structure of the adhesive matrix and the processing conditions. Experimental observations have shown that nonionic surfactants may be insufficient in terms of dispersion adequacy in some systems, while anionic surfactants, especially sodium dodecyl sulfate, provide superior dispersion performance.
The effectiveness of additive use in adhesives does not only depend on the type of additive, but also directly depends on how this additive is distributed in the system. By ensuring homogeneous distribution of additive particles with optimum surfactant selection, the performance of adhesive systems can be significantly increased. Therefore, it is of great importance to comprehensively analyze dispersion behaviors and evaluate system compatibility in detail when developing additive formulations. In PU-calcium carbonate blend systems, the choice of surfactant is a direct determining factor on the dispersion quality and therefore the final properties of the adhesive.
Although nonionic surfactants may offer advantages in certain applications, in systems where filler-matrix compatibility is critical, as in this study, the use of sodium dodecyl sulfate, an anionic surfactant, provides superior results. The homogeneous dispersion provided by SDS increases both the consistency in material performance and offers significant advantages in terms of long-term stability. Therefore, in the selection of surfactants, the filler surface properties, the interaction capacity with the polymer matrix and the dispersion dynamics in the system should be carefully evaluated.
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