Isophorone Diisocyanate (IPDI) Microencapsulation for Mono-Component Adhesives: Effect of the Active H and NCO Sources

A Study of Hydroxyl-Terminated Block Copolyether-Based Binder Curing Kinetics

In order to determine the curing reaction model and corresponding parameters of hydroxyl-terminated block copolyether (HTPE) and provide a theoretical reference for its practical application, the non-isothermal differential scanning calorimetry (DSC) method was used to analyze the curing processes of three curing systems with HTPE and N-100 (an aliphatic polyisocyanate curing agent), isophorone diisocyanate (IPDI), and a mixture of N-100 and IPDI as curing agents. The results show that the curing activation energy of N-100 and HTPE was about 69.37 kJ/mol, slightly lower than the curing activation energy of IPDI and HTPE (75.60 kJ/mol), and the curing activation energy of the mixed curing agent and HTPE was 69.79 kJ/mol. The curing process of HTPE conformed to the autocatalytic reaction model. The non-catalytic reaction order (n) of N-100 and HTPE was about 1.2, and the autocatalytic order (m) was about 0.3, both lower than those of IPDI and HTPE. The reaction kinetics parameters of the N-100 and IPDI mixed curing agent with HTPE were close to those of N-100 and HTPE. The verification results indicate a high degree of overlap between the experimental data and the calculated data.

Microcapsules of Poly(butylene adipate-co-terephthalate) (PBAT) Loaded with Aliphatic Isocyanates for Adhesive Applications

This work introduces the encapsulation of hexamethylene diisocyanate derivatives (HDI, TriHDI, and PHDI) with the biodegradable polymer poly(butylene adipate-co-terephthalate) (PBAT) through a solvent evaporation method. These microcapsules (MCs) were then employed in adhesive formulations for footwear. Moreover, MCs containing PHDI were produced in a closed vessel, demonstrating the potential for recovering and reusing organic solvents for the first time. The MCs were achieved with an isocyanate payload reaching up to 68 wt %, displaying a spherical shape, a core-shell structure, and thin walls without holes or cracks. The application of MCs as cross-linking agents for adhesives was evaluated following industry standards. The adhesives' strength surpassed the minimum requirement by a significant margin. Creep tests demonstrated that the formulation with MCs exhibits superior thermostability. Furthermore, the formulation with MCs-PHDI presented the best results reported to date for this type of system, as no displacement was observed in the bonded substrates. Environmental assessment indicates that adhesives with MCs have higher global warming potential (+16.2%) and energy consumption (+10.8%) than the standard commercial adhesives, but under alternative realistic scenarios, the differences can be insignificant. Therefore, adhesive formulations incorporating MCs promise to be on par with traditional adhesive systems regarding environmental impacts while providing benefits such as improved and safe handling of isocyanates and excellent bonding effectiveness.

Design of Experiment for Optimizing Microencapsulation by the Solvent Evaporation Technique

We employed microemulsion combined with the solvent evaporation technique to produce biodegradable polycaprolactone (PCL) MCs, containing encapsulated isophorone diisocyanate (IPDI), to act as crosslinkers in high-performance adhesive formulations. The MC production process was optimized by applying a design of experiment (DoE) statistical approach, aimed at decreasing the MCs' average size. For that, three different factors were considered, namely the concentration of two emulsifiers, polyvinyl alcohol (PVA) and gum arabic (GA); and the oil-to-water phase ratio of the emulsion. The significance of each factor was evaluated, and a predictive model was developed. We were able to decrease the average MC size from 326 μm to 70 µm, maintaining a high encapsulation yield of approximately 60% of the MCs' weight, and a very satisfactory shelf life. The MCs' average size optimization enabled us to obtain an improved distributive and dispersive mixture of isocyanate-loaded MCs at the adhesive bond. The MCs' suitability as crosslinkers for footwear adhesives was assessed following industry standards. Peel tests revealed peel strength values above the minimum required for casual footwear, while the creep test results indicated an effective crosslinking of the adhesive. These results confirm the ability of the MCs to release IPDI during the adhesion process and act as crosslinkers for new adhesive formulations.

Emulsion Stabilization Strategies for Tailored Isocyanate Microcapsules

We report on the stabilization of an oil-in-water (O/W) emulsion to, combined with interfacial polymerization, produce core-shell polyurea microcapsules (MCs) containing isophorone diisocyanate (IPDI). These will act as crosslinkers for mono-component adhesives. The emulsion stabilization was evaluated using three types of stabilizers, a polysaccharide (gum arabic) emulsifier, a silicone surfactant (Dabco^®DC193), a rheology modifier (polyvinyl alcohol), and their combinations. Emulsion sedimentation studies, optical microscopy observation, and scanning electron microscopy enabled us to assess the emulsions stability and droplet size distribution and correlate them to the MCs morphology. Fourier transform infrared spectroscopy and thermogravimetric analysis revealed the MCs composition and enabled us to evaluate the encapsulation yield. All stabilizers, except DC193, led to spherical, loose, and core-shelled MCs. The rheology modifier, which increases the continuous phase viscosity, reduces the emulsion droplets sedimentation, keeping their size constant during the MCs' synthesis. This allowed us to obtain good quality MCs, with a smaller average diameter, of approximately 40.9 µm mode, a narrower size distribution and 46 wt% of encapsulated IPDI. We show the importance of the emulsion stability to tune the MCs morphology, size, and size distribution, which are critical for improved homogeneity and performance when used, e.g., in natural and synthetic adhesive formulations industry.

Hydroxyl-Decorated Diiron Complex as a [FeFe]-Hydrogenase Active Site Model Complex: Light-Driven Photocatalytic Activity and Heterogenization on Ethylene-Bridged Periodic Mesoporous Organosilica

A biomimetic model complex of the [FeFe]-hydrogenase active site (FeFeOH) with an ethylene bridge and a pendant hydroxyl group has been synthesized, characterized and evaluated as catalyst for the light-driven hydrogen production. The interaction of the hydroxyl group present in the complex with 3-isocyanopropyltriethoxysilane provided a carbamate triethoxysilane bearing a diiron dithiolate complex (NCOFeFe), thus becoming a potentially promising candidate for anchoring on heterogeneous supports. As a proof of concept, the NCOFeFe precursor was anchored by a grafting procedure into a periodic mesoporous organosilica with ethane bridges (EthanePMO@NCOFeFe). Both molecular and heterogenized complexes were tested as catalysts for light-driven hydrogen generation in aqueous solutions. The photocatalytic conditions were optimized for the homogenous complex by varying the reaction time, pH, amount of the catalyst or photosensitizer, photon flux, and the type of light source (light-emitting diode (LED) and Xe lamp). It was shown that the molecular FeFeOH diiron complex achieved a decent turnover number (TON) of 70 after 6 h, while NCOFeFe and EthanePMO@NCOFeFe had slightly lower activities showing TONs of 37 and 5 at 6 h, respectively.

Preparation and Self-Healing Application of Isocyanate Prepolymer Microcapsules

In this study, we successfully manufactured polyurethane microcapsules containing isocyanate prepolymer as a core material for self-healing protection coatings via interfacial polymerization of a commercial polyurethane curing agent (Bayer L-75) and 1,4-butanediol (BDO) as a chain extender in an emulsion solution. With an optical microscope (OM) and a scanning electron microscope (SEM), the resulting microcapsules showed a spherical shape and an ideal structure with a smooth surface. Fourier transform infrared spectra (FTIR) showed that the core material was successfully encapsulated. Thermal gravimetric analysis (TGA) showed that the initial evaporation temperature of the microcapsules was 270 °C. In addition, we examined the influence of the concentration of the emulsifier and chain extender on the structure and morphology of the microcapsules. The results indicate that the optimal parameters of the microcapsule are an emulsifier concentration of 7.5% and a chain extender concentration of 15.38%. Microcapsules were added to the epoxy resin coating to verify the coating’s self-healing performance by a surface scratch test, and the results showed that the cracks could heal in 24 h. Furthermore, the self-healing coating had excellent corrosion resistance.

Microencapsulation of reactive isocyanates for application in self-healing materials: a review

Microencapsulation of curing agents is a major strategy for the development of self-healing polymers. Isocyanates are among the most promising compounds for the development of one-part, catalyst free, self-healing materials, but their microencapsulation is challenging due to their high reactivity. To keep the healing agent intact in the liquid state and containing free-NCO groups, the monitoring of several synthesis parameters is essential. This review aims to summarise the outcomes in the microencapsulation of isocyanates, emphasising the efforts reported in the literature to modulate the microcapsule properties. In this regard, the main synthesis procedures are presented, followed by the most relevant characterisation methods used to assess microcapsule properties. The correlation between these properties and synthesis parameters is also discussed, and finally the main potential and challenges for industrial applications are highlighted.

Influence of the Prepolymer Type and Synthesis Parameters on Self-Healing Anticorrosion Properties of Composite Coatings Containing Isophorone Diisocyanate-Loaded Polyurethane Microcapsules

Self-healing anticorrosion composite coatings containing isophorone diisocyanate-loaded polyurethane microcapsules were developed, and comprehensive research on prepolymer and microcapsules synthesis, as well as functional composite coatings preparation and characterization, was performed. The influence of the prepolymer type and the concentration of the stabilizing agent used in the synthesis procedure on the properties of the microcapsules was studied in detail. For this purpose, three different prepolymers were prepared from toluene-2,4-diisocyanate (TDI) and either glycerol, 1,4-butanediol, or 1,6-hexanediol, and their chemical properties were investigated. Microcapsules were synthesized from the obtained prepolymers, according to the oil-in-water polymerization method, where 1,6-hexanediol was used as a chain extender, while the concentration of the stabilizing agent in the synthesis procedure was varied. Microcapsules prepared from TDI-glycerol prepolymer, synthesized in the presence of 10 wt% of the stabilizing agent, showed superior chemical, morphological, and thermo-gravimetrical properties; thus, they were incorporated into the coating in the concentration of 20 wt%. The prepared composite coatings demonstrated self-healing and anticorrosion properties, and thus the developed microcapsules show great potential for the incorporation into the composite anticorrosion coatings at critical points where damage can easily occur, providing longer and more efficient anticorrosion protection.

Review on Adhesives and Surface Treatments for Structural Applications: Recent Developments on Sustainability and Implementation for Metal and Composite Substrates

Using adhesives for connection technology has many benefits. It is cost-efficient, fast, and allows homogeneous stress distribution between the bonded surfaces. This paper gives an overview on the current state of knowledge regarding the technologically important area of adhesive materials, as well as on emergent related technologies. It is expected to fill some of the technological gaps between the existing literature and industrial reality, by focusing at opportunities and challenges in the adhesives sector, on sustainable and eco-friendly chemistries that enable bio-derived adhesives, recycling and debonding, as well as giving a brief overview on the surface treatment approaches involved in the adhesive application process, with major focus on metal and polymer matrix composites. Finally, some thoughts on the connection between research and development (R&D) efforts, industry standards and regulatory aspects are given. It contributes to bridge the gap between industry and research institutes/academy. Examples from the aeronautics industry are often used since many technological advances in this industry are innovation precursors for other industries. This paper is mainly addressed to chemists, materials scientists, materials engineers, and decision-makers.

Polycaprolactone microcapsules containing citric acid and naringin for plant growth and sustainable agriculture: physico-chemical properties and release behavior

Plant growth promoting rhizobacteria (PGPR) is an alternative to chemical fertilizers for sustainable, environment friendly agriculture. There is a need to develop strategies to potentiate the interaction between rhizobacteria and plants. Flavonoids and organic acids (components of root exudates) play specific beneficial roles as carbon sources and signal molecules in the plant - rhizobacteria interactions. The goal of this work is to encapsulate signal molecules, namely citric acid and naringin, an organic acid and a flavonoid, respectively, by a biodegradable polymer, polycaprolactone (PCL), in order to maintain the stability and activity of those signal molecules and enable their slow or controlled release over a selected period of time, according to the needs of the plants. This approach is expected to potentiate food crops, namely peanut crop, in adverse environmental conditions (water deficit), by promoting the beneficial interaction between the peanut plant (A. hypogaea) and rhizobacteria. The microcapsules (MCs) are obtained by an emulsion process combined with solvent evaporation technique and are characterized by scanning electron microscopy, thermogravimetry and Fourier transformed infrared spectroscopy. The kinetics of in vitro release of encapsulated molecules, in a period where the uptake of the compound in plants can occur, is studied. The encapsulation synthesis parameters that lead to the best encapsulation process yield and efficiency, as well as to the best final performance in terms of release, are identified. The effect of pH and molecular weight of PCL is found to mediate the release properties of the molecules for different types of soil. PCL 45000 Mw dissolved at 16% in dichloromethane leads to an encapsulation efficiency of 75% and the resulting MCs containing naringin exhibit a slow release profile for 30 days, unmodified by pH, enabling their use in soils of different characteristics. This research makes possible the manufacturing of smart materials for sustainable agriculture practices.