Synthesis, Thermal Properties and Curing Kinetics of Hyperbranched BPA/PEG Epoxy Resin

CNT-Reinforced Self-Healable Epoxy Dynamic Networks Based on Disulfide Bond Exchange

The design and utilization of polymers with healing capability have drawn increasing attention owing to their enhanced chain mobility and opportunity to heal minor cracks in composites. Rehealable thermoset polymers promise reduction in the maintenance cost and thus prolonged lifetime, reshaping, and recyclability. Introducing reversible covalent bonds is the mainstay strategy to achieve such plasticity in crosslinked polymers. Herein, we report a dynamic epoxy, which includes associative covalent adaptive networks (CANs) based on disulfide exchange bonds. Epoxy resin is chosen to study rehealing, as it is one of the most critical thermosetting polymers for various industries from aerospace to soft robotics. This study enlightens us about not only the consequences of CANs in the epoxy but also various factors such as soft segments and carbon nanotubes (CNTs). Epoxy dynamic networks are investigated in an attempt to explore the synergistic effect of the soft-segmented resins and CNTs on the healing and reshaping characteristics of epoxy networks along with varying stiffness. This research discusses epoxy dynamic networks in three main aspects: crosslink density, CAN density, and CNTs. Introducing soft segments into the epoxy network enhances the healing efficiency due to the increased chain mobility. A higher CAN density accelerates network rearrangement, improving the healing efficiency. It should also be noted that even with a low weight fraction of nanotubes, CNT-reinforced samples restored their initial strength more than neat samples after healing. The tensile strength of dynamic networks is at least 50 MPa, which is significant for their utility in primary or secondary structural components.

Lignosulfonate-Based Polyurethane Adhesives

The feasibility of using lignosulfonate (LS) from acid sulphite pulping of eucalyptus wood as an unmodified polyol in the formulation of polyurethane (PU) adhesives was evaluated. Purified LS was dissolved in water to simulate its concentration in sulphite spent liquor and then reacted with 4,4′-diphenylmethane diisocyanate (pMDI) in the presence or absence of poly(ethylene glycol) with M_w 200 (PEG₂₀₀) as soft crosslinking segment. The ensuing LS-based PU adhesives were characterized by infrared spectroscopy and thermal analysis techniques. The adhesion strength of new adhesives was assessed using Automated Bonding Evaluation System (ABES) employing wood strips as a testing material. The results showed that the addition of PEG₂₀₀ contributed positively both to the homogenization of the reaction mixture and better crosslinking of the polymeric network, as well as to the interface interactions and adhesive strength. The latter was comparable to the adhesive strength recorded for a commercial white glue with shear stress values of almost 3 MPa. The optimized LS-based PU adhesive formulation was examined for the curing kinetics following the Kissinger and the Ozawa methods by non-isothermal differential scanning calorimetry, which revealed the curing activation energy of about 70 kJ·mol⁻¹.

In-Out Surface Modification of Halloysite Nanotubes (HNTs) for Excellent Cure of Epoxy: Chemistry and Kinetics Modeling

In-out surface modification of halloysite nanotubes (HNTs) has been successfully performed by taking advantage of 8-hydroxyquinolines in the lumen of HNTs and precisely synthesized aniline oligomers (AO) of different lengths (tri- and pentamer) anchored on the external surface of the HNTs. Several analyses, including FTIR, H-NMR, TGA, UV-visible spectroscopy, and SEM, were used to establish the nature of the HNTs’ surface engineering. Nanoparticles were incorporated into epoxy resin at 0.1 wt.% loading for investigation of the contribution of surface chemistry to epoxy cure behavior and kinetics. Nonisothermal differential scanning calorimetry (DSC) data were fed into home-written MATLAB codes, and isoconversional approaches were used to determine the apparent activation energy (E_α) as a function of the extent of cure reaction (α). Compared to pristine HNTs, AO-HNTs facilitated the densification of an epoxy network. Pentamer AO-HNTs with longer arms promoted an Excellent cure; with an E_α value that was 14% lower in the presence of this additive than for neat epoxy, demonstrating an enhanced cross-linking. The model also predicted a triplet of cure (m, n, and ln A) for autocatalytic reaction order, non-catalytic reaction order, and pre-exponential factor, respectively, by the Arrhenius equation. The enhanced autocatalytic reaction in AO-HNTs/epoxy was reflected in a significant rise in the value of m, from 0.11 to 0.28. Kinetic models reliably predict the cure footprint suggested by DSC measurements.

Going Beyond the Carothers, Flory and Stockmayer Equation by Including Cyclization Reactions and Mobility Constraints

A challenge in the field of polymer network synthesis by a step-growth mechanism is the quantification of the relative importance of inter- vs. intramolecular reactions. Here we use a matrix-based kinetic Monte Carlo (kMC) framework to demonstrate that the variation of the chain length distribution and its averages (e.g., number average chain length x_n), are largely affected by intramolecular reactions, as mostly ignored in theoretical studies. We showcase that a conventional approach based on equations derived by Carothers, Flory and Stockmayer, assuming constant reactivities and ignoring intramolecular reactions, is very approximate, and the use of asymptotic limits is biased. Intramolecular reactions stretch the functional group (FG) conversion range and reduce the average chain lengths. In the likely case of restricted mobilities due to diffusional limitations because of a viscosity increase during polymerization, a complex x_n profile with possible plateau formation may arise. The joint consideration of stoichiometric and non-stoichiometric conditions allows the validation of hypotheses for both the intrinsic and apparent reactivities of inter- and intramolecular reactions. The kMC framework is also utilized for reverse engineering purposes, aiming at the identification of advanced (pseudo-)analytical equations, dimensionless numbers and mechanistic insights. We highlight that assuming average molecules by equally distributing A and B FGs is unsuited, and the number of AB intramolecular combinations is affected by the number of monomer units in the molecules, specifically at high FG conversions. In the absence of mobility constraints, dimensionless numbers can be considered to map the time variation of the fraction of intramolecular reactions, but still, a complex solution results, making a kMC approach overall most elegant.

Ethoxylated Butoxyethanol-BADGE Adducts-New Potential Migrants from Epoxy Resin Can Coating Material

The acetonitrile extracts of can-coating materials have been analyzed by using high-pressure liquid chromatography/electrospray ionization-mass spectrometry (HPLC/ESI-MS). On the basis of detected ions [M + H]⁺, [M + NH₄]⁺, [M + Na]⁺ and product ions, the ethoxylated butoxyethanol-bisphenol A diglycidyl ether adducts were identified in two of the analyzed extracts. Although the oxyethylene unit-containing compounds are widely used for the production of different kinds of materials, the ethoxylated species have not been earlier detected in epoxy resin can-coatings.

Curing Behavior, Rheological, and Thermal Properties of DGEBA Modified with Synthesized BPA/PEG Hyperbranched Epoxy after Their Photo-Initiated Cationic Polymerization

This paper investigates the photo-initiated cationic polymerization of diglycidyl ether of bisphenol A (DGEBA) modified with bisphenol A (BPA)/polyethylene glycol (PEG) hyperbranched epoxy resin. The relationship between curing behavior, rheological, and thermal properties of the modified DGEBA is investigated using photo-differential scanning calorimetry (DSC) and photo-rheometer techniques. It is seen that the addition of the hyperbranched epoxy resin can increase UV conversion (αUV) and reduce gelation time (tgel). After photo-initiation polymerization (dark reaction) occurred, a second exothermic peak in the DSC thermogram takes place: namely, the occurrence of curing reaction owing to the activated monomer (AM) mechanism. Consequently, the glass transition temperature decreased, and at the same time, UV intensity increased which was due to the molecular weight between crosslinking points (Mc). Furthermore, the radius of gyration (Rg) of the network segment is determined via small-angle X-ray scattering (SAXS). It is noted that the higher the Mc, the larger the radius of gyration proves to be, resulting in low glass transition temperature.

Thermal Analysis of Crosslinking Reactions in Epoxy Nanocomposites Containing Polyvinyl Chloride (PVC)-Functionalized Nickel-Doped Nano-Fe3O4

This work reports on the thermal analysis of epoxy containing polyvinyl chloride (PVC) surface-functionalized magnetic nanoparticles (PVC–S/MNP) and its bulk-modified nickel-doped counterpart (PVC–S/MNP/Bi–B). Nanoparticles were synthesized through the cathodic electro-deposition method. The morphology of particles was imaged on a field-emission scanning electron microscope (FE-SEM), while X-ray diffraction analysis and Fourier-transform infrared spectroscopy (FTIR) were used to detect changes in the structure of nanoparticles. The magnetic behavior of particles was also studied by vibrating sample magnetometry (VSM). In particular, we focused on the effect of the bulk (Ni-doping) and surface (PVC-capping) modifications of MNPs on the thermal crosslinking of epoxy using nonisothermal differential scanning calorimetry (DSC) varying the heating rate. The cure labels of the prepared nanocomposites were assigned to them, as quantified by the cure index. The good cure state was assigned to the system containing PVC–S/MNP/Bi–B as a result of excessive ring opening of epoxy. Cure kinetics parameters of PVC–S/MNP/Bi–B incorporated epoxy was obtained by the use of isoconversional methodology. The activation energy of epoxy was decreased upon addition of 0.1 wt% of PVC–S/MNP/Bi–B due to the reaction of Cl− of PVC by the functional groups of resin.

Monitoring the structure–reactivity relationship in epoxidized perilla and safflower oil thermosetting resins

The reactivity of epoxidized perilla oil and epoxidized safflower oil with two aromatic dicarboxylic acids was studied. The presence of S–S bonding at the β position of the carboxylic group increases the reactivity of the acidic proton toward epoxy ring opening.