Special Issue "Thermosets II"

A special issue of Polymers (ISSN 2073-4360). This special issue belongs to the section "Polymer Chemistry".

Deadline for manuscript submissions: closed (31 January 2020).

Special Issue Editors

Prof. Dr. Angels Serra
E-Mail Website
Guest Editor
Department of Analytical and Organic Chemistry, Faculty of Chemistry, University Rovira i Virgili, C/ Marcel lí Domingo s/n, N4, 43001 Tarragona, Spain
Interests: epoxy thermosets; click-crosslinkable materials; dual curing; curing mechanisms; structural characterization; spectroscopy; synthesis of monomers; ring-opening polymerization; hyperbranched polymers; star polymers; mechanical characterization; thermal characterization
Special Issues and Collections in MDPI journals
Prof. Xavier Ramis
E-Mail Website
Guest Editor
Department of Heat Engines, Thermodynamics Laboratory of Barcelona School of Industrial Engineering, Universitat Politècnica de Catalunya BarcelonaTech, Av. Diagonal 647, ETSEIB, 08028 Barcelona, Spain
Interests: thermosets; click crosslinkable materials; dual curing; kinetics; curing mechanisms; structural characterization; ring-opening polymerization; hyperbranched polymers; star polymers; mechanical and thermal characterization; structure property relationship; thermal analysis
Special Issues and Collections in MDPI journals

Special Issue Information

Dear Colleagues,

Thermoset materials are usually highly crosslinked polymers, produced after a curing process. At present, they have wide technological applications in conventional and advanced materials. They are used in applications such as adhesives, matrices for fiber-reinforced composites, surface coatings, caulks, insulating materials, and electronic encapsulation, among others. Moreover, cutting-edge research in the field is focused on the development of new thermosetting materials for novel applications, such as 3D printing, smart materials and actuators, encapsulation, etc.

The main reason for the widespread use of thermosets is their combination of high strength and stiffness, high resistance to creep, excellent corrosion resistance, and appropriate electrical properties. However, owing to their high crosslink density, thermosets have not only a low impact resistance, but also a high inability to be reprocessed or repaired. A great effort is now focused on the preparation of self-healing or vitrimeric materials. These can be repaired or reprocessed after suffering damage or when their service-life is over.

This Special Issue is the second part of other previous issue, “Thermosets”, which aimed to present new insights into the improvement of all types of thermosets, specially designed to fulfill the requirements for advanced technologies. The goal of this second Special Issue remains the same, but we are now focusing on how the structure and characteristics affect the final thermoset properties and their processing. The issue covers aspects such as morphology, structure, properties, novel and bio-based thermosets, and their nanocomposites and applications. Traditional and new chemical processes will contribute to the preparation of such materials and in new processing technologies, such as dual curing. These allow to obtain stable intermediate materials after the first curing step that can be processed before triggering the second curing step that allows the final material performance to be achieved. This issue covers specially designed chemical structures, which will confer interesting characteristics to the thermosets described.

We cordially invite experts in the field of thermosetting materials to submit papers to this Special Issue. This Special Issue will also include selected papers from Baekeland 2019.

text

Prof. Angels Serra
Prof. Xavier Ramis
Guest Editors

Manuscript Submission Information

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Keywords

  • Thermosets
  • Crosslinking
  • Self-healing
  • Vitrimers
  • 3D-printing
  • Reprocessing
  • Degradation
  • Reworkability
  • Toughness
  • Dual curing
  • Smart materials
  • High performance

Published Papers (23 papers)

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Research

Open AccessArticle
On the Nature of Epoxy Resin Post-Curing
Polymers 2020, 12(2), 466; https://doi.org/10.3390/polym12020466 - 18 Feb 2020
Cited by 1 | Viewed by 818
Abstract
Post-curing is intended to improve strength, elevate glass transition, and reduce residual stress and outgassing in thermosets. Also, experiments indicate post-curing temperatures lead to ether crosslinks and backbone dehydration. These results informed molecular dynamics methods to represent them and compare the resulting thermomechanical [...] Read more.
Post-curing is intended to improve strength, elevate glass transition, and reduce residual stress and outgassing in thermosets. Also, experiments indicate post-curing temperatures lead to ether crosslinks and backbone dehydration. These results informed molecular dynamics methods to represent them and compare the resulting thermomechanical effects. Diglycidyl ether of bisphenol A (DGEBA)-diamino diphenyl sulfone (DDS) systems were examined. Independent variables were resin length, stoichiometry, and reaction type (i.e., amine addition, etherification, and dehydration). Etherification affected excess epoxide systems most. These were strengthened and became strain hardening. Systems which were both etherified and dehydrated were most consistent with results of post-curing experiments. Dehydration stiffened and strengthened systems with the longer resin molecules due to their intermediate hydroxyl groups for crosslinking. Changes in the concavity of functions fit to the specific volume versus temperature were used to detect thermal transitions. Etherification generally increased transition temperatures. Dehydration resulted in more transitions. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Cerium Salts: An Efficient Curing Catalyst for Benzoxazine Based Coatings
Polymers 2020, 12(2), 415; https://doi.org/10.3390/polym12020415 - 11 Feb 2020
Cited by 4 | Viewed by 948
Abstract
The effect of three different cerium salts (Ce(NO3)3·6H2O, CeCl3·7H2O and Ce(OOCCH3)3·5H2O) on the ring-opening polymerization (ROP) of a model diamine-based benzoxazine (4EP-pPDA) was investigated. With the incorporation [...] Read more.
The effect of three different cerium salts (Ce(NO3)3·6H2O, CeCl3·7H2O and Ce(OOCCH3)3·5H2O) on the ring-opening polymerization (ROP) of a model diamine-based benzoxazine (4EP-pPDA) was investigated. With the incorporation of the cerium salts, the curing temperature of 4EP-pPDA is reduced substantially, and the glass transition temperatures of the resulting networks are increased significantly. The three cerium salts exhibit different catalytic activities, which were analyzed by FT-IR, NMR, and energy-dispersive X-ray (EDX). Ce(NO3)3·6H2O was found to exhibit the best catalytic effect, which seems to be related to its better dispersibility within 4EP-pPDA benzoxazine precursors. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Vanillin Acrylate-Based Resins for Optical 3D Printing
Polymers 2020, 12(2), 397; https://doi.org/10.3390/polym12020397 - 10 Feb 2020
Cited by 8 | Viewed by 1297
Abstract
The investigation of biobased systems as photocurable resins for optical 3D printing has attracted great attention in recent years; therefore, novel vanillin acrylate-based resins were designed and investigated. Cross-linked polymers were prepared by radical photopolymerization of vanillin derivatives (vanillin dimethacrylate and vanillin diacrylate) [...] Read more.
The investigation of biobased systems as photocurable resins for optical 3D printing has attracted great attention in recent years; therefore, novel vanillin acrylate-based resins were designed and investigated. Cross-linked polymers were prepared by radical photopolymerization of vanillin derivatives (vanillin dimethacrylate and vanillin diacrylate) using ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate as photoinitiator. The changes of rheological properties were examined during the curing with ultraviolet/visible irradiation to detect the influences of solvent, photoinitiator, and vanillin derivative on cross-linking rate and network formation. Vanillin diacrylate-based polymers had higher values of yield of insoluble fraction, thermal stability, and better mechanical properties in comparison to vanillin dimethacrylate-based polymers. Moreover, the vanillin diacrylate polymer film showed a significant antimicrobial effect, only a bit weaker than that of chitosan film. Thermal and mechanical properties of vanillin acrylate-based polymers were comparable with those of commercial petroleum-derived materials used in optical 3D printing. Also, vanillin diacrylate proved to be well-suited for optical printing as was demonstrated by employing direct laser writing 3D lithography and microtransfer molding techniques. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Preparation and Characterization of UV-Curable Acrylic Membranes Embedding Natural Antioxidants
Polymers 2020, 12(2), 358; https://doi.org/10.3390/polym12020358 - 06 Feb 2020
Viewed by 717
Abstract
We examine the behaviour of acrylic resin-based membranes containing natural anti-oxidants, such as Galla chinensis tea powder extract (TP) and Taiwanese green propolis (TGP), in different concentrations ranging between 5 and 20 wt %. Membrane morphology was investigated by means of Environmental Scanning [...] Read more.
We examine the behaviour of acrylic resin-based membranes containing natural anti-oxidants, such as Galla chinensis tea powder extract (TP) and Taiwanese green propolis (TGP), in different concentrations ranging between 5 and 20 wt %. Membrane morphology was investigated by means of Environmental Scanning Electron Microscopy (ESEM), while the UV-curing reaction was monitored by Fourier-Transform Infra-red (FTIR) spectroscopy. In most cases Thermogravimetric (TG), Differential Scanning Calorimetric (DSC) and Dynamo-mechanical Thermal (DMT) analyses showed that the desirable characteristics of the UV-cured acrylic resin are not substantially altered by the presence of the organic fillers. The release kinetics of polyphenols and flavonoids, determined in water for TP-containing membranes (ETx) and in ethanol/water mixture (7:3 v/v) for TGP-containing ones (EPx), was satisfactory, reaching a plateau after 24 h. Finally, preliminary antibacterial tests against S. epidermidis were performed on the membranes with higher additive amount and gave positive results for ET-type; on the contrary, no inhibitory effect was observed for the tested EP-type membranes. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Novel Bio-Based Epoxy Thermosets Based on Triglycidyl Phloroglucinol Prepared by Thiol-Epoxy Reaction
Polymers 2020, 12(2), 337; https://doi.org/10.3390/polym12020337 - 05 Feb 2020
Cited by 4 | Viewed by 731
Abstract
The pure trifunctional glycidyl monomer from phloroglucinol (3EPO-Ph) was synthesized and used as feedstock in the preparation of novel bio-based thermosets by thiol-epoxy curing. The monomer was crosslinked with different commercially available thiols: tetrafunctional thiol (PETMP), trifunctional thiol (TTMP) and an aromatic dithiol [...] Read more.
The pure trifunctional glycidyl monomer from phloroglucinol (3EPO-Ph) was synthesized and used as feedstock in the preparation of novel bio-based thermosets by thiol-epoxy curing. The monomer was crosslinked with different commercially available thiols: tetrafunctional thiol (PETMP), trifunctional thiol (TTMP) and an aromatic dithiol (TBBT) as curing agents in the presence of a base. As catalyst, two different commercial catalysts: LC-80 and 4-(N,N-dimethylamino) pyridine (DMAP) and a synthetic catalyst, imidazolium tetraphenylborate (base generator, BG) were employed. The curing of the reactive mixtures was studied by using DSC and the obtained materials by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA). The results revealed that only the formulations catalyzed by BG showed a latent character. Already prepared thermosetting materials showed excellent thermal, thermomechanical and mechanical properties, with a high transparency. In addition to that, when compared with the diglycidyl ether of bisphenol A (DGEBA)/PETMP material, the thermosets prepared from the triglycidyl derivative of phloroglucinol have better final characteristics and therefore this derivative can be considered as a partial or total renewable substitute of DGEBA in technological applications. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Highly Crosslinked Polybenzoxazines from Monobenzoxazines: The Effect of Meta-Substitution in the Phenol Ring
Polymers 2020, 12(2), 254; https://doi.org/10.3390/polym12020254 - 21 Jan 2020
Cited by 3 | Viewed by 901
Abstract
It is possible to control the crosslink density of polymers derived from monobenzoxazines by switching the type of substituents in the phenolic ring and their relative position with respect to the phenol group. We prepared several substituted monobenzoxazines in the para and meta [...] Read more.
It is possible to control the crosslink density of polymers derived from monobenzoxazines by switching the type of substituents in the phenolic ring and their relative position with respect to the phenol group. We prepared several substituted monobenzoxazines in the para and meta positions of the phenolic ring and studied how these substituents affected the polymerization temperature of monomers and the thermal stability of the final polymers and, more extensively, how they affected the crosslink network of the final polymers. Gel content and dynamic mechanical analysis confirm that ortho- and para-orienting substituents in the meta position generate highly crosslinked materials compared to para ones. This fact can lead to the design of materials with highly crosslinked networks based on monobenzoxazines, simpler and more versatile monomers than the commercial bisbenzoxazines currently in use. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
New Eco-Friendly Synthesized Thermosets from Isoeugenol-Based Epoxy Resins
Polymers 2020, 12(1), 229; https://doi.org/10.3390/polym12010229 - 17 Jan 2020
Cited by 5 | Viewed by 1355
Abstract
Epoxy resin plays a key role in composite matrices and DGEBA is the major precursor used. With the aim of favouring the use of bio resources, epoxy resins can be prepared from lignin. In particular, diglycidyl ether of isoeugenol derivatives are good candidates [...] Read more.
Epoxy resin plays a key role in composite matrices and DGEBA is the major precursor used. With the aim of favouring the use of bio resources, epoxy resins can be prepared from lignin. In particular, diglycidyl ether of isoeugenol derivatives are good candidates for the replacement of DGEBA. This article presents an effective and eco-friendly way to prepare epoxy resin derived from isoeugenol (BioIgenox), making its upscale possible. BioIgenox has been totally characterized by NMR, FTIR, MS and elemental analyses. Curing of BioIgenox and camphoric anhydride with varying epoxide function/anhydride molar ratios has allowed determining an optimum ratio near 1/0.9 based on DMA and DSC analyses and swelling behaviours. This thermoset exhibits a Tg measured by DMA of 165 °C, a tensile storage modulus at 40 °C of 2.2 GPa and mean 3-point bending stiffness, strength and strain at failure of 3.2 GPa, 120 MPa and 6.6%, respectively. Transposed to BioIgenox/hexahydrophtalic anhydride, this optimized formulation gives a thermoset with a Tg determined by DMA of 140 °C and a storage modulus at 40 °C of 2.6 GPa. The thermal and mechanical properties of these two thermosets are consistent with their use as matrices for structural or semi-structural composites. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
New Epoxy Thermosets Derived from Clove Oil Prepared by Epoxy-Amine Curing
Polymers 2020, 12(1), 44; https://doi.org/10.3390/polym12010044 - 27 Dec 2019
Cited by 5 | Viewed by 972
Abstract
New thermosets from a triglycidyl eugenol derivative (3EPOEU) as a renewable epoxy monomer were obtained by an epoxy-amine curing process. A commercially-available Jeffamine® and isophorone diamine, both obtained from renewable resources, were used as crosslinking agents, and the materials obtained were compared [...] Read more.
New thermosets from a triglycidyl eugenol derivative (3EPOEU) as a renewable epoxy monomer were obtained by an epoxy-amine curing process. A commercially-available Jeffamine® and isophorone diamine, both obtained from renewable resources, were used as crosslinking agents, and the materials obtained were compared with those obtained from a standard diglycidylether of bisphenol A (DGEBA). The evolution of the curing process was studied by differential scanning calorimetry and the materials obtained were characterized by means of calorimetry, thermogravimetry, thermodynamomechanical analysis, stress–strain tests and microindentation. 3EPOEU formulations were slightly less reactive, and the thermosets obtained showed higher Tgs than those prepared from DGEBA, since they had higher crosslinking density than formulations with DGEBA because of the more compact structure and higher functionality of the eugenol derivative. 3EPOEU thermosets showed good thermal stability and mechanical properties. The results obtained in this study allow us to conclude that the triglycidyl derivative of eugenol, 3EPOEU, is a safe and environmentally friendly alternative to DGEBA. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Modified Epoxy Resin Synthesis from Phosphorus—Containing Polyol and Physical Changes Studies in the Synthesized Products
Polymers 2019, 11(12), 2116; https://doi.org/10.3390/polym11122116 - 16 Dec 2019
Cited by 2 | Viewed by 1093
Abstract
Epoxy resins are commonly used to manufacture the molding compounds, reinforced plastics, coatings, or adhesives required in various industries. However, the demand for new epoxy resins has increased to satisfy diverse industrial requirements such as enhanced mechanical properties, thermal stability, or electrical properties. [...] Read more.
Epoxy resins are commonly used to manufacture the molding compounds, reinforced plastics, coatings, or adhesives required in various industries. However, the demand for new epoxy resins has increased to satisfy diverse industrial requirements such as enhanced mechanical properties, thermal stability, or electrical properties. Therefore, in this study, we synthesized new epoxy resin (PPME) by modifying phosphorous-containing polyol. The prepared resin was analyzed and added to epoxy compositions in various quantities. The compositions were cured at high temperatures to obtain plastics to further test the mechanical and thermal properties of the epoxy resin. The measured tensile and flexural strength of epoxy compositions were similar to the composition without synthesized epoxy resin. However, the heat release rates of the compositions exhibited tendencies of a decrease proportional to the amount of PPME. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Influence of Filler Content and Filler Size on the Curing Kinetics of an Epoxy Resin
Polymers 2019, 11(11), 1797; https://doi.org/10.3390/polym11111797 - 01 Nov 2019
Cited by 3 | Viewed by 969
Abstract
In this research, the influences of filler content and filler particle size on the flow-hardening behavior were investigated by a measuring mixer. In order to more reliably assess the observed rheological behavior, isothermal differential scanning calorimetry (DSC) measurements were employed to study the [...] Read more.
In this research, the influences of filler content and filler particle size on the flow-hardening behavior were investigated by a measuring mixer. In order to more reliably assess the observed rheological behavior, isothermal differential scanning calorimetry (DSC) measurements were employed to study the curing kinetics of the compounds. The measured data can be fitted well with Kamal-Sourour’s model modified by the diffusion correlation according to Chern and Poehlein. After that, the influence of filler content and size on the kinetic parameters are presented discussed. The results show that the ultimate glass transition temperature is significantly lower for pure epoxy resin (EP) than for compounds filled with surface-treated glass beads, which have an essential effect on the diffusion-controlled reaction at different curing temperatures. For the surface-treated glass beads used in this study, the reaction speed in the early curing stage is accelerated by increasing filler content or decreasing of filler size. In the later curing stage, the reaction speeds of compounds with higher filler content or smaller fillers reduce more quickly. The study of reaction kinetics indicates that the activation energy Ea1, Ea2, the reaction order m, and n are affected differently by varying filler content and size. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Acetoacetate Based Thermosets Prepared by Dual-Michael Addition Reactions
Polymers 2019, 11(9), 1408; https://doi.org/10.3390/polym11091408 - 27 Aug 2019
Cited by 2 | Viewed by 945
Abstract
A novel set of dual-curable multiacetoacetate-multiacrylate-divinyl sulfone ternary materials with versatile and manipulable properties are presented. In contrast to common dual-curing systems, the first stage polymer herein consists of a densely crosslinked, high Tg network as a result of base-catalyzed multiacetoacetate-divinyl sulfone [...] Read more.
A novel set of dual-curable multiacetoacetate-multiacrylate-divinyl sulfone ternary materials with versatile and manipulable properties are presented. In contrast to common dual-curing systems, the first stage polymer herein consists of a densely crosslinked, high Tg network as a result of base-catalyzed multiacetoacetate-divinyl sulfone Michael addition. A more flexible secondary network forms after base-catalyzed Michael addition of remaining multiacetoacetate to multiacrylate. Curing is truly sequential as the rates of the two Michael additions are significantly different. Curing kinetics were analyzed using differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR). The materials at each curing stage were characterized using dynamic mechanical analysis (DMA) and SEM. Although some phase separation was observed in certain formulations, the incompatibilities were minimized when the molar percentage of the acetoacetate-divinyl sulfone polymer network was above 75%. Furthermore, the environmental scanning electron microscopy (ESEM) images of these materials show that the more flexible acetoacetate-acrylate phase is dispersed in the form of polymeric spheres within the rigid acetoacetate-divinyl sulfone matrix. This unique dual microstructure can potentially render these materials highly resilient in applications requiring densely crosslinked polymer architectures with enhanced toughness. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
On the Dielectric Behavior of Amine and Anhydride Cured Epoxy Resins Modified Using Multi-Terminal Epoxy Functional Network Modifier
Polymers 2019, 11(8), 1271; https://doi.org/10.3390/polym11081271 - 31 Jul 2019
Cited by 7 | Viewed by 1739
Abstract
A range of modified amine- and anhydride-cured epoxy systems based upon diglycidyl ether of bisphenol A was produced, through the systematic incorporation of moieties termed functional network modifiers (FNMs) that serve to change the network structure in controlled ways. Here, the chosen FNM [...] Read more.
A range of modified amine- and anhydride-cured epoxy systems based upon diglycidyl ether of bisphenol A was produced, through the systematic incorporation of moieties termed functional network modifiers (FNMs) that serve to change the network structure in controlled ways. Here, the chosen FNM was trimethylolpropane triglycidyl ether (TTE). The resulting materials were characterized by Fourier transform infrared spectroscopy, thermal analysis, dielectric spectroscopy and measurements of direct current conduction. A progressive reduction in the glass transition temperature of the modified samples was seen with increasing TTE, which is interpreted in terms of changes in the network architecture of the resin. The molecular origins of the dielectric γ and β relaxation processes are proposed. The observed increase in conduction seen exclusively with increasing TTE content in the amine-cured systems is considered in terms of the chemistry of the FNMs, variations in free volume, changes in molecular dynamics and residual unreacted groups retained from the curing reaction. Specifically, we relate the observed increase in conduction to the presence of unreacted amine groups. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Toughening and Enhancing Melamine–Urea–Formaldehyde Resin Properties via in situ Polymerization of Dialdehyde Starch and Microphase Separation
Polymers 2019, 11(7), 1167; https://doi.org/10.3390/polym11071167 - 09 Jul 2019
Cited by 6 | Viewed by 1459
Abstract
The goal of this study is to employ bio-based dialdehyde starch (DAS), derived from in situ polymerization and the resultant microphase separation structure, to improve the strength of melamine–urea–formaldehyde (MUF) resin, as well as enhance the properties that affect its adhesive performance. Thus, [...] Read more.
The goal of this study is to employ bio-based dialdehyde starch (DAS), derived from in situ polymerization and the resultant microphase separation structure, to improve the strength of melamine–urea–formaldehyde (MUF) resin, as well as enhance the properties that affect its adhesive performance. Thus, we evaluated the effects of DAS on the chemical structure, toughness, curing behavior, thermal stability, and micromorphology of the MUF resin. Furthermore, the wet shear strength and formaldehyde emissions of a manufactured, three-layer plywood were also measured. Results indicate that DAS was chemically introduced into the MUF resin by in situ polymerization between the aldehyde group in the DAS and the amino group and hydroxymethyl group in the resin. Essentially, polymerization caused a DAS soft segment to interpenetrate into the rigid MUF resin cross-linked network, and subsequently form a microphase separation structure. By incorporating 3% DAS into the MUF resin, the elongation at break of impregnated paper increased 48.12%, and the wet shear strength of the plywood increased 23.08%. These improvements were possibly due to one or a combination of the following: (1) DAS polymerization increasing the cross-linking density of the cured system; (2) DAS modification accelerating the curing of the MUF resin; and/or (3) the microphase separation structure, induced by DAS polymerization, improving the cured resin’s strength. All the results in this study suggest that the bio-based derivative from in situ polymerization and microphase separation can effectively toughen and enhance the properties that affect adhesive performance in highly cross-linked thermosetting resins. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Thermally Healable and Recyclable Graphene-Nanoplate/Epoxy Composites Via an In-Situ Diels-Alder Reaction on the Graphene-Nanoplate Surface
Polymers 2019, 11(6), 1057; https://doi.org/10.3390/polym11061057 - 18 Jun 2019
Cited by 2 | Viewed by 1296
Abstract
In this work, thermally healable graphene-nanoplate/epoxy (GNP/EP) nanocomposites were investigated. GNPs were used as reinforcement and crosslinking platforms for the diglycidyl ether of bisphenol A-based epoxy resin (DGEBA) through the Diels-Alder (DA) reaction with furfurylamine (FA). The GNPs and FA could then be [...] Read more.
In this work, thermally healable graphene-nanoplate/epoxy (GNP/EP) nanocomposites were investigated. GNPs were used as reinforcement and crosslinking platforms for the diglycidyl ether of bisphenol A-based epoxy resin (DGEBA) through the Diels-Alder (DA) reaction with furfurylamine (FA). The GNPs and FA could then be used as a derivative of diene and dienophile in the DA reaction. It was expected that the combination of GNPs and FA in DGEBA would produce composites based on the interfacial properties of the components. We confirmed the DA reaction of GNPs and FA at the interface during curing of the GNP/EP nanocomposites. This procedure is simple and solvent-free. DA and retro DA reactions of the obtained composites were demonstrated, and the thermal healing properties were evaluated. The behavior of the GNP/EP nanocomposites in the DA reaction is similar to that of thermosetting polymers at low temperatures due to crosslinking by the DA reaction, and the nanocomposites can be recycled by a retro DA reaction at high temperatures. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Rheological and Mechanical Characterization of Dual-Curing Thiol-Acrylate-Epoxy Thermosets for Advanced Applications
Polymers 2019, 11(6), 997; https://doi.org/10.3390/polym11060997 - 04 Jun 2019
Cited by 4 | Viewed by 1562
Abstract
Mechanical and rheological properties of novel dual-curing system based on sequential thiol-acrylate and thiol-epoxy reactions are studied with the aim of addressing the obtained materials to suitable advanced applications. The crosslinking process is studied by rheological analysis in order to determine conversion at [...] Read more.
Mechanical and rheological properties of novel dual-curing system based on sequential thiol-acrylate and thiol-epoxy reactions are studied with the aim of addressing the obtained materials to suitable advanced applications. The crosslinking process is studied by rheological analysis in order to determine conversion at gelation and the critical ratio. These parameters are used to discuss the intermediate material structure for each acrylate proportion and their possible application in the context of dual-curing and multi-step processing scenarios. Results from dynamo-mechanical analysis and mechanical testing demonstrate the high versatility materials under investigation and revealed a wide range of achievable final properties by simply varying the proportion between acrylate and thiol group. The intermediate stability between curing stages has been analysed in terms of their thermal and mechanical properties, showing that these materials can be stored at different temperatures for a relevant amount of time without experiencing significant effects on the processability. Experimental tests were made to visually demonstrate the versatility of these materials. Qualitative tests on the obtained materials confirm the possibility of obtaining complex shaped samples and highlight interesting shape-memory and adhesive properties. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Effect of Curing Agent on the Compressive Behavior at Elevated Test Temperature of Carbon Fiber-Reinforced Epoxy Composites
Polymers 2019, 11(6), 943; https://doi.org/10.3390/polym11060943 - 31 May 2019
Cited by 3 | Viewed by 1076
Abstract
The aim of the underlying research is to understand the effect of elevated test temperatures on the mechanical properties of carbon fiber-reinforced laminates based on three different hardeners. A high-temperature stable adhesive was developed for the end tabs of the specimen. Bifunctional bisphenol [...] Read more.
The aim of the underlying research is to understand the effect of elevated test temperatures on the mechanical properties of carbon fiber-reinforced laminates based on three different hardeners. A high-temperature stable adhesive was developed for the end tabs of the specimen. Bifunctional bisphenol A diglycidyl ether (DGEBA) epoxy cured with triethylenetetramine (TETA), isophorone diamine (IPDA), and 4,4′-diaminodiphenylsulfone (DDS) were cured and tested in a Celanese compressive test up to 250 °C. A model by Feih was applied, and sufficient accordance (R2 > 97%) with the compressive data was found. We showed that the network density and the chemical structure of the thermoset network influenced the compressive behavior. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Semi-Interpenetrating Polymer Networks Based on Cyanate Ester and Highly Soluble Thermoplastic Polyimide
Polymers 2019, 11(5), 862; https://doi.org/10.3390/polym11050862 - 13 May 2019
Cited by 9 | Viewed by 1272
Abstract
Thermoplastic polyimide (TPI) was synthesized via a traditional one-step method using 2,3,3′,4′-biphenyltetracarboxylic dianhydride (3,4′-BPDA), 4,4′-oxydianiline (4,4′-ODA), and 2,2′-bis(trifluoromethyl)benzidine (TFMB) as the monomers. A series of semi-interpenetrating polymer networks (semi-IPNs) were produced by dissolving TPI in bisphenol A dicyanate (BADCy), followed by curing at [...] Read more.
Thermoplastic polyimide (TPI) was synthesized via a traditional one-step method using 2,3,3′,4′-biphenyltetracarboxylic dianhydride (3,4′-BPDA), 4,4′-oxydianiline (4,4′-ODA), and 2,2′-bis(trifluoromethyl)benzidine (TFMB) as the monomers. A series of semi-interpenetrating polymer networks (semi-IPNs) were produced by dissolving TPI in bisphenol A dicyanate (BADCy), followed by curing at elevated temperatures. The curing reactions of BADCy were accelerated by TPI in the blends, reflected by lower curing temperatures and shorter gelation time determined by differential scanning calorimetry (DSC) and rheological measurements. As evidenced by scanning electron microscopy (SEM) images, phase separation occurred and continuous TPI phases were formed in semi-IPNs with a TPI content of 15% and 20%. The properties of semi-IPNs were systematically investigated according to their glass transition temperatures (Tg), thermo-oxidative stability, and dielectric and mechanical properties. The results revealed that these semi-IPNs possessed improved mechanical and dielectric properties compared with pure polycyanurate. Notably, the impact strength of semi-IPNs was 47%–320% greater than that of polycyanurate. Meanwhile, semi-IPNs maintained comparable or even slightly higher thermal resistance in comparison with polycyanurate. The favorable processability and material properties make TPI/BADCy blends promising matrix resins for high-performance composites and adhesives. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessEditor’s ChoiceArticle
One-Pot Synthesis of Amide-Functional Main-Chain Polybenzoxazine Precursors
Polymers 2019, 11(4), 679; https://doi.org/10.3390/polym11040679 - 14 Apr 2019
Cited by 11 | Viewed by 1746
Abstract
Main-chain polybenzoxazines containing amide linkages were successfully prepared in one pot. Three different polymers were synthesized by reacting 3,4-dihydrocoumarine (DHC) and paraformaldehyde with 1,3-diaminopropane or 1,6-diaminohexane or Jeffamine ED-900. The one-pot reaction proceeded through the combination of the ring-opening of DHC with amines, [...] Read more.
Main-chain polybenzoxazines containing amide linkages were successfully prepared in one pot. Three different polymers were synthesized by reacting 3,4-dihydrocoumarine (DHC) and paraformaldehyde with 1,3-diaminopropane or 1,6-diaminohexane or Jeffamine ED-900. The one-pot reaction proceeded through the combination of the ring-opening of DHC with amines, and subsequent Mannich and ring-closure reactions occurring in a cascading manner. The obtained polymer from Jeffamine exhibited good film-forming properties, and free-standing flexible films were easily solvent- casted on Teflon plates. All polymeric precursors were characterized by spectroscopic analysis, and their curing behavior and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Digital Luminescence Patterning via Inkjet Printing of a Photoacid Catalysed Organic-Inorganic Hybrid Formulation
Polymers 2019, 11(3), 430; https://doi.org/10.3390/polym11030430 - 06 Mar 2019
Cited by 3 | Viewed by 1223
Abstract
Accurate positioning of luminescent materials at the microscale is essential for the further development of diverse application fields including optoelectronics, energy, biotechnology and anti-counterfeiting. In this respect, inkjet printing has recently attracted great interest due to its ability to precisely deposit with high [...] Read more.
Accurate positioning of luminescent materials at the microscale is essential for the further development of diverse application fields including optoelectronics, energy, biotechnology and anti-counterfeiting. In this respect, inkjet printing has recently attracted great interest due to its ability to precisely deposit with high throughput and no contact, functional materials on different types of substrates. Here, we present a novel photoacid catalysed organic-inorganic hybrid luminescent ink. The formulation, containing monomers bearing epoxy and silane functionalities, a photoacid generator and a small percentage of Rhodamine-B, shows good jetting properties and adequate wetting of the deposited droplets on the receiving substrates. Ultraviolet exposure of the deposited material triggers the cationic ring-opening polymerization reaction of the epoxy groups. Concomitantly, if atmospheric water is available, hydrolysis and condensation takes place, overall leading to a luminescent crosslinked hybrid organic-inorganic polymeric material obtained through a simple one-step curing process, without post baking steps. Advantageously, protection of the ink from actinic light delays the hydrolysis and condensation conferring long-term stability to the ink. Digital patterning leads to patterned emissive surfaces and elements with good adhesion to different substrates, mechanical and optical properties for the fabrication of optical and photonic elements and devices. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Thermal Conductive Composites Prepared by Addition of Several Ceramic Fillers to Thermally Cationic Curing Cycloaliphatic Epoxy Resins
Polymers 2019, 11(1), 138; https://doi.org/10.3390/polym11010138 - 15 Jan 2019
Cited by 8 | Viewed by 1684
Abstract
Novel composite coatings prepared from 3,4-epoxy cyclohexylmethyl 3,4-epoxycyclohexane carboxylate (ECC) and different ceramic fillers have been prepared to improve the thermal dissipation of electronic devices. As latent cationic initiator, a benzylanilinium salt with triethanolamine has been used, which leads to a polyether matrix. [...] Read more.
Novel composite coatings prepared from 3,4-epoxy cyclohexylmethyl 3,4-epoxycyclohexane carboxylate (ECC) and different ceramic fillers have been prepared to improve the thermal dissipation of electronic devices. As latent cationic initiator, a benzylanilinium salt with triethanolamine has been used, which leads to a polyether matrix. Different proportions of Al2O3, AlN and SiC as fillers were added to the reactive formulation. The effect of the fillers selected and their proportions on the evolution of the curing was studied by calorimetry and rheometry. The thermal conductivity, thermal stability, thermal expansion coefficient and thermomechanical and mechanical properties of the composites were evaluated. An improvement of 820% in thermal conductivity in reference to the neat material was reached with a 75 wt % of AlN, whereas glass transition temperatures higher than 200 °C were determined in all the composites. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessEditor’s ChoiceArticle
Photoinitiator Free Resins Composed of Plant-Derived Monomers for the Optical µ-3D Printing of Thermosets
Polymers 2019, 11(1), 116; https://doi.org/10.3390/polym11010116 - 11 Jan 2019
Cited by 27 | Viewed by 3814
Abstract
In this study, acrylated epoxidized soybean oil (AESO) and mixtures of AESO and vanillin dimethacrylate (VDM) or vanillin diacrylate (VDA) were investigated as photosensitive resins for optical 3D printing without any photoinitiator and solvent. The study of photocross-linking kinetics by real-time photorheometry revealed [...] Read more.
In this study, acrylated epoxidized soybean oil (AESO) and mixtures of AESO and vanillin dimethacrylate (VDM) or vanillin diacrylate (VDA) were investigated as photosensitive resins for optical 3D printing without any photoinitiator and solvent. The study of photocross-linking kinetics by real-time photorheometry revealed the higher rate of photocross-linking of pure AESO than that of AESO with VDM or VDA. Through the higher yield of the insoluble fraction, better thermal and mechanical properties were obtained for the pure AESO polymer. Here, for the first time, we validate that pure AESO and mixtures of AESO and VDM can be used for 3D microstructuring by employing direct laser writing lithography technique. The smallest achieved spatial features are 1 µm with a throughput in 6900 voxels per second is obtained. The plant-derived resins were laser polymerized using ultrashort pulses by multiphoton absorption and avalanche induced cross-linking without the usage of any photoinitiator. This advances the light-based additive manufacturing towards the 3D processing of pure cross-linkable renewable materials. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Cold-Cured Epoxy-Based Organic–Inorganic Hybrid Resins Containing Deep Eutectic Solvents
Polymers 2019, 11(1), 14; https://doi.org/10.3390/polym11010014 - 22 Dec 2018
Cited by 8 | Viewed by 1762
Abstract
The development of improved cold-cured resins, to be used as either adhesives or matrices for FRP (fiber reinforced polymer) composites employed in the construction industry, has become the focus of several academic and industrial research projects. It is expected that the use of [...] Read more.
The development of improved cold-cured resins, to be used as either adhesives or matrices for FRP (fiber reinforced polymer) composites employed in the construction industry, has become the focus of several academic and industrial research projects. It is expected that the use of nano-structured organic–inorganic hybrid materials could represent a realistic alternative to commercial epoxy-based resins due to their superior properties, especially in terms of higher durability against: moisture, temperatures, harsh environments, and fire. In this context, organic–inorganic epoxy hybrids were synthesized by a modified sol–gel method without the addition of water. The experimental formulations were prepared starting from a mixture of a silane-functionalized epoxy resin, alkoxysilane components and a deep eutectic solvent (DES) based on a blend of choline chloride and urea. The latter was added in two different loads in order to analyze in depth its effect as a promoter for an effective dispersion of silica nano-phases, formed through hydrolysis and condensation reactions, into the cross-linked epoxy network. The produced formulations were cold-cured for different time spans in the presence of two hardeners, both suitable for a curing process at ambient temperature. In this first part of a wider experimental program, several analyses were carried out on the liquid (rheological and calorimetric) and cold-cured (calorimetric, thermogravimetric, dynamic-mechanical, flexural mechanical, and morphological) systems to evaluate and quantify the improvement in properties brought about by the presence of two different phases (organic and inorganic) in the same epoxy-based hybrid system. Full article
(This article belongs to the Special Issue Thermosets II)
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Open AccessArticle
Analysis of Network Structures in Thiol-Ene UV Curing System Using Reworkable Resins
Polymers 2019, 11(1), 5; https://doi.org/10.3390/polym11010005 - 20 Dec 2018
Cited by 2 | Viewed by 2002
Abstract
An analysis of the network structures in thiol-ene UV curing resins was carried out using reworkable resins composed of di(meth)acrylate monomers having tertiary ester linkages. The effect of the functionality of the thiols, the functional ratio of the thiol and ene functions, their [...] Read more.
An analysis of the network structures in thiol-ene UV curing resins was carried out using reworkable resins composed of di(meth)acrylate monomers having tertiary ester linkages. The effect of the functionality of the thiols, the functional ratio of the thiol and ene functions, their conversions and curing atmosphere on the chain lengths was discussed. A mixture of (meth)acrylates, thiol compounds, a photoradical initiator, and a photoacid generator was cured by irradiation at 365 nm. The cured samples were degraded by heating after irradiation at 254 nm. Size exclusion chromatography (SEC) and 1H NMR analyses of the degraded samples were carried out after the methylation. The crosslinking conditions strongly affected the network structures. The degraded samples have molecular weights between 250 and 2700. The molecular weights of the degraded resins increased with the functionality of the thiol compounds. The chain length dependence suggests that thiol compounds with a high functionality have a low reactivity due to steric hindrance. The chain lengths of the degraded networks were nearly proportional to the concentration of the (meth)acrylate monomers. The addition of reactive diluents enhanced the reactivity and increased the chain length. Full article
(This article belongs to the Special Issue Thermosets II)
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