Phenol-Crotonaldehyde Resins. II. Effect of Crotonaldehyde Purity on Resin Properties

Acid-catalyzed polycondensation of phenol and crotonaldehyde results in soluble thermoplastic resins over a broad range of compositions. The thermal and curing behavior of the resins are found to vary markedly with the phenol to crotonaldehyde mole ratio and the purity of crotonaldehyde. Infrared analysis of the resins and their fractions separated by column chromatography indicates that all the resins are structurally similar. The number-average molecular weights of the resins fall in the range of 400 to 600. The resins from distilled crotonaldehyde exhibit higher molecular weights than those from crude crotonaldehyde. The thermal properties of the resins are comparable to the Novolak-type phenol-formaldehyde resins. The thermoplastic nature is retained even at higher fraction of crotonaldehyde, unlike for the conventional Novolak resins.     

A phosphate-based epoxy resin for flame retardance: synthesis, characterization, and cure properties

A phosphorus-containing oligomer, bis(3-hydroxyphenyl) phenyl phosphate (BHPP), was synthesized through the reaction of phenyl dichlorophosphate and 1,3-dihydroxybenzene, and characterized by elemental analysis, Fourier transform IR spectroscopy, and ¹H NMR and ³¹P NMR spectroscopy. Consequently, the phosphate-based epoxy resins with a phosphorus content of 1 and 2 wt % were prepared via the reaction of diglycidyl ether of bisphenol-A with BHPP and bisphenol-A, and were confirmed with Fourier transform IR spectroscopy and gel permeation chromatography. Phenolic melamine, Novolak, and dicyanodiamide were used as curing agents to prepare the thermoset resins with the control and the phosphate-based epoxy resins. Thermal properties and thermal degradation behavior of these thermoset resins were investigated by using differential scanning calorimetry and thermogravimetric analysis. The thermoset resins cured with phenolic melamine exhibited higher glass-transition temperatures than the other cured resins owing to the high rigidity of their molecular chain. Thermogravimetric analysis studies demonstrated that the decomposition temperatures of the thermoset resins cured with Novolak were higher than those of the others. A synergistic effect from the combination of the phosphate-based epoxy resin and the nitrogen-containing curing agent can result in a great improvement of the flame retardance for their thermoset resins.     

Multifunctionality in Epoxy Resins

Abstract

Epoxy resin will continue to be in the forefront of many thermoset applications due to its versatile properties. However, with advancement in manufacturing, changing societal outlook for the chemical industries and emerging technologies that disrupt conventional approaches to thermoset fabrication, there is a need for a multifunctional epoxy resin that is able to adapt to newer and robust requirements. Epoxy resins that behave both like a thermoplastic and a thermoset resin with better properties are now the norm in research and development. In this paper, we viewed multifunctionality in epoxy resins in terms of other desirable properties such as its toughness and flexibility, rapid curing potential, self-healing ability, reprocessability and recyclability, high temperature stability and conductivity, which other authors failed to recognize. These aspects, when considered in the synthesis and formulation of epoxy resins will be a radical advance for thermosetting polymers, with a lot of applications. Therefore, we present an overview of the recent finding as to pave the way for varied approaches towards multifunctional epoxy resins.     

Biomass-derived dynamic covalent epoxy thermoset with robust mechanical properties and facile malleability

Biomass-derived dynamic covalent thermoset has been considered as a promising solution to the high dependence on fossil resources and the difficulty in recyclability after curing of conventional bisphenol A epoxy resins. However, the design and preparation of a dynamic covalent biobased epoxy thermoset with both comparable thermal and mechanical performances to bisphenol A epoxy resins and reprocessibility remains a significant challenge. Herein, based on imine chemistry, a novel Schiff base-con...     

Plastic deformation mechanisms in polyimide resins and their semi-interpenetrating networks

The plastic deformation mechanisms in both thermoplastic and thermoset polyimide resins and their semi-interpenetrating networks (semi-IPNs) were investigated. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers was evaluated. In situ polarized light microscopic observation of crack-tip deformation mechanisms in solvent-cast films was conducted with a miniature testing device attached to the microscope stage. NASA LaRC TPI, a thermoplastic polyimide, was found to exhibit combined shear yielding and crazing under plane-stress loading conditions. Contrarily, NR-150B2 thermoplastic polyimide exhibited diffuse shear yeilding; no shear banding or crazing was found near the crack tip. Adding a small amount of thermoplastic component, either NR-150B2 or LaRC TPI, was found to raise considerably the fracture toughness of PMR-15 thermoset PI. PMR-15 thermoset films also showed diffuse shear yielding, albeit with a much smaller deformation zone. This was consistent with its low toughness and could be understood on the basis of a limited extensibility of a highly crosslinked network. Numerical calculations were performed to confirm this low value of network chain draw ratio. The dimensions of the deformation zone ahead of the crack tip in a semi-IPN with a thermoset matrix were increased as a higher weight fraction of the thermoplastic component was added. The deformation zone size of a thermoplastic matrix was found to decrease with an increasing amount of thermoset PMR-15. Deformation also became more diffuse with a higher PMR-15 content in LaRC TPI. Fracture toughness variations can be correlated with deformation size changes in these semi-IPNs.     

晶须及其在高分子材料中的应用

简要介绍了目前高分子材料改性中常用的晶须，如钛酸钾晶须、硼酸铝晶须、氧化锌晶须等，主要对其在橡胶、热塑性弹性体、热塑性树脂以及热固性树脂中的应用作了详尽论述。     

Thermoplastic modification of monomeric and partially polymerized Bisphenol A dicyanate ester

Bisphenol A dicyanate ester (BADCy) was modified with different amounts of an engineering thermoplastic, polysulfone (PS) to improve impact strength of the parent resin. Differential scanning calorimetry of the blends suggested that addition of PS widens the curing exotherm of the BADCy considerably. FTIR of cured neat resins indicated total conversion of cyanate functional groups into triazine rings by cyclotrimerization. The cured neat resins showed phase separated morphology with cyanate ester as the continuous phase. The modified resins were shown to have better thermal, hygrothermal and impact strength properties. However, when glass fiber reinforced composites were made using partially polymerized BADCy and PS, very little or no phase separation in the resin was noticed. Flexural and impact strength measurement of composites showed that PS modification has compromised the flexural properties and only retained the impact strength of the parent resin containing composite. This study thus suggests that improvements realized in thermoplastic modification of monomeric BADCy are not directly transferable to composites using a partially prepolymerized BADCy.     

Thermal stability and dynamic mechanical thermal analysis of epoxy thermosets possessing N,N′-disubstituted pyromellitimide units

Abstract

In this work, three epoxy resins including diglycidyl ethers of N,N′-bis(2-hydroxyethyl)pyromellitimide (DIDGE), bisphenol-A (BADGE), and polyethylene glycol (PEDGE) were isothermally cured by an amine curing agent possessing N,N′-disubstituted pyromellitimide units (denoted by DIDAM). DIDGE resin was synthesized from the reaction of N,N′-bis(2-hydroxyethyl)pyromellitimide with an excess of epichlorohydrin. Also, DIDAM curing agent was prepared from the reaction of pyromellitic dianhydride with an excess of ethylene diamine. Completion of the isothermal curing processes was approved by both Fourier transform-infrared spectroscopy and non-isothermal differential scanning calorimetry (DSC). The DSC traces showed only the phase transitions related to the thermal degradation of the resulting thermosets. According to the thermogravimetric analyses, the DIDGE/DIDAM thermoset showed higher thermal stability at temperatures above 425?°C than the other two thermosets. While BADGE/DIDAM and PEDGE/DIDAM thermosets showed about 70% weight loss in the thermal range of 400–850?°C, DIDGE/DIDAM thermoset was encountered with only about 40% weight loss. The glass transition temperatures (T_g ) of the resulting thermosets were determined using tan δ vs temperature plots obtained from dynamic mechanical thermal analysis. The T_g values of BADGE/DIDAM, DIDGE/DIDAM, and PEDGE/DIDAM thermosets were found to be 211?°C, 189?°C, and 81?°C, respectively.     

Curing kinetics of liquid-crystalline epoxy resins

By endcapping mesogenic rigid rod molecules with reactive epoxy groups a novel class of liquid-crystalline thermoset has been obtained. In fact is has been shown that the nematic molecular arrangement is sustained over the crosslinking reaction of liquid-crystalline epoxy resins when the curing reaction is carried out in the thermal stability range of the liquid-crystalline phase. Calorimetric analysis was used in characterizing the isothermal cure. An unsophisticated model is proposed for evaluating the activation energies of the crosslinking reaction. For liquid-crystalline epoxy resins lower activation energies result with respect to the cure reactions for non liquid-crystalline epoxy resins.     

Interdiffusion at the interface between poly(vinylpyrrolidone) and epoxy

Electron microprobe analysis (EMP) was used to study interdiffusion in bilayer films of thermoplastic poly(vinylpyrrolidone) (PVP) and a thermoset epoxy. The bilayer films were prepared by casting a stoichiometric mixture of the uncured diglycidyl ether of bisphenol A epoxy (DGEBA) and 4,4′-diaminodiphenylsulfone (DDS) on the PVP film and then curing the system in a two-step process under a nitrogen atmosphere. For the EMP studies, the sulfur signal was used as a probe for DDS, while the nitrogen signal served as a probe for both DDS and PVP. The addition of brominated DGEBA to the conventional DGEBA in a 1: 1 weight ratio allowed the bromine signal to be used as a probe for the epoxy phase. It was found that the interfacial thickness was much larger for the film prepared from low molecular weight PVP than that from high molecular weight PVP. Interdiffusion was suppressed when the initial cure temperature in the two-step cure cycle was 130°C compared to 170°C, in which the first stage of the cure reaction dominated the interdiffusion process. More importantly, it was demonstrated that the diffusion front of the curing agent was located closer to the thermoplastic polymer phase as compared to that of the thermoset polymer in the interface region. This tendency was more significant in the system with the larger interfacial thickness. These results have important consequences on interphase structures and properties. They suggest that crosslinking of the epoxy in the interphase may be suppressed because of an insufficient amount of curing agent and that the not-fully-reacted curing agent in the PVP phase may act to plasticize this phase. © 1997 John Wiley & Sons, Inc.     

Synthesis of methacrylic copolymers with nona‐1‐butoxy trititanosiloxane as side groups and its application as a thermosetting resin

Methacrylic copolymers with a hydroxyl group on one end of the main chain and nona‐1‐butoxytrititanosiloxane as side groups (called methacrylic hybrid copolymers) were synthesized for use as baked‐finish‐type coating resins. The chemical structures of the side groups in the methacrylic hybrid copolymers were confirmed with the ash weight of the copolymers after combustion, the elemental ratio analysis of Si and Ti in the ash determined by inductively coupled plasma emission spectrometry, and the characteristic absorption band determined by Fourier transform infrared spectrophotometry. The methacrylic hybrid copolymers were cured at temperatures less than 150 °C in the absence of a curing accelerator. The cured copolymers exhibited a high thermal stability. The curing temperature of the copolymers was determined by the change in the absorption peak strength (peak area) of the 1655 cm⁻¹ band in the IR difference spectrum. The thermal stability of the copolymers was evaluated as the thermal‐degradation temperature measured by thermogravimetric analysis. The methacrylic hybrid copolymers were then be used as effective curing resins. The mixture, consisting of thermoplastic methacrylic terpolymer with hydroxyl and carboxyl groups and the methacrylic hybrid copolymers, were cured at less than 150 °C in the absence of a curing accelerator and exhibited a higher thermal‐degradation temperature than the copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1090–1098, 2001     

Modelling the effect of moisture absorption on the thermomechanical relaxation spectra for thermoset/thermoplastic resin blends

The principles of molecular modelling have been combined with Group Interaction Modelling (GIM) for the prediction of properties of thermoset/thermoplastic resins. The glass transition temperature of the systems was modelled and thermomechanical properties of epoxy resin polyether sulphone blends were discussed. As a result a full envelope of Dynamic Mechanical Thermal Analysis Relaxation (DMTA) curve for wet and dry material was predicted. Differential Scanning Calorimetry (DSC) and DMTA experimental methods were used to validate the modelling results.     

Transport properties of thermoplastic/thermoset blends

Gas transport properties are reported for two series of films prepared from initially miscible thermoplastic/thermoset blends, respectively, polystyrene PS/thermoset and poly(2,6 dimethyl 1,4 phenylene oxide) PPE/thermoset blends. The thermoplastic contents are such that in both cases, after the phase separation, the continuous phase is the thermoplastic‐rich phase and scanning electron microscopic photomicrographs clearly evidenced the dispersion of thermoset‐rich nodules in the continuous thermoplastic‐rich phase with a more tortuous morphology in the case of PPE based films. Permeability measurements were made for O₂ and CO₂ at 20°C and a reduction in permeability coefficients was observed with increased thermoset content. Analysis using Maxwell law suggests that for all thermoplastic/thermoset blends, the thermoset particles can be considered as impermeable to gas and that the diffusion takes place in the continuous phase. In the case of PPE based films, the higher decrease of permeability than that predicted by the law has been related to the morphology of the blends and thus the tortuosity and to a partial miscibility of the thermoset in the thermoplastic. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 473–483, 1999     

Oligo-fluoropolymer Modified Cycloaliphatic Epoxy Resins with Excellent Compatibility,Waterproof and Mechanical Properties for LED Encapsulation

An oligo-fluoropolymer(PFM) with functional cycloaliphatic epoxy and fluorinated groups was obtained via free radical polymerization and applied to the modification of cycloaliphatic epoxy resins(CE). The chemical structure of PFM was characterized by Fourier transform infrared(FTIR) spectroscopy, gel permeation chromatography(GPC) and nuclear magnetic resonance(NMR) spectroscopy, and the effects of different PFM concentrations(0.5%—6%, mass fraction) on the thermal resistance, mechanical properties, surface dewettability, light transmission, refractive index and various cured polymer properties were studied in detail. The DSC and TGA results demonstrate that the modified epoxy resins possess a higher thermal resistance than the neat epoxy resin. The improvements in the surface dewettability and water resistance are caused by the high crosslinking density and the enrichment of the oligo-fluorinated random copolymers dispersed in the matrix. The fracture surface morphologies of the thermosets were investigated by scanning electron microscopy(SEM) and transmission electron microscopy(TEM). It was observed that the optical transmittance of the composites was maintained even though microphase separation occurred during the curing process. With respect to the corresponding properties of the neat epoxy resins, the 2 phr(parts per hundreds of resin) PFM thermoset exhibited relatively better comprehensive properties, making the cured material a good candidate for light-emitting diode(LED) encapsulation.     

Cure study of addition-cure-type and condensation-addition-type phenolic resins

By the incorporation of propargyl and methylol groups on to novolac backbone, a series of addition-curable phenolic resins and condensation-addition dual-cure type phenolic resins (novolac modified by propargyl groups referred as PN, and novolac modified by propargyl and methylol groups simultaneously referred as MPN) were synthesized. The processing characteristics, thermal cure and catalytic cure behavior for both resins were investigated mainly by means of viscosity measurement and non-isothermal differential scanning calorimetry (DSC) techniques. The effect of propargyl and methylol content of PN and MPN, the molecular weight and the configuration of the parent novolac, on the processing and cure behavior was studied in details. Processing parameters and curing kinetic parameters were obtained. Both resins exhibit excellent processing properties. Thermal cure of PN resins possessed one cure mechanism and that of MPN resins possessed two cure mechanisms according to DSC analysis. The dual-cure-type mechanism made MPN resins superior to PN resins in terms of a mild and controllable cure process. Compared with thermal cure, catalytic cure of PN resins showed lower initiation temperature and cure temperature by about 60 °C. These novel resins have a bright prospect of application as matrix for thermal-structural composite materials.     

High flow addition curing polyimides

A new series of high flow PMR-type addition curing polyimides was developed, which employed the substitution of 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (BTDB) for p-phenylenediamine (p-PDA) in a PMR-II formulation. These thermoset polyimides, designated as 12F resins, were prepared from BTDB and the dimethyl ester of 4,4′-(hexafluoroisopropylidene)-diphthalic acid (HFDE) with either nadic ester (NE) or p-aminostyrene (PAS) as the endcaps for addition curing. The 12F prepolymers displayed lower melting temperatures in DSC analysis, and higher melt flow in rheological studies than the corresponding PMR-II polyimides. Long-term isothermal aging studies showed that BTDB-based 12F resins exhibited comparable thermo-oxidative stability to p-PDA based PMR-II polyimides. The noncoplanar 2- and 2′-disubstituted biphenyldiamine (BTDB) not only lowered the melt viscosities of 12F prepolymers, but also retained reasonable thermal stability of the cured resins. The 12F polyimide resin with p-aminostyrene endcaps showed the best promise for long-term, high-temperature application at 343°C (650°F). © 1994 John Wiley & Sons, Inc.     

Dielectric properties of self‐catalytic interpenetrating polymer network based on modified bismaleimide and cyanate ester resins

Bismaleimide (BMI) resins with good thermal stability, fire resistance, low water absorption, and good retention of mechanical properties at elevated temperatures, especially in hot/wet environments, have attracted more attention in the electronic and aerospace industries. However, their relatively high dielectric constant limits their application in the aforementioned fields. In this work, a new promising approach is presented that consists of the formation of a self‐catalytic thermoset/thermoset interpenetrating polymer network. Interpenetrating polymer networks (IPNs) based on modified BMI resin (BMI/DBA) and cyanate ester (b10) were synthesized via prepolymerization followed by thermal curing. The self‐catalytic curing mechanism of BMI/DBA‐CE IPN resin systems was examined by differential scanning calorimetry. The dielectric properties of the cured BMI/DBA‐CE IPN resin systems were evaluated by a dielectric analyzer and shown in dielectric properties‐temperature‐log frequency three‐dimensional plots. The effect of temperature and frequency on the dielectric constant of the cured BMI/DBA‐CE IPN resin systems is discussed. The composition effect on the dielectric constant of the cured IPN resin systems was analyzed on the basis of Maxwell's equation and rule of mixture. The obtained BMI/DBA‐CE IPN resin systems have the combined advantages of low dielectric constant and loss, high‐temperature resistance, and good processability, which have many applications in the microelectronic and aerospace industries. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1123–1134, 2003     

Bioinspired Design Provides High‐Strength Benzoxazine Structural Adhesives

A synthetic strategy to incorporate catechol functional groups into benzoxazine thermoset monomers was developed, leading to a family of bioinspired small‐molecule resins and main‐chain polybenzoxazines derived from biologically available phenols. Lap‐shear adhesive testing revealed a polybenzoxazine derivative with greater than 5 times improved shear strength on aluminum substrates compared to a widely studied commercial benzoxazine resin. Derivative synthesis identified the catechol moiety as an important design feature in the adhesive performance and curing behavior of this bioinspired thermoset. Favorable mechanical properties comparable to commercial resin were maintained, and glass transition temperature and char yield under nitrogen were improved. Blending of monomers with bioinspired main‐chain polybenzoxazine derivatives provided formulations with enhanced shear adhesive strengths up to 16 MPa, while alloying with commercial core–shell particle‐toughened epoxy resins led to shear strengths exceeding 20 MPa. These results highlight the utility of bioinspired design and the use of biomolecules in the preparation of high‐performance thermoset resins and adhesives with potential utility in transportation and aerospace industries and applications in advanced composites synthesis.     

A new class of epoxy thermosets

The reinforcing strategies of epoxy thermosets rely on the control of the phase separation between the additive and the growing thermoset. With standard additives, such as reactive liquid rubbers, the length scale of the resulting domains is the micrometer. Here, we present a route that enable a control of the morphology down to the nanometer scale. This strategy is based upon the self-assembly process of blends of epoxy and SBM triblock copolymers, namely Poly(Styrene-b-1,4 Butadiene-b-Methyl methacrylate). It relies on the respective affinities between the epoxy precursors and each of the three blocks. Liquid epoxy has a strong affinity for PMMA, whilst it is not miscible with polystyrene nor polybutadiene at standard processing temperatures. Thus, within the reactive system, microphase separation leads to a regular network of S-B domains. This nanostructure is governed by thermodynamics. The size and geometry of the dispersed domains are controlled by the concentration and the ratio between blocks lengths. The domain size is of the order of magnitude of the chain length, ranging typically from 10 to 30 nanometers. What controls the blend's morphology throughout the curing process of the thermoset was one topic on which we focused our interest. Nanostructured thermosets have been obtained. These supramolecular architectures yield significant toughness improvements while preserving the transparency of the material. The reinforcing mechanisms are not yet fully understood : it is intriguing to induce significant toughening with elastomer domains smaller than 30 nanometers in diameter. Besides being efficient epoxy tougheners, SBM can broaden the scope of applications of thermosets due to specific rheological behaviors. Thanks to the self assembly process taking place in the blend of the SBM block copolymers with the epoxy thermosets precursors, the reactive solvent can be turned into a reactive gel or solid (before curing). This physical gelation is induced by the microphase separation and is thus thermoreversible. At relatively moderate loadings of block copolymers the reactive blend behaves like a thermoplastic material, with adjustable modulus and tackiness. These results evidence that SBM block copolymers open a broad area for designing new class of thermoset materials.     

Preparation, Curing Reactivity and Thermal Properties of Titanium-doped Silicone Resins

Novel titanium-doped silicone resins were synthesized from low-cost silane monomers and tetrabutyl titanate as raw materials and hydrochloric acid as catalyst, with titanium element as dopant into principal chain of Si-O-Si. The resins were characterized by means of FTIR, 1 H NMR and 13 C NMR spectra, their thermal properties and curing properties were investigated and their corresponding films were determined. The results show that the thermal stability and storage stability of the resins were influenced by the types of silane monomers containing different carbon atomicities of organic group. The thermal stability of the titanium-doped silicone resin with a molar ratio of silane monomer B(n-propyl triethoxysilane) to silane monomer C(n-octyl triethoxysilane) being 1:1 is superior to that of the resin with a molar ratio of silane monomer B to silane monomer C being 1:3. However, the storage stability of the former is inferior to that of the latter.This work also showed that the synthesized titanium-doped silicone resins have the highest thermal stability up to 450―500 °C with an atomicity molar ratio of 1:4 of titanium to silicon in the reactants. But the best storage stability of the resin prepared from the reactants with an atomicity molar ratio of 1:6[n(Ti):n(Si)] was obtained. The effect of the type and content of curing agent on the curing properties of the resin was also studied. Moreover, thermal mechanism and curing mechanism were proposed in this work.