Influence of the curing cycle selection on the thermal degradation of an epoxy-diamine system

Summary Polymers have a great interest for the study and design of new materials. Among these materials are epoxy resins, that have good properties, such as low shrinkage during cure, good adhesion, high water and chemical resistance, etc. They have also fast and easy cure in a broad range of temperatures. TTT diagrams are very helpful to design new epoxy materials as they allow the search for very important final properties, such as thermal stability, conversion or glass transition temperature of a material cured through a selected curing cycle. In this work the dependence of the thermal stability on the selected curing cycle for a DGEBA/1,2 DCH system was studied.     

Novel thermosets obtained by UV‐induced cationic copolymerization of DGEBA with an spirobislactone

The photocuring process of the diglycidyl ether of bisphenol A (DGEBA) with the bislactone 1,6‐dioxaspiro[4,4]nonane‐2,7‐dione (s(γ‐BL)) was studied. Triarylsulfonium hexafluoroantimonate was employed as photoinitiator. FTIR/ATR was used to study the evolution of epoxy, lactone, and intermediate spiroorthoester groups to identify the different reactions that take place during the photocuring process. Photo‐DSC and DSC were used to study the thermal evolution of the photocuring process and to assess the T_g of the fully cured material. Thermogravimetric analysis (TGA) was used to determine the thermal stability of the fully cured material. The thermomechanical properties of the materials were investigated using dynamic mechanical‐thermal analysis. Shrinkage undergone during photocuring and gelation was studied with TMA. A strong influence of the photocuring temperature on the photocuring process of the DGEBA‐ s(γ‐BL) system was observed. Differences in the reactivity of the different species were observed with respect to the thermally cured system using ytterbium triflate as cationic thermal initiator. As a consequence, photocured materials exhibited a superior thermal stability and lower flexibility. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5446–5458, 2007     

Controlled shrinkage polymers: Characterization of epoxy resins cured with spirobislactones

The diglycidyl ether of bisphenol A (DGEBA) was cured with either an aliphatic or an aromatic spirobislactone using a tertiary amine catalyst. The products were characterized by FTIR, TGA, DSC, dilatometry, and single-fiber adhesion measurements, and their performance was compared to that of DGEBA cured with acid anhydrides. Both aliphatic and aromatic bislactones are effective curing agents for DGEBA. FTIR and dilatometry confirm that both lactone rings open early in the curing reaction and initially offset shrinkage caused by polymerization. After the bislactone has been consumed, oxirane reactions proceed in a normal fashion. The final shrinkage of cured DGEBA polymers, with or without addition of bislactones, is 3.0–3.5%. Bislactone-modified materials possess superior thermal properties, when compared to those of anhydride-cured materials.     

化学结构变化对刚棒状环氧树脂相行为和热力学性能的影响

用线性酚醛树脂(PN)和4-氨基苯基氨基砜(SAA)作为固化剂, 与刚性棒状环氧树脂联苯环氧(DGEBP)、四甲基联苯环氧(DGETMBP)和传统双酚A环氧树脂(DGEBA)分别进行固化. 研究了固化剂和环氧树脂化学结构的改变对热固网络相行为和热力学性能的影响. 结果表明, 刚性环氧网络比传统的DGEBA具有更好的热力学性能. DGEBP可形成不同类型的取向网络, 而取向态的类型也直接影响了热固网络的热力学性能. 用扫描电镜(SEM)观察不同网络体系的断裂面结构, 发现取向的刚性棒状环氧网络的断裂面呈韧性断裂, 而其它无定形环氧网络则呈典型的脆性断裂.     

Development and characterization of a novel skeletal modified tetraglycidyl epoxy toughened DGEBA epoxy matrices

The present work investigates the improvement in mechanical properties observed for commercially available diglycidyl ethers of bisphenol-A (DGEBA) with the incorporation of a new type of skeletal modified tetra glycidyl epoxy resin TGBAPB as modifier. Varying weight percentages of TGBAPB have been blended with DGEBA and cured with diaminodiphenylmethane (DDM). The chemical structure of TGBAPB was confirmed by FTIR, NMR, and molecular weight determination was carried out by ESI-MS spectroscopic techniques. The thermal properties were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and mechanical properties like tensile strength, flexural strength, impact strength were also studied by universal testing machine (UTM). Scanning electron microscopy (SEM) investigates the morphological behavior of the neat and blend epoxy resins. The results from different studies indicate that the blend epoxy resin system “B” comprising 75% DGEBA/25% TGBAPB has shown improvements in both toughness and stiffness, despite the fact that it is often found that the enhancement of these two properties together in a material cannot be simultaneously achieved. These aspects of this work are novel.     

Kinetics and thermal properties of epoxy resins based on bisphenol fluorene structure

The fluorene-containing epoxy, diglycidyl ether of 9,9-bis(4-hydroxyphenyl) fluorene (DGEBF) was synthesized by a two-step reaction procedure. In order to investigate the relationship between fluorene structure and material properties, DGEBF and a commonly used diglycidyl ether of bisphenol A (DGEBA) were cured with 4,4-diaminodiphenyl methane (DDM) and 4,4-(9-fluorenylidene)-dianiline (FDA). The curing kinetics, thermal properties and decomposition kinetics of these four systems (DGEBA/DDM, DGEBF/DDM, DGEBA/FDA, and DGEBF/FDA) were studied in detail. The curing reactivity of fluorene epoxy resins was lower, but the thermal stability was higher than bisphenol A resins. The onset decomposition temperature of cured epoxy resins was not significantly affected by fluorene structure, but the char yield and T_g value were increased with that of fluorene content. Our results indicated that the addition of fluorene structure to epoxy resin is an effective method to improve the thermal properties of resins, but excess fluorene ring in the chain backbone can depress the curing efficiency of the resin.     

Studies on epoxy resins cured by cationic latent thermal catalysts: The effect of the catalysts on the thermal,rheological, and mechanical properties

To investigate the effect of catalysts on the thermal, rheological, and mechanical properties of an epoxy system, a resin based on diglycidyl ether of bisphenol‐A (DGEBA) was cured by two cationic latent thermal catalysts, N‐benzylpyrazinium hexafluoroantimonate (BPH) and N‐benzylquinoxalinium hexafluoroantimonate (BQH). Differential scanning calorimetry was used for the thermal characterization of the epoxy systems. Near‐infrared spectroscopy was employed to examine the cure reaction between the DGEBA and the latent thermal catalysts used. The rheological properties of the blend systems were investigated under an isothermal condition with a rheometer. To characterize the mechanical properties of the systems, flexure, fracture toughness (K_IC), and impact tests were performed. The phase morphology was studied with scanning electron microscopy of the fractured surfaces of mechanical test samples. The conversion and cure activation energy of the DGEBA/BQH system were higher than those of the DGEBA/BPH system. The crosslinking activation energy showed a result similar to that obtained from the cure kinetics of the blend systems. The flexure strength, K_IC, and impact properties of the DGEBA/BQH system were also superior to those of the DGEBA/BPH system. This was a result of the substituted benzene group of the BQH catalyst, which increased the crosslink density and structural stability of the epoxy system studied. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 187–195, 2001     

碳纳米管改性环氧树脂的导热和阻燃性能

以双酚A二缩水甘油醚(DGEBA)环氧树脂(Epoxy Resin,EP)为基体、甲基六氢苯酐(MHHPA)为固化剂、以多壁碳纳米管(MWCNTs)为添加剂制备了环氧树脂/碳纳米管纳米复合材料。通过对微观结构、玻璃化转变温度(Tg)、热失重、热导率和锥形量热测试结果分析,研究了质量分数少于1.5%的MWCNTs对环氧树脂的导热和阻燃性能影响,结果表明,MWCNTs质量分数为1.5%时,复合材料发生团聚;纳米复合材料随着MWCNTs质量分数的增加Tg值先增加后降低;失重5%时,对应的温度先增加后降低,残炭量增加;样品的热导率呈现先升高后降低的趋势,当MWCNTs质量分数为1%时,复合材料的热导率最大;MWCNTs加入后环氧树脂的总释热量减少,释烟量增加,阻燃性得到一定程度的提高。     

Thermally reversible nanostructured thermosetting blends modified with poly(ethylene-b-ethylene oxide) diblock copolymer

The main aim of this research was the generation of new intelligent materials, in this case thermoreversible material, based on epoxy matrix modified with semi-crystalline block copolymers. In this study, the epoxy system based on a diglycidyl ether of bisphenol-A (DGEBA), was cured with a stoichiometric amount of an aromatic amine hardener, 4,4’-methylene bis (3-chloro-2,6-diethylaniline) (MCDEA). A diblock copolymer of polyethylene-b-poly(ethylene oxide) (PEOE) was used as self-assembly agent. Optical properties of the samples modified by addition of PEOE were studied by using transmission optical microscope (TOM) equipped with a hot stage. Additionally, morphology generated in the sample was studied by atomic force microscopy (AFM).     

Curing and thermomechanical properties of off-stoichiometric anhydride–epoxy thermosets

In the present work, we report the preparation and characterization of a new family of thermosets based on off-stoichiometric anhydride–epoxy formulations in the presence of an anionic initiator. Diglycidyl ether of bisphenol A (DGEBA) and hexahydro-4-methylphthalic anhydride (HHMPA) have been used as epoxy and anhydride comonomers, respectively, and 1-methylimidazole (1MI) has been used as anionic initiator. The isothermal curing kinetics and the thermal properties of the stoichiometric and the off-stoichiometric systems have been compared. The kinetics of the isothermal curing has been analyzed by differential scanning calorimetry (DSC) using an isoconversional method and the ?esták–Berggren equation to determine the activation energy, the frequency factor and the reaction orders. The materials obtained were characterized by DSC and dynamic mechanical analysis. Gelation during epoxy–anhydride condensation was determined by thermomechanical analysis. At the same curing temperature, the reaction is faster in the system with excess of epoxy groups. However, the glass transition temperatures of the partially cured stoichiometric system are greater. The gelation time of the off-stoichiometric system is shorter than that of the stoichiometric one. The results indicate that the dual-curing character of off-stoichiometric DGEBA/HHMPA thermosets with 1MI as anionic initiator makes them suitable for multistage curing processes with easy control of degree of cure and material properties in the intermediate stage and enhanced final material properties.

     

Evaluation of physico-chemical properties and antimicrobial synergic effect of ceftazidime-modified chitosan

The aim of this work was to study the effect of tris(3-nitrophenyl) phosphine (NPPh3), which showed a good thermal stability and carbon-forming ability, on the flame retardancy and thermal degradation mechanism of epoxy resins. A series of diglycidyl ether of bisphenol A (DGEBA) loaded with tris(3-nitrophenyl) phosphine (NPPh3) were prepared. It was found that NPPh3 can effectively improve the flame retardancy and thermal stability of the composites. When the loading amount of NPPh3 was 14%, the LOI value of the DGEBA composites was 29.2% (about 1.53 times the corresponding value of the original DGEBA resin). Thermal stability was studied by thermogravimetric analysis, and the results showed that the addition of NPPh3 can improve char formation of this system both in nitrogen and in air atmosphere. Specifically, its combustion residue at 800 °C in nitrogen atmosphere was about 4.26 times of the original resin. Differential scanning calorimetry indicated that NPPh3 slightly decreased the glass transition temperature of epoxy resins. Additionally, the gaseous degradation products were analyzed by thermogravimetric analysis/infrared spectrometry, providing insight into the thermal degradation mechanism. Scanning electron microscopy and Fourier transform infrared were brought together to evaluate the morphology and structure of the residual char obtained after combustion.     

Curing studies of epoxy resin with nadic endcapped phosphorylated amines

Polyimides have aromatic moieties in the backbone structure which are responsible for their increased thermal stability. If phosphorus is introduced in the main chain structure of polyimides, there is further improvement in the thermal stability. This has been proved by the work carried out in our group. The polyimide having amine termination can be used for crosslinking of epoxy resins.In the present study amine terminated phosphorus containing nadicimide were taken as curing agent for DGEBA resins. The curing characteristics of DGEBA resin were studied by DSC using different amounts of nadic endcapped phosphorylated amines. DSC thermogram showed the heat of polymerization was lower as compared to system cured with aromatic amines.     

Improvement in fracture behaviors of epoxy resins toughened with sulfonated poly(ether sulfone)

The sulfonated poly(ether sulfone) (SPES) was successfully prepared using chlorosulfonic acid as a sulfonating agent. Diglycidylether of bisphenol-A (DGEBA) epoxy resins were modified with different contents of SPES, and the thermal and mechanical interfacial properties of DGEBA/SPES blends were investigated. As a result, the surface free energy of the blends was increased by the addition of SPES. DSC measurements revealed that the curing reaction was delayed with the increase of SPES content. Whereas, the thermal stabilities of the blends were slightly decreased as the SPES content increased. Meanwhile, the glass transition temperature and fracture toughness of the blends were increased with increasing SPES content, due to the improved intermolecular interactions, such as hydrogen bonding, between the hydroxyl group of DGEBA and the sulfonic group of SPES in the blends. The agreement could be observed by SEM which revealed phase separated morphology of DGEBA/SPES blends.     

Influences of template concentration and epoxy functionality on porous epoxy systems: Characteristics of thermal stability, optical, dielectric, and mechanical properties

Two different multi-porous epoxy thermosets (MPETs), bi-functionality of DGEBA and tri-functionality of TGTPM, were prepared foremost for the interaction of template concentration and epoxy functionality under several physical properties. By performing an automated mercury porosimeter test, we found out template concentration was critical to the amount of voids/pores. Meanwhile, epoxy functionality decided the formation of porous structure through SEM. Comparing with TGTPM MPETs system, the DGEBA MPETs system with 20 wt% template displayed appreciable T_g and tan δ properties while the phenomenon exhibited higher thermal stability property. Additionally, thermal conductivity patterns show the DGEBA MPETs system is a remarkable material of thermal resistance. However, it reduces optical clarity, dielectric permittivity and mechanical strength according to the UV-visible spectroscopy, LCR meter, and DMA, normally. Therefore, we can understand that template concentration and epoxy functionality are key factors of physical degradation and stability in porous epoxy materials.     

Thermal properties and toughness performance of hyperbranched‐polyimide‐modified epoxy resins

An amine‐terminated hyperbranched polyimide (HBPI) was prepared by the condensation polymerization of a commercially available triamine monomer with a dianhydride monomer. The effects of the HBPI content on the thermal and mechanical interfacial properties of diglycidyl ether of bisphenol A (DGEBA) epoxy resins were investigated with several techniques. The thermogravimetric analysis results showed that the thermal stability of the DGEBA/HBPI blends did not obviously change as the HBPI content increased. The glass‐transition temperature (T_g) of the DGEBA/HBPI blends increased with the addition of HBPI. Improvements in the critical stress intensity factor (K_IC) and impact strength of the blends were observed with the addition of HBPI. The K_IC value and impact strength were 2.5 and 2 times the values of the neat epoxy resins with only 4 wt % HBPI. The fractured surfaces were studied with scanning electron microscopy to investigate the morphology of the blends, and they showed that shear deformation occurred to prevent the propagation of cracks in the DGEBA/HBPI blends. These results indicated that a toughness improvement was achieved without a decrease in the thermal stability or T_g. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3348–3356, 2006     

Effect of epoxy and filler concentrations on curing behaviour of isotropic conductive adhesives

Isotropic conductive adhesives (ICAs) are considered as the most promising replacement to lead-based solders due to relatively low melting point, simple processing and fine pitch capability. The study reports on the effect of volume fractions of silver flakes and particles on the curing reaction of ICAs. In addition, the thermal behavior of ICAs formulated with two types of polymeric adhesives: diglycidylether of bisphenol-A (DGEBA) and polyurethane (PU) was also investigated in this work. The increase in silver flakes concentration seem to reduce the melting temperature of the DEGBA and PU-based ICAs, which could be due to retardation of cross-linking of polymers due to silver flakes acting as barrier within the polymer matrix. In addition, the formulation of ICAs containing silver flakes and particles yielded a much higher reaction compared to conventional ICAs. The study showed that use of polyurethane (PU) as opposed to conventional Diglycidylether of bisphenol-A (DGEBA) showed a more stable system.     

Synthesis and curing of amino‐containing benzoxazine as a novel hardener for diglycidyl ether of bisphenol‐A

Benzoxazines containing various additional functional groups have been extensively reported to improve the properties of polybenzoxazines. In this work, a novel amino‐containing benzoxazine (PDETDA‐NH₂) was conveniently synthesized from diethyltoluenediamine (DETDA), 2‐hydroxybenzaldehyde, and paraformaldehyde and was used as a hardener for diglycidyl ether of bisphenol‐A (DGEBA). The curing behaviors of PDETDA‐NH₂ and PDETDA‐NH₂/DGEBA systems were studied by DSC, FT‐IR, and ¹H NMR. When curing, PDETDA‐NH₂ was firstly polymerized to N,O‐acetal‐type polymer and then rearranged to Mannich‐type polymer at elevated temperature, while the addition reaction between amino and benzoxazine was discouraged because of the steric hindrance of alkyl substituents. During PDETDA‐NH₂/DGEBA curing, it was found that the reactions happened in the order of addition polymerization of amino and epoxide, ring‐opening polymerization of benzoxazine, etherification between phenolic hydroxyl of the polymerized benzoxazine, and epoxide. Compared with DETDA cured DGEBA, PDETDA‐NH₂ cured DGEBA showed higher modulus, higher char yield, and much lower water uptake.     

TG/DTG/DTA/DSC as a tool for studying deposition by the sol-gel process on materials obtained by rapid prototyping

In rapid prototyping (RP), building 3D physical prototypes involves the addition of material in layers. The sol-gel route is an alternative to produce multicomponent oxide materials with chemical, physical and thermal properties that cannot be obtained by other processes. The sol-gel method allows for the preparation of coatings on several kinds of materials, directly influencing the materials’ properties. In this work, metal oxides were prepared by the sol-gel process and deposited further by dip-coating technique on ABS and Nylon substrates obtained by RP. The resulting coating presented good adhesion to the substrates. The obtained materials were characterized by scanning electron microscopy (SEM) and thermal analysis (TA).     

Origin of Modulus Improvement for Epoxide-terminated Hyperbranched Poly(ether sulphone)/DGEBA/TETA Systems

It has been experimentally shown that epoxide-terminated hyperbranched polyether sulphone (EHBPES) can significantly improve the mechanical properties of traditional diglycidyl ether of bisphenol A/triethylenetetramine (DGEBA/TETA) systems,but the origin of the improvement is still unclear.In this work,we used molecular dynamics (MD) simulations to gain a thorough understanding of the origin of modulus improvement for EHBPES/DGEBA/TETA systems.It is found that the modulus of EHBPES/DGEBA/TETA systems increases with the increase of EHBPES loading.In addition,the crosslinking density,cohesive energy density (CED),and free volume can be used to understand the modulus for EHBPES/DGEBA/TETA systems.It is shown that the highest modulus is achieved at 7 wt％ EHBPES loading due to the highest crosslinking density and CED.When EHBPES loading is below 7 wt％,the higher CED and crosslinking density are responsible for the higher modulus.At higher loadings (＞ 7 wt％),the decreased modulus is closely related to the decreased crosslinking density and increased fractional free volume.It is expected that our results could be of great implications for designing high-performance epoxy materials.     

Synthesis and thermal degradation behaviors of hyperbranched polyphosphate

A novel epoxy-terminated hyperbranched polyphosphate (E-HBPP) was synthesized by employing an A₂ + B₃ polycondensation and characterized by FTIR, ¹H NMR and GPC. E-HBPP was used as a reactive-type flame retardant for diglycidyl ether of bisphenol-A/m-phenylene diamine (DGEBA/mPDA) system. A series of flame retardant resins were prepared and their flame retardancy was monitored by the limiting oxygen index (LOI). The results showed that the LOI value of the cured samples and the degree of expansion of the formed char after burning increased along with the E-HBPP content. Their thermal degradation behaviors were investigated by thermogravimetric analysis and in situ FTIR and showed that the phosphate group of E-HBPP first degraded to form poly(phosphoric acid)s at around 300 °C, which had a major contribution to form the compact char to protect the sample from further degradation. The dynamic mechanical thermal properties were studied by dynamic mechanical thermal analysis (DMTA) and the results showed a good miscibility between E-HBPP and DGEBA. The mechanical properties of the cured films were also investigated. Less than 20% E-HBPP addition improved both the tensile strength and elongation at break.