Dielectric properties of epoxy composites with modified multiwalled carbon nanotubes

We report here a high dielectric percolative polymer nanocomposite, fabricated by a combination of triethylene-tetramine (TETA) modified multiwalled carbon nanotube (named as TETA-MWNT) within epoxy resin matrix. In this composite system, with various TETA-MWNT volume fractions, the dielectric constant (K) is well fitted by the scaling law of the percolation theory with the percolation threshold f _c is 0.042 and the critical exponent p is 0.786. At 1,000 Hz of room temperature, the value of the dielectric constant is as high as 421 with the TETA-MWNT content of 4.14vol%, which is almost 60 times higher than that of epoxy resin. In contrast, a simple blend of pristine MWNT in epoxy composite shows evident lower dielectric constant and much higher loss with the same volume fraction.     

A dielectric study of water uptake in epoxy resin systems

The influence of epoxy resin formulation on the nature and extent of moisture is studied using dielectric and gravimetric analysis for samples cured at 40, 50, 60, and 70 °C and aged at 50 °C and 70 °C. The equilibrium moisture uptake depends on the difference between the glass transition and aging temperature. Dielectric relaxation data measured from 1 to 3 × 10¹⁰ Hz indicates the presence of water in at least two different environments. The high frequency relaxation ca. 1 × 10¹⁰ Hz is associated with water clustered in nano‐voids, whereas the relaxation at 10⁵ Hz arises from a combination of OH pendant group reorientation motion coupled with that of molecularly dispersed water molecules. A correlation exist between the dielectric permittivity and the amount of moisture absorbed. Water initially resident in voids is re‐dispersed with aging into the resin matrix aiding plasticization and allowing densification of the matrix. The extent to which changes occur depends on the chemical functions forming the matrix and densification leads to a drop in the amount of water absorbed. In the complex resin systems, water interacts with both OH groups and polyether of the amine curing agent which is not possible with the aliphatic diamine in the simple system.] © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44717.     

Dielectric studies of three epoxy resin systems during microwave cure

Dielectric properties of three curing epoxy resin systems at an industrial microwave frequency, 2.45 GHz, were measured over a temperature range lower than the cure temperature. Extent of cure, which is determined by DSC, is used to describe the progress of the polymerization. It has been found that, normally, the real and the imaginary part of the complex dielectric constant increased with temperature and decreased during microwave cure. The changes of the dielectric properties during the reaction are related to the decreasing number of the dipolar groups in the reactants and the increasing viscosity. The Davidson-Cole model can be used to describe the experimental data. The Zong model is applicable to polymeric materials at high microwave frequencies and can be used to calculate the parameters of the Davidson-Cole model. The epoxy resins exhibit one γ relaxation, which can be described by the Arrhenius rate law. The evolutions of the parameters in the models are discussed.     

Effects of titanate coupling agent on the dielectric properties of NiZn ferrite powders–epoxy resin coatings

During ferrite powder–polymer resin slurry preparation, the ferrite particles tend to hold together, forming agglomerates, which lead to the formation of micro-structural defects in the ferrite powder–polymer coatings. These defects affect the quality of magnetic devices. In this study, the titanate coupling agent effects on the NiZn ferrites powder dispersion in an epoxy resin and solvent system and the electromagnetic properties of NiZn ferrite powders–epoxy resin composite coatings are investigated. It was observed that the dispersion of NiZn ferrite powders and the affinity of NiZn ferrites and epoxy resin can be substantially enhanced by coating a titanate coupling agent onto the ferrite powder surfaces. This coating promotes mixing homogeneity and increases the dielectric constants at low frequencies (below 100 Hz) due to the increase in phase boundary between the NiZn ferrite powders and epoxy resin.     

红外光谱法研究环氧类涂料对水汽的吸附

用红外光谱法研究了湿度对环氧类涂料表面吸附水汽的影响,并讨论了不同湿度对环氧类涂料和粘合剂之间的附着力的影响,为涂料的施工提供了快速质量监控的方法。     

Novel nanocomposite of epoxy resin by introduced reactive and nanoporous material

A reactive and nanoporous particle (OG) was introduced to UV-cured epoxy resin to form great low D _k material for electronic industrial. We expected the porous cage of OG to decrease the dielectric constant of UV-cured epoxy resin and multiple reactive functional groups (oxirane ring) of OG reacted with photoinitiator to increase the curing density of UV-cured epoxy resin. The glass transition temperatures (T _g) of epoxy increases with the increase of the OG content up to 10 phr due to the increase of crosslinking density. Excessive aggregation at highest OG content of 15 phr results in the reduced crosslinking density and T _g. The char yield of the composite increases with increase of OG content because stable Si and SiO₂ are formed after thermal decomposition. The presence of OG results in the higher porosity and thus the lower dielectric constant.     

Cure behaviour of epoxy resin matrices for carbon fibre composites

The cure behaviour of two resin formulations (with high and low curing agent content respectively) of an epoxy resin system, used as matrix for carbon fibre composites, was studied through calorimetric analysis. The aim of this work is to investigate the kinetics of this specific epoxy system in order to be able to choose a proper set of processing parameters which will give good composite material properties. The shape of the conversion curves gives evidence of the differences in the cure kinetics of the two systems. Furthermore, the values of the activation energies were determined both for formulation in the conversion range where vitrification occurs, following a phenomenological approach. These values give an indication of the differences in the curing mechanisms, when varying the content of curing agent. In particular, for both systems, the same reaction represents the onset of the cure process, ie the autocatalytic epoxy ring opening through addition reaction to the primary amine. This reaction dominates the entire cure process of the epoxy formulation at high curing agent content. Conversely, in the formulations with a low curing agent content, after depletion of the primary amines, different reactions may take place (with secondary amines and hydroxyl groups), depending on the cure temperature and the resin viscosity. © 1999 Society of Chemical Industry     

Flame retardancy and thermal degradation mechanism of epoxy resin composites based on a DOPO substituted organophosphorus oligomer

A series of flame-retardant epoxy resins (EP) with different content of poly(DOPO substituted dihydroxyl phenyl pentaerythritol diphosphonate) (PFR) were prepared. The PFR was synthesized via the polycondensation between 10-(2,5-dihydroxyl phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-BQ) and pentaerythritol diphosphonate dichloride (SPDPC). The structure of PFR was confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H nuclear magnetic resonance (¹H NMR). The flame retardancy and the thermal stability of the EP/PFR hybrids were investigated by limiting oxygen index (LOI) test and thermogravimetric analysis (TGA) in air. The results showed that the incorporation of PFR into EP can improve the thermal stability dramatically. The mechanical results demonstrated that PFR enhanced failure strain slightly accompanied by a decrease in tensile strength. The thermal oxidative degradation mechanisms of the EP/PFR hybrids were investigated by real time Fourier transform infrared spectra (RTFTIR) and direct pyrolysis/mass (DP-MS) analysis. X-ray photoelectron spectroscopy (XPS) was used to explore chemical components of the residual char of EP and EP/PFR hybrid. DP-MS analysis showed that the degradation process of EP/PFR hybrid was divided into two characteristic temperature regions, attributed to the decomposition of phosphate and aromatic structure.     

Characterization of water in bacterial cellulose using dielectric spectroscopy and electron microscopy

It is shown that only 10% of the 99 wt% water present in bacterial cellulose (BC) gels, produced by Acetobacter xylinum, behave like free bulk water; the majority of the water molecules in the gels is more or less tightly bound to the cellulose. The magnitude of the diffusion coefficients of ions transported in the water phase of the BC gels as well as the information contained in freeze fracture transmission electron microscopic images of the gel structures indicates that the bulk-like water is confined in “lakes” rather than forming a continuous phase throughout the gel. Water desorption isotherms suggest that these “lakes” decrease in size with increasing oxygen concentration used during the biosynthesis process of the gels.     

Synthesis and thermally stimulated current study of molecular relaxations of the grafted polymethylphenylsiloxane segment in thiodiphenol-modified epoxy resin

A series of grafted polymethylphenylsiloxane (PMPS) segments in thiodiphenyl-containing epoxy resin (ESTP) was prepared. The structure was evaluated by IR, ¹H-NMR, and ¹³C-NMR measurements. DSC measurements on the grafted ESTP epoxy resins showed a decreasing trend for T_g with increasing content of PMPS siloxane. The TSC measurements further confirmed this trend. This result suggests that the matrix of ESTP copolymer was less rigid with grafting of PMPS oligomer, due to the Si—O—C linkage at the opened glycidyl ether chain located approximately between the BPA and 4,4′-thiodiphenoxyl chains in the epoxy backbone. A new and broad sub-T_g transition appeared at −60°C in the TSC spectrum was observed for the cooperative motion of this siloxane moiety. All copolymers showed sub-T_g relaxations of γ- and δ-modes, observed at −100 and −130°C, respectively. These two relaxations may be attributed to the motions of BPA and the terminal groups in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1523–1530, 1998     

An impact study of epoxy resin composites reinforced with glass fiber fabrics

The impact properties of epoxy resin composites reinforced with three types of fabrics which are welf-knitted structural fabrics (WKSF), three-dimensional fabrics (3D-3A & 3D-5A) and plain-weave structural fabrics (2D-2A) have been investigated. The results of experiments show that WKSF composite has total impact energy about 1.5 and 3.5 times those of 3D and 2D fabrics composite respectively. The ductility index of WKSF is about 2.2 times of those of 3D and 2D. WKSF composites are very ductile materials and can absorb much more impact energy than 3D and 2D composites. The pushed-out volume of WKSF composites after the impact test was calculated from the photographs and the results show that the volume of the pushed-out zone is proportional to impact energy.     

The relationship between dielectric and rheological behavior of epoxy resin during cure

Measurement of the frequency-dependent, vector voltage (V_c) provided an in-situ and non-destructive technique to measure continuously the rheological change of a resin due to polymerization, and can be used as the basis of real-time control. The vector voltage depends on the degree of polarization of the dipolar molecules and on the change of viscosity during cure; both result from the modified structure of the epoxy resin during cure, The initial stage of curing, represented by the former portion Of the Vc curve (divided at the minimum of the Vc curve), was caused mainly by the effects of temperature and viscosity. During the latter stage of the cure reaction, Vc alters because of the effect of the lightened matrix structure that inhibits alignment of dipoles. The duration of reaction. temperature of curing and degree of conversion all have the same effects on both vector voltage and viscosity, The minimum value of vector voltage is correlated to the minimum viscosity, and there is a nearly quantitative relationship between them, One can determine the viscosity of the epoxy resin during cure from reading of the vector voltage. Various reaction mechanisms may be explained based on the graphs of vector voltage of various types.     

Charing polymer wrapped carbon nanotubes for simultaneously improving the flame retardancy and mechanical properties of epoxy resin

Molybdenum-phenolic resin (Mo-PR) was grafted onto the surface of multi-walled carbon nanotubes (MWCNTs) to obtain modified MWCNTs (CNT-PR). Compared to epoxy resin, epoxy resin/CNT-PR nanocomposites showed the improvements in flame retardancy and mechanical properties. Structural characterization showed that the grafted Mo-PR improved the dispersion of MWCNTs in epoxy resin and enhanced the interfacial interaction between CNT-PR and epoxy resin. On the other hand, the grafted Mo-PR could show high char yield during the process of combustion. Thus the flame retardancy of nanocomposites was improved, especially for the heat release rate and total smoke production. Furthermore, the combination of CNT-PR with melamine dramatically promoted the LOI value and the level of UL-94 rating.     

环氧树脂基介电复合材料的制备和性能研究

以环氧树脂(EP)为基体树脂、经硅烷偶联剂改性后的压电陶瓷钛酸钡(BaTiO3)为增强填料,采用浇铸法制备了有机/无机介电复合材料。研究了填料用量对复合材料介电性能、力学性能和热性能的影响。实验结果表明,BaTiO3能显著提高材料的介电常数,当w(BaTiO3)=60%时,复合材料的介电常数为23.6,比纯EP的介电常数(4.0)提高了近6倍,而且复合材料的介电常数受频率影响较小,具有较好的频率稳定性;随着BaTiO3含量的增加,材料的弯曲强度和冲击强度都呈先增后减的趋势,最大弯曲强度和冲击强度分别为123.8 MPa和26.3 kJ/m2;材料的热稳定性研究表明,材料的起始热分解温度随着BaTiO3含量的增加而提高,材料的耐热性能得到改善。     

Enhanced dielectric permittivity of BaTiO3/epoxy resin composites by particle alignment

Barium titanate (BaTiO₃)/epoxy resin composites with a novel structure, in which the BaTiO₃ particles were directionally aligned in the epoxy resin matrix, were fabricated using the ice-templating method. The effects of the filler particle alignment and the filler fraction on the dielectric permittivity as well as the dielectric loss of the composites were studied. The results show that the aligning filler particles can significantly improve the dielectric permittivity while maintaining the dielectric loss compared with the traditional composite structure (homogeneously distributed). Due to the feasibility of the enhancement of the dielectric properties of the composites, the particle alignment that is achieved via the ice-templating method can be used in the field of high energy density capacitors.     

环氧树脂功能性固化剂的研究现状与进展

介绍了近年来国内外功能性固化剂的研究进展。     

Improved interfacial adhesion in carbon fiber/epoxy composites through a waterborne epoxy resin sizing agent

A series of self‐emulsified waterborne epoxy resin (WEP) emulsions were used as surface sizing for carbon fibers (CFs) to improve the interfacial adhesion between the CF and epoxy matrix. In this work, the hydrogenated bisphenol‐A epoxy resin (HBPAE) was modified by polyethylene glycol (PEG) with molecular weights of 400, 800, 1000, 1500, 2000, 4000, and 6000 g/mol. The properties of the WEP emulsion were examined by Fourier transform infrared spectroscopy, dynamic light scattering, and transmission electron microscopy. The surface characteristics of sized CFs were evaluated using scanning electron microscopy, atomic force microscopy, and X‐ray photoelectron spectroscopy. Afterwards, CF/EP composites were prepared and their fracture surface and interlaminar shear strength (ILSS) were examined. The results indicated that PEG2000 modified HBPAE sizing had the optimum emulsion stability and film‐forming ability. Meanwhile, the results also demonstrated that a continuous and uniform sizing layer was formed on the surface of CFs and the surface sizing was excellent in improving the chemical activity of CFs. Compared with unsized CFs, the O1s/C1s composition ratio was observed to increase from 11.51% to 33.17% and the ILSS of CF/EP composites increased from 81.2 to 89.7 MPa, exhibiting better mechanical property than that of commercial Takemoto S64 sized CFs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44757.     

Toughening of cycloaliphatic epoxy resin by multiwalled carbon nanotubes

The toughness of cycloaliphatic epoxy resin 3,4‐epoxycyclohexylmethyl‐3′,4′‐epoxycyclohexane carboxylate (ERL‐4221) has been improved by using multiwalled carbon nanotubes (MWCNTs) treated by mixed acids. The MWCNT/ERL‐4221 composites were characterized by Raman spectroscopy and their mechanical properties were investigated. A significant increase in the tensile strength of the composite from 31.9 to 55.9 MPa was obtained by adding only 0.05 wt % of MWCNTs. And a loading of 0.5 wt % MWCNTs resulted in an optimum tensile strength and cracking energy, 62.0 MPa and 490 N cm, respectively. Investigation on the morphology of fracture surface of the composites by field emission scanning electron microscopy demonstrated the crack pinning‐front bowing and bridging mechanisms of toughening. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characteristics of epoxy resin cured with in situ polymerized curing agent

In order to improve the heat resistance of a cured epoxy resin together with reducing the viscosity of the resin composition, an epoxy resin was cured with a curing agent formed from the radical copolymerization of vinyl monomers during the cure process of the epoxy resin. N-phenylmaleimide and p-acetoxystyrene were used as vinyl monomers of the curing agent. The epoxy resin was cured by the insertion reaction of the ester group of the in situ polymerized curing agent and the epoxy group of the epoxy resin. In the cure system of the epoxy and the phenol resins, reduction of the viscosity of the resin composition was achieved by replacing some or all of the phenol resin with these monomers. When all phenol resins were replaced by these monomers, the viscosity of resin composition (0.01 Pa s at 70 °C) decreased by about 1/2000 compared with that of the system with only phenol resin (21 Pa s at 70 °C). The glass transition temperature (T_g) of the cured resin with no phenols was 174 °C, an improvement of 17 °C compared with that of the system cured with only phenol resin. The flexural strength of the new resins remained unchanged.