Epoxies, dielectric permittivity

The first attempt to use these ideas in epoxy cure was by Fisch and Hofmann 66), but their assignment of permittivity changes to changes in polar group concentrations was marred by what we interpret as electrode polarization effects. Blyakhman et al. 51 52), examined the post-cure dielectric permittivity and loss tangent of anhydride-... 

Sheppard, J N. F., Senturia, S. D. Molecular contributions to the dielectric permittivity of unreacted epoxy/amine mixtures, p. 22 a, Technical Program Summary, Adhesion Society Annual Meeting, Savannah, GA, 1985... 

Although some competitor resins (e.g., polyimides and cyanate esters) are replacing epoxy resins in some more demanding applications, in which the superior glass transition temperatures or lower dielectric permittivity/low dielectric loss are preferred, brominated epoxies are still widely used. 

Computer controlled systems have been developed that enable investigation of the spatial distribution of impedance within resins and in turn the identification of flaws (62). Water trapped within the composite will cause dielectric permittivity to increase, thus revealing its presence. Detection of voids, variation in the cross section of the adhesive layer, ingress of moisture into a joint structure, and characterization of cure in epoxy resin can be achieved with high frequency dielectric spectroscopy (10" -10 Hz) (63). 

Fig. 3. Dielectric permittivities of a non-filled Bisphenol-A based epoxy resin (C) and its composite filled with 60 wt% wollastonite (C/W, left) and of a non-filled cycloaliphatic epoxy resin (B) and its composites filled with 60 wt% wollastonite (B/W) and quartz (B/Q, right). The high-temperature, low frequency polarization process is most probably due to the matrix/electrode interface in the case of resin C, while it is related to the matrix/filler interface in the case of resin B. (After Ref. [6]).

Dielectric cure monitoring generally relies on measin-ement of the ionic conductivity (a in eq. 19). The conductivity during cure of epoxy-amine systems have been characterized to establish relationships between conductivity and viscosity (103,104), conductivity and Tg (104), and relationships to the conversion of epoxide (103). Recently, models were established to relate changes in the dipole component of the complex permittivity to the advancement of cure through the Tg-con version relationship, expanding the capabilities of dielectric sensing to monitor cure (102). 

The volume resistivity, permittivity, and dielectric loss factor of nanostructured interpenetrating polymer networks based on natural rubber/polystyrene have been found to increase as a function of blend composition, reaching a maximum of 10 -10 Hz dielectric loss factor [27]. Measurements of volume resistivity have also been reported on epoxy resin-polyaniline blends resulting in the establishment of a correlation between a shoulder on the 1583 cm band with the degree of volume resistivity [31]. 

Kochetov et al. (2012) described the dielectric response of a range of nanodielectrics based upon particulate nanofillers dispersed within an epoxy matrix. In all cases, with the exception of nano-silica, the inclusion of a low volume fraction (<5 %) of nanofiller resulted in a reduction in the measured real permittivity, below that of the host matrix, despite S In another epoxy-based... 

Blends of epoxy resins with other types of resins have also been developed. These are used when performance demands exceed the capabilities of even the high-Tg/Td epoxies, but where the costs of the highest performance materials cannot be justified. In many cases, the driving force behind these materials is the need for improved electrical properties versus the standard epoxy offerings. Specifically, improvements in the dielectric constant (permittivity) and dissipation factor (loss tangent) are the properties of interest. Materials with lower values for these properties are needed for circuits that operate at high frequencies. 

Riffle JS, Yilgor I, Tran C et al (1983) Elastomeric polysiloxane modifiers for epoxy network -synthesis of fimctional oligomers and network formation studies. ACS Symp Ser 221 21-54 Risse S, Kussmaul B, BCriiger H et al (2012) Synergistic improvement of actuation properties with compatibilized high permittivity filler. Adv Funct Mater 22 3958-3962 Romasanta LJ, Leret P, Casaban L et al (2012) Towards materials with enhanced electro-meehanical response CaCu3Ti40i2-polydimethylsiloxane composites. J Mater Chem 22 24705-24712 Rosset S, Shea HR (2013) Flexible and stretchable electrodes for dielectric elastomer actuators. Appl Phys A Mater Sci Proeess 110 281-307... 

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