Electrical and Thermo-mechanical Properties of DGEBA ...

J Electr Eng Technol2018 13(6) 2425-2433 httpdoiorg105370JEET20181362425

2425Copyright The Korean Institute of Electrical Engineers

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (httpcreativecommonsorg licensesby-nc30) which permits unrestricted non-commercial use distribution and reproduction in any medium provided the original work is properly cited

Electrical and Thermo-mechanical Properties of DGEBA Cycloaliphatic Diamine Nano PA and SiO2 Composites

Pavel Trnkadagger Vaclav Mentlik Lukas Harvanek Jaroslav Hornak and Libor Matejka

Abstract ndash This study investigates a new organic based material and its dielectric and mechanical properties It is a comprehensive nanocomposite comprising a combination of various types of nanofillers with hydrophobic silica nanoparticles (AEROSIL R 974) as a matrix modifier and a polyamide nano nonwoven textile Ultramid-Polyamide 6 pulped in the electrostatic field as a dielectric barrier The polymer matrix is an epoxy network based on diglycidyl ether of bisphenol A (DGEBA) and cycloaliphatic diamine (Laromine C260) The designed nanocomposite material is an alternative to the conventional three-component composites containing fiberglass and mica with properties that exceed current electroinsulating systems (volume resistivity on the order of 1016 Ωmiddotm dissipation factor tan δ = 47middot10-3 dielectric strength 39 kVmm)

Keywords Dielectric Nanocomposite Nonwoven Polyamide Thermo-mechanical properties

1 Introduction The three-component composite systems currently used in

high-voltage engineering to create a dielectric barrier eg for main wall insulation of rotary machines are heterogeneous and contain macroscopic inorganic components [1] They primarily use glass fabric as a carrier [2 3] PET (polyethylene terephthalate) or PI (polyimide) films are also widely used [4] The second component - a dielectric barrier - is reconstructed mica manufactured into mica paper [5 6] The third component - the matrix - is an epoxy [7] polyester [8] or silicone resin [9] These materials are characterized by a volume resistivity ρv of 1013Ωm dissipation factor tan δ of 0015 dielectric strength Ds of 35kVmm [10] and glass transition temperature Tg of 115degC [11] However the properties vary with the com-ponents and manufacturer making generalization impossible Currently there is no synthetic insulating material that meets all the requirements for high-voltage insulation systems and not containing a macroscopic inorganic component (eg dielectric barrier) A similar situation occurs with electrical insulating systems for F thermal class (155degC and higher) [12] New elements in this area include polymer nanocom-posites consisting of a polymer matrix with a microphase-separated nanofiller enabling improvement of its properties eg a decrease in the accumulated space charge and reduction of the internal electrical stress [13]

The first use of nanofillers in the field of high-voltage EIS (electrical insulating systems) occurred in 1988 when

one of the first patents in this area [14] (US Pat 4760296) describes the benefits of adding a filler of submicron dimensions to an insulation system based on epoxy resin and mica in the main coil insulation generators Similar work [15] was focused on silicon micro- and nanoparticles and the influence of reducing the filler particle size to increase the voltage resistance of polymers The theoretical treatise of Lewis in 1994 was a milestone [16] and the origin of interest in nanodielectrics An increase in worldwide interest in this area and several experiments occurred after the publication of practically oriented research [17] Systems with different polymer matrices and nanofillers are currently studied for use in the field of nanodielectrics The most commonly used matrix polymers are epoxides or polyolefins [18] Thermoplastics such as polystyrene [19] and polyamide [20] are also used Thermoplastic nanocomposites have easier workability and can be recycled [21] However in terms of the mechanical and thermal properties and their resistance to aggressive environments nanocomposites based on epoxides are preferred Regarding nanofillers attention is primarily devoted to layered silicates (clays) [22] and inorganic oxides (SiO2 [23] TiO2 [24] Al2O3 [25] ZnO [26] MgO [27] BN [28]) Other study was focused on investigation of the electrical properties of nanocomposites containing POSS (polyhedral oligomeric silsesquioxanes) [29] The basic requirement in preparation of polymer nanocom-posites is homogenous dispersion of the nanofiller in the matrix reducing the aggregation of large clusters Large clusters usually lead to a deterioration in the final properties of the composite The interaction of polymer and nanofiller plays an important role Weather it is a polar or non-polar substance also affects the resulting electrical properties [30]

Also thermo-mechanical properties of epoxy-based

dagger Corresponding Author Faculty of Electrical Engineering University of West Bohemia Czech Republic (pavelketzcucz)

Faculty of Electrical Engineering University of West Bohemia Czech Republic (mentlik harvy jhornakketzcucz)

Institute of Macromolecular Chemistry Academy of Sciences of the Czech Republic Czech Republic (matejkaimccascz)

Received November 3 2017 Accepted June 4 2018

ISSN(Print) 1975-0102 ISSN(Online) 2093-7423


2426 J Electr Eng Technol2018 13(6) 2425-2433

micro- and nanocomposites were investigated by Dynamic Mechanical Analysis (DMA) or Thermal Gravimetric Analysis in a few studies (eg [31 32]) where the shear modulus Grsquo and mechanical loss factor tan δ were used for estimation of glass transition temperature Tg Mass loss m is also the key parameters for evaluation of thermal stability of dielectric material There are a variation of changes of investigated parameters depending on the filler type and size [33] effect of moisture reduced crosslink density of epoxy matrix or imbalanced stoichiometry between resin and hardener [34]

The aim of this study was to prepare a new electrical insulating material - a complex nanocomposite ndash that lacks a macroscopic inorganic component and has the same or improved properties compared to existing material of the same class

We studied polymeric nanocomposites containing epoxy matrix modified by nanofillers based on nanoparticles and polymeric nanofabric applied as a dielectric barrier of the composite The polymer matrix of nanocomposite forms an epoxy-amine network using a classic epoxy resin - diglycidyl ether of bisphenol A (DGEBA) crosslinked cycloaliphatic diamine 33-dimethyl-44 diaminocyklohexylmetan (Laromine C260) This matrix was chosen due to its excellent properties such as high thermal and chemical resistance high strength environmental resistance excellent adhesion to different surfaces and ease of curing without formation of volatile compounds This resin also helps increase the glass transition temperature Tg because it is reduced by the addition of SiO2 nanoparticles and polyamide (PA) [35] The nanofiller used to improve the electrical properties of the matrix was hydrophobic nanosilica Hydrophobized nanosilica at a low volume concentration was used because there is a negative impact of large volumes of nanoparticles on electrical and other properties [35] A nonwoven nanofibrous layer based on Polyamide 6 created the dielectric barrier of the composite

The main focus is on study of the influence of the composition of the nanocomposite on the electrical and thermo-mechanical properties thermal stability and optimi-zation of the developed material composition

2 Composite Components This section includes details regarding individual

composite components and the preparation of the samples

21 Polymer matrix The polymeric matrix was an epoxy network with a resin

based on diglycidyl ether of bisphenol A (DGEBA) see Fig 1 The epoxide equivalent weight EEW was 187 gmol As a crosslinker cycloaliphatic diamine 33-dimethyl-44diamino-cyclohexylmethane (Laromine C260) was applied The equivalent weight of the NH groups AHEW was 595 geq Resin based on bisphenol A is the most common epoxy resin and is applicable in a wide range of products Its applications in industry include preparation of matrix for composites potting compounds adhesives coatings and lamination mixtures Using a cycloaliphatic diamine as a hardener leads to formation of a network with a high glass transition Tg and very good mechanical properties describes how to prepare each part of the final manuscript more specifically

22 Nanofillers

The hydrophobic silica AEROSIL R 974 is modified

silica (silicon dioxide) with a specific surface area of 200 m2g containing 12 nm nanoparticles The hydrophobic character of the particles has a positive impact on composites in comparison with hydrophilic ones

23 Nanofabric Ultramid ndash Polyamid 6

The nanofibrous layer of polyamide 6 (NAP) with a

molecular weight of 30000 gmol is suitable for this purpose due to its low viscosity The SEM images in Fig 2 show the polyamide nanofiber layer It is most favorable for the formation of nanofibers in an electrostatic field [36] The fibers are formed from a polymer solution consisting of 13 polyamide 6 and 87 formic acidacetic acid at a 21 ratio with a final viscosity of 4363 mPs The produced nanofibers have suitable mechanical properties and advantageous hydrophobicity These are advantageous prerequisites for handling the fiber layers

They also appear to have a relatively favorable starting material cost The melting point of the polyamide (ISO 3146) is 220degC the density (ISO 1183) is 112 to 115 gcm3 the round pellets are 2 - 25 mm and the water absorption after saturation in 23degC water is 85 The

Fig 2 SEM images of the nanofiber

Fig 1 Formula of epoxy resin based on DGEBA

Pavel Trnka Vaclav Mentlik Lukas Harvanek Jaroslav Hornak and Libor Matejka

httpwwwjeetorkr 2427

homogeneity of the nanofiber is evident and the number of defects is minimal The fibers are cylindrical without significant deviations from the mean diameter of 989 nm and have a standard deviation of 208 nm and variation coefficient of 21 The basis weight of the prepared layer is 4 gm2 and the thickness is 84 μm filled up to 25 to 30 with 8 to 11 moisture absorption

24 The reactive diluent

For better homogenization of the system and saturation

of the nanofabric reactive diluent phenylglycidylether (PGE) was used at concentrations from 5 to 20 mol of the total epoxy group content or 4-20 weight percent relative to the DGEBA In the experiments we observed the effect of the diluent on the resulting product properties

25 Sample preparation

Specially made separable molds (see Fig 3) consisting

of two polished steel plates (170acute 170acute 10 mm) were used for the nanocomposite sample preparation To ensure

optimal conditions and prevent the possibility of adhesion between the steel and the samples 130 acute 130 acute 10 mm Teflon plates were inserted

The epoxy network was prepared by reacting DGEBA with a diamine (Laromine) in a stoichiometric ratio of functional groups (weight equivalent) of Cepoxy CNH = 11 In cases of a reactive diluent part of the diepoxide DGEBA was replaced by PGE monoepoxide so that the total concentration of the epoxy groups remained constant The processing technology is important for the quality of the samples particularly the homogeneity of the material and the absence of air inhomogeneities

An overview of the steps of the sample preparation process is shown in Fig 4

When preparing the polymer network the correct weight ratios of the components sequence of the process steps time limits of vacuum during mixing and temperature during curing were precisely monitored

3 Experiment Commonly known phenomenological AC and DC

electric methods were used to determine the basic electrical properties of the nanocomposites

31 Volume resistivity measurement

Dielectric absorption measurements were used to

evaluate the volume resistivity ρv (Ωm) and polarization index pi (-) (ratio of absorption currents in 15 s and 60 s or 60 s and 600 s) of the samples according to standard IEC 62631-3-2 [37] A Keithley 6517 electrometer was used with a Keithley model 8009 three-electrode measuring system with a 50-mm diameter measuring electrode 75-

Fig 4 Overview of the sample preparation

Fig 3 The mold used to cast the samples



mm outer diameter shielding electrode and 1-mm gap The measuring system was placed in a small screen room to eliminate the influence of external electrical fields Before measurement the samples were conditioned and discharged (24 hours) Stable temperature and humidity (45 rh) were ensured during measurement The measurement was controlled using an automatic measuring system with Agilent VEE Pro software

The volume resistivity was calculated using (1)

( )mtimesWtimes

times=

ItVA

c

efVr (1)

where ρv is the volume resistivity (Ωm) V is the applied voltage (V) tc is the average sample thickness in the area of the electrode (m) I is the current at time 6000 s (A) and Aef is the effective electrode surface (m2)

32 Dissipation factor measurement

The dissipation factor tan δ (-) was measured using a

Tettex 28302831 solid dielectrics analyzer with a Tettex 2914 three-electrode measuring system and temperature and pressure control according to standard IEC 60250 [38]

33 Dielectric strength measurement

The dielectric strength Ds (kVmm) was measured

according to standard IEC 60243-1 [39] and determined according to Eq (2)

Ds = Vbr tc (2)

where Vbr is the breakdown voltage (kV) and tc is the average sample thickness in the area of the electrode (m1000)

A special electrode system was constructed for this measurement with 6-mm diameter electrodes in precise planar alignment Maximal use of the sample area results in more measured data (BDV ndash breakdown voltage) The BDV voltage evaluation occurred while the sample was submerged in insulating mineral oil [40] (10 BDV per sample) to compare each set of samples during the experimental phases A 200 kV high-voltage laboratory source was used as the source of the test voltage with 50 Hz AC The effective BDV was measured using a BDV detector

34 Dynamic Mechanical Analysis (DMA)

DMA of nanocomposite samples was conducted using an

ARES G2 (TA Instruments) Oscillatory shear deformation was applied at a frequency of 1 Hz and at 20-250degC with a temperature increase of 3 degCmin-1 The glass transition temperature Tg was determined from the position of the maximum mechanical loss factor tan δ The shear modulus

was determined in a rubbery state at 200 degC

35 Thermogravimetric Analysis (TGA) TGA measurements were performed using a Pyris 1

Perkin Elmer Thermogravimetric Analyzer under air atmosphere The measuring temperature interval was 30-880 degC with a 10 degC temperature rise 50 mlmin-1 gas flow and approximately 10 mg sample

4 Results and Discussions The influences of the individual components (ie silica

nanofillers and polyamide nanofabric (NAP) diluent PGE) of the new nanocomposite were studied to optimize its composition for high-voltage insulation (optimum thermal mechanical and electrical properties low measured data dispersion) The electrical and thermomechanical properties of the material and its thermal stability were studied

The optimum amount of hydrophobic nanosilica in the matrix was 1 Higher and lower volumes of nanosilica did not improve the electrical properties

The fluidity of the epoxy resin (to nanofabric) was adjusted using the reactive diluent phenylglycidyl ether PGE (10)

41 Electrical properties of nanocomposite

The volume resistivity and one-minute polarization

index were evaluated from the dielectric absorption data The volume resistivity data are shown in Table 3

From the results considerable improvement is evident ie an increase of volume resistivity due to inclusion of hydrophobic nanosilica in the matrix from 321015 to 271016 Ωm (three orders of magnitude higher than the conventional material) however there is only a slight increase in the dispersion of values compared to the matrix only) The PGE diluent decreased the resistivity and the dispersion of its values was significantly reduced The resulting resistivity is close to that of the pure resin without additives The DGEBA-Laromine-SiO2-NPA nano-composite (containing hydrophobic silica increasing the volume resistivity and nanofabric with a positive influence on the dielectric strength of the nanocomposite) has a lower volume resistivity than samples containing silica SiO2 only 281016 Ωm and lower dispersion of the values which suggests better consistency of the results The resistivity of this nanocomposite is higher and therefore more favorable than a mixture with the diluent PGE Thus the DGEBA-Laromine-SiO2-NPA nanocomposite is the preferred system

A similar trend can be observed in the polarization index results

Another variable that reveals the behavior of these nano-composites in an electric field is the relative permittivity



Its dependence on temperature was studied and the results are shown in Fig 5 with a comparison of the nanocomposites in Table 1

Table 1 shows the differences in permittivity corres-ponding to a temperature rise of 100degC The DGEBA ndash Laromine - SiO2 - PGE nanocomposite exhibits strong temperature dependence (εr 30degC = 2987 and εr 130 degC = 486 a relative increase of 6254) The other nano-composites exhibit only a slight increase in the relative permittivity in the observed temperature range The DGEBA - Laromine - SiO2 - NPA nanocomposite has the lowest temperature dependence of all samples investigated in this temperature range (the difference between 30 and 130degC is only 1421) We can conclude that nano-composites without PGE have lower thermal dependence εr These samples are relatively stable at a suitable εr

Regarding the dissipation factor tan δ if we look at its temperature dependence from 30 to 130 degC (Fig 6) we can observe the lowest losses for the DGEBA - Laromine - SiO2 - NPA nanocomposite at temperatures above 80 degC

This nanocomposite exhibits stable tan δ (00061ndash00057) in the 30 to 80degC interval This nanocomposite composition is optimal in terms of dielectric losses The negative effect of PGE is notable because although the composite has the lowest tan δ at 30 degC it exhibits a steep increase above 80degC (0045) The other samples exhibit steeper increases and higher dissipation factors at higher temperatures

Table 2 illustrates the dissipation factor curves as the temperature increases 30 to 130degC The DGEBA - Laromine - SiO2 - NPA nanocomposite exhibits the lowest change here The dielectric strengths of the studied nanocomposites

Table 1 The change in relative permittivity with an

increase in temperature of 100 degC

Sample εr (-) at 30 degC

εr (-) at 130 degC

Relative increase ()

DGEBA ndash L 359 419 1671 DGEBA ndash L - SiO2 385 461 1974

DGEBA ndash L - SiO2 - PGE 299 486 6254 DGEBA ndash L - SiO2 - NPA 366 418 1421

DGEBA ndash L - SiO2 - PGE - NPA 373 436 1689

Fig 5 Temperature dependence of the relative permittivity

of the nanocomposites

Table 3 Electrical parameters and their standard deviations of the studied nanocomposites (measured at 500 V)

Sample Ri (Ω) pi1 (-) pi10 (-) ρv (Ωm) tan δ (-) εr (-) Ds (kVmm) DGEBA ndash Laromine 321E+15 341 546 537E+15 770E-03 359 377

Standard deviation 259E+15 111 335 406E+15 136E-03 054 085 DGEBA - Laromine - SiO2 271E+16 399 782 495E+16 736E-03 385 383

Standard deviation 222E+16 015 108 383E+16 152E-04 007 056 DGEBA - Laromine - SiO2 - PGE 628E+15 347 664 103E+16 448E-03 299 358

Standard deviation 123E+15 032 148 148E+15 272E-04 024 063 DGEBA - Laromine - SiO2 - NPA 280E+16 359 75 332E+16 470E-03 366 39

Standard deviation 195E+16 076 166 218E+16 132E-03 019 086 DGEBA - Laromine - SiO2 - PGE - NPA 169E+16 333 813 264E+16 555E-03 373 36

Standard deviation 169E+16 019 335 276E+16 448E-04 013 063

Table 2 The change in relative permittivity with an increase in temperature of 100 degC

Sample tan δ (-) at 30 degC

tan δ (-) at 130 degC


DGEBA ndash L 745E-03 210E-02 18188 DGEBA ndash L - SiO2 549E-03 181E-02 22969

DGEBA ndash L - SiO2 - PGE 375E-03 450E-02 110000 DGEBA ndash L - SiO2 - NPA 610E-03 152E-02 14918

DGEBA ndash L - SiO2 - PGE - NPA 554E-03 234E-02 32238

Fig 6 Temperature dependence of the dissipation factor

tan δ of the nanocomposites



are presented in Table 3 showing the positive impact of NPA - the DGEBA - Laromine - SiO2 - NPA nanocomposite had a Ds of 39 kVmm The results identified a negative effect of the PGE diluent and a positive effect of SiO2 in the matrix Table 3 shows all the electrical parameters

42 Thermomechanical properties and thermal

stability of nanocomposites In addition to optimal electrical properties dielectric

materials must exhibit suitable thermomechanical properties and thermal stability

Components used in the preparation of nanocomposites that improve the electrical properties of the material slightly deteriorate the thermomechanical properties The goal is to find a suitable compromise The mechanical and thermal properties of polymer networks are primarily affected by the chemical structure and network density An important characteristic of the thermomechanical behavior of networks is the elastic modulus in a rubbery state which depends on the network crosslinking density and glass transition temperature Tg Above the Tg there is a significant change in the mechanical properties and the

material loses its stiffness strength and dimensional stability under mechanical tension Further increase in the temperature above a certain critical value leads to thermal degradation of the polymeric material The thermal stability for electrical materials is defined as the temperature T3 at which a 3 mass loss of the material occurs due to thermal degradation (Fig 8)

The studied nanocomposites were characterized using DMA and TGA The elastic modulus Tg and thermal stability (temperature T3) were determined Fig 7 shows the dynamic mechanical analysis results for the nano-composites ie the temperature dependence of the shear modulus Grsquo and the mechanical loss factor tan δ (GldquoGrsquo)

Table 4 Thermomechanical properties and thermal stability of the epoxy matrix and nanocomposites

Sample Tg (degC) G (MPA) at 200 degC T3 (degC)




Fig 7 The shear modulus Grsquo and mechanical loss factor tan δ (GldquoGrsquo) with respect to temperature

Fig 8 Mass loss of nanocomposites with increasing temperature



A summary of the results is given in Table 4 The incorporation of silica into the epoxy matrix results

in a decrease in the Tg and modulus Hydrophobic modified silica acts in the matrix as a plasticizer which is a consequence of the weak interfacial epoxy-silica (hydrophobic) interactions and an enlarged free volume [41] allowing for greater mobility of chains (see [42] for a hypothesis on the involvement of the Ds declining free volume theory) Application of reactive diluent PGE in the preparation of nanocomposites causes a decrease in the Tg and modulus as a substitution of diepoxide for mono-functional epoxide PGE causes a decrease in the network crosslinking density

This deterioration of the thermomechanical properties of nanocomposites is partly compensated by the use of nanofibrous NPA The TGA curves characterizing the mass loss of the material when heated in an air atmosphere are shown in Fig 8 From Fig 8 and Table 4 the silica clearly increases the thermal stability of the material in comparison with the epoxy matrix whereas NPA and PGE reduce it

The nanocomposite containing silica and NPA (the best from an electric point of view) had suitable thermal properties for thermal class F but had a small decrease in the Tg and modulus compared with those of the epoxy matrix Future experiments should focus on mechanical improvement of the material

5 Conclusion A DGEBA Cycloaliphatic Diamine Nano PA and SiO2

composite was prepared as an alternative to conventional electrical insulation systems containing macroscopic inorganic components The proposed material should exhibit similar or improved dielectric and thermal properties The prepared composite exhibits the following electrical properties volume resistivity on the order of 1016 Ωmiddotm dissipation factor tan δ = 4710-3 and dielectric strength 39 kVmm (example parameters of conventional EIS materials for resin rich technology [main wall insulation of rotary machines] internal resistivity ρv of 1013 Ωm dissipation factor tan δ = 0015 and dielectric strength of Ds = 35 kVmm) The thermal class of the material is the same as that of the conventional ndash F (155degC)

The Tg of the composite is worse as expected (173degC vs 140degC) compared with that of the matrix The Tg was reduced by the addition of nanoparticles in eg [35] and Table 4 therefore we used resin with high Tg The problem here is different from that of conventional materials with macro inorganic components in which the matrix is the weak point of the system (resin glass mica) In our case the lowest Tg of the system had NPA

The resulting material exhibits good thermal stability as shown Fig 8 and the material is stable up to approx 300degC

The system comprises a combination of two types of nanofillers hydrophobic silica nanoparticles and nano-fibrous polyamide 6 The inner structure is more homogenous meaning a better inner distribution of the electrical field Therefore we expect lowered inner PD activity which is to be proven by a long-term electrical aging test

We showed that an NPA nano nonwoven layer can act as a dielectric barrier (for high-voltage use of the material) that can replace mica with a resulting dielectric strength of 39 kVmm Hydrophobic silica increases the volume resistivity to approximately 51016 Ωm The resulting dielectric loss tan δ of approximately 0005 at temperatures up to 100 degC is also suitable Hydrophobic silica increases the thermal stability of the material Overall the thermomechanical properties and thermal stability of nanocomposites are slightly deteriorated by incorporation of nanofillers compared to those of the epoxy matrix as a compromise between high dielectric strength for high-voltage purposes In EIS systems such as the one proposed a high Tg of the matrix is required

The complex DGEBA ndash Laromine ndash Hydrophobic SiO2 ndash NPA nanocomposite has some improved properties compared with conventional electrical insulation materials It is an electrical insulating system without an inorganic (mica) dielectric barrier which is beneficial and therefore it is undoubtedly a novel material

Further research must to be done to explain the role of the nanoparticles in the electric breakdown process [41 42] To replace the conventional material with a carrier (eg glass fabric) it is necessary to improve the tensile strength of the proposed material

Acknowledgements This work is supported by the Ministry of Education

Youth and Sports of the Czech Republic under the RICE mdash New Technologies and Concepts for Smart Industrial Systems project NoLO1607 and by the Student Grant Agency of the West Bohemia University in Pilsen grant No SGS-2018-016 Diagnostics and materials in electrotechnics

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[2] C M Laffoon C F Hill G Lee Moses L J Berberich ldquoA new high-voltage insulation for turbine-generator stator windingsrdquo Trans Am Inst Electr Eng vol 70 no 1 pp 721-730 Jul 1951

[3] B Dewimille and AR Bunsell ldquoAccelerated ageing of a glass fibre-reinforced epoxy resin in waterrdquo



Composites vol 14 no 1 pp 35-40 Jan 1983 [4] G C Stone I Culbert E A Boulter H Dhirani

ldquoElectrical Insulation for Rotating MachinesDesign Evaluation Aging Testing and Repairrdquo Wiley-IEEE Press Piscataway 2014 pp 111-131

[5] R L Griffeth E R Younglove ldquoThe manufacture and processing of mica paper for use in electrical insulationrdquo in Proc COI 1951 pp 22-23

[6] N Andraschek A J Wanner C Ebner G Riess ldquoMicaepoxy-composites in the electrical industry applications composites for insulation and investi-gations on failure mechanisms for prospective optimizationsrdquo Polymers vol 8 pp 1-21 Aug 2016

[7] Z Jia Y Hao H Xie ldquoThe degradation assessment of epoxymica insulation under multi-stresses agingrdquo in IEEE Trans Dielectr Electr Insul vol 13 no 2 pp 415-422 Apr 2006

[8] R Goetter M Winkeler ldquoNew developments in unsaturated polyester resins used for electrical insulationrdquo in Proc EEIC 2001 pp 51-56

[9] G H Miller ldquoSilicone resin rich mica paper laminates for class H operation and radiation resistancerdquo in Proc EIC 1975 pp 273-275

[10] L Harvanek ldquoNanomaterials for electrotechnicrdquo Doctoral Dissertation University of West Bohemia Pilsen 2017

[11] V Boucher P Rain G Teissedre P Schlupp ldquoMechanical and dielectric properties of glass-mica-epoxy composites along accelerated thermo-oxidative agingrdquo in Proc ICSD 2007 pp 162-165

[12] International Electrotechnical Commission ldquoElectrical Insulation - Thermal Evaluation and Designationrdquo IEC Standard 600852007 Nov 7 2007

[13] J Dong Z Shao Y Wang Z Lv X Wang K Wu We Li C Zhang ldquoEffect of temperature gradient on space charge behavior in epoxy resin and its nano-compositesrdquo IEEE Trans Dielectr Electr Insul vol 24 no 3 pp 1537-1546 Jun 2017

[14] D R Johnston M Markovitz ldquoCorona-resistant insulation electrical conductors covered therewith and dynamoelectric machines and transformers incorporating components of such insulated con-ductorsrdquo US Patent 4760296 Jul 26 1988

[15] P O Henk T W Korsten T Kvarts ldquoIncreasing the electrical discharge endurance of acid anhydride cured DGEBA epoxy resin by dispersion of nano-particle silicardquo High Perform Polym vol 11 no 3 pp 281-296 Sept 1999

[16] T G Lewis ldquoNanometric dielectricsrdquo IEEE Trans Dielectr Electr Insul vol 1 no 5 pp 812-825 Oct 1994

[17] J K Nelson J Fothergill L A Dissado W Peasgood ldquoTowards an understanding of nanometric dielectricsrdquo in Proc CEIDP 2002 pp 295-298

[18] T Tanaka T Imai ldquoAdvances in nanodielectric materials over the past 50 yearsrdquo IEEE Elect Insul

Mag vol 29 no 1 pp 10-23 Jan-Feb 2013 [19] S Yu P Hing ldquoThermal and dielectric properties of

fiber reinforced polystyrene compositesrdquo Polym Compos vol 29 no 11 pp 1199-1202 Nov 2008

[20] G G Raju ldquoDielectrics in Electric Fieldsrdquo Boca Raton CRC Press Taylor amp Francis Group 2016

[21] R Stewart ldquoThermoplastic composites mdash recyclable and fast to processrdquo Reinf Plast vol 55 no 3 pp 22-28 May-Jun 2011

[22] R B Valapa S Loganathan G Pugazhenthi S Thomas TO Varghese ldquoAn overview of polymerndashclay nanocompositesrdquo Clay-Polymer Nanocomposites Amsterdam Elsevier pp 29-81 2017

[23] A C Biju T A A Victoire D E Salvaraj ldquoEnhancement of dielectric properties of polyamide enamel insulation in high voltage apparatuses used in medical electronics by adding nano composites of SiO2 and Al2O3 fillersrdquo J Electr Eng Technol vol 10 no 4 Jul 2015

[24] S K Singh S Sing A Kumar A Jain ldquoThermo-mechanical behavior of TiO2 dispersed epoxy compositesrdquo Eng Fract Mech vol 184 pp 241-248 Oct 2017

[25] N Loganathan S Chandrasekar ldquoAnalysis of surface tracking of micro and nano size alumina filled silicone rubber for high voltage AC transmissionrdquo J Electr Eng Technol vol 8 no 2 Mar 2013

[26] W Yang R Yi X Yang M Xu S Hui X Cao ldquoEffect of particle size and dispersion on dielectric properties in ZnOepoxy resin compositesrdquo Trans Electr Electron Mater vol 13 no 3 pp 116-120 Jun 2012

[27] T Andritsch R Kochetov P H F Morshuis J J Smit ldquoDielectric properties and space charge behavior of MgO-epoxy nanocompositesrdquo in Proc ICSD 2010 pp 1-4

[28] I A Tsekmes R Kochetov P H F Morshuis J J Smit ldquoAC breakdown strength of epoxy-boron nitride nanocomposites Trend amp reproducibilityrdquo in Proc EIC 2015 pp 446-449

[29] J Boček L Matějka V Mentliacutek P Trnka M Šlouf ldquoElectrical and thermomechanical properties of epoxy-POSS nanocompositesrdquo Eur Polym J vol 47 no 5 May 2011

[30] A S Vaughan G Gherbaz S G Swingler N A Rashid ldquoPolarnon-polar polymer blends on structural evolution and the electrical properties of blends of polyethylene and ethylene - vinyl acetaterdquo in Proc CEIDP 2006 pp 272-275

[31] K Nam J Cho H Yeo ldquoThermomechanical behavior of polymer composites based on edge-eselectively functionalized graphene nanosheetsrdquo Polymers vol 10 no 1 pp 1-11 Jan 2018

[32] M Liang K L Wong ldquoStudy of mechanical and thermal performances of epoxy resin filled with micro particles and nanoparticlesrdquo Energy Procedia



vol 110 pp 156-161 March 2017 [33] M H Alaei P Mahajan M Brieu D Kondo S J A

Rizvi S Kumar N Bhatnagar ldquoEffect of particle size on thermomechanical properties of particulate polymer compositerdquo Iran Polym J vol 22 n 11 pp 853-863 Nov 2013

[34] G Liu G H Zhang D Zhang Z Zhang X An X Yi bdquoOn depression of glass transition temperature of epoxy nanocompositesrdquo J Mater Sci vol 47 no 19 pp 6891-6895 Oct 2012

[35] C Zou J C Fothergill S W Rowe ldquoThe effect of water absorption on the dielectric properties of epoxy nanocompositesrdquo IEEE Trans Dielectr Electr Insul vol 15 no 1 pp 106-117 Feb 2008

[36] E Marsano L Francis F Giunco ldquoPolyamide 6 nanofibrous nonwovens via electrospinningrdquo J Appl Polym Sci vol 117 no 3 pp 1754-1765 Aug 2010

[37] International Electrotechnical Commission ldquoDielectric and resistive properties of solid insulating materials - Part 3-2 Determination of resistive properties (DC methods)rdquo IEC Standard 62631-3-22015 Apr 12 2015

[38] International Electrotechnical Commission ldquoRecom-mended methods for the determination of the permittivity and dielectric dissipation factor of electrical insulating materials at power audio and radio frequencies including metre wavelengthsrdquo IEC Standard 602501969 Jan 1 1969

[39] International Electrotechnical Commission ldquoMethods of test for electric strength of solid insulating materialsrdquo IEC Standard 60243-1 Mar 26 2013

[40] VI Ushakov ldquoInsulation of High-Voltage Equipmentrdquo New York Springer 2004 pp 11-15

[41] J Artbauer ldquoElectric strength of polymersrdquo J Phys D vol 29 no 2 pp 446-56 Feb 1996

[42] J K Nelson Y Huang TM Krentz L S Schadler J Dryzek B C Benicewicz M Bell ldquoFree volume in nanodielectricsrdquo in Proc ICPADM 2015 pp 40-43

Pavel Trnka He was received the MSc and PhD degrees from the University of West Bohemia Czech Republic in 2002 and 2005 respectively He was a student of the University of Applied Science Fachhochschule Regensburg Germany and worked at the Depart-ment of Research Maschienenenfabrik

Reinhausen Germany in 2003-4 During 2006 to 2007 he was a postdoctoral associate in the High Voltage Laboratory Department of Electrical and Computer Engineering Mississippi State University USA In 2008 he graduated dr hab University of Zilina Faculty of Electrical Engineering Slovakia

Vaclav Mentliacutek He was born in Pilsen in 1939 He graduated from the School of Mechanical and Electrical Engine-ering in Pilsen and later worked at Faculty of Electrical Engineering University of West Bohemia in Pilsen In 1985 he recieved a PhD in 1998 he became professor in the field of

Electrotechnology He is head of section of Electro-technolgy at FEE UWB in Pilsen He successfully led 23 projects and was supervisor of 25 PhD He was awarded twice by the Prize of University of West Bohemia

Lukas Harvanek He was born in Klatovy in 1988 He received the MSc and PhD degree from the University of West Bohemia Czech Republic in 2012 He was a student of the Brunel University Institute of Power Systems London England in 2012 - 2013 and also a student of Beijing Institute of

Technology Beijing China in 2015

Jaroslav Hornak He was born in Klatovy in 1989 He received the MSc degree from the University of West Bohemia Czech Republic in 2014 He completed a traineeship at PampG Rakona Rakovniacutek Czech Republic in 2016 and also practical internship at University of Zilina Faculty of Electrical Engine-

ering Zilina Slovakia in 2017

Libor Matejka He was born in Prague in 1947 He graduated at Charles University in Prague and then he has been working in the Institute of Macromolecular Chemistry in Prague Here he received PhD in 1977 and in 2009 he became Doctor of Science (DSc) Since 1994 he has been a head

of Department of Nanostructured Polymers and Composites and since 2010 he is a head of Scientific Centre of Polymer Materials and Technologies He was awarded two times (1981 and 2010) by the Prize of Academy of Sciences of the Czech Republic



micro- and nanocomposites were investigated by Dynamic Mechanical Analysis (DMA) or Thermal Gravimetric Analysis in a few studies (eg [31 32]) where the shear modulus Grsquo and mechanical loss factor tan δ were used for estimation of glass transition temperature Tg Mass loss m is also the key parameters for evaluation of thermal stability of dielectric material There are a variation of changes of investigated parameters depending on the filler type and size [33] effect of moisture reduced crosslink density of epoxy matrix or imbalanced stoichiometry between resin and hardener [34]

The aim of this study was to prepare a new electrical insulating material - a complex nanocomposite ndash that lacks a macroscopic inorganic component and has the same or improved properties compared to existing material of the same class

We studied polymeric nanocomposites containing epoxy matrix modified by nanofillers based on nanoparticles and polymeric nanofabric applied as a dielectric barrier of the composite The polymer matrix of nanocomposite forms an epoxy-amine network using a classic epoxy resin - diglycidyl ether of bisphenol A (DGEBA) crosslinked cycloaliphatic diamine 33-dimethyl-44 diaminocyklohexylmetan (Laromine C260) This matrix was chosen due to its excellent properties such as high thermal and chemical resistance high strength environmental resistance excellent adhesion to different surfaces and ease of curing without formation of volatile compounds This resin also helps increase the glass transition temperature Tg because it is reduced by the addition of SiO2 nanoparticles and polyamide (PA) [35] The nanofiller used to improve the electrical properties of the matrix was hydrophobic nanosilica Hydrophobized nanosilica at a low volume concentration was used because there is a negative impact of large volumes of nanoparticles on electrical and other properties [35] A nonwoven nanofibrous layer based on Polyamide 6 created the dielectric barrier of the composite

The main focus is on study of the influence of the composition of the nanocomposite on the electrical and thermo-mechanical properties thermal stability and optimi-zation of the developed material composition

2 Composite Components This section includes details regarding individual

composite components and the preparation of the samples

21 Polymer matrix The polymeric matrix was an epoxy network with a resin

based on diglycidyl ether of bisphenol A (DGEBA) see Fig 1 The epoxide equivalent weight EEW was 187 gmol As a crosslinker cycloaliphatic diamine 33-dimethyl-44diamino-cyclohexylmethane (Laromine C260) was applied The equivalent weight of the NH groups AHEW was 595 geq Resin based on bisphenol A is the most common epoxy resin and is applicable in a wide range of products Its applications in industry include preparation of matrix for composites potting compounds adhesives coatings and lamination mixtures Using a cycloaliphatic diamine as a hardener leads to formation of a network with a high glass transition Tg and very good mechanical properties describes how to prepare each part of the final manuscript more specifically

22 Nanofillers

The hydrophobic silica AEROSIL R 974 is modified

silica (silicon dioxide) with a specific surface area of 200 m2g containing 12 nm nanoparticles The hydrophobic character of the particles has a positive impact on composites in comparison with hydrophilic ones

23 Nanofabric Ultramid ndash Polyamid 6

The nanofibrous layer of polyamide 6 (NAP) with a

molecular weight of 30000 gmol is suitable for this purpose due to its low viscosity The SEM images in Fig 2 show the polyamide nanofiber layer It is most favorable for the formation of nanofibers in an electrostatic field [36] The fibers are formed from a polymer solution consisting of 13 polyamide 6 and 87 formic acidacetic acid at a 21 ratio with a final viscosity of 4363 mPs The produced nanofibers have suitable mechanical properties and advantageous hydrophobicity These are advantageous prerequisites for handling the fiber layers

They also appear to have a relatively favorable starting material cost The melting point of the polyamide (ISO 3146) is 220degC the density (ISO 1183) is 112 to 115 gcm3 the round pellets are 2 - 25 mm and the water absorption after saturation in 23degC water is 85 The

Fig 2 SEM images of the nanofiber

Fig 1 Formula of epoxy resin based on DGEBA

























( )mtimesWtimes

times=

ItVA

c

efVr (1)








Ds = Vbr tc (2)





























εr (-) at 130 degC























































References




















































































( )mtimesWtimes

times=

ItVA

c

efVr (1)








Ds = Vbr tc (2)





























εr (-) at 130 degC























































References
































































( )mtimesWtimes

times=

ItVA

c

efVr (1)








Ds = Vbr tc (2)





























εr (-) at 130 degC























































References






































































εr (-) at 130 degC























































References




























































































References












































































References











































































































































Electrical and Thermo-mechanical Properties of DGEBA ...

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