2012 IEEE 10th International Conference on the Properties and Applications of Dielectric Materials

July 24-28,2012, Bangalore, India

Effect of coupling agent on PD resistivity of unsaturated polyester - alumina nano-composites

Ram A. Sharma*, Sarojini Swain, Lokesh Chaudhari, Subhendu Bhattacharya,

Department of Advanced Material Process Technology Centre Global R &D Centre, Crompton Greaves Ltd.,

Kanjur Marg, Mumbai, India *Email: ram.sharma@cgglobal.com

Abstract: Partial discharges occur in medium and high voltage insulation systems which result in the degradation of the system and lowers performance life. Research on methods to lower partial discharges or contain harmful effects of this phenomenon is of utmost importance. The use of coupling agents has shown to increase the performance of composite systems with respect to the erosion resistance in the case of epoxy composites as well as increase overall performance of composites. The present study focuses on a nano-alumina unsaturated polyester composite system with respect to the effect of particle and coupling agent concentration on the erosion resistance, mechanical and thermal properties. It was observed that an optimum nano-alumina concentration was observed below 1.0 parts per hundred resin (phr) and concentration of coupling agent at 100 parts per hundred of nano-particle. It was observed that along with an increase in the erosion resistance of the composites the overall mechanical and thermal properties were seen to increase. The decrease in performance of composites at higher concentrations of coupling agent and particles was caused due to a lowering of crosslink density of the resultant composites.

Keywords: Nanotechnology, Surface discharges, Resin

insulation . Introduction: Partial discharges discharges in high voltage equipment occur within the insulation resulting in the formation of electrical trees and the deterioration of the insulation [1,2]. The causes leading to the generation of partial discharges and their effects have been widely studied in order to understand this phenomenon [3]. It has been seen that partial discharges occur at interfaces and in micro-cracks and voids which are formed due to improper processing or as a matter of course of insulation preparation [4]. The deterioration of the insulation is via a thermo-oxidative mechanism and once initiated the degradation by-products accelerate the further deterioration or "tree formation" of the deterioration path. The reasons for the site of the initial point of deterioration have as yet not been determined though it is a topic of current investigation. It is known that initially a nano­hole is formed which act as focus points for discharges to occur and treeing is initiated from this point [5].

The effect of addition of nano-particles to increase the partial discharge resistivity or electrical erosion resistance has

978-1-4673-2851-7/12/$31.00 ©2012 IEEE

been studied with reference to epoxy and unsaturated polyester matrices [6-10]. Further, the effect of coupling agents to increase the mechanical and chemical performance of nano­composites has been proven with respect to a number of nano­particles and resin compositions [11]. The effect of addition of coupling agent to a nano-composite to increase the electrical erosion resistance of epoxy composites has shown that the use of coupling agents have improved the performance of the resultant composites [12].

The present study will evaluate the performance of nano­composites based on nano-alumina. The optimum concentration of nano-alumina, with reference to electrical erosion resistance, will be used to study the effect of coupling agent. Apart from the electrical erosion resistance the effect of the coupling agent on the mechanical and thermal properties of the alumina composites will also be evaluated.

EXPERIMENTAL PROCEDURE A) Materials

The unsaturated polyester used in the study was obtained from Mis Naphtha Resins, Bangalore, India and was used as such. The resin was pre-accelerated with 0.2% cobalt naphthanate (6% Co content), had a solid content of 55% and used styrene as the reactive diluent. The acid value of the composition was 12 mg KOH/g resin and had a viscosity of 330 rnPas (@ 25°C and 50 rpm). The initiator used for the cure of the unsaturated polyester resin was methyl ethyl ketone peroxide (MEKP) obtained from Mis Naptha Resins and used as such. The coupling agent used in the study was a gamrna­methacryloxylpropyl trimethoxy silane, obtained from Mis Momentive Performance Materials Inc., India. The nano­alumina used for the study were procured from Evonik Industries, India sold under the trade name Aeroxide Alu C and has the following properties: average particle size 13nm and specific surface area (m2jg) 100 ±15.

B) Preparation of composite samples

The nano-composites were prepared by adding the required amount of nano-particle and coupling agent to the unsaturated polyester resin under mechanical agitation. The nano-particles were added on a parts per hundred basis (phr) i.e. 1 phr would indicate that 1 gm of the nano-particle was added per 100 g of resin and coupling agent was added on parts per hundred based on the amount of nano-particle added. The dispersion

was then subjected to uItrasonication coupled with mechanical agitation, using an rpm of IS00 ± SO, in a temperature controlled bath maintained at 2SoC for a period of 8 hours. Following the dispersion of the nano-materials in the polymer matrix the mixture was degassed, required amount of initiator was added and stirred. The composition was then poured into Teflon and metal moulds and allowed to cure at room temperature i.e. 2S ± 1°C for 12 hours followed by post-curing at 80 ± 1°C for four hours. The composites were then allowed to stabilize for 7 days at 2S ± 1°C and SO% relative humidity before any testing was carried out.

C) Testing

a. Physical Properties

The unsaturated polyesters containing various nano-particles were evaluated for their viscosity on a Brookfield DV -II Pro viscometer at SOrpm with 600 ml of the sample being maintained at a constant temperature of 2SoC using a constant temperature bath.

h. Electrical erosion evaluation

The erosion of the samples was carried out using the electrodes defined in IEC 60343 with a voltage of 12kV and SOHz for a duration of 8 hours. The samples used for the erosion were cast on hard chromed metal moulds and the smooth surface used for the test. The polymer nano­composites had dimensions of 1 OOx 100x3 mm with a variance of S%. A schematic of the test setup used is shown in Figure 1.

After the samples were subjected to surface flashover they were cleaned using an ultrasonicator using distilled water as medium for IS minutes to remove loosely adhering particles of the degraded unsaturated polyester. The samples were then dried and analysed for their surface roughness using an Ambios XP 2 surface profilometer with a scan length of ISmm from the edge of the electrode with a stylus force of Img.

c. Mechanical Properties

The mechanical properties of the nano-composites were evaluated using a Lloyd SO Universal Testing Machine with a SOkN load cell in the case of tensile properties and SOON for flexural properties with a jaw speed of 2mm/min and a gauge length of Scm. The samples used for evaluation of mechanical properties had dimensions of 100xlOxSmm.

The impact strength of the samples were evaluated on a Denson Avery Impact tester with a striker of 2.7 J with a striking velocity of 3.46 m/s in accordance with ASTM D2S6.

PlANE ELECTRODE

Fig. 1. Schematic of the test set-up used for erosion of samples

d. Thermal properties

The glass transition temperature of the UPR composites were evaluated on a Mettler Toledo DSC822e machine with a sample weight of 1O-20mg and a heating rate of lOoC/min. The sample was cycled from 25-2S0°C and 2S0-25°C and the same repeated. The glass transition was evaluated from the second run to eliminate thermal history from the sample.

e. Chemical properties

The prepared composites were immersed in toluene. Three samples with dimensions of 3x3xO.3 cm were immersed separately in 100 ml of toluene, maintained at 30°C. After the required amount of time the samples were removed and gently dried using a filter paper to remove toluene adhering to its surface. The dried composite samples were further placed in an air circulating oven, maintained at 100°C, for 2 hours and removed, cooled to room temperature in a dessicator and then weighed. The gel content was then estimated [13].

The cross-linking density of the polymer composites was estimated using the solvent swelling method using toluene as the swelling solvent [14]. The crosslinking density (n) is dependant on the molecular weight between cross links as shown in Equation 1.

N =

Pr I Me (Eq. 1) Where, Pr - is the density of the composite and Me - is the molecular weight between crosslinks and is calculated by Equation 2.

(Eq. 2) Where, V s - solvent molar volume X - polymer solvent interaction parameter, which was 0.82 for an unsaturated polyester resin based on maleic anhydride, phthalic anhydride and propandiol with toluene [IS] Vr - is the equilibrium volume fraction of composite and is determined as shown in Equation 3 [14].

V r

(Eq. 3) Where, Ps - is the solvent density and Mr and Ms are the weights of dry composite and absorbed solvent.

RESULTS AND DISCUSSION A) Optimisation of nano-particle concentration

Table 1 shows the effect of addition of alumina on the properties of the unsaturated polyester composite. It could be observed that the electrical erosion resistance was greatest at

concentration levels of 0.5 phr of alumina. The electrical Table 1: Effect of alumina on properties of unsaturated erosion resistance as indicated by the average roughness and polyester composites. erosion path length of the composites after subjecting them to electrical surface flashover. It could be observed that the average roughness increased as the concentration of the nano­particle increased after 0.5phr. This was contrary to what was previously reported in the case of epoxy-alumina composites where an increase in the erosion resistance was observed as the concentration of nano-alumina increased in the composite [8]. The crosslink density of the composites decreased as the concentration of nano-alumina increased. This could be due to the increase in viscosity of the compositions resulting in a lower diffusion rate of reacting species, which resulted in a decrease in the crosslinking density of the composites [16]. The formation of agglomerates could also be a contributing factor to the decrease in crosslink density. Comparing the mechanical properties it was observed that the mechanical properties, with reference to tensile and flexural strength, showed a peak at 0.5 phr, the same concentration at which the nano-composite showed the highest erosion resistance. The increase in the tensile and flexural strength was due to the reinforcing action of the nano-particles in the polymer matrix [17]. The decrease in tensile and flexural properties at higher nano-particle concentrations could be attributed to particle agglomeration which has been shown to have a negative impact on the mechanical properties of polymer composites [18]. At 0.5 phr it was observed that the elongation at break also showed a low value, this was due to the reinforcing action of the filler, which has been previously observed. As the concentration of nano-particle increased the elongation at break increased since agglomeration of the particles would

Property

Viscosity mPas (25C, 50rpm)

Gel time (min)

Peak exotherm CC)

Tensile strength (MPa)

Elongation at break (%)

Flexural strength (MPa)

Impact strength (kJ/m2)

Tg (C)

Crosslink density x 10-6 (g/mol)

Ra value (micron)

Erosion path (mm)

UPR

330

67

115

28.71

3.44

46.32

0.86

73.74

118.8

1.83

7.61

o II

UPR

0.5

376

57

117

29.84

2.09

57.52

1.42

77.43

114.7

1.26

6.85

o'C ........

C-CH I II

3

/CH, CH,

UPR UPR UPR

1.0 1.5 3.0

480 590 750

75 76 117

99 108 90

29.28 28.46 18.19

2.73 2.43 1.75

60.14 56.5 41.98

0.92 1.01 0.95

76.55 76.15 76.59

108.5 98.6 108.1

1.67 1.80 2.01

7.21 8.21 9.51

result in a reduction of the reinforcing action of the filler, this is reflected by the reduction in tensile strength as well. However, at concentration levels above 3.0 phr it was observed that the elongation at break and the tensile strength showed very low values. This could be attributed to excessive agglomeration of the nano-particles, which have been reported to adversely affect both the tensile and elongation properties of composites [19]. The impact strength followed a similar trend wherein the optimum concentration of alumina was found to be 0.5 phr. The glass transition temperature of the nano­composites were also determined. It was observed that the addition of nano-alumina increased the glass transition temperature of the composites. This was due to the incorporation of the nano-particles between polymer chains and segments restricting their motion and hence leading to an increase in the glass transition temperature. In this instance as well it was seen that the highest glass transition temperature was observed at an alumina concentration level of 0.5 phr, at higher concentrations due to agglomeration the dispersion of the nano-particles in the polymer matrix was hampered and hence the restriction to polymer segmental motion was not was effective as at 0.5 phr. From the above discussion it is clear that the optimum concentration of alumina in the unsaturated

H'T OH ?/CH2 + HO�OH + MeO-$i-OMe V

polyester matrix is at 0.5 phr. 1.0 phr alumina also showed a higher erosion resistance based on the average roughness values and length of erosion path. These two concentrations were taken for further study of effect of silane coupling agent.

I OMe

luetacryloxyprop).i himethoxy silane AlumiufI Unsflhwafed polyester resin

Bonding between the fillcr particle and poiymcl" matrix:

Figure 2: Function of silane as coupling agent.

UPR

5.0

958

150

69

18.67

1.22

45.3

1.06

72.8

98.4

1.91

9.57

B) Effect of addition of coupling agent

Silanes are used to enhance the interaction between the polymer and filler through a chemical bonding process, as shown in Figure 2. The reaction is basically an etherification reaction between the hydroxyl groups present on the surface of the filler particle and the reaction with the polymer matrix through the double bond present on the silane and on the unsaturated polyester oligomer. There are a number of methods by which this reaction can be carried out. The most efficient is one in which the filler is treated with the silane in the presence of water and is then filtered and heated to effect the anchoring of the silane to the filler surface. This method is difficult to apply to an industrial or commercial process where a large throughput of filler has to be treated and used. A more applicable method, in this case, is the addition of the silane to the resin mixture followed by addition of filler. This is the method which was followed in the present study keeping in mind practical applicability of the technology. Table 2 and 3 shows the effect of addition of silane to 0.5 and 1.0 phr alumina on various properties. It could be seen that the addition of coupling agent had a negligible effect at 50phr of silane in the case of 0.5phr alumina but in the case of 1.0phr alumina it showed a large increase in erosion resistance. However, further addition of silane resulted in a decrease in the erosion resistance, reflected by an increase in the average roughness and an increase in the erosion path length. The increase in erosion resistance of the composites could be attributed to an increase in the polymer filler interaction while the reduction in the erosion resistance would be due to unreacted silane which acts as a plasticizer or reduces the crosslinking density. Considering the gel content of the composites it could be seen that the addition of silane coupling agent did not adversely affect the gel content. This showed that the silane was incorporated into the polymer network. If the silane was left unreacted it would be leached out during the course of determining the gel content of the sample resulting in a lower gel content. Thus the reduction in erosion resistance was due to the fact that the silane reduced the crosslinking density of the composite, as can be seen in Table 2 and 3.

Considering the mechanical properties, the addition of a coupling agent should increase the tensile and flexural strength of the resultant composites at concentration levels below the quantity required to cover the surface of the particles. At concentration levels of silane higher than that required to cover the surface, the excess silane will decrease the crosslinking density of the polymer matrix. It was observed that the addition of silane to the 0.5phr alumina composites had a negligible effect on the tensile and flexural strengths of the composites, indicating the compatibility of the particle with the resin matrix. Further, it was observed that the addition of silane increased the elongation at break, which is a consequence of the reduction in the crosslinking density of the polymer matrix and has been reported before [17]. The impact strength of the composites were seen to be decreased by the addition of coupling agent. It has been shown in the case of high impact polystyrene that the an increase in the

Table 2: Effect of addition of silane on properties of 0.5 phr alumina based composites.

Silane concentration 1.0Al 1.0Al 1.0Al 1.0Al

Property OS 50S 100S 150S

Coupling agent concentration (phr based on nanoparticle) 0 50 100 150

Gel content (%) 97.56 97.74 98.24 98.29

Tensile strength 29.28 33.44 34.1 29.9 (MPa)

Elongation at break 2.73 3.1 5.28 4.26 (%) Flexural strength 60.14 62.25 51.27 51.38 (MPa)

Impact strength 0.92 1.24 1.21 0.77 (kJ/m2)

Tg (C) 77.55 81.01 79.66 78.25

Crosslink density x 10-6 (g/mol) 114.7 107.2 99.6 98.5 Ra value for 5mm length (microns) 1.67 0.88 2.04 2.14

Erosion path length (mm) 7.21 12.95 10.14 12.19

Table 3: Effect of addition of silane on properties of 1.0 phr alumina based composites.

Silane 0.5Al 0.5Al 0.5Al 0.5Al

concentration OS 50S 100S 150S

Property

Coupling agent concentration (phr based on nanoparticle) 0 50 100 150

Gel content (%) 97.36 97.56 97.97 98.27

Tensile strength 29.84 27.7 29.92 28.7 (MPa)

Elongation at break 2.09 3.81 4.11 5.29 (%) Flexural strength 57.52 52.85 54.18 52.84 (MPa)

Impact strength 1.42 1.28 1.14 1.04 (kJ/m2)

Tg (C) 76.43 73.63 73.71 72.08

Crosslink density x 10-6 (g/mol) 114.7 102.9 95.5 83.2

Ra value (microns) 1.25 1.24 2.06 2.14

Erosion path length (mm) 6.85 8.79 10.8 9.13

compatibility of the butadiene phase in the polymer matrix results in a decrease in the impact resistance [21], a similar theory could be applied in this case as well. The decrease in the crosslink density would also have an adverse effect on the impact strength of the composites. The reduction in covalent

bonds per unit volume of the polymer matrix would require lower amount of energy to break the bonds, leading to a lower impact resistance.

The glass transition temperature of the composites showed an initial increase on addition of coupling agent after which it decreased. A decrease in the crosslinking density would increase the chain mobility of the polymer composite. This increase in the chain mobility of the polymer composite would be reflected by the reduction in the glass transition temperature of the composite, since the glass transition temperature is an indication of the molecular or chain mobility of the polymer matrix.

CONCLUSIONS The addition of coupling agent to the nano-composite

resulted in an increase in erosion resistivity upto a certain

coupling agent concentration. Higher nano-alumina particle

and coupling agent concentrations showed reduced erosion

resistance due to a reduction in crosslinking density of the

polymer composite. The optimum concentration with respect

to the erosion resistance coincided with the highest

performance with respect to mechanical and thermal

properties. The use of nano-alumina as a reinforcing and

partial discharge resistive material in unsaturated polyester

composites could be a useful method to increase the

performance and life of the resultant composite insulation

system

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