Comparative Studies on the Electrical and Mechanical Behavior of Some Soldering and / or Impregnation Lacquers

ADRIANA-MARIANA BORS1, DANIEL LINGVAY2, ALINA-RUXANDRA CARAMITU3*, IULIAN IORDACHE3, IOSIF LINGVAY3 1.ICPE SA, 313 Splaiul Unirii, 030138, Bucharest, Romania 2.Sapientia Hungarian University of Transylvania, Faculty of Sciences and Arts, 4 Calea Turzii , 400193, Cluj-Napoca, Romania 3.National Institute for Research and Development in Electrical Engineering INCDIE ICPE-CA, 313, Splaiul Unirii, 030138, Bucharest, Romania

Depending on the type of polymer, ingredients, polymerization conditions, etc. the physical-chemical characteristics of the lacquers are different and have multiple uses in the most diverse fields such as the furniture industry, coatings for protection, pharmaceutical industry, electrical engineering, etc.
On the other hand, both waste resulting from the application of synthetic lacquers and end-of-life waste products containing polymer and / or synthetic lacquer coatings are hardly biodegradable -they have a pronounced resistance to the moulds action [22][23][24][25].
The electrical and mechanical characteristics of polymers and composite polymer materials are determined by the nature of the polymer and the filler as well as by the polymerization conditions [26][27][28][29][30][31] When using at impregnating of windings power transformers with mixed insulation, it is necessary that the lacquer be compatible with the used electro-insulating fluid, and not degrade or initiate chemical degradation processes of the electro-insulating fluid [32][33][34][35][36][37].
In view of these considerations, the purpose of the paper is to compare the electrical and mechanical behaviour of some lacquers from class H, used for soldering / impregnating some component parts (benchmarks: stator, rotor) of the electric servo motors.

Experimental part
By electrical and mechanical measurements three different types of lacquers were compared. The types of investigated lacquers, which were chosen according to the class of insulation, F (155 o C), H (180 °C) and C (above 180 o C) are shown in table 1. For electrical determinations (dielectric spectroscopy and resistivity), samples were prepared on a cold-rolled silica sheet having the dimensions of 60 x 200 mm and a thickness of 0.35 mm by immersion in the investigated lacquer.
After the leakage of excess lacquer (storage at 22 ± 3°C for 2 h) the polymerization of lacquer was achieved by heat treatment -heating with 1.5 degrees / minute and after 3 hours at 155 ± 5 °C, natural cooling in the oven with thermostat and air circulation.
The thicknesses of lacquer layers obtained -measured with a micrometer -were in the 35 ± 4 µm range.
For the breaking strength determination, samples were made by bonding two pieces of square steel bar of 22 x 22 mm with the investigated lacquers.
The polymerization of lacquer from samples achieved was made at 155 ± 5 °C for 3 h. The image of the samples used to determine the bending strength and their positioning on the test machine is shown in figure 1.
By analyzing the values in table 2 it is found that the hardness of L-S lacquer is systematically higher than that of L-G lacquer (by about 40 %) and that of L-528 lacquer (by about 67 %).
In figure 2 shows the bending force and deformation diagrams, obtained on the investigated samples (by continuous deformation and bending force monitoring) placed on supports with a distance of 2 cm between them.
In figure 2, it is noted that the three investigated materials (3 types of lacquers) respond differently to the bending force. Thus, for L-528 lacquer, when increasing the bending force, the deformation increases monotonously (with an average value of about 0.11 mm/kN), up to a critical value when the material breaks (at about 1350 N).
Type of mechanical tests including compression strength -three-point bending strength at the maximum strength of 30 kN and measurement accuracy of 0.5 % at ambient temperature (22 ± 3 °C).
The electrical resistivity of the lacquers was calculated from the resistance values measured at ambient temperature (22 ± 3 °C) with FLUKE 1550B Mega Ohm Meter. Measurements were performed in DC at 250 V with a circular measuring electrode described in [40,41] having a contact surface of 20 cm 2 .
Three samples with different coating thicknesses were measured for each type of lacquer. Shore hardness determinations were made at ambient temperature (22 ± 3 °C) with Shore A, model HBA 100_0, manufactured by SAUTER (Germany).

Results and discussions
The results of the Shore hardness determinations are shown in table 2. For L-G lacquer, at increasing bending force, the deformation is monotonous (about 0.03 mm/kN) up to a value of about 6000 N when, a level of deformation occurs more pronounced (suggesting a flow of the lacquer, a slight elasticity, respectively), after which, until the material breaks (at about 8500 N) the increase of the deformation is approximately linear 0.04 mm/kN).
The L-S lacquer, shows a particular behavior namely, up to about 150 N is recorded a relatively large deformation of 0.06 mm (about 0.4 mm/kN), after which up to 8800 N the deformation is relatively small (0.01 mm/kN) followed by a marked of flow area with large deformation at the decrease bending force (-0.032 mm/kN) up to about 50 N when the material breaks.
This behavior of the L-S lacquer indicates that in electrical applications it is capable of taking over relatively high deformations of the metal support (due to thermal expansion) without significant increasing the mechanical stress of the insulation system. Scheme in figure 3, shows the parameters for the calculation of the bending resistance σ b, of the investigated materials. With the notations in figure 3, σ b, is: (1) where: M b -bending moment; F max -the maximum value of bending force; W -bending resistance module; (2) (3) The calculated values of σ b, and W for the investigated samples (l = 2 cm; b = h = 2.2 cm) are shown in table 3. up to about 0.0098, compared to L-S lacquer, which up to 10 kHz has a dielectric loss tgδ approximately constant of 0.0045.
As it can be seen in figure 4, at frequencies higher than 2 kHz, dielectric loss tgδ for L-S lacquer is less about twice compared to L-G lacquer and about 2.6 times with the L-528 lacquer. It is also noted that, at frequencies up to 250 Hz dielectric losses, tgδ for L-528 lacquer are about 2.9 times higher than for L-G and L-S lacquers.
These tgδ dielectric losses values indicate that for electric motors the heating due to dielectric losses in insulation can be substantially smaller (up to 2.9 times) when using the L-S or L-G lacquers for the windings consolidation compared to L-528 lacquer, with the remark that, L-S lacquer has dielectric losses tgδ of only 0.0045 including at frequencies in the 250 Hz to 10 kHz range.
The volume resistivity calculation ñ, for the investigated lacquers was performed with the relation (4): (4) where: R -measured insulation resistance; S -area of the measuring probe (20 cm 2 ); d -thickness of the insulation layer (lacquer).
The measured values of R at ambient temperature and the calculated values of ρ ρ ρ ρ ρ are comparatively presented in table 4.
By comparing the data presented in table 4, it is observed that the volume resistivity of the L-S lacquer is about 10 % higher than for the L-G lacquer and about 3.6 times higher than for the L-528 lacquer, values that are consistent and explain the evolution of the dielectric losses at low frequencies (up to 250 Hz) of the lacquers (the contact area S for all samples was of 20 cm 2 ).
By analyzing the values in table 3 it is found that at the ambient temperature the L-S lacquer bending resistance is approximately equal to that of L-G lacquer and about 6.5 times higher than that of L-528 lacquer.
Results of determination by the dielectric spectroscopy technique and respectively, evolutions of dielectric loss and of frequency, are comparatively presented in figure 4.
By analyzing figure 4, it is observed that at the ambient temperature and at frequencies up to 250 Hz there are no differences in the dielectric behavior of the L-G and L-S lacquers, but as the frequency increases, dielectric loss tgδ for the L-G lacquer have a tendency of continues growth

Conclusions
Three different types of lacquers based on alkyl-epoxymelamine copolymer (L-528), epoxy polymer (L-G) and polyurethane (L-S) have been characterized by mechanical (bending resistance and Shore A hardness) and electrical (dielectric spectroscopy -dielectric losses and insulation resistance in DC) determinations.
The main conclusions after processing the experimental results are: -the polyurethane-based lacquer has relatively large deformations of approx. 0.4 mm/kN up to 150 N bending force, so it has the ability to take over relatively high deformations of metal support (due to thermal expansion from various applications) without significant increase of the mechanical stress, unlike the alkyl-epoxy-melaminebased copolymer lacquer (0.11 mm/kN) and the pure epoxy lacquer (0.0 3mm/kN); -at ambient temperature the bending resistance of the L-S polyurethane lacquer is about equal to that of the L-G epoxy lacquer and about 6.5 times greater than that of the L-528 alkyl-epoxy-melamine -based copolymer lacquer; -at frequencies below 250 Hz the dielectric losses in the epoxy and polyurethane lacquers are about equal and are about 2.9 times lower than in the alkyl-epoxy-melamine copolymer lacquer; -at frequencies higher than 250 Hz the investigated lacquers behave differently, namely at the frequency increase up to 10 kHz, the dielectric loss tgδ of the polyurethane lacquer remains constant at about 0.0045, unlike the epoxide lacquer at which dielectric loss tgδ increases monotone up to 0.0095 at 10 kHz, or for the alkylepoxy-melamine-based copolymer lacquer at which the tgδ is about constant 0.012 in the 250 Hz -10 kHz range; -the resistivity of the polyurethane L-S lacquer is about 10% higher than the L-G epoxy lacquer and about 3.6 times higher than the L-528 alkyl-epoxy-melamine-based copolymer lacquer.