ISSN 19954212, Polymer Science, Series D. Glues and Sealing Materials, 2014, Vol. 7, No. 1, pp. 57–60. © Pleiades Publishing, Ltd., 2014. Original Russian Text © E.A. Barzilovich, A.E. Verstakov, V.A. Nikulin, N.V. Sirotinkin, V.A. Sytov, 2013, published in Klei. Germetiki. Tekhnologii, 2013, No. 4, pp. 20–24.
The Influence of the Fractional Composition of a Filler on Thermal Conductivity of a Polymer Composition E. A. Barzilovicha, A. E. Verstakova, V. A. Nikulina, N. V. Sirotinkinb, and V. A. Sytova a
FSUE Special Design and Technological Bureau Tekhnolog (SDTB Tekhnolog), Sovetskii pr. 33a, St. Petersburg, 192076 Russia email:
[email protected] b St. Petersburg State Institute of Technology (Technical University), Moscovskii pr. 26, St. Petersburg, 190013 Russia email:
[email protected] Received January 20, 2013
Abstract—The dependence of thermal conductivity of polymer composites based on Viksint PK68 on the dispersion composition of such fillers as powders of silicon carbide and microdiamonds has been studied. It has been shown that the thermal conductivity of polymeric compositions depends on the gradient of fractions in the powders of fillers. Keywords: Viksint PK68 sealant, thermal conductivity of sealants, powder fillers, silicon carbide, microdia mond powders DOI: 10.1134/S199542121401002X
Currently, development of electrical devices and electronics is being held back, including by a lack of potting compounds and structural adhesives with increased heat conductivity. Miniaturization and increasing specific power consumption of electrical machines and REA requires intensification of heat removal. In this regard, the development of thermally conductive polymer compositions is a promising and popular area of research. Currently, the most common way to increase ther mal conductivity of polymer composites is the intro duction of freeflowing fillers having high thermal conductivity [1, 2]. The total thermal conductivity of the filled composition depends on the volumetric fill ing, i.e., on the packing density of particles (Fig. 1). Packing density can be increased by using powders of different dispersions. The objective of this study was to identify the influ ence of dispersion characteristics of freeflowing fillers on the thermal conductivity of a filled polymer com position. The study was conducted for fillers of differ ent chemical natures. Not only various amounts of dif ferent powders, but also the dependence of heatcon ducting properties on the fillers that combine mixtures of different proportions, were of interest for the study.
the thermal conductivity of the polymer priming com position have been considered using the example of powders of silicon carbide and diamonds. The choice of these materials was made to establish the effect of the chemical nature of the filler on the heatconduct ing properties of the composition. Diamond has a high technical heat conductivity (up to 2000 W/(m K)), is available in a large range on the market, and is processed industrially, which allows standard powder with stable properties to be used. Sil icon carbide is also produced industrially, is repre sented by a wide range, and has a substantially lower price, thereby reducing the cost of the final product in industrial production. At the moment, two types of silicon carbide are industrially produced in Russia—“black” and
OBJECTS OF STUDY In this paper, patterns of the influence of the frac tional compositions of fillers of different chemical natures and various dispersed compositions thereof on
Fig. 1. Models of distribution of filler particles in polymer matrix. 57
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BARZILOVICH et al. 4 tc = const 1
2
5 3 6
tc = const W t1(τ)
t2(τ) t3(τ)
Fig. 2. Thermal circuit of the heatmeasuring cell of the ITSλc20 device: (1) sample, (2) metal core, (3) thin thermal insulating layer, (4) external isother mal media, (5) electric heater, and (6) foundation.
“green.” Due to the fact that thermal conductivity in polymeric compositions of green silicon carbide is higher than that of black, the former was used in the work, which allowed better results and measurement accuracy to be attained. A lowviscosity coldcuring compound was selected as the resin binder for its convenience in working with materials and to simplify the sample pouring technique. Due to the way in which thermal conductivity is measured on an ITSλc20 instru ment, a tight fit of the test sample to the surfaces of the measuring cell is necessary, which entails high demands for the surfaces of the sample. Flexibility of the sample material to a certain extent eliminates sur face defects; therefore, use of elastic coupling is pref erable. These requirements are met by the lowmolec ularweight organosilicon coldcuring compound Viksint PK68. EXPERIMENTAL Thermal characteristics were studied on an ITSλc20 instrument to determine thermal conduc tivity λ, specific heat capacity c, and thermal resis tance R of solid nonmetallic materials under normal conditions. For measurements of thermal conductiv ity, samples were used in the form of discs 30 ± 0.1 mm in diameter and a thickness of 1–5 mm. The permissi ble wedge of surfaces of the sample should not exceed 0.05 mm. The range of values of thermal conductivity of the samples can be 0.15–2.5 W/(m K). The device has a top and consists of two electrically coupled blocks—a heat cell and a controller that con trols the cell and produces automatic processing of an experiment.
Thermal circuit of the heatmeasuring cell is shown in Fig. 2. The essence of the measurement method is as follows. Sample 1 is in thermal contact with metallic core 2, in which electrical heater 5 is placed. Thin insulating layer 3 is located on the oppo site side of the heater and provides a favorable ratio between heat flows through the sample and the layer itself. The upper operating face of the sample is in con tact with external isothermal environment 4. The heat flux of the assigned power is released onto the heater during the experiment and is maintained constant during the experiment. Temperature of the heater t1(τ) and top temperature of the sample t2(τ) are registered by thermocouples. Singlejunction differential ther mocouples made of manganin and constantan are used as temperature sensors. In the initial state, upper unit 4 is tightly pressed by its lower surface to the front surface of base 6, which provides isothermality of the metal core of the heat measuring cell. Just before the experiment, upper unit 4 was raised and sample 1 was placed on the lower block. In the experiment, upper unit 4 with its weight presses sam ple 1 to lower block 6. Before an experiment, the medium–insulating layer–corewithheater–core–sample–medium sys tem is in an isothermal state at room temperature. During the experiment, the heater and the sample are heated by electricity. Changes in the temperature of the sample faces are recorded by temperature sensors with a predetermined time interval. To determine thermal conductivity λ of the sample, patterns of the regular stage of the experiment are used. Its calcula tion is done in accordance with the value of stationary drop on the sample Θsample found in the experiment relative to the ambient temperature. For calculations, calibration constants of the heat cell are used, taking into account the effect of heat loss through thermal insulation 3 of the plate and heat transfer through the side face of the rotary switch. The calibration con stants in the cell are thermal conductivity of the ther mal insulation K (Θ1), W/K, and equivalent radius of the disk R0, m. The values of calibration constants are determined by the manufacturer in calibration experiments on the samples, prepared from reference materials. Fillers were introduced by simple stirring of powder without additional dispersing into the mixture of com ponents 1 (rubber) and 2 (hardener) of Viksint PK68. After thorough stirring for 2 min, the resulting composition was poured into a horizontally mounted form on a plasticfilm substrate (to avoid adhesive sticking of the sample to the form). Samples were held at 15–25°C for 24 h; then, samples were prepared in the form of discs with a size required for measuring heat. Before testing, the samples were incubated in a measurement laboratory for at least 1 h. POLYMER SCIENCE
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THE INFLUENCE OF THE FRACTIONAL COMPOSITION OF A FILLER Table 1. Dependence of thermal conductivity on fractional composition of silicon carbide Amount of Amount of filler, Frac filler, wt parts, vol parts, tion per 100 wt parts per 100 vol parts of binder of binder M5 M28 M40 F180
144 144 144 144
45 45 45 45
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Table 2. Dependence of thermal conductivity on the ratio of F180 and M5 fractions in silicon carbide
λ, W/(m K)
Fraction
0.80 (0.69–1.02) 0.79 (0.67–1.16) 0.71 (0.67–0.78) 0.94 (0.82–1.16)
F180/M5
RESULTS AND DISCUSSION
Amount of Amount of filler, wt parts, filler, vol parts, per 100 wt per 100 vol parts of binder parts of binder 0/144 24/120 48/96 72/72 96/48 120/24 144/0
0/45 7.5/37.5 15/30 22.5/22.5 30/15 37.5/7.5 45/0
λ, W/(m K) 0.80 (0.75–1.30) 0.80 (0.78–0.86) 0.81 (0.79–0.87) 1.01 (0.93–1.11) 1.04 (0.92–1.19) 1.06 (0.93–1.15) 0.94 (0.82–1.16)
Study of SiliconCarbide Powders The maximum degree of filling was selected on the basis of the requirements of composition fluidity. It was found empirically that the introduction of silicon carbide powders into Viksint PK68 in an amount of more than 144 wt parts per 100 wt parts of binder leads to a critical increase in the viscosity and prevents the use of the composition in drilling (Tables 1–3, Figs. 3–5). Marking of siliconcarbide powders in accordance with GOST (State Standard) 3647–80 Dispersity, μm
Marking M5 M28 M40 F180
0–5 20–8 28–40 75–90
Table 3. Dependence of thermal conductivity on the ratio of different fractions in silicon carbide Amount of Amount of filler, wt parts, filler, vol parts, Fraction per 100 wt per 100 vol parts of binder parts of binder F180/M5 F180/M28 F180/M40 M40/M5 M40/M28 M28/M5
120/24 120/24 120/24 120/24 120/24 120/24
37.5/7.5 37.5/7.5 37.5/7.5 37.5/7.5 37.5/7.5 37.5/7.5
λ, W/(m K) 1.06 (0.93–1.15) 1.0 (0.76–1.33) 0.8 (0.68–0.92) 0.84 (0.73–0.91) 0.80 (0.72–0.92) 0.79 (0.71–0.89)
dences of thermal conductivity on the ratio of the data of fractions at the same degree of filling.
It follows from Table 1 that the maximum value of thermal conductivity is achieved by using M5 and F180 fractions. Table 2 shows studies of the depen
It follows from the data presented in Table 2 that the maximum value of thermal conductivity is achieved at a ratio of coarse to fine fractions equal to
Thermal conductivity, W/(m K) 1.0
Thermal conductivity, W/(m K) 1.2
0.8
1.0
0.6
0.8 0.6
0.4
0.4
0.2
0.2
0 M5
M28
M40
F180 Fraction SiC
0 0/144 24/120 48/96 72/72 96/48 120/24 144/0 Ratio of fractions F180|M5
Fig. 3. Dependence of thermal conductivity of poly mer composition on the fractional composition of a filler of siliconcarbide powder. POLYMER SCIENCE
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Fig. 4. Dependence of thermal conductivity of poly mer composition on the ratio of F180 and M5 frac tions in a filler of siliconcarbide powder. 2014
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BARZILOVICH et al.
Table 4. Dependence of thermal conductivity on fractional composition of microdiamond powders Disper Amount of Amount of sion of filler, wt parts, filler, vol parts, fraction, per 100 wt per 100 vol μm parts of binder parts of binder
λ, W/(m K)
1–2
187
53
0.85 (0.68–0.87)
2–5
187
53
0.77 (0.72–0.84)
28–40
187
53
0.92 (0.84–1.00)
120/24, respectively. Table 3 shows the data on the dependence of thermal conductivity of different frac tions in an optimum ratio. Study of Microdiamond Powders To study the dependence of thermal conductivity of the polymer composition using microdiamond pow ders (GOST (State Standard) 9206–80), fractions with dispersions of 1–2, 2–5, and 28–40 μm were used. The maximum degree of filling was also selected based on the requirements for composition fluidity. The thickening properties of microdiamond powders allow one to introduce them into Viksint PK68 in quantities not exceeding 187 wt parts per 100 wt parts of binder (Tables 4, 5). The measurement results shown in Table 3 show the specific effect of fractional composition of silicon carbide powders in polymer compositions on the ther mal conductivity of the latter. There is a statistically significant trend toward an increase in the thermal conductivity of the compositions with an increase in the gradient composition of the filler. The maximum value of the gradient is provided in a mixture of frac tions of F180 and M5, for which there is the maximum value of thermal conductivity. Thermal conductivity, W/(m K) 1.2
Table 5. Dependence of thermal conductivity on the frac tional ratio in microdiamond powders Amount of Amount of Dispersion filler, wt parts, filler, vol parts, of fraction, per 100 wt per 100 vol μm parts of binder parts of binder 28–40/2–5 28–40/1–2 28–40/1–2 28–40/2–5
170/17 170/17 156/31 156/31
0.85 (0.76–0.97) 0.89 (0.82–0.91) 0.82 (0.78–0.91) 0.84 (0.76–0.86)
On the assumption that an increase in the gradient of fractions increases thermal conductivity linearly, it can be assumed that the minimum value of the param eter will be when using a mixture of M40 and M28 fractions. However, it is seen from Table 3 that the thermal conductivities of the M40/M28 and F180/M40 mixtures are same, while that of M28/M5 is significantly lower. Apparently, only combined use of the F180 coarse fraction with a more finely dis persed one leads to an increase in thermal conductiv ity. Fine fractions (M5, M28) do not show a sustained increase in thermal conductivity when used together. Analogous dependences of thermal conductivity on the fractional composition of the microdiamond powders are less pronounced, but have the same nature as in the case of siliconcarbide powders. CONCLUSIONS (1) The effect of the fractional composition of sili concarbide powder on the thermal conductivity of a composition has been established. (2) The thermal conductivity of compositions con taining siliconcarbide powder depends on the gradi ent of its fractions. (3) The influence of the fractional composition of the microdiamond powders on thermal conductivity of polymer compositions has a character similar to the influence of the siliconcarbide powder.
1.0
REFERENCES
0.8 0.6 0.4 0.2 0
48.5/4.8 48.5/4.8 44.5/8.8 44.5/8.8
λ, W/(m K)
F180/M5
F180/M28
F180/M40 Fraction SiC
1. D. P. Volkov and M. V. Uspenskaya, Izv. Vyssh. Uchebn. Zaved., Priborostr. 53 (4), 49–51 (2010). 2. G. N. Dul’nev and Yu. P. Zarichnyak, Thermal Conduc tivity of Composite Materials and Mixtures (Energiya, Leningrad, 1974) [in Russian]. 3. S. N. Gladkikh, N. N. Vekshin, E. V. Kolesnikova, I. V. Tkachenko, T. N. Dreval’, Pol. Sci. Ser. D 5 (3), 145–150 (2012). 4. A. A. Donskoi, N. V. Baritko, O. A. Eliseev, and V. A. Tumanov, Pol. Sci. Ser. D 4 (3), 198–206 (2011).
Fig. 5. Dependence of thermal conductivity of poly mer composition on the ratio of fractions in a filler of siliconcarbide powder.
Translated by Sh. Galyaltdinov POLYMER SCIENCE
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