This experiment was designed to estimate the heat storage ability of SAPC. The aim of this experiment is to measure the thermal property of SAPC to develop its application in the field of health care, for example to develop efficient heating pads for patients suffering from rheumatism and other ailments for application of constant moderate heat is beneficial.
The experiment conditions were as follow. In a glass vessel preheated to a temperature of 54 oC and equipped with a thermometer, one gram of SAPC and 200 ml of water with the same temperature of 54 oC were added separately. The vessel was then positioned on a thermal insulation plate. The temperature of the water was recorded in a preset time schedule. The temperature of air surrounding the vessel was 20°C. For the purpose of comparison, a blank test was carried out under exactly the same condition but without addition of SAPC. The schematic experimental setup is shown in Figure 52.
Figure 52. Experimental setup to measure the heat storage capacity of SAPC
Figure 53. Temperature retaining properties of SAPC. (environmental temperature: 20 °C)
The data shown in Figure 53 was measured with the thermometer II (see Figure 52).
As shown in Figure 53, the addition of SAPC decreased the heat loss and slowed the temperature drop by a factor of 1.5 - 2.0 times compared to pure hot water.
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To compare the heat loss of the two different systems, the temperature above the water level was recorded with thermometer I. Figure 54 shows the temperature difference recorded by thermometer I.
Figure 54. Temperature difference above the water level of pure water and SAPC-water mixture systems (Twater - TSAPC/water)
From Figure 54 it is obvious that at the beginning of the experiment, the temperature above the pure water was higher than the temperature above the SAPC/water mixture system, but, after about 10 minutes, the temperature was lower than that of the SAPC/water mixture system. The reason is probably that at the beginning of the test, although the temperature of the two systems was the same, the temperature above the pure water was higher than that of the SAPC/water system because the pure water system releases heat faster than the SAPC/water system. But, since the temperature drop of the pure water system is faster than that of the SAPC/water system, after a while, temperature in the pure water system dropped which was lower than that of the SAPC-water system, and so did the temperature above the water level.
This suggests that the reason for the SAPC/water system to keep its temperature longer than the pure water system was that the water molecules were blocked in the network of the SAPC and hence their movement was restricted. This prevented heat loss and kept the higher temperature for a longer time. This suggestion is supported by the observation that the temperature in the water system was almost the same from center to surface (homogenous), but in the SAP/water system, the temperature in the center of the glass vessel was much higher than that at the surface of the system. An approximate temperature gradient of 2~3 oC was observed.
5.2.2. Thermostability of SAPC
The purpose of this test was to estimate the thermal stability of the SAPC. In the experiment, the SAPC samples were treated in an oven with pre-set temperature. After treatment, the WAC of the samples were measured using distilled water to see the effect of heat treatment. After the heat treatment, the appearances of some samples were changed. The results are shown in Table 16.
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Table 16. Compositions and the appearances of the SAPCs after the heat treatment Composition (%)
AM Bentonite AANa 120 oC 170
oC
200
oC a 60 40 - NC B B
b 50 50 - NC NC SB
c 25 - 75 NC NC NC d 75 - 25 NC B B NC: no change; B: bloated; SB: slightly bloated.
Heat treatment changed the appearance of some SAPC. Samples with higher sodium acrylate content showed a higher thermostability.
Table 17. Thermostability of SAPC with different AM/AANa composition AM/AANa/Bentonite 170 oC 200 oC
e 1 / 4 / 5 NC NC
f 1 / 1 / 2 NC NC
g 4 / 1 / 5 NC B
Table 17 shows the relationship between thermostability and composition of SAPC clearly, i.e. the sodium acrylate component increased the thermostability.
The effects of heat treatment on the water absorption capacities of SAPC are shown in Figure 55.
Figure 55. Effect of heat treatment on the WAC of SAPC
From Figure 55 it can be concluded that below 170°C there was no effect of the heat treatment on the WAC of SAPC samples. After the temperature has risen to 200°C, the
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WAC of sample d (AM-AANa) rose greatly. This might be caused by the destruction of some of the bonds in the structure during the heat treatment, which decreased the cross-linking density. However, the 200oC temperature treatment had no effect on the other samples.
This means that the SAPC in general has a rather good thermostability.