2.12. Thermogravimetric Analysis
TGA was carried out using thermomicrobalance from NETZSCH (Selb, Germany) with a scan range from 25 °C to 800 °C at a constant heating rate of 10 °C/min in an air atmosphere with nitrogen flow as the purge gas. Samples of 9–10 mg were loaded in an Al2O3 crucible.3. Results and discussions
Table 2 shows the corresponding IR band assignments of illite and modified illite (
Figure 1). The characteristic bands of 3697, 3625, and 3432 cm
−1 correspond to the stretching vibration of the hydroxyl groups (OH). The bands at 1033 and 468 cm
−1 are those of Si-O bonds. The band located at 911 cm
−1 is attributed to the deformation vibration of Al-OH, and the bands at 753 and 532 cm
−1 arise from the deformation vibrations of Al-O-Si. The presence of a doublet at 789 and 777 cm
−1 and a singlet located at 693 cm
−1 is attributed to the quartz vibrations. The band at 1455 cm
−1 developed due to the hydroxyl bending vibration again reflects the presence of bound water [
15,
16].
As shown in the XRD pattern of
Figure 2, the dominant phase is quartz (JCPDS card No. 46–1045), an impurity present in the illite sample, where on the figure Q and I mean respectively quartz and illite. The peaks at 2θ = 17.77°, 19.90°, 27.99°, 26,65 and 35.05°, according to JCPDS card No. 000-26-0911 corresponds to the illite phase. Natural clay, not fractionated, contains a great deal of quartz (the main impurity), and this is a normal phenomenon. In addition, the intensity of the XRD peaks varies depending on the substance being determined and its crystallinity, so peak intensities should not be considered without appropriate calibration. Amine modification has no effect on the crystalline structure of the materials presented; only amorphous impurities may have been removed during the modification—the amine etching. This confirms the identity of the IR spectra of the pure filler and its modified versions. During the modification of the etching, the amines did not graft onto the surface of the filler.
All samples showed similar patterns with an almost continuous and significant mass loss from 25–700 °C (
Figure 3), a combination of water and amine loss resulting from modification. The mass loss increases with the amine concentration, indicating that higher concentrations have increased the amount of amine deposited on the material. The highest loss was observed for the THA 1.0 sample, while the unmodified sample exhibited the lowest.
The base silicone pressure-sensitive adhesive without the filler exhibited good adhesion and tack properties—presented in
Table 3. The cohesion at room temperature and 70 °C was also high. However, the thermal resistance of pressure-sensitive adhesives revealed a relatively low SAFT test result. This, unfortunately, limits the applicability of the adhesive. The goal of the filler addition and its modification was to improve the thermal resistance of the silicone adhesive while maintaining other parameters on a similar level.
Table 4 presents the viscosity changes over time of the adhesives containing 3 wt.% illite. The viscosity of the compositions increased rapidly after filler addition (i.e., the viscosity doubled the value or was too high to be coated on a substrate) when compared to the neat composition. The lowest value was obtained for the sample containing filler without modification. The highest viscosity increase was noted between the fifth and the seventh day. Practically all the systems, after 7 days, became too viscous to be coated.
Comparing the fillers used, the filler modified with amine with a concentration of 0.1 M shows the lowest viscosity value, while the highest viscosity value was obtained for the filler modified with amine of 1.0 M. This may be due to the fact that illite filler was etched with amine with the highest concentration; therefore, the proportion of silicon atoms is the highest [
17]. The desired viscosity of the modified compositions is as low as possible, depending on the type of material to be coated; the upper limit is assumed to be that the viscosity around 50 Pas is already uncoatable (paste consistency).
Figure 4 shows the peel adhesion values of silicone pressure-sensitive adhesives containing modified illite. The blue color shows the results for the adhesive modified with the filler, while the filler was not treated with amine. The red color shows the results for the tape containing a filler modified with amine at a concentration of 0.1. Green-colored tapes are made of adhesive modified with a filler etched with amine at a concentration of 0.5. Moreover, the color purple—a filler treated with amine with the highest concentration equal to 1.0. The adhesive of the system with unmodified illite decreased with filler content. A similar phenomenon was observed for the adhesives containing illite treated with amine. It is a common phenomenon in the technology observed, for example, in adding commercial silicone pressure-sensitive adhesive montmorillonite and its amine modifications [
18]. The increase in adhesion with an increase in the filler concentration for the sample containing the filler modified with amine 0.5 is quite a rare phenomenon that is difficult to interpret. It shows the maximum with a content of 1.0 wt.%, likewise for tapes manufactured using illite filler modified with 1.0 M amine. For these samples, a maximum was obtained at the value of 0.5 wt.% of filler. The adhesion value depends on the compatibility between the filler and the matrix, its miscibility, and its tendency to agglomerate [
19,
20]. However, it seems that each filler’s modification could have a slightly different effect on adhesion value.
The effect of the amount of filler content on the tack is presented in
Figure 5. The adhesive containing neat filler showed a known phenomenon, i.e., tack value decreased with filler content [
21]. For the composition with a filler modified with the lowest concentration of THA, a slight increase was observed at the value of 1.0 and 3.0 wt.%. The system containing illite modified with 0.5 M THA tack value changed only slightly with the filler amount. However, the adhesive with a filler was modified with the highest concentration of amine. Thus, depending on its content, the filler had a different effect on the properties of pressure-sensitive adhesives. It was observed, especially for low and high polymer matrix loads [
19,
22,
23]. Such a phenomenon could be due to the compatibility of the filler with the polymer matrix leading to increased tack for low filler contents. On the other hand, the stickiness value could decrease for higher filler loads as a result of its agglomeration.
Table 5 presents the results of the cohesion at room temperature, 70 °C, and the thermal resistance determined using the SAFT test. In the case of cohesion measurements, all tested compositions showed values higher than required for the industrial strip production (above 72 h). The only exception was lower cohesion (32 h) for the system containing 3 wt.% of neat illite. These results confirm the proper compatibility of the silicone resin with the modified filler.
The prepared self-adhesive tapes were also tested for thermal resistance using the SAFT test. When comparing the result for the neat adhesive (without the filler—
Table 2), i.e., 147 °C, it could be observed that for each composition containing illite, increased thermal resistance was noted, even above the maximum tested 225 °C. However, the thermal resistance decreased with increasing the filler amount, which was easily observed for the system containing illite modified with 1.0 M THA. Thus, the presence of mineral filler improved the thermal resistance, but high organic modifier content could be unfavorable. The highest values were obtained for the samples with the filler unmodified with amine. In the case of amine-etched samples, a decrease in the maximum operating temperature was noted, but not below the values obtained by the samples without fillers. Therefore, the etching of the samples improves their compatibility with the resin but does not increase its thermal resistance, which may be related to the increase in silicon mass in the etched fillers compared to the fillers without modification [
19,
23].
The effect of filler content on the shrinkage of silicone pressure-sensitive adhesives is shown in
Table 6. It could be noted that the higher the filler content in the polymer matrix, the lower shrinkage. This may be due to a better alignment of the polymer mesh or a more compact internal structure of the adhesive film [
24,
25]. In addition, adding a filler not modified with amine has an unfavorable effect on the shrinkage of the pressure-sensitive adhesive. This is because the introduced modifying material affects the speed of the crosslinking reaction of the adhesives in the measured samples, which results in the lack of a compact composite structure and, therefore, compared to the samples modified with amine, they showed less shrinkage [
26,
27]. The best results after stabilization of the composition (7th day), i.e., the lowest shrinkage values, were obtained for the compositions containing neat illite and the filler modified with 0.1 M THA for fillings 1.0 and 3.0 wt.%.