Synthesis mechanism of CTS modified SS/PU composites
Figure 1 schematically shows the primary chemical reactions route that occurs during the SS/PU composites preparation and the CTS effect. The organic component and the modified SS are mixed by using a two-blade paddle mixer. The achieved homogeneous mixed slurry is poured into the mold and cured until theexpected SS/PU composites are achieved (see Fig. 1a). During the process of mixing, the hydrolyzed CTS is oriented at the interface between the organic phase and the inorganic phase, thus forming a smaller and more uniform inorganic dispersion system by the reduction of the organic–inorganic interfacial tension (see Fig. 1b,d). This benefits the acceleration of the curing process and the better dispersion of the stress. Thus, the mechanical properties of the composites are further improved. During the following curing of SS/PU composites, multifunctional polyisocyanate is reacted with the water from the SS, leading to a highly cross-linking network of formed polyurea by urea units in the organic phase (see Fig. 1c)25. More water consumption produces more carbon dioxide into SS, which in turn produces more sodium carbonate, leading to gelation of the SS in the inorganic phase23. Water continuously enters the organic phase from the inorganic phase, while the diffusion path of the produced carbon dioxide is opposite26. The directional mass transfer of water and carbon dioxide together realizes the hardening of the SS to the polysilicate and the cross-linking polymerization of the organic phase, and eventually produces the solidification of of the organic phase and the inorganic phase2. Moreover, a large amount of reaction heat from the amine-isocyanate chemistry will also accelerate this curing process. In addition, the lipophilic group can make part of CTS get into the organic phase well-distributed. Subsequently, the reactive lipophilic groups of the CTS as hard segment will bind with the cross-linking point (e.g. the NCO and urea groups) of the organic phase, forming a chemical bond. Therefore, it is precise because of multiple physical and chemical interactions between the CTS and the SS/PU composites that the SS/PU composites have the outstanding mechanical properties.
Contact angle analysis of the modified SS
To achieve a high mechanical performance of the composites, interface properties between the organic and the inorganic phases need to be improved. Figure 2 shows the static contact angle of the modified SS on the cured organic component surface. Pure SS presents a contact angle of 117.49°, whereas by modifying it is gradually reduced from 117.49° to 77.73°. An improvement of more than 33% is observed in wettability of SS on the cured organic component surface. It is probably ascribed to CTS modification decreasing the surface energy and hydrophilicity index of the SS, so SS becames much more hydrophobic and stronger affinity toward the organic phase27. This improved wetting property is beneficial to the refinement and uniform distribution of the SS in PU matrix and improves the strength of the SS/PU composites.
Mechanical properties of SS/PU composites
Figure 3 shows that the compressive strength obtained form the SS/PU composites with the CTS content of 0.0 wt%, 0.5 wt%, 1.5 wt%, 2.5 wt% and 3.5 wt%. The CTS is observed to enhance the compressive strength of the SS/PU composites significantly. Excellent wettability allows maximum inorganic phase refinement. Accordingly, the curing process of the SS/PU composites becomes faster and more complete. Therefore, the compressive strength of the SS/PU composites varies from 55.07 to 62.19 MPa with the increase of the CTS content from 0.0 wt% to 2.5 wt%. As CTS content is greater than 2.5 wt%, a peak is reached for the SS/PU composites, showing a slight decrease in compressive strength thereafter. This is probably due to the arising interface saturation with the CTS in the SS/PU composites. The outcome therefore indicates that the interface saturation between the inorganic phase and organic phase may start as early as the 2.5 wt% CTS content range, which is adequate to reach high compressive performance.
To further understand the mechanical properties of the SS/PU composites, the flexural strength, flexural modulus and fracture toughness of the SS/PU composites with 2.5% of CTS optimal addition and without CTS are examined respectively, as shown in Fig. 4. From Fig. 4, it is observed that 2.5 wt% of the CTS appropriately improves the average flexural strength of the SS/PU composites from 39.3 MPa to 41.9 MPa. However, the 17.5% and 9.7% enhancement in flexural modulus and fracture toughness are achieved from 2.5 wt% CTS modified the SS/PU composites, respectively. This may be attributed to the active cross-linking effect of the CTS. The CTS can enter and form strong chemical interactions with the organic phase of the SS/PU composites and further strengthen the interior cross-linking structure of the organic phase, thus facilitating the improvement of strength and toughness for the SS/PU composites. Therefore, 2.5 wt% of CTS will be used as the designated amount of modified SS/PU composites. The obtained SS/PU composites will be used for the fracture surface morphology, elemental composition, chemical structure and thermal properties analysis.
SEM–EDS analysis of SS/PU composites
In order to understand constitutes of the SS/PU composites and further explore how the CTS produces physical and chemical interactions to enhance the mechanical performance of the SS/PU composites, SEM and EDS are used together to observe the fracture surface morphology and analyze the elemental composition and content in the SS/PU composites (see Fig. 5). The SEM images show the spherical dispersed phases embedded in the continuous phase. Moreover, the continuous phases exhibit many dome-shaped cavities throughout the fractured surface of the composites. It is obvious that there is an insufficient interfacial bond strength between the continuous phase and the dispersed phase. The EDS analyses show that the continuous phases consist of C (65.54 wt%), O (11.38 wt%), Cl (13.17 wt%) and C (70.11 wt%), O (13.23 wt%), Cl (12.98 wt%) in the non-modified and modified SS/PU composites, respectively. It reveals the continuous phase originated from the organic component. On the other hand, the main elements of the dispersed phase are Si (32.15 wt%), O (44.96 wt%), Na (12.57 wt%), C (10.33 wt%) and Si (30.10 wt%), O (46.65 wt%), Na (10.04 wt%), C (13.21 wt%) in the non-modified and modified SS/PU composites, respectively. The results therefore prove that the dispersed phase originates from the SS.
As shown in Fig. 5A(a),B(a), compared with the non-modified samples, 2.17 wt% of the Si element appears in the continuous phase of the modified sample. This phenomenon illustrates that the CTS will migrate into the organic phase in the modified SS/PU composites. Figure 5A(b) and B(b) show that the element content of C (42.99 wt%), Cl (24.90 wt%) and Si (2.78 wt%) on the cavity surface of the modified composites are much higher than that (C (21.09 wt%), Cl (10.75 wt%) and Si (trace))of the non-modified composites. This confirms that the CTS is widely distributed at the interface between the organic phase and the inorganic phase. Furthermore, it is found that C and Cl elements are greatly increased on the cavity inner surface of the modified SS/PU composites. This demonstrates that the introduction of CTS improves the interface compatibility and affinity between organic and inorganic phases. In addition, it can be clearly seen that the O (46.65 wt%) and C (13.21 wt%) content on the dispersed phase of the modified sample are higher than that (O (44.96 wt%) and C (10.33 wt%)) of the non-modified sample (see Fig. 5A(c),B(c)). This increment is primarily attributable to higher CO2 accumulation rates at the inorganic–organic phase interface caused by the rapid cross-linking polymerization of the organic phase in the modified SS/PU composites.
In order to confirm the influence of CTS on the distribution and morphology of SS in the PU matrix. The cross-sectional SEM images of the non-modified and modified SS/PU composites are shown in Fig. 6a,b. The SEM images confirm that the polysilicate particles are individually dispersed in the PU matrix. Moreover, the SEM images show that the polysilicate particle size and dome-shaped cavities have become smaller and more uniform in the modified SS/PU composites due to the addition of CTS, which is responsible for the relatively high mechanical strength and toughness of SS/PU composites. In addition, Fig. 6c,d directly show the size distribution histograms of the polysilicate particles of the non-modified and modified samples. The size of polysilicate particle varies from 0 μm to 60 μm, and 0 μm to 24 μm for the non-modified and modified samples, respectively. Moreover, the mean size of polysilicate particle for the non-modified and modified samples are also reduced from 11.3 μm to 5.7 μm. The results demonstrate that the CTS has excellent emulsification ability. Additionally, 4.4% of the polysilicate particles with a scale between 24 μm and 60 μm are present in non-modified samples. Adding the size of polysilicate particles leads to larger cavities formation. These particles and cavities as defects can easily cause stress concentration, which deteriorates the mechanical properties of the SS/PU composites. The micro-structural analysis of SS/PU composites ensures the excellent emulsifying ability of the CTS, which is consistent with the results of mechanical performance testing.
IR spectra of SS/PU composites
The intensity of the NCO peak is a very important parameter, expressing the degree of curing of SS/PU composites. The strong absorption peak at 2270 cm−1 corresponds to the NCO group stretching in the SS/PU composites. As shown in Fig. 7, whether modified or not, the NCO peak intensity of the SS/PU composites gradually decreases as the the curing time increases from 0.5 to 28 days. This implies that NCO groups in the organic phase are gradually consumed by the diffused water from the inorganic phase during the curing process of the SS/PU composites. Moreover, by comparison, the NCO peak intensity of the modified SS/PU composites is weaker than that of the non-modified SS/PU composites under the same curing time. This difference indicates that the curing process of the SS/PU composites can be accelerated by the addition of CTS due to the intensity of the peak decreasing progressively as the curing rate increases. In addition, in comparison with the non-modified SS/PU composites, the absorption peak of NCO groups exhibited by the modified SS/PU composites decreases after fully curing. This is due to the fact that the CTS enters the organic phase as the additional cross-linking points form chemical interactions with NCO groups in the organic phase.
The DRIFTS spectrum obtained form the modified SS/PU composites is compared with those form the non-modified SS/PU composites in Fig. 8, which shows that the nearly identical spectrum curves of modified and non-modified SS/PU composites. This indicates that the addition of CTS has no obvious effect on the change of the chemical structure of the SS/PU composites. The absorption peaks at 3280–3360 cm−1 and 1680 cm-1 might contribute to amide N–H stretching peak and the urea carbonyl groups (C=O)28,29,30, which is the essential structures for polyurea from the reaction between NCO groups and water molecules. However, the characteristic peak of NCO at 2267 cm-1 is still observed, indicating that there might be a small amount of residual NCO in the SS/PU composites after fully curing.
Thermo gravimetric analysis of SS/PU composites
In order to fully evaluate the effect of CTS on thermal properties of SS/PU composites, we investigate the course of thermal decomposition of SS and SS/PU composites before and after modification (see Fig. 9a). In the temperature range from 30to 250 °C the free water and the crystallization water are lost from the SS, with maximum rate at 80 °C and 132 °C, respectively. Similarly, the DTG peaks of SS/PU composites within 50–250 °C are due to elimination of water and a small amount of low molecular weight organics release. Between 50 and 178 °C, a slight reduction in degradation rate for modified SS/PU composites compares to non-modified SS/PU composites. It results from the fact the dispersion of SS particles in the PU matrix is higher and finer than that of non-modified SS/PU composites, therefore, the water from the SS can be more easily consumed. This is also the reason for the high thermal stability of the modified composite material between 30 °C and 198 °C. Moreover, a strong degradation rate peak is detected between 178 °C and 220 °C, which should be linked with the evaporation of the CTS present at the inorganic–organic phase interface of the modified SS/PU composites. Besides, three other typical weight loss steps are observed: the first and second mass loss occurring at 330 °C and 412 °C can be attributed to the degradation of hard and soft segment, respectively31. The third weight loss in the range of 480–550 °C might be due to the degradation of the rest of the organic phase32. In this step, the weight loss in the temperature range of 490 °C-506 °C seems to be ascribed to the decomposition of the CTS33. Moreover, the decomposition temperature is notably higher than the boiling point (195 °C) of the CTS because the CTS forms strong chemical bonds with the organic phase.
Figure 9b displays the DSC thermograms of the non-modified and modified SS/PU composites. The DSC thermograms show that there is a complex structure in the SS/PU composites. It will not clearly discern the glass transition temperature (Tg) of the SS/PU composites. This phenomenon is attributed to that Tg is superimposed to thermal evaporation and degradation34. In the non-modified and modified SS/PU composites, the broad endothermic peak at 120–210 °C is ascribed to the endothermic melting peak of hard segment (Tm, hs). The addition of CTS has a notable effect on the Tm, hs value of the SS/PU composites, which can be observed from the incremental change in Tm, hs values. It mainly attributed to the increasing of hard segments contents and crosslinking density by the CTS as the active hard segment crosslinker chemical links with the organic phase. A strong endothermic peak at 190 °C superimposes to the Tm, hs, and the result confirms that the evaporation of the CTS because this temperature is close to the boiling point of free CTS. It also indicates that partially CTS is not bonded with the dispersed or continuous phases in the SS/PU composite, which is in good agreement with the TGA results.