Investigating the Interphase in Hydroxyl-Terminated Polybutadiene (HTPB) Composites via Dynamic Mechanical Analysis 31 To tune the surface chemistry of the glass beads, SigmacoteR⃝ was applied to the particles. SigmacoteR⃝ is a siliconizing reagent for glass and is intended to function in this system as a shielding agent to reduce particle-binder interaction effects and prevent particle-binder covalent bonding. Glass beads were purchased from Potter’s Industries and SigmacoteR⃝ was purchased from Sigma-Aldrich. Mixes were generated with and without coated glass beads in this work. For the remainder of this study, mixes that utilize glass beads with SigmacoteR⃝ are denoted with the suffix “-SC”. Prior to mixing, glass beads were sieved to reduce the span of the particle size distribution. The grades of spheres selected in this work and the U.S. mesh sieves that the beads were collected between are shown in table 2. Table 2 also includes particle size data on the post processed glass beads. The particle size data confirms that the application of SigmacoteR⃝ did not significantly change the particle size distributions of the glass beads. Table 2 Particle size data and surface area estimates for coated and uncoated glass beads Filler U.S. Mesh Dv10 (µm) Dv50 (µm) Dv90 (µm) Average Particle Size (µm) Surface Area per mL of filler (cm2/mL) P4000 325,500 7.14 21.26 39.70 14.35 4181 P4000-SC 325,500 8.78 23.42 41.15 16.75 3582 P0040 120,170 59.66 69.25 81.08 66.5 902.2 P0040-SC 120,170 55.97 68.64 86.4 67.64 530.0 P0080 70,80 151.4 178.0 207.5 176.4 340.1 P0080-SC 70,80 149.1 174.6 205.6 173.3 346.2 P0170 40,45 340.1 378.5 412.5 375.3 159.8 P0170-SC 40,45 331.2 375.1 411.0 370.8 161.8 P0280 25,30 529.9 608.7 680.5 602.0 99.67 P0280-SC 25,30 523.5 606.1 695.8 602.5 99.59 Formulation components were weighed out and hand-mixed for two minutes at ambient temperature. After hand-mixing, the mixture was degassed in a vacuum chamber at ambient temperature for 10 minutes to remove air in the system. Larger sphere sizes showed minimal settling at the end of the degassing step and were gently mixed to resuspend particles to mitigate settling and reintroduction of air. Mixes were then poured into open face 3.2 x 12.5 x 60 mm molds and cured at 60 ◦C for 48 hours. Qualitatively, no settling was observed by eye in the cured samples. Figure 1 shows a picture of cured samples. A color difference is observed between the coated and uncoated beads. At this time, it is unknown what caused the color change. Some potential explanations could be as follows: side reactions/molecular complexing between the Ethyl 702 or DBTDL and SigmacoteR⃝, or refractive index differences between glass beads with and without covalent bonding to the binder. Currently, we have dismissed residual HCl from the coating reaction as the root cause, as coated beads were washed with deionized water and the rinse had a neutral pH. The underlying cause of the color may have an inconsequential effect on the results, since color changes can manifest at ppm levels, but it cannot be discounted when interpreting the results. Methods Dynamic mechanical analysis DMA data was collected with a TA Instruments Discovery Hybrid Rheometer (DHR20) equipped with a rectangular torsional geometry. Samples were tested with a frequency sweep (0.01, 0.1, 1, and 10 Hz) from -110 to 100◦Cwith1◦C temperature steps and a 0.1% oscillatory strain. Samples were verified to be within the linear viscoelastic regime (LVR) prior to selecting the 0.1% oscillatory strain. A tensile axial force of 0.15±0.1 N was maintained throughout the procedure to avoid skewing results from thermal expansion/contraction. Specimens were equilibrated for 45 seconds prior to data collection at each temperature step. Collected DMA data is parsed into tanδ vs temperature curves at each respective test frequency (0.01, 0.1, 1, and 10 Hz) for analysis. DMA data processing Since DMA traces for filled HTPB composites historically exhibit two overlapping tan δ peaks, such as the example shown in figure 2, it is necessary to decompose the traces into distinct peaks to properly quantify the peak heights, widths, and areas.
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