Crystallization Behavior of Virgin TR-55 Silicone Rubber Measured Using Dynamic Mechanical Thermal Analysis with Liquid Nitrogen Cooling

Abstract

SUMMARYDynamic mechanical thermal analysis (DMTA) of virgin TR-55 silicone rubber specimens was conducted. Two dynamic temperature sweep tests, 25 to -100°C and 25 to -70 to 0°C (ramp rate = 1°C/min), were conducted at a frequency of 6.28 rad/s (1 Hz) using a torsion rectangular test geometry. A strain of 0.1% was used, which was near the upper limit of the linear viscoelastic region of the material based on an initial dynamic strain sweep test. Storage (G') and loss (G") moduli, the ratio G"/G' (tan δ), and the coefficient of linear thermal expansion (α) were determined as a function of temperature. Crystallization occurred between -40 and -60°C, with G' increasing from ~6×10 6 to ~4×10 8 Pa. The value of α was fairly constant before (~4×10 -4 mm/mm-°C) and after (~3×10 -4 mm/mm-°C) the transition, and peaked during the transition (~3×10 -3 mm/mm-°C). Melting occurred around -30°C upon heating. MATERIALS AND METHODSThe Dow Corning TR-55 silicone rubber material was made at Kansas City Plant. Specimens were cut by hand from a sheet using a scalpel. Specimens were approximately 45 mm long × 12 mm wide × 2 mm thick. Dynamic mechanical thermal analysis (DMTA) was conducted using the TA Instruments ARES rheometer with liquid nitrogen cooling in B132S R2729; the instrument was controlled by TA Orchestrator software. A torsion rectangle test geometry was used. The auto-tension feature was used to maintain a tensile force of approximately 10 g during the test by automatically adjusting the gap distance to compensate for thermal expansion/contraction of the specimen. Dynamic temperature ramps were recorded for the following two temperature profiles: (1) cool from 25 to -100°C at a rate of 1°C/min and (2) cool from 25 to -70°C at a rate of 1°C/min, hold at -70°C for 2 min, then heat to 0°C at a rate of 1°C/min. One specimen was tested for each of these sweep tests (specimens were not retested). During each temperature sweep the specimen was subjected to a sinusoidally oscillating strain of 0.1% at a frequency of 6.28 rad/s (1 Hz). The 0.1% strain was near the upper limit of the linear viscoelastic region of the material based on an initial isothermal dynamic strain sweep test. Shear storage and loss moduli, G' and G", respectively, and tan δ = G"/G' were recorded as a function of temperature.Taking advantage of the auto-tension feature of the ARES instrument, the change in specimen length, ΔL (i.e., gap distance), was also recorded as a function of temperature. The instantaneous coefficient of linear thermal expansion, α, at a temperature, T, was estimated from the slope of the ΔL versus T curve:where L T is the length of the specimen at the temperature, T, and the derivative of L with respect to T is equivalent to the derivative (slope) of ΔL with respect to T since L = ΔL + L 0 , where L 0 is the original length of the specimen (i.e., gap distance at the start of the test). This calculation is based on the assumption that the change in the gap distance with temperature represents the change in length of an unconstraine...

Dynamic mechanical thermal analysis and dielectric thermal analysis of siloxane rubber-based composites filled with carbon black

“…The decrease in the E 0 values in the temperature interval from À40 C to À30 C as well as the increase in tan d concerning the tested composites is due to the melting of the crystal structure of the siloxane rubber that formed during the cooling of the samples. 9 This was proven by the data of the storage modulus (E 0 ) that were measured during the cooling of the samples as well as during cooling followed by heating. (Figure 3).…”

“…In the first case (indicated on Figure 3 as ''cooling'') the measurements are carried out during the cooling of the sample. In the second case (indicated as ''cooling followed by heating'') the sample is firstly cooled till À100 C and then the measurements are carried out during the heating of the sample till 0 C. Figure 3 shows that the storage modulus (E 0 ) values increase at about À70 C when the measurement is carried out during the cooling of the sample which is an indication of a crystallization phase transition 9 in the tested siloxane rubber-based composites. When the measurement is carried out during the heating of the sample the storage modulus (E 0 ) values decrease at about À40 C, which is due to the melting of the already formed crystal structure.…”