Preparation and characterization of polyurethane damping materials derived from mixed-base prepolymers containing numerous side methyls

2.1 Materials

HPHP, an industrial product, was obtained from Zibo Chemical of China. Polypropylene oxide glycol (PPG-2000) was supplied by Shandong Dongda Chemical of China. Succinic acid (SA) was analytically pure and supplied by Tianjin Bodi Chemical of China. 2,4-Toluene diisocyanate (TDI-100) was supplied by Bayer. 4,4′-Methylene-bis-(2-chloroaniline) (MOCA) was purchased from Suzhou Xiang Yuan Special Fine Chemical of China. Tetraisopropyl titanate (TPT) was analytically pure and supplied by Alfa Aesar (Tianjin) Chemical of China. A release agent (YB-05H) was obtained from Shanghai Billion State Chemical. Poly(HPHP succinate) (PHPS; average molecular weight of 2300) was prepared in our laboratory.

2.2 Preparation of PU damping materials

2.2.1 Preparation of PHPS

SA (823 g) was placed into a four-necked round-bottomed flask equipped with a stirrer, a thermometer, and a fractionating column, and then HPHP (1661 g) and TPT (0.285 g) as a catalyst were added. The reaction mixture was heated to 210°C–220°C and maintained at this temperature. Acid number and hydroxyl number were measured every 2 h, and then the reaction mixture was vacuumed until the acid number dropped to 0.7 mg KOH/g. Finally, PHPS with an average molecular weight of 2300 the molecular weight is achieved by calculate was obtained. The PHPS preparation is schematically presented in Figure 1.

Figure 1:

Preparation of PHPS.

2.3 Measurement and characterization

Dynamic mechanical thermal analysis (DMTA) was carried out on a DMA Q800 analyzer with film tensile mode in a temperature range from -50°C to 60°C at a frequency of 1 Hz and a heating rate of 1°C/min. The tensile strength was measured on a CMT7104 universal testing machine (Shenzhen SANS Test Machine, Shenzhen, P. R. China) with a crosshead rate of 200 mm/min according to ISO 37:2005. The tear strength was tested on the same machine as tensile test according to ISO 34-1:2004. Five replicates of each sample were taken. The hardness of the samples was measured using a hardness tester. The hydroxyl value was determined by acetic anhydride and p-toluene sulfonic acid acylation method. The acid value was measured through toluene and ethanol solution. The Fourier transform infrared (FT-IR) spectra were recorded by a 5DX-B IR spectrometer with the attenuated total reflection accessory. Scanning tunneling microscopy (STM) imaging was performed on a Nanoscope IV with tapping mode, sweep range of 10×10 μm, height of 60 nm, and frequency of 1 Hz.

3.1 FT-IR analysis

Figure 2 shows the FT-IR spectra of PU-1, PU-3, and PU-5. The peaks among 2970 and 2869 cm^-1 were assigned to CH symmetric and asymmetric stretching of -CH₃ and -CH₂ (Figure 2). The peak at 1091 cm^-1 belonged to the stretching peak of the C-O-C ether linkage in PU-1 and PU-3. The peak at 2270 cm^-1 corresponding to the NCO group disappeared, indicating that the NCO had completely reacted. The characteristic peak at 3350 and 1728 cm^-1 was assigned to the stretching of the secondary amino -NH- group and the C=O group, respectively, and this indicates that there were a large number of the -NHCO- group in the polymer molecule chains. The results showed that PUs were successfully synthesized.

Figure 2:

FT-IR spectra of PUs.

3.2 Dynamic mechanical thermal analysis

In this study, the temperature range (ΔT) for effective loss factor (tanδ>0.4) was taken to quantitatively evaluate the damping properties of the PU materials. The temperature dependence of the storage modulus (E′) and tanδ for PUs are shown in Figures 3 and 4, respectively. The effects of the m_PESP/m_PETP ratio on the dynamic mechanical properties of PUs were investigated.

Figure 3:

DMTA curves of the storage modulus E′ vs. temperature for PUs.

Figure 4:

DMTA curves of tanδ vs. temperature for PUs.

Figure 3 shows that E′ decreased as temperature increased. In the -40°C to 20°C temperature range, the E′ of PU-5 was the maximum. This is because the structure of the PESP was more rigid than that of the PETP. The E′ of PU-1 was minimum, and those of other PUs were between PU-1 and PU-5. As the temperature has reached beyond 20°C, the E′ values of PUs tended to the same value.

As shown in Figure 4, the damping performances of PUs from five m_PESP/m_PETP ratios were significantly different. Among the five curves, PU-1 and PU-5 indicated single glass transition temperatures (T_g) of -20°C and 22°C, respectively. Each of the other samples displayed two kinds of relaxations, and the low temperature peaks are related to the T_g of the PETP segment phase CIIR; high temperature peaks correspond to the T_g of the PESP segment phase. This is because the micro-phase separation for PUs varied with the m_PESP/m_PETP ratio. When the m_PESP/m_PETP ratio has reached an appropriate ratio range, the PETP and PESP segments would exhibit a certain degree of micro-phase separation in PUs. As seen in Figure 4, bimodal appeared in PU-2 and PU-3, tanδ was up to 0.65, and the ΔT of tanδ>0.4 was beyond 60°C. The improvement of the ΔT was attributed to a strong interaction between a flexible ether bond in the propoxylated group and rigid neopentyl groups and ester groups of PUs. Meanwhile, the mechanical vibration energy that converted into thermal energy was higher due to the friction within the PU matrix with the side methyl group. As the content of the PESP exceeded 60%, PU-4 exhibits efficient damping property in a narrow temperature range and could only be used at the temperature >0°C. Those results indicated that the combination of PESP and PETP with the side methyl group was effective for increasing ΔT to improve operating temperature and damping capacity.

3.3 Mechanical property analysis

The tensile and tear strength of PUs from different m_PESP/m_PETP ratios were shown in Table 2. The tensile strength and tear strength of the PUs increased slightly as the PESP content increased (Table 2). However, the elongation at break was in contrast. As the content of PESP increased, the relatively rigid segments gradually increased. Therefore, the tensile strength further increased. At the same time, the flexibility of the polymer chain deteriorated, and the tensile deformation was essentially a process of “consume” flexibility of polymer chain. Thus, increasing PESP content led to a decline in the elongation at break compared to net PESP.

3.4 STM analysis

In recent years, STM have been successfully used in microscopic surface morphology and performance analysis, inspection measurement, etc. (10). To understand the morphology of PUs, STM measurements were conducted to investigate the surface morphology of PU-2 and PU-5 films. Two-phase structures were observed in PU-2 and PU-5 from Figure 5. The darker part corresponding to the hard segment in diagram was the low-lying portion of the surface, and the lighter part corresponding to the soft segments in diagram was the convex portion of the surface. In PU-2, polyester segments may be considered as harder segments relative to polyether segments, indicating that PU-2 had more efficient phase separation and more complicated combination of hard and soft segments compared to PU-5. It can be concluded from Figure 5D that the bump height was not consistent but scattered high and low, which was due to the irregular “block” of PESP and PETP.

Figure 5:

STM images of PU-5 and PU-2: (A, C) height and (B, D) three dimensions.