Synthesis and Characterization of Novel Thermoplastic Polyurethane Elastomer Adhesives (II)

4 TPU structural performance characterization

4.1 Characterization of Relative Molecular Mass

The relative molecular weight and distribution of TPU will have an important influence on the mechanical properties and processing performance. In this paper, some of the synthesized TPUs were characterized by GPC method. The obtained spectra are shown in Figure 2.
From the data in Table 1, it can be seen that the number average molecular weight and dispersion index of the synthesized TPU are within the range of data reported in the literature (30 000 to 50 000). In addition, the peak shape of the obtained GPC spectra is basically symmetrical, and there are no impurity peaks, indicating that the reaction of the system is relatively complete.
Figure 2 GPC Curve of TPU

Table 1 TPU GPC data

Sample Mn Mw Dp
PEA60 49 087 99 647 2.03
PEA55 46 868 113 234 2.42
PEA50 41 491 105 387 2.54
PEA45 37 125 81 065 2.18
PEA40 35 470 116 443 3.23

4.2 Infrared (FTIR) Characterization of TPU Structures

FTIR is an important means to characterize the molecular structure qualitatively. The synthesized TPU is qualitatively characterized by using Fourier ATR reflection technology. The obtained spectrum is shown in Figure 3.

The attribution of the spectrum is shown in Table 2. The attribution and qualitative analysis of the spectrum show that the polymer has the characteristics of polyester polyurethane.

Table 2 FTIR spectrum peak assignment of TPU

Peak position/cm-1 attribution
3 200～3 500 ν(CH) and its hydrogen bond peaks
2 800 to 3 000 v (CH2)
1 735 v (C=0) amide band
1 540 δ(NH)+v(CN) amide band
1 420 δ(NH)+v(CN) amide band
1 050 to 1 250 v (COC)

4.3 Mechanical Properties of TPU

Mechanical properties are an important factor in determining whether a material can be used practically. As with other materials, the mechanical properties of TPU are closely related to its molecular structure and microscopic phase separation structure. The TPU with different hard segment content, different hard segments and different chain extenders synthesized was tested using a tensile tester.
The mechanical properties of different hard segment content TPU are shown in Table 3. From Table 3, it can be seen that as the hard segment content increases, the tensile strength of TPU increases significantly, and the elongation at break decreases. It can also be seen from Table 3 that when the hard segment content is about 50%, the polyester polyurethane has the best comprehensive mechanical properties. This is because in the thermoplastic elastomer, the hard segment not only functions as a physical cross-link, but also has a reinforcing function similar to carbon black due to the existence of the hard segment island region. It is precisely because of this that the hard segment content is increased to increase the tensile strength of the elastomer. From Table 3, it can also be seen that when the hard segment content is 50% to 55%, the TPU exhibits the optimum tensile strength and elongation to meet the requirements of the use of the propellant.

4.4 TPU Nitrates Absorption Capacity

The compatibility of TPU and nitrate esters will ultimately determine whether TPU can be used practically in propellants. For this purpose, the absorption capacity of nitrate on TPU was further examined. The results are shown in Table 4.

As can be seen from Table 4, when the hard segment content is less than 50%, the polyester thermoplastic polyurethane elastomer can be completely dissolved in the nitrate ester. When the hard segment content is further increased, the solubility of the polyurethane elastomer in the nitrate ester deteriorates. This is because as the hard segment content increases, the aggregation of the hard segment microdomains increases, and the hydrogen bonding force of the hard segment microdomains further increases, thereby making it difficult for the nitrate ester to destroy the pseudo cross-linking effect of the hard segment microdomains. On the other hand, as the hard segment content increases, this means that the soft segment content decreases, while the portion of the thermoplastic polyurethane elastomer that determines the compatibility with the nitrate ester is a soft segment portion. Therefore, due to the combined effects of these two aspects, the compatibility of the high hard segment polyurethane elastomer with nitroglycerin deteriorates. Despite this, even if the hard segment content is as high as 60%, the polyester polyurethane shows good compatibility with the nitrate ester, and its percentage of weight increase is as high as 585%. Polyester polyurethane elastomers show much better compatibility with nitrates than polyether polyurethanes [5]. This is because the polyester polyurethane contains a large amount of polar ester groups, and the solubility parameter of the polyester polyurethane elastomer is more similar to the solubility parameter of the nitrate ester, so that it has better compatibility with the nitrate ester.

Table 3 Mechanical properties of TPU Sample M σp/MPa εp 50% 100% 200% 300%
PEA40 3.30 4.44 1 004% 1.08 1.33 1.53 1.70
PEA45 4.98 5.26 968% 0.99 1.38 1.66 1.87
PEA50 12.9 10.17 697% 2.36 2.77 3.43 4.22
PEA55 39.72 20.64 653% 5.39 5.71 6.91 8.70
PEA60 50.36 28.73 458% 8.41 9.67 11.02 13.69

Table 4 Absorption capacity of NG of polyester TPU with different hard segment content [Height of test sample/g [Samples end weight/g[Growth percentage]

40% 0.511 8 Dissolution -
45% 0.509 6 Dissolution -
50% 0.511 4 Dissolution -
55% 0.508 2 Partially Dissolved -
60% 0.510 5 2.986 8 585%

5 Conclusion

a) Polyethylene glycol adipate oligomer diol is used as a soft segment, isophorone diisocyanate and 1,4-butanediol as hard segments. A two-step fusion prepolymerization method was used to synthesize a A thermoplastic polyurethane elastomer (TPU) that plasticizes nitrates and meets the requirements for propellant use.
b) It was determined that the prepolymerization temperature was 90 °C, the chain extension reaction temperature was 110 °C, the prepolymerization time was 1.5 h, the post curing time was 15 h, and the R value was selected as 1.05.
c) The results of GPC and FTIR analysis show that TPU has a reasonable number average relative molecular weight and linear polyester polyurethane structural characteristics within the selected hard segment content range.
d) Mechanical properties tests and nitroglycerine absorption test results show that when the hard segment content is 50% to 55%, the thermoplastic polyester polyurethane elastomer has the optimal mechanical properties to meet the requirements of propellant use (σm)>3 MPa, εm>500%), and has good compatibility with nitrates.

Chen Futai Duo Yingquan Luo Yunjun Tan Huimin

(School of Chemical Engineering and Materials, Beijing Institute of Technology

Source: 21st Century Fine Chemicals Network