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

Abstract Polyethylene glycol adipate oligomer glycol was used as the soft segment, isophorone diisocyanate and 1,4-butanediol as hard segments. A two-step fusion prepolymerization method was used to synthesize a kind of A thermoplastic polyurethane elastomer (TPU) that plasticizes nitrate and meets the requirements for propellant use. The synthesis reaction temperature, reaction time and R value were determined. The TPU was characterized by GPC, FTIR, mechanical properties test, nitroglycerin absorption experiment and other analytical testing techniques. The results show that within the selected hard segment content, TPU has a reasonable number average relative molecular weight and structural characteristics of linear polyester polyurethane, and a good compatibility with nitrate, when the hard segment content is 50% to 55. At %, the thermoplastic polyester polyurethane elastomer has mechanical properties satisfying the requirements for propellant use (σm>3 MPa, εm>500%).

Keywords Thermoplastic materials, solid rocket propellant, testing technology.

1 Introduction

Thermoplastic polyurethane elastomers (TPU) are a class of (AB) n-type linear multi-block polymers that are alternately composed of thermodynamically incompatible hard segments and soft segments. Because of their unique microphase separation structure, their physical and mechanical properties The performance is quite excellent [1]. The use of thermoplastic polyurethane elastomers as binders in solid propellants can solve the problems of traditional thermosetting propellants that are difficult to recycle, use in production, and prolonged service propellants, have low production efficiency, and have poor inter-assay reproducibility. High and low temperature mechanical properties [2]. At present, reports on the use of thermoplastic elastomers as adhesives have been reported abroad, but to a certain extent, there are higher processing temperatures (greater than 120 °C), and most of the thermoplastic elastomers used in foreign countries cannot contain energetic energy such as nitrates. Plasticizer plasticization, making the propellant energy level is low [3,4]. For this purpose, a novel ethylene oxide-tetrahydrofuran random copolyether with a good compatibility with nitric acid esters and a soft segment of polyethylene glycol were synthesized. Novel isophorone diisocyanate and small molecule glycols were used as hard segments. Thermoplastic polyurethane elastomers were used as binders to prepare thermoplastic elastomer propellants [5]. In order to further improve the compatibility of thermoplastic polyurethane elastomers with nitric acid esters, a polyethylene glycol adipate soft segment with good compatibility with nitric acid esters was selected to synthesize polyester thermoplastic polyurethane elastomers and obtained with nitric acid. New propellant TPU with better compatibility with esters.

2 experimental part

2.1 Raw materials
a) Polyethylene adipate (PEA). Tianjin Polyurethane Material Factory, with a number average molecular weight of 2 000 and an average functionality of 2, was used after vacuum drying at 90 °C for 2 h.
b) Isophorone diisocyanate (IPDI). Germany's Huls Company offers purity greater than 99.9%.
c) 1,4 butanediol. The molecular sieve was dehydrated and used after redistillation.
d) Nitroglycerine. The laboratory synthesis, Abel test qualified.

2.2 Experimental Methods

Gel Permeation Chromatography analysis was performed using a Waters 150-C gel permeation chromatograph to determine the relative molecular mass and its distribution. Tetrahydrofuran was used as a solvent, the injection volume was 1 ml/min, and the test temperature was 30 °C.
Infrared analysis, infrared analysis using Nicolet560 Fourier infrared spectrometer, using ATR total reflection technology, the incident angle of 55 °.

The mechanical properties were measured using an Instron-6022 universal material testing machine to test the tensile strength, elongation and modulus, the tensile rate was 100 mm/min, and the test temperature was 293 K.

Nitrate compatibility test, the TPU cut into small particles of about 0.5 g, immersed in 5 ml of nitroglycerin, after 48 h, test the hard segment content of TPU after nitroglycerin swelling weight, and calculate the weight gain percentage.

3 Synthesis of TPU

3.1 Synthetic process

The TPU was synthesized by a two-step melt prepolymerization method. The measured polyethylene glycol adipate was vacuum degassed at 90 °C for 30 min. The system was then vented and charged with nitrogen gas. Preheated IPDI was added under stirring. After 60-90 min of reaction, continue heating to 110 °C, add preheated BDO, react for 3 to 5 min under rapid stirring, and finally pour the product into a preheated mold at about 110 °C at about 110 °C in a nitrogen atmosphere. After about 15 hours of ripening, the product was released from the mold. The hard segment content of the TPU was controlled by adjusting the molar ratios of PEA, IPDI, and BDO.

3.2 Determination of synthesis conditions

3.2.1 Determination of temperature

The synthesis reaction temperature has a great influence on the reaction rate, side reactions and system viscosity. According to the Arrhenius equation, the increase in temperature favors an increase in the reaction rate, thereby shortening the reaction time, and also greatly reduces the reaction viscosity and increases the operability of the reaction. However, an excessively high temperature also increases the possibility of side reactions, thereby seriously affecting the performance of the synthesized TPU.

When the system is fully dehydrated, the following reactions will mainly occur during the synthesis of TPU:

a) The reaction of isocyanates with hydroxyl groups to carbamate;
b) Carbamate reacts with di-isocyanate to produce allophanate.
In addition, when the catalyst is present and the temperature is high, the isocyanates also produce dimerization, trimerization, and polymerization to produce urea anhydride, trimerized isocyanates, and linear polymers, but these reactions may occur. Less sexual.

In the above reaction, the reaction of the isocyanate with the hydroxyl group is the main reaction to produce the linear high molecular weight TPU. The reaction of carbamate and diisocyanate will produce cross-linking between the molecular chains, thus losing the thermoplastic properties of the synthesized product. The reactivity of carbamate and diisocyanate is relatively low, and it usually has sufficient reactivity under the action of high temperature (120-140°C) or selective catalyst. This provides people with an upper limit of the reaction temperature. Because of this, the reaction temperature in the industrial production of TPU is generally between 80 and 120 °C. Through experimental exploration, the temperature of each reaction step in the synthesis is shown in the synthesis process.

3.2.2 Determination of reaction time

a) Determination of the prepolymerization time.

The use of di-n-butylamine back titration method to determine the prepolymerization time of the polyester polyurethane, that is, to determine the isocyanate content in the prepolymerization system over time, as shown in FIG. 1, it is considered that when the isocyanate content change is small, the prepolymerization The reaction was basically completed.

As can be seen from Figure 1, for the PEA-IPDI-BDO series TPU, after the prepolymerization reaction is 1.0 to 1.5 h, the percentage change of the isocyanate content in the system is small, and it can be considered that the prepolymerization reaction has basically been completed. The polymerization time was 1.5 h.

b) Determination of post-cooking time.

Curing is the process of making the thermoplastic polyurethane elastomer molecular chain continue to grow. It has a great influence on whether the system is fully reacted or not and the performance of the polymer is good or bad. The short curing time is not conducive to the complete reaction of the system; too long curing time not only prolongs the production cycle and increases the cost, but also may lead to negative results such as degradation of the polymer chain. Therefore, it is very meaningful to optimize the synthesis process of TPU by choosing the appropriate post-cooking time. The degree of reaction of the polymer is closely related to the molecular weight. Therefore, based on the study of the molecular weight, it can be determined that the optimal aging time is about 15 h.

3.2.3 Determination of R value

For the linear polycondensation reaction, when R=[NCO]/[OH]=1.0, the molecular weight of the polymer can reach infinity. However, in the actual reaction, since the raw materials are impure and there are various side reactions, the isocyanate groups are often subjected to a relative excess in the synthesis. According to the literature [6], when preparing TPU, the ratio of [NCO]/[OH] is generally 0.97~1.03. In the literature [7], it is considered that R = 1.0 to 1.05 is more appropriate. Since the melt viscosity of the polymer increases with the increase of the molecular weight, when the molecular weight increases to a certain extent, the fluidity of the molten state is very poor, which makes it difficult to form the polymer. Especially for thermoplastic elastomers used as propellant binders, due to the compression molding process, the processing temperature should not exceed 120°C considering the process safety, and it is therefore more necessary to limit the molecular weight of the polymer. Considering that the molecular weight of the polyurethane elastomer is generally not high, the R value at the maximum molecular weight is used.

Since the molecular weight is most affected by the R value, the R value can be directly evaluated by measuring the TPU molecular weight. In this study, gel permeation chromatography (GPC) was used to determine the optimal R value. When the R value is 1.05, the molecular weight of the thermoplastic polyurethane is the largest, so it is determined that R=1.05.