# Experimental Study on Flexural Properties of Polyurethane-Cement Composites under Temperature Load
## 4 Preparation Process of Polyurethane Cement Composites
### Preparation Process  
The process of preparing polyurethane can be divided into the one-step method and the two-step method according to the process. The one-step method is to mix and shape raw materials such as oligomer polyol, polyisocyanate, chain extender, and catalyst at one time, which is most suitable for producing and processing low-modulus products [46]. The viscosity of MDI-type polyurethanes is large and is usually improved by a two-step method, also known as the semi-prepolymer synthesis method. The effect is improved by retaining some of the oligomeric polyols in the synthesis of prepolymer and synthesizing the semi-prepolymer with lower viscosity first, and then adding a small amount of oligomeric polyol and chain extender in the chain expansion reaction [47]. The method used in this study to prepare polyurethane cement composites is a three-stage method based on the two-step method (semi-prepolymer synthesis method): first, the oligomer is reacted with an excess of isocyanate to form a prepolymer, then the prepolymer is reacted with diol and 1.4-butanediol to prepare high molecular weight polyurethane or polyurea, and finally mixed with cement and stirred and mixed into the mold, as The process is as follows, and the preparation process is shown in Figure 12.  
1. Dry and dehydrate the cement: Put the cement into the frying furnace, ignite and heat it under the frying furnace, and keep the frying furnace rotating at a constant speed, as shown in Figure 12a. The stirring time is about 90 min\(\sim\)120 min. The purpose of frying dry is to eliminate as much free water and bound water from the cement as possible to prevent air bubbles when reacting with the polyurethane glue. 2. Preparation of polyurethane semi-prepolymer: Isocyanate (PM-200) and polyether polyol (ES305) were mixed in the designed ratio to form a viscous flow-like material followed by immediate airtight drying and placing, ready for use. 3. Polyurethane polymer mixing: Add dry chain extender 1,4-butanediol and a small amount of polyether polyol to the step (2) system and add 2% of the total mass of KOSMOS-29 catalyst drop by drop during the mixing process and stir to ensure uniform mixing of the rubber solution. The use of mixers can reduce the mixing time and the entire mixing process by about 2 min\(\sim\)5 min. Whether the two liquids are evenly mixed can be determined by observing whether the color of the glue is the same after mixing isocyanate and polyol. Since the polymerization reaction of isocyanate and polyol is fast, it is necessary to ensure that the interval between steps (2) and (3) is not too long and should be controlled at about 5 min. 4. Preparation of polyurethane cement colloid: Mix the frying-dried and dry cement in step (1) and the uniformly mixed polyurethane glue in step (3), as shown in Figure 12b, and stir at high speed for 3 min to make the polyurethane glue and cement are evenly mixed to form polyurethane cement colloid. 5. Pouring and regeneration molding: Pour the polyurethane cement colloid into the preprepared mold and smooth the residual colloid on the surface of the test mold, the polyurethane cement colloid can be solidified in about 60 min at room temperature. The specimen can be taken out of the mold after 24 h of regeneration at room temperature. The polyurethane cement composite material has good moldability. Var \begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Raw Materials** & **Mix Ratio** \\ \hline
1 & PM-200 isocyanate & 390 \\
2 & ES305 polyether polyol & 390 \\
3 & KOSMOS-29 catalyst & 7.5 \\
4 & HQEE chain extender & 20 \\
5 & Silicate cement & 800 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The mix ratio of each raw material for this test(kg/m\({}^{3}\)). ious kinds of polyurethane cement specimens with different shapes can be prepared according to the specimen molds to meet the tests of different mechanical indexes. The polyurethane cement sample prepared by drying the silicate cement has a smooth and flat surface and no bubbles or holes inside.