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By employing the interpenetrating polymer network technique and using two or more different types of polymer materials, a new material with excellent properties can be developed; this material combines the advantages of various materials while overcoming their respective limitations. Interpenetrating polymer network technology is a new technique that has developed rapidly in the field of blending and modification of polymer materials both domestically and internationally over the past few decades. It involves two or more polymers interpenetrating each other during the polymerization process to form a networked interlocking structure, thereby endowing the polymer with properties superior to those of any individual component. The acrylic epoxy resin and polyurethane interpenetrating polymer network grouting material is based on acrylic epoxy resin; taking advantage of the fact that polyurethane prepolymers contain –NCO groups that can react with water, these groups are incorporated into the grout. This material combines the various excellent properties of unsaturated polyester, epoxy resin, polyurethane, and methacrylate-based materials; it possesses good strength as well as moderate toughness, and can cure at low temperatures and in water. It is a versatile interpenetrating polymer network grouting material worth further development. 1.3.1 Synthesis of polyurethane prepolymers Dehydrated castor oil and dibutyl phthalate were poured into a three-necked flask in the prescribed mass ratio, and mixed thoroughly. TDI was then added gradually while stirring; the temperature was allowed to rise naturally, and cooling might be necessary as well. Once the temperature of the reactants began to drop, heating was resumed slowly using an oil bath, with continuous stirring, to maintain the reaction temperature at 70–80°C for 1.5 hours. Heating was then stopped, and prepolymers with different NCO/OH ratios could be obtained depending on the preparation conditions. 1.3.2 Synthesis of acrylate epoxy resin In a 500-milliliter three-necked flask equipped with a thermometer, a reflux condenser, a dropper funnel, and a stirrer, an appropriate amount of epoxy resin was added according to the specified mass ratio. The mixture was heated to 80–90°C, and then the acrylate mixture containing a catalyst was slowly added drop by drop. The reaction temperature was maintained at 110–120°C; the addition process took about 1 hour, after which the reaction continued for another 1 hour. Once the reactants have cooled to 50°C, the epoxy resin solution of acrylic acid is obtained. 1.3.3 Synthesis of epoxy-acrylic resin and polyurethane interpenetrating polymer network grouting material: Different amounts of polyurethane prepolymer are added to an epoxy-acrylic resin solution containing a certain amount of diluent, according to the designed mass ratio; after mixing evenly, an initiator and a accelerator are added, thereby obtaining the epoxy-polyurethane interpenetrating polymer network grouting material. 1.3.4 Determination of the properties of grouting materials: This is carried out in accordance with the relevant provisions in the Compilation of Standards for the Chemical Industry (Adhesives) and the Code for Testing Hydraulic Concrete issued by the Ministry of Water Resources and Electric Power of the People’s Republic of China (DL/T 5150-2001). The grouting material specimens are formed in water, and the curing method is curing in water. (1) Viscosity: Tested in accordance with the GB/T2794-1995 standard. (2) Adhesive strength: The bonding surface of the old mortar is brushed with a wire brush, and after brushing it is shaped in a small ‘8’ pattern; then, a tensile test is conducted after curing for a certain period of time in accordance with the DL/T 5150-2001 standard. (3) Compressive strength: The specimen size is 40mm×40mm×40mm, and testing is carried out in accordance with the DL/T 5150-2001 standard. (4) Fracture strength and elongation: The specimen size is 2.5 cm × 0.5 cm, and testing is carried out in accordance with the GB1040-79 standard. (5) Flexural strength: The specimen dimensions are 40mm×40mm×160mm, and testing is carried out in accordance with the DL/T 5150-2001 standard. (6) Tensile strength: The test specimen is in an “8” shape with a cross-sectional dimension of 2cm×2cm; testing is carried out in accordance with the DL/T 5150-2001 standard. 2 Results and Discussion 2.1 Synthesis Principle and Preparation 2.1.1 Synthesis Principle and Preparation of Polyurethane Prepolymers The —NCO groups in isocyanates are highly reactive; their oxygen and nitrogen atoms are electronegative, while the electron cloud density on the carbon atoms is lowest, giving them a positive charge. —The oxygen atom of the NCO group has the highest electronegativity and acts as a nucleophilic center; it can attract hydrogen atoms from molecules containing active hydrogen to form hydroxyl groups. However, the hydroxyl groups on unsaturated carbon atoms are unstable and rearrange to form carbamates. Its reaction equation is as follows: n OCN—R—NCO + n HO—R1—OH → ~(CONH—R—NHCO—OR1—O—)n. If there is an excess of –NCO groups in the reactants, that is, if the molar ratio of isocyanate groups to hydroxyl groups is greater than 1, then a polyurethane prepolymer with –NCO groups at the ends is obtained ； If the hydroxyl groups and isocyanate groups in the reactants are present in equal amounts, a polymer with an infinite molecular weight should theoretically be formed; however, due to the influence of impurities, the molecular weight of polyurethanes is generally in the range of tens of thousands to over a hundred thousand ； If there is an excess of hydroxyl groups, polyurethanes with terminal hydroxyl groups are obtained. This experiment aims to prepare polyurethane prepolymers with an —NCO end group, and the main factor determining the properties of these prepolymers is the isocyanate index NCO/OH; if this ratio is too low, the resulting prepolymers have high viscosity and low reactivity ； An excessively high ratio results in too high activity of the prepolymer, as well as excessive foaming upon reaction with water, which reduces performance and is not economical; the optimal NCO/OH ratio can be determined through experiments. 2.1.2 Principles of synthesis and preparation of polyurethane prepolymers: 1 mole of epoxy resin requires 2 moles of acrylic acid; to ensure complete reaction of the acrylic acid, an excess of epoxy resin is needed. In this experiment, a molar ratio of 1:1.2 between epoxy resin and acrylic acid was used. During synthesis, when the reaction temperature is above 130°C, side reactions occur rapidly, leading to gelation ； When the temperature is below 70°C, the forward reaction hardly occurs. Taking all the above factors into account, the reaction temperature should be controlled at 110–120°C. 2.2 Properties of Grouting Materials and Their Influencing Factors 2.2.1 Determination of the Optimal NCO/OH Ratio for Polyurethane Prepolymers During underwater grouting, for grouting materials based on acrylic epoxy resin and polyurethane interpenetrating polymer networks, if the NCO/OH ratio of the PU prepolymer is too high, resulting in excessive reactivity and too much free TDI, it is likely to react with the OH groups in the EA molecules that possess bonding activity. This can cause the resin to gel or reduce its bonding capacity; moreover, it leads to excessive foaming upon contact with water, creating pores at the bonding surface, all of which reduce the bonding strength ； If the ratio is too low, the activity of the prepolymer is too low to effectively break the water film on the surface to be bonded, which also results in a decrease in bonding strength. Therefore, the NCO/OH value has a direct impact on the bonding strength of the bond surface after grouting with pulp. The highest bonding strength in the presence of water is achieved when the NCO/OH ratio is 2.5. 2.2.2 Determination of the optimal amount of polyurethane prepolymer in grouting materials: A polyurethane prepolymer with an NCO/OH ratio of 2.5 was selected. It can be seen that as the amount of polyurethane prepolymer used increases, the initial viscosity of the slurry gradually rises. The wet adhesion strength and compressive strength reach their optimal values at an increased amount of polyurethane prepolymer; however, as the amount used continues to increase, these strengths show a downward trend. 2.2.3 Effect of diluent amount on the physical and mechanical properties of grouting materials. Acrylic epoxy resin can be cross-linked and copolymerized with various olefin monomers; by adding such monomers, the viscosity of the slurry can be adjusted. By examining the effect of different amounts of two types of active diluents used in the experiments on the physical and mechanical properties of the cured material, it was found that as the amount of diluent increases, the viscosity of the slurry decreases, while the compressive strength, adhesion strength, and fracture strength all decline. The main reason for the decrease in mechanical properties is that as the amount of diluent increases, the degree of self-polymerization of the diluent itself rises; nevertheless, the overall mechanical strength remains high. 2.2.4 Influence of initiator and accelerator amounts and temperature on the gelation time of the slurry: Acrylic epoxy resin containing a diluent can be polymerized at room temperature using an oxidation-reduction initiation system. The main factors affecting the gelation time include the type and amount of initiators and accelerators, as well as temperature. The impact of the amounts of initiators and accelerators and temperature on the gelation time of the slurry. The curing process of acrylic epoxy resin mixed with a diluent, initiated by free radicals, is a chain reaction. There is a distinct induction period prior to the setting time; during this period, the viscosity of the slurry remains essentially unchanged at its initial value, ensuring good workability. As can be seen from the graph, by adjusting the amounts of initiator and accelerator at different temperatures, it is possible to control the curing time to meet the requirements of grouting in various situations. 2.3 Formulation design of grouting materials and simulated grouting tests 2.3.1 Formulation design of grouting materials and performance testing As a grouting material, it must possess good workability, meaning that its initial viscosity should be low, the increase in viscosity over time should be slow, and the setting time should be adjustable; in addition, it needs to have good mechanical properties. The reaction between acrylic epoxy resin and polyurethane interpenetrating polymer networks primarily involves free-radical polymerization; during the polymerization induction period, the viscosity of the slurry remains essentially unchanged from its initial value. Therefore, when determining the formula for the grouting material, it is necessary to take into account both the initial viscosity of the slurry and the mechanical properties of the cured material. Based on the experimental results mentioned above, two optimized formulas were developed. 2.3.1 Simulating concrete crack grouting under water conditions To further examine the grouting capability of acrylic-epoxy resin and polyurethane interpenetrating polymer network grouting materials, we split 15cm×15cm×15cm concrete blocks and then reassembled them; a grouting nozzle and an exhaust nozzle were attached at each diagonal corner, and the joints were sealed with epoxy glue. The sealed concrete blocks were filled with water and placed in water for grouting. Once the grout pushed the water out, the flow of grout was stopped. The grouted specimens remained immersed in water until they were removed prior to testing. Experimental measurements showed that the bond-splitting tensile strengths of the concrete crack grouting under simulated wet conditions for the two formulations were 1.21 MPa and 1.10 MPa, respectively. The experimental results indicate that the interpenetrating polymer network grouting materials studied possess good mechanical properties. (1) When the isocyanate index NCO/OH of the resulting polyurethane prepolymer was 2.5, the water-containing adhesive strength of the interpenetrating polymer network grouting material was the highest, reaching 1.4 MPa. (2) For the prepared interpenetrating polymer network grouting material, as the amount of polyurethane prepolymer used increases, the initial viscosity of the slurry gradually rises. When the amount of polyurethane prepolymer is 25%, the water-bonding strength and compressive strength reach their optimal values; further increases in this amount result in a decline in these properties. (3) The interpenetrating polymer network grouting material under study possesses good physical and mechanical properties, and is capable of meeting the requirements for grouting concrete cracks. Last edited by QXZ-1966 on 2009-3-7 at 18:45.]