As the artery project of urban underground facilities, once leakage occurs, the manpower, material and financial resources required for later repair are large, and there is often no repair construction environment, so waterproofing is the first priority in the construction of underground projects. Only by ensuring that the project does not leak, can the normal use and structural safety of the project be guaranteed.
This paper mainly introduces the development method of a special polyurethane waterproof coating for underground pipe gallery, and discusses the effects of curing speed, the amount of coupling agent, the selection of latent curing agent and the amount of adding on the bond strength, tensile strength and elongation at break of the coating.
Preparation process
Add polyether polyol, dispersant and defoamer 1 to 1000 ml flasks in turn, raise the temperature to 90 ℃, add inorganic pigments and fillers, thixotropic agent and antioxidant, heat them to (115 ± 5) ℃, dehydrate them for 4 h under vacuum degree of - 0.095 MPa, then lower the temperature to 60 ℃, add MDI, react for 1.5 h under (78 ± 2) ℃, and add TMXDI to raise the temperature to (85 ± 1) ℃, react for 1 h. Then add catalyst 1 and catalyst 2, keep the temperature for 45 min, add latent curing agent, silane coupling agent, defoamer 2, leveling agent and solvent, mix at (70 ± 2) ℃ for 30 min, cool to 60 ℃, defoaming at vacuum degree (- 0.07 ~ - 0.08) MPa for 10 min, stop vacuum, stir and filter out.
performance testing
The special polyurethane waterproof coating for underground pipe gallery is scraped and applied to the glass plate coated with release agent for three times, and the coating with thickness of (1.5 ± 0.2) mm is prepared. Under the standard test conditions [(23 ± 2) ℃ and relative humidity (50 ± 10)%] film is formed, cured for 96 hours, then demoulded, and then the coating is turned over and cured for 72 hours under the standard test conditions, and the waterproof coating layer is prepared according to GB / T Testing of relevant properties of polyurethane waterproof coating (19250-2013)
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Effect of curing speed on the bond strength of polyurethane coating
Under the long-term dynamic load, the waterproof material must be firmly bonded with the main structure of the pipe gallery to effectively avoid water channeling. Generally, the waterproof coating applied to the underground pipe gallery usually has a very fast curing speed, which can greatly shorten the construction period. However, the extremely fast curing speed makes the coating not wet the base layer well, and the coating does not fully penetrate into the concrete structure and then solidifies to form a film on the surface, resulting in the low bond strength between the coating and the base layer, which is easy to separate and peel off, especially in the spraying construction. The excessive pursuit of waterproof coating on the wettability of the base, ignoring the curing speed, will affect the construction progress.
Fig. 1 Effect of curing speed on adhesive strength of polyurethane coating
Fig. 1 shows the effect of curing speed of the coating on the bond strength at a certain viscosity (the curing speed is characterized by the surface dry time of the coating). It can be seen that with the continuous reduction of curing speed, the surface drying time is gradually extended, the wettability of the coating on the base is getting better and better, the surface of mortar sample block is fully penetrated (Fig. 2), and the bonding strength is gradually increased. At the same time, it is found that when the curing speed of the coating is too fast, with the increase of the tensile strength and hardness of the coating in the later stage, the coating of some sample blocks will have edge warping and peeling phenomenon in the curing shrinkage process. When the surface drying time is about 30 minutes, the coating has good adhesion performance and can also meet the requirements of rapid construction.
Figure 2 permeability display
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Effect of coupling agent content on the properties of polyurethane coating
There are a large number of strong polar groups in the molecular structure of polyurethane, such as ether bond, urethane bond, etc., which can produce chemical bond or hydrogen bond with the active group on the surface of the base layer and have good adhesion. However, at a fast curing speed, the number of chemical bonding between the coating and the active groups on the surface of the base layer is small, which is difficult to meet the demand of the underground pipe gallery for high bonding strength. Therefore, in addition to properly reducing the curing speed of polyurethane, appropriate coupling agent can also be added to enhance the bonding strength of the coating and the base layer.
Fig. 3 Effect of coupling agent content on bond strength and elongation at break
Silane coupling agent is a commonly used adhesion promoter in polyurethane materials. Because the silane coupling agent containing amino group will react with - NCO group, resulting in the decrease of polyurethane system stability and increase of viscosity, which is not conducive to the storage and construction of materials, 3-ISOCYANATE propyltrimethoxysilane was used as the coupling agent in this experiment. It can be seen that after the addition of 3-isocyanate-propyltrimethoxysilane, the bond strength between the coating and the base begins to increase, and increases with the increase of the addition amount, which indicates that the silane coupling agent plays a bridge role in the connection between the cement mortar (inorganic phase) and the polyurethane coating (organic phase), and improves the bond strength; while with the increase of the addition amount of silane coupling agent, the bond strength increases As a result, the crosslinking density of the coating increases, which leads to the decrease of the elongation at break. When the amount of coupling agent is 0.8%, the adhesive strength and elongation at break of the coating are the best.
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Effect of latent curing agent on properties of polyurethane coating
When polyurethane waterproof coating is applied to the underground pipe gallery, the design thickness is generally 2.0mm. If the coating is applied in two times, the thickness of each time should reach 1.0mm. In order to prevent the formation of bubbles in polyurethane coating, a latent curing agent is used in the test, which can react with moisture first to produce active groups, and then the active groups and isocyanate groups are crosslinked and solidified rapidly to form a film, avoiding the phenomenon of foaming of polyurethane coating caused by CO2 generated by the reaction of isocyanate groups with water directly; meanwhile, the strength of urea bond is far greater than that of urethane bond And it is very stable, which can improve the physical properties and aging resistance of the coating, and can meet the requirements of long-term use.
Fig. 4 Effect of adding amount of triazolidine latent curing agent on tensile properties
Fig. 5 Effect of amount of latent curing agent on tensile properties
Two kinds of latent curing agents, oxazolidine and aldimine, were selected for comparative analysis in the test. Figure 4-5 shows the effect of the addition amount of the two latent curing agents on the tensile strength and elongation at break of the film. It can be seen that with the increase of the amount of latent curing agent, the tensile strength of the coating increases first and then decreases, while the elongation at break increases all the time. The tensile strength of the film with oxazolidine is better than that with aldimine at the same amount, and the elongation at break is just the opposite. When the latent curing agent of aldehydes and imines is used and the dosage is 2.5%, the comprehensive performance of the film is the best.
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Comprehensive performance
After a series of tests, when the surface drying time is controlled at about 30 min, the amount of silane coupling agent is 0.8%, and the amount of aldehyde imine latent curing agent is 2.5%, the comprehensive performance of the special polyurethane waterproof coating for underground pipe gallery is the best, and all indexes of the product meet the design requirements. The specific test results of the pilot test are shown in Table 1.
Table 1 performance test results of polyurethane waterproof coating for underground pipe gallery
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conclusion
A special polyurethane waterproof coating for underground pipe gallery was prepared in this study. The effects of curing speed, coupling agent dosage, latent curing agent selection and dosage on adhesive strength, tensile strength and elongation at break of the coating were discussed.
1) When the surface drying time is controlled at about 30 minutes, the coating has good adhesion performance and can meet the requirements of rapid construction;
2) Silane coupling agent can effectively improve the bonding strength of polyurethane coating and base layer, and its addition amount should be controlled at 0.8%;
3) The addition of latent curing agent can greatly reduce the number of bubbles in the film. Under the same amount of addition, the tensile strength of the film with oxazolidine is better than that with aldimine, and the elongation at break is just the opposite. Therefore, when the amount of latent curing agent with aldimine is 2.5%, the tensile property of the film is the best.
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Author: Li Chao, Chen Yang, Fei Ma, Wei Wei, Xu Yuan