Polyurethane thermal insulation material is a kind of polymer with repeating structural units of urethane chain segments, which is made of isocyanate (black material) and combined polyether (white material) by mixing and reacting with high-pressure polyurethane spraying equipment. Because of its good thermal insulation performance and excellent waterproof performance, it is widely used in external wall insulation, roof insulation, cold storage, grain depot, archives, pipelines, doors and windows and other special parts. Polyurethane foam is widely used in roof and wall insulation abroad. The main application forms are composite plate and on-site spraying.
It is difficult to master the spraying process of rigid polyurethane, which easily leads to uneven foam holes of sprayed polyurethane. Enterprises should strengthen the training of spraying construction personnel so that they can master spraying technology and independently solve technical problems encountered in spraying construction. The key technical problems to be solved in spraying construction mainly include the following aspects.
Control of whitening time and atomization effect of polyurethane raw materials
The formation of polyurethane foam needs to go through two stages: foaming and curing. The process from the mixing of black and white materials to the stop of foam volume expansion is called polyurethane foaming. During the foaming process, the system releases a lot of reaction heat. The uniformity of bubbles should be considered when spraying. Cell uniformity is mainly affected by the following factors:
(1) material ratio deviation The density of equipment foaming is quite different from that of manual foaming. Usually, the fixed material ratio of polyurethane spray coating machine is 1∶ 1, but the actual material ratio is different from that of spray coating machine because of the great difference in viscosity of white materials from different manufacturers. When the white material is excessive, the foam density is low, the color is white, the foam strength decreases, the hand feels soft, and it is easy to shrink at low temperature; When there is too much black material, it is characterized by high foam density, dark color, high foam strength and hard and brittle hand feel. In these cases, the material ratio should be checked immediately to see whether the filter is blocked and whether the pressure and temperature indications are normal, so as to ensure the accuracy of the black-and-white material ratio.
(2) Polyurethane foaming at room temperature is greatly influenced by temperature. Foaming depends on heat. If there is no heat, the foaming agent in the system cannot evaporate, so it is impossible to produce foamed plastics. Heat comes from chemical reactions and the environment. The heat of chemical reaction is not affected by external factors, but the heat provided by the environment changes with the change of environmental temperature. When the ambient temperature is high, the environment can provide heat for the reaction system, which can improve the reaction speed and shorten the reaction time. This shows that the foam is fully foamed, and the density of foam surface and core is close. When the ambient temperature is low (such as below 18℃), part of the reaction heat will be released into the environment. Heat loss, on the one hand, prolongs the curing period of foam and increases the molding shrinkage of foam (the lower the temperature, the higher the molding shrinkage); On the other hand, the amount of foam material increases.
(3) The wind speed is required to be below 5m/s during air spraying operation. When the wind speed exceeds 5m/s, the heat generated by the reaction will be blown away, which will affect the rapid foaming reaction of polyurethane foam and make the surface of the product brittle. At the same time, because the polyurethane spraying machine mixes the raw materials and sprays them in an atomized state, if the wind speed is too high, the atomized particles will be blown away, which increases the loss of raw materials and pollutes the environment.
(4) Base temperature and humidity It can be seen from engineering practice that the base wall temperature also has a great influence on the foaming efficiency of polyurethane. In the spraying process, if the ambient temperature and the wall temperature of the building base are very low, the reaction heat will be absorbed by the base quickly after the first spraying of rigid polyurethane, thus reducing the foaming amount of the material. Therefore, in the process of construction, try to shorten the rest time at noon, and arrange the working procedures reasonably during the construction arrangement to ensure the foaming rate of rigid polyurethane. Rigid polyurethane foam is a polymer product produced by the mixed reaction of isocyanate and combined polyether. Among them, the isocyanate component easily reacts with water to generate urea. If the urea bond content in polyurethane increases, the foam will become brittle and the adhesion between the foam and the substrate will decrease. Therefore, the surface of the sprayed substrate is required to be clean and dry, the relative humidity is less than 80%, there is no rust, dust, pollution and humidity, and construction is not allowed in rainy days. If there is dew or frost, remove the dew and dry the surface.
B. Control of foam spraying quantity on site
In the process of construction, we should try to avoid the pollution of rigid foam polyurethane spraying. Polyurethane raw materials are sprayed from the spray gun in the form of droplets under high pressure. In addition, raw materials have low density and light weight, and are easily carried away by the wind, resulting in waste of raw materials and environmental pollution. In the construction of polyurethane spraying, raw materials close to 1/2 can't be sprayed on the wall in the construction of building corner lines and decorative lines, and the waste of raw materials is serious. In the process of construction, the smoothness of the base should also be effectively controlled. Poor flatness of the base wall will also cause a certain waste of raw materials. In addition, if the flatness error of the foundation wall is too large in the process of polyurethane spraying, it is necessary to saw off the part with too large local positive deviation, which not only wastes polyurethane raw materials and labor costs, but also brings difficulties to the subsequent construction.
Management of construction site materials
The principle of polyurethane foaming is that the foaming agent evaporates when heated and enters the foam cell, resulting in the pressure difference between inside and outside the cell, thus completing the foaming process, so the content and effective utilization of foaming agent are also the key to foaming efficiency. Before construction, the foaming agent is mixed into the white material according to a certain proportion. Because foaming agent is a very volatile substance, if the temperature inside the container is too high under the influence of the environment during construction, the foaming agent in the container will start to evaporate, turn into gas in the hollow space at the upper part of the container, or volatilize into the atmosphere through the holes in the container, resulting in the decrease or lower than the predetermined proportion of foaming agent in the material. At this time, the foam is not fully foamed, and the density of the foam becomes larger, so that the same amount of material cannot produce the foam with the expected volume. Therefore, polyurethane should be sealed during storage, and exposure to sunlight is strictly prohibited.
D, safety guarantee measures in the process of polyurethane rigid foam spraying
Before spraying, shielding should be done well. Generally, doors and windows are covered with plastic striped cloth cut into pieces with the same area as the doors and windows. For shelf tube, wrought iron and other irregular parts that need protection, polyethylene plastic wrap should be used for winding protection. Polyurethane foam is easy to splash when spraying, so the construction personnel should do a good job of labor protection. Strictly abide by the safety rules of aerial work, wear safety helmet and safety belt, especially take effective protective measures to prevent the hanging basket from falling. New workers must undergo strict technical training and safety education before taking up their posts. Enclosures should be made around the spraying operation to avoid environmental pollution. When using scaffolding, use a small eye safety net to cover it tightly; When the hanging basket is used for construction, the shielding height around the hanging basket should be no less than 1.8m, and the width should be no less than the spraying range to prevent polyurethane particles from being blown away by the wind. In the process of spraying, it is forbidden to work with naked flame such as electric welding, and sufficient fire extinguishers should be equipped during construction to prevent on-site fire. The installation and operation personnel of spraying electrical equipment shall hold relevant certificates, and the electricity consumption shall be strictly implemented in accordance with the on-site management regulations. Brush polyurethane interface mortar in time after spraying.