Take you to know the knowledge of sealant
One
Basic concept of building sealant
Building sealant is a kind of paste like building sealing material composed of base rubber material, filler, curing agent and other additives. It will be solidified into elastic rubber material after use. It is bonded on the building base material and plays the role of sealing, waterproof and leakproof. It is mainly used for sealing the joints of buildings. As a kind of building adhesive, it is different from other building adhesives in form and application. Other building adhesives are generally fluid, mainly used for bonding and pasting building decoration materials, and have no sealing effect.
Construction sealant can be divided into two types according to its different functions: one is construction structural sealant, the other is non structural sealant. Building structural sealant, also called structural adhesive for short, is used for structural bonding and sealing of glass and other building plates when making curtain wall unit parts. These plates are completely bonded to the frame by structural adhesive, without other fixed connections. Therefore, structural adhesive has strict requirements on strength and adhesion. Non structural sealant is other construction sealant except structural sealant. The function of this kind of sealant is to seal the joint, and it does not play the role of structural bonding. Therefore, there is no strict requirement on the strength, as long as it can better adhere to the base material and play the role of sealing. Because of the influence of the expansion and contraction of the base material on the expansion and contraction of the joint, the sealant must have good elasticity and displacement ability.
Two
Classification of building sealants
Construction sealants can be classified in different ways, such as structural sealants and non structural sealants mentioned above. This is a classification method, which is based on strength requirements combined with application occasions.
The more commonly used classification method is based on chemical composition. According to the chemical composition of the basic sealant, the sealant can be divided into polysulfide, polyurethane, organosilicon, neoprene, butyl rubber, acrylic rubber, etc. At present, organosilicon, polyurethane and polysulfide are widely used in the market.
The construction sealant is divided into single component and two-component packaging according to the packaging method. Single component packaging is to mix all the raw materials that make up the sealant together and seal them in a plastic bottle or soft packaging aluminum film. When using, the product can be printed out of the packaging with a glue gun. The two-component package is composed of two components a and B. generally, component A is the basic rubber material and component B is the curing agent. Fillers and other additives are added into component A and component B as required. When using, special mechanical equipment (two-component gluer) shall be used to mix component A and component B evenly according to the specified proportion. Component A and component B will undergo chemical reaction and solidify. Neither component a nor component B alone can be used.
According to curing characteristics, sealant can be classified into chemical reaction type, solvent volatile type and heat capacity cold curing type. Silicone sealant belongs to chemical reaction type.
Three
3. Reclassification of silicone building sealant
Organosilicon building sealant is cured by condensation of volatile small molecules. According to different chemical reactions, the products can be classified again. At present, there are two kinds of products in circulation on the market: one is acid glue, the other is acid, the small molecule released during curing is acetic acid, which has sour taste, has certain corrosiveness to some metal materials, and has certain stimulation and corrosiveness to human body. Another kind of product is neutral glue, which is divided into alcohol glue and ketoxime glue according to the different small molecules released during curing. The small molecules released in the process of alcohol type adhesive and curing are methanol, without corrosive and irritating smell. When the ketoxime adhesive solidifies, it emits ketoxime, which has a greater smell than the alcohol adhesive. These two kinds of products are different in smell and nature.
Four
Brief introduction of formulation, process and performance characteristics of silicone building sealant
Silicone building sealant mainly consists of base glue, filler, cross-linking agent, catalyst and other additives. Base adhesive is the basic material of sealant, which determines the performance of sealant. The base rubber is 107 room temperature vulcanized silicone rubber, the chemical structure is hydroxyl terminated polydimethylsiloxane, the filler is some inorganic powder, such as silica, calcium carbonate, calcium bicarbonate, etc., the role of the filler is to provide strength, hardness, rheological properties, etc. Crosslinking agent and catalyst are the curing system of sealant. Sealant changes from liquid state to elastomer through the reaction of base glue, crosslinking agent, catalyst and water in air.
The production process of sealant is to mix all components of sealant evenly. The common production method is to mix the base material and filler with kneader, and grind with grinder if necessary, and then mix the crosslinking agent, catalyst and other auxiliaries with planetary machine under vacuum.
Compared with other types of sealant, silicone building sealant has good elasticity, high temperature resistance and low temperature flexibility, good weather resistance, ozone resistance and UV resistance, long service life, but the cost of silicone is relatively high, which is usually a little more expensive than other types of sealant, with poor strength, especially tear resistance. It is introduced that water resistance is better than polymerization Polyurethane sealant is poor, and its oil resistance is inferior to polysulfide sealant.
Five
Curing properties of silicone building sealant
Single component sealant sealed packaging in the container is stable, from the container hit out in the air will be solidified. This is a chemical reaction process. The sealant is originally a paste. After contacting with the air, the base glue, cross-linking agent and catalyst in the sealant will react with the water in the air. The result of this chemical reaction is that the paste will gradually become an elastic solid, which is the curing of the sealant. After the sealant is made, the surface of the sealant is the first to contact the water, so the first to cure is the surface. After the surface is cured, the water will seep in from the cured surface, and the inner layer will be cured again. In this way, the sealant will be cured gradually from the surface to the inside, and the longer the time is, the thicker it will be cured. The thicker the sealant is applied, the longer it will take to fully cure. For example, the 10 mm thick sealant usually takes more than a week to fully cure.
Two component sealant is carried out by chemical reaction between components a and B. A. It is stable to store component B separately and sealed. Once component A and component B are mixed, they will start to react. Therefore, component A and component B should be used immediately after mixing. The two-component curing reaction does not need the participation of water in the air, so its curing is carried out by the internal and the surface at the same time, and the complete curing time has nothing to do with the thickness of the adhesive, so the reaction has nothing to do with whether it contacts the air. As long as the components a and B are mixed together, they will be cured even in the closed state.
The following are several concepts of sealant curing performance:
Surface drying: the sealant is paste like when it is punched out. When touching its surface with fingers or other materials, the sealant will adhere to the fingers or materials. After the sealant is put out, the surface contact moisture begins to solidify the skin. When the skin is formed, and then the fingers or materials are used to contact the surface, there will be no more adhesive on the fingers or materials. This is called surface drying.
Surface drying time: when the sealant is printed out from the container, the time required for it to dry is the surface drying time.
Debonding: after the sealant surface is dry, touch the surface with fingers. Although there is no adhesive on the fingers, there is still a certain adhesion between the adhesive surface and fingers. This phenomenon is called "debonding". This is that the curing reaction on the surface of the adhesive has not been fully performed. With the extension of time, it will further solidify until a layer of skin with certain elasticity and strength is formed on the surface. It feels dry when touching with fingers, and there is no sense of adhesion. We call it anti adhesion.
Debonding time: when the sealant is printed out of the container, the time required for debonding is debonding time.
Six
Factors affecting the curing performance of sealant
Environmental factors have a significant impact on the curing performance of sealant. First of all, it is the effect of temperature. The higher the temperature is, the faster the curing reaction speed will be. The phenomenon is that the surface drying and debonding are relatively fast. If the temperature is very low, such as below 5 ℃, the sealant curing will be very slow. If the temperature is too high, such as 40 ℃, the sealant will be too dry and inconvenient to use. Humidity also has a significant impact on the curing performance of sealant, because the curing reaction of sealant requires water in the air, so too dry weather such as relative humidity lower than 40 ℃ is adverse to the curing of sealant. But it is not that the higher the humidity is, the better it is, because the volatile small molecules should be released when the sealant is cured. If the air humidity is too high, the small molecules are not easy to volatilize, which is not conducive to the sealant curing. Experiments have proved that when the relative humidity is higher than 80 ℃, the debonding and deep curing of sealant will be affected. Sometimes after 2-3 days, the surface of sealant is still sticky. The standard conditions for sealant performance specified in national standards are: temperature (23 ± 2) ℃, relative humidity (50 ± 5) ℃
Seven
Main properties of sealant
In addition to the above curing properties, the following performance items are also important:
(1) Appearance: the appearance of sealant mainly depends on the dispersion of filler in the base glue. Packing is a kind of solid powder. After being dispersed by kneader, grinder and planetary machine, it can be evenly dispersed in the base glue to form fine paste. Sometimes, according to the different properties of the packing itself, it does not exclude the existence of a very small amount of fine particles or fine sand, which is an acceptable normal phenomenon. If the filler is not well dispersed, there will be many very coarse particles. In addition to the dispersion of fillers, other factors can also affect the appearance of products, such as the inclusion of particulate impurities, crusting, etc. All of these conditions will be considered as rough in appearance. The method of appearance observation is to print out the product from the package for direct observation, or to print 1-2g of the product on the white paper, fold the white paper in half, press it flat, and then open it for observation. The term is "butterfly observation". When coarse particles are found, they should be judged.
(2) Hardness: hardness refers to the hardness of sealant after fully curing into rubber body, which belongs to one of the physical and mechanical properties of the product. Hardness is the ability of a material to resist being scratched or pressed onto its surface. According to the different methods of hardness measurement, there are many ways to express hardness, such as Brinell hardness, Rockwell hardness, shore hardness and so on. Shore a hardness is specified by the state. The standard hardness value is produced according to the national standard method and tested by the hardness tester. The hardness of sealant is high, the rigidity of surface sealant is strong, the elasticity and flexibility are insufficient; the hardness is small, on the contrary, the elasticity and flexibility are good, and the rigidity is insufficient. Therefore, sealant is neither the harder the better, nor the softer the better, but according to the actual needs of a certain range of requirements.
(3) Tensile strength: the tensile strength is also one of the mechanical properties of sealant after complete curing. Tensile strength also known as tensile strength, breaking strength, commonly known as tension. It refers to the ability of materials to resist damage when they are under tension. The tensile strength value is also detected according to the method specified in the national standard. Sealant should have certain strength requirements according to the needs of its use, especially for structural adhesives, especially in China