1. Building sealant basic concept


Building sealant is a paste-like building sealing material composed of base rubber, filler, curing agent and other additives. It will be cured into a flexible rubber material after being used, and bonded to the building substrate. To seal, waterproof and leakproof, mainly used for joint sealing of joints in buildings. As a kind of construction adhesive, it is significantly different from other construction adhesives such as glue in form and application. Other construction adhesives are generally fluid, mainly used for bonding and pasting architectural decoration materials. Functioning.


Building sealants can be divided into two categories according to their roles: one is architectural sealant and the other is non-structural sealant. The structural sealant is also referred to as structural adhesive. It is used for structural bonding and sealing of glass and other building panels when the curtain wall unit is made. These panels are completely bonded to the frame by structural glue, and there is no other fixed connection. . Therefore, structural adhesives have strict requirements on strength and adhesion. Non-structural sealant is also a kind of building sealant other than structural adhesive. The role of this kind of glue is joint sealing, and it can not afford structural bonding. Therefore, there is no strict requirement on strength, as long as it adheres well. It is sufficient to seal on the substrate. Due to the influence of the thermal expansion and contraction of the substrate on the expansion and contraction of the joint, the sealant must have good elasticity and displacement capability.


2. Classification of building sealants


Building sealants can be classified according to different methods, such as the structural sealants and non-structural sealants mentioned above. This is a classification method, which is classified according to the strength requirements combined with the application.


The more commonly used classification method is to classify by chemical composition. According to the chemical composition of the base rubber used in the sealant, the sealant can be classified into polysulfide, polyurethane, silicone, neoprene, butyl rubber, acrylic rubber and the like. At present, the most used amount on the market is silicone, polyurethane and polysulfide.


The building sealant is divided into single-component and two-component packaging according to the packaging method. The single-component packaging is to mix all the raw materials that make up the sealant, and seal it in a plastic bottle or a soft-packed aluminum film. It's ok to get the product out of the package. The two-component package is composed of two components, A and B. In general, component A is the base compound and component B is the curing agent. Fillers and other additives are added to components A and B as needed. When using, use special mechanical equipment (two-component glue machine) to mix components A and B in a specified ratio, A, B. The component is chemically cured, and either the individual component A or the individual component B cannot be used.


According to the curing characteristics, the sealant can be divided into chemical reaction type, solvent evaporation type, heat capacity and cold type. Silicone sealants are chemically reactive.


3. Silicone building sealant reclassification


Silicone building sealant is condensed by chemical reaction to remove volatile small molecules. According to the difference of chemical reaction, small molecules can be separated and classified. There are two types of products currently circulating in the market: one is acid gel. These products are acidic. The small molecules released during curing are acetic acid, which has a sour taste. It is corrosive to some metal materials and has certain stimulation to the human body. Sexual and corrosive. The other type of product is neutral glue. These products are neutral and are classified into alcohol type and ketone gum according to the small molecules released during the curing process. The alcohol-type glue and the small molecule released during the curing process are methanol, which is non-corrosive and irritating. The ketone oxime releases ketone oxime when it is cured, which is more odor than the alcohol type glue. These two types of products differ in odor and nature.


4. Introduction to the formulation, process and performance characteristics of silicone building sealants


Silicone building sealant is mainly composed of base rubber, filler, crosslinker, catalyst and other additives. The base rubber is the basic material of the sealant and determines the performance of the 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 white carbon black, calcium carbonate, heavy calcium, etc., the role of the filler is to provide strength, hardness, flow Variable performance, etc. The cross-linking agent and the catalyst are curing systems of the sealant, and the sealant is changed from a liquid state to an elastomer by reacting the base rubber with the cross-linking agent and the catalyst and the moisture in the air.


The production process of the sealant is a process in which the components constituting the sealant are uniformly mixed. The usual production method is to mix the base material and the filler by a kneading machine, and if necessary, grind with a grinder, and then use a planetary machine to uniformly mix the crosslinking agent, the catalyst and other auxiliary agents under vacuum.


Compared with other types of sealant, silicone building sealant has good elasticity, high temperature resistance and low temperature flexibility. It has good weather resistance, ozone resistance and UV resistance, and has a long service life, but silicone. The cost is relatively high, usually more expensive than other types of sealant, the strength is especially poor tear resistance, there is information that the water resistance is worse than polyurethane sealant, oil resistance is not as good as polysulfide sealant.


5. Curing properties of silicone building sealants


The one-component sealant is packaged in a container that is stable and will cure when exposed to air and exposed to air. This is a chemical reaction process. The sealant is originally a paste. After contact with air, the base glue, cross-linking agent and catalyst in the sealant react with the moisture in the air. The result of this chemical reaction is This paste gradually becomes an elastic solid, which is the curing of the sealant. After the sealant is punched out, since the first contact with moisture is the surface of the sealant, the first surface is cured. After the surface is solidified, the water penetrates from the solidified surface, and the inner layer is cured. The sealant is gradually cured from the inside and the inside of the watch. The longer the time, the thicker the cure. The thicker the sealant is, the longer it takes to fully cure. For example, a 10mm thick glue usually takes more than a week to fully cure.


Two-component sealant is carried out by chemical reaction between components A and B. The A and B components are sealed and stored separately. Once the components A and B are mixed, they begin to react, so the components A and B should be used immediately after mixing. The two-component curing reaction does not require the participation of moisture in the air, so its curing is carried out simultaneously with the interior and the surface. The complete curing time is independent of the thickness of the glue, so the reaction is not related to whether or not it is in contact with air, as long as A, The components B are mixed together and solidified even in a sealed state.
 
The following are a few concepts of sealant curing properties:


Dry surface: The sealant is creamy when it is punched out. When it is touched with a finger or other material, the compound will adhere to the finger or material. After the sealant is punched out, the surface contacts the moisture to start to solidify the crust. When the skin is formed, and then the finger or material is used to contact the surface, no glue will adhere to the finger or material. This is called the stem.


Dry time: When the sealant is shot from the container, the time required for the surface to dry is the dry time.
 
Detackification: After the sealant is dry, touch the surface with your fingers. Although there is no glue attached to the finger, you can still feel a certain adhesion between the surface of the glue and the finger. This phenomenon is called No sticky. This is that the curing reaction of the rubber surface has not yet been fully demonstrated. As time goes on, it will cure further until the surface forms a layer of skin with a certain elasticity and strength. It feels dry with finger contact and has no sticky feeling. We call it tack free.


Tack free time: The time required for the sealant to start from the container and the time it takes to tackify the surface is the tack free time.


6. Factors affecting the curing performance of sealant


Environmental factors have a significant effect on the curing properties of the sealant. The first is the effect of temperature. The higher the temperature, the faster the curing reaction. The phenomenon is that the surface is dry and tack free. If the temperature is very low, such as below 5 °C, the sealant will cure slowly. If the temperature is too high, such as above 40 ° C, the sealant will be used because the surface is too dry.


Humidity also has a significant effect on the curing properties of the sealant, because the sealant cure reaction requires moisture in the air, so too dry weather such as relative humidity below 40 °C is detrimental to the cure of the sealant. But not the higher the humidity, the better, because the sealant should release volatile small molecules when it cures. If the humidity of the air is too high, the small molecules will not easily volatilize, which is not conducive to the curing of the sealant. Experiments have shown that when the relative humidity is higher than 80 ° C, the sealant debonding and deep curing will be affected, sometimes after 2-3 days, the surface of the sealant is still sticky.


The standard conditions for the performance of sealant specified by national standards are temperature (23 ± 2) ° C, relative humidity (50 ± 5) ° C


7. Main properties of sealant


In addition to the curing properties mentioned above, some of the following performance items are also important:


(1) Appearance: The appearance of the sealant mainly depends on the dispersion of the filler in the base rubber. The filler is a solid powder. After being dispersed by a kneader, a grinder or a planetary machine, it can be uniformly dispersed in the base rubber to form a fine paste. Sometimes, depending on the nature of the filler itself, the presence of the pole is not excluded. A small amount of fine fine or fine sand is acceptable as normal. If the filler is not well dispersed, many very coarse particles will appear. In addition to the dispersion of the filler, other factors may also affect the appearance of the product, such as the incorporation of particulate impurities, crusts and the like. These situations are considered to be rough. The appearance of the observation method is to take the product out of the package and directly observe it, or put the product on 1-2g on white paper, and fold the white paper to open and observe it. The term is called "butterfly observation". When coarse particles are found, the coarse particles should be judged.


(2) Hardness: Hardness refers to the hardness of the sealant after it has completely cured into a rubber body, and it is one of the physical and mechanical properties of the product. Hardness refers to the ability of a material to resist the scoring or pressing of an attempted substance into its surface. According to the method of measuring the hardness, the hardness is expressed by various methods such as Brinell hardness, Rockwell hardness, and Shore hardness. The state specifies Shaw A hardness. The standard hardness value is measured by a hardness tester prepared according to the national standard method. The hardness of the sealant is high, the surface sealant is rigid, and the elasticity and flexibility are insufficient. The hardness is small, the elasticity and flexibility are good, and the rigidity is insufficient. Therefore, the sealant is neither as hard as possible, nor as soft as possible, but has a certain range of requirements according to actual needs.


(3) Tensile strength: Tensile strength is also one of the mechanical properties after the sealant is completely cured. Tensile strength, also known as tensile strength, tear strength, commonly known as tensile force. Refers to the ability of a material to resist damage when subjected to tension. Tensile strength values are also detected by methods specified by national standards. According to the needs of its use, the sealant has certain strength requirements, especially the structural adhesive. The minimum value of the strength is clearly specified in the national standard. The sealant with too weak strength cannot meet the needs of use. However, it is also inadmissible to over-emphasize the strength of the sealant while ignoring the elasticity.


(4) Elongation: Elongation is the elastic behavior of the sealant after it is fully cured. It is also one of the mechanical properties and refers to the ratio of the ratio of the total elongation to the original length of the material during stretching. A good elastic sealant will have a large elongation. As a minimum requirement for elongation, the sealant must meet the requirements of the national standard for the elongation performance.


(5) Tensile modulus and displacement capacity: Tensile modulus and displacement capacity are the comprehensive performance of the above mechanical properties. The tensile modulus characterizes the strength produced when the sealant is stretched to a certain elongation. Therefore, the method of expressing the amount of the touch is at the same time as the elongation, for example, when the elongation is 25%, the tensile amount is 0.46 MPa. The displacement capability is the ability of the sealant to withstand the displacement of the seal due to the expansion and contraction of the substrate. For example, we say that the sealant has a displacement capacity of ±25%, which means that the seam with this product can withstand 25% of the original stretch and compression. For example, the original seam width is 12mm, it can be compressed up to 9mm, stretching To 15mm. The displacement capacity can be detected by a tensile compression cycle or a cold drawn hot pressure cycle.


(6) Adhesion to the substrate: This is an important property in the actual use of the sealant. The sealant must have good adhesion to the substrate actually used. A convenient way to verify the adhesion is to clean and dry the substrate with a suitable solvent or detergent, and place the sealant on it. After the sealant is cured (about 3-5 days), peel off the sealant by hand. Observe the bonding.


(7) Extrusion: This is a project of the construction performance of the sealant. It is used to indicate the difficulty of the sealant when it is used. The extrusion of the too thick glue is poor, and it is very laborious to use the glue. However, if the glue is made too thin considering the extrudability alone, it will affect the thixotropy of the sealant. Extrusion can be measured by the method specified by national standards.


(8) Thixotropy: This is another item of sealant construction performance. Thixotropy is the opposite of fluidity. It means that the sealant will change its shape only when a certain pressure is applied. If there is no external force, it can maintain its shape. Shape without flowing. The determination of the sag specified in the national standard is the judgment of the thixotropy of the sealant.


8. Simple judgment method of sealant performance


The sealant is punched out of the rubber bottle into a strip, and the following properties can be tested or simply judged during the process of marking the glue and the strip.


(1) Extrusion: The extrudability of the glue can be initially felt by the difficulty of the glue.


(2) Curing performance: After the rubber strip is printed, the dry time, the tack free time and the complete curing time can be determined to determine whether the curing performance is normal.
 
(3) Thixotropy: After the rubber strip is shot, the horizontal placement is not deformed, and the surface sealant is thixotropic. If there is a phenomenon of rogue deformation, it indicates that the thixotropy is not good.


(4) Hardness: After the strip is fully cured (generally about 1-2 days), it can be felt by pressing the strip with a finger. This method can compare the hardness relatively.
 
(5) Strength, Elasticity, and Elongation: Pulling the fully cured strip allows for relative judgment of strength, elasticity, and elongation. Attention must be paid to this experiment: we usually put the strip on the paper. After peeling off the strip from the paper, a layer of paper is often stuck on the strip. If the strip is pulled, the paper is not Elastic, one pull will tear, and the strip stuck on the paper will tear open a hole. Because the silicone sealant is generally low in tear strength, the stress generated by stretching will concentrate on tearing. Cracks, it is easy to pull the strip from here, affecting observation and judgment. The correct way is to put the strip on the plastic film, and pull the strip to judge the performance after the strip is completely cured.


9. Corresponding standard for sealant


(1) National standard GB16776 "silicone structural adhesive for construction": This is a mandatory national standard, which stipulates the basic performance requirements of structural adhesive. All structural adhesives produced, sold and used in China must meet the standard. Requirements.
 
(2) National Standard GB/T14683 "Building Silicone Sealant": Other weather-resistant adhesives, sealants and glass glues other than structural adhesives (except for special purposes) are currently implemented in this national standard.


(3) Building materials industry standard JC/T883-2001 "Building sealant for stone": Stone sealant in addition to GB/T
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