Abstract：The purpose of adding fillers to sealants is mainly to improve the physical properties of the sealants and reduce costs. For example, the use of appropriate fil
The purpose of adding fillers to sealants is mainly to improve the physical properties of the sealants and reduce costs. For example, the use of appropriate fillers can increase the elastic modulus, stability, thermal distortion, and reduce the thermal expansion coefficient of the sealant in a wide temperature range. Fillers are divided into inorganic fillers and organic fillers, and several commonly used inorganic fillers are reviewed below.
Carbon black as a filler is mainly to increase the tensile strength of the sealant and play a reinforcing role, and it has little effect on the elongation. One of the reasons is that there are many micropores on the surface of carbon black, so that the polymer may penetrate into these pores to form a honeycomb claw structure and strengthen the interaction between the interfaces; the second reason is that the surface area of carbon black is large, making the interface between the polymer and carbon black The friction resistance between carbon black is also large; the third reason is that the surface of carbon black contains some polar groups such as carboxyl groups, phenolic groups, and quinone groups, as well as some gases and organic compounds adsorbed, so that there is no physical adsorption between carbon black and polymers. In addition, there is chemical adsorption, which further strengthens the chemical interaction between the interfaces. Due to the strong chemical force between carbon black and polymer interface, sometimes even exceeds the internal friction of the polymer, so as the content of carbon black increases, the ability of the sealant to resist external force damage increases, and the tensile strength also increases. increase. However, it is precisely because of this interaction that carbon black cannot act as an internal lubricant in the polymer and has little effect on elongation.
Calcium carbonate is cheap and comes from a wide range of sources. It is a commonly used filler for polyurethane sealants. Because of its different processing procedures, it has different effects on the properties of the glue. Activated calcium carbonate is the product of light calcium carbonate after chemical treatment. The surface is covered with a layer of low-molecular organic matter. When it is blended with polyurethane prepolymer, it can increase its interfacial adhesion and improve its blendability, but this The frictional resistance of this interface is still smaller than the internal frictional resistance of the polymer, which plays a role of internal lubrication, thereby improving the fluidity of the sealant. In this way, the interface between the active calcium carbonate particles and the polymer becomes a breakthrough for external damage. Therefore, as the content of active calcium carbonate increases, the number of such breakthroughs increases, which makes the tensile strength of the sealant decrease and the elongation significantly decreases.
Light calcium carbonate is formed by adding carbon dioxide into calcium hydroxide aqueous solution. It has uniform particle size and large specific surface area. It can be well dispersed in the base material and can better improve the construction performance and mechanical properties of the sealant.
Heavy calcium carbonate is obtained by mechanical crushing of limestone. It has coarse particles, irregular particle shape, small specific surface area, small oil absorption, and poor blending effect in sealants. It is generally not used.
The change of the amount of talcum powder has obvious effects on the consistency of the rubber compound, and has no regular change on the physical and mechanical properties of the cross-linked sealant. However, talc powder is a natural mineral and is made by mechanical crushing. The particle size dispersion is large, and the dispersion with the base material is not good, which affects the properties of the sealant.
In polyurethane sealants, Silica white is mainly used as a thixotropic agent to adjust the fluidity of the glue, that is, the sag. Good fluidity prevents the glue from sagging.
Combining the functions of several common fillers, we can see that the functions of fillers in polyurethane sealants include:
(1) Improve the mechanical properties of the adhesive and play a reinforcing role. Some polymers have weak interactions between molecules and low cohesive energy, so their mechanical properties are not high. Choosing fillers with appropriate particle size can play a reinforcing role. Since the active surface of the filler particles can combine with several macromolecular chains to form a cross-linked structure. When one of the molecular chains is under stress, the stress can be dispersed and transmitted to other molecules through the cross-linking point. If one of the chains breaks, the other chains still function, and the whole will not be endangered immediately, so the mechanical properties of the adhesive can be greatly improved. Commonly used fillers, such as metal powders and metal oxides, can improve the compressive strength and dimensional stability of the adhesive, but will also reduce some properties of the adhesive, such as peel strength. Adding carbon black, white carbon black, calcium carbonate, etc. to silicone and rubber can improve tensile strength, hardness and wear resistance.
(2) Reduce the thermal expansion coefficient and curing shrinkage of the adhesive. During the curing process, the colloid will cause volume shrinkage due to chemical action. In addition, due to the different thermal expansion coefficients of the adhesive and the glued object, thermal shrinkage will also occur. These two types of shrinkage will generate internal stress in the adhesive layer, resulting in stress concentration, so that the adhesive layer is cracked or the joint is damaged, which directly affects the service life of the adhesive joint. Fillers can adjust the shrinkage rate during the curing process, reduce the difference in thermal expansion coefficient between the adhesive and the glued object, and prevent the crack from extending, so it can significantly improve the bonding strength, especially the shear strength at high temperature.
(3) Increase the viscosity of the glue, improve the thixotropy, and improve the operation process. Adding filler can play a thickening effect, avoiding the lack of glue caused by the flow of the glue during the curing process. It can also improve the thixotropic properties of the glue to control the fluidity of the glue, adjust the curing speed, prolong the service life, and facilitate the operation and construction.
(4) Give the adhesive new functions. Adding good conductive metal powder such as silver powder to the glue can be formulated into a conductive adhesive; thermally conductive powder such as copper powder, aluminum powder, alumina, magnesium oxide, GD thermal conductive agent, etc. can be used as fillers to improve the thermal conductivity of the adhesive; flame retardant Powder such as aluminum hydroxide and other fillers can improve the flame retardancy of the adhesive; when special conductive carbon black, carbon-based iron powder, and magnetic powder are added as fillers, conductive or magnetically conductive adhesives for special occasions can be prepared; epoxy resin Adding zinc chromate can increase the strength retention rate; adding Zr(SO3)2 can reduce the water absorption of epoxy resin. Some fillers can also improve the resistance to humidity and heat aging and salt spray of adhesive joints.
There are tens of thousands of fillers for polyurethane sealants, how should we choose when designing the formula?
Below we summarize from the perspective of the different roles played by fillers in adhesives:
(1) Fillers to improve its impact resistance include: asbestos fiber, glass fiber, calcium carbonate, mica powder, aluminum powder, etc.;
(2) Fillers to improve its hardness and compressive performance: metals and their oxides, quartz sand, chromium oxide powder, porcelain powder, iron powder, cement, boron carbide, etc.;
(3) Fillers to improve wear resistance: graphite powder, talcum powder, calcium carbonate, molybdenum disulfide, etc.;
(4) Fillers to improve its heat resistance: asbestos powder, silica gel powder, phenolic resin, porcelain powder, titanium dioxide powder, etc.;
(5) Fillers to increase its adhesion: alumina powder, porcelain powder, titanium dioxide, etc.;
(6) Fillers that increase their thermal conductivity: metal powders such as aluminum powder, copper powder, iron powder, silver powder, non-metallic powders such as carbon black, graphite powder, alumina, magnesia, composite thermally conductive fillers such as GD of Jin Ge New Materials -S, HAR, FAR series thermal conductive agents, etc.;
(7) Fillers that increase their flame retardancy: compounds containing crystal water, aluminum hydroxide, magnesium hydroxide, aluminosilicate, and modified products of the above substances such as FA, FM series, and FS series of Jinge New Materials Modified products, etc.;
(8) Fillers to increase its conductivity: carbon black, graphite, carbon fiber, carbon nanotubes, silver powder, etc.;
(9) Fillers to increase its lubricity: graphite powder, kaolin, talcum powder, aluminum disulfide powder;
(10) Filler to increase its magnetic permeability: magnetic substances such as magnetite powder.
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