What Are the Different Types of Sealants?

Liquid sealant, also known as liquid gasket, liquid gasket, liquid sealing filler, etc. It is a new polymer static sealing material in a liquid state. The difference between a liquid sealant and a liquid seal putty is that it must give a certain tightening force to the liquid sealant in order to exert its sealing effect, so some people call it a "liquid gasket". But it is different from solid gaskets, such as those made of rubber, asbestos, metal, paper and other materials. It has fluidity, so there is no compressive deformation that must occur when a solid gasket is used as a seal, so there is no internal gasket. Stress, relaxation, creep, and elastic fatigue failure are factors that cause leakage. Because of its fluidity, it can fill the depressions and gaps between the bonding surfaces and eliminate interface leakage, so it is an ideal static sealing material.

Liquid sealant

The glue is made of synthetic rubber as the main component, and added with other resins, fillers and additives. It is a semi-dry, visco-elastic liquid sealing gasket. After coating and drying, a rubber-like elastic body is formed, and it has good coating properties, sealing properties, shock resistance and impact resistance. In addition, the glue also has good resistance to gasoline, engine oil, water, acid and alkali. The glue is used for pressure-resistant sealing of mechanical equipment in the working temperature range of -40 ° C to 200 ° C, such as the sealing of various flanges, cover plates, threaded pipes, joints and other surfaces. The glue is easy to fill the gap of the joint surface of the parts, so as to prevent the "three leaks" (oil leakage, water, gas). Supply of polymer liquid sealant-oil and high temperature resistance. ^[1]

It can be divided into many types according to the chemical composition of the liquid sealant, the shape of the film after coating and the application.

1. Classification by chemical composition

Can be divided into resin type, rubber type, oil modified type and natural polymer type. This classification method can infer the temperature resistance, sealability and adaptability to various media based on the characteristics of polymer materials.

2. Classified by application

Can be divided into heat-resistant, cold-resistant, pressure-resistant, oil-resistant, water-resistant, solvent-resistant, chemical-resistant, and insulating.

3. Classified by its film-forming properties after coating

It can be divided into four types: dry attachment type, dry peelable type, non-drying sticky type, and semi-dry sticky elastic type. This classification is commonly used in China. Convenient for both manufacturers and users.

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The liquid sealant itself is liquid, so it has good fluidity. It can fill the gaps in the narrow gaps of the metal joint surface, forming a uniform, stable and continuous film with stickiness, viscoelasticity or peelability. The joint surfaces of the various components function as a seal.

The liquid sealant has good sealing performance under a certain tightening force, good pressure resistance, heat resistance and oil resistance. It has good stability to the medium (oil, water), does not corrode metals, and it is liquid. It does not need compression deformation when it acts as a seal like a solid gasket, so there is no internal stress and relaxation. , Creep, elastic fatigue and other leakage factors. Due to its fluidity and thixotropy, it can fill the depressions and gaps between the joint surfaces, eliminating the interface leakage phenomenon of solid gaskets in use. Sealant is a substance with good adhesive elasticity. When subjected to vibration, shock and excessive compression, it will not cause damaging leakage such as cracking and falling off like a solid gasket. In addition, the sealant has a simple manufacturing process, cheap price, and convenient storage. Therefore, it is an ideal static sealing material for mechanical products.

Why can the liquid sealant seal in the gap of the mechanical joint surface? We can understand it with some examples that are usually familiar to everyone. For example, coating a layer of vaseline between the glass drier cover and the main body in the laboratory can prevent water and gas from invading.

Before studying the composition of the sealant, we first study what conditions the viscous liquid has. Can be used as a sealant. A practical liquid sealant must meet the following conditions:

(1) Fillability: When the metal surface is rough, it can also fill all the depressions, and has good wettability to the metal surface.

(2) Denseness: The liquid sealant in the gap itself is dense and flexible.

(3) Anastomosis: It has good adhesion to metal surfaces, and still adheres to metal surfaces when subjected to vibration.

(4) Continuity: Film formation must be continuous.

(5) High viscosity: The glue viscosity should be high, and the change with temperature should be small.

(6) Stability: The glue itself is not corrosive to metals, stable to the sealed medium, and does not cause chemical reactions.

(7) Adaptability: adapt to changes in external environmental media and temperature conditions, and do not lose sealing due to changes in external conditions.

In the use of liquid sealant, its sealing performance is not only related to its nature, but also the working conditions have a great impact on it.

1. temperature

The matrix of the liquid sealant is mostly high molecular polymers, and these polymers have different operating temperature ranges, so they must be selected during use. The heat resistance of a liquid sealant can be expressed by the thermal decomposition temperature or softening temperature of the glue. The heat resistance of the sealant depends on the heat resistance of the polymer. The heat resistance of the polymer is high. high.

2. Drying time

Most general-purpose liquid sealants contain solvents. Whether the solvent is completely volatilized or not has a great impact on its sealing performance. After the glue is applied to the joint surface, too late or premature connection and tightening will reduce its sealing performance. Generally speaking, for dry peelable sealants, it should be allowed to dry for 2 to 3 minutes before the connection is tightened. For non-dry adhesive type and semi-dry viscoelastic type containing solvent, it takes 3 to 6 minutes. Both silicone and anaerobic liquid sealants do not contain solvents and do not require drying time. However, when silicone sealants are cured, they must absorb a small amount of water in the air to achieve sealing. This process is called silicone sealants. This is a "hygroscopic" process.

3.Tightening torque

When the joint surface is fastened, the greater the tightening force, the smaller the joint surface gap, the worse the fluidity of the glue in the gap, and the greater the glue's adhesion to the metal, the more pressure resistant the glue is. high.

4. Joint surface shape

The shape, width, and number of bolts used on the joint surface also have a great impact on the sealing performance of the sealant. The following points can be summarized:

(1) If the area of the flange joint surface is constant, the smaller the inner diameter of the flange, the higher the pressure resistance of the liquid sealant.

(2) If the inner diameter of the flange joint surface and the pressure per unit area are constant, the wider the flange joint surface, the higher the pressure resistance of the glue.

(3) The larger the pressure per unit area of any flange, the greater the tightening torque, the higher the pressure resistance of the glue.

(4) If the inside and outside diameters of the flange joint surface change at the same time, the pressure per unit area of the joint surface remains constant. When the change in the outer diameter area of the flange joint surface is greater than the change in the inner diameter area, the higher the pressure resistance of the glue.

(5) The greater the adhesive force of the sealant, the higher the pressure resistance of the adhesive.

Liquid Sealant-5. Surface Roughness

If the connection surface is flat, the lower the surface roughness, the higher the pressure resistance of the glue. However, the surface roughness is too low, the glue is easy to deform and flow, and the glue is easy to be squeezed out under pressure, which is not conducive to sealing, but the pressure resistance is reduced. Therefore, the surface roughness is preferably Ra 6.3 to Ra 0.63. This also reduces processing costs.

6. Unevenness

If the joint itself is not flat, the thickness of the glue between the joints will be different after the connection is fastened. The frictional resistance encountered when the glue at a larger thickness flows is small and easily deformed. Under the action of the medium pressure, it is easy to produce Leaks and pressure resistance of the glue decrease.

7. Holding time and pressure resistance

The liquid sealant can withstand higher pressure in a short period of time during use. With the extension of the holding time, the pressure-resistant sealant can be used. The force has decreased, but the pressure resistance has stabilized after 2 hours.

8. Use of liquid sealant and solid gasket

Under normal circumstances, the joint surface gap is less than 0.1mm, the general-purpose liquid sealant can be used alone; when the gap exceeds 0.1mm or the working conditions are harsh, the general-purpose liquid sealant can be used in combination with a solid gasket, and its sealing effect better. When the two are used in combination, the solid gasket mainly prevents leakage of the joint surface due to an excessively large gap, and the liquid sealant plays a role of filling the joint surface with depression.

9. Medium resistance

When selecting a liquid sealant, it is necessary to consider its stability to the contact medium. This stability can be measured by the weight change rate. Generally, the weight change rate is less than ± 10% can be considered stable.