Introduction and hidden dangers and misunderstandings of silicone sealants for construction

time:2020-11-03 17:48:00source:http://en.scknd.com/news808250.html

1. Introduction of silicone sealant

A. Characteristics of silicone sealant

Silicone sealant refers to the sealant produced with linear polysiloxane as the main raw material, also called silicone sealant.

The polymer main chain of the silicone sealant is mainly composed of silicon-oxygen-silicon bonds. During the curing process, the crosslinking agent reacts with the base polymer to form a networked Si-Q-Si skeleton structure. Compared with other organic sealants composed of macromolecules (such as polyurethane sealants, acrylic sealants, polysulfide sealants, etc.), the most notable feature is excellent high and low temperature resistance and weather resistance.

It can be seen from the above table that the Si-0-Si bond energy is very high, which is not only much larger than that of other polymers (other chemical bond energy is lower than 300nrn UV bond energy), but also Si-0-Si bond energy is higher than that of other polymers. Ultraviolet bond energy, which is the fundamental reason why architectural silicone sealants still maintain good performance under ultraviolet radiation, while other organic polymer materials are prone to aging. Therefore, the silicone sealant for construction has excellent weather resistance.

B. Classification of silicone sealants for construction

Most of the silicone sealants for construction use a room temperature vulcanization (RTV) condensation curing system, that is, no heating is required during curing, and it can be cured at room temperature, and some small molecular substances will be released during the curing process.

1. Classification according to curing system

According to the types of small molecules released during curing, silicone sealants for building construction can be mainly divided into deacetic acid type, deketone depletion type, dealcohol type, dephthalamide type, etc.

2. Classification according to component scores

According to the number of components of the product, silicone sealants for construction can usually be divided into two categories: one-component and two-component. One-component silicone building sealants are generally cured by reacting with moisture in the air. The curing process is gradually carried out from the surface to the deep layer. Therefore, the deep curing speed is relatively slow, and the construction depth, width, and ambient temperature. , environmental humidity, etc. have certain requirements, which are greatly affected by environmental humidity. Under normal circumstances, it takes 5--7 days to have a certain strength, and it takes 7-2l days to achieve the ultimate effect. Single-component is generally suitable for construction on site due to its ease of use and other reasons.

Two-component silicone sealant for building construction has two components, A and B. When using, you need to mix the two components evenly, and then inject the glue into the glue part within a certain period of time, and mix it for a certain period of time to seal The glue will harden and cannot be used. Two-component generally takes 2 to s days to reach strength. Two-component adhesive is generally suitable for sealing of insulating glass in door and window factories because of its fast curing speed and deep curing.

2. Hidden dangers in the production process of silicone sealant for construction

Nowadays, due to the disorderly development of the domestic market leading to vicious competition in the market, and due to rising raw materials and increasingly fierce competition, many companies and even well-known companies, in order to pursue profits, are racking their brains to reduce product costs (cost reduction is mainly based on 1) Addition Plasticizers are used to change the appearance brightness and increase the selling point. 2) Add additives to add more fillers and reduce costs. 3) Using recycled raw materials such as base rubber, silicone oil, etc.) to impact the market at low prices. As a result, at least 80% of the construction silicone sealant products on the market today are more or less added with a plasticizer-white oil.

White oil, namely liquid paraffin oil, is a colorless, odorless, tasteless, non-fluorescent transparent oily liquid obtained from petroleum lubricating oil fractions by high-pressure hydrotreating, which is flammable and volatile;

The physical properties of white oil are added to the silicone sealant for building construction:

Feature 1: It can reduce the amount of silicone base polymer, reduce product cost, and improve extrudability, softness and brightness.

Hazards - slowing down of curing speed, oil seepage pollution, easy wrinkling on the surface, sticky dust accumulation, and gradually low mechanical properties with the continuous precipitation of white oil, and the glue joints will become hard, shrink, debond and crack in half a year. It will bring serious hidden danger to the project quality.

3. What should be paid attention to when purchasing silicone sealant for construction?

1) Identify the brand. Valid registered trademarks, distinctive image recognition, reasonable price positioning, and perfect after-sales service are the identification standards for brand products.

2) Look at the packaging. Look at the carton for product name, factory name, specification, origin, color, date of manufacture, and whether there is a certificate of conformity, quality assurance certificate, and product inspection report in the carton; Clear and complete; thirdly, to see if the net content is accurate, the manufacturer must indicate the specification, model and net content on the bottle.

3) Test glue. First, smell the smell, secondly compare the gloss, thirdly check the particles, fourthly check the bubbles, fifthly check the curing effect, and sixthly test the tensile force and slack.

4. Misunderstandings in the selection and use of silicone sealants for construction

1) The more expensive the silicone sealant for construction, the better?

Many consumers will believe in the phrase "one price, one product", and think that the more expensive things are, the better. In fact, this is not the case; the price positioning of silicone sealants for construction mainly depends on its use, which can be basically divided into structural adhesives, weather-resistant adhesives and neutral adhesives. Prices are basically in this order. Structural adhesives generally have higher requirements for strength and toughness; weather-resistant adhesives have higher requirements for high and low temperature resistance, UV resistance, corrosion resistance, and other properties; while neutral adhesives mainly require gloss and elasticity. Therefore, if it is tested by the standard of weather-resistant adhesive, it is likely to be unqualified. (Note: The state stipulates that other types of building silicone sealants other than structural adhesives cannot be used in structural assemblies) Therefore, it is very important for us to choose the appropriate type of adhesive according to practical needs when choosing silicone adhesive products.

2) What is the curing method of silicone sealant for construction?

Many consumers are unaware of how building silicone sealants cure. The silicone sealant for construction is basically divided into one-component and two-component according to the curing method;

One-component is to absorb the moisture in the air and react with the cross-linking agent to connect the base polymer into a three-dimensional skeleton. Since it is to absorb moisture and cure, it is a process from the surface to the inside, from the shallow to the deep, and the deeper it goes. The more difficult it is to absorb moisture, the longer the curing time will be. It is recommended that the sizing depth should not exceed 5cm

Two-component is a mixture of two components, which does not depend on other substances to cure, and can be deeply cured.

3) Environmental requirements for the use of silicone sealants for construction?

Due to the curing method of silicone sealant for construction, the following usage restrictions must be observed:

.It is not suitable for the surface of building materials that seep out grease, plasticizers or other solvents;

.Not suitable for frosted or damp surfaces, airtight or long-term water immersion occasions;

.It is not suitable for use on the surface of paint, it may cause sealing failure due to cracking or peeling of the paint film;

.It is not suitable for use on surfaces that will directly contact food or places that are prone to mechanical wear;

.It is not suitable for construction at noon or when the surface temperature of building materials is lower than 4°C or higher than 40°C;

.Non-structural adhesives are forbidden to be used in structural assembly;

Various glues have their own relative usage restrictions such as:

Acid glue: Corrosive to copper, brass and other metals, not suitable for all kinds of stone materials and iron carbide matrix materials;

Dealcoholization glue: It is not suitable for construction at noon or direct sunlight, and it is not suitable for fluorocarbon materials;

Degumming: It has trace corrosion to copper, brass, silver and other metals, so it is not suitable for mirror industry.

4) The faster the curing speed of silicone sealant for construction, the better?

In order to pursue the construction period or construction speed, many builders blindly require silicone rubber manufacturers to speed up curing. In fact, this approach is very wrong. The reaction of the crosslinking agent with the base polymer to form a networked Si-O-Si skeleton structure is a complex process. If the reaction time is too short, the skeleton structure may be too small, resulting in a weak and brittle adhesive strength after curing; at the same time, it will also cause the colloid to not completely wet the substrate, resulting in a decrease in the bonding strength with the substrate; it may also be caused by curing. If the speed is too fast, the small molecular gas generated during the curing process cannot volatilize and aggregate from the uncured colloid in time, resulting in bubbles that make the surface of the sizing surface uneven and affect the appearance; (this kind of situation is more common in outdoor construction in summer), if blindly If the requirements are fast, the quality of the project will inevitably be affected. Of course, too slow curing speed can also lead to problems such as contamination of the stone. Therefore, it is recommended that the general surface drying time should be about 20-50min at room temperature.

5) Can the strength of the silicone sealant for construction reach the highest level in 1-2 days after curing?

We often see that many consumers and even construction parties will take the newly cured glue to test its performance, and some will compare it with the glue that has been cured for a few days; in fact, it is very unscientific. We already know that the cross-linking of architectural silicone sealants into a skeleton structure is a complex and lengthy process. Therefore, the construction silicone sealant basically means that it can only reach 60% of the strength after 24 hours of curing, and can reach 75% to 80% of the strength after 72 hours. Sexual glue is longer.

6) Why is acid glue easy to fall off when used on cement and stone?

The curing of acid glue is to produce acetic acid, which will react with the surface of alkaline materials such as cement and stone to form a chalky substance, which will cause shedding.

7) Why is the surface wrinkled after the neutral glue is cured?

The expansion coefficient of the substrate is large, and there is displacement or vibration; the glue is too thin during construction; the catalyst activity is too large, the shrinkage is strong, and the plasticizer is too much.

8) Is the yellowing of neutral transparent glue the reason?

The amine group in the cross-linking agent and plasticizer, the amine group is very easy to cause yellowing; for example, when neutral glue and acid glue are used at the same time, it may cause the neutral glue to turn yellow; it may also be caused by the long storage time of the glue .

9) What is the foaming phenomenon after dealcoholization and sizing?

High temperature sizing, high temperature curing

(1) The curing speed is too fast, the release speed of methanol is greater than the speed of methanol seepage from the colloid, so that the methanol will gasify and generate bubbles after accumulation, and sizing under direct sunlight should be avoided (summer); , Noon curing.

(2) The sizing is too thin, it should be greater than 6mm

(3) Foam rod (strip): Destruction causes foaming (open-cell foam strip); there is moisture; foam strip material: incompatible, chemical reaction occurs

(4) The surface dries too fast and the glue cures too fast

(5) The sizing is not regular, and it is repeatedly scraped

(6) Why do some high-grade silicone glue and some aluminum-plastic panels, stainless steel, aluminum alloys, etc. have poor bonding effect, but low-grade glue sticks more firmly?

Because the structural systems of the two are different, high-grade adhesives are generally 100% silicone adhesives. The material itself is very hard, lacks infiltration, and has poor compatibility with general aluminum-plastic panels, stainless steel, and aluminum alloys. In addition, its curing speed is not good. It is relatively fast, while the surface of materials such as aluminum-plastic panels is smooth and smooth, and the colloid has solidified before it has completely penetrated the surface of the substrate, so the bonding is not strong. The low-grade glue has a slower curing speed and a longer penetration time. In addition, other materials such as plasticizers are added to make it soft, so the low-grade glue is easily compatible with the substrate and has good adhesion. Therefore, we repeatedly stress that customers should conduct self-tests before use to observe whether the materials meet the requirements of use.

7) Is aluminum more difficult to bond in winter than in summer?

In summer, the aluminum surface is relatively dry due to heat absorption, so the building silicone sealant can quickly infiltrate the substrate to achieve a better bonding effect after sizing. However, in winter, due to the difference in specific heat between aluminum and air, when the temperature changes (especially when the aluminum is moved from a place with a low temperature to a place with a high temperature), or because of hand touch, dust, etc. Forms a layer that is imperceptible to the naked eye. Due to the existence of this film layer, a membrane appears between the silicone adhesive and the substrate. The silicone adhesive cannot wet the surface of the substrate, which greatly reduces the adhesiveness of the silicone adhesive, and even leads to the failure of the adhesive.

It is recommended that manufacturers put the substrate in the drying room for a certain period of time (such as 24h) before construction in winter. At this time, the temperature of the substrate is similar to the indoor temperature, and the temperature of the contact surface between the sealant and the substrate surface after sizing is also high, which is beneficial to The coupling agent in the sealant reacts with the groups on the substrate to increase the bond strength of the sealant.

To sum up, the quality problems caused by consumers' choice of glue or improper use are not uncommon. With the country's vigorous promotion of urbanization, silicone sealants for construction will be more widely used. The correct selection and use of construction silicone sealants must attract our attention.

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