Analysis of Common Problems in the Use of Structural Adhesive

1 Introduction

Structural adhesive is high strength, can withstand large loads, and aging resistance, fatigue resistance, corrosion resistance, stable performance in the expected life, suitable for structural bonding adhesive. It is mainly used for bonding between metal, ceramic, plastic, rubber, wood and other materials or different materials, and can partially replace traditional connection forms such as welding, riveting, bolt connection, etc. Silicone structural sealant is the key material used in the full or semi-hidden frame glass curtain wall. By connecting the plate and the metal frame, it can bear the wind load and the self-weight load of the glass, which is directly related to the durability and safety of the building curtain wall structure, and is one of the key links of the safety of the glass curtain wall. It is a structural sealant with linear polysiloxane as the main raw material. In the curing process, the crosslinking agent reacts with the base polymer to form an elastic material with a three-dimensional network structure.

Due to the Si-O bond energy in the molecular structure of silicone gum, the bond energy in common chemical bonds is relatively large (Si-O specific physical and chemical properties: bond length 0.164±0.003nm, thermal dissociation energy 460.5J/mol. Significantly higher than C- O 358J/mol,C-C than 304J/mol,Si-C318.2J/mol). Compared with other sealants (such as polyurethane, acrylic acid, polysulfide sealants, etc.), it has strong ultraviolet light resistance and atmospheric aging resistance, can maintain 30 years in various weather environments without cracking or deterioration, and has 50% deformation displacement resistance in a wide temperature range.

However, with the increase in the use of silicone structural sealant, there will be a variety of problems in practical applications, such: component B has the phenomenon of particle agglomeration and pulverization, component B has the phenomenon of segregation and delamination, the pressure plate cannot be pressed down or turned over, the glue extraction speed of the glue machine is slow, the colloid of butterfly sheet has particles, the surface dry pull-off time is too fast or too slow, the colloid appears crust or vulcanization, the glue cannot be cured normally during the gluing process, the glue cannot be cured normally, the glue sticks after a few days, the hardness after curing is abnormal, and the substrate needle-like pores, bubbles in the glue, poor bonding with the substrate, incompatible with accessories, etc. Below, Lu's Chemical will analyze the possible causes of several common problems in the process of structural adhesive service, and give corresponding solutions, hoping to provide reference for the analysis of practical problems.

2 Analysis of common problems of structural adhesive service

2.1 component B has particle agglomeration and pulverization

If the particle agglomeration and pulverization phenomenon occurs in component B, there are two reasons: first, this phenomenon has occurred in the upper layer before use, which is due to poor packaging sealing, and the crosslinking agent or coupling agent in component B is an active compound, which is easy to react with moisture in the air, and the batch should be returned to the manufacturer. Two is in the use of the process of shutdown, again when the machine appears particle agglomeration pulverization phenomenon, indicating that the pressure plate of the glue machine and glue sealing is not good, should contact with the equipment to solve the problem.

2.2 glue machine glue speed is slow

When the product is used for the first time, the gluing speed of the gluing machine is too slow during the gluing process. There are three possible reasons: ⑴ the poor fluidity of component A, ⑵ the pressure plate is too large, and ⑶ the pressure of the air source is not enough. When it is determined that it is the reason (1) or the reason (3), we can solve it by adjusting the pressure of the glue gun. When it is determined that it is the second reason, ordering a barrel of matching caliber can solve the problem. In the normal use process, if the glue speed slows down, it may be that the mixing core and the filter screen are blocked. Once found, the equipment needs to be cleaned in time.

2.3 pull-off time is too fast or too slow

The breaking time of the structural adhesive refers to the time when the colloid is mixed from the paste to the elastomer, and is generally tested every 5 minutes. There are three factors that affect the dry curing of the rubber surface: ⑴ the influence of the proportion of components A and B, etc.; ⑵ temperature and humidity (of which the influence of temperature is the main one); (3) The formula of the product itself is defective.

The solution for the reason (1) is to adjust the ratio, increase the proportion of B component can shorten the curing time, the adhesive layer becomes hard and brittle; and reduce the proportion of curing agent, will prolong the curing time, the adhesive layer becomes soft, toughness and strength is reduced. Generally, the volume ratio of component A:B can be adjusted between (9~13:1). If the ratio of component B is high, the reaction speed is fast, and the breaking time is short. If the reaction is too fast, the trimming and stopping time will be affected. If the reaction is too slow, the time for the colloid to dry completely will be affected. The breaking time is generally adjusted between 20~60 minutes, and the colloid performance after curing is basically the ratio range. In addition, when the construction temperature is too high or too low, we can appropriately reduce or increase the proportion of B component (curing agent), so as to achieve the purpose of adjusting the surface dry and curing time of the colloid. If the product itself, you need to replace the product.

"Flower glue" appears in the process of 2.4 gluing"

The flower gum is produced by uneven mixing of A/B component colloids, showing local white stripes. The main reasons are: (1) the pipeline of component B of the gluing machine is blocked; ⑵ The static mixer has not been cleaned for a long time; (3) The scale is loose and the glue discharge speed is uneven; (4) The process parameters of the equipment have not been adjusted by changing the manufacturer or brand. According to the reasons (1) and (2), it can be solved by cleaning the equipment. For reason (3), it is necessary to check the proportional controller and make appropriate adjustments.

Colloid crust or vulcanization during 2.5 gluing

When the two-component glue is partially cured during the mixing process, the glue produced by the glue gun will crust or vulcanize. When there is no abnormality in the curing and glue discharging speed, and the glue still has crust or vulcanization, it may be that the equipment has been shut down for a long time, the glue gun has not been cleaned or the gun has not been washed thoroughly, and the crust or vulcanized glue needs to be washed clean before construction.

Bubbles in the 2.6 adhesive

Generally speaking, the colloid itself has no air bubbles, and the air bubbles contained in the colloid are most likely mixed with air during transportation or construction, for example: (1) the exhaust gas is not exhausted when the rubber barrel is replaced; ⑵ The pressure plate of the components is not pressed down after the machine is used, resulting in incomplete bubble discharge. Therefore, in the use of the front bubble to be thorough, the use of the process should be the correct operation of the glue machine, to ensure that the seal to prevent air from entering.

Poor bonding between 2.7 and substrate

The sealant is not a universal glue, so it cannot be guaranteed to bond well with all substrates in practical applications. With the diversification of the surface treatment methods and new processes of the substrate, the bonding speed and bonding effect of the sealant and the substrate are also different.

There are three forms of bond interface damage between structural adhesive and substrate, one is cohesive damage, I .e. cohesive force> cohesive force; The second is bond damage, I .e. cohesive force

The use of the substrate itself is difficult to bond, such as PP, PE, because of its high molecular crystallinity, low surface tension, can not form a molecular chain diffusion and entanglement with most substances, and thus can not form a strong adhesion at the interface;

(2) The product has a narrow bonding range and can only work on some substrates;

(3) The curing time is not enough. Usually two-component structural adhesive action, at least 3 days of conservation, and a single component is to be maintained for 7 days, if the temperature and humidity of the conservation environment is low, you need to extend the curing time.

(4) The proportion of A and B components is not correct. Users in the use of two-component products, must be strictly in accordance with the proportion of the manufacturer's requirements of the proportion of base glue and curing agent, otherwise there may be problems in the early curing, or the use of late in the adhesion, weather resistance and durability problems;

(5) The substrate is not cleaned as required. Because the dust, dirt and impurities on the surface of the substrate will hinder the bonding, it should be strictly cleaned before use to ensure good bonding between the structural adhesive and the substrate.

(6) The primer is not applied as required. Pretreatment with primer on the surface of aluminum profiles can shorten the bonding time while improving the water resistance and durability of the bond [3]. Therefore, in practical engineering applications, we must use the primer correctly and strictly avoid degumming caused by improper use [4].

2.8 incompatible with accessories

The reason for the incompatibility with the accessories is that the sealant has a physical or chemical reaction with the accessories in contact with each other, resulting in the harm of discoloration of the structural adhesive, non-stick with the substrate, reduced performance of the structural adhesive, and shorter service life of the structural adhesive.

3 Conclusions

Silicone structural adhesive has high strength, high stability, excellent aging resistance, high temperature resistance and other excellent performance, widely used in building curtain wall structure bonding. However, in practical applications, due to human factors and the problems of the selected substrate (the construction specifications cannot be strictly observed), the performance of the structural adhesive is greatly affected, and even it fails. Therefore, the compatibility test and adhesion test of glass, aluminum and accessories should be checked before construction, and the construction process should be carried out in strict accordance with the requirements of each link, so as to achieve the effect of structural adhesive and ensure the quality of the project.

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