Structural adhesive is a kind of adhesive with high strength, high load, aging resistance, fatigue resistance and corrosion resistance. It has stable performance in the expected life. It is suitable for the adhesive which can bear the bond of structural parts. It is mainly used for bonding between the same materials such as metal, ceramics, plastics, rubber, wood and other materials, and can partly replace the traditional connection forms such as welding, riveting, bolt connection, etc. Silicone structural sealant is a key material used in the glass curtain wall of full or semi hidden frame. Through connecting the plate and metal frame, it bears wind load and glass self weight load, which directly affects the durability and safety of the curtain wall structure of the building, and is one of the key links of the safety of glass curtain wall. It is a structural sealant with linear polysiloxane as the main raw material. During the curing process, the crosslinker reacts with the base polymer to form elastic material with three-dimensional mesh structure. The Si-O bond energy in the molecular structure of silicone gum is larger in common chemical bonds (specific physical and chemical properties of Si-O: bond length 0.164 ± 0.003nm, thermal dissociation energy 460.5j/mol). Compared with other sealants (such as polyurethane, acrylic acid, polysulfide sealant, etc.), UV light and atmospheric aging resistance are stronger, which can keep the deformation resistance of 30 years and not deteriorate in various weather environments, and have ± 50% deformation displacement resistance in a wide range of temperature.

However, with the increase of silicone structural sealant, there are various problems in practical application, such as: the particle agglomeration and pulverization of component B, segregation and stratification of component B, the phenomenon of pressing plate not going down or turning over the rubber, slow glue out speed of the beater, particles in butterfly gel, too fast or too slow surface drying and breaking time, peeling or vulcanization of colloid During the process of glue beating, there are "flower glue", the colloid can not be cured normally, the hand is stuck after curing for several days, the hardness is abnormal after curing, the needle shaped pores are found on the bonding surface of the base material, the air bubbles are sandwiched in the adhesive, the adhesive is not good with the base material, and incompatible with the accessories, etc. Next, we 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.

Analysis of common problems in structural adhesive service

2.1 particle agglomeration and pulverization of component B

If the particle agglomeration and pulverization of component B occurs, there are two reasons: one is that the phenomenon has occurred in the upper layer before use, which is due to the poor packing and sealing, and the crosslinking agent or coupling agent in component B are active compounds, which is easy to react with water and gas in the air, and the batch shall be returned to the manufacturer. Second, the machine stops during use and the particle agglomeration occurs when starting again. It indicates that the sealing between the pressure plate and the rubber material of the glue beater is not good, and the equipment side should be contacted to solve the problem.

2.2 the glue out speed of the glue beater is slow

During the initial use of the product, the glue speed is too slow during the process of gluing machine, which may be caused by three reasons: 1) the fluidity of component A is poor, the pressure plate is too large, and the pressure of gas source is not enough. When the reason (1) or reason (3) is determined, we can solve the problem by adjusting the pressure of the glue gun; when it is determined to be the second reason, ordering matching caliber barrel can solve the problem. If the glue output speed is slow during normal use, the mixing core and filter screen may be blocked. Once found, the equipment needs to be cleaned in time.

2.3 the breaking time is too fast or too slow

The breaking time of structural adhesive refers to the time when the colloid is mixed from paste to elastomer, and it is generally tested every 5 minutes. There are three factors influencing the dry curing of the adhesive surface: 1. The proportion of components a and B; 2) temperature and humidity (the influence of temperature is the main); 3) the formulation of the product itself has defects.

The solution to the reason (1) is to adjust the proportion of proportion and increase the proportion of component B to shorten the curing time and make the rubber layer hard and brittle; and reducing the proportion of curing agent will prolong the curing time, and the rubber layer will be soft, the toughness will be enhanced and the strength will be reduced. Generally, the volume ratio range of component a:b can be adjusted between (9-13:1). If the proportion of component B is high, the reaction speed is fast and the pulling time is short. If the reaction is too fast, the time of finishing and stopping gun will be affected. If the reaction is too slow, the gel drying time will be affected. Generally, the breaking time is adjusted between 20 and 60 minutes. The colloid performance is basically the same after curing in this proportion range. In addition, when the construction temperature is too high or too low, we can reduce or increase the proportion of component B (curing agent) appropriately, so as to adjust the drying and curing time of colloid surface. If the product itself is a problem, it needs to be replaced.

2.4 "glue" appears during the process of sizing

The gum is produced by the uneven mixing of a/b component colloids, which shows that there are white stripes in the local area. The main reasons are: (1) the pipe of component B of the glue beater is blocked; ⑵ the static mixer has not been cleaned for a long time; 3) the scale is loose and the glue output speed is not uniform; (4) the process parameters of the equipment are not adjusted by changing the manufacturer or brand; for the reasons (1) and (2), the equipment can be cleaned; for the reason (3), the proportion controller shall be checked and adjusted appropriately.

2.5 during the process of sizing, the colloid appears to be crusted or vulcanized

When the two-component glue is cured locally during the mixing process, the glue shot by the glue gun will appear skin or vulcanization. When the curing and discharging speed are normal, and the rubber still has skin or vulcanization, it may be that the equipment will be shut down for a long time, the glue gun is not cleaned or the gun is not thoroughly washed, and the construction shall be carried out after the skin or vulcanization glue is washed.

2.6 bubbles are sandwiched in the adhesive

Generally speaking, the colloid itself is free of bubbles. Some bubbles in the colloid clamp are likely to be mixed with air during transportation or construction, such as: (1) exhaust gas is not discharged when replacing rubber barrel; ⑵ component is not pressed down after the pressure plate is put on the machine, which leads to incomplete bubble discharge. Therefore, in the use of front bubble to be thorough, in the use process should be correctly operated rubber machine, to ensure that the seal to prevent air from entering. If you suspect that the product itself has bubbles, you can judge it by butterfly test.

2.7 poor bonding with base material

Sealant is not a universal adhesive, so it can not guarantee good bonding with all the base materials in practical application. With the diversification of surface treatment and new technology, the bonding speed and effect of sealant and substrate are different.

There are three types of interface failure between structural adhesive and substrate, one is cohesion failure, namely cohesion > cohesion; the other is bond failure, that is, cohesion < cohesion, and the third is that both failure modes are qualified, and the failure area of bond is less than or equal to 20%, and the failure area of adhesive is unqualified when the area of bond failure exceeds 20%; when the bonding failure area is more than 20%, it is not expected to appear in practical application. There are six reasons for the non adhesion of structural adhesive and base material:

(1) the substrate used is difficult to bond, such as PP and PE, because of its high crystallinity and low surface tension, it is unable to form the diffusion and entanglement of molecular chain with most substances, so it is unable to form strong adhesion at the interface;

(2) the product has a narrow bonding range, which can only play a role in some base materials;

(3) the maintenance time is not enough. Generally, after the action of two component structural adhesive, it is necessary to maintain for at least 3 days, while the single component shall be maintained for 7 days. If the temperature and humidity of the maintenance environment is low, the curing time shall be extended.

(4) the proportion of components a and B is not correct. When using dual component products, users must strictly mix the proportion of base adhesive and curing agent according to the proportion required by the manufacturer, otherwise, problems may occur in the early curing or problems in the adhesion, weather resistance and durability in the later stage of use;

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

(6) the primer is not applied as required. The bottom coating is used to pre-treatment the surface of aluminum profile, which can reduce the bonding time and improve the water resistance and durability of the bonding. Therefore, in practical engineering application, we should use the primer correctly, and strictly avoid degumming caused by improper use method.

2.8 incompatibility with accessories

The reason of incompatibility with accessories is that the physical or chemical reaction occurs between the sealant and the accessories, which may cause the color change of structural adhesive, non adhesion with the base material, the decrease of structural adhesive, and the shorter the service life of structural adhesive.

3 conclusion

Silicone structural adhesive has high strength, high stability, excellent aging resistance and high temperature resistance, and is widely used in structural bonding of building curtain wall. However, in practical application, due to human factors and the problems of the selected substrate (construction specifications cannot be strictly observed), it has a great impact on the performance of structural adhesive, even makes it fail. Therefore, before construction, compatibility test and adhesion test of glass, aluminum material and accessories shall be checked. During construction, it shall be carried out strictly according to the requirements of each link, so as to realize the effect of structural adhesive and ensure the project quality.