Knowledge sharing on construction methods of automotive adhesives

1,、 Dip coating method

The dip coating method, also known as the thin flow method, uses equipment consisting of a controlled flow glue funnel, a glue tank for recovering glue, and a glue circulation pump. In order to prevent the intrusion of external dust and solvent evaporation, the entire equipment is enclosed in a specially designed small chamber and can maintain a constant temperature. During work, a gap that can precisely control the amount of adhesive is used to adjust the size of the gap to control the flow of adhesive and obtain the desired coating thickness. In addition, the thickness of the coating can be adjusted by the speed at which the substrate moves. This method requires a relatively high level of adhesive, as the adhesive does not contain any impurity particles. The viscosity must be uniform and consistent. This method cannot be used for adhesives with particularly high viscosity, and is mainly used for resin adhesives.
2、 Spray method

Spray coating is a method of applying glue by using a spray gun as a tool, powered by compressed air, to spray the glue mixture into a mist and evenly coat it on the substrate. When compressed air is sprayed through the nozzle during spraying, due to the extremely fast airflow, the air pressure at the nozzle drops significantly, causing the adhesive in the glue storage tank to be pressed out of the nozzle. When the adhesive encounters the high-speed compressed air airflow, it forms a mist and is sprayed onto the substrate at a certain speed, forming a uniform coating. The commonly used compressed air pressure during operation is 0.4 to 0At a pressure of. 5MPa, the distance between the nozzle array is generally around 25cm, which can result in a spraying area of 13-14cm2. Generally speaking, the greater the distance between the nozzle and the substrate, the larger the spraying area. However, as the distance from the nozzle to the screen increases, the kinetic energy of the adhesive droplets when they reach the substrate is already very small, which affects the quality of spraying. The control of coating thickness depends on the speed at which the substrate moves in the spraying area and the number of spraying repetitions. The uniformity of the spray coating depends entirely on the operator's experience under hand conditions.
The advantages of spraying method are:.
① A coating that is not affected by uneven thickness of the substrate and can be uniformly applied.
② The device is relatively simple, easy to operate, and not subject to any restrictions
③ It can be used for spraying base and composite adhesives, as well as for coating adhesive mixtures.
The disadvantages of spraying method are:.
① The utilization rate of adhesives is relatively low, and many adhesives are diffused into the air.
② Strict ventilation and dust prevention measures must be taken for severe air pollution.
③ Most of them are operated manually, so the efficiency is low and the quality difference is relatively large.
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3,、 Brushing method

The so-called brush coating method refers to the uniform application of adhesive onto the surface of the substrate through a brush (or artificial brush) on a coating roller, and achieving a certain thickness requirement. The most commonly used coating method in production is brush coating method.
The advantages of brushing method:.
① It has a wide range of applications, including animal glue, resin glue, base glue, and composite glue. It has a wide viscosity range for adhesives and can meet different requirements.
② The pressure of adhesive coating is relatively high, which strengthens the adhesion between the adhesive and the substrate. Especially in the process of laminating, due to the high pressure of adhesive coating, the adhesive can be fully coated in the gaps of the component, increasing the adhesion to the substrate.
③ The adhesive surface faces the operator, making the operation relatively convenient. Defects on the adhesive surface are easy to detect, and there are fewer surface defects on the coating.
④ The structure of the device is relatively simple and the thickness adjustment is convenient.
Disadvantages of brushing method:.
When the glue application speed is fast, it is often easy to produce longitudinal patterns on the glue surface. To overcome this drawback, natural leveling, uniform roller gluing, and brush gluing are often used to solve the problem.
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4,、 Scraping method

The so-called scraping method is a method of applying adhesive to the surface of a substrate through a scraper and its auxiliary adhesive roller. This method is only applicable for the application of primer. The coating thickness in the scraping method is controlled by the gap between the scraper and the substrate or by the pressure exerted by the scraper on the substrate. The equipment for the scraping method consists of a scraper, steel roller, adhesive groove, and baffle. The scraper and the adhesive groove are connected together and can move back and forth in the horizontal direction to adjust the gap between the scraper and the steel roller, in order to control the thickness of the coating. The precision of the steel roller is high, and the thickness and uniformity of the coating depend on the ellipticity, taper, and flatness of the scraper. Adjust the gap between the scraper and the substrate surface during work, that is, the coating thickness, and block the baffle. The adhesive can be poured onto the baffle, and the substrate moves clockwise.
The advantage of this method is that its thickness control is relatively accurate and the uniformity of the coating is relatively high. But its disadvantage is that the pressure of the scraper on the adhesive during scraping is very small, which reduces the adhesion ability of the adhesive to the substrate. This method is only suitable for synthetic resin adhesives with good penetration ability to the substrate. The thickness of the adhesive layer is expressed in grams of adhesive per square meter of area. Ensuring the thickness of the adhesive layer is to ensure sufficient adhesion between the components and prevent them from falling off during use. Therefore, it is necessary to determine the coating thickness correctly in order to ensure that the components have strong adhesive properties.
The main factors affecting the thickness of the adhesive layer are:.
① The roughness of the surface of the adhesive material.
  & nbsp; & nbsp;& nbsp; Appropriate roughness can increase the bonding contact area and improve the bonding strength. Excessive roughness can lead to poor interface contact, making it difficult for the adhesive to wet and penetrate, resulting in a lack of adhesive or a thick adhesive layer, which can reduce the bonding strength; Low roughness can also cause poor adhesive infiltration, thin adhesive layer, and decreased bonding strength.
② The quality of the adhesive is good, and the thickness of the adhesive layer can be reduced; On the contrary, the thickness needs to be increased.
③ Different adhesive materials have varying adhesive layer thicknesses.
④ When applying adhesive, the substrate runs at a high speed and the adhesive layer thickness is also large; The degree of tension affects the thickness of the adhesive layer in the scraping method, and a high tension results in a thin adhesive layer.
⑤ The adhesive has a high viscosity and a thick adhesive layer.
Generally, for epoxy resin adhesives, the thickness of the adhesive layer should be ensured to be between 0.1 and OWhen it is 1.5mm, the bonding strength is the highest. If the adhesive layer is too thin, it is easy to produce defects during the curing process. If the adhesive layer is too thick and the cohesive strength of the adhesive is lower than that of the bonded material, it will also reduce the adhesive strength. In order to ensure the thickness of the adhesive layer, the usual method is to add a trace amount of other types of adhesive, such as box sand, which can also maintain the appropriate adhesive layer thickness and adhesive strength. The curing conditions not only affect the initial strength of the adhesive, but also have an impact on the thermal aging performance of the adhesive. After the adhesive is bonded, in order to obtain a uniform adhesive layer thickness and prevent the adhesive layer from shifting, pressure curing is often required. Pressurization is beneficial for removing solvents or low molecular weight by-products from the adhesive layer, and can also prevent poor contact between the adhered objects caused by volume shrinkage of the adhesive. Appropriate curing pressure can ensure close contact between the adhesive and the surface of the adhered material, and ensure the appropriate thickness of the adhesive layer. Excessive assimilation pressure often leads to a thin adhesive layer and causes a lack of adhesive; However, if the curing pressure is too low, volatile small molecules in the adhesive will also remain in the adhesive layer, causing the adhesive layer to be too thick and reducing the bonding strength.
To ensure adhesive performance, it is necessary to heat to a certain temperature during curing. Appropriate curing temperature can fully crosslink and stretch the molecular segments in the adhesive, and evaporate the small molecules generated during the curing process. This not only achieves good shear strength, but more importantly, good peel strength. Low curing temperature often makes it difficult for small molecules in the adhesive to evaporate and even form defects, greatly reducing the bonding strength; At the same time, if the temperature is too low, the molecular segments of the adhesive cannot fully stretch, making it difficult to obtain good adhesive strength, especially with relatively low peel strength. Although a high curing temperature can overcome the problem, due to the fast cross-linking speed of adhesive molecules, it can also cause small molecules to not evaporate from the adhesive before it has already cured, which can also form defects and affect the pulling effect; At the same time, if the temperature is too high, cross-linking occurs before the molecular chain segments have fully extended, which will reduce the adhesive strength of the adhesive. Finally, an appropriate curing time is also necessary to achieve good bonding strength. The curing of adhesives usually requires a certain amount of time to ensure that the adhesive is fully bonded to a certain bonding strength. The curing time is too short, and the adhesive cannot cure completely, resulting in low cross-linking density and reduced bonding strength, or even non curing.