15 min read

Glass components are favored in the field of architecture for their unique transparency and aesthetics, and they have been used for nearly a thousand years. Due to the absolute brittleness of glass materials, the application scope of monolithic glass in the field of construction is greatly limited. In recent decades, with the emergence of laminated glass and the improvement of its laminating process, the use area and application scope of glass in the construction field have been greatly increased and extended. Laminated glass forms two or more layers of glass into a common stress assembly through the good adhesion of the interlayer film adhesive layer, which not only enhances the bearing capacity of the laminated glass component before it is broken but also provides a Certain bearing capacity after cracking. The bonding performance between the interlayer and the glass sheet is a crucial factor affecting the overall performance of the laminated glass. When the laminated glass is degummed, its transparency is reduced in appearance and the visual effect is affected; in function, part or all of the interlayer shear force transmission is lost, which reduces the overall bearing performance of the laminated glass and affects its service safety; After fragmentation, degumming reduces the adhesion of the film to the glass fragments, increases the possibility of the glass fragments splashing, and even causes the cracked glass components to fall and collapse as a whole. 

There are three main types of bonding failure of laminated glass: adherent failure (glass body failure), cohesive failure (adhesive layer failure), and adhesion failure (glass and glue layer detachment). When the force between the film and the glass exceeds its adhesion, the film and the glass are separated, and the appearance is manifested as bubbles, warped edges, white edges, etc. Its essence is adhesion failure, which is often referred to as degumming...

Laminated glass degumming is directly related to the safety, reliability, and durability of laminated glass, and the factors that affect laminated glass degumming run through the selection of laminated glass raw materials, glass deep processing, and its use and maintenance stages. The flatness of the original glass sheet, the adhesion characteristics of the interlayer, and the type of physical and chemical adsorption at the film-glass interface directly affect the interlayer bonding performance of the laminated glass. 

Based on the adhesion thermodynamics, Jinan LIJIANG Glass explored the influence of the original glass, film surface, and interface properties on the interlayer bonding performance of laminated glass, and put forward suggestions for improving the interlayer bonding performance of laminated glass from the raw material processing technology. 

Jinan LIJIANG Glass explored the influence of the physical and chemical properties of the film on the interlayer bonding properties of the laminated glass and suggested that the polyvinyl acetal (PVB) laminated glass should be edge-sealed during use. The deep processing of laminated glass also has a critical influence on the bonding properties of the glass-film interface. Improper operation during processing will seriously reduce the bonding performance of the film-glass interface, and even directly lead to the occurrence of bubbles and warping of the laminated glass during the production process. etc. phenomenon. Environmental factors, such as temperature, humidity, ultraviolet radiation, soaking in cleaning agents, dissolution and corrosion of edge sealing structural adhesives, etc., will affect the bonding properties of laminated glass film-glass interface to varying degrees. 

Jinan LIJIANG Glass tested the bonding strength of PVB laminated glass film-glass interface by the crisscross method, and studied the PVB film thickness, loading speed, and the effects of UV aging and humidity aging on the adhesion of PVB film-glass interface. effect on junction performance. Most of the existing research on the degumming of laminated glass only focuses on the influence of unilateral factors (such as raw material selection and service environment factors) on degumming. various stages. A comprehensive and comprehensive understanding of the factors affecting the degumming of laminated glass is necessary to prevent and reduce the degumming of laminated glass.

Jinan LIJIANG Glass analyzes the degumming mechanism of laminated glass and sorts out the factors that affect the degumming of laminated glass at various stages such as raw material selection, deep processing, and maintenance.

1. Analysis of bonding mechanism of glass-film interface

Laminated glass is made of glass and transparent film as the base material. Through the high temperature and high pressure of the autoclave (temperature range 135~140℃, pressure below 1.2 MPa), the glass and the film form a good bond at the interface and finally form a composite material. The bond between the film and the glass is formed by the combined action of various forces. According to the difference in glass-film interface, the bonding effect between film and glass mainly includes adsorption, chemical bond, and mechanical occlusion. Van der Waals force is the most widely existing adsorption force in the glass-film interface, such as Lewis acid-base interaction, hydrogen bonding, etc. In the high-temperature hot-melt state, the molecules of the glass-film two-phase interface are in close contact and infiltrate with each other, and are rearranged under the action of molecular attraction to form a physical adsorption force. At the same time, some polar groups (such as C-OH groups, and Si-OH groups) interact to form covalent bonds, and these bonding forces together constitute the cohesion of the glass-film interface and "solidify" during the cooling process of the laminate. The adhesion between the PVB film and the glass surface is mainly provided by the hydrogen bonds formed by C-OH in the PVB film and the abundant Si-OH on the surface of the inorganic glass (as shown in Figure 1). 

Figure 1 The schematic illustration of the bonding mechanism between PVB and glass surface 1

The ions or functional groups contained in some films can chemically react with the glass surface to form new ionic bonds to enhance the bonding force between the glass and the film interface, such as the copolymerization of ethylene and methacrylate, a component of the iconic film (SG) It contains 1% sodium ions, which can interact with the ionic bonds formed by the glass.use (as shown in Figure 2)

Figure 2 The schematic illustration of the bonding mechanism between SG and glass surface 1

In addition, there are cracks, step pores, etc. on the microscopic surface of the glass. The film material flows into the microscopic defects on the glass surface in a viscous flow state, which increases the contact surface area and can provide a certain mechanical bite force after molding. PVB Glass SG Si 0H HO CHz OH.HOS chematic illustration of the bonding mechanism between PVB and glass surface.

The bonding properties between glass and film depend on the forces between different molecules at the interface. Ideally, the interface can form relatively stable physical and chemical forces, but in practice, it is difficult to meet all the conditions for physical and chemical reactions on the interface, and the optimal bonding performance state cannot be achieved. At the same time, the low-energy impurities adsorbed on the interface will replace the original The high-energy surface becomes a low-energy surface, which further affects the bonding performance of the interface.

2. The influence of raw materials

2.1 The influence of glass

At present, the flat glass used in construction is mainly soda-lime float glass. Its main chemical components are silicon dioxide, calcium oxide, and sodium oxide. The mass percentage of each component is SiO: 69%-75%, Cao 5%~10%. %, Na 2013%~15%, A1 2030~2.5%. In minerals, Ca ions and Mg ions, Na ions, and K ions are often associated with similar properties. Therefore, soda-lime float glass also contains a small amount of Mg ions and K ions. These substances can have physical and chemical adsorption with the film on the glass surface. The physical adsorption is mainly caused by the van der Waals force, and the chemical adsorption is caused by the broken bond on the glass surface and the chemical bond formed by the electron transfer of the adsorbed molecule. The adsorption effect coexists. The bond strength of the glass-film interface is closely related to the properties of the glass and film itself. The choice of glass and film will directly affect the bond strength between laminated glass layers. Within the forming temperature range, under the combined action of surface tension and gravity, the natural flattened and polished thickness of soda-lime silicate float glass is about 7mm. When the thickness of the production glass is not 7mm, use the edger (Pilkington, Luoyang float method) or the graphite baffle (PPG method) to control the thickness and width of the glass, resulting in deviations in the thickness of the center and edge of the glass plate. 

When producing 10mm thick glass, the edger or baffle will accumulate glass from the edge to the center, resulting in a larger glass edge thickness and a smaller center thickness (see Figure 3); 

Figure 3 Schematic diagram of 10mm thick glass stretching

Figure 3 Schematic diagram of 10mm thick glass stretching

when producing 5mm thick glass, the edger or baffle The plate stretches the glass from the edge to the center, resulting in a smaller glass edge thickness and a larger center thickness (see Figure 4).

Figure 4 Schematic diagram of 5mm thick glass stretching

Figure 4 Schematic diagram of 5mm thick glass stretching

In addition, during the tempering process, the edge of the glass absorbs heat quickly, the middle absorbs heat slowly, and the unevenness of the glass surface has further deteriorated. The poor flatness of the glass surface will cause optical deformation of the glass, and at the same time, optical distortion will occur, and in the interlayer process, the thickness of the interlayer adhesive layer after lamination is likely to be uneven, and the interlayer bonding performance is poor. Flat laminated glass products in the international industry standard "Safety Glass for Construction Part 3: Laminated Glass" standard require that the bow curvature should not exceed 0.3%, and the wave curvature should not exceed 0.2%. Laminated glass has stricter requirements on glass flatness matching than other hollow or monolithic glass. When the glass flatness matching is low, the glass and the film will not bond tightly during the lamination process, and the bubble removal efficiency will be low; It also produces a short-term forced bond between the glass and the film, which not only affects the bond strength but also produces tensile stress between the glass and the film after leaving the autoclave, which can lead to degumming of the glass later, as shown in Figure 5.

Figure 5 Glass debonding caused by warpage mismatch of tempered glass

2.2 Influence of film moisture content

PVB and SG films are the most commonly used film materials for architectural laminated glass. PVB film is composed of about 75% PVB resin and 25% plasticizer and a small number of additives. SG film is composed of a copolymer of ethylene and methacrylate, which contains about 1% of sodium metal ions. Compared with PVB, SG film has higher strength, hardness, and better bonding properties.In the selection process of film, the moisture content is the most important factor that must be controlled. O—H bonds in water molecules will affect the formation of hydrogen bonds at the glass-film interface. The presence of too many water molecules will lead to the transformation of the high-energy surface of the glass into a low-energy surface, which will seriously affect the adsorption of the glass surface and reduce the adhesion between the glass and the film. PVB resin is a hygroscopic material and the process of PVB water absorption is reversible. Its moisture content is related to the environment and the time it is exposed to. According to the principle of phase balance and diffusion, as long as the indoor environment meets the requirements of lamination, the final moisture content of the film can meet the processing requirements. need. Compared with PVB, although the allowable moisture content of SG film is higher, the too high moisture content will seriously reduce the adhesion between SG film and glass, and the water absorption process of SG is not completely reversible. Therefore, in the process of laminated glass lamination, the moisture content of the film should be strictly controlled, and the temperature and humidity of the lamination room should be strictly controlled within a reasonable range. In addition, due to the limitation of the processing equipment of the film production supplier, the maximum width of the film is often limited. When the width of the laminated glass exceeds the width of the film, film splicing is required. Under high-temperature melting, impurities, dust, etc. at the splices of the clips will lead to a decrease in the bonding performance of the glass-film interface.

3. The influence of the deep processing stage

Glass deep processing technology includes glass edging, hole punching, gap treatment, color glaze, tempering, and laminating technology. The edge treatment of the glass, the number and position of openings, the use of colored glaze glass, and the treatment of tempering and lamination processes will directly affect the bonding properties of the glass-film interface.

The edge treatment of glass improves the stress concentration on the edge after glass processing, reduces processing damage and installation damage, and improves the transparency and aesthetics of glass components. The glass edge treatment process generally requires the use of a grinding wheel above 180 mesh to grind the glass, in particular, it is recommended to use a fully automatic glass edging machine for grinding glass edges. Edge processing is easy to causes the edge of the glass to be broken and corners to fall. When it is directly applied to the laminated glass, it will form obvious pits and easily induce edge degumming. When the mesh number of the grinding wheel used in the edge treatment process is too small, it is easy to cause stress concentration on the edge. 

In the tempering process, to avoid the damage caused by the stress concentration, it is necessary to increase the tempering process temperature. Too high tempering process temperature will cause the surface waviness of glass to become larger, thereby reducing the bonding properties of glass and film. Therefore, it is recommended to use a fully automatic tempering furnace to process tempered glass. Many laminated glass products use holes to connect with adjacent structures. The location and size of the openings, glass thickness, hole spacing, and hole margins will affect the surface waviness of the tempered glass, thereby affecting the bonding properties of the area around the holes. In addition, the pores are often deformed greatly under the action of load, and water vapor, light, etc. will further erode the exposed adhesive layer at the pores, resulting in the easier generation of air bubbles or degumming at the pores. Once degumming occurs, the external force and water vapor will be further intensified, which will accelerate the destruction of the bonding interface between the glass and the film (see Figure 6). Colored glazed glass often contains additives of different materials and colors due to its use requirements. The area of the additive used on the glass surface will affect the bonding effect of PVB, SG, and other films and glass. According to engineering experience, 30% of the color glaze coverage area is the upper limit to ensure the bonding effect of PVB laminated glass. In addition, due to the different color glaze formulas and suppliers, when using color glaze glass, the compatibility between the color glaze and the film should be considered, and whether there will be chemical reactions between the two during long-term use. It is recommended to use certified glaze products Or relevant compatibility testing should be done for specific projects.

Figure 6 The degumming at the edge of the hole

Figure 6 The degumming at the edge of the hole

The laminating process of laminated glass mainly includes four processes: raw material processing, lamination, pre-pressing, and autoclave heating, in particular, the production of laminated glass is carried out with a fully automatic laminated glass production line. Before laminating glass, the glass surface must be cleaned. The cleaned glass surface is a high-energy surface. When there are organic and inorganic pollutants such as oil stains and dirt on the glass surface, these pollutants will form an intermediate layer between the glass surface and the film during the lamination process, which will cause the glass surface to be removed from the high-energy surface. The surface is converted into a low-energy surface, which reduces the physical and chemical adsorption capacity of the glass surface and affects the bonding ability between the glass and the film. To ensure the cleanliness of the glass surface, the glass cleaning equipment must be in good working condition, and the temperature of the cleaning water should be kept above 50°C. The calcium content, pH value, and metal ion content of the cleaning water will directly affect the adsorption capacity of the glass surface. When cleaning the glass, the corresponding cleaning water should be selected according to the type of film. In the process of assembling, lay the cut film flat on the bottom glass plate and ensure that there are no wrinkles, then press the upper glass on the film, and then put a rubber ring on it to vacuum. During the vacuum pumping process, attention should be paid to making the surrounding sealing strip evenly stretched. During the cold pumping process, the vacuum degree of the vacuum gas storage tank should be maintained at 0.09~0.1 MPa, and the cold pumping time should not be less than 5 min. During the lamination process, the film is not laid flat, the vacuum degree of vacuuming is not up to standard, and the vacuuming time is not long enough, which will affect the adhesion between the film and the glass during the lamination process.In the pre-pressing process, the flat glass generally adopts the radial pressing process to remove the air between the glass and the film through the hot spoke axis in the radial pressing section, and the curved glass generally adopts the vacuum negative pressure process to extrude the air between the glass and the film through negative pressure. . In the autoclave heating process, the residual air is integrated into the film by fine control of temperature, air pressure and time, and the uniform fluidity of the film is realized, and the bonding force between the glass and the film is strengthened. The cleanliness, temperature, pressure, and time need to be strictly controlled in the pre-pressing and autoclave heating process. Improper procedures and unreasonable operations in this process will affect the adhesion between the glass and the film. Adhesive layer materials such as PVB, SG, etc. are highly sensitive to humidity. In the lamination process, two laminations are often required, that is, pre-pressure degassing and dehumidification, and then lamination at high temperature and high pressure in an autoclave to finally make the glass and film tightly bond. Before the radiation pressure, the laminated glass often needs to be fixed with relevant fasteners. The fasteners can bond the glass and the film together in a short time, but when the clamping force is loosened, the glass and the film at the clamping place will be in a long-term position. In the state of tensile stress, the fastener will also cause the film at the clamp to absorb more during the autoclave exhaust process, reducing the bond strength of the clamp. 

Figure 7 The tempered laminated glass processing factory

Figure 7 The tempered laminated glass processing factory

When the air between the glass and the film is not smooth, and the spoke cylinder will be worn out in the long-term service, the middle part will be worn too much, which will cause the uneven radiation pressure of the laminated glass and lead to the poor air exhaust in the middle of the laminated glass. The laminated glass needs to be trimmed after radiation pressing. If the trimming produces a large pulling force on the film, it will destroy the contact cross-section pattern between the glass and the film, thereby reducing the bonding force between the glass and the film. Therefore, sharp edges should be used in the trimming process. knife and try to avoid pulling on the film. The control of heating temperature and heating time in the autoclave heating process determines whether the glass plate can be heated evenly. Poor heating and heating time control will lead to large temperature stress in the laminated glass, which will seriously reduce the adhesion between the glass and the film. , even directly leading to the separation of glass and film.

4. Use the maintenance stage

During the use and maintenance of laminated glass, the factors affecting its degumming mainly include external forces, environmental effects, cleaning agents, and contact structural adhesives. The debonding of laminated glass is because the tensile stress or shear stress between the glass and the film exceeds the bond strength of the glass-film interface under the action of the external load. The unreasonable value of the design load of the laminated glass (especially the stress concentration places such as openings or gaps) and the improper installation operation (especially the forced extrusion installation of the laminated glass) are the main reasons for the degumming of the laminated glass. In the design process of laminated glass, special attention should be paid to the design of openings or gaps, and strong plugging and hard installation should be avoided during the installation process.

The environmental factors that affect the debonding of laminated glass mainly include temperature, humidity, and ultraviolet radiation. Most of the interlayer films currently used are temperature-sensitive materials, and their mechanical properties and bonding properties with glass have serious temperature dependence. The increase in temperature tends to reduce the adhesion between the glass and the film, and may also cause certain temperature stress inside the laminated glass. Water vapor is also one of the most important factors affecting the bonding performance of glass and film. Water vapor intrusion not only affects the water content of the film but also destroys the hydrogen bond at the glass-film interface, which greatly reduces the interface bond strength. When the water vapor intrusion leads to the degumming of the laminated glass, the degumming further expands the contact interface between the film and the water vapor, forming a vicious circle and accelerating the degumming of the laminated glass. At present, the commonly used interlayer film materials for laminated glass are all polymer materials. Lighting, especially ultraviolet radiation, will often cause adverse effects to varying degrees, especially when combined with other unfavorable factors, it may cause the laminated glass to produce bubbles...Laminated glass is often fixed and connected with structural adhesive, and the structural adhesive is often in direct contact with the laminated glass film.

Therefore, the selection of structural adhesive should fully consider whether it will cause physical and chemical corrosion to the film, and it should be ensured that its components cannot penetrate, dissolve, and corrode the laminated glass film material. Laminated glass often needs maintenance and cleaning during use. When the cleaning agent is not selected properly, the acid and alkali components contained in it may cause physical and chemical reactions such as dissolution and corrosion of the film, which will have a fatal impact on the film and the bonding interface.In the actual service process of laminated glass, its degumming is often caused by a combination of factors, including not only the "congenital defects" caused by the selection of raw materials and the glass's deep processing stage but also the artificial and non-artificial defects caused by the installation and service of the laminated glass. Stress concentration also includes the long-term effect of adverse factors in the external environment.

5. Conclusion

Based on the analysis of the bonding mechanism of the glass-film interface, the factors affecting the degumming of laminated glass at various stages of raw material selection, deep processing, and use and maintenance were explored, and suggestions for reducing or avoiding degumming of laminated glass were put forward in a targeted manner.

The main conclusions of this paper are as follows:

(1) In the selection stage of laminated glass raw materials, the flatness of the glass surface, the matching degree between the original glass sheets, and the moisture content of the interlayer film should be strictly controlled, otherwise it will cause irreversible adverse effects on the bonding performance of the glass-film interface.

(2) In the deep processing stage of laminated glass, the glass opening, edge treatment, and the use of colored glaze glass should be properly handled; in addition, the cleanliness and humidity of the lamination environment and the temperature of the autoclave during the lamination process should be strictly controlled. Rationality and precision of processing equipment.

(3) During the installation process of laminated glass, strong plugging and hard installation should be strictly prohibited; and the adverse effects of temperature, humidity, ultraviolet radiation, structural adhesives, cleaning agents, and external loads that may be suffered by the laminated glass during use should be Appropriate consideration should be given to the design and corresponding control measures should be taken.

(4) The factors affecting the degumming of laminated glass have superimposed and promoting effects on each other. The prevention and control of degumming of laminated glass need to be jointly controlled from raw material selection, deep processing, use and maintenance, and other stages. It is necessary to use suitable raw materials. Strictly control the operation standardization during the processing of laminated glass, and also need its reasonable design and maintenance.


For more information about Jinan LIJIANG Glass insulating glass processing equipment and insulating glass processing accessories, please click here to learn more.   

Comments
* The email will not be published on the website.