Analysis of the function of the hottest adhesive i

  • Detail

Analysis of the role of adhesive in plastic film compounding

"bonding is a state in which two different objects are combined through close interface contact, so that mechanical force or work can be transferred through the interface". Through calculation, it is found that in the case of dispersion force, the force between two surfaces that reach molecular level close contact can reach about 1500MPa. Therefore, some people believe that without the role of adhesive, if the two phases reach molecular level close contact, only the dispersion force is enough to form a strong bond

however, in reality, the ideal molecular level close contact cannot be achieved. Because most surfaces are micro uneven, when they are combined, only a few points can reach molecular level close contact. Therefore, it is necessary to introduce a liquid phase to fill the "potholes" on the surface through the flow of liquid, so as to achieve molecular level close contact between the surfaces (as shown in Figure 1). The liquid phase that plays this role is usually called adhesive. Of course, the liquid adhesive must then be cured in some way in order to have sufficient cohesive strength to "transmit mechanical force or work"

therefore, two basic conditions for forming good adhesion can be summarized: A. introducing liquid adhesive to infiltrate the bonding surface to form close molecular contact between the surfaces; b. Curing of liquid adhesive between bonding surfaces

the curing of adhesive depends on the characteristics of the adhesive itself and the characteristics of the bonded surface. The curing methods mainly include the following:

■ the polymer used as adhesive is coated in a molten state by increasing the temperature. When the temperature is reduced, the melt is cooled into a solid adhesive film. The familiar "hot melt adhesive" works in this way. Common varieties include EVA, polyester, polyamide, etc

■ the polymer used as adhesive is dissolved or dispersed in a suitable volatile small molecular liquid (such as water, organic solvent, etc.) for coating. When the small molecular liquid is absorbed or volatilized, a solid adhesive film is formed. Common varieties include PVA water emulsion, a large number of polyurethane and rubber adhesives

■ the polymer used as adhesive is coated in the form of monomer or oligomer (so the viscosity is very low), and then it is further polymerized and crosslinked by applying appropriate catalyst or heating, radiation, etc. to form a solid adhesive film. The common varieties are unsaturated polyester, epoxy, polyurethane and other adhesives

in industrial practice, one of these basic processes can be used, and several can be used at the same time. The solution type two-component polyurethane adhesive used for the composite of flexible packaging film is to use the latter two at the same time. Its main agent (usually hydroxyl terminated structure) and curing agent (usually isocyanate terminated structure, and the functionality is greater than 2) are dissolved in organic small molecular solvents (such as ethyl acetate, acetone, toluene, etc.), The solvent is coated and dried on the compounding machine, and placed in the curing room for a certain time to make the hydroxyl group and isocyanate group react chemically and crosslink and solidify

2. The pulley on the toothed bar and its track are excessively dusty and rusted. In the whole process of forming the bonding key, the infiltration and adsorption of the adhesive on the bonded surface and the mutual diffusion between them are very critical. The former determines whether "close molecular contact" can be formed and the degree of perfection; The latter two can provide additional interaction besides van der Waals force to further enhance the strength of the bonding bond. These processes are not only affected by the rheological and chemical properties of the adhesive, but also determined by the surface properties of the bonded surface, especially the size and chemical composition of the surface tension

the influence of infiltration on bonding is first shown in that incomplete infiltration can produce interface defects, thus reducing the strength of bonding bond. The driving force of infiltration is the spreading coefficient:


where G1 is the surface tension of phase 1 (adhesive); G2 is the surface tension of phase 2 (adherend); G12 is interfacial tension; L12 is the spreading coefficient of phase 1 on Phase 2. The condition of spontaneous spreading is L12 ≥ 0

this is the basis of thermodynamics. From the perspective of dynamics, the rate formula of automatic infiltration is: where G1 is the surface tension of the adhesive; H1 is the viscosity of adhesive; L is the dynamic characteristic length of the adhesive/adherend pair; θ ∞ is the contact angle when the time is infinite; θ Is the contact angle at any time

due to the infiltration, the two phases reach close contact between molecules, adsorption and diffusion, and the chemical reaction can then occur. The degree depends on the properties of the adhesive and the adherend and the bonding conditions. Adsorption and diffusion are first subject to the principle of "similarity and compatibility". However, both theoretically and experimentally, the smaller the available working chamber volume is, and it has been proved that even between incompatible polymers, the diffusion of local chain segments is easy to occur, with a thickness of about 10~1000. The chemical reaction completely depends on the characteristics of their functional groups. Because its bond energy is much higher than van der Waals force, its contribution to the formation of strong bonding bonds is obvious

interfacial characteristics and adhesiveness of plastic film

among the main film varieties used in composite flexible packaging, there are many famous non sticky materials such as polyethylene and polypropylene. In fact, in terms of dosage, these two materials account for the vast majority of composite flexible packaging materials. The reason why they are difficult to stick is that on the one hand, the low surface tension (35.7dyne/cm for untreated polyethylene and 30.1dyne/cm for polypropylene) makes it difficult for the adhesive to infiltrate its surface; On the other hand, they all have non-polar molecular structures, which are incompatible with general adhesives, so they cannot form strong interactions. Moreover, the opening agent and smoothing agent added in the process of film processing tend to diffuse and precipitate to the surface, forming the so-called "weak boundary layer", which will further reduce the bonding strength

therefore, polymer film materials used in flexible packaging composites should be surface treated, with a year-on-year increase of 58.3% to change their chemical composition, increase surface energy, improve crystal morphology and surface geometric properties, or remove impurities and fragile boundary layers. Polymer surface treatment methods that have been developed and obtained industrial applications include chemical treatment, optical treatment, plasma treatment, polycrystalline nucleation and surface grafting. These methods usually only cause physical or chemical changes in the surface layer (100 to 100mm thick) without affecting its overall properties

film materials used for flexible packaging are usually corona treated in the processing and molding process or before printing and compounding to improve wettability and adhesion. Corona treatment is a process in which the film passes through an electric field of high-frequency and high-voltage discharge. Its essence is a mixed plasma treatment. The research shows that this treatment causes chain cracking, ablation and crosslinking, and its typical oxidation depth is 50~500. There are many reasons for the improvement of film adhesion after corona treatment, which is usually considered to be the introduction of polar groups (that is, the wettability is improved); Crosslink the surface (strengthen the weak boundary layer); Or form a permanently polarized dielectric; And interfacial diffusion (due to reduced surface viscosity and increased molecular fluidity due to chain cleavage), etc. Many of these effects have been verified by experiments: for example, the most obvious is that the surface tension of polyolefin film after corona treatment reaches 38dyne/cm or even more than 40dyne/cm; Under electron microscope, the surface of polyethylene was fuzzed, rough or formed some small round pits after corona treatment; The external spectrum observed that many carbonyl and carboxyl groups were introduced into the surface of polyethylene after corona treatment

effect evaluation of film compounding

flexible packaging film compounding is a typical bonding between polymer materials. Its effect evaluation first obeys the general evaluation standard of bonding structure, that is, the ability to transmit mechanical work or force, and the durability of this ability. In addition, it has its own standards, that is, its visual effect, softness and ability to withstand post-processing (mainly heat resistance, cooking resistance, medium resistance, barrier, etc.), as well as the durability of this ability

the strength of bonding is shown when the bonding structure resists failure, which is numerically equal to the stress on the bonding layer when failure occurs. It should be pointed out that the strength of bonding is often far lower than its ideal (theoretical) strength, because the material may have defects or cracks, and the stress concentration caused by various reasons. As a result, the local stress exceeds the local strength and the fracture of bonding bond occurs. The purpose of bonding is to transfer mechanical work from one object to another. Due to the introduction of adhesive and the formation of a thin bulk phase layer by the adhesive itself, the mechanical work is transferred from one object to the adhesive layer, and then from the adhesive layer to another object. The whole bonding system includes three bulk phases and two interfaces, so the problem becomes complex. If the bond breaks when a small stress is applied, it is usually said that the adhesion of this system is very "poor". However, this statement may be wrong, because the fracture may occur completely at the interface, or at a position very close to the interface in the bulk phase, or it may be completely in the bulk phase, and there may also be a mixed fracture of interface failure and cohesion failure. Therefore, it is very important for theory and practice to correctly determine the fracture location, which is the first step to analyze the bonding problem. If it has been determined that the fracture occurred in the thin layer adjacent to the interface and is a cohesive fracture, the main energy should be focused on strengthening the weak boundary layer; On the contrary, if the fracture occurs at the interface, we should try to increase the interface attraction or strengthen the interface diffusion

flexible packaging film composite has certain requirements for bonding strength. Due to the adhesion to the polymer matrix, and the chemical properties of the adhesive and the adherend material are usually incompatible, their diffusion coefficient is very small, so the interface formed by entrepreneurs to manage and run enterprises with confidence is clear-cut, and its strength is often relatively low. However, due to the formation of a certain number of chemical bonds between the polyurethane adhesive and the corona treated film surface (excessive -nco groups react with hydroxyl groups on the surface of the substrate), this situation has been improved to a certain extent. Figure 2 describes the T-shaped peel fracture model often used in the composite strength evaluation of composite films. It can be seen from this model that the fracture is not completed in one step. Understanding this is very useful for the design and selection of composite adhesives

as a film composite used in flexible packaging, in addition to bonding strength, transparency, heat and hydrolytic resistance, and medium resistance are all important quality indicators. There are many factors affecting transparency. First, the adhesive is required to be non crystalline, so it is optically isotropic (this is different from the requirements of general structural adhesives. The crystallization of structural adhesives will greatly improve the initial viscosity and final bonding strength, so it is beneficial). Secondly, it is the influence of impurities. Solid foreign matters will cause the adhesive to agglomerate with this as the "core", forming the so-called "glue core"; The effect of small bubbles is also very similar. Moreover, the melting point of the adhesive should not be too low, otherwise the so-called "flow pattern" formed by cold flow during the curing process will greatly affect the transparency of the composite film

Copyright © 2011 JIN SHI