"Bonding is the state in which two different objects are brought together by a tight interface and are joined so that mechanical forces or work can be transmitted through the interface." It was found by calculation that in the case of dispersion forces, the forces between the two surfaces that come into close contact with the molecular level can reach about 1500 MPa. Therefore, some people believe that without the need for adhesives, if the two phases reach molecular-level close contact, only the dispersion force is enough to form a strong bond.
However, the ideal molecular-level close contact in reality cannot be achieved. Because most surfaces are microscopically rugged, when they are combined, only a handful of points can be reached in close contact at the molecular level. Therefore, it is necessary to introduce a liquid phase that fills the surface with "dumps" through the flow of liquid, so that the surfaces are brought into molecular-level close contact (as shown in Figure 1). The liquid phase that plays this role is what is usually referred to as an adhesive. Of course, the liquid adhesive must then be cured in some way so that it has sufficient cohesive strength to "transfer mechanical forces or work."
Thus, two basic conditions for forming a good bond can be concluded: a. Introducing a liquid adhesive, infiltrating the adhesive surface to form a close molecular contact between the surfaces; b. Curing the liquid adhesive between the surfaces.
The cure of an adhesive depends on the properties of the adhesive itself and the properties of the surface being bonded. Curing methods are mainly the following:
â– The polymer used as an adhesive is applied in a molten state by raising the temperature, and when the temperature is lowered, the melt is cooled to a solid adhesive film. We are familiar with the "hot melt" that is working in this way, common varieties include EVA, polyester, polyamide and so on.
â– The polymer used as an adhesive is dissolved or dispersed in a suitable volatile small molecule liquid (such as water, organic solvent, etc.) for coating. When the small molecule liquid is absorbed or volatilized, a solid adhesive film is formed. Common varieties are PVA water emulsions, a large number of polyurethane and rubber adhesives.
â– The polymer used as an adhesive is coated in the form of a monomer or oligomer (thus having a low viscosity) and then further polymerized and crosslinked to form a solid adhesive by applying a suitable catalyst or heating, radiation, or the like. membrane. Common varieties are unsaturated polyester, epoxy, polyurethane and other adhesives.
Industrial practice can use one of these basic processes, or it can be used simultaneously. Solution-type two-component polyurethane adhesives used in flexible packaging film compounding are the latter two. The main agent (usually the terminal hydroxyl structure) ) and curing agent (usually isocyanate-based structure, and functionality greater than 2) are dissolved in organic small molecule solvents (such as ethyl acetate, acetone, toluene, etc.), coated on a compounding machine and dried solvent, cured The chamber is left for a certain period of time so that the hydroxyl group and the isocyanate group chemically react and crosslink and solidify.
During the formation of the bond bond, the adhesive is critical to the wetting, adsorption, and interdiffusion between the bonded surfaces. The former determines whether "close molecular contact" can be formed and the perfect degree of formation; the latter two can provide additional interactions other than van der Waals forces to further enhance the strength of the bond. These processes are affected not only by the rheology and chemical properties of the adhesive, but also by the surface properties of the bonded surface, in particular the size and chemical composition of the surface tension.
The effect of wetting on adhesion firstly shows that incomplete impregnation can create interfacial defects, thus reducing bond bond strength. The driving force of infiltration is the spreading factor:
L12= g2 - g1 - g12
In the formula, g1 is the phase 1 (adhesive) surface tension; g2 is the phase 2 (adherend) surface tension; g12 is the interfacial tension; l12 is the spreading factor of phase 1 on phase 2. The conditions for spontaneous spreading are
L12≥0
This is the basis of thermodynamics. From the point of view of kinetics, the rate of automatic infiltration is: where g1 is the surface tension of the adhesive, h1 is the viscosity of the adhesive, and L is the kinetics of the adhesive/adhered pair. Characteristic length; θ∞ is the contact angle when the time is infinite; θ is the contact angle at any time.
Due to the infiltrating action, the two phases reach close contact between the molecules, adsorption and diffusion, and the chemical reaction can then occur. The degree depends on the nature of the adhesive and the adherend and the conditions of the adhesion. Adsorption and diffusion first obey the principle of "similar compatibility". However, both theoretically and experimentally, it has been demonstrated that even incompatible polymers, the diffusion of localized segments is prone to occur, and its thickness is about 10 to 1000 Ã…. The chemical reaction is completely dependent on the characteristics of the functional groups of the two. Because their bond energy is much higher than van der Waals forces, their contribution to the formation of strong adhesive bonds is obvious.
Interfacial Characteristics and Adhesion of Plastic Films
The main types of films used for composite flexible packaging are known as hard-to-stick materials such as polyethylene and polypropylene. In fact, from the amount of use, these two materials occupy the vast majority of composite flexible packaging materials. The reason why they are difficult to adhere is due to their low surface tension (35.7 dyne/cm for untreated polyethylene and 30.1 dyne/cm for polypropylene), making it difficult for adhesives to infiltrate their surfaces; on the other hand, they are All have non-polar molecular structures that are incompatible with common adhesives and therefore cannot form strong interactions. In addition, the opening agent and the slip agent added during the film processing tend to diffuse toward the surface and form a so-called “weak boundary layerâ€, which further reduces the adhesive strength.
Therefore, the polymer film materials used in the packaging of flexible packagings are all surface treated to change their chemical composition, increase surface energy, improve crystalline morphology and surface geometry, or remove impurities and fragile boundary layers. Polymer surface treatment methods that have been developed and industrially applied include chemical treatment, optical treatment, plasma treatment, polynucleation, and surface grafting. These methods usually cause only physical or chemical changes in the surface layer (100 to 100 mm thick) without affecting their overall properties.
Thin film materials for flexible packaging are typically corona treated to improve wettability and bondability during processing or before printing and recombination. Corona treatment is the process of letting a thin film pass through an electric field of high frequency and high voltage discharge. Its essence is a mixed plasma treatment. Studies have shown that this treatment results in chain cleavage, ablation and cross-linking, with a typical oxidation depth of 50 to 500. After corona treatment, there are many reasons why the film adhesiveness is improved, and it is generally considered that polar groups are introduced (ie, the wettability is improved); the surface is cross-linked (the weak boundary layer is strengthened); or It is the formation of a permanently polarized dielectric; and interfacial diffusion (due to reduced surface viscosity and increased molecular mobility due to chain cleavage), and so on. Many of these effects have been verified experimentally. For example, the most obvious is that the corona treated polyolefin film has a surface tension of 38 dyne/cm or even more than 40 dyne/cm. Electron microscopy shows that the surface of polyethylene is fuzzed after corona treatment. Rough or formed small round pits; outside the spectrum observed that after corona treatment of polyethylene, the surface introduces many carbonyl and carboxyl groups and so on.
Film Composite Evaluation
Flexible packaging film compounding is a typical adhesion between high polymer materials, and its effect evaluation is first subject to the general evaluation standards for adhesive structures, namely the ability to transfer mechanical work or force, and the durability of this capability. In addition, it has its own criteria, namely its visual effects, its flexibility and its ability to withstand post-processing (mainly heat, cooking, media, barrier, etc.), and its durability.
The strength of the bond is demonstrated when the bonded structure resists failure, and is numerically equal to the magnitude of the force applied to the adhesive layer at the time of failure. It should be noted that the strength of the bond is often much lower than its ideal (theoretical) strength, because the material may have defects or cracks, and stress concentration due to various reasons, and as a result, local stress is caused to exceed the local strength. Bonding bond breakage occurred.
The purpose of bonding is to transfer mechanical work from one object to another. Due to the introduction of the adhesive, and because the adhesive itself forms a thin bulk phase layer, this causes the mechanical work to pass from one object to the adhesive layer and then from the adhesive layer to the other. The object, the entire adhesive system includes three body phases and two interfaces, and the problem becomes complicated. If a small stress bond is applied, that is, the bond is broken, the bonding of the system is usually said to be "poor". But this statement may be wrong, because the fracture may occur completely at the interface, it may occur in the bulk phase very close to the interface, or it may be completely in the bulk phase, and there may also be a mixture of interface and cohesive failure. Type fracture. Therefore, the correct determination of the fracture site is very important for both theory and practice. It is the first step in the analysis of adhesion problems. If it has been concluded that the fracture occurred in a thin layer adjacent to the interface and is a cohesive fracture, the main effort should be placed on strengthening the weak boundary layer; conversely, if the fracture occurs on the interface, it should try to increase the interface attractiveness or strengthening Interface diffusion.
Flexible packaging film composites have certain requirements for the bonding strength. Due to the adhesion to the polymer matrix, and the adhesive and the adherent material are usually chemically incompatible, their diffusion coefficients are small, so the interface formed is clearly defined and its strength is often compared. low. However, due to the fact that a certain amount of chemical bonds can be formed on the surface of the polyurethane adhesive and the corona treated film (the excess of -NCO groups reacts with hydroxyl groups on the surface of the substrate), this situation has been improved to some extent. Figure 2 depicts the T-peel fracture model often used in the evaluation of composite film composite strength. From this model it can be seen that the fracture is not completed in one step. Understanding this point is very beneficial for the design and selection of composite adhesives.
As a film composite for flexible packaging applications, transparency, heat resistance, hydrolysis resistance, and dielectric resistance are important quality indicators in addition to the strength of the bond. There are many factors affecting the transparency. The first requirement is that the adhesive is amorphous and therefore optically isotropic (this is different from the requirements for general structural adhesives. The crystallization of structural adhesives will greatly increase the initial viscosity. Force and final bond strength are thus advantageous). Secondly, with the influence of impurities, the solid foreign body will cause the adhesive to agglomerate as a "nucleus" to form a so-called "colloid"; the effect of small bubbles is very similar to this. Then the melting point of the adhesive must not be too low, otherwise cold flow during the curing process and the formation of so-called "flow patterns" will greatly affect the transparency of the composite film. Heat resistance, hydrolysis resistance, and medium resistance are mainly aimed at the post-processability and packaging adaptability of flexible packaging products. The influencing factors are similar, that is, the chemical structure of the adhesive, especially the degree of cross-linking.
Source: Global Flexible Packaging Industry