Sealing Technology (I)

Sealing, also known as sealing and sealing, refers to the various closed processes that are carried out after the packaging container is filled with the product to ensure that the contents are retained in the container during transportation, storage, and sale. There are many different methods and materials used in packaging and sealing, such as bonding, capping (plugs, caps, etc.), heat sealing and nail sealing.
First, the adhesion of two solids of the same kind or different types, due to the action of another substance (adhesive) interposed between the two surfaces, the phenomenon of firm bonding is called adhesion. Briefly, bonding is a method of using an adhesive for sealing. This method has the advantages of simple process, high productivity, high bonding strength, uniform stress distribution, good sealing performance, wide application range, and increased insulation, insulation, etc., and is widely used in the packaging industry for paper, cloth, wood, and plastics. Combination of various materials such as metals. It plays an important role in the process of sealing, manufacturing of composite materials, sealing (boxes), stickers, and labeling.
(a) Basic Conditions for Adhesive Bonding Adhesive must have good fluidity. The adhesive must be able to disperse easily and evenly over the entire surface of the adherend, fill the uneven surface, and form a uniform thin layer of adhesive on the entire surface of the adherend. .
2. Adhesive must be able to fully wet the surface of the adherend. Adhesive on the surface of the adherend to a certain degree of infiltration is one of the necessary conditions for the completion of adhesion, fully infiltrated into the adhesive molecule and the surface of the adherend molecules It has created the necessary conditions for attracting and achieving bonding. Usually, a small amount of wetting agent is added to the adhesive component, or the adhered surface is treated beforehand to increase the degree of infiltration of the adhered object by the adhesive.
3. There must be sufficient force between the adhesive and the adherend. There must be a large binding force between the adhesive and the adherend to form a strong bond. This binding force can be: mechanical force - the adhesive penetrates into the adherend, the force in the form of a "glue" form; intermolecular forces - van der Waals forces, orientational attraction between polar molecules, chemical forces - The force formed by chemical bonds such as ionic bonds, covalent bonds, hydrogen bonds, and the like.
(b) How does the bonding force between the binding agent and the adherend actually arise? How to theoretically guide adhesion? Many scholars proceeded from the different experimental conditions for the study of the bonding theory, but to this end, it is impossible to establish a complete and accepted adhesion theory. Here are some of the major bonding theories:
1. Adsorption theory The theory of adsorption is currently supported by many theories. According to this theory, the bonding process is divided into two stages. In the first stage, the adhesive molecule migrates to the surface of the adherend due to Brownian motion. As a result, the polar gene of the adhesive gradually approaches the polar part of the adherend. By the solvent volatilization, heating or pressure, the adhesive molecules can be in close proximity to the adherend. The second stage is that when the distance between the adhesive and the surface of the adherend is very small (less than 0.5nm), an intermolecular interaction occurs. This attraction includes the dispersion force from the energy level of 418.7j/mol to the energy level. For a series of forces up to 4.187×104 j/mol hydrogen bonding force, the bonds thus formed have dipole-dipole bonds (eg, cellulose-polyvinyl chloride bonds), couple-induced dipole bonds (eg, fibers). (adhesion of styrene-polystyrene) and forms of hydrogen bonding (such as cellulose-polyvinyl alcohol). As a result of these effects, the bonding is completed.
Although the adsorption theory is widely supported, it cannot explain some of the bonding phenomena, such as the firm bonding between certain nonpolar polymer compounds.
2. Mechanical Theory Mechanical theory was the earliest proposed adhesion theory. It believed that adhesion was a mechanical process and was the result of the mechanical action of the adhesive on the two adhesive surfaces. The adhesive flows in and fills the uneven surface of the adherend. Once cured, the adhesive and the adherend surface are joined by interlocking and bonding is completed, as is the case with the ship anchors thrown into the seabed mud.
Although this theory was proposed very early, it has not been able to explain the problem of adhesion of objects with smooth surfaces (such as glass).
3. Diffusion Theory Diffusion theory takes the fundamental characteristics of polymer compounds (polymer chain structure and its flexibility) as the starting point. It is believed that the thermal motion of polymer molecules will cause the diffusion of adhesive macromolecules, when the adhesive is in liquid form. When applied on the surface of the adherend, if the adherend is a material that can be swollen or dissolved, the adhesive and the adhered object will diffusely interweave with each other across the interface, and will be connected to form a strong adhesive surface. . This theory is essentially the diffusion, penetration and dissolution of polymers between them.
This theory can explain the problem of adhesion of some macromolecular compounds, but it cannot be explained well for high-polymer adhesives for bonding metals.
4. Electrostatic Theory This is a newer theory of adhesion. In experiments where adhesives were peeled off from some of the adhered objects, the peeled surfaces were found to be electrically charged, emitting light in dark places and having a squeaky sound. After research, it is thought that there is an electric double layer at the bonding interface, and there is electrostatic attraction between them, so that adhesion occurs. The formation of electric double layers may be the result of interfacial molecular interactions. This theory cannot explain the adhesion of non-polar substances that cannot produce double layers.
5. Polarity Theory This theory holds that adhesion is related to the polarity of the adhesive and the material to be bonded. The polar material is bonded with a polar adhesive, and the non-polar material is bonded with a non-polar adhesive. .
6. Chemical Theory This theory holds that a firm bond is formed between the adhesive and the adherend through a chemical reaction.
In summary, various theories have their own experimental basis, and can explain the adhesion phenomenon satisfactorily in some situations, but they also have some practical limitations. In general, there is not yet a complete adhesion theory. This is due to the complexity of the adhesion phenomenon itself and the limitations of experimental conditions.
(c) Classification of adhesives There are many ways to classify adhesives. Packaging adhesives are commonly used in the following categories:
1. According to the type of main adhesive substances Adhesive substances Organic substances Natural glucose derivatives: Amino acid derivatives such as starch, dextrin, gum arabic, etc. Natural resins such as casein, bone glue, hide glue, etc.: Rosin and its derivatives, lacquer, etc. Category: Asphalt and other synthetic resins Thermoplastic: Polyvinyl acetate, Polyacrylate, Polystyrene, Ethylene-vinyl acetate copolymers Thermosetting: Epoxy resin, phenolic resin, urea-formaldehyde resin, polyisocyanate, etc. Rubber neoprene, butadiene Base, styrene butadiene, nitrile, and other composite phenolic - polyvinyl alcohol, phenolic - neoprene, epoxy - polyamide and other inorganic silicate; phosphate; metal oxide gel 2. According to the curing mode, it can be divided into non-reaction curing type and reaction curing type.
(1) non-reactive curing type a. Solvent and water volatilization curing type: Dissolve adhesive substances or the like with a solvent or water, or make an aqueous emulsion, uniformly apply the adhesive to the adhered surface, and the solvent or moisture binding surface volatilizes or is absorbed by the adherent. The adhesive film is formed to exert adhesive force. The adhesive material for this type of curing can be starch, animal and vegetable glue, thermoplastic resin, natural rubber, synthetic rubber, and the like.
b. Hot-melt type: This is a relatively new type of adhesive, which is a solid adhesive with a thermoplastic polymer as its main component and no water or other solvents. The adhesive is melted by heating, applied to the adhered surface, and subsequently cured by cooling in air to complete the adhesion. Many thermoplastic resins can be made into hot melt adhesives.
(2) Reaction Curing Type A curing agent or a catalyst is added to a binder containing a reactive functional group, and an irreversible chemical change occurs in the binder material by heating or not heating to cause curing of the binder.
3. According to the application mode of the adhesive, the application manner of the adhesive can be divided into a solution type, an emulsion (glue) type, a hot melt type, a pressure sensitive type, and a spray type.
4. According to the operating temperature can be divided into cold glue, hot glue two categories. Cold glues include solution type emulsions and adhesive tapes. Hot glues are mainly hot melt adhesives. There are a small number of tapes here that require heating to bond. (To be continued)