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The wonders of pressure-sensitive adhesive: How to create a strong bond without using heat or solvents?
Pressure-sensitive adhesive (PSA) is a non-reactive glue that bonds to a surface when pressure is applied. The beauty of this type of glue is that it requires no solvent, water or heat to initiate the bonding action, making it widely used in products such as tapes, labels, adhesive dots, stickers and sticky notes.
As the name "pressure-sensitive adhesive" indicates, the degree of adhesion is affected by the applied pressure. Factors such as surface smoothness, surface energy and removal of contaminants are also important factors for correct adhesion.
Pressure-sensitive adhesives are typically designed to form a solid bond at room temperature, but they lose viscosity at low temperatures and lose resistance to shear at high temperatures. Therefore, special glues are made to operate at high or low temperatures.
The difference between structural and pressure-sensitive adhesives
Generally speaking, glue can be divided into two categories: structural glue and pressure-sensitive glue. Structural glue usually forms a permanent bond through some chemical reactions, such as solvent evaporation (like white glue), reaction with UV radiation (like dental glue), etc. In contrast, pressure-sensitive adhesives require only slight pressure to bond the glue to the substrate.
The bonding strength of pressure-sensitive adhesive comes from its fluidity and resistance to flow. When the glue comes into contact with the substrate, there are also intermolecular interactions, such as van der Waals forces, which significantly affect the final bonding strength. .
Pressure-sensitive adhesives exhibit viscoelasticity, two properties critical to proper bonding. Unlike the tensile shear strength of structural glues, the properties of pressure-sensitive adhesives are determined by their shear, peel resistance and initial tack. These properties are affected by formulation, coating thickness, kneading and temperature.
The influence of shape
The shape of the tape or label also affects how well it adheres. For example, tape with sharp corners tends to peel off in those corners, while rounded corners improve bonding strength.
Application scope
Pressure sensitive adhesives are designed for permanent or removable applications. Examples of permanent applications include safety labels for electrical equipment, foil tape for HVAC vent ducts, automotive interior assemblies and sound/vibration damping films. Some high-performance permanent pressure-sensitive adhesives have high adhesion values, can support kilogram loads per square centimeter of contact surface, and are durable even at high temperatures.
These bonds take hours or days to establish a permanent bond.
Remove performance
Removable glue is designed to form a temporary bond that ideally can be removed without leaving a trace after months or years. This type of glue is used in applications such as surface protection films, masking tapes, bookmarks, and price tags. Some removable glues are designed to be applied and removed again and again; these products generally have lower adhesion.
Sometimes, removing pressure-sensitive tape can cause damage to the substrate to which it is adhered. One way to reduce surface damage is by pulling at a slower speed and smaller peel angle.
Manufacturing Process
Pressure-sensitive adhesives can be manufactured using a liquid carrier or in a 100% solid form. Items like tapes and labels are made by applying glue to a support and then evaporating an organic solvent or aqueous vehicle.
The dried glue is further heated in a subsequent process to initiate the cross-linking reaction and increase its molecular weight.
And 100% solids glue may be a low-viscosity polymer that reacts with radiation to increase the molecular weight and form the glue (radiation-cured pressure-sensitive adhesive); or it may be a high-viscosity material that is heated to reduce viscosity and then cooled to its final shape (Melted Pressure Sensitive Adhesive, HMPSA).
Composition
Pressure-sensitive adhesives are usually based on rubber mixed with a suitable tackifier (such as resin ester). The rubber formula can be based on acrylic, which can form sufficient adhesion on its own without the need for a tackifier, or a modified polymer (SBC), which is elastic at low temperatures and resistant to high temperatures, and is often used in hot melt adhesives. In application.
The A-B-A form of the structure can promote phase separation of the polymer and provide good cross-linking between the structures at both ends.
Temperature considerations
The performance of pressure-sensitive adhesives is significantly affected by temperature. Tackiness or "quick tack" properties are critical to initial adhesion, and low temperatures can cause pressure-sensitive adhesives to become too strong and lose their tack. Once applied, temperature will also affect how it performs in use.
The most common test method for PSA and tape is at 23 degrees Celsius and 50% relative humidity, but experts in charge of glue formulation often conduct testing at other temperatures to better suit the needs of the end user. It is especially important to identify the "glass transition temperature".
The boundaryless characteristics of pressure-sensitive adhesives not only give us a new understanding of small objects in our daily lives, but also make us think about how many untapped potentials and applications are hidden behind these seemingly ordinary products. ?
