The Basics of Pressure-Sensitive Adhesives
What are Pressure-Sensitive Adhesives?
Definitions of Pressure-Sensitive Adhesives and Bonding Agents
A bonding agent is a material used to join objects together, one that undergoes a state change from liquid to solid before and after bonding.
Pressure-sensitive adhesives are a type of bonding agent that does not change from liquid to solid. Also known as PSA, it can be easily attached to an adherend with light finger pressure.
A pressure-sensitive adhesive cannot be used straight out of the tube, like bonding agents. PSAs must be applied to a base "support" material such as film, volatilize solvent or water, and be used as adhesive tape or film.
Morphology of Pressure-Sensitive Adhesives
These can be divided into three major categories:
- Solvent types
- Emulsion types
- Non-solvent types
The characteristics and morphology of each are summarized below:
*Can be scrolled horizontally
| Morphology | Characteristics | |||
|---|---|---|---|---|
| Solvent type | Adhesive components uniformly dissolved in organic solvents
|
Good coatability | Ventilation equipment required | |
| Emulsion type | Adhesive components dispersed in water | Low environmental impact | Inferior water and weather resistance | |
| Non-solvent type | Hot melt type | Thermal fusion | A thick pressure-sensitive adhesive layer can be created with a single coating | Requires attention to high temperatures |
| UV polymerization type | Adhesive components dissolved in monomer | Polymerization inhibition is likely to occur | ||
Solvent-Based Pressure-Sensitive Adhesives
Solvent-based pressure-sensitive adhesives are adhesives in which the main components and additives are dissolved in organic solvents, and their adhesive properties are manifested as the organic solvents evaporate.
Pressure-sensitive adhesives consist of resins such as acrylics, which dissolve in organic solvents such as toluene and ethyl acetate (hereinafter “solvents”). Solvents are used to reduce the viscosity of the adhesive and improve its coatability. Moreover, because solvents evaporate more quickly than water, it is easier to achieve the dry state necessary for the proper expression of adhesive properties, thus improving the efficiency of the manufacturing process. There is a wide variety of solvents available, allowing for selection based on the solubility of additives or the adjustment of the coating surface, depending on the purpose.
However, because the inclusion of solvents poses the risk of flammability as well as health and environmental risks due to volatile organic compounds (VOCs), there is demand for products with lower VOC content. Also, note that ventilation is required in any work area when using solvent-based pressure-sensitive adhesives.
Emulsion-Type Pressure-Sensitive Adhesives
Emulsion-type pressure-sensitive adhesives are adhesives in which the primary components and additives are dispersed in water. They do not use solvents, resulting in a lower environmental impact. With the recent increase in global awareness of environmental issues, emulsion-type adhesives, which are solvent-free, have garnered attention.
However, because emulsifiers (surfactants) are used during production, these adhesives may have inferior water resistance compared to solvent-based adhesives. Additionally, due to their high surface tension and thixotropic properties, technical expertise is required to achieve stable film formation. Furthermore, it has poor drying efficiency, which may reduce productivity. As such, planning is required for its use.
Non-Solvent Pressure-Sensitive Adhesives
Non-solvent pressure-sensitive adhesives include hot melt and UV curing types, both of which are 100% active ingredient adhesives that contain neither water nor organic solvents. Since they do not contain volatile components, it is possible to achieve a thicker adhesive layer with a single application, making them environmentally friendly adhesives that do not use organic solvents. Additionally, they do not have the issue of water resistance, which is a concern with emulsion-type adhesives.
Hot melt adhesives are rubber-like resins at room temperature but can be melted and applied when heated. Because they melt at high temperatures, it is necessary to select substrates that are heat resistant, and care must be taken for safety during application.
UV curing adhesives can achieve high molecular weights, offering a high degree of design flexibility and durability. However, they also have some issues, such as a tendency for residual monomers to retain odor and the need for equipment solutions to avoid oxygen from interfering with the polymerization reaction.
Types of Pressure-Sensitive Adhesives
Adhesives are broadly divided into three major categories:
- Acrylic pressure-sensitive adhesives
- Rubber pressure-sensitive adhesives
- Silicone pressure-sensitive adhesives
Other types of adhesives include urethane pressure-sensitive adhesives.
The characteristics of each are as follows:
*Can be scrolled horizontally
| Acrylic | Silicone | Rubber | |
|---|---|---|---|
| Solvent type | Solvent type | Solvent type | |
| Adhesive strength | ＋＋＋ | ＋ | ＋＋＋ |
| Cohesive strength | ＋＋ | ＋＋ | ＋＋ |
| Stickiness | ＋＋ | ＋ | ＋＋＋ |
| Water resistance | ＋＋＋ | ＋＋＋ | ＋＋＋ |
| Heat resistance | ＋＋＋ | ＋＋＋ | ＋ |
| Weather resistance | ＋＋＋ | ＋＋＋ | － |
| Solvent resistance | ＋＋ | ＋＋＋ | ＋ |
| Cold resistance | ＋ | ＋＋＋ | ＋＋＋ |
| Price range | Mid | High | Low |
Acrylic Pressure-Sensitive Adhesives
Pressure-sensitive adhesives synthesized primarily from acrylic esters. By appropriately designing a wide variety of monomers, we can achieve excellent performance in terms of strong adhesion, heat resistance, low-temperature adhesion, high transparency, and high durability.
Generally, these pressure-sensitive adhesives have strong adhesive strength and excellent weather and heat resistance, making them more suitable for outdoor use or in humid or high-temperature environments compared to other types of adhesives. Additionally, the use of a wide range of monomers allows for a wide variety of designs, making it possible to tailor the adhesives to specific purposes. Their excellent transparency results in frequent use for optical applications. However, their adhesive properties at low temperatures are inferior to those of other types of adhesives.
Rubber Pressure-Sensitive Adhesives
Pressure-sensitive adhesives composed primarily of rubber, broadly classified into natural and synthetic rubber-based types. They tends to have strong initial stickiness, and their high adhesion and peeling strength allow them to be used on a wide variety of materials. They also boasts excellent cold resistance, allowing them to stick to adherends while maintaining wettability even in a low-temperature environment. Under high temperatures, however, the pressure-sensitive adhesive tends to soften and performance deteriorates. UV degradation is also likely to occur, making them unsuitable for use under heat or outdoor environments.
Silicone Pressure-Sensitive Adhesives
Pressure-sensitive adhesives composed primarily of silicone polymer, which boasts excellent heat resistance, degradation, and chemical resistance. They also demonstrates excellent adhesion to materials with low surface energy that other types of pressure-sensitive adhesives have difficulty sticking to. While they have heat and cold resistance that allows them to be used in any temperature environment, they are considerably more expensive than other types of pressure-sensitive adhesives. As such, they are used in special applications, as opposed to general-purpose tapes. In general, they are soft pressure-sensitive adhesives with weak tack and adhesive strength.