Adhesive Synthesis Technology

Adhesives are either made of natural materials or synthesized polymers. Acrylic adhesives are a typical example of a synthetic adhesive.

Acrylic Adhesive Raw Materials

Acrylic monomers are the raw materials used in acrylic adhesive. Acrylic monomers are also used in contact lenses, plastic glass and water-absorbing polymers. By selecting and polymerizing these acrylic monomers it is possible to synthesize acrylic polymers with certain functions and use them as an adhesive. Acrylic adhesives are widely used in LCD panels, mobile phones and automobiles due to their outstanding transparency and resistance to weather, heat and chemicals.

Synthesis of Acrylic Adhesives　　

Methods of synthesizing acrylic polymers include solution polymerization, emulsion polymerization and UV polymerization. In recent years with the increase in the awareness of global environmental conservation, emulsion polymerization and UV polymerization have been becoming more popular as they do not require the use of organic solvents. The various methods of synthesizing acrylic polymers are shown below together with their relative advantages and disadvantages.

Polymerization type	Advantages	Disadvantages
Solution polymerization	Manipulation of polymerization conditions is relatively simple. Additives can easily be used and there is a broad range of design possibilities. (Various properties can be achieved.) Thin film coating is possible.	Uses organic solvents. Requires the use of solvent recovery equipment. Difficult to use at high concentrations.
Emulsion polymerization	Uses water, making it environmentally friendly. Possible to use at high concentrations (enhanced drying performance). Additives can easily be used and there is a broad range of design possibilities. (Various properties can be achieved.) Thin film coating is possible.	Inferior resistance to humidity and water. Has a low degree of adhesion to the substrate. The adhesive surface is prone to roughness. Inferior transparency.
UV polymerization	Environmentally-friendly as it does not require the use of solvents. Easy to realize high performance (high molecular weight). Easy to make thick products. Easy to make products with strong adhesion characteristics.	Requires the use of specialized equipment, making polymerization difficult. Only a limited range of resins can be used to provide adhesion. Coating speed is relatively slow.

♦ Solution Polymerization　: With this method, radical polymerization of acrylic monomers is carried out in an organic solvent such as toluene or ethyl acetate using an azo or peroxide type initiator that produces radicals under heat in order to synthesize adhesive.
Adhesive with the desired properties is synthesized by selecting the comonomer (to control properties) or the monomer containing the required functional group (to provide the cross-linking point) and determining the amount to be blended at the same time as the acrylic monomer (the main monomer).
With solution polymerization, the molecular weight distribution and molecular weight of the acrylic adhesive to be synthesized changes greatly according to polymerization conditions (the solvent used, monomer concentration, amount of initiator, initiator addition method, polymerization temperature, polymerization time) and influences the properties of the adhesive. Normally, when adhesion is a priority, molecular weight is kept relatively low (Mw: 200,000~400,000) and when heat resistance is a priority the molecular weight is kept relatively high (Mw: 400,000~800,000).

♦ Emulsion Polymerization: Emulsion-type acrylic adhesives have advantages over solvent-type acrylic adhesives, as well as having their disadvantages. As polymerization of acrylic monomers takes place in water a surfactant is used, meaning that resistance to humidity and water is inferior. On the other hand, this method has advantages in terms of both cost and safety. It is also possible to obtain polymers with a high molecular weight that have a high concentration of solids while still maintaining high levels of drying and adhesion performance.
In recent years with the emphasis on resource and energy conservation as well as environmental issues, there has been a move towards processes that do not require the use of solvents. Emulsion polymerization also offers advantages in terms of the cost and range of equipment required.

♦ UV Polymerization: With this method, prepolymers are produced without the use of solvents or water by adding a initiator that produces radicals when exposed to light to an acrylic monomer, which is then irradiating with UV light. Next, a substrate is coated with these prepolymers before once again being exposed to UV light to produce an acrylic polymer (adhesive).

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