Adhesive consists of various raw materials, as we can see from the diagram below. These materials are combined according to the desired application to create an adhesive with various functions, which is then used in an adhesive tape. Adhesive tape is extremely convenient, sticks immediately and can be preprocessed to a designated shape, which is why it is widely used in LCD TVs, mobile phones and automobiles. In applications where items are required to be permanently secured, such as with automotive parts, adhesives must be strong, while on the screens of mobile phones protective film must peel off easily and leave no adhesive residue. The adhesive used with polarizing films must be transparent and weather-resistant and the adhesives used in the interior of vehicles must have only minimal traces of VOCs. With each of these different applications a different type of adhesive is required. We therefore design them to achieve blends that are optimized to the different applications in order to realize the required functions. We use special designs to produce adhesive for use in the processing of silicon wafers and ceramics that enable it to be strong under normal conditions, but to lose its strength when exposed to heat or UV light.
It is possible to create various kinds of adhesive according to how much of certain kinds of materials are added to an elastomer. Determining how much of what kind of material is required in an adhesive is the task of adhesive design technology.
Types of Adhesive
Rubber, acrylic and silicone adhesives are used according to the required application and each of them must be designed using the different properties of the elastomer.
♦ Rubber Adhesive
As natural rubber itself only has a low degree of adhesion, normally some kind of tackifier is added. This makes it possible for it to stick to a wide range of adherends and it is often used in packaging tape.
♦ Acrylic Adhesive
By selecting an acrylic monomer and carrying out copolymerization it is possible to synthesize acrylic polymers with various functions that enable them to be used as adhesives. As it is outstanding in terms of transparency and resistance to weather, heat and solvents it is widely used in LCD panels, mobile phones and automotive applications.
When designing synthetic adhesives (selecting acrylic monomers) it is important to consider the Tg of synthesized acrylic polymers (the temperature at which optimum adhesion to the adherend is achieved), the insertion of cross-linking sites* (durability and heat resistance) and the copolymerizability of the acrylic monomers (the uniformity of the acrylic polymer and cross-linking sites).
* Insertion of cross-linking sites: When manufacturing adhesive tape, a cross-linking agent, such as isocyanate or epoxy, is added to give the acrylic polymer a 3D structure. Acrylic polymers require cross-linking agents and monomers containing functional groups that are capable of cross-linking, such as acrylic acid or hydroxyethyl acrylate.
As acrylic polymers have a high polarity it is difficult for them to adhere to adherends with a low polarity, such as polyolefin. In some cases this can be improved by adding a tackifier.
♦ Silicone Adhesive
Silicone rubber is outstanding in terms of its resistance to extremes of hot and cold and can be used in a wide range of temperatures. As it has only a low degree of adhesion itself, silicone resin is used as a tackifier. By changing the structural ratio of silicone rubber and silicone resin it is possible to obtain the required adhesion characteristics. It is also cross-linked with cross-linking agents, such as benzoyl peroxide, in order to improve heat-resistance.
♦ Urethane Adhesive
This type of adhesive is used in applications such as protective films and fluff removers on account of its outstanding removability.
As a comprehensive manufacturer of adhesive tape, Nitto Denko has every kind of adhesive and has the ability to create the functions required to suit a particular application. In order to make the most of material properties, in addition to designing adhesives based on material adhesion theory and interfacial chemistry, we also use solution-oriented technology to meet demands concerning new adhesive functions.
Adhesive Types and Characteristics
Natural rubber is cheap.
Small selection of adherends.
Only a minor increase in adhesion as it has non polar group.
Deteriorates in terms of resistance to heat and weather.
Acrylic ester copolymer
Polymer itself is adhesive.
Better resistance to heat and weather than rubber adhesives.
Large selection of adherends.
Can be used in a wide range of temperatures.
Outstanding resistance to heat and cold.
Outstanding resistance to chemicals and weather.
Only minor degrees of malodor and skin irritation.
Outstanding moisture permeability.
Difficult to produce strong adhesion and tack.
The Relationship between the Number of Carbon Atoms in the Alkyl Group and Tg
The Tg of polymer synthesized when the (meta) acrylic ester monomer undergoes polymerization differs according to the number of carbon atoms in the (meta) acrylic ester monomer R (alkyl group) (see below). It is possible to synthesize an acrylic adhesive with a Tg correlating to the properties desired in the adhesive.