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Adhesive Design Technology
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.
Adhesive Materials
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
| Adhesive type | Elastomer | Characteristics |
|---|---|---|
| Rubber | Natural rubber |
|
| Acrylic | Acrylic ester copolymer |
|
| Silicone | Silicone rubber |
|
| Urethane | Urethane resin |
|
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.
Monomers used in Acrylic Adhesive
| Monomer | Structure | Tg (℃) | |
|---|---|---|---|
| Main monomer | Ethyl acrylate | CH2=CHCOOC2H5 | -20 |
| Butyl acrylate | CH2=CHCOOC4H9 | -55 | |
| 2-ethylhexyl acrylate | CH2=CHCOOC8H17 | -70 | |
| Isononyl acrylate | CH2=CHCOOC9H19 | ||
| Comonomer (To improve cohesion) |
Vinyl acetate | CH2=CHOCOCH3 | 32 |
| Acrylonitrile | CH2=CHCN | 97 | |
| Acrylamide | CH2=CHCONH2 | 165 | |
| Styrene | CH2=CHC6H5 | 80 | |
| Methyl methacrylate | CH2=C(CH3)COOCH3 | 105 | |
| Methyl acrylate | CH2=CHCOOCH3 | 8 | |
| Monomer containing functional group | Acrylic acid | CH2=CHCOOH | 106 |
| Hydroxyethyl acrylate | CH2=CHCOOC2H2OH | ||
| Acrylamide | CH2=CHCONH2 | 165 | |
| Glycidyl methacrylate |  |
||
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