Choose the right pressure sensitive adhesive after decompression

pressure sensitive adhesive, English name: pressure sensitive adhesives (abbreviation: PSA), its bonding process is very sensitive to pressure so-called pressure sensitive type, pressure sensitive adhesive is generally not directly used in the adhesive, pressure sensitive adhesive is made through a variety of materials pressure sensitive adhesive products (adhesive tape and adhesive label).

Pressure-sensitive adhesive is a kind of viscoelastic body which has both the viscous property of liquid and the elastic property of solid. This kind of viscoelastic body has the influence factors and properties of both the contact process and the failure process of bonding.

Pressure-sensitive adhesive classification:

1. Classification of the main components of the press-sensitive adhesive

① Elastic type pressure-sensitive adhesive

The elastomer used for this kind of pressure-sensitive adhesive was originally natural rubber, and then gradually expanded to a variety of synthetic rubber and thermoplastic elastomer. According to the elastomer used, this kind of pressure-sensitive adhesive can be further divided into natural rubber pressure-sensitive adhesive, synthetic rubber pressure-sensitive adhesive, and thermoplastic elastomer pressure-sensitive adhesive.

• Natural rubber pressure-sensitive adhesive

This is a kind of rubber pressure sensitive adhesive developed the earliest and the output is still large up to now. They are natural rubber elastomer as the main body, with viscosifying resin, softener, antioxidant, makeup filler and crosslinking (curing) agent and other additives complex mixture, because natural rubber has high cohesion strength and elasticity, and can be well miscible with many viscosifying resin, get high viscosity and good wetness to the adhesive material, So natural rubber is a kind of ideal pressure sensitive adhesive body material. Its main disadvantages are the existence of unsaturated double bonds in the molecule, poor light resistance and oxygen aging performance. However, its weatherability and heat resistance can be improved by cross-linking and using antioxidant. Almost all kinds of pressure sensitive adhesive products can be made with natural rubber pressure sensitive adhesive.

• Thermoplastic elastomer pressure sensitive adhesive

Thermoplastic elastomers represented by styrene-butadiene-styrene block copolymer (SBS) and styrene-isopentadiene-styrene block copolymer (SIS) are the main raw materials for manufacturing hot melt pressure-sensitive adhesives. Hot-melt pressure-sensitive adhesive does not use solvents, does not produce environmental pollution, high production efficiency, in today’s society for energy saving and elimination of environmental pollution is increasing, the importance of this kind of pressure sensitive adhesive is also increasing.

② Resin pressure-sensitive adhesive

This kind of pressure sensitive adhesive used resin polyacrylate, polyurethane, polyvinyl chloride, polyethylene ether and so on. Among them, polyacrylate is the most used, and its yield has surpassed that of natural rubber pressure sensitive adhesive.

• Acrylic pressure sensitive adhesive

Acrylate copolymer obtained by copolymerization of various acrylate monomers is one of the most important resin pressure sensitive adhesives. Compared with the rubber type pressure sensitive adhesive, they have many advantages: colorless and transparent appearance and good weather resistance; Generally do not need to use adhesive resin, softener and antioxidant additives can get good pressure sensitive bonding properties, so the formula is simple; By using copolymerization and crosslinking, pressure sensitive adhesives can be prepared to meet different performance requirements. Therefore, in the past 20 years, this kind of pressure sensitive adhesive has developed very rapidly, and has replaced the dominant position of natural rubber pressure sensitive adhesive.

• Silicone and other resin type pressure sensitive adhesives

The pressure sensitive adhesive composed of organic silicone resin and organic silicone rubber has excellent high temperature resistance and aging resistance. It is a kind of more important special pressure sensitive adhesive. Its main use is to manufacture a variety of high-grade pressure sensitive adhesive products.

Polyvinyl ether is a kind of resin pressure sensitive adhesive developed earlier, but its importance has been gradually replaced by acrylate pressure sensitive adhesive.

In addition, ethylene. Vinyl acetate copolymer (EVA), polyurethane, polyester, PVC and other resins can also be prepared into a variety of pressure sensitive adhesives.

2. Press the form classification of sensitive glue

Pressure sensitive adhesive can be divided into solvent pressure sensitive adhesive, aqueous pressure sensitive adhesive, emulsion pressure-sensitive adhesive, hot melt pressure sensitive adhesive and calendering pressure sensitive adhesive five types.

Emulsion, solvent – based and hot – melt pressure – sensitive adhesives are dominant.

3. Press whether the main polymer of the sensitive adhesive is cross-linked

Pressure sensitive adhesives can be divided into crosslinked and non-crosslinked pressure sensitive adhesives. Crosslinked pressure sensitive adhesive can be divided into heating crosslinking type, room temperature crosslinking type, optical crosslinking type, etc. Crosslinked pressure sensitive adhesive has good bonding strength, especially suitable for making permanent pressure sensitive labels.

Bonding mechanism of pressure sensitive adhesive

There is no chemical reaction between the pressure sensitive adhesive and the substrate, so the bonding force between the pressure sensitive adhesive and the substrate is mainly achieved by the intermolecular force between the polymer and the substrate (namely, van der Waals force).

However, the van der Waals force is a short-range force whose force decays rapidly with the sixth power of distance. In order to obtain high strength bonding interface, close contact at the molecular level should be established at the interface, and the molecular spacing should generally be less than 0.5 nm.

However, the solid surface is usually unable to reach the level of 1 nm, which means that the pressure sensitive adhesive needs to have sufficient wettability to spread on the substrate surface, to fill the peak and valley, to promote the two adhesive surfaces to increase the contact area through the adhesive, and to reach the short distance below 0.5 nm of molecular force. So as to achieve a strong adhesive relay.

The relationship between the application performance and the physical performance of the structure

In the use of pressure sensitive adhesive, the main performance includes adhesion and deviscosity, usually evaluated by the following indicators: initial viscosity, holding viscosity, peeling strength and failure mode.

3.1 Initial viscosity

Initial viscosity refers to the adhesion produced in a very short time after pressure sensitive adhesive is subjected to pressure. Good initial adhesion requires strong activity of the adhesive chain segment and good wettability of the substrate. Under small external pressure, it can quickly wet the rough surface of the substrate and reach the molecular level contact, so that countless intermolecular forces can produce sufficient bonding strength.

On the contrary, the bond hysteresis of the polymer with poor chain segment activity is weakened, and the initial viscosity is lower.

From the perspective of dynamics, the pressure sensitive adhesive must produce liquid-like flow under the external pressure of low frequency (about 0.01Hz) to improve the contact area on the rough surface of the substrate, thus improving the bond strength. Therefore, only when the energy storage modulus of the polymer is low, the initial viscosity will be generated.

3.2 Stickiness

Stickiness refers to the ability of a pressure-sensitive adhesive to resist persistent shear creep damage after sticking to an object.

In daily use, you will find that the double-sided adhesive tape of the bonding material will automatically fall off after a long time, and adhesive marks will remain on the surface of the bonded material, which means that the failure mode of the adhesive tape is mainly manifested as the cohesive damage of the pressure sensitive adhesive, which indirectly indicates that the adhesive property of the adhesive tape is mainly determined by the cohesive strength of the pressure sensitive adhesive.

Therefore, increasing the energy storage modulus of polymer at low frequency (frequency of load action) can improve the stickiness of pressure-sensitive adhesive.

3.3 Peeling strength

Peel strength refers to the force required for the adhesive layer to peel off from a standard substrate at a constant rate and Angle, mainly reflecting the size of the adhesion force between the pressure sensitive adhesive and the surface of the adhesive.

Some researchers have found that the peel strength of all pressure-sensitive adhesives is proportional to the ratio of (frequency 435 Hz) to (frequency 1 Hz). It is believed that the mass of pressure-sensitive adhesives can be described by the size (25 ℃) and glass transition temperature at 0.1 and 100 Hz, where the frequency 0.1 Hz corresponds to the formation of cementation. And 100 Hz corresponds to desviscosification. A good pressure sensitive adhesive is lower at low frequencies, indicating that cementation is easy to form, and higher at high frequencies, indicating that it has better resistance to stripping.

3.4 Failure Mode

Failure mode, pressure sensitive adhesive failure mainly has two forms: interface failure and adhesive layer failure. The interface failure is caused by the poor “viscosity” of the substrate, mainly depends on the adhesion strength between the adhesive and the substrate, and the adhesive layer failure is mainly caused by the low cohesion strength of the adhesive itself.

In general, what is expected to happen when stripping is interface failure (no residual glue will be left), and should try to avoid adhesive layer failure (cohesive failure), so when considering the stripping strength, it is often necessary to consider the change of failure mode of pressure sensitive adhesive.