Hot melt pressure-sensitive adhesive resin to increase mechanism and choice

Production of hot melt pressure sensitive adhesive material most of the solid, at room temperature is basically no viscosity. Why do some of the materials mixed with each other will produce sticky? I believe that many people will have the same doubts as me. In particular, when we add the material which we call the increasing viscosity, the initial viscosity and peel strength are obviously increased, and the initial viscosity is closely related to the increase of the viscosity of the resin. It is discussed that the mechanism of initial adhesion formation is consistent with the mechanism of the formation of the bond strength and initial adhesion, which is the same as the Fan Dehua force.

According to the existing data and previous studies. Broadly, there are two theories: a view of the viscous nature is determined by the morphology of the two-phase system of adhesive; also a point of view that viscosity of a viscoelastic deformation characteristics of the decision. At present, the industry is more inclined to the latter view, because the use of viscoelastic theory can explain more experimental facts.

Visco elasticity is a research branch of solid mechanics, and it is called rheology. Based on the elastic and viscous properties of materials, the distribution law of the internal stress and strain and the relationship between them and the external force are studied. The viscous properties of materials are mainly related to the stress and strain rate in the materials. The viscoelastic body can be understood as a mixture of the elastic body and the liquid. The elastic part is assumed to be static, while the liquid part is not static. When the derivative of the strain is not zero, the liquid part is due to the presence of the micro friction, the viscosity, and assume the dynamic stress. Therefore, a static viscoelastic body is equivalent to a pure elastic body.

And the static water has no internal shear stress, which is a limit of the viscoelastic body. Pure elastic bodies in the process of the occurrence of strain with time, without viscosity, strain does not lag behind the stress, which is a viscoelastic body of the other extreme. Therefore, the viscoelastic body is between the solid and liquid. Hot melt pressure sensitive adhesive is a typical viscoelastic body.

C.A.Dahlquist first pointed out that the addition of the adhesive resin can reduce the elastic modulus of the pressure sensitive adhesive. From the point of view of the viscoelastic body, the viscosity of the resin is the main reason of the initial viscosity. Do a lot of research in Japan in this regard by Fu Kanazawa, terpene resin with different softening point to draw the following conclusions:

1, with the increase of the proportion of resin, the apparent viscosity of the system decreased, and the initial viscosity increased;

2, when the apparent viscosity reaches the lowest value, the initial adhesion strength reaches the maximum, that is, the softening point of the resin is lower, the higher the value of the resin concentration is reached;

3, resin softening point is low, the minimum viscosity value is smaller, and the highest initial adhesion force is the biggest.

From the point of view of viscoelastic analysis, according to the calculation, when the viscosity of the system is reduced to the 106~105Pa.s, viscoelastic even in very slight pressure is sufficient to produce larger deformation and flow, a few seconds of time, it also can good wetting is adherend surface, resulting in a viscous force. The smaller the viscosity of the system, the more full, the greater the initial viscosity.

I think that the three big mechanics of hot melt pressure sensitive adhesive (initial adhesion, stripping force, holding capacity) phenomenon is the embodiment of Fan Dehua force. And now we use van Edward to explain the mechanism of increasing viscosity. There are two necessary conditions for the formation of van Edward force, which is close enough to the distance (moisture) and sufficiently large electrostatic force (induced force, dispersion force, orientation force). Increase the viscosity of the resin, the system will reduce the apparent viscosity, the more the more adequate moisture. Under the premise of wetting, the greater the polarity of the molecules (the larger the induction and the orientation force), or the larger the molecular weight (the larger the dispersion force), the greater the van Edward force is formed. This is why we use the hot melt adhesive, often need to add a polarity of the adhesive resin.

So we have to choose how to increase the resin? This depends on your recipe system and customer's requirements. One of the prerequisites for selecting the one of the increasing viscosity resins is that it is completely compatible with other ingredients in the formulation system, especially the elastomer. The compatibility of raw materials is related to their solubility parameters. The more close the solubility parameter, the better. Solubility parameter is defined as the square root of cohesive energy density. The cohesive energy of the material is the sum of the molecules of the material (van Edward), which is determined by the molecular structure, especially the polarity of the molecules. Usually, the ordinary viscosity resin is a mixture of different sizes, so the molecular weight distribution of the resin is also important.

Second, the low smell and color of the product will be the majority of customers will get a good impression, of course, is also an important factor to consider the excellent aging resistance.

On the total, a comprehensive performance of a good viscosity resin should be a narrow molecular weight distribution, with polarity, low odor, color, light, aging resistance, low price. Of course, we can according to different product performance and market demand to give priority to select several indicators.