When we are about to glue two substrates, our purpose is to carry out the bonding successfully. There is no doubt about it. However, to achieve a perfect gluing, it is necessary to analyse the relationship between two variables: surface tension and surface energy. Next, we define these phenomena and we will tell you how they influence on surfaces bonding, using hotmelt adhesives and water based adhesives.
What is adhesion? The Royal Spanish Academy defines this concept in its second acceptance as the “attractive force that holds together molecules of different chemical species”.
In this regard, José Ángel Garde Belza, from the Packaging Technologies department of the Ainia technology center, maintains the Technical Guide to Packaging that there are various theories to interpret the phenomenon of adhesion.
One of them refers to the theory of thermodynamic adsorption or wettability, according to which the adhesion is the result of the physical interaction of the surfaces in contact by the effect of wetting.
This is where another concept comes into: wettability, of great importance for all kinds of surface treatments such as printing, varnishing and gluing, and the dependency on the surface tension of the liquid, in our case hotmelt adhesives and water-based adhesives, and surface energy of the solid, that is, of the substrates to be bonded.
Surface tension and surface energy
Surface tension and surface energy are two phenomena that directly influence the bonding of two substrates.
In physics, surface tension of liquid, in our case of hotmelt adhesive or water-based adhesives refers to the amount of energy needed to increase its surface per unit of area and counteract intermolecular forces on the surface that generate an attraction inland. In other words, it is the resistance that a liquid presents to the penetration of its surface.
Therefore, the lower the surface tension, the more tendency the liquid will have to form a sphere, which is the shape that ensures a lower area / volume ratio.
Surface tension is the amount of energy required to increase the surface of the liquid, that is, of hotmelt adhesive or water based adhesive.
For its part, surface energy of a solid is the degree of attraction or repulsion that surface of one material exerts on another. Thus, as greater surface energy of the substrates to be bonded, as better will be the adhesion.
Surface energy is the degree of attraction or repulsion that surface of one material exerts on another
How to obtain a good bonding?
To achieve an optimal bonding, substrate to be bonded must have a surface energy higher than the adhesive’s surface tension, thus achieving good wettability or wetting; while hotmelt adhesive must have lower surface tension than surface energy of the substrates to help its wetting.
A liquid is considered to wet a solid when the contact angle ß (angle between the surface and the drop side) is less than 90º. If ß is larger, the droplet will have a very round shape, so surface energy of the substrate will be too low and therefore the wettability too. On the contrary, if it is small, the drop will be flat; the surface energy of the substrate will be high and the wettability aswell
Poor wetting Partial wetting Total wetting
For instance, if we want to glue a rigid PVC that has a surface energy of 39 dynes / cm with a water based adhesive (see table below), we must ensure that our adhesive has a surface tension of less than 39 dynes / cm even better if it is at least 3-4 dynes / cm below, otherwise it will not wet well and therefore the bonding will be poor.
Another option would be to increase surface tension of the material to be glued (in the example would be PVC) by applying a corona treatment, which is an electronic bombardment, or a flame treatment, or a laser treatment, etc., what they do is to orient all the electrical charges of the material in the same direction and that causes its surface energy to rise.
How do we measure surface tension?
There are colored solutions or markers on the market which can measure surface tension. In the case of markers, we will apply a line on surface of the material to be measured and, if the liquid remains without retraction for more than 2 seconds, you will know that the surface energy of the material is equal or greater than tension surface of the applied solution.
In this case, we should test higher tension surface solutions to determine surface energy value of the material.
If, on the contrary, drops are formed, it would be necessary to test other solutions with lower surface tension values until finding the value in which the liquid remained without retraction for more than 2 seconds.
As an example! If a drop of water with a surface tension of 72 dynes / cm and a temperature of 25 ° C is placed on a surface losing its spherical shape and spreading out creating a film, the material will have a surface energy of at least 72 dynes / cm.
Surface energy <72 dynes / cm Surface energy ≥ 72 dynes / cm
Therefore, to guarantee a correct bonding of two substrates it is necessary that both have a minimum surface energy of 38 dynes / cm.
Did you know that we ourselves could manufacture some known surface tension solutions from 30 to 56 dynes / cm? To do this, we will apply the ASTM D2578 standard developed by AENOR which allows mix various proportions of formamide and 2-ethoxyethanol (ethyl cellosolve).
As observed in the table below, the most conflictive substrates are usually plastics. Polyethylene and polypropylene, in particular, are the most complicated materials.
|MATERIAL||SURFACE ENERGY (dynes/cm)|
|Stainlees steel||700 – 1.100|
|Crystal||250 – 500|
|Wood||40 – 50|
|Polyester||41 – 48|
|PET||41 – 48|
|ABS||35 – 42|
|Polyethylene (PE)||30 – 31|
|Polypropylene (PP)||29 – 31|
To increase surface energy of the aforementioned materials, it is necessary to subject them to previous treatments whose effectiveness will decreases within time. Therefore, ideally, the surface energy should initially be high enough (greater than 44 dynes / cm) to avoid any problems that may arise.
After reading this article, we will know that:
- Each surface and each liquid have a particular surface tension. Therefore, to ensure good adhesion, surface tension of the material will have to be greater than that the adhesive’s.
- The lower surface tension, the more the adhesive will have to form a sphere, a form that ensures a lower area / volume ratio, therefore, the lower the adhesion.
- The higher surface energy of the substrates to be bonded, the greater the adhesion.
- When substrate that we are going to glue has a high surface energy, it means, it has a great capacity to attract, and hotmelt adhesive or water based adhesive has a low surface tension, (or which is the same thing, low resistance to deformation or breakage), then a correct wetting of the adhesive on the substrate will happen, and it will result at effective bonding.
If you have any questions about these two phenomena, you can contact us at firstname.lastname@example.org We will do our best to solve all your enquiries!