TPUs for solvent based adhesives
Under the PEARLSTICK® TPU trade name, MERQUINSA makes a wide range of linear thermoplastic polyurethane elastomers (TPUs), which have been specially developed for use as raw materials in the manufacture of solvent-based adhesives for the automotive, furniture, footwear, and other industries.
PEARLSTICK® TPUs are mainly crystalline polymers. Their structure is characterized by a small proportion of hard segments, in comparison with their content in soft segments. It is their polyol component (the part traditionally known as the soft segment or amorphous part) that crystallises and imparts them such crystalline characteristics. The resulting polymers show outstandingly high tenacity and stress-strain power.
These TPUs have another interesting feature: a low melting point in their crystalline zones (50-60ºC), which permits the polymer to be softened or melted at relatively low temperatures for the adhesion process, with sufficient thermoplasticity and surface tack to ensure correct union between substrates of different materials.
Using the various types of PEARLSTICK® TPU as the basis, many solvent-based TPU adhesives can be made with a variety of organic solvents, for subsequent application in various fields.
The several PEARLSTICK TPU families are differentiated from each other mainly in their structures (different relationship between hard and soft segments), their polyol types and their respective molecular weights.
Such properties are summarized in the following table:
In this table, we can see that:
All the PEARLSTICK families are easy to dissolve by the usual stirring processes. The admission of a bigger or smaller quantity of diluents will depend on the structure of each family and will be directly related to the proportion of hard segments that can be found in each of them.
Regarding the crystallization rate we can find from high crystallization rates (45-60 line) to low crystallisation rates (45-80 and 45-90 lines).
The PEARLSTICK TPU types based on polycaprolactone-copolyester TPU have higher tack retention, with a special relevance in this respect of the 45-60 product line (hot-tack).
When it comes to mechanical properties (initial and final peel strength), these PEARLSTICK TPU grades show higher values thanks to their higher crystallinity.
Moreover, these show a higher heat resistance and especially the 45-90 product line, due to its structure.
The PEARLSTICK TPU types based on polyester-copolyester TPU offer the best heat resistance.
Regarding freezing resistance, it must be mentioned that all the solutions of crystalline polyurethanes freeze at low temperature and they return to their original state without loosing their properties when the solutions thaw. The freezing temperature will depend mainly on the structure of the considered polyurethane (proportion of hard segments) and the crystallinity of the soft segment. The presence of diluents in the solvent mixture improves the freezing resistance properties. Among all the PEARLSTICK TPU families, the 49-46 product line shows the highest freezing resistance.
The PEARLSTICK items based on polycaprolactone-copolyester TPU show a clearly better hydrolysis resistance.
All the PEARLSTICK lines are compatible with each other.
How to dissolve TPUs
The mechanism, by which a linear TPU changes from solid to liquid form in the presence of a solvent when a solution is prepared, can be described as follows.
The TPU swells first and then progressively, and due to the absorption of the solvent by the solid, it takes the aspect of a viscous solution and finally a homogeneous solution is formed.
The swelling corresponds to the penetration of the solvent in the TPU molecules, followed by the absorption and combination of the TPU with the solvent. Such combination origins a progressive separation of the macromolecules, breaking the intermolecular links (hydrogen bridges) and destroying the forces that form the initial structure, Such forces are much weaker in the crystalline phase (soft segment) than in the amorphous phase (hard segment). When all the links are broken, the macromolecules can move easily and a homogeneous solution is then obtained.
Depending on the solvent mixture and on the given quantity of intermolecular forces, the capacity of separating and breaking the links that join the macromolecules will be different.
If such links are not broken (due either to a faulty stirring, or little solubility of the solvent mixture), the final solution will tend to return to its initial structure, and eventually form a non-homogeneous solution (gel solution).
All PEARLSTICK TPU product lines are available in a wide range of viscosities.
Due to its easy measurement with a viscometer, as a general rule it is accepted to define the TPU average molecular weight of TPU by the viscosity of its solution.
Prior to establishing the relationship between the viscosity and molecular weight measurements, different factors that can affect the determination of the viscosity values should be taken into account, such as the main, following ones:
With standardised factors, the measurement of the resulting viscosity becomes a usual and widely accepted way of TPU identification.
The following figure is an example.
Solvents have a very important role in the preparation of TPU adhesive. The selection of the right solvent is important because it affects:
The solubility of the different PEARLSTICK TPU families in several solvents is shown in the following table.
Maximum solubility in solvent mixtures
The following table shows the maximum ratios of solvent / diluents which are acceptable for a good solubility.
These solutions have been prepared with a concentration of 15 % solids of PEARLSTICK TPU under slow agitation and without any shear force (in a rotary bottle device), at a speed of 50 rpm and during 48 hours (Method MQSA Nº 55 B).
The solubility has been checked after stirring by visual control.
MERQUINSA has used a Tack tester for such purpose. It is formed by an activation unit (flash lamp) and a dynamometer which applies a given pressure on the substrates and also measures the bonding strength.
This device reproduces rather exactly this property (for instance in shoe production, the bonding of a shoe sole to the upper) as production variables such as: activation temperature, time allowed before bonding, kind of materials, bonding temperature and bonding pressure, can be studied.
In the footwear industry, and specifically, in sole bonding, the adhesive must have a high hot tack, so as to guarantee a perfect initial bonding, even at relatively low activation temperatures (50-60ºC).
Studies on several PEARLSTICK TPU are shown below:
A variety of additives can be added to solvent-based TPU adhesives in order to improve a series of properties. Some of the most important ones are described in this section.
The addition of "Fumed silica" increases the crystallization speed of the adhesive, prevents the formation of threads and favours an adequate rheological application.
There are two types of "fumed silica"
The addition of Malonic acid (better than fumaric acid, because of its higher solubility), improves the adhesion on rubber and SBR (1-3% ppw on polyurethane).
The addition of different Resins, (such as coumarone, alkyl-phenol resins, etc.), improves the hot-tack value but reduces the crystallization rate (0-5% ppw on polyurethane).