TPUs for solvent based adhesives

Technical literature

General properties

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).

Viscosity measurement

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:

• TPU concentration
• Kind of solvent
• Measurement temperature
• Viscosimeter (type, spindle, speed, time)
• Procedure followed for obtaining the TPU solution (agitation system)

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.

Solubility tables

Solvents have a very important role in the preparation of TPU adhesive. The selection of the right solvent is important because it affects:

• TPU solubility
• Adhesive viscosity
• Storage stability (particularly at low temperature)
• Surface wettability.
• Evaporation time of solvent

The solubility of the different PEARLSTICK TPU families in several solvents is shown in the following table.

Concentration of solutions: 15% solids
	45-40	45-50	45-60	45-80	45-90	46-10	46-73	49-46
Acetone
MEK
MIBK	X	X	X	X	X		X	X
Cyclohexanone
Ethyl Acetate								X
Isobutyl Acetate	X	X	X	X	X		X	X
Methylene Chloride	X	X	X	X	X		X	X
Trichloroethylene	X	X	X				X	X
1.1.1 Trichloroethane	X	X	X	X	X		X	X
Toluene	X	X	X	X	X		X	X
Xilene	X	X	X	X	X		X	X
	Soluble    X Insoluble    Swelling
This table has been obtained following a stirring process with high shear (COWLES) with a speed of 1330 rpm and during 3 hours (Method MQSA nº 55 A). The solubility has been checked after stirring by visual control.

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.

Hot tack

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:

Material of test specimens		Application (Brush)		Conditions
PVC (standard):	Cleaned with Acetone	PVC:	1 coat	Pressure:	2 Kg/cm²
Split leather:	Roughened	Split leather:	2 coats	Contact time:	4 sec.
ADHESIVE: 15% solution in MEK

Additives

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"

• Hydrophilic silica (functional structure -Si-OH-).
  It tends to form links of hydrogen bridges with the polyurethane polar groups, favouring a viscosity increase and reducing shelf life (0-5% ppw on polyurethane).
• Hydrophobic silica (functional structure -Si-CH3-).
  The addition of hydrophobic silica does not form hydrogen bridges and besides, it increases shelf life (0-10% ppw on polyurethane).

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).