DESIGNING TOMORROW’S SPECIALITY POLYURETHANES

PPEEAARRLLSSTTIICCKK®®TPUs for solvent based adhesives

January 2003

INTRODUCTION From a chemical standpoint, PU elastomers can have a crystalline or amorphous structure, depending on the relationship between the diisocyanate and the chain extender (microdiol) when both react and form a urethane group.

The structure of a PU elastomer contains soft and hard segments. The soft segments are formed by polyol groups that impart flexibility to polyurethanes. The reaction between diisocyanate and chain extender form the hard segments, which have a crystalline and rigid structure, and are bonded by chemical links and hydrogen bridges. MERQUINSA’S PRODUCTS FOR SOLVENT-BASED PU ADHESIVES Under the PEARLSTICK® tradename, MERQUINSA produces in continuous process different types of linear polyurethane elastomers, supplied in pellet form, those are mainly intended as raw materials for the production of solvent based adhesives. We are talking about polyurethane elastomers with a special, well defined characteristic: they are mainly crystalline polymers, which means that, as a result of the small proportion of “hard segments” in their structure, and of the molecular weight of the “soft segment”, they are polymers with strong properties at ambient temperature and a low meeting point of the crystalline areas (50-60ºC). Thanks to it, the polymer melts at rather low temperature during the adhesion process and has sufficient thermoplasticity and tack, in order to guarantee a good and strong bond between substrates. Besides, the low transition temperature (Tg) of the polyurethane elastomer, in spite of the high crystallisation grade, grants good flex properties at low temperature, whereas it is a strong and relatively hard thermoplastic polymer under normal conditions. An important part of the PEARLSTICK® line is obtained from ε-caprolactone and thanks to a special polymerisation process developed by MERQUINSA, polyurethane elastomers based on polycaprolactone-TPU are obtained, that are especially designed for having a higher hydrolysis resistance and an easier activation capacity than those obtained from other polyesters. The PEARLSTICK® line comprises also linear polyurethane elastomers obtained from other polyesters (mainly adipates).

GENERAL PROPERTIES OF OUR PEARLSTICK ® RANGE The several PEARLSTICK ® families differ from each other mainly in their structures (different relationship between hard and soft segments), their polyol types and corresponding, different molecular weights. That is why there is a wide range of possibilities, when we select the product that will be the best choice, considering the properties that are requested. Such properties are summarized in the following table.

GENERAL PROPERTIES OF PEARLSTICK ®

45-40

45-50

45-60

45-80 45-90 46-10

46-45

46-73

Polycaprolactone-based TPU Polyester based TPU EASY TO DISSOLVE

M - H

M

M

M

M

H

M - H

M

CRYSTALLISATION RATE

M

H - VH

VH

L

L

M - H

M

M

THERMOPLASTICITY

M

L - M

L

L

L

H

M

M

TACK RETENTION

H

H

VH

H

H

M - L

M - L

M

INITIAL PEEL STRENGTH (3 minutes)

M

H

H

M - H

H

L

M - L

M

FINAL PEEL STRENGTH (3 days)

H - VH

H - VH

VH

M

M

M

M

M - H

HEAT RESISTANCE (60ºC)

M - H

H

H

H

H - VH

L

M

M - H

FREEZING RESISTANCE

M - L

M

L

H

VH

H

H - M

M

HYDROLYSIS RESISTANCE

H

H

H

H

H

M - L

M - L

M

VH.- VERY HIGH, H.- HIGH, M.- MEDIUM, L.- LOW

In this table, we can see that: All the PEARLSTICK families are easy to dissolve by the traditional 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 crystallisation rate, depending on the polyester our PEARLSTICKs are obtained from (soft segment), we can find from high crystallisation rates (45-60) to low crystallisation rates (45-80 and 45-90), when we consider our PEARLSTICK ® on polycaprolactone based TPU.

The PEARLSTICK ® items based on polycaprolactone TPU have noticeably higher tack retention, with a special relevance in this respect of the 45-60 line. When it comes to mechanical properties (initial and final peel strength), the PEARLSTICK ® items based on polycaprolactone TPU show higher values thanks to their higher crystallinity. The PEARLSTICK ® items based on polycaprolactone TPU show a higher resistance to high temperatures and especially among them the 45-90 line, due to its structure. Regarding freeze 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 freeze resistance properties. Among the polyurethanes based on polycaprolactone-polyester copolymers, the 45-90 grades offer the best results. The PEARLSTICK ® items based on polycaprolactone TPU show a clearly better hydrolysis resistance. As a result of such properties and based on our experience, we can mention that PEARLSTICK ® 45-60 is the line with best balanced properties. Its freeze resistance can be improved, if requested, and with a much reduced alteration of its properties, with PEARLSTICK ® 45-50. The proportion of diluents (Toluene, Ethyl acetate) can be increased, in case of interest, which means in case the solubility has to be improved; formulations should be based on PEARLSTICK ® 45-40. We must mention that all the PEARLSTICK ® lines are compatible with each other.

MEASUREMENT OF THE VISCOSITY OF THE PEARLSTICK ® ITEMS. Every PEARLSTICK ® line is available in a wide range of viscosities. Due to its easy measurement with a viscometer, it is normally accepted to define the average molecular weight of polyurethane by the viscosity of a solution based on such polyurethane. 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:

Polyurethane concentration Kind of solvent Measurement temperature Procedure followed for obtaining the Polyurethane solution (agitation system)

With standardised factors, the measurement of the resulting viscosity becomes a usual and widely accepted way of identification of polyurethane. For a better identification of the product that has to be used, the last 2 figures of the reference number of the different PEARLSTICK ® lines refer to the average viscosity at 20ºC of a 15 % solution in MEK, obtained in accordance with MERQUINSA’s method (MQSA Nº 55A). For instance, for PEARLSTICK ®45-60/18, the average viscosity will be of approx 1800 mPa.s, measured at 20ºC, which is within the specifications of this item (1600-2000 mPa.s at 20ºC).

PEEL STRENGTH AT DIFFERENT ACTIVATION TEMPERATURES A study describing the difference between adhesives formulated with polycaprolactone-based TPU, 1-6 hexanediol adipate based TPU and 1-4 butanediol adipate TPU is shown. Important differences can be observed, in the values obtained in initial (1 minute) and final (3 days) peel-strength, depending on the activation temperature of the adhesives. Practically speaking, this graphic shows that in the bonding process of a shoe sole and upper, at the end of the drying oven (the materials may be at a temperature between 45-60ºC when the bonding takes place, depending on the situation of the operator) the adhesive mainly based on polycaprolactone TPU allows a wide time margin for assuring an optimum bonding, whereas the adhesives made out of 1-6 hexanediol adipates TPU, not to mention those based on 1-4 butanediol adipates TPU, allow a much shorter time margin and with a very low adhesive strength.

PEEL STRENGTH AT DIFFERENT ACTIVATION TEMPERATURES

ACTIVATION TEMPERATURE (º C)

DIFFERENCE BETWEEN PEARLSTICK® LINES Based on the DMTA, we can see the differences in the properties between two PEARLSTICK ® families that are both based on polycaprolactone TPU, such as PEARLSTICK ® 45-60/18 and PEARLSTICK ® 45-40/19. The crystallisation value is given by log. G' (elastic component), that is to say the PU modulus decreases when temperature increases (the soft segments fuse) and it reaches a fused state. The process is the same for both items, in the same temperature range (45-55ºC), but the gradient of the curve is not the same. The hot-tack value is given by log. G'' (viscous component). In this case, when the temperature rises, the PU module grows, until reaching a maximum value, at which stage the tack value is the highest. Thermoplasticity (idea of resistance to temperature) is given by the value of tag. delta, and the higher tag delta, the more thermoplastic the PU.

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 ad also measures the strength needed for unbonding, 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, materials, bonding temperature and bonding pressure, can be studied.

TACK TESTER

In shoe production and specifically, in sole bonding, the adhesive used should have a high hot tack, so as to guarantee a perfect initial bonding, even at relatively low temperatures (50-60ºC). Studies on PU’s based on polycaprolactone (PEARLSTICK ® 45-60) show that they have a higher hot-tack than products of another nature.

Material: PVC/PVC

Conditions: Pressure : 2 Kg/cm2 Contact time : 4 sec.

HOT-TACK (Solvent-based adhesives)

0

20

40

60

80

100

120

140

30 40 50 60 70 80 90

ACTIVATION TEMPERATURE (ºC)

N/c

m2

P- 45-60Competitor

EFFECT OF VISCOSITY ON THE CHANGE IN PROPERTIES As a rule, we could say that for a given PEARLSTICK line (for instance PEARLSTIC ®

45-60), a change in viscosity (molecular weight) of the formulated adhesive will result in a change in its properties. For instance, we can see that, a decrease in viscosity (molecular weight) is accompanied by an increase in solubility (easy solution and increased diluents tolerance), and crystallisation rate and thermoplasticity grow as well. On the opposite, with an increase in viscosity (molecular weight), the initial and final peel strength increase, as well as heat resistance, and a higher activation temperature will be needed.

PU ADHESIVE PROPERTIES COMPARATIVE CHART The following figure depicts the main properties of the different solvent-based PU adhesives, depending on the kind of polyester they have been formulated with, and compares them with what the ideal adhesive would be. Adhesives based on polycaprolactone TPU, 1-6 hexanediol adipate TPU and 1-4 butanediol adipate TPU are compared. We can see that with low activation temperatures (due either to the equipment conditions or to the nature of the materials) the adhesives formulated with polycaprolactone based TPU perform better than those based on 1-6 hexanediol adipate based TPU or 1-4 butanediol adipate based TPU, concerning the properties that can be required to a PU adhesive, such as: initial peel at relatively low temperatures (40-50ºC), easy activation, long hot tack, good thermoplasticity. Compared with 1-6 hexanediol adipate based TPU, polycaprolactone based TPU shows slightly lower heat resistance and crystallisation rate.

HOW TO DISSOLVE TPUs The mechanism, by which a linear TPU elastomer changes from solid to liquid form in the presence of a solvent when a solution is prepared, can be described as follows. The polyurethane 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 a homogeneous solution is eventually formed. The swelling corresponds to the penetration of the solvent in the polyurethane molecules, followed by the absorption and combination of the polyurethane 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, We can say that 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 were 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 (gelled solution).

STIRRING SYSTEMS According to our experience, the best stirring systems are those that have a shearing effect on the polyurethane elastomer (PEARLSTICK ®) when the solution is being prepared. Among them, the COWLES stirrer is especially adequate. In the design of the equipment, special care must be given to the dimensions and the relationship between them. The relationship between the diameter of the stirrer (d2) and the diameter of the reactor or mixing vessel (d1) is important for obtaining quickly a good solution.

STIRRING SYSTEMS

SOLUBILITY OF THE PEARLSTICK ® LINE IN SOLVENT MIXTURES. The most widely used solvents are acetone and MEK. The PEARLSTICK® items have a better solubility in acetone than in MEK. In the adhesive, diluents make the polyurethane molecule swell, which increases the viscosity during shelf-life. They also improve freeze resistance at low temperatures, among the main diluents; toluene and ethyl acetate can be mentioned. The following table shows the maximum solvent/diluents ratios that have to be used when formulating an adhesive based on linear polyurethane (PEARLSTICK ®).

ACETONE/TOLUENE

ACETONE /ETHYL ACETATE

MEK /TOLUENE

MEK /ETHYL ACETATE

45-40 /1545-40 /1945-50 /18 65/35 75/25 75/25 80/2045-60 /1445-60 /1845-80 /16 70/30 85/15 90/10 90/1046-10 /0646-10 /1646-45 /15 60/40 55/45 80/20 75/2546-73 /19

50/50 40/60 70/30 60/4060/40 60/40 80/20 70/30

70/30 80/20 80/20 80/2070/30 80/20 80/20 85/15

10/90 0/100 20/80 0/10015/85 0/100 20/80 0/100

65/35 60/40 80/20 75/25

Concentration: 15% solids Agitation: Rotary bottle device Speed: 50 r.p.m. Time: 48 hours

The solutions have been prepared at a concentration of 15 % solids with a non-shearing stirring device (rotatory bottle device) with a rotating speed of 50 rpm during 48 hours.

ADDITIVES IN SOLVENT BASED PU ADHESIVES A variety of additives can be added to PU adhesives in order to improve a series of properties. Among the most important, we can find:

FUMED SILICAFUMED SILICA

Hydrophilic silica

Hydrophobic silica

1 - 5%/ PU

1 - 10%/ PU

ACIDSACIDSMalonic acidFumaric acid 1 - 3%/ PU

RESINSRESINS

Coumarone-indeneTerpene -phenolicVC/VA

0 - 5% / PU

The addition of “Fumed silica” increases the crystallisation rate of the adhesive, prevents the formation of threads and favours an adequate reological application. There are two types of “fumed silica” Hydrophilic silica (functional structure –Si-OH-) tends to form links of hydrogen bridges with the polyurethane polar groups, favouring a viscosity increase and reducing shelf life. Add 0-5% on polyurethane. Representative items that can be mentioned: AEROSIL 200, CAB-O-SIL M-5, WACKER HDK N-20. Hydrophobic silica (functional structure –Si-CH3-). The addition of hydrophobic silica does not form hydrogen bridges and besides, it increases shelf life. Add 0-10% on polyurethane. Representative items that can be mentioned: AEROSIL R-972, CAB-O-SIL TS 610, WACKER HDK H-20. Malonic acid (better that fumaric acid, because of its higher solubility), improves the adhesion on rubber and SBR. Add 1-3% on polyurethane. The addition of different resins, such as coumarone, alkyl-phenol resins, etc., improves the hot-tack value but reduce the crystallisation rate. The quantity to be added varies, but is of 0-5% on polyurethane. The addition of vinylchloride vinyl acetate improves the adhesion to PVC and metallic materials. The quantity to be added can vary between 0-5% on polyurethane.

ENVIRONMENT-FRIENDLY SOLVENTS The solvents evaporated in the factories are expelled to the atmosphere, usually by mechanical extraction, so as to minimise the flammability and toxicity dangers. We will explain in a few words some of the definitions according to EPA (Environmental Protection Agency of the United States) ODS (Ozone-Depleting Substances) – They are substances that destroy ozone in the high atmosphere layers, which is responsible for the protection of the atmosphere from the UV rays. VOCs (Volatile Organic Compounds) – Besides their technical definition (based on temperature and pressure conditions), there is a general definition that describes them as any carbonate compound that participates in photochemical reactions in the atmosphere. They are known because they generate ozone at earth level, which is one of the main pollution components (smog). This is because such compounds react photochemical with the nitrogen oxides in the presence of sunlight, producing chemical oxidants, of which ozone is the most widely known. The created pollution is toxic and attacks the biological processes, and thus damaging vegetation.

HAPs (Hazardous Air Pollutants) – In this case the word hazardous means that they cause a series of adverse human health and environmental effects. Such solvents are listed in the Toxic Chemical Release Inventory (TRI).

The different laws tend to increasingly control pollution and VOCs emission, mainly in solvents. Three strategies can be adequate for such purpose: 1- Switch to solvent-free adhesives, 2- Store and destroy the solvent vapours, 3- Development of friendly solvent systems. Since June 1995, EPA has removed acetone from the list of TRI Chemicals, because it was considered a non-VOC solvent. Based on this fact, MERQUINSA carried out a study in 1997 and presented a paper on “Environment-friendly solvent-based TPU adhesives”, where the changes in properties of PEARLSTICK 45-60/14 were analysed when it was formulated with acetone and mixtures of different solvents (none of them included in the TRI list) versus conventional Acetone/Toluene and/or Ethyl Acetate mixtures (included in the TRI list). The following table shows the main solvents that can be used for the production of polyurethane adhesives, classified as per the previously mentioned definition criterion.

No VOCs VOCs

No HAPs No HAPs HAPs

Solvents Acetone --- Tetrahydrofurane(THF) Dimethylformamide (DMF) MEK

Diluents Methyl Acetate Oxsol-100 Ter-Butyl Acetate (TBAc)

Cypar 7 Cypar 9 Citrosol V Orange terpenes

Toluene Xylene Ethyl acetate Hexane Cyclohexanone

Oxsol-100: (p-chloro trifluoromethyl benzene) (Occidental Chemical Corp.) Ter-Butyl Acetate (TBAc) (Lyondell Chemical) Cypar 7, Cypar 9: (Shell Chemical Co.) Citrosol V: (Cinene 1,8 (9) p-mentadiene) (Moellhausen Trading SpA)