TPU: the first commercial TPEs.

Otto Bayer and his colleagues at I.G. Farbenindustrie did pioneering work on polyurethanes in the late 1930s (ref. 1). At a later date, Du Pont (ref. 2) and ICI (ref. 3) recognized the elastomeric properties of the polyurethanes which led to production on an industrial scale in the 1940s (Ref. 4). The properties, however, were quite poor. Water was used as the chain extender and the diisocyanate was naphthalene-l,5-diisocyanate (NDI) (ref. 5).

Great progress on the properties of TPUs was made in the 1950s in the laboratories of BF Goodrich by Schollenberger, et al. (refs. 6 and 7). In the early forties, BFG had a keen interest in developing new synthetic rubbers. Bayer% success with polyurethanes stimulated interest in the post-war years leading to an active research program in polyurethanes beginning in l 947. A licensing agreement was reached with Bayer in 1953 dealing with polyester preparation and elastomeric fluid-casting techniques. Another agreement covered technology for isocyanate foam manufacture; however, the Chemical Division was excluded from the agreement. Since no diisocyanate technology was purchased, this technology was developed internally. Early work concentrated on paraphenylene diisocyanate (p-PDI).

Pilot plant scale development of polyesters and diisocyanates were initiated at the Avon Lake Technical Center in 1953. Millable gums which could be processed on conventional rubber machinery were some of the early products of the polyurethane research program. When the product was found to be unacceptable for tire tread stock, the project was terminated. Liquid casting systems based on p-PDI were the next development; however, by the end of 1955, a significant change had occurred in the program.

The rapid expansion of diisocyanate facilities by Du Pont, Mobay and National Aniline made the diisocyanate business unattractive except for p-PDI. Although the casting system showed great promise, considerable technical effort was still required for process development and the commercialization was dependent on the manufacture of p-PDI. When the decision was made not to manufacture p-PDI, the major emphasis of the program shifted to thermoplastic polyurethanes. The thermoplastics were truly unique materials at the time as similar polymers were not being offered by competing companies.

An active market development program was initiated for Estane VC thermoplastic. VC was used to indicate that the polyurethane was virtually crosslinked (ref. 7), i.e. it performed like a crosslinked rubber. In the early phases of the research, the thermoplastic polyurethanes were produced by a batch process. The proposed applications were wire jacketing, fuel storage ceils, leather substitutes, coated fabrics, whitewail tire paint, skin on latex foam, calendered sheet and film applications. The inability to produce gel free extrusion products in the batch process retarded the market development in this area. A breakthrough occurred when one of the softer products (i.e. less urethane) began to be used as a shoe adhesive. The demand for this product led to development of a continuous process.

In the late fifties, the process development work concentrated on a continuous process to produce thermoplastics. With the advent of the continuous process, products with enhanced quality for extrusion applications became available. With the increased market acceptance, the first commercial thermoplastic polyurethane plant was built in the early 1960s.

Chemistry

The urethane reaction is an addition reaction between a hydroxyl group and an isocyanate as shown in figure I. The isocyanate group has several isoelectronic structures shown in figure 2. In the case of aromatic isocyanates, additional resonance forms exist, making these more reactive than aliphatic isocyanates. The considerable electrophilic character of the carbon atom of the isocyanate group is responsible for its reactivity. The reactivity of isocyanates with active Hcompounds is summarized in table 1. Amines are the most reactive, while hydroxyl, carboxyl, water, urea and urethane groups are less reactive. It is noteworthy to point out that the reaction with water consumes two equivalents of the isocyanate, and may result in crosslinking, and possibly gels in the TPU reaction.

TPU polymerization 

The polyurethane reaction, by analogy, is the reaction of a multifunctional (usually difunctional) hydroxyl compound (polyol or diol) with a polyisocyanate, to give a polyurethane. In the case of a difunctional alcohol and diisocyanate, the result is a linear polymer. When the functionality of the alcohol or the polyisocyanate is greater than 2, crosslinking may occur.

Raw materials

There are three basic ingredients in commercial polyurethane materials, the macroglycol, the chain extender and the diisocyanate. Macroglycols can be classified as polyester based, polyether based, polycarbonate based or polycaprolactone based.

The polyester type polyols are prepared by a condensation reaction between a short-chain diol and a diacid, for example, the reaction of 1,4-butanediol with adipic acid to give poly(tetramethylene adipate)glycol. Other diols and diacids can also be used.

* Polyester - poly (tetramethylene adipate) glycol.

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The vast majority of polyether based thermoplastic polyurethanes use poly(tetramethylene ether)glycol as the macroglycol. This is in contrast to polyurethane foams, where poly(propyleneoxide)glycol and poly(ethyleneoxide)glycol predominate.

* Polyether - poly(tetramethylene ether) glycol.

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Polycarbonate polyols are considered somewhat as specialty polyols because of their much higher price. These polyols do have excellent hydrolytic stability.

* Polycarbonate - poly(hexamethylene carbonate) glycol.

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Polycaprolactone polyols are similar to polyester polyols in their properties and benefits, although there is an improvement in some properties, e.g. better hydrolytic stability. The cost of these polyols is, however, somewhat higher.

* Polycaprolactone - poly([Epsilon] - caprolactone) glycol.

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Chain extenders are generally low molecular weight organic diols. Some examples are 1,4-butanediol, neopentyl glycol, hydroquinone bis(2-hydroxy ethyl)ether, ethylene glycol, 1,6-hexanediol and 1,6-cyclohexane-dimethanol.

Diisocyanates can be classified as aromatic (isocyanate group attached to an aromatic ting) or aliphatic. Some commercially important aromatic diisocyanates are 4,4'diphenylmethanediisocyanate (MDI), p-PDI and toluene diisocyanate (TDI). Aliphatic diisocyanates have the advantage of being less sensitive to degradation and discoloration by UV light. However, they are much less reactive than aromatic diisocyanates.

Polymerization method

Thermoplastic polyurethanes can be prepared by a variety of methods. Three common methods are one-shot polymerization, prepolymer method (two-step) and solution polymerization. There are advantages and disadvantages to each method.

In the random, or one-shot, polymerization the reaction streams consist of: 1 ) the diisocyanate, and 2) the macroglycol/chain extender mixture. Stabilizers, lubricants and other additives can also be mixed in with the glycols. All ingredients are mixed simultaneously, resulting in a more random block polymer. The one-shot polymerization can be done either as a batch or a continuous process.

The prepolymer method is a two-step process wherein an isocyanate terminated prepolymer is prepared by reacting together the macroglycol and the diisocyanate. This prepolymer is then, in a second step, further reacted with the chain extender to consume the remaining isocyanate groups. The prepolymer method is usually thought of as a batch process, and gives a polyurethane which is less random, more blocky.

Solution polymerization methods are also practiced, using a variety of common solvents, e.g. toluene, MEK, THF and DMF. The solution process can be done at lower temperatures, and can be done as either a one-shot polymerization or a two-step process.

A variety of additives are available which can be compounded into the TPU formulation. These include ultraviolet stabilizers, antioxidants, hydrolytic stabilizers, lubricants, fillers, pigments, flame retardants, glass fibers, etc.

Polymer physics

The structure of thermoplastic polyurethanes (ref. 9), consists of a blocky, segmented structure made up of hard and soft segments. The hard segments are composed of the reaction product of the chain extender and the diisocyanate. The soft segment consists of the reaction product of the macroglycol and the diisocyanate. The ratio of these two phases and their degree of separation determine, to a great deal, the properties of the resultant TPU.

The TPU hard segments tend to crystalyze together forming virtual crosslinks which are partly responsible for the excellent strength in TPUs. These crosslinks are reversible, however, and can come apart by the action of heat or salvation. Crystallization of the hard segment is due, in large part, to hydrogen bonding.

The TPUs made from polyester polyols show more phase mixing between the hard and soft segments due to the better hydrogen bonding between the polyester groups and the urethane groups as compared to the amount of hydrogen bonding between the polyether oxygen atoms and the urethane groups (ref. 9).

Structure - relationship variables

The number of different variables in the TPU synthesis allows for tailoring of the polymer properties. For example, table 2 shows the effect of the macroglycol structure on the properties of the TPU. Another factor which will influence the final properties is the macroglycol molecular weight. Commonly, polyols have a MW between 750-4,000. Higher MW polyols tend to have lower Tg and better low temperature properties. However, the higher MW polyols may exhibit less phase mixing between the hard and soft segments, which can lead to deleterious effects (i.e. poor processability).

The choice of chain extender can have a dramatic effect on the polymer properties. Since the chain extender ends up in the hard segment, changing its structure can affect the crystallization and phase mixing. Substituting the bulky neopentyl glycol for the unhindered 1,4-butanediol results in a less crystalline TPU. Using hydroquinone bis (2-hydroxy ethyl)ether makes a TPU which is very crystalline.

As mentioned previously, the diisocyanate structure also has an effect on the TPU properties. Commercial MDI is actually a mixture of two isomers (4,4' and 2,4'). The ratio of these two isomers in the MDI can also have an effect on the properties. One of the TPU variables which is most often varied is the stoichiometry, expressed as NCO/OH percentage, or ratio. A diagram of TPU molecular weight vs. NCO/OH ratio is shown in figure 3 (tel. 10). Since this is a condensation polymerization reaction, the ideal stoichiometry to achieve the highest Mw is 1.00. Values below 0.9 or greater than 1.0 give low Mw polymers with only very limited applications (e.g. some adhesives). To produce a TPU with adequate strength, it is felt that the Mw should be above 100,000 (GPC Mw vs. polystyrene standards). To achieve a high MW polymer it is necessary to control the ratio to a very narrow range (figure 4). At stoichiometries greater than 1.00, an excess of isocyanate is present, which can lead to crosslinking via allophanate formation. This reaction is reversible, and free isocyanate can be generated during the heat of processing.

The other most commonly varied parameter is the hard segment/soft segment ratio. This can be expressed in a number of ways, e.g. % hard segment, or molar ratio of monomers, etc. In table 3, the effect of varying the hard segment/soft segment ratio is demonstrated. It can be seen that the increase in hard segment (i.e. increased urethane content) gives a harder polymer with higher modulus, less elongation and decreased solubility.

Environmental resistance

The environmental resistance of TPUs has been an issue since the production of these materials began. The various types of TPUs respond differently to the different types of environmental factors, such as heat, ozone, UV radiation, hydrolysis, fungal attack, etc.

Conclusion

Thermoplastic polyurethanes are very versatile polymers as can be seen by the number of different applications in which they are used. The wide range of properties found in TPUs offers many distinct advantages when using these materials. Some of the advantages are common to all TPUs. Others are only achieved in particular grades of TPUs.

References

1. O. Bayer, H. Rinke, W. Siefken, L. Ortner, H. Schild, (to