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Nonfootwear Elastomer Applications and Methods of Manufacture

Apart from shoe soles, elastomer categorization relative to applications is an exercise in rapidly diminishing returns. Analysis of large sectors such as "transportation" reveals that the applications are myriad, none of which by themselves would merit significant exposition, or are much like other unrelated applications. A partial list of application and application areas is provided in Table 9.6. For instance, the requirements of conveyor belts [34, 35] and tire treads [36] are quite similar but apply to vastly different consumers, and compete with different companies and alternative materials. Rather, these applications are differentiated by their use of specific PU components and the way these components are then used to make parts. A graphical way of depicting these components of construction is shown in Figure 9.10.

Cast Elastomers

From Table 9.6, it is clear that breaking PU elastomer technology down by applications (apart from shoe soles) is futile due to the sheer number of applications, the lack of property uniformity within each application, and the lack of volume domination within applications [37]. The custom nature of the applications and resulting formulations makes for a complex commercial

Tire fill

Instrument panel covers

Pipe line pigs

Print rollers

Shock absorbers

Pipe line insulation

Conveyor belts

Non-pneumatic tires

Roof membranes

Roller skate wheels

Run-flat tires

Rubber tougheners

Cattle tags

Pump impellers

Mechanical polishing pads

Tooth brushes

Track and field surfaces

Suspension bushings

Blood bags

Potting/encapsulation

Railway pads

Medical devices

Water skis

Bicycle helmets

Seals

Bicycle seats

Electrical j acketing/tubing

Gaskets

Tool hand covers

Breathable films for apparel

Spray coatings

Belts and pulleys

Airline emergency slides

Segmentation of polyurethane elastomers not including footwear. Elastomers applied as coatings are also not included here.

FIGURE 9.10 Segmentation of polyurethane elastomers not including footwear. Elastomers applied as coatings are also not included here.

environment. Since the applications are relatively small and dispersed among numerous end users, cast elastomers seems like an ideal commercial space for systems houses to flourish. Indeed, PU formulated system houses do occupy an important distribution channel. However, the availability of raw materials such as polyols, isocyanates, and curatives at low costs invites one-stop-shop distributors to serve as alternative lower cost middlemen, as well as for molders to try and fill the space of these middlemen, and to purchase directly from the large chemical manufacturers (Fig. 9.11). The ability of molders to choose their mode of purchase creates a very competitive market, and results in very rapid commoditization of formulation innovation or methods of production.

The choice of commercial relationship will also depend on the technical sophistication of individual molders and their interest in workplace health and safety controls. For cast PU elastomers, the primary means of production that has created equilibrium relationships among all of the participants have been via the use of prepolymer and stripped (or low-free isocyanate) prepolymer formulations.

For clarity, the prepolymer approach can be contrasted to the so-called one-shot approach. When using a one-shot approach, an elastomer producer will take a predetermined amount of polyisocyanates, polyols, curative (or chain extender),

A simplified representation of polyurethane elastomer market positions.

FIGURE 9.11 A simplified representation of polyurethane elastomer market positions.

and additives, mix thoroughly, and pour them into a mold where reaction proceeds to completion.

In a prepolymer process, polyol is reacted with a stoichiometric excess of isocyanate such that there is a minimal amount of polymerization that can occur since all the available hydroxyl groups are reacted with isocyanates, and there is no available hydroxyl functionality to react with isocyanate (Fig. 9.12) [38^10]. Temperature is controlled to minimize the extent of isocyanate-isocyanate reaction. These fully reacted polyols possessing isocyanate end groups then make up one side of a two-component formulation. The other side will usually constitute curative (chain extender) or polyol and chain extender and other additives.

From this process, four different prepolymer varieties have established themselves: the full prepolymer, the quasiprepolymer, the low free isocyanate prepolymer, and the stripped prepolymer [41]. These prepolymer varieties differ from one another merely in the amount of free isocyanate left in the prepolymer side to then be reacted with curatives or curatives and polyols (see Table 9.7).

In a quasiprepolymer preparation, a relatively small amount of polyol is added to a large stoichiometric excess of polyisocyanate as described in Figure 9.13.

While Figure 9.13 depicts the preparation of a TDI quasiprepolymer, MDI quasiprepolymers are commonly found and predominant in manufacturing environments trying to limit worker chemical exposure to isocyanates. The full prepolymer is characterized by a much lower content of free isocyanate that somewhat simplifies handling, but also limits the number of different materials that can be prepared from a single full prepolymer. The quasiprepolymer allows the molder to vary end products simply by varying the ratio of curative and polyol in the second component. The same can be accomplished to a more limited degree with the full prepolymer; but due to the low free isocyanate available, the molder will usually only add curative as a second component to build hard segment volume. While the full prepolymer and quasiprepolymer approaches have dedicated users in all geographies, it is more common for European molders to request quasiprepolymer systems and North American formulators to request full prepolymer systems.

The prepolymer production concept.

FIGURE 9.12 The prepolymer production concept.

TABLE 9.7 Characteristics of the types of commercially

Amount of free

Prepolymer variety

isocyanate (%)

Comments

Quasiprepolymer

12-28

Requires formulating with polyol and

curative—provides flexibility to molder to vary final elastomer properties

Full prepolymer

<12

Provides very high control to molder for a specific final polymer result, but each specific prepolymer can only make a single elastomer

Low free isocyanate

~1

Less expensive than stripped prepolymer

Stripped prepolymer

<0.1

Most expensive, lowest hazard, usually does not require isocyanate hazard labeling

Example of the quasiprepolymer concept.

FIGURE 9.13 Example of the quasiprepolymer concept.

While full prepolymer and quasiprepolymer formulated systems are predominant in most cast elastomer applications, there is growing awareness, and in all geographies, that free isocyanate can present an undesirable exposure risk to workers in environments that might otherwise have acceptable environmental controls. For these customers, producers have created products having the excess isocyanate removed from the prepolymer. The thoroughness of the distillation will create either the low-free (monomer) prepolymer or the stripped prepolymer. There is not much reliable information on the relative volumes of low-free versus stripped prepolymer,

North American consumption of prepolymers by type.

FIGURE 9.14 North American consumption of prepolymers by type.

and they are often considered as a single class. Further, due to the relatively greater difficulty distilling low vapor pressure MDI from prepolymer systems [42] the prevalence of stripped and low-free TDI prepolymers is much greater than the same for MDI. Figure 9.14 shows that the North American market for stripped/low-free prepolymers (all TDI based) is approximately 30% of the total prepolymer market.

The overall size of the prepolymer market is difficult to estimate with a high degree of confidence and available published market estimates are often unwarranted extrapolations from unrelated data. A reasonable estimate based on analysis of numerous sources is that the overall prepolymer market, irrespective of the isocyanate, is about 400 million pounds split about half between TDI and MDI with a trend toward increasing MDI incorporation. The choice of customers to use a low-free or a stripped isocyanate prepolymer is to some extent arbitrary, although two things are certain: (1) there is a substantial price increase, as much as $1.0 per pound, in reducing free isocyanate from 1 to less than 0.1%, and (2) that stripped prepolymers do not have to possess warning labeling on their packaging, while labeling of low-free prepolymers varies from country to country [43^15]. For customers who are insensitive to labeling issues, the price differential drives them toward low-free prepolymers, since the absolute difference in resulting elastomer properties between low-free versus and stripped prepolymers is generally anecdotal, and likely reflects subjective preference.

The decision to use a quasiprepolymer or a full prepolymer has consequences that the molder must consider before making a process commitment. The basic equipment is common for both quasiprepolymers and full prepolymers except for the possible use of a third tank when working from a quasiprepolymer to vary curative and polyol volumes independently (Fig. 9.15). Other differences are provided in Table 9.8.

In no way representative of all full or quasiprepolymer systems, Table 9.9 provides a simple comparison of formulation and resulting elastomer properties. A representative experimental procedure for making prepolymers and making the cast mold plaque for testing is provided in the caption of Table 9.9. The simplicity of the procedure is largely responsible for wide adoption of cast elastomer techniques;

Simplified diagram of a cast elastomer molding system with two tanks. A quasi prepolymer system might often employ a third tank to allow for easy variation of curative and polyol. Reprinted with permission from Ref. [46]. © John Wiley & Sons, Inc.

FIGURE 9.15 Simplified diagram of a cast elastomer molding system with two tanks. A quasi prepolymer system might often employ a third tank to allow for easy variation of curative and polyol. Reprinted with permission from Ref. [46]. © John Wiley & Sons, Inc.

TABLE 9.8 Differences between quasiprepolymer and full prepolymer systems and resulting elastomers

Differences between quasiprepolymer and full prepolymer systems and resulting elastomers

TABLE 9.9 Cast elastomers formed at between 70 and 80 °C under N2 for 3-6 h at desired NCO content

Cast elastomers formed at between 70 and 80 °C under N2 for 3-6 h at desired NCO content

Extent of reaction determined by amine titration (dirabutylamine) followed by degas at 70 °C. Prepolymers mixed with second part in a speed mixer followed by open-air cure at 100 °C in a heated mold followed by post-cure heating (annealing) at 100 °C in a convection oven.

TABLE 9.10 Applications defined by the typical shore a hardness required

Application

Shore A

Shore D

Papermaking rolls

70

Metal forming wiper dies

65

Mallet heads

95

55

Run-flat tires

95

Solid tires

90

45

Metal forming die pads

85

40

Idler rolls

80

35

Abrasive handling pads

75

Auto tire treads

65-80

Silk screen wiper blades

60

Door seals

55

Inner tubes

50

Printing rolls

25

Rubber bands

20

however, proper protection from exposure to isocyanates should always be employed. Elastomers made with process and formulation optimization can be employed in applications such as those listed in Table 9.10.

 
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