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Thermoplastic Polyurethanes

TPUs are widely regarded as providing the desirable properties of PU elastomers with the desirable processing of thermoplastics. This means that TPUs can be processed by extrusion, injection molding, casting, and casting from solution. TPUs achieve this processing flexibility by avoiding crosslinking of the polymer chains; other PU systems do not achieve this processing flexibility. By and large, all building blocks used for making TPUs have a functionality of 2.0. It has been shown that TPU processing is not noticeably harmed when employing soft segment functionalities as high as 2.05 (by incorporation of small amounts of a three-functional polyol), but soft segment functionalities as low

Global segmentation of the 2012 TPU market by consumption volume.

FIGURE 9.16 Global segmentation of the 2012 TPU market by consumption volume.

as 2.1 cannot be processed as a melt [48]. As discussed in the Section, the market for TPUs is also fractured among applications (Fig. 9.16) and customers. Each application segment has numerous subsegments demanding unique solutions. In keeping with China's dominance in parts manufacturing, and particularly in footwear, it is consistent that China should be the dominant consumer of TPUs, and the growth of its consumer culture coupled with increasing demand for automotive transportation assures that China should continue to see high TPU growth rate (estimated at about 8% per annum) for the near future.

While production of TPUs is not very technologically advanced, it is capital intensive requiring large twin-screw extruders for reactive processing, and a significant amount of specialized processing equipment for before and after the reactive extrusion [49]. Further, since the number of applications and requirements within each application segment are numerous, the requirement for rapid and frequent formulation changeover and inventory storage complicates manufacturing and customer service. North American production of TPU is dominated by Lubrizol Corporation manufacturing about 70% of North American production. The other producers are low-cost global producers of TPU feedstocks (particularly, MDI required for its 2.0 functionality) such as BASF, Bayer, and Hunstman. The same is true for Europe, although they are joined by CODVI, a major polyol producer. These same producers also maintain a footprint in China, often with local partners. Yantai Wanhua, China's largest producer of MDI, is also the largest producer of TPU in China, but for perspective, in a more fractured market, this only reflects about 15% of all Chinese production (Fig. 9.17).

The capital requirements for TPU production are not conducive to service by formulation system houses operating on a low capital asset business model. However, production of TPU grades for large accounts can be made more profitable by selling additional inventory to smaller, more difficult-to-service accounts through distributor channels (Fig. 9.18). The success of this structure is complicated by the drive to

Segmentation of the 2012 global TPU market by region.

FIGURE 9.17 Segmentation of the 2012 global TPU market by region.

Simplified diagram of the commercial relationships between manufacturers and molders of TPUs. Systems houses do not participate in this segment due to the large capital costs associated with polymer production.

FIGURE 9.18 Simplified diagram of the commercial relationships between manufacturers and molders of TPUs. Systems houses do not participate in this segment due to the large capital costs associated with polymer production.

low-cost supply and the competing cost for distributor profit. It is additionally complicated by small consumer's need for technical assistance and the uneven ability of distributors to supply such service.

Simplified illustration of reactive extrusion of TPUs. Reprinted with permis¬sion from Ref. [46]. © John Wiley & Sons, Inc.

FIGURE 9.19 Simplified illustration of reactive extrusion of TPUs. Reprinted with permission from Ref. [46]. © John Wiley & Sons, Inc.

TPU technology is relatively straightforward and highly optimized. TPUs are produced by reactive extrusion in which all of the ingredients are simultaneously introduced to a twin-screw extruder, and under temperature and mixing action within the screw elements, produces a fully formed polymer at the extruder exit (Fig. 9.19). Almost all TPUs are made with MDI and butanediol for the hard segment. Soft segment differences have been the major area of differentiation in performance and price. The reaction is catalyzed using hydrolyzable tin catalysts such as tin octanoate. Hydrolytically stable catalysts such as dibutyl tin dilaurate are not used industrially to avoid complications of a live catalyst in the polymer for long periods. In the past, tin catalysts were preferred to amine catalysts because of lower color formation, lower odor, and specificity for the urethane reaction; however, recent restrictions, particularly in Europe, have generated increased reliance on amines [50].

While it is a simple matter to substitute TPU reactive components, most commercially available TPUs comprise MDI and BDO hard segments. Manufacturers will use different soft segments depending on the application. For hydrolytically sensitive applications, manufacturers will typically use polyether polyols. The vast majority of these use polytetramethylene ether glycol (PTMEG) that is prepared from the polymerization of THF (see Chapter 2). PTMEG has many good attributes contributing to TPU properties. Among these is that as a linear molecule, it is able to accommodate a balance of hard phase separation with hard phase dispersion. The phase separation is accommodated by PTMEG only interacting with hard segment by hydrogen bonding through the ether linkage [51, 52]. This limited soft segment-hard segment interaction minimizes the competition with hard segment-hard segment interaction. In addition, as an unbranched chain, PTMEG does not preferentially compete for free volume in the polymer, optimizing hard segment phase dispersion. Another benefit of PTMEG is that the terminal hydroxyls of PTMEG are all primary, maximizing the reaction rate with isocyanates. PTMEG is also able to strain crystallize at higher molecular weights, and therefore increases the ultimate tensile strength of the polymer. Finally, due to the method of preparation (see Chapter 2), all the polymer chains in PTMEG are bifunctional maximizing network connectivity and elastomer properties. TPUs based on PTMEG are generally regarded as possessing a very good balance of properties but are among the most expensive due to the market price of PTMEG which is tied to the price of tetrahydrofuran as opposed to the less-expensive propylene oxide (see Chapter 2) and a more expensive process (see Chapter 2). PTMEG is a polyether, and therefore thermally limited by a relatively low ceiling temperature. At the ceiling temperature, the polymer will depolymerize back to starting materials—for polyethers usually between 230 and 250°C [53].

Polyethers based on polypropylene oxide (PPO) make poorer TPUs than does PTMEG due to macrophase separation of hard segment caused by the free volume requirement of PPO pendant methyl groups [54]. Equally problematic for TPUs prepared from PPO is the slow reactivity of the terminal secondary hydroxyl groups of these soft segments. The reactivity ratio of secondary to primary hydroxyls is at least a factor of 3 (see Chapter 3), which makes it difficult to complete polymerization during the short reaction time in the reactive extruder, and also drives the equilibrium of hydroxyl and isocyanate to urethane unfavorably back to reactants [55]. Last, due to reaction isomerization, there is a net probability for some of the end groups of PPO to not create terminal hydroxyls, but instead create terminal allylic groups (see Chapter 2 for discussion of "monol" production). Since formation of allylic groups is a statistical process, and they are polymerization chain stoppers, their proportional importance is greater as soft segment molecular weight increases [56, 57].

TPUs prepared with polyethylene oxide soft segments are elastomeric, but phase separation of hard and soft segments is hindered by the large number of ether groups in the soft segment available to hydrogen bond with the hard segment [58, 59]. This hard-soft segment interaction competes with intrahard segment hydrogen bonding promoting phase mixing rather than phase segregation. In addition, polyethylene oxide soft segments are relatively hygroscopic. TPUs with polyethylene oxide absorb up to 15% of their weight in water from the atmosphere [60], whereas TPU with PTMEG soft segments will absorb about 1% water by mass at standard test conditions. Water plasticization of PEG TPUs greatly degrades TPU tensile properties.

TPUs are also commonly prepared with polyester soft segments. Polyester soft segments are preferred for applications where potential hydrolysis of the material is not an issue for long-term performance, and where cost is of paramount concern. Most polyester TPUs employ soft segments made from the condensation polymerization of adipic acid and butanediol. Along with a price that is less than half that of PTMEG, polyester polyol creates acceptable and transparent thermoplastic elastomers. Further, polyesters offer superior thermal stability to polyether polyols but can lose molecular weight in the presence of moisture and heat, reverting to reac-tant acids and alcohols. This degradation pathway can be catalyzed by the presence of acid end groups that are an inevitable artifact of the polyester polymerization. These acid end groups also degrade PU network connectivity by effectively acting as chain stoppers.

The carbonyl groups of polyester polyols are efficient competitors with urethane carbonyls for hydrogen bonding with the urethane N-H. The result of this is that a higher volume percentage of hard segment is required to affect hard segment-soft segment phase separation, and the equilibrium mixing of nonphase separated urethane groups is relatively high [61, 62] (see Chapter 4). This can result in a higher soft segment T in the polymer; however in many cases, this effect is not significant enough to greatly hinder low-temperature toughness of these materials.

With alternative thermoplastic elastomers available, the parts designer must try to evaluate the suitability of a particular material for its role. TPUs are conventionally considered very high-performing elastomers with a very good balance of properties, but not ideal for all applications. Table 9.11 shows comparative properties for TPUs from polyether and polyester soft segments compared with a generic block copolymer polyester elastomer and an olefin elastomer. While many properties are good for each elastomer class, there is no doubt that TPUs distinguish themselves with regard to tensile, dynamic, and abrasion dimensions of performance.

As mentioned earlier, it is a simple matter to prepare new TPU formulations as long as the materials can form a thermoplastic (all components are two functional), are dry, are matched stoichiometrically, and are properly catalyzed for reaction during their residence within the reactive extruder. Table 9.12 provides illustrative formulations for making TPUs of a given hardness. They are conveniently prepared

TABLE 9.11 Comparison of properties for typical thermoplastic elastomer classes

Comparison of properties for typical thermoplastic elastomer classes

The choice of which elastomer to use will depend on performance and price requirements. + Indicates good performance.

0 Indicates adequate performance in limited situations. - Indicates relatively poorer performance.

TABLE 9.12 Example formulations for TPUs of various hardnesses (modulus) and from various soft segments

Example formulations for TPUs of various hardnesses (modulus) and from various soft segments

by their simultaneous introduction into the feed throat of a reactive extruder such as drawn in Figure 9.19. Along with the reactive components, the manufacturer will often introduce catalysts, antioxidants, processing aids such as waxes for easy transit through the process and mitigating interpellet stickiness, and also small amounts of monofunctional alcohols to limit molecular weight if a higher melt index polymer melt rheology is desired.

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