PU Elastomer Fibers
The global changes in the PU elastomeric fiber (referred to generically as "spandex" or "elastane" fibers) industry are emblematic of significant changes in overall global manufacturing, and PU manufacturing specifically. Figure 9.22 shows the rapid growth of spandex manufacturing in China coupled with the tailing off of spandex manufacturing in the United States . The trend for Europe is nearly identical to that for the United States over this time period. The market consumption points to several trends. One is that the overall market for spandex fiber is very strong and usage is expanding into numerous areas of everyday life, including nearly all clothing segments, upholstery, disposable diapers, and medical garments. The growth of consumption was mirrored, and probably caused, by the growth in excess manufacturing capacity and concomitant price pressures. The worldwide manufacturing asset utilization was only about 73% in 2010 matching growth in Chinese production capacity, and at the beginning of an increased rate of consolidation in US and European capacity . During the period of record
FIGURE 9.22 Industrial consumption of polyurethane elastomeric fiber.
for Figure 9.22, the price of spandex fibers went from a high of about $22/kg in 1986 to a low of about $5/kg in 2005. The fluctuating price of building blocks comprising spandex fibers, particularly PTMEG (see Chapter 2) has forced fiber price increases in the past few years. While it is assured that spandex prices will continue to fluctuate, the high prices of the 1980s are unlikely to be seen again. More recent prices have been in the $8-10/kg range. A floor for spandex pricing is created by the costs of component building blocks and manufacturing costs.
Spandex fibers are technically PU-urea polymers employing diamine chain extenders, MDI, and either polyether or polyester soft segments. The need for polyurea hard segments is a result of the requirement for higher thermal stability than that offered by PU hard segments [76-78]. The need for elastomeric fabric stability in clothes dryers and under ironing conditions is critical in this regard. Further, due to the strength of polyurea hard segments, minimization of hard segment volume and maximization of soft segment volume in the polymer are achieved, creating a relatively soft polymer with very good elastomer retractive properties. Equivalent soft segment retracting power would require a larger amount of PU hard segment and result in an overall harder fiber that might reduce the comfort goal that these fibers have.
Production of spandex fibers is via a solution dry spinning process using a prepolymer route . The producers manufacture the prepolymer by reacting the polyols with an excess of MDI (virtually all PU elastomeric fibers are made with 4,4' MDI) (Fig. 9.23). The amount of excess MDI is controlled to minimize chain extension whereby soft segment chains are polymerized with MDI, but not so much that too much hard segment is produced in the next step.
The prepolymer is subsequently dissolved in an aprotic polar solvent, usually dimethylformamide, and reacted with a diamine such as ethylene diamine (Fig. 9.24). The dilute (but still very viscous) solution is subsequently spun through a spinneret into a long heated shroud able to remove the significant amount of solvent involved,
FIGURE 9.23 Synthesis of the prepolymer for making spandex. The soft segment shown is polytetramethylene glycol. Manufacturers may use polyester soft segments, although these potentially suffer from hydrolytic instability.
FIGURE 9.24 Preparation of spandex polyurethane urea polymer from prepolymer and diamine. R = methylene bisphenyl from MDI R' is a linear moiety-like ethylene form ethylene diamine, and R" is the PTMEG soft segment. Reprinted with permission from Ref. . © John Wiley and Sons, Inc.
FIGURE 9.25 Hard segment structure stabilization through germinal hydrogen bond interaction between hard segments on different chains. Reprinted with permission from Ref. . © John Wiley and Sons, Inc.
and leave the elastomeric fiber. The resulting polymer has a high density of interactive N-H units and carbonyl units that are capable of interchain interactions. The isocyanate-diamine hard segment phase separates from the soft segment by virtue of phase incompatibility (see Chapter 4), and hydrogen bond interaction between the N-H and carbonyls. The short distance between amine functions on a chain extender such as ethylene diamine allows for the unpaired electrons of carbonyl groups to simultaneously entertain two hydrogen bonds from the urea moiety. Thus, the intrachain cohesive energy between urea hard segments are as much as 2X that of urethane hard segments, and allows for stronger hard segment interactions allowing less hard segment volume for the same crosslink strength . However, the high cohesive energy density of these hydrogen bonds precludes melt spinning of these materials due to decomposition of polyether soft segments at temperatures below the polyurea hard segment melting point (Fig. 9.25). The phase-separated urea segments act as crosslinks tying the soft segments together. From this standpoint, PU elastomers behavior is well understood by simple applications of the concepts of rubber elasticity.
A trend in spandex technology, besides lower cost, is to produce designed fibers that fill performance gaps in current spandex performance. Particularly, there is effort to produce spandex fibers that show less susceptibility to degradation upon exposure to chlorine. Chlorine exposure is known to greatly reduce the overall strength of elastomer fibers via a substitution of CI for the carbamate hydrogen followed by eventual scission of the urethane bond, or by substitution for hydrogen along the polyether soft segment. Recently, major spandex producers have come forward with mineral-containing fibers improving chlorine resistance [81, 82]. Similarly, there is a push for fiber manufacturers to produce fibers compatible with various wrinkle-free technologies. Another trend is to produce commercial quality elastomeric fibers via a melt-spinning process (i.e. solvent-free) and therefore avoid the cost of solvent, solvent recovery, and environmental, health and safety concerns associated with a spinning solvent [83, 84]. As noted before, PU hard segments suffer from relatively poor thermal stability in comparison with polyurea hard segments, and require more hard segment volume in the polymer to achieve similar properties reducing overall elasticity in the fiber. Nevertheless, melt-spun fiber has been able to make some inroads into the market, primarily in the lower cost/lower performance segment of the market.