TPU polymers are typically made by reacting (1) a hydroxyl terminated polyether or hydroxyl terminated polyester, (2) a chain extender, and (3) an isocyanate compound. Various types of compounds for each of the three reactants are disclosed in the literature. The TPU polymers made from these three reactants find use in various fields where products are made by melt processing the TPU and forming it into various shapes to produce desired articles by processes such as extrusion and molding. An important use of TPU is in the area of making melt spun elastic fibers. The TPU fibers can be combined with other natural and synthetic fibers to make clothing and a variety of other consumer and industrial products.
TPUs are segmented polymers having soft segments and hard segments. This feature accounts for their excellent elastic properties. The soft segments are derived from the hydroxyl terminated polyether or polyester and the hard segments are derived from the isocyanate and the chain extender. The chain extender is typically one of a variety of glycols, such as 1,4-butane glycol.
U.S. Pat. No. 5,959,059 discloses a TPU made from a hydroxyl terminated polyether, a glycol chain extender, and a diisocyanate. This TPU is described as being useful for making fibers, golf ball cores, recreational wheels, and other uses.
One of the typical deficiencies of conventional TPU fibers is their inability to withstand the high temperatures that are normally encountered in manufacturing procedures. For instance, high temperature resistance is important if one wishes to make an article, such as clothing, by combining the TPU fibers with synthetic fibers, such as polyester fibers. This is because synthetic fibers must be dyed and heat set at high temperatures to prevent the clothing made therewith from shrinking during subsequent washing and drying cycles. For this reason, conventional TPU fibers that do not offer resistance to high temperatures are normally used in combination with natural fibers such as cotton, which do not require high temperature heat treatment.
Higher temperature resistance is also highly desirable in cases where TPU is used in fabric coating applications. For instance, in clothing a fabric coating can be employed wherein a TPU liner is used in conjunction with a fluro-polymer coating which is usually heated after application to the fabric to dry and cross-link the fluro-polymer. In such cases, the TPU needs to be capable of withstanding the elevated temperatures encountered in the drying and crosslinking steps of the procedure.
In numerous applications, it would be desirable for the TPU to have a high melting point for good high temperature resistance coupled with a low level of hardness, such as a Shore A hardness of less than 75 and preferably lower. In other words, a soft TPU having good high temperature resistance could be beneficially employed in such applications. However, soft TPUs having high melting points that are free of plasticizers have been elusive. This is because the melting point and the hardness of the TPU typically both increase with the level of chain extender utilized. Thus, it has typically been difficult to make a TPU having a high melting point by increasing the level of chain extender without also increasing the hardness of the polymer.
Plasticizers are commonly added to TPU compositions to make the polymer composition more elastic and softer. However, in many applications it is not desirable to add plasticizers to the TPU composition. For instance, the use of plasticizers in clear TPU compositions should be avoided because the presence of plasticizers can reduce the clarity of the composition. Thus, plasticizers are not desirable for utilization in TPU compositions where clarity is being sought. Plasticizers can also bloom or be extracted from such compositions in which case the hardness of the composition can increase to an undesirable level. The use of certain plasticizers has also come under scrutiny from environmental and toxicological standpoints. Accordingly, eliminating the need for plasticizers in TPU compositions would normally be deemed to be advantageous if desired physical characteristics could be attained without them.
U.S. Pat. No. 7,202,322 discloses a TPU having a high level of thermal resistance and a high moisture vapor transmission rate. These polymers are also impervious to liquid water and have electrostatic dissipating properties. These characteristics make them desirable for use in applications requiring high moisture vapor transmission properties, such as house wrap, roofing underlays, various textile coatings, and melt spun fibers. The TPU compositions described by U.S. Pat. No. 7,202,322 are prepared by reacting a hydroxyl terminated polyether intermediate, an aromatic chain extender glycol, and a polyisocyanate.