what is the differenence between Polyester TPU and Polyether TPU?
Thermoplastic Polyurethane (TPU), also known as PU thermoplastic, is a linear block copolymer composed of soft segments of low-molecular-weight polyols and hard segments of diisocyanates and chain extenders. The properties of TPU largely depend on the type of long-chain diol used. Its hardness can be adjusted by the proportion of hard segments. The photostability of TPU can be improved by adding stabilizers, and it also depends on whether the isocyanate used is aromatic or aliphatic.
Differences Between Aromatic and Aliphatic Isocyanates
Aromatic Isocyanates
Aromatic isocyanates are used in applications where oxidation and discoloration due to UV light exposure are not a concern. Polyurethane coatings made with aromatic polyisocyanates are prone to oxidation and degrade more easily under direct sunlight.
In contrast, Aliphatic Isocyanates are primarily used for light-stable coatings. In applications requiring UV or sunlight stability, such as automotive clear coats and many waterborne formulations, aliphatic products are preferred.
Aliphatic polyisocyanates provide polyurethane coatings with excellent chemical resistance and good aging resistance. The absence of benzene groups ensures durable adhesion under harsh conditions.
Compared to aliphatic derivatives, aromatic isocyanates result in coatings with poorer aging resistance (yellowing) and chemical resistance (especially poor alkali resistance). Therefore, aromatic polyisocyanates are mainly used for indoor applications (floor coatings, tank coatings, etc.) or as primers. Even in the automotive industry, the use of primers is decreasing because primer yellowing can affect the color of the topcoat and cause interlayer delamination.
Generally, aromatic polyisocyanates are not primarily used for coatings. For example, 80% of TDI products are used to make flexible foams, while 65% of MDI products are used to make rigid foams.
Classification of TPU
TPU (Thermoplastic Polyurethane) can be classified according to different criteria:
1. By Soft Segment Structure : Polyester type, polyether type, and butadiene type, which contain ester groups, ether groups, and butadiene groups, respectively.
2. By Hard Segment Structure : Urethane type and urethane-urea type, obtained by chain extension with diols or diamines, respectively.
3. By Crosslinking : Purely thermoplastic (linear structure with no crosslinks) and semi-thermoplastic (containing a small amount of crosslinks such as urethane-urea bonds).
4. By Synthesis Process : Bulk polymerization and solution polymerization. In bulk polymerization, there are pre-polymerization and one-step methods. The pre-polymerization method involves reacting diisocyanates with high-molecular-weight diols for a certain period before adding chain extenders to form TPU. The one-step method involves mixing diisocyanates, high-molecular-weight diols, and chain extenders simultaneously to form TPU. Solution polymerization involves dissolving diisocyanates in a solvent, reacting with high-molecular-weight diols for a certain period, and then adding chain extenders to form TPU.
5. By Product Application : Profiles (various mechanical parts), pipes (sheaths, rod materials), films (sheets, plates), adhesives, coatings, and fibers, etc.