|University Federico II
of Naples, Italy
Reaction between a isocyanate molecule and an alcohol yields a urethan group
Polyurethanes (PU) are synthetic polymers that include urethane groups in their chains. Such groups are formed by reaction of an isocyanate with an alcohol as shown in the figure.
Examples of di- and poly-isocyanates commonly used as monomers
Polyurethanes (PU) are produced through a complex synthesis between isocyanates (with at least 2 -N=C=O groups in the molecule) and polyols (with 2 or more hydroxyl groups, -OH, in the molecule), in the presence of suitable catalysts (systems based on aliphatic amines and/or organic tin salts).
The molecular structures of some di- and poly-isocyanates, commonly used in PU synthesis, are shown in the figure.
The steps of PU synthesis are described below:
- Pre-polymer synthesis - It is generaly performed by reacting polymeric di-ols with di-isocyanates. Polyols may have either a polyether or a polyester chemical structure (see figures below). The resulting PU is called Polyether-PU or Polyester-PU, respectively. The structures of two sample ether and ester polyols are the following:ETHER-POLYOL
Scheme of the reaction leading to formation of PU pre-polymer
A scheme of the reaction forming the PU pre-polymer is depicted aside.
- Chain extension - It is performed using low-molecular weight diols or diamines. Compounds such as 1,4-butandiol, ethylene glycol and 1,6-hexandiol, are typical examples of chain extenders.
PU's belong to the class of alternating block-copolymers, where hard and flexible segments alternate along the chain. These blocks are named A and B in the following scheme, representing the structure of a PU macromolecule:
top: molecular structure of poly(ethylene glycol). bottom: structure of the repeating unit of a polyurethane molecule from this polyglicol and MDI. The polymer is characterized by the sequence of flexible and rigid blocks.
The flexible segments include polyols, that may be either polyethers (generally prepared from a mixture of propylene oxide and ethylene oxide), or polyesters (for instance, from ethylene glycol and adipic acid), while the rigid segments are formed from residues of di-isocyanates and low-molecular weight polyols (chain extenders), as shown in the figure.
In the condensed state, due to strong hydrogen bonds between urethane groups, the rigid segments aggregate, giving rise to a separated micro-phase dispersed in the soft phase. Thus the resulting material is a two-phase system, where the two phases are interconnected and have different properties. In segmented PU's two glass transition temperatures (Tg) are often observed, corresponding to the soft (at lower temperature) and the rigid (at higher temperature) phase.
Structure of segmented polyurethanes. The hard and soft phases are shown, which determine the properties of the bulk material.
At temperatures below the Tg of the hard phase the rigid segments, acting as physical cross-links, provide mechanical strength to the bulk material. At higher temperatures the material is able to flow, so it can be processed and shaped like a common thermoplastic.
The micro-morphology of a segmented PU is schematically represented in the figure.
The figure represents the polyurethane tree, pointing out the wide range of materials with different characteristics, that can be produced with the modern polyurethane chemistry
PU properties are strongly dependent upon the chemical nature of the basic components, their relative abundance and synthesis and processing conditions. The branched tree in the figure schematically depicts the opportunities offered by modern chemistry to produce many types of PU-based materials, often tailored for special and sophisticated applications.
An important area for polyurethane applications is conservation of stone buildings and artifacts.
Polyurethanes are widely used for watercraft like inflatable boats and surfboards. They are also used to make tennis rackets.
There are polyurethane fibers; one example is Lycra, an extremely elastic fiber. Polyurethane fabrics can be made impermeable and are used for windbreakers and shower curtains.
Expanded polyurethane is prepared by adding a blowing agent, which releases gases, to the reaction mixture. Expanded PU is generally an open cell foam, contrarily to expanded polystyrene, that has closed cells. Therefore expanded polyurethane is soft and deformable, and it is suited for making pillows and mattresses. In particular, it is much used for car seats.
Polyurethanes are useful as floor and wall coatings in buildings. Expanded polyurethane is outstanding in insulating buildings from water, heat and noise, so it is used for home insulation.
Also shoes, or parts of them, can be made of polyurethane.
Boats are often constructed by enclosing a spongy polyurethane layer, that makes them light, between two external walls of hard plastics, often containing fiberglass.