2. The Effectiveness of 1,4-Butanediol as a Chain Extender
To understand the popularity of 1,4-butanediol as a chain extender, it's important to recognize an odd-even effect in polyurethane intermediates.
The first criterion for an effective diol urethane intermediate is linearity; the second criterion is that it ideally contains an even number of carbon atoms (i.e., the number of CH2 groups in the repeating chain).
Molecular-level chain packing of the individual hard and soft segments of polyurethanes benefits the overall performance properties of the resulting elastomers. For polyester polyols and diol chain extenders, an odd-even carbon number effect has been observed.
For example, in adipate polyester polyols based on diols with an even number of carbon atoms (EG-2, BDO-4, and HDO-6), the melting points, glass transition temperatures, and overall performance properties are higher than comparable systems based on diols with an odd number of carbon atoms (PrDO-3 and PeDO-5).
Particularly outstanding is the high degree of crystallinity and melt enthalpy (i.e., heat required to melt the crystalline hard segment) in MDI-BDO hard segments of a polyurethane. Please see the chart below for an example.
Melt Enthalpy for MDI/Diol Segments in BDO/Adipate Polyester Polyol based Polyurethanes | ||
Diol in Hard Block | ΔH(J/g)- TM | TM°C |
EG | 0.9 | 175 |
1,3-PDO | 0.4 | 145 |
BDO | 2.8 | 148 |
1,5-PDO | 0.14 | 167 |
1,6-HDO | 0.69 | 156 |
(Source)
The odd-even effect is attributed to the crystal packing of the hard segment and hydrogen bonding between neighboring urethane molecules.
For even-number chain extenders, modelling has shown that the molecular chains align better, and interchain hydrogen bonding is facilitated with chain extenders like BDO. This drives the beneficial phase separation between the hard and soft polymeric segments. In fact, many studies have demonstrated that the properties of a polyurethane are very dependent on the extent and perfection of this phase separation.
While ethylene glycol (EG) is also an even-carbon-number diol chain extender, it is not a good alternative to BDO.
There are two disadvantages observed with EG: poor miscibility with many polyols leading to processing and performance disadvantages and the polar nature of EG leading to a reduction in hydrolysis resistance.
Otherwise, EG is a good chain extender and polyester intermediate, affording polyurethanes with good mechanical properties.
3. The Processing Conditions for 1,4-Butanediol Chain Extenders
As a liquid diol, 1,4-butanediol can be used as a chain extender at room temperature in MDI and aliphatic diisocyanate systems.
Processing temperature and catalyst loading levels impact the pot life of diol/MDI systems. While uncatalyzed systems can have a pot life of 10–20 minutes at 70°C, processing at higher temperatures or using catalysts will significantly reduce the pot life. Tin and titanate catalysts are commonly used to increase the reaction rate of the diol with the isocyanate.
In addition, BDO freezes at 20°C (68°F). Therefore, it's important to maintain BDO above the freezing point to avoid crystallization in the equipment lines and phase separation in a B-Side formulation.
BDO is also hygroscopic and requires maintenance in an inert environment to avoid moisture absorption. With its low moisture content, urethane grade BDO permits processors good control of stoichiometry by avoiding the reaction of H2O with isocyanates and concomitant release of CO2 gas bubbles.