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DryAdd for Polyurethanes

The following examples for polyurethanes illustrate the range of systems, from TPUs to foams, that DryAdd can tackle. With its ability to create one or two new reactive product sites from each reaction, its capacity to model both pre-gel and post-gel regions, and its full range of reports, plots and analysis functions, DryAdd can give you new insights into your own problems.

DryAdd has been validated against detailed experimental work and Miller-Macosko network models. Both linear and crosslinked systems can be studied, and illustrative examples are given below.

Throughout this page, if you click on the pictures, you can expand them to get a better look.

Thermoplastic polyurethanes involve mixing a difunctional isocyanate - like pure MDI - with a diol - usually, a polyester or polyether. Low-molecular weight chain extenders can also be added. The resulting polymers are linear molecules, with hard block (rigid) and softblock segments. Interchain interactions between the hardblock segments form physical crosslinks, which can be destroyed on heating the polymer.

In this example, 1,4-butanediol and a polyether diol (with average molecular weight of 1000) have been mixed with pure difunctional MDI. All hydroxyl groups have been assumed to have equal reactivity. The main chemical reaction is between the hydroxyl group and the isocyanate, NCO. A urethane linkage is formed in this reaction. Provided there are no side reactions, then this system will form linear polymers.

DryAdd's block sequence analysis can be used to look for the frequency with which butanediol-MDI blocks occur; the results are shown in the screenshot below; reasonable lengths must be achieved before phase separation will be seen.

Changing the relative reactivities of the materials can have a significant effect. In the screenshot below, the results were obtained for a system in which the reaction of MDI with butanediol is four times faster than the reaction of MDI with the polyether.

In some cases, a side reaction takes place, in which the urethane group reacts with another isocyanate, to give an allophanate group. The allophanates lead to branching, and some crosslinking, which degrades the processability of the material. The reaction scheme including this side reaction is shown in the screenshot below.

The side reaction of the urethane group with the isocyanate group has been allowed (by creating a phantom urethane group on the MDI) but is relatively slow compared to the main reaction.

Isocyanate groups can react with water to form carbon dioxide. If the evolution of this gas is controlled correctly as the system gels, it can be used to produce a water-blown environmentally-friendly foam.

In this example, we have mixed a polyol with average OH functionality of 2.5 (represented as two materials, with functionality 2 and 3) and molecular weight of 1000, with a polymeric MDI with average functionality of 2.3. Since DryAdd allows only integer numbers for functionality, the MDI is also a mixture of species.

Allowed reactions let the water + isocyanate form amine and CO₂, followed by the rapid reaction of the amine with isocyanate to give a urea linkage. Isocyanate plus polyol will give urethane links. We have included further side reactions to give allophanates and biurets - these could have been omitted, and they are relatively insignificant. Relative rates are taken from The ICI Polyurethanes Book.

If the water molecule is set up with two phantom groups - amine and CO₂ - then the evolution of CO₂ can be monitored, and compared with the molecular weight buildup. Here, the amount of each is displayed alongside the the rise in secondary cycles - a sure indicator of the onset of gelation. By adjusting reaction rates (for example, by using catalysts) the evolution of CO₂ can be controlled with respect to gelation.

Rigid foams are made using polyols with functionality 3 to 5, and MDI of functionality of about 2.7. Again, this is achieved in DryAdd through using a mixture of MDI materials. Generally, polyisocyanurates are also formed especially when, as here, there is an excess of isocyanate. These can be modeled in DryAdd by creating a fast-reacting temporary group which, initially, does not react. As the temperature rises, and at a user-specified conversion, the simulation is paused and these trimerization reactions can be switched on. The plot shows this as a slight discontinuity in the slope for the line for secondary cycles.

Alternatively, in DryAdd-Pro and Pro+ you can use real rate equations, provided you have the pre-exponential and Arrhenius activation energy. The isocyanurate reactions lead to a more highly crosslinked - and hence more mechanically rigid - system.

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