For example, I have read from some papers, that this issue can be done with a simple magnetic stirring of the components (polyol, isocyanate, chain extender) in a beaker at approx 80 ªC and atmospheric pressure.
But in others, they have used more complicated equipment and conditions, like four-neck reactors and nitrogen atmosphere or in a vacuum.
Polyurethanes may be synthesized by one step and by two-step reactions. The first involves mixing the polyol, the diisocyanate and the chain extender (perhaps with catalysts), while the second propares first the so-called isocyanate prepolymer which is chain extended in a second step. The tow-step methods allows more precise, more reliable and more variable reaction-control, thus a more reporducible final product. (The hard segment wil be more uniform, less by-products are formed).
Polyurethanes may be synthesized by one step and by two-step reactions. The first involves mixing the polyol, the diisocyanate and the chain extender (perhaps with catalysts), while the second propares first the so-called isocyanate prepolymer which is chain extended in a second step. The tow-step methods allows more precise, more reliable and more variable reaction-control, thus a more reporducible final product. (The hard segment wil be more uniform, less by-products are formed).
but equally, for the two-step synthesis I have to use the reactor to approach that reaction control?
In the fitst step you react two moles of diisocyanate with one mole macrodiol and the final product will be a diisocynate prepolymer (a soft segment with two unreacted isocyanate gropus at each end). The reaction should be performed under dry nitrogen blanket to protect the isocyanate groups from ambient moisture. Once the reaction is complete you may add the chain extender which react with the isocynatae end grups of the prepolymer. Depending on the reactivity of the isocynate and of the diol and the chain extender you may need various catalysts.
it is noteworthy that the stoichiometry is very important in both steps. In prepolymer synthesis, the stoichiometry has to be set in means of macrodiol end groups. For second step, any excess of chain extender will led to a large reduction in molar mass.
Of couse, the nature of the macrodiol block will determine the PU elasticity.
First step is the more troublesome, if you note that your PU is not growing look carefully to this step. Its also in first step that reticulation can most likely happen.
Complementing. I'm used to produce multicomponent PU using a round-bottomed flask in two steps, without using reactors. However, all reagents and solvents must be free of moisture and must avoid environment moisture (on rainy days it is impossible). Also, my limit temperature is 42 ° C, as the diisocyanate may undergo dimerization reactions above this temperature, which leds to reticulation.
If you refer to the practical preparation of the polyurethane, it can be done in bulk and in solution.
In the preparation is in bulk and one shot, depending on the viscosity of the components, you can use magnetic stirring for the initial mixing of the components and afterwards you fill a mold for the curing. For example, if you macrodiol is low viscosity, as a PPO polyether diol of relatively low molecular weight, your diisocyanate is liquid such as HDI or modified MDI, or pure MDI if the mixing is done as temperatrues higher than 40ºC, and your chain extender is liquid as BDO, and the reaction kinetics are not very fast at the mixing temperature (if catalyst is added to the formulation it should be done at the end of the mixing time), the components can be mixed in bulk for a certain time, before viscosity increases too much, and the mixture poored into a mold (PU can adhere very tightly to mold surfaces thus a teflonated surface is recomended or at least the use of a good demoulding agent applied previously onto the surface of the mold). If one or more of the components are too viscous for magnetic stirring (for example a relatively long polyester macroglycol) you will need mechanical stirring for the mixing of the components. If some of the component is solid at room temperature, obviously you need to mix the components at a temperature where all the components are liquid. The higher the temperature the lower the viscosity but the faster the reaction so you must find the right temperature that gives you the time necessary to mix thoroughly the components and still be able to pour the mixture onto a mold. The mixture in the mold can be cured in an oven at the necessary temperature for the curing of the polyurethane in a reasonable time. If the temperature is high (80ºC or higher), water in the atmosphere should not be a problem.
This method can be used for crosslinked polyurethanes if the gel point is not reached before the pouring of the mixture. Of course, in the case of a crosslinked PU, the mold should have the final form of the polymer.
This procedure can also be used with the prepolymer method if the viscosity of the final prepolymer is not too high, normally if the ratio diisocyanate/macroglycol is 2 or above. In this case, after prepolymerization, you add the chain extender and mix it with the prepolymer before pouring the mixture into the mold.
As pointed out by other authors in previous answers, a dry atmosphere is strongly recommended, specially if mixing of the components take relatively long time.
If the preparation is in solution (only for linear PU), you can use the one shot or the prepolymer method, as already explained in previous answers. The most important point is to use anhydrous solvents. When possible, I prefer to use halogenated solvents because they are very hydrophobic, but if the hard segment of the prepared PU is not soluble in halogenated solvents, the PU will precipitate and therefore it is necessary to use more polar solvents. THF is a very powerful solvent for PU, but if this solvent does not work, I would recommend you DMF, DMAc or NMP. It is important again that these solvent are anhydrous (commercially available), although I prefer to use freshly distilled solvent prepared by myself.
The reaction temperature I use, depends on the reactivity of the monomers, usually in between 80 and 120ºC. Of course, it is also limited by the solvent. For example, if you use 1,2-diclhloroethane or THF, the maximum temperature will be limited by the boiling point of the solvent)
In order to minimize the side reactions that can occur in solution, I use the highest concentration possible that allows magnetic stirring (usually at around 20% wt/volume, but if varies a lot depending on the structure of the PU). When I use the one shot method, I directly prepare the PU at the final concentration of the solution. When I use the prepolymer method, I prepare the prepolymer at very high concentration (as high as 50-100% weight/volume) and after the prepolymerization time I add the chain extender and the rest of the solvent to give the final concentration.
Muy buena explicación Angel Marcos-Fernández... muchas gracias
Otra consulta Ángel.... tienes información sobre el uso de Glycerol propoxylate-block-ethoxylate (f = 2.5) como poliol de un poliuretano?
To allow the participation of the rest of the researchers I will writ in english (if you are more comfortable in spanish it would be better to write directly to me to my e-mail address).
The propylene oxide-ethylene oxide copolymers you mention are the pluronics (information available at http://worldaccount.basf.com/wa/NAFTA/Catalog/ChemicalsNAFTA/pi/BASF/Brand/pluronic). Personally I have never work with these pluronics. Commercial polyether poliols are based on polypropylene oxide (PPO) because they are hydrophobic whereas polyethylene oxide (PEO) is hydrophylic, and for this reason, these polyols are not used in polyurethanes for industrial applications.
The block polyol you mention will have a hydrophobic/hydrophylic behaviour depending on the ratio of both blocks. The PPO block will always be amorphous whereas the PEO block will be crystalline if the chain is long enough (approximately above 1500-2000 g/mol) and both blocks are thermodinamically uncompatible. Polyethers, except polytetramethylene glycols, do not give high mechanical properties. The functionality of the polyol is above 2, so you will obtain a crosslinked material. If my memory is still good enough, I remember that I read somewhere that these pluronics (or at least some of them) have a special behaviour in solution, with a change in solubility with temperature, and that these pluronics have been used in some polyurethanes for biomedical applications.
If you give more details about the polyurethane you want to prepare and the intended application, all the participants interested in your question will try to help you.
I need to prepare nanocomposites of polyurethane and carbon nanotubes, and to synthesize the PU matrix I have MDI, BD and that polyol... Then, my problem here is to predict the behavior of the whole preparation process, because I cannot find information about this kind of use for that poyol...
As stated above by our experienced colleagues, you should apply one-shot or quasi-prepolymer route for your system. In case of similar reactivities of Pluronic's hydroxyl groups and its functionality above 2, the resulting polyurethane is expected to be quite densely crosslinked, hence insoluble and not applicable to solution processing (e.g. fiber spinning or dispersing CNT in the polymer solution). Accordingly, the casting bulk technique should be here utilized.
If you allow, I would advise you to produce PU/CNT nanocomposites according to the following procedure:
(1) dispersion of CNT in the Pluronic macrodiol or a mixture of macrodiol and chain extender at elevated temperature, 80-100 deg C (to lower its viscosity) by ultrasound-assisted mixing,
(2) addition of a chain extender (BDO), if not added during dispersing,
(3) addition of a freshly purified MDI at reaction conditions (purification of MDI is usually needed for a crude monomer or when stored for a prolonged time due to the formation of high contents of MDI dimers)
Without the use of a catalyst, polymerization (polyaddition) at 80 deg C proceeds very fast as observed by a continuous increase in viscosity of the reaction medium. The polymerizing mass should now be quickly poured into a hot mold and allowed to cure for a few hours.
I completely agree with Pawel Sobecki. I will just stress the need for the purification of MDI (because of dimer formation). I purify MDI by sublimation at high vacuum of the long time stored MDI.
A crude MDI can be purified by crystallization, melt filtation, vacuum distillation or sublimation. Crystallization from from n-hexane (after removing insoluble solids) gives MDI of high purity (analytical grade), but the method is suitable for rather small quantities. I agree with Angel Marcos-Fernández, sublimation under high vacuum is a simplest and cleanest way to obtain dimer-free MDI.
Traditionally, polyurethane elastomers can be synthesized via a “one-shot” process or pre-polymer method. While the one-shot process is the quickest and easiest of the manufacturing techniques, preparation via the pre-polymer method imparts greater control over the chemistry of the reaction, influencing the structure, mechanical properties, reactivity and processability of the finished product
The polyurethanes (PUs) can be synthesized in both beaker or a complicated reactor. But what does determine the reaction vessel?
Polyurethanes are synthesized through a polyaddition reaction, therefore, high MWs are only attainable for practically perfect reactions. This means, the viscosity of the reaction mixture will be significantly raised at final steps of PUs formation. If a beaker is used as the reaction vessel, you must use higher solvent contents to prevent gelation. In addition, the purging of an inert gas such as N2 in the beaker is more difficult than a proper designed reactor.
I would like to mention that the block ratio and the functionality of monomers are determining factors for attaining the polyurethane elastomers. The one step or two-step reactions only affects on the final properties of polyurethane elastomers and have no significant effect on the type of the reaction vessel.
What will happen if the gelation point is reached? will the polyurethane stick to the beaker or reactor?
2-step process with formation of isocyanate-ended polyol is the simplest one. Try it
in my opinion, prepolymer method is better than one-stage method becuse the reaction can be highly controlable. Furthermore possibility of gelation is considerably low. In prepolymer method, at first you should mix polyol and diisocyanate together in determined temperature and mechanical stirring. After about 2h add chain extender(in a specific Black Ratio). Then, you should cure the samples in oven.
This is my personal experience to avoid gelation use the two step method
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials Chemistry
- Polymer Chemistry