Polyurethanes come to mind first when one thinks of foam products, and indeed polyurethanes dominate the solid foam market. Such foams may be either rigid or flexible, depending on how the process of manufacture takes place. In fact, polyurethane systems allow enormous variations in the polymerization and fabrication processes; it is this complexity which keeps the υrεthane area a fertile field for development and expansion.

Like many macromolecules, polyurethanes are a general class of materials which can be prepared via many different routes, at least in principle. However, industrial practicalities dictate a preferred approach based on, for example, feedstock availability and ease of processing. For example, ordinary condensation polymerization of bischlorofor ates with dia ines will yield polyurethanes, but the universal large scale practice calls for condensation of diisocyanates with diols. (More generally, the common synthesis involves diisocyanates and polyols, wherein the diol is a special case, where triol species produce

crosslinking) . A typical instance might have 2,4- toluene diisocyanate (TDI) reacting with 1,4- butanediol. In any case, the practical problems show up not at the level of individual chemical molecules but rather with the physical production and molding steps.

P0l3rurethar.es are notoriously defiant regarding fabrication. The production of a good, useful foam object involves precise control over the size and distribution of the hollow voids, or cells in the product. An open cell foam would make a poor life preserver while a closed cell foam would make a poor sponge. Volumes have been written on the problems associated with polyurethane processing, and the subject is generally beyond the scope of this discussion, except as relates to prepolymer stabilization.

Most polyurethanes cannot simply be made into a melt and injected into a mold in the way that polyethylene normally perform. One viable method is the "one shot" approach, whereby all the reactants are combined simultaneously with injection into the mold. The alternative process calls for controlled synthesis of a prepolymer, i.e., a short chain polyurethane intermediate. The use of the intermediate provides a polyurethane which has generally better properties. The prepolymer method is generally more forgiving than the one shot approach, and hybrid techniques are possible, but the present art still has much room for improvements. This patent addresses the practical problem of Drεpolymer stability.

In particular, the present invention provides for treatment of polyols: this process involves the treatment of polyalkylene carbonate polyols, leading to more stable prepoly ers and improved urethane products. Polyalkylene carbonate (PAC) polyols may be made by a base-catalyzed reaction, and some catalyst remains in the product PAC. Accordingly, the prior art has depended on residual acid species, e.g., HC1, in the TDI to neutralize the residual base species in the polyol. Where necessary, it is possible to add an acid chloride to the TD1 (invariably benzoyl chloride) to provide for the neutralization; but the limitation on the prior art is that benzoyl chloride simply does not stabilize PAC prepolymers -- even when added in large excess. Benzoyl chloride may prevent a runaway exothermic reaction, but even so, it is just as objectionable as HC1 for many applications because residual chloride ions remain in the product. Even further, benzoyl chloride does not provide a stable prepolymer. The present invention produces stable PAC prepolymers with dual advantages of longer storage times (before fabrication) and longer gel times (during fabrication). Thus, premature curing does not occur, and the molded products have better physical properties, environmental resistance, etc.

The PAC polyol is typically a diol with an equivalent weight of about 250 to 2000, although triols are available. Addition of a strong acid to the PAC polyol neutralizes the residual base catalyst, preventing side reactions, including tri erization of the TDI.

Specifically, the PAC polyol requires initial characterization with rεsoect to its "CPR" count.

"CPR" represents the phrase "controlled polymerization rate," signifying the amount of residual base in the prepolymer. CPR determination protocol calls for 30 g of PAC in 100 ml of methanol to be titrated with 0.01 N HC1, where the ten times the acid volume is equal to the CPR value. See "Urethane Poiyεther Prepolymers and Foams: Influence of Chemical and Physical variables on Reaction Behavior" by Schotten, Schuhmann, and TenKoor, in J. Chem. Ens;. Data. Vol. 5, No. 3 _> -July 1960. The •key is to achieve a negative CPR value '03/ addition of the strong acid. But a CPR value below -100 would be unnecessary, possibl; even counterproductive and detrimental to the product.

The strong acids used here include ethanesulfonic acid (MSA) and para-toluenesulfonic acid (PTSA). Certainly many other strong acids will also work, but each acid type should be tested experimentally, not to verify its abilit3/ to clean up the PAC polyol, but rather to determine whether unwanted side reactions also occur. For example, as suggested earlier, HC1 has been found to be an undesirable acid. But it is equally clear that virtually any organosulfonic acid will perform satisfactorily.

Furthermore, some acids react di ects with TDI, e.g., H2SO4 AND PTSA; so it is necessary to treat the PAC polyol with the acid prior to its reaction with a polyisocyanate.

The process involves mixing an acid with a selected P0I3/OI, more particularly with PAC, either before or after it is reacted with a polyisocyanate to form a prepolymer. The mixing procedure is best

carried out at 60°F (15°C) to 95°F (35°C), in a closed container. The acid is added to the PAC with stirring. The acid is stirred into the PAC using, for laboratory amounts, a stirring device, to mix acid. The amount of acid is quite small; as an example, for one liter of

PAC, acid is added with stirring in an effective amount of just a few pp , or only a few drops. Since only a small amount of acid is needed, a neutral diluent

(preferably the PAC polyol itself) is added to the acid, perhaps 10 to 50 to one of acid. The acid is added over time with stirring. If the residual base species in the PAC is known before treatment, the amount of acid can be calculated. On the other hand, acid can be ratably added to achieve base neutralization over time to avoid excessive over dosing. Therefore, the preferred procedure is adding acid while stirring the PAC until the requisite neutralization is accomplished. This extent of acid addition varies primarily with the degree of PAC neutralization. Should insufficient acid be added, the step is repeated until a negative CPR value is obtained.

The following examples and comparative run are provided to illustrate the invention but are not intended to limit the scope thereof.

Comparative Run A

This comparative run shows the inefficacy of benzoyl chloride as a stabilizer. A PAC polyol was reacted with toluene diisocyanate to form a prepolymer having an isocyanate content of 5 percent. The prepolymer CPR values were found according to the procedure mentioned above. Viscosity of the prepolymer

after treatment is given in centipoises, as measured with, a Brookfield Viscometer Model RVTD. This machine is rotational viscometer containing various spindles, previously calibrated by the manufacturer. The spindle Is placed in the solution to be analyzed and rotated. The viscosity is calculated by multiplying the RPM by the appropriate spindle calibration factor.

TAE 5LE I

Prepo lyrtier

?repolvm.er Viscos li y, Time Before

CP ^" CΌ {Pa ι-s) Gelation

1.76 instantaneous

1.40 74,200 (74. 2) 1 hour

0.99 40,400 (40. 4) 1 day

0.006 33,000 (33) 1 day

-2.01 27,000 (27) 1-2 days

-4.98 24,000 (24) 1-2 days

-7.95 29,800 (29. 8) 1-2 days

-13.89 41,200 (41. 2) 1-2 d?-ys

-31-35 37,000 (37) 1-2 days

-61.41 26,800 ^* (26. 8) 1-2 days

Various side reactions appear to have occurred, Including trimerization of the isocyanate, resulting in gelation.

Example 1

In a second test described in Table II, PTSA was used to treat a quantity of PAC. The treated PAC was then reacted with an excess of toluene diisocyanate to form a prepolymer containing 5 percent isocyanate groups. Measurements were taken after 24 hours at 80°C.

TABLE II

CPR of PAC PREpolymer Result

5 . , 2 gelation

1 . , 2 gelation

0 . 2 no gelation (still liquid)

-5 . 0 no gelation (still liquid)

The first two runs evidence trimerizaticn with the TDI, while the two runs at lower CPR show stability of the prepolymer made with a properly treated PAC.

ExamDle 2

For a third test described in Table III, more quantitative data was obtained by measuring NC0 loss (a weight percent of the prepolymer). The percentage value is found by carrying out a dibutylamine reaction, followed by back titratin with HC1. Measurements were taken after 24 hours at 80°C.

TABLE III

6.0 Benzoyl chloride gelation

-10 PTSA 0.04 1.7 MSA 0.01

Treatment or neutralization of the polyol.in the latter two cases was sufficient to stop virtually any trimerization of the prepolymer.

As shown from the foregoing tables, PAC polyol neutralization is accomplished to obtain a more useful prepolymer. While variations in the present process may be incorporated, the scope of the present disclosure is determined by the claims which follow.