Dimensionally Stable Polyurethanes and Composites

Abstract

Rigid polyurethanes and composites are made from a reaction mixture containing an aromatic polyisocyanate and a mixture of polyols. The mixture of polyols has an average hydroxyl equivalent weight of 125 to 275 and an average hydroxyl funtionality of 2.5 to 4 hydroxyl groups per molecule. 5 to 33% of the weight of the mixture of polyols is triisopropanolamine. Rigid polyurethanes made from such a reaction mixture have excellent dimensional stability, even when cured at or near room temperature.

Claims

1. A composite comprising a continuous resin phase and 8 to 85% by weight, based on the weight of the composite, of a discontinuous phase comprising filler particles, reinforcing fibers or both filler particles and reinforcing fibers, wherein the continuous resin phase is a cured polyurethane which is the reaction product of a polyurethane-forming reaction mixture characterized by an isocyanate index of 95 to 150, the reaction mixture comprising A) an aromatic polyisocyanate or mixture of aromatic polyisocyanates, the aromatic polyisocyanate or mixture of aromatic polyisocyanates having an isocyanate functionality of 2 to 4 and an isocyanate equivalent weight of 80 to 175; B) a mixture of polyols, the mixture of polyols having an average hydroxyl equivalent weight of 125 to 275 and an average hydroxyl functionality of 2.5 to 4 hydroxyl groups per molecule, wherein triisopropanolamine constitutes 5 to 33 weight percent of the mixture of polyols.

2. The composite of claim 1 wherein the discontinuous phase comprises reinforcing fibers having diameters of 0.5 to 10 μm and lengths of 2 mm to 150 mm.

3. The composite of claim 2 wherein the fibers are glass fibers.

4. The composite of claim 3 wherein the discontinuous phase further comprises filler particles.

5. The composite of claim 1 which contains 2 to 15% by weight reinforcing fibers, 30 to 70% by weight of filler particles and 20 to 50% by weight of the continuous resin phase.

6. The composite of claim 1 which has a void volume of no greater than 65%.

7. A process for preparing a composite of claim 1, comprising (i) introducing reinforcing fibers and/or filler particles and a polyurethane-forming reaction mixture into a cavity of a mold or onto a form, closing the mold or applying mechanical pressure to the form such that the reinforcing fibers and/or filler particles become embedded in the polyurethane-forming reaction mixture and (ii) curing the polyurethane-forming reaction mixture in the presence of the reinforcing fibers and/or filler particles in the mold cavity or on the form to form the composite, wherein the polyurethane-forming reaction mixture is characterized by an isocyanate index of 95 to 150 and comprises A) an aromatic polyisocyanate or mixture of aromatic polyisocyanates, the aromatic polyisocyanate or mixture of aromatic polyisocyanates having an isocyanate functionality of 2 to 4 and an isocyanate equivalent weight of 80 to 175; B) a mixture of polyols, the mixture of polyols having an average hydroxyl equivalent weight of 125 to 275 and an average hydroxyl functionality of 2.5 to 4 hydroxyl groups per molecule, wherein triisopropanolamine constitutes 5 to 33 weight percent of the mixture of polyols.

8. The process of claim 7 wherein the step of curing the polyurethane-forming reaction mixture is performed at a temperature of no greater than 40° C.

9. The process of claim 7 wherein the composite has a void volume of no greater than 65%.

10. The process of claim 7 wherein the polyurethane-forming reaction mixture contains reinforcing fibers having diameters of 0.5 to 10 μm and lengths of 2 mm to 150 mm.

11. The process of claim 7 wherein the composite contains 2 to 15% by weight reinforcing fibers, 30 to 70% by weight of filler particles and 20 to 50% by weight of the continuous resin phase.

12. The process of claim 7, wherein step i) is performed by wetting the reinforcing fibers with the polyurethane-forming reaction mixture, dispensing the reinforcing fibers wetted with the polyurethane-forming reaction mixture into the mold or onto a form, closing the mold or applying mechanical pressure to the polyurethane-forming reaction mixture on the form and curing the polyurethane-forming reaction mixture in the mold.

Description

EXAMPLE 1 AND COMPARATIVE SAMPLES A-H

[0054] Polyurethane composites are made by reacting a formulated polyol blend with a polymeric MDI in a mold with a 1000×30×50 mm cavity. The mold temperature is 22-24° C. in all cases except for Comparative Sample B, in which the mold temperature is 54° C. Fibers are omitted in these experiments to simplify the formulation and minimize the possible effects of uneven fiber distribution when using laboratory equipment.

[0055] All ingredients except the polyisocyanate are combined to produce a formulated polyol blend. The TIPA, which is a room temperature solid, dissolves in Polyol A upon mixing. The mold cavity is lined with a polyethylene film. The formulated polyol blend and polyisocyanate are separately equilibrated to 22-24° C., then combined for 20 seconds on a high-speed mixer to produce a polyurethane-forming reaction mixture. About 740 grams of the reaction mixture are poured into the lined mold cavity. A second piece of polyethylene film is placed atop the reaction mixture in the mold cavity and the mold is closed. The reaction mixture is cured in the mold for 20 minutes while maintaining it at the aforementioned temperatures. The resulting composites are demolded and stored at approximately 22-24° C. for 24 hours. The molded parts have densities in the range of 400 to 650 kg/m.sup.3.

[0056] The reaction mixture in each case is prepared from ingredients as indicated in Table 1.

[0057] Polyol A is a propoxylated glycerin. It has a nominal hydroxyl functionality of 3 and a hydroxyl number of 274 mg KOH/g (205 equivalent weight).

[0058] The Propoxylated TMP is a propoxylate of trimethylolpropane. It has a nominal functionality of 3 and a hydroxyl number of 950 mg KOH/g (59 equivalent weight).

[0059] The surfactant is a silicone foam-stabilizing surfactant.

[0060] TIPA is a 99% triisopropanolamine product.

[0061] Catalyst A is a 33% by weight solution of triethylenediamine in dipropylene glycol.

[0062] Catalyst B is benzyldimethylamine.

[0063] TCPP is tris(1-chloro-2-propyl)phosphate.

[0064] The CaCO.sub.3 is a calcium carbonate powder.

[0065] Polymeric MDI A has an isocyanate content of 31.45% by weight and an average isocyanate functionality of 2.7.

[0066] Polymeric MDI B has an isocyanate content of 30.2% by weight and an average isocyanate functionality of 2.85.

TABLE-US-00001 TABLE 1 Parts By Weight Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ingredient A* B* C* D* E* F* G* Ex. 1 Polyol A 92.8 92.8 92.8 92.8 84.8 89.8 67.8 80.8 Surfactant 1.38 1.38 1.38 1.38 1.38 1.38 1.38 1.38 TIPA 0 0 0 0 0 0 0 12 Glycerin 0 0 0 0 0 3 0 0 Ethylene Glycol 0 0 0 0 8 0 0 0 Propoxylated 0 0 0 0 0 0 15 0 TMP Tripropylene 0 0 0 0 0 0 10 0 Glycol TCPP 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 Water 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 CaCO3 150 150 180 150 150 150 150 150 Catalyst A 1 1 1.33 1 1 1 1 Catalyst B 0 0 0 0.5 0 0 0 0 Polyisocyanate 105 0 105 105 105 100 105 105 A (index) Polyisocyanate 0 105 0 0 0 0 0 0 B (index) Average 204 204 204 204 138 173 133 165 Hydroxyl Equivalent Weight, polyols.sup.1 Average 3.0 3.0 3.0 3.0 2.52 3.0 3.05 3.0 functionality, polyols1 *Comparative. .sup.1The polyols are Polyol A, plus TIPA, glycerin, ethylene glycol, the propoxylated TMP and tripropylene glycol, as present in each particular case, not counting other ingredients.

[0067] Dimensional stability for each of Comparative Samples A-G and Example 1 is assessed as follows: The length of each part is measured after equilibrating it for 24 hours at 22-24° C. The part is then laid horizontally in an oven and heated to 80° C. for 72 hours. Its length is measured immediately upon removing it from the oven. The part is then cooled to 22-24° C. over about one hour, and its length is measured again. Results are as indicated in Table 2.

[0068] The glass transition temperature (T.sub.g) and storage modulus (at 70° C.) are determined for each of Comparative Samples A-G and Example 1, with those results also included in Table 2. T.sub.g is determined by a dynamic mechanical analysis (three point bending test) and is taken as the peak of the tan delta curve.

TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Comp. Comp. Comp. Comp. A* B* C* D* E* F* G* Ex. 1 Initial Length, 1001 1001 1000 1000 1001 1001 1000 1000 mm Length at 80° C., 1014 1017 1016 1017 1012 1010 1009 1005 mm Length after 1010 1014 1013 1014 1009 1007 1005 1003 recooling, mm T.sub.g, ° C. 89 96 81 87 97 78 100 107 Storage 114 69 170 104 80 36 99 209 Modulus, MPa

[0069] Comparative Sample A represents a baseline case. This formulation exhibits excellent dimensional stability when molded at 54° C. When molded at 22-24° C., however, the product undergoes a significant length increase, and does not closely reassume its original length upon recooling. The difference in results between the moldings made at 54° C. and 22-24° C. curing temperatures suggests that dimensional stability is related to the extent of curing that takes place during the molding step.

[0070] Comparative Samples B-G represent various approaches to improving dimensional stability. In Comparative Sample B, a higher functionality polyisocyanate is used to try to promote greater crosslinking during the molding step. Results are worse than the baseline.

[0071] In Comparative Sample C, the amount of filler is increased by 20%, as higher filler levels often impart greater dimensional stability to composites. Again, results are worse than the baseline case.

[0072] An additional catalyst is included in Comparative Sample D, but results are worse than the baseline.

[0073] Comparative Samples E, F and G represent attempts to increase the exotherm of the system (by lowering average equivalent weight of the polyols and increasing the amount of polyisocyanate) thereby increasing the extent of in-mold curing. In Comparative Sample F, a mixture of tripropylene glycol and propoxylated TMP is used due to the high viscosity of the propoxylated TMP, the tripropylene glycol regulates viscosity and also increases exotherm due to its low equivalent weight relative to Polyol A. Comparative Sample E performs comparably to the baseline, whereas Comparative Samples F and G show only small improvements. Comparative Samples E and F also exhibit surface defects upon demold.

[0074] The results obtained with Comparative Samples B-G demonstrate that a variety of approaches intended to promote better in-mold curing at low mold temperatures all fail to significantly improve dimensional stability. Increasing crosslink density via the use of a higher functionality polyisocyanate is ineffective, as is decreasing equivalent weight via the addition of various low equivalent weight polyols and adding an additional amine catalyst.

[0075] Example 1 unexpected has significantly better dimensional stability than the baseline and all of the other comparative samples. Expansion upon the heat treatment is only about one-third that of the baseline case, and only half that of the best of the other comparative samples. The results after recooling are similarly improved. The product demolds well to produce a good quality surface.

[0076] The T.sub.g and storage modulus data show a pronounced increase in both values when triisopropanolamine is included in the formulation. The other attempts to improve in-mold curing have comparatively little if any beneficial effect (compare Comp. B-G with Comp. A), underscoring the unique benefits of TIPA in the formulation.

EXAMPLES 2-5

[0077] Example 1 is repeated, varying the amount triisopropanolamine and adjusting the amount of Polyol A on a weight-for-weight basis. The formulations are as set forth in Table 3. Example 1 is included for reference. The moldings are evaluated for dimensional stability as in the previous samples, with results as indicated in Table 3.

TABLE-US-00003 TABLE 3 Parts By Weight Ingredient Ex. 2 Ex. 3 Ex. 1 Ex. 4 Ex. 5 Polyol A 87.8 82.8 80.8 77.8 72.8 TIPA 5 10 12 15 20 Surfactant 1.38 TCPP 4.6 Water 0.95 CaCO.sub.3 150 Catalyst A 1 Polyisocyanate A 105 105 105 105 105 (index) Initial Length, mm 1000 1000 1000 1000 1000 Length at 80° C., mm 1007 1006 1005 1004 1002 Length after 1005 1004 1003 1002 1000 recooling, mm

[0078] The results indicated in Table 3 show that triisopropanolamine improves dimensional stability when used over a wide range of loadings. Within this range, dimensional stability improves with increased triisopropanolamine loading.

Comparative Samples H-K

[0079] Triethanolamine is commonly used as a crosslinker in polyurethane foam formulations. It is chemically similar to triisopropanolamine in that both contain a single tertiary nitrogen atom and both have three hydroxyl groups. Polyurethane composite Comparative Samples H-K are made formulations corresponding to those of Examples 2-5, except triethanolamine (TEOA) replaces triisopropanolamine on a weight-for-weight basis.

[0080] Comparative Samples A and H-K and Examples 2-5 each are evaluated for cream, gel and tack-free time, in each case by forming a formulated polyol blend of all ingredients except the polyisocyanate, and equilibrating the polyol blend to 40° C. The polyisocyanate is separately equilibrated to 25° C. The polyisocyanate and polyol blend are combined for 12 seconds on a high-speed laboratory mixer and poured into cups. Cream time is the time after pouring at which a visible reaction is observed. A metal stick is periodically pressed to the surface of the curing reaction mixture and removed to evaluate for gel time (the time after mixing at which strings of polymer stick to the spatula). A human finger is periodically pressed to the surface of the curing reaction mixture and removed to evaluate tack-free time (the time after mixing at which the polymer no longer sticks to the finger). Those results are indicated in Table 4.

TABLE-US-00004 TABLE 4 Comp. Comp. Comp. Comp. Comp. A* Ex. 2 H* Ex. 3 J* Ex. 4 J* Ex. 5 K* Polyol A 92.8 87.8 87.8 82.8 82.8 77.8 77.8 72.8 72.8 TIPA 0 5 0 10 0 15 0 20 0 TEOA 0 0 5 0 10 0 15 0 20 Cream Time, s 32 29 25 28 20 27 19 26 19 Gel Time, s 125 115 83 110 60 100 49 104 40 Tack-free time, s 185 170 124 161 85 150 64 140 51 *Comparative

[0081] As the data in Table 4 indicates that adding these amounts of TIPA causes only a small decrease in cream, gel and tack-free times across the entire range of loadings. TEOA, on the other hand, causes very large deceases in each of those times, indicating a very large increase in the reactivity of the system. Because of their lower reactivity, the TIPA-containing systems are better adapted than the TEOA-containing systems for making large moldings in a casting process. The small increase in reactivity of the TIPA systems allows them to be used easily in such processes.