Statistical Studies on One-Package Polyurethane Surf ace
THERE
has been considerable interest recently in polyurethane coating formulations. As part of a continuing research program on coatings at Purdue, statijtical studies were made on idealized polyurethane systems to evaluate the effects of formulation changes on film properties. Four formulations having potential value as polyurethane coatings were given trials in the preliminary investigation :
Sward Hardness Data for the Isocyanate-Adduct System Polymeric Triol
Moles Triol per Mole Isocyanate Adduct 0.8 0.7 0.6 0.5
1.0
0.9
0.4
0.3
Mol. wt. 698
20.0 22.3
27.3 32.3
44.0 40.7
28.7 44.5
49.0 39.5 37.3
38.3 44.0
43.4 43.0
47.6
Mol. wt. 930
2.7 2.0
5.7 5.7
12.7 12.7
17.8
30.6 32.0
38.3
37.3
50.3 41.5
levels of excess toly!ene diisocl-anate and three levels of added polymeric diol, mol. lvt. 1180. l h e hardness was measured u s . time, and the toughness was measured after ahour 3 or 4 days. An analysis of variance of these data shows that hardness decreases very significantly with increasing addition of polymeric diol, increases very significantly with additional isocyanate excess, and that the two effects have a significant interaction. An analysis of these data shows that only the amount of added diol significantly affects the hardness. After 8 days in storage, the condition of the duplicate samples was noted for the low-molecular weight triol. The storage stability was found by an analysis of variance to be significantly increased by an incremental excess of isocyanate. No significant effects were found for the higher-molecular-weight triol. As it was thought that added amounts of polymeric diol should increase the storage stability, additional experiments were made in which excess isocyanate was held a t a constant level. I t is clear that additional diol gives increased storage stability. Ail samples of this system were tough, and could be sharply flexed on metal panels with no apparent damage.
Two-Package System. A polymeric triol was reacted with excess tolylene diisocyanate. The resultant product was cured by adding glycol. This system was not investigated beyond the preliminary stages, because of the inconvenience of the two-package system. Molecular-Sieve Cured System. The product from the reaction of a polymeric triol with excess isocyanate was mixed with a Molecular Sieve loaded with a n amine. The amine was driven off and the system cured by heating. This system was abandoned after it was found to have poor storage life in the preliminary studies. Air-Moisture Cured System. The product from the reaction of a polymeric triol with excess isocyanate was cured at room conditions. This Eystem, and the next, were studied in detail. Isocyanate-Adduct System. A polymeric triol was mixed with a phenol adduct of a trifunctional isocyanate. The system was heated, the phenol driven off, and the cure was effected. Results
Air-Moisture Cured System. The investigation was carried out as two separate 3 X 3 factorial experimc-nts. For each of two different polymeric triols, experiments were run with three
Storage Stability Data Added Polymeric Diol, %
150
Excess Tolylene Diisocyanate, % 175
200
0 4 8
Gelled, gelled Gelled, gelled Gelled, gelled
Gelled, gelled Gelled, gelled Viscous, viscous
Fluid, viscous Fluid, gelled Fluid, fluid
Additional Diol increased Storage Stability Added Polymeric Diol, %
Time until Gelation, Days
0 4
2 4
8 16
11
7
Isocyanate-Adduct System. This investigation was carried out as a 2 X 8 factorial experiment, with the molecular weight of polymeric triol being one factor and the ratio of isocyanate adduct to triol being the other factor. An analysis of these data shows that each of the factors has a very significant effect on hardness, with greater hardness being associated with the triol of lower molecular weight and with lower triolto-isocyanate ratios. All films of this system could be sharply flexed on metal panels without damage. Discussion The hardness of the films varied inversely according to the mean molecular weight of the chains between branch points. Calculations were made of this mean molecular weight for the airmoisture cured system, assuming that all isocyanate groups which did not react with hydroxyl groups became linked together in a trifunctional branch by the action of water. For the moisture-cured system, gelation time was inversely related to the probability of formation of continuing polymer chains in the reaction mixture. I t was theoretically calculated that
The Sward Final Hardness Data Hardness decreases significantly with increasing addition of polymeric diol Added Polymeric Diol Solids,
wt. 70 0 4 8
1382
Polymeric Triol, Mol. Wt. 930
Polymeric Triol, Mol. Wt. 698
-
150
175
51.6,51.8 49.0,49.6 43.3,43.8
53.0,52.3 49.8,49.8 48.4,48.2
INDUSTRIAL AND ENGINEERING CHEMISTRY
Excess Tolylene Diisocyanate, 70 200 370 54.2,53.0 49.3,51.2 47.8,49.8
52.8,50.8 48.0,51.0 46.8,49.2
400
430
55.0,54.2 52.2,49.8 47.5,46.8
56.2,55.5 51.0,48.8 48.0,46.2
POLYURETHANE COATINGS gelation should not have occurred in any of the systems used. As gelation did occur, it appears that undesirable side reactions were taking place. The most probable causes of these reactions are adventitious impurities: including water. The water present could have
reacted with tw’o isocyanate groups to form a urea linkage, which in turn could have reacted with an isocyanate group to form a biuret, and so forth. Thus a very small amount of water left in the resin after drying might eventually have lead to gelation.
S. N. GLASBRENNER Armstrong Cork Corp., Lancaster, Pa.
BRAGE and L, c, CASE School o f Chemical Engineering, Purdue University, Lafaye++e,Ind.
Baked Polyurea Coatings polyurea coatings considered here are one-component coatings that can be applied from either a water or a solvent system. The coatings are formed bv the reaction of urethanes with amine-bearing resins, and are cured by baking at 350” to 400’ C. for one half hour. I n order to be stable at room temperature, the urethane portion of the svstem must be based on the reaction product of aliphatic alcohols with polyisocvanates. Urethanes based on isocyanates and phenolic-hydroxyls will react with aminebeariqg resins at room temperature. T h e mechanism proposed for the formation of polyurea coatings is shown in Equation 1. Properties of polyurea coatings may be varied over a wide range by varving the nature of the reactants. I n general, polvurea coatings have good resistance to acids, alkalies, and water-immersion exposures. They also have good flexural
T H E
Urea
A4kohol
and hardness properties and show excellent adhesion to metal. Polyurea coatings should find use as industrial primers o r as protective coatings. Their ability to be applied from a water-thinned system is an especially attractive feature.
Co.’s experimental product Mondur E-92 and General Mills’ Versamid resins of the 100 series.
The raw materials used in formulating polyurea coatings are Mobay Chemical
M o b a y Chemical Co., New Martinsville, W. Va.
’.
G. A. HUDSON, J. C. HIXENBAUGH, E. R* WELLS, and E. E* HARDY
Urethane Coatings from Castor Polyols applicability of castor polyols to the preparation of urethane coatings has been extensively reported by Metz, Ehrlich, Smith, and Patton ( I ) , Jvho systematically evaluated 17 castor poly01s. The screening work reported established that excellent urethane films could be prepared from castor polyolurethane systems. Exploratory work has been extended in a more detailed investigation of three new promising castor polyols to give a refining of the preparation methods and formulating variables. Because glyceryl monoricinoleate, of the three previously tested monoester castor polyols, had exhibited an optimum balance of over-all properties, it was selected as most suitable for more detailed investigation. Modifications of this basic chemical type were accordingly prepared and evaluated in terms of ultimate performance in urethane vehicles. I t was found that a lower prepolymer reaction temperature (50” C.) compared with that previously used (70’ C.) was desirable from a production standpoint, as better control of the reaction as well T H E
as improved aging properties was obtained. Despite a higher cost factor, it was decided to raise the NC0,’OH equivalent ratio from a previously recommended 1.75/1 to values in the range of 2.1/1 to 2.5/1, because this increase markedly improved the shelf and pot life stability of the urethane vehicle while retaining excellent coating performance. Of the monoglyceride variants prepared (using different ratios of castor oil to glycerol) and tested, one of medium monoglyceride content (Polycin 61 containing 14% monoglyceride) was selected as giving a best compromise of cost and ultimate performance properties in urethane coatings. From an economic standpoint, the reduced isocyanate demand of the Polycin 61 is definitely advantageous, for the diisocyanate chemical is by far the more expensive ingredient. To a large extent, the higher isocyanate equivalent weight of Polycin 61 compensates (costwise) for the increased NCO/OH ratio used in preparing the urethane vehicle. An analysis of performance data for
the two glyceryl monoricinoleate-urethane vehicles reveals a markedly improved shelf and pot life obtained by using the higher NCO/OH ratio. Despite some sacrifice in hardness, there is a significant gain in solvent and chemical resistance. I n the prior investigation, attempts had been made to improve the over-all cost of urethane vehicles by designing castor polyols with high isocyanate equivalents. LTnfortunately, the few castor polyol derivatives developed at the time with isocyanate values higher than castor oil exhibited incompatibility characteristics that discouraged further work along such lines. Subsequently, however, a series of polymerized castor oils (urethane grade blown oils) having isocyanate equivalent weights in the range of 360 =t 15, has shown very acceptable properties. Low viscosity blown castor oils were found suitable for preparing either a prepolymer or polyisocyanate type of urethane vehicle. The high viscosity blown castor oils lent themselves only to VOL. 51, NO. 1 1
0
NOVEMBER 1959
1383
