.dg98fProduct -and Process Development r A section devoted to information on the development of products
'and the processes for making them on any scale with industrial implications, and including economics and market development
Preparation and Properties of Polyurethane Coatings RAIN-EROSION PROTECTION FOR ALUMINUM CHARLES B. REILLY
AND
MILTON ORCHIN
University of Cincinnafi, Cincinnati, Ohio
T
HE leading edges of high speed aircraft are rapidly eroded by flight through a moderate rainstorm, An aluminum panel specimen traveling a t 500 miles per hour through a simulated rainfall of 1 inch per hour shows considerable roughness after 5 to 10 minutes. After about 15 minutes, pitting ensues and increases rapidly until the metal is completely eroded. In order to protect aluminum against this rain erosion, organic coatings of strong, elastic materials which can absorb the energy of impact by reversible deformation are desired. From reports ( 2 ) on the excellent mechanical and elastic properties of the polyurethane polymer, it was believed that this synthetic elastomer could be developed to meet the requirements of an erosion-resistant coating. This article describes the preliminary development and the properties of a successful coating prepared from a polyurethane formed from poly(ethy1ene adipate) and 2,4tolylenediisocyanate ( T D I ) and cross linked with ethanolamine. Polyester prepared from 2,4-tolylenediisocyanate and poly(ethy1ene adipate)
A typical preparation of polyester diol was carried out as follows: A mixture of 186.5 grams (3 moles) of ethylene glycol and 381.7 grams (2.61 moles) of adipic acid was heated in a 1-liter flask equipped with a thermometer well and a Dean-Stark trap, The mixture was heated to 160' C. for 2 hours during which time appreciable water distilled. The temperature was raised to 220' C. and maintained at this level for 22 hours. At the end of this time a total of 93.4 grams of water had distilled. The refractive index of the water was ng = 1.3389. Assuming that the difference in observed index from the value for pure water was owing solely to the presence of ethylene glycol and that the refractive index of the mixture varies linearly with composition, the observed index of refraction of the distillate indicates about 93.5y0 water and the rest glycol. The Dean-Stark trap on the reaction flask was replaced by a Claissen head which led to a cooled collecting flask and the mixture was heated for 7 hours a t 220' to 235' C. under 2.5 mm. of Hg pressure. An additional 10.5 grams of distillate was = 1.3381, corresponding to about 94.5% water. On cooling, the January 1956
polyester solidified to a waxy, nearly colorless solid, melting a t approximately 55' to 58' C. The average molecular weight of the polyester was determined by the isocyanate equivalent method (6). About 6 grams of the molten polyester was accurately weighed and placed in an iodine flask fitted with a side arm for inserting a hypodermic needle. There was then added 10 ml. of toluene and 2 drops of N-methylmorpholine. A condenser carrying a drying tube was fitted to the flask, and the contents were heated to reflux temperature. By means of the hypodermic syringe there was then added 20 ml. of 0.709N phenyl isocyanate solution in toluene. After the contents of the flask had refluxed an additional hour, 20 ml. of 1.045N di-n-butylamine solution in toluene was added by means of the syringe. After 15 minutes of refluxing, the flask was cooled, and 50 ml. of methanol was added through the top of the condenser. About 10 drops of bromophenol blue was added t o the solution, and titration of the excess amine was performed with 0.9608N hydrochloric acid. A blank was run in the same manner with the same quantities of material except that no polyester sample was present. The isocyanate equivalent was calculated from the equation Isocyanate equivalent =
1000 X wt. of sample - ml. HCl (blank)] X N H C ~
[ml. HCl (sample)
Since there are presumably two end (hydroxyl) groups, the isocyanate value was multiplied by 2 to give an average molecular weight of 3040 for the ester. The reaction was repeated to test the procedure with a known weight of pure diethylene glycol and a molecular weight of 105.5 (theory 106.1) was obtained. To secure the acid number of the polyester, approximately 3 grams of sample of molten polyester was accurately weighed into an iodine flask and 25 ml. of neutral chloroform added as solvent. The solution was titrated with 0.1N alcoholic potassium hydroxide to the phenolphthalein end point. The acid number was calculated from the formula Acid No. =
-YbaBe X mol. wt. base X ml. of base
Wt. of sample
INDUSTRIAL AND ENGINEERING CHEMISTRY
59
PRODUCT AND PROCESS DEVELOPMENT
Table I. Solvent Acetic acid Chlorobenzene Acetone Toluene Butyl acetate
Effect of Solvent on Cured Film
}
hexane}
5 % HC1 6% NaOHi
Hz0
I
Observations Considerable swelling: became transparent Swelled, but less t h a n in acetic acid; became transparent Slight swelling; became transparent Very slight swelling; remained opaque
Yo swelling; remained opaque No swelling: turned yellowish N o swelling; remained opaque
The condensation product resulting from the reaction between TDI and PEA when 2 2 TDI/PEA > I is called the “adduct” diisocyanate and abbreviated BDI. I n this work, a molar ratio of TDI/PEA of 1.65 was usually employed. Theoretically, the condensation product which should result would have on the average about 3091, structures in which two T D I molecules are linked to one polyester diol and 70% structures in which three TDI molecules are linked by two polyester diols as in Structure 1
1 If it is assumed that theie is one carboxyl group per mole of polymer, the apparent molecular weight of the polymer would be
/
2CO
1000 X wt. of sample MI. of 1N alkali
0
I1
-SHC-O-CH2CH2-
vhich in this example was 10,860. Since the determined molecular weight vas, however, 3040 there would be on the average 10,860 3040 X 2 = -7 end groups in the 10,860 grams of polymer, and hence on the average in such a polyester of acid number 5.2 about one of every seven end groups is a carboxyl group. Urethane Formation and Chain Extension. A solution of 22.8 grams (0.0075 mole) of poly(ethy1ene adipate) (PEA) 3040 in 80 ml. of chlorobenzene was refluxed, and about 40 ml. of chlorobenzene mas removed as distillate to ensure the removal of all TTater. There was then added 2.16 grams (0.0124 mole) of TDI and the solution refluxed for 1 hour. After cooling to room temperature, 0.254 gram (0.00416 mole) of ethanolamine was added and the solution refluxed 10 minutes. There was a noticeable increase in viscosity a t this point. The solution was cooled and diluted r i t h an equal volume of acetone. The viscosity of this solution remained approximately the same for about 2 weeks, but after a month the solution had completely set t o a solid gel. Curing. The chlorobenzene-acetone solution of the extended polyurethane was poured onto a 4 X 6 inch glass plate and the plate heated in the oven for 2 hours at 120” C. A transparent elastic film, insoluble in acetone, was obtained. It could be peeled off the glass. After 2 days a t room temperature, the film hardened to a stiff translucent sheet. Small strips of the cured film were placed in test tubes containing some rather common reagents. The observation given in Table I were made the next day. Polyesters with Terminal Hydroxyl Groups. Table I1 shows that the most important factor controlling the molecular weight of the polv(ethy1ene adipate) is the heating schedule. Rhen an excess of glycol is employed, the more drastic conditions produce an average polyester with fewer carbovyl groups. As already indicated PEA 3040 with an acid number of 5.2 possesses on the average about one carboxyl group out of seven end groups. For purposes of simplification, the carboyyl group is neglected, and it is assumed that on the average the polyester is a diol of average molecular weight 3040. Control of reaction gives urethane of desired chain length with free isocyanate end groups
CHI
0 \
NCO When this structure is employed in gross structural relationships, it is represented as
Ethanolamine proved satisfactory reagent for chain extension, cross linking, and aluminum application
In the early foreign work on the synthrtic elastomer Vulcollan, chain extension and cross linking 17 ere achieved by treating the adduct diisocyanate such as Structure 1with water. The reaction of an isocyanate with water is familiar to most organic chemists as an obstacle to the derivatization of an alcohol owing to the reactions
+ HOH RNHz + RKCO RNCO
RKH2
+ COz
+ RXHCOSHR
These reactions are taken advantage of in the poljwethane field to give foamed resins because they effect carbon dioxide evolution, chain extension, and curing of the polymer network. ‘ r h e reaction of an active hydrogen compound with an isocyanate molecule probably involves nucleophilic attack (1)a t carbon, and accordingly the rate of reaction of isocyanates with amines, water, alcohols, etc., ought t o be greatly influenced by the electrophilic character of these compounds. The relative rates of reaction of active hydrogen compounds with isocyanates in decreasing order are probably RzNH > RNHz > NH3 > ArNHz
> ROH > HOH > RC02H
Thus, even in the presence of water, a disubstituted urea is formed because of the faster rate of reaction of an amine compared to water. The conversion of an isocyanate group in the adduct diisocyanate to an amine and subsequent reaction of the amine with additional AD1 like Structure 1 cvould result, in the early stages of reaction, in chain extension:
The stoichiometry of the reaction between polyester diol and diisocyanate must obviously be controlled t o give a urethane of desired chain length with free isocyanate groups a t the end. This, of course, requires that the diisocyanate be used in excess. The greater the excess diisocyanate, the shorter the OCN~N-N(N-N~NH~ adduct chain, and a t the maximum of 2 TD1:l PEA, the adduct would contain O C N ~ N - N ~ N - ~ HCON N H two free isocyanate groups separated by two tolyl groups and one PEA unit.
60
+
+I ~
INDUSTRIAL AND ENGINEERING CHEMISTRY
N
-
-
NN - ~N ~ N C o
(I[) Vol. 48, No. 1
PRODUCT AND PROCESS DEVELOPMENT
High speed aircraft can be protected against rain erosion by application of polyurethane coatings to leading edges The substituted urea is an active hydrogen compound in the same sense as an amine and can react further with an isocyanate. It would be expected that such a reaction would proceed more slowly with a urea than with an amine since the electrophilic character of nitrogen in a urea is reduced by the presence of a carbonyl group:
R
R
I
H--N :
H-N
L o H-N
I
+4
R
I: c-oe I
H-A@ c--+
H-N
:
R
I
:
I R
I R
I
L O ’
The slow reaction of a urea with an isocyanate group would proceed as follows:
R’ R--N=C=O
+ R’--Pr”CONHR‘
.+ R - - N H C O L N H R ‘
When water is employed for the cure, this reaction is the principal cross-linking reaction. In a simple example, two moles of Structure 2 may be assumed to react to produce cross links a8 follows: 211-oc
N
ratio of ethanolamine (EA) to adduct diisocyanate (ADI). If one assumes that both the hydroxyl group and the amino groups of ethanolamine react with diisocyanate groups, then the situation is analogous to the bifunctional-bifunctional linear polymerization of the diacid-diol polyester condensation and the polyester diol-diisocyanate adduction. It is further assumed for purposes of this work that the chain extension which results from ethanolamine and adduct diisocyanate interaction produces only one cross-linking site-namely, the substituted urea site. As the number of linked AD1 units is increased by increasing the relative proportions of ethanolamine there is, of course, a corresponding decrease in the number of free isocyanate groups in the mass of the polymer. However, the increasing proportions of the ethanolamine also provide an increasing number of crosslinking sites as shown in Table 111. It must be remembered that the ratio TDI/PEA determines the number of polyester units in the adduct diisocyanate and hence the linear distance between cross-linking sites. As the TDI/PEA ratio approaches unity, the distance between the potential cross-linking sites becomes greater and the number of potential cross-linking sites per unit weight of adduct diisocyanate becomes smaller. For any given TDI/PEA ratio, the closer the molar proportion of ethanolamine approaches the molar excess of T D I over PEA, the fewer the unreacted isocyanate groups left intact in a given weight of linear extended diisocyanate but the greater the number of cross-linking sites for these fewer isoAH cyanate groups. Some of these relationships are C’O I shown in Table IV. In both the 6:3:2 formuI lation and the 9:6:2 formulation, the number of cross-linking sites is just equal to the number of isocyanate groups. The cross-linking reaction is I slower than the chain-extension reaction and probably occurs during the heat cure. No extensive study of the optimum ratio ADI/EA was made during this work, but rough qualitative tests showed that when a TDI/PEA ratio of 1.65 was employed, a molar quantity of ethanolamine of from 75 t o 100% of the excess T D I gave satisfactory results.
ON-NDN-N~~O-NHC~N-N~N-N~ 6.0 AH
I -Nm-NnNCO-NH ON OCN~N-
a~-----N
The new structure contains intact isocyanate groups which can react with additional urea groups in the same or different units to further cross link and t o thus set up the polymer network. This occurs during the heat cure of the polymer. The urethane linkage must obviously react a t a slower rate with an isocyanate than the urea linkage. The urethane linkage is formed during the initial reaction with diisocyanate and is present in Structure I , yet no extensive chain extension or cross linking occurs until the addition of water or other curing agent. The electron pair on nitrogen in the urethane is evidently less available for nucleophilic attack than the nitrogen in a urea, an effect which is probably due to the inductive effect of alkoxy and the greater electronegativity of oxygen as compared to nitrogen.
Table II. Molecular Weights of Poly(ethy1ene Adipate) Molar Ratio, Glycol/ Adipic Acid
1.05:l 1.05:l 1.15:l
Time of Reaction, Hours
Max. Temp. Range, O
c.
180-190 160 (vac.) 230 210 (vac.) 220
4 $3 (vac.) 4 $3 (vac.) 24 $6 (vac.)
HPS: +s I&!-*
1.15:l
220-235
1.15:l
220-240
24 $7 (vac.) 44 $14 (vac.)
.1
Molecular Weight
1390
Acid NO.
30
1990
9.8
2600
8 35
3040
5.2
3400
1.1
OR’ The object of this work was the development of a coating for aluminum, so gas evolution was deemed detrimental since gas bubbles interfere with adhesion and entrained bubbles weaken resistance to impact. After some preliminary evaluation, ethanolamine was found t o be a satisfactory reagent for chain extension and cross linking for the intended application. The degree of chain extension can be controlled by varying the January 1956
Table 111. Effect of Adding Varying Proportions of Ethanolamine to 100 Motes of Adduct Diisocyanate Moles EA 50 67 75 90
No.
Isocyanate Groups
100 66 50 20
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
NO. Cross-Linking
Sites 50 67 75
90
61
PRODUCT AND PROCESS DEVELOPMENT Table IV.
Relative Moles TDI PEA EA 4 2 1
Idealized Structure of Linear Extended Diisocvanates from Various Formulations Ratios CLS/ CLS/ Structures PEA S C O 0.5 0.5
6
3
2
0.67
1.0
8
4
3
0.75
1.5
6
4
1
0.25
0.5
9
6
2
0.33
1.0
12
8
3
0.37
1.5
0 = tolyl group - =
polyester chain 0
x = CLS = -NH
c1OCH2CHJVHCOljH-linkage, which is a cross-linking site
Polyureihane films and coatings show high tensile strength and permanent sef values
The investigation of the physical properties of the polyurethanes, especially the attempts to correlate structure with these properties, is of obvious importance, and many laboratories, including our own, have such studies under way. The preliminary evaluations of the physical properties of the films and coatings developed during this study are presented here. After-Cure. Films of polyurethane, prepared from AD1 and ethanolamine &s described, were prepared by pouring the acetonechlorobenzene solution on a glass plate. After evaporation of the solvent at room temperature and curing at 120’ C. for 2 hours, the films were cooled and pulled off the glass plate. At this point the films were soft, elastic, and transparent. A gradual transition to a hard, tough, translucent sheet occurred over a period of about 48 hours at room temperature. This transformation could conceivably arise from the slow alignment of the polyester chains in the coiled state caused by the strong dipoledipole interaction of the many polar groups in the chain. Stress-Strain Behavior. Data on stress-strain behavior, tensile strength, and elongation at breaking point of the films were secured by means of a Dillon universal testing machine. This machine has a multirange scale, and the rate of deformation can be fixed at any desired speed up to 20 inches per minute. Dumbbell specimens having cross-sectional areas ranging from 0.0003 to 0 00043 square inch were used. Dimensions were secured Kith the aid of a Randall and Stickney micrometer gage. The recording attachment of the Dillon tester was employed to secure stress-strain curves. This attachment consists of a revolving drum which measures the rate of elongation and a moving pen which records the change in stress. A pendulum arm indicates the amount of stress developed in the sample. Since only qualitative data were possible at this stage of the investigation, the shape but not the values of the stress and strain were recorded. A typical stress-strain curve secured with a polyurethane film is shown in Figure 1. This curve s h o w that a very small elongation caused a very rapid increase in stress (stage A). After the development of considerable stress, probably resulting from the breaking of rather strong secondary forces, there was a sudden halt in stress and considerable elongation set in (stage B). This sudden change in behavior was accompanied by a “neckingin’’ of the sample and the strip changed from a translucent to a transparent film. Presumably, the crystallinity disappears when the chains are able to move past one another and to begin
62
to uncoil. When the necking-in stage was complete, the additional stress resulted in gradual elongation (stage C). Presumably, during this stage, the uncoiling chains begin to be oriented in the direction of strain, and gradual crystallinity again begins to occur. The crystallinity is manifest by the develop-
Strain (Elongation)
Figure 1 .
Stress-strain curve for cured film showing early stages (PEA 3040)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 48, No. 1
PRODUCT AND PROCESS DEVELOPMENT sessed considerably less permanent set than the urethane prepared from PEA 3040 Rain-erosion tests exhibit protective action of coatings on aluminum
Plain AI after 60 min.
Figure 2.
Neoprene coating after Polyurethane coat56 min. ing after 434 min.
Test specimens after exposure to rainfall of 1 inch per hour at 500 miles per hour
ment of opacity in regions of highest stress just prior to breaking of the sample. The crystallinity which occurs on stretching the film probably arises from the uncoiled chains assuming a highly oriented arrangement with the dipole forces aligned so as to oppose the flow of chains. This crystallinity differs from that which is present in the unstretched film where the crystallites are randomly located in the coiled chains and obstruct the uncoiling of the chains in the early stages of deformation. The crystallinity of the polyurethane film which develops at high elongation is similar to that which occurs in natural rubber (6). Stage (A) of the polyurethane stress-strain curve is similar to the behavior of natural rubber a t very low temperatures ( -66” C.) ( 3 ) . If the original aftercured polyurethane film is heated to about 40” C., it becomes transparent and flexible and the stress-strain curve when taken in the transparent state shows no horizontal portion characteristic of the stiff, unheated films. Tensile Strength and Elongation. The ultimate tensile strength of the polyurethane prepared from PEA 3040 was almost 10,000 pounds per square inch with an ultimate elongation of 652%. This extremely high tensile strength is one of the outstanding features of the polyurethane film and coating. It seems quite certain that primary bonds are being broken in the rupture. Polyurethanes prepared from lower molecular weight polyesters did not have nearly the tensile strength of this sample. Set Properties. The ASTM method was used to obtain permanent set values. Marks were placed 1 inch apart on a strip of sample, and the sample was elongated 300% (marks 4 inches apart). After 10 minutes in the stretched state, the sample was released and after an additional 10 minutes, the distance between marks was again measured. The difference between this “relaxed” reading and the 1-inch reading is multiplied by 100 to secure the percentage of permanent set. The permanent set of the polyurethane prepared from PEA 3040 was 195%. ”hen the relaxed film was heated a t 45” C. for 1 minute, the film contracted to its almost exact original length. Apparently no permanent displacement of chains had occurred a t 300% elongation, and the deformation was principally a result of chain uncoiling. Qualitative tests showed that the polyurethanes prepared from low molecular weight polyesters posJanuary 1956
The preliminary data on the protection afforded aluminum specimens against rain erosion by the polyurethane coatings described here were indeed encouraging. Alcoa aluminum sheet (24STp) 2.5 x 5 inches was curved to simulate the leading edge of an airplane wing, and the specimen was mounted at the ends of an aluminum propeller. The propeller was spun so that the speed a t the center of the test panel was 500 m.p.h. When the propeller reached this speed, water was sprayed onto the propeller at a rate equivalent to 1 inch per hour and a drop size of 0.9 mm. The propeller was stopped intermittently and the test specimen examined for wear. This apparatus and test method are described more fully elsewhere ( 4 ) . The specimen panels were prepared as follows: The panel was washed with acetone, dried, and then heated in a detergent solution (Sprex A.N.), then rinsed with tap water, distilled water, and dried. The specimen panel was then treated with Alodine 1200 to produce a “chemical film” base for the polymer. The acetone-chlorobenzene solution of the polyurethane was then brushed onto the specimen until a layer of 0.010 to 0.015 inch had been built up. The specimen was airdried and then heat-cured a t 120” C. for 2 hours. The specimen was then allowed t o cool and was stored at room temperature for several days before it was mounted in the rain-erosion test equipment. A polyurethane coating prepared from PEA 2600 with a ratio of TDI/PEA of 1.65 with ethanolamine added in sufficient quantity to just equal the molar excess of T D I gave exceptionally good results. This specimen remained apparently unaffected until 294 minutes had elapsed, a t which time a small pinhole developed, but no essential wear of the coating occurred until 434 minutes had elapsed. A coating prepared from a neoprene copolymer lasted only 56 minutes under identical test conditions. Figure 2 shows an untreated aluminum specimen after 60 minutes, the neoprene coating after 56 minutes, and the polyurethane coated specimen after 434 minutes. Although many of the polyurethane coatings developed pinholes earlier than the one described, in no case was there any appreciable wear until long after neoprene coatings had worn through. The polyurethane coatings became transparent and soft on contact with the rain (probably owing t o some flow), and a shiny, homogeneous surface appeared. This homogeniety of the surface is one of the most important factors affecting resistance to rain erosion. Acknowledgment
The authors wish to thank the Bureau of Aeronautics, Department of the Navy, for their generous financial support. Thanks are also due Kenneth Mecklenborg and John Gibson for technical assistance. literature cited (1) Baker, J.
(2) (3) (4)
(5) (6)
W.,Holdsworth, J. B., J. Chem. SOC. 1947, p. 713; Baker, J. W., and Gaunt, J., Ibid., 1949, p. 9: Raker, J. W., Davies, M. M., and Gaunt, J., Ibid., 1949, p. 24. Bayer, O.,Rubber Chem. & Technol. 23,812 (1950) ; 26,493(1953). Davis, C. C., and Blake, J. T., “Chemistry and Technology of Rubber,” Reinhold, N. Y., 1937. Gibson, John, M. S. Dissertation, University of Cincinnati,June 1955. Monsanto Chemical Co., St. Louis, Mo., Tech. Bull. P-125, 1953. Van Amerongen, G. J., in “Elastomers and Plastomers,” (R. Houwink, editor.), Elsevier, New York, 1950.
ACCEPTED October 14, 1956. RECEIVED for review July 14, 1955. Based on the thesis submitted by C. B. Reilly to the Graduate School of Arts and Sciences, University of Cincinnati, in partial fulfillment of the requirements for an M.S. degiee.
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