J. Coat. Technol. Res. DOI 10.1007/s11998-008-9121-9
B RI E F C O M M U N I C A T I O N
The effect of difunctional acids on the performance properties of polyurethane co
Shailja Awasthi, Devendra Agarwal
? FSCT and OCCA 2008 Abstract Hydroxyl-terminated polyesters are the most common polyols that are crosslinked through an isocyanate group. In this study, the polyester polyol resins were synthesized by using 1,4-cyclohexanedimethanol (1,4-CHDM) with the mixture of different diacids as 1,3-cyclohexanedicarboxylicacid (1,3CHDA), 1,4-cyclohexanedicarboxylicacid (1,4-CHDA), isophthalic acid (IPA), adipic acid (AA), and azelaic acid (AZA). The solubility and viscosity of these polyester polyol resins were determined by using suitable solvent. All the polyester polyols were crosslinked with HDI isocyanurate and IPDI trimer to form polyurethane coating ?lms. These ?lms were evaluated for their mechanical, thermal, and chemical resistance properties. The studies on ?lm characteristics reveal that the cycloaliphatic diacids afforded polyurethane with greater performance properties than that of aromatic and linear aliphatic diacids. Keywords Polyurethane coating, Aromatic, Linear diacids, Cycloaliphatic compounds, Isocyanate trimer ?exibility, chemical and corrosion resistance, and a wide range of mechanical strength. Because of these characteristics, PU coatings have emerged as coatings of choice for application from industrial maintenance to automobile ?nishing to chemical resistant coatings.1,2 Typically, ester-containing polyols offer abrasion resistance and adhesion promotion, while polyether polyols provide low-temperature ?exibility and low viscosity. Industrial polyether polyols are generally limited in monomer composition to propylene oxide, ethylene oxide, butylenes oxide, and tetrahydrofuran. Industrial polyester polyols, many of which are the condensation products of organic acids and alcohols, may be prepared by a great many more combinations of monomers and thus add to the potential versatility of polyurethane products.3 The polyurethane polymers are formed by reaction of isocyanates and polyalcohols. The main reaction occurs on site when the two components are mixed prior to use, the mixture having limited usage time or pot life. Single component polyurethane coatings that cure by the reaction with atmospheric moisture are also available. Polyurethane coatings are characterized by the following properties: good gloss stability, excellent color stability, good chemical and mechanical resistance, and moisture sensitivity during manufacture and application.4 Two-component polyurethane (2K-PU) systems are especially attractive since they offer ?exibility in formulation, which enables one to customize according to the demands of varying requirements. Polyols are major components of PU coating systems and often designed to suit the performance requirements of the intended applications. Among the common commercially available polyols for 2K-PU system are hydroxyl-functional acrylics and polyether polyols. Polyester and acrylics produce very tough polyurethane ?lms under proper curing conditions and are among the most widely used
Polyurethanes have been used in the coatings industry for approximately 35 years. They have shown excellent performance and great versatility in many applications. Polyurethanes (PU) have found extensive applications in the coating industry, mainly because they exhibit excellent abrasion resistance, toughness, low temperature
S. Awasthi (&), D. Agarwal Department of Oil & Paint Technology, Harcourt Butler Technological Institute (HBTI), Nawabganj, Kanpur 208002, India e-mail: firstname.lastname@example.org D. Agarwal e-mail: email@example.com
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H O R NCO + R OH R N C OR
Fig. 1: Formation of polyurethane resin
Table 1: Coating performance of polyester polyols Application condition Viscosity Appearance Hardness Brittleness Gloss retention Solvent resistance Salt water spray resistance Equal High Very good Medium Excellent Fair Fair Very good
Mumbai. The other diacids AA, and azelaic acid (AZA) came from SD Fine Chemicals and IPA came from SISCO in Mumbai. The crosslinkers HDI isocyanurate and IPDI trimer were procured from Bayer Corporation (Desmodur N-3300 and Desmodur Z-4470, respectively). All the reactants were used as received. The chemical structures of HDI isocyanurate and IPDI trimer are illustrated in Fig. 2 (see also Table 2). Synthesis of polyester polyol The formulation of polyester polyols synthesized is given in Table 3. Five formulations consisting of single diol 1,4-CHDM with different diacids (1,3-CHDA, 1,4CHDA, IPA, AA, and azelaic acid were used for studies. The polyester polyols were synthesized at $210– 230°C in a four-necked, round-bottom ?ask equipped with mechanical stirrer, nitrogen purge, and a modi?ed Dean and Stark condenser. The conversion of polyester polyol was monitored by determining the acid value with respect to time until the resin had an acid value between 3 and 6 mg KOH/g resin (see Fig. 3).15 The acid value and hydroxyl value of polyesters were measured according to the ASTM standards D 1639-89 and D 4274-94, respectively. The reaction is shown in Fig. 4. Characterization of polyester polyol The hydroxyl value of polyester polyol was determined via titration. Gel permeation chromatography (GPC) data were obtained to con?rm the molecular weight of polyester polyol formed on a SHIMADZU C-R4A ? Chrotopac by using water (100 A). Columns were calibrated using AlDrich polyethylene glycol standards. The data is shown in Table 4. The viscosity of polyester polyols was determined by Brook?eld Viscometer in methyl ethyl ketone solvent (see Fig. 5). Coating formulation and ?lm preparation Polyurethane coating samples (PUS-1, PUS-2, PUS-3, PUS-4, and PUS-5) were prepared by adding the required amount of HDI isocyanurate (hexamethylene diisocyanate isocyanurate) to the polyester polyol solution, (NCO:OH:1.6:1), which was diluted in MEK and the mixture was stirred well. The ?lms were applied on the sand-blasted steel panels with the help of a ?lm applicator. All efforts were made to maintain a uniform ?lm thickness of 100 lm for their general mechanical and chemical resistance properties. The ?lms were cured at 120°C for 1 h and the cured ?lms were stored for 3 days under ambient atmospheric conditions before testing.
polyols for high performance coatings. Polyether polyols are generally used in highly ?exible systems such as sealants and other noncoating applications.5 Hydroxyl-terminated polyesters are the most common polyols that are crosslinked through isocyanate groups. Generally, polyester polyols can achieve highsolid coatings with great solvent resistance and good adhesion to metals.6,7 Polyester resins for coating applications are usually prepared with both aromatic and aliphatic dibasic acids. Isophthalic acid is the principal aromatic dibasic acid used in coatings, and adipic acid is the principal aliphatic diacid.8 The aromatic diacid compound is used to increase the glass-transition (Tg) temperature, hardness, and chemical resistance. However, the phenyl ring readily absorbs UV light, limiting the photooxidative stability of the polyester.9 The largest volume of urethane coatings are twopackage (2K) coatings. These coatings are typically used for wood, plastics, automotive topcoat, and aircraft topcoat.10–14 Polyurethane resins are derived from the reaction of hydroxyl-functionalized oligomers or polyols with an isocyanate as shown in Fig. 1 (see also Table 1). In this study, the polyester polyols were designed to have a low molecular weight of 800–1000 g/mol to achieve a low viscosity for the high-solid coatings. The polyester polyols were synthesized and the general polymeric properties, including the acid value, the hydroxyl value, the average molecular weight, the polydispersity index, and the viscosity were evaluated. The polyester polyols were crosslinked with HDI isocyanurate and IPDI trimer forming polyurethane ?lms. After crosslinking, the mechanical and chemical resistance properties of the coating ?lms were evaluated.
General information The diacids 1,3-CHDA and 1,4-CHDA and diol 1,4CHDM were obtained from Eastman Chemicals in
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NCO H3C NCO O N O H3C CH3 H3C NCO
O CH3 N
N (H2C)6 NCO O
N (CH2)6 NCO
Fig. 2: Chemical structure of HDI isocyanurate and IPDI trimer
Table 2: Description of HDI isocyanurate Isocyanate used HDI Trimer IPDI Trimer Molecular weight 504.60 763.1 Isocyanate value 21.8 17.5 Functionality 3.5 3.4 Equivalent weight 193 247
Table 3: Formulation of polyester polyol Sample no. 1 2 3 4 5 Sample code PP-1 PP-2 PP-3 PP-4 PP-5 Diol 1,4-CHDM 1,4-CHDM 1,4-CHDM 1,4-CHDM 1,4-CHDM Diacids 1,3-CHDA 1,4-CHDA + 1,3-CHDA 1,4-CHDA + IPA 1,4-CHDA + AA AA + AZA Molar ratio diol:diacid 3:2 3:1:1 3:2:1 3:1:1 3:1:1
1,4-CHDM = 1,4-cyclohexane dimethanol; 1,4-CHDA = 1,4-cyclohexanedicarboxylic acid; 1,3-CHDA = 1,3-cyclohexanedicarboxylic acid; IPA = Isophthalic acid; AA = Adipic acid; AZA = Azelaic acid
Mechanical properties of polyurethane ?lms
PP-1 PP-2 PP-3 PP-4 PP-5
Av mgKOH/gm solid
120 100 80 60 40 20 0 0 30 60 90 120 150 Reaction time (min) 180
The mechanical properties are shown in Table 5. HARDNESS: Hardness is the resistance of material to the indentation of scratching. The most widely used hardness test for coatings is ‘‘scratch hardness.’’ It was measured by using a scratch hardness tester (ASTM D 5178, Sheen Instruments Limited in England). The panels were loaded with different weights until a clear scratch showing the bare metal surface was seen. PERCENT ADHESION: Percent adhesion was determined by using a crosshatch adhesion tester (ASTM D 3359). FLEXIBILITY: Flexibility of coated ?lms was determined by bending the panels to 180°C ?-inch mandrel (Sheen Instruments Limited, England). Thermal properties The thermal stability of the coating samples were determined by a comparison of the onset degradation
Fig. 3: Acid value vs reaction time
Characterization of ?lm properties Polyurethane-coated ?lm panels were evaluated for their mechanical properties, such as scratch hardness, adhesion, ?exibility, thermal properties, and chemical resistance properties.
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O CH2O C
O C OH2C
O CH2O C
O C O
Fig. 4: Synthesis of polyester polyol
Table 4: Properties of polyester polyols Sample code PP-1 PP-2 PP-3 PP-4 PP-5 Acid no. 4.2 3.4 3.5 5.9 5.4 Hydroxyl no. 154 154 156 154 155 Mn 873 892 935 900 873 Mw 1493 1552 1626 1602 1606 PDI
14 12 12.2
1.71 1.74 1.74 1.78 1.84
10 8 6 4 2 0 PP-1
temperature (up to 5% wt. loss) of the cured samples with thermogravimetric analyzer (TGA) of Universal V3.9A TA Instrument at a heating rate of 20°C/min in nitrogen atmosphere from 50 to 200°C (see Table 5). Chemical resistance properties of polyurethane ?lms To evaluate the overall performance of the coatings, the coated ?lms were exposed to the action of various solvents, acids, alkalis, and water. The coated panels were sealed from three sides by using molten paraf?n wax before dipping in various chemicals. Preparation of reagents (a) Seawater This was prepared as per the composition given in IS 1404–1970. (b) Acids The acids were diluted by taking the required quantity of water and acids on a volume basis to achieve the desired concentration. The acids were added slowly in the water to make dilute acids. The following concentrations of various acids were used: sulfuric acid (5%, 10%), hydrochloric acid (10%, 36%), and acetic acid (5%, 15%). (c) Alkalis Sodium hydroxide was dissolved in water to make solutions of 10% and 50% concentration on a weight/volume basis. Two concentrations of 10% and 25% of ammonium hydroxide were taken by diluting with water on a volume basis. These reagents, along with other solvents such as MEK, xylene, toluene, acetone, methanol, and ethanol were taken to determine the chemical resistance of the cured polyurethane ?lms. The panels were observed for a visible change in the condition of the ?lm at regular intervals when immersed in these chemicals at an ambient temperature for a period of 6 months.
Fig. 5: Viscosity vs formulation
Table 5: Mechanical and thermal properties of polyurethane ?lm Coating sample Scratch Flexibility hardness (?-inch) mandrel (g) Glass Crosshatch transition adhesion temp. Tg (°C, (%) DSC) 100 100 100 100 100 100 100 92 100 100 60 59 66 39 19 48 39 61 32 23
PUS-1 PUS-2 PUS-3 PUS-4 PUS-5 PUS-6 PUS-7 PUS-8 PUS-9 PUS-10
1500 1500 1700 1100 900 1400 1600 2300 1200 800
Pass Pass Pass Pass Pass Pass Fail Fail Pass Pass
Results and discussion
Preparation and properties of polyester polyols The polyesters were designed to have a low molecular weight of 800–1000 g/mol to achieve a low viscosity for high-solid coatings. The polyesters were crosslinked by HDI isocyanurate, affording polyurethane ?lms. From
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the data in Fig. 5, it is apparent that the cycloaliphaticbased polyester offered better solubility in MEK than the aromatic and linear diacid polyesters. The IPA/1,4CHDA-based polyester needed to be heated to mix with the crosslinker, and the elevated temperature resulted in a reduced application time. The polyester oligomers based on 1,3-CHDA and a mixture of 1,3CHDE/1,4-CHDA were liquid at room temperature because in both the polyesters, the 1,3 substitution in 1,3-CHDA brought a nonsymmetric structure to the polyester, thus it reduced the intermolecular interaction that led to crystallinity. The difference in the intermolecular interaction also changed the solubility in the common solvent MEK. AA/azelaic acid had a linear symmetric chain and may have provided a stronger interaction between the polyester oligomers. The same effect of diacids on the viscosity can be observed for the series of polyesters. The interaction between the polyester polyol and the solvent had a signi?cant effect on the viscosity of the solution.16 Mechanical properties Hardness The data in Table 5 shows that the polyurethane coatings based on IPA/1,4-CHDA showed the best hardness among the 10 ?lms, even crosslinked with HDI isocyanurate and IPDI trimer, so that it could bear the weight of 1700 and 2300 g, respectively. The polyurethane coatings based on 1,3-CHDA and 1,3CHDA/1,4-CHDA both had harnesses of 1500 g when crosslinked with HDI isocyanurate, so the isomeric mixture dose not effect hardness. Generally the linear structure of AA/AZA in the polyesters provided polyurethane ?lms with a lowest hardness of 900 and 800 g. The rigid phenyl ring increased the hardness. It was presumed that the conformational interchange of the cyclohexane ring (i.e., 1,3-CHDA) decreased the ?lm hardness in comparison with the rigid IPA. Percentage adhesion All the 10 formulations showed a 100% adhesion on mild steel panels except PUS-8, which showed 92% adhesion (see Table 5). Flexibility It can be seen from Table 5 that all the formulations did not pass the ?exibility test. Coatings based on IPDI trimer along with 1,4-CHDA/1,3-CHDA, 1,4-CHDA/ IPA failed in ?exibility with ?-inch mandrel. Thermal properties The polyurethane 1,4-CHDA/1,3-CHDA has the lowest glass transition temperature (Tg) within the cyclohexyl
Table 6: Resistance to watera of polyurethane ?lms Sample Deionized water PUS-1 PUS-2 PUS-3 PUS-4 PUS-5 PUS-6 PUS-7 PUS-8 PUS-9 PUS-10
Water Sea water 5 5 5 5 5 4 5 5 4 3
5 5 5 5 5 5 4 5 5 4
When dipped for 6 months 5 = Film unaffected; 4 = Change in color and loss in gloss; 3 = Blistering of ?lm; 2 = Softening of ?lm; 1 = Partial removal of ?lm from the panel; 0 = Complete removal of ?lm from the panel
Table 7: Resistance to acidsa of polyurethane ?lms Sample Acetic acid 5% (v/v) PUS-1 PUS-2 PUS-3 PUS-4 PUS-5 PUS-6 PUS-7 PUS-8 PUS-9 PUS-10 5 5 5 5 4 4 4 4 4 4 15% (v/v) 5 5 4 4 4 4 4 4 4 3 Acids Sulfuric acid 5% (v/v) 5 5 4 4 3 4 4 4 3 3 10% (v/v) 4 5 4 3 3 3 3 3 2 2 Hydrochloric acid 10% (v/v) 5 5 5 4 3 3 3 3 3 3 36% (v/v) 5 3 4 3 3 3 3 3 2 2
Note: See footnotes of Table 6 for details Table 8: Resistance to alkalisa of polyurethane ?lms Sample Sodium hydroxide 10% (w/v) PUS-1 PUS-2 PUS-3 PUS-4 PUS-5 PUS-6 PUS-7 PUS-8 PUS-9 PUS-10 5 4 5 4 4 4 3 4 3 3 50% (w/v) 5 3 5 3 3 4 2 4 3 2 Alkalis Ammonium hydroxide 10% (v/v) 5 4 5 4 4 4 4 4 3 3 25% (v/v) 4 3 4 4 4 3 3 4 3 3
Note: See footnotes of Table 6 for details
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Table 9: Resistance to solventsa of polyurethane ?lms Sample Toluene PUS-1 PUS-2 PUS-3 PUS-4 PUS-5 PUS-6 PUS-7 PUS-8 PUS-9 PUS-10 5 5 5 5 5 5 5 5 4 4 Xylene 5 5 5 5 4 5 3 4 4 4 Methanol 5 5 5 4 4 4 4 4 4 4 Solvents Ethanol 5 5 5 5 4 4 4 4 4 3 Acetone 5 5 4 5 4 5 4 4 3 3 Methyl ethyl ketone 5 5 5 5 4 4 4 4 3 3
Note: See footnotes of Table 6 for details
diacids series, as indicated by DSC when the molar ratio of 1,4-CHDA to 1,3-CHDA is 1:1. A large difference in Tg was observed for the series of polyurethane with mixed diacids of IPA, 1,4-CHDA, 1,3-CHDA, AA and AZA. The corresponding Tg using DSC measurement ranges from 66°C to 19°C. Generally, the linear structure of diacids in the polyesters provide polyurethane with lower Tg, the aromatic diacid with higher Tg, and the cycloaliphatic diacid with intermediate Tg. The rigid phenyl ring with a planer structure increases the Tg and hardness. It is presumed that the conformational interchange of cyclohexane ring increases the ?lm ?exibility in comparison with the rigid IPA. The 1,4-CHDA/ 1,3-CHDA copolymer, however, provides the polyurethane better adhesion than the 1,4-CHDA/IPA copolymer. In addition, the polyester containing a mixture of 1,4-CHDA/1,3-CHDA possesses better solubility than the 1,4-CHDA/IPA-based polyester. Chemical resistance properties of polyurethane ?lms From the results mentioned in Tables 6–9, it can be seen that all the coating ?lms are almost unaffected by water and sea water, but loss in gloss and change in color is observed in few coatings, crosslinked with IPDI trimer. The coatings based on linear diacids AA/AZA and 1,4-CHDA/AA have shown poor resistance to acid, alkalis, and solvents as compared to coatings based on 1,3-CHDA, 1,3-CHDA/1,4-CHDA, and 1,4-CHDA/IPA. One more thing also observed from the above results is that the coatings that are crosslinked with IPDI trimer have shown comparatively poor resistance against water, acids, alkalis, and solvents than the coatings crosslinked with HDI trimer.
cycloaliphatic diacid provides polyester with better solubility in methyl ethyl ketone (MEK) than aromatic and linear diacids. The 1,4-CHDA-based polyester has shown poor solubility in MEK compared to 1,3-CHDA due to the symmetric structure of 1,4-substitution on the cyclohexane. A mixture of 1,4-CHDA and 1,3-CHDA can reduce the viscosity and increase solubility in MEK. Thus, the cycloaliphatic diacids can also provide polyesters with better solubility than aromatic or linear diacids. The chemical resistance properties show that these coatings also have good resistance to water and other chemicals and can be used safely in exterior applications.
1. Blank, WJ, ‘‘Novel Polyurethane Polyols for Waterborne and High Solids Coatings.’’ Prog. Org. Coat., 20 235–259 (1992) 2. Ming, Z, Denggao, J, Cuihong, H, ‘‘The Effect of Isocyanate Index-NCO/-OH on the Structure of Polyurethane Dispersion.’’ Paint India, 55 59–67 (2005) 3. O’Brien, ME, Hillishafer, DK, Williamson, EH, ‘‘A Hot Formula.’’ Adhes. Age, 44 (11) 20–25 (2001) 4. Sen, A, ‘‘Protective Coating for Maintenance Engineers – One Approach.’’ Paint India, XLIX (11) (1999) 5. Manari, VM, Massingill Jr, JL, ‘‘Two-component High Solid PU Coating System Based on Soy Polyols.’’ J. Coat. Technol. Res., 3 (2) 151–157 (2006) 6. Hood, JD, Blount, WW, Sade, WT, ‘‘Polyester Resin Synthesis Techniques for Achieving Lower VOC and Improved Coating Performance.’’ J. Coat. Technol., 58 (739) 49–52 (1986) 7. Wicks, ZW, Jones, FN, Pappas, SP, ‘‘Organic Coatings Science and Technology.’’ In: Film Formation, Components and Appearance, I, ISBN-0471614068, Chapter 8. Wiley, New York (1992) 8. Ni, H, Daum, JL, Thiltgen, PR, ‘‘Cycloaliphatic Polyesterbased High-solid Polyurethane Coatings. The Effect of Difunctional Acid.’’ Prog. Org. Coat., 45 49–58 (2002). doi:10.1016/S0300-9440(02)00100-5 9. Pilati, F, Toselli, M, Messori, M, Sanders, D (eds.), Waterborne and Solvent-based Saturated Polyesters and Their End User Applications, Chapter 2. Wiley, New York (1999)
The aromatic diacid provides polyester with better hardness and high Tg among all the ?lms. The
J. Coat. Technol. Res. 10. Gregorovich, BV, Hazan, I., ‘‘Environmental Etch Performance and Scratch and Mar of Automotive Clearcoats.’’ Prog. Org. Coat., 24 131–146 (1994). doi:10.1016/00330655(94)85011-9 11. Roesler, RR, Grace SA, Polym. Mater. Sci. Eng., 83 327, Am. Chem. Soc. Div. (2000) 12. Andriu, VJ, Laurent, P, ‘‘Air Convective Drying and Curing of Polyurethane Based Paints on Sheet Molding Compound Surfaces.’’ J. Coat. Technol., 70 (882) 67–76 (1998) 13. Shoemaker, SH, ‘‘Two-Component Isopolyester Urethane Coatings for Plastic.’’ J. Coat. Technol., 62 (787) 49–55 (1990) 14. Kubitza, W, ‘‘Water Based Two-Pack Polyurethane Paints.’’ J. Oil Color Chem. Assoc., 75 340–347 (1992) 15. Hood JD, Blount, WW, Sade, WT, ‘‘Polyester Resin Synthesis Techniques for Achieving Lower VOC and Improved Coating Performance.’’ J. Coat. Technol., 58 (739) 49–52 (1986) 16. Haseebuddin, S, Padmavati, T, Raju, KVSN, ‘‘In?uence of Dibasic Acids on the Properties of Modi?ed Polyurethane Coatings.’’ Surf. Coat. Int., 78 68 (1995)