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The effect of difunctional acids on the performance properties of polyurethane coatings.

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-cyclohexanedi-methanol (1,4-CHDM) with the mixture of different diacids as 1,3-cyclohexanedicarboxylicacid (1,3-CHDA). 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 cross-linked with HDI isocyanurate and IPDI trimer to form polyurethane coating films. These films were evaluated for their mechanical, thermal, and chemical resistance properties. The studies on film characteristics reveal that the cycloaliphatic diacids afforded polyurethane with greater performance properties than that of aromatic and linear aliphatic diacids.

Keywords Polyurethane coaling, Aromatic, Linear diacids, Cycloaliphatic compounds, Isocyanate trimer

Introduction

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 flexibility, 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 finishing to chemical resistant coatings. (1), (2)

Typically, ester-containing polyols offer abrasion resistance and adhesion promotion, while polyether polyols provide low-temperature flexibility 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 flexibility 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 films under proper curing conditions and are among the most widely used polyols for high performance coatings. Polyether polyols are generally used in highly flexible 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 high-solid 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 ([T.sub.g]) 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 two-package (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).

[FIGURE 1 OMITTED]
Table 1: Coating performance of polyester polyols

Application condition        Equal
Viscosity                    High
Appearance                   Very good
Hardness                     Medium
Brittleness                  Excellent
Gloss retention              Fair
Solvent resistance           Fair
Salt water spray resistance  Very good


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 films. After crosslinking, the mechanical and chemical resistance properties of the coating films were evaluated.

Experiment

General information

The diacids 1,3-CHDA and 1,4-CHDA and diol 1,4-CHDM were obtained from Eastman Chemicals in 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).

[FIGURE 2 OMITTED]
Table 2: Description of HDI isocyanurate

Isocyanate used  Molecular  Isocyanate  Functionality  Equivalent
                  weight       value                     weight

HDI Trimer        504.60       21.8          3.5          193
IPDI Trimer       763.1        17.5          3.4          247


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, 1,4-CHDA. IPA, AA, and azelaic acid were used for studies.
Table 3: Formulation of polyester polyol

Sample no.  Sample code    Diol    Diacids              Moral ratio
                                                        diol:diacid

1               PP-1     1,4-CHDM  1,3-CHDA                3:2
2               PP-2     1,4-CHDM  1,4-CHEA + 1,3-CHDA     3:1:1
3               PP-3     1,4-CHDM  1,4-CHDA + IPA          3:2:1
4               PP-4     1,4-CHDM  1,4-CHDA + AA           3:1:1
5               PP-5     1,4-CHDM  AA + AZA                3:1:1

1,4-CHDM = 1,4-cyclohexane dimethanol; 1,4-CHDA = 1,4-
cyclohexanedicarboxylic acid; 1,3-CHDA = 1,3-cyclohexanedi-carboxylic
acid; IPA = Isophtallic acid; AA = Adipic acid; AZA = Azleaic acid


The polyester polyols were synthesized at ~210-230[degrees]C in a four-necked, round-bottom flask equipped with mechanical stirrer, nitrogen purge, and a modified 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.

[FIGURE 3 OMITTED]

The reaction is shown in Fig. 4.

[FIGURE 4 OMITTED]

Characterization of polyester polyol

The hydroxyl value of polyester polyol was determined via titration. Gel permeation chromatography (GPC) data were obtained to confirm the molecular weight of polyester polyol formed on a SHIMADZU C-R4A Chrotopac by using water (100 [angstrom]). Columns were calibrated using A1Drich polyethylene glycol standards. The data is shown in Table 4.
Table 4: Properties of polyester polyols

Sample code  Acid no  Hydroxyl no  Mn    Mw   PDI

PP-1           4.2      154        873  1493  1.71
PP-2           3.4      154        892  1552  1.74
PP-3           3.5      156        935  1626  1.74
PP-4           5.9      154        900  1602  1.78
PP-5           5.4      155        873  1606  1.84


The viscosity of polyester polyols was determined by Brookfield Viscometer in methyl ethyl ketone solvent (see Fig. 5).
PP-1  12.2
PP-2  8.2
PP-3  7.6
PP-4  6
PP-5  2.2

Note: Table made from bar graph.


[FIGURE 5 OMITTED]

Coating formulation and film 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 films were applied on the sand-blasted steel panels with the help of a film applicator. All efforts were made to maintain a uniform film thickness of 100 [micro] for their general mechanical and chemical resistance properties. The films were cured at 120[degrees]C for 1 h and the cured films were stored for 3 days under ambient atmospheric conditions before testing.

Characterization of film properties

Polyurethane-coated film panels were evaluated for their mechanical properties, such as scratch hardness, adhesion, flexibility, thermal properties, and chemical resistance properties.

Mechanical properties of polyurethane films

The mechanical properties are shown in Table 5.
Table 5: Mechanical and thermal properties of polyurethane film

Coating    Scratch     Flexibility  Crosshatch  Glass transition
sample   hardness (g)  (1/4-inch)    adhesion   temp. [T.sub.g]
                        mandrel                 ([degrees]C, DSC)

PUS-1       1500          Pass         100             60
PUS-2       1500          Pass         100             59
PUS-3       1700          Pass         100             66
PUS-4       1100          Pass         100             39
PUS-5        900          Pass         100             19
PUS-6       1400          Pass         100             48
PUS-7       1600          Pass         100             39
PUS-8       2300          Pass          92             61
PUS-9       1200          Pass         100             32
PUS-10       800          Pass         100             23


HARDNESS: Hardness is the resistance of material to the indentation of scratching. The most widely used hardness lest 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 films was determined by bending the panels to 180[degrees]C 1/4-inch mandrel (Sheen Instruments Limited. England).

Thermal properties

The thermal stability of the coating samples were determined by a comparison of the onset degradation 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[degrees]C7min in nitrogen atmosphere from 50 to 200[degrees]C (see Table 5).

Chemical resistance properties of poly are thane films

To evaluate the overall performance of the coatings, the coated films were exposed to the action of various solvents, acids, alkalis, and water. The coated panels were sealed from three sides by using molten paraffin 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 films. The panels were observed for a visible change in the condition of the film at regular intervals when immersed in these chemicals at an ambient temperature for a period of 6 months.

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 films. From the data in Fig. 5, it is apparent that the cycloaliphatic-based polyester offered better solubility in MEK than the aromatic and linear diacid polyesters. The IPA/1,4-CHDA-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 1,3-CHDE/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 significant 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 films, 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 1,3-CHDA/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 films 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 film 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 flexibility test. Coatings based on IPDI trimer along with 1,4-CHDA/1,3-CHDA, 1,4-CHDA/IPA failed in flexibility with 1/4-inch mandrel.

Thermal properties

The polyurethane 1,4-CHDA/1,3-CHDA has the lowest glass transition temperature ([T.sub.g]) within the cyclohexyl 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 [T.sub.g] was observed for the series of polyurethane with mixed diacids of IPA, 1,4-CHDA, 1,3-CHDA, AA and AZA. The corresponding [T.sub.g] using DSC measurement ranges from 66[degrees]C to 19[degrees]C. Generally, the linear structure of diacids in the polyesters provide polyurethane with lower [T.sub.g], the aromatic diacid with higher [T.sub.g], and the cycloaliphatic diacid with intermediate [T.sub.g]. The rigid phenyl ring with a planer structure increases the [T.sub.g] and hardness.

It is presumed that the conformational interchange of cyclohexane ring increases the film flexibility 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 films

From the results mentioned in Tables 6-9, it can be seen that all the coating films 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 cross-linked with IPDI trimer have shown comparatively poor resistance against water, acids, alkalis, and solvents than the coatings crosslinked with HDI trimer.
Table 6: Resistance to water (a) of polyurethane films

Sample              Water

        Deionized water  Sea water

PUS-1          5            5
PUS-2          5            5
PUS-3          5            5
PUS-4          5            5
PUS-5          5            5
PUS-6          5            4
PUS-7          4            5
PUS-8          5            5
PUS-9          5            4
PUS-10         4            3

(a) When dipped for 6 months

5 = Film unaffected; 4 = Change in color and loss in gloss; 3 =
Blistering of film; 2 = Softening of film; 1 = Partial removal of film
from the panel; 0 = Complete removal of film from the panel

Table 7: Resistance to acids (a) of polyurethane films

Sample                     Acids

          Acetic acid  Sulfuric acid   Hydrochloric
                                         acid 10%

          5%     15%    5%      10%    10%     36%
        (v/v)   (v/v)  (v/v)   (v/v)  (v/v)   (v/v)

PUS-1     5       5      5       4      5       5
PUS-2     5       5      5       5      5       3
PUS-3     5       4      4       4      5       4
PUS-4     5       4      4       3      4       3
PUS-5     4       4      3       3      3       3
PUS-6     4       4      4       3      3       3
PUS-7     4       4      4       3      3       3
PUS-8     4       4      4       3      3       3
PUS-9     4       4      3       2      3       2
PUS-10    4       3      3       2      3       2

Note: See footnotes of Table 6 for details

Table 8: Resistance to alkalis (a) of polyurethane films

Sample                   Alkalis

          Sodium hydroxide     Ammonium hydroxide

        10% (w/v)  50% (w/v)  10% (v/v)  25% (v/v)

PUS-1       5         5          5          4
PUS-2       4         3          4          3
PUS-3       5         5          5          4
PUS-4       4         3          4          4
PUS-5       4         3          4          4
PUS-6       4         4          4          3
PUS-7       3         2          4          3
PUS-8       4         4          4          4
PUS-9       3         3          3          3
PUS-10      3         2          3          3

Note: See footnotes of Table 6 for details

Table 9: Resistance to solvents (a) of polyurethane films

Sample                         Solvents

        Toluene  Xylene  Methanol  Ethanol  Acetone  Methyl ethyl
                                                        ketone

PUS-1      5       5        5         5        5          5
PUS-2      5       5        5         5        5          5
PUS-3      5       5        5         5        4          5
PUS-4      5       5        4         5        5          5
PUS-5      5       4        4         4        4          4
PUS-6      5       5        4         4        5          4
PUS-7      5       3        4         4        4          4
PUS-8      5       4        4         4        4          4
PUS-9      4       4        4         4        3          3
PUS-10     4       4        4         3        3          3

Note: See footnotes of Table 6 for details


Conclusion

The aromatic diacid provides polyester with better hardness and high [T.sub.g] among all the films. The 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.

References

(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 Polyester-based 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)

(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, "Influence of Dibasic Acids on the Properties of Modified Polyurethane Coatings." Surf. Coat. Int., 78 68 (1995)

S. Awasthi ([??]), D. Agarwal

Department of Oil & Paint Technology,

Harcourt Butler Technological Institute (HBTI),

Nawabganj, Kanpur 208002, India

e-mail: shailja_2005@rediffmail.com

D. Agarwal

e-mail: dahbti@rediffmail.com
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Date:Sep 1, 2009
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