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Influence of cycloaliphatic compounds on the properties of polyurethane coatings.

Abstract A series of polyester polyol resin was synthesized by using 1,4-cyclohexanedimenthanol (1,4-CHDM) and three different diacids: 1,3-cyclohexanedicarboxylic acid (1,3-CHDA), isophthalic acid (IPA) and adipic acid (AA). The solubility and viscosity of the polyester polyols were determined, by using methyl ethyl ketone (MEK). All the polyester polyols were crosslinked with hexamethylene diiscyanate (HDI) isocyanurate to form polyurethane coating films. These films were evaluated for their mechanical and chemical resistance properties. Studies on the film characteristics revealed that the polyurethane films based on cycloaliphatic diacid generally showed comparatively better performance properties than the polyurethane film based on aromatic and linear aliphatic diacids in general.

Keywords Polyester polyol, Polyurethane coating, Cycloaliphatic compound

Introduction

Polyurethane as a coating medium was introduced in the 1950s and since then has gained ground in terms of applications. Polyurethane coatings as such show superior weatherability, solvent resistance, acid-base resistance, and hydrolytic stability compared with other resins. (1) The largest volume of polyurethane coating is a two-pack coating. These coatings are typically used for plastic, wood, aircraft topcoats, and automotive top-coats. (2-6) Polyurethane resins are derived from a reaction of a hydroxyl-fuctionalized oligomer or polyol with an isocyanate, as shown in Fig. 1.

[FIGURE 1 OMITTED]

Ester-containing polyols offer abrasion resistance and adhesion promotion, while polyether polyols exhibit low-temperature flexibility and low viscosity. Industrial polyether polyols are generally limited in monomer composition to propylene oxide, ethylene oxide, butylene oxide, and tetrahydrofuran, while 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 therefore add to the potential versatility of polyure thane products.(7)

Polyester polyols are generally preferred for harder coatings with better weatherability.(8) The various coating performances of polyester polyols are strongly dependent on the level of branching and the hydroxyl value of the polyol (see Table 1).
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


Hydroxyl-terminated polyesters are the most common polyols, which are crosslinked through isocyanate groups. Generally, polyester polyols can achieve highsolid coatings with great solvent resistance and good adhesion to metals.(9.10) 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.(11) The aromatic diacid compound is used to increase the glasstransition ([T.sub.g]) temperature, hardness, and chemical resistance. However, the phenyl ring readily absorbs UV light limiting the photo-oxidative stability of the polyester.(12)

In the early 1990s, cyclohexyl dibasic acids were proposed as replacements for the aromatic dibasic acids. The cyclohexane diacid monomers, which can be typically used in the preparation of polyesters, are haxahydrophthalic anhydride (HHPA), 1,3-cyclohexanedicarboxylic acid (1,3-CHDA) and 1,4-cyclohexanedicarboxylic acid (1,4-CHDA). The chemical structure of the diacids and anhydride are shown in Fig. 2.

[FIGURE 2 OMITTED]

This cycloaliphatic structure imparts physical properties intermediate between the aromatic and the linear aliphatic polyester except for any yellowing resistance.(13-18) The flexibility of the cycloaliphatic polyester is also found in between the aromatic and linear aliphatic polyester because of the cyclic structure, which can absorb energy through the interconversion of chair and boat conformation. The linear structure imparts flexibility to the polyester resin.(19)

The role of the cycloaliphatic diacid in the polyurethane is an appropriate subject for investigation. A series of hydroxyl-terminated polyesters were prepared using 1,4-cyclohexanedimethanol (1,4-CHDM) with cycloaliphatic, linear, and aromatic diacids. The polyester polyols were designed to have a low molecular weight of 800-1,000 g/mole 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 forming polyurethane films. After crosslinking, the mechanical and chemical resistance properties of the coating films were evaluated.

Experimental

General information

The diacid 1,3-CHDA and diol 1,4-CHDM were obtained from Eastman Chemicals, the adipic acid was obtained from SD Fine Chemicals. and the isophthalic acid (IPA) from SISCO, Mumbai. The crosslinker HDI isocyanurate was procured from Bayer Corporation (Desmodur N-30). All the reactants were used as received. The chemical structure and description of HDI isocyanurate is illustrated in Fig. 3 and Table 2.

[FIGURE 3 OMITTED]
Table 2: Description of HDI isocyanurate

Molecular weight Isocyanate value Functionality Equivalent weight

504.60 25.0 3 168.20


Synthesis of polyester polyols

The formulation of the synthesized polyester polyols are given in Table 3. Three formulations consisting of single-diol 1,4-CHDM with three different diacids--1,3-CHDA, IPA, and adipic acid--were used.
Table 3: Formulation of Polyester polyols

Sample number Product code Diol Diacid Molar ratio
 (diol: diacid)

1 PE-1 1,4-CHDM 1,3-CHDA 3:2
2 PE-2 1,4-CHDM IPA 3:2
3 PE-3 1,4-CHDM AA 3:2


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 & Stark condenser. The conversion of the polyester polyols 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. 4).(20) The acid value and hydroxyl value of polyesters were measured according to the ASTM standards D 1639-89 and D 4274-94, respectively.

[FIGURE 4 OMITTED]

Characterization of polyester polyols

The hydroxyl value of the polyester polyol was obtained via titration. Gel permeation chromatographic (GPC) data were obtained to confirm the molecular weight of the polyester polyol formed on a SHIMADZU C-R4A Chrotopac by using water (100[degrees]A) columns. Samples were dissolved in tetrahydrofuran (THF) at approximately 0.1 g/ml concentration. A 1.0 ml/min flow rate with THF as the mobile phase was used. The columns were calibrated using Aldrich polyethylene glycol standards. The data are shown in Table 4.
Table 4: Properties of polyester polyols

Sample Acid value Hydroxyl value Number average molecular weight
 ([M.sub.n])

PE-1 4.2 154 873
PE-2 3.4 154 935
PE-3 5.7 156 900

Sample Weight average molecular Polydispersity index (PDI)
 weight ([M.sub.w]) ([M.sub.w]/[M.sub.n])

PE-1 1,493 1.71
PE-2 1,625 1.74
PE-3 1,602 1.78


The viscosity of the polyester polyols was determined by a Brookfield Viscometer in methy1 ethy1 ketone solvent (see Table 5 and Fig. 5).
Product code Viscosity (Pa s)

PE-1 12.2
PE-2 7.7
PE-3 5.0

Note: Table made from bar graph.


[FIGURE 5 OMITTED]
Table 5: Viscosity of polyester polyols

Sample Viscosity (Pa s)
PE-1 12.2
PE-2 7.7
PE-3 5.0


Coating formulation and film preparation

Polyurethane coating samples (CS-1, CS-3) were prepared by adding the required amount of HDI isocyanurate (hexamethylene diisocyanate isocyanurate) to the polyester polyol solution (ratio NCO:OH; 2:3), which was then diluted in methy1 ethy1 ketone and the mixture was stirred well. The films were applied on to the sand-blasted steel panels with the help of film applicator. All efforts were made to maintain a uniform film thickness of 100 [micro]m for the general mechanical and chemical resistance properties. The films were cured at 120[degrees]C for 1 h, and the cured films were stored for three 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, and chemical resistance properties.

Mechanical properties of polyurethane films

The mechanical properties of polyurethane are shown in Table 6.
Table 6: Mechanical properties of polyurethane films

Product code Scratch hardness Flexibility Crosshatch
 (g) (1/4-inch) adhesion (%)
 mandrel

CS-1 1,700 Pass 100
CS-2 1,500 Pass 100
CS-3 1,200 Pass 100


HARDNESS: Hardness is the resistance of a material to indentation of scratching. The most widely used hardness test for coatings are scratch hardness. This was measure by using a scratch hardness tester (ASTM D 5178, Sheen Instruments Limited, England). The panels were loaded with different weights until a clear scratch showing the bare metal surface was seen (see Table 6).

PERCENTAGE ADHESION: The percentage adhesion was determined by using crosshatch adhesion tester (ASTM D 3359. Sheen Instruments Limited, England) (see Table 6).

FLEXIBILITY: The flexibility of the coated films was determined by bending the panels to 180[degrees] using a 1/4- inch mandrel (Sheen Instruments Limited, England) (see Table 6).

Chemical resistance properties of polyurethane 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

SEAWATER: This was prepared as per the composition given in IS 1404-1970, as shown in Table 7.
Table 7: Composition of seawater

Salt Weight in grams

Sodium chloride 23.476
Magnesium chloride 4.981
Sodium sulfate 3.917
Calcium chloride 1.102
Potassium chloride 0.664
Sodium bicarbonate 0.192
Potassium bromide 0.096
Boric acid 0.026
Strontium chloride 0.024
Sodium fluoride 0.003


All these salts were dissolved in distilled water and the total volume was made up to 1,000 ml.

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%).

ALKALIS: Sodium hydroxide was dissolved in water to make solution of 10% and 50% concentration on a weight per 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

From the data in Table 5, it is apparent that the cycloaliphatic-based polyester offered better solubility in MEK than the traditional aromatic and linear diacid polyesters. The IPA-based polyester needed to be heated to mix with the crosslinker, and the elevated temperature resulted in a reduced application time. Time polyester oligomers based on 1.3-CHDA were liquid at room temperature because 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 had a linear symmetric chain and may provide a stronger interaction between the polyster 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. (21)

Mechanical properties

HARDNESS: The data in Table 6 shows that the polyurethane coating based on IPA showed the best hardness among the three films, so that it could bear the weight of 1,700 g. The polyurethane coatings based on 1,3-CHDA and AA had hardness of 1,500 and 1,200 g, respectively. Generally the linear structure of the diacids in the polyesters provided polyurethane films with a lower hardness. The rigid phenyl ring increased the hardness. It was presumed that the conformational interchange of the cyclohexane ring (i.e., 1,3-CHDA) decreases the film hardness in comparison with the rigid IPA.

PERCENTAGE ADHESION: All the three formulations showed a 100% adhesion on mild steel panels (see Table 6).

FLEXIBILITY: For this test the resin was coated on tin plates. It was found (see Table 6) that all the formulations passed the flexibility test.

Chemical resistance properties of polyurethane films

RESISTANCE TO WATER: The resistance of the polyurethane films to deionized water and seawater is shown in Table 8. The results indicate that all the three films of polyurethane were completely resistant to the deionized water and the seawater for the entire period of exposure (i.e., for 6 months).
Table 8: Resistance to water (a) of polyurethane films

Sample Water

 Deionized water Seawater

CS-1 5 5
CS-2 5 5
CS-3 5 5

(a) When dipped for 6 months; 5 = Film unaffected


RESISTANCE TO ACIDS: The resistance of the polyurethane film to the different acids is shown in Table 9. The data shows that the polyurethane films based on the aromatic, cycloaliphatic, and linear diacids offered good resistance to the acetic acid (conc. 6%) for the entire period of immersion. Coatings based on the adipic acid the isophthalic acid were hardly affected by the sulfuric acid (5% and 10%, respectively). The films were resistant for up to 3 months, after which the films showed a loss in gloss and a change in color during exposure in the fourth month. The IPA-based coating showed a blister formation and softening of the film at the end of the fourth and the fifth month, respectively. In the exposure to hydrochloric acid, the IPA-based coating showed loss in gloss and a change in color in the fourth month when immersed with 10% (v/v) HCl. The 1,3-CHDA-based coating showed only loss in gloss during the fifth month when exposed to 36% (v/v) HCl, while the coating based on AA showed loss in gloss and a change in color in the third month, and blister formation during the fourth month of their exposure to 36% (v/v) HCls.
Table 9: Resistance to acids (a) of polyurethane films

Sample Acids

 Acetic acid Sulfuric acid Hydrochloric acid

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

CS-1 5 5 5 4 5 5
CS-2 5 5 5 5 5 4
CS-3 5 5 4 2 4 3

(a) When dipped for 6 months; 2 = Softening of film; 3 = Blistering of
film; 4 = Change in color and loss in gloss; 5 = Film unaffected


RESISTANCE TO ALKALIS: The exposure of the polyurethane coatings based on IPA and 1,3-CHDA in sodium hydroxide [10%, 50%, (w/v)] shown in Table 10 that these films were completely unaffected during the entire period of their immersion. The polyurethane coating based on AA showed a loss in gloss and a change in color in the fourth month of their exposure to 10% (w/v) sodium hydroxide. A blister formation was observed in the fifth month when exposed to 50% (w/v) sodium hydroxide. polyurethance coatings based on IPA and 1,3-CHDA were completely unaffected when exposed to 10% (v/v) ammonium hydroxide, but the AA-based film showed a loss in gloss and a change in color in the exposure of their fifth month. The same effect was observed for coatings based on IPA and AA when immersed in 25% (v/v) ammonium hydroxide; both films showed a loss in gloss and a change in color in the fifth and fourth month of their exposure, respectively. The coating film based on 1,3-CHDA was not affected by the alkali during the whole period of immersion in 25% (v/v) ammonium hydroxide.
Table 10: 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)

CS-1 5 5 5 4
CS-2 5 5 5 5
CS-3 4 3 4 4

(a) When dipped for 6 months; 2 = Softening of film; 3 = Blistering
of film; 4 = Change in color and loss in gloss; 5 = Film unaffected


RESISTANCE. TO SOLVENTS: From the data in Table 11, it can be seen that all the polyurethane films showed good solvent resistance. All three films remained unaffected by toluene, xylene, and MEK during the entire period of their exposure (i.e., 6 months). The polyurethane coatings based on IPA and 1,3-CHDA offered good resistance to methanol, while, blistering was observed in the AA-based coating at the end to the fifth month after a loss in gloss and a change in color. The same effect was observed for this coating in ethanol. but the coating based on 1.3-CHDA showed a loss in gloss and a change in color in the sixth month of their exposure to ethanol. The AA-based coating showed a loss in gloss and a change in color in the third month, blister formation in the fourth month, and softening of the film in the sixth month when immersed in acetone.
Table 11: Resistance to solvents (a) of polyurethane films

Sample Solvents

 Toluene Xylene Methanol Ethanol Acetone Methyl ethyl ketone

CS-1 5 5 5 5 5 5
CS-2 5 5 5 4 5 5
CS-3 5 5 3 3 2 5

(a) When dipped for 6 months; 0 = Complete removal of film from the
panel; 1 = Partial removal of film from the panel; 2 = Softening of
film; 3 Blistering of film; 4 = Change in color and loss in gloss;
5 = Film unaffected


From the previous discussion, it is apparent that the polyurethane coatings based on AA were the most affected by the chemicals. The 1,3-CHDA-based coating showed the best chemical resistance properties. The superiority of the coatings based on the cycloaliphatic compound was considered to be because of the presence of the 1,3-substitution. (11) It brought a nonsymmetrical structure to the polyester, thus reducing the intermolecular interactions that lead to crystallinity. and showed the best chemical resistance properties. The IPA-based coating also showed a good performance against the different chemicals, which can be attributed to the presence of a phenyl ring in the compound.

Conclusion

The cycloaliphatic diacid provides polyesters with better solubility in MEK than polyester prepared from aromatic and linear diacids. From the data provided in this paper. it is also confirmed that the cyclo aliphatic diacid provides polyurethane coatings with intermediate mechanical properties between the aromatic and 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 automotive topcoats.

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S. Awasthi, D. Agarwal

Department of Oil & Paint Technology, Harcourt Butler Technological Institute (HBTI), Nawabganj, Kanpur 208002, India

e-mail: shailja_2005@rediffmail.com
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Publication:JCT Research
Date:Mar 1, 2007
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