Carboxylated Polyester Additives for Improving the Adhesion of Coatings

35 Carboxylated Polyester Additives for Improving the Adhesion of Coatings

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0099.ch035

W. J. JACKSON, JR., and J. R. CALDWELL Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, Tenn. 37662

Carboxylated polyesters were prepared by extending hydroxyl-terminated polyester segments with dianhydrides. Carboxylated polyesters which were soluble in common lacquer solvents were effective in improving the adhesion of coatings on a variety of substrates when 1-10% was blended with cellulose acetate butyrate, poly(vinyl chloride), poly(methyl methacrylate), polystyrene, bisphenol polycarbonates, and other soluble polymers.

' T p h e adhesion of many polymeric coatings on various substrates is poor. Adhesion often can be improved, however, by using a primer, which must be applied and dried before applying the top coat. A more convenient and economical process would be the application of a single coat containing an adhesion-promoting component. Incorporation of carboxyl groups i n vinyl polymers ( I ) and polyolefins {1,7) improves the adhesion of these polymers to various materials. However, many of these carboxylated polymers, particularly the carboxylated polyolefins, have limited solubility in volatile, lacquer-type solvents such as butyl acetate or methyl ethyl ketone and thus are limited in their ability to improve the adhesion of coatings applied from solvents. Carboxylated polyesters that are soluble in these solvents can be prepared. W e were therefore interested i n determining the effects of structure and carboxyl content on the adhesion of coatings of various classes of polymers blended with carboxylated polyesters. A

Experimental Inherent viscosities of the polymers were determined at a concentration of 0.23 gram/100 m l in 60/40 phenol/tetrachloroethane at 25°C. Polyesters for solubility determinations were prepared by conventional 562 In Multicomponent Polymer Systems; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

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Table I.

Polyester Solubilities Solubility*

Polyester"


Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Acid Components

0

60/40 I/adipic 60/40 T/phthalic I 60/40 I/T 60/40 I / H 60/40 I/phthalic 50/50 I/phthalic 70/30 H / T 70/30 H / I H 60/40 T / H 50/50 T / H 50/50 T/I 50/50 I/adipic 60/40 I / H

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Adhesion of Coatings

Glycol

Toluene

Methyl Ethyl Ketone

EG EG BD BD BD BD BD BD BD NPG NPG NPG NPG CHDM CHDM

insol. insol. insol. insol. gel. insol. sol. sol. sol. sol. sol. sol. sol. sol. gel

gel. insol. sol. gel sol. sol. sol. sol. sol. sol. insol.

6

Butyl Acetate

insol. insol. insol. gel. sol. sol. sol. sol. sol. sol. insol.

All inherent viscosities are 0.5-1.0. Solids content is 20%. Gel indicates separation of polymer as a gel after solution had cooled and stood overnight. Ratios are molar. I = isophthalic acid, T = terephthalic acid, H = 50/50 cis/ /raws-hexahydroterephthalic acid. E G = ethylene glycol, BD = 1,4-butanediol, NPG = neopentyl glycol, C H D M = 30/70 cis/trans-1,4-cyclohexanedimethanol. a

h

c

d

procedures (5, 8 ) . The solubilities of these polyesters before introduction of carboxyl groups are listed i n Table I. The solubilities were determined at a solids content of 20% i n the solvents. If the polymer d i d not dissolve readily at room temperature, the mixture was heated on a steam bath until the polymer dissolved. The solution was then allowed to cool and stand overnight. A n y separation of the polymer as a gel was then noted. Preparation of Carboxylated Polyesters. Hydroxyl-terminated polyester segments were prepared by conventional procedures and extended with an equimolar amount of dianhydride ( 2 ) . To avoid the possibility of crosslinking the polymer, reaction with the dianhydride was carried out at 175 °C. Depending upon the size of the reaction mixture, about 1 to 3 hours were required for all of the dianhydride to react and for a medium-to-high melt viscosity to be obtained. Inherent viscosities were 0.3 to 0.4. Adhesion of Coatings. Except for K - l polycarbonate [4,4'-(2-norbornylidene)diphenol polycarbonate] ( 4 ) , an experimental polymer (inherent viscosity 0.85), all the coatings were prepared with commercial products: EAB-381-0.5 and EAB-381-20 cellulose acetate butyrates from Eastman Chemical Products, Inc.; V Y H H vinyl chloride ( 8 7 % ) / v i n y l acetate ( 1 3 % ) copolymer from Union Carbide Corp.; Butvar B76 poly(vinyl butyral) from Shawinigan Resins Corp.; Plexiglas V poly (methyl methacrylate) from Rohm and Haas C o . ; Dylene P3I polystyrene from 1

In Multicomponent Polymer Systems; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.


564

MULTICOMPONENT

POLYMER

SYSTEMS

Sinclair-Koppers C o . ; Lexan 125 bisphenol A polycarbonate from General Electric C o . ; Polysulfone aromatic poly(sulfone-ether) from Union Carbide Corp.; and P P O poly(2,6-dimethyl-p-phenylene oxide) from General Electric C o . Initial adhesion tests were made with 3.5- X 1- X 0.064-inch specimens of aluminum (Aluminum Associates No. 2024) and cold-rolled steel (polished A S T M A-415). Later tests were made with strips of copper, chrome-coated steel (Weirton Steel C o . ) , and brass; with molded nylon 66 bars; and with Mylar 300A poly (ethylene terephthalate) film. Before use, the specimens were washed (scrubbed with a soft bristle brush) with a solution of Alconox detergent, rinsed with water, rinsed with acetone, and dried. The specimens were coated with a blend of the carboxylated polyester and second polymer in a solvent (10% solids), which, when possible, consisted of a conventional lacquer solvent (74.2% toluene, 7.4% butyl alcohol, 7.4% Solvesso 100 solvent, 3.7% ethyl acetate, 3.7% butyl acetate, and 3.6 Cellosolve acetate). If the second polymer was not soluble (polysulfone, polycarbonates, P P O ) , chloroform was used. The coatings were dried for 1 hour at room temperature and then for 2 hours in an oven at 115°C. The coating thickness was about 0.5 m i l . The coatings were scored (crosshatched) with a razor, and adhesion was determined b y the conventional cellophane tape test. The tape was pressed firmly against the coating and then jerked up. If any trace of the coating was removed b y the tape on several tests, the coating was considered to have failed the test. The results obtained by the scored cellophane tape test are listed in Tables II and III. A l l coated specimens used for this test were dried for 2 hours at 115°C to ensure complete removal of all solvents, but a 1-hour drying period at 23 °C was sufficient for many of the coatings to pass the cellophane tape test. Table II.

Effect of Carboxyl Content on Adhesion

Polyester Segment Mol. Wt. before Carboxylation

5500 5500 3000 3000 2100 2100 1200 1200

Acid No. 1 1 26 26 39 39 56 56 126 126

Ami. in Blend, % 5

0 10 1 5 1 5 1 5 1 5

K-l Polycarbonate Blends Steel + + + + + + + + +

Al

+ + + + +

° Polyester of neopentyl glycol and equimolar amounts of terephthalic and isophthalic acids extended with pyromellitic dianhydride. Determined by crosshatched cellophane tape adhesion test (+ is pass; — is fail). 6


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Discussion Preparation and Solubility of Polyesters. A number of polyesters were prepared from several diols and dicarboxylic acid esters to determine the effect of structure on the solubility i n typical solvents used i n lacquers. The data i n Table I show that solubility i n the solvents decreased i n the following order: toluene > methyl ethyl ketone > butyl acetate. Polymers that were soluble in all three solvents are examples 9-14. The carboxylated polyesters were prepared by a two-step process: (1) preparation of a hydroxyl-terminated polyester segment and (2) reaction with a dianhydride to extend the polyester and introduce carboxyl groups. When the dianhydride is pyromellitic (II), the equation is as follows:

of Blends Containing Carboxylated T50I(NPG)

a

Adhesion* Cellulose Acetate Butyrate Blends 0

Steel

+ + + + +

Al

—

+ + + +

Vinyl Chloride/Vinyl Acetate Copolymer Blends 4

Steel

+ + + + + +

Al

— —

+ + + +

EAB-381-0.5 from Eastman Chemical Products, Inc. * V Y H H from Union Carbide Corp. • Polyester (inherent viscosity 0.46) was not carboxylated. c


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MULTICOMPONENT

Table III.

SYSTEMS

Adhesion of Blends Containing Carboxylated T50I(NPG) 0

Blend Major Component


POLYMER

Substrate Polyester, wt % Brass

Steel Cu Cr

c

_

0 5 10

— + +

_ + +

Cellulose acetate butyrate ( E A B 381-20)'

0 5 10

— + +

_ + +

V i n y l chloride (87%)/ vinyl acetate (13%) copolymer

0 1 5

+ +

+ + + +

+ +

+ +

P o l y v i n y l butyral)

0 5

+ +

+ + + +

+ +

-

Poly (methyl methacrylate)

0 10 20

+ +

+

Polystyrene

0 10

— +

_ +

Bisphenol A polycarbonate

0 10

— +

_ +

0 1

+ +

Aromatic polysulfone

0 10

— +

_ +

Poly(2,6-dimethyl-pphenylene oxide)

0 10

+ +

_ +

100

+

+

+

+

+

100

+

+

—

+

—

+

+

_ +

_ -

_ -

+ -

+ +

+

— — + +

+

_

_ +

_ +

_ _ — +

_

_ +

_

_

+

_ _ - + _

+

—

+

_ +

_

PET Film* _ -

_

+ +

-

Nylon 66

_ _ + + + +

_ _ + + +

_ +

_ _ + + +

Al

Cellulose acetate butyrate ( E A B 381-0.5)'

K - l polycarbonate

b

_

_ +

_

+ —

— + +

_ +

-

-

— _ —

-

— -

+

— —

T50I(NPG) carboxylated

6

T50I(NPG) uncarboxylated

+

+ —

+

° Determined by cross-hatched cellophane tape adhesion test (+ is pass; — is fail). Prepared by extending the polyester of neopentyl glycol and equimolar amounts of terephthalic and isophthalic acids (segment molecular weight 3000) with pyromellitic dianhydride; acid number was 39. Chrome-coated steel. Polyethylene terephthalate) film (Mylar 300A). EAB-381-0.5 has an appreciably lower molecular weight than EAB-381-20 (falling ball viscosities of 0.5 vs. 20 sec) (3). Passed adhesion test when solvent was chloroform. 6

c

d e

f



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The carboxyl content of the polymer is controlled by the length of the hydroxyl-terminated polyester segment ( I ) . T o extend the hydroxylterminated polyester segment, an equimolar amount of dianhydride was added. N o crosslinking occurred when the reaction temperature was 175°C, but some crosslinking took place at higher temperatures ( 2 0 0 ° C ) , presumably because of esterification and perhaps acidolysis. Esterification could occur between the terminal hydroxyl groups and the carboxyl groups produced from the dianhydride, and acidolysis could occur if the carboxyl groups attacked the polymer chain. In addition to pyromellitic dianhydride ( P M D A ) , three other dianhydrides were used to extend the polyester segments. These anhydrides were prepared by heating trimellitic arihydride ( I V ) with glycol diacetates ( V ) ( 6 ) : O

II

o

IV

V

II

+ 2 CH3COOH

o

VI

CH b.

R

?

—CH2~~ C — C H 2 —

CH

H

3

CH — 2



568

M U L T I C O M P O N E N T

POLYMER

SYSTEMS

F o r convenience, the letters T (terephthalic acid), I (isophthalic acid), H [50/50 cis/frans-hexahydroterephthalic acid (1,4-cyclohexanedicarboxylic a c i d ) ] , and N P G [neopentyl glycol (2,2-dimethyl-l,3- propanediol)] are used to refer to the polyesters prepared from these intermediates; thus, T 5 0 I ( N P G ) is the copolyester from neopentyl glycol and equimolar amounts (50/50 molar ratio) of terephthalic and isophthalic acids. The carboxylated polyesters based on examples 9-14 i n Table I were soluble i n butyl acetate and methyl ethyl ketone, but it was necessary to add u p to 10% of an alcohol (methyl or butyl) to toluene to give solubility. Some of these carboxylated polyesters were also soluble i n plasticizers. Carboxylated T 5 0 I ( N P G ) and T 5 0 H ( N P G ) (3000-molecular weight segments extended with P M D A ) were soluble i n dibutyl phthalate but not in dioctyl phthalate. The similar H ( N P G ) carboxylated polyester was soluble i n both dibutyl and dioctyl phthalates, but it was inferior to the above two polymers i n promoting adhesion. Adhesion of Polymer Blends Containing Carboxylated Polyesters. The adhesion of blends containing a carboxylated polyester was affected by the structure of the polyester segments, the structure of the molecule providing the carboxyl groups, and the carboxyl content. O f the soluble polyesters listed i n Table I, a carboxylated polyester which was particularly effective in imparting adhesion to coatings was obtained by extending hydroxyl-terminated T 5 0 I ( N P G ) (example 13) with P M D A :

VII

The carboxyl content was determined b y the molecular weight (length) of the polyester segment which was extended by the dianhydride—the greater the molecular weight (e.g., the value of m i n formula V I I ) , the lower the carboxyl content. The value of n was about 2 to 4 when the polyester segment molecular weight was about 3000 ( m = 13).


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Table II shows the effect of the carboxyl content on the adhesive characteristics of PMDA-extended polyesters i n blends with K - l polycarbonate [4,4'- (2-norbornylidene) diphenol polycarbonate] (VIII), cellulose acetate butyrate, and poly (vinyl chloride). K - l polycarbonate is an experimental polymer which, like cellulose acetate butyrate and poly (vinyl chloride), is very sensitive to adhesive changes because only 1-2% of a carboxylated

VIII polyester is required to give good adhesion. As shown i n Table II, incorporation of carboxyl groups improved the adhesion significantly. About 1 % of the carboxylated polyester was as effective i n improving the adhesion of the three polymers on steel as 5 - 1 0 % of the uncarboxylated polyester. O n aluminum, 1 % of the carboxylated polyester was more effective than 10% of the uncarboxylated polymer. A l l the coated samples listed i n the tables were heated i n an oven for 2 hours at 115°C to ensure the removal of all solvent. Drying at room temperature, however, was sufficient for many of the coatings to pass the cellophane tape test. The cellulose acetate butyrate blends with 1 % of each of the four carboxylated polyesters in Table II, for instance, passed the adhesion test on steel after the coatings had dried at 23 °C for only 0.5 hour, and the blends with 1 % of the polyesters having acid numbers of 39-126 passed the adhesion test on aluminum. Polyesters extended with dianhydride V i a (from bisphenol A d i acetate and trimellitic anhydride) instead of P M D A gave results similar to those shown i n Table II. Polyesters extended with dianhydrides V I b or V i c gave inferior results. Metal specimens coated with the various blends were immersed i n water i n an accelerated test to determine the effect of high humidity. Cellulose acetate butyrate blends containing carboxylated polyesters prepared with hexahydroterephthafic acid [ H ( N P G ) , T 5 0 H ( N P G ) ] were particularly susceptible to moisture and failed the adhesion test after immersion for only 0.5 hour; similar blends containing T 5 0 I ( N P G ) extended with dianhydride V i a or with P M D A passed the adhesion test after immersion for 16 hours. W h e n coatings on cold-rolled steel of cellulose acetate butyrate (EAB-381-0.5) blends containing 1 % of each of the


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POLYMER

SYSTEMS


four T 5 0 I ( N P G ) / P M D A polyesters of Table II were scored (crosshatched ) with a razor and immersed in water, all four passed the adhesion test after immersion for 8 hours. When these samples were returned to water for 3 days, the least rusting of the steel occurred when the polyester adhesion promoter having an acid number of 39 was used. T 5 0 I ( N P G ) / P M D A was of particular interest because of its adhesive characteristics, oxidative stability, and polymer cost. Table III shows the improvement i n adhesion obtained when various substrates were coated with blends containing this polyester (acid number 39). As indicated i n the table, the ease of obtaining adhesion on the different substrates decreased approximately in the following order: brass > steel > copper > chrome-coated steel > aluminum > nylon 66 > poly (ethylene terephthalate). In spite of the wide differences i n structure and polarity of the various polymers, the carboxylated polyester significantly improved the adhesion of the coatings. The presence of a plasticizer in addition to the carboxylated polyester adversely affected the adhesion of some of the polymers—e.g., cellulose acetate butyrate and poly (vinyl chloride) plastisols. Compatibility of Carboxylated Polyesters in Blends. Compatibility of the carboxylated polyester i n the polymer blends d i d not appear to affect the adhesion significantly. T 5 0 I ( N P G ) (segment molecular weight 1400) extended with P M D A was compatible (clear coating) at a concentration of 20% in cellulose acetate butyrate, whereas the similar polyester with a segment molecular weight of 3000 was incompatible (hazy coating). Coatings of the latter blends had the best adhesion, however. The various carboxylated polyesters were incompatible i n poly (methyl methacrylate) and polystyrene, but they were compatible in the vinyl chloride/ vinyl acetate copolymer. Carboxylated T 5 0 H ( N P G ) was superior in compatibility to carboxylated T 5 0 I ( N P G ) i n some of the blends, but the adhesion was poorer after immersion in water. Oxidative Stability of Carboxylated Polyesters. The polyesters which were extended with dianhydrides are those in Table I which were soluble i n the lacquer solvents. O f these, theoretical considerations indicate that T 5 0 I ( N P G ) should be the most oxidatively and thermally stable because it is the only one with a completely aromatic acid component (terephthalic and isophthalic), and the glycol component has the stable neopentyl structure. When K - l polycarbonate films containing 5 % of this polyester extended with P M D A were heated i n a forced-air oven at 200°C, the film life (time to brittleness when creased) was not lowered appreciably (compared with a control containing no carboxylated polyester). The incorporation of 5 % of the similar carboxylated T 5 0 H ( N P G )



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in the film, however, reduced the film life almost to one-half its normal life of 140 hours for a 2-mil film. Surprisingly, coatings of 10% T 5 0 I ( N P G ) / P M D A i n cellulose acetate butyrate prevented oxidation (discoloration) of the copper when heated i n an air oven for 2 hours at 115°C (accelerated oxidation test). When the polyester was not present i n the coating, the copper darkened under these conditions. Since the carboxylated polyester itself has a very high oxygen permeability (too high to measure), perhaps the copper was protected from oxidation because of the unusually strong adhesion of coatings containing the polyester. Conclusions In blends of 1-10% of the carboxylated polyesters with various coating materials, the incorporation of carboxyl groups was the most important structural feature which affected adhesion. Polyesters with acid numbers of about 25-125 appreciably improved the adhesion of coatings, and an acid number of about 40 was satisfactory for most of the coating materials. A carboxylated polyester which has good oxidative stability and is particularly effective as an adhesion promoter is obtained by extending with pyromellitic dianhydride the polyester of neopentyl glycol and equimolar amounts of terephthalic and isophthalic acids. This polymer shows promise as an adhesion promoter for air-dry automotive primers and is being evaluated for this application on an experimental basis by a number of companies. Acknowledgment W e are pleased to acknowledge the excellent technical assistance of W . C . Cooper, H . F . Kuhfuss, H . G . Moore, and H . R. D . Spears. Literature Cited (1) Brown, H. P., Anderson, J. F., "Handbook ofAdhesives,"I. Skeist, Ed., p. 255, Reinhold, New York, 1962. (2) Caldwell, J. R., U. S. Patents 3,459,584 and 3,484,339 (1969). (3) Eastman Chemical Products, Inc., "Cellulose Acetate Butyrate for Protective Coatings," 2nd ed., pp. 9-10, 1968. (4) Jackson, W. J., Jr., Caldwell, J. R., Ind. Eng. Chem., Prod. Res. Develop. 2, 246 (1963). (5) Kibler, C. J., Bell, A., Smith, J.G.,J.Polymer Sci., Pt. A 2, 2115 (1964). (6) Loncrini, D. F., U. S. Patents 3,182,073 and 3,183,248 (1965). (7) Smarook, W. H., Bonotto, S., SPE Annual Tech. Conf. XIII, May 1967, p. 119. (8) Smith, J. G., Kibler, C. J., Sublett, B. J.,J.Polymer Sci.A-1,4, 1851 (1966). RECEIVED December 22, 1969.


Carboxylated Polyester Additives for Improving the Adhesion of Coatings

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