Cross-Linking Water-Reducible Coatings by Direct Esterification - ACS

Chapter 26

Cross-Linking Water-Reducible Coatings by Direct Esterification 1

1

2

Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch026

Guobei Chu , Frank N. Jones , Rahim Armat , and Stacy G. Bike

2

1

National Science Foundation Industry/University Cooperative Research Center in Coatings, Eastern Michigan University, Ypsilanti, MI 48197 Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109 2

It is demonstrated that direct esterification reactions can be used to crosslink coatings. An acrylic resin with high levels of carboxyl and hydroxyl groups crosslinks to hard, solvent resistant films within 30 minutes at temperatures as low as 140 °C when catalyzed by titanates or strong acids. Tetraisopropyl titanate (TPT) is the most effective catalyst on a weight basis, with p-toluenesulfonic acid (p-TSA) equal or close behind. When small amounts (3 to 6 weight per-cent) of methylolated melamine formaldehyde (MF) resin are added to p-TSA catalyzed formulations, crosslinking is slightly accelerated and surface properties are enhanced. Presumably, such formulations cure partly by MF resin crosslinking and partly by direct esterification. Acrylic resins that crosslink by direct esterification are potentially economical, do not form crosslinking by-products that are hazardous air pollutants (HAP) or volatile organic compounds (VOC), and are readily adaptable to water-reducible formulation. Rheology of such formulations is predictable. Cure rates of films cast from aqueous formulations are similar to those cast from organic solvent.

For decades most thermosetting coatings have been crosslinked by one or more of four types of chemical reactions: [1] transetherification reactions of alcoholated amino-formaldehyde resins, [2] reactions of unblocked or blocked polyisocyanates with polyols to form urethanes, [3] a variety of ring-opening reactions of oxirane (epoxy) groups, and [4] autoxidation, for example of alkyd resins (1). Many potential alternatives to these well-entrenched crosslinking chemistries have been studied, and some have found modest commercial use (2). However, these four chemistries remain the workhorses of the coatings industry.

0097-6156/96/0648-0430$15.25/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


26.

431

Cross-Linking Coatings by Direct Esterification

C H U ET AL.

Formulators have adapted these four chemistries to s a t i s f y today's stringent environmental and workplace safety requirements. However, future regulations w i l l be even more stringent and may constrain the use of one or more of these chemistries. For example, there i s concern about method (1), t r a n s e t h e r i f i c a t i o n of amino r e s i n s , because traces of formaldehyde along with larger levels of alcohols are formed as c r o s s l i n k i n g by-products and emitted to the atmosphere as V o l a t i l e Organic Compounds (VOC) unless incinerated. Methylolated melamine formaldehyde (MF) r e s i n s , the type of amino r e s i n most commonly used in water-reducible coatings, emit substantial amounts of methanol and smaller amounts of formaldehyde. Beside being VOCs, both substances are l i s t e d as a Hazardous A i r Pollutants (HAP) in the U.S. Methanol emissions w i l l be limited by 1997 to 10 tons per year per f a c i l i t y , posing a problem for large operations. MF resins made from alcohols that are not HAP l i s t e d can be substituted, but they are d i f f i c u l t to formulate in waterreducible coatings. C l e a r l y i t behooves the coatings industry to redouble the search for "greener" c r o s s l i n k i n g methods. This paper w i l l describe preliminary experiments suggesting that e s t e r i f i c a t i o n of resins containing carboxyl and hydroxyl groups (Equation 1) may be adaptable as a c r o s s l i n k i n g chemistry for coatings. Lacking a better term, we c a l l i t "direct esterification." RC00H

+

HOR'

catalyst S> RC00R'

+ H0 2

(1)

Direct e s t e r i f i c a t i o n i s one of the oldest and most widely studied reactions in organic chemistry, yet computer l i t e r a t u r e searches back to 1967 revealed no references to i t s use as a method for c r o s s l i n k i n g coatings. It is known that hydroxyl-rich polymers such as starch (3,4) and polyvinyl alcohol (5) can be crosslinked with d i c a r b o x y l i c acids, although in some cases the f i r s t step in the reaction may be formation of a c y c l i c anhydride of the d i a c i d . It is also well known that t i t a n a t e s , used as catalysts in t h i s study, can function as c r o s s l i n k e r s (6-8). It should be noted that other types of e s t e r i f i c a t i o n reactions are used to c r o s s l i n k coatings. A common example is reaction of resins bearing carboxyl and oxirane groups (Equation 2).

I

/ \

RC00H

+

CH —CHR" 2

>

(2)

RC00CH CHR" 2

Crosslinking of coatings by nucleophile catalyzed t r a n s e s t e r i f i c a t i o n was recently described by Craun (9). Crosslinking by d i r e c t e s t e r i f i c a t i o n offers potential advantages: It i s green, and i t is r e l a t i v e l y cheap. On the other hand, potential disadvantages, such as slow cure rates and moisture

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

432

FILM FORMATION IN WATERBORNE COATINGS

sensitive f i l m s , can be envisaged. We w i l l show that, with further research and development, i t might be possible to make c r o s s l i n k i n g by e s t e r i f i c a t i o n feasible for certain types of coatings, thus r e a l i z i n g the potential advantages. Experimental Details


Materials. Materials were the best available grades obtained from the following sources: Methyl methacrylate (MMA) Butyl acrylate (BA) A c r y l i c acid (AA) 2-Hydroxyethyl acrylate (HEA) A z o b i s i s o b u t y r o n i t r i l e (AIBN) 2-Butoxyethanol (2-BE) Ν,Ν-Dimethylamino ethanol (DMAE) p-Toluenesulfonic acid monohydrate (p-TSA) Tetraisopropyl t i t a n a t e ("Tyzor TPT") Tetra-n-butyl t i t a n a t e ("Tyzor TBT") Methyl ethyl ketone (MEK) Diethanolamine (DEA) Hexakis(methoxy methylol) melamine resin ("Cymel 303," HMMM) Type R-36 CRS panels Double d i s t i l l e d water

Aldrich Rohm & Haas Aldrich Rohm & Haas Polysciences Union Carbide Aldrich Aldrich du Pont du Pont Aldrich Aldrich Cytech Q-Panel

A l l materials were used as received. Polymer Synthesis. A dual functional a c r y l i c resin (DFAR) was synthesized under monomer starved conditions using the recipe in Table 1. The f i r s t portion of 2-butoxyethanol was placed in a 500mL four neck breakaway flask equipped with a heating mantle with a thermostatic temperature c o n t r o l l e r , a mechanical s t i r r e r , a thermometer, a nitrogen i n l e t and a dropping funnel. A nitrogen atmosphere was maintained throughout the process. The reaction solution was maintained at 100 °C with the c o n t r o l l e r . A solution of a l l other ingredients except for the second portion of AIBN was added dropwise with continuous s t i r r i n g during 3 hrs. S t i r r i n g was continued at 100 °C for another hr and then the second portion of the AIBN was added. S t i r r i n g was continued for 2 more hrs at 100 °C, and the transparent, colorless resin solution was cooled to 25 °C. Characteristics of DFAR are shown in Table 1. Acid-functional and hydroxyl-functional a c r y l i c resins (AFAR and HFAR, respectively) were prepared by a s i m i l a r procedure using the recipes shown in Table 2. Hydrolyzed Titanium Chelate (HTC) Catalyst.

This material

prepared following Example 2 of Deardorff's patent (10). Tetraisopropyl t i t a n a t e (35.5g, 0.125 mol) was placed in a 100-mL flask equipped with a s t i r r e r , a thermometer, an addition funnel, a reflux condenser, and a nitrogen i n l e t and a heating mantle with a temperature c o n t r o l l e r . The flask was purged with


was

26. CHU ET AL.


Table 1.

433


Composition and Properties of Dual Functional Acrylic Resin (DFAR)

Components

Charge

9. Methyl methacrylate Butyl acrtlate A c r y l i c acid 2-Hydroxyethyl acrylate

61.1 60.0 30.0 48.4

Charge

wt

ml

I

0.616 0.469 0.417 0.417

30.8 30.0 15.0 24.2 100.0

AIBN f i r s t portion second portion 2-Butoxyethanol f i r s t portion second portion

2.0 0.3

0.012 0.002

44.4 88.9

0.377 0.753

Properties "Tg" (Fox eq.) (°C) Nonvolatile (wt*) Viscosity (Brookfield)(Pa s) Acid number (mgKOH/g) calculated measured Hydroxyl number (mgKOH/g) calculated Molecular weight (Mn; GPC) Mw/Mn (GPC)

10 64 51 117 105 117 10300-15100 3.6-6.3


434



Table 2.

Compositions and Properties of Acrylic Resins HFAR and AFAR Hydroxy 1 -Fund iona 1 Acid-Fund iona 1 Acrylic Resin Acrylic Resin (HFAR) (AFAR)

Components

Charge 2

Charge 3

Methyl mathacrylate Butyl acrylate Hydroxethyl methacrylate A c r y l i c acid

120 160 120

30 40 30

160 120

40 30

120

30 100

100 AIBN

f i r s t portion second portion

Methyl ethyl ketone Methyl isobutyl ketone

5.1 1.2

28 4

266.7 266.7

Properties "Tg" (°C) (Fox Eq.) Solids (wt.*) Hydroxl number (mgKOH/g) calculated OH equivalent weight (solids) Acid number (mgKOH/g) calculated COOH equivalent weight (solids) Molecular weight (GPC) Mn Mw/Mn (GPC)

10 60.28

38 60.83

129 435 234 240 15000 2.2

3800 2.6


26.

CHU ET AL.


435


n i t r o g e n and heated to 80 ° C . A s o l u t i o n o f 13.25 g (0.125 mol) o f d i e t h a n o l amine (DEA) and 2.25 g (0.125 mol) o f water was added w i t h v i g o r o u s s t i r r i n g d u r i n g 1 hour at 80 ° C . A g e l formed i n i t i a l l y and then d i s s o l v e d when about 40% o f the s o l u t i o n had been a d d e d . The p r o d u c t was 49 g o f c o l o r l e s s , v i s c o u s o i l ( H T C / i s o p r o p a n o l ) . A 20-g p o r t i o n o f t h i s m a t e r i a l was c o n c e n t r a t e d on a r o t a r y e v a p o r a t o r ( t o remove b y - p r o d u c t i s o p r o p a n o l ) t o y i e l d 8.2 g o f w h i t e s o l i d HTC. HTC was used in two f o r m s , as H T C / i s o p r o p a n o l f o r nonaqueous f o r m u l a t i o n s and as a 3 weight % s o l u t i o n o f s o l i d HTC i n water f o r aqueous f o r m u l a t i o n s .

DFAR Coatings Cast from Solution in Organic Solvent.

Catalyst

s o l u t i o n s were p r e p a r e d i n i s o p r o p a n o l ( f o r T P T , p-TSA and HTC) o r in n - b u t a n o l ( f o r T B T ) . The c a t a l y s t s o l u t i o n s were added t o the above DFAR r e s i n s o l u t i o n i n amounts c a l c u l a t e d t o g i v e the c a t a l y s t l e v e l s shown i n Table 3. These s o l u t i o n s were then d i l u t e d w i t h i s o p r o p a n o l o r n - b u t a n o l t o a v i s c o s i t y t h a t would a f f o r d t h e desired film thickness after application. The r e s u l t i n g c o a t i n g s o l u t i o n s were aged o v e r n i g h t and then were drawn down on Type R-36 p a n e l s o b t a i n e d from the Q - P a n e l Company u s i n g a #60 w i r e - r o u n d b a r . R-36 p a n e l s a r e u n t r e a t e d , matte s u r f a c e , c o l d - r o l l e d s t e e l p a n e l s o f t h i c k n e s s 0.8 mm (0.032 i n ) . T a r g e t d r y f i l m t h i c k n e s s was 20 um; measured d r y f i l m t h i c k n e s s e s v a r i e d from 15 t o 21 um. Panels were baked f o r 30 minutes a t v a r i o u s t e m p e r a t u r e s e t t i n g s i n a f o r c e d a i r oven. In each experiment t e m p e r a t u r e s e n s i t i v e t a p e was p l a c e d on the f r o n t o f each panel t o measure the t e m p e r a t u r e r e a c h e d by the p a n e l . The tape was o b t a i n e d from Paper Thermometer C o . , Box 129, G r e e n f i e l d , NH 03047. These t a p e s c o n t a i n a s e r i e s o f s e c t o r s which t u r n dark when a c e r t a i n t e m p e r a t u r e i s r e a c h e d . Both the oven s e t t i n g t e m p e r a t u r e s and the r e a d i n g s noted on the t e m p e r a t u r e s e n s i t i v e tape were n o t e d . A n o t a t i o n of a s i n g l e temperature f o r the tape means t h a t the tape s e c t i o n f o r t h a t t e m p e r a t u r e had p a r t i a l l y darkened. A n o t a t i o n of a r a n g e , f o r example 127-132 ° C , means t h a t the 127 ° C s e c t o r had f u l l y darkened and the 132 ° C s e c t o r was unchanged. In g e n e r a l the tape r e a d i n g s r a n s i g n i f i c a n t l y below the oven r e a d i n g s . The d a t a r e p o r t e d i n Table 3 a r e from e x p e r i m e n t s performed i n August d u r i n g a p e r i o d o f r e l a t i v e l y h i g h h u m i d i t y i n the laboratory. D u p l i c a t e e x p e r i m e n t s were performed i n F e b r u a r y , a p e r i o d o f low r e l a t i v e h u m i d i t y ; the F e b r u a r y and August r e s u l t s are compared i n Table 4 .

DFAR Coatings Cast from Aqueous Formulations.

Water-reduced

c o a t i n g s were p r e p a r e d by a d d i n g 10 g o f Ν , Ν - d i m e t h y l a m i n o e t h a n o l (DMAE) and c a t a l y s t s o l u t i o n [see Table 5 f o r amounts, g i v e n as p a r t s per hundred ( p h r ) on a s o l i d s b a s i s ] t o 112 g o f c o n c e n t r a t e d DFAR r e s i n s o l u t i o n w i t h s t i r r i n g . The amount o f DMAE added was s u f f i c i e n t t o n e u t r a l i z e 75% o f the c a r b o x y l groups on t h e r e s i n based on the t h e o r e t i c a l a c i d number. D e - i o n i z e d water (78 g) was g r a d u a l l y added w i t h s t i r r i n g . At f i r s t t h e v i s c o s i t y i n c r e a s e d , and as more water was added i t d e c r e a s e d t o g i v e a t r a n s p a r e n t , c o l o r l e s s d i s p e r s i o n o r s o l u t i o n a t 35% NVW. The aqueous c o a t i n g s were s t o r e d o v e r n i g h t and then drawn down and baked as d e s c r i b e d above. Dry f i l m t h i c k n e s s e s were 18-23 um.



436


Table 3. (August)

Properties of DFAR Coatings Cast from Solution in Organic Solvent

Catalyst Catalyst level(phr)

TPT 1.01.52.0

TBT 1.01.52.0

P-TSA 1.01.52.0

121 °C(tape) 129 °C(oven setting) MEK rub resistance Pencil hardness

28 44 60 F F H

6 11 8 HB HB HB

30 40 52 F F H

127-132 °C(tape) 140 °C(oven setting) MEK rub resistance Pencil hardness

61 82 132 H H 3H

19 38 78 HB HB F

50 81 126 H H 2H

HTC 2

None 0

6 2B

84 H

6 2B


200 200 200 3H 3H 4H

152 168 172 F F H

200 200 198 2H 3H 3H

20 B


200 200 200 5H 5H 6H

200 200 200 2H 2H 3H

200 200 200 200 3H 3H 4H 3H

50 Β

Dry film thickness:

18-20 μη.


26. CHU ET AL.

437



Table 4. MEK Rub Resistance of DFAR Coatings Cast from Organic Solution i n Summer and Winter

Temperature (°C) tape oven setting

121 129

127-132 140

149-154 167

February TPT (phr) 1.0 1.5 2,0

17 38 79

93 192 200

200 200 200

TBT (phr) 1.0 2.0

17 29

37 200

200 200

p-TSA (phr) 1.0 1.5 2.0

24 30 38

67 113 200

200 200 200

TPT (phr) 1.0 1.5 2.0

28 44 60

61 82 132

200 200 200

TBT (phr) 1.0 2.0

6 8

19 78

200 200

30 40 52

50 81 126

200 200 200

August

p-TSA (phr) 1.0 1.5 2.0 Dry f i l m t h i c k n e s s :

18-25 μπι.


438


The data reported in Table 5 are from experiments performed in August. Results of experiments performed in February and August, are compared in Table 6.


AFAR/HFAR Solution Coating Formulation. AFAR and HFAR resins were mixed in a weight r a t i o of 40:60 to give a s o l u t i o n with a 1:1 equivalent r a t i o of carboxyl:hydroxyl groups. Coatings were prepared and drawn down e s s e n t i a l l y as described above; r e s u l t s are reported in Table 7. Aqueous DFAR Formulations with Added Melamine-Formaldehyde Resin. Aqueous coating formulations were prepared as described above from 50 g (32.3 g s o l i d s ) of DFAR, 4.5 g (extent of n e u t r a l i z a t i o n 75%) of DMAE, and 38.3 g of DI water. HMMM r e s i n ("Cymel 303") and p-TSA were then dissolved in the formulations with s t i r r i n g . The formulations were kept overnight and then drawn down and baked. Film properties were tested described above. Results are shown in Table 7. DFAR Rheology. DFAR was characterized r h e o l o g i c a l l y using a Bohlin VOR rheometer with a concentric c y l i n d e r geometry. The geometry consists of an outer rotating cup of radius 15.4 mm and an inner stationary bob of radius 14 mm. The sample temperature was maintained at 25 ± 0.2 °C. A solvent trap was used to provide a p a r t i a l l y vapor-saturated space above the sample to greatly reduce the evaporation rates of the 2-BE and water. The rheological measurements consisted of steady shear v i s c o s i t y measurements over the shear rate range of 0.185 to 185 s e c . Since Newtonian flow behavior was observed for a l l compositions at low shear rates, the v i s c o s i t y at 0.185 sec" was chosen as the v i s c o s i t y shown d i l u t i o n curves. To obtain d i l u t i o n curves of DFAR, the resins were p a r t i a l l y neutralized with dimethylamino ethanol (DMAE). They were then progressively d i l u t e d with water, and the v i s c o s i t y was measured after each portion of water was added. The mixing was effected by an a i r driven s t i r r e r . Double d i s t i l l e d water was used to minimize effects of ionic contaminants. Two levels of amine n e u t r a l i z a t i o n were tested, one in which the amount of amine was s u f f i c i e n t to neutralize 75% of the carboxylic acid groups in DFAR (extent of n e u t r a l i z a t i o n , EN = 75%) and the other at EN = 24%. The r e s u l t s are shown in Figures 1 and 2. 1

For comparison, the d i l u t i o n curve of a water reducible a c r y l i c r e s i n , designated WRAC, having conventional levels of carboxyl and hydroxyl groups i s shown in Figure 1. This r e s i n i s composed of MMA, BA, AA, and HEA in a 45/36/8/11 mol r a t i o . It was synthesized at 62 weight % s o l i d s in 2-butoxyethanol by a procedure s i m i l a r to the procedure described for DFAR, as detailed in a forthcoming publication ( i l ) . Panel Heat-up Rate Experiment. Temperature s e n s i t i v e tapes were placed on uncoated Type R-36 panels and the panels were baked in a forced a i r oven set at 140 °C for varying times. The tape reading rose to 132-138 °C within 10 min and remained at that level for 24 hr. In repeat experiments small deviations from these r e s u l t s were noted. They were attributed to variations in oven a i r flow at different locations in the oven.


26. CHU ET AL.

T a b l e 5.

Cross-Unking Coatings by Direct Esterification

439

P r o p e r t i e s o f DFAR C o a t i n g Cast from Aqueous F o r m a t i o n s

(August)

Catalyst and Catalyst Level (phr)


TPT

U.

TBT

P-TSA

L5

2JI

24

46

71

10

HB

HB

HB

Β

LO

L5

2,0

LO

HTC

2J[

L5

1.0

NONE

1.5

2.0

13

20

13

HB

HB

2B

121°C(tape) 129° C(oven MEK rub

setting)

resistance

Pencil

hardness

Impact

resistance

14

12

HB

Β

33

57

HB

HB

11 Β

(in-lb) D

100

100

90

160

160

160 160

100

100

160 160

160

160

R

40

40

40

160

160

160 160

40

40

160 160

160

160

127-132°C(tape) 140° CCoven MEK rub

setting)

resistance

Pencil

hardness

Impact

resistance

81

102

111

61

86

Η

Η

Η

HB

HB

94

35

F F

66

87

F

F

24

27

35

HB HB

8

HB

2B

(in-lb) D

100

80

80

100

100

R

40

20

20

40

40

80 100 20

40

100

80

160 160

160

160

40

10

160 160

120

160

138-143°C(tape) 153° C(oven MEK rub

setting)

Pencil

r e s i s t a n c e 200 hardness 2H

Impact

resistance

200

200

96

132

3H

3H

H

H

143 198 2H

H

200

200

62

78

93

29

2H

2H

F

H

H

B

100

160

10

160

(in-lb) D

80

80

80

100

100

R

10

10

10

10

10

100 100 10

100

100

10

10

10

100 100 10

10

149-154°C(tape) 167° C(oven

setting)

MEK r u b r e s i s t a n c e 200 Pencil

hardness

Impact

resistance

2H

200

200

200

200

4H

4H

2H

3H

200 200 3H

3H

200

200

200

200

200

60

2H

3H

2H

2H

4H

HB

(in-lb) D

80

60

60

80

80

80

80

80

80

80

80

80

160

R

10

10

10

10

10

10

10

10

10

10

10

10

160

Dry f i l m t h i c k n e s s :

18-23 um.


440


Table 6. MEK Rub Resistance of DFAR Coating Cast from Aqueous Formulations i n Sumner and Winter

Month

February


Catalyst (1 phr)

TPT

August

p-TSA

TPT

p-TSA

127-132° C (tape) [140° C (oven setting)]

66

31

81

35

149-154° C (tape) [167° C (oven setting)]

200

200

200

200

Dry f i l m thickness:

18-23 μπι.

Table 7. Properties of Two-Component (AFAR and HFAR) Acrylic Coatings Cast from Solution in Organic Solvent

Catalyst Catalyst

TPT Level

(phr)

1.0

TBT 2.0

1.0

2.0

p-TSA 1.0

2.0

127-132° C (tape) 140° (oven setting) MEK rub resistance Pencil hardness

18 Β

35 Β

9 17 2B Β

18 2B

20 2B

149-154° C (tape) 167° (oven setting) MEK rub resistance Pencil hardness

99 121 HB H

28 61 F Β

47 Β

81 Β

171-177° C(tape) 190° C (tape) MEK rub resistance Pencil hardness

200 200 3H 4H

122 200 2H 2H

Dry f i l m thickness:

184 200 H 2H

25-27μπι.


441



26. CHU ET AL.

]Q-3

1I 10

,

I

,

I

20

!

ι

,II 30

ι

,

,

,

I

40

,

ι

I

,

I

•

!

50

I

!

I

60

I

, , • I 70

Polymer Concentration (Wt. %)

Figure 2. Water Dilution Curves of WRAC at 70% E N and of DFAR at 24% E N .



442


Coating Testing. Film thicknesses were measured using a DeFelsko "MIKROTEST" magnetic film thickness gauge. Solvent resistance was measured by double rubbing with nonwoven paper saturated with MEK using firm hand pressure. The failure criterion was when the first penetration through the coating to the panel was noted. Tackiness of the coating surface was noted during the test, and some irreversible disturbance of the surface occurred with a l l coatings except the ones with 6% HMMM resin added. Pencil hardness was measured by the method of ASTM D 3363-92a. Impact resistance was measured using a Gardner Impact Tester having a maximum impact of 1.84 m-kg (160 in-lb) after 1-3 days aging of the panels. Results Dual functional acrylic resin (DFAR) was easily prepared by a conventional polymerization method using "monomer starved" conditions to promote polymer uniformity. It was made with high hydroxyl and acid numbers to increase its functionality (see Discussion Section). Its only abnormality was its unusually broad molecular weight distribution, M /M , which ranged from 3.6 to 6.3. This broad distribution suggests that esterification reactions may have occurred to some extent during polymerization or that the HEA was contaminated with ethylene glycol diacrylate. w

n

The rheological response of DFAR, with its abnormally high levels of carboxyl and hydroxyl functionality, was compared to WRAC, a conventional water-reducible acrylic resin. As shown in Figure 1, the dilution curve of DFAR at 75 % EN (extent of neutralization) lacks the pronounced viscosity plateau in the dilution curve of WRAC at 70 % EN. As shown in Figure 2, when DFAR is neutralized at 24 % EN using an amount of amine that would be equivalent to 75 % EN for WRAC, the dilution curve has somewhat more inflection, but not as much as WRAC at 70 % EN. These dilution curves are similar in shape to curves reported by Hill and Richards (12) for a family of acrylic resins containing 10 to 50 mol % of acrylic acids. Unpigmented coatings were formulated with various catalysts and films were cast on steel panels and baked. Transparent, glossy films were formed except for films catalyzed by HTC, which had uneven, granular surfaces and a tendency to discolor when used on steel. Two experimental problems were encountered in evaluation of the coatings' cure response. The first was the 9 to 15 °C discrepancy between the oven settings and the temperature tape readings noted in Tables 3-8. Heat-up rate experiments indicate that tape readings are consistently lower than oven settings and that variation of the temperatures actually experienced by panels varies somewhat in this oven. The tape temperature readings in Tables 3-8 are judged to be more reliable than the oven setting readings. A second problem was poor reproducibility of impact resistance


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Table 8. Properties of DFAR Water Reducible Coating Containing HMMM Auxiliary Crosslinker

HMMM (phr) P-TSA

(phr)

121° C (tape) 129° C (oven setting) MEK rub resistance Pencil hardness Impact resistance (in-lb)D R 127-132° (C (tape) 140° C (oven setting) MEK rub resistance Pencil hardness Impact resistance (in-lb)D R 138-143° C (tape) 153° C (oven setting) MEK rub resistance Pencil hardness Impact resistance (in-lb)D R Dry f i l m thickness:

130 H

151 H

177 H

200 3H

20 10

20 10

20 10

20 10

200 3H

200 3H

200 3H

200 3H

30 10

30 10

60 30

60 40

200 4H

200 4H

200 4H

200 5H

50 10

50 10

60 10

70 30

20-25 μπι.


444



results. While impact r e s i s t a n c e t e s t s were performed on a l l p a n e l s , t h e r e s u l t s a r e not a l l d e t a i l e d h e r e . In g e n e r a l , f i l m s w i t h lowest MEK r e s i s t a n c e had h i g h e s t impact r e s i s t a n c e , but d i r e c t impact r e s i s t a n c e f e l l t o the 40 - 80 i n - l b range f o r f i l m s t h a t had 200 MEK rubs and r e v e r s e impact r e s i s t a n c e f e l l even l o w e r . D e s p i t e t h e s e p r o b l e m s , i t i s thought t h a t t h e c r o s s l i n k i n g p o t e n t i a l of DFAR i n s o l v e n t - b o r n e f o r m u l a t i o n s i s c l e a r l y i n d i c a t e d by t h e r e s u l t s shown i n T a b l e 3, and t h a t the r e s u l t s i n T a b l e 5 r e p r e s e n t the c u r e r e s p o n s e of the w a t e r - r e d u c e d f o r m u l a t i o n s . S o l v e n t rub r e s i s t a n c e i s p r o b a b l y t h e b e s t i n d i c a t o r o f r e l a t i v e e x t e n t o f c u r e o f t h e t h r e e t e s t s performed i n t h i s s t u d y . Solvent rub r e s i s t a n c e i n c r e a s e s from 6 - 1 5 d o u b l e rubs f o r the l e a s t c u r e d f o r m u l a t i o n s t o > 200 d o u b l e rubs f o r the most c u r e d . Hardness t e s t s f o l l o w a p a r a l l e l c o u r s e , r i s i n g from 2B f o r the l e a s t c u r e d c o a t i n g s t o 2H-3H when 200 MEK rubs a r e a t t a i n e d and f u r t h e r t o 5H at higher baking temperatures. The r e s u l t s i n T a b l e s 3 and 5 show t h a t DFAR c o a t i n g s can c r o s s l i n k s u f f i c i e n t l y t o produce h a r d , s o l v e n t r e s i s t a n t f i l m s when baked f o r 30 minutes a t 138-143 ° C (oven s e t t i n g 153 ° C ) w i t h 1, 1 . 5 , o r 2 p a r t s per hundred by weight (phr) o f TPT and p-TSA catalysts. F i l m s c a t a l y z e d by TBT and HTC r e q u i r e d h i g h e r t e m p e r a t u r e s t o r e a c h 200 MEK r u b s . There was l i t t l e d i f f e r e n c e between t h e c u r e r e s p o n s e s o f c o a t i n g s c a s t from nonaqueous and aqueous f o r m u l a t i o n s - - i n view of the poor r e p r o d u c i b i l i t y o f the s o l v e n t rub and hardness t e s t s the s m a l l d i f f e r e n c e s noted may be w i t h i n experimental e r r o r . The apparent o r d e r o f e f f e c t i v e n e s s o f the c a t a l y s t s on a weight b a s i s i s TPT > p-TSA > TBT > HTC » no catalyst. The d i f f e r e n c e s between TPT and p-TSA a r e modest and may not be s t a t i s t i c a l l y s i g n i f i c a n t . The d a t a i n T a b l e s 3 and 5 a r e from t e s t s performed i n A u g u s t , a p e r i o d of r e l a t i v e l y high r e l a t i v e humidity in the l a b o r a t o r y . R e s u l t s a r e compared w i t h r e s u l t s o b t a i n e d i n F e b r u a r y , a p e r i o d o f low r e l a t i v e h u m i d i t y , i n T a b l e s 4 and 6. F i l m s c a s t from nonaqueous f o r m u l a t i o n s appeared t o have a s l i g h t l y b e t t e r c u r e r e s p o n s e i n F e b r u a r y than i n August ( T a b l e 4). There was l i t t l e d i f f e r e n c e when f i l m s were c a s t from aqueous f o r m u l a t i o n s ( T a b l e 6). These r e s u l t s s u g g e s t t h a t c u r e may be s l i g h t l y i n h i b i t e d by m o i s t u r e i n humid a i r but i s not f u r t h e r i n h i b i t e d by m o i s t u r e i n aqueous f o r m u l a t i o n s . A g a i n , t h e r e i s a q u e s t i o n o f whether the o b s e r v e d d i f f e r e n c e s a r e l a r g e r than e x p e r i m e n t a l e r r o r . A b l e n d of a c a r b o x y l i c a c i d f u n c t i o n a l a c r y l i c r e s i n (AFAR) w i t h a h y d r o x y l f u n c t i o n a l a c r y l i c r e s i n (HFAR) had a r e l a t i v e l y poor c u r e r e s p o n s e , as shown i n T a b l e 7. The b l e n d s c u r e d , but t h e y r e q u i r e d t e m p e r a t u r e s o f about 175 ° C t o r e a c h 200 MEK r u b s v s . about 140 ° C f o r the r e s i n which i n c o r p o r a t e s both t y p e s o f f u n c t i o n a l groups (DFAR). The l a r g e d i f f e r e n c e i n c u r e r e s p o n s e may be e x p l a i n e d by the a b i l i t y o f DFAR t o undergo i n t e r - and i n t r a m o l e c u l a r c r o s s l i n k i n g , w h i l e the HFAR/AFAR b l e n d i s l i m i t e d t o intermolecular c r o s s l i n k i n g . It

s h o u l d be noted t h a t the

s u r f a c e s o f DFAR c o a t i n g s were


26. CHU ET AL.


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i r r e v e r s i b l y d i s t u r b e d by t h e s o l v e n t r u b t e s t . The s u r f a c e f e l t " t a c k y " d u r i n g t h e t e s t , and a f t e r w a r d s t h e g l o s s was r e d u c e d . In t h i s r e s p e c t t h e y a r e i n f e r i o r t o c o a t i n g s c r o s s l i n k e d w i t h MF r e s i n s , which o f t e n pass 200 MEK rubs w i t h o u t s u r f a c e m a r r i n g . A d d i t i o n o f a f u l l y m e t h y l o l a t e d MF (HMMM) r e s i n t o t h e aqueous DFAR f o r m u l a t i o n s was i n v e s t i g a t e d w i t h t h e r e s u l t s shown i n T a b l e 8. p-TSA c a t a l y s t was u s e d . Comparison o f T a b l e s 8 and 5 shows t h a t a d d i t i o n o f 3 weight p e r - c e n t o f HMMM r e d u c e s t h e bake temperature r e q u i r e d t o r e a c h 200 MEK r u b r e s i s t a n c e by about 10 ° C . A d d i t i o n o f 6 weight p e r - c e n t o f HMMM f u r t h e r improves t h e c u r e response. M o r e o v e r , a d d i t i o n o f 6% HMMM e l i m i n a t e d t h e problem o f surface marring d u r i n g the s o l v e n t rub t e s t .

Discussion Use o f d i r e c t e s t e r i f i c a t i o n f o r c r o s s l i n k i n g o f c o a t i n g s seems obvious. Why have t h e r e been so few p u b l i s h e d r e p o r t s o f i t i n t h e recent l i t e r a t u r e ? One r e a s o n might be t h a t r e s i n s d e s i g n e d f o r t h i s type of c r o s s l i n k i n g are u n s u i t a b l e f o r high s o l i d s c o a t i n g s because they need h i g h f u n c t i o n a l i t y ( c a r b o x y l and h y d r o x y l groups p e r m o l e c u l e ) a n d , t h e r e f o r e , h i g h m o l e c u l a r w e i g h t s and h i g h viscosities. However, r e s i n s d e s i g n e d f o r use i n w a t e r - r e d u c i b l e c o a t i n g s can h a v e , w i t h i n l i m i t s , h i g h v i s c o s i t i e s . As shown i n Figures 1 and 2 , t h e w a t e r - d i l u t i o n b e h a v i o r o f DFAR i s d i f f e r e n t than t h a t o f c o n v e n t i o n a l w a t e r - r e d u c i b l e r e s i n s l i k e WRAC. These d i f f e r e n c e s can be a t t r i b u t e d t o d i f f e r e n t e x t e n t s o f d i p o l e - d i p o l e i n t e r a c t i o n s and o f a g g r e g a t i o n between t h e two r e s i n s y s t e m s . DFAR i s more s o l u b l e i n water due t o t h e g r e a t e r c o n c e n t r a t i o n o f h y d r o p h i l i c groups on t h e c h a i n , i n c l u d i n g s a l t e d c a r b o x y l g r o u p s , f r e e c a r b o x y l g r o u p s , and h y d r o x y l g r o u p s . The h i g h e r c o n c e n t r a t i o n o f a c i d groups i n DFAR and t h e r e s u l t i n g i n c r e a s e i n d i p o l e i n t e r a c t i o n s between i o n i c a c i d - a m i n e s a l t groups as compared t o WRAC l e a d s t o g r e a t e r c h a i n e x p a n s i o n and i n t e r c h a i n i n t e r a c t i o n s . These e f f e c t s g i v e r i s e t o t h e i n i t i a l l y h i g h v i s c o s i t y o f DFAR i n t h e absence o f water (Figure 1). In a d d i t i o n , t h e s e s a l t groups impart a h i g h degree o f water s o l u b i l i t y t o DFAR which a c c o u n t s f o r t h e s o l u t i o n l i k e d i l u t i o n b e h a v i o r o f DFAR as compared t o WRAC. The p l a t e a u i n the d i l u t i o n c u r v e o f WRAC, due t o e l e c t r o s t a t i c e x p a n s i o n o f t h e polymer c h a i n s ( Π ) , i s n o t o b s e r v e d w i t h DFAR. In a d d i t i o n , t h e e x p a n s i o n o f t h e s e a m p h i p h i l i c c h a i n s i n an i n c r e a s i n g l y w a t e r - r i c h medium l e a d s t o c h a i n a g g r e g a t i o n f o l l o w e d by a s h a r p d e c r e a s e i n viscosity. Such a sharp d e c r e a s e i n v i s c o s i t y i s not seen w i t h DFAR a t 75% EN; t h e i n f l e c t i o n i n v i s c o s i t y i s p r o b a b l y due t o a moderate amount o f e l e c t r o s t a t i c c h a i n e x p a n s i o n . The degree o f a g g r e g a t i o n o f polymer c h a i n s can be a d j u s t e d by v a r y i n g t h e e x t e n t o f neutralization. Indeed, when DFAR i s n e u t r a l i z e d t o o n l y 24%, i t s d i l u t i o n b e h a v i o r approaches t h a t o f WRAC a t 70% EN (Figure 2), s u g g e s t i n g some a g g r e g a t i o n o f DFAR a t h i g h e r water c o n t e n t s . While t h e w a t e r - d i l u t i o n c u r v e s o f DFAR a r e d i f f e r e n t than t h o s e o f c o n v e n t i o n a l w a t e r - r e d u c i b l e r e s i n s , t h e s e appear t o be no


446



major o b s t a c l e s t o f o r m u l a t i n g and a p p l y i n g DFAR r e s i n s i n aqueous systems. I f r h e o l o g i c a l c h a r a c t e r i s t i c s pose no major p r o b l e m s , the p e r c e i v e d o b s t a c l e s must be i n the c r o s s l i n k i n g c h e m i s t r y . Perhaps p o t e n t i a l i n v e s t i g a t o r s assumed t h a t d i r e c t e s t e r i f i c a t i o n r e a c t i o n s a r e too slow t o be u s e f u l . Coatings chemists are f a m i l i a r with p o l y e s t e r i f i c a t i o n p r o c e s s e s t o make p o l y e s t e r and a l k y d r e s i n s , which g e n e r a l l y r e q u i r e t e m p e r a t u r e s o f 220-260 ° C and times o f 4-10 hours t o c o m p l e t e . C o a t i n g s , on the o t h e r hand, a r e t y p i c a l l y baked at t e m p e r a t u r e s below 160 ° C in 10 t o 30 m i n u t e s . However, p o l y m e r i z a t i o n p r o c e s s e s have d i f f e r e n t k i n e t i c r e q u i r e m e n t s than c r o s s l i n k i n g p r o c e s s e s . In p o l y e s t e r i f i c a t i o n s the r e a c t i o n s must be d r i v e n t o h i g h c o n v e r s i o n s , g e n e r a l l y above 98%. In the c a s e o f p o l y e s t e r i f i c a t i o n t h i s i s d i f f i c u l t because e s t e r i f i c a t i o n r e a c t i o n s a r e r e v e r s i b l e and a r e o f t e n o f a h i g h k i n e t i c o r d e r , u s u a l l y somewhere between second o r d e r and t h i r d order (8). A consequence o f h i g h k i n e t i c o r d e r i s t h a t p o l y e s t e r i f i c a t i o n r e a c t i o n s t h a t a r e r e l a t i v e l y f a c i l e a t low conversions r e q u i r e f o r c i n g c o n d i t i o n s to d r i v e the r e a c t i o n s to high c o n v e r s i o n . In c o n t r a s t , c r o s s l i n k a b l e polymers can b e , and o f t e n a r e , d e s i g n e d so t h a t adequate c r o s s l i n k i n g can be a t t a i n e d a t l e s s than h i g h c o n v e r s i o n ; i f enough f u n c t i o n a l i t y i s b u i l t i n t o the r e s i n s , c o n v e r s i o n s below 50% may g i v e optimum c r o s s l i n k d e n s i t y . A n o t h e r d i f f e r e n c e i s t h a t p h y s i c a l removal o f water i s d i f f i c u l t in large s c a l e p o l y e s t e r i f i c a t i o n s . Sometimes i t i s the rate limiting step. But water can e a s i l y escape from t h i n c o a t i n g f i l m s , p r o b a b l y o r d e r s o f magnitude f a s t e r than i t can e s c a p e from a mass o f polymer i n a l a r g e r e a c t i o n k e t t l e . An i n d i c a t i o n o f the importance o f water removal r a t e can be found i n a r e c e n t r e p o r t by Jong and Saam ( 1 3 ) , where i t was shown t h a t p o l y e s t e r i f i c a t i o n w i t h a s t r o n g a c i d c a t a l y s t can be a f f e c t e d a t r e a s o n a b l e r a t e s a t 50 t o 80 ° C by m a n i p u l a t i n g a two-phase r e a c t i o n system so t h a t water i s q u i c k l y removed from the p o l y e s t e r . The c a t a l y s t i s c r i t i c a l . Without i t a much h i g h e r temperature i s r e q u i r e d f o r a g i v e n l e v e l of c u r e (See T a b l e 3 . ) . In a d d i t i o n t o s t r o n g a c i d ( p - T S A ) c a t a l y s t , t i t a n a t e c a t a l y s t s were s t u d i e d because the l i t e r a t u r e (8) i n d i c a t e s t h a t they a r e more e f f e c t i v e as e s t e r i f i c a t i o n c a t a l y s t s than c a t a l y s t s based on S n , B i , Z n , P b , etc. T P T , TBT, and p-TSA c a t a l y s t s a r e c o m m e r c i a l l y a v a i l a b l e . The f o u r t h , HTC, i s e a s i l y made ( 1 0 ) . It i s s a i d t h a t i t s s t r u c t u r e can be r e p r e s e n t e d a s :

/

,CH

2

tiO

HN

CH - C H 2 2

HTC

HTC was s t u d i e d because i t i s r e p o r t e d (10) t o have e x c e p t i o n a l c a t a l y t i c a c t i v i t y f o r e s t e r i f i c a t i o n and t o form s t a b l e s o l u t i o n s


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in water. However, as described above, i t did not perform as well as the other catalysts in aqueous formulations, perhaps because of phase separation. Proprietary commercial titanium chelate c a t a l y s t s are a v a i l a b l e but were not investigated. They are reported (f>) to be more stable in water than TPT and TBT. It is u n l i k e l y that tetraisopropyl and tetra-n-butyl t i t a n a t e s (TPT and TBT) are the actual c a t a l y t i c species. As soon as they are added to the coating formulation they probably begin to undergo exchange reactions with the hydroxyl and carboxyl groups of the r e s i n ; a complex equilibrium i s presumably established. Subsequent addition of water l i k e l y perturbs t h i s e q u i l i b r i u m , as an assortment of hydrolysis reactions i s possible ( 7 , 8 ) . One possible reaction i s p a r t i a l hydrolysis to produce titanoxanes species having Ti-O-Ti p a r t i a l structures. In view of the near c e r t a i n t y that exchange and hydrolysis reactions occur, i t i s s u r p r i s i n g that addition of water has n e g l i g i b l e affect on the c a t a l y s t a c t i v i t y (Compare Tables 3 and 5 . ) . Hydrolysis could lead to deactivation of the c a t a l y s t by p r e c i p i t a t i o n of T i 0 . No p r e c i p i t a t i o n from aqueous TPT and TBT formulations was observed, although long-term s t a b i l i t y tests were not performed. With further study i t may be possible to f i n d c a t a l y s t s that are more effective than TPT without the shortcomings of the hydrolyzed titanium chelate c a t a l y s t (HTC). 2

The dual functional a c r y l i c r e s i n (DFAR) was d e l i b e r a t e l y designed with high f u n c t i o n a l i t y , with 21.7 mol per-cent each of AA and HEMA. A t y p i c a l water reducible r e s i n has 10 mol percent of HEMA and 7.8 mol per-cent of AA.(i) The t h e o r e t i c a l hydroxyl number of DFAR i s 117 mg-KOH/g-resin, roughly twice the levels commonly used for water reducible resins designed to be crosslinked with melamine formaldehyde (MF) r e s i n s . Its t h e o r e t i c a l acid number, also 117 mg-KOH/g-resin, i s about three times as high that for as such r e s i n s . Based on t h e o r e t i c a l hydroxyl and acid numbers and on the M measured by HPLC i t s number average f u n c t i o n a l i t y i s about 30 hydroxyl groups and 30 carboxyl groups per molecule; thus DFAR i s capable of forming a highly crosslinked network. If a l l carboxyl and hydroxyl groups were e s t e r i f i e d (probably a physical i m p o s s i b i l i t y because of s p a t i a l constraints as high conversion is approached), about 40 per-cent of the monomer units would be connected by c r o s s l i n k s . A c r y l i c resins for c r o s s l i n k i n g by MF resins with comparable M are t y p i c a l l y designed so that 10 to 15 per-cent of the monomer units can be connected ( I ) . While quantitative comparisons are problematic because of the different chemistries involved, i t seems evident that DFAR can a t t a i n a c r o s s l i n k density comparable to that of contemporary a c r y l i c thermoset coatings i f only a moderate f r a c t i o n (perhaps half or even less) of i t s carboxyl and hydroxyl groups are e s t e r i f i e d . n

n

An effort to measure the extent of c r o s s l i n k i n g (i^e the conversion of the e s t e r i f i c a t i o n reaction) by FT-IR did not y i e l d s a t i s f y i n g r e s u l t s . The hydroxyl and carboxyl groups in the resin give r i s e to a broad, nondescript peak in the 3100 to 3500 cm" region; i t changes during c r o s s l i n k i n g , but no quantitative data could be derived. The FT-IR spectral changes and the observed c a t a l y s t effects indicate that c r o s s l i n k i n g of DFAR occurs p r i m a r i l y 1


448



by d i r e c t e s t e r i f i c a t i o n , but the p o s s i b i l i t y t h a t t r a n s e s t e r i f i c a t i o n r e a c t i o n s c o n t r i b u t e t o i t s c r o s s l i n k i n g cannot be r u l e d o u t . As reviewed by F r e d e t and M a r é c h a l ( 8 ) , d i f f e r e n t a u t h o r s have drawn v a r y i n g c o n c l u s i o n s about the k i n e t i c s and mechanisms o f catalyzed direct e s t e r i f i c a t i o n reactions. It i s g e n e r a l l y agreed t h a t k i n e t i c o r d e r i s h i g h ; t h u s the r e a c t i o n w i l l slow s h a r p l y as conversion (extent of reaction) i n c r e a s e s . F u r t h e r , as c o n v e r s i o n approaches h i g h l e v e l s a second f a c t o r w i l l a l s o slow the r e a c t i o n : r e s t r i c t e d m o b i l i t y w i t h i n the c r o s s l i n k e d network. In view o f t h e s e c o n s i d e r a t i o n s and o f the l a r g e number o f r e a c t i v e groups i n DFAR i t seems p r o b a b l e t h a t c o n v e r s i o n i s w e l l below 100% i n a l l f i l m s made in t h i s s t u d y . If so t h e r e i s a s u b s t a n t i a l c o n c e n t r a t i o n o f u n r e a c t e d c a r b o x y l and h y d r o x y l groups i n the f i l m s even a f t e r h i g h s o l v e n t r e s i s t a n c e and h a r d n e s s have been r e a c h e d . T h e r e a r e p o t e n t i a l d i s a d v a n t a g e s t o d e s i g n i n g r e s i n s such as DFAR w i t h e x c e s s f u n c t i o n a l i t y and c u r i n g them a t f r a c t i o n a l conversion. One p o t e n t i a l problem i s t h a t the c o a t i n g may have a narrow c u r e window and be q u i t e s e n s i t i v e t o overbake and underbake. ["Cure window" i s a term c o i n e d by Bauer t o d e s c r i b e the range o f bake t e m p e r a t u r e s a t a g i v e n time o r bake times at a g i v e n t e m p e r a t u r e what w i l l y i e l d s a t i s f a c t o r y p r o p e r t i e s o f a g i v e n coating (14).] There i s even the p o s s i b i l i t y t h a t c r o s s l i n k i n g might c o n t i n u e a t a slow r a t e d u r i n g s e r v i c e , l e a d i n g t o e v e n t u a l e m b r i t t l e m e n t ; f o r t h i s r e a s o n c o a t i n g s based on t h e DFAR c o n c e p t w i l l be b e s t s u i t e d t o a p p l i c a t i o n s where t h e y a r e baked a t t e m p e r a t u r e s h i g h above t h e i r maximum s e r v i c e t e m p e r a t u r e s . Present d a t a show t h a t c u r e window i s a problem but do not e n a b l e us t o estimate i t s s e v e r i t y . It i s e n c o u r a g i n g t o note t h a t many c o a t i n g s , f o r example most c o a t i n g s f o r m u l a t e d w i t h a l c o h o l a t e d melamine-formaldehyde r e s i n s , are d e l i b e r a t e l y formulated to achieve optimum p r o p e r t i e s a t l e s s than f u l l c o n v e r s i o n , and the i n d u s t r y has l e a r n e d t o d e a l w i t h t h e i r r a t h e r narrow c u r e windows and the r e s u l t i n g s e n s i t i v i t y to overbake. A second p o t e n t i a l d i s a d v a n t a g e i s t h a t u n r e a c t e d h y d r o x y l and c a r b o x y l groups i n the f i l m s may cause the c u r e d f i l m s t o be s e n s i t i v e to m o i s t u r e , b a s e s , and d e t e r g e n t s . The degree of t h i s problem w i l l depend on the c o n c e n t r a t i o n o f u n c o n v e r t e d f u n c t i o n a l groups i n the f i l m and on the p h y s i c a l c h a r a c t e r i s t i c s o f the f i l m . High c r o s s l i n k d e n s i t y and h i g h T can be e x p e c t e d t o m i n i m i z e the problems. The s e v e r i t y o f t h i s problem i s b e s t e v a l u a t e d i n the c o n t e x t o f the r e q u i r e m e n t s f o r a s p e c i f i c a p p l i c a t i o n . q

A f u r t h e r c o n c e r n i s t h a t e s t e r c r o s s l i n k s might be t o o v u l n e r a b l e t o h y d r o l y s i s f o r extended s e r v i c e o u t d o o r s o r i n humid environments. However, e s t e r groups o f t e n have s u f f i c i e n t h y d r o l y s i s r e s i s t a n c e f o r demanding a p p l i c a t i o n s . Powder c o a t i n g s c r o s s l i n k e d by e s t e r i f i c a t i o n o f o x i r a n e f u n c t i o n a l a c r y l i c r e s i n s w i t h c a r b o x y l i c a c i d s have s u f f i c i e n t r e s i s t a n c e t o h y d r o l y s i s t o be used as a u t o m o t i v e t o p c o a t s i n J a p a n . Liquid coatings crosslinked by t h i s method a r e r e p o r t e d t o have good w e a t h e r a b i l i t y and r e s i s t a n c e to a c i d e t c h i n g ( I S ) . E s t e r groups p r e s e n t i n many


26. CHU ET AL.

449


w e a t h e r a b l e p o l y e s t e r c o a t i n g s have s u f f i c i e n t when h i g h l y c r o s s l i n k e d .

hydrolytic

stability


The e x p e r i m e n t s i n which HMMM r e s i n was added t o aqueous DFAR f o r m u l a t i o n s p o i n t toward a p o t e n t i a l l y u s e f u l f o r m u l a t i n g method: A c o a t i n g c o u l d be f o r m u l a t e d t o c u r e p a r t l y by a c o n v e n t i o n a l c r o s s l i n k i n g r e a c t i o n and p a r t l y by d i r e c t e s t e r i f i c a t i o n . This approach c o u l d be used t o reduce VOC and HAP e m i s s i o n s w h i l e p a r t l y r e t a i n i n g the b e n e f i t s o f c o n v e n t i o n a l c r o s s l i n k e r s . R e s u l t s o f t h i s s t u d y r a i s e a q u e s t i o n about c u r r e n t b a k i n g c o a t i n g s t h a t are presumed t o be c r o s s l i n k e d by o t h e r mechanisms: Might e s t e r i f i c a t i o n p l a y a r o l e in the c r o s s l i n k i n g p r o c e s s ? Examples o f c o a t i n g s where t h i s might o c c u r a r e w a t e r - r e d u c i b l e a c r y l i c c o a t i n g s c r o s s l i n k e d w i t h a l c o h o l a t e d melamine-formaldehyde r e s i n s and epoxy c o a t i n g s c r o s s l i n k e d w i t h a n h y d r i d e s o r c a r b o x y l i c acids. In both c a s e s the c a t a l y s t s used a r e a l s o e s t e r i f i c a t i o n catalysts. Thus i t seems p o s s i b l e t h a t , i n a d d i t i o n t o the r e c o g n i z e d c r o s s l i n k i n g mechanisms, e s t e r i f i c a t i o n might p l a y a r o l e , p r o b a b l y s e c o n d a r y but perhaps enough t o i n f l u e n c e f i l m properties. R e c o g n i t i o n o f t h i s p o s s i b i l i t y might be u s e f u l t o d e s i g n e r s of c o n v e n t i o n a l c o a t i n g s .

Summary and Conclusions It i s demonstrated t h a t an a c r y l i c r e s i n w i t h high l e v e l s o f c a r b o x y l and h y d r o x y l groups ("DFAR") can c r o s s l i n k by d i r e c t e s t e r i f i c a t i o n w i t h i n 30 minutes a t t e m p e r a t u r e s as low as 140 ° C when c a t a l y z e d by c e r t a i n t i t a n a t e s o r s t r o n g a c i d s . While o n l y a f r a c t i o n o f the f u n c t i o n a l groups r e a c t , c r o s s l i n k i n g i s s u f f i c i e n t to give hard, s o l v e n t r e s i s t a n t f i l m s . T h i s r e l a t i v e l y unexplored approach seems e s p e c i a l l y w e l l s u i t e d t o w a t e r - r e d u c i b l e c o a t i n g s , where r e s i n s can have h i g h e r m o l e c u l a r w e i g h t s , and t h e r e f o r e h i g h e r f u n c t i o n a l i t y than r e s i n s f o r h i g h s o l i d s c o a t i n g s . Rheology o f DFAR i n d i c a t e s more s o l u t i o n - l i k e b e h a v i o r and much l e s s a g g r e g a t i o n than w i t h c o n v e n t i o n a l w a t e r - r e d u c i b l e r e s i n s . T h i s a p p r o a c h , : L i U the use o f d i r e c t e s t e r i f i c a t i o n t o c r o s s l i n k c o a t i n g s , has the p o t e n t i a l advantage o f e l i m i n a t i o n of the t o x i c h a z a r d a s s o c i a t e d w i t h some c r o s s l i n k e r s and e l i m i n a t i o n o f c r o s s l i n k i n g b y - p r o d u c t s which are VOCs o r a r e l i s t e d as Hazardous A i r P o l l u t a n t s ( H A P s ) . It i s a l s o p o t e n t i a l l y e c o n o m i c a l . Thus i t m e r i t s e x p l o r a t i o n by the c o a t i n g s i n d u s t r y i n view o f impending VOC and HAP r e g u l a t i o n s . Many q u e s t i o n s remain t o be answered b e f o r e the commercial f e a s i b i l i t y of t h i s approach f o r a g i v e n a p p l i c a t i o n can be assessed. P o t e n t i a l problems i n c l u d e the p o s s i b i l i t y o f narrow c u r e windows and m o i s t u r e s e n s i t i v e f i l m s . The purpose o f t h i s paper i s not t o s o l v e t h e s e problems but s i m p l y t o c a l l t o the a t t e n t i o n o f the c o a t i n g s i n d u s t r y the p o s s i b i l i t y o f u s i n g d i r e c t esterification r e a c t i o n s to c r o s s l i n k c o a t i n g s .


450


Acknowledgements

This project was supported by the National Science Foundation Industry/University Cooperative Research Center in Coatings at Eastern Michigan U n i v e r s i t y , North Dakota State University and Michigan Molecular I n s t i t u t e . Helpful discussions with John Saam and Kenneth G. Hahn, J r . are appreciated.


References

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