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---

Part 3: Synthetic Materials

MNL17-EB/Jun
...
Friel I

ranging from a few hundred to a few thousand centipoise
...
The coatings
manufacturer is then able to dissolve the solid-grade acrylic
in a wide range of solvents, thereby having greater control
over the formulation ingredients
...
Some of the important properties
for several acrylic homopolymers are shown in Table 1 [3]
...
Some of the principal applications for
acrylic solution coatings include automotive finishing, factory, and farm implement coatings, general-purpose pigmented coatings, aerosol lacquers, and clear coatings for a
multitude of substrates
...
e
...
) to form a cross-linked network
...

Acrylic emulsion polymers (often referred to as acrylic
latexes) have become one of the major binder types in use in
the coatings industry today
...
The droplets are
typically stabilized by surfactants, and usually no solvent is
present
...
In fact, the use of acrylic emulsions in industrial applications is expanding at the expense of

ACRYLICPOLYMERS, WHICHARE USED as coatings binders, are
comprised chiefly of esters of acrylic and methacrylic acid
that are polyrnerized by additional polymerization, usually
using a free radical mechanism:
H

CH 3

I
(--CH2--C - - ) - -

I
--(--CH2--C--)--

I
C ~0

I

OR
An acrylate

I
C=0

t

OR
A methacrylate

Interest in acrylic technology dates back to the 1920s when
Dr
...
Shortly afterwards, his associate,
Otto Haas, established the first commercial production of
methyl and ethyl acrylate in the United States [1]
...
Probably the highest
profile use of an acrylic began in 1936 with the introduction
of thermoplastic, transparent methacrylate sheet
...
Since it could be formed easily and had excellent
optical properties, the transparent plastic was used for aircraft canopies, bomber noses, and gun turrets [2]
...
The acrylics gained widespread market acceptance as coatings binders due to such outstanding properties as color stability, transparency, and resistance to weathering and aging
...

Acrylic solution polymers (often referred to as acrylic resins) are generally copolymers of acrylate and methacrylate
esters prepared by direct solution polymerization in a solvent
that has a solubility parameter similar to that of the polymer
...
Acrylic resins are typically
supplied at about 30 to 50% solids by weight, with viscosities

TABLE 1--Properties of polymethacrylates and polyacrylates [3]
...
, Research Laboratories, 727 Norristown
Road, Spring House, PA 19477
...
22 = k g / c m 2
...
Copyright1973)
...
astm
...
Over the past 20 years, acrylic emulsion manufacturers have made great strides in improving the
properties of acrylic emulsions so that they now offer performance similar to the solvent-based coatings they are replacing
...
During the past 40 years, it has
been good exterior durability that enabled acrylic emulsions
to replace solvent alkyds as the dominant binder in the exterior house paint market
...
They do not need to be
oxidized or cross-linked to form a hard, resistant finish
...

Acrylic resins are usually supplied in strong solvents such
as toluene, xylene, or methyl ethyl ketone
...
They are often used in
unpigmented form as protective finishes over vacuum metalized plastics and polished metals such as brass
...
No external pigment wetting agents are
required to make finely dispersed pigment grinds for highgloss lacquers
...
They do not discolor powdered
metals, such as aluminum
...

Each acrylic monomer brings to the polymer its own individual performance characteristics based on its molecular structure
...
The Tg of a
polymer is a softening point: it is actually a temperature
range where the polymer undergoes a second-order transition
...
To approximate the Tg for a copolymer composition, it is useful to utilize
the relationship proposed by Fox [4]
...


Since thermoplastic acrylics are not cross-linked to achieve
a desired level of performance, the concept of Tg and the
ability to manipulate Tg as a means to control properties is
crucial in designing polymers that meet the needs of the
coatings market
...
The
free-volume theory states that the Tg for any given polymer
occurs at that temperature where the fractional free volume
(i
...
, unoccupied space contained within the polymer)
reaches some universally constant value that remains unchanged as temperature decreases below Tg
...
In Fig
...
The arrows
( T ) in Fig
...
This is the
Tg
...

Simha and Boyer have independently calculated that at Tg,
free volume accounts for 11% of a polymer's total volume [7]
...
22
1
...
1~
1
...
""
09 ,
...
,

m

C18

0
-

E

/ " "

1
...


1
...
01 , *
'12

0

/

~


...
//


...
-"

0
...
05
0
...
91
C ~ ~ ~ ~ +
~
0
...
87
0
...
83
~ t t t t t t t f t
-80 -60 -40 -20 0 20 40 60 80 100 120 140

Temperature, ~
where
W1

and W2 = the weight ratios of Monomers 1 and 2, respectively,

FIG
...
(Reprinted with permission from the
American Chemical Society
...
The acrylates
have an alpha-hydrogen atom next to the carbonyl group,
giving them more rotational freedom and hence greater segmental chain motion than the methacrylates
...
The methacrylates are therefore
higher in Tg, harder, higher in tensile strength, and lower in
elongation than analogous acrylate polymers
...
As the alcohol moiety
of the ester side chain becomes larger, the polymer chains are
pushed further apart, creating additional free volume,
thereby encouraging greater molecular rotation
...
Table 2 demonstrates the large range of Tg's that exist for the acrylate and
methacrylate family of homopolymers [8]
...
Most dry film properties for thermoplastic
acrylics improve with increasing molecular weight up to a
MW of about 100 000 and then tend to level off
...
This
dependence of strength, elongation, and resistance of the
acrylic polymer on molecular weight is primarily due to the
greater n u m b e r of chain entanglements (which act almost
like cross-links), anchoring the polymer chains at higher molecular weight (i
...
, longer chain length)
...

TABLE

Polymer

n = K(MW) a

(2)

where
91 = solution viscosity
K and a = constants derived experimentally for a specific
polymer/solvent combination (for polymethyl
methacrylate in toluene, K • l0 s = 7
...
73)
...

As molecular weight increases, the solution viscosity also
increases, thereby posing handling and application problems
if the molecular weight becomes too high
...
The lower the molecular
weight of the polymer, the lower the viscosity at a given solids
content, or conversely, the higher the spray solids at spray
viscosity
...

The obvious answer to the viscosity versus film property
dilemma is to reach an o p t i m u m balance by producing an
intermediate molecular weight polymer
...
It is
also helpful to narrow the molecular weight distribution,
reducing the a m o u n t of low-molecular-weight fractions,
which have a deleterious effect on resistance and strength
properties, while also minimizing high-molecular-weight
portions, which increase viscosity and cause application
problems such as cobwebbing of spray-applied acrylic automotive lacquers
...

Tg, ~
Polymer

poly(methyl methacrylate)
poly(ethyl methacrylate)
poly(n-propyl methacrylate)
poly(isopropyl methacrylate)
poly(n-butyl methacrylate)
poly(sec-butyl methacrylate)
poly(isobutyl methacrylate)
poly(t-butyl methacrylate)
poly(n-hexyl methacrylate)
po]y(2-ethylbutyl methacrylate)
poly(n-octyl methacrylate)
poly(2-ethylhexyl methacrylate)
poly(n-decyl methacrylate)
poly(lauryl methacrylate)
poly(tetradecyl methacrylate)
poly(hexadecyl methacrylate)
poly(octadecyl methacrylate)
poly(stearyl methacrylate)
poly(cyclohexyl methacrylate)
poly(isobornyl methacrylate)
poly(phenyl methacrylate)
poly(benzyl methacrylate)
poly(ethylthioethyl methacrylate)
poly(3,3,5-trimethylcyclohexylmethacrylate)

41

105
65
35
81
20
60
53
107
- 5
11
- 20
- 10
- 60
- 65
- 72
- 100
104
170(110)
110
54
-20

poly(methyl acrylate)
poly(ethyl acrylate)
poly(propyl acrylate)
poly(isopropyl acrylate)
poly(n-butyl acrylate)
poly(sec-butyl acrylate)
poly(isobutyl acrylate)
poly(t-butyl acrylate)
poly(hexyl acrylate)
(brittle pt)
poly(heptyl acrylate)
poly(2-heptyl acrylate)
poly(2-ethylhexyl acrylate)
poly(2-ethylbutyl acrylate)
poly(dodecyl acrylate)
(brittle pt)
poly(hexadecyl acrylate)
poly(2-ethoxyethyl acrylate)
poly(isobornyl acrylate)
poly(cyclohexyl acrylate)

Tg, ~
6
- 24
-45
- 3
- 55
- 20
- 43
43
-

57
60
38
50
50

- 30
35
- 50
94
16

79

NOTE:The brittle point measured by a fracture test often approximates Tg
...
From Encylopedia of Polymer Science and
Engineering, Vol
...
, New York, Copyright 1985)
...
To ensure good solubility of the polymer,
it is important to match the solubility parameter of the solvent to that of the polymer
...
The solubility
parameter concept was defined by Hildebrand [10] and applied to coatings by Burrell
...

For polymers, the solubility parameter can be calculated by
knowing the molecular structure of the repeating unit according to Small's method
...

Besides being helpful in estimating solubility, the solubility
parameter concept is helpful in predicting the resistance of
polymers to solvents or other organics
...
More hydrophobic acrylics (with low 8 values) will have better
resistance to polar materials, such as water and alcohol
...
For spray application, moderately fast evaporating solvents are needed to avoid running
and sagging of the low-viscosity paint
...
Since thermoplastic acrylics
dry by evaporation of solvent alone, extremely slow-drying
solvents, which retard development of properties, should be
avoided
...
e
...
While many other acrylate
and methacrylate monomers exist, as indicated in Table 2,
these few are the primary acrylic monomers that are commercially available and that are, therefore, the most economically feasible
...
They are hard enough to dry rapidly to a
tack-free state that allows early handling of the coated product and also hard enough to resist marring, print [see ASTM
Test Method for Print Resistance of Lacquers (D 2091)], block
[see ASTM Test Method for Blocking Resistance of Architectural Paints (D 4946)], and dirt pickup
...


Up to the mid-1950s, nitrocellulose lacquers were the
major automotive coating in use
...
This problem was
eliminated in 1956 when General Motors adopted acrylic lacquers based on poly(methyl methacrylate)
...

The acrylic lacquers generally contain external plasticizers,
such as the phthalates, which contribute to improved chip
resistance and cold crack resistance
...
e
...

Since they are very hard and fairly high molecular weight
(i
...
, 100 000), the thermoplastic solution polymers designed
for automotive use are not capable of the excellent molecular
flow that would be expected of softer/lower-molecular-weight
polymers
...
This disadvantage was
shared by the older nitrocellulose lacquers
...
Since these lacquers are thermoplastic, they are permanently subject to softening by strong
solvents, such as toluene or acetone, if for some reason they
would contact the automotive finish
...


Thermosetting Acrylic Resins
Thermosetting acrylic resins are compositionally very similar to the thermoplastic-type acrylics, with the exception that
they contain functional groups, such as carboxyl or hydroxyl,
that are capable of reacting with another polymeric or monomeric multifunctional material to produce a three-dimensional network structure
...
An alternate
route to improved film properties is to use a thermosetting
acrylic polymer, converting linear, moderate-molecularweight polymer chains to an infinite molecular weight structure
...
" To be truly crosslinked, one of the reactive species must have at least two
reactive sites, while the other species has at least three reactive sites per molecule or chain
...


CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS
Over the years, numerous chemical reactions utilizing various functional groups have been devised as a means to crosslink acrylic polymers
...


Acid-Functional Acrylics Cross-Linked with Epoxy
Resins
Possibly the most resistant acrylic enamels are based on
thermosetting acrylic solution polymers that contain acid
functionality and are reacted with an epoxy resin
...


H
I

CH2--C--CH2-\0 /

~----~

~H~

,--,
--O--(z

7

&,--,
x)--C--(/

\

/
\~/

I

/

&

\ ) - - O - - C H 2 - - C H - - C H2

\

/

\/

O

CH~ \ ~ /

The acrylic solution polymer is made acid functional simply by incorporating acrylic or methacrylic acid into the
backbone; when cured under suitable conditions, it reacts
with the epoxide to form ester linkages between the two
polymers:
E P O X Y - - C H - - C H 2 + ACRYLIC--COOH

\/

O
EPOXY--CH--CH2

I
OH

I
OCO--ACRYLIC

An alternative reactant to epoxies based on Bisphenol
A/epichlorohydrin is another acrylic polymer wherein epoxide sites have been incorporated by polymerizing glycidyl
acrylate, glycidyl methacrylate, or allyl glycidyl ether into the
acrylic backbone
...
Also, in recent years, epoxy manufacturers have developed many aliphatic epoxides in an attempt
to mimic the benefits of the workhorse Bisphenol A based
products (i
...
, resistance properties) without the accompanying disadvantages (i
...
, poor resistance to UV light)
...
Some higher solids acrylics are lower in
molecular weight with higher acid content
...

Styrene or vinyltoluene are often incorporated into the
acrylic polymer as "honorary" acrylic monomers because
their reaction kinetics with acrylic monomers are fairly good
and because they often improve alkali resistance and are low
in cost
...
In some instance, melamine-formaldehyde resins are
sufficiently basic to be used as catalyst and have the additional advantage of entering into the cross-linking reaction
[13]
...
Even with catalyst, the baking or stoving requirements
for acid/epoxide systems are fairly high, with 15 to 30 min at
150 to 190~ being typical
...
To
determine the optimum conditions for a given system, it is
best to cure that system at a variety of baking conditions and
then measure properties to determine the temperature and
time required for m a x i m u m performance
...
Acrylic/epoxy
enamels fill the needs of the appliance industry primarily
because of their excellent alkali/detergent resistance [see
ASTM Practice for Detergent Resistance of Organic Finishes
(D 2248-89)], which is critical to the performance of washing
machine coatings
...
Acrylic/epoxy
enamels, on the whole, offer an excellent balance of these
important properties
...

The disadvantages associated with acrylic/epoxy coatings
are usually concerns brought about by the aromatic nature of
the Bisphenol A based epoxy cross-linker
...
If used outside, the aromatic Bisphenol A portion of the epoxy would degrade rapidly, and early chalking would occur
...
Where this i~ a problem, an aliphatic epoxy can
be substituted for at least some of the Bisphenol A based
epoxy to reduce sensitivity to UV radiation (see Chapter 10
entitled "Epoxy Resins in Coatings")
...
2

Pencil hardness

2H

Solvent resistance, glass
Cellosolve acetate, 15 rain
Cellosolve acetate, 60 rain
Xylol, 15 min

6B
6B
5B

Stain resistance, cold-rolled steel
no stain
trace

Mustard, 30 min
Ink, 30 min
Print resistance, cold-rolled steel
82~

light print

30 min, 2 psi

Detergent resistancea
1% detergent, 74~

200 h

Few--6

Optical properties, CRS (two coats)
Original 60~ gloss
Gloss after 16 h at 178~
Original color
Color after 16 hr at 178~

95
...
4
9
...
0

Microknife adhesion, CRS "H" Value

22
...
, 1/4 in
...

Cold-rolled steel
Bonderite 1000

0-0-0
0-0-0

Reverse impact, inch-lbs (Joule)
Cold-rolled steel
Bonderite 1000

22 (2
...
70)

Direct impact, inch-lbs (Joule)
Cold rolled steel
Bonderite 1000

35 + (3
...
65 +)

~ASTMblister rating
...

~ = no cracks; 9 = delamination
...
Likewise, the
methylol e t h e r (when c a p p e d with alcohol) is m o r e reactive
t h a n a conventional dialkyl ether
...
Since the
acid-methylol r e a c t i o n is relatively slow, it allows significant
self-condensation of the a m i n o resin [16]
...

The hydroxyl-functional acrylics are, therefore, favored
over acid-functional p o l y m e r s a n d are m o s t often used in

c o m b i n a t i o n with a m e t h y l o l a t e d o r b u t y l a t e d m e l a m i n e f o r m a l d e h y d e or b e n z o g u a n a m i n e - f o r m a l d e h y d e condensate
...
Hydroxyl functionality is i n c o r p o r a t e d into the
acrylic p o l y m e r b y c o p o l y m e r i z i n g m o n o m e r s such as hydroxyethyl acrylate (HEA) o r hydroxyethyl m e t h a c r y l a t e
(HEMA)
...
Acrylic/amino t h e r m o s e t t i n g e n a m e l s
were, therefore, very successful in replacing the less d u r a b l e
a l k y d / m e l a m i n e systems in a u t o m o t i v e t o p c o a t applications,
a n d general industrial finishing
...
It also offers significantly higher a p p l i c a t i o n
solids
...

A n o t h e r r e a s o n that acrylic/amino resin t e c h n o l o g y bec a m e so p o p u l a r is b e c a u s e of the versatility of the chemistry,
w h e r e b y p r o p e r t i e s can be readily altered b y varying acrylic
Tg, acrylic m o n o m e r s , acrylic functionality level, a n d crosslinker type a n d level
...
Table 4 briefly d e m o n s t r a t e s the kinds
of variation in p e r f o r m a n c e w h i c h can be o b t a i n e d by a few
m a n i p u l a t i o n s in c o m p o s i t i o n [17]
...
Meth31101 or methylol e t h e r groups c a n be a t t a c h e d to an acrylic
backbone, and the resulting p o l y m e r can self-condensate, resulting in a cross-linked structure without the need for a n
external cross-linking agent
...
The p o l y m e r i z a t i o n is
usually a conventional free-radical, solution p o l y m e r i z a t i o n
c a r r i e d out in alcohol or a c o m b i n a t i o n of alcohol and arom a t i c solvent
...

After the p o l y m e r i z a t i o n is complete, the p o l y m e r is t r e a t e d
with f o r m a l d e h y d e to convert it to the methylol amide
...
The conversion p r o c e e d s as follows

[18]:
P O L Y M E R - - C O - - N H 2 + HCHO
POLYMER--CO--NH--CH2OH

)

P O L Y M E R - - C O - - N H - - C H a O H + ROH
P O L Y M E R - - C O - - N H - - C H 2 O R + H20

)

As an alternative process, the AM m o n o m e r can be methylolated before being polymerized
...

Acrylate
Styrene
HEMA
BA
BMA
St
MeSt
EtSt
Viscosity in 1/1 xylenefoutanol
Solids content, % by wt
Cross-linking agent, % by wtb

27
...
5

...
0

...


...
8
30

Methacrylate
Styrene

Methacrylate
Methyl Styrene

Copolymer Composition,
27
...


...


...

22
...
0

...



...


...



...
4
30


...


% by wt
27
...


...

22
...


...


...
0

Methacrylate
Ethyl Styrene
27
...

22
...



...
4
30


...
0
X+
48
...


Film Properties
Color or yellowness factorb
Initial
After 4 h/219~
Gloss (60~ initial
After 4 h/219~
Front impact, in
...
5% Rinso at 74~ blisters
after 4 days of immersion

- 2
...
2
98
84
2
0
7+

- 4
...
9
98
92
0
0
7

- 5
...
6
94
84
0
0
7

- 3
...
1
98
74
0
0
7

10
10
6
9-

10
10
10
9

10
10
10
9

10
10
3
9

~Allcopolymers were <1 in Gardner Holdt color
...
at 149~ Ratings: 10 = best or no failure; 0 = complete failure
...

(Reprinted with permission of the Journalof Coatings Technology
...
)
further h e a t
>
POLYMER--CO--NH--CH2--NH--CO-POLYMER + H20
If the m e t h y l o l a t e d a m i d e has b e e n etherified b y b u t a n o l
capping, i n s t e a d of w a t e r as a by-product, a mixture of b u t a nol a n d dibutyl ether w o u l d be obtained
...
e
...
The strength of the methylol
a m i d e acrylics is that they have no glaring weaknesses a n d
m a k e very g o o d g e n e r a l - p u r p o s e resins
...
The
isocyanate g r o u p ( - - N = C = O) is extremely reactive a n d will
cross-link with any type of functionality having a labile hyd r o g e n atom
...
Acrylic p o l y m e r s
designed to be cross-linked with isocyanate resins generally
contain hydroxyl functionality i n c o r p o r a t e d b y p o l y m e r i z i n g
hydroxyethyl acrylate (HEA) o r hydroxyethyl m e t h a c r y l a t e
(HEMA) into the acrylic backbone
...
The
two-package polyisocyanate/polyhydroxyl coatings m a k e up
"Type V
...
The hydroxyl functional acrylic reacts with isocyanate resin as follows:

ACRYLIC--OH + R--N = C = O
R--NH--COO--ACRYLIC
(a urethane)

>

The preferred isocyanates are usually aliphatic, such as the
a d d u c t of h e x a m e t h y l e n e d i i s o c y a n a t e (HMDI), b e c a u s e of
the p o o r e r color a n d exterior d u r a b i l i t y a s s o c i a t e d with the
a r o m a t i c types of isocyanates
...
A few such catalysts include
triethylene diamine, zinc n a p h t h e n a t e , a n d dibutyl tin-dil a u r a t e [20]
...
Acrylic u r e t h a n e s are, therefore, invaluable in the
t r a n s p o r t a t i o n i n d u s t r y w h e r e high-quality coatings are
n e e d e d for aircraft, r a i l r o a d cars, trucks, buses, etc
...
The acrylic u r e t h a n e s c o m b i n e
the i n h e r e n t UV resistance a n d exterior d u r a b i l i t y of acrylics
with the a m b i e n t cross-linking ability of aliphatic isocyanates
to p r o d u c e hard, tough, chemical-resistant, high-perform a n c e coatings
...
"
Over the years, the hydroxyl-functional acrylics have b e e n
i m p r o v i n g in p e r f o r m a n c e , a n d n o w the e m p h a s i s is on
higher solids content for lower VOCs
...
I n c r e a s i n g

PAINT AND COATING TESTING MANUAL

46

the solids of the acrylic r e d u c e s solvent levels in the form u l a t e d coating
...

An alternative a p p r o a c h to r e d u c e d solvent o r higher solids
is to m o d i f y the acrylic u r e t h a n e with a reactive diluent w h i c h
is fluid a n d acts like a solvent b u t t h e n reacts to form p a r t of
the cross-linked n e t w o r k [21]
...


TABLE 5--Copolymerization of ethyl acrylate, methyl
methacrylate, and methacrylic acid [23]
...
0 g
5
...
0 g
100
...
0 g
4
...
0 g
0
...
15%)
Ammonium persulfate in 5 mL of water
Sodium formaldehyde sulfoxylate in 5 mL of
water
t-butyl hydroperoxide (70%)

Procedure:

R"
\

O/C

I

H2C

/

H

H

\

/

~N--R--N/C

I

R"

~O

I

I

CH2 H2C

+ 2H20

>

CH2
HO

I

H2C

HN--R--NH

I

CH 2

I

H2C

OH

I

+ 2R'CHO

CH 2

This type of functionality has the advantage that it is one
p a c k a g e stable with isocyanates as long as m o i s t u r e is excluded from the paint
...


ACRYLIC EMULSION POLYMERS
An acrylic e m u l s i o n is a t w o - p h a s e system in w h i c h acrylic
p o l y m e r droplets are d i s p e r s e d in an external w a t e r phase,
usually with the a i d of a n emulsifier (i
...
, surfactant)
...
A typical acrylic e m u l s i o n p o l y m e r i z a t i o n recipe is
given in Table 5 [23]
...
However, the viscosity of an
e m u l s i o n is unaffected b y p o l y m e r m o l e c u l a r weight since
solution principles do not p e r t a i n to e m u l s i o n s (the p o l y m e r
is insoluble in the c o n t i n u o u s w a t e r phase)
...

The particle size of an e m u l s i o n is also very i m p o r t a n t in
d e t e r m i n i n g p e r f o r m a n c e a n d m u s t be carefully controlled
...
Particle size does affect e m u l s i o n viscosity, with large
particle size generally being a s s o c i a t e d with low viscosity
...
0 by adding 50% sodium hydroxide
solution
...
The maximum temperature
of 77~ is attained in 12 to 15 min
...
5 with
28% aqueous ammonia, and filter; the gums amounted to 0
...

The free acid (unneutralized) surfactant can also be used as an
emulsifier for the above copolymerization
...

Filtration of the finished emulsion gave only 0
...
The
properties of these emulsions were:
Surfactant Form

Sodium Salt

Free Acid

Solids Content, %--Calculated
--Found
pH at 25~
Viscosity (Brookfield), cP
Particle size (light scattering), %
Minimum film-forming temperature, ~

35
...
3
5
...
9~
22
...
0
35
...
8
10
...
3r
3tY

~Emulsion adjusted to pH 9
...

bMeasurement at 2% solids
...
4% solids
...
However, in recent years, clean air
regulations have further s t r e n g t h e n e d the p o s i t i o n of acrylic
emulsions, usually at the expense of solvent alkyds
...
At the s a m e
time, the p r o p e r t i e s of acrylic e m u l s i o n p o l y m e r s in the industrial coatings m a r k e t has i m p r o v e d so that they n o w offer
p e r f o r m a n c e s i m i l a r to their solvent-borne counterparts
...
Over the last 40 years, this m a r k e t has evolved
from an entirely oil-based m a r k e t to one d o m i n a t e d b y emulsions
...
The
health, safety, a i r quality, a n d o d o r concerns a s s o c i a t e d with
the solvents in oil-based p a i n t s have m o v e d p e o p l e t o w a r d s

CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS
water-based latex paints whenever there is not a large penalty
in performance
...
Thirdly, emulsion polymers, particularly
quality acrylic emulsions, have surpassed oil-based paints for
long-term performance and protection in most applications
...
Therefore, in segments of the architectural coatings market where
performance is not critical, such as for interior flat paints,
acrylic emulsions
...
However, in
segments where performance and durability are essential,
such as for exterior flat and gloss paints, acrylic emulsions
control a very large portion of the market both in the United
States and abroad
...
Interior
flat paints are aimed primarily at broadwall applications
...
The one resistance property demanded in this
m a r k e t is scrub resistance; in this regard, vinyl/acrylics perform satisfactorily
...

Associative thickeners (also referred to as rheology modifiers)
dramatically enhance flow, brushability, film build, and spatter resistance compared to conventional hydroxyethyl cellulose (HEC) thickeners
...

Typically, they are copolymers of a hard methacrylate monomer such as methyl methacrylate with a soft, commercially
available acrylate m o n o m e r such as ethyl acrylate, butyl acrylate, or ethylhexyl acrylate
...
Acrylic emulsions play a major role in
this market, especially at the high-performance end
...

Acrylic emulsions used in gloss and semigloss paints are
copolymers of acrylate and methacrylate monomers and are
usually harder than emulsions used in flat paints
...
Since gloss and semigloss paints are formulated at low

47

pigment volume concentration (PVC) to obtain gloss, the pigment does not contribute significantly to film hardness; it
must all come from the polymer
...
This may be accomplished by copolymerizing adhesion promoting functional monomers into the acrylic polymer backbone
...
The adhesion promoter particularly is needed to improve adhesion and blister resistance
when the paint film becomes wet, as it might in a bathroom
...
To ensure good water resistance, acrylic
emulsions used in this market often contain hydrophobic
monomers such as styrene
...
This level of
outstanding appearance has been possible in recent years
with the introduction of associative thickeners
...
Associative thickeners
loosely bind to the surface of the latex particles through
hydrophobic interactions, forming a network structure
which accounts for their thickening action
...
Smaller particle-size emulsions have greater surface
area and therefore have more interaction with associative
thickeners
...
Consequently, small-particlesize, hydrophobic acrylic emulsions have been designed specifically for use with associative thickeners
...
In fact, before these new
emulsion/thickener systems, truly high-gloss latex paints
were out of the question
...

Overall, the appearance properties of the newer small particle-size hydrophobic acrylics, when used in combination
with associative thickeners, rivals that of oil-based enamels
...
In the past few years, new
composite acrylic emulsions have been introduced, particularly into the interior gloss area, which are made up of two or
more nonhomogeneous phases
...
The goal of this type of polymerization is to incorporate the best characteristics of the
different phases
...
e
...
This is undesirable both

48

PAINT AND COATING TESTING MANUAL

from a cost and an organic emissions perspective
...
This type of polymerization
can also be used to achieve a desired surface chemistry while
not disturbing the bulk composition of the latex particle
...


Exterior Coatings Applications
By far the most challenging application for any coating is
as an exterior paint required to protect a multitude of substrates in diverse and extreme weather conditions
...
One primary reason for their success, as mentioned earlier for solution acrylics, is their lack of absorption
of ultraviolet light coupled with their inherent hydrolysis
resistance
...

One of the toughest demands facing exterior flat house
paints is the need to withstand the freeze-thaw type of expansion and contraction of dimensionally unstable substrates
such as pine or other soft woods
...
When the coalescent leaves, the acrylic paint
film remains pliable and able to withstand substrate swelling
and freezing, unlike oil-based house paints which become
harder and embrittle on exposure as they continue to crosslink
...
Since flat house
paints contain a fairly high pigment content (i
...
, PVC = 40 to
60%), dirt resistance is enhanced by the pigment loading
...

For exterior flat house paints, the inclusion of an effective
adhesion promoter in the acrylic backbone is crucial for good
adhesion
...
Furthermore, the improved adhesion enhances
crack resistance over dimensionally unstable wood substrates
...
The chalk acts like a powdery barrier, preventing the emulsion binder from penetrating to the real
substrate and establishing an adhesive bond
...

For this reason, many exterior grade acrylic emulsions have
been designed at a fairly small particle size of about 100 nm

the flocculating mechanism of HEC
...
In an attempt to combine these
seemingly mutually exclusive properties, particle-size distributions have been carefully controlled to ensure a tailored
mixture of small particles that give good adhesion to chalky
surfaces and large particles that help to improve flow in
formulations thickened with HEC
...

Wide particle-size distribution acrylic emulsions do n o t
significantly address the low film build associated with
smaller particle-size emulsions when thickened with HEC
...
e
...
This was addressed in the 1980s by the Rohm and
Haas Co
...
2 [26]
...
The lobes of
this polymer are about 350 nm, but it has an effective hydrodynamic volume of about 1000 nm and is, therefore, very
effective at imparting high film build in paints thickened with
HEC
...
Since in its commercial form this technology
also contains small particles, good adhesion characteristics
are retained while film build is optimized
...
These properties are made
worse by the catalytic degradation effects of TiO 2 on the
binder, so that high PVC flat paints are generally poorer than
low PVC gloss paints
...

Among the common acrylic copolymer compositions in use
commercially, MMA/BA polymers are better than MMA/EA
polymers, and higher methacrylate containing binders are

[25]
...
2-Scanning electron micrograph of Multilobe | acrylic
particles (courtesy of Rohm and Haas Co,)
...
Of course, the methacrylate/acrylate levels are
generally determined by the Tg required to achieve the desired balance of crack resistance and dirt pickup
...
Acrylic emulsions have been designed that meet all of these challenges and perform very well
in environments as diverse as the tropical regions of Asia and
the Philippines to the Scandinavian regions of Europe
...
Since exterior gloss paints must have
good dirt resistance at low PVC, the acrylic emulsions used in
these paints are harder than those used in flat paints and
generally have a Tg in the area of 20 to 35~ This Tg range can
provide acceptable dirt resistance while still having good
grain-crack resistance
...
e
...

Since brush marks are much more obvious in a gloss paint
than in a flat paint, good flow and levelling is much more
critical for a gloss paint than for a flat paint
...
The flow of these systems could
best be described as fair, but overall they have demonstrated
an admirable balance of properties and are still popular today
...
e
...

Over the past ten years, the decorative as well as protective
capabilities of exterior gloss paints have improved significantly to the point where acrylic emulsions are rapidly replacing alkyd enamels as the preferred coating for exterior trim
...
Maintenance coatings are generally used to protect
metal surfaces such as bridges, storage tanks, and other industrial facilities, often in harsh chemical and corrosive environments
...
They are similar in hardness and composition
to exterior flat house paint binders with the exception that
they are formulated with reactive pigments and additives
which help prevent rusting
...

These acrylic latex maintenance coatings had the usual
advantages in application of water-based paints over solvent

49

alkyd paints along with expected advantages in chalk resistance, color retention, and other decorative qualities
...
This is partly because the alkyds continue to harden
and eventually crack, leaving the substrate exposed and subject to the elements
...

The one weakness of the acrylic latexes in the area of
maintenance coatings was their low Tg, which reflected a
lower hardness than the alkyd paints
...
These hard latex binders used in interior and exterior gloss paints were fine tuned to maximize corrosion resistance and overall protection
...
The small particle
size of these binders fits perfectly with associative thickener
technology to give tight water-resistant films, which are an
improvement over HEC-thickened paints that can have microscopic defects as a result of the flocculating mechanism of
HEC [27]
...
This is particularly true in
California, which has historically been at the forefront of
clean air legislation
...
Acrylic producers
have responded to the needs of these coatings users by developing more sophisticated emulsions capable of meeting the
demanding performance requirements of many different end
users
...
Also, high
gloss was not possible with these older emulsions
...
Without this evolution in
performance, it is unlikely that industrial coatings users
would switch to latex coatings even with the more severe
emissions regulations
...
The typical Tg range for such
acrylics is about 30 to 70~ The film formation problems
usually associated with such hard emulsions are somewhat
alleviated by the controlled application conditions in the factory, so that low-temperature film formation is generally not
required
...

N e w e r heterogeneous acrylic e m u l s i o n s c o m p r i s e d of two
o r m o r e phases have recently b e c o m e i m p o r t a n t in the industrial coatings industry
...
These p r o p e r t i e s allow the m a n u f a c t u r e r to stack, pack,
a n d ship coated parts m o r e quickly [28]
...
The
m a j o r deficiency of acrylic emulsions in these areas is the
"warmth" of w a t e r - b a s e d coatings c o m p a r e d to solvent-based
materials
...


linked, the infinite m o l e c u l a r weight provides for solvent a n d
c h e m i c a l resistance, along with h a r d n e s s a n d toughness
...

F o r m a n y years, the c o n s t r u c t i o n i n d u s t r y has relied on
hydroxyl functional acrylic e m u l s i o n s r e a c t e d with u r e a o r
m e l a m i n e to coat p r o d u c t s such as h a r d b o a r d , w o o d panels,
shingles, a n d m e t a l coil
...
I n coil
coating applications, the t h e r m o s e t t i n g acrylics offer high
gloss, excellent durability, g o o d c o r r o s i o n protection, as well
as g o o d roll coatability
...
Usually these systems are catalyzed with an a c i d
catalyst to achieve the fastest/lowest t e m p e r a t u r e cure
...

A recent d e v e l o p m e n t in cross-linking acrylic e m u l s i o n
technology is an epoxy cross-linking, a m b i e n t cure system
w h i c h has m a n y a p p l i c a t i o n s b u t has been f o u n d to be particularly useful in m a i n t e n a n c e coatings
...
This n e w
acrylic/epoxy system is c o m p a r e d to an e p o x y / p o l y a m i d e

Thermosetting Emulsions
Just as is the case with solution acrylics, functional groups
can be i n c o r p o r a t e d into the p o l y m e r b a c k b o n e of an acrylic
e m u l s i o n so that it can react with a n o t h e r functional m a t e r i a l
after a p p l i c a t i o n to the substrate, f o r m i n g a cross-linked
polymer
...
Acid functional acrylic e m u l s i o n s can be cross-linked
with emulsified epoxy resins
...
After the e m u l s i o n is cross-

TABLE 6--Properties for an aqueous acrylic/melamine coil coatings enamel over aluminum
and galvanized steel [29]
...
-lbs
Reverse impact, in
...
mandrel bend
Blistering~
White rust
Flat
Blistering~

0
0
...
0
65/89
Very good
9

Mini-Spangle
Galvanized Steel~
0
...
8

-/80 b

Good
9

H

H

B
2-3T
20 to 25
15
Excellent
Excellent
200
Pass

B
3T
35
10
Excellent
Excellent
200
Pass

0
None
1/16 in
...

Mod-Dense, No
...
8


...


4/16 in
...
2, No
...


...

bG1ossdependent on smoothness of substrate
...
Blister density is rated as few, moderate, or dense
...


CHAPTER 6--ACRYLIC POLYMERS AS COATINGS BINDERS

51

TABLE 7--Resistance characteristics of acrylic/epoxy water-borne coatings versus
conventional controls [30]
...

MEK
Toluene
Butyl acetate
Gasoline
Butyl cetlosolve

Solvent Resistance
>300

Mod
...
soft
Lt
...
soft
Lt
...
soft
Mod
...
soft
Lt
...
soft

Lifted
Lt
...
soft
Mod
...
stain
OK
Lt
...
stain
OK
Hvy
...
= light; Mod
...
= medium; Hvy
...

(Reprinted with permission of the American Paint and Coatings Journal
...
)

coating and an alkyd coating in Table 7 [30]
...
No less a key feature is its very good co r r o si o n
p e r f o r m a n c e [31]
...
20
...
R
...
, Chicago, IL,

Industry, Rohm and Haas Company 1909-1959,
Press, R
...
Donnely & Sons Co
...
21
...
H
...
, "Fundamentals of Acrylic Polymers," Paint
and Varnish Production, July 1973
...
G
...
1, 1956,
p
...

[5] Fox, T
...
, Jr
...
J
...
21,
1950, p
...

[6] Rogers, S
...
, Journal of Physical Chemistry,
Vol
...
985
...
and Boyer, R
...
, Journal of Chemical Physics, Vol
...
5, t 962, p
...

[8] Kine, B
...
and Novak, R
...
, "Acrylic and Methacrylic Ester
Polymers," Encyclopedia of Polymer Science and Engineering,
2nd ed
...
F
...
M
...
G
...
Menges,
and J
...
Krosckwitz, Eds,, Vol
...


[9] Rodriguez, F
...
, New York, 1970, p
...

[10] Hildebrand and Scott, The Solubility of Non-Electrolytes, 3rd ed
...
, New York, 1949, pp
...

[11] Burrell, H
...
27, No
...
726
...
A
...
3, 1953, p
...

[13] Solomon, D
...
, The Chemistry of Organic Film Formers, Robert
E
...
, Huntington, NY, 1977, p
...

[14] "Acryloid Thermosetting Acrylic Resins," revised October 1966,
Rohm and Haas Company promotional literature C-170, Spring
House, PA
...
H
...
Krieger Publishing Co
...
277-281
...
and Lestienne, F
...
, Journal of Applied Polymer Science, Vol
...
475
...
C
...
, and Cadwell, L
...
, "Acrylic
Coatings Cross-linked with Amino Resins, Symposium on Thermosetting Acrylic Resins," Official Digest, Vol
...
729
...
H
...
Krieger Publishing Co
...
283
...
M
...
P
...
33, p
...

[20] North, A
...
, Journal of Paint Technology, Vol
...
557, 1971,
p
...

[21] Watson, D
...
and Schall, D
...
, American Paint and Coatings
Journal, 19 Aug
...
58
...
C
...
,
Spring House, PA, 1991
...
, Spring House, PA
...
S
...


52

PAINT AND COATING TESTING MANUAL

[25] Harren, R
...
, Organic Coatings: Their Origin and Development,
R
...
Seymour and H
...
Mark, Eds
...
, Inc
...
297
...

[27] Harren, R
...
, Organic Coatings: Their Origin and Development,
R
...
Seymour and H
...
Mark, Eds
...
, Inc
...
309
...
, Modern Paint and Coatings, November 1991, p
...


[29] Rohm and Haas Co
...
2
...
, American Paint & Coatings Journal, 20 Jan
...
43
...
J
...
9697

---