Natrosol B hydroxyethylcellulose (HEC)
™
Superior Viscosity Stability for Latex Paints
With good chemistry great things happen.™
Natrosol™ B HEC
Superior Viscosity Stability for Latex Paint
Natrosol B HEC —
Superior Viscosity Stability
For Latex Paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Viscosity Stability Of Latex Paints . . . . . . . . . . . . . . . . . . . . . . . . 3
Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 1 Structure of Natrosol HEC —
Typical Substitution Pattern of
Regular-Grade Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2 Structure of Natrosol B HEC —
Typical Substitution Pattern for
Bioresistant Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Viscosity Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Table I Typical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Applications Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Viscosity Stability Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table II Formulation of a Low-Cost Interior
Flat Tint Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Table III Comparative Enzyme Resistance of
Natrosol 250 HBR HEC and
Competitive Thickeners in a LowCost Interior Flat Tint Base . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 3 Comparative Enzyme Resistance of
Natrosol 250 HBR HEC and Competitive
Thickeners in a Low-Cost
Interior Flat Tint Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Table IV Formulation of a Tintable
Eggshell Enamel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2
Natrosol HEC Superior Viscosity Stability for Latex Paints
Table V Comparative Enzyme Resistance of
Natrosol 250 MBR HEC and
Competitive Thickeners in a
Tintable Eggshell Enamel . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 4 Comparative Enzyme Resistance of
Natrosol 250 MBR HEC and
Competitive Thickeners in a
Tintable Eggshell Enamel . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Paint Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Vinyl-Acrylic Flat Paint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Table VI Comparative Performance of
Natrosol 250 HBR HEC and
Competitive Thickeners in a LowCost Interior Flat Tint Base . . . . . . . . . . . . . . . . . . . . . . . . . 11
100% Acrylic Eggshell Enamel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table VII Comparative Performance of
Natrosol 250 MBR HEC and
Competitive Thickeners in a
Tintable Eggshell Enamel . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Recommended Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Product Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix — Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Natrosol B HEC
Superior Viscosity Stability for Latex Paint
Natrosol B HEC is one of Ashland Inc.’s recommended products for
latex paint thickening. It not only provides superior resistance to
biodegradation, but also provides the excellent balance of ease of
incorporation, thickener efficiency, color acceptance, open time,
and in-can stability that regular grades of Natrosol impart. The
data in this booklet demonstrates the excellent viscosity stability
that Natrosol B HEC maintains in latex paints that have been
contaminated with cellulolytic enzymes. In addition, this booklet
acquaints the reader with the available types, the chemistry, and
the typical physical properties of Natrosol B HEC.
Viscosity Stability of
Latex Paints
Water is the continuous phase in latex paints. In addition, many
of the components of a latex paint are manufactured in an
aqueous environment. Whenever water is present, microbial
growth may occur. The growth of these microorganisms
is accompanied by the production of enzymes, including
cellulolytic types. Cellulase enzymes break the bonds between
adjacent unsubstituted anhydroglucose units in the cellulose
thickener backbone. This reduces the molecular weight of
the polymer and it is no longer an effective thickening agent.
The in-can preservatives that are added to a latex paint will
kill the microorganisms that may be present, but there are
no preservatives, either mercurial or nonmercurial, that will
denature the enzymes and render them inactive.
Natrosol B HEC was introduced in the mid-1970s to the
marketplace. This was the first bioresistant cellulosic thickener.
Its use virtually eliminated the problem of viscosity loss in
contaminated paints.
Natrosol HEC Superior Viscosity Stability for Latex Paints3
Chemistry
HEC is manufactured by reacting ethylene oxide with the reactive
hydroxyls of the anhydroglucose units that compose the cellulose
chain. Ethylene oxide will also react with the previously substituted
hydroxyls, forming a side chain. The relative reactivity ratio of
hydroxyls Cx:C6:C2:C3 is 10:10:3:1—that is, ethylene oxide prefers
to react at primary, rather than secondary, hydroxyl groups. The
polymer shown in Figure 1 demonstrates the typical substitution
pattern that results. Such a polymer is susceptible to bond cleavage
by cellulase enzymes.
Figure 1
Structure of Natrosol HEC — Typical Substitution Pattern of
Regular Grade Polymers
By controlling reaction conditions, the relative reactivity ratios of the
hydroxyl groups can be altered so that a more even substitution
pattern, illustrated by the polymer in Figure 2, is achieved. This
polymer will be much less likely to suffer bond cleavage by
enzymes, since fewer β 1-4 bonds are exposed. Natrosol™ B HEC is
manufactured by this controlled process.
Figure 2
Structure of Natrosol B HEC — Typical Substitution Pattern for
Bioresistant Polymers
4
Natrosol HEC Superior Viscosity Stability for Latex Paints
Viscosity Types
The Natrosol B HEC grade is available in viscosity types
widely used in the latex paint industry. These are:
Brookfield Viscosity at 25°C, cps
Viscosity Type
1%
2%
Natrosol 250 H4BR HEC
2,600-3,300
—
Natrosol 250 HBR HEC
1,500-2,500
—
Natrosol 250 MHBR HEC
1,800-1,500
—
Natrosol 250 MBR HEC
—
4,500-6,500
Note that the B product designations all include an R. This
nomenclature refers to the glyoxal surface treatment that is applied
to all Natrosol B HEC products. The surface treatment renders the
material temporarily insoluble so that it can be easily dispersed
in neutral or acidic aqueous media. After a typical hydration time
delay of 4 to 25 min, the Natrosol begins to dissolve and develop
viscosity. More information is available in Ashland booklet 250-11
“Natrosol HEC: Physical and Chemical Properties,” and in technical
data sheet VC-515, “Techniques for Dispersion
and Dissolution.”
Properties
Table I — Typical Properties
Natrosol B HEC is a nonionic polymer that dissolves readily in hot
and cold water. Table I shows the typical properties of Natrosol B
HEC powder and solutions.
Form
white to off-white,
free-flowing powder
Moisture content, as packed, max, %
5.0
Ash, as Na2SO4, max, %
5.5
Bulk density, g/mL
0.6
Density, lb/gal
11.5
Denisty, g/mL
1.38
Bulking value, gal/lb
Particle size on U.S. 40, max, %
pH, 1% water solution
Hydration time (pH 8.0), min
0.087
10
6.0-8.5
4-25
Natrosol HEC Superior Viscosity Stability for Latex Paints5
Application Testing
Viscosity Stability Study
Five cellulosic thickeners representing major commercial types were
used in the study. These were Natrosol™ HEC, both bioresistant and
regular grades; Methocel hydroxypropyl methylcellulose, Cellosize
HEC, and Bermocoll ethyl HEC. High-viscosity thickeners were used
in the vinyl-acrylic flat paint that was tested, and medium-viscosity
thickeners were used in the acrylic eggshell paint. Formulations for
these paints are shown in Tables II and IV, respectively.
The stability tests were conducted by preparing identical paint
samples with the thickener as the sole variable. Each paint was then
inoculated with a fixed weight of a fungal cellulolytic enzyme and
the paint viscosity monitored as a function of time. A comparison of
enzyme resistance, as measured in percent viscosity retained versus
time, appears in Figures 3 and 4 and Tables III and V. A discussion of
the test procedure used can be found in the Appendix.
To simulate a severe enzyme contamination problem, a relatively
high level of enzyme was used—1.0 ppm. In other studies, the level
has been as low as 0.1 ppm. The enzyme concentration will directly
affect the rate of viscosity loss, but not the final (degraded) viscosity
of the paint. Enzymatic degradation of the thickener is evidenced
by a relatively rapid loss in viscosity during the early part of the
testing period. This is followed by a gradual reduction in the rate
of viscosity loss and, finally, a leveling of viscosity at some reduced
value. Once the viscosity has leveled off, it will not drop further with
time. The use of a high enzyme concentration (1.0 ppm) enabled
us to record the rate of viscosity loss and, more importantly, the
final viscosity within a 28-day test period. Similar tests using lower
enzyme levels (0.1 ppm) have been run over a 6-month period,
showing the same ultimate degree of viscosity loss, but at a
considerably slower rate.
Table II — Formulation of a Low-Cost Interior Flat Tint Base
Premix on high-speed disperser at 2,000 rpm for 10 min:
Water
KTPP (FMC Corp.)
Ross & Rowe* 551 (Ross & Rowe, Inc.)
Tamol* 731 (Rohm and Haas Co.)
Nopco NXZ (Henkel Corporation)
Ethylene glycol
Carbitol acetate (Union Carbide Corp.)
Thickener (2.5% solution)(a)
Add and grind at 3,500 rpm for 15 min:
Ti-Pure* R-901 (E. I du Pont de Nemours and Co.)
Camel Carb* (Genstar Stone Products Co.)
Iceburg clay (Burgess Pigment Co.)
1160 Silica (Unimin Specialty Minerals)
Let down (low speed on an air stirrer):
Tergitol NP-10 (Union Carbide Corp.)
Vinyl-acrylic latex, 55%
Merbac 35 (Calgon Corp.)
Water
Thickener (2.5% solution)
NH4OH, 28% ammonium hydroxide`
Total
Tint at 2 oz/gal
Physical Properties
PVC, %
Solids, wt%
Solids, volume %
Density, lb/gal (kg/L)
Stormer viscosity (overnight), KU
pH
See Table III and Figure 3 for types.
(a)
6
Natrosol HEC Superior Viscosity Stability for Latex Paints
lb/100 gal
g/L
200.0
2.0
2.0
5.0
2.0
20.0
10.0
101.0
240.4
2.4
2.4
6.0
2.4
24.0
12.0
121.4
175.0
150.0
125.0
25.0
210.3
180.3
150.3
30.0
3.0
200.0
0.5
40.0
110.0
1.0
1,171.5
3.6
240.4
.6
48.1
132.2
1.2
1,408.0
62.7
49.9
30.3
11.75 (1.41)
88±3
8.0-8.5
Table III — Comparative Enzyme Resistance of Natrosol™ 250 HBR HEC and Competitive Thickeners
in a Low-Cost Interior Flat Tint Base
Enzyme Concentration: 1.0 ppm Cellulolytic Enzyme on Total Paint
Concentration
Thickener
lb/100 gal
g/L
Stormer Viscosity (KU) vs Time (days)
Paint
pH
Initial
14
28
42
168
Viscosity Retained
After 168 Days, %
Natrosol 250 HBR HEC
5.3
6.4
8.2
93
88
88
88
88
95
Natrosol 250 HR HEC
5.0
6.0
8.2
95
70
70
70
68
72
Methocel J12MS(a)
5.3
6.4
8.4
90
79
79
79
79
88
5.0
6.0
8.2
91
65
64
64
62
68
5.3
6.4
—
98
68
67
67
68
69
Cellosize QP30,000
Bermocoll E431FQ
(b)
(c)
Dow Chemical Co.
Dow Chemical Co.
(c)
Akzo Nobel Ind.
(a)
(b)
Figure 3
COMPARATIVE ENZYME RESISTANCE OF NATROSOL 250 HBR HEC AND COMPETITIVE THICKENERS IN A LOW-COST INTERIOR FLAT TINT BASE
Natrosol HEC Superior Viscosity Stability for Latex Paints7
Table IV — Formulation of a Tintable Eggshell Enamel
Premix on high-speed disperser for 10 min:
Lb/100 gal
g/L
Propylene glycol
51.8
62.1
Water
50.0
59.9
Thickener(a)
1.5
1.8
Tamol* 731 (Dow Chemical Co.)
7.0
8.4
Nopco NDW (Henkel Corporation)
2.0
2.4
250.0
299.5
75.0
89.8
83.0
99.4
Add and grind at 2,000 rpm for 20 min:
Ti-Pure* R-902 (E.I. du Pont de Nemours and Co.)
Imsil* A-25 (Unimin Specialty Minerals)
Let down (low speed on an air stirrer):
Water
Nopco NDW
100% acrylic latex, 46.5%
Texanol* (Eastman Chemical Co.)
premix
Propylene glycol
Water
premix
Dowicil* 75 (Dow Chemical Co.)
Triton* N-57
Dow Chemical Co.
Triton X-114
4.8
539.1
12.6
15.1
17.3
20.7
24.9
29.8
1.3
1.6
4.0
4.8
4.0
4.8
2.0
2.4
60.1
72.0
1,100.5
1,318.4
Triton X-207
Thickener (2.5% solution)
Total
4.0
450.0
Tint with Cal/Ink 8800 series colorants at 4 oz/gal
Physical Properties
PVC, %
33.2
Solids, wt%
48.5
Solids, volume %
32.3
Density, lb/gal (kg/L)
11.0 (1.32)
Initial pH
8.2-8.5
Stormer viscosity (overnight), KU
92-96
See Table V and Figure 4 for types.
(a)
Table V — Comparative Enzyme Resistance of Natrosol™ 250 MBR HEC and Competitive Thickeners in a Tintable Eggshell Enamel
Enzyme Concentration: 1.0 ppm Cellulolytic Enzyme on Total Paint
Concentration
Thickener
Stormer Viscosity (KU) vs Time (days)
lb/100 gal
g/L
Natrosol 250 MBR HEC
3.0
3.6
Natrosol 250 MR HEC
3.0
3.6
Methocel J5MS
3.0
3.6
Cellosize QP4400
3.0
3.6
8
Paint
pH
Initial
14
28
42
168
Viscosity Retained
After 168 Days, %
8.4
95
89
88
88
88
93
8.5
96
75
74
74
72
75
8.5
92
82
80
80
80
87
8.5
95
74
72
72
72
76
Natrosol HEC Superior Viscosity Stability for Latex Paints
Figure 4
Comparative Enzyme Resistance of Natrosol™ 250 MBR HEC and Competitive Thickeners in a Tintable Eggshell Enamel
The results of the viscosity stability studies showed that typical latex
paints thickened with Natrosol B HEC retained an average of 94%
of their initial viscosity. At the same time, paints with competitive
thickeners lost significant viscosity and did not have sufficient body
for application.
The Methocel J5MS and J12MS thickeners used in these studies
did show better performance than the Cellosize or Bermocoll
thickeners. Chemically, Methocel J types are cellulose ethers with
a high substitution level of hydroxypropyl and methyl groups.
This high degree of substitution does offer some protection
against enzyme attack. To demonstrate that the superior enzyme
resistance of Natrosol B HEC is independent of molecular weight
grade or enzyme level employed, a second study was performed
using higher molecular weight grades of both thickeners. The
vinylacrylic paint referred to in Table II was thickened with either
Natrosol B HEC or Methocel grades of similar molecular weight,
as determined by comparison of their Brookfield viscosities.
Paints were inoculated with either 0.1 or 1.0 ppm of cellulolytic
enzyme. Table VI and Figures 5 through 8 clearly show that
paints thickened with each of the samples of Natrosol B HEC
retained more viscosity over the 28-day time period than did
the corresponding paints thickened with Methocel. A high level
of substitution is not sufficient to prevent enzyme degradation;
the substituents must be placed along the cellulose chain in a
uniform manner, as is the case with Natrosol B HEC.
Natrosol HEC Superior Viscosity Stability for Latex Paints9
Paint Evaluation
Key paint properties were thoroughly tested in both flat and
eggshell paint systems to characterize fully the possible influence
of each thickener. With so many paint properties involved, it
is important that thickeners of approximately equal molecular
weights be used to obtain truly comparative results.
The results of the following evaluations indicate that typical
vinyl-acrylic and 100% acrylic paints thickened with Natrosol™ B
HEC possess key properties that are fully comparable to those of
the same formulations prepared with competitive commercial
cellulosic thickeners.
Vinyl-Acrylic Flat Paint
In the vinyl-acrylic flat paint, high viscosity (1% solution viscosity
= 1,500 to 2,500 cps, 1.5 to 2.5 mPa•s) thickeners were utilized.
The comparative results for a number of paint properties are listed
in Table VII. In general, all five thickeners produced paints with
similar properties.
The leveling and sag values were virtually identical. There was a 10%
to 15% variation in brush drag, as expected, owing largely to slight
differences in the molecular weight and usage level of the thickeners.
Natrosol B HEC gave slightly better color development with several
of the colorants studied. The colorants chosen were based on their
reputation for promoting color compatibility problems.
All of the thickeners produced paints with similar properties. Sag
and brush drag were essentially equivalent. Natrosol B HEC gave
slightly better color development with most of the colorants tested.
Hiding, gloss, scrubbability, wet-burnish resistance, and stain
removal were similar for all five thickeners.
Recommended Reading
Natrosol HEC and other cellulosic thickeners influence the rheology,
or flow, of latex paints. While beyond the scope of this booklet, the
roles played by Natrosol and interactions of Natrosol with other
latex paint ingredients in modifying rheology are understood.
“Natrosol Controls the Flow of Latex Paint,” Ashland booklet 250-12,
is an excellent introduction to this area and is available by request.
This complete Natrosol HEC product line is composed of many
viscosity types. For more information on the physical and
chemical properties of Natrosol HEC products, read “Natrosol
Hydroxyethylcellulose: Physical and Chemical Properties,” Ashland
booklet 250-11.
Natrosol Plus hydrophobically modified hydroxyethylcelullose
(HMHEC) is a bioresistant associative cellulosic polymer. For
more information on physical and chemical properties, as well
as recommended formulating techniques, read “Natrosol Plus:
Modified Hydroxyethylcellulose,” Ashland booklet 250-18.
All of the thickeners imparted essentially equivalent heat stability,
hiding, sheen, scrubbability, wet-burnish resistance, and ease of
stain removal.
Product Safety
100% Acrylic Eggshell Enamel
In the acrylic eggshell enamel, medium-viscosity (2% solution
viscosity = 4,500 to 6,500 cps, 4.5 to 6.5 mPa•s) thickeners were
tested. The comparative results for a number of paint properties are
listed in Table VIII.
CASRN: 9004-62-0
CAS Name: Cellulose, hydroxyethyl ether
10
Natrosol HEC Superior Viscosity Stability for Latex Paints
Read and understand the Safety Data Sheet (SDS) for Natrosol
before using this product.
Table VI — Comparative Performance of Natrosol™ 250 HBR HEC and Competitive Thickeners in a Low-Cost
Interior Flat Tint Base
Property
Enzyme Resistance (Biostability)
Viscosity retained after 168 days
at 1.0 ppm enzyme on total paint, %
Heat Stability
Stormer viscosity, overnight
Stormer viscosity, 2 weeks at 122°F (50°C)
pH, initial
pH, 2 weeks at 122°F
Leveling
Leneta blade(a)
Brushout rating(b)
Rollout rating(b)
Sag Resistance
Leneta Anti-Sag Index(c)
Applied Viscosity (poises)
12,000 sec-1
Color Development (2 oz/gal)(b)
Phthalocyanine blue
Perylene red
Quinacridone red
Carbazole violet
Lampblack
Hiding Power
Contrast ratio at 3 mils (75 μm) wet (untinted)
Gloss, 60° (Untinted)
Sheen, 85° (Untinted)
Scrubbability
Cycles to failure, average
Wet-Burnish Resistance
Increase in 85° gloss
Stain Removal, %
No. 2 pencil
China marker
K&N stain
Ballpoint pen
Lipstick
Natrosol
250 HBR HEC
Natrosol
250 HR HEC
Methocel
J12MS
Cellosize
QP30,000
Bermocoll
E431FQ
95
72
88
68
69
93
98
8.2
8.0
95
100
8.2
7.9
90
94
8.4
8.1
91
96
8.2
8.0
98
99
8.2
8.0
6
3
3
7
3
2-3
6
3
3
6
2-3
2-3
6
—
—
7
8
8
7
7
0.84
0.81
0.91
0.94
0.85
3
3
3
3
3
3
3
3
2-3
3-4
3
4
4
2-3
4
3
3
3
3
3
3
—
—
—
4
0.996
3
8
0.996
2
8
0.992
2
8
0.995
2
8
—
3
—
74
71
78
58
—
5.2
4.9
3.6
4.0
—
100
100
60
80
60
100
100
60
80
60
100
100
60
80
60
100
100
60
80
60
—
—
—
—
—
0—very poor leveling; 10—excellent leveling.
For these subjective evaluations, the paint containing Natrosol 250 HBR HEC was rated 3 and used as a control. Rating is 1—best; 5—poorest.
(c)
3—extreme sag; 12—no sag.
(a)
(b)
Natrosol HEC Superior Viscosity Stability for Latex Paints11
Table VII — Comparative Performance of Natrosol™ 250 MBR HEC and Competitive Thickeners
in a Tintable Eggshell Enamel
Property
Enzyme Resistance (Biostability)
Viscosity retained after 168 days
at 1.0 ppm enzyme on total paint, %
Heat Stability
Stormer viscosity, overnight
Stormer viscosity, 2 weeks at 122°F (50°C)
pH, initial
pH, 2 weeks at 122°F
Leveling
Leneta blade(a)
Brushout rating(b)
Sag Resistance
Leneta Anti-Sag Index(c)
Applied Viscosity (poises)
12,000 sec-1
Color Development (2 oz/gal)(b)
Phthalocyanine blue
Perylene red
Quinacridone red
Carbazole violet
Hiding Power
Contrast ratio at 3 mils (75 μm) wet (untinted)
Gloss, 60° (Untinted)
Sheen, 85° (Untinted)
Scrubbability
Cycles to failure, average
Wet-Burnish Resistance
Increase in 85° gloss
Stain Removal, %
No. 2 pencil
China marker
K&N stain
Ballpoint pen
Lipstick
Natrosol
250 MBR HEC
Natrosol
250 MR HEC
Methocel
J12MS
Cellosize
QP4400
93
75
87
76
95
101
8.4
8.1
96
99
8.5
8.0
92
96
8.5
8.0
95
99
8.4
8.0
5
3
7
4
6
3
5
2
12
11.8
11.8
12
1.10
1.05
1.05
1.07
3
3
3
3
3
3
4
2-3
3
3-4
3
2-3
3
3
4
1
0.991
16
32
0.991
14
33
0.990
15
33
0.990
14
32
690
671
701
679
3.9
6.4
4.2
2.1
100
100
60
80
60
100
100
60
80
60
100
100
60
80
60
100
100
60
80
60
0—very poor leveling; 10—excellent leveling.
For these subjective evaluations, the paint containing Natrosol 250 HBR HEC was rated 3 and used as a control. Rating is 1—best; 5—poorest.
(c)
3—extreme sag; 12—no sag.
(a)
(b)
12
Natrosol HEC Superior Viscosity Stability for Latex Paints
Appendix
Test Methods
Enzyme Resistance
Introduction
Latex paints containing cellulosic thickeners will lose viscosity
during storage if the paint has been contaminated with cellulolytic
enzymes. These enzymes are extracellular proteins produced by
certain species of bacteria, fungi, and yeasts that catalytically cause
the polymer chains of the thickener to be cleaved at adjacent
positions of unsubstitution. The resulting viscosity loss is due largely
to the thickener’s inability to continue to efficiently bind the water
in the vehicle because of its greatly reduced chain length.
Although the in-can preservative in the formulation may act to
“sterilize” the paint within 4 to 48 hours after incorporation, there
are no known preservatives, mercurial or nomercurial, currently
on the market that, at their normal usage levels, can denature
cellulolytic enzymes or permanently render them inactive.
While the concentration of enzymes produced by the
microorganisms during storage prior to sterilization may or may
not be significant, most of the problem of enzyme contamination
stems from the use of “spoiled” raw materials such as a latex,
thickener stock solution, aqueous colorant, or pigment slurry.
Indirectly, of course, enzymes may ultimately be introduced into the
paint through poor in-plant housekeeping when tanks, valves, lines,
and hoses are not regularly disinfected(1) to prevent a buildup of
actual enzyme-producing agents.
Thus, it is essential that the preservative be incorporated into the
formula as soon as possible, and that sensitive raw materials, such
as the thickener stock solution, be used as soon as possible or be
preserved to prevent the production of enzymes through microbial
contamination. Paint raw materials can be routinely screened for the
presence of cellulolytic enzymes as described in Ashland Bulletin
VC-492, “Test Procedure for Detecting Enzymes in Latex Paint.”
Disinfection can be achieved through steam or other environmentally acceptable means.
Cellulolytic enzymes are essentially inactive at pH 9.5 and above. The paint pH should
preferably be in the range of 7 to 9. Paints based on 100% acrylic latices often have a pH
above 9, partly because of the free ammonia in the latex.
(3)
If thickeners of noticeably different DPs or aqueous viscosities are to be tested, it is essential that the same enzyme weight:DP ratio be maintained to avoid favoring the lower
DP thickener.
(4)
Cellulase enzyme C1424, Sigma Chemical Co., P.O. Box 14508, St. Louis, MO 63178.
(1)
(2)
Among the factors that affect the rate of thickener degradation in
the paint are:
• Enzyme concentration
• Enzyme activity as influenced by paint pH and temperature
• The specific paint formulation utilized
• Concentration of thickener
• The molecular structure of the thickener
Degree of polymerization (DP), or chain length
Degree of substitution (DS)
Uniformity of substitution
Substituent size
Substituent charge
It is because of the many variables involved, especially with
respect to the thickener, that this method has been developed.
It is designed to act as a semiquantitative guide in evaluating the
relative enzyme resistance of cellulosic thickeners in latex paints as
they are affected by the structural factors just described, especially
the first three.
Summary of Method
Samples of fixed weight of the same latex paint having a pH lower
than 9.5(2) and thickened with the series of cellulose derivatives
are each inoculated with a fixed amount of an aqueous enzyme
solution of known concentration. The paint viscosities are then
measured daily, or at least weekly, with a Stormer viscometer in
accordance with ASTM Method D 562 over a period of 6 months.
The test may be terminated whenever one of the paints loses 20%
of its original viscosity, as measured prior to inoculation.
Apparatus
1. A series of the same latex paint of any type (flat, semigloss,
or high-gloss, interior or exterior) having a pH lower than
9.5, stable on storage, and containing a series of cellulosic
thickeners of approximately the same DP or viscosity in
water(3)
2. Distilled water
3. Standardized source of cellulolytic enzyme(4)
4. Half-pint, lined, friction-top cans with lids or, preferably, 8-oz
wide-mouth jars with caps
5. Three 100-mL volumetric flasks
6. Stormer viscometer conforming to ASTM Method D 562
7. Electric or air-driven stirrer
8. Constant-temperature bath
9. Balance capable of weighing to a sensitivity of ±0.0001 g.
10.Balance capable of weighing to a sensitivity of ± 1 g
11.10-mL transfer pipette
12.1-mL transfer pipette
Natrosol HEC Superior Viscosity Stability for Latex Paints13
Reagents and Solutions
Standardized Enzyme Solution(1) — Weigh on an analytical
balance 0.2500 g of enzyme and transfer it quantitatively to a
100-mL volumetric flask by washing with distilled water. Half-fill
the flask with distilled water, stopper, and agitate it gently to
avoid unnecessary foam until the enzyme dissolves completely
(a slight amount of insoluble matter may remain). Add distilled
water to the mark of the flask and again agitate the flask gently
several times to mix the contents. Using a 10-mL transfer pipette,
withdraw 10 mL of stock enzyme solution (which contains 2,500
ppm enzyme) and transfer it to a second 100-mL volumetric flask.
Add distilled water to the mark of the second flask, stopper, and
agitate it gently to mix. This flask now contains 250 ppm enzyme.
Repeat this step, using a third volumetric flask and a clean 10-mL
pipette, by withdrawing a 10-mL aliquot from the second (250ppm) flask. Add distilled water to the mark of the third and final
flask and mix as before. This flask now contains 25 ppm enzyme
and is to be used to inoculate the test paints.
Procedure
1. Measure, and record as the initial reading, the equilibrated
viscosity of each paint with the Stormer viscometer according
to ASTM Method D 562.
2. Measure and record the pH of each paint sample.
3. Using a 1-mL transfer pipette, withdraw 1-mL aliquots of stock
enzyme solution from the third (25-ppm) volumetric flask
and transfer each aliquot to a 250-g sample of each test paint
previously weighed out in either a half-pint can or an 8-oz
widemouth jar. Stir the samples 2 min at moderate speed
to disperse the enzyme homogeneously. The samples now
contain 0.10 ppm enzyme. Samples of each test paint should
also be run as controls, using 1 mL of distilled water as the
inoculum. Store the samples at 73.5 ± 3.5°F (23 ± 2°C) during
the test.
4. Measure and record the viscosity of each sample daily if the
rate of viscosity loss appears to be very rapid, or weekly if
slower. Too rapid a rate of degradation (i.e., <1 month for a
20% viscosity loss in any one sample) may be an indication
that too high an enzyme concentration was used for that
particular test paint, especially if its pH is below 8.5. In such
cases, a lower enzyme level(1) may be necessary to maximize
any differences in enzyme resistance among the test
thickeners. The test may be terminated when one of the test
samples in the series has lost 20% of its original viscosity in 1
or more months.
These directions are for preparing a typical enzyme stock solution, 1 mL of which, when
introduced into 250-g of the test paint, will produce an enzyme level of 0.10 ppm.
Low-pH (<8.5) paints may require a level of only 0.05 ppm, while more alkaline paints,
especially those based on 100% acrylic latices, may require levels as high as 1.0 ppm to
produce degradation. All samples in a series must contain the same ppm level regardless
of the level chosen.
(1)
14
Natrosol HEC Superior Viscosity Stability for Latex Paints
Calculations and Graphs
1. Percent viscosity retained is usually of greater importance in
defining the performance quality of the paint than percent
viscosity lost. The following equation may be used to calculate
this parameter.
n
% nR = t x 100
no
where % nR = % viscosity retained after time t
nt = final paint viscosity at time t
no = initial paint viscosity
2. Data can be graphically illustrated by plotting either the
paint’s Stormer viscosity in KU or percent viscosity retained as
a function of time in days.
Report
1. The percent viscosity retained for each sample at the end of
the test period.
2. The type of enzyme used and concentration in ppm.
3. The pH of each sample.
4. The storage temperature of the samples.
5. The specific procedure (A or B) used to determine the samples’
Stormer viscosities as described in ASTM Method D 562.
6. Any available molecular characteristics of the thickeners, such
as their viscosity in water, molar substitution (MS), degree of
substitution (DS), etc.
Precision
Because of the wide range of diverse ingredients composing latex
paints and their possible individual influence on enzyme activity,
no statement of either repeatability or reproducibility can be made
to cover every formulation. However, provided the sample with
the least resistant thickener has lost 20% of its original viscosity in 1
or more months, differences of greater than 5% between any two
samples can then be considered significant.
Other Paint Properties
Heat Stability
The Stormer viscosity and pH of each sample were determined at
73.5 ± 3.5°F (23 ± 2°C) before and after storing them in an oven for
2 weeks at 122°F (50°C).
Leveling
Instrumental evaluations of leveling, using the Leneta Leveling Test
Blade, were made by a drawdown method as described in ASTM
Method D 4062. All samples were presheared at high speed for 2
min on an air stirrer just prior to drawing them down.
Visual determinations by brush were made after applying a fixed
quantity of paint over a sealed surface of known area, such as
Leneta Form 7B.
Rolled evaluations were made after applying a fixed quantity of
paint over a sealed surface 1 ft2 in area, such as Leneta Form 12H.
Sag Resistance
The Leneta Anti-Sag Index of each sample was determined in
accordance with the manufacturer’s recommended test procedure,
using a 3- to 12-mil Leneta Anti-Sag Meter.
Applied Viscosity
The samples’ applied viscosity by brush or roller was measured at
12,000 sec-1 using a Haake Rotovisco viscometer equipped with the
PK1 cone and plate attachment.
Roller Performance
Roller spatter was evaluated by catching the spatter from a
horizontally moving roller on a black plastic panel placed just below
the substrate, unprimed gypsum wallboard. Ratings were assigned
in accordance with the D/L Laboratories method described in the
Gardner/Sward Paint Testing Manual, 13th ed., 424-5.
Color Development
After shaking each sample 10 minutes on a Red Devil shaker and
allowing it to stand 15 minutes, drawdowns of each were made
side by side with a control paint using a 5-mil (130-μm) clearance
applicator. After drying for several minutes, each drawdown was
gently rubbed with the index finger for 30 seconds to determine if
a color change occurred. Darker colors in the rubbed area indicated
colorant flocculation. Lighter colors resulted from TiO2 flocculation.
Hiding Power
Three-mil (75-μm) wet films of each sample (untinted) were drawn
down over Leneta Form 9B. After the films were dried overnight,
the reflectances over the adjacent black and white areas were
measured. The contrast ratio (CR) was calculated as follows:
CR = Reflectance over black area
Reflectance over white area
Ratios of 0.98 or higher indicate acceptable dry-hiding.
Gloss and Sheen
These properties were measured using a Gardner glossmeter after
drawdowns of each paint, cast using a 7-mil (180-μm) clearance
applicator over a sealed surface, were allowed to dry overnight.
Scrubbability
The scrubbability of each sample was measured in accordance
with ASTM Method D 2486, using the Gardner straight-line and
washability machine.
Wet-Burnish Resistance
Films of each paint were cast using a 7-mil (180-μm) clearance
applicator and allowed to dry for 7 days at 73.5 ± 3.5°F (23 ± 2°C)
and 50 ± 5% RH.
After each sample’s 85° gloss was measured, each was scrubbed for
200 cycles, using a Gardner straight-line and washability machine
equipped with a 1-lb (454-g) abrasion boat and cellulose sponge.
The sponge was soaked to a constant wet weight of 75 g in
tapwater and then stroked 20 times with a bar of hand soap. After
each panel had been rinsed and dried, the 85° gloss of each was
again measured.
Stain Removal
The same test procedure was followed as for wet-burnish
resistance, except that 4 g of Ajax was placed on the soaped
sponge before scrubbing. Percent removal was visually estimated
after each panel was rinsed and dried.
Stormer Viscosity
Determined in accordance with ASTM Method D 562, Procedure A.
Natrosol HEC Superior Viscosity Stability for Latex Paints15
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16
Natrosol HEC Superior Viscosity Stability for Latex Paints