Protection of Light-Sensitive Blue Wool Fabric with Cellulose Acetate Films Containing
Ultraviolet Stabilizers
TYRONE L. VIGO and NANCY E. WYATT
USDA Textiles and Clothing Laboratory, Knoxville, TN 37916
Cellulose acetate films containing various ultraviolet stabilizers were evaluated for their ability to
protect light-sensitive fabric from color change on exposure to an artificial light source at relative
humidities and temperatures simulating warm, moderately humid indoor climates. Stabilizers that
absorb ultraviolet light were the most effective in protecting the wool fabrics while zinc acetate
dihydrate was moderately effective; however, the mechanisms by which the zinc salt offers
photochemical protection are not known. Color differences in the exposed fabrics were determined
by ΔE values computed from L,a,b readings. The most effective stabilizers gave ΔE values of 0.80.9 relative to a 550 kJ/m2 fabric exposure; unprotected fabric gave a ΔE value of 2.7. Cellulose
acetate films containing no stabilizers offered little protection; other film thicknesses or combinations of stabilizers afforded no advantage over using one stabilizer in 8-mil-thick film.
Although the importance of environmental factors (temperature, rela-tive humidity, airborne
contaminants, and oxygen) that affect the fading of dyed textiles exposed to artificial and natural
light indoors is well documented (1,2), relatively little information exists on the effectiveness of UV
stabilizers incorporated into plastic films to minimize or retard such fading. To obtain this
information, light-sensitive blue wool fabric (AATCC L-4 standard) was exposed to light from a
xenon-arc source, with and without protection by clear cellulose acetate films
containing UV stabilizers. This fabric was chosen as a prototype in this study because its fading
characteristics are well known, and because results obtained with this fabric could prove useful for
evaluating other light-sensitive fabrics.
Color changes in the fabrics were quantitatively determined by changes in ΔE—computed from
L,a,b, tristimulus measurements by the Scofield-Hunter color difference equation (3). Unlike results
reported in a previous study (4), it was demonstrated that the cellulose acetate films containing UV
stabilizers did not need to be colored to minimize fading in the exposed fabric. The primary
advantage of using cellulose acetate rather than other polymer substrates as the plastic film is that
the true effectiveness of various stabilizers for protecting fabrics could be determined since
cellulose acetate film without stabilizers offers little photochemical protection. Another advantage
is that cellulose acetate films may be cast from solution in a laboratory and do not require special
polymer processing equipment.
As noted in other studies (5,6), a quantitative assessment of color change in the blue wool or other
dyed fabrics is more reliable than the use of subjective visual assessments that classify fading by
relationship to a grey scale standard (7) and that employ such terminology as "just appreciable
fading." Results with five different stabilizers (four commercial and one experimental) at three
different levels of stabilizer concentration and at three different film thicknesses were investigated.
Fabric exposure times simulated those occurring with 20-100 h of noon sunlight indoors under
warm, moderately humid conditions.
Experimental
Preparation of Cellulose Acetate Films. Commercial cellulose acetate powder (Eastman Cellulose
Acetate 398-3) (15 wt % ) and five commercial and experimental UV stabilizers (0.25-1.0 wt %
cellulose acetate) were dissolved in 35% diacetone alcohol-45% acetone-5% ethanol. All solvents
were reagent grade and the solvent composition used was adapted from recommendations in a
technical bulletin (8). The mixture was stirred in a Waring blender to insure homogeneity, then
poured into glass tubes, and centrifuged for 5 min at 2000 rpm to remove air bubbles. The solution
was then poured onto ¼ -in-thick plate glass and spread with a Gardner knife that has a set of
adjustable micrometers, allowing films of uniform thickness to be cast. A 5-mil film required
micrometer settings of 0.040 in and an 8-mil film required settings of 0.058 in. The 12-mil films
were cast in multiple layers, each layer requiring micrometer settings of 0.047 in. By this procedure,
cellulose acetate films of 5-, 8-, and 12-mil were obtained after the solvents had been allowed to
evaporate for at least 72 h at ambient temperature. During this time, the plate glass was protected
from dust by a sheer muslin cloth supported on a framework above and around the side of the glass.
Ultraviolet Stabilizers. Four commercial UV stabilizers and zinc acetate dihydrate (U.S.P. grade)
were added to various films during their preparation. Two were of the hydroxybenzotriazole type,
one a hydroxybenzophenone, and another a hindered-amine type (Table I).
Exposure to Wool Fabrics to Light. Blue Wool Fabric L-4 (1978 lot—a fabric dyed with two blue
dyes, one fugitive, the other substanta-tive) was used as the test fabric. This particular fabric fades
approximately one unit on the grey scale (7), i.e., "just appreciable fading," when it is exposed for
20 h to an artificial light source. Samples (6.5 X 10 in) of this L-4 fabric were covered with samples
of the various cellulose acetate films prepared as described above (films having different
thicknesses and containing different concentrations and types of UV stabilizers), inserted into
sample holders and exposed in an Atlas Weatherometer (a 2500-W xenon-arc light source) to a light
intensity of 110-550 kJ/m2 for approximately 20-h exposure intervals. The Weatherometer test
conditions were set for 85°F and 50% relative humidity with a black panel temperature of 100°F.
Because the light intensity of a xenon-arc light source varies with the age of the lamp (9), a light
monitor was attached to the light source to insure that light exposure (total radiation dose) was the
same for all fabric samples. The light monitor measures and integrates the radiant energy from the
xenon-arc lamp. Infrared-absorbing inner and outer filters (10) were used for all exposures. The
resultant UV component of the transmitted spectrum closely resembles UV in natural sunlight
except for a peak at 390 nm and an additional energy distribution at 300-320
Table I.
a
UV Stabilizers Incorporated into Cellulose Acetate Films
Trademark of American Cyanamid.
Trademark of Ciba-Geigy.
c
Proprietary product presumed to contain a tetramethylpiperdine moiety.
b
nm. The UV component transmitted to the fabric was about one-third greater in intensity than for an
equivalent 550 kJ/m2 exposure to natural sunlight.
Ultraviolet Absorption/Transmission Curves of Plastic Films. Three 8-mil-thick cellulose acetate
films containing: (a) no stabilizer, (b) 0.5% UV-1A (hydroxybenzotriazole type), and (c) 1.0% UV4 (zinc acetate dihydrate) were inserted into a Cary 17D UV/VIS/NIR Recording
Spectrophotometer and their absorption/transmission curves run in the region of 250-500 nm. The
film containing no stabilizer was run against air as the standard while films containing stabilizers
were run against the unstabilized film as the standard.
Testing of Fabrics for Colorfastness to Light. After each interval of exposure, fabrics protected by
the films were measured for their color change by taking six L,a,b readings on the fabric with a
Hunter color difference meter. By this method, the L,a,b readings were used to calculate the AE
value, which denote the magnitude of total color difference, by the formula
ΔE = (ΔL2 +Δa2 + Δb2) 1/2 (3). An average ΔE for each exposed fabric was calculated with standard
deviations of 0.01-0.10 for fabrics having a ΔE of 1 or less, 0.12-0.20 for fabrics having a ΔE
between 1 and 2, and 0.21-0.38 for fabrics having a AE between 2 and 3.
Testing of Fabrics for Tensile Properties. Unexposed and exposed pieces of L-4 Blue Wool fabrics
were tested in the warp and fill direction for tearing strength by ASTM Method D-1424 on an
Elmendorf tester (11).
Results and Discussion
Cellulose acetate films of the same thickness (8 mil) containing five different stabilizers at three
concentrations were evaluated for their effectiveness in protecting the blue wool fabrics (Table II).
Unprotected wool fabric and wool fabric covered with film containing no stabilizers exposed to the
xenon-arc source under comparable conditions served as primary and secondary controls. Film
without any stabilizer offered little protection throughout the exposure period; after 550 kJ/m2
exposure, fabric protected by such a film had a AE value of 2.38 and unprotected fabric had a value
of 2.70.
Of the five stabilizers, the two hydroxybenzotriazole (UV-1A and UV-1B) and
hydroxybenzophenone (UV-2) compounds were most effective in preventing changes in the color
of wool fabric. Although these stabilizers were effective at concentrations as low as 0.25%, the best
protection at this particular film thickness was achieved with 0.5% of either hydroxybenzotriazole
or 1.0% hydroxybenzophenone. AE values after 550 kJ/m2 were 0.79 for UV-1A, 0.80 for UV-1B,
and 0.83 for UV-2 at these concentrations. Equal parts by weight (a total concentration of 0.5 and
1.0%) of the hydroxybenzophenone (UV-2) and one of the hydroxybenzotriazoles (UV-1A) in the
film did not afford any better protection; indeed, color changes in the fabric were somewhat greater
when combinations of stabilizers were used than when only one stabilizer was used. Since these
hydroxybenzotriazoles and hydroxybenzophenones are known to function as photostabilizers by
absorbing UV light in the 300-360 nm range, it is not surprising that they are effective for protecting the wool fabric from photofading when incorporated into the cellulose acetate films (12).
Absorption/transmission curves of cellulose acetate film containing no stabilizer and one containing
0.5% of a hydroxybenzotriazole (UV-1A) confirmed that protection from photo-fading of the blue
wool fabric by the hydroxybenzotriazole was through maximum absorption of UV light by the
stabilized film over a wide range (the region of 250-375 nm). The film containing no stabilizer
exhibited maximum absorption only in the region of 250-275 nm.
The other commercial stabilizer (UV-3), a hindered-amine type, is presumed to afford
photochemical protection by functioning primarily as a free radical and/or oxygen scavenger (13).
Thus protection should only occur in the primary substrate, the cellulose acetate film, and not in a
secondary substrate such as the blue wool fabric. The results shown in Table II verified this
assumption; the ΔE values obtained, irrespective of the concentration of UV-3 used in the film,
were within experimental error of those observed with the unprotected fabric.
It was not possible to evaluate insoluble pigments such as zinc oxide for their effectiveness as UV
stabilizers in the cellulose acetate films because the centrifugation necessary to remove air bubbles
prior to film casting caused such insoluble materials to be distributed unevenly in the casting
solution. Therefore, a soluble experimental stabilizer, zinc acetate dihydrate (UV-4) was chosen for
this study. Although previous research (14) had demonstrated that it reacts readily with peroxides
(i.e., it functions chemically as a peroxide scavenger), it had not been determined whether or not it
also could function photochemically as a peroxide scavenger or be effective as a UV stabilizer.
However, a recent patent (15) describing zinc acetate as an effective photostabilizer for polyurethanes created additional interest for evaluating it as a stabilizer in the cellulose acetate films for
protecting the blue wool fabrics from photofading.
At concentrations of 0.25 and 0.50%, the zinc acetate behaved like the hindered-amine stabilizer—it
afforded little or no photochemical protection to the wool fabric; ΔE values of 2.47 and 2.21 were
observed at the above concentrations after 550 kJ/m2 exposure. Thus, one could assume it functions
photochemically as an oxygen/peroxide scavenger. At concentrations of 1.0%, zinc acetate did
afford moderate protection to the wool fabrics (ΔE value of 1.55 after 550 kJ/m2 exposure), but its
absorption/transmission curve exhibited a maximum absorption only at 250-300 nm. Thus, the
moderate protection afforded to the wool fabric
Table II. Effectiveness of UV Stabilizers in 8-Mil-Thick Cellulose Light of
a
Calculated from L,a,b values on a Hunter color difference meter (average of six readings on each
sample) ; exposure in Weatherometer is listed in kilojoules per square meter (ambient dry bulb
temperature, 85°F ± 3°; wet bulb temperature. 70°F ± 4°, black panel temperature 100°F; % relative
humidity, 50 ± 8%). Each 110 kJ/m2 corresponds to about 20 h exposure to noon sunlight
conditions.
by films containing 1.0% zinc acetate was not attributable to absorption maxima above 300 nm, and
indicated that additional studies are needed to determine mechanisms by which it functions as a UV
stabilizer.
AE values and light exposure in kilojoules per square meter for the primary and secondary controls,
and for fabrics covered by 8-mil films containing 0.5 and 1.0% stabilizers (extracted from data in
Table II) were given a least-squares treatment to fit an exponential curve y = abx (where y = ΔE and
x = exposure in kJ/m2). In curves generated for stabilizer concentrations of 0.5 and 1.0% (Figures 1
and 2), two general families of curves are observed: (a) those for treatment that effectively protect
the wool fabric (i.e., the films containing hydroxybenzotriazoles
Acetate Film for Protection of Blue Wool Fabrics from Xenon-Arc Different Intensities
b
Based on weight of cellulose acetate powder used for casting film; when more than one stabilizer
was used, equal amounts of each were incorporated into the solution.
c
No film. Blue Wool Fabric L—4. ΔE values for primary and secondary controls (8-mil film with
no stabilizers over wool fabric and wool fabric alone) are mean values based on four samples
subjected to UV.
and the hydroxybenzophenone) and (b) those for treatments that do not protect the wool fabric (the
unprotected control and the film without stabilizer, the hindered-amine stabilizer, and the zinc
acetate dihydrate stabilizer at low concentrations). At 1.0% levels of stabilizer, the zinc acetate
dihydrate curve (Figure 2) is intermediate between the two families of curves, indicating that it
affords moderate protection to the wool fabric.
Correlation coefficients were very good for most of the curves (0.92 or greater) with the exception
of three: UV-1B at 0.5 and 1.0% (correlation coefficients of 0.76 and 0.64, respectively) and UV1A (correlation coefficient of 0.89). Lack of good correlation, particularly with the UV-
Figure 1. Least-squares fit to exponential curve y = abx (y = ΔE; x =kJ/m2) for unprotected blue
wool fabric, and for blue wool fabrics covered with cellulose acetate films (8-mil) containing no
stabilizer, or one or more stabilizers at 0.5% concentration
1B stabilizer, is attributable to reversals in AE observed at 220-440 kj/m2. However, such reversals
are within the standard deviation. Thus, the exponential relationship above generally defines the
color change in the blue wool fabrics as a function of light intensity X time.
To determine whether or not changes in color were accompanied by fiber damage, unexposed fabric
and several fabrics exposed to the xenon-arc source for 550 kJ/m2 (ΔE from 1 to 3) were tested for
tearing strength by the Elmendorf method (11). Before exposure, the tearing strength of the blue
wool fabric was 2040 g in the warp and 1710 g in the fill direction. For all other fabrics tested after
exposure to light, the
Figure 2. Least-squares fit to exponential curve y = abx (y = ΔE, x =kJ/m2) for unprotected blue
wool fabric, and for blue wool fabrics covered with cellulose acetate films (8-mil) containing no
stabilizer, and one or more stabilizers at 1.0% concentration
lowest values observed for tearing strength were 1830 g in the warp and 1490 g in the fill direction.
This 10% reduction in tearing strength is not significant. Thus, even fabrics that underwent the
greatest color change on exposure to the xenon-arc light suffered no perceptible loss of tensile
properties.
To determine whether or not film thickness had any effect on protection of the wool fabrics, various
5- and 12-mil-thick cellulose acetate films were cast and used to cover fabrics exposed in the
Weatherometer. Although it was possible to obtain films of good clarity and a minimum of surface
defects at 5- and 8-mil thickness, two castings were necessary to obtain 12-mil-thick films.
Attempts to prepare a 12-mil-thick film in a single cast resulted in pronounced cratering and poor
clarity.
Table III shows results obtained when three UV stabilizers, alone or in combination, were used in 5and 12-mil films for protecting the blue wool fabrics. The hindered-amine stabilizer (UV-3) was not
tested because it was ineffective in 8-mil films (Table II). In 5-mil-thick films, the trends in A.E
values for the hydroxybenzotriazole (UV-1B), hydroxy-benzophenone (UV—2), and the zinc
acetate (UV—4) stabilizers were the same as they were when these stabilizers were incorporated
into 8-mil-thick films. However, after maximum exposure time, the ΔE values were somewhat
higher than those observed with 8-mil films. Similar trends were observed with films of 12-mil
thickness. Only the hydroxy-benzophenone (UV-2) had ΔE values less than 1.0 after 550 kJ/m2
exposure. The relative effectiveness of a hydroxybenzotriazole (UV—IB), a hydroxybenzophenone
(UV-2), and combinations of each of these stabilizers as a function of film thickness are shown in
Figure 3. All films, even those containing no stabilizers, were most effective in protecting the wool
fabrics from color change when they were 8-mil thick. Again, no advantage was observed in using
combinations of stabilizers instead of a single stabilizer in the film.
Conclusions
1. Cellulose acetate films of 8-mil thickness containing 0.5— 1.0% hydroxybenzotriazole or
hydroxybenzophenone UV stabilizers were the most effective in protecting Blue Wool L-4 fabrics
from undergoing color changes when exposed to a xenon-arc light source that simulated 20-100 h
of noon sunlight indoors under warm, moderately humid conditions.
2. Films containing no stabilizers afforded little protection to the blue wool fabrics when they were
exposed to the light source.
3. Varying film thickness or using combinations of stabilizers afforded no advantage over using
one stabilizer in 8-mil-thick cellulose acetate films with regard to fabric protection.
Table III.
a
Effectiveness of UV Stabilizers in 5- and 12-Mil-Thick Xenon-Arc Light of
Same conditions as Table II, footnote a
b
Same conditions as Table II, footnote b
Cellulose Acetate Films for Protection of Blue Wool Fabric from Different Intensities
c
5-miI-thick film was cast at a micrometer setting of 0.040 in; 12-mil film cast in multiple layers,
each layer requiring 0.047 in setting.
d
Same controls as Table II, footnote c
Figure 3. Plot of AE after 550 kJ/m2 exposure of blue wool fabric with and without cellulose
acetate films of varying thickness, films contained stabilizers at 0.5% concentration
4. The hindered-amine type stabilizer was ineffective, and the zinc acetate dihydrate moderately
effective, in protecting the wool fabrics. Further study is needed to determine the mechanisms by
which the latter compound imparts photo-stabilization.
5. The relation between AE (color change) in the fabrics to exposure intensity (kJ/m2) generally
can be described by an exponential curve (y = abx) derived from a least-square treatment.
6. The results obtained using the L—4 Blue Wool standard fabric with these films could be used to
evaluate protection of other dyed textiles from photofading.
Acknowledgments
The authors thank Dorothy Carter Southerland, Department of Textiles & Clothing, University of
Tennessee, Knoxville for performing parts of the experiment and Dr. James F. Kinstle, Associate
Professor, Department of Chemistry, University of Tennessee, for his helpful suggestions and
discussions on the preparation of plastic films. Mention of a trade name does not constitute a
recommendation or endorsement of that product by the U.S. Department of Agriculture to the
exclusion of other suitable products.
Literature Cited
1. Thomson, G. "Textile Conservation"; Leene, J. E., Ed.; Smithsonian Institution: Washington,
DC, 1972; p. 98.
2. Kajitani, N. In "Preservation of Paper and Textiles of Historic and Artistic Value," Adv.
Chem. Ser. 1977,164, 161.
3. Hunter, R. S. "The Measurement of Appearance"; Hunter Associates Laboratory, Inc.: Fairfax,
VA, 1973.
4. Garrow, C; King, M. G.; Roxburg, C. M. Text. Inst. Ind. 1970, 8(7), 197.
5. Wood, L. A.; Shouse, P. J.; Passaglia, E. Text. Chem. Color. 1970, 2(11), 182.
6. Vaeck, S. V. /. Soc. Dyers Color. 1978, 94(7), 301.
7. Am. Assoc. Text. Chem. Color. Tech. Man. 1977, 53, 132.
8. "Eastman Cellulose Acetate for Coatings," Publication No. E-140A, Eastman Chemical
Products, Kingsport, TN, June, 1976.
9. Am. Assoc. Text. Chem. Color. Tech. Man. 1977, 53, 144.
10. "Atlas Fade-ometer and Weatherometer," Bulletin No. 1300 C, Atlas Electric Devices Co.,
Chicago, 1978.
11. Am. Soc. Test. Mater., Book ASTM Stand. Relat. Mater. 1978, 32, 323.
12. Ranby, B.; Rabek, J. "Photodegradation, Photo-oxidation and Photostabilization of Polymers";
Wiley—Interscience: New York, 1975.
13. Patel, A. R.; Usilton, J. J. In "Stabilization and Degradation of Polymers," Adv. Chem. Ser.
1978,169, 116.
14. Danna, G. F.; Vigo, T. L.; Welch, C. M. Text. Res. }. 1978, 48, 173.
15. Maki, H.; Takamura, Y. Jap. Pat. 78 52 598, 1976; Chem. Abstr. 1978, 89, 181078n.
RECEIVED November 19, 1979.