The Properties and Lifetime of Polymer UV Films
by PETRA VÁVROVÁ, HANA PAULUSOVÁ & IRENA KUČEROVÁ
INTRODUCTION
The use of a UV filter as protection against UV radiation has become an integral part of the
conservation of archive and library funds. The damage caused by light and particularly by its
highest-energy, i.e. ultraviolet component, is usually very extensive and, unfortunately, most often
irreparable1.
The most common means of employing UV filters is to install them on window glass or on light
sources, which do not already have UV filtering. For use in exhibitions, a suitable arrangement can
be employed directly for the exhibition cases, or a sensitive object on display can be covered
individually.
UV filters can be obtained commercially in several forms2:
• UV films
• UV plates
• glass "sandwich" systems with UV film
• UV coatings.
This work is concerned exclusively with the properties and use of UV films.
WINDOW FILMS
Window films constitute an important group of UV filters; these films began to be produced in
1969 to regulate sunlight3. Initially, they had a number of inadequacies, such as the formation of
bubbles or wrinkles during application, low resistance to scratching, fading of their colour or high
mirror effect. These defects have been gradually eliminated. Window films are manufactured for
various purposes. They can prevent the passage of only UV radiation, or they can decrease the
permeability for visible light, or prevent the passage of heat. The latter system is more complicated,
as heat is transferred to a major part of the external environment, where the heated glass is cooled
by a stream of air. It is necessary to consider carefully the conditions under which these films
should be employed.
Films may also be used to increase safety and security, where the strength of the glass is increased
by up to 300% by a layer of film.
Films are manufactured exclusively of rolled polyester material, which retains high permeability for
light radiation without discernible distortion. Various thin or coloured polyester films are laminated
together during production according to requirement to achieve a certain degree of darkness or
strength. An adhesive layer containing UV absorbers is applied to the final form of the film. UV absorbers based on cyclic iminoesters are used most frequently to filter out UV radiation4,5. The
adhesive layer may be sensitive to application pressure or may be applied using water. The adhesive
is then covered with a thin transparent film to protect it against dirt prior to application. The surface
of the film is usually protected against scratching6.
In conservation, clear UV films without a reflective layer tend to be used. When carefully applied,
the UV film becomes practically invisible on the surface of the glass.
One of the basic aspects of interest to restorers is the quality of the UV filter and the time over
which it can be used, without the danger of an increase in the passage of UV radiation through the
film, or the danger of subsequent damage to the preserved object. An ideal UV filter absorbs all the
radiation with a wavelength shorter than 400 nm and allows the passage of all radiation with a
wavelength above 400 nm7. However, materials employed in practice tend to allow small amounts
of radiation in the UV region through as well as absorbing some visible light. By using a suitable
arrangement, a UV film with very good parameters can be prepared, it will, however, have a limited
lifetime. The usual guaranteed lifetime of UV films applied to the inside of window glass is 5-7
years; if it is necessary to place the film on the outside of the window, the guaranteed liietime is
much less, only 2 years3. Films near the end of their life can be removed using scrapers or razor
blades, a suitable solvent or soapy water. After removing the polyester film, the adhesive remains
on the glass, which must be cleaned and de-greased before applying another film. The ease with
which an old film may be removed will depend on how long it has been in place and how extreme
has been its exposure to light. For example, it has been noted that, removal of film was considerably
more complicated and tedious7,8 on the south side of a building where it has been more exposed to
sunlight.
It is known from experience that films are replaced most frequently because of mechanical damage,
e.g. scratching, rather than because of indications of failure in preventing UV filtering through. In
caring for film attached to window glass, it is also necessary to select cleaning methods carefully.
Agents containing ammonia must not be used and, for the same reason, UV films may not be used
in environments where ammonia is employed (e.g. chemical laboratories)3.
UV FILTERS ON LIGHT SOURCES
Light sources emitting UV radiation can be screened using UV polymer films or plates. These films
are most frequently made of polyesters, which absorb scattered UV light, and form a kind of sleeve
around the light source tube. The use of polymer UV filters is limited by the thermal resistance of
the material. For example, tungsten-halogen light bulbs must always be filtered using glass, as the
high working temperature does not permit the use of polymer filters. Plate filters made of
polyacrylate or polycarbonate exhibit satisfactory absorption properties, but are difficult to protect
against scratching. The best UV filters are capable of decreasing UV radiation emitted by light
sources to a value of 0-5 μW·lm -16. In addition to the requirement of filtering detrimental UV
radiation from the light source, it is necessary also to pay attention to the presentation of colour. In
some cases, this requirement is very important, e.g. in exhibiting illuminated manuscripts or
coloured maps, etc. Some filters absorb the blue part of the visible spectrum, or some other part of
the wavelength range in the visible region. The blue part of the spectrum has such high energy, that
it can also cause damage to sensitive materials, e.g. unstable dyes. Unfortunately, these filters affect
the presentation of colours and thus cannot always be used. It is better to protect materials that are
very sensitive to light by exhibiting a copy2.
EXPERIMENTAL9
The experiments were carried out to study the properties of UV films*, that is their ability to absorb
UV radiation and their success of allowing through visible light, following exposure to accelerated
ageing. Simultaneously, the mechanical and chemical properties and any changes in these properties
were studied. The conditions of ageing were chosen so as to include the basic factors acting on
films in practice, i.e. heat, relative humidity, light and a combination of these factors.
Samples
The tested UV filters (Table 1) were selected from a wider set of clear films on the basis of the
spectral characteristics measured in the UV/VIS region The criteria for inclusion in the test were the
highest possible absorption of UV radiation and the greatest permeability of visible light.
* Available on the market in the Czech Republic.
Table 1: Samples of UV filters selected for the experiment.
Table 2: Properties of the radiation source.
Accelerated ageing
• Dry thermal ageing
The samples were subjected to accelerated ageing according to CSN 64 0770, i.e. 55 ± 8 °C and RH
0 ± 5% for a period of 60 days. Aged samples were taken at intervals of 15 days.
• Humid ageing
55 ± 3 °C and RH 65 ± 5% for 45 days. Aged samples were taken at intervals of 15 days.
• Ageing by the effect of light
Ageing occurred in a light chamber using a PHILIPS CLEO 25 W light source for a period of 40
days (2 samples after 20 days). The properties of the radiation source are given in Table 2. This
consisted of a fluorescent light with a high fraction of the UVA component. The spectral
characteristics are shown in Fig. 1. This source allows irradiation of an area measuring 40 x 40 cm
and is part of the irradiation unit. The temperature of the surface of the sample during radiation was
about 22 ± 2 °C; the relative humidity in the chamber was measured as 15 ± 5%.
Fig. 1: Spectral characteristics of the source.
Experimental conditions at the sample position
• intensity of light by visible radiation 863 1x,
• intensity of irradiation by UV radiation 20.16 W•m-2,
• exposure time 480 and 960 hours (2 samples taken),
• share of UV radiation emitted by light source 23.37 •W•lm-1;
• the UV filter samples were placed in the light chamber with the adhesive, the UV absorber on top
and the polymer support at the bottom, to replicate its position on a window.
• Light ageing combined with humid heat.
The light ageing was carried out for a period of 20 days (for the ageing conditions, see above),
followed by ageing by humid heat for a period of 15 days.
Tested properties
• The chemical composition of the matrix of the UV film and its adhesive (adhesive separated
using tetrahydrofuran) using a UV/VIS spectrophotometer and FTIR;
• the transmittance of the UV film, measured using UV/VIS spectrophotometry in the wavelength
range 200-800 nm;
•
•
•
•
the mechanical properties;
the tensile strength according to standard CSN 64 0604,
the ductility;
the colour fastness of the UV films (CIE Lab colour space).
Instruments and measuring methods
The UV/VIS spectra were measured on a UV/VIS UV 4 spectrometer (Unicam, Great Britain).
Measuring parameters:
• wavelength: 200-800 nm
• slit width: 2 nm
• scanning rate: 240 nm•min-1
The spectra were evaluated using Vision 32 software and transferred to the Microsoft Excel 97
program.
The infrared spectra were measured on a Nicolet 740 FTIR spectrometer (Nicolet Instruments Co.,
Madison, USA) with a DTGS detector and a KBr beam splitter. Measuring parameters:
• resolution: 2 cm-1;
• number of accumulated spectra: 32;
• Happ-Genzel apodization.
The spectra were processed by the Omnic v. 3.1 program (Nicolet Instruments Co., Madison, USA)
and identification was carried out by comparison with the database of standard spectra from
Hummel Polymers and Additives.
The colour stability of the UV films was measured on a CM-508d compact portable
spectrophotometer (Minolta, Japan) using a white standard as a support. The measuring conditions
were adjusted to the SCE regime with a characteristic observer and characteristic light 10/D65 and
substitute chromaticity temperature of 6500 K. The L*a*B* coordinates and the total colour
difference ΔE* were measured for the unaged and the aged UV film samples.
Tensile strength and ductility were determined using the Instron 1122 tensile testing machine
(Instron, USA). The measurement was carried out at a clamp velocity of 5 mm•min-1. Prior to the
measurement, the samples were conditioned at a temperature of 23 ± 2 °C and RH of 55 + 5% in an
air-conditioned chamber according to standard ST SEV 885-78.
Fig. 2: UV absorbance of sample A (left) and sample E (right).
RESULTS AND DISCUSSION
Chemical composition of the UV films
The tested films, made for installation on window glass, are all constructed in practically the same
way. The base consisted of polyester film, which may be multi-layered; an adhesive layer is placed
on its upper surface. Chemical analysis indicated that the UV absorbers were mixed into the
adhesives of films A, B, C, D, F and G. The UV/VIS spectra of the pure matrix films indicated that
the film itself absorbed UV radiation to a wavelength of 305-315 nm, while the film together with
the adhesive with the UV absorber permitted the passage of UV radiation only above a wavelength
of 380-390 nm. Only filter E seems to have had an UV filter incorporated into the polymer film
itself (Fig. 2). The ability of the films to absorb UV radiation was checked using methyl violet
(Basic Blue 1).
It followed from FTIR measurements that all the films were made of polyethylene terephthalate
(PETP; Fig. 3). The adhesive layer comprised an adhesive based on common acrylate dispersion.
We found the following types of copolymer
• Sample A: vinyl acetate - 2-ethyl hexyl acrylate - acrylic acid copolymer;
• sample B: 2-ethyl hexyl acrylate - acrylic acid - butyl acrylate copolymer.
More exact determination of the composition of the adhesive would have required the use of
pyrolysis gas chromatography.
The chemical composition of the UV absorbers in the adhesive layer could not be determined
because of their low content and because of the lack of suitable equipment.
Chemical changes in the UV film during light ageing
FTIR was employed to monitor the change in the chemical composition of the UV film. Light
ageing caused changes only in some of the UV films in our study.
Fig. 3: IR-spectra of the films and of pure PETP.
On the basis of the measured IR spectra, samples could be divided into two groups. The first group
(films A, B, C, D and E) did not exhibit any changes in the composition of the adhesive layers with
UV absorbers, in comparison to the unaged samples.
Fig. 4: R spectra of the adhesives of film F and G, before and after light ageing.
The second group consisted of films F and G, where deviations were found in the IR spectra (Fig.
4). The adhesive of films F and G shows the presence of different groups, probably -NH2, which
were affected by ageing.
Fig. 5. UV/VIS spectra of sample A before and after dry (left)different kinds of ageing.
Comparison of the IR spectra of imaged and aged samples indicated that no changes occurred in the
chemical structure of the matrix (i.e. polyethylene PETP). This is probably because the polymer
support was protected during light ageing by the adhesive layer containing an UV absorber. It
seems that the UV absorbers in the adhesives protected not only the exhibited objects, but also the
film matrix.
Spectral characteristics of polymer UV films during accelerated ageing xxx
The transmission curves of the tested UV films initially exhibited very similar behaviour. The
amount of visible light transmitted through the films was about 80-90%. UV radiation was reliably
absorbed to a wavelength of 360 nm and most of the radiation in the 390-400 nm region was
transmitted.
It followed from the measured spectral characteristics that the change in the ability of the film to
absorb UV radiation during ageing was dependent upon the type of artificial ageing employed. Fig.
5 shows the transmittance UV/VIS spectra of UV film A during dry, humid, light and combined
ageing. The shapes of the spectral curves of the other studied films are not given because they are
very similar.
Fig. 6: UV/VIS spectra of sample G before and after dry (left) and combined ageing (right). The
diagram lines of 720 h and 1440 hours are nearly congruent.
Thermal ageing affected the amount of UV radiation transmitted for all the UV films. The effect of
dry heat was greater compared to the other types of ageing. Following 1440 hours of ageing, the
level of transmitted UV increased to 1-3% for UV radiation of wavelengths 290-360 nm and to 3050% for UV radiation of wavelength 380 nm. Following ageing, films A and C transmitted more
than 30% of the UV radiation with a wavelength of 380 nm, while the unaged samples absorbed
almost 95%. Film E was an exception, where the ability to transmit remained practically unchanged
by the effect of accelerated ageing.
There was less change in the amount of visible light the films transmitted. Only film G exhibited
different behaviour; its ability to transmit the visible part of the spectrum decreased by up to 30%
(Fig. 6). The change was visible to the naked eye - the clear film became cloudy.
Surprisingly, ageing in a humid atmosphere had virtually no affect on any of the films tested for UV
transmittance. There was, however, a slight decrease in the amount of visible light transmitted of 13%.
Light ageing had less effect than expected on changes in the spectral characteristics. All the tested
films absorbed UV radiation up to a wavelength of 360 nm even after ageing. The transmittance for
a wavelength of 380 nm increased nearly two-fold for films C and E.
Combined ageing (light and humid heat) affected the transmittance of UV radiation far more than
the separate effects of light and humid heat. While only small changes appeared in the spectra
following light ageing, the action of humid heat led to an increase in the transmittance of UV
radiation and simultaneously to a discernable decrease in the transmittance of visible light (up to
10%).
In the overall evaluation of the tested films from the standpoint of filtering out UV radiation, only
samples B and G retained satisfactory behaviour even after accelerated ageing.
Changes in the mechanical properties of UV films
Specific conditions aside, accelerated ageing significantly decreased the tensile strength of all the
tested UV films. The measured values attained only 7-29% of the original values.
It is difficult to comment on the decrease of this phenomenon, as the exact chemical composition of
the UV films was not known. Experience has confirmed the low resistance of PETP films without
stabilizers to light and UV radiation, while a considerable improvement can be achieved through the
use of UV absorbers. However, in our case, the decrease in the tensile strength occurred also as
result of thermal ageing and ageing in a humid atmosphere. It is possible that the films contained
additives that were highly sensitive to the high temperature (55 °C) of these ageing methods.
Even if the energy strain of accelerated ageing initially provoked the progressive decrease in tensile
strength, after a certain time, depending on the type of film, the residual strength equalized to a
comparable value (Fig. 7). In addition, the initial differences in tensile strength in the longitudinal
and transverse directions equalized during ageing.
The ductility (stretch to break) of the UV films decreased during ageing much more slowly than the
tensile strength. The greatest decrease was recorded for humid heat, followed by light ageing and
the combination of the two.
Colour changes in the UV films
Following thermal and combined ageing, UV film G became discernibly cloudy. This was probably
caused by a chemical change in the structure of the adhesive with the UV absorber. Visible changes
in the colour also appeared for film D through the effect of dry ageing. The other films were slightly
affected by thermal ageing, but the values of ΔE*, ΔL*, Δa* and Δb* of the CIE Lab colour space
are not significant.
Comparison of the effectiveness of UV filtration by the tested UV films using dyes
The dye methyl violet (Basic Blue 1) was employed to compare the ability of UV films to absorb
UV radiation. This was chosen intentionally, as it appears very frequently as a means of making
records in archive documents. Methyl violet forms the basis for ink pencils, purple copying paper,
purple tapes for typewriters and purple copying (hectographic) inks. Methyl violet is a dye that is
very sensitive to light and belongs to the group of aryl methane dyes; its low light stability
Fig. 7: Tensile strength depending upon accelerated ageing.
Fig. 8: Filter paper dyed with methyl violet submitted to light (the conditions of light ageing; cf. p.
239); a: no light (0 hours); b: 10 h; c: 44 h; d: 92 h; e: 140 h. Left diagram: samples without filter;
right diagram: samples covered by filter A.
is well documented. The sensitivity of methyl violet to light is thus an ideal property for comparison
of the quality of UV films in the selected set.
The ability to absorb UV light by the individual films was evaluated using the diffusion reflection
UV/VIS spectra of paper samples (Whatman No. 1) dyed with methyl violet. The samples covered
with the UV films were irradiated under the conditions of light ageing and their UV/VIS spectra
were measured at regular intervals. A comparison sample was irradiated without a UV filter. The
results are given in Fig. 8.
As expected, the UV films significantly retarded the fading of the dye. The shape of the diffusion
reflection spectra was roughly the same for all tested films. It must be emphasized that the tests
were made only with new, unaged UV films, and not with films exposed to accelerated ageing.
CONCLUSIONS
It was found in the study of the optical, chemical and mechanical properties of UV films subjected
to various ageing conditions that:
• The absorption of UV radiation by UV films is affected by accelerated ageing, where the greatest
changes in all the tested films are caused by thermal ageing.
• The effect of the combined action of light and humid heat leads to lowered transmittance of the
visible part of the spectrum in all the UV films.
• The changes in transmittance of UV radiation caused by light ageing are lower than for ageing by
dry and combined ageing.
• Humid ageing has only a minimal effect on the transmittance of the UV and visible components
of light.
• The UV films had high initial tensile strength and ductility (stretch at break); however, after 4060 days of accelerated ageing, these values decreased considerably, especially tensile strength. This
fact can probably be explained by the presence of an additive that had a detrimental effect on this
quality. However, no such additive could be found using FTIR. The considerable increase in the
brittleness of the film will have a detrimental effect particularly on removal of no-longer useful film
from window glass.
• The behaviour of the adhesive layers was monitored during light ageing using FTIR. The results
indicated that there was a change in the chemical structure in samples F and G. The polymer matrix
remained unchanged.
• The colour stability of films A, B, C, D, E and F was very good. Only filter G became
discernibly cloudy during all types of ageing, probably as a consequence of changes in the structure
of the adhesive with the UV absorber.
• It follows from evaluating the monitored properties that only UV films B and D comply with the
strict criteria required for the conservation of sensitive archive materials and can be recommended
for use in practice.
• The rate of fading of a sensitive dye - methyl violet - through the action of light decreased to
approximately one half after 140 hours of irradiation when using UV films.
The lifetime of UV films is apparently limited and will depend on the specific climatic conditions
during use. For example, film attached to window glass will be exposed to much more severe
ageing conditions (heat, light, humidity) than film covering an exhibited object in a display case. It
can be expected that window film on the south side of a building during the summertime will be
exposed to similar conditions as were selected for accelerated ageing.
It follows from the obtained information that periodic control of the spectral characteristics of UV
films is essential.
ACKNOWLEDGEMENTS
We would like to thank the collective of workers of the Scientific Research Section of the State
Central Archives in Prague for support and assistance in resolving problems. We would like to
thank Mrs. Miroslava Novotna of the Central Laboratories of the Institute of Chemical Technology
in Prague and Ing. Dušan Benda from Pardubice for measuring the IR spectra. Part of this work was
carried out with support from research plan MSM 223100002.
SUMMARIES
The Properties and Lifetime of Polymer UV Films
The use of UV filters is very important in the protection of archival and library collections against
light. Basic information on the behaviour and the durability of a number of different UV filters is
summarized. Study of the properties of UV filters provides information on the changes in the UV
absorption characteristics during artificial ageing. A discussion is given of the optical and
mechanical properties of UV filters.
Proprietes et durabilite des films polymeres UV
L'utilisation des filtres UV est une mesure ires efficace pour proteger les collections des archives et
des bibliotheques centre les effets nefastes de la lumiere. Un resume est donne sur Pinforma-lion de
base relative au comportement et a la durabilite d'un certain nombre de differents filtres UV.
L'etude des proprietes des filtres UV fournit des informations sur les changements inter-venus dans
les caracteristiques de 1'absorption UV observes lors du vieillissement accelere. Une discussion a
lieu sur les proprietes optiques et mecaniques des filtres UV.
The Properties and Lifetime of Polymer UV Films
Der Einsatz von UV-Filtern ist ein wichtiges Mittel zum Schutz von Archiv- und Bibliothelsgut vor
der schädlichen Einwirkung von Licht. Nach einem Bericht über das grundsätzliche Verhal-ten und
die Dauerhaftigkcit einer Reihe von UV-Filtern werden Ergebnisse einer Studie über
Veränderungen der UV-Absorption mitgcteilt, die nach verschiedenen Methoden und Stufen der
beschleunigten Alterung an ihnen auftreten. Weiterhin werden die optischen und mechanischen
Eigenschaften der Filter dskutiert.
REFERENCES
1. Thomson, G.: The museum environment. London: Butterworth-Heineman 1986.
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archive documents. Prague. Prague: State Central Archive 2001.
3. Henkenius, M.: A guide to window film. http://popularmechanics.com/home_improvement/
home_improvement/1996/10/window_film/print
4. Pospíšil, J.: Photo-oxidation degradation of polymers: causes, consequences and protection.
Chemicke listy, 85 (1991): 904-923.
5. Head, F. S. H., & G. Lund: New ultraviolet absorbers for improving the stability of cotton
cellulose to light. Journal of the Society of Dyers and Colourists. 85,2 (1969): 61-65.
6. Saunders, D.: Ultra-violet filters for artificial light sources. National Gallery, Technical
Bulletin, 13 (1992): 61-68.
7. Staniforth, S.: Problems with ultraviolet filters. In: Lighting - Preprints of the Bristol
Conference on Lighting in Museum, Galleries and Historic Houses. Bristol 1987: 25-30.
8. Allen, G., L. Black, K. Hallam & S. Staniforth: A preliminary investigation into the effect of
self-adhesive ultraviolet-absorbing films on window glass. In: ICOM Committee for Conservation
12th Annual Meeting, Lyon 1999: 757-763.
9. Vavrova, P.: Studium vlastnosti vybrany'ch UV-filtn't na polymerni podlo'ce (The study of the
properties of selected UV filters on a polymer support]. Thesis. Institute of Chemical Technology in
Prag, Department of Chemical Technology of Monuments Conservation, Prague 2001.
Ing. Petra Vávrová
Ing. Irena Kučerová, Ph.D.
Institute of Chemical Technology in Prague
Institute of the Chemical Technology of Restoration of Monuments
Technicka 1905
166 28 Prague 6
Czech Republic
Tel: +420 224 354 154
Fax:+420 224 311 100
E-mail: Petra.Vavrova@vscht.cz
Irena.Kucerova@vscht.cz
Ing. Hana Paulusová
State Central Archives in Prague
Scientific Research Section
Archivni 4
149 01 Prague 4 - Chodovec
Czech Republic
Tel.:+420 974 847 467
Fax: +420974 874214
E-mail: sualO@mvcr.cz