The Application of Carbopol™ Poultices
on Paper Objects
by AGNES BLÜHER, URSULA HALLER, GERHARD BANIK &
ELISABETH THOBOIS
INTRODUCTION
In conservation practice, various kinds of poultice materials can be applied in order to add humidity
or solvents by means of a controlled and locally restricted technique. The liquid can be applied
either by using soaked compact materials or in form of vapors, such as by means of Gore Tex™
sandwiches.1 Non-swelling inorganic agents, such as talcum powder, magnesium oxide, or hollow
glass balls can be used as poultices or gels can be formed by means of swelling organic polymers.
One gel-forming product is Carbopol™, which was introduced to conservation by R. Wolbers2 in
1988 for varnish removal restricting the interference strictly to surface layer of paintings. In 1992 T.
Petukhova3 described the application of Carbopol/ solvent poultices for varnish removal on
drawings and hand colored maps. The whole palette of organic solvents including non-polar
systems can be thickened by Carbopol and clear gels result. Gel formation implies the addition of a
basic and surface active agent that is assumed to be so well integrated into the gel that no uncontrolled migration and side effects to the substrate result. Therefore investigations until now have
been mainly concerned with the removability of the gels from surfaces.4 However, the advantages
and limits of application of Carbopol poultices have not been fully investigated especially, as far as
migration of alkaline gel-forming additives to the substrate paper is concerned.
CARBOPOL™
Carbopol™ is the trademark for polymers based on acrylic acid manufactured by the B. F.
Goodrich Company. It has been in use in the cosmetics and pharma-
ceutical industries for more than 30 years. If used as additive in cosmetical products it is indicated
under the name Carbomer™. Various types of Carbopol polymers are available that vary according
to molecular weight and the degree of intermolecular cross-links (Table 1). Cross-linking of the
polyacrylic acid is revealed by copolymerization with polyalcenyl polyether. The general structure
of the polyacrylic acid is represented in Fig. 1. Type 980, which has a molecular weight of 4
million, is particularly suitable as it shows the highest gel-forming property and absolute transparent
gels are formed. Carbopol is a white, highly hygroscopic powder and should therefore be stored in
sealed containers.
Preparation of poultice gels from Carbopol
The gel is prepared by stirring the powder in water or in the required blend of solvents. Before the
swelling phase, the Carbopol polymer is a coiled molecule showing only a very limited uncoiling
and thickening effect, as is the case after suspension in nonpolar organic solvents or solvent blends.
The thickening effect occurs when the molecule uncoils so that a stable network is formed — that is,
a gel. This stretching (uncoiling) of the molecules is caused by electrostatic repulsion of the chain
segments arising from the formation of negative charges i.e. ions, along the chain after a
neutralizing agent has been added. A general scheme of the stretching mechanism is given in Fig. 2.
Stretching of the molecules can also be achieved by adding considerable amounts of hydrogen bond
forming compounds such as non-ionogenic tensides, e.g. Triton X-100, which is a non-ionic tenside
on the basis of polyethers (ROHM & HAAS). This possibility is, however, not further discussed
here.
When suspended in water, the Carbopol molecule already starts to uncoil, as some carboxylate
groups are formed as a result of partial dissociation of carboxyl
Table 1. Selected types of Carbopol resins
* Polymerization in benzene. The new types are polymerized in ethyl acetate and cyclohexane. For
the described investigations the old types has been used.
** Brookfield-viscosity at pH 7-7.8, 25°C.
*** P means high purity grade.
Table 2. List of tested Carbopol gels
* maximum and minimum values according to the literature2'5
** deionized water
*** extent of discoloration; - no, + + strong, + + + very strong
groups. Depending on the Carbopol concentration, the pH of the water (preparation) ranges from
2.5 to 3.5. The real stretching and uncoiling of the Carbopol molecules that is essential to reach
maximum viscosity only occurs after addition of a neutralizing substance, whereupon a greater
number of charged carboxylate groups is formed, enforcing the stretching of the molecules.
In aqueous Carbopol gels or in gels based on water solvent blends with a high water content,
sodium hydroxide, ammonia or triethanolamine can be used as neutralizing agents. In systems
based on organic solvents, amine compounds such as Ethomeen C 25 (polyoxyethylene (15)
cocoalkylamine, AKZO Chemicals), and in case of nonpolar solvent systems Ethomeen C 12
(Cocoalkylbis (2-hydroxy-ethyl), amine, AKZO Chemicals) are recommended and have been
successfully applied for gel formation.
Ethomeens are ethoxylized fatty amines with the general structure (Fig. 3) R signifies the
hydrophobic part of the molecule the cocoalkyl residue (mainly C12). The number of hydrophilic
ethylene oxide groups amounts in Ethomeen C 25 x+y=15 and in Ethomeen C 12 x+y=l (compare
formula in Fig. 3). Whereas Ethomeen C 25 dissolves in water, Ethomeen C 12 does not.
Corresponding to their molecular structure, Ethomeens have a definite tenside character. Manufacturers present them in their technical information leaflets as detergents and plasti-
cizers. The amount of neutralizing substances can be assessed according to the desired pH value of
the aqueous gels. Thickening already begins in the pH range from 5 to 6. Neutralizing with amines
such as Ethomeen C-25 or C-12 should be limited to the amount of amine required for maximum
thickening.
The equivalent weight can only serve as a rough orientation for the amount of neutralizing agent, as
actual thickening takes place a long time before the stoichiometric equivalent point is reached. In
this context, therefore, the term neutralization does not imply the addition of a neutralizing agent in
an exact stoichiometric amount, but it is enough to form sufficient salt groups to uncoil the
Carbopol molecule to cause required thickening. Details of gel formation mechanisms are given in
the technical description5 and in U. Haller's thesis.6
The gel-forming components, polyacrylic acids and amines are weak acids or bases respectively. In
the state of equilibrium a greater part of the nondissociated carboxyl groups and free amines are
present in the gelifying system. This means, in other words that, on the one hand, only a part of the
added amine component is attached to the polyacrylic acid in form of ammonia salts, whereas the
majority is not fixed and may move freely. Sodium hydroxide on the other hand as a strong base
establishes strong salt bindings to the polyacrylic acid and movability of alkaline components is
therefore restricted.
Fig. 1. Polyacrylic acid - general structure
Fig. 2. Schematic depicting of Carbopol in uncoiled state
Water retention of Carbopol gels
An important criterion in using a gel is its ability to retain a solvent in the face of capillary forces of
the substrate it is applied to. Comparative examinations of the solvent migration from gel to the
substrate are possible by aid of a dye. A. Southall7 investigated the migration of liquid from gels to
several substrates by staining the gel with Naphthalene black 12B. The dye moves out of the gel together with the liquid - water or solvent blend - and stains the respective substrate (canvas, a 19thcentury test painting and polyester sailcloth) black. The results could demonstrate that the Carbopol
gels tested show a lower tendency of liquid retention with increasing capillarity of the respective
substrate.
For the investigations described in this articles aqueous Carbopol gel preparations were applied on
filter paper homogeneously stained with Ponceau S (C.I. 27 195). Ponceau S is an anionic dye of
the group of triphenylmethane dyes that is very soluble in water. The water retention capacity of the
gel sitting on the paper can be easily judged by the transport of the dye together with the migrating
water spreading out in circles around the applied gel. As the minute dye molecules have almost no
affinity with paper, they are able to move unrestrictedly with the liquid line of the water. Aqueous
gels of the Carbopol types 980 or 954 have a significantly higher ability to retain water than gels on
the basis of cellulose ethers of the same concentration. With a sufficient Carbopol concentration —
nearly 1.5% Carbopol 980 in aqueous systems neutralized with sodium hydroxide - any movement
of the dye, i.e., the solvent in horizontal direction, is nearly completely prevented. In order to gain a
comparable water retention in gels based on high-molecular-weight cellulose ethers such as methyl
cellulose or hydroxypropyl cellu-lose(Klucel™), a solid content in the range of 4-8% is required.
However, some unexpected observations were made during the investigations of water retention
using the colored filter paper:
• Carbopol gel prepared with triethanolamine as neutralizing agent, the pH of the gel was 8.5,
caused bleaching of Ponceau S after one or two days of contact with the gel poultice or in areas
where liquid moved out from the gel was present. The bleaching is observable by a color change
form red to yellow and follows an oxidation mechanism. The speed of the bleaching reaction
depends on the presence of oxygen and can be accelerated by the addition of hydrogen peroxide.
• Gels of similar pH but neutralized with sodium hydroxide do not show this phenomenon.
The oxidizing bleaching of Ponceau S shown here is only possible under alkaline conditions which
means that, together with water, free basic amines are mi-
grating from the Carbopol poultice to the substrate paper. Having thus indirectly pointed out the
presence of amines in the substrate, the migration behavior of amine components from Carbopol gel
was examined in more detail.
VISUALIZATION OF AMINES IN CARBOPOL GELS BY ACCELERATED AGING
Organic amines such as triethanolamine or Ethomeens have a high tendency of turning yellow when
exposed to light or under the influence of elevated temperature. This characteristic of amines can be
exploited for their detection in organic substrates such as paper. In order to examine the movability
of amines in Carbopol gels, the gels neutralized with amines were applied on to various carrier
materials, and the behavior of the amines consequently was examined after their visualization
through artificial ageing by means of light and elevated temperatures.
Experimental
Aliquots of 0.1 g of the gels listed in Table 2 were spread on to various carrier materials covering an
area of 2X1.5 cm. As carrier materials filter paper, Hollytex™ fleece and microscopic glass slides
were used. One drop of the pure amines was also given to each carrier. After application the gels
were air-dried on the carrier materials - the gels containing amines did not reach complete dryness
but remained sticky - and were consequently exposed to the artificial aging processes.
Light ageing was carried out by means of a Heraeus Xenotest 150 S. Samples were treated 115 h at
1.5 kW/m2 until fading of Blue Standard No. 6, sample temperature was approximately 35°C. This
was followed by artificial ageing in a climate chamber (Heraeus 0020) at 70°C and 55% RH for 72
h. These non-standard conditions were chosen as they were sufficient to cause discoloration of
amines and make their presence visible in the substrates under investigation, without changes in
optical appearance of the carrier materials under this mild conditions.
Fig. 3. Ethomeen C 25 rsp. C 12 - general structure
Results of ageing the amine-contaimng Carbopol gels
Gels neutralized with amines as well as pure amines applied directly show heavy discoloration - i.e.,
yellowing on all carrier materials. In order to evaluate the stability of the carrier materials towards
alkaline compounds and an alkaline environment, one drop of 10% NaOH was directly applied to
the respective carrier as a reference. No discoloration of the carrier materials could be observed in
all cases NaOH was used as neutralizing agent, neither with gels G (pH 7.0) and H (pH 8.5) nor if
NaOH 10% was applied directly to the carrier material. It can therefore be concluded that the
discoloration that occurred under the gentle ageing conditions is due to the presence of amines.
It has been shown that not only the amine containing gels turn highly yellow but also the liquid that
has migrated from the gel to the substrate. Around the applied poultices of gel E and F, which are
prepared by means of triethanolamine, humidity tidelines were formed on filter paper, which dried
again, apparently leaving no traces. After the artificial ageing procedure, yellowing was observed in
this area, which increased in accordance with the concentration of the triethanolamine concentration
of the gel (E<F, Fig. 4). This implies that triethanolamine
Fig. 4. Migration of triethanolamine out of gel E (left side) and of gel F (right side). The migrated
triethanolamine becomes visible on the filter paper after accelerated aging as a dark ring around the
residues of gel in the center.
left the gel poultice together with the humidity. In case of Hollytex fleece as carrier material traces
of moisture were only noticeable on the back after application of the poultice, whereas the Carbopol
itself does not penetrate the Hollytex on the condition that a weight of more than 50 g is avoided.
After ageing, heavy yellow stains were found on the reverse side of the Hollytex fleece. These can
be attributed to penetration of the Hollytex by triethanolamine.
Ethomeen C 25 and C 12 can also migrate out of the gels, although not as easily as triethanolamine,
and can also be detected on the reverse side of the Hollytex fleece. In general it can be stated from
the investigations until now that the tendency of amine components to migrate increases strongly if
more than the minimum concentration of amine component is added in order to achieve gel
formation. Because of this fact, the amount of neutralization agent should be limited to the
minimum amount for maximum thickening.
The time required for the migration of amines to the substrate was also examined. Gel B (0.5%
Carbopol, Ethomeen C 25 1:6) was applied to Hollytex and was removed after 30 s or 15 min
respectively. It turned out that only a processing time of 15 s is sufficient to cause migration to the
reverse side of the Hollytex, which becomes visible as light yellowing after the artificial aging of
the samples. After 15 min, the yellow discoloration has almost reached its maximum. It can be
concluded from these results that Ethomeen C 25 starts migration together with humidity from gel
to substrate immediately after gel application. In gels of higher Carbopol concentration, such as a
2% Carbopol gel, on the one hand, the speed of this migration process is limited due to the higher
gel viscosity and the better solvent retention ability of the gel. On the other hand, this effect must be
seen as related to its circumstances, because a greater quantity of amine is present.
AGEING BEHAVIOR OF CARBOPOL GELS REGARDING THEIR APPLICATION ON PAPER
In the light of the results given above, there is a danger of contaminating paper with the amine
compounds. This contamination through migration of amine components takes also place if
Hollytex is used to interleaf gel and paper substrate because the Hollytex can be penetrated by the
amines. A further series of tests should examine whether the gels can be used on paper without risk
when they have been neutralized with sodium hydroxide or ammonia. A further question was
whether these gels can be brought into direct contact with paper or whether an interleaf is
necessary. In order to assess the ageing behavior of Carbopol gels, the artificial ageing conditions
must be intensified in comparison with the mild conditions in the preceding chapter.
Experimental
The gels under examination were coated to a quantity of 0.1 g onto carrier materials over an area of
2 X 1.5 cm. Carrier materials were Hollytex fleece and various types of paper listed in Table 3
together with the gels. The gels remained on the carriers and, after drying, were exposed to artificial
ageing. The extent of yellowing was evaluated by measuring the optical density using a
densitometer (Vipdens 200 P, Viptronic GmbH).
In a further series of experiments, 0.2 g of gel was not applied directly onto the papers but onto a
HoUytex fleece as an interleaf, covered with polyester film (Mellinex) and weighted down with a
glass bowl (50 g). After 15 h the Hollytex with the gels was removed. No gel permeates the
HoUytex provided it is weighted with a maximum of 50 g. After drying, no watermarks or staining
could be seen on the carrier papers.
The prepared samples were exposed to artificial ageing under the following conditions:
• exposure to light: 22 hours (apparatus parameters see under Results of ageing...); and
• 72 h aging in the climate chamber at 90°C with cyclic changes of humidity between 50 and 80%
RH after every 12 h.
Table 3a. Aging performance of Carbopol gels. List of tested Carbopol gels (Carbopol 940 1%)
b. Papers used as support materials
* Cold extract according DIN 53124.
** Mold-made, Biittenpapierfabrik Hahnemuhle, Dassel, Germany
*** Fa. A. Glaser, Stuttgart, Germany
Results of ageing the amine-free Carbopol gels
The direct coats of the gels on to the carrier papers show more and less heavy yellow staining on all
papers. A coat of methyl cellulose (Methocel A4M 3.5%, Dow Chemicals) applied as a reference
material did not discolor at all under the applied ageing conditions.
The degree of discoloring of the gels depends on such factors as composition and pH of the
substrate and the pH of the gels (Fig. 5). It was apparent that yellowing of gels was lowest in case of
application on Hollytex. Among the different papers tested in this series, filter paper resulted in the
highest discoloring of the gels. On the other papers, gels show various degrees of yellowing; see,
for example, paper 5 (old rag paper) in Fig. 6.
It can thus be concluded that the tendency of the gels to yellow strongly depends on the
composition of the carrier material itself. It can be additionally shown that yellowing increases if
gels with pH 9 are applied.
Gels neutralized by means of ammonia are in so far an exception as ammonia vaporizes, and the pH
of the gels therefore is not stable. This might be an expla-
Fig. 5. Discoloration of various gels coated on different support materials after the acclerated aging
procedure. Optical density measured on the front side, paper 4: on the reverse side.
Fig. 6. Discoloration of Carbopol gels K, L, M and N directly applied to paper No. 5 (old rag paper)
after artificial aging (A), prior to artificial aging (B).
nation for the fact that gels neutralized by means of ammonia generally show a lower tendency to
yellow.
Observations of individual papers ascertained several exceptions. Gels K and L containing
ammonia on paper 4 (neutral sized handmade paper, Table 3b) and to a less degree on paper 5 (old
rag paper with gelatin / alum surface sizing, Table 3b) turned dark brown during ageing, the
discoloration permeating to the reverse side of the papers. These observations might well arise from
the formation of yellowing ammonium compounds in the substrate as a result of the application of
gels K and L. On the other hand, gels M and N, which have been neutralized with sodium
hydroxide, reduce the brown staining arising through ageing on the reverse side of paper 4 (see Fig.
5).
In this context, it should be remembered that ammonia as a weak base can leave the gels in the same
way as the amines. On account of its volatility, proof of its existence during the experiments is
therefore only indirectly possible. Migration of alkalinity form gels containing ammonia is however
easy to trace by applying the gel (L) together with Hollytex as interleaf on an indicator paper, which
turns blue, indicating a pH between 8-9 a few minutes after gel application. The same experiment
with gel N (containing sodium hydroxide) shows no
migration of alkaline components to the indicator paper, even 1 h after gel application. It may thus
be concluded that sodium hydroxide makes a stable salt formation with Carbopol.
In case the gels are not directly applied to the paper but on Hollytex as an interleaf, no discoloration
on the papers could be observed after artificial ageing. The gels used for these tests are again gels K
to N (Table 3a), where ammonia and sodium hydroxide were used as neutralizing agents. The
conclusion can be drawn that the interleaf prevents the Carbopol gel from penetrating to the paper,
and the humidity migration does not transport discoloring agents to the substrate.
It can generally be deduced from the results that Carbopol gels should not be applied in direct
contact with paper because of the limited ageing performance. If, however, by means of an interleaf
such as Hollytex the direct contact with paper is prevented it can be used as a poultice material in
order to moisten paper. As the gel has to be neutralized, there is danger of migration of alkalinity,
which is lowest if sodium hydroxide is used as neutralizing agent. In case ammonia is the
neutralizing agent, it could be shown that the pH of the paper is increased by the result of gel
application and in some cases discoloration of the paper was observed. This discoloration could be
explained due to formation of ammonium components with additives present in the paper.
CONCLUSION
Based on the observations already described, the application on paper objects of Carbopol gels that
have been neutralized with organic amines appears somewhat questionable. Even thick intermediary
carrier papers become permeated with amines in due course, which leads to contamination of the
paper, despite the fact that triethanolamine as well as Ethomeen C-25 are soluble in water and can
theoretically, at least, be removed again if thoroughly washed. The question of their retention in the
paper is, however, open and should not be neglected -especially in the case of the presence of acid
groups in the paper. Apart from this, post treatment after local use by means of aqueous immersion
is not generally provided.
If Carbopol is neutralized with an inorganic base such as sodium hydroxide or ammonia, it can be
applied to paper. However, the above observations recommend avoiding direct contact of the gel
with the paper by means of an intermediary carrier, also heavy weighing to prevent the gel
permeating the intermediary carrier. These precautionary measures must be taken if the gels are to
be used locally without aqueous treatment afterwards. Under such conditions it is feasible to
remove adhesives that have been pasted over on papers sensitive to humidity.
MATERIALS
Carbopol resins: BF Goodrich Company, 6100 Oak Tree Boulevard, Cleveland, OH 44131, USA.
BF Goodrich Chemical (Deutschland) GmbH, Gorlitzer StraBe 1, D-4040 Neuss, Germany.
Carbopol 954 and Ethomeen C 25, C 12 can be obtained from G. Kremer, Farbmtihle, D-88317
Aichstetten, Germany.
Triethanolamine puriss. p.a., Fluka Chemie AG, CH-9470 Buchs, Switzerland.
Filter paper: white ribbon 5892, ashless, pH 4.3, Schleicher & Schiill GmbH, Postfach 4, D-37586
Dassel, Germany.
Hollytex fleece type 3265, thickness 0.127 mm, A. K. Diethelm AG, Lascaux Farbenfabrik, CH8306 Brüttisellen, Switzerland.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support of Henkel Austria, Vienna. For technical support
we also thank the Institute for Textile and Fiber Chemistry at the University of Stuttgart.
SUMMARIES
The Application of Carbopol Poultices on Paper Objects
The polyacrylic acid Carbopol has been employed for several years as a gel formative.
Characteristics and the ageing process of Carbopol gels are discussed in the light of their
application on paper. Investigations showed that neutralizing agents which are necessary to achieve
gel formation, are migrating with the moisture to the substrate to which the gel is applied. Special
attention is drawn to the behavior of (organic) amines used as neutralizing agents.
L'application de cataplasmes de Carbopol sur des objects en papier
L'acide polyacrylique Carbopol est employe depuis plusieurs annces commc formateur de gel. Les
caracteristiques et le comportement lors du vieillissement de gels Carbopol sont envisages en
fonction dc leurs applications sur le papier. Lcs recherches ont montre que des agents de
neutralisation, necessai-res pour pcrmettre la formation du gel, migrent avec l'humidite dans le
substrat sur lequel le gel est applique. On attire specialement l'attention sur le comportement des
amines utilisecs comme agents de neutralisation.
Die Anwendung von Carbopol-Gelen auf Papierobjekten
Die Polyacrylsäure Carbopol wird seit einigen Jahren als Gelbildner in der Restaurierung
eingesetzt. Es werden die Eigenschaften und das Alterungsverhalten von Carbopol-Gelen
hinsichtlich einer Ein-setzbarkeit auf Papier diskutiert. Die Untersuchungen ergaben, daß mit der
Feuchtigkeit auch die Neutralisierungsmittel, die fur die Gelbildung erforderlich sind, aus dem Gel
in das Substrat wandern. Besondere Aufmerksamkeit wurde dem Verhalten der Amine geschenkt,
die als Neutralisierungsmittel verwendet werden konnen.
REFERENCES
1. Singer, H., Dobrusskin, S. & Banik, G.: Behandlung wasserempfindlicher Objekte mit Gore-Tex.
Restauro 97 (1991): 102-111.
2. Wolbers, R. C: Notes for workshop on new methods in the cleaning of paintings. Marina del
Rey: The Getty Conservation Institute, 1988, 1990, 1992.
3. Petukhova, T.: Removal of varnish from paper artifacts. The Book and Paper Group Annual
(AIC) 11 (1992): 136-140.
4. Burnstock, A. & White, R.: Cleaning gels: further studies. Conservation Science in the UK.
Preprints Glasgow 1993: 36-39.
5. Carbopol-Resins-Handbook and Polymer-Handbuch fur die industrielle Anwendung. Cleveland:
BF Goodrich Company.
6. Haller, U.: Herstellung und Anwendung von Losungsmittel-Gelen, -Fasten und -Kompressen in
der Restaurierung. Diplomarbeit. Stuttgart: Staatliche Akademie der Bildenden Kiinste, 1994.
Edited by Institut fur Museumskunde der Staatlichen Akademie der Bildenden Kiinste Stuttgart,
No. 11, 1995.
7. Southall, A.: Wolbers' corner Workshop Notes. Conservation News 39, July (1989): 11-12.
Gerhard Banik
Agnes Blüher
Staatliche Akademie der Bildenden Ktinste Stuttgart
Studiengang Restaurierung und Konservierung von Graphik, Archiv- und Bibliotheksgut
Höhenstraße 16
D-70736 Fellbach
Germany
Ursula Haller
Hochschule für Bildende Kiinste
Fachbereich Restaurierung
GüntzstraBe 34
D-01307 Dresden
Germany
Elisabeth Thobois
Graphische Sammlung Albertina
Augustinerstr. 1
A-1010 Wien
Austria