Impregnation via Supercritical Fluids - Principles and ... - ISSF 2012

ABSTRACT
Impregnation via supercritical fluids
Principles and applications
Eckhard Weidner
Ruhr University, Chair of Process Technology, 44780 Bochum and
Fraunhofer UMSICHT (www.umsicht.fraunhofer.de), 46047 Oberhausen, Germany
weidner@vtp.rub.de
Impregnation is defined as the process of imbuing or saturating a material or substance with something. In
technical applications impregnation is applied for modifying properties of bulk substances by physically or
chemically binding functionalizing compounds (impregnates) to a bulk material or surface. A prominent
example is to render wood resistant to water by impregnation with waxes and/or fats and/or resistant to
biological degradation by impregnating it with salts of zinc or copper. Providing impregnates in liquid solutes or
from a gas phase is state of the art, connected with disadvantages like low space yield in case of impregnation
from gas phases, slow diffusion processes and long process times in case of impregnation from liquid solutions.
Exploiting the specific properties of supercritical fluids is resulting in intensified processes with short cycle
times and considerable savings of energy and raw materials. In addition pressure allows modifying properties of
the bulk material during pressurizing or depressurizing.
Impregnates might either be bound physically or chemically in the substrate. In the first case the process is
mainly controlled by diffusion and adsorption, while in the second case chemical reaction rates have to be
considered as well. Based on a principle process scheme of impregnation the role of pressurized gases for both
binding processes is discussed. The principles are exemplified with recent developments such as the colorization
of polymers, the impregnation of hip implants, the impregnation of nuts with antioxidants and the chemical
impregnation (tanning) of leather with either inorganic (e.g. chromium) or organic tanning agents.
The examples illustrate that impregnation via supercritical fluids allows low process temperatures and high depth
of penetration at (very) short process times and avoids the use of (hazardous) solvents, time- and energy
consuming drying procedures, the overdosing of (expensive) impregnates. Cost break-down for an industrial
process for tanning leather indicates that impregnation via supercritical CO2 is highly competitive, with short
times for return on investment.
IMPREGNATION – DEFINITION, MARKETS
Properties of materials are defined by their chemical nature and physical form. Both can be modified in a wide
range by compounding bulk materials with additives. For generating such compounds a variety of mechanical,
physical and chemical processes like extrusion, agglomeration, precipitation, crystallization, spray drying and
grafting are known and applied. Following the definition that characterizes impregnation as process of imbuing
or saturating a material or substance with something, all mentioned unit operations can be addressed as
“impregnation”.
In a more detailed approach impregnation means to contact a bulk material (substrate) either in its final physical
form or as an intermediate material with a functionalizing substance (impregnate), which might be provided
from a gas phase, a liquid phase or a solid phase. The concentration of the impregnates in or on the surface of the
final product is low, typically some percent by weight or even (much) lower. They may be homogeneously or
non-uniformly distributed. Depending on their specific properties and the specific task, impregnates can be
provided as a pure substance, a solution or as a dispersion. In the final product impregnates can either be bound
physically or chemically.
A prominent example is the formation of leather, where an impregnate (tanning agent) is contacted in an
aqueous solution with carefully pretreated animal hides. The tanning agents diffuse in the skin matrix and (in
case of chromium or alumina) reacts chemically with collagens. The thus treated (impregnated) skin is called
leather. By impregnation the mechanical strength is improved by orders of magnitudes and the product is
rendered water- and temperature-resistant. Nowadays over 2 000 km 2 of leather are produced every year. The
turnover of leather as an intermediate product amounts to approx. US $ 45 billion. Approximately 90 % of all
leather is tanned by using chromium-III-salts. Leather manufacturing is extremely intensive with respect to
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consumption of resources. About 7 million tons of skin, 500 000 tons of salt and 500 000 tons of chromium-IIIsalt
are needed. An estimated overall amount of about 14 million m³ of wastewater is generated worldwide [1].
Articles made of plastics have to withstand exposure to heat, light, and humidity without changes in appearance
and mechanical performance. Furthermore neat polymers in most cases do not show satisfactory properties
regarding e. g. colour, surface touch or flammability. Regarding modification of mechanical performance,
appearance and functionality a broad variety of additives and fillers is used. The total global production of
plastics increased to 260 million tonnes in 2007 (Plastics Europe 2008). The average use of additives in plastics
is 5 to 7 % by weight which corresponds to a global annual additive consumption of 13 to 18 million tonnes a
year. Important groups of additives and fillers used in plastics modification are antioxidants, stabilizers,
lubricants, slipping and antifogging aids, flame retardants, blowing agents, crosslinkers, colorants and whitener,
reinforcing fillers and coupling agents [2]. Classically such substances are admixed by extrusion or kneeding
processes. An alternative is to provide the additives as impregnates from liquid dispersion or from gas phases.
The transport in the polymer is than mainly driven by intermolecular forces and is not supported by mechanical
forces, as it is the case in extruders. Shear and connected temperature stress can thus be avoided.
Fruits of oil plants, like nuts are cultivated worldwide and frequently are used for the preparation of food
products, soaps, greases or cosmetics. Well-known examples for fruits of oil plants are soy beans, coconuts,
peanuts, hazelnuts, almonds, walnuts, macadamia nuts, pumpkin seeds and sunflower or rape seeds. In Germany
about 250.000 tons of nuts are imported per year and the per capita consumption was 3,9 kg in 2009. The high
content of unsaturated fats contributes substantially to the nutritional value, but also makes the product sensitive
to oxygen. Storage time is limited as the product turns rancid. Well established processes to protect nut products
from oxygen are applied to prolong shell life. Coating with sugar or fat hinders the diffusion of oxygen. Roasting
reduces moisture so that the sensitivity for oxidation and deterioration is reduced [3]. A new and alternative
process would be to impregnate the nuts with antioxidants. Due to the high fat content, lipophilic antioxidants
are first choice. Those additives are typically either viscous liquids (e.g tocopherol, rosemary extract) or solids
(e.g. carnosic acid concentrates). Impregnation from liquid phases therefore would either be very slow or would
require the use of diluents (solvents), which have to be carefully removed after having helped to carry the
antioxidants into the solid matrix. To the authors best knowledge such a process has not been realized yet.
PROPERTIES OF CLASSICAL PROCESSES
Before impregnating hides with tanning agents the collagen molecules have to be pretreated at high pH-values in
the so-called pickling step. Subsequently tanning takes place in rotating drums over a period of 12 to 20 hours at
low pH-values, starting at pH 2.5 and increased to pH 3.8 at the end of the process. Due to partial hydrolysis of
skin compounds, penetration of the tanning agent at low pH value is facilitated. By steadily raising the pH value
the tanning agent bonds with reactive groups of the collagen (carboxyl groups). Leather of good quality is
achieved if 3.8 to 5 % of Cr2O3 by weight are absorbed and bound in the leather. Subsequently finishing with
colors, oils and specific chemicals brings about different forms of appearance like smooth leather for clothes,
stiff leather for shoes or soft leathers for furniture [1]. The skin can be considered as a chemical reactor, where
the impregnate (e.g. chromium) has to reach the reaction site and then reacts. Furthermore macroscopic
movement of skins in rotating drums has a significant influence on the transport of tanning agents, due to
renewing the tanning liquid between folded or attached skins. Therefore this impregnation process requires a
finely adjusted balance between mixing, diffusion and reaction rate. If the reaction rate is too high, only outer
layers of the skin are tanned and the leather is of very bad quality. If the reaction rate is too low (e.g at low pHvalues),
the skin swells and may even be destroyed. If the skins are not moved (mixing) the tanning agent is not
distributed evenly, which results in bad quality.
In contrast to compounding, impregnation techniques have hardly been used in the polymer processing industry
until now. Two processes have been published recently: the Bayer-Aura-Color-Infusion-Technology and the
Essilor impregnation-process of optical lenses. The processes differ in pretreatment before impregnation. Both
have in common that transparent bodies of polycarbonate are dipped into a bath for several seconds to hours at
elevated temperatures. Subsequently the parts are dried and the solvent has to be recovered. The achieved depth
of impregnation typically is around 20 µm [2].
In both cases the polymer chains have to be mobilised in order to facilitate the diffusion of impregnates. This can
be achieved by temperature and/or interaction with solvents or by partial degradation via radiation. The depth of
impregnation is rather low and the use of solvent requires careful drying.
From the examples can be derived that classical impregnation processes may have the following drawbacks:
• Low diffusion rates
• High temperatures
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• Limited depth of penetration
• (Very) long contact times
• Use of (hazardous) solvents
• Time and energy consuming drying procedures
• Overdosing of impregnates causing pollution and/or requiring recovery
• Sometimes, e.g. in the case of nuts impregnation procedures are not known
• High consumption of consumables (water, salts and so on)
IMPREGNATION PROCESSES VIA SUPERCRITICAL FLUIDS
Overcoming those drawbacks motivates the search for alternatives. One interesting possibility is the use of nearor
supercritical solvents. In certain applications (e.g. in case of wood) they have already successfully found their
way into industrial application [4] or have reached a ready-to-use stage of development, as it is the case for the
colorization of fibres and garments [5]. In recent years a number of new proposals have been made, a selection
of which is highlighted in the following. All processes have more or less similar process steps, which are
illustrated in Figure 1
Impregnate / Additives
Bulk
material
Supercritical Fluid
Conditioning
Supercritical Fluid
Energy
Compression
Mixing
Solution / Dispersion
Sorption
Expansion
Conditioning
(e.g. Drying)
End product
Figure 1: Basic Process Scheme for impregnation via near- or supercritical fluids
Impregnates and possibly additives have to be admixed with a compressed SF, mostly CO2. The phases leaving
the mixing stage can be:
• a homogeneous, molecularly dispersed liquid phase, comprising impregnate(s) and additive(s), that are
containing dissolved CO2
• a liquid phase that may comprise two or more partially miscible liquids (e.g. emulsion)
• a liquid phase that may comprise partially miscible solids (e.g. suspension)
• a gas phase, in which the impregnates are dissolved homogeneously (e.g. supercritical fluid phase)
• a gas phase, where liquid droplets and or solid particles are dispersed (e.g aerosol)
• and of course mixtures of the described phases
In opposition to classical impregnation processes, the use of near-critical fluids allows tuning the properties of
impregnates via pressure, temperature and the kind of gas used. For instance it is well known that viscosities and
interfacial tensions of liquids can be reduced by orders of magnitude, if a near-critical fluid is dissolved. It is also
well known, that near- or supercritical fluids may have a high solubility for large molecules and that this
solutions are characterised by very high diffusion constants. Near-critical fluids may also influence the
agglomeration between solid particles, which may favour their use for carrying suspended nanoparticles.
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Supercritical Fluid
Compression
Recovery
Optional steps
Required steps
Supercritical Fluid
Impregnate /
Additives

In the next step (sorption), the mixture of impregnate, additives and nearcritical fluid is contacted with the
substrate to be impregnated. This can either be done by once filling the vessel, containing the substrate, with the
near-critical mixture containing the impregnate (stagnant impregnation). Alternatively the near-critical mixture
can continuously be renewed (flow impregnation). In this case recovery of non-used impregnate and nearcritical
fluid might be required.
The involved species interact with the substrate via sorptive forces. For designing and understanding the
impregnation process knowledge of the number of phases and the properties of such mixtures and of the
interaction with the substrate is essential. Especially the interaction with the substrate is not entirely understood.
For polymers it is know, that glass transition temperature may be considerably reduced by near-critical fluids,
which results in an increased mobility of the polymer chains. As mentioned this is essential for transporting
impregnates into polymers.
If lipophilic substances such as fats or oils (e.g. in nuts) have to be impregnated, CO2 or propane have a rather
high solubility in such phases. The fat may swell by the dissolved gas and viscosity and interfacial tension will
be diminished. Lipophilic impregnates may be transported fast and effectively into such fat phases by means of
near-critical fluids.
In the presence of water pH-values may be of relevance for diffusion and reaction rates (e.g.in tanning).
Pressurized CO2 will form carbonic acid which, depending on pressure, may be as low as pH 3. In animal skins,
both lipophilic and hydrophilic regions are present. Collagens can be considered as biopolymeric compound,
with specific and not well understood interactions with CO2. If a water soluble tanning agents has to be
transported into the skin, accompanied and/or followed by a pH-triggered reaction with a biopolymer all the
above described transport phenomena may be of relevance.
After the impregnate has been transported into the substrate, gas and possibly other solvents have to be removed.
Here the compressibility of the near-critical fluid has to be considered. Even be a slight reduction of pressure,
solubility of impregnates in near-critical gases may be reduced considerably. Assuming a porous substrate, the
impregnate might precipitate in these pores during expansion and adhere to pore walls. The volume increase of
the expanding gas might induce rather high gas velocities in the pores which might withdraw precipitates. Gas
that is physically dissolved in the substrate under the impregnation pressure may form bubbles during expansion.
Thus mechanical forces are induced, that may cause cracks or even destroy the substrate or could be used for
structuring effects (e.g. foaming). During expansion, the gas cools down considerably, which may result in
formation of ice (if water is present) or dry ice (in case of CO2) or of other solids (e.g. solidification of
impregnates).
This short overview illustrates that nearcritical fluids have unique properties opening ways to new processes and
products. Some examples will be highlighted in the following chapter:
EXAMPLES – POLYMERS
Polyamid 12 in powdery form is used as a material for laser sintering. The polymer has a glass transition
temperature of around 37°C and a melting temperature of around 178°C. Two physically different substrates
(powder and a laser-sintered body) have been impregnated via supercritical CO2 with yellow dye (Teratop
yellow GWL, Huntsman Textile Effects, CH). It turned out that temperature has to be considerably above the
glass transition temperature to yield a satisfying result in reasonable time. The dye was introduced as powder
into the autoclave equipped with windows. From the chemical structure it is unlikely that the dye is soluble in
CO2. But at a pressure of about 19 MPa the gas phase looked uniformely and deep colored. It is assumed that the
dye is dispersed in CO2 and forms a kind of aerosol. Impregnations have been carried out above 19 MPa. While
at 23 MPa, 90 °C and duration of up to 20 minutes the results were not sufficient, an increase of temperature to
110°C at a pressure of 23 MPa resulted in a homogeneous colour distribution in the powder [2], as shown in
Figure 2.
CO2 has a limited solubility in PA 12. In own measurements in a magnetic suspension balance it was found that
under equilibrium 12,5 wt% of CO2 are dissolved in molten PA12 at a temperature of 175°C and pressure of
22MPa. It took several hours to reach equilibrium at a layer thickness of some mm of PA 12. The solubility in
solid PA12 at 110°C is not known, but is assumed to be in the same range. Furthermore it is likely that due to the
low contact time of 25 min equilibrium is not reached by far during impregnation. Due to a low content of CO2,
a rather high expansion rate of 3 bar/s (compared to nuts – see below) did not result in cracks, bubble formation
or changes in morphology of the powder. The colour was homogeneously distributed, stable during washing in
water at 70°C and after drying a homogeneous powder was obtained. The shape of the particles remained
spherical and the impregnated powder has a good flowability, which is a key parameter for laser sintering.
Alternatively a white laser-sintered body was impregnated at similar conditions (Figure 3). Complete
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impregnation was obtained, which is an indication, that the sintered structure has pores that do not form a
separate resistance to mass transfer, but that impregnation of the single particles is still the time limiting step.
Similar impregnations with colour and nanosilver dispersions have successfully been applied to other polymeric
materials such as amorphous thermoplastic polyurethane (TPU), polycarbonate (PC), semicrystalline polyamide
and polypropylene (PP) [2]. In addition to rather deep impregnation it turned out that the crystallinity of the
polymers may be modified. This is believed to be connected with the change in glass transition via pressurized
CO2 and the cooling rates during expansion.
Figure 2: Colorimpregnated Figure 3:High pressure colorized laser-sinter body
PA 12 powder[2]
before and after impregnation [2]
Gamse et al [6] investigated the impregnation of medical products with antioxidants. Hip implants can be made
from specific titanium alloys. Hip cups from polymers like crosslinked polyethylene (PE) are preferred in
combination with metal implants to reduce wear and friction. It is tried to
improve biocompatibility by adding antioxidants like alpha-Tocopherol to the
polymer. It is to be slowly released from the polymer after implantation. If the
antioxidant is added before cross-linking of PE at least a part of the Tocopherol
is destroyed during polymerization by free radicals. As an alternative the
crosslinked polymer is impregnated with antioxidant via supercritical CO2. The
tocopherol is dissolved in the gas and contacted with the polymer. The
supercritical solution of tocopherol in CO2 was contacted with model cubes
with a size of 20*20*20mm. The tocopherol is physically adsorbed. An
intermediate result is shown in Figure 5. If the temperature is in the range of the
glass transition temperature of UHWMPE (around 170°C) concentrations are
rising, but homogeneous distribution is not reached.
By optimizing process conditions uniform concentration of tocopherol in model
cubes was achieved. Finally hip cups were impregnated with tocopherol from
CO2 at 300 bar, 170°C in 12 hours. The absolute concentration and the rather
uniform distribution are shown in Figure 6. Finally it was found that careful
Figure 4: Hip implant from and optimised depressurization is required for a product of high quality.
Polyethylene with ultrahigh Expansion takes minimum 4 hours (average rate 1,25 bar/min) but 12 hours
molecular weight
(approx. 0,5 bar/min) are preferred.
Figure 5: Tocopherol concentration
in model cubes [6]
5
Figure 6: Tocopherol distribution in a cross section of a
hip cup after process optimization [6]

EXAMPLES - NUTS
Impregnation with antioxidants is not only of interest for medical products but also for food. Different types of
peeled nuts (almond, hazelnut, peanut, walnut, macadamia nut) were used in total, chopped, planed, grated or
ground form. As antioxidant two different rosemary extracts were selected. Extract 1 is a lightly yellow
deodorized and decolorized powdery extract obtained by solvent extraction containing 40% of carnosic acid
(CS). Extract 2 is a light orange oily liquid, which was obtained by high pressure extraction with supercritical
CO2 with containing 20% of CA. Different modes of mixing of extracts, nuts and CO2 have been tested. The
powdery extracts where provided in an impregnation autoclave located below a large volume (some liters) of
nuts. The system was pressurized and the extract diffuses through the CO2 phase into the layer of nuts (stagnant
impregnation). In a different approach the CO2 was admixed with the liquid extract before being continuously
introduced into the autoclave (flow impregnation). In a third mode the liquid extract was sprayed onto the nuts
before introducing them into the autoclave. Then the system was pressurized and the impregnation started
(stagnant impregnation).
Satisfying results with significantly prolonged shelf life without affecting the taste was obtained at pressures
between 28 and 32 MPa, temperatures between 50 and 60°C and an impregnation time of 60 min. From the
powdery extract at stagnant impregnation only one third of CS was extracted. With increasing distance to the
extract layer of the powdery extract the content of CS in nuts decreases. The liquid extract with lower content of
CA was significantly better as it is completely dissolvable in supercritical CO2. Stagnant impregnation with
previously contacting the nuts with liquid extract was superior and resulted in homogeneous distribution of
antioxidant, while flow impregnation resulted in reducing concentrations from bottom to top of the nut layer.
The CA-concentration in the impregnated samples was ranged from 20 to 100 ppm. Regarding acceptable taste
of the impregnated nuts concentrations below 50 ppm are recommended.
In Figure 7 the peroxide values during storage of chopped hazelnuts at room temperature are shown. Starting
with a peroxide value of 0, indicating a good quality of nuts, it took more than 240 days until a strong increase of
the peroxide value accompanied by a rising rancid taste occurred. Impregnation with CS results in significantly
better stability to oxygen depending on the amount of CS applied. Expansion has to be carried out carefully to
avoid cracking or grinding. A maximum rate of 3bar/min was found to be applicable. Results found in smallscale
experiments have successfully been verified in industrial trials with batches of several hundred kilograms
of nuts.
Figure 7: Oxidation measured via peroxide value at different concentrations of carnosic acid (CS) [3]
EXAMPLES - LEATHER
About 15 years ago, it was found that the process time for tanning animal skins can be reduced by a factor of 3 to
6 (which means from e.g. 24 hours to 4 to 6 hours) when the process is carried out under CO2 [7] and that the
consumption of water can be reduced by about 80 % by multiple use of tanning solutions in simulated countercurrent
process. In recent developments a method was found to reduce the waste water from tanning to almost
zero [8]. During classical pretreatment (softening, liming, pickling) skins that are fully saturated with water are
obtained. A part of that water (typically 20 to 40%), having a high pH-value (pH 11 to 12) is removed via
pressing or in centrifuges. The partly dewetted skins are contacted with a tanning solution (typically containing
chromium sulfate at a pH of 2,8), which is penetrating into the skin. Those pretreated skins are contacted with
pressurized CO2 (> 3 MPa) in rotating tanning drums. CO2 is partly diffusing into the skin and in the tanning
solution. It is believed that the skin structure is widened by the gas and that the pH-value of the aqueous tanning
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solutions is slightly increased by pressurized CO2 to values between 3 and 4. In developing the process into
industrial scale (tanning drum of some m³) it was found that tanning times can even be shorter than in smaller
equipment (drum < 20 liters). Leather of high quality is obtained already after 2 hours of contact with CO2. This
further reduction is due to the improved mechanical movement of the skins by rods, mounted in the rotating
tanning baskets.
Under classical tanning conditions the skins are subject to hydrolysis at a low pH-value. To reduce the so-called
swelling a rather large amount of salt (NaCl) is added. It was found that salt becomes obsolete, if the process is
carried out under CO2. After expansion at rates of 1 to 2 bar/min the wet skins are removed from the autoclave.
To find out whether the tanning agent is chemically bound in the skin, they are washed with a defined amount of
boiling water. Shrinking of the impregnated material would be a first indication for bad quality due to
degradation of non-reacted proteins. Leather with shrinking temperatures of 100°C and above is obtained.
Chromium content in the hot washing water would be an indication that the chromium is not fully bound. By
ICP-AAS extremely low concentrations of chromium in the washing water are found after CO2-intensified
tanning, indicating that practically all chromium present in the original tanning solutions is chemically bound in
the skin. In classical processes, chromium has to be overdosed in free water, to provide a reservoir and to
maintain a concentration gradient from the free liquid phase into the skin. Such overdosing is obsolete, if the
impregnation process is carried out under CO2 in the above described way. This leads to a considerably reduced
consumption of chromium for tanning. In Figure 8 the consumption of water, salt and chromium for a tanning
batch of typical size (3 t of skin) is compared.
Figure 8: Comparison of consumables in classical and CO2-intensified tanning process
Under the theoretical assumption that all skins in the world would be tanned via a CO2-intensified process, the
savings with respect to raw materials, required energy and Carbon dioxide emissions would be:
• Reduction of waste water containing chromium >95%,
o >14 million m³ per year
• Reduction of tanning time per batch
o from 12 to

sometimes high solubility of impregnates, solid substrates and gases there is a number of advantages of
impregnation via supercritical fluids compared to classical processes
• Temperatures are moderate or low,
• Depth of penetration can be (very) high,
• Contact times are short,
• Use of (hazardous) solvents can be avoided,
• Time and energy consuming drying procedures can be avoided,
• Overdosing of impregnates causing pollution and/or requiring recovery is reduced or avoided,
• Demand for consumables is reduced or avoided.
Process analysis and examples illustrate that exploiting the interaction of nearcritical fluids with (solid)
materials, the impregnating agents and further additives to improve processes and to generate new products
requires specific knowledge. Especially the interaction between solid matrices, impregnating agents and gas is
highly complex and not yet fully understood from the theoretical point of view, especially if solid materials have
directional-dependent properties (anisotropies). Nevertheless by experimental and empirical methods selected
processes and products have successfully been developed and introduced industrially, e.g for impregnating
wood. By experimental studies on the impregnation of other products mostly in research or small technical scale
specific features of the application of near-critical fluids have been discovered (bubble formation, swelling, pHvalue
shift for triggering reactive impregnation, modification of crystallinity, cracking of solids) and have partly
been investigated. The examples were selected to highlight the very high potential for that technique in a variety
of markets like technical polymers, medical applications, food products or natural materials like leather. This
selection is far from being complete as very interesting proposals have been made for drug delivery systems,
impregnation of inorganic materials like stone or concrete, for generating new catalyst systems (e.g. making
water soluble enzymes insoluble via low temperature impregnation) or for so called interpenetrating networks,
where monomers are introduced into a solid via nearcritical fluids and subsequently are polymerised. These
examples illustrate that the potential of this specific impregnation process can hardly be underestimated.
LITERATURE
1 Renner, M.; Weidner, E.; Jochems, B., Geihsler, H., Journal of Supercrit Fluids,
http://dx.doi.org/10.1016/j.supflu.2012.01.007
2 Renner, M., Weidner, E., Bertling, J., Proc 12th Eur. Meeting on Supercrit Fluids; Graz, 2010, Vol. 12,
ISBN 978-2905267-72-6
3 Gruener, S., Otto, F., Weidner, E., Impregnation of oil containing fruits, J. of Supercrit Fluids, accepted
4 http://www.superwood.dk/
5 Bach, E., Cleve, E., Schollmeyer, E., Rev. Prog. Color 32 (1), 2002, 88–102
DOI: 10.1111/j.1478-4408.2002.tb00253.x
6 Gamse, T., Marr, R., Wolf, C., Lederer, K., Chem. Ind. 61 (5), 2007, 229-232
7 Geihsler, H., Weidner, E., DE 197 07 572 A 1, 1995
8 Renner, M.,Geihsler, H., Weidner, E., DE 10 2009 018 323, 2009
8