Natural Thermotropic Materials For SolarSwitching GlazingNatuÈ rliche thermotrope Materialien fuÈ r solargeschaltete Blendschutzverglasungen

J. Schneider and A. Seeboth

Thermotropic polymer materials will permit new possibilities forthe development of intelligent sun protection glazing. In compar-ison to synthetic polymer systems biopolymers offer substantialbenefits. These are e.g. their good availability and environmentalcompatibility, but also their favourable price. This clarifies a pricecomparison for some well tested thermotropic polymer systemspresented in this paper. New investigations on the thermotropicbehaviour of hydroxypropylcellulose (HPC) are reported. Thechanging material characteristics arising from the combinationof HPC with hydroxyethylcellulose (HEC) and gellan gum are ex-plained in detail.

Thermotrope polymere Materialien bieten kuÈnftige MoÈglichkei-ten fuÈr die Entwicklung von Sonnenschutz-Verglasungen. Im Ver-gleich zu synthetischen Polymersystemen besitzen BiopolymereVorteile. Diese sind z. B. ihre gute VerfuÈgbarkeit und Umweltver-traÈglichkeit, aber auch der guÈnstige Preis. Ein Preisvergleich fuÈreinige gut untersuchte thermotrope Polymersysteme wird in die-sem Aufsatz dargestellt. Des weiteren wird uÈber Untersuchungenzum thermotropen Verhalten von Hydroxypropylcellulose (HPC)berichtet. Die sich aÈndernden Materialeigenschaften, die durchdie Kombination von HPC mit Hydroxyethylcellulose (HEC)und mit Gellan Gum entstehen, werden detailliert erlaÈutert.

1 Introduction

In a few years, the application of intelligent, the light entryself adjusting windows and glazing items [1], will be a natu-rally constituent of modern building architecture [2, 3]. Theseglazing utilize the characteristic of thermotropic materials tocloud themselves reversibly when warming up by the sunlightto a paper-white state and thus to reflect a large amount of theincident sunlight. In recent years a lot of polymer blends [4]and synthetically produced polymeric hydrogels [5] with awater content between 10% and 90% were examined as ther-motropic materials. In 1999 in Europe and in the US more then100 million m2 windows were produced [6]. With an esti-mated market share of only 1% in the next years about800 000 m2 per year of thermotropic glazing items could beproduced only in Central and Western Europe. With the grow-ing requirement of thermotropic materials, resulting from thisprediction, beside the purely physical aspects of the thermo-tropic materials e. g. turbidity-temperature, ± intensity and ±homogeneity, economic and ecological aspects will becomemore important.

In opposite to synthetic polymers the application of natural,modified or micro-biologically produced biopolymers offersthe possibility to fulfil both the request for inexpensive rawmaterials as well as the tendency to develop a non-toxic pro-duct, compatible to the environmental. As thermotropic bio-polymers above hydrocolloids [7] like modified or microbio-logically produced polysaccharides and excerpts from plantsand sea algae are subject of research.

The cellulose derivatives hydroxypropylcellulose (HPC)and hydroxyethylcellulose (HEC) examined by us belongsto that kind of chemical substances. Harmless biopolymersare already used for years in the foodstuffs industry [8] toa large extent as thickening and gelling agent as well as forpharmaceutical and cosmetic applications. These substancesvast fulfil the demands on price, availability and environmen-tal compatibility to a thermotropic material corresponding toreal market conditions.

2 Thermotropic polymers for sun-protection application

2.1 Hydrogels and polymer blends

These organic compounds show, in a system dependingtemperature range, a reversible and more or less sharp transi-tion from transparent to cloudy. Glazing filled with a thin layerof thermotropic hydrogel is transparent like glass below avariable cloudy temperature of e. g. 35 8C. Warmed up bythe sunlight, this glazing switch to a cloudy paper-white stateby there self without darkening the inner space of a building.The switching temperature and the intensity of turbidity canbe adapted well to the architectural and climatic conditions byvary the composition of the thermotropic gel. So the glareshield behaviour can be already adapted to the desires andsubjective feelings of the users before the installation ofthe glazing. Without mechanical or electrical control aswell as the energy input and installations necessary for it,these intelligent windows offer an automatic, solar heat con-trolled warming and light adjustment. Thermotropic architec-ture glazing could enable transparency of conventional glaz-ing combined with comfortable sun protection. Bright spacesin the winter, warmed up by the sun, as well as pleasantly coolspaces in the summer with reduced light entry, which enablesglare-free living and working. This intelligent type of glazingis ideally applicable for skylights in industrial buildings, pur-chase centres and museums or also for railing and balconyglazing in office and residential buildings. Also the applica-tion as transparent thermal insulated fronts is possible.Practicing bar becomes this by the use of thermotropic foilsor coatings on the basis of polymer blends or by filling a dou-ble-glazing with thermotropic substances (s. Fig. 1).

For the commercial use of hydrogels as light and heat filtersa set of requirements arise to the thermotropic systems:

Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001) 0933-5137/01/0303-0231$17.50 � .50/0 231Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001

± The system and all of its components should be innocu-ously, non-flammable, biologically degradable, inexpen-sive and insensitive to light.

± The systems should possess a high viscosity over a tem-perature range from 0 8C to 80 8C.

± The turbidity should change homogeneously over the wholearea of the glazing items, without streaks and with an ac-ceptable rate for the human eye.

± The temperature-dependent switching should take place be-tween a clear state with a transmission > 85% and aclouded, light scattering state with a transmission < 15%.

± The switching temperature should be adjustable between30 8C and 80 8C.

± Furthermore the transition must be highly reversible andreproducible during a long period.Polymeric hydrogels are proved systems, which fulfil the

most important of these requirements. They are used so farmainly in pharmacy [9] and medical [10] technology. Hydro-gels are chemical or physical cross-linked macromolecules, inwhich at least a part of the network consists of hydrophilicgroups. These polymer networks pour under accommodationof partially considerable water volumes [11]. The swelling ofa hydrogel results from cooperation between molecular inter-actions in aqueous, partial ionic polymer solutions on the onehand and elastic characteristics of polymer-networks on theother hand.

At low temperatures the macromolecules are homoge-neously mixed in aqueous solution. During rise in tempera-ture an aggregation of the polymers or quenching out of waterfrom the network takes place. If the phases indicate differentrefraction indices, the consequence is a clouding of the sys-tem. With layer thickness within the range of 1 ± 5 mm anextensive dispersion of the incident light with a high partof reflection takes place. Investigations on gelling [12], andon the swelling behaviour [13] were again strengthenedmade in the last years. This particularly applies to the phase

transitions in non-ionic hydrogels, discovered by Tanaka in1984 [14].

2.2 Synthetic thermotropic polymer systems

As suitable thermotropic polymer systems mainly water-soluble mixtures or copolymers from derivatives of poly (ac-rylic acid) [15], as well as systems based on poly (vinyl alco-hol), polyglycol, poly (vinyl acetal)-resins and polyether areexamined. In the year 1977 a thermotropic system of acrylamide polymers and poly (vinyl alcohol) or poly (vinyl capro-lactame) was developed as thermotropic component for thefilling of a double-glazing by R. Rullier (Saint Gobain Indus-tries) [16]. In 1993 D. Chahroudi [17] (Suntec Corp.) devel-oped a thermotropic system on the basis of poly (methyl vinylether) solved in water and cross linked with methylenebis-acrylamide. This system is commercially offered as `CloudGel', however with limited success. Also polymerised, ethy-lenic unsaturated N-substituted lactame, N-vinylsuccinimideas well as vinyl ethers and copolymers of these monomerswith cross-linking comonomers were also developed as ther-motropic gels [18]. The influence of different cross linkers aswell as of non-ionic tensides on the homogeneity of hydrogelswas also well examined, e.g. the gelling by crosslinking ofaqueous polyvinyl alcohol (PVA) with borates is a well exam-ined system, which can be used as gel-basis for the productionof thermotropic systems [19].

Since the first observation of their phase formation, syn-thetic lyotropic LC-polymers like poly(benzyl glutamate)(PBLG) and full-aromatic polyamides have attached consid-erable interest. Common features of these polymers are thegood solubility in water and a more or less rigid polymer back-bone (stiff chain of polymer), which enable the formation ofliquid crystalline phases [20]. In solutions of these polymers atransition from an isotropic phase to a polymer-solvent systemwith defined long range order is observed with increasingpolymer concentration. Lyotropic LC-polymer gels, whichare stable over a large temperature range in the gel state,could be realised by us in a PEG/PVA borax system [21].The temperature-induced transition of the optically anisotrop-ic LC phase into the isotropic phase was effected in this sys-tem without destruction of the gel network.

Sometimes lyotropic gels can posses several maxima andminima of transparency over a wide temperature range asdemonstrated by us on an aqueous system of ethoxylizedpoly(dimethyl siloxane) (ePS)/PVA-borax [22]. While heat-ing up the system a transition from cloudy to clear and againto cloudy can be observed. The reasons for the different statesof transparency are phase transitions in the lyotropic LC-sys-tem on one hand and phase separations at usually higher tem-peratures on the other hand.

2.3 Thermotropic systems from naturalcompounds

Gels and hydrogels on the basis of natural compounds suchas gelatine and polysaccharides are for a long time well knownand well examined, their thermotropic characteristics andtheir applicableness as thermotropic sun protection materialswere however hardly examined.

So far the hydroxypropycellulose (HPC) shows good ther-motropic characteristics. HPC is swelling in cold water and

Fig. 1. Possible construction of smart windows [1]: a: polymer-blend (� 0.2 mm) coated direkt onto the glazing. b: glazing filledwith a thermotropic hydrogel, thickness of the layer is 1 ± 2 mm

Abb. 1. MoÈgliche ,smart windows` [1]: a: Beschichtung der Ver-glasung mit einem Polymerblend (� 0,2 mm). b: Doppelvergla-sung, gefuÈllt mit einem thermotropen Hydrogel, die Dicke derSchicht betraÈgt 1 ± 2 mm

232 J. Schneider, A. Seeboth Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001)

results in a colourless, clear and viscotic colloidal solution,which is stable in presence of nearly all electrolytes. Theuse of HPC for thermotropic windows was already testedby H. Watanabe [23, 24]. In order to prevent the irreversiblesedimentation of polymer-aggregates occurring with thephase separation of HPC, as a further component the amphi-philic polyoxypropylen-2-ether-2-hydroxymethyl-1,3-propa-nediol (MW=400) was added to the thermotropic solutionfrom hydroxypropylcellulose and NaCl in water. The non-io-nic amphiphile should function thereby as a spacer, that pre-vents the complete phase separation.

2.4 Cost estimation for selected thermotropicsystems

Apart from technical and ecological factors the price will beof course a decisive parameter for a success of thermotropicwindows at the market. This price must be oriented at pricesfor high-quality thermal protection windows (two-way andtriple glazing). For an acceptable, marketable product themaximum price for the required filling components per kgshould be below a maximum value of 5 ± 6 Q. At this pricethe thermotropic systems can only consist of commerciallyavailable chemicals. Therefore the thermotropic systemshave to be produced without further synthesis steps at leastup to the successful introduction on the market.

In table 1, some examples for the prices of thermotropicmaterials are given. The comparison shows that most systemsare not suitable as thermotropic filling material because oftheir high prices (Tab. 1).

3 Experimental

Optical transmissionThe temperature dependence of the optical transmission of

the samples was measured at a wavelength of 600 nm in 1 cmcuvettes against water in the reference spectra by using an x-dap UV/VIS-spectrometer. At each temperature the sampleswere kept for about 20 min in order to achieve an equilibriumstate.

Optical reflectionThe temperature dependence of the optical diffuse reflec-

tion of the samples was measured at a wavelength of600 nm in 8 � 8 cm sample-windows with a layer thicknessof 2 mm against a water filled reference window by using anx-dap UV/VIS-spectrometer. The light source was adjustedperpendicular to the window and the detector in an angleof 458. For reference spectra OptisolÒ was used. At each tem-perature the samples were kept for about 20 min in order toachieve an equilibrium state.

Rheological propertiesThe rheological properties of the sample were studied by

using a dynamic stress rheometer SR-200 (Rheometric Scien-tific GmbH) fitted with a peltier heating stage. Samples of athickness between 0,5 and 1,5 mm were placed between par-allel plates and covered with a solvent trap. A dynamic testmode was chosen to characterise the rheological propertiesof the samples (frequency: 1 rad/s and an auto stress adjust-ment to keep the strain in the rate between 1,5% and 2%. Thesamples were heated from 0 8C to 90 8C at a heating rate of0,5 8C/min and data were collected every 10 seconds.

Table 1. Costs of the thermotropic filling material for a glazing with an area of 1 m2 and 1 mm layer thickness. The amount of aqueous,polymeric material, required for the filling, is 1 kg. The prices are oriented at the prices for chemicals with technical qualities of supplieresfor research chemicals. Price discounts for bulk amounts and costs for deionized water are not considered

Tabelle 1. Kosten fuÈr das thermotrope FuÈllmaterial einer Verglasung mit einer FlaÈche von 1 m2 und einer Schichtdicke von 1 mm. DiebenoÈtigte Menge an waÈssrigem polymeren Material ist 1 kg. Die Preise orientieren sich an Preisen von Chemikalien technischer QualitaÈt,wie sie von Anbietern von Feinchemikalien verkauft werden. PreisnachlaÈsse fuÈr Groûmengen und Kosten fuÈr deionisiertes Wasser werdennicht beruÈcksichtigt.

polymersystem compound 1 compound 2 compound 3 hydrogel

n-IPAAm/DMAm*(10%/10%)

n-Isopropylacrylamide1150 Q/kg 6

N,N-Dimethylacrylamide160 Q/kg X

217 Q/kg

n-IPAAm/Aam*(10%/10%)

n-Isopropylacrylamide1150 Q/kg 6

Acrylamide17 Q/kg X

116 Q/kg

PVA/Borax/(ePS)(13,4%/1,6%/6,6%)

PVA125 Q/kg

Ethoxylated Polydimethyl-siloxane (ePS)1000 Q/kg

86 Q/kg

Polyether/LiCl(80%/4%)

Polyether**2,5 Q/kg

LiCl70 Q/kg

4,80 Q/kg

HPC/HEC/NaCl(0,5%/1,5%/2%)

HPC565 Q/kg

HEC52 Q/kg

NaCl8 Q/Kg

4,60 Q/kg

HPC/Gellan/NaCl(0,5%/1%/1%)

HPC565 Q/kg

Gellan-Gum***250 Q/kg

NaCl8 Q/Kg

5,40 Q/kg

*) Costs of starting and crosslinking substances are not considered

**) Bulk price

***)Price estimated, free sample of Monsanto Inc.

Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001) Blendschutzverglasungen 233

MaterialsCommercial samples of HPC and HEC obtained from Al-

drich Co. were used. The Gellan sample (KelcogelÒ) werekindley supplied by Monsanto Inc. , Germany.

4 Results

We show in this paper, that the application of tensides is notnecessary for the deflocculation of suspended HPC, if in ad-dition to HPC a further polysaccharide is constituent of thesystem. We present here the combination of HPC withHEC (s. Tab. 2) in respect of their thermotropic behaviour,as well as the insertion of the HPC in a solid Gellan-Hydrogel(s. Tab. 3).

Contrary to other non-ionogenic cellulose ethers such asHPC/HEC-solutions don't show heat coagulation or a floccu-lation. With high shearing stress HEC-solutions show a tem-porary decrease of their viscosity, which achieves its initialvalue when the shearing stress is removed again (pseudoplas-tic behaviour).

For a solution with 2 wt.% HEC (HE014) a thermoreversi-ble gel point is observed. A separation of the dynamic visc-

osity into an elastic (G 0) and a viscous (G 00) part of dynamicviscosity can be used to detect processes like phase separationand gelling. Fig. 2. shows that at the gel point the elastic partof dynamic viscosity (G 00) prevail over the viscous part (G 0)of the dynamic viscosity (G 00 > G 0). With addition of 2 wt.%NaCl (HE017) to the 2 wt.% HEC-solution a strong shift ofthe gelling temperature range is observed. The gel point ofthe pure 2 wt.% HEC solution at 9 8C disappears (s. Fig. 2).By addition of 2 wt.% NaCl a nearly 100% decrease of thedynamic viscosity is observed. Here only at temperatures

Table 2. Mixing ratios (wt.%) of the examined samples of the system HPC-HEC-NaCl

Tabelle 2. MischungsverhaÈltnisse (wt. %) der untersuchten Proben des Systems HPC-HEC-NaCl.

sample HPC HEC NaCl total polymer content total solids content

HE001 0,30% 1,40% 2,0% 1,70% 3,70%HE002 0,60% 1,40% 2,0% 2,00% 4,00%HE003 0,30% 2,70% 2,0% 3,00% 5,00%HE011 0,30% 1,40% 0,0% 1,70% 1,70%HE013 2,00% 0,00% 0,0% 2,00% 2,00%HE014 0,00% 2,00% 0,0% 2,00% 2,00%HE016 2,00% 0,00% 2,0% 2,00% 4,00%HE017 0,00% 2,00% 2,0% 2,00% 4,00%

Table 3. Mixing ratios (wt.%) and temperature of cloud point ofthe examined samples of the HPC-Gelan-NaCl system

Tabelle 3. MischungsverhaÈltnisse (wt. %) und TruÈbungstempera-turen der untersuchten Proben des Systems HPC-Gelan-NaCl.

Sample* HPC NaCl TCloud

KG012 0,5% 1,0% 35 8CKG011 1,5% 1,0% 42 8CKG014 0,5% 1,5% 35 8CKG015 1,5% 1,5% 41 8C

*) all samples contain 1 wt.% Gellan-gum

Fig. 2. Temperature dependence of the visc-osity of samples with 2 wt.% HEC and 2 wt.%HEC � 2 wt.% NaCl. The dynamic viscosityg* on the left y-axis is separated into a viscous(loss module G 00) and an elastic part (storagemodule G 0) as shown at the right y-axis. Mea-suring parameters: parallel plates (25 mm),heating rate 0,5 K/min, frequency 1 rad/s,strain 1,5 ± 2%

Abb. 2. TemperaturabhaÈngigkeit der Visko-sitaÈt von Proben mit 2 wt.% HEC und2 wt.% HEC � 2 wt.% NaCl. Die dynami-sche ViskositaÈt g* (linke y-Achse) laÈût sichseparieren in einen viskosen (G 00) und einenelastischen Teil (G 0), wie auf der rechten y-Achse aufgetragen. Messparameter: parallelePlatten (25 mm), Heizrate 0,5 K/min, Fre-quenz 1 rad/s, Stress 1,5 ± 2%

234 J. Schneider, A. Seeboth Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001)

above 56 8C thermoreversible gelling occurs, which effects astrong increase of the dynamic viscosity. The effect of salts onthe gelling behaviour of thermotropic hydrogels is also animportant parameter for the range of the turbidity tempera-ture. The salt removes free water from the polymer systemand polar interactions between hydrates and polar fragmentsof the molecules such as oxygen or nitrogen atoms occurs. Thechange of the turbidity temperature can be detected well bymeasurement of the temperature dependent transmission.The pure 2 wt.% HEC solution show in the temperature rangeof the gel phase a decrease of transmission around approxi-mately 15%. Also the 2 wt.% HEC solution with additionof 2 wt.% NaCl show at temperatures aboth the gel point a

decrease of transmission of approximately 8% (s. Fig. 3).Mixing solutions of HPC and HEC results in a strong changeof the transmission behaviour and the viscosity compared withthe pure polymer solutions. With the addition of HEC to HPCsolutions with a constant content of HPC a strong reduction ofthe turbidity temperature is observed. This can be pursuedboth with the temperature dependent transmission and reflec-tion measurements of the sample windows (s. Fig. 4). Also theincrease of the HEC concentration (here nearly doubling) withconst. HPC and NaCl concentrations leads to a turbidity tem-perature lowered by 5 8C like the comparison of (HE001) with(HE003) demonstrates. As one recognizes by the comparisonof HE001 with HE002 this effect is also found if instead of theHEC concentration the HPC concentration is doubled. How-ever, if the NaCl concentration rises in mixtures with constantHPC and HEC portions an increase of the turbidity tempera-ture is observed like the comparison between (HE011) and(HE001) shows. Mixing 2 wt.% HPC with 2 wt.% HEC solu-tions in the ratio 1:4,6 (HE011) leads to a decrease of viscosityaround 60% compared to the purely 2 wt.% HEC solution.The elastic part of viscosity G 0 strongly decreases, so nogel point at 9 8C like for pure 2 wt.% HEC is observed. Asexpected the drastic viscosity change of the pure HPC solu-tion at the cloud point at 43 8C is cushioned. Compared to pure2 wt.% HPC solutions the turbidity takes place without grow-ing of larger aggregates and sedimentation of HPC. Thefurther addition of 2,0 wt.% NaCl (HE001) shows no drasticeffect on the turbidity temperature but a nearly 100% viscosityincrease of the HPC/HEC mixture is achieved, although anNaCl addition to pure HEC solution effects a strong reductionof the viscosity (s. Fig. 5). With temperature decrease the elas-tic part of viscosity G 0 rises relatively strongly by the additionof NaCl and therefore gelling below 1 8C is observed. Above50 8C however neither with nor without NaCl gelling is ob-served. However, if the HEC content in the system HPC-HEC-NaCl is continued to increase (here doubling) respec-tively the content of HPC is further lowered, two gel points

Fig. 3. Temperature dependance of the optical transmission at600 nm of 1 cm thick samples of solutions containing a) 2 wt.%HEC and b) 2 wt.% HEC � 2 wt. % NaCl

Abb. 3. TemperaturabhaÈngigkeit der optischen Transmission bei600 nm und 1 cm Schichtdicke fuÈr LoÈsungen mit einem Gehaltan a) 2 wt.% HEC und b) 2 wt.% HEC � 2 wt.% NaCl

Fig. 4. Temperature dependence of the optical transmission at600 nm of 1 cm thick samples with solutions containing 2 wt.%HPC and mixtures of HPC, HEC and NaCl (s. Tab. 1)

Abb. 4. TemperaturabhaÈngigkeit der optischen Transmission bei600 nm und 1 cm Schichtdicke fuÈr LoÈsungen mit einem Gehaltan 2 wt.% HPC und Mischungen von HPC, HEC und NaCl (s.Tab. 1)

Fig. 5. Temperature dependence of the dynamic viscosity g* ofsamples with mixing ratios as discribed in Tab. 1. Measuring para-meters: parallel plates (25 mm), heating rate 0,5 K/min, frequency1 rad/s, strain 1,5 ± 2%

Abb. 5. TemperaturabhaÈngigkeit der dynamischen ViskositaÈt g*von Proben mit MischungsverhaÈltnissen wie in Tab. 1 beschrie-ben. Messparameter: parallele Platten (25 mm), Heizrate 0,5 K/min, Frequenz 1 rad/s, Stress 1,5 ± 2%

Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001) Blendschutzverglasungen 235

are found. The first gel point at 14 8C is observed close to thetemperature range where the gel point of pure 2 wt.% HECsolution (9 8C) was found. The second gel point at 65 8C isfound close to the range of the gel point of the 2 wt.%HEC solutions with NaCl addition (56 8C) (s. Fig. 6 andFig. 7).

Gellan-Gum [25] is the extracellular polysaccharide, whichis formed by aerobic fermentation from the bacterium Auro-monas elodea. Gellan is a hetero polysaccharide of d-glucose,l-rhamnose and d-glucuronic acid (1,5:1:1). Gellan is disper-sible in cold water; in hot water soluble at 80 8C to 90 8C.Acetylated products give a weak and flexible gel, whosestrength is increased by mono or multi-valued cations. Alsothe temperature of the irreversible gelling increases by addi-tion of salts. These gels show e. g. compared with PVA/Boraxgels during rising the temperature no softening, but melts onlyat temperatures far above 100 8C. We found, that it is possibleto integrate HPC in the solid Gellan-Gum network and there-fore to create a solid temperature-stable thermotropic hydro-gel (Tab 3).

It is well-known, that particularly multi-valued salts as wellas lithium halides are suitable for physical cross-linking. Suit-able experiments with LiCl and citrates however alwaysshowed a lightly opalescent or synaeresis of the gellan-gels. The turbidity transition of all samples lies in the rangebetween 37 8C and 42 8C but the turbidity is however not asintensive as e. g. in the polyether/LiCl systems. The compar-ison of (KG011) with (KG015) shows that an increase of thesalt content causes a reduction of the switching temperature inmixtures in which the HPC-content (0.5 wt.%) is lower thanthe Gellan content.

On the other hand the comparison of (KG012) with(KG014) shows that an increase of the content of salt causesa rise of the cloud temperature in mixtures with higher(1,5 wt.%) HPC content than Gellan content. The low watercontent and the increase of the content of salt causes a strongincrease of the gel strength in this system, whereby the heatcoagulation of the HPC in the Gellan matrix is made moredifficult.

These two effects must be considered for a better under-standing of the influence of salt on the cloud temperature.An increased NaCl concentration lowers on the one handthe turbidity temperature of hydroxypropylcellulose. On theother hand it comes to a strong increase of gel-strength,which makes precipitating for HPC more difficult. This effectovercompensates the reduction of the turbidity temperature,thus it comes to the observed increase of the switching tem-perature.

5 Conclusion

HEC can be used due to the large structural similarity withHPC in HPC solutions as viscosity stabilizer and prevents anirreversible flocculation of HPC. The addition of sodiumchloride causes a sinking of the turbidity temperature inthe mixtures. The viscosity reducing effect of NaCl onHEC solutions disappears in mixtures with HPC. The combi-nation of HEC and NaCl with HPC leads to thermotropicsystems with lowered turbidity temperatures, which do notexhibit a precipitous change of viscosity in the temperaturerange from 0 8C to 70 8C. Also the integration of HPC intoa solid gallan gel matrix is possible by election of suitablemixture temperatures. In these gels the cloud temperaturecan be adjusted by changing the NaCl and total polymer con-tent. The maintained thermotropic gels show a homogeneousand reversible turbidity.

The effect of salts on the transmission behaviour of thermo-tropic hydrogels is an important parameter for the cloud tem-perature. The salt removes free water from the system by hy-

Fig. 6. Temperature dependence of the viscosity of the sample con-taining 0,3 wt.% HPC and 2,7 wt.% HEC � 2 wt.% NaCl. The dy-namic viscosity g* on the left y-axis is separated into a viscous (lossmodule G 00) and an elastic part (storage module G 0) as shown at theright y-axis. Measuring parameters: parallel plates (25 mm), heat-ing rate 0,5 K/min, frequency 1 rad/s, strain 1,5 ± 2%

Abb. 6. TemperaturabhaÈngigkeit der ViskositaÈt g von Proben miteinem Gehalt von 0,3 wt.% HPC � 2,7 wt.% HEC � 2 wt.% NaCl.Die dynamische ViskositaÈt g* (linke y-Achse) laÈût sich separierenin einen viskosen (G 00) und einen elastischen Teil (G 0), wie auf derrechten y-Achse aufgetragen. Messparameter: parallele Platten(25 mm), Heizrate 0,5 K/min, Frequenz 1 rad/s, Stress 1,5 ± 2%

Fig. 7. Temperature dependance of the optical transmission anddiffuse reflection at 600 nm of 1 cm (transmission) and 2 mm (re-flection) thick sample with solution containing 0,3 wt.% HPC and2,7 wt.% HEC� 2 wt.% NaCl. Parameters for the reflection-meas-urement: sample area 8 � 8 cm angle of incidence 908, referenceOptisolÒ

Abb. 7. TemperaturabhaÈngigkeit der optischen Transmission undder diffusen Reflexion bei 600 nm und 1 cm (Transmission) und2 mm (Reflexion) Schichtdicke von LoÈsungen mit einem Gehaltvon 0,3 wt.% HPC und 2,7 wt.% HEC � 2 wt.% NaCl. ParameterfuÈr die Reflektionsmessungen: ProbengroÈûe 8 � 8 cm, Einfallswin-kel 908, Weiûstandard OptisolÒ

236 J. Schneider, A. Seeboth Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001)

dration and it comes to polar interaction between hydrates andpolar molecule fragments such as oxygen or nitrogen atoms.Thereby in addition, prediction of the effect of salts on thecharacteristics of gels showed up is not always possible.

6 Acknowledgement

This work was supported by the Federal Ministry for edu-cation and research (Nr. 3029820B)

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Address: Dr. J. Schneider, WITEGA ± Angewandte-Werkstoff-For-schung-gGmbH, Justus-von-Liebig-Str. 3, 12489 Berlin, Germany,e-mail: a.seeboth@witega.de, j.schneider@witega.de

Received: 6/16/00 [T 293]

Richtlinie VDI/VDE/GESA 2635 Blatt 2 (Entwurf)

Experimentelle Strukturanalyse ± Empfehlungzur DurchfuÈ hrung von Dehnungsmessungen beihohen Temperaturen

Hrsg.: VDI Verein Deutscher Ingenieure, VDI/VDE-GesellschaftMess- und Automatisierungstechnik (VDI/VDE-GMA).Ausgabedatum: MaÈrz 2001, Preis: 104,90 DM.EinspruÈche bis 30. September 2001.Vertrieb: Beuth Verlag GmbH, 10772 Berlin, Tel.: 0 30/26 01-22 60,Fax: 0 30/26 01-12 60, E-mail: postmaster@beuth.de

Dehnungsmessstreifen (DMS) werden in der Industrie und in For-schungsinstituten in breitem Umfang bei der experimentellen Bean-spruchungsanalyse, der BetriebsuÈberwachung, zum Bau von Auf-nehmern und Messelementen fuÈr das mittelbare Messen mecha-nischer GroÈûen sowie in der QualitaÈtssicherung eingesetzt.

Die Richtlinie VDI/VDE/GESA 2635 Blatt 2 bietet Orientierungbei der Auswahl von Hochtemperatur-Dehnungsaufnehmern, die

direkt auf der PruÈfobjekt-OberflaÈche montiert werden und derenSensor-Element die Temperatur der PruÈfobjekt-OberflaÈche misst.Dazu erlaÈutert die Richtlinie die Begriffe der Hochtemperatur-Deh-nungsmesstechnik und fuÈhrt die Auswahlkriterien fuÈr Dehnungs-aufnehmer auf. Von den derzeit uÈblichen Hochtemperatur-Deh-nungsaufnehmern werden das Funktionsprinzip, wichtige Beson-derheiten und typische Anwendungen angegeben.

DaruÈber hinaus gibt die Richtlinie Empfehlungen fuÈr die zweck-maÈûige Vorbereitung, DurchfuÈhrung, Messunsicherheits-AbschaÈt-zung und Auswertung der Dehnungsmessungen bei hoher Tem-peratur, die sich in der Praxis bewaÈhrt haben.

Fachleute aus der Gemeinschaft fuÈr experimentelle Struktur-analyse (GESA), die zur VDI/VDE-Gesellschaft Mess- undAutomatisierungstechnik (GMA) gehoÈrt, treffen im Arbeitskreis¹Mechanische und elektrische Verfahrenª regelmaÈûig zusammen,um Ergebnisse aus Forschung und Praxis auszutauschen und soauf dem neuesten Stand in der DMS-Technik zu bleiben.

Mat.-wiss. u. Werkstofftech. 32, 231±237 (2001) Blendschutzverglasungen 237