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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.2 Concepción jun. 2004

http://dx.doi.org/10.4067/S0717-97072004000200013 

 

J. Chil. Chem. Soc., 49, N 2 (2004), pags.:173-178

WATER VAPOR PERMEABILITY AND MECHANICAL PROPERTIES OF CHITOSAN COMPOSITE FILMS

 

S. Patricia Miranda 1* , Omar Garnica1, Virginia Lara-Sagahon1 and Galo Cárdenas2

1 Laboratorio Biotecnología, Facultad Estudios Superiores Cuautitlán, Avda 1 de Mayo s/n, Cuautitlán Izcalli, UNAM (México)
2 Facultad de Ciencias Químicas, U. de Concepción, Concepción (Chile)

Dirección para correspondencia


ABSTRACT

Chitosan films or membranes can be obtained by casting chitosan solutions in acetic acid at 1 % w/v concentration at room temperature.

The films set were elaborated from 2% chitosan solutions mixed with three plasticizers (polyethylene glycol 200 (PEG), sorbitol and glycerol) and two surfactants (Tween 60 and Tween 80) at two concentrations. The goal was to decrease the water vapor permeability and to improve the mechanical properties of chitosan films.

The effects of plasticizers (PEG, sorbitol and glycerol) and emulsifiers (Tween 60 and Tween 80) at two concentrations from chitosan solution on water vapor permeability and mechanical properties of cast films were evaluated.

Water vapor permeability ranged from 3.64 to 6.56 g mm / h m2 KPa., tension force from 7.23 to 48.3 MPa and elongation percentage from 22.9 to 167.02 were obtained.

Chitosan two percent with 0.3 % Tween 80 produces films that showed the lowest permeability value. Chitosan with 0.6 % glycerol has the highest percentaje of elongation, being 4.5 times higher than chitosan films. Chitosan with 0.6% Tween 80 improved the hydrophobicity of the chitosan film in 1.14 times. The highest value for tension force was found for chitosan with 0.6 % of Tween 60. These values are similar to some polymers based on mineral oil.

Keywords: chitosan, chitosan films, thickness, water vapor permeability, mechanical properties.


INTRODUCTION

Chitosan (b-1,4-D-glucosamine) is an abundant lineal polysaccharide and obtained from chitin deacetylation composed of b-(1,4)- (N-acetylglucosamine) main constituent of crustacean shells. This abundant polymer with capacity of films formation, among many other applications at industrial level, it is employed in textiles, cosmetics, chemicals and medicines as well as in some water treatment processes. It is a material to which have been awarded a great quantity of applications due to its cationic character, biodegradability, non toxicity and biocide activity, with a promissory application in packing and foods covers as a barrier for contaminating agents like microorganisms, as well as the exchange of water and gases with the exterior (1).

The term chitosan describes a series of chitin macromolecules with different molecular weight, viscosity and degree of deacetylation (40-99%). It is a linear polyamine with a number of amino groups that are available for chemical reaction with acids or other active groups. Due to high molecular weight and a linear unbranched structure, chitosan is an excellent viscosity enhancing agent in acidic media. It behaves as a pseudoplastic material showing a decrease in viscosity with increasing rates of cut. The viscosity of chitosan solution increases with an increase in chitosan concentration, decrease in temperature and increase with the degree of deacetylation.(2).

Upon dissolution in organic solvents, the amine groups of the macromolecule become protonated, with a resultant positively charged soluble polysaccharide. Chitosan with a low degree of deacetylation (40%) has been reported to be soluble up to pH 9.0, while chitosan with a degree of deacetylation of about 85 % is soluble only up to pH 6.5 (3).

The promising applications of chitosan are edible films and coatings for foods, fruits and vegetables that would find new markets for existing materials and replace a great amount of non degradable synthetic materials (4).

Several authors (5-11) have studied chitosan films which exhibit selective permeability to gases, but a lack resistance to water transmission was observed. However, the incorporation of lipid molecules to provide hydrophobicity and plasticizers to modify their mechanical properties has been the objective of this work.

Surfactants such as tween 60 and 80 have been incorporated to improve the hydrophobicity. Plastizers like PEG, sorbitol and glycerol were blended to improve mechanical properties.

MATERIALS AND METHODS

Chitin and chitosan preparation. Chitin was prepared from shrimp shells. Demineralization with HCl 1N at 30 °C and deproteinization and NaOH 1N at 100°C was carried out. Chitin was deacetylated to obtain chitosan by chemical treatment using 50 % p/v NaOH at 100 °C for 1-2 h.

Glycerol, Polyethylene glycol 200 and acetic acid were from Baker, D-sorbitol, Tween 60 and Tween 80 from Sigma Chemicals.

Chitosan characterization.

Molecular weight determination of chitosan. An Ostwald viscometer of 0.45 mm, was used for chitosan. A thermostatic bath from Cole-Palmer was used. After getting the intrinsic viscosity from tables K and a, were obtained for HAc/NaAc. K = 0.076, a = 0.76 (12).

From these data and Mark-Houwinks equation the average viscosimetric molecular weight can be obtained. Mv =130,000 g/mol.

Deacetylation degree of chitosan. A pH meter WTW pH 531, combined glass electrode was used. The percentage of deacetylation in chitosan was determined by potentiometric titration (13). A 85 % degree deacetylation was obtained.

Base solution preparation for chitosan films:

Two percent chitosan solutions in 1% v/v acetic acid with stirring were prepared.

Solutions for films:

Chitosan solutions mixed with plasticizers and surfactants were prepared from the base solution; glycerol, sorbitol and polyethylene glycol, Tween 60 and Tween 80 were added to the concentrations of 0.3% and 0.6% w/v, respectively. In all cases, the incorporation of the mixture at 70°C with stirring was done. The air captured in the solutions during the mixture by centrifugation of the solution was eliminated.

Films preparation

Glass plates 10 cm diameter were cleaned and 21 ml of chitosan solution or chitosan solutions with plasticizer or emulsifiers were casted. The films were dried overnight at room temperature (25°C). Once the films are dried, the films are stored at environmental conditions ( 25 °C and 50 % Relative Humidity ) until used.

Film thickness

The film thickness was measured with a digital micrometer Mitutoyo with 0.001mm resolution. The data was taken in several points of the films and the average was calculated.

Water Vapor Permeability determination of chitosan films:

Gravimetric techniques are commonly used to determine WVP (Water Vapor Permeability) of edible films. This test has been standardized by ASTM E96-80 (1987) (14). Two versions of this technique, the desiccant method and the water method are available. In this work, the desiccant method was used.

The apparatus and methodology described in the ASTM (1980) "Water Method" as modified by McHugh et al. (1993) and were used to measure the WVP of the films (15).

The test specimen is sealed over the mouth of a test camera containing desiccant dry (CaCl2), and the assembly is placed in a controlled temperature and relative humidity (RH) chamber which has been previously set up for 24 h at 95 % ± 5 relative humidity, 28 to 32°C and 182 m/min of air flow. Three replicas of each film were measured. There was an air gap of 11 mm between the water and the under side of the film (see figure 1).


 
Fig. 1. Ambient chamber conditions are at 95 % ± 5 relative humidity, 28 to 32°C and 182 m/min of air flow. Test cup contain dry CaCl2. There is a gap of 11 mm.

The weight increase of the test cup was monitored through the time and a minimum of seven values were taken starting from the steady state.

The Water Vapor Transmission Rate (WVTR) was calculated from linear regression of the slope of weight gained in the test cup versus time and then by dividing the slope by exposed area of the films. Permeance (WVTR / P2-P3) was also calculated at 32 °C as described by Mc Hugh (1993) and Gennadios (1994), where P2 and P3 are the corrected partial pressure at the inner surface of the films and the pressure of the surface of dessicant. The correction of the air gap between the surface of dessicant and the inner face of the film was also consider (15,16,17).

An improvement of the technique was carried out by introducing a thermometer in the test chamber to measure the temperature of the stagnant air.

Effective Water Vapor Permeability (WVP) was calculated as the product of the Permeance and average thickness of the film.

The ANOVA-MANOVA (p < 0.05) was used to analyze the data.

Mechanical test of chitosan films:

The mechanical properties were evaluated with the standard method for tension established by ASTM 882-91(18).

The formed films were cut in ribbons of 2.8 cm wide and 10.5 cm long, and stored at 50 ± 2% of relative humidity and 30°C for 48 h. before the test. A texture analyzer TAXT2, (Texture Technologies, Corp., New York), at a transverse speed of 2 mm /s for tension was employed. The exposed film was 6.7 cm long.

Tension force and elongation to the fracture point were evaluated. Four film samples for each tension force were tested and ANOVA-MANOVA p < 0.05 statistical method for data analysis was used.

RESULTS AND DISCUSSION

All the chitosan solutions in the different formulations become transparent and free of bubbles.

The chitosan films obtained were compact, transparent, with smooth surface without pores or fractures at low magnification and they were easy to handle when dried off. They exhibited, same behavior as those reported by Wong for chitosan films (19).

Thickness effect of chitosan films on Water Vapor Permeability.

Table 1, shows the thickness of chitosan films, which increase when some component is added, being the lowest for 2% chitosan film and the highest for 2% chitosan film composite with 0.6% Tween 80. Thickness increases from 3 to 40 % regarding to chitosan films were observed.

This is in agreement with that described by Caner (11) for others chitosan films since he obtained thickness increases from 10 to 34 % when plasticizers were used.

 

Table 1. Chitosan films thickness

Film Thickness*
mm x10-2
 

2% Chitosan 2.82 ± 0.02
2% Chitosan + 0.3% PEG 3.09 ± 0.01
2% Chitosan+ 0.6% PEG 3.43 ± 0.01
2% Chitosan+ 0.3% Sorbitol 3.23 ± 0.00
2% Chitosan + 0.6% Sorbitol 3.39 ± 0.00
2% Chitosan + 0.3% Glycerol 3.41 ± 0.00
2% Chitosan + 0.6% Glycerol 3.68 ± 0.01
2% Chitosan + 0.3% Tween 60 3.23 ± 0.06
2% Chitosan + 0.6% Tween 60 3.73 ± 0.03
2% Chitosan+ 0.3% Tween 80 2.91 ± 0.01
2% Chitosan + 0.6%Tween 80 3.95 ± 0.02

*film thickness were measured with a digital micrometer Mitutoyo . The data was taken in 5 points of the films and the average was calculated.

The thickness effect due to plasticizers and emulsifiers do not follow a particular behavior pattern. But in general we can observe in the Table 1, that for higher solutions concentration there is an increment in thickness.

Ideal polymeric films exhibit no thickness effect on WVP; however, hydrophilic films often but not always exhibit positive slope relationships between thickness and water vapor permeability.

Several explanations have been provided for such anomalous thickness effect to cellulose films to different structures being formed at different thickness. Schultz attributed them to equilibrium moisture relationships at film-air interfaces differing from test cup solution equilibrium conditions (20).

Barrer and Banker are attributing thickness effects to film swelling as a result of attractive forces between films and water. Such film swelling could result in changing film structures (21,22).

The effective WVP correction method provides an explanation for such thickness effects (23).

The effect of film thickness on the corrected percent relative humidity conditions at the upper side of the film in the test cup indicated an exponential relationship between these factors. Without the WVP correction method to calculate the true relative humidity conditions at the film upper side, this relationship was not apparent or effective. As film thickness increased, the film shows an increase resistance to mass transfer across it; consequently, the equilibrium water vapor partial pressure at the inner film surface increased.

In this work, chitosan solutions precursors for films were mixed with plasticizers and emulsifiers. The thickness films were not only attributed to different film morphology but also depends on the nature of the molecules incorporated.

Water Vapor Transmission Rate (WVTR) determination for chitosan films

According to the terminology used by ASTM related to WVTR "is the steady water vapor flow in unit time through unit area of a body, normal to specific parallel surfaces, under specific conditions of temperature and humidity at each surface".

In Table 2 the WVTR values are shown for chitosan composite films. Chitosan films mixed with PEG 0.3% and chitosan films mixed with 0.6% of Tween 60 show significant difference in WVTR values, being the first one the highest transmission rate and the second one the lowest.

The lowest value was for the 2% chitosan film mixed with 0.6% Tween 60, while the higher value was the 2% chitosan film mixed with 0.6% polyethylene glycol. The lowest value is most probably due to the different composition of the emulsifiers, the stearic acid in Tween 60 is less polar than oleic acid in Tween 80, and in the higher values due to the conformational structure of the polymer matrix after mixing with PEG.

However, in the table 2 the small amount of plastifier (PEG and sorbitol) increases the value of WVTR which is probably due to the different morphology of the films due to the different interaction of sorbitol and glycerol with chitosan, glycerol is more polar than sorbitol. On the other hand, the Chitosan-PEG is able to produce partial crosslinking with chitosan at higher concentration, due to the intermolecular interaction between these polymers as it is reported by Jiang (24).


Table 2. Water Vapor Transmission Rate of chitosan films.

Films WVTR
g / h.m2

Chitosan 2% Peg 142.76 c ± 1.01
Chitosan 2% + Peg 0.3% 157.60 ± 1.18
Chitosan 2% + Peg 0.6% 141.42 b c ± 2.71
Chitosan 2% + Sorbitol 0.3% 145.67 c ± 0.10
Chitosan 2% + Sorbitol 0.6% 133.24 a ± 0.55
Chitosan 2% + Glycerol 0.3% 133.22 a ± 0.44
Chitosan 2% + Glycerol 0.6% 142.09 b c ± 1.27
Chitosan 2% + Tween 60 0.3% 140.63 b c ± 8.97
Chitosan 2% + Tween 60 0.6% 124.02 ± 3.73
Chitosan 2% + Tween 80 0.3% 135.07 a b ± 1.18
Chitosan 2% + Tween 80 0.6% 139.68 a b c ± 2.79

ANOVA-MANOVA and Tuckey Test Analysis (p< 0.05) .
Same superscript letters within a column means no significant difference.

The WVTR values obtained were higher, compared to the values for chitosan films reported by Wiles (20) for a 84% RH gradient, who obtained a WVTR average of 50 g / h.m2. However, the WVTR were lower in relation to those reported by Hosokawa (8) for a film made up of chitosan-cellulose.

According to Wiles results, no significant values in WVTR between different types of chitosan film are observed (23). The percentage of deacetylation of chitosan films and the viscosity of the cast solution did not have an effect on the WVTR properties of chitosan film.

Moisture sorption increases rapidly at the low water vapor pressures while increasing moderately at intermediate pressures from 7.0 to 12.5 mm Hg. At high water pressures, the moisture sorption increase becomes exponential (23).

Permeance

Is a performance evaluation and not a property of a material and is defined as "water vapor permeance", the time rate of water vapor transmission through unit area of flat material or construction induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions". This term is frequently used when the film is heterogeneous, unknown composition and thickness.

In heterogeneous materials like covers it is more adequate this terminology because gives more information about the efficacy as a barrier comparing different films of the same type and unknown thickness.

It has been reported an study with films prepared with chitosan solutions at 1% in acetic acid and it was found that relative humidity lower than 0.6 the films keep a low permeability coefficient but at high partial pressure, the gas pressure increases dramatically. This observation is more prominent for CO2 than O2, which is important for the packing of food products that keep breathing (25).

Water Vapor Permeability (WVP) for chitosan films

Water Vapor Permeability of a film is a constant that should be independent of the driving force on the water vapor transmission. When a film is under different water vapor pressure gradients (at the same temperature), the flow of water vapor through the film differs, but their calculated permeability should be the same. This behavior does not happen with hydrophilic edible films where water molecules interact with polar groups in the film structure causing plasticization or swelling.

Another assumption inherent to the calculation of permeability is its independence from film thickness. This assumption is not true for hydrophilic edible films.

Because of that experimentally determined water vapor permeability of most edible films applied only to the specific water vapor gradients used during the testing and for the specific thickness of the tested specimens, it has been proposed the use of the terms "Effective Permeability" or "Apparent Permeability".

In table 3, the results are reported as "effective permeability" for chitosan films, these values were taken under experimental conditions (RH 16% CaCl2 inside the cup and 95% outside the cup and 32 °C).

Table3. "Effective Water Vapor Permeability" of chitosan films

Film Effective Permeability
ASTM 1980
Effective Permeability
Schwartzberg 1986
Effective Permeability
McHugh (1993),
Gennadios (1994)

g.mm.
h.m2.KPa.
g.mm.
h.m2.KPa.
g.mm.
h.m2.KPa.

2% Chitosan 1.28 a ± 0.01 4.46 a b c d ± 0.11 4.17 a b c d ± 0.09
2% Chitosan + 0.3% PEG 1.55 c ± 0.01 7.27 ± 0.25 6.56 ± 0.20
2% Chitosan + 0.6% PEG 1.54 c ± 0.02 5.26 e f ± 0.35 4.93e f ± 0.300
2% Chitosan + 0.3% Sorbitol 1.50 b c ± 0.13 5.49 e f g ± 0.01 5.11 e f g ± 0.01
2% Chitosan + 0.6% Sorbitol 1.44 b ± 0.01 4.29 a b ± 0.05 4.08 a b ± 0.04
2% Chitosan + 0.3% Glycerol 1.44 b ± 0.12 4.32 a b c ± 0.04 4.10 a b c ± 0.03
2% Chitosan + 0.3% Tween 60 1.44 a ± 0.02 4.86 a ± 0.84 4.56 a ± 0.74
2% Chitosan + 0.6% Tween 60 1.47 b c ± 0.13 3.87 a ± 1.18 3.72 a ± 0.27
2% Chitosan + 0.3% Tween 80 1.25 b ± 0.14 3.84 b c d e ± 0.10 3.64 b c d e ± 0.0
2% Chitosan + 0.6% Tween 80 1.75 ± 0.03 5.81 f g h ± 0.38 5.47 f g h ± 0.34

ANOVA-MANOVA and Tuckey Test Analysis (p< 0.05) .
Same superscript letters within a column means no significant difference.

These conditions are completely opposite to most of the data already reported in which the relative humidity inside the cup is higher.

Water vapor transmission were measured and corrected by different calculation methods and because in this work the thickness of all films is known, the calculation of water vapor permeability is obtained by multiplying permeance by thickness.

From the three correction methods used, the Schwarztberg method shows the highest values of WVP, continued by McHugh, Gennadios and ASTM, the last one does not consider the space of air stagnant between the bottom surface of the film and the desiccant.

In the case of water vapor permeability evaluated by the Schwartzberg correction method, higher values were obtained.

The significant differences among the values of WVP for chitosan films are the same for both corrections of hydrophilic films, however, the McHugh (15) and Gennadios (16) methods are most accepted, and they have been used by many authors (26,27,28).

The highest permeability value is shown in this study for 2% chitosan films with 0.3% of polyethylene glycol, while the lowest is for 2% chitosan with 0.3% of Tween 80 as it is observed in table 3, representing a variation of 57% higher and 12% lower, respectively, compared to the permeability values of 2% chitosan films.

The polyethylene glycol and sorbitol behavior in chitosan films can be explained in the following terms, when a plasticizer is added, the free volume increases so does the permeability, but when the plasticizer concentration is increased, the film polymeric matrix structure changes. A similar behavior was observed by Cherian (5) for gluten films added with sorbitol and for Parris (25) for zein films prepared with propylene glycol.

In the case of addition of glycerol to chitosan films, the behavior observed can be explained due to the size and solubility of the molecule compared with sorbitol or polyethylene glycol, consequently a great amount of water is trapped in the matrix and swelling is promoted. This is in agreement with that reported by McHugh (15) for whey films prepared with glycerol.

The mechanism for the prediction of water transport through hydrophilic films like chitosan is more complex. The complexity is due to nonlinear water sorption isotherms and water content dependent diffusivities (17). If films are cationic and strongly hydrophilic water molecules interacts with the polymer matrix and increases permeation for water vapor (23)

With such a large increase in moisture, the films are swelling in some extent. Swelling would cause conformational changes in the microstructure of the film that not only increase moisture sorption, but also creates channels in the polymeric structure to allow the increase in permeant flow. Swelling results in deviation from Fickian behavior. A consequence of swelling is the changes in the polymer structure that occur in response to stresses generated within the film during sorption. This increase of water vapor solubility leads to an increase in water vapor permeability. Water vapor behaves as a plasticizer inside the chitosan film matrix (23).

Regarding to the use of surfactants in chitosan films, these were added with the purpose that the polar side of the surfactant molecule could be bonded to the polar part of the chitosan molecule, and the non polar group was located away from the chitosan molecule, creating a better barrier to water vapor.

Chitosan films blended with 0.3% of Tween 80, had the smallest thickness and the lowest permeability due to a better integration in the matrix, decreasing the free volume and the WVTR.

In general, mineral oil based polymers have a fixed ratio between oxygen and carbon dioxide permeabilities. However, some biobased materials like starch, the permeability of CO2 compared to O2 is much higher than conventional plastics (29).

When comparing the Water Vapor Transmittance (WVT) of several biobased materials to materials based on mineral oil, it becomes clear that it is possible to produce materials with WVT rates comparable to the ones provided by some conventional plastics (30).

Mechanical properties of chitosan films:

The mechanical properties tension force and elongation to the fracture point are summarized in table 4.

Table 4. Mechanical properties of chitosan films
1
Film Tension Force
MPa.
Elongation
%

2% Chitosan 39.47 bcd ± 6.6 37.44 abc ± 2.9
2% Chitosan + 0.3% PEG 14.57 a ± 1.7 49.56 acd ± 3.1
2% Chitosan + 0.6% PEG 7.23 a ± 0.02 31.64 abc ± 3.3
2% Chitosan + 0.3% Sorbitol 38.48 bcd ± 3.3 56.91 dc ± 4.2
2% Chitosan + 0.6% Sorbitol 35.47 bc ± 1.6 107.50 ± 3.7
2% Chitosan + 0.3% Glycerol 35.44 bc ± 3.4 79.20 e ± 0.42
2% Chitosan + 0.6% Glycerol 33.69 b ± 3.7 167.02 ± 3.6
2% Chitosan + 0.3% Tween 60 44.44 cd ± 4.6 39.10 abc ± 2.9
2% Chitosan + 0.6% Tween 60 48.37 d ± 2.5 48.15 acd ± 1.2
2% Chitosan + 0.3% Tween 80 31.54 b ± 4.4 24.03 ab ± 1.7
2% Chitosan + 0.6% Tween 80 18.51 a ± 1.5 22.98 a ± 2.1

ANOVA-MANOVA and Tuckey Test Analysis (p< 0.05) .
Same superscript letters within a column means no significant difference.

The smaller tension force was observed for chitosan films with 0.6% of polyethylene glycol and the higher for chitosan films with 0.6% of Tween 60. The tension force decreased in the 2% chitosan films when polyethylene glycol was added in both concentrations and also with 0.6% Tween 80.

The increase in plasticizer concentration decrease the tension force in the films, this behavior is more evident with PEG which not only is more permeable but also induces more fragile films.

The elongation data are shown in table 4, chitosan- 0.6% glycerol film present the higher value, being 4.5 times higher compared with the control.

For plasticized chitosan films with polyethylene glycol a tendency to decrease the elongation is observed.

The elongation results of plasticized films with glycerol and sorbitol, show a significant elongation increase when increasing the plasticizer concentration, being the higher values for chitosan-glycerol films and this is probably attributed to the fact that these films contains more plasticizer per mol that those containing sorbitol, since the molecular weight of sorbitol is twice than glycerol. Therefore, the molecular weight of the plasticizer affects the elongation properties. The glycerol molecules possess a very high dielectric constant and due to this higher polarity in solution than polyethylene glycol and even more than sorbitol who exhibits thicker films.

The different mechanical behavior of films containing sorbitol and glycerol, agrees with that reported by some authors (16, 31) who reported an increase in elongation when adding a plasticizer and/or an increase of concentration of the same one. However, it seems to disagree with that obtained for chitosan films with polyethylene glycol.

CONCLUSIONS

Chitosan films obtained were compact and transparent and their thickness increased when some component is added, but do not follow a particular behavior pattern with plasticizers or emulsifiers.

Chitosan solutions precursors for films were mixed with plasticizers and emulsifiers and their thickness is not only attributed to different film morphology but also depends on the nature of the molecule incorporated.

The lowest water vapor transmission rate was for the 2% chitosan films mixed with 0.6 % Tween 60, and stearic acid in Tween 60 is less polar than oleic acid in Tween 80.

Water vapor transmission were measured and corrected by different calculation methods, among them Mc Hugh and Gennadios methods are more used than Schwartzberg method.

The highest permeability value was for 2% chitosan films with 0.3% of PEG and the lowest was for chitosan with 0.3 % Tween 60. The last film improved their hydrophobicity and mechanical properties.

The more interesting result was the fact that during the measurements the temperature inside the test chamber was measured. An increase in the temperature was observed due to the reaction of water vapor with calcium chloride.

 

ACKNOWLEDGEMENTS

The authors would like to acknowledge the finantial support of Grant Cátedras 1.06 Biotecnología (S. P. Miranda) from Facultad de Estudios Superiores, Cuautitlán,UNAM.

 

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Correspondencia a: E-mail: mircas@servidor.unam.mx

(Received: June 4, 2002 ­ Accepted: February 5, 2004)

 

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