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Polymer Degradation and Stability 92 (2007) 244e252
www.elsevier.com/locate/polydegstab
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Degradation of agar films in a humid tropical climate: Thermal,
mechanical, morphological and structural changes
a
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Y. Freile-Pelegrı́n a,*, T. Madera-Santana a, D. Robledo a,
L. Veleva b, P. Quintana b, J.A. Azamar b
Department of Marine Resources, CINVESTAV-IPN, Carretera Antigua a Progreso Km 6, A.P. 73 Cordemex, 97310 Merida, Yucatan, Mexico
Department of Applied Physics, CINVESTAV-IPN, Carretera Antigua a Progreso Km 6, A.P. 73 Cordemex, 97310 Merida, Yucatan, Mexico
b
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Received 22 August 2006; received in revised form 16 November 2006; accepted 18 November 2006
Available online 26 December 2006
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Abstract
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Agar films were subjected to natural weathering exposure in a humid tropical climate for 90 days to determine their biodegradation behavior
and functionality. Exposed samples were taken at 15, 30, 45, 60 and 90 days. Mechanical, thermal, structural and morphological properties were
determined using tensile test, differential scanning calorimetry (DSC), attenuated reflectance infrared spectroscopy (ATR-FTIR), X-ray diffraction and environmental scanning electron microscopy (ESEM). The photodegradation process and temperatureerelative humidity fluctuations
promoted a decrease in agar mechanical properties in early exposure stages (30e45 days) caused by a reduction in agar molecular size and
a decrease in the number of sulfate groups. These changes alter agar crystallinity, causing contraction that leads to formation of micro-fractures
and embrittlement, and promote microbial attack. Accelerated weathering exposure of agar films showed that outdoor climate parameters play an
important role in their degradation. These results will aid in further research to determine the potential use of agar as an environmentally friendly
solution to the problem of biodegradable composites disposal.
Ó 2006 Elsevier Ltd. All rights reserved.
r's
Keywords: Agar; Biodegradation; Mechanical properties; Thermal properties; FTIR, DRX; Tropical humid climate
1. Introduction
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Biodegradable polymers have been developed recently in
response to public concern over the growing environmental
problem of plastic wastes. These wastes originate from the
use of non-renewable raw materials and accumulation of
this non-biodegradable packaging represents an environmental threat. The search for low-cost, environmentally friendly
materials has led to the development of different biodegradable plastics incorporating natural polymers (i.e. starch, cellulose) into conventional plastic formulations. When these
plastic blends are placed in biologically active environments
the natural polymers in them accelerate the degradation rate
by microbial attack and/or exposure to atmospheric agents
* Corresponding author. Tel.: +52 999 1242159; fax: +52 999 9812917.
E-mail address: [email protected] (Y. Freile-Pelegrı́n).
0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymdegradstab.2006.11.005
such as solar radiation, wind, rain and humid conditions,
among others [1]. Chemical degradation may also occur in
these blends when molecular bonds break due to the material’s inherent instability. All these processes aid in break-down
(degradation) of materials and consequent natural recycling
processes. However, one important reason why some natural
polymers are not incorporated into consumer products is their
degradation times that range from months to several years
(3e4). Also, these polymers are not as strong as synthetic
polymers. Many studies have focused on the search for biopolymers capable of substituting synthetic polymers or filled
polymers in packaging film, food or pharmaceutical coating
applications [2]. Several biopolymers (e.g. lipids, protein
and polysaccharides) have proved extremely interesting since
they meet a number of specific functional requirements
(moisture barrier, solute and/or gas barrier, water or lipid
solubility, color and appearance, good mechanical and
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
2. Experimental
py
The agar used was extracted by AGARMEX (Mexico) from
Gelidium robustum, the main agar source in Mexico [12]. The
agar films were prepared as follows: 3 g of agar powder were
dissolved in 200 mL distilled water at 98 C for 0.5 h; the hot
agar solution was poured into square plastic moulds, left at
room temperature for 24 h to gel and then dried at 60 C in
a vacuum oven for 36 h.
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2.1. Test site and sample exposure
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The test site was located in the ruraleurban humid atmosphere of Merida city (21 010 N, 89 370 W), 30 km from
the Caribbean coast. The Yucatan Peninsula tropical humid
climate is characterized by two seasons: dry (winter) and rainy
(summer). Temperatures follow a cycle of warming beginning
in February, a peak in May and a gradual decrease during the
rainy season until reaching minimum levels in Decembere
January due to cooler northerly winds.
The agar films were exposed on a stationary rack [10] fixed
to the laboratory roof (Fig. 1) at the site latitude angle (22 ).
Outdoor conditions were accelerated by covering samples
with glass (3 mm thick), with 5 cm of space between the glass
and samples, to increase temperature, simulating a greenhouse
effect. Triplicate samples were taken at 15, 30, 45, 60 and 90
days during the experimental period (JanuaryeApril 2004).
On-site temperature and irradiance were recorded every 30
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rheological characteristics, non-toxic) more effectively than
conventional synthetic materials [2].
Red marine seaweeds (Rhodophyta) are the source of
some promising biopolymers since they contain considerable
amounts of the polysaccharide agar, which has a unique
structure. In seaweeds, agar fulfils a function analogous to
that of cellulose in terrestrial plants, although it differs because marine seaweeds require a more flexible structure to
resist currents and wave motion [3]. It is extracted from
the cell matrix of seaweeds of the Gelidiaceae and Gracilariaceae families. Agar is a hydrophilic colloid consisting of
polysaccharides that have the ability to form reversible
gels simply by cooling a hot aqueous solution. It is composed of alternating 1,3-linked-D-galactose and 1,4-linked
3,6-anhydro-L-galactose units. This disaccharide can be
substituted by sulfate esters and methoxyl, and may also
carry pyruvic acid residues [4]. The type, amount and location of these substitutes strongly affect the physical properties of the gel and, therefore, its functionality [5,6]. Agar
gel melts on heating and resets on cooling. This cycle can
be repeated for an indefinite number of times without compromising gel mechanical properties. Cooling causes agar
thermal effusivity to increase to a maximum that coincides
with gelation [7], which is produced exclusively by hydrogen
bonds.
Because of its ability to form very hard gels at very low
concentrations, agar has been used extensively as a gelling
agent in the food industry and in other applications such
as microbiology and molecular biology techniques. More recent uses of agar include dental moulds, casting of archaeological pieces and sculpture moulds [8]. Due to its
combination of renewability and biodegradability, its enormous gelling power, and the simplicity of the extraction process [3], agar has been singled out as a promising candidate
for future use in plastic materials. Despite its promise, however, only one study has been done using agar in combination with synthetic polymers to produce materials with
degradable properties [9], and no research has been done
on its degradation behavior. The accelerated outdoor weathering test of degradation has been used recently in a standardized form [10], and has been employed increasingly in
response to growing industrial demand for decision-making
data. In these tests, outdoor weathering is done using the
sun as the irradiance source, but other studies have demonstrated that more than one weathering parameter can influence the rate of deterioration under these circumstances.
For example, it has been reported that environmental temperature of humid tropical climate seems to be the key factor
that impacts the rate of ageing of composites more than
the ambient humidity [11].
The present study objective was to determine the biodegradation behavior and functionality of agar, as an innovative biodegradable material, under accelerated outdoor weathering in
the humid tropical climate of the Yucatan Peninsula in southeast of Mexico. Morphological changes on agar films, as well
as any mechanical, thermal and structural changes were
characterized.
245
Fig. 1. Stationary rack for accelerated weathering test (for details see Ref.
[10]).
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
246
minutes by a data logger (HOBOÒ Onset Computer Corp., Pocasset, MA, USA) placed under the glass and close to the samples (Fig. 2A,B). Location air temperature and relative
humidity data during the experimental period were obtained
from the National Water Commission (Consejo Nacional de
Agua e CAN; http://www.cna.gob.mx) (data for days 7 and
80 are shown in Fig. 3A,B).
and 100 scans. The technique applied was attenuated total reflectance (ATR) with an Avantar multibounce HATR accessory with ZnSe crystal at 45 .
2.4. X-ray diffraction analysis
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Powder X-ray diffraction (XRD) patterns were recorded
with a Siemens D-5000 diffractometer (Cu Ka wavelength
of 1.5418 Å) operated at 35 kV and 25 mA. Agar films were
placed in a zero background rotary (15 rpm) Si sample holder
to avoid any background interference. Measurements were
made over the 5e60 (2q) angular range, at a step time of
8 s and at a step size of 0.02 (2q).
2.2. Molecular weight determination
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Molecular weight of the tested agar films was calculated
from their intrinsic viscosity values [h]. Intrinsic viscosity of
the agar samples in an aqueous 0.75 M NaSCN solution was
measured using a capillary viscometer (CannoneUbbelohde,
State College, PA) at 35 0.5 C. The weight-average molecular weight ðMwÞ was calculated with the MarkeHouwink
equation [13]:
2.5. Mechanical properties’ characterization
Tensile strength, strain at break and elastic modulus of the
agar films were tested using an Instron (model 4442) universal
tensile machine. The tensile procedure was done according to
ASTM method D882-00 [14], and each tensile testing was calculated with the Series IX ver. 5.1 program (Instron Engineering Corp., Canton, MA, USA).
2.3. Spectroscopic characterization
A
40
35
30
r's
Temperature (ºC)
45
Enthalpy (DH) and melting temperature (Tm) were measured with a differential scanning calorimeter (DSC, Perkin
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50
2.6. Thermal characterization
rs
The agar films’ FTIR spectra were determined using an
infrared spectrometer with Fourier transformation (Nicolet
model Nexus 870). Measurement range was 4000e
650 cm1, with a 4 cm1 resolution, 0.475 cm1/s scan speed
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h ¼ 0:07Mw0:72
25
20
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15
10
Irradiance (W m-2)
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50
B
40
30
20
10
0
0
15
30
45
60
75
90
Days
Fig. 2. In situ daily average (A) temperature and (B) irradiance of agar films during accelerated weathering exposure on stationary rack. Bars represent standard
deviation.
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
A
100
30
80
20
70
60
100
B
90
80
50
py
90
70
40
60
50
30
40
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30
20
2
4
6
8
10 12 14 16 18 20 22 24
Hours
rs
Fig. 3. Daily changes in air temperature (-), relative humidity () and in situ
agar film temperature (,) after 7 days (A) and 80 days (B) of accelerated
weathering exposure.
on
20
0
(p < 0.01). The AMW decreased drastically from 105,000
g/mol to w25,000 g/mol after 30 days of weathering exposure
(Fig. 4). Beginning at 45 days, AMW showed a slight but continuous decrease, reaching a minimum of w8000 g/mol at 90
days. Depending on the agar source, chain molecular weight
varies from 80,000 g/mol to 140,000 g/mol [13].
After exposure, agar film functional groups exhibited
changes in the FTIR spectra (4000e2500 cm1, Fig. 5A;
1800e800 cm1, Fig. 5B) in comparison to the typical spectrum of pristine agar film (0 day). Absorption bands between
3400 and 3200 cm1 (associated with OeH stretching [15]),
decreased notably, indicating water loss. At 60 and 90 days,
the peaks at 2850e2820 cm1 (associated with methoxyl
groups [5]) and 1640 cm1 (caused by stretching of the conjugated peptide bond formed by amine (NH) and acetone (CO)
groups [16]) decreased in intensity. Similar behavior was observed for the peak at 1370 cm1, which is associated with ester sulfate [5]. In contrast, the characteristics agar bands at
1070 and 930 cm1 (associated with the 3,6-anhydro-galactose bridges [17]), and at 890 cm1 (attributed to the CeH
of residual carbons of b-galactose [18]) showed no changes
in intensity with increased exposure time.
These results suggest that the agar photodegrades in sunlight. Photodegradation is the chemical transformation of
a compound into smaller compounds caused by absorption
of UV, visible, or infrared radiation, including irreversible alterations such as changes in molecules, protein denaturing,
and bonding with or cleavage from other atoms or molecules
[19]. The agar spectra observed here showed no significant
changes in galactose structure during exposure and bands
characteristics of this structure remained permanent. Galactose
chain length, however, was modified, as manifest in the agar
film AMW behavior at different exposure times. Changes in
AMW were attributed to chain scission, which produces a drastic reduction in AMW at 15e30 days exposure. Molecular
chain scission, however, does not open the galactose ring, suggesting that no major functional group transformation
co
40
Relative humidity (%)
Temperature (ºC)
50
247
2.8. Statistical analysis
Au
Film mechanical and thermal variables were tested for normality (KolmogoroveSmirnov) and homogeneity of variances
(Bartlett test). Weathering effects on mechanical properties
were determined with an ANOVA (p < 0.05). All statistical
calculations were run with the Statistica ver.6 (Statsoft, Inc.)
program.
105000
90000
Average Mw (g
th
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Morphological changes were described based on images of
the agar films. Differences in surface film morphology were
observed by environmental scanning electron microscopy
(ESEM, Phillips XL 30). Average film thickness was determined by measuring 10 different zones with an electronic
gauge (Mitutoyo, Japan) with a precision range between 0.1
and 1% as a function of thickness value (0e100 mm).
mol-1)
2.7. Morphological characterization
pe
Elmer model DSC-6). Samples of 10 mg agar film were heated
to 200 C at a rate of 10 C/min and then cooled to room
temperature.
75000
60000
45000
30000
15000
3. Results and discussion
0
3.1. Structural changes
0
15
30
45
60
75
90
Days
Significant changes in average molecular weight (AMW) as
a function of exposure time were observed in the agar films
Fig. 4. Agar film average molecular weight after 0, 15, 30, 45, 60 and 90 days
of accelerated weathering exposure.
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
248
1400
A
90
1200
60
1000
Counts
30
15
0
800
60
600
py
45
45
0
400
30
15
co
200
90
4000
3500
3000
2500
10
Wavenumber (cm-1)
B
20
30
40
50
60
Fig. 6. XRD analysis of agar films at 0, 15, 30, 45, 60 and 90 days of accelerated weathering exposure.
al
90
60
high-angle reflections recorded at this exposure time suggests a lower molecular state order that may be related to
the agar chain scission apparent in the AMW decrease.
The reason there being only minor changes in the diffractogram at 15 days exposure remain unclear.
on
45
30
15
rs
0
1600
1400
1200
1000
Wavenumber (cm-1)
800
r's
Fig. 5. ATR infrared spectra of agar films in the wavelength ranges of 4000e
2500 cm1 (A) and 1800e800 cm1 (B) at 0, 15, 30, 45, 60 and 90 days of
accelerated weathering exposure.
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occurred during exposure and that only free radical chain scission may have taken place. Dehydration and depolymerization
are generally considered as two main processes in the polysaccharides degradation mechanism [20]. The decrease in the methoxyl and sulfate groups observed in the longest exposure
period (60 and 90 days) was mainly due to depolymerization
of the agar. Similar results have been reported during degradation of carrageenan, another sulfated anionic polymer from red
seaweeds [21].
The XRD pattern for pristine agar film (0 day) showed an
orderly structure and a high degree of crystallinity indicated
by a peak at 19.9 2q and a slight shoulder at 13.83 2q
(Fig. 6). This peak was only slightly affected at 15 days exposure, but its diffraction intensity decreased drastically at
longer exposure times, reaching a minimum at 90 days.
The XRD pattern behaved similarly at 30, 45 and 60 days,
and the low 2q reflection becomes progressively more evident in the diffractograms. At 90 days the peak at 19.9
2q had narrowed, indicating that crystalline order in the
agar molecules had been altered. The lower number of
The stressestrain curve pattern indicated that agar mechanical properties changed as exposure time increased (Fig. 7). At
45 and 60 days, the agar films had become brittle, exhibiting
25e30% reduction in their strain values. By 90 days the samples could not be tested due to their extreme brittleness. The
films also experienced a progressive decay in tensile strength
(Fig. 8A) and strain at break (Fig. 8B), with a 50% overall reduction in both properties and minimum values recorded beginning on day 45. No significant difference (p > 0.05) was
observed between values for 45 and 60 days. Elastic modulus
3
Stress (MPa)
1800
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3.2. Mechanical changes
2
Exposure
time, days:
0
15
30
45
60
1
0
0
1
2
3
4
5
6
Strain (%)
Fig. 7. Stressestrain curves of agar films at 0, 15, 30, 45, 60 and 90 days of
accelerated weathering exposure.
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
115
A
110
300
2.0
1.5
B
250
95
200
90
85
80
75
6
70
0
15
30
45
150
100
50
60
75
90
Days
Fig. 9. Changes in agar film melting temperature and heat of fusion at 0, 15,
30, 45, 60 and 90 days of accelerated weathering exposure.
2
3.3. Thermal changes
on
C
Agar melting temperature (Tm) ranged from 74 to 97 C,
with no significant differences (p > 0.05) during the first 45
days of exposure (Fig. 9). It then increased and remained relatively constant, with no significant differences (p > 0.05) between longer exposure times. Heat of fusion DHmelt ranged
from 177 to 306 J/g, decreasing at 15 days and reaching a minimum at 30 days. This parameter then reached higher values at
45 and 60 days exposure, with no significant differences between values during the last 30 days of exposure. These increases in Tm and DHmelt of the exposed agar films may be
due to changes in molecular structure associated with ageing
behavior, such as greater crosslinking and chain scission. In
polymers exposed to solar radiation, side chain bonds break
and become crosslinking sites. This increased crosslinking inhibits relative chain motion, strengthens the polymer and
makes it more brittle [27]. These changes in crystallinity
with greater exposure time were confirmed by the melting enthalpy values shown in the DSC curves and the X-ray analysis.
Similar results have been reported in a study of starch, another
biopolymer widely used as biodegradable additive, in which
the authors state that biodegradation tends to increase thermal
stability of this polysaccharide [28].
rs
0
100
al
4
80
60
pe
Strain at break (%)
100
py
2.5
105
co
Melting temperature (ºC)
3.0
1.0
Elastic modulus (MPa)
350
Heat of fusion (J g-1)
Tensile strength (MPa)
3.5
249
40
20
15
30
45
r's
0
60
Days
th
o
Fig. 8. Agar film changes in tensile strength (A), strain at break (B) and elastic
modulus (C) at 0, 15, 30, 45 and 60 days of accelerated weathering exposure.
Au
(Fig. 9C), however, showed an increase at 15 days and reached
a maximum at 30 days. It then decreased at 45 and 60 days,
but still had values significantly higher (p < 0.05) than those
observed at 0 and 15 days. This increase may be explained
by crosslinking after the chain scission reactions [22], which
could increase film rigidity. The overall decay in agar film mechanical properties during exposure observed here may be associated with a reduction in molecular weight produced by
chain scission which is promoted in turn by photodegradation.
Studies on the effect of molecular weight on the characteristics
of agar [23,24] and other polymers [25,26] have shown that
the mechanical properties decrease as molecular weight decreases. Therefore, the decay in agar film mechanical properties observed here indicates a decrease in plastic
characteristics, which coincide with results reported for other
polymers [27].
3.4. Morphological changes
As exposure time increased, agar surface morphology
changed from translucent and smooth to irregular and
rough with bent edges. The ESEM images before degradation
(Fig. 10B1) and after weathering exposure show small spots at
30 days (Fig. 10B2) followed by randomly distributed fungus
growth and micro-fractures, and large unaltered areas (Fig.
10B3,B4). At 90 days, fungal and bacterial colonizations are
much more evident on the film surface and a network of
micro-fractures and holes in different patterns has clearly
degraded the surface (Fig. 10B5).
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
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250
Fig. 10. Surface morphology changes in agar film during accelerated weathering exposure. (A) Macro-morphological observations; (B) Micro-morphological observations; numbers indicate exposure time: 1 ¼ 0 day; 2 ¼ 30 days; 3 ¼ 45 days; 4 ¼ 60 days; and 5 ¼ 90 days.
Y. Freile-Pelegrı́n et al. / Polymer Degradation and Stability 92 (2007) 244e252
3.5. Agar degradation behavior
co
py
daily temperature and humidity led to deterioration in agar
film morphological, structural and mechanical properties during the early stages (30e45 days) of weather exposure as a result of decreases in molecular size and the number of sulfate
groups. These changes alter agar crystallinity, cause it to contract and lead to consequent formation of micro-fractures and
polymer embrittlement. These chemical and morphological
conditions promote microbial and fungal attacks. The agar
degradation process data reported here will be important in
further research on potential uses of agar as an environmentally friendly solution to the problem of biodegradable composite disposal.
Acknowledgements
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This research was financed by SAGARPA-CONACYT under project contract no. 2002-C01-1057. The authors thank C.
Chávez Quintal and D. Aguilar for technical assistance during
the experimental work. Mechanical and thermal properties
were determined at the FIQ-UADY and ITM-Merida.
References
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[7] Freile-Pelegrı́n Y, Bante J, Alvarado-Gil JJ, Yáñez-Limón M. Photothermal characterization of the gelation process in Gelidium robustum agar. J
Phys IV 2005;125:821e4.
[8] Armisén R, Galatas F. Handbook of hydrocolloids. In: Phillps G,
Willians P, editors. USA: Woodhead Publishing UK; 2000. p. 450.
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[10] Veleva L, Valadez-González A. Stationary rack and black under glass exposures of mineral filled polyethylene in inland and marine tropical climates. In: Durability 2000: accelerated and outdoor weathering testing,
vol. 1385. ASTM International STP; 2000. 61e72.
[11] Valadez-González A, Veleva L. Mineral filler influence on the photooxidation mechanism degradation of high density polyethylene. Part II:
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[12] Robledo D. The seaweed resources of Mexico. In: Critchley AT, Ohno M,
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[13] Rochas C, Lahaye M. Average molecular weight and molecular weight
distribution of agarose and agarose-type polysaccharides. Carbohydr
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[14] ASTM D882. Standard test method for tensile properties of thin sheeting.
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[15] Tako M, Higa M, Medoruma K, Nakasone Y. A highly methylated agar
from red seaweed, Gracilaria arcuata. Bot Mar 1999;42:513e7.
Au
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The present results indicate that during accelerated weathering exposure agar films absorb solar energy, mainly inducing
photodegradation, consequent chain scission and a decrease in
sulfate and methoxyl groups. Diminished mechanical properties caused by a reduction in molecular weight make the material more rigid and lead to the formation of surface cracks.
The drastic weather changes recorded in Yucatan during the
trial period enhanced the agar film biodegradation rate. As it
has been stated, the prevailing weathering process in the tropical humid climate of the Yucatan Peninsula can be extremely
complex, and involves a number of weathering factors acting
in conjunction to produce polymer degradation [29]. The extreme fluctuations in daily temperature and high humidity recorded during the present accelerated weathering exposure
trial promoted agar degradation. Similar results were found
by Jakubowicz et al. [30], who reported that moisture can
have a strong accelerating effect on polyethylene film degradation. Based on the weather data used here and previously reported data for the Merida ruraleurban environment [29],
temperature in the area generally peaks around noon (38e
41 C), when relative humidity reaches about 40%, and falls
at night, when relative humidity is about 100%. According
to ISO 9223, time of wetness (TOW) is defined as the time
during which a layer of moisture appears on a material surface
exposed to the environment. This phenomenon occurs when
relative humidity 80% at temperatures above 0 C [29].
These extreme daily changes in temperature and humidity
cause surface cracking on agar films. Moreover, agar has
a high hygroscopic character, which is important to maintain
its structure, meaning its water content increases as humidity
rises during the night. Agar is an aqueous gel the entire structure of which consists of polymer molecules linked only by
hydrogen bonds; this network can store a large amount of water and allows it to move freely through the macroreticulum.
Each molecule maintains its structure independently, so the
gelation process in agar is not a polymerization but rather
a simple electrostatic attraction [8]. Gelation of agar involves
its conversion from a fluctuating disordered coil formation in
solution to a rigid, ordered structure (co-axial double helix)
which forms the junction zones of the gel network [8]. This
pattern may explain why galactose rings in agar do not open
and it can therefore maintain its structure, despite molecular
chain scission during photodegradation. The decrease in molecule sulfate groups may promote microbial degradation on
the damaged agar films. This is based on the fact that sulfation
of a hydroxyl group in galactose residues can prevent enzymatic hydrolysis [31], and that reports on agar properties
have demonstrated the antimicrobial character of these groups
[32,33].
4. Conclusions
Accelerated weathering exposure of agar films suggests that
outdoor climate parameters play a significant role in the degradation process. Both photodegradation and fluctuations in
251
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