Environmental Stress Cracking of Poly(3-hydroxibutyrate) Under Contact with Sodium Hydroxide

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Environmental stress cracking (ESC) is one of the most important causes of polymer premature failure, occurring when a combination of mechanical load and an aggressive fluid is applied. The phenomenon is well know by polymer producers and product designers but its mechanisms are not very well understood. Although the ESC effects of many commercial polymers are well known, this type of failure in biopolymers were not studied yet. In the current work, the stress cracking behaviour of poly(3-hydroxybutyrate) (PHB) with 4,0 and 6,2% of hydroxyvalerate (HV) was investigated in injection-moulded bars under contact with sodium hydroxide (NaOH) solutions. The experiments were conducted using two different types of stress arrangements: (i) an ordinary tensile testing and (ii) a relaxation experiment. In both situations the injection-moulded bars were exposed to the NaOH solution and some testing conditions where varied, like the cross-head speed of the tensile test and the maximum load of the relaxation arrangement. The results showed that NaOH acted as a strong stress cracking agent for PHB, causing surface cracking and reducing significantly the mechanical properties. Catastrophic failure with an extensive surface damage was also observed by photographed and scanning electron microscopy (SEM) images. The magnitude of the effects increased with decreasing crosshead speed and increasing loading level.

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DOI: http://dx.doi.org/10.1590/1516-1439.286114

Environmental Stress Cracking of Poly(3-hydroxibutyrate) Under Contact

with Sodium Hydroxide

_Rômulo Freitas Farias_ _a_ _, Eduardo Luís Canedo_ _b,c_ _,_

_Renate Maria Ramos Wellen_ _c_ _, Marcelo Silveira Rabello_ _c_

_aServiço Nacional de Aprendizagem Industrial – SENAI CIMATEC, Salvador, BA, Brazil_
_bInstituto Federal de Educação, Ciência e Tecnologia de Pernambuco – IFPE, Recife, PE, Brazil_

_cUniversidade Federal de Campina Grande – UFCG, Campina Grande, PB, Brazil_

Received: April 7, 2014; Revised: March 9, 2015

Environmental stress cracking (ESC) is one of the most important causes of polymer premature
failure, occurring when a combination of mechanical load and an aggressive fluid is applied. The
phenomenon is well know by polymer producers and product designers but its mechanisms are not
very well understood. Although the ESC effects of many commercial polymers are well known, this
type of failure in biopolymers were not studied yet. In the current work, the stress cracking behaviour
of poly(3-hydroxybutyrate) (PHB) with 4,0 and 6,2% of hydroxyvalerate (HV) was investigated in
injection-moulded bars under contact with sodium hydroxide (NaOH) solutions. The experiments
were conducted using two different types of stress arrangements: (i) an ordinary tensile testing and
(ii) a relaxation experiment. In both situations the injection-moulded bars were exposed to the NaOH
solution and some testing conditions where varied, like the cross-head speed of the tensile test and the
maximum load of the relaxation arrangement. The results showed that NaOH acted as a strong stress
cracking agent for PHB, causing surface cracking and reducing significantly the mechanical properties.
Catastrophic failure with an extensive surface damage was also observed by photographed and scanning
electron microscopy (SEM) images. The magnitude of the effects increased with decreasing crosshead
speed and increasing loading level.

Keywords: _PHB, stress cracking, mechanical properties, failure_

1. Introduction

Most polymer products are made from non-renewable
resources like oil-based chemicals. In recent years, polymers
made from natural products are gaining increasing interest
in several areas, including biomedical, packing, agricultural
and conventional consumer products. Among these
polymers, poly(3-hydroxybutyrate) (PHB), a semicrystalline
thermoplastic, biodegradable and biocompatible (nontoxic)
resin, is a promising material. PHB is obtained from renewable
resources (mainly sugar cane) by biotechnological processes
of low environmental impact. Ongoing research, intended
on establishing its properties, extending its applications,
and optimizing its processing conditions is reported in the
literature 1-6 .

During their lifetime, polymer products may be subjected
to aggressive environments, like high temperature, ultraviolet
radiation and certain chemicals, with consequences like the
deterioration of mechanical properties and/or poor surface
appearance. The investigation of the nature of these damages
is of great importance to reduce failure probability and hence
to design products with extended lifetime 7,8 . The reduction
of polymer properties by the action of an active chemical
agent under simultaneous stress is a phenomenon defined
as environmental stress cracking (ESC) 9 .

ESC is one of the main reasons for shortening the lifetime
of polymers, accounting for about 25% of the failures 10, where
the formation of surface crazes and cracks result in lower
properties. The magnitude of the damage caused by ESC
is related to size and distribution of the crazes and cracks
formed, together with their capacity for energy absorption,
and with the dissolution of the active agent in the polymer
(or polymeric compound) 11 . Even though the effects of stress
cracking on polymers is known for many decades 12,13, very
few investigations are reported in the scientific literature.
One of the authors of this paper has some contribution in this
field, with works on the combined effect of stress cracking
and chemical degradation of poly(ethylene terephthalate)
(PET) 14,15, poly(methyl meta acrylate) (PMMA) 16,17,
polycarbonate (PC) 18 and polystyrene (PS) 19 .

To the best of the authors knowledge, no work was
done on the investigation of ESC behavior of PHB. This
is considered to be an important gap in this field due to
increasing importance of this polymer in, for example,
medical prosthesis. In this application the product may be
subjected to body fluids and mechanical loads that might
cause ESC. For example, when using PHB implants in cats,
failure was reported at very low levels of stresses 20, but the
relation to ESC was not considered by the authors.

This work is concerned with the environmental stress

ki b h i f PHB i i h

solution of sodium hydroxide (NaOH) 3M (12%) acting as an
aggressive agent. The effect of NaOH in ESC mechanisms were
studied by tensile experiments under different arrangements.
Thermal properties and crystallinity were determined by
differential scanning calorimetry (DSC) whereas the effects
on surface cracking, and fractography of injection molded
bars were investigated by macrophotography and scanning
electron microscopy (SEM) analyses.

2. Experimental

Two grades of PHB were used, namely (i) FE-141, with
3-hydroxyvalerate (3HV) content 4.0%, MFR = 23-25 dg/min
(ASTM D 1238, 190 °C/2.16 kg), hereinafter called PHB-1; and
(ii) FE-112, with 3HV content 6.2%, MFR = 30-33 dg/min, called
PHB-2. Both resins have a density of 1.2 g/cm 3 (ASTM D 729)
and were supplied by PHB Industrial S/A (Brazil). Analytical
grade of sodium hydroxide (NaOH) was used as stress cracking
agent in concentration of 3M (12g/100 mL). The chemical
structure of 3HB and 3HV structural units are given in Figure 1.

Type I (ASTM D 638) tensile test bars were produced in a
Romi 130 injection molding machine, operating with a barrel
temperature ranging between 150-190 °C and the mold at 20
°C. Mechanical properties under tension (modulus, strength,
deformation at break) were measured at room temperature
(23 °C) with an EMIC DL2000 universal testing machine
with load cell of 5 kN; tests were performed with crosshead
speeds ranging between 0.1 and 5.0 mm/min. In addition, a
tensile relaxation test was conducted, in which a load (from
231 N to 1099 N) was pre-applied and its decay monitored as
a function of time for up to 20 minutes. In both cases, tensile
and relaxation conditions, the fluid was applied on the surface
of the samples during testing, following a procedure described
elsewhere 15 . After testing, the fracture and molded surfaces
were photographed with an Olympus 10 MP digital camera
and by scanning electron microscopy using a Shimadzu
SSX 550 Superscan equipment. Fractured surfaces were
sputtered with gold prior to SEM inspection to avoid charging.

Thermal properties and degree of crystallinity were
determined using a differential scanning calorimeter
DSC Q20 TA Instruments, with ~5 mg samples, at constant
heating rate of 10 °C/min, from 0 °C to 200 °C under nitrogen
flux of 50 mL/min. From DSC scans two melting endotherms
were observed, and melting temperature of peak I (lower
endothermic peak temperature) and peak II (higher endothermic
peak temperature), ( _TP_ )I and ( _TP_ )II, were determined, as well
as latent heat of melting, Δ _HM_, and crystallinity degree. The
crystallinity degree was computed from the melting peak
latent heat as:

Xc = ∆ H0m

∆ H
=

∆∆ HH0mm (1a)

c = ∆∆ HH0mm

where:

∆ _H_ _m_ = ∫

_mS_

t1

_H_ _m_ = ∫ J(t) - _mS_

_m_

tt12 0
J(t) -J (t)dt (1b)

1

where _J_ ( _t_ ) is the heat flow (W/g) between the sample and
the surroundings, _J_ 0( _t_ ) is suitable virtual baseline during the
event, _t_ 1 and _t_ 2 are the initial and final times of the event, _mS_ is
mass of the sample (mg), and ∆ Hm0 is the latent heat of fusion

mass of the sample (mg), and ∆ Hm0 is the latent heat of fusion

f h 100% lli l 0 146 / [13]

0 146 / [13]

3. Results and Discussion

_3.1. Differential scanning calorimetry (DSC)_
_measurements_

It is very well know that morphology and crystallinity
have a great influence on polymer performance 21 . To assess
the effect of HV content on crystallinity, thin films were
removed from the moulded surfaces and tested by DSC.
Since PHB is a high crystallinity polymer, well defined
melting peaks were observed with double melting for both
types of PHB, as shown in Figure 2. Complex melting
peaks were observed for both materials with the main ones
occurring in the range 142–179 °C and 137–176 °C for
PHB-1 and PHB-2, respectively. Multiple melting peaks
are commonly observed in many semicrystalline polymers,
including polyesters such as poly(ethylene terephtalate)
(PET) 22 and poly(trimethylene terephalate) (PMT) 23,
polyethers such as poly(ether ether ketone) (PEEK) 24,
and polyolefins such as polypropylene (PP) 25 . Multiple
melting peaks may be attributed to the melting of crystals
of different sizes and perfection. Specifically, small and less
perfect crystals melt at lower temperature relative to the
larger and more perfect ones. Multiple peaks may be also
attributed to the existence of different crystal modifications,
or to melting/recrystallization processes occurring during
the DSC scan 26,27 . The degree of crystallinity calculated
according to Equation 1 was 49.8% and 47.0% for PHB-1
and PHB-2, respectively. It is known that high crystallinity
is related to a compact and dense phase; it is also known
that hydroxyvalerate units (HV) inserted within PHB chains
tend to decrease its crystallinity 28 . Table 1 summarizes the
results obtained from DSC thermograms.

Figure 1. Structural units of 3-hydroxybutyrate 3HB (a) and
3-hydroxyvalerate 3HV (b). Commercial PHB´s used in the present
work are random copolymers of 3HB and 3HV.

Figure 2. DSC scans of PHB-1 and PHB-2 during heating at

_3.2. Stress cracking under tensile testing_

Figure 3 shows stress-strain plots for PHB-1 unexposed
and exposed to the NaOH solutions, with crosshead speeds
ranging between 0.1 and 5.0 mm/min. Even in the absence of
NaOH, PHB failed in a fragile way due to the combination
of high crystallinity and high glass transition temperature.
Figure 3b shows a strong effect of the crosshead speed when
NaOH solutions are applied during tensile testing. The
lower the crosshead speed, the longer is the contact of the
aggressive solution with the polymer surface, increasing the
effects of stress cracking. This caused in a great reduction
in both tensile strength and maximum elongation and,
therefore, showed that NaOH is a strong stress cracking
agent to PHB. Even a brief exposure to NaOH during the

Table 1. Melting parameters of PHB-1 and PHB-2 determined
during heating at 10 °C/min.

Parameter PHB-1 PHB-2

Melting range (°C) 125.9 – 182.6 118.3 – 171.9
( _TP_ )I (°C) 166.0 153.8
( _TP_ )II (°C) 172.5 167.6
Δ _HM_ (J/g) 101.2 87.8
_XC_ (%) 69.3 60.1

Figure 3. Stress-strain curves of PHB tested under several crosshead
speeds (indicated) with specimens unexposed (a) and exposed (b)

The surface of PHB sample bars, tested at several
crosshead speeds, unexposed and exposed to NaOH,
were analyzed in photographic images. Figure 4 shows
unexposed and NaOH exposed PHB-1 bars after tensile
testing at 2 mm/min. In Figure 4a some micro-fissures on
the shell region of the bar are observed. This is the region
most susceptible to the occurrence of ESC due to stress
concentration effects. During ordinary tensile testing, microfissures or cracks can be originated by chain stretching and
slipping of the crystal planes 30 . When NaOH solution is
applied during testing, the effects of stress cracking appear
as crazes and cracks at the sample surface as a result of
the physical and/or chemical attack 8,31,32 . The presence of
tiny crazes in a brittle polymer may be a way do absorb
energy before fracture, acting as toughening agent 33 . This
was not the case here, since the tensile properties, including
maximum elongation, were reduced in contact with the
active fluid. Instead, the crazes and cracks act as stress
risers, causing a deterioration in mechanical properties.
Figure 4b shows the macrophotograpy of PHB-1 exposed
to NaOH, which acted aggressively and led to the formation
of deeper micro-fissures and cracks.

In Figure 4c the formation of a greater number of deeper
crazes is observed when lower crosshead speeds are used,
which coincides with lower mechanical properties (Table 2).
The slower the crosshead speed, the longer is the time that
the sample remains under the stress, favoring the surface
damage. When this type of specimen was tested under
NaOH solution much more cracks is noted (Figure 4d),
including the formation of deeper cracks, unable to absorb
a high amount of energy before failure and thus causing a
more brittle fracture. According to Bernier & Kambour 12,
when the ESC is highly aggressive there is a tendency to
form just one crack on the specimen surface that propagates
very fast into the interior and no other surface crazes are
observed. Similar tendencies were observed before by one

h h f hi k 14 15 17

test resulted in a remarkable effect on the stress-strain
behavior. This chemical has already shown to attack PET 14 .

The final results of tensile modulus, tensile strength,
and deformation at break of PHB-1 and PHB-2, unexposed
and exposed to NaOH are summarized in Table 2. Firstly,
an increase in tensile strength with increasing crosshead
speed for the unexposed bars is evident, and this is due to
viscoelastic effects 29 . Exposure to NaOH did not affect the
elastic modulus significantly, meaning that the solution did not
cause plasticization in an extent to soften the bulk of polymer
structure. The most acceptable theory of stress cracking
mechanism considers that the action of the fluid is restricted
to sites of high stress concentration without widespread
plasticization 10 . The remarkable effectof NaOH was seen
in the ultimate properties, tensile strength and maximum
elongation, which reduced by 30% and 40% respectively
when compared to the unexposed samples. The effect was
stronger at low crosshead speeds, when reduction of these
properties of 60 and 70% was observed. We suggest that
stress and exposure times to the aggressive agent promoted
formation of crazes, cracking and, possibly, breakage of
molecular chains 14 .

_3.3. Analysis of macroscopic images_

Figure 5 shows images of PHB-2 exposed to NaOH and
tested at 1.0 mm/min (Figure 5a) and at 0.1 mm/min
(Figure 5b). It is clear that cracking is notably more severe
when the sample is tested at 0.1 mm/min when compared
to the one tested at 1.0 mm/min, following the same trend
showed in Figure 4. The pattern of surface cracks, however,
are different between the two types of PBH´s. It is not clear
the reasons why these two polymer behave differently,

but the stiffer chemical structure of the valerate units in
PBH‑2 may be a reason.

In the present work similar values of the crystallinity of
PHB-1 and PHB-2 were obtained (see Table 1). However, it
is possible that changes of the inner structure were caused
by the addition of valerate units, which lead to a more open
or irregular structure in PHB-2, as a result of the ethyl side
group of the valerate compared to the methyl group of the
butyrate (different polarities and energy status).

Table 2. Mechanical properties of PHB unexposed and exposed to NaOH 3M.

Mechanical

Property

Crosshead Speed

PHB-1 PHB-1 + NaOH PHB-2 PHB-2 + NaOH
(mm/min)

Tensile
Modulus

Tensile 0.1 2393 ± 142 2114 ± 115 2478 ± 29 2332 ± 137
Modulus 1.0 2294 ±  29 2447 ±  41 2224 ± 71 2379 ± 252

(MPa)

2.0 2436 ±  94 2243 ± 270 2179 ±  4 2172 ± 134
5.0 2195 ±  96 2106 ±  23 1959 ± 21 2139 ± 251
Tensile 0.1 24.51 ± 0.54 8.32 ± 0.74 20.03 ± 0.46 7.80 ± 0.93
Strength 1.0 25.35 ± 0.85 19.37 ± 2.05 22.77 ± 0.44 18.14 ± 0.41

(MPa)

Tensile 0.1 24.51 ± 0.54 8.32 ± 0.74 20.03 ± 0.46 7.80 ± 0.93
Strength 1.0 25.35 ± 0.85 19.37 ± 2.05 22.77 ± 0.44 18.14 ± 0.41

(MPa)

2.0 26.50 ± 0.12 19.33 ± 0.06 21.79 ± 0.08 20.61 ± 0.71
5.0 27.18 ± 0.02 24.67 ± 0.38 27.23 ± 0.21 21.08 ± 2.27
Deformation 0.1 1.76 ± 0.32 0.47 ± 0.01 0.95 ± 0.00 0.43 ± 0.00
at Break (%) 1.0 1.68 ± 0.06 0.97 ± 0.08 1.47 ± 0.19 0.90 ± 0.05

(MPa)

0.1 1.76 ± 0.32 0.47 ± 0.01 0.95 ± 0.00 0.43 ± 0.00
1.0 1.68 ± 0.06 0.97 ± 0.08 1.47 ± 0.19 0.90 ± 0.05
2.0 1.57 ± 0.00 1.10 ± 0.03 1.39 ± 0.14 1.09 ± 0.12
5.0 2.25 ± 0.18 1.85 ± 0.12 1.65 ± 0.04 1.15 ± 0.05

Figure 4. Macrophotographic images of the surface of PHB-1 samples after tensile testing: (a) unexposed to NaOH at 2 mm/min,
(b) exposed to NaOH at 2 mm/min, (c) unexposed to NaOH at 0.1 mm/min, (d) exposed to NaOH at 0.1 mm/min.

Figure 5. Macrophotographic images of the surface of PHB-2 samples after tensile testing, (a) exposed to NaOH at 1.0 mm/min,

_3.4. Stress cracking under stress relaxation_

In this experiment, test bars of PHB-1 and PHB-2 were
preloaded to 25%, 50% and 75% of the maximum stress
measured in the standard tensile tests, and the machine
stopped at a constant deformation. During time, the stress
relaxation takes place, which is a typical effect of the
viscoelastic behavior of polymers 29 . During this procedure,
NaOH was applied to the bar surface and the decay of load
was monitored with time up to a maximum of 20 minutes.
Typical results are given in Figures 6 and 7 as plots of load
vs. time. It was observed that both PHB-1 and PHB-2 not
exposed to NaOH withstand the stress without fracturing
during the pre-established 20 minutes. The decay in load
was similar in both polymers and was more intense when
higher initial loads were applied. When the experiment was
conducted with the bars exposed to NaOH the decrease in
the load was significantly higher and, very important, in
many samples the experiment did not reach the end of the
exposure time of 20 minutes since the bar fractured before
that. From these curves, the relaxation rate was determined
as the ratio between the decay of load and the exposure time
(20 minutes or until fracture). Table 3 shows the effect of
the preloads on the relaxation rate of PHB-1 and PHB-2.
There is an increase in the relaxation rate with the initial
load for all the systems investigated.

Figure 6. Stress relaxation curves for PHB-1 samples tested without

All samples tested under stress relaxation and in contact
with NaOH failured before the end of the defined exposure
time of 20 minutes. The effect of the preload on the time
of failure is shown in Figure 8. The higher the preload the
more rapid the sample breaks. The effect is more intense
with PHB-2, indicating that, under these conditions, this
grade is more susceptible to the stress cracking effect under
the contact with NaOH solutions.

Aggressive external environment reduces the time that
the material can withstand high stresses without failure.
The formation of surface crazes and cracks is the most
visible effect of stress cracking that result in fracture during

Figure 7. Stress relaxation curves for PHB-2 samples tested without
(a) and with (b) NaOH.

Figure 8. Time to failure of PHB-1 and PHB-2 as a function of

stress relaxation experiments. Breaking of linkages of tie
molecules, dislocation and slips of crystalline planes, are
also possible factors that may lead to the formation of
open (less dense) structures that may facilitate the access
of NaOH to the inner layers of PHB. This condition could
promote local plasticizing and lead to craze initiation,
growing and, ultimately, to fracture. Since ESC depends
on the diffusion of the chemical agent into the polymer
structure, the rate of fluid absorption is a critical parameter
affecting the rate of craze initiation and extension. The faster
the chemical agent is absorbed, more rapidly the polymer
will be subjected to crazing and subsequent failure 10 . In
a previous work 34 PHB-2 was observed to absorb higher
quantities of NaOH than PHB-1.

Figures 9 and 10 show images of the surface of test
specimens subjected to stress cracking during stress relaxation
tests. The appearance of surface damage is evident, which
become more visible and intense as load increases, which
also leads to shorter times until fracture. The magnitude
of these defects is more intense in PHB-2 than in PHB-1,
following results shown before (Figures 4 and 5).

Hydrolysis reactions may occur between the water
(H2O) present in NaOH 3M solution and the ester group
(COO) of PHB, releasing reaction products containing
acid and alcohol groups. The hydrolysis is a chemical
degradation reaction very common in polyesters like
PET and polycarbonate 35 and is of major importance in
defining their lifetime. Since the hydrolysis rate increases

significantly with increasing temperature 36, and this work
were performed at ambient temperature (23 °C), hydrolytic
reactions may be a contributing factor, but probably not
the decisive one, for the failure of exposed samples. This,
however, is not discharged since it took place with PET
in contact with NaOH solutions 14 .

Localized plasticizing effects certainly take place in
samples exposed to the NaOH solution and is associated to
the main reason for ESC deterioration. NaOH diffuses into
the polymer structure in sites where stress raisers are present
and initially promote the formation of a gel (solubilization),
which facilitates the formation of a swollen structure and
allows cracks and crazes to develop easily, by breaking
chemical bonds in the macromolecular chains and tie
molecules (disentanglement) leading to plasticization. The
imposition of an external load contributes to plasticizing
by facilitating the formation of voids and pores and hence
the diffusion of NaOH.

Saponification reactions between NaOH and PHB-1 or
PHB-2 occur primarily on the surface. The ion OH − of the
NaOH reacts with the carbonyl carbon of the PHB, which
is positively polarized, leading to the formation of alcohol
and sodium salt. The reaction is irreversible and promotes
the polymer degradation. In the existence of pores on the
samples shell, the entrance of OH - is facilitated and promotes
the breaking bonds of inner layers leading to degradation
and decreasing the mechanical strength of PHB.

Table 3. Relaxation rate under several preloads of PHB-1 and PHB-2 samples unexposed and exposed to NaOH.

Preload (N)

PHB-1 PHB-1 + NaOH PHB-2 PHB-2 + NaOH

Relaxation Rate (N/s)

275 (25%*) 0.047 ± 0.005 0.530 ± 0.160 231 (25%*) 0.061 ± 0.007 0.292 ± 0.035
549 (50%*) 0.117 ± 0.021 3.198 ± 0.537 461 (50%*) 0.111 ± 0.002 2.082 ± 0.103
824 (75%*) 0.191 ± 0.009 8.738 ± 1.760 692 (75%*) 0.144 ± 0.008 8.581 ± 1.140

(*) Corresponding percentages of the the breaking stress during tensile experiment of the unexposed sample.

Figure 9. Macrophotographic images of sample surfaces of PHB-1 after tensile testing at 2 mm/min exposed to NaOH, preloads: (a)
275N, (b) 549N, (c) 824N.

Figure 10. Macrophotographic images of sample surfaces of PHB-2 after tensile testing at 2 mm/min exposed to NaOH, preloads: (a)

_3.5. Scanning electron microscopy (SEM)_

The fracture surfaces of PHB-1 bars broken under tensile
testing are shown in Figure 11 for samples tested under
a strain rate of 0.1 mm/min. The images (a), (c) and (e)
correspond to the upper border, middle and lower border,
respectively, of PHB-1 bar not exposed to NaOH whereas
the images (b), (d) and (f) are the corresponding areas of the
sample tested under contact to NaOH solution. In general,
unexposed samples showed a typical behavior of a fragile
material, which is consistent with the mechanical behavior
(Figure 3), and not many rough zones. Rough areas are
related to ductile behaviour (with energy consumption
before fracture) and smooth areas to brittleness (without
energy consumption before fracture). Several mirror zones
are evident, indicating that fracture initiation occurred
from more than one spot. Cracks and void (indicated
by arrows) are also seen. Porosity may be related to the
existence of internal defects (pre-existing and originated
during processing), which are places of stress concentration.
In Figure 11b a large area with whitening characteristic
is prominent, and is related to the ESC, which causes
crazing that alter optical properties due to light scattering
by the voids. Even though the stress cracking is a surface
phenomenon, the propagation of crazes and cracks took
place into the sample interior. Higher magnification images
of the samples in Figure 11 are given in Figure 12, showing
in better detail features like voids, mirror zones and rough
areas.Figure 13 presents SEM images of PHB-1 fractured
bar after stress cracking under stress relaxation tests and

Figure 11. Scanning electron microscopy of fractured surface of
PHB-1 bars, during stress _versus_ strain tests at 0.1 mm/min. Images
(a), (c), (e): upper border, middle and lower border, respectively,
of unexposed bars. Images (b), (d), (f): upper border, middle and

Figure 12. Scanning electron microscopy of fractured surface of
PHB-1 bars, during stress _versus_ strain tests at 0.1 mm/min. Images
(a), (c), (e) of unexposed bars, images (b), (d), (f) of bars exposed to
NaOH. Images (a) and (b): low magnification; images (c) and (d):
intermediate magnification; images (e) and (f): high magnification.

Figure 13. Scanning electron microscopy of PHB-1 fractured bar
surface exposed to NaOH, during stress relaxation tests. Images (a),
(c), (e): upper border, middle and lower border of the bar, respectively,
with applied load of 275 N. Images (b), (d), (f): upper border, middle

exposed to NaOH during the experiment. The images of
Figure 13a, c and e correspond to the upper border, middle
and lower border, respectively, of the PHB-1 fractured bar
under the effect of 275 N as preload; whereas (b), (d) and
(f) the corresponding images of a sample tested under a
load of 824 N. The images for the test with 275 N show a
larger amount of whitened areas compared to one tested at
824 N, indicating a more intensive crazing effect. Under a
higher load, smoother areas were observed, suggesting that
high initial stresses lead to catastrophic failure by brittle
fracture. It seems that craze and crack propagation is more
likely to take place when lower levels of stress are applied
whereas catastrophic failure is more probable when the
experiment is done under a higher load. In images (b) and
(d) of Figure 13 cracks and porosities inside the bar are
noted (indicated by arrows), which may be regarded as
start sites of rupture.

4. Conclusions

This work investigated the environmental stress cracking
(ESC) of injected samples bars of two PHBV samples with
HV content of 4.0% and 6.2%, respectively. NaOH solution
3M was used as the active fluid. The tensile mechanical
behavior of these polymers was examined using crosshead

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speeds ranging from 0.1 mm/min to 5.0 mm/min. During the
stress relaxation tests preloads were used corresponding to
25%, 50%, and 75% of the maximum load obtained during
the standard tensile tests. Results showed that the effect of the
aggressive agent NaOH was remarkable and all experiments
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Action of NaOH on surface layers of PHB, together with the
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failure. We also analyzed the effect of ESC in PHB unexposed
and exposed to NaOH 3M applying fatigue tests, the results
will be presented in the forthcoming paper.

Acknowledgements

The authors wish to thanks Prof. Agnelli (UFSCar,
São Carlos, SP, Brazil) for molding the test samples. PHB
Industrial SA (Brazil) for the PHB resins. The authors are
also grateful to the _Conselho Nacional de Desenvolvimento_
_Científico e Tecnológico_ (CNPq/Brazil) and _Fundação de_
_Amparo à Ciência e Tecnologia do Estado de Pernambuco_
(FACEPE/Brazil) for financial support.

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