Journal of Ceramic Processing Research. Vol. 13, No. 5, pp. 662~665 (2012)
662
J O U R N A L O F
CeramicProcessing Research
Deterioration of mechanical properties by the curing of phenolic resin during fab-
rication of porous PSZ ceramics under high temperatures and high humidities
Sanghyeon Yoona, Janghoon Kimb, Hyun Parkc, Juntae Leed and Heesoo Leea,*aSchool of Materials Science & Engineering, Pusan National University, Busan 609-735, KoreabNational Core Research Center for hybrid materials solution, Pusan National University, Busan 609-735, KoreacGlobal Core Research Center for Ships and Offshore Plants (GCRC-SOP), Pusan National University, Busan 609-735, KoreadMaterial Testing Center, Korea Testing Laboratory, Seoul, 152-718, Korea
Mechanical and microstructural analyses of partially stabilized zirconia (PSZ) ceramics maintained under high temperatureand a high relative humidity were investigated to elucidate the relation between the curing of the phenolic resin as a binderand the deterioration of the ceramics. The PSZ powders containing the binder were exposed to the high humidity (85%, 50 οC)and the high temperature (25%, 100 οC) atmospheres for 1 h before compaction. The green densities of the specimensdecreased from 3.54 g/cm3 to 3.45 g/cm3 and to 3.35 g/cm3, respectively. The flexural strengths decreased about 3% and 8%compared to the specimen exposed to a lower humidity and temperature condition (25%, 50 οC). Rough surfaces and someagglomerated particles with a large volume of voids and internal spaces were observed in the fracture surfaces of the greenbodies. These agglomerates were generated by the curing of the phenolic resin with the formation of methylene bridges. Thereaction occurred by the decomposition of hexamine as a curing agent in the high temperature and the high humidityatmospheres.
Key words: Porous PSZ ceramics, Deterioration, High temperature/humidity, Phenolic resin, Methylene bridges.
Introduction
Zirconia-based materials have attracted noticeable interest
because of their excellent mechanical and thermal
properties currently used as refractories [1]. In order to
prevent the phase transformation from tetragonal to
monoclinic accompanying a large volume expansion of
3 ~ 5%, representative stabilizers such as, CaO, MgO,
and Y2O3, have been added to maintain the tetragonal
phase during cooling from the sintering temperature [2,
3]. Calcia partially stabilized zirconia (CaO-PSZ) has
been particularly used in submerged entry nozzles, and is
one of the most important refractories for the continuous
casting process in steel making [4].
Organic binders are an essential additive to provide
the good flowability of ceramic particles and to enhance
the strength of a green body during the fabrication
process of porous ceramics. To obtain a uniform green
density and microstructure at a given compaction
pressure, it is required that the binder is weak enough
to allow deformation and must also have enough
adhesion between the primary particles to provide
sufficient green strength [5]. The properties of porous
ceramics depend on the internal defects during
compaction being present in the final product so it has
been emphasized that the defects in the green body
need to be minimized [6, 7].
Phenolic resin is typically used in the submerged
entry nozzle due to its outstanding chemical, thermal,
and dimensional stability as a thermosetting binder.
Extensive scientific research has been carried out to
investigate the curing behavior of phenolic resins [8,9].
In order to avoid inconsistency of the properties of the
porous ceramics by atmospheric conditions, a study of
the curing mechanism of phenolic resin should be
carried out. In our previous study, the curing behavior of
a phenolic resin according to various relative humidity
levels and their influence on the properties of porous
PSZ were investigated [10]. A further investigation of
the mechanical properties of the PSZ kept under high
temperature and high humidity has been conducted. In
the present study, the effects of the curing of a phenolic
resin in high temperature and in high humidity
atmospheres on the deterioration of porous CaO-PSZ
ceramics were demonstrated by changes in mechanical
properties and microstructural analysis. The thermodynamic
behavior and chemical structure of the phenolic resin
as a binder were also analyzed to verify its curing
mechanism using DSC and 13C NMR.
Experimental procedure
Partially stabilized zirconia (PSZ), containing 4 wt%
for CaO as a dopant to stabilize the zirconia with an
*Corresponding author: Tel : +82-51-510-2388Fax: +82-51-512-0528E-mail: [email protected]
Deterioration of mechanical properties by the curing of phenolic resin during fabrication of porous PSZ ceramics... 663
average particle size of approximately 10 µm, was used
as a raw material. Novolac type phenolic resin (Kangnam
Chemical Co., CB-8081) was used to prepare the porous
PSZ ceramics as a binder. The PSZ powders were
mixed with 5 wt% of the phenolic resin using a ball mill
with zirconia balls for 24 h. The mixture was maintained
under three different temperature/relative humidity
conditions (50 οC/25%, 50 οC/85%, and 100 οC/25%)
for 1 h by using a constant temperature and humidity
chamber (Aero Tech, TATC-150). The mixtures were
formed into a cylindrical (25 mm in diameter) and a
rectangular (60 mm in length and 7 mm in width) steel die
under a uniaxial pressure of 20 MPa and subsequently
sintered in air at 1550 οC for 1h.
The apparent porosity and bulk density of the
specimens were determined via water immersion based
on the Archimedes method [11]. The flexural strengths
of the specimens were evaluated by three point bending
tests using a universal testing machine (UTM, DUT-
3000CM) with a span length of 20 mm and a crosshead
speed of 0.5 mm minute-1 [12]. A microstructural analysis
was performed on the fracture surfaces of the green bodies
using a scanning electron microscope (SEM, Hitachi, S-
4700). A differential scanning calorimetry (DSC, TA
Instruments Ltd, Q200) was used to clarify the degree of
cure of the phenolic resin, and the thermograms were
obtained over a temperature range of 90-200 οC at a
heating rate of 10 K minute-1 in an inert atmosphere. The
curing behavior of the phenolic resin was demonstrated
by the analysis for the changes in chemical structure of
the binder using nuclear magnetic resonance spectroscopy
(NMR, Agilent). The solution state 13C NMR spectra
were recorded immediately after dissolving the samples
in acetone-d6 (99.9%) solvent with the spectrometer at a
resonance frequency of 600 MHz.
Results and Discussion
Table 1 shows the changes in green and sintered
density of the PSZ specimens kept under different
relative humidities and temperatures before compaction.
The green density decreases from 3.54 g/cm3 to 3.45 g/
cm3 in the specimen exposed at 85% relative humidity
and the specimen exposed at 100 οC exhibits a lower
green density. There is a large deviation in green density
of the specimen maintained under a high humidity, and
this seems to be an inhomogeneous stacking of the PSZ
particles due to moisture absorption in the humid
atmosphere. The sintered density of the PSZ ranges from
3.69 g/cm3 to 3.53 g/cm3 in response to a variation of the
green density. This variation represented a significant
portion of porous ceramics being sampled because it
might cause poor durability of a sintered body [13]. It
is predicted that the high temperature and high humidity
lead to an inferior compaction of the PSZ because of the
differences in the deformation characteristics of the
phenolic resin molecules as a binder.
The average results of the flexural strength and the
apparent porosity measurements are presented in Fig.
1. The specimen exposed to 100 οC exhibites the lowest
flexural strength (22.2 ± 0.48 MPa) with deterioration of
8% compared to the specimen exposed to 50 οC. In the
case of 85% relative humidity, the strength decreases
about 3% in comparison with the normal condition
(50 οC, 25%). The apparent porosity increases from
33.8 ± 0.51% to 37.0 ± 0.37% in response to a variation
of the strength. There are larger variations of the
mechanical properties in a high tempeature than in a
high humidity. This result indicates that when the PSZ
Table 1. Green and sintered density of the PSZ maintained underdifferent environmental conditions.
SpecimenNormal
conditionHigh
humidityHigh
Temperature
Temperature/Relative humidity
50 ο
C / 25% 50ο
C / 85% 100ο
C / 25%
Green density(g/cm3)
3.54 ± 0.018 3.45 ± 0.045 3.35 ± 0.028
Sintered density(g/cm3)
3.69 ± 0.015 3.61 ± 0.036 3.53 ± 0.017Fig. 2. SEM images of fracture surfaces of green bodies : (a)exposed at 50
ο
C, 25%, (b) 50ο
C, 85%, (c) 100ο
C, 25% and athigher magnifications (d), (e), (f).
Fig. 1. Apparent porosity and flexural strength of porous PSZspecimens exposed to different temperatures and relative humidities.
664 Sanghyeon Yoon, Janghoon Kim, Hyun Park, Juntae Lee and Heesoo Lee
powders containing phenolic resin were kept at a high
temperature or a high humidity, they became degraded
and their properties were changed by a decrease of
their compactibility.
SEM images in Fig. 2 show the fracture surfaces of
green bodies exposed to different relative humidities
and temperatures. The specimen prepared by the PSZ
powder maintained normal condition (50 οC, 25%) exhibits
a dense microstructure and a relatively homogeneous
particle size ranging from 0.5 to 3 µm in Fig. 2 (a, d).
For the high humidity condition, the microstructures
(Fig. 2 (b, e)) show a rough surface and an irregular
particle size due to the formation of agglomerates with
a large volume of voids and internal spaces. The
reduction of green density and the wider scatter of data
as shown in Table 1 are interpreted in terms of the
existence of agglomerates. Agglomerated particles above
5 µm are also observed in the specimen exposed at
100 οC (Fig. 2 (f)). It seems that the curing of phenolic
resin in a high temperature and in a high humidity
produces hard agglomerated particles which can reduce
the mechanical strength of the porous PSZ because of
the defects such as the voids and the internal spaces.
The DSC curves in Fig. 3 describe the curing
behavior of phenolic resins maintained under different
environmental conditions. The phenolic resins have a
broad exothermic peak around 150 οC and the peaks
become smaller with increasing temperature or relative
humidity. These peaks were attributed to a condensation
reaction of methylol groups, and the area under the
peak enclosed by a baseline was associated with the
amount of the heat reaction to the curing of the
phenolic resin [14]. The thermograms indicate the
degree of cure for the phenolic resin, and it is defined
by the equation [15]:
α = (QT - QR) / QT (1)
where QT is the reference value for the total heat of
curing for the phenolic resin with no precure, QR is the
residual heat of curing for the partially precured
phenolic resin in a high temperature or in a high
humidity, and α is converted into the percentage of
curing. From equation (1), the degrees of cure for the
phenolic resins are 38.5% and 42.3% in the high
humidity and the high temperature conditions, respectively.
It is proposed that the curing reaction of the phenolic
resin in the high temperature may be more prominent
than in the high humidity condition.
The 13C NMR spectra in Fig. 4 provides information
for changes in the chemical structure of the phenolic
resin kept under different environmental conditions.
The resonances corresponding to the para-para, para-
ortho, and ortho-ortho methylene linkages appear
around 40, 35, and 30 ppm, respectively. The resonances
increase in the high humidity condition, and higher
resonances are observed in the phenolic resin maintained
at 100 οC. This result indicates that the curing reaction
of phenolic resin is closely associated with the
chemical reaction between the phenolic novolac resin
and hexamethylentetramine (hexamine) as a curing
agent. The reaction is enhanced with increased methylene
cross-linking by the decomposition of hexamine due to
a high temperature and high humidity.
It should be noted that the strongest resonance
exhibits at the para-para linkage and increases more
than those at the para-ortho and ortho-ortho linkages as
the humidity and temperature increases. This result was
interpreted in that the para-linked intermediates were
less stable and more easily decomposed in a high
temperature and/or a humid atmosphere to form the
methylene linkages [16]. There is a noticeable
phenomenon for the curing of phenolic resin as shown
Fig. 4. It has been reported that the curing reaction of a
phenolic resin started at 90 οC [17]. However, the
curing of a phenolic resin is also observed in the high
humidity condition although the temperature is lower
than 90 οC. Tonogai et al. [18] asserted that moisture in
Fig. 3. DSC thermogram curves of phenolic resins maintainedunder different environmental conditions.
Fig. 4. 13C solution NMR spectra of the phenolic resin exposed todifferent environmental conditions.
Deterioration of mechanical properties by the curing of phenolic resin during fabrication of porous PSZ ceramics... 665
a humid atmosphere promoted the decomposition of
hexamine, and Zhang et al. [16] reported that a lower
temperature required forming the methylene linkages with
a lower amount of the hexamine. This consequence can be
explained in that the decomposition of hexamine by the
moisture reduces the temperature for curing reaction.
Conclusions
The deterioration of mechanical properties of the
porous PSZ containing phenolic resin maintained under
a high temperature and a relative humidity was
investigated. The degree of cure for the phenolic resin
was increased by the formation of methylene bridges
due to the decomposition of hexamine as a curing
agent in a high temperature and in a high humidity.
This curing generated the hard agglomerated particles
which led to the formation of defects such as voids and
internal spaces by the poor compaction of the PSZ.
Accordingly, it could be confirmed that the green and
the sintered density of the PSZ decreased by the curing
of the phenolic resin in a high temperature and in a
high humidity. The flexural strength decreased by the
increase of the apparent porosity due to the defects.
Considerable changes in the mechanical properties
were exhibited in the specimen kept under the high
temperature condition because there were more
methylene bridges than with the high humidity condition.
Acknowledgements
This research was supported by a grant from the
Fundamental R & D Program for Core Technology of
Materials funded by the Ministry of Knowledge Economy
(M-2009-01-0028) and the National Research Foundation
of Korea (NRF) grant funded by the Korea government
(MEST) through GCRC-SOP (No. 2011-0030668).
References
1. N. Bamba, Y.H. Choa, T. Sekino, K. Niihara, J. Eur.Ceram. Soc. 23 (2003) 773-780.
2. S.P.S. Badwal, Solid State Ion. 52 (1992) 23-32.3. W.C. J. Wei and Y.P. Lin, J. Eur. Ceram. Soc. 20 (2000)
1159-1167.4. M.O. Suk and J.H. Park, J. Am. Ceram. Soc. 92 (2009)
717-723.5. S. Balasubramanian, D.J. Shanefield, and D.E. Niesz, J.
Am. Ceram. Soc. 85 (2002) 749-754.6. R.D. Carneim and G.L. Messing, Powder Technol. 115
(2001) 131-138.7. J. Nam, W. Li, and J.J. Lannutti, Powder Technol. 133
(2003) 23-32.8. M.O. Abdalla, A. Ludwick, and T. Mitchell, Polymer 44
(2003) 7353-7359.9. F.Y. Wang, C.C. M. Ma, and W.J. Wu, J. Appl. Polym. Sci.
80 (2001) 188-196.10. Y.H. Cho, S.H. Yoon, J.Y. Kim, Y.S. Koo, and H.S. Lee, J.
Ceram. Process. Res. 13 (2012) 97-100.11. M. Chen, C. Lu, and J. Yu, J. Eur. Ceram. Soc. 27 (2007)
4633-4638.12. L. Shen, M. Liu, X. Liu, and B. Li, Mater. Res. Bull. 42
(2007) 2048-2056.13. T.A. Deis and J.J. Lannutti, J. Am. Ceram. Soc. 81 (1998)
1237-1247.14. A.W. Christiansen and L. Gollob, J. Appl. Polym. Sci. 30
(1985) 2279-2289.15. X.-M. Wang, B. Riedl, A.W. Christiansen, and R.L.
Geimer, Polymer 35 (1994) 5685-5692.16. X. Zhang, M.G. Looney, and D.H. Solomon, Polymer 38
(1997) 5835-5848.17. M.H. Choi, I.J. Chung, and J.D. Lee, Chem. Mater. 12
(2000) 2977-2983.18. S. Tonogai, Y. Sakaguchi, and S. Seto, J. Appl. Polym. Sci.
22 (1978) 3225-3234.
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