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: heesoo@pusan.ac.kr

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).

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