Pressed compacts were made of spray dried alumina containing water soluble polyvinyl alcohol or cellulose derivative binder. The burning out of organic binder on gradual heating was investigated by visual and microscopic observations of the cross section and by thermogravimetry. Burning out proceeds inward from the peripheries, gradually reducing the size of the black core, which first consists of a dark boundary layer and later turns uniformly black with a sharp boundary. A detailed mechanism of the burning out process between and within the spray dried granules is observed under the microscope. Oxygen atmosphere accelerates the burning out process

Locher, C.

Pfaff, E.

Schulz, P.

Zografou, C.

English

NASA Technical Reports Server

'_ NASA TECHNICAL MEMORANDUM NASA TM-77065
w
INVESTIGATIONS INTO THE BURNiNG-OUT OF
ORGANIC SUBSTANCES IN THE CERAMIC BODY
_| NATIONAL AERONAUTICS AND SPACE ADMINIStrATION
_ASHINGTON, D.C. 20546 JUNE 1983
i i_
] 984002248
https://ntrs.nasa.gov/search.jsp?R=19840002248 2020-03-21T00:49:58+00:00Z
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*_ INVESTIGATIONS INTO THE BU_,.ING-OUT OF ,TrTN_ lq_':z
:* ORGANIC SUBSTANCES IN THE CERAMIC BODY L P_f._0_O,o.,0,e,_C.d.
_" "'7-_ A..ke4o| |. Po, fo,m.. I O,lenloetleA leped.o Pie. -
i_ C. Locher, E. Pfaff, P. Schulz and .C. Zografou IO. w.,&Urn0He.
,_ .. II. CoA0,oewe. G,e,. No.
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: Translation of "Untersuchungen zum Ausbrennen organischer
, Substanzen im keramischen Scherben", Keramische Zeitschrift,
_I Vol. 34, No, 7, 1982, pp. 361-364.
• • | | | |
ii I&, At._..._'0"Pressed compacts were made of spray-dried alumina containing
water-soluble polyvinylalcoh_l or cellulose derivative
binder. The burning out of organic binder on gradual heating
was investigated by visual and microscopic observations of
the cross section _ d by thermogravimetry. Burning out proceed ,
inward from the per pheries, gradually reducing the size of the :_**
black core, which first consists of a dark boundary layer and
later turns uniformly black with a sharp boundary. A detailed
mechanism of the burning-out process between and within the
spray-dried granules can be observed under" the microscope.
Oxygen atmosphere accelerates the burning-out process, pp. _
/
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"1984002248-002
INVESTIGATIONS INTO THE BURNING-OUT OF
ORGANIC SUBSTANCES IN THE CERAMIC BODY
C. Locher, E. Pfaff,
P. Schulz, C. Zografou
The process of burning off organic substances at
I various temperatures is demonstrated with the aid of
! investigations of body structure. These results are
augmented by thermo-gravimetric investigations. The
comparative discussion forms a contribution to the
understanding of the possible formation of "black cores".
i. Introduction
The production process of ceramic articles often requires /361"
the addition of organic substances to the raw material, which
must again be driven out of the molding in the subsequent burn.
These additives are basically reguired by the chosen shaping
process, which sometimes is only possible (e.g. injection molding)
or becomes optimized and rationalized through such additives.
Thus, for example in the production of slips, organic
additives are used that extensively influence the liquefaction
4,
properties, formation and strength of the body, and the shaping
properties. Binders are a_ded in spray drying in order to enhance '
the strength of the granules and the green product. Waxes are
rather often used as lubricants, which for example reduce the
friction between the die wall and also between the primary _
particles in molding and thereby lead to a more uniform density
of the green product. In this manner it is attempted to reduce
the density gradients in the body in order to obtain a homogeneous
texture.
Numbers in the margin indicate pagination in the foreign text.
1
i
1984002248-003
!These organic additives must be uniform dziven out of the
_ body as soon as possible during the burn, as they may hinder the
sintering.
Thus the burnout process, as well as the method of the
burn, acquires an important influence on the properties of the
end product.
•
i The burn out of organic additives can be initially describedin three simplified stages:
- thermal decomposition of the organic additives (e.g.
Moviol, uylose).
, - diffusion of gases through the available porosity into the
furnace space.
- oxidation of these gases in the furnace space and in the
ceramic body in dependence on the available pore space. 1
Depending on the type of organic component, the thermal
decomposition takes place at various temperatures and can begin
as early as 200°C. Thus the burnout process falls within the
heat-up phase of the burn, which generally may take place quickly
(fast burn over). The completeness of the burnout larqely
depends on the diffusion resistance of the molding and the
available time. As the burn proceeds, the porosity diminishes
and the pores become smaller. Thus, with increasing temperature,
an increasing resistance is offered to the diffusion of gas.
If molded ceramic objects with large green density are being
z
burned, it is possible that the pore channels necessary for the
diffusion processes have already been closed before the burnout
is finished. The application of a glaze to the green body inten-
sifies this process, so that swelling and cracks can result from
1984002248-004
ORI_NAL PAGE IS
OF POOR QUALITY
the buildup of gas pressure. If the burnout process is only
incomplete, so-called "black cores" remain in the fired structure.
2. Investi@ations
The formation of black cores in ceramic bodies was investi-
gated by the example of uniaxial-molded, spray-dried alumina
(Table i). Small cylindrical specimens (d = 20 mm, h = 7.5 mm)
of sample 1 were heated in a Simon-M_ller furnace and taken outi
at different temperatures. After cooling to room temperature,
the objects were embedded in synthetic resin, cut out, and photo-
graphed.
Table i. Investigated samples (al.mina: Alcoa A16)
Specimen Percentage Moisture Bulk
of Binder Content Density,
(Moviol, % g/cm 3
tylose)
%
I 3 _1.2 1,3g
, 5 _ _ 1,(_?
. ].-
In parallel a thermogravimetric investigation was made on
an additional specimen. Fig. 1 shows the burnout of the binder
as a function of the temperature. In this figure the curve has
been supplemented with macro-photographs of the sectioned
specimen.
At about 200°C the curve displays a sharp inflection. We
may conclude from this that the removal of the residual humidity
of 1-1.2% is complete and that the burnout of the binder now .
begins.
11
It becomes evident that the burnout of the binder proceeds
1984002248-005
ORIGINAL P_G£ |_
OF POOR QUALITY
7- heatin_ I
5 - , , !
4 l t /, i
_ , / i .
3, F'----! , speci
o
I.- I i
200 z,O0 600 800 _OOC¢t
_E_ Temper°t ur / °C
Fig. I. Thermogravimetric analysis
of specimen I, showing the specimen
cross sections.
from the outer toward the inner margin. As the bottom edge of /362
the specimen is little exposed to convection on account of the
necessary backing, the burnout is impeded at that place and is
only incomplete at first. Under closer consideration it appears
that the core of the specimen is separated by a dark ring from
the outer rim of the body, which is already burned white. We
shall discuss this phenomenon further below.
:
The observations allow the following initial conclusions.
The organic components decompose into gases, as mentioned. With
rising temperature, pure amorphous carbon splits off and can
result in formation of black cores, so that it must be oxidized
by inward-diffusing oxygen. It can then leave the body as CO
and CO 2. This has been described by Otto in similar form [I].
The formation of black cores thus depends significantly on the
partial oxygen pressure of the furnace atmosphere.
2.1. Experimental Firin 9 with Nitrogen and Oxygen Injection .,
If the formation of black cores is to be attributed, as
assumed, to the deposition of carbon, i.e. an incomplete oxida- ':'
4
] 984002248-006
ORIGIINAL P_GE f_
OF POOR QUALITY
; tion of this carbon, the entire body would therefore be uniformly
!;
" black in color during a firing under protective gas.
Specimens heated to various temperatures in a silit tube
__ furnace with N 2 spray revealed a uniform black coloration. On
i the contrary, during a firing with oxygen spray no formation of
black cores was observed. This also shows that the deposits are
; in fact attributable to carbon. Oxygen injection is therefore
a prospect for avoiding the formation of black cores. The slow
and early burnout of the binder at low temperature is a more
[ interesting and economical possibility for the practical preven-
tion of deposits of organic additives.
2.2. The Burnout of Binder at Low Temperature with Lengthy
Holding Time (Specimen i)
--}
._ In the Simon-MUller oven, six molded objects of alumina
'.',"I No. 1 were heated to 270°C and taken out after various holding
i
|_ times. In parallel, the loss in mass was measured on a thermal ibalance for one specimen (Fig. 2). After a holding time of 67 h,
. the speciren was again heated. At 1000°C no further mass loss
could be detected.
% --t
"N (1) I " "_° ..... °--".....
o :t
-' [• m3 _ ',
!i[ ® 2- -;" o 20 _b 6o0 I ...../; "• "_ ]1_ holding ttme/'h
'" _ O _" -_-......
m O 200 "-00 600 800 1000
¢: Temoeratur I op .,,,.
Fig. 2. Thermogravimetric analysis {_
of specimen i. Holding time 67 h
•. at 270°C.
- 5 '_
1984002248-007
" ORIGINAL PAGE |_l
OF POOR QUALITY
The specimens taken out after various holding times show
that no black core was formed. Except for a minor blackening at
the outer margin, the binder is quite uniformly burned out in
this case. After a holding time of 5 h, the majority of the
binder is already burned out.
This investigatior shows that even at relatively low tempera-
ture the majority of the binder can be burned out.
2.3. Burnout of Binder and Observation of Black Cores in Specimen 2
Similar studie_ were done on specimen 2 as well. For this,
five specimens were prepared and fired in the Simon-M_ller furnace.
At 200°C and thereafter, a sample was taken for every i00 K,
vacuum-impregnated after cooling, and prepared for study by
light microscope [2]. The macro-photographs are shown in Fig. 3.
For the sample taken at 400°C, an enlarged section under the
reflected light microscope was also prepared. This is shown by
Fig. 4.
Fig. 3. Section of specimen 2:
a - 200°C; b - 300°C; c - 400°C;
d - 500°C; e - 600°C.
1984002248-008
iOF POOR QUALITY
.._ : _.
, ,
_ . •
OdlD"
0
0°
i
Fig. 4. Detail magnification of
Fig. 3c.
The burnout of binder begins at the granule margins, which
become visible to the naked eye by deposition of carbon. This
picture (Fig. 4) of the granulates in the green texture further
allows an assessment of its deformation and orientation during
the compaction process [3]. At a further temperature rise to
300°C the formation of a dark ring becomes clear, as also observed
in the study of specimen i. At a temperature of 400°C, completely
black granules with white border precipitate in the outer rim of
the body, which is already burned white. This is especially clear
in the magnified section of Fig. 4
These pictures allow a more extensive interpretation of the
burnout process. For this, the properties of the granules must
be especially consulted. In the interpretation of the findings
given here it became evident that it is necessary to examine
more closely the influence of oxygen on the decomposition reac-
tion. Therefore, a further thermogravimetric investigation was ,
made. /36____3
2.4• Investi@ation of the Oxygen Influence
The aim of this investigation was to prove that the oxygen
produces a pre-oxidation of the binder and that this decomposes _E
, .
L !
1984002248-009
_- ORIQINAL PA_k' _
OF POOR Q'JALrl'Y
more easily as a result of the consequent instability.
Two thermogravimetric investigations of alumina No. 1 were
done on the thermal balance. The specimens weighed 50 mg and
were heated as loose powder in a platinum crucible. The heating
dm3/minrate was 5 K/min and the gas sweep was 2.5
Specimen "a" was heated to 200°C under oxygen sweeping and
then held at this temperature for 65 min. Then, at the same
temperature, a switch was made to nitrogen sweeping with holdingI
for an additional 45 min. After this, it was again heated to
1500°C. Specimen "b" was processed under nitrogen alone as a
reference sample. The TGA curve (thermogravimetric analysis)
and the respective temperature curve are shown in Fig. 5 and 6.
600-
. _ o2 42sweeping .....
" _ sweeping / /I.-
";. - o'233- -- / I"
k O_ ............... i
i_. 0 _ 80 120 ,_c 2_./holding time
_ tlme/mln .... _" at 2_°'-....
_./_ 30 60 80 '
0 3- _ J ./._"
2- "OE_ >.
al
m=, _ . _ _.' .1"• 0 _ _'-_U'-'f--_,-- • _....... t
0 100 200 ]OC 1.00 503 "
"remperat ur /eC -.,-
Fig. 5. Thermogravimetric analysis
and temperature curve of specimen "a".
The heating to 200°C under nitrogen results in a mass loss i[
of 1.2%, which can be explained by the evaporation of the residual ,,
moisture. A similar heating to 200°C under oxygen produces a
lesser mass loss of only 0.6%. Sinc_ we must assume that a
residual moisture of 1.2% is also evaporated in this case, the
measurement finding can only be explained by an accumulation of
oxygen.
8
1984002248-010
I
f
; ORIGiMAL PA.-.. _
"_ OF POOR QUALITy.!
_ _. 6oo /-"
! s._2°°!_ eeping _ ......"
' _ O_ ..... /
i_ o _o 8o 12o _o 2oo..."holding time
m" time/mi_-_ J-'" at 2oooc_
03T I ®
2T . ['r':"L.... . .......
0 ,d 4_ t , -"
............
0 100 200 300 _00 500
Temperolur /oc _-
Fig. 6. Thez_ogravimetric analysis
: and temperature curve of specimen "b".
' When 200°C is reacheC, the binder starts to decompose in
both specimens. Under oxygen sweeping the mass loss takes place
more quickly than the nitrogen sweeping. After changing from
oxygen to nitrogen sweeping in experiment "a", there is a larger
rise in the curve, explained by the now-absent overlay caused
_ by the oxygen uptake.
Corparison of both curves shows that in experiment "a"
! after a holding time of 80 min a larger amount of binder is
J-J burned out and the later burnout process takes place more quickly.
i The 0.5% smaller mass loss of specimen "b" indicates the much
! r
. ! more intense deposition of carbon.
The finding is significant for practical purposes, as it
shows that a heating under sufficient oxygen excess produces a
>
later _ccelerated decomposition of binder even Pt low temperatures. {
In this manner, formation of black cores can be avoided. !
_ 3. Summary and Interpretation ,.
The heating of the specimen produces a thermal decomposition
of the binder, starting at about 200"C. The liberated gases i
9 t'"
1984002248-011
diffuse from the specimen into the furnace space. At increasing
temperature, carbon is liberated.
A second reaction, having decisive influence on the burnout ¢
process, is superimposed on this decomposition when there is
sufficient oxygen in the furnace space. Depending on the avail-
able partial pressure, oxygen diffuses into the specimen and is
deposited on the binder. This reaction begins at quite low
temperatures. As a result of this oxygen uptake, the organic
chain becomes unstable and breaks down more quickly. It is to
be assumed that the decomposition products also take up oxygen
and further decompose. The xesulting carbon is oxidized to CO
and CO_ and diffuses out of the specimen.
The oxygen diffusing into the body encounters a diffusion
resistance that varies in time and space. It is very dependent
on the porosity. This can explain why the thermal decomposition
is at first more intense at the granule boundaries than in the
granules themselves. This was observed with special intensity
in the case of specimen 2. The oxygen required for oxidation of
the binder can be supplied more quickly across the grain boun-
daries than from #" outer shell into the grain interior. This
is explained by _ _ensified porosity between the grains,
which can still b_ _etected in part in the fire,] texture.
Under stronger heating the decomposition and oxidation of
the carbon take place more quickly and also extend to the grain
_nterior in the area_ of the specimen where the partial oxygen
pressure is larger. The oxygen requ_ed for these reactions is
at _irst available only in th£ boundary layer of the body, with
the exception of the bottom surface, which is only slightly
exposed to convection. The drop in the oxygen pressure in the
marginal zone is shown schematically in Fig. 7.
-4
i0
1984002248-012
i#;
I ,
Fig. 7. Schematic representation
of the partial oxygen pressure
at (a) 300°C and (b) 400°C.
i
The oxygen is instantly consumed at the o,lter margin of the
specimen. Tt settles on the binder. Thus, in the oxygen-free
boundary zone there is an intensified cracking of the binder and
also an intensified deposition of carbon. The oxidation of the /3_4 '
carbon also takes place from the margin of the specimen toward
the interior, but at low temperatures not to the same extent as
the oxidation and decomposition of the binder. Thus, at the
margin of the specimen a dark ring is formed.
r
After the completed burnout of binder from the marginal
zones, the diffusional resistance becomes smaller in that place
and the oxygen pressure rises, as there is no more oxidation in
this zone. The boundary zone ax therefore shifts toward the
interior of the specimen, as shown in Fig. 7b.
The dark-col&red grains, located in the specimens pressed
from alumina No. 2 shortly in front of the dark ring in the
margin of the body that is already burned white, reflect these
processes. If the grain surface is free of carbon, there is an
intensified diffusion of oxygen into the core. The binder will
now also decompose in the core of the grain, liberating carbon.
Due to the improve diffusion pathways a_ong the boundaries of
the granulate, the carbon in the interior of large grains is
oxidized more £1owly, and the dark spots in the picture are
formed. -
ii i
i-
1984002248-013
m_.T _ t • _
I
The difference between a dif _sion across grain boundaries
and into the interior of the grains is the larger as the grains
are more dense. A perfect sphere (approximately) only obtains
in the case of specimen 2, however, as studies of the grains have
shown [4]. Spray-grain No. 1 consists for the most part of open
hollow spheres. This is attributable to the larger proportion
of binder. For this reason, there is no distinct difference
between the margin and interior of the granules in specimen I.
The oxygen is supplied almost equally to both the outer and inner
surface of these hollow spheres. Therefore the carbon is burned
more uniformly out of the granules.
References
i 'x. J. Otto, "Black Cores in Ceramic Products", Keram. Z. ,
_| 31, 1979, [2] 77. J
2. P. Schulz, P. Reynen, A New Method of Investigation for
the Structure of Nonfired Molded Ceramic Objects, Report at the
DKG Annual Conference, Wiesbaden, 1980.
J 3. E. Pfaff, C. Locher, Unpublished Graduate Thesis, _,WTH-
Aachen.
i 4. P. Schulz, Dissertation, RWTH-Aachen, 1981.
1 ," I
t
I
-,_
12
1984002248-014


Investigations into the burning-out of organic substances in the ceramic body

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