'_ NASA TECHNICAL MEMORANDUM NASA TM-77065

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INVESTIGATIONS INTO THE BURNiNG-OUT OF

ORGANIC SUBSTANCES IN THE CERAMIC BODY

_| NATIONAL AERONAUTICS AND SPACE ADMINIStrATION

_ASHINGTON, D.C. 20546 JUNE 1983

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_ ITANOMIO Y01r_.|PAr,l[

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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. Pressed _..._'0" 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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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
!

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 described in 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 intensifies this process, so that swelling and cracks can result from
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 investigated 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 out i

at different temperatures. After cooling to room temperature,

the objects were embedded in synthetic resin, cut out, and photographed.

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.

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It becomes evident that the burnout of the binder proceeds
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- ':'

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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 prevention 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. balance Infor parallel, one specimen the loss (Fig. in2).massAfter was measured a holding ontimea thermal of 67 h, ii

. 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 6o 0 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 '_
" 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 temperature 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.
i

OF POORQUALITY

.._ : _.

, ,

_ . •

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 reaction. 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 !
_- ORIQINALPA_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

rate was 5 K/min and the gas sweep was 2.5 dm3/min

Specimen "a" was heated to 200°C under oxygen sweeping and

then held at this temperature for 65 min. Then, at the same

I temperature, a switch was made to nitrogen sweeping with holding

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 42 sweeping .....

";. - "o'233- _ sweepin -- g // I" /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• =0,_ _ _'-_U'-'f--_. ,-- _ •__....... .' .1" 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
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'"
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 available 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 boundaries 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
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

im_.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, _,WTHAachen.

i 4. P. Schulz, Dissertation, RWTH-Aachen, 1981.

"1 ,I

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