Debinding processes- - · PDF fileFriedherz H. Becker Riedhammer GmbH Klingenhofstrasse 72...

Friedherz H. Becker

Riedhammer GmbH

Klingenhofstrasse 72

90411 Nürnberg

Debinding processes-

physical and chemical conclusions

and their practical realisations

CFI_debinding_processes3.doc 1 /21

Introduction

More than 10000 years ago people already understood how to shape special raw materials

and to strengthen the formed body by thermal treatment. The plasticity or ductility of a

material is an important precondition for manufacturing vessels, building materials or tools.

Typical resources of kneadable materials are clay, china clay or loam, which are used

commercially in the traditional ceramic industry nowadays.

Often these resources do not comply with the strong demand of the technical ceramic

industry, where very pure, synthetic and high temperature resistant materials have to be used

in order to reach the desired properties of the finished product.

Synthetic raw materials as well as feldspar, Cornish stone or quartz have to be classified as

non-ductile resources, but they can be used in kneadable shaping processes, if they are

mixed with plastic raw materials or with organic substances. These organic substances are

polymers, also called “binder”.

The use of polymer varies according to the ceramic processing method. They are added in

different concentrations to suspensions for slip casting - or spray drying purposes and are

applied as binder, plastification agents and slip additives means as well as liquefier.

Binders, which are used in the slip casting process or in pressing, give the green body a

sufficient strength by gluing together particles at their boundary surfaces. Usually those

binders are used, which are based on polyvinyl alcohols (PVA), polyacrylate or cellulose.

High-polymeric compounds such as cellulose and polysaccarides work as plastification

agents. They make possible the flow of ceramic masses during extruding, however, they can

also have binder functions.

Slip additives are organic additives, whose mode of action is based on the interaction of its

hydrophilic function groups, e.g. the O-atoms of the aether group -C-O-C-, with the H +

ions of the Me-OH-groups at the surface. Uniformly loaded particles, which can easily slip at

each other, result from this. During dry pressing a higher green density can be obtained,

because of an improved tension tranfer, i.e. a better pressure gateway is reached. A typical

slip additive is polyethylene glycol (PEG).

Short chained acids of polyacrylics (PAA) are used as agents of dispersion, as a stabilizer and

as a liquefier for slips.

Before sintering ceramic green bodies the debinding process of organic additives must be

performed. The temperatures for these processes vary between 150°C and 600 °C. Organic

polymers have to be removed completely from the green body, since carbon delays can

influence the sinter process and the qualities of the final product negatively.

The debinding process is a time intensive step in the complete production process. The speed

of decomposition of the polymers should not exceed the transport velocity of the products of


pyrolysis, since an excess pressure of the gaseous pyrolysis products can lead to rips and to

the destruction of the body.

Task of binders

During raw material preparation the binder has to give an optimum binding to the solid. A

minimum of binder quantity should realize the desired plasticity of the compound.

Demands can be concluded as follows:

• The structure of the binder must allow a preparation of the compound with low

abrasion of the equipment.

• The binder should be processable without decomposition and without smell nuisance

in the temperature range of 20 °C to 200 °C.

• In order to protect the operating personal toxic substances should be avoided.

• The melting range of thermo-plastic binders should be as big as it is demanded by the

forming machine. During the process of injection molding or extrusion these binders

must be melted rapidly.

• The binder must stick on the surface of the powder. A tendency for decomposition

specially appears during injection molding processes because of the high velocity of

the mass flow and the difference of density of both components.

• Stabilisation against deterioration of microorganisms is necessary in aqueous masses

and of oxidation or light ageing in thermo plastic systems.

• The binder should grant a sufficient stability of the formed product for non-destructive

transport or for mechanical finishing.

Types of binders

Binders can be classified into :

• Slip additives

• Binding agents and

• Plastification agents.

In the ferrite industry slip additives are used to reduce the internal friction of materials during

pressing and to allow a non-destructive and fast release of the moulding from the die. Slip

additives are added as aqueous solutions in corresponding concentrations or as powder,

which will be mixed with the mass.

For the production of ferrite mouldings with synthetic resin as a binder it is necessary to

homogenise or to granulate the ferrite powder with a solution of alcohol and resin.

Binding agents are added to increase the flexural strength of the pressed body and

plastification agents should increase the plasticity of the mass, specially when the forming will

be done in piston presses or in screw extrusion presses. The amount of plastification agents


varies between 0.2% and 1% and depends inter alia on the grain size of the mass, on the

dimension of the moulding, on the pressure of the press.

Organic plastification systems have to be distinguished between [1]:

1. aqueous systems,

2. solvent containing systems,

3. thermoplastic systems.

Aqueous plastification systems consist of dispersions or solvents of polymers, where the water

has the function of deflocculant or solvent. The effectivity of plastification is not only caused

by the structure of polymers but also supported by the water content.

Cellulose ether is important for the processing of aqueous plastic ceramic masses, which do

not consist of plastic raw materials.

Depending on the kind of ether groups and their quantity cellulose ether is colloidally soluble

in water and in organic solvents. The hydrated macro molecules allow the production of

highly viscous solutions at low concentrations (<5%) already, which can be used as a

plastification agent in ceramic masses. In most cases these solutions must be supported with

a water soluble binder, because the binding forces of cellulose ether might be to low. Such a

plastification system, based on cellulose ether is used for the extrusion of ceramic masses for

the production of extruded profiles mostly. The forming of ceramic bodies by injection

moulding (CIM) with cellulose ether only is not possible, because the stability of the green

body is not sufficient. To increase the stability of plastic masses additionally the use of

polyethylene glycol, polyvinyl alcohol or polyacrylate recommended

Solvent containing systems are disappearing in ceramic production facilities because of the

increasing demands of environment protection, workplace hygiene and safe working.

Specially condiments containing and chlorinated solvents are effected by these demands.

In ceramic masses solvent containing systems are preferred, if oxidation processes of the raw

materials or low vapour pressures are required. A typical application of these substances is

the doctor blade process, which is used for the production of ceramic foils.

The organic agents mostly consist of epoxy resin, polyvinyl acetate or polyvinyl pyrrolidone.

Solvent containing masses as well as aqueous plastic masses are difficult to store, because

storage leads to fluctuation in liquid level and as a consequence to alterations in viscosity,

which hampers the dosing procedure.

Thermo plastic systems were originally developed for injection moulding machines in the

plastics industry. Also the stellite industry especially for extruding processes uses thermo plastic

systems. Ceramic masses have to be adopted to the forming technology, which mostly refers

to injection moulding.

Ceramic masses with thermo plastic additives are “dry” at room temperature, therefore their

viscose behaviour is constant over a long period.


Thermo plastic systems consist of:

• paraffines

• montan waxes

• polyolefin waxes

• polyolefin thermoplastic

• polypropylenes (PP)

• polyethylenes (PE)

• polyacetals

• polyoxymethylenes (POM)

Molecular chains of polyolefine thermoplastics, polypropylenes (PP) and polyethylenes (PE)

are much longer than those of waxes. This difference arises in higher binding forces of

thermoplastics and as a consequence a higher melting viscosity and melting point. However

more difficult is the decomposition process in low oxygen atmospheres. Also the filling degree

in the mould is smaller.

Polyacetals are homopolymeres or copolymeres of aldehydes or cyclic acetals.

Polymeres of formaldehyde belong to this group, they are named as polyoxymethylenes

(POM). Because of its chemical structure their thermal resistance is strong in comparison to PE

or PP. Polyoxymethylenes can separate formaldehydes already during processing. In the

presence of acids like nitric acid or formic acid a depolymerisation of POM to formaldehyde

below its melting point can be reached.

This depolymerisation below the processing temperature makes POM´s very interesting for

injection moulding of ceramic articles or their manufacturing by extrusion. However the

condensation of nitric acid must be strongly considered as a corrosion problem for the

thermal treatment of the ceramic product. Also protection against contamination of the

environment by the formaldehyde must be kept in mind. The maximum concentration at

working place (MAK) of formaldehyde is fixed to 0.6 mg/m³. A proper method to realize these

legal instructions consists of an after firing equipment.

The thermal treatment of the debinding process destroys the polymers by oxidation or

combustion in oxygen containing atmosphere. Very often it is an uncontrolled reaction of

high reaction rate inside the shaped part creating a high gas pressure, which can lead to

ruptures of the compact. It is useful to transfer reactive thermoplastics into a modification of

radical decomposition, which is easier to oxidise. This is a way to transfer polymeres of high

viscosity into substances of oily consistency. The radical decomposition will start with a

defined temperature and continues as a chain reaction. Also in hydrogen atmospheres such

a de-waxing process can be accomplished, but of course instead of an oxidation a

hydrogenation of decomposition products will occur.

The physics of thermal debinding


The defining physical procedures of thermal debinding are [2]:

• The capillary flow

• The low pressure diffusion process

• The high pressure permeation process

The capillary forces involve liquid extraction, while the other two require the binder to be a

vapour.

Slightly elevated temperatures influence the viscosity and surface tension of the organic

liquid; capillary forces start with the transport of the liquid phase from big to small pores. As

soon as it arrives at the surface it will be vaporized, if its vapour pressure is larger than the

ambient pressure. With increasing temperature the kinetics of volatilisation increases too.

Above a certain temperature the capillary forces cannot saturate the demand of

volatilisation of the liquid at the surface and the interface of both the vapour and the liquid is

pulled back to the inside of the body.

The binder may be thermally decomposed into low molecular weight species, such as H2O,

CH4, CO2, CO etc. and subsequently removed by diffusion and permeation. The difference

between diffusion and permeation depends on the mean free path of the gas species. The

mean free path varies with the pressure, molecular weight of the gas and pore dimensions.

Generally, diffusion will be dominant at low pressures and small pore sizes; permeation would

be expected to control debinding with large pore sizes and high vapour pressures, where

laminar flow controls the rate of gas exit from the compact.

Typically the pressure of a debinding process varies between 10-3 bar and 70 bar and the

grain sizes between 0,5 and 20 µm.

According to the debinding model in figure 1 the time of the 3 above mentioned debinding

processes can be calculated [3]:


P

P0 gas surface of sample

porosity

binder

solid

Figure 1: debinding model

Time t1 in which the organic liquid moves to the surface of the body, debinding by capillary forces:

)(******)1(*5,4

3

22

1S

S

DDDWEHDGEt

−−

=

Time of the diffusion process t2 :

UEPPDTkMHt

**)(**2)**(*

20

5,02

2 −=

Time of the permeation process t3 :

)(*****)1(**5,222

0223

22

3 PPFDEGPEHt

−−=

D = diameter of grain mDS = average diameter of grain mE = porosityF = mole fraction of binder in the vapourG = viscosity of the vapour Pa *sk = Boltzmann constant 8,32J/(K*mol)H = height of the sample mM = molar mass of vapour kg/molP0 = ambient pressure PaP = pressure at interface vapour/liquid Pa U = molar volume of the solid binder m³/molW = energy of interface J/m²

The above mentioned equations give an idea about the influence of parameters on the

debinding time. The increase of capillary forces raises the debinding process and minimizes

the tension in the green body. This can be done by increasing the surface tension and at the

same time by decreasing the viscosity.

A uniform grain size dimension enhances all debinding processes; however, agglomerates or

areas of higher density slow down this process. The use of fine powder accelerates the

sintering velocity, but reduces the speed of debinding.


The chemistry of thermal debinding

The thermal decomposition of polymers takes place by radical splitting of their chain. A

homolytic decomposition of a C-C-bond leads to radical cracked products. Homolytic

means the symmetric decomposition of the duplet. The following intermolecular transfer of

hydrogen and the continuous decomposition of the chain create saturated and unsaturated

fractions consisting of monomers ( small, but very reactive molecules) and oligomers

( combination of identical reactive molecules).

Appearance of aromatic compounds [4]

Aromatic compounds are determined by cyclic organic compounds with a special

electronic structure.

They will be created via

• propagation and condensation

• radical demerisation

• trimerisation of species of acetylen

• Diels-Alder-reaction

Radical propagation reactions, which create stepwise prolongations of the hydrocarbon

chains, may lead to closed ring reactions with 5 or 6 rings. In thermal systems the carbon rings

arrange themselves in their most stable formation:

C5-rings have a cyclopentadien structure, while C6-rings become the benzene structure [5]

Radical dimerisation (special formation of polymerisation, e.g. agglomeration of 2 units of

atoms or molecules, so called monomers) of C3-alphates is the reason for the ring formation.

Aliphates are nonpolar hydrocarbons, which are soluble in fat and oil or can dissolve them

(lipophil); all organic compounds which are not aromatic are aliphatic.

Trimerisation of 3 species of acetylen creates benzene, but an adequate catalyst for this

reaction is necessary.

Oxidised aliphatic and cyclic compounds

During thermal decomposition the attack of oxygen molecules happens inside the polymers

creating alkyl-hydroperoxides. The break of the C-C-compound at the most fragile point and

the splitting of the O-O-compound pass to radicals of alkyl, formaldehyde and hydroxy

radicals. The alkyl radical can form with oxygen an alkyl peroxy radical, which alters from the

former polymer through hydrogen transfer to a hydroperoxide. The results of decomposition

of the hydroperoxide are hydroxy radicals and aldehydes. Hydroxy radicals react with the

short chained ethylene and propene to formaldehyde, acetaldehyde propenaldehyde [6].


Conclusion of the chemistry of debinding

According to the a.m. mechanisms of thermal debinding of polymers the following products

of polymers can be expected [4]:

Additive: polyvinyl alcohol (PVA)

products empirical of pyrolysis formular

formaldehyde CH2O

acetaldehyde CH3CHO

cotonaldehyde C4H6O

benzene C6H6

phenol C6H5OH

benzaldehyde C7H6O

toluene C7H8

styrol C8H8

cresol C7H8O

benzofuran C8H6O

naphthalene C10H8

Figure 2: Polymers used as binder


Additive: polyethylene glycol (PEG)

products empirical of pyrolysis formularformaldehyde CH2O

acetaldehyde CH3CHO

valeraldehyde C5H10O

mono-,di- and trimers of ethylene

glycolC2H4(OH)2

methyl-1,3-dioxolane C4H6O3

benzene C6H6

Figure 3: Polymers used as slip agent

Additive: acid of polyacrylics (PAA)

products empiricalof pyrolysis formular

formaldehyde CH2O

acetaldehyde CH3CHO

propotionaldehyde C3H6O

2-butanone C4H8O

benzene C6H6

Xylene C8H10

phenol 6H5OH

cresol C7H8O

Figure 4: Polymers used as dispergent agent or stabilizer

Nearly all products of pyroysis have a certain toxicology, which is listed in the recommendations of the EPA (Environmental Protection Agency of the United States of America ) and in the list of maximum concentration at working place (MAK).


Thermal analytical examination of binders.

With methods of thermal analysis the burning out of binders can be studied systematically.

Thermogravimetry (TGA) registers the mass changes of a sample which is subject to a mostly

linear heating-up. The course of the mass change is mostly represented as a relative value in

% in comparison with the mass at the starting point. Derivative with respect to time the

derivation of the TGA curve - the DTG curve - is often useful because of better clearness. The

DTG-curve shows the velocity of the mass change in %/min.

The temperatures of decomposition of the three organic substances PVA, PEG and PAA vary

generally between 150 °C and 550 °C [3]. The maximum rate of decomposition of PVA, PEG

and PAA are shown in the table of figure 5.

organic substance range of decomposition [°C]

PVA 200 - 300

PEG 150 - 250

PAA 250 - 350

Figure 5: maximum rate of decomposition of PVA, PEG and PAA

Typical for PVA and PAA are their step-by-step decomposition process, due to their side

chains, which have a lower bond strength than the carbon skeleton.

Al2O3 can have a catalytic effect on the decomposition process of polymers and therefore

on their range of decomposition temperature [7].

Al2O3 may increase the decomposition of the main and side chain of the polymers and

therefore move the reaction to lower temperatures. This can be seen in figure 6 as a

comparison between the pure PVA and together with Al2O3.


Figue 6: Decomposition of pure PVA and an Al2O3-PVA-mixture.

Similar effects can be observed in the decomposition of PEG, see figure 7.

Figure 7: decomposition of pure PEG and an Al2O3-mixture.

Half of the pure PEG is pyrolised at 330 °C, but in the green body of the Al2O3-mixture

with PEG the same status is already reached at a temperature 100 °C lower.


The shift of debinding of PAA as a mixture with Al2O3 to higher temperatures is presumably a

matter of high affinity of the carboxylgroup to the surface of Al2O3, see figure 8.

Figure 8: decomposition of pure PAA and an Al2O3-mixture.

The reason of different procedures of the debinding process of pure substances and ceramic

compounds can be explained by the distribution of particles, the compaction during

forming, electrochemical interaction of polar molecules and reactions with the atmosphere.

The delay of reactions in mixed and pressed bodies appears due to aggravated oxygen

transport mechanisms in the ceramic matrix [8].

The catalytic debinding process

The precondition for a catalytic debinding process in a kiln is a flow of nitrogen, which will be

dosed with a liquid catalyst, mainly HNO3, The catalyst will be vaporized, due to the

temperature in that part of the kiln. The consumption of nitric acid (HNO3) is very low: 20 ml

are useful for a kiln volume of 50 dm³ [9].

It is recommended to use polyacetal as a binder, because this polymer depolymerises

completely under the influence of an acid or a base.

The catalyst attacks the side OH-groups and creates molecules of formaldehyde.

Formaldehyde has a very low dew point (-21 °C) and it will be directly vaporized from solid to

vapour at the typical temperature of debinding (110 °C). The molecules are small, therefore

the velocity of the diffusion process is high and the debinding process is fast. The gain of time


in comparison to normal thermal debinding processes can reach the factor 10. There is no

danger of distortion for the ceramic body, because the liquid phase of the binder is missing.

Also the internal gas pressure is low, which minimizes the danger of cracks. But as already

mentioned, it is necessary to realize an intensive circulation of the kiln atmosphere in the cross

sections.

The debinding time depends on the quantity of catalyst, and of nitrogen and temperature.

An increase of the mentioned factors shortens the time of the process.

The depolimerisation of polyacetal leads to formaldehyde, whose quantity of

0,1 kg/h and concentration of 20 mg/m³ are not allowed to be exceeded in exhaust systems

due to reasons of environmental protection.

An after firing system preferably two-staged equipment is strongly recommended. In the first

stage the formaldehyde-nitrogenoxide-exhaust gas will be burned by a support burner in

reducing atmosphere; existing nitric oxides will be reduced to N2. The second stage consists of

a firing system, where the hydrocarbons and formaldehyde will be transferred to water and

carbon dioxide in an oxidising atmosphere at around 800 °C.

Kilns for debinding purposes

Similar to kilns, which are used for sintering applications, there are also two kinds of kilns for the

debinding process:

• Tunnel kiln,

• Periodic kiln.

In a tunnel kiln the material will be continuously transported through the kiln. On its way it will

be heat treated and complied with atmospherical conditions, according to an adjusted

temperature and atmospheric curve, which is controlled by an extensive control system. The

binder burn-out sections are an integral part of the kiln and erected in front or adjacent to it.

Depending on the kind of binder and the process of debinding, - thermal or catalytic

debinding – an oxidising process or a burn-out in nitrogen atmosphere has to be performed in

kilns, which have a gas tight design.

Types of tunnel kilns are:

• Roller hearth kiln

• Pusher type kiln

• Belt kiln

• Tunnel kiln with car conveyance

All of them can be fuel fired or heated electrically.

The periodic kilns can be distinguished in:


• Chamber kilns

• Shuttle kilns

• Elevator kilns

If the capacity is lower and more flexibility is demanded, then a periodic kiln has to be used.

Burn-out and sinter process are performed during one firing cycle. Special atmospheres are

necessary, therefore these kilns must have a gas tight design; they are heated electrically or

with radiant tubes, which can be heated with fuel.

Continuous debinding processes in tunnel kilns

A typical configuration of binder burnout kiln and a sintering kiln consists of a roller hearth kiln

as the debinding part and a pusher type kiln for sintering.

The binder burnout kiln is composed of several sections, each approx. 2m long. The casings

are made of a steel sheet structure, the inside lining consists of high grade refractory fibre

materials, standard bricks and shaped sheet-metal parts according to the temperature

profile.

The binder burnout kiln is heated with electricity. The principle of a burnout section will be

explained according to the drawing of a cross section of a roller hearth burnout kiln in figure

9.


Figure 9: cross section of a binder burnout kiln

The rather low temperature in the burnout kiln allows the installation of wire heating elements.

They are installed in the air circulation channel below the transport area. The temperature in

the channel is controlled automatically. The heating elements can be sheathed, so there will

be no direct contact with the binder gases. Each of these air circulation zones is equipped

with a directly driven hot gas fan. This fan sucks the air through the loading, where it will be

enriched with binder volatiles; then the mentioned hot gas fan presses the waste gases

through channels, which are built to both front sides of each binder section, and then

directed over the sheathed heating elements, where the gases will be heated up.

The fresh air flow, which is required for each air circulation section, is adjusted manually and

measured by measuring orifice. The fresh air is a mixture of hot air, which is supplied by a heat

exchanger from the post combustion system and of cold air, mixed down to the desired


exhaust

fresh air

heating elements

transport rollers

temperature. The pressure in the fresh air supply will be kept constant by automatic pressure

control.

The same mass flow - as waste gases enriched with binders - escapes through the exhaust of

the binder burnout kiln in the centre of the cross section’s channel and will

be conducted via a heat exchanger, where the gases will be heated up to around 500 °C, to

the burner of the fuel heated post combustion system. The exhaust flow of each air

circulation section is adjusted manually, but the total exhaust gas flow is automatically

controlled by the measurement of the kiln pressure.

With this system – see figure 10 - the energy required for the post combustion of the binders as

well as the energy contained in the binders is used for heating the binder burnout kiln.

hot air

cold air fan

exhaust fan

fresh air fan

burner

heat exchanger

waste gas

section of binder burnout kiln

post combustion chamber

Figure 10: schematic drawing of a binder burnout system

The post combustion system is divided into 3 parts. The first part is a heat exchanger to

preheat the waste gas followed by the combustion chamber as the second part. The last

part consists of the second heat exchanger in which the fresh air will be preheated.

The waste gas is increased to approx . 500 °C in the first heat exchanger and then lead to the

combustion chamber, where it will be completely burned out at more than 800 °C. The


developed heat in the combustion chamber is transferred to the fresh air, which is led to the

input of the binder burnout kiln.

The binder containing waste gases are measured by a gas detector in the binder burnout

kiln, in the post combustion system and in the waste gas pipe. This gas detector gives an

alarm, if the waste gases reach the ignitable limit. To avoid an uncontrolled ignition of the

gas mixture in case of current failure the binder burnout sections will be flooded by argon

gas.

Debinding processes in periodic kilns

In periodic kilns the binder burnout and the sintering process are performed in one firing

cycle. The most used periodic kiln for these applications is the top-hat kiln, see figure 11.

figure 11: top-hat kiln


The top-hat kiln consists of a solid steel frame structure with welded-on steel sheet cladding. It

is lined with high grade fibre modules designed for the intended application temperature of

the sintering process. The kiln cars are also provided with a light weight application insulation,

which features an excellent insulation property and a low thermal mass. The products to be

fired are set in piles. Kiln dimensions are designed according to customer’s indicated amount

of piles; possible number of product piles are 1, 2, 4, 6, 8, 10, 14 or16. The useful width with 350

mm or 700 mm refers to the width of one or two piles and the length to the amount of rows of

piles with the space for SiC-heating elements in between each row. Kiln’s height is available

with 300 mm, 600 mm or 1000 mm.

The kiln car is lifted and lowered by spindles and pressed to the top hat hydraulically. A

flexible silicone profile packing is provided as sealing between the kiln car and the kiln casing.

A second car can be discharged and charged during the kiln operation.

The controlled heating up of the kiln is realized with SiC-rods, which are arranged horizontally

between the setting piles and the outer kiln walls. They are subdivided in several control

groups in order to get an excellent temperature uniformity.

The kiln atmosphere is controlled automatically throughout the entire binder burnout and

sintering process. For binder burnout purposes air streams are conducted over the heating

elements before they enter into the kiln. By flowing over the heating elements the air will be

preheated. With this system the energy consumption is decreased and the temperature

uniformity increased.

The cleaning of the waste gases is realized by the downstream thermal post combustion

equipment. Normally the post combustion plant is erected on the top of the periodic kiln, it is

heated with fuel.

The schematic drawing of figure 12 gives an overview over all functions of the periodic top-

hat kiln.


figure 12: scheme of a top-hat kiln

This type of kiln is preferred because of its

• high flexibility and good reproducibility of debinding and sintering processes

due to an adapted process control system

• variable adjustment of temperature curves optimally adapted to the product

requirements

• high temperature uniformity

• possibility to adjust different atmospheric conditions throughout the entire

debinding and sintering process

• direct connected thermal post combustion for cleaning the waste gases.

Conclusion

Debinding methods can be catalogued in solvent, catalytic and thermal processes. Only

both last mentioned procedures need thermal treatment. While the catalytic process can be

performed at lower temperatures around 120 °C, the thermal debinding process can reach

600 °C.

The most used binders for thermal debinding processes are based on PVA, PEG and PAA.

They have a characteristic debinding temperature performance, which can be analysed in

thermal gravimetric assays, however ceramic matrices smear the reaction, which might start

earlier but end later.

The products of pyrolysis are diversified and in some cases endanger health.

A post combustion system is recommended, which is explained in connection with the roller

hearth binder burnout kiln and the periodic top-hat kiln.


Literature:

[1] Bayer, M.: Organische Hilfsstoffe zur Herstellung plastischer Keramikmassen für die

Spritzguss- und Strangpressformgebung. Technische Keramische Werkstoffe. Deutscher

Wirtschaftsdienst, Juni 1992, Kap. 3.2.3.2.

[2] German, R.M.: Theory of thermal debinding; Int. Journal of Powder Metallurgy,

(23),1987, No. 4, page 237 – 245.

[3] Ferrato, M.; Cartier,T.; Baumard,J.F.; Coudamy,G.: Der Bindemittelabgang in

keramischen Scherben, cfi/Ber. DKG 71 (1994), No.1/2, page 8-12.

[4] Ziegler, G.; Willert-Porada, M.: Schadstoffreduzierung durch Prozessoptimierung bei der

thermischen Zersetzung organischer Additivsysteme für die keramische Formgebung

unter Einbeziehung der Mikrowelleneinkopplung. Report to research project no. 12068

N

[5] Froese,K.L.: Pathways in the formation of chlorinated aromatic compounds through

heterogeneous combustion reactions of C2-hydrocarbons. Dissertation an der Fakultät

für Biologie, Chemie und Geowissenschaften der Universität Bayreuth.

[6] Hucknall,D.J.: Chemistry of hydrocarbon combustion. Capman & Hall, London, New

York, 1985

[7] Masia et al.: Effects of oxides on binder burnout during ceramics processing.

Journal of Materials Science, (1989), P. 1907-1912.

[8] Emmerich,W.-D.; Janoschek,J.; Kaisersberger,E.: Thermoanalytische

Untersuchung des Bindemittelausbrennes und des Dichtsinterns.

[9] Blörnacher, M.; Weinand,D.: Fortschritte in der Binderentwicklung;

Pulvermetallurgie in Wissenschaft und Praxis, lectures at symposium in Hagen,

Germany, 1991, VDI-Verlag


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