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Cross‐Linking via Electron Beam Treatment ofa Tailored Polysilazane (ABSE) for Processingof Ceramic SiCN‐FibersSylvia Kokott a & Günter Motz aa Ceramic Materials Engineering , University of Bayreuth , Bayreuth,GermanyPublished online: 05 Jun 2007.

To cite this article: Sylvia Kokott & Günter Motz (2007) Cross‐Linking via Electron Beam Treatment of aTailored Polysilazane (ABSE) for Processing of Ceramic SiCN‐Fibers, Soft Materials, 4:2-4, 165-174, DOI:10.1080/15394450701309881

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CROSS-LINKING VIA ELECTRON BEAM TREATMENT OF

A TAILORED POLYSILAZANE (ABSE) FOR PROCESSING

OF CERAMIC SiCN-FIBERS

Sylvia Kokott and Gunter Motz A Ceramic Materials Engineering, University ofBayreuth, Bayreuth, Germany

A Cost-efficient ceramic SiCN fibers were produced by the precursor route consisting of fourprocessing steps: synthesis of the polymer, melt-spinning, curing via electron beam (e-beam)and subsequent pyrolysis at 11008C in a nitrogen atmosphere. A special solid and meltablefiber polymer, the so-called polycarbosilazane ABSE has been developed for this purpose, whichis easily curable by e-beam. In this article the influence of the curing dose and the molecularweight of the used precursor on polymer cross-linking are discussed. The degree of cross-linkingof the e-beam treated precursor ABSE is characterized by the gel fraction and the pyrolysis beha-vior. The gel fraction increases with the e-beam dose as well as with higher molecular weight of theused precursor fraction. By an optimal adjustment of the e-beam dose on the molecular weight ofthe precursor the curing treatment leads to unmeltable polymer fibers at reduced costs as well as toan improved flexibility of the green fibers which is very important for the continuous pyrolysisprocess to ceramic fibers.

Keywords Precursor ceramic, SiCN-fibers, Radiation curing, Gel fraction

INTRODUCTION

During the last decades research in fiber reinforced ceramic matrixcomposites (CMCs) has been of increasing interest. Themanifold opportu-nities of CMCs for high temperature applications in oxidizing and/or cor-rosive environments arise from the combination of the positive propertiesof ceramic matrix and fibers as well as the elimination of the negativematrix properties like brittleness. But for an extensive adoption of theCMCs the properties and the price of the fibers play a decisive role. Unfor-tunately, until now there is no ceramic fiber commercially available atreasonable costs, which offers high strength and Young’s modulus, goodcreep resistance and oxidation stability beyond 12008C. Though carbonfibers are available in various qualities with different properties and

Received 12 October 2006; Accepted 18 December 2006.Address correspondence to Gunter Motz, Ceramic Materials Engineering, University of Bay-

reuth, Bayreuth, Germany. E-mail: guenter.motz@uni-bayreuth.de

Soft Materials, 4(2–4): 165–174, (2007)Copyright # Taylor & Francis Group, LLCISSN 1539-445X print/1539-4468 onlineDOI: 10.1080/15394450701309881

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prices, they are all susceptible to oxidation already at temperatures ofabout 4008C (1–5). In contrast, oxidic ceramic fibers are stable in air butthey are prone to creep at temperatures above 11008C (1, 3, 4, 6, 7).Creep is the plastic deformation of a material that is subjected to a stressbelow its yield stress. There are three basic mechanisms that can contributeto creep: (i) dislocation slip and climb, (ii) grain boundary sliding and (iii)diffusional flow. But also deformation occurs by preferential growth oforiented elongated particles (8). SiC fibers show a lot of favorable proper-ties, however, they suffer from their relatively low oxidation stability andtheir high price of up to 12000 US$/kg (1, 3, 7, 9, 10).

Ceramics in the system SiCNmade by the precursor route (11–13) fea-tures good expectances to close the above mentioned gaps (14–18).Besides the good corrosion resistance in acids and bases the resultingnon-oxide, amorphous ceramic SiCN fibers possess a good oxidation stab-ility at up to 15008C in an oxidizing environment, caused by the nitrogencontent, which inhibits oxidation (19). Furthermore, production ofceramic SiCN fibers by the precursor route is comparably cheap (20).The cost efficiency is realized by the opportunity to use cheap eductsand a reproducible four-step continuous process. The first step is thesynthesis of the specially tailored, solid and relatively air stable polycarbo-silazane ABSE (21). Due to the viscoelasticity of the melt the precursor ismelt-spun in a second step to endless green fibers in a multifilamentmodus with adjustable diameters between 20 and 150 mm. To pyrolysethe meltable green fibers, curing by e-beam is necessary, leading to theformation of a three-dimensional network with new covalent bondsbetween the macromolecules and thus to unmeltable fibers. In the finalstep, non-oxide ceramic SiCN fibers are achieved by continuous pyrolysisin a furnace up to 11008C in nitrogen atmosphere. All steps formanufacturing the ceramic SiCN fibers from the ABSE precursor arewell-established and feasible at a comparably low financial expense(21–23). Nevertheless, in particular the expensive curing process is stillnot optimized yet. Especially the very high price for commercially availableSiC-fibers produced via the precursor route is caused by the high e-beamcuring doses between 7500 and 20000 kGy (2, 24, 25). Therefore, systema-tic tests with irradiation at lower beam doses and the subsequent examin-ation of the ABSE precursor properties are worthwhile to determine theminimum necessary e-beam dose.

However, it is very difficult to characterize the state of cross-linkingafter e-beam curing of a polysilazane like the irradiated ABSE polymer.Conventional microscopic methods like scanning electron microscopy(SEM) or atom force microscopy (AFM) cannot be applied, since the amor-phous microstructure of the ABSE precursor is not affected by irradiation.As well, the small number of formed bondings for cross-linking in relationto the total amount of bondings does not influence the characteristic

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spectra of the polymer, so that neither infrared spectroscopy (IR) nornuclear magnetic resonance spectroscopy (NMR) can provide informationabout changes in the polymeric network. Furthermore, as a result ofirradiation the polymer becomes insoluble, so that changes in the molecu-lar weight of the polymer are not detectable by gel permeation chromato-graphy (GPC), which requires the solubility of the polymer. Otherwise, theformation of insoluble species can be used to estimate the degree of cross-linking by measurement of the gel fraction. Additional melting tests byknown gel fraction value of the cured polymer enables a reliable fibercuring. To characterize the degree of cross-linking after e-beam curingonly the measurement of the gel fraction in combination with meltingtests yield suitable results to estimate the curing conditions to make thepolymer fibers infusible.

EXPERIMENTAL PROCEDURE

The so-called ABSE polycarbosilazane was synthesized by co-ammono-lysis of bis(dichloromethylsilyl)ethane and dichloromethylsilane (molecu-lar ratio 10 : 1) in toluene as described elsewhere (23, 26) and can betransferred easily to a pilot plant (batch size 40 l). After filtration fromthe precipitated ammonium chloride the solvent and oligomeric bypro-ducts were removed by distillation at 1808C under reduced pressure.The synthesis yield of the colorless, brittle and meltable solid is about75 wt.%.

The definition of the gel fraction is in accordance to the German stan-dard DIN 16892 (27). Therefore, cylindrical samples with a thickness of1 cm and a diameter of 3 cm of several ABSE-precursor charges withdifferent average molecular weights (3700, 14000, and 27000 g/mol)were manufactured by melting. The introduction of oxygen into thepolymer would lead to chemical cross-linking. Therefore, the sampleswere handled for every processing step in an inert atmosphere. Theso-prepared samples were irradiated by an electron accelerator with differ-ent e-beam doses (300, 400, 600, 800 and 1000 kGy) and an accelerationvoltage of 10 MeV. The cured samples were chopped into small pieces.Some of the small pieces were used for the melting tests and the rest wasmilled to coarse powder. The powder was weighted and subsequentlydissolved in toluene. The unsolved parts were separated by filtration,dried and weighted for a second time. The gel fraction was calculated byequation 1:

G ¼m2 � 100

m1ð1Þ

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G ¼ gel fraction in %

m1 ¼ weight of the powder before solution in toluene

m2 ¼ weight of the dried insoluble parts after filtration (27).

For the melting and pyrolysis tests small specimen of the cured ABSEprecursor were transferred into the furnace and pyrolyzed with a heatingrate of 5 K/min up to 10008C in a nitrogen atmosphere. After heat treat-ment the samples were examined regarding observable signs of melting.

To verify the results of the melting tests, green fibers made from anABSE charge with an average molecular weight of 3700 g/mol werecured with the same doses of irradiation like the ABSE specimen and con-tinuously pyrolyzed as shown in Fig. 1. The green fiber bundle was drawnthrough the cooled stovepipe and, via a special set-up to keep the fibers intension, fixed between the decoiling and coiling unit. This can also beregarded as a simple selection method: only completely cured greenfibers can be pyrolyzed continuously without rupture. After heating thefurnace up to the maximum pyrolysis temperature of 11008C (5 K/min,N2-atmosphere) the fibers were pulled through the furnace with a constantvelocity of 0.5 cm/min.

The pyrolysis behavior and the ceramic yield of the different precursorsamples were also investigated by thermo-gravimetric analysis (TGA,Linseis STA) with a heating rate of 3 K/min up to a temperature of11008C in nitrogen atmosphere. Gaseous products formed during heattreatment were identified by infrared spectroscopy (FTIR, Vector 22,Bruker) coupled with the TGA equipment.

RESULTS AND DISCUSSION

Besides a small amount of Si-H groups the used ABSE polycarbosila-zane contains no reactive groups like Si-vinyl. The resulting low reactivity

FIGURE 1 Processing of ceramic SiCN fibers by continuous pyrolysis in a horizontal furnaceequipment.

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enables the short-time handling of the precursor in air, but chemicalcuring of the polymer ABSE fibers in a reasonable time is impossible.Therefore, the most promising method to provide complete curing inshort time is the treatment with e-beam. The penetration depth of thee-beam with an acceleration voltage of 10 MeV in a polymeric material islarger than 3 cm. Following this way a complete coiled roll of greenfibers can be cured en bloc, and a decoiling of the brittle green fibers forthe curing step is not necessary. As well, up to a thickness of more than3 cm the shape of the precursor samples is of negligible influence forour test. To simulate the thickness of a green-fiber coiled roll the cylindri-cal precursor specimen were chosen for the measurement of the gel frac-tion. In order to find out the optimum e-beam dose at a starting averagemolecular weight of Mw ¼ 3700 g/mol, a special ABSE precursor chargewas irradiated with different e-beam doses (400, 600, 800, and1000 kGy). The determined gel fraction G (Eq. 1) is increasing almost lin-early with the applied e-beam dose from G ¼ 2% after irradiation with400 kGy up to G ¼ 18% after treatment with 1000 kGy (Fig. 2). This obser-vation is explained by the formation of free radicals during irradiation ofthe precursor. The subsequent recombination of the radicals leads to inter-molecular cross-linking reactions and the formation of covalent bondingsbetween different polymer molecules without the use of chemicalinitiators. Without deformation by melting during the irradiationprocess the polymer formed a partly insoluble gel. The higher thee-beam dose, the higher the number of generated free radicals and thehigher the degree of cross-linking. Despite the relatively high irradiationdose of 1000 kGy the gel fraction of 18% indicates that no degradationof the precursor occurred.

FIGURE 2 Gel fraction of the ABSE precursor depending on the e-beam dose (Mw precursor: 3700 g/mol). The photographs show the specimen after irradiation and subsequent pyrolysis at 10008C(N2-atm.).

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For the most applications the irradiation of plastics should lead to abetter strength and resistance against creep as well as to an improvedchemical stability (28–30), whereas the cross-linking of the ABSE precur-sor via e-beam should mainly result in an unmeltable polymer (thermoset)to enable the transformation of the ABSE polymer fibers into ceramicSiCN fibers and to facilitate their handling by improved flexibility.Usually, the meltspun polymer ABSE fibers are too brittle for severalcoiling and decoiling steps (Fig. 3a). But already the curing of the ABSEgreen fibers with a low irradiation dose of 300 kGy leads to more flexiblefibers (Fig. 3b).

To avoid melting of the cured green fibers during pyrolysis the deter-mination of the necessary degree of cross-linking (gel fraction) is required.The comparison of the melting tests on the irradiated ABSE specimen withthe related gel fraction measurements leads to the result that a gel fractionvalue of 18% is necessary to ensure shape stability (Fig. 2). The sampleswith lower gel fraction melt during pyrolysis, whereas only the samplewith a gel fraction of about 18% is completely unmeltable. This resultwas confirmed by continuous fiber pyrolysis experiments. The ceramiza-tion of polymer ABSE fibers with an initial average molecular weight of3700 g/mol was only successful after curing with a dose of 1000 kGy.

In the second test series ABSE samples with various average molecularweights were irradiated with a uniform e-beam dose of 300 kGy. Evalu-ation of the samples revealed that the gel fraction increases with higher

FIGURE 3 a) Brittleness of the ABSE green fibers directly after the melt spinning process.b) Improved flexibility of an ABSE green fiber after e-beam curing with a dose of 300 kGy.

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molecular weight of the used precursor (Fig. 4) up to an average molecularweight of 15000 g/mol. Beyond this value the increase of the gel fraction isgetting weaker. The results in Fig. 3 indicate that at low initial molecularweights, i.e., about 5000 g/mol, the applied e-beam dose of 300 kGy istoo small to cross-link enough of these relatively small molecules to forman insoluble polymer with the required gel rate. In the case of biggerinitial molecules (higher average molecular weight) the same number ofnew bondings caused by the e-beam treatment is sufficient to build up inso-luble cross linked regions, accompanied with a strong increase in the gelfraction and leading to an unmeltable three-dimensional network(Fig. 4). It appears that there is saturation due to cross-linking by for-mation of new intermolecular bondings for precursor charges with mol-ecular weights higher than 15000 g/mol after treatment with an e-beamdose of 300 kGy. The resulting gel fraction stagnates at 22%. Obviously,for higher initial molecular weights the probability that new bondingswere built up intramolecularly within the macromolecules is much higher.

Another explanatory approach is that during the irradiation not only across-linking process is initialized but also degradation. From a certaininitial concentration of long chains, cross-linking to insoluble networksand degradation are well-balanced, i.e. the formed content of gel fractionstagnates (28–30). However, a gel fraction of about 18% is sufficient for asuccessful pyrolysis of the ABSE green fibers into ceramic SiCN fibers.

The e-beam curing also affects the pyrolysis behavior and the ceramicyield of the ABSE precursor (Fig. 5). Even a low dose of 400 kGy leads toan increase in ceramic yield from 65% for the uncured polymer of up to70% after pyrolysis up to 10008C (N2-atmosphere). A further increase inthe e-beam dose up to 1000 kGy enhances the ceramic yield up to 72%.The pyrolysis process can be divided into 3 steps. In the temperature

FIGURE 4 Gel fraction depending on themolecular weight of the used ABSE precursor (e-beam dose:300 kGy). The photographs show the specimen after irradiation and subsequent pyrolysis at 10008C(N2-atm.).

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range up to 4508C further thermally induced cross-linking occurs charac-terized by the separation of ammonia and evaporation of oligomers. Attemperatures higher than 4508C the transformation from the polymerinto the amorphous ceramic is initiated by the separation of mainlymethane. The third step starts at about 8008C and is characterized bystructural rearrangements of the amorphous SiCN ceramic (18, 31–35).Fig. 3 shows that the e-beam curing reduces the first mass loss up to4508C remarkably because a part of the oligomers are cross-linked tobigger molecules. The higher degree of cross-linking of the used precur-sor, for example because of e-beam treatment, reduces the content of vola-tile oligomers and lowers the mass loss. In the case of the uncured polymera further cross-linking is only thermally induced by condensation reactionsof ammonia at temperatures where the evaporation of oligomers starts.This leads to an additional mass loss and finally to a lower ceramic yield.

CONCLUSION

The determination of the gel fraction is a very good and useful methodto characterize the state of cross-linking in a polymer material. In the caseof the ABSE precursor the gel fraction after e-beam curing depends onboth the molecular weight of the polymer and the e-beam dose.Knowing these two parameters it is possible to adjust the necessary gel frac-tion by a minimal e-beam dose for a successful subsequent pyrolysis.

Independent from the initial molecular weight of the used ABSE pre-cursor a gel fraction of 18% after curing is sufficient to avoid melting of the

FIGURE 5 TG-FTIR coupled measurements of the ABSE precursor (Mw 3700 g/mol) in dependencyon the e-beam curing as well as characterization of the pyrolysis behavior and identification of the gas-eous pyrolysis products.

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green fibers during pyrolysis. On the basis of this result an irradiation doseof only 300 kGy is enough to stabilize the shape of fibersmelt-spun from anABSE precursor with an average molecular weight of about 14000 g/mol.A higher e-beam dose probably leads to an increased formation of intramo-lecular bonds which have only little affect on the gel fraction. In the case ofa lower average molecular weight of about 3700 g/mol the necessaryirradiation dose increases up to 1000 kGy. Compared to the Hi-Nicalonwor other SiC fibers which are cured with e-beam doses from 7.5 up to20 MGy (2, 24, 25), it is possible to save more than 95% of the irradiationcosts by using a high molecular ABSE precursor.

ACKNOWLEDGMENTS

The technical assistance of Beta Gamma Service Company for carryingout the curing by e-beam is thankfully appreciated.

Thanks to the staff members of the chair Keramische Werkstoffe undBauteile of the University of Bremen for measurement the mechanicalproperties of the fibers.

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