Oxidation Reaction of Polycarbosilane - American Chemical Society

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Downloaded by MICHIGAN STATE UNIV on November 29, 2013 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0224.ch034

Oxidation Reaction of Polycarbosilane Hiroshi Ichikawa, Fumikazu Machino, Haruo Teranishi, and Toshikatsu Ishikawa Research and Development Laboratory, Nippon Carbon Company, Ltd., Shin-urashima-cho, Kanagawa-ku, Yokohama 221, Japan

Polycarbosilane (PCS) is an organic polymer with a skeleton of Si-C bonds. SiC fiber (Nicalon) is produced from PCS in three steps: spinning, curing (rendering the material infusible), and heating. SiC fiber has high tensile strength, high elastic modulus, and excellent thermostability in high-temperature oxidizing atmospheres and is generally cured via oxidation of PCS by heating in air. The structure and mechanical characteristics of SiC fiber are largely affected by the conditions of PCS curing. The mechanism of PCS oxidation has not been clarified yet, and to clarify this point, the changes in PCS powder after oxidation in air were studied. The oxidation of PCS involves the following reactions: (1) oxidation of two Si-Η groups followed by condensation to give Si-O-Si, (2) oxidation of Si-Η to give Si-OH, (3) oxidation of Si-CH to give Si-OH and formalde hyde, and (4) dehydration condensation of two Si-OH groups to give Si-O-Si. PCS becomes infusible by the cross-linkage of Si-O-Si groups. However, infusible PCS contained more than 7 wt % of oxygen, because it had more Si-OH than Si-O-Si. The condensation reaction of Si-OH groups also gives Si-O-Si after heat treatment in nitrogen. By applying this method, we obtained infusible PCS con taining only 3-6 wt % of oxygen, which is less than that contained in fibers prepared by the conventional method. 3

STRUCTURAL MATERA ILS

made offiber-reinforcedcomposites with light weight and high strength have come into use in space rockets, aircrafts, automobiles, and sports articles in recent years. Silicon carbide (SiC) fiber, which was first developed by Yajima et al. (I, 2), is drawing particular attention as a reinforcing fiber for composites. SiC fiber with β-SiC as the 0065-2393/90/0224-0619$06.00/0 © 1990 American Chemical Society

In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

620

SILICON-BASED POLYMER SCIENCE: A COMPREHENSIVE RESOURCE


main component may be obtained by spinning, curing, and heating poly carbosilane (PCS), an organosilicon compound (Figure 1). SiCfiberhas been commercialized successfully and is presently on the market with the trade name of Nicalon (3, 4). SiC fibers are continuous fibers bundled in groups of500 filaments, with each filament having a diameter of 15 μπι. The fibers have high strength,

dimethyldichlorosilane h-NaCi

dechlorination

polysilane

/i

H

l

thermal decomposition & polymerization polycarbosilane

-melt spinning Spun fiber

curing in air Infusible fiber

heat treatment in N

2

SiC fiber (Nicalon)

Figure 1. Production process for PCS and SiC fiber (Nicalon).

high modulus, high thermostability in a high-temperature oxidizing at mosphere (5), and good compatibility with metal or glass. These character istics are not found in carbon fibers. Therefore, practical developmental research for applications of the fiber as a heat-resistant material for space rockets and as a reinforcing fiber forfiber-reinforcedplastics (FRP), metals (FRM) (6, 7), and ceramics (FRC)(8) is in progress. In the production of SiC fiber, one of the most important steps is the curing process, by which melt-spun PCS fiber is heated without fusion or adhesion and converted to SiC fiber. The structure and mechanical char acteristics of the SiC fibers are largely affected by the curing conditions (9). The curing of PCS fiber generally involves oxidation by heating in air.


34.

ICHIKAWA ET AL.

Oxidation Reaction of Polycarbosilane

621

The reaction polymerizes PCS and and renders it infusible. However, the mechanism of the reaction has not been clarified yet. PCS structural changes during heat treatment in vacuum have been reported by Hasegawa et al. (10), and studies related to oxidation in air during SiC fiber production have been partially reported (11). This chapter will try to clarify the reaction mechanism of PCS oxidation during curing to produce SiC fiber.


Experimental Procedures PCS Synthesis. Poly(dimethylsilane) (400 g) was synthesized by the dechlo rination reaction of 1066 g of dichlorodimethylsilane with 390 g of fusing sodium in 2.5 L of xylene at 100-140 °C for 12 h (12). Poly(dimethylsilane) (300 g) in a 2-L autoclave was subjected to airtight pressure pyrolysis for 4 h at 470 °C. The pressure during the reaction was 0-62 kg/cm 2 (gauge pressure). A light-yellow solid PCS (160 g) was produced as follows: (1) A 200-g sample of reactant was dissolved in 1 L of hexane. (2) The insoluble substance was removed by filtration. (3) The low-molecularweightfractionwas removed by vacuum distillation at 133-400 Pa. The PCS product was analyzed by gel permeation chromatography (GPC), IR spectroscopy, thermogravimetric analysis, (TGA), and differential scanning calorimetry (DSC). The specific gravity and melting point of the product were determined. Oxidation. PCS powder was used to gain basic knowledge about the curing process. PCS was crushed, pulverized in a mortar, and filtered through a sieve measuring less than 325 mesh. About 2 g of the sample was weighed into a glass measuring bottle (60-mm diameter by 40 mm), which was filled by vibration to make the powder settle evenly. The amount of powder at this time was —70 mg/cm 2 . The PCS-powder-filled measuring bottle was placed in a hot-air-circulating drier and subjected to oxidation for 1-8 h, with the temperature being raised by 10 °C incre ments (10 °C/h) from 150 °C to a fixed temperature (150, 160, 170, 180, or 190°C). After oxidation, the weight change of the sample was measured, and the IR spectrum and solubility in hexane and tetrahydrofuran (THF) were determined. Melting points were determined for and GPC analyses were performed on the measurable samples. Heat Treatment under Nitrogen Atmosphere. Among the oxidized PCS pow ders, those with weight gains of 3-9.5% (six samples) were heated in a nitrogen atmosphere. The oxidized PCS powder (1 g) was placed in a heat-resistant glass container (28-mm diameter by 200 mm) and heated with a cylindrical band heater under a gaseous-nitrogen flow of 2 mL/min. The heating temperature was 260-480 °C. Samples with oxidation weight gains of 8.2% were heated at a rate of 100 °C/h. After heat treatment, the samples were examined by IR spectroscopy, solubility in THF, and visual observation to distinguish whether they had become infusible. Measurement Techniques. PCS melting points were measured by light permeability with a Mettler automatic melting-point apparatus (model PF-61). The powdered sample, which was filtered through a sieve of less than 325 mesh, was introduced into a 1-mm-diameter glass capillary tube (filling density 0.54-0.57 g/cm 3 ; height 4.0-4.5 mm), and the temperature was raised at a rate of 10 °C/min. The temperature at which light began to be transmitted was named the



622


melting-starting temperature (Ts), and the temperature at the inflection point at which the sample fuses and the light transmission curve rapidly rises was named the melting-finishing temperature (Tm). For GPC analysis, a high-speed liquid chromatograph (Toyo Soda Company, model HLC-801 A) was used. The columns (G 2000 H and G 4000 H , two by two) were connected in series. The detector was a differential refractometer. The sample was dissolved in T H F (65 mg of PCS per 3 mL of THF) and analyzed at a current speed of 1 mL/min and a pressure of 8 kg/cm 2 . The average molecular weight was calculated on the basis of the elution of polystyrene molecular weight standards. TGA and DSC were performed with a TGA-DSC equipment (Rigaku-denki Company) in air, with sample weights of 3 mg and temperature increments of 5 °C/min. IR spectra were obtained with a spectrophotometer (model TRA-1) man ufactured by Nippon-bunko Company. KBr tablets with a density of 1 mg of PCS per 200 mg of KBr were used. Solubility in hexane or THF was determined by adding —100 mg of PCS powder to 20 mL of hexane or THF. Methanol was added to any sample that became a gel or was insoluble. Samples that became cloudy were determined as partly soluble, and samples that settled to give a transparent supernatant liquid were determined as insoluble. Oxidation weight gain was calculated by precisely weighing both treated and untreated PCS samples after leaving them in a silica gel desiccator for more than 1 h and using equation 1 Wo - W ,

àw/w

=

\

T7

1

X

100

(1)

in which Aw/w is the oxidation weight gain (in percent), W i is the weight before oxidation (in grams), and W 2 is the weight after oxidation (in grams). Oxygen analysis was performed by using an oxygen-nitrogen analyzer (model EMGA-2200) manufactured by Horiba-Seisakushyo. A 3-mg sample was placed in a graphite crucible and heated to >2500 °C in He gas. Carbon dioxide emissions were measured by using an IR detector, and the amount of oxygen was calculated. Silicon was measured by a gravimetric method after alkali fusion. Carbon and hydrogen were measured by volumetric burning, which applies to JIS M 8813 of coal analysis (Liebig method).

Results and Discussions PCS Characteristics. The characteristics of PCS powder are given in the list on page 623. In this list, D 4 2 5 is density, and M w and M n are the weight-average and number-average molecular weights, respectively. M n s were calculated from GPC data. The decomposition of PCS was studied by TGA and DSC in air. Results are shown in Figure 2. The starting temperature in exotherm was 170180 ° C , and weight increase started at around 210 °C. A slight difference in the starting temperature was observed. This difference may be due to the rapid rise in temperature (5 °C/min). The weight increased with heating, but it reached a limit at >275 ° C , which is the exothermic peak of DSC.


ICHIKAWA ET AL.


Characteristics of PCS D4 Melting points ( °C)

1.116

2 5

176 194

Τ x

Τ

s

Molecular weights M Mw IR absorbance ratios

1470 2980


D

2.6 0.35

• ^ 2 1 0 0 ^ 2950 ^ 1 3 5 5 ^ 2950

Elemental analysis (wt %) Si 0 C

H

Φ

Ο) c σ _c U

45.9 0.63 41.4 8.36

Weight Increase

TGA

Χιο%

Weight Decrease

Exotherm

Ό u

1 cal s

DSC 100

Endotherm 300

200

400

t/°C Figure 2. TGA-DSC

curves for PSC oxidation in air. The heating rate 5 °C/min.


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PCS Weight Gain by Oxidation. PCS powder was oxidized for 18 h in air at 10 ° C intervals from 150 to 190°C. The weight gain (Aw/w) was measured at each level (Figure 3). The weight gain Aw/w directly increased with oxidation temperature and time. In particular, the rate of change of Aw/w directly increased with oxidation temperature. The weight gain per hour from 0 to 3 h was calculated for each oxidation temperature (Figure 4).

1


14 ι

Time/h Figure 3. Dependence of Aw/w on time of oxidation in air at various temper atures.

The weight gain per hour is proportional to the oxidation reaction rate. However, the reaction rate of PCS reached an inflection between 170 and 180 ° C and increased at temperatures of >180 °C. At temperatures of >180 °C, the reaction mechanism must be different from that at temperatures of 300 °C for the sample with àw/w < 6.2% and for the sample with àw/w = 8.2% until -400 °C. This result means that during oxidation treatment PCS with a large amount of remaining S i - Η starts to decrease at 300 °C and PCS with a few remaining S i - Η starts to decrease at 400 °C. Figure 11c shows that A 1 2 6 0 /A 2 95o also tended to decrease. Figure l i d shows that A 1 0 2 0 / A 8 3 0 increased as the heating temperature rose. As previously stated, the peak at 1020 cm" 1 is caused by Si-O-Si stretching vibration and C H 2 deformation in Si-CH 2 -Si. Also, the absorp tion at 830 c m - 1 is caused by the S i - C H 3 stretching vibration. Therefore, this result indicates that Si-O-Si and Si-CH 2 -Si groups are increasing relative to S i - C H 3 groups. The Si-CH 2 -Si group also has an absorption at 1355 cm" 1 for a different angle of vibration. The Si-CH 2 -Si absorption was hardly changed; the value of A 1 3 5 0 / A 2 9 5 0 for the sample was 0.3-0.4 and did not change during the experiment as the temperature rose. Because the S i - C H 3 absorption, which was the ref erence, was almost unchanged until 300 °C, the increase in A1020/Ά 830 until 300 ° C is attributed to the increase in Si-O-Si groups. Figure 11 also shows the T H F solubility of heat-treated PCS samples. The solubility in T H F is closely related to the degree of infusion. The value of A 1 0 2 0 / A 8 3 0 had the highest correlation with T H F solubility (Figure lid). The samples for which A 1 0 2 0 / A 8 3 0 < 0.63 were soluble in THF. When A 1 0 2 0 / A 8 3 0 = 0.66-0.67, the samples became gels (partially soluble), and when A 1 0 2 0 / A 8 3 0 > 0.69, the samples became insoluble. This trend proves that PCS becomes infusible with cross-linkage of Si-O-Si groups and polymerization.


3400

2

2

5

Proposed Mechanism of PCS Oxidati

just discussed, the oxidation of PCS by heating to 500 ° C in a nitrogen



τ PC)

τ CC) 2

295

T(

Figure 11. Changes in Av/Av> of PCS oxidized by heat treatment in nitrogen: (a) A 3 4 0 0 / A 2 9 5 0 , (b) A ioo/A o, (c) A12m/A2950, and (d) A10JA830. Key for values of Aw/w; · , 2.9%; Δ , 3.2%; V, 4.9%; • , 6.2%; O, 8.2%; and Θ, 9.5%. Key for solubility in THF: φ, A, and soluble; V, B, and Φ, gei; and Δ , V, • , O , and Θ, insoluble. (1) indicates the position of the original oxidized PCS.

T(°C)


636


atmosphere probably proceeds by the following mechanism: \ / -SiOH + HOSi/ \


\ / -SiOH + H S i / \ \ / -SiOH + C H 3 S i / \ \

-SiH

/

+

/

CH3S1-

\

/

\

—» -Si- -O-Si/

+ H20

\

\ / -Si- -O-Si- + H 2 / \ \ / -Si- -O-Si- + C H 4 / \ \

/

—> -Si- - C H - S i - + H /

2

2

The main reaction of PCS oxidation by heating in a nitrogen atmosphere involves the formation of Si-O-Si by a dehydration condensation reaction of Si-OH bonds (equation 3). This reaction can be deduced from the fact that the number of Si-OH bonds decreases notably compared with those of other groups. The decrease in S i - Η (àw/w < 6.2%) and S i - C H 3 bonds at >300 ° C may be explained by equations 6 and 7. At >400 ° C , a decrease in S i - Η and S i - C H 3 bonds was noted for every PCS sample. This finding suggests that Si-CH 2 -Si bond formation from S i - Η and S i - C H 3 proceeds by equation 8. The previous results indicate that the Si-OH groups introduced by the oxidation reaction proceed to form Si-O-Si bonds through dehydration con densation even with heating in a nitrogen atmosphere. As shown in Table II, even a sample with àw/w = 3.2%, which was fusible before treatment, became infusible without adhesion with gradual heating in nitrogen. This fact means that the Si-O-Si content required for infusion is equal to an oxygen content of 3 wt %. Therefore, the actual oxygen content is less than one-half of the minimum oxygen content (7 wt %) in the infusible PCS. This conclusion indicates that SiC fibers with less oxygen content (3-6 wt %) compared with conventional SiC fiber can be produced.

Conclusion The characteristics of PCS were studied after oxidation in air or nitrogen under various conditions. The results were used to elucidate the mechanisms of the oxidation and curing reactions. The following conclusions were reached: The melting point and molecular weight of PCS increases with an increase in weight during oxidation. PCS becomes infusible In Silicon-Based Polymer Science; Zeigler, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

34.

ICHIKAWA ET AL.


637

when the weight gain during oxidation is >7%. The weight gain caused by oxidation is almost the same as the amount of oxygen introduced. • The oxidation of PCS proceeds according to the following steps: 1. oxidation of two S i - Η bonds followed by condensation to give Si-O-Si,


2. oxidation of S i - Η to give Si-OH, 3. oxidation of the S i - C H 3 bond to give Si-OH and formal dehyde, and 4. dehydration condensation of two Si-OH to give Si-O-Si. • PCS becomes infusible by cross-linkage of Si-O-Si groups. However, the infusible PCS samples contain large amounts of Si-OH groups that do not contribute to curing. • Si-O-Si bond formation by dehydration condensation of Si-OH groups continues during gradual heating in a nitrogen atmosphere. PCS becomes infusible even with oxidation weight gains of 3-6%. On the basis of experimental findings, the amount Si-O-Si groups re quired for the curing of PCS is equivalent to a PCS oxygen content of 3 wt %. Therefore, SiC fibers with less oxygen content compared with conven tional SiC fibers can be prepared.

References 1. Yajima, S.; Hayashi, J.; Omori, M . Chem. Lett. 1975, 931. 2. Yajima, S.; Okamura, K.; Hayashi, J. Chem. Lett. 1975, 1209. 3. Ishikawa, T.; Nagaoki, T. Recent Carbon Technology; JEC PressInc.:Cleveland, OH, 1983; p 348. 4. Teranishi, H . ; Ichikawa, H . ; Ishikawa, T. New Mater. New Proc. 1983, 2, 379. 5. Ishikawa, T.; Ichikawa, H . ; Teranishi, H . Proc. Electrochem. Soc. 88-5; Proc. 6. 7. 8. 9. 10. 11.

Symp. High Temp. Mater. Chem. 1987, 4, 205-217.

Yajima, S.; Okamura, K.; Tanaka, J.; Hayase, T. J. Mater. Sci. 1980, 15, 2139. Ishikawa, T. CHEMTECH 1989, 19, 38. Prewo, K. M.; Brennan, J. J. J. Mater. Sci. 1982, 17, 2371. Ichikawa, H . ; Teranishi, H . ; Ishikawa, T. J. Mater. Sci. Lett. 1987, 6, 420-422. Hasegawa, Y.; Okamura, K. J. Mater. Sci. 1983, 18, 3633. Taki, T.; Maeda, S.; Okamura, K.; Sato, M . ; Matsuzawa, T. J. Mater. Sci. Lett. 1987, 6, 826-828.

12. Yajima, S.; Hasegawa,Y.;Hayashi, J.; Iimura, M . J. Mater. Sci. 1978, 13, 2569. RECEIVED for review May 27, 1988. ACCEPTED revised manuscript May 11, 1989.


Oxidation Reaction of Polycarbosilane - American Chemical Society

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