The crosslinking of eutectic mixtures of bismaleimides

Fur. Polym. J. Vol. 29, No. 2/3, pp. 357-363, 1993 Printed in Great Britain. All rights reserved

0014-3057/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd

THE CROSSLINKING OF EUTECTIC MIXTURES OF BISMALEIMIDES GEOFFREYPRITCHARDand MARTIN SWAN School of Applied Chemistry, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KTI 2EE, England (Received I0 April 1992; accepted 16 June 1992)

Abstract--The high reactivity of the maleimide groups in bismaleimide resins is attributable to the electron withdrawing effect of the carbonyl groups causing the carbon carbon double bonds to become electron deficient. Consequently, bismaleimides homopolymerize readily to form network polymers without the aid of catalyst at temperatures between 150 and 300°. This paper is concerned with the effect on the reaction rate of the bridging group between the maleimide end groups. Observations of the crosslinking of a eutectic mixture of three oligomers containing different bridging groups suggest that the reactivity is similar for all three. Nevertheless when one of the three oligomers was crosslinked by itself, it reacted much more rapidly than the mixture. These observations are believed to reflect the steric hindrance effect of one or both of the other bismaleimide oligomers on the rate of reaction of the eutectic. Similar observations were found with rubber-toughened bismaleimide systems. The probability of steric hindrance from the aliphatic and the aromatic oligomers is discussed.

INTRODUCTION

Several techniques have been used to monitor the curing reaction of bismaleimides. They include i.r. spectroscopy [1--4], electron spin resonance (ESR) [5], ~3C nuclear magnetic resonance (NMR) [6], dielectric measurements [7], and thermal techniques such as differential scanning calorimetry (DSC) [8, 9] and dynamic mechanical thermal analysis (DMTA) [10, 11]. The various techniques do not all measure the same parameters; some determine the extent of reaction of the bismaleimide groups, while others measure the changes in properties induced by network formation. The apparent cessation of such changes does not necessarily imply the complete reaction of all the bismaleimide groups. Bismaleimides can be crosslinked directly through their maleimide double bonds but the majority of investigators have concentrated on the two-stage process in which they are first reacted with aromatic diamines, by chain extension via a Michael addition, and then homopolymerized. The Michael addition reaction (Fig. 1) occurs at lower temperatures than the homopolymerization, and hence it is practicable to chain-extend the bismaleimides prior to complete crosslinking. This is done to improve the mechanical properties. Hummel et al. [1] monitored the crosslinking reaction in a series of aliphatic bismaleimides with different chain lengths (6-12 methylene groups) between the maleimide end groups. They used i.r. spectroscopy and differential thermal analysis (DTA), and found that the activation energy for crosslinking and the reaction rate constant fell with increasing chain length. They discounted as very unlikely the explanation that the change in reaction kinetics was caused by the inductive effect as a result of variations in the length of the methylene bridges. Instead they discussed the observation in terms of the mobility of the

reactive sites in the crosslinking systems. They argued that the longer the bridge, the lower the initial viscosity, and the slower the rise in viscosity with crosslinking. So the rate of crosslinking was less affected by diffusion control in the later stages of reaction than it would be during the (initially rapid) cure of short bridge bismaleimides. Acevedo et al. [8] carried out a kinetic study of flexible oligomeric terephthalate bismaleimides, They were chosen because their flexible structures could be crosslinked to form network polymers with such low glass transition temperatures that diffusion control of the reaction rate would not occur through vitrification. They reported closely similar activation energies but concluded that the chain length between maleimide end groups in flexible systems does not affect the rate of polymerization. The kinetic parameters were consistent with a first order crosslinking reaction. The present work compares the cure rate of a commercially available eutectic mixture containing two aromatic oligomers and one aliphatic bismaleimide oligomer, with the reaction rate when one of the aromatic oligomers was used alone. The curing reaction was monitored by sol-gel analysis and DSC. Soluble extracts obtained from the sol-gel analysis were injected onto a high performance liquid chromatography (HPLC) column to determine the extent of reaction of each monomer in the matrix. EXPERIMENTAL PROCEDURES

Materials

The eutectic mixture of bismaleimide oligomers was supplied by Shell Technochemie GmbH (Germany) under the trade name Compimide® 353. It contains 55% by weight of BDM, i.e. 4,4' bismaleimido diphenylmethane, 30% 2,4' bismaleimido toluene and 12% 1,6 bismaleimido-2,2,4trimethylhexane (Fig. 2). A separate sample of the oligomer BDM was obtained from British Petroleum. 357

358

GEOFFREY PRITCHARD and MARTIN SWAN O

O H

II

N--R~N

H/C'~

II

/

'i ~C~H 0

0

H2N

H

.\

o

I_@

o

II

c/C\ I_1 . N - - R - (S~.~" t5

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/N

C--N--NH

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H

il

I

I

o

c--N--N\ II

/C--- C . N I/n \ Is -IC

c~C\ H~I N--R--N /~ C - - . - t5/

H 0

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II / .

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li

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Polymerization

H

o

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N--R--N

0

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H

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j

I/H \C.--C~

H~] H/C"-i

N--R--N /

I \C--'C'--

0 0 0 Fig. I. Michael addition reaction between a bismaleimide resin and an amine.

Hycar® 1300X 13 was obtained from B. F. Goodrich UK Ltd. It is a carboxyl-terminated butadiene acrylonitrile copolymer rubber with an approximate molar mass 3200-3500 g/mol and an acrylonitrile content of about 27 wt%.

Resin curing The bismaleimide resin was heated until liquid and poured into glass vials. Each glass vial was sprayed on the inside with a PTFE release agent (Rocol®, MRS) to aid removal of the resin at a later stage. A silicone oil bath was heated to the desired temperature using a Eurotherm controlling device, and held to within 1° of that temperature. Seven glass vials were immersed in the oil. A glass vial was removed at a predefined time and quenched in liquid nitrogen to stop any further reaction from taking place. The resin was then removed from the vials and ground to a fine powder (average particle size about 50-75 #m) ready for sol-gel or DSC analysis.

Rubber addition The experiments with the eutectic mixture were repeated with the addition of three levels of Hycar® 1300XI3. The required amounts of bismaleimide eutectic and rubber were mixed into evaporating basins and transferred to a preheated vacuum oven at 140°. The mixture was left until all the euteetic had melted, then removed for stirring followed re-insertion and the application of a vacuum to remove any residual volatiles. It was left to co-react for 2 hr, with stirring every 15min to form a copolymer [12] which was then crosslinked as before. Copolymerization was assumed to

have taken place when a homogeneous mixture of molten resin and rubber formed. At first the rubber and molten resin were immiscible, forming two layers with the rubber on top, but after stirring at 140° for 2 h r the amount of immiscible rubber reduced until the mixture appeared homogeneous, suggesting that copolymerization had taken place.

Sol-gel analysis Each finely ground sample was accurately weighed and placed in a large boiling tube. 50 cm 3 dimethylformamide (DMF) was pipetted into the boiling tube and stirred to ensure that the powder was in intimate contact with it. Each tube was then placed in a dark cupboard. Every 24 hr the boiling tubes were removed from the cupboard and shaken. After two weeks a sample of the DMF solution (10 cm 3) above each insoluble sample was pipetted into a weighed evaporating basin. The evaporating basins were placed in a muffle furnace at a temperature just below the boiling point of DMF (153°), and left until they achieved constant weight as a result of solvent evaporation. The remaining solution and the insoluble powders were filtered through weighed porosity grade 4 sinter funnels, and washed with dichloromethane to remove any remaining DMF. Blockage of the sinters with swollen particles was overcome prior to filtration by addition of silica gel. The sinters were placed in a vacuum oven and dried to constant weight at 140°.

Differential scanning calorimetry (DSC) The DSC cell was first calibrated using an indium, lead, zinc sample. A weighed sample of each bismaleimide powder

Crosslinking of bismaleimide eutectic mixtures

359

was then placed in an aluminium pan and heated from 120400 ° at 10°/min in a static air environment in a Mettler DSC 20 cell. The resulting thermogram was transferred from a Mettler TCIOA processor to an IBM PC to analyse the data. The residual exotherm was measured in triplicate for each powder and averaged.

but entirely unreacted molecules, and in the case of long-chain bismaleimides, intramolecular cyclization reactions which fail to contribute crosslinks.

HPLC High pressure liquid chromatography was carried out by means of a Spectra Physics SP8700XR instrument using a Bondapak C 18 column with 50: 50 acetonitrile: water solvent as an eluent. Small samples of the finely ground powders used in the sol-gel analysis were placed in vials and acetonitrile added. The vials were left for two weeks at 50° to dissolve any residual monomers. The supernatant liquid was pipetted off from the undissolved powder and an equal volume of distilled water added. Samples (5 #1) were injected onto the column and the separated components detected using an u.v.-visible detector at 220 nm.

Figure 3 shows the sol-gel results for the isolated BDM oligomer at three temperatures. The final extent of insolubilization appears to be governed by the cure temperature. At 195 ° the oligomer became almost completely insoluble after 10-15 hr. Curing at 169 ° achieved an almost constant state of about 80% insolubility after 24 hr. This is to be expected since chemical reaction between large and rigid molecular segments in a network polymer is increasingly restricted by the lack of mobility of the reacting species themselves [10]. When the network has become sufficiently dense for the glass transition temperature to reach the cure temperature, little or no further crosslinking can occur until the temperature is raised and a postcure applied. Later, a point is again reached at which the mobility is reduced by diffusion control of the reaction, which eventually stops. The effect is not as obvious when the sample is cured at 182 ° and a significant increase in insolubility is still occurring after 24 hr at this temperature. We can deduce that almost all the oligomeric molecules are eventually either reacted at one or more ends, or intricately entangled, provided that the cure temperature is well above 190 ° . Figure 4 shows the corresponding results for the eutectic. The whole reaction was much slower; achievement of 50% insolubility at the lowest cure temperature, 169 °, took roughly ten times as long as with the single isomer. The eutectic did not reach the point of diffusion control within the 24 hr timescale, even at 195 ° .

Glass transition temperatures The glass transition temperatures of the final crosslinked products were determined by a needle penetration technique using a Mettler Thermomechanical Analyser (TMA 40). The temperature at which the rate of increase of needle penetration depth sharply increased was taken as the glass transition temperature. RESULTS AND DISCUSSION

Degree of cure The extent of reaction of the bismaleimide groups is the fundamental definition of degree of cure. The crosslinking reactions associated with cure cause changes in physical and mechanical properties. The sol-gel analysis determines the extent to which the oligomeric molecules have become connected to the network, but some molecules are connected by only one reactive bismaleimide chain end, leaving the other unreacted. There might also be some entangled,

Sol-gel results

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0

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0

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Ctt 3 0 0 Fig. 2. Monomers present in Compimide® 353: (1) 4,4' bismaleimido diphenylmethane; (2) 2,4' bismaleimido toluene; (3) 1,6' bismaleimido-2,2,4-trimethylhexane.

360

GEOFFREY PRITCHARD a n d MARTIN SWAN

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400

600 aoo TIMF./MINS

1000

, 1200

, 1400"

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200

400

600 800 TIME/MIN8

1000

i 1200

t 1400

Fig. 3. Sol-gel analysis of 4,4" bismaleimido diphenylmethane.

Fig. 5. Residual exotherm analysis using DSC on 4,4 bismaleimido diphenylmethane.

Differential scanning calorimetry

rubber present. Further increases in the quantity of rubber lowered the Tg, as would be expected when random copolymerization with a polymer of very low Tg takes place. The reaction leading to the formation of a rubber copolymer was not necessarily complete, and some of the rubber will have remained unreacted, although scanning electron microscopy showed only traces of phase separation [13]. In the event of partial phase separation, two separate glass transition temperatures would be detectable, one for the copolymer resin and one for the rubber phase. The values in Table 1 were determined over a range from 50 to 275 ° and could not include any transitions associated with phase-separated rubber. If the rubber had been entirely phase-separated, the bismaleimide phase would be virtually unaffected for crosslinking purposes.

Analogous DSC results are given in Fig. 5 (for the single oligomer) and Fig. 6 (for the eutectic mixture). The results are similar to the sol-gel ones but they show a lower conversion, especially during the initial stages of cure at 169 °. This is because DSC measures the residual exothermic reaction of any remaining maleimide bonds, whereas sol-gel analysis measures the soluble and insoluble fractions of the matrix. Insoluble segments of the matrix may well still contain some unreacted maleimide double bonds. Comparison of the sol-gel and DSC data suggests that there were relatively few unreacted maleimide bonds in the single oligomer after 24 hr at 195 °, but about 15% in the eutectic under the same conditions, and about 45% in the eutectic cured at 169°. Unlike sol-gel, the DSC method failed to discriminate clearly between the single oligomer samples cured at 182 ° and at 195 ° except over a very small time range.

Glass transition temperature The glass transition temperatures are given in Table 1. The samples cured at the same cure temperature all showed broadly similar Tg values. Addition of the first 33 parts of rubber actually increased the T~ slightly, because the bismaleimide crosslinking reaction became easier in the more open meshed network resulting from copolymerization. There was therefore less unreacted bismaleimide than when there was no

Relative reactivity of the oligomers Three structural features of the three oligomers affect the cure rate: (a) the inductive effect of the bridging unit on maleimide reactivity; (b) the mobility of the oligomeric molecules up to the point when they become attached to the network; (c) the steric effect of those oligomeric molecules already attached to the network on the ease of access of other reactive species. It has already been argued [1] that the inductive effect is the weakest. The mobility of the oligomer molecules is more important, but the steric restrictions caused by incorporating short chains into

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Fig. 4. Sol-gel analysis of Compimide® 353.

I 200

L169 C I-t- 182 C -x-1915 C 400

600 800 TIME/MIN8

1000

i

i

1200

1400

Fig. 6. Residual exotherm analysis using DSC on Compimide® 353.

Crosslinking of bismaleimide eutectic mixtures Table 1. Glass transition temperatures of BMI resins after 24 hr cure Oligomer

Cure temp (°C)

Rubber content (w/w)

BDM Eutectic Eutectic Eutectic Eutectic Eutectic

169 169 169 169 169 195

0 0 33 66 100 66

Tg (°C) 253 232 251 245 235 287

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the network are probably the most important and the longest acting. An HPLC chromatogram is shown in Fig. 7 for the eutectic mixture cured at 182° for 24 hr. The relative amounts of the remaining oligomers are consistent with the values given in the data sheet [14] for Compimide® 353, and with that found experimentally for the unreacted Compimide® 353. Figure 8 shows the quantities of each oligomer estimated by HPLC to remain unreacted in the sol fraction, as a function of reaction time at 169°, alongside the rate of development of the gel fraction. The relative amounts of all three oligomers remained much the same throughout cure, and the proportions of aliphatic and aromatic bismaleimides at the start and finish of cure were similar; no one oligomer reacts preferentially with the network during the curing reaction. Specifically, there was no suggestion that the aromatic bismaleimides reacted preferentially with each other or themselves rather than the aliphatic oligomer, because of their higher reactivity. The long range inductive effect of aromatic groups might have been assumed to lead to increased electron withdrawal from the maleimide bonds, making the latter more reactive. The addition of the reactive rubber to the matrix has no effect on the reaction of the monomers, and consequently the steric effect

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169"C -x- Monomer 1 -"- Monomer 2 -x- Monomer 3 I Fig. 8. Relative amounts of unreactive monomers as a function of reaction time at 169°. attributed to rubber has an equal effect on all the bismaleimide monomers. The present observations accord with the previously mentioned conclusions of Acevedo et al. [8] who suggested that the chain length between maleimide end groups did not significantly affect the rate of polymerization, although in their case the polymers remained rubbery throughout, avoiding the question of steric control. It can be concluded from the HPLC results that the bridging group between reactive maleimide groups had no effect on the rate of polymerization of the oligomer in the eutectic mixture used here. However the addition of two more oligomers drastically reduced the rate of polymerization of the BDM, as demonstrated by the sol-gel results, which show that the initial rate of reaction of 4,4' BDM was much faster than that of Compimide® 353. These results are confirmed by DSC analysis. Consider first why the addition of two oligomers slows down the rate of crosslinking of the BDM. The 2,4' oligomer has a very short distance between maleimide groups and results in a dense network. It might be expected that the steric hindrance to further crosslinking reactions involving any of the three oligomers would be especially severe when this

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Fig. 7. HPLC chromatogram of soluble extracts from Compimide® 353 cured at 182° for 24 hr.

200

400

__z__

-~-182"C -x-195"C 600

800

1000

1200

1400

TIME/MINS Fig. 9. Sol-gel analysis of Compimide® 353 containing 33 pphr CTBN.

362

GEOFFREY PRITCHARD and MARTIN SWAN

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100

100

80

iq, 00 == ..J

~

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i

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C~ 20O

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TIME/MINS

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1000

,

,

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1400

TIME/IIIN$

Fig. 10. Sol-gel analysis of Compimide® 353 containing 66 pphr CTBN.

Fig. 12. Residual exotherm analysis using DSC on Compimide® 353 containing 33 pphr CTBN.

oligomer is present, although other oligomers would have similar effects to different extents. Whatever the species approaching the network, many of those pendant unreacted maleimide groups already attached to the network, by virtue of the other maleimide group on the same oligomer molecule having reacted, would be rendered relatively inaccessible by the existence of very short crosslinks. In contrast, the 1,6' aliphatic oligomer would produce a looser mesh and facilitate crosslinking. It is also necessary to explain why the aliphatic oligomer polymerized at much the same rate as the aromatic ones in the eutectic. The aliphatic oligomer (species 3 in Fig. 2) melts at 60 ° and its molecular mobility must therefore be much greater than that of either of the aromatic oligomers, which melt at 155-157 ° (in the case of the 4,4' BDM) and 174-176 ° (in the case of the 2,4' compound) respectively. This should offset the reduced reactivity caused by the weaker inductive effect of aliphatic bridging groups. The 2,4' oligomer would be expected to react more slowly than the 4,4' on both mobility and steric grounds, but these factors are compensated to some extent by the more favourable inductive effect.

weight of bismaleimide resin respectively. In all cases, the gel fraction was much greater after 24 hr than in the eutectic without rubber, and it approached the values obtained with BDM. The effect of cure temperature was almost completely removed after 24 hr; even the samples cured at 169 ° came close to 100% insolubility. There was a small increase in the gel fraction with rubber content at the 12 hr stage. There must be a possibility not only of phase separation of the rubber, as discussed above, but of reaction between the maleimide double bonds and the residual unsaturation of neighbouring Hycar molecules, whether copolymerized or not. This is especially likely for samples with 66 and 100 pphr rubber, since they have an excess of available butadiene double bonds. The reaction of rubber-modified BMIs cannot therefore be compared directly with the crosslinking of unmodified Compimide 353. However, the corresponding DSC results (Figs 12, 13 and 14) showed the same convergence as the sol-gel ones towards 100% reaction at the 24 hr stage regardless of cure temperature, whereas the unmodified Compimide showed a marked temperature effect and achieved only about 80% insolubility.

Rubber toughened bismaleimides Figures 9, 10 and 11 show sol-gel graphs for the crosslinking of the rubber-modified bismaleimides containing 33, 66 and 100 parts per hundred by 100

CONCLUSIONS

I. The crosslinking of 4,4' BDM occurs much more rapidly than the crosslinking of the eutectic 100

x ~

Yh

T

80

00

00

~_ 60

40

~ 40

.J

|

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C~

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800 000 TIME/MINS

1000

, 1200

I 1400

Fig. 11. Sol-gel analysis of Compimide® 353 containing 100 pphr CTBN.

0

200

400

600

000

1000

1200

1400

TIME/MIN8

Fig. 13. Residual exotherm analysis using DSC on Compimide® 353 containing 66 pphr CTBN.

Crosslinking of bismaleimide eutectic mixtures 100

sition temperature helps to overcome steric hindrance problems. The rate at which the maximum gel fraction is achieved still depends on cure temperature. The steric effect of the rubber has an equal effect on all three oligomers, and consequently there is no change in the relative amounts of unreacted oligomers during the crosslinking reaction as a result of Hycar addition.

80

6O

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363

/ i 200

I ~169"C 400

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600

800

-X-195"C i 1000

, 1200

Acknowledgement--The authors wish to thank Peter Keefe of BF Goodrich (UK) Ltd for supplying samples of reactive rubbers for this work. i 1400

Tlme/mlns

Fig. 14. Residual exotherm analysis using DSC on Compimide~ 353 containing I00 pphr CTBN.

mixture known as C o m p i m i d e ® 353. The most likely explanation is that one of the other oligomers in the eutectic mixture causes steric hindrance to the crosslinking reaction. 2. Despite differences in the nature of the bridging group, the three oligomers in C o m p i m i d e ® 353 crosslink at much the same rate. This is believed to be because the difference in inductive effect between the aromatic and aliphatic bridging units is offset by the much greater mobility of the aliphatic oligomeric molecules. The similar rates of reaction for the two aromatic oligomers is attributed to mobility similarities. Any differences between the two aromatic oligomers such as the short bridge length of one of them leading to a dense network and consequent diffusion control of the crosslinking reactions, would affect the rate of consumption of both oligomers to much the same extent. This is not to say that the aromatic monomers would react at the same rate if crosslinked in isolation. 3. Addition of H y c a r ® C T B N rubber increases the gel fraction and after 24 hr, there is little difference between the gel fractions of samples cured at different temperatures. This observation is consistent with the lowering of the glass transition temperature by copolymerization with rubber. A low glass tran-

REFERENCES 1. D. O. Hummel, K.-U. Heinen, H. Stenzenberger and H. Siesler. J. Appl. Polym. Sci. 18, 2015 (1974). 2. C. M. Tung. Polym. Preprints 2.8, 7 (1985). 3. C. L. Leung, T. T. Liao and C. M. Tung. Polymeric Mater. Sci. Engng 52, 134 (1985). 4. C. Di Giulio, M. Gautier and B. Jasse. J. appl. Polym. Sci. 29, 1771 (1984). 5. I. M. Brown and T. C. Sandreczki. Macromolecules 23, 94 (1990). 6. C. G. Fry and A. C. Lind. New Polym. Mater. 2, 235 (1990). 7. Applications Report "Cure of BMI Resins" AR0051, Micromet Instruments, Inc. Cambridge, MA02139, U.S.A. 8. M. Acevedo, J. de Abajo and J. G. de la Campa. Polymer 31, 1955 (1990). 9. K. N. Ninan, K. Krishnan and J. Mathew. J. appL Polym. Sci. 32, 6033 (1986). 10. M. S. Chattha and R. A. Dickie. J. appl. Polvm. S¢i. 40, 411 (1990). I1. C. L. Leung, T. T. Liao and C. M. Tung. Polymeric Mater. Sci. Engng 52, 139 (1985). 12. A. J. Kinloch, J. Shaw and D. A. Tod. Advances in Chemistry Series 208 Rubber Modified Thermoset Resins (edited by C. Keith Riew and John K. Gillham), Chap. 8. American Chemical Society, Washington DC (1984). 13. G. Pritchard and M. Swan. J. mater. Sci., Lett. 11, 1443 (1992). 14. Compimide® 353 Data Sheet, Technochemie-GmbH, Verfahrenstechnik, Gutenbergstrabe 2, Postfach 40, D6915, Dossenheim, Germany (1990).