Processable Phthalonitrile Resins with High-Thermal and Oxidative

Chapter 29

Processable Phthalonitrile Resins with High-Thermal and Oxidative Stability

Downloaded by NORTH CAROLINA STATE UNIV on September 27, 2012 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch029

Matthew Laskoski, Dawn D. Dominguez, and TeddyM.Keller Materials Chemistry Branch, Code 6127, Naval Research Laboratory, Washington, DC 20375-5320

A processable multiple aromatic ether-linked phthalonitrile with high thermal and oxidative stability has been synthesized and characterized. The oligomeric phthalonitrile monomer was prepared in a two-step, one-pot procedure using an excess amount of resorcinol and 4,4'-difluorobenzophenone in a N , N -dimethylformamide/toluene mixture and K CO as the base followed by end-capping with 4-nitrophthalonitrile. The monomer was thermally crosslinked in the presence of bis(4-[4-aminophenoxy]phenyl)sulfone forming a void-free thermoset. Thermal, oxidative, and mechanical measurements were performed on the resulting polymer. The polymer exhibits excellent thermal and oxidative stability, superior mechanical properties, and low water absorption 2

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U.S. government work. Published 2006 American Chemical Society

In Fire and Polymers IV; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


379 Thermosetting polymers containing terminal phthalonitrile units are a unique class of high temperature materials having a variety of potential uses in the adhesive (/) and electronic (2,3) industries and as a matrix resin in structural applications (4). Phthalonitrile composites have superior flame resistant properties when compared to other polymeric composites. Phthalonitriles are the only thermosetting materials that meet the Navy's stringent requirement of MIL-STD-2031 (6) for use as the matrix resin in polymeric composites aboard Naval submarines (4a, b). In the past twenty years, a variety of high temperature materials have been developed by incorporating aromatic units within polymeric systems containing phthalonitrile (5) end units. To date, the majority of phthalonitrile resins have high melting points and short processing windows and result in somewhat brittle thermosets once thermally cured. This limits the use of these resins in a broad variety of applications. Recent research in this area has focused on the incorporation of aryl ethercontaining linkages between the terminal phthalonitrile units (7). It has been determined that the necessity of using low cost organic reactants, a short reaction synthetic scheme, and low temperature processing of the resin are essential to the viable use of organic resins in various composite applications. New phthalonitrile monomers have been prepared utilizing a two-step, one-pot reaction (5b). This synthetic approach results in a phthalonitrile system with a lower melting point and a broader processing window for easy conversion to crosslinked polymers, which will allow phthalonitrile resins to compete with other high performance polymeric systems. The synthesis, polymerization, and material properties of a newly developed phthalonitrile resin based on resorcinol will be described. The thermal and physical properties of this material will be compared to those of a phthalonitrile system based on bisphenol A.

Experimental All starting materials were of reagent grade and used without further purification. The synthesis of the oligomeric multiple aromatic ether-containing phthalonitrile 6b prepared from bisphenol A and 4,4'-difluorobenzophenone has been described previously (7b,8). Differential scanning calorimetric (DSC) analysis was performed using a TA Instruments DSC 2920 modulated thermal analyzer at a heating rate of 10 °C min" and a nitrogen purge of 50 cm min" . Thermogravimetric analysis (TGA) was performed using a TA Instruments SDT 2960 Simultaneous DTA-TGA at a heating rate of 10 °C min' under a nitrogen or air purge of 50 cm min" . Infrared (IR) spectra were recorded as films deposited on NaCl plates using a Nicolet Magna FTIR 750 spectrometer. HNMR was performed using a Brttker ADVANCE 300 spectrometer. A TA Instruments AR-2000 Rheometer, in conjunction with an environmental testing chamber for temperature control and torsion fixtures, was used to monitor the response of samples (50 mm x 13 mm x 2 mm) to oscillatory testing. The measurements were made in nitrogen over the temperature range of 1

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~ 40 to 450 °C. A temperature ramp of 3 °C min" was used to determine the storage modulus and damping factor (tan 8) of the material at a frequency of 1 Hz and a strain of 2.5 x 10" %. Normal force control was utilized throughout the tests to keep the samples taut. 2


Preparation of the Oligomeric Multiple Aromatic Ether-Containing Phthalonitrile (6a). To a 100 mL, three-necked flask fitted with a thermometer, a Dean-Stark trap with condenser, and a nitrogen inlet were added resorcinol (la) (10.0 g, 90.8 mmol), 4,4'-difluorobenzophenone (2) (9.90 g, 45.4 mmol), powdered anhydrous K C 0 (9.40 g, 68.1 mmol), toluene (10 mL), and N,Ndimethylformamide (DMF) (60 mL). The resulting mixture was degassed with argon, the Dean-Stark trap was filled with toluene, and the mixture was heated to reflux at 135 - 145 °C for 12 -18 h or until no more water was collected in the Dean-Stark trap. The toluene was then removed by distillation and the reaction mixture was cooled to 50 °C. At this time, 4-nitrophthalonitrile (5) (15.7 g, 90.7 mmol) was added in one portion and the reaction mixture was heated at 80 °C for 6 - 8 h. The mixture was cooled to ambient temperature and poured into a 5 % aqueous KOH solution resulting in the formation of a solid. The material was broken up and collected by filtration at reduced pressure. The white solid was dissolved in chloroform (200 mL) and washed with 200 mL of a 5 % aqueous NaOH solution, 200 mL of distilled water, 200 mL of a 5 % aqueous HCl solution, and finally 200 mL of water until neutral. The solvent was removed in vacuo and the solid vacuum dried to yield 6a (27.3 g, 92 %). H-NMR (300 MHz, CDC1 ): 8 7.85-7.70 (m, aromatic-H), 7.57-7.25 (m, aromatic-H), 7.126.97 (m, aromatic-H), 6.92-6.78 (m, aromatic-H). IR [cm" ]: v 3074 (C=CH), 2232 (CN), 1650 (C=0), 1587 (C=C), 1477 (aromatic), 1307 (aromatic), 1307 (C-O), 1244 (CH ), 1162 (C-O), 928 (C-O), 842 (aromatic). 2

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Formulation of 6 and Aromatic Amine Composition To the melt of 6 at 200 °C was added 3 weight % of bis(4-[4aminophenoxy]phenyl)sulfone (p-BAPS). Once the curing additive had been evenly dispersed by stirring, the sample was cooled and used in the DSC and TGA cure studies.

Polymerization and TGA Studies on Thermoset 7 Solid samples of 6 containing 3 weight % of p-BAPS were added to TGA pans. The mixtures were cured under nitrogen by heating at 270 °C for 12 h,


381 300 °C for 3 h, 350 °C for 6 h, 375 °C for 8 h and 425 °C for 8 h. Polymer 7 formed as a film on the bottom of the pan. The thermal and thermo-oxidative properties of the polymer film were then determined from 50-1000 °C.


Rheometric Measurement Sample Preparation Samples for rheometric measurements were prepared by degassing 6 under vacuum at 275 °C for 4 h in a mold with cavity dimensions of 65 mm x 13 mm. The temperature of the mold was reduced to 220 °C, /?-BAPS (3 weight %) was added with stirring and the resulting mixtures were degassed for an additional 30 min. The samples were cooled, placed in an oven, and heated at 270 °C for 12 h, 300 °C for 3 h, 350 °C for 6 h, 375 °C for 8 h and 425 °C for 8 h. The cured samples were removed from the mold and sanded to a thickness of approximately 2 mm.

Results and Discussion Synthesis The reaction of 4,4'-difluorobenzophenone 2 with a bisphenol 1 has been utilized to incorporate aryl ether linkages into the interconnecting unit of a multiple aromatic ether phthalonitrile 6. (Scheme 1) The oligomeric phthalonitriles 6 were prepared by a two-step, one-pot procedure from the reaction of 1 and 2 in the presence of potassium carbonate as the base, followed by the reaction of the potassium diphenolate-terminated intermediate 3 via a nitro displacement reaction involving 4-nitrophthalonitrile 5. The oligomeric phthalonitriles were isolated in 92 - 94 % yield. Phthalonitriles 6 were readily soluble in common organic solvents such as toluene, DMF, acetone, dichloromethane, and chloroform. The reaction was performed in DMF and a minimal amount of toluene to allow azeotropic distillation of the water that formed as a by-product in the reaction. The reaction mixture was stirred at solvent reflux (135 - 145 °C) until no more water was collected in the DeanStark trap. The length of the spacer between the terminal phthalonitrile groups can be varied by simply changing the ratio between 1 (excess) and 2. IR analysis was used to monitor and determine the progress of the synthetic reactions producing monomers 6. The spectra of thin films of the hydroxylterminated intermediates 4, which were isolated by acidic workup of 3, were examined. During the reaction involving 3 and 5 resulting in the formation of 6, the most notable features were the disappearance of the hydroxyl functionality at approximately 3420 cm" and the appearance of the nitrile functionality at 1


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about 2232 cm" . Once the hydroxyl peak was fully diminished, the reaction was essentially complete.


O

Curing Additive

if THERMOSET 7

Scheme 1. Preparation of oligomers 6 and thermosets 7.

The neat curing of phthalonitrile resins has been shown to proceed very slowly even during extended periods at elevated temperatures. Consequently, the resulting monomers 6 (Figure 1) were cured by the incorporation of a minute amount of the highly thermally stable curing additive, p-BAPS, to allow the 14


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thermoset formation reaction to proceed quickly and at a low temperature. The incorporation of aromatic ether spacers between the reactive terminal phthalonitrile units generated oligomeric monomers 6a and 6b which exhibited softening temperatures around 60 and 75 °C, respectively. Both monomers were completely free flowing around 130 - 150 °C, and therefore had a long processing window before reaction with the curing additive occurred at around 250 °C. Cost effective composite fabrication techniques such as filament winding, resin infusion molding, and resin transfer molding (RTM) may be feasible with these systems at low initial resin processing temperatures.

6b

Figure J. Structure of oligomeric phthalonitriles 6a and 6b.

Characterization Polymerization of 6 where n » 1 was achieved and studied using DSC analysis in the presence of small quantities of p-BAPS (9) to afford 7. For the study, 6 was cured with 3 weight % of p-BAPS. The DSC thermograms obtained by heating at 10 °C min to 400 °C revealed endothermic transitions at approximately 60 and 75 °C for 6a and 6b, respectively. These transitions corresponded to a softening of 6 from the amorphous phase. Both resins also displayed an exothermic transition commencing around 250 °C and ending at approximately 350 °C, which was attributed to the reaction of 6 with /?-BAPS. Rheometric measurements were performed on samples of 7 cured to 425 °C with 3 weight % p-BAPS under identical conditions. Figure 2 shows a plot of the storage modulus for samples of 7 up to 500 °C. The storage modulus for 7a varied from 1625 to 830 MPa when heated from 30 to 500 °C, respectively. The storage modulus for 7b changed from 1290 to 725 MPa over the same



384 temperature range. In addition, 7a exhibited a higher storage modulus than 7b at any given temperature. The lower modulus for 7b could be a result of the longer aromatic ether spacer and the reduced crosslinking density in 7b relative to 7a. Since the phthalonitrile end units are closer together for 7a, it is easier for the curing additive to find the ends. Figure 3 shows the damping factor (tan 8) for polymer 7 cured to 425 °C. When heated to 500 °C, 7a and 7b both exhibited peaks commencing at 400 °C. The observed peak maxima appeared near the sharpest decline of the storage modulus. At 500 °C, polymers 7a and 7b maintained 51 and 56 % of their initial storage modulus, respectively. It is important for these materials to maintain structure integrity if they are to be used at elevated temperatures (4c).

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200 300 400 Temperature (°C)

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Figure 2. Storage modulus for polymer 7 cured with 3 weight % p-BAPS: (A) polymer 7a, (B) polymer 7b.

The thermal and thermo-oxidative properties were investigated between 50 and 1000 °C in a TGA chamber. Figure 4 shows the TGA thermograms for 7 cured to 425 °C with 3 weight % /?-BAPS. Polymer 7a and 7b displayed similar thermal properties dispite the fact that 7b contains approximately 7 % by weight of aliphatic moieties. When heated under inert conditions, polymers 7a and 7b retained about 95 % weight at 545 °C and exhibited char yields of 77 % and 79 %, respectively, at 1000 °C. Polymer 7b exhibited a slightly higher char yield relative to 7a. Further studies are being performed to determine the effect of heating to such high temperatures on the mechanical and other physical properties of 7a and 7b. When polymers 7a and 7b were heated in air, a weight retention of 95 % was observed at 550 and 535 °C, respectively, with decomposition occurring between 600 and 900 °C for both polymers.


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Figure 3. Damping factor (tan S) for polymer 7 cured with 3 weight % p-BAPS: (A) polymer 7a, (B) polymer 7b.

Temperature (°C) Figure 4. Thermogravimetric analysis of polymer 7 heated to 1000 V under inert and oxidizing environments: 7b under N (A) and air (A') and 7a under N (B) and air (B'). 2


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The oxidative stability for polymer 7 was examined over extended periods. Figure 5 shows an oxidative aging plot for samples of 7a and 7b heated stepwise to 375 °C under air in 8 h temperature intervals. Table 1 indicates the percent of total weight loss over the various temperature intervals. Upon heating to the maximum temperature of 375 °C, a total weight loss of around 5 and 8 % for 7a and 7b, respectively, was observed. The majority of the weight loss occurred on the final segment at 375 °C for both polymers.

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Processable Phthalonitrile Resins with High-Thermal and Oxidative

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