WO1998005699A1

WO1998005699A1 - Non-catalytically cured bisoxazoline-phenolic copolymers having improved thermo-oxidative stability

Info

Publication number: WO1998005699A1
Application number: PCT/US1997/013860
Authority: WO
Inventors: Marvin L. Deviney; James D. King; Joel L. Kampa
Original assignee: Southwest Research Institute SwRI
Current assignee: Southwest Research Institute SwRI
Priority date: 1996-08-06
Filing date: 1997-08-06
Publication date: 1998-02-12
Anticipated expiration: 1999-02-06
Also published as: AU3910597A

Abstract

The present invention provides a method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote curing, the method comprising exposing the resin to an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize and to produce substantially uniform heating throughout the resin, wherein said curing opens a number of oxazoline rings sufficient to create cross-linking which produces a rigid polymer composition. A preferred mechanism for curing is by pultrusion. In a preferred method, the resin is mixed before curing with an antioxidant selected from the group consisting of 2,4,8,10-tetraoxa-3,9,diphosphaspiro[5,5]undecane, 3,9-bis[2,4-bis(1,1-dimethylethyl)phenoxy]- and an acetone-diphenyl amine condensate product.

Description

TITLE: NON-CATALYTICALLY CURED BISOXAZOLINE-PHENOLIC

COPOLYMERS HAVING IMPROVED THERMO-OXIDATIVE STABILITY

Field of the Invention

The present invention relates to thermoset bisoxazohne phenolic copolymers having improved thermo-oxidative stability, and to methods for producing such copolymers Backeround of the Invention Copolymers of non-modified 2,2'-(l,3-phenylene)bιs (2-oxazohne) and phenolic novolak resins in which the bisoxazohne monomer acts as a cross-linking agent (and secondarily as a chain extending agent) have been reported in the literature These copolymers offer high values of modulus, strength, toughness, heat deflection temperature and terlaminar shear (1 e strong bonding to carbon fibers and fiberglass) The reported bisoxazohne and phenolic novolak copolymers also have much better thermo-oxidative stability than epoxy resins and almost all other thermoset resins, with the exception of polyimides

Polyimides have higher thermo-oxidative stability than reported bisoxazohne and phenolic novolak copolymers, however, polyimides do not have the necessary strength for high performance airframe (non-engine) applications Because of microvoids, microcracking, etc , a thermoset polyimide would not be strong enough for adequate service life of large supersonic jet parts, such as wings and tail sections

The matenal used to manufacture such parts must have thermo-oxidative stability at temperatures of about 177°C (350°F) because new products require such features For example, the new supersonic NASA HSCT (high speed civil transport) will operate for many hours (per flight) at speeds of about Mach 22 with skin temperatures of about 177°C (350°F) or higher Supersonic military fighter aircraft also may operate with skin temperatures of about 177°C (350°F) or higher, but for shorter times than in the HSCT case Other applications requinng thermo-oxidative stability at temperatures above about 177°C (350°F) include high temperature electrical applications, such as transformer housings, submersible vessel support structures (i.e., near nuclear reactors), certain steel manufacturing applications (i.e., for high temperature gaskets), etc.

Unfortunately, known thermoset resins that have the necessary strength for such parts do not also have the increased, long term, thermo-oxidative stability required for components used in high performance, high temperature applications. Bisoxazoline and phenolic novolak copolymers possess the requisite strength and have much better thermo- oxidative stability than many resins; however, known bisoxazoline and phenolic novolak copolymers do not have sufficient thermo-oxidative stability for many of these high performance, high strength applications. Consequently, a need exists for a thermoset resin which has increased, long term, thermo-oxidative stability, as well as other desirable characteristics, such as high values of modulus, strength, toughness, heat deflection temperature, and interlaminar strength. Summary of the Invention

The present invention provides a method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote curing, the method comprising exposing the resin to an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize and to produce substantially uniform heating throughout the resin, wherein said curing opens a number of oxazoline rings sufficient to create crosslinking which produces a rigid polymer composition

Brief Description of the Drawings

Fig. 1 is a schematic diagram illustrating the basic concept of electromagnetic dielectric heating for insulating materials, or electrically non-conducting materials.

Fig. 2 is a schematic illustration of a cylindrical electrode arrangement for heating material in the annular space between the electrodes 1 and 2

Fig. 3 is a schematic representation of a multi-segment electrode or capacitor arrangement to heat material in the E-field which will exist in the space adjacent to and between the electrodes.

Fig. 4 is a table showing the structures of the antioxidants used in Example 4 Fig. 5 is a chart of results obtained using the antioxidants shown in Example 4 Fig. 6 is a chart of time vs. % wt of original resin remaining, which illustrates the thermo-oxidative aging of a cured, basic HCBOX resin sample containing 2%Ultranox 636 (no catalyst used) in air at 200° C.

Fig. 7 is a chart of time vs. % wt of original resin remaining, which illustrates the thermo-oxidative aging of a cured, basic HIBOX resin sample containing 2% Aminox (Naugard) (no catalyst used) in air at 200 ^CC

Fig. 8 is a chart depicting the results of the experiments described in Example 4 for a number of other antioxidants tested. Detailed Description of the Invention Research has revealed that the principal catalysts used (or contemplated for use) to speed the curing of l,3-phenylene-bis-(2-oxazoline) have a deleterious effect on the thermo-oxidative stability of the cured thermoset From these results, it was determined that l,3-phenylene-bis-(2-oxazoline) should be cured without the aid of a catalyst in order to avoid thermo-oxidation at temperatures at or near 200 °C (392°F) or higher Unfortunately, when no catalyst is present, the resulting thermoset resins must be cured for a period of between 16-24 hours at 177°C (350°F) Post-curing for 6-12 hours at about 225 °C (437 °F) also is required to achieve the maximum glass transition temperature (about 210-225 ° C or 410-437 ° F) The resulting resins have better thermo- oxidative stability and minimal cure-shrinkage (about 0 5% of less); however, the long curing periods render noncatalytically cured bisoxazoline-phenolic resins undesirable as a practical matter

According to the present invention, the rate of non-catalytic curing of bisoxazoline phenolic resins is increased using a high-voltage, high-frequency electromagnetic field, preferably an electric field (E-field), to produce heating by means of the dielectric loss properties of the resin The use of an electromagnetic field having substantially any frequency or substantially any wavelength should operate according to the present invention; however, certain frequencies or wavelengths will be preferred in certain circumstances, as outlined herein

A preferred frequency range for curing bisoxazoline phenolic resins is that which is sufficiently high to maximize the heating but sufficiently low to produce uniform heating throughout the entire volume of the material For small items, microwave may be preferred, but for items of greater dimensions, the short wavelengths of microwave energy may introduce problems in obtaining uniform heating throughout the volume of material. It is preferred to make the wavelength of the electromagnetic field such that the largest dimension of the material to be simultaneously heated is less than about 0.1 wavelength. The "wavelength" of the electromagnetic field is equal to the velocity of light (or propagation velocity) divided by the frequency of the electromagnetic field. The electromagnetic E-field may be used to quickly raise the temperature of the material to a desired point, and then to maintain the material at that temperature for the desired cure period using either the same field or using auxiliary thermal heating.

In order to make the noncatalytically cured bisoxazoline-phenolic copolymers of the present invention, a desired amount of bisoxazoline and phenolic resin should be melted together. The wt% of the oxazoline should be between about 10-80 wt%, preferably between about 50-70 wt%, most preferably between about 60-65 wt% of the resulting mixture. The wt% of the phenolic resin should be, respectively, between about 20-90 wt%, preferably between about 30-50 wt%, most preferably between about 35-40 wt% of the mixture. A relatively high level of oxazoline is preferred because the heat distortion temperature or Tg (glass transition temperature) of the resulting product increases as the wt% of oxazoline increases. A preferred phenolic resin is a novolak resin, most preferably Alnovol PN 3201, available from Hoescht Celanese. Bisoxazoline is commercially available from Pear Development Company, Toronto, Canada and Takeda Ltd. of Osaka, Japan.

In a preferred embodiment, which combines the use of an antioxidant and electromagnetic curing, the mixture also should contain at least about 2 wt% of an antioxidant, preferably either Ultranox™ U626, available from General Electric Specialty Chemicals, or Aminox™, also known as Nauguard A, available from Uniroyal Chemical Co. Both of these antioxidants resulted in substantially increased thermooxidative stability at temperatures of about 200°C (392^CF) and above compared to the other antioxidants tested in the study. (See Figs. 6-8) The active ingredient in the antioxidant Ultranox™ U626 is 2,4,8,10-tetraoxa-3,9- diphosphaspiro[5,5]undecane, 3,9-bis[2,4-bis(l, l-dimethylethyl)phenoxy]-, which has the following structure:

Antioxidants having similar characteristics also should substantially increase thermooxidative stability of the resulting resin at about 200°C (492°F).

The active ingredient in Aminox™ is a secondary amine comprising an acetone- diphenyl amine condensate product with a melting point of 85-95 °C (185-203 °F) and a flash point of 179°C (354°F) Antioxidants containing similar compounds also should substantially increase thermooxidative stability of the resulting resin at about 200 °C (392°F).

The mixture of bisoxazoline, phenolic novolak resin, and antioxidant (preferred) should be stirred at 155°C (310°F) until homogeneous (approximately 1 minute). The mixture then should be degassed and poured into a suitable mold known in the art. The mold or the cavity of the mold is manufactured from Teflon™, or a similar low dielectric material that will not tend to absorb energy in preference to the bisoxazoline-phenolic resin. In a preferred embodiment, reinforcement materials, such as carbon fiber or glass cloth, are inserted into the mold cavity during filling and impregnated with the liquid resin. Once filled, the lid of the mold should be put in place and excess resin should be squeezed out of the mold to ensure a void free product. The use of the preferred antioxidants herein should increase the thermooxidative stability of a resin regardless of how the resin is cured. However, in a preferred embodiment, the preferred antioxidants are used with electromagnetic curing If curing time is not a concern the resin may be cured without using an electromagnetic field at about 177°C (350°F) for about 12-16 hours, and post cured at about 225 °C (437°F) for about 8-12 hours. The total cure time for non-catalytic curing should be at least about 16 to 24 hours.

A preferred embodiment reduces the curing time by exposing the filled mold to electromagnetic energy, preferably by placing the mold inside of a suitable cavity, and irradiating the mold with high frequency electromagnetic radiation for more rapid curing Substantially any high frequency electromagnetic frequency generator may be used. The mold and the sample part should be irradiated electromagnetic radiation having a frequency in the range of from about 27 Mhz to about 2 5 GHz, preferably in the range of from about 100 MHz to about 250 MHz depending upon the dielectric of the material The required wattage depends upon several factors, namely (1) the mass of material to be cured, (2) the curing temperature, (3) the specific heat of the material, and, (4) the allowable curing time For material initially at room temperature, between about 10-30 watts per gram should be applied for a cure time of between about 1-2 minutes, or until visual observation indicates that the material has been cured The cured part should exhibit a Tg between about 175°-230°C (350°-446°F) preferably about 200-210°C (392-410°F), demonstrating a high level of cure

Fig 1 shows the basic concept of electromagnetic dielectric heating for insulating materials, or electrically non-conducting materials An electromagnetic generator EM is connected in sequence with an inductor L and a capacitor C to form a tuned circuit that is resonant at the frequency of the electromagnetic generator EM This L-C circuit may be used to control the frequency of the electromagnetic generator EM The dielectric material to be cured 10 is placed between the plates or electrodes 12a and 12b of the capacitor C where there is a maximum voltage (E) of the EM field frequency Materials of larger loss tangent heat more rapidly in an electric field of a given magnitude and frequency The electrodes 12a and 12b which form the capacitor may be shaped as required to create a field of desired shape and intensity distribution in the material

A preferred "molding" process for use in conjunction with the present invention is pultrusion Pultrusion or "pulforming" techniques are known in the art, and are described in detail in "Pulforming" in 6 INTERNATIONAL ENCYCLOPEDIA OF COMPOSITES (Stuart M Lee, Ed 1991) pp 331-358, incorporated herein by reference. Substantially any microwave generator could be used during pultrusion, however, the microwave cavity would need to be designed so that the material would be evenly irradiated as it was pultruded A preferred embodiment would not require an electrode in the center of material being pultruded One way to accomplish electromagnetic curing during pultrusion is shown in Fig

2, in which like parts are given like numbers to those in Fig 1 Fig 2 depicts a cylindrical electrode arrangement for heating material in the annular space 10 between the electrodes

12a and 12b, which form a capacitor, C This capacitor in combination with inductor, L, is resonant at the EM field frequency The annular ring of material may be caused to move through the electrode region, e.g. by extrusion or pultrusion, to provide a means for continuous heating of extended lengths of pipe or tubing made of this insulating material

Another way to accomplish electromagnetic curing during pultrusion would be to use a multi-segment electrode or capacitor arrangement to heat material in the E-field which will exist in the space adjacent to and between the electrodes This embodiment is depicted in Fig 3, in which like parts again are given like numbers to those appearing in

Figs 1 and 2 The parallel electrodes 12a and 12b may be of extensive length and curved around the periphery of a tube shaped dielectric material 10 to provide heating without the need for an electrical connection within the interior of the pipe or tubing The E-field required for electromagnetic curing also can be produced using a resonant cavity having an appropriate E-field distribution Such resonant cavities are well known in microwave technology This method is very appropriate for higher frequencies extending into the microwave region

The invention will be better understood with reference to the following examples EXAMPLE 1

The following procedures were used to prepare bisoxazoline phenolic resins using the catalysts shown in Table I, below, which were obtained from Aldrich Chemical Co The numbers shown next to the catalysts in Table I are sample identification numbers

TABLE I 21 Sodium Hexafluoro Phosphate, 33 Phosphorus Pentoxide, 38 Benzyl Trimethylammonium Chloride and α,α-Dichloro p-Xylene,

43 Tetrabutylammonium Hexafluoro Phosphate,

44 Tetrabutylammonium Tetrafluoro Borate, 16: Diphenyl Phosphate; 47: No catalyst.

Approximately 60 wt% of bisoxazoline, obtained from Pear Development Company, Toronto Canada, and approximately 40 wt% of Alnovol PN 3201, obtained from Hoescht Celanese, were melted at about 150°C (302°F). The mole ratio of oxazoline:phenolic -OH functional groups was 1 : 1. Approximately 1 wt% of the catalyst was added, based on the total weight of the copolymer composition. After degassing, the melt was poured into a suitable mold and cured at about 180°C (356°F) for about 8 hours, and post-cured at about 225 °C (446°F) for about 2 hours.

The final samples were exposed to air in a muffle furnace at 200 °C (392°F) for the listed time periods. As shown in Fig 5, Sample 47 (which was thermally cured without the aid of a catalyst) exhibited zero weight loss after 3344 hours of such exposure. In contrast, the best performing catalyst-cured sample was Sample 38, which was cured with 2% of a co-catalyst system (benzyl trimethylammonium chloride and α,α-dichloro xylene in a 1 : 1 wt ratio). Sample 38 exhibited a 0.52 wt% loss after 3818 hours in the muffle furnace. The measured weight losses on all of the samples are presented in Table 2, which also shows the total time spent in the hot air muffle furnace corresponding to the measured weight loss for each sample.

TABLE 2

SAMPLE NO. WEIGHT LOSS TOTAL HOURS IN

200 °C (392 °F)

MUFFLE FURNACE

44dm 1.5% 2688

21 1.8% 1272

33 1.3% 1272

38 0.52% 3818

43 1.0% 1272

44 1.0% 1272 SAMPLE NO. WEIGHT LOSS TOTAL HOURS IN

200°C (392°F)

MUFFLE FURNACE

16 1.2% 1970

47 0.00% 3344

EXAMPLE 2 Nine grams of 1,3-phenylene bisoxazoline obtained from Ashland Chemical Co. and

6 g of Alnovol PN 3201, obtained from Hoescht Celanese, were melted together at 150°C (302°F) and stirred until homogeneous (approximately 1 minute) The liquid resin was then quickly degassed and poured into a mold

The mold consisted of a Teflon™ block which was a well milled out to accommodate the desired product shape. The mold lid was a flat slab of Teflon™ serving only to enclose the part and maintain its dimensional stability Two piles of glass cloth were inserted into the mold cavity during filling and impregnated with the liquid resin. Once filled, the lid of the mold was put in place, squeezing out excess resin to ensure a void free product. The filled mold was placed inside of a microwave cavity with a suitable microwave generator The mold and sample part then was irradiated at 210-220 MHz at a power level of 40 Watts for 7 minutes. The cured part exhibited a Tg of 200° C, demonstrating a high level of cure. For comparison, an essentially identical sample, which was thermally cured in an oven for 22 hours, exhibited a Tg of 196°C EXAMPLE 3

The cured part from Example 2 is exposed to air in a muffle furnace at 200 °C (392 °F) for a prolonged period. The part exhibits zero weight loss after 3500 hours of such exposure.

EXAMPLE 4 60 wt % of 1 ,3-phenylene bisoxazoline, obtained from Ashland Chemical Co., 40 wt% of _ lnovol PN 3201, obtained from Hoescht Celanese, and 2 wt% or less of each of the antioxidants listed in Table 3 (sequentially), whose structures are given in Figs. 4a-4p, were melted together and the mixture was stirred until homogeneous (approximately 1 minute). The mixture then was degassed and poured into a Teflon™ mold. Once filled, the lid of the mold was shut and excess resin was squeezed out of the mold to ensure a void free product.

TABLE 3

Manufacturer Compound Figure

Hoeschst Celanese Hostanox SE 10 Fig. 4A

Hostanox PAR 24 Fig. 4B

Hostanox 03 Fig. 4C

Uniroyal Naugard 445 Fig. 4D

Naugard 76 Fig. 4E

Naugard 524 Fig. 4F

Naugard Super Q Fig. 4G

Naugard P Fig. 4H

Monsanto Flectol n/a

Santonox Fig. 41

Great Lakes Lowinox CPL Fig. 4J

Anox 20 Fig. 4K

Ciba Geigy Irganox B1171 Fig. 4L

Albemarle Ethanox 330 Fig. 4M

Ethanox 702 Fig. 4N

Cytec Cyanox l790 Fig. 4O

GE Ultranox U626 Fig. 4P

BASF 2003 Uvinul 2003 n/a ^• i l ¬

Manufacturer Compound Figure

Uniroyal Aminox or Naugard A Acetone-diphenyl amine, condensate product, MP

= 85-95 °C, Flash Point

= 179°C

The resin was cured at about 180°C (356°F) for between about 12-16 hours, and

post cured at about 225°C (437°F) for about 8-12 hours. The total cure time was at least

24 hours.

The final samples were exposed to air in a muffle furnace at temperatures of either 177°C (350.6°F), 200°C (392°F), or 230°C (446°F). The weight of the original sample

was noted, and the weight of the sample remaining after being heated for various periods of time was recorded and compared to the weight of the original sample. The data for the testing at 230°C is given beow. The overall results at all temperatures are shown in Figs. 6-8

Sample No. Time Wt% of sample remaining

38 0 100

504 95.66

936 95.38

1272 95.19

1440 94.83

1560 94.67

47 0 100

504 95.85

946 94.86 Sample No. Time Wt% of sample remaining

1272 94.38

48 0 100

168 97.28

336 96.33

504 95 94

64 0 100

168 97.14

336 96.11

504 95.8

66 0 100

168 97.12

336 96.15

504 95 72

67 0 100

168 98 06

336 97 25

552 96 81

68 0 100

158 97.77

336 96.83

552 96 5

The resins made using Ultranox™ U626 and Aminox™ (also known as Nauguard A) exhibited very little weight loss (about 0 3 wt%) after 2540 hours, as reflected in Figs 6 and 7 The resins made using the other antioxidants exhibited a much greater weight loss, as shown in Fig. 8 Persons of skill in the art will appreciate that many modifications may be made to the embodiments described herein without departing from the spirit of the present invention. Accordingly, the embodiments described herein are illustrative only and are not intended to limit the scope of the present invention

Claims

WE CLAIM:

1. A method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote said curing, said method comprising exposing said resin to an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize heating and to produce substantially uniform heating throughout said resin, wherein said curing opens a number of oxazoline rings sufficient to create crosslinking which produces a rigid polymer composition.

2. A method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote said curing, said method comprising exposing said resin to an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize heating and to produce substantially uniform heating throughout said resin, wherein said curing opens a number of oxazoline rings sufficient to create crosslinking which produces a rigid polymer composition, and wherein said resin comprises a structure having a largest dimension to be heated which is less than about 0.1 wavelength of said electromagnetic field.

3. The method of claim 1 wherein said electromagnetic field is an E-field.

4. The method of claim 2 wherein said electromagnetic field is an E-field.

5. The method of claim 1 wherein said electromagnetic field comprises microwaves.

6. The method of claim 2 wherein said electromagnetic field comprises microwaves.

7. The method of claim 1 wherein said resin comprises an initial temperature; said electromagnetic field raises said initial temperature to a second temperature which is greater than said initial temperature; and said electromagnetic field maintains said resin at said second temperature during a cure period.

8. The method of claim 2 wherein said resin comprises an initial temperature; said electromagnetic field raises said initial temperature to a second temperature which is greater than said initial temperature; and said electromagnetic field maintains said resin at said second temperature during a cure period.

9. The method of claim 1 wherein said frequency is in the range of from about 27 MHz - 2.5 GHz; and, said wattage is in the range of from about 10-30 watts per gram.

10. The method of claim 9 wherein said frequency is in the range of from about 100 MHz to about 250 MHz.

11. The method of claim 2 wherein said frequency is in the range of from about 27 MHZ to about 2.5 GHz; and, said wattage is in the range of from about 10-30 watts per gram.

12. The method of claim 11 wherein said frequency is in the range of from about 100 MHZ to about 250 MHZ.

13. The method of claim 3 wherein said frequency is in the range of from about 27 MHz to about 2.5 GHz; and, said wattage is in the range of from about 10-30 watts per gram.

14 The method of claim 13 wherein said frequency is in the range of from about 100 MHZ to about 250 MHZ.

15. A method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote said curing, said method comprising pultruding said resin through an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize heating and to produce substantially uniform heating throughout said resin, wherein said curing opens a number of oxazoline rings sufficient to create crosslinking which produces a rigid polymer composition

16. The method of claim 2 wherein said exposing comprises pultruding said resin through said electromagnetic field.

17 The method of claim 3 wherein said exposing comprises pultruding said resin through said electromagnetic field

18 A method for curing a resin comprising unreacted phenolic OH groups and bisoxazoline monomers in the absence of a catalyst effective to promote said curing, said method comprising exposing said resin to an electromagnetic field comprising a voltage, a frequency, and a wattage sufficient to maximize heating and to produce substantially uniform heating throughout said resin, wherein said curing opens a number of oxazoline rings sufficient to create crosslinking which produces a rigid polymer composition, wherein said resin further comprises an antioxidant selected from the group consisting of 2,4,8, 10- tetraoxa-3,9,diphosphaspiro [5,5]undecane, 3,9-bis[2,4-bis(l, l-dimethylethyl)phenoxy)- and an acetone-diphenyl amine condensate product

19. The method of claim 2 wherein said resin further comprises an antioxidant selected from the group consisting of 2,4,8, 10-tetraoxa-3,9,diphosphaspiro [5,5]undecane, 3,9-bis[2,4-bis(l, l-dimethylethyl)phenoxy)- and an acetone-diphenyl amine condensate product.

20. The method of claim 3 wherein said resin further comprises an antioxidant selected from the group consisting of 2,4,8, 10-tetraoxa-3,9,diphosphaspiro [5,5]undecane, 3,9-bis[2,4-bis(l,l-dimethylethyl)phenoxy)- and an acetone-diphenyl amine condensate product.

21. The method of claim 15 wherein said resin further comprises an antioxidant selected from the group consisting of 2,4,8,10-tetraoxa-3,9,diphosphaspiro [5,5]undecane, 3,9-bis[2,4-bis(l,l-dimethylethyl)phenoxy)- and an acetone-diphenyl amine condensate product.

22. A method for increasing thermooxidative stability of a thermoset copolymer comprising unreacted phenolic OH groups and bisoxazoline monomers comprising, before curing said resin, mixing with said resin an antioxidant selected from the group consisting of 2,4,8, 10-tetraoxa-3,9,diphosphaspiro [5,5]undecane, 3,9-bis[2,4-bis(l,l- dimethylethyl)phenoxy)- and an acetone-diphenyl amine condensate product.