WO1997001584A1

WO1997001584A1 - Chemical modification of thermoplastic elastomers

Info

Publication number: WO1997001584A1
Application number: PCT/US1996/011027
Authority: WO
Inventors: H. James Harwood; Steven W. Jolly
Original assignee: University of Akron
Current assignee: University of Akron
Priority date: 1995-06-27
Filing date: 1996-06-27
Publication date: 1997-01-16
Anticipated expiration: 1997-12-27
Also published as: AU6342496A

Abstract

This invention describes chemical modifications of aromatic groups in end-blocks of triblock thermoplastic elastomers, e.g. Kraton® G, to increase the upper service temperature and to decrease the amount of compression set when compared to the unmodified parent copolymer. The polystyrene end-block of Kraton® G were nitrated yielding materials having Tg's as high as 148 °C, compression set values as low as 68 % and stress to break values as high as 10.5 MPa when measured at 65 °C. The nitrated polymers exhibit self-curing characteristics during molding.

Description

Chemical Modification of Thermoplastic Elastomers

Technical Field The invention described herein pertains generally to a process by which thermoplastic elastomers can be chemically modified (e.g., nitrated) to obtain materials with improved high temperature utility.

Background of the Invention

Commercially available thermoplastic elastomers are generally free from extractable additives and are used for many applications. In contrast to conventional vulcanizates, which contain undesirable extractables, they are valuable for applications in which an elastomer must come in contact with medicinals, living tissue or agricultural chemicals. Unfortunately, they have the disadvantage of being easily deformed when submitted to steam-sterilization. Efforts have therefore been made to prepare thermoplastic elastomers with softening points high enough to enable them to be steam-sterilized. One method for obtaining such materials is to modify the chemical structures of commercially available thermoplastic elastomers so as to increase their softening points sufficiently to render them steam-sterilizable. The invention described herein concerns the nitration of aromatic monomer units in thermoplastic elastomers to increase the softening points ofthe elastomers. In addition, nitro groups that are introduced into the elastomers by nitration confer other valuable properties to them, such as solubility in polar solvents and an ability to self- vulcanize upon heating.

Thermoplastic elastomers are a special class of polymers of practical and theoretical interest. They were introduced by the Shell Chemical Company (U.S.A.) in 1965. Thermoplastic elastomers are novel because they can be formed into useful articles by common, rapid, thermoplastic processing techniques. They exhibit rubber-like properties such as high resilience, high tensile strength and reversible elongation. Good abrasion resistance can be obtained, but without chemical vulcanization. Thermoplastic elastomers consist of block copolymers ofthe A-B-A structure, -(A-B)_n- structure or even -[B_x-C(A)-]_n-, where A is a polymeric segment having a high glass transition temperature and B is an elastomeric polymer segment, n is the number of repeating block sequences in the polymer, x is the number of repeating units of one monomer in the polymer and C represents a graft point in the polymer backbone. There are four factors which influence the elastomeric performance of these polymers. They are choice of monomers, block lengths of A and B, weight fractions of A and B, and the molecular weight distribution ofthe elastomeric mid-block.

Polymers derived from diene monomers are often hydrogenated to obtain materials that have improved thermal and chemical stability. For example, the polymer that results from butyl lithium initiated polymerization of butadiene contains 1,4-butadiene and 1,2-butadiene repeating units and it reacts easily with oxygen, ozone and other chemicals. When this polymer is hydrogenated, it becomes a copolymer of ethylene and butene- 1. This polymer has much better chemical resistance than the parent polybutadiene. Similarly, hydrogenation of polyisoprene yields a copolymer containing ethylene and propylene repeating units. This polymer also has much better chemical resistance than the parent polymer. Hydrogenation of unsaturated units in copolymers is also an important way to improve chemical and thermal stability. Hydrogenation of a statistical styrene-butadiene copolymer, for example, yields a polymer containing styrene, ethylene and butene- 1 units.

Kraton® G-1652 is an example of an ABA triblock copolymer containing polystyrene (A) and hydrogenated polybutadiene (B) segments. Its upper temperature of usefulness depends on the softening point of its polystyrene segments, which can vary from about 65 °C to about 90°C depending on the molecular weight ofthe polystyrene segments in the material. It has been demonstrated previously, that the polystyrene segments in Kraton® G-1652 can be modified by benzoylation and naphthoylation and that the softening points ofthe modified polystyrene segments can be as high as 121, 141 and 151 °C for the benzoylated, 2-naphthoylated and 1-naphthoylated polymer, respectively, the reaction schematic for which is shown in Figs. 13 and 12 respectively for the latter two polymers. It has also been recognized that acylated Kraton® G-1652 polymers have enhanced softening points, improved strength at all temperatures and improved compression sets compared to the unmodified polymer. It has also been demonstrated that the polystyrene segments in Kraton®

G-1652 can become arylsulfonylated by reaction with arylsulfonyl chlorides in the presence of Lewis acids and that arylsulfonation confers higher softening points, higher strengths and retention of elastomeric properties at higher temperatures upon the polymers, just as benzoylation and naphthoylation do. Although modification of Kraton® G-1652 and similar thermoplastic elastomers by benzoylation, naphthoylation or arylsulfonylation confers many desirable properties upon them; the modified polymers have the disadvantage of having high initial moduli, undergoing irreversible yielding on extension and having poor compression sets. This is because the modification reactions raise the volume fractions ofthe modified polystyrene segments to an undesirably high level.

Other types of thermoplastic elastomers that are amenable to modification by acylation or arylsulfonylation include A-B-A triblock copolymers containing polystyrene (A) and hydrogenated polyisoprene (B) segments. In addition, it is possible to prepare A-B-A triblock copolymers where the A end blocks are poly(α- methylstyrene), poly(4-biphenyl), poly( vinyl naphthalene), poly(styrene-co-l,l- diphenylethylene), poly(styrene-co- α-methylstyrene) or poly(styrene-alt-stilbene) and the B blocks are hydrogenated polydiene segments such as hydrogenated polybutadiene, hydrogenated polyisoprene or hydrogenated polypiperylene. Polyisobutylene is also a possible candidate for a (B) block. All such thermoplastic elastomers can benefit from modification by acylation or arylsulfonylation, but the modified polymers will all have increased initial moduli because ofthe high volume fraction ofthe modified polymer segments that will be present.

It can be mentioned that the styrene end-blocks in thermoplastic elastomers containing polystyrene and blocks have also been modified by chloromethylation, arylsulfonylation, benzylation and benzhydrylation reactions, but that most of these modifications will not increase the softening points ofthe polymer appreciably and most result in substantial increases in initial modulus.

It is not surprising that the prior art approaches have resulted in modified polymers having high initial moduli, irreversible yielding on extension and poor compression sets. The molar volumes of prior art benzoylated, naphthoylated or arylsulfonylated styrene units can be estimated to be 2-2.5 times the molar volume of a styrene unit. The volume fraction ofthe modified polystyrene segments thus becomes so large that such segments are obliged to become a continuous phase in the sample and this is what confers the above-mentioned undesirable characteristic to the samples. Thus, it can be seen that what is needed is a chemical modification process which would raise the softening points of thermoplastic elastomers, such as Kraton® G-1652 to a satisfactory level without the attendant negative effect on final product properties. This can be achieved through the use of modification reactions that incorporate substituents having low volume requirements onto the polymers. This invention describes the use of nitration as a means to accomplish this and thereby improve the high temperature performance of thermoplastic elastomers. The nitration reaction will also confer other desirable properties to the polymers, as will be discussed subsequently. With nitration, the molar volume of a nitrated styrene unit can be estimated to be only about 1.25 times that ofa styrene unit and therefore be very effective at increasing the glass transition temperature ofthe polystyrene end-segments without affecting the volume ofthe end-segments appreciably. The nitrated polymers exhibit better physical properties than the parent polymer but do not exhibit the high initial moduli, irreversible yielding on extension, and poor compression sets of benzoylated, naphthoylated or arylsulfonylated polymers prior art polymers. Through the use of this chemical modification, it is possible to obtain materials with acceptable softening points, relatively low volume fractions ofthe modified segments, and acceptable initial moduli. The presence of nitro groups will increase the compatibility ofthe modified polystyrene segments with other polymers and with polar solvents. This is expected to make the polymers compatible with some polar polymers and that this compatibility may result in the polymers conferring impact resistance to polymer blends. Examples might include blends ofthe nitrated polymers with styrene- acrylonitrile copolymers. The nitrated polymers would be more easily combusted than the parent polymers and find applications as impact stabilizers or binders for explosives or solid fuels (propellants). In addition, it was discovered that the nitrated polymers, although initially thermoplastic and moldable or processable, become crosslinked under molding conditions. Thus, the nitrated polymers are in fact, self- vulcanizing elastomers. As such, they may find utility in sealing compositions and adhesives. Whatever chemistry is responsible for self-vulcanization may enable the nitrated polymers to be self-compatibilizing when blended and heated with other polymers. They might also serve as compatibilizers for blending hydrogenated thermoplastic elastomers with other monomers. If they could combine with another polymer during blending and heating, the product would be a graft copolymer that could be a compatibilizer.

Summary ofthe Invention

In accordance with the present invention, there is provided a method for nitrating the aromatic substituents in a thermoplastic elastomer and thereby increasing its softening point in addition to imparting other useful characteristics to thermoplastic elastomers.

It is an object of this invention to increase the softening point of the thermoplastic elastomer through a chemical nitration process and obtain products without attendant high initial moduli.

It is yet another object of this invention to increase strength at elevated temperatures thereby increasing the utility of the polymers at elevated temperatures. It is still another object of this invention to nitrate thermoplastic elastomers chemically without increasing the volume fraction of modified segments of the thermoplastic elastomer to an undesirable extent.

These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims.

Brief Description of the Drawings The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein:

Fig. 1 is a 200 MHz Η-NMR spectra of Kraton® G-1652 and a 32% nitrated derivative;

Fig. 2 is a 200 MHz Η-NMR spectra of Kraton® G-1652 and various nitrated derivatives; Fig. 3a is a Size Exclusion Chromatogram of Kraton® G-1652;

Fig. 3b is a Size Exclusion Chromatogram of a 41% nitrated derivative of Kraton® G-1652;

Fig. 4 is a plot of osmotic pressure/concentration (PaL/g) vs. Concentration (g/L) for Kraton® G-1652 and a 27% nitrated derivative; Fig. 5 is a plot of DMTA data of a 56% nitrated derivative of Kraton® G-

1652;

Fig. 6 is a plot of DMTA data obtained from Kraton® G-1652 and various nitrated derivatives;

Fig. 7a is a plot of Stress vs. Strain Curves for Kraton® G-1652 and various nitrated derivatives;

Fig. 7b is a plot of Stress vs. Strain Curves for Kraton® G-1652 and various 1 -naphthoylated derivatives;

Fig. 8 is a plot of Permanent Set vs. Nitration Percent of Kraton® G-1652 and various nitrated derivatives; Fig. 9 is a plot of Stress vs. Strain Curves for Kraton® G-1652 and various nitrated derivatives;

Fig. 10 is a plot of Permanent Set after 200% strain at 65°C vs. Nitration Percent of Kraton® G-1652 and various nitrated derivatives; Fig. 11 is a plot of Compression Set after 1 hour at 121 °C vs. Nitration

Percent of Kraton® G-1652 and various nitrated derivatives;

Fig. 12 is a reaction schematic of the 1 -naphthoylation of Kraton® G-1652; Fig. 13 is a reaction schematic of the 2-naphthoylation of Kraton® G-1652; and Fig. 14 is a reaction schematic of the nitration of Kraton® G-1652.

Detailed Description of the Invention Referring now to the drawings wherein the showings are for purposes of illustrating the preferred embodiment of the invention only and not for purposes of limiting the same, the Figures show various physical measurement characteristics for both parent elastomer and nitrated derivatives.

The best mode for carrying out the invention will now be described for the purposes of illustrating the best mode known to the applicant at the time. The examples are illustrative only and not meant to limit the invention, as measured by the scope and spirit of the claims. Experimental Procedures

Example #1

Nitration using CCl_φ HNO_j, and acetic anhydride

Kraton® G-1652, Shell Chemical Co., (1.0 g, 2.79 mmol of styrene units) was dissolved in 15.0 mL of CC1₄ in a 25 mL Erlenmeyer flask which was purged with N₂. Acetic anhydride (1.20 mL , 12.6 mmol) was slowly added to the stirred carbon tetrachloride solution over 5 minutes. Concentrated nitric acid (0.28 mL, 41-9 mmol) was then carefully added over 5 minutes. After 10 minutes, the water-white solution acquired a yellow tint that persisted throughout the course ofthe reaction. The reaction was continued for a specified length of time which varied from 1-4 hours and was then stopped by precipitating the polymer into 50 mL of methanol with rapid stirring. The yellow tinted crumbs that formed were collected on a Buchner funnel lined with Whatman #1 filter paper. The collected crumbs were dissolved in 20 mL of tetrahydrofuran and re-precipitated into 60 mL of methanol. The yellow tinted crumbs were collected, air dried and dried further in a vacuum oven at 25 °C for 24 hours. The recovery procedure for the other nitrated derivatives was the same as described above and is not repeated in the descriptions ofthe other nitration procedures.

Example #2 Nitration using ammonium nitrate, chloroform and trifluoroacetic anhydride Pulverized ammonium nitrate was dried in a 130° C oven for 1.0 hour. A portion of this material (0.223 g, 2.79 mmol) was added to a clean, dry, N₂-purged, 50 mL 2-neck reaction flask along with 5.0 mL of chloroform. Kraton® G-1652 (1.0 g, 2.79 mmol of styrene units) was dissolved in 15 mL of chloroform and the solution was added in one portion to the stirred nitrate salt suspension in the reaction flask. A water-cooled condenser, fitted with a drying tube filled with Drierite® was fitted onto the 50 mL reaction flask. Trifluoroacetic anhydride (1.18 mL 3.78 mmol) was carefully added to the stirred reaction flask contents in one portion. The reaction proceeded for 72 hours, at room temperature, typically 23 °C. The nitration reaction was stopped by the addition of 10 mL of tap water, and the chemically modified polymer was recovered as described above.

Example #3 Nitronium Trifluoromethanesulfonate (Nitronium Triflate) Preparation

Trifluoromethanesulfonic acid (10.0 mL, 113 mmol) was dissolved in 30 mL of methylene chloride in a 500 mL 3-neck reaction flask equipped with a mechanical stirrer, N₂ inlet and a dropping funnel. Anhydrous nitric acid (2.30 mL, 53.3 mmol) was added drop-wise to this stirred solution. As the nitric acid was added a white, crystalline solid was continually formed. The white, crystalline solid was a mixture of nitronium trifluoromethanesulfonate ( 53.3 mmol) and hydronium trifluoromethanesulfonate (53.3 mmol). Nitration using Nitronium Triflate

A solution of Kraton® G-1652 (20.0 g, 55.8 mmol of styrene units) in 300 mL of methylene chloride was added slowly over 5 minutes to the stirred suspension of nitronium triflate (53.3 mmol) prepared above. The reaction was carried out at various temperatures (-28 to 25 °C) for reaction times which varied from 0.5-2 hours. The reaction was stopped by the slow addition of 25 mL of tetrahydrofuran over a 5 minute period. There was evidence of gel particles in the reaction flask and these were removed by filtering the reaction solution through borosilicate glass wool. Approximately 3.75 g of insolubles were collected and the nitrated polymer was obtained by adding the filtrate to methanol. Typically, 17.5 g of dry nitrated derivative was obtained after the first precipitation into methanol.

Discussion When the nitration procedure of Example #1 was followed, the results depended on the scale ofthe reaction. On a small scale (1 or 2 grams of polymer), a 30% nitration level was attained and the product was soluble in THF and CHC1₃. However, when the reaction was conducted on a larger scale (20 to 30 grams of polymer) the product was a crosslinked gel. In order to obtain a derivative with a larger percentage of nitration, the molar ratio of acetyl nitrate to polystyrene residues was increased from 0.85 to 1.5. In an initial experiment 45% nitration of Kraton® G-1652 was obtained, but efforts to reproduce this result generally resulted in only approximately 25% nitration. The results are shown in Table I. Table I

Nitration of Kraton® G-1652 with Acetyl Nitrate in CC1₄

Molar Ratio Values

Cone.

Sample S Acetic Time Percent fert residues HNO₃ Anhydride Remarks (hrs) Nitration

B321A1 100 0.85 2 1.5 26 no gel

B322A1 100 0.85 2 2.25 26 no gel

B323A1 100 1.5 4.5 0.5 45 no gel

B324A1 100 1.5 4.5 1.25 44 no gel

B333A1 50 3 9 2.25 56 no gel

B335A1 50 3 9 16 N/E moderately x-linked

B337A1 50 3 9 4.75 85 moderately x-linked

B338B1 50 1.5 4.5 2.25 22 no gel

B338C1 50 1.5 4.5 4.75 32 no gel

B341A1 50 1.5 4.5 1.25 17 no gel

B342A1 50 1.5 4.5 18 24 no gel

B393A1 50 3 9 10 N/E moderately x-linked

N/E = not evaluated x-linked = crosslinked

When the acetyl nitrate to polystyrene residue ratio was further increased to 3 and the reaction time was also increased, 85% nitration occurred. The nitrated materials were generally cross-linked, however, when pot times were greater than 2.25 hours.

It is known that rates can be affected by choice of solvents. Therefore, in efforts to increase nitration rate over cross-linking rate, other solvents were investigated. The acetyl nitrate to polystyrene residue molar ratio was kept at 3 but different solvents were employed for the nitration. When carbon disulfide was employed, the product was cross-linked and when hexanes were employed a low percentage of nitration (2%) occurred. The results of these investigations are summarized in Table II.

Table II Nitration of Kraton® G-1652 with Acetyl Nitrate

Molar Ratios

Cone PS Acetic Time Percent

Sample HNO, residue Anhydride (hrs) Nitration Solvent Remark

B367A1 50 1 3 9 1 < 2 hexanes no gel

B367B1 50 1 3 9 1 N/E CS₂ mod. x-linked

N/E = not evaluated

Reaction temperature was varied using different acetyl nitrate to polystyrene residue ratios in efforts to increase the percentage of nitration and avoid the formation of crosslinked product. Increasing the temperature to 40 °C did not lead to improved results because the products were either less nitrated when compared to those obtained under similar conditions at 25 °C or the products were crosslinked. The results are shown in Table III.

Table III Nitration of Kraton® G-1652 with Acetyl Nitrate

Molar Ratios

Cone PS Acetic Time Percent

Sample HNO_j residue Solvent Anhydride (hrs) Nitration Remark

B367A1 50 1 1.5 4.5 1 21 CCl, no gel

B367B1 50 1 3 9 1 N/E CC1₄ mod x-linked

N/E = not evaluated The nitronium source was changed from concentrated nitric acid to anhydrous nitric acid. Under the conditions where anhydrous nitric acid was employed 30% nitration was achieved and the product was soluble in both THF and CHC1₃. The change from concentrated to anhydrous nitric acid produced a derivative which was

5% more nitrated and still soluble in organic solvents. The results are shown in Table

IV.

Table IV

Nitration of Kraton® G-1652 with Acetyl Nitrate using Anhydrous Nitric Acid

Molar Ratios

Time Percent

Sample CottC PS HNO_j Aeetie fefc⁾ residue Anhydride φrs) Nitration Solvent Remark

B345C1 50 1 1.5 4.5 1.25 32 HNO_j no gel

B345B1 50 1 1.5 4.5 2.25 31 HNO_j no gel

B345A1 50 1 1.5 4.5 4.75 37 HN0₃ no gel

The use of nitronium triflate as a nitration reagent gave higher degrees of nitration than were achieved with acetyl nitrate on the 1-2 gram scale. When Kraton® G-1652 was nitrated with nitronium triflate at 20 °C, 30-45% nitration was obtained. An unexpected result was the formation of gel in addition to the soluble nitrated derivative. The gel amounted to 15-20% ofthe nitrated product. The results are shown in Table V.

Table V

Nitration of Kraton® G-1652 with Nitronium Triflate @ 20°C

Molar Ratio Values

Cone.

Sample PS residues CF₃SO₃H HNO, Percent Nitration Remarks

Cone

B344B1 40 8 4 0.17 15 HNO, mod.

B345D1 29 0.8 20 0.17 N/E x-linked anhy. HNO, mod.

B347A1 29 4 2 0 9 N/E x-linked anhy. HN0₃ mod

B347B1 29 3 1.5 0.9 N/E x-linked anhy. HN0₃

16% gel

B347C1 29 2 1 0.9 56 anhy. HN0₃

21% gel

B358A1 50 4 2 0.17 28 anhy HN0₃

1 1% gel

B359A1 50 2 1 1 35 anhy. HN0₃

16% gel

B397A1 60 2 1 1 44 anhy. HN0₃

18% gel

B399B1 60 2 1 1 41 anhy. HN0₃

N/E = not evaluated x-linked = crosslinked

In an effort to eliminate the formation of gel, the nitration was performed at -28°C for various times. Subsequently, the solution was warmed to 20°C and the nitration was allowed to continue at 20 ^CC for various reaction times. The same general trends observed under this set of reaction conditions were observed when the nitration was performed at 20^CC. The results are shown in Table VI. Table VI

Nitration of Kraton® G-1652 with Nitronium Triflate @ -28 °C

Molar Ratio Values

Rxn.

Cone. PS Percent

Sample CFiSO,H H Oi Time Remarks

⁽sfc⁾ residues {hrs) Nitration

B352A1 40 07 8 1 56

B353A1 50 1 8 1 27 22% gel

B360B1 50 2 1 1 46

B363A1 50 2 1 1 < 2

B363B1 50 2 1 0 5 N E 0°C

B401A1 50 1 1 3 41 22% gel

N E = not evaluated

In Example #2, both KNO₃ and NH₄NO₃ were used with trifluoroacetic anhydride (TFAA) in the nitration of Kraton® G-1652. Higher percentages of nitration were achieved with NH₄NO₃ (37%) as compared with KNO₃ (25%). This can be attributed to solubility differences ofthe two nitrate salts in CHC1₃ with NH₄NO₃ being more soluble than KNO₃. Approximately 35% nitration was observed when NH₄NO₃ was employed on a small scale (2 grams of polymer). When the reaction scale was larger (20 grams) the product was a gel. The results of these investigations are provided in Table VII.

Table VII

Nitration of Kraton® G-1652 with Nitrate Salts and Trifluoroacetic Anhydride

N/E = not evaluated x-linked = crosslinked

Kraton® G-1652 was 35% nitrated and the product obtained was soluble in organic solvents when mixtures of nitrate salts and TFAA were used. However, when large batches of polymer were nitrated, a cross-linked material resulted. The results for large batches could be due to an increase in the ratio of amounts of TFAA to nitrate salt used during the reaction.

Several methods were used to characterize Kraton® G-1652 and the various nitrated derivatives. These methods included 'H-NMR spectroscopy, size exclusion chromatography (SEC), membrane osmometry, dynamic mechanical thermal analysis (DMTA) and elemental analysis. Compression moldings ofthe polymers were prepared at 160-190°C during 5-10 minutes and pressures of 20 tons per square inch. Standard ASTM procedures were followed for the various physical property evaluations. However, special microdumbell test specimens were used for the stress - strain studies.

The 200 MHZ 'H-NMR spectra of Kraton® G-1652 and the 32% nitrated derivative are shown in Figure 1. It was observed that the only new proton resonances observed in the nitrated derivative occur in the aromatic region at 8.0 ppm. These proton resonances were attributed to the protons adjacent to the nitro group on the phenyl ring of substituted polystyrene residues. In Figure 2 are shown

200 MHz 'H-NMR spectra for various nitrated derivatives of Kraton® G-1652.

The relative molecular weights, M„, of several nitrated derivatives were measured by size exclusion chromatography, SEC, using a polystyrene calibration.

The M„ values appeared to be smaller than those expected for each derivative based on the M„ of the parent polymer and the extent of nitration obtained.

However, when the absolute M_n of sample B353A1 was measured by membrane osmometry, a value of 55,200 was obtained, which was larger than the expected value of 45,000 and considerably larger than the value obtained by size exclusion chromatography (SEC) analysis. This indicated that the nitrated derivatives were tightly coiled in THF, the solvent used for the SEC measurements and that this would cause the polystyrene calibration used for the SEC measurements to be incorrect. It was therefore decided to apply a correction factor to the M,, values obtained by SEC. When this was done, the nitrated polymers were found to have molecular weights larger than expected. This indicates that branching reactions accompanied the nitration reactions. The molecular weight data are summarized in Table VIII. Table VIII

Size Exclusion Chromatography Results for Kraton® G-1652 and Nitrated Derivatives

Percent M„*

Sample Nitration (by SEC) M Mm (expected) (corrected)

KG 1652 0 43500 1.09 — —

B353A1 27 27700 1.86 45000 55200

B339A1 37 38100 1.44 45500 76200

B401A1 41 41700 2.35 45700 83400

B399B1 41 48800 2.09 45700 97600

* The corrected Mn for samp le B353A1 w. is measured b¹ y membrane osmometry. The values for the other samples were calculated by applying a correction factor of (55,200/27,700). The SEC chromatograms of the parent copolymer along with the 41% nitrated derivative are shown in Figs. 3a and 3b respectively. It can be seen that the SEC chromatogram of the nitrated derivative is broader than that of the parent copolymer. The M M_n ratios of the nitrated derivatives were approximately 2 whereas the M. M. ratio for the parent polymer was 1.09. The increase in M_^/M. that occurred upon nitration is also consistent with the occurrence of some chain branching during the nitration reaction.

T_g measurements of Kraton® G-1652 and the nitrated derivatives were made using a Dynamic Mechanical Thermal Analyzer (DMTA). The storage modulus, loss modulus and tan δ values for the 56% nitrated derivative at various temperatures are shown in Figure 5. The peak in tan δ occurs at 148°C and this value is taken as the T_g ofthe nitrated polystyrene end-segments. Figure 6 shows the tan δ versus temperature curves for Kraton® G-1652 and the 27 and 41% nitrated derivatives.

The T_g's determined from tan δ measurements for the various nitrated derivatives are listed in Table IX. The T_g's ofthe nitrated polystyrene segments were found to increase with increasing substitution while the T_g's ofthe mid-segments remained substantially the same which indicates that the mid-segment remained elastomeric.

Table IX

T_g's for Various Nitrated Derivatives Determined by Maxima in tan δ vs. Temperature Curves

Percent T_g (E-co-B) T₈ (styrene)

Sample Substitution CQ CQ

KG 1652 0 -30 95

B322A1 26 ~ 111

B353A1 27 ~ 113

B323A1 45 -30 125

B333A1 56 — 148 The mechanical properties ofthe nitrated polymers, i.e., tensile stress and strain, initial moduli, permanent set, and Shore A-2 hardness values, changed with the extent of nitration. The stress vs. strain curves ofthe nitrated polymers at 20 °C are shown in Figure 7 along with the curve for Kraton® G-1652. It can be seen that stress to break values for the nitrated derivatives are approximately 3 MPa less than that of Kraton® G-1652 and that the nitrated derivatives have larger elongations than the parent polymer. In fact, there is an inverse relationship between the strain to break and percent nitration. The curves also indicate that the moduli at 100% elongation decrease with the extent of nitration. The nitrated derivatives have smaller moduli than the parent polymer. An exception is the 41% nitrated sample. Its 100% modulus was larger than that observed for Kraton® G-1652. Table X lists the 100% moduli of nitrated derivatives and Kraton® G-1652.

These values are below 3 MPa, which is considerably below the 100% moduli of benzoylated, naphthoylated or arylsulfonylated derivatives of Kraton® G- 1652, such values generally ranging from 3-5 MPa. Fig. 7b shows stress vs. strain curves that have been measured for naphthoylated Kraton® G-1652 samples. The curves show a sharp initial rise in stress at low strains. This characteristic is not evident in the strain curves depicted for the nitrated polymers in Fig. 7a. This sharp initial rise is not desirable. Since it is not evident in Fig. 7a, this demonstrates one of the advantages of using nitration to increase the softening point ofthe polymers as opposed to other modification reactions such as benzoylation, naphthoylation or arylsulfonylation.

Table X

100% Strain Modulus for Kraton® G-1652 and Nitrated Derivatives @ 20 °C

Percent 100% Modulus

Sample Nitration (MPa)

KG1652 0 2.9

B353A1 27 2.3

B401A1 41 2.6

B399B1 41 3

As shown in Figure 8, there is a direct relationship between the permanent set and the amount of nitration. Also, the Shore A-2 hardness measured for the nitrated derivatives was smaller than that measured for Kraton® G-1652. Data are shown in

Table XI.

Table XI

Shore A-2 Hardness Values for Kraton® G-1652 and Nitrated Derivatives (α>. 20 °C

Percent

Sample Nitration Shore A-2

KG1652 0 81

B353A1 27 71

B401A1 41 71

B399B1 41 76

The mechanical properties ofthe nitrated polymers at 65 °C are influenced dramatically by the extent of nitration. Stress vs strain curves ofthe nitrated polymers at 65 ^βC are shown in Figure 9 along with the curve for Kraton® G-1652. It can be seen that parent polymer, Kraton® G-1652 has practically no strength at 65 °C and can be easily drawn to nearly 500% elongation with a stress of only 2 MPa. In contrast, the nitrated derivatives show appreciably higher strengths, the ultimate tensile strengths increasing with the extent of nitration. Thus, nitration increases the strength ofthe thermoplastic elastomers and their utility at elevated temperatures. Also of interest in Fig. 9, is the fact that the initial parts ofthe stress- strain curves ofthe parent and nitrated polymers are very similar. In contrast to benzoylation, naphthoylation or arylsulfonylation reactions, nitration of Kraton® G- 1652 does not result in a derivative with a high initial modulus. The low percent moduli ofthe nitrated derivatives and Kraton® G-1652 are listed in Table XII. The nitrated derivatives also exhibit smaller permanent set values than the parent polymer, as seen in Figure 10.

Table XII

100% Strain Modulus for Kraton® G-1652 and Nitrated Derivatives @ 65 °C

Percent 100% Modulus

Sample Nitration (MPa)

KG 1652 0 2.1

B353A1 27 1.7

B401A1 41 2.4

B399B1 41 2.6

Compression set was measured after 1 hour at 121 °C and 25% compression. The compression set ofthe 41% nitrated derivatives was smaller than that of Kraton® G-1652. This can be attributed to the hard block phase ofthe nitrated derivatives resisting flow at the test temperature due to their T_g's being higher than that of Kraton® G-1652. Thus, the end-segments appear to be effective in anchoring the mid-segments in the 41% nitrated derivatives. A difference was noted in the compression sets observed for nitrated derivatives that had different molecular weight distributions. The derivative with a M_w/M_n of 2.09 exhibited a smaller compression set than that with a M^/M,, of 2.34. It is believed that the samples contained different amounts of diblock and that the sample with the broader molecular weight distribution contained a higher proportion of diblock. It therefore exhibited a greater tendency to flow. Compression set values ofthe nitrated derivatives and Kraton® G-1652 versus percent nitration of end-segments are shown in Figure 11.

The gel contents ofthe molded nitrated derivatives were determined using toluene as solvent after 168 hours extraction. The 41% nitrated derivatives contained approximately 70% gel after molding while the 27% nitrated derivative contained only 3% gel. Since neither nitrated sample contained gel prior to molding, it is clear that cross-linking occurs during molding. The gel contents ofthe nitrated derivatives and Kraton® G-1652 are shown in Table XIII. The fact that the polymer with 41% nitration were partially converted to insoluble gels under the conditions ofthe molding experiments indicates that they could have been completely insolubilized if the extent of nitration had been higher or if molding conditions have been somewhat more severe. In other words, the gel measurements indicate that the nitrated polymers are self-curing. This is a valuable characteristic since the initially thermoplastic polymers can be converted into thermoset vulcanized elastomers without the need of any vulcanization agents being present. This self-curing feature is totally unexpected, but very desirable since completely cured polymers can be expected to have better tensile strengths and higher solvent resistance and better creep properties than the uncured samples.

Table XIII

Gel Contents of Kraton® G-1652 and Nitrated Derivatives

t

Sample Percent Percen Nitration Gel Sol

KG 1652 0 0 100

B353A1 27 3 97

B401A1 41 67 33

B399B1 41 73 27

While the experimental work described relates primarily to Kraton® G- 1652, there is no need to limit the invention to this particular elastomer or even class of elastomers. The reaction is generic to thermoplastic elastomers which have aromatic or styrenic-like hard blocks and rubbery soft blocks. A brief list of non-limiting examples for the elastomeric part of the thermoplastic elastomer would include hydrogenated polybutadiene, hydrogenated polyisoprene, hydrogenated polydienes, ethylene-α-olefin copolymers, polyisobutylene, neoprene, polysiloxanes, polyacrylates, etc. Similarly, non-limiting examples for the hard block would include polystyrene, poly(vinyl naphthalene), poly(vinyl biphenyl), poly(styrene-co-α-methylstyrene), poly(styrene-co- 1 , 1 -diphenylethylene), poly(4- vinyl biphenyl), polyindene, etc. Additionally, while most of the examples presented herein have less than 60% of the aromatic units nitrated, this does not represent an upper limit on the extent of the nitration reaction and none is so intended.

This invention has been described with reference to preferred and alternate embodiments. Obviously, modifications and alterations will occur to others upon the reading and understanding of the specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

Wltat is Claimed is:

1. A process to raise the glass transition temperature of a thermoplastic elastomer above 100°C while essentially maintaining the initial moduli of the elastomer comprising the step of nitrating at least one vinyl aromatic monomer within the elastomer with a nitrating agent.

2. The process of claim 1 wherein the step of nitrating occurs in a solvent.

3. The process of claim 2 wherein the elastomer is at least partially soluble in the solvent.

4. The process of claim 3 wherein the nitrating agent is selected from the group consisting of acetyl nitrate, trihaloacetyl nitrate, nitronium trihaiomethanesulfonate, nitric acid and salts thereof.

5. The process of claim 4 wherein the solvent is selected from the group consisting of carbon tetrachloride, chloroform, methylene chloride, hexanes, carbon disulfide and ethylene chloride.

6. The process of claim 3 wherein a ratio of the nitrating agent to vinyl aromatic monomer is at least 0.85/1.

7. The process of claim 6 wherein the ratio is at least 1.5/1.

8. The process of claim 3 wherein the step of nitrating occurs below room temperature.

9. The process of claim 3 wherein the step of nitrating occurs at a temperature of room temperature or above.

10. The process of claim 1 wherein the nitrating agent increases the molar volume of a nitrated vinyl aromatic monomer no more than 2 times the molar volume of the vinyl aromatic monomer.

11. The process of claim 10 wherein the nitrating agent increases the molar volume of a nitrated vinyl aromatic monomer no more than 1.5 times the molar volume of the vinyl aromatic monomer.

12. The process of claim 1 1 wherein the nitrating agent increases the molar volume of a nitrated vinyl aromatic monomer no more than 1.25 times the molar volume of the vinyl aromatic monomer.

13. The process of claim 1 which further comprises the step of heating the elastomer thereby converting the elastomer into a thermoset.

14. The process of claim 7 which further comprises the step of heating the elastomer thereby converting the elastomer into a thermoset.

15. The process of claim 10 which further comprises the step of heating the elastomer thereby converting the elastomer into a thermoset.

16. The process of claim 1 1 which further comprises the step of heating the elastomer thereby converting the elastomer into a thermoset.

17. The process of claim 12 which further comprises the step of heating the elastomer thereby converting the elastomer into a thermoset.

18. The product of the process of claim 1.

19. The product of the process of claim 7.

20. The product of the process of claim 10.

21. The product of the process of claim 1 1.

22. The product of the process of claim 12.

23. The product of the process of claim 13.

24. The product of the process of claim 14.

25. The product of the process of claim 15.

26. The product of the process of claim 16.

27. The product of the process of claim 17.