WO1984003883A1

WO1984003883A1 - Stabilised rubber compositions

Info

Publication number: WO1984003883A1
Application number: PCT/GB1984/000098
Authority: WO
Inventors: Gerald Scott; Lakshman Premal Nethsinghe
Original assignee: Gerald Scott; Lakshman Premal Nethsinghe
Priority date: 1983-03-30
Filing date: 1984-03-26
Publication date: 1984-10-11
Also published as: JPS60500957A; EP0169201A1

Abstract

A C-substituted or unsubstituted aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) nitrone having anti-fatigue and/or antiozonant properties in rubber.

Description

STABILISED RUBBER COMPOSITIONS This invention relates to stabilised rubber compositions ; more particularly, this invention relates to additives which have static antiozonant properties when incorporated in organic rubbers ; and to stabilised rubber compoundings and vulcanised rubber incorpo rating these additives, possibly by chemical binding.

At the present time, the most successful class of antiozonant additive for rubbers comprises the N-aryl-substituted p-phenylenediamines, notably the N-aryl , N '-alkyl-substituted p-phenylenediamines . The pre-eminent example is N-phenyl, N'-isopropyl-pphenylenediamine (IPPD) commercially available as SANTOFLEX IP

("SANTOFLEX" is a registered Trade Mark of Monsanto Company) . A major disadvantage of these additives is that, in service , a small fraction thereof becomes converted into an intensely coloured by-product which not only prevents their use in white or coloured rubber compositions but also prevents carbon black-containing rubber compositions containing them being used contiguous with white or coloured polymer compositions into which the discoloration could migrate .

We, accordingly, attempted to provide improved antiozonant additives (in which the tendency to create discoloration in rubber compositions containing them was reduced) by preparing the analogous nitroxyls or hydroxylamines thereby obviating the presence of the free diamine which was believed to lead to the discoloring species. While many of the novel addit ives prepared (especially 4 , 4 ' -dime thoxy diphenyl nitroxyl which , at 1% concentration , was more than twice as ef fective an antiozonant as IPPD) were highly efficient antiozonant additives , they too unfortunately caused discoloration. Indeed , there appeared to be a direct relationship between the tendency to discolour and antiozonant activity in these additives.

We have now prepared a novel class of additive which exhibits antiozonant activity in rubbers with a reduced tendency to discoloration. According, therefore, to one aspect of this invention we provide a C-substituted or unsubstituted aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) nitrone having antifatigue and/or antiozonant properties in rubber. Particularly effective such compounds are those aldonitrones of the formula:

wherein:

R₁ represents a (+ I) or a (+ M) group;

R₂ and R₃, which may be the same or different, each represent a substituted or unsubstituted (alkyl or alkoxy) group;

R₄ and R₅, which may be the same or different, each represent a hydrogen atom or a substituted or unsubstituted (alkyl or alkoxy) group; and

R₆ represents a substituted or unsubstituted (branched alkyl or cycloalkyl) group.

For example R₁ may represent an unsubstituted or alkyl substituted hydroxy group or an unsubstituted or mono- or di-alkyl substituted amino group; or R₂ and R₃, which may be the same or different, each represent an unsubstituted C₁ to C₅ alkyl or alkoxy group, preferably each represents an unsubstituted C₁ to C₄ alkyl group.

Examples of suitable substituents include a halogen atom, for example a fluorine, chlorine, bromine or iodine atom, a hydroxyl group, an alkoxy group, an alkyl carboxylic ester group, a mercapto group, a thioether group, a substituted or unsubstituted amino or amido group.

For ease of synthesis, it is preferred that R₂ and R₃ are the same. It is also preferred that at least one of R₄ and R₅ represents a hydrogen atom. R₆ may suitably be a branched alkyl group , preferably a C₃ to

C₆ branched alkyl group. It is particularly preferred that R₆ represents an isopropyl , sec . butyl or tert. butyl group .

This invention also provides the compound α-C-4-hydroxyphenylN-tert. butyl nitrone melting at 212º -215ºC ; the compound α-C-4hydroxy-3 , 5-dimethylphenyl-N-tert . butyl nitrone melting at 178º 180ºC ; the compound α-C-phenyl-N-tert. butyl nitrone melting a t 75º -77ºC; the compound α-C-4-hydroxy-3 , 5-dimethylphenyl-Nisopropyl nitrone melting at 192º -195º C ; α-C-4-methoxyphenyl-Nisopropyl nitrone (as a red , glassy solid) ; and the compound α-C-4-chlorophenyl-N-isopropyl nitrone melting at 183º -185ºC .

The compounds of the invention may be prepared in a variety of ways ; for example :

(a) by oxidation of the corresponding C-substituted or unsubstituted aryl-N-substituted or unsubstituted

(branched alkyl or cycloalkyl) hydroxylamine;

(b) by reaction of a substituted or unsubstituted aryl ketone or aldehyde, especially an aldehyde, with a primary N-substituted or unsubstituted (branched alkyl or cycloalkyl) hydroxylamine (there materials are

Schiff 's bases ) ;

(c) by N-alkyla tion of the corresponding oxime ;

(d) by reaction of the corresponding ketimine and primary hydroxylamine ; and (e) by oxida tion of the corresponding N-sub stituted imine.

Further details of these processes may be elucidated by analogy with those disclosed by Hammer et al in Chem. Rev . 64 , 474 (1964) .

According to a further aspect of this invention, there is provided a compounding which comprises at least one compound of the invention as herein described in intimate admixture with a rubber . The rubber may comprise natural rubber, polybutadiene , polyisoprene, chloroprene , butyl rubber, ABS, nitrile rubber, ethylene-propylene rubber , the toughening phase in high-impact polystyrene or polyacrylonitrile . The compound may comprise from 0.1 to 10, preferably, 0.5 to 6, especially 1 to 4 pph of the rubber.

In accordance with a preferred feature of this invention, the compound is chemically bound to the rubber, preferably by the method disclosed in UK 1503501. It is especially preferred to premill the additive, in the absence of oxygen, with the rubber. In accordance with a particularly preferred feature of this invention there is provided a process for preparing a compounding as aforesaid, which process premilling a compound of the invention with the rubber for a time not exceeding 10 minutes, preferably not exceeding 6 minutes, particularly preferably not exceeding 2 minutes. Preferably the premilling is effected in the presence of oxygen. Where it is desired to utilise this feature with the abovementioned chemical binding it is necessary to effect the former procedure first.

This invention further provides a compounding prepared by this process.

This invention also provides a rubber compounding as herein described which has been vulcanised. This invention provides a synergistic mixture of a nitrone of this invention and a rubber antioxidant.

The following Examples illustrate the invention.

EXAMPLE 1 3,5-Dimethyl-4-hydroxybenzaldehyde was synthesised by the formylation of 2,6-xylenol according to the procedure described by Nikiforov et al (Bull. Acad. Sci. USSR div. Chem. Sci. p. 559 (1965)). 12.2 g (0.1 moles) of 2,6-xylenol was used together with 35 g of boric acid, 25 g of hexamethylenetetraamine and 100 ml ethylene glycol. The yield was 10.5 g (70% of theoretical). Melting point: 110-113°C.

2-Methyl-2-nitropropane was prepared by the oxidation of tertiarybutylamine using hydrogen peroxide according to the procedure described by Stowell (J. Org. Chem., 36, p. 3055 (1977)). 36.6 g (52 ml); 0.5 moles) of tertiarybutylamine was used. The yield was 25.8 g (0.293 moles; 59% of theoretical). Boiling point 126-128°C. The corresponding hydroxylamine was then synthesised by the reduction of the 2-methyl-2-nitropropane using zinc dust and ammonium chloride according the method described by Greene et al (J. Org. Chem., 34, p. 2269 (1969)). 25 g (0.28 moles) of the 2-methyl-2-nitropropane was used. The yield was 18.2 g

(0.204 moles; 73% of theoretical). Melting point: 60-62ºC.

The nitrone α-C-4-hydroxy-3,5-dimethylphenyl-N-tert. butyl nitrone was synthesised by the condensation reaction of the aldehyde with the hydroxylamine. N-tertiarybutyl-hydroxylamine (3.0 g; 0.033 mole) and 3,5-dimethyl-4-hydroxybenzaldehyde (4.95 g; 0.033 mole) were dissolved in the minimum volume of absolute ethanol and allowed to stand at room temperature for a few days. Colourless crystals were formed. The crystals were separated by filteration and recrystallised from ethanol. Yield: 4.2 g (0.190 mole; 58% of theoretical). Melting point: 178-180°C. Analytical data: (i) calculated for C₁₃H₁₉NO₂ element C H N expected 70.6 8.6 6.3 found 71.0 9.2 6.5

(ii) Mass spectroscopy:

Molecular ion at m/e = 221. (iii) Infrared analysis (KBr disc)

3100-3300 cm^-1 (broad OH stretch); 1590 cm^-1 (aromatic C-C stretch); 1570 cm^-1 (C=N stretch); 1160 cm^-1 (N-O stretch); disappearance of the carbonyl stretch of the parent aldehyde at 1680 cm^-1.

N.B. typical literature values C=N of nitrones appear at 1560-1580 cm^-1(84); and the N-O stretch appear at 1150-1270 cm^-1(84).

(iv) N.M.R. analysis: (in acetone d ).

1.5 δ(S; 9H; N- Butyl protons); 2.2 δ(S; 6H; ring methyl protons); 7.5 δ(S; 1H; -CH = N(O)-); 8.0 δ(S; 2H; aromatic protons). S = singlet. EXAMPLES 2 to 6 The following nitrones were prepared in essentially the same manner .

Example 2 α-C-4-hydroxyphenyl-N-tert. butyl nitrone. Yield: 30% ; melting point; 212-215°C.

Example 3 α-C-phenyl-N-tert . butyl nitrone . Yield: 70%; melting point; 75-77ºC .

Examples 4 α-C-4-hydroxy-3 ,5-dimethylphenyl-N-isopropyl nitrone. Yield: 58%; melting point: 192-195ºC. Example 5 α-C-4-methoxyphenyl-N-isopropyl nitrone .

Example 6 α-C-4-chlorophenyl-N-isopropyl nitrone. Melting point : 183-185ºC .

EXAMPLE 7

100 g of natural rubber (SMR, 10) was premilled on a two roll open mill for 2 minutes with 1 g of the nitrone of Example 1; and was then compounded, in order shown, with: zinc oxide: 5.0 g stearic acid: 3.0 g sulphur: 2.5 g CBS: 0.6 g

The initial Wallace Plasticity (Po) was next determined at 120ºC using a Wallace Plastimeter.

The experimental run was repeated premilling for 4 minutes, and also for 6 minutes. In all cases the premilling and compounding time totalled 15 minutes.

The vulcanising, antifatigue and antiozonant properties are shown, and compared with those of a control and a sample comprising IPPD, in the following Tables. Referring to Table 3, the colour rating (arbitrary scale) of the sample containing the additive of Example 2, at 20% strain, was 2, compared with 8 for the control and IPPD samples at the same strain.

EXAMPLE 8 Example 7 was essentially repeated using the nitrone of Example 4. The maximum torque was 60 lbs^-1; the number of hours to break was 130; and the time of ozone exposure to failure was 70 hours. The colour rating was 3.

EXAMPLE 9 Example 7 was essentially repeated using the nitrone of Example 4 in admixture with 1 g of Nonox WSP:

The antifatigue and static antiozonants results are shown in Table 4.

This Example illustrates the complementation effect of added thermal antioxidant: while this addition somewhat reduced the time of exposure to failure, it will be seen that it considerably enhanced the time to break.

Claims

1. A C-substituted or unsubstituted aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) nitrone having antifatigue and/or antiozonant properties in rubber.

2. A compound according to Claim 1 of the formula:

wherein:

R₁ represents a (+ I) or a (+ M) group;

R₆ represents a substituted or unusubstituted (branched alkyl or cycloalkyl) group.

3. A compound according to Claim 1 or 2 wherein R₁ represents an unsubstituted or alkyl substituted hydroxy group or an unsubstituted or mono- or di-alkyl substituted amino group.

4. A compound according to any preceding claim wherein R₂ and

R₃, which may be the same or different, each represent an unsubstituted C₁ to C₅ alkyl or alkoxy group.

5. A compound according to Claim 4 wherein R₂ and R₃, which may be the same or different, each represent an unsubstituted C₁ to C₄ alkyl group.

6. A compound according to any preceding claim wherein R₂ and R₃ are the same.

7. A compound according to any preceding claim wherein at least one of R₄ and R₅ represents a hydrogen atom.

8. A compound accoridng to any preceding claim wherein R₆ represents an isopropyl, sec. butyl or tert. butyl group.

9. The compound:

10. The compound:

11. A compounding which comprises at least one compound according to any preceding claim in intimate admixture with a rubber.

12. A rubber compounding according to Claim 11 wherein the rubber comprises natural rubber, polybutadiene, polyisoprene, chloroprene, butyl rubber, ABS, nitrile rubber, ethylene-propylene rubber, the toughening phase in high-impact polystyrene or polyacrylonitrile.

13. A compounding according to Claim 11 or 12 wherein the nitrone comprises from 0.1 to 10 pph of the rubber.

14. A compounding according to any of Claims 11 to 13 wherein the nitrone is chemically bound to the rubber.

15. A process for preparing a compounding according to any of Claims 11 to 14 which process comprises premilling a compound according to any of Claims 1 to 10 with the rubber for a time not exceeding 10 minutes.

16. A compounding prepared by the process of Claim 15.

17. A rubber compounding according to any of Claims 11, 12, 13, 14 or 16 which has been vulcanised.

18. A mixture of a nitrone according to any of Claims 1 to 10 and a rubber antioxidant.