STABILISED RUBBER COMPOSITIONS

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 incor- 05 po 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 ^-phenylene- diamines,. notably the N_-aryl , N_' -alkyl-substituted £-phenylene- diamines . The pre-eminent example is N--phenyl, N -isopropyl-p_- 10. phenylenediamine (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 15 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 0 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 5 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 0 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 anti- fatigue and/or antiozonant properties in rubber. Particularly effective such compounds are those aldonitrones of the formula: wherein: represents a (+ I) or a (+ M) group; R„ and R , which may be the same or different, each represent a substituted or unsubstituted (alkyl or alkoxy) group; R, and R , which may be the same or different, each represent a hydrogen atom or a substituted or unsubstituted (alkyl or alkoxy) grou ; and R represents a substituted or unsubstituted (branched alkyl 6 or cycloalkyl) group. For example may represent an unsubstituted or alkyl substituted hydroxy group or an unsubstituted or mono- or di-alkyl substituted amino group; or and R , which may be the same or different, each represent an unsubstituted C to C alkyl or alkoxy group, preferably each represents an unsubstituted C, to C. 1 4 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 and R are the same. It is also preferred that at least one of R, and represents a hydrogen atom. O PI R may suitably be a branched alkyl group , preferably a C to 6 3 C branched alkyl group. It is particularly preferred that 6 6 represents an isopropyl , sec . butyl or tert. butyl group . This invention also provides the compound αr-C-4-hydroxyphenyl- N--tert. butyl nitrone melting at 212 -215 C ; the compound α-£-4- hydroxy-3 , 5-dimethylphenyl-N--tert . butyl nitrone melting at 178 - 180 C ; the compound α-O-phenyl-N-tert. butyl nitrone melting a t 75 -77 C; the compound α-C_-4-hydroxy-3 , 5-dimethylphenyl-N_- isopropyl nitrone melting at 192 -195 C ; α- -4-methoxyphenyl-N- isopropyl nitrone (as a red , glassy solid) ; and the compound α-£-4-chlorophenyl-N_-isopropyl nitrone melting at 183 -185 C . The compounds of the invention may be prepared in a variety of ways; for example : Ca) by oxidation of the corresponding £-substituted or unsubstituted aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) hydroxyla ine; (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 _e_t_ 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 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 pro¬ cedure 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. OMPI 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 e_t al. (J. Org. Chem., 34_, p. 2269 (1969)). 25 g (0.28 moles) of the 2-meth l-2-nitropropane was used. The yield was 18.2 g (0.204 moles; 73% of theoretical). Melting point: 60-62 C. The nitrone α-£-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-dimeth l-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 c l3 H 19 N0 2 element C H N expected 70.6 8.6 6.3 found 71.0 9.2 6.5 (ii) Mass spectroscop : 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~ (C=N stretch); 1160 cm " (N-0 stretch);, disappearance of the carbonyl stretch of the parent aldehyde at 1680 cm N.B. typical literature values C=N of nitrones appear at 1560-1580 cm " ; and the N-0 stretch appear at 1150-1270 cm "1(84) . (iv) N.M.R. analysis: (in acetone d ). t 6 1.5 δ(S; 9H; N- Butyl protons); 2.2 <5(S; 6H; ring methyl protons); 7.5 δ(S; 1H; -CH = * N(0)-); 8.0 δ(S; 2H; aromatic protons). S = singlet. OMPI EXAMPLES 2 to 6 The following nitrones were prepared in essentially the same manner . Example 2 α-£-4-hydroxyphenyl-N-tert. butyl nitrone. Yield: 30%; melting point; 212-215° . Example 3 c_-£-phenyl-N-tert. butyl nitrone . Yield: 70%; melting point; 75-77 C . Examples 4 α-£-4-hydrσxy-3 ,5-dimethylphenyl-N-isopropyl nitrone. Yield: 58%; melting point: 192-195 C. Example 5 α-£-4-methoxyphenyl-N-isopropyl nitrone . Example 6 α-C-4-chlorophenyl-N-isoprop l nitrone. Melting point : 183-185 C . 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 120 C using a Wallace Plasti eter. 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. TABLE 1 TABLE 2 OMP- i/Λ , IPO " - > TABLE 3 ADDITIVES TIME OF OZONE EXPOSURE (hr) PRE¬ MILLIN Sub-scripts CONC 10 15 20 25 30 35 40 45 50 55 60 65 70 75 (min) refer to (phr) strain (%) Control 0 0 0 1 4.5 5 0 10 Control 0 0 0 0 2 5 0 20 Control 0 0 0 0 0 0 1 2 3 5 0 30 Ex.2 1.0 0 0 0.25 0.25 0.25 00..55 2. 5 4.0 5 2 10 Ex.2 1.0 0 0 0 0.5 0.5 0.5 0.5 2.5 3 3 4 - 5 2 20 Ex.2 1.0 0 0 0 0 0 0 0 0 0 0.5 0.5 1.5 30 3.5 2 Ex.2 1.0 0 0 0.25 0.25 0.25 0.5 2.5 4.0 5 4 10 Ex.2 1.0 0 0 0 0.5 0.5 1.5 2 3 3 3.5 5 4 20 Ex.2 1.0 0 0 0 0 0 0 0 0 0 0.5 0.5 2 3.5 4 30 Ex.2 1.0 0 0 0.25 0.25 0.25 3 5 6 10 Ex.2 1.0 0 0 0 0.5 0.5 2 2.5 3.5 5 6 20 Ex.2 1.0 0 0 0 0 0 0 0 0 0 1.5 3 3.5 3.5 6 30 Notation 0 No cracks 1 Fine cracks (ca. 0.5 or 1.0 mm) visible under x5 magnification EXAMPLE 8 Example 7 was essentially repeated using the nitrone of Example 4. The maximum torque was 60 lbs ; 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. 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. 11B

Claims: CLAIMS 1. A £-substituted or unsubstituted aryl-N-substituted or unsubstituted (branched alkyl or cycloalkyl) nitrone having anti¬ fatigue and/or antiozonant properties in rubber. 2. A compound according to Claim 1 of the formula: wherein: represents a (+ I) or a.(+ M) group; R and R , which may be the same or different, each represent a substituted or unsubstituted (alkyl or alkoxy) group; R and R , which may be the same or different, each represent a hydrogen atom or a substituted or unsubstituted (alkyl or alkoxy) group; and R represents a substituted or unusubstituted (branched alkyl 6 or cycloalkyl) group. 3. A compound according to Claim 1 or 2 wherein R. 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 R and R , which may be the same or different, each represent an unsubstituted C to C alkyl or alkoxy group. 5. compound according to Claim 4 wherein R and R , which may be the same or different, each represent an unsubstituted C, to C, 1 4 alkyl group. 6. A compound according to any preceding claim wherein and R are the same. 7. A compound according to any preceding claim wherein at least one of R, and R,. represents a hydrogen atom. 8. A compound accoridng to any preceding claim wherein R fi represents an isopropyl, sec. butyl or tert. butyl group. 9. The compound: melting at 195 C 10. The compound: melting at 180 C 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. OMPI 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.

Inventors: NETHSINGHE LAKSHMAN PREMAL, SCOTT GERALD, NETHSINGHE, LAKSHMAN, PREMAL, SCOTT, GERALD
Applicants: NETHSINGHE LAKSHMAN PREMAL, SCOTT GERALD, NETHSINGHE, LAKSHMAN, PREMAL, SCOTT, GERALD
Application year: 1984
Main classification: C07C135
IPC codes: C07C67, C07C239, C07C291, C08K5, C08L7, C08L21