GB2310858A - Inhibiting nitrosamine formation in rubber - Google Patents

Description

INHIBITING NITROSAMINE FORMATION IN RUBBER This invention relates to inhibiting nitrosamine formation during the vulcanisation of rubber.

Aliphatic N-nitrosamines are known to be carcinogenic in some animals. Significant levels of volatile nitrosamines have been detected in factories manufacturing rubber products both in the products themselves and in the ambient air. Since workers in the rubber industry have been shown to suffer from an increased incidence of certain types of cancer, it has been suggested that nitrosamines contribute to this increase. Concern has also been expressed about nitrosamine levels in certain types of rubber product, more particularly those which come into physical contact with users and those which are used in confined spaces.

Legislation already exists restricting nitrosamine levels in rubber products used in baby bottle teats and comforters. There are also environmental guidelines about the maximum permissible levels of nitrosamines in rubber manufacturing premises.

In these circumstances, there is an evident need to reduce as much as possible the formation of nitrosamines during rubber vulcanisation. It is believed that nitrosamines are generated by the reaction of nitrogen oxides (NOx) with secondary amines (or secondary amine residues) present in the rubber formulation. It appears likely that the NOX is already present in the ambient atmosphere (produced in vehicle exhausts and/or by industrial combustion). The nitrogen oxides are absorbed by the fillers customarily used in rubber (carbon black or silica) and are thus brought into contact, during the vulcanisation, with secondary amines generated by many common accelerators used in rubber vulcanisation. Examples of such accelerators containing secondary amine residues include: N-Morpholinyl-benzothiazole-2-sulphenamide N, N'-di-isopropyl-benzothiazole-2-sulphenamide 4-Morpholinyl-2-benzothiazole disulphide N-Oxydiethylenethiocarbamyl-N'-oxydiethylene suiphenamide Tetramethylthiuram disulphide Tetraethylthiuram disulphide Zinc dimethyldithiocarbamate Zinc diethyldithiocarbamate, and Dithiodimorphol ine Other constituents of rubber formulations may also give rise to secondary amines capable of forming nitrosamines during vulcanisation.

There have been various proposals of means for reducing or preventing nitrosamine formation during rubber vulcanisation. For example, it should be possible to produce a vulcanisable formulation which does not include any ingredient capable of giving rise to secondary amine residues. While this approach seems to provide the best long term chance of success, it has been found to be difficult to implement because some accelerators containing secondary amine residues have found no satisfactory replacements.

Removal of the nitrosating species NCX from the industrial environment also reduces or eliminates nitrosamine formation, but while it is possible to reduce NOx level, for example by keeping internal combustion engines and their exhaust products well away from the vulcanisation zone, it is not practical to reduce NOX levels to zero. More particularly, the fillers used in rubber are capable of adsorbing NOX during their production, normally in an environment entirely outside the control of the rubber manufacturer.

Similarly, improved ventilation in the rubber vulcanisation zone can reduce the exposure of the rubber workers to nitrosamines. However, this approach does not improve the position of the end user of the rubber product.

In these circumstances, the possibility of including in the rubber formulation products which inhibit the formation of nitrosamines has been studied. Such products may act by removing the NOX or the secondary amine from the formulation.

Thus vitamin E (Ronotec 200 of Hoffmann-La Roche) is sold as a rubber additive for this purpose; however, this can have a disadvantageous effect on the physical properties of the rubber on ageing. It has also been proposed (USP 5070130 assigned to B.F. Goodrich Co.) to incorporate alkaline earth metal oxides or hydroxide into vulcanisable formulations to reduce nitrosamine formation.

It has now been found that alkaline earth metal carboxylates and phenates can advantageously be incorporated into vulcanisable rubber formulations to reduce nitrosamine formation during vulcanisation. Such carboxylates and phenates should desirably have a melting point below 1300C in order to be fully compatible with the rubber formulation during the vulcanisation process.

The present invention accordingly provides vulcanisable rubber formulations comprising an alkaline earth metal carboxylate or phenate in an amount sufficient to reduce or inhibit nitrosamine formation. As already indicated, such rubber formulations may comprise a vulcanisation accelerator containing secondary amine residues such as one of those mentioned above.

Currently preferred alkaline earth metal carboxylates include carboxylates derived from aliphatic and alicyclic acids with 4 to 22 carbons, especially 8 to 20 carbons, or from aromatic acids with 7 to 12 carbons, and from calcium, barium and magnesium, especially barium. Suitable phenates include those derived from aromatic alcohols of 6 to 12 carbons.

Preferred materials include calcium, barium and magnesium stearate, especially barium stearate, and barium neodecanoate as well as magnesium and calcium phenate. The carboxylate or phenate employed should generally have a melting point not exceeding 1300C.

It has also been found that the addition of calcium oxide as well as the carboxylate/phenate enhances the effect of the latter significantly. Thus an about 40% reduction in nitrosamine concentration due to the addition of barium carboxylate can be increased to about 70% with the addition of calcium oxide as well.

The rubber formulation may be of any known kind and based on, for example, natural rubber, styrene butadiene rubber, nitrile rubber, polybutadiene, polychlorophene or EPDM rubber, or any mixture of these, especially EPDM rubber.

The alkaline earth metal carboxylate or phenate is suitably incorporated in the vulcanisable formulation at a level of 0.05 to 5 parts, especially 0.5 to 2 parts, per 100 parts by weight of rubber with, optionally, 1 to 20 parts, especially 2 to 10 parts, per 100 parts by weight of the rubber, of calcium oxide. They may be incorporated as pure compound, diluted in a process oil, or supported on a high surface area solid, e.g. silica filler. The manner of providing the alkaline earth metal carboxylate or phenate and, optionally, calcium oxide is chosen to provide ease of handling and mixing with the other ingredients of the formulation.

The vulcanisable formulation also contains other ingredients conventionally present in vulcanisable formulations including especially the usual carbon black or silica filler and other materials such as zinc oxide, stearic acid, antioxidant....

The vulcanisation itself is brought about in the usual manner by incorporating in the formulation an accelerator/ sulphur mixture and curing the formulation at elevated temperature in the usual way.

The following Examples illustrate the invention.

Example 1 A rubber master batch was prepared by mixing together: Natural Rubber (SMR CV) 100 parts by weight Carbon Black (FEF 550) 45 Enpar 16 4 Zinc Oxide (Red Seal) 5 Stearic Acid 2 Flectol Pastilles 1 Santoflex 13 3 Antilux 600 3 These ingredients were mixed on a Banbury mixer to a temperature of at least 1200C. The accelerator/sulphur mixture (sulphur 0.33 phr, MOR 3 phr, Tetramethylthiuram disulphide 2 phr) was then added on a 2-roll mill and the formulation was then vulcanised.

Calcium stearate, magnesium stearate, barium stearate or calcium phenate was added to the formulation in a proportion of 2.0 phr. For comparison vulcanisation was also carried out with no additive and with 0.5 phr of Ronotec 200. Nitrosamine levels in the cured product were measured by the following procedure using a thermal energy analyser: After vulcanisation a portion of the vulcanised material was cut into 1 x 1 cm squares. A 3 g portion of the squares was weighed and placed in a cellulose thimble for extraction. The sample was then extracted with methanol as follows:

Step 1: Place the cellulose thimble containing the vulcanisate into a Soxhlet Extractor containing 50 ml of methanol containing 0.1 % ascorbic acid. Fit the extractor onto a 150 ml round bottom flask. Step 2: After 24 hours extraction in cold methanol, add a further 30 ml through the Soxhlet. Then reflux over a water bath for 1 hour. Step 3: After cooling, transfer the extract into a volumetric flask and make up to 100 ml. Step 4: Pipette 25 ml of the extract into a glass vial and seal with a crimped cap and PTFE lined rubber septum and were stored in the dark at OOC prior to their analysis.

Two portions of the extract were subjected to nitrosamine analysis as follows: The methanolic extract (10 ml) was diluted with water (90 ml) and treated with an internal standard (N-nitrosodipropylamine, 41 ng in ethanol, 0.5 ml), sodium chloride (10 g) and dichloromethane (20 ml)). The mixture was shaken and allowed to separate. The dichloromethane layer was recovered, dried (sodium sulphate) and evaporated to convenient small bulk. A part of the concentrate was subjected to gas-chromatography on a glass column packed with Carbowax 20M on Chromosorb P and a chromatogram developed at 1450C (carrier gas argon, ca.

20 ml/min) and the exit gas flow was examined by a thermal energy analyser (TEA). In this detector the eluting species were pyrolysed at 4750C, substances other than nitric oxide were frozen out, and the nitric oxide was mixed with ozone to give a chemiluminescent reaction which was measured by means of a photomultiplier tube. The output of the detector was recorded and the ratio of the response area due to any nitrosamine to that due to the internal standard is calculated. A series of analyses was made in which known quantities of individual nitrosamines were added to water containing 10% of methanol and the solutions subjected to the analysis described above. The response area ratios for each nitrosamine are linear with respect to mass of nitrosamine. From the lines so obtained, the mass of each nitrosamine corresponding to its observed response was calculated.

The results obtained are shown in Table 1.

TABLE 1 Compounds pp billion Nitrosamines NDMA NMOR Total %Reduction (increase) Blank 303 59 362 Ronotec 200 (0.5 phr) 150 30 180 50.3 Calcium stearate (2.0 phr) 235 87 322 11.0 Calcium Oxide (2.0 phr) 431 46 477 (31.7) Magnesium Stearate (2.0 phr) 195 83 278 23.2 Magnesium Oxide (2.0 phr) 352 95 447 (23.5) Barium Stearate (2.0 phr) 129 51 180 50.3 Barium Oxide (2.0 phr) 213 98 311 14.1 Barium Hydroxide (2.0 phr) 228 106 334 7.3 Calcium Phenate (2.0 phr) 225 38 263 27.3 NDMA = nitrosodimethylamine NMOR = nitrosomorpholine In further experiments barium stearate was used at different levels with the following results: TABLE 2 Compounds pp billion Nitrosamines NDMA NMOR Total %Reduction Control 156 88 244 Barium Stearate (0.5 phr) 62 18 80 67.2 Barium Stearate (2.0 phr) 81 25 106 56.5 Barium Stearate (6.1 phr) 67 15 82 66.4 It is to be noted that the absolute values of nitrosamines measured vary from experiment to experiment but the percentage reduction in nitrosamine level caused by the presence of the alkaline earth metal carboxylate or phenate is essentially constant.

Example 2 An EPDM compound was prepared using the following basic formulation: Parts EPDM 100 Carbon Black N550 25 Zinc Oxide 5 Stearic acid 2 Sulphur 2 MBT 2 TMTD 1.25 Varying amounts of calcium oxide and barium neodedanoate were incorporated separately or together. The formulation was prepared as follows: - The carbon black, stearic acid, zinc oxide, EPDM (and calcium oxide and/or barium neodecanoate if used) were mixed together in an internal mixed.

- The sulphur, MBT and TMTD were added to the previously mixed compound on a 2-roll mill and mixed.

The formulation was vulcanised at 1700C with a cure time of Tw as measured on a Monsanto Rheometer.

The vulcanised sample was extracted and analysed for nitrosamines as in Example 1.

The results are shown in Table 3. Since we have shown that simply increasing the amount of inhibitor does not lower the nitrosamine level (Table 2) there is no reason to expect the effect of the calcium oxide and barium neodecanoate to be additive. Thus the increase from -40% to -70% inhibition when the two compounds are used together is unexpected.

Table 3 Compound pp billion Nitrosamines NDMA %Reduction Blank 104 Calcium oxide (10 phr) 57 45.2 barium neodecanoate (0.5 phr) 65 37.5 Calcium oxide + barium neodecanoate 34 67.3 (10 phr + 0.5 phr, premixed) Calcium oxide (10 phr) + barium neodecanoate (0.5 phr) 30 71.1 The alkaline earth metal carboxylates or phenates used in the present invention may be produced in known manner and some are commercially available. Barium searate may be, for example, produced as follows: Stearic acid (273 g; 1 mole) was added slowly with stirring to a mixture of barium oxide (51.1 g; 0.33 mole), xylene (500 g), water (5 g) and hexyl capitol (50 gj heated at 1000C. The temperature was then raised to 1500C as the aqueous distillate was collected in a Dean & Stark trap. Once the water was removed the temperature was increased to -1700C as the remaining xylene was removed, finally under vacuum. The product was allowed to cool to give a waxy solid, melting point 900C.

Barium neodeconoate may be prepared as follows: Barium hydroxide ocathydrate (252.4 g; 0.8 mole) was added quickly with stirring to a mixture of neodecanoic acid (287.0 g; 1.64 mole), dioxitol (36.6 g) and mineral oil (300 g) heated to 1000C. The temperature was then raised to 1800C under vacuum (28" Hg) to remove water formed in the reaction. Once the water was removed the temperature was decreased to 70-800C and CO2 was bubbled through the product (7.5 litres/hour for 8 hours = 2.68' mole). The product was filtered under pressure and then allowed to cool to give a clear, dark amber, viscous liquid.

Claims (11)

1. A vulcanisable rubber formulation comprising an alkaline earth metal carboxylate or phenate in an amount sufficient to reduce or inhibit nitrosamine formation.

2. A formulation according to claim 1 comprising a vulcanisation accelerator containing secondary amine residues.

3. A formulation according to claim 2 comprising one or more of N-Morpholinyl-benzothiazole-2-sulphenamide N,N' -di-isopropyl-benzothiazole-2-sulphenamide 4-Morpholinyl-2-benzothiazole disulphide N-Oxydiethylenethiocarbamyl-N' -oxydiethylene sulphenamide Tetramethylthiuram disulphide Tetraethylthiuram disulphide Zinc dimethyldithiocarbamate Zinc diethyldithiocarbamate, and Dithiodimorpholine.

4. A formulation according to any one of claims 1 to 3 comprising an alkali metal carboxylate or phenate having a melting point not exceeding 130"C.

5. A formulation according to claim 4 comprising a calcium, barium or magnesium stearate or phenate.

6. A formulation according to any one of claims 1 to 5 comprising 0.05 to 5 phr of the said carboxylate or phenate.

7. A formulation according to any one of claims 1 to 6 comprising calcium oxide.

8. A formulation according to claim 7 comprising up to 10 phr of calcium oxide.

9. A formulation according to any one of claims 1 to 8 comprising a carbon black or silica filler.

10. A formulation according to any one of claims 1 to 9 in which the rubber is natural rubber, styrene-butadiene rubber, nitrile rubber, polybutadiene, polychloroprene, EPDM rubber or a mixture of any of these.

11. Vulcanised rubber obtained by vulcanising a vulcanisable formulation as claimed in any one of the preceding claims.