GB2102413A - Bisperoxycarbamates, their preparation and use as crosslinking agents - Google Patents

Abstract

The invention provides compounds of the general formula <IMAGE> in which R1 represents a carbon-based linking group which may incorporate one or more hetero atoms and R2 and R3, which may be the same or different, each represents an aliphatic or cycloaliphatic group which may be unsaturated or saturated, unsubstituted or substituted, a straight chain or a branched chain an aryl group which may be unsubstituted or substituted or an araliphatic or aliphaticaryl group which may be unsubstituted or substituted and the aliphatic group of which may be unsaturated or saturated, a straight chain or a branched chain, a process for their preparation and their use as crosslinking agents, e.g. for plastics and rubbers. The compounds also act as chemically-bound antioxidants. <IMAGE>

Description

SPECIFICATION Bisperoxycarbarnates, process for their preparation and use thereof The present invention relates to bisperoxycarbamates, a process for their preparation and their use.

The use of organic peroxides in the crosslinking of unsaturated rubbers is common when, for example, maximum heat stability is required in the vulcanisate. The use of peroxide and other free radical type chemical reactions, for example high energy radiations, has also been proposed for the vulcanisation of many saturated rubbers. Free radical reactions are also often the only practical method of crosslinking plastics which usually have saturated chemical structures. Anti-oxidants also have to be used in conjunction with the crosslinking agents; the antioxidants have to be carefully selected so as not to interfere with the curing process.

In the case of rubbers crosslinked with peroxides and the like, the resulting vulcanisates often have disadvantages of poor strength and resistance to cyclic deformation fatigue when compared with comparable vulcanisates obtained by sulphur, metal oxide, diamine and other crosslinking reactions.

Decomposition products produced during vulcanisation often remain undesirably free in the prepared rubbers and plastics; no bonding to the polymer chains occurs. Unpleasant odours are also associated with the use of peroxides.

Dicumyl peroxide is the main peroxide in use in the vulcanisation of rubbers etc. and is the only crosslinking agent that can be used for curing carbon black-filled silicone rubber. In the latter case, the peroxide cures the rubber very slowly and causes the production of porous vulcanisates.

Most studies using peroxycarbamates have been concerned with their synthesis, decomposition and use as initiators in vinyl or diene polymerisation. A monoperoxycarbamate, N-di-methyltertbutylperoxycarbamate, has been suggested by Obrien eft at J. Amer. Chem. Soc., 81, (1959), 1506; for use as a free radical generator, in combination with free radical acceptors in crosslinking a thermoplastic vinylidine fluoride copolymerised with hexafluoropropane.

The present invention provides a compound of the general formula

in which R, represents a carbon-based linking group which may incorporate one or more hetero atoms and R2 and R3, which may be the same or different, each represents an aliphatic or cycloaliphatic group which may be unsaturated or saturated, unsubstituted or substituted, a straight chain or a branched chain an aryl group which may be unsubstituted or substituted or an araliphatic or aliphaticaryl group which may be unsubstituted or substituted and the aliphatic group of which may be unsaturated or saturated, a straight chain or a branched chain.

The compounds of the present invention have been found to be suitable for use as vulcanising agents such as for vulcanising unsaturated rubbers or diene rubbers.

The radical represented by R, is suitably a carbon-based linking group comprising one or more aliphatic, cycloaliphatic, aliphaticcycloaliphatic, cycloaliphaticaliphatic, aromatic, aliphaticaromatic and aromaticaliphatic groups. Each group may be unsubstituted or substituted and/or uninterrupted or interrupted by one or more hetero atoms, for example nitrogen, oxygen and sulphur atoms. Each group may also be unsaturated or saturated and the aliphatic group, where present, may be straight chained or branched chained.

The radical R1 may be a carbon-based chain linking the two peroxycarbamate groups. Radical R, may also be in the form of a carbon-based ring or rings of an aliphatic or aromatic nature linking the two groups or a mixture of chain and ring linking groups. Where a mixture of groups is present they suitably form a linking group which is symmetrical in nature. A carbon-based chain, where present, may incorporate one or more carbonyl groups.

Suitably the number of carbon atoms in the radical R, does not exceed 20; preferably radical R, contains from 6 to 8 carbon atoms.

Aliphatic radicals R, are suitably straight-chained, saturated alkylene radicals; hexylene may be especially mentioned. Cycloaliphatic-based radicals R, are suitably saturated, substituted cycloalkylene radicals; cyclohexylene-based radicals are preferred. Aromatic-based radicals R, are suitably aryl radicals, preferably phenyl radicals.

As substituents suitable for radicals R1, R2 and R3, alkyl radicals and aryl radicals are preferred.

Especially preferred are methyl and phenyl substituents. One or more of the same or different substituents may be present.

Radical R, is preferably a hexylene, cyclohexylene, trimethyl-substituted cyclohexylenemethylene or methylene bis(cyclohexylene) radical.

The radicals represented by R2 and R3 are suitably the same. Each preferably represents an unsubstituted or substituted, straight chain or branched chain alkyl group. It is especially preferred that radicals R2 and R3 are the same and each represents a tert-butyl group which is unsubstituted or substituted by a phenyl group.

The following Table indicates suitable compounds of the invention.

TABLE

Methylene-bis(cyclohexyl)-N,N'-bis(tert-butylperoxycarbamate), A above, is an especially preferred compound of the invention.

The compounds of the invention differ in action from the peroxide free radical crosslinkers, typified by dicumyl peroxide, by chemically combining with the rubber molecule to form a direct crosslink whose relatively high molecular weight and long linear structure provide greater flexibility, in respect of cracking and vibration, and dynamic fatigue resistance than the short chain C-C peroxide crosslinks. They have no unpleasant odour associated with their use and they can be used to cure rapidly carbon black-filled silicone rubber and allow the use of non-pressurised hot air tunnels for vulcanisation.

Surprisingly, the bisperoxycarbamates of the invention play a dual role during vulcanisation. By bonding to the polymer chains during crosslinking and, in fact, forming the crosslinks, the bisperoxycarbamates provides anti-degradent properties for the polymer. The compounds of the invention act as a chemically-bound anti-oxidant and impart a longer life to rubbers and plastics cured with them without the need for the addition of anti-oxidants as is required with other crosslinking agents.

Other advantages found when using the compounds of the invention as crosslinking agents are that the polymers have a better tensile strength and have better heat ageing properties.

It has been found useful to use the bisperoxycarbamates of the invention in conjunction with one or more absorbents capable of absorbing any carbon dioxide and water which may be formed during crosslinking. Such an additive further reduces the porosity in the crosslinked product.

Accordingly, the present invention also provides a composition suitable for use as a crosslinking agent which comprises a compound of the invention together with a suitable carrier. Preferably the composition contains one or more compounds capable of absorbing carbon dioxide and/or water.

The present invention further provides a method of crosslinking polymer chains which comprises using a compound or a composition of the invention as a crosslinking agent.

The present invention also provides a process for the preparation of a compound of the invention which comprises reacting a diisocyanate of the general formula OCN-R1-NCO (II) in which R, is as defined above with a hydroperoxide of the general formula HOOR2 (Ill) wherein R2 is as defined above or a hydroperoxide of the general formula HOOR3 (IV) wherein R3 is as defined above or a mixture of two such compounds wherein the molar ratio of diisocyanate to hydroperoxide or hydroperoxide mixture is at least 1:2.

The process of the invention is suitably carried out in the presence of an organic solvent and/or a catalyst. Suitable organic solvents are benzene and trichlorobenzene. Suitable catalysts are triethylamine and pyridine. Hydroperoxide in excess of the specified ratio may be used.

The following Examples illustrate the invention. Unless otherwise specified, percentages are given on a weight basis. The abbreviations "NR" and "IR" used in the Examples denote natural rubber and polyisoprene rubber respectively.

Examples of preparation A. Methylene-bis(cyclohexyl )-N,N'-bis(tert-butylperoxyearbamate) 262 g of methylene-bis(4-cyclohexyl isocyanate) were first dissolved in approximately 3.5 litres of benzene. 26.5 g of triethylamine were then added to the solution. 21 6 g of tertiary butyl hydroperoxide were added slowly to the solution from a dropping funnel. The dropping funnel was then washed out with 0.5 litres of benzene. The reaction solution was kept in a nitrogen atmosphere maintained by the passage of dry nitrogen over the surface of the reactants at room temperature.

The progress of the reaction of the diisocyanate with the hydroperoxide was followed by observing the disappearance of the characteristic-N=C=O absorption band (i.e. 2270 cm~1) with time of reaction. The total reaction time normally observed was 142 hours at room temperature (230C).

After the reaction was complete, benzene was removed at room temperature under reduced pressure. The white precipitate obtained was washed twice with n-heptane and filtered through a Buchner filter using a water-pump. The filtered product was then dried at room temperature in a dessicator under vacuum. The yield obtained was 71% by weight.

The characteristic infrared spectrum of the bisperoxycarbamate has the following characteristic peaks: 0--0 860 cm-' C-O 1192 cm-1 C-(CH3)3 1370 cm-1 and 1393 cm-1 C=O 1720 cm-1 CH stretching 2860 cm-1,2935 cm-l and 2985 cm-l N-H 3360 cm-' Other bisperoxycarbamates were prepared in analogous manner using corresponding starting materials. The compounds are identified as in the Table above.

Compound Starting materials B 1,6-hexamethylene diisocyanate and cumune hydroperoxide C 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate and tert-butyl hydroperoxide D cyclohexyl diisocyanate and tert-butyl hydroperoxide E 1,6-hexamethylene diisocyanate and tert-butyl hydroperoxide F polyurethane-type prepolymer based on 1,4-butane diol and excess 3-isocyanate-3,5,5-tri methylcyclohexyl isocyanate (IPDI) and tert-butyl hydroperoxide G 2,4-tolylene diisocyanate dimer and tert-butyl hydroperoxide H polyester diol/2,4-tolylene diisocyanate (TDI) prepolymer and tert-butyl hydroperoxide J polycaprolactone/methylene-bis(4-cyclohexyl isocyanate) prepolymer and tert-butyl hydroperoxide Bisperoxycarbamate from 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl-isocyanate (IPDI) and tertiary butyl hydroperoxide The total reaction time required for complete disappearance of the-N=C=O peak was 70 hours.

The yield obtained was 69% by weight. The infrared peaks were 0--0 854 cm-' C-O 1187 cm-1 C-(CH3)3 1365 cm-1 and 1385 cm-1 0=0 1715 cm- CH stretching 2920 cm-1, 2940 cm-' and 2964 cm-' N-H 3340 cm-' This compound exhibits two decomposition points. It begins to meit at 11 3 C and shows two decompositions peaks at 1 670C and 1 770C. These peaks were thought to correspond to the cleavage of the 0--0 bonds of the aliphatic peroxycarbamate and the cyclic peroxycarbamate.

Bisperoxycarbamate prepared from a polyurethane prepolymer based on 1,4 butane diol reacted with excess IPDI (block ratio 1 this polyurethane type was then reacted with tertiary butyl hydroperoxide to form the bisperoxycarbamate In the case of this bisperoxycarbamate a prepolymer was initially prepared. 1 ,4 butane diol was reacted under nitrogen with excess IPDI for 2.5 hours at 1 200C to form the prepolymer. The excess diisocyanate monomer was removed by washing several times the reaction product with dry petroleum ether and the product dried. The dry product was reacted with tertiary butyl hydroperoxide using a similar procedure to that described before. The total reaction time required for complete disappearance of the -N=C=O peak was 89 hours. The yield obtained was 88% by weight.This compound begins to melt at 11 70C and shows one decomposition peak at 1 630C.

O-O 850 cm-1 C-O 1193 cm-1 C-(CH3)3 1363 cm-1 and 1383 cm-' 0=0 1714cm-' CH stretching 2948 cm-' N-H 3327 cm-1 Bisperoxycarbamate from 2,4,tolylene diisocyanate dimer and tertiary butyl hydroperoxide The total reaction time for complete disappearance of the -N=C=O peak was about 48 hours.

The yield obtained was 26% by weight. The compound begins to decompose at 660C.

O-O 833 cm-1 C-O 1200cm-1 C-(CH3)3 1410 cm-1 C=O 1720 cm-1 CH stretching 2870 cm-1 and 2932 cm-1, 2980 cm-1 CH aromatic 3040 cm-1 and 3083 cm-1 NH 3328 cm-1 Bisperoxycarbamate from polyester/TDI prepolymer (Solitane 790) and tertiary butyl hydroperoxide The total reaction time for complete disappearance of the-N=C=O=peak was 54 hours. The yield obtained was 2.8 g. The compound begings to decompose at 64 C.

O-O 830 cm-1 C-O 1172 cm-1 C-(CH3)3 1392 cm-1 C-O 1735 cm-1 CH stretching 2890 cm-1 and 2977 cm-1 CH aromatic 3070 cm-1 NH 3230 cm-1 Bisperoxycarbamate from polycaprolactone/methylene-bis-bis-(4-cyclohexyl isocyanate) prepolymer and tertiary butyl hydroperoxide The total reaction required for complete disappearance of the -N=C=O peak was very slow. The yield obtained was 7.1 g. Almost similar to the bisperoxycarbamate (d), the prepolymer bisperoxycarbamate begins to decompose at 138 C.

O-O 857 cm-1 C-O 1185 cm-1 C-(CH3)3 1377 cm-1 and 1400 cm-1 C=O 1737 cm-1 CH stretching 2880 cm-1 and 2953 cm-1 N-H 3390 cm-1 Examples of crosslinking properties In the following Examples,

Aliphatic diisocyanate backbone

Aliphatic/cyclic diisocyanate backbone

cyclic diisocyanate backbone V 9 N3 V 9H3 W-c-o-o--cH3 iv ,CH3 I H9C-'"' -(CH2)s-O-F--CHT:-VCH3 IV - 3 4 2 R o 2 4 ≈ aCH Prepolymer vdiisocyanate backbone Dicumyl peroxide is supplied by AKZO Chemie U.K. Limited under the trade name Perkadox SB (Crystalline solid of 95% purity).Natural Rubber of the grade DPNR is obtained from the Malaysian Rubber Producers' Research Association whilst Polyisoprene Rubber, grade Cariflex IR 305 is suppiied by Shell Chemicals (U.K.) Limited.

Mixes are prepared using a two-roll mill at room temperature. The minimum activation temperatures to give a satisfactory practical cure are obtained by DSC observation using a Du Pont 990 Thermal Analyser, whilst the vulcanisation conditions for specimen preparation are based on cure rate data from the Monsanto Oscillating Disc Rheometer. Samples are cured between two Melinex sheets. The crosslink density of the vulcanisate is determined by a swelling method and calculated using the Flory-Rehner Equation.

where Mc is the number average molecular weight between crosslinks p is the density of rubber V0 is the molar volume of solvent Vp is the volume fraction of rubber in swollen vulcanisate X is the polymer solvent interaction constant.

1 2Mc is the crosslink density.

The determination of tensile properties of the vulcanisates is based on micro-ring samples of dimensions: internal diameter 23 mm, external diameter 27 mm, and thickness of approximately 1.0 mm. Stress relaxation and hot air oven ageing data is obtained by Tension Stress Relaxometer (Wallace) and Multi-Cell Ageing Block (Wallace) respectively.

Cure characteristics These are shown in figures 1 and 2 for the four bisperoxycarbamates which are compared with dicumyl peroxide as a control. In DPNR and IR gum stocks as rubber bases the minimum temperatures at which a satisfactory practical cure rate is found possible for the bisperoxycarbamates is shown in table 1.

Table 1 Minimum temperatures for practical cure rates with bisperoxycarbamates

Minimum cure activation temperature (as determined Bisperoxycarbamate by Du Pont DSC) I 116 C II 125 C III 130 C IV 130 C dicumyl peroxide 140 C These temperatures are similar to that of the traditional sulphur systems.

The cure rates of all four bisperoxycarbamates in both NR and IR are found to be faster than that of the dicumyl peroxide system. For example, maximum ODR torque occurs within 10 minutes at 1600C for the bisperoxycarbamate Ill whereas with the dicumyl peroxide about 3 hours at 1 500C is required for the complete development of maximum cross-linking. Bisperoxycarbamate IV exhibits only low cure state efficiency.

Crosslink density Figures 3 and 4 illustrate the effect of curing agent type and concentration on crosslink density.

The crosslink efficiencies of the bisperoxycarbamates in the two rubbers is seen to be dependent on the structures of their diisocyanate backbone: cyclic > aliphatic/cyclic > aliphatic > prepolymer.

In NR for a given concentration of curing agent the dicumyl peroxide vulcanisates give higher crosslink density than their corresponding bisperoxycarbamate vulcanisates. Also, the Polyisoprene Rubber (IR 305) vulcanisates have higher crosslink densities for a given concentration of curing agent than the equivalent Natural Rubber. This is considered due to the presence of vinyl structures (5% vinyl content) in the Polyisoprene macromolecules.

Cure chemistry The decomposition of these bisperoxycarbamates and their crosslinking mechanism are thought to be represented by the following equations (1)-(7):

CH CH 2 I 2-C=CH-CB"" C H2-C=CH-C (Note a C-C crosslink is formed) (4) MCH2-C=CH-CR CH3 + + + CH2 --CH- Cf2-C H CH2- & H-CH~ ~CH2-C-C}I-CH~ H? yy " CH2 /- ~ 2 -H N-R CH H CH2CH N-H CH E > -C=H (Note a -NH-R1-NH- crosslink is formed) (5)

It is considered that a bisperoxycarbamate decomposes thermally with formation of free radicals, as shown in equations 1 and 2, and these vulcanise the rubber. Porosity is observed in these vulcanisates, especially in those with a high concentration of bisperoxycarbamate. This can be explained if equation 2 takes place as CO2 is generated. This interpretation is further substantiated by the addition of additives which absorbed the liberated carbon dioxide and resulted in vulcanisates free from porosity. Also the addition of reactive monomers (coagents) to the rubber mixes is found to eliminate this porosity presumably by reacting with the free radical, (A) of equation 1, before it liberates the carbon dioxide of equation 2. In our investigation it was observed that suitable reactive monomers were Trimethylol Propane Trimethacrylate (Sartomer SR-350) and N,N'-m-phenylene dimaleimide (HVA-2) as both eliminate this porosity in the rubber vulcanisate.

It is considered than an RzO radical abstracts hydrogen from the a-methylene group of the polyisoprene unit (equation 3) in an analogous manner to that of the free radicals produced during the well known decomposition of dicumyl and ditertiary butyl peroxides in their crosslinking reactions. Two polymer macroradicals resuiting from this reaction can couple giving rise to a carbon-carbon crosslink (equation 4). Experiments by other workers with 2,6-diene (a model compound that simulates polyisoprene) suggested that equation 4 is the preferred mode in the C-C crosslinking reaction.

However, there is known to be a choice of three types of hydrogen atoms in the isoprene unit that can effect hydrogen abstraction. The order of reactivity of these three types of hydrogen atoms, identified below, has been found to be a > b > c.

Since the equation 4 is considered the predominant 0-0 forming reaction, the other two types of macroradicals are not considered in this paper.

Previous workers have shown that based on dibenzoyl peroxide or on alkyl perbenzoate as curing agents for Natural Rubber, the benzyloxy groups are proved to attach themselves to the rubber main chain either at the site of a crosslink as in (A) below, or at isolated points in the chain as in (B) below

Using the above attachments (A) and (B) as proven models it is postulated that the amino group in the bisperoxycarbamate could become grafted to the rubber main chain as shown in equations 5 and 6.In the case of the Polyisoprene Rubber it was experimentally established that an amino group has attached itself to the polymer since investigation showed the presence of chemically bound nitrogen in the vulcanisate which had been previously extracted with solvent (acetone) to remove any excess and u n reacted bisperoxyca rba mate cross-linker.

It is considered that the amino radical can also, in principle, abstract hydrogen from the amethylene group of the isoprene unit as in equation 7. Besides the reactions quoted above, other side reactions may occur.

Tensile properties (T.S. and E.B.) Figures 5 and 6 show the effect of concentration of these bisperoxycarbamates and of dicumyl peroxide on the tensile strength of gum vulcanisates of NR and IR. In the case of NR, the tensile strength of the vulcanisate cured with bisperoxycarbamate Ill does not reach its optimum with 4 phr of the curing agent. However, increasing the level of this bisperoxycarbamate, in DPNR, beyond 4 phr was observed to give rise to porosity in the vulcanisate. As a result of this observation absorbing systems were then investigated to remove the liberated carbon dioxide.

Three systems were examined and found to eliminate porosity: (i) Sartomer SR350 (ii) HVA-2 (iii) CaO+Ca(OH)2 Of these the CaO+Ca(OH)2 system are considered the most suitable with respect to versatility and cost as the Ca(OH)2 absorbed the reaction produced CO2 and, the CaO absorbed the H20 by means of reactions 8 and 9.

Increasing the level of bisperoxycarbamate Ill in combination with the above additives in DPNR results in vulcanisates of superior strength e.g. 8.8 MPa in Table 2, compared with vulcanisates cured with dicumyl peroxide, e.g. 7.2 MPa in Figure 5.

In Natural Rubber the two bisperoxycarbamates (I and II) demonstrate poorer strength than the vulcanisate cured using dicumyl peroxide. However, in the case of the Polyisoprene Rubber, the bisperoxycarbamate I vulcanisates have much better strength than the dicumyl peroxide vulcanisates.

On observation of all the cured samples, it is found that vulcanisates cured with dicumyl peroxide have a strong and unpleasant smell, whilst the odour in some of the bisperoxycarbamate vulcanisates is hardly perceptible.

For a given crosslink density, especially at high levels, the bisperoxycarbamate vulcanisates are considered more flexible i.e. their breaking strain is higher by about 37% than the equivalent dicumyl peroxide vulcanisates, see figures 7 and 8. This is at the crosslink density of 40x 1 0-3 kg mole-' for the NR cured with the cyclic bisperoxycarbamate. This is considered a direct consequence of the crosslink structures involved. Dicumyl peroxide is well established as forming short C-C crosslinks between rubber chains whilst the bisperoxycarbamates are considered to form a predominance of the much larger and hence more flexible

crosslinks.

Thermal stability properties Stress relaxation As shown in figures 9 and 10 the order of thermal oxidative stability of the bisperoxywarbamate vulcanisates is found to be cyclic > aliphatic/cyclic > aliphatic. Of the three bisperoxycarbamates in NR, Ill demonstrates comparable stability to DPNR cured with dicumyl peroxide. In the case of IR 305, all three bisperoxycarbamate vulcanisates are more stable than their comparable dicumyl peroxide vulcanisates. The lower scission rate in the NR vulcanisate, cured with dicumyl peroxide, when compared with the IR vulcanisate is considered due to the presence of naturally occuring anti-oxidants in the NR (19), these being proportionally greater than those available for protection in the IR 305.

Optimisation of the level of bisperoxycarbamate Ill in DPNR As discussed earlier, the tensile strength of the bisperoxycarbamate III NR vulcanisate have not reached its optimum level at 4 phr as increasing the level of bisperoxycarbamate III results in vulcanisate porosity. Therefore an experiment was designed using fixed concentration of absorbers to try to eliminate this porosity whilst the concentration of the bisperoxycarbamate Ill was varied from 5 phr to 8 phr. The tensile properties of the vulcanisates thus obtained are shown in Table 2.

Table 2 Formulations and tensile properties for optimisation of the level of the cyclic bisperoxycarbamate (III) in DPNR DPNR 100 100 100 100 Ca(OH)21 15 15 15 15 CaO(2) 15 15 15 15 Concentration of bisperoxycarbamate 5 6 7 8 Ill (phr) Cure conditions 30 minutes/1400C Tensile strength (MPa) 8.6 8.2 8.1 9.1 Modulus at 300% strain (MPa) 1.70 1.81 2.02 2.35 Elongation at break (%) 550 520 500 410 ') 98% Analar, BDH 2) Pure, low in chlorine and sulphur, BDH As shown in Table 2, the optimum for strength and economy reasons would be 5 phr. However, this is achieved using an unnecessary high level of absorbers and a second step to complete optimisation is to try and reduce the concentration of absorber.An additional problem is that CaO absorbs moisture from the atmosphere and also Ca(OH)2 absorbs CO2 from the atmosphere, both of which would render CaO/Ca(OH)2 absorber system inert after a period of storage. Furthermore both are difficult to disperse in rubber. To solve this problem and to aid the dispersion of CaO and Ca(OH)2 in rubber the common aromatic processing oil, Shellflex 729 UK, is utilised to make a CaO/Ca(OH)2 paste and the ratio used is CaO/Ca(OH)2/Sheliflex 729UK of 3/3/4 by weight.

The tensile properties of vulcanisates prepared using this oil dispersed absorber system are shown in Table 3. The minimum level suitable for elimination of porosity is between 5-7.5 phr absorber dispersed in oil.

Table 3 To reduce and optimise the level of absorber system (5 phr of the cyclic bisperoxycarbamate (III) present)

Concentration of absorber 5 7.5 10 12.5 15 system in oil (60% active) (phr) Cure conditions # 30 minutes/140 C # Tensile strength (MPa) 8.8 8.5 7.8 7.2 7.2 Modulus at 300% strain 1.20 1.25 1.21 1.13 1.14 (MPa) Elongation at break (%) 640 630 630 650 640 Porosity Visible # no visible porosity porosity A theoretical calculation of the quantity of the absorber system required can be made as follows: Theory 5 5 grams of bisperoxycarbamate Ill = Moles 442 5 No. of moles of CO2 liberated during cure = - x2 (see equation 2) 442 5 Weight of Ca(OH)2 required to absorb the CO2 = x2x74.08=1 .68 g 442 5 Number of Moles of H2O liberated (see equation 8) = ---- x2 442 5 Weight of CaO required to absorb the liberated H2O = x2x56.08=1.27 g 442 The above calculation shows that the amount of Ca(OH)2 added in the mix (i.e. 2.25 g of Ca(OH)2 in the case of 7.5 phr of 60% paste) is close to the minimum theoretical value required to absorb all the CO2 produced. The weight of CaO added in the mix is based on the same weight of Ca(OH)2 for convenience.However, it is to be noted that the calculation above does not take into consideration a side reaction known to occur where an amount of free radicals are consumed through reaction with the aromatic processing oil.

During the vulcanisation step, an additional reaction product is produced i.e. Ca(OH)2 from the CaO and H2O generated during crosslinking and this would also be available for use in absorbing the liberated CO2. Since the amount of H2O liberated during the reaction is low when compared to the naturally occurring H2O present in rubber and fillers we can in theory eliminate the use of CaO in the bisperoxycarbamate formulation. In technological practice it would probably always be desirable to add some additional CaO to cope with side reactions such as water present in fillers and the raw rubber.

However, the relationship between practical formulation need and chemical stoichiometry is good and remarkable in the history of rubber compounding technology.

Applications of the cyclic bisperoxycarbamate (III) in silica and black-filled DPNR The original investigations were performed on unfilled gum rubbers used as models to eliminate any possible cure reaction interference. However, practical systems contain fillers which are now considered. The cyclic bisperoxycarbamate (III) is selected as an example as its gum vulcanisates possess the highest strength properties of those investigated. The formulations used for bisperoxycarbamate Ill are shown in Table 4. Precipitated silica (20 phr) is chosen as the reinforcing filler as this proportion approximates to the highest tensile strength available from a dicumyl peroxide NR vulcanlsate of this type.The bisperoxycarbamate lil curing system exhibits superiority in tensile strength of about 20% over the dicumyl peroxide one and it also requires a much shorter time to reach optimum cure state.

Table 4 Formulations and tensile properties of silica-filled DPNR (Bisperoxycarbamate III compared with dicumyl peroxide)

DPNR 100 100 Silica (UltrasilVN3) 20 20 Dicumyl peroxide 1.5 Bisperoxycarbamate lil 5 Ca(OH )2 60% dispersion in 6 CaO # Shellflex 729UK Cure conditions 60 min/150 C 30 min/1400C Tensile strength (MPa) 10.0 1 2.4 Modulus at 300% strain (MPa) 2.26 2.41 Elongation at break % 500 540 The properties of the carbon blacks and their NR vulcanisate properties, based on bisperoxycarbamate Ill as curing agent, are shown in table 5 and 6 respectively.

Bisperoxycarbamate Ill gives satisfactory vulcanisates of good strength with carbon blacks of medium to large particle size as represented by SRF, MT, and Acetylene types. However, at present, only relatively low strength vulcanisates have been obtained with the more highly reinforcing blacks typified by SAF (N 110) and HAF (N330). From Table 5, for bisperoxycarbamate vulcanisates, it would seem that the purity (i.e. non-carbon content) of the black plays a predominant role in their strength as both SAF and HAF blacks respectively contain 2.2% and 1.7% of organic groups based on H, 0 and S and these blacks give low strength rubbers, whereas the higher carbon content blacks, SRF, MT and Acetylene, give high strength vulcanisates. It is this organic content factor that seems to predominate in this instance and not the more usual pH and surface absorption factors.

Table 6 Properties of the carbon blacks

Actual % composition Mean particle density pH values Carbon black type size diameter (mm) (g.cm ) (*) C H O S Ash SAF (1) (Vulcan 9) 22.5 1.80 97.4 0.4 1.1 0.70 0.50 HAF(') (Vulcan 3) 32.0 1.80 7.7 97.9 0.4 0.7 0.60 0.40 Regal(1) SRF 83.0 1.80 7.3 99.2 0.4 0.2 0.01 0.02 Sevacarb2 MT 300.0 1.80 4.7 99.3 0.3 0.1 0.01 0.30 Shawinigan(3) acetylene 42.0 1.95 5.5 99.7 0.1 0.2 0.01 0.04 *pH values of the carbon blacks were determined according to ASTM D15/2 Part 38, July 1975.

1)Cabot Carbon Ltd.

(2)Phillips.

3)Cairn Chemicals Ltd.

Table 7 Tensile properties of carbon black vulcanisates(a) crosslinked with the cyclic bisperoxycarbamate (III)

Normalised concentration (phr) (based on the surface area of U.T.S. (MPa) Carbon black type 25 parts of SAF) (ring samples) E.B.(%) SAF 25.0 4.1(c) 260 HAF 35.5 6.8(c) 270 (Regal) SRF 92.1 9.0 160 (Sevecarb) MT 120.0(b) 9.3 150 Acetylene (Shwinigan) 50.5 9.0 190 a)Formulation used-DPNR 100, Ca(OH)2 1.8, CaO 1.8, Bisperoxycarbamate Ill 5, and black as shwon in Table 6. Samples were cured for 60 min/130 C.

(b)Maximum processabilitv level on the two roll mill.

(c)Slight porosity.

Curing ability of bisperoxycarbamate Ill in other synthetic rubbers The effectiveness of bisperoxycarbamate Ill as a curing agent in other synthetic rubbers has been investigated. As shown in Figure 11, both CR and SBR require a lower concentration of bisperoxycarbamate Ill to achieve a technical state of cure than NBR and EPDM. The bisperoxycarbamate Ill has particularly fast rate of cure in the polychloroprene rubber.

No absorber system is used as only low levels of bisperoxycarbamate III are involved.

Claims (27)

Claims

1. A compound of the general formula

in which R, represents a carbon-based linking group which may incorporate one or more hetero atoms and R2 and R3, which may be the same or different, each represents an aliphatic or cycloaliphatic group which may be unsaturated or saturated, unsubstituted or substituted, a straight chain or a branched chain an aryl group which may be unsubstituted or substituted or an araliphatic or aliphaticaryl group which may be unsubstituted or substituted and the aliphatic group of which may be unsaturated or saturated, a straight chain or a branched chain.

2. A compound as claimed in claim 1, wherein R, represents a carbon-based linking group comprising one or more of the following groups as part of the linking group an aliphatic group which may be unsaturated or saturated, unsubstituted or substituted, a straight chain or a branched chain, uninterrupted or interrupted by one or more nitrogen and/or oxygen atoms a cycloaliphatic, aliphaticcycloaliphatic or cycloaliphaticaliphatic group which may be unsaturated or saturated, unsubstituted or substituted, uninterrupted or interrupted by one or more nitrogen and/or oxygen atoms, the aliphatic groups of which may be a straight chain or a branched chain an aromatic, aliphaticaromatic or aromaticaliphatic group which may be unsaturated or saturated, unsubstituted or substituted, uninterrupted or interrupted by one or more nitrogen and/or oxygen atoms, the aliphatic group of which may be a straight chain or a branched chain.

3. A compound as claimed in claim 2, wherein R1 represents an unsubstituted or substituted alkylene radical, an unsubstituted or substituted cycloalkylene radical, an unsubstituted or substituted cycloalkylenealkylene radical or an unsubstituted or substituted alkylene-bis(cycloalkylene) radical.

4. A compound as claimed in claim 3, wherein R, represents a hexylene, cyclohexylene, methylsubstituted cyclohexylenemethylene or methylenebis(cyclohexylene) radical.

5. A compound as claimed in any one of claims 1 to 4, wherein R2 and R3 are the same and each represents an unsubstituted or substituted, straight chain or branched chain alkyl group.

6. A compound as claimed in claim 5, wherein R2 and Ra are the same and each represents an unsubstituted tert-butyl group or a tert-butyl group substituted by a phenyl group.

7. A compound as claimed in claim 6, wherein R2 and R3 each represents an unsubstituted tertbutyl group.

8. A compound as claimed in claim 1, which is specified herein.

9. Methylene-bis(cyclohexyl )-N,N'-bis(tert-butylperoxycarbamate).

10. A process for the preparation of a compound as claimed in claim 1 , which comprises reacting a diisocyanate of the general formula OCN-R1-NCO (II) in which R, is as defined in claim 1 with a hydroperoxide of the general formula HOOR2 (Ill) wherein R2 is as defined in claim 1 or a hydroperoxide of the general formula HOOT, (IV) wherein Ra is as defined in claim 1 or a mixture of two such compounds wherein the molar ratio of diisocyanate to hydroperoxide or hydroperoxide mixture is at least 1:2.

11. A process as claimed in claim 10, which is carried out in the presence of a catalyst.

12. A process as claimed in claim 11, wherein the catalyst is triethylamine or pyridine.

13. A process as claimed in any one of claims 10 to 12, which is carried out in an organic solvent.

14. A process as claimed in claim 13, wherein the organic solvent is benzene.

1 5. A process as claimed in claim 10, which is carried out substantially as described herein.

16. A compound as claimed in any one of claims 1 to 9 whenever prepared by a process as claimed in any one of claims 10 to 1 5.

1 7. A composition suitable for use as a crosslinking agent which comprises a compound as claimed in any one of claims 1 to 9 and 16 and a suitable carrier.

1 8. A composition as claimed in claim 17, which also contains one or more compounds capable of absorbing carbon dioxide and/or water.

1 9. A composition as claimed in claim 18, which also contains one or more compounds selected from trimethylol propane trimethacrylate, N,N'-m-phenylene dimaleimide, calcium hydroxide and calcium oxide.

20. A composition as claimed in claim 19, which also contains a mixture of calcium hydroxide and calcium oxide.

21. A composition as claimed in claim 20, which also contains an aromatic processing oil.

22. A composition as claimed in claim 1 7 which is substantially as described herein.

23. A method of crosslinking polymer chains which comprises using a compound as claimed in any one of claims 1 to 9 and 16 or a composition as claimed in any one of claims 17 to 22 as a crosslinking agent.

24. A method as claimed in claim 23, which is used for crosslinking plastics and rubber materials.

25. A method as claimed in claim 24, which is used in the curing of natural and polyisoprene rubber materials.

26. A method as claimed in claim 24, which is used in the curing of carbon black-filled silicone rubber.

27. A method as claimed in any one of claims 23 to 26, which is carried out substantially as described herein.