Low temperature process for crosslinking rubber

Abstract

The present disclosure relates to a process for crosslinking rubber, the process comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1.7 (Mw and Mn determined by GPC, polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol (Mw and Mn determined by GPC, polybutadiene standards); ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10; and b) curing the composition of step a) at a temperature of less than 150°C, optionally in an open-air curing environment.

Description

[0001] LOW TEMPERATURE PROCESS FOR CROSSLINKING RUBBER

[0002] Technical Field

[0003] The present disclosure relates to a low temperature process for crosslinking rubber, preferably in an open-air curing environment, and compositions suitable for use in said process.

[0004] Liquid rubbers (like polybutadiene or polyisoprene with low to medium molecular weight) are becoming more increasingly applied in the elastomer application field, in large part because replacing unreactive oils with these reactive liquid rubbers is environmentally advantageous.

[0005] Because of the high level of unsaturated carbon functionalities in these rubbers, they can be efficiently cured by peroxides. However, the typical curing temperature for such unsaturated rubbers when using peroxides is 150°C or above. Such high temperatures are undesirable for many applications, e.g., because of temperature sensitive substrates and / or because of environmental / economic concerns (high energy consumption).

[0006] US 2011 / 224382 discloses a thermosetting composition comprising (A) a dienebased polymer having two or more unsaturated bonds, (B) a polythiol derived from mercaptocarboxylic acid and (C) a thermally radical-generating agent. The compositions required a large amount of curing agent and a high curing temperature of 150-170°C.

[0007] EP 2420535 discloses a thiol-containing liquid rubber composition comprising a liquid styrenebutadiene copolymer and a polythiol derived from mercaptocarboxylic acid. This composition is shown to be UV-curable (using a photoinitiator) or curable at a temperature of at least 150°C (using a peroxide). The compositions disclosed in EP 2420535 required less curing agent than US 2011 / 224382, but the system was shown to be successful only for liquid styrene-butadiene rubbers (according to EP2420535 other liquid rubbers, notably butadiene and isoprene rubbers, could not be successfully cured using the system disclosed therein).

[0008] The curing systems of US 2011 / 224382 and EP 2420535 evidently have substantial limitations and disadvantages, most notably the high temperature required for curing (150°C or above), the requirement of a large amount of curing agent (>15phr), the lack of flexibility in the type of polythiol that can be used (which could cause compatibility problems), and the low versatility in terms of rubber type that can be cured by the systems.

[0009] US 11518828 discloses processes for making macromolecular networks by cross-linking saturated rubbers that are end-functionalized (“telechelic polymers”) with polythiols in the presence of a peroxide. The processes were shown to be successful at low temperatures (70°C), but the processes were unacceptably slow and required an accelerator comprising an amine moiety (curing required 14 days to reach completion, even in the presence of the accelerator). Slow curing times and the need for additional amine-functionalized compounds are highly undesirable traits for industrial processes of this kind.

[0010] US 2022056161 relates to curable compositions for use in 3D printing (i.e., open-air curing). Worked examples 2 and 3 of US 2022056161 provided a low temperature (100°C) thermal casting process wherein an unsaturated rubber (OH-terminated polybutadiene; Poly bd® R- 45HTLO) was crosslinked with a bis-thiol (3,6-dioxa-1 ,8-octanedithiol) using a peroxide. The cured rubber product obtained was very soft (shore A hardness of 5-10), which is of very limited real-world practical application. To determine whether the disclosed shore A values were genuine empirical results or merely a reporting error, the inventor of the present disclosure repeated Example 3 of US 2022056161 using an equivalent unsaturated rubber and found that not only was the resulting cured rubber soft, but the exposed surfaces of the cured rubber were also very tacky, neither of which are desirable properties in many end-use applications that require open-air curing systems (such as spray-applications, wherein the uncured composition is sprayed onto a substrate and then thermally cured in an open-air environment). Worked examples 4 and 5 of US 2022056161 show that the shore A hardness could be increased by switching from a thermal cure system to a UV cure system, however UV cure systems also have numerous drawbacks (e.g., would require specialist equipment and relatively expensive curing reagents).

[0011] There remains a need for a low temperature process for thermally curing unsaturated rubbers at low temperature, particularly in an open-air environment, and preferably a process that does not face the same limitations as the previously known processes. Summary

[0012] It has now been found that a specific process using specific radical initiators and polythiols provides for successful crosslinking of a wide variety of unsaturated rubbers at lower temperatures and in open-air curing conditions. Accordingly, the present disclosure may be summarized by the following Aspects:

[0013] Aspect 1 . A process for crosslinking rubber, the process comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7 (Mw and Mn determined by GPC using polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol (Mn determined by GPC using polybutadiene standards); ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10; and b) thermally curing the composition of step a) at a temperature of less than 150°C, optionally in an open-air curing environment.

[0014] Aspect 2. The process of Aspect 1 , wherein the at least one unsaturated rubber ia) has a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .5, preferably less than 1 .3, and most preferably less than 1.1 .

[0015] Aspect 3. The process of Aspect 1 , wherein the at least one unsaturated rubber ib) has a number average molecular weight (Mn) of at least 15,000 g / mol.

[0016] Aspect 4. The process of any one of Aspects 1 to 3, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25°C, 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.

[0017] Aspect 5. The process of any one of Aspects 1 to 4, wherein the at least one unsaturated rubber i) comprises a solid long chain rubber, optionally selected from solid butadiene rubber, solid styrene-butadiene rubber, solid natural rubber, and mixtures thereof (solid at 25°C, 1 atmosphere).

[0018] Aspect 6. The process of any one of Aspects 1 to 5, wherein the at least one radical initiator ii) is selected from aliphatic peroxyesters, aliphatic diacylperoxides, aliphatic percarbonates, aliphatic perketals, aliphatic azo compounds, or mixtures thereof. Aspect 7. The process of any one of Aspects 1 to 6, wherein the at least one radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxy isopropyl carbonate, t-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl (4,4-di(tert-butylperoxy)valerate, 1 ,1-di(tert-butylperoxy)- 3,3,5-trimethylcyclohexane, 1 ,1 -di(tert-butylperoxy)-cyclohexane, 2,2'-Azodi(2- methylbutyronitrile), or mixtures thereof.

[0019] Aspect 8. The process of any one of Aspects 1 to 7, wherein the at least one polythiol iii) is a dimercaptan, a trimercaptan, a tetramercaptan, or mixtures thereof.

[0020] Aspect 9. The process of any one of Aspects 1 to 8, wherein the at least one polythiol c) is selected from polythiols derived from mercaptocarboxylic acid, a C2-C10 dithiol, a C3-C10 trithiol, a C4-C10 tetrathiol, or mixtures thereof.

[0021] Aspect 10. The process of any one of Aspects 1 to 9, wherein the composition of step a) comprises about 1 -10 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i), preferably about 1-5 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i).

[0022] Aspect 11 . The process of any one of Aspects 1 to 10, wherein the composition of step a) comprises about 0.5-10 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i), preferably about 0.5-5 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i), preferably about 1 -5 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i).

[0023] Aspect 12. The process of any one of Aspects 1 to 11 , wherein the composition of step a) further comprises a reinforcing filler, preferably selected from silica, calcium carbonate, talc, carbon black, clays, or mixtures thereof, more preferably selected from calcium carbonate, talc, or mixtures thereof.

[0024] Aspect 13. The process of any one of Aspects 1 to 12, wherein the weight ratio of ii) to iii) is from about 5:1 to about 1 :5, preferably about 3:1 to about 1 :3, and more preferably about 2:1 to about 1 :2.

[0025] Aspect 14. The process of any one of Aspects 1 to 13, wherein the curing temperature in step b) is from about 90°C to about 140°C, preferably about 95°C to about 135°C, more preferably from about 100°C to about 130°C, and most preferably from about 105°C to about 125°C.

[0026] Aspect 15. The process of any one of Aspects 11 to 14, further comprising controlling the crosslink density of a cured rubber by first determining a constant of proportionality (k) between the crosslink density (Nm) of cured rubber obtained after step b) and molar equivalents of SH groups ([SH]) in the composition of preceding step a) (Nm = k*[SH]), and then using said constant (k) to adjust the molar equivalents of SH groups ([SH]) in the composition of step a) to control the crosslink density (Nm) of the cured rubber obtained after step b).

[0027] Aspect 16. A cured rubber obtainable by the process of any one of Aspects 1 to 15.

[0028] Aspect 17. A composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7 (Mw and Mn determined by GPC using polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol (Mn determined by GPC using polybutadiene standards); ii) about 1 to about 10 parts by weight per 100 parts by weight i) of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; iii) about 0.5 to less than 10 parts by weight per 100 parts by weight i) of at least one polythiol; and iv) optionally at least one reinforcing filler; wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10.

[0029] Aspect 18. The composition of Aspect 17, wherein the at least one unsaturated rubber ia) has a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .5, preferably less than 1 .3, and most preferably less than 1.1.

[0030] Aspect 19. The composition of Aspect 17, wherein the at least one unsaturated rubber ib) has a number average molecular weight (Mn) of at least 15000 g / mol.

[0031] Aspect 20. The composition of any one of Aspects 17 to 19, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25°C, 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer. Aspect 21 . The composition of any one of Aspects 17 to 20, wherein the at least one unsaturated rubber i) comprises a solid long chain rubber, optionally selected from solid butadiene rubber, solid styrene-butadiene rubber, solid natural rubber, and mixtures thereof (solid at 25°C, 1 atmosphere).

[0032] Aspect 22. The composition of any one of Aspects 17 to 21 , wherein the at least one radical initiator ii) is selected from aliphatic peroxyesters, aliphatic diacylperoxides, aliphatic percarbonates, aliphatic perketals, aliphatic azo compounds, or mixtures thereof.

[0033] Aspect 23. The composition of any one of Aspects 17 to 22, wherein the at least one radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5- di(2-ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxy isopropyl carbonate, t-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl (4,4-di(tert-butylperoxy)valerate, 1 , 1 -di(tert-butylperoxy)- 3,3,5-trimethylcyclohexane, 1 ,1 -di(tert-butylperoxy)-cyclohexane, 2,2'-Azodi(2- methylbutyronitrile), or mixtures thereof.

[0034] Aspect 24. The composition of any one of Aspects 17 to 23, wherein the at least one polythiol iii) is a dimercaptan, a trimercaptan, a tetramercaptan, or mixtures thereof.

[0035] Aspect 25. The composition of any one of Aspects 17 to 24, wherein the at least one polythiol c) is selected from polythiols derived from mercaptocarboxylic acid, or a C2-C20 polythiol, such as a C2-C10 dithiol, a C3-C10 trithiol, a C4-C10 tetrathiol, or mixtures thereof. Aspect 26. The composition of any one of Aspects 17 to 25, wherein the composition of step a) comprises about 1 -5 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i).

[0036] Aspect 27. The composition of any one of Aspects 17 to 26, wherein the composition of step a) comprises about 0.5-5 parts polythiol iii) per 100 parts unsaturated rubber i), preferably 1 -5 parts polythiol iii) per 100 parts unsaturated rubber i).

[0037] Aspect 28. The composition of any one of Aspects 17 to 27, wherein the combined total amount of radical initiator ii) and polythiol iii) in the composition of step a) is less than 15 parts by weight per 100 parts by weight of the unsaturated rubber i).

[0038] Aspect 29. The composition of any one of Aspects 17 to 28, wherein the composition of step a) comprises the at least one reinforcing filler, and wherein the at least one reinforcing filler is selected from silica, calcium carbonate, talc, carbon black, clays, or mixtures thereof. Aspect 30. The composition of Aspect 29, wherein the at least one reinforcing filler is selected from calcium carbonate, talc, or mixtures thereof.

[0039] Aspect 31 . Use of a composition according to any one of Aspects 17 to 30 for open-air curing applications.

[0040] Detailed Description

[0041] As mentioned above, it has now been found that a specific process using specific radical initiators and polythiols provides for successful crosslinking of a wide variety of unsaturated rubbers at lower temperatures and in open-air curing environments.

[0042] Accordingly, in a first aspect, the present disclosure relates to a low temperature process for crosslinking rubber, preferably in an open-air curing environment, the process comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7; or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol; ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10; and b) thermally curing the composition of step a) at a temperature of less than 150°C, preferably from about 90°C to about 140°C, and preferably in an open- air curing environment.

[0043] Contrary to what would have been expected from either US 2011 / 224382 or EP 2420535, the inventors have determined that the combination of the above components and process steps led to the successful cure of a wide range of unsaturated rubbers, including isoprene and butadiene rubbers, at much lower curing temperatures (<150°C) and in an open-air curing environment.

[0044] In a second aspect, the present disclosure relates to a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7; or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol; ii) about 1 to about 10 parts by weight per 100 parts by weight i) of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) about 0.5 to less than 10 parts by weight per 100 parts by weight i) of at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10.

[0045] It should be noted that the composition does not require accelerators for quick and correct cure at lower temperatures and in open-air curing environments, such as an accelerator comprising an amine moiety as required by the compositions of US 11518828. Compounds comprising amine moieties are generally undesirable and thus preferably avoided in cured rubbers because they can form and / or release potentially toxic byproducts. The accelerator N- phenyl-2-naphthylamine, for example, as used in the worked examples of US 11518828, is known to undergo metabolic dephenylation to produce 2-naphthylamine, a known human carcinogen. As such, in a preferred embodiment, the composition disclosed herein does not contain an accelerator comprising an amine moiety, such as those disclosed in US 11518828. i) at least one unsaturated rubber

[0046] As shown in the worked examples below, the present process for crosslinking rubbers is applicable to a wide variety of unsaturated rubbers, both natural and synthetic. “Unsaturated rubbers” as used herein has its ordinary meaning, namely rubbers with unsaturation in the rubber backbone (e.g., polybutadiene is an unsaturated rubber because it contains C=C double bonds in the rubber [polymer] backbone). “Unsaturated rubbers” does not mean saturated rubbers that have been end-functionalized with vinyl or alkynyl moieties, such as those disclosed in US 11518828 which are excluded from the scope of the term “unsaturated rubbers” as used herein.

[0047] That said, further investigations into open-air curing found that the curing system disclosed herein was unsuccessful with certain unsaturated rubbers. What has been observed is that unsaturated rubbers with a low Mn (<10,000 g / mol) and a high polydispersity value (Mw / Mn >1 .7) resulted in very poor cure or no cure at all under open-air curing conditions. Conversely, unsaturated rubbers with a low Mn (<10,000 g / mol) and a low polydispersity value (Mw / Mn <1 .7) resulted in excellent cure under open-air curing conditions. Furthermore, unsaturated rubbers with a high Mn (at least 10,000 g / mol) and a high polydispersity value (Mw / Mn) could be cured successfully under open-air curing conditions. Without wishing to be bound by theory, it is thought that the presence of significant quantities of low molecular weight species within the unsaturated rubber may be negatively interfering with the open-air curing process. Such low molecular weight species would be present in very small amounts for the unsaturated rubbers that cured successfully, whereas the unsaturated rubbers with a low Mn and a high polydispersity value (which failed to cure properly or at all) would contain significant levels of low molecular weight species.

[0048] For the avoidance of doubt, “open-air curing environment” as used herein has its ordinary / literal meaning, i.e., the composition is exposed (i.e., open) to an air environment (e.g., the air atmosphere of the laboratory or equivalent space) during the curing process. In general, this will be performed by thermally curing the composition in an open-air environment (i.e., the composition is not placed under a special and / or inert atmosphere, e.g., a N2blanket) such that the surface of the composition is in contact with the air environment surrounding the composition during the curing process. As noted above, an example of an "open-air curing environment" is spray-applications, wherein the uncured composition is sprayed onto a substrate and then thermally cured in an environment that is open to the air atmosphere (e.g., in a factory production line, or in an oven with hot air circulating). As used herein, the term “polydispersity value” (PDV, also known as the “polydispersity index”) is the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn), i.e., PDV = Mw / Mn. For example, for a polymer with a Mw of 3500 and a Mn of 3000, the PDV for that polymer is 1 .17 (3500 / 3000 = 1.17).

[0049] “Mw” as used herein is shorthand for “weight average molecular weight” and “Mn” as used herein is shorthand for “number average molecular weight”. For the present disclosure, both Mw and Mn are determined by gel permeation chromatography (GPC) in accordance with the following procedure (GPC is a type of size exclusion chromatography (SEC) that is mainly used to measure the molecular weight of polymer compounds). The GPC samples and standards are prepared by weighing the samples and adding mobile phase to a concentration of 2.5 mg / mL. Thereafter the standards and samples are shaken on a laboratory shaker for 30 minutes at 300 RPM. The samples are then filtered and injected into the system. The conditions used during the analysis are shown in the table below. For the calibration of the system, polybutadiene standards are used.

[0050] Accordingly, for the open-air curing system disclosed herein it has been determined that the unsaturated rubber must have a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7, or the unsaturated rubber must have a number average molecular weight (Mn) of at least 10,000 g / mol. If the unsaturated rubber does not meet at least one of these requirements, then correct cure at low temperatures cannot be guaranteed in an open-air curing environment (empirical studies indicate that failure to meet at least one of these requirements results in either an undesirably soft and tacky “cured” product, or no cure at all). This is an important consideration for the compositions disclosed herein, as a key objective behind the present disclosure was the provision of a rubber composition that is capable of curing correctly at low temperature and in an open-air environment (e.g., in spray applications). In a preferred embodiment, the unsaturated rubber ia) has a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .5, preferably less than 1 .3, and most preferably less than 1.1. Such rubbers are typically (but not exclusively) liquid rubbers (liquid at 25°C, 1 atmosphere), preferably a liquid butadiene homo or copolymer, a liquid isoprene homo or copolymer, or mixtures thereof. Accordingly, in a preferred embodiment, the unsaturated rubber is selected from a liquid rubber (liquid at 25°C, 1 atmosphere) having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7, preferably less than 1 .5, preferably less than 1 .3, and most preferably less than 1.1.

[0051] In another preferred embodiment, the unsaturated rubber ib) has a number average molecular weight (Mn) of at least 15,000 g / mol, such as at least 20,000 g / mol, or at least 25,000 g / mol. Such rubbers are typically (but not exclusively) solid long chain rubbers, preferably selected from solid butadiene rubber, solid styrene-butadiene rubber, solid natural rubber, and mixtures thereof (solid at 25°C, 1 atmosphere). Accordingly, in a preferred embodiment, the unsaturated rubber is a solid rubber (solid at 25°C, 1 atmosphere) having a number average molecular weight (Mn) of at least 10,000 g / mol, preferably at least 15,000 g / mol, such as at least 20,000 g / mol, or at least 25,000 g / mol.

[0052] Optionally, the unsaturated rubber i) may be functionalized (as long as the rubber backbone remains unsaturated), such as terminally functionalized. Preferred functionalization includes, but is not necessarily limited to, terminal hydroxy functionalization of the unsaturated rubber i). ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition

[0053] As shown in the worked examples below, the present process for crosslinking rubbers has been found to work only with a specific type of radical initiator - those capable of forming at least one alkyl radical upon thermal decomposition:

[0054] R is an (optionally substituted) aliphatic group (for tertiary alkyl radicals (-CR3) one Fl may be a radical stabilizing group, e.g., CN). The term “alkyl radical” as used herein includes branched and linear alkyl radicals. The term also includes substituted alkyl radicals (e.g., the alkyl radical formed by thermal decomposition of 2,2'-Azodi(isobutyronitrile) is a propyl radical substituted at the 2-position, i.e., 2- cyanopropyl radical) and unsubstituted alkyl radicals (e.g., the alkyl radical formed by thermal decomposition of dioctanoyl peroxide is an unsubstituted heptyl radical).

[0055] Preferred radical initiators include aliphatic peroxyesters, aliphatic diacylperoxides, aliphatic percarbonates, aliphatic perketals, aliphatic azo compounds, or mixtures thereof. Examples of preferred radical initiators include, but are not limited to, dioctanoyl peroxide, didecanoyl peroxide, dilauroyl peroxide, di(3,5,5-trimethylhexanoyl) peroxide, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butyl peroxybenzoate, t-butyl peroxyacetate, t-butyl peroxy isopropyl carbonate, tert-butyl peroxy-2- ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxydiethylacetate, tert-butyl peroxyisobutyrate, 1 ,1 ,3,3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2- ethylhexyl carbonate, tert-amyl peroxy-2-ethylhexyl carbonate, butyl (4,4-di(tert- butylperoxy)valerate, 1 ,1 -di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1 , 1 -di(tert- butylperoxy)-cyclohexane, 2,2'-Azodi(2-methylbutyronitrile), 2,2'-Azodi(isobutyronitrile), or mixtures thereof.

[0056] Most preferred are dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxy isopropyl carbonate, t-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, 2,2'-Azodi(2-methylbutyronitrile), butyl (4,4-di(tert- butylperoxy)valerate, 1 ,1 -di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1 , 1 -di(tert- butylperoxy)-cyclohexane, or mixtures thereof.

[0057] The radical initiator is preferably an organic peroxide, preferably at least one organic peroxide selected from any of the organic peroxides of the above lists.

[0058] It is preferred to use from 1 to 10 parts by weight of the at least one radical initiator ii) per 100 parts by weight of the unsaturated rubber i), preferably in an amount of about 1 to less than 10 parts per hundred parts of the unsaturated rubber, more preferably from 1 to 5 parts by weight of the at least one radical initiator ii) per 100 parts by weight of the unsaturated rubber i). It was found that the rubber could be successfully cured with up to about 10 phr radical initiator, with limited additional benefits being observed beyond 5 phr (phr = parts by weight per 100 parts by weight unsaturated rubber i)). Hi) at least one polythiol

[0059] The low temperature process disclosed herein requires a polythiol to effect curing. The type of polythiol used is not considered to be a limiting element of the process disclosed herein.

[0060] Preferably, the polythiol is a dimercaptan, a trimercaptan, a tetramercaptan, or mixtures thereof. Examples of preferred polythiols include, but are not limited to, polythiols derived from mercaptocarboxylic acid (such as pentaerythritol tetrakis(3-mercaptopropionate) and trimethylolpropane tris(3-mercaptopropionate)), and C2-C20 polythiols, such as a C2-C10 dithiol (such as 1 ,8-octanedithiol), a C3-C10 trithiol, a C4-C10 tetrathiol, or mixtures thereof. For ease of handling, the polythiol is preferably a liquid polythiol (liquid at 25°C, 1 atmosphere).

[0061] It is preferred to use from 0.5 to less than 10 parts by weight of the at least one polythiol iii) per 100 parts by weight of the unsaturated rubber i), more preferably from 0.5 to 5 parts by weight of the at least one polythiol iii) per 100 parts by weight of the unsaturated rubber i). It was found that at 10phr and above the benefits afforded by the increased amount of thiol were generally insignificant (i.e., did not benefit the curing system), and that the unsaturated rubber could be successfully cured at low temperatures when using about 0.5 to about 5 phr polythiol.

[0062] Weight ratio of ii) to iii)

[0063] As shown in the worked examples below, the weight ratio of ii) to iii) of from 10:1 to 1 :10 is a key factor for ensuring that the rubber will cure properly at lower temperatures. Outside this range, it has been found that the quality of the cured rubber is substantially reduced. It is preferred if the weight ratio of ii) to iii) is from 5:1 to 1 :5, preferably 3:1 to 1 :3, and more preferably about 2:1 to about 1 :2.

[0064] In a preferred embodiment, the composition of step a) comprises from 1 to 10 parts by weight of the at least one radical initiator ii) per 100 parts by weight of the unsaturated rubber i), more preferably from 1 to 5 parts by weight of the at least one radical initiator ii) per 100 parts by weight of the unsaturated rubber i), and from 0.5 to less than 10 parts by weight of the at least one polythiol iii) per 100 parts by weight of the unsaturated rubber i), more preferably from 0.5 to 5 parts by weight of the at least one polythiol iii) per 100 parts by weight of the unsaturated rubber i), wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10, preferably from 5:1 to 1 :5, preferably 3:1 to 1 :3, and more preferably about 1 :1 . Thus, in a preferred embodiment, the composition of step a) comprises i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7; or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol; ii) about 1 to about 10 parts per hundred parts by weight i) of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) about 0.5 to less than 10 parts per hundred parts by weight i) of at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10, preferably from 5:1 to 1 :5, preferably 3:1 to 1 :3, and more preferably about 2:1 to about 1 :2.

[0065] It is preferred if the combined total amount of radical initiator ii) and polythiol iii) in the composition of step a) is less than 15 parts per 100 parts by weight of the unsaturated rubber i), preferably 11 parts or less per 100 parts by weight of the unsaturated rubber i).

[0066] Thus, in another preferred embodiment, the composition of step a) comprises: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7; or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol; ii) about 1 to about 10 parts per hundred parts by weight i) of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) about 0.5 to less than 10 parts per hundred parts by weight i) of at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10, preferably from 5:1 to 1 :5, preferably 3:1 to 1 :3, and more preferably about 2:1 to about 1 :2, and wherein the combined total amount of radical initiator ii) and polythiol iii) in the composition is less than 15 parts per 100 parts by weight of the unsaturated rubber i), preferably 11 parts or less per 100 parts by weight of the unsaturated rubber i). Temperature

[0067] As shown in the worked examples below, an unexpected finding was that the composition had to be cured at the correct temperature to ensure that the unsaturated rubber cured properly.

[0068] At 150°C - the standard cure temperature for such compositions - it was found that the composition did not cure properly, instead producing a turbid product which is indicative of incorrect cure.

[0069] As such, in step b) of the process, the composition of step a) is cured at a temperature of less than 150°C, preferably from about 90°C to about 140°C, preferably from about 95°C to about 135°C, more preferably from about 100°C to about 130°C, and most preferably from about 105°C to about 125°C. Not only does this achieve the desired outcome of a low temperature process for crosslinking rubber, but it does so in a manner that ensures the production of a cured rubber of excellent quality.

[0070] Reinforcing Fillers

[0071] Cured rubbers often require reinforcing fillers to increase the strength and hardness of the cured rubber.

[0072] A surprising finding was that not only could a variety of reinforcing fillers be successfully incorporated into the cured rubber using the low-temperature process disclosed herein, but the resulting cured rubber had improved mechanical properties across all categories tested (hardness, tensile strength, rebound, crosslink density). Improving the mechanical properties of the rubber whilst simultaneously lowering the curing temperature is a substantial technical development.

[0073] Suitable reinforcing fillers include, but are not limited to, carbon black, silicas, clays, talc, and calcium carbonate. For completeness, talc and calcium carbonate are not usually considered as “reinforcing fillers”, however a further surprising finding was that in this curing system both talc and calcium carbonate were found to reinforce the cured rubber (and are thus herein considered a “reinforcing filler”). Both talc and calcium carbonate are very cheap fillers, therefore the finding that they act as reinforcing fillers in the thiol-peroxide curing system disclosed herein is highly beneficial from a cost perspective. Accordingly, in a preferred embodiment, the reinforcing filler is selected from talc, calcium carbonate, or mixtures thereof.

[0074] In view of the above, the present disclosure also relates to a composition comprising: i) at least one unsaturated rubber selected from: la) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7; or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol; ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition, preferably in an amount of from 1-10 phr, preferably about 1 -5 phr; iii) at least one polythiol, preferably in an amount of about 0.5 to less than 10phr, more preferably about 0.5-5 phr; and iv) at least one reinforcing filler, preferably selected from carbon black, silicas, clays, talc, calcium carbonate, or mixtures thereof, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10, preferably from 5:1 to 1 :5, preferably 3:1 to 1 :3, and more preferably about 1 :1 .

[0075] Per the above (and as demonstrated in the worked examples), this curable composition gives rise to rubber products with unexpectedly improved mechanical properties. Accordingly, the present disclosure also relates to a cured rubber obtainable by curing the above composition, preferably at a temperature of less than 150°C, preferably from about 90°C to about 140°C, preferably from about 95°C to about 135°C, more preferably from about 100°C to about 130°C, and most preferably from about 105°C to about 125°C.

[0076] The reinforcing filler may be used in any suitable amount, such as, but not limited to, up to about 200 parts per hundred parts by weight unsaturated rubber i), preferably about 1 to about 100 parts reinforcing filler per hundred parts by weight unsaturated rubber i).

[0077] Cured Rubber

[0078] In a second aspect, the present disclosure relates to a cured rubber obtainable by the process disclosed herein. As shown in the worked examples, the cured rubbers obtained from the process disclosed herein have very low to zero visible turbidity, indicating proper cure under the low temperature process conditions.

[0079] In a preferred embodiment, the cured rubber obtainable by the process disclosed herein contains less than 10 parts by weight polythiol crosslinking agent per 100 parts by weight of the unsaturated rubber i), preferably 5pbw or less polythiol crosslinking agent per 100 parts by weight of the unsaturated rubber i). The cured rubber preferably also contains a reinforcing filler, preferably selected from carbon black, silicas, clays, talc, calcium carbonate, or mixtures thereof. Talc and calcium carbonate are preferred because they are cheap yet highly effective reinforcing fillers in the thiol-peroxide system disclosed herein.

[0080] Applications

[0081] The presently disclosed curing system is applicable in a wide range of applications that require a low temperature-curable rubber, particularly applications that require the rubber to cure correctly in an open-air environment (i.e., open-air curing applications), including, but not limited to:

[0082] • Liquid Applied Sound Deadener / Damping applications (LASD)

[0083] • Rubber Printing Blankets

[0084] • Hose

[0085] • Belting

[0086] • Zero-leak Seals (for tyres, e.g., run-flat tyres)

[0087] • 3D-printing and low temperature curing (where liquid rubbers can be applied)

[0088] • Impregnated fabric gloves

[0089] • Adhesives and coatings

[0090] • Plasticizer: Low viscosity rubbers blending with high consistency rubbers (e.g., oils in rubber compounds)

[0091] • Binders (e.g. as additive for bitumen for roofing or asphalt applications)

[0092] • Plastic and rubber injection moulding (moulding of rubber on top or coextrusion of rubber next to a thermoplastic polymer)

[0093] Accordingly, the present disclosure also relates to the use of the presently disclosed low- temperature curing system for any of the above-mentioned applications that require a low temperature-curable rubber. The present disclosure also relates to the use of a composition as described above for open-air curing applications (which said open-air curing applications require a low cure temperature, i.e., less than 150°C).

[0094] Controlled Crosslinking

[0095] Another unexpected and advantageous finding was that, under the low temperature process conditions disclosed herein, the molar equivalents of SH groups in the composition of step a) (i.e., the molar amount of SH groups per 100pbw unsaturated rubber) correlated almost linearly with the crosslink density of the cured rubber (irrespective of the polythiol used). It was found that, for a given combination of radical initiator and unsaturated rubber, the crosslink density of the subsequently cured product could be manipulated (i.e., fine-tuned) in a generally predictable manner (i.e., within acceptable experimental error) by controlling the molar equivalents of SH groups in the composition of step a) (the linear correlation appears to be independent of the type of polythiol used, up to 10phr polythiol).

[0096] For example, for the combination of LBR305 (a liquid butadiene homopolymer) and 1 phr Trigonox 141 (2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane), the crosslink density was found to be substantially directly proportional to the molar equivalents of SH groups in the composition (worked Example 1 , below). The same trend was observed for the combination of LIR 390 (liquid butadiene-isoprene copolymer) and 2phr Perkadox SE8 (Dioctanoyl peroxide) (worked Example 7, below). The constant of proportionality (k) would seem to be somewhat dependent upon the specific combination and relative amounts of components i) and ii), however it would be straightforward to determine said constant of proportionality in a laboratory (i.e., by plotting crosslink density (Nm) of cured rubber against a range of SH molar equivalents for a given combination of components i) and ii) and cured under identical curing conditions, and then derive the constant of proportionality from the gradient of the linear trendline). For example, plotting the crosslink density (Nm) against the SH molar equivalents ([SHequiv, mmol]) for worked Examples 7A-7C below returns a substantially linear trendline with a k value of about 0.037 (i.e., Nm = 0.037*[SHeqUiv, mmol]). If, say, it was desired to increase the crosslink density of the cured rubber of Example 70 from 0.31 Nm to about 0.61 Nm (per Example 7A), then it could be predicted from the k value (approx. 0.037) that the amount of 1 ,8-octanedithiol would need to be increased such that the SH molar equivalents in composition a) would be about 16.4mmol (i.e., 0.61 (Nm) / 0.037(k)=16.4([SHeqUiv, mmol])). As shown in Example 7G (and Fig. 5), making such an adjustment to the SH content of Example 7C resulted in a cured rubber with the desired crosslink density (within acceptable experimental error).

[0097] What this demonstrates is that the curable composition comprising components i) to iii) as set out above can be easily manipulated to reliably control the crosslink density of the cured rubber with an acceptable level of accuracy when the composition is cured at a temperature of less than 150°C, such as from about 90°C to about 140°C.

[0098] Accordingly, in a preferred embodiment, the present disclosure relates to a process for controlling the crosslink density of a cured rubber, the process comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7 (Mw and Mn determined by GPC using polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol (Mn determined by GPC using polybutadiene standards); ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) about 0.5 to less than 10 parts per hundred parts by weight i) of at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10; and b) thermally curing the composition of step a) at a temperature of less than 150°C, optionally in an open-air curing environment. wherein the crosslink density of the cured rubber obtained after step b) is controlled by first determining a constant of proportionality (k) between the crosslink density (Nm) of cured rubber obtained from step b) and molar equivalents of SH groups ([SHequiv, mmol]) in the composition of preceding step a) (Nm *= k*[SHeqiUv„ mmol]), and then using said constant to adjust the molar equivalents of SH groups in the composition of step a) to control the crosslink density of the cured rubber obtained after step b).

[0099] Figures

[0100] Figure 1 shows cured rubber products obtained at a polythiol to peroxide ratio of 10:1 and 20:1.

[0101] Figure 2 shows rubber products that were cured at 150°C and at 120°C.

[0102] Figure 3 plots crosslink (gel) % against phr polythiol and phr peroxide.

[0103] Figure 4 plots the rheometer results of Examples 7A-7D.

[0104] Figure 5 plots the rheometer results of Examples 7E-7H.

[0105] Worked Examples

[0106] The present disclosure will be elucidated by the following examples without being limited thereto or thereby.

[0107] Mn and Mw according to the present disclosure are determined by the detailed GPC procedure set out above. Rheometer: PREMIER MDR from Alpha Technologies. This apparatus is able to provide, inter alia, calculated values of ML (minimum torque), MH (maximum torque), and T90 (time to 90% state of cure) as defined by International Standard ISO 6502-1991 .

[0108] To determine the % of crosslinking, the cured rubber was weighed, then extracted in boiling xylenes for 22h. After drying in an air ventilated oven at 125°C for 4 hours (to remove residual xylenes), the dried gel is weighed, and the gel wt.% remaining after extraction is reported (i.e., ([weight after extraction] / [weight before extraction])*100). Scale: 100 wt.% gel remaining = 100% crosslinked; 0 wt.% gel remaining = 0% crosslinked.

[0109] The below considerations apply to the following Examples:

[0110] • For liquid elastomers (e.g., LBR305), the peroxide, polythiol and liquid rubbers were mixed by hand at room temperature.

[0111] • For solid peroxides (e.g., Perkadox SE8 or Laurox), the liquid rubber is preheated in an oven for 10 minutes at 60°C, the peroxide is added after removal of the rubber from the oven, and the mixture prepared by stirring the composition by hand. The polythiol can be added to the liquid rubber before or after preheating.

[0112] • For rigid / highly viscous liquid rubbers (e.g. LIR390), the rubber is preheated in an oven for 10 minutes at 60°C (to soften it), the peroxide and polythiol are added after removal of the rubber from the oven, and the mixture prepared by stirring the composition by hand.

[0113] • For long-chain (solid) rubbers (e.g., natural rubber), the mixing in of the peroxide and polythiol is performed on a two-roll-mill which is heated to 50°C. This is a standard way of mixing in components to “solid” elastomers.

[0114] • All polythiols tested were liquids. However, if the polythiol would have been a melting solid, then the same preheating treatment could be applied.

[0115] Example 1

[0116] 100 parts by weight (pbw) of a liquid butadiene homopolymer (LBR305) was mixed with varying amounts of Pentaerythritol tetrakis(3-mercaptopropionate) (1 -20 parts by weight per hundred parts by weight rubber, phr) and 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane (Trigonox 141 ) (1 -20 phr). The resulting compositions are set out in Table 1 and were cured at 120°C in a closed mold (i.e., not open-air curing). The T90 value for each example was reached in under 15 minutes (the T90 value being the time required for the torque to reach 90% of the maximum achievable torque as measured by a rheometer).

[0117] Table 1.

[0118] 1LBR305 = a liquid butadiene homopolymer, Mn 26,000 [reported value], commercially available from Kuraray®

[0119] 2PEMP = Pentaerythritol tetrakis(3-mercaptopropionate)

[0120] 3Trigonox 141 = 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane, commercially available from Nouryon®

[0121] 4Crosslink density is determined as the difference in the calculated values of ML (minimum torque) and MH (maximum torque) (MH-ML).

[0122] 5wt.% of crosslinked rubber remaining after extraction in boiling xylenes for 22h

[0123] As shown in Table 1 , increasing the ratio of polythiol to peroxide beyond 10:1 resulted in a visually turbid rubber product. This indicates improper cure of the rubber, hence an inferior rubber product (see Fig. 1 , which shows the cured products obtained at a polythiol to peroxide ratio of 10:1 and 20:1 - the 20:1 product is of much poorer quality). Similarly, increasing the ratio of peroxide to polythiol beyond 10:1 also resulted in a visually turbid / hazy rubber product. Again, this indicates improper cure of the rubber, and therefore an inferior rubber product.

[0124] Thus, whilst each example resulted in a cured rubber, it was only possible to produce a cured rubber of excellent quality with polythiol to peroxide weight ratio within the range of 10:1 to 1 :10. This was an unexpected finding, as US 2011 / 224382 provided a worked example with a PEMP to peroxide ratio of 20:1 and indicated that the rubber was properly cured at 150°C. It was concluded that the weight ratio of the polythiol to peroxide was critical for the low temperature curing system disclosed herein.

[0125] It was also found that there was very little benefit to adding more than 10 phr peroxide or polythiol, with maximum crosslinking occurring at about 5-10 phr of either component (Fig. 3). Accordingly, it was determined that less than 10 phr of each component was more than sufficient for producing high quality cured rubbers at lower temperatures, which is much less than is considered necessary in US 2011 / 224382. As such, the process disclosed herein not only provides for lower temperature cure, but also provides a curing process that requires much less curing agent to achieve full and proper cure of the rubber.

[0126] Example 2

[0127] The results from Example 1 were surprising in view of Comparative Examples 20 and 21 of EP2420535. In those Comparative Examples 20 and 21 , the PEMP to peroxide weight ratio was 2:1 , however this was shown to be ineffective for curing a liquid butadiene rubber. The butadiene rubber tested in those Comparative Examples was very similar to that tested in Example 1 above, and Trigonox 121 (used as the radical initiator in those Comparative Examples) is very similar to Trigonox 141 used in Example 1 above. The only notable difference was the curing temperature (150°C in CE 20 and 21 of EP2420535; 120°C in Example 1 above). It was not expected that the difference in curing temperature would have any notable effect, however, to confirm this, two identical curing reactions were performed, one at 120°C (in accordance with the process disclosed herein) and another at 150°C (which is representative of CE 20 of EP2420535).

[0128] 100 pbw of a liquid butadiene homopolymer (LBR305) was mixed 10 phr Pentaerythritol tetrakis(3-mercaptopropionate) and 5 phr 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane (Trigonox 141 ) (1 -20 parts). The resulting compositions are set out in Table 2 and were cured at either 120°C or 150°C in a closed mold (i.e., not open-air curing). The T90 value for each example was reached in under 6 minutes (the T90 value being the time required for the torque to reach 90% of the maximum achievable torque as measured by a rheometer).

[0129] Table 2.

[0130] 1LBR305 = a liquid butadiene homopolymer, Mw 26,000 [reported value], commercially available from Kuraray®

[0131] 2PEMP = Pentaerythritol tetrakis(3-mercaptopropionate)

[0132] 3Trigonox 141 = 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane, commercially available from Nouryon®

[0133] As shown in Figure 2, the rubber composition cured at 150°C was very turbid, which, as noted above, is indicative of improper cure of the rubber and therefore an inferior rubber product. Conversely, the same rubber composition cured at 120°C was clear, indicating proper cure and thus a superior rubber product. It was surprising to find that the reduced curing temperature had such a marked effect on the quality of the final product.

[0134] Example 3

[0135] In addition to the above, it was also found that the low temperature cure only occurred with certain radical initiators.

[0136] 100 pbw of a liquid butadiene-isoprene copolymer (LIR390) was mixed 2 phr Pentaerythritol tetrakis(3-mercaptopropionate) and 2 phr radical initiator. The resulting compositions are set out in Table 3 and were cured in a closed mold (i.e., not open-air curing) at the temperature given.

[0137] O = cured to solid

[0138] X = no cure Table 3.

[0139] 1Laurox = Dilauroyl peroxide, commercially available from Nouryon ®

[0140] 2Tx42S = tert-Butyl peroxy-3,5,5-trimethylhexanoate, commercially available from Nouryon® under the trade name Trigonox 42S

[0141] 3Tx21 S = tert-Butyl peroxy-2-ethylhexanoate, commercially available from Nouryon® under the trade name Trigonox 21 S

[0142] 4Perkadox SE8 = Dioctanoyl peroxide, commercially available from Nouryon®

[0143] 5Perkadox PM50-S-ps (50%) = Di(4-methylbenzoyl) peroxide, commercially available from Nouryon®

[0144] 6Perkadox 24-FL = Dicetyl peroxydicarbonate, commercially available from Nouryon®

[0145] 7Tx141 = 2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane, commercially available from Nouryon® under the trade name Trigonox 141

[0146] 8AMBN = 2,2’-Azodi(2-methylbutyronitrile), commercially available from Nouryon® under the trade name Perkadox AMBN

[0147] 9Perkadox LT50 (50% BPO) = dibenzoyl peroxide, commercially available from Nouryon®

[0148] 10Trigonox BPIC-75C (75%) = tert-Butylperoxy isopropyl carbonate, 75% solution in isododecane, commercially available from Nouryon®

[0149] 11Trigonox C C-75 = tert-Butyl peroxybenzoate, 75% in odorless mineral spirits, commercially available from Nouryon®

[0150] 12Trigonox 1 17S = tert-Butylperoxy 2-ethylhexyl carbonate, commercially available from Nouryon®

[0151] 13Trigonox 17 = butyl (4,4-di(tert-butylperoxy)valerate, commercially available from Nouryon® 14Trigonox 29 = 1 ,1 -di(tert-butylperoxy)-3,3,5-trimethylcyclohexane

[0152] It was found that curing was successful only when the radical initiator was able to generate at least one alkyl radical upon decomposition; dibenzoyl peroxide and di(4-methylbenzoyl) peroxide generate phenyl radicals upon decomposition, and dicetyl peroxydicarbonate generates O-radicals upon decomposition. This was, again, an unexpected finding, as both US 2011 / 224382 and EP2420535 explicitly identify dibenzoyl peroxide as being a suitable initiator for the curing systems disclosed in each teaching. Example 4

[0153] As demonstrated in Examples 1 -3, the key elements to the low temperature curing system disclosed herein are:

[0154] • The weight ratio of polythiol to radical initiator needs to be in the range of 10:1 to 1 :10 to ensure proper cure;

[0155] • the curing temperature needs to be less than 150°C, preferably about 90-140°C, to ensure proper cure; and

[0156] • the radical initiator needs to form at least one alkyl radical upon thermal decomposition, otherwise no curing is observed at lower temperatures.

[0157] As shown below, this combination of features gives rise to a much more flexible curing system for unsaturated rubbers. This system is not limited in the same way as that of US 2011 / 224382 (which requires large amounts of a specific curing agent [polythiol derived from mercaptocarboxylic acid] and a high curing temperature) or EP2420535 (which was disclosed as being suitable for styrene-butadiene rubbers only, required a specific curing agent [polythiol derived from mercaptocarboxylic acid], and required a high curing temperature). It should also be noted that much less curing agent was required by the present curing system for full cure (<10phr curing agent; cf. >15phr curing agent in both US 2011 / 224382 and EP2420535). It could not have been predicted from either prior art teaching that such marked technical improvements could be achieved by such subtle technical modifications.

[0158] 100 pbw of an unsaturated rubber was mixed with a polythiol and 2 phr radical initiator. The resulting compositions are set out in Table 4 and were cured in a closed mold (i.e., not open- air curing) at the given temperature.

[0159] O = cured to solid

[0160] X = no cure

[0161] Table 4.

[0162] 1LIR390 = a liquid butadiene-isoprene copolymer, Mn 48,000 [reported value, polystyrene standard], commercially available from Kuraray®

[0163] 2LBR305 = a liquid butadiene homopolymer, Mn 26,000 [reported value, polystyrene standard], commercially available from Kuraray®

[0164] 3CIS BR 40 = Europrene Neocis CIS BR 40, a polybutadiene rubber4sSBR VSL = sSBR VSL 4526-0 HM, a Styrene-Butadiene Rubber

[0165] 5NR LAC10 = a natural rubber (polyisoprene)

[0166] 6Polyvest HT = a low molecular weight hydroxyl-terminated liquid butadiene rubber (Mn 2900 [reported value, polybutadiene standard])

[0167] 7LBR361 = a low molecular weight liquid butadiene homopolymer, Mn 5,500 [reported value, polystyrene standard], commercially available from Kuraray®

[0168] 8Butyl 3-mercaptopropionateaLaurox (Dilauroyl peroxide)bTrigonox 141 (2,5-Dimethyl-2,5-di(2-ethylhexanoylperoxy) hexane)cPerkadox SE8 (Dioctanoyl peroxide)

[0169] * Examples 4C, 4D and 4E contained 2phr reinforcing filler (HISII 315D)

[0170] Full cure was observed for different classes of unsaturated rubbers of varying molecular weight. It was also not essential to use a polythiol derived from mercaptocarboxylic acid. The versatility provided by the present curing system could not have been predicted from US 2011 / 224382 or EP2420535. For completeness, no cure was observed when either the radical initiator was absent (Comparative Example 4J) or the polythiol was absent (Comparative Example 4K). Additionally, no cure was observed when using a monothiol (Comparative Example 4L).

[0171] Example 5

[0172] In Example 4, Polyvest HT (an unsaturated rubber not meeting the molecular weight requirements of the unsaturated rubber i) of the presently disclosed compositions; C.Ex.4F) reached complete cure, and the cured rubber product was of sufficient hardness and did not have a tacky surface. Polyvest HT is reported as being almost identical to the OH-terminated liquid polybutadiene as used in Example 3 of US 2022056161 (Poly bd® R-45HTLO); the following table sets out the technical data for each of those polymers as obtained from the technical data sheets provided for each of those polymers:

[0173] *The ratio (%) of 1 ,2-vinyl, 1 -4 trans and 1 ,4-cis double bonds in the polybutadiene rubber backbone is the same for both (approx. 20 : 60 : 20)

[0174] It would therefore have been expected that the results obtained for Polyvest HT should have mirrored those obtained in Example 3 of US 2022056161 . The significant discrepancy in the results was therefore surprising. US 2022056161 , of course, is directed to compositions for use in 3D printing (i.e., open-air curing), whereas C.Ex.4F was prepared in a closed mold. As such, Example 3 of US 2022056161 was repeated (identical amounts and conditions, with open-air curing instead of a closed mold), using Polyvest HT in place of Poly bd® R-45HTLO. It was found that curing in an open-air environment (C. Ex. 5B, Table 5) produced a soft rubber (shore A <30) with very tacky exposed surfaces (the surface was of sufficient tackiness that it adhered to a nitrile rubber glove when the inventor touched the surface of the cured rubber with a gloved hand). A direct comparison was then performed in a closed mold (C. Ex. 5A, Table 5; i.e., not open-air), and the resulting cured rubber was of sufficient hardness (shore A 43-45) and did not have a tacky surface. The substantial loss of mechanical performance could thus be directly attributed to the open-air curing environment. A further comparison was performed in an open-air curing environment, with the exception that the Polyvest HT rubber was replaced with LBR361 (a low-molecular-weight non-terminal- functionalized polybutadiene; Ex. 5C, Table 5). The cured rubber in that instance had excellent hardness properties (shore A 70) and did not have a tacky surface. It was initially thought that it may have been the OH-terminal functionalization that gave rise to the difference in curing properties. Accordingly, a further comparison was performed with another low- molecular-weight non-terminal-functionalized polybutadiene, Polyvest 110 (C. Ex. 5D, Table 5). That composition, however, completely failed to cure (it remained a liquid).

[0175] Table 5. a Cured in an air ventilated oven at 100°C for 3.5 hours in a closed (sealed) container b Cured in an air ventilated oven at 100°C for 3.5 hours in an open container wherein the surface of the curing composition is exposed to hot air (“open-air curing environment”). c Shore A and surface feel not applicable as the composition remained a liquid

[0176] In view of the seemingly inconsistent results, each of the tested polymers were analyzed by GPC (per the detailed method set out above, using polybutadiene standards) to confirm the correctness of the molecular weight data provided in the technical data sheets. The results of those analyses are set out in Table 6: Table 6.

[0177] ‘Values obtained from the technical data sheet for Poly bd® R-45HTLO

[0178] §Calculated using Mw = PDV*Mn (rearranging PDV = Mw / Mn)

[0179] For the unsaturated rubber with a Mn of less than 3000 g / mol (Polyvest 110), no cure was observed, whereas for the unsaturated rubbers with an Mn of at least 3000 g / mol open-air cure was observed (at least to some extent). This indicated a minimum Mn of about 3000 g / mol was required for curing in open-air conditions. For the unsaturated rubber with a Mn of at least 3000 g / mol and a high polydispersity value (Polyvest HT; measured PDV = 1 .87), the open-air curing was not successful (the “cured” product was an unacceptably soft rubber with a tacky surface), whereas for the unsaturated rubber with a Mn of at least 3000 g / mol and a low polydispersity value (LBR361 ; measured PDV = 1 .04), the open-air curing was successful (the cured product was hard and non-tacky). Polydispersity (in this technical context) is a measure of the distribution of molecular mass in a given sample, so a low Mn with a high polydispersity will equate to a broad molecular weight distribution with a large number of low- molecular mass components within the unsaturated rubber sample, whereas a low Mn with a low polydispersity will equate to a narrow molecular weight distribution with a low number of low-molecular mass components within the unsaturated rubber sample. This data suggested that the unsaturated rubber required a low amount of low-molecular mass components to ensure successful cure in open-air conditions. This curing issue is not observed for unsaturated rubbers with a high Mn (e.g., at least 10,000 g / mol), which would make sense (in view of the above data) as such high Mn unsaturated rubbers inherently have very low quantities of low molecular weight species.

[0180] Example 6

[0181] The flexibility of the low temperature cure system disclosed herein is further exemplified by the ability to successfully incorporate various reinforcing fillers in various cured rubbers (Table 7, wherein the listed components were mixed together in the stated amounts and then cured at the stated temperature). These should be understood as being exemplary in nature only and are provided as a few non-limiting examples to demonstrate the ability of the curing system disclosed herein to successfully incorporate reinforcing fillers into the cured rubber across a range of unsaturated rubbers.

[0182] O = cured to solid X = no cure

[0183] Table 7.

[0184] 1Hi-Sil 315D is a reinforcing grade of silica It should be noted that calcium carbonate is not usually considered a “reinforcing filler”, however in this curing system it was found to reinforce to the cured rubber in the above example (the resulting rubber had a Shore A Hardness value of 57).

[0185] Furthermore, an unexpected outcome was that the material properties of the rubber produced by the low-temperature process disclosed herein were improved when compared to the standard curing process (high temperature crosslinking using peroxides). As shown in the data in Table 8, the mechanical properties of the rubber produced using the composition disclosed herein were improved across all categories tested (hardness, tensile strength, rebound, and crosslink density). Table 8.

[0186] Example 6B: 100 parts LBR305 + 100 phr Imercarb 36 (CaCOs) + 2 phr Tx141 + 2 phr PEMP Comparative Example 6C : 100 parts LBR305 + 100 phr Imercarb 36 (CaCOs) + 1 .4 phr Perkadox BC M50 / M100 = stress at 50% and 100% elongation, respectively

[0187] In view of the very low cost of calcium carbonate, the unexpected improvement in mechanical properties as provided by calcium carbonate in the thiol-peroxide cure system is clearly advantageous from a cost perspective. Curiously, it was found that talc (another low-cost material not generally considered as being a reinforcing filler) also reinforced the rubber under the thiol-peroxide cure system (Example 6D) but was found to be incompatible with a standard curing process (no cure was observed when talc was used in high temperature crosslinking using peroxides; Comparative Example 6E). Table 9.

[0188] This unexpected result was not unique to the type of talc used, with equivalent results being observed when using different talcs in the thiol-peroxide system disclosed herein. To confirm the reinforcing effects of the calcium carbonate and talc fillers in the thiol-peroxide system, the rubbers (from Ex. 6B & 6D) were subjected to swelling tests. As described in Kraus, J. Applied Polymer Science, 1963 (7), pp. 861 -871 , a filler with a very strong fillerrubber interaction (i.e., a reinforcing filler) will restrict swelling (for the purposes of this disclosure, Kraus provides a method for determining whether a filler is a reinforcing filler - a value of less than 1 represents a reinforcing filler). The swelling tests showed that the calcium carbonate and talc fillers significantly restricted swelling of the rubber (both fillers consistently generated a ct’o t’ value of less than 1 ; if values drop below 1 this is indicative of a restriction of swell by the filler addition, due to strong reinforcing interactions between the filler and the rubber, extending into the bulk of the sample). The swelling tests thus confirmed what was observed in the mechanical tests, that the talc and calcium carbonate fillers reinforce the rubber in the thiol-peroxide curing system disclosed herein. For completeness, the swelling test was also performed on the standard cure system (CEx.6C), and in that case the calcium carbonate filler was not found to be reinforcing (<t>0 / ct> > 1 ; values above 1 are indicative of a lack of adhesion between the filler-surface and the rubber, with desorption of the rubber from the filler surface generating cavities in the swollen sample). This proves that it is the combination of the calcium carbonate / talc with the cure system disclosed herein that gives rise to the unexpected reinforcement of the rubber when using those fillers.

[0189] Example 7

[0190] A further advantage of the present process is controllability of the crosslink density in the cured rubber. Under the process conditions disclosed herein, it was found that the crosslink density could be controlled by controlling the molar amount of SH groups in the composition of step a), irrespective of the type of polythiol employed.

[0191] 100 pbw of a liquid butadiene-isoprene copolymer (LIR390) was mixed with polythiol and 2 phr dioctanoyl peroxide (Perkadox SE8). The resulting compositions are set out in Table 10 and were cured at 110°C.

[0192] Figure 4 provides the rheometer results of Examples 7A-7D. A lower molar amount of SH groups correlated with lower crosslink density.

[0193] Figure 5 provides the rheometer results of Examples 7E-7H. The final crosslink density (Torque, Nm) was almost identical for each example when the amount of polythiol was adjusted to ensure equivalent molar amounts of SH groups in the starting composition. Table 10.

[0194] 1TEMP = Trimethylolpropane tris(3-mercaptopropionate)

[0195] 2Butyl 3-mercaptopropionate

[0196] These results demonstrate that the crosslink density of the cured product can be reliably controlled using the low temperature process disclosed herein simply by manipulating the molar amount of SH groups in the curable composition. This, in essence, provides for cured rubber products with customizable crosslink properties. This unexpected finding is highly beneficial to the technical field, as certain end uses require cured rubbers with specific levels of crosslink density.

[0197] As expected, the monothiol resulted in zero cure in each instance (Comparative Examples 7D and 7H).

[0198] In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than to mean ‘consisting of’. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Europe or elsewhere at the date hereof.

Claims

CLAIMS1 . A process for crosslinking rubber, the process comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7 (Mw and Mn determined by GPC using polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol (Mn determined by GPC using polybutadiene standards); ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and iii) at least one polythiol, wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10; and b) thermally curing the composition of step a) at a temperature of less than 150°C, optionally in an open-air curing environment.

2. The process of claim 1 , wherein the at least one unsaturated rubber ia) has a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .5, preferably less than 1 .3, and most preferably less than 1 .1 .

3. The process of claim 1 , wherein the at least one unsaturated rubber ib) has a number average molecular weight (Mn) of at least 15,000 g / mol.

4. The process of any one of claims 1 to 3, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25°C, 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.

5. The process of any one of claims 1 to 4, wherein the at least one unsaturated rubber i) comprises a solid long chain rubber, optionally selected from solid butadiene rubber, solid styrene-butadiene rubber, solid natural rubber, and mixtures thereof (solid at 25°C, 1 atmosphere).

6. The process of any one of claims 1 to 5, wherein the at least one radical initiator ii) is selected from aliphatic peroxyesters, aliphatic diacylperoxides, aliphatic percarbonates, aliphatic perketals, aliphatic azo compounds, or mixtures thereof.

7. The process of any one of claims 1 to 6, wherein the at least one radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxy isopropyl carbonate, t-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl (4,4-di(tert-butylperoxy)valerate, 1 ,1-di(tert-butylperoxy)- 3,3,5-trimethylcyclohexane, 1 ,1 -di(tert-butylperoxy)-cyclohexane, 2,2'-Azodi(2- methylbutyronitrile), or mixtures thereof.

8. The process of any one of claims 1 to 7, wherein the at least one polythiol iii) is a dimercaptan, a trimercaptan, a tetramercaptan, or mixtures thereof.

9. The process of any one of claims 1 to 8, wherein the at least one polythiol c) is selected from polythiols derived from mercaptocarboxylic acid, or C2-C20 polythiols, such as a C2-C10 dithiol, a C3-C10 trithiol, a C4-C10 tetrathiol, or mixtures thereof.

10. The process of any one of claims 1 to 9, wherein the composition of step a) comprises about 1 -10 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i), preferably about 1 -5 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i).11 . The process of any one of claims 1 to 10, wherein the composition of step a) comprises about 0.5-10 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i), preferably about 0.5-5 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i), preferably about 1 -5 parts by weight polythiol iii) per 100 parts by weight unsaturated rubber i).

12. The process of any one of claims 1 to 11 , wherein the composition of step a) further comprises a reinforcing filler, preferably selected from silica, calcium carbonate, talc, carbon black, clays, or mixtures thereof, more preferably selected from calcium carbonate, talc, or mixtures thereof.

13. The process of any one of claims 1 to 12, wherein the weight ratio of ii) to iii) is from about 5:1 to about 1 :5, preferably about 3:1 to about 1 :3, and more preferably about 2:1 to about 1 :2.

14. The process of any one of claims 1 to 13, wherein the curing temperature in step b) is from about 90°C to about 140°C, preferably about 95°C to about 135°C, more preferably from about 100°C to about 130°C, and most preferably from about 105°C to about 125°C.

15. The process of any one of claims 11 to 14, further comprising controlling the crosslink density of a cured rubber by first determining a constant of proportionality (k) between the crosslink density (Nm) of cured rubber obtained after step b) and molar equivalents of SH groups ([SH]) in the composition of preceding step a) (Nm = k*[SH]), and then using said constant (k) to adjust the molar equivalents of SH groups ([SH]) in the composition of step a) to control the crosslink density (Nm) of the cured rubber obtained after step b).

16. A cured rubber obtainable by the process of any one of claims 1 to 15.

17. A composition comprising: i) at least one unsaturated rubber selected from: ia) an unsaturated rubber having a number average molecular weight of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .7 (Mw and Mn determined by GPC using polybutadiene standards); or ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol (Mn determined by GPC using polybutadiene standards); ii) about 1 to about 10 parts by weight per 100 parts by weight i) of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; iii) about 0.5 to less than 10 parts by weight per 100 parts by weight i) of at least one polythiol; and iv) optionally at least one reinforcing filler; wherein the weight ratio of ii) to iii) is from 10:1 to 1 :10.

18. The composition of claim 17, wherein the at least one unsaturated rubber ia) has a number average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1 .5, preferably less than 1 .3, and most preferably less than 1.1.

19. The composition of claim 17, wherein the at least one unsaturated rubber ib) has a number average molecular weight (Mn) of at least 15000 g / mol.

20. The composition of any one of claims 17 to 19, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25°C, 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.21 . The composition of any one of claims 17 to 20, wherein the at least one unsaturated rubber i) comprises a solid long chain rubber, optionally selected from solid butadiene rubber, solid styrene-butadiene rubber, solid natural rubber, and mixtures thereof (solid at 25°C, 1 atmosphere).

22. The composition of any one of claims 17 to 21 , wherein the at least one radical initiator ii) is selected from aliphatic peroxyesters, aliphatic diacylperoxides, aliphatic percarbonates, aliphatic perketals, aliphatic azo compounds, or mixtures thereof.

23. The composition of any one of claims 17 to 22, wherein the at least one radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxy isopropyl carbonate, t-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl (4,4-di(tert-butylperoxy)valerate, 1 ,1 -di(tert-butylperoxy)- 3,3,5-trimethylcyclohexane, 1 ,1 -di(tert-butylperoxy)-cyclohexane, 2,2'-Azodi(2- methylbutyronitrile), or mixtures thereof.

24. The composition of any one of claims 17 to 23, wherein the at least one polythiol iii) is a dimercaptan, a trimercaptan, a tetramercaptan, or mixtures thereof.

25. The composition of any one of claims 17 to 24, wherein the at least one polythiol c) is selected from polythiols derived from mercaptocarboxylic acid, or C2-C20 polythiols, such as a C2-C10 dithiol, a C3-C10 trithiol, a C4-C10 tetrathiol, or mixtures thereof.

26. The composition of any one of claims 17 to 25, wherein the composition of step a) comprises about 1 -5 parts by weight radical initiator ii) per 100 parts by weight unsaturated rubber i).

27. The composition of any one of claims 17 to 26, wherein the composition of step a) comprises about 0.5-5 parts polythiol iii) per 100 parts unsaturated rubber i), preferably 1 -5 parts polythiol iii) per 100 parts unsaturated rubber i).

28. The composition of any one of claims 17 to 27, wherein the combined total amount of radical initiator ii) and polythiol iii) in the composition of step a) is less than 15 parts by weight per 100 parts by weight of the unsaturated rubber i).

29. The composition of any one of claims 17 to 28, wherein the composition of step a) comprises the at least one reinforcing filler, and wherein the at least one reinforcing filler is selected from silica, calcium carbonate, talc, carbon black, clays, or mixtures thereof.

30. The composition of claim 29, wherein the at least one reinforcing filler is selected from calcium carbonate, talc, or mixtures thereof.31 . Use of a composition according to any one of claims 17 to 30 for open-air curing applications.

Images