Low temperature process for crosslinking rubbers

Abstract

The invention relates to a method for crosslinking rubbers, said method comprising: a) obtaining a composition comprising: i) at least one unsaturated rubber selected from the group consisting of: 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 as determined by GPC using a polybutadiene standard); or ib) an unsaturated rubber having a number average molecular weight ((Mw and Mn) of at least 10,000 g / mol (Mn as determined by GPC using a polybutadiene standard); ii) at least one radical initiator which 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) thermocuring the composition of step a) at a temperature of less than 150 DEG C, optionally in an open curing environment.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a low temperature method for crosslinking rubber, preferably carried out in an open cure environment, and a composition suitable for said method. BACKGROUND OF THE INVENTION

[0002] Liquid rubbers such as low to medium molecular weight polybutadiene or polyisoprene are increasingly being used in the field of elastomer applications, largely because replacing non-reactive oils with these reactive liquid rubbers is environmentally beneficial.

[0003] Due to the high content of unsaturated carbon functional groups in these rubbers, they can be efficiently cured by peroxides. However, when using peroxides, the typical cure temperature for such unsaturated rubbers is 150 °C or higher. Such high temperatures are not ideal for many applications, for example due to temperature-sensitive substrates and / or environmental / economic considerations (high energy consumption).

[0004] US 2011 / 224382 discloses a thermosetting composition comprising (A) a diene-based polymer having two or more unsaturated bonds, (B) a polythiol derived from a mercapto carboxylic acid, and (C) a thermal free radical generator. This composition requires a large amount of curing agent and a high cure temperature of 150 - 170 °C.

[0005] EP 2420535 discloses a thiol-containing liquid rubber composition comprising a liquid styrene-butadiene copolymer and a polythiol derived from a mercapto carboxylic acid. This composition has been shown to be curable by UV (using a photoinitiator) or at a temperature of at least 150 °C (using a peroxide). The composition disclosed in EP 2420535 requires less curing agent than US 2011 / 224382, but this system has been shown to be successful only for liquid styrene-butadiene rubber (according to EP 2420535, other liquid rubbers, especially butadiene and isoprene rubbers, cannot be successfully cured using the system disclosed therein).

[0006] The cure systems of US 2011 / 224382 and EP 2420535 clearly have significant limitations and drawbacks, most notably the need for high temperature (150 °C or higher) curing, the need for a large amount of curing agent (≥15 phr), a lack of flexibility in the type of polythiol that can be used (which can lead to compatibility issues), and a low generality of the types of rubbers that can be cured by the system.

[0007] US 11518828 discloses a method for preparing a macromolecular network by crosslinking a terminally functionalized saturated rubber (“telechelic polymer”) with a polythiol in the presence of a peroxide. Although this method has been proven successful at low temperatures (70 °C), it is unacceptably slow and requires an accelerator containing an amine moiety (even in the presence of the accelerator, it takes 14 days to complete curing). The slow curing time and the need for an additional amine-functionalized compound are highly undesirable characteristics for such industrial methods.

[0008] US 2022056161 relates to curable compositions for 3D printing (i.e., open curing). Working Examples 2 and 3 of US 2022056161 provide a low-temperature (100 °C) hot casting method in which an unsaturated rubber (OH-terminated polybutadiene; Poly R-45HTLO) is crosslinked using a peroxide via a dithiol (3,6-dioxaoctane-1,8-dithiol). The resulting cured rubber product is very soft (Shore A hardness of 5 - 10), and the practical applications of this product are very limited. To determine whether the disclosed Shore A hardness values are true empirical results or merely reporting errors, the inventors of the present disclosure repeated Example 3 in US 2022056161 using an equivalent unsaturated rubber and found that not only was the resulting cured rubber soft, but the exposed surface of the cured rubber was also very sticky, and these two characteristics are not ideal properties in many end-use applications that require an open curing system (such as spray applications, where the uncured composition is sprayed onto a substrate and then thermally cured in an open environment). Working Examples 4 and 5 in US 2022056161 show that the Shore A hardness can be increased by switching from a thermal curing system to an ultraviolet curing system, but the ultraviolet curing system also has numerous drawbacks (e.g., requires specialized equipment and relatively expensive curing agents).

[0009] There is still a need for a low-temperature method for thermally curing unsaturated rubbers at low temperatures (especially in an open environment), and preferably, the method does not have the same limitations as previously known methods. Summary of the Invention

[0010] It has now been found that a specific method using a specific radical initiator and a polythiol can successfully crosslink various unsaturated rubbers at lower temperatures and under open curing conditions. Accordingly, the present disclosure can be summarized in the following aspects:

[0011] Aspect 1. A method for crosslinking a rubber, the method comprising:

[0012] a) obtaining a composition comprising:

[0013] i) at least one unsaturated rubber selected from the following:

[0014] 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 are determined by GPC using polybutadiene standards); or

[0015] ib) an unsaturated rubber having a number-average molecular weight (Mn) of at least 10,000 g / mol (Mn is determined by GPC using polybutadiene standards);

[0016] ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and

[0017] iii) at least one polythiol,

[0018] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10; and

[0019] b) thermally curing the composition of step a) at a temperature below 150 °C, optionally in an open curing environment.

[0020] Aspect 2. The method according to Aspect 1, wherein the number-average molecular weight (Mn) of the at least one unsaturated rubber ia) is at least 3000 g / mol and the polydispersity value (Mw / Mn) is less than 1.5, preferably less than 1.3 and most preferably less than 1.1.

[0021] Aspect 3. The method according to Aspect 1, wherein the number-average molecular weight (Mn) of the at least one unsaturated rubber ib) is at least 15,000 g / mol.

[0022] Aspect 4. The method according to any one of Aspects 1 to 3, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25 °C and 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.

[0023] Aspect 5. The method according to 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 and 1 atmosphere).

[0024] Aspect 6. The method according to any one of Aspects 1 to 5, wherein the at least one radical initiator ii) is selected from aliphatic peroxyesters, aliphatic diacyl peroxides, aliphatic percarbonates, aliphatic ketone peroxides, aliphatic azo compounds or mixtures thereof.

[0025] Aspect 7. The method according to any one of Aspects 1 to 6, wherein the at least one free radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl (4,4-bis(tert-butylperoxy)pentanoate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2'-azobis(2-methylbutyronitrile) or a mixture thereof.

[0026] Aspect 8. The method according to any one of Aspects 1 to 7, wherein the at least one polythiol iii) is a dithiol, trithiol, tetrathiol or a mixture thereof.

[0027] Aspect 9. The method according to any one of Aspects 1 to 8, wherein the at least one polythiol c) is selected from polythiols derived from mercapto carboxylic acids, C2-C10 dithiols, C3-C10 trithiols, C4-C10 tetrathiols or a mixture thereof.

[0028] Aspect 10. The method according to any one of Aspects 1 to 9, wherein the composition of step a) comprises about 1-10 parts by weight of the free radical initiator ii) per 100 parts by weight of the unsaturated rubber i), preferably about 1-5 parts by weight of the free radical initiator ii) per 100 parts by weight of the unsaturated rubber i).

[0029] Aspect 11. The method according to any one of Aspects 1 to 10, wherein the composition of step a) comprises about 0.5-10 parts by weight of the polythiol iii) per 100 parts by weight of the unsaturated rubber i), preferably about 0.5-5 parts by weight of the polythiol iii) per 100 parts by weight of the unsaturated rubber i), more preferably about 1-5 parts by weight of the polythiol iii) per 100 parts by weight of the unsaturated rubber i).

[0030] Aspect 12. The method according to 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, clay or a mixture thereof, more preferably selected from calcium carbonate, talc or a mixture thereof.

[0031] Aspect 13. The method according to any one of Aspects 1 to 12, wherein the weight ratio of ii) to iii) is 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.

[0032] Aspect 14. The method according to any one of Aspects 1 to 13, wherein the curing temperature in step b) is 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.

[0033] Aspect 15. The method according to any one of Aspects 11 to 14, further comprising controlling the crosslink density of the cured rubber by first determining the proportionality constant (k) between the crosslink density (Nm) of the cured rubber obtained after step b) and the molar equivalent of SH groups ([SH]) in the composition of the previous step a) (Nm ≈ k * [SH]), and then using said constant (k) to adjust the molar equivalent of SH groups ([SH]) in the composition of step a) to control the crosslink density (Nm) of the cured rubber obtained after step b).

[0034] Aspect 16. A cured rubber obtained by the method according to any one of Aspects 1 to 15.

[0035] Aspect 17. A composition comprising:

[0036] i) at least one unsaturated rubber selected from:

[0037] 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 are determined by GPC using polybutadiene standards); or

[0038] ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol (Mn is determined by GPC using polybutadiene standards);

[0039] ii) from about 1 to about 10 parts by weight, per 100 parts by weight of i), of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition;

[0040] iii) from about 0.5 to less than 10 parts by weight, per 100 parts by weight of i), of at least one polythiol; and

[0041] iv) optionally at least one reinforcing filler;

[0042] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10.

[0043] Aspect 18. The composition according to Aspect 17, wherein the number average molecular weight (Mn) of the at least one unsaturated rubber ia) is at least 3000 g / mol and the polydispersity value (Mw / Mn) is less than 1.5, preferably less than 1.3 and most preferably less than 1.1.

[0044] Aspect 19. The composition according to aspect 17, wherein the number average molecular weight (Mn) of the at least one unsaturated rubber ib) is at least 15,000 g / mol.

[0045] Aspect 20. The composition according to any one of aspects 17 to 19, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25 °C and 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.

[0046] Aspect 21. The composition according to 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 and 1 atmosphere).

[0047] Aspect 22. The composition according to any one of aspects 17 to 21, wherein the at least one free radical initiator ii) is selected from aliphatic peroxy esters, aliphatic diacyl peroxides, aliphatic percarbonates, aliphatic ketone peroxides, aliphatic azo compounds, or mixtures thereof.

[0048] Aspect 23. The composition according to any one of aspects 17 to 22, wherein the at least one free radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2'-azobis(2-methylbutyronitrile), or mixtures thereof.

[0049] Aspect 24. The composition according to any one of aspects 17 to 23, wherein the at least one polythiol iii) is a dithiol, trithiol, tetrathiol, or mixtures thereof.

[0050] Aspect 25. The composition according to any one of aspects 17 to 24, wherein the at least one polythiol c) is selected from polythiols derived from mercapto carboxylic acids, or C2-C20 polythiols such as C2-C10 dithiols, C3-C10 trithiols, C4-C10 tetrathiols, or mixtures thereof.

[0051] Aspect 26. The composition according to any one of aspects 17 to 25, wherein the composition of step a) comprises about 1-5 parts by weight of free radical initiator ii) per 100 parts by weight of unsaturated rubber i).

[0052] Aspect 27. The composition according to any one of aspects 17 to 26, wherein the composition of step a) comprises from about 0.5 to 5 parts, preferably from 1 to 5 parts, of polythiol (iii) per 100 parts of unsaturated rubber (i).

[0053] Aspect 28. The composition according to any one of aspects 17 to 27, wherein the total amount of the combination of free 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 unsaturated rubber (i).

[0054] Aspect 29. The composition according to 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, clay, or mixtures thereof.

[0055] Aspect 30. The composition according to aspect 29, wherein the at least one reinforcing filler is selected from calcium carbonate, talc, or mixtures thereof.

[0056] Aspect 31. Use of the composition according to any one of aspects 17 to 30 for open cure applications. Detailed Description

[0057] It has now been found that a specific method using specific free radical initiators and polythiols can successfully crosslink various unsaturated rubbers at lower temperatures and under open cure conditions.

[0058] Accordingly, in a first aspect, the present disclosure relates to a low temperature method for crosslinking rubber, preferably in an open cure environment, the method comprising:

[0059] a) obtaining a composition comprising:

[0060] i) at least one unsaturated rubber selected from:

[0061] 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

[0062] ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol;

[0063] ii) at least one free radical initiator that forms at least one alkyl radical upon thermal decomposition; and

[0064] iii) at least one polythiol,

[0065] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10; and

[0066] b) The composition of step a) is thermally cured at a temperature below 150 °C, preferably from about 90 °C to about 140 °C, preferably in an open curing environment.

[0067] Contrary to the expectations of US 2011 / 224382 or EP 2420535, the inventors have determined that the combination of the above components and method steps can successfully cure a variety of unsaturated rubbers, including isoprene and butadiene rubbers, at much lower curing temperatures (<150 °C) and in an open curing environment.

[0068] In a second aspect, the present disclosure relates to a composition comprising:

[0069] i) at least one unsaturated rubber selected from:

[0070] 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

[0071] ib) an unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol;

[0072] ii) from about 1 to about 10 parts by weight, per 100 parts by weight of i), of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition; and

[0073] iii) from about 0.5 to less than 10 parts by weight, per 100 parts by weight of i), of at least one polythiol;

[0074] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10.

[0075] Notably, the composition can cure rapidly and correctly at lower temperatures and in an open curing environment without the use of an accelerator, such as an accelerator containing an amine moiety required by the composition of US11518828. Such compounds containing an amine moiety are generally undesirable and are therefore preferably avoided in cured rubbers, as they may form and / or release potentially toxic by-products. For example, it is known that the accelerator N-phenyl-2-naphthylamine used in the working examples of US11518828 undergoes metabolic de-phenylation to produce 2-naphthylamine, which is a known human carcinogen. Thus, in a preferred embodiment, the compositions disclosed herein do not contain accelerators containing an amine moiety, such as those disclosed in US11518828.

[0076] i) at least one unsaturated rubber

[0077] As shown in the following working examples, the method of the present invention for crosslinking rubber is applicable to various natural and synthetic unsaturated rubbers. As used herein, "unsaturated rubber" has its ordinary meaning, i.e., a rubber containing unsaturated groups in the rubber backbone (e.g., polybutadiene is an unsaturated rubber because its rubber [polymer] backbone contains C═C double bonds). "Unsaturated rubber" does not refer to saturated rubbers end-functionalized with vinyl or alkynyl moieties, such as those disclosed in US 11518828, which are not within the scope of the term "unsaturated rubber" as used herein.

[0078] That being said, further investigation of open curing revealed that the curing systems disclosed herein are not suitable for certain unsaturated rubbers. It was observed that unsaturated rubbers with low molecular weight (Mn) (<10,000 g / mol) and high polydispersity values (Mw / Mn > 1.7) cure poorly or not at all under open curing conditions. In contrast, unsaturated rubbers with low Mn (<10,000 g / mol) and low polydispersity values (Mw / Mn < 1.7) cure excellently under open curing conditions. Additionally, unsaturated rubbers with high Mn (at least 10,000 g / mol) and high polydispersity values (Mw / Mn) can be successfully cured under open curing conditions. Without wishing to be bound by theory, we believe that the presence of a large amount of low molecular weight material in the unsaturated rubber may have a negative impact on the open curing process. For successfully cured unsaturated rubbers, the content of such low molecular weight material is very low, while for unsaturated rubbers with low Mn and high polydispersity values (which do not cure properly or not at all), the content of low molecular weight material is quite high.

[0079] For the avoidance of doubt, as used herein, "open curing environment" has its ordinary / literal meaning, i.e., during the curing process, the composition is exposed (i.e., open) to an air environment (e.g., the air environment of a laboratory or equivalent space). Typically, this will be achieved by thermally curing the composition in an open environment (i.e., the composition is not placed in a special and / or inert atmosphere, such as a nitrogen protective layer), such that the surface of the composition is in contact with the air environment surrounding the composition during the curing process. As described above, an example of an "open curing environment" is a spray application, where the uncured composition is sprayed onto a substrate and then thermally cured in an environment exposed to an air atmosphere (e.g., in a factory production line or in an oven with hot air circulation).

[0080] As used herein, the term "polydispersity value" (PDV, also known as "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 Mw of 3500 and Mn of 3000, the PDV of the polymer is 1.17 (3500 / 3000 = 1.17).

[0081] As used herein, "Mw" is an abbreviation for "weight-average molecular weight", and "Mn" is an abbreviation for "number-average molecular weight". For the present disclosure, both Mw and Mn are determined by gel permeation chromatography (GPC) (which is a size exclusion chromatography (SEC) mainly used for determining the molecular weight of polymeric compounds) according to the following procedure. GPC samples and standards are prepared by weighing the samples and adding mobile phase to a concentration of 2.5 mg / mL. Then the standards and samples are shaken on a laboratory shaker at 300 RPM for 30 minutes. Then the samples are filtered and injected into the system. The conditions used during the analysis are shown in the table below. To calibrate the system, polybutadiene standards are used.

[0082]

[0083] Thus, for the open curing systems disclosed herein, it has been determined that the number-average molecular weight (Mn) of the unsaturated rubber must be at least 3000 g / mol and the polydispersity value (Mw / Mn) must be less than 1.7, or the number-average molecular weight (Mn) of the unsaturated rubber must be at least 10,000 g / mol. If the unsaturated rubber does not meet at least one of these conditions, sufficient curing at low temperatures cannot be guaranteed in an open curing environment (empirical studies have shown that not meeting at least one of these conditions results in the "cured" product being undesirably soft and sticky, or not curing at all). This is an important consideration for the compositions disclosed herein, as a key objective of the present disclosure is to provide a rubber composition that can cure sufficiently at low temperatures and in an open environment (e.g., in spray applications).

[0084] In a preferred embodiment, the number-average molecular weight (Mn) of the unsaturated rubber ia) is at least 3000 g / mol and the polydispersity value (Mw / Mn) is 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 and 1 atmosphere), preferably liquid butadiene homopolymers or copolymers, liquid isoprene homopolymers or copolymers or mixtures thereof. Thus, in a preferred embodiment, the unsaturated rubber is selected from liquid rubbers (liquid at 25 °C and 1 atmosphere) having a number-average molecular weight (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) less than 1.7, preferably less than 1.5, preferably less than 1.3 and most preferably less than 1.1.

[0085] In another preferred embodiment, the number average molecular weight (Mn) of the unsaturated rubber ib) is 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 limited to) solid long-chain rubbers, preferably selected from solid butadiene rubbers, solid styrene-butadiene rubbers, solid natural rubbers, and mixtures thereof (solid at 25 °C and 1 atmosphere). Thus, in a preferred embodiment, the unsaturated rubber is a solid rubber (solid at 25 °C and 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.

[0086] Optionally, the unsaturated rubber i) can be functionalized (as long as the rubber backbone remains unsaturated), such as end-functionalized. Preferred functionalizations include but are not limited to end-hydroxyl functionalization of the unsaturated rubber i).

[0087] ii) at least one radical initiator that forms at least one alkyl radical upon thermal decomposition

[0088] As shown in the working examples below, it has been found that the present method of crosslinking rubbers is only applicable to certain types of radical initiators, namely those radical initiators that are capable of forming at least one alkyl radical upon thermal decomposition:

[0089]

[0090] R is an (optionally substituted) aliphatic group (for a tertiary alkyl radical (·CR3), one R can be a radical stabilizing group, such as CN).

[0091] 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 the thermal decomposition of 2,2'-azobisisobutyronitrile is a 2-substituted propyl radical, i.e., 2-cyanopropyl radical) and unsubstituted alkyl radicals (e.g., the alkyl substituent formed by the thermal decomposition of dioctanoyl peroxide is an unsubstituted heptyl radical).

[0092] Preferred radical initiators include aliphatic peroxy esters, aliphatic diacyl peroxides, aliphatic percarbonates, aliphatic ketone peroxides, 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 3,5,5-trimethylhexanoate peroxide, tert-butyl peroxybenzoate, tert-butyl peroxyacetate, tert-butyl peroxyisopropyl carbonate, tert-butyl 2-ethylhexanoate peroxide, tert-amyl 2-ethylhexanoate peroxide, tert-butyl peroxy diethylacetate, tert-butyl peroxyisobutyrate, 1,1,3,3-tetramethylbutyl 2-ethylhexanoate peroxide, tert-butyl 2-ethylhexyl carbonate peroxide, tert-amyl 2-ethylhexyl carbonate peroxide, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobisisobutyronitrile, or mixtures thereof.

[0093] Most preferred are dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, tert-butyl 3,5,5-trimethylhexanoate peroxide, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxybenzoate, tert-butyl 2-ethylhexanoate peroxide, tert-butyl 2-ethylhexyl carbonate peroxide, 2,2'-azobis(2-methylbutyronitrile), butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, or mixtures thereof.

[0094] The radical initiator is preferably an organic peroxide, more preferably at least one organic peroxide selected from any of the organic peroxides in the above list.

[0095] Preferably, 1 to 10 parts by weight of the at least one radical initiator ii) is used per 100 parts by weight of the unsaturated rubber i), preferably in an amount of about 1 to less than 10 parts per 100 parts by weight of the unsaturated rubber, more preferably 1 to 5 parts by weight of the at least one radical initiator ii) is used per 100 parts by weight of the unsaturated rubber i). We have found that rubber can be successfully cured using up to about 10 phr of the radical initiator, and only limited additional benefits are observed when exceeding 5 phr (phr = parts by weight per 100 parts by weight of the unsaturated rubber i)).

[0096] iii) at least one polythiol

[0097] The low-temperature method disclosed herein requires polythiols to achieve curing. The type of polythiol used is not considered a limiting factor in the method disclosed herein.

[0098] Preferably, the polythiol is a dithiol, trithiol, tetrathiol or a mixture thereof. Examples of preferred polythiols include, but are not limited to: polythiols derived from mercapto carboxylic acids (such as pentaerythritol tetra(3-mercaptopropionate) and trimethylolpropane tri(3-mercaptopropionate)), and C2-C20 polythiols, such as C2-C10 dithiols (such as 1,8-octanedithiol), C3-C10 trithiols, C4-C10 tetrathiols or mixtures thereof. For ease of handling, the polythiol is preferably a liquid polythiol (liquid at 25 °C and 1 atmosphere).

[0099] Preferably, 0.5 to less than 10 parts by weight of the at least one polythiol iii) is used per 100 parts by weight of the unsaturated rubber i), more preferably 0.5 to 5 parts by weight of the at least one polythiol iii) is used per 100 parts by weight of the unsaturated rubber i). We have found that when the amount of polythiol used is 10 phr or higher, the benefits of increasing the amount of polythiol are generally not significant (i.e., no benefit to the curing system), and when using about 0.5 to about 5 phr of polythiol, the unsaturated rubber can be successfully cured at low temperature.

[0100] The weight ratio of ii) to iii)

[0101] As shown in the following working examples, a weight ratio of ii) to iii) of 10:1 to 1:10 is a key factor in ensuring normal curing of the rubber at lower temperatures. Outside this range, a significant decrease in the quality of the cured rubber has been found. The weight ratio of ii) to iii) is preferably 5:1 to 1:5, preferably 3:1 to 1:3 and more preferably about 2:1 to about 1:2.

[0102] In a preferred embodiment, the composition of step a) comprises: 1 to 10 parts by weight of the at least one free radical initiator ii) per 100 parts by weight of the unsaturated rubber i), more preferably 1 to 5 parts by weight of the at least one free radical initiator ii) per 100 parts by weight of the unsaturated rubber i), and 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 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 10:1 to 1:10, preferably 5:1 to 1:5, preferably 3:1 to 1:3 and more preferably about 1:1.

[0103] Thus, in a preferred embodiment, the composition of step a) comprises

[0104] i) at least one unsaturated rubber selected from the following:

[0105] 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

[0106] ib) An unsaturated rubber having a number average molecular weight of at least 10,000 g / mol;

[0107] ii) From about 1 to about 10 parts of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition per 100 parts by weight of i); and

[0108] iii) From about 0.5 to less than 10 parts of at least one polythiol per 100 parts by weight of i),

[0109] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10, preferably from 5:1 to 1:5, preferably from 3:1 to 1:3 and more preferably from about 2:1 to about 1:2.

[0110] Preferably, the combined total of the radical initiator ii) and the 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).

[0111] Thus, in another preferred embodiment, the composition of step a) comprises:

[0112] i) At least one unsaturated rubber selected from:

[0113] 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

[0114] ib) An unsaturated rubber having a number average molecular weight of at least 10,000 g / mol;

[0115] ii) From about 1 to about 10 parts of at least one radical initiator that forms at least one alkyl radical upon thermal decomposition per 100 parts by weight of i); and

[0116] iii) From about 0.5 to less than 10 parts of at least one polythiol per 100 parts by weight of i),

[0117] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10, preferably from 5:1 to 1:5, preferably from 3:1 to 1:3 and more preferably from about 2:1 to about 1:2, and

[0118] wherein the combined total of the radical initiator ii) and the 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).

[0119] Temperature

[0120] As shown in the following working examples, an unexpected finding was that the composition had to be cured at the correct temperature to ensure sufficient curing of the unsaturated rubber. At 150 °C (the standard curing temperature for such compositions), we found that the composition was not sufficiently cured, but instead produced a cloudy product, indicating incorrect curing.

[0121] Thus, in step b) of the method, the composition of step a) is cured at a temperature below 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. This not only achieves the desired results of the low-temperature crosslinked rubber method, but also does so in a manner that ensures the production of a cured rubber of excellent quality.

[0122] Reinforcing filler

[0123] Cured rubber typically requires a reinforcing filler to increase the strength and hardness of the cured rubber.

[0124] Surprisingly, it has been found that using the low-temperature method disclosed herein, not only can a variety of reinforcing fillers be successfully incorporated into the cured rubber, but also the mechanical properties of the resulting cured rubber are improved in all test categories (hardness, tensile strength, resilience, crosslink density). Improving the mechanical properties of rubber while reducing the curing temperature is a significant technological advancement.

[0125] Suitable reinforcing fillers include, but are not limited to, carbon black, silica, clay, talc, and calcium carbonate. For completeness, talc and calcium carbonate are not typically considered "reinforcing fillers", however, another unexpected finding was that in this curing system, both talc and calcium carbonate were found to be able to reinforce the cured rubber (and are therefore considered "reinforcing fillers" herein). Both talc and calcium carbonate are very inexpensive fillers, and therefore, from a cost perspective, their use as reinforcing fillers in the thiol-peroxide curing system disclosed herein is highly advantageous. Thus, in a preferred embodiment, the reinforcing filler is selected from talc, calcium carbonate, or a mixture thereof.

[0126] In view of the above, the present disclosure also relates to a composition comprising:

[0127] i) at least one unsaturated rubber selected from:

[0128] 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

[0129] ib) an unsaturated rubber having a number average molecular weight of at least 10,000 g / mol;

[0130] ii) at least one free radical initiator that forms at least one alkyl radical upon thermal decomposition, preferably in an amount of 1-10 phr, more preferably about 1-5 phr;

[0131] iii) at least one polythiol, preferably in an amount of about 0.5 to less than 10 phr, more preferably about 0.5-5 phr; and

[0132] iv) at least one reinforcing filler, preferably selected from carbon black, silica, clay, talc, calcium carbonate or mixtures thereof,

[0133] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10, preferably from 5:1 to 1:5, preferably from 3:1 to 1:3 and more preferably about 1:1.

[0134] According to the above (and as demonstrated in the working examples), the rubber products prepared from the curable composition have unexpectedly improved mechanical properties. Accordingly, the present disclosure also relates to a cured rubber obtainable by curing the above composition, preferably cured at a temperature below 150 °C, preferably about 90 °C to about 140 °C, preferably about 95 °C to about 135 °C, more preferably about 100 °C to about 130 °C and most preferably about 105 °C to about 125 °C.

[0135] The reinforcing filler can be used in any suitable amount, such as but not limited to up to about 200 parts per 100 parts by weight of the unsaturated rubber i), preferably about 1 to about 100 parts of the reinforcing filler per 100 parts by weight of the unsaturated rubber i).

[0136] Cured rubber

[0137] In a second aspect, the present invention relates to a cured rubber obtainable by the method disclosed herein. As shown in the working examples, the cured rubber obtained by the method disclosed herein has a very low to even zero visible turbidity, indicating good curing under low temperature process conditions.

[0138] In a preferred embodiment, the cured rubber obtainable by the method disclosed herein contains less than 10 parts by weight of a polythiol crosslinking agent per 100 parts by weight of the unsaturated rubber i), preferably contains 5 parts by weight or less of a polythiol crosslinking agent per 100 parts by weight of the unsaturated rubber i). The cured rubber preferably further contains a reinforcing filler, preferably selected from carbon black, silica, clay, talc, calcium carbonate or mixtures thereof. Talc and calcium carbonate are preferred because they are inexpensive and efficient reinforcing fillers in the thiol-peroxide systems disclosed herein.

[0139] Applications

[0140] The curing system disclosed by the present invention can be widely applied to applications that require low-temperature curing of rubber, especially applications that require the rubber to be fully cured in an open environment (i.e., open curing applications), including but not limited to:

[0141] ● Liquid Applied Sound Deadening / Damping (LASD) applications

[0142] ● Rubber printing blankets

[0143] ● Hoses

[0144] ● Belts

[0145] ● Zero-leakage seals (for tires, such as run-flat tires)

[0146] ● 3D printing and low-temperature curing (wherein liquid rubber can be applied)

[0147] ● Impregnated fabric gloves

[0148] ● Adhesives and coatings

[0149] ● Plasticizers: low-viscosity rubbers blended with high-consistency rubbers (such as oils in rubber blends)

[0150] ● Binders (such as additives for roofing asphalt or bitumen applications)

[0151] ● Plastic and rubber injection molding (rubber is molded on top of a thermoplastic polymer, or rubber and thermoplastic polymer are co-extruded adjacent to each other).

[0152] Therefore, the present disclosure also relates to the use of the low-temperature curing system of the present disclosure in any of the above applications that require low-temperature curing of rubber. The present disclosure also relates to the use of the above compositions in open curing applications (the open curing applications require a lower curing temperature, i.e., below 150 °C).

[0153] Controlled crosslinking

[0154] Another unexpected and advantageous finding is that under the low-temperature process conditions disclosed herein, the molar equivalent of SH groups in the composition of step a) (i.e., the molar amount of SH groups per 100 parts by weight of unsaturated rubber) is almost linearly correlated with the crosslink density of the cured rubber (regardless of the polythiol used). It has been found that for a given combination of free radical initiator and unsaturated rubber, the crosslink density of the subsequent cured product can be controlled (i.e., fine-tuned) in a generally predictable manner (i.e., within an acceptable experimental error range) by controlling the molar equivalent of SH groups in the composition of step a) (this linear correlation seems to be independent of the type of polythiol used, up to 10 phr of polythiol).

[0155] For example, for the combination of LBR 305 (liquid butadiene homopolymer) and 1 phr Trigonox 141 (2,5 - dimethyl - 2,5 - bis(2 - ethylhexanoylperoxy)hexane), it was found that the cross - link density was essentially directly proportional to the molar equivalent of SH groups in the composition (see Working Example 1 below). The same trend was also observed for the combination of LIR 390 (liquid butadiene - isoprene copolymer) and 2 phr Perkadox SE8 (dioctanoyl peroxide) (see Working Example 7 below). The proportionality constant (k) seems to depend to some extent on the specific combination and relative amounts of components i) and ii), however, it is simpler and more straightforward to determine said proportionality constant in the laboratory (i.e., by plotting the cross - link density (Nm) of the cured rubber against a range of SH molar equivalents for a given combination of components i) and ii) and curing under the same curing conditions, and then obtaining the proportionality constant from the gradient of the linear trend line). For example, for Working Examples 7A - 7C below, a plot of cross - link density (Nm) against SH molar equivalent ([SH equivalent, mmol]) showed a substantially linear trend line and a k value of approximately 0.037 (i.e., Nm ≈ 0.037 * [SH 当量 , mmol]). That is to say, if it is desired to increase the cross - link density of the cured rubber of Example 7C from 0.31 Nm to approximately 0.61 Nm (according to Example 7A), then it can be predicted from the k value (approximately 0.037) that the amount of 1,8 - octanedithiol needs to be increased such that the SH molar equivalent in composition a) is approximately 16.4 mmol (i.e., 0.61 (Nm) / 0.037 (k) ≈ 16.4 ([SH 当量 , mmol])). As shown in Example 7G (and Figure 5 ), such an adjustment of the SH content in Example 7C enables the cured rubber to reach the desired cross - link density (within acceptable experimental error).

[0156] This demonstrates that when the composition is cured at a temperature below 150 °C, such as from about 90 °C to about 140 °C, the curable composition comprising components i) to iii) as described above can be readily manipulated to reliably control the cross - link density of the cured rubber at an acceptable level of precision.

[0157] Accordingly, in a preferred embodiment, the present disclosure relates to a method for controlling the cross - link density of a cured rubber, the method comprising:

[0158] a) obtaining a composition comprising:

[0159] i) at least one unsaturated rubber selected from:

[0160] 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 are determined by GPC using polybutadiene standards); or

[0161] ib) An unsaturated rubber having a number average molecular weight (Mn) of at least 10,000 g / mol (Mn is determined by GPC using polybutadiene standards);

[0162] ii) At least one free radical initiator that forms at least one alkyl radical upon thermal decomposition; and

[0163] iii) At least one polythiol in an amount of about 0.5 to less than 10 parts per hundred parts by weight of i),

[0164] wherein the weight ratio of ii) to iii) is from 10:1 to 1:10; and

[0165] b) Thermally curing the composition of step a) at a temperature below 150 °C, optionally in an open curing environment;

[0166] wherein the crosslink density of the cured rubber obtained after step b) is controlled by first determining the proportionality constant (Nm ≈ k * [SH 当量 , mmol]) between the crosslink density (Nm) of the cured rubber obtained from step b) and the molar equivalent of SH groups ([SH eqiuv , mmol]) in the composition of the previous step a), and then using said constant to adjust the molar equivalent of SH groups in the composition of step a) to control the crosslink density of the cured rubber obtained after step b). Description of the Drawings

[0167] Figure 1 Shows cured rubber products obtained at ratios of polythiol to peroxide of 10:1 and 20:1.

[0168] Figure 2 Shows rubber products cured at 150 °C and 120 °C.

[0169] Figure 3 Plots the relationship between the crosslink (gel) % and the phr of polythiol and the phr of peroxide.

[0170] Figure 4 Plots the rheometer results of Examples 7A - 7D.

[0171] Figure 5 Plots the rheometer results of Examples 7E - 7H.

[0172] Working Examples

[0173] The present disclosure will be illustrated by the following examples, but is not limited thereto or thereby defined.

[0174] The Mn and Mw according to the present disclosure are determined by the detailed GPC procedure listed above.

[0175] Rheometer: PREMIER MDR from Alpha Technologies. This device can provide calculated values such as ML (minimum torque), MH (maximum torque), and T90 (time to reach 90% cure state) as defined in the international standard ISO 6502 - 1991.

[0176] To determine the cross - linking percentage (%), the cured rubber is weighed and then extracted in boiling xylene for 22 hours. After drying in a ventilated oven at 125 °C for 4 hours (to remove residual xylene), the dried gel is weighed, and the weight percentage of the remaining gel after extraction is recorded (i.e., ([weight after extraction] / [weight before extraction]) * 100). Scale:

[0177]

[0178] A gel residue rate of 100 wt% = a cross - linking degree of 100%; a gel residue rate of 0 wt% = a cross - linking degree of 0%.

[0179]

[0180]

[0181] The following considerations apply to the following examples:

[0182] · For liquid elastomers (such as LBR305), peroxide, polythiol, and liquid rubber are manually mixed at room temperature.

[0183] · For solid peroxides (such as Perkadox SE8 or Laurox), the liquid rubber is pre - heated in an oven at 60 °C for 10 minutes. After removing the rubber from the oven, peroxide is added, and the mixture is prepared by manually stirring the composition. The polythiol can be added to the liquid rubber before or after pre - heating.

[0184] · For rigid / high - viscosity liquid rubbers (such as LIR390), the rubber is pre - heated in an oven at 60 °C for 10 minutes (to soften it). After removing the rubber from the oven, peroxide and polythiol are added, and the mixture is prepared by manually stirring the composition.

[0185] · For long - chain (solid) rubbers (such as natural rubber), the mixing of peroxide and polythiol is carried out on a two - roll mill heated to 50 °C. This is the standard method for mixing components into a "solid" elastomer.

[0185] ● All of the polythiols tested were liquids. However, if the polythiol is a molten solid, the same preheating treatment can be employed.

[0186] Example 1

[0187] 100 parts by weight (pbw) of a liquid butadiene homopolymer (LBR305) was mixed with different amounts of pentaerythritol tetrakis(3-mercaptopropionate) (1 - 20 parts by weight per 100 parts by weight of rubber, phr) and 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane (Trigonox 141) (1 - 20 phr). The resulting compositions are listed in Table 1 and cured in a closed mold at 120 °C (i.e., non-open cure). The T90 value for each sample was reached within 15 minutes (the T90 value is the time required for the torque to reach 90% of the maximum achievable torque measured by a rheometer).

[0188] Table 1.

[0189] Component Ex.1A Ex.1B Ex.1C Ex.1D Ex.1E Ex.1F Ex.1G Ex.1H Ex.11 <![CDATA[LBR305 1 > 100 100 100 100 100 100 100 100 100 <![CDATA[PEMP 2 > 20 10 5 2 1 1 1 1 1 <![CDATA[Trigonox 141 3 > 1 1 1 1 1 2 5 10 20 <![CDATA[Crosslink density 4 (Nm)]]> 3.56 3.13 1.88 0.5 0.13 0.33 1.19 1.83 2.1 T90 9.05 6.29 6.43 10.74 13.76 14.98 14.86 13.43 11.62 <![CDATA[% Crosslinking 5 > 100 99 100 92 68 83 95 95 93 Transparency after curing <![CDATA Turbid > Transparent Transparent Transparent Transparent Transparent Transparent Transparent <![CDATA Turbid >

[0190] 1 LBR305 = liquid butadiene homopolymer, Mn is 26,000 [reported value], available from obtainable.

[0191] 2 PEMP = pentaerythritol tetrakis(3-mercaptopropionate).

[0192] 3 Trigonox 141 = 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, available from obtainable

[0193] 4 The crosslink density was determined by the difference between the calculated values of ML (minimum torque) and MH (maximum torque) (MH - ML).

[0194] 5 wt.% is the weight percentage of the crosslinked rubber remaining after extraction in boiling xylene for 22 hours.

[0195] As shown in Table 1, when the ratio of polythiol to peroxide exceeded 10:1, the resulting rubber product was visually cloudy. This indicates that the rubber was not cured sufficiently, and thus the quality of the rubber product was poor (see Figure 1 , which shows the cured products obtained at polythiol-to-peroxide ratios of 10:1 and 20:1 - the product at 20:1 has much poorer quality). Similarly, when the ratio of peroxide to polythiol exceeded 10:1, the resulting rubber product was also visually cloudy. This also indicates that the rubber was not cured sufficiently, and thus the quality of the rubber product was poor.

[0196] Thus, although a cured rubber was obtained in each example, a cured rubber of excellent quality could only be produced when the weight ratio of polythiol to peroxide was in the range of 10:1 to 1:10. This was an unexpected finding because US 2011 / 224382 provided working examples with a weight ratio of PEMP to peroxide of 20:1 and showed that the rubber cured well at 150 °C. It was thus concluded that the weight ratio of polythiol to peroxide was crucial for the low-temperature curing system disclosed herein.

[0197] We also found that adding more than 10 phr of peroxide or polythiol provided little to no benefit, and maximum crosslinking occurred when each component was present at about 5 - 10 phr ( Figure 3 ). Thus, it was determined that using less than 10 phr of each component was sufficient to produce a high-quality cured rubber at a lower temperature, which was much lower than the amount considered necessary in US 2011 / 224382. Thus, the method disclosed herein not only achieved low-temperature curing but also provided a curing method that required much less curing agent to achieve complete and sufficient curing of the rubber.

[0198] Example 2

[0199] The results of Example 1 were unexpected compared to Comparative Examples 20 and 21 of EP 2420535. In these Comparative Examples 20 and 21, the weight ratio of PEMP to peroxide was 2:1; however, it was shown that this was ineffective for curing liquid butadiene rubber. The butadiene rubber tested in these comparative examples was very similar to the butadiene rubber tested in Example 1 above, and Trigonox 121 (used as a free radical initiator in these comparative examples) was very similar to Trigonox 141 used in Example 1 above. The only significant difference was the curing temperature (150 °C in Comparative Examples 20 and 21 of EP 2420535; 120 °C in Example 1 above). It was expected that the difference in curing temperature would not have any significant effect, but to confirm this, two identical curing reactions were carried out, one at 120 °C (according to the method disclosed herein) and the other at 150 °C (which represented the conditions of Comparative Example 20 of EP 2420535).

[0200] 100 pbw of liquid butadiene homopolymer (LBR305) was mixed with 10 - 10 phr of pentaerythritol tetra(3-mercaptopropionate) and 5 phr of 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane (Trigonox 141) (1 - 20 parts). The resulting composition was as shown in Table 2 and cured in a closed mold (i.e., non-open curing) at 120 °C or 150 °C. Each sample reached the T90 value (the T90 value is the time required for the torque to reach 90% of the maximum achievable torque measured by a rheometer) within 6 minutes.

[0201] Table 2

[0202]

[0203] 1 LBR305 = liquid butadiene homopolymer, Mw 26,000 [reported value], available from obtainable

[0204] 2 PEMP = pentaerythritol tetra(3-mercaptopropionate)

[0205] 3 Trigonox 141 = 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, available from obtainable

[0206] As Figure 2 shown, the rubber composition cured at 150 °C is very turbid, as described above, indicating insufficient rubber curing and thus poor quality of the rubber product. In contrast, the same rubber composition cured at 120 °C is transparent, indicating proper curing and thus good quality of the rubber product. Surprisingly, it was found that lowering the curing temperature has such a significant effect on the quality of the final product.

[0207] Example 3

[0208] In addition to the above, it was also found that low-temperature curing occurs only in the case of using certain radical initiators.

[0209] 100 pbw of liquid butadiene-isoprene copolymer (LIR390) was mixed with 2 phr of pentaerythritol tetra(3-mercaptopropionate) and 2 phr of a radical initiator. The resulting composition is shown in Table 3 and cured in a closed mold at a given temperature (i.e., non-open curing).

[0210] O = cured to solid

[0211] X = uncured

[0212] Table 3

[0213]

[0214] 1 Laurox = dilauroyl peroxide, available from obtainable

[0215] 2 Tx42S = tert-butyl peroxy-3,5,5-trimethylhexanoate, available from obtainable under the trade name Trigonox 42S

[0216] 3 Tx21S = tert-Butyl peroxy-2-ethylhexanoate, available from obtainable under the trade name Trigonox 21S

[0217] 4 Perkadox SE8 = Dioctanoyl peroxide, available from obtainable

[0218] 5 Perkadox PM50-S-ps(50%) = Bis(4-methylbenzoyl) peroxide, available from obtainable

[0219] 6 Perkadox 24-FL = Dicetyl peroxydicarbonate, available from obtainable

[0220] 7 Tx141 = 2,5-Dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, available from obtainable under the trade name Trigonox 141

[0221] 8 AMBN = 2,2'-Azobis(2-methylbutyronitrile), available from obtainable under the trade name Perkadox AMBN

[0222] 9 Perkadox LT50(50%BPO) = Benzoyl peroxide, available from obtainable

[0223] 10 TrigonoxBPIC-75C(75%) = tert-Butyl peroxyisopropyl carbonate, 75% isododecane solution, available from obtainable

[0224] 11 TrigonoxC C-75 = tert-Butyl peroxybenzoate, 75% odorless mineral spirits solution, available from obtainable

[0225] 12 Trigonox 117S = tert-Butyl peroxy 2-ethylhexyl carbonate, available from obtainable

[0226] 13 Trigonox 17 = Butyl 4,4-bis(tert-butylperoxy)valerate, available from obtainable

[0227] 14Trigonox 29 = 1,1 - bis(tert - butylperoxy) - 3,3,5 - trimethylcyclohexane

[0228] We found that curing can only be successful when the free - radical initiator decomposes to produce at least one alkyl radical; benzoyl peroxide and bis(4 - methylbenzoyl) peroxide decompose to produce phenyl radicals, while dicetyl peroxydicarbonate decomposes to produce O - radicals. This is another unexpected finding because both US 2011 / 224382 and EP 2420535 clearly state that benzoyl peroxide is a suitable initiator for the curing systems taught therein respectively.

[0229] Example 4

[0230] As confirmed in Examples 1 - 3, the key elements of the low - temperature curing system disclosed herein are:

[0231] · The weight ratio of polythiol to free - radical initiator needs to be in the range of 10:1 to 1:10 to ensure sufficient curing;

[0232] · The curing temperature needs to be below 150 °C, preferably about 90 - 140 °C to ensure sufficient curing; and

[0233] · The free - radical initiator needs to form at least one alkyl radical upon thermal decomposition, otherwise no curing will be observed at lower temperatures.

[0234] As shown below, this combination of features results in a much more flexible curing system for unsaturated rubbers. This system is not as restricted as US 2011 / 224382 (requiring a large amount of a specific curing agent [polythiol derived from mercapto - carboxylic acid] and a high curing temperature) or EP 2420535 (which is publicly stated to be applicable only to styrene - butadiene rubber, requires a specific curing agent [polythiol derived from mercapto - carboxylic acid], and requires a high curing temperature). It should also be noted that the curing system of the present invention requires much less curing agent (<10 phr curing agent; see ≥15 phr curing agent in US 2011 / 224382 and EP 2420535) to fully cure. It could not be predicted from the teachings of the prior art that such a minor technical improvement could achieve such a significant technical improvement.

[0235] 100 pbw of unsaturated rubber was mixed with polythiol and 2 phr of free - radical initiator. The resulting composition is listed in Table 4 and cured in a closed mold (i.e., non - open - air curing) at a given temperature.

[0236] O = cured to solid

[0237] X = uncured

[0238] Table 4.

[0239]

[0240] 1 LIR390 = liquid butadiene - isoprene copolymer, Mn is 48,000 [reported value, polystyrene standard], available from purchased

[0241] 2 LBR305 = liquid butadiene homopolymer, Mn is 26,000 [reported value, polystyrene standard], available from purchased

[0242] 3 CIS BR 40 = Europrene Neocis CIS BR 40, a polybutadiene rubber

[0243] 4 sSBR VSL = sSBR VSL4526 - 0HM, a styrene - butadiene rubber

[0244] 5 NR LAC10 = a natural rubber (polyisoprene)

[0245] 6 PolyvestHT = low - molecular - weight hydroxyl - terminated liquid butadiene rubber (Mn is 2900 [reported value, polybutadiene standard])

[0246] 7 LBR361 = low - molecular - weight liquid butadiene homopolymer, Mn is 5,500 [reported value, polystyrene standard], available from purchased

[0247] 8 butyl 3 - mercaptopropionate

[0248] a Laurox (dilauroyl peroxide)

[0249] b Trigonox 141 (2,5 - dimethyl - 2,5 - bis(2 - ethylhexanoylperoxy)hexane)

[0250] c Perkadox SE8 (dioctanoyl peroxide)

[0251] *Examples 4C, 4D and 4E contain 2 phr of reinforcing filler (HiSil 315D)

[0252] It was found that different types of unsaturated rubbers with different molecular weights were all fully cured. In addition, it is not necessary to use polythiols derived from mercapto carboxylic acids. The universality of the curing system of the present invention cannot be predicted from either US 2011 / 224382 or EP 2420535.

[0253] For completeness, no curing was observed when there was no radical initiator (Comparative Example 4J) or no polythiol (Comparative Example 4K). In addition, no curing was observed when a monothiol was used (Comparative Example 4L).

[0254] Example 5

[0255] In Example 4, Polyvest HT (an unsaturated rubber that does not meet the molecular weight requirements of the composition disclosed in the present invention; Comparative Example 4F) was fully cured, and the cured rubber product had sufficient hardness and a non-sticky surface. It was reported that Polyvest HT was almost the same as the OH-terminated liquid polybutadiene (Poly R-45HTLO) used in Example 3 of US 2022056161; the following table lists the technical data of each polymer obtained from the technical data sheets of each polymer:

[0256]

[0257] * For both, the ratio (%) of 1,2-vinyl, 1,4-trans, and 1,4-cis double bonds in the polybutadiene rubber backbone is the same (about 20:60:20)

[0258] Therefore, it was originally expected that the results of Polyvest HT should be consistent with those obtained in Example 3 of US 2022056161. Therefore, the significant difference in the results was unexpected. Of course, US 2022056161 relates to a composition for 3D printing (i.e., open curing), while Comparative Example 4F was prepared in a closed mold. Therefore, we repeated Example 3 of US 2022056161 (with the same amounts and conditions, but using open curing instead of a closed mold), and used Polyvest HT instead of Poly R-45HTLO. We found that curing in an open environment (Comparative Example 5B, Table 5) yielded a soft rubber (Shore A hardness < 30) with a very sticky exposed surface (the surface was sticky enough that when the inventor touched the cured rubber surface with a hand wearing nitrile rubber gloves, it adhered to the gloves). Then a direct comparison was made in a closed mold (Comparative Example 5A, Table 5; i.e., non-open), and the resulting cured rubber had sufficient hardness (Shore A hardness of 43 - 45) and a non-sticky surface. Thus, the significant loss of mechanical properties can be directly attributed to the open curing environment. Further comparison was made in an open curing environment, with the difference being that Polyvest HT rubber was replaced with LBR361 (a low molecular weight non-terminal functionalized polybutadiene; Example 5C, Table 5). In this case, the cured rubber had excellent hardness properties (Shore hardness A of 70) and a non-sticky surface. It was initially thought that the hydroxyl end-group functionalization might have caused the difference in curing performance. Therefore, a further comparison was made with another low molecular weight non-terminal functionalized polybutadiene, Polyvest 110 (Comparative Example 5D, Table 5). However, this composition did not cure at all (remained liquid).

[0259] Table 5.

[0260] Component C.Ex.5A C.Ex.5B Ex.5C C.Ex.5D Polyvest HT 5 5 LBR361 5 <![CDATA[Polyvest 110 a > 5 3,6 - dioxaoctane - 1,8 - dithiol 0.41 0.41 0.41 0.41 Trigonox 117 0.27 0.27 0.27 0.27 Curing conditions <![CDATA[Sealed mold a > <![CDATA[Open mold b > <![CDATA[Open mold b > <![CDATA[Open mold b > Curing Yes Yes Yes No Shore A hardness 43-45 <30 70 N / A Surface feel Non - sticky Very sticky Non - sticky N / A

[0261] a Cure for 3.5 hours in a ventilated oven at 100 °C in a closed (sealed) container.

[0262] b Cure for 3.5 hours in a ventilated oven at 100 °C in an open container, with the surface of the composition being cured exposed to hot air (“open curing environment”).

[0263] c Shore hardness A and surface feel are not applicable as the composition remained liquid.

[0264] Given the seemingly inconsistent results, each test polymer was analyzed by gel permeation chromatography (GPC) (using polybutadiene standards according to the method detailed above) to confirm the correctness of the molecular weight data provided in the technical data sheet. The results of these analyses are presented in Table 6:

[0265] Table 6.

[0266]

[0267] * Obtained from Poly Value from the technical data sheet of R-45HTLO

[0268] Calculate using Mw = PDV * Mn (rearranging PDV = Mw / Mn)

[0269] For unsaturated rubbers (Polyvest 110) with Mn less than 3000 g / mol, no curing was observed, while for unsaturated rubbers with Mn of at least 3000 g / mol, curing was observed (at least to some extent) under open conditions. This indicates that for curing under open conditions, a minimum Mn of approximately 3000 g / mol is required. For unsaturated rubbers with Mn of at least 3000 g / mol and a high polydispersity value (Polyvest HT; measured PDV = 1.87), curing was not successful under open conditions ("cured" product was an unacceptable soft rubber with a sticky surface), while for unsaturated rubbers with Mn of at least 3000 g / mol and a low polydispersity value (LBR361; measured PDV = 1.04), curing was successful under open conditions (cured product was hard and non-sticky). Polydispersity (in this technical context) is a measure of the molecular weight distribution in a given sample. Thus, low Mn and high polydispersity will be equivalent to a wider molecular weight distribution and a larger number of low molecular weight components within the unsaturated rubber sample, while low Mn and low polydispersity will be equivalent to a narrower molecular weight distribution and a smaller number of low molecular weight components within the unsaturated rubber sample. This data indicates that the unsaturated rubber requires a small amount of low molecular weight components to ensure successful curing under open conditions. For unsaturated rubbers with high Mn (e.g., at least 10,000 g / mol), no such curing problems were observed, which is reasonable (based on the above data) since such high Mn unsaturated rubbers inherently have a very small amount of low molecular weight material.

[0270] Example 6

[0271] The flexibility of the low-temperature curing system disclosed herein is further demonstrated by the ability to successfully incorporate various reinforcing fillers into various cured rubbers (Table 7, where the listed components are mixed together in the specified amounts and then cured at the specified temperature). These examples should be understood as being of an illustrative nature only and are provided in the form of several non-limiting examples to demonstrate that the curing system disclosed herein is capable of successfully incorporating reinforcing fillers into the cured rubbers of various unsaturated rubbers.

[0272] O = cured to a solid

[0273] X = not cured

[0274] Table 7.

[0275] Component Example 6A Example 4C Example 4D Example 4E Example 6B Tx21S 2 Tx141 2 Perkadox SE8 2 2 2 PEMP 2 2 2 2 2 LIR390 100 CIS BR 40 100 sSBR VSL 100 NR LAC10 100 LBR305 100 Carbon black 5 Hi - Sil 315D 2 2 2 Calcium carbonate 100 Curing temperature (°C) 120 110 110 110 120 Curing O O O O O Crosslink density (Nm) 1.62 1.05 0.52 0.15 1.58

[0276] 1 Hi-Sil 315D is a reinforcing grade silica.

[0277] It should be noted that calcium carbonate is generally not regarded as a "reinforcing filler". However, in this curing system, it was found that it has a reinforcing effect on the cured rubber in the above examples (the Shore A hardness value of the obtained rubber is 57).

[0278] In addition, an unexpected result is that when compared with the standard curing process (high-temperature cross-linking using peroxides), the material properties of the rubber produced by the low-temperature method disclosed herein have been improved. As shown by the data in Table 8, the mechanical properties of the rubber produced using the compositions disclosed herein have been improved in all test categories (hardness, tensile strength, resilience, and cross-link density).

[0279] Table 8.

[0280]

[0281] Example 6B: 100 parts of LBR305 + 100 phr of Imercarb 36 (CaCO3) + 2 phr of Tx141 + 2 phr of PEMP

[0282] Comparative Example 6C: 100 parts of LBR305 + 100 phr of Imercarb 36 (CaCO3) + 1.4 phr of Perkadox BC

[0283] M50 / M100 = stress at elongation of 50% and 100% respectively.

[0284] In view of the very low cost of calcium carbonate, the unexpected improvement in mechanical properties provided by calcium carbonate in the thiol-peroxide curing system is clearly advantageous from a cost perspective.

[0285] Strangely, we found that talc (another low-cost material, generally not regarded as a reinforcing filler) also reinforced the rubber in the thiol-peroxide curing system (Example 6D), but it was found to be incompatible with the standard curing process (no curing was observed when using talc during high-temperature cross-linking with peroxides; Comparative Example 6E).

[0286] Table 9.

[0287]

[0288] This unexpected result is not unique to the type of talc used, and the same result is observed when using different talcs in the thiol-peroxide system disclosed herein.

[0289] To confirm the reinforcing effect of calcium carbonate and talc fillers in a thiol-peroxide system, a swelling test was conducted on the rubber (from Examples 6B and 6D). As described by Kraus in J. Applied Polymer Science, 1963(7), pp. 861 - 871, fillers with a very strong interaction between the filler and the rubber (i.e., reinforcing fillers) will limit swelling (for the purposes of the present disclosure, Kraus provided a method for determining whether a filler is a reinforcing filler, i.e., a Φ o / Φ value less than 1 indicates a reinforcing filler). The swelling test showed that the calcium carbonate and talc fillers significantly limited the swelling of the rubber (the Φ o / Φ values of both fillers were consistently less than 1; if the value decreased to less than 1, this indicated that the addition of the filler limited the swelling and extended into the bulk of the sample due to the strong reinforcing interaction between the filler and the rubber). Thus, the swelling test confirmed the results observed in the mechanical tests, i.e., the talc and calcium carbonate fillers enhanced the rubber in the thiol-peroxide curing system disclosed herein. For completeness, the swelling test was also conducted on a standard curing system (Comparative Example 6C), and in this case it was found that the calcium carbonate filler did not exhibit a reinforcing effect (Φ o / Φ > 1; a value greater than 1 indicates a lack of adhesion between the filler surface and the rubber, and the rubber desorbs from the filler surface, thus forming cavities in the swollen sample). This demonstrated that it is the combination of calcium carbonate / talc with the curing system disclosed herein that produces an unexpected rubber reinforcing effect when using such fillers.

[0290] Example 7

[0291] Another advantage of the method of the present invention is the controllability of the crosslink density in the cured rubber. Under the process conditions disclosed herein, it was found that the crosslink density can be controlled by controlling the molar amount of SH groups in the composition of step a), regardless of the type of polythiol used.

[0292] 100 pbw of liquid butadiene-isoprene copolymer (LIR390) was mixed with a polythiol and 2 phr of dioctanoyl peroxide (Perkadox SE8). The resulting composition is listed in Table 10 and cured at 110 °C.

[0293] Figure 4 Rheometer results for Examples 7A - 7D are provided. A lower molar amount of SH groups is associated with a lower crosslink density.

[0294] Figure 5 Rheometer results for Examples 7E - 7H are provided. When the amount of polythiol is adjusted to ensure an equal molar amount of SH groups in the starting composition, the final crosslink density (torque, Nm) for each example is almost the same.

[0295] Table 10.

[0296]

[0297] 1 T EMP = trimethylolpropane tris(3-mercaptopropionate)

[0298] 2 butyl 3-mercaptopropionate

[0299] These results show that by simply controlling the molar amount of SH groups in the curable composition, the crosslink density of the cured product can be reliably controlled using the low-temperature method disclosed herein. This essentially provides customizable crosslinking properties for cured rubber products. This unexpected finding is very beneficial to this technical field because certain end uses require cured rubber with a specific crosslink density.

[0300] As expected, in each case the monothiol resulted in zero cure (Comparative Examples 7D and 7H).

[0301] In this specification, unless otherwise clearly stated, the word "or" refers to an operator that returns a true value when one or both of the stated conditions are met, rather than an "exclusive or" operator, which requires only one of the conditions to be met. The word "comprising" means "comprising", rather than "consisting of". All prior teachings admitted above are incorporated herein by reference. Any reference to any prior published document herein should not be taken as an admission or indication that its teachings were common general knowledge in Europe or elsewhere as of the date hereof.

Claims

1. A method for crosslinking rubber, the method 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 are 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 is 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 below 150 °C, optionally in an open curing environment.

2. The method according to claim 1, wherein the number average molecular weight (Mn) of the at least one unsaturated rubber ia) is at least 3000 g / mol and the polydispersity value (Mw / Mn) is less than 1.5, preferably less than 1.3 and most preferably less than 1.

1.

3. The method according to claim 1, wherein the number average molecular weight (Mn) of the at least one unsaturated rubber ib) is at least 15,000 g / mol.

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

5. The method according to 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 and 1 atm).

6. The method according to any one of claims 1 to 5, wherein the at least one radical initiator ii) is selected from aliphatic peroxy esters, aliphatic diacyl peroxides, aliphatic percarbonates, aliphatic ketone peroxides, aliphatic azo compounds or mixtures thereof.

7. The method according to 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, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2'-azobis(2-methylbutyronitrile) or mixtures thereof.

8. The method according to any one of claims 1 to 7, wherein the at least one polythiol iii) is a dithiol, trithiol, tetrathiol or mixtures thereof.

9. The method according to any one of claims 1 to 8, wherein the at least one polythiol c) is selected from polythiols derived from mercapto carboxylic acids, or C2-C20 polythiols such as C2-C10 dithiols, C3-C10 trithiols, C4-C10 tetrithiols, or mixtures thereof.

10. The method according to any one of claims 1 to 9, wherein the composition of step a) comprises from about 1 to about 10 parts by weight of a free radical initiator ii) per 100 parts by weight of the unsaturated rubber i), preferably from about 1 to about 5 parts by weight of a free radical initiator ii) per 100 parts by weight of the unsaturated rubber i).

11. The method according to any one of claims 1 to 10, wherein the composition of step a) comprises from about 0.5 to about 10 parts by weight of a polythiol iii) per 100 parts by weight of the unsaturated rubber i), preferably from about 0.5 to about 5 parts by weight of a polythiol iii) per 100 parts by weight of the unsaturated rubber i), preferably from about 1 to about 5 parts by weight of a polythiol iii) per 100 parts by weight of the unsaturated rubber i).

12. The method according to 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, clay, or mixtures thereof, more preferably selected from calcium carbonate, talc, or mixtures thereof.

13. The method according to 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 from about 3:1 to about 1:3, and more preferably from about 2:1 to about 1:

2.

14. The method according to any one of claims 1 to 13, wherein the curing temperature in step b) is 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.

15. The method according to any one of claims 11 to 14, further comprising controlling the crosslink density of the cured rubber by first determining the proportionality constant (k) between the crosslink density (Nm) of the cured rubber obtained after step b) and the molar equivalent of SH groups ([SH]) in the composition of the previous step a) (Nm ≈ k * [SH]), and then using the constant (k) to adjust the molar equivalent 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 obtained by the method according to 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 (Mn) of at least 3000 g / mol and a polydispersity value (Mw / Mn) of less than 1.7 (Mw and Mn are 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 is determined by GPC using polybutadiene standards); ii) from about 1 to about 10 parts by weight of at least one free radical initiator that forms at least one alkyl radical upon thermal decomposition per 100 parts by weight of i); iii) from about 0.5 to less than 10 parts by weight, per 100 parts by weight of 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 according to claim 17, wherein the number-average molecular weight (Mn) of the at least one unsaturated rubber ia) is at least 3000 g / mol and the polydispersity value (Mw / Mn) is less than 1.5, preferably less than 1.3 and most preferably less than 1.

1.

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

20. The composition according to any one of claims 17 to 19, wherein the at least one unsaturated rubber i) comprises a liquid rubber (liquid at 25 °C and 1 atmosphere), preferably a liquid isoprene homopolymer or copolymer, or a liquid butadiene homopolymer or copolymer.

21. The composition according to 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 and 1 atmosphere).

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

23. The composition according to any one of claims 17 to 22, wherein the at least one free radical initiator ii) is selected from dioctanoyl peroxide, dilauroyl peroxide, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, tert-butyl peroxy-3,5,5-trimethylhexanoate, tert-butyl peroxyisopropyl carbonate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexyl carbonate, butyl 4,4-bis(tert-butylperoxy)valerate, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(tert-butylperoxy)cyclohexane, 2,2'-azobis(2-methylbutyronitrile) or mixtures thereof.

24. The composition according to any one of claims 17 to 23, wherein the at least one polythiol iii) is a dithiol, trithiol, tetrathiol or mixtures thereof.

25. The composition according to any one of claims 17 to 24, wherein the at least one polythiol c) is selected from polythiols derived from mercapto carboxylic acids, or C2-C20 polythiols such as C2-C10 dithiols, C3-C10 trithiols, C4-C10 tetrathiols or mixtures thereof.

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

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

28. A composition according to any one of claims 17 to 27, wherein the total amount of the combination of free 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 unsaturated rubber i).

29. A composition according to 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, clay or mixtures thereof.

30. A composition according to 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 cure applications.

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