Description:This application is the patent of addition under section 54 and Rule 13(3) of Indian Patent Act 1970, having parent application 202414082427 Dated 28/10/2024 (PRIORITY DATE: 30/10/2023).
CROSSLINKING SYSTEM SAFE PROCESS FOR FLUOROELASTOMERS
FIELD OF THE INVENTION
The present disclosure relates to an improved process for the preparation of novel, safer and sustainable crosslinking system for fluoroelastomers. Particularly, present disclosure relates to a process for producing bisphenol free, safe crosslinking system for fluoroelastomers with improved characterization.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
FKM is a family of fluorocarbon-based fluoroelastomer materials defined by ASTM International standard. FKM is the American standard (ASTM) short form name for Fluoroelastomers or fluoro rubber material. F stands for Fluoro; the K is an abbreviation of the German word Kohlenstoff, meaning Carbon; and the M is the designation of saturated backbone rubber from ASTM.
FFKM are perfluoroelastomeric compounds containing an even higher amount of fluorine than FKM fluoroelastomers.
Phr or phr (parts per hundred rubber) refers to a unit of measure used by rubber chemists to depict what amount of certain ingredients are needed.
BACKGROUND OF THE INVENTION
The background information hereinbelow relates to the present disclosure but is not necessarily prior art.
Fluoroelastomers are an important class of high-performance elastomers because of their versatility and unique combination of relevant properties. They have a wide range of applications in strategic materials, automobiles, aerospace, electronics, and energy sectors and the like. Fluoroelastomers have excellent thermal stability, oil resistance, excellent mechanical properties and abrasion resistance properties. These properties are mainly linked to the low polarizability and the strong electronegativity of the fluorine atom to its small van der Waals radius (1.32 Å) and to the strong C–F bond (485 kJ mol−1). Hence, fluoroelastomers exhibit high thermal, chemical, ageing and weather resistance, excellent inertness to solvents, hydrocarbons, acids and alkalis, low dielectric constants, low flammability, low refractive index, low surface energy and low moisture absorption.
Commercially available fluoroelastomers are mainly of two types: FKM elastomers and FFKM elastomers. FKM elastomers are highly fluorinated (perfluorinated), polymers in which vinylidene fluoride (VDF) is used as a partially fluorinated co-monomer. FFKM perfluoroelastomers are fully fluorinated hydrocarbons in which there are no C-H bonds, but they have ether linkage. FFKM can withstand exposure to almost any chemicals.
Fluoroelastomers, particularly fluoroelastomers of the FKM type, are widely used in industrial applications requiring high-performance materials, such as automotive, aerospace, oil and gas, and chemical processing industries. Their superior thermal stability, chemical resistance, and mechanical durability make them indispensable for sealing, gasket, and lining applications in harsh environments.
The curing reactions in FKM elastomers are associated with the strong polarity of C-F bonds that leads to polarization of molecules leading to the elimination of hydrogen fluoride (HF) under the influence of several factors such as incorporation of additives. Existing crosslinking systems are based on diamine derivatives, bisphenol derivatives, peroxides with co-agent and high energy radiation. Among them, bisphenol curing system is the most widely used system for crosslinking of fluoroelastomers. The bisphenol curing system has several safety issues, such as it causes endocrine disruption, reproductive disorders and other health hazards.
Traditionally, fluoroelastomers have been crosslinked using bisphenol-based curing systems. While bisphenol curing imparts desirable mechanical and thermal properties, the use of bisphenols presents significant health and environmental hazards. Bisphenol derivatives, including bisphenol A (BPA) and bisphenol AF, are known endocrine disruptors and have been associated with reproductive toxicity and other adverse health effects. Furthermore, their persistence in the environment raises sustainability concerns, leading to increasing regulatory restrictions on their usage.
To address these issues, alternative curing systems have been explored in the prior art. For example, Tanaka in the US 2013/0150503 A1 disclosed peroxide-based curing systems for fluoroelastomers, which eliminates bisphenols but often compromises compression set resistance and long-term heat stability, limiting their suitability for elevated temperature sealing applications.
Hung et al. in US 6,359,089 B2 describes ionic curing agents and onium salts to facilitate crosslinking. They have disclosed a novel class of fluorovinyl ether monomers which may be useful as cure site monomers in fluoroelastomers. These systems may face limitations in achieving a balance between mechanical strength, thermal resistance, and processability besides other characteristics.
More recently, KAWASAKI et al. in WO 2022/264837 A1 provides alternative chemistries aiming to reduce toxicity while retaining performance. The disclosure provided a fluoroelastomer crosslinkable composition, containing a polyol-crosslinkable fluoroelastomer (a) and a cross-linking agent (b), wherein the cross-linking agent (b) is 3,6-dihydroxy-9H-xanthan-9-one and a salt of the compound with an alkali metal, an alkaline earth metal, or an onium compound. However, challenges remain in simultaneously achieving safety, sustainability, and superior physico-mechanical properties comparable to or better than conventional bisphenol-cured systems.
Accordingly, there exists a need for a safer and sustainable crosslinking system for fluoroelastomers, especially FKM, that overcomes the drawbacks of bisphenol curing while maintaining or enhancing the elastomer’s mechanical, thermal, and chemical resistance properties.
The present invention comprises an improvement of the invention claimed in the specification of our main patent application no. 202414082427 applied for. It is directed to provide a safer process for bisphenol-free crosslinking system. The invention offers a sustainable and less toxic alternative that eliminates reliance on hazardous bisphenols while delivering improved or comparable physico-mechanical performance to FKM fluoroelastomers relative to traditional curing systems.
OBJECT OF THE INVENTION
The main object of the present invention is to provide a bisphenol-free, safe process for crosslinked fluoroelastomers.
Another object of the present invention is to provide safe, environmentally friendly, scalable process for producing bisphenol free, safe crosslinking system for fluoroelastomers with improved characterization.
Yet another object of the present invention is to provide a crosslinking system that results in fluoroelastomers having improved cure characteristics, improved mechanical properties, improved tensile properties, improved compression set resistance, improved crosslink densities and lesser cytotoxicity.
Still another object of the present invention is to provide a process for preparation of a crosslinking system for fluoroelastomers.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY OF INVENTION
In one aspect according to the present invention, it relates to a bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive to the fluoroelastomer
ii. mixing at a temperature ranging between 30-80O C
iii. allowing the mixture for maturing to obtain the cross-linked fluoroelastomers.
In a particular aspect according to the present invention, it relates to the process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium as dimethyl sulphoxide (DMSO); d) a processing aid; and e) at least one additive to the fluoroelastomer to obtain a first mixture, wherein the first mixture comprising a phase transfer catalyst; a non-bisphenol curative agent; and a dispersion medium;
ii. adding a processing aid agent and an additive to the first mixture; and
iii. mixing at a temperature ranging between 30-80O C carried out at a rotor speed of 40-100 rpm
iv. allowing the step ii. mixture for maturing to obtain the cross-linked fluoroelastomers.
Further aspects of the present invention are detailed in the description and examples sections below, wherein many changes can be made in the invention embodiments without departing from the scope of the disclosure.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is an illustrative example of cure characteristics analysed via RPA at 1770C bisphenol AF as well as Phloroglucinol.
Fig. 2 is an illustrative example of the stress-strain plot of bisphenol AF and phloroglucinol-cured fluoroelastomer vulcanizates.
Fig. 3 is an illustrative example of SEM images of FKM vulcanizates cured with phloroglucinol, prepared with DMSO as a dispersion agent and bisphenol AF.
Fig. 4 is an illustrative example of SEM images of FKM vulcanizates cured with phloroglucinol, prepared without and with DMSO as a dispersion agent.
Fig. 5 is an illustrative example of MTT assay demonstrating phloroglucinol having significantly lower cytotoxicity in comparison to bisphenol AF.
DETAILED DESCRIPTION OF INVENTION
The present disclosure provides an improved process for the preparation of novel, safer and sustainable crosslinking system for fluoroelastomers.
Embodiments are provided in order to thoroughly convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components and methods to provide a complete understanding of the embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used in the present disclosure is only for the purpose of explaining a particular embodiment, and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having” are open-ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.
In one embodiment according to the present invention, it provides a process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive to the fluoroelastomer
ii. mixing at a temperature ranging between 30-80O C
iii. allowing the mixture for maturing to obtain the cross-linked fluoroelastomers.
According to the present invention as per the above embodiment of process of preparing bisphenol-free, safe crosslinking system for fluoroelastomers, wherein the phase transfer catalyst used in step a) may be selected, but not limited to, quaternary ammonium salt or quaternary phosphonium salt. They are used for purpose of enhancing the reaction rates and improves crosslinking efficiency in combination with the novel crosslinking system as per the parent invention in our pending application no. 202414082427.
In further elaboration for the above embodiment, inventors have performed experiments using phase transfer catalyst as quaternary ammonium salt which are selected from cetyl trimethyl ammonium chloride, cetyl triethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, lauryl trimethyl ammonium chloride, dodecyl trimethyl ammonium chloride, hexadecyl trimethyl ammonium iodide, cetyl trimethyl ammonium bromide, and cetyl trimethyl ammonium iodide or quaternary phosphonium salt which are selected from benzyl triphenyl phosphonium chloride, triphenylphosphonium chloride, trimethyl (4-vinyl benzyl) 10 phosphonium chloride, and (methoxymethyl) triphenyl phosphonium chloride.
According to the present invention as per the above embodiment of process of preparing bisphenol-free, safe crosslinking system for fluoroelastomers, wherein the non-bisphenol curative agent used in step b) may be selected, but not limited to, phloroglucinol (1,3,5-trihydroxy benzene), pyrogallol, hydroxyquinol, catechol, hydroquinone, resorcinol or mixture thereof. A non-bisphenol curative agent replaces bisphenol compounds from the conventionally known curative agents to ensure effective crosslinking without compromising safety in combination with the novel crosslinking system as per the parent invention in our pending application no. 202414082427.
In further elaboration for the above embodiment, inventors have performed experiments using a non-bisphenol curative agent using phloroglucinol (1,3,5-trihydroxy benzene), pyrogallol, hydroxyquinol, catechol, hydroquinone, resorcinol or mixture thereof.
In another embodiment according to the present invention, it provides a process for preparing a crosslinking system for fluoroelastomers, wherein the quantities of non-bisphenol curative agent used is in the range of 0.1 phr to 4.0 phr.
Inventors have observed that phloroglucinol (1,3,5-trihydroxy benzene) exhibit IC50 value about 863.6 ±0.06 μM, which is apparently more than 11 times safe and better compared to Bisphenol AF having reported IC50 value of about 73.04 ±0.07 μM making the crosslinking system highly safe and bisphenol free with equivalent or improved desirable elastomer characteristics and are illustrated in figure 4.
Inventors further observed that phloroglucinol (1,3,5-trihydroxy benzene) exhibit Crosslink densities about 3.2 x 10-4 (mol/cc), which is apparently 1.6 times higher and safer compared to Bisphenol AF which reported 2 x 10-4 (mol/cc) value making the crosslink system safe and bisphenol free with equivalent or improved desirable elastomer characteristics.
The overall difference observed by the inventors of this present invention was to provide a safer alternative to bisphenol, which resulted in the non-toxic, economic and providing superior or similar physico-mechanical properties along with compression set resistance (C-set) for the present invention of crosslinking system for fluoroelastomers.
According to the present invention as per the above embodiment of process of preparing bisphenol-free, safe crosslinking system for fluoroelastomers, wherein a dispersion medium may be selected, but not limited to, dimethyl sulphoxide (DMSO), diacetone alcohol, polyethylene glycol 400 (PEG 400), dimethyl formamide and cyclohexanol. It maintains uniform distribution of components during the processing within the fluoroelastomer matrix in combination with the novel crosslinking system as per the parent invention in our pending application no. 202414082427.
In a particular embodiment, inventors of the present application surprisingly found that DMSO improves the performance besides crosslinking uniform distribution in the fluoroelastomers. The dispersion of curative agent phloroglucinol within the FKM elastomer matrix was examined using scanning electron microscopy (SEM), as illustrated in figure 4 which displays SEM images of FKM vulcanizates cured with phloroglucinol, prepared without and with DMSO as a dispersion agent, respectively. It is evident that the presence of dispersion agent as DMSO, results in a more uniform dispersion of phloroglucinol. This improvement can be attributed to dipole-dipole interactions between DMSO and phloroglucinol. By fully dissolving phloroglucinol, DMSO as dispersion medium appears to transform into a molecularly dispersed phase, facilitating its evenly distribution throughout the FKM matrix. Subsequently, DMSO enhances the wettability of phloroglucinol on the FKM elastomer during mixing which minimizes particle agglomeration and promotes stronger interaction with the FKM elastomer chains, leading to uniform dispersion. Furthermore, the low viscosity of DMSO allows it to effectively transport the dissolved phloroglucinol deep inside the FKM matrix during compounding process while ensuring that the curative agent is evenly distributed rather than concentrated in specific areas or forming clusters.
According to the present invention as per the above embodiment of process of preparing bisphenol-free, safe crosslinking system for fluoroelastomers, wherein a processing aid may be selected from a group comprising carnauba wax, beeswax, and candelilla wax in the range between 0.5 phr to 5.0 phr, which facilitates processing, ensuring consistent material properties of elastomer in combination with the novel crosslinking system as per the parent invention in our pending application no. 202414082427.
Processing aid viz. Carnauba wax may be sourced from Nanjing Tianshi New Material Technologies Co., Ltd. No.29, Caofang Road, Luhe Development Zone, Nanjing, China, candelilla wax may be sourced from Longchang Chemical Co. Ltd., Shibu Development Zone, Changyi City, Weifang City, Shangdong Province, China.
In a particular embodiment, inventors have used a processing aid as carnauba wax in quantity of about 1 phr.
According to the present invention as per the above embodiment of process of preparing bisphenol-free, safe crosslinking system for fluoroelastomers, wherein at least one additive magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), magnesium hydroxide Mg(OH)2, calcium hydroxide Ca(OH)2 and barium hydroxide Ba(OH)2, which are used in the range between 2.0 phr to 12.0 phr. It provides additional performance benefits, such as improved thermal stability or chemical resistance in combination with the novel crosslinking system as per the parent invention in our pending application no. 202414082427.
In a particular embodiment, inventors have used a combination of magnesium oxide and calcium hydroxide in the ratio of about 1:2, respectively.
According to the above embodiment, the above-mentioned step ii. of mixing process of the crosslinking system may be carried out at a rotor speed ranging between 40-100 rpm, however, it may vary suitably as per the need of characteristics desirable in the fluoroelastomers.
In another embodiment as per the present invention of a process of preparing bisphenol-free, safe crosslinked fluoroelastomers as per the embodiments, wherein step a) a phase transfer catalyst; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive are mixed together and subsequently added in the fluoroelastomers mixture.
The fluoroelastomers employed in the present invention for crosslinking incorporation is a known fluoroelastomer, particularly, a fluoroelastomer having less or no cross-linkable functional groups. In the present invention, the cross-linkable functional group may, be a carbon-carbon unsaturated bond, iodine, bromine, a cyano group or a hydrolysable silyl group. Specific examples of the fluoroelastomer include a tetrafluoroethylene/propylene copolymer, a tetrafluoroethylene/propylene/vinylidene fluoride terpolymer, a vinylidene fluoride/hexafluoropropylene copolymer, and vinylidene fluoride / hexafluoropropylene / tetrafluoroethylene terpolymer. Among them, a tetrafluoroethylene/propylene copolymer and vinylidene fluoride/hexafluoropropylene copolymer are poor in the cross-linking reactivity are preferred.
The tetrafluoroethylene/propylene copolymer may be one obtained by copolymerization of tetrafluoroethylene (hereinafter referred to as TFE) and propylene (hereinafter referred to as P) only, or may be one obtained by copolymerization of TFE and Propylene with other monomers. Such other monomers may, for example, be hexafluoropropylene, vinylidene fluoride, a perfluorovinyl ether of the formula CF2═CF—O—Rf (wherein Rf is a C1-8 saturated perfluoroalkyl group or a perfluoro(alkoxyalkyl) group), an α-olefin such as ethylene or butene (excluding propylene) and a vinyl ether such as methyl vinyl ether or ethyl vinyl ether. Such other monomers may be used alone or in combination as a mixture of two or more of them.
The composition of the TFE/P or HFP/VDF copolymer preferably has a ratio of repeating units based on TFE or HFP / repeating units based on P or VDF = 30/70 to 70/30 (molar ratio). Within such a compositional range, the obtainable cross-linked rubber will be excellent in the cross-linked rubber physical properties, and the heat resistance and chemical resistance will be good. The content of repeating units based on other monomers is preferably from 0 to 20 mol %, more preferably from 0 to 10 mol %.
In yet another embodiments as per the present invention, wherein step a) a phase transfer catalyst; b) a non-bisphenol curative agent; c) a dispersion medium are mixed together to obtain first mixture and step d) a processing aid; e) at least one additive are mixed together to the first mixture to form cross-linked fluoroelastomers.
In yet another embodiments as per the present invention of a process of preparing bisphenol-free, safe crosslinked fluoroelastomers as per the embodiments, wherein step d) processing aid and step c) dispersion medium in the step i. are mixed together and subsequently added in the fluoroelastomers mixture.
In another embodiment according to the present invention, it provides a process for preparing a crosslinking system for fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive to the fluoroelastomer to obtain a first mixture, wherein the first mixture comprising a phase transfer catalyst; a non-bisphenol curative agent; and a dispersion medium;
ii. adding a processing aid agent and an additive to the first mixture; and
iii. mixing at a temperature ranging between 30-80O C
iv. allowing the step ii. mixture for maturing to obtain the cross-linked fluoroelastomers.
In a particular embodiment in line with the earlier embodiment according to the present invention, it provides a process for preparing a crosslinking system for fluoroelastomers, wherein the process comprises the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent as phloroglucinol or resorcinol; c) a dispersion medium as dimethyl sulphoxide (DMSO); d) a processing aid as Carnauba wax or Candella wax or Beeswax; and e) at least one additive selected from Calcium hydroxide Ca(OH)2 or magnesium oxide (MgO) or barium hydroxide Ba(OH)2 to the fluoroelastomer to obtain a first mixture, wherein the first mixture comprising a phase transfer catalyst; a non-bisphenol curative agent; and a dispersion medium;
ii. adding a processing aid and an additive to the first mixture; and
iii. mixing at a temperature ranging between 30-80O C
iv. allowing the step ii. mixture for maturing to obtain the cross-linked fluoroelastomers.
In yet another particular embodiment according to the present invention, it provides a process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent as phloroglucinol or resorcinol; c) a dispersion medium as dimethyl sulphoxide (DMSO); d) a processing aid as Carnauba wax or Candella wax or Beeswax; and e) at least one additive selected from Calcium hydroxide Ca(OH)2 or magnesium oxide (MgO) or barium hydroxide Ba(OH)2 to the fluoroelastomer
ii. mixing at a temperature ranging between 30-80O C
iii. allowing the mixture for maturing to obtain the cross-linked fluoroelastomers.
In yet another embodiments of the present invention, the cross-linked fluoroelastomers produced by using the crosslinking system of the present disclosure may be used in aerospace applications, automotive application, sealing, O-ring applications and the like.
In an embodiment, the crosslinking system doesn’t cause acute health hazards and is non-toxic.
In an embodiment, the cross-linked fluoroelastomers have significant curing characteristics as compared to the existing crosslinking system.
In an embodiment, the cross-linked fluoroelastomers have significant and superior physico-mechanical properties as compared to the existing crosslinking system.
In an embodiment, the cross-linked fluoroelastomers have a significant compression set resistance (C-set) as compared to the existing crosslinking system and a comparative C-set resistance is disclosed in the disclosure.
In an embodiment, the cross-linked fluoroelastomers have a significant cytotoxicity study as compared to the existing crosslinking system and a comparative IC50 value has been disclosed in the disclosure along with comparative MTT assay graph showcasing better and higher value as compared to the existing crosslinking system is disclosed in figure 5.
The present invention is more particularly described by a process that is intended as illustration only, since numerous modifications and variations within the scope of the present invention will be apparent to those skilled in the art. Unless otherwise noted, all parts, percentages, and ratios reported in the following examples are on a weight basis, and all reagents used in the examples were obtained or are available from the chemical suppliers.
The mechanism involved during the crosslinking system for fluoroelastomers according to the present invention may be illustrated in below three stages:
a. in the first step, additive selected from calcium hydroxide Ca(OH)₂ reacts with phase transfer catalyst selected from BTPPC (R3R’P+Cl-) to form R3R’P+-OH and then R3R’P+-OH reacts with phloroglucinol (Ar(OH)3) to form R3R’P+-OAr(OH)2 (reaction intermediate).

b. In the second step, HF elimination and rearrangement happens in the presence of inorganic base (MgO) to generate the formation of olefinic bond (C=C) which was analysed by FT-IR analysis that showed distinct peak at 1645 cm-1.
c. In third step, the olefinic bond formed in step two is reacted with the reaction intermediate (obtained in the first step) to form final crosslink.

d. The final crosslink product is allowed for maturation for a time period of 10 – 24 hours which will consume 3 molecules of hydrofluoric acid and a final crosslink product will be formed.

The crosslinking system developed by the inventors of the present application using curative agent as phloroglucinol in combination with dispersion medium DMSO have been observed consistently in the quality of the FKM crosslinking system, either better or equivalent to conventional bisphenol-AF based curative agent.
This crosslinking system as well as its process has exceptional merits for its commercial importance besides safe use for environment and health.
The following examples illustrate the nature of the invention and are provided for illustrative purposes only and should not be construed to limit the scope of the invention.
Reference Example: Using Bisphenol AF as Curative agent and phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC)
In the Step -1, the curative agent as bisphenol AF (2.0 phr) is added in the experimental batch, along with the phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC) (0.55 phr) into FKM elastomer as VDF-HFP copolymer (100 phr) for 5-10 min, the additive as magnesium oxide (3.0 phr), calcium hydroxide (6.0 phr), MT carbon black (30 phr) at 50oC for 10-15 min mixing which was performed assisted with Brabender Plasticorder PLE 330 (Viscosity test machine), operating at a rotor speed of 40-100 rpm, to produce a final compound.
Step-2 mixing is performed at a temperature of 50 ± 5OC
Step-3 allowing the mixture for maturing for 24 hours to obtain the cross-linked fluoroelastomers.
Step-4 The cure characteristics of the compound were assessed using a rubber process analyser (D-RPA 3000).
The assessed results as per the analysis performed is summarized below:
Property Value – using Bisphenol AF as curative agent
Maximum torque (MH) 30.2 dN.m
Minimum torque (ML) 1.9 dN.m
Scorch time (ts2) 1.5 min
Optimum cure time (tc90) 3.8 min
State of cure (MH- ML) 28.2 dN.m

Example: 01
In the Step -1, the curative agent as phloroglucinol (1,3,5-trihydroxy benzene) (0.5 phr) is added in the experimental batch, along with the phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC) (0.7 phr) with dispersion medium as dimethyl sulphoxide (DMSO) (0.5 phr) into FKM elastomer as VDF-HFP copolymer (100 phr) for 5-10 min, the additive as magnesium oxide (3.0 phr), calcium hydroxide (6.0 phr), processing aid as Carnauba wax (1 phr) MT carbon black (30 phr) at 50oC for 10-15 min mixing which was performed assisted with Brabender Plasticorder PLE 330 (Viscosity test machine), operating at a rotor speed of 60 rpm, to produce a final compound.
Step-2 mixing is performed at a temperature of 50 ± 5OC
Step-3 allowing the mixture for maturing for a time period of 24 hours to obtain the cross-linked fluoroelastomers.
Step-4 The cure characteristics of the compound were assessed using a rubber process analyser (D-RPA 3000).
The assessed results as per the analysis performed is summarized below:
Property Value – using phloroglucinol as curative agent
Maximum torque (MH) 31.7 dN.m
Minimum torque (ML) 2.1 dN.m
Scorch time (ts2) 1.4 min
Optimum cure time (tc90) 3.9 min
State of cure (MH- ML) 29.6 dN.m

Example: 02
In the Step -1, the curative agent as pyrogallol (0.5 phr) is added in the experimental batch, along with the phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC) (0.7 phr) with dispersion medium as dimethyl sulphoxide (DMSO) (0.5 phr) into FKM elastomer as VDF-HFP copolymer (100 phr) for 5-10 min, the additive as magnesium oxide (3.0 phr), calcium hydroxide (6.0 phr), processing aid as Carnauba wax (1 phr) MT carbon black (30 phr) at 50oC for 10-15 min mixing which was performed assisted with Brabender Plasticorder PLE 330 (Viscosity test machine), operating at a rotor speed of 60 rpm, to produce a final compound.
Step-2 mixing is performed at a temperature of 50 ± 5OC
Step-3 allowing the mixture for maturing for a time period of 24 hours to obtain the cross-linked fluoroelastomers.
Step-4 The cure characteristics of the compound were assessed using a rubber process analyser (D-RPA 3000).
The assessed results as per the analysis performed is summarized below:
Property Value – using Pyrogallol as curative agent
Maximum torque (MH) 13 dN.m
Minimum torque (ML) 2.0 dN.m
Scorch time (ts2) 0.75 min
Optimum cure time (tc90) 6.68 min
State of cure (MH- ML) 11 dN.m

Example: 03
In the Step -1, the curative agent as resorcinol (0.65 phr) is added in the experimental batch, along with the phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC) (0.7 phr) with dispersion medium as diacetone alcohol (0.5 phr) into FKM elastomer as VDF-HFP copolymer (100 phr) for 5-10 min, the additive as magnesium oxide (3.0 phr), calcium hydroxide (6.0 phr), processing aid as Carnauba wax (1 phr) MT carbon black (30 phr) at 50oC for 10-15 min mixing which was performed assisted with Brabender Plasticorder PLE 330 (Viscosity test machine), operating at a rotor speed of 60 rpm, to produce a final compound.
Step-2 mixing is performed at a temperature of 50 ± 5OC
Step-3 allowing the mixture for maturing for a time period of 24 hours to obtain the cross-linked fluoroelastomers.
Step-4 The cure characteristics of the compound were assessed using a rubber process analyser (D-RPA 3000).
The assessed results as per the analysis performed is summarized below:
Property Value – using Resorcinol as curative agent
Maximum torque (MH) 24 dN.m
Minimum torque (ML) 2.1 dN.m
Scorch time (ts2) 1.6 min
Optimum cure time (tc90) 3.3 min
State of cure (MH- ML) 22 dN.m

Example: 04
In the Step -1, the curative agent as hydroxyquinol (0.65 phr) is added in the experimental batch, along with the phase transfer catalyst benzyl tri-phenyl phosphonium chloride (BTPPC) (0.7 phr) with dispersion medium as diacetone alcohol (0.5 phr) into FKM elastomer as VDF-HFP copolymer (100 phr) for 5-10 min, the additive as magnesium oxide (3.0 phr), calcium hydroxide (6.0 phr), processing aid as Carnauba wax (1 phr) MT carbon black (30 phr) at 50oC for 10-15 min mixing which was performed assisted with Brabender Plasticorder PLE 330 (Viscosity test machine), operating at a rotor speed of 60 rpm, to produce a final compound.
Step-2 mixing is performed at a temperature of 50 ± 5OC
Step-3 allowing the mixture for maturing for a time period of 24 hours to obtain the cross-linked fluoroelastomers.
Step-4 The cure characteristics of the compound were assessed using a rubber process analyser (D-RPA 3000).
The assessed results as per the analysis performed is summarized below:
Property Value – using Hydroxyquinol as curative agent
Maximum torque (MH) 19.1 dN.m
Minimum torque (ML) 1.74 dN.m
Scorch time (ts2) 1.29 min
Optimum cure time (tc90) 4.6 min
State of cure (MH- ML) 17.36 dN.m
Further the comparative studies for cure characteristics were analysed via RPA at 1770C, towards which the resulting cure curves are illustrated in figure 1. The figure illustrates FKM-BPAFa which represents FKM cured with Bisphenol AF as curative agent and FKM-PGb represents FKM cured with Phloroglucinol as curative agent. The curves represents as the initial decrease in torque is attributed to the softening of the FKM matrix, followed by an increase in torque due to the formation of cross-links among elastomer chains. The cure traces of bisphenol AF (FKM-BPAFa) and phloroglucinol (FKM-PGb) vulcanized FKM are represented by the black and red solid lines, respectively. Further below table provides the data from comparative curing study between bisphenol AF and phloroglucinol vulcanized FKM. It is evident that all the cure parameters, including maximum torque (MH), minimum torque (ML), scorch time (ts2), optimum cure time (tc90), state of cure and cure rate index (MH- ML), are comparable between the two systems and shows Phloroglucinol cured crosslinking system as better results as compared to Bisphenol cured crosslinking system.
Parameters Bisphenol system (FKM-BPAF) Phloroglucinol system
(FKM-PG)
Maximum torque (MH) 30.2 dN.m 31.7 dN.m
Minimum torque (ML) 1.9 dN.m 2.1 dN.m
Scorch time (ts2) 1.5 min 1.4 min
Optimum cure time (tc90) 3.8 min 3.9 min
State of cure (MH- ML) 28.2 dN.m 29.6 dN.m

Further, the inventors of the invention, through multiple trials and errors observed that by choosing Dimethyl sulfoxide (DMSO) as the dispersion medium for phloroglucinol to improve compatibility with the FKM matrix, allowing the same or even better cure properties at lower BTPPC as phase transfer catalyst, resulting in an increase in scorch time and other characteristics. An additional advantage of using DMSO as a dispersion medium is its non-toxic nature, polar characteristics, and high boiling point (1890C). Carnauba wax as processing aid was chosen to reduce compound viscosity for better processing ability.
Further, to observe the mechanical properties of the crosslinking system, the compounded crosslinking system by comparing of tensile properties between press-cured FKM and post cured FKM cured by bisphenol AF and phloroglucinol curatives respectively were press-cured at 1700C for 10 min and subsequently post curing was done at 2300C for 24 h. The resultant values observed are as below:
Properties Press curing by
Bisphenol AF Press curing by Phloroglucinol Post curing by
Bisphenol AF Post curing by Phloroglucinol
Hardness
72 shore A
73 shore A
75 shore A
75 shore A
Tensile strength
10 MPa
9.9 MPa
13.4 MPa
13.8 MPa
Elongation at
break
225 %
220 %
167 %
162 %
Modulus at 100% elongation
5.3 MPa
5 MPa
7.5 MPa
7.7 MPa
The comparative mechanical properties (as described in above table) of FKM vulcanizates that underwent press curing and post curing, using bisphenol AF and phloroglucinol crosslinking systems. The data clearly indicates that all tensile parameters, including hardness, tensile strength, elongation at break, and modulus at 100% elongation, exhibit improvement after the post-curing process for both curing systems. This observation underscores the enhancement of FKM vulcanizates’ tensile properties through post-curing. Additionally, it is evident that the mechanical parameters for both crosslinking systems are comparable, whether after press curing or post curing. It is evident that the mechanical properties of the post-cured samples are significantly higher than the same by the press cured one.
Further, the hardness of the cured samples was analyzed by Shore A hardness tester using ASTM D2240 and the tensile properties were evaluated using a Universal Testing Machine (UTM) using ASTM D412. The testing was conducted in accordance with ASTM D2240 and ASTM D412, respectively.
Below table represents the data obtained from hardness and tensile properties using Universal Testing Machine (UTM) for Bisphenol AF and phloroglucinol-cured fluoroelastomer vulcanizates. The results demonstrate close similarity in all mechanical properties like hardness, tensile strength, elongation at break, and modulus at 100% elongation.
The hardness values for both bisphenol and phloroglucinol-cured FKM vulcanizates are 75 Shore A. Figure 2 illustrates the comparative stress-strain plot of bisphenol AF and phloroglucinol-cured fluoroelastomer vulcanizates. The tensile strength of bisphenol and phloroglucinol-cured fluoroelastomer vulcanizates are 13.4 MPa and 13.8 MPa, respectively.
The elongation at break for bisphenol and phloroglucinol-cured fluoroelastomer vulcanizates is 167% and 162%, respectively, while the modulus at 100% elongation is 7.5 MPa for bisphenol AF and 7.7 MPa for phloroglucinol-cured fluoroelastomer vulcanizates.
Properties FKM-BPAF FKM-PG
Hardness 75 shore A 75 shore A
Tensile strength 13.4 MPa 13.8 MPa
Elongation at break 167 % 162 %
Modulus at 100% elongation 7.5 MPa 7.7 MPa
Further, to observe the compression set test was conducted as per ASTM D395 and is representing the results indicating that the compression set values for fluoroelastomer vulcanizates crosslinked with bisphenol AF and phloroglucinol are 21% and 19%, respectively.
Referring to the table for compression set, it can be affirmed that the compression set percentages are comparable for FKM vulcanized with phloroglucinol in comparison to conventionally bisphenol-cured FKM.
Properties FKM-BPAF FKM-PG
Compression set (%) at 2000 C for 70 h according to ASTM D395 21 % 19 %

Figure 3 illustrates the comparative scanning electron microscope (SEM) micrographs of the fractured surfaces of FKM vulcanizates cured with bisphenol AF and phloroglucinol. In both cases, a uniform dispersion of MT carbon black (N990) in the FKM elastomer matrix is observed which confirms the homogeneous mixing of filler in FKM matrix.
Further, to observe a comparison of crosslink density between fluoroelastomer vulcanizates cured with bisphenol AF and phloroglucinol. The crosslink densities for bisphenol AF and phloroglucinol-cured FKM vulcanizates are recorded as 2 × 10-4 mol/cc and 3.2 × 10-4 mol/cc, respectively. Notably, the crosslink density is significantly higher for phloroglucinol-cured FKM when compared to bisphenol AF-cured FKM which may be due to the presence of tri-functional –OH in phloroglucinol.
Type of Vulcanizates Crosslink density (mol/cc)
FKM-BPAF 2 × 10-4
FKM-PG 3.2 × 10-4
Further, to observe the IC50 value represents the concentration of a substance required to induce 50% cell death. The results from the MTT assay demonstrate that phloroglucinol has significantly lower cytotoxicity, with an IC50 value of 863.6 ± 0.06 µM, in comparison to bisphenol AF, which has an IC50 of 73.04 ± 0.07 µM, as shown in below table and figure 5.
Curative Agent IC50 value (µM)
Phloroglucinol 863.6 ± 0.06
Bisphenol AF 73.04 ± 0.07

This indicates that only 73 µM of bisphenol AF is needed to cause 50% cell death, whereas a much higher concentration of 863.6 µM is required for phloroglucinol. Therefore, phloroglucinol (an exploratory curative) is considerably safer than bisphenol AF (a conventional curative). The comparative IC50 curve is shown in figure 5.
The unique bisphenol free, safe crosslinking system for fluoroelastomers developed according to the present invention comprising the components, a phase transfer catalyst; a non-bisphenol curative agent; a dispersion medium; a processing aid; and an additive may be usable for commercial 15 scale purposes for safe crosslinking fluoropolymer / fluoroelastomers development compared from reference example demonstrating use of bisphenol AF, which is often referred to as unsafe for polymers used in food grade polymers as their coatings.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted in order to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments so fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations, and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions, or quantities fall within the scope of the disclosure unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the scope of the invention. These and other changes in the preferred embodiment, as well as other embodiments of the invention, shall be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation. , C , C , Claims:1. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive to the fluoroelastomer
ii. mixing at a temperature ranging between 30-80O C
iii. allowing the mixture for maturing to obtain the cross-linked fluoroelastomers.
2. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 1, wherein step ii. of mixing process is carried out at a rotor speed of 40-100 rpm.
3. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 1, wherein non-bisphenol curative agent is selected from phloroglucinol, pyrogallol, hydroxyquinol, catechol, hydroquinone, resorcinol or mixture thereof.
4. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 1, wherein processing aid and dispersion medium in the step i. are mixed together and added in the fluoroelastomers mixture.
5. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 1, wherein additive is selected from magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), magnesium hydroxide Mg(OH)2, calcium hydroxide Ca(OH)2 and barium hydroxide Ba(OH)2.
6. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 4, wherein the processing aid is selected from Carnauba wax or Beeswax or Candelilla wax or Paraffin wax, and dispersion medium is selected from dimethyl sulphoxide (DMSO), diacetone alcohol, polyethylene glycol 400 (PEG 400), dimethyl formamide and cyclohexanol.
7. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent; c) a dispersion medium; d) a processing aid; and e) at least one additive to the fluoroelastomer to obtain a first mixture, wherein the first mixture comprising a phase transfer catalyst; a non-bisphenol curative agent; and a dispersion medium;
ii. adding a processing aid agent and an additive to the first mixture; and
iii. mixing at a temperature ranging between 30-80O C
iv. allowing the step iii. mixture for maturing to obtain the cross-linked fluoroelastomers.
8. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent as phloroglucinol or resorcinol; c) a dispersion medium as dimethyl sulphoxide (DMSO); d) a processing aid as Carnauba wax or Candella wax or Beeswax; and e) at least one additive selected from Calcium hydroxide Ca(OH)2 or magnesium oxide (MgO) or barium hydroxide Ba(OH)2 to the fluoroelastomer to obtain a first mixture, wherein the first mixture comprising a phase transfer catalyst; a non-bisphenol curative agent; and a dispersion medium;
ii. adding a processing aid and an additive to the first mixture; and
iii. mixing at a temperature ranging between 30-80O C
iv. allowing the step iii. mixture for maturing to obtain the cross-linked fluoroelastomers.
9. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers comprising the steps –
i. mixing components of cross-linking system comprising a) a phase transfer catalyst selected from quaternary ammonium or phosphonium salt; b) a non-bisphenol curative agent as phloroglucinol or resorcinol; c) a dispersion medium as dimethyl sulphoxide (DMSO); d) a processing aid as Carnauba wax or Candella wax or Beeswax; and e) at least one additive selected from Calcium hydroxide Ca(OH)2 or magnesium oxide (MgO) or barium hydroxide Ba(OH)2 to the fluoroelastomer
ii. mixing at a temperature ranging between 30-80O C
iii. allowing the mixture for maturing to obtain the cross-linked fluoroelastomers.
10. A process of preparing bisphenol-free, safe crosslinked fluoroelastomers according to claim 1, wherein a non-bisphenol curative agent used in the range of 0.1 phr to 4.0 phr.