WO2022229752A1

WO2022229752A1 - Curable fluoroelastomer compositions containing quaternary phosphonium hydroxybenzoate catalysts

Info

Publication number: WO2022229752A1
Application number: PCT/IB2022/053214
Authority: WO
Inventors: Leon M. LILLIE; Michael H. MITCHELL; Tatsuo Fukushi
Original assignee: 3M Innovative Properties Company
Priority date: 2021-04-30
Filing date: 2022-04-06
Publication date: 2022-11-03
Also published as: TW202302746A

Abstract

Quaternary phosphonium hydroxybenzoate catalysts that can be used to cure nitrile-group containing fluoropolymers are described. Compositions containing such compounds as catalysts with fluoroelastomers containing nitrile curesite groups, cured compositions and articles comprising cured compositions are also described.

Description

CURABLE FLUOROELASTOMER COMPOSITIONS CONTAINING QUATERNARY PHOSPHONIUM HYDROXYBENZOATE CATALYSTS

FIELD

[0001] The present disclosure relates to curable fluoroelastomer compositions containing compounds having a quaternary phosphonium cation and an anion comprising a hydroxybenzoate. The use of such compounds as catalysts for perfluoroelastomers containing nitrile curesite groups and cured compositions are also described.

SUMMARY

[0002] Briefly, in one aspect, the present disclosure provides compositions comprising an uncured highly fluorinated elastomer comprising nitrile groups and a quaternary phosphonium hydroxybenzoate compound according to Formula (I)

wherein each R1 group is a linear, branched or cyclic hydrocarbon comprising 1 to 8 carbon atoms, and the R2 groups are either both H or both OH.

[0003] In other aspects, the present disclosure provides cured elastomers prepared from such compositions, including articles comprising cured elastomers.

DETAILED DESCRIPTION

[0004] Fluoroelastomers, particularly perfluorinated elastomers, can have excellent solvent and temperature resistance. Fluoroelastomers have been cured by a wide variety of mechanisms including the use of cure site monomers and a corresponding catalyst, sometime referred to as a curative. For example, peroxide curing systems have been used with bromine- and iodine-containing cure site monomers.

[0005] In high temperature applications, nitrile-containing cure site monomers have been used. In some cases, onium catalysts have been used to cure such perfluoroelastomers. Commonly used onium catalysts include a low molecular weight fluorinated or perfluorinated anion. It was believed that these fluorinated anions were required to achieve the compatibility with highly fluorinated elastomers needed to allow them to act as effective crosslinking catalysts.

[0006] U.S. Patent No. 6,888,388 B2 (“Fluoropolymer Compositions Containing a Nitrogen Cure Site Monomer”) describes compositions that include a fluoropolymer having interpolymerized units derived from (a) a nitrogen-containing cure site monomer and (b) non-fluorinated onium catalysts, including non- fluorinated quaternary onium catalysts. [0007] Surprisingly, the present inventors discovered that the non-fluorinated, quaternary phosphonium hydroxybenzoate compounds of the present disclosure are compatible with highly fluorinated elastomers and can provide curing properties comparable to the traditional fluorinated onium catalysts. Also, in some embodiments, the quaternary phosphonium compounds of the present disclosure can provide improved compression set at high temperatures as compared to quaternary ammonium catalysts. In addition, the catalysts are solid at typically compounding conditions, leading to significant processing benefits.

[0008] Generally, the non-fluorinated quaternary phosphonium compounds of the present disclosure are low molecular weight quaternary phosphonium hydroxybenzoate compounds, as shown in Formula (I)

wherein each R1 group is a linear, branched or cyclic hydrocarbon comprising 1 to 8 carbon atoms. In some embodiments, both R2 groups are H, i.e., the anion of 4-hydroxybenzoic acid. In some embodiments, both R2 groups are OH, i.e., the anion of 3,4,5-trihydroxybenzoic acid (also referred to as gallic acid).

[0009] In some embodiments, each R1 group is a linear or branched alkyl group, e.g., in some embodiments, al R1 groups are linear alkyl groups. In some embodiments, each R1 group comprises 1 to 4, e.g., 2 to 4 carbon atoms. In some embodiments, all R1 groups are the same, e.g., in some embodiments, each R1 group is butyl group.

[0010] Despite using a non-fluorinated anion, these quaternary phosphonium hydroxybenzoate compounds are compatible with highly fluorinated elastomers. As they are solids at typical compounding conditions, they are easier to blend with the elastomer then liquid catalysts.

[0011] As used herein, a “highly fluorinated” polymer is one in which at least 80 mole%, e.g., at least 90 or even at least 95 mole% of the hydrogen atoms of the polymer backbone have been replaced with fluorine atoms. In some embodiments, 0 to 20 mole%, e.g., 5 to 15 mole% of the hydrogen atoms of the polymer backbone may be replaced with chlorine atoms. In some embodiments, these highly fluorinated polymers may include one or more hydrogen-containing end groups or pendant groups, which are not considered to be part of the polymer backbone.

[0012] In some embodiments, the highly fluorinated elastomer is a copolymer of tetrafluoroethylene (TFE) and one or more perfluorinated comonomers. In some embodiments, at least one perfluorinated comonomer is a perfluoroalkylvinyl ether (PAVE), including perfluoroalkyl alkoxy vinyl ethers. Suitable PAVEs include perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), and perfluoropropyl vinyl ether (PPVE-1 and PPVE-2). In some embodiments, at least one perfluorinated comonomer is a perfluoroalkyl allyl ether (PAAE), including perfluoroalkyl alkoxy allyl ethers. Suitable PAAEs include perfluoromethyl allyl ether (MA-1), perfluoroethyl allyl ether (MA-2), and perfluoropropyl allyl ether (MA-3).

[0013] In some embodiments, at least one perfluorinated comonomer is a perfluorinated alpha-olefin other than TFE, such as hexafluoropropylene (HFP). Additional optional comonomers include partially fluorinated alpha olefins (e.g., CH2=CF2, VDF), F and Cl containing olefins such as chlorotrifluoroethylene, and non-fluorinated alpha olefins such as ethylene or propylene.

[0014] In some embodiments, the polymer is a perfluorinated, i.e., at least 99 mole% or even 100 mole% of the hydrogen atoms of the polymer backbone have been replaced with fluorine or chlorine atoms, preferably fluorine atoms. If chlorine atoms are present, they comprise no greater than 20 mole%, e.g., no greater than 10 mole% based on the total moles of fluorine and chlorine atoms on the polymer backbone. In some embodiments, the perfluorinated polymer comprises no greater than 5 mol%, e.g., no greater than 1 mole%, or even no greater than 0.01 mole% chlorine atoms based on the total moles of fluorine and chlorine atoms on the polymer backbone.

[0015] Generally, the highly fluorinated elastomer contains a sufficient number of nitrile groups (CºN) to achieve the desired degree of cure (i.e., crosslinking). In some embodiments, the elastomer contains at least 0.1 mole % of nitrile groups, e.g., at least 0.5 or even at least 1 mole % of nitrile groups. In some embodiments, the fluorinated elastomer contains no greater than 5, e.g., no greater than 2 or even no greater than 1.5 mole % of nitrile groups.

[0016] In some embodiments, the nitrile groups are introduced by copolymerizing a nitrile-group containing curesite monomer with the other comonomer, e.g., the TFE, PAVE, and PAAE comonomers. Although not particularly restricted, suitable nitrile-containing curesite monomers include CF₂=CF0CF2CF(CF3)0CF2CF₂CN, CF2=CFOCF₂(CF2)3CF₂CN, and CF2=CF0CF₂(CF2)3CF(CF₃)CN.

[0017] The quaternary phosphonium hydroxybenzoate compounds of the present disclosure can be used to cure highly fluorinated, nitrile-group containing elastomers. Generally, the quaternary phosphonium hydroxybenzoate compounds act as catalysts to form triazine crosslinks from the pendant nitrile groups, crosslinking the elastomer to form a cured compound.

[0018] Generally, the quaternary phosphonium hydroxybenzoate compounds can be blended with the highly fluorinated elastomer using commonly known equipment and methods. However, the present inventors determined that benzoate compounds that are solid at the compounding conditions are easier to combine into the fluoroelastomers resulting in a more uniform dispersion using less aggressive processes. [0019] Generally, the blends of the quaternary phosphonium hydroxybenzoate compounds in highly fluorinated elastomers can be formed into articles such as sheets and O-rings using commonly known equipment and methods. For example, the compounded elastomer composition can be press cured at temperatures of at least 175 °C, e.g., at least 185 °C. In some embodiments, the press curing temperature is no greater than 225, or even no greater than 200 °C. [0020] The press cure conditions (e.g., time and temperature) can be selected to achieve a desired level of cure. The extent of cure can be determined according to the Cure Rheology Procedure described below and used in the Examples.

[0021] Cure Rheology Procedure. Cure rheology tests were carried out using uncured, compounded samples using a rheometer marketed under the trade designation PPA 2000 by Alpha technologies,

Akron, OH, in accordance with ASTM D 5289-93a at 188 °C, no pre-heat, 30 minutes elapsed time and a 0.5 degree arc. Both the minimum torque ( p) and the maximum torque (M_j-[) were measured in units of inch_*pounds and converted to deciNewtommeters (dN_*m). If no plateau or maximum torque was obtained, the highest torque attained during the specified period of time was reported as M_jp The time for the torque to reach a value equal to Mp + 0. l(M_jq - Mp), (t’10), the time for the torque to reach a value equal to Mp + 0.5(M_jq - Mp), (t’50), and the time for the torque to reach Mp + 0.9(M_jq - Mp), (t’90) were also measured. The ratios of the viscous torque over the elastic torque were determined at the minimum and maximum torque and are reported as the tan d.

[0022] Generally, adequate cure is indicated by an M_jq value of at least 4 dN_*m, as determined according to the Cure Rheology Procedure. In some embodiments, M_jq is at least 5, or even at least 6 dN_*m. The level of cure can also be evaluated from the Tan d at Mpp In some embodiments, the Tan d at M _[ is no greater than 0.4, e.g., no greater than 0.3, or even no greater than 0.2, as determined according to the Cure Rheology Procedure.

[0023] In some embodiments, the press cured samples may also be post cured. Although not particularly limited, typical post curing temperatures are at least 200, e.g., at least 250 °C and no greater than 325, e.g., no greater than 300 °C. Post curing times can depend on a variety of factors including the press curing conditions and the post curing temperature. In some embodiments, post curing times are at least 10 hours, e.g., at least 15 hours. In some embodiments, post curing times of no greater than 30 hours, e.g., no greater than 25 hours are desired.

[0024] Examples

[0025] Reference Example REF-1 was prepared using a conventional fluorinated onium catalyst. A curable composition was prepared by combining 94 parts by weight of a perfluoroelastomer containing nitrile cure site groups (3M™ DYNEON™ PFE 131 TZ fluoroelastomer from 3M Company) and 7.5 parts by weight of a fluorinated onium catalyst (3M™ DYNEON™ PFE 01CZ catalyst from 3M Company). As this catalyst product consists of 20% by weight of the fluorinated onium catalyst in 80% by weight of a perfluorinated elastomer, the result was 100 parts by weight of the perfluorinated resin and 1.5 parts by weight of the catalyst per 100 parts of the perfluorinated resin (i.e., 1.5 phr).

[0026] The catalysts summarized in Tables 2A and 2B were prepared following the procedure outlined in US 8,906,821 B2 - “Example Preparation of Tetrabutylphosphonium 2-(p-toluyl)-l, 1,1, 3,3,3- hexafluoroisopropoxide (TBPTHI2), C H 3 C H 4 C ( C F 3 ) 2 O ⁺ P ( C 4 H 2 ) without Methanol” (Column 14, Line 40-60) using materials listed in Table 1. Table 1: Summary of materials used in the preparation of the examples.

Table 2A: Non-fluorinated quaternary phosphonium benzoate curatives

Table 2B: Non-fluorinated quaternary ammonium benzoate curatives

[0027] Samples P-1 to P-4 were prepared from a curable composition containing 100 parts by weight perfluoroelastomer (3M™ DYNEON™ PFE 131 TZ) and 1 phr of various non-fluorinated quaternary phosphonium benzoate compounds summarized in Table 2A. In each case, the quaternary phosphonium was atetrabutyl phosphonium. Samples N-l to N-3 were prepared using 1 phr of similar tetrabutyl ammonium benzoate compounds shown in Table 2B.

[0028] The samples were press cured and the torque as a function of time was recorded according to the Cure Rheology Procedure described above. The results are shown in Table 3.

Table 3: Cure rheology results

[0029] As shown in Table 3, except for Samples P-2 and N-2, the samples using the non-fluorinated quaternary phosphonium and quaternary ammonium benzoate compounds cured the highly fluorinated elastomer, as indicated by M_jq values greater than 4 dN_*m and Tan d at M_jq values of less than 0.2. In contrast, the samples prepared using 2-hydroxy benzoate (P-2 and N-2) had a Tan d at M_jq of about 0.4 and M_[-[ values less than 2 dN_*m, indicating poor cure.

[0030] The samples were also evaluated according to the following Compression Set Procedure. The results are summarized in Table 4, reported as a percentage.

[0031] Compression Set Procedure. O-rings (214, AMS AS568) were molded and press-cured at 188 °C for 30 minutes. The press-cured O-rings were then post-cured at 250 °C for 16 hours. The post-cured O-rings were subjected to compression set testing for 70 hours at 250 °C or 300 °C in accordance with ASTM D 395-03 Method B and ASTM D 1414-94 with an initial deflection of 25 percent.

Table 4: Compression set results (* 1 of 3 samples had bubbles)

[0032] In each case, the quaternary phosphonium hydroxybenzoate compounds provided substantially better compression set performance relative to the corresponding quaternary ammonium hydroxybenzoate compound. In addition, the compression set performance using some of the phosphonium hydroxybenzoate compounds having a 4-hydroxy group (P-3 and P4) were substantially better than the compression set achieved with the fluorinated onium curative, especially at higher temperatures. The compression set obtained using the 2-hydroxy benzoate compound (P-2) was comparable to the compression set achieved with the fluorinated onium compound.

[0033] Liquids are notoriously hard to mill into perfluoroelastomers. They must be added slowly, drop-by-drop with significant time between drops to ensure the liquid is mixed into the elastomer. If too much liquid is added, the polymer will either crumble or fall off the mill and it will be exceptionally challenging to re-band (i.e., to get the elastomer back onto the mill). This process can take hours (about 1- 3 hours was typical for the liquid curatives used in the present examples).

[0034] Typically, liquids have been added with the aid of a solid carrier (e.g., DLCs - Dry Liquid Concentrates) or added to the solid fdler package (e.g. carbon black) prior to milling. However, in some applications, e.g. semiconductor, require high purity and must have minimal potential for particle generation. DLC’s or solid fillers are often significant sources of particle generation. [0035] In contrast, curatives that are solid can be incorporated within 15-30 minutes allowing for better confidence in the levels of the curative (liquids can soak into the paper that is used to catch elastomer that has fallen off the mill, while solids can be added back to the banded elastomer if they fall on the paper) and imparting less thermal history to the material.

[0036] Samples generate heat during the compounding process and specific compounding temperatures can vary. However, typical compounding temperatures are greater than 20 °C, e.g., at least 50 or even at least 60 °C. In some embodiments, the compounding temperatures may be as high 80 °C. [0037] In some embodiments, the quaternary phosphonium hydroxybenzoate compound is a solid at typical processing conditions, e.g., a solid at 80 °C. The melting points of the curatives are show in Table 5.

Table 5: Melting point of catalysts

[0038] Although P-1 provided the best performance in both cure (Table 3) and compression set (Table 4); this compound is a liquid at room temperature and typical compounding conditions. As the compounds of P-3 and P-4 are solids at typical compounding conditions e.g., 80 °C, they are preferred for some applications.

[0039] The Compression Set Procedure and the Cure Rheology Procedure recited in the attached claims refer to the test methods described in this specification.

Claims

What is Claimed is:

1. A composition comprising

(i) an uncured highly fluorinated elastomer comprising nitrile groups, wherein at least 80 mole% of the hydrogen atoms of the polymer backbone of the highly fluorinated elastomer have been replaced with fluorine atoms; and

(ii) a quaternary phosphonium hydroxybenzoate compound according to the formula

wherein each R1 group is a linear, branched or cyclic hydrocarbon comprising 1 to 8 carbon atoms, and either both R2 groups are H, or both R2 groups are OH.

2. The composition of claim 1, wherein each R1 group is a linear or branched alkyl group comprising 1 to 4 carbon atoms.

3. The composition of claim 2, wherein each R1 group is a butyl group.

4. The composition of any one of claims 1 to 3, wherein both R2 groups are H.

5. The composition of any one of claims 1 to 3, wherein both R2 groups are OH.

6. The composition of any one of the preceding claims, wherein quaternary phosphonium hydroxybenzoate compound is a solid at 80 °C.

7. The composition of any one of the preceding claims, wherein at least 95 mole % of the hydrogen atoms of the polymer backbone of the uncured highly fluorinated elastomer have been replaced with fluorine atoms.

8. The composition of any one of the preceding claims, wherein the uncured highly fluorinated elastomer comprises repeating units derived from a perfluorinated monomer comprising a nitrile group, tetrafluoroethylene (TFE), and at least one comonomer selected from the group consisting of a perfluoroalkyl vinyl ether (PAVE) and a perfluoroalkyl allyl ether (PAAE).

9. The composition of any one of the preceding claims, wherein the uncured highly fluorinated elastomer comprises at least 1 mole % and no greater than 5 mole % of nitrile groups.

10. The composition of any one of the preceding claims, wherein the composition has a M _j value of at least 4 dN/m as measured according to the Cure Rheology Procedure.

11. The composition of any one of the preceding claims, wherein the composition has a Tan d at M _j of no greater than 0.3 as measured according to the Cure Rheology Procedure.

12. A cured elastomer prepared from the composition according to any one of claims 1 to 11, wherein the uncured highly fluorinated is cured by the quaternary phosphonium benzoate compound to form triazine crosslinks.

13. The cured elastomer of claim 12, having a compression set of no greater than 50% after 70 hours at 300 °C, as measured according to the Compression Set Procedure.

14. An article comprising the cured elastomer according to claim 12 or 13.