A unique type of fluorocarbon elastomer.

A unique type of fluorocarbon elastomer

Aflas TFE elastomer is a fluorocarbon elastomer based on the monomers tetrafluoroethylene (TFE) and propylene (P). The copolymer structure consists of a regularly alternating arrangement of the two monomers. It remains non-crystalline despite this regular arrangement due to the random orientation of the propylene methyl group involved in the polymerization.

Aflas TFE elastomer differs from conventional fluorocarbon elastomers (FKM type) in that it does not contain hexa-fluoropropylene (HFP) or vinylidene fluoride (VDF) in the polymer backbone. Because of this difference it is referred to as a TFE elastomer to distinguish it from other fluorocarbon elastomer types. It will be referred to as TFE/P throughout the remainder of this article.

The structure of TFE/P provides a physical and chemical resistance property profile which is unique among fluorocarbon elastomers. While it maintains many of the outstanding properties expected from a fluorocarbon elastomer such as heat, oil and chemical resistance, there are several areas in which TFE/P differs significantly.

TFE/P will provide long term service at a temperature of 200 [degrees]C and progressively shorter term service up to a practical limit of 275 [degrees]C based on retention of physical properties (figure 1). TFE/P has a lower fluorine content (57%) than FKM type elastomers (65-70%). The result of lower fluorine is higher volume swell in oils and fuels particularly those of a high aromatic content. From a practical standpoint, TFE/P has satisfactory volume swell resistance to oils and lubricants for most applications although excessive swell in most types of fuels may eliminate TFE/P as a candidate material in these applications. TFE/P does not have an official ASTM D-2000 designation at this time, but would be designated an HJ material by type and class (figure 2).

Areas where TFE/P may provide advantages versus FKM type elastomers include improved resistance to high pH environments, high temperature steam and water, amine corrosion inhibitors, some types of polar solvents, many types of acids, gamma radiation and electrical resistance properties. Examples of TFE/P properties in these specific environments will be shown later.

TFE/P has excellent physical and mechanical properties as well. Tensile strength exceeding 3,000 psi and compression set as low as 25% after testing for 70 hours at 200 [degrees]C on o-rings can be achieved with proper compounding. The practical hardness range for TFE/P compounds ranges from 60 to 95 type A durometer. The glass transition temperature of TFE/P is relatively high (+3 [degrees]C), however the material is very tough at low temperatures as indicated by its brittle point of -40 [degrees]C. Suitability for use at low temperatures will be dependent on specific application requirements.

TFE/P elastomer products

There are presently five products of TFE/P copolymers commercially available. These products differ principally by Mooney viscosity. Selection of a specific product is dependent on specific processing or application requirements.

In general the lowest viscosity products, FA-150L (ML 1+10, 35 [+ or -] 13), and FA-150E (ML 1+10, 60 [+ or -] 10) are used for extrusions and injection moldings where optimum flow characteristics are needed.

FA-150P (ML 1+10, 95 [+ or -] 10) is considered a general purpose product used for all types of molding, extrusion and calendering applications.

FA-100S (ML 1+10, 160 [+ or -] 15) provides the best balance of physical properties of all the products including resistance to compression set. It is recommended for most compression molding applications.

FA-100H is the highest viscosity product available and is typically used in applications where maximum extrusion resistance or explosive decompression resistance is needed, such as in down hole oil well applications.

Blends of various TFE/P products are commonly used to modify the flow characteristics of compounds to meet specific processing requirements.

Properties of the different TFE/P products are compared in a typical 75 type A durometer, medium thermal black filled formulation (table 1). The most notable difference in the various grades is viscosity as indicated by minimum torque values measured during Mooney scorch and rheology testing. As mentioned before, the FA-100S product provides the best compression set resistance, as well as highest tensile strength.

Table : Table 1 - TFE/P product comparison

Formulation

  Polymer                                     100 PHR
  MT black                                       35
  a,a'-bis(t-butylperoxy)-diisopropylbenzene      3
  TAIC                                            4
  Sorbitan monostearate NF                        1
                   FA-100H   FA-100S   FA-150P   FA-150E   FA-150L

Mooney scorch MS @ 121 [degrees]C

Minimum               56        42        30        13.5      10.5
Pts. rise in 25'       0         0         0         0         0

ODR @ 177 [degrees]C 3 [degrees] Arc

Min. torque           32.0      24.0      15.9       6.4       6.0
ts2                    1.4       1.4       1.7       1.9       1.9
tc50                   2.9       3.1       3.9       4.0       4.1
tc90                   7.1       7.3       8.5       8.3       8.6
Max torque            75.5      75.5      59.0      45.5      42.0

Post cure 16 Hrs. @ 230 [degrees]C

Tensile (psi)        2,110     2,380     2,015     1,740     1,640
Elongation %           305       275       280       255       230
100% Modulus           665       695       630       685       695
Shore A                 76        76        75        76        76

Compression set .139" o-ring

70 Hrs. @ 23 [degrees]C  28       28        32        39        42
70 Hrs. @ 200 [degrees]C 52       44        45        50        48

Compounding TFE/P elastomers

Formulations of TFE/P typically consist of polymer, reinforcing fillers, peroxide curing agent, coagent and process aids. Other ingredients may include inorganic bases, cure activators, tackifiers and plasticizers, depending on the specific application (table 2).

Table : Table 2 - compounding TFE/P

Ingredient                  Level (PHR)
Elastomer                       100
Filler                         0-80
Peroxide                        1-5
Coagent                        3-10
Process aids                    0-2
Cure activators                 0-1

Polymer selection

As has been discussed, the choice of TFE/P polymer will be dependent on both processing and application requirements. All products have similar chemical and heat resistance characteristics, but the mechanical properties and processability of the different products varies significantly. In most cases the FA-100S or FA-150P products will satisfy the processing requirements (table 3).

Table : Table 3 - TFE/P polymer selection

Polymer grade   Uses
FA-150L         Extruded, calendered goods
FA-150E         Extruded, calendered goods; polymer blends
FA-150P         General purpose grade
FA-100S         Compression molding; best physical properties
FA-100H         Compression molding; highest molecular weight

Reinforcing fillers

The choice of filler used in the TFE/P compound can have a great influence on performance and properties. As with FKM type elastomers, medium thermal (N-990) carbon black is the most common filler used in TFE/P. MT black provides a good balance of physical properties and can be used at higher loadings than more reinforcing type fillers.

For optimum compression set resistance the use of a mineral black is recommended. The use of mineral black in combination with MT black can improve compression set resistance by 5 to 10% over a straight MT black filled compound.

For harder, high tensile strength, high modulus compounds more reinforcing SRF, FEF or HAF carbon blacks or non-black mineral fillers such as surface modified fumed silicas may be used (table 4).

Table : Table 4- TFE/P filler comparison

Formulation                 A      B      C      D
FA-100S                    100   100    100    100
TAIC                        4      4      4      4

a,a'-bis(t-butylperoxy)- diisopropylbenzene 4 4 4 4

Sorbitan mono-

stearate NF                 1      1      1      1
Mineral black              25     --     --     --
N-990                      --     30     --     --
N-762                      --     --     20     --
N-550                      --     --     --     20
Tensile, psi             1,800  2,600  2,900  3,100
Elongation, %             260    250    250    240
Hardness, type A           73     74     72     75

% Compression set -214 size o-rings

70 hrs. @ 200 [degrees]C 27 41 49 40

Many types of fillers, including clays, magnesium silicate, small particle size blacks, etc., may severely retard the cure of TFE/P compounds. This retarding effect can be overcome by the use of cure activators which will be discussed later.

Peroxide vulcanizing agents

TFE/P compounds are vulcanized by use of a peroxide/coagent type cure system. Most conventional peroxides used in the rubber industry can be used with TFE/P. A study comparing various peroxides indicates that a,a'-bis(t-butylperoxy)-diisopropylbenzene provides the best balance of rheological and physical properties including compression set resistance. Selection of a particular peroxide is also dependent on processing requirements. Peroxide level will vary depending on type used and properties desired.

Coagents

A coagent is required to vulcanize TFE/P compounds in combination with a peroxide. Triallylisocyanurate (TAIC) has been found to provide excellent physical and rheological properties. Other coagents such as Ricon 153 (high vinyl 1,2 polybutadiene from Colorado Chemical) or Silane A-174 (Union Carbide) can be used in conjunction with the TAIC to enhance specific properties such as modulus. The level of TAIC used will vary from 3-7 phr depending on desired properties.

Process aids

Process aids are used to improve flow in molding and extrusion operations and provide improved release from both mill roll and mold surfaces. Many types of process aids can be used with TFE/P. Particularly effective for improving release characteristics are sodium stearate (1-2 phr), polyethylene glycol (0.5-2 phr), oxidized polyethylene (2 phr), silicone oil (0.5-1 phr) and blends with low viscosity EPR or EPDM elastomers (2-5 phr). To improve flow properties, carnauba was (1-2 phr), sorbitan monostearate NF (1-2 phr) and liquid EPDM (2-3 phr) are recommended.

Miscellaneous compounding ingredients

Depending on the application, there are a number of other ingredients which may be added to enhance specific properties of the TFE/P compounds (table 5). Inorganic bases such as litharge or dibasic lead phosphite may improve acid resistance to some specific acid types. Calcium hydroxide or magnesium oxide may improve long term heat resistance, as well as improve adhesion properties.

Table : Table 5 - compounding TFE/P for specific properties

* Improve heat resistance - Inorganic bases (i.e. Ca [(OH).sub.2], MgO) (1-5 PHR) * Improve compression set - Mineral black; higher peroxide/ coagent levels; cure activator (i.e. PEG, Na stearate) * Improve acid resistance - Litharge, dyphos (1-5 PHR) * Improve building tack - Blends with polyisobutylene or EVA elastomers

Polyethylene glycol and sodium stearate act as cure activators in carbon black and mineral filled compounds, in addition to being good process aids. Silane coupling agents also act as cure activators and adhesion promoters. Plasticizers may be used to soften, improve flow and increase tack in TFE/P compounds not intended for high temperature service.

Processing TFE/P elastomers

TFE/P is processed on conventional rubber equipment and is handled much like any other fluorocarbon elastomer. With proper compounding, TFE/P may be processed using compression, injection or transfer molding, extrusion or calendering techniques.

Mill mixing

Mixing of TFE/P is usually done on a conventional two roll water cooled rubber mill. The TFE/P is first banded on the mill roll. To help with initial breakdown of the polymer, it may be desirable to preheat the TFE/P just prior to milling. If blending TFE/P products or other elastomers, the highest viscosity polymer should be banded first. Blends of polymers should be well dispersed before adding other compounding ingredients. Once a smooth band has been formed on the mill, the remaining compounding ingredients may be added. For easiest incorporation of TAIC, peroxide, lubricants, etc., premixing all ingredients prior to addition is recommended. Mill rolls should be kept as cool as possible (less than 70 [degrees]C) to prevent sticking.

Internal mixing

TFE/P can also be mixed in an internal mixer. As in mill mixing, the polymer is broken down before adding the remaining premixed ingredients. A drop temperature of 105-125 [degrees]C is usually needed for the batch to come together. Addition of the peroxide curing agent on the finishing mill or in a second mixer pass is suggested to insure scorch safety.

Molding

TFE/P can be molded in compression, injection and transfer molding processes. Processing conditions will be dependent on rheological properties of the compound. Typical molding temperatures will range from 160-200 [degrees]C for the FA-100 products and 145-170 [degrees]C for FA-150 products. TFE/P generally has excellent flow properties compared to other comparable viscosity elastomers. However, the hot tear properties of TFE/P like fluorocarbon elastomers in general is relatively poor. Proper mold design to avoid severe undercuts along with the use of good internal and/or external mold release aids will provide for satisfactory demoldability.

Extruding and calendering

The processability of TFE/P in both extruding and calendering operations is excellent because of the good flow characteristics of TFE/P at elevated temperatures. The lower viscosity FA-150 products in particular are recommended for use in these processes. Process conditions will be dependent on compound formulation, but smooth extrudates can be obtained typically at temperatures of 60-75 [degrees]C. For calendering operations the top and middle rolls should be kept below 70 [degrees]C with a cool bottom roll to prevent sticking.

Post curing

A post cure is recommended for TFE/P compounds to achieve optimum properties. In most cases a post cure of four hours at 175 [degrees]C will be adequate although longer or higher temperature post cures should not be detrimental to properties. If post curing thick cross section parts (greater than 1/4" thick) a step post cure is recommended starting at 125 [degrees]C up to 175 [degrees]C to prevent fissuring. For bonded parts a step post cure is also recommended and maximum temperature should not exceed 175 [degrees]C.

Mold shrinkage

The mold shrinkage of TFE/P will vary depending on formulation, but generally for a 75 type A durometer compound a 2.5% shrinkage can be expected after press cure and 3-3.5% after post cure.

Bonding

Bonding of TFE/P to many types of substrates can be achieved with proper compounding. Commonly used adhesives include silane types or epoxies. Baking of the adhesive onto the substrate to be bonded will significantly enhance adhesion.

When bonding carbon black filled TFE/P compounds it is beneficial to include 6-10 phr of an inorganic base such as magnesium oxide or calcium hydroxide in the compound. Non-black fillers such as fumed silica or diatomaceous silica provide improved adhesion versus carbon black filled compounds. Increased levels of TAIC or addition of a silane coupling agent to the compound also increase adhesion strength.

Solution coatings

Because TFE/P has such a broad range of chemical resistance there are relatively few solvents which TFE/P is soluble in. For making solutions of TFE/P, tetrahydrofuran (THF) or trichlorotrifluoroethane are typically used in combination with ethyl acetate. The FA-150 grades are most soluble. A typical TFE/P solution would consist of 15% (by weight) TFE/P compound, 15% trichlorotrifluoroethane and 70% ethyl acetate.

Applications for TFE/P elastomer

Usage of TFE/P first gained acceptance in corrosive oil-field environments because of its resistance to a wide variety of fluids and gases including aggressive amine based corrosion inhibitors, sour oil and gas, steam, acids and high pH completion fluids. Because of its broad chemical resistance profile TFE/P is now being used in other areas.

Aerospace/defense applications

The unique capability of TFE/P to provide resistance to most fluids used in both military and commercial aircraft is helpful in areas where contact with a variety of fluid types may occur. TFE/P has good resistance to all types of hydraulic fluids, lubricants and in particular the aggressive additive packages used in newer types of jet turbine oils. It also provides fair resistance to jet fuels. TFE/P is also resistant to oxidizers and fuels used in rocket propulsion systems and to cleansers used to remove chemical warfare agents.

Automotive/off-highway applications

The additives used in today's high temperature rated engine oils or lubricants are more aggressive towards elastomers than those previously used. These effects can be seen in the new SF and SG rated engine oils, power steering and transmission fluids, EP gear lubricants, and in corrosion inhibited engine coolants (tables 6-8). TFE/P does not embrittle or exhibit surface cracking after exposure to any of these fluids, has good resistance to all types of brake fluids, but is not recommended for most fuel applications because of the high volume swell obtained after exposure to these fluids.

Table : Table 6 - comparison of fluorocarbon elastomers in automotive fluids

Formulations tested      TFE/P     FKM 1     FKM 2
Polymer                   100       100       100
N-990 black                30        30        30
TAIC                        4       2.5        --
Peroxide                    4       2.5        --
Ca(OH)2                    --         3         6
MgO                        --        --         3
Carnauba wax                1         1        --

Original properties

Tensile, psi            2,318     2,067     1,982
Elongation, %             258       219       286
100% Modulus, psi         643       771       524
Hardness, type A         74.5      76.5      74.5

FKM 1 - 69% fluorine, terpolymer FKM 2 - 65% fluorine, copolymer

Table : Table 7

Aged 168 hours at 163 [degrees]C in SAE 10W30 SG/CD

motor oil

                       TFE/P   FKM 1   FKM 2
% Tensile, change        -3     -42     -66
% Elongation, change     -3     -48     -75
% 100% Modulus, change   +4     +34      --
Pts. hardness, change    -7     -2      +3
  % Volume change        +9     +2      +1
  Surface cracking       No     Yes     Yes

Aged 168 hours at 150 [degrees]C in SAE 90W EP gear lubricant

% Tensile, change        -9     -57     -48
% Elongation, change     +13    -61     -73
% 100% Modulus, change   -18     --      --
Pts. hardness, change    -6     -2      +4
  % Volume change        +5     +1      +1
  Surface cracking       No     Yes     Yes

Table : Table 8

Aged 168 hours at 163 [degrees] C in automatic transmission fluid 

                         TFE/P   FKM 1   FKM 2
% Tensile, change         +4      -38     -64
% Elongation, change      -8      -47     -62
% 100% Modulus, change    +12     +43     +31
Pts. hardness, change     -7       0      +4
  % Volume change         +11     +3      +2
  Surface cracking        No      Yes     Yes

Aged 168 hours at 150 [degrees]C in rust inhibited engine coolant 

% Tensile, change         -2      -30     -64
% Elongation, change      +13     -17     -56
% 100% Modulus, change    -12     -14     +46
Pts. hardness, change     -2      -4      +6
  % Volume change         +1      +7      +5
  Surface cracking        No      Yes     Yes

Aged 168 hours at 150 [degrees]C in DOT 3 brake fluid

% Tensile, change         -10     -14     -73
% Elongation, change      -6      -2      -51
% 100% Modulus, change    -5      -4      -23
% Volume change           +5      +5      +25

Chemical industry applications

The wide range of chemical resistance properties in combination with its excellent heat resistance properties make TFE/P an ideal material for many applications in the chemical industry (tables 9 and 10). While it may not provide the best resistance to a specific chemical, it often times may be the only elastomer that can exhibit satisfactory resistance to a combination of chemicals and environmental conditions.

Table : Table 9 - TFE/P chemical resistance

  Formulation tested                  Original properties
FA-100S              100 PHR       Tensile, psi         2,610
N-990 black               35       Elongation, %          195
TAIC                       4       100% Modulus, psi    1,130
a,a'-bis(t-butylperoxy)-           Hardness, type A        73
diisopropylbenzene         4

Aged 70 hours at 23 [degrees]C in methanol

  % Tensile change                                         -1
  % Elongation change                                      +2
  Pts. hardness change                                      0
  % Volume change                                          +1

Aged 70 hours at 100 [degrees]C in 50% sodium hydroxide 

  % Tensile change                                         -3
  % Elongation change                                      -5
  Pts. hardness change                                      0
  % Volume change                                           0

Aged 1 week at 200 [degrees]C in steam

  % Tensile change                                        -16
  % Elongation change                                     +10
  Pts. hardness change                                     +1
  % Volume change                                           0

Table : Table 10 - TFE/P acid resistance

  Formulation tested                   Original properties
FA-150P                   100 PHR   Tensile, psi      2,110
N-990 black                    30   Elongation, %       215
TAIC                            4   100% Modulus, psi   770
a,a'-bis(t-butylperoxy)-            Hardness, type A     70
diisopropylbenzene              4

Sorbitan mono- stearate NF 1

Aged 168 hours at 100 [degrees]C in 25% sulfuric acid 

  % Tensile change                                       -5
  % Elongation change                                   +14
  Pts. hardness change                                   +2
  % Volume change                                         0

Aged 96 hours at 90 [degrees]C in 37% hydrochloric acid 

  % Tensile change                                       -3
  % Elongation change                                    -9
  Pts. hardness change   -5
  % Volume change   +11

Aged 168 hours at 150 [degrees]C in 85% phosphoric acid 

  % Tensile change                                       +6
  % Elongation change                                   -16
  Pts. hardness change                                   +3
  % Volume change                                         0

The types of applications TFE/P may be specified for in this industry include heat exchange gaskets, hose innerliners, o-rings, pipe gaskets, diaphragms for pumps and valves, and chemical tank liners.

Oilfield applications

The ability of TFE/P to resist a wide range of chemicals has been particularly beneficial in the oilfield/petroleum industry. Oilfield elastomeric parts can be exposed to many fluids and gases, including oil, sour oil and gas, amine corrosion inhibitors, steam/hot water, acids, carbon dioxide, diesel and water based drilling and completion fluids, and high pH completion fluids. This diversity of fluids and gasses, along with the uncertainty about when they might be encountered, has placed a burden on oilfield elastomers. TFE/P has the potential to resolve these difficulties since it is resistant to all previously mentioned oilfield fluids and gases. It is the ability to resist combinations of these chemicals that makes TFE/P an attractive oilfield elastomer (table 11).

Table : Table 11 - TFE/P resistance to oilfield media

Formulation tested

                                   Sorbitan mono-
FA-100H                 100 PHR    stearate NF             1.0
N-550 black                  20
Aerosil R972                 15    Original properties
TAIC                        5.6    Tensile, psi          3,000
a,a'-bis(t-butylperoxy)-      7    Elongtion, %             90
diisopropylbenzene                 100% Modulus, psi     1,800
1,2 polybutadiene           5.3    Hardness, type A         95
Carnauba wax                1.5

Electrical applications

TFE/P has excellent electrical resistance properties which are maintained even at high temperatures. The electrical resistance properties, combined with the chemical and heat resistance of the material, make it useful for cable insulation and electrical connectors in severe service applications.

Summary

TFE/P is a unique type of fluorocarbon elastomer capable of withstanding a broad range of chemicals, along with having excellent heat and electrical resistance properties. They are being used to advantage in several areas including the oilfield, chemical processing, automotive and aerospace.

PHOTO : Figure 1 - retention of ultimate tensile strength and elongation

PHOTO : Figure 2 - heat, oil resistance of elastomers ASTM D-2000 designations