Reinforcement with fluorplastic additives.

Reinforcement with flouroplastic additives

The use of high molecular weight polytetrafluoroethylene (PTFE) as a reinforcing additive to improve the tear strength of elastomers was studied in silicone rubber[1-6] by the mid-1950s and in fluoroelastomers[7,8] by the late-1960s. Although the PTFE is added as a powder, the shear developed during compounding into an elastomer fibrillates the powder into a continuous network of nodes and fibers. This network structure effectively reinforces elastomers but it also leads to distortion of finished parts and unacceptably high hardness and modulus of vulcanizates. These problems limit the practical level of addition of high molecular weight PTFE powders and, therefore, tend to limit their usefulness to applications where elasticity and smooth surfaces are not important, such as in propellants, pyrotechnics and explosives. Special compounding[9] procedures have been developed to reduce the effects of fibrillation and aid in the dispersion of high molecular weight PTFE into elastomers. Techniques to modify the surface[10,11] of PTFE powder to improve compatibility with hydrocarbon elastomers and reduce fibrillation have also been developed. At the present time, these techniques find limited application.

Efforts[11] to control fibrillation by reducing the PTFE molecular weight by electron beam irradiation or by copolymerizing tetrafluoroethylene (TFE) with other monomers, such as hexafluoropropylene (HFP) or perfluoro(propylvinyl ether) (PPVE) to produce the low molecular weight, melt processable fluoroplastic copolymers FEP (TFE/HFP) or PFA (TFE/PMVE) are not successful. Such low molecular weight PTFE homopolymers and copolymers do allow easy compounding of the powders into elastomers but, since fibrillation is completely eliminated, elastomer vulcanizates are not reinforced and they frequently have reduced tensile and tear properties. Some commercial use is made of these low molecular weight fluoroplastics and copolymers in elastomers at levels of about 10-30% to improve abrasion resistance and reduce coefficients of friction. The melt processable fluoroplastics FEP and PFA can be melt blended into perfluoroelastomers, which have sufficient thermal stability to withstand processing temperatures above 300 [degrees] C, to improve processing behavior[12].

A new high molecular weight TFE/HFP fluoroplastic micropowder has recently been developed (Teflon MP1500, Du Pont) which forms short fibers, ribbons or platelets when compounded with sufficient shear into elastomers. The controlled structure developed during compounding allows high levels of incorporation of the micropowder into elastomers with uniform dispersion and results in significant improvements in tear strength and abrasion resistance, as well as reduced coefficients of friction.

Experimental

Compounding ingredients and definitions

Formulations of compounds evaluated in this study are shown in the appendix. Typical commercial formulations representative of o-ring sealing devices were used. Three TFE based fluoroplastics differing in properties were compared, consisting of a conventional high molecular weight TFE homopolymer (Teflon 6C), a conventional low molecular weight irradiated TFE homopolymer (Teflon MP1200), and a new high molecular weight TFE/HFP fluoroplastic micropowder, (Teflon MP1500) designated here as MicroPowder 1500 or MP1500. A variety of elastomers were selected for study, consisting of two fluoroelastomers, silicone, fluorosilicone and several hydrocarbon elastomers.

Test samples

All test samples were prepared using standard rubber processing techniques. Compounds were mixed either in a laboratory internal mixer or on a two roll mill. Low viscosity elastomers, such as the silicone and fluorosilicone elastomers, were compounded by first preparing a masterbatch consisting of 50 parts MP1500 copolymer to 100 parts elastomer and then reducing the concentration of copolymer to the desired final level by further addition of the elastomer. The higher viscosity of the masterbatch allowed sufficient shear to be developed during compounding to adequately cold draw the copolymer so that optimum reinforcement of the final composition was achieved.

Test specimens were prepared from ASTM slabs that were compression molded. All data were obtained on test pieces that were cut so as to be tested in a direction parallel to the rotation of the mill rolls during the final sheeting out of the compounded stock. Testing parallel to the mill direction tends to maximize the effects of fibrillation when anisotropic behavior occurs. Materials that required a post-cure were finished in an air circulating oven. Times and temperatures for molding and post-cure are given in the appendix.

Test procedure

All testing procedures conformed to appropriate ASTM test methods. Physical properties were tested using the following ASTM procedures: Hardness, durometer A (ASTM D 2240); Tensile strength, elongation and modulus (ASTM D 412); Tear strength, die B (ASTM D 624); Compression set, plied specimens (ASTM D 395, method B); Abrasion resistance, Taber abraser (ASTM D 3389); Peel adhesion (ASTM D 429 method B, steel coupons).

Discussion

Table 1 shows tensile, tear and abrasion data for 30 phr carbon black filled vulcanizates of a typical commercial FKM (vinylidene fluoride/hexafluoropropylene) fluoroelastomer formulation containing various levels of the three fluoroplastics as additives. In table 1, [M.sub.100] = stress at 100% elongation, [T.sub.B] = tensile stress at break, [E.sub.B] = elongation at break. At a level of only 10 phr, high molecular weight PTFE is highly reinforcing due to the formation of a continuous fiber network during compounding. Tear strength is increased significantly, however, there is also a large increase in hardness and modulus and the vulcanizate becomes stiff and boardy or leather-like. Low molecular weight irradiated PTFE has relatively little effect on tensile or tear properties even at levels as high as 20 phr, however, there is observed a significant improvement in abrasion resistance, as measured by weight loss. Lower levels of the low molecular weight PTFE show similar small effects. At levels of 10 and 20 phr, MP1500 copolymer shows significant increases in tear strength and abrasion resistance with a moderate increase in modulus or hardness. Increases in modulus and hardness can be compensated for in these vulcanizates, if required, by a reduction in the level of carbon black, with little effect on tear strength.

Table 2 shows tensile and tear data at 25 [degrees] C for a 40 phr carbon black filled FKM-GFLT (vinylidene fluoride/tetrafluoroethylene/perfluoro(methylvinylether/cure site) fluoroelastomer containing 5 and 10 phr of MP1500. Tear data are also presented for press cured samples at 177 [degrees] C. It is seen that MP1500 significantly increases tear strength even at relatively low levels and at high temperatures. Modulus and hardness are increased only slightly. In addition, it is found that mold fouling and sticking, frequent problems with peroxide cured elastomers, are reduced by the addition of these relatively low levels of the fluoroplastic copolymer, while other important physical properties, such as compression set, heat resistance, resistance to oil swell and cold temperature properties are practically unaffected. From table 2 it is also seen that peel adhesion to steel is not affected. The fluoroelastomer FKM-GFLT is a specialty high performance polymer and is frequently used to make small parts with very thin cross sections. Tear strength at high temperatures and reduced mold fouling and sticking are important in removing fragile press cured articles from molds. Many articles are reinforced with steel inserts, so that good adhesive peel strength must be maintained.

Table 2 - tensile and tear data for FKM-GFLT vulcanizates

Sample                                    2.A      2.B      2.C
PTFE type                                None   MP1500   MP1500
PHR                                         0        5       10

Physical properties @ 25 [degrees] C

  [M.sub.100] (MPa)                       9.7     10.5     11.8
  [T.sub.B] (MPa)                        11.9     13.7     14.1
  [E.sub.B] (%)                           131      146      128
Die B tear(*) @ 25 [degrees] C (kN/m)    23.9     29.5     35.7
Die B tear(**) @ 177 [degrees] C (kN/m)   7.5      9.3     10.5
Hardness, A (pts)                          79       80       80

Compression set

  22 hrs/200 [degrees] C (%)               20       22       23
Peel adhesion (kN/m)                      5.3      6.4      6.8

(*)Press and post cured. (**)Press cured. No post cure.

Table 3 shows tensile and tear data for a silicone elastomer, VMQ poly(dimethylsiloxane/methylvinylsiloxane) copolymer, and a fluorosilicone elastomer, FVMQ poly(methyl-3,3,3-trifluoropropyl/methylvinylsiloxane) copolymer, containing silica filler and 10 phr MP1500. Very large increases in tear strength are obtained when the fluoroplastic is compounded with sufficient shear into these elastomers. In general, when a conventional high molecular weight PTFE homopolymer is mill mixed into silicone or fluorosilicone elastomers, even at very low levels, an anisotropic fibrillated layered structure is obtained. Delamination occurs readily when one attempts to tear the vulcanizate. A laminated structure is not obtained with MP1500, even at very high levels, and vulcanizates appear to be isotropic.

Table 3 - tensile and tear data for VMQ and FVMQ vulcanizates

Sample                                  3.A      3.B    3.C      3.D
Elastomer type                          VMQ      VMQ   FVMQ     FVMQ
PTFE type                              None   MP1500   None   MP1500
PHR                                       0       10      0       10

Physical properties @ 25 [degrees] C

  [M.sub.100] (MPa)                     2.8      3.5    3.6      3.8
  [T.sub.B] (MPa)                       5.1      5.2    5.1      5.9
  [E.sub.B] (%)                         170      160    158      171
Die B tear (kN/m)                         9       15     12       16
Hardness, A (pts)                        60       68     72       80

In general, it is found that elastomers which have relatively low tear strengths, such as silicone, fluorosilicone and special purpose fluoroelastomers, benefit more from the addition of low levels of MP1500 than do elastomers which already have relatively high tear strengths, such as natural rubber (NR) and neoprene (CR) elastomers. However, since the tear strengths of all elastomers decrease with increasing temperature, it is found that the improvements in tear strengths afforded by the addition of MP1500 increase at elevated temperatures.

Table 4 shows tensile and tear results at 177 [degrees] C for typical carbon black filled vulcanizates of an EPDM (ethylene/propylene/hexadiene), CR (neoprene or polychloroprene), CSM (chlorosulfonated polyethylene) and AEM (ethylene/acrylic) elastomers.

Table 4 - die B tear data at 177 [degrees] C for typical hydrocarbon elastomer vulcanizates

PTFE type          None        MP1500
PHR                   0            40
Elastomer   Tear (kN/m)   Tear (kN/m)
EPDM                4.9             8
CR                 12.3          16.1
CSM                 4.0           8.6
AEM                 9.5          12.8

Table 5 shows static and dynamic coefficients of friction at 25 [degrees] C for EPDM vulcanizates containing 20 phr of an irradiated low molecular weight PTFE and MP1500.

Table 5 - coefficients of friction for EPDM

PTFE type                  None  Low MW   MP1500
PHR                           0      20       20

Coefficients of friction

  Static                    3.0     0.5      0.4
  Dynamic                   1.6     0.3      0.4

The normal formation of a continuous fiber network, which results when conventional high molecular weight PTFE is compounded into elastomers, is believed to be due to unfolding of the highly developed PTFE crystalline structure. Particle-to-particle interactions are apparently also involved since the network is continuous. Low molecular weight PTFE apparently has an insufficient chain length to create fibers or any other type of drawn structure. The tendency for the new TFE/HFP fluoroplastic micropowder, MP1500, to fibrillate is reduced significantly in a manner not totally understood. However, it is believed that instead of a continuous long fiber network, short fibers or elongated platelets or ribbons are formed by the shear stresses during compounding. The tendency of the copolymer to fibrillate is controlled by the level of HFP comonomer and by the molecular weight. High molecular weight is believed to be important in developing the platelet structure by allowing some unfolding of the crystalline PTFE polymer chains while the HFP units in some way limit the extent of unfolding. The copolymer contains a sufficiently low amount of hexafluoropropylene and is of sufficiently high molecular weight that, unlike conventional FEP fluoroplastics, which have low molecular weight and high HFP content, the new TFE/HFP fluoroplastic copolymer is not melt processable. Since the tendency for fibrillation is significantly reduced, the level of shear during compounding must be adequate to sufficiently deform the copolymer particles, especially when they are compounded into low viscosity elastomers. It is found that the resulting structure significantly improves tear strength without causing the usual unacceptable increase in hardness, modulus or distortion of vulcanizates.

Figure 1 shows electron micrographs of blends of 10 phr of a high molecular weight PTFE homopolymer (A), 30 phr of an irradiated low molecular weight PTFE homopolymer (B) and 50 phr of a high molecular weight TFE/HFP fluoroplastic copolymer, MP1500, (C) mill mixed into an KFM (vinylidene fluoride/hexafluoropropylene) fluoroelastomer. It is seen that high molecular weight PTFE is dispersed in the elastomer in the form of a continuous fiber network, low molecular weight PTFE is dispersed mostly in the form of distinct particles approximately 0.2 [Mu] in diameter, while MP1500 is dispersed in the form of elongated plate-like agglomerates of distinct particles approximately 10-20 [Mu] in length, 5-10 [Mu] in width and 2-5 [Mu] thick.

Conclusions

Either low molecular weight irradiated PTFE or high molecular weight MP1500 copolymer can be easily dispersed into elastomers at high concentrations. Both materials improve abrasion resistance and lower the coefficients of friction of elastomers. However, MP1500 copolymer resin reinforces a broad range of elastomers and affords significant improvements in tear strength, especially at high temperatures. Improved high temperature tear strength is important in reducing production costs by increasing output, since complex parts (i.e. with undercut) can be removed from hot molds with reduced scrap rate. The addition of the fluoroplastic copolymer also decreases mold fouling and sticking, while having no adverse effect on adhesion of elastomers to metal. The optimum level of MP1500 as an additive depends on the final physical properties required of the elastomeric vulcanizate and some adjustments in compound ingredients, such as carbon black or silica, may be desirable. Sufficient shear must be generated during compounding to develop the MP1500 short fiber, platelet or ribbon structure which accomplishes the reinforcement. [Table 1 Omitted]

PHOTO : Figure 1 - electron micrographs of blends of fluoroplastics in FKM elastomers A - high molecular weight PTFE @ 10 phr

PHOTO : B - low molecular weight PTFE @ 30 phr

PHOTO : C - high molecular weight TFE/HFP copolymer (MP1500) @ 50 phr

References

[1]W.H. Crandell, Rubber World, Nov., 236 (1955). [2]M.M. Safford and A.M. Bueche, U.S. patent 2,710,290 to General Electric (1955). [3]G.M. Konkle and T.D. Talcott, U.S. patent 2,927,908 to Dow Corning (1960). [4]E.V. Wilkus, U.S. patent 3,132,116 to General Electric (1964). [5]W.W. Foster, U.S. patent 3,449,290 to Union Carbide (1969). [6]J.D. Blizzard and C.M. Monroe, U.S. patent 4,010,136 to Dow Corning (1977). [7]M.H. Kaufman and J. Gonzales, Rubber Chem. Technol. 41,527, (1968). [8]L.M. Magner and J.O. Punderson, U.S. patent 3,484,503 to Du Pont (1969). [9]D.B. Kitto, U.S. patent 4,520,170 to Du Pont (1985). [10]A.A. Khan and C.W. Stewart, U.S. patent 4,469,864 to Du Pont (1984). [11]C.W. Stewart, U.S. patent 4,596,855 to Du Pont (1986). [12]A.L. Logothetis and C.W. Stewart, U.S. patent 4,713,418 to Du Pont (1987).