C . A.
HOMSY
Flaarocar,bon
I N CORROSION PREVENTION
20
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
Cost reductions, increased processibilty, and continued research have extended application ofjuorocarbon resins to a wide range of corrosion problems in Juids conveying, static and dynamic load support, and antistick uses
Resins
ince the discovery of polytetrafluoroethylene (TFE)
S' 25 years ago and subsequent commercialization in 1946, research has produced a whole family of fluorocarbon resins: T F E molding powders; melt-processible FEP (fluorinated ethylene-propylene) resin and dispersion; and films and filament forms of both resins. Processing techniques for TFE, similar to those for powder metallurgy, have matured into highly developed commercial procedures, and made these resins available to solve a wide range of corrosion problems. Pr0p.rti.s
Fluorocarbon resins, trademarked Teflon, offer several useful properties (Table I) : -High-temperature
resistance :TFE, 500' F. ;FEP, 400°
-High chemical resistance. Only molten alkali metals, fluorine, and a few complex halogenated compounds at high temperatures have serious effects -Resistance to cryogenic temperatures -Excellent dielectric and insulating properties, retained under severe environmental conditions and maximum use temperature
I I I 1111 I I
-
IIIII 1 I IY
-Low coefficient of friction without stick-slip phenomenon. Almost nothing will adhere to the untreated resin surfaces -Excellent bearing wear performance when formulated with suitable reinforcing agents Although each of these properties considered separately is useful, their combination makes fluorocarbon resins outstanding engineering materials. These properties are inherent in the resins Certain others, however, particularly for T F E resins, are controllable by processing and fabricating techniques, notably permeability, flexural fatigue life, elongation, and impact strength. Crystallinity and void content in a finished item of T F E resin are two variables which have particular effect on these properties. Per cent crystallinity may be varied between the approximate limits of 50 to 65%, lower crystallinity being obtained by more rapid cooling from sintering temperatures. Flexural fatigue life, tensile strength, elongation, and impact strength are improved by low crystallinity. Flexural modulus or stiffness and
Chemical resistance of piping cornpmed. Black mem in&& full resisfance; dmk yellow mcm indic& resisfance to low cotlCbnhntiOm. PVC = polyvinyl chloride; ABS = Milomhila-burodimd-rtyrmc; Epoq = glms-reinforced; Phenolic = mbestas-jUed; RL = rubber; VL = vinyl chloride-vinylidene chloride copolymer; GL = glass; A-7 = Ni-Cu, 67-29; A-2 = Nicr-Fc, 77-73-6; A-3 = FcCr-Ni, 43-79-28; A-4 = Ni-Cr-Mo, 58-77-5
VOL 56
NO. 3
MARCH 1964
21
resistance to permeation are improved markedly by high crystallinity or slow cooling. Specific gravity, the property generally measured to indicate crystallinity, usually varies between about 2.1 4 and 2.20 for low and high crystallinity, respectively. The presence of voids is determined by dielectric strength measurements and by dye penetrant tests. Voids markedly reduce tensile strength, elongation, resistance to permeation, and flexural fatigue life. They are minimized by proper preforming and sintering techniques. I n parts found to have many voids, apparent specific gravity is not a measure of crystallinity. Where control of these properties and, therefore, void content and crystallinity are important, the engineer should follow two general approaches : SPI industry standards of quality for specification purposes ; and detailed consultation with the supplier or fabricator. I n addition, extensive and detailed technical data on fluorocarbon resins, developed by producers of these materials, are available for materials specification and design. Uses of these resins to prevent corrosion seem to fall into three categories : -Leak-free conveying of fluids. Process-equipment lining and components such as gaskets, packings, seals, filters, and laboratory ware-e.g., beakers and bottles -Bearings for static and dynamic load support-friction or sleeve-type, and antifriction or roller-type -Coatings for solids and slurry-handling equipment and sealing (packaging) machines Packings a n d Gaskets
Gaskets and packings, the simplest of seals, are essential components of fluid-handling systems. Use of TFE has meant sharply improved performance in terms of maintenance, reliability, and therefore economy. Du Pont has saved about $ million in the last few years through use of these products in applications suchas: -Solid gaskets of filled and unfilled resin for piping, towers, and heat exchangers -The more resilient envelope gasket for glass-lined pipe and equipment. FEP is used also for this purpose. -Cup
and cone packing in all-alloy valves
-Packing of woven asbestos impregnated with resin dispersion for pumps and valves -Packing of braided yarn for pumps and valves in corrosive and high temperature conditions More than $75,000 per year is being saved by standardization on T F E impregnated asbestos stem packing for valves other than stainless. Reliability has been increased and maintenance has been virtually eliminated. Moreover, the need for large inventories of valves with different packings has been eliminated. Whereas in conventional packings, the grease, graphite, or oil impregnants are washed out in time, leaving a lifeless, dry, and abrasive asbestos, ‘TFE resins provide lubrication which is permanent, even under extreme con22
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
ditions such as 550’ F. steam service in power plants. I n a comparison test, T F E asbestos and conventional packings were placed in 300’ F. 150-p.s.i steam service. After 14 months, the conventional packing was dry and brittle. T h e TFE-asbestos packing was as good as new and could have been returned to effective service. In the packing field, TFE yarn is the newest de\,rlopment. I t is a braid of continuous filaments of resin impregnated with a dispersion of the resin. Having the chemical resistivity of pure resin and flexibility of the braided construction, it is often used in conjunction with solid packing rings of T F E resin. I n one instance, such packing has given trouble-free service in an agitator stuffing box where the reactor was handling organic material in 507, nitric acid at 185’ C. As one operator commented: “Everything else on this reactor has let go but the packing.” Annual savings to Du Pont through use of this packing have been a minimum of $2000 for valves, 610,000 in pump applications, and $8000 on reaction vessels. Back-up rings of TFE are commonly spiral skived from cylindrical bar stock. This straightforward procedure yields simple annuli which are the rings. Such rings are widely used in hydraulics to protect rubber O-rings from distortion due to heat and pressure. Finally, in aerospace propulsion which has much in common with chemical processing, more sophisticated gasket designs, such as the spiral-wound configuration, have given reliability engineers a bench mark for superior performance. The spiral-wound gasket consists of alternative concentric circles of highly resilient steel and filled T F E resin; in this way highest resiliency is coupled with the sealing power of a fluorocarbon resin. Seals
As seals for process equipment, the fluorocarbon resins, especially T F E resin, are found in many configurations. Valve and P u m p Seals. Plug valves incorporate a molded sleeve of TFE which eliminates the need for lubrication and provides a tight seal even when wear or corrosion occurs on the plug. The cost increment for T F E is very small in view of the increase in valve reliability. Similarly, gate, ball, butterfly, and diaphragm values perform better with fluorocarbon seals. Thc entire diaphragm may be of molded resin, or, for greater resiliency, TFE is often laminated to rubber. Mechanical Seals. T F E can be used effectively for mechanical seals, both stationary and dynamic. One type of bellow seal, almost completely of TFE resin, is the only seal to our knowledge that has been used for nitric acid service. Some have been in operation for two to three years. The rubbing face is made of graphitefilled T F E and the flexible portion from unfilled resin. Pipe Thread Sealant. TFE unsintered tape and dryor liquid-based T F E powder as pipe-thread scalants provide corrosion-proof sealing of any threaded joint, and maintain lubricity so the joint may be made and
C. ‘4. Homsy is Technical Representative in tfze T e f lon Marketing Section, E. I . du Pont de Nemours & Co., Inc., Wilmington, Del.
AUTHOR
TABLE 1.
SELECTED PROPERTIES OF T F E AND FEP FLUOROCARBON RESINS
Thermal conductivity, B.t.u./hr./sq. ft./" F./in.
Coefficient of thermal expansion, 10-5 in./in./' F. Specific heat, B.t.u./lb./O F. Heat distortion, temp.,
- 200 to 360 O F. -423" F. 24 hr. ;
Water absorption 7 0 wt. increase Dimensional change, in./in.
FEP
2.13-2.20
Cenco Fitch Cenco Fitch
1
2.14-2.17
1.7
1.4
0.9
0.8
D570-547 D570-547
2
>lo= 2.2
I
D149-59
~
480
Does not track
VOL. 5 6
NO. 3
I1
x
1018
2.2 2.2 500-600 Does not track
MARCH 1964
23
TABLE 11.
PROPERTIES OF
General Phfial Ropertics
Filler load in^, vol. % ,
1
I
BvlL dencity, Ib./eu. in.
._.
I
13.3
0.0129
0.0103
2.22 2.ao
S@fk gravity
I
22.2
I
14.6
1
tzb
27.1 I17.6.5.0 113.3,2.4
0.0107
0.0087
0.0103
0.0104
0.0107
2.22
2.12
3.85 3.97
2.25 2.18
2.28 2.27
0.013
0.000
0.019
0.016
0,010
2.51
2.25
7.73 2.61 7.73 3.01 8.12 3.22 9.24 3.88
8.33 3.51 8.77 3.55 9.61 3.85 11.11 4.45
I :::I :::I ;::I
4.9 3.9
6.9 7.1 3.8 3.9
Measurcd ThmnticaJ
A S T M D1457-66T
2.18
Water absorption
A S T M D570-69T
0.000
0.015
coeff. of thermal mnductivity, B.t.u./hr./sq. fL/ (' F./in.)
Cenco Pitch
1.7
2.55
3.12
3.12
3.25
8.02 2.94 8.40 2.96 9.05 3.42 10.31 4.15
6.97 4.19 7.33 4.19
6.97 4.38 7.46 4.67 8.14 5.12 9.50 5.97
5.40 4.38 5.72 4.37 6.35 4.99 7.82 5.75
caa.of linear therm. ULp. ( X 10-9, rn./in./"F. 78'-200 ' F.
78'400
'F.
ASTM D696-44 ,MD CD
MD CD
78e400' F.
MD CD
78'-500
' F.
%deformation, 78' F., 2000 p.a.i., 24 hr. 76 permanent deformation'
MD CD
MD CD
%deformation, 78' F., 2,OOO p.s.i., 100 br. % pumanent deformation
6.81' 7.04
I
7.60
I
9.12
I
7.9 8.4
I
;:
II
4.72 9.35 5.55
II
II
1 -
MD CD MD CD
16 3 18:7 8.8 9.1
12.6 14.9 6.0 7.9
8.9 9.4 4.4 5.6
10 1 11.5 6.4 7.3
6.1 6.4 2.5 2.5
7.6 8.7 5.8 5.9
7.8 8.1 5.6 5.5
MD CD
30.1 32.8 17.4 19.2
16.6 27.7 11.9 16.2
10.6 27.8 4.9 17.9
16.0 15.4 12.0 10.8
10.6 8.4 7.1 4.9
11.3 12.2 8.4 8.4
9.6 10.9 6.4 6.8
MD CD
unmade repeatedly. Contamination of pipe lines with conventional pipe thread dope is eliminated and tape eliminates cans, brushes, and rags that are housekeeping and personal cleanliness problems. Du Pont has one installation of steam and condensate piping containing 6000 screwed joints, in sizes ranging from '/p to 2 inches, made with T F E tape and put on stream without a single l e a k s o m e t h i n g unheard of with ordinary thread sealants. This tape as a thread sealant is rapidly becoming standard a t our plants. Piston Rings and Rod Packings. Reinforced T F E resins are being used increasingly as piston rings and rod packing in unlubricated compressors and those handling corrosive gases. For example, on a two-stage com24
II
8.00
I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
pressor pumping gas contaminated with H2S from 165 to 185 p.s.i. and up to 350" F., glass-filled T F E rings average eight month's life us. four to six weeks' for lubricated cast iron rings; in addition, compressor capacity was increased 20%. Glass-filled T F E resin rings have also given over seven years' satisfactory service in inert gas compressors where carbon rings usually failed after only a few weeks. Carbon-filled TFE resin rings continue to give trouble-free service after four years in chlorine compressors. Leading manufacturers now offer compressors equipped with this resin a t the same cost as with carbon, both a t a small premium over lubricated cast iron. Other seals for reciprocating shafts or cylinders are:
TYPICAL FILLED TFE COMPOSITIONS
UltimatetmsikarrsRh, psi.
I
-
ASTMDl45756T 3500-4500
-
2,100 2,800 3,600 - 2,700
ASTM D1457-56T
MD 30CMOO
,CD
ASTM D790-59T CD
c3J
817 0.5o-O. 90
capnpn*ivcmu@, p i . ASTM D695-54
630
MD
l%rmin
CD 25% .train
MD
4,485
MD
1,100
m
0.2%0lfsct
CD
MD
ModulIu ( X 10)
6.0
CD
320 330
300 270
569 3.12
2.39
- -
1,m
850 5,200 3,950 1,650 1,270 9.8 8.5
-
~
606
1,200 1,120 5,610 4,520 1,870 1,270 11.8 10.0
1%
80
220
280
-
90 -
220
280
2.78
1,213 2.41
240
859 2.03
1,143 1.97
1,030 1,090
1,m
1,130
5,000 5,200 1,420 1,710 9.6 8.9
1,120 1,170 7,720 7,300 1,700 1,740 11.0 11.4
970 5,120 4,260 1,590 1,420 9.5 9.2
1,100 6,350
57 53 46 42
65 64 52 51
56 57 43 44
59 44 46
2.25
2.97
5,160 1,m
1,810 11.0
11.9
SborCDummber,
HardaQ.
scakD 78O If.
MD CD
51
54 53
300' F.
MD
37
40
39
CD
I d impaq Ib.Afm
ASTM D256-56
Diekmic a h m s m , v . / d
ASTM D149a'
Air
3.0
2.70
1,500
448
m
p
.
o
p
44 -
2.18 -
2.56
1.96 -
57
921
327 866
63 69
X X
63 187
690 932
2.2 22
2.50 2.35
2 63 2.85
X X
X X
3.38 3.25
2.71 2.68
lo"
lip seals, O-rings, and X-rings. Graphite-filled lip seals of various sizes are used for reciprocating shafts; appropriate stresses can be built into this design to improve sealing efficiency. O-rings and X-rings are slotted along the outer circumference for metal spring inserts which increase mechanical memory. Such features are built into seals of TFE resin for applications in hydraulic and pneumatic systems. Interestingly, simple seals for high speed rotating shafts cut from l/s-inch thick sheeting of TFE resin have performed for almost three years in combined carbon disulfide vapor and sulfuric acid service at 195' F.; successful sealing is accomplished despite accumulation ofsolid sulfate crystals. An unusual example of a seal molded from fluorocar-
10'8
10"
bon resin is the V-shaped seal incorporated in a single nut coupling for glass drain lines. Chemical inertness, temperature resistance, resiliency, and surface finish promote leak-free sealing. These seals have been molded from T F E and FEP resin. Lined Pipe. Performance of piping systems are of major importance because these comprise from about 25 to 33% of total investment in a chemical process plant. Pipe lined with T F E or FEP offers almost universal chemical resistance-evcn better than glass. SPI commercial specifications for these products are being developed to ensure uniform product quality. As a class, nonmetallic linings in steel have the broadest range of chemical resistance; second in breadth of range VOL 56
NO. 3
MARCH 1964
25
TABLE 1 1 1 .
FRICTION, P V , AND WEAR PERFORMANCE OF TYPICAL FILLED TFE COMPOSITIONS
Property Limiting P V , 1b.-ft./sq. in.-min. At 10 f.p.m. At 40 f.p.m. At 100 f.p.m. At 400 f.p m. At 1000 f.p.m.
PV for 0.005 in. waar per 1000 hr. (nonlubricated) Wear factor, K , cu. in.-min./lb.ft.-hr. Coefficient of friction Static load = 500 p.s.i. Dynamic load, P V = 8:000 to 10,000 10 f.p.m. 100 f.p.m. 1000 f.p.m. a
1 I
L'nzlled
TFE
11
75% glass
jiber
,
i
~.
1
15y~ glass fiber, 5%
,
1
~
1
MoSg
1 200 1,400 1,800 2,100 2,500 ~
1
I
1
I
15,000
x
10-10
~
16 X 10-10
I
10 X 10-10~ 34 X 10-101 6 X 10-10
I
0.05-0.08
0.10-0.13
, 0.10-0.13
0.08--0.10
0.08-0.10
~
l~
15 X 10-101 9 X 10-'0 0.08-0 . l o
0.10 0.13 Unstable operation
0,20-0,22 0.27-0.40 0.3?-0.50
0.17-0.21 0.26-0.29 0.30-0.45
0.12-0.16 0.20-0.26 0.30-0.31
0.14-0.22 0.35-0.50 0.16-0.24
'
0.08-0.10 1
I
0.12-0.15 0.24-0.50 0.24-0.37
'
1
0.12-0.13 0.32-0.35 0.19-0.24
Compositions are those detailed in Table II ( 4 ) . PV = product of unit load and relative velocity.
are the unsupported plastics. Metal alloys are very selective in chemical resistance except to solvents. I n terms of installed cost, steel lined with these resins is less costly than glass-lined steel or Schedule 10 Hastelloy C. It is equal to, or lower in cost, than heavier schedules of nickel and high nickel alloys and slightly more expensive than Type 316 stainless, Schedule 40. I n general, when process considerations require material more costly than Schedule 10 stainless steel, steel lined with T F E or FEP is cost competitive. I n one case, installation of a T F E lined system saved $700,000. Conditions involved HC1 and organic solvents at 150' C . and 100 p.s.i. By replacing glass-lined steel, high nickel alloys, and phenolic piping, downtime for unscheduled maintenance was reduced more than 80%; production loss was minimized and plant capacity increased. I n another instance, a n installation of 2-inch pipe lined with FEP resin has handled 30 million pounds of 7oy0 sulfuric acid at 250' C. for 14 months. Dip pipes completely protected by fluorocarbon resin are now available; in fact, a FEP resin lined and covered dip pipe has been in successful use for 16 months feeding a 50 to 30 sulfuric-nitric acid mixture at 200' F. Externally the dip pipe is exposed to concentrated " 0 3 vapors at 230' F. No deterioration has occurred. This performance has led to the installation of a completely lined system at a cost of only 70y0of glass-lined steel which had a history of erratic performance. Lined Hose. Companion product to lined piping are elastomer and metal overbraided hose of T F E and FEP resin. Industrial rubber hose lined with these resins offers serviceability to 150 p.s.i. and 300' F. with practically any chemical. Thus, this hose lends versatility to process operations regardless of the product being handled ; key applications are for tank truck, tank car, and barge unloading. One of the earliest applications was for loading cylinders with NZO and 26
I ~
Filled Compcunds,a Balance TFE (Wt. 70) 2070 glass 25% glass 15% 60% fiber, 5% fiber graphate bionre grajhzte
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
COZ. The hose has been used for almost two )-ears without failure in a service involving repeated flexing and impulse at loading temperature of - 5' F . Flexible metal tubes previously used lasted only three months. Unlined rubber lasted less time. Metal overbraided hose of T F E resin, the most frequently used, offers the full temperature potential of T F E resin in a flexible conduit. This is particularly useful for flexible steam lines, hot oil lines, etc. However, this hose should not be used for liquid chlorine which is under high pressure. Permeation may cause failure of the steel overbraid and resultant pressure failure of the fluorocarbon resin liner. In all hose applications, particular attention must be paid to end connections. Overbraided hose usually has a metal connector that comes in contact with the fluid being handled. Chemical transfer hoses have flanged ends with the liner flared over the flange face so that only fluorocarbon resin is in contact with the fluid. Where frequent connect and disconnect operations are encountered, it is advisable to install a replaceable adaptor on the hose end so that damage to the gasket face does not require replacement of the entire hose. Such details as these make the difference between economic success and failure of lined flexible conduits. Bellows. Bellows of TFE resin are used with and without reinforcing coatings in many piping systems to accommodate expansion and contraction, to dampen vibration: or to compensate for misalignment and angular deflection. For satisfactory results, it is important that the correct design of bellows be used for each of these conditions. Prototype testing has been found the most adequate way to predict the quality of bellows of a given design and manufacture. Lined Valves. Very recently, commercially produced ball and butterfly valves completely lined with TFE or FEP have appeared. One manufacture of ball
valves has provided reliable performance in field tests for almost two years in sulfuric-hydrochloric acid and nitrobenzene service at 180' F. Alloy valves had previously lasted only one month in this service. Lined Vessels. The latest contribution to corrosionfree fluid-handling systems is the FEP-lined vessel. A long-term developmental program for the lining of vessels with sheeting of this resin has led to successful field tests. This novel technology does not necessarily require adhesives and is expected to provide linings for use over the full range of resin utility. A typical installation is a 20-inch diameter by 14-foot lined tower which has proved successful in handling 200' F . HC1 a t 25 p s i . FEP lined storage tank, pipe, and fittings were also part of this particular installation. Vessels designed for such linings are equal or lower in cost than other lined vessels where materials such as exotic alloys and glass are used. I n addition, important savings should result because of sharply reduced maintenance costs. These expenses are a serious burden in any chemical process plant, amounting to about $250,000 per year in a single Du Pont plant. Another new development is filter elements of both T F E and FEP made by several techniques: filaments and wire mesh coated with resin dispersion , woven wire coated with resin, and porous molding of TFE. Such filters are proving themselves in applications such as distillation tower demisters and process stream filters replacing precious metal. With all these developments in lined process equipment, corrosion-free fluid-handling systems entirely lined with fluorocarbon resins can now be assembled. Static and Dynamic Support of loads
For supporting static and dynamic loads, bearings of alloyed metal and grease or oil lubrication can, under good environmental conditions, perform well for long periods of time. Generally, however, bearings on equipment in the chemical industry are subject to environments which torture a bearing by impregnation of abrasive matter, leaching of lubricant, or both. Bearing components made from reinforced T F E resin have come into widespread use; the resin itself is a n excellent lubricant necessitating no hydrocarbon greases and oils and exhibiting no. stick-slip. Indeed, after a short time in use, a fluorocarbon bearing has transferred a small coating of resin to the mating surfaces. T h e interfacial boundary thereby becomes resin us. resin, a condition ideally suited to minimize friction and wear. For these reasons such bearings are used in a t least 10 application areas: -Near substances that must not be contaminated-e.g., textiles, paper, food, and drugs -Near or in liquids which are solvents for hydrocarbons -With corrosive substances such as water, acids, and alkalies -At cryogenic temperatures where normal lubrication is impossible -At elevated temperatures (to 500' F.) where common lubricants degrade or vaporize
-At low humidity where other dry bearings cannot be used -For heavy load, slow-speed duty that tends io squeeze oil out of ordinary bearing surfaces -In
areas where there is danger of fretting or galling
--In applications where slip-stick motion is undesirable or where lowest possible static friction is needed-e.g., reciprocating or oscillating systems -In locations relatively inaccessible for routine lubrica t ion T h e art of reinforcement to increase hardness and stiffness and decrease cold flow with little or no increase in coefficient of friction is rapidly becoming a science. I t is achieved in several ways : -Filling. Mixing discrete organic or inorganic particles or fibers with virgin resin. Glass fiber, graphite, and bronze powders are used with T F E and FEP resins; also, aluminum silicate fiber with T F E resin. -Embedding. Any filled or virgin fluorocarbon resin may utilize a metallic or nonmetallic backing or embedment such as screens, perforated metals, or fabrics. Unfilled and glass-filled T F E resins have been used with perforated metal embedment. --Impregnated compositions. Any porous structure in which the pores or interstices have been completely or partially filled with dispersion of T F E or FEP resins. These include base structures such as porous metals, glass fabric, T F E fluorocarbon fabric, and asbestos. Recent work has contributed importantly toward establishing the physical properties of reini-orced T F E compositions and delineating the parameters of loadvelocity capability [so-called PVlimit, the product of unit load ( p s i . ) and relative velocity (f.p.m.)] and wear in bearing service. Wear rate may be expressed as the product of a wear constant, K , and P V product (Table 111). Percentages of filler are weight per cent. T h e best combination of P V and wear performance seems to be 60y0 by weight bronze-filled T F E resin. Work is continuing, however, to determine optimum concentration of a particular filler. Some fillers are more costly to incorporate and environmental conditions may preclude some types. For example, best wear performance with bronze powder filler occurs in the range 60 to 7ooj, by weight. Since transition to greater wear is sharp at the 7oojl, level, 60% by weight is considered the practical optimum. However, optimum wear does not coincide with highest PV limit, which increases to a maximum at about 75y0 by weight. Therefore, economic choice of a particular type and level of filler should be based on judicious evaluation of environmental and PV conditions and allowable wear. Recent work has shown 25% by weight glass-filled FEP resin to be unexcelled among the thermoplastic materials tested for bearing utility. Since that research, Delrin acetal resin filled with T F E fluorocarbon fibers (Delrin AF) has become available as another excellent injection moldable candidate for bearings. VOL. 5 6
NO. 3
MARCH 1964
27
Friction Bearings
Sleeve Bearings. In friction bearings-e.g., sleeve bearings-sliding friction occurs between bearing and mating surfaces. Such bearings are widely used in chemical pumps and compressors. I n one centrifugal pump operated in a 60y0 nitric acid environment, conventional bearings were impossible because of corrosion and contamination of the process stream; a sleeve bearing of glass-filled T F E resin replaced carbon at a premium of about $50 after various combinations of steel shafts and bearing material had been tried. T h e submerged pump has been troublefree for more than 2l/2 years; the carbon bearing had lasted only a few months. Maintenance savings are about $1000 annually, with concomitant benefits in production output and safety. Split-Curled Bearings. Economy and performance advantages have been achieved by the split-curled bearing which incorporates a thin tape of filled TFE resin as the bearing surface. This development emphasizes the rewards from designing load support elements to take full advantage of the properties of a new engineering material. There are two basic rvays to retain split-curled bearings: install with an undercut on the shaft or housing; place in a metal “can” and insert same into the housing. I n either case, the load-carrying tape is free to rotate, thus preventing concentration of wear on any one spot. Moreover, thinness of the tape allows rapid heat dissipation which raises PVlimits. Slip-Joint Bearings. The simplest form of a fluorocarbon friction bearing is two flat plates in opposition. This configuration ma): be used to support relatively intense unit loads where relative motion is small in amplitude and frequency. Such bearings, commonly referred to as slip joints in the chemical industry, allow accommodation of motion caused by thermal expansion or wind to occur at support joints of large structures such as towers, vessels, and long lengths of piping. I n fact, such bearings as bridge supports (so-called bridge-bearing pads) are coming into wide use both in the United States and Europe. Antifriction Bearings. In antifriction bearings, relative motion between bearing and mating surfaces involves rolling friction predominantly. Ball bearings are a good example. Antifriction bearings are normally lubricated and represent about 80% of the some $1 billion-per-year bearing market in this country. The special properties of fluorocarbon resins have promoted their use, particularly reinforced T F E resin, as retaining cages and races in ball bearings to eliminate need for lubrication. These developments, relatively recent, have been spurred by the needs in military and space programs for antifriction bearings which are fully operable in cryogenic and high vacuum conditions. Corrosion Free Antistick Coatings
A unique property of fluorocarbon resins in the solid state is the virtual impossibility of adhering substances to them. This property very early led to surface release 28
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
applications in the chemical and process industries. However, good receptivity to common adhesives can be obtained by etching with suitable solutions of alkali metals such as sodium. In addition, film of FEP resin is available with one or both surfaces treated for cementability. This film can also be melt-sealed to itself and substrates to yield permanent joints. Recent theory has concurred with practice in pointing out that fluorocarbon resins in the melt state, particularly FEP resins, are universal adhesives. Their low intermolecular forces is the main reason for this behavior and results in modest bond strengths of about 10 to 30 pounds per inch in 180’ peel, 2 inches per minute, from clean, roughened substrates. Many manufacturers offer pressure-sensitive tape of filled, impregnated (glass cloth), and unfilled T F E and FEP resins ; some products use adhesive formulations which are suitable for temperatures up to 500’ I?, Antistick surfaces can also be obtained by applying dispersion coatings of T F E and FEP resins, marketed in the form of primers and finishes, which can be sprayed or brushed. Fluidized bed coating techniques using powder of FEP resin have also been developed. In one economic application of anti-stick coatings of fluorocarbon resins, stainless steel dryer trays for a polymeric material were coated .iz-ith approximately 3 mils of T F E dispersion. The necessity of scraping and hammering to empty the trays w a s eliminated. Moreover, the coating has increased the life of the trays fivefold and has reduced labor required €or emptying. A second example is the lining of a mixer made of steel. Process material clung tenaciously to the steel; discharging the batch was a manual and time-consuming operation. Mixers lined with cementable T F E tape bonded to the steel eliminated this difficulty.
E lec tr ie al As a ne\\ development, heating cable of TFE and PEP resins for tracing process vessels and lines are available. Several successful test installations are in plant areas where environmental conditions w-ould attack any metal used for steam heating. REFERENCES (1) Boes, D. J . , “Long-Term Operation and Practical Limits of DIV Srlf-1.nbricated Bearings a t Pressures from 1 X 10-6 Mm. Hg. to Atmosphkric” Sci. Paper 62-123-545-P1,\\estinghouse Research Idxxatories, Churchill B;rough, Pittsburgh, Pa., April 2 5 , 1962. (2) Bowen, P. H., “.4nalyticaI and Experimental Studv of Adapting Brarinsa for Use in a n Ultrahigh Vacuum Environment,” Contract AF 40 (6O0)-115, ‘Task 77794, Project V 7 G , Aerospace Environments Ofice, .4rnold AFS, T r n n . , August 1961. (3) E. I. d u Pont de Nemours & Go., Plastics Drpt , Subzero F. D a t a De\cloped Rocketdvne Div., h-orth American Aviation, Contract AF 04 (611)-6354, Edwards Air For& Base. Calif. (4) Lewis, R . B., E . I. du Pont de Nemours 8; Go.: Engineering Dept., prriondl communication, January 1963. (i) Murphy, T. P., Plastics Derign Process. (January 19G3). ( 6 ) O‘Rourke, J. T., J.Teflon 3, No. 8 (September-Octolxr 1 9 6 2 ) (7) U.S. Patent 2,230,654.
This i s fhefirsf in a series of articles on plastics in corrosion confrol based on a symposium presented by the ACS Division OF Organic Coatings and Plasficr Chamisfry at the N e w York Nafional Meeting, September 1963. The entire series will be available in reprinfs.
