Ind. Eng. Chem. Res. 1992,31, 275S2764
ll = second invariant of rate of deformation tensor, s-2 = unknown parameter, eq 18 = potential 9 = stream function Q = = fRe'
Superscripts
+ = dimensionless quantity
* = quantity evaluated from trial profiles Subscripts r, B = r and 8 components 11 = parallel flow i = cross flow
Literature Cited Adams, D.; Bell, K. J. Fluid friction and heat transfer for flow of sodium carboxymethyl cellulose solutions across banks of tubes. Chem. Eng. h o g . Symp. Ser. 1968,64 (No. 82), 133. Barboza, M.; Rangel, C.; Mena, B. Viscoelastic effects in flow through porous media. J . Appl. Polym. Sci. 1979,23, 281. Bud, R. B.;Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1960. Bud, R. B.; Armstrong, R. C.; Hassager, 0. Dynamics of Polymeric Liquids Vol. I: Fluid Mechanics; Wiley: New York, 1987. Chhabra, R. P.; Manjunath, M.Flow of non-Newtonian power law liauids through packed and fluidized beds. Chem. Eng. Commun. 1991,106,33: Chmielewski. C.: Pettv. C. A.: Javaraman. K. Cross flow of Elastic Liquids t d r o k h G a y s of 'Cyfinders. J. Non-Newtonian Fluid Mech. 1990,35,309. Christopher, R. H.; Middleman, S. Power law flow through a packed tube. Ind. Eng. Chem. Fundam. 1965,4,422. Drummond, J. E.; Tahir, M. I. Laminar Viscous Flow Through Regular Arrays of Parallel Solid Cylinders. Int. J. Multiphase Flow 1984,10,515. Ganoulis, J.; Brunn, P.; Durst, F.; Holweg, J.; Wunderlich, A. Laser Measurements and Computations of Viscous Flows Through Cylinders. J . Hydraul. Eng. 1989,115,1223. Gummalam, S.;Narayan, K. A.; Chhabra, R. P. Rise Velocity of a Swarm of Spherical Bubbles through a Non-Newtonian Fluid:
2769
Effect of Zero Shear Viscosity. Int. J . Multiphase Flow 1988,14, 361. Happel, J. Viscous Flow in Multi-particle Systems: Slow Motion of fluids Relative to Beds of Spherical Particles. AIChE J . l958,4, 197. Happel, J. Viscous Flow Relative to Arrays of Cylinders. AIChE J. 1959,5, 174. Happel, J.; Brenner, H. Low Reynolds Number Hydrodynamics; Prentice Halk Englewood Cliffs, NJ, 1965. Jaiswal, A. K.; Sundararajan, T.; Chhabra, R. P. Simulation of Non-Newtonian fluid flow through fixed and fluidized beds of spherical particles. Numer. Heat Transfer 1992,21,275. Jarzebski, A. B.; Malinowski, J. J. Drag and Mass Transfer in Multiple Drop Slow Motion in a Power Law Fluid. Chem. Eng. Sci. 1986,41,2569. Kemblowski, Z.; Michniewicz, M. A new look at the laminar flow of power law fluids through granular beds. Rheol. Acta 1979,18,730. Kuwabara, S . The Forces Experienced by Randomly Distributed Parallel Circular Cylinders or Spheres in a Viscous Flow at Small Reynolds Numbers. J. Phys. SOC.Jpn. 1959,14,596. Kyan, C. P.; Wasan, D. T.; Kintner, R. C. Flow of Single Phase Fluids through Fibrous Beds. Ind. Eng. Chem. Fundam. 1970,9, 596. LeClair, B.P.; Hamielec, A. E. Viscous Flow Through Particle Aasemblagea at Intermediate Reynolds Numbers: Steady State Solutions for Flow Through Assemblages of Cylinders. Znd. Eng. Chem. Fundam. 1970,9,608. Prakash, 0.;Gupta, S. N.: Mishra, P. Newtonian and Inelastic Non-Newtonian Flow Across Tube Banks. Ind. Eng. Chem. Res. 1987,26,1365. Slattery, J. C. Mass,Momentum and Energy Transfer in Continua; McGraw-Hilk New York, 1972. Srinivas, B. K.; Chhabra, R. P. An Experimental Study of NonNewtonian Fluid Flow in Fluidized Beds: Minimum Fluidization Velocity and Bed Expansion Characteristics. Chem. Eng. Process. 1991,29,121. Vossoughi, S.;Seyer, F. A. Pressure Drop for Flow of Polymer Solution in a Model Porous Medium. Can. J. Chem. Eng. 1974,52, 666.
Received for review April 6,1992 Revised manuscript received August 20, 1992 Accepted September 10,1992
Environmentally Durable Elastomer Materials for Windshield Wiper Blades Ares N. Theodore,* Marsha A. Samus, and Paul C. Killgoar, Jr. Research Staff, Polymer Science Department, Ford Motor Company, Dearborn, Michigan 48121
The primary cause for the poor performance of current windshield wiper blades is the inherently poor resistance of natural rubber to attack by ozone, oxygen, and sunlight causing degradation of the elastomer. EPDM (ethylene-propylene terpolymer) is a low-cost elastomer with excellent resistance to attack by these environmental agents. A significant deterrent to using EPDM has been its high rubber-to-glass friction. Unlike natural rubber, the surface cannot be easily chlorinated t o lower the friction. It was found that the friction of the EPDM loaded with graphite could be controlled and the other physical properties needed in a windshield wiper blade maintained at the same time. Critical parameters identified in this work are the size and loading of the graphite and the size of the carbon black. The correct choice of EPDM and cure system is important in mnnimiaing the physical properties of the compound. Prototype wiper blades based on EPDM exhibit superior environmental resistance, frictional properties, compression set, and tensile set when compared to production natural rubber windshield wiper blades.
Introduction Developing an environmentally stable, durable elastomeric windshield wiper blade is important to provide added quality to the automobile and consequently increase customer satisfaction. Windshield wiper blades should have good wipe quality under all environmental conditions, not make noise during operation, and be resistant to envi-
ronmental attack.' Factors which affect wipe quality are the sharpness of the blade edge, the ability of the blade to conform to the glass surface, the ease with which the blade flips from side to side during operation, and the level of glass to rubber friction. Noise is believed to be caused by excessive friction between the rubber and the glass. This is most noticeable during light rain and is charac-
0888-5885/92/ 2631-2759$03.O0/0 0 1992 American Chemical Society
2760 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 Table I. Compound Recipes for Developmental EPDM Compounds
Royalene EPDM 585 Statex M-70 N650 N347 ZnO stearic acid Circo Lt. RPO Circosol4240 Dixon 1176 Dixon 200-42 Captax Methyl Tuads Ethyl Tellurac sulfur Sulfads CaO
R-602 100
R-1713 100
119
119
R-1738 100 119
compound R-1739 R-1743 100 119
100 119
R-1759
R-1789
R-1814
100
100
100
70 5
50 5
11
1 11
1 11
49
70 5
5
5
5
5
1 11
1 11
1 11
1 11
1 11
100
75
100
75
75
49 75
49 75
1
1 0.8
1
1
0.8 0.8 0.8 0.8
3 0.8 0.8 0.8 0.8
3
0.8 0.8 0.8 0.8
3 0.8 0.8 0.8 0.8
0.8 0.8 0.8 0.8
0.8 0.8 0.8
terized as a high-frequency squeal. Another consequence of high friction is blade chatter, stick/slip behavior on the windshield, which lowers the wipe quality. Windshiled wiper blades are exposed to reletively high temperature (>125 "C), oxygen, ozone, and W radiation. These agents taken together or individually can degrade the elastomer in the blade. Natural rubber (NR)is currently the material of choice for wiper blades because of ita good physical properties, dynamic mechanical properties, and low-temperature properties (which affect low-temperature flipping act i o ~ ~ )In . ~addition, -~ the surface can be easily chlorinated to reduce the friction and improve wear. However, natural rubber, an unsaturated polymer, has poor resistance to attack by ozone, oxygen, and light.w Thus NR blades tend to take large compression seta. They also continue to cross-link and become brittle, producing uneven wear of the blade edge and a reduction in the wipe quality. Ethylene-propylene terpolymers (EPDMs) are amorphous copolymers of ethylene and propylene containing a low percentage of a diene monomer pendant to the molecular backbone. The resulting elastomer is curable by conventional methods, yet its saturated backbone structure provides inherent resistance to attack by heat, light, oxygen, and 020ne.~The outstanding weather resistance and the relatively low cost of the polymer makes EPDM very attractive for elastomeric automobile components exposed to the environment such as the windshield wiper blade. There has long been interest in making EPDM wiper blades. The problems with using EPDM have been 2-fold first, unlike natural rubber the surface cannot be easily chlorinated to reduce the friction, and second, the physical properties of the material are not as good as the corresponding properties of NR. Work done previously by Rusch et al." suggests that addition of particulate graphite to EPDM can be an effective method to reduce friction. On the other hand, a patent issued to Symbolik" teaches that if enough graphite to sufficiently reduce the friction on glass is added to EPDM, the physical properties will be too poor for use as wiper blades. In spite of these potential problems, the advantages for changing to EPDM are so significant that a program was established to develop an EPDM wiper blade compound with performance characteristics and service life superior to the present NR production component. The objective of this work was to develop an EPDM compound that had lower friction, better compression set, better abrasion resistance, and better environmental resistance than NR. This paper will describe initial efforts
5 1
0.8
0.8 0.8 0.8
75 2 0.8 0.8 0.8 0.8 2
to develop a practical graphite-containing EPDM compound for wiper blades. Resulta of the material evaluation will be discussed with emphasis on identifying property deficiencies and processing problems.
Experimental Section Materials and Material Preparation. The EPDM used in this study was provided by Uniroyal Chemical Co. (Royalene 502) and by Polysar, Inc. (EPDM 585). A production natural rubber compound manufactured by Tridon, Ltd.,was used for the program baseline (NR(82)). The experimental compounds were mixed in a Banbury mixer (Model BR) using a 6-min mix cycle with curatives added during the last minute, prior to sheeting on a cooled, 200 X 400 mm two-roll mill. Vulcanization parameters were determined by means of an oscillating disk rheometer. The compounds were molded and cured at 160 "C to 100% optimum. Optimum cure was chosen as the point at which the slope of the rheometer curve becomes a constant. Complete formulations for the compounds made in this study are given in Table I. Sample sheeta (150 X 150 X 2 mm) were molded according to ASTM D3182, and compression set buttons (28 mm diameter X 13 mm thick) were molded according to ASTM D395. Tensile and tear specimens were cut from the cured sheets using a die and punch press. Squeegees and slab stock for evaluation were molded at 160 "C for 10 min. Compression set buttons were molded at 160 "C for 20 min. Test blades were either compreaion molded or extruded. However, the majority of the work was done with compreasion-molded blades. Extruded blades were made using production extrusion equipment so that processing variables might be explored. Test Methods. Tensile strength and elongation at break were determined at room temperature according to ASTM D412 (die C), and tear strength was determined according to ASTM D624 (die B). An electromechanical tester was used at a test speed of 500 mm/min. Compression set testing was done according to ASTM D395 (method B). The test specimens were under 25% compression for 22 h at 85 "C in a ventilated, air-circulating oven. Hardness was measured according to ASTM D2240 on the unaged samples using a Shore A durometer. A 10-5 relaxation was allowed before the final reading. Wipe quality was assessed using a flat, black glass which is sprayed with water and wiped with the test blade. The blade is stopped at the extremes of its sweep, and the
Ind. Eng. Chem. Res., Vol. 31, No. 12,1992 2761 Table 11. Comparison of Properties of NR and EPDM Blade Materials comDound properties NR(70) N R W NR(87) R-602 tensile strength, MPa 21.4 7.8 17.7 22.0 355 ultimate elongation, % 470 455 513 72.7 55.4 tear strength, kN/m 3.2 4.8 2.0 100% modulus, MPa 3.5 300% modulus, MPa 14.1 12.7 9.7 7.3 56 52 68 70 hardness, Shore A 14.4 19.0 % cornpression set, 70 "C 22.0 26.0 20.2 % compression set, 85 O C 52 41 resilience, % 11.0 3.0 tee6at 160 O C , min 12.0 10 20 cure time at 160 "C ~
number and thickness of water streaks left behind are determined. From this determination the blade is given a rating from 1to 10, with 10 representing a perfectly clean glass. In our laboratory an Escort windshield was used as a test platform for evaluating the wipe quality. The windshield was smeared with a slurry of water and Arizona road dust and the wiper was activated while the slurry was wet. The number of streaks left after one pass of the blade was used as the measure of wipe quality. Comparable results were obtained by the two techniques, making it possible to correlate our results. Rubber to glass friction was evaluated indirectly by measuring the current draw of the wiper motor during operation. The total current measured is the sum of the effort to overcome the rubber/glass friction and the internal friction of the windshield wiper mechanism. The technique cannot distinguish differences between materials on wet glass but is reasonably discriminating on dry glass if the moduli of the rubber elements are about the same. Dynamic mechanical properties were determined using a Polymer Laboratories DMTA in dual-cantilever bending mode (simple tension) at a strain of 62 pm. Data was collected at 1Hz from -90 to 50 "C at a rate of 1"C/min.
Results and Discussion In the study by Rusch et al.,'O it was shown that an EPDM compound with 100 phr (parts per hundred rubber) of graphite might be a good candidate material for wiper blades. This conclusion was based upon a comparison of friction data and limited initial physical properties (tensile strength, elongation at break, and hardness) of the EPDM and NR. However, the feasibility of using such a compound was not determined and appeared to be a proper starting point for this study. In the intervening years since the work of Rusch was performed many modifications in wiper materials have occurred. These were driven by changes in the design of windshields and wiper drive mechanisms. One of the more significant changes was the development of more highly curved glass, with changing contour across the wipe path, for improved aerodynamics. The changes in the material needed to accommodate the new windshield design are lower moduli and hardness. Lower modulus was needed to improve the wiper blade flipping action and contact with the glass. ALSO, a new emphasis was placed on developing materials with low compression set. The evolution in hardness and moduli is easily seen by comparing two production NR compounds, one from the early 1970s (NR(70)) and the other a current production material (NR(87)). As can be seen from Table 11, a reduction in the 300% modulus and Shore A hardness of approximately 30% has occurred since 1970. This reduction in modulus and hardness was realized without sacrificing tensile strength. Tear strength and compression set were not
studied in the previous work, and therefore no data for comparison with the current materials are available. The EPDM compound developed earlier (R-602) was tailored to the requirementa of the 1970s. As seen in Table 11, the hardness and modulus closely match that for NR(70). However, a significant difference between NR(70) and R-602 is the 56% lower tensile strength of the EPDM compound. The low tensile strength caused a concern about the utility of this material for a windshield wiper application. The low tensile strength raised additional concerns about the tear strength of the material. Tear strength is important because of the thin cross sections found in windshield wiper blades and the possibility, in cold, wet weather, of the blade freezing to the glass. (If the strength is too low when the wiper motor is activated, the blade could tear or rip.) Additionally, it was not obvious that the modulus and hardness could be reduced without further reductions in tensile and tear strength. Consequently, the first stage of this work was to prepare R-602 and perform a complete evaluation of ita physical properties and behavior as a wiper blade. R-602 and the production natural rubber NR(82) were compression molded into test specimens and wipers. The cure conditions are given in Table 11. Wiper blades of the natural rubber were chlorinated for comparison of frictional properties. The physical property evaluation (see Table 11) showed that R-602 was harder than NR(82), as expected, with less resilience and lower tensile strength. In contrast, R-602 offered improved compression set and lower friction. Blades made from R-602 exhibited 28% less friction than the surface-chlorinated NR(82) (3.8 vs 5.3 A). The low friction of the EPDM compound remained fairly constant during the 100OOO test cycles, whereas the friction of the NR compound increased as the test progressed. It is postulated that the NR friction rises because the chlorinated surface is worn away. Laboratory durability evaluation of blades made from R-602 indicated that the low tensile properties were adequate for a windshield wiper blade. During the course of this work, however, every effort was made to improve the tensile strength. The evaluation of R-602 revealed also two potential problem areas. The elastomer compound was slow curing and the blades exhibited poor wipe-quality characteristica. The poor wipe quality was attributed to poor slitting, which is the operation that produces the sharp edge on the blade required for good wipe quality. The poor slitting could be caused by poor ingredient dispersion, excessive graphite loading, inappropriate filler (graphite and carbon black) particle size, and the slitting operation itself. The slow cure speed of the compound posed a potential problem with the economica of manufacturing wiper blades from EPDM. The primary method used to form wiper blades is compression molding with a typical cure cycle of 12 min, although some manufacturers use extrusion to form blades. The EPDM compound as formulated (R-602) was slower curing to tclm (20 vs 1 2 min at 160 "C) than the production NR. This is within an acceptable range for compression molding but is unacceptable for an extrusion process. In an attempt to accommodate the extrusion production process, methods to increase cure speed were studied. One approach taken was to increase the functionality of the elastomer by replacing Royalene 502 (R-602) with EPDM 585 (R-1738). The compounds are listed in Table I. The two parameters used to assess the impact of compound modification on cure speed were scorch time, a measure of processing safety, and cure rate. Scorch time is defined as the time for the torque to increase 2 dN m
2762 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 loo
100 I L
-
I
80
E z d
60
3
P
e
- 1
'=Ax
40
'
20 0
5
10
15
20
0
Table 111. Effects of EPDM Type and Graphite Loading on Cure and Physical Properties compound properties R-602 R-1738 R-1713 R-1739 502 502 585 EPDM type 585 75 graphite loading, phr 100 100 75 21.8 18.3 23.0 minimum torque, dN m 28.8 88.2 97.3 maximum torque, dN m 106.8 96.0 2.4 2.4 2.4 2.3 ts2,min 4.1 4.1 3.8 4.3 t,,, min 14.4 9.4 11.4 9.0 td6,min 18.0 16.0 18.0 17.2 tclW, min 18.2 21.0 18.8 24.0 rate,, dN m/min 6.4 9.0 9.8 8.9 tensile strength, MPa 298 413 349 ultimate elongation, % 285 17.6 17.4 18.1 22.0 compression set, % 67 68 66 72 Shore A durometer
above the minimum torque value. The cure rate is defined as the slope of the tangent line to the torque curve taken at the time for the compound to reach 50% cure. These definitions are graphically represented in Figure 1. The change to EPDM 585 had no adverse effect on the scorch time, 2.4 min for each compound. The cure rate for R-1738 was 23% faster than the cure rate for R-602. The cure data along with a comparison of the physical properties of the compounds are presented in Table 111. The tensile strength increased, elongation at break decreased, and hardness increased. These differences are consistent with a tighter, more highly cross-linked elastomer network. The effect of polymer change on cure speed was confirmed with a different pair of compounds, R-1713 (Royalene 502) and R-1739 (EPDM 585), which contained 25 phr less graphite than R-602 and R-1738. R-1739 has a cure rate 12% faster than R-1713, not as dramatic an effect as observed with the preceding two compounds. This difference is attributed to the reduction in graphite loading. It is well-known that some carbon blacks can accelerate the cure of elastomeric compounds.sJ2 The acceleration effect of different c a r b n blacks has been correlated with the pH of aqueous carbon black slurries. Blacks that produce basic slurries accelerate the cure; blacks that produce acidic slurries retard the cure. It was postulated that the graphite acta in a manner similar to a furnace black, which forms a basic slurry. To further c o n f i i the effect of graphite on the cure rate, three EPDM 585 compounds were made: (i) EPDM 585 and curatives; (ii) EPDM 585, curative, activators, carbon black, and oil; and (iii) a formulation identical to ii, except it contained the graphite. In Figure 2, the three rheometer curves are shown along with the calculated cure rate for each. The incorporation of black increased the cure speed (curve B) over the polymer/cure package alone. The addition of
Rate B = 17.3 dNmlminuta Rate 5o C = 29.1 dMmlminute . .
I
1 ' ' 1 ' ' ' 1 1 ' ' 1 ' 1 1 1 " ' " ' 1 1 1 ' 1 1 1 1 '
0
Time, [minutes]
Figure 1. Cure curve parameters. tclWis the time for the torque to reach MH,ts2is the time for the torque to rise 2 dN m above ML, and t , is the time to achieve x % of the torque difference (MH- Md.
I
2
4
6
8
10
12
14
I6
Tlme. (minutes]
Figure 2. Effect of graphite on cure rate of EPDM elastomer. C w e A is EPDM and curatives alone; B is EPDM, curatives, oil, and carbon black; and C is EPDM, curative, oil, carbon black, and graphite.
Sweep Direction
Streaks
Figure 3. View of windshield wiper blade edge. Incomplete dispersion of graphite or carbon black can leave imperfections in the slit edge which produce streaking on the windshield.
graphite further accelerated the cure of the compound by an additional 69%. There was no observable acceleration of the cure by graphite with compounds made using Royalene 502 (R-602 vs R-1713). It is known that acceleration of EPDMs with low functionality, e.g., Rayalene 502, is very difficult even using ultmaccelerat~rs.'~ The accelerating effect of carbon black, and by inference graphite, is considerably lower than an ultraaccelerator, thus explaining the results seen with the Royalene based compounds. The poor wipe quality, i.e., poor slitting of the blades, seen with R-602 needed to be improved. The parameters most likely to contribute to improving the slitting are graphite loading and particle size, and to a lesser extent carbon black particle size. Filler loading and particle size can impact wipe quality in the following manner. Wiper blades are molded as a doublet which are subsequently separated into two individual blades by slitting using a sharp knife or blade. As the blade cuts through the rubber, it can impinge on a filler particle. Because of the hardnees of the particle, it will not cleave. Therefore, the particle is generally pulled from the rubber, leaving a void. The location, Le., center vs edge (Figure 3), and dimensions of the hole w i l l determine if there is an effect on wipe quality. If the loading of filler is high, the probability of encountering a particle during slitting is greatly increased, while the particle size will determine the size of the void left. If the filler is not fully dispersed during mixing, then aggregates will be present which are larger than the primary particle size of the filler. If an aggregate is encountered during slitting and is removed, an even larger void can be formed. Carbon black particle size was not considered a significant contributor to the poor wipe quality because the
Ind. Eng. Chem. Res.,Vol. 31, No. 12, 1992 2763 Table IV. Cure and Physical Property Comparison for Carbon Black Series compound properties R-1743 R-1759 R-1789 N650 N347 type of black N568 tensile strength, MPa 8.7 9.0 10.7 344 302 ultimate elongation, % 289 68 70 Shore A durometer 68 29.0 18.0 compression set, % 26.1 89.0 91.0 96.5 maximum torque, dN m 22.6 23.5 minimum torque, dN m 28.3 8.3 8.2 8.5 td5,min Table V. Effect of Graphite Particle Size on Physical Promrties compound DroDerties R-1789 R-1812 R-1790 R-1798 grade of graphite 1176 200-42 1176 200-42 5 45 5 particle size, pm 45 11.4 11.7 10.7 tensile strength, MPa 10.7 314 254 302 246 ultimate elonation, % 18.5 19.1 18.0 22.6 compreaaion set, % 72 70 72 hardness, Shore A 70 27.1 22.9 27.3 24.6 ratew 103 96 94 maximum torque, dN m 96
particle size of the carbon black used in R-602 (N568) was about 0.049 pm. Changing to a smaller carbon black, such as N347 with a particle size of 0.028 pm, might provide a small improvement in wipe quality. More importantly, using a smaller particle size black would be expected to improve the tensile strength, tear strength, and abrasion reistance of an EPDM c ~ m p o u n d . ' ~ The black used in R-602 (N568) was removed from commercial production during the course of this study, and N650 was chosen as a replacement for it. The properties of carbon blacks N568 and N650 are very similar with the exception of particle size: N568 is 0.049pm and N650 is 0.053 pm. Compounds R-1743 and R-1759 based on these carbon blacks have similar physical properties, indicating that the two carbon blacks are essentially equivalent (Table IV). Decreasing the particle size to 0.028 pm (N347), leaving all other properties of the black the same, did improve the tensile strength slightly and made a major improvement in compression set. The unexpectedly large improvement in compression set alone was sufficient justification for changing to the smaller particle size carbon black in further compound studies. Wipe-quality evaluations were not performed on this series of compounds. Compounds R-602 and R-1713 (Table I) were used to compare the effect of graphite loading on wipe quality. The 25 phr reduction (25%) in graphite produced a 100% improvement in wipe quality. R-602 was rated 2-3, on a scale of 1-10, and R-1713,4-5. Although the improvement was significant, 4-5 wipe qualities are unacceptable for commercial application. Removing graphite improved the tensile strength, increased the elongation at break, decreased the compression set, and lowered the hardness slightly (Table 111). The blade friction was unaffected by removing 25 phr graphite, but removal of an additional 25 phr did have an adverse effect on the friction while further improving the wipe quality to =7. With Dixon graphite 1176, it appeared that the loading could not be reduced to a level needed to produce good wipe quality and still have low friction. As a result, changing to a smaller particle size graphite appeared to be the next step in this study. A comparison of physical property data for two pairs of compounds using Dixon graphite 1176 and 200-42 is preaented in Table V. The amount of primary accelerator is slightly higher in the f i t set of compounds. Dixon 1176
Table VI. Comparison of Baseline NR with Optimized
EPDM comDound NR(82) R-1814 18.7 9.9 499 269 72.7 38.1 26.0 16.7 56 66 45 43 10 8-8.5 ~~
properties tensile strength, MPa ultimate elongation, % tear strength, kN/m compression set, % Shore A durometer friction, A wipe quality
-100
BO
-60 -40 -20 Temperature, "C
0
~
M
Figure 4. Tan 6 versus temperature for the baseline natural rubber (NR(82)) and EPDM compound R-1814. The glass transition temperature of the EPDM is 15 deg higher than that of the NR.
has an average particle size of 40 pm, lo00 times larger than carbon black. Dixon 200-42 has an average particle size of 5 pm and is about the smallest graphite commercially available. Compounds with the smaller graphite have slightly higher tensile strength, lower elongation at break, and increased hardness. The compression set, although a little worse in the compounds with the smaller graphite, was still much better than that of NR. To evaluate the effect of the smaller size graphite on wipe quality, a new compound was made using EPDM 585, reduced graphite loading, and smaller particle size carbon black. In addition, the modulus was lowered by reducing the carbon black loading to produce a compound approaching NR(82). This compound, R-1814,also contained 2 phr CaO as a desiccant to reduce H20 vapor during processing. A summary of the physical properties of the compound, friction behavior, and wipe quality of prototype wiper blades is presented in Table VI. The new compound has considerably better compression set and lower friction than the NR, as well as improved tensile strength over R-602. The wipe quality was improved to 8-8.5. These properties were sufficient for vehicle evaluation of EPDM windshield wiper blades. Initial vehicle evaluations were conducted during the winter and spring quarters. The blades performed well during thisperiod with some indication of stiffening at low temperatures. There was no noise observed with these blades, supporting our belief that lower friction should reduce noise. However, subsequent testing indicated that wipe-related noise was not totally eliminated under all service conditions. Durability, as determined by changes in wipe quality with use, was satisfactory, but longer duration tests are needed to confirm the environmental durability and long-term abrasion resistance. There were no failures of blades resulting from the lower tensile strength of EPDM. TWO areas were identified in this work for further study. First, the low-temperature properties of EPDM were not as good as those of NR. This behavior was quantified by comparing the dynamic mechanical properties of EPDM
Ind. Eng. Chem. Res 1992, 31, 2164-2168
2764
k
9.80
durability testing is needed. Preparation of the prototype 1 blades was done using production methods for mixing and
9.00
-
molding the material. Two deficiencies of EPDM were identified for improvement. First, it would be desirable to improve the low-temperature properties, and second, a reduction in the cost would make it more competitive with NR. Registry No. Ethene-ethylidenenorbornene-propene co-
= 8.20 -
w
7.40
-
6.60 -100
polymer, 25038-36-2; graphite, 1782-42-5.
Literature Cited 80
-60 -40 -20 Temperature. O C
0
20
Figure 5. Storage modulus versus temperature for the baseline natural rubber compound (NR(82)) and EPDM compound R-1814. The EPDM compound starts to get stiffer a t higher temperatures than the natural rubber.
and NR. Figure 4 shows that the dynamic Tgof the EPDM compound is about 15 "C higher than that of the NR. In Figure 5, we observed that the elastic modulus (log E3 of EPDM compound is at a level that could affect the flipping action of the wiper blade. Second, the material cost of the EPDM compound was higher than that of NR. Resolution of these issues will be discussed in a subsequent technical paper.
Conclusion EPDM elastomers have excellent environmental resistance. Replacement of the current NR materials by EPDM would be expected to produce a longer lived blade. In this work it has been shown that the rubber-to-glass friction of EPDM can be controlled while maintaining the physical properties needed in a wiper blade by using relatively high loadings of graphite. Factors governing the wipe quality of the EPDM blades are carbon black size, graphite particle size, and graphite loading. The cure speed of the compound was accelerated by increasing the functionality of the elastomer. Lowering the modulus to accommodate changes in windshield curvature had no adverse effect on physical properties. Prototype blades have been made and have undergone preliminary vehicle evaluations. The results of the study indicated that EPDM blades could be a viable alternative to NR with further development, although longer term
(1) Batt, R. A. Windshield Wiper Systems Expand Plastic Use.
Automot. Eng. 1972,87 (3), 74-78. (2) McLellan, J. The Development of Windscreen Wipers. Rubber Deu. 1971,24 (41, 110-116. (3) Batt, R. A. "Plastic Components for Windshield Wiper Systems"; SAE Paper 790201; March 1979. (4) Nypaver, D. Competition Rouses Slumbering Wiper Giants. Rubber Plast. News 1983, Oct 24, 11-12. (5) Overman, G. R.; Davis, R. G. Squeegee. US.Patent 3,036,297, May 22, 1962. (6) Criegee, R. The Ozonolysis of Olefins and Acetylenes. Presented at the 120th Meeting of the American Chemical Society, Sept 10, 1951. (7) Criegee, R. The Course of Ozonization of Unsaturated Compounds. Rec. Chem. h o g . 1957,18, 111-120. (8)Dunn, J. R. Aging and Degradation. In The Stereo Rubbers; Saltman, W. M., Ed.; Wiley-Interscience: New York, 1977; Chapter 9, pp 511-582. (9) Borg, E. L. Ethylene/Propylene Rubber. In Rubber Technology, 2nd ed.; Morton, M., Ed.; International Thomson Education Publishing, Inc., reprinted by Robert E. Krieger, Co.: New York, 1973; Chapter 9. (10) Ruech, K. C.; Forrester, J. R. Frictional Characteristics of EPDM Windshield Wiper Blades. Rubber World 1971, 165, 54-56. (11)Symbolik, W. S. Squeegee Type Windshield Wiper Blade. U.S. Patent 3,080,596, March 12, 1963. (12) Studebaker, M. L. The Rubber Compound and its Composition. In Science and Technology of Rubber; Eirich, F. R., Ed.; Academic Press: New York, 1978; Chapter 9. (13) Hoffman, W. Sulfur Vulcanization. In Vulcanization and Vulcanizing Agents; Maclaren and Sons: London, 1967; pp 73-352. (14) Boyers, J. T. Fillers Part I: Carbon Black. In Rubber Technology, 3rd ed.; Morton, M., Ed.; Van Nostrand Reinhold: New York, 1987; Chapter 3. Received f o r review April I , 1992 Revised manuscript received August 10, 1992 Accepted August 31, 1992
Pyrolysis/Gasification of Wood in a Pressurized Fluidized Bed Reactor Guanxing Chen,* Krister Sjostrom, and Emilia Bjornbom Department of Chemical Technology, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden
The paper deals with pyrolysis and steam gasification of wood in a pressurized fluidized bed reactor. The objective is to study the effect of the treatment conditions on the yield and the reactivity of char. The work shows that in the studied range of the experimental conditions the yield of char is influenced by neither the treatment temperature nor the pressure (650-710 "C and 0.34-1.0 ma). The principal interest is focused on the reactivity of the char in the reaction with steam. It is shown that prolonged exposure of char to high temperature has a negative effect on ita reactivity in s t e a m gasification. Char produced by pyrolysis of wood in nitrogen is much less reactive in the following gasification reaction with steam compared to the char produced in simultaneous pyrolysis/gasification of wood in the presence of steam.
Introduction Gasification of wood is a feasible approach for production of gaseous fuels from biomass. The steam-char reaction is the rate-limiting step in gasification of wood. The
reactivity of char in steam gasification is strongly influenced by the treatment conditions under the pyrolysis and the gasification (Reed et al., 1980;Ekstrom and Rensfelt, 1980; van Dan et al., 1985; Katta and Keairns, 1989).
0888-588519212631-2164$03.00/0 0 1992 American Chemical Society
