INDUSTRIAL AND ENGINEERING CHEMISTRY
2338
4CKhOWLEDGMENT
Vol. 41, No. 10
BIBLIOGRAPHl
The authors wish to express their gratitude t o C. F. Bonilla of the Johns Hopkins University for his assistance in this investigation. In addition, the writers thank P. Borgstrom and A. L. Alexander of the Kava1 Research Laboratory who made it possible For the major portion of the experimental work to be done a t the Saval Research Laboratory. The kind cooperation of P. N. Arnold of the Sound Division, the excellent aid of C. Bloedorn in the design and construction of electronic systems, and the fine machine work of R . Chambers, all of the Naval Research Laboratory, are hereby gratefully acknowledged.
(1) Gaines, N., Physics, 3, 209-29 (November 1932). ‘2) Gardner, H., “Physical and Chemical Examinations
01
Paint$.
Varnishes, Laquers, and Colors,” 10th ed.,pp. 175-81.Retheada. Md., H. A. Gardner Laboratory, Inc., 1946. 13) Moses, Saul, IXD. EXQ.CHEM., 41, 2338 (1949). (4) New York Production Club, OficiuE Digest Federation Paint & Varnish Production Clubs, p. 141 (October 1939) ; p. 167 ( O r tober 1940). ( 5 ) St. Clair, H. W., Rev. Sci. Instrurncm.3, 12,No. 5, 250 (1941). RECEIVXD July 9, 1948. Presented before t h r Division of Paint, Varmsh. a n d Plastics Chemistry a t t h e 114th hlwtin. I f t.he AMERICAS CHEMICAL SOCIETY.Wa-hineton. D. C.
THE NATURE OF ADHESION Analysis of the data on the adhesion of polystjrene.
VYHH, and methyl methacrylate systems to aluminum alloy has shown that adhesion in the cases investigated depends on the presence of a fluid, or quasi fluid, or mobile state at or near the film-metal interface. The data. obtained frnm the new technique in measuring adhesion
I
N A precedirig paper ( 1j, the ultrasonic niethod for nieasuring the adhesion of organic coatings to metal substrata was described. This paper presents results of adhesion measurements and a discussion of the correlations leading to a partial explanation of adhesive forces. The following tables of adhesion data were obtaiiied by experimental procedures as outlined ( I ) . The voltage column indicates the voltage generated by the vibrator a t the time of separation of the film from the metal. The amplitude column i. obtained from this voltage reading as shown in the sample calculation ( 1 ). The colunins mass, area, and maximum height record the dimensions of the film removed a t the corresponding voltage. The column F I B is the calculated force of adhesion (of necesjity, an average value) for the particular film whose dimensionappear to the left in the corresponding horizontal ron Groupings A , B, etc. call attention to differences in F/A under the ham(’ !drying conditions. 8, ip the related point, streqs R Q defined in a Inter section.
by ultrasonic vibrations, represent adhesion as defined in a previous paper ( I ) . An explanation of simple example+ is offered in support of the concept of mobility. The general picture of mobility is now being developed for all types of adhesion between the variou- nrganic system. a n d -~ihstrata.
Polystyrene (mo1ecular weight 80,OOU to 90,OOO) with depuaiteti from a solution of benzene (about 11% solids) in one thin film The mass was permitted to dry for 20 hours a t 25’ C. The d of condenser separation was 0.007 inch. The frequencv of the dural was 25,000 cycles per second: \lass, Gram 0,0028 0,0026 0.0027 0,0026 0,0028 0,0027 0.0027 0,0026 0,0026 0 0029
\-olcage
Volt 1 ) . 136
0,140
0.135 Cl.160 0.155 0. I55 0.145 0.145 12, 1.55 I). 4 1
1.5,; 1l i
Polysty~ene(molecular weight 80,000 t o 90,000) n as deposited From a solution of toluene in two or three layers to form the film. The mass was permitted to dry for 80 hours a t 45’ C. I n this qeries, the d of condenser separation was 0.007 inch with the exception of the first run in which d was 0.005 inch. The frequency i f the dural cylinder was 23,600 cycles per qecond:
0.640
0.515 0.463 0.405 0,400 0,440
0.530 0.355 0.345
n
4113
.Ires, Sq. Cm.
Height. Cm. 0.636 0.22 0,0468 0.24 0,0646 0.534 0.22 0.0560 0.560 0,518 0.29 0.0520 0,639 0.21 0,0400 0.215 0.602 0,0450 0.595 0.23 0.0481 0.24 0.0522 0.601 0.573 0.25 0.0582 0.595 0.236 0.0510 0.611 0.23 0.0468 0.560 0.24 0.0830 0,550 0.24 0,0542 0.0571 0.27 0.606 Mean 0.238 Standard deviation 0.02 1Ia.a. Gram
0.010?
\lean Standard deriarinn
iInpiiruae
Cm. 0.000698
0 000557 0,000581 0.000546 0,000686 0.000686 0,000648 0.000648 0 000688 0 000648 o oon648
I A Lh. Sq. In 2.52 1.86 2.02 1.85 2.4i 2.36 2.2:,
2.15 2.20 2.58 2.52 2.B 0 . 2.5
0 00s-
n nrl
Alethy1 iriethaciylate polyme~wan depwted from a solutioii
d acetone and methyl ethyl ketone. The mass applied in several
DATA
Voltage. Volt 0.580 0,290 0,300 0,380
0.282 0.281 0,280 0.282 0,282 0.282 0.281 0,283 0,283 0.283
0 0030
~
Max.
Max, Height Cm. 0.0102 0,0064 0.0089 0.0064 0,0085 0.0089 0.0086 0.0076 0.0076 0.0103
Area,
Sq. Cm. 0,282
FIA, implitude, Lb./Sq. Cm. In. 1).00222 69.8 0.00164 63.4 52.2 0,00181 $7.2 0.00196 0,00328 64.7 63.9 0.00268 0,00234 60.2 0.00209 58.1 67.5 0.00208 62.1 0.00227 68.2 0.00279 0,00186 56.3 0.00181 57.0 n.no213
64.2
61.8 4.9
lavers TIas permitted to dry for 100 hours at 40” C. The d of contlenwr separation was 0.007 inch. and the frequencx o i t h p i111r;il n as 22,500 cycles per second: \oltage Volt U.430 10.330
s.,
0.4Al)
In.
17.405
Lh./Sq.
157 j31 ~ 7 2 180 229 191 i79 167 173 177 213 147 145 191
0.326 0.42.5 0.335 0,370 0,340
o RZO
Xlssi Gram
Area 8q. Cm
0.0220 0,0365 0.016: 0,0270 0,0240 0,0280 0,0293 0,0282 0,0300 0.0310
0,402 0.578 0.400 0.430 0,475 0.492 0.460 0.474 0,470 0.469
Mean
3tandard deviation
Max. Height Cm. 0.2i
0.24 0.14 0.24 0.23 0.23 0.26 0.25 0.26 0.26 0.24 0.03
41np11
tude Cm IJ.0022t 0,00172 0,00233 0.00165 n.00221 0.00211 0.00171 0.00191 0.00178 11 n
m~n
FQA, Lh./Sc,. In. 35.6 31.5 28.0 30.2 32.4 34.7 31.6 32.8 33.1
32.3 32.2 2.0
&m.
Lb./Bq
In. 207 141
112 136 174 18i 1.52 16i 156 151
VYHH (cupolymer of vinyl acetate and v111yl chloride) was deposited from a solution of acetone in several thin layers. The mash naq permitted to dry for 72 hours a t 40” C. In these series. the d of condenser separation was 0.007 inch, and the frequency of the dural rvlinder was 23,600 cycleq per second:
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1949 Mass, Gram
.%rea, % .I Cni
Height, Cm.
0.455 0.348 0.380 'J 340 U.270 11.315
0,0084 0.0095 0.0076 0 0106 0,0097 0 0110 0.0104 0.0101 0.0096 0.0093 0,0091 0,0085
0.302 0.272 0.278 0 29Q 0.282 0 272 0 275 0,280 0.284 0.282 0 282 0 27rI
0.14 0.145 0.12 0.15 0.15 0.155
Volt
o -. m n. 0 320 ,II 300
320 0 300 1 300 I)
0.0103 0,0082 0,0087 0.0068 0.0067 0.0175 0,0096 0.0087 0.0112 0.0109 0,0091 0.0109 0.0107
I50 0.145 ' J . 155 0,140 0 . 145 i) 120 0 , 155 0 155 0 . 140 0 , 145 0,145 n.170
(1
'1
180
F/A
Lb./&. In. 21 .o 20.1 17.8 19.9 15.1 20.7 18.3 19.2 16.t 16.8 16.: 15., 18.1 2.1
0.00202 0,00175 0.00137 0.00161 0.00152 0.00167 0,00156 0,00163 0.00156 0.00156
0.15
0.145 0.14 0. 13.5 0.135 0.14 I). 14 0,009
Mean 3tandard devlatioL ti
Amplitude, Cm. 0,00201 0.00180
Max.
\ UlLag6,
0.09 0.09 0.125 0.08 0.120 0.11 0 07 0.095 0.115 0.130 0.105 0.09 0.09
0,374 0,358 0.278 0,346 0,322 0 440 0,280 0.351 0 290 0 275 0.310 0,348 0.364
0.000698 0,000694 0.000701 0.00069
6.1 5.2
0.000471 0.000712 0.000683 0.000844 0.00069 0.00069 0.00072 0,00069
6.0 7.8 5.4 6.7 8.1 5.8 6.8 6.4
121 112 104 113 89 107 100 104 94 95 92 94
27.1 26.9 37.8 23.9 35.7 22.4 21.5 28.0
7.0
4.3
0.00069
4.6
26.6 35.9
28.2 27.9 26.9
Mass,
Volt
Gram
u . 890
.&mplitude, Cm. 0.00287 0.00291 0.00308 0.00257 0.00320 0,00292 0.00321 0.00323 0.00279 0.00271 0.00282 0.00261 0.00284
.\lax. Height, Cm. 0.18 0.36 0.23 0.21 0.16 0.20 0.21 0.23
Are&: Bq. Cm. 0.350 0.180 0.347 0,440 0.382 0.352 0.380 0.341 0,325
0.026 0,690 0.014 0,630 0,029 0,500 0.041 0.640 0.027 0.027 0.580 0,030 0,635 0.025 0.640 0,029 0.535 0.360 0.521 0.033 0.400 0.036 0.540 0.335 0.031 0,500 o 341, 0.550 0,029 Mean Standard deviatioi
df = d a p t d A where p = density; t = height of fiber; w = 2n X resonant frt quency; and a = amplitude. The maximum stress occurs a t the maximum height of thr the stress becomes film. Replacing t by t,,
d f / d A = d a p t , = S,
L YHH (copolymer oi viuyl acetate and vinyl chloride) wat it.posited from a solution of acetone in thick layers. The mass was permitted to dry for 90 hours a t 45" C. The d of condenser yeparation was 0.007 inch for this series with the exceptions of the erst three runs which had the following d of 0.0067, 0.0061, 0.0068 inch, respectively, The frequency of the dural cylinder w w 22,500 cycles per secon(l: Voltage,
chosen as the maximum height. Experimentally, the average height is difficult to determine; the maximum height is a memurable physical quantity that indicates the trend of the height dimension. It is possible to define a quantity, S ,, a point stress that indicates the limiting value of stress that can be exceeded before rupture occurs. T o derive this maximum stress quantity, a a m of random topography is assumed. (The film can have any outline shape, or form, regular or irregular in height.) A small fibei (mathematically speaking) of the film is chosen and assumed tc be homogeneous-that is, the density is considered to be constanl along the length. If F is the total force on the entire film, df ithe portion of the total force acting on the fiber of area d 9 During motion
6.1 1.1
0.10 0.02
Mean Standard deviation
SWL,
Lb.,/Sri In.
0.24 0.22 0.24 0.26 0.24
I'/.%. &n, Lb./Sq. Lb./Sq In. In. 62.2 203 68.2 409 277 74.9 69.7 213 67.8 202 65.2 229 264 73.6 291 68.7 72.4 262 233 71.9 265 73.2 69.9 266 26: 70.3 69.8 3 4
0 23 n 04
2339
Lu building up the layers t o increase the mass, the drops \rere permitted to take up an equilibrium position and dry. Each layer varied in area from the preceding one, so that the film as*iinipd a mounded shape The point of highest peakedness was
For a series, A , of polystyrene runs (data nut shown in tht tables), correlation coefficients have been calculated. These coefficients were used to derive statistical equations of the best fitting straight line for the data. The results were checked for reliability and confidence limits established by accepted statistical procedure. One determination was made for a combined series 01 polystyrene runs A to D,all having a 144hour dry a t 40' C These latter data seem to denote a curvilinear relation, but no efforl was made to derive this index of correlation with its resultant plot The best fitting straight line was chosen to show the trend. In Figure 1, F / A is plotted against the maximum height for rui A of a polystyrene series. Under the conditions of the experiments, the correlation factor is 0.86 and the confidence limit of this statistic is 99%. An increase in height was proportional tcl an increase in adhesion for a given dry time. This trend is made more illustrative in Figure 4 in which the combined (A to D )data for 144-hour dry are correlated. I t is this fact which has led tc the postulate on adhesion. Figure 2 shows that F / A varies inverse11 nith the area; t h e correlation coefficient is 0.84 and the confidence limit of the statistic is 05%. Figure 3 gives the relation between the mass and F / A for this run. All these variables are interrelated, for the mass is proportional to the area and the height factor, D a t a on the other films show the same trend of higher F / A with greater height of film. For comparable heights, an increase in dry time resulted in a decrease in adhesion. Many qualitative observations were made that confirmed these findings. Compara-
34
I
\
30
.-F A
LBS til
2 -
(-'
-p/ I
25
'
1
'
'
1
8
'
30
25 WAX
'
1
35
HT--CMS
Figure 1. Adhesion us. Itlaximum Height for Polystyrene A Dry time, 144 hours a t 40n C.
t I
t
d
5.3 o
.5o
40 2
A~iEA-ChrS-
Figure 2.
Adhesion us. Area for Polystyrene A Dry time, 144 hours at 40' C .
INDUSTRIAL AND ENGINEERING CHEMISTRY
2340
t
25j 025
'
!
I
'
'
Vol. 41, No. 10
I
030
035
M A S S-GRAMS
Figure 3.
Adhesion
1;s.
&lassfor Polystyrene A
Dry time, 144 hours a t 40° C.
tive measures of accelerations vcre shown by the v o l t a g s : ~ n dby the size of the w v e on the oscilloscope. Some qualitative findings n-ei'e traiislated into quantitative data after a system for nipawring amplitude had becn devised.
'cA-..a -___i-23 MAX
Figure 4. DISCUS SIOA
From the standpoint of the physics of coordinate trxiishiiun! a variation in the thickness or height of tlic film does not affect the value of recorded adhesion as long a,? the thickness is less than a quarter n-ave length of the frequency used. (For the frequencies in use, the upper limit of height is of tlie order of 1 cm.) Increased thickneEs adds mass t o the film and extends the limits of the equipment under the preseiit magnetic and vibration system. If the mass of the film is increased by the addition of material other than the base polymer, the force to remove tlie film coin1)ination from a surface is normally unaffected. Thus:.a metal ball, cylinder, or plate can be placed on the wet polymer t o form a sandwich or laminated combination. Experiments such as these were completed on a semiquantitative basis and the results led to th? same conclusions on adhesion as the free films did. Significantly, the use of the external mass enables the equipment to serve as an excellent means for measuring the adhesive or bonding qualities of organic materials on all sorts of substratametal to metal, metal to wood, metal to plastic, and plastic to glass. I n the simple system of polymer and solvent, the polymer film dries by solvent evaporation, a process of diffusion through the body of the film and a phase cliange a t or near tlie air-film interface. The solvent diffuses through the film a t a rate determined by the nature of the polymer, the minute colloidal capillaries that provide passageways for the solvent, and the residual bonds that tend to associate the solvent niolecules with tlie polymer configuration. Thermal conditions, of course, influence tlie diffusion. With all other factors the same, the diffusion rate is inversely proportional to the path leiigth; the longer the path, tlic mow tiinc requircd. I n systems under study, variations in hcigllt dcnote differences in diffusion path length and accordingly, differences in evaporation are espected. The greater the heiglit, tlie more solvent is present in tlie film and near the interface; the greater the height, the higher are the values of adhesion. I t appears likely that a correlation exists between the higher adhesion values and the presence of solvent. The conclusion that confronts 3 s is: adhesion in these cases depends on tlie presence of a solvent at or near the interface. The more solvent within the film and therefore the more near the film-metal interface, the higher the adhesion value. Lxtending the foregoing analysis of the data, a simple statement can be proposed on the nature of adheeion in these cases:
I
24
28
32
31
HI-CMS
Adhesion t's. >laximum IIeight f o r Poljstyrene (A to D) Dr] time. 144 hours at 40' C .
.Idliesioli depends on the presence of a fluid, or quasi fluid, or xilobile state a t or near the iiiterface of the film and the metal surface. .is the cvaporation proceeds, a state is reached in which some parts of the metal surface are in contact with the fluid moleculer; (force fielclsj. whereas other parts arc not. This variation of surince contact becomes evident as the loss of adhesion or the reduction in tlie amount of mobile contacts. In the data presented, i r tms shown that films of the same material and approximately the same height, but different dry times, had different values of adhesion. .It this point it would be advisable to clarify the conteiitioil that adhesion as measured dppendi on the presence of a iiiohilr 4:rte. It appears likely that the nornial force of attraction betFvren the film ~noleculesand metal surface atom resides in the molecular ilomain.---van der \T-aal, residual valency, polar, or hydrogen bonding forces. However, in the physical situation of placing LL film upon a metal surface, several competing forces appear. The presence of the mobile state shifts the resultant of the forcc, vectors to the normal direction keeping the film molecules and the 1 surface in contact. This can wise in the following mnnner:
If H piece of resin, such as rosin or ester gum, is melted on a metal, i t displays adhesion until the temperature equilibrium can no longer permit the fluid or mobile state. K h e n this point 1s reached, the resin snaps away from the surface. If a film of solvent is applied to the rosin, it will stick t o the metal. On sufficient drying, the solvent is removed and the adhesion force is again reduced to a small value. The solid piece of resin exhibits macroscopic cohesive forces forces which are coincident with and develop on solidification. These forces are oftentimes considered as the shrinkage and contractile forces of liquid-solid phase change. Fusion of the rosin provides a mobile interface in nhich the CDhesive bonds have been broken or weakened-that is, the separation of the adjacent groups has been cvtended by the theImal motion. At this point the resultant normal attractive force between the film and metal is predominant as compared to the forces between the film molecules. A shift of equilibrium, such as lowering the temperature, brings into play the cohesive and surface energy components. When the resultant of these cohesive elements approaches or exceeds the normal attraction at the interface, the resin exhibits poor adhesion. Let A , Figure 5, represent the molten Tosin on a metal surfare
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1949
Moltmdes pictured as circles have a random arrangement 111 thr flu~dor mobile state. The resin loses thermal energ\- b j conduction to the metal bur'ace and by convection and radlatlon to the air. The crosshatched portion in B represents the molecules whose cohesire and -urface bonds are being restored as a result of the 11qu1d-sold phase change. Xormally, the metal plate IS essentially at the same tempera'ure as the air and the transfer of heat does not appreciabl? thange the temperature of the metal a t the interface. Since thc neat transfer coefficient at the metal side 15 greater than a t the ail ntarfacc, more heat ~ i 1 be 1 transferred in the direction denoted b\ the airoTv in B. Except io1 a small amount of skin hardening, the phase change occurs irom the bottom up
518:
2341
h common case is that of the cellulose ester&. These do 1101 adhere to a surface unless solvent or a fluid molecule such as il plasticizer (which is nothing more t h a n a high boiling, low vapor pressure solvent) is present. I n time. a film of cellulose :icetatc deposited from an acetone solution lost adhesion and 1iftc.d from the surf:iee. The loss of adhesion was coincident with the loss of solvent as s1lun.n in constant weight determinations. By lightl? \vetting the surface of the disengaged film will1 acetone, tht, filni :rgxin adhered t o the metal. The sl-stem of forces irivolvrrl in t h i iinple is illuatrnted in the follon-ing Let .I, Figure 6, represent a freshly applied film of polynier and solvent. The molecules pictured as circles di,note the binder molecules. In R the arrow gives the direction of the diffusion gradient. atched portion represents the case or skin hardening. remainder of the film is still mobili:-that is, lionrigid hange in this case occurs from the top down. the portion of the film a t its extreme boundary as i t ) C. T h r skin \\-ill norinally be thicker a t the junction point of thc. metal and film near this boundary. -4sthe evaporation proceeds. the film attempts to increase its density, decrease its volume arid keep the surface area at a minimum. I n doing this, tensions, T . the internal stress exerted by the top section on the bottom scction, are set up in the more rigid skin. I n the rnobile portion, only a small hydrostatic pressure is transmitted. The tensions set up couples which oppose 1 hose of the iiormal attractive forces a t the less mobile (more rigid) portion of the film-metal interface. If the resultant of the cohesive and purface forces exceeds the normal attractive forces, the film will lift from the surface in the direction given by the resolution of the competing couples. It is easy to see that once the edge of the film is uot in contact with the metal, more solvent can diffuse quickly along the metal boundary, and this in t,urn progressively rrduces thc adhesion.
Figure 5
In C' consider the film to be separated into two sections fur H rnomcnt. As the mean temperature of each section drops, thr film attempts to increase its density and decrease its volume. Tensions, T , set up in the nonmobile or rigid contractile skiti oi The upper section act on the lower portion as indieatid. rhe highly fluid or mobile portion transmits only a small fluid preasurr' which is everywhere normal to the boundary. The tensions, I", set up a bending moment that is opposed to the normal attractivr force at the interface. If the resultant of this moment exceeds .he normal force of attraction a t the interface, the film separatw from the metal. The compressive force. F... acati o n the lower portion as t h f density changes. 111 this idealized esplanatiori,
iio
6 iA)
differentiation is made betweeri
the potential energy levels and geometric configuration of tlie
filni intrrface molecules in ph3.r tr:triFforination. Figure 5 is a iimplifird schematic reprc~~c~ntutirrii of the liquid-solid phase t-+liangc. K l i t ~ r solverit i or plasticizer u r even riinhile Inn. ~nolecularweight fraction is :iddetl to a solid or liigli inolrcu1:ir n.eiglit portion of reeiii. t!ie siroiig cc?lic.ive horids bet\vrcii the ndjacerit resin groups :ire tlistortcd :ind ~\-cwkerit~ii.The rnobile additives proi-irlr ir~~iiliorno~erieity a n d , i i i the *iiiiplest c:iie, iiicreasr t h r srp:ir a t i o n 1,f the rii:iin re-iii grl !up.. Tiit, iiiotiile film displays adhr.ion t o the iiietal surfncr f i ~ rt h i . ~~csnltaiit of the attractive force-. L t tlir: iiiterf:icc is greater ttian t h e iioiiii;il curnpoiiciit of t h e cutiebiw xnd the surface f o i w > trntliiig to keep tile :wea a. miriiinuin. If wine cstern:il coiitlitioii rcti u p a clrivi~igforce or gradient which ninvcs the mobilc c ~ o n ~ t i t u e ~:in-ny i t c from t h p surface (such as diffusion, plasticizer migration, i~ijlyiiieiiz:itioii,chernicnl reaccion, oi' teniprrature) tlie conipcting f'orcc.-. reach a nc\v equilibrium, .idllc+ioii has berii rc,tluced for tlie normal rejultaiit of the cohesive, mid sui,face forces of tlie film nl)pi~oachesthp value of thP sttr:ictivc force :it the intrrface.
C ? . * ? 6'G!
Figure 6
bor clarity) the : L ~ O T cheniatic repreapiitatioii of tlica liquidwlid phase clinnge has heen simplified. In this ide:rlizr!ti csplanation, no differeirti:itiori is madc betn-ecii tlie 1)oteriti:il ciicrgy lrvt tic filni i i i t c r f : m inolrculc~i i j
lien
])ortioil of tlir. vo1:itile solverit is rr~placcdhy a plastiioneiit of polyiiier, the coinpclt riding :~n twer-present mobilr tlit, surroundings sct up R driving grucliciit, !he filiii continues to :id!iei~~. It i.: true t!i:it the p i ~ e ~ i ~of n cniol)ilit>e at tlie iiiterf';icc. such ar wlvent lirtwct~ntlie layer of film nioleculea arid the metal surface. c:in prnvidp anchoring through wrface effvct- o f I:uiiinnr film. I w :I
2342
INDUSTRIAL AND ENGINEERING CHEMISTRY
tween two flat plates, much the same as liquid between two metal plates. This behavior which probably plays a part in the over-all adhesion adds a net resultant to the normal attraction. This behavior would exhibit poor shear resistance and in the usually practiced testing procedure would show up as poor adhesion. I t is reasonable to visualize that excessive polymerization or gelling or other phase changes from the mobile state set up competing forces whose resultant is sufficient to overcome the remaining normal attractive interfacial forces. A general picture is now being developed for all types of adhesion between the various organic systems and substrata. No effort is being made a t this writing to define the explicit physicalchemical nature of the forces that contribute t.n adhesion and its loss.
Vol. 41, No. 10
SUMMARI
This discussion has provided, by virtue of some quantitative data obtained from a new technique in measuring adhesion, an insight into the nature of adhesion. 9 simple statement is proposed on the nature of adhesion. Measured adhesion depends on the presence of a fluid, or quasi fluid, or mobile state a t or near the film-met a 1 interface. LITERATURE CITED
1) Moses, Saul, and mitt. R . K., IND,E m . CHEM.,41, 2334 (1949). K t ; a m v ~ uJuls 0, 1948. Presented before the Division of Painr, VarmaL and Plastics Chernktry at the 114th Meeting of t h e AMERICAKCnmiIr.4i 3 o r I F r I W a - h i n g t o n , D. C.
Antiknock Quality Requirements High Compression Ratio Passenger Car Engines R.
W. SCOTT,
C;. S. TOBIAS. A W
P. L. HAINE3
Standard Oil Development Company. Elizabeth, N . J .
'1'0
determine the antiknock quality requirements of high compression ratio gasoline engines, road antiknock studies employing the Borderline technique ha\ e been conducted on sixteen gasolines varying in composition, octane number, and tetraethyllead content in four cars having engines specifically designed to operate at conipression ratios of 8.0, 8.0, 10.0, and 12.5 to 1, respectively. Similar evaluations were also made in two cars of presentdaj design, one equipped with a 7.5 and the other with a 9.0 to 1 compression ratio engine head. The data indicate that the antiknock quality requirements of the engines in terms of laboratory octane numbers may be generalized as a function of compression ratio varying at sea level from 93 research octane number for 8.0 to 1 compression ratio to 102 research octane number for 12.5 to 1 compression ratio. Furthermore, in high compression ratio engines the research octane number requirement is limiting, and fuels of greater sensitivity apparently can be tolerated as compression ratio is increased.
T
HE, uw oi higher winpression ratio enginea repreaentc a basic means of obtaining improved over-all engine efficiencq in automotive equipment. However, the extent to Rhich the compression ratio of gasoline engines can be increased may be limited hy fuel antiknock quality. In the past, automotive manufacturers have progressively increased cornpression ratio as higher antiknock quality gasolines were made available. Fox example, from 1927 to 1941, the motor octane number of the average regular grade gasoline marketed in the United States increased from the vicinity of 55 to 75. During this samri period the average compression ratio of new automotive engines increased from 4.4 to 6.6 to 1. The utilization of highcr compiesjion ratios to date has been primarily to achieve increased performance, whereas improved mileage has been a somen hat less important consideration. It appears that present-day cars nou have about all the get away and top speed which the public requires or is feasible within the limits of currently designed cars and highways. For the most part then, any future increases in compression ratio will presumably be directed primarily toward an improvement in fuel economy. This seems to be highly desirable, particularly as long &s the higher antiknock qualit) gasolines required to permit this further increase in compresqion
i'itic) c u i be made available ac a price ahich will enable the cai owncar to travel more miles for his dollar. .4s cited by Ketterinp ' d ) , an improvement of as much as 40% in miles per gallon mal be realized by raising the compression ratio of an engine from 6.5 t o 12.5 to 1. It has been estimated that an improvement of only l C c in fuel economy (miles per gallon) would result in a yaving of approximately 7.0 gallons of gasoline annually per passenger car, since the average car cowumes 714 gallons of gasolinc in traveling about 10,000 miles per year a t 14.0 mile5 per gallon. Thus, on a nationwide basis with 31,000,000 passenger cars in use, a 1% saving in fuel would reduce gasoline COII.umption by 217,000,000 gallons per year and would result in H saving of about $49,000,000 per year in gasoline cost to the public n-suming a retail price of 22.5 cents per gallon for gasoline. To determine the fuel antiknock quality which will be required by high compression ratio passenger car engines, sixteen expeririiental fuels were evaluated at the General Motors proving ground in four General Motors cars specifically designed to operate a t high compression ratio. These tests were made in cooperation with the Research Laboratories Division of General Uotors Corporation who performed the road antiknock evalualion3 of these fuels in the four cars. In addition, these sanie fuels c're evaluated in the Elizabeth, K.J., area by the Standard Oil Dr-velopment Company in two cars of present-day design hut rquipprd nith high compre-sion ratio engine cylinder head5
DESCRIPTION OF FUELS, C4KS, i Y D rES'I METHOD5
I'hr histpen evperimental fuels evaluated for antiknock perrorniancix in the six cars were of varying composition, laboraton octane number and, to some extent, tetraethyllead conteni Pertinent laboratory inspection data on these fuels, which art designated by the numbers 1 through 16, are presented in Table I. The antiknock quality of the fuels studied covered a range o? roughly 79 to 102 by the motor method and of 90 to 100 by the research method. Fuels 1 through 6 were pure hydrocarbon blends, whereas the remaining 10 fuels were high octane number gasolines representative of possible future rommercid production Qr products of typical refinery processes. Four of the six cars employed were powered by engines specifically designed to operate a t high compression ratio. These arc designated as cars A, B, C, and D with compression ratios of 8.0, 8.0, 10.0, and 12.5 to 1, respectively. The additional cars designated as E and F, were of present-day design equipped. respectively, with a 7.5 and 9.0 to 1 compression ratio engine wljnrlrr hi3ad. Thp distributor spark advance with increasing
