Temperature Dependence of the
Adhesion of High Polymers to Cellulose CHARLES H. HOE'RICHTER, JR., AND A. DOUGLAS MCLAREN' Rayon Technical Division, E. I . d u Pont de Nemours & Company, Znc., Buflalo, N. Y . B y mixing a vinyl chloride-vinyl acetate polymer with a second polymer in which a portion of the vinyl acetate was replaced by maleic acid, it was possible to vary the concentration of the carboxyl group through a wide range. Experiments showed that adhesion to regenerated cellulose which was assumed to be a measure of the COOH + OH dipole-dipole bonds formed increases in a manner expressible by
adhesion = k(C0OH)" reminiscent of the Freundlich sorption isotherm. At any given carboxyl concentration adhesion increases with temperature. This result can be explained by a high endothermic activation energy for viscous flow in combina-
E
ARLIER work on the adhesion of high polymers has suggested further study on the contributions of molecular structure and the polar nature of adhesive and substrate. McBain and Lee (11)examined a large number of pure chemical substances as adhesives for metals and found that often the melting points of pure substances run parallel with joint strength. Furthermore, a connection exists between adhesion and molecular structure: Many of the stronger joints were given by substances containing hydroxyl and carbonyl groups and, in general, aromatic compounds tend to give stronger joints than aliphatic compounds. Their conclusion was that the force field of a solid material is in some way transmitted through films of adhesive to a depth of perhaps more than one hundred molecules thick. They have also considered the role of deformability of an adhesive (10) and concluded that an adhesive must be adaptable to volume changes accompanying setting, aging, shrinking, swelling, temperature, and humidity fluctuations. In addition, these workers contend that unmistakable although imperfect relations exist directly 'between the strength of joints and such properties as internal pressure, tensile strength, elasticity, and hardness; and inversely between compressibility and atomic volume of the materials joined. Gerngross (6, 7) in considering the various glues and cements,
alloys, cement-asbestos board, plaster, paper, glassine, and cellophane. Resins having widely different proportions of combined maleic acid were studied for their adhesiveness, and it was found that about 0.25 to 0.30% maleic acid represented the minimum needed for good air-drying adhesion to steel, although improvement was noted with as little as 0.1% of the acid. This acid content could be obtained by blending resins containing much higher proportions of acid, to reduce the over-all acid content to this range. This investigation examined the influence of both temperature and concentration of polar groups on the adhesion of certain vinyl polymers to regenerated cellulose with the view of developing a concept of the nature of the processes contributing to the formation and stabilization of the adhesive bond in such systems. EXPERIMENTAL ' ADHESION MEASUREMENTS. Regenerated cellulose sheeting containing 16% glycerol was coated on both sides with solutions of polymer compositions containing various concentrations of carboxyl groups. After conditioning at 35% relative humidity the coated films were heat-sealed as follows:
Two Pieces of film, each 1.5 x 3 inches, were fused a t a temperature 30" C. above the tack temperature (defined as the lowest temperature a t which two films of polymer will just fuse under a pressure of 20 pounds per square inch which is applied for 2 seconds). The area of the seal was approximately 0.75 x 1.5 inches, extending completely aCrOSS the short width of the sample. Sealed films were again allowed to come to equilibrium with air at 35% relative humidity and 240 c. Such samples were then pulled at room temperature on a Suter tester, registering in grams, a t speed of 0.185 inch Per second- The relative adhesion Of the polymers to the base sheet was Obtained in terms Of grams pull per linear inch and one half as the polymer peeled (not the initial break) from one side or the other of the cellulose film.
including starch, dextrin, proteins, caoutchouc, cellulose ether, phenol-formaldehyde, polymeric acrylic acid esters, and polymeric vinyl acetate, observed that they are polymeric substances consisting of long chains and sometimes cross-linked by secondary valences. These long chains are capable of orienting themselves with their long axes parallel to the applied load; under continued heating, both orientation and adhesion disappear. Gerngross postulates the chains oriented a t right angles to the plane of the cement and believes that adhesion takes place between reactive groups a t each end of the chain and the surface. de Bruyne (8) put forward what he describes as an adhesion rule: Strong joints can never be made to polar adherents with nonpolar adhesives or to nonpolar adherents with polar adhesives. Doolittle and Powell (4) have shown that certain polar groups may be used to improve the adhesion of vinyl chloride-vinyl acetate copolymers to steel, concrete, aluminum-magnesium 1
tion with a smaller exothermic heat of sorption. The former energy is a measure of the process of the chain segment motion which is necessary to permit more dipoles within the polymer to approach the vicinity of the active centers in the substrate. Application of the Guzman equation to the rheological measurements on the polymer composition shows the heat of fluidity to be of the order of +30,000 cal. (endothermic). Application of the Clausius-Clapeyron equation to the measurements on adhesion gives the heat of adhesion involved in the combined processes to be +12,500 cal. (endothermic). The heat of sorption per mole of maleic acid residues on cellulose is calculated to be approximately -17,500 cal. (exothermic).
The polymers studied were Vinylite VYHH (85y0 vinyl chloride-15% vinyl acetate) and a special sample of Vinylite VMCH (85% vinyl chloride-9.3% vinyl acetate-5.7% maleic acid) which had approximately the same molecular weight, tack temperature, and viscosity. By mixing these two polymers writh a constant concentration of mixed plasticizers and measurng the adhesion it was possible to demonstrate the effect of carboxyl concentration on adhesion, as shown in Table I. In studying the
Present address, Polytechnic Institute of Brooklyn, Brooklyn, N. Y .
329
INDUSTRIAL AND ENGINEERING CHEMISTRY
330
where I(, is a fractional exponent,. Since the adhesion of the experimental composition does not, progress linearly or its a highcr power of the carboxyl content, it is evident that, adhesiori here is not the result of direct chemical combination. Equatioii 1 , however, is analogous to the typical Freundlich (6) sorptiori isot,herm. The Langmuir isotherm might be made applicdilr a:, rcgards intcrpretation of the data, but the authors believe Lhitt the errors of measurement are too large to warrant a distinction. Consequently, the more simple Freundlich equation is used her(:. Iralucs for log K and n for the isotJherms given in FiRuw 2 n-ere determined by the me1,hotf of lea quai'es and arc s h o n 1 1 in T a h l ~11.
R H E O L O G I C A L B E H A V I O R OF A
VINYLIT E C O A f l N G
\
C O M POSI TlON
FIG, I
3.0 h
0 W
a c, 4
2.0
a
s
a,
1.0
TABLE 11. 'rPl,lperatll,e,
-30 - 20 -10
o
2;5
2.6
I/T
10 20
x
30 40
103
30
fin
effect of temperature on adhesion, seals vxre treated, as before, except that 5 minutes before pulling they were subjected to the desired temperatuie.
ASD CARBOXYL TABLE I. EFFECTOF T~XIJERAKRI: COSCEXTR.~TION OS PEELSTRCSGTH
Weight ratio VYHH/\,.iICH Coating weight, g./sq. m. hfoles (COOHI/
99.5/0.5 99.0/1.0 9 8 . 5 i l . 5 98.0/'2.0 97.0/3.0 94.0;G.O
4.44
3.89
4.23
5.53
4.77
4.06
1OOOp.polyrner 0,00491 0,00983 0.01474 0.01973 0,02958 0.05916 Peel strength,
g./l.,5 inches -30° C. -200 -100 c.
e. 00 c. 10" c. 200 c. 30' C. 40' C. 500 c. 60' C.
85 78 93 125 119 150 352 588 853 695
...
58
60 70 , . .
4i6
570 507
88 85 88 132 190 260 536
105 123 127 I35 200 300 570
9130 855
880 1200 1000
. .
105
165
153
245
199 243 428 685 995
303 470
...
...
...
990
... ... ...
...
" (2.
COEFFICIEX rs
"
n
Log ti 2.64 3.10 3.07 8 3.28 3.51 3.51 33.48 48 3 . 68 68 3.81 3.93 3.80
0 367
0 614 0 571 632 711 605 542 52; 504 476
0 0 0 0 0 0 0
r . l h ( , expression of adhesion as ii furiiction of carboxyl ~ O J I W ; I I tratiori i s supported by the fact, that compositions cont,ainiiig only Vinylite VYHH slio~vlittle 01' no adhesion to regenerakd ccllulose (maximum 50 grams at 50' C.), whereas those contaiiii t i # only small amounts exhibit adhesion up t o 1200 grams. Within the range -20 O to 50 C. the exponents are reasoilably close to the theoretical value of 0.67 which is predicted by Gyaiii (9) for surface sorption-i.c., since the ratio of area to volume in any geometrical figure varies as 12 to 1.3, the distribution eque tion
C? = kClfi
(2)
of surface horytion if expressed in the for111
cp== A(:laj3
(31
hccording to Gyani, t,hc magnitude of the exponent for iiiterfacial sorption phenomena, obsprved expc:rimentally, is in t'hn range of 0.50 to 0.75. At the extremes of the tempcraturi. range investigated in this work, t,he values of n deviated front these limits. Further, failure did not, occur by simple st,ripping, ption was no longer the conleading to the conclusion 1hat trolling process. ADHESION AS A COVBINATION SOWTIOX-VISCOSITY PIIEZIOII b;NON. I n view of the above infnl,rnat,ion on the tempsrxtui~tL
For a theoretical analysis of RHEOLOGICAL ~IEASUREMESTS. the data it was necessary to establish the temperature coefficient of viscositv for the Dolrmer compositions under study. Viscosities were determined using a modification of A.S.T.X. RIethod D553-42 for estimating the viscosity of rubber solutions ( 1 ) . About 10 grams of LOG ADHESION V 9 LOG(CO0H) AT a plasticized Vinylite composition containing 3% of the 85-9.3-5.7 polymer with VYHIl were placed in VARIOUS TEMPERATURES a tube heated in an oil bath. The time required for -3.0 a needle (No. 18 copper viire) surmounted by a 10gram weight to penetrate a n arbitrary distance, z 12 mm., into the plastic mass, after first being im0 cn mersed to a depth of about 4 mm., >vasobserved as w a function of temperature from 120" to 150" C. I n A plot of log, time against reciprocal temperaU .25 ture is shown in Figure 1. 0 I
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$0'
//
0 DISCUSSION AND RESULTS
ADHESIONAS A FUNCTION OF CONCENTRATIOS. A plot of the log of peel strength (adhcsion) us. the log of carboxyl concentration is given in Figure 2. Analysis shows these data conform to an equation of the type adhesion = k(CO0H)" ( 1)
-I
L
/
, eo-
I N D U S T R I A:L A N D E N G IN E E R IN G C H E M I S T R Y
February 1948
331
which inrreases with temperature. Therefore, the temperature dependence of adhesion-Le., whether adhesion increases or decreases with temperature-depends on the relative magnitudes of the molar heats If the heat of sorption derived from the sorption process is less than the heat of fluidity of the polymer at the interface, the net value of H , will be opposite in sign from H , and adhesion will increase with increasing temperature. Figure 2 illustrates that this phenomenon occurs for the polymer vinyl chloride-vinyl acetate-maleic acid sorbed on cellulose. Examples of such behavior in classical sorption experiments are rare but not without precedence. The sorption of liquid mercury by charcoal is affected in this unusual way with incwasing temperature ( 3 ) . It is apparent from the form of Equation 6 that if log (COOH) at constant adhesion is plotted against 1/T the slo: es of the resultant “isosteres” are a direct measure of Ha.
r
I
c C
c:
c
Ha
c c
= slope
of adhesion isostere X 2.303 X 1.987
(7)
From Figure 2 a t a given adhesion level the relationship of temperature and concentration may be obtained (Figure 3). Substituting the measured slopes in Equation 7, one finds the. values of Ha to be 12,500 cal. (standard deviation = 390 cal.). A similar treatment of Equation 5 shows that a plot ot the natural logarithm of time against reciprocal absolute temperature will give the slope required to calculate Hi directly, sinre fluidity is directly related to time by the equation
-
In t = -In $ , -In C1
‘AN7 LOG ADHESION 35 % Ri H* 3,l
32
33
VT
3.4
x
35
3.6
104
dependence of adhesion, one may explain adhesion as being the resultant of two separate processes. The extent of sorption for any given number of available dipoles decreases with increasing temperature, as predicted by Freundlich. Acting against this normal sorption behavior, however, increasing temperature accelerates the microbrownian movement of the chain segments to which substituent groups are attached, permitting more favorable orientation and therefore a larger number of dipoles to be available for sorption. The total adhesion is the resultant of these two processes. Thermodynamically the relationship can be expressed by the equatipn
Ha
=
Ha
+ Hj
(4)
where H , = heat of adhesion, H f = heat of fluidity, and H . = heat of sorption. The heat of fluidity (8),Hj,is given by the expression d- In + tdT
Hj RT2
(5)
where + is the reciprocal of viscosity. The heat of adhesion H,,’, can be expressed by means of a modified form of ClausiusClapeyron equation
dlnc dT
5
I
RT2
where c is the concentration of maleic acid residues in contact with the cellulose substrate. The expression is a modification, si&e H. includes both a term Hj (analogous to a heat of vaporization) and a term H,. Thus H , is seen to be a net value for the entire adhesion process of fluidity and sorption. Sorption is an exothermic process which decreases with increasing temperature, while fluidity is a n endothermic process
(8)
Here C1 is the constant of integration depending upon the experimental conditions a t which the viscosity was measured. Actually the fluidity measurement was contaminated by some elasticity effects, but the principle is not invalidated by what is perhaps a too large value of H/ (12). The plot shown in Figure 1 gives a slope of 1.51 X lo4which, when multiplied by R, gives a value of 30,000 cal. per mole for H,, which is the right order of magnitude. Substituting in Equation 4,
H, = 12,500 cal. -30,000 cal. H. = -17,500 cal. The thermodynamic evidence implies that while the behavior is of the nature to be expected if the bonds involved only van der Waals forces, the calculated molar heat is consistent only in the order of magnitude. Ester formation can hardly be considered, since the phenomenon is almost instantly reversible. ACKNOWLEDGMENT
Acknowledgment is extended to the Carbide and Carbon Chemicals Corporation for providing special vinylite resins used in this study. LITERATURE CITED
Am. 800. Testing Materials, Designation D553-42, p. 683, 1944. Bruyne, N. A. de, Flight, 51 (1939). Coolidrre. A. S.. J . Am. Chem. Soc.. 49.708 (1927). Doolitce; A. K., and Powell, G. M., >a&, Oil Chem. Rev., 107, No. 7,9-11 (April 6, 1944). (5) Freundlich, H., “Colloid and Capillary Chemistry,” pp. 112-13, New York, E. P. Dutton & Co., 1926. (6) Gerngross, O.,2.angew. Chem., 44, 232 (1931). (7) Gerngross, O., and Goebel, E., “Chemie und Technologie der Leim und Gelatin Fabrikation,” Dresden, T. Steinkopff, 1933. (8) Guzman, J. de, A n d e s SOC. espafi.jis. qutm., 11,353 (1913). (9) Gyani, B. P., J . Phys. Chem., 49,442 (1945). (10) McBain, J. W., and Lee, W. B., J . SOC.Chem Ind., 45 No 30, 321T (1927); IND.ENG.CHEM.,19,1005 (1927) (11) McBain, J. W., and Lee, W. B., Proc. Roy. SOC.,A113, 606 (1) (2) (31 (4j
(1927). (12) Tuckett, R. F., Trans. Faraday Soc., 40,455 (1944). RECEIVED September 23, 1946. Presented before the Division of Physical and Inorganic Chemistry at the 110th Meeting of the AMERICAS CHFMICAL
SOCIETY,Chicago, 111.
