Penetration of water into films deposited from emulsions leads to softening, whitening, and loss of adhesion. The mechanism of penetration proposed is that the driving force is provided by the difference between the vapor pressure of the film and the vapor pressure to which the
film is exposed. The opposing force is the resistance of the film to expansion to accommodate the water “cells,” and thus it is a function of the elastic modulus of the film. Capillarypenetration is unimportant in a well-formed film, for no water is absorbed from a saturated salt solution.
GEORGE L. BROWN A N D JAMES P . SCULLIN1 R o h m & Haas Co., 5000 Richmond St., Philadelphia 37, P a .
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ILMS deposited from polymer emulsions undergo several changes upon exposure to water (or water vapor). Among these are: 1. Swelling of the film, accompanied by plasticization and destruction of film continuity. Bubbles appear in response to stresses created by swelling where the film is attached t o a rigid surface, and these allow easier disruption of the film when it is subjected to abrasion. 2. Whitening of the free film or change in color value of a pigmented film resulting from the introduction of inhomogeneity in t h e refractive index. 3. Loss in adhesion to surfaces resulting from destruction of epecific adhesion forces through the presence of water.
Water absorption of polymer films may be followed by observing the weight of water absorbed or changes in physical properties of the polymer. The present investigation has been concerned chiefly with the whitening of emulsion films immersed in water. Optical densities of films of various thickness immersed i n water have been determined as a function of time. Lowry and Kohman (8) have demonstrated that the absorption of water by natural rubber sheets follows Henry’s law (absorption proportional to vapor pressure) for vapor pressures below a certain level-about 75% relative humidity. This absorption is the result of true solubility of water in the polymer. Above this critical level, absorption rises sharply with rising vapor pressure. Later investigators have demonstrated the same effect for many other polymers, and this deviation from linearity is one of t h e factors responsible for the pronounced departures from Fick’s law for the permeability of polymers measured under varying humidity conditions (3). The structure of the films deposited from emulsion is different from that of solution cast or milled films and is influential in determining the absorption of water. As water evaporates from the emulsion particles, they approach and eventually touch t o form a continuous polymer network. The spheres deform, flow, and interpenetrate, leaving small deposits of water containing the soluble substances originally present in the emulsion. As the last water diffuses through the polymer network and evaporates, small cells of the water solubles remain behind. These water solubles are catalyst and catalyst residue, emulsifier, and salts introduced as impurities. Much of the emulsifier will remain adsorbed on the emulsion particles, and these molecules may serve as further loci for accumulation of water. If the polymer flows sufficiently, the particles will coalesce t o form a continuous film, substantially free of voids space or capillaries. I n this work, a number of soft polymers that formed films of good continuity have been examined, 1
Present address, Heyden Chemical Corp., Garfield, N. J
.April 1953
The two possible mechanisms for water penetration in the films are capillary flow, which would occur in channels representing imperfections in the film, and diffusion. On the basis of the data presented here, diffusion alone can satisfactorily account for the flow of water into the polymer during initial stages of exposure. Capillaries are “apparently opened during later stages. A detailed exposition of the proposed mechanism for water penetration may be given. Water enters the polymer, in which it is soluble t o a slight extent, and diffuses into small pockets of salt and other water-soluble materials. The driving force for penetration is provided by the osmotic pressure. The driving pressure which results in the accumulation of water in the film is the difference between the vapor pressure to which the film is exposed and the vapor pressure inside the film. The vapor pressure in the film is initially t h a t of a saturated solution of the salts (and other water-soluble materials, if present) deposited between latex particles during the course of the water evaporation necessary for film formation. Opposition to water penetration results from the increasing hydrostatic pressure on a water cell formed in the film, this force being supplied by the resistance of the polymer to deformation. This resistance is a function of the elastic modulus of the polymer under the existing specific conditions of temperature and water content. Thus water absorption reaches equilibrium when the hydrostatic pressure on the water cell in the polymer is equal to the osmotic pressure difference. EXPERIMENTAL
Films were formed by drying the chosen polymer emulsion on a leveled glass plate. Films were dried hour at 100” C. Light transmission was measured by placing the film on a slide fitted into a specially constructed rectangular glass cell. The cell was filled with water,, placed in a photometer, the sample inserted; transmission readings taken at appro riate intervals. Readings are a function of the geometry of t%e instrument used so the values reported are relative but not reproducible between instruments of various design. The rates of whitening (optical density versus time) for a number of acrylate polymers are shown in Figure 1, and a number of other commercial latices in Figure 2. A description of the samples is given in Table I. RESULTS
In many cases the optical density initially increases linearly with time, but the rate falls as saturation is approached. For Rhoplex WN-80, the rate depends on film thickness, but for each thickness the rate is linear over a substantial range. I n other cases, this functionality does not apply. An exponential relationship is often valid, but no general function has been found.
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represent the time of observation. factory over the range
0.6-
If the two parametric equations for whitening and water absorption of Rhoplex WN-80 are equated, one obtains the surprising relationship O.D. = kQ?, where O.D. is optical density. If the postulated mechanism for water penetration is correct, certain aspects of behavior should be predictable. The removal of salts from the emulsion or polymer film should materially reduce the rate of whitening and equilibrium swelling. With Rhoplex KN-80, both the emulsion and film have been dialyzed. The emulsion was placed in cellophane dialysis tubing, washed for 3 days, and a film made. Dialysis of an unmodified Rhoplex film was accomplished by washing for a similar period of time, then redrying the film. A comparison of the initial rates of whitening is as follows:
RHOPLEX FRN
/@Gi?r RHOPLEX ER RHOPLEX WG-9
0.1I-
Rhoplex Wi-SO Rhoplex WK-80, dialyzed emulsion Rhoplex WX-80, dialyzed film
RHOPLEX WN-80
15
30 45 TIME, MINUTES
60
Figure 1
It is peculiar that although the optical density of a Rhoplex WN-80 film in water is a linear function of time over an extended region, the weight of water absorbed is proportional to the square root of time (Figure 3). Barrer ( 1 ) refers to this relationship as the parabolic diffusion law, but it appears to be empirical and not a solution to the more exact derivations. This relationship is valid for many systems during initial stages of diffusion. This relationship may be represented as
where Q is the weight absorbed, and the subscripts t, 0, and
Barrer finds this to be satis-
m
Optical Density, 1 Hour 0.100 0 000 0 013
Equilibrium swelling values of emulsion films are also markedly decreased with removal of salts. Dialysis of the emulsion was conducted under conditions such that removal of emulsifier was unlikely, and the treated emulsion was stable. It is surprising that such a marked improvement was accomplished merely by removing residual salts. The fact that the film could be washed free of salts indicates that when swelling has become pronounced, channels are opened through the structure. It is necessary to redry this expanded film before these improvements are achieved. Alternate leaching and drying of emulsion films would be expected to be beneficial, if film structure was not impaired during these cycles. The use of water repellents in small amounts might be expected to delay water absorption, by slowing diffusion through the film. The equilibrium value should be little affected, however. This was found to be true. The use of lyOof several rosin-type water repellents slows initial whitening of Rhoplex WN-80 t o about one half. Effects of temperature should be threefold: 1. Permeability of the polymer is changed (usually increased with increased temperature) which will change the rate of water absorption. 2. Modulus is usually decreased with increased temperature. 3. Osmotic pressure is changed as a result of changes in both the external and internal vapor pressures.
The rate of whitening appears to be very temperature dependent, particularly in the region where the modulus of the polymer
Table I . Product Rhoplex F R N Rhoplex WN-80) Primal S-1 Rhoplex MR Rhoplex ER Rhoplex WC-9 Dow513K . Neoprene 571 Polyblend 550 X 20 Hycar OR-25 Geon 652
1'5
30 4'5 TIME,MHUTES
60
Saran X-646
Materials Used in Tests
Polymer Type
Emulsifier Supplier Nonionic Rohm & Haas Co.
~4cry1ic Acrylic Styrenebutadiene Chloroprene Sitrile rubber vinyl-chloride Nitrile rubber (butadieneacrylonitrile) Vinyl chloride (plasticized) Vinylidene chloride (copolymer?)
Anionio
Rohm & Haas Co.
Anionic Anionic
Dow Chemical CO. Du P o n t
Anionic
Goodrich Chemical Co.
-4nionic
Goodrich Chemical Co.
Anionic
Goodrich Chemical CO.
Bnionic
Dow Chemical Co.
Figure 2
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TI/: Figure 3
is changing rapidly-that is, i n the region of the second order transition temperature. In view of the complexity of these changes, the effects of temperature have not been studied in detail as yet. I n general, polymers of higher modulus whiten more slowly, which is in accord with the mechanism proposed. Exact comparisons are not allowable because of wide variations in emulsifiers and the permeability of the various polymers. For certain copoIymer systems, water resistance changes markedly in the narrow concentration region where the modulus changes rapidly and is little affected by equivalent alterations in composition where the modulus is essentially unaffected. This effect would seem t o dissociate the changes which can be attributed to hardness and those attributable t o the change in monomer composition. These effects are being investigated further. Films of increasing thickness whiten more slowly, at least
initially. Increasing film thickness is equivalent t o increasing polymer modulus, since it opposes expansion of a water cell near the polymer surface. This effect is illustrated in Figure 4, where the intial rate of whitening of Rhoplex WN-80 films are plotted against film thickness. Values presented are averages of two or more determinations on films of the same thickness. Similar effects have been demonstrated for the rate of swelling of a polymer in solvent. Although t h e rate of water absorption of Rhoplex WN-80 could be accounted for by either capillary penetration or difI 5 IO 15 20 25 30 f u s i o n , b o t h of FILM T”Ess MLS which bear the Figure 4 same dependence on t h e aauare root of time, capillary penetration is clearly unimportant. This is best proved by rates of absorption of water by films immersed in salt solutions. A Rhoplex WN-80 film does not swell or whiten when exposed t o a saturated sodium chloride solution (75% relative humidity). LITERATURE CITED
(1) Barrer, R. M., and Ibbitson, D. A,, Trans. Faraday Soc., 40, 206 (1 944). (2) Lowry, H. H., and Kohman, G. T., J. Phys. Chem., 31,23 (1927). (3) Taylor, R. L., Herrman, D. B., and Kemp, A. R., IND. ENG. CHEM.,28,1255 (1936). RECEIYBD for review OCTOBER 17, lS52
ACCEPTED January 29, 1953.
F. A. DICIOIA A N D R. E. NELSON General Latex & Chemical Corp., Cambridge, Mass.
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Aqueous latex emulsions are frequently irreversibly coagulated by freezing. This paper reports a study of freeze-thaw properties of several polymers being used or proposed for use in latex paints. The effects of particle size, emulsifying agents used in the latex polymerization, and variations in monomers utilized are reported. A
latex which in itself has excellent freeze-thaw properties may be altered in this respect by compounding for paint. The effect of varying the freeze-thaw method has also been studied and observations are reported. Some suggestions are given regarding compounding to obtain good freezethaw in a latex paint.
A
be functions of the protective colloids on the particle. I n certain instances, advantage has been taken of the phenomenon of coagulation of latices by freezing. I. G. Farben was the first to try this method. Patents were issued t o Lecher et al. (9) for coagulation of isoprene latex by freezing. Konrad (8) described coagulation of a polyisoprene or polybutadiene sodium soap-emulsified latex by freezing at 15 ’ to -20’ C. Neoprene is normally produced in the solid form b y a freezing technique (%, 5 ) . Synthetic latices have been microflocculated by freezing as part of a procedure for solids concentration (11,18).
QUEOUS latex emulsions are frequently irreversibly coagu-
lated by freezing (10). Normal natural Hevea latex is very difficult t o freeze, b u t it too can be irreversibly coagulated by freezing, especially after being compounded. A concentrated latex made from a freeze-resistant normal latex, by a method t h a t involves removal of natural protective colloids-Le., by centrifuging-becomes very much less resistant t o freezing. On the other hand, a normal latex concentrated b y evaporation retains its stability against low temperature. The properties governing low temperature resistance may therefore be said to April 1953
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