sions made by the authors (4, that the carbonyl functional groups present in the oxidized asphalt consist principally of aldehydes, ketones, and acids, with a minor portion as esters. The carbon-oxygen single-bond components consist of alcohols, peroxides and/or hydroperoxides, and possibly ethers, although the present work does not distinguish among these classes. Acknowledgment
The authors thank V. E. Gray for technical assistance in the development of the colorimetric methods, and A. J. Turner for conducting many of the experimental determinations. The Kuwait asphalt sample was obtained through the courtesy of P. M. Jones, National Research Council of Canada. Literature Cited
(1) Beitchman, B. D.,J. Res. Notl. Bur. Std. 63A, 189 (1959). (2) Bellamy, L. J., “The Infrared Spectra of Complex Molecules,” 2nd ed., pp. 179,182,Wiley, New York, 1962. (3) Campbell, P. G., Wright, J. R., IND. ENC. CHEM.,PROD. RES.DEVELOP. 3,186 (1964). (.4 ,) Camohell. P. G.., Wrizht. J. R.. J. Res. Nofl. Bur. Std. 68C. 115 (1564): (5) Dugan, P. R., Anal. Chcm. 33,696(1961). (6) Zbid., p. 1630. (7) Gray, V.E,, private communication. I
I
\*,”*,.
(15) Palit, S . R., Ghosh, P., J. PdymcrsCi. 58, 1225 (1962). (16) Robertson, A., Waters, W. A., Tram. Porodq Soc. 42, 201
(1946). (17) Schriaheim.. A,.. Greenfeld, S. H.. A S T M Bull. 220. 43 11“‘ \.__i 7). (18) Iiergiyenko, S. R., Garbalinskiy, Acta Chim. Hung. 37, 213 I*\ (191>, ,. imith, C. D., Schuetz. Schuetz, C.. C., Hodeson. Hodgson,’ R. S.. S., IND. ENC.CHEM. (19) Iimith. P R OD. RES. RES.YEL LOP. 5, 5,153 i53 (1966). (19667 (20) s:kcwart, J. E., J. Res. Notl. Bur. Std. 58, 265 (1957). (21) F:trieter, 0. G., Snoke, H. R., Ibid., 16,481 (1936). (22) “right, J. R., Campbell, P. G., J. Appl. Chcm. (London) 12, 256 (1962).
RECEIVED for review June 28, 1965 ACCEPTED July 26, 1966
Division of Petroleum Chemistry, 151st Meeting, ACS, Pittsburgh, Pa., March 1966. Certain commercial instruments arc identified in this D ~ L Xin~ order to soecifv the exoerimental or~cediire r~ - - ~ - ~ ~ adequately.’ i n no c&d&s’sucd identification imply recommendation or endorsement by the National Bureau of Standards, nor that the equipment identified is necessarily the best available for the purpose. ~~~
~
~
.
DRYING AND OXIDATION RATES FOR LINOLEIC MODIFIED ALKYD RESINS E. G. B O B A L E K , ’ E . R. M O O R E , * A N D J. R . S H E L T O N E@neerigP Division,Cora Imtitntd of Technology, Clewland, Ohio
The hardening rates and volumetric rates of catalyzed oxygen absorption during film formation at 21 C. were examined a s o function of molecular weight for a n alkyd resin prepared by a solvent process reaction from glycerol, phthalic anhydride, and linoleic acid in the molar ratio of 1 :1 :0.4. The rate of oxygen uptake up to 150 hours a t 22” C. increased with increasing molecular werght. It is hypothesized that a diffusion limitation was imposed on the termination step of the free radical oxidation process. This diffusional limitation is related to current gelation theory. studies reported by Bohalek, Moore, Levy, and Lee (7, 2) suggest that gelation of an alkyd resin involves three distinct steps (see Figure 1): (1) formation of a stable colloidally dispersed microgel phase; (2) coalescence of microgel into much larger molecular aggregates which remain stably dispersed; and (3) phase inversion where the dispersed gel particles rapidly coagulate into a continuous structure network which encapsulates the original suspending liquid into separated domains. If a resin synthesis is carried past stage 3, the resin becomes solvent-insoluble and infusible, and loses fluidity, or is gelled. Carrying a reaction past stage 3 is undesirable during synthesis, but the polymer must go through stage 3 as early as is feasible in the film formation process. It is suggested here that physical gelation of a paint vehicle to form a dried protective film proceeds via the same sequence as would be the case in the resin hulk during synthesis if the process there were extended long enough, except that the polymerization mechanism causing the different colloidal states is different. Most vehicles having a useful drying time, as a result of oxidative polymerization in films, have been carried very close to the brink of step 2 during synthesis by polyesterification.
P W S E INVERSION. PHYSICAL
ECENT
R
Present address, Univasity of Maine, Orono, Maine.
* Present address, The Dow Chemical Co., Midland, Mich.
MOLECULAR W E l O H i INCREASE
-
COURTESYJournaCo(Applied PoCmevScience
Figure 1.
Gelation model
The drying of a paint film can then be visualized as a planned destabilization of the dispersed microgel phase. The stability of this dispersed gel phase is dependent on both the number and the size of particles in the dispersed gel phase, analogous to a latex. Destabilization of this dispersion can be enhanced by addition of a heavy metal drier soap such as lead tallate, which promotes physical coagulation or flocculation and sedimentation of dispersed colloidal gel particles, or by a cobalt salt which, in addition, favors oxidation which VOL. 5
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induces crosslinking or further molecular growth so as to increase the number, size, or concentration of the insoluble dispersed component of the polyphase polymeric colloid. One of the factors which promote the rate of physical coagulation of colloid can be controlled effectively by regulating the extent of reaction during the resin synthesis, so as to produce more or less of this microgel which acts as a “nucleating” component. This can be anticipated if the hypothesis regarding the gelation mechanism is correct. The normal expectation is that the speed of the oxidation mechanism would depend on the average olefinic functionality of the aliphatic oil-acid radicals, and not upon how many of such are fixed per molecule of polymers by esterification. This paper deals primarily with a study of the rate of this oxidative chemical reaction via an oxygen-absorption technique. Particularly, it aims to report the unexpected observation that samples from the same parent alkyd species show an acceleration of the initial rate of oxygen absorption when the molecular weight of the alkyd polymeric ester is increased before exposure to the oxidation-induced film-formation process. Experimental Techniques
The apparatus has been described by Shelton and McDonel ( 3 ) . I t was used in a room controlled to 21 O i 1O C. Samples for drying studies were dissolved in a mixture of equal weights of xylol and butanol to a concentration of 40% resin solids. Metallic driers were added (as tallate) in sufficient quantity to adjust the total metal composition up to 1% lead, 0.02% manganese, and 0.02% cobalt, the percentage being based upon polymer content of the solution. After addition of drier, a weighed strip of glass fiber cloth, previously heated in a muffle furnace to remove organic contaminants, was impregnated with this solution, then placed on a tin plate. Samples were then placed in a desiccator, purged with COS,and evacuated for 24 hours to remove all traces of solvent. The sample was weighed to determine polymer pickup and quickly placed in a chamber of the oxidation apparatus, which was immediately evacuated. When all samples were in their separate chambers, the entire system was purged three times by flooding with 0 2 and drawing a vacuum. Then the apparatus was filled with 0 2 at atmospheric pressure, each chamber was isolated, and oxygen uptake was followed volumetrically.
proceeds most rapidly at the film surface, which explains why a tough skin forms over residual liquid when cobalt catalyst is used above, or in excessive quantities with reference to, the optimum cobalt-lead ratio. Manganese is less efficient than cobalt, but it has a markedly positive effect in mixture with lead and cobalt. Resins with increasing extents of reaction might be expected to exhibit surface drying to a greater and greater degree. This can be simply demonstrated by allowing solutions of formulated resin to set in lightly covered containers in the order of increasing molecular weight and observing the skin thickness as a function of time. Decreasing stabihty of the microgel dispersion through increased extent of reaction would lower the amount of oxidation necessary to cause phase inversion near the film surface. Such accelerated surface drying would create a membrane barrier which retards the diffusion of oxygen into the film, thus limiting the amount of “through dry.” Likewise, the time required to reach a certain degree of surface dry would decrease with increasing molecular weight. The latter expectation was shown to be correct by Bobalek et al. (7) (Figure 2). Drying time decreased rapidly as the theoretical gel point (D.P.*. = 11.25) was approached. The reference alkyd previously proposed (7), consisting of glycerol, phthalic anhydride, and linoleic acid (in molar ratios of 1 : 1:0.4), was used to investigate oxygen-absorption rates. The effect of increasing degree of polymerization on the oxygen-absorption rate is shown in Figure 3. Figures 2 and 3 involve the same samples and may be compared directly to relate oxygen uptake and surface dry time.
600
500
400 v)
Discussion of Results
The first step in film formation involves the removal of solvent through evaporation. After this, gelatification causes hardening to a dried film. The sequence of steps described ( 7 , 2 ) for the gelatification process is hypothesized as applicable to film drying. The drying process is partially controlled through regulation of factors that affect the stability of the colloidally dispersed microgel phase, such as use of lead drier to accelerate the microgel coagulation step. The most important factor affecting stability of the microgel dispersion is the molecular weight, which depends on extent of the polymerization reaction of the base resin before it is subjected to further oxidative polymerization by the drying oil mechanism. Small changes in the molecular weight can cause great changes in the resulting stability of the microgel phase (7, 2) near the gel point (see Figure 1). Further control can be exercised through choice of the drier system. Metallic driers such as iead seem to promote a physical destabilization of the dispersion, resulting in what the coatings art calls a “through-dried” film. Others, such as cobalt, promote destabilization by catalyzing the oxidation which activates a new polymerization mechanism, causing further growth in size and complexity of polymer chains. Oxidation, however, 324
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
K
3
0 I
300
W
zI(3
5 200 0
0.P.N Figure 2. Effect of advancing degree of polymerization on drying time of model linoleic alkyd with lead-cobalt-manganese drier Drying at 2 2 ’ C. from 4-mil cast fllm as reported b y Bobalek et a/. ( 1 )
70
TERMINATION (6) 2 R . ( 7 ) Re
60
-
+
+ ROO.
(8) 2 ROO.
Figure 3. Oxygen absorption as a function of time and number average degree of polymerization at 22OC. linoleic alkyd with lead-cobalt-manganese drier
Contrary to expect ation, increasing the molecular weight increased rather than decreased the initial rate of oxygen consumption, even a t high molecular weight, where the skindrying barrier effect is most likely to occur. This suggests that retarded diffusion of oxygen into the drying film a t early levels of oxidation is not the rate-controlling step, as might be expected. However, after approximately 150 hours a t 22’ C. all the curves of Figure 3 level off at the same rate of oxygen uptake, indicating diffusion control a t advanced levels of surface dry. Higher initial degrees of polymerization would restrict mobility of molecular chains with a resulting limitation on the rate of the bimolecular termination step of oxidation. Initiation and propagation steps would be affected to a lesser degree based on the following reaction sequence:
-
+ C O + ~ RO. + C O O H + ~ R O O H + C O O H + ~ ROz. + HzO +
INITIATION (1) ROOH (2)
+
c o +z
(3) RO. f RH PROPAGATION (4) R .
+
(5) R O O .
0 2
-C
ROH
+ R.
+ ROO.
+ RH + ROOH + R *
R-R
-+
+ ROOR
stable products
If diffusion of oxygen is not a rate-controlling process (and Figure 3 indicates that it is not during the initial stage of oxidation), increased molecular weight could affect the propagation rate only in step 5, where the rate should be dependent on the chain segment mobility. With the great abundance of “RH” groups on adjacent chains, it may be assumed that the segmental movement of ROO. groups ensures contact, which is independent of whole chain mobility. Segment mobility, and thus propagation rate, is relatively independent of molecular weight. Steps 1 and 2 of initiation are dependent upon the mobility and concentration of ROOH groups and metal ions, and while chain mobility of the ROOH groups is lowered as molecular weight increases, as long as metal ion concentration is high compared to ROOH concentration, initiation will be relatively independent of molecular weight. While only one ROOH is involved in each step of the metal-catalyzed peroxide decomposition, two radicals must get together for termination to occur, requiring movement of whole molecules, which is dependent on molecular weight. A decreased rate of termination with a lesser effect upon initiation a t higher degrees of polymerization would result in longer kinetic chains and increased absorption of oxygen as the propagation reactions repeat their cycle a greater number of times for each initiation. As indicated in Figure 3, the increase in drying rate as molecular weight increases is not simply a physical phenomenon The presence of a gel phase promotes an increased rate of oxygen absorption. Thus as molecular weight is increased, the drying process is speeded by decreasing the distance on the extent of reaction coordinate which must be transversed before gelation is completed, and by increasing the rate of molecular growth via the oxidative crosslinking mechanism. The latter event occurs because gelation retards termination more than the initiation and propagation steps of the free-radical chain reaction. Diffusional limitation of oxygen penetration does not overcome these effects of increasing molecular weight or molecular complexity u p to approximately 150 hours in atmospheres of pure oxygen a t 22’ C.
literature Cited
(1) Bobalek, E. G., Moore, E. R., Levy, S. S., Lee, C. C., J . Appl. Polymer Sci.8 , 625 (1964). ( 2 ) Moore, E. R., “Significance of the Gel Point to Design of Alkyd Polymers,” Ph. D. thesis, Case Institute of Technology, 1962. (3) Shelton, J. R., McDonel, E. T., J. Appl. Polymer Sci. 1, 336-9 (1959). RECEIVED for review December 13, 1965 ACCEPTED July 26, 1966
Division of Organic Coatings and Plastics Chemistry, 141st Meeting, ACS, Washington, D. C., March 1962.
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