138
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 138-144
Environmental Aging of Epoxy Resins: Synergistic Effect of Sorbed Moisture, Temperature, and Applied Stress Antonio Aplcella’ and Luigl Nlcdais Istltuto di Principi di Ingegneda Chimlca, Unhrersh ’ @Ii Studi di Napoli, 80 125 Napks, Ita&
The exposure to a humid environment at relathrely high temperatues produces a reduction of the physical properties of the epoxies, due both to a moisture-induced plasticization and microcavlties formation. Equilibrium moisture sorption levels are found to be represented, for the same polymer and in the same temperature and humidity conditions, both by linear and upward isotherms depending upon the humidity history to which the system has been subjected. Such humidity history dependence is progressively lost as the temperature of the experlment is decreased. Sorption data are analyzed in the light of a transport model in which part of diffusing molecules are completely immobilized in the newly formed microcavities. The nature of the hypothesized damaging process is in agreement with the diffusion coefficient depression and with the solubllity increase experimentally found and theoretically predicted by a previously presented model. A crazing criterion has been used qualitatively to test the nature of the damaging process invoived in the synergistic effect of the applied stresses, moisture, sorption, and temperature. Some aspects further indicate crazing as the main phenomenon responsible for the damaging process.
Introduction Fiber reinforced plastics are being increasingly utilized for structural applications where their long-term properties are of primary importance. As a result, the problem of environmental aging on mechanical performance is attracting a great deal of attention. In the case of epoxy composites it has been shown that their elevated mechanical properties are strongly affected by moisture absorption from high humidity environments (1-9). This effect, especially at higher temperatures, has been associated with moisture induced plasticization and/or micromechanical damaging (63. Sorbed moisture acta both as a plasticizer and crazing agent for the epoxies. The physical and mechanical integrity of the epoxies is deteriorated by certain thermal-moisture exposure combinations, but while the plasticization is a reversible phenomenon the microcavitation is not recoverable. The damaging process, governed by the synergistic effect of sorbed moisture and temperature, is particularly evident on water solubility for which additional weight gains are if samples are exposed to cycling condiobserved (1,3,6) tions of environment and temperature (thermal spikes). This additional weight gain has been attributed to moisture entrapment during microcracking of the resin, since glass transition temperature changes were not observed (6).Often, when the diffusing species have high affinity with the polymer, the sorption is coupled with molecular relaxation and crazing (10-16) wherein morphological modifications of the polymer are involved. Anomalous sorption behavior has been reported for numerous polymer-diluent systems (13,17~9-22).In the case of the thermoplastic polymers, a sharp advancing swollen front has been observed and both the propagation kinetics and morphologicalmodifcations due to the solvent have been extensively studied (1820-22). Low cross-linked polymers have shown almost similar behavior (15);however, the cross-linked structure of these resins reduces the amount of relaxation due to solvent sorption,which namely may take place in the regions with low cross-linking density (4,23-25). Moreover, for moisture and water sorption, localized cavitation has been also associated with the tendency of water to form clusters (12,25). In recent years the interest has been focused primarily on the effects of moisture sorption on the properties of 0 196-432 1181I 1220-01 38$0 1.OO/O
thermosetting resins during “thermal spikes” in real-life simulation testa (1,2,5,6,26,27).The consequent reduction of ultimate properties has been associated both with irreversible damage (microcrackformation) and with reversible damage (water plasticization) (6). The formed microcavities may trap additional moisture without modifying the total amount of water actually dissolved in the bulk material. Equilibrium weight gains has been, indeed, o b served (10) to be progressively affected by microcavity formation as the temperature was increased. A history-dependent solubility model has been previously presented in a generalized form in accordance with the Dual Mode Sorption Theory (28) to take into account the hygrothermal history dependence of the effective diffusion coefficients. Furthermore, the hypothesized morphological changes in the form of microcavities have been correlated to the diffusion parameters. Sorption kinetics of water and moisture in epoxy resina were, $ fact, analyzed (29) in the light of a complete immobilization model for the trapped species. Equilibrium moisture sorptions, usually reported to be reresented by phenomenological power law functions of the relative humidity with exponents ranging from 1 to 2 or higher values (1,3#,31,27), were also interpreted in the light of the proposed model. On the other hand, since crazing has been associated in glassy polymer with the initiation of brittle fracture, the knowledge of the nature of the morphological changes occurring in “real life” test conditions is of great interest in the life assessment of composites using epoxies 88 matrices. The role of sorbed organic solvents in craze and crack initiation has been pointed out by several authors (32,33).Surface energy reduction and plasticization are mainly involved to explain void nucleation and growth under the action of a tensile stress. The influence of an applied stress is here considered when coupled with the exposure to a humid environment in severe conditions of temperature. A general crazing criterion (34,351 has been, indeed, used to discuss the nature of the damage induced in an epoxy when exposed in a humid environment under different conditions of temperature and applied stresses. Experimental Section Materials. Specimens were prepared from bisphenol A diglycidyl ether (DGEBA) Epikote 828 (kindly supplied 0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 139 Table I. The Effect of Temperature and Prior History on the Apparent Sorption Equilibria (I 0) %, samples Si4s,%, samples calcda water test temp, "C si, % first equil at 75 "C first equil at 4 5 "C excess, % 23 45 75 a
S a 3= 3.92 S45= 3.90 S75= 4.12
Sa',,, = 4.86 S4s,5= 4.60 S7s,, = 4.12
SaJ4,= 4.15 S454,= 3.90
calcd dissolved water, Sd, % 3.92 3.63 3.18
0 0.27 0.94
___
Referred to 23 "C.
by Shell It.) using commercial triethylene-tetramine (TETA) (Montedison Spa) as curing agent. Distilled water was used in the sorption experiments. Dissolved gases were removed by repeated freeze-thaw cycles, under vacuum, using liquid nitrogen as refrigerant. The epoxy samples were prepared following the same procedures previously described (IO). Sorption Kinetics Experiments. Moisture sorption kinetics and apparent equilibria were determined by means of a McBain (36) quartz, helical spring microbalanceserved by a standard vacuum system. The quartz springs with a 0.50 mg/mm sensitivity were obtained from the Ruska Corporation, Worden Quartz Products Division, Houston, Texas. The sample temperature was maintained constant by circulating thermostated water through a water jacket surrounding the sorption cell. Different activities have been maintained in the system by imposing and controlling, by means of a mercury differential manometer, different pressures of water vapors. Gravimetric liquid sorption measurements were performed by weighting 3.0 X 3.0 X 0.05 cm3 samples repeatedly on a Galileo analytical balance following immersion in water maintained at constant temperature. The samples were removed from the water, blotted, placed in a weighing bottle, weighed, and finally replaced in the constant temperature water bath. Sorption data are indicated as C (percentage of weight gain referred to the dry weight) and plotted as a function of N2/1, where 1 is the thickness of the samples ranging from 0.2 to 0.4 mm for vapor sorption and from 0.4to 0.6 for liquid sorptions. Liquid sorptions on specimens subjected to a different "stress history" have been performed by suspending in thermostated water dumbell-shaped samples which were kept in tension by using a dead load. Unloaded reference samples were also equilibrated in the same pool. When the reference samples had reached water saturation, the central portion of the loaded dumbell was cut, weighed in the wet state, and dried under vacuum at the same temperature of the sorption test. Liquid water resorptions were then followed by gravimetric measurements on the previously loaded and unloaded samples. Environmental Aging Dependence on Temperature History. A reproducible and well-defined influence of thermal history in presence of sorbed moisture has been experimentally determined in a previous work (IO) for the water sorption in epoxy resins. It was observed that the saturation levels were determined by the higher temperature of the thermal cycle. The differences in solubilities of samples with different hygrothermal histories were explained in terms of hypothesized induced microcavities that can be formed by solvent crazing in the plasticized system exposed to high temperatures. The dependence of microvoiding on the temperature was in fact experimentally found to be progressively more relevant at higher temperatures where lower energies for the craze formation are required (33). Figure 1 shows the liquid water sorption behaviour of a sample equilibrated at 75 "C and then brought back to a lower temperature (23 "C). The sample previously satu-
I
10
20
e-I.10:
0
4-5"
10
20
I
Figure 1. Moisture uptake kinetics at 75 "C (left side) and 23 "C (right side) for liquid water sorption in epoxy resina: PY5, apparent solubility at 23 "C of a sample previously exposed to 75 "C;P, apparent solubility at 23 "C of a sample experienced only at 23 "C. Table 11. Plasticization Effect of the Sorbed Water on the Glass Transition Temperature of the Epoxy Resina ( I 0 ) glass trans from Clash & samples si, % sd, % Berg, T,, "C _ _ _ 135 dry saturated S T 5= 4.12 sa" = 3.18 108 at 75 "C S45= 3.90 = 3.63 91 saturated at 4 5 "C
rated at 75 "C sorbs additional water even if the saturation level, achieved in a test carried out exclusively at 23 OC (fullline on the right side of Figure l),is apparently lower (dotted line on the right side of Figure 1). The difference between the two asymptotes has been considered to be the excess water trapped in the formed voids. The results of such an analysis (IO) are summarized in Table I. The actually dissolved water Sd was found to be a decreasing function of temperature. Lower glass transition temperatures were, indeed, found from torsional stiffness testa for samplea saturated at lower temperatures (Table 11). Dependence on Relative Humidity Histories. Isothermal sorptions have been carried out on the same sample at progressively higher humidities (Figure 2a). Once the sample was conditioned at the highest humidity, a second set of sorptions was performed (Figure 2b). In Figure 2 polymer weight gains have been plotted as a function of square root of time normalized to the sample thickness, 1 for tests performed at 60 "C. Numbers on the curves refer to the activity of water at which the specific test has been run. The activity is defined as the ratio between the water vapor pressure in the sorption cell and the saturated water pressure at the temperature of the experiment. In Figure 3a equilibrium solubilities at different water activities obtained from the asymptotic values of the sorption curves of Figure 2, are reported. Open circles refer to the progressive moisture equilibrium weight gains which have been reached in the first set of sorptions. The isotherm is celarly nonlinear (upward) showing a positive deviation from linearity at activities higher than 0.50. Once
140
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
7:60 C
>
1
/
411 .?o’,
A‘=,,,‘
Figure 2. Moisture uptake kinetics at 60 O C from environments at progressively higher humidity (I); second set of sorption (II)kinetics at 60 “C of samples previously exposed to a = 0.99.
the maximum value of the activity was experienced by the sample, i.e., a = 0.99, the equilibrium moisture weight gains were linear with the activity (full circles). The difference in the sorption behavior in the same environmental conditions may be associated with progressive damage produced in the material equilibrated at increasingly higher moisture contents. If maximum water content has been reached and temperature is held constant, no additional damage can be induced in the resin, and the polymer water system behaves linearly since the internal state of the material is not changing during the experiment. In thiscase, an apparent Henry’s law constant can be defined as the slope of the fiial linear isotherm or the secant of the upward isotherm at the specific activity for samples conditioned at humidities near saturation or at lower values, respectively. For a fixed internal state of the polymer, sorption becomes a reversible phenomenon; the equilibrium values of the solubility in a third set of sorption experiments, full triangles in Figure 3a, show, inf act, good agreement with previous data. The isotherms relative to experiments performed by using the same procedures at temperatures both higher (75 “C) and lower (45 and 30 OC) than the previous one are reported in Figure 3b, c, and d. The damage induced in the resin at low relative humidities seems to be irrelevant for each investigated temperature since the nonlinear character of the sorption isotherms always becomes evident only at activities higher than 0.50 (see Figure 3). The sorption parameters obtained on the “as cast” polymer at low activities will be referred to in the following as relative to the undamaged state. The overall apparent Henry’s law constants of samples equilibrated at high humidities are greater than those obtained
on samples in the undamaged state (initial slope of the upward isotherms). Furthermore, the differences between the linear and upward isotherms are less pronounced at low temperatures (Figure 3). In fact, as previously discussed for the temperature dependence of the damage, a low-temperature environment is inducing lower damage than a higher, even in the same conditions of moisture content. Sorbed moisture per se is not effective in producing any microcavitation in the resin but, as already pointed out in the literature (1,6),the synergistic effect of moisture and temperature is really effective in the damaging process. The overall apparent Henry’s law constants KT are plotted in a van’t Hoff diagram in Figure 4 (full circles). In the same figure, the apparent Henry’s law constantsfor the undamaged resins, obtained from the initial slope of the upward isotherms, have been reported as open circles. Specimens equilibrated at high humidities and different temperatures are not expected to show apparent Henry’s law constants that can be correlated by straight lines in the van’t Hoff diagram (fullcircles in Figure 4), since they are characterized by different degrees of microvoiding. Conversely, a straight line fits the solubility constants for the resins referred as “undamaged” and those obtained at low temperatures (also at 20 and 2 “C from liquid water sorptions), confirming the expectation that the upward shape of the isotherms is due to the hygrothermal history dependence of the induced damage. Linear sorption isotherms with higher apparent Henry’s law constants are obtained at lower temperatures from samples which have been exposed to more severe environment conditions (higher temperatures and/or humidities). The presence of microcavitiea in the damaged specimens has been related to the effective diffusion coefficients by means of arguments taken from the Dual Mode Sorption Theory (28)using history-dependent hole saturation constants for the Languimir population (29). For sorption of gases and vapors in glassy polymers the Dual Mode Sorption Theory has been successfully developed to correlate the presence of hypothesized “preexisting holes” or “free volume elements” frozen in the glassy state to characteristic anomalous sorption behavior. Michaels et al. (37) discussed its effect on the diffusion process and Vieth and Sladek (38) elaborated the theoretical aspects of the problem in the case of complete immobilization of the species adsorbed in the “holes”. If changes of the apparent water solubilities are attributed to a microcavitational damage (of the dimensions of the nucleating crazes (32)),the increase of the number of sites in which diffusing molecules may be trapped and immobilized should be evident in a reduction of the effective diffusion coefficient providing an increase of the Languimirian capacity (28). However, it could be questioned that the linear character of the previously described sorption isotherms seems to be in contrast with the typical concave dual mode shape. The presence of “preexisting” or newly introduced finite microvoid capacity would, in fact, produce a concave isotherm shape as such capacity becomes saturated. Nevertheless, the initial linear character of the dual mode isotherm could be maintained, in our case, over the entire range of studied relative humidities, if cavity saturation is achieved in the vicinity of the upper limit of the investigated pressures where water condensation occurs. Penetrant diffusion has been described (28,37)to follow ordinary diffusion laws also in systems showing two modes of sorption in the range of pressures where linear isotherms are observed (very low or very high pressures). At low
Ind: Eng. Chem. Prod. Res. Dev., Vd. 20, No. 1, 1981 141
3
a
a
Figure 3. Equilibrium moisture uptakes w.external activity: a, T = 60 "C;( 0 )f i t sorption at progressively higher activities, (0)second sorption,after exposure to a = 0.99, (A)third sorption; b, T = 75 "C; (0)first sorption, (0)second sorption; c, T = 45 O C ; (0)first sorption, (0)second sorption; d, T = 30 "C,(0)f i t sorption, (0)second sorption.
pressures an effective diffusion coefficient is used, which is related to the real sorption parameters by (37)
De€f Drealkd/KT
(1)
where Dredis the diffusion coefficient of the dissolved molecules, k d is the Henry's law constant for the dissolved population, and KT is the overall apparent solubility constant. Solubility increases and effective diffusion coefficient depressions in damaged systems may be correlated by means of eq 1 as
Dlefi/D1lefi
K"T
/ K'T
(2)
where D&'s are the effective diffusion coefficients and KT's are the apparent solubility constants for two samples of different histories. The primes and double primes indicate tests performed in the same environmental conditions but on samples with a higher ('1 and lower ("1 damage level, respectively. Sorption kinetics, obtained in the same external conditions (a = 0.60 and T = 75 "C),on an "as cast" sample and on a previously exposed to high humidity sample, for which no additional morphological changes are expected, are compared in Figure 5. For a high moisture uptake (I3 > A) a lower diffusion coefficient is, in fact, observed (ta > t3. The effective diffusion coefficient depression, LYIL?",
and the apparent solubility increases, K"T/ K'T, experimentally found (29)in teats performed at different external activities (from 0.20 to 0.99) and temperatures (from 30 to 75 "C)are reported in Figure 6 as open circles and are compared with eq 2, full line. Diffusion coefficients for damaged (2' = 60 "C,a = 0.99) and undamaged resins have been cross-plotted with solubility data in Figure 7 as a function of the reciprocal of the temperature. As for the solubility data,open triangels in Figure 7,the diffusion coefficients,calculatedfrom testa performed both at low temperatures (2 and 20 "C)in liquid water and high temperatures at low activities (referred to as undamaged), are fitted by a straight line in the Arrhenius plot (open circles in Figure 7). For the damaged samples lower diffusion coefficients (full circlea) and higher solubilities (full triangles) than in the undamaged state have been found in the range of temperatures studied. An activation energy for the diffusion procw of about 13.5 kcalfmol has been calculated both for damaged and undamaged samples. The correspondingenthalpy of sorption for both materials has been found to be of -11.0 kcal/mol. The nature of the hypothesized damaging process is in agreement with the diffusion coefficient depression experimentally found and theoretically predicted by an analysis (29) based on the Dual Mode Sorption transport model for completely immobilized trapped species (28).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
142
1.a
r” E E
.9
a m 0
D
:
\
.e
0
-> 0
0
m
L
.7
e’
/
0 initially und.mmgmd
3
trmpecmturm
I
i
I
.6
0 dmmmg4datte.t
K,
K /:
Figure 6. Diffusion coefficients reduction Dr/D” vs. apparent solubility increasw K N ~ / Kfrom ’ ~ sorption tests performed in the same environmental conditions but on samples with different previous hypothermal history.
6
I
I
I
1
I
3.4
3.2
3.O
3.6
3
1 ,’T. TO’, ‘ K
,
1
80
1 0
I
I
40 T. ‘c
60
20
Figure 4. Solubility contants of linear isotherms vs. reciprocal of the temperature.
m
8
m
f
ix”
M
,
, =\,
I
!,;
, 0
,
, 6
,
, E
,
,
,
0’
10
Figure 5. Effect of previous humidity history on equilibrium uptakes (E > A) and effective diffusion coefficients (Db < Da).
In conclusion, sorbed moisture may induce a different degree of irreversible damage in the epoxy resin depending upon the temperature and humidity levels imposed on the sample. For low values of the temperature (2’ < 20 “C) or for low relative humidities (R.H. < 0.601,subsequent sorptions are not affected by the previous temperature and humidity histories (29). Applied Stresses and Damaging Process in Presence of Sorbed Moisture. Epoxies have been described to undergo solvent crazing in the presence of sorbed water (4,IO). Crazing is a process of plastic deformation in tensile stress direction without lateral contraction involving significant cavitation and localized fibrillation (32). Recognition that crazing requires void formation led to the suggestion that a dilatational component of stress must be involved. Sternstein and Ogechin concluded that the proper criterion for crazing should contain both the dilational and flow stress C‘b
= A(T) + B(T)/I,
(3)
I
I
3.0
I
I
38
I
- . ‘K.. ’
1
I
0’
3.4
I/ i . l O ’ .
1
a0
,
I
80
40 T,’c
e0
I 0
Figure 7. Cross-plot diffusion coefficients ( D ) and apparent aoluw. the reciprocal of temperature for undamaged sample bilitiea (KT) (0,A)and damaged at 60 OC and a = 0.99 (.,A).
where bb, the flow stress, is the scalar difference of the principal stresses, Il is the first stress invariant, defined as ul + u2 + c3,and A(T) and B ( T ) are temperature-dependent constants (34). In ordinary conditions of temperature and under uniaxial tension, the term B / I l is twice as large as A, which illustrates the primary role of dilation in the crazing process. The same authors find the shear envelope to be consistent with a three-dimensional analogue of the Mohr-Coulomb criterion and illustrate the secondary role of dilation in shear yielding. Crazing, then,
Id.Erg. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 145 Table 111. Apparent Solubility Differences between Loaded and Unloaded Samples Exposed to Liquid Water a t T = 40 “C water uptake, c, g of solv/ applied stress, 100 g dry U. kdmm* Polymer 0 3.89 0.30 4.54 ~
AC. % 0 16.3
cannot occur unless a finite hydrostatic tensile component of the stress exists, while shear yielding may occur also under hydrostatic compression conditions. The stress bias (i.e., the stress vector with magnitude equal to the major shear stress but with direction of the major principal stress) may then be viewed as the driving force for the fibrillation and orientation, whereas the first stress is invariant as the cavitational driving force. This suggests that as the stress field becomes nondilational the cavitation process is the limiting factor and crazing becomes difficult to initiate (35). The above criterion has been qualitatively used to confirm the nature of the damaging process associated with moisture sorption. In fact, if craze formation is associated to the changes of the apparent solubilities of samples subjected to different hygrothermal histories, the application of an external stress field should influence the sorption behavior of water in the resin. Sorbed solventinduced osmotic stresses (39)or differential swelling strains in regions of different cross-linking densities (423) may produce the “internal” stress field responsible for the craze nucleation. The two major hypotheses for the action of sorbed agents in craze and crack initiation, on the other hand, have been reported as surface energy reduction (significativewhen craze cavities are less than 100 A) and plasticization. Plasticization reduces the resistance to craze and crack initiation since the swelling agents reduce the viscosity of the glass. The application of an external stress field will further favor or decrease this tendency to craze, depending on the variations of the resulting first-stress invariant. For example, the application of a tensile stress will increase II and a more crazed and damaged material should be obtained, while the application of a hydrostatic compressive stress field, conversely, will decrease the tendency of the material to craze. Sorption experiments have been performed on samples subjected to different “stress histories” during the expoaure to the same environment. Liquid water sorption has been previously carried out at T = 40 O C on a uniaxially loaded dumbbell sample and on an unloaded reference sample. After a period three times longer than is usually needed to reach equilibrium, the dumbbell specimens were cut into rectangular shapes and weighed in the wet state. The dry weight was determined after a drying procedure carried out under vacuum at 40 “C.Weight gains for the subsequent resorption in liquid water of the two samples with the different stress histories are reported in Figure 8. The previously loaded and water penetrated sample clearly sorbs more water than the unloaded sample. In Table III are reported the applied stress, equilibrium moisture uptakes, and percent solubility increases for the loaded and unloaded samples. An increase of about 16% of the apparent solubilities has been found after the application of a stress that is only 7% of the yielding stress of saturated samples (40). Local yield in the form of craze may then occur since the applied stress is well inside the linear elastic region of the material (40). The application of a tensile stress in our glassy system results in an effective dilatation of the sample (Poisson ratio 0.3). The increase in
01 0
10
20
3
d2“ Figure 8. Liquid sorption behavior at 40 O C of specimens with different previous loading histories in presence of sorbed moisture. Reported sorption testa were performed when unloaded. JE/1.10:
“crazeability” is then expected as a consequence of the increase of the driving force for cavitation and is evident in an apparently higher solubility of water in the previously loaded sample which retains part of the dilatation imposed during the aging. In conclusion, more information on the nature of the damaging process occurring in the epoxy resins exposed to humid environments can be obtained by analyzing the effect of external stress fields on the sorption behavior. Further work on the subject is in progress.
Literature Cited (I) McKague, E. L., Jr.; Halklas, J. E.; Reynolds, J. D. J. Compar. Meter. 1975, 9 , 2. (2) W u n g ; Springer, 0. S. J. Compos. Meter. 1976, 70, 2. (3) Weltsmen, Y. J. Compos. Meter. 1976, 70, 193. (4) Magan, R. J.; ONsel, J. J. Meter. W . 1977, 72, 1966. (5) Ishal. 0.;Amon, U. ASTM STP 658, Vlnson, J. R., Ed.; American Society for Testing and Materials, 1978 pp 267-276. (6) Browkrg, C. E. pdym. €ng. Scl. 1976 78, 16. (7) L m , A. C.; Springer, 0. S. J. Compos. Meter. 1979, 73, 17. 18) . . Keenan. J. D.: Seferis. J. C.: Qulnilvan. J. F. J. ADD/. polvm. Sd. 1980, in press. (9) Ishal, 0.;Amon, U. J. rest. €vel. 1977, 5(4), 320. (IO) Aplcella, A.; Nlcoleis. L.; Asterlta, 0.;Drbli, E. Porymer, 1979, 20, 9, (11) Michaels, A. S.; Bixler, H. J.; Hopfenberg, H. B. J. App. folym. Scl. i.a- -a-,. ~.-Q, -Q- .I .. (12) P-ny, (3. A. P e m r 1976, 17, 690. (13) Le &and, P. 0.;Kambow, R. P.; Haaf, W. R. J . polym. Scl., Part A 1972. 2. 1565. (14) GwnLA.C.-pdLmer 1975, 76, 2. (15) Kwei, T. K.; Zupko, H. M. J. polvm Scl. Pae A2 1969, 7 , 876. (16) Berens. A. R.: HoDfenbera. H. B. &&mer. 1978. 79. 489. i17j Hopfenberg, H. B.; wieY, R. H.; siannet, v. i. ~ 0 , y m .~ n g .sci. 1969, 9 , 242. (18) Nlcolals, 1137 L.; Drbli, E.; Hopfenberg, H. B.; l”, D. polLmer1977, 78,
..
(19) Thomas, N.; Wlndle, A. H. polvmer1978, 79, 255. (20) Nlcdals, L.; [Mdi, E.; Hopfenberg, H. 8.; Cariceti, G. J. Membr. &/. 1978, 3, 231. (21) Nlcolals, L.; Drbll, E.; Hopfenberg, H. 8.; Aplcella, A. Po/ymr 1979, 20, 459. (22) Kambow, R. R.; Ramagosa E. E.; &umr. C. L Mecromolecu~1972, 5, 335. (23) Kreibich. U. T.; Schmid, R. J . Polym. Scl., Polym. Symp. No. 53 1975, 177. (24) Kenyon, A. S.; Nlelsen, L. F. J. M e c m l . Sci. Chem. A 1969, 3(2), 275. (25) Gordon, G. A. Poiymr 1977, 78, 958. (26) L m , A. C.; Springer, 0. S. J . Compos. Meter. 1979, 73, 17. (27) McKague, E. L., Jr., Reynolds. J. D.; Haklas, J. E. J . Appl. m m . SCI. 1978, 22, 1643. (28) Vieth, W. R.; Howell, J. M.; Hoselh, J. H. J. MMr. Scl. 1976, 7, 177. (29) Aplcella, A.; Nicolais, L.; Astarlta, 0.; M,E. Poiym. Eng. SA. 1980, in press. (30) Shen, C. H.; Springer, 0. S. J . Compos. Meter. 1976, 70, 36. (31) McKague, E. L.. Jr.; Reynolds. J. D.; Hakias, J. E. J . Eng. Meter. Techrol. Ser. H 1978, 48,92. (32) Kamboca, R. P. Macfomol. Rev. 1973. 7 , 1.
144
Ind. Eng. Chem. Prod, Res. Dev.
(33) Andrews, E. H.; Levy, G. M.; Wlllis, J. J. Meter. Sd. 1975, 8 , 1000. (34) . . Sternsteln. S. S.: Oaechln. L.: Sherman. A. ADD/. . . Po&”. . Svm~. 1966, 7 , 175. (35) Sternsteln, S. S.; Ogechin, L. polym. h p r . 1969, 10, 1117. (36) I l k s , K. J. Mekromol. Chem. 1969, 127, 1. (37) M(chaels, A. S.; Vieth, W. R.; Barrle, J A. J. Appl. Phys. 1969, 34,
~.
13.
(38) Vleth, W. R.; Sladek, K. J. J. CollOidScl. 1965, 20, 1014. (39) Sarti, G. C. Polymer 1979, 20, 827.
1981,20, 144-147
(40) Nicolels, L.; Drkll. E.; Aplcelle. A,; Albanese, 0. Interbn Sci, Rep. AFOSR TR 78-1429. 1978.
Received for review March 5, 1980 Accepted July 16, 1980
The work was sponsored by the U.S.Air Force Office of Scientific Research, Contract AFOSR-77-3369.
Kinetics and Mechanism in Formation of Sodium Insoluble Metaphosphate and Its Application as a Threshold Agent Chung Y. Shen Detergents and phosphetes D/visbn, Monsanto Indimtrlel Chemicals Company, St. Louis, Missouri 63166
In investigathrg thermal condensatbnreactions cowmonosodknnetto insoluble metaphosphetes (IMP) and various intermediates and side products, an amorphous intermedlate has been detected by electric resistance and X-ray measurements and characterized by b x c h a n g e chromatographic analyses. Kinetlcs and activation energies in producingvarious intermediates have been determined. To prevent formath of undesirable soluble byproducts, the rate of crystalllzeth of the desired IMP 8881715 to be the rate-controlling step. Conversion of IMP to soluble trimetaphosphate has been shown io be catalyzed by water vapor and cation impurities. IMP has a finite low solubinty in water making it ideal for water threshold treatment to prevent calcium carbonate and
sulfate scale formation.
Introduction Sodium insoluble metaphosphate (IMP), NaP03-II, or Maddrell salt, has been known for a long time (Van Wazer, 1958) and has been shown to be a chain polymer containing more than 1000 NaP03 units (Strauss and Day, 1959). Based on Stanford Research Institute’s Chemical Economics Handbook, IMP was produced at a rate of more than 9000 metric tons per year in 1973 for the dentifrice use alone. Several procedures for producing IMP were described in the patent literature (Taylor and Erdman, 1943; Kern and Heymer, 1969,1972),but little information is available about the difficulty to convert monosodium orthophosphate to IMP and about the many side-reaction products which are soluble and must be limited to a few percent in commercial products. The kinetics of the conversion appear to be complicated and are influenced by many impurities and water vapor in contact with the conversion material. The purposes of this work are to elucidate the conversion mechanism and to determine rates of conversion through each conversion step. The causes of formation of soluble salts will be described. Finally, a potential use of IMP as a threshold agent will be shown. Mechanism and Kinetics Experimental Section. Monosodium orthophosphate, NaH2P04or MSP, was obtained by recrystallization of commercial product from the Monsanto Co. until analyses shown by titration (Van Wazer, et al., 1954),wet chemical analysis and ion exchange (Kolloff, 1959), and paper chromatographic (Karl-Kroupa, 1953) analyses were essentially 100% pure (Shen et al., 1965). The crystals were ground to pass 100 U.S.standard mesh screen for studies. A 316 stainless steel box of 6.3 mm thickness was constructed to fit into the cavity of a Hoskins FD204C muffle Ol96-432l/8l/ 122O-O144$Ol.OO/O
furnace. This stainless steel box served to keep the temperature variation inside the entire box within 3 O C . Test samples of 10 g size contained in a porcelain crucible were suspended by a 0.08-mm stainless steel wire passing through a hole from the top of the furnace and the box, attached to an analytical balance so that the weight losses could be measured. A 0.08-mm chromel-alumel thermocouple was placed at the center of the test sample through the same hole to measure the temperature of the sample. The accuracy of the balance is A0.5 mg with the thermocouple in place. The furnace temperature was controlled by another chromel-alumel thermocouple placed inside the box but outside the crucible. The furnace temperature could be increased linearly with a temperature program controller from a thermogravimetric analytical instrument manufactured by Robert L. Stone Co., Austin, Texas. The heating rate mentioned in the paper is the rate of increasing the muffle furnace temperature with time. All other temperatures are temperatures of the converting test material. Two holes were provided at the ends of the box. Air with controlled humidity passed through two water-bubblers maintained at the appropriate temperature to give the desired water vapor pressure and was preheated from a copper coil between the box and the wall of the furnace. The preheated and humidified air entered from the rear end hole and left from the front end hole of the box. The air rate was controlled by a rotameter at 3 mL/min. The humidified air was used to study the effect of water vapor on converting rates. Mechanism and Rate of Conversion of MSP to IMP by Thermal Dehydration. Earlier publications indicated that MSP can be converted to acid pyrophosphate and 0 1981 American Chemical Society
