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Particle size measurements and properties of AVIAMIDE®-6 microcrystalline nylon
Particle Size Measurements and Properties of AVIAMIDE®-6 Microcrystalline Nylon F R A N K J. K A R A S I N S K I AND C H A R L E S F. F E R R A R O Central Research Department, FMC Corporation, Princeton, New Jersey 08540
Received December 10, 1970; accepted December 29, 1970 Aviamide 6 microcrystalline nyloi1 is a colloidal form of polyeaprolactam prepared from Nylon-6 by controlled hydrolysis of the amide linkages followed by appropriate mechanical action which frees the particles from the polymer matrix. The particles termed mieroerystals form stable dispersions in water over a wide range of concentration without the need of dispersing aids. The particle size distribution of Aviamide6 aqueous gel was measured by centrifugal and gravitational sedimentation techniques over the range of 0.1 to 59 it. The applicability of this technique was substantiated by electron microscopy studies. Depending on the grade, Aviamide-6 was found to contain as much as 90% by weight of the particles _
mineral silicates. These materials possess physical properties closely related to those of microcrystalline cellulose, because of the presence of colloidal particles common to all. Aviamide-6 microcrystalline nylon is a colloidal form of polycaprolactam as discrete particles prepared by the controlled hydrolysis of the amide linkages. B y appropriate mechanical action in an aqueous medium, colloidal microcrystals having a length to diameter ratio of approximately 1 are produced to form a stable gel. Much larger particles comprising aggregates of these also are in suspension. The particle size characterization of Aviamide-6 is important as a method of evaluating products for specific end-use applications. In this paper, the application of classical sedimentation techniques as a rapid and reproducible method to characterize the particle size of Aviamide-6 is described and the physical properties of several grades are compared. Journal of Colloid and Interface Science, ¥ol. 36, No. 2, J u n e 1971
195
196
KARASINSKI AND FERRARO EXPERIMENTAL
1. Materials. Three grades of Aviamide-6 microcrystalline nylon were prepared for this study: (1) Regular, (2) Fine, and (3) Superfine grades. Each grade forms stable gels in an aqueous medium and they are differentiated by their particle size distributions. 2. Particle Size Determination. Stokes (6)
derived the equation for the force of resistance to motion of a sphere suspended in a continuous medium. Hall (7) first applied Stokes' equation to the measurement of particle size. Although Stokes' frictional coefficient is applicable only to spherical particles, it has been used for particles the maximum to minimum diameter ratio of TABLE II EFFECT OF P E R
TABLE I
CENT
SOLIDS OF AVIAMIDE®-6
(ScFEnFINE) DISPERSION ON PARTICLE
EFFECT OF TIME OF DISPERSION OF AVL%MIDE®-6 (FINE) ON PARTICLE SIZE RESULTS Dispersion time (rain)
Wt % Aviamide-6 particles <1 ~
Wt % Aviamide-6 particles <0.2 /L
10 20 30
75.2 75.5 75.6
49.2 49.7 49.6
SIZE RESULTS % Solids Aviamide 6 Wt % Aviamide-6 dispersion particles ~1 ~
0.23 0.52 0.76 1.50
88.3 86.1 87.7 85.2
FIG. 1. Electron micrograph of AVIAMIDE®-6 (200 rex). Journal of Colloid and Interface ~cience, VoI. 36, No. 2, June 1971
Wt % Aviamide 6 particles _~0.2
92.2 85.3 85.8 83.5
AVIAMIDE-6 MICROCI~YSTALLINE NYLON
197
FIG. 2. Electron micrograph of ~0.1 t~ fraction of AVIAMIDE®-6 (50 rex). which does not exceed 4. Sedimentation techniques are shown to be valid for the particle size characterization of Aviamide-6 if the method is considered as a fractionation based on size rather than an absolute m e a s u r e m e n t of particle sizes. For sedimentation in a gravitational field, the following equation was used: t -
18 X 108@ d~(o~ - p2)g'
[1]
where t = time of settling in seconds; v = viscosity of water at 25°C (0.00894 poise) ; h -- total depth of settling (15 cm); d = diameter of particle in microns. Upon
substitution of a given value of d into Eq. [i], particles above this diameter settle out and particles equal to or below this diameter remain in suspension; pl = density of Aviamide-6 (1.14 g m / c c ) ; p2 = density of water (1.00 gm/cc) ; g = 980 cm/see 2, gravitational constant. For centrifugal sedimentation, the equation below was used: t =
18 X 10avh d2(pl -- p2)g (RCF) '
[2]
where R C F is the relative centrifugal force and h the total settling depth is equal to 8 era. An International Centrifuge Model Journal of Colloid and Interface Scie~we, Vol. 36, No. 2, June 1971
198
KARASINSKI AND FERRARO
FI~. 3. Electron micrograph of _<0.2 t~ fraction of AVIAMIDE®-6
H T with an eight place angle head (33 °) was used for the centrifugal sedimentation measurements, and the relationship between rotation of the head ( R P M ) and RCF is: R C F = 12.508 × 10-~ (RPM) 2.
[3]
Aviamide-6 mieroerystalline nylon gels are normally produced as high solids pastes (40 %-55 %). 1.5 % solids dispersions of Aviamide-6 were prepared for centrifugal sedimentation from the pastes by mixing for 10 min at high speed in a Waring Blendor. The powders in Table VII were also redispersed by this method (25 min mixing). Dispersion was complete under these conditions since longer times of mixing did not affect the results as shown in Table I. Journal of Colloid and Interface Science, ¥oi. 36, No. 2, J u n e 1971
(50 mx).
The effect of concentration of the dispersions analyzed for particle size determination is shown in Table II. The variation in the measured particle sizes for the latter three concentrations is only several per cent. A eoneentration of 1.5% solids was therefore chosen for all subsequent measurements since greater accuracy in weighing can be attained. The reproducibility at the 1.5% solids level was in the order of 1% or less. 1.5 % solids dispersions were also used for the gravitational sedimentation experiments on the Regular and Fine grades of Aviamide-6. For centrifugal sedimentation analysis of a sample, two 50-ml Stainless steel eentri:
AVIAMIDE-6 MICROCRYSTALLINE NYLON
!99
FIG. 4. Electron mierograph of --<0.5t~ fraction of AVIAMIDE®-6 (50 mx). fuge tubes were filled with 40 ml of the 1.5 % solids Aviamide-6 dispersion. T h e centrifuge is run for the appropriate amount of time (e.g., for determination of the weight per cent Aviamide-6 particles <_ 1 ~, the centrifuge is run for 20 min at 2500 R P M ) and allowed to come to a complete free stop so t h a t the sediment at the b o t t o m of the tubes is not disturbed. The effects of acceleration and deceleration of the centrifuge were neglected for the purposes of this study. T e n milliliters of the top fraction from each of the two centrifuge tubes are combined in a tared weighing dish. The percentage solids is determined b y complete removal of the aqueous phase (6 hours at 90°C). The weight per cent Aviamide-6 less than or equal to the calculated diameter is then determined.
The same dispersion and analytical procedures were used for gravitational sedimentation analyses. 3. Electron Microscopy. T h e electron micrographs of Aviamide-6 were prepared using a Philips EM-300 electron microscope at magnifications from 10,000 to 200,000 X. Dilute suspensions were evaporated on a collodion fihn supported by a 200 mesh copper screen and air dried. The screen was then placed in a high v a c u u m evaporator and P t shadowed at a 30 ° angle. 4. Viscometry. Viscosity measurements of aqueous gels of Aviamide-6 were carried out at 25°C using the H a a k e Rotovisco, a rotational viscometer. The M V I I Couette system was used. The ratio of diameters of cup to cylindrical bob is 41.5 m m to 36.8 m m (1.128). The width of the gap is 2.35 Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
200
KARASINSKI AND FERRARO
FIG. 5. Electron micrograph of _< 1 # fraction of AVIAMIDE®-6
mm. The length of the bob is 60 mm. The viseometer has a basic drive system covering a range of 3.6-583.2 rpm in ten steps. Apparent viscosity as a function of shear rate was calculated at various rpm utilizing the appropriate constants supplied with the instrument. 5. Electrophoretic Mobility. Electrophoretie mobilities of dispersions at various pH values were measured at 25°C with a Zeta Meter (Zeta Meter Inc., New York). The dispersions of 0.001% concentrations were prepared from 0.1 weight % stock solutions by mixing for 5 rain. Separate dispersions were prepared for each p i t adjustment. The pH of the dispersions were adjusted with dilute NH4OH and H3PO4. The standard electrophoresis cell supplied Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
(50 mx).
with the instrument was used for the measurements. A fixed de voltage of 200 volts was maintained across the platinum electrodes. 6. Surface Area. The surface area of Aviamide-6 powders prepared by various methods was measured on a Perkin-Elmer Sorptometer, Model 212 D. RESULTS AND DISCUSSION The properties of microcrystalline nylon-6 are related to its small particle size. During its manufacture, the degree of attrition utilized following the hydrolysis step of the process determines the particle size distribution of the final product. This, in turn, controls the functional properties required for specific end-use applications. Although
AVIAMIDE-6
MICROCRYSTALLINE
NYLON
201
Fro. 6. Electron micrograph of <5 ~ fraction of AVIAMIDE®-6 (10 mx). many methods are available for the determination of particle sizes, sedimentation techniques, especially centrifugal, offer a rapid and accurate method when applied to Aviamide mieroerystalline nylon-6. The measurement of several points by a rapid technique (e.g., wt % _< 1 and 0.2 ~) provides useful data for characterizing the product.
1. Applicability of Centrifugal Sedi~nentation Technique. The smallest particle diameter of Aviamide-6 appears to be in the region of 50-100 A. A few of these ultimate particles can be seen in the electron micrograph 200,000X in Fig. 1. It is also observed that the particles are approximately spherically shaped. The particles are composed of aggregates of moteeu!es which are joined by strong lateral bonding forces so as to form
discrete colloidal particles (8). In aqueous medium, the particles are in a solvated state and tend to flatten on air drying. This increase in diameter, however, is counteracted by hydrogen bond forces as the water phase leaves. Figures 2-6 are electron micrographs of the particles in diluted top fractions of Aviamide-6 dispersions after centrifugal sedimentation for various specified diameters. The maximum diameter of the particles in each does not exceed the calculated diameter programmed for the centrifugal sedimentation test. Therefore, the use of Eq. [2] appears to be essentially valid for the determination of the weight per cent undersize. Replication of the electron microscopy studies has shown Eq. [2] to be valid down to approximately 0.1 t* Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
202
KARASINSKI AND FER~ARO
particle diameter and up to a maximum value of about 15 ~. The method measures weight per cent undersize both of the ultimate particle or of aggregates of two or more which have not been separated during the mechanical attrition step. 2. Particle Size Distribution. The cumulative particle size distributions of three grades of microcrystalline nylon-6 are shown in Table III and Figure 7. The points above 15 , particle diameter were determined by gravitational sedimentalion. It is apparent that the particle size distribution can be controlled from narrow to broad depending on the degree of mechanical attrition utilized in the manufacture of the grade.
A comparison of the weight per cent of mierocrystalline nylon-6 particles less than 1 and 0.2 ~ has proven to be very informative. The measurement of these two values by centrifugal sedimentation can serve as useful data for the rapid characterization of changes made during the preparation of microerystalline nylon-6. As an example, the effect of milling of two samples of Aviamide-6 using a Premier Colloid Mill is shown in Table IV. The maximum buildup of colloidal particles is seen to occur after two passes for Sample 1. Milling can, therefore, be discontinued at this point. For Sample 2, however, four passes are necessary to achieve this purpose. 3. Dilution Effects. Aviamide nylon-6 gels are prepared as high solids pastes (40 %55 % solids). For some applications such as eleetroeoating, it is necessary to dilute these gels to low solids dispersions with a minimum of particle-to-particle association. This has been found especially important in forming thin films (0.1 rail or less) on metal substrates. For the purpose of dilution, the use of low shear mixing equipment is more desirable than high shear. The effect of the type of mixing equipment was characterized by the measurement of the weight per cent particles less than 1 and 0.2 after the direct dilution to 1.5 % solids of high solids gels by both types of equipment. Table V contains the results of this study. The 40 % solids gel is completely dispersible to low solids by a low shear method. This
TABLE III PARTICLE SIZE DISTRIBUTION OF THREE GRADES OF AVIAMIDE-6 Wt % <
Regular
0.2 0.5 1.0 5.0 11.0 15.0 25.0 35.0 50.0 60.0 75.0 100.0
17.0 28.7 36.0 56.4 66.4 72.4 78.3 86.8 91.4 95.4 97.4 98.7
Fine
Superfine
49.5 68.0 75.5 88.4 98.6 99.3 100.0
86.0 88.4 90.0 95.4 99.3 .100.0
I00
o
~9o ~8o
o
o
o
o
uJ 70 -~ 6O >.50 m o
.~c
SUPERFINE FINE
.J
I0 I
I
I
I
I I III
I
I
I
I
1.0 SIZE,
I
I I II IO
I
MICRONS
FIG. 7. P a r t i c l e size d i s t r i b u t i o ~ of AVIAMIDE®-6. Journal of Colloid and Interface Noienze, Vol. 36, No. 2, June 1971
]
I
I
I
I
]1
]00
AVIAMIDE-6 MICROCI~XSTALLINE NYLON TABLE IV PASSES T H R O U G H
EFFECT OF N U M B E R OF ON THE W E I G H T % No. of passes
--
t
n i ITTII
i
I~--L
~ n It
A MILL
OF PARTICLES _<1 U
Sample 1
Sample 2
71.9 81.7 80.4 81.0 81.5
81.4 81.4 86.4 88.8 88.9
1 2 3 4 5
n
203 I
n
1 n I ~
A- SUPERFINE IO,O00
TABLE V EFFECT
OF METHOD OF DILUTION OF 40070 AND 5 0 % SOLIDS MICROCRYSTALLINE N Y L O N - 6 G E L S (SUPERFINE GRADE) ON THE FRACTION OF SUBMICRON PARTICLES
% Solids gel
50
Method of dilution
4
6
72.0 89.6 91.8 90.1
2
4
S
I00
S
4
6
IO00
FIG. 8. Effect of particle size distribution oE the apparent viscosity of microerystalline Nylon-6. TABLE VII EFFECTS
60.0 86.7 84.4 86.8
I0
RATE OF SHEAR(sec-I]
Wt % particles Wt % particles ~ 1u <0.2 #
Low shear High shear Low shear High shear
40
[00
OF DRYING
OF AVIAMIDE
~YLON-6
GELS
ON P A g T I C L E SIZE AND SURF.~CE A R E A ! Surface
Sample •
(mS/gin)
I--
VI
TABLE APPARENT VISCOSITY 8%
VERSUS
SOLIDS SUPERFINE
SHEAR RATE
AND F I N E
FOR
~RADES
OF AVIAMIDE NYLON-6 Shear rate (sec-1)
3.2 6.5 9.8 19.6 29.4 58.8 88.2 176.0 265.0 529.0
Apparent viscosity (cps) Superfine
20520 11010 6775 3950 2760 1380 920 520 380 210
Apparent viscosity (cps) Fine
2540 1180 830 380 260 125 69 35 42 24
is not the case for the 50% solids. On the basis of this study, it was concluded that product of no more than 40% solids can be conveniently "let-down" using low shear mixers such as a Lightning Mixer.
4. Rheological Properties of Aviamide Nylon-6. The theological properties of microcrystalline gel systems have been previously reported (9). These are dependent on their particle size distribution and in particular on the fraction of particles less than 1 u in the maximum dimension. Gen-
1. Aqueous gel spraydried powder 2. Aqueous gel freezedried powder 3. Aqueous gel alcohol d e h y d r a t e d and v a c u u m dried
1.2
89 56 89 78
86 40 86 64
-0.9 127
erally, the viscosity of the gel increases as the fraction of submicron colloidal particles increases. The effect of particle size distribution on the apparent viscosity of 8% microcrystalline nylon-6 gels is shown in Table VI and Fig. 8 for Superfine and Fine grades. This plot shows that the rheology of these pseudoplastic systems obeys the power law (10). The particle size distributions are shown in Fig. 7. The Regular grade is not included since the fraction of submicron colloidal particles is not sufficient for gelation at 8 % solids. The Superfine product has approximately ten times the viscosity of the Fine grade. Both grades show highly dependent shear thinning behavior. 5. Effects of Drying. After the removal of water from Aviamide gels, hydrogen bonding causes aggregation of the colloidal particles. The degree of bonding and, Journal of Colloid and Interface Science, Vol. 36, No. 2, June 1971
204
KARASINSKI AND FERRAR0 TABLE VIII ELECTROPHORETICMOBILITY (EM) VEnSVS PH OF THREE GRADES OF AVIAMIDE Regular
Fine
Superfine
pH
EM (,ucm se¢-x volt-l)
pit
EM (,acre sec-1 volt-x)
pH
EM (#cmse( "x volt'- 0
3.00 4.00 4.90 5.50 6.00 6.52 7.10 8.10 8.50 9.00 10.00 10.50 10.70
+0.63 +1.50 +1.75 +2.05 +1.90 +1.95 +2.00 +1.80 +1.00 +0.40 --1.25 -- 1.10 -- 1.15
2.90 4.00 5.00 -5.80 6.55 7.10 7.90 8.50 8.85 9.95 10.50 10.70
+0.55 +1.15 +1.37
3.00 4.00 5.00 5.70 -6.55 6.95 8.00 8.50 9.00 10.00 10.50 10.75
+0.59 +0.96 +2.00 +1.90 -+1.85 +1.80 +1.35 +1.20 --0.35 --0.70 -- 1.40 -- 1.40
oA ff 1.5
A
A
-
electric point at p H 8.7. On the acid side, the dispersions are positively charged owing to NHs + groups and on the basic side they are negatively charged owing to the presence of COO- groups.
A oo
+0,5
o -t
- E.C Z
o
X x A
A-REGULAR GRADE
-
+1.95 +1.86 +1.80 +0.80 +0.83 +0.47 --0.97 --0.75 -- 1.25
:E
o
NYLON-6
ACKNOWLEDGMENT
A
SUPERFINE GRADE
oH
FIG. 9. Eleetrophoretic mobility (EM) vs. pH of microcrystalline Nylon-6.
We wish to express our appreciation to Mr. George Raynor of the Avicel Department of American Viscose Division, FMC Corporation for his consultations and guidance in use of the centrifugal sedimentation techniques. REFERENCES
therefore, the resulting particle size and surface area depends on the method of drying. The effect of method of drying of Aviamidc-6 gels is shown in Table VII. Also included are the measured surface area of the powders. Spray and freeze-drying both result in varying degrees of hydrogen bonding of the particles as the water phase evaporates. Alcohol dehydration minimizes this effect; therefore, the particles are almost completely redispersible in aqueous medium. 6. Electrophoretic Mobility vs. p H . The values of electrophoretic mobility vs. p H of three grades of Aviamide nylon-6 are shown in Table V I I I and Fig. 9. Within experimental error no essential difference was found between the three samples; therefore, a single curve can be drawn to represent the mobility of the three samples. Aviamide nylon-6 dispersions have an isoJournal of Colloid and Interface Science, Vol. 36, No. 2, June 1971
1. BATTISTA, O. A. AND SMITH, P. A., U. S.
Patent 2,978,446 (April 4, 1961). 2. B~TTISTA,O. A., AND SMITH, P. A., Ind. Eng. Chem. 54, 20 (1962). 3. B~TTISTA, O. A., Amer. Scientist 53, 151 (1965). 4. BATTIST& O. A., A. V. Tobolsky, Ed., In "Structure and Properties of Polymers" pp. 135-155. (J. Polymer Sei. C, 9), Interscience, New York, 1965. 5. B~TTISTA, O. A., ERDI, N. Z., FERRARO, C. F., AND KARASINSKI, F. J., J. Appl. Polym. Sci.
11,481-498 (1967). 6. STOKES,G. G., Cambridge Phil. Soc. Trans. 8, 287 (1849). 7. HALL, A. D., J. Chem. Soc. Trans. 85, 950 (1904). 8. BATTISTA,0. A., J. Polym. Sci. Part C 9,135155. 9. ERDI, X. Z., CRUZ, M. M., AND BATTISTA, 0. A., J. Colloid Interface Sei. 28, No. 1, 3647 (1968). 10. HERSCHEL, W. H . , AND BULKLEY~ R., Kolloid Z. 39,291 (1926).
Received December 10, 1970; accepted December 29, 1970 Aviamide 6 microcrystalline nyloi1 is a colloidal form of polyeaprolactam prepared from Nylon-6 by controlled hydrolysis of the amide linkages followed by appropriate mechanical action which frees the particles from the polymer matrix. The particles termed mieroerystals form stable dispersions in water over a wide range of concentration without the need of dispersing aids. The particle size distribution of Aviamide6 aqueous gel was measured by centrifugal and gravitational sedimentation techniques over the range of 0.1 to 59 it. The applicability of this technique was substantiated by electron microscopy studies. Depending on the grade, Aviamide-6 was found to contain as much as 90% by weight of the particles _
mineral silicates. These materials possess physical properties closely related to those of microcrystalline cellulose, because of the presence of colloidal particles common to all. Aviamide-6 microcrystalline nylon is a colloidal form of polycaprolactam as discrete particles prepared by the controlled hydrolysis of the amide linkages. B y appropriate mechanical action in an aqueous medium, colloidal microcrystals having a length to diameter ratio of approximately 1 are produced to form a stable gel. Much larger particles comprising aggregates of these also are in suspension. The particle size characterization of Aviamide-6 is important as a method of evaluating products for specific end-use applications. In this paper, the application of classical sedimentation techniques as a rapid and reproducible method to characterize the particle size of Aviamide-6 is described and the physical properties of several grades are compared. Journal of Colloid and Interface Science, ¥ol. 36, No. 2, J u n e 1971
195
196
KARASINSKI AND FERRARO EXPERIMENTAL
1. Materials. Three grades of Aviamide-6 microcrystalline nylon were prepared for this study: (1) Regular, (2) Fine, and (3) Superfine grades. Each grade forms stable gels in an aqueous medium and they are differentiated by their particle size distributions. 2. Particle Size Determination. Stokes (6)
derived the equation for the force of resistance to motion of a sphere suspended in a continuous medium. Hall (7) first applied Stokes' equation to the measurement of particle size. Although Stokes' frictional coefficient is applicable only to spherical particles, it has been used for particles the maximum to minimum diameter ratio of TABLE II EFFECT OF P E R
TABLE I
CENT
SOLIDS OF AVIAMIDE®-6
(ScFEnFINE) DISPERSION ON PARTICLE
EFFECT OF TIME OF DISPERSION OF AVL%MIDE®-6 (FINE) ON PARTICLE SIZE RESULTS Dispersion time (rain)
Wt % Aviamide-6 particles <1 ~
Wt % Aviamide-6 particles <0.2 /L
10 20 30
75.2 75.5 75.6
49.2 49.7 49.6
SIZE RESULTS % Solids Aviamide 6 Wt % Aviamide-6 dispersion particles ~1 ~
0.23 0.52 0.76 1.50
88.3 86.1 87.7 85.2
FIG. 1. Electron micrograph of AVIAMIDE®-6 (200 rex). Journal of Colloid and Interface ~cience, VoI. 36, No. 2, June 1971
Wt % Aviamide 6 particles _~0.2
92.2 85.3 85.8 83.5
AVIAMIDE-6 MICROCI~YSTALLINE NYLON
197
FIG. 2. Electron micrograph of ~0.1 t~ fraction of AVIAMIDE®-6 (50 rex). which does not exceed 4. Sedimentation techniques are shown to be valid for the particle size characterization of Aviamide-6 if the method is considered as a fractionation based on size rather than an absolute m e a s u r e m e n t of particle sizes. For sedimentation in a gravitational field, the following equation was used: t -
18 X 108@ d~(o~ - p2)g'
[1]
where t = time of settling in seconds; v = viscosity of water at 25°C (0.00894 poise) ; h -- total depth of settling (15 cm); d = diameter of particle in microns. Upon
substitution of a given value of d into Eq. [i], particles above this diameter settle out and particles equal to or below this diameter remain in suspension; pl = density of Aviamide-6 (1.14 g m / c c ) ; p2 = density of water (1.00 gm/cc) ; g = 980 cm/see 2, gravitational constant. For centrifugal sedimentation, the equation below was used: t =
18 X 10avh d2(pl -- p2)g (RCF) '
[2]
where R C F is the relative centrifugal force and h the total settling depth is equal to 8 era. An International Centrifuge Model Journal of Colloid and Interface Scie~we, Vol. 36, No. 2, June 1971
198
KARASINSKI AND FERRARO
FI~. 3. Electron micrograph of _<0.2 t~ fraction of AVIAMIDE®-6
H T with an eight place angle head (33 °) was used for the centrifugal sedimentation measurements, and the relationship between rotation of the head ( R P M ) and RCF is: R C F = 12.508 × 10-~ (RPM) 2.
[3]
Aviamide-6 mieroerystalline nylon gels are normally produced as high solids pastes (40 %-55 %). 1.5 % solids dispersions of Aviamide-6 were prepared for centrifugal sedimentation from the pastes by mixing for 10 min at high speed in a Waring Blendor. The powders in Table VII were also redispersed by this method (25 min mixing). Dispersion was complete under these conditions since longer times of mixing did not affect the results as shown in Table I. Journal of Colloid and Interface Science, ¥oi. 36, No. 2, J u n e 1971
(50 mx).
The effect of concentration of the dispersions analyzed for particle size determination is shown in Table II. The variation in the measured particle sizes for the latter three concentrations is only several per cent. A eoneentration of 1.5% solids was therefore chosen for all subsequent measurements since greater accuracy in weighing can be attained. The reproducibility at the 1.5% solids level was in the order of 1% or less. 1.5 % solids dispersions were also used for the gravitational sedimentation experiments on the Regular and Fine grades of Aviamide-6. For centrifugal sedimentation analysis of a sample, two 50-ml Stainless steel eentri:
AVIAMIDE-6 MICROCRYSTALLINE NYLON
!99
FIG. 4. Electron mierograph of --<0.5t~ fraction of AVIAMIDE®-6 (50 mx). fuge tubes were filled with 40 ml of the 1.5 % solids Aviamide-6 dispersion. T h e centrifuge is run for the appropriate amount of time (e.g., for determination of the weight per cent Aviamide-6 particles <_ 1 ~, the centrifuge is run for 20 min at 2500 R P M ) and allowed to come to a complete free stop so t h a t the sediment at the b o t t o m of the tubes is not disturbed. The effects of acceleration and deceleration of the centrifuge were neglected for the purposes of this study. T e n milliliters of the top fraction from each of the two centrifuge tubes are combined in a tared weighing dish. The percentage solids is determined b y complete removal of the aqueous phase (6 hours at 90°C). The weight per cent Aviamide-6 less than or equal to the calculated diameter is then determined.
The same dispersion and analytical procedures were used for gravitational sedimentation analyses. 3. Electron Microscopy. T h e electron micrographs of Aviamide-6 were prepared using a Philips EM-300 electron microscope at magnifications from 10,000 to 200,000 X. Dilute suspensions were evaporated on a collodion fihn supported by a 200 mesh copper screen and air dried. The screen was then placed in a high v a c u u m evaporator and P t shadowed at a 30 ° angle. 4. Viscometry. Viscosity measurements of aqueous gels of Aviamide-6 were carried out at 25°C using the H a a k e Rotovisco, a rotational viscometer. The M V I I Couette system was used. The ratio of diameters of cup to cylindrical bob is 41.5 m m to 36.8 m m (1.128). The width of the gap is 2.35 Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
200
KARASINSKI AND FERRARO
FIG. 5. Electron micrograph of _< 1 # fraction of AVIAMIDE®-6
mm. The length of the bob is 60 mm. The viseometer has a basic drive system covering a range of 3.6-583.2 rpm in ten steps. Apparent viscosity as a function of shear rate was calculated at various rpm utilizing the appropriate constants supplied with the instrument. 5. Electrophoretic Mobility. Electrophoretie mobilities of dispersions at various pH values were measured at 25°C with a Zeta Meter (Zeta Meter Inc., New York). The dispersions of 0.001% concentrations were prepared from 0.1 weight % stock solutions by mixing for 5 rain. Separate dispersions were prepared for each p i t adjustment. The pH of the dispersions were adjusted with dilute NH4OH and H3PO4. The standard electrophoresis cell supplied Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
(50 mx).
with the instrument was used for the measurements. A fixed de voltage of 200 volts was maintained across the platinum electrodes. 6. Surface Area. The surface area of Aviamide-6 powders prepared by various methods was measured on a Perkin-Elmer Sorptometer, Model 212 D. RESULTS AND DISCUSSION The properties of microcrystalline nylon-6 are related to its small particle size. During its manufacture, the degree of attrition utilized following the hydrolysis step of the process determines the particle size distribution of the final product. This, in turn, controls the functional properties required for specific end-use applications. Although
AVIAMIDE-6
MICROCRYSTALLINE
NYLON
201
Fro. 6. Electron micrograph of <5 ~ fraction of AVIAMIDE®-6 (10 mx). many methods are available for the determination of particle sizes, sedimentation techniques, especially centrifugal, offer a rapid and accurate method when applied to Aviamide mieroerystalline nylon-6. The measurement of several points by a rapid technique (e.g., wt % _< 1 and 0.2 ~) provides useful data for characterizing the product.
1. Applicability of Centrifugal Sedi~nentation Technique. The smallest particle diameter of Aviamide-6 appears to be in the region of 50-100 A. A few of these ultimate particles can be seen in the electron micrograph 200,000X in Fig. 1. It is also observed that the particles are approximately spherically shaped. The particles are composed of aggregates of moteeu!es which are joined by strong lateral bonding forces so as to form
discrete colloidal particles (8). In aqueous medium, the particles are in a solvated state and tend to flatten on air drying. This increase in diameter, however, is counteracted by hydrogen bond forces as the water phase leaves. Figures 2-6 are electron micrographs of the particles in diluted top fractions of Aviamide-6 dispersions after centrifugal sedimentation for various specified diameters. The maximum diameter of the particles in each does not exceed the calculated diameter programmed for the centrifugal sedimentation test. Therefore, the use of Eq. [2] appears to be essentially valid for the determination of the weight per cent undersize. Replication of the electron microscopy studies has shown Eq. [2] to be valid down to approximately 0.1 t* Journal of Colloid and Interface Science, Vol. 36, No. 2, J u n e 1971
202
KARASINSKI AND FER~ARO
particle diameter and up to a maximum value of about 15 ~. The method measures weight per cent undersize both of the ultimate particle or of aggregates of two or more which have not been separated during the mechanical attrition step. 2. Particle Size Distribution. The cumulative particle size distributions of three grades of microcrystalline nylon-6 are shown in Table III and Figure 7. The points above 15 , particle diameter were determined by gravitational sedimentalion. It is apparent that the particle size distribution can be controlled from narrow to broad depending on the degree of mechanical attrition utilized in the manufacture of the grade.
A comparison of the weight per cent of mierocrystalline nylon-6 particles less than 1 and 0.2 ~ has proven to be very informative. The measurement of these two values by centrifugal sedimentation can serve as useful data for the rapid characterization of changes made during the preparation of microerystalline nylon-6. As an example, the effect of milling of two samples of Aviamide-6 using a Premier Colloid Mill is shown in Table IV. The maximum buildup of colloidal particles is seen to occur after two passes for Sample 1. Milling can, therefore, be discontinued at this point. For Sample 2, however, four passes are necessary to achieve this purpose. 3. Dilution Effects. Aviamide nylon-6 gels are prepared as high solids pastes (40 %55 % solids). For some applications such as eleetroeoating, it is necessary to dilute these gels to low solids dispersions with a minimum of particle-to-particle association. This has been found especially important in forming thin films (0.1 rail or less) on metal substrates. For the purpose of dilution, the use of low shear mixing equipment is more desirable than high shear. The effect of the type of mixing equipment was characterized by the measurement of the weight per cent particles less than 1 and 0.2 after the direct dilution to 1.5 % solids of high solids gels by both types of equipment. Table V contains the results of this study. The 40 % solids gel is completely dispersible to low solids by a low shear method. This
TABLE III PARTICLE SIZE DISTRIBUTION OF THREE GRADES OF AVIAMIDE-6 Wt % <
Regular
0.2 0.5 1.0 5.0 11.0 15.0 25.0 35.0 50.0 60.0 75.0 100.0
17.0 28.7 36.0 56.4 66.4 72.4 78.3 86.8 91.4 95.4 97.4 98.7
Fine
Superfine
49.5 68.0 75.5 88.4 98.6 99.3 100.0
86.0 88.4 90.0 95.4 99.3 .100.0
I00
o
~9o ~8o
o
o
o
o
uJ 70 -~ 6O >.50 m o
.~c
SUPERFINE FINE
.J
I0 I
I
I
I
I I III
I
I
I
I
1.0 SIZE,
I
I I II IO
I
MICRONS
FIG. 7. P a r t i c l e size d i s t r i b u t i o ~ of AVIAMIDE®-6. Journal of Colloid and Interface Noienze, Vol. 36, No. 2, June 1971
]
I
I
I
I
]1
]00
AVIAMIDE-6 MICROCI~XSTALLINE NYLON TABLE IV PASSES T H R O U G H
EFFECT OF N U M B E R OF ON THE W E I G H T % No. of passes
--
t
n i ITTII
i
I~--L
~ n It
A MILL
OF PARTICLES _<1 U
Sample 1
Sample 2
71.9 81.7 80.4 81.0 81.5
81.4 81.4 86.4 88.8 88.9
1 2 3 4 5
n
203 I
n
1 n I ~
A- SUPERFINE IO,O00
TABLE V EFFECT
OF METHOD OF DILUTION OF 40070 AND 5 0 % SOLIDS MICROCRYSTALLINE N Y L O N - 6 G E L S (SUPERFINE GRADE) ON THE FRACTION OF SUBMICRON PARTICLES
% Solids gel
50
Method of dilution
4
6
72.0 89.6 91.8 90.1
2
4
S
I00
S
4
6
IO00
FIG. 8. Effect of particle size distribution oE the apparent viscosity of microerystalline Nylon-6. TABLE VII EFFECTS
60.0 86.7 84.4 86.8
I0
RATE OF SHEAR(sec-I]
Wt % particles Wt % particles ~ 1u <0.2 #
Low shear High shear Low shear High shear
40
[00
OF DRYING
OF AVIAMIDE
~YLON-6
GELS
ON P A g T I C L E SIZE AND SURF.~CE A R E A ! Surface
Sample •
(mS/gin)
I--
VI
TABLE APPARENT VISCOSITY 8%
VERSUS
SOLIDS SUPERFINE
SHEAR RATE
AND F I N E
FOR
~RADES
OF AVIAMIDE NYLON-6 Shear rate (sec-1)
3.2 6.5 9.8 19.6 29.4 58.8 88.2 176.0 265.0 529.0
Apparent viscosity (cps) Superfine
20520 11010 6775 3950 2760 1380 920 520 380 210
Apparent viscosity (cps) Fine
2540 1180 830 380 260 125 69 35 42 24
is not the case for the 50% solids. On the basis of this study, it was concluded that product of no more than 40% solids can be conveniently "let-down" using low shear mixers such as a Lightning Mixer.
4. Rheological Properties of Aviamide Nylon-6. The theological properties of microcrystalline gel systems have been previously reported (9). These are dependent on their particle size distribution and in particular on the fraction of particles less than 1 u in the maximum dimension. Gen-
1. Aqueous gel spraydried powder 2. Aqueous gel freezedried powder 3. Aqueous gel alcohol d e h y d r a t e d and v a c u u m dried
1.2
89 56 89 78
86 40 86 64
-0.9 127
erally, the viscosity of the gel increases as the fraction of submicron colloidal particles increases. The effect of particle size distribution on the apparent viscosity of 8% microcrystalline nylon-6 gels is shown in Table VI and Fig. 8 for Superfine and Fine grades. This plot shows that the rheology of these pseudoplastic systems obeys the power law (10). The particle size distributions are shown in Fig. 7. The Regular grade is not included since the fraction of submicron colloidal particles is not sufficient for gelation at 8 % solids. The Superfine product has approximately ten times the viscosity of the Fine grade. Both grades show highly dependent shear thinning behavior. 5. Effects of Drying. After the removal of water from Aviamide gels, hydrogen bonding causes aggregation of the colloidal particles. The degree of bonding and, Journal of Colloid and Interface Science, Vol. 36, No. 2, June 1971
204
KARASINSKI AND FERRAR0 TABLE VIII ELECTROPHORETICMOBILITY (EM) VEnSVS PH OF THREE GRADES OF AVIAMIDE Regular
Fine
Superfine
pH
EM (,ucm se¢-x volt-l)
pit
EM (,acre sec-1 volt-x)
pH
EM (#cmse( "x volt'- 0
3.00 4.00 4.90 5.50 6.00 6.52 7.10 8.10 8.50 9.00 10.00 10.50 10.70
+0.63 +1.50 +1.75 +2.05 +1.90 +1.95 +2.00 +1.80 +1.00 +0.40 --1.25 -- 1.10 -- 1.15
2.90 4.00 5.00 -5.80 6.55 7.10 7.90 8.50 8.85 9.95 10.50 10.70
+0.55 +1.15 +1.37
3.00 4.00 5.00 5.70 -6.55 6.95 8.00 8.50 9.00 10.00 10.50 10.75
+0.59 +0.96 +2.00 +1.90 -+1.85 +1.80 +1.35 +1.20 --0.35 --0.70 -- 1.40 -- 1.40
oA ff 1.5
A
A
-
electric point at p H 8.7. On the acid side, the dispersions are positively charged owing to NHs + groups and on the basic side they are negatively charged owing to the presence of COO- groups.
A oo
+0,5
o -t
- E.C Z
o
X x A
A-REGULAR GRADE
-
+1.95 +1.86 +1.80 +0.80 +0.83 +0.47 --0.97 --0.75 -- 1.25
:E
o
NYLON-6
ACKNOWLEDGMENT
A
SUPERFINE GRADE
oH
FIG. 9. Eleetrophoretic mobility (EM) vs. pH of microcrystalline Nylon-6.
We wish to express our appreciation to Mr. George Raynor of the Avicel Department of American Viscose Division, FMC Corporation for his consultations and guidance in use of the centrifugal sedimentation techniques. REFERENCES
therefore, the resulting particle size and surface area depends on the method of drying. The effect of method of drying of Aviamidc-6 gels is shown in Table VII. Also included are the measured surface area of the powders. Spray and freeze-drying both result in varying degrees of hydrogen bonding of the particles as the water phase evaporates. Alcohol dehydration minimizes this effect; therefore, the particles are almost completely redispersible in aqueous medium. 6. Electrophoretic Mobility vs. p H . The values of electrophoretic mobility vs. p H of three grades of Aviamide nylon-6 are shown in Table V I I I and Fig. 9. Within experimental error no essential difference was found between the three samples; therefore, a single curve can be drawn to represent the mobility of the three samples. Aviamide nylon-6 dispersions have an isoJournal of Colloid and Interface Science, Vol. 36, No. 2, June 1971
1. BATTISTA, O. A. AND SMITH, P. A., U. S.
Patent 2,978,446 (April 4, 1961). 2. B~TTISTA,O. A., AND SMITH, P. A., Ind. Eng. Chem. 54, 20 (1962). 3. B~TTISTA, O. A., Amer. Scientist 53, 151 (1965). 4. BATTIST& O. A., A. V. Tobolsky, Ed., In "Structure and Properties of Polymers" pp. 135-155. (J. Polymer Sei. C, 9), Interscience, New York, 1965. 5. B~TTISTA, O. A., ERDI, N. Z., FERRARO, C. F., AND KARASINSKI, F. J., J. Appl. Polym. Sci.
11,481-498 (1967). 6. STOKES,G. G., Cambridge Phil. Soc. Trans. 8, 287 (1849). 7. HALL, A. D., J. Chem. Soc. Trans. 85, 950 (1904). 8. BATTISTA,0. A., J. Polym. Sci. Part C 9,135155. 9. ERDI, X. Z., CRUZ, M. M., AND BATTISTA, 0. A., J. Colloid Interface Sei. 28, No. 1, 3647 (1968). 10. HERSCHEL, W. H . , AND BULKLEY~ R., Kolloid Z. 39,291 (1926).