I n d . E n g . C h e m . Res. 1987,26,681-684
68 1
Production and Characterization of Stable Emulsions of Polyethylene and Hydrocarbon Oils Jyh-Ping Chen Department of Chemical Engineering, T h e Pennsylvania State University, University Park, Pennsylvania 16882
E. Earl Graham* Department of Chemical Engineering, Cleveland State University, Cleveland, Ohio 44115
It was demonstrated that stable emulsions of polyethylene in hydrocarbon oils can be formed by a high-speed thin-film-coating process. These emulsions which can be used as electrical sealants are very stable and have a very high yield stress (greater than 10000 dyn/cm2) and a high initial viscosity. At increasing shear rates, the emulsions are pseudoplastic with a power-law index of 0.2-0.3 (at room temperature). Further improvements in the stability of the emulsions can be made by using a hydrocarbon which is highly saturated and increasing the concentration of the polyethylene. Correlations were developed relating the viscosity of the emulsions to temperature and polymer concentration. Specialized sealants are used in the electronics industry. One type of telecom sealant is a waxy, greaselike material at room temperature. It consists of an emulsion of greater than 10% branched-chain polyethylene in a naphthenic hydrocarbon oil. In the production process, the polymer is completely dissolved in hot mineral oil. The solution is then cooled down rapidly to room temperature to stabilize the sealant. Such sealants are found to be pseudoplastic (shear-thinning) fluids and are well represented by a power-law model with a power-law index of about 0.2-0.3 a t room temperature. Problems have been observed with the storage stability of these sealants, consistency from batch to batch, and the drift of sealant viscosities with storage time. These problems appear to be related to the morphology of the polymer phases which slowly precipitate out of the solutions, decreasing the stability of the oil-based emulsions. The tendency for phase separation increases with storage time, especially at temperatures of 20-60 "C. This results in the failure of the sealants to meet functional specifications. It is, therefore, important to develop better formulations or processing methods to improve the properties of polymer-in-oil sealants. The rheological properties of these sealants have been studied as a function of temperature, temperature history, and composition. A plate and cone mechanical spectrometer and a Mini-Rotary viscometer were used to measure the viscosity and the yield stress of the samples. Oil separation tests were performed to characterize the sealants' stability.
Apparatus and Experiment Procedures A plate and cone viscometer with an environmental chamber including a built-in heater and fan providing temperature control for viscosity measurements above room temperature was used to measure viscosity as a function of shear rate. A rotary concentric viscometer containing nine small viscometric cells within a thermostated aluminum block was used to measure the yield stress of the sealants. The yield stress can be obtained from the minimum weight which starts the movement of the rotor by calculating the resultant shear stress at the rotor surface. A computer was used to control a built-in heater and cooling system for
* Author to whom correspondence
should be addressed.
0888-5885/87/2626-0681$01.50/0
measurements at different temperatures. An oil separation test was used to determine the tendency of the oil in the sealant to separate at elevated temperatures. The apparatus consists of a 60-mesh stainless steel cone. About 10 g of sample was placed in the cone which was hung in a clean and weighed beaker. After heating in the oven for 24 h at 77 "C, the cone was removed and the beaker weighed again. The percentage of the separated oil of the original sample could then be calculated from the weight of the oil collected in the beaker. A simple but useful film cooling method was used in preparing the samples. A small amount of the hot solution containing oil, polymer, and an antioxidant was added to a stainless steel beaker (74 mm in diameter) with a flat bottom surface. A t 110 "C, the beaker was immersed in a well-stirred cold bath, and the solution was cooled as a thin film at the bottom of the beaker. The final product was scraped out from the beaker for further study. The cooling rate experienced by the samples during the cooling process was adjusted by varying either the film thickness (sample volume in the beaker) or the temperature of the cold bath. The materials used were a high density polyethylene of molecular weight 5000 and two hydrocarbon oils: oil A which was 99.4% saturated and had a pour point of -12 "C and oil B which was 97.5% saturated and had a pour point of -9 "C.
Results and Discussion Cooling Process. The temperature profiles in the film during the cooling process were calculated (Crank, 1975) by assuming one-dimensional heat transfer only in the direction perpendicular to the bottom surface of the beaker and by assuming that the temperature of the surface of the beaker was the same as that of the batch immediately following the immersion. The results of these numerical calculations are shown as temperatures vs. positions and time in Figure 1for a sample volume of 8 mL (2-mm film thickness) and bath temperatures of -20,0, and 16 "C. In these calculations, the latent heat of the sample has been neglected when compared to the sensible heat. The positions at different points on the film are represented by y l b , where b is the film thickness and y is the vertical direction pointing downward. The position y = 0 corresponds to the upper open surface of the film, where y = b is the lower surface. The viscosities of these samples are 0 1987 American Chemical Society
682 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 106
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-
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Commercial Sealant Film Cooling, 2mm Film Film Cooling, lmm Film
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Figure 4. Oil separation test results for sealants at a storage temperature of 50 "C.
t Shear Rate (set.-' ) Figure 2. Viscosityshear rate data measured at 23 "C for sealants from film cooling of a 2-mm film.
shown in Figure 2. As can be seen from the results, colder bath temperatures resulted in faster cooling rates and higher viscosities for the sealants. For larger sample volumes, thicker films were formed. With a 12-mL sample, the film would be 3 mm, while with a 16-mL sample, the film would be 4 mm. A dramatic decrease in cooling rates with increased film thickness was observed, resulting in a decrease in the sample viscosities as shown in Figure 3. The results of yield stress measurements are summarized in Table I for all the samples. As evident from those values, a faster cooling rate also produced samples with higher yield stress. Oil separation tests have also been carried out for the samples prepared using various film thicknesses cooled in
Table I. Properties of Sealants from Film Cooling for Oil A 13% Polymer bath temp, O C -20 0 16 sample vol, mL 8 8 12 16 8 12 16 yield stress, 11200 10150 7700 5950 8750 5600 4550 dyn/cm2
+
a 0 "C bath. Data has been obtained for times up to 6 weeks for the sample stored a t 50 "C. The results are shown in Figure 4. It can be seen that the oil separation was significantly reduced by using the f i i cooling method. For samples prepared using a 2 mm film, the fraction of oil separation was 2.8% when extrapolated to 1 year, which is well below a typical industrial specification limit of 4%. The effect of the cooling rate on the particle size of the sample was studied with a scanning electron microscope (SEM). Samples were prepared with three different cooling rates: fast film cooling with a 1-mm film in a 0 "C bath, intermediate film cooling with a 2-mm film in a 0 "C bath, and slow bulk cooling in ambient air with continuous stirring.
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 683 A
0
70001 t
13% Polymer, Oil A 13% Polymer, Oil B
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Table 11. Yield Stress in Different Temperatureso temD. "C 22 30 40 50 60 oil A + polymer 10 150 4900 2800 1400 oil B +polymer 11200 6000 4200 2400 1600
100 1
Table 111. Properties for Sealants with Different Polymer 13% Polymer Concentration for Oil A polymerconcn,wt 5 7 9 11 13 17 19
+
1050 2800 3150 5600 10150 14700 17500
The particle sizes of those samples were obtained by observing dried films of sealant in toluene, diluted to 1% solids, with the SEM at a magnification of 400X. The particle size was found to be smaller for a sample produced by using a faster cooling rate. This result correlated well with the observed oil separation rate and the viscosity of the sealant. Formulation. The results of the viscosity measurements and oil separation tests for sealants containing different hydrocarbon base oils are shown in Figures 5 and 6. Yield stress values at different temperatures are reported in Table 11. These samples all contain 13% polymer and were prepared from the film cooling method with a 2-mm film and a 0 "C bath. It can be seen that sealants containing oil A have comparable viscosity and yield stress but better stability than the sealants containing oil B. The main difference between the two oils is the fraction of paraffinic content, which is 68% for oil A and 70% for oil B. It, therefore, seems advantageous to use a base oil of higher paraffinic content to decrease the oil separation rate of the sealant. Concentration and Temperature Effects. Seven sealant samples with polymer concentrations of 5-19 % (weight) were prepared by cooling films of 2-mm thickness in a 0 OC bath. The results of viscosity and yield stress measurements at room temperature are reported in Figure 7 and Table 111. As shown in Figure 7 , the power-law relationship between the viscosity and the shear rate is valid for all samples with a narrow range of power-law index from 0.11 to
2
3
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5
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Oil Separation (wt%) Figure 6. Oil separation for sealants from film cooling of a 2-mm film using a 0 "C bath a t a storage temperature of 50 "C.
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"Polymer concentration at 13%, 0 "C bath, 2-mm film, yield stress in dyn/cm2.
% yield stress, dyn/cm2
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Figure 7. Effect of polymer concentration on viscosity measured at 22 O C for oil A and polymer.
0.16. The viscosity increases at a given shear rate as the polymer concentration is increased. A useful linear relation is obtained at a fixed shear rate, if the logarithm of viscosity is plotted against the logarithm of polymer concentration as shown in Figure 8 for a shear rate of 1 s-l. If we assume the lines in Figure 7 are parallel to each other in view of the narrow range of power-law index and experiment errors, a master curve can be obtained by moving the lines as Figure 7 horizontally and superimposing them into a single line. This can be done by plotting the logarithm of the viscosity against the modified shear rate, i/ f C2, where C is the polymer concentration in weight percent. The results are shown in Figure 9. As can be seen, this superimposes all the lines for different concentrations into a single line for viscosity vs. modified shear rate. The modified shear rate is a variable combining both the effects of shear rate and polymer concentration. The power of 2 comes from the slope of the straight line in Figure 8. The effects of temperature on the viscosity of the sealant were investigated for several sealants with different formulations.
684 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 r
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Figure 9. Correlation of viscosity vs. modified shear rate for oil A and polymer.
From an extension of the free-volume theory of polymer systems (Bohdanecky and Kovar, 1982), it was found that a linear relationship results when the reciprocal of the logarithm of the viscosity is plotted against the absolute temperature. Figure 10 shows the excellent correlation of straight, parallel lines obtained by this method for 17% polymer in oil A. This correlation allows the viscosity of a sealant to be determined at any temperature for a given sealant formulation. With the above two simple but powerful correlations for concentration and temperature effects, one can interpolate or extrapolate viscosity data a t different shear rates for sealants a t different temperatures or with different polymer concentrations.
Conclusions The emulsions of hydrocarbon oil and polyethylene were all found to be very non-Newtonian (shear-thinning) and behaved as power-law fluids at room temperature within the range of shear rates used in the experiments. The properties of the emulsions were also found to be directly related to the wax morphology and consequently the cooling rate which resulted in better properties for the sealants, including high viscosity, high yield stress, and less oil separation (more stable). Slow cooling methods produced coarser crystals and produced less desirable properties. Fast (thin) film cooling which gives a very fast cooling rate is a desirable processing method in view of the drastic reduction of the oil separation of the product compared to commercial sealants. Hydrocarbon oils with the highest paraffinic content should be used in the formulation. The viscosities and yield stresses of the sealants increased with increasing polymer concentration. The power-law indexes were within a narrow range for sealants with polymer concentrations from 5% to 19%. Viscosity vs. shear rate correlations at different temperatures and with different polymer concentrations have been demonstrated within the range of the variables used in the experiments. Registry No. Polyethylene, 9002-88-4.
Literature Cited Bohdanecky, M.; Kovar, J. Viscosity of Polymer Solutions; Elsevier Science: New York, 1982. Crank, J. The Mathematics of Diffusion; Oxford University Press: London, 1975.
Received for review April 25, 1986 Accepted November 3, 1986
