Anomalous behavior of aerosol produced by atomization of monodisperse polystyrene latex

Anomalous behavior of aerosol produced by atomization of monodisperse polystyrene latex

JOURNAL OF COLLOID SCIENCE 15, 357--360 (1960) A N O M A L O U S B E H A V I O R OF A E R O S O L P R O D U C E D BY A T O M I Z A T I O N OF M O N ...

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JOURNAL OF COLLOID SCIENCE 15, 357--360

(1960)

A N O M A L O U S B E H A V I O R OF A E R O S O L P R O D U C E D BY A T O M I Z A T I O N OF M O N O D I S P E R S E P O L Y S T Y R E N E LATEX I G. Langer and A. Lieberman Armour Research Foundation of Illinois Institute of Technology, Chicago, Illinois Received April 18, 1960

ABSTRACT A study was made of the uniformity of aerosol particles generated by the atomization of monodispersed polystyrene latices. It was found that the stabilizer associated with the latices formed extraneous particles as well as increased the size of the polystyrene particles. This was due to the fact that the stabilizer binds water strongly even in the presence of dry air. Heat drove the bound water off but adversely affected the polystyrene particles. During development of an electrostatic classifier for submieron aerosol particles, it was decided that the monodisperse polystyrene latices produced by the Dow Chemical Company would produce ideal standardizing aerosols. These materials are available in a wide range of particle diameters from 0.09 to 1.1 microns; they are spherical; the diameter of a specific batch is accurately known. I n addition, aerosols made from these latices have been used by other organizations as standard test aerosols for filter evaluation (1). Therefore several batches of polystyrene latices were obtained for use as test materials. Aerosol was prepared by diluting the latex to a convenient concentration and dispersing it as an aerosol by means of a simplified Lauterbach generator (2). The aerosol was diluted with dry air to evaporate the water from the latex spheres. The aerosol was then passed to the classitier and precipitated. I t was noted that the precipitation pattern was that of a polydisperse aerosol. Analysis of the classifier operation indicated that it was operating satisfactorily and that a closer examination of the aerosol was in order. The Armour Research Foundation Electronic Particle Counter (3) was used for rapid analysis of the aerosol with the equipment shown in Fig. 1. D a t a are obtained within seconds of sampling the aerosol and the aerosol 1 This work was performed as part of Contract No AF 19(604)2411 under the sponsorship of the Air Force Cambridge Research Center. 357

358

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FIO. 1. Test equipment for studying aerosols produced by atomization of m ° n ° dispersed polystyrene latex particles. •

TABLE I

ParticleSizeDistribution ~ PolystyreneLatex Aerosols Madeby Atomizaton Diam.of Dilutionof Rel.ham. sphere 0~..57 latex particle suspensmn in (%) 0.138 0.814 0.814

1:500 1:100 1:10

4 4 4

72 12 3

Sizedistribution(%) Conc. (parttcles/c.c.) 0.7-1.0 1.01.4 1.4-2.0 2.02,8 2.8-4.0 Meas. Calcd. 19 24 13

5 52 36

3 12 33

1 0 13

0 0 2

15 109 500

0 2 20

Calculated concentrations refer to the stated size range for the particle counter for the latex particles only. The particle counter was set to obtain correct size data for water aerosol with refractive index 1.33. Although the polystyrene has a refractive index of 1.60; the error in the stated sizes is less than 25%. is e x a m i n e d in situ b y s c a t t e r e d light m e a s u r e m e n t . E x a m i n a t i o n w i t h the particle c o u n t e r verified the wide size d i s t r i b u t i o n of t h e aerosol. T h e aerosol was s a m p l e d from a 34 in. d i a m e t e r Lucite sphere where it was m i x e d w i t h d r y d i l u t i n g air. T a b l e I shows the d a t a which were o b t a i n e d w i t h two latices t h a t were dispersed a n d d i l u t e d with d r y air a t r o o m t e m p e r a t u r e . T h e o n l y possible conclusion is t h a t , e v e n at 4 % relative h u m i d i t y , all the w a t e r drops were n o t e v a p o r a t e d . I n f o r m a t i o n from D o w C h e m i c a l C o m p a n y is t h a t t h e original solids c o n t e n t of t h e latices is 7 % p o l y s t y r e n e a n d 2 % u n i d e n t i f i e d stabilizer of a

B E H A V I O R OF ATOMIZED M O N O D I S P E R S E P O L Y S T Y R E N E L A T E X

359

TABLE I I Size Distribution of Polystyrene Latex Aerosols Diluted with Heated Air Size distribution (%) Diameter of latex particles

0.138 0.138 0.814 0.814 0.814

DiCtion of suspension

Rel. bum. Air temp. in sphere (°F.) in (%) heater

1:500 1:500 1:100 1:100 1:10

4 4 4 4 4

) (particles/c.c. Conc. )

'4-12"0-12'8-1

75 330 75 330 75 25O 330 34O 360

8 25 24 42 13 31 30 34

1 0 52 0 36 36 12 11

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o r o/o /

12 / o, o / lO9 OJ ~3 6 1 0 0

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OI 13 1 0 0 0

0 2 0 0 0 0

0.2 500 147 3.2 2.4 0

Calcd.

0 0 2 2

20 20 20 20 20

TABLE I I I Effect of Humidity on Size Distribution and Concentration of Heated 0.814 ~ Polystyrene Latex Aerosol ~

(%)

4 19 100

0.5-0.7 0.7-1.0 1.0-1.4 1.42.0 340 350 340

Conc.

Size distribution (%)

Rel. hum. Air temp. (°F.) in sphere in heater

55 33 30

34 24 37

11 29 22

0 4 5

(particles/c.e.)

2.0- 2.0-4.0 2.8 0 0 0

0 0 0

Meas.

CaJcd

2.4 52 100

20 20 20

Aerosol from atomizer was diluted with dry air, passed through heater, and then entered the sphere which was at different humidities. s o a p n a t u r e . T h e r e f o r e , some e x p e r i m e n t s were p e r f o r m e d in w h i c h t h e d i l u t i n g a i r s t r e a m was h e a t e d in a n a t t e m p t to e v a p o r a t e w a t e r a n d / o r d e c o m p o s e t h e s t a b i l i z e r o n l y . T a b l e I I shows t h e d a t a o b t a i n e d in t h i s series of e x p e r i m e n t s . T h e first t w o lines in T a b l e I I show t h a t m e a s u r a b l e p a r t i c l e s were p r e s e n t in t h e s p h e r e e v e n a f t e r b e i n g h e a t e d to 350°F. T h e p a r t i c l e c o u n t e r w h i c h does n o t c o u n t b e l o w 0.5 m i c r o n s h o u l d n o t h a v e d e t e c t e d a n y p a r t i c l e s if o n l y p o l y s t y r e n e w a s p r e s e n t . Lines 3 a n d 4 show t h a t m o s t of t h e w a t e r w a s e v a p o r a t e d f r o m t h e n u m e r i c a l l y m o r e d i l u t e 0.814 m i c r o n s u s p e n s i o n a t 350°F. T h e r e m a i n d e r of t h e t a b l e shows t h e effect of inc r e a s i n g a i r t e m p e r a t u r e on b o t h t h e r e s u l t a n t size d i s t r i b u t i o n a n d conc e n t r a t i o n of p a r t i c l e s . I t a p p e a r s t h a t a t e m p e r a t u r e of a p p r o x i m a t e l y 340°F. is n e c e s s a r y to effectively e v a p o r a t e t h e w a t e r d r o p l e t s . O b s e r v a t i o n of t h e e n t i r e t a b l e i n d i c a t e s t h a t t h e r e q u i r e d t e m p e r a t u r e is d e p e n d e n t

360

LANGER AND LIEBERMAN

on the amount of stabilizer present in the diluted latex suspension. It was surmised that the stabilizer holds water very tenaciously. If so, the polystyrene particles will retain a water film even in very dry air. In the case of the 0.814 micron suspension diluted 1:100 and under the condition of atomization used, only 1 in 50 drops should contain a polystyrene particle. The remainder of the droplets apparently do not evaporate completely because of the residual stabilizer until a temperature of 340 ° to 350°F. is reached. The stability of the water drops alone indicates that at least some of the stabilizer is distributed throughout the liquid rather than attached only to the particles. Some further experiments were performed and are shown in Table III. These data show that aerosol can be regrown by exposing it to sufficient water vapor. In other words the heat just dried out the stabilizer but decomposed the polystyrene at 340°F. and above. It was concluded from these experiments that atomization of monodisperse polystyrene latices does not necessarily produce a monodispersed test aerosol. These preliminary experiments did not solve the problem of proper treatment to obtain monodisperse aerosols. One difficulty was that neither the stabilizer nor the water appeared in ordinary sampling procedures with microscope slides, because these materials spread out in a thin film. However, some electron microscope photographs by Dautrebande (4) show the presence of extraneous material from a latex aerosol collected on an electron microscope screen. Presumably, the extraneous particles are due to the stabilizer, which is also shown between closely spaced particles from a drop of evaporated latex. Further work is planned to show the presence of the stabilizer with the light microscope and to develop a procedure for effective removal of the stabilizer in order to obtain a true, monodisperse aerosol. I~EFERENCES i. STERN, S., "Simple technique for generation aerosols." J. Appl. Phys. 30,952 (1959).

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2. LAUTERBACH,I~. E., Arch. Ind. Health 13, 156 (1956). 3. FISHER,M. A., Proc. 3rd Natl. A i r Pollution Symp., pp. 112-119 (April, 1955). 4. DAUTR:FBANDE,L., "Studies in Aerosols," p. 55, VR-530, University of Rochester AEC Project, 1958.