Nonuniform Emulsion Polymers - American Chemical Society

23 Nonuniform Emulsion Polymers Process Description and Polymer Properties

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D. R. BASSETT and K. L. HOY Technical Center, Union Carbide Corporation, South Charleston, WV 25303

Because of its heterophase nature, emulsion polymerization is generally more complicated than simple solution polymerization in which monomers and polymers are soluble in a suitably chosen solvent. In emulsion polymerization the different relative solubilities of monomers in water and in the polymer particles lead to different reaction locales and to different particle structures. Another complicating factor is the need to achieve and maintain colloidal stability throughout the polymerization and subsequent handling of the dispersions. Emulsion polymers can properly be called products by process since the process details exert such a powerful effect on the properties of the particles and resultant films. Consequently, an emulsion polymer is far more than a product defined by a simple polymer composition. One way of altering the properties of latex particles is to change the monomer feed composition during the polymerization. Much work has been carried out on multistage processes in which the composition of each stage differs from that of the preceding stage (1). Film properties as well as filming properties of latexes can be altered in this manner. Not only are multistage processes cumbersome to carry out in practice, but often incompatibility of the copolymers produced in the various stages leads to poor end use properties, especially in thin films. We report here a process for continuously changing the composition of the monomer mix fed into a reactor producing, thereby, copolymers whose instantaneous compositions vary as the polymerization proceeds. The exact nature of the variation in composition is controlled by suitable choices of simple variables. Characterization of several model emulsion polymers produced in this manner serves to highlight the polymer differences induced by the process. PROCESS DESCRIPTION A simple arrangement for gradually changing the monomer mix composition of the feed stream entering the reactor is shown in Figure 1. In this arrangement, the monomer mixture in the far tank is continuously added to the monomer mixture in the well-stirred 0097-6156/81 /0165-0371 $05.00/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


372

EMULSION POLYMERS AND EMULSION

POLYMERIZATION

near tank. The continuously changing mixture in the near tank i s simultaneously fed into the reaction vessel in the usual manner of emulsion polymerization. Assuming monomer starved conditions, i . e . , no build up of unreacted monomers, the composition of the growing particles varies as the polymerization takes place, thereby avoiding the abrupt changes in composition encountered in multistage pro cesses. In effect, each p a r t i c l e is an alloy of polymers of a nearly i n f i n i t e compositional variety within set compositional limits (2). An expression for the instantaneous composition of the mixture entering the reactor can be developed by assuming perfect mixing and writing the material balance for monomer A in the near tank: Input (A) - Output (A) = Accumulation (A)

(1)

At constant flow rates, R-j and R^, this equation becomes C R dt - C R dt = d i W ^ ) 2

2

1

(2)

]

where Co is the concentration (weight fraction) of monomer A in the far tank (constant), C-j is the concentration of monomer A in the near tank, and W-j i s the weight of the monomer mixture in the near tank at any given time. Rearrangement of Equation 2 and integration yields the following expression: W" +

(Rg-R^t

Ί

ln C°

w

(3)

-C°

where W-| is the i n i t i a l weight of the monomer mix in the near tank, C^ is the i n i t i a l concentration of monomer A in the near tank. If we require both monomer mix tanks to empty simultaneously, Equation 3 becomes C° -C (4) 2 h W C° -C° 2 h where χ = - R / ( R 2 " l ) · shown that χ i s also the ratio of the i n i t i a l monomer weight in the far tank to that in the near tank. Since the quantity-(R2~R])t/W° i s r e a l l y the fraction of the monomer fed, c< , at time t , the final general equation for the process can be given: 1 +

(Ro -Ri) * . t 1

L

L

R

l

t

c a n

D e

2

Ci Ί

- (C^

-C?)(l-


(5)

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Equation 5 expresses the v a r i a t i o n i n the concentration of monomer A i n the feed stream entering the r e a c t o r as a f u n c t i o n of time. Since t h i s v a r i a t i o n i s a power f u n c t i o n of time, the process has been named "power feed." G e n e r a l l y , the feed r a t e into the r e a c t o r , R-|, i s f i x e d by the t o t a l feed time d e s i r e d f o r a given p o l y m e r i z a t i o n . The feed r a t e from the f a r tank i s then:

Another useful power feed equation permits the c a l c u l a t i o n of f r a c t i o n of a given component, A , i n the f i n a l polymer,

F

C

A =i l

d

=

C

2 -

the

7

(x+1)

Normalized p l o t s of several power feed o r o f i l e s are shown in Figure 2. For the s p e c i a l case where χ = 1, (W-| =W R] =2R2) > the feed p r o f i l e i s l i n e a r with time. Curvature i s introduced by s u i t able changes of the i n i t i a l monomer weights i n the two tanks: when χ > 1, the curve i s concave to the a b s c i s s a ; when χ < 1, the curve i s convex. With proper mixing in the near tank, these feed p r o f i l e s can be v e r i f i e d e x p e r i m e n t a l l y . The a d d i t i o n of a t h i r d monomer tank to the basic powerfeed arrangement expands the p o s s i b l e feed p r o f i l e s a v a i l a b l e f o r i n v e s t i g a t i o n . As i l l u s t r a t e d i n Figure 3 , one such arrangement i n v o l v e s a s t i r r e d middle tank which r e c e i v e s a monomer mix from the f a r tank and pumps a v a r y i n g mixture to the near tank. The arrangement i s e s s e n t i a l l y a power feed on top of a power feed and can be analyzed i n the same manner as c a r r i e d out with the two tank systems, except that C , the c o n c e n t r a t i o n of monomer A i n the second (middle) tank i s not constant but i s given by 29

2

C

2

=

C

3

"

( C

3

" Φ

(

1

(

"

8

)

Where y = W3/W2. The e a s i e s t way to handle t h i s problem i s by computer i n t e g r a t i o n , but an e x p l i c i t equation can be developed (_3) f o r C ] , the c o n c e n t r a t i o n of monomers A in the feed stream e n t e r i n g the r e a c t o r :

C,

= C°

+

y-x

(C° - C ° )

;i

x

- « ) o-oO-

v

+ (c° -c°)(i - o( )

x

In Equation 9 , C3 i s the i n i t i a l c o n c e n t r a t i o n o f monomer A i n the f a r tank of Figure 3 , and


(9)

EMULSION POLYMERS

AND EMULSION POLYMERIZATION


374

Figure 2. Power feed concentration profiles of a single monomer in the feed stream entering the reactor as a function of time. The curvature is controlled by the value vf x, the ratio of monomer weight in the far tank to that in the near tank.


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W° + W° χ =^ - ^ Ί

Γ

375

(10)

The monomer feed rates are related as follows, R

2 -

R

l

R - R (^) Downloaded by PENNSYLVANIA STATE UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch023

3

2

(12)

Whereas the two-tank arrangement permits monomer feed profiles which vary smoothly in one d i r e c t i o n , the three-tank scheme leads to inflections and concentration reversals as i l l u s trated in Figure 4. Such reversals are useful in preparing hardsoft-hard, hydrophilic-hydrophobic-hydrophilic polymer variations and the l i k e . In addition, three tank power feed has been useful as a means of calculating monomer inventory in copolymerization experiments (4·). While other monomer tank arrangements can be devised, the two-and three-tank configurations described here can generate most of the monomer feed profiles l i k e l y to be of interest. Variable feed rates can also be employed to generate similar p r o f i l e s , but the use of constant feed rates simplifies laboratory and plant operations. EXPERIMENTAL Model latexes were prepared using a conventional semibatch technique in which the reactor i n i t i a l l y contained only water and an anionic surfactant, Aerosol 0T (American Cyanamid), for p a r t i c l e generation. Generally the polymerizations were carried out at 80-85°C using ammonium persulfate as the i n i t i a t o r , although some redox polymerizations were also carried out. Uniform staged, and two-tank power feed techniques were used to introduce the monomers into the reactor. Samples were taken p e r i o d i c a l l y during the three-hour feed time for residual monomer analysis. Generally, not more than 3-5% unreacted monomer was detected at any time during the reaction, thus assuring monomer starved conditions. The finished latexes were cast or molded into films for testing. RESULTS AND DISCUSSION The properties of polymers prepared by different processe can be studied in a variety of ways, the choice often depending on the intended end use of the polymer. For coatings applications, mechanical testing of films can lead to an understanding of polymer structure as well as to a prediction of end use performance ( 5 j . As an i l l u s t r a t i o n of the influence of monomer feed profiles on


EMULSION POLYMERS


376

Far Feed Tank Figure 3.


Middle Feed Near Feed Tank Tank

Latex Reactor

Three-tank monomer feed arrangement in which the near and middle tanks are stirred during operation

Figure 4. Monomer concentration profiles of a single monomer in the feed stream entering the reactor as a function of time for the three-tank arrangement


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the p r o p e r t i e s of latex f i l m s , a s e r i e s of emulsion polymers was prepared and examined in t h i s manner. Examples o f mechanical measurements included dynamic mechanical, s t r e s s - r e l a x a t i o n , and b r i t t l e - d u c t i l e experiments. P a r t i c l e s t r u c t u r e was i n v e s t i g a t e d by p a r t i c l e expansion measurements u t i l i z i n g a sedimentation technique. Stiffness-Temperature - One simple t e n s i l e t e s t of polymers involves the measurement of a modulus as a f u n c t i o n of temperature. Figure 5 i l l u s t r a t e s the c h a r a c t e r i s t i c s of such a measurement. The modulus in t h i s case i s the secant modulus at 1% s t r a i n . Both polymers had the same composition: 39/59/2 - methyl methacrylate/ butyl a c r y l a t e / a c r y l i c a c i d . The power feed example was prepared such that butyl a c r y l a t e v a r i e d 0.83 — 0.30 and methyl meth a c r y l a t e v a r i e d 0.15 — * 0.68 as the polymerization proceeded, with χ = 0.83. While both examples show the usual trend of high modulus at low temperature to low modulus at higher temperature, the power feed polymer e x h i b i t s a much broader t r a n s i t i o n region than the uniform example. Stress Relaxation - The a b i l i t y to p r e d i c t long term p r o p e r t i e s of polymers using short term t e s t s i s of obvious benefit in develop ing high performance coatings from l a t e x e s . Relaxation t e s t s have been developed f o r t h i s purpose and serve as a useful means of i n v e s t i g a t i n g the v i s c o e l a s t i c p r o p e r t i e s of polymers. In a s t r e s s r e l a x a t i o n experiment, a constant s t r a i n i s a p p l i e d to the specimen and the r e s u l t i n g s t r e s s i s measured as a f u n c t i o n o f time. The time-dependent r e l a x a t i o n modulus can be extended to long times by making measurements over a wide temperature range and using the p r i n c i p l e of time-temperature s u p e r p o s i t i o n (6) to s h i f t the moduli to form a master curve as i l l u s t r a t e d in Figure 6. In t h i s case two polymers o f the same composition (62/38-methyl methacrylate/ butyl a c r y l a t e ) are compared at a reference temperature of 2 6 ° C . The power feed example u t i l i z e d a feed p r o f i l e with i n c r e a s i n g butyl a c r y l a t e , 0 —*» 0.65, decreasing methyl methacrylate con c e n t r a t i o n , 1 . 0 — * · 0.35, and χ = 1.3. The s i m i l a r i t y o f the s t r e s s r e l a x a t i o n r e s u l t s i n Figure 6 and the s t i f f n e s s - t e m p e r a t u r e r e s u l t s i n Figure 5 i s q u i t e apparent While the uniform (or random) copolymer e x h i b i t s the usual abrupt t r a n s i t i o n between b r i t t l e and rubbery regions with i n c r e a s i n g time, the power feed example undergoes a much more gradual change. T h i s broadened t r a n s i t i o n region may be an important f a c t o r in the a b i l i t y of a f i l m to r e l i e v e s t r e s s g r a d u a l l y to avoid c r a c k i n g over long periods of time. Dynamic Mechanical T e s t i n g Film p r o p e r t i e s such as impact r e s i s t a n c e and the cure response of thermosetting r e s i n s are con v e n i e n t l y i n v e s t i g a t e d by dynamic measurements i n which an o s c i l l a t o r y or t o r s i o n a l s t r a i n i s a p p l i e d to the sample with the s t r e s s and phase d i f f e r e n c e between the a p p l i e d s t r a i n and measured s t r e s s being determined. In the present study, a Rheovibron Viscoelastometer was used which employed a s i n u s o i d a l s t r a i n at a


378

EMULSION POLYMERS

1

1

ι

r


1

1

1

1

6 -

\

-

POWER FEED

CO UNIFORM FEED


-

3 9 / 5 9 / 2 - M MA/BA/AA -30

1 -20

1 -10

L_ -10

1 0

1

1

1

20

30

40

1 50

TEMPERATURE, °C

Figure 5. Stiffness-temperature comparison of two emulsion polymers having the same overall composition. In the power feed example, the methyl methacrylate concentration varied (0.15 0.68) and the butyl acrylate concentration varied (0.83 0.30), with χ = 0.83.

0

2

6

4

Log

t

8

10

12

(MIN)

Figure 6. Stress relaxation comparison of two emulsion polymers having the same overall composition. The power feed example utilized a feed profile with increasing butyl acrylate (0 -> 0.65), decreasing methyl methacrylate (1.0 —» 0.35), and χ = 1.3. The time axis has been shifted to a reference temperature of 26°C.



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frequency of 11 Hz and a heating rate of 1°C per minute. Using the standard treatment of data from this type of experiment (7J, the storage modulus, E' (a measure of e l a s t i c response), and the loss modulus, E" (a measure of the viscous response), were c a l culated and displayed as a function of temperature in Figures 7-10 for a series of four latexes having the same composition: 50/50 styrene/ethyl acrylate. Figure 7 shows the dynamic mechanical response obtained with the latex prepared with a constant monomer feed composition. As expected, a single sharp t r a n s i t i o n i s observed characteristic of a reasonably uniform copolymer. Figure 8 shows the dynamic mechanical spectrum for a two-stage process in which the f i r s t stage feed was ethyl acrylate and the second stage feed was styrene. This time, two well-defined transitions are observed characteristic of the hard and soft homopolymers. The dynamic mechanical responses of the two power feed latexes are quite different from either of those discussed above. In Figure 9, the polymer was prepared via a linear power feed pro f i l e in which the near tank contained only ethyl acrylate and the far tank contained only styrene. In Figure 10, the polymer was prepared with the tanks reversed: the monomer feed began with styrene and ended with ethyl acrylate. In both cases, the transition regions are much broader than those observed with the uniform feed or staged feed examples. The broadened transition response i s a general character i s t i c of power feed polymers as evidenced by the results of stiffnesstemperature, stress relaxation and dynamic mechanical measurements. The broadening phenomenon i s probably a result of an alloying effect caused by the wide variety of sequencing induced by the continuously changing monomer feed composition. Indeed, sequence d i s t r i b u t i o n analysis i s an important aspect of the characterization of non-uniform emulsion polymers ( 4 ) . The broadening effect may also result from compositional differences in polymer domains that form at different stages in the reaction. In terms of practical film properties, this broadened response suggests a wider usetemperature range which i s of importance in many coatings a p p l i cations. B r i t t l e - D u c t i l e - F l e x i b i l i t y i s a mechanical property of great interest in polymer design especially for polymers intended for use in films. Impact tests provide information about the a b i l i t y of a film to withstand a high rate of deformation. Bending and drawing operations generally occur at lower strain rates and test the a b i l i t y of a film to withstand severe elongations. The analysis of stress-strain behavior of polymers over a range of temperatures shows that â b r i t t l e specimen generally breaks at low elongations without exhibiting y i e l d , whereas a ductile specimen exhibits a y i e l d point which permits greater elongation before f a i l u r e occurs. The temperature at which a sample changes from b r i t t l e to ductile can be called the b r i t t l e - d u c t i l e transition temperature ( Τ ρ ) . β



380

EMULSION POLYMERS

Figure 7 .


Dynamic mechanical properties of a model latex made with a uniform monomer feed composition

130

-90

-50

-10

30

70

T E M P , DEG C

Figure 8. Dynamic mechanical properties of a model latex made with a two-stage monomer feed: Stage I contained ethyl acrylate, Stage II contained styrene.



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381

Figure 9. Dynamic mechanical properties of a model latex made with a linear power feed process; ethyl acrylate varied 1-^0 and styrene varied 0 -> 1.

Figure 10. Dynamic mechanical properties of a model latex with a linear power feed process; ethyl acrylate varied 0 — > 7 and styrene varied 1 —» 0.


382


POLYMERIZATION

Although the exact nature of the B-D transition i s not well under stood, Wu (8) has shown that T is related to the a b i l i t y of plastics to be deep-drawn. Since the implications of such behavior to coatings are obvious, we desired to learn i f non-uniform polymers offered any advantage in this respect. A convenient wav of measuring Τβπ i s to determine the ultimate strain (at break) and the y i e l d or a specimen as a function of temperature. The intersection of the two curves defines T p. A constant strain rate of 10% per minute was employed. Polymers were isolated by a i r drying or freeze drying and molded at 150°C into dumbbell speciments for the tensile measurements. Glass transition temperatures (Tg) were measured by differential scanning calorimetry. The relationship between f l e x i b i l i t y and copolymer com position was explored by measuring Tg and Τβρ f ° series of copolymers consisting of styrene or methyl methacrylate paired with several "softer" comonomers. An example of the kind of results obtained is i l l u s t r a t e d in Figure 11 with the methyl methacrylate/ ethyl acrylate pair. Included in this plot i s a d u c t i l i t y , or toughness, parameter, q, defined by Wu as, B D


B

r

a

q=^lBD

( 1 3 )

g with temperatures in °K. In general, the objective i s to minimize Τβρ relative to Tg so that q i s as large as possible, i . e . , design a polymer which i s hard but f l e x i b l e over a wide range of tempera tures. As shown in Figure 11, the MMA/EA compositions yielded a f a i r l y smooth variation in T but a much more erratic variation in Τβρ. The two most f l e x i b l e composition ranges (highest q values) occurred at the extremes of the series, homopolymers of methyl methacrylate or ethyl acrylate, with inconvenient Tg's for most practical uses. The composition styrene/ethyl acryiate series yielded q values of less than 0.05 indicating less f l e x i b i l i t y over the entire composition range. This prediction was borne out by impact tests on the polymers of the series. Some of the con clusions of this study are: i ) no simple relationship between T and Tgp was found; i i j no simple correlation was detected between T ana sub-T transitions in the dynamic mechanical spectra of the samples, and i i i ) copolymer pairs containing methyl methacrylate were generally more f l e x i b l e (lower Τβρ) than the corresponding copolymers containing styrene. The effect of non-uniform polymer composition i s shown in Figure 11 for the case of a 50/50-methyl methacrylate/ethyl acrylate copolymer made by a linear power feed process in which the near tank i n i t i a l l y contained only ethyl acrylate and the far tank only methyl methacrylate. Compared with i t s uniform counterpart, the non uniform polymer had a T approximately 25° lower and a respectable q

g

BD

g

R n



BASSETT AND HOY

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373

Figure 11. Brittle-ductile behavior of a series of methyl methacrylate/ethyl acry late copolymers.

Wt % METHYL METHACRYLATE

The "toughness" parameter q is defined by Equation 13. The effect of nonuniform polymerization is shown for a. 50/50 co polymer in which the power feed profile involved a decreasing ethyl acrylate concen tration 1), with χ = 1 (linear). ((· ) power feed, q = 0.19)


384


POLYMERIZATION


q value of almost 0.20. Indeed, most of the power feed polymers, as well as the multi-staged polymers examined, were characterized by a significant lowering of Τβη· This behavior could be another manifestation of the alloying effect discussed e a r l i e r . Particle Structure - So far, the discussion of non-uniform emulsion polymers has centered around mechanical properties, interpreted in terms of gradations of sequences between set compositional l i m i t s . But latexes are aqueous dispersions, and an exploration of p a r t i c l e morphology i s equally interesting and important. For instance, can polymer particles be constructed such that particle sequence compositions are located in a desired region of the p a r t i c l e , and what evidence exists that such particle morphologies have been realized? We have carried out extensive investigations of the swelling properties of carboxylic latex particles with increasing PH (£>10). Particles expand on neutralization due to electrostatic repulsion of the charged carboxylate groups and subsequent absorption of water. The major factors which control p a r t i c l e expansion are: the type and concentration of copolymerized acid, the stiffness (Tg) and hydrophilicity of the backbone, and the polymerization process employed. The process variable of most interest was the use of varying monomer compositions in stepwise changes. It was found that the expansion properties of carboxylic particles were controlled to a large extent by the order of addition of the different monomer compositions. P a r t i c l e expansion, then, may be useful as a means of exploring the structures of emulsion polymers made in various ways. The expansion behavior of carboxylic latex particles can be studied by several methods (]_0). The present comparison was made using a sedimentation method which involved the measurement of p a r t i c l e sedimentation rates in an ultracentrifuge at various degrees of neutralization. Assuming the change in p a r t i c l e volume is equal to the volume of water absorbed, an expanded particle settles slower, as i t s density decreases, according to the equation: ^o_r+x S r

(14)

Where S is the sedimentation coefficient of the p a r t i c l e at an adjusted pH, S i s the sedimentation coefficient of the unswollen particle (low pH), r i s the unswollen particle radius, and χ is the increase in radius of the swollen p a r t i c l e . In this study, the model latexes were diluted with d i s t i l l e d water to 1 percent solids by weight. Individual samples were adjusted to various pH values with sodium hydroxide and allowed to equilibrate for at least 24 hours. Sedimentation rates were obtained at 30°C using a Beckman Model Ε analytical ultracentrifuge. Two latexes were prepared with the composition: 47.5/47.5/5-styrene/ethyl acrylate/methacrylic acid. The power feed example had the monomer feed p r o f i l e shown in Figure 12, with 0



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the styrene concentration increasing with time and ethyl acrylate decreasing as the reaction proceeded. The methacrylic acid con centration in the monomer feed was held constant in both cases. The particle expansion characteristics of the two latexes are shown in Figure 13. With 5 percent incorporated acid, both latexes have considerable potential for expansion upon neutralization. Not only did the power feed example f a i l to show the substantial expansion exhibited by the uniform feed example, but particle con traction was observed at the higher pH's characteristic of latex particles having no incorporated acid (9). Assuming carboxyl groups tend to locate in the surface regions of p a r t i c l e s , the r e s t r i c t i o n in the expansion of the power feed example may be explained by the preferential polymerization of the styrene-rich segments near the sruface of the growing particles as opposed to polymerization in the particle i n t e r i o r . The "onion skin" growth mechanism i s supported by filming experiments in which film formation i s greatly effected by the nature of the monomer composition added last in the polymeri zation. In power feed examples, as well as in staged feeds, hard and hydrophobic compositions hinder film formation while softer and more hydrophilic compositions aid film formation. Curiously, in this respect, i t was found that the filming characteristics of a l l - a c r y l i c latexes responded to non-uniform polymerization techniques much more dramatically than did their styrene-acrylic counterparts. Molecular Weight Control - In addition to the control of monomer reaction sequences and particle morphology, the power feed process lends i t s e l f to molecular weight manipulation in ways not possible with uniform polymerizations. Molecular weight modifiers can be used separately or in combinations to produce unusually broad molecular weight d i s t r i b u t i o n s . One p a r t i c u l a r l y useful technique involves the use of a multifunctional monomer in the near tank and a chain transfer agent in the far tank to produce latex particles with very high molecular weight capable of forming smooth, glossy films (11 ). The response of particle coalesence to small amounts of a chain transfer agent added late in a polymerization is another indication of the surface growth mechanism mentioned above and is consistent with other film studies of heterogeneous latexes (12). CONCLUSIONS A simple multi-tank arrangement for continuously changing the composition of a monomer feed stream into a latex reactor has been described and analyzed. The process i s useful for preparing controlled non-uniform emulsion polymers. Such polymers exhibit broad transitions in dynamic mechanical and stress-relaxation measurements which imply a broad range of sequence distributions within the polymers and possibly polymer domains that form during the changing feed composition. The process can also be used to control the p a r t i c l e morphology of emulsion polymers to affect particle coalescence, v i s c o s i t y control and functional group


EMULSION POLYMERS



386

Figure 13. The effect of nonuniform polymerization on the expansion behavior of carboxylic emulsion polymers. The power feed example was prepared using the monomer feed profile illustrated in Figure 12 ((Φ) uniform feed; (J^) power feed).



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Nonuniform Emulsion Polymers

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location. Finally, the multiple tank arrangement offers unique opportunities for molecular weight control through the use of molecular weight modifiers. ABSTRACT A novel polymerization technique has been developed by which non-uniform emulsion polymers can be produced in a con trolled manner. The technique involves the continuous addition of one monomer mixture into a stirred tank containing another monomer mixture. This continuously changing mixture is then fed into a reaction vessel producing, thereby, polymers whose instantaneous copolymer composition varies as the polymerization proceeds. A general equation is developed which expresses the composition of the feed stream as a function of time. The process, call "power feed", is used to prepare emulsion polymers which exhibit dynamic mechanical spectra having broad transition ranges. Stress relaxation measurements also show a broadened transition range compared with analogs prepared using a uniform monomer feed profile. The characteristics of power feed latexes are interpreted in terms of an alloying of a wide range of sequence distributions and/or polymer domains of differing compositions superimposed on a particle morphology in which the surface properties of the particles re flect the characteristics of the polymer formed last. Practical applications include the control of particle expansion of carboxylic emulsion polymers on neutralization, the increased flexibility of relatively hard polymers and the control of molecular weight distributions for specific latex applications. ACKNOWLEDGMENTS The authors gratefully acknowledge the following people for their experimental contributions to this work: R. W. Cal lard, L. C. Cantley, Y. P. Chang, S. L. Hager and R. E. Kelchner. LITERATURE CITED 1. See for example: Ryan, C. F . , U.S. Patent 3,562,235; Dickie, R. Α.; and Newman, S., U.S. Patent 3,787,522. 2. Bassett, D. R.; Hoy, K. L . ; U.S. Patent 3,804,881. 3. Whelan, J . M.; Union Carbide Corporation. 4. Johnston, J. E . ; Bassett, D. R.; MacRury, T. B.; This Volume, following paper. 5. H i l l , L. W., Prog. Org. Coatings, 1977, 5, 277. 6. Aklonis, J . J.; MacKnight, W. J.; Sher.,Μ.,"Introduction to Polymer Viscoelasticity," Wiley-Interscience, New York, 1972. 7. Nielsen, L. E., Mechanical Properties of Polymers and Composites, Marcel Dekker, New York, 1974, Volume 1. 8. Wu, S., J . Appl. Polymer S c i . , 1976, 20, 327. 9. Bassett,. D. R.; Hoy, K. L.; "Polymer Colloids II,", Fitch, R. M., ed., Plenum, New York, 1980, p.1. 10. Bassett, D. R.; Derderian, E. J.; Johnston, J . E., MacRury, Τ. Β . , This Volume. 11. Bassett, D. R.; Hoy, K. L.; U.S. Patent 4,039,500. 12. Eliseeva, V. I., Acta Polymerica, 1979, 30, 273. RECEIVED April 6, 1981.


Nonuniform Emulsion Polymers - American Chemical Society

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