Suspension Polymerization

1

2

Polymer Reaction Engineering �

•  Polymers a brief market overview �

•  Introduction to polymerization processes�

•  Coordination polymerization �

•  Free radical polymerization �

•  Suspension polymerization �

•  Emulsion polymerization �

•  Step-growth polymerization �

•  Control of polymerization reactors�

3

Suspension Polymerization�

•  The suspension polymerization process is typically carried out in well-

stirred batch reactors�

•  The volume of the reaction vessel can be up to 150 m3 �

•  The monomer(s) is (are) initially dispersed in the continuous phase

(commonly water) by the combined action of surface active agents

(inorganic or/and water-soluble polymers) and agitation �

•  All the reactants (monomer(s), initiator(s), etc.) reside in the organic or

“oil” phase�

4

Suspension Polymerization Reactor�

5

Suspension Polymerization�

•  The polymerization occurs in the monomer droplets that are progressively

transformed into sticky, viscoelastic monomer–polymer droplets and finally

into rigid, spherical polymer particles in the size range of 50–500 µm�

•  The polymer solids’ content in the fully converted suspension is typically

30–50% w/w �

•  In the inverse suspension polymerization, the hydrophilic monomer(s) (e.g.,

acrylamide, acrylic acid) and initiator are dispersed in the hydrophobic

continuous organic phase (e.g., hexane, paraffin oil)�

6

Suspension Polymerization�

7

Bead and Powder�suspension polymerization�

“Bead” suspension polymerization: �

•  The polymer is soluble in its monomer and smooth spherical particles are

produced�

•  The most important thermoplastic produced by the “bead” suspension

polymerization process is PS�

•  In the presence of volatile hydrocarbons (C4−−C6), foamable beads, the

so-called EPS, are produced�

8

Bead and Powder�suspension polymerization�

“Powder” suspension polymerization: �

•  The polymer is insoluble in its monomer and, thus, precipitates out leading

to the formation of irregular grains or particles�

•  PVC is an example of the “powder” type suspension polymerization �

9

Advantage of suspension polymerization�

The main advantages of suspension polymerization compared to the bulk

process are�

•  Easier control of the reaction temperature due to the presence of the

dispersion medium (e.g., water)�

•  Milder reaction conditions�

•  Product homogeneity, especially for monomers having a very low solubility

in the continuous phase�

•  Higher purity than those produced by emulsion polymerization �

10

Disadvantage of suspension polymerization�

•  Low reactor productivity due to the presence of the dispersion medium

(e.g., 50% v/v)�

•  The required post-treatment of the dispersion medium for removing all the

undesired impurities (e.g., suspending agents, etc.)�

•  Difficulty in the production of homogeneous copolymers, especially when

the monomers have different reactivities and solubilities in the continuous

phase�

11

Control of PSD�

•  In general, the initial monomer droplet size distribution (DSD) as well as

the polymer PSD depends on the type and concentration of the surface

active agent, the quality of agitation (e.g., reactor geometry, impeller

type, power input, etc.) and the physical properties (e.g., densities,

viscosities, interfacial tension) of the continuous and dispersed phases�

•  The dynamic evolution of the droplet/PSD is controlled by the rates of

two physical processes, namely, the drop/particle breakage and

coalescence�

12

Control of PSD�

13

Control of PSD�

Droplet breakage: �

•  Mainly occurs in regions of high shear stress (i.e., near the agitator

blades) or as a result of turbulent velocity and pressure fluctuations along

the drop’s surface�

Drop/particle coalescence�

•  Can be increased / decreased by the turbulent flow field�

•  At sufficiently high concentrations of surface active agents, it can be

assumed to be negligible for very dilute dispersions�

14

Control of PSD�

The suspension polymerization process can be divided into three stages�

•  At low monomer conversions (i.e., low viscosity of the monomer–polymer

phase, stage one), drop breakage is the dominant mechanism�

•  During the second sticky-stage of polymerization, the drop breakage rate

progressively decreases while drop/particle coalescence becomes the

dominant mechanism�

•  At higher monomer conversions, the particles are sufficiently hard so the

collisions between them are elastic and, thus, the particle coalescence

ceases (identification point)�

•  After this point, the PSD has been established�

15

Surface active agents�

•  Play a very important role in the stabilization of liquid–liquid dispersions�

•  They can be water-soluble copolymers (e.g., poly(vinyl alcohol) (PVA) and

cellulose ethers) or colloidal inorganic powders (Pickering dispersants, e.g.,

tricalcium phosphate, barium sulfate, calcium carbonate, etc.)�

•  These stabilizers reduce the drop/particle coalescence�

•  Water-soluble substituted celluloses are mainly used in the manufacture

of PVC�

16

Droplet size �

Dependence of the steady-state Sauter mean diameter on the agitation speed for (a) various PVA grades and (b) concentrations�

17

Bead suspension polymerization�

•  The polymer is soluble in its monomer and, thus, the monomer–polymer

mixture is homogeneous�

•  Polystyrene for injection molding �

•  Poly(methyl methacrylate) and its copolymers containing small amounts of

acrylate esters�

•  Styrene–acrylonitrile copolymers azeotropic monomer/comonomer

composition to minimize copolymer compositional drift �

18

EPS�

•  Suspension polymerization in the presence of a blowing agent (e.g.,

pentane)�

•  It is also possible to introduce the blowing agent to the polymer after

polymerization and allowing it to diffuse into the beads�

19

EPS�

•  Once the beads are hard, the reaction mixture is heated to a

temperature above the glass transition temperature of the PS, 100°C�

•  During heating, the reactor is pressurized with a blowing agent (usually n-

pentane) at 5–8% w/w �

•  Subsequently, the reactor is pressurized with nitrogen at 7–9 bars and

the so-called impregnation stage starts�

•  n-pentane diffuses into the “beads”�

•  The system is cooled down to 20–30◦C, so that no bead expansion can take

place during the discharge�

•  In the next stage, the excess of stabilizer is chemically removed�

20

EPS�

•  In the final processing,

the EPS beads are

warmed up to 80–110°C,

generally with steam

that causes the beads

to expand by foaming

and their volume to

increase by a factor of

30–50�

21

Powder suspension polymerization�

•  The “powder” suspension polymerization is the most important

polymerization process for manufacturing PVC�

•  The main advantage of this process is that large (e.g., 300–500 µm),

porous polymer particles can be produced�

•  Fast residual monomer removal rate�

•  Large plasticizer uptake capacity�

•  The production of polymer particles with desired PSD and porosity can be

achieved by changing the quantities and types of stabilizers as well as

the agitator speed�

22

Powder suspension polymerization�

•  The polymerization is

commonly carried out

isothermally�

•  Temperatures in the

range of 45–70°C

(depending on MW)�

23

PVC suspension polymerization�

•  The main difference between the “bulk” and the suspension process is

that agitation is used to control not only the aggregation of the primary

particles but also the size distribution of the final grains�

•  Above a critical monomer conversion (i.e., xc ∼ 30%) the volume

contraction of the polymerizing particles stops, which partially explains

the appearance of internal particle porosity�

24

PVC suspension polymerization�

In the VCM suspension polymerization, two types of stabilizers, primary and

secondary are used�

•  The main function of the primary surface active agents is to control the

grain size (grain porosity)�

•  Secondary stabilizers are surface active agents with a higher lipophilic

content (e.g., PVA stabilizers with low degree of hydrolysis and cellulose

ethers with high degree of substitution of the hydroxyl-groups)�

•  Decrease of the primary particles aggregation rate�

25

Suspension Polymerization Reactor�

26

Scale-up of suspension polymerization�

•  The scale-up of suspension polymerization reactors (i.e., from lab to pilot

and then to industrial scale) is not straightforward�

•  The most significant problem in scale-up occurs when different physical

processes become limiting at different scales�

•  Commercial-scale suspension reactors have to perform several functions

simultaneously�

Dispersion, reaction and heat transfer (do not scale-up in the same

manner)�

27

Scale-up of suspension polymerization�

•  Heat removal can become a limiting factor for reactor performance at

large scales while it is rarely a problem for lab-scale reactors�

•  In suspension polymerization, scale-up of an agitated tank reactor should

keep unchanged the particle morphology (e.g., PSD, porosity, bulk density) �

•  The reactor design can guarantee the heat removal�

•  Thus, the problem reduces to the scale-up of a liquid–liquid dispersion in

an agitated vessel�

(criteria: constant power input per unit volume, the impeller tip speed, the

Weber number, the Reynolds number)�

28

Evolution of PSD�

Dynamic evolution of the PSD with respect to polymerization time for VCM suspension polymerization �

TP: 56.5◦C�Impeller speed: 330 rpm�Dispersed phase volume fraction: 40%�

29

Evolution of PSD�

Dynamic evolution of the Sauter mean diameter of PVC particles with respect to polymerization time�