suspension polymerization
DESCRIPTION
Suspension PolymerizationTRANSCRIPT
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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�
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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�
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Suspension Polymerization Reactor�
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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)�
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Suspension Polymerization�
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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�
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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 �
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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 �
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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�
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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�
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Control of PSD�
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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�
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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�
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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�
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Droplet size �
Dependence of the steady-state Sauter mean diameter on the agitation speed for (a) various PVA grades and (b) concentrations�
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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 �
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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�
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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�
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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�
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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�
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Powder suspension polymerization�
• The polymerization is
commonly carried out
isothermally�
• Temperatures in the
range of 45–70°C
(depending on MW)�
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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�
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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�
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Suspension Polymerization Reactor�
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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)�
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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)�
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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%�
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Evolution of PSD�
Dynamic evolution of the Sauter mean diameter of PVC particles with respect to polymerization time�