Polymer devolatilization, also known as polymer degassing, is the process of removing low-molecular-weight components such as residual monomers, solvents, reaction by-products and water from polymers.

Motivation

When exiting a reactor after a

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

reaction, many polymers still contain undesired low-molecular weight components. These component may make the product unusable for further processing (for example, a

polymer solution Polymer solutions are solutions containing dissolved polymers. These may be (e.g. in ), or solid solutions (e.g. a substance which has been plasticized). The introduction into the polymer of small amounts of a solvent (plasticizer) reduces the te ...

cannot directly be used for plastics processing), may be

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

, may cause bad sensory properties such as an unpleasant smell or worsen the properties of the polymer. It may also be desirable to recycle monomers and solvents to the process.

Plastic recycling Plastic recycling is the processing of plastic waste into other products. Recycling can reduce dependence on landfills, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions. Recycling rates lag beh ...

can also involve removal of water and volatile degradation products.

Basic process types

Devolatilization can be carried out when a polymer is in the solid or liquid phase, with the volatile components going into a liquid or gas phase. Examples are: * Solid polymer, liquid phase: Extraction of

caprolactam Caprolactam (CPL) is an organic compound with the chemical formula, formula (CH2)5C(O)NH. This colourless solid is a lactam (a cyclic amide) of caproic acid. Global demand for this compound is approximately five million tons per year, and the vast ...

from polyamides with water. * Solid polymer, gas phase: Removal of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

from

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...

via air or nitrogen in silos. * Liquid polymer, gas phase: Removal of

styrene Styrene is an organic compound with the chemical formula C6H5CH=CH2. Its structure consists of a vinyl group as substituent on benzene. Styrene is a colorless, oily liquid, although aged samples can appear yellowish. The compound evaporates easi ...

from

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It i ...

via vacuum. It is usual for different types of devolatilization steps to be combined to overcome limitations in the individual steps.

Physical and chemical aspects

Thermodynamics

The

thermodynamic activity In thermodynamics, activity (symbol ) is a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on conc ...

of volatiles needs to be higher in the polymer than in the other phase for them to leave the polymer. In order to design such a process, the activity needs to be calculated. This is usually done via the

Flory–Huggins solution theory Flory–Huggins solution theory is a lattice model (physics), lattice model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecule, molecular sizes in adapting the usual expression (mathematics), exp ...

. This effect can be enhanced via higher temperatures or lower partial pressure of the volatile component by applying an inert gas or lower pressure.

Diffusion

In order to be removed from the polymer, the volatile components need to travel to a phase boundary via

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

. Because of the low diffusion coefficients of volatiles in polymers, this can be the rate-determining step. This effect can be enhanced by higher temperatures or by small diffusion lengths due to its higher

Fourier number In the study of heat conduction, the Fourier number, is the ratio of time, t , to a characteristic time scale for heat diffusion, t_d . This dimensionless group is named in honor of J.B.J. Fourier, who formulated the modern understanding of h ...

Heat transfer

Because polymers and polymer solutions often have a very high viscosity, the flow in devolatilizers is

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, leading to low

heat transfer coefficient In thermodynamics, the heat transfer coefficient or film coefficient, or film effectiveness, is the Proportional (mathematics), proportionality constant between the heat flux and the thermodynamic driving force for the Heat transfer, flow of heat ...

s, which can also be a limiting factor.

Chemical stability

Higher temperatures can also affect the chemical stability of the polymer and thus its use properties. If a polymer's

ceiling temperature Ceiling temperature (T_c) is a measure of the tendency of a polymer to revert to its constituent monomers. When a polymer is at its ceiling temperature, the rate of polymerization and depolymerization of the polymer are equal. Generally, the ceilin ...

is exceeded, it will partially revert to its monomers, destroying its usability. More generally,

polymer degradation Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycl ...

also occurs during devolatilization, limiting the temperature and residence time available for the process.

Foam vs. film devolatilization

There are two basic forms of devolatilization to a vacuum. In foam devolatilization, bubbles inside the polymer solution nucleate and grow, finally bursting and releasing their volatile content to the surroundings. This requires sufficient vapor pressure. If possible, this is a very efficient method because the volatiles only need to diffuse a short way. Film devolatilization occurs when there is no longer sufficient vapor pressure to generate bubbles, and requires on sufficient surface area and good mixing. In this case, stripping agent such as nitrogen may be added to the polymer to induce improved mass transfer through bubbles.

Types of devolatilizers for polymer melt

Devolatilizers for polymer melts are classified as static or moving, also called "still" and "rotating" in the literature.

Static devolatilizers

Static devolatilizers include: * Falling strand devolatilizers: Polymer is partitioned into many individual strands which fall down in a vacuum chamber. Diffusion moves volatiles into the gas phase, which are then collected via a vacuum system. This is usually the last stage of a devolatizing process, when vapor pressure is low. * Falling film evaporator: Polymer falls down vertical walls, volatiles diffusing on the side that is not in contact with the walls. * Tube evaporators: A boiling polymer solution flows downward in a vertical

shell and tube heat exchanger A shell-and-tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this ...

into a separator. Polymer is collected at the bottom, vapor is collected via a vacuum system and condensers. * Flash evaporators: A polymer solution is preheated and brought into a separator, where pressure below the vapor pressure of the solution leads to a part of the volatiles evaporating.

Moving devolatilizers

* Co-rotating twin screw extruders: The polymer solution is brought into a co-rotating twin screw extruders, where it is subjected to shear and mechanical energy input and where vapors are drawn off. This type of machine allows different pressures in different zones. An advantage is the self-cleaning action of those extruders. * Single-screw extruders: In principle similar to co-rotating twin screw extruders, without the self-cleaning action. * Wiped-film evaporators: Polymer solution is brought into a single large vessel, where a rotor agitates the product and creates surface renewal. Only a single pressure level is possible in these machines. * Large-volume kneaders: A polymer solution is brought into a large-volume kneader and subjected to shear at longer residence times than in an

extruder Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

Devolatilizers for suspensions and latexes

Removal of monomers and solvents from

latex Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latices are found in nature, but synthetic latices are common as well. In nature, latex is found as a wikt:milky, milky fluid, which is present in 10% of all floweri ...

and suspensions, for example in the production of

synthetic rubber A synthetic rubber is an artificial elastomer. They are polymers synthesized from petroleum byproducts. About of rubber is produced annually in the United States, and of that amount two thirds are synthetic. Synthetic rubber, just like natural ru ...

, is usually done via stirred vessels.

References

{{Reflist Chemical engineering Process engineering Polymers