Removal of Volatile Organic Compounds from Bulk and Emulsion

Review Cite This: Ind. Eng. Chem. Res. 2019, 58, 11601−11623

pubs.acs.org/IECR

Removal of Volatile Organic Compounds from Bulk and Emulsion Polymers: A Comprehensive Survey of the Existing Techniques Alicia De San Luis,†,‡ Catherine C. Santini,† Yvan Chalamet,*,‡ and Veŕ onique Dufaud*,† †

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Université de Lyon, CNRS, UMR 5265 Laboratoire de Chimie, Catalyse, Polymères, Procédés (C2P2), 43 bd du 11 Novembre 1918, F-69616 Villeurbanne, France ‡ Université de Lyon, CNRS, UMR 5223, Ingénierie des Matériaux Polymères (IMP), F-42023 Saint-Etienne, France ABSTRACT: Volatile organic compounds (VOCs) are mostly toxic and hazardous substances that generally have a strong odor and can cause health and environmental problems. It is thus of upmost importance for the polymer manufacturing industry to eliminate these volatiles from the synthesized polymers. The properties and the application of the polymer, as well as the nature and the concentration of volatiles, govern the choice of the most appropriate elimination technique to avoid any modification or degradation of the polymer. In this review, we take stock of the most suitable purification methods currently used to remove VOCs from polymers in bulk and in emulsion with particular emphasis on the equipment and experimental requirements for each method, as well as the theoretical considerations that motivate the use of the method. In addition to conventional techniques, we are also presenting our efforts to identify potentially promising alternatives for reducing the VOC content from polymers. odors and flavors, and these impurities are often toxic, such as vinyl chloride, present in polyvinyl chloride (PVC) or acrylates present in paints or organic solvents. Although these monomers and other impurities are present in residual concentrations in the final products, the removal of VOCs from polymers has become a critical issue because the polymer manufacturers must meet current market needs. To fulfill the existing and future legal and social requirements on its products, the polymers’ industry is striving to find more effective and less costly solutions to remove VOCs from their products to ensure compliance with the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulations and environment rules.7 Depending on the polymer of interest, the synthesis process (bulk, emulsion, solution, suspension), and the final application of the polymer, different purification methods have been developed, mostly at the industrial scale, as witnessed by the rapid growth of the patent literature on this specific subject. Review articles and/or other academic literature on this topic remain, however, scarce and date to 2005 at the latest.8,9 The aim of the present report is to provide a comprehensive survey of the existing techniques used for the reduction of VOCs content from polymers, including bulk and waterborne emulsion polymers, by analyzing the past and more recent patent and open literature data. As the choice of the technique to be applied

1. INTRODUCTION Polymers are omnipresent in modern life, in personal care products, clothes, vehicles, and food packaging. A dizzying variety of polymers with various physicochemical properties, which have been synthesized by different methods that include bulk polymerization, emulsion polymerization, or polymerization in solution among others, are used for those applications. However, most polymerization reactions do not run to completion leaving unreacted monomers in the final polymer products. Those monomers are generally toxic; additionally, they have strong odors and may form explosive mixtures leading to safety and environmental issues. In addition to residual monomers other volatiles can also be present in a polymer, mainly nonpolymerizable compounds such as solvents or additives whose potential harmfulness may present additional toxicological risks. According to the European Union, all those residual volatile compounds can be treated as volatile organic compounds (VOCs). A VOC is defined as “any organic compound having an initial boiling point less than or equal to 250°C measured at a standard atmospheric pressure of 101.3 kPa”.1 Over the years, legislation concerning the VOCs, especially in terms of environment and health protection, has been increasingly restrictive with respect to the nature and the concentration of the permitted compounds in the final product.1−7 In addition to legislation, consumers are increasingly demanding in terms of health and risks associated with products such as food packaging, surgical prostheses, and healthcare products. All may contain impurities leading to unpleasant © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11601

February 19, 2019 May 22, 2019 May 27, 2019 May 27, 2019 DOI: 10.1021/acs.iecr.9b00968 Ind. Eng. Chem. Res. 2019, 58, 11601−11623

Review

Industrial & Engineering Chemistry Research is very case specific, the review has been organized according to the type of polymer and polymerization process focusing, for each case, on the theoretical aspects of the techniques, the mechanisms involved, and the equipment used. The advantages and limitations regarding VOCs removal efficiency of each method are also discussed through representative examples. Finally, future research directions as promising alternatives to more conventional state-of-the-art techniques are presented.

2.1.1.1. Mass Transfer. Generally speaking, devolatilization is a mass transport process. The volatiles present in the polymer melt diffuse to the polymer/vapor interface, and then they are transported to a gas stream to be removed and collected. The mass balance of a volatile organic compound under steady state conditions is given by eq 1:

2. PURIFICATION OF POLYMERS IN BULK The purification of polymers in bulk has been widely studied in order to remove the VOCs present in the final product whether they are residual monomers, solvents, or additives. Several physical and chemical techniques are currently employed with different degrees of success in terms of removal efficiency. The most common and reported purification method, the devolatilization procedure, is first deeply analyzed, focusing on the physical mechanisms involved, the parameters that govern the process, and a description of the equipment. Additionally, less common techniques for the removal of volatiles from bulk polymers, namely postpolymerization, temperature increase, or addition of a reactive comonomer are also discussed. 2.1. Devolatilization. In the devolatilization process, the removal of VOCs is based on the diffusion of VOCs from the polymer to the polymer/vapor interface for their posterior transport out of the devolatilization equipment. Therefore, the diffusion coefficient, the thermodynamic equilibrium, and the interfacial area are crucial parameters that can be tuned in order to implement the technique and achieve good purification results.8−13 The devolatilization mechanisms, the parameters influence in terms of purification efficiency and the main equipment will be discussed in detail in the following subsections. 2.1.1. Mechanisms. In the purification of a polymer melt by devolatilization, there are two dominant mass transfer mechanisms, diffusion and bubble nucleation−foam formation (Figure 1). Additionally, the rate of devolatilization can be

where win is the VOC mass fraction coming into the equipment, Qin is the mass flow coming into the equipment, wout is the VOC mass fraction going out from the equipment, Qout is the mass flow going out from the equipment, wv is the VOC mass fraction evaporated and Qv is the evaporated VOC mass flow. The removal rate of VOCs (wvQv) can be expressed in terms of the interfacial area (A) and the flux of VOCs through the interface (k), polymer melt/vapor or bubble/vapor (eq 2).

winQ in = woutQ out + wvQ v

(1)

wvQ v = kA

(2)

The total interfacial area (A) is the sum of the interfacial area of the polymer melt surface and the interfacial area of each bubble. In the case of a diffusion mechanism, the interfacial area of each bubble is zero; therefore the only term that influences the VOCs removal rate is the polymer melt interfacial area with the VOC vapor. In the case of a bubble-foaming devolatilization mechanism, the interfacial area between the bubbles and the polymer melt and between the bubbles and the VOC vapor have to be taken into account to calculate the removal rate. In conclusion, the interfacial area is a key parameter in the devolatilization process as it has a direct influence on the removal rate of the VOCs present in a polymer melt independently from the mechanism taking place. To maximize this parameter it is necessary to renew the surface of the polymer melt, which can be achieved by mechanical agitation, and/or by generating bubbles and foaming in the polymer melt. This will be explained in more detail in the following sections. Polymer melts consisting of a homogeneous mixture of polymer and VOC (or solute) are not ideal solutions from a thermodynamic point of view. Therefore, the constraints generated by the macromolecules, as well as the polymer− VOC interactions have to be taken into account. For this, the Flory−Huggins equation is used (eqs 3 and 4):

or

ΔG = RT[n1 ln ϕ1 + n2 ln ϕ2 + n1ϕ2x12]

(3)

ji P zy lnjjj 2 zzz = ln(1 − ϕ2) + ϕ2 + x12 ϕ22 j P* z k 2{

(4)

where n is the number of moles of each component (1 referring to the polymer and 2 to the solute) and ϕ is the volume fraction of each component, the term x12 is the Flory−Huggins parameter representing the interdispersion energy of the polymer and solute molecules, P2 is the solute vapor pressure in equilibrium with the polymer−solute mixture and P2* is the pure solute vapor pressure. At low solute (VOC) concentration, ϕ2 → 1, and eq 4 can be simplified and related to the densities of the polymer and solute as shown in eq 5 and eq 6.

Figure 1. Schematic representation of the devolatilization process. Polymer melt diffusion (left) and bubble-foaming (right) mechanisms with A representing the interfacial area.

improved if a stripping agent is introduced in the system. First, the mass transfer principles and the equations driving the process are going to be presented.12 Then, the diffusion and bubble nucleation and foam formation mechanisms are developed. Finally, the effects of introducing a stripping agent to improve the devolatilization process are explained.

ij P yz lnjjj 2 zzz = ln(1 − ϕ2) + 1 + x12 j P* z k 2{

11602

(5)

DOI: 10.1021/acs.iecr.9b00968 Ind. Eng. Chem. Res. 2019, 58, 11601−11623

Review

Industrial & Engineering Chemistry Research P2 =

ρ1 ρ2

ω2P2*e(1 + x12)

(6)

where ρx is the density of the polymer (1) or of the solute (2) and ω2 is the weight fraction of the solute (VOC) in the polymer. From eq 6 it can be concluded that the reduction of the vapor pressure minimizes the mass of solute present in the polymer matrix. Moreover, an increase in the temperature also reduces the weight fraction of the solute, decreasing the viscosity of the polymer. This conclusion is illustrated in Figure 2 for a better

Figure 3. Schematic representation of the diffusion process of VOCs from a polymer melt.

shows a schematic representation of the process. The polymer melt containing the VOCs enters into the devolatilization device, and then the VOCs diffuse through the polymer melt to the vapor phase and are removed up to a concentration that depends on the efficiency of the equipment. The polymer melt is then evacuated from the devolatilizer. The diffusion of compounds in polymer melts is complex because of the evolution of the system between the liquid and the solid state depending on the temperature. The diffusion coefficient depends on the composition of the polymer−VOCs mixture, its swelling, and on the temperature of the system. An increase in the temperature of the system results in a decrease of the viscosity of the polymer melt, thus enhancing the overall devolatilization process. The rate of the devolatilization process is controlled by the diffusion of the volatiles throughout the polymer melt. The concentration of volatiles at the liquid/vapor interface is in equilibrium with the concentration of the volatiles in the gas phase.14 The theories and physical models of diffusion may help us to understand and predict the diffusion behavior of small molecules into a polymer system. One of the major contributions in the physical description of the diffusion phenomena of solutes in polymers was brought by Vrentas and Duda in the 1980s.15−17 They studied different polymer−solute systems under a wide range of temperature and concentration. Their model is based on the concept of free volume in a polymer (eq 9), which is defined as the volume not occupied by the polymer backbone (Vhole).

Figure 2. Theoretical calculation of the weight fraction of different solutes in polymer solutions as a function of the temperature under 100 Pa and fixing x12 = 0.3.

understanding. The theoretical evolution of the weight fraction of different solutes in polymer−solute mixtures is plotted as a function of the temperature, under a pressure of 100 Pa, and an interaction parameter fixed at x12 = 0.3. The vapor pressure of the pure solute (P2* or Psat) was calculated using Antoine’s eq 7 and Wagner’s eq 8 equations. Moreover, tabulated data for parameters A, B, and C, specific for each compound, can be found in several sources such as NIST Chemistry WebBook. T=

B −C A − log P

ij P yz Aτ + Bτ1.5 + Cτ 2.5 + Dτ 5 lnjjj sat zzz = j Pcrit z 1−τ k {

(7)

ij y jjτ = 1 − T zzz jj Tcrit zz{ k

(8)

From Figure 2, one observes that for polymer−solute mixtures containing water or methyl acrylate it is difficult to predict the weight fractions going below 100 ppm, while those values can be attained for ethylene glycol and vinyl acetate. One can also notice that it is possible to get weight fractions below 100 ppm at moderate temperatures (100−150 °C) for some of the solutes. In the case of ethylene glycol, weight fractions of around 40 ppm can be obtained at 300 °C, whereas for vinyl acetate the decrease of the weight fraction with temperature is faster attaining values below 10 ppm at 250 °C. Furthermore, if not fixing the Flory−Huggins parameter, the interdispersion energy of the polymer and solute molecules would increase, resulting in a decrease of the compatibility between the polymer and the solute. 2.1.1.2. Diffusion Mechanism. In most of the devolatilization processes bubbles are produced; however, some reports claim that conventional diffusion may occur at low concentrations of VOCs. Diffusion of volatiles through the polymer for their removal takes place due to a concentration gradient. Figure 3

V = Vs + Vhole

(9)

where V is the total volume, Vs is the specific volume of the skeleton of the macromolecules, and Vhole is the free volume. The free volume is temperature-dependent. It increases with the temperature as the motion of the macromolecules increases thus creating interstitial spaces in which the solvent molecules are free to move by free volume jumps. In addition to the free volume concept, Ventas and Duda’s model takes into account various other parameters such as the polymer concentration, the polymer−solute interactions, and the size and molar mass of the solute molecules. For example, for a binary system in which the solute molecules diffuse into the polymer matrix, the diffusion coefficient is expressed as follows (eq 10): 11603


ÄÅ ÅÅ ÅÅ Å expÅÅÅÅ− ÅÅ ÅÅ ÅÅÇ

ÉÑ ÑÑ ÑÑ ÑÑ ÑÑ K12ω2(K22 − Tg 2 + T ) Ñ ÑÑ ÑÑ + γ2 ÑÑÖ

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Industrial & Engineering Chemistry Research D = D01 e−E / RT

ω1V1̂ * + ω2ξV 2̂ * K11ω1(K21 − Tg1 + T ) γ1

(10)

where D01 is the diffusion coefficient of the solute alone, E is the activation energy of the volumetric free volume heats of the solute, ωi is the mass fraction of the component i, V î * is the smallest volume that the compound i requires to perform a free volume jump, ξ is the ratio of the critical molar volume of jumping unit of solute to the critical molar volume of jumping unit of polymer, γi is the overlap factor for free volume, (the same free volume can be occupied by more than one molecule), Tgi represents the glass transition temperature of the component i, K11 and K21 are free volume parameters for the solute, and K12 and K22 are free volume parameters for the polymer. For instance, in the case of a toluene−polystyrene mixture, Duda et al. studied the evolution of the diffusion coefficient of toluene in polystyrene as a function of composition and temperature based on the free-volume theory and obtained a value using theoretical calculations.15 As shown in Table 1, at

Figure 4. Schematic representation of a bubble−foam devolatilization process.

For bubbles to be created and nucleation started, the liquid has to be superheated. Superheating or boiling retardation means that the solute or VOC does not boil at a temperature that exceeds its boiling temperature. This occurs when the temperature is increased above the values at which the equilibrium vapor pressure is equal to the pressure around the polymer melt. The degree of superheat can also be tuned depending on the pressure of the system; it increases when decreasing the pressure of the system.14,19,20 Nucleation of the bubbles can be homogeneous or heterogeneous. Homogeneous nucleation occurs when the vapor phase is created in the bulk of the liquid, which is in the melt. The bubbles created by homogeneous nucleation have two possibilities: to grow by vaporization of volatiles or to disappear through their condensation. The bubbles grow by diffusion of the volatiles from the polymer melt to the gas phase. Heterogeneous nucleation occurs when the vapor phase is generated at the solid interface. Note that the superheat needed for nucleation can be decreased if, before increasing the temperature of the polymer, the pressure of the system is increased. This makes that the cavities at the surface that might contain gas act as bubble precursors decreasing the temperature needed to get superheated conditions.12 Furthermore, foam is usually created by reduction of the pressure of the system, as it happens, for example, in flash evaporation. In the case of devolatilization using nonrotating equipment, three successive regimes can be defined for a boiling−foaming mechanism.9,13 The first one, called free boiling, takes place when the ratio between the equilibrium partial pressure of the volatile in equilibrium with the melt and the total pressure of the chamber is high. It is based on the creation and fast growth of the bubbles enhancing the mass transfer by creating a convective mixture. This causes a fast decrease of the temperature of the melt, in the case where no energy is supplied, and leads to an increase in the viscosity of the polymer and a decrease in the bubble speed formation. During the second regime, the vapor bubbles grow, meaning that the volatiles are going out of the melt, thus originating an increase in the viscosity of the system. In the last step, the third regime, almost all the volatiles have been removed, so bubbles barely grow and the evaporation rate is very slow. At this point, the removal of volatiles is controlled by the molecular diffusion at the polymer/vapor interface, whereas in the previous steps the removal was controlled by the bubble formation and growth. This decrease in the removal rate during the third step explains why nonrotating devolatilizers are not suitable for removing low concentrations of VOCs. However, neither homogeneous nor heterogeneous nucleation phenomena completely explain the boiling−foaming mechanism. According to theory, the degree of superheat of

Table 1. Diffusion Coefficients at Different Temperatures for Different Weight Fractions of Toluene in a Toluene− Polystyrene Mixture15 diffusion coefficients (m2/sec) × 10−6 weight fractions of toluene temp (°C)

0.05

0.1

0.4

0.6

110 140 160 170 178

0.03 0.4 0.8 1 1.6

0.2 0.9 2.5 4.4 5.3

1 3.7 5.6 6.8 7.5

0.7 1.0 1.8 2.5 3.1

low weight fractions, the temperature has a significant effect on the diffusion coefficient; this effect levels at higher weight fractions. This sensitivity to temperature is related to a treatment temperature close to the glass transition temperature of polystyrene (90 °C). On the other hand, at constant temperature, the diffusion coefficient increases with the weight fraction to a maximum followed by a slight decrease at higher weight fraction. More recent reports, once diffusivity values could be accurately measured, have demonstrated that devolatilization cannot be explained by the sole diffusion mechanism even at low concentration of volatiles as other mechanisms such as bubble nucleation and foam formation take place.12,18,19 Those mechanisms are explained in the next section. 2.1.1.3. Bubble Nucleation and Foam Formation Mechanism. Boiling-foaming is the other mechanism driving devolatilization processes, deeply studied by Biesenberger et al.20−23 In this mechanism, the interfacial area between the polymer and the gas phase depends on the rate of volatiles removal from the polymer. Figure 4 shows a schematic representation of the process. Briefly, bubbles are nucleated and grow as a result of the diffusion of the VOCs through the polymer melt to the gas phase. Those bubbles coalesce forming foam at the polymer melt-gas interface. This foam can stay there for more or less time depending, among other parameters, on the viscosity of the sample. Finally, the VOCs are removed from the polymer melt pool by applying a vacuum. 11604


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which leads to an increase of the diffusion coefficient of the VOC. To take into account the presence of the stripping agent, Vrentas and Duda reformulated the free volume theory for a ternary system in a polymer film.26 They included an additional term in their calculation of the diffusion coefficient of the VOC related to the volume occupied by the second solute molecules, the stripping agent. This way, in equations, 11, 12, and 13, index 1 corresponds to the VOC in the polymer, index 2 to the stripping agent, and index 3 to the polymer.

the sample has to be much higher than the processing temperature needed for the nucleation rates observed.12 Additionally, the viscosity of the sample would be too high to allow the bubbles traveling from the walls to the melt surface for their removal. So, adequate shear forces, such as the ones produced by mechanical rotation, are needed to favor the movement of the bubbles. However, it has to be taken into account that if the rotational speed is very low, the bubbles remain longer in the polymer melt from which they are expelled by action of the polymer pool motion. Regarding the influence of the viscosity, at high viscosities even a high rotational speed does not allow for the growth of the bubbles. Therefore, an appropriate shear rate is necessary to cause a deformation of the bubbles, as bubbles are more rapidly nucleated in the area where the deformation is higher. Hence, the pressure and the rotational speed have to be controlled in order to get the optimal conditions for the removal of the volatiles. The bubbles formed are transported to the lower pressure region where they coalesce forming foam. At high volatile concentrations, the higher is the rotational speed and the faster is the foam formation, leading to an improvement in the devolatilization efficiency.19 As seen above, the bubble foaming mechanism has some special features depending on the devolatilization equipment (rotating or nonrotating). Albalak et al.18 describe how this mechanism is applied to the devolatilization of polystyrene and low density polyethylene using a falling strand devolatilizer. A detailed description of this publication can be found in section 2.1.2.1 of this review where falling strand devolatilizers are described. 2.1.1.4. Stripping Operation. Along the previous sections concerning the theoretical aspects of the devolatilization process, the importance of the temperature was highlighted as a major factor in the implementation of the VOCs removal process. However, there are many polymers that are thermally sensitive and therefore could be degraded if the temperature is increased to the levels required for an efficient VOC removal. Additionally, the diffusion rate decreases when low VOCs concentrations are achieved as observed in Table 1. Consequently, in many industrial polymer purification processes, stripping agents (water, nitrogen, air, etc.) are introduced to the polymers to improve the rate of devolatilization. Ravindranath and Mashelkar24 analyzed the effect of a stripping agent in the devolatilization of polymers. They examined, for example, the devolatilization of ethylene in polyethylene using water as a stripping agent based on the results reported by Werner in 1980.25 Werner showed that, by adding 3% wt of water, the residual ethylene content in polyethylene could be reduced from 600 to 200 ppm when the devolatilization takes place in a twinscrew extruder. Ravindranath and Mashelkar24 also pointed out the dependence among the concentration of water, the pressure of the system, and the final concentration of residual ethylene. At high pressure, a marked decrease in the final concentration of ethylene while increasing the amount of water was observed, whereas at low pressure no major differences were noticed with respect to devolatilization carried out in the absence of a stripping agent. Note that in all the cases, a lower ethylene content was achieved when the pressure is decreased, the greater effect being observed in the absence of water. As explained previously in this report, in the absence of the expansion of the polymer by foaming, the VOCs are removed from the polymers by diffusion. In this case, the improvement in the devolatilization efficiency due to the presence of a stripping agent is related to the increase in the free volume of the polymer,

ÄÅ ÅÅ i ÅÅ jjω V ̂ * + ÅÅ jj 1 1 Å D1 = D01 expÅÅÅÅ− k ÅÅ ÅÅ ÅÅ ÅÇ

ω2V 2̂ *ξ13 ξ23 ̂ VFH γ

É yz ÑÑÑÑ ̂ * z + ω3V 3 ξ13zz ÑÑÑ { ÑÑÑ ÑÑ ÑÑ ÑÑ ÑÑ ÑÖ

ÅÄÅ i ̂ * ÑÉ ÅÅ j ω1V1 ξ23 ̂ * + ω V ̂ *ξ yzz ÑÑÑÑ ÅÅ jjj + ω V z 2 2 3 3 23 z ÑÑÑ ÅÅ ξ13 { ÑÑ D2 = D02 expÅÅÅÅ− k ÑÑ ̂ VFH ÅÅ ÑÑ ÅÅ ÑÑ γ ÅÅ ÑÑ ÅÇ ÑÖ

(11)

(12)

̂ VFH K K = 11 (K 21 + T − Tg1)ω1 + 12 (K 22 + T − Tg 2)ω2 γ γ γ K13 + (K 23 + T + Tg 3)ω3 γ (13)

where D0x is the pre-exponential factor for component x, ωx is the mass fraction of component x, V ̂x* is the smallest volume that ̂ is the compound x requires to perform a free volume jump, VFH the average hole free volume per gram of mixture, γx is the overlap factor for free volume (the same free volume can be occupied by more than one molecule), ξx3 represents the ratio of the critical molar volume of jumping unit of component x to the critical molar volume of jumping unit of polymer, Tgx is the glass transition temperature of the different components, and Kij is the free volumes parameters of the solutes (VOC and stripping agent) and the polymer. From these equations, Ravindranath and Mashelkar24 calculated the effect of the various parameters on the devolatilization efficiency in a molten polymer film. The calculation is based on the resolution of the transient mass balance in a finite film. Thus, they have shown that an increase in the amount of stripping agent enhanced the concentration of VOC desorbed from the polymer film. However, the stripping agent must not diffuse too rapidly during the devolatilization phase under low pressure, as it might be extracted before the VOC. Generally, the thermodynamic properties of the stripping agent are fairly close to those of the volatile compound. Thus, when the concentration, temperature, and pressure conditions required for the boiling of the VOC are reached, they also make it possible to reach the boiling point of the stripping agent, and consequently the foaming of the stripping agent. This foaming is interesting in several ways. It allows for a quicker generation of a network of bubbles that will coalesce and break for the rapid release of the volatile compound. Additionally, it accelerates the growth of the VOC bubbles by reducing the partial pressure in the system, and as well, it increases the exchange surface, as the VOCs can diffuse either through the created and nucleated bubbles or through the stripping agent bubbles (Figure 5).27 In conclusion, the use of a stripping agent while performing devolatilization enhances the removal capacity of the equipment 11605


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Thermoplastics, low density polyethylene, polypropylene, polystyrene, and related copolymers or diorganopolysiloxanes are examples of polymers that can be purified by flash evaporation. For instance, carrying out the purification of polystyrene at 133 Pa and 260 °C reduced the styrene content from more than 1.8 × 103 ppm to around 26 ppm.28 For diorganopolysiloxanes, with a viscosity range between 104 cps and around 2 × 105 cps at 25 °C, purification using steam as the stripping agent has been reported. A partial vacuum and a temperature set below the evaporation temperature of the cyclic polysiloxanes were applied to samples containing between 9 and 18 wt % of cyclic polysiloxanes, achieving a final concentration of 2 wt %.29 Falling Film Devolatilizer. This technique is mainly implemented in the case of heat sensitive and barely viscous polymers. The feed introduced at the top of the tank forms continuous and thin films along the walls taking advantage of gravity. Volatiles are taken out from the sample applying vacuum, and the purified polymer is recovered at the bottom of the tank (Figure 6). Obvious advantages of this technique over

Figure 5. Scheme of a stripping operation.

at lower temperatures by increasing the diffusion coefficient of the VOCs. This is due to the increase of the free volume of the polymer that directly affects the diffusion coefficient as shown in eqs 11 and 12.26 2.1.2. Equipment. In this section the different types of equipment existing for the devolatilization of bulk polymers, melts, and solutions are described. These types of equipment are commonly divided into nonrotating (or still) and rotating devolatilizers, in which mechanical agitation is incorporated.19 2.1.2.1. Nonrotating Devolatilizers. Nonrotating devolatilizers are mainly used for polymers presenting a low viscosity and containing a high concentration of volatile compounds. This is due to, as explained in section 2.1.1.3, the low mass transport efficiency that this equipment presents at reasonable residence times for very low VOCs’ concentrations. Some examples of nonrotating equipment are flash chambers, falling-strand, and falling-film devolatilizers. In the case of still devolatilizers (no mechanical rotation), bubble nucleation, growth, and ultimately separation from the polymer melts and foam formation are the mechanisms taking part. To improve these phenomena and thus enhance the release of VOCs, stripping agents (steam, nitrogen, water, etc.) are commonly introduced in the devolatilization chamber. Flash Devolatilizer. This equipment is used for shear sensitive, not heat sensitive, and barely viscous (≪1 Pa·s) polymers. First, the polymer is preheated under controlled pressure to superheat the polymer melt in order to avoid any cool-down evaporation. Then, the polymer enters into the flash chamber in which the pressure and the temperature are tuned depending on the VOCs to be extracted, so that the volatiles boil. The removal of the VOCs is carried out in a continuous vacuum, and the purified polymer is collected at the exit of the chamber using gravity as driving force. Apart from vacuum, stripping agents such as nitrogen or steam can be added to enhance the extraction efficiency. This is a widely used technique, both at the laboratory and industrial scale, mainly due to the simplicity of the process and its low cost. However, the long residence times (several hours) and the high temperatures (>220 °C), due to the low heat and mass transfer, can modify the properties of the polymer and even degrade it. Additionally, the bubbles formed are entrapped in the polymer melt and the volatiles redissolved in the polymer.

Figure 6. Falling-film devolatilizer.

flash devolatilization are based on a higher heating surface favoring the diffusion of VOCs through the polymer, as well as a higher heat transfer at lower temperatures, hence reducing the overall cost of the process. However, recirculation of the polymer melt is needed to efficiently reduce the concentration of the VOCs. Polysiloxanes, polyvinyls, and polyesters can be purified using this technique.30 However, no experimental data regarding the extent of purification have been found in the literature reviewed. Falling Strand Devolatilizer. Viscous polymers (around 1 Pa·s) containing a lower concentration of VOCs than in the previous cases can be advantageously purified by using this technique, owing to the maximization of the exposed surface to vacuum resulting in higher heat and mass transfer. The falling strand devolatilizer is a specific configuration of a flash devolatilizer, in which the entrapment of the bubbles containing the volatiles in the polymer melt is avoided. Likewise in the falling film devolatilization, the polymer is fed by the top of the chamber. However, in the present case, after the preheater, the 11606


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time distribution, and the fast surface removal of the volatiles. However, it is more expensive, and compared to other rotational equipment, it has a low rotational speed. The polymer is fed by the top of the equipment and the purified product is recovered from the bottom. Regarding the vapor containing the volatiles, it can leave the tank either from the top (in counter-current) for the more volatile compounds or from the bottom (in cocurrent) for the less volatile ones, normally under vacuum. The rotor has blades along it allowing for the transport of the polymer films formed on the walls along the tank (Figure 8). Simultaneously, bow waves produce a turbulent flow that enhances the heat and mass transfer.

polymer strands are extruded into the tank to fall into a pool from where the polymer is recovered. Then, the volatiles are extracted by applying vacuum to the system (Figure 7). The need of the preheater in this kind of devolatilizers to generate superheat produces a significant amount of oligomers that can alter the final properties of the polymer.12,31

Figure 7. Falling-strand devolatilizer.

Compared to the falling film technology, the devolatilization process when producing polymer strands is much more complex as volatiles not only just come out of the strands by diffusion but also by forming foam and volatile bubbles within the core of the melt. Albalak et al. studied the devolatilization of polystyrene and of low density polyethylene enriched with styrene and hexane.18 They concluded that the devolatilization mechanism involved is based on the formation of micro- (1−3 μm diameter) and miniblisters (10−15 μm diameter) on the surface of the polymer melt strands and on the surface of the volatile bubbles formed in the core of the polymer strands. Heterogeneous bubble nucleation plays a major role in the rate of devolatilization. This nucleation is mainly governed by the degree of superheat of the polymer and the geometry of the nucleation sites. Polystyrene, polyethylene, and polysiloxanes are examples of polymers purified using a falling-strand devolatilizer. For instance, polystyrene containing 10−40 wt % of styrene was treated at 200−280 °C and under a pressure between 6.7 × 103 and 26.7 × 103 Pa in a first tank and 400 to 2.7 × 103 Pa in a second tank. A final VOC content was less than 0.1 wt %, and in most of the cases less than 0.05−0.01 wt % was achieved.32 2.1.2.2. Rotating Devolatilizers. In the case of highly viscous samples containing low amounts of volatiles, rotating devolatilizers are more suitable for the removal of VOCs from bulk polymers. Thanks to the rotation system, the polymer is better distributed along the devolatilization tank, and also the polymer/vapor interphase along the melt is renewed increasing the heat and mass transfer for optimized removal efficiency. Examples of the most common rotating equipment are wipedfilm evaporators, screw extruders, and corotating disk-packs. Wiped-Film Evaporator.30 This equipment is well-suited for heat sensitive, viscous (4 × 104 Pa·s) containing a very low concentrationof VOCs. There are single or twin screw extruders. Both are composed of a feed hopper, two metering zones, one after the feeding zone and one at the end of the screw, and a vent zone where the polymer is devolatilized by vacuum. As there is a small space between the 11607


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equipment, the sample enters from the top into a processing chamber formed by two disks attached to a shaft surrounded by a barrel. The outlet is placed at the same height as the inlet but separated by a channel block (Figure 10). Many designs can be

screw and the wall of the extruder, the polymer is deposited on the wall while the melt flows. This brings to a removal of volatiles from both the surface of the films and the surface of the rotating polymer melt (Figure 9). The viscosity of the sample has an

Figure 9. Single-screw extruder (top) and twin-screw extruder (bottom) scheme.

Figure 10. Disk processor devolatilizer.

made depending on the desired function, devolatilization, mixing, melting, or pumping. In the case of devolatilization, the polymer melt is spread on the walls forming a film, the thickness of which can be controlled by the operating parameters and feed of the sample. As in the case of the screw extruders, a rotating melting pool is created enhancing the circulatory speed and surface renewal. The main advantage of this equipment is that it can be easily modified allowing for boiling or foaming42 devolatilization mechanisms to take place for greater efficiency in the release of VOC. Additionally, as the polymer melt is deposited on the walls, the vent problems are eliminated. A wide range of polymers can be purified using this technology, such as polycarbonates, polyesters, polyamides or thermoplastics.38 However, not many experimental details are given for most of the cases. Wenzel et al.43 described the purification of oxymethylene based polymer using the following conditions: 160−220 °C in the first and stabilization stages and 160−190 °C in the second stage. The pressure used was of 1.33 × 10−5 to 0.04 MPa in the first stage, 0.08 to 0.13 MPa during the stabilization stage and 1.3 × 10−5 to 0.013 MPa in the second stage. The purification process lasted between 10 and 120 s for obtaining a final VOC concentration lower than 150 ppm. 2.2. Temperature Increase. Raising the temperature is an easy means to convert the residual monomer present in the polymer, and to remove any other volatile organic compounds that may remain within the polymeric matrix. Nevertheless, increasing the temperature can affect the properties of the polymer, such as its molecular weight, or even degrade it. This kind of treatment is usually carried out in the same reaction vessel in which the polymerization has taken place and at high monomer conversions. Polymers applied in medical applications, such as acrylonitrile-co-methyl methacrylate,43 are an example of polymers purified with this method. In this particular case, the concentration of residual monomer dropped from 3 × 103 ppm to 40 ppm after a heating treatment of 8 h increasing the temperature from 120 to 140 °C. Vallo et al. studied the influence of the cure temperature on the residual monomer content in poly(methyl methacrylate)

impact on the bubble creation and diffusion through the polymer melt. However, thanks to the rotation of the melt, the bubbles can be exposed to the surface increasing the removal efficiency. Twin screw extruders, as shown in the bottom panel of Figure 9, are composed by two screws that can work in corotation or counter-rotation. The presence of two interpenetrated screws allows for a better mixing of the polymer melt compared to single screw extruders. Therefore, more polymer film is exposed to the extruder walls, leading to an exceptional interchange between the melt and the film. Moreover, the exposition of the bubbles to the surface is enhanced compared to single screw extruders. This also avoids stagnation problems under the vent area. Additionally, and according to Nichols and Lindt, polymer isolation (devolatilization, drying, and coagulation) from solutions, suspensions, and emulsions, can be carried out using counter-rotating nonintermeshing twin-screw (CRNI) extruders in several steps. Furthermore, this kind of twin-screw extruders can be used for reactive polymer processing and for finishing synthetic rubber polymers. This is possible thanks to a high free volume and a low shear enhancing the mixing and the surface renewal enabling a multistage feeding.13,36 Screw extruders can be used for the purification of, for example, polypropylene, polyethylene, polystyrene, polydimethylsiloxanes, and related copolymers. Biesenberger and Kessidis37 reported on the removal of styrene from polystyrene using a single screw extruder. They investigated the most suitable conditions in terms of length of the vent zones, screw speed, pressure, and measuring the styrene concentration by gas chromatography. Under the optimized conditions, complete removal of styrene from a polystyrene sample containing 5.4 × 103 ppm of styrene was achieved in vacuum at 204 °C. Disk Processor.38−41 In the context of improving the mixing capacity of the equipment, some modifications have been made to the already existing devices, such as in single and twin screw extruders. However, no significant changes in the principle of operation were made, until Valsamis et al.38 designed the corotating disk processor at the end of the 1970s. In this kind of 11608


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a 10 h half time temperature below 90 °C-100 °C. Following this idea, Wenzel et al.43 patented the synthesis of acrylonitrile copolymers in bulk by free radical polymerization with a very low residual monomer content. The authors described the postpolymerization process of an acrylonitrile-co-methyl methacrylate (60−80%/40−20%) using two initiators, one for the main polymerization reaction with a half-life of 1 h at 60 to 100 °C and a second one for the postpolymerization step with a halflife of 1 h at 100 to 140 °C. Both initiators were added at the beginning of the polymerization reaction. For instance, tertiary butyl perbenzoate, tertiary butyl-per-3,5,5-trimethylhexanoate, 2,2-bis(tert-butyl peroxy)-butane, or dicumylperoxide, used as high decomposition temperature initiators, were activated only after the polymerization reaction reached a conversion higher than 75%. Then, the temperature was increased to around 130 °C, affording a final residual monomer content below 50 ppm. Other acrylonitrile copolymers containing radical polymerizable monomers, such as vinyl monomers as styrene43 and other polymers such as poly(methyl methacrylate),47,48 and polyols49 could also be purified using this process. In the case of nvinylpyrrolidone, the continuous addition of an oil soluble initiator allowed reducing the residual monomer concentration to levels below 0.1%wt.50 Postpolymerization is widely used owing to the simplicity and low cost of the method as no further equipment is needed. However, the presence of residual amounts of initiator can have a harmful effect on the properties of the polymer, so the deactivation of the radicals is imperative. 2.4. Reactive Comonomer. The addition of a comonomer is a less common technique for removing volatile organic compounds from bulk polymers. This consists of the addition of a second monomer (comonomer) compatible and highly reactive with the main one, that is, with a high reactivity ratio. Consequently, the comonomer acts as a scavenger of the monomer to be removed. Moreover, the comonomer should feature a lower boiling point than the principle monomer in order to be readily removed in case of partial conversion. The addition time of the comonomer to the reactor has to be precisely determined to prevent any detrimental modifications of the polymer properties, for example, the viscosity. As in the case of the previous purification methods, temperature increase, and postpolymerization, this technique is usually applied at high monomer conversion and does not require additional equipment. Guo reported in a patent the reduction of the residual allyl monomer from acrylates to concentrations lower than 1% by the addition of hydroxyalkyl(meth)acrylate at the end of the polymerization reaction in an appropriate concentration (ratios of 2:1 to 3:1 hydroxyalkyl(meth)acrylate/allyl monomer).51 Lindsey and Simms reported the removal of a highly toxic isocyanate monomer from an isocyanate methacrylate polymer. This was done by adding butyl acrylate as a scavenger monomer at the end of the polymerization reaction (98% conversion of isocyanate) in a proportion at least equal to the concentration of residual monomer. The authors claimed a decrease of 20−50 wt % in the concentration of residual monomer leading to a final isocyanate concentration between 100 and 500 ppm.52

bone cement.44 Samples were first cured at room temperature, followed, for some of them, by an additional postcuring treatment at 140 °C for 2 h to induce complete conversion of the monomer thus allowing an investigation of the effect of the residual monomer concentration on the mechanical properties of the polymer. Monomer conversion was monitored by dynamic scanning calorimetry (DSC) and gas chromatography (GC). However, the complete removal of volatiles was hampered by the partial vitrification of the polymer which resulted in a decrease in the mobility of the molecules through the polymer chains. This occurs when the Tg of the polymer (∼110 °C) is equal to the curing temperature. In summary, the removal of VOCs by increasing the temperature of the reactor is a simple, inexpensive implementation technique requiring no additional and complex equipment; however, to reduce the temperature necessary for the removal of volatile substances, and thus reduce the risk of degradation of the polymer, it is commonly applied together with the addition of an initiator. This latter technique, also referred to as postpolymerization, is briefly presented in the following section. 2.3. Postpolymerization. Postpolymerization is currently employed for the purification of emulsion polymers and thus will be described in more detail in section 3.1. Nevertheless, this methodology can also be applied to some polymers synthesized in bulk, notably at high monomer conversions, for the removal of polymerizable residues. It is based on the addition of an initiator, thermal or redox, usually different than the one used in the main polymerization reaction, either to the reactor or the storage tank in order to promote the conversion of the residual monomers. If present in the reactor, the postpolymerization initiator may be included among the polymerization compounds, or added after the main polymerization reaction. In the first case, two initiators would be present in the reactor which could cause undesirable side-reactions. This is overcome by activation of the postpolymerization initiator at higher temperatures than those usually used for the main polymerization reaction. Often, the temperature must be increased to allow the initiator to function properly and thereby promote faster monomer conversion. Nevertheless, the temperature must be carefully controlled to prevent any damage of the polymer and the occurrence of harmful reactions caused by the initiator. Kamath published two patents regarding the synthesis of vinyl polymers using this latter approach.45,46 In these two patents, the polymerization of vinyl monomers, including styrene was carried out in the presence of at least two free radical thermal initiators activable at different temperatures. Two methods were investigated. On the one hand, the reaction temperature was gradually increased during the polymerization in a range between 50 and 150 °C. On the other hand, the main polymerization reaction was first conducted at a constant temperature before increasing it for the postpolymerization stage. In both cases, the idea was first to activate the polymerization initiator and then the “finishing catalyst” used for the removal of the residual monomer. Using both methods and different combinations of initiators, Kamath showed that concentrations between 2.6 and 0.06 wt % of the residual monomer could be attained at the end of the polymerization of styrene. The reaction times varied between 4 h in the case of a progressive increase of the temperature and 4 h + 3 h for a two-temperature, two-step reaction. The best results (

Removal of Volatile Organic Compounds from Bulk and Emulsion

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