Photoinitiated Polymerization - American Chemical Society

Chapter 37

Continuous Photopolymerization in a Novel Thin Film Spinning Disc Reactor

Downloaded by UNIV OF MINNESOTA on July 10, 2013 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch037

Kamelia V . K. Boodhoo, William A . E. Dunk, and Roshan J. Jachuck Process Intensification and Innovation Centre (PIIC), Department of Chemical and Process Engineering, University of Newcastle Upon Tyne, Newcastle Upon Tyne NE1 7RU, United Kingdom

n-Butyl acrylate has been polymerized in bulk by UV initiation using a novel spinning disc reactor (SDR). Very high conversions were achieved in seconds, whilst the polymer so produced was unbranched. This observation contradicts the accepted view that acrylates give insoluble network polymers even at low conversions when bulk polymerization is attempted. We present here the principles underlying the development of the SDR, and its application to photo-polymerization.The results of our work withn-butylacrylate and the effects of disc rotational speed, monomer feed rate, and UV radiation intensity are presented and qualitatively discussed. Application of the SDR to continuous polymerization processes, and its potential in reducing plant space i.e. process intensification will also be discussed briefly in this chapter.

© 2003 American Chemical Society

In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction The observation that certain compounds could be affected by sunlight to give materials having the same chemical composition but very different physical properties was made as long ago as 1845. Blyth and Hofmann (1) noted that styrene was converted from a liquid to a glassy solid when exposed to sunlight and it was shown by Kopp (2) that the two materials were chemically the same. Similar observations were reported for other vinyl compounds, but it was not until 1910 that the chain reaction nature of the process was shown (3). In 1912 Ciamician reviewed the field of photochemistry and foresaw possible applications in the chemical industry (4). However the potential was never exploited in any significant way for the large scale production of liner soluble polymers although in the 1920s I.G.Farben used sunlight to polymerize vinyl chloride and vinyl acetate. Inability to scale up laboratory light sources and the seasonable uncertainties of sunshine presence led to abandonment of these plans (5). Subsequently photopolymerization was confined to laboratory studies and no viable industrial process has emerged although the patent literature discloses ideas for reactors. Many of these are based on having films of monomer on a moving belt that passes under a bank of UV lamps (6,7). Since there is no agitation, mixing is minimal thus the polymerization proceeds in what is essentially a static film. In addition, the removal of the heat of polymerization is limited to the dimensions of the belt. Photochemical initiation has several advantages over initiation by freeradicals obtained by the thermal dissociation of chemical initiators: • • • • •

Polymerization may be carried out over a very wide range of temperatures, even as lowas-100°C. Production of primary radicals is essentially independent of temperature. Rate of polymerization is much less affected by temperature than when thermal initiators are used. Polymerization of monomers having low ceiling temperatures becomes feasible. Compared to redox initiator systems that exhibit low activation energy, photo-initiation may be finely tuned.

With these advantages it seems surprising that no real progress has been made in the industrial utilization of photopolymerization since the abortive attempts at IG Farben. Of course the problems of radiation penetration and uniformity in bulk reactions is not trivial, whilst reactor design and development have done little to address these issues.


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Novel Spinning Disc Photopolymerizer The novel spinning disc reactor (SDR) has previously been shown to dramatically increase the rates of step-growth (8) and radical chain (9) polymerizations. The ability of the SDR to continuously generate thin, sustainable films under the action of the centrifugal field represents an important advancement towards the design of a viable continuous reactor system applicable to bulk photopolymerization reactions. Not only would the thin film (50-300 μιη) allow efficient penetration of UV light used to initiate the polymerization, but the shear forces developed within the film would also promote excellent mixing conditions. The intense fluid dynamics environment on the rotating disc is expected to enhance initiation and propagation steps. We believe that the applied centrifugal field extends polymer chains that have been disentangled by shear, causing them to grow radially. Translation diffusion of the active chain ends may then be restricted so that bimolecular termination reactions are reduced (10) An this event, we feel that there may be tendency towards a "living" mechanism of propagation. A study of the kinetics involved in polymerizations on the spinning disc is currently underway in our laboratory to elucidate the mechanism of chain formation and growth. Bulk polymerization of monomers by the free radical mechanism has the advantage that only the monomer and an initiator are involved, thus the polymer produced will be free of contaminants. However the highly exothermic nature and rapid rates of many monomers, combined with the high viscosities at high monomer conversion, severely limit the use of this technique in conventional polymer reactors. These problems are particularly relevant to the esters of acrylic acid, and to a lesser extent to methacrylates, both of which find wide industrial application. These issues are addressed in the SDR by the greatly enhanced heat transfer characteristics achievable on the rotating disc (11) and the intense mixing levels generated within the film even at high viscosities. These SDR features would allow a good control over the reaction exotherm so that the bulk polymerization process proceeds safely in the SDR to yield polymer with tightly controlled molecular weight distribution (MWD). η-Butyl acrylate was chosen as the test monomer to evaluate the performance of the SDR for photopolymerization since, with a propagation rate constant of ca 16,000 L/(mol.sec) at 30°C (12) and a heat of polymerization of 77.4 kJ/mol (13), the bulk polymerization of η-butyl acrylate presents significant challenges as a fast, highly exothermic reaction system. Furthermore, n-butyl acrylate polymerization is normally complicated by the ease in which branching occurs by tertiary hydrogen abstraction, resulting in insoluble polymer networks. This is common to most acrylic esters but particularly so for the η-butyl ester. This problem has recently been suggested to be at the root of the inability to determine by extrapolation accurate k values above 30°C by pulsed laser p


440 techniques (14). The high intensity mixing in the SDR offers the potential to control the extent of such transfer reactions and therefore reduce polymer network formation.

Experimental


Materials η-Butyl acrylate (care: lachrymator) was obtained from Aldrich and freed from inhibitor by washing with 0.5% chilled sodium hydroxide solution, then with distilled water. The monomer was dried over-night with anhydrous magnesium sulfate and filtered before use. The photo-initiator Irgacure 651 (2,2dimethoxy-2-phenyl acetophenone) was obtained from Ciba Speciality Chemicals and used as received. The monomer was de-aerated and polymerizations performed using oxygenfreegrade nitrogenfromBOC Gases.

Equipment A schematic of the SDR is shown in Figure l.The reaction surface is a 200 mm diameter stainless steel smooth disc which is supported on an internal cooling chamber mounted on the rotating shaft. Waterfroma thermostatically controlled bath circulates in this chamber to maintain the disc temperature. The monomer/photo-initiator mixture was fed to the centre of the disc by means of a Watson Marlow 505S/RL peristaltic pump. The UV source (1000 W metal halide flood lamp from UV Light Technology Ltd) was clamped over the disc surface with the beam directed towards the centre of the disc. The UV lamp and the top section of the SDR were enclosed in box made of UV absorbing acrylic sheets to contain the UV radiation when the lamp was in operation. Suitable eye and skin protection gear were also used. The intensity of the lamp was measured by a UV-A meter (from UV Light Technology Ltd). A flat glass plate transparent to 365 nm wavelength of the UV spectrum separates the disc surface and the UV lamp to contain any splashing of the monomer/initiator mixture.

Procedure Inhibitor-free η-butyl acrylate containing the desired concentration of photo initiator was placed in a flask protected from light by a black shield, then


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sparged with nitrogen using a sintered glass distributor for 30minutes. The SDR was flushed with nitrogen prior to and throughout the polymerization. The UV lamp was allowed to warm up for Sminutes before commencing the disc irradiation. The intensity of the UV radiation was controlled by adjusting the distance of the lamp from the surface of the disc. When the chosen disc temperature and disc speed had been set the monomer/photo-initiator mixture was pumped onto the disc under the UV radiation. On completion of the run the lamp was switched off, the disc stopped and polymer collected from the reactor housing and, where possible, the disc surface.

Monomer/ photo-initiator feed

M

UV lamp source

UV transparent glass cover

Vent

^

Cooling water jacket

Internally cooled spinning disc system

Product sample

Figure J. Schematic ofspinning disc reactor for photopolymerization

Characterization Molecular weights, polydispersities and monomer conversions were obtained by gel permeation chromatography using two PL gel 5 m Mixed-C columns (Polymer Laboratories), each of dimensions 300mm χ 7.5mm, set at 30°C. Tetrahydrofuran was used as solvent at a flowrate of 1.0 ml/min. An RI


442 detector was used to detect polymer and monomer peaks. Molecular weight data were calibrated against Easi-Cal polystyrene standards whilst conversion was measured against calibration samples of pure η-butyl acrylate and pure poly(nbutyl acrylate). The conversion and molecular weight analyses were performed by PL Caliber LC-GC and GPC/SEC softwares respectively. Branching in the polymer formed in the SDR was assessed by analysis of C NMR spectra obtained using a Varian Associates Unity 500 spectrometer operating at 125.8 MHz. l 3


Results and Discussion The two most important variables affecting the performance of the SDR are the disc residence time (t ) and the film thickness (δ) which are governed by operational parameters such as the rotational speed (N) of the disc and the feed flowrate (Q). For a smooth disc (8) res

9

where ν is the kinematic viscosity of the liquid, ω= 2πΝ/60 and r is the radial distance from the centre of the disc, with subscripts ο and i representing outer and inner respectively. Plots showing the effect of disc rotational speed and feed flowrate on the mean residence time in the SDR and mean film thickness across the disc, based on equations (1) and (2) above respectively, are presented in Figure 2. Changes in the two variables will have a direct influence on the extent of η-butyl acrylate photopolymerization in the SDR as will be demonstrated by thefindingsof the present study.

Influence of disc rotational speed and feed flowrate As the rate of rotation of the disc is increased from 200 to 1000 rpm at constant flowrate of 1 ml/s of monomer/initiator feed mixture, conversion of nbutyl acrylate falls quite sharply from about 90% ( t » = 2.1 s) to about 30% (W= 0.7 s) as shown in Figure 3 below. This may be explained by a decreasing



Figure 2. Theoretical profiles of mean residence time andfilmthickness across a smooth disc surface at various rotational speeds andfeedflowrates



Figure 3. Effect ofdisc rotational speed on conversion of η-butyl acrylate in the SDR at differentfeed flowrates


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residence time on the disc with rotational speed which results in reduced exposure to the UV illumination. It would seem that the decrease in film thickness (from 125 μπι at 200 rpm to 43 μιη at 1000 rpm), which would tend to encourage more efficient UV penetration into the film, has less of an influence on the conversion achieved than the residence time. Therefore residence time is the controlling factor for this particular set of operating conditions. The drop in conversion with rotational speed is less significant at the higher flowrate of 5 ml/s. A comparison between Figures 2 and 3 reveals that the trend in conversion at different flowrates follows closely the corresponding residence time profiles. This is further indication that residence time is more influential on the conversion achieved in the SDR thanfilmthickness.

Influence of UV intensity 2

Within the range of UV intensities tested (11-125 mW/cm ), the results show that the extent of photopolymerization in the SDR is significantly enhanced as UV intensity is increasedfrom11 mW/cm to about 30-40 mW/cm for all the rotational speeds studied (Figure 4). Beyond the 30-40 mW/cm UV intensity range, conversion drops sharply in all cases. These results suggest that an optimum UV intensity exists for each rotational speed at which conversion is highest, as shown in Figure 4. In contrast, Decker and co-workers (15) found that increasing the light intensityfrom10 to 600 mW/cm in the UV curing of multi functional acrylates caused the final conversion to rise from 80% to nearly 100%. This was attributed, in part, to increased sample temperature at high UV intensity which promoted molecular mobility of reacting species. However, in the SDR, the temperature of the polymerizing film flowing across the disc is maintained at the set value of 40°C by the highly efficient heat removal system. It would seem therefore that temperature effects are minimised in the SDR. The trend in Figure 4 may be explained as follows. As intensity is increased, a larger number of initiator radicals is generated in the flowing film which allows more monomer molecules to be consumed, resulting in a rise in conversion. However, beyond a certain optimum intensity, the number of radicals is so exceedingly high that they are more likely to terminate growing chains rather than initiate new chains so that, overall, there are less active chain ends available for addition of monomer molecules. 2

2

2

2

Molecular weight properties and branching effects The average molecular weights (M„, M ) and the polydispersity (PDI) of the poly(butyl acrylate) formed in the SDR are presented in Table I below. It is w



L Effect of UV intensity on conversion of η-butyl acrylate in SDR for a Figure4. range of disc rotational speeds


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2

clearly seen that as intensity is increased from 11 mW/cm to 125 mW/cm , there is a significant drop in both M„ and M values for 200 and 500 rpm. This predictable effect can be attributed to the large number of initiator radicals formed at more intense UV irradiation which increases the probability of termination reaction reactions and hence shorter chains being formed. This observation fits in with the explanation offered in the previous section whereby conversion drops at the highest intensity due to increased chain termination in the presence of increased number of radicals. w


Table I. Molecular weight properties of poly(butyl acrylate) in SDR Disc speed (rpm) 200 300 200 500 1000* 200 500 1000*

Feedflowrate= / ml/s UV intensity M M (mW/cm ) 11 140000 300000 490000 11 260000 68725 25 33761 25 58626 31005 25 67807 238696 125 25394 50529 125 41735 20900 125 198376 73803 n

w

PDI

2

2.14 1.88 2.03 1.89 3.55 1.99 2.00 2.69

Polymer from the SDR at 200 and 500 rpm has narrow molecular weight distribution (MWD) as indicated by the polydispersity index (PDI) in the range of 1.8 to 2.1 (Table I). This observation indicates that a rapid rate of initiation can be sustained in the SDR which is comparable to the rate of propagation. Also, under the influence of intense mixing in the film on the disc, the propagation rate constant is likely to remain as high as it was in the initial stages of the polymerization. It is to be noted, however, that molecular weight properties (M„, M and PDI) at 1000 rpm (*) are higher than expected. The data presented are for samples takenfromthe walls rather thanfromthe disc as virtually no polymer was left on the disc surface. The growing chains may be joining up in bimolecular termination reactions in the film flowing down the wall. In sharp contrast, we believe that disentangled, extended growing chains in the SDR flow environment (10) would be less likely to combine and hence molecular weights of polymer on the disc would be lower. The extent of branching in the polymer formed in the SDR was also of significant interest. It is well known that when acrylate monomers are polymerized in bulk, extensive branching occurs even at low conversions due to transfer reactions resulting in highly branched, insoluble polymer networks (13). w


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However, much to our surprise, C NMR spectra analysis (16) revealed that poly(butyl acrylate) formed by bulk polymerization on the rotating disc was entirely linear. The absence of branching may be linked to the extensional flow regime in the high centrifugal fields of the rotating disc which somehow suppresses any transfer reactions in the film. In using the SDR for bulk polymerization the auto-acceleration effect, arising as a result of the inability of active polymer chains to terminate thus allowing uncontrolled monomer addition, appears to be eliminated. It is our belief that conditions on the spinning disc lead to a controlled addition of monomer, rather similar to that which is observed in anionic and cationic polymerizations i.e. molecular weight increases linearly with conversion.

Static film vs. SDR film The effect of mixing in the highly sheared thin film in the SDR is better understood by comparing the SDR performance data with static film data at comparable film thicknesses. Measured data for an average SDR film thickness of about 125 μπι (from equation 2 above, with disc speed of 200 rpm, feed flowrate of 1 ml/s and η-butyl acrylate viscosity of 7.0x10" Pa.s at 40°C) are compared against those for a 200 μπι static film at 25 mW/cm UV intensity as shown in Table II below. 4

2

Table II. Comparison of static film and SDR film at 25 m W/cm Film type Exposure time (s) Static 10 SDR 2.1 (dynamic)

Conversion Mw (%) 52,000 30 90 70,000

Mn

PDI

28,000 33,000

1.8 2.1

2

From Table II, it is observed that the photopolymerization proceeds at a much faster rate in the dynamic SDR film as a result of the enhanced mixing levels promoted by the shear forces. The concentration of primary radicals at the same UV intensity would be similar in both the static and the SDR film but it is likely that the efficiency factor f of these radicals in initiating polymer chains is greater in the SDRfilmdue to the increased mixing. Thus more polymer chains are generated in the SDR film, which consume more monomer molecules in the propagation reactions to give a higher conversion in a shorter exposure time. We have also noted that in order to achieve comparable conversions of ~90% in the static film, it needs to be exposed for 40 s to a UV intensity of 75 mW/cm . 2


449 However, due to the higher UV intensity and exposure times, the resulting molecular weights (Mw~35,000 and Mn~l 8,000) are far less than those in the SDR film. Hence, the SDR is a more efficient reactor system at moderate UV intensities.


Opportunity for process intensification The thin film spinning disc reactor is an example of a compact, intensified reactor which uses the centrifugal forces to promote mixing and transport properties (heat and mass transfer) in the polymerizing film. The fluid dynamics generated in the fluid film provide the ideal environment for inherently fast reactions such as photopolymerization to take place in a controlled manner. Also, with the use of thinfilmsand therefore reduced amount of inventory in the reactor at any one time, the intrinsic safety of processes requiring the handling of hazardous materials is greatly improved. The safety of polymerization processes is largely dependent on the extent to which the exotherm of the reaction can be controlled, which has a direct influence on the molecular weight distribution. With its enhanced heat transfer rates, the SDR has the ability to closely control reaction temperature in the film thus preventing runaway reactions and producing polymer with a narrow distribution. The compactness of the SDR combined with the additional benefits offered by the use of the spinning disc technology highlight the potential for achieving intensification of photopolymerization processes.

Conclusions The novel thin film spinning disc reactor presents a tremendous opportunity for performing continuous photopolymerization of monomers in bulk. With its enhanced mixing, high heat removal capabilities, sustainable thin film flow for efficient UV penetration and short, controllable residence time, the SDR has the potential of producing high conversion, high molecular weight polymers with narrow MWD and hence improved product quality. The SDR also appears to have the ability to control the gel effect thus giving tighter control over polydispersity.

References 1. 2.

Blyth, J.; Hoffmann, A W . Ann. 1845, 53, 292. Kopp, E.; Compt. Rend. 1845, 21, 1378.


450 3. 4. 5.


6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Ostromislensky, I.; J. Russ. Phys. Chem. Soc. 1911, 44, 204. Ciamician, G., Science 1911, 36, 385. Morawetz, H. Polymers: The Origins and Growth of a Science; John Wiley & Sons: New York, 1985; p.95. Boutin, J.; Neel, J. German Patent 2,716,606, 1977. Arndt, P.J. et al. German Patent, 3,208,369, 1983. Boodhoo, K.V.K.; Jachuck, R.J.J. Green Chem. 2000, 2, 235. Boodhoo, K.V.K.; Jachuck, R.J.J. Appl. Therm. Eng. 2000, 20, 1127. Boodhoo, K.V.K.; Dunk, W.A.E.; Jachuck, R.J.J. J. Appl. Polym. Sci. 2001, in press. Jachuck, R.J.J.; Ramshaw, C. Heat Rec. Sys. & CHP. 1994, 14(5), 475. Beuermann, S.; Paquet, D A . (Jr); McMinn, J.H.; Hutchinson, R.A. Macromolecules 1996, 29, 4206. Concise Enclyclopedia of Polymer Science and Engineering; Kroschwitz, J.I., Ed.; Wiley-Interscience: New York, 1990, p. 16-20. Van Herk, A.M. Macromol. Rapid Commun. 2001, 22(9), 687. Decker, C. Polym. Internat. 1998, 45(2), 133 and references therein. Ahmad, N.M.; Heatley, F.; Lovell, P.A. Macromolecules 1998, 31, 2822.


Photoinitiated Polymerization - American Chemical Society

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