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Terf-butyl peroxyacetate initiated semibatch polymerization of 1,1-difluoroethylene in supercritical carbon dioxide.

INTRODUCTION

Due to being nontoxic, low cost, and an excellent solvent, supercritical carbon dioxide (scC[O.sub.2]) is used as an environmentally friendly replacement solvent for hydrocarbons. Successful applications of scC[O.sub.2] include extraction of caffeine from coffee beans and tea, hops extraction, and as a reaction medium [1-3]. Since scC[O.sub.2] has similar solvent characteristics compared to fluorocarbons [4], it has been investigated as a replacement solvent for processes involving fluorinated solvents and surfactants for synthesizing fluoropolymers, such as Teflon(tm) resins [5], Supercritical carbon dioxide may also be used to synthesize other fluoropolymers such as polyvinylidene fluoride (PVDF).

PVDF is used in pipes, heat exchangers, in UV-resistant, and acid-resistant applications [6]. The monomer of PVDF is 1,1difluoroethylene or vinylidene fluoride (VF2) and PVDF is conventionally manufactured by either emulsion or suspension polymerization in aqueous or non-aqueous systems [7]. Many conventional manufacturing processes for fluoropolymers have been scrutinized due to environmental concerns. In aqueous polymerization processes, perfluorooctanoic acid (PFOA) is used as a surfactant in emulsion and suspension polymerization. Perfluorinated compounds, such as PFOA, are endocrine disrupting chemicals, correlated to several diseases, and due to their stability persist in the environment [8]. As a consequence, PFOA has been found throughout the environment and in blood and tissue samples of the general human population, worldwide [8-11]. In addition to producing waste water, emulsion polymerization is energy-intensive since the polymer product must be dried prior to further processing. PVDF was also produced in non-aqueous systems using chlorofluorocarbons (CFC's) such as Freon-113, which was phased out by the Montreal Protocol for ozone depletion.

Given the environmental concerns with using PFOA or CFC's in the production of fluoropolymers, there has been considerable research performed in producing PVDF and polytetrafluoroethylene in alternative emulsion mediums, such as scC[O.sub.2]. Carbon dioxide has a critical point of 304.12 K and 7.374 MPa [12], VF2 has a comparable critical point of 302.80 K at 4.458 MPa. Supercritical carbon dioxide exhibits both liquid and gas-like properties, which are dependent upon temperature and pressure. Density, heat capacity, dielectric constant, viscosity, and other properties may be varied by changes in the temperature and pressure of scC[O.sub.2] [2, 3]. Supercritical carbon dioxide is a suitable replacement solvent for VF2 polymerization since it is relatively inert to highly electrophilic radicals resulting in no chain transfer to scC[O.sub.2] as a solvent during free radical initiated polymerization [3, 5, 13]. Supercritical carbon dioxide can also plasticize and swell polymeric materials [14], Plasticization by scC[O.sub.2] leads to increased free volume and chain mobility [15], In addition, scC[O.sub.2] facilitates diffusion-controlled reactions [16, 17], Since acid surfactants are not used in production, polymers synthesized in scC[O.sub.2] have significantly fewer acid end groups leading to high molecular weight materials [18], In addition, following polymerization, PVDF may be isolated directly in the reactor from carbon dioxide resulting in little additional post-treatment of the PVDF product, eliminating the use organic solvents and decreasing the amount of waste water treatment. This reduces the amount of energy used for drying.

Supercritical carbon dioxide has unique solvent properties that can affect the synthesis of PVDF. Supercritical carbon dioxide and VF2 are completely soluble in one another over the range of temperatures and pressures typically considered for polymerization in carbon dioxide [5, 13]. Amorphous fluorinated macromolecules and low melting point fluoropolymers are soluble in scC[O.sub.2] [19, 20]. The high solubility of amorphous fluoropolymers in carbon dioxide may be due to the solute-solvent interaction between the quadrapole moment in carbon dioxide and the multipoles along the CF2 polymer chain [20]. However, the higher the molecular weight, the higher the required pressure for a polymer to remain soluble [16]. Once the molecular weight is sufficiently high, PVDF is immiscible or essentially insoluble in carbon dioxide and it takes pressures greater than 1600 bar at temperatures between 130 and 215[degrees]C to dissolve PVDF [14, 21]. While this could be a potential drawback in fluoropolymer synthesis, scC[O.sub.2] swells and plasticizes polymers and carbon dioxide can transport VF2 into the polymer phase during the reaction maintaining high concentration of monomer near active chain ends [18].

Synthesis of PVDF in scC[O.sub.2] has been studied with a variety of initiators [16, 20], Most of the pioneering work and kinetic models were developed using diethyl peroxydicarbonate (DEPDC) as a free radical initiator at temperatures between 60 and 90[degrees]C and pressures between 200 and 305 bar in both batch and continuous stirred tank reactors [5, 13, 22-25], Several models have been developed evaluating the kinetics of free radical initiated polymerization of VF2 using DEPDC as an initiator as well [13, 24, 26-28], Free radical initiators are selected for polymerization reactions based on the reaction conditions and the reactivity of the initiator, which is dependent upon the type of initiator and temperature. The reactivity of the initiator may be considered on the basis of the temperature at which it takes half the initiator to decompose over time ([t.sub.1/2]), such as 1 h [29], Initiators that have been previously evaluated and published include diisopropyl peroxydicarbonate (ti/2[104[degrees]C] = 20 h), tm-butyl peroxypivalate, tert-amyl peroxypivalate, di-tm-butyl peroxide (DTBP), diakylperoxiydicarbonate, rm-butylperoxidy butyrate, and DEPDC [6], For this article, the effects of initiator amount, reaction time, mixing rate, and monomer fraction upon molecular weight, polydispersity, and amount of PVDF produced were evaluated using tert-butyl peroxyacetate (TPBA) as an initiator. TPBA is compared to DTBP and published data using DEPDC as an initiator. An extensive literature review of articles on the polymerization of VF2 has revealed that the use of tert-butyl peroxyacetate (TBPA) as an initiator in supercritical carbon dioxide has not been previously published. A dispersion agent was not used in this study.

EXPERIMENTAL

Chemicals

The free radical initiators DTBP, trade name Luperox[R] DI, and TBPA, trade name Luperox[R] 7M75, were supplied by Arkema. Luperox[R] 7M75 is 75 wt-% TBPA in aliphatic hydrocarbons. The self-accelerating decomposition temperature (SADT) and decomposition half-life temperature at 10 and 1 h for the initiators are listed in Table 1. The initiators were diluted using 1,1,1,2,3,4,4,5,5,5-decafluoropentane (CAS # 138495-42-8) supplied under the trade name Vertrel[R] XF Solvent from Dow Chemical. The carbon dioxide was instrument grade, 99.99% purity, from Praxair. The 1,1-difluoroethene was 99.8% purity supplied by Arkema. All chemicals were manufactured in the United States.

Apparatus

A process How diagram of the semibatch experimental system is shown in Fig. 1. The system is divided into three sections: reactor, gas delivery, and initiator delivery. The reactor section consisted of a reactor, agitator, reactor heater, feed preheater, pressure gauge, safety head assembly (SHA), and a computer controlled back-pressure regulator (BPR). The reactor was a bolt-closure, 300 mL, agitated Autoclave Engineers reactor with a SHA, thermowell, and cooling coil. Reactants were fed through a tube passing through the head leading to the impeller. The reactor was heated using a clamp heater. Carbon dioxide and VF2 were preheated using a heat tape. A computer data acquisition and control system was used to record the reactor pressure and control the depressurization of the reactor using the BPR. The reactor was protected from over-pressure by an independent emergency depressurization valve and SHA.

Carbon dioxide and VF2 had independent gas delivery systems and were mixed near the reactor inlet. Both feed gases were compressed using pneumatically driven booster pumps plumbed with stainless steel sample cylinders that acted as pulse dampeners. A forward pressure regulator was used to maintain a relatively constant VF2 pressure in the reactor during experiments. The VF2 cylinder was kept on a balance with reading increments of 6 g to provide a crude mass balance. Initiator was fed into the reactor using a single piston Eldex Laboratories micrometering pump.

Procedure

Experiments were carried out batch-wise with initiator and C[O.sub.2] and semibatch with VF2. The apparatus was prepared by purging the C[O.sub.2] and VF2 delivery systems of air. The reactor was then purged of air with carbon dioxide and initially charged to 2.86 MPa of C[O.sub.2]. The reactor and feed preheater were then heated to the experimental reaction temperature. While the reactor was heating, the agitation rate was set to 500 rpm, which is near the maximum agitation rate for the system. Once at experimental temperature, C[O.sub.2] was fed into the reactor to the desired partial pressure and the temperature was allowed to stabilize. Then the feed lines for the reactor were purged of C[O.sub.2] with VF2. The reactor was then charged to the experimental operating pressure with VF2. While the reactor temperature and pressure stabilized, the initiator feed lines were purged with Vertrel solvent, then with solvent-initiator solution. The initiator solution consisting of Vertrel solvent and initiator for a total of volume 3 mL was then fed to the reactor, which also denoted the start of the experimental run. At this point, the rate of agitation was adjusted, as needed, if mixing speed was evaluated. To insure that all of the initiator was in the reactor, the initiator feed system was used to add 0.7 mL of solvent into the reactor following the addition of the initiator. The total volume of solvent with initiator fed to the reactor was held constant throughout all experiments with a volume of 3.7 mL. The reaction was evaluated semibatch with VF2. Additional VF2 was added to make-up for VF2 consumed by the polymerization reaction. At the conclusion of the experiment, the reactor was gradually depressurized over 30 min, allowed to cool, then opened and the polymer product was removed. The polymer's mass, bulk density and morphology were recorded. The polymer molecular weight and molecular weight distribution were then analyzed using high temperature gel permeation chromatography at Arkema.

RESULTS AND DISCUSSION

Comparison between Initiators

Initially, both DTBP and TBPA were evaluated as free radical initiators to produce PVDF using supercritical carbon dioxide as a reaction medium. To compare DTBP and TBPA, experiments were performed at the 10 h half-life temperature (cf. Table 1) for each respective free radical initiator, at similar concentrations of C[O.sub.2] and VF2 and pure initiator loadings of 1 mL, or 5.4 mmol of DTBP and 6.2 mmol of TBPA. The agitation rate was 500 rpm and the semibatch reaction time was 180 min. The concentrations of C[O.sub.2] and VF2 at operating pressure and temperature were estimated using the Peng-Robinson equation of state with a binary interaction parameter of -0.0204 [30]. At these conditions, PVDF produced using either DTBP or TBPA was a freely flowing white powder with a few small agglomerated particles. At these conditions, DTBP was the less reactive initiator compared to TBPA, yielding less PVDF by mass. DTBP produced PVDF with a higher molecular weight and greater polydispersity than TBPA, Table 2. Due to lower reaction temperatures and greater quantity of PVDF, TBPA was selected for further evaluation.

Effect of Process Conditions upon Molecular Weight using TBPA

The effects of changing VF2 concentration, reaction time, initiator loading, and agitation rate upon the molecular weight of PVDF produced using TBPA as an initiator at its 10 h half-life temperature was evaluated. The TBPA initiated reactions were evaluated at a constant temperature of 104[degrees]C and carbon dioxide concentrations ranging from 6.59 to 10.0 mol/L.

Initially, the amount of initiator was varied with a C[O.sub.2] concentration of 9.55 mol/L, initial VF2 concentration of 3.13 mol/ L, an agitation rate of 500 rpm, and reaction time of 180 min. Total system pressure was maintained at 27.0 MPa at 104[degrees]C for this evaluation. While this pressure is comparable to published studies using DEPDC as an initiator at 75[degrees]C, the C[O.sub.2] concentration for this work is roughly half due to the increased temperature [22]. It was found that as the amount of initiator was decreased, the molecular weight increased, Fig. 2. The average molecular weight ranged from 65,000 g/mol with a polydispersity of 1.8 at an initiator loading of 2.35 x [10.sup.-3] (moles TBPA/ moles VF2) to 398,000 g/mol with a polydispersity of 3.3 at an initiator loading of 2.57 x [10.sup.-4] (moles TBPA/moles VF2). This result is expected for radical polymerization: the kinetic chain length of a polymer, which is related to the polymer molecular weight, is a function of the monomer concentration, [M], and initiator concentration, [I], as follows [M]/[[I].sup.0.5] [31]. When the results are linearized, it was found that molecular weight was linearly related to [M]/[[I].sup.0.5] with a correlation coefficient ([R.sup.2]) of 0.938. A corresponding decrease in average molecular weight with increased initiator amount was also observed by others using DEPDC initiated polymerization of VF2 using scC[O.sub.2] at 75[degrees]C and 27.5 MPa [22].

The next effect evaluated was agitation rate, Fig. 3. These experiments were conducted at 104[degrees]C, 7.26 mol/L C[O.sub.2] and 4.42 mol/L VF2. Agitation rate was varied from 0 to 500 rpm with initiator loadings of 1.25 x [10.sup.-4] (moles TBPA/moles VF2) and 2.14 x [10.sup.-4] (moles TBPA/moles VF2) with a reaction time of 180 min. To perform experiments without agitation, the initiator was fed to the reactor with agitation at 500 rpm and after 5 min agitation was stopped. This allowed for the initiator to be thoroughly mixed in the reactor. For an initial initiator amount of 2.14 x [10.sup.-4] (moles TBPA/moles VF2), agitation has a small effect on polymer molecular weight with molecular weight increasing by 7%, from 0 to 500 rpm. However, the amount of PVDF produced increased by 24% with increasing agitation. Agitation had a greater effect upon polymer molecular weight and production with 1.25 x [10.sup.-4] (moles TBPA/moles VF2) resulting in a 27% increase in molecular weight and 32% increase in PVDF produced. In studies using DEPDC as an initiator, mixing rate did not have a pronounced effect upon either amount of PVDF produced, polydispersity, or average molecular weight [13, 23]. However, the DEPDC was evaluated at a higher rate of agitation, between 1300 and 2700 rpm, which resulted in an average PVDF molecular weight between 52,000 and 62,100 g/mol [23]. The reason agitation rate may have an effect upon molecular weight and amount of PVDF produced using TBPA in a semibatch reactor is that the reactor is not perfectly mixed at lower rates of agitation. Without agitation, some of the polymer may diffuse or settle down to the bottom of the reactor due to gravity. Given that the reaction was carried forth semibatch with respect to the monomer, inadequate mixing could lead to concentration gradients of both the monomer throughout the reactor as well as localized concentration gradients of monomer near the active sites of the polymer that may have started to settle near the bottom of the reactor. This could lead to depletion of monomer and earlier termination resulting in lower mass amounts of PVDF produced and molecular weights. Polymer molecular weight is dependent upon monomer and initiator concentration. In other published studies, increasing the rate of mixing may lead to an increased rate of polymerization in the case of synthesis of PVDF. Imperfect mixing may also lead to bimodal molecular weight distributions as well as the inhibition of the polymerization reaction in both the production of PVDF as well as high-pressure polyethylene production [23, 27],

As reaction time increased from 90 to 180 min, the molecular weight of PVDF, as well as amount produced, increased, Fig. 4. The maximum average molecular weight was 840,000 g/mol with 1.25 x [10.sup.-4] (moles TBPA/moles VF2) and 810,000 g/mol with 2.14 x [10.sup.-4] (moles TBPA/moles VF2) both at 180 min of reaction time. The mass of PVDF produced increased with increasing reaction time since the monomer, initiator, as well as active chain ends had more time to react. It is surprising that the average molecular weight increased with increasing reaction time since the free radical initiated polymerization of VF2 in supercritical carbon dioxide is typically reported and modelled as a chain-growth mechanism [13, 16, 26] However, increasing molecular weight with increasing reaction time is not completely unexpected. Increasing molecular weight, as well as increasing polydispersity was also observed with increasing residence times using a continuously stirred tank reactor (CSTR) with DEPDC initiated polymerization with a VF2 concentration of 2.8 mol/L, molecular weight and polydispersity was not observed to increase with a monomer concentration of 0.78 mol/L [23]. For this work, polydispersity index (PDI) increased from 3.7 at 90 minutes to a PDI of 4.3 at 180 min for 1.25 x [10.sup.-4] (moles TBPA/moles VF2). With 2.14 x [10.sup.-4] (moles TBPA/moles VF2), the PDI was observed to increase from 3.7 at 120 min to 4.7 at 180 min of semibatch reaction time. In a study using DEPDC as an initiator with a CSTR, with residence times up to 50 min, the maximum reported molecular weight of PVDF was 441,000 g/mol. In the study with DEPDC, at an initial VF2 concentration of 2.51 mol/L, polydispersity and molecular weight were reported to increase with increasing residence times as well, however neither molecular weight nor polydispersity were observed to increase with an initial VF2 concentration of 0.78 mol/L [23]. The reported increases in molecular weight and polydispersity with increased reaction time are thought to be due to a couple of possibilities. The first possibly is that chain termination of the polymerization reaction may be due to combination, which could result in higher molecular weights [23]. The second thought is it may be due to crosslinking [24], A third possibility is that polymerization is heterogeneous, taking place in a polymer phase and supercritical phase within the reactor with transport of polymer radicals between the phases [23, 24], The fourth possibility is that the polymerization is homogenous with the chain-transfer-to-polymer reaction as well as the termination reaction being either diffusion or kinetic limited, of which a publish model exists that can predict the dependence of molecular weight upon residence time [26], In comparison, this reaction was performed semibatch with respect to VF2 with TBPA as an initiator. A difference between this study and batch reactors is that the VF2 concentration is held relatively constant with respect to time, comparable to a CSTR. It is noteworthy in both Figs. 3 and 4 that as the amount of initiator is decreased, the average molecular weight increased and the amount of polymer produced decreases.

Lastly, the effect of monomer and initiator concentration upon the average molecular weight and polydispersity were evaluated at a reaction time of 180 min at a constant temperature of 104[degrees]C with an agitation rate of 500 rpm. As VF2 concentration was increased and initiator amount decreased, the average molecular weight increased to a maximum molecular weight of 1.3 million g/mol with 1.24 x [10.sup.-4] (moles TBPA/moles VF2) and 4.68 mol/L of VF2, Fig. 5. As initiator concentration decreases, the molecular weight is expected to increase. Since this reaction was evaluated semibatch with respect to VF2, the monomer concentration was held relatively constant with the initiator being depleted over time, which would result in higher molecular weight polymers in comparison to batch polymerization where both the monomer and initiator concentration decreases with time. Over identical experimental conditions for TBPA initiated polymerization of VF2, 104[degrees]C 500 rpm agitation, 8.86 mol/L C[O.sub.2], the PDI increased from 1.4 at the condition of 2.19 x [10.sup.-3] (moles TBPA/moles VF2) with 2.15 mol/L VF2, which corresponds to 0.0047 mol/L TBPA, to a PDI of 5.4 with 1.24 x [10.sup.-4] (moles TBPA/moles VF2) with 4.68 L/mol VF2, which corresponds to 0.00058 mol/L TBPA, Fig. 6. In other works using 0.03 mol/L of DEPDC as an initiator in a CSTR with a residence time of 21 min and a reaction temperature of 75[degrees]C, the PDI was observed to increase from 1.5 to 5.5 over VF2 concentrations from 0.75 to 3.5 mol/L [26], This experimental study using TBPA as an initiator uses less initiator at a higher temperature compared to DEPDC. Overall, TBPA initiated polymerization of VF2 to produce PVDF resulted in higher molecular weight PVDF with a lower polydispersity at comparable monomer concentrations that than PVDF produced using DEPDC as reported in the literature [13, 23, 26],

CONCLUSIONS

1,1-Diflurorethene was reacted using tert-butyl peroxyacetate (TBPA) as a free radical initiator to produce PVDF at TBPA's ten hour half-life temperature using carbon dioxide as a solvent in a semibatch reactor. It was found that PDVF can be produced with average molecular weight up to 1.3 million g/mol and polydispersity of 5.4. Increased agitation, up to 500 rpm, and reaction times up to 180 minutes resulted in greater amounts of PVDF and higher molecular weights. As the amount of initiator was decreased and monomer concentration increased, the average molecular weight increased.

ACKNOWLEDGMENTS

The authors are grateful for the financial assistance and molecular weight characterization services of Arkema, Inc and the advice of Dr. H. Bryan Lanterman.

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Jonathan Wenzel, (1) Sunggyu Lee (2)

(1) Department of Chemical Engineering, Kettering University, Flint, MI, 48504

(2) Department of Chemical and Biomolecular Engineering, Ohio University, Athens, Ohio, 45701

Correspondence to: J. Wenzel; e-mail: jwenzel@kettering.edu

Contract grant sponsor: Arkema, Inc.

DOI 10.1002/pen.24269

TABLE 1. Chemical formulas, trade names, and decomposition
temperatures of the tested free radical initiators.

                                                           CAS registry
Species                               Formula                 number

Di-tert-butyl peroxide     [C.sub.8][H.sub.18][O.sub.2]      110-05-4
Ten-butyl peroxy acetate   [C.sub.6][H.sub.12][O.sub.3]      107-71-1

Species                    Arkema brand name   SADT/[degrees]C

Di-tert-butyl peroxide       Luperox[R] DI           82
Ten-butyl peroxy acetate    Luperox[R] 7M75          79

                           10 h [t.sub.1/2]   1 h [t.sub.1/2]
Species                      ([degrees]C)      ([degrees]C)

Di-tert-butyl peroxide           129                149
Ten-butyl peroxy acetate         102                123

TABLE 2. Effect of initiator type upon molecular weight and yield.

            [T.sub.rxn]            [[rho].sub.CO2]    [[rho].sub.FV2]
Initiator   ([degrees]C)   P/MPa   (mol [L.sup.-1])   (mol [L.sup.-1])

DTBP            117        28.28        10.00               2.05
TBPA            104        26.32        10.22               2.15

            Polymer                     [[bar.M].sub.W]/
Initiator     (g)     [[bar.M].sub.W]   [[bar.M].sub.N]

DTBP          7.9         125,100             1.8
TBPA         20.5         49,900              1.4

Pure initiator volume: 1 mL, reaction time 180 min, agitation rate
500 rpm.
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