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A UV-initiated reactive extrusion process for production of controlled-rheology polypropylene.

INTRODUCTION

Polypropylene (PP) is one of the world's most important thermoplastic materials, and it is used in numerous applications in the plastics industry. Because of its desirable properties such as low-cost, high-melting point, low-density, high-strength and stiffness, and excellent chemical resistance, its market share is growing fast. To achieve the diversity in polymer grades suitable for the different applications of PP, the molecular weight and molecular weight distribution (MWD) must be tailor-made to fit the performance requirements of each application.

The free radical promoted degradation of PP during extrusion is now a well recognized manufacturing process. The product is termed as controlled-rheology polypropylene (CRPP). CRPPs have been produced industrially for years using peroxides as free radical initiators. Such reactive extrusion (REX) processes convert the low-melt flow rate commodity resins to polymers with higher melt flow rates that have diverse processing properties because of the reduced viscosity and elasticity. There are many publications that describe the process of peroxide-initiated degradation of PP. These publications describe experimental and modeling studies that highlight the effects of processing conditions on the molecular weight, rheological, and thermal properties of CRPPs (1-22). Although peroxides have dominated as free radical initiators for CRPP production, nitroxide free radical generators have been recently introduced (23), (24), and their performance has been briefly compared with routinely used difunctional initiators (25). In addition, the use of a tetra-functional peroxide initiator has been explored for the production of CRPP (26).

Besides using chemical initiators, free radicals for controlled PP degradation can be generated by using irradiation methods, such as electron beams and [gamma]-rays, which have been chosen to modify the structure and performance of PP in some reports (27-29). Ultraviolet (UV) irradiation is one kind of high-energy irradiation that has been used as a modification method in polymer research for a long time. A considerable amount of work has been devoted to UV-irradiated polymerization, degradation, surface grafting, and crosslinking reactions (30-38). Allen et al. (32) presented an in-depth account of the complex mechanisms involved in both thermal and photochemical oxidation of polyolefins, with particular emphasis on PP. Tang et al. (33) investigated the effects of chemical structure and synthesis method on the photodegradation of PP and found that copolymerization with an amount of ethylene monomer is an effective approach to obtain high stability of PP to UV-irradiation. Rabell and White (34), (35) investigated the role of physical structure and morphology in the photodegradation behavior of PP and found that the initial physical structure of PP, which include the degree of crystallinity, crystal size, and molecular orientation, influence the photooxidation by affecting the oxygen permeability and UV absorption characteristics. Qu et al. (36) provided a new mechanism of benzophenone (BP) photoreduction in photoinitiated crosslinking of polyethylene and its model compounds. Pan et al. (37) explained the efficient photografting, sensitized by BP, by two possible mechanisms: the sensitization of the formation of the excited triplet state of maleic anhydride (MAH) by BP and electron transfer followed by proton transfer between MAH and the benzopinacol radical, which may operate together.

In this article, we have developed a REX process for the UV-initiated degradation of PP. This process employs BP as a photoinitiator, and the generation of free radicals is achieved by irradiating the molten PP in a short-open section of the barrel. A couple of characteristics of this process are: (i) the initiator is very well-mixed with the molten PP prior to the free radical initiation and (ii) the produced CRPP does not exhibit any of the characteristic odors accompanying the traditional peroxide-initiated degradation processes.

EXPERIMENTAL

Materials

A commercial PP homopolymer (Petrothene PP31KK01) from Equistar Chemicals was used in this study, which had a MFR of 5.0g/10 min (230[degrees]C, 2.16 kg). BP was purchased from Fisher Scientific Company and was used as received.

Equipment

The REX experiments were carried out in a Leistritz LSM 30.34 corotating twin-screw extruder with LID = 40/1 (ten heating zones). The UV source adopted was a UV Developer kit from UV Process Supply. The power of the mercury lamp was 3.0 kW and the UV power output used in the experiment was 300 W/in. of that lamp.

Procedures

To facilitate homogeneous distribution of BP in the PP resin, the required amount of BP was dissolved in acetone, before mixing with the PP pellets. The concentration of BP in the PP matrix varied between 0-0.5 wt%. After evaporation of acetone, the PP pellets impregnated with BP were added to the twin-screw extruder at the first zone. UV irradiation was achieved in the eighth and ninth barrel zones, which had two windows. The irradiation area on the two barrels was 40 X 75 mm and 33 X 60 mm, respectively. The screw configuration and lamp setup used in the experiments is shown in Fig. 1, These open barrel sections were placed after the kneading block to ensure complete mixing of the BP before irradiation. Two open barrels were used to ensure sufficient exposure for photo initiation.

[FIGURE 1 OMITTED]

Experiments were carried out at three different temperatures (170, 200, and 230[degrees]C) and three different polymer flow rates (24.8, 40.7, and 72.0g/min). The screw rotation speed was fixed at 150 rpm. The strand extrudates were cooled through a water bath and pelletized for further analysis.

Characterization

Melt flow rate (MFR) of the virgin and degraded PP were measured using a Kayeness melt indexer, according to ASTM 1238 at 230[degrees]C/2.16 kg. Steady-state shear viscosity was measured at 200[degrees]C using a Kayeness Galaxy V capillary rheometer with a die of LID = 40 (D = 0.030 in.).

A constant stress rotational rheometer AR2000 (TA Instrument) was used to measure the dynamic rheological behavior of PP samples. The samples obtained from the extruder were compression-molded at 210[degrees]C for 5 min, and disk-shaped specimens with a thickness of 1 mm and a diameter of 25 mm were prepared. The measurements of the dynamic rheological properties were performed with a parallel-plate fixture (diameter = 25 mm), with a gap distance of 1 mm, and the strain was kept at 1% to ensure linear viscoelastic response. The frequency range was 0.01-100 Hz, and the temperature was 200[degrees]C. Tests were run under nitrogen purge at a flow rate of 5 ml/min.

MWD of the modified PP were determined by size exclusion chromatography (Polymer ChAR) equipped with a triple detector system. Samples were dissolved in 1,2,4-tricblorobenzene at 145[degrees]C and injected in the column with a flow rate of 1.0 ml/min. Universal calibration using polyethylene standards (SRM 1475a, NIST) were applied to determine the molecular weights.

FT-IR spectra of the virgin and degraded PP were obtained using an ABB Fourier-transform infrared spectrometer. Films were compressed at 210[degrees]C for 2 min.

Thermal properties of the samples were measured with a TA Instrument Q2000 differential scanning calorimeter. The temperature and heat flow area were calibrated with indium before the analysis. The samples (ca. 6 mg) were sealed in an aluminum pan and heated or cooled in a nitrogen atmosphere. Initially, the samples were heated at room temperature to 210[degrees]C at a rate of 20[degrees]C/min to erase the thermal history and cooled at a rate of 10[degrees]C/min to obtain the nonisothermal crystallization. The degree of crystallinity Xc was calculated by the relative ratio of the enthalpy of crystallization per gram of sample to the heat of fusion of the PP crystal (209 J/g) (34).

RESULTS AND DISCUSSION

According to previous studies on the BP photoreduction mechanism (36), when the BP photoinitiator absorbs UV radiation, it is excited to the singlet state 1(BP) *, and then rapidly relaxes to the more stable triplet state 3(BP) * through intersystem crossing as shown in Fig. 2. The triplet 3(BP) * can abstract a tertiary hydrogen from the PP chain to form a ketyl radical and a polymer alkyl (P*) radical. The polymer alkyl radical (P*) undergoes a chain-scission reaction to produce a shorter polymer chain ([P.sub.1]) and another polymer radical ([P*.sub.2]). This chain scission is similar to that promoted by peroxide radicals and results in molecular weight reduction and narrowing of the MWD (2).

[FIGURE 2 OMITTED]

Figure 3 shows the effect of photoinitiator BP concentration on the MFR of the degraded PP at three different reaction temperatures. When using UV radiation without any BP (data points at 0% BP concentration), the MFR values achieved at 170, 200, and 230[degrees]C were 7.8, 12.1, and 39.9 respectively. The corresponding MFR values obtained under the same extrusion conditions without UV radiation and without photoinitiator were 4.2, 4.9, and 7.5 at 170, 200, and 230[degrees]C respectively. Comparison of these MFR data indicates that the effect of UV initiated degradation on MFR is more significant than that of thermal degradation. From this figure, it can be seen that MFR increases with BP concentration and temperature. For the lower temperatures, it seems that the MFR levels off at BP concentrations above 0.1%. However, at the higher reaction temperature, the MFR keeps increasing but at a lower rate. Figure 4 shows the effect of PP throughput on MFR of the degraded PP. The MFR of degraded PP increased with decreasing PP throughput. For a given BP concentration, lower throughput results in longer residence times in the extruder and therefore more exposure time to UV irradiation that leads to higher radical concentration. In addition, lower throughput results in a lower degree-of-fill and thinner-melt stream that UV irradiation can penetrate to a greater depth and thus increase the radical concentration (39).

[FIGURE 3 OMITTED]

Figures 3 and 4 show that the MFR of UV degraded PP increases slowly when BP concentration exceeds 0.1 wt%, especially at reaction temperatures 170 and 200[degrees]C. This suggests that the UV irradiation, and not the BP concentration, was the key factor for this degradation reaction. Similar results have been achieved in photodegradation of PP and starch-filled polymer (40).

[FIGURE 4 OMITTED]

Figure 5 shows the MWDs of virgin and some degraded PP. After the UV-initiated degradation, there was a shift of the MWD to lower molecular weights. The shift in the distribution was accompanied by an apparent removal of the high-molecular-weight molecules and a narrowing of the distribution. In agreement with previous peroxide-initiated degradation studies (2), this indicates that the high-molecular-weight molecules are the ones to predominately degrade in such a UV irradiation and extrusion process. The corresponding average molecular weights are listed in Table 1.

[FIGURE 5 OMITTED]
TABLE 1. SEC results of UV degraded  polypropylene (extrusion
conditions at 200[degrees]C, throughput = 72.0g/min).

                     Virgin PP  UV irradiation  UV irradiation and
                                                     0.05% BP

[M.sub.n] (g/mol)      66,600        57,150            51,000
[M.sub.w] (g/mol)     242,200       202,000           159,300
[M.sub.z] (g/mol)     545,700       428,700           367,300
[M.sub.w]/[M.sub.n]     3.64          3.54              3.13


Figure 6 shows the steady shear viscosity of degraded PP at various BP concentrations. It is apparent that UV irradiation and BP concentration have dramatic effects on the rheological behavior of degraded PP. Increasing the BP concentration leads to lower viscosity and more Newtonian behavior. These results were consistent with the MFR results and prove that UV energy was the key parameter in such a REX.

[FIGURE 6 OMITTED]

Figures 7 and 8 show the flow curves of degraded PPs at different reaction temperatures without BP and at fixed BP concentration (0.1 wt% of PP). Under these conditions, the shear viscosity decreased with an increase of reaction temperature. Compared with long hundreds of hours photo irradiation on solid PP, UV induced PP during melt extrusion has high efficiency. It takes only several seconds of irradiation to get degraded PP with high MFR. However, because thermal-degradation cannot be totally avoided for such a process, it enhances the PP degradation especially at the higher reaction temperature (230[degrees]C).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Figures 9 A and B show the double logarithmic plots of complex viscosity (|[eta]*|) and storage (G') and loss (G") moduli as functions of frequency ([omega]) for degraded PP at different BP concentrations. As expected, similar decreases in complex viscosity, storage modulus, and loss modulus could be seen for UV degraded PP with increasing BP concentration. Viscosity is a very sensitive way to detect the subtle change of topological structure of macro-molecules. The complex viscosity curves of virgin and degraded PP are shown in Fig. 9A. When PP was degraded by UV with and without BP, the zero-shear complex viscosity decreased significantly, indicating a reduction of the weight-average molecular weight of PP. As seen in Fig. 9A, the storage modulus decreases with BP concentration, which indicates that the degraded PP is less elastic than the virgin material.

[FIGURE 9 OMITTED]

Rheological techniques could be used to relate molecular structure to linear viscoelastic properties. According to the previous reports (7), (41), it is possible to correlate the breadth of the MWD for the linear PP resins with the value of the "crossover modulus," [G.sub.c]. The "crossover modulus" is defined as the modulus value at the "crossover frequency," [[omega].sub.c], where the storage and loss moduli are equal. Subsequently, the rheological polydis-persity index, PI, is defined as follows:

PI = [[10.sup.5]/[G.sub.c]]. (1)

In addition, the "modulus separation" (Modsep) technique proposed by Yoo (42), can also be used to compare the MWDs of linear PPs. From the same frequency sweep data, the distance between the G' and G" curves is measured at a constant modulus value, namely, 1000, 500, or 100 Pa depending on the resin MFR. Then, the modulus separation (Modsep) is defined as:

Modsep = [[omega]'/[omega]"] (2)

where [omega]' = [omega] (G' = 1000, 500, or 100 Pa) and [omega]" = [omega] (G" = 1000, 500, or 100 Pa).

Table 2 summarizes experimental results and calculated values for the virgin and degraded PP. It can be seen that the crossover modulus and crossover frequency increases with the BP concentration. PI decreases and Modsep increases with BP concentration, indicating that degraded PP have narrower MWD than virgin PP. When the BP concentration reaches 0.1 wt%, the effect of degradation reaction on the MWD seems to diminish.
TABLE 2. Rheological polydispersity of virgin and degraded
polypropylene calculated from dynamic rheological properties.
(Extrusion conditions at 200[degrees]C, throughput = 72.0g/min).

Samples    [G.sub.c]  [[omega].sub.c]   PI    Modsep   Modsep @500 Pa
             (kPa)        (rad/s)            @1000 Pa

Virgin PP    25.71         31.49       3.89    3.83         4.63
0% UV        24.94        125.4        4.00    4.00         4.83
0.05% UV     28.45        250.1        3.51    4.22         5.16
0.1% UV      30.20        314.9        3.31    4.34         5.36
0.3% UV      30.20        396.4        3.31    4.37         5.38
0.5% UV      32.20        396.4        3.11    4.33         5.31


Figure 10 shows the FT-IR spectrum of the virgin and degraded PP at different concentrations of photoinitiator. After UV degradation, it can be seen that the peaks corresponding to carbonyl groups are almost the same with those of virgin PP. This observation differs from that on solid PP irradiated by UV at room temperature (33-34). For the long term degradation of PP by UV irradiation at room temperature, the main products of degradation, carbonyl group, and hydroperoxides are normally in the wavelength ranges 1700-1800 [cm.sup.-1] and 3300-3600 [cm.sup.-1]. It can be deduced that oxidative degradation was not important during this UV-initiated REX degradation process. The shoulder peak at 888 [cm.sup.-1], which appears in the degraded PP (indicated by the arrow in Fig. 11), is characteristic of the terminal double bonds in PP molecular structure (13). In addition, no evidence of residual BP could be found in these spectra. However, a more detailed evaluation of residual BP should be performed in the future, because any residual BP could induce further degradation during service of the PP manufactured using this technique.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The crystallization behaviors of virgin and UV degraded PPs are shown in Fig. 12, and the thermal properties are listed in Table 3. The crystallization temperature, the onset temperature, and crystallinity of PP decrease slightly after UV irradiation and with increasing BP concentration. The crystallization behavior of PP is affected by the chain length of molecules. Because of chain scission of PP molecules during UV irradiation, shorter molecules of PP are produced, and they can only be "frozen" at lower temperatures compared with longer molecules of PP, thus decreasing the crystallization temperature when cooling. The decrease in crystallinity of the degraded PP is probably because of the inability of some of the degraded molecules to crystallize when the temperature is reduced. The crystallization behavior observed in this work is similar to that of controlled-rheology PP grades produced by peroxide initiation, and it depends on MWD as described in the literature (43).

[FIGURE 12 OMITTED]
TABLE 3. Crystallization behaviors of UV degraded PP at 200[degrees]C
with throughput 72.0g/min.

Samples         [T.sub.c]   [T.sub.onset]  [H.sub.c]  Crystallinity
              ([degrees]C)  ([degrees]C)     (J/g)         (%)

Virgin PP         118.9         122.3        105.3        50.4
BP = 0% UV        116.7         120.8        104.1        49.8
BP = 0.3% UV      116.4         120.4        102.9        49.2
BP = 0.5% UV      116.2         120.5        101.2        48.4

[T.sub.c], crystallization peak temperature; [T.sub.onset],
crystallization onset temperature; [H.sub.c] enthalpy of
crystallization.


CONCLUSION

A reactive process on the basis of UV initiation has been developed for the production of controlled rheology PPs. The MWD can be significantly modified even at very low-irradiation times. BP was used as free radical generator, and it was found that its concentration has a significant effect even at low-levels. The reduction in molecular weight and rheological properties was more significant at higher reaction temperatures. The degraded materials exhibited reduced viscosity and elasticity as expected because of the reduced molecular weight and narrower MWDs. After degradation, MFR increased and viscosity decreased with BP concentration. FTIR analysis showed a reduction in oxidative degradation compared with traditional free radical processes. Crystallinity levels and crystallization temperatures of the degraded PPs were lower than those of virgin PP.

ACKNOWLEDGMENTS

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged by the authors.

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Correspondence to: Costas Tzoganakis; e-mail: ctzogan@uwaterloo.ca

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2010 Society of Plastics Engineers

Guangjian He, (1), (2) Costas Tzoganakis (1)

(1) Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

(2) National Engineering Research Centre of Novel Equipment for Polymer Processing, South China University of Technology, Guangzhou 510641, China

DOI 10.1002/pen.21732
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