Rheology and Melt Strength of Long Chain Branching Polypropylene Prepared by Reactive Extrusion With Various Peroxides
Feng-Hua Su, Han-Xiong Huang Center for Polymer Processing Equipment and Intellectualization, College of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China
Long Chain Branching Polypropylenes were prepared in an extruder by a melt grafting reaction in the presence of various peroxides and a polyfunctional monomer of 1,6-hexanediol diarylate. Fourier Transformed Infrared spectra and the rheological characteristics indicated that the grafting reaction added long branched chains to linear polypropylene (PP). In comparison to the initial PP, the branched samples exhibited higher melt strength, lower melt flow index, and enhancement of crystallization temperature. The branching number of the modified samples agreed well with their melt viscoelasticity and the improved degree of their melt strength. The branching level in modified PPs could be controlled by the property and structure of the peroxide used. Peroxides with lower decomposition temperature and more stable radicals after decomposition promoted the branching reaction, leading to the modified PPs with higher branching level and melt strength. POLYM. ENG. SCI., 50:342–351, 2010. ª 2009 Society of Plastics Engineers
INTRODUCTION Compared with many other commercial polymers, polypropylene (PP) has many desirable properties, such as higher melting point, low density, excellent chemical resistance, higher tensile modulus, and low cost. However, commercial PP produced with Zieglar-Natta or metallocene catalysts has a linear molecular structure with a narrow molecular weight distribution. Linear PP has relatively low melt strength and exhibits no strain hardening. The low melt strength results in local instability in thermoforming, blow molding, extrusion coating, and foaming. Correspondence to: Feng-Hua Su; e-mail:
[email protected] Contract grant sponsor: Research Fund for the National Natural Science Foundation of China; contract grant number: 20904012; contract grant sponsor: China Postdoctoral Science Foundation; contract grant number: 20080440749. DOI 10.1002/pen.21544 Published online in Wiley InterScience (www.interscience.wiley.com). C 2009 Society of Plastics Engineers V
‘‘High melt strength PP’’ can be produced by introducing long chain branching (LCB) into linear PP [1–3]. A LCB structure can improve the processing ability of PP under melt conditions, including strain hardening and shear thinning, and, thus broadening the end uses for PP [4–6]. At present, two techniques have been developed to prepare LCB polypropylene (LCB-PP): postreactor treatment [7–16] and in situ polymerization [17, 18]. Postreactor treatments, including electron beam irradiation [7–10] and reactive extrusion processes [1, 11–16], are the more popular methods because they can be easily used in industry. Compared to electron beam irradiation, the reactive extrusion has many merits, including simple operation, low cost, and high productivity. For this reason, the preparation of LCB-PP by reactive extrusion processes has generated increasing interest over the past decades. Lagendijk et al. [1] produced LCB-PP by reactive extrusion in the presence of peroxydicarbonate (PODIC) with various structures. They found that all modified samples showed enhanced strain hardening, slightly lower melt flow index (MFI), increased extrudate swelling, and improvements in melt strength. They suggested that the amount of LCB can be controlled by the type and the amount of PODIC used for the modification, indicating that the peroxide structure had a direct influence on the branching level of modified PPs. PP has a tendency to undergo b-scission because of the nature of its molecular structure, and this competes with grafting and cross-linking reactions during the reactive extrusion process. The use of polyfunctional monomers can decrease the degradation and improve the degree of branching for PP [11, 12]. A polyfunctional monomer in the presence of free radicals can produce more stable macroradical sites, which increase the likelihood of further reactions, because of a decrease of the probability of fragmentation. Rheological properties of a melt are strongly affected by the presence of long chain branches [1, 5, 7–20]. Thus, rheology has proven to be a reliable and easy to implement method for the verification of the existence of long branches on the polymeric chain [11, 13, 15–17]. Long POLYMER ENGINEERING AND SCIENCE—-2010
TABLE 1. Physical and chemical characteristics of peroxides used for modifying PP.a
Chemical name dicumyl peroxide
di-4-tert-butylcyclohexyl peroxide
Mw (kg/mol)
Tt1/2 ¼ 1 h (8C)
EA (kJ/mol)
DCP
270
132
129.3
BCHPC
398
64
128.1
146
141
130.7
Abridged name
Formula
OO C(CH3)3 (CH3)3C
di-tert-butyl peroxide
DTBP
cumene hydroperoxide
CHP
152
166
132.5
dibenzoyl peroxide
BPO
242
91
122.3
a
Tt1/2 ¼ 1 h (8C) represents the temperature at which the half-life time of peroxide is hour.
branched chains increase the possibility of entanglements in the polymeric melt, and thus affect its elasticity. Yu and coworkers [11] confirmed the branching chain structure of modified PPs by small-amplitude oscillatory shear experiments. The detection of the branching number is very difficult because the degradation, grafting, and crosslinking reactions take place simultaneously during the reactive extrusion process leading to a complex product. Compared to NMR and GPC, rheology is a more appropriate and reliable technique for verifying the existence of long branches on polymeric chains, especially for low level LCB. Tsenoglou and Gotsis [21] determined the extent of LCB based on the rheological characteristics of these types of samples. Garcı´a-Franco et al. [22] suggested that the level of LCB on polyethylene can be quantified by small amplitude oscillatory shear experiments, with analysis predicated on the use of so-called Van Gurp-Palmen plots (Van Gurp plots). The main objective underlying the introduction of long branched chains onto a linear PP backbone is to increase the melt strength and improve the processing ability of the linear PP. Extensibility of the polymer melt is of great importance for polymer processes, such as polymer fiber spinning, film blowing, coating, and sheet casting, and this can be determined by use of an ‘‘extension diagram.’’ This diagram depicts the drawdown force needed for elongation of an extruded strand, measured as a function of a slowly increased drawdown speed [23, 24]. In the present contribution, our efforts focused on the preparation of LCB-PP by reactive extrusion. We evaluated the efficiency of various peroxides used for modification in the presence of 1,6-hexanediol diarylate (HDDA) and provide an explanation for the mechanism by which different peroxides cause the modification of melt strength and rheology. Various rheological plots are used to DOI 10.1002/pen
differentiate the viscoelasticity of initial PP and the modified samples.
EXPERIMENTAL Materials and Sample Preparation Isotactic PP was supplied in powder form by Maoming Petrochemical Corporation, China. The number average molecular weight (Mn) was 57.992 g/mol, the weight average molecular weight (Mw) was 386.786 g/mol, and the polydispersity (Mw/Mn) was 6.67. The PP powder was stabilized by the addition of 0.1 wt% Irganox 1010 antioxidant (Jinhai Albemarle, China). Polyfunctional monomer (1,6-hexanediol diarylate, HDDA) was purchased from Tianjintianjiao Chemical, China. Various peroxides were purchased from Lanzhou Auxiliary Agent Plant, China. The chemical name, molecular structure, and properties of these are shown in Table 1. The peroxides and HDDA were used as received. For even dispersion of peroxide and HDDA in the PP powder, these were dissolved in 50 ml of acetone, and this solution was added to 1000 g PP powder during mixing with a SHR10A high speed mixer. The concentrations of peroxide and HDDA relative to PP powder were 1.2 mmol/kg and 120 mmol/kg, respectively. The mixing of linear PP with antioxidant, peroxide, and HDDA in the mixer was maintained for 10 min and was followed by modification of the linear PP in a TE35 co-rotating twinscrew extruder. The temperature of the extruder zones was maintained at 160–2108C from hopper to die. The throughput and the screw speed were 4.8 kg/h and 60 rpm, respectively. During the extruding process, a devolatilization zone was established before the pumping zone, POLYMER ENGINEERING AND SCIENCE—-2010 343
TABLE 2. Concentration of peroxide and HDDA relative to PP powder (mmol/kg) and the type of peroxide used for modifying PP.
Samples
HDDA concentration
Peroxide type
Peroxide concentration
— — 120 120 120 120 120
— BPO DCP BCHPC CHP DTBP BPO
— 1.2 1.2 1.2 1.2 1.2 1.2
PP0 PP1 PP2 PP3 PP4 PP5 PP6
close to the die, to remove small molecules, such as peroxide decomposition products. Extrudates were cooled in water and then pelletized. The nature and concentrations of peroxide and HDDA relative to PP powder are shown in Table 2.
Characterization FTIR spectra of all samples were measured at room temperature using a Bruker model Tensor 27 Fourier transformation infrared spectroscope (FTIR). The modified PP was added to xylene and heated to 1408C. The solutions were then charged into acetone at room temperature. Unreacted HDDA monomer and co-polymerized HDDA remained soluble, whereas PP and grafted PPs precipitated out. The modified PPs were separated by filtration and dried at 808C under vacuum for 48 h. The purified samples were pressed into films for FTIR analysis. The crystallization temperatures of the initial and modified PPs were investigated by DSC204F1 differential scanning calorimetry (NETZSCH, Germany). Specimens were heated to 2008C at a rate of 108C/min, held for 3 min to eliminate the thermal histories, and then cooled down to 258C at a rate of 108C/min to determine the crystallization temperature. All rheological experiments were conducted with a Bohlin Gemini 200 Rheometer equipped with a parallel-plate fixture (25 mm diameter). Disk samples were prepared by compression molding to a thickness of 2.0 mm and diameter of 25 mm. The gap between the two parallel plates was maintained at 1.75 mm for all measurements. Small-amplitude oscillatory shear tests were performed as a function of angular frequency (x) ranging from 0.01 to 100 rad/s at 1908C. A fixed strain of 1% was used to ensure that meas-
urements were carried out within the linear viscoelastic range of the materials investigated. Steady model measurement was conducted as a function of shear rate to determine the zero shear viscosity. At sufficiently low shear rate, the shear viscosity of a tested sample will become steady and is then independent of shear rate, allowing determination of the zero shear viscosity, as follows: lim Zðg˙ Þ ¼ Z0 :
(1)
g!0 ˙
The melt strength was measured using a four-wheeled G[mac]ottfert Rheotens 71.97 tester in combination with a G[mac]ottfert Rheograph 25 High Pressure Capillary Rheometer. The Rheotens 71.97 was located close to the exit of the capillary. The additional pair of pull-off wheels was integrated to prevent sticking of the elongated polymer strand, once it had passed the first pair of pulleys during the test. Table 3 lists the test conditions for the elongation experiments used to measure the melt strength and velocity at break. Plastic pellets were melted in the heated test cylinder and pressed with a test piston at constant speed out through a capillary. The melt strand was continuously drawn down at a linear exponentially accelerating velocity between the two counter-rotating wheels of the device, which were mounted on a balanced beam. The drawdown was continuously measured as a function of the angular speed of the wheels as the melt strand was pulling. The force and velocity resulting from the melt strand breakage were deemed as ‘‘melt strength (Ms)’’ and ‘‘velocity at break (Br.Vel)’’ for the tested condition.
RESULTS AND DISCUSSION FTIR Spectroscopy Figure 1 shows the FTIR spectra of the initial PP and the modified samples after purification. A band at about 1735 cm21 was ascribed to the stretching vibration of carbonyl group of the ester. For all samples modified with various peroxides in the presence of HDDA, there are bands at about 1735 cm21, indicating that HDDA had been grafted onto the PP backbone. The spectrum area at 1735 cm21 and 841 cm21 were different for different modified samples, indicating that grafting efficiency was different for different peroxides. To distinguish the grafting efficiency of different peroxides used for modification,
TABLE 3. Test conditions for the elongation experiment used to measure the melt strength and velocity at break for initial PP and modified PPs. Capillary rheometer parameter
Rheotens 71.97 Parameter
Die geometry Length ¼ 30 mm Diameter ¼ 1 mm Piston Speed ¼ 0.1 mm/s Diameter ¼ 15 mm Shear rate ¼ 180 s21, Temperature ¼ 1808C, Melt time ¼ 30 min
The gap between two wheels ¼ 0.3 mm V0 ¼ 20 mm/s Acceleration ¼ 2.4 mm/s2
344 POLYMER ENGINEERING AND SCIENCE—-2010
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FIG. 1. FTIR spectra (800–2000 cm21) of initial PP and PPs modified with different peroxides, after purification.
the carbonyl index (CI) from the FTIR spectra was calculated as follows: CI ¼ A1753 cm1 =A841 cm1 ;
FIG. 3. DSC cooling thermograms of the initial PP and PPs modified with different peroxides.
unable to differentiate the structure of long- versus shortbranched chains. The linear viscoelastic behavior was very sensitive to the LCB of the polymer.
where A1735 cm21 is the spectrum area at 1735 cm21, which is characteristic of the carbonyl groups of the HDDA ester; and A841 cm21 is spectrum area at 841 cm21, which is characteristic of the CH3 groups in the PP backbone. The CI data for the modified PPs shown in Fig. 2 indicate that the grafting degree in the modified samples is different and that the grafting efficiency varied with the structure and property of the chosen peroxide. The order of grafting efficiency was as follows: dibenzoyl peroxide (BPO) [ cumene hydroperoxide (CHP) [ di-4-tert-butylcyclohexyl peroxide (BCHPC) [ dicumyl peroxide (DCP) di-tert-butyl peroxide (DTBP). The value of CI was used to evaluate the grafting efficiency but was
Crystallization Temperature
FIG. 2. Comparison of CI (A1735 cm21/A841 cm21) of modified PPs with different peroxides, after purification.
FIG. 4. Crystallization temperature (Tc) of the initial PP and PPs modified with different peroxides.
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The modification of PP with peroxide and a polyfunctional monomer will result in a change in the molecular weight and its distribution, and also in a change in chain irregularity. These variations in the microstructure affect the crystallization behavior of the system. Figure 3 shows the DSC cooling thermograms of the initial PP (PP0) and the modified samples. The corresponding crystallization temperatures are shown in Fig. 4. The crystallization exotherm was an indication of the bulk crystallization rate; overall, a higher crystallization temperature (Tc) resulted in a more rapid crystallization. For PP1, its higher crystallization temperature and elevated crystallization rate, compared to PP0, was because of the degradation of the
POLYMER ENGINEERING AND SCIENCE—-2010 345
FIG. 5. G0 versus x of initial PP and PPs modified with different peroxides at 1908C.
linear PP chain induced by the BPO peroxide. This degradation of linear PP led to enhancement of the mobility of the molecular chain segment in favor of diffusion and arrangement of macromolecules into a crystal cell; therefore, the crystallization rate was elevated and the crystallization temperature increased. With the addition of HDDA, the crystallization temperatures of modified samples were higher than those of initial PP0 or degraded PP1. Tang et al. [24] have reported that the LCB structure can act as a nucleating agent for the crystallization of PP, which results in a higher crystallization temperature of the sample. The branching structure was most probably generated as peroxide and HDDA were added to the reaction mixture, so that these were responsible for the increased crystallization temperature and the accelerated crystallization. The crystallization temperatures of modified PPs with different types of peroxides were different, which might be attributed to the differences in their molecular structures. The grafting and degradation reaction between different peroxides, HDDA, and PP were different that resulted in the different proportions of LCB and degraded molecules in the modified samples. The LCB and degraded molecules affected the crystallization behavior simultaneously, which resulted in their different crystallization temperatures.
the terminal behavior. The G0 of modified samples in the presence of HDDA was higher than that of initial PP at low frequency, and their terminal slopes decreased compared to that of the initial PP. This result was attributed to the form of the long branched chains in their backbone, which led to the increased dynamic modulus at low frequency and the longer relaxation time [11]. However, as the curves of G0 versus x varied with the type of peroxide, this suggested that the elastic response and the level of LCB in modified PPs were affected by the structure and property of the peroxide. Comparison of these curves indicated that the best branching efficiency was obtained with BPO, followed by CHP and then BCHPC. To further confirm the existence of LCB on the modified PP backbone, the complex viscosity (g*) versus sweeping frequency (x) of PP0 and the modified PPs are shown in Fig. 6. A plateau in the curves for PP0 and PP1 was seen at low frequency, which was because of their linear chain structure. An increase in g* at low frequency and shear-thinning occurred as HDDA was added to the reaction mixture independent of peroxide type and was attributed to the existence of LCB in PP backbone [11, 14, 15]. The shear-thinning behavior changed as peroxide type was varied, indicating that the structure and property of peroxide affected the branching efficiency. The shearthinning behavior of PP modified with BPO in the presence of HDDA was the most distinct among all evaluated cases, demonstrating that a large number of LCBs were grafted in the PP backbone in the presence of this peroxide. It has recently been reported that plots of the tangent of the loss angle (d) versus sweeping frequency (tan d versus x) show changes in response to the presence of long chain branched structures in polymers [11, 13]. The curves of tan d versus x for PP0 and the modified PPs are shown in Fig. 7. For PP0 and PP1, rapidly descending curves with increasing frequency were observed, which is
Linear Viscoelastic Properties The storage modulus (G0 ) plotted as a function of sweeping frequency (x) of initial PP and modified PPs is shown in Fig. 5. Both PP0 and PP1 exhibited typical terminal behavior, because of their linear chain structures. The terminal behavior of PP1 was more obvious than that of initial PP because the chain scission process is very severe in the absence of a polyfunctional monomer. With the addition of HDDA, the modified PPs deviated from 346 POLYMER ENGINEERING AND SCIENCE—-2010
FIG. 6. g* versus x of initial PP and PPs modified with different peroxides at 1908C.
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FIG. 7. tan d versus x of neat PP and PPs modified with different peroxides at 1908C.
typical terminal behavior for liquid-like materials and is because of their linear chain structure. tan d values for PPs modified with different peroxides in the presence of HDDA were substantially lower than that for PP0 at low sweeping frequency, and a platform at a higher frequency regime was also observed. The curves for PPs modified by each different peroxide were also different from each other, which is because of different levels of LCB in their molecules. The PPs modified with BPO, CHP, or BCHPC showed behavior typical of solid-like materials, indicating that the levels of LCB in their molecules were high. As reported by many researchers, the Van-Gurp plot 1 ðd log G ; jG j ¼ ðG02 þ G002 Þ =2 Þ can be used to gain insight into long-chain branching [7, 22, 25, 26]. Hatzikiriakos [25] has found that upon increasing the level of LCB in polyethylene, the Van Gurp curve shifts to a smaller value of the phase angle. Auhl et al. [7] have also suggested that this phase angle tendency toward smaller values at fixed G* can be caused by an introduction of LCB in PP. The same method was also used in the present work for assessing the existence and the level of LCB in PPs modified with different peroxides in the presence of HDDA. As shown in Fig. 8, the Van Gurp plots of PP2PP6 shifted to smaller values of the phase angle in comparison to the initial PP0 and PP1, indicating that all peroxides could produce LCB in a PP backbone in the presence of HDDA. The shifting of Van Gurp plots to smaller values of phase angle varied with the type of peroxide, suggesting that the branching efficiency of different peroxides was dissimilar. The shifting of Van Gurp plots indicated that the levels of LCB in modified PPs were as follows: PP6 [ PP4 [ PP3 [ PP2 [ PP5. This also suggested that the branching efficiency of peroxide used for modification was as follows: BPO [ CHP [ BCHPC [ DCP [ DTBP. The relaxation process can be estimated by a ColeCole plot (g00 –g0 ) [11]. Figure 9 shows the Cole-Cole DOI 10.1002/pen
FIG. 8. The Van Gurp plots of initial PP and PPs modified with different peroxides at 1908C.
plots for PP0 and the modified PPs. Very clear differences were apparent, as Cole-Cole plots of PP0 and PP1 exhibited almost a semicircular pattern because of their linear chain structure. A higher molecular weight resulted in an increased radius of the semicircle. For PP1, the degradation reaction in the absence of HDDA resulted in a lower curve of PP1 compared to PP0. The Cole-Cole plots of the modified PPs were higher than that of the initial PP and showed more evident upturning at high viscosity, indicating that these samples had longer relaxation times. Several rheology plots were used to confirm the existence of LCB in the modified PPs. A number of rheological properties consistent with of LCB-PP, such as higher G0 at low frequency, increased shear-thinning behavior, plateaus in tan d–x plots, G* shifting to smaller values of the phase angle in the Van Gurp plots, and upturning at high viscosity in Cole-Cole plot, were
FIG. 9. Cole-Cole plots of initial PP and PPs modified with different peroxides at 1908C.
POLYMER ENGINEERING AND SCIENCE—-2010 347
found in the modified samples. However, the viscoelastic properties of the modified PPs varied with the type of peroxide used, indicating that the molecular chain structures of the modified samples were affected by the structure and property of the peroxide. By analyzing the viscoelasticity of the modified PPs, the branching efficiency was determined to be: BPO [ CHP [ BCHPC [ DCP [ DTBP. This result agreed well with the analysis from FTIR.
TABLE 4. Rheological parameters for initial PP and PPs modified with different peroxides. Sample PP0 PP1 PP2 PP3 PP4 PP5 PP6
g0 (3104 Pa s)
Bn
0.68 0.49 0.78 1.28 1.48 0.74 1.96
— — 0.09 0.48 0.61 0.06 0.80
Quantitative Analysis of LCB Although viscoelastic behavior can be used to estimate the level of LCB in modified PPs, it is difficult to determine the exact amount of LCB without incorporation of certain macromolecular theories. Figure 10 shows the shear viscosity (g) of the initial PP and modified PPs as a function of shear rate (_c). As illustrated in the experimental section, zero shear viscosity (g0) can be obtained from Fig. 10, and these values are listed in Table 4. Tsenoglou and Gotsis [21] have attempted to determine the level of LCB in PP modified with PODIC. They assumed that the linear polymer chain was broken through scission and that some of their fragments would crosslink with neighbors to form the three-arm branched molecules. The reacted product was a blend consisting of linear molecules and three-arm branched molecules. The weight fractions of branched chains (Bn) can be expressed as follows: Bn ¼
lnðZBL =ZL Þ ; aðML =MC Þ 3 lnðML =MCÞ
(2)
where ZBL is the zero-shear viscosity of branched linear blend, ZL and ML are the zero-shear viscosity and the weight average molecular weight of the linear precursor, and a is an adjustable parameter (a ¼ 0.42). MC is the molecular weight between two successive entanglements and is roughly equal to 11.2 kg/mol for PP. To obtain the
value of Bn, ML is calculated from the relationship between Z0 and Mw of the linear PP, as follows: Log Z0 ¼ 15:4 þ 3:5 log Mw :
(3)
It is clear that Eq. 2 can be used only when gBL is higher than gL. For this reason, the branching level of PP1 cannot be calculated by Eq. 2. The values of Bn for the modified PPs were calculated using Eq. 2 and are listed in Table 4. It is evident that the branching number is different for samples modified with different peroxides. The branching level of PP modified with BPO was the highest, followed by CHP and then BCHPC. The results in Table 4 also suggest that the peroxides DCP and DTBP do not produce a large degree of LCB in the PP backbone. A very crude approximation can be made when deriving Eq. 2, assuming that breakage of the linear chain occurs in the middle and this result in branched chains of arm molecular mass.
Melt Flow Behavior The influences of the molecular structure on the MFI of polymers have been studied extensively [27]. In the case of PPs modified with PODIC reported by Lagendijk et al. [1], their melt strengths increased strongly with decreasing MFI. The MFIs of PP0 and the modified PPs from the present study are shown in Fig. 11. The MFI of PP1 was higher than that of PP0, indicating a decrease in the molecular weight, because of the degradation reaction in the absence of a polyfunctional monomer. The MFIs of the modified PPs were lower than that of PP0, a result of LCB in their backbone. The MFI also changed with the type of peroxide because of their different branching efficiencies. The PP modified with BPO had the lowest MFI of all modified samples, indicating it had the highest branching efficiency, as shown in Table 4.
Melt Strength and Velocity at Break FIG. 10. g versus c_ of initial PP and PPs modified with different peroxides at 1908C.
348 POLYMER ENGINEERING AND SCIENCE—-2010
The existence of LCB can enhance the melt strength of the modified PPs in a melt extensional experiment at the same test conditions and, conversely, the branching level DOI 10.1002/pen
FIG. 13. Comparisons of the melt strength (Ms), velocity at break (Br.Vel), and branching number of initial PP and PPs modified with different peroxides. FIG. 11. Melt flow Indexs (MFIs) of initial PP and PPs modified with different peroxides.
can be determined by with the value of the melt strength. The Rheotens curves, i.e., the drawdown force versus draw speed, of PP0 and the modified PPs are shown in Fig. 12. The force and velocity when the melt strand was broken were designated as ‘‘melt strength (Ms)’’ and ‘‘velocity at break (Br.Vel)’’, respectively, at the test condition. As shown in Fig. 12, the melt strength of PP1 was lower than that of PP0; however, the velocity at break greatly increased. The chain scission process is severe for PP1 leading to decrease in its molecular weight, which results in the decrease of viscosity and melt strength. The increase of drawability for PP1 might result from its decreased viscosity. As the polyfunctional monomer of HDDA was added to the reaction mixture, the various peroxides improved the melt strength of the modified PPs. The melt strength of PP can be improved by increasing the molecular mass, by broadening the distribution, or by introducing branches. The most successful way to
improve the melt strength of PP is by the addition of LCB [1]. In our research system, peroxides and a polyfunctional monomer were used to modify PP. Therefore, it can be inferred that the improved melt strength of the modified PPs in the present system was indeed caused by the introduction of LCB structure. The values of melt strength of the modified samples were affected by the level of LCB. PPs modified with BPO, CHP, and BCHPC in the presence of HDDA showed the significant improvement in melt strength in comparison to PP0, which is attributed to a larger number of LCBs in their backbone. PP modified with BPO had the highest melt strength, followed by PP modified with CHP and then BCHPC. The use of DCP and DTBP also resulted in considerable improvement of melt strength, but their efficiencies were much smaller than BPO or CHP. The degree of LCB for PPs modified with DCP and DTBP were moderate, as shown in Fig. 13. Comparisons of the melt strength, the velocity at break, and branching number of the PP0 and modified PPs are shown in Fig. 13. The velocity at break decreased in general as more LCB occurred on the PP backbone. The melt strength was not proportional to the velocity at break for these samples. The velocities at break of the PPs also varied with the type of peroxide, which again was a reflection of the properties and molecular structures created by the different peroxides. The improvements in melt strength in the modified PPs followed the same trend as the branching number that further confirmed that the improvement of melt strength was caused by the introduction of a LCB structure in the PP backbone. Modifications that resulted in more branches per chain also resulted in higher melt strength. Mechanism
FIG. 12. Rheotens curves (drawdown force as a function of the draw speed) of initial PP and PPs modified with different peroxides at 1808C.
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Lagendijk et al. [1] have systemically investigated the extensional flow properties and melt strengths of PPs modified with different PODIC in the absence of monomer and found that a bulkier structure or a higher length POLYMER ENGINEERING AND SCIENCE—-2010 349
FIG. 14. Chemical reaction of the preparation of LCB-PP by reactive extrusion in the presence of peroxide and polyfunctional monomer.
of the alkyl-carbonate group is more effective at improving the melt strength and branching level of modified PPs. However, the type of peroxide used for modification in their report was only restricted to PODIC. In our system, different types of peroxide were used to modify PP in the presence of a polyfunctional monomer of HDDA to produce LCB. The viscoelasticity, the level of LCB, and the melt strength of the modified PPs were affected by the type of peroxide used for modification. The chemical reactions involved in the preparation of LCB-PP by reactive extrusion in the presence of peroxide and monomer are shown in Fig. 14 [28]. The unique action of these peroxides in inducing LCB on a linear PP chain is that they reacted with PP in the extruder and formed PP radicals, as shown in step 1 of Fig. 14. The formation of a PP radical is the foundation for subsequent branching reactions, which will be affected by the structure and property of the peroxide. From the value of half-life time of different peroxides, as shown in Table 1, it can be inferred that the order of the decomposition temperature for investigated peroxides is as follows: BCHPC \ BPO \ DCP \ DTBP \ CHP. For peroxides with lowest decomposition temperature, such as BCHPC or BPO, the chemical reactions occur at lower temperatures in the extruder, compared to the reactions because of peroxides with higher decomposition temperature. Lower temperature is known to be more beneficial to the branching reactions but not to the degradation reaction. As a result, the PP modified with BPO or BCHPC had a large number of LCBs on their backbone. However, the reason why the branching level is also higher for PP modified with CHP is unclear, but may be a consequence of the stability of the radical produced by 350 POLYMER ENGINEERING AND SCIENCE—-2010
this peroxide. The radicals produced by BPO, CHP, or DCP are more stable in comparison with the radicals produced by DTBP and BCHPC, because of the existence of the phenyl group in their radical molecule that can share the radical. A more stable radical will have a longer quench time; therefore, the time of the extrusion reaction between PP and peroxide with a stable radical, such as BPO, CHP, or DCP, will be longer, which is beneficial for the subsequent grafting reaction. As a result, more LCB will occur in a PP modified with BPO or CHP in the presence of HDDA. However, this explanation does not seem to hold for PP modified with DCP, which might be attributed to the higher molecular weight of DCP in comparison to CHP. In comparison to DTBP, however, the grafting efficiency of DCP is better. All of these factors affect the branching efficiency simultaneously and eventually determine the ultimate branching level of the modified samples. In general, the results from the present study indicate that a peroxide with a lower decomposition temperature and a more stable radical after decomposition will be more beneficial to the grafting reaction, leading to a modified sample with a higher branching level and a greater melt strength. CONCLUSION LCB PPs were prepared by modifying linear PP with different types of peroxide in the presence of a polyfunctional monomer of HDDA. FTIR measurements directly confirmed the grafting reaction and differentiated the grafting efficiency of the different peroxides. DSC results showed that the crystallization temperatures of modified DOI 10.1002/pen
PPs were higher than those of initial and degraded PP, which might be attributed to the existence of LCB in the modified PP backbone. A number of rheological characteristics, such as higher G0 at low frequency, increased shear-thinning behavior, plateau in tan d–x plot, G* shifting to smaller values of the phase angle in the Van Gurp plots, and upturning at high viscosity in Cole-Cole plot, were found for the modified PPs, which confirmed the existence of LCB in the modified PPs. The degree of branching could be estimated from the rheological characteristics and the value of the melt strength of these samples. The branching number was calculated and differentiated for various modified samples. It was found to be affected by the type of peroxide used for modification and could be controlled by peroxide properties and structure. Peroxides with lower decomposition temperatures and more stable radicals after decomposition favored the branching reaction and led to the formation of modified PPs with higher branching levels and greater melt strengths. Based on the rheology and melt strength test results for the modified PPs, it was concluded that the branching efficiencies of the investigated peroxides were as follows: BPO [ CHP [ BCHPC [ DCP [ DTBP. The measured value of branching number agreed well with the melt viscoelasticity and the improvement of melt strength of modified PPs with different peroxides.
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