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The rheological modification of talc-filled polypropylene by epoxy-polyester hybrid resin and its effect on morphology, crystallinity, and mechanical properties.

INTRODUCTION

Polypropylene (PP) is an attractive candidate for many engineering applications because of its excellent chemical resistance, acceptable range of tensile strength and modulus, good impact strength and processability, and low price (1).

Mineral fillers are widely used in PP resins (2). The structural features of fillers such as particle shape, particle size, aspect ratio, and particle size distribution greatly influence the properties of composites. The finer particle size has greater tendency to agglomeration. Agglomerates can be regarded as defects that reduce the impact strength of composites (3), (4) and increase the viscosity of the melts. In is generally the intention to use highest filler fraction with lowest viscosity as dictated by the processing requirements.

Talc is the most frequently used plate-like mineral filler for stiffening of PP resins, increasing dimensional stability, and reducing the production cost (5), (6); however, it shows adverse effects on the ductility, strength, substantial increase in melt viscosity, and deteriorate the processability (7-9).

There are various types of additives that may be used to improve certain properties of composites. The selection of the type of additives for a certain composite depends on the application, the required properties, and the processing method. In producing the complex parts by injection molding process, it is necessary to increase the melt flow of highly filled PP. Lubricating agents are normally used to improve the melt flowability of filled plastics. Lubricants are low molecular weight chemicals, which facilitate the melt flow by migration to outer layer of melt and reduce the friction coefficient with flow channel wall. The migrated lubricant to the surface of parts has diverse effect on the processes such as adhesive bonding, welding, and painting.

In this study, the epoxy resin cured by polyester is used to improve the melt flowability of PP-talc composites. Epoxy resins are polar materials with weak adhesion strength to PP. They inherently interact with the mineral filler particles surface and hence affect the flowability and also thermal stability of the composite by deactivating the metallic impurities on filler surface.

Epoxy resins have been used as modifier with various types of polar (4), (10), (11) and nonpolar polymers (12), (13). The combinations of PP with thermosetting polymers such as epoxy and polyester resins were the subject of some researches (12-14). The reactive melt modification of PP with polyester resin cross-linked by peroxide was studied by Wan and Patel (12). Jiang et al. (13) investigated the mechanical properties, crystallization, and rheological properties of dynamically cured PP/epoxy blends.

Up to now, there are no available released reports or publications considering the using a thermosetting epoxy resin in the talc-filled PP. In this work, the composites of PP with talc, in the presence of the epoxy-polyester hybrid resin, were prepared through reactive mixing in a corotating twin screw extruder. We looked at the need for high stiffness, and therefore high filler fraction with improved melt flowability and improved rheological behavior as dictated by the processing requirements was employed.

Because the melt flow behavior of composites is strongly affected by the presence of mineral filler, the rheological studies can assist in the development of formulations designed to facilitate industrial processability. It is found, in a quite novel way, that the use of epoxypolyester hybrid resin allows to the dispersion of large quantities of talc mineral fillers into PP and increase fluidity of melt at higher shear. The morphology of composites and mechanical performance of formulations were evaluated at different amount of epoxy resin.

EXPERIMENTAL

Raw Materials

The materials used in this study are surveyed in Table 1. PP was a standard injection grade homopolymer, with MFR of about 15 g/10 min (ASTM D 1238 @ 230 [degrees] C/2.16 kg). The talc used in the experiments was highly lamellar with mean particle size of 10 [micro]m and specific surface area of 3.2 [m.sup.2]/g. The maleic anhydride-grafted polypropylene (MPP) with trade name of Fusabond [R] P M613 with MFR (ASTM D 1238 @ 190 [degrees] C/2.16 kg) of 120 g/10 min was used as adhesion promoter. The epoxy resin was a solid resin with equivalent weight of 750 and curing time of 6 min at 190 [degrees] C for 50/50 epoxy/polyester resins ratio. As a processing stabilizer, 0.2 parts of Irganox 1010 per one hundred part of compound was added to all formulations.
TABLE 1. Material used in the study.

Materials        Grade and description    Company

PP-homopolymer       Ceetec-J801 R        Hyundai
Tale                 1445                 Luzenac
Coupling agent       Fusabond (R) P M613  Dupont
Antioxidant          Irganox 1010         Ciba
Epoxy resin          Kukdo YD013          Kukdo
Polyester resin      Uralac 5142          DSM


Sample Preparation

The compounding of the composites was performed on a Coperion Werner Pfleiderer, ZSK25-WLE twin screw extruder. It is a corotating twin screw extruder, with 25 mm, L/D = 40, modular screw. The twin screw extruder was equipped with a side feeder, which was used to intrude the talc and epoxy resin in the barrel. The ingredients of each formula were weighted and fed to the compound by four Brabender gravimetric dosing units. The antioxidant and PP pellets, according to recipes of Table 2, fed into main hopper and the talc was fed to side feeder. As the amount of epoxy-polyester hybrid resin was different in various formulations, one feeder was allocated to dose this resin. The weight mix ratio of epoxy/polyester resins was 1/1. The epoxy and polyester resins were premixed first, and then the particles were passed through a 20-[mu]m sieve used in the formulations. To study the effect of epoxy-polyester hybrid resin, 2.5, 5, and 10 wt% of resin were added to PP + talc composites with 20, 30, and 40% of filler content. To improve the interfacial interaction of PP + talc and epoxy resin, the MPP were used in an amount of 1.5 wt% to the formulations of PP + 30 wt% talc and PP + 30 wt% talc + 2.5 wt% resin. The epoxy resin curing reaction with the polyester resin release water (15) as shown in Fig. 1. The moisture of talc powder and the water byproduct, generated due to resin curing reaction, were removed efficiently by two free devolatalizing ports followed by one vacuum degassing port. The mixing operation was carried out at 750 rpm screw speed, with approximate residence time of about 4.5 min of the material in the barrel. The temperature profile on the barrel was 170, 170, 180, 185, 190, 200, 195, 195, 195, 190, and 190[degrees]C from the hopper to the die head. The melt temperature was about 215[degrees]C on screw tip.
TABLE 2. The recipes of composites used in this study.

Material                    PP (wt%)  Talc (wt%)  Epoxy resin (wt%)

PP + 20% Talc                  80         20              0
PP + 20% talc + 2.5% resin     77.5       20              2.5
PP + 20% talc + 5% resin       75         20              5
PP + 20% talc + 10% resin      70         20             10
PP + 30% talc                  70         30              0
PP + 30% talc + 2.5% resin     67.5       30              2.5
PP + 30% talc + 5% resin       65         30              5
PP + 30% talc + 10% resin      60         30             10
PP + 40% talc                  60         40              0
PP + 40% talc + 2.5% resin     57.5       40              2.5
PP + 40% tlac + 5% resin       55         40              5
PP + 40% talc + 10% resin      50         40             10

0.2 part of Irganox 1010. per 100 parts of materials was added to all
formulations.


[FIGURE 1 OMITTED]

Measurement of the Properties

Test specimens for tensile and impact tests were prepared by screw injection molding machine, at the temperature of 185-210[degrees]C, from hoper to nozzle. The tensile tests were carried out on MTS10/M universal testing machine, on five sample bars (type I) at 50 mm/min according to ASTM D 638 test method. The impact strength of the blends was measured according to ASTM D 256A, by a Zwick/Roell B5102 pendulum impact tester, on five samples.

The melt flow index of the composites was determined by a Zwick melt-flow analyzer, according to ASTM D 1238 on the pre dried granules. To evaluate the shear rheological behavior of the samples, two loads of 2.16 and 10 kg at 230[degrees]C were used in the measurement.

The injection molded samples were fractured in the liquid nitrogen. The morphology of fractured and gold-coated surfaces of the samples were studied by Cambridge 360, SEM (scanning electron microscopy), using a voltage of 15 kV.

The thermal analysis of samples were performed by DSC Q1000 of TA with the samples of about 5 mg, in aluminum pans under nitrogen atmosphere. To remove the thermal history, all samples were heated at 50[degrees]C/min, from room temperature to 250[degrees]C. To study the crystallization behavior, the samples were cooled down to room temperature by a rate of 5[degrees]C/min after holding 5 min in 250[degrees]C. The samples were reheated to 250[degrees]C by a heating rate of 10[degrees]C/min, and then their melting behavior was evaluated.

RESULTS AND DISCUSSION

Rheology of Composites

The rheological behavior of composites was studies in the linear viscoelastic range of deformation in this section. The complex viscosity [eta]* and storage modulus G' of PP and composites containing 20, 30, and 40 wt% of talc are shown in Fig. 2.

[FIGURE 2 OMITTED]

The complex viscosity curve of pure PP and PP-talc composites without epoxy resin against frequency have shown a Newtonian plateau ([[eta].sub.0]) at very low range of shear rates (Fig. 2a). The zero shear viscosity [[eta].sub.0] in dynamic measurement can be represented by (16):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

By increasing the talc content, the viscosity of composites were increased distinctly at low range of shear rates. The nonlinear change of viscosity on filler content implies the significant rise in interparticle interaction (Fig. 2b). By increasing the talc content to 40 wt%, the complex viscosity [eta]* at very low shear rates was sharply increased, which may be due to the presence of agglomerates in the melt. The agglomerates confine part of matrix material in their interparticle voids and decrease the volume fraction of the matrix around and increase the viscosity. The filler reduce the mobility of macromolecules in the interphases and this increases the melt stiffness.

At low and medium range of frequencies, the complex viscosity showed a sharp increase in the vicinity of 30 wt% of talc in the composite. The general behaviors of viscosity-frequency curve for the composites without resin are similar to pure PP resin with a plateau region at very low range of shear rates. Addition of resin to the composites showed a pronounced effect on the rheological behavior of PP-talc system, the Newtonian plateaus disappear and exhibit yield stress. The complex viscosity and storage modulus of composites at various concentrations of talc and resin are shown in Fig. 3.

[FIGURE 3 OMITTED]

The existence of yield stress, which must be exceeded for flow to occur, has been indicated as a common characteristic of highly filled polymer melts, associated with interaction between the filler particles (17). The origin of yield stress in highly filled polymer dispersions is reviewed by Malkin (18). The influence of filler type, size, concentration, and geometry on shear yielding in highly loaded polymer melt have considered in several studies (19-22), The observed yield stress in the viscosity behavior of melt revealed the presence of particle-particle interaction in the system that is directly dependent on the size of cured resin particles.

Tanaka and White (23), according to cell model theory (24), showed that for a concentrated suspension of interacting sphere in the matrix with power law type behavior, the viscosity of system directly depends on existence of interparticle interaction energy and hydrodynamic viscose dissipation. It is showed in some research works that (23), (25), (26) when particle size decreases, the yield stress increases at higher filler content.

It is believed that there are not considerable interactions between PP resin and the cured epoxy resin particles. The effect of the solid particles of cured epoxy resin on theological parameters may be attributed to their hydrodynamic effect and also talc-resin interaction.

To evaluate the resin-talc interactions, the viscosity of melts at a reference frequency of 10 ([s.sup.-1]) were selected and are shown in Fig. 3d. It is found that small amount of resin has a significant effect on viscosity of composites. Using 2.5 wt% of resin with PP + 40 wt% talc composite increased the complex viscosity [eta]* from 3800 to 7100 (Pa s).

The comparison of complex viscosity of PP + 40 wt% talc (3800 Pa s) and PP + 30% talc +10% resin (4100 Pa s) in Fig. 3d revealed the stronger effect of cured resin particles on increasing the viscosity. This may be observed due to hydrodynamic viscose dissipation effect of fillers.

The storage modulus G' of composites containing epoxy resin were increased in all range of frequencies. The storage modulus showed greater independency to frequency at lower range of shear rates. A minor amount of resin showed a significant effect on storage modulus. This was more pronounced at higher loading level of talc (40 wt%), which depicts the more solid-like behavior with high elastic energy of composites treated by resin. The loss modulus and damping factor (tan [delta]) of composites at various amount of resin are shown in Fig. 4.

[FIGURE 4 OMITTED]

Reactive mixing of resin with PP-talc composite increased the elasticity of melt, and tan [delta] showed a sharp decrease especially at low and medium rang of frequencies. The tan [delta] of PP-talc composites without epoxy resin decreased constantly by increasing frequency and cross the reference line of tan [delta] = 1 at cross over points of 92, 60, 24 ([s.sup.-1]), respectively. Incorporation small amount of 2.5 wt% epoxy resin decreased the tan [delta] by a vertical shift to lower values at crossover points of 38 and 26 ([s.sup.-1]) for 20 and 30% talc-PP composites. For 40 and wt% talc, the storage modulus is ever higher than loss modulus and no crossover point were observed.

By incorporating the epoxy resin, the tan [delta] showed a sharp decrease with a minimal variation in wide range of frequencies. This type of behavior is the evidence of predominance of elastic deformation nature of melts in wide range of frequencies.

The interaction of talc particles with resin for PP + 40 wt% talc composite can be seen in Cole-Cole plot (27), (28) of Fig. 5. The change in overall behavior of G'-G" plot of composites treated by resins is the evidence of interparticle interaction and formation of network structure in the PP matrix.

[FIGURE 5 OMITTED]

Modeling of Rheological Behavior

The experimental data of complex viscosity against frequency showed linear Power-Law-like behavior and can be approximated with a line with different slopes in low and high frequency ranges. The following equation is proposed for this type of behavior (14) and fit properly on the experimental data:

[eta]* = [K[[omega].sup.n']/(1 + [[omega].sup.2])[n'-n"/2]] (2)

where K is the [eta]* at [omega] = 1 (rad/sec); and at n' = n", n' is the slope of [eta]*--[omega] plot in the regions of 0 < [omega] < 1, and n" is slope of [eta]*--[omega] in the regions of 1 < [omega] < 100.

The model parameters of K, n', and n" for the composites are shown in Table 3.
TABLE 3. Model parameters for the PP + 40 wt% talc composites.

                    Model parameters

Resin content      K       n'      n"

2.5%            29,000   -0.581  -0.617
5%              40,000   -0.712  -0.641
10%            140,000   -0.818  -0.746


The solid lines in Fig. 3c are model predicted trends, according to parameters given in Table 3. It is seen that Eq. 2 can predict properly the complex viscosity behavior of PP-talc composites mixed with various amount of epoxy resin in wide range of frequencies by relevant viscose parameters.

Effect of Filler Volume Fraction on Suspension Viscosity

The measurement of suspension viscosity can be used to characterize the microstructure state of dispersion. The Einstein relationship is well known in the study of dilute suspension of solid spherical particles in Newtonian fluid. Generally, when [phi] > 0.1 ([phi] = volume fraction of filler), the suspensions are considered to be concentrated and unlike the dilute suspensions, the size of the filler drastically change the viscosity behavior of concentrated suspensions.

At very high concentration, Mooney proposed an Arrhenius type equation for the suspension's viscosity. The Maron-Pierce type equation as an empirical expression has been evaluated to fit the entire range of volume fraction by Kataoka et al. (29), Kitano et al. (30), and used by Poslinski et al. (31), (32). This equation is as below:

[[eta].sub.r] = [[eta].sub.c]/[[eta].sub.p] = [1/[(1 - [empty set]/[[empty set].sub.m]).sup.2]] (3)

where [[eta].sub.r], [[eta].sub.c], and [[eta].sub.p] are relative viscosity, the viscosity of composite, and polymer melt viscosity, respectively, [phi] and [[phi].sub.m] are the volume fraction and maximum volume fraction of filler.

When the suspending medium is non-Newtonian, its viscosity change by shear rate and the concentration dependency of relative viscosity could be correlated by modified form of equations and using power law form of equation for [[eta].sub.p]. It is indicated in previous section that the melt behavior of PP and also the composites at higher shear rates can be described by the power law model. In the modified form of Eq. 3, the non-Newtonian behavior of viscosity of suspending medium is considered in the calculations.

[[eta].sub.c] = [1/[(p - [empty set]/[[empty set].sub.m]).sup.2]] x [[bar.k][[gamma].sup.n - 1] (4)

where [bar.k], n are the Power-Law constants of suspending medium and [gamma] is shear rate. The measured viscosities were fitted using Eq. 4 and are shown in Fig. 6. The [[phi].sub.m] and p values were determined and are surveyed in Table 4. The nonlinear dependency of [eta] on [phi] on [phi] implies that the interactions give rise to a significant effect on viscosity.
TABLE 4. [[PHI].sub.m] and P-values calculated according to Eq. 4.

Material                [[PHI].sub.m]   P

PP + talc                   0.46       0.8
PP + talc + 2.5% resin      0.6        0.6


[FIGURE 6 OMITTED]

It is found that the value of [[phi].sub.m] for the composites mixed with 2.5% resin is bigger than PP-talc composites without resin, which can be due to the effect of epoxy resin on the talc-PP interaction and filler particle arrangement.

p is an adjustable parameter, which its value changes by composition and shear rate. The value of [[phi].sub.m] is a measure of the packing of filler in polymer matrix and of the thickness of immobilized polymer adsorbed on the filler surface (33).

Effect of Filler and Resin on Crystallization and Melting

The nonisothermal crystallization curves of composites are shown in Fig. 7. The crystallization of the matrix affects significantly on mechanical properties of composites (34), (35).

[FIGURE 7 OMITTED]

The crystallization and melting data of composites are summarized in Table 5. The degree of crystallinity of PP has been determined by:
TABLE 5. The DSC crystallization and melting data of composites.

Material     [T.sub.m]     [T.sub.onset]     [T.sub.p]     [DELTA]T]
           ([degrees]C)     ([degrees]C)   ([degrees]C)  ([degrees]C)

PP + 30%       166.5            137           129.5           7.5
talc

PP + 30%       164.4            130           123.8           6.2
talc +
2.5%
resin

PP + 30%       163.5            129.5         123.5           6
talc +
5.0%
resin

PP + 30%       161              130           123.8           6.2
talc + 10
% resin

Material                    [DELTA][H.sub.c] (Cal/g)     %

PP + 30% talc                          88              37

PP + 30% talc + 2.5% resin             96              41.3

PP + 30% talc + 5.0% resin             98.2            42.7

PP + 30% talc + 10 % resin             96              45.6

[T.sub.m], melting temperature; [T.sub.onset], onset temperature of
crystallization; [T.sub.p], crystallization peak temperature;
[DELTA]T], [T.sub.onset]-[T.sub.p]; [DELTA][H] [T sub.c], heat of
crystallization; D.C., degree of crystallization.


D.C. (%) = [DELTA][H.sub.m]/[DELTA][H.sub.T] x 100/[chi] (5)

where D.C. is degree of crystallinity, [DELTA][H.sub.m] is the specific heat of melting, [DELTA][H.sub.T] is the total heat of melting of 100% crystalline isotactic PP, and x is weight fraction of PP in the composites. The [DELTA][H.sub.T] is taken 40.5 cal/g in the calculations (36). The DSC study of pure PP used in this study showed a crystallization temperature of about 113.5[degrees]C with about 40% crystallinity. Talc as a strong nucleating agent increased the crystallization temperature to 137[degrees]C and the degree of crystallinity decreased to 37%. The incorporation of epoxy resin increased the degree and also rate of crystallinity. The range of temperature that crystallization occurred ([DELTA]T) can be used as a measure of crystallization rate. These results revealed that the crystallinity and crystallization rate increased by using epoxy resin in the formulations. The increase in the heat of crystallization may be attributed to the facilitated molecular movement of PP due to the presence of epoxy resin, and sliding in to the lamellas. As it can be seen in Fig. 7, the crystallization curve of composites modified by epoxy resin is located between the curves of PP and PP + 30% talc composite. The epoxy resin interacted with talc powder during the reactive mixing process and changed partially their surface chemistry. This is the epoxy resin particles that play the main role in nucleation of PP.

Using epoxy resin leads to decrease in the melting temperature of composites (Table 5), and the melting temperature [T.sub.m] of composites decreased by increasing resin content. The decrease in [T.sub.m] is the evidence of decrease in crystal size (37), The decrease in crystal size and increase in heat of melting (higher D.C.) can be related to higher crystallization rate (38).

Melt Flowability

Flow behavior of thermoplastic polymer melts is essential for all forms of production and processing methods. It directly depends on viscosity of melt that can be measured by various methods (39). For industrial applications, MFR is widely use as a measure of melt fluidity, while it is usually measure in a certain range of low shear rate. The measurement of MFR by higher applied load than standard provides another data on the flow curve of polymer and will interpret the flowability of melt.

The MFR of the materials at low and high load of 2.16 and 10 kg are shown in Table 6. At 2.16 kg load, the fluidity of melt decreased significantly by increasing the talc and epoxy resin in the composite. There was negligible flow for the composite containing 40% talc and 10% epoxy resin in MFR test and no data were obtained. At high load of 10 kg, the composites treated by epoxy resin showed a substantial increase in MFR.
TABLE 6. The melt flow rate of composite at 230[degrees]C and two loads.

Material                     [MFR.sub.230     [MFR.sub.230
                           [degrees]C, 2.16  [degrees]C, 10
                            kg] (g/10 min)   kg] (g/10 min)

PP + 20%talc                      13               174
PP + 30%talc                      12               161
PP + 40%talc                     10.7              153
PP + 20%talc 4- 2.5%resin         9.8              180
PP + 30%talc + 2.5% resin         7.9              189
PP + 40%talc + 2.5% resin         6.1              208
PP + 20%talc + 5% resin           6.5              184
PP + 30%talc + 5% resin           3.3              196
PP + 40%talc + 5% resin           0.7              217
PP + 20%talc + 10% resin          2.6              232
PP + 30%talc + 10% resin          0.6              240
PP + 40%talc + 10% resin          --               260


It has been reported that for compositions comprising a linear polymer and a certain quantity of a filler, one observes a virtually jump-like increase of the volumetric flow rate within a very narrow range of shear stresses or the so-called flow separation effect (40).

The epoxy resin showed considerable promise as viscosity modifier for highly filled PP-talc composites and even thin-walled complex parts can be easily and completely filled in injection molding process. At low shear rate, the interparticle network between the filler particles strongly increase the viscosity. It is believed that at high shear rates the applied shear energy could breakdown the interparticle network structure, which is weakened due to talc new surface chemistry. The weak interfacial interaction of PP-epoxy resin and new surface chemistry of talc progressively reduce the friction force between PP melt and filler particles and affect the orientation of filler platelets in the direction of flow (41).

Morphology of the Composites

The SEM images of fractured surfaces of the injection molded samples are shown is Figs. 8 and 9. The SEM images were prepared from the cross-sectional areas close to outer surface of samples, because the talc usually aligns itself in the direction of flow in the middle parts of cross section and keeps the random form in the areas close to surfaces (8). The fracture surface of PP-talc composite without epoxy resin is shown Fig. 8a. As it can be seen in this figure, the talc particles are randomly distributed along the cross section close to outer surface and no flow-induced orientation was observed. Adding 2.5 wt% of resin to the formulation changed the rheology of melt, and the talc platelets were aligned in the direction of flow all over the cross section (Fig. 8b). These images confirm the observation in melt flow analysis of melt and jump-like increase of the volumetric flow rate in MFR test with high load. Most of particles in the fractured surface have platelet-like structure oriented in flow direction with various particle sizes. The sizes of big particles are close to mean particle size of talc, which is about 10 [mu]m. There are many other smaller, nanosize, particles with average diameter of about 200 nm, visible in the SEM micrographs of Fig. 9, which are cured epoxy resin particles. Jiang et al. (13), in their study on dynamically cured epoxy-PP blends, reported that the epoxy resin in 70/30 PP/epoxy blend dispersed as spherical particles with 3-4 [micro]m diameter in the PP matrix. The observed spherical shape particles may be because of lower viscosity/equivalent weight of resin that has been used in their study. The shape and size of the cured epoxy resin particles are clear in Fig. 9.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The resin softens and disperses in PP matrix because of applied heat and shear rate in the extruder. The dispersed epoxy resin in PP matrix changed from viscous material to solid particles, through a chemorheological curing process. The resin may react and adhere partially to talc particle surfaces and leads to a good dispersion of filler in PP matrix. The resulting PP compounds are homogeneous and very flowable at high range of shear rate. It is believed that the resin was interacted with talc particle surfaces and changed its surface chemistry. The resin filler interaction and new surface chemistry of talc can be the reason of changing the crystallization behavior and exhibiting an increased MFR at high shear rates. The SEM study revealed that using the resin with PP-talc system improved the dispersion of talc in PP and leads to orientation of filler platelets in the direction of flow. The variation in the morphology may be because of influence of talc on rheology of epoxy-talc mixture. The distinct and clear boundaries of dispersed particles and PP matrix are the evidence of weak interfacial interaction, which leads to lower mechanical properties of composite without using any compatibilizers.
(a)

            PP+20%  PP+30%  PP+40%

No Resin     36.4    34.6    33.5
2.5% Resin   31.6    30.5    29.5
5%           29.7    29.0    27.3
10% Resin    24.8    24.4    21.5

(b)

No Resin     3540    3950    4850
2.5% Resin   3200    3800    4800
5% Resin     3200    3920    4900
10% Resin    3100    4420    5000

(c)

No Resin     50      43.5   30
2.5% Resin   50      38      3.0
5% Resin     38.5    32.5   26
10% Resin    33      29     23.5

          30% Talc  30 Talc1-1  30% Talc+2.5%  30% Talc+2.5%
                      5% MPI        Resin      Resin+15% MPI

Tensile       34.5       37.8         30.5           38.5
Strength
(MPa)

Elastic     3950       3900         3850           3920
Modulus
(MPa)

Impact        43         58           38             56
Strength
(j/m)

Note: Table made from bar graph.


[FIGURE 10 OMITTED]

Mechanical Properties

The incorporation of filler having a weak interaction with matrix show diverse effect on tensile and impact strength of the composites. Talc is a magnesium silicate hydroxide mineral and does not anticipate interacting with nonpolar PP resin. The epoxy-polyester hybrid resin is a polar material with an inherent tendency to adhere to talc particles, while having poor interaction with PP. The results for tensile strength, elastic modulus, and impact strength of composites are shown in Fig. 10a-c.

Addition of 2.5 wt% resin to PP + 20% talc composite leads to 13.5% decrease in tensile strength, while the tensile strength of PP + 40% talc (33.5 MPa) is only 8% lower than PP + 20% talc composite (36.4 MPa). This distinct effect on tensile strength should not be because of filling effect of the cured resin particles in PP matrix. It is believed that the resin act as stress concentrators and promoted the failure of composites. In multiphase-filled polymer compositions, which may contain mixed filler types or proportions of filler (42) and a fiber or secondary modifying polymer, the spatial distribution of the phases has a direct bearing on the properties of the composite (43), (44). The epoxy resin may adhere to talc particles and be present as discrete droplets within the thermoplastic matrix or coexist in both structural forms.

It has been shown that the spatial location of the secondary modifying polymer can have a profound effect on mechanical properties (430) and may be influenced by the relative chemical affinity of modifying polymer and plastic matrix toward the filler, and also the shear imposed during mixing (17). The aforementioned results revealed different mechanism in stiffening of PP composite in the presence of resin. It is believed that some part of talc can be mixed with epoxy resin in the early stages of reactive mixing in the extruder and two interface formed. One of the interfaces is related to talc with PP, and the other one is connected to cured epoxy resin containing talc with PP. In the end stages of the mixing operation in long twin screw extruder, the resin mixed partially with talc particles dynamically cures and changes to a thermoset. filled resin and thus develops a new solid phase in the composites.

The effect of MPP on mechanical properties of composites were studied and are shown in Fig. 10d. It is found that MPP improved the interfacial interaction of talc and resin with PP in PP + 30% talc and PP + 30% talc + 2.5% resin and increased the tensile strength and elastic modulus. The impact strength of composites has not shown noticeable change in the presence of MPP.

CONCLUSIONS

Reactive mixing of epoxy-polyester hybrid resin with PP and talc were carried out on a twin screw extruder. The incorporated epoxy-polyester hybrid resin was cured and changed to solid particles in the materials. Addition of even 2.5 wt% of resin to the composites showed a pronounced effect on the rheological behavior of PP-talc system, the Newtonian plateaus disappeared and exhibited yield stress. A new form of rheological model was used to model the rheological behavior. This model predicts efficiently the complex viscosity of composites with a good correlation with experiment results.

By incorporating the epoxy resin, the tan [delta] showed a sharp decrease with a minimal variation in wide range of frequencies. This type of behavior is the evidence of predominance of elastic deformation nature of melts in wide range of frequencies.

The dynamically cured resin showed a significant effect on flowability of melt at higher range of shear rate, which is attributed to resin-talc interaction and reducing the friction force. The talc particles align parallel to the mold surface all over the cross section from molds surface to center part of molded object. The epoxy resin in the PP-talc composites changed the talc surface chemistry and due to improved mobility of the PP molecules, the rate and also the degree of crystallinity increased. The crystallization study of 30 wt% talc-filled composite revealed that the hybrid resin system increased the degree of crystallinity to about 56.5%.

The cured epoxy resin particles were played the role of stress concentrators and decreased tensile strength, elastic modulus, and impact strength. It is found that MPP improved the interfacial interaction of talc and resin with PP and increased the tensile strength and elastic modulus. The optimum percentage of epoxy resin in the PP-talc composite system, considering mechanical properties and rheological modification, is about 2.5 wt%.

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Yousef Jahani, Morteza Ehsani

Iran Polymer and petrochemical Institute, Faculty of Polymer Processing, Tehran, Iran

Correspondence to: Y. Jahani: e-mail: y.jahani@ippi,ac.ir

Contract grant sponsor: Iran National Science Foundation: contract grant number: 85026/02.

DOI 10. 1002/pen.21294

Published online in Wiley InterScience (www.interscience.wiley.com).

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