Introduction

Polypropylene (PP) is a high performance thermoplastic material. To become cost-effective, its toughness and Young’s modulus need to be improved, along with other aspects like barrier properties and thermal resistance when loaded with nanoclay or other nanofiller. Consequently, its application can be expanded in the automotive and packaging industries, as a replacement for engineering plastics with higher added value [1]. However, due to its hydrophobic nature, polypropylene is mostly incompatible with nanofillers. A remedy discussed in the literature for this problem is the use of compatibilizing agents to enhance adhesion between nanofiller and the polymer matrix, especially by introducing polypropylene modified with maleic anhydride, which is widely used in melt intercalation for the preparation of PP nanocomposites. Another strategy employed, together or not with the first one, is the functionalization of nanoclays through the introduction of ammonium salt with long carbon chains on the surface of nanofillers. Unfortunately, those strategies are very often not successful [2].

Another approach being used to prepare nanocomposites of polymer/clay is in situ polymerization [3–5]. In this technique, the polymer is synthesized in the presence of clay to allow intercalation or even exfoliation. This technique is employed to prepare several polymers with liquid monomers, using a number of different clays.

A very interesting type of naturally occurring clay is halloysite nanotubes (HNTs), Al2Si2O5(OH)4·nH2O (n = 0 or 2), which are similar in chemical composition to kaolinite, with unique characteristics, abundance and commensurate low cost. They have high aspect ratios with nanoscale diameters and long lengths (typically these hollow cylinders have outer diameter around 30–50 nm and length of 100–2000 nm). Among several applications is the fabrication of polymer nanocomposites, where the clay has remarkable reinforcing effects. The resulting composite also has enhanced flame retardancy and high dimensional stability, making it excellent for structural and functional applications [6, 7].

Due to its hydrophilic nature, surface modification of this clay is important. Examples of surface treatment on HNTs are grafting maleated polypropylene (PP-MA) on the nanotubes; and adding organic substances with hydrogen-bonding functionalities to form a network structures with the hydroxyls and oxygen atoms from Si–O–Si on the HNT surfaces. Those processes facilitate the dispersion of HNTs in the matrix, and more importantly, transfer the stress from the matrix to the filler, improving mechanical performance of the composites as well as transparency, which is attributed to the excellent dispersion of the nanotubular clay.

HNTs can also be added to polymers to produce low-density nanocomposites due to their hollow structures, replacing large quantities of macro or microcompounds. Flexural, tensile strength and impact strength can be noticeably improved with the incorporation of highly dispersed HNTs. This effect is due to the rod-like morphology and high aspect ratio of HNTs.

It is also potentially advantageous to employ these HNTs because of the results from using other synthetic nanotubular solids such as carbon nanotubes (CNTs). However, it is well known that the preparation of nanoparticles is relatively complex and costly. In addition, for naturally layered clay minerals, even the generally required organic treatment is not sufficient to generate well-intercalated and/or exfoliated morphologies in polymer matrices. As for the CNTs, their relatively higher price and tendency to pigment polymers will restrict their application as fillers. As a consequence, easily available and effective flame retardant nanofillers like halloysite are highly desired.

Using halloysite, series of halloysite nanotubes/polystyrene (HNTs/PS) nanocomposites were successfully prepared by the in situ bulk polymerization of styrene with the organo-HNTs as macromonomers. The thermogravimetric results also showed that the thermal stabilities of the HNTs/PS nanocomposites prepared via bulk polymerization were better than pure polystyrene. Moreover, the maximum thermal degradation temperature of the nanocomposites increased with the rise of the amount of HNT fillers added [8].

High-impact polystyrene (PS) nanocomposites filled with individually dispersed halloysite nanotubes (HNTs) were also prepared by emulsion polymerization of styrene in the presence of HNTs with sodium dodecyl sulfate (SDS) as the emulsifier. The emulsion polymerization resulted in the formation of polystyrene nanospheres separating individual HNTs. The HNTs were uniformly dispersed in the PS matrix. The impact strength of the PS/HNT nanocomposites was much higher than that of the neat PS [9].

Other polymers, like polyaniline, were coated with halloysite nanotubes (PANI/HNTs) by in situ emulsion polymerization of anilinium chloride adsorbed halloysite nanotubes (HNTs), obtained by the dispersion of HNTs in an acidic aqueous solution of aniline, using ammonium persulfate (APS) as oxidant. The morphological analyses showed that the polyaniline coated halloysite nanotubes via the in situ chemical oxidation polymerization with ultrasonic irradiation had better defined structures. The conductivities of the PANI/HNT hybrids increased with rising amounts of HCl dopant added in the emulsion polymerization [10].

On the other hand, in the synthesis of polyolefin/clay nanocomposites by the in situ polymerization technique, the polymerization catalyst has to be fixed on the surface of the clay, so that the polymer chains grow from its surface.

Besides this, it must be considered that the use of filler with some chemical modifier can result in poisoning of the catalysts which reduces or decrease their polymerization performance [11].

As regards the method of in situ polymerization, it was reported that the propylene polymerization using metallocene catalysts and post-metallocene embedded between layers of silicate caused exfoliation. Comparison of silicate dispersions showed that in situ polymerization was more effective in forming nanocomposites than the method of melt intercalation. More recently, the use has been reported of this technique for conventional Ziegler–Natta catalysis, or with catalysts based on titanium and aluminum alkyl as cocatalyst [12].

The key point is that the active catalyst must be anchored in the nanofiller. Generally, the surface of fillers contains few hydroxyl groups that can react with active catalysts. Therefore, it is possible to anchor the catalyst MgCl2/TiCl4 on the nanoclay surface [13].

Few reports in the literature discuss the production of PP nanocomposite/clay by in situ polymerization using Ziegler–Natta catalysis. It is more usual to find works related to this topic for the synthesis of polyethylene. One problem encountered when this technique is applied to PP is the difficulty in controlling the isotacticity of the polymer, which plays a very important role in the crystallinity, and as a consequence, in the mechanical strength of PP. This is probably the reason for the scarcity of literature on this topic [14].

In this study, halloysite was used in its original form and also modified with vinyltrimethoxysilane (VTMS) or silicon chloride (SiCl4) for the synthesis of Ziegler–Natta catalysts using both magnesium chloride and halloysite as supports. The polypropylenes synthesized with these catalysts were analyzed in terms of thermal and dynamic mechanical properties.

Experimental section

All the procedures involving the catalyst system and polymerization were performed under nitrogen atmosphere, with the use of the standard Schlenk and glove bag techniques.

Materials

Ultrafine halloysite H (kindly provided by Imerys Tableware, New Zealand) was dried for 24 h in a vacuum oven, and then cooled under nitrogen flow. The original clay is called here HO (original halloysite). Chemically modified halloysites (HV and HS, respectively, reacted with VTMS or SiCl4) were used in the synthesis of bisupported Ziegler–Natta catalysts. The polypropylenes obtained with halloysite were submitted to thermogravimetric and dynamic mechanical analyses and their properties were compared with those of PP obtained without clay, using a standard catalyst prepared with an MgCl2/TiCl4 system. The chemical modifications of halloysite and the catalytic synthesis are described below.

Halloysite modification with silane

A suspension of halloysite in xylene with drops of water (supplied by Vetec Fine Chemicals Ltd.) was kept in a closed system at 80 °C for 24 h, then 20 mmol of VTMS (Sigma-Aldrich, USA) per gram of clay and 0.5% by mass of dibutyltin dilaurate catalyst (Miracema-Nuodex Chemical Industry Ltd.) were added. The system was kept under stirring without cover to enable release of methanol. The product was washed with xylene and then with ethanol (Vetec Fine Chemicals Ltd.), and dried at 120 °C.

Modification with SiCl4

SiCl4 (Sigma-Aldrich, USA) was added to halloysite and kept under stirring for 2 h. Then the material was washed with hexane (Vetec Fine Chemicals Ltd.) and dried at 120 °C to constant mass.

Preparation of Ziegler–Natta catalysts

Under nitrogen, magnesium ethoxide (Degussa, Germany) was complexed with carbon dioxide in anhydrous ethanol at low temperature (−60 °C) for 3 h. In the case of bisupported catalysts, halloysite was suspended for 24 h in anhydrous ethanol. After 8 h of contact, dried isoparaffin (Unipar commercial S/A) was introduced into the system and the temperature was raised to 100 °C to remove the excess alcohol. Then, the system was brought to chlorination temperature and titanium chloride (TiCl4, Sigma-Aldrich, USA) was added under stirring. The system was maintained for 3 h to complete chlorination, forming MgCl2.

After precipitation of the desired activated magnesium chloride as catalyst support, the supernatant was removed and the support was washed with dry hexane. On the support, n-butyl phthalate (Elekeiroz SA) was added at 100 °C under stirring, and the system was maintained in this condition for 2 h.

For impregnation of the support, the pro-catalyst TiCl4 was added at 110 °C and kept under stirring for 2 h under nitrogen. The supernatant was then removed and the catalytic solid was washed with dry hexane until no residual hydrochloric acid was observed. The catalyst obtained was dried to constant mass under nitrogen.

Polymerization of propylene

The polymerization reactions were performed in Büchi reactor with mechanical stirrer (650 rpm) and cased glass beaker of 1 L. The reactor was connected to a Haake model N6 thermostatic bath. The glass reactor was previously dried at 120 °C for one hour and mounted on the metal support under flowing nitrogen. After cooling, hexane, the cocatalyst triethylaluminum (Akzo Nobel, USA) and the external electron donor dimethoxy-diphenyl-silane (Tokyo Kasei Kogyo Co., LTD, Japan) were introduced and then the stirring system was switched on. After propylene was introduced to saturation at 1 bar manometric pressure and 70 °C, the catalyst was injected and pressure was immediately increased to 4 bar, and maintained for 1 h.

The polymerization was stopped by releasing the monomer pressure and pouring the polymer into a beaker containing a 5% solution of ethanol/HCl to remove the catalyst residue. After stirring overnight, the polymer was filtered and washed first with sodium bicarbonate solution for neutralization, and subsequently with ethanol. Then, the polymer was dried in a vacuum oven at 60 °C to constant mass.

Characterization of halloysite and polypropylenes

Halloysite was characterized by X-ray diffraction using a Rigaku Miniflex diffractometer working with a potential difference of 30 kV and 15 mA. The scan was performed in the 2θ range of 2°–30°, goniometer speed of 0.05°/min, with CuKα radiation of λ = 1.5418 Å.

Scanning electron microscopy (SEM) was carried out with an FEI Quanta 200 microscope. The catalysts were fixed on the holder with carbon adhesive tape under nitrogen in a glove bag and metallized with gold, while the polymer specimens (20 × 8 × 0.2 mm) were cryofractured with liquid nitrogen and the surfaces were fixed on the holder and also metallized with gold.

The polypropylenes were made into films by compression at 180 °C for 1 min. Specimens with dimensions 20 × 8 × 0.2 mm were obtained and characterized with a TA Instruments DMTA Q800 dynamic mechanical thermal analyzer in voltage mode, using the tension film clamp, with the isotherm −40 °C for 10 min, scanning in the temperature range of −40 to 140 °C, frequency 1 Hz, 0.1% strain and heating rate of 3 °C/min.

Differential scanning calorimetry (DSC) was used to obtain further information about the thermal properties of the polymer, such as melting temperature (T m), crystallization temperature (T c) and melting enthalpy (ΔH m). The polymer’s degree of crystallinity (X c) was determined by the melting enthalpy value of the sample and the theoretical 100% crystalline PP (195 J/g). Samples were analyzed as powder and heated from 30 to 200 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The equipment employed was a TA Instruments Model Q 1000. The first run was carried out to eliminate the polymer thermal history and the second run for the determination of material properties. Thermogravimetric analyses (TGA) were performed with a TA Instruments RT Series Q 500. Samples were heated in the range from room temperature to 700 °C in an inert atmosphere, at a heating rate of 10 °C/min. The isotactic index (I.I., in wt%) of the obtained PP was calculated after extraction of atactic PP in a Soxhlet apparatus under boiling heptane, by the relation between the remaining mass (isotactic PP) and the initial sample mass, considering the amount of clay in the sample.

Results and discussion

Chemical modifications of the original halloysite were analyzed by X-ray diffractometry. Figure 1 shows the diffractograms of the original halloysite (HO) as well as the modifications with VTMS (HV) and SiCl4 (HS). The XRD profile of the original halloysite presents reflections at 2θ = 7.2° and 13.2°. The first one at 7.2° belongs to the intact portion of halloysite, having a monolayer of water between the double layers of the clay, while the diffraction at 2θ = 13.2° is related to dehydrated halloysite. This diffraction pattern confirms the multiwall nanotubular structures of halloysite [15].

Fig. 1
figure 1

X-ray diffractograms of original halloysite (HO) and its modifications with VTMS (HV) and SiCl4 (HS)

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The characteristic diffraction peak at 2θ equal to 13.2° (d = 0.67 nm) in the original halloysite was shifted to 2θ equal to 13.1° (d = 0.68 nm) and 12.8° (d = 0.70 nm) for the VTMS- and SiCl4-modified halloysites, HV and HS, respectively. Moreover, peak intensity in HV sharply decreased. This means that both treatments increased the basal spacing of halloysite.

Catalyst with halloysite

The SEM micrographs of the standard Ziegler–Natta catalyst (cat-Ti80) and cat-HV3 containing the VTMS-modified halloysite at clay:MgCl2 mass ratio equal to 3:1 are presented in Fig. 2. Spheroidal particles were obtained with sizes around 12 μm containing nanotubes of diameter 350–400 nm and length of 2.5 μm. The surface of these particles can also be observed. The notable feature is the large porosity and irregularity of the surface of the catalyst particle compared to the standard one. The morphology of the catalysts containing the original and modified halloysite with SiCl4 was very similar to the cat-HV3 showed here.

Fig. 2
figure 2

SEM micrographs of the conventional catalyst cat-Ti80 (a) and of the cat-HV3 with magnification of 7000× (b) and 10,000× (c)

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In this case, we chose to study two halloysite/MgCl2 mass ratios in preparing catalysts with halloysite, 3:1 and 5:1. Table 1 shows the titanium content in the prepared catalysts and Table 2, the results of propylene polymerizations with these catalysts. In the catalysts containing halloysite, the Ti contents were much higher than the original one without clay. Furthermore, in the catalysts whose clay:MgCl2 ratio was 3:1, the titanium content was higher than in catalysts having clay:MgCl2 ratio of 5:1. Therefore, increasing the content of the halloysite catalyst caused a decrease in the amount of fixed Ti. This shows that this clay allows low fixation of titanium sites on its surface. This is important because it does not markedly increase the amount of residual Ti in the polymer. The increase in Ti content of catalysts with halloysite may have occurred due to the increased porosity of the catalyst, as seen in the SEM micrographs.

Table 1 Standard catalyst and those obtained with halloysite
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Table 2 Results of polymerization with the standard catalyst and those containing halloysite
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Furthermore, the Ti fraction impregnated in the catalyst containing VTMS-modified halloysite sharply increased. As will be seen later, this catalyst was less active than the others, indicating that part of the fixed Ti is inactive for polymerization. In fact, Ti must have reacted through the hydroxyl groups resulting from hydrolysis of VTMS since, as known, Ti fixed to OH groups results in Ti–O species, which are inactive for polymerization [12].

Comparing the yields of the Ziegler–Natta bisupported catalysts with that of the traditional one (Table 2), the yield was always higher for the latter. Moreover, higher the clay content in the catalyst was, lower the catalytic performance was. Finally, the chemical modification with VTMS in clay resulted in decreased activity of the catalyst.

The DSC results showed that the melting temperature (T m) of the polypropylenes obtained from catalysts with clay:MgCl2 ratio equal to 3:1 was generally slightly higher than those with the ratio 5:1 (endotherms in Fig. 3). The PP-HS3 and PP-HO3 presented T m of 158 °C, which is as high as that of PP without clay (PP-Ti80). In turn, PP-HV3 showed T m of 153 °C, with wider endotherm peak than those of the other polymers in this group.

Fig. 3
figure 3

DSC endotherm of PPs synthesized with catalyst containing clay:MgCl2 = 3:1 (a) and clay:MgCl2 = 5:1 (b)

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Both polypropylene PP-HO5 and PP-HV5 also exhibited broadened peaks characteristic of T m at 154 °C, as shown in Fig. 3b. This indicates again that the high clay content did not allow the polymer chains to organize themselves with good regularity, so they formed crystals of different sizes requiring different amounts of energy to melt.

In turn, the degree of crystallinity (X c) of the PPs obtained with catalysts containing lower clay ratios was higher than for those obtained with catalysts in higher proportion. In the first set of catalysts, X c remained in the range between 36.2% for PP-HV3 and 46.8% for PP-HO3, while the last set was in the range between 10.5% in PP-HV5 and 16.5% in PP-HO5. In general, polymers with lower clay content have chains with lesser hindrance to organize and they form larger, better organized crystals.

The thermal degradation of the polypropylene obtained with the catalysts containing both clay:MgCl2 ratios (3:1 and 5:1) are illustrated in the TG thermograms of Fig. 4A, B, respectively.

Fig. 4
figure 4

TG thermograms of PPs obtained from catalysts with clay:MgCl2 = 3:1 (A) and clay:MgCl2 = 5:1 (B). (a) TG and (b) DTG

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These materials showed degradation temperature T onset in the range of 443 °C (PP-HS3) and 456 °C (PP-HO3), and for polymer PP-HVS, T onset was 450 °C. Therefore, they were more thermally stable than PP-Ti80 (without clay), with T onset equal to 435 °C. The temperature of maximum degradation rate (T max) of these materials were also higher than that of PP-Ti80 (456 °C), where PP/clay of this series showed T max at 458, 462 and 469 °C in PP-HS3, PP-HV3 and PP-HO3, respectively.

Therefore, the clay protected the polymer against thermal degradation, probably shielding it against the action of heat. In other words, the clay acted as a thermal insulator. The literature attributes the retardant effect of halloysite on thermal degradation to the encapsulation of the initial products of decomposition inside the tubes (lumens), which are then eliminated with increasing temperatures [16].

Unlike the previous series, degradation of the last set of PPs occurred in at least two stages. That is, for PP-HO5 degradation occurred with T onset and T max equal to 380 and 408 °C, respectively, and another stage of degradation with T onset and T max equal to 444 and 456 °C, respectively. In turn, PP-HV5 presented a first stage of thermal degradation where T onset and T max were 343 and 411 °C, and a second stage at 443 and 460 °C, respectively. These polymers contain very high amounts of clay, and therefore, halloysite nanotubes allowed a higher accumulation of degraded material in its interior. With increasing temperature, this material was eliminated in the second stage of degradation.

Polypropylene containing halloysite

Figure 5A–C shows SEM images of polypropylenes PP-HO3, PP-HV3 and PP-HS3, respectively. The images reveal, after successive amplifications in Fig. 5A, the well-dispersed original halloysite nanotubes in the matrix PP-HO3. These tubes have length from 0.5 to 1.5 μm and diameters between 100 and 500 nm. In Fig. 5B, isolated tubes of halloysite modified with VTMS can be seen through the SEM images with the same size.

Fig. 5
figure 5figure 5

SEM micrographs of polypropylene PP-HO3 (A), PP-HV3 (B) and PP-HS3 (C) employing SEI at different magnifications

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In turn, SEM micrographs of PP-HS3 revealed the presence of tubes of SiCl4 modified halloysite with lengths of less than 5 μm and diameters less than 1.5 μm, forming a cluster of tubes 30 μm in length by 10 μm in width (Fig. 5C).

Figures 6, 7, and 8 show XRD analysis of the catalysts and polypropylenes obtained with halloysite. Figure 6a shows that in PP-HO3, containing 5.8% clay, besides the peaks characteristic of polypropylene, a characteristic diffraction peak of halloysite also appears. This peak position at 12.05° (d = 0.74 nm) shows there was a shift relative to the original halloysite, shown at 2θ equal to 13.20° (d = 0.67 nm), indicating the PP chains grew between the tubes of halloysite, separating them. In turn, Fig. 6b shows that in the cat-HO3 this peak shifted to 2θ 12.45° (d = 0.71 nm), i.e., it was possible to insert MgCl2 between the tubes of natural halloysite in cat-HO3.

Fig. 6
figure 6

X-ray diffractograms of cat-HO3 and PP-HO3 (a) and of cat-HO5 (b)

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Fig. 7
figure 7

X-ray diffractograms with different intensities of a cat-HV3 and PP-HV3. b cat-HV5 and PP-HV5

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Fig. 8
figure 8

X-ray diffractograms of cat-HS3 and PP-HS3

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Figure 6 shows the comparison of X-ray diffractograms of the original halloysite and of catalyst cat-HO5. A small amount of polypropylene PP-HO5 containing 76.3% halloysite was obtained. In other words, a high load of clay in the catalyst prevented the growth of polymer chains. However, the catalyst the characteristic peak of halloysite, shifted from 2θ = 13.20° (d = 0.67 nm) to 12.20° (d = 0.73 nm), confirming the insertion of MgCl2 between halloysite tubes.

Figure 7a shows that for polypropylene PP-HV3, obtained with 5.0% VTMS- modified halloysite, there was also a shift of the diffraction peak characteristic of the original clay to 2θ = 12.45° (d = 0.71 nm) which was the same displacement observed in catalyst cat-HV3. Again, there is an indication of formation of polymer between the halloysite nanotubes. In Fig. 7b, diffraction peaks characteristic of polypropylene in PP-VH5 obtained with 77.2% halloysite are observed with low intensity.

In Fig. 8 it can be seen that the PP-HS3 obtained with 3.9% SiCl4-modified halloysite presented, besides the characteristic diffraction peaks of PP, the clay peak at 2θ of 12.25° (d = 0.73 nm). This indicates that polypropylene chains were produced between the halloysite nanotubes, as this is slightly greater than that observed in the original clay.

Table 3 and Fig. 9 present the results of dynamic mechanical analyses (DMTA) for polypropylene obtained without clay, as well as those synthesized with catalysts containing halloysite.

Table 3 Results of DMTA analyses of pure PP and PPs with halloysite
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Fig. 9
figure 9

Curves of storage modulus (a), loss modulus (b) and tan δ (c) of the PPs obtained

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Figure 9a shows that in all temperature ranges investigated for polypropylenes PP- HS3 and PP-HV3, storage moduli E′ were higher than for the polymer without clay PP-Ti80, showing that the presence of halloysite in the polymeric matrix enhanced the mechanical properties of the nanocomposites, possibly due to good dispersion of this filler in the matrix.

At 25 °C, the values of E′ were 77 and 44% higher for PP-HV3 and PP-HS3, respectively, in comparison with PP-Ti80. In turn, PP-HO3 presented modulus very near PP-Ti80, i.e., the original halloysite did not influence the value of E′, unlike the VTMS- or SiCl4-modified halloysite.

The glass transition temperature T g values (calculated by E′ and tan δ), presented in Fig. 9b, c, respectively, were higher for all polypropylenes with halloysite compared with the PP obtained without clay (PP-Ti80), showing the effect of clay is to restrict the molecular motion of the polymer chains, which leads to increased T g.

The tan δ max values indicate that the relaxation capacity of the polymer chains of sample PP-HS3 was higher than that of PP-Ti80, whereas the polymers PP-HO3 and PP-HV3 had lower relaxation capabilities. This shows that the molecules of the amorphous phase of PP are interwoven into the clay.

Sample PP-HS3 obtained with 3.9% halloysite, modified by SiCl4 has spherical morphology with 0.25 cm radius as disclosed in the photograph shown in Fig. 10, where only a few fine particles of polypropylene can be observed. The morphology of these two polymers exemplifies what is desired from the production of polyolefins, in general, namely promotion of the smooth flow of the polymer in the production reactor, pipes and storage silos, and reduced scale formation (fouling), avoiding unwanted stops in continuous industrial production of polypropylene. This shows that the technique used in this work resulted in spherical catalysts, and through the replica phenomenon, it was possible to produce spherical polymer particles.

Fig. 10
figure 10

Photo of polymer PP-HS3

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Conclusion

Halloysite was reacted with vinyl silane and SiCl4 and both treatments increased the basal spacing of the clay. The Ziegler–Natta catalysts prepared with those halloysites showed spheroidal morphology of the particles like that of the standard catalyst, with much higher titanium content, although increase the amount of halloysite in the catalyst reduced the amount of Ti fixed on its surface. The activity decreased in the catalyst samples containing halloysite, especially in those with higher amounts of clay. PP/halloysite nanocomposites obtained through in situ polymerization employing these catalysts have in general melting temperature as high as PP without clay, but much higher degradation temperatures, as shown in the TGA. Moreover, the degree of crystallinity of PP/clay was higher. SEM micrographs of the nanocomposites revealed isolated and well-dispersed nanotubes of halloysite in the polymer matrix. Regarding the mechanical properties, measurements by DMA revealed that polymers containing halloysite without treatment and those with VTMS- halloysite showed similar storage moduli as the PP without clay, but PP containing SiCl4-halloysite has sharply increased E′. The presence of halloysite also increased the glass transition temperature of PP.