DISPERSION AND REINFORCEMENT OF NANOCLAYS ON THERMOPLASTIC COMPOSITES USING RHEOLOGICAL TECHNIQUES

G.A. Soberanis-Monforte, J.L. Gordillo-Rubio, P.I. Gonzalez-Chi*Unidad de Materiales, Centro de Investigación Científica de YucatánCalle 43, No. 130, Chuburná de Hidalgo, Mérida, Yucatán, México.

*ivan@cicy.mx

ABSTRACT

Melt compounding was used to prepare thermoplastic composites reinforced with nanoclays; the extrusion parameters and the chemical modification of nanoclays where used to promote a good reinforcement/matrix interface. The nanoclay dispersion and the interfacial interaction between the nanoclay and the thermoplastic matrix affected the rheological properties of the mixture, since the melt viscoelastic behaviour is sensitive to changes of the molecular structures at the nanoscale and mesoscale.

The clay used on this research was palygorskite (attapulgite) extracted from the southeast of the state of Yucatan, Mexico; the palygorskite was subjected to a purification process to eliminate impurities and then superficially modified with 3-aminopropiyltrimetoxiysilanoe. From these processes, three types of specimens were generated: unpurified, purified and silanized clays; they were incorporated to on the a matrix of polypropylene by melt compounding.

Frequency sweeps were used to evaluate the interaction between the matrix and the nanoreinforcement. The master curves of storage modulus (G’), loss modulus (G”) and complex viscosity (η*) vs angular frequency (ωx) show an increase of the interfacial interaction polypropylene/palygorskite when the surface of the clay was chemically modified.

1 INTRODUCTION

The palygorskite clay is a fibrous silicate which structure was proposed by Bradley in 1940 [1]:

[Si8Mg5O20 (OH )2 ] (H 2O )4 ∙4H 2O. The palygorskite structure contains three different types of

water molecules: adsorbed water, bonded water to the octahedral cations and water bonded to the hydroxyl groups.

Due to its rheological and adsorption properties, palygorskite is widely used in different applications like animal waste adsorbent, pesticide carrier, decolorizing agent, catalyst and catalyst support, etc.

The adsorbent and/or catalytic applications of a nanoclay depend from their surface physico-chemical properties, which have been modified by chemical and thermal treatments, to control their surface activity [2-4]. Even though the palygorskite has been studied from different points of view [5-8], the literature concerning towards this mineral is less abundant than that addressing to sepiolite or different smectites.

The clays are being used to create new materials with unique characteristics, among the most interesting ones are the composites where the clay is mixed to the nanometer order with a polymer; this materials are known as polymer-silicate nanocomposites (PSNs) which have high mechanical, thermal, and barrier properties with only a small amount of silicates. [9-12 (1-22)]. One of the real challenges posed by the PSNs is the distribution of nanoreinforcement in the matrix which could limit the applications of the PSNs. Depending on the chemical nature of the constituents and the interactions between the nanoreinforcement particles, their dispersion can generate agglomerated, intercalated and exfoliated structures [XX/].

The molten dispersion of the nanoclay is recognized as a promising technique for manufacturing PSNs, due to their ease of operation, low cost and low negative impact on the environment, since it is based on a conventional process of polymer blend by twin screw extrusion. When this processing method is used for polymers of non-polar nature, such as the polypropylene (PP), compatibilizing agents are required because the shear stresses generated during the processing are not sufficient to exfoliate or disperse the clay in the matrix; that is why a generally accepted practice is to add to the formula maleated polypropylene (MaPP) as a compatibilizer. However, to succeed some parameters must taken into account: the molecular weight of the MaPP and the content of maleic anhydride grafted (MA). It has been found that a high content of MA generally increases the melt intercalation of PP oligomers in the clay structure but this may generate immiscibility with the PP matrix affecting the mechanical properties of the nanocomposites [3]. The effect of molecular weight is less clear; most authors used low molecular weight MaPP [3-5], but high molecular weight have improved mechanical properties, on the other hand, the presence of MaPP of different molecular weights not only affects the final morphology of the nanocomposite but also the melt behavior of the matrix [6].

OIn the n melt matrix, the size, shape, structure and surface characteristics of the disperse phase affect the rheological properties, often negatively, however, with the nanoparticles, the interaction between the reinforcement and the matrix takes place at molecular level, directly affecting the molecular motion and flow, for this reason the dispersion degree and the interfacial interaction between nanoclay and a thermoplastic matrix affect the rheological properties of a nanocomposite. In addition, the knowledge of the rheological behavior of a nanocomposite permits the optimization of material’s processing (extrusion or injection).

A considerable number of works have been made to study the mechanical, rheological and thermal properties of the PSNs, however, few of them are focused on the study of the morphological effects of the nanoclay dispersion over the rheological properties of the PSNs [22, 23], Hoffmann et al [30], and Lim & Park [32] used rheological techniques to establish that the linear viscoelastic properties of nanocomposites are highly affected by the final estate of dispersion of clay in the polymeric matrix, particularly in nanocomposites where the polymer chains and the clay surface were are chemically bonded [26, 27].

2 EXPERIMENTAL

2.1 Materials

The clay used as reinforcement of the PSNs was palygorskite from a quarry located in Ticul, Yucatan, Mexico. The matrix used was polypropylene (PP) from Valtec Indelpro Company. The

compatibilizer was maleated polypropylene 426512 (MaPP) from Aldrich. For the clay purification process it was required: hydrogen peroxide at 30 % reagent grade from Chemicals Monterrey, hydrogen peroxide at 3.95 % curing grade from Belmen, glacial acetic acid reagent grade from Merck, hydroxylamine chloride reagent grade from Mallinckrodt Chemicals and sodium hexametaphosphate reagent grade from Tecsiquim. For the silanization process, 3-aminopropyltrimethoxysilane from Aldrich was used.

2.2 Extraction and purification of the clay

The purification process of the clay consisted of five treatments: 1) 30 g of clay was diluted in 400 ml of distilled water for 3 h or until total dispersion, the solution was filtered using a mesh number 80 (0.180 mm) to remove the retained stony debris. 2) 200 ml of hydrogen peroxide at 30 % was added and stirred until the bubbling ceased and then 300 ml of hydrogen peroxide at 3.95 % was added, once the bubbling ended, the solution was heated at 100 ºC to evaporate the water; the addition of the hydrogen peroxide at 3.95 % was repeated until no change of color was observed on the solution, then 3 ml of glacial acetic acid was added and then the solution was evaporated, finally the clay was dried in an oven for 8 h at 100 ºC. 3) 240 ml of a solution 1 M of sodium acetate adjusted to pH 5 and 300 ml of distilled water was added to the clay and stirred for 3 h, then the mixture was centrifuged at 1500 rpm for 15 min periods, then the sediments were decanted and dried in a oven at 100 ºC for 8 h. 4) 450 ml of hydroxylamine chloride 0.04 M and 150 ml of glacial acetic acid were added to the clay and the mixture was heated at 96 ± 3 ºC with occasional stirring for 6 h; 600 ml of distilled water were added to the clay and stirred and evaporated for 30 min, then it was centrifuged at 1500 rpm during 15 min periods until the liquid is colorless and odorless; the clay was dried in a oven at 100 ºC for 8 h [56]. 5) 1 l of the dispersant solution of sodium hexametaphosphate at 0.5 % was added to the clay and stirred for 8 h, then the solution was placed per 60 min in an ultrasonic bath, and then centrifuged at 2000 rpm for 60 min, decanted and the clay was dried in a vacuum oven at 80 ºC with at -635 mm Hg. Finally, the clay was grinded and stored.

2.3 Silanization process of the purified clay

The silanization of the clay was performed using the following methodology: 1) 500 ml of distilled water was added to 10 g of purified clay and sonicated for 1 min; the solution was heated between 40 and 45 ºC and stirred for 10 min. 2) 2.5 ml of 3-amoinopropyltrimethoxysilane was added drop by drop and the stirring was keep for 60 min; then the mixture was centrifugated at 2000 rpm for 20 min during 20 min and decanted. 3) 25 ml of ethanol were added to each centrifuge tube and placed in anthe ultrasonic bath for 10 min, then 25 ml of distilled water were added to each tube and centrifugated at 2000 rpm for 10 min. 4) 40 ml of distilled water were added to each centrifuge tube and then sonicated for 10 min and centrifugated for 10 min at 2000 rpm. The solid material at the centrifuge tubes was dried in a vacuum oven at 80 ºC at -635 mm Hg. Finally, the clay was grinded and stored.

2.4 Infrared spectroscopy

3 mg of clay were mixed with 130 mg of KBr and the mixture was subjected to a pressure of 10 ton in a hydraulic press Carver to mold a tablet that was dried in a vacuum oven at -635 mm Hg and 100 ºC for 24 h. The spectrophotometer used was a Fourier Transform Infrared (FTIR) Nicolet model Protege 460. The spectra were obtained in the range from 4000 to 400 cm-1 with a resolution of 4 cm-1 and 100 scans.

2.5 Scanning Electron Microscopy

Microscopic examination was performed to establish the morphology of the palygorskite before and after each one of the treatments from the purification process. The scanning electron microscope (SEM) used was a JEOL model JSM-6360-LV. The scans were performed with 20 kV in high vacuum mode with magnifications of 10,000x and 20,000x. The samples were coated with gold in a Denton Vacuum Desk-II cold sputtering machine, with a deposition time of 18 s.

2.6 Extrusion and molding of the PSNs

The PP/nanoclay blends were prepared in a double-screw extruder with three heating zones (model CTSE-V/MARK-II, Brabender) equipped with a drive unit Plasti-corder (model 813402). The length of the screw is 400 mm with an L/D ratio of 13:1. The extrusion conditions used were: temperatures of the heating zones and die, 180, 200, 210 and 210 °C respectively, speed of the spindle, 150 rpm. The extruder was fed with three dispensers calibrated to generate the formula: 0.5 % clay, 10 % MaPP and 89.5 % PP.

The extrudates were molded into sheets of 150 x 150 x 0.9 mm in a semiautomatic press Carver at 210 °C with a progressive compression force of 4500 kgf.

2.7 Rheometry frequency sweeps

The rheological analysis was performed in a rheometer TA instruments, model AR-2000 fitted with parallel plates of 25 mm of diameter. The frequency sweep experiments were performed on specimens disks with an average diameter of 25 mm diameter and 0.9 mm thickness; a pneumatic pressure of 30 psi and 10 % strain was used, the test temperatures at the environmental chamber of the rheometer were 170, 190 and 210 ºC. Two methods were user for the data acquisition: 1) ascending frequency sweeps, where the frequency was increased from 0.1 to 200 rad/s at the three different temperatures, 2) descending frequency sweeps, where the frequency was decreased from 200 to 0.1 rad/s at the three different temperatures (six frequency sweeps per sample). Each frequency sweep was performed 2 min after the chamber reached the working temperature (170, 190, and 210 ºC) and the storage modulus (G’), loss modulus (G”) and complex viscosity (η*) vs angular frequency (ωx) were reported. The results of the six frequency sweeps from each sample and the principle of time-temperature superposition (TTS) were used to generate the master curves, which were calculated at a reference temperature of 170 ºC.

The time-temperature superposition principle is used to study of the behavior of materials to a wide range of frequencies and/or temperatures. This principle transfers the information obtained to a single reference temperature, so it is possible to compare the results from the different temperatures of all samples. The superposition principle is valid only when all relaxation times

aτ (T ), in the Maxwell equation have the same temperature dependence; mathematically, this

principle can be expressed as [19, 21]:

G´ (ω,Tr )=ρ (T r )Trρ (T )T

G´ (aτω ,T ) (1)

G´ ´ (ω,T r )=ρ (Tr )T rρ (T )T

G ´´ (aτω,T ) (2)

where:

r represents the reference temperature,

aτ is the relation between two relaxation times at two temperatures

ρ (T ) is the density of the polymer at a temperature T

3 RESULTS AND DISCUSSION

3.1 Purification and silanization of the clay

The purification of the clay was focused on the removal of stony waste and organic matter, and also the ion exchange calcium per sodium; the clay collected from a mine, naturally contains some contaminants which adversely affect the silanization process and therefore the compatibility with the thermoplastic matrix.

Figure 1 shows the FTIR spectra from the various steps of the clay purification process. The absorption bands at 3700–3200 cm−1 correspond to the stretch vibrations of structural OH and OH of the bound and hygroscopic waters. The absorption band at 1700–1600 cm−1 is attributed to the bend vibration from the hygroscopic, adsorbed and bound water or from structural OH. The bands between 1300 and 400 cm−1 are a combination of the stretch and bend vibrations of the Si (or Al)–O structure and octahedral M–OH (M represents metal element). Table 1 list the band position corresponding to the functional groups of the palygorskite. [article FTIR Palygorskite]

Figure 1. FTIR spectra of the different treatments from the palygorskite purification process.

Table 1. FTIR band position for the palygorskite clay [article: FTIR palygorskite].

Position (cm-1) Assignation3615 Al2-OH3581 Mg2-OH3578 Al,Fe3+-OH3551 Mg,Fe3+-OH3395 Si(OH)Si3283 Si(OH)Al1655 OH2

1635 H2O1200 SiU-O-SiD1120 Si-O1093 Si-O1035 Si-O985 Si-O914 Al2-OH646 Si-O584 Si-O512 Octahedral deformation484 Si-O (parallel to layer)444 MgO6 (Rotation)

The FTIR spectra from the different treatments of the purification process show no substantial changes compared to the spectrum from the untreated clay. The only differences were observed at 1655 and 1635 cm-1 which correspond to water due to the hydrophilic nature of the clay. The spectrum from treatment 2 shows some difference at 1650 y 900 cm -1 which corresponds to the bending vibration mode of the hygroscopic water and Si–O structures, however, the subsequent treatments show no difference at all. The bands identified at the figure 1 are similar to the ones reported by Cai, Xue and Polya at their FTIR study of palygorskite [FTIR Palygorskite], the clay did not show chemical damaged related to its purification.

Figure 2 compares the FTIR spectra of the purified/silanized clay and purified clay. The spectrum of the clay after silanized with 3-aminopropyltrimetoxysilane shows some differences in the regions corresponding to the silane, which probes the silane grafting to the clay (after the silanization

process, the clay was heavily washed with alcohol). Table 2 shows the band positions of the purified/silanized palygorskite.

Table 2. FTIR bands from the purified/silanized palygorskite [52 aut??]

Position (cm-1) Assignation3615 Stretch Mg—OH dioctahedral3516 Stretch OH of coordinated water with Al, Mg3390 Stretch OH of zeolitic and absorbed water3212 Stretch N—H 3050 Stretch N—H de amino (—NH3

+)2930 Stretch C—H 1625 Deformation OH of water1500 Symmetric deformation N—H of amino (—NH3

+)1470 Deformation C—H1194 Stretch Si–O

Figure 2. FTIR spectra from the palygorskite silanized with 3-aminopropiyltrimetoxyxisilane.

The bands at 3212 cm-1 could be assigned to the N—H stretching from the silane, the vibration at 1608 cm-1 also belongs to a silane amine group and it is overlapped with the 1628 cm-1 band characteristic of the clay’s water absorption, finally, the band at 1500 cm-1 corresponds to the absorption of an amino group which is absent at the clay spectrum. When the clay reacted with the silane, the band intensity of 2930 cm-1 assigned to the C—H stretching gets increased due to the organic part contribution of the organic portion of silane. (this is a particularly intense vibration of the silane); it is On additionalso, probable that a secondary reaction of the silane amine groups took place as evidenced by the broad band at 3050 cm-1 that most probably corresponds to vibrations of -NH3

+. groups of silane instead of –NH2 groups. [ , ] COMO LO SABES?

3.2 Microscopy analysis of the clay

The SEM analysis was made on six different samples: clay without treatment (as mined) and five clay samples from the five different treatments of the purification process. Figure 3a clearly shows the fibrillar morphology of the clay without treatment at 20,000x. Figures 3b to 3e show the treated nanofibers, their morphology, length and diameter seem to be similar to the untreated fiber. However, figure 3f shows nanofibers with substantial length reduction; apparently, the last treatment of the purification process damaged the clay, probably because this treatment involves a long period of centrifugation (60 min) and sonication (60 min).

Figure 3. SEM micrographs of the palygorskite without treatment and after the five treatments of the purification process.

3.3 Rheology analysis of the PP/MaPP/Clay formulations

The rheology properties were measured to five different samples: PP, a mixture of PP and MaPP and three formulations containing different types of clays (Table 2).

Table 2. Samples for the rheology test; the formulations were prepared by twin-screw extrusion

FormulationPP%

MaPP%

Palygorskite %

PP 100 0 0%

PP/MaPP 90 10 0%

PP/MaPP/UC 89.5 10 0.5 % (Unpurified Clay)

PP/MaPP/PC 89.5 10 0.5 % (Purified Clay)

PP/MaPP/PSC 89.5 10 0.5 % (Purified and Silanized Clay)

The master curves of the PP frequency sweeps are shown in figure 4; a long relaxation time

τ∼ (1/ωx )∼ (0.05−9.552 ) was observed, corresponding to the frequency interval were the G’ and

G” intersect (ωx∼10.469−20 rad / s). Some frequency curves showed a small fluctuation of the

data acquired at low frequencies, this was probably caused by the low torque signal which is near the resolution limit of the rheometer.

Figure 4. PP master curve from the frequency sweeps: 0.1 to 200 and 200 to 0.1 rad/sat 170, 190 and 210 ºC. Reference temperature 170 ºC.

Figure 5 shows the rheological behavior of the mixture PP/MaPP; the relaxation time corresponding

to the intersection of G’ and G” (ωx∼16.618−17.875 rad / s) is τ∼ (1/ωx )∼ (5.594−6.018 ). This intersection interval is shorter than the one from PP. The loss modulus, the storage modulus and the complex viscosity at high frequencies of the mixture PP/MaPP are similar to the ones from PP (figure 4); that is expected since the mixture PP/MaPP contains only 10 % of MaPP (with 0.6 % by weight of grafted maleic anhydride), therefore, the rheological behavior is dominated by the PP.

Figure 5. PP/MaPP master curve from the frequency sweeps: 0.1 to 200 and 200 to 0.1 rad/sat 170, 190 and 210 ºC. Reference temperature of 170 ºC.

Based on the results from table 3, it can be said that all the formulations initially showed a similar linear behavior, since in all cases the G’, G” and η*, slopes are similar. And the frequency interval in which G’ and G” curve intersect is different for each formulation.

The rheological pattern of the PP/MaPP/UC formulation (figure 6) is similar to PP, this could be caused by the low clay content used (0.5 % by weight), as postulated by Jin Zhao et al [19]: "The number of particles per unit volume is a key factor to establish the characteristic rheological response of a clay nanocomposites". In addition, the clay used did not had any surface treatment, so the clay particles did not fully dispersed, consequently did not substantially contribute to the strengthening of the nanocomposite.

Table 3. Summary of the rheological properties from the five different samples studied.

Properties SAMPLE

PP PP/MaPP PP/MaPP/UC PP/MaPP/PC PP/MaPP/PSC

Intersection interval

ω (rad/s)11.69 – 16.67 16.21 – 17.41 13.18 – 19.06 24.61 – 47.44 10.06 – 14.61

Slo

pe L

ow

freq

.

G´ 1.17 1.17 1.22 1.28 1.18

G´ ´ 0.84 0.85 0.86 0.88 0.85

η¿-0.15 -0.15 -0.12 -0.12 -0.14

Hig

h fr

eq.

G´ (Pa ) 86340.37 81450.98 82969.69 51861.50 102606.69

G´ ´ (Pa ) 49049.31 47776.72 48402.53 37018.50 54267.88

η¿ (Pa . s ) 498.03 460.28 471.98 319.87 537.60

Low

freq

.

G´ (Pa ) 40.19 41.37 30.34 14.54 49.55

G´ ´ (Pa ) 308.76 301.99 254.37 144.89 335.81

η¿ (Pa . s ) 12174.49 11488.99 10808.26 5337.88 14138.63

Figure 6. PP/MaPP/UC master curve from the frequency sweeps: 0.1 to 200 and 200 to 0.1 rad/sat 170, 190 and 210 ºC. Reference temperature of 170 ºC.

The rheological properties of the PP/MaPP/PC and PP/MaPP/PSC mixtures showed considerable differences when compared to PP. The PP/MaPP/PC (figure 7) showed a decrease of G’, G” and η* both at high and low frequencies; this could be result of the damage caused to the nanofibers of the clay during the purification process (figure 3) affecting their reinforcement ability due to the reduction of the fiber’s aspect ratio.

Figure 7. PP/MaPP/PC master curve from the frequency sweeps: 0.1 to 200 and 200 to 0.1 rad/sat 170, 190 y 210 ºC. Reference temperature of 170 ºC.

By contrast, PP/MaPP/PSC (figure 8) showed a substantial increased of G’, G” and η*, both at high and low frequency; though the fibers from the purified clay had a reduced aspect ratio, the silanization added functional groups to the fiber’s surface resulting in a better interfacial compatibility between the matrix and the reinforcement. The amino groups grafted onto the surface of the clay provides a chemical bond to the polymeric matrix; during the extrusion process, part of the polymer chains undergo some degradation generating chemically active carboxylic groups able to react with the amino groups of the grafted silane [50].

Figure 8. PP/MaPP/PSC master curve from the frequency sweeps: 0.1 to 200 and 200 to 0.1 rad/sat 170, 190 and 210 ºC. Reference temperature of 170 ºC.

Figure 9 shows the relaxation times from each sample (table 3); the relaxation time of the first three samples show no substantial changes, however, the last two samples show a considerable change. PSNs (Polymer-Silicate Nanocomposites) with purified clay had a small relaxation time, consequently the system became less elastic and therefore the interaction matrix/reinforcement is low and the reinforcing particles created areas of stress concentrations affecting the rheological properties; most likely caused by the damage suffered by the clay during the purification process. The PSNs prepared with purified and silanized clay became more elastic; this formulation reported the largest relaxation time, which proves the existence of an interaction between matrix and reinforcement and therefore an improvement of the rheological properties of PSNs.

Figure 9. Relaxation times of the five formulations studied.

4. CONCLUSIONS

The infrared spectrum of the purified clay is consistent with the spectrum of the palygorskite reported at the literature. The clay’s chemical structure was not damaged during the extraction and purification.

The silanization method designed was able to chemically attach to the nanofiber surface the 3-aminopropiyltrimethoxysilane, as demonstrated by the infrared spectra. The presence of the silane improved the interface between the polar clay and the nonpolar polypropylene influencing the rheological properties of the composite when melted.

The SEM Micrographs proved that the last treatment of the purification process, involving sodium hexametaphosphate, centrifugation and sonication, caused a morphological change to the nanoclay: the aspect ratio was reduced.

The formulation PP/MaPP/PC prepared with the purified clay reported the lowest values of storage modulus probably caused by the low reinforcement content (0.5%) and the damage caused to the clay by the last treatment of the purification process. However, the PSNs with purified/silanized clay (PP/MaPP/PSC) improved their rheological properties compared to PP, despite the damage suffered by the clay during the purification process.

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XX. [Mecanical and Rheological properties of the Maleated Polypropylene-layered silicate anacomposites].