Carmen Albano ^1,2,*, Yanixia Sánchez ¹, Nohemy Domínguez ¹, Arquímedes Karam ¹

¹ Instituto Venezolano de Investigaciones Científicas, Centro de Química, Laboratorio de Polímeros. Caracas, Venezuela.

² Universidad Central de Venezuela, Facultad de Ingeniería. Caracas, Venezuela. * E-mail: calbano@ivic.ve

Publicado On-Line: 27-Ene-2009

Disponible en: www.polimeros.labb.usb.ve/RLMM/home.html

Resumen

Las condiciones de mezclado tienen un  efecto importante sobre la dispersión y el tamaño de partícula de la carga en la elaboración de compuestos. Con el propósito de promover una mejor adhesión interfacial entre los residuos marinos (RM) y el polipropileno (PP), se realizaron dos modificaciones en la interfase: el tratamiento de la carga con un agente acoplante del tipo zirconato (NZ12) y la adición de acido acrílico (AA) empleando irradiación gamma.  Se encontró que la incorporación de residuos marinos (RM) produce un incremento en la estabilidad térmica y que a una concentración de 20 ppm  los RM producen un efecto nucleante, observado a través del incremento en el porcentaje de cristalinidad. La adición de AA y NZ12 a las mezclas ocasionó un mejoramiento de las interacciones polímero- carga.  Se encontró que el uso de ambos agentes tiene un efecto sinergístico  tanto en el incremento de las propiedades mecánicas como en la reducción del efecto degradativo ocasionado por la radiación gamma.

Palabras Claves: Polipropileno, Residuos marinos, Compuestos, Irradiación, Agente acoplante, Funcionalización

Abstract

Keywords: Polypropylene, Seaweed residues, Composites, Irradiation, Coupling agent, Grafting.

Recibido: 06-Jun-2008; Revisado: 21-Nov-2008; Aceptado: 14-Ene-2008

1. INTRODUCTION

The current tendency in research into human bone replacements is to use biomaterials prepared from composites. Composites based on a polymeric matrix and ceramic fillers appear to be good bone and teeth substitute materials due to their greater mechanical and thermal properties than polymers alone, and their low cost and density. In addition, they can induce bioactivity at the composite surface when biocompatible inorganic particles are used [1]. Composites based on polyolefins and biocompatible ceramics have gained interest in the biomedical field due to their potential application as substitutes, especially when hydroxyapatite (HA) is used because it is one of the major constituents of human bone [2-4]. Studies have also shown that coralline structures are a good alternative filler, because implants made with corals are absorbed and progressively replaced by growing bone, indicating that corals are biocompatible and osteoconductive materials [5-6]. Similarly, seaweeds residues (SR) could allow the development of new biomaterials, due to their composition being similar to corals i.e. containing 90% CaCO₃ in an aragonite structure.

Previous works have been carried out with the purpose of characterize and evaluate the seaweed residues composites, finding that due to porous structure can be useful as an osteoconductive material, beside to promote mechanical, physical and thermal properties that make this composite suitable for biomaterial applications [7-8]. However, likely other composites, polymer-SR compounds are accompanied by filler-polymer compatibility problems, as the formation of agglomerates. Therefore, it is necessary to add, if it is possible, some coupling agent or coupling methodology, in order to enhance surface interactions [9-14].

In this context, the aim of this work was to study the effect the mixing conditions on the filler distribution and the thermal, mechanical, morphological and rheological properties of the composites, as well as to study the addition of coupling agents neopentyl (diallyl)oxy, tri(dioctyl) phosphate zirconate (NZ12) and acrylic acid (AA), with the purpose to obtain a better interfacial adhesion. Additionally gamma radiation was tested in the composites with AA, in order to induce the grafting of AA molecules into the PP matrix to improve compatibility between filler and matrix.

2. Experimental Part

2.1 Materials

PP J600 with a melt flow index (MFI) of 7.0 dg/min supplied by Propilven C.A. was used as received. NZ12 from Kenrich Chemicals dissolved in toluene as coupling agent. Seaweed residues (SR) were obtained from the North West Venezuelan coastlines (Falcon State). SR were washed with fresh water and sieved to remove sand particles from the residues. They were dried in an oven at 100˚C for 2 days and then pulverized in a cross mill Glen Creston, in order to decrease the particle size distribution. Then, the obtained SR powder was again treated in a ball ceramic milling Norton Oivision. Finally, SR was passed through a 100-mesh sieve to delimit particle size distribution. Average particle size distribution of SR was determined by laser ray diffraction; which was found to be 33.1 mm.

The internal mixer conditions were optimized. Different temperatures, mixing times and rates conditions were tested (Table 1) in order to obtain the best distribution SR particles and the best mechanical properties in the composite, using as reference a SW particle of 30 pph.   Once the optimum conditions were selected, PP/SR composites were prepared varying the SR content from 0 to 30 pph. It should be noted that the control sample without SR, was prepared throughout using the same conditions.

Table 1. Mixing conditions of PP-SR composites.

The effect of coupling and grafting agents on the composites compatibility properties (it was studied for the best particle size distribution and properties). The classification of the samples was: I) Untreated materials. II) Composites with filler treated by spraying a solution of NZ12 in hexane at different concentrations (0.3, 0.5, 0.7 and 1 wt %). III) Blends with acrylic acid in 2 wt% with respect to the polymer. And IV) Composites with both coupling and grafting agents.

Samples were irradiated with  g-rays from a 60 Cobalt source in air at a dose rate of 4.8 kGy h-1. Integral dose employed was 25 kGy. The 60Co source was a MDS Nordium (IR-216) with a Harwell- Red Perspex dosimeter.

 2.3 Composites Characterization

MFI measurements were made from the irradiated and non-irradiated pellets of the PP and composites PP/SR. A Ray-Ran Advanced melt flow systems with a weight of 2.16 kg was used at 210˚C, according to ASTM D-1238.

Mechanical properties were carried out in an Instron 4204 universal testing machine at 50 mm/min. The probes were moulded by compression and cut according to ASTM-638 standard procedure. Each experimental point represents an average of at least seven samples tested under identical conditions.

Morphological studies of cryogenically fractured samples of PP/SR composites. The surface of the fracture was coating with gold by the ionic deposition method. Micrographs were carried out using a scanning electron microscope (SEM), Hitachi S-2400.

Differential scanning calorimetric (DSC) measurements were performed with a Mettler Toledo DSC 821. Samples were heated under nitrogen atmosphere from 25 to 200°C at 20ºC/min and kept at 200°C for 3 minutes, in order to erase the previous thermal history. Then, the sample was cooled at 10 °C/min from 200 to 25°C. Finally a second heating scan was performed from 25 to 200°C at 10 °C/min. The crystallinity (X_c) was calculated using the enthalpy of a perfect crystal as reference (207.1 J/g).

Thermogravimetric Analysis (Mettler Toledo TGA851) were carried out, using a heating rate of  20°C/min up to 700°C under N₂. The activation energy (E_a) and initial decomposition temperatures (T_id) were calculated from the data obtained, using the McCallum-Tanner method [15].

Gel permeation chromatography (GPC) was performed in a Water GPCV 2000 equipment with a refractometer and viscosimeter detector, using 1,2,4-trichlorobenzene as mobile phase at 135˚C at 1 ml/min. Molecular weights averages were determined by an universal calibration curve made with polystyrene standards.

Figure 1 shows the SR morphological structure. It can be observed the presence of non-continuous and interconnected oriented cavities [10-12], which may be of benefit to cellular invasion. It is an important asset, considering our interest to use SR as reinforcement, specifically for PP matrices, in order to propose a composite for biomedical applications, such as human implants.

3.1 Optimization of the Mixing Conditions

Considering the importance of an optimal filler distribution in the polymer matrix, at first, blending conditions were optimized for 30 pph of filler content, testing the mixing condition shown in  Table 1.

The mechanical properties of the samples obtained were evaluated. It was found that no occurred significant variations in mechanical behaviour when temperature, time or rate conditions were modified. The PP/SR composites showed a tensile strength (σ_b) of  25.7 ± 0.6 MPa, a deformation at break (ε_b) of 8.8 ± 1.1% and a Young’s Modulus (E) of  690 ± 35 MPa approximately.

However, SEM micrographs (Figure 2) allowed observing the effect of mixing condition on composite dispersion and the poor adhesion between SR particles and PP matrix. Figure 2d showed that at higher mixing rates and times with the lowest temperature produced better filler dispersion with a decrease in particle average size. This is due to an increase in the shear stress with the mixing rate and the increase of the viscosity with the temperature decrease, which contribute to the destruction of SR agglomerations in the matrix. This effect increased with time.

It was found that the optimal mixing conditions were 80 rpm, 180 ˚C and 3 min. Once the optimal conditions were established, the filler content into the composite was evaluated, varying the SR content from 10 to 30 pph. As expected, SR addition caused a decrease in MFI, from 7.89 ± 0.24 g min^-1 without SR to  6.57 ± 0.19 g min^-1 with 30 pph, due to the viscosity increase.

Additionally, it was found that the incorporation of SR induced a slight increase in both crystallization temperature and melting enthalpy (Table 2). This could suggest a nucleating effect. This behaviour is observed at levels of up to 20 pph of filler, while at higher SR content the formation of agglomerates avoids this effect (Figure 2a).

 On the other hand, the thermodegradative behaviour of theses composites showed that the initial decomposition temperatures (T_id) and activation energies (E_a) augmented with filler content, which means that the SR was able to produce an increase in thermal stability.

Regarding the mechanical properties, it was found that the inclusion of SR did not modify significantly the Young’s Modulus of PP, (around 630-700 MPa) However, as Table 3 shows, there was a non-negligible decrease in tensile strength and elongation at break with increasing the concentration of SR into PP from 0 to 30% wt. This mechanical behaviour was attributed to the poor interfacial adhesion between polymer and filler.

Table 3. Tensile Properties of PP-SR composites

Comparing all the characterization results, it is possible to note that the better composites were obtained using 10 and 20 pph of SR, related to the particle distribution and mechanical properties. Even though, the results did not display significant

changes. The composite selected was the one with 20 pph to continue the study. That is because not only is it important to obtain good mechanical properties but also that the composites acquire osteoconductive properties, which will be better if the SR content is higher.

3.2 Coupling System Evaluation

According to the results showed above, it was evident that the SR particles have a poor adhesion with the polypropylene matrix. This was expected due to the different chemical nature of SR and PP, which only allows physical interfacial interactions between them. For this reason, the attempted problem resolution involved the use of coupling techniques, in order to improve the SR-PP interactions.

Table 4 summarizes the mechanical, rheological and thermal properties obtained for the composites at 20 pph SR, which were treated with the NZ12 coupling agent at the better mixing condition (80 rpm, 180 ˚C and 3 min). As can be noted in these results,  the addition of a coupling agent did not affected in a significant way the rheology and the mechanical behaviour of the composites, which implied no lubricant effect or improvement in interfacial adhesion as a consequence of NZ12 addition. These results were different to previous ones obtained by Albano et al [2,3], where a 0.3-0.5 % addition of NZ12 at HDPE-HA composite produced a 25 % increase of Young Modulus and the stress at break, approximately. This is an evidence of the influence of the composites chemical nature, which can or not contribute to improve of the interfacial interactions.

Table 4. Effect of NZ12 content in PP-20 pph SR composites.

Concerning the thermal properties were studied (Table 5), no significant variations were obtained for both crystallization and melting temperatures. Nevertheless, a decrease in melting enthalpy with NZ12 content was observed. This could be attributed to the effect of the coupling agent on the crystal size and chains arrangements.

Table 5. Thermal behaviour of NZ12 coupled PP/SR composites.

PP/SR composites did not show enhancements in mechanical performance, when a zirconate as coupling agent was used. Therefore, the inclusion of acrylic acid (AA) as grafting agent was proposed, using additionally gamma radiation, in order to introduce functional groups in polymer chains and to promote filler-polymer interactions.

Figure 3 shows the IR spectra of materials with AA, before and after irradiation. In these spectra can be appreciated the intensity increase in the region of 1650-1750 cm^-1, corresponding to the carbonyl groups and the increase of 1190 cm^-1 characteristic to the C-O stretch, which can be attributed to the insertion of the acid into the polymer by the  radiation effects.

On the other hand, it was observed that presence of AA before irradiation did not produce variation in the PP viscosity. However, the degradation observed in the composites grafting with AA was minor, as demonstrated by the lower MFI with respect the irradiated PP without chemical modification (Figure 4). This fact, allowed inferring that the AA insertion diminish the radiation effect on the composite. This could be a consequence of insertion of polar groups, inducing a better superficial interaction between PP and SR surface [16], which could produce a higher stability of the composite.

Table 6. Acid acrylic and irradiation effect on mechanical of PP/SR composites.

an improvement in interfacial adhesion between filler and polymer that conferred rigidity to the material, even when the material was irradiated.

Thermal essays showed an important decrease in crystallization and melting temperatures, as well as melting enthalpy (Table 7), which was attributed to changes in crystalline structure due to chain scission reactions produced by the radiation effect. In the case of AA incorporation, it can be noted that the presence of these agent (grafted or not) produces crystal disruptions. This effect is more remarkable when the AA is incorporated to the PP chains due to ramifications produced during the grafting. 

Table 7. Acid acrylic and irradiation effect on thermal properties of PP/SR composites.

Additionally, thermal results allowed proving that the grafting of AA conferred radiation stability to the composite, because the effect of gamma rays (in PP in relation to the crystallization temperature decrease) is lower when this agent is incorporated to the PP, still at lower concentration.

In order to evaluate the possibility of a synergistic effect both agents coupled and grafted (0.5% NZ12, 2% AA) were studied.

As can be noted in Table 8, when NZ12 and AA are used together, there was a synergistic effect (group IV), which caused a raise in Young modulus and stress at break values, even for non irradiated materials. That means that AA is acting like coupling agent too. The slight decrease in mechanical behaviour observed in the group V was attributed to degradation of PP caused by the gamma ray exposition. ]]> Table 8. Mechanical properties of coupled composites (acrylic acid and NZ12) irradiated at 25 kGy

Regarding thermal properties (Table 9), it was found that the presence of the agents interfere the crystallization process, which was evidenced by the decreasing on crystallization temperature and melting enthalpy, compared to untreated material. Irradiated composites showed a similar tendency.

Table 9. Thermal properties of coupled composites (acrylic acid and NZ12) irradiated at 25 kGy

3. CONCLUSIONS

The mixing conditions have an important influence on filler dispersion and particles size into the polypropylene matrix.  The incorporation of SR into PP matrix did not produce an increase in the tensile modulus and hardness. It was observed a decrease in the tensile strength and elongation at break as a consequence of poor filler-matrix interaction. Additionally this addition not only confers better thermal stability, but also has a nucleating effect at 20 pph of filler content, observed in a slight increase of crystallinity.

The incorporation of a NZ12 did not show the expected coupling effect; however it was found that the grafting AA can induce improvements in the composite properties, such as Young Module, stress at break and the notable reduction of the radiation effect on the composite.

When both agents were included into the composites, a synergistic effect was observed, which produced an important increase in Young modulus as well as stress at break.

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