Microwave Enhanced Foaming of Carbon Black Filled Polypropylene

Transcription

1 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene Microwave Enhanced Foaming of Carbon Black Filled Polypropylene Aleksander Prociak 1, Tomasz Sterzynski 2, Slawomir Michalowski 1, and Jacek Andrzejewski 2 1 Department of Chemistry and Technology of Polymers, Cracow University of Technology, Cracow, Warszawska 24, Poland 2 Institute of Materials Technology, Poznan University of Technology, Poznan, Piotrowo 3, Poland Summary Microwave heating has a number of advantages in comparison to the conventional method due to the ability to heat a part of composite material directly through specific interaction of electromagnetic radiation with selected types of materials. Most thermoplastics are relatively transparent for microwave irradiation and they do not absorb microwaves to a sufficient extent to be heated. Enhanced microwave heating can result from the use of fillers such as carbon black. Different types of carbon black were used to increase the susceptibility of polypropylene (PP) for microwave processing. Measurements were carried out on PP filled with carbon black of different grades. Relative temperature rises and heat efficiency versus the content of carbon black in PP were analyzed. The temperature changes of different mass samples of these materials under microwave irradiation with various power were also investigated. Moreover, polypropylene composites with additive of chemical blowing agents were blown under microwave irradiation and the influence of foaming conditions on cell structure and selected properties of porous products were estimated. Key words: polypropylene, carbon black, microwave irradiation, foaming INTRODUCTION In order to improve selected properties of polymer materials different types of structures are prepared including composites and foams. In the preparation process, heat is needed to liquefy thermoplastic polymer and Smithers Rapra Technology, 2011 Cellular Polymers, Vol. 30, No. 4,

2 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski to cure monomers or prepolymers. The possibility of very rapid curing of heterogeneous materials and the formation of unique structure has been presented by many researchers. Radiation processing, including microwave heating, is an economical and applicable method of physical and chemical modification of polymeric materials [1-3]. The comparison of thermal and microwave cure assumes a new dimension when the temperature distribution inside the sample is considered, and that is where the scientific challenge lies. The fundamental difference in the heat transfer during the polymer processing in thermal and microwave fields is that microwave energy, in contrast to thermal heating, is supplied directly to a large volume, thus avoiding the thermal lags associated with conduction and/or convection. Consequently, temperature gradients and the excessive heat build-up during thermal processing could be reduced by a microwave power control [4]. Microwave heating has a number of advantages in comparison to the conventional method due to the ability to heat a part of composite materials directly through specific interaction of electromagnetic radiation with selected types of materials. Therefore, the various techniques that use microwave irradiation are employed for forming, foaming, welding, joining, and bonding polymers and composites [3, 5, 6]. Through proper material selection, microwaves are able to penetrate the substrate materials and cure the adhesives in situ. Selective heating with microwaves is achieved by incorporating interlayer materials that have high dielectric loss properties relative to the substrate materials [5]. It was found that the morphology of syntactic foams could be differently affected when cured under thermal and microwave conditions. Under microwave irradiation, due to faster energy transfer, more efficient crosslinking effects were observed at the interface, leading to higher intermolecular crosslinking density at the particle interface. As a consequence, slight but observable differences of mechanical behavior arise when curing the hollow-glass microsphere-filled epoxy resin composites in thermal and microwave conditions. The microwave-cured syntactic composites were found to be less ductile and a little more rigid due to a greater homogeneity [7]. Most thermoplastics are relatively transparent for microwave irradiation and they do not absorb microwaves to a sufficient extent to be heated. Enhanced microwave heating can result from the use of fillers such as carbon black, ferrites, or conducting polymers and chiral microinclusions [8]. The application of inherently conducting polymers was reported for welding thermoplastics. The conductivity of selected composites is shown in Table 1 [9]. 202 Cellular Polymers, Vol. 30, No. 4, 2011

3 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene Table 1. Substrate conductivity of selected thermoplastics filled with conducting polymers [9] Thermoplastic substrate Conducting polymer* Electrical conductivity (S/cm) Polypropylene A 1.0 ± 0.1 Polypropylene B 12.0 ± 3.0 Polypropylene C (5.7 ± 0.3)x10-2 Polyethylene PPPTS 22.0 ± 2.0 Polyethylene PAPTS 9.0 ± 1.0 Polyethylene A 1.0 ± 0.1 Polyethylene B 12.0 ± 3.0 * PPPTS - polypyrrole p-toluenesulfonate; PAPTS - polyaniline p-toluenesulfonate; A - nonwoven polyester tape impregnated with PPPTS; B - microporous polyethylene impregnated with PPPTS; C - nonwoven polyester impregnated with carbon black Different fillers such as talc, zinc oxide, and carbon black are used to increase the susceptibility of common polymers to microwave processing. The test on unfilled HDPE gave no significant temperature rise. Among these fillers carbon black was the most effective filler for imparting microwave heat ability to HDPE and the efficiency was directly proportional to its surface area [10]. Among thermoplastics the most popular materials applied for manufacturing cellular products are polyethylene, polypropylene, poly(vinyl chloride), polystyrene and polyurethane. These materials are foamed using physical and chemical blowing agents. Durable and stable at room temperature chemical blowing agents release different gases (most often nitrogen and carbon dioxide) during the chemical decomposition at higher temperatures. The choice of chemical blowing agent depends on many different factors including the type of polymeric materials and processing method [11, 12]. Microwave irradiation as an effective method of polymer heating can be applied for preparation of cellular poly(vinyl chloride) and polyurethane materials using chemical blowing agents. The type and mass of polymeric materials and chemical blowing agent have important influence on the character of foaming process and the quality of cells structure [3]. In this paper, the foaming results of the polypropylene filled with carbon black of different grades under microwave irradiation are discussed. The effects of different parameters on the ability of these composites to be heated were analyzed. Moreover, the influence of the type of carbon black fillers on apparent density and cell structure of obtained porous materials was investigated. Cellular Polymers, Vol. 30, No. 4,

4 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski EXPERIMENTAL The samples of carbon black filled polypropylene composites were prepared in the form of granulated products. Polymer used in this investigation was Moplen HP500J from BasellOrlen Polyolefins. The basic properties of this material are: density: 0.9 g/cm 3 and melt flow index (230 C/2.16 kg) 3.2 g/10 min. The first extrusion process was made to obtain the necessary compositions of PP and carbon black. This first compounding process was performed on the twin-screw extruder (Leistritz MICRO 27 GL/GG-44D), the extruded rod was quenched and pelletized. Second stage of extrusion was preceded by mixing the pellets and a blowing agent inside the chamber of cylindrical mixer. Final extrusion was made on the single screw extruder (MetalChem D22, L/D 34). Extrusion process was run as the function of barrel temperature and screw speed. The intersection of the die channel was rectangular with the dimensions of 2x20 mm. The applied temperature profiles of the extruder are shown in Table 2 and the flow parameters depending on the screw speed for the temperature profile no. 2 are mentioned in the Table 3. Table 2. Temperature profiles in extruder Profile number Zone temperatures [ C] (die) Table 3. Flow parameters for the temperature profile no 2 from Table 2 Screw speed [rpm] Mass flow [g/min] Vol. flow [cm 3 /min] Linear speed [m/min] Two types of carbon black fillers Printex 60 and Corax 339 were applied with the specific surface 115 and 92 m 2 /g respectively. Porous composite were obtained in the second extrusion process with the additive (1 wt%) of chemical blowing agent (CBA) Celogen RA (p-toluenesulfonyl semicarbazide). 204 Cellular Polymers, Vol. 30, No. 4, 2011

5 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene Molecular structure and properties of Celogen RA are shown in Table 4. Temperature of second extrusion process was lower than the decomposition temperature of the applied CBA. Such prepared materials were irradiated in the microwave reactor. Table 4. Chemical blowing agent used for foaming PP composites Name of CBA Celogen RA Molecular structure Decomposition temperature, [ C] Decomposition gases Nitrogen, Carbon monoxide, Carbon dioxide, Ammonia The samples of granulated PP composites (with carbon black fillers) in the amount of 100 g were heated in the glass batch with a diameter ca. 6 cm. Temperature of respective PP composites was measured at the different depth of 1 to 4 cm from the upper material surface using an optical temperature detector that was placed in the quartz tube. Moreover, the surface temperature of PP composites was measured using an pyrometer. The method of temperature measurement is shown in Figure 1. Figure 1. Scheme of temperature measurement of composites in microwave reactor Cellular Polymers, Vol. 30, No. 4,

6 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski Figure 2. Scheme of the set-up for extrusion and microwave enhanced foaming of PP composites Polypropylene composites with different content of various types of carbon black fillers and 1-2 wt% of Celogen RA as a chemical blowing agent were extruded in the shape of a tape using single screw extruder. Before cooling the tape of each PP composite was pull broached through the microwave cavity, where the material was additionally heated and the blowing agent was decomposed. The scheme of the preparation method of porous PP composites is shown in Figure 2. The foamed materials were conditioned at 22 C and 50% relative humidity for 24 hours, before being cut to analyze the cell structure and measure their physical properties. The slices of PP foam samples were cut (perpendicularly to the extrusion direction) using a microtome. Several photos of each PP foam structure were taken (using optical and scanning electron microscopy) in order to estimate the cell parameters (cross-section area and anisotropy index of cells). The foam morphology was analyzed in each photo using the same procedure of Aphelion software. The anisotropy index expresses the circularity of the cells. It is set as the ratio of the height and width of the rectangular, in which a cell is inscribed. The apparent density (kg/m 3 ) was measured in accordance with ISO Standard tests: ISO 845. Results and discussion The ability of carbon black filled polypropylene to heat under microwave irradiation was confirmed carrying out the trials in the microwave reactor NOVA 204. The heating effects of the samples with the power of 500W in the granulate form are shown in Figure 3. The measurements of temperature 206 Cellular Polymers, Vol. 30, No. 4, 2011

7 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene Figure 3. Temperature of the sample at 1 cm depth from surface vs. time of microwave heating with power 500W. PP3P, PP6P, PP3C and PP6C polypropylene with 3 or 6 wt% of carbon black Printex or Corax respectively profile shows that the samples with Corax 339 most effectively absorbs the microwave irradiation, especially with 6 wt% of this carbon black. Considerable gradient of temperature was observed between the core and surface of samples. The temperature measurements at different depths of investigated samples were realized and the results are shown in Figure 4. Such effects take place due to the low heat capacity of polypropylene and the heat transfer from the irradiated PP composite to the sample surroundings that is not heated by microwaves. The temperature gradient in PP composites was observed to be higher as the consequence of temperature increasing. However, the gradient significantly depends on the type and content of carbon black fillers in PP composites. The changes of temperature gradient in various PP composites are reflected by the changes of the standard deviation of average temperature that was calculated on the base of the temperature values measured at different depths from sample surface (Figure 5). Firstly, the samples of PP filled with deferent type and content of carbon black were extruded and blown under microwave irradiation using 2 wt% of Celogen RA. Influence of the type and content of carbon black fillers on the apparent density and cell structure of obtained products is shown in Table 5 and Figure 6. The foaming process of each PP composite was carried out Cellular Polymers, Vol. 30, No. 4,

8 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski Figure 4. Temperature of the sample (PP with 3 wt% of Corax 339) at different depth from surface vs. time of microwave heating with power 500W Figure 5. Standard deviation of average value of sample temperature measured at different depths (1, 2, 3 and 4 cm) from surface. PP3P, PP6P, PP3C and PP6C polypropylene with 3 or 6 wt% of carbon black Printex or Corax respectively. Microwave power - 500W 208 Cellular Polymers, Vol. 30, No. 4, 2011

9 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene otherwise. The most effective heating of the PPC6 sample as a consequence of microwave absorbing gave the benefits like the lowest apparent density and fine cells of this PP composite. The apparent density of PP6C samples is nearly twice lower and the cells are significantly finer than in the case of PP6P composites. Although, the applied carbon black fillers have similar specific surface and the extrusion conditions were the same. The shape of cells in all porous PP composites was generally spherical what is confirmed by the values of anisotropy index in the range of The cells were also observed close to the sample surface, however they were frequently smaller in comparison to the cells in the composite core. In the case of the composites extruded without application of microwave irradiation cells were found only occasionally. However, this is evidence that during extrusion a part of blowing agent can be decomposed, although the decomposition temperature is considerably higher than the extrusion temperature. In the next stage of investigation the influence of extrusion conditions on the cell structure of blown PP composites was analysed. Extrusion conditions and the parameters of cell structure are presented in Table 6. Table 5. Structure parameters and apparent density of PP composites blown with Celogen RA under microwave irradiation Sample symbol Type of carbon black Average surface area [mm 2 ]x10-3 Anisotropy index Apparent density [kg/m 3 ] PP3P Printex PP6P Printex PP3C Corax PP6C Corax (a) (b) Figure 6. Microscopic images of cell structure in sample core of carbon black filled PP: (a) PP - Corax 3 wt%, (b) PP - Corax 6 wt% Cellular Polymers, Vol. 30, No. 4,

10 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski Table 6. Structure parameters of PP composites with carbon black Corax 339 as the effect of different extrusion conditions Sample symbol Die temperature [ o C] Extrusion output [g/min.] Average surface area [mm 2 ]x10-3 Anisotropy index PP3C PP3C PP3C PP3C PP3C PP6C PP6C PP6C PP6C PP6C The samples were prepared using Celogen RA (1 wt%), different content of Corax 339 and applying various die temperature and extrusion outputs. SEM images of cell structure of sample with 3 and 6 wt% content of carbon black (extrusion output 9.43 g/min, die temperature 180 C) are shown in Figure 7. Higher content of carbon black in the PP composite has caused better microwave absorption and higher efficiency of foaming what is reflected by higher number of cells and lower apparent density of sample PP6C (587 kg/ m 3 ) in comparison to sample PP3C (808 kg/m 3 ). Considerable effects of die temperature on the cell structure of PP6C composites extruded with the same output (9.51 g/min) are shown in Figure 8. The increase of die temperature from 170 to 190 o C allowed to obtain the product with higher number of smaller cells. (a) (b) Figure 7. SEM images of cell structure of sample with different content of carbon black (extrusion output 9.43 g/min., die temperature 180 C): (a) PP3C, (b) PP6C 210 Cellular Polymers, Vol. 30, No. 4, 2011

11 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene The impact of extrusion output on the number and size of cells in the samples of PP6C composites is shown in Table 6 and Figure 9. (a) (b) (c) Figure 8. SEM images of cell structure of PP6C samples obtained with extrusion output 9.51 g/min., applying different die temperature: (a) 170 C, (b) 180 C, (c) 190 C (a) (b) (c) Figure 9. Microscopic images of cell structure of PP6C samples obtained with different extrusion output (die temperature 190 C): (a) 9.51 g/min., (b) g/min., (c) g/min Cellular Polymers, Vol. 30, No. 4,

12 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski The increase of extrusion output significantly affected the cell number and size causing the joint of small cells into larger ones. However, the apparent density of the materials obtained with extrusion outputs 9.51 and g/min. were similar and lower than 600 kg/m 3, while the increasing extrusion output to g/min gave the samples with apparent density above 700 kg/m 3. Conclusions Microwave irradiation is an effective method of polymer heating that can be applied for the preparation of cellular thermoplastic materials using chemical blowing agents. The type and content of the filler capable to absorb microwave irradiation play important role in the foaming process and considerably influence the cells structure and physical properties of prepared composites. In the carbon black filled polypropylene, so called hot-spots effects are observed what can be reflected by dependence of the temperature value on the measurement position. Apparent density and cell structure of carbon black filled polypropylene composites significantly depend on microwave power as well on the temperature and extrusion output. In the future, the application of microwave irradiation for foaming of carbon black filled polypropylene may allow to design new porous composites with tailored properties. ACKNOWLEDGMENT This work was financed in the frame of science support in the years as the Research Project No. N N References 1. Bogdal D. and Prociak A., Microwave-Enhanced Polymer Chemistry and Technology, Ames, USA (2007). 2. Prociak A. and Lasoń M., Global Congress on Microwave Energy Applications, 197, Otsu, Japan (2008). 3. Prociak A., Sterzynski T., Bogdal D., Michalowski S., and Safian D., The 12th International Conference Blowing Agents and Foaming Processes 2010, Paper 3 (on CD), Cologne, Germany (2010). 212 Cellular Polymers, Vol. 30, No. 4, 2011

13 Microwave Enhanced Foaming of Carbon Black Filled Polypropylene 4. Mijovic J., Corso W.V., Nicolais L., and d Ambrosio G., Polymers for Advanced Technologies, 9 (1998) Thostenson E.T. and Chou T.-W., American Society of Mechanical Engineers, 58 (1999) Bogdal D., Penczek P., Pielichowski J., and Prociak A., Advances in Polymer Science, 163 (2003), Palumbo M. and Tempesti, E., Acta Polymerica, 49 (1998) Bogdal D., Prociak A., and Michalowski S., Current Organic Chemistry, 15 (2011) Kathirgamanathan P., Polymer, 34 (1993) Harper J.F. and Price D.M., 10th International Conference on Microwave and High Frequency Heating, 298, Modena, Italy (2005). 11. Lee S.T. and Ramesh N.S., Polymeric Foams, Mechanisms and Materials, Boca Raton, USA (2002). 12. Gendron R., Thermoplastic Foam Processing, Principles and Development, Boca Raton, USA (2005). Cellular Polymers, Vol. 30, No. 4,

14 Aleksander Prociak, Tomasz Sterzynski, Slawomir Michalowski, and Jacek Andrzejewski 214 Cellular Polymers, Vol. 30, No. 4, 2011