Investigation of Foaming Behavior of Thermoplastic Polyolefin (TPO) Blend. Ungyeong Peter Jung

Transcription

1 Investigation of Foaming Behavior of Thermoplastic Polyolefin (TPO) Blend Ungyeong Peter Jung A thesis submitted in partial fulfillment of the requirements for the degree of BACHELOR OF APPLIED SCIENCE Supervisor: C. B. Park Department of Mechanical and Industrial Engineering University of Toronto March, 2007

2 Abstract Thermoplastic Polyolefin (TPO) blend is a mixed material of a thermoplastic and an elastomer by various types of foaming technologies. Although it has the inherent advantages from both thermoplastic and elastomer such as higher impact strength, lighter material weight, better manufactuability, and better recyclability, its morphology and foaming behavior have not been extensively studied yet. The main reason is because it is a multi-phase system, which consists of dispersed elastomer particles in continuous polymer matrix. In case of foaming TPO blend, a foaming agent (or blowing agent) is involved and it makes the multi-phase system of TPO to form more complex system. Thus, this research project focuses on foaming behavior of TPO blends depends on various compounding equipment settings, weight compositions, and talc contents.

3 i Acknowledgements I would like to acknowledge my supervisor Prof. Park for all his supports and excellent supervisions, and also my immediate supervisor, Ryan S.G. Kim, who has been a great mentor and spent a great number of hours to help me throughout this research project.

4 ii Table of Contents Acknowledgement List of Figures List of Tables i iii iv 1. Introduction 1 2. Literature Review Plastic Foaming Previous Studies 4 3. Statement of Objectives 7 4. Hypotheses 8 5. Methods and Approaches Major experimental apparatus and equipments Foaming behavior of TPO blends with different weight compositions Foaming characteristics of TPO blends with different compounding methods Changes of foaming behavior of TPO blends with various talc contents Results and Discussion Foaming behavior of TPO blends with different weight compositions Foaming characteristics of TPO blends with different compounding methods Changes of foaming behavior of TPO blends with various talc contents Conclusion Figures and Tables References 32 Appendix A 34

5 iii List of Figures 1. Schematic diagram of a single screw extrusion injection foaming system 2. Schematic diagram of foaming simulation system 3. Picture of actual batch foaming system 4. Picture of batch mixer system 5. Schematic diagram of twin-screw extruder 6. SEM images of TPO blend samples with 5000x of magnification 7. Serial images of foaming process of samples with N 2 blowing agent 8. Cell density of samples against foaming process time 9. Linear relationship between cell density and number density of elastomer 10. SEM images of TPO70 samples with different compounding methods in 2000x magnification 11. Serial images of foaming process of TPO70 sample with different compounding methods 12. Cell density of samples against foaming process time 13. Linear relationship between cell density and number density of elastomer 14. Serial Images of TPO foaming with N 2 with increasing the talc content at the high concentration (~1.5 wt%) of N Effect of talc content on cell density at the high concentration (~1.5 wt%) of N 2 (P sat =2000 psi, 180 C, dp/dt=35 MPa/s) 16. Serial Images of TPO foaming with N 2 with increasing the talc content at the low concentration (~0.4 wt%) of N Effect of talc content on cell density at the low concentration (~0.4 wt%) of N 2 (P sat =500 psi, 180 C, dp/dt=10 MPa/s)

6 iv List of Tables 1. Classification of foamed polymers based on cell sizes and cell densities 2. Different weight compositions of TPO samples 3. Different weight compositions of TPO blends with talc content

7 1. Introduction In general, TPO blends are the physical mix of polypropylene (PP) with elastomer. Polypropylene is one of the most versatile commodity polymers because of its excellent chemical and moisture resistance, superior ductility and stiffness, and fine manufacturability. Due to its poor impact strength at low temperature, however, its applications are often limited [1]. This is the main motivation of TPO blends, so the purpose of blends is to improve the impact strength of polypropylene by mixing with elastomeric modifiers. The typically used elastomers for TPO blends are Ethylene- -olefin copolymer (EOC), Ethylene-propylene copolymer (EPR), and Ethylene-propylene diene monomer (EPDM). The properties of TPO blends are typically in between those of current plastic polymers and thermo-set rubbers because the blends are consist of continuous matrix of polyolefin and dispersed phase of elastomer. The availability of TPO formulations containing metallocene catalyzed polyolefin elastomers (EOCs) has recently provided a cost effective way to balance material performance [2]. In the current field of industry, the TPO blends can be usually divided into two categories; one is a simple blend of thermoplastic polyolefin and another is a vulcanized TPO or thermoplastic Vulcanizate (TPV), which has vulcanized rubber phase. The usage of TPO blends has been increasing recently due to the following characteristics. First, fewer manufacturing steps are required to process TPO blends, which leads to less energy consumption. Second, the cycle time is much shorter than that of thermoset rubber, typically measured in seconds whereas the cycle time of thermo-set rubber is measured in minutes. Third, TPO can be reprocessed due to its inherent thermoplastic properties [3]. Thus, because of these characteristics, TPO materials have been utilized for many different applications, especially in the automotive industry [4]. They are typically

8 2 used for soft, durable, exterior body parts to replace sheet metal: for example, bumper fascia, exterior trim, door trim and instrumental panel. In most cases, plastic foaming process is used to manufacture TPO blends. Plastic foaming process is a process to create polymer with foams by utilizing foaming agent, which is usually an inert gas. This process will be further discussed in details in the literature review section. Although many scholars recently have studied TPO blends and plastic foaming technology, there has not been an extensive research on the foaming behavior of TPO blends because most of them treated those two subjects separately. Therefore, the main focus of this research is to study chemical and physical characteristics of TPO blends in relation to the foaming process. 2. Literature Review 2.1. Plastic Foaming Since TPO blends are typically generated by plastic foaming technology, this section explains the process in details. Plastic foams are simply polymers with presence of cells or bubbles throughout the material. This cellular structure can be foamed with foaming agents (or blowing agents) such as volatile liquids, inert gas, or chemical compounds to generate gases during the process. This foamed polymer material is able to provide wide range of material density, which is from g/cm 3 to 0.96 g/cm 3 [5]. The foamed polymers are typically categorized to three based on their cell sizes and cell densities as it is shown in Table 1 in Figures and Tables chapter [6]. Since the foaming technology inserts cells in the foamed structure, this can reduce the material weight significantly. This directly leads to save the material cost, which is often

9 3 treated as one of the most critical parts of manufacturing cost. In addition, foamed polymer structures have an excellent strength-to-weight ratio, superior insulating abilities, energy absorbing capability against shock, vibration, and sound [6]. In the last few years, many studies have been done on reducing the size of cell and improving its uniformity because the foamed polymers with smaller and uniformly distributed cells exhibit better mechanical and thermal properties [7]. Plastic foam structures can be produced by using the thermodynamic instability characteristics of a polymer/gas solution to endorse high cell density in the polymer matrix [8]. The microcellular polymer foaming process consists of four basic steps, which are polymer/gas solution formation, micro-cell nucleation, cell growth, and stabilization. The formation of a polymer/gas solution is typically accomplished by dissolving an inert gas such as CO 2 and N 2 into a polymer matrix under high pressure, and this process creates a foamed structure with high gas concentration. During the micro-cell nucleation, it is required to reduce the gas solubility in the solution via a temperature and/or pressure control. The cell nucleation process governs not only the cell morphology of material, but also the properties of foamed material. After the cell nuclei are formed, they have tendency to grow and reduce the total polymer density as the gas in the polymer matrix continues to diffuse into the formed cells. This is cell growth process, which is very important to achieve the desired foam density. The growth rate of cells depends on the properties such as diffusion rate of gas, stiffness of the polymer/gas solution, availability of gas inside the solution, and the time allowed for cells to grow [9].

10 Previous Studies In this section, the past studies related to this research are presented in the chronological order to review how the technology has been improved and what efforts have been made so far to achieve current technology. The first extensive study for foaming TPO blends was conducted in 1992 by Dutta and Cakmak [10]. They have investigated the foaming process of an olefinic thermoplastic elastomer; consisting of polypropylene and ethylene-propylenediene-terpolymer. They also have used the chemical blowing agents that release N 2 gas. It was observed that the cell structures could be varied as the blend composition changes. As the concentration of elastomer increased, the cells changed their shapes from spherical to highly elongated oval shapes and the shapes were very inconsistent. They also have noticed that the foam structure was non-homogeneous, and a wide range of cell size distribution was also obtained. The investigation of foaming technology, which involves a chemical blowing agent (CBA), was conducted by Wang et al [11, 12]. The main applications for these foams are for seals, doors, windows, and hoods in the automotive applications. The advantage of using CBA is that it does not require a special foaming extrusion system. However, it is usually expensive so it increases the TPO manufacturing cost from additional 10% to 15%. In addition, CBA-blown TPO foams usually have an open-celled structure that can lead to more water absorption that is not favorable for the sealing application due to unexpected reactions during decomposition of CBA and foaming. They recommended applying high head pressure and low die temperature to prevent premature foaming of the polymer inside of extruder. The lowest density can be achieved by higher temperature and lower screw speed. Although the foams were evaluated in the terms of hardness, cell size, cell size distribution, and foam

11 5 density, however, they did not describe how material characteristics of TPO blends affect their foaming behaviors. New TPO foaming technology was introduced by Sahnoune, which uses water as a physical blowing agent (PBA) [13, 14]. The water foaming process was able to achieve the foam density as low as 0.15 g/cm 3 ; however, the size of cell was relatively large that is approximately from 200 μm to 300 μm. The bubbles have shown the tendency to join rather than to disperse at a certain water level, which makes it impossible to reduce the weight further. It was also noticed that the cell density becomes higher as the size of die becomes smaller. However, he did not provide any extensive study on effects of elastomer particle size and distribution throughout the TPO blends. Spatiael et al. studied the foaming behavior of several TPO formulations containing various amounts of branched polypropylene resin with water as blowing agent, whereas the extensional viscosity of the materials with different formulations was not measured and considered [15]. They indicated that the replacement of a small amount of linear polypropylene with branched polypropylene enhanced the density of foam and cell structure. As the concentration of branched polypropylene was increased, however, it was realized that the increased concentration reduces its foam-ability. They believed that this was led by strong extensional strain hardening, and there is an optimal amount of branched content. Yet, they did not extend their study to find an optimized composition. Thus, it is required to find an optimal formulation to analyze the foaming behavior of TPO blends more accurately. Kropp et al. researched foaming behavior of three types of TPE materials with CO 2 as their blowing agent and hydrocerol as a nucleating agent [16]. Those TPE materials were thermoplastic polyurethane (TPU), a styrene based TPE (SEBS), and a PP/EPDM TPO blend.

12 6 They have observed that as the amount of nucleating agent was increased, the density of foam was decreased whereas the cell density was increased. In other words, more active nuclei provided more growing cells simultaneously. They have noticed that the foaming process of PP/EPDM TPO blend was the most difficult to control and as a result, the foam structure was not homogeneous. In their study, they did not include any observation or analysis on the effect on cell structure and foaming property parameters of TPO blends. However, they indeed recommended a useful information regarding the foaming process of TPO blends such as proper foaming temperature and CO 2 concentration for each studied material. Lee et al. researched the extrusion foaming of PS/HDPE blends with using CO 2 as the blowing agent, and observed that the addition of CO 2 decreased the size of dispersed phase of elastomer [17]. They also concluded that the cell morphology of the foam was directly related to that of blend phase, which depends on process environments. In their study, however, they did not conduct any experiment with TPO blends although they are typical immiscible polymer-elastomer blends and their morphology can be varied by the processing conditions. Therefore, it is required to study how blend phase morphology can affect the cell morphology of TPO blends. Park et al. found that completing dissolution of the blowing agent in the molten polymer is the most critical stage in microcellular foam processing [8]. The parameters that govern this stage are solubility of the blowing agent, the saturation pressure, the degree of mixing, and the residence time [5]. In other words, the entire injected blowing agent into molten polymer can be dissolved in the polymer matrix as long as the amount of blowing agent is below the solubility limit and enough residence time with good mixing is provided. Although

13 7 many researches have been completed on the solubility and diffusivity of CO 2 and N 2 in a single phase system (i.e. homo polymers), their solubility and diffusivity in TPO blends have not been studied yet in the literatures. The development of optimal formulation of TPO blends, which is suitable for the current polymer foaming process technique, is a very interesting topic because it can open up more possibilities in the high-volume market such as automotive industry. TPO foams have not been utilized in the industry yet because foaming TPO with extrusion molding has not been developed and there has not been extensive research directly on foaming of TPO blends. This was also contributed by the fact that TPO blends are relatively new. At last, the main and more fundamental reason for the lack of studies on foaming of TPO blends is because they are multi-phase system structures. Since TPO blends have more complex microscopic structures, it is very difficult to control their foaming process. Thus, it is necessary to obtain more knowledge of foaming behavior of TPO blends and how they are different comparing to those of currently used thermoplastics. Furthermore, it is critical to establish the structure, characteristics, and processing condition parameters that can produce a good quality of TPO blends. 3. Statement of Objectives This research project was largely aimed to improve understanding of foaming behavior of TPO blends and to find an optimal processing condition. Among various aspects of foaming behavior, however, this research project focused on three specific aspects that are critical and have not been studied previously. First, it is to observe how foaming behavior of the blends reacts as their weight composition varies. Second, it is to analyze how the foam can be

14 8 changed according to various compounding equipment settings. Third, it is to study foaming behavior reacts to talc content changes. 4. Hypothesis 4.1. TPO blends compositions strongly affect the cell nucleation during process In general, the characteristics of polymer blends are greatly rely on the composition of blends. Especially, the cell nucleation behaviors in TPO materials strongly depend on their weight compositions between polypropylene and elastomer because of the heterogeneous nucleation scheme. Therefore, the different weight compositions will show different bubble nucleation results The foaming behavior pattern of TPO blends changes based on their compounding methods. The previous studies have been performed with their own compounding methods, so there is no specific research that has investigated foaming behavior of TPO blends with different compounding methods. In order to find an optimal formulation and processing conditions of the blends, its compounding method should be optimized as well. Although the identical weight composition of the experimented blend will be used throughout the experiment, the foaming behaviors will be different because the principle behind each compounding method varies.

15 Talc particles provide a higher number of nucleating sites in TPO matrix, so more cells are created and the average size of cells becomes smaller. There are two types of nucleation, which are homogeneous and heterogeneous nucleation. Homogeneous nucleation is resulted by thermodynamic instabilities while heterogeneous nucleation takes place at an interface between the polymer and the phase of another material. Since TPO blends are multi-phase system, the heterogeneous nucleation is occurred in the blend. Nucleating agents play very critical roles to control the morphology of cells being formed during the foaming process. They are usually finely powdered to serve as solid surfaces for heterogeneous cell nucleation, and remain thermally stable during the process. Hence, the condition for heterogeneous nucleation depends on the surface geometry of nucleating particles and surface energy. The heterogeneous nucleation on the solid phase is occurred when the free energy for gas cluster formation on the surface of a nucleating agent is less than that of homogeneous nucleation in polymer melt. Therefore, the presence of an efficient nucleating agent will facilitate the production of a larger number of smaller cells. Talc (hydrous silicate of magnesium) is one of the blowing agents that are typically used for polymer foaming process. This blowing gas is adsorbed on the surface of the particulate during phase separation, and the accumulated gas becomes an embryo that is an unstable intermediate state. If the size of embryo exceeds the critical size to survive, a cell will be formed. This leads to a finer cell structure than simply blowing a part without any nucleating agent; however, it is difficult to utilize the nucleating agents for microcellular structures because of their size.

16 10 5. Method and Approach This section of report is divided into three sub-sections based on three separate experiments that have been performed over the course of project. They were to observe and analyze foaming characteristics of TPO blends with various weight compositions, compounding methods, and talc content. Each sub-section will discuss details of each independent experiment including theoretical backgrounds, experimental apparatus and equipments, and experimental procedures Major experimental apparatus and equipments Prior to discuss details for each experiment analysis, it is required to explain of several experimental equipments that have been used for all three experiments. First equipment is a single-screw extrusion foaming system. It consists of a single-screw extruder, gas-injection equipment, a gear pump, a dissolution-enhancing device, two heat exchangers, a filament die, and a cooling sleeve. The gear pump controls the flow rate of molten polymer and is independent from temperature and pressure changes. The dissolution-enhancing device guarantees the homogeneity of polymer/gas solution. The heat exchangers are to provide uniform cooling to the polymer. In the filament die, the polymer is shaped and experiences cell nucleation while the cooling sleeve governs the die temperature accurately. Figure 1 in Figures and Tables chapter shows the brief diagram of this extrusion injection foaming system. Second equipment is a foaming simulation system, and it was to observe early stages of foaming process such as cell nucleation and growth. The system is explained in details in the next section of first experiment. Third apparatus is a scanning electron microscope (SEM),

17 11 which was used to visually observe cell density of foamed sample. The particular system used throughout the project was a JEOL 6400 SEM system. It was also used to measure the cell sizes and analyze the distribution of cell. For SEM system, an opening voltage of 10 kv, and a magnification of 2000X and 5000X were used. Lastly, a micro-scale was used to measure the weight and material density of foamed sample Foaming behavior of TPO blends with different weight compositions Introduction The objective of this experiment was to evaluate the foamability of TPO blends with different compositions of elastomer and optimize its composition to maximize its foamability and mechanical performance. The evaluation categories were cell nucleation and growth. This analysis was performed with using the foaming simulation system because it can provide more accurate control on the foaming process condition parameters. In order to observe how the TPO blend behaves with the changes of actual process parameters, it was required to develop an optimal formulation of TPO blends independent from the actual foaming process. The simulation system was designed to accommodate higher-pressure drop rates (up to 2.5 GPa/s) and higher frame rates (up to 120,000 frames/s) to capture the foaming behavior of polymer [18]. The high-pressure drop rate in the high-pressuretemperature chamber of the system was measured experimentally and simulated in theoretical terms using TPO blends. The simulation system is a batch foaming device that can record the data of cell nucleation and cell growth behaviors during the foaming process. In addition, the system enables a user to visually observe the cell nucleation and its growth.

18 Experimental apparatus and equipments Figure 2 illustrates the schematic diagram of foaming simulation system, and figure 3 shows the picture of actual system itself. The system consists of a high-pressure - temperature chamber, a pressure drop rate control system, a data acquisition system (ACAD) for pressure measurements, a gas supplier system, an objective lens, a light source, and a high-speed CCD camera. A highly sensitive camera with a high frame rate (Photron USA Inc., model: Ultima Apx Fastcam) was selected to record the rapid foaming process. Since the size of cell polymer is very small, which is less than 10 m, it was required to have optical lenses with low and high magnifications, which can provide 27.5 times and 125 times magnifications respectively. Software called Labview (National Instruments, Inc) was used to control the pressure drop rate and obtain the data of pressure decay and the foaming images. N 2 was used as a blowing agent throughout the experiment, and it was supplied by BOC Canada. In terms of material, PP7805 (Polypropylene) was used as a polymer that was supplied by Exxon while Engage 8130, a metallocene-catalyzed ethylene-octene copolymer (POE), was used as an elastomer that was supplied by Dow Chemical. The melt flow rate of PP7805 is 80g/10min, and its material density of 0.91g/cm 3. Meanwhile, the melt flow rate of Engage 8130 is 13g/10min and it has the material density of 0.863g/cm 3. The temperature of extrusion was 200 C, and the TPO blends with different weight compositions were prepared and classified according to the weight percentage of polypropylene. The samples are shown in Table 2.

19 Experimental procedure For each type of foam sample, the pressure decay and foaming images were obtained from the computer with the following procedure. 1. Pump a gas at a given pressure into the high-pressure-temperature chamber using a syringe pump 2. Adjust the temperature of gas inside the chamber with using a thermostat 3. Open the solenoid valve and record the pressure decay 4. Analyze the pressure drop rate based on the recorded pressure decay data and evaluate cell nucleation and growth behaviors using Image Pro-plus software After the foaming images were obtained, the samples were cryogenically fractured with using liquid nitrogen in order to take images from the SEM system. Then, the several aspects of morphology of samples were analyzed and measured Theoretical background The images captured with the high-speed CCD camera were analyzed to calculate the number of cells observed from the images taken at a given moment in time with equation 1 in Appendix A. In order to use this equation, it is required to calculate the expansion ratio first with using equation 2. In the equation 2, the volume-average radius of all counted cells at a given moment can be defined by equation 3. Thus, the cell density can be calculated with using the volume-average radius, and this is shown in equation 4. The cell nucleation of TPO blends can be treated as heterogeneous foam nucleation since TPO blends consist of polymer phase and elastomer additives. The efficiency of producing bubbles depends on the several characteristics such as type and shape of nucleating particle

20 14 and the interfacial tension of solid and solid-gas interface. The presence of small particles and bubbles can reduce the required activation energy for cell nucleation. This is the reason why uniformly distributed particles are critical to the foaming behavior of TPO blends Foaming characteristics of TPO blends with different compounding methods Introduction The goal of this experiment is to observe how foaming characteristics of TPO blends are changed as their compounding techniques are varied. The identical evaluation categories and foaming simulation system were implemented from the previous experiment in order to isolate the compounding technology among other processing condition variables. The samples were mixed by three different types of compounding equipments such as a batch mixer, a lab-scale twin-screw extruder, and an industry-scale twin-screw extruder Experimental apparatus and equipments Throughout this experiment, TPO70 sample was commonly used for all three mixing methods. Figure 4 shows the actual picture of a batch mixer, while Figure 5 describes the schematic diagram of twin-screw extruder system. Although two twin-screw extruders were operated with the same fundamental principles, the size and other processing parameters were different. For the lab-scale extruder, it consists of 10mm diameter of screws with the length to diameter ratio of 10 to 1, and its screw speed is 70rpm. On the other hand, the industry-scale extruder has 30mm diameter screws with the length to diameter ratio of 38 to 1. Its screw speed is 250rpm, which is approximately four times faster than that of lab-scale extruder. In addition, the size of industry-scale is significantly larger than the lab-scale

21 15 extruder. The identical N 2 was used as the foaming agent for this experiment as well Experimental procedure In the beginning, TPO70 samples were foamed by using each compounding methods. Then, the samples have been gone through the identical procedure for foaming simulation as the previous experiment in order to obtain the simulation results. Lastly, the morphology of each TPO foam samples was observed and analyzed based on its SEM images of cryogenically fractured surface Theoretical background The same theoretical background from the previous experiment was applied to analyze the morphology of samples since this experiment utilized the identical evaluation criteria Changes of foaming behavior of TPO blends with various talc contents Introduction This experiment focuses on analyzing the foaming behavior of TPO blends depends on the amount of talc blended within the blends. As it was mentioned previously, the presence of talc helps foamed polymer to have a finer cell structure, which means a larger number of smaller cells. Since the cell morphology is a critical characteristic of foamed TPO that directly affects its material density, it is quite necessary to study how the cell morphology changes with the respect of the amount of talc.

22 Experimental apparatus and equipments In terms of material composition of sample, the same polypropylene and elastomer were used, but new weight compositions were introduced for this experiment only. The new weight compositions were experimented to accommodate different amount of talc within TPO blends. The samples are listed in Table 3. The talc was JetFil700C supplied by Luzenec with the material density of 2.8g/cm 3 and an average particle size of 1.5 µm. The same blowing agent, N 2, was used as two previous experiments. The industry-scale twin-extruder was implemented as the compounding method and the same foaming simulation system was employed to observe early stages of cell nucleation and growth Experimental procedure The identical experimental procedure was adopted as the first experiment, which was observing the foaming behavior of blends with different weight compositions with the foaming simulation system. The schematic diagram of system is described in figure Theoretical background The theoretical background from the first experiment was implemented for this experiment as well. 6. Results and Discussion This section of report describes the results and analyzes them from each experiment. Then, their significance was interpreted and evaluated based on the predetermined categories. Three sub-sections illustrate the findings of three experiments independently. A number of

23 17 figures and tables were employed and they can be referred to the figures and tables chapter later on Foaming behavior of TPO blends with different weight compositions Figure 6 shows the SEM images of cryogenically fractured and etched surfaces of three TPO samples. As it is indicated in the figure, the dark (black) parts are POE elastomer particles and the gray parts are continuous matrix of PP. The pictures clearly show that the number and size of elastomer particles are increase as the weight composition of elastomer increases. In addition, it is observed that the particles are more uniformly distributed as the amount of elastomer increases. Figure 7 shows the images from the foaming simulation system for the samples including pure PP material and pure POE material. Since pure PP has a higher solubility than pure POE, its foaming process produced more cell nucleation than the pure POE. More importantly, it is noticed that the TPO blends produce more nucleated cells than the pure material samples. Furthermore, the cell nucleation activity increases as the content of POE increases based on the images as shown in figure 8. Based on figure 9, the cell density always has a linear relationship with the number density of elastomer. Thus, it is noticed that the dispersed elastomer particles act as preferential nucleation locations in the continuous PP matrix, which eventually results a higher number of cells created in the TPO foam samples Foaming characteristics of TPO blends with different compounding methods There are three different types of compounding methods that were evaluated in this experiment. The TPO70 weight composition was commonly used for all of three methods to

24 18 analyze the results comparatively. The three technologies are using a simple batch mixer, a lab-scale twin-screw extruder, and an industrial-scale twin-screw extruder. As it is shown in the SEM images of figure 10, the twin-screw extruders generated more uniformly distributed cells than the batch mixer. In addition, the sizes of cells are significantly smaller than those from the batch mixer. Since uniform distribution of dispersed elastomer particles ensures the homogeneity of TPO blends, it is realized that the twin-extruder type, especially the industrial-scale extruder, is more novel technology to compound TPO blends to achieve better foam-ability. The industrial-scale extruder produced the narrower dispersed elastomer particles in smaller sizes, and this was mainly because the industrial-scale extruder was able to provide greater shear energy with a larger screw diameter and longer residence time with greater L/D ratio. The foaming process images of samples are shown in figure 11. It is visually shown that the industrial-scale extruder yielded more bubble nucleation activities, and this agrees with the results of morphology analysis. The foam-ability of TPO blends is strongly influenced by their interfacial structure between polymer and elastomer because poorly bonded interfacial regions require lower activation energy for bubble nucleation [19]. Therefore, the smaller-sized and uniformly distributed elastomer particles are preferred for TPO foaming structure in order to achieve more homogeneous material as it is shown in the figure 12. In addition, the figure 13 shows the cell density is linearly proportional to the number density of elastomer Changes of foaming behavior of TPO blends with various talc contents Figure 14 shows the foaming simulation images of the TPO-talc blends with the respect of time. The images illustrate the dynamic behavior of cell nucleation and growth in the very

25 19 early stage of physical foaming process. In addition, figure 15 shows both the cell density change against time and the maximum cell density with increasing talc content. All the pictures clearly indicate that bubble nucleation and growth occur simultaneously. The cell density curve shows a sigmoid function shape in all experiments; having an induction time, the nucleation starts and the number of cell increases. During this growth process, the nucleation activity dramatically increases, but it ceases at a certain moment. Then, the cell density curve reaches a steady state, and this is called the maximum cell density. However, the maximum cell density might not be equal to the cell density of the final foam product because of the cell coalescence. Although it was observed that the cell density increases as the talc content increases, the effect of talc content was not significant in the range of N 2 content, which is approximately 1.6wt%, and pressure drop rate of 35MPa used for this experiment. The difference between 0% and 1% was really insignificant, but there was a noticeable difference from 1% to 10% in terms of cell density. Based on this observation, it is realized that the sensitivity of cell density with respect to the talc content was not high in case of high concentration of N 2 because the homogeneous nucleation by a dissolved gas into polymer matrix was dominant comparing to the heterogeneous nucleation by the talc particles. Figure 16 describes the foaming simulation images of TPO-talc blends in case of lower concentration of N 2 gas, which is 0.4 wt%. The saturation pressure was 500 psi and the pressure drop rate was 10MPa/s. According to the pictures, the cell density increases significantly as the talc content increases. Thus, it is realized that not only homogeneous nucleation is occurred with the presence of blowing agent, but also heterogeneous nucleation is occurred actively with the help of talc. This indicates that the addition of talc was very

26 20 effective for nucleation when using N 2 as a blowing agent with only low concentration. Furthermore, the speed of cell nucleation and growth was improved as the talc content was increased. Consequently, it can be concluded that the cell density and nucleation were strongly affected by both N 2 and talc content in case of low concentration of N 2. When either one of N 2 and talc content was significantly higher than another, the cell nucleation activity was governed by the higher concentration agent regardless of the content of another. This observation is consistent with the conclusion drawn by Park et al [20], which was the effect of talc was not observed due to the large amount of gas dissolved into the polymer matrix. Based on the research from Park et al [20], an increase of talc content above 10 wt% did not increase the cell density. The excessive talc content might lead to an agglomeration of nucleated cells, which is not an optimal result. Figure 17 shows both the cell density change against time and the maximum cell density with increasing talc content. 7. Conclusion In this research project, the main objective was to improve the understanding and knowledge of foaming behavior of TPO blends in various aspects. In order to achieve this goal, the research was carried out to analyze the foaming behavior with three different aspects. First experiment was performed with using batch foaming simulation and N 2 as a blowing agent. Its objective was to find the relationship between foaming behavior and weight composition of TPO blends. According to the results, it was observed that the increase of weight percentage of elastomer improves the cell density. Second experiment was to investigate an optimal compounding method; therefore, three different compounding technologies were implemented such as a batch mixer, a lab-scale twin-screw extruder, and

27 21 an industrial scale twin-screw extruder. According to the results from the foaming simulation, the industrial scale twin-screw extruder was able to generate the most uniform distribution of smaller cells. Last experiment was to analyze the relationship between the foaming behavior of TPO and talc content. It was realized that the cell density of TPO blend increases as the talc content increases only when the concentration of N 2 is significantly low. Based on the observation, the talc content became ineffective to the cell density at a high concentration of N 2.

28 22 8. Figures and Tables Figure 1. Schematic diagram of a single screw extrusion injection foaming system Figure 2. Schematic diagram of foaming simulation system

29 23 Figure 3. Picture of actual batch foaming system Figure 4. Picture of batch mixer system Figure 5. Schematic diagram of twin-screw extruder

30 24 Figure 6. SEM images of TPO blend samples with 5000x of magnification (a) TPO90 (b) TPO70 PP matrix POE (elastomer) phase (c) TPO50 Figure 7. Serial images of foaming process of samples with N 2 blowing agent

31 Cell Density (cells/cm 3 ) Cell Density (cells/cm 3 ) 25 Figure 8. Cell density of samples against foaming process time 1.6x x x10 6 TPO50 TPO70 TPO90 PP Elastomer 8.0x x Time (sec) Figure 9. Linear relationship between cell density and number density of elastomer sec 0.44 sec 0.40 sec 0.34 sec 0.30 sec Number Density of Elastomer (#/cm 3 )

32 26 Figure 10. SEM images of TPO70 samples with different compounding methods in 2000x magnification (a) Batch mixer (b) Lab-scale extruder (c) Industrial-scale extruder Figure 11. Serial images of foaming process of TPO70 sample with different compounding methods BTPO = TPO70 mixed with a batch mixer TTPO = TPO70 mixed with a lab-scale twin-screw extruder ITPO = TPO70 mixed with an industrial-scale twin-screw extruder

33 Cell Density (cells/cm 3 ) Cell Density (cells/cm 3 ) 27 Figure 12. Cell density of samples against foaming process time 2.4x10 6 ITPO (Uniform and very fine elastomer) TTPO (Uniform but reasonably fine elastomer) 2.0x10 6 BTPO (Non-uniform and large elastomer) 1.6x x x x Time (sec) Figure 13. Linear relationship between cell density and number density of elastomer 10 7 ITPO 10 6 BTPO TTPO sec 0.44 sec 0.40 sec 0.34 sec 0.30 sec Number Density of Elastomer (#/cm 3 )

34 Cell Density (cells/cm 3 ) 28 Figure 14. Serial Images of TPO foaming with N 2 with increasing the talc content at the high concentration (~1.5 wt%) of N 2 (a) 0 wt% talc (b) 1 wt% talc (c) 5 wt% talc (d) 10 wt% talc Figure 15. Effect of talc content on cell density at the high concentration (~1.5 wt%) of N 2 (P sat =2000 psi, 180 C, dp/dt=35 MPa/s) 2.0x wt% Talc 1.6x wt% Talc 1 wt% Talc 0 wt% Talc 1.2x x x Time (sec) (a) Change in cell density against time

35 Maximum Cell Density (cells/cm 3 ) Talc Content (wt%) (b) Maximum cell density vs. talc content Figure 16. Serial Images of TPO foaming with N 2 with increasing the talc content at the low concentration (~0.4 wt%) of N 2 (a) 0 wt% talc (b) 1 wt% talc (c) 5 wt% talc (d) 10 wt% talc

36 Maximum Cell Density (cells/cm 3 ) Cell Density (cells/cm 3 ) 30 Figure 17. Effect of talc content on cell density at the low concentration (~0.4 wt%) of N 2 (P sat =500 psi, 180 C, dp/dt=10 MPa/s) 2.0x wt% Talc 5 wt% Talc 1 wt% Talc 1.5x wt% Talc 1.0x x Time (sec) (a) Change in cell density against time Talc Content (wt%) (b) Maximum cell density vs. talc content

37 31 Table 1. Classification of foamed polymers based on cell sizes and cell densities Category Cell Size [ m] Cell Density [# of cells/cm 3 ] Conventional Foams >100 < 10 6 Fine-celled Foams 10 to to 10 9 Microcellular Foams <10 >10 9 Table 2. Different weight compositions of TPO samples Foam Sample PP (wt%) POE (wt%) TPO TPO TPO Table 3. Different weight compositions of TPO blends with talc content Foam Sample PP (wt%) POE (wt%) Talc (wt%) TPO TPO TPO

38 32 9. References 1. M. Kontoupolou, M. Bisaria, and J. Vlachopoulous, Intern. Polym. Process., 12, 165 (1997) 2. L. A. Meiske, S. Wu, K. Sehanobish, and J. Dibbern, SPE, ANTEC, 2001 (1996) 3. J. A. Hudson, J. Inject. Mold. Technol., 1,2, 117 (1997) 4. R. Eller, IISRP Annual Meeting (2002) 5. D. Klempner, and K. C. Frisch, Handbook of Polymeric Foams and Foam Technology, Hanser, New York (1991) 6. J. L. Throne, Thermoplastic Foams, Sherwood Technolgoies, Inc. Sherwood Publishers, Ohio (1996) 7. D. F. Baldwin, and N. P. Suh, SPE, ANTEC Tech. Papers, 38, 1503 (1992) 8. C. B. Park, and N. P. Suh, Polym. Eng. Sci., 36, 34 (1996) 9. N. S. Ramesh, and S. T. Lee, Foam Extrusion Principles and Practice, Chap. 5, Technomic Pub. Co., Lancaster (2000) 10. A. Dutta, and M. Cakmak, Rubber Chem. Technol., 65, 932 (1992) 11. R. Brzoskowski, Y. Wang, C. L. Tulippe, B. Dion, H. Cai, and R. Sadeghi, SPE, ANTEC, Tech. Paper, 3204 (1998) 12. Y. Wang, New Low Density TPV Foam for Extrusion Profiles, Foamplas 98, May 19-20, Teaneck, New Jersey, 161 (1998) 13. A. Sahnoune, J. Cell. Plast., 37, 149 (2001) 14. A. Sahnoune, SPE, ANTEC, Tech. Paper, 665 (2000) 15. P. Spitael, C. W. Macosko, and A. Sahnoune, SPE, ANTEC, Tech. Paper, 493 (2002)

39 D. Kropp, W. Michaeli, T. Herrmann, and D. Schroder, SPE, ANTEC, Tech, Paper, 43, 3473 (1997) 17. C. H. Lee, K. J. Lee, H. G. Jeong, and S. W. Kim, Adv. Polym. Tech., 97, 19(2) (2000) 18. Q. Guo, J. Wang, C. B. Park, and M. Ohshima, SPE, ANTEC, Tech. Paper, #510 (2004) 19. N. S. Ramesh, D. H. Rasmussen, and G. A. Campbell, Polym. Eng. Sci., 34, 1698 (1994) 20. C. B. Park, L. K. Cheung, and S. W. Song, Cell. Polym., 17, 221 (1998)

40 34 Appendix A Equation 1. Calculation of cell density N N 0 A 3 2 N = calculated cell density N 0 = number of cells in an area A of an image Φ = expansion ratio Equation 2. Calculation of expansion ratio N R 1 N R 3 3 i i avg R i = radius of each counted cell R avg = volume-average radius of all counted cells at a given moment Equation 3. Calculation of volume-average radius of all counted cells R avg 1 1 N 3 N Ri Ri i 1 i 1 N N 0 Equation 4. Calculation of cell density using volume-average radius N 1 N A N0 A R 3 avg