Foam injection molding of thermoplastic elastomers: Blowing agents, foaming process and characterization of structural foams

Transcription

1 Foam injection molding of thermoplastic elastomers: Blowing agents, foaming process and characterization of structural foams S. Ries, A. Spoerrer, and V. Altstaedt Citation: AIP Conference Proceedings 1593, 401 (2014); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Foam injection moulding of a TPO/TPC-blend and the effect of different nucleating agents on the resulting foam structure AIP Conference Proceedings 1593, 367 (2015); / Foam injection molding of poly(lactic acid) with physical blowing agents AIP Conference Proceedings 1593, 397 (2015); / Prediction of bubble growth and size distribution in polymer foaming based on a new heterogeneous nucleation model Journal of Rheology 48, 439 (2004); / Mould temperature control during injection moulding process AIP Conference Proceedings 1664, (2015); /

2 FOAM INJECTION MOLDING OF THERMOPLASTIC ELASTOMERS: BLOWING AGENTS, FOAMING PROCESS AND CHARACTERIZATION OF STRUCTURAL FOAMS S. Ries 1 *, A. Spoerrer 1 and V. Altstaedt 1 1 Neue Materialien Bayreuth GmbH, Germany stefan.ries@nmbgmbh.de; andreas.spoerrer@nmbgmbh.de; volker.altstaedt@nmbgmbh.de; Abstract - Polymer foams play an important role caused by the steadily increasing demand to light weight design. In case of soft polymers, like thermoplastic elastomers (TPE), the haptic feeling of the surface is affected by the inner foam structure. Foam injection molding of TPEs leads to so called structural foam, consisting of two compact skin layers and a cellular core. The properties of soft structural foams like soft-touch, elastic and plastic behavior are affected by the resulting foam structure, e.g. thickness of the compact skins and the foam core or density. This inner structure can considerably be influenced by different processing parameters and the chosen blowing agent. This paper is focused on the selection and characterization of suitable blowing agents for foam injection molding of a TPE-blend. The aim was a high density reduction and a decent inner structure. Therefore DSC and TGA measurements were performed on different blowing agents to find out which one is appropriate for the used TPE. Moreover a new analyzing method for the description of processing characteristics by temperature dependent expansion measurements was developed. After choosing suitable blowing agents structural foams were molded with different types of blowing agents and combinations and with the breathing mold technology in order to get lower densities. The foam structure was analyzed to show the influence of the different blowing agents and combinations. Finally compression tests were performed to estimate the influence of the used blowing agent and the density reduction on the compression modulus. Keywords: Thermoplastic Elastomer, Blowing Agent, Foam Injection Molding, Structural Foam. Introduction Plastics take in our daily life an important part and appear in almost all areas. They can be found as commodity plastics in disposable articles, as well as in technical components as high-performance polymers. Due to the continuously rising oil prices the cost of the materials increase, and therefore the plastics industry tries to reduce the production cost further and further by the steadily increasing price pressure. With regard to the used material a cost reduction is possible. Expensive technical plastics for example can be replaced with commodity plastics such as polyolefins. Another approach, to reduce costs is the foam injection molding processes (FIM). With this method it is possible to produce technical components with cellular foam core using a blowing agent. In this process a structural foam is molded which consists of compact skin layers with a cellular foam core. The advantages of this method include the reduction of material use, the reduced component weight, the elimination of sink marks, warping and internal tensions. This method is mainly of great interest for the automotive industry to reduce the component weight and thereby reducing fuel consumption and emissions. Another point of interest is the haptic and compression behavior of foamed thermoplastic elastomers. In the foam injection molding process parts with a soft touch surface can be produced. Furthermore, the foam injection molding process has some process-related advantages. This includes, for example, that the holding pressure is in contrast to the conventional injection molding process omitted, as the expanding gas generates a quasi-intrinsic homogeneous pressure resulting in relatively low cavity pressure and clamping force requirements. Another advantage is that the use of blowing agents reduces the viscosity of the polymer melt, this leads to a lower injection pressure and an extension in flow-length. Through the lower melt viscosity the processing temperature can be reduced. Altogether the lower processing temperature, the better heat transfer between polymer and mold wall and the minor component weight causes a cooling time reduction, which decreases the costs [1, 2, 3]. Mold technology For the FIM process two types of mold technologies can be used. In the low-pressure process the mold is filled with 80 to 95 % polymer gas melt. By the nonvolumetric filling of the cavity (80 95 %) and the pressure drop in the mold the melt can expand and fill up the remaining mold volume. It is called the lowpressure process, since the cavity pressure remains relatively low. The degree of foaming is in the range 5 to 20 %. In the high-pressure process (also known as "precision mold opening" or "breathing mold technology ) structural foams with lower density can be produced by molds with variable cavity (vertical flash face). The mold cavity is completely filled with polymer melt and then immediately opened by a few millimeters. Through this opening of the cavity a pressure drop occurs and the melt is able to foam. The Proceedings of PPS-29 AIP Conf. Proc. 1593, (2014); doi: / AIP Publishing LLC /$

3 opening takes place by the pull-back of the clamping unit. For the fabrication of structural foams different blowing agents have to be used. The propellants can generally be divided into three classes: Chemical blowing agents (CBA), physical blowing agents and microspheres (MS). Chemical blowing agents are organic or inorganic substances which carry out a decomposition reaction above a certain temperature and split off gaseous and solid elements. The required temperature for the reaction is supplied during processing the polymer, whereas a certain amount of activation energy is needed to start the reaction. The released amount of gas during the chemical reaction depends on the temperature profile, the residence time in the plasticizing unit, the system pressure and the melt strength of the polymer. The chemical blowing agents can both have an endothermic or an exothermic decomposition course. For the TPE, we only take the endothermic chemical blowing agents into consideration, since the decomposition course can be better controlled; the decomposition temperature is lower and no bad smelling by-products such as hydrogen sulfide are released. An advantage of chemical blowing agents is the easy insertion to the injection molding process. In addition, foaming with chemical blowing agents is usually cheaper because no additional license fees for gassing systems (e.g. Texel s MuCell process) and no machine changes are needed. Disadvantages are the non-volatile by-products of the carrier polymer (masterbatch), which remain in the device. Therefore the mechanical resistance can be lowered and a premature failure of the component can occur. [5, 6] Physical blowing agents are compounds used as organic liquids or as gases in the supercritical state. They are directly injected into the polymer melt. There are several groups of physical blowing agents: The short-chain hydrocarbons, the halogenated hydrocarbons (HCFC, CFC) and the inert gases (N 2, CO 2 ). Nowadays, mainly non-flammable, inert gases are used because of their environmental friendly properties. They are used although the solubility of these gases in the melt is significantly worse and thus the required mass pressure to release the propellant is much higher. The advantage of physical blowing agents is the much higher gas dosing possibility and the closed-cell structure of the structural foams. The disadvantage is the high cost factor generated by the license costs (e.g. MuCell process) and the special machine design. [7] The third type of blowing agents are the so called microspheres. These are microscopic spheres (diameter 12 microns) with a thermoplastic shell and a hydrocarbon filling. An increasing temperature leads to an extent of the hydrocarbon and the pressure in the balls increases. The thermoplastic shell softens which results in a volume expansion. Through this expansion, the initial density of polymers can be reduced up to 30 %. The diameter of the balls is increased by a factor of 3 to 4 and the shell thickness of the microspheres is reduced to a value of about 0.1 microns [8]. Depending on which thermoplastic is used for the shell, the mechanical properties of the foam might be impacted. The shell can for example consist of acrylonitrile-containing polymers and the core materials are hydrocarbons such as propane, butane, pentane, isobutane, isopentane and combinations [9]. The advantage of the expanded microspheres compared to chemical or physical blowing agents is a closed-cell foam structure with a defined cell size by the size of the expanded spheres. After cooling, the expanded microspheres remain in the polymer and act as filler, so the foam stability is increased. The stability of the microspheres by cooling is reached significantly earlier than chemical blowing agents, so the use of these microspheres results in a lower cycle time. The use of microspheres also brings disadvantages. These are the lower degree of expansion compared to other propellants. A specific temperature window must stick to the processing, because a too low temperature results in a lower foaming and a too high temperature in a collapsing of the foam cells. Furthermore, the price compared to the other blowing agents is relatively high. Experimental DSC Measurements The measurements were performed with a DSC/SDTC 821E instrument from Mettler Toledo. For the experimental procedure 10 mg of the examined blowing agent masterbatch were weighed in aluminum crucibles and then encapsulated. The capsules in the DSC were exposed to a temperature program. This temperature program involved in each case a heating and a cooling curve. The samples were heated from 25 C to 250 C, held for five minutes at 250 C and then cooled from 250 C then to 25 C. The heating and cooling rate was 10 K/min. TGA/FTIR Measurements The TGA measurements were performed with a TGA/SDTA 851e from Mettler Toledo. For this, about 10 mg of the propellant masterbatches were filled in an alumina crucible and then heated from 25 C to 400 C (N 2 flow: 50 ml). By evaluation software the weight loss was pursued with the built-in ultra-micro scale in dependence of the temperature. In addition to the TGA an FTIR spectrometer was connected (Nicolet Nexus 470) to analyze the released gases of the blowing agent. Expansion Measurements The expansion measurements were used to determine the expansion start, the maximum expansion and the temperature stability of the microspheres. For these measurements about 0.5 g of the microspheres was weighed in a 25 ml graduated cylinder. Subsequently, this cylinder was heated in an oven (company 402

4 Memmert) from 25 C to 220 C with a heating rate of 5 K/min. During this heating phase, the degree of expansion of the microspheres was followed and recorded using the scale on a graduated cylinder. Blowing agents In the following Table 1 the characterized blowing agents are shown. Table 1 Overview over the characterized blowing agents. Manufacturer Lehmann und Voss Lehmann und Voss Akzo Nobel Akzo Nobel Akzo Nobel Trade name Luvopor 9654 Luvopor 9674 Expancel 930 MB 120 Expancel 950 MB 120 Expancel 980 MB 120 Blowing agent chemical chemical microspheres microspheres microspheres KCD CellMix a60/950 microspheres Foam injection molding The production of the structural foams was made in thermoplastic foam injection molding process. The experiments were performed on a 150 t hydraulic injection molding machine (type Battenfeld BA /400 BK). The injection unit has a physical direct gassing unit from Trexel (MuCell process) and a screw with 35 mm diameter. The controls of the machine allow a precision opening ("breathing") of the mold cavity after the injection process. The time between the injection and the opening of the tool can be set (delay time). The tool has a vertical flash face and in combination with the precision opening a variation of the wall thickness of specimens is possible. For the filling of the mold cavity, the plastic melt is turned around through a hot runner channel with pneumatic needle valve nozzle. The prepared specimens have the dimensions 220 mm x 64 mm with different wall thickness. Materials and blowing agents for the FIM process For the foam injection molding tests a TPE of the company PTS Plastic Technologie Service was used. As blowing agents one chemical blowing agent and one type of microspheres were used. An overview over the properties of the used TPE and the used foaming agents are given in following tables. (Table 2 and 3) Table 2 Overview of the properties of the used TPE. Material TPE-Blend Material designation A7000/82 Hardness [Shore A] 83 Density [g/cm 3 ] 0,99 Tensile strength [MPa] 15 Elongation at break [%] 980 Processing temperature range [ C] Table 3 Overview of the used blowing agents. Manufacturer Lehmann und Voss Trade name Luvopor 9674 Blowing agent chemical KCD CellMix a60/950 microspheres Density Measurements The density of the foam samples was determined with the lift scale of Mettler Toledo AG (type 245) in the so-called Archimedes principle. The Archimedes principle states that each body that is immersed into a liquid becomes as much easier, as the amount of liquid displaced by it. In each test series five samples were measured. First, the weight of the molded part in air (m air ) was determined and then in a liquid (m water ) of known density 0 (e.g. distilled water). For these contexts, the density of the component can be calculated after the following equation 1: mair 0 Eq.1 mair mwater Prerequisite for the measurement of the density based on Archimedes principle is a closed-cell foam structure, otherwise the liquid can penetrate in the foam and affect the results. Shore hardness The measurement of the hardness of the compact and foamed samples was based on the standard DIN The measurements were performed at three different points in the sample. Five samples were measured per run. The Shore Hardness Tester (Shore A durometer) consists of a spring-loaded indenter. The elastic penetration depth is a measure for the appropriate Shore hardness of the material, which is measured on a scale from 0 to 100. Large values indicate a high hardness. The thickness of the Specimen must be at least 6 mm. The specimens can be constructed from several thin layers. It should be noted that the layers have uniform contact. The measurements were made at 23 C and a relative humidity of 50 %. Compression test To determine the mechanical properties of compact and foamed specimen, the compression test were carried out under standard conditions (23 C, 50 % humidity) on a Zwick Z050 universal testing machine (DIN EN ISO 604). The crosshead speed was constant 1 mm/min. As sample geometry, cylindrical specimens with a diameter of 10 mm are used. The compression modulus was determined as secant modulus in the linear-elastic range from 2 to 3 % strain. Structure analysis The microscopy images were recorded with a light microscope MZM 1 of the company Askania. The images of the foams were taken with different magnifications to determine the cell size and the 403

5 compact skin layer thickness to obtain an impression of the foam structure. For the samples, which had a cell size of less than 150 microns (outside the magnification range), SEM images were recorded to evaluate the cell size. For these recordings, samples were taken from the samples, cooled in the liquid nitrogen and then broken. The fracture surface was then sputtered with an approximately 13 nm thick gold layer (sputtering: Cressington Coater Type 108 Auto) and investigated with the scanning electron microscope Jeol JSM-IC with different magnifications and an accelerating voltage of 15 kv. The evaluation of the structural foam structure was performed with the software Image J. The skin layers and the cell sizes were analyzed. also noticed that Luvopor 9674 releases a much higher amount of heat (higher heat flow), what would suggest higher active substance content in the masterbatch. TGA/FTIR Measurements In addition to the DSC measurements TGA/FTIR measurements were performed. With these measurements, the released quantity of gas for the chemical blowing agents should be determined. By coupling the TGA and FTIR measurement, the composition of the released gases can be analyzed. The following diagram shows the TGA curves from 25 C to 400 C of Luvopor 9654 and Luvopor Results and Discussion DSC Measurements The chemical blowing agents were investigated with the Differential Scanning Calorimetry. The melting points of the masterbatch supporting materials and the start of decomposition of the propellant should be analyzed. These parameters are important, because they have to be compatible with the foamed polymer (thermoplastic elastomer). The decomposition beginning of the foaming agent is crucial for the processing. The temperature in the feed section of the screw must always be below the decomposition temperature of the selected blowing agent so that a premature loss of gas can be excluded. The following diagram shows the DSC curves (1st heating curves) of the blowing agent Luvopor 9654 and Luvopor Figure 2 TGA curves of Luvopor 9654 and Luvopor The TGA curves (Fig. 2) show the same decomposition course as it was analyzed in the DSC measurements. Luvopor 9654 also shows in the TGA a two-step decomposition course from about 140 C to 235 C. Above 300 C the decomposition of the carrier polymer probably already starts (third stage). The Luvopor 9674 shows a single-stage decomposition course from 130 C to 190 C. The released amount of gas varies for both propellants. Luvopor 9654 has a maximum gas rate of 13 % and Luvopor %. The amount of gas will be related to the starting weight of the blowing agent masterbatch. Due to the higher gas content of the Luvopor 9674 a higher density reduction in the foam injection molding tests is possible. The gases released during the decomposition reaction were transferred from the TGA into an IR spectrometer and analyzed with regard to their components. Figure 1 1st DSC heating curve of Luvopor 9654 and Luvopor In the diagram (Fig. 1) both propellants show an endothermic decomposition process. Peaks at about 100 C to 105 C can be identified as the melting point of the carrier material (polyethylene). In case of Luvopor 9654 a two-step decomposition curve is recognized. The two peaks occur at about 150 C and 220 C, what is a propellant combination of sodium bicarbonate and citric acid. For Luvopor 9674, a single-stage decomposition reaction can be identified. The decomposition peak also occurs at about 150 C, which suggests sodium bicarbonate as propellant. We Figure 3 IR spectra of Luvopor 9654 and Luvopor

6 The IR spectra of the chemical blowing agents (Fig. 3) show the same gas composition. Both propellants set carbon dioxide at about 700 and 2350 cm -1 and water at about 1600 and 3700 cm -1 free. The CO 2 transmission peaks of Luvopor 9654 show a transmission of 70 % and the CO 2 transmission curves of Luvopor 9674 however have a minimum value of 50 %. So it can be seen, that Luvopor as in the TGA and DSC measurements expressed - releases more gas, which can be observed in the IR spectrum as a lower transmission for the CO 2 and H 2 O peaks. From the data of the DSC, TGA and FTIR measurements Luvopor 9674 was selected for the foam injection molding trials since it has the lower decomposition temperature and a higher gas release. Expansion Measurements The expansion measurements of the microspheres are used to determine the expansion start, the maximum expansion and the thermal resistance. Figure 4 Comparison of the expansion measure-ments of the four types of microspheres. The expansion curves in Figure 4 show that the four different types of microspheres have a different expansion starting temperature. The expansion of Expancel 930 MB 120 begins at a temperature of 125 C. The expansion of the other microspheres starts at 130 C (Expancel 950 MB 120), 140 C (CellMix a60/950) and 150 C (Expancel 980 MBX 120). The expansion starting temperature is crucial in the foam injection molding process. If the expansion starts too early, the microspheres are more vulnerable to a collapse during the continuous manufacturing process. Another difference between the microspheres is the maximum expansion. The largest expansion provides Expancel 930 MB 120 and CellMix a60/950 with about 2125 %, followed by Expancel 950 MB 120 with a maximum expansion of 1875 %. The lowest expansion shows Expancel 980 MBX 120 with a maximum dilation of the factor 16.5 (1650 %). The most important point for the production of homogeneous and closed-cell structural foams with microspheres is the maximum temperature the microspheres can be suspended before they collapse (rupture of the shell) and shrink (see Fig. 5). Figure 5 SEM images of microspheres before (left) and after (right) the collapse. Figure 4 shows that three microsphere types (Expancel 930 MB 120, Expancel 950 MB 120 and CellMix a60/950) have the same limit temperature of 200 C, so they must be fabricated in the foam injection molding process with maximum 200 C nozzle temperature. The thermoplastic elastomers which can be processed with these three types of microspheres are for example TPS, TPO and TPV. Only Expancel 980 MBX 120 has a higher temperature resistance up to 215 C and they can therefore also be used for TPE s with higher processing temperatures (e.g. TPC or TPU). The different values obtained for the expansion start, maximum expansion and maximum processing temperature can be explained by the varying structure and composition of the microspheres. The polymer shells each having a different softening temperature (different thermoplastic shell), which affects the beginning of the expansion. The tensile strength of the microsphere shell influences the various maximum expansion and thermal resistance. Furthermore, the microspheres are filled with various types of hydrocarbons, which have a different vapor pressure. Therefore the start of expansion and the maximum expansion of the microspheres differs depending on the type of microsphere. From these data, one type of microspheres can be chosen for the manufacturing process. One type of microspheres shows a maximum fabrication temperature of 200 C with the largest expansion in the relevant temperature range. So CellMix a60/950 is selected form these four microspheres-types because it has a higher expansion starting temperature (140 C to 125 C) compared to Expancel 930 MB 120 and thus this type is less susceptible to destruction during the injection molding process. Characterization of integral foams The integral foams produced in FIM process were characterized in terms of structure, density and mechanical properties. Parameters such as homogeneity, skin layer thickness, cell size, foam density, hardness and young s modulus will be in particular discussed. The investigated integral foams are all based on an initial wall thickness of 3 mm. Structural analysis The structural analysis of structural foams is carried out to determine the homogeneity of the foam. The 405

7 morphology is examined with regard to the compact skin layer thickness and the average cell size. The results of the microscopic investigations provide a qualitative impression of the foam structure, depending on the blowing agent and the process conditions. In the following pictures examples of the foam structures of the TPE blend are shown. The Structural foams were produced with a precise mold opening (volume expansion) of 3 mm to 5 mm. The solid lines in the images of the structural foams show the differentiation between the compact skin layer and the cellular foam core. Figure 6 Microscopy images of the TPE blend foam structures; left: foamed in MuCell process (0.3 % N 2 ), right: foamed in MuCell process (0.3 % N 2 ) with talcum (5 %). Figure 7 Microscopy images of the TPE blend foam structures; left: foamed with chemical blowing agent (3 % Luvopor 9674), right: foamed with a combination of MuCell process (0.3 % N 2 ) and chemical blowing agent (3 % Luvopor 9674). Figure 8 SEM images of the TPE blend foam structures; left: foamed with microspheres (3 % CellMix a60/950), right: foamed with a combination of microspheres (3 % CellMix a60/950) and chemical blowing agent (3 % Luvopor 9674). Figure 6 right shows that in the MuCell process structural foams with relatively large foam cells and inhomogeneous foam morphology are formed. This is due to the too low nucleation (homogeneous nucleation). When a bubble is formed through homogeneous nucleation, this bubble expands very fast due to the high diffusion rate of the used nitrogen in the MuCell process. As a result only a few cells are formed that grow rapidly and lead to a coarse foam. In order to avoid the lack of heterogeneous nucleation in the MuCell process talcum as nucleating agent was directly mixed with the TPE granules (dry blend). The addition of talcum in the melt leads in addition to the homogeneous nucleation to heterogeneous nucleation. So more nucleating points are formed, which reduces the cell size of the structural foams and the homogeneity of the foam is improved (see Fig. 6 right). The foam structure of the structural foams produced with chemical blowing agent (Fig. 7), are similar to the foam structures that were produced in the MuCell process. The structural foams have relatively large foam cells, which are distributed irregularly. The nucleation should be guaranteed by the solid decomposition residues of the chemical blowing agent. The gas solubility of CO 2 is higher and the diffusion of the gas out of the melt is slow compared to N 2. Due to the increased nucleation rate many small bubbles are formed, but by the lower diffusion rate of the CO 2 they grow very slowly, because the CO 2 has the tendency to separate just at lower ambient pressure with the polymer melt. The combination of MuCell process and chemical blowing agents will combine the advantages of the two blowing agents. The N 2 of the MuCell process ensures by the high diffusion rate the driving force for the cell growth and the chemical blowing agent provides the necessary nucleation by the solid decomposition residues. The additional released CO 2 of the chemical blowing agent increase the available amount of gas for the cell growth. These assumptions set out in the theory are reflected in the microscopy images of the structural foam. The prepared test specimens have a relatively fine-celled foam structure with homogeneously distributed foam cells (Fig. 7 right). Foaming with microspheres delivers generally a very homogeneous foam structure with defined cell size through the expanded microsphere size. If a volume expansion is performed during the process, a structural foam with fine foam structure is formed (Fig. 8 left). The precision mold opening associated a pressure drop and the expansion of the microspheres is facilitated. Higher volume expansions than 3 mm to 4 mm (30 % density reduction) are not possible because the expansion pressure of the microspheres compared to N 2 or CO 2 is relatively low. To improve the limited expansion capacity of the microspheres, structural foams with a combination of microspheres and chemical blowing agent were produced. With this blowing agent combination very fine foam cells are formed and a large volume expansion (3 mm to 6 mm) could be reached (Fig. 8 right). The cell size increases slightly with increasing volume expansion. The reason for the higher degrees of expansion is the blowing agent combination. The chemical blowing agent leads to a lower viscosity of the polymer melt and creates cavities in the melt, making the expansion of the microspheres easier and so the synergy effect of the foaming agents can be used. Analysis of the top layer thickness Out of the showed foam structures the top layer thickness was determined. This parameter was measured directly on the microscopy images. The value for the top layer thickness of the TPE blend is the average of a top layer. 406

8 Figure 9 Overview of the top layer thickness of the TPE blend integral foams. Generally no clear trend appears with regard to the different blowing agents (Fig. 9). The top layer thickness of the structural foams produced with the foaming agent combination MuCell /chemical blowing agent have the lowest values. This is explained by the reduction of melt viscosity. Due to the reduced melt viscosity the melt has more time (longer delay time) to foam, whereby the proportion of the polymer that solidifies on the mold wall is lower. Due to the additional nucleation by talcum the top layer thickness could also be reduced. This could be due to the lowering of melt viscosity, as the crystallization temperature is lowered and thus the proportion of the foam core is larger. The heterogeneous nucleation probably takes place at a higher pressure and therefore the foaming during the mold opening occurs earlier. In general, factors such as the delay time of the precision mold opening, the melt and mold temperature and the injection speed have a larger amount on the formation of the top layer as the used blowing agent. Investigation of cell size The determination of the cell size can be made directly out of the microscopy images. The effects of different types of blowing agents on cell size were analyzed (Fig. 10). The structural foams made in the MuCell process or with chemical foaming agents, have the largest foam cells due (from μm) to the low nucleation (MuCell ) and the low diffusion rate of CO 2 (chemical propellant). By the combination of N 2 (MuCell ) and chemical blowing agent an additional heterogeneous nucleation takes place. This leads to smaller cell sizes of the TPE blend (400 μm). Microspheres form structural foams with the finest foam cells, because the size is determined by the expanded microspheres ( μm). The cell sizes of the integral foams are dependent of the precision mold opening distance. If the polymer melt in the mold already has a too high viscosity, the foam cells are pulled apart by this movement and enlarged. In addition at higher expansion the foam cells coalescence because the cell walls are broken by the high elongation. Density of structural foams The characterization of the structural foam density was made with the Archimedes method. Figure 11 Density of TPE blend structural foams. In Figure 11 the density of the TPE blend structural foams in different settings is shown. It is confirmed that the density decreases with increasing mold breathing (volume expansion). The theoretical density reduction by the precision mold opening correlates with the actual density reduction. This theoretical density reduction can be calculated from the thickness of the injected melt and the executed precision mold opening. Thus, a theoretical density reduction of 25 % (3 4 mm), 40 % (3 5 mm) and 50 % (3 6 mm) is achieved. The maximum density reduction is generally dependent on the specific amount of blowing agent/material combination, the type of blowing agent and the nucleation. Hardness of the structural foams The hardness measurement gives an impression of the mechanical properties of the structural foams. Figure 10 Overview of the cell size of the TPE blends structural foams foamed with different blowing agents. 407

9 sequence of measured values can be approximated by a linear equation. Due to this linear equation, it is now possible to calculate the hardness as a function of density reduction. Minor deviations from the linear trend emerge from the various process parameters and blowing agents which effects different properties like the top layer thickness. Figure 12 Shore A hardness of the TPE blend structurl foams. The hardness decreases with increasing degree of foaming (Fig. 12). This is due to the higher proportion of the foam core and the lower density. Prerequisite for this assumption is that the top layer thickness of the structural foams is approximately the same, which is dependent on the processing parameters. The hardness of the structural foams is thus due to the construction of Shore hardness measurement mainly dependent of the structure of structural foam (thickness of top layer and foam core). The blowing agent combination of N 2 (MuCell ) and chemical blowing agent delivers the highest values for hardness reduction. This can be explained by the lower top layer thickness of these structural foams. As explained in chapter before, by this foaming agent combination, the melt viscosity is lowered. Thus, the melt has more time to foam, whereby the proportion that can solidify at the mold wall is reduced. In the hardness measurement, the top layer thickness is crucial because it takes up most of the force exerted by the specimen to the component. The direct comparison of the hardness with the density reduction shows a correlation for these two parameters and so a prediction of hardness for specific density reductions can be made. Compression test To determine the elastic mechanical properties of the structural foams compression tests were performed. These studies should show the effect of the different blowing agents on the elastic properties of the components. The stress-strain curves of the compact TPE blend and one TPE blend structural foam are shown in Figure 14. Figure 14 Comparison of the stress-strain curves of compact and foamed TPE blend. With the same initial wall thickness (3 mm) and so the equal weight the mechanical performance and the young s modulus decrease. The foaming of TPEs have a significant influence on the stress-strain behavior. Through the foam core of the specimens can be further compressed with a smaller force. For a better comparison of the foams the compression young s modulus values were determined from the stress-strain-curves in the linear-elastic range at a compression of 2 to 3 %. Figure 13 Hardness-density reduction-correlation of the TPE blend structural foams. This hardness-density-correlation is shown in Figure 13. It reveals a nearly linear progression, so this Figure 15 Overview of the determined compression young s modulus values of the TPE blend structural foams. 408

10 Figure 15 shows generally a clear trend in the young s modulus development in relation to the volume expansion. By increasing the precision mold opening the portion of the foam core increases more and more and so a smaller resistance is opposed during the compression test and the young s modulus values decrease. In contrary the young s modulus values show no clear trend in terms of used blowing agent. Thus, the structure of the structural foams (homogeneity) influences the values most. These includes the parameters of the top layer thickness, the cell size, the density of the foam core and the homogeneity of the foam core. Another point that should be analyzed on the basis of compression tests was the course of the compression young s modulus in relation to the density reduction. Figure 16 Trend of compression young s modulus of the TPE blend structual foams as a function of density reduction. The measured young s modulus values can be approximated to an exponential decay (Fig. 16). Thus it is possible to calculate the young s modulus in relation to the density reduction from the curve equation. Minor deviations from the exponential function emerge from the various process parameters which results in different values (e.g. top layer thickness) of the test specimen. Different to the trend of hardness in the course of the density reduction it can be assumed that the force is distributed over the entire specimen surface and therefore it is a purely elastic deformation of the entire sample. On the basis of the compression tests a model for the calculation of the compression young s modulus of the foam core can be set up. Figure 17 Schematic layout of a compression test on a structural foam. The experimental setup can be seen in Figure 17. It is a series conection of the acting force. Hence, various assumptions are made. Firstly, it is assumed that the compression of the structural foam consists of the deformation of the foam core and the two compact top layers. Moreover, the compression is calculated from the change in length in relation to the initial length. The tension acting in the whole specimen, is the same as in the top layers and in the foam core of the structural foam. This experimental setup, the assumptions and the derivation of the used formula are shown below. Table 4 Parameters for the calculation of the foam core young s modulus. s = Change in thickness SF = Compression top layer s 0 = Initial thickness SF = Tension structural foam s SF = Thickness sructural FC = Tension foam core foam s TL = Thickness top layer TL = Tension top layer s FC = Thickness foam core E SF = Young s modulus structural foam SF = Compression E FC = Young s structural foam modulus foam core FC = Compression foam core Assumption: 0 0 = TL Ecompact E TL = Young s modulus top layer TL1 TL2 2 TL SF 2 TL FC s s s and E s s s E E E s 2 s s SF ssf TL stl FC sfc 2 with E E E ssf stl s 2 E E E E SF TL1 FC TL2 SF TL FC SF TL FC FC SF TL FC FC s E SF SF sfc s 2 E TL TL 0 SF TL FC Eq.2 For the values above and the previous compression measurements the compression young s modulus of the foam core can be determined. So also a trend of the compression young s modulus for the foam core with respect to the density reduction can be made. Conclusions By the DSC measurements of the blowing agents, the decomposition temperatures and the melting points of the carrier material of the masterbatches could be determined. Luvopor 9654 has a carrier of polyethylene. The blowing agent is a combination of sodium bicarbonate and citric acid. Luvopor 9674 is based on polyethylene and sodium bicarbonate. The 409

11 released gas and the gas composition of the chemical blowing agent were investigated using a combination of TGA and FTIR measurements. Luvopor 9654 delivers 13 % and Luvopor % carbon dioxide and water. For the used microspheres, the expansion starting temperature, the maximum expansion and the highest possible processing temperature are of interest. These parameters can be found out by expansion measurements. The four types of microspheres, which were tested, showed a variation in the expansion begin (125 C to 150 C), in the maximum expansion (1650 % to 2125 %) and in the highest possible processing temperature (200 C to 215 C). Out of these measurements one chemical blowing agent (Luvopor 9674) and one type of microsphere (CellMix a60/950) were selected for the foam injection molding tests. Luvopor 9674 was selected because of the higher gas amount (TGA measurements) and the so bigger potential for higher density reductions compared to Luvopor CellMix a60/950 microspheres were selected because of the highest expansion at 200 C (processing temperature of the TPE) and the higher expansion starting temperature. The structural foams, which are produced with the foam injection molding process and breathing mold, were analyzed by structure, density, hardness and compression behavior. The foam structures show a dependency of the used blowing agent. The MuCell process and the chemical blowing agent (CBA) form, due to lack of nucleation (MuCell) and low gas pressure (CBA), only foams with relatively large cells (> 800 m). By linking MuCell -process and CBA, it is possible to eliminate the disadvantages of both methods. So more homogeneous and fine-celled foams (cell size: 200 to 500 m) can be produced. Microspheres and a combination of microspheres and CBA provide foams with homogeneous foam structure and the finest foam cells (< 200 m). In the analysis of the top layer thickness no clear trend in relation to the different blowing agents could be found. Only the MuCell - process/chemical blowing agent combination creates the lowest top layer thicknesses. This can be explained by the decreasing viscosity of the polymer-gas-melt. In general, factors like the delay time, polymer and mold temperature and injection speed affect the top layer thickness more than the used foaming agents. The density of the integral foams shows a dependence of the executed volume expansion and solidification kinetics of the polymer-gas melt. The determination of the mechanical properties was carried out by measuring the hardness according to Shore A and by compression tests. The hardness measurements demonstrated a dependence of the values of the expansion level. The structural foams that were produced with the combination MuCell - process/cba have the lowest hardness values because of the lowest top layer thickness. There was also a direct correlation between the hardness and the density reduction. Due to the linear equation of the hardness with the density reduction, it is possible to calculate one of the two factors from the other. The compression tests provide the same trend as the hardness measurements. With increasing mold opening the amount of the foam core increases more and more. So a smaller resistance for the deformation during the compression tests is available. In contrast to the hardness measurement the force is distributed over the entire sample surface and the complete structural foam is debited. Similar to the hardness-density correlation a trend for the young s modulus values and the density reduction could be established. Another important point of the study was to establish a model out of the compression tests for the calculation of the mechanical properties (young s modulus) of the foam core. The calculated young s modulus of the foam core also shows a tendency related to the density reduction. Thus, a prediction of the mechanical characteristics as a function of the structure is possible. Acknowledgements The authors thank STMWIVT (Programm Neue Werkstoffe in Bayern, Projekt Hotmold for elastic foams, NW Cluster Neue Werkstoffe) and the companies Lehmann & Voss, KCD and AkzoNobel for providing the blowing agents and the company PTS Plastic Technologie Service for providing the TPE blend. References 1. Müller N., Spritzgegossene Integralschaumstrukturen mit ausgeprägter Dichtereduktion. Dissertation, Universität Erlangen-Nürnberg, Alstädt V., Mantey A., Thermoplast- Schaumspritzgießen, Carl Hanser Verlag, N. N. Clariant Masterbatches; So fein kann spritzig sein. 4. Michaeli W., Lettowsky C., Medizintechnik Life Science Engineering, Springer Verlag, Mergenhagen T., Chemische Treib- und Nukleierungsmittel. In: Altstädt V., editor. Fachtagung Polymerschäume Würzburg: SKZ, Zhou Q., Cong C. B., Effect of Exo- and Endothermic Blowing and Wetting Agents on Morphology, Journal of Cellular Solids, 41 (3), 2005, S Wegner J.-E., Additiv-Masterbatches für Schaumfolien, Kunststoffe 1/ Rosskothen K.-R., Expancel microspheres, the rather different blowing agents for the foaming of thermoplastic elastomers, TPE Magazine, 01/ Kobe J. J., Laperre J. D., Zhou Z., Durch Dehnung lösbare Schaumstoffe, darauf basierende Gegenstände und Verfahren zu deren Herstellung, Deutsches Patent, DE T2,