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A direct method for evaluating polymer foamability
Polymer Testing 30 (2011) 118–123
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
A direct method for evaluating polymer foamability Chenyang Yu, Ya Wang, Bingtian Wu, Wenguang Li* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu, Sichuan 610065, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2010 Accepted 4 November 2010
In this study, we proposed a direct method to rapidly evaluate polymer foamability by using a new self designed foaming apparatus with nitrogen (N2) as a blowing agent. Two parameters, the foaming temperature window and the correlation factor of the foam density with foaming temperature were used to characterize the foamability. Low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) resins were selected as samples to compare their foamability. The results showed that LDPE had a foaming temperature window as wide as at least 52 C, whereas that of LLDPE was only 8 C. Moreover, the foam density and cell morphology were insensitive to foaming temperature for LDPE but sensitive for LLDPE. The inherent capability of LDPE for gas foaming was much better than that of LLDPE. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Method Evaluate Foamability LDPE LLDPE Gas foaming
1. Introduction Physical gas extrusion foaming is an advanced and efficient technology used to produce polymeric foams in industry. In the past decades, researchers investigated the critical processing parameters and equipment setup in this continuous extrusion foaming process [1–5]. To date, various polymers, including polyethylene (PE) [6], polypropylene (PP) [7–9], poly (vinylidene fluoride) (PVDF) [10], poly (ethylene terephthalate) (PET) [11], polycarbonate (PC) [12] and poly (l-lactide) (PLA) [5,13] have been foamed on a laboratory scale. However, it is still difficult to make such investigations for many other polymers due to the absence of knowledge about polymer foamability for the initial processing setup. At the same time, it is time consuming and expensive to identify the critical processing parameters in the continuous extrusion foaming process. Although some polymers are successfully foamed at laboratory scale, they have not reached the optimizing stage for commercial production. Obviously, we need to develop a simple,
* Corresponding author. Tel.: þ86 028 85460817; fax: þ86 028 85402465. E-mail address: [email protected] (W. Li). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.11.001
efficient and reliable method to rapidly accumulate experimental data as well as to study the critical parameters for successful gas foaming. In a previous paper [14], we reported a new gas foaming apparatus self designed in our laboratory. It is a batch gas foaming process with short cycle time (<30 min), easy to operate and suitable for direct foaming with a small amount of raw materials. The critical processing parameters such as temperature, pressure and pressure drop rate can be well controlled. Instrumental and experimental assessments showed that it could be an ideal lab tool for rapid screening the polymer resins for gas foaming. The question then is: What standards should we use to indicate if the polymer foamability is good or not in the results? In past studies of microcellular foams, the foamability was meant for the microcellular foaming ability for polymers under a certain processing condition [15,16]. It was found that the processing parameters such as temperature, pressure and depressurization rate were most crucial. Either high gas pressure at a low foaming temperature or low gas pressure at a high foaming temperature was favorable for generating microcellular foams. The high pressure drop rate was preferable for spontaneous cell nucleation and growth, leading to microcellular foams with a high cell density. It should be mentioned that the focus was on how to generate
C. Yu et al. / Polymer Testing 30 (2011) 118–123
micro-cell structure, the foam expansion ratio was small and negligible (<2 times) in most cases. In fact, it has been a challenge to produce low density microcellular foams [17]. As recently revealed, the size of the cells mainly depends upon the foaming temperature [18,19], i.e. the state of the polymer for a given sample. This means that it is possible to obtain microcellular foams for almost all polymers under a certain processing condition (low foaming temperature and high depressurization rate). Thus, the definition of foamability from the microcellular foaming results is meaningless and ambiguous. It is known that for the gas foaming process, once the high pressure gas is in the polymer matrix through either a batch or continuous process, the foam can always be obtained by a sudden pressure drop and cooling. The foam quality such as cell structure and expansion ratio is not only dependent upon processing conditions but also upon the polymer itself. For example, low density polyethylene (LDPE) is easy to foam to a low density, but high density polyethylene (HDPE) is not. Thus, polymer foamability should relate to the inherent ability of a polymer to be foamed at a given foaming condition. This inherent ability is a characteristic of the polymer, similar to mechanical properties. In order to characterize the inherent gas foaming ability (foamability), it is necessary to develop some criteria. In the literature, tremendous efforts have been paid to learn the rheology of polymer/gas mixtures for a better understanding of the foaming process and polymer foamability [20–24]. Since the cell walls are stretched in the foaming process, the nucleation stage and the following steps of cell growth and stabilization are controlled by the extensional rheology [1]. It was found that polymers with a high degree of long chain branching exhibited strain hardening behavior in the rheological test. These polymers usually have high melt strength and good foamability due to the strain hardening. However, the quantitative relationship between the melt strength and polymer foamability is still unclear. Although the melt strength is measurable in a melt extensional test and it is then used to judge the polymer foamability, the criteria is indirect and inaccurate, particularly for polymers without chain branching. Therefore, it is very important to have direct and accurate criteria to characterize the polymer foamability. As mentioned before, for a given polymer, the state for foaming is mainly determined by the foaming temperature, which is most critical among the processing parameters. For instance, for amorphous high Tg polymers, Krause et al. [25] found the lower and upper bound foaming temperatures and determined the microcellular foaming temperature range of 80–100 C at different foaming pressures. For a crystalline PP, Xu et al. [26] found that the foaming temperature range was only about 4 C. These results show that the wide or narrow foaming temperature range differing from one polymer to another may be an indication of different inherent foaming ability. Therefore, we propose to use the foaming temperature window as the first criteria to evaluate polymer foamability. We will introduce how to determine the foaming temperature window for a given sample in the following section. In order to determine whether the product quality is easy to control or not, we propose to use the correlation factor of the
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foam density-foaming temperature curve as the second criteria. As for cell structure control, it is reasonable to observe the variation of cell size with temperature and to further elaborate the evaluation of polymer foamability by considering foam microstructure. In the current study, we used our self designed foaming apparatus to develop a fast, reliable and direct method to evaluate the polymer foamability. It is well known that LDPE is generally better than LLDPE for foaming because of higher molecular entanglement density and higher molecular chain interactions leading to increased melt strength. But, we still do not know how much better it is. Therefore, we chose LDPE and LLDPE as samples to further illustrate how to evaluate the foamability for given polymers. The results showed that LDPE had a foaming temperature window as wide as at least 52 C, while that of LLDPE was only 8 C. Moreover, the foam density of LDPE was insensitive to foaming temperature whereas that of LLDPE was very sensitive. We hope that this direct evaluating method of polymer foamability could be useful for gas foaming research in academia and helpful for foam production in industry. 2. Experimental 2.1. Materials LDPE (trade name: IF7B, melt flow index ¼ 7.0 g/10min) was from Yanshan Petrochemical Co., SINOPEC. LLDPE (trade name: 7042, melt flow index ¼ 2.0 g/10min) was from Lanzhou Petrochemical Co., SINOPEC. The pellets as received were directly used for foaming experiments. The N2 (purity 99.99%) was purchased from Taiyu Air product Co., LTD, Chengdu, China. 2.2. Foaming experiments We used a self designed foaming apparatus to do the foaming experiments. The foaming apparatus consists of a compression molding machine, high-pressure mold, high pressure gas supply system, pressure gauge and highpressure needle valve. The features, work principle and foaming procedure were systematically introduced in a previous paper [14]. In this study, about 20 polymer pellets were used for each batch at predetermined saturation time and foaming temperature. To determine the equilibrium time of saturation, a foaming pressure of 12 MPa was applied to the samples, saturating for 1–20 min at the foaming temperature. For all other foaming experiments, a foaming pressure of 20 MPa was used at a fixed saturation time of 10 min. When the foaming pressure was 12 and 20 MPa, the pressure drop rate was at 17.56 and 28.86 MP/s, respectively [14]. After foaming, the samples were quickly removed for the subsequent characterization. The cycle time from batch to batch was less than 30 min. 2.3. Foam characterization The foamed samples were sliced by a sharp blade and then characterized by an Aigo-GE5 digital viewer (purchased from Beijing Huaqi Information Digital
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Technology Co., Ltd), which is an optical microscope with a maximum magnification of 540 times. The photographs were analyzed by the software of this digital viewer. The density of the foamed pellets was measured using Archimedes’ principle, in which the measuring vessel was calibrated with a metal sinker. A known mass of foam was attached to the sinker and immersed in water, and the volume of the water displaced due to the immersed specimen was measured. It is nearly impossible for the foamed sample to absorb any water as the measurement time was very short. This method for measuring foam density is the same as that reported in literature [10]. 2.4. Evaluation of polymer foamability Polymer foamability was evaluated by two measured parameters, foaming temperature window and the correlation factor of foam density with foaming temperature. The foaming temperature window was determined from the lower and upper bound foaming temperatures at a given foaming condition. In this study, the lower foaming temperature was the temperature at which the expansion ratio (r ¼ dp/df, dp and df are the density before and after foaming) was at least two. The upper foaming temperature was the temperature where the sample was not successfully foamed due to cell collapse. A series of foaming experiments were done through varying the foaming temperature while the other foaming parameters (saturation time, foaming pressure and pressure drop rate) were fixed. The foamed sample densities were measured and plotted with the foaming temperatures. The foaming temperature window where the sample was capable of gas foaming was determined from the curve. The slope of the curve, an indication of the foaming process’s sensitivity to temperature, was also calculated. Both parameters were used to represent the foamability for a given sample. 3. Results and discussion 3.1. Assessment of an appropriate N2 saturation time The first step for foaming is the gas dissolving into the polymer, and the first question to answer in this paper is how long the high pressure N2 can reach the equilibrium concentration. We tried to use a digital pressure transducer (CY200, purchased from Chengdu Test Electronic Information CO., LTD, with an accuracy of 0.01 MPa) inserted into the mold to detect how long the high pressure N2 could saturate the polymer. It was found that the change of pressure with time was too small. Perhaps the dissolved gas in the small sample was too little and unnoticeable as compared to a gas pressure of 12–20 MPa. So, we tried to detect the equilibrium time by an indirect means. After choosing a certain foaming temperature and pressure, we deduced the equilibrium time from the foam density curves as a function of saturation time. This method was also used by other researchers [18]. Fig. 1 shows the variation of foam density as a function of saturation time under a foaming pressure of 12 MPa. Two foaming temperatures of 125 C and 135 C were used for LDPE, 119 C and 123 C for LLDPE samples. As seen in the
Fig. 1. The change of foam density as a function of saturation time at the foaming pressure of 12 MPa.
figure, the foam densities decreased dramatically with increasing the saturation time from 1 to 8 min, and then remained unchanged thereafter. Thus, the equilibrium time of gas saturation for both LDPE and LLDPE was less than 10 min when the foaming pressure was fixed at 12 MPa. As high temperature (see the LDPE in Fig. 1) and high pressure were preferable for gas dissolving in the polymer matrix, it was reasonable to believe that increasing the foaming pressure to 20 MPa would further decrease the equilibrium time. The following experiments conducted for evaluating the foamability of LDPE and LLDPE were under circumstances such that the gas had attained equilibrium concentration in the polymer matrix.
3.2. The foamability of LDPE and LLDPE Fig. 2 is the foaming temperature window of LDPE and LLDPE when the foaming pressure and saturation time are fixed at 20 MPa and 10 min, respectively. From Fig. 2, we can see that the foaming temperature window for LLDPE was from 115 to 123 C, whereas LDPE had a foaming temperature window from 108 to 160 C. It is should be
Fig. 2. The foaming temperature window of LDPE and LLDPE under the foaming pressure of 20 MPa and at the saturation time of 10 min.
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noted here that the LDPE can be still foamed even above 160 C. The wide foaming temperature window for LDPE was expected because LDPE had a high degree of long chain branching, leading to higher macromolecular entanglement density and higher macromolecular chain interactions, which made the growth of cells easier to stabilize in the rapid gas expansion foaming process. It can be also noted from Fig. 2 that the minimum foam density of LLDPE and LDPE was 0.090 g/cm3 and 0.052 g/ cm3, respectively. Under the same processing condition, LDPE expanded about 20 times whereas LLDPE expanded about 11 times relative to their virgin resins. Lower expansion ratio for LLDPE foam might be caused by some crystal residues remaining at the foaming temperature around its melting point (122.09 C). This is necessary for LLDPE to have a high enough melt strength in the gas foaming process. In comparison, LDPE was foamed in a molten state (its melting point is 106.12 C). Fig. 2 shows that the foam density of LLDPE was very sensitive to the foaming temperature. For example, the sample density obtained at 115 C was 0.372 g/cm3, which was much higher than 0.090 g/cm3 obtained at 123 C. On the contrary, the LDPE was insensitive to the foaming temperature. The density was almost identical when increasing the foaming temperature from 115 C to 160 C. The LDPE had a slope of 1.71 104 (g/cm3)/ C in densityfoaming temperature curve whereas LLDPE had a slope of 3.48 102 (g/cm3)/ C. Fig. 3 shows the minimum foam densities obtained for LDPE and LLDPE under foaming pressures of 12 MPa and 20 MPa. It can be seen that the foam density of LDPE (0.105 g/cm3) at 12 MPa was also insensitive to foaming
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Fig. 3. Effect of foaming pressure on the foam density of LDPE and LLDPE at the saturation time of 10 min.
temperature, whereas that of LLDPE was sensitive. We could also see that with lowering of the foaming pressure the foam density was increased, because the gas content in the polymer matrix was determined by the gas pressure. Figs. 4 and 5 show the cell morphologies of foams obtained at different foaming temperatures for LDPE and LLDPE, respectively. From Fig. 4, it can be seen that the cell morphology of LDPE obtained at 115 C had no significant difference compared with the sample obtained at 135 C. The cell morphology of LDPE was not sensitive to foaming temperature. On the contrary, the cell morphology of LLDPE was very sensitive to the foaming temperature (see Fig. 5). For example, when at lower foaming temperature
Fig. 4. Cell morphology of LDPE obtained from different foaming temperatures when the foaming pressure and saturation time were 20 MPa and 10 min, respectively. (1)115 C, (2)120 C, (3)125 C, (4)130 C, (5)135 C
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Fig. 5. Cell morphology of LLDPE obtained from different foaming temperatures when the foaming pressure and saturation time were 20 MPa and 10 min, respectively. (1)115 C, (2)117 C, (3)119 C, (4)121 C, (5)123 C
of 115 C, the cells were much smaller than that at 123 C. In addition, the cell sizes were not uniform at lower foaming temperatures due to the crystal residues remaining in the LLDPE melt. At higher foaming temperatures, thinner cell walls would be generated, because the melt strength of LLDPE was very low. When the foaming temperature was over 123 C, the melt strength was not high enough to hold the growing cell structure in the rapid gas foaming process. The bubbles would rapidly collapse at the end. Thus, LLDPE can not be foamed thereafter. It should be noted that the melt strength of LDPE was high enough to hold the growing cell structure even though some open cells were generated at temperatures over 140 C (which was not shown here). From the foaming temperature window, the correlation factor of foam density with foaming temperature and variation of cell structure with temperature, we can see that the LDPE has outstanding foamability as compared to LLDPE. The wide foaming temperature window, foam density and cell structure insensitivity to foaming temperature all ensured the LDPE foam can be manufactured on a commercial scale with high process and product quality control. No wonder that LDPE foam has been produced since the early 1940s and is still a major commercial foam product with wide applications. On the contrary, LLDPE is not a competitive resin for gas foaming even though it can be foamed under certain conditions. Based on the above results, we can see that the method for evaluating the polymer foamability works directly for raw materials. No nucleating agents and processing aids were added in the experiments. The results purely show inherent polymer ability for gas foaming under a given processing condition.
4. Conclusions In this paper, we used a self designed batch foaming apparatus to develop a method to rapidly evaluate polymer foamability. Two parameters were proposed as criteria. The first was the foaming temperature window, and the second was the correlation factor of the foam density with foaming temperature. By selecting raw materials of LDPE and LLDPE as examples, we further illustrated how to use this method for evaluating the foamability. The results showed LDPE had a foaming temperature range of more than 52 C, whereas that of LLDPE was only 8 C. The foam density and cell structure of LDPE were insensitive to foaming temperature, whereas that of LLDPE was sensitive. LDPE indeed had an excellent foamability. This is a simple, quick, direct and accurate method for evaluating polymer foamability. It is quite suitable for all shapes of materials, particularly for raw polymer resins. Therefore, it can also help foam production manufacturers to select the raw materials, to do initial foaming setup, to optimize the processing parameters and to develop new products. References [1] S.T. Lee, Foam Extrusion: Principles and Practice. Technomic Publishing Company, 2000. [2] X. Xu, C.B. Park, D. Xu, Effects of die geometry on cell nucleation of PS foams blown with CO2, Polymer Engineering and Science 43 (2003) 1378–1390. [3] H.E. Naguib, C.B. Park, S.W. Song, Effect of supercritical gas on crystallization of linear and branched polypropylene resins with foaming additives, Industrial and Engineering Chemistry Research 44 (2005) 6685–6691.
C. Yu et al. / Polymer Testing 30 (2011) 118–123 [4] P.C. Lee, J. Wang, C.B. Park, Extruded open-cell foams using two semicrystalline polymers with different crystallization temperatures, Industrial and Engineering Chemistry Research 45 (2006) 175–181. [5] S.T. Lee, L. Kareko, J. Jun, Study of thermoplastic PLA foam extrusion, Journal of Cellular Plastics 44 (2008) 293–305. [6] J.W.S. Lee, C.B. Park, Use of nitrogen as a blowing agent for the production of fine-celled high-density polyethylene foams, Macromolecular Materials and Engineering 291 (2006) 1233–1244. [7] C.B. Park, D.F. Baldwin, N.P. Suh, Effect of the pressure drop rate on cell nucleation in continuous processing of microcellular polymers, Polymer Engineering and Science 35 (1995) 432–440. [8] C.B. Park, L.K. Cheung, A study of cell nucleation in the extrusion of polypropylene foams, Polymer Engineering and Science 37 (1997) 1–10. [9] H.E. Naguib, C.B. Park, N. Reichelt, Fundamental foaming mechanisms governing the volume expansion of extruded polypropylene foams, Journal of Applied Polymer Science 91 (2004) 2661–2668. [10] S. Siripurapu, Y.J. Gay, J.R. Royer, J.M. DeSimone, R.J. Spontak, S.A. Khan, Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process, Polymer 43 (2002) 5511–5520. [11] L.D. Maio, I. Coccorullo, S. Montesano, L. Incarnato, Chain extension and foaming of recycled PET in extrusion equipment, Macromolecular Symposia 228 (2005) 185–200. [12] J.W.S. Lee, K. Wang, C.B. Park, Challenge to extrusion of low density microcellular polycarbonate foams using supercritical carbon dioxide, Industrial and Engineering Chemistry Research 44 (2005) 92–99. [13] L.M. Matuana, C.A. Diaz, Study of cell nucleation in microcellular poly (lactic acid) foamed with supercritical CO2 through a continuous-extrusion process, Industrial and Engineering Chemistry Research 49 (2010) 2186–2193. [14] C.Y. Yu, B.T. Wu, Y. Wang, W.G. Li, A new self designed apparatus for foaming, Polymer Testing 29 (2010) 1014–1020. [15] S.K. Goel, E.J. Beckman, Generation of microcellular polymeric foams using supercritical carbon dioxide. I: effect of pressure and
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temperature on nucleation, Polymer Engineering and Science 34 (1994) 1137–1147. D. Miller, P. Chatchaisucha, V. Kumar, Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide I. Processing and structure, Polymer 50 (2009) 5576–5584. H.E. Naguib, C.B. Park, U. Panzer, N. Reichelt, Strategies for achieving ultra low-density polypropylene foams, Polymer Engineering and Science 42 (2002) 1481–1492. L. Zirkel, M. Jakob, H. Münstedt, Foaming of thin films of a fluorinated ethylene propylene copolymer using supercritical carbon dioxide, The Journal of Supercritical Fluids 49 (2009) 103–110. T. Mori, H. Hayashi, M. Okamoto, S. Yamasaki, H. Hayami, Foam processing of polyethylene ionomers with supercritical CO2, Composites Part A: Applied Science and Manufacturing 40 (2009) 1708–1716. P. Micic, S.N. Bhattacharya, Elongational behavior of polyethylene melts-effect of deformation, Polymer Engineering and Science 40 (2000) 1571–1580. P. Spitael, C.W. Macosko, Strain hardening in polypropylenes and its role in extrusion foaming, Polymer Engineering and Science 44 (2000) 2090–2100. G.T. Nam, J.H. Yoo, J.W. Lee, Effect of long-chain branches of polypropylene on rheological properties and foam-extrusion performances, Journal of Applied Polymer Science 96 (2005) 1793–1800. E.T. Kabamba, D. Rodrigue, The effect of recycling on LDPE foamability: elongational rheology, Polymer Engineering and Science 48 (2008) 11–18. R.G. Liao, W. Yu, C.X. Zhou, Rheological control in foaming polymeric materials: I. Amorphous polymers, Polymer 51 (2010) 568– 580. B. Krause, R. Mettinkhof, N.F.A. van der Vegt, M. Wessling, Microcellular foaming of amorphous high-Tg polymers using carbon dioxide, Macromolecules 34 (2001) 874–884. Z.M. Xu, X.L. Jiang, T. Liu, G.H. Hu, L. Zhao, Z.N. Zhu, W.K. Yuan, Foaming of polypropylene with supercritical carbon dioxide, The Journal of Supercritical Fluids 41 (2007) 299–310.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
A direct method for evaluating polymer foamability Chenyang Yu, Ya Wang, Bingtian Wu, Wenguang Li* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu, Sichuan 610065, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2010 Accepted 4 November 2010
In this study, we proposed a direct method to rapidly evaluate polymer foamability by using a new self designed foaming apparatus with nitrogen (N2) as a blowing agent. Two parameters, the foaming temperature window and the correlation factor of the foam density with foaming temperature were used to characterize the foamability. Low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) resins were selected as samples to compare their foamability. The results showed that LDPE had a foaming temperature window as wide as at least 52 C, whereas that of LLDPE was only 8 C. Moreover, the foam density and cell morphology were insensitive to foaming temperature for LDPE but sensitive for LLDPE. The inherent capability of LDPE for gas foaming was much better than that of LLDPE. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Method Evaluate Foamability LDPE LLDPE Gas foaming
1. Introduction Physical gas extrusion foaming is an advanced and efficient technology used to produce polymeric foams in industry. In the past decades, researchers investigated the critical processing parameters and equipment setup in this continuous extrusion foaming process [1–5]. To date, various polymers, including polyethylene (PE) [6], polypropylene (PP) [7–9], poly (vinylidene fluoride) (PVDF) [10], poly (ethylene terephthalate) (PET) [11], polycarbonate (PC) [12] and poly (l-lactide) (PLA) [5,13] have been foamed on a laboratory scale. However, it is still difficult to make such investigations for many other polymers due to the absence of knowledge about polymer foamability for the initial processing setup. At the same time, it is time consuming and expensive to identify the critical processing parameters in the continuous extrusion foaming process. Although some polymers are successfully foamed at laboratory scale, they have not reached the optimizing stage for commercial production. Obviously, we need to develop a simple,
* Corresponding author. Tel.: þ86 028 85460817; fax: þ86 028 85402465. E-mail address: [email protected] (W. Li). 0142-9418/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.11.001
efficient and reliable method to rapidly accumulate experimental data as well as to study the critical parameters for successful gas foaming. In a previous paper [14], we reported a new gas foaming apparatus self designed in our laboratory. It is a batch gas foaming process with short cycle time (<30 min), easy to operate and suitable for direct foaming with a small amount of raw materials. The critical processing parameters such as temperature, pressure and pressure drop rate can be well controlled. Instrumental and experimental assessments showed that it could be an ideal lab tool for rapid screening the polymer resins for gas foaming. The question then is: What standards should we use to indicate if the polymer foamability is good or not in the results? In past studies of microcellular foams, the foamability was meant for the microcellular foaming ability for polymers under a certain processing condition [15,16]. It was found that the processing parameters such as temperature, pressure and depressurization rate were most crucial. Either high gas pressure at a low foaming temperature or low gas pressure at a high foaming temperature was favorable for generating microcellular foams. The high pressure drop rate was preferable for spontaneous cell nucleation and growth, leading to microcellular foams with a high cell density. It should be mentioned that the focus was on how to generate
C. Yu et al. / Polymer Testing 30 (2011) 118–123
micro-cell structure, the foam expansion ratio was small and negligible (<2 times) in most cases. In fact, it has been a challenge to produce low density microcellular foams [17]. As recently revealed, the size of the cells mainly depends upon the foaming temperature [18,19], i.e. the state of the polymer for a given sample. This means that it is possible to obtain microcellular foams for almost all polymers under a certain processing condition (low foaming temperature and high depressurization rate). Thus, the definition of foamability from the microcellular foaming results is meaningless and ambiguous. It is known that for the gas foaming process, once the high pressure gas is in the polymer matrix through either a batch or continuous process, the foam can always be obtained by a sudden pressure drop and cooling. The foam quality such as cell structure and expansion ratio is not only dependent upon processing conditions but also upon the polymer itself. For example, low density polyethylene (LDPE) is easy to foam to a low density, but high density polyethylene (HDPE) is not. Thus, polymer foamability should relate to the inherent ability of a polymer to be foamed at a given foaming condition. This inherent ability is a characteristic of the polymer, similar to mechanical properties. In order to characterize the inherent gas foaming ability (foamability), it is necessary to develop some criteria. In the literature, tremendous efforts have been paid to learn the rheology of polymer/gas mixtures for a better understanding of the foaming process and polymer foamability [20–24]. Since the cell walls are stretched in the foaming process, the nucleation stage and the following steps of cell growth and stabilization are controlled by the extensional rheology [1]. It was found that polymers with a high degree of long chain branching exhibited strain hardening behavior in the rheological test. These polymers usually have high melt strength and good foamability due to the strain hardening. However, the quantitative relationship between the melt strength and polymer foamability is still unclear. Although the melt strength is measurable in a melt extensional test and it is then used to judge the polymer foamability, the criteria is indirect and inaccurate, particularly for polymers without chain branching. Therefore, it is very important to have direct and accurate criteria to characterize the polymer foamability. As mentioned before, for a given polymer, the state for foaming is mainly determined by the foaming temperature, which is most critical among the processing parameters. For instance, for amorphous high Tg polymers, Krause et al. [25] found the lower and upper bound foaming temperatures and determined the microcellular foaming temperature range of 80–100 C at different foaming pressures. For a crystalline PP, Xu et al. [26] found that the foaming temperature range was only about 4 C. These results show that the wide or narrow foaming temperature range differing from one polymer to another may be an indication of different inherent foaming ability. Therefore, we propose to use the foaming temperature window as the first criteria to evaluate polymer foamability. We will introduce how to determine the foaming temperature window for a given sample in the following section. In order to determine whether the product quality is easy to control or not, we propose to use the correlation factor of the
119
foam density-foaming temperature curve as the second criteria. As for cell structure control, it is reasonable to observe the variation of cell size with temperature and to further elaborate the evaluation of polymer foamability by considering foam microstructure. In the current study, we used our self designed foaming apparatus to develop a fast, reliable and direct method to evaluate the polymer foamability. It is well known that LDPE is generally better than LLDPE for foaming because of higher molecular entanglement density and higher molecular chain interactions leading to increased melt strength. But, we still do not know how much better it is. Therefore, we chose LDPE and LLDPE as samples to further illustrate how to evaluate the foamability for given polymers. The results showed that LDPE had a foaming temperature window as wide as at least 52 C, while that of LLDPE was only 8 C. Moreover, the foam density of LDPE was insensitive to foaming temperature whereas that of LLDPE was very sensitive. We hope that this direct evaluating method of polymer foamability could be useful for gas foaming research in academia and helpful for foam production in industry. 2. Experimental 2.1. Materials LDPE (trade name: IF7B, melt flow index ¼ 7.0 g/10min) was from Yanshan Petrochemical Co., SINOPEC. LLDPE (trade name: 7042, melt flow index ¼ 2.0 g/10min) was from Lanzhou Petrochemical Co., SINOPEC. The pellets as received were directly used for foaming experiments. The N2 (purity 99.99%) was purchased from Taiyu Air product Co., LTD, Chengdu, China. 2.2. Foaming experiments We used a self designed foaming apparatus to do the foaming experiments. The foaming apparatus consists of a compression molding machine, high-pressure mold, high pressure gas supply system, pressure gauge and highpressure needle valve. The features, work principle and foaming procedure were systematically introduced in a previous paper [14]. In this study, about 20 polymer pellets were used for each batch at predetermined saturation time and foaming temperature. To determine the equilibrium time of saturation, a foaming pressure of 12 MPa was applied to the samples, saturating for 1–20 min at the foaming temperature. For all other foaming experiments, a foaming pressure of 20 MPa was used at a fixed saturation time of 10 min. When the foaming pressure was 12 and 20 MPa, the pressure drop rate was at 17.56 and 28.86 MP/s, respectively [14]. After foaming, the samples were quickly removed for the subsequent characterization. The cycle time from batch to batch was less than 30 min. 2.3. Foam characterization The foamed samples were sliced by a sharp blade and then characterized by an Aigo-GE5 digital viewer (purchased from Beijing Huaqi Information Digital
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Technology Co., Ltd), which is an optical microscope with a maximum magnification of 540 times. The photographs were analyzed by the software of this digital viewer. The density of the foamed pellets was measured using Archimedes’ principle, in which the measuring vessel was calibrated with a metal sinker. A known mass of foam was attached to the sinker and immersed in water, and the volume of the water displaced due to the immersed specimen was measured. It is nearly impossible for the foamed sample to absorb any water as the measurement time was very short. This method for measuring foam density is the same as that reported in literature [10]. 2.4. Evaluation of polymer foamability Polymer foamability was evaluated by two measured parameters, foaming temperature window and the correlation factor of foam density with foaming temperature. The foaming temperature window was determined from the lower and upper bound foaming temperatures at a given foaming condition. In this study, the lower foaming temperature was the temperature at which the expansion ratio (r ¼ dp/df, dp and df are the density before and after foaming) was at least two. The upper foaming temperature was the temperature where the sample was not successfully foamed due to cell collapse. A series of foaming experiments were done through varying the foaming temperature while the other foaming parameters (saturation time, foaming pressure and pressure drop rate) were fixed. The foamed sample densities were measured and plotted with the foaming temperatures. The foaming temperature window where the sample was capable of gas foaming was determined from the curve. The slope of the curve, an indication of the foaming process’s sensitivity to temperature, was also calculated. Both parameters were used to represent the foamability for a given sample. 3. Results and discussion 3.1. Assessment of an appropriate N2 saturation time The first step for foaming is the gas dissolving into the polymer, and the first question to answer in this paper is how long the high pressure N2 can reach the equilibrium concentration. We tried to use a digital pressure transducer (CY200, purchased from Chengdu Test Electronic Information CO., LTD, with an accuracy of 0.01 MPa) inserted into the mold to detect how long the high pressure N2 could saturate the polymer. It was found that the change of pressure with time was too small. Perhaps the dissolved gas in the small sample was too little and unnoticeable as compared to a gas pressure of 12–20 MPa. So, we tried to detect the equilibrium time by an indirect means. After choosing a certain foaming temperature and pressure, we deduced the equilibrium time from the foam density curves as a function of saturation time. This method was also used by other researchers [18]. Fig. 1 shows the variation of foam density as a function of saturation time under a foaming pressure of 12 MPa. Two foaming temperatures of 125 C and 135 C were used for LDPE, 119 C and 123 C for LLDPE samples. As seen in the
Fig. 1. The change of foam density as a function of saturation time at the foaming pressure of 12 MPa.
figure, the foam densities decreased dramatically with increasing the saturation time from 1 to 8 min, and then remained unchanged thereafter. Thus, the equilibrium time of gas saturation for both LDPE and LLDPE was less than 10 min when the foaming pressure was fixed at 12 MPa. As high temperature (see the LDPE in Fig. 1) and high pressure were preferable for gas dissolving in the polymer matrix, it was reasonable to believe that increasing the foaming pressure to 20 MPa would further decrease the equilibrium time. The following experiments conducted for evaluating the foamability of LDPE and LLDPE were under circumstances such that the gas had attained equilibrium concentration in the polymer matrix.
3.2. The foamability of LDPE and LLDPE Fig. 2 is the foaming temperature window of LDPE and LLDPE when the foaming pressure and saturation time are fixed at 20 MPa and 10 min, respectively. From Fig. 2, we can see that the foaming temperature window for LLDPE was from 115 to 123 C, whereas LDPE had a foaming temperature window from 108 to 160 C. It is should be
Fig. 2. The foaming temperature window of LDPE and LLDPE under the foaming pressure of 20 MPa and at the saturation time of 10 min.
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noted here that the LDPE can be still foamed even above 160 C. The wide foaming temperature window for LDPE was expected because LDPE had a high degree of long chain branching, leading to higher macromolecular entanglement density and higher macromolecular chain interactions, which made the growth of cells easier to stabilize in the rapid gas expansion foaming process. It can be also noted from Fig. 2 that the minimum foam density of LLDPE and LDPE was 0.090 g/cm3 and 0.052 g/ cm3, respectively. Under the same processing condition, LDPE expanded about 20 times whereas LLDPE expanded about 11 times relative to their virgin resins. Lower expansion ratio for LLDPE foam might be caused by some crystal residues remaining at the foaming temperature around its melting point (122.09 C). This is necessary for LLDPE to have a high enough melt strength in the gas foaming process. In comparison, LDPE was foamed in a molten state (its melting point is 106.12 C). Fig. 2 shows that the foam density of LLDPE was very sensitive to the foaming temperature. For example, the sample density obtained at 115 C was 0.372 g/cm3, which was much higher than 0.090 g/cm3 obtained at 123 C. On the contrary, the LDPE was insensitive to the foaming temperature. The density was almost identical when increasing the foaming temperature from 115 C to 160 C. The LDPE had a slope of 1.71 104 (g/cm3)/ C in densityfoaming temperature curve whereas LLDPE had a slope of 3.48 102 (g/cm3)/ C. Fig. 3 shows the minimum foam densities obtained for LDPE and LLDPE under foaming pressures of 12 MPa and 20 MPa. It can be seen that the foam density of LDPE (0.105 g/cm3) at 12 MPa was also insensitive to foaming
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Fig. 3. Effect of foaming pressure on the foam density of LDPE and LLDPE at the saturation time of 10 min.
temperature, whereas that of LLDPE was sensitive. We could also see that with lowering of the foaming pressure the foam density was increased, because the gas content in the polymer matrix was determined by the gas pressure. Figs. 4 and 5 show the cell morphologies of foams obtained at different foaming temperatures for LDPE and LLDPE, respectively. From Fig. 4, it can be seen that the cell morphology of LDPE obtained at 115 C had no significant difference compared with the sample obtained at 135 C. The cell morphology of LDPE was not sensitive to foaming temperature. On the contrary, the cell morphology of LLDPE was very sensitive to the foaming temperature (see Fig. 5). For example, when at lower foaming temperature
Fig. 4. Cell morphology of LDPE obtained from different foaming temperatures when the foaming pressure and saturation time were 20 MPa and 10 min, respectively. (1)115 C, (2)120 C, (3)125 C, (4)130 C, (5)135 C
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Fig. 5. Cell morphology of LLDPE obtained from different foaming temperatures when the foaming pressure and saturation time were 20 MPa and 10 min, respectively. (1)115 C, (2)117 C, (3)119 C, (4)121 C, (5)123 C
of 115 C, the cells were much smaller than that at 123 C. In addition, the cell sizes were not uniform at lower foaming temperatures due to the crystal residues remaining in the LLDPE melt. At higher foaming temperatures, thinner cell walls would be generated, because the melt strength of LLDPE was very low. When the foaming temperature was over 123 C, the melt strength was not high enough to hold the growing cell structure in the rapid gas foaming process. The bubbles would rapidly collapse at the end. Thus, LLDPE can not be foamed thereafter. It should be noted that the melt strength of LDPE was high enough to hold the growing cell structure even though some open cells were generated at temperatures over 140 C (which was not shown here). From the foaming temperature window, the correlation factor of foam density with foaming temperature and variation of cell structure with temperature, we can see that the LDPE has outstanding foamability as compared to LLDPE. The wide foaming temperature window, foam density and cell structure insensitivity to foaming temperature all ensured the LDPE foam can be manufactured on a commercial scale with high process and product quality control. No wonder that LDPE foam has been produced since the early 1940s and is still a major commercial foam product with wide applications. On the contrary, LLDPE is not a competitive resin for gas foaming even though it can be foamed under certain conditions. Based on the above results, we can see that the method for evaluating the polymer foamability works directly for raw materials. No nucleating agents and processing aids were added in the experiments. The results purely show inherent polymer ability for gas foaming under a given processing condition.
4. Conclusions In this paper, we used a self designed batch foaming apparatus to develop a method to rapidly evaluate polymer foamability. Two parameters were proposed as criteria. The first was the foaming temperature window, and the second was the correlation factor of the foam density with foaming temperature. By selecting raw materials of LDPE and LLDPE as examples, we further illustrated how to use this method for evaluating the foamability. The results showed LDPE had a foaming temperature range of more than 52 C, whereas that of LLDPE was only 8 C. The foam density and cell structure of LDPE were insensitive to foaming temperature, whereas that of LLDPE was sensitive. LDPE indeed had an excellent foamability. This is a simple, quick, direct and accurate method for evaluating polymer foamability. It is quite suitable for all shapes of materials, particularly for raw polymer resins. Therefore, it can also help foam production manufacturers to select the raw materials, to do initial foaming setup, to optimize the processing parameters and to develop new products. References [1] S.T. Lee, Foam Extrusion: Principles and Practice. Technomic Publishing Company, 2000. [2] X. Xu, C.B. Park, D. Xu, Effects of die geometry on cell nucleation of PS foams blown with CO2, Polymer Engineering and Science 43 (2003) 1378–1390. [3] H.E. Naguib, C.B. Park, S.W. Song, Effect of supercritical gas on crystallization of linear and branched polypropylene resins with foaming additives, Industrial and Engineering Chemistry Research 44 (2005) 6685–6691.
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