low density hms polypropylene foam: controlling foam...

LOW DENSITY HMS POLYPROPYLENE FOAM: CONTROLLING FOAM DENSITY AND CELL MORPHOLOGY

Steven M. Krupinski*, Kimberly M. McLoughlin

Product Development, Braskem NA, Pittsburgh, PA, USA

Abstract Linear isotactic polypropylene (PP) is used in a vast array of applications because it provides mechanical strength, chemical resistance, and thermal stability. However, semi-crystalline linear PP has limited use in low-density foam applications, which are dominated by amorphous polymers, such as polystyrene. This paper discusses technical challenges that have limited the use of PP in low-density, extruded foams. Specifically, the challenge of controlling foam density along with closed cell percent and cell count is addressed. The rheological properties have been evaluated in terms of viscosity, elasticity and melt strength which show good foaming potential. Interactions between the HMSPP polymer, linear PP blend polymers, blowing agent type, additive formulation, and process variables are investigated here for a new, developmental HMSPP grade. Braskem has developed a proprietary technology to produce High Melt Strength Polypropylene (HMSPP), branded as the Amppleo family, with a specific long chain branching configuration that helps overcome the limitations of linear PP when foaming to low densities of 150-50kg/m3.

Introduction Isotactic polypropylene (PP) is a semi-crystalline polymer that is widely used in packaging, automotive, and industrial applications due to its balance of mechanical properties, heat resistance, and favorable carbon footprint. Polypropylene foam expands the range of PP performance properties and provides opportunities for material reduction, light-weighting and sustainability. However, foam processing of PP is more challenging than foaming amorphous polymers, such as polystyrene and polyurethane for two critical reasons. First, because PP is semi-crystalline, melt processing requires heating PP into a temperature range well above its glass transition temperature, where its melt viscosity, especially extensional viscosity, is relatively low compared to the viscosity ranges of amorphous polymers that are processed at temperatures just above their glass transition temperatures (Tg). Standard linear polypropylene grades cannot provide low-density foam that truly competes with that made from amorphous polymers because the extensional viscosity (often called “melt strength”) of the polymer melt is too low to retain a

significant volume of gas blowing agent while the PP expands. Low melt strength leads to higher open cell content and higher density foams. Historically, multiple technologies have been developed to improve the melt strength of polypropylene, enabling PP foam extrusion processes to achieve significant density reductions. [1, 2] The second challenge associated with the semi-crystallization nature of PP is more complex. Polypropylene crystallization from the melt induces foam cell nucleation. [3] Therefore, while the PP melt is simultaneously foaming and cooling, crystallization starts, and it competes with the foam nucleation mechanism provided by the addition of external foam cell nucleators such as talc, sodium carbonate and calcium carbonate. As a result, PP crystallization complicates the control of foam cell morphology. Control of PP cell morphology requires balancing contributions from the polymer with contributions from the foam process, including the additive formulation. This paper discusses the critical factors that control both density and foam cell morphology for a novel, developmental HMSPP. The effects of isobutane and CO2 blowing agents are compared, and the role of cell nucleating agent is demonstrated. Interactions between polymer, blowing agent type, additive formulation, and process variables are investigated. The range of cell morphologies accessible in some density ranges is extended using blends of linear and branched PP.

Background

The performance properties of polymer foams depend strongly on both density and cell morphology. In the case of extruded foam sheet, density depends on the amount of blowing agent that is mixed with the molten polymer during extrusion and retained by the cooling polymer during expansion after exiting the foam die. The effects of blowing agent loading, polymer melt temperature, and die pressure on polypropylene foam density have been demonstrated in previous literature reports. [4,5,6] In additional to density, polymer foam performance depends on cell morphology. The sizes and shapes of cells, thickness of cell walls, and whether the walls form

SPE ANTEC® Anaheim 2017 / 2481

closed or open cells decide critical attributes that determine the way a foam performs in an application. For example, in both food packaging and protective packaging, closed cells are required to provide energy management or leak resistance. In other applications, an optimal ratio of open to closed cells is needed to provide thermal or sound insulation. In still other applications, open cells are required to enable vacuum forming. Because foam cell morphology is so important, many studies have been conducted to understand how cell morphology is developed and controlled. The fundamental mechanisms that drive cell size and cell count have been studied and reported. Technologies have been developed specifically to control foam cell morphology, including specialized foam extrusion die geometries and foam cell nucleation agents. In this paper, we investigate the relative roles of polymer blends, blowing agent loadings, additive formulation, and foam process variables in balancing these attributes for a novel, developmental high melt strength polypropylene grade; Amppleo HMSPP.

Materials and Methods

A developmental, high-melt strength polypropylene Amppleo (HMSPP) provided by Braskem was used for the foam extrusion experiments. The properties of the HMSPP are shown in Table I. For comparison, the properties of Braskem’s D036W6 grade are also shown. The D036W6 is a commercially available, standard, linear PP with similar melt flow rate (MFR).

Table I Properties of Braskem HMSPP

Property Unit HMSPPStdPP

MFR g/10min 3.5 3.6Tm °C 164 162Tc °C 131.5 115meltstrength@190⁰C cN 36 6meltelongation@190⁰C mm/sec 165 117flexuralmodulus MPa 1945 1650HDT ⁰C 122 107

Compared to standard, linear polypropylenes, the HMSPP exhibits both high melt strength and high melt elongation, as measured by a Göttfert Rheotens melt rheometer. The Rheotens instrument consists of a capillary barrel which is heated to a constant set temperature (190⁰C), a piston to deliver a steady volume of polymer melt, a capillary die through which a molten strand is extruded at fixed speed, and a set of take-up wheels which draw the molten strand at increasing velocity. The torque on the strand take-up wheels is measured and then converted to force (centinewtons). This force is then plotted versus the

drawdown velocity in Figure 1. The peak force is referred to as the melt strength, an indication of extensional viscosity. The velocity at peak force is reported as the melt elongation.

Figure 1. Rheotens melt strength data for Braskem HMSPP. In addition to the HMSPP, several linear polypropylenes (LPP) were used in small concentrations to form linear/ HMSPP blends for foam extrusion. These resins were mixed and compounded using the 40-mm KMB co-rotating twin screw extruder that was the primary extruder within the tandem direct injection foam process. The properties of the linear PP’s are listed in Table II.

Table II Linear PP Blend Agents

polymer type MFR wt%C2HMSPP(1) branchedPP 3.5 N/ALPP2 low-C2randomcopolymer 0.5 0.5LPP3 high-C2randomcopolymer 0.5 6LPP4 heterophasiccopolymer 0.3 8

Direct-Injection Foam Process Method Foams were produced using a laboratory pilot-scale, tandem DI extrusion foam line at KraussMaffei Berstorff GmbH in Hanover, Germany. The KMB foam extrusion system consists of a primary, 40-mm diameter, twin screw compounding extruder (TSE) for melt mixing and a secondary, 90-mm diameter, single screw extruder for cooling. The blowing agent gas is injected directly to the primary extruder. The extruder system pressure is maintained significantly above the partial pressure of the blowing agent at that temperature to maintain the polymer melt-blowing agent mixture as a single phase. The pilot line throughput rate capacity is 80-100 kg/hr. The


secondary extruder was fitted with a 50-mm annular die, and then the foamed extrudate stretched over a cylindrical cooling mandrel before being slit to flatten out the foam sheet and finally fed into a takeup winder. The ratio of mandrel diameter to annular die diameter (blow up ratio) was 2:1 for lower density foams and 3:1 for higher density foams. Liquid isobutane (2 wt%, 4 wt% and 6 wt%) and liquid carbon dioxide (1.2 wt%) were tested as physical blowing agents. The blowing agent was added to the twin screw extruder using a positive displacement pump. The secondary extruder barrel was controlled by three separate oil zone heaters. Melt cooling was provided at the die using an oil zone heater/cooler. The melt temperature and pressure were monitored, respectively, by thermocouple and transducer located at the inlet to the annular die. Prior to extrusion, polymer pellets were dry-blended with other additives, as noted in Table II. Glycerol monostearate (GMS) was added as a processing aid (0-1 wt% range) to help slow the diffusion of blowing agent through the foam cell walls. FinTalc M30 was used as a foam cell nucleating agent (0-1 wt% range). Ecocell 20P, which is supplied by Polyfil, was added as a chemical blowing agent (0-0.15 wt% range). A solid volumetric metering feeder was used to feed the materials into the hopper of the TSE.

Test Methods Density Foam density, r, was determined by using a 100 mm2

square foam sheet sample. The mass (m) measured on an analytical balance was divided by the volume calculated from the dimensions, length (l), width (w) and height (h) of the sample measured with a caliper, as the equation

Closed cell content Closed cell content was measured using a Quantichrome Ultrafoam 1200e pycnometer (V5.04.) Each sample was cut into three pieces with approximate area 3 in2 each. The exact dimensions of the pieces were measured by caliper and entered as input the equipment to calculate the sample external geometric volume, .

Figure 2. Pycnometer method used to measure closed cell content. The gas pycnometer process is illustrated in Figure 2. The sample is placed in the cylinder, the cylinder is sealed, the calibrating amount of gas is introduced, and the resulting pressure is measured. The pressure difference between the empty cylinder and the cylinder holding the sample is proportionally related to the volume occupied by the closed cells of sample present, because the gas diffuses into the open cells. The working equation for the gas pycnometer is; V pycnometer = Vc + Vr /(1-Pinitial/Pfinal) Where Vc is the volume of the empty sample chamber, Vr

is the volume of the reference chamber and Pinitial is the initial pressure within the sample chamber and Pfinal is the sample chamber pressure once the interchamber valve is opened. The closed cell content is calculated according to the expression:

Cell shape and size Foam cell morphology was analyzed through high definition images acquired using an Optical Microscope (Hirox) with magnification 35X or 50X, depending on the cell density. A small sample of foamed sheet was cut with a surgical blade along a diagonal relative to the machine direction. The cut surface was colored using a blue ink marker to enhance visual contract, and the sample was placed on the microscope stage. The micrograph area, A (µm), and cell count, NA, were recorded. The number of


cells per unit area, NA, was used to calculate the number of cells per volume, N, using this equation:

ResultsandDiscussion

CO2 vs Isobutane Blowing Agent: Effects on Foam Process Performance and Density The effect of gas feed concentration and type on density is demonstrated in Figure 3 below. For the foams produced using butane, density decreases as gas feed concentration increases over the range of 2-6 weight percent. This is consistent with literature reports. [6] The higher density demonstrated here (150 kg/m3) can be reached using CO2 instead of isobutane, but the weight of CO2 required is lower than that of isobutane since the molar mass of CO2 is 44 versus 58; respectively.

Figure 3. Effect of blowing agent on foam density. Effect of Melt Temperature Previous literature reports have also demonstrated that extruded foam density depends on melt temperature as well as blowing agent concentration, and the melt temperature which provides minimum density depends on blowing agent concentration[6]. For a given concentration of blowing agent, density decreases as polymer melt temperature decreases. This is due to the increase in polymer viscosity upon cooling, which enables retention of a higher amount of gas, which allows for a greater expansion ratio of the foam. The optimum foaming temperature shifts lower as the blowing agent concentration increases and the polymer

solution viscosity decreases. For each of the blowing agent concentrations tested here, the set temperature of the extrusion zone nearest the die was adjusted to provide a minimum foam density (Figure 4).

Figure 4. Effect of die zone set temperature on foam density. (Broad range of densities.) The higher density foams (150 kg/m3) produced with isobutane and those produced with CO2 exhibited minimum density at approximately the same optimum temperature of 176⁰C. However, the CO2 foam density decrease had a much steeper slope, as shown in Figure 5, suggesting a narrower foam processing window.

Figure 5. Effect of die zone set temperature on foam density. (Focusing on the high density foam range) At the same temperatures and in the same density range of 145-152 kg/m3, the die pressure was much higher for the CO2 blowing agent compared to the butane (Figure 6).


Figure 6. Effect of die zone set temperature on die pressure. Higher die pressure has been correlated with increased cell density, [7] so it was expected that the CO2 foams (which had higher die pressure) would exhibit higher cell counts than the butane foams. This seems to be the case, as discussed below. Foam produced using CO2 had a significantly higher cell count (2.8 million cells/in3) compared to foam with the same density produced using butane (1.7 million cells/in3.) Effect of Cell Nucleation Agents on Cell Count and Closed Cell Content The effect of cell nucleation agents on cell count and closed cell content are shown in Table III. For one series of foam, a chemical foaming agent, Ecocell 20P was added. Ecocell also acts as a foam cell nucleating agent in addition to releasing carbon dioxide. For another series, talc was used as a nucleating agent. The number of cells per unit volume (cell count) and the percentage of closed cells both increase as density increases, regardless of whether talc or CBA is added as a nucleating agent. However, at a given density, a much higher cell count can be obtained (with the same closed cell content) by using the CBA instead of the talc.

Table III Effects of Blowing Agents

on Cell Count and Closed Cell Content Talc CBA (Ecocell 20P)

wt% blowing

agent density, kg/m3

cell count,

cells/in3

closed cell

content density, kg/m3

cell count, cells/in3

closed cell

content 6% iC4 45.3 3.8E+04 54 43.4 1.2E+05* 52*

4% iC4 63.7 1.1E+05 63 60 5.8E+05 59

2% iC4 148 2.8E+06 71

1% CO2 149 1.7E+06 74

*CBA concentration decreased from 0.075 to 0.025 wt% for 6% butane The optical micrographs below (Figure 7) were both obtained at 35x magnification. They show a comparison between foams at with density about 150 kg/m3, both produced using the same concentration of CBA. The foam on the left was produced with CO2 blowing agent, and the one on the right with isobutane. The circles on the micrograph are used to count the number of cells in a unit area. They are included here to highlight variability in cell density. As noted in Table III, the CO2 foam has a wider distribution of cell sizes compared to the butane foam, which has more uniform cell size.

Figure 7. Optical micrographs of foams produced using CO2 and butane.


Effect of Linear Polymer Blend Agents The effect of blending HMSPP with linear polymers is demonstrated in Figure 8 below for CO2 foams with density about 150 kg/m3. Adding only a small amount of linear polymer (5 wt%) led to readily observable changes in cell morphology and cell size distributions.

Figure 8. Optical micrographs of foamed blends produced using CO2. As shown in Table IV, the foams produced from the HMSPP/ linear PP blends had similar cell counts and slightly higher closed cell content compared to the HMSPP-only foam. However, the micrographs illustrate that the foams produced from blends had more uniform cell sizes, larger average cell sizes, and fewer of the very small cells observed near the surface of the HMSPP foam.

Table IV Effect of Linear Blend Agents

on Cell Count and Closed Cell Content

density, kg/m3 cell count, cells/in3

closed cell content, %

HMSPP only 145 1.7E+06 74 5 wt% LPP2 147 1.7E+06 74 5 wt% LPP3 147 9.5E+05 76 5 wt% LPP4 147 1.4E+06 77

Blending the HMSPP with LPP2 (polypropylene copolymer with 0.5% C2 and fractional melt flow rate) or with LPP4 (polypropylene impact copolymer with fractional melt flow rate) increased the uniformity of cell sizes without significantly affecting cell count. However, blending the HMSPP with LPP3 (higher-C2 random copolymer with fractional melt flow rate) led to some cell coalescence, which reduced cell count. The synergistic effects of blending linear and branched polymers to provide low density foams with high cell density have been demonstrated in literature reports. [7]

Conclusions A new HMSPP produced by Braskem can be used to extrude low-density foams with high cell count and closed cell content, using either CO2 or butane as a blowing agent. To achieve this, one must control the cell morphology by carefully balancing contributions from the polymer, foaming agents, additives, and process variables. This will enable production of low-density PP foam for a wide variety of applications.

Acknowledgements The authors wish to thank the following researchers for valuable insights and contributions. Dr. Marcelo Farah conducted the foam extrusion trials at KMB, and Cintia do Prado conducted foam testing.

References

1. A. DeNicola, U. S. Patent 5, 047, 466 (1991.) 2. M Raetzsch, H. Bucka, A. Hesse, U Panzer, and N.

Reichelt, U. S. Patent 6,204,348 (2001.) 3. S. Doroudiani, C. Park, and M. Kortschot, Polym.

Eng. & Sci. 36(21), 2645 (Nov 1996.). 4. P. Lee, W. Kaewmesri, J. Wang, C. Park, J.

Pamchusak, R. Folland, and A. Praller, J. Appl. Poly. Sci. 109, 3122 (2008.)

5. H. Naguib and C. Park, Polym. Eng. & Sci., 42(7), 1481 (July 2002.)

6. C. Park and L. Cheung, Polym. Eng. & Sci., 37(1), 1 (1997.)

7. P. Spitael and C. Macosko, Poly. Eng. & Sci., 44(11), 2090 (2004.).


low density hms polypropylene foam: controlling foam...

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