Use of Inert Gases in Extruded Medium Density Polypropylene Foams

S . K. DEY, P. NATARAJAN. and M. XANTHOS

Polymer Processing Institute Castle Point on the Hudson

Hoboken, New Jersey 07030

and

M. D. BRAATHEN

Borealis AS N-3960, Stathelle, Norway

Carbon dioxide gas was used a s a physical blowing agent to produce medium density polypropylene foam sheets using a single screw extruder. The mechanical properties of the foam were similar in the machine direction and in the transverse direction. A better surface finish and a lower density foam was produced by using a commercial wrapping film as a cap layer. The process conditions and die design data are presented in an attempt to relate to product characteristics.

INTRODUCTION the polymer, lower molecular weight and less freezing

ypes of polyolefin foams, on the basis of their den-

density (lower than 50% voids); 2. medium density (between 50% and 90% voids); and 3. low density (higher than 90% voids).

Each of these types of foams fiids different applica- tions in the consumer market. The low density foams are primarily used for insulation purposes (sound or heat) and high density foams are primarily used for structural applications ( 1 I. The medium density polypropylene based foams, which are the topic of this paper, may be used in the automotive and packaging industry. The advantage of this type of foam is its higher specific strength which is the ratio of tensile strength over den- sity. This type of foam is primarily produced using chemical blowing agents. There may be two problems with the use of chemical blowing agents a s related to recycling of industrial scrap: 1. The solid residual de- composition products may affect the processability of the recycled polymer. 2. The residual undecomposed chemical blowing agent may impair recycling of noncon- forming parts, edge trim, etc.

For the last eight years, the Polymer Processing Institute (PPI] has been conducting research on the replacement of CFC's and volatile organic compounds with 'environmentally friendly" gases (nitrogen. car- bon dioxide, argon etc.) for extruded polymer foams. Some of the reasons for not using the "friendly gases" a t the first place may have been their low solubility in

T sity. can be divided into three categories: 1. high

point depression of the solution (2). However, by choosing the process, polymer and additives judi- ciously and through proper process tuning, both high and low density polymer foams were successfully pro- duced at PPI. Amorphous polymers, i.e. polystyrene, were found to be foamable to a density of 50 kg/m3 with carbon dioxide gas (3). PVC structural foam of medium density was also produced using carbon di- oxide gas and argon gas (4). The semicrystalline ma- terials, because of their morphology. are more difficult to foam compared to amorphous polymers. At the melt to solid transition, the crystalline phase can act a s a site for the nucleation of bubble. This nonhomog- enous nucleation phenomenon may start different bubbles a t different times followed by coalescence of the bubbles and collapse of the structure. Foam sta- bility is also dependent on the viscoelastic character- istics of the polymer near the die exit. In the present work, polypropylene homopolymer foams were pro- duced by extruding through a ribbon die. Carbon di- oxide was used a s blowing agent. The objective was to produce foams with density ranging between 0.3 and 0.5 g/cc and eliminate the use of chemical blowing agents that may impair recyclability.

EXPERIMENTAL

Materials

For this work, a polypropylene homopolymer (grade HC 1155) from Borealis AS. Norway, was used. The

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 1996, Vol. 2, No. 4 339

S . K . Dey et al.

Solid Metering n

_. ...

I I Cylinder V k ; ;-- - -

Serial Interface Fg I Schernatir of the expenmental setup

melt flow index of the material a s measured at PPI was found to be 3.0 (230°C and 2.16 kg). The melt elastic- ity of this material was also measured at PPI with the Melt Elasticity Indexer (CSI) and was found to be 1 .O at 190°C. This is the equilibrium recoverable strain for the material which was strained 11 strain units at a rate of 1.0 strain unit per second. Bone Dry grade of carbon dioxide supplied by JWS was used for all the experiments. Talc and Safoam FP (from Heedy Inter- national) were used a s nucleating agents. Talc with a mean particle size of 1.2 pm was supplied by Luzenac. A master batch of 1 wt% talc was precompounded and diluted to 0.1 % talc before use. An endothermic nu- cleating agent, 0.5% Safoam F'T, was dry blended with the resin.

Processing Equipment

A 32 mm (1.25 inch) diameter. Killion segmented single screw extruder was used for all experiments. The extruder was configured to a length to diameter ratio of 40 for optimum processing. A K-tron single screw feeder was used to meter feed the polymer into the hopper of the extruder. The screw was configured to completc plastication within 19 diameters. Carbon dioxide gas. directly from the cylinder, was fed into the barrel of the extruder at this point. The injection pres- sure of the gas was controlled by a manual control

90 mm

Hg. 2. Scheniatic of the ribbon die.

valve. The rest of the screw was configured to provide optimum cooling and conveying with low viscous heat generation. A schematic of the experimental setup is shown in Fig. 1.

A 90 mm wide ribbon die with 1.27 mm opening was used. The original die, manufactured by C. W. Bra- bender. was 127 mm wide and had adjustable open- ing. The die was modified at PPI to suit foam extrusion (Fig. 2). A small three stack roller was used for cooling and polishing the extruded ribbon. Only the middle

340 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 1996, Vol. 2, No. 4

Use of Inert G a s e s in Extruded MDPE Foams

roller was driven by a DC motor. The top roller and the bottom roller were driven by friction only (Fig. 3).

Data Acquisition

The melt temperature and pressure at the die were digitized using Keithley Metrabyte's M-Modules. The modules are 15 bit accurate. The M-Modules were connected to the serial port of a IBM PS2. A computer program was developed to log these data and plot them on the screen on a real time basis.

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Product Characterization

The extruded foamed ribbon was allowed to mature a t room temperature for several days before charac- terization. The density of the product was measured by both volume displacement in water, a s well as by weighing the samples and measuring their dimen- sions. Tcnsile strength, tensile modulus and elonga- tional properties were measured on microtensile bars. which were machined from the ribbons in both ma- chine and transverse directions.

RESULTS AND DISCUSSION

The Die Design

Initial experiments were done with a standard rib- bon die from C. W. Brabender. In the absence of the gaseous blowing agent, very smooth and uniform sheets of polypropylene were produced using this die and the takeoff roller. In the presence of the blowing agent, it was observed that the ribbon thickness was higher in the center. This nonuniformity of the thick- ness was found to be an increasing function of the gas injection pressure. This was due to two reasons. First, the flow resistance at the edge is higher because there are three stationary walls compared to two stationary walls at the center of the die; second, the flow path for the fluid is longer for the edge. To nullify these effects, the land length near the edge was reduced (see Fig. 2). With the modified die, the ribbon thickness in the transverse direction was found to be acceptable.

Gas Injection Pressure

The experimental results are presented in Table 1. I f all conditions are the same, the foam density would be expected to be a decreasing function of blowing agent injection pressure. But owing to interactions between variables, foam density may not show this predictable variability, which was the case in this work. Post ex- trusion tooling, especially for profile extrusion, may have complicated the situation further.

Nucleating Agents

Both Safoam and talc were found to be suitable for polypropylene foam extrusion for the density range of 0.5 g/cc to 0.3 g/cc. At this time, sufficient data is not

Table 1. Material: HC115J from Borealis AS, Norway Blowing agent: Carbon Dioxide Gas Die: 90 mm x 0.38 mm Ribbon Die

Gas Injection Pressure, Melt Pressure Melt Temp. Foam Density,

Run # Additives MPa at Die, MPa at Die, "C dcc Comments

8019526 Safoam FP 0.69 1.38 167 0.5 Density too high 8179532 0.5% Safoam FP 3.45 4.1 159 0.3 Surface too rough 9129538 0.1 YO Talc 5.52 4.09 143.4 0.3 Surface rough 9199539 0.1 YO Talc 3.45 2.69 156 0.3 Commercial wrapping film was used and the

91 99540 0.1 % Talc 3.45 2.67 155 0.3 No wrapping film was used and Rollers at 95°C 921 9541 0.1 % Talc 3.45 3.55 148.7 0.3 Commercial wrapping film was used and the

rollers were kept at 95°C

rollers were kept at 11 5°C

JOURNAL OF VINYL 8 ADDITIVE TECHNOLOGY, DECEMBER 1 S 6 , Vol. 2, No. 4 341

S . K. Dey et al.

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342 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, DECEMBER 1996, Yo/. 2, No. 4

Use of Inert Gases in Extruded MDPE Foams

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available to compare the advantages or disadvantages of the two.

was wound on the two ends of the middle roller to obtain a 1.27 mm clearance between the rollers (see

The Takeoff Rollers Rg. 3). The temperature control of the rollers was found to be important. An unacceptable product (very

An increase in the density of the foam was observed when the takeoff rollers were used to cool and polish the foamed ribbon. This is because the rollers were spring loaded and were squeezing out the blowing agent from the polymer in the nip region. To nullify this effect. a commercial adhesive tape (Scotch tape)

rough surface) was obtained when the- rollers were chilled with process water. Circulating hot water at 95°C through the rollers produced a far better prod- uct. However, the product density was found to be higher than the theoretically calculated one based on the gas amount added. To reduce the gas escaping in

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, DECEMBER 1 H , Vol. 2, No. 4 343

S. K. Dey et al.

Fig. 8. PP./ourrt with commercial wrappirig.rdrn (Run #9219541J 36.7x.

the nip region of the rollers. a commercial wrapping film (Saran) was used a s cap layers, both a t the top and the bottom, of the ribbon. Substantial improvc- rnents in the product density and surface smoothness werc achieved (Table I ) .

Mechanical Properties

Since the product densities in Table I are not thc same, one may not directly compare the mechanical properties of the various foams. This variability was taken care of by dividing the mechanical property value by the respective density of the foam. The rc- suits are presented in Figs. 4- 6. The machine and the transverse direction data of the foams were found to be isotropic from tensile modulus and tensile strength points of view. Similar conclusions. however, could not be drawn for the clongation data. Specific tensile modulus and specific tensile strength were also im- proved whcn the wrapping film was used a s a cap layer. Again, similar conclusions could not be drawn for the elongation.

MORPHOLOGY

The morphologies of the foamed samples are pre- sented in Figs. 7 and 8. An optical microscope was used to achieve 36.7x magnification. I t is clear that more uniform cell size and much thickcr sample was obtained when a commercial wrapping film was used (Fig. 8). The approximate size of the bubbles are 500

pm X 250 pm. The skin thickness appears to be about the same order of magnitude for both the samples.

CONCLUSIONS

A polypropylene homopolymer was successfully foamed with carbon dioxide to densities between 0.5 and 0.3 g/ml in a single screw extruder. 'I'he choice of takeoff equipment was critical since it was found to be important to avoid any kind of squeezing action on the foamed melt. Slow cooling of the foamed mass was found to provide a smoother surface and more rcpro- ducible results. A cap layer of commercial wrapping film was found to improve both density and specific tensile modulus and strength. Further work is in progress to optimize the process conditions.

ACKNOWLEDGMENTS

This work was supported by Borealis AS. Norway. We would also like lo thank Dr. Tan of PPI and his associates for helping with the characterization work.

REFERENCES

1. S. Semerdjiev. Introduction to Structural Foam. SPE Pro-

2. S. K. Dey. C. Jacob, and J . A. Biesenberger. SPEANTEC

3. C. Jacob and S. Dcy. J . Cell. Plast., Jan . 1995. p. 38. 4. S. K. Dey. C. Jacob, and M. Xanthos. SPE ANTEC Tech.

cessing Series Brookfield. Conn. ( 1 982).

Tech. Papers. 40. 2197 (1994).

Papers. 41.4138 (1995).

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