inert-gas extrusion of rigid pvc foam

Inert-Gas Extrusion of Rigid PVC Foam

S. K. DEY, C . JACOB, and M. XANTHOS

Polymer Processing Institute Castle Point on the Hudson

Hoboken, New Jersey 07030

A novel approach to the extrusion of high-density, rigid PVC foam uses commercial RPVC compounds with inert-gas physical blowing agents (carbon dioxide and argon). The process was developed on a segmented single-screw extruder with L/D of 40. On-line monitoring of process variables was also carried out. This technique provides an alternative to conventional processing methods using chemical blowing agents.

INTRODUCTION

urrent technology for extrusion processing in pro- C duction of high-density RPVC foam profiles involves the use of chemical blowing agents (CBAs), based on azodicarbonamide and sodium bicarbonate, together with other additives ( 1 ) .

Some limitations associated with use of CBAs are:

1) On-line adjustment of foam densities is difficult to achieve without changing CBA concentration.

2) Residual CBA in the extrudate can hinder recycling of scrap into the process.

3) Migration of CBA in the dry-blended mix during transport may cause density variations in the product.

4) Relatively high cost of CBAs.

While physical blowing agents (PBA) are not widely used in vinyl foam production, their use for other polymers, notably polystyrene and polyolefins, make it possible to overcome these problems. However, conventional PBAs such as halocarbons (CFCs and HCFCs) and hydrocarbons are subject to strict environ- mental regulations today. A n alternative group of inert- gas blowing agents such as carbon dioxide (CO,), nitro- gen (N,) and argon (Ar) are currently generating much interest since they are more “environmentally friendly” and are not subject to the same regulations.

This work investigated the use of two inert-gas PBAs-CO, and Ar-in the development of a process for the extrusion of RPVC foam profiles. Experimenta- tion was done with commercially available RPVC compounds to achieve densities between 600 kg/m3 and 1000 kg/m3. Special attention was also given to the development of cell size and surface texture in the extruded foam profile.

BACKGROUND

General advantages offered by foaming polymers include raw material savings, weight reduction, high specific modulus, insulating properties and shock ab-

sorption capabilities. The rising demand of WVC foam profiles is primarily due to increased “wood replace- ment” applications, primarily from economic and maintenance points of view (2).

CBAs are primarily used in applications where a high-density product is required, PBAs can be used in both high- and low-density product lines. PBA foam extrusion essentially involves the following steps- mixing and dissolving the blowing agent into the polymer melt, cooling down the gas-polymer solution, expansion of gas in the melt as it exits the die forming a cellular structure, and finally, stabilization of the cell walls to form a foam.

Inert-gas extrusion processes developed at the Poly- mer Processing Institute have produced C0,-based low-density foams with polyethylene and polystyrene. For polyethylene, the process utilized a tandem system of two extruders that separated the melting and mixing stages from the cooling stage (3). An instru- mented, tightly controlled system is an integral part of this tandem process for these low-density foams (48 kg/m3/3 lb/ft3). A process has also been developed for foaming polystyrene to low densities (50 kg/m3/3 Ib/ ft?) that uses a single-screw extruder alone (4, 5). An- other process for polystyrene uses a twin-screw extruder alone (6) for a foamed product with density of 32 kg/m3 (2 lblft‘)).

EXPERIMENTAL

Materials

Two commercially available RPVC compounds (identified as RPVC-V and RPVC-W) were used in the process. RPVC-V was a compound based on a resin with a K-value of 53, while the resin in RPVC-W had a K-value of 58.

Bone-dry grades of CO, and Ar were used as physical blowing agents.

Nucleating agents included talc with a mean par- ticle diameter of 1.2 p.m from Luzenac and Safoam-

48 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1


PT, a proprietary nucleating agent from Reedy Inter- national.

nected to the serial interface of an IBM PS/2 personal computer. The 1 / 0 Pak was used to monitor gas mass

Processing Equipment flowrate and temperatures and pressures atdifferent locations. It was also used to control gas injection -

A 32 mm (1.25 inch) diameter, Killion segmented single-screw extruder was configured with a length to diameter (L/D) ratio of 40 for optimum processing.

pressure.

Product Characterization - RPVC pellets were metered u s k g a K-%-on, twin- screw, volumetric feeder to starve-feed the extruder. Gas was injected into the barrel through a nozzle in a Dynisco type port. An electronic gas-pressure control- ler was used to regulate the gas injection pressure, and the mass flow rate of the blowing agent was moni- tored with a Matheson gas mass flow meter. Two rod dies were used in the process, 4.8 and 6.4 mm in diameter, and a breaker plate was used before the die. A schematic representation of the setup is shown in Fig. 1 ,

The foaming process adopted was the “free-foaming” mode with the product expanding outwards into the calibrator sleeve as it exited the die. A 12.7 mm (0.5 inch) calibrator mounted on a 610 mm (24 inch) long, acrylic sizing-box equipped with vacuum and

The extruded foam rod samples were characterized for various physical properties. Densities were ob- tained by weighing each sample and determining its volume from dimensional measurements made with an electronic Vernier caliper. The surface texture was evaluated through visual inspection and graded on a 5-point scale from very rough to very smooth. An op- tical microscope was used for determining the cell size distribution across a cross section of the sample and the presence of a skin, if any was noted. The flexural modulus and flexural stress a t 5% strain of the sample were determined by use of a 3-point bending fix- ture, a t a test speed of 5.1 mm/s (0.2 inch/sl with a span of 51 mm (2 inches).

water lines was used to size the product. A profile- puller was used to draw the extrudate a t controlled RESULTS AND DISCUSSION

rates through the sizing-box.

Data Acquisition

A 16 channel data-acquisition board with 12 bit accuracy (I/O Pak from Action Instruments) was con-

Processing

In Table 1 , the following four groups of experiments are discussed. Groups I and I1 refer to experiments that did not employ vacuum sizing. The extruded pro-

Fig. 1 . Schematic of RPVC foam extrusion setup.

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 49

S. K. Dey, C. Jacob, and M. Xanthos

Table 1. Properties of Extruded RPVC Foam Rod.

Group No. Compound (MPa) Agentlphr Sizing (kglm3) (mm) Surface Experimental RPVC GaslPress. Nucleating Vacuum Density Cell Size

I RPVC-V co2/1 .4 - no 420 2.1 rough RPVC-V Ar/l.4 - no 480 1.5 rough

I I RPVC-V c0,/2.1 none no 620 1.9 medium RPVC-V c0,/2.1 Safoam/O. 1 no 350 1.2 v.rough

111 RPVC-W co,/1.9 Safoam/l Yes 750 0.31 v. smooth RPVC-W co2/1 .4 Tald l Yes 800 0.38 v. smooth

IV RPVC-V C0,/2.5 Safoam/0.05 Yes 600 0.9 smooth RPVC-W co*/1.9 Safoam/0.05 Yes 750 0.7 v. smooth

1600 1 1 0

E \ w Y

1200 4- .- rn c a

800 E m 0 L

400 0 0.4 0.8 1.2 1.6

C02 Injection Pressure, MPa Fig. 2 . Egect of gas injection pressure on foam density.

files in groups I11 and IV were sized using a vacuum- sizing box, thus resulting in smoother surfaces.

I. Blowing Agent: Both CO, and Ar performed well as blowing agents, yielding products with densities of under 500 kg/m3 (Table 1 ) . However, Ar produced slightly finer cell distribution in the product, because its lower solubility in the polymer resulted in better bubble nucleation characteristics. No attempt was made to size the extruded foam rod profiles and hence surface finish was very rough.

II. Nucleating Agent: The presence of nucleating agent (0.1 phr Safoam-PT) had a twofold effect on the extruded product. The primary outcome was the gen- eration of finer cells in the product as the Safoam provided more sites for the growth of bubbles. Fur- ther, the density of the product was significantly de- creased since the increased number of nucleation sites reduced the residual gas in the melt. Table 1 provides a comparison of these two products. Again, these results apply to a n unsized rod profile.

III. Type of Nucleating Agent: A set of experiments was carried out to compare the two different nucleating agents, talc and Safoam-PT, a t a concentration of 1 phr. Safoam-PT gave slightly better characteristics in the product in terms of density and cell size (Table 1 ).

IV. RPVC Resin: Rheological characterization showed the two compounds to differ in viscosity and MFI values with RPVC-W exhibiting a lower melt flow rate and higher shear viscosity than RPVC-V. Evalu- ation in the extrusion process (with 0.05 phr Safoam-PT as nucleating agent) showed that RPVC-W,

200 Melt Temperature

30

2 l a 20

m I- $10 +

6000 7000 8000 9000 10000

Time, sec

Fig. 3. Process uariations for an experimental run.

based on a 58 K resin, was the most promising formu- lation. The product had the best surface quality and the process displayed excellent stability. Some surg- ing in die pressure was noted with the RPVC-V, which was attributed to variation in resin feed rate. More importantly, the surface quality was not as good as

Experimental groups V and VI (not presented in Table 1 ) attempt to identify the effects of other parameters.

V. Blowing Agent Injection Pressure: CO, injection pressure was increased in stages, unsized samples were collected and densities measured. The variation of density with the injection pressure is presented in Fig. 2. The observed reduction in density with increase in injection pressure is due to greater expansion of the extrudate emerging from the die, which itself results from its higher gas content. However, there is a min- imum density that can be achieved, and further increase of gas concentration should increase the foam density (6).

VI. Extruder L I D Ratio: Experimentation was also conducted for the reduction of the LID ratio. A change from 40 L /D to a 36 L/D extruder configuration ad- versely affected product quality of the FWVC foam. The reduced length did not provide sufficient homogeniza- tion of the melt, resulting in gas pockets in the extruded rod.

with RPVC-W.

Stability

Process Stability: Use of a data-acquisition system made it possible to study variations in the process, as

50 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1


- - - - -

evidenced by the melt temperature and pressure at the die, melt pressure at a n intermediate point in the barrel and the gas injection pressure. Figure 3 tracks the response of the sensors placed at these locations. The degree of conformity of data over the extended time-period shows that the process is reasonably stable.

Product Stability: Variations in product quality, as represented by density and weight measurements, were determined through sampling over a period of time. Figure 4 does show some variation in these parameters within an approximate band of ?5%. One possible reason for this is the fluctuation of the melt front within the gas injection section of the barrel, resulting in varying amounts of gas dissolving into the polymer. A final determination of the impact of these deviations should be made by the processor, though it might be possible to optimize the process to eliminate the;:;.

Flexural Properties

Figure 5 shows flexural properties of the foam sample as dependent variables of its density. The reduction in these mechanical properties arising from the density reduction is expected, since void content is correspondingly increased. However, a simple rela- tionship should not be expected since these properties are dependent on various other parameters, e.g., skin layer thickness, sample dimensions, and cell size and geometry.

Scale-Up and Commercialization Considerations

The 32 mm diameter single-screw extruder used to develop this process has throughput limitations from a production standpoint, and the process needs scal- ing to meet larger capacity requirements of continuous operation. For long residence times in the extruder, to promote uniform temperature and gas distribution, the production-size extruder requires a L/D ratio of about 40, which is greater than the 24 L/D conventionally used. Another factor in commercialization is the development and optimization of die designs for complex profiles, which are related to pressure drop during the extrusion process and to the surface characteristics of the foamed product. Consis- tency and monitoring of processing conditions and introduction of gas require a greater degree of control

30 P" B

25 $ 2

20 fj - 15 f

X Q) 10

0.03 I 1500

Mean Wt. I .02534, Std. Dev. = 0.00053

A Mean Denslty I 980. Sld. Dev = 50

0 500 0 600 1200 1800 2400

Time, sec

Fig. 4. Study on product uariation.

600 1 35 A Flexural Modulus a Flexural Stress 0 5% Strai 3'

A

A 0

0 0

a

100 I 1 5 500 750 1000

Foam Density, kg/m3

Fig. 5. Flexural properties as afunction of density.

than normally required for extrusion of foamed profiles with CBAs. This factor may necessitate updating of employee skills by proper training, since new technology often needs more sophisticated treatment than current procedures.

CONCLUSIONS

While high-density RPVC foams have typically been produced using CBAs, a novel process is presented in this paper that uses low-pressure injection of inert- gas blowing agents, such as CO, and Ar, for the continuous extrusion of foamed profiles. The effects of gas injection pressure, nucleating agent and PVC resin type on product quality, as defined by density, surface texture and cell size, were also studied. The process showed reasonable stability and measurements indi- cated that variations in the product were minimal.

During the experimentation, it was confirmed that certain processing variables earlier identified were important to the stability of the process and good product quality. These are:

A steady feed rate of polymer into the extruder. A good melt seal on the screw before the point of gas injection into the barrel. Precise control of injection pressure and monitoring of gas flow rate into the extruder. Accurate control of barrel temperature. A long residence time in the extruder to deliver ho- mogeneous melt a t the die. Monitoring of melt temperature and die pressure.

In estimating the relative costs of products prepared by switching from the CBA to the CO, foaming process, one should consider not only the additional equipment and equipment modification costs (barrel extension or new cooling extruder, gas metering con- troller, data acquisition system), but also the blowing agent cost savings, estimated at about 2.5B/kg for a 600 kg/m3 product. Advantages of the inert-gas injection technology that could also lead to economic advantages are: [a) rapid, on-line changes in product density by controlling the gas injection pressure; (b) use of a single base compound to make products of different densities; (c) enhanced reuse of scrap, now

JOURNAL OF VINYL &ADDITIVE TECHNOLOGY, MARCH 1996, Vol. 2, No. 1 51

S. K. Dey, C. Jacob, and M. Xanthos

dictated by the RPVC compound stability rather than the presence of residual CBA in it: (d) product quality is more uniform over a long extrusion period. Finally, processing and blending equipment will not require thorough cleaning and purging to eliminate all traces of CBAs, which becomes important when changing product lines or materials.

ACKNOWLEDGMENTS This work was supported by the Technology

Transfer Merit Program of the New Jersey Commis- sion on Science and Technology through a contract with the New Jersey Polymer Extension Center and the Polymer Processing Institute. We would also like to thank Dr. Victor Tan and his associates, Ming-Wan

Young and Andy Ponnusamy of PPI for the characterua- tion tests.

REFERENCES 1. I. B. Page, SPE ANTEC Tech. Papers, 40, 3485 (1994). 2. J . R. Patterson and J. L. Souder, SPE ANTEC Tech. Pa-

pers, 40, 3480 (1994). 3. S. K. Dey. C. Jacob, and M. Xanthos, Proc. SPE RETEC

“Continuous Compounding in the go’s,” p. 118, Somer- set, N.J. (Nov. 30-Dee. 2, 1993).

4. C. Jacob and S. K. Dey, SPEANTEC Tech. Papers, 40. 1964 (1994).

5. C. Jacob and S. K. Dey, Proc. Polymer Processing Society Regional Meeting for the Americas, Morgantown, W.Va.

6. G. Szamborski and J. Pfennig, J. Vinyl. Technol., 14, 105 (AUg. 5-6, 1993).

( 1992).

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inert-gas extrusion of rigid pvc foam

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