Thermoplastic Polyurethane Foaming through Extrusion …leaders.4spe.org/spe/conferences/ANTEC2017/papers/397.pdf · THERMOPLASTIC POLYURETHANE FOAMING THROUGH EXTRUSION USING A BLOWING

THERMOPLASTIC POLYURETHANE FOAMING THROUGH EXTRUSION USING A BLOWING AGENT

Qingping Guo and Andrew Kenny, EHC Canada, Inc., Oshawa, ON

Shahid Ahmed and Ghaus Rizvi, University of Ontario Institute of Technology, Oshawa, ON

Abstract

This study investigated a process of making

thermoplastic polyurethane (TPU) foam using Expancel® as blowing agent during an extrusion process. For this purpose, different let down ratios (LDRs) of blowing agent were implemented in TPU. The study employs X-ray micro computed tomography (µCT) in order to see effect of the changing LDR on expansion ratio and cellular structure of the foamed TPU. The results reveal that LDRs and pressure significantly influence both the expansion ratio and the morphology of the phases present in the foamed TPU. Also the viscosity of TPU at different LDRs was measured using a custom-made in-situ capillary rheometer, which was mounted to an extruder.

Introduction

Due to excellent physical properties, chemical resistance, abrasion resistance, good adhesion and ease of processing, TPUs find application in a wide variety of industrial applications, such as engineering materials [1-5]. EHC Canada, Inc. is the largest escalator handrail manufacturer in the world and develops custom urethane products for many industries, such as escalator handrails, composite steel belts. EHC Canada mainly extrudes these TPU products.

However, because of high TPU cost, a TPU foaming

technology was developed at EHC Canada, Inc. for reducing handrail weight and cost. Foamed thermoplastic polyurethane (TPU) was made by introducing a chemical or physical blowing agent into TPU polymer melt during processes such as extrusion. The morphology of foamed TPU not only depends on type and LDR of the blowing agent used, but also on its rheological behavior and the process conditions implemented.

Thermoplastic foam processing deals with heterogeneous mixtures of polymer melts and dispersed gas bubbles. A good understanding of rheological behavior of these mixtures is extremely important for optimizing the existing process conditions of handrail production line to employ on a handrail foaming production line. All foamed TPU samples at different LDRs of blowing agent was measured using a custom-made in-situ capillary rheometer, which was mounted to an extruder. At the same time, the viscosity of TPU at

different LDRs of blowing agent was measured. Figure 1 shows the schematic of viscosity measurement system. Figure 2 shows 3D drawing of custom-made capillary rheometer.

PT - Pressure transducer

Figure 1: Viscosity Measurement System

Figure 2: Geometry of Custom-made Capillary Rheometer

The internal structure architecture of a foamed

polymer mainly determines its properties including physical, mechanical, thermal and acoustic, and subsequently the end use application. Therefore, analyzing the morphology of foamed polymers is crucial. A 3D imaging technique such as X-ray micro computed tomography (µCT), has proved to be superior to 2D imaging techniques as it provides more realistic results. The structures of foamed TPU samples under various processing conditions are studied using 3D CT scanning (SkyScan 1172, Bruker Corp. Belgium).

From processing point of view, the measurement of

polymer melt viscosity during foaming process is more challenging. Conventional rheological instruments such as a rotational rheometer cannot be helpful because the rheometer must be placed in an enclosed chamber under a sufficiently high pressure, so that formation of bubbles

PT

Single Screw Extruder

Melt Pump Capillary Die

Rheometer Ada

pter

Hopper Side feeder

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can be suppressed at a specified measurement temperature, and the gaseous component cannot escape from the polymer melt. Similarly, if a plunger type rheometer is used, it must be attached with a high pressure chamber, so that at the die exit the polymer melt with gaseous component can exceed the critical pressure for bubble nucleation. A continuous-flow custom-made capillary or slit rheometer as shown (in Figure 2) was employed in-situ on an extrusion line to measure the viscosity of TPU melts with dispersed gas bubbles [6].

Experimental Materials

Several chemical blowing agents (XO-459 from Bergen International, iD Endo 80 MFC and iD Endo 80 MFC from iD additives, Inc., and Expancel® from AkzoNobel) were screened to fabricate foamed TPU having a uniform cellular structure and the desired expansion ratio, among those Expancel® was found best to meet our requirement.

A type of thermoplastic polyurethane (TPU) of EHC Canada currently using for handrail manufacturing was used for this study. The blowing agent as mentioned in previous discussion was Expancel® (Grade: 950 MB 80, make: Akzo Nobel Pulp and Performance Chemicals Inc.). MB Expancel® grade consists of thermoplastic particles in the form of dry unexpanded microspheres mixed with a carrier such as EVA – Ethylene Vinyl Acetate. Each microsphere consists of a polymer shell, which encapsulates gases such as hydrocarbons: isopentane and isooctane. Upon heating internal pressure of these hydrocarbons increases causing the polymer shell softened that eventually significant increase in volume of the microspheres as shown in Figure 3 [14].

Before to be used, TPU was dried over 4 hours in a dryer (CD400, Conair, US) to ensure moisture content less than 0.03% measured on a high performance analyzer (Computrac Max-2000 XL).

Figure 3: Expansion of the Expancel® Microspheres[14] Experimental Setup

The experimental setup used for making of TPU foam and obtaining experimental viscosity data is shown in Fig. 1. A gravimetric feeder (model: Maguire MGF-ST) was mounted beside the main hopper to setup various LDRs of blowing agent. A TPU grade was tested for foaming and its viscosity at a setting temperature of 180°C and shear rates from 24(1/s) to 110 (1/s) that corresponds to a melt pump speed of 4rpm to 18rpm respectively. The melt pump delivering the polymer melt at a mass flow rate was calibrated. As explained earlier, for polymer melt viscosity measurement the use of capillary rheometer involves two types of end corrections: Bagley and Rabinowitsch. The former calculates the entrance pressure drop on L/D axis and determines the true wall stress, whereas the later determines true shear rate at wall [7-13]. Procedure A brief description of procedure is given as follows: 1. Calibrate the pressure transducers and thermocouples 2. Turn on the extruder and set the setting temperatures

for various zones along the extruder, melt pump and capillary die

3. Set the gear pump rpm to control melt flow rate through die

4. Prime the side feeder with blowing agent used, and set the LDR for foaming.

5. Record pressure readout once it gets stable for viscosity calculations

6. Collect foamed TPU samples in a water filled container for characterization on an X-ray CT scanner

7. Under one setting temperature, change flow rate by setting the gear pump to 4, 6, 9, 13, and 18 rpm, repeat step 4 for each flow rate setting.

8. Change the setting temperature, repeat step 3 to 5. Viscosity Calculation

The procedure followed for the viscosity calculation

is described as: (i) calculation of the wall shear stress using the measured pressure drop from pressure transducers, and the apparent wall shear rate under various flow rate and temperature settings; (ii) Determination of the Bagley correction factor by extrapolating the graph between pressure drop linearity analysis of the pressure profiles obtained at various pressure transducer locations; (iii) true shear rate estimation using Weissenberg-Rabinowitsch correction [1]; (iv) Calculation of viscosity using wall shear stress and true shear rate values.

Results and Discussions Blowing Agent LDR Effect on TPU Foaming

Foamed TPU was fabricated using Expancel® as blowing agent for letdown ratios (LDR) of 0.1, 0.4, 0.7,

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and 1.0 weight percent at a setting temperature of 180°C. Foamed samples collected under each let down ratio were characterized on an X-ray micro CT. A 3D visualization of the morphology, and the expansion ratios measured, are presented in Fig. 4 to 5. It reveals that increase in blowing agent weight percent results in an increase in the expansion ratio and percent porosity while the bubbles size and distribution are uniform for all samples.

(a) 0.1 wt%

(b) 0.4 wt%

(c) 0.7 wt%

(d) 1.0 wt%

Figure 4: X-ray CT images with Expancel®

Figure 5. Blowing Agent LDR Effect on Expansion Ratio

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Pressure Effect on TPU Foaming

To see the pressure effect on expansion ratio, foamed TPU samples were collected for three pressure stages: L/D=10, L/D=20, L/D=30, representing low to high pressure conditions in sequence, and characterized with X-ray CT. The CT scan results are shown in Fig. 6 and Fig. 7 revealing that an increase in pressure would cause a higher expansion ratio and better distribution of microspheres, higher let down ratios make this effect more visible (Figure 6).

An explanation to this trend is that the polymer melt exits the melt pump at a specified volume flow rate (cm3/rpm) and it carries dispersed expanded micro-spheres. When it reaches adapter mounted near to the capillary entrance it gets squeezed to pass through the capillary hole. In this process dispersed microspheres are compressed too by the pressure developed in the adapter. A higher L/D ratio of the capillary die will require a higher pressure in the adapter to let the polymer melt squeeze and pass through capillary hole, which causes the Expancel® blowing agent microspheres only could expand a little bit from their original size. After TPU extrudates get out of die, the microspheres can grow much more to full size at atmosphere pressure and room temperature. On the contrary, a lower L/D ratio of the capillary die will require a lower pressure, which causes the Expancel blowing agent microspheres could expand to a little bigger. After TPU extrudates get out of die, the microspheres only can grow a little bit less to full size at atmosphere pressure and room temperature. Therefore, the melt pressure after melt pump has a significant effect on expansion ratio of foamed TPU. If the LDR is pretty low such as 0.1 wt%, the effect may not be that significant.

Figure 6. Pressure Effect on Expansion Ratio and Porosity Shear Rate Effect on TPU Foaming

Foamed samples were also collected to see the effect of changing shear rate on the morphology of

foamed polymer. Shown below in Fig. 8 is the graph reflecting change in expansion ratio and percent porosity

(a) L/D=30

(b) L/D=20

(c) L/D=10

Figure 7. Pressure Effect on Porosity Distribution:

a) LD30, b) LD20, c) LD10 versus shear rate for a LDR of 0.4%. It reveals from the graph that shear rate does affect the expansion ratio and percent porosity. However the effect is not that significant as a change in shear rate from 20 (1/s) to 110 (1/s) could cause just little change from 1.07 to 1.104 in expansion ratio and 6.5 to 9.0 in percent porosity. At higher shear rate(s), viscous heating effect may cause a rise in temperature and bubbles shape of the polymer melt across capillary hole along the length, which can affect the expansion ratio. However, it seems the viscous heating effect for shear rate(s) studied is not that significant.

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Figure 8. Shear Rate Effect on Expansion Ratio and Percent Porosity

Blowing Agent Effect on Foamed TPU Viscosity

A good understanding of rheological behavior of

these mixtures is extremely important to process die design and optimization of process conditions.

TPU viscosity was measured using pressure drop data

recorded during making of foamed TPU for LDRs ranging from 0.1% to 1.0% and shear rates from 20 (1/s) to 110 (1/s). The viscosity curves obtained are shown in Fig. 9, which reveals that higher LDR results in a lower viscosity of the foamed TPU. Under high shear rate, the viscosity of TPU with various Expancel® weight percentages are not significantly different, the extrusion system and processing parameters don’t need to be changed if the Expancel percent is less than 1%.

Figure 9. Effect of LDR on Foamed TPU Viscosity

Conclusions

This study concludes that foaming of the

thermoplastic polyurethane (TPU) can successfully be executed using Expancel® as blowing agent and extrusion as foaming process. The study reveals X-ray µCT is a powerful high precision tool that not only allows 3D visualization of the foamed polymer cellular structure but

also 3D evaluation of the porosity and expansion ratio. Analysis of X-ray µCT results clearly indicates that higher pressure and higher LDR conditions result in higher expansion ration and more uniform distribution of the porosity. Based upon findings of the morphology analysis and viscosity measurements, foaming of TPU can be employed in actual handrail production provided it meets the necessary mechanical performance criteria. This would not only reduce the handrail weight but also the material cost.

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