processing and characterization of autoclave-based epp · pdf fileprocessing and...

University Of Toronto Mechanical & Industrial Engineering Department

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Processing and Characterization of

Autoclave-based EPP Beads

…………………………………………………

ABHISHEK PATEL

MUHAMMAD KASHIF FAROOQUI

…………………………………………………..

M.Eng. PROJECT REPORT

DEPARTMENT OF MECHANICAL AND INDUSTRIAL

ENGINEERING

UNIVERSITY OF TORONTO


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© Copyright by Abhishek Patel & Muhammad Kashif Farooqui (2010)

ACKNOWLEDGEMENTS

We would like to express our earnest gratitude to Dr. Chul Park for his leadership,

support and cooperative advice throughout the tenure of this project. This project

would not have been successfully completed without his guidance.

A big thank you to all members of Microcellular Plastics Manufacturing Laboratory

group: Nemat Hossieny for his advice and support in our experiments, Yanting Guo

and Dr. Wenli Zhu for their recommendations towards this project and assistance in

form of managing the DSC experiments, Anson Wong and Raymond Chu for their

constant inputs and guidance regarding the experimental setups.

Special note of appreciation to Ryan Mendell and all the machine shop crew for their

patience and tremendous help in assisting us with the setup in the shortest time

possible.

We also extend our gratitude to the Department of Mechanical and Industrial

Engineering, University of Toronto, Donna Liu and Brenda Fung for their aid in the

successful completion of our M.Eng program.


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Table of Contents

1.1 E.P.P 4

1.2 History of E.P.P 6

1.3 Properties of EPP Plastic Foaming 6

1.4 EPP Manufacturing Process 9

2.1 Objective of Study 12

2.2 Thermoplastic Foaming 13

2.3 EPP Pre-Expansion 13

2.4 Double Peak Mechanism 14

3.1 Solubility Analysis via PVT 18

3.2 PVT Relationship 18

3.3 PVT Experimental Design 19

3.4 PVT Experimental Results and Discussion 21

4.1 Solubility Analysis via MSB 23

4.2 MSB Experimental Apparatus and Procedure 24

4.3 MSB Experimental Results and Discussions 26

5.1 Autoclave Apparatus Setup 28

5.2 Procedure 30

5.3 Initial Results and Discussions 32

5.4 2nd Generation Chamber 35

5.5 Chamber and Propeller Modification 36

6 References 39


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CHAPTER 1

INTRODUCTION

1.1 E.P.P (EXPANDED POLYPROPYLENE)

Expanded polypropylene (EPP) is an engineered plastic foam material, highly

versatile closed-cell bead foam that provides a unique range of properties, including

outstanding energy absorption, multiple impact resistance, thermal insulation,

buoyancy, water and chemical resistance, exceptionally high strength to weight ratio

and 100% recyclability. [1]

1.2 HISTORY OF EPP

EPP was first developed in the 1970's by JSP, as a result of research into new

forms of polypropylene. The material's first applications were for automotive

products in Japan in 1982. Demand for EPP has since increased dramatically in

all regions of the world based partly on the need of auto makers to improve

energy management functions whilst reducing weight and improving

environmental benefits. The first automotive application for EPP was for an


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energy absorbing component in a bumper system. EPP is now widely utilized for

numerous other automotive parts and systems, including seating and other

interior components. [1]

Polypropylene resin is now increasingly utilized in various fields because of excellent

mechanical strengths, heat resistance, machinability, cost balance, combustibility

and recyclability thereof. Foamed, non-cross linked resin moldings of abase resin

including a polypropylene resin (hereinafter referred to simply as "PP"), which retain

the above excellent properties and which have exceptional additional characteristics

such as cushioning property and heat resistance are thus utilized for various

applications as packaging materials, construction materials, heat insulation materials

etc.

Recently, there is an increasing demand for PP moldings having higher rigidity and

lighter weight than the conventional ones. For example, in the field of vehicles such

as automobiles, PP moldings have been used in various parts such as bumper

cores, door pats, pillars, tool boxes and floor mats. In view of protection of

environment and saving of energy, light weight and high rigidity PP moldings

retaining excellent cushioning and shock absorbing properties are desired. In the

field of containers and boxes for storing and transporting foods such as fish, molded

polystyrene foams have been hitherto used. Because of inferior shock and heat

resistance, however, molded polystyrene foams are not suitably reused. Therefore,

the need for light weight and high rigidity PP moldings are also increasing in this

field. [2]


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1.3 PROPERTIES OF EPP PLASTIC FOAMS

• Lightweight with very high strength to weight ratio

• Withstands multiple impacts without significant damage

• Highly durable

• Resistant to water, chemicals and most oils

• Resistant to temperature extremes from -35°C (-31°F) up to 130°C

(265°F)

• 100% recyclable with environmental benefits [3]

• EPP is approved for use in conjunction with food products. Its thermal

insulation properties and structural strength make it appropriate for

containers such as food delivery containers and beverage coolers and the

like. EPP does not support microbial growth and can be made sterile with

steam cleaning. [10]

1.3.1 Unique performance characteristics

• Porous EPP is comprised of cylinder-shaped polypropylene beads, which

adds air space between the beads in the final molded form, which

enhances beneficial acoustical insulating effects and reduces weight.


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• EPP is commonly colored black for automotive applications, though is

often found in white for packaging products. EPP is available from some

suppliers in vibrant colours suitable for presentation-grade textured

surface. [10]

1.3.2 EPP Properties

Physical Properties

EPP density range, from 20 g/l through 200 g/l

Tensile strength (kPa) 270 to 1930

Tensile elongation (%) 21 to 7.5

Compressive strength (kPa)

25% strain 80 to 2000

50% strain 150 to 3000

75% strain 350 to 9300

Compression set (%)

25% strain, 22H, 23°C 13.5 to 10.5

Burning rate (mm / min) 100 to 12 [10]


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1.3.3 Applications

EPP is widely utilized by automotive manufacturers because of its performance

benefits for energy management, lightweight, enhanced functionality, durability

and recyclability. Applications include seating, bumpers, stowage systems, door

panels, pillars, floor levelers, parcel shelves, head rests, tool kits, sun visors and

myriad filler parts.

Reusable industrial packaging, known as dunnage, is frequently made from EPP

due to its durability and its inherent ability to absorb energy in transit. EPP is

used increasingly in furniture, toys such as model aircraft and other consumer

products due to its versatility as a structural material and its light weight, as well

as other performance characteristics.

EPP is approved for use in conjunction with food products. Its thermal insulation

properties and structural strength make it appropriate for containers such as food

delivery containers and beverage coolers and the like. EPP does not support

microbial growth and can be made sterile with steam cleaning. [10]


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1.4 EPP MANUFACTURING PROCESS

The manufacturing process is complex, requiring both technical expertise and

specialized custom equipment. Polypropylene resin is combined with other

ingredients in a multi-step proprietary process. Under tightly controlled

conditions, extruded pellets expand to become consistently shaped beads of

expanded polypropylene foam. Other specialized manufacturing techniques may

be employed to produce variations in the final product form.

EPP foam beads are then injected into molds. In many cases, cost-effective

multi-cavity aluminum molds are used. Pressure and steam heat fuse the beads

into a finished shape. The finished EPP foam part becomes a key component in

sub-assemblies incorporated in the original equipment manufacturer’s product.

[1]


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Fig. 1 Generic Overview of EPP Manufacturing Process [1]

The processing of all beaded foam resins generally involves two primary stages;

pre-expansion and steam chest moulding.

1.4.1 Pre-Expansion

In the pre-expansion stage raw beads containing a blowing agent are exposed to

a heat source, usually steam, causing the beads to increase by up to 50 times

their original size.

1.4.2 Moulding

The steam chest moulding stage begins when the pre-expanded beads are

injected into a mould. In the steam chest molding process, steam (water

vapor) is used as both a heating medium and a blowing agent in polymer foam

processing. The molding process consists of a series of operations:


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steaming, depressurizing, cooling, and ejection[Ref]. The steaming process

softens and fuses the polymeric beads, and the steam penetrates and

condenses in the polymeric bead foams. When depressurized, the water

condensed in the cells begins to gasify and expand the foams. With

subsequent water cooling, the interfaces of the beads solidify, and

efficient bonding between the beads occurs. The two important advantages of

steam chest molding are (1) good temperature uniformity throughout the

samples due to the use of steam as a heating medium for foaming, and (2)

three-dimensional (3D) shaping ability. The above advantages may be

beneficial in shaping fairly large parts with a 3D shape. The cycle time of

this final moulding stage is dependent on the mould configuration, part

density and the type of resin. In certain designs, this process allows to

mould around inserts placed within the tool. This allows for the manufacture

of ready-to-use components or for packaging and materials handling products

to be used as part of a system.


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CHAPTER 2

2.1 OBJECTIVE OF THE STUDY

2.1.1 Current State

Current market practice of EPP production and molding imposes excessive

transportation cost to bead purchasers (bead molders) because molders are

purchasing expanded foam beads, light weight with huge volume. In an attempt

to ease the burden that bead molders bear, a new EPP production method that

enables an alternative EPP manufacturing practice is suggested.

The research scope includes:

• Extensive experiments on the relationship between expansion ratio of

PP beads and various content of BA, foaming temperature and pressure.

• Understanding of mechanism of the double melting peaks' formation

during EPP manufacturing process.

• Solubility and diffusivity measurements of various content of BA

(Blowing Agent), temperature and pressure in the system of

polypropylene/water. [6]


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2.2 THERMOPLASTIC FOAMING

Foaming of thermoplastic is gaining interest because of its potential to decrease

the weight of automotive parts and its superior mechanical and acoustic

properties for packaging as well as building materials. Thermoplastic foamed

products can be distinguished from their unfoamed counterparts by virtue of their

cellular structure. A typical foaming process has three stages: (1) polymer/gas

solution formation, (2) cell nucleation, and (3) cell growth and stabilization. The

polymer/gas solution formation is achieved by saturating a polymer with high-

pressure gas, typically above its melting temperature. The solution formation is

governed by gas diffusion in the polymer matrix. Cell nucleation occurs when

thermodynamic instability is introduced in the polymer/gas solution system

through a rapid change in the gas solubility. After cell nucleation, the gas from

the polymer matrix diffuses into the cells promoting cell growth. The cells grow

and reduce the total polymer density as the gas molecules diffuse into the

nucleated cells from the polymer matrix. Cell growth is primarily controlled by the

time allowed for the cells to grow, the temperature of the system, the states of

the supersaturation, the hydrostatic pressure of stress applied to the polymer

matrix and the viscoelastic properties of the polymer/gas solutions [5].

2.3 EPP PRE-EXPANSION

Expanded polypropylene resin beads are generally obtained by the dispersion

method in which the resin particles are dispersed in a dispersing medium, such

as water, in an autoclave together with a physical blowing agent and heated


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under a pressure to a temperature higher than the softening point of the resin

particles to impregnate the resin particles with the blowing agent. The resulting

dispersion is discharged from a bottom portion of the autoclave into a lower

pressure atmosphere while maintaining the pressure within the autoclave at a

pressure higher than the vapor pressure of the blowing agent, so that the resin

particles are foamed and expanded to obtain the expanded beads. Because of

the surface tension of the dispersing medium acted on the softened resin

particles during the impregnation thereof with the blowing agent, the resin

particles are smoothed and rounded. Therefore, the expanded beads obtained

from such rounded resin particles are generally round in shape. The diameter of

the through holes of the resin particles generally increases as a result of the

expansion and foaming. Thus, the expanded beads generally have a greater

diameter than their raw material resin particles.

The blowing agent used in the dispersion method may be an organic physical

blowing agent such as an aliphatic hydrocarbon, e.g. propane, butane etc. or an

inorganic physical blowing agent such as nitrogen, carbon dioxide, argon or air.

From the standpoint of costs and environmental problem, the use of an inorganic

blowing agent, particularly nitrogen, air or carbon dioxide is preferred. [?]

2.4 DOUBLE PEAK MECHANISMS

A common way of promoting good bead fusion is to develop two melting peaks in

the bead materials [7], [8]. When the steam temperature is chosen between the


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two melting peaks, the higher temperature peak will maintain the expanded

shape of the beads, while the lower-temperature melting peak contributes to the

sintering of the beads. An alternative way to promote good bead fusion is to

generate one broad melting peak, but it is not as good as having two peaks.

Even with two peaks, providing a constant steam temperature is not easy

because heat transfer takes place when steam enters the mold. Therefore, two

distinct melting peaks with a wide temperature difference are needed for

expanded bead foam processing.

Fig. 2 Typical DSC Curve for EPP Bead [9]


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2.4.1 Material Used:

A random PP co-polymer (SEP-550) from Honam Petrochemicals was selected

as the material primarily because of its wide application capability and physical

properties.

Applications: Foam beads for energy absorption systems (bumper core)

Attributes: Good heat welding property, High impact resistance

Datasheet for SEP-550 [11]

Properties Test

Method Units Properties

Melt Flow Rate 230℃, 2.16kg D-1238 g/10min 7.5

Density D-1505 g/㎠ 0.9

Mold Shrinkage HPC

Method % -

Tensile Strength at Break D-638 kg/㎠ 255

Ultimate Elongation D-638 % 500 <

Flexural Modulus of Elasticity D-790 kg/㎠ 9,500

Hardness D-648 R -

IZOD Impact Strength 23℃ HPC

Method kg.cm/cm 11


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IZOD Impact Strength -10℃ HPC

Method kg.cm/cm -

DU-POINT Impact Strength 23℃

HPC Method kg/㎝ -

DU-POINT Impact Strength -10℃

HPC Method kg/㎝ -

Softening Point D-1525 ℃ 129

Weatherability W-O-M Hour -

Heat Resistance 130℃ Air Oven Hour -

Heat Distortion 4.6kg/㎠ D-648 ℃ -

HAZE(2mm) D-748 % -

Flammability UL


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CHAPTER 3

3.1 SOLUBILITY ANALYSIS via PVT

3.1.1 Determination Of CO2 Solubility in EPP & Water

The initial step was to attempt to determine the amount of solubility of CO2

in the material dispersed in water. There have been numerous experiments

conducted and sufficient data available to gauge the amount of carbon

dioxide soluble in EPP at various pressure-temperature combinations.

There though is not adequate experimental data to support how the

addition of water to the equation impacts the solubility of the gas in the

polymer.

Any liquid in which the resin particles are insoluble may be used as the

dispersing medium. Examples of the dispersing medium include water,

ethylene glycol, glycerin, methanol, ethanol and mixtures thereof. The

dispersing medium is preferably water or an aqueous dispersing medium.

Primary reason for selecting water was due to the known fact that water

acts as an economic dispersion medium for polymer autoclaving

processes. Expanded polypropylene resin beads are generally obtained by

a dispersion method which includes dispersing particles (or pellets) of a

polypropylene resin in water in an autoclave and impregnating the

polypropylene resin particles with a blowing agent at a temperature higher

than the softening point of the resin under pressurized conditions. The

dispersion is then discharged from the autoclave to the atmosphere so that

the resin particles are foamed and expanded [27].

3.2 PVT RELATIONSHIP (PP & Carbon Dioxide)

In polymer/gas solution system, when gas dissolves into a molten polymer,

the polymer swells (or dilates) due to gas sorption. The amount of polymer

swelling or dilation is characterized by its PVT properties, which can be


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obtained by measuring the equilibrium state volume of a polymer/gas

solution at any specific temperature and pressure. In polymer foaming

applications, cell nucleation and growth are governed by physical

properties such as solubility [12, 13], diffusivity [14] and surface tension

[15–17]. However, the determination of these properties relies on the PVT

data (i.e.), the polymer swelling caused by gas dissolution. For example,

currently, the most commonly used technique for gas solubility

measurement in polymer melts is the use of a magnetic suspension

balance system [18–21], in which the accurate determination of gas

solubility depends on the buoyancy correction of the equilibrium

polymer/gas solution volume, i.e., the swelling of the polymer due to the

dissolved gas. Therefore, knowing their PVT properties (the polymer

swelling by dissolved gas) is critical for understanding and controlling foam

processing

3.3 PVT EXPERIMENTAL DESIGN

3.3.1 PVT system

The overall system has two major functional traits (viz.) the software attribute and

the hardware attribute. The software attribute comprises of an integration of

modules with each function module implemented to perform individual tasks. The

individual tasks are as follows: image capture, image reconstruction, and volume

integration. The hardware attribute is the actual constructed and assembled

apparatus through which the function modules work. Fig. 5 shows a schematic of

the overall PVT system. Those two major functional attributes are integrated to

construct the complete apparatus. The software algorithm such as the camera

movement and image capture, etc. were developed and implemented in the


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actual system to facilitate the functions such as stage movement and camera

triggering for image capturing, etc. through the apparatus. [5]

Fig. 5 PVT System/Experimental Setup [5]

3.3.2. PVT Experimental setup

The experimental apparatus consists of the following components: a high-

pressure chamber with a sapphire visualization Y.G. Li et al. / Fluid Phase

Equilibria 270 (2008); a 2024×2024 resolution JAI Pulnix TM4100 CL camera

with a control software (Easy Grab); Schneider 4/80 lens and extension tubes; a

temperature controller (Omega CN132) with thermocouple (Omega RTD); four

cartridge heaters; an automatic high-precision XY stage with Galil motion

controller and control board; a manual 1 in. XYZ stage to adjust the position of


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the light source; a syringe pump connected to the gas tank; and a backlight

source with a light equalizer/diffuser [5]

3.4 PVT EXPERIMENTAL RESULTS AND DISCUSSIONS

3.4.1 PVT Behaviors for PP/CO2 & Water System

The current system setup for the PVT system in MPML Laboratory did not

adequately support the experimental conditions of attempting to determine

solubility of CO2 in EPP in an aqueous based medium. The PVT system was

used to predict the amount of gas solubility in the autoclave wherein the EPP

beads are suspended in water. Though water is primarily used as a dispersion

medium in the process of bead expansion it certainly affects the amount of CO2

that would dissolve in the EPP beads during saturation. Further experiments

need to be conducted with modified experimental setup to allow the water to

remain stable and aid determines the gas solubility in EPP. The scope of these

experiments exceeds the current project scope and has been dedicated as a part

of future work.

3.4.2 PVT Behaviors for PP/CO2 Only System

Under experimental conditions wherein the EPP beads are subjected to

saturation via Gas (CO2) only, under high pressure and temperature, sufficient

previous experimental data has been collected to help determine the solubility of

gas in polymer. When a polymer melt is exposed to a high pressure gas, the


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dissolved gas under high pressure will cause the polymer to swell and increase

the specific volume. This means that the presence of gas evidently enhances the

overall activity of the polymer/gas system, and thus, creates more free volume for

the CO2 molecule to penetrate into the polymer matrix after the gas fills the

existing free volume. Therefore, the specific volume will be increased under a

high pressure gas. The increased free volume causes an increase in solubility

and diffusivity [22]. The dissolved CO2 causes a plasticization effect to reduce

the viscosity of the polymer/gas mixtures and increases the chain mobility [23].

This increased specific volume also decreases the surface tension of polymer

[24]. In summary, despite the hydraulic compression effect, the dissolved gas

increases the specific volume and thereby, affects the solubility, diffusivity,

viscosity and surface tension. All these fundamental factors are decisive in

determining the foaming behavior of a polymer and hence determining the

solubility and the swelling caused by gas dissolution is of utmost importance,

since it governs all of these parameters.


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CHAPTER 4

4.1 SOLUBILITY ANALYSIS via MAGNETIC SUSPENSION BALANCE

Inconclusive PVT Experimental results for determination of CO2 solubility in EPP

suspended in an aqueous medium propagated the need to use a different

experimental medium to determine the same. Magnetic Suspension Balances

(MSB) is able to perform kinetic and equilibrium measurements and solubility

data (e.g. absorption and adsorption isotherms) can be examined gravimetrically

with high accuracy [25].


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4.2 MSB EXPERIMENTAL APPARATUS AND PROCEDURE

A schematic of the MSB is shown in Fig. 6

Fig. 6 Magnetic Suspension Balance [25]


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The MSB consists of a sorption chamber that is used to expose the sample to

gas at elevated temperatures and pressures, and a microbalance, which is

isolated from the sample and exists at ambient conditions. An electromagnet

connected to the microbalance is adjusted so that a permanent magnet

connected to a rod–rod–basket assembly and located within the sorption

chamber is kept in suspension. Thus, the microbalance measures a weight that

is proportional to the electromagnetic force, which keeps the rod–rod–basket

assembly in suspension [25].

In a typical experiment, the polymer is placed in a sample basket as powder or is

directly attached as a film (i.e., PSF) to the hook of the rod–rod assembly. After

closing the sorption chamber the polymer is degassed by evacuating the sorption

chamber at 10−2 Torr until the weight measured by the microbalance remains

unchanged over time. A heating circulator is used to control the temperature of

the chamber, which is measured with a calibrated platinum resistance

thermometer to an accuracy of ± 0.5 °C. The sample weight, read from the

microbalance under vacuum and at temperature T, is recorded as ws(vac,T).[25]

The electromagnetic force measured by the microbalance increases to keep the

rod–rod–basket assembly in suspension. In this manner, the mass of the gas

absorbed by the polymer is measured directly by monitoring the increase of the

electromagnetic force. Eventually, the equilibrium sorption (the solubility) is

reached and the weight of the sample stops increasing. At this final saturation


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stage, the weight reading from the microbalance at pressure P and temperature

T is recorded as ws (P, T).

The mass of the gas dissolved in the polymer, wfl (P, T) is calculated using the

following equation:

wfl(P,T)=ws(P,T)−ws(vac,T)+ρfl(P,T)(Vr–b+Vp+Vsw) [25]

where ρfl(P,T) is the density of the gas at P and T, Vr–b and Vp the volumes of the

rod–basket assembly and of the original polymer, respectively, and Vsw is the

volume change of the original polymer due to swelling. The last term of the above

equation, ρfl(P,T)(Vr–b + Vp + Vsw), represents the buoyancy force caused by the

compressed gas, which lifts the rod–rod–basket assembly and thus decreases

the electromagnetic force necessary to keep the rod–rod–basket assembly in

suspension [25]. The density of CO2, ρfl(P,T) is calculated from the Bender EoS

[26], which is accurate for the purposes of this work.

4.3 MSB EXPERIMENTAL RESULTS AND DISCUSSIONS

Multiple experiments were conducted at different pressure and temperature sets

for pre-weighed EPP beads dispersed in water. The aim was to attain

experimental data to support claims for determining an optimum pressure-

temperature combination wherein peak solubility of CO2 could be achieved in

EPP beads dispersed in water.


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Similar results to PVT were attained for solubility data of CO2 in EPP beads

dispersed in water. Lack of apparatus capability to retain water during the

saturation phase was a major deterrent for the inconclusive results. The

conclusion in short though was that once again the apparatus needed to be

modified to adapt to the experimental set of conditions required. These

modifications exceeded the current project scope and have been delegated as

part of future work.


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CHAPTER 5

5.1 AUTOCLAVE APPRATUS SETUP:

A pre-designed autoclave was initially selected as the pressure chamber for bead

expansion. Additional features like pressure gauge (Range: 0 – 1000 psi) and

four 250 W cartridge heaters from McMaster Carr (machined and embedded into

the chamber along the periphery) were added for experimentation. A side

discharge valve from Swagelok was appended to provide rapid discharge of gas

after saturation and provide conditions for bead expansion and foaming. A safety

feature in the form of an 8” x 8” x 11” plexiglass cuboid with a 1” Diameter orifice

was affixed at the end of the valve outlet for secure release of steam-gas mixture

from the chamber. An additional cylindrical Stainless Steel (SS-316) vessel was

machined in the UofT Machine Shop to create a medium for holding water and

EPP bead mixture.


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Fig. 3 Initial Chamber With Side Discharge Valve

A temperature controller unit was designed with an 8-pin SS (Solid State) relay,

20 Amp General purpose fuse and micro-controller (from Love Controls) to

provide a closed loop feedback system for temperature control. The circuit

diagram for the controller was illustrated below:


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Fig. 4 Temperature Controller Circuit Diagram

5.2 PROCEDURE

The expanded beads are prepared from resin particles by the dispersion method

in which the resin particles are dispersed in a dispersing medium, such as water,

in an autoclave together with a physical blowing agent and heated under a

pressure to a temperature higher than the softening point of the resin particles to

impregnate the resin particles with the blowing agent. The resulting dispersion is

discharged from a the autoclave into a lower pressure atmosphere while

maintaining the pressure within the autoclave at a pressure higher than the vapor

pressure of the blowing agent, so that the resin particles are foamed and

L N

L

L

N

Relay

Controller

Fuse

N L Heater

AC Supply


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expanded to obtain the expanded beads. Because of the surface tension of the

dispersing medium acting on the softened resin particles during the impregnation

with the blowing agent, the resin particles are smooth and rounded. The diameter

of the resin particles increases as a result of the gas absorption and the

expanded bead attain a greater diameter than their raw material resin particles

[27].

Various experiments were conducted at different temperatures and various

saturation times. 50 Grams of unexpanded particles formed of the SEP-550

polymer were charged into an autoclave and was mixed with 500 ml of water to

form dispersion. The dispersion was heated to foaming temperature of 150 C. A

blowing agent (carbon dioxide) was then fed to the autoclave until the pressure

within the autoclave had the value of 600 psi and the dispersion was maintained

at that condition for 60 min. Then the autoclave was opened to discharge the gas

and the dispersion was recovered after opening the autoclave. The same set of

experiment was conducted albeit with the saturation time now at 120 min. Similar

experiments were conducted for another a couple of temperature – saturation

time combinations (viz.) 155 C and 160 C, both @ 60 mins and 120 mins.


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5.3 INITIAL RESULTS AND DISCUSSIONS

Fig. 4 Results from initial experiments


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Initial experimental data was directed towards attaining an optimum temperature

– saturation time blend to enable maximum expansion ratio for the polymeric

material. All initial observations consisted of low to negligible expansion ratios

and involved substantial agglomeration of the beads.

The conjoint beads made it very arduous to estimate the exact expansion of each

individual pellet. The following possibilities were researched and looked into in an

attempt to resolve the agglomeration issue:

� Pre-coating the beads before the foaming

� Setting a stirrer in the chamber.

• Adding a suspension stabilizer to the dispersion with the following ratios:

o Water: 220 parts

o PP pellets: 100 parts

o Suspension stabilizer: 0.2 parts (ex. Sodium dodecylbezene

suffocate, aluminum sulfate


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The addition of suspension stabilizer was determined to be the most economic

and likely possible alternative as the pre-coating of beads was not only

expensive but also could impact the gas absorption capabilities of the polymer

bead. The addition of stirrer would involve a complete re-design of the entire

autoclave chamber and involve additional financial costs.

.

Fig. 6 2nd generation of Chamber setup


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Fig. 7 2nd generation of Chamber

5.4 2nd GENERATION CHAMBER RESULTS AND DISCUSSIONS

A new chamber was designed with bottom discharge option for both gas and

liquid together with the pallets. After conducting some test experiments with the

same conditions previously used except adding the stirring with the help of a

100RPM motor, the following observations have been noted.

Molten Pellets Stuck Inside Chamber

• Low Stirring RPM – Not enough force to eject polymer beads out of the

chamber

• Insufficient discharge port Diameter


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• Temperature is too high

Some simulation experiments were performed using a glass beaker of same size

of chamber and magnetic stirrers of different diameters and lengths to observe

the effect of increased speed of stirring, a better vortex was formed which was

required in order to provide enough force to push the beads out of the chamber.

Results showed that stirring speed and structure of the stirrer have significant

effects on the water vortex

5.5 CHAMBER AND PROPELLER MODIFICATIONS

New propeller was designed in order to get enough vortex to push the beads out.

Different propellers were tested and a 3 fin 2.5” diameter propeller was acquired

for future experiment. The previous rotary seal failed as we raised the stirring

speed to above 100rpm

New rotary seals were purchased which can be operated under the following

conditions

• Pressure = 2000 psi (max)

• Speed = 1500 – 1750 rpm

• Temperature = 200 C


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This requires increase of hardness of the shaft (55-65Rc) and with 8-12 mils

plunge-ground finished and a retainer ring to hold the seal in place will be

applied.

Fig. 8 Modification of shaft of propeller


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Fig. 9 Modification to the propeller

Fig. 10 New high speed motor


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REFERENCES

[1] http://www.bpf.co.uk/Plastipedia/Polymers/Moulding_EPP.aspx

[2] Hidehiro Sasaki, Akinobu Hira, Hashimoto, Tokoro, “Process Of Producing

Foamed Molding From Expanded Polypropylene Resin Beads and Process Of

Producing Expanded Polypropylene Resin Beads”, United States Patent , Patent

# U.S 7,560,498 B2

[3] www.epp.com

[4] http://www.morval.com/03process/03index.html

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