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Extrusion Foaming of PlastidWood-Fiber Composites Using a Chemical Blowing Agent

Ghaus Muhammad Rizvi

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Mechanical & Industrial Engineering University of Toronto

O Copyright by Ghaus Muhammad Rizvi ( 1998)

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Extrusion Foaming of PlastidWood-Fiber Composites using a Chemical Blowing Agent

Ghaus Muhammad Rizvi M.A.Sc., 1998, Department of Mechanical & Industrial Engineering

University of Toronto

ABSTRACT

This Thesis presents a study on feasibility of continuous extrusion processing for the

manufacture of fine-celled plasiic/wood-fiber composite foams. The main benefits of

incorporating wood-fibers in plastics are the increased stiffness and lowered cost of the

resulting composites. However, these improvements are usually accompanied by loss in

the ductility and impact resistance of the composites. These shortcomings can be

removed by effectively foaming and incorporating a fine-ceIl structure in the composites.

,Moisture removal is an important requirernent for improving the foam structure. Various

schemes of onIine moisture removal were explored and a promising process for the

production of fine-celled plastic/wood-fiber composites foams has been developed,

HDPWwood-fiber composite foams, with significantly improved ce11 structure and a high

volume expansion ratio up to nine-fold. have been successfully produced. For less

stringent requirements. the tèasibility of using the water, inherently present in the wood-

fiber, as a blowing agent was also explored. The addition of a nucleating a,oent

significantly improved the ce11 structure of PS/wood-fiber composite foams.

I would like to thank my supervisor Professor Chui B. Park for his guidance and

encouragement in the research. 1 would also like to thank Materials and Manufacturing

Ontario (MMO), CertainTeed, CRF Technologies Group Ltd. and Dupont Canada for

their funding and support in this project.

1 wish [O acknowledge the help and support of my past and present colleagues in

the Microcellular Plastics Manufacturing Laboratory at the University of Toronto. They

include: L. Matuanri. A. Behravesh, A. Yueng, D. Fernandes, F. Liu, Dr. Y. Liu. G. Liu.

H. Naguib. D. Ladin. S. Park, H. Zhang. R. Pop-Iliev, A Xie, X. Guan and J. Xu. I ais0

want to acknowledge the professional & technical support froin Mike Smith. Jeff

Sansorne and Dave Esdaile in the Machine Tool Laboratory.

Above all, 1 would like to acknowledge the immense support. patience and

understandin,o extended by my wife Sameena and my children Rem, Ghazi and Rarizi.

Wi thout their sacrifice. it would have been impossible to continue studies at this stage of

rny lifc.

Abstract

Acknowledgments.. .

Table of Contents

Nomenclature

List of Tables

List of Fi, cures

Chapter 1 Introduction

1 . 1 PlastidWood-Fiber Composites

1.2 Foams

1.3 Motivation of Thesis and Objectives

1.4 Microcellular Foams

1.5 Design of Experiments

1.6 Ovsrview of the Thesis

Chapter 2 Literature Review

2.1 Pimtic/Wood-Fiber Composites

2.1.1 FiIlers in Polymers

2. i -2 Processing and Nature of Fibers

2.1.3 Coupling Agents and Surface Modification

2.2 Thennoplastic Foams

2.2.1 Conventional Continuous Foarn Processing

2.2.2 Batch Microcellul~ Processing

2.2.3 Semi-Continuous Microcellular Processing

2.2.4 Continuous Microcellular Processing

2.3 Foaming of plastic/Wood-Fiber Components

1 V

vii

Chapter 3 Detailed Design & Construction of the Experimental Equiprnent

3.1 Design of Plastic/Wood-Fiber Extrusion Systern

3.1.1 Overview of the System

3.1.2 Extruder

3.1.3 Diffusion Enhancing Device

3.1.4 Heat Exchanger

3.1 -5 Force- Fed Hopper

3.2 Design of the Vented Extrusion Equipment

3.2.1 Overview of the System

3.2.3 Gear Pump

3.2.3 Modification of Diffusion Section.

Chapter 1 Foaming of PSIWmd-Fiber Composites in Extrusion Using

Moisture as a Blowing Agent

4.1 Introduction

4.2 Experimental

4.2.1 mate riais

4.2.2 Thermogravimetric Analysis of Wood-Fibers

4.2.3 Differential Scanning Calimetry

4 -24 Blending

4.2.5 Extrusion Setup

4.2.6 Extrusion Foaming of the Composites

4.3 Results and Discussion

4.3.1 Thermogravimetric Analysis of Wood-Fibers

4.3.2 Effect of Moisture on Volume Expansion

4.3.3 Cell Nucleation Behavior of Plastic/Wood-fiber Composites

with Moisture, HC and Mineral Oil

3.4 Summary and Conclusion

Chapter 5 Foaming of P W d - f i b e r composites

5.1 Introduction

5.1.1. Requirement for Coupling Agent

5.1.2 Polyethylene (PE) Foams

5.1.3 Batch Process vs. Continuous Extrusion

5.2 Experimentation with Various System Configurations

5.2.1 Processing of LLDPWWood-Fiber Composite Foams using a

Single Screw Extruder

5.2.2 Processing of HDPE/Wood-Fiber Composite Foams using a

Single Screw Extruder

5.2.3 Processing of LLDPENood-Fiber Composite Foam using a

Vented Extruder

5.2.4 Processing of LLDPEi'Wood-Fiber Composite Foam using a

Master Pellet

5.2.5 Experiments of HDPE/Wood-Fiber Composite Foams Using a

Tandem Extruder Setup

5.3 Summary and Conclusions

Chapter 6 Summary, Conclusion and Future Work

6.1 Summary

6.2 Conclusions

6.3 Future Work

References

= diffusivity (crn2/s)

= diffusivity coefficient constant (cm2/s)

= activition energy for diffusion (I)

= universai gas constant ( JK)

= solubility of gas in the polymer ( ~ r n ~ / ~ or g(gas)/g(polymer))

= Henry's law constant (cm3 [STP]/g-Pa)

= saturation pressure (Pa)

= rate of nucleation (#/cm3s)

frequency factor of gas molecules joining the nucleus ( I l s )

concentration of gas molecules in solution (#/rn3)

Gibbs free energy of bubble nucleation (J)

Boltzman constant (J/K)

temperature (K)

inrerfacial tension (mllm')

gas saturation pressure or pressure drop (Pa or psi)

Reynolds number

flow rate (@/min)

specific gravity

viscosity or melt viscosity (cP)

= inside pipe diameter (in)

= volume flow rate (m3/s)

vii

D

No

RPiM

v f

Vpo~~rncr

qb[ouinp agrni

@

R f

HC

TGA

LLDPE

HDPE

PS

HIPS

diameter of the cell (m)

cell-population density (cells/cm3)

rotational speed (roundmin)

theoretical maximum volume expansion ratio

specific volume of the polymer

blowing agent efficiency (5%)

surface to volume ratio pçr unir volume (m-')

radius of the extrudate (m)

Hydrocerol Compound

Thermal Gravimetric Andyzer

Linear Low Density Polyethylene

High Density Polyethylene

Polystyrene

High Impact Polystyrene

LIST OF TABLES

Table 4.1 : Difierent fomrilrrtions of composites . . . . . .. . ... ...... ..

Figure 2.1 :

Figure 2.2:

Figure 2.3:

Figure 2.4:

Figure 2.5 :

Figure 2.6:

Figure 3.1 :

Figure 3.2:

Figure 3.3:

Figure 3.4:

Figure 4.1 :

Figure 4.2:

Figure 4.3:

Figure 4.4:

Figure 4.5:

Figure 4. 6:

Figure 4.7:

Figure 4.8:

Figure 4.9:

Figure 5.1 :

Schematic of the Celuka process.

Schematic of free-rise foaming extrusion

Schematic of the batch process.

A schematic of a Microcellular Continuous Process

Modeling of gas escape and cell coalescence in extruded foams at a high temperature of polymer melt

Ce11 coalescence phenomenon caused by rupture of the thin membrane separating two ceils

Schematic of the mixing stage

The designed heat exchanger.

Force feed hopper.

Schematic of the gear pump

Experimental Set-up.

TGA thermograms for the devolati lization of wooc i fibers.

Effects of material compositions and extrusion foaming temperature on the expansion ratio of foamed composites

Effects of temperature variations on the specific volume of the foamed composites

DSC Thermograms of PS & HE'S

Maximum achievable volume of wood-fiber composites.

Effects of marerial compositions and extrusion foaming temperature on the ce11 population densities of foamed composites.

SEM micrographs of PS/wood-fiber composite foams

SEM micrographs of HIf S/wood-fiber composite

Effect of nozzle temperature on the expansion ratio of LLDPE/wood-fiber compositç foams processed with 1 wt% HC. Series A represents composites produced with undried wood- fibers. and series B A represents composites produced with dried wood-fi bers.

Figure 5.2:

Figure 5.3:

Figure 5.4:

Figure 5.5:

Figure 5.6:

Figure 5.7:

Figure 5.8:

Figure 5.9:

Figure 5.10:

Figure 5.1 1

SEM micrographs of a) LLDPEYwood-fiber composite foams (a) using undried and (b) dried wood-fibers

Theoretical expansion ratio for a composite foam having 20 wt% wood-fiber.

Volume expansion behavior of HDPE/wood-fiber composite foams using HC as a CBA

SEM microgragh of HDPE/wood-fi ber composite foam wi th ten- fold expansion

Venting extruder with gear pump

modification in the extrusion setup with a gear pump

Volume expansion behavior of LLDPE/wood-fiber composites while using a venting extruder with a gear pump

Volume expansion behavior of HDPE/wood-fiber composite foams in a tandem extrusion setup

SEM rnicrogragh of HDPEhood-fiber composite foam from a tandem extrusion system with nine-fold expansion.

Schernatic of a devolatilization tandem extrusion system

Introduction

1.1 PlastidWood-Fiber Composites

During the last few decades, thermoplastics have gained e ver-increasing

ricceptance as an important family of engineering materials and are steadily replacing

metals in a wide variety of applications. The commercial consumption of therrnoplastics

hris grown at an average rate of more than 6% per year during the last decade and this

trend is expected to continue in the foreseeable future. with the consequent increase in

their prices [1,2]. This situation has created an impetus for cost reduction by utilizing

fillers in thermoplastics [3]. Composite materials also offer an opponunity for utilizing

the favorable propenies of the constituent components.

Presentl y, the fi ber-reinforced plastic composites market, which represents one

third of al1 performance plastics, is dominated by glass fiber (93% of reinforcement

materials) [4] and some other inorganic fillers such as talc, mica, clay. and calcium

carbonate. These high-density conventional fillers offer wide property changes in the

composites but on a volumetric bais, their use is not cost-effective [ 5 ] .

In the last decade, natural organic rein forcements such as lignocellulosic-fibers

Oes over most have penetrated slowly into this market, because they offer many advanta,

common minera1 fillers. They are abundantly available, have lower cost and density,

resu l t in reduced Wear of processing equipment, and are renewable, recyclable, non-

tiazrirdous and biodegradable. The replacement of inorganic fillers with comparable

cellulosic-fibers provides weight savings and decreases the cost without reducing the

rigidity of the composite [Il. Wood-fibers are predominantly the most extensively used

fibers among al1 the cellulosic-fibers used as filiers.

The cost and weight of wood-fibers is substantially lower than the other

commonly used filler materials. Plasticlwood-fiber composites can be a cost-effective

alternative to many filled plastics or metals in tems of bending stiffness or weight [3] .

The wood-fibers are non-abrasive so that reiativeiy large concentrations can be

incorporated into plastics without causing serious machine Wear during blending and

processing. Although plastic/wood-fiber composites have been commercialized. and can

compete with certain plastic and wood products, their potential industrial applications have

been limited because of their low impact strength and high density compared to natural

wood and certain plastics [6].

Impact strength and ductility of plastics can be significantly improved by

incorporating a fine ceIl structure into them [7]. Additional benefits of having a foarned

structure are the reduction in weight and cost. Therefore, it is reasonable to expect that, if

a fine ce1 led structure is successfully produced in plasticlwood-fiber composites. the

problems with impact strength and weight mentioned earlier cm be reduced or

eliminated. This would significantly increase the potentiai of industrial application for

these composites.

1.2 Foams

Foamed materials are plastics having voids or cavities, or in other words they

have a ce1 lular structure. Thennoplastic foams are produced by deliberately adding at

least one gas-generating substance such as a chemical blowing agent. a soluble gas. or

volatile liquid under pressure to the polymer melt, then altering the environment to cause

the gas-generating substance to yield discrete bubbles [8].

Foarns or cellular materials offer many advantages over traditional materials and

non-celluIar polymers. Forims are very cost-effective and have unique insulating

properties, impact-resistant characteristics. buoyancy. and outstanding strength-to-weight

rarios, among other attributes [8]. Plastic foams have important economic impact on

nearly every aspect of life today. High-density celluIar plastics are used in fumiture.

transportation, and building products. Low-density foams are used for shock mitigation.

insulation. and rigid packaging.

The ceIl morphology of the foams influences their characteristics. such as

strength. thermophysical, dielectric and other properties. Conventional foam production

methods generally yield non-uniform cells with ce11 size greater than t 00 -300 pm with a

wide ceIl size distribution. This causes a deterioration in their properties. Foarns having ri

uniformly distributed fine-celled structure exhibit superior insulation and impact

resistance when compared to conventional foarns.

1.3 MotivationofThesisandObjectives

As mentioned earlier, the ductility and impact strength are substantirill y improved

by incorporating a fine-celi structure into the plastic/wood-fiber composites. However. if

the cell structure is couse. the properties of the composite foams will be significantly

reduced and such composites will exhibit low tolerance to a repeated nailing or screwing

operation. This will reduce their ability to compete with wood in the lurnber market. It is

believed that very small and uniformly distributed cells will also prevent the ïüptuic of

composite during nailing and screwing operations, This provides the basic motivation of

this thesis, which is to produce fine celled plastic/wood-fiber composites in a continuous

cost-effective extrusion process. Increasing the expansion ratio, within the limits of the

envisaged application, can lead to further cost reduction.

At present. most commercial manufacturers are using chemical blowing agents

(CBA's) for making composite foams [9]. Therefore. this study focuses on the use of

CBA's, for foaming of plastic/wood-fiber composites, as an evolutionary step. Best

results are more likely to be obtained by direct injection of the physical blowing agent

(PBA) directly into the extruder barre1 at high pressure and temperature. This method will

also be more cost-effective as CBA's are expensive and have low gas yield. The

composite foams currently being produced in the industry have a very coarse structure

and the bubble size is of the order of a couple of millimeters. Therefore. if foamed

plastic/wood-fiber composites with a uniforrn distribution of cells smaller than 0.5 mm

are successfully produced. it would be of considerable importance and will provide

significant insight into the development of a still finer foarn structure.

Therefore, the objective of this research was to develop a system capable of

produci ng plasticlwood-fiber composites having a cell density of the order of 1 OE'

cells/cm3 and expansion ratio of up to 10.

1.4 klicrocellular Foams

The tcchnology for producing plastic foarns having extremely small ceIl size, Iess

than 10 Pm, and a large cell population, of the order of 10' cells/cm3, has been steadily

evolving over the last decade. These foams have k e n named microcellular foams. A similar

üpproach is adopted for producing a fine-celled structure in plastic/wood-fiber composites.

The concept of creating microcellular structure in a polymer was invented by Suh

et al. [IO] at the Massachusetts Institute of Technology in the early 1980's. The

development of these materials is based on the idea that the creation of a very large

number of microbubbles, smaller than the pre-existing natural flaws in a polymer. can

reduce the material cost and consumption in mas-produced plastic parts without

compromising mechanical properties.

The mechanism for producing a very fine cell structure is that a gas is first

dissolved into the polymer matrix and then a large number of bubbles are nucleateci by

inducing a therrnodynamic instability. This instability c m be created by rapidly dropping

the pressure across the extrusion die [IO- 151.

1.5 Design of Experiments

Since there is a very large number of primary variables including the processing

conditions. which affect the properties of the composites, a large number of experiments

would be rcquired to study them al1 at once. This would entail numerous experiments. a

substantia1 portion of which might prove to be of minimal value with regard to

information content. In order to reduce the number of experiments, a number of material

and processing parameters were kept constant, for example only 20 wt% of one grade of

wood-fiber was used in al1 experiments.

1.6 Overview of the Thesis

Chapter 2 contains the literature survey. it starts with a review of the composites and

their fillers and the usage of coupling agents. Conventionai foarns and their production

process are bnefly reviewed followed by a review of microcellular foarns and their

production.

Chapter 3 describes the actual design of different cornponents in the single screw

extrusion system. the vented extruder with a gear pump and finally a tandem exvusion

system. These include the extruders, a diffusion enhancing device, a heat exchanger, dies.

and other componen ts.

Chapter 4 describes the experiments conducted for studying the feasibility of using

the inherent moisture present in the wood-fiber as a blowing agent. The devolatilizing

characteristics of wood-fibers were studied using TGA and important processing limitations

were iden ti fied for plastic/wood-fi ber composite extrusion in general. PS and HPS/wood-

fiber composites were processed in extrusion and foamed successfulIy. The effect of using a

nucleating agent on the foam structure was also studied.

Chapter 5 describes the efforts made for producing PE/wood-fiber composite f o m s

using ri CBA. The key issue was the removai of moisture for achieving a good foam

structure. Efforts were made to do away the vacuum-oven batch drying process and to

dcvelop a continuous, online process for moisture removal. For this purpose, various

processing systems were developed and finally a promising system was identified.

Chapter 6 provides a surnmary and the conclusions of the research, along with

suggestions for future work based on the research results presented in Chapter 5.

Chapter 2

Literature Review

The use of wood-fibers in thennosetting resins is well established in products

such as particleboard and fiberboard. However, in therrnoplastics their use ris filler

material h a s been limited due their weak adhesion and poor dispersion [ 1,161. Recent

investigations into bondingkoupling agents that reduce the incompatibility between the

hydrophillic wood-fibers and the hydrophobic polymers, has provided an impetus for

producing plastic/wood-fibe r composites to make use of the rnany advantages offered by

these composites [!. 161.

2.1 PlastidWood-Fiber Composites

The products of cellulose materials, such as wood-fiber. combined with synthetic

plastics are known as plastic/wood-fiber composites. One of the advantages of combining

wood-fiber with plastic is to combine desirable properties of both components. hence

enhancing or extending their usefulness [ 1,6,8,17].

2.1. J Filfers in Plastics

The primary purposes for using a filler in ri thermoplastic composite are their

potential ability to modify the properties and to reduce the material cost. The fillers can

be either solid particulate or fibrous materials. The modification in properties can lead to

reduced tackiness. improved stiffness and scuff resistance, and enhanced electrical

properties and dimensional stability [ 18,193.

iMineral Fillers

Mineral filler-filled plastic composites have been used for many decades.

Calcium carbonate (&CO3) has been widely used in plastic composites because of the

cost advantages and other appealing properties including low plasticizer absorption and

strong resistance to thema1 degradation during processing. Silicates, silicas (clay, talc.

mica. etc.) and sulfates of the alkine-earth metals [barium (BaSOA) and anhydrous

calcium (CaS04) sulfates] constitute other inorganic filfers widely used. Ctays are useful

for some specific applications such as electric iiisulator products. In general, al1 the

fiIlers are mainly utilized with the aim of lowering material cost [18].

Another class of filler materials used in plastics can be termed as "functional

fillers". They include glass fibers and glass spheres, carbon fibers, fine-particle calcium

carbonates, etc. [19]. These fillers are mainly used for improving the performance of the

composite rather than for reducing its cost. Although these inorganic fillers are low-cost

on a weight basis, their high densities (specific gravity about 2.5) can increrise the cost on

a volume basis and make processing more difficult [2].

Organic Fillers

Organic fiilers such as wood-fibers, pulp-fibers. and sugar-cane husks are aiso

used for making polymer composites. They possess a low specific gravity (about 0.9),

and when used to extend resins, they offer a significant cost saving [2-4.6,7,17]. The use

of cellulosic fillers is more common in polyolefins (polypropylene and polyethylene)

than in ot her thermoplastics. Most of the available literature on cellulosic-fiber fi lled

plastics can be classified into three groups: ( 1 ) study of the processing parameters. (2)

effect of the nature or type of fibers, and (3) effect of coupling and adhesion promoters

on the performance of the composites.

2.1.2. Processing and Nature of the Fibers

The available literature indicates that, from among the many forms of available

cellulosic fillers, wood-fibers are the rnost favored form of fillers in most publications.

Yusko~~a et al. [20] studied the interaction energy between polymer and different forrns of

cellulosic-fibers (lignin, cotton linter, and wood-fiber) and found that wood-fiber

contributed the most desirable strength properties to the composite matrix due to its

highest adhesion interaction. Their study has also demonstrated the promising potential

of cellulosic-fibers to replace common inorganic fillers.

The incorporation of fibrous materials derived from cellulose into a plastic matrix

has also been studied extensively [3.7,21-241. The main reason for using tïbrous

cellulose materials lies in the tàct that these fibers are more able to resist degradation of

their aspect ratio during processing, which is essential for fibers to transfer stresses

effectively in the composites.

The effect of the nature of cellulosic-fibers in a plastic matrix has been studied by

.Maldas et al. [25]. The cellulosic-fibers used were derived from cotton. softwood and

hardwood in the forrns of different pulps (CTiMP. kraft, and sawdust). Based on

mechanical property testing, the authors concluded that the softwood fibers are preferable

to hardwood fibers in improving properties. Such disparity was explained by the

morphological difference between the more flexible softwood and stiffer hardwood fibers

as well as the aspect ratio of the fibers from the two wood species.

2.1.3. Coupling Agents and Surface Modifications

The strength performance of the cellulosic fiber/plastic composites is generally

low compared to unfilled polymer. The decreased strength is likely a result of poor

adhesion and/or dispersion of fibers due to the natural incompatibility of phases during the

mixing of the hydrophilic cellulosic fibers with the hydrophobic polymeric matrix. To

improve the adhesion between fibers and polymer, coupling agents are usually used.

Coupling agents are able to react chemically on both ends during processing, with the

fibers on the one side and the polymer on the other, thus creating a chemical btidge at the

interface. The resultant chemical bonding increases the ability of the matrix to transfer

stress to the fibers [26-281. Although coupling agents c m chemically react with both the

polymer and the fibers, there is some evidence that the reaction with the polymer is not

necessaril y a prerequisite.

In order to overcome the incompatibility between cellulosic fibers and plastics,

Kokta et al. have assessed the effects of various types of isocyanates [26], mateic

anhydride, and silanes [29] as coupling agents. The mechanical properties of hardwood

aspen in the form of CTMP pulp and sawdust [29] as well as Cotton fiber [26] with two

types of PVC resins (Baron and Goodrich) were investigated. The results showed chat

the type and concentration of coupling agents used influenced the performance of the

composites to a considerable extent. For example, the tensile and flexural properties of

the isocyanate treated wood-fibers composites were increased compared to the

composites with untreated fibers. When used with a moderate concentration (0.5 to 5 %

by weight), isocyanate acted as an adhesion promoter and offered the maximum

enhancement of mechanical properties whereas with greater concentration the properties

deteriorated.

The main drawbacks of cellulosic-fibers filled polymer are their relatively low

degradation temperature (about 200°C) and their water sorption. which weakens their

adhesion with the hydrophobic matrix.

Goettler [30] studieci the extrusion of PVC hose products reinforced with short

cellulose fibers at the temperature range of 138 to 160°C. His study revealed that with

intimate interfacial bonding between fiber and matrix, a composite's tensile strength

could be doubled over thar of a poorly bonded composite. In addition, desirable

dispersion of cellulose fibers in plasticized PVC could be accomplished if sufficient

shearing stress was generated during compounding. High shearing stress helped not only

break up fiber agglomerate. but also minimize structural defects caused by fiber

agglomerates.

2.2 Thermoplastic Foarns and their Processing

Foamed plastics are materials having voids or cavities. or in other words they

have a cellular structure. Thermoplastic foams are produced by deliberately adding at

lerist one gas-generating substance such as a chernical blowing agent, a soluble gas. or

volatile liquid under pressure to the polymer rnelt, then altering the environment to cause

the gas-generating substance to yield discrete bubbles [8].

2.2.1 Con ventional Continuous Foam Processing

A good review of conventional polymeric foams and their processes is given in

[3 1 1. Polymeric foams can be produced in such processes as extrusion, injection molding

and compression molding. The selectior! and design of an appropriate process are

governed mainly by the material characteristics, and thus it is impossible to offer a single

process for al1 polymeric materials. However, fundamentally, al1 foaming processes are

sirnilrir.

In genera1, a foaming process consists of three main steps: formation of ü

polymer/blowing agent solution/mixture, ce11 nucleation, and cell growth. One of the

conventional foam processes is the "Celuka Process" [32] (a schernatic is s h o w in

Figure 2.1 [33]). In this process, the polymer is meited in an extruder. and a blowing

agent is then injected which is mixed with the polymer and is dissolved in it. In some

foam processing. the injected blowing agent does not completely dissolve in the polymer

melt. The rnelt containing the blowing agent is forwarded to the head of the extruder.

which is partiaIly plugged with a torpedo or mandrel. The melt then flows into a cooled

shaper connected to the head of the extruder. The outer layer of the extrudate therefore

cools and forms a skin Iayer. The foaming proceeds inward from the outer layer to the

ccnter of the extrudate. In this process, virtually no cell nucleation occurs in the meIt

before it flows into the shaper. Ce11 nucleation is initiated in the shaper, where ceil

growth also proceeds. In fact, in ail the conventional processes, ce11 nucleation and ceIl

growth takes place simultaneously.

The Celuka process is a "controlIed foam extrusion ( C E ) " process in which the

foam product consists of a solid skin and a cellular core. In a CFE process. the extrudate

is mechanical1y constrained by a cooled shaper and thus is forrned to the shaper profile.

Moreover, since the outer layer of the foam product is cooled and a solid skin is formed,

the diffusion of the blowing agent from the outer surface to the atmosphere (or blowing

agent escape) is prevented. However, the blowing agent can be still lost through the hot

inner surface where foam growth occurs. This process can be used to produce foam parts

with ri wall thickness of over 6 mm [3 I l .

In contrat to a CFE process, a conventional "free foaming extrusion (FFE)"

technique allows the foaming article to expand without constraint. A schematic of an FFE

process is shown in Figure 2.2. The temperature of the melt rit the die exit plays an

important role in determining the fina1 properties and the amount of expansion [34]. This

process is suitable for products with a wall thickness in the range of 2-6 mm [3 1 1 .

In a conventional foam process, foam products of various densities can be

produced. However, the state-of-the-an foams have a full y-grown ceIl size greater than

100 Pm. a ce11 population density lower than 106 cells/cm3. and a non-uniform ceil size

distribution [3 1,351. Hence. the mechanical properties of the conventional foams are

poor.

In conventional foam processing, the most commonly used blowing agents are

FCs, CFCs. n-pentane. and n-butane [36]. These agents can dissolve into the polymer

rcsin in large quantities due to their high solubility [37]. For instance. the solubility of

FC-1 14. a commonly used blowing agent. in polystyrene is above 20% by polymer

weight at a pressure of 1000 psi and a temperature of 200°C 1371. This suggests that a

foam product with a high void fraction can be produced at a relatively low pressure in the

system. In addition, the loss of blowing agent from the extrudate during expansion is

small since the diffusivities of such agents are low due to their large rnolecular size 1381.

This allows the extrudate to expand significantly, Therefore, the final product hm a low

foam densi ty. Despi te the favorable properties of the conventional blowing agents. there

are major concerns in utilizing them. For instance, CFCs are known to deplete the ozone

layer and their use has been stopped by the Montreal Protocol [39]. The other long chain

blowing asents such as n-pentane and n-butane are also hazardous because of their high

flarnmability. Therefore. aitemative biowing agents should be developed in polymer

foam processing.

2.2.2 Batch Microcellular Processing

,Microcellular polymers were first produced in a batch process 1401. A schematic

of the batch process is shown in Figure 2.3. In this process, a polymer sample is placed in

a high- pressure chamber connected to a gas reservoir. The gris can be either nitrogen

(Nz) or carbon dioxide (CO?). The poIymer sample absorbs the gas and. after a sufficient

time, reaches a saturation state. The amount of gas dissolved in the polymer plays an

important role in the final foam quality. When the sampIe is fully saturated with the p.

the pressure is rapidly decreased to cause a sudden drop in the solubility of the gas in the

pol ymer. This initiates a thermodynarnic instability, which drives nucleation of billions of

microcells. These rnicrocells cannot expand significantly, as a large amount of gas is lost

during cell growth. The foaming of the sample in the batch process is initiated by heating

through the surface of the sample. Since the temperature of the foam skin is always the

highest during the foaming process, the gas can easily escape through the hot skin to the

atmosphere. Therefore, the sample expands to a lesser degree, which depends on the

amount of gas left in the sample and used for ceil growth. However, the foam products

have a small ce11 size on the order of 10 pm due to the presence of a large number of

nuclei generated in the polymer matrix.

In a batch process, ceil nucleation is governed mainly by the saturation pressure

(or pressure drop), and ce11 growth is governed by the heating temperature and time.

Hence. the nurnber of nucleated cells and the amount of expansion can be independently

controlled. It musc be noted that in a batch process, the foaming temperature is in general

chosen to be the lowest to make the ceIl growth step easily controllable f22]. When the

nuclei are generated. their growth is retarded due to the high stiffness of the polymer

matrix at the Iow temperature. However, this prevents the degradation of the foam

product due to coalescence of the cells. Growth of cells can be controlled by modulating

the temperature and the time of exposure to heat.

A major disadvantage of the batch process is that a very long time is required for

saturation of gas in the polymer. This is due to a low rate of gas diffusion into the

polymer at room temperature. For instance, the diffusivity (D) of CO2 in polystyrene at

room temperature is 6x 10-%rn2/s [41]. The diffusion time can be estimated using the

fol lowing equation:

where lr is the thickness of the sample. For a sample of 1 mm thickness, the diffusion time

will be about i2 hours. As a consequence, the total processing time is too long, and thus

the batch process j not cost-effective.

iMatuana [2 1 -23,421 has reponed successful microceIlular foaming of ce1 lulosic-

fibers filled thermoplastics. The same process as described above was used, however, the

wood-fibers were treated to increase their adhesion with PVC. A ten-fold expansion was

re ported.

2.2.3 Sem i-Continuo 14s Microcellular Processing

Kumar and Schirrner f43] developed a semi-continuos process for production of

solid state PET foams. In this process, layers of polymer sheet are interleaved with gas

permeable materials (such as gauze, porous paper sheet, etc.). The roll of the interleaved

polymer and gas permeable material is saturated with an inert gas in a high-pressure

vessel. The polymer sheet is taken out from the vessel, unrolled. and separated from the

gas penneable material. The saturated sheet is drawn through a heating station with a

suftkient residence time to achieve a desirable foam density. In this process, the gas

tends to escape from the sheet after its removal from the vessel. Therefore. the foaming

must be processed promptly to avoid excess gas loss. In other words, only a finite length

of the sheet c m be foamed at one time, and hence. the process is called "semi-

continuoust'.

2.2.4 Cori finnous Microcellular Processing

In order to overcome the shortcomings of the batch process, a cost-effective,

continuous microcellular process has been developed in extrusion [ l 1-15]. A schematic

of the microcellular extrusion process is shown in Figure 2.4. In this process, a much

shorter time is needed for saturation of the polymer with gas. When the polymer is melted

in the extrusion barrel, a metered amount of gas is delivered to the polymer melt [13].

Three strategies are apptied to assist the complete dissolution of the injected gas

in the polymer melt. The first strategy is to inject and dissolve the gas at an elevüted

temperature. This can be explained by the following equation (4 11:

4 D = D,, exp(--), RT

where D = diffusivity, cm%.

Do = diffusivity coefficient constant. crn'ls,

.aD = activation energy for diffusion. J . and

R = universal gas constant, J/K.

-4s s h o w in the equation, the diffusion coefficient increases as the temperature increases.

Thus, the diffusion rate can be increased by processing the polymer/gas mixture at a

higher temperature.

The second strategy is to use convective diffusion [IO]. The idea of convective

diffusion is to bring the poIymer melt with low gas concentration into contact with the

bubbIes with high gas concentration. To achieve this, the bubbles are stretched by the

shear field generated by the screw motion: the stretched bubbles have a larger interfacial

area and a lower striation thickness in the polymer melt. The diffusion distance is greatly

rcduced. and thus, the gas can be diffused into the polymer more easily. Static mixers

can be added downstream of the extruder to enhance the shear mixing and ensure the

homogeneity of the polymedgas solution.

The third strategy is to increase the system pressure to increase the solubility of

gas in the polymer. The relationship between pressure and gas solubility can be

expressed by the following equation [41]:

where c, = solubility of gas in the polymer, cm3/& or g(gas)/g(polymer).

H = Henry's law constant. cm3 [STPIIg-Pa. and

p, = saturation pressure, Pa.

As shown in the equation above, the solubility of gas increases with pressure.

The diffusivity of CO2 in polystyrene at a temperature of 200°C is estimated to be

10-5 cm'ls [ 1 1.121. Thus, the time for gas diffusion into a 1 mm thick melt (or a diffusion

distance of 0.5 mm) is about 4 min (Equation 2.1 ). This represents a significant reduction

in the diffusion time which results in a much shorter processing time.

There are three essential steps in a continuous rnicrocellular extrusion process: i )

uniform formation of polymerlgas solution, ii) ce11 nucleation. and i i i ) ce11 growth. The

formation of a uniform solution of polymer and gas is essential in microcellular

processing, since pockets of undissolved gas can generate undesirabl y large voids. A

uniform solution can be achieved if the amount of gas injected is below the solubility

limit [13]. and dissolution of gas into the polymer is enhanced [ I 1,121. It must be noted

that evcn if an appropriate amount of gas is injected into the polymer melt. this does not

necessarily guarantee that a11 the gas will be completely dissolved into the polymer.

Moreover. complete dissolution of gas into the polymer melt must be ensured. before the

melt reaches the nucleation die. This can be achieved by reducing the gas diffusion

distance in the polymer (i.e., the striation thickness of the polymer and gas). The

diffusion distance is reduced due to the shear field generated by the motion of the screw.

In a shear field. the bubbles are stretched, giving a larger interfacial area and a lower

striation thickness. Cornplete diffusion can be ensured by using a diffusion-enhancing

device in which static mixers are placed to enhance shear-mixing [ l l ] . Microcell

nucleation occurs in a rapid-pressure-drop nucleation nozzie [12,14]. The rapid drop in

pressure causes a rapid drop in the solubility of gas in the polymer melt. This induces a

thennodynamic instability, as in the case of batch processing, which drives nucleation of

billions of microcells. The nucleated cells continue to grow at the die exit. The final foam

structure is determined by the conditions under which the three steps of the process are

perf'ormed.

In microcelIular processing, nucleation is induced by a thennodynamic instability,

which is achieved by using a rapid pressure drop device. Colton and Suh [Ml developed

a model of bubbie nucleation rate:

where I'? = rate of nucleation. #/cm3s,

f, = frequency factor of gas molecules joining the nucleus, I/s.

Co = concentration of gas molecules in solution. #/m3,

AG = Gibbs free energy of bubble nucleation. J.

k = Boltzman constant. J/K. and

T = temperature. K.

and AG can be expressed as:

AG = 1 4 v , h 3AP'

w here = interfaciai surface tension. rni/rn2, and

A P = gas saturation pressure. Pa.

Therefore, a higher nucleation rate can be achieved by increasing the saturation pressure

or blowing agent concentration. In addition, another important factor that affects the

nucleation rate is the pressure drop rate. The investigation of Park et. al [14] has shown

that the nucleation rate varies directly in proportion to the pressure drop rate. They

explained this phenomena by nucleation/cell growth competition for the dissolved g a . If

the pressure drop rate is high, there is relatively less time for the gas to diffuse into the

already nucleated cells, and the gas tends to nucleate an additional cefl. ffence, a greater

number of nuclei can be obtained by inducing a higher pressure drop rate.

Another important step in the formation cf microcellular foams is ceIl growth.

The nucleated cells grow as long as gas is availabte in the polymer matrix to diffuse into

the celIs, and the ce11 walls are pliant enough to expand- The rate of ceIl growth is

controlled by the diffusion rate and the stiffness of the polymer melt. Cell growth is

governed by the time allowed for the cells to grow, the temperature of the system. the

hydrostatic pressure or stress applied to the poiymer matrix. the viscoelastic properties of

the polymerlgas solutior?, and the degree of supersaturation 145.461. Cell growth must be

cürefully controlled because it affects not only the final foam density, but also the ce11

density, by preventing cell coalescence. Nitrogen or carbon dioxide, which are the

chosen blowing agents for the continuous process, have a small molecular size. They can

easily escape from the polymer foam to the environment. This will lead to colhpsed cells

and the foam density will be increased Figure 2.5. If the melt strength of the polymer is

too Iow, the ceIl walls will rupture due to the surface tension induced by cell growth [46].

A schematic of this phenomenon is shown in Figure 2.6. Consequently. the adjacent cells

will join together to f om one single cell. and the ceIl density will \je greatly reduced.

A great deal of research has been conducted on continuous microcellular foaming

[ 1 1 - 151. Most of it has focused on the nucleation phenornenon. Behravesh et al. 146-491

developed a process that can prevent cell coalescence and gas escape in the cell growth

stage. Cell coalescence was suppressed by cooling the poIymer/gas solution

homogeneously to increase the melt strength. The gas loss to atmosphere was controlled

by cooling the surface of the extrudate to forrn a solid skin layer, thereby preventing the

gas from escaping out of the polymer. The PS filarnentary foarn produced by this process

has a 40 fold volume expansion and a cell density of 5 x 10' cells/cm3.

2.3 Foaming of PlastidWood-Fiber Composites

There is not much published literature about continuous processing of

plristic/wood-fiber composite foarns. In fact, even the extrusion process for composites is

mostIy discussed in the context of injection molding. Only a few articles could be found

on production of plastic/wood-fiber composite foams, and invariably the processes are

based on injection molding. where foaming takes place in the mold 150.5 11. Pabedinskas

et al. [ 52 ] describe a continuous Celuka process for producing plastic/wood-fiber

composite foams. However, the ce11 structure achieved is extremely coarse.

A study, of the fundamental issues involved in the continuous extrusion of the

thermoplastic/wood-fi ber composite foams, is required in order to determine the

parameters for achieving the desired density reduction and a fine-celled structure.

I l Polyrner 1

j Blo~ving Agen t l

Diffusion

!.la to r - - - . d L . l A - - - L -

Sucleat ion Yozzle

Figure 2.4: A schematic of a Microcellular Continuous Process

coalesœd ceiis

gas escape

Figure 2.5 : ~Modeling of gas escape and cell coalescence in extruded foams at a h i t h temperature of polymer melt [39]

ccll ccll

Figure 2.6: Cell coalescence phenornenon caused by rupture of the thin membrane separating two cells

Chapter 3

Detailed Design & Construction of the Experimental Equipment

This chapter describes the design and construction of a new system dedicated to

the processing of polymer/wood-fiber composite foams. An extruder with variable drive,

a gear pump with variable drive, electronic transducers and controllers were purchased

from their respective suppliers. Al1 the interconnecting and other mechanical parts were

fabricatedhachined in the departmental machine shop at the University of Toronto. The

electrical control panel and rdated wiring were configured in the Microcelfular Plastic

.Manufacturing Laboratory of the department.

3.1 Design of PlastidWood-Fiber Extrusion System

3.1.1 OverviewoftheSystem

As the objective of this study is to develop fine-celled structure in plastic/wood-

fiber composite foams, it was decided to adopt an approach similar to that used for

producing microcellular foams. The first microcellular extrusion system was designed by

Park [53] for filament processing of styrenic polymers. Behravesh [47] used a sirnilar

system to produce microcellular foams of H P S and HI'PE. It consisted of a single screw

extruder. gas injection equipment, a diffusion enhancing device, a heat exchanger

(cooling system), and a filament die. It was decided to utilize a similar configuration

during this study for controlling the pressure and temperature of the plastic/wood-fiber

composite.

The objectives of the design are: 1 ) to plasticate the polymer/wood-fiber

composite in the extruder: 2) to decompose completely the CBA in the last stage of the

extruder and the diffusion device; 3) to ensure the hornogeneity of the composite/blowing

agent solution by using the diffusion enhancing device; 4) to avoid ceIl coalescence by

cooIing the meIt in the heat exchanger: and 5) to induce ce11 nucleation in the composite

melt in the die; and 6) to promote volume expansion outside the die. The detailed design

of each component is presented in the following sections.

3.1.2 Extruder

The extruder used in the single screw extrusion system consists of a ?4" laboratory

extruder (Brabender: 05-25-OOO), a 5 hp motor (Allen Bradley: 1329 Invener Duty

Motor) with a variable speed drive unit (Allen Bradley 1336 Impact). The screw is a

sinzle stase mixing screw (Brabender: 05-00-144) with a 30:l UD ratio. The purpose of

the mixing stage is to enhance the mixing of the polymer, wood-fiber and blowing agent.

A schematic of the mixing section is s h o w in Figure 3.1.

3.1.3 Diffzdsion Enliancing Device

A diffusion enhancing device is used to ensure that the constituents of the

composite and the decomposed gasses from the chemicai blowing agent are mixed

homogeneously. It was originally designed by Park [53] and modified by Behravesh

1471. It consists of an in-house design, rnild steel body, a static mixer (Omega:

FMX844 1 S), and a band heater (Omega: MB 1G 1 J 1 A 1 ). The rationale behind this design

is to use the static mixer to promote shear rnixing, and to maintain a high melt

temperature to promote a high diffusion rate of the blowing agent into the polymer melt.

In order to determine the required number of static mixer elements, a calculation

was done based on an equation provided by the manufacturer's technical brochure [54]:

where Re = Reynolds number,

Q = flow Rate, gal/min,

S = specific gravity,

p = viscosity, cP, and

D = inside pipe diameter, in.

A typical flow rate (Q) of the system is in the range o f 5-12 cm3/min ( 1 . 3 - 3 . 2 ~ 1 0 - ~

gal/min). The specific gravity (S) is approximately 1 for polystyrene, and the inside pipe

diameter (D) is 0.0 127m (0.5 in).

The viscosity is influenced by the shear rate and temperature. The apparent shear

rare ( ) in a circular channel can be calculated by the foilowing equation [54]:

With a channei diameter (D) of 0.07 m (0.28 in) and a flow rate (Q) of 10 cm3/min

( 2 . 6 4 ~ 10" @/min), the apparent shear rate is approximately equal to 4.95 l/s. From the

manufacturer's data sheet (Nova), the apparent viscosity of PS-IO1 is 3438.93 Pa's

(3438930 cP). With these values, the Reynolds number can be calculated using Equation

(3.1):

The pressure drop across the static mixer c m be determined using the following equation

[54] :

A P = Q x p x L , (3.4)

where L = larninar Factor (from the manufacturer), and

AP = pressure Drop, psi.

With a flow rate (Q) of 0.303 m3/rnin (2.64~10-3 gallmin), a viscosity (p ) of 3438930

cP. and a larninar factor (L) of 0.0588, the value of head loss is:

AP = ( 2 . 6 4 ~ 10-"x (3438930) x (0.0588) = 533.83 psi (3.68 MPa) (3.5)

This value is reasonable compared with a typical systern operating pressure over 27.58

iMPa (4000 psi). Based on the calculated results, it can be concIuded that a static mixer

with six elernents can fulfiil the mixing requirements. Additional mixing of the polyrner

meIt is provided by the rnixing stage of the extruder's screw.

3.1.3 Heat Exchanger

Ce11 coalescence is suppressed by cooling the polymer composite melt, which

increases the melt strength. It is important to cool the polymer melt homogeneously

because non-uniform temperature distribution could induce non-uniform ce11 growth,

which would result in a non-uniform ceIl structure. The heat exchanger was designed by

Behravesh [47]. It consists of an in-house design of a mild steel body and a static mixer

(Labcore: H-01669-12). This static mixer is different from the one used in the diffusion-

enhancing device. The mixer used in the diffusion-enhancing device does not promote

material transport in the radial direction. The static mixer used in the heat exchanger

directs the polymer fiow in the radiai direction such that the core material is constantly

being exchanged with the boundary materiai. According to the calculation by Behravesh

[47], four mixing elements can be used without causing too high a pressure drop.

Precise control of temperature is important for the uniform foaming of the

polymer. Use of band heaters with temperature controllers does not provide adequate

controt because of Iack of a cooiing source. To achieve better control, high temperature

oil (Labcore: iMode1 H-01294-40) was circulated in the outer channel of the heat

exchanger. The temperature of the oïl was controiled with an oil bath controller (Grant:

Mode1 W6-KD) which is equipped with a pump to circulate the oil to the heat exchanger.

A drawinz of the heat exchanger is shown in Figure 3.2.

3.1.5 Force- Feed Hopper

Becriuse of its Iow bulk density and its hydrophilic nature, wood-fiber has a high

propensity for adhesion. This tendency of adhesion makes it very difficult to feed the

plastic/wood-fiber mixture through the hopper opening into the bmel. Secondly, due to

the hygroscopic nature of wood-fibers in the composite mixture. it tends to absorb

moisture from the atmosphere during its long residence in the hopper.

To resolve these problems, a force-feed hopper was designed and constructed in-

house in the laboratory. Fig 3.3 shows a cross section of this hopper. It is provided with a

3/1" auger bit for forcing the dry blended mixture into the extruder barrel. The auger bit or

screw is connected to one half of a flexible and detachable coupling also s h o w in figure

3.3. The other half of the coupling is connected to a 12 V DC motor. with wonn and

wheel gears, mounted on the hopper lid. Two stiff vanes collect the mixed rnaterials and

guide it towards the rotating auger bit, which forces it down into the extruder barrel.

After filling the hopper with the dry blended mixture, the lid is closed so that the two

halves of the coupling engage. The material is forced into the barrel due to the motion of

the screw. The hopper is continuously purged with dry nitropen gas to prevent the

ambient moisture from entering it, thereby maintaining dry conditions within the hopper.

3.2 Design of the Vented Extrusion System

3.2.1 Overview of the System

The vented extrusion system consists of an extruder with a vent opening after the

second of the three heating zones of the barrel. The vent is normally used for removing

the volatile rnaterials from the polymers and additives. It was proposed to utilize this vent

for removing the water vapors present in the wood-fibers of the composite mixture.

As already described in chapter 1, the dissolution of the blowing gasses is a

nscessary condition for obtaining a large nurnber of nucleated bubble sites. This in tum

necessitates a high pressure in the dissolution section to increase the solubility of the gas

in the polymer matrix. During initial trials, the pressure at the extruder outlet could not

be increased substantially, as an attempt to do so resulted in excessive material Ioss

tlirough the vent. Therefore, it was decided to use a gear pump at the extruder outlet to

increase the pressure to the required level. The gear purnp, being a positive displacement

device, was not only expected to increase the pressure but also stabilize the flow rate, and

hence, improve the over al1 process control.

3.2.2 The Gear Pump

As suggested in section 3.2.1, a gear pump (Zenith: PEP-1 l ) , with a '/t hp motor

drive (Pacific Scientific: Model SR), a speed control unit (2enith: ZeDrive) and a

temperature controller (Eurothem Controls: Model 94). was used in the extrusion system

to control the pressure and polymer flow rate. The gear pun~p consists of two closely

intemeshing gears that rotate in a counterrotating motion to convey the polymer melt

frorn one end to the other. A schematic of the gear purnp is shown in Figure 3.4.

3.2.3 Modification of Dijjùsion Section

The results that were expected from the gear pump did not fully materialize,

although there was some decrease in the material loss through the vent. On reviewing the

design, it was noted that the molten plastic entering the gear pump contained decomposed

gasses from the CBA. Even if al1 the gasses were dissolved into the polymer before

reaching the gear pump, a substantial amount would rigain evolve into the gaseous phase

due to the reduced pressures encountered at the gear pump suction. This would render the

gear pump ineffectual.

To address the problem mentioned above, the process was redesigned. A new

decomposition/diffusion section, sirnilar to the heat exchanger described above [see

Figure 3-21, was constructed and added after the gear pump. For the new process, the

temperature from the vent up to and including the gear pump was kept under 160°C to

prevent CBA decomposition. After the gear pump, the temperature was increased to

205°C in the decomposition/diffusion section. This section was also provided with a

static mixer in order to improve the temperature homogeneity and effective

decomposition/diffusion. The extrudate was then cooled down to 150°C in the cooling

section and finally extruded through the nozzle die.

Figure 3.4: Schematic of the =car pump (courtcsy of Zcnith).

Chapter 4

Foaming of PS/Wood-Fiber Composites in Extrusion Using

Moisture as a Blowing Agent

4.1 Introduction

Foamed polymers can be produced by utilizing either a physical or chemical

blowing agent. The former can be either a gas (e-g., COz and Nz) or a liquid (e.g., CFCs

and other aIiphatic hydrocarbons) whereas the latter is usudly a solid [IO]. Nearly al1

these blowing agents have some problems associated with their usage. The use of CFCs

is banned in many countries because of their detrimental effect on the ozone layer,

whereas common aliphatic hydrocarbons are highly flammable. CBAs are known to be

expensivc, and yield low gas concentration during their decomposition.

As a result of persistent public pressure, the government continues to legislate

regulations aimed at enhancing environment protection and worker safety. The

development of an environmentally friendly, non-toxic, safe and low-cost blowing agent

may help the industry in coping with these regulations and simultaneously Save on the

cost of CBA. Since water, which is freely available, non-toxic and environment friendly,

c m be completely transfomed into a güseous state at tempentures above 100°C. i t is

belicved that it can potentially be uscd as a blowing agent in the production of foams in

an extrusion process. In this context, it is the purpose of this work to investigate the

feasibility of using the moisture in the wood-fiber 'as an alternative blowing agent for

producing foamed plastic/wood-fiber composites.

Water can be injected into the polymer melt in the extrusion foam processing like

other physical blowing agents [40]. However, since the sohbility of water in a

hydrophobic plastic is very low [ 3 6 ] , it may not be possible to completely dissolve al1 the

injected water in the plastic matrix during processing. Therefore, it is suggested that the

foam structure be developed by finely dispersing the water bubbles in the plastic matnx

followed by decreasing the pressure at a high temperature. However, the dispersion of

writer bubbles in the thermoplastic matrix would not be good because of the high surface

tension of water [69]. As a result, the ce11 structure would be neither fine nor

homogeneous. But since the dispersion of wood-fiber in the plastic can be independently

promoted by utilizing an appropriate coupling agent [2,7,59-6 1 1, md/or by intensive

mixing [60.6 1 1, the hy,oroscopic wood-fibers used as a filler can be utilized as a carrier to

distribute water in the polymer matrix. Although it is known that the coupling agents

have no significant effect on mixing of styrenic materials and wood-fibers [6 1 ] used in

this study, the well-known appropriate coupling agents for polyethylene, polypropylene.

and PVC can be effectively used to distribute widely the wood-fibers in the plastic matrix

[3.7.57.59,60].

This study investigates the feasibility of using moisture in the wood-fiber as an

alternative and cost-effective biowing agent in the production of plastic/wood-fiber

composite foams. Since water is different from other blowing agents in that it does not

dissolve to a significant degree in the polymer matrix, the foaming mechanism with water

would be different. The gradually released moisture from the wood-fiber during

processing can result in good dispersion of the released gas in the plastic matrix.

Therefore. a considerably better ce11 structure can be obtained by using the bound water

and/or the water of constitution released from the wood-fiber, rather than injecting the

water directly in the extrusion barre1 into the plastic melt. Another major difference of

writer compared to other blowing agents is that it condenses at about 100°C during

cooling of foam, thereby, creating low pressure in the cells. The possible contraction of

volume after foaming due to the vacuum created by the condensation of blowing agent is

discussed. A chemical-blowing agent is also used together with water, for comparison

purposes, to investigate the effect of each blowing agent on the foaming behavior of

wood-fi ber cornposi tes.

1.2 EXPERIMENTAL

42.1 Materials

The PS and HIPS used in this study were Nova 10 1 and Nova 3350. respectively,

supplied by Nova Chernicals. The wood-fibers were standard softwood (pine) grade

l2O?O, supplied by American Wood Fibers. 50 wt% of these fibers passed through the

sieve of 120 mesh s i x ( 1 25 microns) and were retained on 140 mesh (106 microns). The

moisture content and initial specific gravity of the fibers were 8% and 0.4, respectively.

A chemical blowing agent, Hydrocerol Compound (HC) suppl ied from Boehringer

Ingelheim was used with the intention of improving the ceII-population density of forimed

product. 1 wt% concentration of HC was chosen as a nucleating agent according to Ref.

[62] . Since HC was in a powder form, a small amount of commercial mineral oil (Life,

Phamaprix) was used, in order to uniformiy distribute the chemical blowing agent

particles into the plastic pellets [62] . On the other hand, no coupling agent was used for

processing the styrenic materials and wood-fibers as described above. All the materiais

were uscd as received,

4.2.2 Th ermogra vitneinTc Analysis of Wood Fibers

Therrnogravimetric analysis (TGA) was conducted to study thermal

devolatitization of moisture contained in wood-fibers using a TGA 2050 (TA

Instruments). A heating rate of 100°C/min was used. A typical arnount of material used

for the TGA experiments was 15 mg. The sample fibers were not oven-dried. The fibers

were initially heated up to 1 10°C and this temperature was maintained for 150 minutes.

The temperature was then increased to 200°C and maintained at this level for the

rernaining duration of the experiment (150 min). The oven was continuously purged with

nitrogen during the experiment to remove the released moisture and maintain dry

conditions. The purging also prevented the degradation of wood-fibers.

42.3 Differetltial Scanning Calorirnetry

Differcntial scanning calorimetry (DSC) was conducted to mesure the glass

transition temperatures (Tg) of PS and HIPS using a DSC 2910 (TA Instruments). A

typicd amount of material used for the DSC experiments was 5 mg. The plastic materials

were initially heated up to 770°C and this temperature was maintained for 20 minutes to

erase the thermal history. The temperature was then decreased to 25°C at a cooiing rate

of 1O0C/min.

4-24 Blending

Plastic petlets and a small amount of mineral oil (0.5 wt%) were mixed in a dry

blender. This was followed by the addition of 1 wt% of HC. The mixture was further

blended to ensure uniforrn distribution of the nucleating agent throughout the mixture.

FinalIy, 20 wt% of wood-fibers were added and further dry-blended to achieve

homogeneity. For both PS and WPS, three different types of mixtures were prepared and

the detailed description of the composition of each mixture is listed in Table 4.1.

Composition A was as described above, composition B was the same as A without HC,

and composition C had only polymer and wood-fibers.

Table 4.1 : Different formulations of composites.

- ---

Compositions (wt%)

A B C

4.2.5 Extrusio rr Setup

Figure 4. ï shows the experirnental setup used in this study. A force-fed hopper

for feeding plastic pellets and wood-fibers was designed and constructed at the University

of Toronto, as described in Chapter 2. The 3/4" single-screw extruder (Brabender 05-25-

PS or HPS

Wood-fi bers

Hydrocerol Compound

;Mineral oil

78.9 79.7 80.0

19.6 19.8 20.0

1 .O O .O 0.0

0.5 0.5 0.0

000) was driven by a 3.7 kW (5 HP) DC motor. At the extruder outlet is a diffusion-

enhancing device containinz a static mixer (Omega FMX8441S) to ensure the complete

dissolution of the gas generated from HC into the plastic matrix and water. This is

connected to the second heat exchanger, which uniforrnly cools the polymer/wood-fiber

composites using another type of static mixer (Omega P-04669-12) and an oïl bath

temperature controller. The elements of this static mixer are configured to transport the

core flow to the outer region and vice versa to impart a homogeneous distribution of

temperature across the radial cross-section. The extrudate finally passes through a nozzle

die having a diarneter of 1.27 mm (0.050") and length of 2.54 mm (0.100"). The nozzle

die is also cooled by circulation from an oil bath. A control panel equipped with

temperature controllers (Omega CN9000A) and pressure indicators (Omega DP25-S) was

used to monitor and control these parameters. Therrnocouples used were J type (Omega)

and the pressure transducers were Dynisco PT462E- 1OiM-6/ 18.

-1.2.6 Extriision Foaming of tJie Composites

The dry-blended mixtures were fed into the hopper. through which it was force-

fed into the extruder, The maximum temperature i

wris raised to 20j°C, to ensure the decomposition

cooling section, the temperature of the melt was uni:

n the barre1 and the diffusion section

of the hydrocerol compound. In the

fonnly reduced to 150°C. The nozzle

temperature was reduced from 150°C ( 1-2OC at a time). At different stable conditions,

samples were coIlected. The samples were dipped in liquid N2 and fractured. The

fractured samples were gold plated and SEM micrographs were triken for ceIl

chriracterization.

4.3 Results and Discussion

This section describes the experimental results of TGA analysis of wood-fiber and

foaming of plastic/wood-fiber composites with the moisture in wood-fiber. The

influences of HC on the volume expansion ratio and the ce11 population density are also

discussed.

4.3.1 Thermogravimetric Analysis of Wood Fibers

The amount of residual moisture in wood-fibers as a function of temperature was

investigated using a TGA to gain a fundamental understanding of the devolatilization

mechanisms of the free water. the bound water, and the water of constitution. Since the

wood-fibers are subjected to a high temperature for decomposing HC during the foaming

extrusion process, the devolatilization behavior of moisture from the wood-fibers was

investigated up to 205°C. A thermogram of devolatilization of wood-fibers obtained by

TGA analysis is illustrated in Fis. 2. It is clearly seen that at 1 10°C. the initially undried

wood-fibers start Iosing weight as soon as the experiment starts. Approximately 8% of

the moisture was removed from the fibers maintained at this temperature for 150 minutes.

However, it should be noted that most of the moisture loss (7.1 %) occurred during the

first 10 min of the experiment whereas an insignificant loss (0.9%) occurred during a

long period of heating time (more than 140 min). This loss of moisture is mainly due to

the drying of wood-fibers because the fibers were heated up abovc the boiling

temperature of water. The weight percent loss in moisture observed by the TGA analysis

azrees well with the 8% value of moisture content detemined by the oven-dry method

using the ratio of the weight of water to the weight of oven-dry wood. converted to a

percentage. This result implies that, during the extrusion process, some of the moisture

contained in wood-fibers may be released as soon as the compounded mixture enters the

hot extruder barrei. As a result, a substantial amount of water vapor escapes through the

hopper opening before being trapped in the molten mixture and pushed further

downstream into the extruder.

After heating up the wood-fibers at 1 10°C for 150 min. the temperature was

quickly raised to 205°C and maintained at that temperature for 150 minutes while

conti nuously purging with nitrogen. The thermogram of devolatilization of wood-fibers

shown in Fig. 2 clearly shows that even after wood-fibers are considered oven-dry, they

stili release water. An additional 3% moisture was released when the temperature was

raised from i 10°C to 205°C. This moisture is believed to be the water of constitution or a

small amount of remained absorbed-water that requires higher energy to dislocate. Since

the typical extrusion foam processing temperature and time are 200°C and less than 10

minutes, respectively, it is believed that the absorbed water and the water of constitution

can be dislodged during the foaming process and this water vapor could be utilized ris a

blowing agent.

43.2 Effect of Moistrrre on Volume Expansion

The feasibiiity of using water as a biowing agent in the production of foamed

plastic/wood-fiber composires was investigated. A chemical blowing agent was also used

together with water to investigate the effect of each blowing agent on the foaming

behavior of wood-fiber composites. Figure 3 shows the effects of material compositions

and extrusion foaming temperatures on the expansion ratio of foamed composites.

The experimentd results show that the inherent wood-fiber moisture can be

effectively used as a blowing agent for both PSIwood-fiber and HIPS/wood-fiber

composites. Depending on the type of the polymer, an expansion ratio of up to 20 was

successfull y achieved in the composites made with plastic and wood-fibers only

(composition C). However. it should be noted that without rnintral oil and HC. the

highest expansion ratio was obtained when PS was used as the polymeric material in the

composites. At the same extrusion foaming temperciture. the expansion ratio achieved in

PShood-fiber composite foams was significantly higher than that of HIPS/wood-fiber

composite foams. The maximum expansion ratios achieved in PSIwood-fiber and

HiPS/wood-fiber composite foams were 20 and 9 (for composition C), respectively, when

the extrusion foarning temperatures were in the range of 130°C to 140°C.

The difference of the volume expansion ratios obtained from the two composites

may be due to the stiffness of the plastic materials. When a foam structure is developed

by the moisture vapor at the die exit, volume expansion would occur quickly because the

moisture evaporates immediately at a lowered pressure. It should be noted that the cell

growth mechanism in the foam processing with water vapor is quite different from that

with other blowing agents. Most blowing agents, either physical o r chemicd. dissolve in

the polymer matrix before the onset of foaming at the die exit [63 ] . and therefore. the

nucleated cells grow as the blowing agent diffuses out of the polymer melt to the nuclei

[22.64]. But in the case of foaming with the moisture absorbed by the wood-fibers. the

blowing asent does not dissolve in the plastic matrix. and therefore, diffusion is not

involved in cell growth. As a result, it is expected that ceIl growth with moisture is faster

because the slow diffusion process is not involved although the rate of moisture release

from the wood-fibers is unknown yet. Since the volume of water vapor decreases as the

ten-iperature decreases as shown in Fig. 4, the foarned volume tends to decrease over time

as the extruded foam is cooled. Heat is lost through the foam skin to the environment

during cooling, and therefore, the temperature of the foam skin will be the lowest in the

foarn. As the foam skin is frozen at Tg, the total foam structure and volume will be

deterrnined by the conditions at this moment because of' the solidification of polymer. As

the foam is further cooled. al1 the polymeric materials will be solidified. However. the

water vapor inside the cells will start condensing to the liquid phase at 100°C, crenting a

vacuum in the cells, and the foam tends to colIapse. If the forrned plastic foam structure

is stiff enough, then the foam will not shnnk much even with the vacuum created in the

celis. But if the polymer is not stiff enough, then the volume expansion ratio will

decrease. It should be noted that HiPS is less stiff than PS due to the presence of mbber

particles. Consequently, the volume expansion ratio of HiPS/wood-fiber composite

foams blown by moisture would be lower than that of a PS/wood-fiber composite foams.

Since the onset of glass transition temperature, Tg was measured to be 105°C for both PS

and HIPS according to the DSC therrnograms s h o w in Fig. 5 , i t is not clear whether the

polymer matrix in the foamed composite was completely soiidified at the moment of

moisture condensation (100°C). However, since large expansion ratios were obtained

from the foams of these materials, at least the plastic material in the skin area of foam

must have been solidified first before condensation of water began. No significant effect

of mineral oil was observed on the volume expansion behaviors of PS/wood-fiber and

HIPS/wood-fiber composite foams.

When the chernical blowing agent was added, the volume expansion ratios of

HIPS/wood-fi ber composite foams were improved signi ficantly (Fig. 3b, Composition A)

whereas there was no significant change in the expansion ratios of PS/wood-fiber

composite foarns (Fig. 3a, Composition A). According to the manufacturer of the

chernical blowing agent, 1 wt% HC produces approximately 0.18 wt% CO2 and 0.18 wt%

water [65]. The specific volumes of CO2 at 100°C and room temperature are 703 cm31g

and 250 crn3/g respectively [70]. Therefore. the addition of î wt% HC will generate 1.27

c m h a d 0.99 c m h f COz volume per unit weight of composite at atmospheric pressure.

at 100°C and room temperature, respectively. Although the aniount of generated COz is

only sufficien( for about 100% volurnetric expansion, it helps significantly in reducing the

vacuum effect caused by condensation of water. As a result, the HPSlwood-fiber foams

are subjected to Iower compressive pressure differential, which improves the expansion

ratio substantially. in other words, although the HC was added as a ce11 nucleating agent.

i t also contributed to the volume expansion ratio of wood-fiber composite foams by

preventing volume contraction caused by water condensation. On the other hand. the

expansion ratios of the PSIwood-fiber composites were not affected much by adding 1

wt% HC. This implies that the PS/wood-tïber composite foams had not been contracted

by water condensation because of the high stiffness of PS.

The maximum achievable volume expansion ratio (a) of the composite foams

with the moisture in wood-fibers and the added HC can be theoretically calcuiated using

Eq. 4.1 [48] based on the assumption that al1 the gas is used for ce11 growth:

where v,, is the specific volume of blowing agent at the freezing temperature of polymer

and Vpolurnçr is the specific volume of polymer at room temperature; and nt,, and m,,iy,,r

are the mass flow rate of the gas and polymer respectively. It should be noted that the

specific volume of blowing agent is taken at the freezing temperature because the plastic

matrix structure is solidified at this point. It is assumed in this calculation of the

maximum volume expansion ratio that the air eventually diffuses into the vacuumed cells

after the foams are cooled to the room temperature and the foam shape is restored. For

wood-fiber composite foams, total volume expansion is caused by the moisture released

from the wood-fibers and the CO2 and water generated from the HC. The contribution of

moisture from the wood-fibers to the expansion is first calculated. If the amount of

moisture from the wood-fibers is assumed to be I I 93 of wood-fiber weight (Fig. 2), then

the total amount of moisture in the composite with 20 wt70 wood-fiber would be 2.2

wt%. Considering the specific volume of saturated water vapor to be 1873 crn3lg at 100°C

[67] , the maximum achievable expansion ratio is calculated to be 42.2, including its own

volume. If it is assumed that the wood-fibers are oven-dried or lose most of the water

vapor in the extrusion barre1 during plastication of polymer, the level of residual rnoisture

that can be used for foaming would be 3% of wood-fiber weight (Fig. 2). In this case. the

maximum achievable expansion ratio including its own volume is calcuiated to be 12.2.

The contribution of added HC can also be calculated. Considering the specific volumes

of CO2 (703 cm3/$) and saturated water vapor (1873 cm3/@ at 100°C. the maximum

achievable expansion ratio by 1 wt% HC (or equivalently 0.18 wt% CO-, and 0.18 wt%

water) is calculated to be 4.6. This value c m be added to the previously calculated

volume expansion ratio from the moisture in the wood-fibers for the foam processing

with composition A. Figure 6 shows the maximum achievable volume expansion ratio of

the composite foams with the moisture in wood-fibers and the added HC.

43.3 Cell Nucleation Behavior of PIustic/Wood-fiber Composites with Moisture, HC

and Mineral Oil

The ce11 nucleation behaviors of plasticlwood-fiber composites in foam

processing with moisture, HC. and minera1 oil were quite different from the expansion

behaviors. Figure 7 shows the cell-population densities of foamed composites as a

function of the materiais' compositions and extrusion foaming temperatures. Except for

the PSIwood-fiber composites foarned with moisture only (composition C), the measured

ce11 densities of PSlwood-fiber and HIPSlwood-fiber composite foams ranged from lo5

cells/cm3 to 106 ce11s/cm3. Since the processing temperature range was iow enough for

PS and HIPS, it is believed that ce11 coalescence did not occur [68]. Therefore, the final

ceil density of the foams would represent the number of cells nucleated in the early stage

of foaming.

The cell densities of HIPSIwood-fiber composite foams with moisture only

(composition C) were one order of magnitude higher than those of PSIwood-fiber

composite foams (Fig. 7). it seems that the mbber particles dispersed in the polymer

matrix enhanced ce11 nucleation although the cell nucleation mechanism in the foam

processing with water as a blowing agent is quite different from the foam processing with

other blowing agents [663.

When only minera1 oil was added in the formulation of the PSIwood-fiber

composites (composition B)? the ce11 density was dramritically increased (Fig. 7) although

there was no visible change in the volume expansion ratio of the PS/wood-fiber

composite foams (Fig. 3). This increased ceIl density in PSIwood-fiber composite foams

implies that the minera1 oil enhanced ceIl nucleation just as the rubber particles did in

HIPS. However, there was no significant effect of minera1 oil on the cell density of the

HIPS/wood-fiber composite foams.

The effect of adding HC on the ce11 density of the composites was also

investigated. To our surprise, there was no noticeable effect on the cell density for both

PSIwood-fiber and HiPS/wood-fiber composite foams although significant enhancement

of the ceIl density was expected with the addition of lwt70 HC [62] . This indicates that

the nucleation mechanism with water for plastic/wood-fiber composites is dorninrired by

the moisture in the wood-fibers and that the added chemical blowing agent rarely affects

cell nucleation. A further study is required to clearly identify the nucleation mechanism

in the foam processing with water as a btowing agent. Figures 8 and 9 show the typical

ce1 l morphologies of PS/wood-fi ber and ff IPS/wood-fiber composite foams. respectively,

for compositions A. B. and C.

4.4 Summary and Conclusions

This study examined the possibility of using the moisture in the wood-fiber as a

blowing agent for producing PS/wood-fiber and HIPS/wood-fiber composite foams. For

each plastic. three types of mixtures were prepared and processed: one mixture with HC

distributed using a minerd oil; another mixture with mineral oil but without the

nucleating agent to identify the effect of the mineral oil: and Iastly the plastic/woof-fiber

mixture without any additives.

Based upon the experirnental results, the following conclusions can be drawn:

Even from the oven-dried wood-fibers, a certain amount of moisture is generated in

the processing of plastic/wood-fiber composites. Thi