Density graded LMDPE foams produced under a temperature gradient : morphology and properties

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JIAOLIAN YAO

DENSITY GRADED LMDPE FOAMS PRODUCED

UNDER A TEMPERATURE GRADIENT:

MORPHOLOGY AND PROPERTIES

Mémoire présenté

à la Faculté des études supérieures de l’Université Laval dans le cadre du programme de maitrise en génie chimique

pour l’obtention du grade de Maître ès sciences (M.Sc.)

DÉPARTEMENT DE GÉNIE CHIMIQUE FACULTÉ DES SCIENCES ET DE GÉNIE

UNIVERSITÉ LAVAL QUÉBEC

2011

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Résumé

Dans ce travail, du polyéthylène linéaire de densité moyenne (LMDPE) et des microcapsules Expancel ont été utilisés pour produire des mousses polymères avec un gradient de densité. En contrôlant indépendamment les plaques supérieure et inférieure d'un moule de compression à des températures différentes et en contrôlant le temps de moulage, des mousses symétriques et asymétriques ont été produites. Aussi, l’effet du type et de la concentration des agents gonflants sur le profil de densité et la morphologie (taille des bulles et la densité des bulles) des mousses a été étudié. Finalement, on rapporte et on discute les propriétés mécaniques en flexion et en tension en relation avec le profil de densité et la morphologie.

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Abstract

In this work, linear medium density polyethylene (LMDPE) and Expancel microbeads were

used to produce density graded polymer foams using compression molding. By controlling

independently the top and bottom plate temperatures in the mold, different temperatures and molding times were used to produce symmetric and asymmetric foams. The effect of blowing agent type and content were also studied to control the density profile and foam morphology (cell size and cell density) across thickness. Finally, the mechanical behavior in flexion and tension is reported and discussed in relation with foam morphology and structure.

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Foreword

This master thesis consists of three articles, each chapter representing an article which was presented or will be presented at the international conference on blowing agents and foaming processes sponsored by RAPRA (iSmithers).

In the first part of this thesis, asymmetric LMDPE foams with graded density were produced by compression molding under a temperature gradient and using a single Expancel 950 DU 80 microbead as a blowing agent. The effect of processing parameters, such as molding temperature and Expancel content, on density profile, morphology and flexural property of the foams was studied. My contribution in this work was to perform all the experimental work, data collection and analysis (including calculations) and to write the first draft of the paper. The paper was published as:

D. Rodrigue, M.R. Barzegari, J. Yao, Compression molding of polyethylene foams under a temperature gradient: Morphology and properties, Blowing Agents and Foaming Processes Conference 2009, Hamburg, 19-20 May, paper 9, 8 pages (2009).

In the second part, compression molding with different temperature of both plates was used to produce asymmetric LMDPE foams with graded density using two types of blowing agents: Expancel 009 DU 80 and Expancel 950 DU 80 microbeads which have similar initial particle sizes, but different optimum blowing temperatures. We investigated the effect of molding temperature, as well as type and content of blowing agent on the density profiles, morphology and mechanical properties in tension and flexion. My contribution was to perform all the experimental work and to do data collection and analysis. The paper was published as:

J. Yao, M.R. Barzegari, D. Rodrigue, Polyethylene foams produced under a temperature gradient with Expancel and blends thereof, Blowing Agents and Foaming Processes Conference 2010, Cologne, 19-20 May, paper 17, 16 pages (2010).

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The third paper was submitted and will be presented at the next international conference on blowing agents and foaming processes (2011). In this work, we studied the effect of molding temperature, molding time, type and content of Expancel microbeads on the density profile, morphology and mechanical properties in flexion and tension of LMDPE foams produced under a temperature gradient using Expancel 009 DU 80 and Expancel 950 DU 120 which have different initial particles sizes and different optimum blowing temperatures. My contribution in this work was to perform all the experimental work, data collection and analysis (including calculations) and to write the first draft of the paper. The paper will be published as:

J. Yao, M.R. Barzegari and D. Rodrigue, Density Graded Linear Medium Polyethylene Foams Produced Under a Temperature Gradient With Expancel Microbeads, Proc. of Blowing Agent and Foaming Processes, Düsseldorf, 10-11 May, (2011).

I was also involved as main contributor in other articles studying the morphology and mechanical properties of symmetric and asymmetric foams with graded density:

J. Yao, S. Lepage, M.R. Barzegari, D. Rodrigue, Polymer foams produced under a temperature gradient, ANTEC 2009, SPE Proceedings of the 67th Annual Technical Conference & Exhibition, Chicago, June 22-24, pp. 408-412 (2009).

J. Yao, M.R. Barzegari, D. Rodrigue, Asymmetric and symmetric composite foams, ANTEC 2010, SPE Proceedings of the 68th Annual Technical Conference & Exhibition, Orlando, 16-20 May, pp. 2360-2363 (2010).

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Acknowledgements

I would like to thank my supervisor professor Denis Rodrigue. He was a great source of support and guidance throughout my research at Laval University. Working with him was a wonderful and rewarding experience. I will always be grateful for his help.

I would like to thank the Departments of Chemical Engineering and Wood Science and Forestry, as well as CERMA (CERSIM) for technical support and help during my master period.

My family deserves credits for their never ending love and affection. Without their support and encouragement, I would not have made it this far.

Finally, I am thankful to all my office collaborators and technician Mr. Yann Giroux for their help and suggestions. My work would never be completed without their kind collaboration.

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“The best preparation for good work tomorrow is to do good work today” Elbert Hubbard

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List of Tables

Table 3.1 Description of the experimental conditions tested ... 37

Table 3.2 Average cell size (D, microns) and cell density (Nf, 105 cells/cm3) for the samples produced. ... 42

Table 3.3 Apparent flexural modulus for the samples produced. ... 44

Table 4.1 Moulding conditions used for the samples produced. ... 53

Table 4.2 Average cell size (D, microns) and cell density (Nf, 105 cells/cm3) for symmetric samples ... 66

Table 4.3 Average cell size (D, microns) and cell density (Nf, 105 cells/cm3) for asymmetric samples. ... 67

Table 4.4 Apparent flexural modulus of the foams produced. ... 69

Table 4.5 Apparent tensile modulus of the produced samples. ... 70

Table 5.1 Moulding conditions for the samples produced using a single Expancel grade. ... 80

Table 5.2 Moulding conditions used for the samples produced using a blend of Expancel. ... 81

Table 5.3 Average cell diameter (D, microns) and cell density (Nf, 107cells/cm3) for samples with a single Expancel grade. ... 94

Table 5.4 Average cell diameter (D, microns) and cell density (Nf, 107cells/cm3) for samples with a blend of Expancel. ... 96

Table 5.5 Apparent flexural modulus for foams produced using a single Expancel grade. ... 98

Table 5.6 Apparent flexural modulus for foams produced using a blend of Expancel. ... 99

Table 5.7 Apparent tensile modulus for the foams produced using a single Expancel grade. ... 100

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List of Figures

Figure 1.1 Cross-section of bamboo [1]. ... 2

Figure 1.2 Examples of closed cell (right) and opened cell (left) foams [5]. ... 4

Figure 1.3 Schematic diagramme of the extrusion process [16]. ... 11

Figure 1.4 Schematic diagramme of the rotational molding process [18]. ... 12

Figure 1.5 Schematic diagramme of the injection molding process [19]. ... 12

Figure 1.6 Schematic of the compression molding process [20]. ... 14

Figure 2.1 Cross-section of two foams sonicated at different positions in an ultrasonic standing wave [35]… ... 19

Figure 2.2 Schematic representations FGSF with four types of micro-balloons [36]. ... 20

Figure 2.3 Load applied for the tensile testing. ... 21

Figure 2.4 Load applied for flexural testing. ... 22

Figure 2.5 Shear inside of test sample for the flexure test ... 23

Figure 2.6 Deformation of sandwich structures in three-point bending tests. ... 23

Figure 3.1 Density profiles of symmetric structural foams at different molding temperatures: 140o_{C (A-1), 170}o_{C (A-2) and 200}o_{C (A-3). ... 39}

Figure 3.2 Density profiles of asymmetric structural foams at different molding temperature gradients: 140-170o_{C (A-4), 170-200}o_{C (A-5) and 140-200}o_{C (A-6). ... 40}

Figure 3.3 Density profiles of asymmetric structural foams at different blowing agent concentrations: 1.5wt% (A-7), 3wt% (A-6) and 4.5wt% (A-8). ... 41

Figure 3.4 Density profiles of asymmetric structural foams at different waiting times: 5 min (A-9), 6.5 min (A-6) and 8 min (A-10). ... 41

Figure 3.5 Typical SEM micrographs of the structural foams produced. ... 42

Figure 4.1 Density profiles of symmetric structural foams with 3% blowing agent (Expancel 009 DU 80) at different moulding temperatures: 150oC (E009-0), 160oC (E009-1), 170o_{C (E009-2), 180}o_{C (E009-3), 190}o_{C (E009-4) and 200}o_{C (E009-5). ... 56}

Figure 4.2 Density profiles of symmetric structural foams with 3% blowing agent (Expancel 950 DU 80) at different moulding temperatures: 160o_{C (E950-1), 170}o_C (E950-2), 180o_{C (E950-3), 190}o_{C (E950-4) and 200}o_{C (E950-5). ... 57}

Figure 4.3 Density profiles of symmetric structural foams with 1.5% Expancel 950 DU 80 and 1.5% Expancel 009 DU 80 at different moulding temperatures: 150o_{C (EM-27),} 160oC (EM-2), 170oC (EM-3), 180oC (EM-4), 190oC (EM-5) and 200oC (EM-6). ... 58

Figure 4.4 Density profiles of asymmetric structural foams with 3% Expancel 009 DU 80 at different moulding temperature gradients: 160-190o_{C (E009-21), 170-190}o_{C (E009-25)} and 180-190o_{C (E009-23). ... 59}

Figure 4.5 Density profiles of asymmetric structural foams with 3% Expancel 950 DU 80 at different moulding temperature gradients: 160-190o_{C (E950-21), 170-190}o_{C (E950-22)} and 180-190o_{C (E950-23). ... 60}

Figure 4.6 Density profiles of asymmetric structural foams with mixtures of expandable beads (1.5% Expancel 950 DU 80 and 1.5% Expancel 009 DU 80) at different moulding

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temperatures gradients: 150-190oC (EM-29), 160-190oC (EM-24), 170-190oC (EM-25), 180-190o_{C (EM-26), 150-200}o_{C (EM-28) and 160-200}o_{C (EM-30). ... 61}

Figure 4.7 Density profiles of symmetric structural foams with different type and content of microbeads for a constant moulding temperature of 160o_{C: E009-1 (3%), E950-1}

(3%), EM-2 (1.5%/1.5%) and E009-6 (1.5%). ... 62

Figure 4.8 Density profiles of symmetric structural foams using different type and content of microbeads for a constant moulding temperature of 190o_{C: E009-4 (3%),}

E950-4 (3%), EM-5 (1.5%/1.5%), E009-6 (1.5%) and E950-6 (1.5%). ... 62

Figure 4.9 Typical SEM micrographs of the produced structural foams. ... 63

Figure 4.10 Typical cell size distribution of the structural foams produced. ... 65

Figure 5.1 Density profiles for structural foams with 1.5% blowing agent (Expancel 009 DU 80) at different moulding temperatures: 150o_{C (EE009-11), 160}o_{C (EE009-12),}

170o_{C (EE009-13), 180}o_{C (EE009-14) and 190}o_{C (EE009-15). ... 84}

Figure 5.2 Density profiles for structural foams with 1.5% blowing agent (Expancel 950 DU 120) at different moulding temperatures: 180o_{C (E120-56), 190}o_{C (E120-57), 200}o_C

(E120-58), 210o_{C (E120-59) and 220}o_{C (E120-60). ... 85}

Figure 5.3 Density profiles for symmetric structural foams with a blend of 1.5% Expancel 950 DU 120 and 1.5% Expancel 009 DU 80 at different moulding

temperatures: 150o_{C (ME-37), 160}o_{C (ME-31), 170}o_{C (ME-32), 180}o_{C (ME-33), 190}o_C

(ME-34), 200oC (ME-35), 210oC (ME-36) and 220oC (ME-8). ... 86

Figure 5.4 Density profiles for samples with 3% Expancel 009 DU 80 at different moulding temperature gradients: 150-190o_{C (EE009-22), 160-190}o_{C (EE009-21),}

170-190o_{C (EE009-23) and 180-190}o_{C (EE009-24). ... 87}

Figure 5.5 Density profiles for samples with 3% Expancel 950 DU 120 at different moulding temperature gradients: 160-230oC (E120-25), 170-230oC (E120-24),

180-230o_{C (E120-22), 190-230}o_{C (E120-21) and 200-230}o_{C (E120-23). ... 88}

Figure 5.6 Density profiles for samples produced with a mixture of expandable beads (1.5% Expancel 950 DU 120 and 1.5% Expancel 009 DU 80) at different moulding temperatures gradients: 150-200o_{C (ME-14), 160-200}o_{C (ME-16), 170-200}o_{C (ME-13),}

180-200o_{C (ME-18), 150-220}o_{C (ME-26), 160-220}o_{C (ME-25), 170-220}o_{C (ME-23) and}

180 -220o_{C (ME-22)………. ... 89}

Figure 5.7 Density profiles for samples with 3% Expancel 009 DU 80 at different moulding times: EE009-28 (160-200o_{C, 4.5 min), EE009-26 (160-200}o_{C, 5 min),}

EE009-25 (160-200o_C,_{5.5 min) and EE-009-27 (160-200}o_{C, 6 min). ... 90}

Figure 5.8 Density profiles for samples with 1.5% Expancel 950 DU 120 using different moulding times: E120-42 (180-230o_{C, 5 min), E120-41 (180-230}o_{C, 5.5 min), EE009-38}

(180-230o_{C, 6 min) and E120-40 (180-230}o_{C, 6.5 min). ... 91}

Figure 5.9 Density profiles for samples using different types and contents of microbeads under similar moulding temperatures of both sides of the mould: EE009-31

(160o_C;160o_{C;3%), EE009-12 (160}o_C;160o_{C;1.5%), ME-31(160}o_C;160o_{C;1.5%;1.5%),}

ME-31(200o_C;200o_{C;1.5%;1.5%), E120-54 (200}o_C;200o_{C;3%) and E150-58}

(200o_C;200o_{C;1.5%)… ... 92}

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Chapter 1.

Introduction

Generally, materials can be divided into different typologies according to their composition. For example, we are aware of metals and alloys, ceramics, plastics, semiconductors, composite materials, etc. Nowadays, progress in science and technology continuously brings about demands for new materials with improved properties, bringing in the forefront further development in technology and development in new materials. In engineering applications, there are two groups of conventional advanced materials used. Macroscopically homogeneous, fiber reinforced or particulate composites are being used since 1940s, and ―advanced composites‖ with their very high strength and stiffness are usually used in various applications such as automobiles, aircraft, space vehicles, offshore, etc. The second group is the layer or sandwich type composite materials. For example, a protective coating responsible for corrosion resistance or a thermal barrier coating is used in combustors, vane and blade of platforms and aircraft engines.

The layer or sandwich type composites materials have some disadvantages. For example: at high temperatures, the coating bond oxidizes due to the diffusion of oxygen and a thermally grown oxide layer forms, leading to delaminationof the layer. Also, local stress occurs as a result of thermal mismatch between the layers leading to a degradation of sandwich type composite materials. A new method to overcome the disadvantages of these materials has been proposed to be functionally graded materials. This method is described next.

1.1

Functionally Graded Materials

Functionally graded materials (FGM) are materials or structures in which the material properties vary with location. Generally, the variation is continuous instead than being discontinuous like in laminate and standard sandwich composites.

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Nature supplies many examples of functionally graded materials. In many of these cases, ―natural‖ functionally graded structures were evolved based on some mechanical functions: bones give a light and stiff frame to the body, wood supports the tree under environmental loadings, while leaves transport fluids. Bamboo is one of the best examples of a structurally smart plant as presented in Figure 1.1 [1].

Figure 1.1 Cross-section of bamboo [1].

The concept of functionally graded materials (FGM) was first applied in a new composite for the Japanese Space Program. The properties of the composite on both sides were different. One side was in contact with high temperatures and the composite was a heat resistant ceramic, while the other side was in contact with low temperatures and composed of a though metal with high thermal conductivity. In between, the material had a gradual composition variation from the ceramic to the metal. This composite could endure higher temperatures than most of the available ceramic or metal materials taken alone [2].

Since then, the concept of FGM was used for different types of materials including polymers and composites. Usually, FGM can be made from metal/ceramic, metal/alloy, non-metal/non-metal, non-metal/ceramic and ceramic/ceramic materials. However, only a few studies focused on polymeric graded materials. Since 1990, several researches have been done on the processing methods and the structural characterization of polymeric graded materials. These methods include centrifugal force field, fibers stacked gradually in a matrix, temperature gradient and so on. The composition continuously changes across thickness, because the volume fraction of the materials continuously changes, which is caused by using reinforcement materials with different shapes, sizes or properties, by modulating the cell structure or the material density, or by interchanging the roles of

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reinforcement and matrix phases in a continuous manner. Since mechanical and thermal properties are strongly function of composition, they are also changing continuously with position.

With these special properties, FGM can be used in aerospace, automotive, telecommunication and biomedical applications. According to the definition of functionally graded materials, polymer structural foams having cell size and/or cell density change as a function of position are a sub-group of FGM.

1.2

Polymeric Foams

Polymeric foam consists of at least two phases: the solid polymer phase and the gaseous phase. In other words, this is a material which has a large amount of gas bubbles in a polymer matrix. The gas bubbles in the foams are also referred to as voids or cells. A polymeric foam is also called a cellular polymer. Because of the gas bubbles present in the materials, polymer foams have less material per unit volume than the neat polymer. So the weight and the amount of material used can be reduced significantly by producing polymer foam materials.

Foam materials can be divided into three categories: convectional foams, fine-celled foams and microcellular foams. These categories are divided by the cell size and cell density of the foams. In general, convectional foams have cell sizes higher than 300 microns and a cell density less than 106 cells/cm3. Fine-celled foams have cell sizes between 10 and 300 microns and a cell density between 106 cells/cm3 and 109 cells/cm3. Microcellular foams have cell sizes less than 10 microns and a cell density higher than 109 cells/cm3 [3]. In addition, foamed materials are also dived by their cell structure: opened or closed cell foams [4]. Opened cell foams have a microstructure where the cells are connected to each other. On the other hand, closed cell foams have cells which are completely isolated from each other in the matrix. Closed cell foams are used in structural and load-bearing applications, while opened cell foams are used in cushioning and acoustic applications. Examples of these structures are presented in Figure 1.2.

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Figure 1.2 Examples of closed cell (right) and opened cell (left) foams [5].

With their special properties, such as light weight, excellent strength/weight ratio, superior insulating abilities, energy absorbing performance (including shock, vibration and sound), and comfort features, polymer foams can be found in several day to day applications. Their main field of application includes automobile, aviation, packaging, sports applications, toys and novelties, insulation, etc.

Polymer foaming is now a mature industry. It is reported that 23 billion pounds of foamed products were used each year in the world in the early 2000s and this number was expected to increase considerably in the future as more applications are discovered [6]. The total US production of foamed plastics, widespread among the many applications, was estimated to be more than 7.4 billion pounds in 2001, which accounts for about 10% of the total commodity resin consumption [7,8]. This production was projected to grow nearly 3% annually to 8.5 billion pounds in 2006, which represents more than $18 billion [9].

1.2.1

Fundamental principles of polymeric foam formation

First, a gas must be dissolved in the polymer solution or the melted polymer to form a saturated solution at high pressure. Then, pressure is released and the gas concentration exceeds the gas’s solubility. An unstable supersaturated state is created and the gas forms

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numerous tiny bubbles (called bubble nucleus). The bubble nucleus grows into a full bubble which can be stabilized and remain in the plastic to form a polymeric foam. The formation of bubbles can be divided into three steps: 1) bubble formation, also called cell nucleation, 2) bubble growth and 3) bubble stabilization [10]. Each step is described next.

1.2.1.1 Bubble formation

Among all three steps, cell nucleation is the most important step to control cell morphology because in this first step, the cell number density is decided and this will influence the cell sizes and cell density of the final product [11,12].

To produce the gas cell, blowing agents are added to the molten polymer or the polymer solution, to form a gas-liquid solution. With increasing gas concentration and system pressure, the solution will become supersaturated. When releasing the pressure, the gas tries to escape from the solution by forming bubble nuclei. This nucleation step can be divided into two types: homogeneous nucleation and heterogeneous nucleation. Spontaneous bubble nucleus in the medium is called homogeneous nucleation. If there is a second phase (interface) used to create the bubble nucleus, this is called heterogeneous nucleation. If the second phase is a finely solid particles (high specific surface area), the bubbles will form more easily at the liquid-solid interface and the particles are called nucleating agents.

There are also other theories to explain the formation of bubble nuclei. One of them is the presence of ―hot spots‖. These hot spots can become a nucleus point due to thermal and kinetic energy lowering the potential energy that needs to be overcome for a bubble to be created such that the oversaturated gas easily escapes to form bubble nuclei at the hot spot. Another theory is shear nucleation: bubble nucleation can be caused by flow or local shear stresses. Bubble nucleation can also occur under unsaturated conditions in a shear flow field if enough energy is generated (viscous dissipation) to overcome the nucleation energy barrier. So mechanical agitation of the liquid can also be used to form bubble nuclei, as in the frothing of latex systems.

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Gas sources can be of different types: 1) Unsaturated dissolved gases in the polymer solution or molten polymer are changed to supersaturated state by increasing temperature or decreasing pressure, 2) low boiling liquids can be used as blowing agents (forced into the gas phase by increasing temperature or decreasing pressure), and 3) chemical blowing agents (CBA) can decompose thermally to form gases.

The formation of bubbles in a liquid requires a change in the free energy (F) of the system as:

F A

  (1.1)

where  is the surface tension of the liquid and A is the total interfacial area.

Bubble nucleation is affected by several parameters such as temperature, pressure and possibly humidity (in some cases). For heterogeneous nucleation, surface characteristics of the particles, concentration and rate of gas generation (CBA) also have effects on this process.

1.2.1.2 Bubble growth

Once the cell generated, bubble growth is due to gas solubility decreasing with the pressure drop in the molten polymer or polymer solution. Gas molecules diffuse from the solution into the bubble nuclei. Viscosity, diffusion coefficient, gas concentration and the number of bubble nuclei will determine the growth kinetics of the bubbles. The concentration gradient of dissolved gas in the liquid solution promotes the diffusion of gases and promotes the growth of the bubbles.

At first, the bubble nucleus is mainly spherical and grows as a result of the interaction of the differential pressure (∆p) between the gas pressure in a spherical bubble and the pressure in the surrounding fluid. The equilibrium is related to:

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r p 2 

(1.2)

where ―r‖ is the radius of the bubble.

The differential pressure is larger for a small bubble, so that the gas pressure in a small bubble is higher than in a large bubble. So the gas will tend to diffuse from a small bubble into larger bubbles as:

1,2 1 2 1 1 2 p r r     _  _   (1.3)

∆p1,2 is the pressure difference between the bubbles of radius r1 and r2, respectively. Equations (1.1-3) indicate that the system has a tendency towards cell coalescence, so to minimize the interfacial area and thus reduce the required free energy change. Accordingly, the system will be more stable with fewer larger cells than with several smaller ones for a given foam volume. Equations (1.1-3) all predict that surface tension is the main parameter determining bubble size and bubble growth.

1.2.1.3 Bubble stabilization

The morphological properties such as cell size, cell size distribution, cell geometry, the type of cells, and cell density have important effects on the mechanical properties of foam. For this reason, it is necessary to control the structure during foam growth to get the desired mechanical properties. When the structure is produced, it must be stabilized.

Bubble stabilization is a physical process. Usually, it is done by cooling the molten polymer: viscosity increases and mobility gradually decreases until complete solidifying and shaping. The gas-liquid system coexistence is unstable during the process from bubble growth to bubble stabilization. The bubbles may continually expand until the polymer solidifies or the bubbles may combine (coalescence) or break (rupture). To minimize

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bubbles combination effect, the average interface distance of adjacent bubbles must be increased so the interfacial area of the gas is minimized. In order to prevent bubble break-up, improving the viscoelastic properties of the molten polymer is important so the bubble walls have sufficient strength to stabilize the bubble. In order to let the bubble structure reach maximum expansion, the appropriate solidification time (kinetics) must be chosen so the solidification rate is optimum.

1.2.2

Blowing agent

The gaseous phase in any polymeric foam material is obtained by the use of blowing agents in the foam manufacturing process. There are two main types of blowing agents: chemical blowing agents (CBA) and physical blowing agents (PBA) [13].

1.2.2.1 Chemical blowing agents

Chemical blowing agents (CBA) release gas by their decomposition. The reaction can be thermally activated or can be activated by other components (activator or catalyst). Chemical blowing agents can be individual compounds or mixtures of compounds. Most commercial CBA are solids are room temperature, so they are easy to handle.

CBA can be divided into two categories: inorganic blowing agents and organic blowing agents [14]. Solid inorganic CBAs release gas with a reversible thermal decomposition reaction. Using inorganic blowing agents, it is difficult to fabricate high-quality foams because inorganic CBA are difficult to disperse in polymer solutions or molten polymers. On the other hand, organic blowing agents have more advantages: 1) the reactions producing the gases are irreversible. 2) The reactions releasing gases are easy to control because the decomposition temperature range is narrow. 3) They are easy to disperse in polymer solutions or molten polymers. 4) The reactions mainly produce nitrogen where the diffusion rate of nitrogen in polymer solutions or molten polymers is small, so the gas does not escape easily from the foam and foaming efficiency is higher.

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The common chemical blowing agent for polyolefins is azodicarbonamide (ACA or ADCA). This compound has the ability to release a large volume of gas per unit mass (220 ml/g) and a relatively high decomposition temperature (205-215°C). Nevertheless, modified azodicarbonamide can have a relatively wide decomposition temperature range, but a lower induction temperature.

1.2.2.2 Physical blowing agents

Physical blowing agents (PBA) produce gases by physical processes such as evaporation, desorption at high temperatures or pressure drops. PBA do not experience chemical reaction, they are inert gases, low-boiling liquids (nitrogen, pentane, etc.) and easy-sublimation solid. The main disadvantage of PBA is equipment investment which is expensive because foam processing requires high-pressure and security (no leaks). Furthermore, some of the low-boiling liquids (CCl3F, CCl2F2, CCl2FCClF2, etc.), used as blowing agents in the past, were found to affect the atmospheric ozone layer. Today, the use of physical blowing agents has been limited to low density foams [15].

1.2.2.2.1 Physical blowing microcapsule

Physical blowing microcapsules are thermally expandable particles. They consist of a shell and a core. A low-boiling point organic solvent is encapsulated in a thermoplastic shell. Depending on the different purposes, particle diameter range varies from 5 to 100 microns. The thermoplastic shell is called the envelope or wall material and the embedded low-boiling point organic solvent is known as the core material. With increasing temperature, the core material rapidly vaporizes to produce high internal pressures. At the same time, the wall material softens with heating so the whole particle expands under the internal pressure to form large bubbles. In general, the diameter of the microcapsule can increase to several times its initial value and the volume can easily increase from 10 to even 100 times. These expandable microcapsules have relative shape stability and do not retract after cooling. Thus, the microcapsules can restrain the occurrence of surface deterioration. That deterioration is often found in conventional physical foaming processes, as well as

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chemical foaming. However, the current microcapsules have some disadvantages. For example, the capsules are difficult to use in injection molding and extrusion at high temperatures (over 200oC) due to possible break-up and/or collapse of their shell.

The factors controlling thermal expandability are volatility of the encapsulated low-boiling point organic solvent, gas permeability, viscoelasticity of the thermoplastic shell and thickness of the envelope. In this work, some EXPANCEL microbeads will be used.

1.2.3

The manufacturing methods of polymeric foams

Most polymeric foams are produced by one of the several known foaming techniques which include: extrusion, compression molding, injection molding and rotational molding. A brief review is presented here.

1.2.3.1 Extrusion

Extrusion is a continuous process used to produce polymer parts. There are several different types of configurations for the equipments: single-screw extruders having a long screw, twin-screw extruders and tandem-extruder lines. There are usually three distinct zones in an extrusion barrel (Figure 1.3). The first zone is called the feed zone, which used to carry pellets and melt them. The second zone is called the compression zone. This area is characterized by a place where the space between the sleeves and the screw decreases which leads to an increase in pressure as to compress the polymer particles. The third zone is called the metering zone and is characterized by a constant diameter screw for effective homogenization of the polymer and makes quantitative material flow, pressure uniform flow from the die out. Finally, the polymer comes through the die, which is used to give shape to the product such as spaghetti, film, foil or duct.

The extrusion process can be used directly to produce polymer foams. The technique consists of adding a specific amount of blowing agent and to adjust the temperature which is in the range of the decomposition temperature of blowing agent, and then to choose

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temperatures, speed and optimal pressures to produce a good foam in terms of cell size and density reduction.

Figure 1.3 Schematic diagramme of the extrusion process [16].

1.2.3.2 Rotational molding

Rotational molding is a process that can be used to mold very large empty parts in one operation (see Figure 1.4). It usually consists of molds, ovens, cooling chambers and mold spindles. The spindles are installed on a rotating axis, which provides a uniform coating of the polymer inside the mold. Rhttp://en.wikipedia.org/wiki/Rotational_molding - cite_note-2otational molding is used to produce polymer foams by introduction of the polymer and blowing agent into a mold which closes tightly. Then, the mold is inserted in a convection oven and rotated in two directions. Finally, the mold is cooled down, usually by a fan and/or water spray. The cooling rate must be controlled within a certain range to avoid part defects (residual stresses, warping, etc.). At the end, the part is removed from the mold and start a new cycle. It is possible to obtain very large hollow foamed parts without any weld lines by this process. To produce good quality foam, the blowing agent, whose decomposition temperature is about 50°C higher than the melting temperature of the polymer, must be used [17].

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Figure 1.4 Schematic diagramme of the rotational molding process [18].

1.2.3.3 Injection molding

Injection molding is a semi-continuous process to prepare polymer materials. It is used to produce high volume production. Generally, injection molding machines consist of a material hopper, an injection ram or screw-type plunger and a heating unit (Figure 1.5). Actually, injection molding is a process between compression and extrusion. With injection molding, the granular polymer is fed from a material hopper into a heated barrel by gravity or forced feeding. Then the granules with CBA are slowly moved forward by a screw-type plunger, the granular polymer is pushed into a heated chamber, where the polymer is melted. With the plunger moving forward, the molten polymer is pushed through a nozzle, and then enters into the mold cavity. The mold is maintained cold so that the molten polymer becomes solid rapidly as the mold is filled.

Figure 1.5 Schematic diagramme of the injection molding process [19].

1.2.3.3.1 Foam injection at low-pressure

This technique can be used to produce polymer foams. During this process, injection of the molten polymer and the blowing agent do not completely fill in the mold. After injecting in the mold, the molten polymer with blowing agent expands to fill in the mold. As a result, polymer injected only partially fills the mold, the blowing agent decomposes, which

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generates pressure in the cavity and then the mold is completely filled. Because the pressure produced by the decomposition of the blowing agent is lower than that of the other injection process, this method is called low-pressure injection.

1.2.3.3.2 Foam injection at high-pressure

The high-pressure injection is mainly used to produce the lower-density foam. Unlike the foam injection at lower-pressure, the mold is completely filled by the molten polymer with the blowing agent during the injection process. So the pressure of the mold is the same as that of the other injection process and this foaming process is called foam injection at high pressure. After a relatively short time, some parts of the mold are opened at a certain distance in order to supply space for material expansion. The expansion is done at atmospheric pressure until the polymer meets the surface of the movable parts of the mold. This technique has some disadvantages, such as obtaining imperfect surfaces.

1.2.3.4 Compression molding

The principle of foam production using the compression process is to make a premix of polymer and blowing agent, usually by extrusion, and then to place it between two hot plates (Figure 1.6). High pressure between both plates is used to shape the material and high temperature is used to activate the blowing agent. The blowing agent decomposes to produce the gas. After a few minutes, the pressure is removed. With a rapid drop in pressure, the phenomenon of bubble nucleation occurs and bubble expansion takes place. Finally, the mold is cooled, usually with cold water channels passing through the mold, to get a solid part.

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Figure 1.6 Schematic of the compression molding process [20].

The process of compression molding was studied by Faruk et al.[21]. They found that the type and concentration of chemical blowing agent have an important effect on cell morphology, density and mechanical properties of the foams. This is why it is very important to control these parameters to get good foams.

Compression molding has the advantage of avoiding the orientation phenomena as a result of the shear stress at the output of extrusion and injection. The orientation of cells has an effect on mechanical properties since it causes anisotropy of the material. In addition, this method has fewer parameters which are easy to control. On the other hand, this method has a clear disadvantage: the process is not continuous and production rates are low [22]. Nevertheless, compression molding will be used in this work to produce polymer foams.

1.3

Objectives and Thesis Organization

Much work has been done to study the production and to predict the mechanical behavior of symmetric integral foams (constant density profile). Especially, models for flexural and tensile modulus were developed. Nevertheless, newer materials like FGM have a continuous variation of composition (density) across thickness. Since the mechanical and thermal properties also change with position, it becomes very difficult to relate the macroscopic properties of a part with the microscopic properties unless a complete knowledge on how the material is distributed is obtained. It is clear that polymeric foams

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are a sub-group of FGM when cell size and/or cell density change with position. Based on this observation, the main objectives of this thesis can be summarized as:

1) To use a simple technique such as compression molding to produce symmetric and asymmetric foams in which the morphology changes continuously across thickness from one side to the other side, thus producing density graded polymer foams.

2) To study the effect of blowing agent concentration on the morphology and density profile of symmetric and asymmetric polymer foams.

3) To study the effect of molding temperature and molding time on the morphology and density profile of symmetric and asymmetric polymer foams.

4) To study the effect of density profile and foam morphology on the mechanical properties (tension and flexion) of symmetric and asymmetric polymer foams.

This thesis presents the effect of processing parameters on cell morphology, density profile and mechanical properties of density graded polymer foams. It is composed of six chapters and a brief description on each one is listed here.

Chapter 1 presents a general overview of FGM and polymer foams with the objectives of this research and the structure of this thesis. These topics include the definition of FGM and foaming technology. Chapter 2 provides a literature survey and a theoretical background to the relevant topics used for this work. The basics of polyethylene foaming, the processing method for FGM and studies about the mechanical and morphological properties of these materials are introduced. Chapters 3, 4, 5 describe in detail the experiments performed, the materials used, the methods and equipments, as well as the results obtained based on three conference proceedings. In all cases, linear medium density polyethylene (LMDPE) foams with different density profiles (symmetric, asymmetric) and morphological structures are produced under different processing conditions. A detailed discussion on the effect of foaming temperature, molding time, as well as the type and content of blowing agents on

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cell morphology and density profile are provided. Also, the effects of density profile and morphology on the mechanical properties (tension and flexion) are investigated. Finally, Chapter 6 presents the major conclusions of this research and suggestions for future works.

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Chapter 2.

General Literature Review

2.1

Polyethylene Foams

Polyethylene is produced through the polymerization of ethylene. Polyethylene with different densities or material properties is created by using different catalysts during the polymerization process and/or different polymerization techniques. For example, linear medium density polyethylene (LMDPE) has good stiffness and impact strength and also has a lower melting temperature.

The history of the science and technology of polymeric foams began in the late 1920s with latex foam [23]. Two major processes, Dunlop and Talalay, were developed [24]. In 1942, a process developed by Johnston at DuPont, used nitrogen as physical blowing agent to fabricate polyethylene foams [25]. Among the earliest patents issued for the preparation of polyethylene foam, carbon dioxide was used in 1945 as blowing agent. Although these methods were not commercially successful, similar methods using solvents with low boiling points or chemical agents found acceptance in the market place.

Polyethylene foam is a durable, lightweight, resilient, closed-cell material. It is often used for packaging and material handling applications due to its excellent vibration dampening and insulation properties, as well as to its high resistance to chemicals and moisture.

The commercial production of polyethylene foam began as insulation of electric cables in the early 1950s [26]. Polyethylene foam with a density of 0.5 g/cm3 was produced using direct extrusion with chemical blowing agents. Since 1958, insulation with polyethylene foam was mainly based on low density polyethylene (LDPE). Subsequently, high density polyethylene (HDPE) provided more rigid materials [27].

Homogeneous polyethylene foams with no dimensional variation of properties are traditionally used for most applications. But some situations however require materials with

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different properties. Recently, a number of new processes were developed to produce asymmetric structural foams. For example, Tovar-Cisneros et al.[28] fabricated HDPE asymmetric structural foams by injection molding using different mold temperatures. They found that mold temperature affect strongly foam morphology. Chen and Rodrigue [29] produced LDPE asymmetric structural foams with different skin thicknesses using compression molding.

2.2

Functionally Graded Materials Processing Methods

Today, several technologies have been developed to produce FGM. Some manufacturing methods are presented here.

Fukui et al.[30,31] used high-speed centrifugal casting method to grade the mass densities in the radial direction. A melted Al-Ni alloy was molded into a thick-walled tube, which was rotated at high speed, and so the molten metal underwent an acceleration, thereby producing a composition gradient, (i.e. phase gradient). Quantitative optical microscopy was used to measure the volume fraction of the A13Ni phase.

Ultraviolet radiations were used in poly(ethylene co-carbon monoxide) (ECO) to make FGM materials [32]. Because the properties of this polymer change rapidly under UV light, irradiated of ECO became stiffer, stronger and more brittle with time and position from the surface. Lambros et al. found that the relationships between elastic modulus and irradiation time can be used to adjust the exposure time and the properties.

Sarkar et al.[33] proposed another manufacturing method, which was called electrophoretic deposition (EPD). This method is made up of two processes: electrophoresis and deposition. Electrophoresis describes that the charged particles in a suspension move under a graded electric field. The coagulation of particles into a dense mass is called the deposition process. By this technique, it was possible to synthesize stepwise FGM, as well as continuous profile FGM. The profile can be controlled precisely by controlling the deposition current density, components flow rates, suspension concentrations, etc.

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El-Hadek and Tipper [34] produced functionally graded syntactic foam sheets in which the micro-balloons were dispersed with a linear composition gradient (graded volume fraction) and dispersed in epoxy. They studied the Young’s modulus and the density of syntactic foams with homogenous dispersion of the micro-balloons by using the wave speed. They found that the syntactic foam sheets have a nearly constant Poison’s ratio.

Torres-Sanchez et al.[35] produced functionally graded polyurethane foams using ultrasound as a porosity-tailoring agent. They found that ultrasound had an effect on the cell architecture and cell size distribution. So, when stable cavitation conditions are established, depending on the acoustic pressure subjected, the ultrasounds can create porosity gradation by producing bubbles of different sizes across thickness, as presented in Figure 2.1.

Figure 2.1 Cross-section of two foams sonicated at different positions in an ultrasonic

standing wave [35].

Chittineni et al.[36] fabricated functionally gradient syntactic foams (FGSF) by using four layers of different types of micro-balloons, each one having different wall thickness (see Figure 2.2). The different layers were integrated before matrix solidification occurred. The mechanical properties, such as compressive strength and energy absorption, were studied and it was found that the compressive strength and energy absorption changed with the

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layer’s organization. The yield strength and energy absorption of these integrated FGSF were higher than that of adhesively bonded FGSF.

Figure 2.2 Schematic representations FGSF with four types of micro-balloons [36].

In this work, we will focus on polymer foams. As a special case, a temperature gradient will be used to produce density graded polyethylene foams using compression molding with a chemical blowing agent.

2.3

Mechanical Properties

Mechanical properties are related to deformation of a material undergoing mechanical loads or forces. These properties include strength, modulus and elongation at break. Several types of tests are available to study the mechanical properties of materials. This is why standard tests were developed. In North America, these standardized testing techniques are developed by the American Society for Testing and Materials (ASTM) [37-38].

The next section presents a brief review of some important tests that can be performed to study the mechanical properties of polymeric foams. Tensile and flexural tests will be introduced.

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2.3.1

Tensile characterization

Tensile stresses developed when a material is subjected to a pulling load. Tensile tests impose forces in opposite directions to create a unidirectional elongation on the material (Figure 2.3). It is mainly used to characterize material’s ductility. To observe fracture mechanisms, this test is better than compression. On the other hand, the main disadvantage of this technique is that the sample must be well anchored to a mounting system (no slipping). In some cases, this can result in local deformation of the test sample.

Figure 2.3 Load applied for the tensile testing.

Sun et al.[39] produced microcellular polysulfone, polyethersulfone and polyprenylsulfone foams by a two-stage batch process. They studied the effect of cell morphology on mechanical properties of these foams and found that the tensile modulus of polysulfone foam was directly proportional to the square of their relative densities. On the other hand, tensile strength was found to be proportional to relative foam density.

Matuana et al.[40] studied the effect of cell morphology on tensile properties of polyvinyl chloride (PVC) foams. They found that, although the tensile strength and modulus increased with increasing relative density, the specific tensile modulus and the specific tensile strength were mostly independent of relative foam density (almost constant). However, the specific elongation at break increased as relative density decreased.

Zhang et al.[41,42] studied the tensile properties of closed-cell structural polyethylene foams using four different molecular weights of HDPE. They investigated the effect of molecular weight and foam density on the mechanical properties during tensile tests up to the breaking point. For all their foams, tensile modulus and strength were found to follow

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the square power-law relationship using relative properties, but the elongation at break was very low since the foams were brittle. On the other hand, for ductile foams, the normalized yield strength also followed a square power-law relation with relative density, while the yield strain was almost constant (close to the value of the unfoamed polymer material). Finally, strain at break increased with increasing HDPE molecular weight.

Barzegari et al.[43] studied the tensile modulus of symmetric LDPE foam with different density profiles prepared by compression and injection molding. For compression molded foams, density profile did not change with position and the tensile modulus followed a simple power-law relation with relative density. For injection molded foams, the density profiles showed a complex behaviour with a high density skin and a low density core. The most important observation was the presence of a transition zone between the skin and core with a gradual variation of density. In this case, the best prediction for the tensile modulus was obtained using the complete density profile which includes the density of the skin, core and transition zones (continuous variation in density between each zone).

2.3.2

Flexural characterization

Flexure characterizes the behavior of a slender sample subjected to an external load applied perpendicularly to a longitudinal axis of the sample. There are two common types of tests: three-point or four-point bending as shown in Figure 2.4.

Load Load

Three-point Four-point

Figure 2.4 Load applied for flexural testing.

There are at least two factors that must be considered for choosing the specimen size and test configuration. The first of these factors is the effect of transverse shear deformation. This phenomenon is caused by two types of forces present in the sample. Compression is

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exerted on the top, while a tensile force is present in the lower part and shear occurs between both extremities (Figure 2.5). When the length-to-thickness ratio of the sample is small or when the material has a low shear modulus, the effect of the transverse shear must be considered. The second factor is related to the supports which can deform locally the sample material at the load locations. Care must be taken to use rounded heads when applying the load to limit local deformation.

Figure 2.5 Shear inside of test sample for the flexure test

As described earlier, two types of tests can be performed: four-point and three-point bending. It was found that the effect of transverse shear is much lower in four-point than in three-point tests. On the other hand, four-point tests need higher loadings to get a good deformation. For its simplicity, it is more appropriate to use the three-point geometry when the length-to-depth radio is greater than 5 [44] and the shear stresses can be neglected when the ratio is higher than 16 [37].

The problem of localized deformation in the sample was studied by Iida et al.[45]. They produced rigid polyurethane sandwich structures with high density skin layers enclosing a low density core layer. It was found that a deformation of the upper skin and local concave (buckling) can occur at the point of load application if the modulus is very low (Figure 2.6). On the other hand, if the modulus is high, a clean break is possible on the tensile side.

Figure 2.6 Deformation of sandwich structures in three-point bending tests.

Zhang et al.[42] studied the flexural properties of closed-cell high-density polyethylene foams prepared by compression molding. They found that the flexural modulus could be

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predicted very well using the Gonzalez model and the I-beam model of Hobbs (average deviation of only 5.8%) when the void volume fraction was in the range of 0-55%. They also showed that even very thin skins (1-5%) have an important effect on the flexural modulus.

Tovar-Cisneros et al.[28] studied the effect of mold temperature on the flexural properties of injection molded symmetric or asymmetric HDPE structural foams. For symmetric foams, they found that the flexural modulus increased with increasing skin thickness. For asymmetric foams, the flexural modulus was higher when the load was applied on the side where the skin was thicker. Similar conclusions were obtained by Chen and Rodrigue for LDPE structural foam prepared by compression molding [29].

Barzegari et al.[46] studied the effect of density profile on the flexural properties of structural foams. They used several models to predict the flexural modulus and found that a single continuous equation fitting the complete density profile (including the skin, the core and the transition zone) can be used in combination with the simple square power-law to predict with high accuracy (average 3% deviation) all their flexural modulus data.

Rodrigue [47] also used density profiles to predict the flexural modulus of structural LDPE foams produced by compression molding. Different density profiles were obtained by changing the blowing agent concentration and the thickness of the core. A simple mathematical equation based on generalized Fourier series with only three fitting parameters was used to adjust for skin thickness, slope in the skin-core transition zone and core density. The model was able to predict the flexural modulus with very high accuracy (maximum 2% deviation).

In order to explain the mechanical properties and the density profiles, a more microscopic analysis of the structure must be done. This is presented next.

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2.4

Morphological Properties

Morphological properties explore the details of the structures commonly encountered in materials. Morphological characterizations can be done by direct observation using techniques such as scanning electron microscopy (SEM), scanning tunnelling microscopy (STM), transmission electron microscopy (TEM), etc.

Since the beginning, morphological properties of polymeric foams have been studied by several authors. We present here some important results based on parameters that were found to affect the final structure and properties of the foams.

Yamaguchi and Suzuki [48] studied the rheological properties and foam processability of polyethylene blends. They found that, when a small amount of cross-linked linear low-density polyethylene (LLDPE) was added, the elastic properties of the matrix were improved, but the steady-state shear viscosity was almost constant. The foams using these blends showed higher expansion ratios and more homogeneous cell size distributions.

Stafford et al.[49] investigated the effects of molecular weight and polydispersity on polystyrene (PS) foam processing. They found that molecular weight and polydispersity had a negligible effect on the foaming process itself, but low molecular weight fractions (<4%) had an important effect on cell diameter and the final structure of the foam.

Chong et al.[50] studied polyolefin foams produced by extrusion. They studied the effect of melt temperature on the foaming degrees and cell sizes and found that cell size increased with increasing melt temperature, but cell size was almost unchanged with increasing the temperature of the cooling water. With the melt temperature increasing, cell size and foaming degrees increased because bubble growth began under a condition of low melt viscosity of the polymer (less resistance to gas expansion). When the viscosity of the polymer increased, bubble growth was reduced leading to smaller cell size and lower foaming degree.

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To improve cell structure, Rodrigue and Gosselin studied the effect of calcium carbonate as a nucleating agent on foam morphology of low density polyethylene foams [51]. They found that the concentration and particle size of the nucleating agent have a strong effect on the final morphology of the foam: with increasing calcium carbonate concentration and decreasing particle size, more surface area was available for bubble nucleation leading to cell density increase.

In 2003, Zhang et al.[52] studied the morphology of symmetric structural foams of high density polyethylene prepared by compression process. They found that the degree of foaming (density reduction) decreased with increasing molecular weight and increased with increasing blowing agent (ACA) concentration. This was related to higher matrix viscosity with increasing polymer molecular weight and higher number of gas molecules available at higher blowing agent content.

Gosselin and Rodrigue [53] studied the cellular morphology of high-density polyethylene foams prepared by extrusion and injection molding. They used different methods to calculate the cell surface and to convert surface data into volumetric properties (density) by taking into account cell deformation via different cell shapes: spherical, ellipsoid of revolution or others. They found that all the methods proposed gave similar estimate of cell density when the cells are nearly spherical (injection), but the results diverged when cell deformation was present: elongated cells in one direction like in the machine direction for extrusion processes.

Tovar-Cisneros et al.[28] investigated the effect of mold temperature on morphology (average cell dimension, cell density and skin thickness ratio) of injection molded HDPE structural foams. They found that symmetric structural foams were obtained when a homogeneous mold temperature was applied and foam skin thickness increased with decreasing mold temperature (faster cooling rate to solidify the polymer in contact with the cold mold walls). Asymmetric structural foams were obtained when both sides of the mold were set at different temperatures. Also, lower mold temperature produced larger average cell diameter and lower cell density.

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Finally, Barzegari et al.[54] studied the effect of injection molding conditions on the morphology of LDPE structural foams. In this case, blowing agent concentration, mold and melt temperature, injection pressure and back pressure were varied at four different levels to determine their respective effect on skin thickness, cell size, cell density and foam density. They found that blowing agent content and injection pressure had the biggest influence on foam density, while mold temperature affected mostly skin thickness. They also found that the back pressure had negligible effect on skin thickness, cell size and foam density.

Based on the information gathered in literature, the next chapters present the materials, methods, results and discussion of LMDPE foams produced by compression molding using Expancel microbeads as the blowing agent. In our work, the effect of processing conditions (mold temperature and molding time) and material composition (Expancel type and content) are related to foam morphology (density profile, cell size distribution and cell density) and mechanical properties (tensile and flexion).

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