Thermoset Nanocomposites E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Serie: Polymer Nano-, Micro- and Macrocomposites
• Sprache: Englisch

Seitenzahl: 408

Veröffentlichungsjahr: 2013

Beschreibung

The book series "Polymer Nano-, Micro- and Macrocomposites" provides complete and comprehensive information on all important aspects of polymer composite research and development, including, but not limited to synthesis, filler modification, modeling, characterization as well as application and commercialization issues. Each book focuses on a particular topic and gives a balanced in-depth overview of the respective subfield of polymer composite science and its relation to industrial applications. With the books the readers obtain dedicated resources with information relevant to their research, thereby helping to save time and money.

Thermoset polymers are a class of materials with many superior properties than thermoplastic materials. Nanocomposites with a large variety of thermoset polymers have been explored and vast knowledge on the synthesis methodologies as well as properties has been generated. The goal of the book is to assimilate these research findings on the many thermoset polymer based nanocomposites systems comprehensively so as to generate better insights into the design, performance and optimization of thermoset nanocomposites.

Leseprobe

Table of Contents

Series Page

Title page

Copyright page

Preface

List of Contributors

1: Epoxy–Vermiculite Nanocomposites

1.1 Introduction

1.2 Experimental

1.3 Results and Discussion

1.4 Conclusions

Acknowledgment

2: Polymer Nanocomposites with UV-Cured Epoxies

2.1 Introduction

2.2 Photopolymerization of Epoxides

2.3 Limits in Curing Epoxy Composites by UV Irradiation

2.4 Top-Down UV-Cured Epoxy Nanocomposites

2.5 Bottom-Up UV-Cured Epoxy Nanocomposites

2.6 Conclusions

3: Influence of Organic Modification and Polyurethane Structure on Clay Dispersion in Polyurethane–Clay Nanocomposites

3.1 Polymer Nanocomposites: An Introduction

3.2 Polyurethane–Clay Nanocomposites

3.3 Influence of Organic Modification of Clay and Structure of PU on PU/Clay Nanocomposites Structure

3.4 Conclusions

4: Thermal Properties of Formaldehyde-Based Thermoset Nanocomposites

4.1 Introduction

4.2 Theoretical Background of Thermal Kinetics

4.3 Thermal Properties of Nanocomposites

4.4 Mechanical Properties of the Nanocomposites

4.5 Summary

Acknowledgment

5: Mechanical Performance of Thermoset Clay Nanocomposites

5.1 Introduction

5.2 Viscoelasticity Analysis: Dynamical Mechanical Thermal Analysis (DMTA)

5.3 Rigidity – Young’s Modulus

5.4 Strain at Break

5.5 Stress at Break – Fracture Toughness

5.6 Conclusion

6: Unsaturated Polyester Resin Clay Hybrid Nanocomposites

6.1 Introduction

6.2 Reinforced Unsaturated Polyester Composites

6.3 Clay Minerals

6.4 Mechanical and Thermal Properties of Clay–UP Nanocopomposites

6.5 Flame Retardance

6.6 Bio-Derived Unsaturated Polyester–Clay Nanocomposites

7: Hyperbranched Polymers as Clay Surface Modifications for Nanocomposites

7.1 Introduction

7.2 Hyperbranched Polymers for Antimicrobial Surface

7.3 Hyperbranched Polymers on Adsorbents for Cr(VI) Water Treatment

Acknowledgment

8: New Methods for the Preparation of Metal and Clay Thermoset Nanocomposites

8.1 Introduction

8.2 Thermoset Nanocomposites Based on Nanoclays

8.3 Thermoset Nanocomposites Based on Metal Nanoparticles

8.4 Concluding Remarks

9: Bio-Based Epoxy Resin/Clay Nanocomposites

9.1 Introduction

9.2 Bio-Based Epoxy Resins and Hardeners

9.3 Bio-Based Epoxy Resins/Clay Nanocomposites

9.4 Conclusion

10: Electrical Properties and Electromagnetic Interference Shielding Response of Electrically Conducting Thermosetting Nanocomposites

10.1 Introduction

10.2 EMI Shield and Shielding Effectiveness

10.3 Conclusions

Acknowledgments

Index

Polymer Nano-, Micro- & Macrocomposites Series

Mittal, V. (ed.)

Surface Modification of Nanotube Fillers

Series: Polymer Nano-, Micro- & Macrocomposites (Volume 1)

2011

ISBN: 978-3-527-32878-9

Mittal, V. (ed.)

In-situ Synthesis of Polymer Nanocomposites

Series: Polymer Nano-, Micro- & Macrocomposites (Volume 2)

2012

ISBN: 978-3-527-32879-6

Mittal, V. (ed.)

Characterization Techniques for Polymer Nanocomposites

Series: Polymer Nano-, Micro- & Macrocomposites (Volume 3)

2012

ISBN: 978-3-527-33148-2

Mittal, V. (ed.)

Modeling and Prediction of Polymer Nanocomposite Properties

Series: Polymer Nano-, Micro- & Macrocomposites (Volume 4)

2013

ISBN: 978-3-527-33150-5

Related Titles

Mittal, V. (ed.)

Polymer Nanotubes Nanocomposites

Synthesis, Properties and Applications

2010

ISBN: 978-0-470-62592-7

Mittal, V. (ed.)

Miniemulsion Polymerization Technology

2010

ISBN: 978-0-470-62596-5

Mittal, V. (ed.)

Optimization of Polymer Nanocomposite Properties

2010

ISBN: 978-3-527-32521-4

Galimberti, M. (ed.)

Rubber-Clay Nanocomposites

Science, Technology, and Applications

2011

ISBN: 978-0-470-56210-9

Fink, J.

A Concise Introduction to Additives for Thermoplastic Polymers

2010

ISBN: 978-0-470-60955-2

Xanthos, M. (ed.)

Functional Fillers for Plastics

Second Edition

2010

ISBN: 978-3-527-32361-6

Thomas, S., Joseph, K., Malhotra, S.K., Goda, K., Sreekala, M.S. (ed.)

Polymer Composites

Volume 1

2012

ISBN: 978-3-527-32624-2

The Editor

Dr. Vikas Mittal

The Petroleum Institute

Chemical Engineering Department

Bu Hasa Building, Room 2204

Abu Dhabi

UAE

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Print ISBN: 978-3-527-33301-1

ePDF ISBN: 978-3-527-65967-8

ePub ISBN: 978-3-527-65966-1

Mobi ISBN: 978-3-527-65965-4

oBook ISBN: 978-3-527-65964-7

ISSN: 2191-0421

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Preface

Thermoset polymers are an important class of materials with many superior properties as compared with thermoplastic materials. Generation of thermoset polymer nanocomposites by the incorporation of layered silicates and other nanofillers in the polymer matrices has led to enhancement of the property profiles of the thermoset materials significantly. Nanocomposites with a large variety of thermoset polymers have been explored, and vast knowledge on the synthesis methodologies as well as properties has been generated. The goal of the book is to assimilate these research findings on many thermoset polymer-based nanocomposite systems comprehensively so as to generate better insights into the design, performance, and optimization of thermoset nanocomposites.

Chapter 1 reports the synthesis and properties of epoxy vermiculite nanocomposites. Vermiculite platelets were surface modified to enhance interfacial interactions with the polymer, and the nanocomposites were generated using in situ polymerization method. Interesting insights into the permeation properties of the nanocomposites have been reported. Chapter 2 presents photoinduced polymerization as an efficient technique for rapid formation of highly cross-linked networks from liquid epoxy resins. The reaction involves a cationic mechanism and is usually initiated by UV light. Chapter 3 reports the synthesis and properties of polyurethane (PU) nanocomposites, and it was observed that the key to superior properties of nanocomposites is critically dependent on the choice of the organic modifier used to modify the surface of clay as well as the nature of the polymer itself. Chapter 4 reviews recent progresses on thermal properties of formaldehyde-based thermoset/cellulose nanocomposites such as melamine-formaldehyde (MF) resin/clay/cellulose, phenol-formaldehyde (PF) resin/clay/cellulose, or PF resin/multiwalled carbon nanotube (MWCNT)/cellulose nanocomposites, particularly by focusing on thermal curing or degradation kinetics of these nanocomposites. Chapter 5 reports a review of mechanical properties of thermoset polymer nanocomposites. It is reported that the nanoclay particles provide to thermosets interesting mechanical properties when the constituents and the processing parameters are adequately selected. These properties are highly improved in the case of rubbery matrix than in glassy matrix. Chapter 6 demonstrates the unsaturated polyester clay nanocomposite systems. These composites are of high importance as the unsaturated polyester resins are the most widely used thermoset matrix resin in the coatings and composites industry, and constitutes about three-fourth of the total resins used. In Chapter 7, recent advances on applications of hyperbranched polymers as clay surface modifications are presented, with particular reference to the preparation of antimicrobial surface and adsorbents for Cr (VI) water treatment. Chapter 8 focuses on new methods to generate metal- and clay-reinforced nanocomposites. The development of in situ methods has clearly facilitated the major advances in the synthesis in a one-pot manner combining polymerization processes leading to the network formation with intercalation/exfoliation or nanoparticle formation, respectively. In Chapter 9, the preparation and properties of the bio-based epoxy resin/hardener/layered silicate nanocomposites are described. The replacement of petroleum-based epoxy resin/hardener with bio-based epoxy resin/hardener is very important from the viewpoint of the conservation of limited petroleum resources and the protection of global environment. In Chapter 10, comprehensive account of electrical and electromagnetic interference (EMI) shielding properties of thermosetting nanocomposites has been provided with special reference to those based on conducting additives like intrinsically conducting polymers and carbonaceous fillers like carbon nanotubes and graphene.

Vikas Mittal

Abu Dhabi

June 2013

List of Contributors

Concepción Abrusci

Universidad Autónoma de Madrid-UAM

Departamento de Biología Molecular

Facultad de Ciencias

Cantoblanco

28049 Madrid

Spain

Nourredine Aït Hocine

ENI Val de Loire

Laboratoire de Mécanique et Rhéologie (LMR)

3, rue de la Chocolaterie

BP 3410, 41034 Blois Cedex

France

Muthukaruppan Alagar

Anna University

Department of Chemical Engineering

AC-Tech Campus

Chennai 600025

India

Muhammad Atif

Politecnico di Torino

Department of Applied Science and Technology

Corso Duca degli Abruzzi 24

10129 Torino

Italy

Saïd Azem

Université Mouloud Mammeri de Tizi-Ouzou (UMMTO)

Laboratoire d’Elaboration et de Caractérisation des Matériaux et Modélisation (LEC2M)

15000 Tizi-Ouzou

Algeria

Roberta Bongiovanni

Politecnico di Torino

Department of Applied Science and Technology

Corso Duca degli Abruzzi 24

10129 Torino

Italy

Fernando Catalina

Instituto de Ciencia y Tecnología de Polímeros

Polymer Photochemistry Group

C.S.I.C. Juan de la Cierva 3

28006 Madrid

Spain

Teresa Corrales

Instituto de Ciencia y Tecnología de Polímeros

Polymer Photochemistry Group

C.S.I.C. Juan de la Cierva 3

28006 Madrid

Spain

Kübra Doğan Demir

Istanbul Technical University

Faculty of Science and Letters

Chemistry Department

Maslak

34469 Istanbul

Turkey

Subramani Deveraju

Anna University

Department of Chemical Engineering

AC-Tech Campus

Chennai 600025

India

Kanniyan Dinakaran

Anna University

Department of Chemistry

MIT Campus

Chennai 600044

India

Hanaya Hassan

Centre d’Etude et de Recherche sur les Matériaux Elastomères (CERMEL)

LMR

29, rue des Martyrs

37300 Joué-lès-Tours

France

Manolya Kukut

Istanbul Technical University

Faculty of Science and Letters

Chemistry Department

Maslak

34469 Istanbul

Turkey

Iñigo Larraza

Instituto de Ciencia y Tecnología de Polímeros

Polymer Photochemistry Group

C.S.I.C. Juan de la Cierva 3

28006 Madrid

Spain

Shroff R. Mallikarjuna

National Chemical Laboratory

Division of Polymer Science and Engineering

Dr. Homi Bhabha Road

Pune 411 008

India

Gema Marcelo

Instituto de Ciencia y Tecnología de Polímeros

Polymer Photochemistry Group

C.S.I.C. Juan de la Cierva 3

28006 Madrid

Spain

Vikas Mittal

The Petroleum Institute

Department of Chemical Engineering

Room 2204, Bu Hasa Building

Abu Dhabi 2533

UAE

Byung-Dae Park

Kyungpook National University

Department of Wood Science and Technology

Daegu 702-201

Republic of Korea

Parveen Saini

CSIR-National Physical Laboratory

Conducting Polymers Group

Polymeric and Soft Materials Section

New Rajender Nagar

New Delhi 110 012

India

Marco Sangermano

Politecnico di Torino

Department of Applied Science and Technology

Corso Duca degli Abruzzi 24

10129 Torino

Italy

Said Seghar

Centre d’Etude et de Recherche sur les Matériaux Elastomères (CERMEL)

LMR

29, rue des Martyrs

37300 Joué-lès-Tours

France

Mitsuhiro Shibata

Chiba Institute of Technology

Department of Life and Environmental Sciences

Faculty of Engineering

2-17-1, Tsudanuma

Narashino

Chiba 275-0016

Japan

Swaminathan Sivaram

National Chemical Laboratory

Division of Polymer Science and Engineering

Dr. Homi Bhabha Road

Pune 411 008

India

Mehmet Atilla Tasdelen

Yalova University

Faculty of Engineering

Department of Polymer Engineering

77100 Yalova

Turkey

Yusuf Yagci

Istanbul Technical University

Faculty of Science and Letters

Chemistry Department

Maslak

34469 Istanbul

Turkey

and

King Abdulaziz University

Faculty of Science

Chemistry Department

Jeddah 21589

Saudi Arabia

1

Epoxy–Vermiculite Nanocomposites

Vikas Mittal

1.1 Introduction

Epoxies form a special class of thermosetting polymeric materials having high thermal and environmental stability. They are well known as creep-resistant materials with very high stiffness properties [1–3]. Owing to these properties, a wide spectrum of epoxy applications is available, which includes the use of epoxies as adhesives, coatings, printed circuit boards, electrical insulators, and so on. One of the major areas where epoxy adhesives find tremendous use are packaging laminates where their sole use is to hold together the various polymeric foils used in these commercial packaging laminates. To save the material costs, an overall decrease in the thickness of the packaging laminate can be achieved if the adhesive can also be made to contribute to the properties required for a packaging material apart from its function of being an adhesive. The common properties required being the permeation barrier, mechanical performance, transparency, suitability for food contact applications, ease of printability, and so on. Permeation barrier to oxygen and water vapor form the most important property needed in the packaging materials. This can be achieved by altering the polymer network structure obtained by crosslinking of the epoxide groups with amines or other crosslinking agents [4, 5]. The use of epoxy polymer with stiff rod-like units in the backbone can help to enhance the required properties. The other alternative includes the incorporation of inorganic fillers in the polymer matrix, this approach being easier to monitor and control. As the filler shape, size, and interfacial interactions affect the polymer prop­erties greatly, organically treated plate-like inorganic aluminosilicate particles can be incorporated in the polymer matrix to achieve polymer nanocomposites for improvement in barrier performance. By incorporating impermeable, transparent, plate-like nanoparticles in the polymer matrix, the permeating molecules are forced to wiggle around them in a random walk, hence diffusing through a tortuous pathway [6–8]. Besides, the decrease in transmission rate of the permeant is a function of the aspect ratio of the inclusions, their volume fraction, and orientation.

The synthesis of epoxy–clay nanocomposites has been extensively studied; how­ever, majority of these studies focused on enhancing the mechanical properties with the incorporation of organically modified fillers [9–17], thus largely neglecting the permeation properties. Only a few recent studies have discussed these properties in detail [18–23]. Apart from that, montmorillonite has been the most commonly used aluminosilicate in these studies. Owing to the low charge density (0.25–0.5 equiv mol−1), a larger area per cation is available on the surface, which leads to a lower basal plane spacing in the clay after surface ion exchange with alkyl ammonium ions. On the other hand, minerals with high charge density (1 equiv mol−1), such as mica, and hence subsequent smaller area per cation, do not swell in water and thus do not allow the cation exchange. However, aluminosilicates with medium charge densities of 0.5–0.8 equiv mol−1, such as vermiculite, offer a potential of partial swelling in water and cation exchange, which can lead to a much higher basal plane spacing in the modified mineral if optimum ion exchange is achieved. In the pristine state, vermiculite particles are composed of stacks of negatively charged 2 : 1 aluminosilicate layers (ca. 0.95 nm thick) with one octahedral sheet sandwiched between two opposing tetrahedral sheets and the resuting regular gap in between (interlayer). The chemical constitution of its unit cell is (Mg,Al,Fe)3(Al,Si)4O10(OH)2Mgx(H2O)n [24, 25]. Due to isomorphic substitutions in the lattice, the layers have permanent negative charges that are compensated mainly by hydrated Mg2+ as interlayer cations. Owing to the higher basal plane spacing in the modified mineral, the electrostatic interactions holding the layers together can be expected to be lower than similar montmorillonite counterparts thus increasing the potential of better properties of the hybrid nanocomposites.

The goal of this investigation was to synthesize epoxy–vermiculite nanocomposite coatings and to study their microstructure development as well as their oxygen and water vapor barrier properties in comparison with already reported epoxy–montmorillonite system [18]. Vermiculite platelets modified with two different ammonium ions were prepared for the purpose. The epoxy matrix and the curing agent were chosen to achieve polymer matrix, which meets the requirements of the food and health regulations and has low gas permeability on its own. The nanocomposite coatings were drawn on polyamide and polypropylene substrates and the curing temperatures were kept low in order to avoid the thermal damage to these substrate foils.

1.3 Results and Discussion

On the one hand, as the lateral dimension of the montmorillonite is limited by the weathering and purification operations, the resulting aspect ratio is also limited to lower values. On the other hand, if vermiculite is optimally modified and hence is made to exfoliate in the polymer matrix, a much higher aspect ratio can be generated. As the permeation properties are significantly affected by exfoliated platelets owing to the enhancement of tortuity in the permeant random path through the membrane, better barrier performance can be expected in the vermiculite system. However, the presence of a greater number of cations in a specific area as compared to montmorillonite also makes it difficult to swell and modify and the platelets are held together by much stronger electrostatic forces. Therefore, only a combined effect of these factors can be expected on the final composite properties.

Table 1.1 shows the mean size (D0.5) and cation exchange capacity values of the pristine vermiculite along with mineral after milling, de-agglomeration, H2O2 treatment as well as delamination. The as-received vermiculite mineral had a low CEC value of 150 μeq g−1. The milled vermiculite was sieved and the fraction with <160 μm size was observed to have a mean diameter of 36.9 μm and a CEC of 240 μeq g−1, which was higher than the pristine mineral. However, after de-agglomeration of the mineral, the size was further reduced to 23.6 μm, but a similar CEC value of 242 μeq g−1 was observed, which indicated that the de-agglomeration process was not effective. The D0.9 value of 78 μm also indicated the broad distribution in particle size. The fraction in the size range of 160–250 μm had a mean diameter of 92.9 μm. A rough estimate of the aspect ratio was also generated with the particle size analyzer, and the vermiculite fraction with <160 μm fraction had an aspect ratio of 5.21 as compared to 6.48 for the fraction >160 μm. Though the higher sized fraction had slightly higher aspect ratio, it was however a result of higher tactoid or stack thickness as well as higher diameter in the mineral. It should also be noted that due to large distribution in the size of the particles, a distribution in the aspect ratio also existed. The fraction in the range of 160–250 μm was re-milled and an average size of 29.9 μm was measured. A much higher diameter D0.9 of 101.8 μm for this fraction indicated that the particle size distribution was still broad. In another trial, the hydrogen peroxide-treated vermiculite was milled and a mean size of 35.5 μm was observed for the fraction <160 μm. A cation exchange capacity value of 300 μeq g−1 was obtained for this fraction, indicating that the H2O2 treatment of vermiculite prior to milling improved the CEC further as compared to both the pristine vermiculite flakes and the milled vermiculite without H2O2 treatment. The fraction above >160 μm had mean size of 78.2 μm and its CEC value of 200 μeq g−1 was higher than the pristine mineral. The mean size and aspect ratio values of the particles decreased when the milling time was enhanced to 2 h, which indicated that the lateral dimensions of the minerals were impacted by the longer milling periods.

Table 1.1 Mean size, aspect ratio, and cation exchange capacity of vermiculite samples after milling (1 h), de-agglomeration, and delamination.

The findings from Table 1.1 confirm that both milling type and time affected the overall dimensions of the mineral particles. Though the mineral particles have average dimensions in the μm scale and various milling and sieving steps lead to minor changes in the overall size, it should be noted that these particles form precursors to the high aspect ratio primary platelets, thus their handling during the milling and de-agglomeration processes has immense significance. Reduction of lateral dimensions signifies reduced aspect ratio, which does not lead to the optimal enhancements in composite properties when such minerals are incorporated into the polymer matrices. It has been often reported that mechanical and permeation properties of the materials are strongly impacted by filler volume fraction as well as aspect ratio. In fact, only exfoliated platelets were reported to contribute to the barrier performance of the polymer-layered silicate nanocomposites [18]. Pretreatment of the pristine minerals (e.g., H2O2 treatment) provided additional advantages in achieving the goal of higher aspect ratio minerals. Enhancement of the CEC of the minerals after treatment suggested that higher extent of mineral surface was exposed due to penetration of the H2O2 molecules into the mineral interlayers and possibly reduction in the stack thickness as a consequence. Such treatments are also advantageous to remove any impurities present in the minerals.

Figure 1.1 demonstrates the evolution of CEC of vermiculite as a function of number of delamination cycles and delamination time. Vermiculite used for delamination was <160 μm sample, which was treated with H2O2 before milling. The first 10 days of delamination of vermiculite with NaCl allowed partial exchange of the Mg2+ ions with Na+ ions generating Na–vermiculite with a CEC of 1050 μeq g−1. Second delamination cycle for further 12 days increased the CEC relatively slowly to 1440 μeq g−1. It signified nearly a tenfold increase in CEC as compared to the pristine mineral. The mean size and aspect ratio of the delaminated minerals in Table 1.1 were similar to the milled materials indicating that although the interlayers were swollen and access to exchangeable cations was enhanced in the delamination process, loose stacks of platelets may still exist. It has also been reported that during the cleaving process, the particle diameter also reduced slightly along with the thickness, thus, justifying the observed values of mean particle size. However, it also confirmed that the mean size of the starting materials was suitable as it allowed the penetration of the NaCl and LiNO3 in the interlayers during the delamination process. Though a partial access was gained to the available surface cations after the cleavage process, the minerals obtained were readily dispersible in water and their enhanced cation exchange capacity was suitable to carry out the surface modification processes to render them compatible with organic polymer matrices.

Figure 1.1 CEC values of vermiculite as a function of delamination time with NaCl and LiNO3, respectively.

To achieve compatibility of the mineral and to reduce the surface energy, the sodium ions were exchanged with benzyldimethylhexadecylammonium chloride (BzC16) and benzyl(2-hydroxyethyl)methyloctadecylammonium chloride (BzC18OH). The two ammonium ions were chosen in order to analyze the effect of different chemical architecture of the ammonium ions on the final composite properties. Depending on the interactions of the swelling solvent and the epoxy prepolymer with the ammonium ion, more chemical interaction of the epoxide groups with the hydroxyl groups present in the ammonium ions can be expected, thus leading to chemical grafting of the epoxy chains on the surface. Similarly, benzyl group present on the surface is also expected to aid in generating stronger van der Waals attraction forces with the epoxy polymer. Figure 1.2 shows the schematic of the modified vermiculites as well as the chemical structures of the system constituents. It is also worth mentioning here that other modifications such as benzyldibutyl(2hydroxyethyl)ammonium chloride, which have been found to significantly exfoliate the montmorillonite clay platelets in the epoxy matrix, were also tried to be exchanged on the vermiculite surface. However, no exchange could be achieved even when the system was constantly stirred for 2 days or higher amounts of ammonium salts corresponding to the mineral CEC were used.

Figure 1.2 (a) and (b) Schematic of modified vermiculites and the chemical architecture of ammonium ions; (c) and (d) chemical structure of the epoxy resin and the crosslinker tetraethylenepentamine.

The presence of a local bilayer during the cation exchange was avoided as the presence of excess unattached modifier molecules can negatively interact with the epoxy prepolymer, thus inducing system instability and subsequent loss of composite properties. Figure 1.3 shows the TGA thermograms of the modified vermiculites. Absence of any low-temperature degradation peak in the thermograms indicates that the excess of the modifier molecules could be successfully washed off. However, it was also important to achieve satisfactory extent of surface ion exchange in order to completely organophilize the mineral. Exchange of >95% of the surface ions was achieved in the present system. As the higher CEC of the mineral leads to the presence of a large number of organic chains on the surface leading to a lower density of the modified vermiculite as compared to the unmodified state, therefore these values should be taken into account for calculating the true volume fraction of the filler. Table 1.2 details these values for the modified filler and the composites. The inorganic volume fraction of 3.5% was used for composites as it was found to be an optimum value for achieving the enhancement in the permeation properties.

Figure 1.3 TGA thermograms of the surface-modified vermiculites.

Table 1.2 Density of the organo-vermiculites and their 3.5 vol% composites.

The basal plane spacing in the nonmodified vermiculite was found to be 1.22 nm, which was enhanced to 3.25 nm for BzC16-modified vermiculite and 3.40 nm for BzC18OH-modified vermiculite. The absence of the peak corresponding to 1.22 nm in the diffractograms of the modified minerals also confirmed that the surface cations were fully exchanged during the exchange process. In contrast, montmorillonite modified with BzC16 and BzC18OH had, respectively, the basal plane spacing of 1.87 and 2.06 nm [18]. It clearly indicates that owing to higher CEC (880 μeq g−1 for montmorillonte), the platelets were pushed to much larger distances in vermiculite than in montmorillonite for the same ammonium ions exchanged on the surface. Table 1.3 describes the basal plane spacing values of the dry fillers, the suspension of fillers in DMF, the suspension of fillers in DMF and epoxy prepolymer, and the final composites. DMF has been chosen for these experiments because of its low volatility for the time scale of the X-ray scanning experiments. The presence of an X-ray peak in the solvent suspensions of modified fillers indicates that the fillers were not completely exfoliated in the solvent itself indicating the presence of residual electrostatic forces. The suspensions of fillers in solvent and prepolymer also confirmed that the prepolymer itself was also unable to delaminate the platelets, although there was no decrease in the basal plane spacing as sometimes observed in the montmorillonite system. However, no change in the basal plane spacing on the addition of prepolymer would suggest that no or very weak intercalation of the prepolymer took place. The basal spacing was enhanced to some extent after curing and nanocomposite generation indi­cating that intercalation occurred during the polymerization leading to further pushing apart of the platelets. However, the presence of diffraction peak in the composite diffractograms also indicated that full exfoliation of the platelets could not be achieved indicating the presence of residual attraction forces. Figures 1.4 and 1.5 show the X-ray diffractograms of the modified fillers and their 3.5 vol% composites. As it is evident from the diffractograms that the intensity of the diffraction peak was reduced after incorporation in the polymer, but owing to the dependency of the intensity on sample preparation and mineral defects, the analysis of the microstructure of the composites was also performed by TEM. Figure 1.6 shows the TEM micrographs of the 3.5 vol% composites. The micrographs showed the presence of mostly mixed morphology with single layers and intercalated tactoids of varying thicknesses. The platelets were also observed to be bent and folded and no specific alignment of the tactoids could be observed at any magnification. One should also note here that the misalignment of platelets leads to reduction in the composite properties, especially gas permeation properties. It has been recently reported by using finite element models of the misaligned platelets that the completely misaligned platelets (as seen in the current system) are roughly one-third effective as barrier materials as compared to the completely aligned platelets thus confirming the need to generate more alignment [28]. The TEM micrographs not only confirmed the X-ray findings about the intercalated morphology, but also showed the presence of some exfoliated layers totally missed in the WAXRD.

Table 1.3 Basal plane spacing of the fillers, their suspensions, and epoxy composites.

Figure 1.4 X-ray diffractograms of the (I) BzC16-modified vermiculite and (II) the filler in epoxy composite.

Figure 1.5 X-ray diffractograms of the (I) BzC18OH-modified vermiculite and (II) the filler in epoxy composite.

Figure 1.6 TEM micrographs of the 3.5 vol% (a) BzC16–vermiculite epoxy and (b) BzC18OH–vermiculite epoxy composites. The dark lines are cross-sections of aluminosilicate platelets.

The gas permeation properties through the composite films containing 3.5% filler volume fraction were measured and are reported in Table 1.4. The oxygen permeation through the pure epoxy films was recorded as 2 cm3 μm/(m2 d mmHg), which in itself is much lower than the other adhesives used in the packaging laminates. The oxygen permeation through BzC18OH and BzC16 composites was, respectively, observed as 1.5 and 1.4 cm3 μm/(m2 d mmHg), which indicates that in spite of very high oxygen resistance of the matrix, further improvements in the oxygen barrier performance could be achieved by the incorporation of the nano-platelets. These values for the montmorillonite fillers were reported as 2.2 and 1.6, respectively. It also indicates that the oxygen permeation was unaffected by the chemical nature of the ammonium ion exchanged on the surface and relied more on the polymer filler interactions. The water vapor permeation through the composite films with unmodified vermiculite was observed to be 37 g μm/(m2 d mmHg), which was much higher than 10 g μm/(m2 d mmHg) observed for the pure polymer itself. A value of 7.5 and 9.7 g μm/(m2 d mmHg) was observed for the BzC18OH and BzC16 composites, respectively, indicating that the polarity of the hydrophilic interlayers was significantly reduced after the cation exchange, but interlayers are still partially polar to attract the molecules of water. Thus the composite properties can be represented as a combined effect of positive factors such as potential higher aspect ratio of the platelets, intergallery polymerization, and enhanced basal plane spacing as well as negative factors such as residual electrostatic forces. Thus, attaching much longer alkyl chains as well as increasing the grafting density in the ammonium ions could be expected to further eliminate the electrostatic interactions.

Table 1.4 Oxygen and water vapor transmission rates through 3.5 vol% organo-vermiculite nanocomposites at 23 °C.

CompositeOxygen permeability coefficienta)Water vapor transmission ratea)cm3 μm/(m2 d mmHg)g μm/(m2 d mmHg)Neat epoxy2.010.0Na–vermiculite1.737.0BzC18OH1.57.5BzC161.49.7

a) Relative probable error 5%.

1.4 Conclusions

Vermiculite platelets modified with alkyl ammonium ions contained much higher basal plane spacing than the modified montmorillonites. The clay interlayers were further pushed apart in the composites owing to interlayer polymerization and the polymer chains were not squeezed out during solvent evaporation and curing. However, just having large d-spacing does not constitute the sole requirement for filler exfoliation, because owing to higher number of cations available per unit area in vermiculite, the residual electrostatic forces were still present although the area is covered with a large number of chains. The oxygen permeation through the composite films was further reduced by 30% in 3.5 vol% BzC16 composites, even though the permeation through the pure matrix was also very low. Owing to the polar interlayers, the water vapor permeation through the nonmodified vermiculite composites exponentially increased, which was significantly reduced in modified vermiculite composites. Grafting of large alkyl chains and ammonium ions with higher chain densities can be a possible approach to further reduce the electrostatic forces in these high CEC minerals.

Acknowledgment

Major portion of the presented work has been published in Journal of Composite Materials, 2008, 42, 2829, copyright Sage Publishers.

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2

Polymer Nanocomposites with UV-Cured Epoxies

Roberta Bongiovanni, Muhammad Atif, and Marco Sangermano

2.1 Introduction

Epoxy nanocomposites are heterophasic systems with at least one of the dispersed phase dimensions in the order of a few nanometers, keeping its nature different from the continuous polymeric phase.

Epoxy nanocomposites can be classified according to different criteria:

a) The number of dispersed phase dimensions lying in the nanometric range.1)

b) The type of interactions between the two phases, that is, the nature of the interface or interphase.

c) The type of process selected to make the nanostructure.

Depending on benchmark (a), three types of nanocomposites can be distinguished:

Type I: All three dimensions of the dispersed phase are in the order of nanometers.Type II: Two dimensions of the dispersed phase are in the nanometer scale.Type III: Only one dimension of the dispersed phase is in the nanometer range.

Typical examples of Type I are polymeric matrices with nanoparticles embedded isodimensionally, for example, metallic particles (e.g., Au, Pt, and Ag) or ceramic particles (e.g., CdS, SiO2, ferrites, and graphite nanoparticles).

For nanocomposites of Type II, typical dispersed phases are made up of particles having an elongated structure, such as nanotubes and nanowhiskers. Examples are sepiolite, palygorskite, and carbon nanotubes (inorganic fillers) and cellulose and chitin nanowhiskers (organic fillers).

Type III nanocomposites contain fillers in the form of sheets. These sheets are one to a few nanometer thick and hundreds to thousands nanometer long. Examples are exfoliated graphite (EG), poly(muconic) acid crystals (organic fillers), and the well-known layered silicates, that is, clays and layered double hydroxides. This family of nanocomposites is better known as polymer-layered crystal nanocomposites.

Rendering to class (b)2), on the basis of the nature of the interface (interphase) between the polymer matrix and the dispersed phase, the nanocomposites are divided into two distinct groups [1]:

1) In Group I, the matrix embeds the dispersed phase. Only weak bonds (hydrogen, van der Waals bonds) are present at the interface and give cohesion to the whole structure

2) In Group II, the two phases are linked together through strong chemical bonds (covalent or ionic-covalent bonds). Obviously, within many Type II materials, organic and inorganic components can also interact via the same kind of weak bonds that define Type I.

Implying to class (c), polymer nanocomposites can be obtained by different approaches. Usually these are a top-down and a bottom-up approach. In a top-down process, the phase to be dispersed in the epoxy matrix is subjected to a mechanical or physicomechanical treatment to reduce its size. In other words, macro- or micro-phases are fragmented into nano. A main drawback of this process is that the addition of fillers at high concentration induces an undesirable viscosity increase, thus influencing the processability. Typically inorganic particles are milled, homogenized, dispersed in the epoxy precursors so that aggregation is lost and primary particles forming the aggregates are adequately separated. The key point is the achievement of a full deagglomeration of the nanoparticles within the epoxy matrix: even though the particles might be well dispersed in the prepolymer solution, aggregation might occur in the matrix, especially during the curing process. To achieve a molecular dispersion and to avoid macroscopic phase separation, the interactions between the organic and the inorganic domains need to be stronger than the agglomeration tendency of the inorganic component [2]. Surface functionalization of the filler is often necessary in order to increase their compatibility, optimize their dispersion, and assure the optimum performance.

Usually a pretreatment of the inorganic fillers is achieved by mixing slurry of the inorganic materials into the solution of a proper surface-modifying agent, obtaining a highly uniform surface treatment. In the case of oxides the coupling agent is an alkoxysilane: it can react with water to form silanol groups, which immediately form covalent bonds by dehydration and condensation on the inorganic particle surface (see Scheme 2.1). The alkoxysilane is selected so that R is compatible with the epoxy matrix and/or contains functional groups, which can participate in the epoxy matrix formation. The surface modification of the ceramic fillers using a coupling agent improves the interfacial adhesion and lowers the viscosity. The hybrids obtained can be Group I or Group II, depending on the nature of R.

Scheme 2.1 Surface modification of a ceramic filler.

In the case of clays, ion-exchange reactions with ammonium salts are conventionally employed to make the clays organocompatible: they are referred to as organoclays or organophilic clays.