Editorial With the publication of Vol. 15 the editors and the publisher would like to take this opportunity to thank authors and readers for their collaboration and their efforts to meet the scientific requirements of this series. We appreciate the concern of our authors for the progress of "Advances in Polymer Science" and we also welcome the advice and critical comments of our readers. With the publication of Vol. 15 we would also like to refer to a editorial policy: articles publishes review critical incited, series this of new developments ni all areas of polymer science ni English (authors may naturally also include workes of their own). The responsible editor, that means the editor who has invited the author, discusses the scope of the review with the author on the basis of a tentative outline which the author is asked to provide. The author and editor are responsible for the scientific quality of the contribution. Manuscripts must be submitted in content language and form satisfactory to Springer-Verlag. Figures and formulas should be reproducible. To meet the convenience of our readers, the publisher will include "volume index" which characterizes the content of the volume. The editors and the publisher will make all efforts to publish the manuscripts as rapidly as possible, i.e., at the maximum six months after the submission of an accepted paper. Contributions from diverse areas of polymer science must occasionally be united in one volume. In such cases a "volume index" cannot meet all expectations, but will nevertheless provide more information than a mere volume number. Starting with Vol. 51, each volume will contain a subject index. Editors Publisher Preface This volume of ADVANCES IN POLYMER SCIENCE contains the first part of a series of critical reviews on selected topics concerning epoxy resins and composites. The last decade has been marked by an intense development of applications of epoxy resins in traditional and newly developing ar, as such as coatings, ad- hesives, civil engineering or electronics and high-performance com- posites. The growing interest in applications and requirements of high quality and performance has provoked a new wave in funda- mental research in the area of resin synthesis, curing systems, properties of cured products and m, thods of their characteriza- tion. The collection of reviews to be published in ADVANCES IN POLYMER SCIENCE is devoted just to these fundamental problems. The epoxy resin-curing agent formulations are typical thermosetting systems of a rather high degree of complexity. Therefore, some of the formation-structure-properties relationships are still of empirical or semiempirical nature. The main objective of this series of articles is to demonstrate the progress in research towards the understanding of these relationships in terms of current theories of macromolecular systems. Because of the complexity of the problems discussed, the theoretical approaches and interpretation of results presented by various authors and schools may be somewhat different. It may be hoped, however, that a confrontation of ideas may positively contribute to the knowledge about this important class of poly- meric materials. In view of the wide range of this area, it was impossible to publish all contributions in successive volum s of ADVANCES IN POLYMER SCIENCE. Part I is published in this Vol. 72; Part II will appear in Vol. 75. Part III and Part IV will follow in the beginning of 1986. The reader may appreciate receiving a list of all contributions to the series EPOXY RESINS AND COMPOSITES to appear in ADVANCES IN POLYMER SCIENCE: M. T. Aronhime and J. K. Gillham (Princeton University, Prince- ton, N.J., USA) The Time-Temperature-Transformation (TTT) Cure Diagram of Thermosetting Polymeric Systems X ecaferP A. Apicella and L. Nicolais (University of Naples, Naples, Italy) Effect of Water on the Properties of Epoxy Matrix and Compo- sites (Part I, Vol. 72) J. M. Barton (Royal Aircraft Establishment, Farnborough, UK) The Application of Differential Scanning Calorimetry (DSC) to the Study of Epoxy Resins Curing Reactions (Part I, Vol. )27 W. Burchard (University of Freiburg, Freiburg .i Br., FRG) Branching in Epoxy Resins Based on Diglycidyl Ethers of Bis- phenol A L. T. Drzal (Michigan State University, East Lansing, MI, USA) The Interphase in Epoxy Composites (Part II, Vol. 75) K. Du~ek (Institute of Macromolecular Chemistry', Czechoslovak Academy of Sciences, Prague, Czechoslovakia) Network Formation in Curing of Epoxy Resins M. Fedtke (Technical University, Merseburg, GDR) Elucidation of the Mechanism of Epoxy Curing by Model Reactions A. Gupta (Jet Propulsion Laboratory, Caltech, Pasadena, CA, USA) Mechanism and Kinetics of the Cure Process in Tetragty- cidytmethane Dianiline-Diaminodiphenyl Sulphone Thermoset System T. Kamon and H. Furukawa (The Kyoto Municipal Research Institute of Industry, Kyoto, Japan) Curing Mechanism and Mechanical Properties of Cured Epoxy Resins J. L. Kardos and M. P. Dudukovi6 (Washington University, St. Louis. MO, USA) Void Growth and Transport During Processing of Thermosetting Matrix Composites A. J. Kinloch (Imperial College, London, UK) Mechanics and Mechanisms of Fracture of Thermosetting Epoxy Polymers E. S. W. Kong (Hewlett-Paekard Laboratories, Palo Alto, CA, USA) Physical Aging in Epoxy Matrices and Composites J. D. LeMay and F. N. Kelley (University of Akron, Akron, OH, USA) Structure and Ultimate Properties of Epoxy Resins F. Lohse, and H. Zweifel (Ciba-Geigy, Basle, Switzerland) Photocrosslinking of Epoxy Resins J. A. Manson, R. W. Hertzberg, G. Attalla, D. Shah, J. Hwang and J, Turkanis (Lehigh University, Bethlehem, PA, USA) Fatigue in Neat and Rubber-Modified Epoxies E. Mertzel and J. L. Koenig (Case Western Reserve University, Cleveland, OH, USA) ec~lerP 1X Application of FT-IR and NMR to Epoxy Resins (Part II, Vol. )57 R. J. Morgan (Lawrence Livermore National Laboratory~ Liver- more, CA, USA) Structure-Properties Relations of Epoxies Used as Composite Matrices (Part I, Vol. )27 .E F. Oleinik (Institute of Chemical Physics, Academy of Sciences of USSR, Moscow, USSR) Structure and Properties of Epoxy-Aromatic Amine Networks in the Glassy State .B A. Rozenberg (Institute of Chemical Physics, Academy of Scien- ces of USSR, Moscow, USSR) Kinetics, Thermodynamics and Mechanism of Reactions of Epoxy Oligomers with Amines (Part II, Vol. )57 .S D. Senturia and N. F. Sheppard (Massachusetts Institute of Technology, Cambridge, MA, USA) Dielectric Analysis of EpoxCyu re R. G. Schmidt and J. P. Bell (University of Connecticut, Storrs, CT, USA) Epoxy Adhesion to Metals (Part II, Vol. )57 .E M. Yorkgitis, N. .S Eiss, Jr., .C Tran, G. L. Wilkes and J. E. McGrath (Virginia Polytechnic Institute, Blacksburg, ,AV USA) Siloxane ModifiedE poxy Resins (Part I, Vol. )27 The editor wishes to express his gratitude to all contributors for their cooperation. Prague, August 5891 Karel Du~ek Editor Table of Contents Structure-Property Relations of Epoxies Used as Composite Matrices R, J. Morgan . . . . . . . . . . . . . . . . . . . . Mechanics and Mechanisms of Fracture of Thermosetting Epoxy Polymers A. J. Kinloch .... : . . . . . . . . . . . . . . . 45 Effect of Water on the Properties of Epoxy Matrix and Composite A. Apicella, L. Nicolais . . . . . . . . . . . . . . . . 69 Siloxane-Modified Epoxy Resins E. M. Yorkgitis, N. .S Eiss, Jr., C. Tran, G. L. Wilkes, J. E. McGrath . . . . . . . . . . . . . . . . . . . . 79 The Application of Differential Scanning Calorimetry (DSC) to the Study of Epoxy Resins Curing Reactions J. M. Barton . . . . . . . . . . . . . . . . . . . . . 111 Author Index Volumes 1-72 . . . . . . . . . . . . . . 551 Subject Index . . . . . . . . . . . . . . . . . . . . . 165 Structure-Property Relations of Epoxies Used as Composite Matrices* R. J. Morgan Lawrence Livermore National Laboratory, L-338 University of California, P.O. Box 808, Livermore, California, 94550 U.S.A. The structure-deformation~failure process-mechanical property relations of epoxies used as matrices in high performance fibrous composites are presented. Such composites are fabricated either from carbon fiber-epoxy prepregs or by filament winding. Thep arameters that affect the processing, cure reactions and the resultant chemical and physical structure of the epoxies are discussed. The deformation and failure processes of these glasses are described, The structural parameters that control the deformation and failure processes, mechanical the response and aging of epoxies are addressed and means of improving their processing and performance are described. 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Processing and C~al Structure of Epoxies Used in Filament Wound Composites . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . 3 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Cure Reactions . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Improved Filament Winding Epoxies . . . . . . . . . . . . . . . . 4 3 Processing and Chemical Structure of TGDDM-DDS Epoxies Used in Composites Fabricated from Prepregs . . . . . . . . . . . . . . . . . 6 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 Starting Materials-Impurities . . . . . . . . . . . . . . . . . . 7 3.3 NMR Characterization of BF3 :Amine Catalysts . . . . . . . . . . 7 3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . 7 3.3.2 Chemical Composition . . . . . . . . . . . . . . ..... 8 3.3.3 Thermal Stability . . . . . . . . . . . . . . . . . . . . . 9 3.3.4 Hydrolytic Stability . . . . . . . . . . . . . . . . . . . . 11 3.3.5 Interaction of BF3 :NH2C2Hs with DDS and TGDDM ..... 11 3.3.6 Catalyst Composition in Prepregs . . . . . . . . . . . . . . 12 3.3.7 Catalytic Species and Activity . . . . . . . . . . . . . . . . 14 4 DSC Studies of the Cure Reactions . . . . . . . . . . . . . . . . . . 15 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2 Prepreg Mixtures and Their Components . . . . . . . . . . . . . 15 * Thwiosr k performed undert he auspices of the U. S. Department of Energy by the Lawrence Liver- more National Laboratory under Contract W-7405-Eng-48 2 R.J. Morgan 4.3 Environmental Sensitivity of BF : 3 NH2C2Hs Catalyst . . . . . . . . 18 4.4 Commercial Prepregs . . . . . . . . . . . . . . . . . . . . . . 18 5 FTIR Studies of the Cure Reactions . . . . . . . . . . . . . . . . . . 18 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2 ~FGDDM Epoxide Homopolymerization . . . . . . . . . . . . . . 19 5.3 TGDDM-DDS Cure Reactions . . . . . . . . . . . . . . . . . . 22 5.4 Rates and Chemistry of Cure Reactions . . . . . . . . . . . . . . 28 5.5 Prepreg Processing Viscosity . . . . . . . . . . . . . . . . . . . 30 6 Physical Structure . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.2 Macroscopic Inhomogeneities . . . . . . . . . . . . . . . . . . 31 6.3 Free Volume . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.4 Network Structure . . . . . . . . . . . . . . . . . . . . . . . 32 7 Deformation and Failure Modes . . . . . . . . . . . . . . . . . . . 35 8 Structural Parameters that Control Mechanical Properties . . . . . . . . 38 9 Service Environment Aging . . . . . . . . . . . . . . . . . . . . . 39 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Relations Structure-Property of Matrices Composite as Used Epoxies 3 1 Introduction The increasing use of high-performance fibrous composites in critical structural applications has led to a need to predict the lifetimes of these materials in service environments. To predict the durability of a composite in service environment requires a basic understanding of (1) the microscopic deformation and failure processes of the composite; (2) the significance of the fiber, epoxy matrix and fiber-matrix inter- facial region in composite performance; and (3) the relations between the structure, deformation and failure processes and mechanical response of the fiber, epoxy matrix and their interface and how such relations are modified by environmental factors. In this paper we review our studies on the structure-property relations of epoxy matrices used in high-performance fibrous composites that are fabricated either from C fiber-epoxy prepregs or by filament winding. We consider the parameters that affect the processing, cure reactions and resultant chemical and physical structure of these crosslinked glasses. The structural parameters that control the deformation and failure modes, mechanical response and service environment aging of these epoxies are addressed. We, also, discuss epoxy systems that exhibit superior processing, thermal and mechanical properties relative to those presently utilized as composite matrices. 2 Processing and Chemical Structure of Epoxies Used in Filament Wound Composites 2.1 Introduction Epoxy resins utilized in forming filament-wound composites must possess low viscosities (q's) and long gel-times at 23 °C. To minimize unreacted starting materials in the finally cured composite requires the chemical cure reactions of the epoxy system must be simple. Furthermore, the number of chemical starting components in the resin must be small to minimize mixing problems that would result in variable thermal and mechanical properties. The toxicity of the resin chemical starting materials must be low. Also, the epoxy system must attain full cure at relatively low post-cure temperatures, < 150 °C, to minimize the development of fabrication strains in the composite. Amine-cured diglycidyl ether of bisphenol-A (DGEBA) epoxies are the principal matrices used in filament wound composites. Pure DGEBA, DER332 (Dow) 1 epoxide cured with an aliphatic polyethertriamine, Jeffamine T403 (Jefferson) is a commonly used epoxy for filament wound Kevlar 49 composites. The chemical structures of the amine and epoxide monomers are shown in Fig. .1 We have studied the structure- property relations of the DGEBA-T403 epoxy in some detail )1 company to a 1 Reference or approval not imply does name product or recommendation of eht pro- duct the by University of California or the U.S. Department of Energy to the noisulcxe of others that suitable be may 4 R,J. Morgan O CH 3 O / \ t---, I ~ / \ CH2--CH -- ca 2- O--O~--C-(~--O--CH2--CH--CH 2 CH 3 Diglycidyl ether of bisphenol A (DOW DER )233 H2C -[-O CH 2 CH (CH3)~ x NH 2 / CH3CH2~CH2--[-O CH 2 CH 2 (CH3)~--NH x + y + z _~ 5.3 ! H2C 40 CH 2 CH (CH3)~z NH 2 Polyether triamine nosreffeJ( T403) Fig. 1. Chemical structure of DGEBA epoxide and T403 polyethertriamine curing agent 2.2 Cure Reactions The amine-cured DGEBA epoxies utilized as matrices for filament wound composites generally form exclusively from epoxide-amine addition reactions (1). + 2 R,CI-I~CH R2NH2 ~ R,CH(OH)CH2NHR 2 )1( O The nature of the cure reactions in these epoxies can be confirmed by monitoring the epoxide consumption via near infra-red spectroscopy for a series of epoxide-amine mixtures containing a range of amine contents. A plot of % epoxide consumption vs. amine concentration for DGEBA-T403 epoxies is illustrated in Fig. 2. This plot confirms that the DGEBA-T403 epoxy system forms exclusively from epoxide-amine addition reactions, because (i) % 100 epoxide consumption is attained at the stoichio- metric amine concentration associated with exclusive epoxide-amine addition cure reactions and (ii) extrapolation of this plot to zero amine content indicates there is no epoxide consumption i.e. there are no epoxide homopolymerization reactions. Characterization of the epoxy cure reactions ensures that a composite can be fabricated and the epoxy is fully cured, assuming that the epoxide and amine starting components are initially homogeneously mixed. 2.3 Improved Filament Winding Epoxies The toughness andmechanical performance of a filament wound composite component is enhanced by crack deflection mechanisms and/or molecular flow occurring in the Structure-Property Relations of Epoxies Used as Composite Matrices 5 120 I I L 100 - o 80- E .I ° o 6o- 1 / x / ~ 411- // / / / 20-- // / / o ,e / 1 t I t q t I 0 10 20 30 40 50 60 70 80 T403 concentration (phr) Fig. 2. Epoxide consumption vs. T403 amine concentration for DGEBA-T403 epoxies cured at 85 °C, 24 h epoxy matrix. The inhibition of crack propagation through a bundle of fibers in a composite can occur by deflection of the crack parallel to the fiber axis by either propagation along the fiber matrix interface and/or through the fiber itself. A poor fiber-matrix interfacial bond and/or microscopic fiber failure by splitting will both enhance these crack deflection toughening mechanisms. The composite performance can also be enhanced under load by molecular flow occurring in the epoxy matrix. Molecular flow is enhanced as gT is approached because the glassy-state free volume is increased. However, the epoxy matrix cannot be too soft, otherwise the composite will readily buckle in compression. In the case of filament-wound Kevlar 49-epoxy composites the poor fiber-interfacial strength, the microscopic splitting of the fibers and matrix ductility all enhance composite mechanical performance ,2 .)3 However, for C-fiber-epoxy filament wound composites the fiber-matrix interface is generally stronger than for Kevlar 49 composites and the C-fiber fails without longitudinal fiber splitting. Hence, for C-fiber epoxy composites the matrix is the principal compo- nent that affects composite toughness, and this matrix must be tough through a wide temperature range and possess a gT > 021 °C. This requires that epoxy matrices for filament-wound C-fiber structures are (i) processible at 23 °C, (ii) fully reacted upon post-curing < 051 °C, (iii) simple chemical and physical systems with limited toxicity and (iv) tough from 23-125 °C. To attain the requirements of an epoxy matrix utilized in filament-wound C-fiber- epoxy composites we have considered the characteristics required of the amine curing agent molecule. To ensure long gel times at 23 °C requires that the primary amine- epoxide (P.A.-E) reaction rate is considerably greater than the rate of the secondary amine-epoxide (S. A.-E) reaction, and that the S.A. reaction does not occur at low temperatures. Furthermore, to attain low 23 °C rl'S and low post-cure temperatures