Polymer Engineering

Bartosz Tylkowski, Karolina Wieszczycka, Renata Jastrzab, Bartosz Tylkowski, Karolina Wieszczycka, Renata Jastrzab

• Publisher: De Gruyter
• Year: 2017
• Print ISBN:
  9783110468281
• eBook ISBN:
  9783110468342
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Polymer Engineering focuses on the preparation and application of polymers in several hot topics such as artificial photosynthesis, water purification by membrane technologies, and biodiesel production from wastewater plants. The authors not only describe the latest developments in polymer science, but also support these experimental results by computational chemistry and modelling studies.

Cristina Acebo, Xavier Ramis and Angels Serra

1Improved epoxy thermosets by the use of poly(ethyleneimine) derivatives

Abstract: Epoxy resins are commonly used as thermosetting materials due to their excellent mechanical properties, high adhesion to many substrates and good heat and chemical resistances. This type of thermosets is intensively used in a wide range of fields, where they act as fiber-reinforced materials, general-purpose adhesives, high-performance coatings and encapsulating materials. These materials are formed by the chemical reaction of multifunctional epoxy monomers forming a polymer network produced through an irreversible way. In this article the improvement of the characteristics of epoxy thermosets using different hyperbranched poly(ethyleneimine) (PEI) derivatives will be explained.

Keywords: adhesives, epoxy, dendrimers

1.1General introduction

Epoxy resins are commonly used as thermosetting materials due to their excellent mechanical properties, high adhesion to many substrates and good heat and chemical resistances. This type of thermosets is intensively used in a wide range of fields, where they act as fiber-reinforced materials, general-purpose adhesives, high-performance coatings and encapsulating materials. These materials are formed by the chemical reaction of multifunctional epoxy monomers forming a polymer network produced through an irreversible way [1]. In this article the improvement of the characteristics of epoxy thermosets using different hyperbranched poly(ethyleneimine) (PEI) derivatives will be explained.

1.2General concepts of epoxy thermosets

The term epoxy or epoxide refers to compounds characterized by the presence of an oxirane or epoxy ring, a three-member ring containing an oxygen atom that is bonded with two carbon atoms as shown in Figure 1.1.

From the structural point of view of chemistry, epoxy resins are monomers or oligomers containing two or more epoxy groups in their structure. P. Castan [2] developed the first system based on the well-known diglycidyl ether of bisphenol A (DGEBA). The commercialization of this resin dates back to 1940 [3]. It is obtained by the reaction of bisphenol A with epichlorohydrin in the presence of a strong base such as NaOH. Depending on the ratio between reactants, the resulting molecular weight can be tuned in order to have different resins being possible to obtain liquid, waxy or solid DGEBA resins [4]. The reaction, still used nowadays, is depicted in Figure 1.2.

Figure 1.1: Chemical structure of epoxy ring.

Figure 1.2: Synthetic scheme for the preparation of Bisphenol A epoxy resins.

There are other types of resins with different structures that lead to materials with different characteristics. Some of the most typical resins are collected in Figure 1.3. The non-epoxy part of the molecule can have an aliphatic, cycloaliphatic or aromatic structure and the functionality, related to the number of epoxide groups, can also be varied.

Figure 1.3: Structures of several commercially available epoxy resins.

Epoxy resins are capable of reacting with different active compounds known as curing agents (with or without catalyst) or with themselves (via an initiator) to form solid, crosslinked materials. This transformation is generally referred to as curing.

From a fundamental point of view, thermosetting epoxy polymers may be defined as polymer networks obtained by a chemical reaction of monomers, which contain two or more epoxy groups per molecule (a functionality equal to or higher than 2) [5]. The functionality of an epoxy monomer is defined by the number of arms (bonding sites) that participates in the formation of the polymer network. The functionality of the epoxy monomers depends on the curing system used and will be discussed below, but a necessary condition for the formation of a network is that at least one of the monomers involved in the reaction has a functionality higher than two, since the global functionality of the system to reach a network structure is a minimum of 4.

1.3Curing agents

Crosslinked epoxy polymers are obtained by the reaction of epoxy monomers with curing agents (co-monomers or initiators). Epoxy polymers can be produced by step or chain growth polymerizations or, eventually, by a combination of both mechanisms [5].

Step growth polymerization (polycondensation) proceeds via a step-by-step succession of elementary reactions between reactive sites. Each independent step causes the disappearance of two co-reacting sites and creates a new covalent bond between a pair of functional groups. In this case curing agents like amines, alcohols, anhydrides, isocyanates or acids have been used in stoichiometric ratio [6].

Chain growth polymerization (ring opening) is characterized by the occurrence of initiation, propagation, chain transfer and termination steps. In the case of epoxides, the initiation step produces an ion (either an anion or a cation) that is called the active center of the polymerization. The ion may be generated by thermal treatment or by an adequate source of irradiation. Once active centers are generated, they produce primary chains by the consecutive addition of monomers through the propagation step of the reaction.

Polycondensation curing mechanisms require an accurate knowledge of the stoichiometry of the system. Among them, the most used curing agents for epoxy resins are primary and secondary amines. In this system, DGEBA is bifunctional and a requirement for the amines is that they must be multifunctional (more than two reacting groups). Considering that primary amines account for a functionality of two and secondary for a functionality of one, usually primary diamines are used. The reaction of epoxy with amines is depicted in Figure 1.4[6].

Primary and secondary aliphatic amines react rapidly with epoxy groups at low temperature to form three-dimensional crosslinked structures. However, they can also be cured at higher temperatures to provide a more densely crosslinked structure with better mechanical properties, elevated-temperature performance and chemical resistance. Other amines, such as aromatic or cycloaliphatic, are less reactive and generally require higher curing temperatures.

Figure 1.4: Reaction of a primary amine with epoxides.

Figure 1.5: Chemical structure of anhydrides used as curing agents of epoxy resins.

After amines, acid anhydrides are the second most used group of curing agents. Among the most common anhydrides, phthalic anhydride (P...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contents
• List of contributing authors
• 1 Improved epoxy thermosets by the use of poly(ethyleneimine) derivatives
• 2 Developments in the use of rare earth metal complexes as efficient catalysts for ring-opening polymerization of cyclic esters used in biomedical applications
• 3 BioArtificial polymers
• 4 Recent advances in “bioartificial polymeric materials” based nanovectors
• 5 Polymer additives
• 6 Technological solutions for encapsulation
• 7 Natural and synthetic polymers in fabric and home care applications
• 8 Polymers in separaion processes
• 9 Polymer application for separation/filtration of biological active compounds
• 10 Polymers application in proton exchange membranes for fuel cells (PEMFCs)
• 11 Membrane contactors for CO2 capture processes – critical review
• 12 Modeling and simulation of membrane process
• 13 Applications of silver nanoparticles stabilized and/or immobilized by polymer matrixes
• 14 Spectroscopic properties of polymer composites
• 15 Combining catalytical and biological processes to transform cellulose into high value-added products
• Index