Polyurethane Polymers: Composites and Nanocomposites

Sabu Thomas, Janusz Datta, Jozef T. Haponiuk, Arunima Reghunadhan, Jozef Haponiuk

• Publisher: Elsevier
• Year: 2017
• Print ISBN: 9780128040652
• eBook ISBN: 9780128041024
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Polyurethane Polymers: Composites and Nanocomposites concentrates on the composites and nanocomposites of polyurethane based materials. Polyurethane composites are a very important class of materials widely used in the biomedical and industrial field that offer numerous potential applications in many areas. This book discusses current research and identifies future research needs in the area. - Provides an elaborate coverage of the chemistry of polyurethane, its synthesis, and properties - Includes available characterization techniques - Relates types of polyurethanes to their potential properties - Discusses composites, nanocomposites options, and PU recycling

Chapter 1

PU Polymers, Their Composites, and Nanocomposites

State of the Art and New Challenges

Józef T. Haponiuk and Krzysztof Formela, Gdansk University of Technology, Gdansk, Poland

Abstract

The chapter is a review of the polyurethane(PU)-based composites and nanocomposites. The chapter details different components in the manufacturing of PU polymers, composites, and nanocomposites. The different types of glycols, isocyanates, and catalysts that are used for the synthesis of different polyurethanes are discussed. The applications and specialties of polyurethane composites and nanocomposites are also elaborated on.

Keywords

Polyurethane; composites; nanocomposites; elastomers

Contents

1.1 Polyols 3

1.2 Isocyanates 3

1.3 Chain Extenders 5

1.4 Catalysts 6

1.5 Synthesis of Polyurethane Elastomers 6

1.6 Perspectives for Polyurethanes, their Composites, and Nanocomposites 10

References 13

The global production of polyurethanes was estimated as 18 Mt for the year 2016, valued at around 53 billion euros, which gives it sixth place among all polymers, based on annual worldwide production [1]. Polyurethanes (PUR) are a unique class of polymers with versatile properties and numerous applications. Their properties can be broadly tailored due to the possibility of a wide selection of appropriate raw materials and available processing technologies. Their synthesis is based on reaction of isocyanate compounds and active hydrogen atoms bearing compounds, mostly with hydroxyl functionality. Isocyanates were first obtained in 1849 by Wurtz [2] by reacting a salt of alkylsulfuric acid with alkali metal cyanates, while the base of chemistry and technology of polyurethane materials was developed 1937 by Otto Bayer [3], who also 10 years later presented the first polyurethane foams, while the thermoplastic polyurethanes were first developed in 1952. Subsequent years have seen the development of production technologies of polyurethanes based on polyester or polyether polyols, which significantly reduced the final cost of polyurethanes.

The versatility of the applications of polyurethanes stems from the wide range of adjustable molecular and supramolecular structures of PUR in the composition, which, in most cases, is characterized by the presence in the polymer chain of rigid (hard) and flexible (soft) segments, which are thermodynamically immiscible and can separate into domains, which is a feature of block copolymers, where the hard segment domains act as physical crosslinking. As such, the degree of phase separation is dependent on the hard and soft segment chemical structure [4]. Phase separation is also influenced by the processing history and improves with increased mobility of the soft segment [5]. The glass transition temperature and melting point of the hard segments are well above the serving temperature [6]. An associated significant number of hydrogen bonds, occurring mainly in the hard segment domains, is possible to combine flexibility with considerable mechanical strength and wear resistance [7,8].

The melting point of the hard segments of PUR increases with their size and with the increasing degree of order (crystallinity). The more the content of hard segments in the polyurethane, the higher the Young's modulus, hardness, wear resistance, and maximal temperature of use. The more the polyurethane elastomer contains flexible segments, the greater the flexibility, elongation, resilience at low-temperature and a smaller hardness, elastic modulus, and resistance to scratching and abrasion. In the domains of rigid polyurethane segments physical crosslinking occurs, they meet at time functions similar to the reinforcing filler. In this way resulting properties of polyurethanes are affected by a number of factors including phase separation and phase mixing, chemical structure and mass proportions of hard and soft segments, and overall molecular weight. The phase-separated structure is essential for polyurethane elastomers. Rigid polyurethanes in the form of foams, coatings, or binders are usually single-phase products.

The polyurethanes are present on the market as foams, elastomers, plastics, adhesives, coating agents, fibers, or leather-like materials. PUR is still counted among the most innovative materials. It is used in areas such as construction, vehicles, furniture industry, production of sports equipment, medical technology, and many others.

Polyurethane reactions belong to the category of step growth or condensation polymerization, where bifunctional monomers react in a stepwise manner creating long chains of the reacting monomers. The chemistry of polyurethanes is well recognized and described in detail in many monographs [9–16].

The main raw materials for polyurethanes include polyols, isocyanates, and low-molecular-weight compounds, in most cases with hydroxyl or amine functionality, called chain extenders. The hard segments are formed in the reaction of isocyanates (IC) and chain extenders, both containing two or more reactive groups. The soft segments are formed by the reaction of isocyanates and polyols.

1.1 Polyols

A characteristic feature of polyols is flexible chains with at least two hydroxyl end groups, where the molecular weight ranges from 1000–6000 g mol⁻¹. The chemical structure of polyols has a significant impact on the flexibility and low-temperature properties of polyurethanes. Polyurethane soft segments consist of a methylene ether or ester. The glass transition temperature of soft segments is typically less than –30°C. The introduction of an aromatic ring affects the stiffness of the structure, increasing the hardness and mechanical properties of polyurethanes. The introduction of ether groups causes the opposite effect: lower melting point, decreased mechanical strength, and increased flexibility. A similar effect is achieved by reducing the symmetry of the molecules, for example, by incorporating chain branching. The type of polyol and its molecular weight play a large role in the resulting physical properties, ranging from soft, low-modulus, and flexible polyurethanes to rigid products. Polyester polyols are often produced by polycondensation of a dicarboxylic acid (adipic, sebacic) with glycols (mostly ethylene, 1,2-propylene, 1,4-butylene, or diethylene) [17]. Superior properties are known for polyurethanes with polytetramethylene glycol (PTMG) or a polycarbonate soft segment [18–21]

1.2 Isocyanates

Isocyanates are characterized by a specific distribution of electron density in the isocyanate group (NCO), which makes the carbon highly susceptible to attack by nucleophiles, and oxygen and nitrogen by electrophiles. Therefore, isocyanates react easily with almost all the compounds containing active hydrogen, such as alcohols, amines, carboxylic acids, and water [9]. The most common isocyanates are:

• 4,4-diphenylmethane diisocyanate (MDI) as a pure product or a mixture of polymeric MDI (PMDI) obtained in the process of obtaining pure MDI;

• toluene diisocyanate (TDI) obtained usually as a mixture of 2,4-TDI and 2,6-TDI 65/35 or 80/20 composition;

• 1,5-naphthalene diisocyanate (NDI);

• isophorone diisocyanate (IPDI);

• 1,6-hexamethylene diisocyanate (HDI);

• 4,4'-dicyclohexylmethane diisocyanate (HDMI);

• p-phenylene diisocyanate (PPDI);

• and various isomers and/or derivatives thereof.

Polyurethanes obtained from aromatic isocyanates show better mechanical properties compared to those obtained with aliphatic isocyanates. Aliphatic diisocyanates are more often used for polyurethanes applied in medicine and food packaging industries because they do not contain carcinogenic aromatic moieties and are not yellowing under exposure to sunlight. Reactions of the isocyanate groups are shown in the Fig. 1.1. Urethane or urea groups are formed by the reaction of an isocyanate group with a hydroxyl or amino group, respectively. At elevated temperatures, isocyanate groups can react with urethane groups, which results in branched allophanate moieties. This slow reaction is catalyzed by organic salts of Pb, Co, Zn, and Sn. It is also a possible reaction of NCO groups with water, by which the CO₂ is released. The reaction proceeds via the carbamic acid derivative and is catalyzed by tertiary amines. It is the primary reaction of polyurethane foaming. The resulting amino group can react with an excess of NCO groups to form urea groups. This reaction is much faster than with hydroxyl groups and does not require the use of catalysts. In the reaction of NCO groups with urea a moiety branching biuret group is formed. The reaction is catalyzed by triethylamine and organic compounds of tin or zinc. It is also possible for the dimerization or trimerization of isocyanate groups themselves. Trimerization to isocynuric ring is applied at synthesis of thermally stable polyisocyanurate foams [9].

Figure 1.1 Reactions of the urethane group.

1.3 Chain Extenders

Extending agents are low-molecular-weight compounds which increase the size of hard segments, the density of hydrogen bonding and molecular weight of the polyurethane. The impact of chain extenders on the properties of polyurethanes is very important, although their proportion by weight is usually only a small part of the polymer [22]

Their reaction with isocyanate takes place in a short time period and causes a quick extension of the polymer chains, which is seen by the sharp increase in viscosity of the reacting system.

Chain extenders include essentially three main groups:

• Diols and their derivatives;

• Diamines and derivatives thereof;

• Other extenders containing groups with active hydrogen.

Polyurethanes extended with diols have lower hardness and strength than polyureas obtained with the participation of the amine extension. The best physical properties show those segmented polyurethanes in which the rigid segments are symmetrical and of large volume. Higher hard segment content increases hardness and modulus and decreases elongation at breakage of a polyurethane elastomer.

1.4 Catalysts

Catalysts used at the synthesis of polyurethanes affect the overa...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• List of Figures
• List of Tables
• List of Schemes
• Chapter 1. PU Polymers, Their Composites, and Nanocomposites: State of the Art and New Challenges
• Chapter 2. Micro- and Nanomechanics of PU Polymer-Based Composites and Nanocomposites
• Chapter 3. Engineering of Interface in Nanocomposites Based on PU Polymers
• Chapter 4. Nanocomposites of PU Polymers Filled With Spherical Fillers
• Chapter 5. Polyurethane Nanocomposites of Layered Silicates
• Chapter 6. Nanocomposites of Polyurethane Filled with CNTs
• Chapter 7. Composites and Nanocomposites of PU Polymers Filled With POSS Fillers
• Chapter 8. Composites and Nanocomposites of PU Polymers Filled with Natural Fibers and Their Nanofibers
• Chapter 9. Polyurethane Nanocomposite Foams: Correlation Between Nanofillers, Porous Morphology, and Structural and Functional Properties
• Chapter 10. Nanocomposites of PU Polymers with Nano Chitin and Nano Starch
• Chapter 11. Self-Healing Properties of PU and PU Nanocomposites
• Chapter 12. Conducting Polyurethane Composites
• Chapter 13. Nonlinear Viscoelastic Properties of Polyurethane Nanocomposites
• Chapter 14. Vegetable Oil-Derived Polyurethane Composites with Graphite as Electrode Materials for Electroanalysis
• Chapter 15. Modeling and Simulation in PU-Based Composites and Nanocomposites
• Chapter 16. Polyurethane Composites and Nanocomposites for Biomedical Applications
• Chapter 17. Flame Retardancy of Composites and Nanocomposites Based on PU Polymers
• Chapter 18. Polyurethane-Based Biocomposites
• Chapter 19. Aging Behavior of Composite- and Nanocomposite-Based Polyurethane
• Chapter 20. Applications of Polyurethane Based Composites and Nanocomposites
• Index