Polyester and Alkyd Resins E-Book

Ulrich Poth

• Herausgeber: Vincentz Network
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Veröffentlichungsjahr: 2020

Beschreibung

Polyester and alkyd resins belong to the most diverse and important material classes of paint chemistry and their usage as binders has been established for a long time. This standard work goes into detail on the composition, structure and properties of these important binder groups and subjects previous findings in that field to a critical review. It shows different precise calculation approaches in modern coatings development, ways to formulate polyester and alkyd resins in experimental designs and how to vary them systematically. A practice- and future-oriented reference book that should not be missing in any laboratory!

Leseprobe

Ulrich Poth

Polyester and Alkyd Resins

Technical Basics and Applications

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Ulrich Poth

Polyester and Alkyd Resins: Technical Basics and Applications

Hanover: Vincentz Network, 2020

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European Coatings Library

Ulrich Poth

Polyester and Alkyd Resins

Technical Basics and Applications

Foreword

Polyesters and alkyd resins have been the most varied class of binders for coating applications for quite some time. They are the subject of many reports and publications. So, is another one necessary? Perusal of the various publications gave the impression that the calculation proposals provided there were not precise enough to allow the development of systematic trial plans. Therefore, we developed our own calculation procedures. These formed the basis for the development of numerous polyesters and alkyds, some of which became large-scale products which were produced for long periods or are still in production. In addition, there was a desire to commit all my experience to paper. However, the next question was: If polyesters and alkyd resins are well established on the coatings market would there even be a need to develop new products using the latest calculation principles? The answer is: We believe that developments are still needed. Finally, friends and colleagues asked me to write down the knowledge that I have gained over the years. The determining factor was that even current publications state that the feasibility limit for polyesters and alkyd resins lies somewhere between the definitions of Carothers [26] and Flory [32]. As the two definitions diverge significantly, there is a conception that the formulation of polyesters and alkyd resins still follows empirical approaches.

Chapter 3 presents theoretical approaches which not only contain evaluations of molecular weights, and functionalities of resins and conclusions about molecular weight distributions and feasibility limits, but in addition can be used to formulate systematic trial plans. The goal is to explain the basis for the underlying systematic structure/property relationship. The calculations have been designed to cover all grades of polyesters and alkyd resins, initially without regard for the influence of individual building blocks. There then follows a description of the influences exerted by building blocks (Chapter 4). Descriptions of the different grades of polyesters and alkyd resins are provided. There are some binders which would not spontaneously be deemed polyesters. Therefore, besides the primary categorisation of product classes by chemical compositions, the products are also described from the application point of view.

To clearly reveal the structure/property relationship, formulation examples are provided in the form of tables, including molar composition, composition by weight, and characteristic values resulting from the calculation methods. The examples are based on patent information and model binders from basic trial programmes which did not materialise as large-scale products. While patent examples (and also the commercial products) can have rather complex compositions, the model examples are simpler, and therefore go a long way towards representing the principles behind the various product classes and could well prove to be the optimal starting point for further developments.

Some typical examples of commercial products and their suppliers are listed. The application areas mentioned are derived from the resins’ different properties. For more information on the binders, the reader should refer to the data sheets issued by the resin manufacturers.

The third question is: What is the future of polyesters and alkyd resins? Alkyd resins now play a lesser role than in the past. The main reason is that alkyd resins are relatively more susceptible to saponification and therefore are not the first choice for water-borne coating formulations, which are mandated by environmental considerations. On the other hand, alkyd resins for oxidative-cure contain renewable raw materials and curing by atmospheric oxygen is essentially non-hazardous. Unfortunately, the curing process is too slow for it to be implemented in industrial coating processes. However, in the future, a concept may be found which could supply a missing property.

Saturated polyesters are enjoying substantial market growth, even in the case of solvent-borne systems. However, saturated polyesters are important for powder coatings and for the UV-curable coatings segment. Saturated polyesters will also play an important role in the future for high-solids (including 100 % systems) and water-borne coatings.

This book is mainly directed at persons working in resin development and production, and in the development and manufacture of resin raw materials – not just experts but also students and newcomers. However, it is also bound to appeal to anybody else in the coatings industry who deals with the application of polyesters and alkyd resins.

Ulrich Poth

Editor's note:

Ulrich Poth passed away in July 2018 and could no longer complete the work on the English manuscript of this present book. Dr. i.R. Toine Biemans thankfully fulfilled the work on the basis of Mr Poth's original German publication.Ulrich Poth

ISBN 3-86630-735-7

ISBN 978-3-86630-735-3

Foreword

1 Definitions

2 History of polyester resins

3 Formation and structure of polyesters and alkyd resins

3.1 Reactions that produce polyesters

3.1.1 Fundamental reactions

3.1.2 Structure of polyesters

3.2 Determination of and limitations on the size of polyester molecules

3.2.1 Dependencies regarding molecular weight

3.2.2 Derivation of gel-point equations

3.3 Methods of calculating average molecular weights of polyesters

3.3.1 Factors that influence molecular weights

3.3.2 Influence of molar ratios of polyol and polycarboxylic acid molecules on molecular weight

3.3.3 Calculating the influence of the degree of condensation on the molecular weight

3.3.4 Sample calculations of molecular weights and related characteristics

3.4 Molecular weight distribution of polyesters

3.4.1 Definitions of average molecular weights

3.4.2 GPC analysis

3.4.3 Influences on the molecular weight distribution

3.5 Formation and structure of alkyd resins

3.5.1 Special aspects of the preparation of alkyd resins

3.5.2 Calculation of molecular weights of alkyd resins

3.5.3 Molecular weight distribution of alkyd resins

3.6 Functionality of polyesters and alkyd resins

3.7 Exceptions and their influence on the molecular weight distribution

3.8 Explanation of symbols in definitions and equations

3.9 Index of equations

4 Influence of building blocks on properties of polyesters and alkyd resins

4.1 Selection criteria for the different building blocks

4.2 Influences on solubility and compatibility

4.3 Influences on film properties

4.4 Classification of polyesters and alkyd resins

5 Saturated polyesters

5.1 High-molecular weight, saturated polyesters

5.2 Polyesters as plasticisers

5.3 Polyester hard resins

5.4 Polyester segments for other resins

5.4.1 Polyester segments for polyurethane elastomers

5.4.2 Polyester polyurethanes for crosslinking in the presence of atmospheric moisture

5.4.3 Polyester acrylates

5.5 Saturated polyesters containing OH groups for crosslinkable, solvent-borne coatings

5.5.1 Structure and composition of OH polyesters for solvent-borne coatings

5.5.2 OH polyesters for crosslinking with amino resins

5.5.3 OH polyesters for crosslinking by polyisocyanates

5.5.4 OH polyesters for crosslinking with blocked polyisocyanates

5.5.5 OH polyesters for high-solid coatings

5.6 Polyesters for water-borne systems

5.7 Polyesters for powder coatings

5.4.1 Thermoplastic polyesters

5.7.2 COOH polyesters

5.7.3 OH polyesters

5.8 Self-crosslinkable polyesters

5.9 Silicone polyesters

6 Unsaturated polyesters

6.1 Crosslinking of unsaturated polyesters

6.2 Unmodified unsaturated polyesters – “wax polyesters”

6.3 “Gloss polyesters”

6.4 UV crosslinking of unsaturated polyesters

6.5 Other unsaturated polyesters

7 Alkyd resins

7.1 Classification of alkyd resins

7.2 Alkyd resins for oxidative crosslinking

7.2.1 Crosslinking reactions

7.2.2 Long-oil alkyd resins for oxidative crosslinking

7.2.3 Medium- and short-oil alkyd resins for oxidative crosslinking

7.2.4 Anti-corrosive alkyd resins

7.2.5 High-solid alkyds for oxidative crosslinking

7.2.6 Styrenated and acrylated alkyd resins

7.2.7 Urethane-modified alkyd resins

7.2.8 Thixotropic alkyd resins

7.2.9 Other modified alkyd resins for oxidative crosslinking

7.2.10 Water-thinnable alkyd resins and alkyd emulsions for oxidative crosslinking

7.3 Alkyd resins for co-crosslinking

7.3.1 Alkyd resins for stoving enamels

7.3.2 Alkyd resins for acid-cure systems

7.3.3 Alkyd resins for polyisocyanate crosslinking

7.3.4 Alkyd resins for high-solid reactive coatings

7.3.5 Alkyd resins for water-borne reactive coatings

7.3.6 Other alkyd resins for reactive coatings

7.4 Comparison of OH alkyd resins and OH polyesters with other resins

7.5 OH alkyd resins – combination partners with physically drying binders

8 Special polyesters

8.1 Polycarbonates

8.2 Polycaprolactones

8.3 Polyesters based on diene adducts

8.4 Stand oils

9 Literature

9.1 General literature

9.2 References

Author

Stichwortverzeichnis

1Definitions

In the chemical sense, polyesters are a class of polymer in which the repeating units are linked together by ester groups. Polyesters used in coatings are also referred to as resins, by analogy with natural resins (rosins) which were the first products to be used to form coating films. In DIN 55958 and DIN 55183 polyesters are defined as: “synthetic resins which are based on polyesters and whose structural units in the polymer chain consist of ester groups”.

Resins for coatings must be capable of transformation into a form ready for application (solutions in organic solvents, solutions or dispersions in water, non-aqueous dispersions, and aerosols).

The applied coating subsequently dries (initially by the evaporation of solvents or water) or coalesces by melting (in the case of powder coatings) to yield a film with a specific performance profile.

Chemical reactions may also take place during this film-forming and drying process and which contributes to an enhanced performance profile.

The term “polyester” has had various meanings in the past. The first polyesters produced industrially (glyptal resins) played a secondary role in the coatings industry because they had poor solubility in common solvents and poor compatibility with other coating components.

In 1927, Kienle founded the chemistry of oil-modified polyesters, by introducing the terms alkyd which is a contraction of the words alcohol and acid.

By “alkyd” is meant the product of the reaction of an alcohol and an acid. By that definition, it should be applied to all polyesters. However, it is mostly reserved for oil- or fatty-acid-modified polyesters because they were the first polyesters to find widespread use in the coatings industry.

So, until the 1960s, the term “alkyd” stood for polyesters composed almost exclusively of phthalic anhydride (contributing the polycarboxylic acid component) and polyalcohols, and which were modified either with the fatty acids of natural oils or with the fats or oils themselves. However, the current definition of alkyds is: “polyester resins prepared by the polycondensation of polyfunctional carboxylic acids, polyfunctional alcohols and oils or fatty acids.” Therefore, the terms often found in the literature, namely “oil-modified alkyds” or “fatty-acid-modified alkyds” are incorrect because they are redundant. In line with this definition, polyesters modified with synthetic, aliphatic monocarboxylic acids (synthetic fatty acids) are also alkyds. In this book, it is assumed that, because fatty acids are structural units which are integral to and have a fundamental impact on all polyesters, any polyester modified with any kind of monocarboxylic acid will be considered an alkyd resin. The term alkyd is therefore extended to include resins which contain benzoic acids or monocarboxylic acids from natural rosins.

By “modified alkyds” are meant only those alkyds which contain components other than polycarboxylic acids, polyalcohols and monocarboxylic acid. These other components may be styrene, acrylics, polyamides, urethanes, epoxies, or siloxanes.

Next to alkyd resins, unsaturated polyester resins played a dominant role in the coating resins market (particularly for furniture) for a considerable period of time. Their name was shortened to polyester resins and the coatings were called polyester coatings.

But, of course, the correct term is unsaturated polyester resin, the definition of which is given in DIN 53184: “Unsaturated polyester resins (UP resins) are polyester resins, in which at least one of the polyfunctional components (polycarboxylic acid or polyalcohol) is unsaturated (olefinic unsaturation) and is capable of reacting by copolymerisation with polymerisable monomers”. This definition does not include the alkyd resins, which are modified with unsaturated monocarboxylic acids.

It was only when other, more complex polyalcohols and new carboxylic acids became available on the raw materials market that polyesters consisting only of polycarboxylic acids and polyalcohols (saturated), i.e. polyester resins in the original meaning of the term, became important to the coatings industry. As a way of distinguishing this class of resins from the other polyesters, they were called “oil-free alkyds” or “saturated polyesters”. While the term “oil-free alkyds” should be avoided, the term “saturated polyesters” is extremely common and is therefore also used in this book. Saturated polyesters are defined as “polyester resins whose polyfunctional components (polycarboxylic acids and polyalcohols) do not contain double bonds capable of reacting by polymerisation.”

There are some special coating resins which would not intuitively be considered polyester resins, namely polycarbonates, polycaprolactones, resins of diene adducts of natural rosins (maleic resins, acrylic resins), oils modified with maleic anhydride, and stand oils. However, in the chemical sense, all these resins contain ester groups or are obtained by esterification and all the rules governing the other polyester resins apply to these classes of resin as well. They are therefore covered in this book as well.

The title of this book is “Polyesters and Alkyd Resins”. Strictly speaking it is needlessly redundant, because alkyds are but a specific class of polyester resins. The thinking behind the title is to draw attention to the importance of alkyd resins as a class.

Ulrich Poth: Polyester and Alkyd Resins

© Copyright 2020 by Vincentz Network, Hanover, Germany

2History of polyester resins

Since the early Middle Ages, if not before, drying oils (e.g. linseed oil) have served as binders for paints and coatings and as “solvents” for waxes, rosins and bitumen in various decorative coatings. Even back then, there were attempts to speed up the rate of drying (which today we would call a crosslinking reaction with atmospheric oxygen). One observation was that this could be achieved by the addition of certain metal oxides, one of them being lead(II)oxide or litharge, which was used as a pigment. Another was that drying oils stored in glass containers became more and more viscous when standing in sunlight.

The drying properties of such stand oils, as they were called, proved to be far superior to those of the original oils (in this book, stand oils are deemed to be a special type of polyesters). It transpired that the entire drying process could be accelerated by combining oils or stand oils with natural rosins (which increased the extent of physical drying). The best mixing results were obtained by employing a heating process known as “cooking”. One particular combination of cooked drying oils, stand oils, rosins and certain amounts of metal oxides yielded varnish, whose properties satisfied most requirements at that time.

In the 17th century, an influx of coating knowledge from the Far East and the first throes of industrialisation gave rise to coatings manufacturers which brought further developments in coating systems. It was discovered that, besides bituminous paints, oil-based systems could be improved by combining “drying oils” with high-melting natural rosin products. Typical products were amber and the copals later found in colonial countries. These were combined with drying oils in an elaborate process known as hot blend.

The next stage of industrialisation in the second half of the 19th century required larger quantities of raw materials for coatings. However, these were unavailable, especially in those countries which had few colonies (like the German states), or when military conflicts severely hindered the exchange of goods. As this period coincided with the founding of the chemical industry, efforts were undertaken there to find alternatives to natural products, mainly on the basis of coal tar.

In 1846, J. J. Berzelius[1] was the first to describe the product which is formed by the reaction between tartaric acid and glycerol and which is considered by all modern authors to be the first polyester. Somewhat later, in 1853 and 1854, P.E M. Berthelot[2, 3] characterised products of glycerol with sebacic acid and with camphoric acid. In 1856, J. M. van Bemmelen[4] described the products of the reaction of glycerol with succinic acid, with citric acid and with mixtures of succinic acid and benzoic acid. He also mentioned the reaction of succinic acid with mannitol. He was the first to observe that, by varying the mixing ratios of these raw materials and the degree of condensation, it was possible to obtain products which were no longer soluble or meltable. In 1901, W. Smith[5] was the first to prepare polyesters of phthalic anhydride and glycerol. He suggested using such products for moulding compounds.

However, it was not until 1914 that the first technical application of such “glyptal resins” for plastic parts was described by M. J. Callahan[6] and L. Weisberg[7]. These resins of phthalic anhydride and glycerol, for the most part with rather high acid values, were and still are used as raw materials for special coating applications. They are soluble only in ketones and lower alcohols and were incompatible with most other coating raw materials known at that time.

After several empirical attempts to make mixtures of glyptal and drying oils (hot blend) in the manner of the process involving copals and oils [8], R. H. Kienle succeeded in preparing polyesters from phthalic anhydride, polyols (mainly glycerol) and natural oils [9]. He named these products alkyds (a contraction of the words alcohol and acid). Alkyds became a very important class of binders for coatings and found widespread application and distribution.

Initially, alkyd resins mainly gained importance due to their ability to oxidatively crosslink as a result of the presence of unsaturated fatty acids having two or more double bonds. Later alkyd resins were also used as a plasticising component in combination with fast, physically drying binders, e.g. cellulose nitrate. These alkyds contain fatty acids of “non-drying” oils and fats (castor oil, peanut oil, coconut fat).

When stoving enamels based on phenol resins and amino resins were introduced into the coatings industry in the 1930s, they were also combined with alkyd resins for plasticising. At a surprisingly late stage – in the 1950s – it was reported that short-oil and medium-oil alkyd resins containing free OH functionalities crosslink by reacting with the functional groups of the amino resins. The same applies to the combination of alkyd resins with amino resins in “acid-curable” lacquers, which were developed and introduced onto the market at around the same time.

Initially, alkyd resin formulations, which had small quantities of fatty acids, contained free OH groups more or less by accident. However, from the early 1930s onwards, they were introduced systematically for detailed theoretical studies [10][11][12]. All of these started by calculating stoichiometric ratios of the three classes of building blocks for alkyd resins. During this period, efforts focused on formulations that would yield the highest-possible molecular weights, and thus deliver the optimal film properties. An additional goal was to create materials which were suitable for moulding compounds. The idea was to prepare soluble and meltable precursors which, after application (e.g. by filling of the moulds), would then continue to react to yield crosslinked and therefore rugged polymers (comparable to phenol and amino resins). Surprisingly, it was thought for a long time that, during stoving, polyesters and alkyds would crosslink by themselves in a continuation of the esterification reaction. However, it should have been known that it was impossible for this reaction to go to completion under such conditions.

Wherever short- and medium-oil alkyds for stoving enamels were formulated stoichiometrically, the condensation reaction had to be interrupted at lower degrees of condensation to avoid gelation. As a result, studies and publications from 1930 to 1965 focused on determining gel points. Subsequently, lower degrees of condensation and combination with a stoichiometric excess of OH groups [13][14] led to alkyd resins containing substantial quantities of residual free OH groups and also free carboxyl groups. Some time elapsed before it was realised that lower-molecular weight alkyd resins – containing free OH groups – were suitable for crosslinking reactions. Combination with crosslinkers followed by application and the subsequent film forming process yielded coatings with optimal properties.

In the USA from the late 1950s on, attention shifted more to acrylic resins containing OH groups thereby replacing alkyd resins, but in Europe the optimisation of alkyd resins continued. Up to the 1980s, such optimised alkyd resins were the preferred binders for industrial coatings (primer surfacers, topcoats, one-coat-enamels for vehicles, apparatus, and machines) in Europe and the rest of the world, apart from the USA and Japan. Only clearcoats were formulated worldwide with acrylic resins, and only since the 1970s.

With the advent of water-borne coating systems during that period, usage of alkyd resins declined, in Europe too. The decline stemmed from the fact that alkyd resins are not ideal for water-borne coatings as they have limited resistance to saponification. However, there are some specialty products which are suitable for water-borne formulations. For example, linseed alkyds, the oldest class of alkyd resins, are still used in the form of aqueous emulsions.

After the introduction of alkyd resins, it was not long [15] before patents relating to the formulation of unsaturated polyesters (UPs) were granted (1930). However, UP resins only became widespread in the coatings industry after 1950, mainly for furniture coatings. Until the late 1960s, closed-pore clearcoats and also coloured coatings (UP varnishes, flatting varnishes) were fashionable. However, tastes changed and open-pore, more natural-looking wood coatings became popular. For most other applications, less expensive systems were used.

As a result, market volumes of UP resins in the furniture segment declined. Now, more or less the only uses for UP resins there are, are for coating pianos and automotive wooden dashboards. Outside the traditional coatings segment, UP resins are mainly used for automotive repair systems (putties) and for plastic parts and sheets (gel coats).

The rise of unmodified saturated polyesters began with the invention of two-pack coatings based on binders containing OH groups and polyisocyanates at Bayer in the 1930s [16][17][18]. These OH group containing binders were mostly saturated polyesters and alkyd resins initially, but later polyethers were introduced as well. The resins needed to have sufficient quantities of OH groups for effective crosslinking with polyisocyanates. Although the formulations were calculated stoichiometrically, the polyester and alkyd resins actually had relatively low molecular weights, because the high excess of OH groups ultimately curbed the average molecular weights.

Saturated polyesters did not become important until the raw materials currently employed for saturated polyesters, e.g. isophthalic acid, dimethyl terephthalate, neopentyl glycol, hexanediol, and trimethylol propane, became available in industrial quantities at more affordable prices. As mentioned earlier, polyesters were initially used in two-pack coatings but, in the 1970s, their range was extended to other industrial coating systems. Because of these developments, saturated polyesters consisting solely of polyols and polycarboxylic acids, came to prominence. The use of common diol combinations led to adequate solubility in common solvents for coatings and better compatibility with other raw materials, properties which are essential for widespread application. Saturated polyesters in the coatings industry continue to enjoy volume growth. Also, two-pack coatings for plastic coatings based on saturated polyesters are rising continuously in volume. Unlike classic alkyd resins, saturated polyesters are more suitable for water-borne coating systems. However, it must be remembered that these polyesters, too, naturally need to have good resistance to saponification. Next to solid epoxy resins, saturated polyesters are the most important class of materials for powder coatings. And the market share of powder coatings, too, is still growing. Furthermore, saturated polyesters play a key role as building blocks in other resin classes. For example, there are polyester acrylates for UV crosslinking and polyester soft segments for aqueous polyurethane dispersions.

The latest books and other publications still refer to formulation rules published in the 1960s. These references often include the caveat that, although the calculation rules are suitable, empirical values still need to be taken into consideration. As polyesters are considered to be an exhaustively defined class of substance, there have been no new theoretical calculations in the intervening period.

Ulrich Poth: Polyester and Alkyd Resins

© Copyright 2020 by Vincentz Network, Hanover, Germany

3Formation and structure of polyesters and alkyd resins

In the literature, polyesters and particularly alkyds are usually described in terms of their building blocks. However, to facilitate an evaluation of the different factors that govern the properties of polyesters and alkyd resins, this chapter deals with general structural influences, as opposed to those exerted by the choice of building blocks. Although there exist overlapping interrelations, first the basic molecular structure of polyesters and alkyd resins and the relevant influencing variables are discussed, which generally apply to the most different material compositions. Building block selection is discussed in Chapter 4.

3.1Reactions that produce polyesters

3.1.1Fundamental reactions

6.1.1.1Esterification of alcohols and carboxylic acids

Esterification of alcohols with carboxylic acids is a classic example of a condensation reaction and useful for describing chemical equilibrium reactions. It is usually the reaction chosen to explain the law of mass action. In the conventional view, which provides a good model of how polyesters are prepared, esterification consists in addition of the electrophilic H atom of the alcoholic OH group to the nucleophilic O atom of the carboxyl group to form an intermediate (see Figure 3.1). The steric effect of this intermediate structure with its three oxygen atoms causes the adduct to decompose either into the starting substances or into the ester and water. The ratio of reactants to products in the equilibrated reaction mixture depends in part on the “R” groups of the reactants. Other factors are the concentrations of reactants and the temperature. The chemical equilibrium stems from the fact that the ester on the product side can polarise itself to the extent that it can revert back to the intermediate by reacting with the water. This reaction is defined as saponification. Evidence for this has been provided by radioactively doping the oxygen atoms in the alcohol, which are subsequently found only in the ester molecule. When a carboxyl group oxygen is radioactively doped, it is found in both the ester and the water molecules, because the two OH groups of the “ortho structure” of the adduct are wholly equivalent [19].

Figure 3.1: Conventional model of the mechanisms behind esterification and saponification

The Figure 3.2 shows the overall equilibrium equation for esterification and saponification.

Figure 3.2: Overall equilibrium equation for esterification and saponification

Equation 3. 1

In other words, for quantitative ester preparation, the equilibrium must be shifted to the product side. This is usually accomplished by removing the water of reaction through distillation. The standard production processes for polyesters are based on this approach. From the point of view of the distillation process, this is an example of “residue recovery”.

6.1.1.2Transesterification

As the saponification reaction shows, the ester group can be polarised by water. But it can also be polarised by alcohols. Thus, ester groups can react with mobile hydrogen atoms from alcohols to form an intermediate structure. This intermediate structure can decompose into its reactants, but it can also decompose into the ester formed with alcohol 2 and into the free alcohol of ester 1 (see Figure 3.3 and overall equation in Figure 3.4).

Figure 3.3: Mechanism of transesterification

Figure 3.4: Overall equation for transesterification

The chemical equilibrium involved in transesterification is also subject to the law of mass action.

Equation 3.2

Here, again, the equilibrium state is influenced by the “R”-groups in the carboxylic acid as well as the alcohol. Naturally, it is also influenced by the concentration of reactants and by the temperature.

If the goal is to prepare one of the esters in high yield, the reaction equilibrium needs to be shifted to the correct side. This is usually accomplished by removing one of the products through distillation. This means that polyesters can be produced from polycarboxylic esters of lower alcohols. Full transesterification with polyols is then achieved by distilling off all the low boiling alcohol from the starting product.

Transesterification also plays a role in the preparation of alkyd resins. Starting products are then natural oils or fats (triglycerides) which are transesterified with polyols. Because of the excess of hydroxyl groups, reaction equilibrium is reached when the fatty acids are distributed on all polyol molecules (partial glycerol esters). These serve as intermediates for a second step in which dicarboxylic acids or anhydrides (e.g. phthalic anhydride) are added to continue the polycondensation and form the final alkyd resin.

Although transesterification in an industrially accepted production process has been described in publications, it is usually ignored when it comes to theoretical descriptions regarding the preparation of polyester molecules. The methods for determining molecular weights and molecular weight distributions or defining gel points assume that only esterification reactions occur during the preparation of polyesters. However, transesterification happens throughout the production process and is not restricted to the primary products. Transesterification mainly affects the molecular weight distribution.

6.1.1.3Reaction catalysis

Many authors believe that the order of the reaction for esterification, saponification and transesterification (mostly 2nd order reactions) requires the use of catalysts. The addition of such acid catalysts or Lewis acids causes polarisation of the carboxyl group on acids and esters by protonation, and that subsequently the alcohol adds to an oxonium complex or a carbonium ion (see Figure 3.5).

Figure 3.5: Acid catalysis at the start of esterification

However, carboxylic acids themselves can liberate protons and so are able to polarise carboxyl groups by themselves. Support for this is provided by the formation of dimers of carboxylic acids (see Figure 3.6).

Figure 3.6: Dimerisation of carboxylic acids

Thus, the reactions of carboxyl groups or ester groups with alcohols are quite complex, especially those between carboxyl groups and tertiary alcohols or phenols – which are completely different from those with primary and secondary OH groups. For this reason, common polyesters are not usually prepared with the aid of tertiary alcohols and phenols.

6.1.1.4Anhydride addition

Due to ease of handling, more often anhydrides (e.g. o-phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, pyromellitic anhydride, succinic anhydride, maleic anhydride) are used in industrial processes than their corresponding diacids. Owing to the molecular stress in the anhydride ring and the exposed position of the nucleophilic oxygen atom, anhydrides readily react with electrophilic active hydrogen atoms, such as those of alcohols. The addition reaction yields an ester group and a free carboxyl group (see Figure 3.7). The latter is then able to form a further ester group. The reaction rate for the formation of the second ester group – especially in the case of aromatic polycarboxylic acids – is lower than that for isolated carboxyl groups due to steric hindrance. Higher temperatures can also cause reversion to the anhydride. This mainly happens in the case of aromatic 1,2-carboxyl groups (e.g. phthalic, trimellitic, and pyromellitic esters).

Figure 3.7: Anhydride addition

6.1.1.5Epoxy addition

Formally, 1,2-epoxies are anhydrides of 1,2-diols. Ring stress in epoxies makes them readily polarisable to yield nucleophilic oxygen and electrophilic carbonium ions. The electrophilic carbonium ion can add nucleophilic oxygen from carboxyl groups to form ester groups and secondary OH groups (see Figure 3.8).

Figure 3.8: Epoxy addition to form esters

Under the influence of strong acids, epoxies can react with themselves to form polyethers in a secondary reaction.

6.1.1.6Other reactions

Several other reactions are known in organic synthesis that can yield polyesters. Two particular reactions are described to prepare a polyester and a polyester-like product.

Polycarbonates are prepared by reaction of phosgene or other derivatives of carbonic acid with alkaline alcoholates. Alkaline phenolates may also be used for this reaction. The high enthalpy of formation of alkaline halogenates ensures that there is a very high yield of carbonates, which are otherwise difficult to prepare. The carbonates formed (see Figure 3.9) are surprisingly stable.

Figure 3.9: Preparation of aryl carbonates

Cyclic esters (lactones) can, to an extent depending on the number of atoms in the ring, undergo ring-opening reactions with carboxylic acids or alcohols to form chain esters. While the chemical equilibrium of cyclic esters of lactic acid, γ-butyrolactone, and δ-valerolactone is on the side of the cyclic ester (five- and six-membered rings), the equilibrium of ε-caprolactone (seven-membered ring) favours the formation of polyester chains (see Figure 3.10).

Figure 3.10: Ring-opening reaction of ε-caprolactone

3.1.2Structure of polyesters

6.1.2.1Formation of linear polyesters

The formation of polyester resins requires the presence of polyfunctional building blocks, i.e. with at least two functional groups. If a diol reacts with a dicarboxylic acid, the first entity formed is a single ester, as described above (see Chapter 3.1.1). This single ester still contains both reactive groups. The OH group of the single ester can react with the carboxyl group of a dicarboxylic acid or of another single ester, while the carboxyl group of the single ester can react with the OH group of a diol or of another single ester. This “head-to-tail” reaction yields chains of oligomers and finally polymers, i.e. linear polyesters (see Figure 3.11).

Figure 3.11: Initiation and formation of linear polyester molecules

If n moles of diol react with n moles of dicarboxylic acid, theoretically, linear polyester molecules with an average number of (n-1) structural units (monoesters) are formed, all of which are terminated with both types of functional groups (see Figure 3.12). There is the further possibility that there will be a mixture of molecules bearing two terminal OH groups and molecules bearing two carboxyl groups.

Figure 3.12: Formation of polyester molecules containing n moles diols and n moles dicarboxylic acids

In the past, a simplified way of representing such structures was found and this is now encountered in the literature as well. Polycarboxylic acids are symbolised by rings and lines, because most polycarboxylic acids in polyesters intended for coatings are aromatic compounds. By contrast, the usually aliphatic polyols are represented simply by lines. The number of line ends represents the functionality of the building blocks. The simplified representation for the formation of linear polyester molecules is shown in Figure 3.13.

Figure 3.13: Simplified way of depicting the formation of linear polyesters

6.1.2.2Formation of branched polyesters

Whenever building blocks contain more than two functional groups, branched polyesters will be formed (see Figure 3.14).

Figure 3.14: Incipient formation of branched polyesters

While there is a possibility during the formation of linear polyesters that molecules of different structure will be formed, the likelihood of this is all the greater during the formation of branched polyesters. A multitude of structural isomers are formed. Their number increases even further because some of the polyester molecules formed are terminated solely with OH groups while others are terminated solely with carboxyl groups. In addition, the number of structural isomers increases exponentially as the molecular weight of the polyesters increases. It is theoretically possible for linear molecules to result from the preparation of polyesters based on building blocks bearing more than two functional groups. However, the quantity of linear molecules will decline as the polyester molecules become larger. The idea that polyesters based on building blocks bearing more than two functional groups of different reactivity will primarily yield linear molecules needs to be viewed critically. A classic example of such an idea is the preparation of polyesters with the aid of glycerol as building block. This postulates that linear polyesters are the first to form and that branched polyesters form only at the end of the reaction, if at all. Of course, the primary OH groups of glycerol will react faster than the secondary ones and, what is more, there are two of them. However, as soon as some of the primary OH groups have formed esters, the concentration of secondary OH groups grow and will be available for reactions that yield branched polyesters.

The use of building blocks of different functionalities (two and more) increases the potential number of structural isomers of polyester molecules.