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- Herausgeber: Vincentz Network
- Kategorie: Wissenschaft und neue Technologien
- Serie: European Coatings TECH FILES
- Sprache: Englisch
- Veröffentlichungsjahr: 2013
The eBook describes the latest knowledge about acrylic resins in solvent-borne and water-borne systems, and for radiation curing in an understandable and descriptive manner. It discusses the different chemical and physical aspects of the production methods and the related application properties. Furthermore the reader receives an overview of the production, properties and application of acrylic resins and their distinctive features.
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Ulrich Poth | Reinhold Schwalm | Manfred Schwartz | Roland Baumstark
Acrylic Resins
Poth/Schwalm/Schwartz/Baumstark: Acrylic Resins © Copyright 2011 by Vincentz Network, Hanover, Germany ISBN: 978-3-86630-809-1
Cover: BASF SE
Bibliographische Information der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.ddb.de abrufbar.
Ulrich Poth, Reinhold Schwalm, Manfred Schwartz, Roland Baumstark Acrylic Resins Hanover: Vincentz Network, 2011 European Coatings Tech Files ISBN 3-86630-809-4 ISBN 978-3-86630-809-1
© 2011 Vincentz Network GmbH & Co. KG, Hanover Vincentz Network, P.O. Box 6247, 30062 Hanover, Germany This work is copyrighted, including the individual contributions and figures. Any usage outside the strict limits of copyright law without the consent of the publisher is prohibited and punishable by law. This especially pertains to reproduction, translation, microfilming and the storage and processing in electronic systems. The information on formulations is based on testing performed to the best of our knowledge. The appearance of commercial names, product designations and trade names in this book should not be taken as an indication that these can be used at will by anybody. They are often registered names which can only be used under certain conditions.
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Layout: Vincentz Network, Hanover, Germany
European Coatings TECH FILES
Ulrich Poth | Reinhold Schwalm | Manfred Schwartz | Roland Baumstark
Acrylic Resins
Preface
Acrylic polymers are essential products for various industrial application fields. Particularly, they play important roles as binders (acrylic resins), dispersion resins and polymer thickeners in the coatings industry.
Since acrylic resins were launched onto the coatings market, they have distinguished themselves by meeting various quality requirements and replacing other resins. Acrylic resins are used in eco-friendly coating systems, for example. Thanks to their broad range of properties and to ongoing developments within this product class, they also serve as ingredients for many different coating systems.
The present book presents an overview of the production, properties and application of acrylic resins and their distinctive features. It covers acrylic resins prepared by solution polymerization and emulsion polymerization, as well as reactive acrylic resins which form films by radiation curing. Besides a general description of these three product classes, there are chapters dealing with particular properties for the diverse application fields.
The goal of the book is to convey the latest knowledge about acrylic resins in solvent-borne and water-borne systems, and for radiation curing in an understandable and descriptive manner. Given the breadth of applications for acrylic resins, the book discusses the different chemical and physical aspects of the production methods and the related application properties. There are numerous literature citations for further reading on specific issues.
The book is aimed at students and newcomers to the field of coatings technology, and also at well-versed experts in coatings applications and other related industries. The essential background information herein will underpin decisions concerning the choice and use of acrylic resins.
Münster, Ludwigshafen,
São Paulo, March 2011
Ulrich Poth
Dr. Reinhold Schwalm
Dr. Manfred Schwartz
Dr. Roland Baumstark
Contents
Ulrich Poth
1 Definitions
Ulrich Poth
2 General composition and structure
2.1 Free-radical polymerization
2.1.1 Polymerization reactions
2.1.2 Kinetics of free-radical chain polymerization
2.1.3 Influences on polymerization reactions
2.2 Monomers
2.2.1 Esters of acrylic acid
2.2.2 Esters of methacrylic acid
2.2.3 Functional monomers
2.2.3.1 Hydroxyl-functional monomers
2.2.3.2 Carboxy-functional monomers
2.2.3.3 Amino-functional monomers
2.2.3.4 Amide-functional monomers
2.2.3.5 Epoxy-functional monomers
2.2.4 Ether acrylates and methacrylates
2.2.5 Polyunsaturated acrylic and methacrylic compounds
2.2.6 Comonomers
2.2.7 Copolymerization
2.2.8 Characterization of monomers
2.2.8.1 Glass transition temperature
2.2.8.2 Material properties
2.2.9 Handling of monomers
2.3 Production processes – generally
2.3.1 Bulk polymerization
2.3.2 Suspension polymerization
2.3.3 Solution polymerization
2.3.4 Emulsion polymerization
2.4 Literature
Ulrich Poth
3Solution polymerization products
3.1 Definition
3.2 History of acrylic resins made by solution polymerization
3.3 Solution polymerization process
3.3.1 Influence of the process on the properties of acrylic resins
3.3.2 Production procedure
3.3.3 Influence of the process conditions
3.3.4 Alternatives to solution polymerization
3.4 Composition of acrylic resins, influences on properties
3.4.1 Influence of monomer types
3.4.2 Initiators
3.4.3 Regulation agents
3.4.4 Process solvents
3.5 Types, properties and application of acrylic resins
3.5.1 Acrylic resins for solvent-borne coatings
3.5.1.1 Thermoplastic acrylic resins
3.5.1.2 Acrylic resins with methylol acrylamides
3.5.1.3 Hydroxy-functional acrylic resins crosslinked by amino resins
3.5.1.4 Hydroxy-functional acrylic resins for crosslinking with isocyanates
3.5.1.5 Comparison of hydroxy-functional acrylic resins with other resins
3.5.1.6 Acrylic resins and alternative crosslinking reactions
3.5.2 Acrylic resins prepared by solution polymerization for water-borne coatings
3.5.2.1 Water as solvent and dispersing agent
3.5.2.2 Production of secondary dispersions of acrylic resins
3.5.2.3 Properties and use of aqueous, secondary acrylic dispersions
3.5.2.4 Comparison of aqueous acrylic resins in secondary dispersion with other resins
3.5.3 Acrylic resins for powder coatings
3.5.3.1 Powder coatings based on acrylic resins and blocked polyisocyanates
3.5.3.2 Powder coatings based on epoxy-functional acrylic resins
3.5.3.3 Powder slurries based on acrylic resin
3.6 Outlook
3.7 Literature
Manfred Schwartz and Roland Baumstark
4Primary dispersions of acrylic resins
4.1 Binder classes, polymerization and polyacrylates
4.1.1 Polyacrylates by polymerization
4.1.1.1 Free-radical polymerization
4.1.1.2 Emulsion polymerization
4.1.2 Polyacrylates; straight acrylics and styrene-acrylate copolymers
4.1.3 Film formation by polymer dispersions
4.1.4 Parameters and properties of coatings binders
4.2 History
4.2.1 Chronological development
4.2.2 Technological development
4.3 Composition of acrylates and their influence on performance
4.3.1 Parameters influencing binder properties during latex preparation
4.3.2 Raw materials
4.3.2.1 Monomer selection
4.3.2.2 Auxiliaries
4.4 Emulsion polymerization processes
4.4.1 Polymerization control
4.4.2 Multi-phase systems
4.4.3 Seed polymerization
4.5 Combinations of acrylic dispersions with other binders
4.5.1 Combinations with other dispersions
4.5.2 Combinations with water-soluble binders
4.6 Applications of acrylate primary dispersions
4.6.1 Emulsion paints
4.6.1.1 Primers
4.6.1.2 Exterior paints
4.6.1.3 Interior paints
4.6.1.4 Gloss emulsion paints
4.6.1.5 Wood coatings
4.6.2 Polymer dispersions in silicate systems
4.6.2.1 Moisture protection
4.6.2.2 Resistance to hydrolysis
4.6.2.3 Water absorption
4.6.2.4 Interactions between dispersion and water glass
4.6.2.5 Demands imposed on an optimum dispersion
4.6.2.6 Silicate dispersion system
4.6.3 Polymer dispersions as binders in silicone resin systems
4.6.4 Elastic coating systems
4.6.4.1 Effective protection against moisture
4.6.4.2 Principal requirements imposed on coating systems for renovating façades
4.6.4.3 Mechanical properties of dispersion films
4.6.4.4 Dirt pick-up resistance
4.6.5 Synthetic resin plasters and exterior insulation and finish systems
4.6.5.1 Classification of synthetic resin plasters and technical requirements
4.6.5.2 Exterior insulation and finish systems
4.6.5.3 Formulation scheme for synthetic resin plasters
4.6.5.4 Typical binders for synthetic resin plasters
4.6.6 Adhesives
4.6.6.1 Theories of adhesion
4.6.6.2 Polymer dispersions as adhesives
4.6.7 Construction chemicals
4.6.8 Fibre bonding/non-wovens
4.6.9 Floor polishes
4.7 Comparison of acrylate primary dispersions with other binders
4.7.1 Whitestone test
4.7.1.1 Comparison of artificial weathering methods
4.7.1.2 Comparison of binders
4.7.2 Comparison of acrylic dispersions with acrylate/styrene dispersions
4.7.2.1 Effect of the binder type and pigment volume concentration
4.7.2.2 Effect of pigment/extender ratio
4.7.2.3 Effect of the extender type
4.7.3 Comparison of acrylic dispersions with vinyl ester dispersions
4.7.4 Comparison of acrylic dispersions with polyolefin dispersions
4.7.5 Comparison of acrylic dispersions with styrene/butadiene dispersions
4.8 Outlook
4.9 Literature
5Reinhold Schwalm Acrylate resins for radiation curable coatings
5.1 Introduction and definitions
5.2 History
5.3 Basics of the radiation curing technology
5.4 Resins for radiation curing
5.4.1 Acrylates – preferred monomers
5.4.2 Acrylate functional reactive diluents
5.4.2.1 Monofunctional acrylates
5.4.2.2 Multifunctional acrylates
5.4.3 Acrylate functional resins
5.4.3.1 Acrylate functionalized standard resins
5.4.3.2 Acrylate functionalized specialty resins
5.4.4 UV curable acrylate functionalized dispersions
5.4.5 Influence of chemical structure on formulation properties
5.4.5.1 Viscosity
5.4.5.2 Reactivity
5.4.5.3 Surface tension – interface tension
5.5 Structure and properties of coating films
5.5.1 Network characteristics
5.5.1.1 Network formation
5.5.1.2 Functionality
5.5.1.3 Crosslink density and molecular weight between crosslinks
5.5.1.4 Glass transition temperatures in highly crosslinked coatings
5.5.1.5 Brittle-ductile transitions in networks
5.5.1.6 Oxygen inhibition
5.5.2 Coating films: structure – property relationships
5.5.2.1 Influence of chemical structure on curing conversion
5.5.2.2 Glass transition temperature: influence on hardness and flexibility
5.5.2.3 Scratch resistance: influence of the crosslink density
5.5.2.4 Photochemical yellowing
5.5.2.5 Thermal yellowing
5.5.2.6 Weathering stability
5.5.2.7 Performance-temperature-energy diagrams
5.6 Applications and formulations
5.6.1 Graphic applications
5.6.1.1 UV overprint varnishes
5.6.1.2 UV printing inks
5.6.2 Wood coatings
5.6.3 Electronics
5.6.4 Other industrial applications
5.6.4.1 UV adhesives
5.6.4.2 Optical glass fibres
5.6.4.3 Stereolithography
5.6.4.4 Dental materials
5.6.5 UV coatings for exterior applications
5.6.5.1 UV curable coatings for automotive applications
5.6.5.2 UV curable coatings for construction applications
5.6.6 UV curing within alternative coating technologies
5.6.6.1 UV powder coatings
5.6.6.2 Dual Cure systems
5.6.6.3 UV film coating technology
5.6.7 Opportunities for UV coatings in the “new” applications
5.7 Literature
Authors
Acknowledgement
Index
1 Definitions
Ulrich Poth
Acrylic resins for coatings are binders (polymers) composed mainly of esters of acrylic acid or methacrylic acid. By analogy with the salts of inorganic acids, the esters are called acrylates or methacrylates. The IUPAC systematic name for acrylic acid is prop-2-enoic acid while that for methacrylic acid is 2-methylpropenoic acid.
Formula 1.1: Esters of acrylic acid and methacrylic acid
Although acrylic esters and methacrylic esters have quite different properties, polymers made from either are called acrylic resins. Numerous products contain mixtures of both esters.
Binders are the film forming components contained in all coating systems. They are so-called because of their ability to wet pigments and to bind coating layers to substrates.
The term resin is derived from natural resin (rosin), which historically were used as a binder for coatings. The term alludes to the physical state of the products. Resins consist of polymers or oligomers that exhibit glass-like behaviour. Physically, they are liquids that have extremely high viscosities, i.e. they are solidified melts. But there are also binders which have the appearance of being true liquids at ambient temperatures.
Esters of acrylic acid and methacrylic acid are distinguished by the reactivity of their double bonds. Among others, the presence of these double bonds renders the esters amenable to polymerization, which may be initiated either by free-radicals or by ions.
Acrylic resins for coatings fall into two groups:
The first comprises the polyacrylates, which are prepared by polymerizing acrylic or methacrylic esters via their double bonds.
Polyacrylates are made by various polymerization processes. The choice of polymerization process critically determines the resultant properties of the acrylic resin.
One process is solution polymerization. In this process, the polymers are prepared in organic solutions, which may be used directly in coatings formulations (see Chapter 3). In addition, such polymers can be transformed into secondary aqueous dispersions or into powder coatings resins. Some binders for the aforementioned applications are produced by bulk polymerization, or pearl polymerization.
Another process is emulsion polymerization, which is employed in the production of primary aqueous acrylic dispersions (see Chapter 4).
Esters of acrylic acid and methacrylic acid which act as building blocks for polymers are called monomers. Further building blocks capable of forming polymers in conjunction with acrylic esters and methacrylic esters are called comonomers.
The second group of acrylic resins for coatings comprises acrylic or methacrylic ester resins which still contain double bonds. These binders are called reactive acrylic resins. Addition or condensation reactions are employed to incorporate the acrylic or methacrylic ester into polymer or oligomer molecules. The resultant binders are capable of forming films by polymerization after application, and are notable for their resistance properties. The polymerization reactions are initiated mainly by energy-rich radiation (e.g. UV light), which yields three-dimensional crosslinked macromolecules.
These reactive acrylic resins, which form films through polymerization of the double bonds of acrylic or methacrylic esters, are classified and named for the reactions by which they are incorporated into polymers or oligomers (see Chapter 5).
Figure 1.1: Classification of acrylic resins for coatings
2 General composition and structure
Ulrich Poth
The composition and structure of acrylic resins determine the application properties of the acrylic binders prepared from them.
2.1 Free-radical polymerization
2.1.1 Polymerization reactions
Polymerization reactions at the double bonds of acrylic monomers [1–6] may be initiated by free-radicals or ions. The most important and common reaction is free-radical initiation. The initiators employed (peroxy and azo compounds) decompose spontaneously into free-radicals when the temperature is increased (initiator reaction). The decomposition rate is influenced by the type of initiator and the temperature.
Formula 2.1: Initiator reaction (e.g. for a peroxy compound)
The resultant free-radicals react with the π-electrons of the double bonds on the monomers. The monomer molecule forms a new single bond and a single electron, i.e. a new free-radical. This is the start reaction in free-radical polymerization.
Formula 2.2: Start reaction
The planar double-bond system becomes a tetrahedral molecule of lower bond energy. Consequently, a substantial amount of energy is released in the form of heat (exothermic reaction). As the activation energy of the free-radical reaction is relatively low, the free polymerization enthalpy of the monomer reaction is markedly negative at -60 to -80 kJ/mol (examples: acrylic acid -75 kJ/mol, methyl methacrylate -58 kJ/mol at 298 K).
The free-radical monomer formed by the start reaction can then add a further monomer molecule, to which it transfers its free-radical nature. This reaction is repeated as long as monomer molecules are available. Such a reaction is called chain propagation.
Figure 2.1: Model of the reaction at the double bond
Formula 2.3: Chain propagation
The free-radical chains may grow until two free-radicals collide, forming a single σ-bond. The second free-radical may be another free-radical chain or an initiator free-radical. This chain-termination reaction is called recombination.
Formula 2.4: Chain-termination reaction by recombination
Propagation may also be terminated by the reaction between a free-radical at the end of a chain and movable atoms or atom groups. This reaction, which creates a chain end and a new free-radical on the partner molecule, is called chain transfer. On the one hand, the reaction partner might be another chain with active hydrogen atoms. In that case, the free-radical formed on this chain would act as the starting point for renewed chain propagation and lead to the generation of branched polymer molecules. On the other hand, chain transfers also take place by reaction between free-radical chains and other molecules in the reaction mixture, e.g. with solvent molecules. In turn, these free-radicals, formed on the partner molecules, can initiate new polymer chains. Some compounds are ideal for chain-transfer reactions, e.g. mercaptans. Such compounds are called regulators; they are added in tiny amounts to control chain propagation.
Formula 2.5: Chain termination by chain transfer
An alternative chain-termination reaction is disproportionation. This occurs when the ends of two free-radical chains react by transferring a hydrogen atom. The driving force for this reaction is the high energy content of free-radical molecules. In a disproportionation reaction, two neighbouring molecular states of lower energy are created. Of the two free-radicals, one becomes a saturated chain end and the other, a chain end with a new double bond.
Formula 2.6: Chain termination by disproportionation
The polymer molecule bearing the new double bond at the end of its chain can act as a macro-monomer. If it is incorporated into a further chain propargation, the second way to form branched polymer molecules is created.
The relative proportions of the various chain-termination reactions depend on the type of polymerization process and the reaction conditions. The predominant chain-termination reactions are believed to be recombination with other polymer molecules or chain transfer to solvent molecules or regulators. These favour the generation of linear polymer molecules. However, it must be remembered that linear acrylic polymers are composed of extensively coiled molecules. The formation of branched acrylic polymers, by chain transfer to other polymer chains or by disproportionation, confers markedly different application properties on such acrylic polymers in coating formulations.
The various polymerization processes and the chosen reaction conditions can give rise to side reactions. These exert a marked influence on the properties of the polymer molecules and coating systems prepared from them (see description of polymerization processes).
When reactive acrylic resins undergo film formation – usually under the influence of high-energy radiation (see Chapter 5) – the reactions involved are the same as those described above. For the most part, the reactive acrylic resins employed contain molecules bearing two or more double bonds. Thus, it is possible for each molecule to participate in more than one chain-polymerization process. That is how polymer molecules become crosslinked in the film matrix.
2.1.2 Kinetics of free-radical chain polymerization
The initiator reaction is a first-order decomposition reaction that yields two freeradicals [7]. The reaction rate (v1) varies with the concentration of initiator (cI) and the temperature. The rate of the initiator reaction is shown in the following Formula 2.7[2]:
Formula 2.7: Velocity of initiator reaction
The velocity of start reaction (v2) varies with the concentration of initiator freeradicals (cR•), the concentration of monomers (cM) and, of course, the temperature.
Formula 2.8: Velocity of start reaction
Comparison of velocity constants for initiator and start reactions show that the constant of start reaction [k2] is much larger than that of initiator reaction [k1]. That is why the velocity of start reaction [vst] is ultimately determined exclusively bei the efficience of initiator decomposition [k1].
Formula 2.9: Comparison of velocity constants
After a very short time, the chain propagation reaction reaches equilibrium with respect to chain termination. The rate of chain termination (vct) varies with the concentration of chains bearing free-radicals (cRnM•).
Formula 2.10: Velocity of chain termination
At equilibrium, the change in concentration of chain free-radicals (cRnM•) is zero. The rate of chain propagation (vP) is then equal to the rate of chain termination (vct).
Formula 2.11: Equilibrium
The propagation velocity (vP) is then given by:
Formula 2.12: Propagation velocity
The molar propagation velocity (nM) varies with the propagation constant (kP) and the monomer concentration (cM) as a function of time.
Formula 2.13: Molar propagation velocity
Examples of kinetic data for monomers are presented in the Table 2.1 for methyl acrylate and methyl methacrylate [02].
Substitution of the data for methyl acrylate into the propagation rate formulas above yields a figure of 165 · 10-5 mol/(l s), i.e. 20,380 molecules per second. For methyl methacrylate, the corresponding propagation rate computes to 17 · 10-5 mol/(l s), i.e. 3,120 molecules per second. For a polymer with an average molecular weight of 10,000g/mol, the average time needed to form single molecules is 0.007s in the case of acrylic monomer, and 0.03s in the case of methacrylic monomer.
Table 2.1: Kinetic data for monomer examples
In other words, esters of acrylic acid polymerize 10 times as fast as esters of methacrylic acid. That is why acrylic resins bearing reactive double bonds consist mostly of esters of acrylic acid (see Chapter 5).
In contrast to polycondensation reactions, where the molecules are formed over many hours, individual molecules of acrylic polymers prepared by free-radical initiated polymerization process are formed in fractions of seconds. Furthermore, again in contrast to polycondensation, free-radical-initiated polymerization is not an equilibrium process. Therefore, the molecular-weight distribution as a function of the average molecular weight obeys statistical laws for the given reaction conditions. Naturally, however, the polymerization reaction depends on the energy conditions. The Gibbs-Helmholtz equation is valid.
Formula 2.14: Simplified version of the Gibbs-Helmholtz equation
This equation states that a reaction takes place only if the free reaction enthalpy (G), which is the difference between the value of the total reaction enthalpy (H) and the product of the temperature (T) and the reaction entropy, is negative. As the temperature rises, the value of the free reaction enthalpy becomes smaller and smaller. If the value reaches zero or the free reaction enthalpy becomes positive, polymerization of monomers is no longer possible. This temperature limit is called the ceiling temperature. For example, the ceiling temperature for methyl methacrylate is 373K (200°C, 328F). Generally, the ceiling temperatures for methacrylates are lower than those for acrylates. This relationship between polymerization efficiency and temperature is what makes low-temperature radiation curing of reactive acrylic resins so advantageous (see Chapter 5).
2.1.3 Influences on polymerization reactions
To summarize, polymerization reactions are induced by the use of initiators to raise the temperature and form free-radicals. Thereafter, all subsequent reaction steps (start reaction, chain propagation, and recombination) are highly exothermic. All polymerization processes must be technically capable of meeting these conditions. As a consequence, there is little scope for variation in production processes; ultimately, these restrictions influence the properties of the polymer products. Above a monomer-specific temperature, no polymerization reaction takes place.
As molecular propagation is not an equilibrium reaction, the polymerization obeys statistical laws [2]. That is the reason that molecular-weight distribution of acrylic resins prepared by free-radical polymerization is much broader than that of polymers prepared by polycondensation. Special techniques are available for achieving lower average molecular weights and narrower molecular-weight distributions. Some of the free-radically initiated polymerization processes also enable particularly large, non-crosslinked polymer molecules (average molecular weights up to more than 106g/mol) to be made.
As already mentioned, acrylic polymers prepared by the usual methods are mostly made up of linear molecules. Those molecules are extensively coiled, a fact which significantly influences the application properties of the resultant coatings, namely the solubility and viscosity of solutions, fastness of physical film formation (drying) of solutions and dispersions, wetting and flow during film formation, efficiency of crosslinking reactions. If, on account of the polymerization process conditions, the proportion of branched molecules increases, the tendency to coil decreases, the solution viscosity rises, and the functional groups become more amenable to crosslinking.
2.2 Monomers
2.2.1 Esters of acrylic acid
Currently, acrylic acid is almost exclusively made from propene by one-step or twostep catalytic oxidation process [8]. Acrylic acid esters are prepared by esterifying acrylic acid with the corresponding mono-alcohols in the presence of specific catalysts [9]. Enzymatic processes that consume less energy and have high yields have also become available [10]. Some esters are made directly. For example, ethyl acrylate can be prepared directly from acrylonitrile and ethanol in the presence of sulphuric acid, or from acetylene, carbon monoxide and ethanol [13]. In addition, acrylic esters of higher mono-alcohols can be prepared by transesterifying low-molecular esters.
In industry, acrylic acid esters for the production of polymers are formed with methanol, ethanol, n-butanol, iso-butanol, tert.-butanol, 2-ethylhexanol, n-dodecyl alcohol, and cyclohexanol, all of which are abundantly available. The other esters of acrylic acid play only a minor role. Chain length and the branching of alcohol side-chains exert a significant influence on the properties of acrylic resins and the resultant coatings (see Chapter 2.2.6).
2.2.2 Esters of methacrylic acid
Methacrylic acid is made by different processes. The most common utilises acetone und hydrocyanic acid, obtained via acetone cyanohydrin. The acetone cyanohydrin is dehydrated with sulphuric acid to methacrylamide.
Table 2.2: Physical data of industrially made acrylic esters [19-23]
The latter is then saponified either with water to form free methacrylic acid or with alcohols to yield different methacrylic esters [12].
Methacrylic acid is also obtained via methacrolein by oxidation of isobutene [18].
Propene and carbon monoxide yield isobutyric acid, which can be dehydrogenated to methacrylic acid.
Ethene and synthesis gas (hydrogen and carbon monoxide) combine to form propionaldehyde, which reacts with formaldehyde to yield a methylol compound that can be dehydrated and oxidised to methacrylic acid.
Polymers can be produced industrially from a swathe of different esters of methacrylic acid and mono-alcohols, namely esters of methanol, ethanol, n-butanol, iso-butanol, tert.-butanol, iso-decyl alcohol, iso-tridecyl alcohol, cyclohexanol.
Table 2.3: Physical data of industrially available methacrylic esters [19-23]
The esters of methacrylic acid with benzyl alcohol, p-tert.-butylcyclohexanol, norboneol, dicyclopentadienylic alcohol and fatty alcohols serve as specialty monomers.
The esters of methacrylic acid are prepared by standard esterification processes, while the esters of high-molecular alcohols may be also made by transesterification, e.g. starting with methyl methacrylate.
As is the case for the various acrylic esters, the type and size of the ester sidechains of methacrylic esters significantly influence the properties of the polymers and resultant coating systems (see Chapter 2.2.6).
As already mentioned, all esters of methacrylic acid polymerize at a much lower rate than esters of acrylic acid.
2.2.3 Functional monomers
Acrylic resins containing functional groups (see Chapter 3) and reactive acrylic resins containing double bonds (see Chapter 5) are made from monomers which bear various functional groups in addition to double bonds.
2.2.3.1 Hydroxyl-functional monomers
When just one hydroxyl group in a polyalcohol is esterified with acrylic acid or methacrylic acid, the resultant monomer still contains free hydroxyl groups. Industrially, such monomers are produced mainly by the addition reaction of epoxy compounds to acrylic acid or methacrylic acid. That method – conversion of ethylene oxide or propylene oxide with acrylic acid or methacrylic acid – is used to prepare the monomers 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate, or 2-hydroxypropyl acrylate and 2-hydroxypropyl methacrylate, respectively [23].
Formula 2.15: Production of 2-hydroxyethyl acrylate
Other hydroxy-functional monomers are prepared by partial esterification of polyols. One mole of acrylic acid and one mole of 1,4-butanediol yields butanediol monoacrylate.
Glycerol monoacrylate is a specialty monomer which is prepared from acrylic acid and glycidol.
Acrylic resins containing free hydroxyl groups are suitable for coatings which form films by chemical reaction. They are crosslinked with different reaction partners: polyisocyanates bearing free or blocked isocyanate groups, and amino resins bearing functional groups (methylol groups or methylol ether groups). The hydroxy-functional monomers differ in their structure and the reactivity conferred by the type of hydroxyl group. These differences determine their use in the production of acrylic resins.
2.2.3.2 Carboxy-functional monomers
Carboxyl groups are simply incorporated into acrylic resins by copolymerizing defined quantities of free acrylic or methacrylic acid with other monomers.
Like hydroxyl groups, the carboxyl groups lend themselves to crosslinking reactions. The preferred reaction partners are compounds containing epoxy groups.
Carboxyl groups in acrylic resins can be neutralized with alkalis or amines. The resultant carboxylate ions can form hydrophilic centres, so that it is possible to transform the polymers into water-borne colloidal solutions or into secondary dispersions which are suitable for water-borne coating systems.
Furthermore, certain quantities of carboxyl groups in acrylic resins exert a catalytic effect. This can be sufficient to effect reaction between hydroxyl groups and functional groups of amino resins (methylol groups, methylol ether groups). The crosslinking reaction is accelerated by the acidity of the carboxyl groups.
In addition, diacrylic acid (carboxyethyl acrylate) serves as a vehicle for incorporating carboxyl groups into special acrylic resins.
Other ways of introducing carboxyl groups into acrylic resins utilise the esters of a diol formed with acrylic acid or methacrylic acid and succinic acid or maleic acid, each of which ultimately bears one free carboxyl group per molecule.
2.2.3.3 Amino-functional monomers
Primary and secondary amines easily enter into an addition reaction with the double bonds of acrylic or methacrylic acid and their esters (Michael addition). Therefore stable monomer compounds are not formed. Initially, monomers bearing tertiary amino groups are formed which do not contain active hydrogen atoms. Such monomers are produced by converting tertiary-amino alcohols and acrylic acid or methacrylic acid into esters. The most important monomers in this class are N,N-dimethylaminoethyl acrylate and N,N-dimethylaminoethyl methacrylate.
However, there also exists a monomer containing a secondary amino group, namely N-tert.-butylaminomethacrylate. This monomer is stable only because steric hindrance by the tertiary-butyl group preventing a Michael addition reaction.
There are also monomers available which consist of quaternary ammonium salts, e.g. 2-(trimethyl ammonium) ethyl methacrylate chloride.
Polymers which have a significant content of amino groups can be neutralized by adding volatile acids. The resultant cationic carrier groups serve as a vehicle for preparing stable, aqueous colloidal solutions (comparable to the effect of anionically stabilized carboxylate ions).
Polymers bearing amino groups are noted for their outstanding adhesion on different surfaces (metals and plastic parts). They are also suitable for making paper and are used as ingredients for adhesives.
Monomers containing tertiary amines can act as catalysts. On one hand, these groups accelerate the reaction between hydroxyl groups and isocyanates (see Chapter 3). On the other, the tertiary amino groups force the crosslinking reaction of reactive acrylic resins by UV light (see Chapter 5).
2.2.3.4 Amide-functional monomers
Hydrolysis of acrylonitrile or methacrylonitrile leads to acrylamide or methacrylamide, respectively. Polymers with significant amounts of acrylamide or methacrylamide are highly polar, and are mostly used in the paper and textile industries. They are also suitable for water treatment.
Amide-functional monomers are also used in the manufacture of self-crosslinking acrylic resins for coatings applications (see Chapter 3). The resins are produced by making the amides react with formaldehyde and mono-alcohols, or the corresponding semi-acetals. These reactions can be performed in polymer analogeous reaction. There are also monomers that already consist the described modification, namely methylol acrylamide, N-(n-butoxymethyl) acrylamide, and N-(iso-butoxymethyl) acrylamide and the corresponding methacrylamides.
Other monomers available contain alkylated amides, e.g. N-ethylmethacrylamide and N,N-dimethylacrylamide.
In addition, there are (meth)acrylamides whose amide groups have been replaced by tertiary-aminoalkyl groups. Polymers containing such monomers are suitable for flocculants, paper coatings and textile finishing agents.
2.2.3.5 Epoxy-functional monomers
Of the various epoxy-functional monomers, glycidyl methacrylate (2,3-epoxypropyl-1-methacrylate) is made industrially. Acrylic resins containing specific quantities of glycidyl methacrylate can be crosslinked with compounds containing primary and secondary amino groups at ambient temperatures, or with compounds containing carboxyl groups mainly at elevated temperatures. The crosslinkers are polyamines or polycarboxylic acids or their derivatives. The most important application of acrylic resins containing the monomer glycidyl methacrylate is that of powder-coating resins.
Table 2.4: Physical data of industrially produced functional monomers [19–22]
2.2.4. Ether acrylates and methacrylates
Not only diols are partially esterified with acrylic acid or methacrylic acid, but also oligomeric etherdiols. If only one hydroxyl group is esterified, the resultant monomers contain an ether side-chain and a hydroxyl group. Examples are diethylene glycol acrylate and triethylene glycol methacrylate.
Glycol monoethers, too, are esterified with acrylic acid or methacrylic acid. Acrylates and methacrylates of methoxyethanol, ethoxyethanol, methyldiglycol and methyltriglycol are produced industrially. Furthermore, there are esters of acrylic acid with phenoxyethanol.
Polyethers are prepared by adding ethylene oxide or propylene oxide to monoalcohols or polyalcohols. There are also block copolymers which contain both cyclic oxides. The various polyethers may be esterified with acrylic acid or methacrylic acid. A number of monomers with ether side-chains of different chain lengths are available industrially.
Acrylic esters or methacrylic esters of such polyethers containing only one double bond per molecule serve as molecular building blocks for acrylic resins. The polyether side-chains perform a marked plasticizing effect. This increases with increase in length of the polyether chain. If the side-chains consist of polyethylene oxide, the polymers become distinctly hydrophilic. Acrylic resins with significant amounts of polyethylene oxide side-chains afford a way of preparing aqueous colloidal solutions or secondary dispersions featuring steric (non-ionic) stabilization.
Furfuryl and tetrahydrofurfuryl acrylate and methacrylate are ether monomers as well.
2.2.5 Polyunsaturated acrylic and methacrylic compounds
Polyunsaturated acrylic and methacrylic monomers result from the esterification of more than one hydroxyl group of a polyol with acrylic acid or methacrylic acid. As all the double bonds of such monomers are amenable to free-radical-initiated polymerization, the monomers build bridges between different polymer chains. The use of just relatively small quantities of such polyunsaturated monomers for copolymerization yields branched acrylic resins. However, larger quantities cause crosslinking of the polymers. This possibility is exploited in the preparation of aqueous and non-aqueous microgel dispersions.
The possibility of crosslinking is mainly employed in the production of reactive acrylic paint systems, mostly for radiation curing (see Chapter 5). Polyunsaturated monomers are also described there.
2.2.6 Comonomers
A series of unsaturated compounds easily copolymerize with esters of acrylic acid or methacrylic acid (see Chapter 2.2.7). Such compounds are called comonomers. They are added to the resin composition to achieve special properties.
Such comonomers are firstly derivatives of acrylic acid or methacrylic acid, mainly acrylonitrile and methacrylonitrile. Nitrile monomers perform outstanding adhesion properties on the resultant polymers, particularly on metallic substrates.
However, the most important comonomer is styrene (vinyl benzene). Also vinyl toluenes and the various methyl styrenes can copolymerize with esters of acrylic acid and methacrylic acid. Copolymers containing these aromatic monomers are noted for their hardness, and show rapid physical drying in film forming of solvent-borne paints.
Copolymerization of olefins with acrylic esters or methacrylic esters is not possible without further ado. This also applies to vinyl halides, vinyl ethers, and vinyl esters.
Also the copolymerization of acrylic esters or methacrylic esters with maleic acid or its derivates requires special proceedings.
By contrast, N-vinyl compounds copolymerize easily with esters of acrylic acid or methacrylic acid. Such comonomers, which are available industrially, are N-vinyl pyrrolidone, N-vinyl imidazole, N-vinyl caprolactam, and N-vinyl carbazole. These monomers perform excellent wetting and penetration on metal surfaces, plastic parts and paper. They are used to prepare polymers, but a much more important use is in reactive acrylic systems which are crosslinked by UV light (see Chapter 5). These monomers are more difficult to handle as the products solidify at ambient temperatures or higher.
Table 2.5: Physical data of industrially available comonomers [19–22]
Other special monomers contain fluorinated alkyl side-chains, e.g. hexafluorobutyl methacrylate. Such fluorine-containing monomers introduce special surface properties on the resultant polymers.
Surface-active polymers are also prepared by using derivatives of acrylic acid or methacrylic acid with siloxanes; e.g. trimethylsiloxyethyl methacrylate, polydimethylsiloxane methacrylate.
2.2.7 Copolymerization
If the velocity constants for the reaction between the different monomers are much higher than the velocity constants for that between monomers of the same kind (k[1.2] and k[2.1] >> (k[1.1] and k[2.2]), the resultant copolymers have an strictly alternating composition, which is independent of the monomer concentration [2]. This copolymer is formed until one monomer is consumed totally.
The copolymerization behaviour can be determined from the differential change in monomer concentration over time as a function of the quotients of the velocity constants and the actual monomer concentrations. This is shown in Formula 2.16:
Formula 2.16: Calculation of copolymerization behaviour
Clearly, it is very difficult to calculate the copolymerization behaviour of just two combined monomers. The calculation becomes much more complex when more than two monomers are combined. Most acrylic resins consist of more than two monomers. Numerous trials are underway to quantify the copolymerization behaviour of monomers individually [24]. One method consists in fragmenting the velocity constants of the monomers into the effects of the resonance stability of the free-radicals (Q) and the polarity (e) of each monomer. Both terms are then expressed in terms of the values for styrene. For styrene, Q is defined as 1.0 and e as -0.8. Formula 2.17 shows how the Q and e values are calculated.
Formula 2.17: Calculating the Q and e values
Plotting the Q and e values for the various monomers yields the following diagram (Figure 2.2):
Figure 2.2: Q-e diagram
The Q-e diagram makes it possible to estimate the tendency of different monomers to copolymerize efficiently or not. Generally, monomers with nearly comparable values for Q and e will yield highly random copolymers. For most acrylic resins, a random distribution of monomers along the polymer chain is desirable.
2.2.8 Characterization of monomers
2.2.8.1 Glass transition temperature
There are different ways to quantify the properties of acrylic monomers with respect to the influences of the different side-chains. The most important is the glass transition temperature (Tg). The glass transition temperature of polymers is the temperature at which the molecule composite changes from a glassy state into an elastic state. In the glassy state, the molecules of polymers are highly associated – coiled – and have high resistance to mechanical deformation. As the temperature rises, the molecules start to become mobile. In the elastic state, molecules respond to mechanical influence, but return to their former configuration as soon as it ceases. Finally, at even higher temperatures the molecules start to uncoil and will no longer return to their former composite state. The polymer becomes plastic and transfers into a melt. The temperature at which the transition from the glassy state to the elastic state occurs is influenced by the degree to which the polymer molecules are associated. The molecular associations themselves are due to physical chain-to-chain interactions (e.g. due to polar groups) and the stiffness of the chains.
Figure 2.3: Elastic modulus of a polymer as a function of temperature
The glass transition temperature can be determined in various ways. The most informative of these is dynamic thermomechanical analysis (DMTA). It measures the response of polymer films to periodic mechanical forces as a function of temperature. The polymer response is plotted as a chart of elastic modulus (E’’, storage modulus) over temperature. Figure 2.3 shows the change in elastic modulus of a non-crosslinked polymer with change in temperature. The temperature at the point of inflection between the glassy and the elastic states is the glass transition temperature (Tg).
Another method for determining glass transition temperatures is dynamic scanning calorimetry (DSC) [26]. The precise value of the glass transition temperature varies with the measurement method and the measuring conditions. Above a certain molecular weight, the value of the glass transition temperature is independent of the molecular weight and is influenced only by the structure and building blocks in polymer chains.
Generally, for acrylic resins: the stiffer the polymer chain is, the shorter the side-chains are; further, the more polar the building blocks are, the higher is the glass transition temperature. Figure 2.4 shows the glass transition temperatures of homopolymers of different monomers, ordered by the number of C atoms in the side-chains.
The chart shows that methacrylic esters confer much higher glass transition temperatures than the corresponding acrylic esters. The reason is that the methyl group – located on the same C atom as the carboxyl group – restricts the mobility of the chain. This contrasts with the behaviour of methyl side-chains in the case of polyester building blocks (e.g. neopentyl glycol).
The glass transition temperatures show a marked decrease with increase in the length of the side-chains. The effect is more pronounced for methacrylic esters than for acrylic esters. The difference is 95°C for the two methyl esters and only 37°C for the two n-hexyl esters.
Surprisingly, the glass transition temperature of relatively long-chain, linear acrylic esters increases with increase in chain length. The reason is that the longer linear side-chains can associate physically (waxy behaviour of long, linear aliphatic chains). This increase is more pronounced for acrylic esters than for methacrylic esters, because the methyl group on the polymer chain acts as a spacer. Branched side-chains generally confer higher glass transition temperatures than the corresponding linear side-chains. Aromatic groups in side-chains (e.g. styrene) generate relatively high glass transition temperatures due to the association effect of π-electron systems in the aromatic ring structure.
On account of their polarity, hydroxy-functional monomers lead to higher glass transition temperatures than ester monomers having the corresponding number of C atoms. However, butanediol-1,4-monoacrylate generates very low glass transition temperatures. Other polar monomers, such as acids, amides and nitriles, yield very high glass transition temperatures due to the additional scope for molecular association of their polar groups.
Figure 2.4: Glass transition temperature as a function of the number of C atoms on the side-chains
As most acrylic resins consist of a couple of different monomers, the question arises as to the glass transition temperatures of monomer mixtures. The answer is that the glass transition temperature of copolymers is determined by the mass fractions of the different monomers.
However, the resultant glass transition temperature is not the linear mean value of the mass fractions of monomers and their individual glass temperatures. The calculation is based on reciprocal mean values, as shown in Formula 2.18.
Formula 2.18: Glass transition temperature of copolymers
Figure 2.5 shows the glass transition temperature of polymers containing different quantities of styrene and n-butyl acrylate. The chart shows measured glass transition temperatures (determined by DSC) and curves of the linear and reciprocal mean values.
Figure 2.5: Glass transition temperature of different copolymers of styrene and n-butyl acrylate
It is clear that even small fractions of n-butyl acrylate lower the glass transition temperature of styrene copolymers significantly, as they keep the rigid styrene chains apart. Conversely, small fractions of styrene in n-butyl acrylate copolymers are unable to associate and hence unable to increase the glass transition temperature. Mathematically, this behaviour is described relatively well by reciprocal mean values. There are some exceptions, e.g. as when molecule segments interact with each other.
The value of the glass transition temperature represents a great deal of polymer properties. High glass transition temperatures confer high mechanical resistance at ambient temperatures, resulting in polymers of high hardness. A related effect is high diffusion density, which leads to better chemical and solvent resistance. However, a corollary of a high glass transition temperature is low flexibility, and such polymers are brittle at ambient temperatures. The brittleness cannot be compensated until the elastic component of the flexibility has been improved, e.g. by crosslinking reactions. (Flexibility is defined as the sum of plasticity and elasticity.)
Low glass transition temperatures signify high flexibility, as conferred by the plasticity component, but also low hardness and less chemical and solvent resistance. Flexible polymers possess better scratch resistance, because the polymer surface can escape the mechanical stress and because of the ability to compensate surface damage by virtue of their special thermoplasticity (cold plastic flow).
Crosslinking reactions raise the glass transition temperatures of polymers. Optimum crosslinking is associated with better flexibility (elastic component of flexibility), better mechanical strength, and chemical and solvent resistance.
2.2.8.2 Material properties
The nature of the side-chains on monomers in acrylic resins influences not only the balance of hardness and flexibility. Acrylic resins containing monomers with short side-chains are relatively polar polymers. The production of stable solutions therefore requires polar solvents. This also applies to acrylic resins which contain significant fractions of functional monomers and to monomers containing aromatic side-chains. Generally, acrylic polymers containing mainly methacrylic derivatives are more polar than those containing acrylic derivatives.
Acrylic polymers containing long aliphatic side-chains are particular nonpolar, and so are readily soluble in non-polar solvents. In addition, the low polarity leads to optimum application behaviour, better spraying properties, efficient wetting of different surfaces, optimum flow and levelling during film-forming, and to more homogeneous films from aqueous dispersions.
Acrylic polymers containing long aliphatic side-chains have significantly lower viscosities in organic solutions and in melts.
Acrylic resins consisting mostly of methacrylic monomers usually yield higher solutions viscosities than resins made from the corresponding acrylic monomers.
Monomers containing cycloaliphatic compounds confer special effects. Although the glass transition temperatures are relatively high – near those of the corresponding aromatic compounds – and although high hardness values are achieved, the solution viscosities are considerably lower than those of polymers containing short side-chains or aromatic compounds. Cycloaliphatic monomers generate optimum balance of hardness and resistance on one hand, but low viscosity and sufficient flexibility on the other. Unlike the aromatic building blocks, cycloaliphatic monomers generate permeability to UV light, a fact which is an advantage for clear coat formulations. However, these monomers are more expensive than other products.
Functional monomers are introduced in acrylic resins mainly for crosslinking reactions. However, they can additionally act as carrier groups for water-borne secondary dispersions. Furthermore, they support wetting and adhesion and some have a catalytic effect on various crosslinking reactions, as mentioned above.
Monomers containing ether side-chains confer higher plasticity than the corresponding monomers with alkyl side-chains. Polymers containing long side-chains of polyethylene oxides offer scope for preparing non-ionically stabilized waterborne secondary dispersions. The ether groups in such polymers are not resistant to UV light.
2.2.9 Handling of monomers
Acrylic monomers and comonomers can polymerize spontaneously under catalytic influences and at elevated temperatures. For this reason, they are stabilized for storage and transportation. The additives for stabilization are reducing compounds. Historically, hydroquinone was the most important stabilizer. Currently, the most common product is methyl hydroquinone, due to its broader solubility. Other suitable stabilizers are hydroquinone monomethyl ether (4-methoxyphenol), 4-tert.-butyl catechol and 2,5-di-tert.-butyl-4-methylphenol (BHT, Ionol).
The optimum quantity of added stabilizer depends on the monomer type and transport destination (e.g. greater quantities are needed for shipments to the tropics). The quantity varies from 15 to 500ppm. Monomers containing acids, amides, nitriles, or hydroxyl groups require more stabilizer than simple ester monomers. Methacrylic monomers require less stabilizer than acrylic monomers.
As monomers are highly reactive compounds, there are health risks associated with handling them. These vary substantially from monomer to monomer [19–22].
For example, N,N-dimethylaminoethyl acrylate is classified as very toxic (T+), but N,N-dimethylaminoethyl methacrylate is classified “only” as harmful and irritant (Xn and Xi).
The following monomers are classified as toxic (hazard symbol: T): 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, acrylonitrile, methacrylonitrile, acrylamide, N,N-dimethyl acrylamide, N-iso-butoxymethyl acrylamide and a polyethylene glycol methyl ether acrylate.
The monomers acrylic acid, methacrylic acid, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate und N,N-dimethylaminoethyl acrylate are classified as corrosive (hazard symbol: C). The low-boiling esters of acrylic acid and methacrylic acid, the acids themselves and the nitriles are classified as highly inflammable or extremely inflammable (hazard symbols: F and F+).
Acrylic esters containing side-chains of up to four C atoms, and 4-hydroxybutyl acrylate (butanediol-1,4 mono acrylic ester) are classified as harmful (Xn); in addition, 4-hydroxybutyl acrylate is defined as dangerous for the environment (N).
All derivatives of acrylic acid and methacrylic acid are classified as irritant (Xi).
All esters of acrylic acid with side-chains of up to eight C atoms and all esters of methacrylic acid with side-chains of up to four C atoms, and also most of the polyunsaturated monomers, esters of polyols, are deemed sensitising. Derivatives of methacrylic acid are less hazardous than the derivatives of acrylic acid. This becomes particularly apparent for the functional monomers of the both groups.
In summary, the appropriate safety regulations must be observed when monomers are handled. Acrylic polymers have to be checked to avoid that the amounts of residue free-monomers do not exceed the given limiting values. For the use of reactive acrylic resins, free-monomers are placed into circulation. Application of these coating systems requires the adherence of particular safety regulations.
2.3 Production processes – generally
The processes for producing acrylic polymers are designed to efficiently dissipate the heat of polymerization (enthalpy) and to guarantee product reproducibility.
By reproducibility here is meant the molecular weights and molecular-weight distributions and the optimum random distribution of monomers along the copolymer chains.
Suitable processes are the mass polymerization, the suspension polymerization, the emulsion polymerization, and the solution polymerization.
Reactive acrylic resins are produced by reactions of functional monomers with different oligomers, without reaction of double bonds. These processes employ the common methods of polyaddition and polycondensation (see Chapter 5).
2.3.1 Bulk polymerization
For bulk polymerization, monomer compositions are mixed with suitable initiators and heated to the requisite polymerization temperature. The exothermic energy of the polymerization must be compensated by external cooling equipment. The resulting polymers have a relatively broad molecular-weight distribution, and very high molecular weights may be achievable. Total conversion of all monomers is difficult to accomplish. As the reaction is highly exothermic, only small batches are produced. It is more easily to prepare bulk polymerization product in thin layer. Plastic parts are produced in that way, e.g. the socalled acrylic glass, consisting of poly methyl methacrylate. This polymerization process is preferred if the target is to prepare 100 % polymers, e.g. for solvent-free secondary dispersions or resins for powder coatings – but the aforementioned restriction has to be taken in mind.
One variant of bulk polymerization is incomplete polymerization in a continuous process. In this, monomer mixtures and suitable initiators are fed at relatively high temperatures through a tube reactor or a flow-through container, where they are partially polymerized. Free monomer is then separated from the solid polymer by distillation. The residual monomers can be recycled. The process conditions must be selected so as to guarantee constant polymer composition and reproducible molecular weights and molecular-weight distributions. The risk of side reactions at high temperatures must be compensated. The polymerization temperatures must, of course, be lower than the ceiling temperature of the monomers. Principially, this process affords polymers of lower molecular weight and narrower molecularweight distribution than those of common bulk polymerization. This socalled SGO process (solid grade oligomer) can be used to make acrylic resins for high-solid coatings [29] which are dissolved in appropriate solvents. The products are also suitable for preparing solvent-free, water-borne secondary dispersions.
2.3.2 Suspension polymerization
In suspension polymerization [30], monomer mixtures are converted into an aqueous emulsion and a free-radical polymerization process is started by initiators which are soluble in organic solvent. The emulsion consists of rather large particles. To prevent fusion of the particles, protective colloids (e.g. polyvinyl alcohol) and specific emulsifiers are added. The particle size and the particle size distribution depend on the shearing conditions (type of stirrer and shear rate).
Suspension polymerization is mostly performed with monomers that generate high glass transition temperatures.
The process is principially the same as that of bulk polymerization, but takes place in individual droplets. The surrounding water readily absorbs the heat of polymerization. When polymerization is complete, the resultant polymer particles are filtered and washed thoroughly. The target is to remove as much of the protective colloids and emulsifier as possible. The polymer particles look like pearls and are mostly delivered as such. Most of these polymers have relatively high molecular weights and a broad molecular-weight distribution, just like bulk polymers. The resultant products are mainly used for the production of physically drying coatings.
2.3.3 Solution polymerization
In solution polymerization, the monomers and organophilic initiators are reacted in organic solvents. The monomers as well as the polymers are soluble in the selected solvents, which absorb the reaction enthalpy. The resultant solutions are mainly the form in which the acrylic resins are shipped. Consequently, the selected solvents must meet the application requirements of the resultant acrylic coatings (see Chapter 3).
2.3.4 Emulsion polymerization
In emulsion polymerization, monomer mixtures are dispersed in water with the aid of suitable emulsifiers. The polymerization reaction, which takes place at elevated temperatures, is initiated by water-soluble initiators and occurs in micelles. The outcome is stable aqueous dispersions which are used for wall paints and house paints (for craftsmen and for the do-it-yourself sector), and for adhesives, printing inks, and textile treatments (see Chapter 4).
2.4 Literature
[1] J. C. Bevington: Journ. Chem. Soc. (1956)
[2] Bruno Vollmert: Grundriss der Makromolekularen Chemie, E. Vollmert Verlag, Karlsruhe (1979)
[3] P. J. Flory: Principles of Polymer Chem.; Cornell Univ. Press; Ithaka (1986)
[4] K. Matyjaszewski, T.P. Davis: Handbook of Radical Polymerization, J. Wiley & Sons, N. Y. (2002)
[5] B. Tieke: Makromolekulare Chemie, Wiley-VCH (2002)
[6] H. G. Elias: Makromoleküle, Wiley-VCH (2002)
[7] A. M. North: The Kinetics of Free Radical Polym.; Pergamon Press; Oxford (1965)
[8] F. T. Maler, W. Bayer: Encyclopedia Chem. Process Des. 1 (1976)
[9] D. J. Hadley, E. M. Evans: Propylene and its Derivates, J. Wiley & Sons, New York (1973)
[10] Specified in DE 102004033555 and US 2006084779, BASF SE
[11] E. H. Riddle: Monomeric Acrylic Esters; Reinhold; New York (1954)
[12] H. Rauch-Puntigam, T. Völker: Acryl- und Methacrylverbindungen. Springer, Berlin (1967)
[13] W. Reppe 1939 at Röhm & Haas
[14] D. J. Hucknell: Selective Oxidation of Hydrocarbons, Academic Press, London (1974)
[15] Acetoncyanhydrinprozess 1933 at Röhm & Haas, Darmstadt
[16] J. W. Crawford: Chem. Abstr. 28 (1934)
[17] Specified in GB 405 699 (1934) from ICI
[18] Isobutene process: J. W. Nemec, W. Bayer: Acrylic and Methacrylic Polymers, Encycl. Polym. Science and Engineering; J. Wiley & Sons, New York (1985)
[19] Technical data sheets from Evonic (Röhm)
[20] Technical data sheets from BP
[21] Technical data sheets from BASF SE
[22] Technical data sheets from International Speciality Chemicals (Bisomer)
[23] Specified e.g. in DE 1568487 (1966) from Bayer
[24] Alfrey and Price, derivation of Arrhenius equation
[25] W. F. Hemminger, H. K. Cammenga: Methoden der Thermischen Analyse. Springer Verlag Berlin
[26] Höhne, G. Hemminger, W. and Flammersheim, H.-J. (1996): Differential Scanning Calorimetry – An introduction for Practioners. Springer-Verlag Berlin
[27] Fox equation
[28] H. F. Mark: Encyclopedia of Polymer Science and Engineering, J. Wiley & Sons, London (1985)
[29] specified in US 6552144 (2000) from Johnson Polymer
[30] C. E. Schildknecht und I. Skeist: Polymerisationsprozesses; Polymerisation in Suspension J. Wiley & Sons, London (1977)
3Solution polymerization products
Ulrich Poth
3.1Definition
This chapter discusses acrylic resins which are made by polymerization in organic solution. Most organic polymer solutions produced in this way serve directly as resin solutions for all kinds of solvent-borne coating materials.
They may also be used in the production of water-borne coatings. In that event, the solvent used for the polymerization process also serves as co-solvent for the aqueous formulation. Or, after the resin has been transformed into the aqueous phase, the solvent is distilled off, to yield a solvent-free water-borne system. The aqueous colloidal solutions or secondary dispersions of resins are stabilized by ionic or non-ionic carrier groups. Acrylic resins prepared by solution polymerization can also be converted into secondary aqueous dispersions by adding emulsifiers. In most of these cases, the process solvent is distilled off. Combinations of both processes are possible, too.
Removing solvent by distillation also affords a route to solid resins. Special solid resins are used for powder coatings.
3.2History of acrylic resins made by solution polymerization
From the very beginning, it was believed that the use of solvents would be advantageous to the production of acrylic resins for coatings. First, the solvent is able to absorb and dissipate the heat of exothermic polymerization reactions for exploitation in reflux cooling. The second advantage is, the resultant solutions can be used directly in the preparation of coatings.
Acrylic resins dissolved in organic solvents were initially used for paint systems that dry physically. They were in competition with coatings based on cellulose nitrate. The one particular advantage they offered was superior weatherability.
At that time – in the 1940s – the fact that the attainable molecular weights of solution polymers were relatively low due to the regulating effect of the process solvents was considered a drawback. Only when it became clear in the early 1950s that very high molecular weights arose from crosslinking reactions after application of functional acrylic paints did acrylic resins for coatings became more important. In those days, the prime application of acrylic resins was for stoving enamels, predominantly in the U.S.A. Acrylic monomers were less expensive there than in Europe, where the dominant class of resin for that application was that of alkyd resins. The first commercially important acrylic resins for coatings prepared by solution polymerization were paints based on resins containing etherified methylol acrylamides. Such resins can self-crosslink at elevated temperatures. On account of the films’ particularly good chemical resistance, the products were used to make coatings for household appliances (washing machines, refrigerators), so-called “white goods”. In the 1960s, the market for automotive OEM (original equipment manufacturing) finishes in the U.S.A., mainly topcoats, was dominated by stoving enamels based on hydroxyfunctional acrylic resins and melamine resins as crosslinkers. In the 1970s, acrylic resins made the great breakthrough in two coatings application fields in Europe. Automotive OEM clear coats were now formulated on acrylic resins and melamine resins because they offered better weatherability than their alkyd counterparts. And, automotive repair topcoats were increasingly reformulated to products based on hydroxy-functional acrylic resins which were crosslinked with polyisocyanate adducts (two-component coatings). Meanwhile in the U.S.A., the aromatic solvents in common use were (and still are) considered critical due to their photolytic effect in the atmosphere. They had to be replaced [1]. This spurred the development and application of, among others, acrylic resins in the form of non-aqueous dispersions (NADs) in aliphatic hydrocarbon solvents. These have the additional advantage of allowing higher application solids, a fact which stems from different thinning behaviour on the part of dispersions. Since the late 1970s, there had also been numerous attempts in Europe to reduce or avoid emissions of volatile organic compounds (VOCs) from coatings during application processes. First to come onto the market, in the 1980s, were so-called high-solid acrylic resins. Their purpose was to yield paints with the maximum-possible application solids. Such products dominate the market of industrial coatings to this day. Special solid acrylic resins for high-solid coatings are made by a continuous high-temperature polymerization process [2]. At the same time, intense research was being done on new ways to crosslink acrylic resins. However, even now, the well-established crosslinking principles are still in use, even though there has been progress in this regard.
Although the principles behind the production of water-borne secondary dispersions of acrylic resins have been widely known since the advent of acrylic polymers, the field of application is still limited. The main problem with such dispersions is that they have difficult application behaviour.
Since the early 1990s, powder coatings based on acrylic resins made by solution polymerization are in development. On account of their typical properties, these powder coatings are mainly used for weatherable topcoats and clear coats. However, due to the relatively high costs of the raw materials and production process, such powder coatings are not widely used. An additional constraint is that powder coatings must be applied in relatively thick layers if they are to yield smooth and glossy coating films. But other solventless coating materials are launched onto the coatings market have gained ground in several application fields, namely reactive acrylic resins, which are liquid, 100% systems based on unsaturated oligomers and acrylic monomers, and are cured by radiation, mainly UV light. They are enjoying continuous growth rates in the coatings market (see Chapter 5).
