Cure Reactions of Polymer Coatings
Piotr Czub and Anna Sienkiewicz
Cracow University of Technology
The basic component of any coating system is the binder, in the case of polymer coatings consisting of an oligomer/polymer and a suitable curing agent or a monomer and an initiator for its polymerization. The cross-linking of the polymer used as the basis for the binder is a necessary step in the formation of the coating film. At the same time, the cross-linking is an equally important as the process responsible for the integrity of the coating (cohesion strength), and even for its adhesion to the substrate or to a subsequent layer in a multilayer coating. The resulting three-dimensional network structure of the cross-linked coating combines the binder with pigments, organic or inorganic fillers and all applied non-reactive additives (e.g. rheological, like a defoamer), thinners, antioxidants, etc. On the other hand, reactive additives (e.g. diluents) taking part in the cross-linking process can be built into the structure of the coating. For example, aliphatic diepoxides, commonly used as the reactive diluents for the bisphenol А-based low-molecular-weight epoxy resins, not only reduce high viscosity of these resins, but they also act like a plasticizer. The cross-linking of the binder provides the coating with the desired mechanical strength, chemical resistance and thermal stability. Depending on the used polymeric binder, the process of cross-linking may occur according to various mechanisms. The most often, this is done by reaction of functional groups present in an oligomer and in a cross-linker. The cross-linking of epoxy, novolac and polyester resins proceeds mainly in this way. In turn, in the case of many important polymer coating materials, such as acrylates, vinyl, alkyd and unsaturated polyester resins, the cross-linking process is carried out by polymerization or copolymerization, usually radical, but also ionic polymerization as well as photopolymerization. A specific process, also involving the reaction of functional groups, is the cross-linking polymerization carried out using of multifunctional monomers (i.e. the synthesis of polyurethanes and polyesters).
Cross-Linking via Functional Group Reactions
Monomers used in polyaddition and polycondensation reactions as well as the resulting oligomers or prepolymers contain active functional groups, usually terminal. These end groups can be successfully used to build the polymer chain, but also in construction of the spatial structure in the cross-linking process. The cross-linking density of such structures (which has the greatest impact, among others, on their mechanical strength, water and solvent absorption, and thermal properties) can be controlled by the length of the oligomeric chains of the materials constituting the basis of the binder, as w'ell as the functionality and structure of the cross-linking agents. Depending on the selected type of the polymer binder and the cross-linker, it is possible to prepare and use the one- and two-component coating systems, as will be shown in the examples below.
The epoxy resins, most commonly used in the commercial coating systems, are the products of the reaction of bisphenols (usually bisphenol A) with epichlorohydrin. As can be seen in Figure 8.1, the oligomeric products of this reaction, which are diglyc- idyl ethers, contain two epoxy groups at the ends of the chains and a different number (depending on the degree of polymerization) of secondary hydroxyl groups. Both types of functional groups can be used to cross-link the resins, although they differ in their reactivity. However, due to the strained structure of the oxirane rings, they are characterized by exceedingly reactivity towards both nucleophilic and electrophilic species and therefore easily react with a wide range of chemicals that can be used as cross-linking agents. In practice, mainly polyfunctional amines (aromatic, aliphatic or cycloaliphatic), polyaminoamides and carboxylic acid anhydrides are used to cross-link the liquid low-molecular-weight epoxy resins (и=0.1—1.2). Generally, aliphatic or cycloaliphatic polyamines, especially with primary groups, however
FIGURE 8.1 The chemical structure of the bisphenol А-based epoxy resin.
hazardous to handle, are very convenient cross-linking agents because the curing process using them can be carried out at ambient temperature. After the first step of the reaction of primary amine and epoxy groups, the secondary amine group is created, as shown in Figure 8.2. It reacts with the next epoxy group at the same time cross-linking different oligomeric chains. It should be remembered that cross- linking with very reactive primary amine groups is accompanied by the release of a large amount of heat, and also the volumetric shrinkage of the cured composition appeared. The structure of the industrially most commonly used polyamines and polyaminoamides is shown in Figure 8.3. Aromatic amines require an elevated curing temperature, but provide a higher glass transition temperature value as compared to aliphatic polyamines. Usually, the cross-linking of the composition is carried out in two stages, first at a temperature of up to 60°C and then even up to 120°C-140°C, depending on the type of amine used. Epoxy resins cured with aromatic amines are particularly resistant to thermal ageing, and chemical resistance of such cured resins exceeds that of those cross-linked with aliphatic polyamines or organic carboxylic acid anhydrides. Polyaminoamides, most often obtained from unsaturated fatty acids and polyamines, contain nitrogen-related active hydrogen atoms (in primary and secondary amine groups), thanks to those they can react with epoxy resins like all amines. They are non-volatile and less toxic than amine cross-linking agents (although they may also contain free amines). Polyaminoamides easily mix with the
FIGURE 8.2 The cross-linking reaction of epoxy resins using primary and secondary amines.
FIGURE 8.3 Examples of aliphatic, aromatic and cycloaliphatic polyamines and polyami- noamides used as curing agents for epoxy resins.
epoxy resins, the curing process can be carried out at room temperature, and shrinkage during cross-linking is very small. They are a kind of internal plasticizers of the cross-linked resin increasing its impact strength. The presence of long fatty acid chains in the vicinity of the amino group gives to polyaminoamides the nature of a surfactant resulting in an increase in a positive effect on the wetting of the substrate as well as the adhesion of the coating film (Czub, Bortcza-Tomaszewski, Penczek and Pielichowski, 2002). In turn, lower cross-linking density may, however, cause a decrease in the chemical resistance of the coating.
The above-mentioned secondary hydroxyl groups, present in the structure of the epoxy oligomers, play an important role in the cross-linking with carboxylic acid anhydrides, mostly cyclic anhydrides of diccarboxylic acids and dianhydrides of tet- racarboxylic acids. The cyclic rings of anhydrides first must be opened, and it is realized just in the reaction of them with the secondary hydroxyl groups, which is shown in Figure 8.4. Additionally, tertiary amines allow cross-linking of the epoxy resins with carboxylic acid anhydrides, acting as the anhydride ring-opening catalysts, which is also shown in Figure 8.4; and they can also limit a possible side reaction that may occur between the epoxide and hydroxyl groups. Examples of the acid anhydrides used to cure epoxy resins are shown in Figure 8.5. Although the cross-linking with carboxylic acid anhydrides requires heating for a long time at the high temperature (usually above 120°C), the curing process is characterized by a low exothermic thermal effect and a low volumetric shrinkage. Anhydride-cross-linked epoxy resins are free of internal stress and resulting mechanical defects. Therefore, they are characterized by high thermal stability and are perfect for the industrial tooling and for insulation materials in the electrical engineering.
Among the group of nucleophilic compounds, the phenolic resins, e.g. phenol- formaldehyde resins, and above all, thermoplastic novolac resins, are also used as cross-linking agents for epoxy resins. As can be seen in Figure 8.6, in addition to the
FIGURE 8.4 The cross-linking reaction of epoxy resins using carboxylic acid anhydrides.
FIGURE 8.5 The structure of exemplary acid anhydrides used for curing of epoxy resins.
FIGURE 8.6 The cross-linking of epoxy resins with phenolic resins.
main reaction between the phenolic and epoxy groups, other reactions that contribute to cross-linking of the composition take place in parallel. The most important of them is the reaction of epoxy groups with secondary alcohol groups - contained in the initial epoxy resin and formed in the main reaction. An ether bond is formed in this reaction, and a secondary hydroxyl group is formed, which further participates in the cross-linking process. The compositions made of the high-molecular-weight epoxy resins based on bisphenol A cured with the novolacs are the coating materials with excellent adhesion, mechanical strength, chemical resistance and flexibility.
Tertiary amines are commonly used as accelerators of cross-linking of epoxy resins with aromatic amines and carboxylic acid anhydrides. However, tertiary amines can also be used as catalysts in the anionic ring-opening polymerization of epoxides, as shown in Figure 8.7. Hydroxyl groups are necessary to initiate and accelerate polymerization, and they may be present in the resin itself, in an amine catalyst or could be introduced with additives, such as alcohol or phenol. A minimum amount of hydroxyl groups, e.g. those present in moisture from the air, is sufficient to initiate the reaction. The structure of tertiary amines suitable for ring-opening polymerization of epoxy resins is presented in Figure 8.8.
For the high-molecular-weight epoxy resins (n=4-14), typical binder for powder coatings, that have oxirane rings only at the ends of long chains, another method of cross-linking must be applied. For such resins, the amount of epoxy groups is too small to cross-link the resin conventionally, and for this purpose, secondary hydroxyl groups present in large amounts in the chains should also be used. Dicyandiamide and 2-methyl imidazole are used for this purpose. During the cross-linking process using dicyandiamide, two reactions take place in parallel: epoxy groups with the four active
FIGURE 8.7 The mechanism of the tertiary-amine-catalysed polymerization of epoxy resins. (Sufficiently adapted - modified and redrawn based on Czub, Boncza-Tomaszewski, Penczek and Pielichowski, 2002.)
FIGURE 8.8 The structure of tertiary amines suitable as catalyst for ring-opening polymerization of epoxy resins.
FIGURE 8.9 The cross-linking of high-molecular-weight epoxy resins with dicyandiamide. (Sufficiently adapted - modified and redrawn based on Ellis, 1993.)
protons of each dicyandiamide molecule and homopolymerization of epoxy resin, initiated by the tertiary amine (as shown in Figure 8.9) (Ellis, 1993). The next stage of the process is the joining of polymer network fragments, resulting from the reactions described above, proceeding through the reaction of secondary hydroxyl groups present in the partially hardened resin with the cyano groups in the dicyandiamide. Imidazoles are applied as efficient accelerators for the dicyandiamide (as well as anhydride) cross-linking. Additionally, they also act as catalytic epoxy curing agents at moderate-to-high temperatures. Curing with 2-methylimidazole proceeds according to the mechanism shown in Figure 8.10 (Ellis, 1993), which assumes that the epoxy group first reacts with the hardener’s amino group to form an equimolar adduct. Then, the next epoxy group reacts with the resulting adduct, forming a compound containing in its structure a highly reactive alkoxylate ion, initiating rapid anionic polymerization of the epoxy groups. Secondary hydroxyl groups present in the chains of the high-molecular-weight epoxy resins can also be applied to cross-link the resins using blocked isocyanates. After unblocking at elevated temperatures, the released isocyanates typically react with hydroxyl groups, giving the cross-linked product.
The basic reaction during polyurethane synthesis occurring between difunctional isocyanates and alcohols (usually diols, glycols, polyesterols or polyetherols) leads to the formation of oligomers or polymers with a linear structure as shown in Figure 8.11. With an excess of isocyanates and at elevated temperatures, the resulting urethane groups (or urea groups in the case of the reaction of isocyanates with amines) can react with isocyanate groups to give allophanates (or biurets). These reactions lead to
FIGURE 8.10 Ring-opening mechanism of epoxy resin polymerization initiated by 2-methylimidazole. (Sufficiently adapted - modified and redrawn based on Ellis, 1993.)
FIGURE 8.11 Reactions occurring during the synthesis of polyurethanes.
FIGURE 8.12 The reaction of isocyanates with water and subsequent reactions.
FIGURE 8.13 The multifunctional raw materials for the polyurethane synthesis.
the formation of a branched and cross-linked structure. Nevertheless, the reaction of isocyanate groups with atmospheric moisture is the oldest method of cross-linking of polyurethanes. This reaction leads to formation of an unstable carbamic acid, which quickly dissociates to carbon dioxide and a primary amine, as shown in Figure 8.12. The resulting amine reacts also very quickly with isocyanate groups forming ureas. Analogously, biurets and allophanates are also formed as a result of follow-up reactions. Cross-linking of polyurethanes, in both one-step and two-step syntheses, may be achieved by using reagents with a higher functionality than two. Examples of raw materials enabling the synthesis of cross-linked polyurethanes are shown in Figure 8.13.
Phenol- and Amino-Formaldehyde Resins
Depending on the conditions of reaction of phenol with formaldehyde, two different products are obtained: resole or novolac resins. Both products, although differing in the structure, are generally oligomers and require cross-linking. The cross-linking makes the coating based on phenol-formaldehyde resins insoluble, tasteless and odourless as well as mechanically strong, and resistant to exposure to high temperature, chemicals (except alkalis), solvents and hot water (Dodiuk and Goodman,
FIGURE 8.14 The structure and cross-linking reaction of phenol-formaldehyde resins.
2014). Resole phenolic resins are methylol-bearing products, while novolacs are non- methylol-bearing products, as can be seen in Figure 8.14. Therefore, the resole resins are ready for self-curing, which is carried out at elevated temperatures with or without an acid catalyst. The creation of the spatial structure occurs at the expense of reactive methylol and hydroxyl groups, in the condensation process leading to creation of methylene bridges. The novolac resins don’t have methylol groups, and they are not able to self-cure. Hexamethylenetetramine (urotropin) is typically used for cross-linking of the novolac resins (as shown also in Figure 8.14). It is a condensation product of formaldehyde and ammonia, and therefore, the cured products contain the methylene and dimethylene amino bridges. The novolac resins may be cross-linked also with formaldehyde or methylol-bearing compounds, such as bismethylol cresol and tetramethylol bisphenol A.
Amino resins (the products of formaldehyde condensation mainly with urea or melamine, but also with benzoguanamine or glycoluril) contain, in their structure, methylol groups, which undergo condensation reactions with the liberating water and forming of dimethylene ether linkages (as can be seen in Figure 8.15).
Polycondensation of polyols with polycarboxylic acids (or their anhydrides or esters) leads to formation of saturated polyesters terminated with hydroxyl or carboxyl groups, depending on the molar ratio of reagents. Usually, diols (glycols) and dicarboxylic acids (phthalic and terephthalic acids or their anhydrides) are applied; therefore, linear chains of oligomeric/polymeric products are formed, as can be seen in Figure 8.16.
FIGURE 8.15 The cross-linking of a melamine-formaldehyde resin.
FIGURE 8.16 The saturated polyester synthesis and commonly used raw materials.
However, reactants with functionality higher than two (shown also in Figure 8.16) are also used, giving branched or cross-linked polyesters, dependent on the polycondensation conditions. The trifunctional alcohols (glycerol and trimethylpropane), the tetrafunctional pentaerythritol or the hexafunctional sorbitol and trimellitic acid, the tetrafunctional citric acid or pyromellitic dianhydride are most commonly used for this purpose. Also, the functionality of the reagents may depend on the reaction conditions. Thus, glycerol is the bifunctional compound at 180°C, because only the primary hydroxyl groups are active at this temperature. But already at 220°C, the secondary hydroxyl group is involved in the reaction, due to which glycerol becomes a trifunctional compound. The high-molecular-weight polyester resins terminated with carboxyl groups usually were previously cross-linked using 1,3,5-triglycidyl isocyanurate (shown in Figure 8.17), which provides very good flexibility, improved UV-light resistance and high thermal stability for the cured resin. However, due to mutagenic effects, its use is recently restricted for occupational health and safety reasons. It can be successfully replaced by such compounds as tris(oxiranylmethyl) benzene-1,2,4-tricarboxylate and N,N,N',N'-tetrakis(2-hydroxyethyl)hexanediamide (shown also in Figure 8.17). These compounds also provide very good properties for cross-linked coatings. However, it should be remembered that cross-linking with the first of them may begin already at 40°C, and when curing with the second one, moisture is released, which can cause coating defects. The blocked isocyanates may also be successfully applied. Additionally, oligomeric polyesters terminated with hydroxyl or carboxylic groups can be cross-linked using other resins, such as epoxides, phenol- and amino-formaldehyde resins or polyisocyanates, selected taking into account their end groups.
FIGURE 8.17 The cross-linking agents for curing of the high-molecular-weight polyesters.