Carboxy-functionalized resins

In order to immobilize the linker, the hydroxy group is either treated directly with a carboxy functionalized resin or is coupled to a dicarboxyl-ic acid spacer on aminomethyl resin. 

COOH HOOC xAr COOH branched, low molecular mass, carboxy-functional 1000- 5000 triglycidylisocya-nurate, epoxy resins, melamine resins powder coatings, waterborne paints... 

The reaction system consisting of acid (i.e., carboxy-functional) polyester resins and epoxy resins is important for powder coatings [2.84]. 

Carboxy-functional (acid) resins for combination with triglycidyl isocyanurate or... 

Glycidyl Esters. Glycidyl ester resins were originally developed for electrical applications. Glycidyl esters of phthalic acid, hexahydro phthalic acid, terephthalic acid or trimellitic acid (e.g. Araldite PY 284, PT 910) cured with carboxy functional polyesters or polyacrylates at elevated temperatures give coating with both excellent colour stability and outdoor resistance. 

The most important combinations of epoxy resins and curing agents are summarized in Table 2.6. Depending on their molecular mass, bisphenol A epoxy resins can be cured by polyaddition via their epoxy or hydroxyl groups. Polyamines, poly thiols, and polyisocyanates are suitable for room temperature cure. Polyanhydrides, polyphenols, acids, and carboxy-functional polyesters are suitable for hot cure. 

Acids and Carboxy-Functional Polyesters. Curing of epoxy resins with acids and carboxy-functional polyesters requires heat. Industrially, these systems are most important in the formulation of powder coatings more than 70% of powder coatings are based on epoxy resins and carboxy-functional polyesters (see also Sections 3.4.2 and 3.4.3). 

Plasticized epoxy resins can be produced by reacting low molecular mass epoxy resins with dimeric fatty acids. These resins (e.g., Epikote 872, Shell) generally exhibit better substrate wetting than the unmodified epoxy resins. This reaction is, however, associated with the formation of ester bonds, and the resins therefore have a poor resistance to alkaline solutions. Instead of fatty acids, carboxy-functional polybutadiene-acrylonitrile elastomers can also be used to flexibilize epoxy resins. 

Powder coatings based on aromatic epoxy resins have high chemical and mechanical resistance, but are not resistant to weathering. To obtain weather-resistant powders, triglycidyl isocyanurate (Araldite PT 810, Ciba-Geigy Tepic, Nissan) must be used as the epoxy component with carboxy-functional polyesters. 

On-line coupling of pyrolysis, gas chromatography, and mass spectrometry is a quick and elegant method for the qualitative detection of monomer units in many resins (e.g., polyesters, polyurethanes, phenolic resins, and polyacrylates). Identification of comonomers of polyacrylates, including hydroxy-functional and carboxy-functional monomers, is facilitated if the sample is silylated before pyrolysis [10.30]. 

C. Wang, G. Lin, J. H. Pae, F. N. Jones, H. Ye and W. Shen, Novel synthesis of carboxy-functional soybean acrylic-alkyd resins for water-reducible coatings , J Coat Technol, 2000, 72, 55-61. 

The stepwise introduction of N -protected amino acids in solid phase synthesis normally involves in situ carboxy activation of the incoming amino acid or the use of pre-formed activated amino acid derivatives. In order to drive the acylation to completion an excess of activated amino acid derivative is utilized, typically 2-10 times the resin functionality. The excess used depends on the nature of the activated species and on the resin loading and void volume of the reaction vessel employed. The most important consideration is to maintain as high an elective concentration of reagents as possible. In small-scale synthesis and with low-functionality resins, large excesses may need to be used as the dead volume of such systems tends to be quite high conversely with resins of high functionality lower excesses can be utilized. 

Oligomerization methods for preparing carboxy-functionalized liquid oligomers have been described in the patent and technical literature [143-156]. The main chemical agent employed here is maleic anhydride (MA) thus, the products obtained are called maleic oils or maleic resins [157]. Also, beta unsaturated mono-or dicarboxylic acids can be used as well as some of their functional derivatives such as acrylates, itaconic anhydride, citraconic anhydride, and fumaric and meth-arcrylic esters [155,156]. 

In single pack heat cured coating systems, carboxy functional acrylics can be crosslinked at elevated temperatures with amino and epoxy resins, as shown in Figure 4-5. 

Crosslinking Reactions for Carboxy Functional Acrylic Resin... 

However, the major use for heat cured, carboxy functional acrylics is in combination with either Bisphenol A based glycidyl ether epoxy resins, or hexamethoxy methyl melamine resins, for metal decorating applications, where their flexibility and deep draw properties are essential. 

In the early days, although paint manufacturers flirted with acrylamide chemistry for automotive topcoats, their higher curing temperatures of 150 C and above precluded commercialisation in this end use. Carboxy functional acrylics have inferior weathering performance. Consequently, thermosetting acrylic topcoats are almost exclusively based on hydroxyl functional polymers. These are crosslinked with either alkylated melamine formaldehyde resins or as 2 component (2K) systems, with aliphatic isocyanate adducts. The 2 component systems are an extremely important class used mainly in automotive repair applications and warrant a chapter in their own right (see Chapter V on component isocyanate curing systems). 

It is advisable to include some carboxy functionality also as the cure reaction between acrylic and melamine resins is acid catalysed. In the more sophisticated industrial markets for acrylics, stoving temperatures are well controlled and adequate cure cycles are achievable. However, in the competitive general metal finishing market, low temperatures and short times are common, so whilst it is possible to externally catalyse using PTSA (para toluene sulphonic acid), this polar material remains in the film after curing to act as a pathway for water. Blistering on exposure to humidity is a common problem in acrylic-melamine systems which have been catalysed by sulphonic acids. 

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