Application principle of amino resin crosslinking agent in coatings

The main role of amino resin crosslinking agent
application
amino resin
(melamine-formaldehyde, benzene melamine formaldehyde and urea formaldehyde (urea) resin) in thermoclicated coatings is to connect the main film-forming material molecules,
through chemical reactions into a three-dimensional (three-dimensional) mesh structure.
this mesh structure is obtained by the reaction of amino resin molecules with the official energy groups on membrane-forming material molecules, and by the aggluent reaction of other amino resin molecules at the same time.
amino resins are easily reacting with polymers with burxyl and mid-hydroxyl, pyrethroid, and alamide-based, so amino resins are commonly used in paint systems based on acrylic, polyester, aolic acid, or epoxy resins .
are also used in polyurethane systems as coating additives to improve the overall performance of certain uses of coatings.principle of amino resins:
amino resins are far more important in paint than they occupy in coatings. Understanding how to design coating formulations using the chemical properties of amino resins has become increasingly important. For example,
coating formula designers are not satisfied with some of the properties of the coating film, can be adjusted by the following methods:



1,
film resin itself improvement or re-selection
;
4,
the choice of catalyst (plus and without, or how much).
Above 4 in addition to Article 1 are related to
amino resins
,
and the performance of amino resins depends on their own energy groups and their activity,
therefore it is important to understand the structure of amino resins. However, before we know about amino resins, we must first have a preliminary understanding of the main resin with amino resins.
mentioned earlier in the report that amino resins are mainly used with
alcoholate resins, acrylic resins, polyester resins, epoxy resins
. Aliclicic resins are mainly synthesized by polyols and polyacic acid resins by esterification reaction, the synthesis process of general alcohols will be appropriately excessive; The number of carp and hydroxyls is usually indicated by acid and hydroxy values. Acid value refers to the number of milligrams of KOH required to be staaled in 1g solid resin with KOH titration. Hydroxy value refers to the number of milligrams of KOH required for the complete titration of KOH in 1g solid resin. Similarly, polyester resins, acrylics, and amino resins also contain certain amounts of carboxyl and hydroxyl. Only synthetic resins use different raw materials, such as acrylic resin from acrylic, hydroxypropyl from hydroxypropyl acid, amino resin contains different amounts of carboxyl and hydroxyl. Acid value, hydroxyl value and viscosity are important indicators of resin, which directly affect the performance of resin.
back to the theme of amino resins, let's first look at the structure of amino resins: figure
is a partially alkylized amino resins that contain alkyds, absinthy, and hydroxymethylene. If the six-way ring between carbon and nitrogen atoms is regarded as a skeleton, the sub-frame or branch derived from it can be visually said to be three heads and six arms. The variety of amino resin properties is the result of the different "arms" and the intricate arrangement between them.
figure 2 shows an extremely symmetrical HMMM structure, a fully methyl etherized amino resin, with only one official group: methyloxy, which is idealized. Since etherization is unlikely to reach 1:6 (maximum) in actual production, the ammonia resins referred to as whole methyl etherization will always have a bit of ammonia and hydroxymethylene.
follows the principle of amino resins to understand its nature: the first step in
synthetic resins is to make melamine react with formaldehyde in the presence of a catalyst to form polyxymethane. All active hydrogen atoms on the tricyclic ring can be converted to hydroxymethane, but in fact two to six moles of formaldehyde are reacting to the tricyclic ring, and those remaining unresponsive active hydrogen atoms are represented by subaminides. We will see later that these groups play an important role in the curing reaction process through the self-amerization reaction.
polyhydroxymethylene melamine is unstable and has only a limited solubility in conventional paint solvents. Amino resin in the coating is mainly cross-curing effect, in order to create a suitable crosslinker for coatings, it is generally necessary to hydroxymethylene and a short chain of alcohol etheric reaction, in order to reduce its reaction activity, and improve its solubility with conventional film-forming materials and fat family solvents. Short-chain alcohols generally use methanol and butanol to control the addition of methanol or butanol and other conditions, can be obtained with different degrees of amino resin.
only the site that reacts with formaldehyde (hydroxymethane) can be alcohol-sealed, and the unresponsive hydrogen atom (subaminin) does not react with short-chain alcohols. In addition, this reaction showed that all six hydroxymethylene reacted with alcohol to produce hexane oxygen methyl melamine, which can actually control the reaction of one to six hydroxymerethyl and alcohol. So there are such different kinds of amino resins.self-polymerization of
amino resins
: The molecular weight of
amino resins is determined by the degree of self-shrinking or bridging between the official energy groups (subamyl, hydroxymeryl, alkymethylene) and melamine molecules in the triamgen ring. In the final application, the amino resin molecular weight affected by bridge polymerization has a great influence on the performance of coating film.
of amino resins can occur in the following ways:
where the reaction identified on the left produces a Methylene bridge, and the right reaction produces a METHYLENE bridge. The degree to which amino resins are bridged is usually expressed in the form of polymerity (DP): DP - molecular weight / weight of each triammolecine ring. Early production of amino resins are basically self-polymerized, DP>3.0. With technological advances, it is possible to minimize the amount of shrinking in amino resin finished products. At present, the commercialized melamine resin has as low as DP-1.1.
the molecular weight of amino resins can be reflected in the viscosity of coatings. DP> 2.0 melamine resin must be released with solventene to 50%-80% solid, in order to achieve the viscosity that can be applied. Single-body DP between 1.1 and 1.5 melamine resins can usually be supplied in 100% effective solid form, the effect of additional solvents on the completed coating VOC is very large. The molecular weight of amino resins also affects the curing reaction and film properties of coatings. A coating system that uses a high DP amino resin will take a shorter time to reach the specified crosslink density than a coating system that uses the same structure, but a lower DP amino resin, so coatings containing high DP crosslinkers require fewer catalysts or weaker acid catalysts to achieve the same curing state. The influence of molecular weight on film coating is mainly in the flexibility range. Coating film cured with high DP amino resin, containing a higher percentage of amino-amino bonds and fewer amino-paint bonds. This type of cross-linking network structure forms a coating with good hardness, but may be brittle. Sometimes it can be compensated by selecting a more flexible paint resin. However, it is generally required that the use of highly flexible coating film requires a monotype of amino resin.
polyester containing a carboxyl group may react with melamine-formaldehyde to produce a useful thermosolystical surface coating with a wide range of physical properties.
many butylated melamine-formaldehyde resins have commercial benefits, starting with different initial polymerization (molecular weight) and different ratios of alkygen to hydroxymethane-free groups and amino-free hydrogen. These differences will affect the viscosity of the liquid, the matching of melamine with polyester, and the curing speed of the magnetic paint. Traditional melamine tree esters are mainly cross-linked to polyester molecules because they react with side hydroxyl groups. Since crosslinking reactions are acid catalytic, when the curing temperature is between 120 and 150 degrees C, polyester resins usually affect crosslinking reactions in strong acids, however, some polyesters in very weak acids require acid catalysis to cure the magnetic paint system. the following phenomena exist: in addition to melamine-polyester cross-linking reaction, butyl etherized melamine-formaldehyde resin also carried out a self-shrinking reaction. In other words, amino resins occur in self-crosslinking to form a melamine mesh structure. This reaction occurs at the same time as the melamine-polyester reaction and is a competitive reaction. The reaction occurs because the butyl etherized melamine-formaldehyde resin contains free hydrocarbon methyl groups and subamino hydrogen in addition to butyrogenic groups, all of which react with each other. Once the amino resin occurs self-intersecting, it will lose some function.
, although the self-crosslink often makes the coating have greater hardness and chemical resistance, but the elastic loss is very large. To make the polyester paint flexible enough. HMMM is a completely hydroxymethylated and fully methylated monomer amino resin. Similar to butyl etherized melamine-formaldehyde, it reacts cross-linked with hydroxyl groups of polyester resins to produce non-softened solids when heated. In essence, HMMM self-interlinking does not occur when the acid-free catalyst is acting, even if the time becomes longer or the temperature increases. However, a self-interlinking reaction occurs when the bulk HMMM is present at 150 degrees C and a strong acid catalyst is present. Conversely, even in the absence of strong acids, traditional butylized melamine and urea resins have a strong self-interlinking reaction as the temperature increases. curing reaction of
amino resins:
Since amino resins are used to cross-link the molecules of the main film-forming material into a mesh structure, it is interesting to see the copolymerization reaction between amino resins and lacquer resins, typically the
(exchange) reaction of
hydroxyxyl and alkyl resins on paint resins: shown in the figure below.
From the image above, one can imagine how the melamine molecule (identified by M in the microcosm) is pulled together with hydroxyl from different film-forming polymers to form a three-dimensional network structure that determines the performance of the paint film.
in the presence of thermal and acid catalysts (usually curing conditions), crosslinking occurs quickly, connecting all available hydroxyls on the paint. In fact, when the polymer network structure is formed, the fluidity of the reactants decreases, and some hydroxyl remains unresponsive. In general, when there is an excess amino resin in the coating than the ideal ratio, the remaining alkyds can participate in other reactions or remain in the coating film without reaction. It was mentioned earlier that amino resins are easily self-connected to each other, with the result that the molecular weight increases in production. These reactions also occur when the coating film is cured. This is not so much a negative factor as a degree of self-interlinking of amino resins, but rather an essential factor for obtaining a good durability, tight polymer mother. All three types of amin resins are involved in self-interlinking reactions, and there is evidence that these reactions occur after exchange with coating resin ethers in highly acid-catalyzed, fully alkylated melamine resin coatings. In the absence of an adjulytic catalyst or weak acid catalyst, these self-interlinking reactions occur to a higher degree in a melamine resin system with high serotonin/or hydroxymethyl performance. In both cases, a slight self-cohesion reaction is critical to the formation of a good network structure.
other reactions that occur when the film is cured in the crosslinking of amino resins are deformaldehyde and hydrolysis reactions. The deformaldehyde reaction is easy to occur at the usual curing temperature, which is almost the only cause of formaldehyde release when the amino resin cures, and the other formaldehyde is free formaldehyde.
amino resin cross-linking film curing will occur some hydrolysis reactions, some of which are converted to hydroxymethylene, high amino or hydroxymeryl content of melamine resin hydrolysis reaction can be catalyzed by alkali, even at room temperature can slowly hydrolysis, so that amino resins are more easily self-linked, and the paint in storage increased viscosity phenomenon. In order to avoid this phenomenon, a fully methyl etherized melamine resin or solvent can be used in water-based coatings that are resistant to alkali hydrolytic reactions. Fully alkylated melamine resins are resistant to alkali-catalytic hydrolysis in water-based systems. Fully alkylated and partially alkylated melamine resins are not acid-resistant to catalytic hydrolysis in water-based systems, so closed acid catalysts must be used in water-based systems.
main function group activity sequence (reactivity) in amino resins is as follows, the main reactions in the curing process are as shown in Figure V:
>NH (subaminide) >>N-CH
2
OH (hydroxymethane) >>NCH
2

3
(methoxygen) >>NCH
2
OC
4
H
9
(alkygenic)
amino resin primary function group on the performance of coating film
: