How to judge the basic indicators of resin?

Appearance of resin: Most of them can be made to be water-white and transparent, and those modified by a small amount of special monomers are light yellow or milky white. For example, it is modified with E-10 (tertiary glycidyl carbonate) as a low viscosity high solid resin to improve gloss, fullness and weather resistance. Modified with (meth)acrylonitrile or (meth)acrylamide to improve adhesion and pigment dispersion, there will be a small amount of yellow. Modification with chlorinated polypropylene to improve adhesion to PP plastic produces milky whiteness. Under normal circumstances, the modified E-10 will only be used on PU paint, and a few are used on baking paint (because it contains -OH value). (Meth)acrylonitrile or (meth)acrylamide are used on stoving varnishes that require better adhesion to metals or inks with better wetting and dispersing pigments.

Therefore, we can simply judge its properties from the appearance of the resin. 

1. Solid content: mostly 50% to 70%, 40% and 80% are also produced, according to user requirements, can be made into different solid content. Under normal circumstances, high solid content is used to make varnishes and varnishes. The solid content is used as solid color paint and topcoat. Low solids are mostly used on silver paints and primers.

2. Viscosity: The size of viscosity reflects the size of molecular weight. In order to ensure the performance of the resin after painting, the viscosity of the thermoplastic resin should be higher (because it is volatile and drying), and the viscosity of the hydroxy acrylic resin, especially the thermosetting acrylic resin should be lower (because it is cross-linked and cured, the later molecular weight will be The control of viscosity depends entirely on the use and performance requirements of the resin, such as resin for ink, especially screen ink, the resin viscosity should be small, and the drying rate should be slow to prevent the screen from being blocked. For transparent putty, it requires a large molecular weight and a high viscosity. Because it needs to be applied thickly to prevent the precipitation of pigments and fillers. Under normal circumstances, under normal circumstances, low viscosity is used to make varnishes and varnishes. Medium viscosity to do solid color paint, topcoat. High-viscosity ones are mostly used on silver paints and primers. Many users only seek immediate benefits, hoping to make the resin viscosity as large as possible, so that more solvent can be added to reduce the cost of paint. However, gloss, leveling, workability and miscibility may be sacrificed.

3. Acid value: The acid value of the resin is also one of the important indicators of the resin. Generally speaking, the larger the acid value, the stronger the adhesion to the metal substrate, the better the wetting and dispersibility of the pigment, and the cross-linking of the paint. The higher the degree and speed, the more unstable the storage. Different uses of resins also have strict requirements on acid value. For example, resins for aluminum powder paints require lower acid values, especially domestic aluminum powders. A low acid value of resins can avoid aluminum powder and resin during storage. Acid reaction, affecting whiteness and even gelling. Therefore, the acid value of the resin used for aluminum powder paint is generally controlled below 2mgKOH/g. When used as sealing varnish on the surface of electroplated metal, it requires a high acid value to achieve better adhesion and electrostatic spraying effect. 

4. Hydroxyl content: Hydroxyl is an active functional group on the side chain of acrylic resin used for post-crosslinking. In general, hydroxyl acrylic resin and thermosetting acrylic resin contain -OH group, and some thermoplastic acrylic resins also contain a small amount of -OH, mainly Increase the polarity of the resin, improve the adhesion and wetting and dispersibility of the paint film. The amount of -OH has a great influence on the crosslinking density of the coating. If the -OH is large, the crosslinking density is large, and the chemical and mechanical properties of the paint film are good, and vice versa. The general OH content of thermosetting acrylic resins is between 1.5% and 2.5%. Because the molecular weight increases rapidly after baking, there is no need for too much -OH value; the general OH content of hydroxy acrylic resins is between 2% and 4%. , too much will increase the cost of curing agent. Since the hydroxyl group is used as a functional group for later crosslinking, the distribution of its main chain is very important to its performance. It is necessary to require at least two -OH values ​​on each molecular chain, so that it can become a spatial network structure after crosslinking. 

5. Glass transition temperature: Tg represents the critical temperature at which the paint film transfers from plasticity to rigidity. All acrylic resins have a Tg point. Generally, only the Tg of the thermoplastic resin is marked, because the thermoplastic resin is an air-drying resin, and the Tg does not change after drying. The Tg value of the cross-linkable resin will increase a lot after drying, so the Tg of the thermosetting and hydroxy acrylic resins is not a reference, and is generally not marked. Thermoplastic acrylic resin, when the Tg value is too low, the paint film is difficult to dry, or cannot be dried; if the Tg is too high, the hardness of the paint film is too high, which will cause the paint film to be brittle. Therefore, the Tg value of the resin must be carefully designed to the best value in strict accordance with the requirements and characteristics of the product. For example, the Tg value of solvent-based exterior wall coatings is generally designed to be 55 ° C ~ 60 ° C. If it is too high in winter, especially in the north, the paint film Cracking will occur, and if it is too low, it will easily cause sticking back in summer, and the stain resistance is poor. If the Tg of thermosetting and hydroxy acrylic resin is too high, the Tg will increase too much after cross-linking, and the paint film will be very brittle. The Tg of acrylic resin is determined at the time of formulation and will not change with process changes. Sometimes when matching paint, resins with different Tg levels will be used in combination to learn from each other to achieve the desired effect. For example, in order to prevent the migration of plasticizers, plastic paints are mixed with high and low Tg resins.

 

6. Solvent system: The type and type of solvent system have a great influence on the performance of the resin. Therefore, the choice of the solvent should also be very strict and elegant. There are three principles for choosing a solvent:

First, according to the polarity and solubility of the resin,

The second is to match the solvent used in the later paint distribution.

The third is according to the volatilization gradient of the solvent. For example, thermoplastic acrylic resins for plastics often choose toluene, butyl ester and butanol because they correspond to thinners for plastics. Hydroxy acrylic resins generally use xylene and butyl esters, which are also required for PU paints.