Basic concept of water-based paint and overview of water-based acrylic resin (2)

Formula design of water-based acrylic resin

Water-based acrylic resins and their coatings have a wide range of applications, so their formulation design is very complicated
.
The basic principles mainly follow the following points:

a.
Determine the resin dosage form for different substrates and products-water-diluted or water-emulsion or dispersion type;

b Select monomer composition and glass transition temperature (Tg);

c Select the type and amount of water-based monomer;

d Select the type and amount of initiator;

E choose the solvent composition;

f Choose the type and amount of emulsifier;

g Select the polymerization process
.

Finally, through experiments, inspections and corrections are carried out to determine the best product technology and formulation coating online coatingol.
com
.

Choice of monomer

a monomer overview

For the convenience of application, polymerized monomers are usually divided into three categories: hard monomers, soft monomers and functional monomers
.

Hard monomers: methyl methacrylate (MMA), styrene (ST), isobornyl methacrylate (IBOMA);

Soft monomers: ethyl acrylate (EA), butyl acrylate (BA), isooctyl acrylate (2-EHA)
.

Weakly polar styrene, long-chain acrylic and methacrylate (such as lauryl ester, octadecyl ester) have good alcohol resistance and water resistance
.

Functional monomers include hydroxyl-containing acrylate and methacrylate, and carboxyl-containing monomers include acrylic acid and methacrylic acid
.

Other functional monomers include: acrylamide (AAM), methylol acrylamide (NMA), diacetone acrylamide (DAAM) and ethyl acetoacetate methacrylate (AAEM), glycidyl methacrylate (GMA), methacrylate, dimethylaminoethyl methacrylate (DMAEMA) and the like
.

The amount of functional monomer is generally controlled at 1% to 6% by mass
.

Due to the steric hindrance effect of isopropyl group, vinyl triisopropoxy silane monomer has a slower hydrolysis of Si-O bond, and its amount can be higher in emulsion polymerization (10)
.

The introduction of carboxyl groups into monomers containing carboxyl groups can improve the resin's moisturizing properties on pigments and fillers and its adhesion to substrates, and it is reactive with epoxy groups and has catalytic activity for the curing of amino resins
.

B monomer copolymerization activity

Because the monomer structure is different, the copolymerization activity is different, and the composition of the copolymer is usually different from the composition of the monomer mixture.
For binary and ternary copolymerization, they can be related by the copolymer composition equation
.
For more diversified copolymerization, there is no good correlation equation available, only experimental research and specific analysis of specific problems can be done
.

In actual work, the monomer mixture "starved state" feeding method (that is, monomer feeding rate <copolymerization rate) is generally used to control the composition of the copolymer
.
If the polymerization rate of mixed monomers is too different, it is difficult to copolymerize styrene with vinyl acetate, vinyl chloride, and acrylonitrile
.
When it is necessary to copolymerize with monomers with a large difference in activity, a monomer can be supplemented for transition, that is, a monomer is added, and the polymerization rate of this monomer and other monomers is relatively close, the copolymerization is good, and styrene is the same Acrylonitrile is difficult to copolymerize.
Adding acrylate monomers can improve their copolymerization
.

The reactivity ratios of the monomer pairs in the following table can be used to evaluate the copolymerization activity of the monomers
.

c hydroxyl type acrylic resin

When synthesizing water-based hydroxy acrylic resin, the type and amount of hydroxy monomer have an important influence on the performance of the resin
.

Commonly used primary hydroxyl monomers for the hydroxy acrylic component of the waterborne two-component polyurethane system: hydroxyethyl acrylate (HEA) or hydroxyethyl methacrylate (HEMA);

The hydroxy acrylic component of water-based amino baking paints commonly used secondary hydroxy monomers: β-hydroxypropyl acrylate (HPA) or β-hydroxypropyl methacrylate (HPMA)
.
Primary hydroxyl monomers have higher activity, and the hydroxypropyl resin synthesized from them will affect the storage of the finished paint when used as the hydroxyl component of the amino baking paint, and secondary hydroxypropyl ester monomers can be selected
.

In addition, hydroxyl type chain transfer agents (such as mercaptoethanol, mercaptopropanol, 2-hydroxyethyl mercaptopropionate) can introduce hydroxyl groups at the ends of macromolecular chains to improve the distribution of hydroxyl groups, increase hardness, and narrow the molecular weight distribution.
Reduce system viscosity
.

Tg design

The glass transition temperature reflects the transition temperature of the amorphous polymer from the brittle glass state to the highly elastic state
.
Coatings for different purposes have very different glass transition temperatures of their resins
.
The elastic emulsion used for exterior wall paint is generally lower than -10°C, and it should be lower in the north; while the resin used for thermoplastic paint is generally higher than 60°C
.
Cross-linked acrylic resin is generally at -20～400℃
.

The commonly used FOX formula for glass transition temperature design:

Where, Wi is the mass fraction of the i-th monomer, corresponding to the i-th monomer the homopolymer of glass transition temperature in K with
.
The glass transition temperature of some monomers can be checked in the table
.

The calculated value of this formula has a certain reference value, but its accuracy is related to the monomer composition and is not certain
.

The choice of initiator

Water-dilutable acrylic resin and acrylic resin secondary dispersion mainly use peroxy and azo initiators

The initiation activity of commonly used peroxygen initiators is shown in the following table:

Dibenzoyl peroxide (BPO) is one of the most commonly used peroxygen initiators.

Tert-butyl peroxybenzoate is an important application initiator in recent years.

There are few types of azo initiators, and the commonly used ones are mainly azobisisobutyronitrile (AIBN) and azobisisoheptanitrile (ABVN)

Persulfates are used as initiators for emulsion polymerization or aqueous polymerization, including ammonium persulfate, sodium persulfate, and potassium persulfate

Azobisisobutylamidine hydrochloride (V50) initiator for cationic emulsion polymerization