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Basic positioning of each functional monomer

1. Monofunctional monomers (such as IBOA, THFA, LA, EOEOEA, etc.) only have one C=C, and after polymerization, they only hang on the network without forming cross-linking nodes, acting as active diluents.   ✅  Advantages:

Low viscosity and strong ability to reduce viscosity;

The lowest curing shrinkage rate;

The best flexibility reduces the brittleness of the paint film;

Significant improvement in substrate adhesion, with many polar groups (hydroxyl, cyclic ether) present;

Adjustable Tg to reduce internal stress in the system.   ❌  Disadvantages:

The curing rate is the slowest;

The cross-linking density cannot be improved, but it is solvent resistant, scratch resistant, and has poor hardness;

High addition levels are prone to migration, stickiness, precipitation, strong odor, and poor weather resistance;

Low upper limit of double bond conversion rate and high risk of residual monomers.   ✅  Usage: Flexible UV, plastic substrate UV, adhesion promotion, shrinkage reduction, matte system, soft film.

2. Two double bonds of bifunctional monomers (such as HDDA, TPGDA, DPGDA, NPGDA, etc.) form a bridging bridge that connects two molecular chains. It is the most commonly used “balanced monomer” in the formula.   ✅  Advantages:

Moderate curing speed;

Moderate shrinkage;

Balancing hardness and toughness, internal stress will not be as extreme as high functionality;

Good dilution ability and wide compatibility;   ❌  Disadvantages:

Hardness, chemical resistance, and wear resistance are not as good as those of grade 3 or above;   ✅  Usage: Universal UV topcoat, wood, plastic, paper UV, most basic formulas mainly dilute monomers.

3. The first truly high crosslinking starting point of trifunctional monomers (such as 3-membered, TMPTA, TMP3EOTA, etc.) forms a three-dimensional network branch.   ✅  Advantages: significantly improved curing speed, increased hardness, wear resistance, solvent resistance, and improved water resistance;   ❌  Disadvantages: Increased shrinkage, increased brittleness, higher viscosity ratio than 2, easy cracking and decreased adhesion when added;

4. Four/five/six functional monomers (4: PETTA; 6 official: DPHA is a high cross-linking density ace with extremely high double bond density ✅  Advantages: extremely fast curing, high hardness, high wear resistance, chemical resistance, scratch resistance, good thermal stability, suitable for high wear and corrosion resistant UV coatings; Can be used to improve surface quality;   ❌  Disadvantages:

The curing shrinkage rate is the highest, the internal stress is huge, and it is prone to edge warping, cracking, paint peeling, and adhesion collapse;

High viscosity;

Very brittle;

Multiple double bonds, high heat release during free radical polymerization, and easy bubble formation during thick coating;

Some high-ranking officials have a higher tendency towards yellowing of pentaerythritol derivatives;

There are many residual double bonds (due to rapid cross-linking, chain segments are locked and double bonds are embedded), which increases the risk of weathering and hydrolysis resistance in the later stage.

3、 Several crucial rules that are prone to pitfalls in research and development

The curing speed is not linear and only depends on the functional degree, but also on the double bond equivalent and substitution type. The functional degree is the same: methyl methacrylate is much slower than acrylic ester; Cyclic monomers (IBOA) cure slower than linear LA.

Misconception: It is not necessarily true that a 6-official system is infinitely faster than a 3-official system. If the viscosity of the system is too high, the diffusion of free radicals will be hindered. After exceeding the critical point, the curing speed will slow down, and even the surface will be sealed first, and the bottom layer will not be oxygen permeable and hinder polymerization, resulting in surface dryness but internal dryness.

1. Shrinkage and adhesion are the core contradictions. High functionality leads to high cross-linking, high shrinkage, and high internal stress. Plastic, film, and thin substrate UV are most prone to stepping on this pit: adding DPHA for wear resistance results in direct cracking, edge shrinkage, and adhesion loss of level 0. Countermeasure: High officials can use it to adjust surface performance, combined with 1/2 officials to release internal stress, rather than using it alone in large quantities.

2. The difference in oxygen blocking polymerization results in a higher concentration of functional monomer double bonds, stronger resistance to oxygen blocking polymerization, and better surface dryness; Monofunctional systems are most prone to surface sticking. This is also the reason why adding a small amount of DPHA to many topcoats can improve surface dryness, but not too much.

3. The problem of residual double bonds (C=C) is that the functional system solidifies too quickly, the network freezes rapidly, and some double bonds are trapped inside and unable to react, known as “embedded double bonds”. Short term testing shows good durability, but under long-term outdoor and humid aging, these latent double bonds will continue to slowly oxidize, yellow, and absorb water, resulting in a decline in the performance of the coating in the later stages. The problem with single functional groups is that small monomer molecules do not react and migrate and precipitate; High functionality refers to the residual double bonds within the macromolecular network, and the two types of drawbacks have different sources.

4. Adding viscosity to the same skeleton, the higher the functionality, the stronger the intermolecular forces, and the higher the viscosity; So the viscosity of DPHA is significantly higher than that of TMPTA, and TMPTA is higher than HDDA. To achieve low viscosity and high cross-linking, it is preferred to use ethoxy/propoxy modified multifunctional monomers (TMP3EOTA), which introduce flexible ether bonds to reduce viscosity and slightly reduce brittleness.

4、 Simple differentiation of cationic UV (epoxy/vinyl ether)

Cationic UV does not have the characteristic of “higher functionality leads to shrinkage explosion” like free radicals: cationic ring opening polymerization has very little shrinkage or even slight expansion, so multifunctional epoxy monomers are not afraid of shrinkage cracking. The entire set of rules on this system is not applicable, and judgment logic should not be mixed.