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1. Organic silicon modified copolymerization mechanism

Silicone acrylic lotion is generally a copolymer of silane coupling agent containing double bonds (vinyl triethoxysilane, etc.) and acrylate (BA, MMA, St, AA, etc.) through free radical lotion copolymerization. The C=C double bond on the silicon monomer undergoes free radical addition with the acrylic monomer, linking the siloxane side to the main chain of the acrylic molecule; The OR alkoxy group on silane has hydrolytic activity and undergoes hydrolysis in an aqueous environment to form silanol SiOH. Dehydration condensation can occur between silanols to form a SiO Si cross-linked network.

There are two parallel reactions in this system:

① Carbon carbon double bond free radical copolymerization; ② The hydrolysis and condensation reaction of siloxane. Free radical copolymerization aims to graft silicon monomers into polymer chains; Hydrolysis polycondensation is easy to cross link molecules in the aqueous phase and directly generate gel particles. This is the core contradiction between silicone acrylic lotion and ordinary styrene acrylic lotion and pure acrylic lotion.

Mechanism of viscosity increase and viscosity change of silicone acrylic lotion

The viscosity evolution law of silicone acrylic lotion is more complex than that of pure acrylic and styrene acrylic lotion. It does not belong to a single physical particle size thickening or carboxyl ion thickening system, but a comprehensive result of the coupling of the rheological mechanism of conventional acrylic system+silane hydrolysis cross-linking thickening mechanism+post hysteresis cross-linking thickening mechanism. The viscosity of ordinary acrylic lotion basically tends to be stable after the end of the reaction, while silicone acrylic lotion has the dual characteristics of “dynamic viscosity change during polymerization+long-term viscosity drift of finished product storage” due to the continuous reaction characteristics of siloxane active functional groups, which is also the core theoretical source of silicone acrylic lotion viscosity that is difficult to control, poor batch stability and easy to thicken after storage. This section systematically disassembles the exclusive viscosity increasing and viscosity evolution mechanism of silicone acrylic lotion from the four dimensions of physical structure, chemical hydration, in-situ crosslinking and hysteresis reaction.

(1) Physical tackifying mechanism of latex particle size and specific surface area: This mechanism is consistent with that of conventional acrylic lotion, and is a decisive factor for the basic viscosity of silicone acrylic lotion. When the dosage of emulsifier is increased, the pre emulsification effect is enhanced, and the number of nucleations increases, the system generates a large number of small-sized latex particles, and the overall specific surface area of the particles significantly increases. The adsorption effect between latex particles and continuous phase water molecules, as well as the frictional resistance between particles and fluid drag resistance, increase synchronously, and the apparent viscosity of the system steadily increases. On the contrary, moderately increasing the latex particle size and constructing a bimodal grading structure can increase the free water volume between particles, reduce the packing density, and effectively reduce the base viscosity. This mechanism belongs to the physical reversible rheological law, does not change continuously with storage time, and determines the initial basic viscosity level of lotion.

(2) Carboxyl ion hydration network chemical thickening mechanism: Hydrophilic functional monomers such as acrylic acid and methacrylic acid are copolymerized on the surface of latex particles, and after alkaline neutralization, they ionize to form a large number of carboxylate anions, constructing a continuous hydration adsorption network, greatly increasing the hydrodynamic radius of latex particles, and producing significant static thickening and thixotropic effects. The higher the carboxyl content, the more complete the neutralization degree, the thicker the hydration layer, the higher the static viscosity of the system, and the stronger the anti sagging property. This mechanism is a universal tackifying mechanism for water-borne lotion, but there is a performance balance in the silicone acrylic system: excessive carboxyl group tackifying will improve the stability and adhesion of lotion, but will introduce a large number of hydrophilic channels, weaken the hydrophobic modification effect of silicone, resulting in a decline in the water resistance of the film.

(3) Exclusive viscosity increasing mechanism of silane hydrolysis in-situ mild crosslinking: this is the core viscosity increasing source of silicone acrylic lotion, which is different from pure acrylic and styrene acrylic lotion. In the system, the vinyl silane coupling agent is grafted onto the side chain of acrylic acid molecules, and the alkoxy groups carried by it continuously hydrolyze in the aqueous environment to generate highly active silanol (Si OH); Molecular dehydration condensation can occur between silanol groups, forming a large number of uniform and mild Si-O-Si cross-linking nodes inside and at the particle interface of latex particles. Different from the flexible tackifying of acrylic molecular chain entanglement, silica crosslinking is a rigid network tackifying, which can directly bind the free water of the system, improve the structural strength of latex particles, increase the flow resistance of the system, and significantly increase the apparent viscosity of lotion. The higher the polymerization temperature, the greater the silicon monomer content, the alkalinity of the system pH, the more intense the silane hydrolysis polycondensation, the more obvious the effect of in-situ crosslinking and viscosity increasing, and in serious cases, it will directly lead to the scrapping of the system gel.

(4) Mechanism of thickening and viscosity drift after storage lag crosslinking: The most prominent viscosity problem of silicone acrylic lotion is irreversible viscosity drift during storage and post continuous thickening. After polymerization, there are still a large number of incompletely condensed active silanol groups and ungrafted free silane in the lotion system. These active groups can undergo slow low temperature polycondensation under normal temperature storage conditions, and continue to generate new Si-O-Si cross-linking networks. With the extension of storage time, the cross-linking density of the system gradually increased, the structural rigidity of the latex particles continued to increase, and the free water body continued to be bound, which was manifested by the steady increase of lotion viscosity with time, the continuous decline of liquidity, and finally thickening and semi gel failure. The irreversible delayed reaction is the fundamental reason for the poor long-term storage stability of high silicon content silicone acrylic lotion. Ordinary styrene acrylic lotion and pure acrylic lotion have no such delayed crosslinking and viscosity increasing behavior.

(5) Multi factor coupling rheological law and viscosity imbalance mechanism: the final viscosity of silicone acrylic lotion is the superposition balance of four mechanisms. The viscosity of the system with low silicon content is mainly dominated by particle size and carboxyl hydration, and the rheological law is stable and similar to that of ordinary acrylic lotion; The viscosity of the medium high silicon content system is mainly increased by silicon cross-linking, with extremely high viscosity sensitivity and low process tolerance; Under high temperature, high pH, and high silicon content conditions, the crosslinking and thickening effect of silane will exponentially increase, making it highly susceptible to viscosity exceeding the standard. At the same time, excessive hydrolysis of silane can cause latex particle aggregation and deterioration of particle size distribution, further adding physical thickening effects, forming a dual surge of “cross-linking thickening+particle size deterioration thickening”, resulting in complete loss of viscosity and construction performance failure of the system. Mastering the coupling mechanism is the core theoretical basis for realizing the “stable viscosity during polymerization, controllable viscosity of finished products, and long-term storage locking” of silicone acrylic lotion.

The viscosity of silicone acrylic lotion is coupled by multiple factors: latex particle size, carboxyl ion hydration and viscosity increase, and mild crosslinking caused by silane hydrolysis.

1) Particle size effect: with the increase of emulsifier, the particle size becomes smaller, the specific surface area increases, and the viscosity of the system increases, with the same rule as that of acrylic lotion;

2) Carboxyl hydration enhances viscosity: AA/MAA ionizes to form a hydration network, improving static viscosity and thixotropy;

3) Silane hydrolysis cross-linking tackifying: Silane hydrolysis produces SiOH, intermolecular condensation occurs, and a slight cross-linking network is formed inside or at the interface of the latex particles, which will significantly increase the apparent viscosity of lotion; The higher the degree of hydrolysis and condensation, the more significant the increase in viscosity; Overhydrolysis polycondensation direct gel scrap. During the storage of high silicon system, the residual silanol will continue to slowly condense, and it is very easy to have the phenomenon of viscosity drift and post thickening in the later storage period, which is a unique stability problem of silicone acrylic lotion.

Mechanism of hydrophobic and weather resistant modification of 3 coating films

The bond energy of silicon oxygen bond is much higher than that of carbon carbon bond, and it has excellent resistance to UV and thermal oxidative aging; During the film formation process, low surface energy silicon oxygen chain segments migrate and accumulate towards the surface of the coating, forming a hydrophobic surface layer, significantly reducing the surface energy of the coating and improving its water resistance and stain resistance. When the silicon content is within an appropriate range, the migration and enrichment effect of silicon segments is significant; Low silicon content results in weak modification effect; Excessive silicon content leads to excessive cross-linking inside, causing the film to become brittle and adhesion to decrease.

4 Main failure defect mechanism of silicone acrylic lotion

(1) During polymerization, gel and a large number of condensates: alkoxysilane is hydrolyzed and condensed too quickly when encountering water; Too high temperature, pH deviation from neutral, early addition of silicon monomer, and too high content of silicon monomer will accelerate the condensation of silicone alcohol and produce a lot of gel.

(2) After storage, the viscosity continues to rise and thicken: there are residual silanol groups inside the latex particles that have not fully condensed. During storage, intermolecular dehydration condensation continues, and the degree of crosslinking slowly increases, manifested as a continuous increase in viscosity with storage time.

(3) Silicon phase separation leads to whitening and loss of gloss in the coating: Silicon units are not effectively grafted, resulting in phase separation and the precipitation of siloxanes alone. The transparency and gloss of the coating decrease. Main reason: Silicon monomers did not participate in free radical copolymerization, only physically blended, with poor compatibility.

(4) Contradiction between coating water resistance and adhesion: Silicon modification enhances hydrophobicity, but excessive crosslinking reduces the adhesion of the coating to the substrate; Carboxyl groups improve adhesion, but introducing hydrophilic groups damages water resistance. The silicone acrylic formula needs to find a balance between the degree of silicon crosslinking and the amount of carboxyl groups used.

Rheological properties and thixotropy mechanism of 5 silicone acrylic lotion

Silicone acrylic lotion is different from conventional acrylic lotion and has unique non Newtonian pseudoplastic rheological characteristics and controllable thixotropic properties. Its rheological behavior is the result of three-dimensional coupling of particle structure, hydration network and silicon oxygen light crosslinking, which directly determines the coating application leveling, anti sagging, coating thickness and molding flatness. The rheological behavior of conventional pure acrylic and styrene acrylic lotion mainly depends on carboxyl hydration and particle stacking, with narrow rheological range and weak adjustability; The micro particle bridging structure formed by silane hydrolysis of silicone acrylic lotion can build a flexible three-dimensional network without significantly improving the initial viscosity, and achieve the excellent thixotropic characteristics of “low shear construction, low viscosity leveling, high shear resistance to sagging, and rapid structural recovery after standing”.

From the perspective of micro rheological mechanism analysis, in the static state, the silanol residues on the surface of latex particles, carboxyl hydration layer, weak hydrogen bonds between particles, and secondary cross-linking of silicon oxygen jointly construct a loose and reversible network. The static structure viscosity of the system is high, which can effectively resist pigment filler settling, storage stratification, and application flow hanging; Under the external force of shear mixing, roller coating and spraying, the reversible network is rapidly deconstructed, the sliding resistance between molecular chains and latex particles is reduced, and the apparent viscosity of lotion is rapidly reduced, giving the system excellent leveling and construction smoothness; After the external force is removed, the weak cross-linking of silicon oxygen and the rapid reconstruction of the hydration hydrogen bond network lead to a rapid increase in viscosity, effectively preventing construction defects such as thick coating flow and edge hanging flow.

The silicon content has significant regulation laws on the rheological properties of silicone acrylic lotion: the rheological characteristics of low silicon system are close to those of ordinary acrylic lotion, with weak thixotropy and poor anti sagging; Moderate silicon content can construct a uniform reversible cross-linked network with optimal rheological properties; The high silicon system has excessive silicon oxygen cross-linking, and the network has changed from “flexible reversible” to “rigid irreversible”, manifested as high viscosity, flow adjustment, shear thinning irreversible, and rough construction texture. At the same time, the pH, neutralization degree and hydrolysis degree of the system directly affect the rheological threshold. The thixotropic window of weakly acidic polymerized and moderately neutralized silicone acrylic lotion is wider, and the construction adaptability is stronger.

Mechanism of weather aging resistance and yellowing resistance of 6 silicone acrylic lotion

Exterior wall coatings and outdoor protective coatings have very high requirements on the ultraviolet aging resistance, thermal oxygen aging resistance, and rain erosion performance of lotion. The core performance advantage of silicone acrylic lotion compared with styrene acrylic and pure acrylic lotion stems from the intrinsic stability of siloxane structure. The main chain of traditional acrylic polymer is mainly composed of C-C and C-O bonds, with low bond energy. Under long-term UV irradiation and high temperature and humid environment, it is prone to bond breakage, molecular chain degradation, and chain rearrangement, resulting in coating powdering, loss of gloss, cracking, yellowing and peeling. The introduction of a large number of Si-O-Si bonds into the silicone acrylic copolymer molecular chain results in bond energies much higher than carbon bond structures, providing strong UV shielding ability and thermal stability, making it difficult to be broken by UV photon excitation, and significantly improving the weather resistance life of the coating from a molecular perspective.

In addition to the bond energy advantage, the surface spontaneous enrichment effect of silicon segments is a key mechanism for long-term weather resistance. Siloxanes have extremely low surface energy and strong hydrophobicity. During the film-forming and drying process, they spontaneously migrate and accumulate towards the gas-liquid interface of the coating, forming a dense, continuous, and pore free inorganic enrichment layer on the coating surface. This inorganic silicon layer can effectively block the penetration of ultraviolet radiation, oxygen, water vapor, acidic and alkaline corrosive media into the interior of the coating, protect the underlying acrylic organic matrix from aging and degradation, and achieve a composite synergistic effect of “the organic matrix provides mechanical film-forming properties, and the inorganic silicon layer provides protective weather resistance”.

At the same time, moderate crosslinking of silane can improve the density of the coating, reduce the free volume, decrease the diffusion channels of aging media, and further enhance the weather resistance. However, silicon modification has a significant threshold effect: when the silicon content is too low, the surface silicon enrichment layer is discontinuous, the protective shielding effect is weak, and the weather resistance improvement is limited; When the silicon content is too high, excessive cross-linking leads to stress concentration and increased brittleness in the coating, and microcracks are prone to occur under long-term temperature to temperature alternation, which in turn reduces long-term weather stability and adhesion. In addition, soap free silicone acrylic lotion has eliminated the residual defects of small molecular emulsifiers, and its film has higher compactness, weather resistance, yellowing resistance, and aging resistance, which are significantly better than conventional silicone acrylic lotion emulsions.

7 Difference mechanism and performance comparison of silicone acrylic/pure acrylic/styrene acrylic lotion system

The three types of acrylic lotion have similar skeleton structures, but different modification systems, resulting in essential differences in polymerization stability, viscosity characteristics, film forming performance, weather resistance and water resistance, which is also the core basis for engineering material selection and formula design. Styrene acrylic lotion introduces styrene monomer, which has strong rigidity, low cost and good hiding power. However, the styrene ring structure has poor UV resistance, is prone to yellowing and aging, and the molecular chains are tightly stacked, poor hydrophilicity, weak adhesion and storage stability. It is only suitable for ordinary indoor painting. Pure acrylic lotion all acrylate copolymerization has excellent system compatibility, flexible film formation, strong adhesion, and yellowing resistance better than styrene acrylic, but there is no inorganic protective structure, water resistance, pollution resistance, and weather resistance are limited, and it is easy to lose luster and powder after long-term outdoor service.

Silicone acrylic lotion belongs to organic-inorganic hybrid modification system, which combines the excellent film-forming property, flexibility and adhesion of organic acrylate with the advantages of high bonding energy, hydrophobic, weather resistance and pollution resistance of inorganic silicon oxygen structure, completely making up the performance shortcomings of traditional acrylic lotion. In the dimension of viscosity and stability, the styrene acrylic and pure acrylic lotion only have physical particle size and carboxyl group hydration and viscosity increase, the system is stable, the viscosity is controllable, and there is no post thickening defect; Due to the dual reaction of silane hydrolysis and polycondensation, silicone acrylic lotion has the risk of polymerization gel and the defect of storage viscosity drift. It has higher process sensitivity, but it can achieve excellent thixotropic rheological properties through controlled crosslinking, and has better construction adaptability.

In the service performance dimension, the hydrophobic angle of silicone acrylic lotion is significantly higher than that of ordinary lotion, the surface is not easy to absorb dust, the rain can clean itself, and the pollution resistance is outstanding; The Si-O inorganic network is resistant to ultraviolet, heat and oxygen aging, and its outdoor service life is more than 1.5~2 times that of ordinary styrene acrylic and pure acrylic lotion. Its only shortcomings are complex formula and process, low fault tolerance, high brittleness of high silicon system, and high cost, which require precise matching of formula and process to achieve optimal comprehensive performance.

8 Silicone acrylic lotion formula performance balance and synergistic mechanism

The core difficulty of R&D and mass production of silicone acrylic lotion lies in the mutual balance and trade-off of multiple performance parameters. It is impossible to improve a single performance, but it must achieve comprehensive balance through multi-component coordinated regulation. First, the balance between water resistance and adhesion: hydrophilic functional monomers such as carboxyl and hydroxyl groups can improve the dispersion stability of lotion, wetting adhesion of substrate, and calcium ion stability, but will form hydrophilic channels inside the film, significantly reducing the water resistance, moisture resistance, and salt spray resistance; Silane modification can block hydrophilic channels and enhance hydrophobicity, but excessive silicon crosslinking will reduce the resin’s wetting ability on the substrate and weaken adhesion.

Secondly, balancing high solid and low viscosity with storage stability: increasing the solid content can improve construction efficiency and reduce VOC emissions, but the particle packing density increases and the system viscosity skyrockets; Moderate particle size distribution can reduce viscosity, but too wide particle size distribution will reduce the storage stability of lotion and make it easy to settle and delaminate; Slight crosslinking of silane can stabilize particle structure and improve storage stability, but excessive crosslinking will directly thicken gel and destroy liquidity.

Thirdly, balance between flexibility and hardness weather resistance: Acrylic soft monomers can enhance the flexibility, crack resistance, and deformation resistance of the coating, but being too soft can lead to poor wear resistance, poor stain resistance, and high temperature rebound of the coating; Hard monomers enhance hardness and rigidity, but the coating is brittle and prone to cracking at low temperatures; Silicon crosslinking can improve surface hardness and weather resistance, but excessive crosslinking further exacerbates brittle defects. The design logic of high-performance silicone acrylic lotion is to precisely balance the above three checks and balances to achieve an integrated match of “high stability, low viscosity, strong adhesion, high hydrophobic, high weather resistance, flexibility and wear resistance”.

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