Re-dispersible polymer powder (RDP) is a kind of functional powder material made of polymer emulsion by spray drying, which is widely used in the fields of dry mortar, tile adhesive, external wall insulation system and so on.Its core function is that it can be re-dispersed into emulsion when it meets water, giving mortar excellent flexibility, bonding strength and weather resistance.In this paper, from the raw material selection, emulsion synthesis, spray drying to post-processing process, in-depth analysis of the industrialized production technology of RDP.
I. Selection of raw materials and pretreatment
1.1 Basic monomer
Main monomer: vinyl acetate (VAc), ethylene vinyl acetate copolymer (EVA), acrylate (such as MMA, BA), etc., accounting for 60%-85%.
Functional monomer: add 1%-5% of methacrylic acid (MAA), acrylic acid (AA) and other carboxyl-containing monomers to enhance the alkali resistance of latex powder.
Crosslinking monomer: double acetone acrylamide (DAAM), N-hydroxymethacrylamide (N-MA), etc., accounting for 0.5%-2%, used to enhance the water resistance after film formation.
1.2 Additive system
Protective colloid: polyvinyl alcohol (PVA1788, PVA2488), the amount of 2%-8%, to determine the re-dispersibility of latex powder and film-forming strength.
Emulsifier: non-ionic (such as TX-10) and anionic (sodium dodecyl sulfate, SDS) compound, HLB value control at 12-16.
Initiator: Ammonium persulfate (APS) or oxidation-reduction system (e.g. APS/NaHSO3), dosage 0.2%-0.8%.
Ⅱ.Emulsion synthesis process
2.1 Pre-emulsification stage
Pre-emulsion preparation: add monomer, emulsifier and deionized water into the pre-emulsification tank proportionally, and emulsify with high speed shear (2000-4000 rpm) for 30 minutes to form a stable pre-emulsion with particle size of 50-200 nm.
Temperature control: keep the temperature of the pre-emulsification system at 25-35 ℃ to avoid early polymerization of monomers.
2.2 Seed emulsion polymerization
Initiator addition: add 20% of the pre-emulsion and 1/3 of the initiator to the reactor, raise the temperature to 75-80 ℃ to initiate polymerization, the formation of "seed" emulsion.
Dropping process: The remaining pre-emulsion and initiator are added continuously by metering pump, the reaction temperature is maintained at 80±2℃, and the dropping time is 2-4 hours.
Heat preservation and maturation: after the completion of drop-addition, keep warm for 1 hour, so that the monomer conversion rate > 99%.
2.3 Post-adjustment process
pH adjustment: add ammonia or NaOH solution, adjust the emulsion pH to 6.5-7.5.
Plasticizer addition: mix with dibutyl phthalate (DBP) or citrate plasticizer (2%-5%), to improve the low-temperature film-forming properties of latex powder.
Ⅲ.Spray drying process
3.1 Design of drying tower parameters
Inlet air temperature: 160-200 ℃, according to the emulsion solid content (usually 40% -50%) dynamic adjustment.
Outlet air temperature: 70-90 ℃, to prevent overheating leading to PVA degradation.
Atomizer selection: centrifugal atomizing disc (speed 15000-25000 rpm) or pressure nozzle (pressure 4-8 MPa), control atomization particle size 80-120 μm.
3.2 Key control points
Matching of solid content: the solid content of the emulsion should be precisely matched with the flow rate of the hot air in the drying tower to avoid walling or over-fine powder.
Anti-caking agent addition: Spray 0.5%-2% of nano-silica or calcium stearate at the outlet of the drying tower to prevent the powder from absorbing moisture and agglomeration.
Tail gas treatment: cyclone separation + cloth bag dust removal, dust recovery rate should be >99.5%.
Ⅳ.Post-treatment and modification
4.1 Sieving and mixing
Vibratory sieving: 80-120 mesh screen is used to remove large particles to ensure uniform particle size distribution of the finished product (D50≈100 μm).
Functional modification: mix with hydrophobic agent (such as organosilicon powder), early-strength agent (aluminum sulfate), etc., to improve the special performance of latex powder.
4.2 Packaging moisture
Multi-layer composite film: aluminum foil/PE composite bag, filled with nitrogen protection, moisture content <0.5%.
Temperature and humidity control: humidity <40%, temperature 20-25℃ in packaging workshop.
V. Quality control system
5.1 Key Indicator Testing
redispersibilityFilm-forming tensile strengthglass transition temperature
Testing Program | Test Methods | Scope of Standards |
Ash (PVA content) | GB/T 9345-2008 | 8%-15% |
Laser particle size analysis | D90<5 μm | |
ISO 527-2 | ≥8 MPa | |
DSC | -10℃~+25℃ |
5.2 Solutions to common problems
Caking problem: Check whether the temperature at the outlet of the drying tower is too high, or the proportion of anti-caking agent added is insufficient.
Film cracking: Adjust the polymerization degree of PVA (preferably 1788 type) and increase the amount of crosslinking monomer.
Short storage period: optimize packaging sealing, add antioxidant (such as BHT).
VI. Application Trends and Process Innovation
Environmentally friendly RDP development: use of bio-based monomers (e.g. lactate) instead of petroleum-based raw materials with VOC content <50 ppm.
Low-temperature spray drying technology: Reduce the risk of PVA degradation through vacuum spray drying (temperature <100°C).
Intelligent encapsulation technology: use microencapsulation process to encapsulate functional additives to achieve controlled release of performance.
Conclusion
The manufacture of dispersible polymer powder is a synthesis of polymer synthesis, drying engineering and powder technology.In the future, with the urgent demand for green building materials, the RDP process will develop in the direction of low energy consumption and high functionality to provide better quality core additives for the dry mortar industry.
