1. Test results and analysis
Four kinds of suspending agent: HB type suspending agent, Henan Xinyang bentonite, Hebei Zhangjiakou bentonite, Hubei marshan bentonite.
Bentonite is the most commonly used suspension agent for water-based coatings. The most important function of bentonite is to form the structure of colloidal dispersion (coating) due to the ion adsorption, so that the coating has suspension stability, yield value and thixotropy.
In bentonite, the main mineral composition is montmorillonite, in addition to the associated impurities.
The bentonite is calcium based before use, and Na2CO3 is added for activation.
The optimal addition of Na2CO3 is 3.5% when three kinds of bentonite are activated, and Xinyang bentonite has the best expansion value and suspension property, but the high temperature induced thermal cracking property is poor.
Generally speaking, with the increase of bentonite content, except for the increase of coating thickness, suspension and leveling, the other process properties are worse. However, the amount of bentonite is small, and the suspension stability of the coating is poor;
In addition, Hb type suspending agent is an excellent suspending agent suitable for the preparation of flow coating.
2. HB type suspending agent suitable for preparing flow coating
HB type suspending agent is different from bentonite, which is made of special LT clay by modification. It not only has good suspension, not easy to harden, good dispersion, but also has high fire resistance. As an excellent suspension agent for flow coating, it can also be used as a high temperature binder, which can effectively improve the high temperature strength of the coating.
The expansion value, gas evolution and fire resistance of HB type suspending agent are compared with bentonite.
(1) Swelling value: the swelling values of HB suspension agent and activated bentonite in Xinyang, Zhangjiakou and marshan were 100%, 96%, 92% and 82% respectively.
(2) Gas generation: HB type suspending agent has the lowest gas generation rate and the slowest gas generation rate among the four suspending agents. Therefore, the possibility of porosity will be reduced when HB type suspending agent is used to prepare coating.
(3) Fire resistance: the fire resistance of HB suspension is between 1630-1650 ℃, and that of three kinds of bentonite is about 1250 ℃. HB type suspending agent can not only be used as zircon powder coating suspending agent for steel casting coating, but also can be used to prepare quartz powder and bauxite powder coating for steel casting production to replace zircon powder or corundum powder coating, which can greatly reduce the cost.
3. Effect of binder type on properties of zircon powder flow coating
The addition of PVAc emulsion increased the leveling property and increased the coating thickness, but the uniformity, fluidity and drip mark resistance of the coating became worse. That is to say, the increase of polyvinyl acetate emulsion increased the trend of flow. For gas generation, when the dosage is 5 phr, the maximum gas generation is only 7.5ml/g.
Water soluble phenolic resin is used as binder. The paint has good fluidity and low gas volume, and its performance is better than that of polyvinyl acetate emulsion. However, with the increase of the amount of water-soluble phenolic resin, the fluidity and leveling of the coating become worse, but there is a tendency to increase the drop mark.
With the increase of the content of polyphosphate, the effect of flow coating is not significant.
With syrup and starch as binder, the coating is bright and smooth, and has good leveling property. However, after the coating is formed, the structure of the coating recovers slowly, the uniformity of the coating is poor, and the amount of binder added in the drop mark year increases and becomes more and more serious, which is easy to produce defects such as accumulation, unclear outline, etc.
It can be seen that the type and amount of binder have a great influence on the performance and flow coating effect of the coating.
4. Effect of CMC on properties of zircon powder flow coating
The suspension property of the coating with CMC is better than that without CMC, the thickness of the coating is increased, and the leveling property is better, but the uniformity and fluidity of the coating are worse, and it is easier to produce drop marks. With the increase of CMC content, the suspension, leveling and thickness of the coating become better, but the uniformity, drip mark resistance and fluidity of the coating become worse.
CMC has three functions in coating: one is to form a network solid structure by polymer chain; the other is to form a solvated layer by adsorbing water molecules by polymer chain; the third is to have shear dilution effect.
5. Effect of other additives on properties of zircon powder flow coating
Generally, adding cosolvent will make the coating suspension, fluidity and adhesion effect between coating and mold cavity worse. Adding a small amount of n-pentanol and n-octanol in the preparation not only has a good defoaming effect, but also can change the wettability of the coating. For water-based zircon powder flow coating, in order to improve the solid content of the coating, alkali water agent is often added to ensure that the coating has good process performance and improve the density of the coating.
