In recent years, with vigorous development of wall board industry in our country, the production scale of enterprises of wall board has been constantly expanded, and the demand in glue has correspondingly risen. Glue plays a very important role in wall board production, the quality of which always determines the quality and grade of wall board products, and it also plays a decisive role in decreasing the cost of products. The quality of glue and the cost of products have already been major factors directly influencing the market competitive strength of wall board products and the benefit of manufacturer. In order to ensure normal production and the quality of products, most of the wall board enterprises in our country have their own glue producing workshops and the workshops are always making intermittent production. How to reduce the unsteady factors in resin production, produce good glue, secure the quality of wall board products and reduce the cost of products has been paid great attention by more and more enterprises. The following will make a brief analysis to the causes of wall clinging (skim coating) which is often occurred in resin production, and put forward some measures to reduce wall clinging in reaction vessel for reference.
  1 Technologic factors
  1.1 Reaction temperature and time
When the temperature of reactive liquid is lower than 80℃, if the catalyzer is ammonia chloride, and because the reaction of ammonia chloride is speedy, the value of PH cannot display and quickly drops after temperature rising, then the reaction becomes more faster, the polycondensation will be too acute, causing the glue condensed and sticking the wall. In addition, if the polycondensation time is too long and the molecular weight of resin is big, and the viscosity degree is too high, it will also cause skim coating phenomenon. Therefore, in operation, we shall correctly control the temperature and time of polycondensaiton and timely terminate the reaction. Generally the suitable temperature of reactive liquid shall be controlled within 95℃.
　  1.2  Temperature difference in wall of the vessel
If the temperature of cooling medium is too low or suddenly drops down will make the temperature difference between wall of the vessel and the material too big, causing the glue liquid clinging the wall. Therefore, no matter in heating up or in cooling down, the temperatures should within a reasonable scope and normally the steam temperature should be less than 180℃, the temperature difference thermal shock less than 120℃, and cooling shock less than 90℃. At the same time, should pay attention to confirm reasonable temperatures in the outlet and inlet of cooling medium, so as to keep the balance in operation.
  1.3 Balance degree in operation
    If the glue making technological indexes such as temperature, pressure are steadily controlled or the fluctuation is too excessive, the resin polycondensation process will go unevenly, easily causing skim coating. Therefore, in the production operation should slowly increase the pressure, and temperature. Generally, slowly increase the pressure and temperature in about 2~3 minutes after supplying 0.15Mpa steam. Reasonable upgrading speed will be 0.1~0.15Mpa per-minute.
  1.4 Material factor
Because the content of sulfate is too high in carbamide, adding carbamide at the last stage of resin polycondensation is like adding curing agent, making the resin speedily unite as net structure. If not treating it in time, the resin would be cured in reaction vessel, so in production, we should choose standard industrial urea material, securing the content of sulfate in carbamide to be strictly below 0.01%.
  2. Equipment factors
  2.1 Flowing characteristic
The flowing status of the material in the reaction vessel is also influencing the degree of skim coating, so improving the material flowing efficiency could greatly reduce the degree of skim coating. In designing, we adopt new-type turbine blending mode which can make the material do radial flow and improve the skim coating degree.  
  2.2 Treatment to internal wall of vessel
Skim coating easily occurred inside the coarse wall. Therefore, we do buffing and polishing treatment to the internal wall of the reaction vessel, heat exchanger, welding lines, etc in processing, consequently improve the smooth finish degree inside the vessel and reduce skim coating. What’s more, improving the technique to reduce the welding points inside the vessel can reduce the skim coating phenomenon to a certain extent.
  2.3 Eliminate dead angle
Skim coating will easily occurred at the dead angels caused by unreasonable equipment structure. Therefore, we try every means to eliminate the dead angels in equipment structure designing. The larger exchanging area of new-type heat exchanger and the better material flow will make skim coating reduce (See the Technological Characteristics and Improvement in High-capacity Reaction Vessel on details).
2.4 Position of feeding tube     
Because the place in feeding tube is a dead angel in which the material is difficult to be blended and concentration in part of its nozzle is big, skim coating will easily occurs. Therefore, we shall reasonably confirm the position of feeding tube. We adopt side tube in tube nozzle designing, making the material slowly down to the liquid surface with a definite angle, avoiding the glue liquid splashing to the internal wall of the vessel as well as blending evenly, making the material distribute as even as possible, reducing the skim coating.
In addition, when the skim coating occurred inside the vessel, can adopt high-pressure water jetter to clean the internal vessel.
Because all synthetic resin used in wood industry has a certain of viscosity, skim coating phenomenon inevitably occurs in glue production process. If it does not be treated in time, it will influence the heat exchanging coefficient and increase the energy consumption in glue production. What’s more, for most skim coating phenomena inside the reaction vessel caused by gel phenomena are fundamentally due to the techniques, the quality of glue liquid may be directly influenced if we do not timely find and adjust the technical and technological indexes. In a word, the adoption of above-mentioned measures will maximally reduce skim coating and secure the high efficiency running in glue making.