Study On The New Producing Technique And Curing Mechanism Of CO₂-Cured Phenolic Resin Binder

Foundry is very important in the development of mechanical industry and it is urgent to prevent the heavy pollution from the production of foundry. The conventional cold-box process used in foundry such as tertiary-amine-set process, SO₂-set process,β-Set process with gaseous catalysts or hardeners are so poisonous or harmful that it is necessary to develop a new cold box process with harmless gas to human body and environment. Researching on the new producing technique and curing mechanism of CO₂-cured phenolic resin binder in this paper is developed to be meet the requirement.The new resin binder is composed of a resol phenolic resin,a dispersant an cross-linking agent and a intensifier etc. The synthetic process of resol phenol-aldehyde resin has been studied at first, the synthetic process parameters of phenolic resin is optimized as followed : NaOH is choosed as a catalyst, NaOH/phenol=0.06/l(mole-ratio), phenol/formaldehyde=1/2.5(mole-ratio), The reacting temperature is divided into two steps and the formaldehyde is added by dropping. The reaction time is controlled by the viscosity of resin to get a suitable molecular weight with a good binding strength. The synthesis is an important base for the next binder production.The producing technique of binder is a critical point in this paper. According to the affection on the strength of binder, the optimum results has been achieved by the experiments of orthogonal design about the amounts of phenolic resin,alkali,borax and silane to form a steady binder system with high strength. The proportion of above factors is as followed: the phenolic resin: KOH : borax: KH-550=100: 45: 16: 1, To add two kind of organic active agents into the binder can improve the strength of binder greatly, the original strength (σ₀) is above 1.8 MPa and final strength(σ₂₄) is above 4.0MPa with the organic active agent A 5% and B 15% to the binder. The successful addition of organic active agents is a technical key to the production of binder.The research on the mechanism of binder can reveal the essential of curing process and can raise the research level to a higher standard. The CO₂-cured mechanism of phenolic resin is analyzed by MRI,IR,SEM and pH measure. The binder which can be cured by carbon dioxide gas, consists of an alkaline aqueous solution of a resol of phenolic resin and an oxyanion capable of forming a stable complex with the resin and sufficient of alkali to substantially prevent stable complex formation between the resin and the oxyanion. While passing carbon dioxide gas through the sands ,the pH of alkaline aqueous solution becomes lower and lower and the hydroxymethyl and hydroxy of the phenolic resin are linked with the oxyanions to form a stable complex and thereby to cure the resin. With the bonding bridge fracture analyzed by SEM, the organic active agents is believed to play an important role in cross-linking of binder too.A cheap curing-promoter which consists of Ca(OH)₂,42.5^#cement and two kind of metal-oxide is specially prepared for the binder. The optimization of the proportion of each part is confirmed by orthogonal test: Ca(OH)₂:5.0 g,42.5^#cement:3.0 g,ZnO:2.0g,MgO:1.5g and the right addition of curing-promoter is about 30% to the amount of binder. The curing-promoter can increase the strength of binder greatly and shorten the time of gassing .The original strength (σ₀) is above 1.8MPa and final strength(σ₂₄) is above 5.2MPa with 3.0% binder . The curing-promoter can increase the strength of binder so successfully that it is quite helpful to the application of CO₂-cured phenolic resin binder.A new and potential cross-linking agent which is a kind of silicate called R is researched for the CO₂-cured phenolic resin to decrease the cost of binder further more. The proportion of main parts of this cross-linking system is as followed: phenolic resin: NaOH: R: silane=100: 15:45: 1 with a high final strength(σ₂₄). A certain amount of acrylic acid,sodium polyacrylate,Ca(OH)₂ is used to get a original strength(σ₀) 0.8MPa.The cross-linking system with a low cost and a high final strength(σ₂₄) 4.0MPa is worthy for a further research.Finally, research has been carried out on the phenolic resin binder industrial production and some performance index of phenolic resin sand have been tested. The CO₂-cured binder has been used successfully for making aluminium-alloy,iron and steel castings. Through the production test, the binder can meet the requirement of the foundry production.Compared with tertiary amine-set process,the cost of the binder is much lower for a beneficiai use in foundry.