Fundamental Research Of Foam Forming Technology

Nowadays,synthetic fibers especially long fiber materials,are used in papermaking industry.The environmental problems of water resources,energy and green resources are becoming more and more serious.The traditional papermaking enterprises are faced with series of problems,such as high energy consumption and low utilization rate of raw materials,so the foam forming technology has come into being.Foam forming technology is different from traditional wet or dry forming processes.It is to use foam as a carrier to disperse fibers in a water-based foam suspension.When the foam is ruptured,the fibers are deposited on the forming wire of the paper machine to form a paper sheet.In this paper,polyvinyl alcohol?PVA?foam system was used to study the key factors affecting the stability of foam,such as surface active agent,foaming condition,fiber properties and so on.The mechanism of foam stability,such as liquid film drainage and gas diffusion,was perfected.This article firstly used the Waring-Blender stirring method to prepare the foam and discussed the effect of polyvinyl alcohol?PVA?as a surfactant on the apparent performance of the foam.The experimental results show that the PVA surfactant solution,which exhibits low surface tension and moderate degree of hydrolysis,can effectively produce foam,resulting in a fast foaming rate and a high foam volume.In addition,foams with an air content of 60-80%can be produced using a sufficient amount of PVA surfactant under the condition of high agitation speed.The effects of surfactant type,salt(Na+,Ca²⁺,Mg²⁺,Al³⁺)and pH,ambient temperature and humidity on the apparent performance of the foam and the physical properties of the paper were studied.The experimental results show that:1)under the neutral condition of pH=7,the foam is more stable,and when PVA surfactant system changes from acid to alkaline,the foam is unstable,bubble size become smaller and its distribution become narrower.2)When the salt content is added to the foaming system,the filtration rate of the foam increases,and the viscosity of the system decreases after the addition of NaC l salt in the PVA foam system.Also,the foam half-life decreases with the increase of the amount of NaCl.At the same time,with the addition of Ca²⁺,Mg²⁺and Al³⁺ions,the stability of the foam is obviously decreased.3)When the temperature rises,the foam stability will decrease;on the contrary,raised humidity will increase the stability of the foam.Second,we use N₂,CO₂ and pressure measurement devices to study the mechanism of foam stability?gas diffusion,liquid film drainage?,to explore the effect of gas diffusion,pressure changes on foam stability.The infrared spectroscopy?FT-IR?method was also used to characterize the liquid and gas diffusion properties of the liquid film.The stability and attenuation of the foam were studied.The experimental results show that gas diffusion is an important part of the mechanism of foam stability.The gas diffusion effect of the foam can lead to the change of foam structure,foam volume,bubble size and the rupture of foam.Under the same conditions,the stability of N₂ foam is better than that of CO₂ foam.After foam foaming,the volume of N₂ foam decreases slowly from 480 ml to 295 ml,and the diffusion rate of N₂ through the membrane is about 0.62 ml/s,its half-life time is over 4.5min.It is because the CO₂ molecules are relatively active and the gas diffusion effect is more obvious,resulting in the rapid coalescence of the foam.Finally,the practical application of foam forming method in papermaking industry is discussed.The experimental results showed that the foam in softwood fiber suspension was more stable than that in PP and PET fibers,whose contact angles were 80.04 degrees and130.26 degrees respectively.For softwood fiber foams,its foam is small and mainly concentrated in the range of 0-50 microns.In this paper,the basic scientific problems in foam forming process are studied,and the principles of foam generation and foam stability are clarified.Foam forming technology can reduce water consumption,improve energy efficiency and control the structure of paper product.It is a disruptive technology and will have a good application prospect.