Ultrasonic Synergistic Alkali Urea(thiourea) Modification And Mechanism Study Of Pulp Fiber For Air Filter Paper

As an indispensable functional paper,air filter paper requires paper with the characteristics of low filtration resistance,high dust holding capacity,high filtration precision and high efficiency.Due to its own characteristics,plant fibers are difficult to fully meet the production requirements of air filter paper.In the traditional production process,the planting fiber is generally treated by mercerizing,the interweaving ability of the plant fiber is reduced,the bulk of the paper is increased,and the effect of low filtration resistance and high dust holding capacity is achieved.The shortcoming of the mercerizing process is that the amount of alkali used is high,and the environment is highly polluted after the wastewater is discharged.The fiber is modified by a green solvent to obtain a paper-based material fiber having a high bulk,a high air permeability and a uniform pore size distribution,and is used as a raw material for air filter paper and the like,which is important for improving the utilization value of the fiber and environmental protection.In this paper,bleached sulphate softwood pulp is used as raw material,and alkali urea（thiourea）system（NaOH-urea-thiourea,NaOH-urea,NaOH-thiourea）is used to gently treat pulp fibers at low temperature to prepare air filter paper with long service life,low filtration resistance,good air permeability and uniform pore size distribution.By comparing the effects of the three treatment methods on fiber morphology and paper-forming properties,a cost-effective treatment method was selected.On this basis,the influence of the synergistic effect of ultrasonic waves on the modification process and fiber was further explored,and the mechanism of ultrasonic modification of the modified cellulose with alkali urea（thiourea）system was clarified.The modified fiber is further modified with a silane coupling agent to impart good compatibility between the fiber and the resin when the air filter paper resin is coated.The pulp fibers were modified by NaOH-urea-thiourea,NaOH-urea and NaOH-thiourea aqueous solution at low temperature（-15°C）.On the basis of keeping the treatment process mild and saving materials,comprehensive consideration is given to the changes in fiber morphology and paper-forming properties after treatment,and the economical and effective treatment methods and ratios are selected after comparing the three methods.Finally,8wt%NaOH-8wt%thiourea aqueous solution was used for low temperature treatment for 20min for subsequent research.At this time,the fiber length decreased by 13%,the fine fiber content decreased by 60%,and the crimp index increased by 63%.At the same time,the surface roughness of the fiber increased by 158%and the crystallinity decreased by 2.17%.After the paper is formed,the bulk reaches 6cm³/g,which is 39.53%higher than that of the untreated fiber,the air permeability is increased by 105%,and the average pore diameter is increased from 28.79μm to 49.62μm,but the strength is decreased by 45%.Ultrasonic waves were combined with NaOH-thiourea aqueous solution to treat pulp fibers.The effects of ultrasonic power and treatment time on the modified fiber of NaOH-thiourea aqueous solution were investigated.The results show that during the treatment,the fiber and the treatment solution are uniformly mixed by using ultrasonic instead of the constant speed agitator（mixing time is 5 min）,and the effect is better when the ultrasonic power is 30%.At this time,the paper bulk reached 6.75 cm³/g,the relative increase was 10.6%,the air permeability was 920 mm/s,and the crystallinity decreased from 77.89%to 63.24%.After ultrasonic mixing and low temperature treatment with NaOH-thiourea aqueous solution,the fibers were sonicated again to improve paper strength and pore size uniformity.The best treatment is to use 40%ultrasonic power for 20 min.At this time,the paper bulk is 6.68 cm³/g,the tensile index is 1.6 N·m/g,the air permeability is 825 mm/s,and the average pore size distribution is changed from 80μm²10μm to 50μm⁸0μm.The micro-jet shock wave generated by ultrasonic cavitation will damage the surface and internal structure of the fiber,and the structure will become loose,so that the solvent molecules of cellulose treatment will penetrate into the fiber interior.It is beneficial for NaOH-thiourea to treat fibers at low temperature and increase the degree of reaction.In the aqueous solution of NaOH-thiourea and cellulose,OH^-in NaOH has a strong hydrogen bond with thiourea-NH₂.At low temperature,the solvent molecules in the solution are very easy to interact with C6on the cellulose chain.The hydroxyl groups on the association and the formation of a relatively stable tubular inclusion complex,the thiourea hydrate is located at the outermost layer of the tubular clathrate.The formation of the tubular inclusion compound destroys the hydrogen bond between the fiber molecules,and the cellulose chain is dispersed and dissociated,partially destroying the crystallization zone of the fiber,thereby causing the fiber to exhibit a state of bending and kink.The air-filter paper base paper was prepared by ultrasonically combining the fiber treated with the NaOH-thiourea aqueous solution as a raw material,and then resin coating was performed.In order to improve the compatibility of the resin with the fiber,the fiber was modified with a silane coupling agent（KH570）,and then the acrylic resin was sprayed on the surface of the base paper at a coating amount of 20%.Infrared spectrum shows that KH570 is attached to the cellulose surface by chemical bonds.The paper tensile index of spray resin before and after KH570 modification was increased from 1.6N·m/g of the base paper to 3.39 N·m/g and 3.5 N·m/g,respectively;the average internal bonding force of the paper was increased to 30 J·m² and 35 J·m²respectively;the paper bending stiffness is increased from 75 mN of the base paper to312 mN and 320 mN respectively;the paper air permeability and bulk have almost no change.