Investigation On Heat Transfer Characteristics Of Rubber In Microwave Vulcanization Process

Compared to conventional vulcanization process of rubber,in microwave vulcanization process,the energy is directly introduced into the rubber resulting in a rapid and volumetric heating process with reduced thermal gradients.It is advantageous to improve the vulcanization quality of the rubber product and shorten the vulcanization time,especially for the vulcanization of the thick-walled product,which can reduce the vulcanization time of 1/3 or more.In this paper,isoprene rubber,tread rubber,sidewall rubber,lining rubber and ply rubber are the object of study.Thermal conductivity,specific heat,dielectric constant and dielectric loss tangent of each mixing rubber were measured and the feasibility of the microwave vulcanization scheme was verified by utilizing experimental method.In this study,the dimensions of block rubber was 40mm(length)X 40mm(width)X 60mm(height),40mm X 40mm X 40mm,and 40mm X 40mm X 20mm,respectively.The regularity of temperature rising and temperature distribution of rubber was obtained.The effect of rubber rotation state and microwave antenna on temperature rising regularity,temperature distribution and microwave heating efficiency of rubber was researched during microwave vulcanization process,the effect of dielectric constant and dielectric loss tangent on temperature distribution and microwave heating efficiency of rubber was also researched,the effect of microwave frequency and microwave power on temperature field and microwave heating efficiency of rubber was explored.Conclusions are as follows:(1)The relationship between thermal conductivity and temperature can be expressed as ? ?a + ?bt;the relationship between specific heat and temperature can be expressed as Cp =a+bt-ct2.(2)For isoprene rubber,sidewall rubber and ply rubber,the relationship between dielectric constant and microwave frequency can be expressed as?' =a-bf+cf2-df3,for tread rmbber,the relationship between dielectric constant and microwave frequency can be expressed as ?' =a-bf-cf2-df3,for lining rubber,the relationship between dielectric constant and microwave frequency can be expressed as?'?a-bf-cf2+df3.For sidewall rubber,the relationship between dielectric loss tangent and microwave frequency can be expressed as tan?-=a-bf-cf2+df3,for isoprene rubber,tread rubber,lining rubber and ply rubber,the relationship between dielectric loss tangent and microwave frequency can be expressed as tan?=a-bf+cf2-df3.(3)For five kinds of block rubber,the temperature distribution of block rubber is uneven after being heated by microwave energy,hot spots are concentrated in the center of the block.In same microwave vulcanization process,the relationship between temperature of measuring point 2#,5#and 8#of block rubber and heating time can be expressed as t = ?1+?2?-?3?2,the relationship between temperature of other temperature measuring points of block rubber and heating time can be expressed as t =?1+?2?+?3?2.(4)Rotation can improve its temperature distribution uniformity and microwave heating efficiency,the maximum reduction rate of temperature difference is 72.47%,the maximum increase rate of average temperature is 25.19%.The position and area of hot spot was influenced by microwave antenna,microwave antenna rotation can reduce the temperature difference of microwave vulcanization of rubber block,the maximum reduction rate is 54.62%,meanwhile,microwave antenna rotation can improve the temperature distribution uniformity of block rubber and microwave heating efficiency,the average temperature of block rubber can be increased nearly by 100%.The rotation direction of microwave antenna has little effect on temperature distribution uniformity of block rubber and microwave heating efficiency,the impact can be ignored.The location and area of the hot spot are affected by the angle of microwave antenna,the relationship between temperature difference of microwave vulcanization and antenna angle can be expressed as ?t-?1-?2?+ ?3?2-?4?3,the relationship between average temperature of rubber and antenna angle can be expressed as t =?1-?2?+?3?2+?4?3,the relationship between microwave heating efficiency and antenna angle can be expressed as ?=?1-?2?+?3?2-?4?3.(5)The temperature difference of microwave vulcanization of block rubber in counterclockwise rotation state is less than that in clockwise rotation state,the temperature difference is minimum,when the rotation rate is 300r/min.The relationship between counterclockwise rotation rate and temperature difference of block rubber can be expressed as ?t = ?1-?2r + ?3r2,the relationship between counterclockwise rotation rate and average temperature of rubber can be expressed as t =?1??2r.The average temperature and microwave heating efficiency of block rubber increased with the value of ?'×tan? increasing,the larger the value of?'×tanb?,the more uneven the temperature distribution of block rubber,the relationship between temperature difference of microwave vulcanization and ?'×tan?can be expressed as ?t = ?1+?2?'×tan?,the relationship between average temperature of rubber and ?'×tan? can be expressed as t =?1+?2?'×tan?,the relationship between microwave heating efficiency and ?'×tan? can be expressed as?=?1+?2?'×tan?.(6)The maximum temperature and minimum temperature of block rubber increase first and then decrease with frequency increasing.The microwave heating efficiency is maximum in the microwave frequency of 2450MHz,but the non-uniformity of temperature distribution is serious,however,more uniform temperature distribution presented in other microwave frequencies,but the microwave heating efficiency is low.For the cuboid and cylindrical rubber block,the uniformity of the temperature distribution of the small volume block rubber is better than that of the large volume block rubber,but for the large volume block rubber,the microwave heating efficiency is maximum.For the circle cylindrical block rubber,the uniformity of the temperature distribution of the small volume and large volume block rubber is better than that of the medium volume block rubber,but for the medium volume block rubber,the microwave heating efficiency is maximum.(7)The smaller the microwave power,the more uniform the microwave heating,microwave power has little effect on microwave heating efficiency,the impact can be ignored.For the block rubber of 60mm height,the uniformity of microwave vulcanization temperature distribution of the cuboid block rubber and the circle cylindrical block rubber is better than that of the cylindrical block rubber,however,the microwave heating efficiency of the solid cuboid and cylindrical block rubber is higher than that of the hollow circle cylindrical block rubber.For the block rubber of 40mm height and 20mm height,the non-uniformity of microwave heating temperature distribution of circle cylindrical block rubber is serious,but its microwave heating efficiency is maximum.