Research On Infrared Emissivity And Thermochromic Properties Of Triphenylmethane Phthalein Materials

With the rapid development of modern detection technology,military target survivability on the battlefield is facing a great threat,so it is important to develop smart adaptive camouflage materials for modern warfare.Adaptive camouflage is a type of technology capable of producing a―chameleon‖effect to adapt to the surroundings.Its aim is to make military targets more movable and better protected from being detected by infrared detection equipment.At the same time,the emissivity property is required to be no difference between military targets surface and the surroundings.Therefore,the smart adaptive camouflage materials with reversible changes in color and infrared emissivity property under temperature fluctuations,have recently received noticeable attention.Triphenylmethane phthalide compounds are a kind of famous thermochromic compounds,which have high absorbance in different bands of visible light and high reflectivity in near-infrared region.Among them,Bromocresol Purple（BCP）can reversibly show orange and tan,and Bromocresol Green（BCG）can reversibly show green and khaki-yellow.CaCl₂,as a typical alkaline earth metal inorganic salt,can accept the surplus electrons from BCP or BCG due to the empty 3d orbital outside the core.In this paper,Bromocresol purple（BCP）and Bromocresol Green（BCG）were used as color former,calcium chloride（CaCl₂）was use as developer,N-16 alcohols and graft copolymers polyethylene glycol-g-cellulose diacetate with solid-solid phase transition properties were used as solvent compounds,respectively.Pure BCP has no thermochromic phenomenon and appears light purple,but when it reacts with CaCl₂,the color of orange is presented in the powder of BCP/CaCl₂.It is found that the color of BCP/CaCl₂ is related to the absorption spectrum in UV-Vis.At high temperature,the color of BCP/CaCl₂ powder turns brown.Additionally,the emissivity of pure BCP almost does not change with rising temperature,and the value is about 0.7.While the emissivity of BCP/CaCl₂ powders decreased from 0.65 to 0.53 with the increase of temperature.Most importantly,when the N-16alcohol is added into BCP/CaCl₂ powders,the thermochromic temperature and the temperature of variable emissivity can be greatly reduced.In order to change the bright color of powders and solid-liquid phase change,BCG was adopted as the color former and PEG,CDA were use as solvent.At room temperature,the color of BCG/CaCl₂/PEG-g-CDA is green.When the temperature increases,the color turns to khaki-yellow.The thermochromic process is reversible.Its emissivity can be changed from 0.58 to 0.93,which is an ideal powder with thermochromism and variable emissivity.Generally speaking,the powder prepared above can only be used after being coated.When different resin matrix and BCG/CaCl₂/PEG-g-CDA powders are mixed,the properties of resin will have different degrees of influence on the properties of powders.It is found that the coating prepared by acrylic acid can retain the properties of powders with thermochromism and variable emissivity.While epoxy modified organic silicon has a great influence on the properties of powder,and the color of coating is quite different from that of powders.In conclusion,acrylic resin matrix is the most suitable for BCG/CaCl₂/PEG-g-CDA powders.