The key to improving the application performance of flaky boron nitride: hydroxyl modification

Hexagonal boron nitride (h-BN) has the title of "white graphite" because it is similar to graphite to some extent, such as flake shape, good lubricating performance, good adsorption performance, and thermal stability It has good properties, and can also show looseness, lubricity, light weight and softness.
It has strong workability and is widely used in the fields of optoelectronics, environmental protection and daily chemicals.

The reason why h-BN has such superior performance is that its atoms are hybridized in sp2 mode, there is a strong covalent bond, and there is only a weak van der Waals force between the lamellae.
At the same time, because the electrons in the structure are bound to polar atoms, it is difficult to move, so it can show excellent electrical insulation, coupled with a lower thermal expansion coefficient, higher heat transfer coefficient, and high chemical stability , High oxidation resistance, making h-BN nanosheets (BNNSs) an effective lamellar filler, and research in the field of anti-corrosion coatings has also progressed in recent years .
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h-BN nanosheet

However, it is not so simple to fully utilize the performance advantages of h-BN.
Take an example of "anti-corrosion coating": the size of currently commercially available h-BN powder is usually in the range of hundreds of nanometers to tens of micrometers.
The layers are stacked on top of each other, and there are more layers.
However, even if BNNSs are used, due to their higher surface energy and fewer surface functional groups, there are problems such as easy agglomeration and poor compatibility in the coating system, which limits their application in the coating system.
Therefore, in order to enhance the application advantages of h-BN in coating systems, it must be functionally modified.

Application of BN in anti-corrosion coatings

At present, there are many methods of functional modification of h-BN.
For example, the use of ball milling and ultrasound can modify h-BN with —OH, ether bond (—OR), —NH2, alkyl group (—R), halogen (— X) and heteroatoms (C and O) and other groups.
The modifying group can promote the dispersion of h-BN in the polymer resin matrix, and can also be used as a reactive group to continue the subsequent graft modification of h-BN.
Compared with other group modification methods, there are many types of hydroxyl modification methods, which are considered to be the most promising h-BN modification method, so I will mainly introduce it in the following.

01 Hydroxylation modification of boron nitride

In the process of hydroxylation modification on the surface of h-BN, the —OH group preferentially binds to the B site on the surface of h-BN.
When multiple different types of groups are simultaneously modified to h-BN, these groups are also preferentially bonded to the B atom.
Therefore, the focus and difficulty of the hydroxylation of h-BN lies in the realization of the chemical activation of the B atom, that is, the breaking of the B-N bond.

Studies have proved that the rupture of the B-N bond is easier to achieve under special conditions such as strong mechanical force, high temperature and pressure, strong acid and strong alkali, or strong oxidation.

①Preparation of hydroxylated h-BN by physical method

When the bulk h-BN is processed by physical means such as ultrasound and ball milling, the h-BN nanosheets will be peeled and broken from the original bulk h-BN under strong physical action, so that the B atom sites are fully exposed and easy to have.
High chemical potential -OH combination, so as to realize the hydroxylation modification of h-BN.

Principle of Ball Milling

Example: Lee et al.
added concentrated NaOH solution to h-BN and ball milled it to prepare hydroxylated h-BN nanosheets.
The average lateral size of the obtained nanosheets is 1.
5 μm, the in-plane structure is regular, and the yield is as high as 18%.
The product can form a stable dispersion in a variety of solvents.
The shearing force generated by high-speed ball milling makes the h-BN nanosheets fall off and break, and the active site is exposed to combine with —OH to realize the hydroxylation of h-BN.
The whole process is completed by mechanical shearing and chemical peeling.

Lin et al.
used ultrasonic method to achieve the hydroxylation of h-BN.
After h-BN was treated in a water bath ultrasonic for 8~24h and the supernatant was centrifuged, the bulk h-BN was successfully peeled off into a single layer or a few layers of h-BN nanometers.
sheet.
In the ultrasonic environment of a water bath, h-BN nanosheets are broken, and the B-N bond at the defect site may be attacked by oxygen atoms in the water to complete the hydroxylation.

② Preparation of hydroxylated h-BN by chemical method

In addition to physical methods, h-BN peeling, lamella fracture and hydroxylation modification can also be achieved through the chemical interaction between some special chemical reagents and h-BN.
Chemical reagents are usually molten hydroxide, hydrogen peroxide, etc.
Under the action of these reagents, the edges of the h-BN sheet will self-curl, and the ions in the system such as OH—will intercalate into the curled h-BN Between the layers, the h-BN nanosheets are gradually peeled off from the bulk h-BN, and hydroxylation modification is realized at the same time.

Example: Li et al.
found that molten hydroxide can achieve the hydroxylation of h-BN.
They grind the solid mixture of KOH and NaOH, mix it with h-BN, and put it in an autoclave at 180°C for 2 hours.
The reaction product can be filtered and centrifuged to obtain hydroxylated h-BN, and the yield is as high as 19%.

Hydrogen peroxide treatment of h-BN can also achieve its hydroxylation.
Zhi et al.
found that the oxygen radicals generated in the hydrogen peroxide solution can react with the B site on h-BN to hydroxylate it.
On this basis, Sainsbury et al.
first covalently grafted the alkoxy group to the B site, and then hydrolyzed and defunctionalized the grafted h-BN with a mixed solution of hydrogen peroxide/concentrated sulfuric acid, and finally successfully detected The existence of hydroxyl group realizes the hydroxylation modification of h-BN.

02 Application of hydroxylated h-BN in composite materials

The solubility of the hydroxylated modified h-BN in water can reach 0.
2mg/mL, and the dispersibility in the polymer has also been significantly improved, which greatly expands the application potential of h-BN in polymers .
For example, the high thermal conductivity of h-BN can transfer the heat of the resin stack faster and enhance the thermal conductivity of the composite material; the rigidity of h-BN can allow greater stress to be transferred from the matrix to the filler, thereby improving the composite material In addition, the bond bridge between the inorganic filler and the organic matrix will lead to interface polarization, thereby enhancing the dielectric constant of the composite material.

Thermally conductive silicone sheet is one of the important application areas of boron nitride

Compared with pure resin, the thermal conductivity, tensile strength and Young's modulus of the resin matrix composites enhanced by hydroxylated h-BN have been improved to a certain extent, but due to the hydroxylation modified h-BN The polarity difference between the B—N and B—O bonds and the carbon chain of the polymer is too large.
There will be gaps and cracks between the h-BN and the resin in the polymer, which will form a thermal boundary resistance, resulting in the inability to form a continuous material in the material.
Heat conduction path.
Therefore, the enhancement range of the thermal conductivity of the composite material modified by hydroxylated h-BN is not obvious.
Studies have pointed out that, in order to give full play to the excellent thermal conductivity of h-BN, subsequent modification of the hydroxyl group can be used to obtain more structures and types of h-BN derivatives to achieve good dispersion in the resin matrix.

source:

Research progress of hydroxylated modified boron nitride nanomaterials, Zong Tong, Zhang Youwei, Liu Pinggui.

Research progress in the application of hexagonal boron nitride in anticorrosive coatings, Yao Jiuti, Wei Ming, Liu Xiaofang, Hu Ke, Ye Tong, Dong Qunfeng, Yang Lifeng.