In order to give full play to the mechanical properties of aramid, it is necessary to carry out surface modification treatment.

  　Aramid fiber is a high crystallinity and high orientation material formed by rigid molecular chain developed by DuPont of the United States. It has a series of excellent properties such as low density, fatigue resistance and shear resistance. It is in the rubber industry and other fields. Widely used in aramid fiber reinforced composites. The properties of the composite materials are related to the matrix phase, the reinforcement phase and the interface state of the two phases. A good interface bond can make the composite material better perform its mechanical properties. Aramid has a rigid molecular structure, high molecular symmetry, weak lateral inter-molecular force, weak intermolecular hydrogen bonds, low lateral strength, which is easy to break under compression and shear forces; due to its high crystallinity, The surface of the fiber is smooth and non-reactive, resulting in poor interfacial adhesion to most of the matrix. Therefore, it is necessary to improve the interfacial bonding between the aramid fiber and the composite material, and to give full play to the excellent mechanical properties of the aramid fiber. The surface of the aramid is modified.

Surface modification method of aramid short fiber:

The surface modification of aramid can be carried out by chemical methods such as plasma or ultrasonic or chemical methods such as nitration/reduction and chlorosulfonation, and introduce polar or reactive groups such as hydroxyl groups and carbonyl groups on the surface of the fiber to form a reactive covalent bond with the matrix. The bond is combined to increase the bond strength between the fiber and the substrate.

1. Copolycondensation modification

By introducing a third monomer having a different structure into the molecular chain of the aramid fiber, the solubility and fatigue resistance of the aramid fiber are improved under the premise of maintaining the original excellent performance.

Bernhard et al. used a combination of p-phenylenediamine and dichloroterephthaloyl copolycondensation to prepare different rigid rod-shaped aromatic polyamides. The main crystal structure is similar to para-aramid, except that it does not occur in heat treatment. Structural changes, steric hindrance and electronic effects of benzene ring substitution lead to different fiber solid-state structures.

2. Using chemical reaction, a reactive group is introduced on the surface of the fiber to chemically combine the fiber with the matrix to form more chemical bonds and increase the interfacial compatibility of the material. Mainly by coupling agent modification, surface etching and surface grafting.

(1) Coupling agent modification: The coupling agent has a bifunctional group in the chemical structure and acts as a “bridge” in the composite material, one end reacts with the surface of the fiber, and the other end reacts with the matrix to increase the compatibility of the interface. Chen Wei et al. used a silane coupling agent to surface-treat the aramid to form a coupling agent bridging and entanglement between the fiber and the rubber matrix, obtaining a good interfacial transition zone, improving the interface structure and eliminating stress mutations. The transverse tensile strength of the composite material is significantly improved, and the high temperature resistance is increased.

(2) Surface etching method: etching modification technology is to treat aramid by chemical or physical technology, which changes the morphology, structure and polarity of the surface layer, which is beneficial to the infiltration and adhesion of the matrix resin of the composite material. , thereby increasing the bond strength of the interface.

Composite materials such as aramid/epoxy are commonly used in chemical etchants such as acid chlorides (methacryloyl chloride, etc.) and acids and bases (acetic anhydride). This can not only erode the surface structure of the aramid, form a rough surface, increase the adhesion of the matrix resin to the surface of the aramid; it can also form polar groups such as -COOH, -OH, etc. on the surface of the aramid by hydrolysis reaction. A covalent bond can be formed between the fiber and the matrix to improve the wettability of the resin matrix to the surface of the aramid.

Penn et al. nitrated Kevlar-29 with nitric acid or ammonium nitrate to introduce a nitro group onto the benzene ring and then reduced the nitro group to an amine group. The surface morphology and surface energy of the modified surface did not change, but the interfacial shear strength of the Kevlar-29/ epoxy composite after nitrification was greatly improved.

(3) Surface grafting method: There are two main grafting reactions on the benzene ring: one is to introduce an amino group by a nitration reduction reaction, and the other is to introduce a chlorosulfonic acid group by a chlorosulfonation reaction to further introduce an activity. Group. Kevlar is treated with chlorosulfonic acid, which first introduces a chlorosulfonyl group on the surface of the fiber, and further converts it into an active group such as a hydroxyl group, a carboxyl group or an amine group.

Yuan Haigen et al. first cleaned Kevlar29 with dichloroethane, absolute ethanol, etc., and then treated with dimethyl sulfoxide (SMSC) in dimethyl sulfoxide (DMS0) solution, and finally the NaA+ fibers were separated. It can be reacted in chloropropene and epichlorohydrin for 10 minutes. This is grafted on the Kevlar29 fiber surface.

CH2=CH-R- group structure, the hydrogen atom on the secondary amide on the surface layer molecule has been replaced by an allyl group. The tensile strength and shear strength of the interface were improved and the shear strength of the interface was improved.

By the action of sputtering, the surface of the fiber becomes rougher, and the contact area of the fiber is increased, thereby increasing the friction between the fiber and the polymer matrix, and improving the interfacial adhesion.