text| Xiao Shanqing
Edit| Xiao Shanqing
●—≺ High strength polyethylene fiber ≻—●
High-strength polyethylene fiber is the third generation of high-performance fiber after carbon fiber and aramid fiber, which is not only the current high-performance fiber, the specific modulus, the highest specific strength fiber, and has good wear resistance, good impact resistance, good chemical resistance, non-absorbent, good biocompatibility, good electrical properties and light specific gravity.
At the same time, because the raw material polyethylene is easy to obtain, if it is applied on a large scale, its production cost is expected to be low, especially the application of melt spinning and liquid spinning technology, which is expected to greatly reduce its production costs.
Therefore, high-strength polyethylene fiber reinforced composite materials are a highly competitive variety in many fields.
Although the softening point of high-strength polyethylene fiber is low and creep is easy to occur under heavy load, which limits its application in the field of temperature-resistant and structural composites, it still has broad application prospects in the fields of bulletproof composites, high-impact composite materials, explosion-proof composites, offshore composites, biomedical composites, and composite materials with unique electrical properties.
Especially in recent years, with the rapid growth of the application of high-cost carbon fiber composites in the civil industry, it is an inevitable trend to find and produce high-performance fibers with lower costs as substitutes for carbon fibers. High-strength polyethylene fiber is one of the potential competitors.
However, because the high-strength polyethylene fiber itself is composed of simple methylene groups, the fiber surface not only does not have any reactive points, but cannot form a chemical bond with the resin.
Moreover, the non-polarity of methylene and the highly crystallined, highly oriented smooth surface of high tensile forming make its surface energy extremely low, not easy to be wetted by resin, and no rough surface for the formation of mechanical meshing points.
Therefore, the interfacial adhesion of high-strength polyethylene fiber reinforced composites has become the primary issue in the production process of this composite.
Since the eighties, many research groups have carried out research in this area, and the methods proposed are: chemical oxidation, chemical collation, radiation grafting, ozone oxidation, etc.
Among them, plasma treatment can achieve the best effect, its interlayer shear strength (ISS value is still much lower than carbon fiber reinforced composites, and due to the difficulty of industrial production of plasma treatment equipment, most of the current industrialized goods are only simple corona treatment.
Strictly control the oxidation conditions of polyethylene fiber, so that the fiber strength will not be lost too obviously, and at the same time can introduce appropriate functional groups on the fiber surface, improve the wettability, coarse pulp and chemical bonding point of the fiber surface, thereby improving the adhesion between high-strength polyethylene fiber and epoxy resin, and increasing the interface bonding strength by 2-4 times.
However, if the degree of oxidation is deepened, the strength of the fiber will be greatly lost, which will inevitably damage the performance of the composite material in the fiber axial direction. Therefore, how to improve the surface properties of fibers while maintaining the strength of fibers has always been the goal pursued by fiber reinforced composite interface workers.
Surface treatment of glass fibers with coupling agents is a very successful example. The coupling agent molecule reacts with the light group on the surface of the glass fiber to form a chemical bond, and the functional groups on the coupling agent molecule, such as amino groups, can chemically bond with the matrix resin to form an interfacial phase with a chemical bonding structure.
Can similar chemical bonding structures be available on other reinforcing fibers such as carbon fibers, aramid fibers and polyethylene fibers? The following are some of the experimental results of our introduction of polyfunctional compounds on the oxidized surface of polyethylene fibers as preliminary exploration of the design of this interface bridge bond.
●—≺ Experimental process ≻—●
The oxidized HSPE fiber and PCI ethyl fermentation solution were reacted in a 60C constant temperature tank for 20 minutes, and after the reaction was completed, the fiber was quickly moved to ether for 5 minutes and washed, and the fiber was directly put into the reaction solution for 5 hours at 60C, and the reaction solution was DMF solution of diethylenetriamine and pentaerythritol, respectively.
The changes of wettability were determined by sedimentation method, the changes of surface functional groups were characterized by FTTR photoharmonics, the changes of surface functional group content were determined by methylene blue adsorption method, the interfacial shear strength between monofibers and epoxy resin matrix was determined by microbound, and the monofilament strength of fibers was measured.
After the high-strength polyethylene fiber is oxidized by potassium dichromate, due to the fracture of the main chain and further oxidation into a base compound, a large part of which is a group functional group, with PCI is easy to convert this surface group into highly active cool chlorine, and then react with polyfunctional amines or alcohol compounds, polyamines or polyol compounds can be introduced into the surface of the fiber, this process can be expressed in the following way.
Figure 1 is the FTIR photoharmonic diagram of PE fiber, oxidized PE fiber, diethylenetriamination-ated PE fiber and pentaerythritol PE fiber, and the changes at 1725cm-11710cm-1 and 3000~3400cm-1 clearly prove the progress of the above chemical reactions.
Table 1 shows the value of the active hydrogen disk on the surface measured by the methylene blue isothermal adsorption method, and the active hydrogen functional group on the surface of the fiber after oxidation increases significantly, and the value of the mogener group is greatly increased after converting it into polyamines or polyol compounds, which further proves the above chemical reaction.
The more sedimentation percentage in the aqueous solution of the alcohol (specific gravity of 0.97), the more the fiber surface is more easily wetted by the liquid, so this value can be used as a relative comparison of the wettability of the fiber surface.
The values in the table show that the wettability of introducing polyfunctional compounds into the fiber surface can be greatly improved, which will facilitate its interfacial fit. In this experiment, the interfacial adhesion between fiber and ring resin was measured by microbound.
Figure 2 shows the relationship between the disbond force and the diameter of the pellets obtained by measuring a series of resin pellets of different diameters, and the interfacial shear strength can be calculated by the following equation:
where F is the average radius of the de-force dimension with a small diameter. It can be seen that the experimental results obtained by many other researchers are similar, and the data is highly dispersed.
We believe that this dispersion is related to the inhomogeneity of fiber diameter and fiber surface functional groups. In the sedimentation experiment, the fiber is cut into lcm long segments with 100 such fiber segments, some sinking liquid, some floating on the surface of the liquid, which shows that the polarity of the fiber surface is different, the sinker polarity is stronger, and the floating polarity is weaker.
And we found in experiments that some fiber segments, straight in the liquid, take a long time to reach stability, which indicates that the surface of this centimeter-long fiber segment is also uneven, and the inhomogeneity of this fiber surface, although it has been mentioned, but there is no direct evidence. The above-mentioned ups and downs experiments can serve as good evidence, and further research work in this area is continuing.
A large part of the dispersion of the above micro-debonding experiment results may be derived from the dispersion of the fiber diameter, as shown in Figure 3 is a statistical chart of hundreds of fiber diameters taken from different winding simplifications, which shows that it has greater dispersion. So we rephrase (1) as:
The interfacial bond strength value is directly calculated from Equation (2), thereby eliminating the influence of fiber diameter dispersion. In order to reduce the impact of other aspects, the experimental results of at least 25 or more different bead diameters are still taken.
Table 3 shows the flat result calculated from equation (2), and the fiber strength values are also listed in the table. The g/o value is used to indicate the degree of fiber strength loss, and the z/o is used to indicate the degree of improvement of interfacial bonding strength.
●—≺ Proof result ≻—●
In order to comprehensively evaluate the adverse factors of fiber strength loss and the favorable factors of interfacial bond strength improvement, we roughly measure the net result with A/AO*/TO. From this value, it is clear that fibers with polyfunctional compounds have certain advantages over oxidized fibers.
In particular, the value of diethylenetriamine increased by a factor (8 times) and the interfacial adhesion of fibers after diethylenetriamination and pentaerythritol in Table 3 when plasma treatment was reported in the current literature2] Table 3.
This may be due to the fact that diethylenetriamine is a room-temperature curing agent for epoxy resins, and the remaining amine groups can still be crosslinked with epoxy resins after bonding to the fiber surface to form chemical bonds. The release group on pentaerythritol is more difficult to react with epoxy resin at room temperature, especially in the presence of room temperature curing agent, and it is more difficult to compete to obtain the reaction point.
Therefore, the introduction of pentaerythritol on the fiber surface may mainly increase the wettability of the fiber surface and improve the van der Waals force between the fiber and the resin, thereby improving the interfacial adhesion strength.
The introduction of diethylenetriamine fibers not only improves surface wettability and increases van der Waals forces, but also adds many chemical bonding points. This proves the importance of chemical bonding at the interface of composite materials.
The introduction of polyfunctional compounds on oxidized high-strength polyethylene fibers improves the interfacial adhesion between fibers and epoxy resins, which is a surface treatment method that does not further lose fiber strength. In particular, the introduction of polyamine compounds can form chemical bonds between fibers and resins, which greatly improves the interfacial bonding strength.
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