| As a new species of high-performance organic fiber,polyimide(PI)fiber has excellent mechanical properties,heat resistance,environmental stability and low dielectric constant due to its high molecular structure,compact structure and high degree of orientation.PI fiber has advantages of low density and high service temperature compared with high performance fibers widely used at present,such as glass fibers and aramid fibers.Therefore,it is expected to be widely applied in high technology industries like advanced polymer matrix composites,high performance ropes and cables,etc.However,the surface properties and interfacial properties of PI fibers were not investigated fully as a new type fiber,which was particularly important for the preparation of PI fiber reinforced composites with excellent properties.The surface of PI fibers is smooth,which resulted in poor interfacial bonding property when compounded with resin.As a result,It is necessary to study the surface modification of PI fibers.In this paper,four kinds of high performance nitrogen-containing heterocyclic polyimide fibers(PI-1 and PI-2 are copolymerized type fibers,and the main chain of PI-2 contains active hydrogen,compared with that of PI-1;PI-3 and PI-4 are homopolymer type fibers,and the main chain of PI-4 contains hydroxyl groups compared with that of PI-3)were modified.First,the surface modification of PI-1 and PI-2 by five different surface treatment methods(coating method,03 treatment,plasma treatment,atomic oxygen treatment,and coupling agent treatment)were studied.Then,on the basis of coupling agent treatment,the nadic anhydride(NA)secondary treatment method was used to modify these four kinds of PI fibers.The changes of the surface characteristics of the fibers before and after modifications and the interfacial bonding ability with resins(epoxy resin and polyimide resin)were studied.The details are as follows:1.Surface modification of PI fibers by coating treatment method(PI-1 and PI-2)The surface modification of PI-1 and PI-2 by SiO2 sol-gel with different concentrations(0?2.50 wt%)was studied.When the treatment concentration was 2.50 wt%,the surface roughness and oxygen content of PI-1 and PI-2 increased significantly;the surface roughness of PI-1 and PI-2 increased from 1.67 nm and 3.23 nm to 13.50 nm and 14.82 nm;the oxygen content of PI-1 and PI-2 increased from 11.02%and 15.08%to 31.35%and 29.86%.And the original mechanical properties of these two fibers did not decreased.Compared with the those of untreated PI fibers,when the coating concentration was 1.25 wt%,the interfacial shear strength(IFSS)and interlaminar shear strength(ILSS)of PI-1 to epoxy resin were 45.88 MPa and 34.72 MPa,increased by 18%and 19%,the IFSS and ILSS of PI-1 to polyimide resin were 30.26 MPa and 43.21 MPa,increased by 23%and 17%;the IFSS and ILSS of PI-2 to epoxy resin were 42.36 MPa and 33.37 MPa,increased by 7%and 19%,the IFSS and ILSS of PI-1 to polyimide resin were 30.09 MPa and 42.88 MPa,increased by 18%and 15%,respectively.The heat(300?)resistance of the polyimide resin-based composite was not affected by the modification.After SiO2 treatment,the surface roughness of PI-1 and PI-2 was increased,which increased the contact surface between the fiber and the resin in the composite materials,and bites each other,thereby increasing the IFSS and ILSS of the composite material.2.Surface modification of PI fibers by O3 treatment(PI-1 and PI-2)The surface modification of PI-1 and PI-2 by O3 treatment with different time(0?40 min)was studied.For PI-1,after O3 treatment for 20 min,the surface oxygen content increased from 14.40%to 21.35%,the water contact angle decreased to 93.9°.Compared with those of pristine fiber,the IFSS and ILSS to epoxy resin of PI-1 were 44.77 MPa and 32.67 MPa,increased by 15%and 12%,the IFSS and ILSS to polyimide resin of PI-1 were 29.29 MPa and 41.23 MPa,increased by 19%and 12%;For PI-2,after O3 treatment for 20 min,the surface oxygen content increased from 9.98%to 23.06%,the water contact angle decreased to 85.2°,compared with those of pristine fiber,the IFSS and ILSS to epoxy resin were 41.38 MPa and 23.98 MPa,increased by 4%and 17%,the IFSS and ILSS to polyimide resin of PI-2were 28.67 MPa and 40.67 MPa,increased by 12%and 9%.After O3 treatment,the number of oxygen-containing functional groups on the surface of the fiber increased,and the polarity of modified PI-2 increased,which was beneficial for the bonding between the fiber and the resin,and also improved the IFSS and ILSS of the composite material.3.Surface modification of PI fibers by using other methods(PI-1 and PI-2)PI-1 and PI-2 were modified by plasma with different time(0?20 min).After 10 min treatment,the surface oxygen of PI-1 and PI-2 contents increased from 16.62%and 13.60%to 21.90%and 22.53%,the water contact angle of PI-1 and PI-2decreased to 86.8o and 82.6o;the IFSS of PI-1 and PI-2 to epoxy resin was from 38.97 MPa and 39.64 MPa to 45.22 MPa and 42.33 MPa,increased by 16%and 7%,the IFSS of PI-1 and PI-2 to polyimide resin was from 24.65 MPa and 25.54 MPa to 29.33 MPa and 28.91 MPa,increased by 19%and 13%.PI-1 and PI-2 were modified by atomic oxygen with different time(0?40 min).After 30 min treatment,the surface oxygen contents of PI-1 and PI-2 increased from 11.02%and 15.08%to 17.64%and 18.02%,the water contact angle of PI-1 and PI-2 decreased to 82.20 and 72.3°;the IFSS of PI-1 and PI-2 to epoxy resin was from 38.97 MPa and 39.64 MPa to 45.58 MPa and 43.01 MPa,increased by 16%and 8%,the IFSS of PI-1 and PI-2 to polyimide resin was from 24.65 MPa and 25.54 MPa to 29.02 MPa and 27.99 MPa,increased by 17%and 8%.After the plasma and atomic oxygen treatment,the surface of the fiber was etched to increase the surface roughness,and the surface oxygen content was increased,which was beneficial to the resin infiltration of the fiber and the interfacial bonding strength between the fiber and the resin.4.Surface modification of PI fibers by using a silane coupling agent(PI-1 and PI-2)The surface modification of PI-1 and PI-2 by 3-aminopropyltriethoxysilane(KH550)with different concentrations(0?6 wt%)was studied.When the treatment concentration was 6 wt%,the surface roughness of PI-1 and PI-2 increased from 1.97 nm and 3.23 nm to 21.90 nm and 32.20 nm,the water contact angle of PI-1 and PI-2 decreased to 82.3° and 77.2°,and the mechanical properties of PI-1 and PI-2 were not damaged.When the concentration of KH550 was 4 wt%,the IFSS and ILSS of PI-1 to epoxy resin were 46.24 MPa and 35.68 MPa,increased by 19%and 22%,the IFSS and ILSS of PI-1 to polyimide resin were 29.57 MPa and 43.13 MPa,increased by 20%and 17%;the IFSS and ILSS of PI-2 to epoxy resin were 42.98 MPa and 34.88 MPa,increased by 8%and 24%,the IFSS and ILSS of PI-2 to polyimide resin were 28.89 MPa and 42.32 MPa,increased by 13%and 14%,respectively.The heat resistance(300 0C)of the polyimide resin-based composite was not affected by the modification.After KH550 treatment,the surface roughness,the oxygen-containing group,and the polarity of the fiber increased,which was beneficial to the infiltration of the resin onto the fiber.KH550 was connected to the fiber through chemical bonds.When the composite material was subjected to an external force,the fiber and the resin were hard to slide relative to each other,thereby increasing the IFSS and ILSS of composites.5.Surface modification of PI fibers by secondary treatment with nadic anhydride(NA)(PI-1,PI-2,PI-3 and PI-4)The four kinds of polyimide fibers were surface-modified by the combination of KH550(4 wt%)and NA(10 wt%).After the secondary treatment with NA,the bonding properties of these four kinds of fibers to the polyimide resin were further improved.Compared with pristine fiber,the IFSS of PI-1,PI-2,PI-3 and PI-4 to polyimide resin were 30.24 MPa,29.68 MPa,25.77 MPa and 33.06 MPa,increased by 23%,16%,11%and 22%;the ILSS of these four fibers was 44.35 MPa,43.96 MPa,37.28 MPa and 44.12 MPa,increased by 20%,18%,8%and 13%,respectively.After the secondary treatment,the nadic anhydride group was introduced on the surface of the fiber,which participated in the crosslinking reaction during the process of curing of the polyimide resin,thereby further increasing the IFSS and ILSS to polyimide resin. |
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