| Polyethylene terephthalate (PET) is a kind of material with low cost and high strength, meanwhile polybutylene terephthalate (PBT) is a kind of material with high crystallization rate and short molding time. Structural difference between PET and PBT is very small, and therefore they have good compatibility. High-performance and low-cost PET/PBT alloy prepared by the means of melt blending has been widely used in automotive, electronics and other fields where both the mechanical and flame-retardant properties are highly required. But the relationship between the structure and properties of the alloy is not thoroughly investigated. In this work, we try to establish the relationship between crystallization/phase morphology and mechanical properties, improve the mechanical properties of the alloy by adding new ester-exchange inhibitors NaH2PO4 and LDH, and further improve the flame retardant properties of the alloy by surface modification of LDH. The main contents are as follows:1. The crystallization and melting behaviors of PET/PBT alloys with different proportions are investigated. The results of DSC show that PET and PBT can interfere with the crystallization of PET/PBT alloys, wherein PET has a little effect on crystallinity of PET-based polyester alloy but has a greater impact on crystallization temperature. Besides, phase separation of PET/PBT occurs when the ratio is about 50/50 w/w, but phase separation is not significant because of the transesterification reaction. The relationships between mechanical properties and crystallinity or compatibility of PET/PBT alloy are analyzed, and ultimately the best ratio of PET/PBT alloy is determined based on the tensile strength and bending strength. When the ratio of PET/PBT is 40/60 w/w, the tensile strength reaches 54.28 MPa, and the flexural strength reaches 84.90 MPa, which were higher than that of PBT.2. A new ester exchange inhibitor NaH2PO4 is added to PET/PBT (40/60 w/w) alloy, and a comparision with conventional ester exchange inhibitor TPP and TPPi has been made. DSC shows that NaH2PO4 effectively enhances the crystallization temperature of the system, melting peaks of PBT and PET are obviously separated, and melting enthalpy (?Hm) reaches 41.08 J/g, which effectively inhibits the transesterification reaction. The optimal amount of NaH2PO4 is determined.1% NaH2PO4 improves 6 MPa of the bending strength of the system. And the system with NaH2PO4 and epoxy compatilizer can improve the mechanical property effectively. Combining 3% ADX (about 8% content of GMA) with 1% NaH2PO4 significantly increases the tensile strength and bending strength of the alloy by 2.3 MPa and 2.4 MPa respectively.3. In order to reduce the negative influence of NaH2PO4 on the tensile strength of PET/PBT (40/60 w/w) alloy, the lamellar LDH is compounded with NaH2PO4. We find that when NaH2PO4/LDH mixing ratio is 0.5/0.5 w/w, 1%, the alloy can maintain its tensile strength and increases bending strength for about 7 MPa, which shows a synergistic effect on PET/PBT (40/60 w/w) alloy. The melting temperature of the compound system is low, which can achieve co-extrusion with resin with low melting temperature. The synergistic mechanism is analyzed by XRD, SEM and DSC. The addition of NaH2PO4/LDH mixture (0.5/0.5 w/w,1%) induce microscopic crosslinking among the molecular chains of PET/PBT (40/60 w/w), which ultimately enhance the mechanical properties of the system.4. Though LDH severely reduces the mechanical properties of PET/PBT (40/60 w/w) alloy, it can enhance the oxygen index by 1.5 units, and improve the UL-94 test grade to V-2 level. Therefore, LDH is modified with KH-560 in order to promote the mechanical and flame retardant properties of PET/PBT alloy. FTIR, XRD results show that KH-560 can react with hydroxyl groups on the surface of LDH to generate M-LDH. The tensile strength increases 3 MPa, the bending strength increases 5 MPa, the oxygen index increases 3.8 units, and UL-94 reaches V-2 level by the presence of 3% M-LDH. |
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