Study Of Acetylene-terminated Polyimide Resin And Carbon Fibre Reinforced Composites

Aromatic polyimides are well known as high performance material with outstanding mechanical properties and high temperature capabilities.They are utilized for a wide range of applications: as matrices for high performance advanced composite materials, as thin films in electronic appications, as structure adhesives and sealants, and as high temperature insulators for aircraft wire coatings. However, the high performance/high temperature polymers typically exhibit high melt viscosities and insoluble in common solvent, as a consequence, the processability by compression and injection moulding for use as composite matrices and adhesive is often difficult or unarchivable. There has been a strong demand for polyimides having good processability, high thermal stability, and high solvent resistance. One promising approach is to use crosslink sites that react by thermal curing during processing or after processing. Acetylene was chosen as the reactive end group because among the compounds commonly used, it offers a major advantage in terms of stability, curing temperature and thermal resistance of the final crosslinked material.Herein, we synthesized two series of acetylene-terminated polyimide prepolymer. One serie was derived from 2, 2-bis [4-(3, 4-dicarboxyphenoxy) phenyl] propane dianhydride (BPADA) and 3-ethynylaniline (APA). Another series was from the reaction of 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), bis-(3-phthalyl anhydride) ether (ODPA) or 2, 2-bis [4-(3, 4-dicarboxyphenoxy) phenyl] propane dianhydride(BPADA) with 1, 3-Bis (3-aminophenoxy-4'-benzoyl) benzene (BABB) and 3-ethynylaniline (APA). It is expected that introduction of flexible structure units such as isopropylidene, ether and carbonyl can greatly enhanced the flexibility of polyimide chain and reduce the prepolymer melt temperature and melt viscosity to broaden the process window.The processability of prepolymer was characterized by differential scanning calorimetry (DSC) and rheometer. The results showed that the prepolymer had a relatively wide processing window and low melt viscosity. At the same time, The theoretic model based on the viscosity-temperature-time relationship was set up, which could forecast process window for high-performance composite materials and optimize the processing parameters. To better control processing parameters, it is critical to understand the thermal crosslinking behavior of polyimide. The reaction can be effectively monitored by FTIR spectroscopy using the medium absorption intensity of the acetylene functional groups (3286cm-1) in the mid-IR spectral region. The percentage conversion of acetylene cross-linking is less than 80% but the curing time is more than 3h when the curing temperature is between 180-220 oC. A remarkable increase in the conversion percentage and reduction of the cross-linking reaction time is observed in the range of 240-270oC. Above 280oC, the cross-linking reaction in the air flow is completed in a few minutes but in the nitrogen flow the conversion of the acetylene groups is nearly 95%, which is nearly 100% only at 320oC. Based on the FT-IR characterization the cured imide oligomer films were prepared by molding the oligomer powder at 250oC and 280 oC for 1 h and 10min under pressure. The polyimide film based on BPADA and APA displayed a high glass transition temperature (Tg) 363oC which was obtained from DMA and high modulus about 3.5GPa. The thermal aging tests were carried in an oven at 177oC for 1000 hours. The thermal stability of films obtained from cured oligomers with different times aging as evaluated by TGA in N2 and air atmospheres. The temperatures of 5% weight loss were all above 477oC in N2, the char yields reported at 700 oC in N2 are above 54%. However, Td5 of the resin after aging 1000h under nitrogen and air reduced only 2.6% and 4.9% respectively. The cured films exhibited excellent thermal stability and thermo oxidative stability.The films based on dianhydride (s-BPDA, ODPA, BPADA) and diamine (BABB) all showed good mechanical properties. The films displayed high modulus between 2.1 and 3.0GPa and elongation at break between 1.3% and 8.4%. The films were evaluated by TGA which showed weight loss before 300oC. The reason of weight loss was that imidization of the cured films were not completely.Carbon fibre reinforced composites (CFRC) were prepared by polyamic acid and carbon fibre. Carbon fiber were two type, one was with sizing agent on the surface of carbon fiber (I) the other was remove sizing agent on the surface of the carbon fiber (II). The composites were used for primary evaluation and long term thermal stability test. The mechanical properties of composite materials were performed at room temperature. The strength of composite materials I and II reached 335MPa and 309MPa. The elongation at break was 9.1% and 9.7%. The mechanical properties of Composite materials also tested at 177 oC and 250 oC. Compared to the room temperature, the mechanical properties of composites reduced, but that reduced slightly. The conclusion is that the composite materials at room temperature and high temperature had better mechanical properties. The thermal aging tests of composites were carried out at 177oC for 1000h. At room temperature tensile test results showed that the composites based on carbon fiber (I) and remove the surface sizing agent (II) both have excellent mechanical properties, which indicated that surface sizing agents on the surface of carbon fiber did not effect the preparation of the composite. After aging for 100h, the elongation at break of composites reduced 12.1% and 14.4%, respectively, and the remaining time, they were essentially remained. After aging 1000h, the strength of composites tested at 177oC can keep 85% of that tested at room and the modulus and elongation at break were essentially unchanged. The composites exhibited excellent thermal stability and thermo oxidative stability.The failure surface of composites were tested by SEM. Fracture of the fiber was tidy, which indicated that fiber wetness of the resin was better. The surface of carbon fiber drawn out was still covered by resin, which reflecting that the interface between matrix resin and fiber was stable. The cross-section scans of composite I and composite II were compared, they had similar appearance. The epoxy sizing agent did not affect the preparation of the composite. After 1000h aging, the sections of composite materials were unchanged which illustrated that the long-term aging almost did not affected the interface between resin and carbon fiber.