Synthesis Of Different Structure Hyperbranched Polyurethanes And Study Of Modified Epoxy Resins

As one of the most important thermosetting materials, epoxy resins have been widely used in various technological applications, such as surface coatings, electrical laminates, adhesives and molding compounds, due to the excellent thermomechanical properties and superior processability. However, the usage of epoxy resin is restricted due to poor impact resistance which is related to the high cross-linking density and chain stiffhess of the cured network. As one of the new efficient modifier of epoxy resins, hyperbranched polymer (HBP) is becoming a research hotspot. Due to their special chemical and physical properties, such as globular structure, large amount of terminal groups and highly branched structure, HBPs have higher reactivity, lower viscosity and higher solubility than their linear analog polymers with similar molecular weight. In this paper, hyperbranched polyurethane (HBPU), with flexible segments between the branching points, was prepared firstly. And the structure of HBPU was modified by grafting hydroxyl-containing rigid cyclodextrin (CD) to get HBPU-CDs. Then, these two polymers were used as modifiers for toughening epoxy resin to study the influence of HBPU's structure in modifying epoxy resin. And the toughening mechanism is also studied. The results are as follows:(1) Synthesis of two different structures of hyperbranched polyurethane. Hyperbranched polyurethane with poly(tetrahydrofuran) (PTHF) between the branching points (HBPU-PTHF), was synthesized from isophorone diisocyanated (IPDI), PTHF and diethanol amine (DEA) via Aa+CB2 method. The HBPU-CDs was prepared through the carboxylation of hydroxyl groups in the HBPU-PTHF with succinic anhydride and the subsequent esterification reaction with cyclodextrins. The structure and properties of the polymers were characterized by FTIR, GPC, NMR, DSC and TGA. The results shows that the DB of the HBPU-PTHF are about 0.67. The grafting ratio of HBPU-CDs was 30.89%, and the glass transition temperature (Tg) increased from 0 to 45?, while the thermal stability decreased slightly. Moreover, both HBPU-PTHF and HBPU-CDs has good solubility.(2) The study of HBPU-PTHF toughened epoxy resin was carried out and compared with the epoxy/linear polyurethane (LPU-PTHF) system. The introducing of PTHF into the HBPU could improve the compatibility with epoxy resin. The HBPU-PTHF didn't affect the favorable processability of the epoxy blends. It was proved that a good phase separation structure with strong interface interaction was formed in the DGEBA/HBPU thermosets. After adding 10 wt% HBPU, the impact strength of samples was increased 2-fold compared with that of neat epoxy sample, from 9.87 kJ/m2 for the neat resin to 29.87 kJ/m2. And the impact strength of DGEBA/LPU-PTHF (10 wt%) thermoset was 23.93 kJ/m2, increased about 142%. However, the flexural strength and glass storage modulus of the DGEBA/LPU-PTHF (10 wt%) thermosets were decreased 15.30% and 22.46%, respectively. But unconspicuous decreases were observed in the DGEBA/HBPU thermosets. Especially, after the addition of HBPU-PTHF, the Tg of the epoxy resin was increased by 5.0-8.6, and the thermal decomposition temperature increased by 19.9-24.3?.(3) The study of HBPU-CDs toughened epoxy resin was carried out and compared with the epoxy/HBPU-PTHF system. Due to the introduction of cyclodextrin, HBPU-CDs plays a dual role of toughening and strengthening in the epoxy resin. Curing at room temperature, a transparent and excellent epoxy thermosets was obtained. The HBPU-CDs was more effective than HBPU in toughening epoxy resin. After adding 10 wt% HBPU, the impact strength and the stress intensity factor (Kic) of samples were 92.40 kJ/m2 and 166.1 MPa?m, increased by 50.10% and 60.95%, respectively, compared to the neat epoxy, but DGEBA/HBPU-PTHF in situ modified thermosets were only improved by 20.99% and 29.65%, respectively. Meanwhile, the tensile strength, flexural strength and glass storage modulus of the thermosets were decreased with the addition of HBPU-PTHF. But after adding 5 wt% HBPU-CDs, the tensile strength of the thermosets reached 63.38 MPa, increased by 10% compared with that of neat epoxy. When adding 10 wt% HBPU-CDs, flexural strength was increased by 23.33%, from 105.9 MPa to the 130.61 MPa. In addition, the Tg of DGEBA/HBPU-CDs thermosets also showed a slight increase, and the thermal decomposition temperature is also increased by 3.33-16.66?.In general, two kinds of HBPU with different structures were designed and synthesized, and the effects of these two kinds of polymers on the properties of the epoxy materials were studied. A good phase separation structure with strong interface interaction was formed in the DGEBA/HBPU thermosets, and the toughness of this material is improved obviously without affecting other properties to a large extent. For the HBPU-CDs in-situ modified epoxy resin, a transparent and excellent epoxy thermosets was obtained. These results indicate that the structure of HBPU has a great influence on the modification of epoxy resin. So the structure of modifiers can be adjusted according to the actual needs, which could meet the various requirements of the epoxy materials. Meanwhile, it could promote the systematic study of HBP modifying epoxy resin.