Ind. Eng. Chem. Res. 2006, 45, 6413-6419
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Ultraviolet (UV)-Curable Amide Imide Oligomers Joseph D. DeSousa and Igor V. Khudyakov* Bomar Specialties Company, Winsted, Connecticut 06098
We have prepared two blends of ultraviolet (UV)-curable oligomers, based on a reaction of cyclic dianhydrides with ω-hydroxy-substituted acrylate and with isophorone diisocyanate (IPDI). Each reaction was solventless and performed in one pot at a temperature no greater than 90 °C. We used benzene-1,2,4,5-tetracarboxylic acid dianhydride (PMDA) and benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) as cyclic anhydrides. Dianhydrides were used in both reactions, in the same molar concentration. Oligomers based on PMDA are designated as P, whereas oligomers based on BTDA are designated as B. Caprolactone-modified acrylate Tone M100 (from Dow) was used as a hydroxyl acrylate. Infrared (IR) and nuclear magnetic resonance (1H and 13C NMR) allowed the structure of each blend of oligomers P and B to be determined. These two oligomers are poly(imide amide) end-capped by Tone M100. There is also an ester link formed between the anhydride and the alcohol (Tone M100). Oligomer B has a remarkable property: it is self-initiating, i.e., it does not require an addition of any photoinitiator to be cured by UV light. This property apparently is due to the presence of a benzophenone fragment in its structure. Moreover, B can serve as an initiator for other freeradical UV-light-induced curing of acrylated and/or methacrylated oligomers being added in a concentration of 5.0 wt % and, in some cases, as low as 1.0 wt % to a formulation. Mechanical properties of cured P and B were studied. Because of the presence of an imide fragment, both P and B demonstrate good high-temperature performance; they manifest an increase of a tensile strength at break after aging at high temperature. Introduction
Chart 1
Aromatic polyimides are well-known thermally stable polymers with good mechanical properties; however, they have a drawback of poor processability.1-3 To improve their processability, several attempts have been made to prepare mixed polymers (oligomers) with the incorporation of flexible segments into polyimides,3 and even make such oligomers ultraviolet (UV)-curable.4 Similarly, the implementation of imide or other rigid cyclic fragments into flexible polyurethanes improves the heat resistance of polyurethanes.5,6 The goal of this work was to prepare UV-curable acrylated oligomeric amide imide or poly(amide imide). Polymers with amide and urethane (carbamate) links that have been formed as a result of UV curing or in dark reactions usually demonstrate good elasticity.2,7 We have used the reaction between a cyclic anhydride of aromatic diacids and isocyanate group, which leads to the formation of an imide as a product of the synthesis at relatively low temperature.5,6 We studied the products of such reactions that lead to a blend of oligomers, as well as the properties of liquid and such cured blends of oligomers, which have turned out to be valuable coatings. Experimental Section Reagents. We used the following reagents: benzene-1,2,4,5tetracarboxylic acid dianhydride (pyromelitic dianhydride, PMDA), from Aldrich; benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA), from Aldrich; and isophorone diisocyante (IPDI), from Rhodia. Chart 1 presents the chemical structures of the dianhydrides. We used ω-hydroxy-substituted acrylate, which has the common name caprolactone acrylate Tone M100 (of Dow). (For the sake of brevity, it will be referenced hereafter simply as Tone.) Tone is an oligomer with 1-3 opened -caprolactone fragments. In our estimation, based on gel permeation chromatography (GPC) traces, the Tone that was used had an average number of 1.8 opened fragments with OH terminal groups. We used Tone with OH# 166.8, which means
the average molecular weight of this compound with reactive OH groups was 336.0 g/mol. We added an inhibitor of spontaneous polymerization of acrylates (Irganox 1010, obtained from Ciba Specialty Chemicals). The acrylated oligomer, BR990, was obtained from Bomar Specialties. The catalysts of isocyanate reactions were dibutyltin dilaurate (Fascat 4202, obtained from Atofina and abbreviated as DBTDL) and K-Kat 348 of King Industries. We used reactive diluents from Sartomer: isobornyl acrylate (IBOA), tripropylene glycol diacrylate (TRPGDA), 1,6-hexanediol diacrylate (HDDA), which have Sartomer abbreviations of SR-506, SR-306, and SR-238, respectively. We used photoinitiator (PI) Irgacure 184, obtained from Ciba Specialty Chemicals. We used co-initiator CN-383, from Sartomer. Tetrahydrofuran (THF) used was obtained from EMD. All reagents were used as received. Cure of Samples. Irgacure 184 was dissolved in warm samples in a typical concentration of 2.0 wt %. Samples were cured with Fusion 300 W/in processor with D-bulb in the air. Two passes under the lamp at a conveyor speed of 20 ft/min were set performed for all cure experiments. The completeness of the cure was confirmed by a partial or, in some cases, complete disappearance of the acrylate group absorption band (∼810 cm-1) in the IR spectra of the cured films.8 Some samples were cured without PI (self-initiation) (cf. later discussion). A co-initiator, CN-383, in an amount of 1-5 wt %, was added to the self-initiated oligomer. Using a cured sample, we reveal the formation of a film, which can be delaminated from a support (glass, Mylar) and possesses certain tensile strength and elasticity.
10.1021/ie060591n CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 19, 2006
Figure 1. Gel permeation chromatography (GPC) trace of P.
Figure 2. GPC trace of B.
High-Temperature Experiments. We prepared samples based on two synthesized oligomers and cured them. The cured samples have the following dimensions: length, ∼5 in.; width, ∼0.5 in.; and thickness, 0.25 mm. Samples were freely hanging in the oven, clipped at the top for 24 h in an air atmosphere at 180 °C. Properties of the samples before and after aging at high temperature were evaluated. Devices. Properties of the products were analyzed via GPC, which gave molecular weights as weight average and number average (Mw and Mn) values, and the molecular weight distribution (MWD) was given as
MWD )
Mw Mn
We used a Polymer Laboratories PL-GPC 50 chromatograph with a RI detector, along with two Polymers Laboratories mixed-D columns, and set the column temperature to 40 °C.
We used stabilized THF as an eluent, running at a rate of 1.0 mL/min. We used Polymer Laboratories Cirrus Software, version 3.0, to analyze the data, and we used a 10-point calibration curve that was based on polystyrene (Easical PS-2 standards from Polymer Laboratories). The IR spectrometer was Perkin-Elmer Spectrum One model with a diamond-crystal universal attenuated total reflectance (UATR) accessory. The viscosity (η) was measured with a Brookfield RVT viscometer with a small adapter (spindle SC415 and cup 7R) that was connected to a Neslab circulating water bath at 50 °C. The tensile properties of cured samples (elongation to break, tensile strength at break, tensile modulus) were measured with a Cheminstruments Tensile Tester-1000 system. The test method was designed to be in compliance with ASTM Standard D 882. The tester was controlled by the Cheminstruments EZ-Lab system program. At least five samples of each cured formulation were studied at ambient temperature, to verify reproducibility of the data.
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Figure 3. Infrared (IR) spectra of (a) P and (b) B.
The hardness of the cured films was measured with a Type D durometer (Shore D, model 307L, PTC Instruments). Nuclear magnetic resonance (NMR) spectra were taken with a Varian VXRS 300 MHz NMR spectrometer that was operated at 299.94 and 75.43 MHz for 1H and 13C, respectively, with typical acquisition parameters. NMR spectra were obtained at ambient temperature in CDCl3. 13C NMR spectra were obtained in the presence of paramagnetic chromium(III) acetylacetonate (Cr(acac)3), to ensure quantization of the carbon signals. The relative concentration of imide and amide fragments was obtained by comparing integrals of corresponding signals in 13C NMR spectra (cf. discussion below). We used ACD/Labs NMR software to predict the NMR spectra and Sadtler database. Dynamic mechanical properties were measured with a TA Instruments DMA 2980 controlled stress rheometer. After measurement of the linear viscoelastic region for each material, strain was selected at 1%. The temperature interval was -50 °C to 100 °C, with a ramp of 2 °C/min. The glass transition temperature (Tg) values of the cured oligomers were of interest. In accordance with a common practice, we identified Tg using the relation tan δ ) G′′/G′, where G′′ is a shear loss modulus and G′ is a shear storage modulus. Thermogravimetric analysis (TGA) was performed with TA Instruments TGA Q-500 in the air. The temperature ramp was 10 °C/min, from room temperature to 700 °C. Synthesis. We prepared oligomers with PMDA (reaction products are abbreviated as P) and oligomers with BTDA (abbreviated as B) in a very similar way. We used 500-mL reactors with a heating mantle, stirrer, and dry nitrogen sparge. The reactions were accompanied by a release of CO2, and the solutions foamed. Foaming was minimized via the slow and careful addition of Tone. We have presented a sequence of reagent additions below, as well as the typical amount of the reagents used in a synthesis. We present data for P and, in parentheses, the proper amount for B. A reactor is loaded with 123.4 g (the same amount for B) of IPDI (1.11 equiv), and dry nitrogen sparge is started. PMDA (75.2 g, or 0.66 equiv) (110.0 g, or 0.66 equiv, of BTDA) is promptly added to the reactor with IPDI. Dianhydrides are
Table 1. Properties of Liquid Oligomers P and Ba oligomer
Mw (g/mol)
MWD
η at 50 °C (cP)
imide:amide ratiob
P B
1680 2560
1.4 1.6
4000 15500
45:55 34:66
a Determination error of values is 10%. b Molar ratio obtained from 13C NMR spectra (cf. text).
hydroscopic, and the proper precautions should be taken. The temperature is set at 85 °C. Catalysts DBTDL (0.2 g) and K-Kat 348 (0.2 g) were added when the temperature of the reaction mixture reached 77 °C. The reaction mixture was maintained at 85 °C for 5 h. The evolution of gas (CO2) was noticed. The temperature then was reduced to 80 °C and the system was placed under dry air sparge. The total amount of Tone required for P was 390.9 g (or 1.16 equiv) (394.0 g or 1.17 equiv for B). A portion (25%) of the total amount of Tone was added initially; 0.6 g of Irganox 1010 was added after the first addition of Tone. The mixture then slowly began to foam. We allowed the foam to dissipate before the next charge. The temperature of the reaction mixture was kept at
