Effects of polyol molecular weight on properties of benzoxazine-urethane polymer alloys.
Polybenzoxazine is a novel class of thermosetting thermosetting,
adj having the property of becoming irreversibly rigid or hardened with the application of heat. In dentistry the term is used in connection with resins. polymer which has various outstanding properties such as high thermal stability, easy processability because of its low melt viscosity, low water absorption, near zero shrinkage after processing with excellent mechanical properties, and its ability to alloy with various types of resins. The monomers of this resin can be synthesized by a solventless method and produce no by-product during polymerization polymerization
Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. . Other advantage of this new resin is an ability to alloy with several kinds of resins. Alloying of benzoxazine resins with other resins was reported by many researchers. For example, the addition of epoxy to the polybenzoxazine network greatly increases the cross-link density of the thermosetting matrix and strongly influences on its mechanical properties (1). A ternary (programming) ternary - A description of an operator taking three arguments. The only common example is C's ?: operator which is used in the form "CONDITION ? EXP1 : EXP2" and returns EXP1 if CONDITION is true else EXP2. system of benzoxazine, epoxy, and phenolic phe·no·lic
Of, relating to, containing, or derived from phenol.
Any of various synthetic thermosetting resins, obtained by the reaction of phenols with simple aldehydes and used as adhesives. resins was also developed as a new class of electronic packaging materials. In this system, a glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). as high as 170[degrees]C and considerable thermal stability at 5% weight loss up to 370[degrees]C can be obtained (2). The benzoxazine--urethane alloy can be tailored to be not too rigid like the neat polybenzoxazine.
Recently Rimdusit et al. reported the comparison of benzoxazine alloying with isophorone diisocyanate (IPDI IPDI Isophorone Diisocyanate
IPDI Institute for Politics, Democracy and the Internet (George Washington University) )-based urethane urethane (yoor´ithān´),
n ethyl carbamate used as an anesthetic agent for laboratory animals, formerly used as a hypnotic in humans. prepolymers and with flexible epoxy (3). Alloying with urethane prepolymer can improve the flexibility of the rigid polybenzoxazine. Interestingly, the positive deviation on the glass transition temperature ([T.sub.g]) of the benzoxazine-urethane alloys was clearly observed, i.e. [T.sub.g] of the benzoxazine-urethane alloys were significantly greater ([T.sub.g] beyond 200[degrees]C) than those of the parent polymers ([T.sub.g] of polybenzoxazine = 160[degrees]C; [T.sub.g] of urethane = -70[degrees]C). Furthermore, the glass transition temperature of benzoxazine-urethane alloy was found to increase with an increase of the urethane content (3). The poly (benzoxazine-urethane) alloy also showed excellent resistance to solvents such as THF THF tetrahydrofolic acid.
tetrahydrofolic acid. , and the thermal stability of polyurethane was greatly enhanced even with the incorporation of a small amount of the polybenzoxazine (4). Furthermore, there are some investigations about the effect of molecular weight of soft segment (polyol) in polyurethane. Those studies suggested that [T.sub.g] decreased with increasing the soft-segment molecular weight. The lower molecular weight of polyols presented more miscibility miscibility (miˈ·s·biˑ·l between the hard and soft segment (5). The study about the effect of soft segment length on the properties of polyurethanes suggested that the phase separation was enhanced with increasing soft segment length (6). From the literature (7), the mechanical properties of polyurethane based on poly ([epsilon]-caprolactone) and 1,4-butane diisocyanate by varying molecular weights of polyol with uniform hard segment revealed that the lower molecular weight of the polyol yields higher rubber plateau but lower modulus. The chain distance between junction points would affect the movement of the network, the number of crosslinked density, long heterogeneous network behavior, and ability to absorb the impact energy. In this work, the molecular weights of polyols used to synthesize urethane prepolymer were varied to investigate their effect on thermal and mechanical properties of the benzoxazine-urethane alloys.
The materials used in this research are benzoxazine resin and urethane resin. Benzoxazine resin is based on bisphenol A, aniline aniline (ăn`əlĭn), C6H5NH2, colorless, oily, basic liquid organic compound; chemically, a primary aromatic amine whose molecule is formed by replacing one hydrogen atom of a benzene molecule with an amino , and paraformaldehyde paraformaldehyde: see formaldehyde. . The bisphenol A (polycarbonate grade) was supplied by Thai Poly-carbonate Co., Ltd. (TPCC). Para-formaldehyde and aniline were purchased from Merck Ltd. (Thailand) and Panreac Quimica S.A., respectively. Urethane prepolymer was prepared using toluene diisocyanate (TDI TDI - Transport Driver Interface ) and polyether pol·y·e·ther
A polymer in which the repeating unit contains two carbon atoms linked by an oxygen atom. polyol. The TDI was obtained from South City Group (Thailand) whereas the poly(propylene glycol) with the number average molecular weights ([M.sub.n]) of 1000, 2000, 3000, and 5000 g/mol was supported by TPI (Tracks Per Inch) The measurement of the density of the storage channels on a disk or tape. Track density on magnetic disks has reached 125,000 tpi (125 Ktpi). See bpi, areal density and magnetic disk. Polyol Co., Ltd.
Benzoxazine Preparation. Bisphenol A, aniline, and paraformaldehyde at the molar ration of 1:2:4 were used for the synthesis of benzoxazine monomer following the method reported by Ishida (8). These three reactants were continuously mixed at about 110[degrees]C for ~ 60 min. The benzoxazine monomer was obtained as clear-yellowish solid at room temperature. The product was then ground into fine powder and kept in a refrigerator for future-use.
Urethane Resin Preparation. The urethane prepolymer terminated with isocyanate i·so·cy·a·nate
Any of a family of nitrogenous chemicals that are used in industry and can cause respiratory disorders, especially asthma, if inhaled. group was prepared from TDI and poly(propylene glycol) at a molar ratio of 2:1 using various [M.sub.n] of the poly(propylene glycol), i.e., 1000, 2000, 3000, and 5000 g/mol. The two reactants were directly mixed without catalyst in a four-necked round bottomed flask and the mixture was continuously stirred under nitrogen atmosphere at about 80[degrees]C for 2 h to yield a light yellow prepolymer (4). Then, the prepolymer was cooled to room temperature and kept in a refrigerator for future-use. The urethane prepolymers were labeled as PU1K, PU2K, PU3K, and PU5K according to the [M.sub.n] of the poly (propylene glycol) of 1000, 2000, 3000, and 5000 g/mol, respectively.
Benzoxazine: Urethane Binary Mixture Preparation. The benzoxazine monomer was mixed with PU1K, PU2K, PU3K, and PU5K at a desirable mass fraction. The mixtures were heated at 150[degrees]C in an aluminum pan and were mixed mechanically for about 15 min to obtain a homogeneous mixture. To ensure complete curing, the mixture was then poured onto an aluminum mold and cured in an air-circulating oven using a step heating profile as follows: 160, 170, 180, and 200[degrees]C for 2 h each. The densities of the polymeric alloys were determined by a water displacement method, ASTM ASTM
American Society for Testing and Materials D792-00 (Method A). The dimension of each specimen was 25 X 50 X 2 [mm.sup.3].
Gel Permeation Chromatography Gel permeation chromatography (GPC) is a separation technique based on hydrodynamic volume (size in solution). Molecules are separated from one another based on differences in molecular size. This technique is often used for polymer molecular weight determination. . The molecular weights of the polyol for the urethane prepolymer preparation were measured by gel permeation chromatography (GPC (1) A PC that uses the Linux-based gOS operating system. See gOS.
(2) (GPC Group) Originally the Graphics Performance Characterization committee of the NCGA, the GPC Group is now part of Standard Performance Evaluation Corporation (SPEC) and oversees the following ). The analysis was performed at 40[degrees]C on a Waters 600 using three Waters Styragel[R] HT columns (Styragel[R] HT 0.5, Styragel[R] HT 1, and Styragel[R] HT 4). The detector is Waters 2414 refractive index measurement (RID). Molecular weights are relative to monodisperse A collection of objects are called monodisperse if they have the same size - i.e. their size distribution is effectively a delta function. A sample of objects with a broader size distribution is called polydisperse. In practice, exactly monodisperse collections rarely exist. polystyrene standards. GPC measurements were performed with tetrahydrofuran tetrahydrofuran: see furfural. (THF). Sample solutions were prepared at a concentration of 0.25% (w/v) by dissolving the polyol (polypropylene glycol) in THF mobile phase at room temperature.
Fourier Transform Infrared Spectroscopy. Fourier transform infrared spectroscopy (FT-IR) spectra of all samples under various curing methods were acquired by using a Spectrum GX FT-IR spectrometer from Perkin Elmer. The apparatus was equipped with a KBr beam splitter and a debuterated triglyclin sulfate sulfate, chemical compound containing the sulfate (SO4) radical. Sulfates are salts or esters of sulfuric acid, H2SO4, formed by replacing one or both of the hydrogens with a metal (e.g., sodium) or a radical (e.g., ammonium or ethyl). (DTGS DTGS Deuterated Triglycine Sulfate (IR detector material) ) detector. All spectra were taken with 32 scans at a resolution of 4 [cm.sup.-1] and a spectral range of 4000-400 [cm.sup.-1]. For urethane prepolymer samples, about 1.0 mg of a viscous liquid sample was swept on a potassium bromide (KBr) window.
Differential Scanning Calorimetry Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. . Differential scanning calorimeter calorimeter: see calorimetry.
Device for measuring heat produced during a mechanical, electrical, or chemical reaction and for calculating the heat capacity of materials. (DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. ) model 2910 from TA Instruments was used to study the curing behaviors of the binary mixtures. The mass of the sample was in the range of 3-5 mg. The experiment was performed at a heating rate of 10[degrees]C/min under nitrogen purging. The glass transition ([T.sub.g]) of the alloys can be obtained using a DSC scan in the range of 30-300[degrees]C at a heating rate of 10[degrees]C/min under nitrogenpurging.
Thermogravimetric Analysis. Thermal stability and thermal decomposition of the cured polymer alloys were studied using a Simultaneous DSC-TGA Q600 SDT SDT Soldat
SDT Sigma Delta Tau (sorority)
SDT Signal Detection Theory (cognitive science)
SDT Service Description Table (Digital Video Broadcast data) from TA Instruments. The experiment was done using a heating rate of 20[degrees]C/min under nitrogen atmosphere. The temperature was ramped from 30[degrees]C to 900[degrees]C using a sample mass of about 10 mg. The degradation temperature at 5% weight loss and the char yield at 800[degrees]C were record for each specimen.
pertaining to the flexure of a joint.
fixation of joints in flexion. In the newborn called contracted calves or foals. Property Measurement. The flexural properties of the BA: U alloy specimens were determined using a universal testing machine A Universal Testing Machine is used to test the tensile and compressive properties of materials. Such machines generally have two columns but single column types are also available. (model 5567) from Instron Co., Ltd. Following ASTM D 790M-93, the test was carried out in three point bending mode with a support span of 32 mm using a constant crosshead cross·head
A beam that connects the piston rod to the connecting rod of a reciprocating engine.
Noun 1. crosshead - a heading of a subsection printed within the body of the text
crossheading speed of 0.85 mm/min. The sample dimension was 25 X 25 2 [mm.sup.3]. Five samples were used to determine the average property values.
The polymer alloys at a mass of ~ 1 g were immersed in 20 ml of chloroform chloroform (klôr`əfôrm) or trichloromethane (trī'klôrōmĕth`ān), CHCl3 to determine the nature of cross-linking network formation. The mass of the residual solid was weighed after 30-day immersion.
RESULTS AND DISCUSSION
Determination of Polyol Molecular Weight
Table 1 shows the number average molecular weights ([M.sub.w]) of utilized polyols measured by GPC in comparison with the values from the monomer/initiator (M/I) determination given by the supplier. The results show that these values are in good agreement with one another. The polydispersity index (PDI PDI Protein Disulfide Isomerase
PDI Personal Docente e Investigador (Spanish: Personal Educational and Investigating)
PDI Pre Delivery Inspection
PDI Professional Development Institute ) of the polyols at each molecular weight was found to be relatively narrow with the value close to unity. This behavior suggested that most of chain lengths of all types of polyol molecules used were nearly equal.
TABLE 1. Number average molecular weights and weight average molecular weights of the polyols determined via GPC. GPC Type of polyol [M.sub.n] (M/I [M.sub.n] [M.sub.w] PDI ratio) (a) (g/mol) (g/mol) (g/mol) Polyol 1010 1000 1028 1098 1.07 Polyol 1020 2000 2022 2137 1.06 Polyol 3003 3000 2887 3061 1.06 Polyol 3009 5000 4396 5564 1.27 (a) (M/I ratio: monomer/initiator ratio).
Fourier Transform Infrared Spectroscopic spec·tro·scope
An instrument for producing and observing spectra.
The chemical structures of neat resins and their formation reaction were studied by FT-IR spectroscopic technique. The urethane prepolymer used in this work was synthesized by a reaction between TDI and polyether polyol with various number average molecular weights i.e. 1000, 2000, 3000, and 5000 g/mol. the important functional groups of the PU prepolymer are N = C = O, C = O, [CH.sub.2] and [CH.sub.3] which were used to characterize the presence of the prepolymer in the polymerization reaction. Figure 1 shows the spectra of the urethane prepolymer at various molecular weights. In our systems, the peaks were observed at 1729 [cm.sup.-1] (C = O stretching of urethane) and 2274 [cm.sup.-1] (N = C = O stretching of unreacted-isocyanate group). It can be observed that the peak height at 1729 [cm.sup.-1] trended to decrease with increasing the molecular weight of polyol based on the same mass of the resin. Moreover, the spectrum of the reactant mixture before synthesis showed one strong peak at 2274 [cm.sup.-1] of an unreacted-isocyanate group. From the figure, all spectra of the prepolymer indicated that the C = O absorption peak of urethane increased whereas the N = C= O peak significantly decreased with the progress of the reaction to form the urethane prepolymer. The reaction condition followed that reported in the literatures (3), (4), (9), (10).
[FIGURE 1 OMITTED]
Thermal Properties of BA: PU Alloys
DSC for Curing Condition Observation. The investigation of the curing condition of the BA: PU alloys at various compositions, i.e., 100:0, 90:10, 80:20, 70:30 mass ratios was performed in DSC experiment. Figure 2 shows the DSC thermogram thermogram /ther·mo·gram/ (ther´mo-gram)
1. a graphic record of temperature variations.
2. the visual record obtained by thermography.
n. of curing exothermic exothermic /exo·ther·mic/ (-ther´mik) marked or accompanied by evolution of heat; liberating heat or energy.
ex·o·ther·mic or ex·o·ther·mal
1. peaks of the neat benzoxazine resin and the binary mixtures between benzoxazine and urethane prepolymer. From this graph, the exothermic peak of the neat benzoxazine resin was located at 232[degrees] C. When the urethane prepolymer was presented in the alloys, exothermic peak was shifted to higher temperature. The temperatures at the exothermic peaks of BA: PU at 90:10, 80:20, and 60:40 were 232, 236, 240, and 242[degrees] C, respectively. The curing retardation was attributed to the dilution effect of the urethane prepolymer (3). Takeichi et al. reported that initially phenolic hydroxyl group from the ring opening of the benzoxazine monomer was produced after that the reaction between phenolic hydroxyl group on the polybenzoxazine and the isocyanate group was expected to proceed (4). The thermogram also showed the decrease of the area under the curing peak of the binary mixtures when the amount of the urethane resin increased. The systematic decrease of the exotherms with the PU implied that the BA: PU interaction possessed a lower heat of reaction per mole of the reactant than that of the BA: BA interaction. Excessive amount of the PU in the binary mixtures might also lead to the presence of the unreacted PU in the fully cured alloys.
[FIGURE 2 OMITTED]
Figure 3 exhibits the DSC thermograms of the mixtures of the benzoxazine resins and urethane prepolymer ([M.sub.n] = 2000) at the mass ratio of 70:30 at various curing conditions. For determining the fully cured condition of every composition used in this experiment, the specific mixture i.e. BA: PU at 70:30 was selected based on the ratio that required the most thermal energy for curing. The heat of reaction determined from the area under the exothermic peak is 273.6 J/g for the uncured 70:30 BA: PU mixture. It was reduced to 125.2 J/g after curing at 160[degrees] C for 2 h and decreased to 10.8 J/g after further curing at 170[degrees]C, 180[degrees]C for 2 h each, and post curing 200[degrees]C for 1 h. Furthermore, after post-curing at 200[degrees]C for 2 h, the exothermic peak disappeared, corresponding to 100% conversion of the mixture. The degree of conversion of the sample was determined according to the following relationship:
[FIGURE 3 OMITTED]
% conversion = (1 - [H.sub.rxn]/[H.sub.0]) X 100
where [H.sub.rxn] is the heat of reaction of the partially cured specimens.
[H.sub.o] is the heat of reaction of the uncured resin.
The obtained curing conversion indicated that the curing reaction of the BA: PU polymer alloys could rapidly occur at higher temperature. Figure 4 showed that the exothermic peaks of BA: PU at 80:20 alloys at various molecular weights were located at the same temperature of about 240[degrees]C.
[FIGURE 4 OMITTED]
Figure 5 shows the glass transition temperatures ([T.sub.g]) of the fully cured BA: PU alloys using polyol with number average molecular weight of 1000 g/mol were 170[degrees]C in 90:10 BA: PU, 205[degrees]C in 80:20 BA: PU, and 240[degrees]C in 70:30 BA: PU The [T.sub.g] of the binary mixture increased with increasing the urethane mass ratio. The similar trend in the [T.sub.g] enhancement from other polyol molecular weights, i.e., 2000, 3000, and 5000 was also observed. Figures 6 to 8 indicated the [T.sub.g] s of the fully cured BA: PU alloys using the polyol molecular weight of 2000 were 175[degrees]C in 90:10 BA: PU, 200[degrees]C in 80:20 BA: PU, and 245[degrees]C in 70: 30 BA: PU Also with the polyol molecular weights of 3000 and 5000, the glass transition temperature were 170[degrees]C and 168[degrees]C in 90:10 BA: PU, 195[degrees]C and 190[degrees]C in 80:20 BA: PU, and 250[degrees]C and 245[degrees]C in 70:30 BA: PU, respectively. Figure 9 illustrates the corresponding [T.sub.g]s obtained from the thermograms of the fully cured BA:PU alloys at various molecular weights. It was observed that the [T.sub.g]s were unaffected by the molecular weights of the urethane polyol comparing based on the same BA:PU mass ratio.
[FIGURE 5 OMITTED]
The effect of the urethane mass fraction on the [T.sub.g] of the BA:PU polymer alloys was previously reported (3) that the glass transition temperatures ([T.sub.g]s) from DMA (1) (Digital Media Adapter) See digital media hub.
(2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases. thermograms showed a synergistic behaviour, i.e., the [T.sub.g]s) from DMA thermograms showed a synergistic behavior, i.e., the [T.sub.g]s of the polymer alloys were found to be higher than the those of both parent polymers with the mass fraction of the PU. The [T.sub.g]s of the BA:PU alloys in this study were also increased with the mass fraction of the PU confirming our previous results (3). The [T.sub.g]s of the IPDI-based polyurethane elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. and the BA-a based polybenzoxazine were reported to be about -70[degrees]C to -20[degrees]C and 160[degrees]C to 170[degrees]C, respectively whereas the BA:PU ratio of 70:30 exhibited the [T.sub.g] of 220[degrees]C (3). The observed increased in the cross-linked density of the binary systems with PU is one possible reason for the enhancement in the [T.sub.g] of the resulting alloys though PU is a softer molecular species having much lower. [T.sub.g] and was expected to lower the [T.sub.g] of the binary alloys. The ability of the urethane prepolymer to enhance the cross-linked density of the polybenzoxazine is thus attributed to the observed phenomenon. The crosslinking was reported to be caused by the reaction between an isocyanate group on a urethane monomer and a hydroxyl group on polybenzoxazine after the phenolic hydroxyl group from the ring opening of benzoxazine monomer was produced (4).
[FIGURE 6 OMITTED]
Thermal Degradation and Thermal Stability Investigation
The thermogravimetric analysis (TGA See TARGA.
TGA - Targa Graphics Adaptor ) thermogram of the polybenozoxazine and BA:PU2K alloys at various compositions is shown in Fig. 10. Degradation temperature ([T.sub.d]), at 5% weight loss, is one of the key parameters needed to be examined for temperature stability of polymers. From the figure, the degradation temperatures of the BA:PU polymer alloys were found to be similar to the neat polybenzoxazine. The TGA curves of the binary mixture at various mass ratios of urethane prepolymer, e.g. 10,20 and 30%, suggested that an addition of up to 30 wt% of urethane resin into the b enzoxazine resin marginally enhanced the thermal degradation temperature of the obtained alloys. The degradation temperature of the polybenzoxazine homopolymer at 5 wt% loss was determined to be about 330[degrees]C which was consistent with the value reported previously by Rimduist et al. (3). The degradation temperatures of BA:PU alloys at the urethane mass ratio of 10 wt%, 20 wt%, and 30 wt% were found to be similar with the values of 336[degrees]C, 334[degrees]C, and 344[degrees]C, respectively. Therefore, one advantage of mixing the urethane resin into the benzoxazine network was to maintain or slightly improve the thermal stability of the polybenzoxazine. This result was attributed to the cross linking density enhancement as explained earlier. On the other hand, the char yield, i.e., the residual weight reported at 800[degrees]C of the polymer alloys was found to decrease with increasing the PU fraction in the binary system. The char yield at 800[degrees]C of the polybenzoxazine was determined to be 25 wt% which is consistent with the value of 25-30 wt% reported in the literatures (3). The TDI-poly(thylene adipate Adipate (-OOC-(CH2)4-COO-) is the ionized form of adipic acid.
As food additives, adipates are used as acidity regulators. Examples are sodium adipate (E356) and potassium adipate (E357). External links ) based polyurethane possessed a smaller value of char yield of only about 8 wt% at 800[degrees]C (4). As a result, the increase of the PU fractions expectedly reduced the char yield of the alloys as seen in Fig. 10. The char yields of BA:PU alloys at 10, 20, and30% mass fractions of urethane were determined to be 23.6 wt% 20.6 wt%, and 18.2 wt%, respectively. This can be explained as the chemical structure of the polyurethane composed of a less thermally stable aliphatic aliphatic /al·i·phat·ic/ (al?i-fat´ik) pertaining to any member of one of the two major groups of organic compounds, those with a straight or branched chain structure.
adj. structure of the polyol compared with the more stable benzene rings in the structure of polybenzoxazine. Therefore, the addition of the urethane resulted in lowering the char yield of the polymer alloys.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Furthermore, the observation of the thermal degradation of the BA:PU alloys (at a fixed mass ratio of 80:20) at various molecular weights of the polyol is illustrated in Fig. 11. Increasing the number average molecular weights of the polyol resulted in only marginal enhancement of the [T.sub.d] at 5% weight loss of the fully cured specimens, i.e. 332[degrees]C in average. The effects of the molecular weight of amino alcohol used for encapping a poly(urea urethane)s were studied by Ubaghs et al. They reported that the degradation temperature of the polymer did not increase with increasing the molecular weights of the encapping moieties beyond certain values (11). Moreover, the char yields of these alloy systems were observed to increase with the molecular weights of the polyol. Figure 11 shows the char yields of the binary mixtures using PU1K, PU2K, PU3K, and PU5K that were determined to be 18.3 wt%, 20.6 wt%, 22.6 wt%, and 22.9%, respectively. In these alloys, the increase in char yield at a greater number average molecular weight of the polyol implied higher flame resistance of the binary systems obtained.
[FIGURE 11 OMITTED]
Mechanical Properties of BA: PU Alloys
Flexural Property Characterization. Figures 12 and 13 show flexural properties of the BA:PU alloy specimens. Figure 12 illustrates flexural modulus of the binary system specimens at various urethane mass fractions as well as at different number average molecular weights of the polyols. The maximum modulus value of 6.2 GPa belongs to the neat polybenzoxazine specimen. At a fixed molecular weight of the polyol, the alloy with 30 wt% of the PU showed the flexural moduli to be in the range of 2.2-2.8 GPa. The modulus of the binary systems tended to decrease in a linear manner with the composition of the PU in the alloys following the rule of mixture. The storage modulus of 1.8 GPa for poly(ether polyol)-based polyurethane was reported in Ref. 12. The phenomenon was due to the basic principle that the addition of the rubbery urethane polymer into the adamantine adamantine /ad·a·man·tine/ (ad?ah-man´tin) pertaining to the enamel of the teeth.
pertaining to the enamel of the teeth. polybenzoxazine was able to lower the stiffness of the resulting polybenzoxazine alloys as clearly seen in Fig. 12. On the other hand, the effects of the number average molecular weights of the polyol on the flexural moduli of the alloys were not significant.
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
Flexural strength of the BA:PU alloys was also evaluated as exhibited in Fig. 13. The strength of the binary systems did not show a linear relationship with the compositions of the alloys but exhibited the synergistic behavior with the maximum at the BA:PU ratio of 90:10 for all molecular weights of the polyol. The behavior was consistent with that reported previously by Rimdusit et al. (3) The flexural strength of the neat polybenzoxazine was determined to be 145.9 MPa. The BA: PUIK provided the ultimate flexural strength of 163.9 MPa at the BA:PU ration of 90:10. The addition of PU1K for 20 wt% and 30 wt% systematically lowered the flexural strength to 140.2 MPa and 89.7 MPa, respectively. The ultimate values of the flexural strengths of BA:PU1K, BA:PU2K, BA:PU3K, and BA: PU5K were determined to be 163.9 MPa, 162.2 MPa, 150.5 MPa, and 149.4 MPa, respectively. The enhancement of the crosslink density of the alloys with the addition of the PU was likely to be related with the synergistic behavior in their flexural strengths as mentioned above.
Solvent Extraction of BA:PU Alloys. Solvent extraction of the BA:PU binary systems was studied using chloroform as a solvent. This experiment investigated the ability of network formation of the BA:PU alloys at different compositions and at various number average molecular weights of the polyol. Table 2 reported the data of BA:PU2K alloy specimens at different compositions before and after 30 days of chloroform immersion. The specimen with the BA:PU2K ratio of 70:30 was found to change from its original state. The color of the chloroform was changed from colorless to deep yellow after immersing for 30 days whereas the solvent of the other three compositions remained colorless throughout the whole evaluation period. As a result, the network of the binary system at 30 wt% of PU tended to be the weakest network; therefore, percent extraction in the solvent was rather high, i.e. 20.4%. The percents of solvent extraction of other specimens were determined to be 0.5% and 0.6% for the specimens with the BA:PU2K ratio of 100:0 and 90:10, respectively. The opposite behavior of the specimens with the ratio 80:20 was observed. The mass of the BA:PU2K specimen after solvent immersion slightly increased even after drying for 48 h at 105[degrees]C. The BA:PU2K alloys may trap the solvent inside their infinite networks thus the weight gain was observed.
TABLE 2. The solvent extraction data at various mass fractions of PU. Mass of specimer (mg) Composition Initial After immersion Percent Percent (BA:PU2K) for 30 days extraction (%) swell (%) 100:0 1364 1357 0.5 -- 90:10 1114 1408 0.6 -- 80:20 1146 1192 -- 3.8 70:30 1088 865 20.4 --
The solvent extraction data of BA:PU (80:20) alloy specimens at various molecular weights of the polyol before and after chloroform immersion for 30 days are shown in Table 3. The BA:PU5K sample was slightly swelled. The swelling degree of the edges of this specimen is corresponded to the highest percent swell of 11.3%. The percent swell was reduced to 3.8% when the number average molecular weight of the polyol was decreased to 2000. This phenomenon is possibly due to the fact that the network of the binary systems tended to have large space between network junction points due to an incorporation of the higher molecular weight of the polyol. The percent extraction of only 0.6% in the BA:PU1K was observed and was attributed to the rather short chain length of the polyol. This may result in a rather tight network structure for solvent penetration. No color change of the chloroform was observed in the specimens having different diol diol
an organic compound containing two hydroxy groups, a dihydric alcohol. Called also glycol. molecular weights (based on BA:PU at 80:20).
TABLE 3. The solvent extraction data at various number average molecular weight of polyol. Mass of specimer (mg) Number average Initial After immersion Percent Percent molecular weight of for 30 days extraction (%) swell Polyol (BA:PU = (%) 80:20) 1000 1122 1115 0.6 -- 2000 1146 1192 -- 3.8 3000 1196 1247 -- 4.1 5000 1209 1363 -- 11.3
The DSC experiment revealed the fully cured condition of binary mixture to be at 160[degrees]C, 170[degrees]C, 180[degrees]C, and 200[degrees]C for 2 h at each temperature. When the urethane resin was added in the range of 0-30% by weight, the glass transition temperature of the BA:PU1K, BA:PU2K, BA:PU3K, and BA:PU5K alloys were found to be in the range of 165-240[degrees]C, 165-245[degrees]C, 165-250[degrees]C, and 164-245[degrees]C, respectively. For BA:PU alloy systems, the degradation temperature at 5% weight loss at various polyol molecular weights were observed to be around 332[degrees]C independent of the molecular weights of the polyol. In addition, the flexural strength of the BA:PU alloys exhibited a synergistic behavior with the ultimate value in BA:PU = 90:10 for every molecular weight of the polyol used.
The authors also greatly acknowledge the Center of Excellence in Catalyst and Catalytic Reaction Engineering (Prof. Piyasan Praserthdam) Chulalongkorn University for TGA measurement. Bisphenol A, toluene diisocyanate, and polyols are kindly supported by Thai Polycarbonate Co., Ltd. (TPCC), south Groups Co., Ltd., and TPI Polyol Co., Ltd., respectively.
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Sarawut Rimdusit, (1) Tharathon Mongkhonsi, (1) Pakawan Kamonchaivanich, (1) Kuljira Sujirote, (2) Sunan Thiptipakorn (1)
(1) Department of Chemical Engineering Laboratory, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
(2) National Metal and material Technology center, National Science and Technology Development Agent Ministry of Science and Technology, Pathumthani 12120, Thailand
Correspondence to: S. Rimdusit; e-mail: firstname.lastname@example.org
Contract grant sponsors: National Metal and Materials Technology Center, CU Graduate Thesis Grant of Chulalongkorn University, Research Grant for Mid-Career University Faculty of the Commission on Higher Education Commission on Higher Education can refer to
DOI (Digital Object Identifier) A method of applying a persistent name to documents, publications and other resources on the Internet rather than using a URL, which can change over time. 10.1002/pen.21171
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2008 Society of Plastics Engineers