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EPL-0001081 article Effect of brominations

Fire, smoke and toxicity are of significant concern for composite materials used in marine applications. Bromination of vinyl ester resin imparts fire retardancy as manifested by a reduction in the amount of smoke, carbon monoxide, and corrosive
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  1. Introduction Thermoset vinyl ester matrices are becomingincreasingly important in industrial applicationsdue to their enhanced mechanical properties. Theyexhibit characteristics similar to epoxy resins, aswell as unsaturated polyester resins. Advantagesinclude high tensile strength and stiffness, low cost,process versatility and good chemical resistance.However, vinyl ester still has some challenges likepoor resistance to crack propagation, brittlenessand large shrinkage that occurs during polymeriza-tion. Therefore, introducing good interfacial bond-ing between nanofillers and the resin is often usedto alleviate volume shrinkage, void formation andimproving surface dispersion along with toughness.Methods of incorporating nanoparticles into poly-mer matrices could be ex-situ , like dispersion of thesynthesized nanoparticles into resin solution, or in-situ monomer polymerization process in the pres-ence of the nanoparticles [1]. The interactionbetween the nanoparticles and matrix for the ex-situ fabricated composites are normally van der waals 724 * Corresponding author, e-mail:© BME-PT Effects of bromination on the viscoelastic response of vinyl ester nanocomposites  A. Almagableh 1*  , P. R. Mantena 2  , A. Alostaz 3  , W. Liu 4  , L. T. Drzal 4 1 University of Mississippi, PO. Box 3967, MS 38677, USA 2 University of Mississippi, 201 D Carrier Hall, MS 38677, USA 3 University of Mississippi, 202 Carrier Hall, MS 38677, USA 4 Composite Materials and Structures Center, Michigan State University, 2100 Engineering Bldg, East Lansing, MI 48824, USA  Received 25 May 2009; accepted in revised form 1 September 2009 Abstract. Fire, smoke and toxicity are of significant concern for composite materials used in marine applications. Bromi-nation of vinyl ester resin imparts fire retardancy as manifested by a reduction in the amount of smoke, carbon monoxide,and corrosive combustion products. In this research, the viscoelastic properties, modulus (stiffness) and damping (energydissipation), of 1.25 and 2.5wt. percent nanoclay and exfoliated graphite nanoplatelet (xGnP) reinforced non-brominatedand brominated vinyl ester have been studied over a range of temperature and frequency. Effects of frequency on the vis-coelastic behavior were investigated using a Dynamic Mechanical Analyzer (DMA) by sweeping the frequency over threedecades: 0.01, 0.1, 1 and 10Hz, and temperature range from 30–150°C at a step rate of 4°C per minute. Master curves weregenerated by time-temperature superposing the experimental data at a reference temperature. The nano reinforced compos-ites showed a drop in initial storage modulus with bromination. Nanocomposites with 1.25 and 2.5wt. percent graphite hadthe highest storage modulus among brominated specimens. Bromination was also found to significantly increase the glasstransition temperature ( T  g ) and damping for all nanocomposites. Among the brominated specimens, 1.25wt. percentgraphite platelet reinforced vinyl ester exhibited the best viscoelastic response with high damping and glass transition tem-perature, along with superior storage modulus over a longer time period.  Keywords: nanocomposites, viscoelastic properties, bromination eXPRESS Polymer Letters Vol.3, No.11 (2009) 724–732  Available online at DOI: 10.3144/expresspolymlett.2009.90  forces, steric interaction. However, the in-situ syn-thesis methods may create strong chemical bondingwithin the composite.Optical and mechanical properties of vinyl esterpolymer reinforced with ZnO nanoparticles, func-tionalized with a bi-functional coupling agentmethacryloxypropyl-trimethoxysilane (MPS) wereinvestigated by Guo et al . [2]. The existence of MPS at the interface between the matrix and parti-cles results in improved interfacial interactionwhich in turn improves UV shielding, modulus andstrength significantly.The physical properties of vinyl ester reinforcedwith unmodified CuO nanoparticles and thosefunctionalized with a bi-functional coupling agentmethacryloxypropyl-trimethoxysilane (MPS) werestudied by Guo et al . [3]. Increase in both thermalstability and mechanical properties were attributedto good nanoparticles dispersion at the interfaceand the resulting chemical bonding between thefunctionalized nanoparticles and the matrix.Vinyl ester thermosetting nanocomposites rein-forced with iron oxide nanoparticles were preparedand characterized by Guo et al . [4]. Iron oxidenanoparticles functionalized with a bi-functionalcoupling agent was observed to increase the adhe-sion and dispersion of the nano filler into the matrixresulting in increased thermal stability, lower cur-ing temperature and improved mechanical proper-ties. The nanocomposites became also magneti-cally stronger and were independent of particlesfunctionalization.Schroeder et al . [5] analyzed morphologically ther-moset materials obtained from styrene/vinyl esterresins of different molecular weights modified withpolymethyl methacrylate (PMMA). It was foundthat different morphologies including dispersion of thermoplastic rich particles in a thermoset resin,continuity of network structure were highly depend-ent on molecular weight of vinyl ester, curing tem-perature and concentration of the PMMA additives.The addition of the thermoplastic PMMA increasedthe fracture resistance without significantly affect-ing both volume shrinkage reduction and the ther-mal-mechanical properties of the modified ther-mosets.Recent interest in the use of organic-matrix com-posite materials in US Navy submarines and shipshas generated the requirement for significantimprovement in the flammability performance of these materials including reduction in the amountof smoke, carbon monoxide, and corrosive com-bustion products. New fire retardant approaches fororganic-matrix composite materials are needed toaddress the smoke issues and to further reduce theflammability of these composites. Focus of ourresearch is on developing stronger, safer and morecost-effective structures for the new generationnaval ships; especially nanoparticle reinforcedglass/carbon polymeric based composites andstructural foams for blast/shock/impact mitigation.Fire, smoke and toxicity are of significant concernin ship structures. The US Navy is currently usingbrominated vinyl ester matrix resin with glass rein-forcement for composite applications in topsidesurface ship structures [6]. This matrix resin wasselected due to its good corrosion resistance andtoughness. Bromine is an effective flame retardant,especially when combined with antimony oxide.Bromination of vinyl ester resin imparts fire retar-dancy as manifested by flame spread and lowerheat release rates. However, this fire-retardant sys-tem functions primarily in the gas phase causingincomplete combustion. As such, brominated resinsproduce dense smoke, an increase in the yield of carbon monoxide, and hydrogen bromide.The work reported here is an extension of previouswork [7] on the viscoelastic behavior of non-bromi-nated vinyl ester nanocomposites. DMA measure-ments are usually carried out under constantdisplacement amplitude in a fixed-frequency defor-mation mode, in which the mechanical propertiesare function of temperature only. Other measure-ments that provide more information may includefrequency sweep with temperature steps, to whichtime-temperature superposition (TTS) applied topredict the long-term time dependent properties of the material [8]. An attempt has been made toexperimentally characterize the dynamic storagemodulus (  E  ′ ) and damping of brominated and non-brominated vinyl ester reinforced with 1.25 and2.5wt. percent nano-clay and exfoliated graphitenanoplatelets (xGnP) as a function of temperatureand frequencies. Dynamic mechanical testing hasbeen used to perform multi-frequency (acceleratedtemperature measurements) and theoretical time-temperature superposition treatment of the data.Effects of bromination on the viscoelastic responseof these vinyl ester nanocomposites are discussed. 725 Almagableh et al. – eXPRESS Polymer Letters Vol.3, No.11 (2009) 724–732  2. Theory The time-temperature superposition principle isbased on the fact that processes involved in molec-ular motion occur at larger rates at elevated temper-atures. The change in property which occurs rela-tively quickly at higher temperatures can be madeto appear as if they occurred at longer times orlower frequencies simply by shifting the data withrespect to time (1/frequency) [8]. By shifting thedata with respect to frequency to a reference curve,a master curve is generated, which covers time (fre-quencies) outside the accessible range.The shifting mechanism used to shift a set of dataupon a reference curve follows WLF [8] model.This model assumes that the fractional free volumeincreases linearly with respect to temperature in thetransition region, and when the free volumeincreases, its viscosity decreases. In this model, thedegree of shifting was calculated according toEquation(1);(1)For both resin systems (with and without bromina-tion), C  1 and C  2 were found to be around 103.9 and399K, respectively. Relationship between theshifting factor ( a T  ) versus T  is plotted in Figure1for pure and brominated vinyl ester. 3. Experimental 3.1. DMA setup Dynamic measurements were carried out using theTA Instrument model Q800 DMA on prismaticspecimens deformed in a single-cantilever clamp-ing mode, with a span length of 17.5mm. Stressand strain with the single-cantilever clamp used inmodel Q800 DMA are calculated with Equa-tions(2) and (3), respectively [8], assuming linearviscoelastic behavior.(2)(3)where  L –clamp span length t  –sample thickness w –width of the specimen  ν –Poisson’s ratio F  c –clamping correction factor σ  x  –stress ε  x  –strain P –applied force δ –amplitude of deformation 3.2. Test description The 1.25 and 2.5wt. percent nanocaly and xGnPreinforced non-brominated and brominated vinylester nanocomposites were characterized by per-forming a multi-frequency isothermal mode, inwhich the sample is equilibrated at different tem-peratures and subjected to a series of frequencies.Specimens with dimensions of 35 × 10 × 1.6mmwere subjected to frequencies of: 0.01, 0.1, 1 and10Hz with a temperature step rate of 4°C perminute starting from 30°C (RT) to 150°C. A verysmall displacement amplitude (25  µ m was appliedsince the analysis assumes linear viscoelastic char-acterization, and two specimens were tested fromeach configuration. The raw data was then fed tothe Rheology data analysis software to generate themaster curves. 3.3. Materials and sample preparations The polymeric matrix used was a vinyl ester resin(manufactured and supplied by Ashland specialtychemical, Division of Ashland INC (Columbus,OH)). DERAKANE 411-350-(non-brominated) isa mixture of 45wt.% styrene and 55wt.% vinylester. Styrene allows the chain extension because of its single unsaturated carbon-carbon double bond,while the vinyl ester resin with two reactive vinyl      υ++δ=ε 22 )1(51213  Lt  LtF  c x  2 6 wt PL  x   =σ )()(log 0201 T T C T T C a T  −+−−= 726  Almagableh et al. – eXPRESS Polymer Letters Vol.3, No.11 (2009) 724–732 Figure 1. Relationship between shifting factor ( a T  ) andtemperature for pure and brominated vinyl esterbased on the WLF model  end groups enables the crosslinking for network.DERAKANE 510A-40 (brominated) vinyl esterresin is a brominated bisphenol-A based vinyl esterconsisting of 38 wt.% styrene, and modified to pro-duce the maximum degree of fire retardancy com-bined with enhanced chemical resistance andtoughness. These additives are Butanone peroxide,N,N-Dimethylaniline, Cobalt Naphthenate, and 2-4-Pentanedione, all supplied from Sigma Aldrich(St. Louis, Mo).Exfoliated graphite nanoplatelets (xGnP) were pro-duced according to the method described in [9].The nanoclay was Cloisite 30B from Southern ClayProducts, Inc (Gonzales, TX). Figures2a and 2bshow a morphology using TEM and SEM for bothedge and lateral views of xGnP inside a polymer.These xGnP nanocomposites have exfoliated anddispersed graphite platelets with 1nm thicknessand several hundred nanometers widths. Distancebetween layers is in the range of 10~30Å and sizeof the layered graphite extends from several hun-dred nanometers to several microns.The samples were prepared by dispersing about3000g of epoxy vinyl ester resin solution with dif-ferent percentages of nanoclay or nanographite in a1gal container for 4hours, followed by 4passesthrough a flow cell connected to a 100W sonicator.1% Butanone peroxide, 0.2% of 2-4 Pentanedione,0.1% N,N-Dimethylaniline, and 0.2% CobaltNaphthenate were added to the mixed vinyl esterresin solution in order and mixed for 10min. Theabove mixed resin solution was mixed for 2minwith FlackTek speed mixer at 3000 RPM. Thewell-mixed vinyl ester resin solution with nanoclayor nanographite was poured into a 13 × 13 × 0.4”mold, let stand for 30minutes at room temperatureand then was post cured at 80°C for 3hours. Pris-matic samples with nominal dimension of 35 × 10 × 1.6mm size were prepared from these plates andtested in a DMA using the single-cantilever clampfixture. 4. Results and discussion 4.1. Densities As shown in Figure3, the densities of brominatedvinyl ester nanocomposites are greater than that of the non-brominated samples. It should be noted thatbromine is a heavy atom and there are four bromineatoms bonded in one molecule, which results indensity being higher for brominated specimens.Specific gravity of 510A-40 brominated vinyl esteris about 1.23 while that of the non-brominated ver-sion 411-350 is 1.046. 727  Almagableh et al. – eXPRESS Polymer Letters Vol.3, No.11 (2009) 724–732 Figure 2. Morphology of xGnP using (a) edge view (TEM), and (b) Lateral view (SEM) Figure 3. Densites of non-brominated (411-350) andbrominated (510A-40) vinyl ester nanocompos-ites  4.2. Modulus The storage modulus versus temperature curve pro-vides valuable information about the stiffness of amaterial as a function of temperature, and it is sen-sitive to structural changes such as molecularweight, fiber-matrix bonding and degree of crosslinking density. Crosslink density, typicallygiven as the average molecular weight betweencrosslinks (  M  c ), is an important factor governingthe physical properties of cured thermoset resins.Moreover, it can be changed by adjusting thestyrene content in the resins, molecular weight of vinyl ester oligomers, altering the state of conver-sion, and control of the cure conditions [10].Crosslinking densities of the two resin systems(non-brominated and brominated) resulting fromdifferent styrene contents were calculated [10] as1116 and 597mol/m 3 , respectively, based on Equa-tion(4):(4)where V  –crosslinking density  R –gas constant G 0 n –plateau modulus from master curve of the shear modulus versus frequency.Calculation of crosslinking density for nanocom-posites using this equation is invalid, because con-tribution of nanoparticles in the crosslinkingmechanism is unknown.Higher initial storage modulus in fiber reinforcedcomposite materials are in general attributed togood fiber/matrix bonding (cross linking), or higherinitial molecular weight [11]. Figure4 shows initialstorage modulus (30°C) for vinyl ester nanocom-posites, with and without bromination. Initial mod-ulus was observed to increase with addition of nanoparticles in the non-brominated system. On theother hand, bromination resulted in a reduction of the initial storage modulus with addition of nanoparticles. Among the brominated specimens,gain in glassy (initial) modulus was observed withxGnP reinforcement, and a loss in glassy moduluswas associated with the addition of nanoclay parti-cles. Loss in storage moduli with the brominatednanoclay composites could be due to weak interfa-cial bonding between the nanoclay particles andbrominated resin. However, the 1.25wt.% xGnPappears to be bonding better with the brominatedresin resulting in higher storage modulus comparedto pure brominated vinyl ester.Figures5–7 show the storage modulus (  E  ′ ) evolu-tion with temperature for some of the vinyl esternanocomposites, with and without bromination.Bromination effect modifies the dynamic mechani-cal behavior for VE and its nanocomposites. Drop  RT GV  n 0 = 728 Almagableh et al. – eXPRESS Polymer Letters Vol.3, No.11 (2009) 724–732 Figure 4. Initial storage modulus for non-brominated andbrominated nanocompsoites at 1Hz frequency Figure 5. Storage modulus for pure vinyl ester with andwithout bromination Figure 6. Storage modulus for 2.5wt. percent nanoclayreinforced vinyl ester with and without bromina-tion
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