Effect of Filler Choice on a Binary Frontal ... - ACS Publications

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Effect of Filler Choice on a Binary Frontal Polymerization System Veronika Viner*,† and Gloria Viner‡ †

United State Navy—Naval Air Systems Command (NAVAIR), Naval Air Warfare Center, Weapons Division (NAWCWD), Research Department, Chemistry Division, 1900 North Knox Road Stop 6303, China Lake, California 93555, United States ‡ Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States ABSTRACT: Binary frontal polymerization is a process that involves two different systems polymerizing simultaneously but independently of each other. Various factors including filler choice and initiator concentration can affect front temperature and velocity. Like thermal frontal polymerization systems, binary frontal polymerization of a cyanate ester system and multifunctional acrylate is affected by initiator (amine) concentration and filler choice. Systems with higher viscosities and higher initiator concentrations resulted in higher velocities. Front temperature was rarely affected by filler choice. Aniline concentration and initial monomer ratios had a greater effect on front temperature than filler choice does.

1. INTRODUCTION Frontal polymerization is a process that is dependent upon thermal diffusion and involves a reaction zone or wave that propagates through unreacted monomer converting it into polymer.1 For highly reactive liquid monomers, large thermal gradients between the exothermic propagating front and cool, unreacted monomers can occur, making fronts vulnerable to convection.2,3 Although convection can result in accelerated front velocities for liquid monomers forming solid polymer products, too much heat loss from convection can quench a front, particularly for liquid fronts with liquid polymer products.4 Addition of filler like Polygloss 90 (ultrafine kaolin clay) or CabO-Sil (fumed silica) increases the viscosity of a system so that convection is minimized; up to a point, an increase in viscosity, which is temperature dependent, results in higher front velocities.5,6 Thus, a putty with a defined thickness is needed in order to minimize buoyancy-driven convection and prevent a front from quenching.7 6 6 However, too much filler can also quench a propagating front. An ultrafine kaolin clay was selected because of its cohesiveness and ability to produce a putty when mixed with a thermal initiator and monomer. Cab-O-Sil M-5, fumed silica, has different rheological properties than Polygloss 90. Thus, if 1.0 g of Cab-O-Sil M-5 and 1.0 g of Polygloss 90 were weighed out and mixed with 10.0 g of TMPTA-n, then the fumed silica sample would have a putty-like consistency whereas the kaolin clay sample would have a liquid-like consistency as shown in Figure 1. Thus, to achieve similar viscosities/consistencies, less filler is required for fumed silica systems, which is less likely to quench the front. The higher surface area, smaller particle size, and much lower bulk density results in Cab-O-Sil (200 m2/g for surface area, 0.2
0.3 μm particle size, and 0.048 g/mL for bulk density)8,9 being able to absorb more liquid than an equivalent mass of Polygloss 90 (surface area of 22 m2/g, bulk density of 0.160 g/mL, and 0.4 μm particle size),9,10 so that a smaller loading of Cab-O-Sil M-5 can be added and still have a front propagate. r 2011 American Chemical Society

Binary frontal polymerization occurs when two different reaction systems undergo reactions simultaneously but independently of each other. Studies on binary frontal polymerization of epoxy
acrylate systems1112 have been done, but cyanate esters have never been cured before using thermal frontal polymerization or binary frontal polymerization. Pojman et al. studied binary frontal polymerization utilizing triethylene glycol dimethacrylate, TGDMA, for the free-radical polymerization and diglycidyl ether of bisphenol A, DGEBA, for the epoxy resin.12 They found that a minimum dependence on front velocity occurred when the initial composition of the monomers was at an intermediate value, e.g., 50%:50% or 1:1. In subsequent studies of another binary frontal polymerization using ethylene glycol dimethacrylate with Luperox 231 and the epoxy resin diglycidyl ether of bisphenol F with EPICURE 3271 or with a BCl3 amine complex,11 Pojman et al. found that a minimal dependence on front velocity did not occur at an initial intermediate (50%:50%) composition of monomers for the cationically cured epoxy resin system because a component in the BCl3 amine complex was affecting the rate of decomposition for the peroxide initiator. Thus, the two polymer reactions were not completely independent of each other. Although studies on binary frontal polymerization of epoxy
acrylate systems1112 have been done, cyanate esters have never been cured before using thermal frontal polymerization or binary frontal polymerization. Herein, we studied a free-radical polymerization system and an amine-catalyzed cyanate ester system for binary frontal polymerization. Cyanate esters have been cured using amines and being heated at elevated temperatures.13,14 In one paper by Bauer and Bauer, cyanate esters resins were studied for snap-cure applications.14 By encapsulating a hardener or amine, Bauer and Bauer were able Received: December 29, 2010 Revised: April 11, 2011 Published: May 10, 2011 6862

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The Journal of Physical Chemistry B to achieve pot lives (length of time it takes before spontaneous polymerization occurs) of up to 3 months. If left unencapsulated, such systems gelled after 30 min at ambient temperatures.13 With increasing amine concentration, glass transition temperatures were reduced.14 They found that a reaction rate of 200 °C was necessary for curing neat cyanate ester (Primaset LeCy or 4,40 -ethylidenediphenyl dicyanate).14 Thus, front temperatures of 200 °C and over were necessary to achieve complete curing of the cyanate ester. Herein, we focus on achieving front temperatures over 200 °C by adding multifunctional acrylates, which can have front temperatures as high as 250 °C. Additionally, these acrylates have a much higher front velocity than monofunctional acrylates, so that heat loss due addition of a possibly nonreactive monomer will have less effect on front velocity and will be less likely to quench the propagating front. Aniline concentration was varied to verify that the reaction (amine-catalyzed cyanate ester cyclotrimerization and thermally initiated homopolymerization of the acrylate) was frontally polymerized, verifying the dependence of the front velocity on initiator concentration. For thermal frontal polymerization, front velocity is proportional to initiator concentration with increasing initiator concentration, resulting in higher front velocities.1 For thermal frontal polymerization and copolymerization, front temperature is the dominant factor in controlling front

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velocity.15 However, according to the mathematical binary frontal polymerization model of Perry and Volpert, other factors including initial monomer mass ratio and the kinetic parameters of the monomers and initiator predominantly influence front velocity. Thus, we studied the effects of both front temperature and velocity to determine whether a difference in trends for both occurred, thereby possibly indicating that binary frontal polymerization rather than copolymerization or homopolymerization of only monomer occurred.

2. EXPERIMENTAL SECTION All reagents were used without further purification. Primaset LeCy (4,40 -ethylidenediphenyl dicyanate) was obtained from Lonza. Polygloss 90 and Cab-O-Sil (M-5) were obtained from Huber Materials and Cabot Corp., respectively. Aniline, TMPTA-n, and Luperox 231 were obtained from Sigma Aldrich. Figure 2 contains the structures of all tested monomers and initiators. Preparation of 10.4 phr Luperox 231 in TMPTA-n Systems. A 10.4 g amount of Luperox 231 was dissolved in 100.0 g of TMPTA-n and stirred by spatula until a clear solution was obtained. Preparation of Systems. Varying amounts of Primaset LeCy, 10.4 phr Luperox 231 in TMPTA-n, and aniline with a total mass of 10.0 g were mixed together before being added to 0.80 g of Cab-O-Sil M-5, 4.50 g of Polygloss 90, or 0.50 g of Cab-O-Sil M-5 and 2.00 g of Polygloss 90. The resulting mixture was mixed together until a putty was formed. Ignition of Propagation in Polymerizable Systems. Strips with dimensions of 2 cm  4 cm  5
7 mm were placed on a 2 cm thick piece of wood and surrounded by wooden barriers. A front was ignited at one end of the strip with a soldering iron. The front temperature was measured using an OMEGA reader and type “K” thermocouple wire. Movies of the propagating front were recorded using a camera before being transferred to a computer. Velocity was calculated by plotting distance vs time and taking the slope of linear regression as the velocity. 3. RESULTS/DISCUSSION

Figure 1. Image of 1.0 g of fillers (Polygloss 90 on left, Cab-O-Sil M-5 on right) mixed with 10.0 g of TMPTA-n.

3.1. Impact of Filler Choice on Front Velocity and Temperature for Different Initial Monomer Ratios. The impact of

filler choice on front velocity and temperature was studied at

Figure 2. Structures of monomers and liquid additives. 6863

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The Journal of Physical Chemistry B three different initial monomer ratios: 30:70% mass, 40:60% mass, and 50:50% mass Primaset LeCy:TMPTA-n. For 3.6 phr aniline in Figure 3, front velocity and temperature were always affected by filler choice due to differences in the viscosities of the various systems and differences in the properties of the fillers. Increasing front temperature and velocity increased with increasing percentage mass cyanate ester. However, for mixed filler and fumed silica systems, this impact was within experimental uncertainty (10%) for all of the tested initial monomer ratios. For both of these systems, the consistency of the system remained putty-like (similar to that of Play Doh) whereas the consistency of the kaolin clay system changed from its initial putty-like consistency to a more fluid-like consistency (like cake batter) or low consistency within minutes of

Figure 3. Front temperature and velocity as a function of % mass 10.4 phr Luperox 231 in TMPTA-n for 3.6 phr aniline in Primaset LeCy systems.

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being prepared. This abrupt change in viscosity occurred due to a slight elevation in temperature from room temperature to about 5
10 °C higher.6 At low initiator concentrations (3.6 phr aniline), no front would propagate completely through the strip for the kaolin clay system with an initial monomer ratio of 50%:50% mass Primaset LeCy:TMPTA-n due to incomplete frontal polymerization. The viscosity of this system was low or almost liquid-like, so that heat loss from buoyancy-driven and surface tension-driven convection eventually quenched the propagating front. Observation of fluid flow at the free interface or surface of the strip confirms the presence of surface tensiondriven and buoyancy-driven convection. However, fluid flow was observed only for kaolin clay-containing systems. At higher aniline concentrations, the viscosity of the kaolin clay system at the same filler loading (4.50 g) was much higher or more putty-like. The difference in the two viscosities is illustrated in Figure 4. The “oozing” or more fluid-like behavior of the lower aniline concentration system is in contrast to the higher aniline concentration system where all of the material is cohesive and sticks to the tongue depressor, thus demonstrating a more putty-like or thicker viscosity. By increasing the viscosity of the system, heat loss was reduced so that the front could not be quenched.16 Heat loss can hinder (or even quench) a propagating front due to the front’s dependence on thermal diffusion.1,6 Thus, although liquidto-solid fronts normally result in an increase in front velocity, the effect of heat loss was greater and resulted in lower front velocities and incomplete frontal polymerization of the strip. Normally, buoyancy-driven convection (and surface-tension driven convection) is eliminated by addition of enough filler to form a putty.17 Since enough filler for each of the three chosen filler systems was added to form a putty, it would be expected that buoyancy-driven convection was eliminated. However, the change in viscosity for the kaolin clay system containing 3.6 phr aniline in Primaset LeCy with an initial monomer:monomer mass ratio of 50:50 for TMPTA-n:Primaset LeCy indicates that this was not the case for kaolin clay systems. The difference in physical properties between kaolin clay and fumed silica as well as the chemical structure of the fillers may explain the differences in viscosities for the different filler systems. For example, aluminum is a known catalyst for cyclotrimerization of cyanate esters.18 Fluid flow was observed on the surface of the propagating front

Figure 4. Image of the difference in viscosities for 3.6 phr aniline in Primaset LeCy (a) and 10 phr aniline in Primaset LeCy (b). 6864

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Figure 6. Front temperature as a function of aniline concentration for different filler systems. Figure 5. Plot of front temperature and velocity as a function of % mass 10.4 phr Luperox 231 in TMPTA-n for 10 phr aniline in Primaset LeCy systems.

only for lower aniline concentrations and only for kaolin clay systems. When a fine powder such as pepper was placed on the surface of the strip, pepper was displaced and did not melt, thus proving the possibility that surface tension-driven and buoyancydriven convection were observed and could have occurred. A high temperature gradient at the surface with air occurs and creates surface tension, which can and did increase heat loss. Although the different filler systems contain the same amount of reactive material, they contain different amounts of filler. Because a higher filler loading is needed for the kaolin clay systems to have a putty-like consistency, more filler (than mixed systems or fumed silica systems) is available to absorb heat from the propagating front so that lower front temperatures occur. With lower front temperatures, lower velocities will occur. Thus, differences in filler loading are the primary reason for the difference in front velocity and temperature for mixed filler and fumed silica systems. This difference in filler loading for different fillers also accounts for some difference in front temperature and velocity for the kaolin clay system and the other systems, but a difference in viscosity is the other primary reason accounting for the sharp difference in front velocity and temperature for kaolin clay systems compared to other systems. Increasing the percentage mass 10.4 phr Luperox 231 in TMPTA-n slightly increased the viscosity of the system no matter what filler was used, resulting in higher front velocities and temperatures. With an increase in viscosity, no fluid flow was observed in the kaolin clay system, thus indicating that buoyancy-driven and surface tension-driven convection had been eliminated. Also, binary frontal polymerization should demonstrate a minimal dependence on front velocity at median initial monomer ratios such as 50%:50% mass TMPTA-n:Primaset LeCy.11,12 In Figure 5, a plot of 10 phr aniline in Primaset LeCy with varying initial monomer ratios and different fillers demonstrates how filler choice affected front velocity without impacting the relationship between front velocity and initial monomer ratio.

Generally, filler choice had no or little correlation with front temperature for systems with an aniline concentration of 10 phr and thermal initiator concentration of 10.4 phr Luperox 231 for the mixed filler and fumed silica systems. Kaolin clay systems, which had lower viscosities or less putty-like consistencies than corresponding systems with either mixed fillers or fumed silica, generally had lower front temperatures due to higher filler loadings than corresponding systems with other types of filler. Front velocity was another matter. No matter what amount of 10.4 phr Luperox 231 in TMPTA-n was added, fumed silica systems always had the highest front velocities due to differences in filler loadings. With a lower filler loading, less material was available to act as a heat sink and absorb heat from the propagating front. Lower filler loading was needed for the fumed silica systems due to increased surface area and low bulk density. Thus, the impact of filler loading is first observed on front velocity before having a smaller impact on front temperature. Thus, at higher aniline concentrations, filler choice impacts front velocity but not front temperature. At low aniline concentrations (3.6 phr), differences in consistencies or viscosities occur, particularly with kaolin clay systems. Yet only front velocity is still affected for all three systems. Slight differences in the trend for front temperature and velocity are an indication of binary frontal polymerization because front temperature is the dominant factor in controlling front velocity for frontal polymerization or copolymerization.19 In contrast, according to Perry and Volpert, for binary frontal polymerization, front velocity can be affected more by the initial monomer mass ratios and kinetic parameters of monomer and initiator than front temperature.19 This conclusion helps to explain the slight disconnect between front velocity and front temperature for our own experimental results. 3.2. Effect of Amine Concentration on Front Velocity and Temperature for Different Filler Systems. Because two different amine concentrations resulted in similar but different results for the three different filler systems at different initial monomer ratios, a wider range of amine concentrations was studied at an initial TMPTA-n:Primaset LeCy ratio of 50%:50% mass. At an initial TMPTA-n:Primaset LeCy ratio of 50%:50% mass, front temperatures increased with increasing aniline concentration no 6865

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concentrations but is starting to develop the distinct plateau shape that is evident for the plotted data for kaolin clay systems. Although the mixed filler systems contain both fumed silica and kaolin clay, the systems tend to replicate the trends of the fumed silica systems than those of the kaolin clay systems. For the mixed filler systems, 63% of the original amount of fumed silica and 44% of the original amount of Polygloss 90 were added. Although more Polygloss 90 was added, more of the original amount of fumed silica than kaolin clay was added. Thus, the percent of the minimal amount of filler needed to form a putty rather than actual filler loading tends to control which filler behavior the mixed filler system will exhibit. Thus, the mixed filler system tends to replicate the trends of fumed silica rather than kaolin clay. Figure 7. Front velocity as a function of aniline concentration for three different filler systems.

matter what filler or mixture of fillers was used. The plot in Figure 6 demonstrates this finding. Eventually, a plateau in front temperature is reached and no longer increases despite increasing the aniline concentration. Previous work done with increasing thermal initiator found a plateau in front velocity was reached with 5% mass Luperox 231 for a TMPTA-n system, and with increasing initiator concentration,7 front velocity no longer increased at such high initiator concentrations.7 Unlike front temperature, front velocity was affected by both aniline concentration and filler choice. As aniline concentration increased, front velocity increased as shown in Figure 7. Because frontal polymerization has a correlation between initiator concentration and front velocity,1 increasing aniline concentration resulting in higher front velocities is an indication that the cyanate ester is polymerizing via frontal polymerization rather than acting as a diluent or reactive additive for acrylate homopolymerization. This finding agrees with the mathematical binary frontal polymerization model of Perry and Volpert.19 Addition of a thiol (a reactive additive) for copolymerization with the acrylate results in lower front velocities and temperatures.20 Addition of an inert diluent results in lower front temperatures.2 They can lower front temperature by acting as a heat sink and can lower the reaction rate so that front velocity is decreased.19 With systems containing only kaolin, the aniline concentration reaches a plateau at high initiator (aniline) concentrations. This result has been seen previously with a thermal initiator and TMPTA-n system.7 However, this plateau or maximum front velocity was not observed in either mixed filler or fumed silica systems. We propose that differences in filler loading and changes in consistency or viscosity for the kaolin clay systems at different aniline concentrations versus no difference in consistency for the other two filler systems cause the difference in trends. These differences in consistency or viscosity are due to fumed silica’s inertness, small particle size, large surface area, and smaller bulk density. With higher filler loadings such as those with kaolin clay, more heat is absorbed from the propagating front so that changes in front velocity are more apparent. With kaolin clay systems, more filler than the other two systems is available to absorb heat from the propagating front. As such, the plateau or maximum front velocity is observed earlier than the other two systems. Yet, the shape of the line for the mixed filler systems is not entirely linear with the higher initiator

4. CONCLUSIONS Binary frontal polymerization can be used to cure cyanate ester resins. A slight disconnect in trends for front temperature and velocity for 3.6 pr and 10 phr aniline at different initial monomer mass ratios is an indication of binary frontal polymerization rather than copolymerization or homopolymerization of only one monomer. Increasing amine concentration resulted in higher front velocities and temperatures, indicative that frontal polymerization is occurring with the cyanate ester. Further proof is that dilution of the acrylate monomer with a reactive additive (such as a thiol, which can undergo copolymerization) or nonreactive additive (such as a plasticizer) results in lower front velocities and temperatures. In the case of substituting acrylate monomer with cyanate ester, front temperature is barely affected. The front velocity decreases with increasing substitution of acrylate monomer with cyanate ester due to the minimal dependence on front velocity on the initial monomer ratio at a median value. Filler choice also affected front velocity, as well as front temperature to a limited extent. Use of fumed silica in the form of Cab-O-Sil M-5 generally resulted in higher front temperatures and velocities than corresponding systems utilizing ultrafine kaolin clay (Polygloss 90). Use of mixtures of kaolin clay and fumed silica resulted in front temperatures within experimental uncertainty of the corresponding pure fumed silica systems and lower front velocities due to use of a higher % of original amount of fumed silica than kaolin clay. Differences in front velocity occurred for different filler systems primarily because of differences in filler loading, which is a result of the difference in physical properties of the fillers. However, under certain reaction conditions for only kaolin clay systems, differences in viscosity resulted in the lowest front temperature and velocities of all tested systems due to buoyancy-driven and surface tension-driven convection. Thus, choice of filler is critical in ensuring that buoyancydriven and surface tension-driven are eliminated entirely. ’ AUTHOR INFORMATION Corresponding Author

*Phone: (760) 939-5322. Fax: (760) 939-1617. E-mail: [email protected].

’ ACKNOWLEDGMENT This research was funded by China Lake Naval Air Warfare Center Weapons Division. 6866

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