Delayed Addition Foaming of Bio-epoxy Blends: Balancing

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Delayed Addition Foaming of Bio-epoxy Blends: Balancing Performance Requirements and Sustainability Srishti Shukla, Shubh Agnihotri, Sai Aditya Pradeep, and Srikanth Pilla ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04415 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Delayed Addition Foaming of Bio-epoxy blends: Balancing Performance Requirements and Sustainability #,‡Srishti

Shukla, #,‡Shubh Agnihotri, #,†Sai Aditya Pradeep, #,†,||,*Srikanth Pilla #Department of Automotive Engineering, Clemson University, Greenville, South Carolina 20607, United States. ‡Department of Mechanical Engineering, Indian Institute of Technology (B.H.U.), Varanasi, Uttar Pradesh 221005, India. †Clemson Composites Center, Clemson University, Greenville, South Carolina 20607, United States. ||Department of Material Science and Engineering, Clemson University, Clemson, South Carolina 29634, United States. *Corresponding Author: [email protected]; Ph: 864-283-7216

Abstract In this work two material systems, Epoxidized Pine Oil (EPO) and Acrylated Epoxidized Soybean Oil (AESO) both previously individually experimented on were blended in varying ratios, foamed and characterized with respect to their microstructure, physical properties, mechanical performance and thermal stability. Intermediate systems were created bearing a broad range of properties derived from both the parent material systems. The structure property relations were studied and the effect of process parameters on the resulting thermomechanical behavior was analyzed. Finally, the degradation kinetics of the developed foams were studied, and activation energies were calculated for a comprehensive understanding of behavior of foams at high temperatures. The obtained foams exhibited high thermal stability and mechanical strength varying from 0.4 to 11.3 MPa for densities ranging from 0.76 to 0.50 g/cm3 respectively, which enables them to be employed in a variety of applications based on product requirements. Keywords: Epoxy, biobased foams, thermosetting blends, Pine oil, Soybean oil, green chemistry.

Introduction Amidst the multifarious applications of epoxy resins, the burgeoning market for epoxy foams can be ascribed to the acute requirement of materials with high strength to weight ratio. In the automotive and aerospace industry for example, fuel efficiency is a pressing concern and lightweighting presents a viable solution. Lighter, stronger and more durable packaging materials are required to cut down transportation costs. In addition to high specific strength, epoxy foams offer superior thermal and chemical stability along with excellent adhesive strength and dielectric properties. The most commercially available and indispensable epoxies however, such as Bisphenol A Diglycidyl Ether (DGEBA) are based on petroleum byproducts and do not conform with global environmental sustainability.1 The escalating concerns regarding limited petroleum reserves have surged the drive towards exploring biobased material systems with properties comparable to their petroleum-based alternatives.2,3,4 It has however been observed that foams based on pure bio-resins exhibit poor mechanical properties in general. Acrylated Epoxidized Soybean Oil (AESO) foams with 96% biobased content by weight were produced by Bonnaillie and Wool using pressurized CO2, the mechanical strength and modulus of which was poor.5 Dworakowska et al. proposed the formulation of foams based on epoxy derivatives from cardanol foamed using Polymethylhydrosiloxane (PMHS), the resulting foams though exhibited high thermal stability, the maximum glass transition temperature attained was 20°C, which implies that the foams would remain in a rubbery state at room temperature and beyond, rendering them unsuitable for most structural applications.6 Altuna et al. studied the effect of foaming agent (NAHCO3) on the mechanical properties of anisotropic foams based on Epoxidized Soybean Oil, cured using Methyl tetrahydrophthalic anhydride.7 They observed that cell structure changed from closed cell to open cell, followed by an anticipated fall in density and mechanical properties, as the foaming agent was increased. A novel epoxy system based on oils extracted from Schizochytrium microalgae was developed by Negrell et al (2017) and tapped for development of biobased foams based on epoxidized algal oil, and another biobased aromatic epoxy comonomer.8 The effect of aromatic epoxy was studied on thermal and dynamic mechanical properties, however it was chosen only as a minor phase and a full

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spectrum of blends was not investigated. The properties of biobased foams in all the antecedent studies, despite having undergone significant improvement, cannot be juxtaposed with the broad range of properties available in conventional foams, which restricts the number of applications the former can be industrially exploited for. Thus, in order to proliferate the application of biobased foams to allow for sustainable development, it is necessary that such systems be available in all combinations of properties, specific to each individual application, which is lacking in contemporary bioderived cellular systems. Although a few attempts have been made at blending bio-based and synthetic polymers with the help of compatibilizers to yield products with intermediate properties,9 the limited perimeter of fossil fuels tips the desideratum towards finding a solution within the scope of resins derived from bioresources. In a paucity of biobased foams capable of replacing standard synthetic foams used industrially, while several resins derived from bioresources are being assiduously explored in the quest for material systems with desired tailored properties, blending of different bio-resins to arrive at intermediate properties tenders a new, efficacious and viable solution. While multitudinous thermoset copolymer systems have formerly been developed and characterized, limited attempts have been made at developing cellular systems from blended bio-resins. Moreover, the potential of biobased blends derived foams and that of blending as a process to obtain a wide tailored intermediate property spectrum have not been investigated to the best of our knowledge. The forerunning biobased cellular systems foamed chemically were based on an orthodox processing methodology wherein the foaming agent is added along with the resin and the hardener. This allows significant time for cell coalescence before gelation resulting in an undesirable distribution of large cells. A new processing technique developed by Brown et al,10 which was based on delaying the addition of foaming agent to two minutes before gelation utilized the high viscosity of resin near gelation to optimize efficient entrapment of gas bubbles and providing for significant reaction between the foaming agent and the hardener. This resulted in microstructure with smaller cells, which effected a significant improvement in mechanical properties. Glass transition temperatures of the obtained DA foams also reflected improvement. One of the most astonishing aspects of delayed addition foams is the deviation from Ashby Gibson models, the defined structure property relations which have been followed by almost all the preceding cellular systems. In contrast with the other previously used strengthening mechanisms in foamed systems, which require reinforcements and complicated processing routes, delayed addition method mandates a minor and undemanding change, which makes it an attractive and cost-effective processing route for high strength biobased foams. Thus, in this work we utilize the optimization attained in delayed addition to study the behavior of blends under foaming, made from a partially biobased Pine oil derived epoxy resin (EPO) and fully biobased Acrylated Epoxidized Soybean oil (AESO) in varying proportions, in order to evaluate the effect of blending two material systems with disparate behavior on the thermal, physical and mechanical properties of the derived foams and to explore the efficacy of blending in general for optimizing the properties of any cellular system. EPO and AESO have both been individually developed into partially and fully biobased foams, however their blends have not been employed in development of intermediate foams in literature. The cell morphology as well as the mechanical and thermal characterization of foams developed by delayed addition were investigated with respect to the blend ratio and process parameters and structure property relationships were explored. The effects of viscosity in blending and its influence on microstructure as well as properties have been analyzed. In addition to this, degradation patterns of the different blend systems have been studied in accordance with Friedman’s method and the effect of blend composition on the final properties of foams has been analyzed.

Experimental Section Materials Epoxidized Pine Oil (EPO), under the trade name Super Sap 100, was purchased from Entropy Resins (Hayward, CA) while Acrylated Epoxidized Soybean Oil was purchased from Sigma-Aldrich (MO). The polymer blends were cured using biobased polyacrylates, modified by polyamines, provided by Entropy Resins under the commercial name Super Sap 1000. The foaming agent used for this work is Polymethylhydrosiloxane (PMHS), also


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procured from Sigma-Aldrich. EPO and the hardener have 37 wt% and 41 wt % biobased content respectively. Biobased content of epoxy resin was mainly composed of α-pinene oxide, a modified monoterpene, while the synthetic content consisted of diglycidyl ether of bisphenol A (DGEBA). AESO on the other hand was constituted by more than 99 wt% biobased content. The molecular composition of the constituents of commercial EPO, AESO and PMHS is shown in figure 1. EPO resin, AESO resin and curing agent were commercially determined to have densities of 1.14g/cm3, 1.04 g/cm3, and 0.975 g/cm3.All the materials were used as received for foaming.

(a)

(b)

(c)

(d)

Figure 1: Molecular composition of AESO (a), α-pinene oxide (b), DGEBA (c) and PMHS (d)

Processing EPO and AESO resins were mixed in a beaker at room temperature using an overhead laboratory stirrer at 2000 rpm in varying compositions– 20, 40, 60, 80 and 100 vol. % of AESO (see Supporting Information for ratio of components in blends) – for 60 s, post which curing agent was added and these components were further mixed for 90 s. A digital laboratory balance with a precision of 0.1g was used to maintain accuracy in the mass percentage of the various constituents. The blend was then allowed to cure at 25⁰C for the respective calculated minimum gelation time of the blends. The approximate value of gelation time (tGEL) was measured by gently pulling upon the thin wires of the thermocouple and quoting the time at which the wire becomes entrapped in the gel.11 Subsequently, PMHS and 5 vol. % curing agent were added two minutes prior to gelation and mixed to the above-mentioned solution for 60s, following which the mixture was poured in square prism shaped silicone molds measuring 70 and 30 cm in side and depth respectively. The samples were cured for 24 h at room temperature. The final cast foams were subsequently cut to suitable dimensions for characterization in accordance with relevant ASTM standards.

Analysis The apparent density of the foams was calculated by determining the volume and weight of each sample. The mass of the samples was measured by means of a digital laboratory balance. The length, width, and height of all samples were measured using vernier calipers with a precision of 0.01mm. Three readings of each dimension were taken, which were then averaged in order to compute the volume of each sample. Temperatures during crosslinking and foaming were measured by the means of a submerged thermocouple in the sample during the curing process. The microstructure of the foams were characterized using scanning electron


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microscope Hitachi S-4800 at 5 kV accelerating voltage. The surface was fractured at liquid Nitrogen temperature and thereafter coated with platinum via Hummer 6.2 Sputtering System for imaging. Analysis of the SEM images was performed using the software ImageJ. Cell distribution and morphology was studied. The average cell diameter was obtained by using:

𝐷𝑝 =

∑𝑛𝐷

(1)

∑𝑛

where D is the cell diameter and n is the number of bubbles. The glass transition temperature (Tg) was determined using a differential scanning calorimeter TA Instruments DSC Q1000 under helium atmosphere (flow rate 20 mL/min). The samples were heated from 25°C to 130°C at 10 °C/min, cooled to 25°C at 20 °C/min, reheated to 130°C at 10 °C/min. Thermogravimetric Analysis was performed employing TA Instruments TGA 2950. The sample weight was 4-6mg for all compositions and each sample was heated at 10 °C/min from 20°C to 550 °C. TGA tests were performed under nitrogen atmosphere at flow rate 40 mL/min. Integral Procedural Decomposition Temperature (IPDT) was calculated via procedure developed by Doyle (1961), simplified for single step degradation processes by Stefani et al.12 The equations are as follows:

𝐼𝑃𝐷𝑇(℃) = 𝐴 ∗ 𝐾 ∗ (𝑇𝑓 ― 𝑇𝑖) + 𝑇𝑖 𝐴 ∗ = (𝑆1 + 𝑆2)/(𝑆1 +𝑆2 +𝑆3) 𝐾 ∗ = (𝑆1 +𝑆2)/𝑆1

(2) (3) (4)

where A* is the area ratio of total experimental curve divided by the total TGA thermogram, K* is the coefficient of A*, Ti and Tf are the initial and final experimental temperatures respectively. Compression tests were carried out in accordance with ASTM D1621-10 at crosshead speed of 2.5mm/min on Instron 5985 Testing System equipped with a 250 kN 2580 Series Static Load Cell.13 Stress strain plots, compressive moduli, compressive strengths and strain energies were determined in accordance with the standard.

Results and Discussion Analysis of microstructure Figure 2 shows the SEM micrographs of specimens foamed with varying percentage of AESO. The microstructure obtained in all the compositions exhibits multimodal cell size distribution, with a varying density of different cell sizes. A better perspective can be obtained by looking at the normalized cell size distribution curve (Figure 3), It is observed that the average cell size decreases from 0.217 to 0.095 mm with increase in AESO concentration except at 80% AESO, where minimum cell size is attained. The decrease may be due to obstruction to movement of bubbles caused by increasing viscosity of parent resin, which prevents them from coalescing.14,15 At 20% AESO, the distribution is more obtuse, indicating the presence of a variety of cell sizes, however as the percentage of AESO increases, the curves become sharper, due to high concentration of small cells and limited variation in cell size. The average pore diameter is small in case of most cells. This is a consequence of limited isotropic thermal expansion of air bubbles within the matrix, due to high viscosity of resin and limited foaming time allowed in delayed addition method.16 Nonetheless, the small pore diameter is observed to allow for improved performance under compression, and hence is desirable. Since the multimodal cell sizes observed in this case, in foams based on pure AESO and blends was also prominent in pure EPO delayed addition foams formulated by Brown et al ,10 such cell morphology must be a characteristic of the processing method and not the resin. It is well established that multimodal foams offer enhanced toughness and insulation capabilities as compared to unimodal foams based on similar resin systems,17,18 due to which several methods have been investigated upon to produce such cellular distribution in foamed polymers. Most of these methods involve presence of absorbent clays or water to initiate nucleation at different sites and are thus either complicated or limited to thermoplastics.18,19,20 Delayed addition offers an opportune, cost effective method which produces multimodal foamed thermosetting systems by exploiting the effect of changes in the rheology of resin during curing on cell growth and coalescence, and does not involve additives.


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Figure 2:- SEM micrographs of foams at 30X resolution (0% AESO10 (a), 20% AESO (b), 40% AESO (c), 60% AESO (d), 80% AESO (e), 100% AESO (f) )

The minimal eccentricity present in the cells in all compositions can be attributed to restriction in volume expansion in all directions except one, by the silicone molds used. A similar phenomenon was observed by Altuna et al while foaming epoxidized soybean oil in aluminum molds allowing unidirectional volume expansion.7 The cells are sparsely interconnected, which is seen as a small concentration of pore throats which could either be due to incomplete coalescence at gelation or due to contraction of cell wall caused by a conversion of monomer present in the cell wall to polymer. The concentration of interconnected cells decreases with increase in the concentration of AESO, at 80% AESO however, they are exceptionally low in population. Cells with common boundaries can be seen in all compositions which is a characteristic of high density foams, albeit an extremely low density of these as well was seen in 80% AESO foams.


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Figure 3:- Normalized cell distribution: Normalized frequency of cells vs. cell size

A uniform rise in cell density was observed from 10 to 61.5 cells/mm2 (see Supporting Information), except at 20% AESO. The unusually high cell density of 20% AESO composition is a result of high temperature attained during foaming reaction, which in case of 20% AESO was approximately 20 degrees higher than the rest (see Supporting Information). This would have resulted in volume expansion of gases evolved during foaming, however due to a more abrupt gelation in 20% AESO as compared to the other compositions; the cells would have received lesser time for migration and collapse. This is further evidenced by most cells sharing boundaries with adjacent cells, which indicates a stage prior to lamellar rupture of common cell wall, frozen in place by gelation.16 It is reasonable that post curing, when foams return to room temperature, the gas present inside cells would contract effectuating a residual tensile stress in the surrounding walls. This is substantiated by a sharp decline in mechanical properties in compression from 20% to 40% AESO, a gradual decline beyond 40% AESO.21 Conclusively, in cases of foams with poor performance under compression, a possible improvement could be sought by a deliberate introduction of residual tensile stress in cell walls. Explicit heating of the gas involved in foaming, or such other methods with relevant optimization might yield the desired effect in foamed systems of similar kind.

Physical Characteristics The foams produced by blends had bulk densities ranging from 0.55 to 0.763 g/cm3, which is higher than the density of pure EPO foam (0.50 g/cm3). There was a continuous increase in density with increasing AESO content. However, the bulk densities of unfoamed blends decrease from 1.08 to 1.02 g/cm3 as the AESO content increases. Thus, as the concentration of AESO increases porosity and volume expansion ratio decrease since increasing AESO content leads to formation of smaller bubbles and with more uniform cell distribution (Figure 3). The temperatures obtained while curing and foaming processes were analyzed and the maximum temperatures recorded were plotted (see Supporting Information). The maximum temperature attained during curing and foaming is indicative of the amount of energy released during the reactions. Crosslinking and gas evolution transpire as concurrently occurring reactions with separate kinetics which in case of chemical foaming, usually also have a common reactant. This makes the individual reactions difficult to study in terms of the effect of physical parameters on these. It is therefore the variation in difference between the maximum temperatures attained during curing and foaming that must provide some insight into the characteristics of the individual reactions, since the temperature obtained during foaming involves heat released in gas evolution, while that obtained in curing does not. It can be observed in the case of our blend systems that as the AESO concentration increases, the aforesaid difference decreases and the curves overlap towards 100% AESO (see supporting information). The reaction of PMHS with curing agent, which results in evolution of dihydrogen is exothermic and hydrogen evolution peaks at 5.2 minutes,


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which drives the addition of foaming agent 5-6 minutes prior to gel point in delayed addition method.10 The minimal increase in the maximum temperature attained in foaming as compared to curing in higher AESO concentrations, thus suggests that the gas evolution reaction is somewhat inhibited. A possible reason for this, could be the high viscosity of AESO resin. Usually smaller gas bubbles contain higher pressure than larger bubbles and the internal pressure acts as the driving force for bubble growth, until the pressure is equilibrated by the resin viscosity.22 Higher resin viscosity, which implies higher pressure inside bubbles must also thus inhibit foaming reaction due to Le-Chatelier’s effect. This is evidenced in less release of energy and less difference in curing and foaming temperatures at higher AESO concentration. In this and similar cases, to exploit the superior properties of resins which characteristically inhibit foaming reactions and cell growth, blending offers a key solution. Clearly similar to how low viscosity of EPO counterbalances the inhibition of foaming reaction in intermediate resins, the effect of parameters such as high viscosity can be diluted using relevant resins for optimization. It has been established that processing methodologies alter the pore structure and resultant thermomechanical behavior in cellular systems and are extensively used to modify them for specific applications, however in cases where a pure resin does not conform with a foaming methodology due to its physical properties prior to curing, which is also a frequent observation in case of thermoplastics,23,24 blending presents a simple and effective answer.

Mechanical Properties Refer to Supporting Information for the graph showing Stress-Strain response of foams under compression in the direction of rise. For values of strain from 0.03 to 0.06, there was linear elastic deformation, which reflects elastic stretching of cell walls. Post elastic limit, the presence of plateau reflects the characteristic of elastomeric foams. Conclusively, there should be elastic collapse in foams owing to the buckling of cell walls.25 However, as the AESO content is increased, the collapse plateau becomes less prominent and is hardly discernible in 80% and 100% AESO foams, which is unusual for polymeric foams. This mandates an extremely gradual transition from stretching of cell walls to elastic buckling, where the overall behavior of foams with higher AESO concentration is still predominantly elastic. Similar transition trends were obtained in cellulose nanofiber foams by Sehaqui,26 where these were assumed to indicate the high load bearing capacity of cell walls. In this case however, due to an increase in bulk density and smaller cell size at higher AESO concentrations, the cell walls become shorter and more resistant to buckling.27,28 Thus yielding and direct collapse must immediately follow the minimum elastic buckling in walls, causing the plateau to diminish. The compressive strength, modulus and strain energy are listed in Table 1. Strength of the foams decreased from 8.00 to 0.37 MPa when the concentration of AESO was increased. Similar trends were shown by modulus and strain energy. This can be imputed to presence of aromatic rings in DGEBA, which impart rigidity to the structure and aliphatic chains in AESO, which in literature are shown to contribute to overall flexibility.29 Since EPO used in this work is a semi biobased epoxy which has excellent mechanical properties but AESO, on the other hand, is a biobased polymer which possesses comparatively poor mechanical properties, there was an overall decline in properties as the bulk density increased which is inconsistent with literature.11,12,30 Such inconsistency could be attributed to the decrease in density of unfoamed blends with increase in AESO concentration, while that of foams increases. This is due to the high viscosity of AESO which resists formation, growth and coalescence of cells, and thus inhibits the foaming in general. Since in chemical foaming methodologies, a foaming agent reacts with other constituents to release the gas, an increase in the quantity of foaming agent is required to produce the necessary foaming effect in blends containing AESO. In this case however, the rise in foam density due to limited foaming is superimposed on the anticipated fall in mechanical properties, due to which the correlation observed between these disagrees with other antecedent foamed systems in literature. Mechanical properties under compression follow power law relationship with cell size and volume expansion ratio. A negative power law relation was observed with cell density. Material selection procedures for most applications are based on agreement between cost constraints, performance requirements and sustainability concerns, where in most cases, one of the three parameters have to be compromised due to lack of available material systems. It must be emphasized that in the results obtained in our case, a wide range of properties are obtained which is a characteristic of blending. This distinctly allows the freedom to


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attain the precise combination of properties required without necessitating complicated processing routes or investment in locating novel systems. Cellular systems based on bioderived resins answer the sustainability concerns in all aspects. Furthermore, akin to how metal alloying between expensive superior metals with more cost effective lower performance material systems has for long been sought after as the solution, blending in biobased foams might be the answer to the balance between superior properties and cost constraints as well. Parameters such as production cost and time can also be optimized by blending desired resins with those that allow the blends to enter appropriate mass production routes, which the former resin might not conform to. The compelling number of possibilities associated with foaming of biobased blend systems advocate it more as an undemanding commercial solution to material development than one based on exhaustive research and investment. Table 1:- Physical and Mechanical properties of foams. Sample (% AESO) 0 20 40 60 80 100

Density (g/cm3) 0.505±0.012 0.550±0.009 0.603±0.006 0.703±0.015 0.752±0.017 0.763±0.023

ɸ

φ

Strength (MPa)

0.53 0.48 0.43 0.32 0.27 0.25

2.15 1.95 1.75 1.48 1.37 1.33

11.3±2.3 8.01±1.8 5.19±0.57 1.52±0.22 0.37±0.06 0.37±0.07

Modulus (MPa) 130±27 104.0±12.1 39.2±4.3 11.3±1.9 2.7±0.7 2.4±0.6

Strain Energy (MPa) 0.646±0.014 0.546±0.092 0.290±0.034 0.082±0.027 0.020±0.004 0.019±0.005

Thermal Characterization The TGA and DTG curves of the blends are shown in Figure 4. All the foam compositions exhibit single step degradation with maximum weight loss occurring between 375-390°C. An initial rise in the rate of degradation at approximately 220°C is manifested in small peaks observed in DTG curves. Similar peaks were earlier distinguished in pure EPO foams with varying PMHS content at 145°C and attributed to the decay of additives present in either in EPO or the hardener by Brown et al.10 Since in this study, the same grade of EPO has been utilized for blending, although the explanation is further evidenced by absence of such peaks in the DTG curves of 100% AESO foams, the difference in the temperatures at which such a rise is observed might indicate a possible reaction between the aforementioned additives and AESO molecules. The shoulder observed in all the DTG curves between 420-440°C is indicative of decomposition of polyamines based on polysiloxane (PMHS) present in the matrix. Since the siloxane (Si-O) bond retains 37-51% ionic character along with partial double bond character due to pπ-dπ bonding between Si and O,31 and thus possesses higher bond dissociation energy (108 kcal/mol) as compared to C-C, C-O, and even Carom-C bonds, it allows polysiloxanes to sustain for short durations at very high temperatures.32 It is also validated by the small quantity of PMHS used for foaming, which is not enough to generate a peak and appears as a shoulder in the curve.


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Figure 4:- TGA and DTG curves of blends Different thermal parameters signifying the extent of degradation of samples and overall thermal stability have been put together in Table 2 for easy reference. Evidently, all the compositions exhibit high thermal stability, with approximately 10% weight loss at 250°C. It is observed that the onset of degradation begins at higher temperatures for blends with more AESO content. The highest values of T1, T5 and T10 are observed for 80% AESO blend, entailing maximum thermal stability at the latter. Nonetheless, the difference between the aforementioned values between 80% and 100% AESO is little, which suggests that the small rise obtained in temperatures could be due to the slightly less concentration of interconnected cells obtained in 80% blends, which could have resulted in lower thermal conductivity.33 It is possible that due to marginally inefficient conduction of heat to the entire sample, the protected section of the sample only began degrading later while temperature increased. Despite the aforementioned regular increase in thermal stability, the IPDT values unexpectedly decrease with increasing AESO content. Since IPDT values are indicative of half volatilization, sometimes the overall thermal stability is expressed in terms of IPDT. Although it might seem contradictory with the earlier conclusion, Heitor et at describes IPDT as a parameter which relates to the apparent activation energy of the system more than its thermal stability.34 A more comprehensive explanation can possibly be concerned with the nature of bonds in the system. As the compositions near 20% AESO are subjected to heat, the breaking of bonds begins at lower temperature, however as the temperature is increased, the rate of breaking of bonds does not increase as rapidly. On the contrary, bonds in blends near 80% AESO begin breaking at higher temperature, but the rate of collapse increases with temperature and the IPDT consequently falls. Chetan et al illustrated two competing effects of aromatic rings on thermal stability of the resin.35 An increase in substitution in the aromatic ring reduces thermal stability while an increment in the aromatic content in the resin augments it. In this work, the substitution on the aromatic rings present in DGEBA is not affected during foaming, though the aromatic content reduces as AESO content is increased. The thermal stability, described by T1, T5 and T10 is affected directly by the additives present in EPO, since the additives decompose first near the initial peak as discussed before. Thus as the EPO content decreases, the thermal stability increases. However, blends with more aromatic content must require more energy for disintegration of bonds, consequently the rate of breaking of bonds at higher temperatures increases with AESO content. Table 2:- Thermal properties evaluated from TGA and DSC Sample (%AESO)

T1 (°C)

T5 (°C)

T10(°C)

Tpeak(°C)

IPDT (°C)

Tg(°C)


Residue (%)

Ea (kJ/mol)

R2

7


0 20 40 60 80 100

104.58 87.08 88.64 99.12 119.90 114.07

190.10 175.58 193.92 202.63 212.77 203.40

303.65 231.35 244.01 242.95 263.19 261.74

374.79 376.45 391.61 388.70 385.87 389.45

402.32 396.99 392.21 384.41 387.22 384.70

47.43 38.69 41.33 37.10 35.73

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11.20 5.554 4.562 4.003 3.567 2.925

126.74±2.19 96.15±1.38 74.40±1.61 63.49±2.40 52.39±1.07 68.11±1.09

0.970 0.984 0.959 0.881 0.958 0.968

The commercial grade of EPO utilized in this study is a combination of DGEBA, α-pinene oxide and additives. While the molecular mass of both DGEBA and pine oil epoxy is significantly less compared to AESO, the char residue obtained by the decomposition of foam compositions in nitrogen atmosphere anomalously decreases with increase in AESO content. Since the absolute quantity of curing agent and PMHS content is the same in all foam compositions, this atypical variation must arise due to the additives present in EPO. It is however interesting that sudden reduction in char residue is observed by introducing AESO in EPO matrix, whereas a very gradual decline is obtained by increasing AESO content subsequently. A possible hypothesis for this could be concerned with the several side reactions that are expected with the additives, one of which is with the curing agent. An otherwise volatile additive which upon reacting with the curing agent becomes a part of the matrix must contribute in char residue. As the AESO content is increased, the consumption of curing agent must increase, since there are many reactive sites on AESO, which leave limited curing agent to stabilize the volatile additives in the matrix. Due to this, a large quantity of additives is eliminated at lower temperature in TGA in AESO containing foams, and lesser contribute to residual mass at higher AESO concentrations. Degradation kinetics of the system were studied in under non-isothermal conditions. The iso-conversional method developed by Friedman was used for the calculation of activation energies for all the blends.36 The degree of degradation, or the fractional extent of reaction α, at a given temperature T was calculated from:

(5)

α = (mo ―m)/(mo ― m∞)

where m is the weight remaining at temperature T, mo and m∞ represent the initial and final weights respectively. The rate of degradation was subsequently calculated and applied to the fundamental rate equation used for kinetic analysis of degradation in precursory literature:34 𝑑𝛼 𝑑𝑡

(6)

= 𝑘(𝑇)𝑓(𝛼)

where, k(T) is the temperature dependent rate constant, the value of which is defined by the Arrhenius equation:

(7)

𝑘(𝑇) = 𝐴 𝑒𝑥𝑝 ( ― 𝐸𝑎/𝑅𝑇)

The pre-exponential factor A depends on geometrical parameters. Ea, R and T represent apparent activation energy, universal gas constant and temperature respectively. Upon simultaneous solution of the aforementioned equations and taking logarithm, we obtain:

ln

Ea

(dαdt ) = ln [Af(α)] ― (RT)

(8)

A plot between ln(dα/dt) and 1/T was obtained, the slope of which provided the values of apparent activation energy (see Supporting Information). The calculated Ea values have been mentioned in Table 2. The apparent activation energy decreases in general with increase in AESO content, which is consistent with the IPDT values discussed before. DSC curves were used to analyze the Glass Transition Temperatures of the blends. Tg uniformly declines from 47.43OC at 20% AESO to 35.73OC at 80% AESO. Stutz discussed the effect of the various parameters on glass transition in a polymer.37 He defined backbone glass temperature, which is governed by the parent structure, the nature


8

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and concentration end groups, which cause a reduction in Tg and crosslink density. The glass transition temperature of AESO is less than EPO, due to which a gradual drop in Tg values is expected as concentration of AESO increases. However, a sharp decline in Tg values from 20% AESO to 40% AESO, and an increase from 40% to 60% thereupon can be a possible indication of less crosslinking density in 40% AESO. Taking into consideration that the end groups and composition of parent structure is only changing proportionally with increase in AESO content, a non-proportional change at 40% AESO suggests crosslinking density as an explanation for it. Notably, the minimum Tg obtained is still enough to support applications at room temperature. This in addition to the high thermal stability of the developed foams makes these suitable for application in automobiles and packaging, wherein the foams usually operate at room temperature, but high thermal stability is expected for accidental cases.

Conclusion Blended systems created from EPO and AESO were successfully formulated, foamed and characterized. The optimization obtained in delayed addition to maximize mechanical performance was utilized for foaming of blends. All the foamed systems were shown to possess high thermal stability and good mechanical properties while simultaneously maintaining a very high percentage of biobased content. The foams were shown to exhibit multimodal cell distribution. Several advantages of blending as an excellent technique for developing new biobased cellular material systems were explored and possible commercial applications of blending were suggested. The domain of research in biobased thermosetting foams for structural applications is relatively new and unexplored. Limited foams have been developed so far with satisfactory properties, most of which have major drawbacks. In such a situation foaming of blends made from separate biobased materials proposes a judicious solution towards development of a new class of materials: cellular biobased blends, which have been shown in this work to have the potential to dovetail performance requirement, cost criterion and sustainability.

Acknowledgements The authors are grateful to Kimberly Ivey for her assistance in conducting DSC and TGA tests. The authors wish to acknowledge the financial support by Robert Patrick Jenkins Professorship, and Dean’s Faculty Fellow Professorship.

Supporting Information Mixing ratios of components in blend compositions; cell density and cell size variation with AESO content; temperature variation during foaming of blend compositions; maximum temperatures obtained during curing and foaming reaction; stress strain curves for foamed blends in compression; variation of log of degradation rate with inverse of temperature for foam compositions.

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For Table of Contents use only: Table of Contents/Abstract graphic:

Synopsis: Biobased foamed blends of Epoxidized Pine oil and Acrylated Epoxidized Soybean oil, exhibiting a broad range of thermo-mechanical properties are synthesized, and the potential of blending is demonstrated in obtaining tailor-made properties.


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Delayed Addition Foaming of Bio-epoxy Blends: Balancing

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