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Aug 29, 2017 - Department of Automotive Engineering, Clemson University, Greenville, South Carolina 20607, United States. ‡ ... class. KEYWORDS: Pin...
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Research Article pubs.acs.org/journal/ascecg

The Power of Processing: Creating High Strength Foams from Epoxidized Pine Oil Nathaniel F. Brown,†,‡ Sai Aditya Pradeep,† Shubh Agnihotri,†,§ and Srikanth Pilla*,†,∥ †

Department Department § Department ∥ Department ‡

of of of of

Automotive Engineering, Clemson University, Greenville, South Carolina 20607, United States Mechanical Engineering, Clemson University, Clemson, South Carolina 29634, United States Mechanical Engineering, Indian Institute of Technology (BHU), Varanasi, Uttar Pradesh 221005, India Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634, United States

S Supporting Information *

ABSTRACT: The present work reports on the synthesis of foams from epoxidized pine oil (EPO) with polymethylhydrosiloxane (PMHS) used as a foaming agent. The effect of two different processing methods involving modifications of curing agent and foaming agent addition timings was also analyzed. Resultant foams were characterized via density, mechanical and thermal testing, and microstructure. Foams produced using a modified processing method displayed properties that deviate from the Ashby−Gibson models, resulting in superior compressive strengths over many synthetic and biobased epoxy foams, ranging from 6.1 to 11.3 MPa. The impact of the method on cellular microstructure was also significant, with 20- and 30-fold increase in cell density from the original processing method for the same levels of foaming agent in both the methods. Glass transition temperatures of the foams ranged from 61.8 to 97 °C, higher than those of many foams in their class. KEYWORDS: Pine oil, Polymethylhydrosiloxane, PMHS, Epoxy, Biobased, Foams, Ashby−Gibson

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INTRODUCTION Given the increasingly volatile petroleum market as well as growing concerns over limited global petroleum reserves, biobased thermosets are being gradually encouraged in place of petroleum-based thermosets. To replace synthetic epoxies, specifically, many biobased epoxies are synthesized using materials derived from existing food sources, causing concerns to be raised over their sustainability and triggering the debate of food vs sustainability. It is also critical that biobased epoxies have similar mechanical and thermal properties to their synthetic counterparts, many of which have aromatic backbones. However, a large number of current biobased epoxies are composed primarily of triglycerides, which tend to result in lower mechanical and thermal properties.1−3 It is in response to these concerns over properties as well as sustainability that epoxy resins derived from natural rosins present a unique solution. One of the chief methods for obtaining these rosins is via a waste byproduct from wood pulping processes, particularly the kraft pulping process. In this process, only about 50% of the raw wood material is actually used in the creation of the final product, leaving numerous byproducts such as tall oil and turpentine that can be used to synthesize epoxy and curing agent precursors.4−6 Compounding the demand for biobased materials, a demand for lightweight materials has been on the rise in many industries, particularly the automotive, aerospace, construction, © 2017 American Chemical Society

and packaging industries. In the automotive industry, for example, the urgency for these materials is particularly high, considering the reaffirmation of the Corporate Average Fuel Economy (CAFE) standards by the Environmental Protection Agency. These standards, which mandate an average fuel economy of 54.5 miles per gallon by 2025, will primarily be achieved by lightweighting the overall structure of the vehicle. Thus far, polymeric foams have met and will continue to meet these material demands, as their structure can be carefully tailored to the specific end application. Methods involving chemical foaming, physical foaming, and emulsion templating have been explored with respect to cellular epoxy foams with varying degrees of success. Stefani et al. performed some notable work in epoxy foams and utilized a polysiloxane as a foaming agent in conjunction with an amine curing agent.7 A reaction between the latter two was proposed for the evolution of hydrogen gas, which permeated the crosslinked polymer matrix, generating foams with densities below 0.49 g/cm3. Wang also performed work with petroleum-based epoxies, taking advantage of the siloxane reaction to generate even lower density foams, down to 0.097 g/cm3.8 In this case, polymethylhydrosiloxane (PMHS) was utilized as the foaming Received: April 22, 2017 Revised: August 27, 2017 Published: August 29, 2017 8641

DOI: 10.1021/acssuschemeng.7b01253 ACS Sustainable Chem. Eng. 2017, 5, 8641−8647

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ACS Sustainable Chemistry & Engineering agent. Wang observed that the number of active amine groups available from the curing agent dictated the intensity of the foaming reaction. On this knowledge, he developed an alternative processing method in which only part of the curing agent was added upon initial mixing, with the remaining quantity of curing agent added later. However, he analyzed only density and not other critical properties (i.e., glass transition temperature, compressive strength) that resulted from these differences in processing. Biobased foams have also been produced using the three aforementioned methods. However, many of these foams, particularly those that are composed primarily of vegetable oilbased compounds, have very low compressive strengths and moduli, primarily due to the poor properties provided by the cross-linked triglyceride matrix. Bonnaillie et al. synthesized a thermosetting foam using acrylated epoxidized soybean oil with carbon dioxide as a physical foaming agent, but compressive strengths of the resultant foams were only about 1 MPa for a foam density of 0.23 g/cm3.9 Dogan et al. utilized malonic acid to cross-link epoxidized soybean oil, and during the reaction, the decarboxylation of malonic acid monoester produced CO2, which acted as a physical foaming agent.10 Despite having densities near 0.5 g/cm3, foams produced had compressive moduli below 0.3 MPa due to the very low cross-linking density. In this work, we present, to the best of our knowledge, the first synthesis of epoxidized pine oil foams. Given the effectiveness of polysiloxanes in foaming synthetic epoxies, this work sets out to evaluate their effectiveness in foaming a partially biobased polymer instead. Additionally, this work intends to demonstrate improved processing methodologies for the creation of biobased epoxy foams and evaluate the impact of processing on properties, an area which has been neglected in past works.

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Figure 1. (a) DGEBA, (b) polyacrylates modified by polyamines, and (c) polymethylhydrosiloxane.

EXPERIMENTAL SECTION

Materials. The partially biobased matrix used for the synthesis of foams in this work was a pine oil-based epoxy resin (under trade name Super Sap 100), procured from Entropy Resins. That epoxy was cured using an amine hardener (under trade name Super Sap 1000), also procured from Entropy Resins. The manufacturer reports the epoxy resin to have an epoxy equivalent weight (EEW) of 200 g/mol and the curing agent to have an amine equivalent weight (AEW) of 96 g/mol. The epoxy resin was determined to have a density of 1.117 g/cm3, and the curing agent was determined to have a density of 0.975 g/cm3. The epoxy resin is a mixture of DGEBA, epoxidized pine oils, waste and nonfood grade vegetable oils, and benzyl alcohol, while the curing agent is composed primarily of polyacrylates modified by polyamines (Figure 1). Pine oil and pine oil derivatives compose 38% of the epoxy resin and 41% of the curing agent. These pine oil components serve as part of the epoxy and as minor reactive diluents added into the epoxy resin. The foaming agent utilized in this work is polymethylhydrosiloxane (PMHS), purchased from Sigma-Aldrich, and had a density of 1.006 g/cm3 (Figure 1). Processing. Two primary processing routes were developed to produce epoxidized pine oil foams and were designated Immediate Addition (IA) and Delayed Addition (DA). In both processes, PMHS was added in 1 of 4 volume concentrations, with respect to the total blend volume: 0.25, 0.50, 0.75, and 1.00% (see Supporting Information for experimental compositions). These processes are depicted in a flowchart in Figure 2. In both processes, curing began at a starting temperature of approximately 21 °C (as determined from thermocouple measurements of the blends). In the IA process, the epoxy resin and curing agent were added into a beaker in a 2:1 ratio by volume and then mixed for 1 min using an overhead laboratory stirrer at a rate of 2200 rpm. Immediately

Figure 2. Flowchart of IA and DA processes. following this initial mixing, PMHS was added in one of the four volume concentrations and mixed for 150 s. The resultant mixture was then poured into cuboid silicone molds and cured for 24 h prior to removal. In the DA method, the epoxy resin and curing agent were added to a beaker in a 2:0.95 ratio by volume and mixed for 1 min in the same manner as before. Following this mixing, the epoxy and curing agent were cured for 20 min. This time is 2 min less than the gelation time for a 150 g sample cured at 25 °C, according to the manufacturer. The 8642

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ACS Sustainable Chemistry & Engineering remaining 5% of the curing agent needed to attain the 2:1 ratio, along with the appropriate quantity of PMHS, were then added to the partially cured blend. The blend was mixed for approximately 90 s, poured into the molds, and cured for 24 h. Foaming in both processes is based upon reactions proposed by Wang, in which the primary amines of the curing agent react with the central repeating unit of PMHS to evolve hydrogen gas (Figure 3).8

Figure 4. SEM micrographs of IA foams.

Figure 3. Reaction of PMHS with primary amine to evolve hydrogen gas. To support this reaction, an evolved gas analysis via mass spectroscopy (MS) was performed by Netzsch Instruments on a foaming sample of PMHS and the curing agent (Figure 3). MS ion-current measurements for hydrogen gas increased significantly from reaction initiation and peaked around 5.2 min, indicating the rapidity of the foaming reaction (see Supporting Information for MS graph). Analysis. Foams were characterized through a determination of their bulk density. Due to their cuboid geometry, computation of volume was significantly simplified. Measurements of the length, width, and height of each sample were made using digital calipers, averaged, and used to calculate the volume of the sample. The mass of each sample was then measured using a digital laboratory balance. Samples were also characterized via dynamic mechanical analysis (DMA). Testing was performed in a TA Instruments Q800 DMA in a dual-cantilever mode. Samples were cut to dimensions of 70 × 8 × 10 mm. The samples were subjected to a heating condition from −100 to 150 °C at a ramp rate of 3 °C/min. Strain oscillation was held constant at 1 Hz and strain amplitude at 0.1%. Thermogravimetric analysis was performed utilizing a TA Instruments Q5000. Samples were heated from 0 to 550 °C at a rate of 10 °C/min in a nitrogen atmosphere. Compression testing of the samples was completed using an Instron 5985 Testing System with a 250 kN 2580 Series Static Load Cell. Per ASTM D1621-10, samples were compressed at a crosshead rate of 2.5 mm/min. Compressive strength and compressive modulus were obtained utilizing the methods of the standard. Foam microstructure was analyzed via scanning electron microscopy (SEM) using a Hitachi S-4800 scanning electron microscope. Samples for analysis were cut from the center of the foams and oriented for imaging on the horizontal plane. Prior to imaging, samples were platinum sputter coated for 2 min using a Hummer 6.2 Sputtering System. Imaging of the samples was performed with the SEM using a 5 kV accelerating voltage and 30 times magnification. Cell diameter was measured from the images using ImageJ 1.49 with either all cells in the image measured or a minimum of 85 cells measured per image.

Figure 5. SEM micrographs of DA foams.

IA and 0.25-DA (see annotation on 0.25-DA). Pore throats in 0.50-IA through 1.00-IA as well as 0.50-DA occupy significant surface area on the cell wall. However, 0.25-IA and the remaining DA foams have very small pore throats with respect to the overall cell diameter. This phenomenon may be explained by a mechanism proposed by Lau et al. in which lamella layer rupture causes the formation of these pore throats.11 Such rupture of the thin liquid material between the gaseous bubbles would be particularly prominent in the IA foams, as the viscosity of the polymer matrix would be low as the foaming process begins. However, as the degree of cure for the DA foams at the time of PMHS addition is significantly higher than that for the IA foams, the viscosity will have increased to higher levels. This decreases the probability of lamella rupture as foaming progresses. IA foams had a nearly linear relationship between PMHS content and cell diameter with the latter parameter increasing with increase in the former (Table 1). This can be attributed to the ease of coalescence of cells during bubble growth due to low matrix viscosity. These cells will proportionally get larger and coalesce into larger units with increases in the volume of hydrogen evolved by the PMHS−amine reaction. A similar increase in cell diameter for polysiloxane content has been reported by Stefani et al. for epoxy foams.7 While there was an increase in the cell diameter from 0.25-DA to 0.50-DA, cell diameter in higher PMHS content DA foams instead decreased and plateaued for increasing PMHS content. In this case, the combined effects of high viscosity and limited time for foaming in the matrix of the DA foams significantly restricted cellular growth and coalescence, promoting the formation of small cells. Accordingly, cell densities for 0.75-DA and 1.00-DA are 60.2 and 65.1 cell/mm2, 20- and 30-fold increases in cell densities, respectively, over their 0.75-IA and 1.00-IA counterparts. Concurrently, the cell density for 0.75-DA is approximately 8fold higher than that of 0.50-DA, an increase that has never been observed in literature for such a minimal addition of foaming agent.12 It would appear, therefore, that after a certain quantity of PMHS was added, nucleation events peaked in the DA foams, a phenomenon augmented by the addition of

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RESULTS AND DISCUSSION Microstructure. SEM images of foam microstructures for both IA and DA foams indicate a high proportion of elliptical cells with eccentricities approaching unity (Figure 4 and Figure 5). Apart from 0.25-IA, IA foams have a high percentage of cells with visible pore throats or shared connection between cells (see annotation on 1.00-IA). This suggests that a percentage of the foam structure is open cell as opposed to closed cell. The microstructures of the DA foams are formed primarily of closed cells, with visible pore throats connecting a small portion of the cells. Furthermore, dimples on the sides of the cells can be observed where pore throat formation began, such as for 0.758643

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ACS Sustainable Chemistry & Engineering Table 1. Physical, Mechanical, Thermal, and Microstructural Properties of IA and DA Foams sample 0.25-IA 0.25-DA 0.50-IA 0.50-DA 0.75-IA 0.75-DA 1.00-IA 1.00-DA

ρ (g/cm3)

ϕ

φ

± ± ± ± ± ± ± ±

0.51 0.44 0.66 0.61 0.74 0.69 0.75 0.76

2.04 1.80 2.96 2.57 3.87 3.26 3.95 4.11

0.531 0.600 0.364 0.420 0.279 0.331 0.274 0.263

0.008 0.031 0.004 0.008 0.008 0.002 0.007 0.004

σ (MPa)

E (MPa)

± ± ± ± ± ± ± ±

226 ± 28 130 ± 27 158 ± 14 63 ± 12 81 ± 9 87 ± 24 68 ± 11 101 ± 13

12.7 11.3 6.5 6.1 3.4 7.2 3.2 7.5

0.6 2.3 0.4 1.0 0.4 1.5 0.2 1.4

φ=

61.8 71.7 69.1 70.7 74.6 93.7 89.8 97.0

0.06 0.14 0.03 0.06 0.03 0.15 0.05 0.08

d (mm) 0.219 0.217 0.349 0.314 0.536 0.104 0.664 0.097

± ± ± ± ± ± ± ±

0.006 0.016 0.016 0.017 0.025 0.005 0.032 0.003

κ (cells/mm2) 12.0 10.0 6.2 7.6 3.1 60.2 2.0 65.1

± ± ± ± ± ± ± ±

0.4 0.6 0.9 0.7 0.3 5.6 0.1 10.1

(3)

where E is the compressive modulus, C is a coefficient, and constant n is equal to 2 for most materials.14 Figure 6 shows

Figure 6. Application of eq 3 to density and moduli measurements. (1)

plots of the relative moduli and densities for both IA and DA foams, with Em determined experimentally as 784 MPa from cross-linked epoxidized pine oil. It is evident upon observation that the DA foams do not have moduli consistent with this power law relationship. However, parameter n for the IA foams was determined to be 1.75, though the model is by no means a perfect fit. This discrepancy from the value of 2 given by Ashby and Gibson is likely due to the IA foams not being composed solely of open cells. However, even the expanded Ashby− Gibson model for closed cells is insufficient for explaining the significant discrepancy observed for DA foams, suggesting that factors other than density, such as cross-link density, cell density, and cell diameter, have an impact on the moduli. Indeed, the data presented indicates an almost inverted relationship from that presented for the IA foams. Compressive strengths of the foams, reported in Table 1, ranged from 3.2 to 12.7 MPa. The same trends reported for compressive modulus across PMHS content and between processes still apply. However, given the continued decrease in density from 0.50-DA to 1.00-DA, it was anticipated that lower compressive strengths would result. Instead, compressive strengths increased from 0.50-DA to 1.00-DA, something not

ρm ρf

± ± ± ± ± ± ± ±

0.89 0.64 0.52 0.34 0.16 0.47 0.18 0.55

⎛ ρ ⎞n Ef = C ⎜⎜ f ⎟⎟ Em ⎝ ρm ⎠

ρf ρm

Tg (°C)

for the DA foams (see Supporting Information for graphs relating parameters to compressive modulus). The Ashby−Gibson model for open cell foams can be applied to this data as well. The model describes the relationship between the foam and matrix moduli and their densities via

unreacted amines. However, the plateau present in the DA foam cell densities and diameters suggests a depletion of available amine groups in the reaction, preventing the reaction from proceeding further. Quantities of PMHS are sufficiently low at 0.25% PMHS to negate these effects, as nucleation events and cellular growth appeared to be similar for both IA and DA foams at that level. Physical Characteristics. The epoxidized pine oil foams produced had bulk densities between 0.263 and 0.600 g/cm3 with increasing PMHS content resulting in decreasing density (Table 1). Between 0.25 and 0.75% PMHS content, DA foams are observed to have densities higher than those of the IA foams due to the limited time for the evolution of hydrogen gas from the PMHS−amine reaction. However, IA and DA foams had approximately the same density at 1.00% PMHS loading, suggesting an increased rate of reaction for the DA foams at higher PMHS levels that compensates for the aforementioned restriction, as quantities of the reagents used are otherwise the same between IA and DA foams. Given the minimal change in density between 0.75-IA and 1.00-IA, the amine depletion proposed earlier may have a more significant impact on IA foams due to the lower availability of amine groups for the PMHS reaction. Given the neat epoxidized pine oil matrix of 1.08 g/cm3, it is possible to calculate the porosity of the foams and the volume expansion ratio of the foams by the following formulas:

ϕ=1−

strain energy (MPa)

(2)

where ϕ is the porosity, φ is the volume expansion ratio, ρ is the density, subscript f represents the foam, and subscript m represents the neat polymer matrix. From eq 1, it was determined that the porosity ranged from to 0.44 to 0.76, indicating a significant void fraction of the polymer matrix even at minimal loadings of PMHS (Table 1). Mechanical Properties. Foams produced exhibited a range of compressive properties, though it will be shown that not all of these properties follow typical models for cellular plastics. Compressive moduli of the foams ranged from 63 to 226 MPa, with IA foams having higher moduli than DA foams for the 0.25 and 0.50% PMHS loadings (Table 1). IA foams displayed a power law relationship between cell diameter and compressive modulus as well as a linear relationship between cell density and compressive modulus. Furthermore, the density dependence of the compressive modulus was shown for the IA foams, an observation consistent with literature.7,9,13 Notably, moduli dependence on these three parameters could not be established 8644

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ACS Sustainable Chemistry & Engineering typically observed in foams with decreasing density. The impact of the high cell density (∼60−65 cells/mm2) and very low cell diameter, approaching that of microcellular foams, is likely one potential cause of this phenomenon. As with compressive modulus, the dependence of compressive strength on cell density, cell diameter, and bulk density was analyzed, resulting in trends similar to those observed for compressive modulus. It was found that relationships between these parameters existed only for IA foams (see Supporting Information for analysis and graphs). Ashby and Gibson also developed a model relating the elastic yield stress of foams to their relative density, as given by ⎛ ρ ⎞p σ = C ⎜⎜ f ⎟⎟ Em ⎝ ρm ⎠

Utilizing data from compressive tests, the strain energy at 10% core deformation could also be found. This was calculated by taking the area under the curve by u=

∫0

εcd

σ (ε)d ε

(5)

where u is the strain energy, ε is the strain, and subscript cd signifies the value of strain at 10% core deformation. Strain energy is shown to be higher in 0.25-IA and 0.50-IA than DA foams, though 0.75-DA and 1.00-DA have strain energies approximately three times greater than those of their IA foam counterparts (Table 1). This change is likely due to the increased energy absorption properties imparted by the high cell density found in the 0.75-DA and 1.00-DA foams. However, it is difficult to determine the exact relationship between the microstructure of the DA foams and their mechanical properties due to their lack of following any established models. It is evident that there is some correlation between the high cell density and low cell diameter and the superior compressive properties of the DA foams over the IA foams at 0.75 and 1.00% PMHS loading. These superior mechanical properties may also be caused by an increase in cross-linking density of the DA foams over the IA foams, as significant cross-linking will have been completed by the time PMHS and the remaining curing agent is added. Such a change could then be reconciled with the Ashby−Gibson equations, as the matrix compressive modulus and yield strength would be altered rather than remaining constant, as the model assumes. As these foams are synthesized from pine oil-derived epoxy, it is necessary to compare their properties with other foams, both biobased and synthetic. All foams in the following studies demonstrated behavior that approximates the Ashby−Gibson models. With respect to synthetic foams, those of Stefani et al. had similar densities (between 0.15 and 0.55 g/cm3) to the IA and DA foams, enabling comparison of mechanical properties.7 At 0.26 g/cm3, those foams had compressive strengths of approximately 1.5 MPa. However, both IA and DA foams, which are only slightly denser, had compressive strengths at 1.00% PMHS content of 3.2 and 7.5 MPa. This is despite the significantly larger cell diameter of the IA foams, with values up to three times those of Stefani et al. At every density, both IA and DA foams are demonstrated to have a higher compressive strength. This behavior, not generally seen in biobased materials, may be in part due to the aromatic structure found within the epoxy resin used for the manufacture of the IA and DA foams. Several studies of biobased foams with similar density ranges can be found in literature. One such work by Lau et al. focused on the development of macroporous foams with densities between 0.224 and 0.293 g/cm3.11 The highest compressive strength of these foams was 4.9 MPa (at 0.290 g/cm3), which is lower than the compressive strength of 1.00-DA at 7.5 MPa and 0.263 g/cm3. However, the IA foams closely approximated compressive strengths of some of the lower density macroporous foams. Compressive moduli of the IA and DA foams were lower than most of the macroporous polymers, which may be in part due to the macroporous foams being solely closed cell. In another work, Altuna et al. synthesized epoxidized soybean oil foams using sodium bicarbonate as a foaming agent, with resultant densities between 0.19 and 0.5 g/cm3.12 At all comparable density levels, IA and DA foams displayed superior compressive strengths, with superior compressive moduli in IA foams at all levels and in DA foams at most. This is likely due to

(4)

where σ is yield stress, C is a constant, and p is a constant that Ashby and Gibson state is equal to 2 for open cell foams and 3 for closed cell foams.14 While the values for compressive strength obtained via ASTM D1621-10 are not values for foam yield strength, they do provide a close approximation for use with eq 4. Applying this equation to the data for IA and DA foams from Table 1, it is found that the constant p has a value of 2.09 for IA foams, closely approximating open cell behavior. As with compressive modulus, compressive strengths for DA foams do not follow the Ashby−Gibson model, instead appearing to show behavior that would correspond to an inverted model of eq 4 for some lower values of relative density (see Supporting Information for graph). Perhaps one of the most notable aspects of this study is this deviation in the behavior of the DA foams from the wellestablished Ashby−Gibson models. While the IA foams follow those models rather closely, their method of manufacture by mixing all components at once is quite common.7,12,15 In a deviation from that method, Wang proposed the addition of a portion of the total quantity of curing agent at a delayed stage so as to decrease foam density.8 The foaming agent (PMHS) in that study was all added upon the initial mixing of the epoxy resin and curing agent in a 1:0.3 ratio. The remaining curing agent was added at a later stage to bring the ratio of epoxy to curing agent to 1:0.35. Accordingly, simultaneous foaming and cross-linking reactions still occurred in Wang’s procedure, making them similar to the IA foams in that respect. For this reason, Wang’s foams do in fact follow the Ashby−Gibson models and show a negative correlation between cell diameter and foam density, comparable to the trend seen for the IA foams. Wang’s foams also display the plateau in density seen for both the IA and DA foams with increasing PMHS content. The primary difference between the DA process and Wang’s modified procedure proceeds from an understanding of the speed of the reactions involved in the foam production. As was determined via mass spectroscopy, rate of reaction between PMHS and the curing agent is highest at approximately 5.2 min. Accordingly, minimal cross-linking would have occurred in Wang’s method by the time that reaction had peaked. This would allow for a significant amount of time for bubble coalescence, resulting in very large cells. In the DA process, however, because significant cross-linking will have occurred by the time PMHS and the remaining curing agent are added, the matrix has a very high viscosity and is approaching gelation, resulting in the evolved hydrogen gas forming small cells and higher cell densities in the final foams. 8645

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ACS Sustainable Chemistry & Engineering

are either comparable to or lower than those for the IA and DA foams.11 This may be due to a difference in matrixes, with the foams of Lau et al. likely synthesized from an epoxy with a triglyceride backbone. Loss moduli and storage moduli curves for IA and DA foams show that while PMHS content dictates the relative moduli for each processing type, differences in the curves between IA and DA methods are otherwise negligible. Loss moduli and storage moduli graphs for IA and DA foams are given in the Supporting Information.

the stiffer matrix and much higher cell densities of the pine oil foams. Accordingly, the properties of the foams with respect to both synthetic and biobased ones indicate that they may be suitable replacements for those foams, particularly when higher compressive strength is required. Thermal Properties. Differential thermogravimetric (DTG) curves for IA foams are presented in Figure 7. Due

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CONCLUSIONS It was demonstrated in this paper that the epoxidized pine oil foams synthesized using the Immediate Addition and Delayed Addition methods can serve as sustainable alternatives to typical petroleum-based foams without sacrificing mechanical and thermal performance. Also, by modifying the timing of reagent additions, a new and simple manufacturing method was developed that improves foam properties at higher foaming agent levels. While the mechanical properties of the IA foams can be modeled and predicted using the Ashby−Gibson equations, the DA foams manufacturing using this new method do not show evidence of following known models for foams. Further work is required to explain the unusually high cell density and compressive strength for the DA foams and the exact way in which the DA method impacts the foaming reaction. Nonetheless, the partially biobased pine oil foams synthesized indicate the possibility of ones from purely biobased epoxidized pine oil being synthesized in the future.

Figure 7. DTG curves for IA foams.

to the similarity of virtually all curves, only those from IA are presented here (TGA and DTG curves may be found in Supporting Information). From an examination of these curves, it is evident that PMHS content has negligible effect on either the thermal decomposition or rate of thermal decomposition. All foams have major mass loss occurring at approximately 370−390 °C, a variation that can be attributed to testing and experimental error. Notably, minimal mass loss (approximately 1%) occurs in the range from 0 to 100 °C, indicating high thermal stability of the foamed polymer for those temperatures. There is, however, an increased degradation of the foamed polymer centered around 145 °C, which may be caused by the decomposition of additives in either the curing agent or epoxy resin, not by a breakdown of the cross-linking. From DMA, loss modulus, storage modulus, and tan δ curves were obtained (see Supporting Information). From the tan δ curves, the glass transition temperature (Tg) of the foams could be estimated from the temperature of the curve peaks. Tabulated values for Tg are given in Table 1 with the Tg of EPO cross-linked with the partially biobased curing agent determined to be 64.7 °C for reference. From these values, it is evident that as the PMHS content increases, there is a resultant increase in Tg, potentially due to the presence of the modified PMHS structure formed from the primary amine−PMHS reaction (Figure 3). The difference in Tg between IA and DA foams further reinforces this possibility, as the simultaneous reactions occurring in the formation of IA foams inhibits the complete reaction of PMHS with the primary amines, as many of those amines will be involved in cross-linking. Concurrently, those simultaneous reactions may inhibit cross-linking to some extent, also resulting in lower Tg. This may be the reason why 0.25-IA has a lower Tg than cross-linked EPO. However, in the DA foams, PMHS can react almost exclusively with non-crosslinked amines, thus minimizing the effect of the aforementioned simultaneous reactions. IA and DA foams at high PMHS content had similar Tg values to those of Stefani et al., whose synthetic foams had Tg near 98 °C.7 Tg values of comparable biobased foams synthesized by Lau et al. range from 67.7 to 85.3 °C, which

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01253. Experimental compositions for IA and DA foams; compressive strength and modulus dependence on cell density, cell diameter, and bulk density; the Ashby− Gibson model for yield strengths; TGA and DTG for IA foams and DA foams; DMA for IA and DA foams; and a mass spectroscopy curve for the reaction of PMHS and the curing agent (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: 1-864-283-7216. ORCID

Srikanth Pilla: 0000-0003-3728-6578 Author Contributions

N.F.B. carried out all experiments and analyses. S.P. conceived the experiments and directed the research. S.A.P. and S.A. assisted with the Delayed Addition experiments. All authors participated in discussions of the research and N.F.B. and S.P. wrote the manuscript. The authors have all given approval to the final version of the manuscript. Funding

The authors are grateful for the funding support of the Clemson EUREKA! Program and Clemson Creative Inquiry. One of the authors (S.A.P.) would also like to thank Sonoco for the support of its fellowship. 8646

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ACS Sustainable Chemistry & Engineering Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Kimberly Ivey for her support of this project’s thermal and mechanical analysis via TGA and DMA performed on dozens of samples. The authors also appreciate Dr. Fadi Abu-Farha for allowing the use of his Instron testing machine for conducting mechanical testing. The authors would also like to thank Netzsch Instruments for their analysis of several samples via TGA-MS and Entropy Resins for chemical information regarding the resins used in this research.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b01253 ACS Sustainable Chem. Eng. 2017, 5, 8641−8647