Polysulfides Synthesized from Renewable Garlic Components and

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Polysulfides Synthesized from Renewable Garlic Components and Repurposed Sulfur Form Environmentally Friendly Adhesives Cristina Herrera, Kristen J. Kamp, and Courtney L. Jenkins ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11204 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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ACS Applied Materials & Interfaces

Polysulfides Synthesized from Renewable Garlic Components and Repurposed Sulfur Form Environmentally Friendly Adhesives Cristina Herrera, Kristen J. Kamp, and Courtney L. Jenkins*† Department of Chemistry, Ball State University, 2000 W. University Ave, Muncie, IN 47306 United States KEYWORDS adhesion, sulfur, inverse vulcanization, garlic, petroleum waste, polysulfides, natural monomer

ABSTRACT Natural materials have been used as glues throughout human history. Over the last century, society has come to rely heavily on synthetic, petroleum-based adhesives instead, consuming ~ 14 million tons per year. In recent years, however, there has been a resurgence of glues formed with renewable materials. This work seeks to integrate the two to form strong adhesives. Here, elemental sulfur was combined with diallyl sulfide (DAS), diallyl disulfide (DADS), and garlic essential oil (GEO) to form adhesive polymers from recycled petroleum waste and renewable monomers. The labile sulfur bonds in DADS and GEO allowed these monomers to be homopolymerized, forming

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polysulfides entirely from renewable monomers. Heating these materials causes them to transition from viscous liquids to hardened solids. A family of copolymers containing different garlic components and varying sulfur to monomer ratios were synthesized, characterized, and tested for this study. Polymer structures were confirmed by 1H NMR. Changes to the polysulfide material properties upon curing were examined by gel permeation chromatography and differential scanning calorimetry. Characterization data of cured polymers were used to choose the optimal cure temperature for adhesion studies. The adhesion strength of polysulfides with varying compositions was determined by single lap shear testing. Strong bonding was obtained for all garlic based polysulfides with strengths three times higher than commercial hide glue.

INTRODUCTION From the animal-based collagenous adhesives of early humans, to the wide ranging use of natural products by ancient Egyptians, glues have been used prolifically for millennia.1-2 Industrialization of adhesives began in 1690 with hide and starch glues dominating the field into the early 1900s.1-2 Even into the 1920s, the adhesive industry was composed almost exclusively of natural glues.2 However, with the development of phenol formaldehyde, synthetic adhesives began to displace natural materials as glues, with dozens on the market by the early 1940s.2 The unmatched durability and water resistance of these adhesives led to the rapid transition to synthetics, which are mainly composed of petroleum sourced components.2 In recent years there has been a concerted effort to return to natural polymers without losing the high strength and durability provided by synthetic glues. Many groups have begun utilizing renewable materials to develop glues suitable for a wide range of applications. Protein-based products have shown promise as tissue adhesives.3-4 Both polyurethanes and epoxies, which have


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more widespread applications, have seen a growth in the incorporation of natural components.5-9 Wood glues have begun to utilize a variety of plant-based materials to improve bonding.10-11 Mussel mimic chemistry in conjunction with renewable monomers have also provided strong bonding materials.12-16 Polysulfides have been prominently used as sealants since the 1960s.17 Despite their primary use as sealants, polysulfides can also serve as adhesives, and are known for being both flexible and resistant to water and solvents.18 Since 2013 there has been renewed interest in utilizing elemental sulfur (S8) in polymer matrices due to the development of inverse vulcanization.19 During petroleum refinement, sulfur is stripped from the organic molecules in crude oil to prevent the formation of sulfur dioxide gas when burned, which leads to the formation of acid rain.20 Limited applications for S8 has led to millions of tons of sulfur waste.20 Inverse vulcanization employs the ring opening of S8 at high temperatures (>159 °C) forming diradicals capable of initiating polymerization to form polysulfides.19 These sulfur-based polymers have been used in a variety of applications including materials for infrared optics,21-23 cathodes,19,24-25 and water purification,26-30 among others,31-36 with a variety that utilize the dynamic sulfur bonds for repair or recycling.37-39 Although inverse vulcanization began by primarily utilizing aromatic hydrocarbons, the field has grown substantially to include a wide range of monomers.40 A subset of renewable monomers have been explored including cardanol benzoxazine derived from cashew agro waste,41 limonene from citrus,42 myrcene from thyme,24 botryococcene from algae,43 eugenol from clove oil,44 perillyl alcohol from lavender,45 farnesol from acacia tree flowers,30 terpinolene from a variety of plant oils,46 squalene from shark liver oil,45 and vegetable oils.28, 46-47 In an effort to create adhesive copolymers from natural components and repurposed sulfur waste, organosulfur compounds from garlic were explored in this study. The distinct smell of garlic


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is due to a variety of sulfur containing compounds. Alliin, which accumulates naturally in stored garlic cloves, is converted to diallyl thiosulfinate (allicin) when the bulb is damaged.48-50 Allicin rapidly decomposes to sulfides including diallyl sulfide (DAS), diallyl disulfide (DADS), and diallyl trisulfide (DATS), among others.48 The monomers DAS and DADS were chosen to provide purified model systems of the more complex garlic essential oil (GEO) due to their low cost and commercial availability. Components in garlic have relatively high boiling points making them compatible with inverse vulcanization. Although allyl groups can be challenging to polymerize, prior work has demonstrated the successful synthesis of poly(S-DADS) by inverse vulcanization.24, 51 The polymer showed promise as a cathodic material for Li-S batteries, adding a sustainable material to the list of beneficial cathodic materials formed by inverse vulcanization.24, 52-54

Here we seek to develop adhesives from renewable monomers and elemental sulfur that has been repurposed from the petroleum industry. This work examines the formation of polysulfides from DADS and DAS as well as the less processed GEO. Distillates used to create GEO contain a variety of sulfides with DADS and DATS as the primary components.48 Polymerization requires short reaction times with no additional solvent needed to form strong, environmentally friendly glues.

RESULTS AND DISCUSSION Synthesis and Characterization The allyl groups present in many garlic components are challenging to polymerize using standard radical polymerization techniques.55 Inverse vulcanization, however, works by generating a large number of sulfur radicals that are able to readily form polysulfides with allyl groups.24, 51,


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Garlic components diallyl sulfide (DAS), diallyl disulfide (DADS), and garlic essential oil

(GEO) were individually combined with elemental sulfur (S8) and heated at 180 °C for one hour (Figure 1A). Sulfur radicals can bind directly to double bonds forming cross-links and sulfur loops or initiate radical propagation along the hydrocarbon backbone.19 This dual mechanism allows rapid polymer formation of dark orange-black copolymers, poly(S-DAS), poly(S-DADS), and poly(S-GEO). A. S

B.

S

S

S

S

S S

poly(S70%-GEO30%)

S

S

S

S

S

S

S

S

S

S

+

diallyl sulfide

poly(S70%-DADS30%)

180 C

S

S

S

n

S

S

S

m

S

S

poly(S70%-DAS30%)

S S

S

S

m

S S 8

7

6

5

4 ppm

3

2

1

0

Figure 1. (A) Formation of poly(S-DAS) from elemental sulfur and diallyl sulfide. Diallyl disulfide and garlic essential oil were also used to form polysulfides. (B) 1H NMR spectra of poly(S-DAS) (red), poly(S-DADS) (green), and poly(S-GEO) (blue) with the 0.8 to 4 ppm region enlarged to show more detail.

Polymer structures were confirmed by 1H NMR spectroscopy (Figure 1B). All three polymers resulted in very similar spectra showing the presence of both propagation (~1.0 to 1.5 ppm) and the appearance of additional C-S bonds from the reaction between sulfur and allylic carbons (~2.5 to 3.5 ppm). Poly(S-DADS) had complete monomer conversion for all samples as indicated by the lack of allyl peaks at 5.0 and 5.7 ppm. Trace amounts of monomer were detected for poly(S10%GEO90%). Incomplete monomer incorporation was also observed for poly(S-DAS) at low sulfur


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contents (10% and 20% S) (Figure S1). Very high sulfur contents, above 80%, created solid materials with limited solubility that made characterization by NMR difficult. A.

DAS

DADS

GEO

B.

S S

S

S

S

S

diallyl disulfide

Figure 2. (A) Image of diallyl sulfide (DAS), diallyl disulfide (DADS), and garlic essential oil (GEO) after heating at 180 ˚C for 1 hr. (B) Schematic showing how the dynamic sulfur bonds in DADS allow homopolymers poly(S-DADS) and poly(S-GEO) to form.

All three garlic-based monomers were subjected to the same reaction conditions as the copolymers in the absence of sulfur. Both DADS and GEO were successfully homopolymerized. There was some monomer remaining in poly(GEO), but poly(DADS) reacted completely based on 1H NMR analysis (Figure S2). Diallyl disulfides readily undergo thermal rearrangement to thiosulfoxides57 and homolytic cleavage forming sulfur radicals at higher temperatures (Figure 2).58-59 These labile disulfide and trisulfide bonds present in DADS and GEO enable polymerization. The thioether linkage present in DAS, however, limits the reactivity of this monomer preventing homopolymerization.58 Although these monomers have only been used to initiate homopolymerization here, the dynamic sulfur bonds in polysulfides have been used to initiate the polymerization of additional monomers.60 Successful formation of poly(GEO) and poly(DADS) indicate that these natural monomers may be used to initiate the polymerization of other monomers.


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Analysis by gel permeation chromatography (GPC) revealed the garlic-based polymers had relatively low molecular weights ranging from 1,200 to 6,400 g/mole (Table S1). In general, a slight decrease in molecular weight was observed as the sulfur content increased. In an attempt to increase the molecular weight, polymers underwent additional curing for 24 hrs at 80 °C, 100 °C, and 160 °C. Cure temperatures were chosen to represent the temperature range at which radicals form either once sulfur is extended into a linear chain (80-100 °C) or when it is in the cyclic S8 form (160 °C).59, 61 B.

A. no cure cured at 80 °C cured at 100 °C cured at 160 °C

refractive index signal

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poly(S30%-DAS70%) poly(S70%-DAS30%)

Time (min) D.

C.

cure:

RT

80 oC 100 oC 160 oC

30% S

70% S

Figure 3. (A) Gel permeation chromatography curves of poly(S30%-DAS70%) before cure (black), and after curing for 24 hrs at 80 ˚C (red), 100 ˚C (blue), and 160 ˚C (green). (B) Percent solubility of poly(S30%-DAS70%) (black) and poly(S70%-DAS30%) (red) at various cure temperatures in dichloromethane (DCM). (C) Images of poly(S70%-DAS30%) dissolved in DCM at 25 mg/mL. (D) Soluble portion of poly(S-DAS) with 30% S (left) and 70% S (right) when cured at room temperature, 80, 100, and 160 °C from left to right. Curing polymers at 80 °C caused an increase in the molecular weight for polymers containing both 30% and 50% S. Poly(S30%-DAS70%) demonstrated a substantial increase from a weight


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average molecular weight (Mw) of 5,720 g/mol to 24,190 g/mol (Figure 3). When the sulfur content was increased to 70%, a decrease in solubility upon curing made solvent-based analysis difficult. Further increasing the cure temperature of poly(S30%-DAS70%) to 100 °C, decreased the molecular weight, likely due to the concurrent decrease in solubility in dichloromethane (DCM) (Figure 3B). Since polymer molecular weight is inversely related to solubility,62 higher molecular weight samples may no longer be soluble in DCM, which would prevent the GPC from providing a complete picture for these samples. This effect was observed more prominently when the cure temperature was increased to 160 °C. Solubility was quantified by dissolving each polysulfide at 25 mg/mL in DCM, resulting in a soluble portion and insoluble portion (Figure 3C). Even with additional DCM, the insoluble portion did not dissolve any further. Other inverse vulcanized polymers in prior work have also demonstrated this heterogeneity.60 There are likely two factors affecting solubility, sulfur content and cross-linking. Previous studies have shown that high sulfur content polymers have limited solubility.19, 60 Cross-linking that is possible due to the use of difunctional monomers would also lead to insolubility. However, sulfur can also bind to form sulfur loops rather than forming crosslinks, which has been proposed to explain the high solubility observed in some inverse vulcanized systems.19, 43 Curing may lead to dynamic sulfur bonds rearranging to form more cross-links rather than S loops. Additionally, curing may also drive off the more volatile, oligomeric species. As the polymer molecular weight increases, there tends to be a decrease in polymer solubility.62 The change in solubility limits the validity of the molecular weight data for polymers cured at 100 °C and 160 °C. There has been evidence that hydrogen sulfide is produced during the synthesis of some inverse vulcanized polymers.65 Although there is a malodor upon curing, this is likely due to the presence


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of mercaptans which have been observed upon polysulfide curing.18,66 However, further testing would need to be conducted for confirmation. To avoid challenges with solubility, infrared spectroscopy and differential scanning calorimetry (DSC) data were collected. Infrared spectroscopy revealed the loss of allyl peaks at 3082 and 1634 cm-1 for all copolymers with very similar spectra regardless of sulfur content (Figure S3). However, small allyl peaks could still be detected for poly(DADS) despite the 1H NMR data showing complete monomer conversion likely due to slight differences in sensitivity (Figure S2). When analyzed by DSC, copolymers had very low glass transition temperatures (Tg) ranging from -24.1 to -9.2 °C, prior to curing (Table S2). Varying the sulfur content had an impact on the viscosity of the polymers with low sulfur feeds producing viscous liquids ranging from the consistency of mineral oil (20% S) to honey (50% S) to putty (80% S). Samples consistently showed complete S8 incorporation by lack of melting peaks at 119 °C. A. Heat Flow (mW)

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cured at 180 °C cured at 160 °C cured at 100 °C cured at 80 °C no cure

B.

C.

Temperature (°C)

Figure 4. (A) Differential scanning calorimetry curves of poly(S70%-DAS30%) before cure (black), and after curing for 24 hrs at 80 ˚C (red), 100 ˚C (blue), 160 ˚C (green), and 180 ˚C (orange). Poly(S-DAS) (B) before cure and (C) after a 24 hr cure at 160 oC. Curing copolymers for 24 hrs produced materials with higher Tg values (Figure 4A). An increase in Tg was observed as the cure temperature increased up to 160 °C. At low cure


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temperatures, 80 °C and 100 °C, the Tg remain quite low. These data are similar to prior results obtained for poly(S-DADS), which were subjected to similar cure conditions.24, 52 At higher cure temperatures, polysulfides transitioned from viscous liquids to solid materials (Figure 4B and C). At the structural level, curing may cause sulfur bonds to rearrange, forming more cross-links rather than S loops. High cure temperatures would also cause the evaporation of low molecular weight species. These changes, especially in combination, would cause an increase in the Tg.63 Similar Tg values of 56.1 and 52.6 °C were observed when cured at 160 °C and 180 °C respectively, indicating that these materials were fully cured at both temperatures. Therefore, 160 °C cure was used for further testing. Adhesion Testing Liquid polysulfides were spread onto Al adherends for mechanical testing. The adherends were sanded to increase surface roughness, allowing the low molecular weight polymers to easily wet and interlock with the surface. Upon curing, these polysulfides transition from viscous liquids to solids, corresponding to an increase in Tg and Mw (Figures 3 and 4), which typically corresponds to improved material strength.64 The adhesion for all samples was tested in lap shear using an Instron Materials Testing System. The maximum force at break was divided by the overlap area to obtain the adhesion strength in Pascals (Pa). The S to monomer content was varied to determine the impact on adhesion strength. The most substantial change in adhesion strength was observed for poly(S-DAS) (Figure 5A). When no sulfur was present, the sample was primarily composed of monomeric DAS, so the low adhesion strength obtained was expected. Even at 20% S, there was still a substantial amount of monomer present (Figure S1), which likely limits the adhesion strength. Upon complete monomer


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incorporation at 30% S, major gains in adhesion strength were observed followed by a gradual increase in adhesion strength up to a maximum of 1.61 MPa at 70% S (Figure 5B).

A.

B.

p(S-garlic essential oil) p(S-diallyl disulfide) p(S-diallyl sulfide)

Sample Adhesion Strength (MPa) poly(S50%-GEO50%)

1.90 ± 0.19

poly(S70%-DADS30%)

1.86 ± 0.23

poly(S70%-DAS30%)

1.61 ± 0.11

hide glue

0.54 ± 0.17

Figure 5: (A) Adhesion strength of garlic based polysulfides with varied S content. (B) Maximum adhesion strength at break for poly(S-DAS), poly(S-DADS), poly(S-GEO), and commercial hide glue and the 95% confidence interval.

Poly(S-DADS) and poly(S-GEO) have more similar trends in adhesion strength (Figure 5A). This is likely due to DADS being one of the most abundant compounds in GEO.49 Additionally, many other compounds in GEO contain labile sulfur49 leading to similarities in polymer formation. Some of the homopolymer samples had regions with voids between the polymer and the substrate, possibly due to remaining monomer being driven off during cure, leading to less consistent samples and the large error for these particular samples. There are no considerable improvements in strength with poly(S-DADS) until there is 50% S present with a maximum adhesion strength of 1.86 MPa at 70% S (Figure 5B). Poly(S-GEO), however, exhibits a steady increase in strength up to 1.90 MPa with 50% S. The presence of more labile S present in GEO, along with the added elemental sulfur may lead to poly(S-GEO) reaching its highest adhesion strength at lower added sulfur content than polymers synthesized with DAS and DADS.


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Typically, polysulfides are limited by their cohesive bonding (bulk strength) rather than adhesive bonding (surface interactions).67 However, there can be challenges in bonding to high energy surfaces such as aluminum, which have an adsorbed water layer on the surface when exposed to air.68 Relatively nonpolar polysulfides can have difficulty removing this water layer to create strong interfacial bonds. This phenomenon was observed here with polymers demonstrating more adhesive failure at low sulfur contents. As the sulfur content was increased, these materials began to exhibit a mixture of both failure modes. Including more sulfur may provide additional sites for molecular interactions such as hydrogen bonding to the sulfur atoms. There may also be some oxidation of the sulfur atoms since both the reaction and cure occur in air. In either case, increased polarity of the polymer may help displace any adsorbed water providing better interaction between the polymer and aluminum surface thus increasing bond strength. All copolymers had a substantial decrease in adhesion strength when composed of 80% S. Other scientists have reported crystalline regions due to long sulfur loops present in inverse vulcanized polymers at high sulfur contents.19, 24, 53 Although this was not observed by DSC for these polymers, it is possible that some crystalline regions exist in very small quantities. This would lead to increased brittleness in these materials allowing cracks to form and propagate more easily causing material failure. Polysulfides composed of 80% sulfur demonstrated cohesive failure exclusively. However, all other materials demonstrated mixed failure modes. Typically, this balance of cohesive and adhesive strength is required to develop strong bonding glues.69 Despite the dynamic nature of S-S bonds, bonding could not be repaired for these adhesives. The high temperature cure leads to materials being quite brittle. It is possible that alternate curing methods may have the potential to produce self-healing materials. However, further testing is required.


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To examine the versatility of these polysulfides, the strongest formulations [poly(S70%DAS30%), poly(S70%-DADS30%), and poly(S50%-GEO50%)] were also tested on pine wood and polytetrafluoroethylene (PTFE) adherends. Much weaker bonding was observed on these substrates with all bond strengths ≤ 0.2 MPa. The thick formulations that worked well on aluminum may have limited penetration and interlocking with the pine surface, which often aids in adhesion to wood. Low surface energy plastics including PTFE are notoriously challenging to stick to. Poly(S50%-GEO50%) obtained 0.1 MPa on PTFE. Although this adhesion strength is much lower, even materials that demonstrate very strong bonding to aluminum, 11 MPa, demonstrate weak bonding, 0.2 MPa, to PTFE.70 At the ideal sulfur to monomer ratio, each of the garlic-based polysulfides demonstrated adhesion more than 3 times stronger than commercial hide glue (Figure 5B). Even when comparing completely natural systems, poly(GEO) versus hide glue, nearly double the strength is observed for glue made from 100% garlic essential oil. Other polysulfides have reported lap shear strength on aluminum of ~0.2 to 0.5 MPa,71-72 indicating that these garlic based polysulfides demonstrate substantial gains in strength relative to some synthetic systems as well.

CONCLUSIONS As society seeks to improve the environmental friendliness of materials, bio-based adhesives have become the goal of the future instead of the past. This work demonstrates the successful development of solvent-free adhesives from renewable monomers and recycled elemental sulfur. Short reaction times and the absence of additional solvents make this synthesis highly attractive. The low Tg and Mw of the garlic-based polymers allows them to be easily spread onto Al adherends without the need for solvents. Copolymers transition from viscous liquids to hardened materials


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as they cure, which is commensurate with an increase in Tg and molecular weight providing mechanical interlocking that makes these polysulfides viable as adhesives. Adhesives made from garlic components and elemental sulfur display stronger adhesion than commercial hide glue. These polymers utilize S8 waste formed in abundance during petroleum refining and incorporates renewable monomers to create adhesive polymers. From start to finish, these polysulfides offer an environmentally friendly glue that delivers the high strength and durability expected of synthetic glues.

EXPERIMENTAL SECTION Materials Sulfur, 99.5-100% was obtained from Sigma-Aldrich. Diallyl disulfide (DADS), 80%, and diallyl sulfide (DAS), 97%, were purchased from ACROS Organics. Garlic essential oil (GEO) from allium sativum was obtained from Plant Therapy. Aluminum sheets, 6061-T6, and scrim cloth (thickness of 0.18 mm) were purchased from McMaster-Carr. ScotchTM Magic tape (thickness of 0.053 mm) was used to control the adhesive application area. Titebond® Hide glue was purchased at a local hardware store to use as a commercial comparison. Polysulfide Preparation Sulfur and one diene (DAS, DADS, or GEO) were combined in a 1 dram vial equipped with a magnetic stir bar. Samples were placed in an oil bath at 180 oC for one hour. All samples were made on a 1.25 g scale. The S:monomer ratio varied from 0:100 to 80:20. Some polymers were then cured in a Precision 45EG oven for 24 hours at 80, 100, 160, or 180 oC. Characterization Methods


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400 MHz JEOL proton nuclear magnetic resonance (1H NMR) spectroscopy was used to confirm the polymer structure. Gel permeation chromatography (GPC) was used to determine the molecular weight of the polymers both before and after curing. An Agilent 1260 infinity series was used with dichloromethane (DCM) eluent. Polymer solubility was tested by measuring 25 mg polysulfide into a pre-weighed vial and dissolving it in 1.0 mL DCM. Samples were vortexed and allowed to dissolve overnight. The soluble portion of each sample was removed. The insoluble portion was weighed once dried to determine the percent solubility. The glass transition temperatures (Tg) were examined by differential scanning calorimetry (DSC) using a Perkin Elmer Pyris 1 DSC with a 2P intra cooler. Polymers were heated from -50 oC – 160 oC at 10 oC per minute to measure the Tg. Adhesion Testing Aluminum adherends were cut by water jet to 8.9 x 1.2 x 0.32 cm. The aluminum surface was scuffed using P150 sandpaper. ScotchTM tape was placed on each Al adherend marking off 1 cm x 1.2 cm. Poly(S-DAS), poly(S-DADS), and poly(S-GEO) polymers were used for adhesion testing. The sulfur content in each of these was varied from 0% to 80% by weight with the garlic component making up the remainder of the material. Garlic-based polymers were spread onto Al adherends using a razor blade until the polymer was even with the tape to control the quantity of adhesive on each sample. Scrim cloth (1.0 cm x 1.2 cm x 0.018 cm) was placed atop the polymer on one adherend to control the bond thickness. Adherends were then overlapped in a lap shear configuration and cured at 160 °C for 24 h. Samples were tested following a modified ASTM Standard D1002 method in tensile at 2 mm/min on an Instron Materials Testing System after 24 hours. Commercial hide glue was applied, cured, and tested under the same conditions. Adhesion strength in MPa was determined by dividing the maximum strength at break by the overlap area.


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Adhesion values are an average of at least 5 samples with 95% confidence intervals shown as error bars.


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ASSOCIATED CONTENT Supporting Information. Proton NMR of poly(S-DAS) with varied S content, proton NMR of DAS, DADS, and GEO monomers and homopolymers, IR spectra of DADs, poly(DADS), and poly(S-DADS) at varying sulfur contents, table of molecular weight and polydispersity index data for polysulfides, and table of glass transition temperature values (PDF)

AUTHOR INFORMATION Corresponding Author *Email:[email protected] Present Addresses †Department of Chemistry, Idaho State University, 921 South 8th Ave, Pocatello, ID 83209. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Many thanks are owed to Heather Siebert and Dr. Jonathan Wilker at Purdue University. Heather Siebert prepared aluminum adherends and coordinated access to the Instron Materials Testing System. Dr. Jonathan Wilker provided guidance and the instrumentation necessary to conduct adhesion testing. An additional thank you is owed to Nicole Anderson who helped finalize the collection of a few pieces of characterization data.


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FOR TABLE OF CONTENTS ONLY S

S

allyl sulfides

S

+

S S

S

S S

S

n

S

m

S

S

S

S

S

S

m

petroleum waste

adherend

S S

S S

garlic essential oil

S

S S

glue adherend


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Polysulfides Synthesized from Renewable Garlic Components and

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