Towards a sustainable waste to eco-friendly grease pathway: A biorefinery proposal for the silk and food industries

doi:10.21203/rs.3.rs-4331431/v1

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Research Article

Towards a sustainable waste to eco-friendly grease pathway: A biorefinery proposal for the silk and food industries

https://doi.org/10.21203/rs.3.rs-4331431/v1

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published 16 Oct, 2024

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The concerning related to climate change, sustainability and residue destination, as evidenced by the United Nation’s Sustainability Development Goals, fosters among others the biorefinery application development. Amidst such circular economy possibilities is the implementation of residual oil biorefineries, imbuing higher value into residues and resulting in eco-friendly products. One of the most abundant residual oils, with very low reusage ratio, is the waste cooking oil, a food industry residue. While other residual oil can be extracted from lipid rich silkworm pupae, a residue from the silk processing industry. Thereby, in this work we propose a biorefinery for the usage and modification of waste cooking oil and silkworm pupae oil into eco-friendly greases based on circular economy concepts. The silkworm pupae oil was modified through epoxidation and hydrolysis, yielding a dense polyol which was used in grease compositions without further modifications (common for this sort of materials). The prepared compositions were elaborated using partial saponification (simplifying the process), under two distinct methodologies, with LiOH and NaOH. The prepared greases were also compared against simple soybean oil and ricin oil greases to identify the polyol addition effect and the frying oil reusage effect. All the greases prepared using methodology B were stiffer with overall higher values for yield point (15.7-56.9 Pa) and flow point (259.2-810.95 Pa), while the thixotropic hysteresis area fluctuated with greater time dependency for polyol containing compositions (1.85-9.16 10⁴ Pa s^-1). The ion change from lithium to sodium using methodology A on polyol compositions resulted in diminished flow points (from 457.35 to 70.31 Pa). The same change on waste cooking oil, on methodology A, resulted in increased values of flow point (from 32.06 to 96.90 Pa). The addition of polyol effect resulted in flow point increase (from 259.2 to 810.95 Pa), while the frying oil reusage effect resulted in increased yield (from 1.19 to 2.02 Pa) and flow points (from 38.44 to 96.90 Pa). The obtained results are on par with other eco-friendly grease examples and therefore corroborate the proposed value enhancing of our tested residual oils. The main perspective for this biorefinery proposal is the usage of the two residual oils together into a blend composition, chaining the silk and the food industries and thereby resulting in a single biorefinery applicable to any machinery dependent industry.

Waste cooking oil

Silkworm

Eco-friendly grease

waste-to-biomaterials

Biorefinery

circular economy

This study provides a novel utilization of silkworm pupae oil as polyol additive on grease compositions with ricin oil. The materials were characterized as eco-friendly greases. The findings justify the biorefinery application for residual oils.

The biorefinery concept, or the value enhancing of residues through modification, is strongly bound to the concept of circular economy, in which residues are reintroduced to the agroindustrial production chain. [1–6] The circular economy application, on the other hand, is one of the main objectives highlighted by the United Nations for the sustainable development and fight against climate change (UN’s 17 goals for sustainable development). [7, 8] The fatty residues are among the eligible materials with biorefinery applications. [9, 10] As residue this materials are mostly burn for fuel, applied on low value products such as soap, or improperly discarded. [11]

Fatty residue applications are diverse but mainly comprised of low value final products, while directly dependent on the composition and impurities, best exemplified by the soap industry. [12] For higher value products, residual oils and fats can be cracked, esterified, transesterified, isomerized, oxidized, deoxygenated, hydrolyzed, hydrogenated, hydroformylated, carbonylated, aminated, dimerized, polymerized, epoxidized and others. [13–18] The reusage, modification and reapplication of fatty residues, therefore, are aligned with United Nation’s sustainable goals 7 (Affordable and clean energy), 11 (Sustainable cities and communities), 12 (Responsible consumption and production), 13 (Climate changing) and 15 (Life and land). [19, 20]

Waste cooking oils (WCO), for instance, are very environmentally pollutant due to low solubility of fatty compounds in water, and therefore a question to be tacked. [11] It is estimated that in Brazil, over 9 billion liters of WCO are discarded annually, with only a small proportion being reintegrated into the production cycle, while most of the WCO is reintegrated as biofuels. [11, 21] The WCO, as most triglycerides, is comprised by fatty saturated and unsaturated chains linked by ester carbonyls groups and glycerol, with varying amounts of free fatty acids (produced by the frying process). [22, 23] Due to its composition, WCO can be effortless modified to obtain lubricants, greases, paraffins, soaps, resins, biodiesel and others. [11, 24]

Another residual oil with similar applications can be obtained by extraction from silkworm pupae (Bombyx mori), an agroindustrial residue from silk processing, to obtain extracted protein rich materials proper for animal feed. [25] In 2020, Brazilian farmers produced 377 tons of silk, placing Brazil as the sixth larger producer of silk. [26] The production of 1 kg of silk requires between 6–8 kg of cocoons, [27] and therefore, a significant amount of residual silkworm pupae, which has next to none industrial application. The silkworm pupae (SP) contains 26% of fatty compounds, from which 38% are linolenic acid, 25% oleic acid and 22% palmitic acid fatty chains. [28] Based on exoteric reasons, the silkworm pupae oil (SPO), also known as chrysalis oil, is used in China, India and others on the composition of cosmetics. [27] On the other hand, linolenic fatty chains (C18:3) present on the SPO composition justify its application as unsaturaton rich substrate.

Unsaturation rich substrates can be easily modified through classical epoxidation conditions with in situ generated organic peracids. The resulting epoxidized material can be hydrolysed with aqueous acid solutions for the production of polyols, frequently used on polyurethanes, lubricants, surfactants composites and others. [29–33] The polyol application on the composition of greases, on the esterified form, elevates the viscosity index of such materials. [34]

Greases are semi-solid materials comprised of an oil base, a thickening agent and additives, with properties directly dependent on oil viscosity, thickening type and thickening concentration. The thickening agents are frequently alkaline soaps based on lithium, calcium, sodium and aluminum hydroxides. Alternative thickening agents are based on clays, silica, polyurethanes and others. [35, 36] Most commercial greases are synthetic/mineral oil based and therefore dependent on petroleum market oscillations, environmental hazardous and non-renewable. The preparation of greases using vegetal oils results in eco-friendly materials with increased lubricity, higher viscosity index, biodegradability, renewability and lower oxidative stability. [36]

The terms green greases, biogreases and eco-friendly greases are all used to designate materials prepared from vegetal and/or residual oil bases with classical, or alternative, thickener agents. All three terms are also used to designate sustainable or renewable materials. [36–42] Among such materials, polyol greases are often described. [43] Polyol greases are normally prepared from fatty substrates which are modified into polyols and further modified into ether or ester materials. This polyol esters, or ethers, can be used as oil bases or as thickening agents. [43–45] Although the simple non-functionalized polyol application is often neglected.

This work is supplementary to other studies developed by our group, where the silkworm pupae (with and without oil) were enhanced by torrefaction and applied as a solid fuel with outstanding results. The oil extraction from silkworm pupae results in protein rich extracted residue, proper for torrefaction and animal feed production, while the polyunsaturated oil can be used for the preparation of high-value products, such as green renewable greases.

Therefore, here we provide a biorefinery application (production of eco-friendly greases) for the oil extracted from silkworm pupae, modified by epoxidation and hydrolysis (into a fatty polyol) and waste cooking oil, in contrast with refined soy oil and ricin oil based on circular economy concepts. The polyol based in silkworm pupae oil is, to the best of knowledge, a novel application of such materials. The greases were also prepared by partial saponification, with NaOH, or LiOH, following two distinct methodologies in contrast with most greases where the thickener agent is prepared and later on mixed with the base oil. The prepared materials were characterized using thixotropic and rheological techniques to access the impact of cations, base oil and preparation methods on the final greases. Thus, this study aims to advance the residual oil biorefinery and eco-friendly grease preparation possibilities, while investigation the effect of fatty polyols on the composition of such looking for their industrial applications.

Materials

The waste cooking oil (WCO) was collected from diners around the administrative regions of Brasilia – Brazil and paper filtered prior to usage, the food grade soy oil (SO) was purchased locally. Sodium hydroxide and Lithium hydroxide, analytical grade, were purchased from Synth. Dehydrated silkworm pupae, was purchased locally. Hexane (95%) was purchased from Allchem and distilled prior to usage. The hydrogen peroxide (50%), chloridric acid (37%) and formic acid (85%) were purchased from Merck, the industrial grade nitrogen gas was purchased from White Martins, the ricin oil (RO) was purchased from Quimisul and the food grade soy oil was purchased locally.

Silkworm pupae oil extraction

The lipidic content of silkworm pupae (silkworm pupae oil - SPO) was extracted with hexane on a soxhlet apparatus, using well established methodologies. [46] Prior to the extraction, 800 g of pupae samples were grinded and dried at 100°C. After the extraction, the hexane on the lipidic extract was evaporated, the resulting oil (180 g) was characterized and stored under inert atmosphere, below 0°C.

Oil and modified oil characterizations

The NMR spectra were acquired using a Bruker NMR 600 MHz, equipped with a Bruker 5 mm broad band (BBFO) probe for ¹H and ¹³C nuclei. Deuterium chloroform with 0.03% tetramethylsilane (CCl₃D + TMS) was used as solvent, with 16 scans for ¹H, 2000 scans for ¹³C and 5 s delay. Infrared (FT-IR) spectra were acquired using a Shimadzu FT-IR Prestige, equipped with an ATR Miracle cell, within a window of 650–4000 cm^− 1, with 32 scans and resolution 4 on a Happ-Genzel apodization method.

Density and viscosity data were obtained using an Anton Paar Stabinger Viscosimeter model SVM 3000 at 40°C. The acidity index were measured by KOH titration with a Gehaka PG2000 pHmeter, and following ASTM D465-96. The samples were dissolved in isopropyl alcohol and analyzed in triplicates. Saponification value was determined using AOCS Cd 3c-91 specifications.

Silkworm pupae oil modification

The obtained SPO was modified though an epoxidation reaction, using formic acid (85%) and hydrogen peroxide (50 vol.). The methodology, adapted from previous studies was described elsewhere. [47, 48] The weight proportion was 1:1.35:0.79 (SPO:formic acid:hydrogen peroxide). The SPO was heated to 60°C and to the oil a mixture of formic acid and hydrogen peroxide was slowly dripped using an addition funnel. After the addition, the mixture was heated to 80°C and kept under reflux for 5 h. Past the reaction time, the mixture was washed with distilled water until a near neutral pH discard water. The mixture was immediately dried under reduced pressure and the resulting material (silk worm pupae epoxide - SPE) was stored, frozen, under inert atmosphere.

The silkworm pupae polyol (SPP) was obtained by simple hydrolysis of SPE with HCl solution. On the hydrolysis step, 200 g of SPE and 300 mL of HCl 3 mol L^− 1 were mechanically stirred at room temperature for 24 h. The hydrolyzed material was washed with distilled water until a near neutral pH discard water. The mixture was immediately dried under reduced pressure and the resulting material (SWP) was stored, frozen, under inert atmosphere.

Grease preparation methods

The grease preparation methods were based on partial saponification and consisted in two distinct procedures, using molar ratios and molar mass adjusted by fatty composition SO (881.42 g mol^− 1), WCO (885.45 g mol^− 1), RO (917.45 g mol^− 1) and SPP (1001.52 g mol^− 1):

Method A (Ma) – Following a methodology adapted from elsewhere [49], an oil base (soy oil, cooking waste oil, ricin oil. or a mixture of SPP and ricin oil*) and an hydroxide solution of NaOH, or LiOH, were placed into a becker, on a molar proportion of 1:0.75 (base:hydroxide), and magnetically stirred at 70°C for 3 h. After the partial saponification, the obtained grease was cooled to room temperature and stored for characterization. Method A’ (Ma’) - Assembled similarly to method A, but with altered proportion of hydroxide, on a molar proportion of 1:1.5 (base:hydroxide). * 1:3 (w w^− 1) of SPP and ricin oil respectively.

Method B (Mb) – Following a methodology adapted from elsewhere [50], an oil base (soy oil, cooking waste oil, ricin oil, or a mixture of SPP and ricin oil*) and an hydroxidene solution of NaOH, or LiOH, were placed into a becker, on a molar proportion of 1:1.5 (base:hydroxide), and mechanically stirred (300 RPM) at 50°C for 24 h. After the partial saponification, the obtained grease was cooled to room temperature, placed into a NaOH based desiccator for 24 h and stored for characterization. Method B’ (Mb’) - Assembled similarly to method B, but with altered proportion of hydroxide, on a molar proportion of 1:0.45 (base:hydroxide). * 1:3 (w w^− 1) of SPP and ricin oil respectively.

The greases obtained received acronyms related to the source oil base, therefore SOG, WCOG, ROG and SPP/ROG for soy oil grease, cooking waste oil, ricin oil and silkworm pupae/ricin oil grease respectively. The preparation method was specified as XX(Li or Na)-Ma, XX(Li or Na)-Ma’, XX(Li or Na)-Mb and XX(Li or Na)-Mb’ for methods A, A’, B and B’ respectively. The grease designations with base oil and hydroxide contents are described in Table 1.

Table 1

Prepared greases designation table.
Grease	Base oil	Molar ratio (Base oil:Hydroxide)	Hydroxide	Preparation method
SPP/ROG(Li)-Ma	SPP/RO (1:3)	1:0.75	LiOH	Ma
SPP/ROG(Li)-Mb		1:1.5	LiOH	Mb
SPP/ROG(Na)-Ma		1:0.75	NaOH	Ma
SPP/ROG(Na)-Mb		1:1.5	NaOH	Mb
WCOG(Li)-Ma	WCO	1:0.75	LiOH	Ma
WCOG(Li)-Mb		1:1.5	LiOH	Mb
WCOG(Na)-Ma		1:0.75	NaOH	Ma
WCOG(Na)-Mb		1:1.5	NaOH	Mb
SOG(Na)-Ma	SO	1:0.75	NaOH	Ma
SOG(Na)-Ma’		1:1.5	NaOH	Ma’
SOG(Li)-Ma’		1:1.5	LiOH	Ma’
ROG(Li)-Mb	RO	1:1.5		Mb
ROG(Li)-Mb’		1:0.45		Mb’
ROG(Na)-Mb’		1:0.45	NaOH	Mb’

Grease thixotropic characterization

The thixotropic data for prepared greases were evaluated using an Anton Paar MCR 72, using parallel-plates model PP40 with 40 mm diameter with 1 mm gap. Viscosity and shear stress were acquired under a three part rotational thixotropic experiment: i) an increasing shear ramp 0-100 s^− 1; ii) a constant shear rate of 100 s^− 1; iii) a decreasing shear ramp 100-0 s^− 1. The experiments were conducted at 25°C, using freshly prepared greases kept in thermal equilibrium for 10 min. The hysteresis area were obtained from fitted for hysteresis curves using Bingham plastic model on OriginPro software.

Grease rheological characterization

The rheological, including SPP kinematic viscosity at 40°C, data for prepared greases were evaluated using an Anton Paar MCR 72, using parallel-plates model PP40 with 40 mm diameter with 1 mm gap. Storage modulus (G’), loss modulus (G”) and loss tangent (tan δ) were acquired under oscillatory shear for two experiments: i) amplitude sweep - an increasing shear strain ramp 10^− 2-10²%, at constant 10 rads^− 1 angular frequency, to determine the LVE region limit and flow-point and; ii) frequency sweep - a increasing angular frequency shear ramp 10^− 2-10² rads^− 1, at constant 0.01% shear strain. The experiments were conducted in duplicates, at 25°C, using freshly prepared greases kept in thermal equilibrium for 10 min.

Oil characterization

Prior to usage the oils were characterized using FT-IR and NMR (¹H and ¹³C) techniques. The FT-IR (Fig. 1) display a similar overall profile across all the oil samples. All the samples presented bands νC-H sp² at 3002 cm^− 1, ν C-H sp³ between 2800 and 3000 cm^− 1 from fatty unsaturated chains. The νC = O band at 1743 cm^− 1 is assigned to ester carbonyl groups with different degrees of symmetry, higher on SO and lower for RO. Higher symmetry for νC = O in oil samples indicates lesser carbonyl signal overlay, and therefore, lesser acidity or lesser contamination with different esters. Below 1500 cm^− 1 is the fingerprint region with overlap of different signals, slightly different for every sample. The RO FT-IR spectra displays a different band at 3410 cm^− 1 attributed to νO-H, while the carbonyl signal was less symmetrical, thus indicating the expected presence of ricinoleic fatty chains, and also the presence of free fatty acids.

To corroborate the structural assumptions made using FT-IR data, the ¹H and ¹³C NMR spectra were acquired (S-Fig.s 1–8). The NMR spectra display a similar overall profile across all the oil samples. Comparing ¹H NMR spectrums, all samples presented unsaturation signals at δ 5.3 ppm, internal glycerol hydrogens at δ 5.2 ppm, external glycerol hydrogens between δ 4.0 and 4.4 ppm, followed by a typical fatty compound profile. All spectrums were normalized using the external glycerol peaks which allowed the comparison of unsaturation hydrogen integrals. An increasing order of unsaturation hydrogen amount was verified, with SO < WCO < RO < SPO, revealing that our refined SO contained less unsaturations than the collected waste cooking oil. The ¹³C NMR spectrums allowed to identify greater unsaturation complexity between 135 − 125 ppm with the same unsaturation hydrogen amount order, as expected based on known fatty compositions, accompanying a typical fatty compound profile.

The extracted SPO, is an unsaturated rich fatty substrate as seen on FT-IR and ¹H and ¹³C NMR spectra. SPO is comprised mainly by linolenic acid chains (C18:3) with 38.18%, followed by oleic acid chains (C18:1) and palmitic acid chains (C16:0). [28] This fatty composition of SPO, polyunsaturated, is very distinct from fatty subtracts used previously, such as Caryocar brasiliense (pequi), Magonia pubescens (tingui) and sewage sludge, [51–53] which were used for fuel. Based on the results, the SPO, given the oil chemistry possibilities, was functionalized into polyol, and used on the composition of higher value eco-friendly greases looking for better possibility applications according to circular economy concepts.

The extracted SPO was epoxidized using a simple homogeneous peracid, resulting on the formation of oxiranic rings and an epoxidized material (Silkwormm Pupae Epoxide - SPE). The generated SPE was simultaneously hydrolyzed by a similarly simple acid hydrolysis methodology followed by washing. The resulting solid material, was hydroxyl rich and called Silkworm Pupae Polyol (SPP), as represented in Fig. 2.

The comparison between SPO and SPP FT-IR spectra (Fig. 3) allow to visualize the unsaturation consumption by the disappearance of band νC-H sp² at 3002 cm^− 1, the polyol formation, by the increase in νO-H band at 3410 cm^− 1 and the symmetry loss in νC = O band due to superposition between ester and carboxylic signals. The ¹H NMR spectra superposition of SPO and SPP (Fig. 4) shows the consumption of the SPO’s unsaturation peak at δ 5.3 ppm, the decrease in intensity of internal (δ 5.2 ppm) and external (δ 4.0–4.4 ppm) glycerol hydrogens due to hydrolysis and washing and also the disappearance of unsaturation vicinal hydrogens at δ 2.7 and δ 1.9 ppm. On the SPP it is also possible to identify superposition of labile hydroxyl groups at δ 4.91 ppm and hydroxyl vicinal hydrogens uncoupled at δ 3.29 ppm.

The ¹³C NMR spectrums of SPO and SPP (S-Figs. 8 and 10) support the mentioned hydrolysis by the presence of carboxyl acid peaks in δ 177 and 178 ppm the in SPP spectra, the unsaturation consumption by the disappearance of signals between δ 135 − 125 ppm and the formation of signals between δ 60–75 ppm attributed mostly to carbons linked to hydroxyl groups.

Grease preparation

The reusage and modification of residual oils into high value products is a biorefinery model in expansion. [54–57] Our proposition of biorefinery, represented in Fig. 5, involves two main residual oils undergoing chemical modifications (partial saponification on both, and epoxidation/hydrolysis on SPO) for the production of high value green greases. Most polyols applied to lubricant compositions are further modified into epoxides, ester or ether and may or may not be used with thickener agents. [58–61] There are almost no studies on the utilization of simple unmodified polyol as additives or thickener agents. Whilst our prepared SPP was too solid to be applied alone on the composition of greases (Kinematic viscosity at 40°C available in Table 2), it could be used as additive (based on intermolecular interactions), allowing the investigation of interaction effects between the polyol and the ricinoleic chains of RO, with its final effects on grease properties.

The grease preparation methodologies were all dependent on the oil base partial saponification. Thus, the saponification value, acidity index and kinematic viscosity (Table 2) were investigated. The saponification value is inversely proportional to the fatty chain average length and therefore, higher saponification values represent how easily fatty chains can be modified through saponification. [62] The acidity index, on the other hand, represents the amount of free fatty acids present on an oil sample, a direct indicative of natural hydrolysis. [63] The kinematic viscosity at 40°C oriented which stirring method would be used. From simple comparison between samples SO and WCO, is possible to access the effects of usage on spent soy oil (WCO). Prior to usage, SO has smaller average fatty length and lower acidity. After usage, WCO presents longer average fatty length and higher acidity, possibly through hydrolysis, thermal cracking and thermal polymerization. [64, 65]

Table 2

Saponification value, acidity index and kinematic viscosity for base oils.
Base oil	Saponification value [mg(KOH) g^− 1]	Acidity index [mg(KOH) g^− 1]	Kinematic viscosity at 40 C (mm² s^− 1)
SO	310.61	0.499	31.42*
WCO	132.32	2.51	39.41**
RO	16.78	4.61	226.2***
SPP	15.68	120.72	16315.8

* [66]

** [67]

*** [68]

The comparison between WCO, RO and SPP shows that RO has even longer average fatty chain length and higher acidity, while SPP has similar saponification value (in relation to RO), but higher acidity index. For RO and SPP samples, there are two competing parameters for saponification, the lower saponification value represents the difficult saponification of such, whilst the higher acidity favors the saponification process.

The Ma methodology, with hydroxide proportion, 1:0.75, under magnetical stirring at 70°C for 3 h, were compared against methodology Mb, with hydroxide proportion 1:1.5, mechanically stirred at 50°C for 24 h, to access the preparation effects over the final greases. The SPP/ROG and WCOG were prepared with both Li and Na hydroxides, on methodologies Ma and Mb and analyzed without complications.

The SOG greases were prepared with both Li and Na hydroxides, on method Ma due to higher saponification value and lower acidity, allowing the use of gentle magnetical stirring. The ROG greases were prepared with both Li and Na hydroxides, on method Mb due to lower saponification value and higher acidity, requiring the stronger mechanical stirring. Both SOG and ROG presented some complications, the SOG(Li)-Ma sample was too liquid for our rheometer plate accessory, while ROG(Na)-Mb sample was too solid. This limitation spawned the modification of methodologies Ma into Ma’ (1:1.5 hydroxide proportion, magnetical stir at 70°C for 3 h), and Mb into Mb’ (1:0.45 hydroxide proportion, mechanical stir at 50°C for 24 h).

Grease rheological and thixotropic characterization

The material rheological characterization for greases is mainly comprised of two complementary experiments, the amplitude and the frequency sweeps. On the amplitude sweep experiment, samples sustain shear strain sweeps and have its G’ and G” recorded, allowing the identification of elastic (G’ > G”) and viscous (G” > G’) behaviors, yield points (τ_r), flow points (τ_f) and the identification of viscoelastic limit region (LVE).

The τ_r value represent the point after which the grease begins to flow as a lubricant oil. [69] The τ_f is the intersect point of G’ and G” curves, representing the point in which the grease the viscous component is equal to the elastic component and therefore, the point beyond which the grease is more viscous than elastic. [70] The obtained data for τ_r and τ_f were compiled into Table 3. The LVE value of 0.01% for all the prepared greases were obtained graphically (Fig. 6) from amplitude sweeps and considering a deviation of 3% from starting values.

The obtained amplitude sweep curves (Fig. 6) allow the identification of a typical commercial grease viscoelastic behavior, whereas G’ > G” under lower shear strains and G” > G’ under higher shear strains. [36] Comparing SPP/ROG and WCOG starting values of G’, directly related to how solid the material is under resting conditions, demonstrates that overall Mb results in stiffer greases. Among SSP/ROG and WCOG, Mb greases resulted in higher values of τ_r and τ_f, The reason is both the higher hydroxide content and the stronger mechanical stirring, resulting in a steady emulsion.

The ion substitution from Li to Na, using Ma, in SPP/ROG results in decreased τ_r (from 6.225 to 3.41 Pa) and τ_f (from 457.35 to 70.31 Pa) values, while the Li to Na substitution on greases prepared with Mb presented almost constant τ_r value (from 27.85 to 27.95 Pa) with decreased τ_f (from 810.95 to 565.7 Pa). Comparison between SPP/ROG(Li)-Mb and ROG(Li)-Mb allow to investigate the polyol addition effect. The polyol addition resulted in a constant τ_r value (from 27.35 to 27.85 Pa), while greatly increasing τ_f (from 259.2 to 810.95 Pa).

The ROG relation between Mb and Mb’ allows to investigate the hydroxide proportion effect on commercial ricin oil. Comparing ROG(Li)-Mb with ROG(Li)-Mb’, the decrease in hydroxide proportion also decreased the τ_r (from 27.35 to 0.75 Pa) and τ_f (from 259.2 to 27.35 Pa) values. The ROG Mb’ substitution of Li for Na resulted in increase on τ_r.(from 0.75 to 49 Pa) and τ_f (from 27.35 to 2489 Pa), an indicative of possible ion size effect on the structure of ROG.

The ion substitution from Li to Na, using Ma, in WCOG results in slight decrease in τ_r (from 2.95 to 2.02 Pa), with three times increase in τ_f (from 32.06 to 96.90 Pa). The Li to Na ion substitution on WCOG, greases prepared with Mb, showed a strong increase in τ_r (from 15.7 to 56.9 Pa) with almost double increase in τ_f (from 395.15 to 645.4 Pa). Comparison between SOG(Na)-Ma and WCOG(Na)-Ma allow to investigate the usage effect (acidity increase and saponification value decrease). The usage effect resulted in slight increase in τ_r value (from 1.19 to 2.02 Pa), with three times increase in τ_f (from 38.44 to 96.90 Pa).

The SOG(Na)Ma and SOG(Na)-Ma’ comparison allow to investigate the hydroxide proportion effect on refined soy oil. The increase in hydroxide proportion, SOG(Na)-Ma to SOG(Na)-Ma’, results in slightly decreased τ_r (from 1.19 to 0.94 Pa) with three times increase in τ_f (from 38.44 to 106.25 Pa). The change of ion from Na to Li, SOG(Na)-Ma’ and SOG(Li)-Ma’ comparison, results in τ_r increase (from 0.94 to 53.4 Pa) and doubling in τ_f (from 106.25 to 233.05 Pa) values.

Saxena and coworkers described a preparation of eco-friendly nano-greases based on soybean oil and organoclay as thickener and CaCO₃ nano-powder as additive. The obtained values for τ_r ranged between 497–992 Pa, while τ_f ranged between 118.2-292.7 Pa, at 25°C, with a commercial all-purpose presented τ_r and τ_f of 1100 Pa and 289.1 Pa, respectively. [36] Padgurskas et al. described biogreases based on rapeseed oil and beeswax as thickener with calculated yield points ranging between 47–301 Pa at 20°C. [71] Roman et al. described ricin oil based and cellulose nanofiber thickened greases, with τ_r of 300 Pa at 25°C. [72] While classical aluminum, lithium, calcium and polyuria thickened greases displays τ_r values between 500 and 1000 Pa. [73] Our prepared materials is therefore comparable with other green eco-friendly greases examples, but with lower yield point values if compared with classical greases. But, they show a lot of benefits to environmental benefits, as waste disposal reduced, renewable materials, circular economy and green chemistry concepts.

On the frequency sweep experiment (Fig. 7), samples sustain angular frequency sweeps within the LVE region (constant 0.01% shear strain), while it’s G’ and G” are recorded, allowing to comprehend the grease’s time dependency behavior. Typical commercial greases display G’ higher than G” which run parallel at lower frequencies and slight increase in G’ and G” at higher frequencies with increase rate slightly higher for G”. [36, 74] The higher rate approach between G’ and G” is an indicative of structural breakdown. [69]

Among SSP/ROG samples, Mb greases showed higher values for G’ and G” within the 10^− 2-10² rad s^− 1 range. The samples SSP/ROG(Li)-Mb and SSP/ROG(Na)-Ma displayed commercial like behavior, while SSP/ROG(Li)-Ma and SSP/ROG(Na)-Mb were deviant. The SSP/ROG(Li)-Ma and SSP/ROG(Na)-Mb G’ and G” increased at higher and lower frequencies, creating a concavity like profile for G’ and G” values. The loss tangent graph shows greater stability for SSP/ROG(Na)-Ma, with considerable linearity for SSP/ROG(Li)-Mb and SSP/ROG(Na)-Ma. The SSP/ROG(Li)-Ma displayed greater loss tangent, with considerable instability at lower frequencies.

The ROG greases displayed commercial like behavior. The comparison between SSP/ROG(Li)-Mb and ROG(Li)-Mb G’ and G” curves shows very similar profiles. The decrease in hydroxide content (Mb to Mb’) results in lower G’ and G” curves and instability below 10 rad s^− 1 as displayed by the ROG(Li)-Mb’ loss tangent. The ion change in ROG, from Li to Na, results in greater stability as shown by the ROG(Na)-Mb’.

Among WCOG samples, Mb greases (WCOG(Li)-Mb and WCOG(Na)-Mb) showed higher values for G’ and G”, within the 10^− 2-10² rad s^− 1 range, and also greater stability as shown by the loss tangent. The Ma greases (WCOG(Li)-Ma and WCOG(Na)-Ma) displayed lower stability at lower frequencies. The SOG greases displayed commercial like behavior. The comparison between WCOG(Na)-Ma and SOG(Na)-Ma G’ and G” curves shows very similar profiles, with slightly higher values for the WCOG, explainable by it’s higher acidity and therefore more effective saponification. The increase in hydroxide content (Ma to Ma’) results in similar G’ and G” curve profiles. The ion change in SOG, from Na to Li, results in greater stability as shown by the SOG(Li)-Ma’ loss tangent curve despite instabilities in middle range frequencies.

The thixotropic experiments for all the samples displayed profiles consistent with expected commercial greases (Fig. 8), which were treated for thixotropic hysteresis area (THA), also compiled in Table 3.

The thixotropic behavior represents the structural recovery within increase and decrease of shear rates and time. [75] To identify thixotropic behavior, a simple hysteresis loop technique can be used resulting in the quantifiable value of thixotropic hysteresis area (THA). [76] High THA values represents greater structural time dependency, while low THA values represent how easily a material recovers its structure post-shear. [77] The THA values are, therefore, the kinetic difference between network breakup (faster) and network buildup (slower). [75]

The Li based SSP/ROG samples displayed higher THA values indicating greater time dependency. The comparison between ROG(Li)-Mb and SSP/ROG(Li)-Mb reveal increased time dependency on occasion of polyol addition (from 2.70 10⁴ to 9.16 10⁴ Pa s^− 1). The ROG(Li)-Mb and ROG(Li)-Mb’ correlation shows the hydroxide content effect on THA, diminishing hydroxide content results in lesser time dependency (from 2.70 10⁴ to -1.05 10⁴ Pa s^− 1). The negative signal on THA for ROG(LI)-Mb indicates that not only the material displayed fast structural recovery, but also achieved greater organization post shear. The ion change from Li to Na in ROG results in great time dependency (from − 1.05 10⁴ to 1.29 10⁵ Pa s^− 1), another indicative of possible ion size effect on the structure of ROG.

Table 3

Rheological and thixotropic relevant values obtained for the studied greases.
Grease	Yield Point, τ_r (Pa)*	Thixotropic Hysteresis Area (Pa s^− 1)	Flow Point, τ_f@G’=G” (Pa)
SPP/ROG(Li)-Ma	6.225	3.83 10⁴	457.35
SPP/ROG(Li)-Mb	27.85	9.16 10⁴	810.95
SPP/ROG(Na)-Ma	3.41	2.32 10⁴	70.31
SPP/ROG(Na)-Mb	27.95	1.85 10⁴	565.7
WCOG(Li)-Ma	2.95	-1.89 10⁴	32.06
WCOG(Li)-Mb	15.7	4.61 10⁴	395.15
WCOG(Na)-Ma	2.02	2.18 10³	96.90
WCOG(Na)-Mb	56.9	1.59 10⁴	645.4
SOG(Na)-Ma	1.19	1.55 10³	38.44
SOG(Na)-Ma’	0.94	-9.44 10¹	106.25
SOG(Li)-Ma’	53.4	3.55 10⁴	233.05
ROG(Li)-Mb	27.35	2.70 10⁴	259.2
ROG(Li)-Mb’	0.75	-1.05 10³	27.35
ROG(Na)-Mb’	49	1.29 10⁵	2489

* Parameters directly provided by Anton Paar MCR 72 software.

The WCOG samples prepared on Mb displayed greater THA. On WCOG the change from Li to Na, using Ma, resulted in great time dependency (from − 1.89 10⁴ to 2.18 10³ Pa s^− 1), while using Mb resulting in decreased time dependency (from 4.61 10⁴ to 1.59 10⁴ Pa s^− 1). The comparison between SOG(Na)-Ma and WCOG(Na)-Ma reveal a slight increased time dependency with usage effect (from 1.55 10³ to 2.18 10³ Pa s^− 1). The SOG(Na)-Ma and SOG(Na)-Ma’ relation shows the relation between hydroxide content and THA, increasing hydroxide content results in lower time dependency (from 1.55 10³ to -9.44 10¹ Pa s^− 1). The ion change from Na to Li for SOG results in greater THA (from − 9.44 10¹ to 3.55 10⁴ Pa s^− 1).

Saxena et al. described THA values ranging between 9.91-24 10³ Pa s^− 1 for soybean organoclay greases [36] Ren et al. described the preparation of synthetic oil (polyalphaolefin - PAO 40) greases with lithium complex thickener with THA values ranging between 2–6 10⁴ Pa s^− 1. [78] Our prepared greases are, therefore, within range with both eco-friendly greases and classical greases.

A direct application for our residual oil prepared greases depends on desired lubrication properties which are intrinsic to each system. For instance, on SSP/ROG, greases displayed low and high τ_r and τ_f values, some presented commercial like frequency sweep profiles, but only slow post-shear recovery (high values of THA). Based on the high values of τ_r and τ_f Mb greases would be better for more intensive applications, to be confirmed by tribological tests. Based on flow point data, SSP/ROG(Na)-Ma seems to be in range with microcristanile and paraffin waxes. [79] On the other hand, WCOG also displayed low and high τ_r and τ_f values, presented almost commercial like frequency sweep profiles and also varying THA values, with applications well discussed. [24]

In this study WCOG despite been well known was prepared and analyzed similarly to SPP/ROG aiming to access possible compatibilities between WCO and SPP. Our focus now on it to study the polyol effect on WCO compositions, linking two different residues into a common biorefinery application on an industrial scale, effectively aligning with UN SDG’s, while advancing the TRL (Technology Readiness Level), as summarized in Fig. 9.

Nevertheless, despite using each residual oil separately, the residual oils biorefinery proposed herein enhances the overall value of the applied residual oils. The grease compositions were prepared by partial saponification, removing unitary operation and simplifying the preparation process (relative to classical grease preparation). Our prepared greases uses no further modified polyol bases, is considerable eco-friendly based on the main renewable components, while also describing reusage/ion effect on grease composition. The prepared grease properties are also on par with other eco-friendly greases and therefore present feasible commercial application.

In this work we proposed a residual oil biorefinery for the production of higher value eco-friendly greases based on waste cooking oil and silkworm pupae oil based on the circular economy concepts. The waste cooking oil was filtered and tested on the compositions for greases (WCOG). The silkworm pupae oil was modified into polyol and tested as additive in ricin oil greases (SPP/ROG). The prepared greases on two different methods with two different hydroxides, through partial saponification, were compared against simpler soy and ricin oil greases. All the materials were characterized for rheological and thixotropic data allowing to understand the polyol addition, reusage and ion effects.

For instance, considering silkworm pupae, the extracted protein rich bagasse was the higher value product, which now may add higher value to the traditional industry of silk. The prepared eco-friendly materials show the viability of such applications, which may be incorporated into any machinery based industry following circular economy principles, exacerbating further perspectives.

As subsequent possibilities there are: the study of SPP on blends (with ricin oil and also with WCO) using RSM (Response Surface Methodology) for optimization, increasing the TRL looking for the industrial applications and the biodegradability study for all prepared greases.

WCO –Waste Cooking Oil

WCOG –Waste Cooking Oil Grease

SO – Soy Oil

SOG –Soy Oil Grease

RO – Ricin Oil

ROG – Ricin Oil Grease

SPO – Silkworm Pupae Oil

SPE – Silkworm Pupae Epoxide

SPP – Silkworm Pupae Polyol

SPP/ROG – Silkworm Pupae Polyol/Ricin Oil Grease

G’ – Storage Modulus

G” – Loss Modulus

τ_r – Yield Point

τ_f – Flow Point

LVE – Viscoelastic Region Limit

THA – Thixotropic Hysteresis Area

Acknowledgements

Authors acknowledge CAPES, CNPq, FAP-DF, IFB and UnB for financial support and fellowships.

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Graphical Abstract Our Graphical abstract compiles the main substrates, methodologies and product (eco-friendly greases), representing the proposed biorefinery for residual oils.
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published 16 Oct, 2024

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Towards a sustainable waste to eco-friendly grease pathway: A biorefinery proposal for the silk and food industries

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Abstract

Figures

Statement Novelty

Introduction

Materials and Method

Materials

Silkworm pupae oil extraction

Oil and modified oil characterizations

Silkworm pupae oil modification

Grease preparation methods

Grease thixotropic characterization

Grease rheological characterization

Results and discussion

Oil characterization

Grease preparation

Grease rheological and thixotropic characterization

Conclusions

Abbreviations

Declarations

Acknowledgements

References

Supplementary Files

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Journal Publication

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