Erythrina Stavila
a,
Frita Yuliati
a,
Azis Adharis
b,
Joddy Arya Laksmono
a and
Muhammad Iqbal
*c
aResearch Center for Polymer Technology, Research Organization for Nanotechnology and Material (ORNM), National Research and Innovation Agency (BRIN), Gedung 460 KST B. J. Habibie/Puspiptek, Jl. Raya Puspiptek, Tangerang Selatan 15315, Banten, Indonesia
bDepartment of Chemistry, Faculty of Science and Computer Science, Universitas Pertamina (UPER), Jl. Teuku Nyak Arief, RT.7/RW.8, Simprug, Jakarta Selatan 12220, Daerah Khusus Ibukota Jakarta, Indonesia
cDepartment of Chemistry, Faculty of Mathematics and Science, Institut Teknologi Bandung (ITB), Jl. Ganesha No. 10, Bandung 40135, Jawa Barat, Indonesia. E-mail: m.iqbal@itb.ac.id
First published on 15th May 2023
Palm oil is a versatile bio-renewable resource for consumer products, oleochemicals, and biofuels. The utilization of palm oil in polymer production as a bio-based polymer is considered a promising alternative to conventional petrochemical-based polymers due to its non-toxicity, biodegradability, and vast obtainability. Triglycerides and fatty acids in palm oil and their derivatives can be utilized as bio-based monomers for synthesizing polymers. This review summarizes the recent advancement in using palm oil and its fatty acids for polymer synthesis and their applications. Moreover, this review will overview the most commonly used synthesis pathways for producing palm oil-based polymers. Therefore, this review can be used as a reference for designing a new approach to synthesizing palm oil-based polymers with desired properties.
Bio-based polymers are environmentally friendly materials and potential alternatives to conventional petrochemical-based polymers. By definition, bio-based polymers are sustainable polymeric materials that can be synthesized from renewable feedstocks and are preferably produced via biological and biochemical methods.3 Bio-based polymers are classified into two categories: naturally obtained polymer and polymer synthesized from bio-derived substances.3 Protein, carbohydrates, terpenes, lignin, vegetable oils and their derivatives are examples of renewable feedstocks that can be utilized to produce bio-based materials.4,5 Among other renewable resources, vegetable oils have been considered as potential resources for sustainable chemicals and polymeric materials because of their low toxicity, biodegradability, and vast obtainability.6
Vegetable oils are extracted from various plants and termed after their plant origin, for instance, palm oil, olive oil, rapeseed oil, sesame oil, sunflower oil, cottonseed oil, corn oil, soybean oil, linseed oil, etc.6 These vegetable oils and their derivatives can be used in industrial applications such as in the synthesis of polymers such as polyols, polyesters, polyurethanes, polyamides, epoxides, etc. The uses of various types of vegetable oils for monomers preparation, polymer synthesis, and their applications have been reviewed elsewhere.6–12 However, to the best of our knowledge, there is no available review that focuses only on the use of palm oil as raw bio-renewable feedstock for polymer synthesis. Therefore, in this review, we will focus on and discuss the recent advance in the utilization of palm oil and its fatty acids for polymer synthesis and its application.
The growing request for edible oil led to increase palm oil production since the oil palm tree is a highly profitable crop, thus significantly influencing Indonesia's oil palm venture expansion.17 Areas with oil palm cultivation in Indonesia, namely North Sumatera, Riau, and Central Kalimantan, show lower average rural poverty and higher gross regional product (GRP).18 The area with oil palm plantations benefits in their regional economic development and poverty alleviation. However, the positive aspect regarding the oil palm expansion is also followed by the negative impact on the environment, especially concerning deforestation and climate change.18 In some cases, deforestation somehow was involved as an irresponsible act on plantation expansion. Deforestation and land conversion contribute to global carbon emissions,19 which consequently play a major role in global climate change.15 Unfortunately, this land conversion and deforestation for oil palm plantations development in Indonesia and Malaysia have emitted CO2 of ∼500 million tons every year,20 which deems the oil palm industry unsustainable. Due to environmental concerns, the EU, one of the world's main importers of palm oil, insists on reducing and stopping the usage of palm oil-based biofuels altogether. They believe that the palm oil industry does not fulfill the standards for EU's biofuel raw material production, i.e., without deforestation, generating low carbon emission, and avoiding conversion of areas full of biodiversity.15,21 Based on these developments, further research and study on sustainability in the palm oil industry has to be improved and established to eliminate the downside of this industry to the environment. The growth in regional economic and poverty alleviation cannot be used to justify the environmental damage due to the irresponsible practice. Thus, it is important to make this industry sustainable and more environmentally friendly. Especially in Indonesia, a lot of people's lives depend on this industry. One of the efforts that can be done is utilizing the oil palm plantation product as a renewable material feedstock for polymer production, which also give significant contribution in polymer chemistry field. Therefore, this article will review the research that have been performed in synthesis of polymer from palm oil and its fatty acids, so then it can be used as a reference for designing new approach in polymer synthesis from palm oil and exploring new possibilities of application of the obtained polymers.
Palm oil mainly consists of triglyceride with its unique fatty acid profile (see Fig. 1) and minor components like tocotrienols and tocopherols (vitamin E), carotenoids (precursor for vitamin A), phytosterols, squalene, chlorophyll, and low levels of phenolic compounds.22 The fatty acids in palm oil consist of about 44% palmitic acid (saturated), 5% stearic acid (saturated), 39% oleic acid (unsaturated), and 10% linoleic acid (unsaturated).23,24 The composition of saturated fatty acids (palmitic and stearic acid) may differ from each country.22 The stereospecific distribution of the fatty acyl residues in triglyceride of palm oil is shown in Table 1. The low linoleic acid content is beneficial for oil stability towards oxidative deterioration.24 The double bond content from the unsaturated fatty acids moiety of palm oil can be determined by iodine value (IV) which is 50–55 mg per 100 g.25 The higher IV specifies more double bonds or unsaturation content in the vegetable oils. In terms of IV, vegetable oils can be categorized into three types: drying oils (IV > 130), semi-drying oils (IV 90–130), and non-drying oils (IV < 90).25,26 Thus, palm oil with low iodine value of 50–55 mg per 100 g is considered as a non-drying oil.
Palm oil is known as the most fractionated oil, and its fraction products have been used in many different applications.27 Fractionation is a selective separation of multi-component mixtures into two or more fractions with different chemical and physical properties; by physical and thermomechanical processes.27 By utilizing the distinction in the crystallization performance of triglycerides, palm oil fractionation produces a liquid of palm olein (65–70%) and a solid fraction of palm stearin (30–35%).22,27 Palm olein has melting points of 18–20 °C and contains a higher amount of oleic acid (39–45%) and linoleic acid (10–13%) than palm oil, which is used as a deep frying oil.24 Whereas, palm stearin has melting points of 48–50 °C and is composed of a higher portion of saturated fatty acids (49–68% palmitic acid) and triglycerides, which is utilized as vanaspati.24 Further fractionation from palm olein (deep frying oil) or palm stearin (vanaspati) can be done to obtain more fraction products, such as super olein (cooking oil), top olein (salad oil), super stearin (animal feed), soft palm middle fraction (margarine), and hard palm middle fraction (confectionery).22,27 In the following section, we will discuss selected articles that reported the application of epoxidized palm olein and other fractionation products as sources of palm oil-based monomers.
Besides directly using palm oil, fatty acids derived from palm oil may also be utilized as a monomer for polymer synthesis. Fig. 3 shows several options for enzymatic modification of fatty acids to make them suitable as bio-based monomers for polymer synthesis. In Fig. 3, oleic acid is used as a model compound for bio-based monomer formation from fatty acids. In Fig. 3(a), conversion of oleic acid to its α,ω diacid functional monomer can be carried out via whole-cell bio-transformation using Candida tropicalis ATCC20962.32 This diacid is an appropriate monomer in polycondensation for producing polyester, polyesteramide, etc. In Fig. 3(b), the double bond in oleic acid can be modified via chemo-enzymatic epoxidation to produce a bio-based monomer with the oxirane ring.33 This oxirane ring will be available for further functionalization. Another alternative for preparing bio-based monomers from fatty acids is shown in Fig. 3(c). Esterification of oleic acid with 2-hydroxyethyl acrylate using lipase from Candida antarctica immobilized on an acrylic resin as a catalyst.34 The obtained monomer will have a vinyl functional group suitable for radical polymerization. These three enzymatic synthetic routes can be used to prepare fatty acid-based monomers. The utilization of enzymes as a catalyst will be an added value to the green process.
There are advantages to using palm oil or fatty acids in polymer production, as summarized in Table 2. Information from Table 2 can be considered in choosing between palm oil or fatty acids as raw material feedstock for making bio-based monomers. The decision to use palm oil or fatty acid will affect the synthetic pathways needed to prepare bio-based monomers and, consequently, the properties of the resulting polymers. Depending on the type of reactive functional groups available in the palm oil or fatty acid-based monomers, the polymerization techniques can be achieved via polycondensation, free radical polymerization, atom transfer radical polymerization (ATRP), reversible addition–fragmentation chain transfer polymerization (RAFT), or cationic polymerization. The following section will discuss the type of polymers synthesized from palm oil or its derivatives and their applications. Polymers based on palm oil and its fatty acids consist of polymers from fatty acid-based vinyl monomers, polyester, polyols, polyurethanes, polyepoxides, composites, polyamides, polyesteramides, etc.
Palm oil | Fatty acids |
---|---|
Cost efficient | Effective formulation |
• Vastly abundant and easy to get. | • Effective modification to only saturated or unsaturated fatty acids35 |
• Palm oil is cheaper than fatty acids, so it might decrease polymer production expenses35 | • Lessen unwanted by-products or impurities |
• Oil is non-corrosive compared to fatty acids; therefore, no anticorrosion equipment is required35 | • Easier to achieve desired polymer products with specific properties |
• Oil showed better resistance to discoloration during storage than fatty acids35 |
As for adding vinyl groups to fatty acids, various conversion methods of fatty acids from vegetable oils and their derivatives into functionalized monomers for effective radical polymerization have been thoroughly reviewed.37 The development of radical polymerization techniques of fatty acid-(meth)acrylate monomers and their polymer applications are also discussed.37 Another review is reported on plant oil-based acrylic monomers (POBMs) made from modification of triglycerides from various plant oils such as high oleic soybean, olive, sunflower, canola, soybean, corn, high oleic sunflower, and linseed oil.38 Moreover, Since reviews on fatty acid-vinyl monomers from vegetable oil have been available, thus in our review, we present only selected reports on fatty acid-based vinyl monomers specially derived from palm oil. Especially fatty acids such as palmitic, stearic, oleic, and linoleic acids are part of the component in palm oil triglycerides. So, we can have a general idea of how palm oil and its fatty acid have been utilized in producing palm oil-based polymer. Finally, this information can be used to design a new synthetic approach to the following practical step for utilizing palm oil and its fatty acids in polymer synthesis.
Here are examples of the synthesis of fatty acid/ester-based vinyl monomers from palm oil, as shown in Fig. 4. Maiti et al. reported on the synthesis of a series of fatty acid-based methacrylate monomers (FAMA) via Steglich esterification of 2-hydroxyethyl methacrylate (HEMA) with saturated fatty acids, see Fig. 4(a).39 They obtained high-yield monomers of 74–85% indicating an efficient way to produce novel fatty acid-based vinyl monomers. Polymerization of FAMA via RAFT resulted in fatty acid-based polymers (PFAMA) with controlled molecular weights, as shown in Fig. 4(a). However, in the Steglich esterification for FAMA formation, they used dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP), known for its toxicity toward humans and the environment, thus limiting the scale-up probability.37 Using a more environmentally friendly catalyst, transesterification of methyl laurate and methyl oleate with HEMA was performed with Candida antarctica lipase B immobilized within a porous polymethacrylate (CAL-B, Novozymes A/S) at optimal reaction condition of 60 °C for 24 h in toluene.40 Interestingly, they also reused the CAL-B for ten batches of the same reaction, which revealed that the enzyme retained its activity at the yield of 88% for methyl laurate with HEMA and yield of 76% for methyl oleate and HEMA. Another report on the use of enzymes as bio-catalyst is in the preparation of oleic acid-based epoxy monomer. The reaction was performed between 2-hydroxyethyl acrylate and epoxy stearic acid using lipase from immobilized Candida antarctica on an acrylic resin as a catalyst in toluene at 37 °C for 24 h that resulted in a yield of 87%.34 Oleic acid-based epoxy monomer was subsequently polymerized by radical polymerization using AIBN in toluene for 24 h. Moreover, palm oil methyl ester (from unsaturated fatty esters) is reported as a monomer for polymeric surfactant preparation. Palm oil methyl ester was modified to methyl ester sulfonate (MES) via a reaction of the methyl ester with NaHSO3, and then, aqueous radical polymerization of MES and acrylamide was initiated by potassium persulfate.41 The obtained polymeric methyl ester sulfonate (PMES) can be used as a substitute surfactant for enhanced oil recovery (EOR) since it can decrease the interfacial tension (IFT) and improve the viscosity control, two beneficial features for EOR application.
Kalita and coworkers synthesized palm oil (PO)-based vinyl ether monomer via transesterification of triglyceride with 2-(vinyloxy)ethanol, see Fig. 4(b).31 Through cationic polymerization, only the vinyl ether group from the monomer was selectively polymerized for producing its PO-based polyvinyl ether with number average molecular weight (Mn) of ∼20000 g mole−1. Kalita et al. also reported the preparation of plant oil-based polyvinyl ethers from linseed, soybean, and palm oil to be compared with commonly used drying oil as binders for artist paints.42 Plant oil was converted to plant oil-based vinyl ether monomers that consisted of 2-vinyloxy ethyl soyate (2-VOES), 2-vinyloxy ethyl palmitate (2-VOEP), and 2-vinyloxy ethyl linseedate (2-VOEL). These vinyl ether monomers from plant oil were further polymerized to produce homo- and copolymers. Applying these polymers in paint binders showed excellent mechanical properties and reduced cure shrinkage better than drying oils.
Preparation of vinyl monomer by acrylation can also be done directly to palm oil triglycerides. Synthesis of acrylated palm olein (APO) was conducted through ring-opening oxirane groups of the epoxidized palm olein (EPOo) using acrylic acid, which resulted in APO with a high yield of 86% and molecular weight of 1750 Da as shown in Fig. 4(c).29 APO has hydroxy groups that are beneficial for surface modification to increase the drug transport performance to the targeted organ. Su et al. reported on the acrylation of different types of vegetable oils for UV-curable coatings application.43 Vegetable oils, including palm, peanut, olive, canola, corn, rapeseed, and grapeseed oil, were reacted with acrylic acid and boron trifluoride ether to form vegetable oil-based acrylate prepolymers. The cured film derived from grapeseed oil showed the best mechanical and thermal properties among all the prepared cured films. Furthermore, Wu et al. reported on the preparation of a vinyl and UV-responsive monomer via modification of palm oil to palm oil fatty acid-ethyl acrylamide (POFA-EA).44 The obtained POFA-EA was used to formulate palm oil-based inks by blending them with acrylic acid (AA) at different weight ratios. To produce Zn2+-containing elastomers, ZnO or ZnCl2 at different weight content and a photo-initiator was added to the blend solution of POFA-EA/AA and then used as palm oil-based inks for photocuring 3D printing. A series of thermoplastic elastomers from palm oil with dynamic physical crosslinks (noncovalent interaction, see Fig. 5) were produced by photocuring 3D printing. Noncovalent interactions improved tensile strength up to 4.2 MPa and fracture elongation of 851% of the elastomers. Moreover, these interactions promote more abilities in the elastomers, like self-healing, recovery, and reconfiguration. This approach of physical crosslinking-enable printability might propose a new method for producing thermoplastic polymers via photocuring 3D printing.
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Fig. 5 Illustration of plausible noncovalent (hydrogen bond) interactions between AA and POFA-EA in palm oil-based thermoplastic elastomers. Reproduced with permission from ref. 44 Copyright 2021 Elsevier. |
As shown in the abovementioned paragraph, we have selected reports showing fatty acids or palm oil as starting materials for polymer production. As for fatty acids as a renewable feedstock for polymer synthesis, there are options for chemical reaction or enzymatic synthesis. Based on the reported studies, enzymatic synthesis in monomer preparation of fatty acid-vinyl monomers was highly effective. Moreover, the immobilized enzyme can be reused for several reactions, and the reaction results in a good yield. However, as far as we know, there are no reports on the enzymatic conversion of triglyceride to fatty acid-based vinyl monomers. So, we suggest that researchers in this field try this triglyceride transesterification using an enzyme as a catalyst to attach vinyl functional groups to fatty acids moieties. As a consideration, since triglyceride is a bigger molecule than a fatty acid, perhaps enzymatic reaction using triglyceride will be more complex and less effective than fatty acid as feedstock.
Palm oil-based alkyds for UV-curable coatings were prepared from palm stearin.48 Alkyds were prepared via alcoholysis using Ca(OH)2 and glycerol at 230 °C to form monoglycerides. Further esterification between monoglycerides and phthalic anhydride was performed to produce alkyds, as shown in Fig. 6(b). A further modification was carried out by combining phthalic acid and maleic acid during polyesterification, see Fig. 6(c). The incorporation of maleic acid increased the unsaturation in the alkyds chain and made it more UV-curable. Increasing maleic acid content in alkyds resulted in coatings with better film properties that were shown by better water and alkali resistance, higher pencil hardness, as well as improved thermal stability. Moreover, adding a small amount of trimethylolpropane triacrylate (TMPTA) as a trifunctional crosslinker into the coating formulation enhanced the film properties significantly. The same trend was also observed from alkyds prepared by combining phthalic anhydride and different maleic anhydride content during polyesterification.35 Furthermore, alkyds modification was carried out by incorporating fumaric acid or maleic acid during alkyds formation, which produced fumarized alkyd (AlkF) or maleated alkyd (AlkM), respectively, as shown in Fig. 6(c).49 AlkF produced coating with better film hardness and higher Tg than AlkM, which meant that the fumarization process was a better approach for increasing alkyd unsaturation groups. Moreover, to improve the coating performance of alkyd resins, there are reports on combining two different oils (palm stearin/dehydrated castor oil or palm stearin/linseed oil)50 or blending palm stearin-based alkyd resin with commercial ketone resin.51
Ang et al. reported on the utilization of palm oil-based alkyd as a polymeric plasticizer in polyvinyl chloride (PVC).52 By incorporating 20 wt% palm oil-based alkyd to PVC, Tg value reduced from 85.3 °C to 66.7 °C, indicating a successful plasticizing process. Moreover, PVC contained 20 wt% palm oil-based polyester showed better migration resistance than PVCs with the addition of two commercially available plasticizers, acetyl triethyl citrate (ATEC) and diethylhexylphthalate (DEHP). Furthermore, the obtained palm oil-based alkyd was used as a co-plasticizer to di-octyl phthalate (DOP) or di-isononyl phthalate (DiNP) as the primary plasticizer for flexible PVC.53 The incorporation of palm oil-based alkyd into plasticized-PVC resulted in an improvement of mechanical and thermal properties. Further studies on the effect of different molecular weights of palm oil-based alkyds for PVC plasticizer was performed.54 Blending 20 wt% of palm oil-based alkyds to PVC resulted in reduced Tg value. The smaller size plasticizer showed better plasticizer ability than the high molecular weight plasticizer because the small size plasticizer penetrates better to PVC chains. Meanwhile, in n-hexane and water/ethanol as solvent, films of PVC plasticized with high molecular weight alkyd displayed better migration and leaching resistance as compared to that of PVC plasticized with low molecular alkyds and commercially available plasticizer of DEHP; thus, high molecular weight alkyd showed less toxicity compared to DEHP. Moreover, a significant improvement in tensile strength and elongation at break was observed on plasticized-PVC film with high molecular weight palm oil-based alkyds.
Ramli and coworkers reported on the incorporation of palm oil-based alkyd resin in polyaniline (PANI) to prepare films of PANI/Alkyd with good adhesion properties on fluoride-doped tin oxide (FTO) for solar panel application.55 Three alkyds with different acid numbers were blended with PANI at various ratios of PANI/Alkyd and then cured on the FTO glass. PANI/Alkyd film with the lowest acid number of alkyd displayed the shortest curing time compared to those with a high acid number. All PANI/Alkyd films exhibited good adhesion, while the PANI film showed poor adhesion performance. Unfortunately, blending PANI with palm oil-based alkyd decreased the film's conductivity since alkyd was not a conducting polymer. Furthermore, the blending process of PANI and alkyd was carried out during coating preparations by mixing various alkyd concentrations with acrylate monomers, M-cresol, and different loading of PANI, then followed by benzophenone for UV curing. The alkyd/PANI coating films of 20% PANI presented the highest conductivity of 1.467 × 10−1 S cm−1.56
Palm fatty acid distillate (PFAD) is a side product from the physical refining process of crude palm oil (CPO), which mostly consist of free fatty acids (∼45% palmitic acid and 33% oleic acid) with small numbers of glycerides, bioactive compounds like phytosterols, tocopherols, squalene, tocotrienols, other hydrocarbons.57,58 There are reports on the use of PFAD for alkyd resins preparation. Chiplunkar et al. prepared alkyd resins from palm oil and PFAD as an additive in liquid detergent.57 The results showed that the PFAD-based alkyd resin was comparable to palm oil-based alkyd resin and commercial liquid detergent. Moreover, Teo and coworkers reported utilizing PFAD in synthesizing hydroxyl-terminated macromer to produce UV-curable urethane acrylate resin for wood-coating applications.58 The highest Mw observed at the PFAD content of 45% was considered the optimal concentration of PFAD. These macromers were further modified to make PFAD urethane acrylate.
From the abovementioned reported articles, palm oil-based alkyds have the potential to be applied as surface coating, plasticizers, and additives. For coating application, further modification by adding more unsaturated groups to alkyd resins is necessary to increase the UV-curable ability of the resins. However, no additional modification is needed to prepare alkyds for plasticizer and additives application. Thus, it is more beneficial from an economic point of view—significantly, the utilization of PFAD as the raw material feedstock for producing alkyd liquid detergent additive. Since PFAD is a by-product of the CPO refining process, producing PFAD-based alkyd will be more environmentally friendly and economically beneficial. As for the alkyd for plasticizers, this application demonstrated more agreement with the nature of the alkyd resins. Palm oil-based alkyd resins have more saturated fatty acids moieties that serve as side chains in the polymer backbone. This saturated fatty acid contributed to more flexibility in the polymer, which is consequently suitable for decreasing glass transition temperature (Tg).
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Fig. 7 (a) Ring-opening of oxirane group from monoepoxidized and (b) diepoxidized palm olein using phthalic acid, and (c) other diacids that were used to ring-opening epoxidized palm olein. |
Oxirane rings in triglyceride are valuable functional groups for further modification. This section discussed the ring-opening polymerization of oxirane functional groups to produce polyester. In a few reports mentioned above, catalysts and solvents are unnecessary in the reactions. However, high-temperature reactions (>100 °C) are needed to perform ring-opening reactions from oxirane rings. This high-temperature reaction is in line with high energy consumption. The ring-opening polymerization of oxirane rings will be preferable if it can be done at a low temperature and short reaction times by adding an environmentally friendly and non-toxic catalyst.
Several reports have been available on the enzymatic modification of fatty acids to fatty diacids and their polymerization. Yang and coworkers reported on the conversion of oleic, erucic, and 9,10-epoxy stearic acids to their corresponding α,ω-diacids functional monomers of 1,18-cis-9-octadecenedioic, 1,22-cis-9-docosenedioic, and 1,18-cis-9,10-epoxy-octadecanedioic acids by whole-cell bio-transformation using Candida tropicalis ATCC20962 as the catalyst that in good yield and with intact double bond or oxirane group, as shown in Fig. 8.32 Modification of oleic acid to 9,10-epoxy stearic acid was done by a chemo-enzymatic method using oleic acid, hydrogen peroxide, and lipase Novozym 435 as the catalyst, see Fig. 8(b). This method was adopted from the previously reported article on lipase-mediated chemo-enzymatic epoxidation of linoleic acid.33 The obtained α,ω-diacids functional monomers were polymerized via enzymatic polycondensation of α,ω-diacids monomer with diols (1,3-propanediol, 1,8-octanediol, or 1,16-hexadecanediol) using Novozym 435 as a catalyst in bulk or diphenyl ether at 90 °C, as shown in Fig. 8(a) and (b), which led to the formation of low melting points (23–40 °C) polyesters with molecular weight (Mw) ranging from 25000–57
000 g mol−1.32 Yang and coworkers also reported on the polycondensation of 1,18-cis-9-octadecenedioic acid and glycerol using either enzymatic (Novozym 435 at 90 °C) or chemical (dibutyltin oxide, DBTO at 150 °C) catalyst and without solvent addition.66 Novozym 435-catalyzed polycondensation of oleic diacid and glycerol with a ratio of 1
:
1 produced polyester with Mn value of 9000 g mol−1 after 24 h reaction, see Fig. 8(c), whereas using DBTO after 6 h reaction resulted in polyester with Mn value of 1750 g mol−1 and increasing reaction time to 8 h led to gel formation that was due to the formation of crosslinking product. Using Novozym 435 as the catalyst, crosslinking and gel formation were not observed. The reason was a steric hindrance at the CAL B's active site that limits the branching formation and crosslinking reactions.66 Another study on oleic diacids reported the copolymerization of oleic diacids, glycerol, and crude linoleic acid using Novozym 435 as the catalyst at 90 °C and bulk reaction condition to produce polymeric triglyceride mimetic structures, as shown in Fig. 8(d).67 By using a monomer ratio of 1
:
1
:
0.67 for oleic diacids, glycerol, and crude linoleic acid, resulting in a product with Mn ∼9500 g mol−1 and 64% trisubstituted glycerol unit after 8 h reaction. The trisubstituted glycerol unit increased by nearly 100% when the proportion of crude linoleic acid increased to 1.33.
From the reports mentioned above, using an enzyme as a catalyst benefited in milder reaction conditions and lower reaction temperature than using a chemical catalyst. Moreover, in enzyme catalysis, unwanted side products such as gel formation and crosslinking can be avoided since the pocket of the active site in the enzyme has specificity on the type of substrate and substrate acceptance limitation due to the steric hindrance. The synthesis of bio-based polymers will be more environmentally friendly, not only through the use of bio-renewable resources as feedstocks but also by using green processes, like using an enzyme as the catalyst.68 However, there is also a limitation in using an enzyme as a catalyst in polymerization, which is the relatively long reaction time (up to days of reaction) to produce a high molecular weight of polymers. It will be better if, soon, the enzyme can be used as a catalyst in a short reaction time (less than one day) to produce high molecular weight polymers.
Kalita and coworkers reported that triglycerides in palm oil were modified to fatty acid-based vinyl monomers before epoxidation and alcoholysis, as shown in Fig. 9(b).31 Double bonds from the vinyl group were polymerized by cationic polymerization. The obtained polymer had unsaturated and saturated fatty acids as side chains. The double bond in the unsaturated fatty acids was further modified via epoxidation to form oxirane rings. Subsequently, the oxirane group was ring-opened via an alcoholysis reaction with methanol to form polyols.
Slightly different methods in polyol preparation, as mentioned above, transesterification was performed prior to the epoxidation process, see Fig. 9(c). Arniza et al. prepared transesterified palm oil-based polyol via three-step reactions involving: (1) transesterification of palm olein using glycerol and catalyst, (2) epoxidation of transesterified palm olein using performic acid, and (3) ring-opening of oxirane group from the epoxidized product using ethylene glycol and boron trifluoride as shown in Fig. 9(c).30 Mohammed et al. reported the preparation of polyols from palm oil and soybean oil by transesterifying triglycerides using glycerol and CaO as a catalyst to form monoglycerides.75 The produced monoglycerides act as a polyol to prepare thermoplastic and thermosetting polyurethanes. Glycerol and CaO were also used in the transesterification of palm olein to produce palm oil-based polyol, as reported by Lumcharoen and coworkers.76 Moreover, Mohd Tahir and coworkers reported on the synthesis of polyol from purified-waste cooking oil (WCO).77 The purification of waste cooking oil was performed by filtration and further adsorption using sugarcane bagasse (SCB) activated carbon. WCO-based polyol was prepared by first making a polyhydric solution, which contained a mixture of diethanolamine, monoethylene glycol, and potassium acetate with a ratio of 90:
7
:
3.
The transamidation process also can be used to prepare polyol from palm oil, see Fig. 9(d). Transamidation of refined bleached deodorized palm oil (RBDPO) using diethanolamine without the addition of catalyst and solvent free resulted in the formation of diethanolamide.78 Using the amidation method, palm oil-based diethanolamide was prepared from the reaction of palm oil with diethanolamine and sodium methoxide as the catalyst.79 Transesterification-alcoholysis reaction using palm oil-based diol, triethanolamine, and lithium hydroxide was carried out to synthesize polyol ester for polymeric nanoparticles in drug delivery system application.80
Instead of using whole palm oil as feedstock, epoxidation and alcoholysis can also be performed on fatty acid methyl ester (FAME) to produce fatty-acid methyl ester-based polyol (PolyFAME-EG).81 FAME (biodiesel) was derived from palm oil, which contains C12:
0 (0.1%), C14
:
0 (0.6%), C16
:
0 (5.7%), C18
:
0 (3.6%), C18
:
1 (72.7%), and C18
:
2 (17.1%). Moreover, empty fruit bunch (EFB) from oil palm trees can also be used as a source for polyol preparation. Amran et al. reported on the liquefaction of oil palm EFB and EFB pulp sheets (cellulose) via the microwave-assisted method for polyols production.82 Further studies on the liquefaction of oil palm EFB and EFB cellulose to produce polyols were carried out in sulfuric acid and polyethylene glycol-glycerol as co-solvent at different designated temperatures and reaction times.83
Several methods are selected to prepare polyols and may result in different types of polyols in terms of their hydroxyl value. OHV of polyols prepared via various methods is summarized in Table 3. The OHV may vary from 78 to 749 mg KOH per g sample. We can conclude that using more reaction steps in fragmenting the triglycerides into its fatty acid or fatty ester will result in better access to the double bonds in the fatty acid moieties. The direct use of palm oil for epoxidation and alcoholysis resulted in low OHV (<100 mg KOH per g sample). We can assume that it will be much harder to access double bonds in a fatty acid moiety of triglyceride than in the fatty acid moiety of monoglyceride, or even better if the source of the double bond is from free fatty acid or fatty ester. Using conventional reaction as shown in Fig. 9, the reaction scheme (c) produced polyol with an OHV of 330 mg KOH per g sample,30 which was the highest compared to other palm oil-based polyols. Moreover, a microwave-assisted method can achieve OHV up to 749.22 mg KOH per g from liquefaction oil palm EFB.82 This information on hydroxyl value is important whether the polyols are further used for producing polyurethanes (PUs). The low and high hydroxyl values of polyols do not align with the quality of polyols but with the number of hydroxyl groups involved in making urethane linkages. A low hydroxyl value of polyol meant fewer hydroxyl groups for making urethane linkages than the high hydroxyl value of polyol. There are specifications for polyols in the polyurethane industry, such as polyols used in the production of flexible foam/elastomers PU should have a molecular weight of 1000–6500, OHV of 28–160 mg KOH per g sample, and functionality of 2–3, whereas, polyols utilized in the production of rigid/structural foam PU should have molecular weights of 400–1200, OHV of 250–1000 mg KOH per g sample, and functionality of 3–8.84 Therefore, PUs from polyol with low hydroxyl value will have different properties with PUs from polyol high hydroxyl value. The next section will discuss the utilization of palm oil-based polyols for PUs production.
Source of triglyceride or fatty acid | Methods in polyol preparation | Hydroxyl value (OHV) mg KOH per g sample | Ref. |
---|---|---|---|
a This OHV had a unit in mL NaOH per g sample. | |||
• Palm oil | • Epoxidation using performic acid | 102 | 69 |
• Hydroxylation using H2O and H2SO4 | |||
• Palm oil | • Epoxidation using peracetic acid | 147.1 | 70 |
• Alcoholysis using diethylene glycol | |||
• Residual palm oil (RPO) | • Epoxidation using performic acid, sulfuric acid, and catalyst | 78.525 | 72 |
• Alcoholysis using methanol and water | |||
• PMTAG | • Epoxidation using perchloric acid | 155 | 73 |
• Hydroxylation using H2O and HClO4 | |||
• EPOo | • Alcoholysis using isobutanol and K10 montmorillonite as a catalyst | 124.71 | 74 |
• Palm olein | • Transesterification using glycerol and catalyst | 330 | 30 |
• Epoxidation using performic acid | |||
• Alcoholysis using ethylene glycol and BF3 | |||
• Palm oil | • Transesterification of palm oil | 233 | 75 |
• Soya oil | • Transesterification of soya oil | 251 | |
• Palm olein | • Transesterification using glycerol and CaO | 140 | 76 |
• Purified waste cooking oil (WCO) | • Formation of polyhydric solution | 148.79 | 77 |
• Transesterification between WCO and polyhydric solution | |||
• RBDPO | • Transamidation using diethanolamide | 254.20a | 78 |
• Palm oil-based diol | • Transesterification and alcoholysis using triethanolamine and LiOH | 182.51 | 80 |
• Biodiesel (FAME) | • Epoxidation using performic acid | 166.5 | 81 |
• Alcoholysis using ethylene glycol and BF3 | |||
• Oil palm EFB | • Microwave liquefaction using ethylene glycol and sulfuric acid | 749.22 | 82 |
• EFB cellulose | 639.91 | ||
• Oil palm EFB | • Liquefaction using sulfuric acid and polyethylene glycol-glycerol at different temperature | 228.08 | 83 |
• EFB cellulose | 270.49 |
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Fig. 10 (a) Synthesis of polyurethane between diol/polyol with isocyanate and (b) different types of isocyanates. |
Synthesis of PUs can be performed by the one-shot or prepolymer method. The one-shot process is the mixing directly of all components for making PUs. In the prepolymer method, a polyurethane prepolymer with –NCO end group is produced via the reaction of diols/polyols with isocyanates, which is subsequently chain extended to form PUs.85 Since polyols as the source of hydroxyl groups and isocyanates for making polyurethanes can be synthesized from vegetable oil. Thus, the synthesis of PUs may also be prepared from raw renewable feedstock such as vegetable oil. A review of the synthesis, characterization, and properties of polyurethanes from various vegetable oils has been reported.9,25,84 This section discusses the recent advancement of palm oil-based polyurethanes.
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Fig. 11 Flexible PU foams SEM micrographs of (a) reference foam from 100% petrochemical-based polyether polyol; foams from blends of the petrochemical-based polyol with (b) 10%, (c) 20%, (d) 30%, and (d) 40% palm oil-based polyol. Reproduced with permission from ref. 76 Copyright 2014 Trans Tech Publications. |
A high hydroxyl value (OHV) of polyols can be used for producing PU rigid foam. However, by using blended polyol of transesterified palm olein-based polyol (OHV of 330 mg KOH per g sample) and petrochemical-based polyol (polyether polyol with OHV of 430–470 mg KOH per g sample) as the source of polyols, resulted in semi-rigid PU foams with lower reactivity of the foaming profile, lower density, and compressive strength than that of 100% petrochemical-based polyol.30 Semi-rigid PU foams were also produced by blended palm oil-based polyol with OHV of 102 mg KOH per g sample up to 70 wt% with commercially available polyether polyol (OHV of 449 mg KOH per g sample).69 PU foam was also prepared from 100% palm oil-based polyol. However, the obtained PU foam morphology had a heterogenous sample with big pores (collapsed cells) zones and others with smaller cells, as shown in Fig. 12(a). Moreover, rigid PU foams were obtained by blending palm oil-based polyol with OHV of 147.1 mg KOH per g sample up to 70 wt% with commercially available polyether polyol (OHV of 449 mg KOH per g sample).70 Incorporation of palm oil-based polyol resulted in deceleration of the foaming and gelling reactions, decreased density, mechanical, and dynamic mechanical performance with the increased palm oil-based content in PU foams. Furthermore, Septevani et al. reported on a study of the preparation of rigid polyurethane by substituting petrochemical-based polyether polyol with palm kernel oil-based polyester polyol and still maintaining the thermal and mechanical performances comparable to PUs prepared from petrochemical-based polyether polyol.88 Rigid PU foams incorporated with palm oil-based polyol up to 30% displayed comparable mechanical and thermal performances as well as dimensional stability as compared to that of reference PU foam from petrochemical-based polyether polyol.
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Fig. 12 PU foams picture of (a) cross-section from semi-rigid PU foam derived from palm oil-based polyol (reproduced with permission from ref. 69 Copyright 2017 Elsevier) (b) rigid and flexible PU foams produced from PMTAG polyol (reproduced with permission from ref. 93 Copyright 2016 Elsevier), and (c) rigid PU foams prepared from liquefied oil palm EFB polyol at various NCO indexes (reproduced with permission from ref. 95 Copyright 2021 MDPI). |
Hydroxy telechelic natural rubber (HTNR) was used as a polyol source with the combination of palm oil-based polyol to produce flexible PU foams. A series of PU foams (PUFs) were prepared by varying the mixture ratios of bio-based polyols (palm oil-based polyol and HTNR), 1,4-butanediol (BDO) as chain extender, catalysts, surfactant, H2O as a blowing agent, dichloromethane (DCM), and diisocyanate (TDI or p-MDI).28 All the PUFs displayed single Tg, which indicated good miscibility between palm oil-based polyol and HTNR. p-MDI-based PUFs exhibited higher storage modulus (E′), hardness, and compressive strength than that TDI-based PUFs.28 Rigid and flexible PU foams in Fig. 12(b) were prepared from polyol derived from PMTAG polyol.93 The rigid and flexible PU foams had small-sized closed cells with cell sizes of 270 ± 40 μm and 386 ± 55 μm, respectively. The closed cell structure and the thermal stability of these rigid and flexible foams indicated the appropriateness for application in thermal insulation. Regarding the structure and physical properties of the foams, the PMTAG polyol might be able to compete with the commercially available bio-based and petrochemical based-polyol for producing industrial-scale PU foams. Moreover, Mohd Tahir et al. reported on the preparation of rigid PU foam waste cooking oil (WCO)-based polyol.77 Chavarro Gomez and coworkers studied the characterization of rigid PU foam prepared from polyol of mixtures of various ratios of residual palm oil (RPO) and algae oil (AO).94 The rigid PU foam with 50% AO content showed improvement in homogeneous cell structure, slightly lower in thermal stability and conductivity than that of industrial insulation materials. Conversely, bio-based PU foams with higher RPO content demonstrated better flexural stress, which could be used in applications requiring abrasion resistance.
Polyols produced by the liquefaction of oil palm empty fruit bunch (EFB) and EFB cellulose were used to produce rigid PU foams.83 The PU foam from oil palm EFB-based polyol displayed larger cell sizes and distribution than from EFB cellulose-based polyol. Thus, PU foam from EFB cellulose-based polyol displayed better thermal and mechanical performance than PU foam from oil palm EFB-based polyol. Further studies on the preparation of rigid PU foams from oil palm EFB-based polyol and p-MDI with the variation of isocyanate indexes (100, 110, 120, 130, 140, and 150) as shown in Fig. 12(c).95 The normalized compressive strength of EFB-based PU foams with a 120 NCO index was comparable to PU foam from petrochemical-based polyol (as reference PU foam). Moreover, pictures of PU foams prepared using oil palm-based polyols are shown in Fig. 12(a)–(c). The appearance of PU foams was also affected by the polyol's color's physical state. Thus, depending on PU applications, in PU formulation, it is crucial to consider the type of polyol, isocyanate (aliphatic or aromatic), the isocyanate index, the amount of water (blowing agent), and other chemicals needed to prepare the PU foams with desired chemical, mechanical, and thermal properties.
Velayutham et al. reported on the properties of palm oil-based PU coating influenced by oleic acid and chemical crosslinks content.96 PU film with 65% oleic acid content displayed the highest thermal stability at 213 °C. All the PU films showed constant Tg values, regardless of the NCO/OH ratio increase. PU films with 28% oleic acid content demonstrated the highest rupture strength of 36 MPa and possibly a very robust coating. Variations content of oleic acid and NCO/OH ratios led to a high-quality coating with superior performance and unfailing long-term application. Moreover, the emulsion type of paint was prepared using palm oleic acid-based polyol (Mw of 950) with 4,4′-MDI.97
PU films were prepared via the pre-polymerization method using palm kernel oil-based monoester polyol (PKO-p) and 2,4′-MDI with different ratios of NCO/OH.98 Prior to the addition of MDI, diethylene glycol (DEG) as a chain extender was mixed with PKO-p. All PUs before the curing process appeared as sticky film. However, after curing, the film with a higher NCO ratio had a rigid physical state, while the film with a higher polyol ratio had a flexible physical form. The PU film from PKO-p was clear and semiflexible, which can be utilized in coating applications, particularly onto the fiberboard, since PU was hydrophobic. Moreover, Teo and coworkers reported on UV-curable urethane acrylate derived from palm fatty acid distillate (PFAD). Five different concentrations of PFAD (15, 25, 45, 55, and 70% w/w) were used to prepare PFAD hydroxyl-terminated macromers.58 Each obtained macromer was reacted with 2-hydroxyethyl acrylate (HEA) and TDI to form PFAD-based urethane acrylate resins with acrylate as side chains. UV-cured PFAD-based urethane acrylate resins with 15–55% PFAD resulted in hard films with Tg values range of 35–62 °C. UV-cured PFAD-based urethane acrylate resin containing 70% PFAD formed a soft film with Tg < 30 °C. However, the UV-cured of PFAD-based urethane acrylate resins with 45% PFAD film displayed the best chemical and physical properties, which would be useful for wood coating applications. In another study, Kosheeladevi et al. presented the application of 9,10-dihydroxystearic acid (DHSA) from palm oil as an internal emulsifier or isocyanate-reactive compound for waterborne polyurethane dispersions (WPUDs) productions.99 DHSA is a promising candidate to fully or partially substitute 2,2-bis(hydroxymethyl)-propionic acid (DMPA) in the preparation of WPUDs. The lower melting temperature of DHSA than DMPA benefited in reducing the reaction time and temperature in the early phase of the synthesis of producing a homogenous mixture with polyol. In terms of film hardness, the WPUDs film with a higher DMPA content had a better hardness value than those DHSA-contained films. The reason was due to the hard property of DMPA as a smaller compound compared to DSHA, a big compound with a C-18 carbon chain length, as shown in Fig. 13. Consequently, DHSA contributes to softness and flexibility to the films. The WPUDs films and coatings with the mixture of 0.5/0.5 of DMPA/DHSA content exhibited very good properties in tensile strength, hardness, elongation at break, and water resistance.
The reported articles show that PUs can be used in adhesive and coating applications. For adhesive applications, palm oil-based PUs demonstrated better mechanical strength than commercially available wood adhesives Titebond™ and Weldbond™.59,60 These results showed the potential of palm oil-based PUs to replace petrochemical-based PUs for adhesive application. Moreover, the utilization of PFAD as raw material feedstock and DHSA as an isocyanate compound for waterborne polyurethane dispersions (WPUDs) production will increase the sustainability and environmentally friendly process.
Somarathna et al. studied palm-based PU elastomers properties for potentially being applied as a protective coating for strengthening structures of concrete and from impact loading.100 The palm-based PU elastomers were prepared via the pre-polymerization method from a reaction of palm-based polyol and 4,4′-MDI with the addition of different content of PEG as a plasticizer at ambient temperature and free catalyst. The PEG content in PU elastomers significantly influenced the mechanical performances of PUs. Palm-based PU elastomers with 2–8% w/w PEG potentially be used for strengthening concrete applications in regards to their overall performances of high strain characteristics, good moduli, and strength properties. Moreover, Mohd Norhisham et al. reported on the preparation of soft PU elastomers with adhesion properties from polyols of palm olein and palm oil fatty acid methyl ester.81 Bio-based polyols were used as the source of polyols for PUs preparation, which consists of a polyol of palm oil fatty acid methyl ester-ethylene glycol (PolyFAME-EG) and commercially available palm olein polyols (Pioneer M-60 and Pioneer E-135). The obtained PU elastomers had high renewable content of more than 70%. PU elastomers with an NCO/OH ratio of 0.73 displayed adhesion properties with peel adhesion in the range of 1.98–2.27 N per inch, which was potentially used as a pressure-sensitive adhesive with high renewable content. Furthermore, Liu and coworkers reported on the study of bio-based elastic PU (EPU) for controlled-release urea fertilizer produced from the waste of frying palm oil.71 Production of PU was performed using polyol, MDI, DBTL, and acetone. EPU was prepared using a similar formulation for making PU with the addition of acrylonitrile (AN as a second monomer). The acrylonitrile created an interpenetrating polymer network for EPU via physical crosslinking. The PU and EPU were further coated with various concentrations of urea fertilizer and subsequently cured, which resulted in palm oil-based PU-coated urea fertilizer (PCU) and EPU-coated urea fertilizer (ECU). The surface of ECU displayed more hydrophobicity than PCU. In terms of the cumulative N release rates, the ECU displayed slower release than PCU, which was excellent for controlled-release fertilizer characteristics. Therefore, the modification of PU using AN was crucial to make the coatings denser and more hydrophobic. Due to its elasticity, the ECU exhibited different swelling volumes on diverse release days as compared to PCU, as shown in Fig. 14(a) and (b). The volume of ECU was twice bigger than the initial volume on day 49th and shrank on day 77th when close to the complete release of nutrients, as shown in Fig. 14(c).
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Fig. 14 Swelling rates of (a) PCU, (b) ECU, and (c) picture of swollen ECU coated with 7% urea fertilizer. Reproduced with permission from ref. 71 Copyright 2019 Elsevier. |
Gomez et al. reported on the synthesis of PUs from residual palm oil (RPO) polyol for biodegradable polyurethanes.72 RPO has a much lower iodine value (IV) number than that refined palm oil but a higher number of free fatty acid (FFA) contents. Also, 10% of jatropha oil (JO) or algae oil (AO) was added to the RPO. Among all the synthesized RPO-based PUs, thermal properties of RPO-based PU with the addition of AO > RPO-based PU with the addition of JO > RPO-based PU. Moreover, Rasli Rosli et al. reported on the synthesis of shape-memory PU (SMPU) via two-step polymerization of polycaprolactone (PCL), isocyanates, and palm kernel oil-based polyol.101 Different ratios of 4,4-methylenebis (cyclohexyl isocyanate) (HMDI) and IPDI were used to prepare SMPU. The SMPU with high IPDI content resulted in better shape-memory behavior; meanwhile, SMPU with high HMDI content led to SMPU with better chain flexibility.
The natural behavior of palm oil-based polymer will be more flexible due to the higher saturated fatty acid content in palm oil. Thus, the production of thermoplastic and elastomers materials will be in accord with the natural properties of palm oil-based polymers. In terms of preparing thermosetting PUs, it will be easier to choose vegetable oils with more unsaturated fatty acids content. Therefore, less modification will involve in the preparation of PUs thermosetting.
Table 4 summarizes the types of polyols, isocyanates, chain extenders, and charge carriers that have been used in the preparation of PUs for electrolyte applications. Based on Table 4, the incorporation of ionic salt as a charge carrier significantly improves the conductivity properties of PUs. Among all the ionic salt, the inclusion of 25 wt% MPII to palm-based PU presented the highest conductivity of 9.07 × 10−4 S cm−1. From these reported studies, the conductivity properties of PUs are affected by the type and concentration of charge carriers, the isocyanate to polyol ratio, and the type of isocyanate selected in PU formulation.
Type of polyol and isocyanate | Chain extender | Charge carrier | Tg (°C) | The highest conductivity (S cm−1) | Ref. |
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a NA: not available. The data was not available in the reference. | |||||
Palm kernel oil-based polyol (PKO-p) and 2,4′-MDI | PEG | Iodopropane | 17 | 2.38 × 10−9 | 102 |
PKO-p and 4,4′-MDI | PEG | LiClO4 | 89.65 | 1.19 × 10−7 at 298 K | 103 |
5.01 × 10−5 at 373 K | |||||
PKO-p and 2,4′-MDI | — | LiI | 30 | 5.33 × 10−5 | 104 |
PKO-p and 2,4′-MDI | — | LiCF3SO3 | NAa | 1.6 × 10−5 | 105 |
Waste cooking oil-based polyol and 4,4′-MDI | — | LiTFSI | 361.3 | 5.76 × 10−6 | 106 |
Waste cooking oil-based polyol and HDI | — | LiTFSI | 313.6 | 6.03 × 10−7 | 106 |
PKO-p and 2,4′-MDI | — | LiI | NAa | 7.6 × 10−4 | 107 |
PKO-p and 2,4′-MDI | — | 1-methyl-3-propylimidazolium iodide (MPII) | 58 | 9.07 × 10−4 | 108 |
PKO-p and 4,4′-MDI | PEG | — | 78.1 | NAa | 109 |
PKO-p and 4,4′-MDI | PEG | LiClO4 | NAa | NAa | 110 |
Interestingly, this palm kernel oil-based polyurethane for screen-printed electrode (SPE) was applied for histamine detection in mackerel fish.110 Histamine is an essential chemical compound that can be used as a parameter in allergic reactions and the freshness of food. The PU-LiClO4 film on the SPE surface had pores to entrap histamine and produce an anodic peak at the specific potential. A reasonable limit of quantification (LoQ) and limit of detection (LoD) was observed in the histamine concentrations range of 0.015–1 mmol L−1. The interfering compounds, i.e., cadaverine and putrescine, did not disrupt the signal from histamine. This method was validated for histamine detection in mackerel fish, which resulted in good accuracy with acceptable relative standard deviation (RSD). SPE-PU-LiClO4 demonstrated a simple yet good selectivity, posing the promising potential for histamine determination in mackerel fish.
A study on the effect on morphology and properties of blending 10 wt% epoxidized palm oil (EPO) with epoxy resin (based on DGEBA) was reported by Sarwono et al.115 Incorporating 10 wt% of EPO into epoxy resin resulted in lower Tg and mechanical properties due to the decrease in crosslinking density and the plasticizer effect from EPO. Fernandes and coworkers reported on blends of EPO and DEGBA-based resin.116 A series of epoxy resin blends of commercially available epoxy resin diglycidyl ether of bisphenol-A (DEGBA-based resin) and epoxidized vegetable oil-based resin (epoxidized neat, waste, and purified vegetable oils) with various loading (5–30 wt%) was prepared.116 The vegetable oil was a blend of palm and rapeseed oil with an approximate ratio of 3:
1. The waste vegetable oil (WVO) was used in deep fat fryers for 4 days. WVO was purified by single117 and multiple extractions and subsequently purified with activated carbon.118 Moreover, Issam et al. reported on the study of properties of blending various ratios of palm oil-based alkyd to epoxy resins.113 In general, alkyd resin and epoxy resin showed good compatibility. The higher composition of epoxy resin resulted in smoother surface morphology due to more effective crosslinking between oxirane functional groups of epoxy resin to the hydroxyl functional groups of alkyd resin. In terms of hardness and surface smoothness, the optimal blending alkyd/epoxy resin ratio was 30
:
70. The blended resins of higher epoxy resins content displayed better water and chemical resistance and better adhesion compared to the sole alkyd resin.
Thermoset from the bio-based resin was prepared from modification of epoxidized palm oil via acrylation and then maleinization reaction, as shown in Fig. 15.119 The acrylation reaction of epoxidized palm oil added a reactive double bond onto triglyceride, as shown in Fig. 15(a).120 Meanwhile, the maleinization reaction was done to introduce acid functionality onto triglyceride (see Fig. 15(b)), thus resulting in more crosslink sites.119 Further studies on the acrylated epoxidized palm oil (AEPO) were carried out in preparation for hybrid thermosets derived from vinyl ester resin and AEPO.121 Variations of content loading of 5–20 wt% of AEPO were used to replace vinyl ester petrochemical-based resin. Increasing the content of AEPO in the hybrid thermosets resins led to lower flexural modulus and strength, whereas the flexural strain increased. The incorporation of 5 wt% AEPO displayed a proper balance of stiffness/toughness, which was considered a suitable matrix resin for further use in composite applications. Moreover, a study on the degree of conversion (DOC) of maleinated acrylated epoxidized palm oil (MAEPO) with isobornyl methacrylate (ISBMA) for UV-curable pressure-sensitive adhesives (PSA) was reported by Tugiman et al.122 Furthermore, Salih and coworkers reported on the synthesis of epoxidized palm oil acrylate (EPOLA) for coating applications.120 The EPOLA-cured film with Irg-187 displayed higher conversion, hardness percentages, and crosslinking density, accompanied by better thermal and mechanical properties compared to EPOLA film cured with D-1173. Meanwhile, a comparative study of acrylated epoxidized jatropha oil and acrylated epoxidized palm oil was reported by Aung et al.123 The acrylated epoxidized jatropha oil displayed better scratch resistance, glossiness, hardness, and shorter UV curing time than that acrylated epoxidized palm oil. This observation was due to jatropha oil having higher unsaturated content in its triglyceride than palm oil. Moreover, Sofian Alias and coworkers reported on the preparation of palm oil-based urethane acrylate (POBUA) as a green anticorrosion coating.124 The acrylic groups in POBUA provide CC linkages to be crosslinked by UV irradiation, which acts as a barrier coating to avoid attacks from corrosive agents on the surface of the mild steel surface. More studies on radiation-curable palm oil-based products have been reviewed by Tajau et al.125
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Fig. 15 Modification of epoxidized palm oil (EPO) via (a) acrylation using acrylic acid to form AEPO or EPOLA and (b) maleinization using maleic anhydride to produce MAEPO. |
Mu and coworkers reported on the preparation of palm oil-based epoxy resins with recyclability, high mechanical strength, and reconfigurable shape-memory properties.126 Initially, palm oil was converted to palm oil-based methyl methacrylate (PMA) monomer by the amidation and esterification method. The PMA monomer was modified into epoxy monomer (EPMA) and further free polymerized. The produced polymer was cured with citric acid (CA) in various feed ratios of acid/epoxy at 90 °C without using a catalyst to form palm oil-based epoxy resins (EP-CA). The EP-CA resins had ester and hydroxyl functional groups in the oxirane crosslinking network that can perform the transesterification process at elevated temperatures, which leads to rearrangement capability. Therefore, EP-CA resins could be reshaped, recycled, and welded without losing the resin's mechanical properties. Furthermore, this dynamic chemical crosslinking contributed to EP-CA resins for shape-memory properties. Both temporary and permanent shapes can be controlled at different temperatures of ≥120 °C, as shown in Fig. 16.
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Fig. 16 Pictures of EP-CA with a ratio acid/epoxy of 40 undergoing permanent and temporary shape reconfigurations. Reprinted with permission from ref. 126 Copyright 2020 American Chemical Society. |
In summary, using palm oil as a feedstock for polyepoxide synthesis has shown promising results based on the product's general thermal and mechanical properties and potential applications. The reported derivatizations reactions to obtain compounds with more oxirane ring functional groups, which are essential for the formation of the resin, are relatively straightforward. In addition, the polymerization reaction (curing process) is relatively simple. Therefore, further exploration of palm oil-based epoxy resins would be an engaging research route, especially for the upscaling means.
Mohd Sari and coworkers reported on the preparation of lightweight concrete incorporated with palm kernel oil (PKO)-based polyurethane for non-load bearing structural application.130,131 Lightweight-concretes were prepared by two different methods: mixing fresh concrete (mixture of cement, water, and fine sand) with PKO-based polyol and crude MDI130 or by adding ground PKO-based PU to fresh mix concrete.131 At the optimum loading of PKO-based PU, the produced lightweight concrete showed a density of 1600 kg m−3 with compressive strength of 7.8 (ref. 130) and 17.5 (ref. 131) MPa, respectively. Moreover, Alamawi et al. reported on the use of PKO-based polyol (PKO-p) as a partial replacement for bituminous binder.132 They described that the incorporation of PKO-p as a bio-binder (BIB) resulted in similar chemical and thermal properties compared to conventional binders, and the BIB can be adjusted for suitable applications. Another report on bitumen modification was done by Vural kök and coworkers.133 Modifying bitumen by adding 7 wt% of palm oil-based PU with an NCO/OH ratio of 4:
1 resulted in the same or even better performance in elasticity as well as high and low-temperature properties compared to bitumen modified with 4 wt% SBS.
Thermosets polymer can also be applied as a polymeric matrix in the production of polymer composites. Nik Salleh and coworkers reported on radiation-curable hybrid coatings that used silico-organic nanoparticles as filler and epoxy acrylates or epoxidized palm oil acrylate (EPOLA) as a polymeric matrix, which resulted in polymer nanocomposites with excellent resistance to abrasion properties as well as scratch performance compared to that of pure acrylates.134 Moreover, Wu et al. prepared a series of palm oil-based thermosets for fiber-reinforced polymer composites that were synthesized by free radical copolymerization of a palm oil fatty acid-ethyl acrylamide (POFA-EA) and natural phenolic (NP) crosslinkers.135 NPs consist of methyl gallate-methacrylate (GM), eugenol-methacrylate (EM), and tannic acid-methacrylate (TM). The thermosets of POFA-EA/GM and POFA-EA/TM with high bio-based content displayed superior mechanical properties and Tgs, better than other vegetable oil-based analogous. The obtained thermosets were then used as the matrix for producing bamboo fibers-reinforced bio-composites via compression molding technique, which displayed comparable mechanical properties and Tgs to their petrochemical-based composite counterparts. Further studies on these palm oil (PO)-based thermosets for fiber-reinforced polymer composites were carried out by preparing copolymer from POFA-EA with isosorbide-methacylate (IM).136 The composites of palm oil-based thermosets reinforced with kenaf, glass, bamboo, and carbon fibers displayed higher Tgs of 136–193 °C compared to their petrochemical-based counterparts. Moreover, the PO-based thermosets matrix also exhibited chemical degradation in an alkali solution that benefited from recycling high-performance reinforcement like carbon fibers. Improvement from their previous studies, Wu et al. reported on the utilization of the photo-curing 3D printing method to prepare highly-customizable PO-based thermoset composites reinforced with micro-scale bamboo fibers (MBFs).137 The PO-based composite inks were formulated by mixing POFA-EA/EM, various loading of MBFs (neat or treated with methacrylic anhydride), and Irgacure 819 (a photo-initiator). Treating MBFs with methacrylic anhydride increased interfacial adhesion between matrix and fibers, improving composites' mechanical properties. This photo-curing 3D printing method provided a more straightforward technique for customizing the shape of polymer composite, as shown in Fig. 17(a). Moreover, PO-based composites fully degraded in alkaline solution and then MBFs were recovered by filtration and drying process, see Fig. 17(b). In Fig. 17(c), the mechanical properties of PO-based composites of original and reprinted using recycled MBFs demonstrated recovery efficiencies of tensile and flexural strength of 68.7% and 80%, respectively. Reprinted PO-based composite using recycled MBFs preserved high shape reliability, as shown in Fig. 17(d).
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Fig. 17 (a) PO-based composites printed in the shape of a hexagon and a house, (b) complete degradation of printed PO-based composite in alkaline solution (3 wt%) after 3 h for MBFs recovery, (c) recovery efficiencies of the tensile and flexural strength of PO-based composites original and reprinted using recycled MBFs after alkaline degradation, and (d) reprint of PO-based composite reinforced with recycled MBFs. Reproduced with permission from ref. 137 Copyright 2022 Elsevier. |
Koay et al. compared the properties of composites from cocoa pod husk (CPH)-filled polypropylene (PP) with the addition of different green coupling agents (GCA) from coconut oil (GCA-C) and palm oil (GCA-P), respectively.138 The variation loadings of GCA-C and GCA-P were added in PP/CPH composites to enhance adhesion between matrix surfaces and filler. At the optimum loading of GCA-C or GCA-P at 3 wt%, the PP/CPH composites displayed significant increases in tensile strength and tensile modulus. GCA-C performed better in increasing interfacial adhesion between the PP matrix and CPH than GCA-P. Furthermore, GCA-P of 3 wt% was further used in the study on the torque rheological performance of PP/CPH composites.139 The PP/CPH composite's rheological behavior depended on the change in viscosity from the variation loading of filler and adhesion between the polymer matrix and CPH. At higher filler loading, higher melt viscosity was observed due to CPH agglomeration and better adhesion of matrix filler in the addition of GCA-P.
Islam et al. reported on the preparation of various cured composite films from palm oil-based polyalkyds with multiwalled carbon nanotubes (MWCNTs),140 montmorillonite nanoclays (MNCs),141 or a combination of MWCNTs with or without ZnO.142 The polyalkyds were prepared by alcoholysis using glycerol and CaO, and subsequently esterification using phthalic anhydride or maleic anhydride.140–142 The addition of nanomaterials with or without ZnO was carried out during sonication or in situ during the esterification reaction. The concentration of 1.0 wt% was the optimal concentration for all the cured films. The addition of nanomaterial via in situ was the best way to incorporate it into the polyalkyds matrix since it resulted in better interfacial interaction and entanglement between the nanoparticles and the polymeric chains. As we compared the mechanical properties of the polyalkyd composites films, the film incorporated with MNCs141 showed better mechanical properties than the film incorporated with MWCNTs140 or a combination of MWCNTs with or without ZnO.142 However, polyalkyd composites film incorporated with MWCNTs140 displayed the highest Tmax at 392 °C during thermal degradation.
There are several reports on the preparation of polymer composites from palm oil-based polyurethane with various types of reinforcements or fillers. In polymer composite, polyurethane act as a polymer matrix. There are several types of reinforcements or fillers, such as multi-walled carbon nanotubes (MWCNTs),143 diaminopropane-montmorillonite (DAP-MMT) nanoclay,144 organoclay montmorillonite (OMMT),145 clay nanocomposites,146 cellulose nanocrystal (CNC),147 aluminum (Al) powder,148 bentonites and chitosan,149 halloysite nanotubes (HNTs),150 empty fruit bunch (EFB) from the oil palm tree,151 and reclaimed rockwool fibers from discarded industrial piping insulation wastes.152 Moreover, Jasmi et al. reported on the polyurethane-graphene nanocomposite for application as an optical fiber Bragg grating (FBG) temperature sensor.153 PU-graphene nanocomposite displayed better thermal stability at 217 °C and conductivity of 1.39 × 10−9 S cm−1 than neat PU at 204 °C and 9.43 × 10−11 S cm−1. The incorporation of reinforcement/filler into the PU matrix influenced the PU morphology and the polymer composite's mechanical and thermal properties.
Based on the reported work of palm oil-based polymer composites discussed above, it is revealed that most of the explored works are intended to improve mechanical properties, thermal stability, chemical resistance, and physical resistance. Indeed, the composites showed improvements compared to their precursor materials counterparts. These composites should be promising materials for structural application, e.g., constructions, coatings, containers, etc. On the other hand, research for different types of applications is still yet to be explored. For instance, it is already widely known that polymer composite has been developed for biomedical applications, separation and purification technology, and catalysis. Therefore, with a similar approach, it is expected that those fields of applications are also possible for palm oil-based polymer composites.
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Fig. 18 FESEM picture of PMUF microcapsule at (a) 500× and (b) 100![]() |
Ling and coworkers reported on the synthesis and characterization of poly(alkyd-urethane)s (PAUs) from various vegetable oils.165 Five types of poly(alkyd-urethane) were consist of palm oil-based poly(alkyd-urethane) (POPAU), sunflower oil-based poly(alkyd-urethane) (SFPAU), palm-sunflower oil-based poly(alkyd-urethane) (POSFPAU), soy oil-based poly(alkyd-urethane) (SOPAU), and palm-soy oil-based poly(alkyd-urethane) (POSOPAU). SFPAU exhibited drying superiority with the highest pencil hardness and impact resistance results, following this order: SFPAU > SOPAU > POSFPAU > POSOPAU > POPAU. There are also acrylated palm oil-based polyurethanes. Physical properties of acrylated palm oil-based polyurethanes using different reactive diluents were reported by Oon et al.166 Acrylated palm oil-based PU was formulated with various reactive diluents (monoacrylate, diacrylate, and triacrylate) with various content in the presence of photo-initiator (Irgacure) and film formation agent (NMP). PU film using monoacrylate displayed better properties than other films using diacrylate or triacrylate. Moreover, Mas'ud et al. reported on the direct polymerization of double bonds from the unsaturated fatty acids in palm oil triglycerides without any triglyceride modification.167 Palm oil-based polymer was prepared by cationic polymerization of waste-used cooking palm oil using boron trifluoride-etherate as a catalyst under microwave irradiation methods with various power settings and reaction times. Polymerization was considered complete by treating the reaction mixture under 600 or 800 watts for 5 minutes of microwave irradiation since a solid product was formed.
Hoong et al. reported on the preparation of high molecular weight polyols by cationic ring-opening copolymerization of epoxidized natural oils (ENO) with tetrahydrofuran (THF).168 Epoxidation of the respective natural oils resulted in epoxidized methyl oleate (EMO), epoxidized soyabean oil (ESO), epoxidized cocoa butter (ECB), and epoxidized palm oil (EPO). Polyols were produced from the copolymerization of the respective epoxidized natural oil with THF. Moreover, modification of triglycerides from red palm olein was carried out by reaction of epoxidized palm olein with 5-norbornene-2-carboxylic acid to produce norbornene palm olein (NPO) with one norbornene (NBE) unit per NPO.169 The copolymerization of NPO and NBE was performed via ring-opening metathesis polymerization (ROMP) with 2nd generation Grubbs (G2) catalyst at 30 °C. The produced copolymer also had crosslinked structure since it had poor solubility in acetic acid, water, and many organic solvents. Ring-opening copolymerization of norbornene-based palm olein (NPO) monomer with norbornene was monitored in real-time by time-domain 1H NMR (TD-NMR).170 Another study about copolymers from palm oil was the copolymerization of palm oil with sulfur via the inverse vulcanization method.171 The elemental sulfur has an octet ring structure that will be opened if it is heated higher than its floor temperature (>159 °C) to form sulfur atoms with a biradical linear chain, accompanied by color changes from yellow to orange. The biradical linear chain of sulfur will homopolymerize to form polysulfide, which is not stable and subsequently depolymerize leading to the reformation of elemental structure crystals. Therefore, by adding palm oil that has unsaturated fatty acids to the biradical linear chain at 170 °C resulted in poly(S-Palm oil). This poly(S-Palm oil) can potentially be applied in oil spill removal, mercury removal, palladium capture, and slow-release fertilizers applications.
Type of polymers | Application | Ref. |
---|---|---|
Polymeric methyl ester sulfonate (PMES) | Polymeric surfactant for enhanced oil recovery (EOR) | 41 |
Plant oil-based polyvinyl ethers | Paint binders | 42 |
Acrylated palm olein (APO) | Drug delivery application | 29 |
Vegetable oil-based acrylate prepolymer | UV-curable coating | 43 |
Palm oil fatty acid-ethyl acrylamide (POFA-EA) | Thermoplastic polymer | 44 |
Polyesters/alkyds | Coatings | 47, 50 and 51 |
UV-curable resin/coating | 35, 48 and 49 | |
Plasticizer in polyvinyl chloride (PVC) | 52–54 | |
Additive in polyaniline (PANI) | 55 and 56 | |
Additive in liquid detergent | 57 | |
Polyol ester | Polymeric nanoparticles for drug carrier | 80 |
Polyurethanes (PUs) | PU foams for tissue engineering in biomedical application | 61–63 |
PU foams | 28, 77, 83, 93–95 | |
Adhesives and coatings | 58–60, 96–98 | |
Thermosetting | 31 and 75 | |
Thermoplastic or elastomers | 75, 79, 81, 100 and 101 | |
Controlled-release urea fertilizer | 71 | |
Polymer electrolytes | 102–110 | |
Polyepoxides | Polymeric matric for bio-based composite | 119 and 121 |
UV-curable pressure-sensitive adhesives (PSA) | 122 | |
Coatings | 120, 123 and 124 | |
Thermoset with reconfigurable shape-memory properties | 126 | |
Polymer composites | Lightweight-concrete | 130 and 131 |
Bitumen binder and modified bitumen | 132 and 133 | |
Coatings | 134, 140–142 | |
Fiber-reinforced polymer (FRP) composites | 135–137 | |
Green coupling agent | 138 and 139 | |
PU foams reinforced with nanoparticle | 143–145, 147 and 150 | |
PU foam with aluminum filler | 148 | |
PU foam with reclaimed rockwool fibers for thermal-insulating application | 152 | |
PU reinforced with nanoparticles or filler for surface-coating application | 146 and 149 | |
Optical fiber Bragg grating (FBG) temperature sensor | 153 | |
Polyacid | Elastomers or hydrogels for biomedical application | 154 |
Chitosan-palmitic acid copolymer | Polymeric micelles for drug delivery of tamoxifen | 157 |
Palmitic acid-based polyamide | Drug delivery of methylprednisolone | 158 |
Stearic acid-based polyesteramide | Polymer carrier in solid dispersion for mefenamic acid | 159 |
Carbamoylethyl pullulan-grafted-palmitic acid diblock copolymer | Drug delivery of raloxifene to breast tumor | 160 |
Copolymer of higher fatty acids, glycidyl methacrylate, and p-anisic acid | Polymeric cold flow improver in blends of bio-diesel and diesel | 161 |
Alkyd microencapsulation | Palm oil-based alkyd as a self-healing agent for epoxy matrix | 163 and 164 |
Poly(alkyd-urethane)s | Surface coating or binder in composites | 165 |
Acrylated palm oil-based PU | Coating | 166 |
Poly(S-Palm oil) | Mercury removal, palladium capture, slow-release fertilizers, and oil spill removal | 171 |
Palm oil and its fatty acids are known as non-toxic, biodegradable, and bio-renewable resources.156 These environmentally friendly characteristics are essential properties for materials in biomedical applications. These should be a good enough reason to urge more research on developing polymers derived from palm oil or its fatty acids for biomedical applications. Moreover, this bio-based polymer should be prepared via a non-toxic and green process, such as using water as a dispersant or enzyme as a catalyst. The use of bio-renewable resources and the green process will guarantee the sustainability of the preparation of this bio-based polymer. Therefore, environmental damage and toxic waste can be avoided as far away as possible.
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