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Plasticizers Derived from Biomass Resources: a Short Review

Plasticizers Derived from Biomass Resources: a Short Review

Review Derived from Biomass Resources: A Short Review

Puyou Jia 1,*, Haoyu Xia 2, Kehan Tang 2 and Yonghong Zhou 1,*

1 Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry (CAF); Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University; Key Lab of Biomass Energy and Materials, 16 Suojin North Road, Nanjing 210042, China 2 College of Chemical Engineering, Nanjing Tech University, 30 Pu Zhu Road, Nanjing 211800, China; [email protected] (H.X.); [email protected] (K.T.) * Correspondence: [email protected] (P.J.); [email protected] (Y.Z.); Tel.: +86-025-85482520 (P.J.); +86-025-85482777 (Y.Z.)  Received: 1 November 2018; Accepted: 22 November 2018; Published: 24 November 2018 

Abstract: With rising environmental concerns and depletion of petrochemical resources, biomass-based chemicals have been paid more attention. (PVC) plasticizers derived from biomass resources (vegetable oil, cardanol, vegetable , glycerol and citric acid) have been widely studied to replace petroleum-based o- plasticizers. These bio-based plasticizers mainly include epoxidized , plasticizer, macromolecular plasticizer, flame retardant plasticizer, citric acid plasticizer, glyceryl ester plasticizer and internal plasticizer. Bio-based plasticizers with the advantages of renewability, degradability, hypotoxicity, excellent solvent resistant extraction and plasticizing performances make them potential to replace o-phthalate plasticizers partially or totally. In this review, we classify different types of bio-based plasticizers according to their chemical structure and function, and highlight recent advances in multifunctional applications of bio-based plasticizers in PVC products. This study will increase the interest of researchers in bio-based plasticizers and the development of new ideas in this field.

Keywords: plasticizer; polyvinyl chloride; biomass resources; review

1. Introduction Plasticizers are among the most important additives required for the processing of materials, especially polyvinyl chloride (PVC) , which accounts for more than 60% of the total yield of auxiliaries [1,2]. Traditional petroleum-based phthalate plasticizers are the most widely used globally. The yield and consumption of traditional phthalate plasticizers account for a large proportion of the total plasticizer production and sales, but they are gradually limited due to potential threats to human health and environment. Strict regulations on environmental protection and safety have been formulated and carried out. The development of environmentally-friendly non-toxic plasticizers and biodegradable bio-based plasticizers to replace has been a research hot spot. Non-toxic green plasticizers with high performance, oil resistance, extraction and migration resistance used in electrical insulation, food packaging, and medical and health products are constantly being developed, produced and applied. Plasticizers are functional additives, which are used to improve flexibility, plasticity, processability, and elongation of polymers, especially in PVC products [3]. When a plasticizer is added to a polymer, the intermolecular subvalent bond force is weakened, the crystallinity is lowered, the relative movement between the molecular segments is increased and the plasticity of the material is improved. Therefore, plasticizers are mainly used to decrease hardness, softening temperature, elastic modulus, and temperature of

Polymers 2018, 10, 1303; doi:10.3390/polym10121303 www.mdpi.com/journal/polymers Polymers 2018, 10, 1303 2 of 27 Polymers 2018, 10, x FOR PEER REVIEW 2 of 28 polymers,include citric while acid improving ester, , their flexibility , and halogenated elongation. Non-phthalatealkanes, and plasticizer compounds alternatives [4–7]. includeWith the citric increase acid ester, in people’s phosphates, awareness polyesters, of environmental halogenated protection, alkanes, and especially epoxy compounds the discovery [4– 7 of]. Withpotential the increasethreats to in human people’s health awareness by phthalate of environmental plasticizers and protection, environmental especially pollution the discovery problems, of potentialthe global threats hygienic to human requirements health by for phthalate plastic plasticizers additives are and increasing environmental. The pollution use of n problems,on‐phthalate the globalplasticizer hygienic alternatives requirements has been for gradually plastic additives introduced are increasing. [8–10]. The use of non-phthalate plasticizer alternatives has been gradually introduced [8–10]. 2. Plasticizer Derived from Biomass Resources 2. Plasticizer Derived from Biomass Resources 2.1. Biomass Based Epoxidized Plasticizer 2.1. Biomass Based Epoxidized Plasticizer Epoxidized plasticizer is a kind of environmentally-friendly plasticizer used in , rubberEpoxidized industry, plasticizerand coatings is aand kind new of environmentally-friendlypolymer materials [11,12]. plasticizer Compared used with in other plastics plasticizers, industry, rubberthe epoxy industry, group andin the coatings epoxidized and new plasticizer polymer structure materials could [11,12 absorb]. Compared and neutralize with other the plasticizers, hydrogen thechloride epoxy released group in by the PVC epoxidized during plasticizer light or thermal structure de couldgradation, absorb which and neutralizerestricts or the delay hydrogens the chloridecontinuous released decomposition by PVC during of PVC light, endow or thermals PVC degradation, products with which good restricts light and or delaysthermal the stability continuous and decompositionextends their service of PVC, life. endows Besides, PVC it has products been approved with good for light food and packaging thermal stability and medical and extends equipment their servicematerials life. in Besides, many cou it hasntries been and approved regions fordue food to the packaging extremely and low medical toxicity equipment of epoxidized materials plasti incizers many, countrieswhich has andmade regions its production due to the and extremely price grow low in toxicityrecent years. of epoxidized Biomass-based plasticizers, epoxidized which plasticizers has made itsmainly production include and epoxidized price grow vegetable in recent oils, years. epoxidized Biomass-based fatty acid epoxidized and plasticizers epoxy group mainly containing include epoxidizedcardanol derivatives vegetable.oils, Currently, epoxidized vegetable fatty acid oils esters and e andpoxidized epoxy group fatty containingacid esters cardanolhave been derivatives. used on Currently,the market vegetable [13,14]. oils and epoxidized fatty acid esters have been used on the market [13,14]. Epoxidized soybean soybean oil oil (ESO) (ESO) is a collection is a collection of organic of compoundsorganic compounds obtained from obtained the epoxidation from the ofepoxidation soybean oil of thatsoybean have oil been tha widelyt have usedbeen aswidely plasticizers used as and plasticizers heat stabilizers and heat of PVC stabilizers materials. of PVC The chemicalmaterials. structure The chemical of ESO structure is shown of inESO Figure is shown1. Ferrer in Figure et al. found 1. Ferrer that et formulations al. found that based formulati on PVCons withbased different on PVC amounts with different of ESO amounts (from 30 of to 50ESO wt (from %) increased 30 to 50 compatibility wt %) increased and improved compatibility thermal and stabilityimproved [15 thermal]. Park etstability al. investigated [15]. Park the et al performances. investigated of the epoxy perform resinsances plasticized of epoxy with resins ESO plasticized [16]. They foundwith ESO that [16]. the They thermal found stability that the and ther glassmal transitionstability and temperature of epoxy temperature resins reduced of epoxy with resins the additionreduced with of ESO. the addition This phenomenon of ESO. This occurs phenomenon due to the occurs decreased due to density the decreased of the epoxydensity network. of the epoxy The additionnetwork. ofThe ESO addition causes theof ESO increase causes of stress the increase intensity of factor stress along intensity with thefactor flexural along strength. with the Zhao flexural et al. investigatedstrength. Zhao the et plasticizing al. investigated effect the of ESOplasticizing on effect of ESO succinate on polybut (PBS)ylene [17]. The succinate results (PBS) show [17] that. ESOThe results efficiently show improves that ESO elongation efficiently atimprove break ofs elongation PBS. The elongationat break of atPBS. break The reaches elongation a maximum at break valuereaches when a maximum the content value of ESOwhen is the 5 wt content % of PBS, of ESO 15 times is 5 wtmore % thanof PBS the, 15 elongation times more of purethan PBS. the Moreover,elongation ESOof pure enhances PBS. Moreover, the elongation ESO aten breakhances of the poly(lactic elongation acid) at break (PLA) of by p 63%.oly(lactic Xu et acid) al. studied (PLA) theby 63%. performances Xu et al. studied of PLA the plasticized performances with ESOof PLA [18 ].plasticized The results with show ESO that [18]. 9 wt The % results ESO increased show that the 9 elongationwt % ESO increase at breakd of the PLA elongation by about at 63%. break PLA of PLA blends by containingabout 63%. 6PLA wt %blends ESO showcontaining maximum 6 wt % tensile ESO strengthshow maximum and melt tensile strength. strength Zhu et andal. added melt maleic strength anhydride. Zhu et (MA)al. added to enhance maleic PLA’s anhydride reactivity (MA) with to ESO,enhance obtaining PLA’s anreactivity elongation with at ESO, break obtaining of about 140%an elongation [19]. This at can break be explained of about by140% the [19] grafting. This content can be ofexplained MA, which by the determines grafting thecontent properties of MA, of thewhich blends. determines Xu et al. the investigated properties the of effectthe bl ofends. ESO Xu on et melt al. rheologicalinvestigated properties the effect of such ESO as on shear melt viscosity rheological and properties melt strength such of as PLA shear blends viscosity [20]. and They melt chose strength melt flowof PLA index blends (MFI) [20] as. measuringThey chose rule. melt The flow results index show (MFI) that as PLA/ESO measuring blends rule. possessThe result mores show MFI than that purePLA/ESO PLA. blends Under possess the temperature more MFI rangethan pure of 160–180 PLA. Under◦C, PLAthe temperature blends with range 6 wt %of ESO160–180 achieve °C, P theLA maximumblends with melt 6 wt strength. % ESO achieve the maximum melt strength.

Figure 1. Chemical structure of epoxidizedepoxidized soybean oil.

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 3 of 28 Polymers 2018, 10, 1303 3 of 27 Our group synthesized a series of epoxidized castor oil polyol esters, epoxidized soybean oil polyol esters, and epoxidized tung oil methyl ester via alcoholysis and epoxidation, and investigated Our group synthesized a series of epoxidized castor oil polyol esters, epoxidized soybean oil their plasticizing effect on PVC films [13,21–23]. Figure 2 shows the synthesis route of epoxidized polyol esters, and epoxidized tung oil methyl ester via alcoholysis and epoxidation, and investigated castortheir oil plasticizing polyol ester. effect on Dynamic PVC films thermomechanical [13,21–23]. Figure2 shows analysis the synthesis (DMA) route and of differential epoxidized castor scanning caloroilimeter polyol (DSC) ester. Dynamic results show thermomechanical that these epoxidized analysis (DMA) vegetable and differential polyol esters scanning present calorimeter excellent plasticizing(DSC) results effect show on PVC that and these can epoxidized be used vegetableas main plasticizers polyol esters to present completely excellent replace plasticizing DOP in effect flexible PVCon films. PVC and can be used as main plasticizers to completely replace DOP in flexible PVC films.

FigureFigure 2. 2.SynthesisSynthesis of of epoxidized epoxidized castorcastor oil oil polyol polyol ester. ester. Vieira et al. investigated the effect of polyesterification of rice fatty acid on properties of natural Vieira et al. investigated the effect of polyesterification of rice fatty acid on properties of natural polymeric plasticizer with the usage of monopropylene glycol, octanol and [24]. polymTheeric experiment plasticizer shows with that the elongation usage of atm breakonopropylene of PVC materials glycol, plasticizedoctanol and with diethylene natural polymeric glycol [24]. The plasticizerexperiment reaches shows 371.2%. that elongation To learn more at break about of rice PVC fatty materials acid, Machado plasticized et al. obtainedwith natural two naturalpolymeric plasticizerepoxidized reaches plasticizers 371.2%. fromTo learn peracetic more acid about (NP-Ac) rice fatty and acid, peroctanoic Machado acid et (NP-Oc). al. obtaine Evend two though natural epoxidizedboth enhance plasticizers the performance from peracetic of PVC acid materials, (NP‐Ac) natural and epoxidizedperoctanoic plasticizers acid (NP‐Oc). derived Even from though NP-Ac both enhancepossess the better performance ability than of NP-Oc. PVC This materials phenomenon, natural may epoxidized result from a plasticizers higher degree derived of epoxidation from NP‐Ac in possessthe processbetter ability of reaction than with NP‐Oc. acid [This25]. Chaudharyphenomenon et al. may found result that from almost a allhigher kinds degree of epoxidized of epoxidation fatty in theacid process esters of (EFAE) reacti areon compatible with acid with[25]. PVCChaudhary and the efficiencieset al. found increase that almost with decreasingall kinds of molecular epoxidized fattyweight acid esters[26]. The (EFAE) alternatives are compatible achieve the balance with PVC of flexibility and the retention efficiencies and mechanical increase properties with decreasing after heat aging, especially compared with phthalate and trimellitate plasticizers. Espinosa et al. found that molecular weight [26]. The alternatives achieve the balance of flexibility retention and mechanical fatty acids obtained from rapeseed oil could serve as another cheaper raw source to produce plasticizers properties after heat aging, especially compared with phthalate and trimellitate plasticizers. Espinosa via thiol-ene addition and epoxidation reaction [27]. Silverajah et al. reported a comparative study et al. found that fatty acids obtained from rapeseed oil could serve as another cheaper raw source to on PLA/epoxidized blend about their mechanical and characterization [28]. They found producethat 1p wtlasticizers % of epoxidized via thiol palm-ene addition oil (EPO) and is enough epoxidation to enhance reaction the flexibility [27]. Silverajah and strength et al. ofreported PLA a comparativealong with study the flexural on PLA and/epoxidized impact properties. palm oil To b obtainlend about the best their performance, mechanical EPO(3), and characterization a mixture of [28]. EPOThey and found soybean that oil1 wt is added.% of epoxidized Pure PLA presents palm oil a tensile(EPO) modulusis enough value to enhance of 1054 MPathe flexibility while that and strengthof EPO(3) of PLA is 1214 along MPa. with Mulla the et flexural al. studied and the impact reaction properties. between PLA To andobtain EPO the by bestcasting performance, process EPO(3),at different a mixture weight of EPO ratio and and foundsoybean that oil the is elongationadded. Pure at break PLA reachespresents its a best tensile (about modulus 210%) when value of 1054the MPa ratio while of PLA/EPO that of EPO(3) blend isis 80/20 1214 MPa. [29]. Chieng Mulla etet al.al. studied studied the the cold-crystallization reaction between temperature PLA and EPO by castingwhen PLAprocess is blended at different with EPO weight [30]. ratio The resultsand found show that thethe mobilityelongation of the at polymericbreak reaches chains its is best (about 210%) when the ratio of PLA/EPO blend is 80/20 [29]. Chieng et al. studied the cold- crystallization temperature when PLA is blended with EPO [30]. The results show that the mobility of the polymeric chains is enhanced. Furthermore, mild interfacial adhesion between PLA and EPO is obtained after adding 1 wt % epoxidized palm oil. Benzene groups containing bio-based plasticizer

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 4 of 28 is also investigated. Chen et al. synthesized and used epoxidized castor oil based diglycidyl ester to replacePolymers DOP2018 ,[31].10, 1303 The synthesis route of epoxidized castor oil based diglycidyl ester is4 ofshown 27 in Figure 3. The addition of epoxidized castor oil based diglycidyl ester significantly enhances thermal Polymers 2018, 10, x FOR PEER REVIEW 4 of 28 stability, compatibility, and flexibility of PVC films, presenting plasticizing effect and thermal enhanced. Furthermore, mild interfacial adhesion between PLA and EPO is obtained after adding stabilizationis also investigated. of PVC films. Chen PVCet al. synthesizedfilms plasticized and used with epoxidized epoxidized castor castor oil based oil baseddiglycidyl diglycidyl ester to ester 1 wt % epoxidized palm oil. Benzene groups containing bio-based plasticizer is also investigated. showreplace good elongationDOP [31]. The at breaksynthesis (332.9%) route aofnd epoxidized remarkable castor increase oil based in flexibility diglycidyl and ester plasticizat is shown ionin than Chen et al. synthesized and used epoxidized castor oil based diglycidyl ester to replace DOP [31]. The Figure 3. The addition of epoxidized castor oil based diglycidyl ester significantly enhances thermal DOP.synthesis In addition, route of the epoxidized tung‐maleic castor oiltriglycidyl based diglycidyl esters, ester which is shown has similar in Figure chemical3. The addition structure of with stability, compatibility, and flexibility of PVC films, presenting plasticizing effect and thermal epoxidizedepoxidized castor castor oil oil based based diglycidyldiglycidyl, ester is also significantly reported enhances [32]. Figure thermal 4 stability,shows the compatibility, chemical andstructure stabilization of PVC films. PVC films plasticized with epoxidized castor oil based diglycidyl ester andflexibility synthesis of route PVC of films, epoxidized presenting castor plasticizing oil based effect diglycidyl, and thermal which stabilization shows similar of PVC plasticizing films. PVC effect show good elongation at break (332.9%) and remarkable increase in flexibility and plasticization than andfilms thermal plasticized stabilization with epoxidized on soft PVC castor films. oil based diglycidyl ester show good elongation at break DOP. In addition, the tung‐maleic triglycidyl esters, which has similar chemical structure with (332.9%) and remarkable increase in flexibility and plasticization than DOP. In addition, the tung-maleic epoxidized castor oil based diglycidyl, is also reported [32]. Figure 4 shows the chemical structure triglycidyl esters, which has similar chemical structure with epoxidized castor oil based diglycidyl, is and synthesis route of epoxidized castor oil based diglycidyl, which shows similar plasticizing effect also reported [32]. Figure4 shows the chemical structure and synthesis route of epoxidized castor oil and thermal stabilization on soft PVC films. based diglycidyl, which shows similar plasticizing effect and thermal stabilization on soft PVC films.

Figure 3. Synthesis of epoxidized castor oil based diglycidyl ester. Figure 3. Synthesis of epoxidized castor oil based diglycidyl ester. Figure 3. Synthesis of epoxidized castor oil based diglycidyl ester.

Figure 4. Synthesis of tung-maleic triglycidyl esters (TMTE).

Figure 4. Synthesis of tung-maleic triglycidyl esters (TMTE). Figure 4. Synthesis of tung-maleic triglycidyl esters (TMTE).

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EpoxidizedEpoxidized sunflower sunflower oil oil is well is well miscible miscible with withpolylactic (PLA) acid and (PLA) PVC and [33,34] PVC. Benaniba [33,34]. Benanibaet al. studied et al. the studied stabilizing the stabilizing effect of ESO effect on of the ESO thermal on the degradation thermal degradation of PVC [35] of PVC. The [ 35res].ults The suggest results suggestthat ESO that improves ESO improves the thermal the thermal degradation degradation of PVC. of PVC. Atek Atek et al. et investigated al. investigated the the migration migration of ofepoxidized epoxidized sunflower sunflower oil oil from from plasticized plasticized PVC PVC into into food food simulants [[36]36].. Chromatography–massChromatography–mass spectrometryspectrometry (GC-MS)(GC-MS) isis carriedcarried outout toto investigate the migrationmigration of ESO. Linoleic acid (C18:2) is used as external standard. The study provides new method to detect the loss of plasticizerplasticizer in plasticizedplasticized PVC materials. L Lardjaneardjane et et al. reported reported that that both both nature nature and and the content of plasticizers acted as influentialinfluential factorsfactors inin thethe migrationmigration ofof epoxidizedepoxidized sunflowersunflower oiloil [[37]37].. Cardanol is is an an important important chemical chemical raw raw material, material, which which has been has widely been widely used to used prepare to preparevarious variousplasticizers. plasticizers. Figure Figure 5 shows5 shows the chemical the chemical structure structure of of various various of of cardanol cardanol based based plasticizers. plasticizers. Epoxidized cardanol glycidyl , as shown in Figures 55 jand and6 ,6, is is synthesized synthesized from from cardanol cardanol via via substitution reactionreaction and epoxidation reaction [[33].33]. It It is used to p partlyartly replaced DOP in soft PVC films.films. Epoxidized cardanol glycidyl ether improves flexibilityflexibility and toughen soft PVC filmsfilms and shows similar volatility, volatility, extraction extraction and and exudation exudation resistance resistance compared compared with DOP. with Our DO groupP. Our also synthesized group also asynthesized kind of epoxidized a kind cardanol of epoxidized based plasticizercardanol using based quaternary plasticizer ammonium using quaternary phosphotungstate ammonium as catalystphosphotungstate [34]. Its chemical as catalyst structure [34]. isIts shown chemical in Figures structure5 and is7 .shown Epoxidized in Figures cardanol-based 5b and 7. plasticizersEpoxidized exhibitcardanol similar-based plasticizing plasticizers efficiency exhibit similar with DOP. plasticiz In addition,ing efficiency excellent withsolvent DOP. In resistance addition, and excellent lower volatilizationsolvent resistance of epoxidized and lower cardanol-based volatilization of plasticizers epoxidized maintain cardanol the-based long stabilityplasticizers property maintain of PVC the products.long stability The property obtained of cardanol-based PVC products.plasticizer The obtained is biologically cardanol-based safe withoutplasticizer acute is biologically toxicity, which safe makeswithout it acute possible toxicity, to be usedwhich in makes food packingit possible and to toys. be used in food packing and toys.

Figure 5. Chemical structure of cardanol based plasticizer.

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Figure 6. Synthesis of epoxidized cardanol glycidyl ether. Figure 6. Synthesis of epoxidized cardanol glycidyl ether. Figure 6. Synthesis of epoxidized cardanol glycidyl ether.

FigureFigure 7. 7.SynthesisSynthesis routes routes of ofepoxidized epoxidized cardanol cardanol based based plasticizer plasticizer (ECP). (ECP). Figure 7. Synthesis routes of epoxidized cardanol based plasticizer (ECP). 2.2. Polyester Plasticizer and Macromolecular Plasticizer 2.2. Polyester Plasticizer and Macromolecular Plasticizer 2.2. PolyesterThe polyester Plasticizer plasticizer and Macromolecular is usually preparedPlasticizer by polycondensation of dibasic acid and . The polyester plasticizer is usually prepared by polycondensation of dibasic acid and diol. It is It is called “permanent plasticizer” because of its large molecular weight, better water resistance, called “permanentThe polyester plasticizer” plasticizer because is usually of its prepared large molecular by polyconden weight,sation better of water dibasic resistance, acid and excellent diol. It is excellent oil and solvent extraction and longer service life [38]. Polyester plasticizer also has the oilcalled and solvent “permanent extraction plasticizer” and longer because service of its life large [38] molecular. Polyester weight, plasticizer better also water has theresistance, characteristics excellent characteristics of low toxicity and safety. It is suitable for application in the fields with high health of oil low and toxicity solvent and extraction safety. and It is longer suitable service for application life [38]. Polyester in the fieldsplasticizer with also high has health the characteristics and safety and safety requirements for plastic products such as beverage , interior decoration, children’s requirementsof low toxicity for plastic and safety. products It is such suitable as beverage for application hoses, interior in the fieldsdecoration, with highchildren’s health toys, and wire safety toys, wire and cable. In addition, polyester plasticizers are especially suitable for surface finishing andrequirements cable. In addition for plastic, polyester products plasticizers such as arebeverage especia hoses,lly suitable interior for decoration, surface finishing children’s of floor toys, tiles wire of floor tiles that are resistant to pollution, water or solvents due to its good durability. Traditional thatand are cable. resistant In addition to pollution,, polyester water plasticizers or solvents are dueespecia to lly its suitable good durability. for surface Traditional finishing ofp olyesterfloor tiles polyester plasticizers can be classified into esters, sebacic acid esters and esters plasticizersthat are resistant can be classifiedto pollution, into water adipic or aci solventsd esters due, sebacic to its acidgood esters durability. and phthalicTraditional acid p olyesteresters according to the type of dibasic acid structure. However, these polyester plasticizers are all derived accoplasticizersrding to the can type be of classified dibasic acid into structure. adipic aci However,d esters, sebacicthese polyester acid esters plasticizers and phthalic are all acid derived esters from petrochemical resources and are against requirements of sustainable development. Renewable fromacco petrochemicalrding to the type resources of dibasic and acidare against structure. requirements However, theseof sustainable polyester development. plasticizers are Renewable all derived biodegradable polyester plasticizers have been paid more attention by industry and academia due to biodegradablefrom petrochemical polyester resources plasticizer ands haveare against been paid requirements more attention of sustainable by industry development. and academia Renewable due to its good research and application prospects [39]. Moreover, compared with DOP, polyester plasticizers itsbi goododegradable research polyester and application plasticizer s prospectshave been [39]paid. more Moreover, attention compared by industry with and DOP academia, polyester due to still have the disadvantages of poor plasticizing efficiency, poor processability and low temperature plasticizersits good still research have the and disadvantages application prospectsof poor plasticizing [39]. Moreover, efficiency, compared poor processability with DOP, and polyester low performance [40,41]. Lindström et al. examined the influence of molecular weight and branching on temperatureplasticizers performance still have the [4 disadvantages0,41]. Lindström of pooret al. plasticizingexamined the efficiency, influence poor of molecular processability weight and and low the performance of polyester [42]. They chose linear and branched poly(butylene )s (PBA) to branchingtemperature on theperformance performance [40,41] of . polyesterLindström [42] et .al. They examined chose thelinear influence and branch of moleculared poly(butylene weight and study the relationship between plasticizing effect and chemical structure. They prepared the samples adipate)sbranching (PBA) on to the study performance the relationship of polyester between [42] .plasticizing They chose effect linear and and chemical branch edstructure poly(butylene. They with molecular weight ranging from 2000 to 10,000 g/mol, along with the content of branching agent preparedadipate)s the (PBA) samples to study with the molecular relationship weight between ranging plasticizing from 2000 effect to 10,000 and chemical g/mol, along structure with. theThey between 0% and 1.8%. They concluded that the degree of branching is most decisive in plasticizing contentprepared of branching the samples agent with between molecular 0% and weight 1.8%. ranging They concluded from 2000 that to 10,000the degree g/mol, of alongbranching with is the efficiency. They also investigated the migration of polyester plasticizer for PVC. content of branching agent between 0% and 1.8%. They concluded that the degree of branching is

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 7 of 28 most decisive in plasticizing efficiency. They also investigated the migration of polyester plasticizer for PolymersPVC. 2018, 10, x FOR PEER REVIEW 7 of 28 PolymersGlycidol2018, 10can, 1303 be derived from glycerol, which is a major compound generated during7 of 27 the most decisive in plasticizing efficiency. They also investigated the migration of polyester plasticizer production of biodiesel from the of plant oils and -rich surplus streams from for PVC. slaughterhouses and the rendering industry [43]. Recently, hyperbranched esters deriving from Glycidol can be derived from glycerol, which is a major compound generated during the glycidolproductionproductio via na of ofgreen biodiesel biodiesel one- potfrom from process the the transesterification transesterification using neither oftoxic plant of plantsolvents oils oilsand no andlipidr expensive lipid-rich-rich surplus surpluscatalysts streams streams have from been usedfromslaughterhouses to slaughterhouses replaced and DOP the and as rendering the main rendering plasticizer industry industry [43] [44,45].. Recently, [43]. Figures Recently, hyperbranched 8 hyperbranched and 9 show esters estersthe deriving two deriving kindsfrom of hyperbranchedfromglycidol glycidol via a esters: viagreen a green one alkyl-pot one-pot terminal process process usinghyperbranche using neither neither toxicd polyglycerol toxicsolvents solvents nor (alkylexpensive nor expensive-HPG) catalysts and catalysts b utylhave- esterified havebeen highlybeenused branched used to replaced to replacedpolycaprolactone DOP DOP as main as main. PVC plasticizer plasticizer plasticized [44,45]. [44 with,45 Figures]. alkyl Figures -HPG 8 8 and and presents 99 show enhanced the the two two kinds transparency kinds of of andhyperbranched thermal stability esters: compared alkyl terminal to PVC/DOP hyperbranchedhyperbranche blends.d polyglycerol Besides, alkyl (alkyl-HPG)(alkyl-HPG-HPG) plasticizer and butyl-esterifiedbutyl ia-esterified biologically safehighly without branched acute polycaprolactone. toxicity and has. PVC a lower p plasticizedlasticized degree with of alkyl alkyl-HPGmigration-HPG presentsin leaching enhanced tests .transparency Butyl-esterified highlyand branched thermal stability poly(ε compared-comparedcaprolactone) toto PVC/DOPPVC/DOP (PCL) with blends. various Besides, degree alkyl alkyl-HPG of- HPGpolymerizations plasticizer ia are biologically prepared by ringsafe-opening without multibranching acuteacute toxicity and polymerization has a a lowerlower degreedegree using of ofglycidol migrationmigration as inina single leachingleaching mon tests.testsomer. Butyl-esterifiedButyl and-esterified subsequent endhighlyhi-groupghly branched modification poly(ε poly(ε,-caprolactone)- caprolactone)with structure (PCL) of hyperbranched with various degree esters of polymerizationsshow ing similar are plasticizing prepared by effect ring-opening multibranching polymerization using glycidol as a single and subsequent on ringPVC-opening and solvent multibranching extraction polymerizationresistance as alkyl using-HPG glycidol. In theas acase single of monthe elongationomer and subsequent at break, PVC end-groupend-group modification,modification, with structure of hyperbranched esters showingshowing similar plasticizing effect plasticized with highly branched poly(ε-caprolactone)/glycidol copolymeric plasticizer shows the on PVC and solvent extraction resistance as alkyl-HPG.alkyl-HPG. In the case of the elongation at break, PVC largest value of 397%, which is even greater than that of PVC/DEHP by 21%. plasticized with highlyhighly branched branched poly(poly(εε-caprolactone)/glycidol-caprolactone)/glycidol c copolymericopolymeric p plasticizerlasticizer shows the largest value of 397%,397%, whichwhich isis eveneven greatergreater thanthan thatthat ofof PVC/DEHPPVC/DEHP by by 21%. 21%.

Figure 8. Synthesis of alkyl terminal hyperbranched polyglycerol (alkyl-HPG). Figure 8. Synthesis of alkyl terminal hyperbranched polyglycerol (alkyl-HPG).(alkyl-HPG).

Figure 9. Synthesis of butyl-esterified highly branched polycaprolactone. Figure 9. Synthesis of butyl-esterified highly branched polycaprolactone. Figure 9. Synthesis of butyl-esterified highly branched polycaprolactone. Similar to epoxidized plasticizers, polyester plasticizers also serve as an alternative to phthalates Similar to epoxidized plasticizers, polyester plasticizers also serve as an alternative to phthalates because they may migrate from PVC products into human bodies or environment [46]. Materials such becauseSimilar they to epoxidized may migrate plasticizers, from PVC products polyester into plasti humancizers bodies also or serve environment as an alternative [46]. Materials to phthalates such as long-chain linear polyester, poly(butylenes 2-methylsuccinate) (PBM) and aromatic becauseas long-chain they may linear migrate polyester, from poly(butylenes PVC products 2-methylsuccinate) into human bodies (PBM) or environment and aromatic [46] hyperbranched. Materials such hyperbranched polyesters (HBPEs) are considered as substitute of phthalates [47–49]. Sunny et al. as polyesters long-chain (HBPEs) linear are polyester,considered as poly(butylenes substitute of phthalates 2-methylsuccinate) [47–49]. Sunny (PBM) et al. studiedand aromatic the hyperbranched polyesters (HBPEs) are considered as substitute of phthalates [47–49]. Sunny et al. Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, 1303 8 of 27 performances of the PVC cooperated with di-(2-ethyl hexyl) phthalate (DEHP), (NBR), carboxylated nitrile rubber (XNBR) and epoxidized (ENR) [50]. The results show that NBR enhanced the stability of the blends. When it comes to food degree PVC, the migration of plasticizersPolymers into 201 food8, 10, raisesx FOR PEER severe REVIEW concerns [51,52]. 8 of 28

Differentstudied starchthe performances sources of the and PVC plasticizers cooperated with influence di-(2-ethyl filmshexyl) phthalate differently, (DEHP), especially nitrile the physical-chemicalrubber (NBR), and carboxylated mechanical nitrile properties. rubber (XNBR) Zullo and etepoxidized al. prepared natural severalrubber (ENR) types [50] of. The plasticizers (glycerol, urearesults and show formamide) that NBR enhanced and examined the stability the of influencethe blends. ofWhen it comes sources to food (maize, degree potato PVC, the and wheat) on physical-chemicalmigration of plasticizers and mechanical into food propertiesraises severe of concerns [51,52]. starch films [53]. The research shows Different starch sources and plasticizers influence films differently, especially the physical- that the compositionchemical and mechanical of formamide properties. and Zullo urea et enhances al. prepared physical-chemical several types of plasticizers and mechanical (glycerol, urea properties of thermoplasticand formamide) starch and films. examined Pushpadass the influence et al.of starch used sources native (maize, corn potato starch and as wheat) raw material on physical to- produce 0.4–0.6 mmchemical thick and films mechanical [54]. Amylopectin properties of thermoplastic and amylose starch account films [53] for. 76.9%The research and 23.1%shows that of native the starch but the percentagecomposition only of formamide changes toand 71.3–76.6% urea enhanc andes physical 23.4–28.7%,-chemical respectively, and mechanical during properties the extrudationof thermoplastic starch films. Pushpadass et al. used native corn starch as raw material to produce 0.4– process, which means that the degradation is not severe. 0.6 mm thick films [54]. Amylopectin and amylose account for 76.9% and 23.1% of native starch but Diproticthe percentage acid for the only synthesis changes toof 71.3polyester–76.6% andplasticizers 23.4–28.7%, can respectively, be obtained during by alcoholysis the extrudation of vegetable oil. Figureprocess, 10 shows which the means synthesis that the routedegradation of bio-based is not severe. polyester plasticizer from palm oil. As shown in Figure 10, palmDiprotic oil monoglyceride acid for the synthesis (POM) of is polyester obtained plasticizers by alcoholysis can be obtained of palm by oil alcoholysis [55]. Palm of oil-based vegetable oil. Figure 10 shows the synthesis route of bio-based polyester plasticizer from palm oil. As polyester plasticizer (POMP) is produced from POM and maleic anhydride via esterification. The shown in Figure 10, palm oil monoglyceride (POM) is obtained by alcoholysis of palm oil [55]. Palm obtained polyesteroil-based polyester plasticizer plasti showscizer (POMP) excellent is produced compatibility from POM with PVCand maleic and significantly anhydride via improves thermal stabilityesterification. of PVC The blends. obtained When polyester DOP plasticizer is replaced shows by POMP excellent gradually compatibility in thewith PVC PVC blends, and tensile strength decreasessignificantly from improves 12.6 tothermal 6.1 MPa, stability and of the PVC elongation blends. When at DOP break is decreasedreplaced by fromPOMP 210.81% gradually to 60.96%, illustratingin that the P plasticizingVC blends, tensile effect strength of POMO decreases is lower from than 12.6 to DOP. 6.1 MPa, This and study the elongation provides at general break method decreased from 210.81% to 60.96%, illustrating that plasticizing effect of POMO is lower than DOP. for producingThis study vegetable provides oil-based general method polyester for producing plasticizer. vegetable oil-based polyester plasticizer.

FigureFigure 10. Synthesis 10. Synthesis bio-based bio-based polyesterpolyester plasticizer plasticizer from from palm palmoil. oil.

FakhouryFakhoury et al. pointed et al. pointed out thatout that non-degradable non-degradable materials materials damage damage the env theironment environment daily, thus daily, thus with the addition of glycerol or are increasingly important [56]. The mechanical, bioplasticsphysicochemical with the addition and physical of glycerol properties or sorbitolof blends are of maniocincreasingly starch and important gelatin are [56 investigated]. The mechanical, physicochemicalduring the and process. physical As a propertiesresult, glycerol of and blends sorbitol, of manioc as plasticizers, starch increase and gelatin the elongation are investigated at break during the process.and As water a result,vapor permeability glycerol and (WVP) sorbitol, of manioc as plasticizers, starch and gelatin increase blends the. Moreover, elongation sorbitol at break, and water vapor permeability (WVP) of manioc starch and gelatin blends. Moreover, sorbitol, poly(glutaric acid-glycerylPolymers monooleate) 2017, 9, x; doi: FOR (PGAGMO), PEER REVIEW poly(succinic acid-glycerylwww.mdpi.com/journal/ monooleate) (PSAGMO)polymers and PMMA are also modified and used as bio-based plasticizer of hydroxyl polymer [57–59]. Polymers 2018, 10, 1303 9 of 27

2.3. Flame Retardant Plasticizer plasticizer is a functional plasticizer, which has both plasticizing effect and flame retardant effect on PVC products. Phosphate esters have good compatibility with various resins and such as PVC, cellulose, (PE) and (PS). They have been used as cellulose plasticizers for more than 100 years. With the high density of urban construction and the rapid development of transportation, the flame-retardant performance requirements of various products are increasing. Phosphate ester plasticizers have been widely used, especially in fireproof and non-flammable products such as the preparation of rubber, plastics, military products, textiles, electrical appliances, conveyor belts and various building materials. The largest producer and consumer country of flame retardants is the United States. -based flame retardants in China have a large gap compared with the United States in terms of production capacity, output and variety. The proportion of phosphorus-based flame retardants in Chinese plastic additives is low. Therefore, adjusting the product structure of plasticizers and flame retardants, and increasing the proportion are essential conditions for the development of phosphate ester plasticizers in the future. Phosphate plasticizer is the main plasticizer for PVC and has the following characteristics: good flame-retardant performance, better compatibility, good mold resistance, poor weather resistance, high toxicity and high price. Phosphate esters are normally prepared by esterification of phosphorus oxychloride or with an or a . Commonly used phosphate esters are triethyl phosphate, tributyl phosphate, o-crepe phosphate, tri-p-tolyl phosphate, , and xylene diphenyl phosphate [60,61]. Phosphate esters are usually used as flame retardant plasticizers, especially in PVC products, because PVC products will not present self-extinguishing when the PVC products contain less than 20 wt % of phosphate plasticizer [62]. Phosphate esters are also used to improve flame retardancy of epoxy resins combined with nitrogen, silicon or boron [63–66]. There are many kinds of phosphate type plasticizers: tributyl phosphate (TBP), tris(2-chloroethyl) phosphate (TCEP), triphenyl phosphate (TPP), tris(2-butoxyethyl) phosphate (TBEP), tris(2-ethylhexyl) phosphate (TEHP), tricresyl phosphate (TCP), tris(2-chloro-isopropyl) phosphate (TCPP), tris(1,3-dichloroisopropyl) phosphate (TDCP), and triaryl phosphates [67,68]. Epoxidized cardanol diethyl phosphate is a kind of phosphate esters, is synthesized from cardanol and enhances thermal stability of PVC films. Figures5 and 11 show its chemical structure and synthesis route. Glass transition temperature (Tg) of PVC films containing 40 wt % of epoxidized cardanol diethyl phosphatePolymers 201 reach8, 10, x FOR 33.20 PEER ◦REVIEWC[69 ]. 10 of 28

Figure 11. Synthesis of epoxidized cardanol ethyl phosphate. Figure 11. Synthesis of epoxidized cardanol ethyl phosphate. Feng et al. used castor oil as basis to synthesize another novel flame-retardant plasticizer based Feng et al. used castor oil as basis to synthesize another novel flame-retardant plasticizer based on castor oilon (FRC), castor oil as (FRC) shown, as shown in Table in Table1a. They1a. They prepared prepared FRC FRC by bya three a three-step-step procedure procedure of alcoholysis, of alcoholysis, epoxidationepoxidation and ring and opening ring opening reaction reaction and and used used moldingmolding machine machine to obtain to obtain the blends the of blends PVC and of PVC and FRC [70]. TheFRC addition [70]. The ofaddition FRC of reduces FRC reduces torque torque by 33.6%by 33.6% and andenhances enhances the the flame flame-retardant-retardant properties properties.. LOI LOI js enhanced by 31.9%. The addition of FRC causes a significant decrease of 61.94% in tensile js enhancedstrength by 31.9%. and Theapproximately addition 60% of FRC in elongation causes at a significantbreak for PVC decrease materials, of which 61.94% indicates in tensile that FRC strength and has plasticizing effect on PVC. Our group synthesized a series of flame-retardant plasticizers based on vegetable oil [71–76]. A flame-retardant plasticizer based on castor oil (PPC) is synthesized by castor oil, H2O2, phosphate and diethyl phosphate [71]. Figure 12 and Table 1b show its chemical structure and synthesis route. The results show that the glass transition temperature increases from 26.5 to 52.6 °C with the content of PPC in 0 to 20 g in 100 g of PVC while the thermal stability is maintained by PPC until 350 °C. In addition, the leaching test indicates that solvent extraction resistance of PPC is better than DOP.

Figure 12. Synthesis of castor oil phosphate ester.

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 10 of 28

Figure 11. Synthesis of epoxidized cardanol ethyl phosphate.

Feng et al. used castor oil as basis to synthesize another novel flame-retardant plasticizer based on castor oil (FRC), as shown in Table 1a. They prepared FRC by a three-step procedure of alcoholysis, Polymers epoxidation2018, 10, 1303 and ring opening reaction and used molding machine to obtain the blends of PVC and 10 of 27 FRC [70]. The addition of FRC reduces torque by 33.6% and enhances the flame-retardant properties. LOI js enhanced by 31.9%. The addition of FRC causes a significant decrease of 61.94% in tensile approximatelystrength and 60% approximately in elongation 60% at breakin elongation for PVC at materials,break for PVC which materials, indicates which that indicates FRC hasthat plasticizingFRC effect onhas PVC. plasticiz Ouring group effect synthesized on PVC. Our a seriesgroup ofsynthesized flame-retardant a series plasticizers of flame-retar baseddant onplasticizers vegetable based oil [71–76]. A flame-retardanton vegetable plasticizeroil [71–76]. basedA flame on-retardant castor oilplasticizer (PPC) isbased synthesized on castor oil by (PPC) castor is oil, synthesized H2O2, phosphate by and diethylcastor phosphate oil, H2O2, phosphate [71]. Figure and 12 diethyl and Table phosphate1b show [71] its. Figure chemical 12 and structure Table 1b and show synthesis its chemical route. The results showstructure that and the synthe glasssis transition route. The temperature results show increasesthat the glass from transition 26.5 to 52.6temperature◦C with increases the content from of PPC 26.5 to 52.6 °C with the content of PPC in 0 to 20 g in 100 g of PVC while the thermal◦ stability is in 0 to 20maintained g in 100 g by of PPC PVC until while 350 the °C. thermal In addition, stability the is leaching maintained test indicates by PPC until that solvent 350 C. extr Inaction addition, the leachingresistance test indicates of PPC that is better solvent than extractionDOP. resistance of PPC is better than DOP.

Polymers 2018, 10, x FOR PEER REVIEWFigureFigure 12. 12.Synthesis Synthesis ofof castor oil oil phosphate phosphate ester. ester. 11 of 28

ChlorineChlorine andand phosphorusphosphorus containingcontaining vegetable-basedvegetable-based plasticizerplasticizer isis alsoalso synthesizedsynthesized fromfrom castorcastor oil,oil, asas shownshownPolymers 201 inin7 Figure,Figure 9, x; doi: 13FOR13 and andPEER Table TableREVIEW1 c1c [ 72[72].]. LOILOI ofof PVCPVC containingcontaining 4040www.mdpi.com/journal/ wtwt %%of of DOP DOP isispolymers 23.6%. 23.6%. WhenWhen halfhalf thethe DOPDOP isis replacedreplaced withwith chlorinatedchlorinated phosphatephosphate esterester basedbased onon castorcastor oil,oil, LOILOI ofof PVCPVC blendsblends reachesreaches 35.4%, 35.4%, peak peak heat heat release release (pHRR) (pHRR) decreases decreases from from 379.00 379.00 to to 289.00 289.00 kW/m kW/m22,, andand totaltotal heatheat releaserelease (THR)(THR) decreasesdecreases fromfrom 31.7831.78 to to 19.12 19.12 MJ/m MJ/m22.

FigureFigure 13.13. SynthesisSynthesis ofof chlorinatedchlorinated phosphatephosphate esterester basedbased onon castorcastor oil.oil.

Flame-retardant plasticizer based on soybean oil containing phosphaphenanthrene groups (PSPE) is also prepared [73], as shown in Figure 14 and table 1d. The results show that PVC blends with PSPE possess better flame retardancy and higher thermal stability, and the LOI value reaches 36.25. PSPE releases PO2, HPO2 and long fatty acid chains during burning. As shown in Figure 15, groups containing soybean oil polyol ester have flame-retardant effect on PVC blends by promoting formation of char residue. Phosphaphenanthrene groups-containing soybean oil-based plasticizer (PSOPE) is prepared [74]. Table 1e shows its chemical structure. The results show that LOI of PVC blends plasticized with 40 wt % of PSOPE reaches 36.2%, indicating that flame-retardant performance of PSOPE is similar to PSPE.

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 15 of 28 PolymersPolymers 20120188,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 1515 ofof 2828 Polymers 2018, 10, x FOR PEER REVIEW 15 of 28

Polymers 2018, 10, 1303 11 of 27

Figure 18. Synthesis of phosphonate groups containing bio-based plasticizers. Table 1. Flame-retardantFigureFigure 18.18. SynthesisSynthesis elements ofof phosphonatephosphonate (phosphate, groupsgroups chlorinecontcontainingaining bb andioio--basedbased nitrogen) plasticizers.plasticizers. containing vegetable Figure 18. Synthesis of phosphonate groups containing bio-based plasticizers. oil-based plasticizers.Table 1 Flame-retardant elements (phosphate, chlorine and nitrogen) containing vegetable oil-based TableTable 11 FlameFlame--retardantretardant elementselements (phosphate,(phosphate, chlorinechlorine andand nitrogen)nitrogen) containingcontaining vegetablevegetable oiloil--basedbased plasticizers.Table 1 Flame -retardant elements (phosphate, chlorine and nitrogen) containing vegetable oil-based plasticizers.Table 1 Flame -retardant elements (phosphate, chlorine and nitrogen) containing vegetable oil-based Flameplasticizers.plasticizers. Retardant Elements (Phosphate, Chlorine and Nitrogen) Containing Vegetable plasticizers. References No. Oil Based Plasticizers References No. References No. Referenceses No. O O References No. O P O O P O OOO OO O OPP O COO O OOO O C O OPOOO O OH P OH OHOO COC OH O O OHO O OH [70] a O C OHOH OH [70] a OOHOHCOCO OH [70] a OOHCO OH O OH [70] a O C OH O OH [70] a OHO OC OHPO OHOH [70] a O C OHOOPOOO OH OHOH [70] a O C OHOPOOPOO OOO OH OHOPOO OH OPOO O O O O O O OP O O O O O OPOO O OP O O O O O O PO O O POO O O CO O OOP O O PO O O O O OC O OOHP O OOP O O CO O OO OCC OHO OOHP O [71] b O P O O OC O O C OHO [71] b O P OHO O OCC OO O C OHOH [71] b OH O O CC OHOH OH [71][71] bb OH C O O C OOHP O [71] b OH C O O OC O P OOH OH O OC OOHOP O [71] b OH O OCO OOP O O O OPOO O O OP O O

[72] c [72][72] [72]cc c [72] c

[73] d [73][73] [73]dd d [73] d

O P O O P O O OCOCH [74] e OO P OOCOCH3 3 [74] e P OOCOCH3 O OCOCH3 O P O CO O OCOCH3 [74][74] ee OCOCHOCOCH33 COO OCOCH3 OOCOCHP OOCOCH O OCOCH 3 [74] [74]e e OCOCH3 3 COCOO OCOCH3 [74] e OCOCHOCOCH3 3 C O 3 OCOCH33 C O OCOCHOCOCH33 O OCOCH3 OCOCH O O OCOCH OCOCH3 Cl O Cl O 3 3 Cl PO Cl O O Cl OP Cl Cl PO Cl O O Cl OPP Cl Cl PO Cl O O O Cl OP Cl ClCl OPP ClCl O O CO O Cl O Cl Cl O Cl O O OC O PO P O OO O OC O Cl O Cl OPO Cl OO OC O O C ClO ClO O C O O C Cl Cl [75] f Cl O OCC OO O CC Cl [75] f O Cl O C O O C ClCl Cl [75][75] ff ClCl C O OO CC ClO Cl [75] f Cl O OC ClO [75] f Cl O O [75] f O OCO Cl PO Cl O Cl OP Cl O ClCl OPP ClCl Cl OP Cl Polymers 2018, 10, x FOR PEER REVIEW Cl OPO Cl 16 of 28 Polymers 2018, 10, x FOR PEER REVIEW O 16 of 28 O Polymers 2018, 10, x FOR PEER REVIEW O 16 of 28 PolymersPolymers 20120188,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW O 1616 ofof 2828 Polymers 2018, 10, x FOR PEER REVIEW CH2CH2OC 16 of 28 Polymers 2018, 10, x FOR PEER REVIEW CH2CH2OC O 16 of 28 Polymers 2018, 10, x FOR PEER REVIEW N O 16 of 28 O C N C O O O P O O C C O O O P O CH CH OCO O N N2 2 O O O CH2CH2OOC [76] g H2CH2C N CNCHN2CHCH2OO2CCH2 O [76] g Polymers 2017, 9, x; doi: FOR PEER REVIEWH CH CO CCH 2CHCHO2OCCH O PO O www.mdpi.com/journal/[76] g polymers 2 2 C NCHC2CH2OC2 2 O PO O Polymers 2017, 9, x; doi: FOR PEER REVIEWOO C OCHN 2CCHO2OOCO OOOP O www.mdpi.com/journal/polymers O O C N C O O O OP O PolymersPolymers 20120177,, 99,, x;x; doi:doi: FORFOR PEERPEER REVIEWREVIEWNC ON NC O P O O OP O wwww.mdpi.com/journal/ww.mdpi.com/journal/polymerspolymers HOCHOC C NC C CHO CH O PO O [76] g Polymers 2017, 9, x; doi: FOR PEER REVIEWO P2 O 2 CNN CNN O2 P 2O O www.mdpi.com/journal/[76] g polymers Polymers 2017, 9, x; doi: FOR PEER REVIEWOHP2CHO2C N C N CH2OCH2 O www.mdpi.com/journal/[76] g polymers OH2CH2C N OC N CHO2OCH2 O [76] g H2OCH2C N C N CH2CH2 [76] g HO2CH2C OC CH2OCH2 [76] g H2CHO 2C CO CHO2PCHO O2 O PO O O O PO O O PO O ROO ONOP O OR OOP O O O OP O O OP O RO ONPO O OR O O O PO O O PO O O RO NO OR O RORO N NN N OROR RO N N N OR RO N OR N N [77] h N N [77] h HO N N N N [77] h N N N [77] h HO N N N N N HO N N OH [77][77] hh HO N N N OH [77] h HO N N N OH [77] h HOHO N N NN HOHO N N N OH OHOH HO N OH OH OH HOHO OH HORO N OROHOH HO RORO NN OROHOR RO N OROH RORO NN OROR RO N N N OR N NN NN NN [77] i N N [77] [77] i i HO N N N N [77] [i77 ] i HOHO NN NN N [77] i HO N N NN [77] i HOHO NN NN NN OROR [77] i HO N N N OROR HO N N N OR HOHO OR HOHO OHOH OR HOHO OHOH HO OHOH RORO N OROHOR RORO N OROR RORO NN OROR RO N OR N N N NN NN NN N N [77] [77] j j RO N N [77] [77] [77j ]j j RORO NN N N [77] j RORORO NNN NN NNN OR [77] j RO N N NN OROR N OROROR HO OR HOHOHO OH HOHO OH HO O OHOHOH O OHOH OO O OO OCH3 O H3CO OP OCH O H CO P OCH33 [78] k OO H33COCO PP OCHOCH3 3 [78] k OO H3CO PP OCHOCH3 3 [78][78] [k78k ] k O H3CO PP [78] [78][78] k k OO O O OO O OO O H3CH2CO OPO OCH2CH3 H CH CO OP OCH2CH3 O H33CH22CO PO OCH2CH3 O H3CH2CO P OCH2CH3 [78] l O H CH CO P OCH2CH3 OO HH33CH22CO PP OCHOCH2CH2CH3 3 [78][78] ll O H3CH2CO P OCH2CH3 [78] l O [78] [78] [l78 ]l l OO [78] l O O O O OO O H CO OP OCH O H3CO OP OCH3 O H33CO OP OCH33 O H3CO PO OCH3 O O H3CO P OCH3 [78] m O O H3CO P OCH3 [78][78] m OO H3CO P OCH3 [78] m OO H3CO P OCH3 [78] [m78 ] m O H3CO P OCH3 [78] m H3CO OP OCH3 O H3CO P OCH3 [78] m O H3CO OP OCH3 [78] m O3 OO 3 HO3CO OP OCH3 O HO3CO P OCH3 O O H3CO OP OCHO 3 O O HH3COCO PP OCHOCHO 3 O [78] n O H33CO PO OCH33 O [78] n O H3CO P OCH3 O [78][78] nn O H3CO O 3 O [78] n OO P OCH OO [78] n OO H3CO P OCH3 OO O 3 3 O [78][78] n 2.4. Citric Acid EsterO Plasticizer O 2.4.2.4. Citric Acid Ester Plasticizer O 2.4. Citric Acid Ester Plasticizer Citric acid ester plasticizers are important, environmentally-friendly plasticizers because of their 2.4. CitricCitric Acid acid Ester ester Plasticizer plasticizers are important, environmentally--friendlyfriendly plplasticizersasticizers becausebecause ofof theirtheir 2.4.safety, CitricCitric nonAcid acid-toxicity Ester ester Plasticizer and plasticizers precipitation are important, resistance. environmentally They have been-friendly approved plasticizers in the United because States, of their the safety,safety, nonnon--toxicitytoxicity andand precipitationprecipitation resistance.resistance. TheyThey hahaveve beenbeen approvedapproved inin thethe UnitedUnited States,States, thethe safety,European non Union-toxicity and and other precipitation developed resistance. countries toThey be usehaved inbeen plastic approved products in the in closeUnited contact States, with the EuropeanEuropeanCitric acid UnionUnion ester andand plasticizers otherother developeddeveloped are important, countriescountries environmentally toto bebe useusedd inin plasticplastic-friendly productsproducts plasticizers inin closeclose contactcontact because withwith of their Europeanthe human Union body and and other meet developed high hygiene countries requirements to be use suchd in plastic as for foodproducts packing, in close children’s contact toys,with safety,thethe humannon human-toxicity body body andand meetprecipitation meet high high hygienehygiene resistance. requirements requirements They ha such suchve been as as for for approved food food packing, packing, in the children’s children’s United States, toys, toys, the themedical human equipment body and and meet sanitary high products. hygiene Therefore, requirements citrate such plasticizers as for food have packing, become children’s the first choice toys, Europeanmedical Union equipment and andother sanitary developed products. countries Therefore, to be citrate use dplasticizers in plastic have products become in the close first contact choice with medicalfor safe andequipment non-toxic and plasticizer sanitary products. products Therefore,in the plastics citrate industry plasticizers globally. have There become are the m orefirst than choice 50 the forfor human safesafe andand body nonnon and--toxictoxic meet plasticizerplasticizer high productshygieneproducts in requirementsin thethe plasticsplastics industry industry such as globally.globally. for food ThereThere packing, areare mm oreore children’s thanthan 5050 toys, the forkinds human safe of and citric body non acid and-toxic plasticizers, meet plasticizer high and productshygiene about 15 in requirements kinds the plastics of products industry such have as globally. been for foodproduced There packing, are at largem ore children’s- scalethan by50 toys, medicalkindskinds equipment ofof citriccitric acidacid and plasticizers,plasticizers, sanitary andand products. aboutabout 1515 Therefore, kindskinds ofof productsproducts citrate plasticizers havehave beenbeen producedproduced have become atat largelarge the--scalescale first byby choice medicalkindsthe industrialized equipmentof citric acid and manufacturingplasticizers, sanitary and products. industry. about 15 Therefore,A kindscetyl oftributyl products citrate citrate plasticizershave (ATBC) been produced and have tributyl become at large citrate the-scale (TBC)first by choice for thesafethe industrializedindustrialized and non-toxic manufacturingmanufacturing plasticizer products industry.industry. inAcetylcetyl the plasticstributyltributyl citratecitrateindustry ((ATBC) globally. and tributyl There citrateare m ore(TBC) than 50 for thehavesafe industrialized andbeen nonstudied-toxic manufacturingin -plasticizerdepth and industria productsindustry.lized Aincetyl thedue tributylplasticsto excellent citrateindustry performances (ATBC) globally. and. ATBC tributyl There has citrateare good m ore(TBC)water than 50 havehave beenbeen studiedstudied inin--depthdepth andand industriaindustrializedlized duedue toto excellentexcellent performancesperformances.. ATBCATBC hashas goodgood waterwater kindshaveand of light beencitric resistance,studied acid plasticizers, in- depth good thermaland and industria about stabilitylized 15 kinds without due to of excellent color,products and performances have maintains been good.produced ATBC flexibility has atgood large at water low-scale by andand light light resistance, resistance, good good thermal thermal stability stability without without color, color, and and maintains maintains good good flexibility flexibility at at low low the andtemperatures.industrialized light resistance, TBC manufacturing has good good thermal compatibility industry. stability with A withoutcetyl cellulose tributyl color, resin, andcitrate PVC, maintains (PP,ATBC) and goodits and plasticizing flexibilitytributyl citrate effect at low i s(TBC) temperatures.temperatures. TBCTBC hashas goodgood ccompatibilityompatibility withwith cellulosecellulose resin,resin, PVC,PVC, PP,PP, andand itsits plasticizingplasticizing effecteffect iiss havetemperatures.remarkable. been studied In TBC addition, in -hasdepth good TBC and c hasompatibility industria antibacteriallized with and duecellulose flame to excellent- retardantresin, PVC, performances properties, PP, and its which plasticizing. ATBC further has effectexpand good is water haveremarkable.remarkable. been studied InIn addition,addition, in-depth TBCTBC and hashas industria antibacterialantibacteriallized andand due flameflame to excellent--retardantretardant performances properties,properties, whichwhich. ATBC furtherfurther has expandexpand good water andremarkable.its light applicability. resistance, In addition, However, good TBC thermalthe has price antibacterial stabilityof the plasticize and without flamer is relatively color,-retardant and high, properties, maintains so it is which mainly good further used flexibility in expand fields at low anditsits light applicability.applicability. resistance, However,However, good thermalthethe priceprice of stabilityof thethe plasticizeplasticize withoutrr isis relatively color,relatively and high,high, maintains soso iitt isis mainlymainly good usedused flexibility inin fieldsfields at low itswith applicability. high requirements However, for thenon price-toxic of safety the plasticize of productsr is suchrelatively as food high, packaging so it is mainly materials, used children’s in fields temperatures.with highhigh requirementsrequirements TBC has good forfor non noncompatibility--toxictoxic safetysafety with ofof productsproducts cellulose suchsuch resin, asas foodfood PVC, packagingpackaging PP, and materials,materials, its plasticizing children’schildren’s effect is withtoys, highmedical requirements equipment for and non packaging.-toxic safety of products such as food packaging materials, children’s remarkable.toys,toys, medicalmedical In addition, equipmentequipment TBC andand has packaging.packaging. antibacterial and flame-retardant properties, which further expand toys, Cmedicalitric acid equipment has been commercially and packaging. produced through large-scale fermentation, thus undoubtedly its applicability.Citricitric acidacid However, hashas beenbeen commerciallycommercially the price of produced producedthe plasticize throughthroughr is large largerelatively--scalescale fermentation, fermentation,high, so it is thus thusmainly undoubtundoubt usededlyedly in fields its applicability.Citric acid However, has been commercially the price of produced the plasticize throughr is large relatively-scale fermentation, high, so it is thus mainly undoubt usededly in fields is consideredCitric acid a hasraw been material commercially for producing produced plasticizers through for large PLA,-scale PVC fermentation, and other materials. thus undoubt Shi etedly al. withisis high consideredconsidered requirements aa rawraw materialmaterial for non forfor-toxic producingproducing safety plasticizersplasticizersof products forfor such PLA,PLA, as PVCPVC food andand packaging otherother materials.materials. materials, ShiShi etchildren’set al.al. withis highconsidered requirements a raw material for non for-toxic producing safety plasticizers of products for such PLA, as PVC food and packaging other materials. materials, Shi etchildren’s al. toys, medical equipment and packaging. toys,Polymers medical 2017, equipment9, x; doi: FOR PEER and REVIEW packaging. www.mdpi.com/journal/polymers PolymersPolymers 20120177,, 99,, x;x; doi:doi: FORFOR PEERPEER REVIEWREVIEW www.mdpi.com/journal/polymerspolymers PolymersCitric 201 acid7, 9, hasx; doi: been FOR PEERcommercially REVIEW produced through large-scale fermentation,www.mdpi.com/journal/ thus undoubtpolymers edly

is considered a raw material for producing plasticizers for PLA, PVC and other materials. Shi et al.

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Flame-retardant plasticizer based on soybean oil containing phosphaphenanthrene groups (PSPE) is also prepared [73], as shown in Figure 14 and Table1d shows its chemical structure. The results show that LOI of PVC blends plasticized with 40d. The results show that PVC blends with PSPE possess better flame retardancy and higher thermal stability, and the LOI value reaches 36.25. PSPE releases PO2, HPO2 and long fatty acid chains during burning. As shown in Figure 15, groups containing soybean oil polyol ester have flame-retardant effect on PVC blends by promoting formation of char residue. Phosphaphenanthrene groups-containing soybean oil-based plasticizer (PSOPE) is prepared [74]. Table1e shows its chemical structure. The results show that LOI of PVC blends plasticized with 40 wt % of PSOPE reaches 36.2%, indicating that flame-retardant performance of Polymers 2018, 10, x FOR PEER REVIEW 12 of 28 PSOPE is similar to PSPE.

Figure 14. Synthesis of DOPO groups-containing soybean oil polyol ester. Figure 14. Synthesis of DOPO groups-containing soybean oil polyol ester. Synergistic flame retardance can be obtained by combining nitrogen, phosphorus and chlorine in one molecular structure of vegetable oil-based flame-retardant plasticizer [75,76]. Table1f,g shows the chemical structure of synergistic flame retardant plasticizer based on vegetable oil. Figure 16 shows the synthesis route of 1,3,5-tris(2-hydroxyethyl)cyanuric acid groups and diethyl phosphate groups containing castor oil-based plasticizer (THEIC-MA phosphate). THEIC-MR-phosphate enhances the flame-retardant performances of PVC blends by promoting the formation of char residue in solid phase, and dilution and isolation of oxygen in gas phase. Cone tests show that the time to ignition increases, combined with the decrease of heat release rate (HRR) value, indicating that THEIC-MR-phosphate effectively improves flame-retardant performance of PVC materials and delays fire process.

Figure 15. Flame retardant mechanism of DOPO groups-containing soybean oil polyol ester.

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Polymers 2018, 10, 1303 13 of 27 Figure 14. Synthesis of DOPO groups-containing soybean oil polyol ester.

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Synergistic flame retardance can be obtained by combining nitrogen, phosphorus and chlorine in one molecular structure of vegetable oil-based flame-retardant plasticizer [75,76]. Table 1f,g shows the chemical structure of synergistic flame retardant plasticizer based on vegetable oil. Figure 16 shows the synthesis route of 1,3,5-tris(2-hydroxyethyl)cyanuric acid groups and diethyl phosphate groups containing castor oil-based plasticizer (THEIC-MA phosphate). THEIC-MR-phosphate enhances the flame-retardant performances of PVC blends by promoting the formation of char residue in solid phase, and dilution and isolation of oxygen in gas phase. Cone tests show that the time to ignition increases, combined with the decrease of heat release rate (HRR) value, indicating that THEIC-MR-phosphate effectively improves flame-retardant performance of PVC materials and delays fire process. Figure 15. Flame retardant mechanism of DOPO groups-containing soybean oil polyol ester. Figure 15. Flame retardant mechanism of DOPO groups-containing soybean oil polyol ester.

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FigureFigure 16. 16. SynthesisSynthesis of of THEIC THEIC-MA-MA phosphate. phosphate.

Hydroxyl and nitrogen rich group-containing tung oil-based ester (GEHTMA-1, GEHTMA-2, GEHTMA-3 and GEHTMA-4) plasticizers are prepared and used to replace (DOTP) [77]. Figure 17 and Table 1h,i,j show synthesis route and their chemical structure. GEHTMA- 3 displays better mechanical properties and endows PVC resins with well-balanced properties of flexibility and strength, which are attributed to simultaneous introduction of hydroxyl, epoxy, benzene ring, ester and nitrogen rich groups into GEHTMA-3 structures.

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Hydroxyl and nitrogen rich group-containing tung oil-based ester (GEHTMA-1, GEHTMA-2, GEHTMA-3 and GEHTMA-4) plasticizers are prepared and used to replace dioctyl terephthalate (DOTP) [77]. Figure 17 and Table1h,i,j show synthesis route and their chemical structure. GEHTMA-3 displays better mechanical properties and endows PVC resins with well-balanced properties of flexibility and strength, which are attributed to simultaneous introduction of hydroxyl, Polymers 2018, 10, x FOR PEER REVIEW 14 of 28 epoxy, benzene ring, ester and nitrogen rich groups into GEHTMA-3 structures.

Figure 17. Synthesis of hydroxyl and nitrogen rich group-containing tung-oil-based ester plasticizers. Figure 17. Synthesis of hydroxyl and nitrogen rich group-containing tung-oil-based ester plasticizers. Lapinte et al. used methyl oleate, methyl linoleate, and oleic diacid as raw materials to prepare a Lapinteseries of flame-retardantet al. used methyl plasticizers: oleate, dimethyl methyl (methyl linoleate, oleate)phosphonate and oleic diacid (PMO), as raw diethyl materials (methyl to prepare a seriesoleate)phosphonate of flame-retardant (PMO2), plasticizers: dimethyl (methyl dimethyl linoleate)phosphonate (methyl oleate)phosphonate (PML), and dimethyl (PMO), (dimethyl diethyl (methyl oleate)phosphonate (PDE) [75] (Figure 18 and Table1k,l,m,n). PVC materials plasticized with these oleate)phosphonate (PMO2), dimethyl (methyl linoleate)phosphonate (PML), and dimethyl flame-retardant plasticizers show similar thermal properties compared with (dimethylblends oleate)phosphonate and excellent flame-retardant (PDE) performances. [75] (Figure Phosphonate 18 and Table groups 1k,l,m,n). promote PVC the formation materials of aplasticized with charthese layer, flame protecting-retardant the material plasticizers against show fire. similar thermal properties compared with diisononyl phthalate blends and excellent flame-retardant performances. Phosphonate groups promote the formation of a char layer, protecting the material against fire.

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Figure 18. Synthesis of phosphonate groups containing bio-based plasticizers. Figure 18. Synthesis of phosphonate groups containing bio-based plasticizers. 2.4. Citric Acid Ester Plasticizer TableCitric 1 Flame acid ester-retardant plasticizers elements are (phosphate, important, environmentally-friendlychlorine and nitrogen) containing plasticizers vegetable because oil- ofbased their safety,plasticizers. non-toxicity and precipitation resistance. They have been approved in the United States, the European Union and other developed countries to be used in plastic products in close contact with Flame Retardant Elements (Phosphate, Chlorine and Nitrogen) Containing References No. the human body and meetVegetable high Oil hygiene Based Plasticizers requirements such as for food packing, children’s toys,

medical equipment and sanitary products.O O Therefore, citrate plasticizers have become the first choice P O O O for safe and non-toxic plasticizerO C products in the plastics industry globally. There are more than OHO OH OH [70] a 50 kinds of citric acid plasticizers,O C and about 15 kinds of products have been produced at large-scale O OH OH P O by the industrialized manufacturing industry.O O Acetyl tributyl citrate (ATBC) and tributyl citrate

(TBC) have been studied in-depth and industrialized dueO to excellent performances. ATBC has good O O O P O O water and light resistance,O P O good thermalO stability without color, and maintains good flexibility at low O O C OH temperatures. TBC has good compatibilityC O with celluloseOH resin, PVC, PP, and its[71] plasticizing effectb is OH O C remarkable. In addition, TBC has antibacterialO and flame-retardantO P O properties, which further expand O its applicability. However, the price of the plasticizer is relatively high, so it is mainly used in fields with high requirements for non-toxic safety of products such as food packaging materials, children’s toys, medical equipment and packaging. [72] c Citric acid has been commercially produced through large-scale fermentation, thus undoubtedly is considered a raw material for producing plasticizers for PLA, PVC and other materials. Shi et al. prepared a novel plasticizer named glycerol co-plasticized thermoplastic starch (CGTPS) by melting and blending citric acid (CA) [79]. The results show that esterification occurs when the content of CA increases. In comparison with commonly used glycerol plasticized thermoplastic[73] starch (GTPS),d the CGPTS shows partial esterification and lower molecular weight. Moreover, the interaction between plasticizers and starch becomes stronger, thus resisting retrogradation and enhancing its mechanical properties. According to Chabrat et al., the presence of citric acid has positive effects on mechanical O P O [74] e OCOCH3 O OCOCH3 and morphological properties of blends madeC O from wheat flour and glycerol [80]. They claimed OCOCH that citric acid functioned as compatibilizer,3 enhancingOCOCH3 blend compatibilization. Schilling et al. O studied the suitabilityO of citric acid (CA) for Eudragit RSCl P POCl system [81]. The results show that O Cl P Cl O O the monohydrate form apparently benefitsO the extrusion of Eudragit RS PO, while the anhydrous form O O O C Cl is less effective. Flexibility of EudragitC O RS PO films increasesCl with the addition of[75] CA monohydratef Cl O C O O Cl P Cl O

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(CAMH). Thus, CAMH serves as a great assistant for Eudragit RS PO system. Olivato et al. obtained starch/poly films by using maleic anhydride (MA) and CA [82]. The results show that CA enhances esterification/transesterification reactions better than MA and more CA can increase the elongation at break of films. In addition, this process produces films that may become the substitute of non-biodegradable plastics. Shi et al. examined the influence of CA on swelling degree, crystallinity and cytotoxicity of PVA/starch films [83]. FT-IR spectroscopy shows esterification between CA and starch occurs at 140 ◦C; esterification is much easier between PVA and CA. The tensile strength of the film reaches its maximum when the content of CA is 30 wt % of PVA/starch films. Besides, the results show that there is no apparent toxicity on cells when the content of CA is less than 20 wt % of PVA/starch films. Crosslinking is another precious character of citric acid. Seligra et al. took CA as crosslinking agent to produce starch–glycerol films, which are biodegradable and non-retrogradable [84]. The obtained films remain amorphous for more than 45 days. The method avoids retrogradation of starch–glycerol films and maintains the degradability. Garcia et al. changed amounts of critic acid to examine the crosslinking effect [85]. The calculation results of water vapor permeability (WVP) show that the increase of content of citric acid would reduce the diffusion coefficient and WVP. In other words, such reduction apparently suggests the occurrence of crosslinking. Thus, crosslinking is an effective approach to improve the properties of starch products. Zuraida et al. used citric acid and water as secondary additive to prepare bio-plastic starch (BPS). FT-IR results show that the addition of CA at 30 wt %, intensifies the properties of BPS to the degree 40% higher than that of BPS with additional water [86] because the addition of CA disrupts molecular hydrogen bond effectively and avoids the appearance of poor starch chain samples with cavities. Ghanbarzadeh et al. studied the effect of CA on corn starch-based edible films [87]. The results show that CA increases the water vapor barrier property (WVBP) and the ultimate tensile strength (UTS) of corn starch-based edible films. However, when the percentage of CA increases from 10 to 20 wt %, the UTS decreases from 6.57 to 1.80 MPa. Thus, triethyl citrate plasticizer can be used to prepare “Green” films, which possess better intercalation and an exfoliated structure [88].

2.5. Glyceryl Ester Plasticizer Glycerol, as a by-product in biodiesel production, is widely applied in microbial production, biomedical applications and plasticizers [89–92]. Palacios et al. synthesized glycerol-based plasticizers and applied them as an alternative of phthalates [93]. They prepared ight kinds of glycerol triesters from butanoic (butyric), propanoic, isobutanoic (isobutyric), isopentanoic (isovaleric), and benzoic acids. Figure 19 shows their synthesis routes. These low-molecular-weight glycerol triesters are all compatible with PVC and have effective plasticizing effect on PVC. Sahu et al. studied the of rosin-glycerol ester derivative [94]. They explored the biodegradation mechanism of rosin glycerol ester and applied it as a biodegradable material. Thomazine et al. found that the addition of glyceryl ester enhances the elongation at break and water vapor permeability (WVP) of gelatin blend [95]. McHugh et al. also reported that sorbitol and glycerol together significantly reduce tensile strength, intensifying elongation of protein edible films [96]. Glyceryl ester is also an ideal addition for starch plasticizers [97–99]. Chang et al. prepared glycerol plasticized wheat starch (GPS)/CN nanocomposite through a casting process [99]. As the content of such nanoparticles is increased from 0 to 5 wt %, the tensile strength also increases from 3.15 to 10.98 MPa. The effect of glyceryl ester on performance of thermoplastic starch (TPS) blends is investigated; the results illustrate that the shear viscosity of thermoplastic starch (TPS) blends decreases by 20% when the percentage of glycerol increases from 36% to 40% [100,101].

2.6. Other Plasticizers Apart from plasticizers mentioned above, there are also other bio-based plasticizers such as ester, traffic waste oil, , ricinoleic acid and cardanol [101–106]. Yin et al. prepared glucose from liquefaction of cellulose and utilized laser desorption ionization–mass spectrometry Polymers 2018, 10, 1303 17 of 27

(LDI-MS) method to evaluate the performance [107]. The results show that glucose esters have great miscibility with PVC and the maximum elongation at break is reached when the content of glucose ester is 40 wt %. Cataldo et al. claimed that biodiesel can be used as an effective low-temperature plasticizer, which can be applied in winter [108]. Isosorbide dioctoate as plasticizer for PLA presents a better miscibility than dioctyl terephthalate. Figure 20 shows the synthesis of isosorbide dioctoate. The light transmittance reduces when the content of isosorbide dioctoate increases in PLA blends but still shows high transparency. The plasticizing performances indicates that isosorbide dioctoate has Polymers 2018, 10, x FOR PEER REVIEW 18 of 28 great potential to replace traditional phthalate plasticizers [109]. The effect of oligo(isosorbide adipate) (OSA),increased oligo(isosorbide from 0 to suberate)5 wt %, the (OSS) tensile and strength isosorbide also increases dihexanoate from 3.15 (SDH) to 10.98 on MPa. PVC T plasticizershe effect of are investigated;glyceryl the ester blends on performance plasticized ofwith thermoplast SDH areic similar starch to (TPS) DIOP blends blends, is investigated; while a film the plasticized results by OSA orillustrate OSS exhibits that the higher shear viscos glassity transition of thermoplastic temperature starch (TPS) [110]. blends Thus, decreases these synthesized by 20% when isosorbide the plasticizerspercentage serve of as glycerol potential increases substitutes from 3 of6%DOP. to 40% [100,101].

Figure 19. Synthesis of glycerol triesters and mix glycerol triesters. Polymers 2018, 10, x FOR PEER REVIEWFigure 19. Synthesis of glycerol triesters and mix glycerol triesters. 19 of 28

2.6. Other Plasticizers Apart from plasticizers mentioned above, there are also other bio-based plasticizers such as glucose ester, traffic waste oil, isosorbide, ricinoleic acid and cardanol [101–106]. Yin et al. prepared glucose from liquefaction of cellulose and utilized laser desorption ionization–mass spectrometry (LDI-MS) method to evaluate the performance [107]. The results show that glucose esters have great miscibility with PVC and the maximum elongation at break is reached when the content of glucose ester is 40 wt %. Cataldo et al. claimed that biodiesel can be used as an effective low-temperature plasticizer, which can be applied in winter tires [108]. Isosorbide dioctoate as plasticizer for PLA presents a better miscibility than dioctyl terephthalate. Figure 20 shows the synthesis of isosorbide dioctoate. The light transmittance reduces when the content of isosorbide dioctoate increases in PLA blends but still shows high transparency. The plasticizing performances indicates that isosorbide dioctoate has great potential to replace traditional phthalate plasticizers [109]. The effect of oligo(isosorbide adipate) (OSA), oligo(isosorbide suberate) (OSS) and isosorbide dihexanoate (SDH) on PVC plasticizers are investigated; the blends plasticized with SDH are similar to DIOP blends, while a film plasticized by OSA or OSS exhibits higher glass transition temperature [110]. Thus, these synthesized isosorbide plasticizers serve as potential substitutes of DOP.

Figure 20. Synthesis of isosorbide dioctoate. Figure 20. Synthesis of isosorbide dioctoate.

More and more plasticizer products derived from cardanol, a renewable main by‐product of the cashew industry have been applied in manufacturing [111]. Greco et al. did many excellent works on producing various of bio-based plasticizers using cardanol as raw material. They studied the effect of cardanol basedPolymers 201plasticizers7, 9, x; doi: FOR PEERon PVC REVIEW by scanning the miscibility [112]www.mdpi.com/journal/. The results polymersshow that partial miscibility occurs between PVC and cardanol acetate, as shown in Figure 5a, while epoxidized cardanol acetate (Figure 7h) leads to complete miscibility with PVC. Therefore, cardanol acetate is used as second plasticizer to replace toxic phthalate plasticizers. Greco also studied the mechanical, solubility and durability properties of cardanol-derived plasticizers and they exhibit better performance [113,114]. Besides, he also found that cardanol derivatives are innovative plasticizers for poly(lactic acid) (PLA) and claimed that the suitability of cardanol based plasticizers would be wider in the future [115,116]. Apart from PLA or PVC, cardanol is widely used for alumina/toluene tape casting slip and silica-filled natural rubber [117,118]. Li et al. used a tung oil-derived epoxidized dimethyl ester (C21 dicarboxylic acid ester) as plasticizer and auxiliary stabilizer for PVC [119]. Figure 21 shows its synthesis route. It is synthesized from tung oil via transesterification, Diels–Alder reaction and epoxidation. PVC blends plasticized with C21 dicarboxylic acid ester as main plasticizer and auxiliary stabilizer show excellent thermal stability and plasticizing performance, combined with improved migration and volatility stabilities compared to dioctyl terephthalate (DOTP) samples.

Figure 21. Synthesis of C21 dicarboxylic acid ester.

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Polymers 2018, 10, 1303 18 of 27 Figure 20. Synthesis of isosorbide dioctoate.

MoreMore and and more more plasticizer plasticizer products products derived derived from from cardanol, cardanol, a renewablea renewable main main by-product by‐product of of the the cashewcashew industry industry have have been been applied applied in in manufacturing manufacturing [111 [111]]. Greco. Greco et et al. al. did did many many excellent excellent works works on on producingproducing various various of bio-basedof bio-based plasticizers plasticizers using using cardanol cardanol as raw as raw material. material. They They studied studied the effectthe effect of cardanolof cardanol based based plasticizers plasticizers on PVC on PVC by scanningby scanning the the miscibility miscibility [112 [112]]. The. The results results show show that that partial partial miscibilitymiscibility occurs occurs between between PVC PVC and cardanol and cardanol acetate, acetate as shown, as in shown Figure in5a, Figure while epoxidized 5a, while cardanolepoxidized acetatecardanol (Figure acetate7h) leads (Figure to complete 7h) leads miscibility to complete with miscibility PVC. Therefore, with PVC. cardanol Therefore, acetate iscardanol used as secondacetate is plasticizerused as second to replace plasticizer toxic phthalate to replace plasticizers. toxic phthalate Greco plast alsoicizers. studied Greco the mechanical,also studied solubilitythe mechanical, and durabilitysolubility properties and durability of cardanol-derived properties of plasticizers cardanol- andderived they exhibit plasticizers better and performance they exhibit [113 ,114 better]. Besides,performance he also [113,114] found that. Besides, cardanol he derivativesalso found that are innovativecardanol derivatives plasticizers are for innovative poly(lactic plasticizers acid) (PLA) for andpoly(lact claimedic thatacid) the (PLA) suitability and claimed of cardanol that the based suitability plasticizers of cardanol would based be wider plasticizers in the future would [115 be,116 wider]. Apartin the from future PLA [115,116] or PVC,. cardanol Apart from is widely PLA usedor PVC, for alumina/toluenecardanol is widely tape used casting for alumina/toluene slip and silica-filled tape naturalcasting rubber slip and [117 silica,118].-filled natural rubber [117,118]. LiLi et et al. al used. used a tunga tung oil-derived oil-derived epoxidized epoxidized dicarboxylic dicarboxylic acid acid dimethyl dimethyl ester ester (C21 (C21 dicarboxylic dicarboxylic acidacid ester) ester as) as plasticizer plasticizer and and auxiliary auxiliary stabilizer stabilizer for for PVC PVC [119 [119].]. Figure Figure 21 shows 21 shows its synthesis its synthesis route. route. It is It synthesizedis synthesized from from tung tung oil viaoil via transesterification, transesterification, Diels–Alder Diels–Alder reaction reaction and and epoxidation. epoxidation. PVC PVC blends blends plasticizedplasticized with with C21 C21 dicarboxylic dicarboxylic acid acid ester ester as as main main plasticizer plasticizer and and auxiliary auxiliary stabilizer stabilizer show show excellent excellent thermalthermal stability stability and and plasticizing plasticizing performance, performance, combined combined with with improved improved migration migration and and volatility volatility stabilitiesstabilities compared compared to to dioctyl dioctyl terephthalate terephthalate (DOTP) (DOTP) samples. samples.

Figure 21. Synthesis of C21 dicarboxylic acid ester. Figure 21. Synthesis of C21 dicarboxylic acid ester. Vegetable fatty acids such as ricinoleic acid are also used to produce bio-based plasticizers. Ester-amide of ricinoleic acid, as shown in Figure 22, is synthesized and used as primary plasticizer to replace phthalate plasticizers in PVC materials [105]. Tg of PVC materials blended with 40 wt % of ester-amidePolymers 2017 of, 9 ricinoleic, x; doi: FOR acid PEER reaches REVIEW− 13.5 ◦C. www.mdpi.com/journal/polymers A series of cardanol-based plasticizers were produced by Yang group [111]. The chemical structure and synthesis route of cardanol esters are shown in Figure5a,f,g,h and Figure 23. These caedanol esters decrease Tg of PVC and show excellent biodegradability and migration stability. Biodegradability of cardanol esters are better than cardanol, which provides potential methods to solve environmental problems produced by non-degradability of cardanol. Esterification reaction of vegetable oil fatty acid and benzyl alcohol is used to synthesize bio-based plasticizers (benzyl ester of DCO fatty acid) [120]. Figure 24 shows synthesis route of benzyl ester of DCO fatty acid (BE), which is used as auxiliary plasticizer to prepare soft PVC materials. Tensile strength, percentage elongation and thermal degradation properties are improved after blending BE in PVC formulations. Good anti-leaching properties and plasticizing performances of BE make it an environmentally-friendly plasticizer. Polymers 2018, 10, x FOR PEER REVIEW 20 of 28

Vegetable fatty acids such as ricinoleic acid are also used to produce bio-based plasticizers. Ester- amide of ricinoleic acid, as shown in Figure 22, is synthesized and used as primary plasticizer to replace phthalate plasticizers in PVC materials [105]. Tg of PVC materials blended with 40 wt % of ester-amide of ricinoleic acid reaches −13.5 °C.

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Vegetable fatty acids such as ricinoleic acid are also used to produce bio-based plasticizers. Ester- amide of ricinoleic acid, as shown in Figure 22, is synthesized and used as primary plasticizer to Polymers 2018replace, 10, phthalate 1303 plasticizers in PVC materials [105]. Tg of PVC materials blended with 40 wt % of19 of 27 ester-amide of ricinoleic acid reaches −13.5 °C.

Figure 22. Synthesis of ester-amide of ricinoleic acid.

A series of cardanol-based plasticizers were produced by Yang group [111]. The chemical structure and synthesis route of cardanol esters are shown in Figures 5a,f,g,h and 23. These caedanol esters decrease Tg of PVC and show excellent biodegradability and migration stability. Biodegradability of cardanol esters are better than cardanol, which provides potential methods to solve environmental problems produced by non-degradability of cardanol. Figure 22. Figure Synthesis22. Synthesis of of ester-amide ester-amide ofof ricinoleic acid. acid.

A series of cardanol-based plasticizers were produced by Yang group [111]. The chemical structure and synthesis route of cardanol esters are shown in Figures 5a,f,g,h and 23. These caedanol esters decrease Tg of PVC and show excellent biodegradability and migration stability. Biodegradability of cardanol esters are better than cardanol, which provides potential methods to solve environmental problems produced by non-degradability of cardanol.

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Esterification reaction of vegetable oil fatty acid and benzyl alcohol is used to synthesize bio- based plasticizers (benzyl ester of DCO fatty acid) [120]. Figure 24 shows synthesis route of benzyl ester of DCO fatty acid (BE), which is used as auxiliary plasticizer to prepare soft PVC materials. Tensile strength, percentage elongation and thermal degradation properties are improved after blending BE in PVC formulations. Good anti-leaching properties and plasticizing performances of BE make it an environmentally-friendly plasticizer. Figure 23. Synthesis of cardanol esters. Figure 23. Synthesis of cardanol esters.

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Figure 23. Synthesis of cardanol esters.

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Figure 24. Synthesis of benzyl ester of DCO fatty acid. Internal plasticizer is the part of polymer that has plasticizing effect on polymer by increasing Internal plasticizer is the part of polymer that has plasticizing effect on polymer by increasing distance of polymer chains, decreasing the interaction force of main chains of polymer and distance of polymer chains, decreasing the interaction force of main chains of polymer and increasing increasing the possibility of mutual movement of polymer chains [121,122]. Self-plasticization the possibility of mutual movement of polymer chains [121,122]. Self-plasticization PVC via PVC via substitution reaction with minnich base of cardanol butyl ether is obtained, and shown substitution reaction with minnich base of cardanol butyl ether is obtained, and shown in Figure 25 [123]. Minnich base of cardanol butyl ether, as shown in Figure 5d, is internal plasticizer in this study. Tg of self-plasticization PVC deceases from 85.6 °C to 49.3 °C. The decrease of tensile strength and increase of elongation at break for self-plasticization PVC illustrates the internal plasticizing effect. Nonmigrating plasticized PVC plasticized with mannich base of waste cooking oil methyl ester is also investigated via the same method [124]. No plasticizer loss for self-plasticization PVC materials in leaching tests gives these materials potential to be used in food packaging and toys.

Figure 25. Synthesis of self-plasticization PVC via substitution reaction with minnich base of cardanol butyl ether.

Click reaction of functionalized PVC and alkynyl group containing bio-based derivatives produces internally plasticized PVC materials [125–128]. Alkynylation EAMR-DOPO (Figure 26), propargyl ether cardanol (Figure 5i), and hexyl-terminated hyperbranched polyglycerol (Figure 27)

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Esterification reaction of vegetable oil fatty acid and benzyl alcohol is used to synthesize bio- based plasticizers (benzyl ester of DCO fatty acid) [120]. Figure 24 shows synthesis route of benzyl ester of DCO fatty acid (BE), which is used as auxiliary plasticizer to prepare soft PVC materials. Tensile strength, percentage elongation and thermal degradation properties are improved after blending BE in PVC formulations. Good anti-leaching properties and plasticizing performances of BE make it an environmentally-friendly plasticizer.

Figure 24. Synthesis of benzyl ester of DCO fatty acid.

Polymers 2018Internal, 10, 1303 plasticizer is the part of polymer that has plasticizing effect on polymer by increasing20 of 27 distance of polymer chains, decreasing the interaction force of main chains of polymer and increasing the possibility of mutual movement of polymer chains [121,122]. Self-plasticization PVC via in Figuresubstitution 25[ 123]. reaction Minnich with base minnich of cardanol base of cardanol butyl ether, butyl as ether shown is obtained, in Figure and5d, shown is internal in Figure plasticizer 25 ◦ ◦ in this[1 study.23]. MinnichTg of base self-plasticization of cardanol butyl ether, PVC as deceases shown in Figure from 5d, 85.6 is internalC to plasticizer 49.3 C. in The this decreasestudy. of tensileT strengthg of self-plasticizat and increaseion PVC of elongationdeceases from at break85.6 °C for to 49.3 self-plasticization °C. The decrease PVC of tensile illustrates strength the and internal increase of elongation at break for self-plasticization PVC illustrates the internal plasticizing effect. plasticizing effect. Nonmigrating plasticized PVC plasticized with mannich base of waste cooking oil Nonmigrating plasticized PVC plasticized with mannich base of waste cooking oil methyl ester is methyl ester is also investigated via the same method [124]. No plasticizer loss for self-plasticization also investigated via the same method [124]. No plasticizer loss for self-plasticization PVC materials PVC materialsin leaching in tests leaching gives these tests materials gives these potential materials to be potentialused in food to packaging be used in and food toys. packaging and toys.

FigureFigure 25. Synthesis 25. Synthesis of self-plasticization of self-plasticization PVC PVC viavia substitution reaction reaction with with minnich minnich base baseof cardanol of cardanol butyl ether.butyl ether.

Click reaction of azide functionalized PVC and alkynyl group containing bio-based derivatives ClickPolymers reaction 2018, 10, ofx FOR azide PEER functionalized REVIEW PVC and alkynyl group containing bio-based22 derivatives of 28 producesproduces internally internally plasticized plasticized PVC PVC materials materials [[125125–128128].]. Alkynylation Alkynylation EAMR EAMR-DOPO-DOPO (Figure (Figure 26), 26), propargyl ether cardanol (Figure 5i), and hexyl-terminated hyperbranched polyglycerol (Figure 27) propargylare grafted ether cardanol onto PVC (Figure chain. 5Thei), and obtained hexyl-terminated PVC materials hyperbranched show lower Tg polyglyceroland improved (Figureflexibility 27 ) are Polymerscompared 2017 to, 9, PVC.x; doi: NoFOR p PEERlasticizer REVIEW migration is found in leaching tests. Thew excellentww.mdpi.com/journal/ solvent extractionpolymers grafted onto PVC chain. The obtained PVC materials show lower Tg and improved flexibility compared to PVC.resistance No plasticizer and volatility migration resistance is found give in these leaching inter tests.nally Theplasticized excellent PVC solvent materials extraction potential resistance for applications in products with a high migration resistance requirement. and volatility resistance give these internally plasticized PVC materials potential for applications in products with a high migration resistance requirement.

FigureFigure 26. Synthesis 26. Synthesis internally internally plasticized plasticized PVCPVC with with alkynylation alkynylation EAMR EAMR-DOPO.-DOPO.

Figure 27. Synthesis of hexyl-terminated hyperbranched polyglycerol.

3. Conclusions Biomass resources have been frequently used to produce PVC plasticizers as low-cost raw materials. Bio-based plasticizers designed for different application are mostly based on chemical structure of biomass resources. Cardanol is a promising alternative to traditional petrochemical- based o-phthalate plasticizers due to its benzene ring structure and active hydroxyl group. Vegetable oil contains flexible long fatty acid chains and rich unsaturated bonds and can be used to produce epoxy plasticizer. Epoxidized soybean oil (ESO) has been industrialized and used as primary plasticizer in food packing materials. Vegetable oil-based flame-retardant plasticizers provide rich carbon combined with flame-retardant elements to improve flame retardancy of PVC product by promoting the formation of char residue. Cardanol and vegetable oil have been the two most important biomass feedstocks for producing plasticizers. Internally plasticized strategy produces

Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 22 of 28

are grafted onto PVC chain. The obtained PVC materials show lower Tg and improved flexibility compared to PVC. No plasticizer migration is found in leaching tests. The excellent solvent extraction resistance and volatility resistance give these internally plasticized PVC materials potential for applications in products with a high migration resistance requirement.

Polymers 2018, 10, 1303 21 of 27 Figure 26. Synthesis internally plasticized PVC with alkynylation EAMR-DOPO.

Figure 27. Synthesis of hexyl-terminated hyperbranched polyglycerol. Figure 27. Synthesis of hexyl-terminated hyperbranched polyglycerol. 3. Conclusions 3. Conclusions BiomassBiomass resources resources have have been been frequently frequently used used to to produce PVC PVC plasticizers plasticizers as low as- low-costcost raw raw materials.materials. Bio-based Bio-based plasticizers plasticizers designed designed for for different different application application are are mostly mostly based based on chemical on chemical structurestructure of biomass of biomass resources. resources. Cardanol Cardanol is a promisingis a promising alternative alternative to traditional to traditional petrochemical-based petrochemical- o-phthalatebased plasticizerso-phthalate plasticizers due to its due benzene to its benzene ring structure ring structure and and active active hydroxyl hydroxyl group.group. Vegetable Vegetable oil containsoil flexiblecontains long flexible fatty long acid fatty chains acid chains and rich and unsaturated rich unsaturated bonds bonds and and can can be usedbe used to to produce produce epoxy plasticizer.epoxy Epoxidized plasticizer. soybean Epoxidized oil soybean (ESO) has oil been (ESO) industrialized has been industrialized and used and asprimary used as plasticizer primary in plasticizer in food packing materials. Vegetable oil-based flame-retardant plasticizers provide rich food packing materials. Vegetable oil-based flame-retardant plasticizers provide rich carbon combined carbon combined with flame-retardant elements to improve flame retardancy of PVC product by with flame-retardantpromoting the elements formation to of improve char residue. flame Cardanol retardancy and of vegetable PVC product oil have by promoting been the two the formationmost of charimportant residue. biomassCardanol feedstocks and vegetable for producing oil have plasticizers. been the Internally two most plasticized important strategy biomass produces feedstocks for producing plasticizers. Internally plasticized strategy produces excellent polymer materials with Polymers 2017, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/polymers plasticity and flexibility but without plasticizer migration. Thermal stability, migration stability, biodegradability and plasticizing efficiency of bio-based plasticizers have been widely investigated and discussed in the literature. However, research on biological toxicity of bio-based plasticizers on human body and animal such as experimental mice and fish is scant. From an industrialization point of view, biological toxicity of bio-based plasticizers should be taken into account. Considerable efforts should be devoted to investigating the relationship between biological toxicity and chemical structure of bio-based plasticizers. Then, we can design non-toxic and environmentally-friendly biomass-based plasticizers to replace reproductive toxicity of o-phthalate plasticizers. Biomass raw materials, synthesis strategy, performances of PVC materials plasticized with bio-based plasticizers, and recent development in plasticizer field, as discussed in this article, might be used for the commercialization of bio-based, low-cost, environmentally-friendly plasticizers for numerous applications.

Author Contributions: P.J. and Y.Z. offered the idea and research design. H.X. searched the literature. P.J., H.X. and K.T. wrote the paper. P.J. made the table and figures. P.J. and H.X. contributed equally to this paper. Funding: This research was funded by the [National Natural Science Foundation of China] grant numbers [31700499, 31670577 and 31670578]; the [Fundamental Research Funds for the Central Non-profit Research Institution of CAF] grant number [CAFYBB2018QB008] and the [Fundamental Research Funds from Jiangsu Province Biomass and Materials Laboratory] grant number [JSBEM-S-2017010). Conflicts of Interest: The authors declare no conflict of interest.

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