Synthesis of a polyester plasticizer from rubber seed oil for polyvinyl chloride

A polyester plasticizer (RSOP) for polyvinyl chloride based on rubber seed oil (RSO) was synthesized. Firstly, RSO monoglyceride were synthesized by the transesteri ﬁ cation of RSO with glycerol at 220–240 o C. Secondly, RSOP was synthesized from RSO monoglyceride and adipic acid by esteri ﬁ cation. The polyester plasticizer was characterized by GPC, FT-IR, 1 H NMR and DSC. Plasticized polyvinyl chloride (PVC) materials with RSOP and dioctyl phthalate (DOP) in varying ratios were prepared via thermal melting process, RSOP was used as a second plasticizer. The properties of the plasticized PVC materials were characterized by a universal testing machine, TGA, DMA and solvent extraction resistance. The obtained PVC materials showed improved thermal stability and lower glass transition temperature than PVC. Solvent extraction resistance and plasticization of plasticized PVC were also improved. This study provides a new strategy for preparing bio-based polyester plasticizer from RSO.


INTRODUCTION
The work on producing bio-based plasticizer to replace phthalates is a signifi cant challenge in food packing, medical devices, automotive products and storage bags, because recent research work shows that phthalates have adverse impacts on people's health and environment 1-4 .A series of alternative plasticizers such as epoxidized soybean oil, castor oil-based polyester, citrate, vegetable oil-based polyol ester, and tung oil-based hyperbranched ester derived from biomass resources have been widely investigated and applied in polyvinyl chloride(PVC) materials [5][6][7][8][9] .Polyester plasticizers are important additives used in PVC materials.There are many advantages for polyester plasticizers compared phthalate plasticizers, such as improved solvent extraction resistance and thermal stability, low toxic or non-toxic 16 .Recently, many researchers reported nature-based polyester plasticizers derived from oleic acid, citric acid, castor oil, itaconic acid and rice fatty 10- 15 .
Rubber seed oil(RSO) is a kind of woody vegetable oil extracted from the seeds of rubber trees.The main components of rubber seed oil are palmitic acid, stearic acid, oleic acid, linoleic acid and linolenic acid.It can be used to synthesize a variety of chemicals and polymer materials.Using rubber seed oil to synthesize plasticizer is an important strategy to increase the added value of rubber seed oil.Bio-based plasticizer from RSO can be synthesized via multiple chemical reactions of triglyceride.The polar groups of RSO can improve the compatibility of RSO-based plasticizers and PVC resin.Recent studies have reported that epoxidized rubber seed oil and rubber seed oil epoxy fatty acid methyl ester were synthesized from rubber seed oil and used as plasticizer for PVC and polylactic acid 16 .
The purpose of this work was to synthesize a novel polyester plasticizer using RSO, glycerol and adipic acid as raw materials by transesterifi cation, esterifi cation and polycondensation.RSO-based polyester (RSOP) was used as the main plasticizer for PVC to produce medium-scale samples for characterization.The properties of plasticized PVC with RSOP and DOP were studied.

Synthesis of RSO monoglyceride
The process of the reaction is shown in Figure 1.Glycerol (184 g, 2 mol) was heated at 220-230 o C for 2 h in N 2 atmosphere.RSO (900 g, 1.06 mol) was gradually added in the reaction for 1h.The mixture was stirred for 5.5 h at 230-240 o C. Then the fl ask was cooled down to room temperature.RSO monoglyceride was obtained after removing excess glycerol by washing with water and distillation [17][18] .

Synthesis of RSOP
Monoglyceride (300 g, 0.83 mol), adipic acid (146 g, 1 mol) and TBT (0.14 g, 0.42 mmol) were placed in a round bottom fl ask.The mixture was heated to melt in N 2 atmosphere.The mixture was stirred at 220-230 o C for 6-8 h to fi nish esterifi cation.The product was obtained after dissolving in acetone, washing from water and drying.RSOP samples obtained at 6, 8 and 10 h were labeled with RSOP-1, RSOP-2 and RSOP-3.The process of the reaction was shown in Figure 1.

Preparation of PVC materials
Plasticized PVC materials were prepared via thermoplastic blending at 165 o C for 5 min.The formulations are shown in Table 1.

Characterization
The intrinsic viscosities of RSOP were measured with NDJ-1 rotary viscometer.Chemical structures of RSOP Polish Journal of Chemical Technology, 24, 4, 1-6, 10.2478/pjct-2022-0023 ceride spectra presented a large increase of the absorption band located at 3337 cm -1 due to -OH presence in the glycerolysis product.The peak of the -C=O shifted from 1740 cm -1 to 1738 cm -1 , confi rming the occurrence of the glycerolysis reaction.In the spectrum of RSOP, characteristic peaks were also shown: -OH (3466 cm -1 ), -CH 3 and -CH 2 CH 3 (2923 and 2854 cm -1 ), and -C=O (1738 cm -1 ) 23-26 .The broad -OH at 3466 cm -1 in the RSOP samples was observed.The peak at 1045 cm -1 decreased because of the disappearance of -OH in the esterifi cation reaction to form -C=O. were studied with AV500 NMR spectrometer and Nicolet iS10 FT-IR.The molecular weights were determined by gel permeation chromatography Model 1515 GPC.TGA was carried out in a TG209F1 TGA thermal analysis instrument (heating rate: 10 o C /min.Temperature range: ambient temperature to 600 o C. DMA was performed via a DMTA Q800 (temperature range: -100 -100 o C; heating rate: 3 o C/min).The mechanical property was determined using E43.104Universal Testing Machine.The leaching tests of plasticizers were tested according to the recent papers 19- 22 .

RESULTS AND DISCUSSION
The GPC results and viscosity of the RSOP are shown in Table 2.When the reaction time increased from 6 to 10 h, Mn increased from 18700 to 22200 g/mol, Mw increased from 18500 to 23500 g/mol, and viscosity increased from 2.2 to 2.9 Pa•s.
The obtained FT-IR curves of RSO, monoglyceride and RSOP are shown in Figure 2. The -CH 3 and -CH 2 CH 3 on the fatty acid chains (around 2923 and 2853 cm -1 ), and the -C=O (1740 cm -1 ) can be observed in the FTIR of RSO.The -OH (3387 cm -1 ), -CH 3 and -CH 2 CH 3 (2923 and 2851 cm -1 ), and -C=O (1738 cm -1 ) can be observed in the spectrum of the monoglyceride [23][24][25][26] .The monogly-  Figure 3 shows 1 H NMR spectra of the RSOP.The peak at around 0.7 ppm was attributed to protons of -CH 3 .The integral of this peak should result from three protons per fatty acid chain in monoglyceride.In the spectrum of RSOP, the peak at 1.35 ppm corresponds to the methylene groups of monoglyceride backbones connecting to the ester structure (-CH 2 -(CH 2 )n-CH 2 -).
Glass transition temperature (T g ) of RSOP was investigated via DSC (Figure 4).The results showed that T g of RSOP increased from -53 to -27 o C with the increase of reaction time.There is only one T g for all RSOPs, which indicated that all RSOPs are homopolymer.
was investigated (Figures 6 and 7).Table 3 summarizes the TGA and DTG parameters T d , T 10 (Temperature at 10% weight loss), T 50 (Temperature at 50% weight loss), and T P (Temperature at weight loss peak)).PVC plasticized with RSOP-3 did not lose any weight below Figure 5 shows that torque values changed with mass of plasticizer in PVC materials.When more DOP was replaced with RSOP-3, the torque increased from 12.7 N • m to 19.0 N • m.RSOP-3 with high relative molecular weight is more diffi cult to be fully miscible with PVC than DOP, which caused the increase of torque.The torque increment will cause an increment of melt viscosity, which will increase energy consumption in processing PVC materials.
To determine the effect of RSOP-3 on the thermal stability of the PVC materials, TGA of PVC material When plasticizer (DOP or RSOP-3) and PVC are blended, the small molecules of the plasticizer such as DOP will be inserted between the polymer molecular chains, which will reduce the attraction between the polymer chains and increase the distance between them, the movement of the molecular chains will be increased, and the entanglement between the polymer molecular chains will be reduced, thereby the plasticity of plastics was be improved [27][28][29] .Therefore, plasticizers with small molecular weight (such as DOP ) have stronger plasticizing ability than polyesters (such as RSOP-3).3), but T 50 and T p were almost unchanged.The char residue of PVC/20wt% RSOP-3, PVC/30wt% RSOP-3, PVC/40wt% RSOP-3 is 14, 17 and 18% with the addition of RSOP-3 at 600 o C.
T g of polymeric matrix can be used to evaluate plasticizing effect of plasticizer.T g derived from DMA and DMA curves are shown in Figure .8 and Table 3. T g of PVC/20wt% RSOP-3, PVC/30wt% RSOP-3, PVC/40wt% RSOP-3 were 32, 45 and 49 o C, which indicated all materials are homogeneous systems 19-20 .The increase of T g indicated that plasticizing effi ciency of RSOP-3 is lower than DOP.When more RSOP-3 was added to PVC materials, PVC/40wt% RSOP-3 showed the lowest elongation at break (190%) and the highest tensile strength (27.6 MPa), while PVC/20wt% RSOP-3 showed the highest elongation at break (280%) and the lowest tensile strength (16.2 MPa).The results indicated that plasticizing perfor-Figure 9 shows the weight loss of plasticized PVC materials with DOP and RSOP-3 in N-hexane, N-hexane can accelerate the migration of plasticizer, and then its migration performance can be evaluated in a short time.With the gradual decrease of DOP and increase of RSOP-3, the weight loss of PVC materials signifi cantly decreased from 7.7 to 3.8%.RSOP-3 showed excellent solvent extraction resistance than DOP, because RSOP-3 with larger relative molecular weight has better migration resistance than DOP.

CONCLUSION
A bio-based polyester plasticizer for polyvinyl chloride was synthesized from RSO.When the reaction time increased from 6 to 10 h, Mn of RSOP increased from 18700 to 22200 g/mol, Mw increased from 18500 to 23500 g/mol, and viscosity increased from 2.2 to 2.9 Pa • s.PVC materials plasticized with RSOP-3 and DOP in varying ratios were prepared via the thermal melting process.When more DOP was replaced with RSOP, the torque increased from 12.7 to 19.0 N • m.T d and T 10 increased, but T 50 and T p were almost unchanged.In addition, RSOP-3 enhanced thermal stability of PVC .The char residue and T g of PVC/20wt% RSOP-3, PVC/30wt% RSOP-3, PVC/40wt% RSOP-3 is 14, 17 and 18% at 600 o C and 32, 45 and 49 o C. PVC/40wt% RSOP-3 showed the lowest elongation at break (190%) and the highest tensile strength (27.6 MPa).The results indicated that plasticizing effi ciency of RSOP-3 is lower than DOP, which is consistent with the DMA results.However, RSOP-3 showed excellent solvent extraction resistance than DOP, because RSOP-3 with larger relative molecular weight has better migration resistance than DOP.This study provides a new strategy for preparing bio-based polyester plasticizer from RSO.

Figure
Figure TGA curves of plasticized PVC 200 o C. When the temperature reached 230-380 o C, the high temperature led to dehydrochlorination of PVC 19-21 .Aromatic compounds were formed at above 380 o C. When the quality of RSOP-3 increased in PVC materials, T d and T 10 increased (Table

Figure 8 .
Figure 8. DMA curves of plasticized PVC materials

Table 1 .
Formulations of PVC materials

Table 2 .
The GPC properties and viscosity of RSOP(25 o C)

Table 3 .
The thermal properties of PVC materials

Table 4 .
Mechanical properties of PVC materials