sign in sign up
<<
Home , Polyurethane

Article Fabrication and Properties of a Bio-Based Biodegradable Polyurethane

Zhaoshan Wang 1, Jieqiong Yan 1, Tongyao Wang 1, Yingying Zai 1, Liyan Qiu 1 and Qingguo Wang 1,2,*

1 Key Laboratory of Rubber- of Ministry of Education, Qingdao University of Science and Technology, Qingdao 266042, China 2 Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao 266042, China * Correspondence: [email protected]; Tel.: +86-532-8402-2418

 Received: 16 April 2019; Accepted: 19 June 2019; Published: 2 July 2019 

Abstract: Using the melt polycondensation of five bio-based aliphatic (succinic acid, sebacic acid, fumaric acid, 1,3-propanediol, and 1,4-butanediol), we first synthesized the more flexible and biodegradable (BPD) with an average molecular weight of 3825. Then, the BPD was polymerized with excessive 4,40-diphenylmethane diisocyanate (MDI). Finally, the molecular chain extender of 1,4-butanediol (BDO) was used to fabricate the biodegradable thermoplastic polyurethane elastomer (BTPU), comprising the soft segment of BPD and the hard segment polymerized by MDI and BDO. Atomic force microscope (AFM) images showed the two-phase structure of the BTPU. The tensile strength of the BTPU containing 60% BPD was about 30 MPa and elongation at break of the BTPU was over 800%. Notably, the BTPU had superior biodegradability in lipase solution and the biodegradation weight loss ratio of the BTPU containing 80% BPD reached 36.7% within 14 days in the lipase solution.

Keywords: polyurethane; thermoplastic; polycondensation; biodegradability; bio-based

1. Introduction Thermoplastic polyurethane (TPU) is an important that combines the softness and of rubber with the easy processability of [1]. Simultaneously, TPU has many advantages [2,3], such as simple processing technology, recyclability, superior mechanical and thermal properties [4,5], excellent wear resistance [6,7], and biocompatibility [8,9]. TPU is irreplaceable in various fields, including automotive, screens, roller systems films, medical, sports products, aerospace and electronics, and electronic applications [10–13]. Unfortunately, traditional preparation methods of TPU rely on fossil resources, which undoubtedly exacerbates the depletion of fossil resources [14,15]. Moreover, most TPUs are not biodegradable [16]. Thereby, improving the biodegradability of TPU and reducing the use of nonrenewable resources have become urgent and significant subjects in the field of polymer materials. Biomass residues, such as corncob [17], cocopeat [18] and sugarcane bagasse [19], are important resources for preparing some bio-based monomers. Bio-based monomers or polymers have many advantages, including low pollution, low cost, and superior biodegradability [20–22]. Therefore, efficiently developing and utilizing biomass residues will play positive roles in solving energy and ecological problems [23–26]. In recent years, the application of biomass residues in preparing bio-based materials has become an important focus in the polymer field [27,28], in which biodegradable thermoplastic polyurethane (BTPU) has also attracted a lot of attention. Sit-Foon Cheng et al. [29], using palm oil-based polyester , synthesized water-blown porous biodegradable polyurethane

Polymers 2019, 11, 1121; doi:10.3390/polym11071121 www.mdpi.com/journal/polymers Polymers 2019, 11, 1121 2 of 13 with a mass loss ratio in the range from 10 to 40% in two weeks under enzymatic conditions. Meanwhile, the tensile strength of the polyurethane foams was only in the range from 0.05 to 0.3 MPa and the elongation at break of the polyurethane foams was in the range from 52 to 227%. Kucharczyk, P. et al. [30] synthesized a (PLA)-based polyesterurethane with an elongation at break of about 3% and good biodegradability. Gokhan Acik et al. [31] synthesized a series of soybean oil-based biodegradable rigid polyurethane films (PU-Fs), which had a glass transition temperature (Tg) in the range from 55 to 62 ◦C. After 12-week enzymatic experiments, the mass of the rigid PU-Fs was lost by about 40 to 50%. Being different from TPUs, which cannot have good biodegradability and good elasticity simultaneously, this paper—using bio-based aliphatic monomers [32,33] and polyester esterification reaction and chain extension technologies—designed and prepared a novel bio-based BTPU, which demonstrates both good biodegradability and good elasticity. BTPU contains a novel soft segment of aliphatic polyester and a hard segment. Notably, compared to some soft segments with 1–3 polyester repeat units in some traditional TPUs [29–31], the novel soft segment of the novel BTPU comprises six kinds of aliphatic polyester repeat units and has good biodegradability and flexibility. Also, the use of fumaric acid is explored to synthesize the novel soft segment because the carbon-carbon double bond in the main molecular chain has the effect of improving the molecular chain relaxation rates [34], leading to a more flexible soft segment. This paper also studied the effects of the molecular structure, molecular weight and soft segment ratio on the biodegradability, mechanical property and thermal property of BTPU. This work can provide new research insights for developing environmentally friendly and functional biodegradable polymer materials.

2. Experimental Section

2.1. Materials

1 1,3-propanediol (PDO) with a molecular weight of 76.10 gmol− and a purity of 99% was supplied by Hunan Hainabaichuan Bioengineering Co., Ltd., Yiyang, China. 1,4-butanediol (BDO) with a 1 molecular weight of 90.12 gmol− and purity of no less than 99% was supplied by Shouguang Jinyu 1 Chemical Co., Ltd., Shouguang, China. Succinic acid (SU) with a molecular weight of 118.09 gmol− and purity of no less than 99% was supplied by Shandong LanDian Biological Technology Co., Ltd., 1 Shouguang, China. Sebacic acid (SA) with a molecular weight of 202.25 gmol− and purity of no less than 98.5% was supplied by Inner Mongolia Weiyu Biological Technology Co., Ltd., Tongliao, China. 1 Fumaric acid (FA) with a molecular weight of 116.07 gmol− and purity of no less than 99.5% was supplied by Anhui Sealong Biotechnology Co., Ltd., Bengbu, China. 4,4-diphenylmethane diisocyanate 1 (MDI), with a molecular weight of 250.26 gmol− was supplied by Wanhua Chemical Group Co., 1 Ltd., Yantai, China. Para- sulfonic acid with a molecular weight of 172.20 gmol− and purity of no less than 99.0% was supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. 1 Chloroform with a molecular weight of 119.38 gmol− and purity of no less than 99.0% was supplied by Yantai Sanhe Chemical Reagent Co., Ltd., Yantai, China. Anhydrous methanol, with a molecular 1 weight of 32.04 gmol− and purity of no less than 99.5% was supplied by Tianjin Damao Chemical Reagent Factory, Tianjin, China. Lipase, 20000 U, was supplied by Henan Xinyu Food Additives Co., Ltd., Zhengzhou, China. Mixed phosphate (pH buffer), was supplied by Shanghai Leici Chuangyi Instrument Co., Ltd., Shanghai, China. Antioxidant-168 (AT-168) and antioxidant-1010 (AT-1010) were supplied by Beijing HWRK Chemical Co., Ltd., Beijing, China.

2.2. Experimental Instruments The ATR-IR analyzer (VERTEX 70) was bought from Bruker Corp., Leipzig, Germany. The nuclear magnetic resonance (NMR) spectrometer, AVANCE, was from Bruker Corp., Leipzig, Germany. The gel permeation chromatograph (GPC), HLC-8320, was manufactured by Corp., Tokyo, Japan. The differential scanning calorimeter (DSC), 204F1, was supplied by Netzsch Corp., Selb, Polymers 2019, 11, x FOR PEER REVIEW 3 of 13

Germany.Polymers 2019 The, 11, 1121gel permeation chromatograph (GPC), HLC-8320, was manufactured by Tosoh Corp.,3 of 13 Tokyo, Japan. The differential scanning calorimeter (DSC), 204F1, was supplied by Netzsch Corp., Selb, Germany. The thermogravimetric analyzer (TGA), 209F1, was manufactured by German Germany. The thermogravimetric analyzer (TGA), 209F1, was manufactured by German Netzsch Netzsch Corp., Selb, Germany. The atomic force microscope (AFM), Multimode8, was from Bruker Corp., Selb, Germany. The atomic force microscope (AFM), Multimode8, was from Bruker Corp., Corp., Leipzig, Germany. The electronic tensile machine, AI-7000, was bought from Taiwan Gotech Leipzig, Germany. The electronic tensile machine, AI-7000, was bought from Taiwan Gotech Testing Testing Machines Inc, Taichung, Taiwan. Machines Inc, Taichung, Taiwan.

2.3.2.3. Experimental Experimental Methods Methods

2.3.1.2.3.1. Synthesis Synthesis of of Biodegradable Biodegradable Polyester Polyester Diols Diols (BPD) (BPD) TheThe monomers monomers of of PDO, PDO, BDO, BDO, SA, SA, SU, SU, and and FA FA were placed into a four-necked round-bottom flaskflask (500 (500 mL), mL), which which was was equipped equipped with with magnetic magnetic stirring, stirring, thermometer, thermometer, and and nitrogen nitrogen gas gas entrance. entrance. TheThe mole mole ratio ratio of of PDO/BDO/SA/SU/FA PDO/BDO/SA/SU/FA was was 5.9/5.9/5.6/1.4/3, 5.9/5.9/5.6/1.4/ in3, which in which the themole mole ratio ratio of alcohol of alcohol to acid to wasacid 1.18:1 was 1.18:1 and andFA accounted FA accounted for for30% 30% of the of the moles moles of ofthe the dibasic dibasic acid. acid. Under Under the the protection protection of of a a nitrogennitrogen atmosphere,atmosphere, the the reaction reaction mixture mixture was was slowly slowly heated heated to 190 ◦toC until190 the°C monomersuntil the monomers completely completelymelted. Then, melted. the esterification Then, the esterification reaction was reaction retained was for retained 2 h and for the 2 h water and the produced water produced was removed was removedwith the helpwith ofthe nitrogen help of nitrogen gas. The gas. catalyst The catalyst (para-toluene (para-toluene sulfonic sulfonic acid with acid a weightwith a weight ratio of ratio 0.25%) of 0.25%)and antioxidants and antioxidants (AT-168 (AT-168 and AT-1010 and AT-1010 with awith weight a weight ratio ofratio 0.1%) of 0.1%) were were subsequently subsequently added added into intothe abovethe above polyester polyester prepolymer, prepolymer, followed followed by decompressing by decompressing the pressure the pressure of the of reactor the reactor below below 200 Pa 200and Pa increasing and increasing the temperature the temperature of polyester of prepolymer polyester fromprepolymer 190 ◦C tofrom 220 ◦190C. The °C polycondensationto 220 °C. The polycondensationreaction was not terminated reaction was until not the terminated acid value until of the the reaction acid value system of the was reaction below system 1 mg KOH was/ g,below then 1the mg biodegradable KOH/g, then the polyester biodegradable (BPD) polyester was fabricated. diol (BPD) was fabricated.

2.3.2.2.3.2. Fabrication Fabrication of of BTPU BTPU AsAs listed in TableTable1 ,1, BPD BPD and and MDI MDI with with a certain a certain weight weight ratio ratio were were added added into ainto four-necked a four-necked flask flask(250 mL)(250 equippedmL) equipped with magneticwith magnetic stirring stirring and a thermometer.and a thermometer. Under vacuumUnder vacuum conditions, conditions, the reaction the reactionwas continued was continued at 80 ◦C forat 80 120 °C min. for 120 Then, min. a certain Then, quantitya certain ofquantity BDO was of BDO slowly was added slowly into added the reactor into theas areactor chain extender as a chain under extender a fast under stirring a speed.fast stirri Afterng 12speed. h of agingAfter 12 in ah vacuum-dryingof aging in a vacuum-drying oven at 100 ◦C ovento 120 at◦ C,100 BTPU-1, °C to 120 BTPU-2, °C, BTPU-1, and BTPU-3 BTPU-2, were and obtained.BTPU-3 were Figure obtained.1 shows Figure the schematic 1 shows illustration the schematic for illustrationthe formation for ofthe BTPU formation from theof BTPU monomers. from the monomers.

esterification MDI polycondensation end capping Prepolymer BPD BDO

chain extension BTPU

Hydroxyl (-OH) Isocyanato (-NCO)

Amido bond (-NHCO-) Carboxyl (-COOH) FigureFigure 1. 1. SchematicSchematic illustration illustration of of bi biodegradableodegradable thermoplastic thermoplastic polyurethane polyurethane (BTPU) (BTPU) formation. formation.

Table 1. Compositions of various biodegradable thermoplastic polyurethanes (BTPUs).

Hard Segment Content (wt%) Sample Name Soft Segment (BPD) Content (wt%) MDI BDO

Polymers 2019, 11, 1121 4 of 13

Table 1. Compositions of various biodegradable thermoplastic polyurethanes (BTPUs).

Hard Segment Content (wt%) Sample Name Soft Segment (BPD) Content (wt%) MDI BDO BTPU-1 60 30.45 9.55 BTPU-2 70 23.27 6.73 BTPU-3 80 16.09 3.91 Note: NCO/OH = 1.

2.4. Characterization and Testing Methods

2.4.1. Analysis by Fourier Transform Infrared (FTIR) Spectrometer The BPD and BTPU synthesized above were dissolved in chloroform and precipitated with cooling methanol. After drying, the purified BPD and BTPU were obtained for testing. The purified BPD and BTPU samples were tested using the attenuated total reflectance mode, 1 1 1 with a resolution of 4 cm− and scanning wave numbers ranging from 4000 cm− to 600 cm− .

2.4.2. Analysis by Proton Nuclear Magnetic Resonance (1H-NMR) Spectrometer

Purified BPD and BTPU were tested at a frequency of 300 Hz and a temperature of 25 ◦C using tetramethylsilane as the internal standard and deuterated chloroform as the solvent.

2.4.3. Analysis by Gel Permeation Chromatograph

The synthesized BPD and BTPU with a concentration of 0.2–0.5 wt% were characterized at 35 ◦C for the detector and column using (THF) as the solvent, with a flow rate at 0.35 mL/min. Shodex- was used as the standard for calibration.

2.4.4. Atomic Force Microscope (AFM) Observation The micromorphological structure of the synthesized BTPU was observed by AFM in the tapping mode. The scope of the scanner was 10 µm 10 µm. The spinning speed of spin coater was 1000 r/min × and the time was 60 s. The method of making the sample was as follows. The BTPU was dissolved in the solvent THF to make a dilute solution with a concentration of about 0.02 wt%. Then, the solution was dropped onto the newly exfoliated sheet and the film was formed by spinning. The solvent was evaporated for a period of time before testing and the dried spin-coated sample film was then tested by AFM.

2.4.5. Mechanical Property Test The tensile properties were tested according to ISO 37-2017 with a stretching speed of 200 mm/min. The thickness of the narrow portion was 1.0 mm 0.1 mm for dumbbell specimens. The test length ± was 10 0.5 mm. ± The hardness was tested according to ISO 48-4:2018. For the determination of hardness using a type A durameter, the thickness of the test piece was at least 6 mm. The other dimensions of the test pieces were sufficient to permit measurement of at least 12 mm away from any edge for the type A durameter. The standard test time was 15 s for thermoplastic rubber.

2.4.6. Thermal Property Test The synthesized BPD and BTPU were measured with a differential scanning calorimeter. The measurement conditions are as follows. With a blanket of nitrogen air for protection, the dried sample was heated from room temperature to 250 ◦C and maintained for 5 min. Then, the sample was cooled down to 90 C and heated again to 250 C. The heating and cooling rates were both 10 C/min. − ◦ ◦ ◦ Polymers 2019, 11, 1121 5 of 13

The synthesized BPD and BTPU were tested with the thermogravimetric analyzer at the following conditions: The temperature was heated from room temperature to 900 ◦C under a nitrogen atmosphere at a heating rate of 20 ◦C/min and a flow rate of 50 mL/min.

2.4.7. Biodegradation Test The lipase degradation method was as follows [35,36]. Approximately 0.5 g of the dried BTPU sample, weighed as W0, was placed into a tube. Subsequently, approximately 0.01 g of lipase was added in the test tube and the total mass (sample, tube, and lipase) was denoted as W1. An appropriate amount of mixed phosphate buffer solution (about 8 mL) with a pH value of 6.8 was added into the sampling tube. This solution was then placed into a thermostatic water bath at a constant temperature of 37 ◦C. The tube was taken out every 48 h. After centrifugal separation and discarding the solution, the residual in the tube was dried in a vacuum-drying oven to a constant weight. The final total mass was denoted as W2. Finally, the mass loss rate was calculated using Equation (1):

Mass loss rate = [(W W )/W ] 100% (1) 1 − 2 0 × Approximately 0.01 g of lipase and a specific amount of mixed phosphate buffer solution with a pH of 6.8 were weighed and added into the sampling tube to repeat the test, which ensured that similar biodegradation conditions were supplied for the biodegradation tests of the remaining BTPUs in the following biodegradation tests.

3. Results and Discussion

3.1. Structural Characterization of BPD and BTPU

3.1.1. FTIR Analysis of BPD and BTPU Figure2 shows the infrared spectra of the purified BPD and BTPU. In the infrared spectrum 1 of BPD, the stretching vibration absorption peak of hydroxyl (-OH) occurred at 3553 cm− . The characteristic peak of the carbonyl group (C=O) and the stretching vibration absorption peak of the 1 linkage (C-O-C) appeared at 1723 and 1153 cm− , respectively. Moreover, they were both strong. The above three peaks indicated that the molecular structure of the synthetic BPD contained a large number of hydroxyls (-OH) and ester groups (-COO-). Being different with the spectrum of BPD, 1 the -OH characteristic peak at 3553 cm− disappeared in the infrared spectrum of BTPU, and the 1 -NCO absorption peak disappeared at 2270 cm− , which indicated that the -OH of BPD completely 1 reacted with the group (-NCO) of MDI. An absorption peak occurred at 1597 cm− , which corresponded to the carbon-carbon double bond (C=C) stretching vibration on the aromatic ring. The result proved the existence of the aromatic ring structures of MDI in the molecular chain of BTPU. 1 Moreover, the N-H stretching vibration absorption peak occurred at 3332 cm− , meanwhile, the N-H 1 bending vibration absorption peak occurred at 1531 cm− , which suggested that the new N-H groups were generated by the reaction between -NCO and -OH. The appearance of two absorption peaks at 1 1722 (C=O) and 1153 cm− (C-O-C) indicated that structures of the amido bond (-NHCO-) existed in the molecular chain of BTPU. Polymers 2019, 11, 1121 6 of 13 Polymers 2019, 11, x FOR PEER REVIEW 6 of 13

1723

1153

2930

3553 3455 BPD Absorbance Absorbance 2855 1531 3332 1597 BTPU

4000 3500 3000 2500 2000 1500 1000 500 −1 Wave number (cm 1)

FigureFigure 2. 2.FTIR FTIR spectra spectra ofof biodegradablebiodegradable polyester polyester diols diols (BPD) (BPD) and and BTPU. BTPU. 3.1.2. H-NMR Analysis of BPD and BTPU 3.1.2. H-NMR Analysis of BPD and BTPU 1 FigureFigure3 shows 3 shows the the molecular molecular structural structural formulas formul ofas BPDof BPD and and BTPU. BTPU. Figure Figure4 presents 4 presents the theH-NMR 1H- 1 spectraNMR ofspectra BPD andof BPD BTPU. and In BTPU. the H-NMR In the 1H-NMR spectra ofspectra BPD, of the BPD, peak the j (δ peak= 3.43 j (δ ppm) = 3.43 was ppm) the was resonance the absorptionresonance peak absorption of H in peak the hydroxyl-terminated of H in the hydroxyl-terminated absorption peak. absorption The peaks peak. at The 6.82(i) peaks ppm at could6.82(i) be attributedppm could to be the attributed carbon-carbon to the carbon-carbon double bond double of FA. bond Moreover, of FA. noMoreover, hydroxyl-terminated no hydroxyl-terminated absorption peakabsorption of H was peak found of H in was the found1H-NMR in the spectra 1H-NMR of BTPU.spectra However,of BTPU. However, a resonance a resonance absorption absorption peak of H inpeak -NH- of appeared H in -NH- at appeared position mat (positionδ = 7.31 m ppm) (δ = and7.31 appm) resonance and a resonance absorption absorption peak of H peak on theof H carbon on 2 directlythe carbon connected directly to theconnected -(C=O)-O-CH to the 2-(C=O)-O-CH- appeared at2- position appeared p (atδ =position4.10 ppm) p (δ in = the4.10 spectra ppm) ofin BTPU.the Atspectra the same of time,BTPU. the At peak the same n (δ =time,7.09 the ppm) peak corresponded n (δ = 7.09 toppm) the resonancecorresponded absorption to the resonance of H in the benzeneabsorption ring andof H the in peakthe benzene l (δ = 3.80 ring ppm), and which the peak corresponded l (δ = 3.80to ppm), the resonance which corresponded absorption peak to the of H resonance absorption peak of H in -CH2- between two benzene rings appeared. The other absorption inresonance -CH2- between absorption two benzene peak of H rings in -CH appeared.- between The two other benzene absorption rings appeared. peaks of The BTPU other were absorption coincident peaks of BTPU were coincident with the peaks of BPD or had some small deviations, which showed withpeaks the of peaks BTPU of were BPD coincident or had some with small the peaks deviations, of BPD whichor had showedsome small that deviations, the molecular which structure showed of that the molecular structure of BTPU was generated by the reaction among BPD, MDI, and BDO. BTPUthat wasthe molecular generated structure by the reaction of BTPU among was genera BPD,ted MDI, by the and reaction BDO. among BPD, MDI, and BDO.

O O O O CCH CH (CH ) CH CH COCH CH CH O HO (CH2)4 O CCH2CH2(CH2)4CH2CH2COCH2(CH2)2CH2O CCH2CH2(CH2)4CH2CH2COCH2CH2CH2O 2 4 2 2 2 4 2 2 2 2 2 2 y x y j c b a f e

O O O O O O O O

C(CH2)2COCH2(CH2)2CH2O C(CH ) COCH CH CH O CCH CHCOCH2(CH2)2CH2O CCH CHCOCH CH CH O H C(CH2)2COCH2(CH2)2CH2O C(CH2)2COCH2CH2CH2O CCH CHCOCH2(CH2)2CH2O CCH CHCOCH2CH2CH2O H z o p q z o p k h q j dik h j BPD O O O * BPD OC NH CH NH C O CH CH CH O * * BPD OC NH CH2 NH C O CH2 CH2 CH2 O * 2 p p 2 X n l m p q X

Soft segment Hard segment BTPU

FigureFigure 3. 3.Molecular Molecular structuralstructural formulas formulas of of BPD BPD and and BTPU. BTPU.

Polymers 2019, 11, 1121 7 of 13 Polymers 2019, 11, x FOR PEER REVIEW 7 of 13

a

c b

f i e k d h j BPD

86420

m n p q l

BTPU 86420 Figure 4. 11H-NMRH-NMR spectra spectra of of BPD BPD and and BTPU. BTPU.

3.1.3. Molecular Molecular Weight Weight and Molecular Weight Distribution ofof BTPUBTPU Table 22 shows shows thethe molecularmolecular weightweight ofof severalseveral BTPUsBTPUs withwith didifferentfferent BPDBPD contents.contents. When the content of BPD (soft (soft segment) segment) in BTPU BTPU increased increased from 60 wt to 80 80 wt%, wt%, the the molecular molecular weight weight of of BTPU BTPU increasedincreased and and the the molecular weight distribution beca becameme wider. This could be explained, as with the increasingincreasing ofof the the BPD BPD content, content, the numberthe number of hydroxyl of hydroxyl groups in groups BPD increases in BPD and increases the corresponding and the correspondingisocyanate groups isocyanate in MDI groups decrease. in MDI This decrease. results in This the ratioresults of thein the isocyanate ratio of the groups isocyanate in MDI groups to the inhydroxyl MDI to groupsthe hydroxyl in BPD groups being closein BPD to being 1, which close led to to 1,the which increase led to of the the increase molecular of the weight molecular of the weightprepolymer. of the Then, prepolymer. the average Then, molecular the average weight mole andcular the weight weight and average the molecularweight average weight molecular of BTPU weightare both of increased BTPU are by both using increased BDO to by extend using the BDO molecular to extend chains. the molecular chains.

Table 2. Molecular weight and molecular weight distribution of BTPU. Table 2. Molecular weight and molecular weight distribution of BTPU.

Sample Name Soft Segment (BPD) CONTENT (wt%) Mn Mw Mw/Mn Sample Name Soft Segment (BPD) CONTENT (wt%) M M M M BTPU-1 60 40,004n 90,809w w 2.27 n BTPU-1BTPU-2 60 70 40,004 51,236 90,809 167,541 2.27 3.27 BTPU-2BTPU-3 70 80 51,236 63,545 167,541 240,200 3.27 3.78

BTPU-3 Note: M80n of the BPD in Table2 is 3825.63,545 240,200 3.78

Note: M n of the BPD in Table 2 is 3825. 3.2. Micromorphological Structure of BTPU 3.2. Micromorphological Structure of BTPU Figure5a–c indicates the 2-D atomic force microscopic images of BTPU-2. Obvious bright lumpy bulgesFigure could 5a–c be foundindicates in the the phase 2-D atomic image force and themicroscopic inerratic bulges,images correspondingof BTPU-2. Obvious to the bright phase lumpy image, bulgescould becould found be found in the in height the phase image image and and amplitude the inerratic error bulges, image. corresponding Thereinto, the to bright the phase convex image, part couldwas the be hardfound segment in the height of BTPU image due and to amplitude its high surface error image. energy Thereinto, and modulus, the bright and the convex dark part concave was theportion hard was segment the soft of BTPU segment due of to BPD. its high The surface interface energy between and the modulus, hard and and soft the segments dark concave was clear, portion the wasphase the domain soft segment was large of BPD. and the The phase interface separation between was the apparent. hard and Figure soft segments5a–c shows was the clear, 3-D imagesthe phase of domainBTPU-2. was The darklarge partand distributedthe phase inseparation the trough was of theapparent. wave was Figure the soft 5a–c segment shows ofthe BTPU, 3-D whichimages was of BTPU-2.distributed The as dark a continuous part distributed phase. Meanwhile,in the trough the of hard the segmentwave was was the the soft light-colored segment of convexBTPU, partwhich of wasthe image,distributed which as was a continuous the dispersed phase. phase. Meanwhile, Figure5 accuratelythe hard segment reflects thewas microphase the light-colored separation convex of part of the image, which was the dispersed phase. Figure 5 accurately reflects the microphase separation of BTPU-2, from which we could see that the hard segments were more evenly distributed in the soft segments in BTPU-2.

Polymers 2019, 11, 1121 8 of 13

BTPU-2, from which we could see that the hard segments were more evenly distributed in the soft segmentsPolymers 2019 in BTPU-2., 11, x FOR PEER REVIEW 8 of 13

(a) (b) (c)

(A) (B) (C)

FigureFigure 5. 5.Atomic Atomic force force microscopic microscopic images images of of BTPU. BTPU. ((aa)) 2-D2-D phasephase image,image, (b) 2-D height height image, image, ( (cc)) 2- 2-D amplitudeD amplitude error error image. image. (A) 3-D(A) phase3-D phase image, image, (B) 3-D (B) height3-D height image, image, and (andC) 3-D (C) amplitude3-D amplitude error error image. image. 3.3. Mechanical Properties of BTPU 3.3. Mechanical Properties of BTPU Table3 lists some mechanical properties of the three kinds of BTPUs. When the soft segment contentTable in BTPU 3 lists increased some mechanical from 60 wt properties to 80 wt%, of the th tensilee three strengthkinds of ofBTPUs. BTPU When decreased the soft from segment 30.2 MPa to 13.5content MPa, in theBTPU elongation increased at from break 60 increased wt to 80 fromwt%, 802 the totensile 1405%, strength and the of hardness BTPU decreased gradually from decreased 30.2 fromMPa 87 to HA 13.5 to MPa, 60 HA. the elongation at break increased from 802 to 1405%, and the hardness gradually decreased from 87 HA to 60 HA. Table 3. Some mechanical properties of various BTPUs. Table 3. Some mechanical properties of various BTPUs. Soft Segment Tensile Strength Elongation at Sample Name Hardness (A) Sample Soft Segment(wt%) Tensile(MPa) Strength ElongationBreak (%) at Break Hardness NameBTPU-1 (wt%) 60(MPa) 30.2 1.5 802 (%)15 87 1 (A) ± ± ± BTPU-1BTPU-2 60 70 30.2 21.3 ± 1.51.2 1008 80217 ± 15 80 1 87 ± 1 ± ± ± BTPU-2BTPU-3 70 80 21.3 13.5 ± 1.21.2 1405 100820 ± 17 60 1 80 ± 1 ± ± ± BTPU-3 80 13.5 ± 1.2 1405 ± 20 60 ± 1 The reasons for these changes are as follows. The hard segment of BTPU is prone to oriented The reasons for these changes are as follows. The hard segment of BTPU is prone to oriented crystallization, which forms the crystalline region. The BPD in an amorphous state surrounds this crystallization, which forms the crystalline region. The BPD in an amorphous state surrounds this region. Thus, the crystalline region forms the physical crosslinking point of the BTPU material. Under region. Thus, the crystalline region forms the physical crosslinking point of the BTPU material. Under thethe action action of of an an external external force, force, the the tensile tensile strengthstrength of the crystalline crystalline region region is is higher higher than than that that of ofthe the amorphousamorphous region region and and the the elongation elongation atat breakbreak is smallersmaller than than that that of of the the amorphous amorphous zone. zone. Therefore, Therefore, thethe relative relative hard hard segment segment content content decreased decreased and theand orientational the orientational crystallization crystallization ability ability of the of segment the weakenedsegment withweakened the increase with the of increase the BPD of contentthe BPD incont BTPU,ent in whichBTPU, resultedwhich resulted in a decrease in a decrease in the in tensile the strengthtensile ofstrength BTPU. of Meanwhile, BTPU. Meanwhile, the density the density of the of physical the physical crosslinking crosslinking point point network network consisted consisted of crystallizingof crystallizing points points decreased decreased with with the the decrease decrease in in the theproportion proportion of the the crystalline crystalline phase phase (or (or the the numbernumber of crystallineof crystalline dots). dots). This This condition condition weakened weakened the binding the binding force to force the surrounding to the surrounding amorphous regionamorphous soft segments, region soft which segments, was reflected which inwas the reflected lower tensile in the strength lower tensile and larger strength elongation and larger at the breakelongation of BTPU. at the break of BTPU.

Polymers 2019, 11, x FOR PEER REVIEW 9 of 13

3.4. Thermal Properties of BTPU

3.4.1. Glass Transition Temperature (Tg) and Melting Temperature (Tm) of BTPU

PolymersFigure2019, 116 ,illustrates 1121 the differential scanning calorimetry heating curves of the three kinds9 of of 13 BTPUs. The Tg values of the three BTPUs are below −36 °C and the Tm values of the three BTPUs are above 160 °C. The Tg and the Tm decreased with the increasing soft segment content because the Tg 3.4. Thermal Properties of BTPU and Tm of BTPU depend on the macromolecular structure of the soft and hard segments and the ratio of the soft segment to the hard segment. The better the flexibility the molecular chain of the soft 3.4.1. Glass Transition Temperature (Tg) and Melting Temperature (Tm) of BTPU segment was, the lower the crystallinity and the easier the relative movement. With the increase of soft segmentFigure6 illustratescontent, the the flexibility di fferential of the scanning molecu calorimetrylar chains of heating the synthesized curves of product the three increased. kinds of Furthermore,BTPUs. The T theg values segment of thecould three move BTPUs at lower are belowenergy, 36which◦C andwas thereflectedTm values in the of decrease the three of BTPUsthe Tg. − Theare abovemolecular 160 ◦ chainC. The regularityTg and the andTm crystallizationdecreased with ca thepability increasing of BTPU soft decreased segment contentdue to the because increase the Tofg theand softTm segmentof BTPU content. depend Therefore, on the macromolecular the Tm also decreased. structure of the soft and hard segments and the ratioFigure of the 6 soft shows segment two melting to the hard peaks segment. in the BTPU The samples better the with flexibility 60 wt% theand molecular 70 wt% BPD chain contents, of the whichsoft segment were probably was, the lowercaused the by crystallinity the presence and of thetwo easier crystal the forms relative in movement.the hard segment. With the Given increase the changeof soft segmentof soft segment content, contents, the flexibility some changes of the molecular in the peak chains shape of and the synthesizedpeak value may product have increased.occurred. WithFurthermore, the decreasing the segment soft segment could move ratio, at the lower peak energy, value whichof BTPU was increased. reflected This in the case decrease may be of due the T tog. Thethe decrease molecular in chainthe soft regularity segment and content, crystallization the corresponding capability increase of BTPU in decreased the hard duesegment to the content increase and of athe relative soft segment increase content. in the interaction Therefore, of the theTm hardalso segments. decreased.

205.3 °C

exo 218.0 °C BTPU-1 198.0°C

BTPU-2 211.3 °C -36.2 °C

-37.0 °C 160.1°C Heat flow BTPU-3 -40.0 °C

-100 -50 0 50 100 150 200 250 ° Temperature ( C) Figure 6. ThermalThermal properties properties of BTPU with different different BPD contents.

3.4.2.Figure Thermal6 shows Stability two of melting BTPU peaks in the BTPU samples with 60 wt% and 70 wt% BPD contents, which were probably caused by the presence of two crystal forms in the hard segment. Given the Figure 7 shows the thermal gravimetric analysis (TGA)/differential thermal gravimetric (DTG) change of soft segment contents, some changes in the peak shape and peak value may have occurred. analysis curves of the BTPU sample with 70 wt% BPD contents. The results indicated that the With the decreasing soft segment ratio, the peak value of BTPU increased. This case may be due to the synthesized BTPU had typical decomposition features of carbon chain polymeric materials. The decrease in the soft segment content, the corresponding increase in the hard segment content and a initial decomposition temperature of the BTPU was 323 °C, which is higher than those of some relative increase in the interaction of the hard segments. traditional TPUs [37,38] with an initial decomposition temperature below 300 °C, demonstrating that the3.4.2. novel Thermal BTPU Stability had good of BTPUthermal stability. The weight loss of the BTPU sample was low when the temperature was less than 323 °C due to low water and some molecular residues in the sample. The Figure7 shows the thermal gravimetric analysis (TGA) /differential thermal gravimetric (DTG) TGA curve shows that the BTPU had a higher weight loss rate within 323 °C to 500 °C and the final analysis curves of the BTPU sample with 70 wt% BPD contents. The results indicated that the solid residue was only 4.03%. synthesized BTPU had typical decomposition features of carbon chain polymeric materials. The initial decomposition temperature of the BTPU was 323 ◦C, which is higher than those of some traditional TPUs [37,38] with an initial decomposition temperature below 300 ◦C, demonstrating that the novel BTPU had good thermal stability. The weight loss of the BTPU sample was low when the temperature was less than 323 ◦C due to low water and some molecular residues in the sample. The TGA curve shows that the BTPU had a higher weight loss rate within 323 ◦C to 500 ◦C and the final solid residue was only 4.03%. Polymers 2019, 11, 1121 10 of 13 Polymers 2019, 11, x FOR PEER REVIEW 10 of 13

T 5%d:323℃, 95% 100 0

mass change: -36.87% 80 -2

60 -4

40 mass change: -59.10% -6

20

Weight remained (%) remained Weight ° 1st peak value: 352 C derived (%/min) Weight residual mass: 4.03% -8 -6.91%/min 0 2nd peak value: 426 °C -8.45%/min -10 0 200 400 600 800 ° Temperature ( C) Figure 7.7. ThermalThermal gravimetric gravimetric analysis analysis/differential/differential thermal thermal gravimetric gravimetric analysis analysis curves curves of BTPU of sample. BTPU sample. As shown in Figure7, two peaks are in the DTG curve at 352 ◦C and 426 ◦C. This indicated that the BTPUAs shown macromolecule in Figure 7, consists two peaks of two are kindsin the of DTG molecular curve at structures: 352 °C and The 426 soft °C. segment This indicated of BPD that and the BTPU hard segment macromolecule synthesized consists from of BDO two and kinds MDI. of molecular This finding structures: further confirmed The soft segment that the of structure BPD and of theBTPU hard is ofsegment the alternating synthesized block from type. BDO The and low-temperature MDI. This finding decomposition further confirmed peak in the that DTG the curve structure was ofmainly BTPU caused is of the by alternating the decomposition block type. of the The soft low-temperature segment of BPD. decomposition However, the peak soft segmentin the DTG and curve hard wassegment mainly decompositions caused by the weredecomposition not isolated of processesthe soft segment because of the BPD. free However, radicals and the energysoft segment produced and hardduring segment the decomposition decompositions of soft were segments not isolated of BPD activatedprocesses the because decomposition the free ofradicals the hard and segments. energy produced during the decomposition of soft segments of BPD activated the decomposition of the hard segments.3.5. Degradation Property of BTPU As seen from the degradation loss curves of BTPU with different BPD contents in a lipase solution 3.5.(Figure Degradation8), BTPU-3 Property exhibited of BTPU a weight loss ratio of 36.7% after 14 days in the lipase solution. Compared to someAs traditionalseen from TPUthe degradation [39], which do loss not curves have significant of BTPU degradationwith different in aBPD lipase contents solution, in thea lipase novel solutionBTPU had (Figure superior 8), BTPU-3 biodegradability. exhibited Thisa weight can be loss explained, ratio of 36.7% as the after soft segment14 days in (BPD) the lipase comprising solution. six Comparedkinds of aliphatic to some polyester traditional repeat TPU units [39], and which a carbon-carbon do not have double significant bond haddegradation good flexibility in a lipase and solution,many ester the bonds, novel so BTPU the macromolecular had superior biodegradabilit main chain of they. This BTPU can was be easily explained, attacked as the and soft decomposed segment (BPD)by lipase. comprising six kinds of aliphatic polyester repeat units and a carbon-carbon double bond had goodMeanwhile, flexibility and the degradationmany ester lossbonds, rate so of the BTPUmacromolecular in the lipase main solution chain showed of the a distinctBTPU was increasing easily attackedtrend with and the decomposed increase of the by degradationlipase. time, and the degradation loss rate of BTPU increased with the increase of the BPD content. The hard segment was the major part responsible for crystalline formation in the BTPU. The increased BPD content decreased the content of the hard segment, resulting in the increased polymer molecular chain flexibility and decreased BTPU crystallinity. Due to the decrease of the physical crosslinking points formed by the hard segment, the lipase easily penetrated the molecule interior. This condition resulted in a substantial increase in the degradation degree.

Polymers 2019, 11, 1121 11 of 13 Polymers 2019, 11, x FOR PEER REVIEW 11 of 13

40 BTPU-3 35 BTPU-2 BTPU-1 30

25

20

15

Weight loss (%) 10

5

0 2 4 6 8 101214

Degradation time (d) Figure 8. WeightWeight loss of BTPU with different different BPD contents after enzymatic degradation.degradation.

4. ConclusionsMeanwhile, the degradation loss rate of the BTPU in the lipase solution showed a distinct increasingA novel trend BTPU with with the controllableincrease of the biodegradability, degradation time, good and thermal the degradation properties, loss and rate mechanical of BTPU propertiesincreased with can bethe successfully increase of fabricated.the BPD content. First, theThe more hard flexiblesegment BPD was (soft the segment)major part was responsible synthesized for bycrystalline using the formation melt polycondensation in the BTPU.of The five increased bio-based BPD aliphatic content monomers decreased (SU, the SA, content FA, PDO, of and the BDO).hard Second,segment, excessive resulting MDI in wasthe reactedincreased with polymer the BPD, molecular which led chain to a new flexibility functional and group decreased of -NHCO- BTPU at thecrystallinity. end of BPD Due macromolecules. to the decrease Finally,of the physical the BDO cr asosslinking the extender points reacted formed with by the the functional hard segment, group the of -NHCO-lipase easily in the penetrated BPD, whereas the molecule the BDO interior. also reacted This condition with excess resulted MDI to in form a substantial the hard segmentincrease ofin the BTPU.degradation The BTPU degree. containing 60 wt% of BPD content with a Tg of approximately 36 C and a Tm of − ◦ about 200 ◦C had a tensile strength of about 30 MPa, whereas its elongation at break exceeded 800%. With4. Conclusions the soft segment decreasing, the tensile strength of the BTPU increased and the elongation at breakA of novel the BTPU BTPU decreased. with controllable The BTPU biodegradability, also had a superior good degradation thermal properties, rate in lipase and solution mechanical and theproperties degradation can be rate successfully increased fabr withicated. rising First, BPD the content. more flexible BPD (soft segment) was synthesized by using the melt polycondensation of five bio-based aliphatic monomers (SU, SA, FA, PDO, and Author Contributions: Data curation, Z.W., J.Y. and T.W.; Formal analysis, Y.Z. and L.Q.; Validation, Q.W.; Writing—originalBDO). Second, excessive draft, Z.W.; MDI Writing—review was reacted and with editing, the BPD, Q.W. which led to a new of - NHCO- at the end of BPD macromolecules. Finally, the BDO as the extender reacted with the Funding: This research is supported by National Natural Science Foundation of China (51773104, 51373085), and Keyfunctional Research group and Development of -NHCO- Projectin the BPD, in Shandong whereas Province the BDO of China also reacted (2017GGX20138). with excess MDI to form the hard segment of the BTPU. The BTPU containing 60 wt% of BPD content with a Tg of approximately Conflicts of Interest: The authors declare no conflict of interest. −36 °C and a Tm of about 200 °C had a tensile strength of about 30 MPa, whereas its elongation at Referencesbreak exceeded 800%. With the soft segment decreasing, the tensile strength of the BTPU increased and the elongation at break of the BTPU decreased. The BTPU also had a superior degradation rate 1.in lipaseEceiza, solution A.; Larrañaga, and the degradation M.; De La Caba, rate K.;increased Kortaberria, with G.;rising Marieta, BPD content. C.; Corcuera, M.; Mondragon, I. Structure-property relationships of thermoplastic polyurethane based on diols. AuthorJ. Appl.Contributions: Polym. Sci. Data2010 ,curation,108, 3092–3103. Z.W., J.Y. [CrossRef and T.W.;] Formal analysis, Y.Z. and L.Q.; Validation, Q.W.; 2.Writing—originalEceiza, A.; Martin, draft, Z.W.; M.; DeWriting—review La Caba, K.; Kortaberria,and editing, Q.W. G.; Gabilondo, N.; Corcuera, M.; Mondragon, I. Thermoplastic polyurethane elastomers based on polycarbonate diols with different soft segment molecular Funding: This research is supported by National Natural Science Foundation of China (51773104, 51373085), and weight and chemical structure: Mechanical and thermal properties. Polym. Eng. Sci. 2008, 48, 297–306. Key Research and Development Project in Shandong Province of China (2017GGX20138). [CrossRef] 3.ConflictsKultys, of Interest: A.; Rogulska, The authors M.; Głuchowska, declare no conflict H. The eofff ectinterest. of soft-segment structure on the properties of novel thermoplastic polyurethane elastomers based on an unconventional chain extender. Polym. Int. 2011, 60, References652–659. [CrossRef]

Polymers 2019, 11, 1121 12 of 13

4. Buckley, C.P.; Prisacariu, C.; Martin, C. Elasticity and inelasticity of thermoplastic polyurethane elastomers: Sensitivity to chemical and physical structure. Polymer. 2010, 51, 3213–3224. [CrossRef] 5. Qi, H.J.; Boyce, M.C. Stress-strain behavior of thermoplastic polyurethanes. Mech. Mater. 2005, 37, 817–839. [CrossRef] 6. Hill, D.J.T.; Killeen, M.I.; O’Donnell, J.H.; Pomerya, P.J.; St John, D.; Whittaker, A.K. Laboratory wear testing of polyurethane elastomers. Wear 1997, 208, 155–160. [CrossRef] 7. Kwiatkowski, K.; Nachman, M. The abrasive wear resistance of the segmented linear polyurethane elastomers based on a variety of as soft segments. Polymers 2017, 9, 705. [CrossRef][PubMed] 8. Xu, C.; Huang, Y.; Tang, L.; Hong, Y. Low-initial-modulus biodegradable polyurethane elastomers for soft tissue regeneration. ACS Appl. Mater. Interfaces 2017, 9, 2169–2180. [CrossRef] 9. Kucinska-Lipka, J.; Gubanska, I.; Janik, H.; Sienkiewicz, M. Fabrication of polyurethane and polyurethane based composite fibres by the electrospinning technique for soft tissue engineering of cardiovascular system. Mater. Sci. Eng. C 2015, 46, 166–176. [CrossRef][PubMed] 10. Schollenberger, C.S.; Dinbergs, K.; Stewart, F.D. Thermoplastic polyurethane elastomer melt study. Rubber Chem. Technol. 1982, 55, 137–150. [CrossRef] 11. Im, H.G.; Ka, K.R.; Kim, C.K. Characteristics of polyurethane elastomer blends with poly(-co-butadiene) rubber as an encapsulant for underwater sonar devices. Ind. Eng. Chem. Res. 2010, 49, 7336–7342. [CrossRef] 12. Feng, F.; Ye, L. Morphologies and mechanical properties of polylactide/thermoplastic polyurethane elastomer blends. J. Appl. Polym. Sci. 2011, 119, 2778–2783. [CrossRef] 13. Tan, L.-C.; Su, Q.-X.; Zhang, S.-D.; Huang, H.-X. Preparing thermoplastic polyurethane/thermoplastic starch with high mechanical and biodegradable properties. RSC Adv. 2015, 5, 80884–80892. [CrossRef] 14. Akindoyo, J.O.; Beg, M.; Ghazali, S.; Islam, M.R.; Jeyaratnam, N.; Yuvaraj, A.R. Polyurethane types, synthesis and applications—A review. RSC Adv. 2016, 6, 114453–114482. [CrossRef] 15. Zhang, C.; Kessler, M.R. Bio-based polyurethane made from compatible blends of vegetable-oil-based polyol and petroleum-based polyol. ACS Sustain. Chem. Eng. 2015, 3, 743–749. [CrossRef] 16. Cregut, M.; Bedas, M.; Durand, M.J.; Thouand, G. New insights into polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process. Biotechnol. Adv. 2013, 31, 1634–1647. [CrossRef] 17. Zhang, M.; Wang, F.; Su, R.; Qi, W.; He, Z. Ethanol production from high dry matter corncob using fed-batch simultaneous saccharification and fermentation after combined pretreatment. Bioresour. Technol. 2010, 10, 4959–4964. [CrossRef] 18. Lee, Y.; Park, J.; Ryu, C.; Gang, K.S.; Yang, W.; Park, Y.K.; Jung, J.; Hyun, S. Comparison of biochar properties

from biomass residues produced by slow pyrolysis at 500 ◦C. Bioresour. Technol. 2013, 148, 196–201. [CrossRef] 19. Tuck, C.O.; Pérez, E.; Horváth, I.T.; Sheldon, R.A.; Poliakoff, M. Valorization of biomass: deriving more value from waste. Science 2012, 337, 695–699. [CrossRef] 20. Jang, Y.-S.; Kim, B.; Shin, J.H.; Choi, Y.J.; Choi, S.; Song, C.-W.; Lee, J.; Park, H.G.; Lee, S.Y. Bio-based production of C2–C6 platform chemicals. Biotechnol. Bioeng. 2012, 109, 2437–2459. [CrossRef] 21. Lin, Y.; Tanaka, S. Ethanol fermentation from biomass resources: current state and prospects. Appl. Microbiol. Biotechnol. 2006, 69, 627–642. [CrossRef][PubMed] 22. Christiane, N.; Yves, P.; Somerville, C. Plant polymers for biodegradable plastics: Cellulose, starch and . Mol. Breed. 1995, 1, 105–122. [CrossRef] 23. Williams, C.K.; Hillmyer, M.A. Polymers from renewable resources: a perspective for a special issue of polymer reviews. Polym. Rev. 2008, 48, 1–10. [CrossRef] 24. Amass, W.; Amass, A.; Tighe, B. A review of biodegradable polymers: uses, current developments in the synthesis and characterization of biodegradable , blends of biodegradable polymers and recent advances in biodegradation studies. Polym. Int. 1998, 47, 89–144. [CrossRef] 25. Schmidt, S.; Ritter, B.S.; Kratzert, D.; Bruchmann, B.; Mülhanupt, R. Isocyanate-free route to poly (carbohydrate–urethane) thermosets and 100% bio-based coatings derived from feedstock. Macromolecules 2016, 49, 7268–7276. [CrossRef] 26. Guo, W.-H.; Shen, Z.-L.; Guo, B.-C.; Zhang, L.-Q.; Jia, D.-M. Synthesis of bio-based copolyester and its reinforcement with zinc diacrylate for shape memory application. Polymer 2014, 55, 4324–4331. [CrossRef] Polymers 2019, 11, 1121 13 of 13

27. Ferreira-Leitao, V.; Gottschalk, L.M.F.; Ferrara, M.A.; Nepomuceno, A.L.; Molinari, H.B.C.; Bon, E.P. Biomass residues in Brazil: availability and potential uses. Waste Biomass Valoriz. 2010, 1, 65–76. [CrossRef] 28. Garcia-Nunez, J.A.; Ramirez-Contreras, N.E.; Rodriguez, D.T.; Silva-Lora, E.; Frear, C.S.; Stockle, C.; Garcia-Perez, M. Evolution of palm oil mills into bio-refineries: literature review on current and potential uses of residual biomass and effluents. Resour. Conserv. Recycl. 2016, 110, 99–114. [CrossRef] 29. Ng, W.S.; Lee, C.S.; Chuah, C.H.; Cheng, S.F. Preparation and modification of water-blown porous biodegradable polyurethane foams with palm oil-based polyester polyol. Ind. Crop. Prod. 2017, 97, 65–78. [CrossRef] 30. Kucharczyk, P.; Pavelková, A.; Stloukal, P.; Sedlarík, V. Degradation behaviour of PLA-based polyesterurethanes under abiotic and biotic environments. Polym. Degrad. Stabil. 2016, 129, 222–230. [CrossRef] 31. Acik, G.; Kamaci, M.; Altinkok, C.; Karabulut, H.F.; Tasdelen, M.A. Synthesis and properties of soybean oil-based biodegradable polyurethane films. Prog. Org. Coat. 2018, 123, 261–266. [CrossRef] 32. Xu, J.; Guo, B.-H. Microbial Succinic Acid, Its Polymer Poly (Butylene Succinate), and Applications. M. Plastics from Bacteria; Springer: Berlin, Germany, 2010; pp. 347–388. [CrossRef] 33. Chen, G.-Q.; Patel Martin, K. Plastics derived from biological sources: present and future: A technical and environmental review. Chem. Rev. 2011, 112, 2082–2099. [CrossRef][PubMed] 34. Rey, A.; Kolinski, A.; Skolnick, J.; Levine, Y.K. Effect of double bonds on the dynamics of hydrocarbon chains. J. Chem. Phys. 1992, 97, 1240–1249. [CrossRef] 35. Wang, Q.-G.; Zai, Y.-Y.; Yang, D.-J.; Qiu, L.-Y.; Niu, C.-Q. Bio-based elastomer nanoparticles with controllable biodegradability. RSC Adv. 2016, 6, 102142–102148. [CrossRef] 36. Hoshino, A.; Isono, Y. Degradation of aliphatic polyester films by commercially available lipases with special reference to rapid and complete degradation of poly(L-lactide) film by lipase PL derived from Alcaligenes sp. Biodegradation 2002, 13, 141–147. [CrossRef][PubMed] 37. Rosu, D.; Tudorachi, N.; Rosu, L. Investigations on the thermal stability of a MDI based polyurethane elastomer. J. Anal. Appl. Pyrolysis 2010, 89, 152–158. [CrossRef] 38. Askari, F.; Barikani, M.; Barmar, M.; Shokrolahi, F.; Vafayan, M. Study of thermal stability and degradation kinetics of polyurethane– by thermogravimetry. Iran. Polym. J. 2015, 24, 783–789. [CrossRef] 39. Umare, S.S.; Chandure, A.S. Synthesis, characterization and biodegradation studies of poly(ester urethane)s. Chem. Eng. J. 2008, 142, 65–77. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).