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Synthesis and characterization of a novel aliphatic polyester based on itaconic acid.

INTRODUCTION

Plasticizers have long been known for their effectiveness in preparing flexible plastics [1-4]. Phthalate esters, the most significant group of polyfvinyl chloride) (PVC) plasticizers, are added in concentrations up to 50% of the weight of the products. They have been found in the most environments, ranging from construction materials, food packaging, toys, medical devices to soil, marine ecosystem, and indoor air [5-8]. However, the materials contact with biological fluids accelerate the migration of phthalate esters, which have adverse effects on animals' liver, kidney, heart, lungs, and other organs [9]. In 1999, six phthalate esters (diisononyl phthalate, diisodecyl phthalate (DIDP), bis(2-ethylhexyl) phthalate (DEHP or DOP), dibutyl phthalate, benzyl butyl phthalate, and di-n-octyl phthalate) are forbidden to be used in childcare articles and toys that are intended to be placed in the mouths of children under the age of 3 in Europe. The use of phthalate esters, diisononyl phthalate, DIDP, and DOP in toys and childcare products is now strongly restricted.

Polymeric plasticizers have attracted considerable attention and do show decreased migratory aptitude [10] in comparison to low molecular weight phthalate plasticizers. The molecular weights of polymeric plasticizers range from 1000 to 10,000. The migration resistance improves with increasing molecular weight, whereas the processability decreases. The PVC blends with aliphatic polyesters, such as polycaprolactone [11-13], poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [14], and poly(butylene adipate) [15-17], have been reported. These polymeric plasticizers are well miscible with PVC and reduce migration aptitude in comparison to low molecular plasticizers, in which plasticizing effects are useful for specific applications [18], However, the raw materials are petrochemicals and depend on oil resources. Therefore, developing a "green" polyester from renewable resources is an inevitable trend as oil prices get higher. Therefore, renewable polymers and plastics are being developed to replace petrochemical plastic in a wide array of applications from fibers and resins to mechanical property flexible foams [19], The structure of 2-methylsuccinate acid (MSA) is similar to aliphatic succinic acid. Moreover, MSA can be produced by catalytic hydrogenation of itaconic acid, which can be prepared via bioconversion of renewable resources such as starch [20]. Akinori et al. [21, 22] developed polyesters derived from MSA, 1,3-propanediol, and 1,4-butanediol (BDO) using rare-earth catalysts, scandium trifluoromethanesulfonate (Sc[([NTf.sub.2]).sub.3]) and scandium trifluoromethanesulfonimide (Sc(NTf2)3). However, that reaction required a large amount of the expensive catalyst (ca.l mol%) and a long reaction time (100 h).

The purpose of this research work is to investigate straightforward preparation of a novel potentially biobased aliphatic polyester poly(butylene 2-methylsuccinate) (PBMS) using traditional catalyst and normal method-direct esterification route, and structure and thermal properties of PBMS polyester based on MSA, and BDO were studied. PBMS was used as plasticizer for PVC and the plasticization efficiency was also investigated.

EXPERIMENTAL

Materials

MSA (99%) was synthesized by catalytic hydrogenation of itaconic acid according to the literature [23]. BDO (A.R. grade) and dioctyl phthalate (DOP, A.R. grade) were purchased from Tianjin Tiantai Refine Chemical Co., Ltd., China. Tetrabutyl titanate (TBT, A.R. grade) was purchased from Tianjin Kemiou Chemical Reagent Co., China. 2-Methylenesuccinic acid (99%) was purchased from Qingdao Langyatai Group Co., China. Polyvinyl chloride (PVC, SG-3) was purchased from QingDao HyGain Chemical Industrial Group Co., China. All above the chemicals were used without further purification.

Synthesis of PBMS

PBMS was synthesized through a two-step procedure of esterification and subsequent polycondensation. Typically, MSA (26.42 g, 0.20 mol) and BDO (19.83 g, 0.22 mol) were put into a 150 mL three-necked round-bottom flask which was equipped with a mechanical stirrer, water separator, and [N.sub.2] inlet pipe. The mixture was heated to 150[degrees]C and kept at this temperature until all the monomers melted completely under nitrogen atmosphere. The mixture was then stirred at 170-175[degrees]C for 6 h to finish esterification, then the (0.0681 g, 0.2 mM) TBT was introduced into the flask, and the polycondensation was carried out at 220[degrees]C with vacuum of 100 Pa for 3 h. The obtained products were dissolved in chloroform, washed with 1 M HCl aqueous solution, and further washed three times with deionized water (DIW). The organic layer was separated, the solvent was evaporated in vacuo, and PBMS was dried at room temperature for 12 h under reduced pressure before further characterization.

Characterization

[sup.1]H NMR Spectroscopy. Chemical structures and compositions of polymers were characterized by [sup.1]H NMR, which was implemented on a BRUKER AV 500 MHz spectrometer at ambient temperature, using CD[Cl.sub.3] and tetramethylsilane (TMS) as the solvent and the internal standard, respectively.

Intrinsic Viscosity Measurement. Intrinsic viscosity [[eta]] was measured at 30 [+ or -]0.1[degrees]C by using an Ubbelohbe viscometer. All the polymers were dissolved in the solvent of phenol and acetylene tetrachloride (w:w = 1:1) at a concentration of ca.0.5 g/dL. [[eta]] was calculated according to Eq. 1:

[[eta]] = [square root of (2 ([t/[t.sub.0]] - 1 ln [t/[t.sub.0]])]/C (1)

where C is concentration of the polymer solution, t is flow time of solution, and [t.sub.0] is flow time of pure solvent.

Esterification Ratio. Acid value ([A.sub.v]) was determined by titration method. A sample of reactant (weight [M.sub.1], ca. 0.50 g) was dissolved in 25 mL chloroform and then titrated with KOH/ethanol solution (c = 0.1 mol/L, volume [V.sub.1] and [V.sub.2] for sample and blank titration, respectively) using phenolphthalein as an indicator (from colorless to red). [A.sub.v] was calculated according to Eqs. 2 and 3, and esterification ratio was calculated according to Eq. 4:

Av1 = 56.1 x [(V2 - V2) x c / M1] (2)

Av2 = 56.1 x [(V3 - V4) x c / M2] (3)

E = [Av1 - Av2 / Av1] x 100% (4)

where E is the esterification ratio, [A.sub.v1] (mgKOH/g) is acid value of reactant and [A.sub.v2] (mgKOH/g) is acid value of product, weight [M.sub.2] was product, volume [V.sub.3] and [V.sub.4] for sample of product and blank titration, respectively.

Gel Permeation Chromatography. The molecular weights and polydispersity indices of the polymers were determined by permeation chromatography at 40[degrees]C with Waters Model 2695 GPC gel permeation chromatography (GPC) using polystyrene as a standard. Tetrahydrofuran (THF) was used as the eluent at a flowing rate of 1.0 mL x [min.sup.-1], and the sample concentration was 2.5 mg x [mL.sup.-1] .

Differential Scanning Calorimetry. A TA DSC-Q200 differential scanning calorimeter (DSC) was utilized to study the glass transition temperature ([T.sub.g]). The samples were first quickly heated to 60[degrees]C and kept at this temperature for 3 min to eliminate any thermal history and then quenched to -80[degrees]C, and after 3 min the samples were reheated up to 60[degrees]C at a heating rate of 10[degrees]C /min. The values of glass transition temperature ([T.sub.g]) was obtained from the heating scans.

Thermogravimetric Analysis. The thermal stabilities of polymers were determined through thermogravimetric analysis (TGA). The thermograms were recorded on a TG/DTA7300 thermogravimetric analyzer from room temperature to 600[degrees]C at a heating rate of 10[degrees]C /min under [N.sub.2] atmosphere.

Tensile Test. The blends of PBMS/PVC were premixed in high-speed mixer for 5 min, and the blends were prepared on two-roll mill at 160[degrees]C. Then the sheets were molded for 2 [+ or -] 0.1 mm thickness by compression molding machine. Tensile testing were carried out on a Zwick/Roell Tensile Tester with a tensile speed of 50 mm/min at 23 [+ or -] 2[degrees]C.

Migratory Aptitude. Migration stability was characterized by the amount of plasticizer migrated out of samples to the gaseous phase (volatility) and liquid phase (extractability) under the harsh condition. The specimens were prepared with dimension with 10 X 10 X 1 [mm.sup.3].

RESULTS AND DISCUSSION

Synthesis and Characterization

PBMS polymers were synthesis from MSA, BDO by a two-step melt polycondensation procedure shown in Scheme 1. The route of direct esterification and polycondensation to synthesized PBMS was selected, because this route is usually characterized by simpler and easier separation of low molecular weight byproducts in industrial production.

The PBMS polymer was characterized by [sup.1]H NMR. The results were shown in Fig. 1. In PBMS spectra, the peaks around 2.3-3.1 and 1.2 ppm were assigned to the underlined protons in -C[[H.bar].sub.2]-, -C[H.bar](C[H.sub.3])-, and -CH(C[[H.bar].sub.3])- from MSA, which were shifted towards higher values in comparison to the [sup.1]H NMR spectrum of MSA (peaks around 2.3-3.0 and 1.0 ppm). The peak around 4.1 and 1.7 ppm was attributed to the protons signal of esterified -COOC[[H.bar].sub.2]- and -COOC[H.sub.2]C[[H.bar].sub.2]- groups in BDO segments. The peak around 3.67 ppm was attributed to the proton signal of the hydroxyl group terminated PBMS. The results suggest that PBMS molecular chain was successfully synthesized.

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In fact, in addition to the main esterification reaction, these were two THF-forming side reactions during esterification stage (Scheme 1). Through these reactions, BDO or even the esterification product (terminal hydroxybutyl ester) was converted into THF as a byproduct, which was distilled out of the reactor together with water. The side reactions were catalyzed by raw material MSA. The amount of distillate depended on the esterification time were shown in Fig. 2. In synthesis of PSMS, the amount of distillate reached the theoretical volume of byproduct water (5.4 mL, level A) in 4 h, and then it increased slowly to 5.62 mL within the next 2 h. It is supposed that about 1.5% BDO was converted into THF. In addition, more THF formation was reported in esterification of succinic acid and BDO in previous literature [24, 25],

Effect of Feed Ratio of BDO to MSA on the Synthesis

The feed ratio of diol to diacid is an important parameter in esterification process. If the feed ratio is too low, the esterification ratio tends to lessen and results in the low molecular weights of the prepared polymers. However, too much excessive diol may give rise to the overmuch occurrence of side reactions and the waste of diol. Therefore, the feed ratio of diol to diacid should be determined appropriately. Figure 3 depicts the effects of the feed ratio of diol to diacid on the esterification ratio and intrinsic viscosity of the products. It was clearly seen that the esterification ratio increased sharply as the BDO/MSA feed ratio increased from 1.02 to 1.12. However, as the BDO/MSA feed ratio exceeded 1.12, the esterification ratio slightly increased. Furthermore, the intrinsic viscosity of the polymers was found to initially increase with increasing the BDO/MSA feed ratio, and then shift to decrease. When the BDO/MSA feed ratio reached 1.08:1, the maximum intrinsic viscosity of 0.0978 dL/g was obtained. The increasing amount of diol raised the esterification ratio, which helped to increasing the intrinsic viscosity of the prepared products. However, as the feed ratio exceeded 1.08, decrease in the intrinsic viscosity of PBMS was found. This can be interpreted that too much excessive BDO may give rise to the occurrence of side reaction and generate more short chain hydroxyl-ended prepolymer leading to obtain low molecular weight PBMS in the polycondensation reaction. Therefore, the proper feed ratio of BDO to MSA was determined to be 1.08:1 through balancing the reaction efficiency and the cost of reactants.

Effect of Catalyst Content on the Synthesis

As to the synthesis of polyesters, TBT was found to be an effective catalyst both in esterification and polycondensation process [26]. Figure 4 shows the dependence of TBT content on the esterification ratio and intrinsic viscosity of PBMS as the BDO/MSA feed molar ratio is 1.08. The esterification ratio was found to initially increase with increasing the catalyst content, and then shift to decrease. When the TBT content reached 0.1 mol% to the amount of MSA, the maximum esterification ratio of 98.05% was obtained. This meant that low content of catalyst tended to render the reaction rate slow and the reaction time long. The variation of intrinsic viscosity of the prepared products at various catalyst contents was presented in Fig. 4.

Obviously, the intrinsic viscosity exhibited a noticeable increase as the TBT content varied from 0.02 mol% to 0.1 mol%, thereafter a decrease in the intrinsic viscosity was found. The polycondensation reaction mechanism of PBMS was similar to that of polyfbutylene terephthalate) [27], It was generally described in terms of coordination of the transition metal to the carbonyl group, or alternatively of basis catalysis. That is to say, the complex reaction was determined to be dominant in the polycondensation reaction. Scheme 2 represents the reaction route for preparing PBMS. The catalyst TBT reacted firstly with carboxy-ended prepolymer formed by esterification reaction of diol to diacid to generate the unstable mid-compounds of TBT*prepolymer. Subsequently, the mid-compounds were decomposed to produce stable PBMS adducts and BDO byproduct discharged by the vacuum system. The reaction occurred repeatedly to prepare higher molecular weight PBMS. The catalyst, TBT may also contribute to the coloration, as it is well-known that the use of TBT often results in yellowing of common polyesters [28], Under the conditions of proper BDO/ MSA feed molar ratio (1.08:1) and content of TBT to the amount of MSA (0.1 mol%), the polycondensation reaction time lasted 10, 20, and 30 h, respectively. The number average molecular weight ([M.sub.n]), weight average molecular ([M.sub.w]), and polydispersity index (PDI) of PBMS polymers were characterized by GPC, and the results are listed in Table 1. According to Table 1, with the extension of the reaction time, the [M.sub.w], [M.sub.n], and inherent viscosity increase obviously. The slight shifting of polydispersity index of PBMS was mainly caused by the shifting vacuum degree, which was difficult to be controlled at the same value in different operations of lab experiment.

Thermal Properties of PBMS

DSC was implemented to study the thermal transition behavior of the PBMS. Figure 5 shows the heating scans of PBMS. The [T.sub.g] increases continuously with polymerization time. The [T.sub.g] of polymerization 30 h (-46.3[degrees]C) is higher than polymerization 10 h (-47.8[degrees]C) and 20 h (-46.8[degrees]C), suggesting high molecular weight, in agreement with the [M.sub.n] and [M.sub.w] validated by GPC.

The thermal stability of PBMS was evaluated by TGA. Figure 6 shows the TGA thermograms of PBMS and DOP. It is apparent that all the PBMS samples have similar thermal decomposition behavior, and they show nearly overlapped TGA curves. The onset decomposition temperature [T.sub.d.0.05] (-5 wt%) of PBMS polycondensation time of 10, 20, and 30 h was 270.3, 272.6, and 282.4[degrees]C, respectively; and the maximum decomposing temperature ([T.sub.d,max]) of above three samples was 406.2, 402.8, and 405.1[degrees]C, respectively. It suggests that higher molecular weight can enhance the thermal stability of PBMS to some extent. A similar phenomenon has been found for poly(triethylene terephthalate) (PTT): the [T.sub.d,0.05] and [T.sub.d,max] in nitrogen also increased with molecular weight [29]. As mentioned in the experiment section, the terminal groups of PBMS are hydroxyl, which the short chains have more the end hydroxyl groups in the material. The thermal stability of end hydroxyl groups in polyesters and copolyesters is relatively poor. Thus, high molecular weight enhances the thermal stability of PBMS.

Efficiency of Plasticizer for PVC

The PBMS can be used as plasticizer for PVC, and its plasticization efficiencies and migration resistant compared with dioctyl phthalate (DOP) for PVC. The tensile and impact properties of PBMS/PVC (50/100 w/w) blend, PBMS/PVC (80/100 w/w) blend, and as well as DOP/PVC (50/100 w/w) blend were shown in Fig. 7. PBMS/PVC (50/100 w/w) blend was typically rigid and brittle. It had high tensile modulus and strength, as well as limited elongation at break. With the increase of PBMS concentration, the tensile strength and modulus of the blend dropped, whereas the elongation at break significantly increased. Figure 7 presented that the toughness of PBMS/PVC (80/100 w/ w) blend was comparable to the DOP/PVC (50/100 w/w) blend. Although comparing with the widely used plasticizer DOP, PBMS which was synthesized from renewable source and can be biodegraded into carbon dioxide and water without causing any environmental pollution, overcomed the chemical toxicity and refractory of DOP. In addition, the PVC films with different concentration of PBMS can show a series of mechanical properties rang from brittleness to toughness. The mobility of the DOP/PVC and PBMS/PVC is listed in Table 2. It can be seen that the degree of migration of PBMS/PVC in cyclohexane, ethanol, and activated carbon is 2.24, 0.54, and 0.08%, respectively. The migration degree of DOP was higher than that of PBMS in above all mediums. The migration depends on the plasticizer diffusion. It is the macromolecular structure of polyester plasticizer that reduces the diffusion in the PVC. PVC and polyesters form intermolecular force from hydrogen bonding between the carbonyl group of the ester and a [beta]-hydrogen in PVC, or from a dipole-dipole interaction between the carbonyl group of the ester and the chlorine atom in PVC [30, 31]. Therefore, the polyester PBMS was concerned a kind of environmental friendly plasticizer, and was expected to be a better substitute of DOP for PVC.

[FORMULA NOT REPRODUCIBLE IN ASCII]

CONCLUSIONS

In this article, novel aliphatic polyesters were successfully synthesized from MSA and BDO by a two-step procedure, direct esterification and polycondensation using TBT as catalyst. MSA was produced from catalytic hydrogenation of itaconic acid which is renewable product. As the balance of the reaction esterification efficiency and the costs of reactants, the proper feed ratio of diol to diacid was determined to be 1.08:1. TBT as catalyst was proved to be efficient and its optimized concentration was 0.1 mol% to the amount of diacid based on analyzing the esterification ratio and intrinsic viscosity of PBMS. Furthermore, the molecular weight of PBMS polymers was governed by the polycondensation time. The obtained polymers have the expected chemical structure. They have excellent thermal stability. The [T.sub.g] increases continuously with molecular weight. With the extension of the reaction time, the molecular weight, and decomposition temperatures increased.

As new potentially biobased polyester plasticizer, desirable mechanical properties were achieved at the weight ratio of PBMS was 80/100 and 50/100. In addition, the PBMS/PVC blends had superior migration resistant property to the low-molecular weight plasticizer DOP for PVC.

ABBREVIATIONS

BDO            1,4-butanediol
DOP            dioctyl phthalate
DSC            differential scanning calorimeter
GPC            gel permeation chromatography
[sup.1]H NMR   nuclear magnetic resonance spectroscopy
MSA            2-methylsuccinate acid
PBMS           Polyfbutylene 2-methylsuccinate)
PDI            polydispersity index
PVC            polyfvinyl chloride)
TBT            tetrabutyl titanate
TGA            thermogravimetric analysis
[T.sub.g]      glass transition temperature


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Yumin Wu, Qingwei Xie, Chuanhui Gao, Ting Wang, Chuanxing Wang

College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, China

Correspondence to: Yumin Wu; e-mail: wuyumin005@l63.com

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 20876081; contract grant sponsor: Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shangdong Province; contract grant number: BS20I2CL016; contract grant sponsor: Joint Funds of the Qingdao Municapal Natural Science Foundation, China; contract grant number: 12-1 -4-3-(33)-jch.

DOI 10.1002/pen.23798

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. General characterizations of PBMS polymers.

Sample     Polycondensation   [eta](dL/g)     [M.sub.n] (a)
               time (h)                     (g x [mol.sup.-1])

PBMS-IO           10             0.18              3090
PBMS-20           20             0.21              4730
PB MS-30          30             0.23              5970

Sample       [M.sub.n] (b)        [M.sub.w] (b)      PDl (b)
           (g x [mol.sup.-1])   (g x [mol.sup.-1])

PBMS-IO           2860                 5810           2.03
PBMS-20           5710                 7750           1.33
PB MS-30          6020                 8730           1.45

(a) Determined from COOH titration.

(b) Determined from GPC calculation.

TABLE 2. Comparison of mobility between PBMS and DOP
plasticizer.

           Cyclohexane   Ethanol   Activated
                                    carbon
Plasticizer             Degree of
                      migration (%)

PBMS          2.24        0.54       0.08
DOP           16.39       10.16      1.64

FIG. 7. Tensile properties of PBMS/PVC (50/100 w/w) blend (a), PBMS/
PVC (80/100 w/w) blend (b), and DOP/PVC (50/100 w/w) blend (c).

(a)

Sample

PBMS/PVC(50/100)      144.33
PBMS/PVC(80/100)       18.48
DOP/PVC(50/100)        20.25

(b)

Sample

PBMS/PVC(50/100)       22.02
PBMS/PVC(80/100)       16.31
DOP/PVC(50/100)        19.54

(c)

Sample

PBMS/PVC(50/100)      202.49
PBMS/PVC(80/100)      302.71
DOP/PVC(50/100)       303.21

Note: Table made from bar graph.
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