Modification of Poly(butylene succinate) with Peroxide:Crosslinking, Physical and Thermal Properties, andBiodegradation

D. J. KIM,1 W. S. KIM,1 D. H. LEE,1 K. E. MIN,1 L. S. PARK,1 I. K. KANG,1 I. R. JEON,2 K. H. SEO1

1 Department of Polymer Science, Kyungpook National University, Taegu 702-701, South Korea

2 Department of Industrial Chemistry, Kyungil University, Kyungsangbuk-do 712-701, South Korea

Received 23 June 2000; accepted 27 October 2000

ABSTRACT: Prior to curing, we evaluated thermal stability of poly(butylene succinate)(PBS). Above 170°C, PBS was severely degraded and the degradation could not besuccessfully stabilized by an antioxidant. PBS was crosslinked effectively by DCP at150°C, and the gel fraction was increased as DCP content increased. The majorstructure of crosslinked PBS is supposed to consist of an ester and an aliphatic group.The tensile strength and elongation of PBS were improved with increasing content ofDCP, but tear strength was only slightly affected. The higher the crosslinking, thelower the heat of crystallization (DHc) and heat of fusion (DHf). However, the meltcrystallization temperature (Tc) of crosslinked PBS was higher than that of PBS. Theviscosity of crosslinked PBS increased and exhibited rubbery behavior as the content ofcuring agent increased. The biodegradability of crosslinked PBS did not seriouslydeteriorate. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1115–1124, 2001

Key words: poly(butylene succinate) (PBS); crosslinking; physical and thermal prop-erties; biodegradation

INTRODUCTION

In the last quarter of the 20th century, plasticproducts gained universal use not only in food,clothing, and shelter, but also in the transporta-tion, construction, medical, and leisure indus-tries. Annual production of synthetic polymer isabout thousands of millions tons1 and syntheticplastics have been developed as durable substi-tute products. Recently, however, the extensiveuse of bioresistant synthetic polymers is consid-ered to be one of the major causes of environmen-tal pollution and there is growing demand for

biodegradable plastics as a solution to problemsconcerning global environment and solid-wastemanagement.

Although several polymers have been identi-fied as degradable in model aqueous media or inanimal bodies, at the moment the family of ali-phatic polyesters appears to be the most attrac-tive and promising.2,3 However, commercial useof high molecular weight aliphatic polyesters hasbeen limited to polyesters produced by microor-ganisms,4 ring-opening polymerization of lac-tones,5 and ring-opening polyaddition of cyclicdimers6 because of cost and their poor propertiessuch as low melting point, low resistance to heat,tendency to hydrolyze, and mechanical strength.

Much of the investigative work has been gen-erated because of the inherent difficulty in syn-

Correspondence to: K. Seo.Journal of Applied Polymer Science, Vol. 81, 1115–1124 (2001)© 2001 John Wiley & Sons, Inc.

1115

thesizing high molecular weight aliphatic polyes-ters through the polycondensation of diols anddicarboxylic acids.7 Polycondensation alone, how-ever, will not produce polyester with such proper-ties that will make it suitable for practical use asa biodegradable plastic. It is necessary to furtherincrease its molecular weight, which can be ac-complished through (1) a polycondensation reac-tion with a multifunctional group such as trim-ethylolpropane, pentaerythritol, and diepoxide; or(2) a coupling reaction of polyester end groupswith a compound that possesses two or more re-active functional groups in a molecule.8–12 On theother hand, many researchers tried to introducecrosslinks into aliphatic polyesters to improveproperties such as mechanical strength and elas-ticity. Han et al.13 prepared biomedical materialsby thermal crosslinking of unsaturated polyestersderived from a mixture of glycolic acid, lactide,fumaric acid, and maleic anhydride with radicalinitiator. Pramanick and Ray14,15 synthesizedcrosslinked polyester from a mixture of glycerol,citric acid, and aspartic acid. Up to now, however,the crosslinking of saturated aliphatic polyestershas rarely been reported. In the present study, weintroduced crosslinking into saturated aliphaticpolyester, poly(butylene succinate) (PBS), to im-prove properties such as mechanical strength andelasticity by simple addition of an organic perox-ide, dicumyl peroxide (DCP). Moreover, we inves-tigated both the thermal properties and the bio-degradation behavior of crosslinked PBS.

EXPERIMENTAL

Materials

PBS chips (SG1111) were kindly supplied by SKChemicals (Korea). To minimize moisture effectsand hydrolysis, PBS was dried in a vacuum ovenat 80°C for 12 h before use. Primary antioxidant(Irganox 1010) and secondary antioxidant (Ir-gafos 168) were purchased from Ciba SpecialtyChemicals (Summit, NJ). Dicumyl peroxide(DCP) was from Nippon Oil and Fats (Japan).Chloroform and 1,1,2,2,-tetrachloroethane werefrom Aldrich Chemicals (Milwaukee, WI). Allchemicals were used without further purification.

Processing Temperature

To establish the processing temperature, thethermal stability of PBS was investigated. PBS

chips were placed in a convection oven at fourdifferent temperatures and change in the reducedviscosity of PBS was measured with respect tooven-residence time. Reduced viscosity of poly-mers was measured by using a Ubbelohde viscom-eter in 1,1,2,2,-tetrachloroethane solvent: 0.05 gPBS/10 mL solvent at 25°C. Gel permeation chro-matography (GPC) was done on an HPLC (Waters150cv; Waters Instruments, Rochester, MN) at40°C with O-chlorophenol/chloroform (1 : 3 v/v) assolvent; molar mass is expressed as polystyrene-equivalent molar mass.

Crosslinking

PBS was crosslinked by simply mixing DCP in aBrabender Plasticorder (PLE331, Germany)equipped with a cam-type rotor (Model No. W50).PBS was first molten at 150°C for 3 min, afterwhich DCP was added. A rotor speed of 40 rpmwas used and mixing torque change was recordedfrom the instant when DCP was added. Thecrosslinked samples were weighed and extractedby chloroform for 24 h using a Soxhlet extractor.Prior to the final weighing, the samples weredried in a vacuum at 70°C for 24 h. Gel fractionwas calculated as the weight of the dried ex-tracted sample divided by the original sampleweight. Solid-state 13C–NMR spectra of the PBSand extraction residual of PBS cured by 4-phrDCP were obtained using a Varian Unity Inova300WB (Varian Associates, Palo Alto, CA).

Mechanical Properties and Rheometric DynamicSpectroscope Examination

PBS was mixed with DCP in a Brabender Plasti-corder at 118°C and subsequently compression-molded into sheets (thickness 0.25 6 0.02 mm formechanical properties, 2 mm 6 0.1 for RDS) us-ing hydrodynamic press (150°C). During the mix-ing, no change of torque was observed. Tensileand tear tests were performed on an Instronmodel 4465. Korean Industrial Standards proce-dures were used for measuring tensile strength,elongation at break, and tear strength (KS M3503) of PBS and PBS cured by DCP. Viscoelasticproperties of PBS and crosslinked PBS were per-formed on an RDS (Rheometrics Scientific, Pisca-taway, NJ) using parallel plate by frequencysweep. A frequency range of 0.16–100 rad/s and2% strain were used.

DSC Measurement

The calorimetric measurements were carried outon a Perkin–Elmer Pyris 1 differential scanning

1116 KIM ET AL.

calorimeter (DSC; Perkin–Elmer, Palo Alto, CA)operating under nitrogen flow. The samples werefirst heated at a rate of 10°C/min from 30 to 120°Cand held at this temperature for 5 min to allowthe complete melting of the crystallites. The sam-ples were then cooled at a rate of 10°C/min from130 to 0°C. The values of crystallization temper-ature Tc and enthalpy of crystallization DHc werecalculated during these cooling runs. The sampleswere then heated up to 120°C at 10°C/min andthe results of melting temperature Tm and en-thalpy of melting DHf referred to these traces arereported.

Biodegradability

To evaluate biodegradability, two methods wereapplied: the enzymatic degradation [total organiccarbon (TOC)] test and aerobic gas (CO2) evolu-tion test. For the enzymatic degradation test,compression-molded films (thickness 45 6 2 mm,diameter 6.2 mm) of poly(ethylene terephthalate)(PET), PBS, and crosslinked PBS were introducedinto small bottles containing 2 mL of phosphate-buffered solution (pH 5 7) at 37°C and then addedwith 4 mL of solution of lipase from Rhizopusarrhizus (200 units). The enzymatic hydrolysiswas carried out for 48 h. After filtration (0.2-mmmembrane filter), a small amount of 1N hydro-chloric acid was dropped onto the filtrate. Thewater-soluble TOC concentration in the filtratewas measured as an indication for the biodegrad-ability with a Shimadzu 500 TOC analyzer (Shi-madzu, Japan). As controls, the TOC value for theenzyme itself was measured under the same con-ditions. The TOC data were the average of twomeasurements and corrected appropriately withthe blank levels. As another method, the aerobicgas evolution test was used and similar equip-ment was previously reported in the literature.16

A powdered sample (10 g) was mixed with 190 g ofcompost. The entire experimental assembly wasplaced in a cultivation room at 50°C for 180 days.CO2 evolved from the compost-containing sampleand the blank test. The amount of CO2 evolvedwas determined by titrating the remainingBa(OH)2. Percentage biodegradability was calcu-lated from dividing the amount of CO2 deter-mined by the theoretical value.

RESULTS AND DISCUSSION

Thermal Stability

Virtually all polymeric materials, either of syn-thetic or natural origin, undergo thermal degra-

dation. Technically, it is important for manufac-turers to evaluate thermal stability to establishprocessing conditions and service temperatures.Thus, prior to curing, we evaluated thermal sta-bility of PBS to establish a processing tempera-ture window.

Figure 1 shows the change in reduced viscosityof PBS as a function of oven-residence time. Asshown in this figure, the reduced viscosity changeof PBS with time can be neglected at 170°C. Butas temperature increased, its reduced viscositydecreased with time, which means that PBS isdegraded thermally over 170°C. One of the majormechanisms of thermal degradation is oxidation.The typical manifestations of oxidation are sum-marized by the term aging phenomena. Funda-mentally, there are various means available toretard thermal oxidation. One of them is additionof stabilizing additives, antioxidants; addition ofantioxidants is the most commonly used methodof stabilization. In the case of PBS, however, aswe can see in Figure 1 and Table I, antioxidantcould not entirely prevent thermal degradation.The possible justification is as following. Thethree main decomposition modes of polyester—thermal, hydrolytic, and thermooxidative degra-dation—are shown in Scheme 1 .17

The hydrolytic degradation of polyester is at-tributed to water. In this study, we dried PBS ina vacuum oven at 70°C for 12 h before use and sohydrolytic degradation may be disregarded. The

Figure 1 Change in reduced viscosity of PBS withrespect to oven-residence time. AO 0.4, PBS 1 primaryantioxidant 0.2 phr 1 secondary antioxidant 0.2 phr;arrow points, GPC measurement.

MODIFICATION OF PBS WITH PEROXIDE 1117

oxidation mechanism was developed originally byBolland and Gee.18,19 It is generally assumed thatprimary radicals are formed through the action ofheat or through the combined actions of heat andmechanical stress. The fixation of an oxygen mole-cule onto a carbon atom–centered free radical isgenerally a very fast reaction if the concentration ofoxygen in the polymer is sufficient. It transformsalkyl radicals Pz rapidly into peroxy radicals PO2

●.The primary antioxidant stabilizes alkyl radicalsand peroxy radicals and inhibits the propagationreaction. The secondary antioxidant decomposeshydroperoxides without intermediate formation offree radicals. Commonly, primary and secondaryantioxidants are mixed up and used for synergyeffect. But, in this study, PBS was degraded se-verely at 210°C and degradation could not be suc-

cessfully stabilized by antioxidants, which is be-cause thermal decomposition is concerned with deg-radation. As shown in Scheme 1, Plage andSchulten20 reported that polyester forms a six-membered cyclic transition state and b hydrogen istransferred to oxygen to yield a carboxylic and vinylend group. That is to say, whereas PBS in thisexperiment is degraded both through thermal oxi-dation and thermal decomposition, antioxidants areinvolved only in thermal oxidation; so, they couldnot entirely stabilize degradation. From these re-sults, we could conclude that the processing temper-ature of PBS should not be over 170°C.

Crosslinking Behavior

The use of an organic peroxide as a crosslinkingagent was first reported for the first time in 1915.

Table I Change in Molecular Weights of PBS with Respect to Oven Aging

Sample Oven Aging hred Mn Mw PDIa

PBS Non 1.159 18,750 119,750 6.39PBS 190°C, 20 min 1.110 17,470 112,960 6.47PBS 210°C, 30 min 1.008 14,290 86,850 6.08PBS 230°C, 10 min 0.886 12,590 64,600 5.13PBS 230°C, 40 min 0.688 11,680 54,810 4.69PBS 1 AO 0.4b 190°C, 20 min 0.943 15,980 79,120 4.95

a Polydispersity index.b PBS 1 primary antioxidant 0.2 phr 1 secondary antioxidant 0.2 phr.

Scheme 1 Hydrolytic, thermal, and thermooxidative degradation of polyester.

1118 KIM ET AL.

Since around 1950, interest in the industrial useof peroxides as crosslinking agents has been in-creasing, partly as a result of the introduction ofsaturated rubbers such as ethylene-propylenerubber (EPM) and silicone rubber, which cannotbe vulcanized with the usual sulfur system. Up tonow, however, the crosslinking of thermoplasticsfor technical applications has been mainly limitedto polyethylene and its copolymers. In this study,we sought to introduce crosslinking to PBS, bio-degradable saturated aliphatic polyester, by us-ing organic peroxide.

To be suitable for technical applications, a perox-ide crosslinking agent must fulfill a number of con-ditions.17 One of the major requirements is that thenature of its decomposition products must be suchthat rapid crosslinking takes place at the desiredtemperature, without a tendency to prematurecrosslinking (prevulcanization, scorch). Organicperoxides have various decomposition tempera-tures and the range commences at a temperaturebelow 0°C and finishes at nearly 200°C. Dicumylperoxide (DCP) is one of the alkyl peroxides andits decomposition range is approximately 170°C.Taking decomposition temperature into consider-ation, DCP might be the most suitable for poly-mers melting at 100–120°C.21 Figure 2 shows thechange in Plasticorder torque with time whenDCP is added to PBS.

In the early stage, torque value was slightlydecreased as DCP was added to PBS. It should bethe result of melting of solid DCP. Then after30–40 s, torque was rapidly increased, such that

the higher the DCP content, the higher the torquevalue. This means PBS is crosslinked effectivelyby simple addition of the organic peroxide DCP.The gel fraction of crosslinked PBS with respectto DCP content is shown in Figure 3.

Gel fraction was augmented as the content ofDCP increased and at above 5 phr, a slight changewas seen. Takshima and Nakayama22 reportedthat the gel content of polyethylene (PE),crosslinked by 2-phr DCP at 140–180°C and thenextracted by xylene, was 80–90%. Comparing ourresults with those reported by Takshima, PBSshows relatively lower gel content than that ofPE. The possible justifications are as follows.First, PBS may have fewer crosslinking sites thanthose of PE. The crosslinking reactions by DCPare as follows23: DCP cleaves thermally to pro-duce two cumyloxy radicals (1), which abstracthydrogen atoms from polymer chains (2). A cu-muyloxy radical can also lead to the formation ofa methyl radical and of phenylmethyl ketone (3).The methyl radical is likewise able to abstracthydrogen (4). These polymer radicals then coupleto crosslink. Taking the crosslinking mechanisminto consideration, it is supposed that PBS has alower content of methylene group in the mainchain than that of PE, such that relatively smallcrosslinking sites make it difficult to crosslink.Second, polymer–solvent solubility parameterdeference could be concerned. That is to say, al-though Takshima used xylene as a Soxhlet ex-traction solvent for crosslinked PE, we used chlo-

Figure 2 Brabender plastograms for PBS as a func-tion of DCP content (150°C, 40 rpm).

Figure 3 Gel fraction of crosslinked PBS with respectto DCP content.

MODIFICATION OF PBS WITH PEROXIDE 1119

roform for crosslinked PBS. Thus this differencemay be one of the possible reasons for the gap ingel content between DCP-cured PE and PBS.

(1)

(2)

(3)

(4)

(5)

In the meantime, many investigators have beendoing research on aliphatic polyesters because ofits biodegradability, which results from ester andflexible aliphatic groups in the polymer mainchain. By the way, if ester group of PBS werebroken down in the course of crosslinking, biode-gradability would be deteriorated. If a radical pro-duced by decomposition of DCP attacks the estergroup (CAO) in the PBS main chain, a carbonatom is placed between two oxygen atoms

(–OOCOO–); this carbon resonance at 95–100ppm can be obtained.24 So, we investigated solid-state 13C–NMR spectra to identify the existence ofthis carbon atom. Figure 4 shows solid-state 13C–NMR spectra of PBSD and the extraction residualof PBS cured by 4-phr DCP (EXXPBS4D).

The ester group carbon “a” resonance at 173.7ppm, and carbon “b,” carbon “c,” and carbon “d” at28.6, 66.1, and 25.9 ppm, respectively, were ob-tained. There was little difference between thetwo spectra and there was no trace of resonanceat 95–100 ppm, which may imply that the basicstructure of crosslinked PBS consists mainly of anester and an aliphatic group and that we cananticipate that crosslinking may not deterioratethe biodegradability of PBS.

Mechanical Properties

One of the reasons for crosslinking PBS is toimprove the mechanical strength of aliphaticpolyester. Tensile strength, elongation, and tearstrength of PBS and PBS cured by DCP areshown in Table II.

Tensile strength and elongation were en-hanced with an increase of DCP content added toPBS. Nielsen25 and Flory26 illustrated that atvery low molecular weights, the tensile stress tobreak, sb, is near zero and as the molecularweight increases, the tensile strength rapidly in-creases, then gradually levels off. A major point ofweakness at the molecular scale is the chain end,

Figure 4 Solid-state 13C–NMR spectra of PBS and extraction residual of PBS curedby 4-phr DCP (EXXPBS4D).

1120 KIM ET AL.

which does not transmit the covalent bondstrength; in the case of crosslinking of PBS in ourstudy, the number of chain ends was not reducedbecause the molecular weight of polymer was in-creased, not polymerization but interchaincrosslinking. However, supposing that two poly-mer chains of the same molecular weight arecrosslinked to produce one crosslinking point, amolecule is made and the far off end-to-end dis-tance is increased, although the number of chainends is four. As a result, number-average molec-ular weight is double what it was before. Accord-ingly, it is supposed that molecular weight aug-mentation by curing enhanced the tensile proper-ties. On the other hand, tear strength was notaffected by crosslinking. We guess that tearstrength of a polymer is concerned with breakingof main-chain covalent bonds. Therefore, it is sup-posed that tear strength depends mainly on thebasic chemical structure of the polymer, not mo-lecular weight or crosslinking.

Thermal and Viscoelastic Properties

DSC thermograms for both cooling and heatingruns of PBS and PBS cured by DCP were inves-tigated and crystalline temperature (Tc) and heat

of crystallization (DHc) values are summarized inTable III.

As can be seen in Table III, crystallinity waspresent in both crosslinked PBS and the ex-tracted sample. Generally, it is known that thecrystallinity of a polymer disappears with the in-troduction of crosslinking, although Albertssonand Eklund27 did an interesting study, in whichthey obtained methylene segment crosslinks bycopolymerizing adipic anhydride with 1,2,7,8-diepoxyoctane (DEO). The percentage gel fractionincreased up to 95 wt % with an increasingamount of crosslinking agent, up to 20 mol %DEO, and these crosslinked polymers showedcrystallinity. However, no crystallinity could bedetected at DEO contents over 20 mol %. Takingtheir report into account, the crosslinking densityof PBS cured by DCP is low, so that polymerchains have mobility to crystallize. Meanwhile, itis noteworthy to see the effect of crosslinking oncrystallization temperature of PBS. Crystalliza-tion temperature of crosslinked PBS is higherthan that of PBS. We surmise that the crystalli-zation temperature of crosslinked PBS probablyincreased as a result of some substances acting asnucleating agents.

Table II Mechanical Properties of PBS and Crosslinked PBS

Material Tensile Strength (kgf/cm2) Elongation (%) Tear Strength (kgf/cm)

PBS 287 57 184PBS 1 0.5Da 280 77 181PBS 1 1D 304 87 214PBS 1 2D 316 152 192PBS 1 3D 307 192 182PBS 1 4D 339 252 206

a Content of DCP (phr).

Table III Thermal Properties of PBS and Crosslinked PBS

Property PBS

PBS 1

0.5Da 1D 2D 3D 4D 4DEXb

Tc (°C) 79.6 89.8 92.4 93.8 93.2 93.4 92.3DHc (J/G) 279.6 281.9 278.5 276.7 275.0 271.1 255.0Tm (°C) 114.3 112.9 112.4 111.4 110.4 110.7 108.9DHf (J/G) 80.1 75.9 74.3 72.3 71.9 63.3 54.6

a Content of DCP (phr).b Extraction residual of PBS cured with 4-phr DCP.

MODIFICATION OF PBS WITH PEROXIDE 1121

Jensen17 insisted that so-called spontaneousnucleation, that is, nucleation without an inten-tional introduction of a nucleating agent, is gen-erally believed to be attributed to foreign sub-stances such as catalyst residues, oxidatively de-graded polymer, or other, processing-determinedimpurities about whose physicochemical naturevery little is known. The nucleating agent shouldbe wetted or absorbed by the polymer and itshould be homogeneously dispersed in the poly-mer, in as fine a form as possible. Considering hisinsistence, it may be possible that a by-product ofcuring reaction or crosslinking point could act asan impurity that can initiate crystallization. Inthe meantime, the melting temperature and heatof fusion of crosslinked PBS were lower than thatof PBS in the heating scan. This intimates thatsize and perfectness of cured PBS crystals arerelatively low because crosslinks restrict chainmotions.

Figure 5 shows storage modulus (G9) and lossmodulus (G0) change of PBS with frequency. G0surpassed G9 in most frequency ranges and thegap between G9 and G0 was decreased with in-creasing frequency, which is the characteristi-cally time-dependent viscoelastic behavior ofthermoplastic polymer melts: the higher thestrain rate, the higher the elasticity. In Table IV,crossover frequency in the G9 and G0 of PBS andPBS cure by DCP is summarized. In the case ofPBS cured by 0.5-phr DCP, G0 exceeded G9 over40 rad/s at 150°C. Crossover frequency shifted to

low frequency with increasing crosslinking andlower temperature. In the case of PBS cured with2-phr DCP, G9 surpassed G0 in the all-frequencyrange (Fig. 6).

This may be explained as follows. First, gener-ally, the higher the molecular weight of a poly-mer, the higher the elasticity, because chain en-tanglement restricts chain slippage, which is ir-reversible deformation. Second, enhancement ofentropy elasticity28 by crosslinking could be con-sidered like that in the rubber. On the other hand,the test procedure of melt strength usually usedinvolves increasing the wind-up speed and noting

Figure 5 Modulus change of PBS as a function offrequency at various temperatures.

Table IV Crossover Frequency in the G* and G(of PBS Cured by DCP (Frequency Range: 0.16–100 rad/s)

Material

Temperature (°C)

150 160 170

PBS Non Non NonPBS 1 0.5Db 40 rad/s Non NonPBS 1 1D 6.3 rad/s 100 rad/s NonPBS 1 2D All rangec All range All rangePBS 1 3D All range All range All rangePBS 1 4D All range All range All range

a G0 surpassed G9 in the all-frequency range.b Content of DCP (phr).c G9 surpassed in the all-frequency range.

Figure 6 Modulus change of PBS cured with DCP(2-phr) as a function of frequency at various tempera-tures.

1122 KIM ET AL.

the force level when the filament breaks.29 If meltstrength of a polymer is not sufficient, it is diffi-cult to process the polymer. Melt strength of apolymer depends on viscosity, G9, and G0. Muchinvestigative work has been generated because ofthe inherent difficulty in synthesizing high molec-ular weight aliphatic polyesters through the poly-condensation of diols and dicarboxylic acids. Oneof the major purposes of this works was to im-prove the melt strength of aliphatic polyesters.Accordingly, RDS results suggest that simple ad-dition of organic peroxide will be able to controlviscoelastic properties of PBS without adding anyfunctional group into the polymer main chain.

Biodegradability

The biodegradability of crosslinked PBS was eval-uated by measurement of TOC values (Fig. 7) andthe aerobic gas evolution test (Fig. 8).

As Figure 7 shows, TOC of PBS is much higherthan that of PET, and that of crosslinked PBS israrely deteriorated. Moreover, the percentage bio-degradability measured by the aerobic gas evolu-tion test of PBS cured with 2-phr DCP is almostequal to that of cellulose. This may be becausecrosslinked PBS still consists of an ester and analiphatic group (as considered in Fig. 4). Biode-gradability of polyesters is highly dependent onglass-transition temperature, crystallinity,30–35

and balance between hydrophilic and hydropho-

bic groups36–39 in the backbone. PET shows lowbiodegradability by reason of high glass-transi-tion temperature and low chain mobility attrib-uted to the rigid aromatic group. This is the rea-son that most of the biodegradable polyesters arealiphatic. Consequently, we may be able to saythat crosslinking caused by organic peroxide ex-erts only a slight effect on the biodegradability ofPBS, aliphatic polyester.

CONCLUSIONS

The processing temperature of PBS should not beover 170°C and antioxidants cannot entirely pre-vent thermal degradation. By simple addition ofDCP, PBS was crosslinked effectively and gelfraction increased as DCP content increased. 13C–NMR spectra revealed that the major structure ofcrosslinked PBS is still an ester and an aliphaticgroup. Crosslinking introduced to PBS enhancedtensile properties, although tear strength wasonly slightly affected by curing. Crosslinking den-sity of PBS cured by DCP may be low so thatpolymer chains have mobility to crystallize. It issupposed that the crystallization temperature ofcrosslinked PBS increased for the help that asubstance produced in the course of crosslinkingacts as nucleating agent. Elasticity of crosslinkedPBS was increased as content of the curing agentincreased, which indicates that processibility of

Figure 7 Change of TOC on enzymatic hydrolysis ofPET, PBS, and DCP-cured PBS. PBS05D, PBS curedwith 0.5-phr DCP; PBS1D, PBS cured with 1-phr DCP;PBS2D, PBS cured with 2-phr DCP; PBS3D, PBS curedwith 3-phr DCP; PBS4, PBS cured with 4-phr DCP.

Figure 8 Results of compost test for cellulose, PE,and PBS cured with 2-phr DCP (PBS2D).

MODIFICATION OF PBS WITH PEROXIDE 1123

PBS can be regulated by simple addition of DCP.The biodegradability of crosslinked PBS wasrarely deteriorated and similar to that of cellu-lose.

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