soft pvc foams: study of the gelation, fusion, and foaming processes. ii. adipate, citrate and other...

Soft PVC Foams: Study of the Gelation, Fusion, andFoaming Processes. II. Adipate, Citrate and Other Typesof Plasticizers

Agnes Zoller, Antonio Marcilla

Chemical Engineering, University of Alicante, P.O. Box. 99, E-03080, Alicante, Spain

Received 19 November 2010; accepted 29 December 2010DOI 10.1002/app.34108Published online 6 July 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Plasticized poly(vinyl chloride) (PVC) isone of the most useful polymeric materials on an industrialscale because of its processability, wide range of obtainableproperties, and low cost. PVC plastisols are used in the pro-duction of flexible PVC foams. Phthalates are the most usedplasticizers for PVC, and in a previous article (part I of thisseries), we discussed the influence of phthalate ester typeplasticizers on the foaming process and on the quality of thefoams obtained from the corresponding plastisols. Becausethe use of phthalate plasticizers has been questionedbecause of possible health implications, the objective of thiswork was to undertake a similar study with 11 commercialalternative plasticizers to phthalates. The evolution of thedynamic and extensional viscosity and the interactions andthermal transitions undergone by the plastisols during theheating process were studied. Foams were obtained by rota-tional molding and were characterized by the determinationof their thermomechanical properties, density, and cell size

distribution. Correlations were obtained between the molec-ular weight and structure of the plasticizer and the behaviorof the corresponding plastisols. After the characterizationof the final foamed product, we concluded that foams ofrelatively good quality could be prepared with alternativeplasticizers for replacing phthalates. Several plasticizers{Mesamoll (alkylsulfonic phenyl ester), Eastman 168 [bis(2-ethylhexyl)-1,4-benzenedicarboxylate], Hexamoll [di(iso-nonyl) cyclohexane-1,2-dicarboxylate], Citroflex A4 acetyltributyl citrate (ATBC), and Plastomoll (dihexyl adipate)}were found to be interesting alternatives in the productionof soft PVC foams because they provided very good qualityfoams with properties similar to, or even better than, thoseobtained with phthalate plasticizers. VC 2011 Wiley Periodicals,Inc. J Appl Polym Sci 122: 2981–2991, 2011

Key words: differential scanning calorimetry (DSC);molding; poly(vinyl chloride) (PVC); viscoelastic properties

INTRODUCTION

In recent years, the use of foamed plastics has beenincreasing in all kinds of different applicationsbecause they can be manufactured from many differ-ent types of polymers, and their properties can becustomized by the addition of additives1,2 to specificrequirements. Their outstanding properties, such aslight weight, low density, excellent strength/weightratio, superior insulating ability, and energy absorp-tion capabilities, make polymer foams especiallyattractive in areas such as packaging, biomedicine,building, automotive, carpet underlays, textiles, fur-niture, and the production of toys.

In a previous work,3 an extensive review of thescientific literature on polymeric foams,4 theirapplication,5 and production6 and the types anddescription of foaming processes7 was provided.Furthermore, a deep introduction to plastisol rheol-

ogy (gelation and fusion8,9 processes, swelling,10 andextensional viscosity11,12), calorimetric behavior ofthe plastisol,13 and foaming agents14 and a compre-hensive exposition of the correlations found betweenall of these processes and their importance in thefoaming process and foam quality was given. In thatstudy, the rheological and thermal behaviors of theplastisols prepared with nine phthalate ester typeplasticizers and the corresponding foam qualitywere studied. Phthalates are the most used plasticiz-ers in flexible foam production; however, their appli-cation has been questioned lately because of possibleenvironmental and health15,16 implications. A furtherrelated issue is the migration17,18 of these plasticiz-ers, which is very important to be considered. Forthese reasons, the study of alternative plasticizers isconvenient and highly recommended to find lessquestioned substitutes to produce the same or evenbetter quality foams. Persico et al.19 compared thebehavior between dietylhexyl phthalate (DOP) andalternatives, such as dietylhexyl adipate (DOA) andacetyl tributyl citrate (ATBC). Furthermore, a shortreview of the application and properties of some al-ternative plasticizers has been found in the litera-ture, dealing with different types of plasticizers,20

Correspondence to: A. Zoller ([email protected], [email protected]).

Journal of Applied Polymer Science, Vol. 122, 2981–2991 (2011)VC 2011 Wiley Periodicals, Inc.

such as trimellitates, alkyl sulfonates [e.g., Mesamollalkylsulfonic phenyl ester (ASE), benzoates (e.g.,Benzoflex 2088), citrates (e.g., Citrofoll BII, known asATBC)], and carboxylates [e.g., Hexamoll di(iso-nonyl) cyclohexane-1,2-dicarboxylate (DINCH)].

A very interesting review of the state of the art con-cerning the health issues of plasticizers can be foundin the work of Wypich,21 where three chapters aredevoted to the health and safety issues of plasticizersand their environmental fate and regulations. How-ever, not all plasticizers have been studied in a simi-lar extension, and the availability of data is not thesame, depending on the specific compound. Thus,whereas plasticizers such as DOP or DINP (phtha-lates in general) have been extensively studied foryears from all the considered points of view, veryfew data have been obtained for other plasticizers.

To replace phthalates, a comparison should be madewith other commercially available plasticizers withregard to their process performance and potentialhealth issues. Other studies22 have shown, for example,that DINCH possesses no environmental or reproduc-tive hazards, and citrates have been approved for usein applications such as pharmaceutical tablet coatings,medical devices, and food packaging. Furthermore, ithas also been emphasized and reported that all non-phthalate plasticizers are safe to use.

Thus, we chose 11 commercially available plasti-cizers of different chemical families to cover a widerange of properties (including two low-migrationpolymeric plasticizers), which have not been ques-tioned from the point of view of health or environ-mental risks, to carry out a study of their suitabilityfor foam applications, in a similar way as we did inthe previous study with nine phthalate plasticizers.3

In other previous works, we studied the rheological23

and thermal13 behavior of 20 plasticizers of differentfamilies, such as linear and branched chained phtha-lates, monomeric and polymeric adipates, citrates, pen-taerythrytols, carboxylates, and an alkyl sulfonate.

In the production of foams, several techniques canbe used, such as extrusion or rotational molding.24

The foaming process generally occurs at elevated tem-peratures at about 180–200�C and involves the curingof the plastisol (gelation and fusion) and the decom-position of the chemical blowing agent; this generatesgases and then bubbles. To obtain good quality foams,all of these dynamic processes have to be adequatelysynchronized with each other. The development ofthe melt strength,12,25 a property that indicates a com-pound’s ability to withstand drawing without break-ing, also plays a significant role in the foamingbecause the polymer matrix has to withstand thestresses evolved during the gas generation and bubblegrowth and stabilizes the foam structure. A high meltstrength is fundamental for the production of foamedplastics with low density and good cell structure.

All of these parameters are influenced by the plasti-cizer type, structure, molecular weight (i.e., the com-patibility with the resin), and the addition of otheradditives. For this reason, a proper selection of theadequate plasticizer and additives for the plastisolformulations is of crucial importance to obtain therequired balance of properties in the final product.The main objective of this work was to study the

relationship among the evolution of the complexand extensional viscosities of the plastisols and thethermal processes occurring during their heatingwith the properties and quality of the foamsobtained in the foaming process of such plastisolsprepared with 11 commercial plasticizers of differentchemical family alternatives to phthalates.

EXPERIMENTAL

Materials

Resin

The poly(vinyl chloride) (PVC) resin ETINOX 400(a vinyl chloride–vinyl acetate copolymer with anominal 5% of vinyl acetate; typical values providedby the supplier were 4.8% of comonomer and a Kvalue of 70; where K is an indirect measure of themolecular weight of PVC, based on the viscosity of aPVC solution, and it is generally determined accord-ing to the standard test method DIN 53726) by AIS-CONDEL (Barcelona, Spain) was used to preparethese plastisol formulations.

Plasticizers

In the previous study,3 9 different commercialphthalate ester plasticizers were studied. In thisstudy, 11 different commercial plasticizers from sixdifferent families were chosen. Among them were

• Two monomeric adipates [diisonoyl adipate(DINA) and dihexyl adipate (DHA)].

• Two polymeric adipates (PM 632 and PM 652).• Two citrates [ATBC and acetyl trihexyl citrate(ATHC)].

• Two pentaerythrytol esters of fatty acids (H 600and H707).

• Two carboxylates [DINCH and bis(2-ethyl-hexyl)-1,4-benzenedicarboxylate (EHBDC)].

• One ASE.

Table I shows the plasticizers selected, their abbre-viations, their commercial names, their densities,their molecular weight, and their producers.

Reagents

Table II shows the stabilizers, costabilizers, kickers,and chemical blowing agents used, along with the

Journal of Applied Polymer Science DOI 10.1002/app

2982 ZOLLER AND MARCILLA

commercial names, chemical components, andsuppliers.

Methods3

Plastisol preparation

Eleven PVC plastisols were prepared by the mixtureof 100 phr (parts per hundred resin) of ETINOX 400,100 phr of 1 of the 11 plasticizers, 2 phr of ReagensCL4 commercial Zn/Ca–stearate stabilizer, 6 phr ofLankroflex 2307 epoxidized soybean oil as a costabil-izer, and 2 phr of zinc oxide. After mixing, the pasteswere subjected to a degassing process for 15 min witha maximum vacuum of �1 mbar for air removal.These plastisols (not including the foaming agent)were used to study the rheological properties of thepolymer matrix [i.e., in the complex viscosity (g*) andthe extensional viscosity measurements]. For differen-tial scanning calorimetry (DSC) measurements, foamproduction, and foam characterization, 2 phr chemi-cal blowing agent [azodicarbonamide (ADC)] wasalso added to the same plastisol formulations.

Plastisol characterization

Evolution of g* by a Bohlin CS 50 rheometer (Malvern,United Kingdom). Evolution of g* of the plastisols wasdetermined bymeasurement ofg* in dynamic oscillatorytests between 40 and 180�C at a 5�C/min heating ratewith 20-mm-diameter parallel plates with a gap of 0.5mm, an oscillation frequency of 1 Hz, and a controlleddeformation of 5� 10�3 in a Bohlin CS 50 rheometer.Evolution of the extensional viscosity by an advancedrheometer expansion system (TA Instruments, Barcelona,Spain). The extensional viscosities of the 10 � 18 � 1

mm3 samples, previously cured at 180�C for 10 min,were measured at 160, 170, and 180�C, with the appli-cation of a 0.1-s�1 prestretch rate and 5-s�1 exten-sional rate with the extensional viscosity fixture acces-sory at each temperature.

Thermal behavior and decomposition ofthe chemical blowing agent by DSC

Thermal transitions, including the decomposition ofthe chemical blowing agent, was studied in a PerkinElmer Pyris 6 DSC instrument (Rodgau, Germany)between 40 and 220�C at a 5�C/min heating rate in anitrogen atmosphere with a 20 cm3/min flow at 20�C.

Foam production

Rotational molding. The plastisols were poured intoa cylindrical mold and placed into a Rotospeed RL1-400 rotational molding machine, Ferry IndustriesINC, Ohio, USA. Two cycles were made: the firstcycle (curing) occurred at 210�C for 8 min with a10:2 rpm arm-to-wheel speed ratio, whereas the

TABLE IProperties and Producers of the Plasticizers Studied

Plasticizer AbbreviationCommercial

nameDensity(g/cm3)

Molecularweight(g/mol) Producer

Monomeric adipatesDihexyl adipate DHA Plastomoll DHA 0.935 314 BASF, Barcelona, SpainDiisonoyl adipate DINA Plastomoll DNA 0.922 398 BASF, Barcelona, Spain

Polymeric adipatesPolymeric esters of adipatic acid PM 652 Palamoll 652 1.050 3300 BASF, Barcelona, SpainPolymeric esters of adipatic acid PM 632 Palamoll 632 1.145 7000 BASF, Barcelona, Spain

CitratesAcetyl tributyl citrate ATBC Citroflex A4 1.050 402 Morflex, Bracelona, SpainAcetyl trihexyl citrate ATHC Citroflex A6 1.050 486 Morflex, Barcelona, Spain

Pentaerythrytol estersPentaerythrytol esters of fatty acids H 600 Hercoflex 600 1.000 604 Hercules, Tarragona, SpainPentaerythrytol esters of fatty acids H 707 Hercoflex 707 1.000 750 Hercules, Tarragona, Spain

CarboxylatesBis(2-ethylhexyl)-1,4-benzenedicarboxylate EHBDC Eastman TM 168 0.984 391 Eastman, San Roque, SpainDi(isononyl) cyclohexane-1,2-dicarboxylate DINCH Hexamoll DINCH 0.949 425 BASF, Barcelona, SpainAlkylsulfonic phenyl ester ASEMixture of ASE ASE Mesamoll ASE 1.055 368 Bayer, Barcelona, Spain

TABLE IIReagents Used

Reagenttype

Commercialname Composition Producer

Stabilizer CL 4 Ca/Zn stearate ReagensCostabilizer Lankroflex

2307Epoxidizedsoybean oil

AkcrosChemicals,Barcelona, Spain

Catalyst(kicker)

Zinc oxide Zinc oxide Pankreac,Barcelona, Spain

Foamingagent

D 200 A ADC Unicell

SOFT PVC FOAMS. II 2983


second cycle (cooling) involved a 2-min airflow, a10-min water flow, and finally, a 2-min airflow.

Foam characterization

Determination of the foam density. A Mettler-ToledoDensity Kit (Barcelona, Spain) for Analytical Balan-ces was used. Appropriate sampling was crucial;thus, all the foam samples were cut out from themost homogeneous part of the entire foam, andreplicated samples were measured.Determination of thermomechanical properties by thermo-mechanical analysis (TMA). The penetration resistanceof the foam was determined with a Setaram 92-16.18TMA instrument (Caluire, France). Two heatingcycles were applied: the first heating cycle wasbetween 30 and 100�C at a 5�C/min heating rate ina nitrogen atmosphere without the application offorce, and the second heating cycle was between 30and 100�C at a 5�C/min heating rate in a nitrogenatmosphere with a 0.04-N force applied.Determination of the average bubble size and standarddeviation. Photographs of the cross sections of thefoam parts prepared were taken and analyzed withvarious imaging programs (GIMP, Image J, Paint,and Photoshop) and statistical steps. Reliable resultswere obtained for the average radius and the stand-ard deviation of the corresponding distributions.

RESULTS AND DISCUSSION

This section is organized in the same way as in theprevious publication;3 that is, it is divided into twoparts, that is, plastisol characterization and foamcharacterization. In the first part, the rheological andthermal behaviors of the plastisol formulations areconsidered, whereas in the second part, the foamcharacterization is discussed.

Plastisol characterization

Evolution of the g*

The results obtained for the temperature dependenceof the g* of the 11 plasticizers selected are shown in

Figures 1 and 2. Figure 1 represents the curves forthe plastisol formulations prepared with the twomonomeric adipates (DHA and DINA), the twopolymeric adipates (PM 632 and PM 652), and thetwo citrates (ATBC and ATHC) plasticizers, whereasFigure 2 shows the corresponding curves of the plas-tisols prepared with the two pentaerythrytols (H 600and H 707), the two carboxylates (EHBDC andDINCH), and an ASE plasticizer. It can be observedthat the gelation process of the formulations pre-pared with DHA, ATBC, and ASE occurred at lowtemperatures (>30�C lower than those of the otherplasticizers) and developed higher viscosities thanthe rest of those studied. This fact may have been,on the one hand, a consequence of the higher com-patibility of these plasticizers, which interacted in amore effective way with the PVC resin at this tem-perature range. On the other hand, it could havealso been a consequence of possible plasticizer evap-oration. The DHA, ATBC, and ASE plasticizers,which had the lowest boiling point values (i.e., 136,171, and 200�C, respectively), were the more likelyto evaporate under the process conditions. This factmay have resulted in a final product having a loweramount of plasticizer than the nominal one (i.e., thatcorresponding to the initial composition), which inturn, may have also resulted in higher developedviscosities; this was in good agreement with the lit-erature with regard to the viscosity as a function ofthe plasticizer concentration.26–28

In a previous work,23 we observed that the adi-pate-type plasticizers presented higher maximumviscosity temperatures, whereas the citrates devel-oped lower maximum viscosity temperatures thanthe phthalates for a given molecular weight. Addi-tionally, the adipates developed lower maximumviscosities than the citrates; in other words, the adi-pates seemed to be less compatible than the phtha-lates, and the phthalates were even less compatiblethan the citrates. These results were in good agree-ment with results reported by Persico et al.,19 whostudied ATBC, DOA, and DOP plasticizers. It was

Figure 1 Evolution of g* (Pa s) of the adipate and citrateplasticizers.

Figure 2 Evolution of g* (Pa s) of pentaerythrytol,carboxylate, and the alkyl sulfonate plasticizers.



concluded that the tetrahedral conformation of theATBC and the presence of four polar groups causeda fast incorporation of this plasticizer into the resin.In the case of DOA, the aliphatic linear structure didnot seem to compensate for the lack of significantpolarity and resulted in the slowest gelation plasti-cizer among them. The intermediate polarity of DOPseemed to justify its intermediate gelation tempera-ture. In this study, the two carboxylate type plasti-cizers studied (DINCH and EHBDC) behaved simi-larly to the adipates (they had only two polargroups, whereas ASE presented similar compatibilityto the citrates).

In previous studies,3,23 when the rheology of thephthalate ester type plasticizers was examined,the data obtained was analyzed accordingly with themolecular weight of the plasticizer used. It wasfound that at elevated temperatures, after the maxi-mum of each gelation curve, the plastisol g* tendedto become lower when lower molecular weight plas-ticizers were used. Studying the developed viscos-ities at several temperatures between 140 and 180�C,we obtained interesting conclusions regarding theplastisol evolution, which led to a reliable interpreta-tion of the results and also to a better understandingof the development of the final properties of eachplastisol. It was found that at 140�C, not all of theplastisols had developed their final structure andproperties. Only the most compatible (i.e., lower mo-lecular weight) ones were capable of developing thefinal structure at 140�C, whereas at higher tempera-tures, all of the plasticizers had completed theirstructural changes to a larger extent. Each formula-tion, depending on the structure and molecularweight of the plasticizer used, may have been in adifferent stage of development of its properties and

new structure. Furthermore, the developed g*became a linear function of the molecular weight ofthe plasticizer in the case of the phthalate esterswhen their properties were fully developed. Thedeep knowledge of these processes is important forunderstanding the foaming behavior of such plasti-sols. In this case, the study of the 11 plasticizers ofsix different families made the comparison morecomplicated within each group, as only two plastisolformulations from each group could be considered.However, they also followed the same behaviorwithin the corresponding family.23 For this reason,we applied the conclusions of the previous publica-tion for better interpretation of the results.One of the main objectives of this study was to an-

alyze the plasticizer effect on the foam quality. Thus,the analysis of the g*’s at the processing tempera-tures and also at the decomposition temperatures ofthe chemical blowing agent (i.e., at the maximumdecomposition rate) was convenient for discussingthe dynamic behavior and eventual foaming proc-esses of such pastes.For these reasons, further data was considered to

be important, which included the difference betweenthe decomposition temperature of the azodicarbona-mide (TADC) and the temperature of the maximumcomplex viscosity (Tg*max). This temperature differ-ence provided information about how the plastisoldeveloped its viscosity, structural changes, andproperties during the processing. Thus, both theTADC and TADC � Tg*max values are presented inTable III. The wider this temperature range was, themore developed the properties and melt strength12,25

of the plastisol formulations should have been; thisis of crucial importance in the understanding of thedynamic foaming system. A developed melt strength

TABLE IIISummary of the Results of the Evolution of g* and TADC

Plasticizerg*max

(Pa s)Tg*max

(�C)g* (Pa s)at 140�C

g* (Pa s)at 150�C

g* (Pa s)at 160�C

g* (Pa s)at 170�C

g* (Pa s)at 180�C

g* (Pa s)at TADC

TADC

(�C)TADC � Tg*max

(�C)

Monomeric adipatesDHA 6,740 116 3,320 1,620 584 232 154 246 170 54DINA 1,820 149 1,460 1,800 1,110 423 219 326 173 24

Polymeric adipatesPM 652 6,230 138 6,170 5,200 3,480 1,750 842 2,254 167 29PM 632 7,290 138 7,260 5,880 3,750 1,900 950 3,517 161 23

CitratesATBC 8,900 121 5,620 3,430 1,730 783 396 941 167 46ATHC 3,990 140 3,990 3,370 2,010 842 401 403 173 33

Pentaerythrytol estersH 600 2,730 148 2,260 2,680 2,000 979 418 930 170 22H 707 2,750 154 1,240 2,690 2,540 1,470 676 1,322 172 18

CarboxylatesEHBDC 3,880 142 3,850 3,180 1,850 841 366 503 175 33DINCH 2,630 149 1,980 2,605 1,900 876 409 510 176 27

ASEASE 12,400 114 5,610 3,160 1,550 701 372 508 175 61



helps plastisols withstand the stresses evolvedcaused by the gas released during the decompositionof chemical blowing agent (CBA); this leads to foamsof better quality and more homogeneous bubble sizedistribution.

The corresponding data are shown in Table III,along with the g* values at the different temperaturesstudied. It was also observed (see the foam qualitydiscussion section of this article) that the best qualityfoams were generally provided by plastisols present-ing the highest TADC � Tg*max values among all of theplasticizers studied, for example, DHA (54�C), ATBC(46�C), and ASE (61�C). Considering these tempera-ture difference values in the case of phthalates, weobserved the same trend, as the best quality foamswere provided by plastisols of diethyl-phthalate(DEP) and di-isobutyl-phthalate (DIBP) (with differ-ence values of 95 and 84�C, respectively).

Figure 3 shows the TADC � Tg*max temperaturedifference values against the molecular weight forall 20 plasticizers studied in the previous part I ofthis series and in this part, except for the polymericadipates. A linear correlation was observed between

these two parameters (especially when the differentplasticizer families were considered). The two poly-meric adipate plasticizers departed from this trendmore strongly than the two pentaerythytols esters offatty acids.Figure 4 shows the maximum complex viscosity

(g*max) values against the TADC � Tg*max of the cor-responding formulations. The trends discussedwithin each family could be clearly seen.In Figure 5, the same parameter is shown, how-

ever, in this case, against the molecular weight of theplasticizer, and the polymeric adipates were alsoexcluded for better comparison. The correlation withthe difference of temperatures TADC � Tg*max seemedto be clearer than with the molecular weight, andeven the polymeric adipate type plasticizers werecloser to the general trend in this plot as comparedwith the molecular weight plot. It was apparent thatthe TADC � Tg*max temperature difference was aparameter more representative of the behavior of thesystem than the molecular weight itself because itcombined two other parameters related to the degreeof development of the structure of the plastisolsmore closely than the molecular weight did.

Evolution of the extensional viscosity

Studying the extensional flow29,30 of polymer meltsis extremely important because of the industrialrelevance in most of the polymer transformationprocesses31 (e.g., blow molding,32 fiber spinning,33

extrusion,34,35 injection molding,36 foaming7,12,37,38).In a previous article,3 we studied and discussed theplasticizer influence on the elongational behavior ofplastisol formulations prepared with nine phthalateester type plasticizers. It was found that plasticizersfollowed similar behavior with the molecular weightas described for g*; the plastisols of higher molecu-lar weight plasticizers developed higher extensional

Figure 3 TADC � Tg*max temperature difference (�C) ver-sus the molecular weight (g/mol) for the commercial plas-ticizers studied (data for phthalates from ref. 3), excludingthe polymeric adipates.

Figure 4 g*max (Pa s) values at the TADC � Tg*max tem-perature difference (�C) for the 20 commercial plasticizersstudied (data for phthalates from ref. 3).

Figure 5 g*max (Pa s) versus the molecular weight (g/mol)for the commercial plasticizers (data for phthalates fromref. 3) studied excluding the polymeric adipates.



viscosities. However, this relation was not as clearas in the case of g*. Furthermore, the extensionalviscosity decreased with temperature, as expected,and the shear thickening39 seemed also to decreasewith the temperature. This was in good agreementwith the results reported by Sugimoto40 in the studyof the rheological properties of PVC-plasticized sys-tems and the relation with the extensional viscosity.Figure 6 shows the extensional viscosities of the pre-viously cured (at 180�C for 10 min) samples pre-pared with ATBC under the following measuringconditions: a 0.1-s�1 prestretch, a 5-s�1 extensionalrate, and three temperatures, 160, 170, and 180�C.This is shown as an example of the trends shown bythe 11 plasticizers studied. In this case, when westudied 11 different plasticizers of six chemical fami-lies, the same conclusions could be deduced, as inthe case of the phthalates. Because all of the curvesobtained presented similar trends, to compare anddiscuss the results, we decided to analyze the exten-sional viscosity values obtained at 0.2 s in the exten-sional experiment.

Table IV shows the summary of these data, andFigures 7–9 show the corresponding graphs at thethree temperatures studied and for all of the plasti-cizers (monomeric adipates, citrates, carboxylates,pentaerythrytols, and the alkyl sulfonate) studied.

In Figures 7, 8, and 9, it can be observed that gen-erally, the extensional viscosity decreased with thetemperature. Furthermore, the extensional viscosityincreased with the molecular weight of the plasti-cizer used. All of the plasticizers followed this trend;however, ATBC and DHA had abnormal behavior,which can clearly be seen in Figures 8 and 9. Thiscould have been due to the high volatility of thesetwo plasticizers, as discussed in the case of g*.ATBC and DHA were the two plasticizers with thelowest boiling points (327 and 315�C, respectively)of the 11 studied in this work. Consequently, plasti-

cizer losses were likely to occur when we obtainedthe different test specimens studied, depending onthe processing conditions. In the extensional viscos-ity measurements, the plasticizer evaporation waseven more probable because the samples preparedto such measurements were precured at 180�Cfor 10 min in an open mold. A further reason wasthat these samples were measured at elevatedtemperatures (160, 170, and 180�C) during a longerperiod of time (including the preconditioning of thesamples). A similar situation was observed in a pre-vious study3 in the determination of the extensionalviscosity of plastisols prepared with DIBP and DEP,although in that case, the effect observed was evenmore marked than in this case. This was ingood agreement with the very low boiling points ofthese two plasticizers (i.e., DIBP at 327�C and DEPat 299�C).Thus, to obtain reliable results, the actual amount

of plasticizer in the test specimen should be known.(This may not be an easy task because evaporationof the more volatile plasticizers may occur when theexperiment is carried out.)

Thermal behavior studied by DSC

The DSC thermograms of the studied plastisol for-mulations always presented two exothermic peaks,as in the previous systems studied.3,13 The first one,at about 60–80�C, was probably due to the swellingof the resin, and the second one, at about 140–160�C,corresponded to the decomposition of the chemicalblowing agent (TADC). In Table V, the two peaktemperatures (Ts and TADC), along with the corre-sponding involved heats, are presented.

TABLE IVSummary of the Results of the Extensional Viscosity

Plasticizer

Extensionalviscosity(Pa s) at

160�C/0.2 s


170�C/0.2 s


180�C/0.2 s

Monomeric adipatesDHA 6,544 8,009 5,610DINA 6,446 3,940 1,749

Polymeric adipatesPM 652 15,904 7,847 4,169PM 632 23,505 11,397 4,313

CitratesATBC 9,189 7,167 5,188ATHC 12,183 4,249 2,417

Pentaerythrytol estersH 600 8,481 5,351 2,972H 707 15,240 8,376 3,355

CarboxylatesEHBDC 6,944 2,685 1,711DINCH 9,442 3,630 2,390

ASEASE 6,021 3,322 1,401

Figure 6 Evolution of the extensional viscosity (Pa s) ofthe plastisol prepared with the ATBC plasticizer at 160,170, and 180�C measuring temperatures.



Both processes and their corresponding tempera-tures were strongly influenced by the type of plasti-cizer and its molecular weight. In the foamingprocess, the decomposition of the CBA had to beadequately synchronized with the viscosity evolu-tion of the plastisol to obtain good quality foams.The temperature of the swelling of the resinincreased with the molecular weight of all of theplasticizers (except for the polymeric adipates). Thisshowed the higher compatibility of the lighter plasti-cizers. TADC was influenced by the nature of theplasticizer and increased with the molecular weightof the plasticizer (again, except for the polymericadipates), as we have shown in previous articles.However, samples presenting the same TADC mayhave led to foams of very different quality. This canbe explained with the different stages of the evolu-tion of g* and the melt strength of the correspond-ing plastisol. Some plastisols at the moment ofdecomposition might have already developed theirproperties and, therefore, had the required meltstrength and final structure (completely gelled) and

were able to provide good quality foams. Others(e.g., the plastisols of less compatible plasticizers)may not yet have finished their development, andthe samples could not withstand the pressure devel-oped by the releasing gases and, consequently,yielded poor quality foams.

Foam characterization

In this section of the article, the TMA penetrationresults, foam density and morphology, and bubblesize distribution are presented and discussed withconsideration of the plasticizer influence type on thefoam formation and quality to thus characterize thefinal foamed product.

Resistance to penetration as studied by TMA

The resistance to penetration of the foams obtainedby rotational molding was measured under the sameconditions described in part I of this series.3 As thefoam production occurred in a closed mold, thefoams presented similar values of the nominal den-sity, although they differed in quality. Many of themshowed very good uniformity in bubble size distri-bution (e.g., ASE, EHBDC, DINCH), whereas otherswere of poor quality and showed several types ofdefects (e.g., PM 632, PM 652). Uniform foams had auniform density close to the nominal one, whereaspoor foams had a nonuniform density and showedlarge bubbles and denser parts with larger densitiesthan their nominal one. Large bubbles significantlyaffected the density of the sample. Table VI showsthe density of the uniform part of the obtainedfoams, the measured average bubble size, and thestandard deviation. These density values were usedto normalize the reported TMA curves.Figures 10 and 11 show the TMA curves of the

obtained foams, normalized with the corresponding

Figure 8 Extensional viscosity (Pa s) values at a 170�Cmeasuring temperature at 0.1-s�1 prestretch and a 5-s�1

extensional rate against the molecular weight (g/mol) ofthe monomeric adipates, citrates, carboxylates, pentaery-thrytols, and alkyl sulfonate plasticizers.







foam density. The softest foams (i.e., those present-ing the larger dimension changes) were producedfrom the formulations prepared with DHA, EHBDC,and DINA. A correlation of these curves wasobserved with the molecular weight. Generally, thehigher the molecular weight was, the higher the re-sistance of the sample was, as expected. However,the forthcoming foams did not follow this trendwith the molecular weight, which could also beunderstood when we considered the fact that theyhad different functional groups. Nevertheless, themonomeric and polymeric adipates, as well as thecarboxylates, did follow the trend within the same

family. It could also be observed that citrates pre-sented quite similar resistance to penetration, andASE followed the same behavior as the pentaery-thrytols and had similar resistance as the citrates.

Bubble size distribution

The bubble size distribution was obtained asdescribed in the Experimental section. The averagebubble size and the standard deviation are two pa-rameters that are widely used to characterize thedistributions and are shown in Table VI.Figure 12 shows the average bubble size versus

TADC � Tg*max It could be observed that plasticizersshowing higher TADC � Tg*max temperature differen-ces yielded smaller bubble sizes and more uniformfoams. This could have been a consequence of theevolution of the properties of the plastisol with tem-perature; that is, more compatible plasticizers pro-vided higher TADC � Tg*max values; thus, they had

Figure 10 Resistance to penetration: adipate and citrateplasticizers.

TABLE VIMeasured Foam Density of the Final Foam, Average

Bubble Size, and Standard Deviation

Plasticizer

Measuredfoam

density(g/cm3)

Averagebubble

size (mm2)

Standarddeviation(mm2)

Monomeric adipatesDHA 0.352 0.268 0.032DINA 0.326 0.349 0.035

Polymeric adipatesPM 652 0.615 0.455 0.056PM 632 0.611 0.574 0.068

CitratesATBC 0.325 0.253 0.027ATHC 0.328 0.283 0.027

Pentaerythrytol estersH600 0.366 0.321 0.031H707 0.360 0.376 0.045

CarboxylatesEHBDC 0.247 0.243 0.022DINCH 0.318 0.246 0.019

ASE 0.224 0.020ASE 0.350 0.349 0.035

TABLE VSummary of the Results from DSC

PlasticizerFirst peak

temperature (�C)First peakheat (J/g)

Second peaktemperature (�C)

Secondpeak heat (J/g)

Monomeric adipatesDHA 67.6 0.99 169.8 11.04DINA 70.7 0.35 173.3 11.91

Polymeric adipatesPM 652 74.0 0.78 166.8 8.94PM 632 69.2 0.75 161.3 8.72

CitratesATBC 66.7 1.03 167.2 8.91ATHC 72.1 0.50 172.9 11.02

Pentaerythrytol estersH 600 72.4 0.52 170.4 11.94H 707 76.8 0.62 171.9 12.49

CarboxylatesEHBDC 72.0 0.18 175.4 11.49DINCH 75.2 0.32 176.2 11.96

ASEASE 66.6 0.58 174.7 10.79



probably already developed their final structure andproperties, such as melt strength; this is importantin the foaming process for the ability to withstandthe pressure caused by the released gases.

Foam morphology

In Figure 13, photographs of the foams show theirmorphology, the used plasticizer, the average bubblesize. and the standard deviation values. As a refer-ence, also shown is a photo of foam prepared from awidely used phthalate ester type plasticizer (DINP),which is marked with a black frame. The photo-graphs are arranged according to the quality of thefoam obtained. It could be observed that foams ofthe best quality (i.e., smallest average size and distri-bution) were those prepared with ASE, EHBDC,DINCH, DINP (from a previous work3), ATBC, andDHA. ASE and the two carboxylate-type plasticizersled to foams of very similar quality, and all of themprovided even better foams than a widely used com-mercial phthalate, DINP. Thus, they can be used asplasticizer alternatives to phthalates for the process-

ing of good quality foams. Polymeric adipates aredefinitely inadequate for producing uniform foams.

CONCLUSIONS

Plastisol gelation and fusion and thermal transitionsof the paste and blowing agent decomposition pro-cess are strongly influenced by the type, chemicalstructure, chain structure, compatibility, and molecu-lar weight of the plasticizer.Studying the extensional viscosity of the plastisols

may provide complementary information to g* forunderstanding the results of the foaming process.The temperature difference TADC � Tg*max proved

to be a very useful parameter for studying the foam-ing process because it provided a measure of theextent of evolution of the plastisol propertiesreferred to the decomposition of the blowing agent.Plasticizers showing the largest temperature differ-ence TADC � Tg*max yielded better foams.Characterizing the final foamed product, we con-

cluded that the polymeric adipates were not the bestoption to choose for the production of uniform foams.

Figure 13 Photographs of the foams obtained, presenting the plasticizer, the average cell size, and its distribution (mm2),showing a reference scale of 1 mm, and a photograph of a foam prepared with the DINP commercial phthalateplasticizer.

Figure 12 Average bubble size (mm2) versus TADC �Tg*max (�C) in the case of all the 20 plasticizers studied.

Figure 11 Resistance to penetration: pentaerythrytol, car-boxylate, and alkyl sulfonate plasticizers.



Nevertheless, it can be emphasized that foams of verygood quality were prepared with ASE, EHBDC,DINCH, ATBC, and DHA, with very similar proper-ties to those obtained with the phthalates. Thus,according to the specific requirements for the end-useproperties and depending on price and availability,these nonphthalate plasticizers can be used as valuablealternatives, especially in applications where healthyand environmentally friendly products are needed.

Consequently, as a final conclusion, we can statethat knowledge of the evolution of g* with tempera-ture and the blowing agent decomposition, alongwith the extensional viscosity, are key factors thathave to be considered when one chooses a plasti-cizer (either phthalate or alternative) for the produc-tion of foams of required properties.

The authors thank the Program of Doctoral Grants for East-ern Europe (Programa de Becas de Doctorado Europa delEste) of the University of Alicante, Department for Interna-tional Relations and Cooperation (Vicerrectorado de Rela-ciones Internacionales y Cooperacion).

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