Melt Compounding Based Rotational Foam Molding Technology ...static.tongtianta.site/paper_pdf/efcc6b78-23c9-11e... · However, like its predecessor, the rotational foam molding technology

Melt Compounding Based RotationalFoam Molding Technology for

Manufacture of Polypropylene Foams

REMON POP-ILIEV AND CHUL B. PARK*Microcellular Plastics Manufacturing Laboratory

Department of Mechanical & Industrial EngineeringUniversity of Toronto

Toronto, Ontario, Canada M5S 3G8

ABSTRACT: This paper is intended to provide an engineering understanding of the tech-nological potentials for processing polypropylene (PP) foams in rotational foam molding. Aprocess proposal, based on the melt compounding material-preparation approach, capable ofproducing completely foamed, single-layer, single-piece PP products in rotational foammolding, is disclosed in detail. It comprises dispersing a chemical blowing agent (CBA) inthe PP matrix using a twin-screw compounder, pelletizing the obtained expandable composi-tion, and then producing foams in an uninterrupted rotational foam molding cycle by usingthe pre-compounded foamable pellets. Several PP grades were deliberately selected to covera wide range of melt flow rates (MFR), starting from 5.5 up to 35 dg/min. After the raw mate-rials participating in the study were characterized using thermal analysis instrumentation,different foamable compositions were formulated in order to prepare both 3-fold and 6-foldfoamable pellets from each PP grade. The optimal foam processing strategies were identifiedvia a systematic experimental parametric search. Foams with the best cell morphologieswere obtained out of the high melt strength PP grades. In addition, the experimental resultsrevealed that the cell morphology of the processed PP foams is not as good as that of respec-tive PE foams. However, the cell morphologies of the PP foams processed by using the meltcompounding-based approach demonstrated significant improvements in comparison withthose processed by using the dry blending-based approach.

INTRODUCTION

PLASTIC FOAMS COMPRISE at least two phases, a solid polymer matrix, and a gas-eous phase originating from at least one gas-generating substance such as a

blowing agent [1–3]. The cellular structure of plastic foams enhances the perfor-mance of foamed plastic articles by generating unique properties, such as load bear-

101Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 21, No. 2/2002

0731-6844/02/02 0101–20 $10.00/0 DOI: 10.1106/073168402024285© 2002 Sage Publications

This revised paper was presented in its original form at the ANTEC 2000 Conference Proceedings, May 7–11, 2000, Or-lando, FL, and the copyright is held by the Society of Plastics Engineers.

*Author to whom correspondence should be addressed. E-Mail: [email protected]

http:\\www.sagepublications.com

ing, cushioning, impact resistance, insulation, and buoyancy. Since a proportionalrelationship exists between the mechanical-strength properties of a foamed plasticarticle and the density of its foam [1,4], foamed plastic articles can often be strongerthan their non-foamed analogues. In addition, due to the substantially reducedweight, foamed plastic articles can achieve outstanding cost-to-performance andfavorable strength-to-weight ratios [5]. For these reasons, recently, the rotationalmolding technology, although originally designed for producing hollow plastic ar-ticles, has been deliberately redesigned to serve as a fabrication process for plasticfoams [5]. The process that was thereby rendered capable of creating a foam layeror core in the interior of the molded article is commonly referred to as rotationalfoam molding. It is effectively used for producing completely or partially foamedsingle-piece plastic articles with or without a non-foamed skin surrounding thefoamed core or layer. Since the rotational foam molding process inherited the capa-bility of delivering very large products while maintaining a low tooling budgetfrom the conventional rotational molding, it should be regarded as the process ofchoice for economic fabrication of very large hollow articles that possess improvedstiffness-to-weight ratios and high surface quality without added finishing ex-penses.

The same four-step production principle applies for both the original and themodified rotational molding process. It comprises four consecutive productionstages that include (1) mold charging, (2) mold rotation in two perpendicular axeswhile heating the mold, (3) mold rotation in two perpendicular axes while coolingthe mold, and (4) part release. Therefore, no major investments in hardware areneeded to use the currently available conventional rotational molding equipmentfor rotational foam molding production [6,7,12]. However, if the molds were to beused for rotational foam molding, a few marginal modifications would be neces-sary. In this context, since fully foamed parts normally will not shrink as much asconventionally rotationally molded parts, greater mold draft angles should be con-sidered for easier part release. Also, the rotational foam molding process requiresextended total cycle times (from 25 to 100%) to allow for complete foaming be-cause a greater amount of material is molded and the heat transfer is retarded by thefoam structure [12].

In order to produce plastic foams in rotational foam molding, a foamable resinmust be introduced into the mold so that melt sintering becomes replaced by a se-quence of plastic foaming stages that result in products with a cellular structure.However, like its predecessor, the rotational foam molding technology is based ona low-pressure (atmospheric) process, and therefore, the use of physical blowingagents for plastics foaming in rotational foam molding is not feasible. As a conse-quence, the foamable resin can be obtained only by introducing a CBA into thepolymer by means of a suitable polymer mixing technique [6–11]. With respect tothis mixing technique, rotational foam molding methods can be based on a dryblending or a melt compounding approach [7,10]. By decreasing the blowing

102 REMON POP-ILIEV AND CHUL B. PARK

agent to polymer ratio in the foamable resin, partially foamed single-layer hollowarticles can be manufactured in rotational foam molding [7,10,11]. In order to pro-duce a non-foamed skin surrounding completely the foamed core or layer, twotypes of resins should be charged into the mold: a non-foamable and a foamable. Ifa skinless foam is desired, only a foamable resin should be used [1,5].

It is important to note that this paper mainly deals with skinless foams that oc-cupy the mold to the full capacity. The reasons for this lie in the intention to mini-mize the number of independently controllable processing parameters participat-ing in the rotational foam molding process, and thereby simplify the investigationof their effects on the foam production. However, the concepts put forward hereinare equally applicable for partially foamed, hollow, skinless, articles, and can bealso used as a foundation for understanding and describing the processing of arti-cles having a distinct unfoamed skin.

PROCESS CONCEPT

Figure 1 illustrates the detailed process diagram of the proposed concept for amelt compounding based rotational foam molding process for producing finecelled PP foams. It comprises two distinct, but technologically strongly relatedstages, a material preparation stage and a foam production stage.

The role of the material preparation stage is to provide a foamable PP composi-tion in a form that would be suitable for use in the subsequent foam productionstage. The material preparation stage should normally comprise three consecutivesteps: grinding, dry blending, and melt compounding. In fact, the grinding stepwould be necessary in case the PP resins are not available in a powder form, but ifthey are, it should be omitted. Thus, once the PP resins would be pulverized in thegrinding step, they would have to be premixed with the CBA in the dry blendingstep, and then the obtained PP/CBA mixture would be transferred to themelt-compounding step. If necessary, during the dry-blending step, the PP pow-ders could be also premixed with other additives (e.g., activator, antioxidant, andothers) which are usually supplied in a powder form. The principal role of the meltcompounding step would be to disperse thoroughly the CBA particles into the PPmatrix in a molten state by means of extrusion compounding while simultaneouslykeeping the CBA particles inactivated (non-decomposed). The obtained foamablePP extrudate in such a way would be then conserved into pellets by first beingcooled while passing through a cooling bath, and then chopped into pellets by us-ing a pelletizer. The presence of non-decomposed CBA particles in the pelletswould be the crucial condition for their subsequent successful foaming. Hence,production of PP/CBA compounded pellets while minimizing the decompositionof the CBA would be the principal issue of concern in the material preparationstage. The intrinsic nature of the basic PP resin and the desired quality of the foamwould determine if the precompounded foamable PP pellets could be used in the

Melt Compounding Based Rotational Foam Molding Technology 103


Figure 1. Detailed diagram of the proposed process concept for melt compounding basedrotational foam molding of fine celled PP foams.

foam production stage with or without an intermediate regrinding intervention.This indicates that regrinding of the foamable PP pellets prior to using them in ro-tational foam molding would be optional. The obtained foamable PP compositionscould be then either stored or used immediately which indicates that the facilitiesfor material preparation and foam production would not have to be physicallyclose or time related in any way.

The latter part of the process comprises the foam production stage. It would be-gin with pouring a predetermined shot size of foamable PP compositions into anopen hollow metal mold after which it would be closed and inserted into a hot ovenwhile rotating. Because of the temperature gradient, the rotating mold, thefoamable compositions, and the air inside the mold, would become graduallyheated. Consequently, a sequence of thermally driven events would take place in-side the mold. Thus, the foamable compositions closest to the mold’s wall wouldbegin to sinter first, and then sintering would continue to extend towards themold’s center. The CBA dispersed inside the polymer should decompose shortlyafter the sintering of the entire material in the mold is completed. The gases re-leased in the melt due to the CBA decomposition would cause the foaming of thePP material. The polymer foaming should first take place at the mold’s wall andthen spread towards its center. The mold would be then cooled and the foamed PPproduct removed, which would render the processing cycle accomplished.Normally, the ultimate goal of this process concept would be to ensure qualitycompliance of the PP foams produced in the foam production stage with certainanticipated or given criteria.

Proposed PP Foaming Model

In order to achieve a cellular PP structure by using the foamable PPpre-compounded pellets in rotational foam molding, a special sequence of pro-cessing steps should be maintained. However, the narrow gap between the meltingtemperature of PP and the onset decomposition temperature of the CBA, togetherwith the low melt strength of PP at elevated temperatures, often represent thegreatest obstacles in the foaming of PP. Therefore, they render the desired foamingsequence of PP in rotational foam molding to be achievable only in rare, almostideal cases. Figure 2 illustrates the proposed ideal processing sequence of steps forachieving PP foams in rotational foam molding.

STEP #1: MOLD CHARGINGAt cycle time t0 � 0, a predetermined amount of foamable PP pellets which in-

clude uniformly distributed CBA particles should be charged in the mold. Themold is then closed and rotated while heated in the oven. Here, the assumed crite-rion for the achieved degree of uniformity of the CBA particle dispersion through-out the pellets is based on the CBA inter-particle distances. Thus, if the CBA inter-


particle distances are in the range between 0.50 �m, the CBA dispersion is consid-ered uniform.

STEP #2: SINTERINGAt cycle time t1 (t1 > t0) because of the temperature gradient, the pellets should

gradually begin to sinter. The pellets closest to the mold’s wall begin to sinter first,and then the sintering extends towards the mold’s center. At cycle time t2 (t2 > t1),shortly prior to the commencement of the decomposition of the CBA, a continuouspolymer matrix is formed.

STEP #3: DECOMPOSITION OF CBAAt cycle time t3 (t3 > t2), the temperature of the melt is further increased and

reaches the decomposition temperature of the CBA. This is the point in time whenthe CBA particles dispersed in the molten polymer matrix should start to decom-pose and thereby generate gases.

STEP #4: FOAMINGCell Nucleation: Simultaneously with the decomposition of the CBA parti-

cles, at cycle time t3, because of the generated gases near the CBA particles, bub-bles begin to appear. This is how cell nucleation sites are created. The distributionof the cell nucleation sites will follow the distribution pattern of the CBA particles[6]. This is why a uniform dispersion of CBA is required for fine-celled PP foam-ing. Because the temperature near the internal mold wall surface is the highest, thefoaming occurs first at the mold’s wall and then spreads towards its center.


Figure 2. Desired sequence of events in rotational foam molding of fine-celled PP foams(ideal PP foaming model).

Cell Growth: At cycle time t4 (t4 > t3), once the cell nucleation has started, cellscontinue to grow until the CBA gas-generation ability is exhausted.

Stabilization: In general, the cell stabilization strategy should be related to thestability of the cell wall and the drainage of the material from the wall that sepa-rates the cells. Increasing the viscosity of the fluid reduces the drainage effect. Theviscosity increase may be caused by a temperature reduction.

STEP #5: COOLINGThe cooling cycle begins immediately after the heating cycle is completed. Be-

cause of the cooling action, the polymer gradually begins to solidify starting fromthe mold’s wall and eventually freezes. The actual moment of freezing is alwaysdelayed for a certain period relative to the moment when the cooling is applied.

STEP #6: PART RELEASEThe mold is opened and the part removed.

EXPERIMENTAL

Materials

RESINSFour PP resins, two branched and two non-branched, were subjected to experi-

mentation. The respective manufacturers describe the mentioned PP resins as fol-lows: PP1 is a high melt strength (HMS) PP homopolymer resin (Montell, PF633),PP2 is a HMS PP medium impact copolymer resin for extrusion coating (Montell,SD812), PP3 is a PP copolymer resin of a rotomolding grade (Millennium,MT4390 HU), PP4 is a nucleated, high flow, medium impact copolymer resin(Montell, SD242) [13,14]. Since PP3 was the only resin supplied in a powderform, it was used as supplied. The remaining three were supplied in a pellet formand were therefore subjected to grinding prior to further usage. Table 1 presentsthe typical properties of the PP resins participating in this study [13,14].

CBAThe modified azodicarbonamide that was selected to participate in the experi-

mental part of this study as a CBA is highly recommended by its manufacturer forrotational foam molding applications that involve PP resins yielding a fine and uni-form cell structure [15]. For convenience, throughout the next part of this paperthis CBA will be referred to as AZ. AZ was supplied in a form of a yellow fine pow-der with an average particle size of 2.4–3.0 �m. The operating temperature rangeof AZ is reported to be between 199 and 232�C [16]. Its decomposition tempera-ture range is from 205 to 215�C, while its gas-yield is around 220 cm3/g [16]. The


gaseous product released by the decomposition of AZ comprises 65% N2, 24%CO, 5% CO2, and 5% NH3, while its solid decomposition products, constitutingapproximately 68% of its original weight, comprise Urazol, Biurca, Cyamelideand Cyanuric Acid. AZ may be activated by metal organic salts, bases, and acidsreducing its decomposition temperature to as low as 165�C [16]. More details areavailable in Reference [6].

CBA ACTIVATORZinc oxide (ZnO) was selected as an AZ decomposition promoter for the pur-

pose of this study. The supplied ZnO was in a powder form, with a purity of 99.9%and a guaranteed particle size of less than 1 �m, according to its manufacturer’sdata [17].

Analytical Instruments

The analytical instruments used in this study include a thermogravimetric ana-lyzer (TGA, TA Instruments, TA2050), and a differential scanning calorimeter(DSC, TA Instruments, TA2910).

Experimental Setup

MATERIAL PREPARATION STAGEThe experimental setup for this experimentation segment comprises regu-

lar-scale industrial equipment. Accordingly, the three types of PP resins suppliedin a pellet form were pulverized using a grinder (Wedco, SE-12-SP), while the PPresin powders were dry blended with the additives using a high-speed mixer (Gun-ther Papenmaier K.G, TGAHK35). The melt compounding operation of thedry-blended mixture obtained in such a way was carried out by using a twin-screwintermeshing co-rotating extruder (Werner & Pfleiderer, ZSK-30) mentioned ear-lier. Finally, an Automatic Apparate Machinenbau pelletizer (ASG 100) was usedto chop the exrudate into pellets after it had been cooled in the cooling bath. Figure3 represents a schematic of the melt compounding experimental setup.


Table 1. Typical properties of the PP resins used.

Typical Resin PropertiesASTM

Method PP1 PP2 PP3 PP4

Melt flow rate, 230�C/2.16 kg, dg/min D1238 5.5 16 20 35Density at 23�C, g/cm3 D792B 0.90 0.90 0.90Density g/cm3 D1505 0.90Homopolymer/Copolymer H C C CBranched/Non-Branched B B NB NB

FOAM PRODUCTION STAGEFigure 4 illustrates the uni-axial lab-scale rotomolding machine used for rota-

tional foam molding experimentation. It consists of a sliding assembly plate, anelectrically heated oven, water-cooling installation, and control units for the oventemperature and the mold rotation. The sliding plate carries the uniaxial rotatingarm assembly that includes a motor, gearbox, transmission, a hollow shaft, and aremovable cylindrical mold. The rotating speed of the arm, which is driven by themotor via the gearbox and the belt-transmission pulleys, can be electronically con-trolled from 0–30 RPM. The water installation provides the means for mold cool-ing after completion of the heating cycle. A thermocouple, inserted into the oven,provides data to the oven temperature control unit. The hollow design of the shaftmakes possible the insertion of another thermocouple inside the center of the moldin order to measure the in-mold temperature changes during processing. By con-necting the thermocouple to a data acquisition device, the temperature changesover time occurring inside the mold can be recorded with a desired sampling fre-quency, and subsequently plotted. The diameter of the cylindrical mold used was31.75 mm (1.25�) and its length was 101.6 mm (4.00�).

RESULTS AND DISCUSSION

Material Characterization

PP RESINSFor successfully processing the selected PP resins in the proposed compound-


Figure 3. Experimental setup for melt compounding of PP foamable resins.

ing-based rotational foam molding process, the knowledge of the transition tem-peratures for each PP resin is of multiple importance. Table 2 provides the resultsobtained from the DSC thermal analysis tests conducted in order to determine thetransition temperatures of PP1, PP2, PP3, and PP4, respectively.

EFFECT OF ZnOA simple calculation can easily prove that the magnitude of the heating rate dur-

ing PP compounding usually reaches 100�C/min and beyond. Therefore, it wouldbe desirable to analyze its effects on the decomposition and activation behaviors ofAZ. Therefore, a series of thermal analysis experiments on samples of variousAZ/ZnO combinations were conducted by using a TGA. In this context, samplesof pure AZ and samples of mixtures between AZ and ZnO in five various concen-


Figure 4. Experimental setup for rotational foam molding.

Table 2. Transition temperatures of the participating PP resins (DSC).

PP ResinOnset Melting

Temperature, CPeak Melting

Temperature, CPeak Crystallization

Temperature, C

PP1 144.6 159.4 127.2PP2 141.3 163.8 127.2PP3 152.4 160.6 111.2PP4 149.2 164.5 132.4

trations, ranging from 0 to 100 phr of ZnO (phr � parts per hundred parts of CBA),were subjected to TGA experiments. In order to simulate the actual heating ratesduring PP compounding in a most appropriate way, three series of TGA experi-ments were conducted by using three different heating rates (50, 100, and150�C/min). Figure 5 presents the obtained TGA spectra that represent the effectof ZnO on the onset decomposition temperature of Celogen AZ-3990 for eachheating rate, respectively. The experimental results revealed that: (1) by increasingthe concentration of ZnO in the sample the onset decomposition temperature ofAZ decreases, and (2) the onset decomposition temperature of AZ additionally de-creases by increasing the heating rate.

Rotational Foam Molding

A series of parametric search rotational foam molding experiments were con-ducted to determine the effect of processing parameters on the cell morphology ofPP foams using the compounding based method.

COMPOUNDING FOAMABLE PP COMPOSITIONSThe presented experiments included 3-fold and 6-fold expansion; the calcu-

lated AZ amount needed for 3-fold expansion (VER � 3) was 0.73%, while for6-fold expansion (VER � 6) it was 1.83% [6]. By using the determined amounts ofCBA that should be dry-blended with the PP powders prior to compounding for3-fold and 6-fold expansion foams, 16 different formulations for foamable PPcompositions have been prepared. The formulations included one type of pelletsfor 6-fold and one type for 3-fold expansion with no activator and two additionaltypes of 6-fold expansion pellets with ZnO, out of each resin. The latter two typesof 6-fold formulations contained ZnO in a concentration of 10 and 50 parts perhundred parts of CBA (phr), respectively. By using these formulations and by fol-lowing the processing strategies proposed and by using the experimental setuppresented in Figure 3, sixteen different foamable PP compositions have been com-pounded.

All PP resins, except PP3, were ambient ground by using an industrial grinderafter being frozen at �40�C. The obtained powders were dry-blended for 60 sec-onds with the prescribed amounts of AZ (with or without ZnO) according to thepreviously prepared formulations, in 4 kilo batches by using an industrialhigh-speed mixer. The processing temperature during compounding was keptaround 150�C by setting the controls of the six heaters along the extruder barrel at150�C. But, the temperature of the melt measured during compounding and dis-played on the extruder’s control board was fluctuating in the range of 170–175�C,due to the additional heat generated by friction and shear. The screws were rotatingat 101 RPM. The extrudate was cooled in a cooling bath and then pelletized by us-ing an industrial pelletizer. The size of the obtained foamable PP pellets was 31 to



Figure 5. Effect of ZnO on the decomposition temperature of AZ at high heating rates: (a)50�C/min, (b) 100�C/min and (c) 150�C/min.

33 pellets per gram. Table 3 presents the compounded foamable PP compositionsand their formulations.

The melt compounding experimental results revealed that, except for the 4types of pre-compounded pellets originating from formulations including 50 phrZnO (P102P, P202P, P302P, and P402P), no signs of CBA pre-decomposition wereregistered among the 12 remaining foamable compositions. This indicates that anamount 50 phr ZnO reduces the decomposition temperature of AZ during com-pounding so much that decomposition-free compounding of PP becomes practi-cally impossible to achieve. Therefore, further experimentation was proceededwith only the three remaining foamable compositions out of each of the four PPresins. This indicated a very important conclusion regarding the melt compound-ing strategy that was earlier proposed, which is that the amount of ZnO in the for-mulation represents an additional constraint that has to be accommodated in orderto achieve decomposition-free compounding of PP. Consequently, the domain ofthe compounding strategy should be restricted to PP/AZ foamable formulationswithout or with low concentrations (up to 10 phr) of ZnO, since the production ofsuch foamable compositions has been experimentally verified.

ROTATIONAL FOAM MOLDING EXPERIMENTSBy using the already described experimental setup (see Figure 4), rotational

foam molding experiments have been conducted on the 12 foamable PP composi-tions the compounding of which has been described in the previous section. The


Table 3. Foamable PP formulations used and respective foam cellmorphology parameters obtained.

# Code PP Pellet Formulation Ver.Average Cell Size

(microns)

1 P101P PP1 + 1.83% AZ +10 phr ZnO 6 400–5002 P102P PP1 + 1.83% AZ +50 phr ZnO 6 pre-decomposition3 P103P PP1 + 1.83% AZ 6 350–4504 P104P PP1 + 0.73% AZ 3 450–550




optional regrinding stage of the process has been deliberately omitted. The mold,with a volume of 80 cm3, for 6-fold expansion needs a shot size of 12 g, while for3-fold expansion 24 g. During the experiments, the mold was inserted into theoven, preheated at Toven � 300�C. After a predetermined processing time, the moldwas removed from the oven and cooled with tap water. Then, the mold was openedand the obtained foamed PP sample removed.

FOAMING PROCESS CHARACTERIZATIONBy using the foamable PP compositions, and by using the thermocouple in-

serted in the center of the mold via the hollow shaft, time-temperature profileswere recorded for various foamable PP compositions. The temperature was mea-sured at the center of the mold while processing the materials at Toven � 300�C. Aspecial acquisition device for temperature data was used. The duration of the rota-tional foam molding heating cycle for the temperature profile data acquisition ex-periments was 30 min. while the mold cooling cycle was 10 min long. Since the se-lected sampling interval was only 0.5 seconds long, the obtained temperatureprofiles with respect to the elapsed processing time possess a high level of accu-racy. Figures 6 and 7 present four comparative temperature profiles, each of whichcombines characteristic thermograms pertaining to foamable formulations of eachPP basic resin under different experimental conditions at a fixed oven temperature(Toven � 300�C). In fact, Figures 6 and 7 illustrate the effect of the foamable PP for-mulation, i.e., the presence of ZnO and the desired VER (defined by the amount ofAZ and the shot size) on the temperature profile in rotational foam molding, foreach basic PP resin carrier, respectively. By using these temperature profiles, it be-comes possible to determine the heating rate inside the mold during the rotationalmolding experiments for each foamable PP composition. This is a sufficientlygood input information in order to estimate the temperature at the mold internalsurface, since its direct measurement in a continuous manner is not possible withthe present experimental setup. By carefully analyzing the obtained time-tempera-ture profiles, in was found that heating rates in the range of 15 to 30 �C/minoccur during the rotational foam molding process. The greatest heating rates occurduring the CBA decomposition. The sudden temperature peaks, shown on the tem-perature profiles, represent these heating rates. These peaks are caused by theself-heating effect of the AZ during decomposition due to its exothermic nature.Another benefit of the temperatureprofiles is the possibility to acquire informa-tion of the approximate time and temperature of the decomposition of AZ duringthe rotational foam molding process by analyzing the location of the temperaturepeaks on the plots.

PARAMETRIC SEARCHBy varying the value of a particular process variable of interest, while keeping

all the remaining variables participating in the process fixed, the effect of that vari-



Figure 6. Effect of foamable PP resin formulation and shot size on temperature profiles of PP1and PP2.


Figure 7. Effect of foamable PP resin formulation and shot size on temperature profiles of PP3and PP4.

able on the process with reference to the operating point can be accurately identi-fied. Therefore, experimental parametric search over time has been conducted byusing the foamable PP compositions available. The parametric search over time isintended to investigate the effect of the processing time while keeping all otherprocessing parameters fixed (e.g., type of foamable PP resin, oven temperature,mold RPMs), and eventually determine the optimal processing time for obtainingthe best quality of the foam. In addition, the effect of the magnitude of the shot sizeon PP foaming in rotational foam molding was investigated by a series of experi-ments using the same foamable PP resin in both 3-fold and 6-fold experiments byusing a shot size of 12 g and 24 g, respectively.

The experimental results presented in Figures 8 and 9 show typical representa-tives of the cell morphologies obtained for the respective VER and foamable for-mulation. As it can be inferred from the data representing the corresponding aver-age cell size presented in Table 3, the 3-fold expanded foams obtained by using3-fold low-viscosity foamable resins (PP2-, PP3-, and PP4-based and indicated as“3 � for 3�” in Figure 8) manifested coarser cell size and less satisfactory cell pop-ulation density in comparison with those obtained by using high-viscos-ity/branched foamable resins (PP1-based). Cell coalescence occurring at the partof the sample closest to the mold wall, which created an insulation zone to the cen-ter of the mold, characterized these experiments. Using 6-fold formulatedfoamable resins based on low-viscosity PP resins for 6-fold expansion (6 � for 6�)resulted in even poorer cell morphology and less satisfactory volume expansion.These results are also presented in Table 3. Severe cell coalescence characterizedthese samples. It is believed that due to the lower heat capacity of themold-material system caused by the reduced amount of material (12 g instead of24 g) charged into the mold for 6-fold expansion, the critical temperature of cell


Figure 8. Typical representatives of cell morphologies of three fold expanded low-viscosityPP foams in compounding-based rotational foam molding: (a) P204P � PP2 + 0.73% CBA1; (3� for 3�; Toven � 300�C; W � 24g; tprocessing � 22 min.), (b) P304P � PP3 + 0.73% CBA1; (3 � for3�; Toven � 300�C; W � 24g; tprocessing � 20 min.) and (c) P404P � PP4 + 0.73% CBA1; (3 � for3�; Toven � 300�C; W � 24g; tprocessing � 22 min.).

coalescence was reached much more quickly. Consequently, due to the unfavor-able combination of low melt strength of the basic PP resins (PP2, PP3, and PP4),and the high temperature, cell coalescence occurred during foaming.

However, using 6-fold formulated foamable materials for 3-fold expansion ex-periments (charging the mold with 24 g of 6-fold foamable resins) improved boththe cell morphology parameters and the volume expansion uniformity in all in-stances. It is also important to note that by using a 6-fold resin for 3-fold expansion(indicated in Figure 9 as “6 � for 3�”) the relative amount of CBA charged into themold is twice greater (0.73% vs. 1.83%). This indicates that further study is re-quired to determine the appropriate CBA amount needed for each VER. In addi-tion, it would be particularly important to determine why the “6 � for 6�” foamableformulations did not expand as much as expected. It is believed that the reasonmight be one or more of the following occurrences: (1) gas loss during compound-ing, (2) incomplete decomposition of the CBA during foaming, or (3) blowing gasloss during polymer sintering in the foam processing.

Clearly, the experimental results revealed a significant advantage in the cellmorphologies of the foams obtained by processing the highest-viscosity branchedfoamable PP compositions. The PPI-based foamable compositions almost reacheda fine-celled morphology and satisfactory expansion uniformity by achieving anaverage cell size in the range of 350 to 450 �m. The best cell morphology was ob-tained by processing P103P (PP1 +1.83% AZ without ZnO) when using 6-fold res-ins for 3-fold expansion [see Figure 9 (b)]. We believe that this could be explainedby the higher melt strength of PP1 due to the branching and high viscosity (MFR �

5.5 g/10 min). However, the results obtained by the similar foamable PP resin(P1O1P) containing 10 phr ZnO are satisfactory as well, due to the low concentra-tion of ZnO that cannot influence dramatically the onset decomposition tempera-ture of AZ.


Figure 9. Typical representatives of six and three fold cell morphologies of high-viscosity PPfoams in compounding-based rotational foam molding: (a) P103P � PP1 + 1.83% CBA1; (6 �

for 6�; Toven � 300�C; W � 12g; tprocessing � 21 min.), (b) P103P � PP1 + 1.83% CBA1; (6 � for 3�;Toven � 300�C; W � 24g; tprocessing � 22 min.) and (c) P101P � PP1 + 1.83% CBA1 + 10 phr ZnO;(6 � for 3�; Toven � 300�C; W � 24g; tprocessing � 19 min.).

SUMMARY AND CONCLUSIONS

Although the experimental results successfully proved the feasibility of theproposed compounding based process for rotational foam molding of PP foams,further research and process improvements are needed. The achieved cellmorphologies in the present study of compounding based rotational foam moldingof PP are less good than the respective results with PE [18], however they are sig-nificantly better if compared with those obtained in a dry-blending based rota-tional foam molding processing of PP foams [19]. Since to date compoundingbased rotational foam molding of PP foams has not been reported, the researchpresented in this paper paves the way towards developing a new improved technol-ogy for rotational foam molding of PP. A major benefit of the compounding is thereduced sensitivity of the process on powder quality inconsistencies. Since thepre-mixed blend is being remelted, remixed and reshaped into pellets during com-pounding, the negative effects that might have been caused by the inconsistenciesin powder quality in terms of particle size and shape are being removed. However,the properties of the pre-compounded pellets are constrained by the required abil-ity to foam while being heated and to expand to a predetermined volume by form-ing a uniform fine-cell PP cellular structure while foaming. Therefore, the idealpellet should be appropriately sized and should contain a sufficient amount of in-activated CBA particles commensurate with the required VER. In addition, theCBA particles should be thoroughly and uniformly dispersed among the other ad-ditives, if any, throughout the pellet polymer matrix at close inter-particle dis-tances. Yet, another advantage of the proposed process is the fact that, except forthe rotational foam molding trials, all experimentation has been conducted by us-ing industrial full-scale equipment. This indicates that for industrial implementa-tion of the process a minor modification of the rotational foam molding processingparameters would be necessary.

ACKNOWLEDGEMENTS

The authors are grateful to the WedTech Inc. for their financial support of thisresearch and for allowing the usage of the corporate industrial equipment for theexperiments. The authors are also thankful to Montell Canada and Network Poly-mers, Inc. for supplying the resins used in the experimental work.

REFERENCES

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