EXTRUSION FOAMING OF PET/PP BLENDS
C. Wan, M. Xanthos*, S. Dey* and Q. Zhang*
Multilifecycle Engineering Rearch Center
*Also: Polymer Processing Institute
施雨欣
New Jery Institute of Technology
Newark, NJ 07102
Abstract
In order to develop new applications for recyclable commingled resin streams, blends containing PET and PP resins with different rheological characteristics were dry blended or compounded at different ratios and subquently foamed by using PBAs and CBAs. Properties of the foamed blends were compared with tho of similar products obtained by foaming the individual PET and PP components in the abnce of compatibilizers/rheology modifiers. Foamed polymer blends with fine cell size and low density could be produced in the prence of suitable compatibilizer systems consisting of functionalized polyolefins or their combinations with reactive coagents
Introduction
By contrast to the commonly ud PS and LDPE resins, extrusion foaming of PET to low densities, (<0.2 g/cc), by injection of physical blowing agents (PBA) is a relatively new and increasingly active area prenting veral challenges. Difficulties are mostly related to the required high processing temperatures and the particular rheological characteristics, rate of crystallization and process stability of the resin. Similarly, foaming of conventional polypropylene grades to low densities, also prent difficulties due to the resin low melt strength that prevents uniform gas expansion and formation of stable cells. Significant developmental work has been conducted over the past fifteen years by resin producers and converters to develop reactor or post-reactor modified resins with high melt strength for low density foaming in single or tandem lines.
Both PET and PP can be foamed to higher than >0.5 g/cc densities by lecting suitable chemical blowing agents. In our earlier work with PET (1) we have shown that resin rheology is not as important for CBA foaming as for PBA foaming and stable foams can be produced with commodity PET or PP grades as long as they meet certain minimum melt viscosity and melt elasticity requirements. For example, standard thermoforming grades of PET with IV’s ranging from 0.83 and 0.96 could not, in general, be foamed to densities lower than 0.7 g/cc in flat sheet dies when carbon
dioxide was ud as a blowing agent. (2). Such density values are not that different from the lowest density values attainable with certain CBAs ud with the same resins. However, lower density foams, (0.2 g/cc density), could be produced with carbon dioxide blowing agent in the same equipment, only when the PET was modified with reactive multifunctional compounds containing for example, acid, anhydride, epoxy or hydroxyl functionality (2,3).
In this article, an attempt is made to produce extrusion foamed PET/PP blends at different resin weight ratios with either PBAs or CBAs. Virgin resins were ud to prepare compositions mimicking mixtures of post-industrial, post-consumer materials. The ultimate objective is to asss the possibility of using recyclable mixed waste streams containing resins with different rheological characteristics in order to produce novel low-density items. The inherent lack of miscibility of the PET/PP pair that would lead to technologically incompatible systems is addresd through the u of reactive additives that are expected to affect, not only morphology but, also rheology of the complex, multipha, multicomponent systems (4).
Experimental
Foaming with Carbon Dioxide
Resins ud in the foaming experiments were predried PET (Shell Traytuf 9506, 0.96 IV) and Polypropylene PP (EXXON Escorene PD 9374 MED). A commercially available polyolefin bad copolymer containing acrylic acid (Primacor 3460) was ud as potential compatibilizer, in some cas in the prence of
a multifunctional coagent that was expected to react with the compatibilizer and the PET end groups, thus, acting as a bridging agent. A 34 mm diameter 40 L/D long co-rotating intermeshing twin-screw extruder was ud. Dry blended material with no additional nucleating agent was meter fed into the hopper. The screw of the extruder was designed to achieve melting within 10 L/D length with the gaous blowing agent injection ction immediately after. The rest of the length of the screw was ud for mixing the blowing agent with the melt, pressurizing and cooling the gas laden melt to an optimum level of pressure and temperature. The melt was extruded through a 3 mm diameter rod die, expansion taking place thereafter. Density and cell size, and thermal properties of the foamed extrudates were measured.
气排球比赛规则
Foaming with Chemical Blowing Agents
Resins ud in the foaming experiments, combined with vendors’ information, are listed in Table 2.
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Prior to extrusion foaming, material combinations were evaluated in a Brabender batch mixer at 270o C and 60 rpm under nitrogen. PET/PP blends were compounded in a Werner&Pfleiderer co-rotating twin-screw extruder with or without compatibilizers. Extrusion foaming was carried out in a Brabender 19 mm single screw extruder equipped with 1 inch slit die in the prence of 1% CBA (Expandex 5PT, Uniroyal Chem.). Conditions are listed in Table 4. Samples approx. 1-1.5 mm in thickness were extruded at screw speeds ranging from 30 to 80 rpm, and die temperatures ranging from 200 to 250o C; die pressure was monitored throughout the experiments (Table 4). The extrudates were characterized for density, processability such as melt sagging, appearance and thermal properties.
Results and Discussion
Foaming with Carbon Dioxide
The PET homopolymer did not produce good quality foam (Table 1). The average density of the foamed rod was 0.41 g/cc with uneven diameter, rough skin and non-uniform cell size. The die pressure was also found to fluctuate widely. In preliminary experiments with binary blends of PP and PET, it was obrved that the density of foams went through a minimum at 20% by wt. PP to a value
of about 0.26 g/cc. For this composition, operating conditions were stable and extrudate surface was relatively smooth. However, cells were coar and the average cell size of the foam incread from about 0.95 mm for PET to about 1.5 mm at 20 wt% PP.
In subquent runs, a dry blend of 80% PET, 15% PP and 5% compatibilizer was ud and a foam of density of 0.25 g/cc was obtained (Fig. 1). The extrusion process was also found to be very stable (Table 1). Though the decrea in density was marginal, the average cell size of the compatibilized foamed blend was found to be about 900 µm versus about 1500 µm for the uncompatibilized foamed blend (Fig. 2). The u of the reactive coagent to enhance compatibility had a great effect on both density (0.17 g/cc) and cell size (180 µm) as shown in Figs. 1 and 2. The peak melting temperatures of PET and PP and the % crystallinity of PET were not significantly affected by the addition of compatibilizer as shown in Table 2. However, when the compatibilizer or compatibilizer/coagent combination was ud, the crystallization peak temperatures of PET and PP were shifted to much lower values. This is presumably the result of enhanced interactions between the blend components delaying their ability to crystallize. The results are in agreement with DSC data reported earlier on PET/PP and PET/PP-g-AA unfoamed blends (5). Foaming with Chemical Blowing Agents
The viscosity and elasticity/melt strength of the PET ud in this study could be upgraded through th
e addition of the reactive copolymers promoting chain extension/branching reactions (Table 3). The effects are more pronounced for the E-GMA copolymer; torque, in this ca, kept increasing even at 9 min. after addition of the modifier. By contrast, following an initial increa upon the addition of Primacor 3460, torque tended to decrea thereafter. The PP ud in the experiments had much lower viscosity than the PET at process conditions and appeared to degrade rapidly at the high melt temperatures. For example, the torque ratio of PET/PP (25/75) was 5.5 after three minutes and incread to a value of 9 after mixing for nine minutes. Comparison of the Brabender torque values, (data not shown), for combinations of PET/PP/compatibilizer (25/70/5) and PET/PP (25/75) suggest that certain viscosity/elasticity changes resulting from the addition of the modifier are translated into higher torque values. The high concentration of the low viscosity PP appears, however, to dilute the effects.
Data shown in Table 5 suggest that although no significant changes in foam density were obrved in the ca of compatibilizer containing blends, melt strength as related to sagging tendency improved. Similarly, appearance and dimensional stability were judged to be superior to tho of the uncompatibilized PET/PP blends.
Table 5 shows thermal property data for the foamed homopolymers and blends. As shown earlier in
石岩水库
the ca of PBA foaming, the prence of the reactive copolymers suppress and/or delay crystallization of the PET. In fact, at 5% Primacor, no crystallization peak temperature is obrved. This is significant information in designing extrusion foaming process, since in addition to the viscoelastic properties of the resins, their ability to crystallize, the rate of crystallization and the interference of crystal nucleation with the bubble nucleation process are known to be of importance.
Conclusions
野鸡习性
The preliminary results prented in this article show that a low density foam with fine cell size can be obtained by blending PET, PP, compatibilizer and a suitable coagent followed by foaming with carbon dioxide in a twin screw extruder. The u of PP or polyolefinic bad compatibilizer did not affect the melting peak temperature of PET or the PET crystallinity in the foam, although it decread its crystallization temperature. As expected, densities of PET/PP blends produced with CBA’s were higher. For such blends containing 25% PET in a polyolefin matrix, the u of reactive compatibilizer was shown to improve overall process stability and product characteristics.
Acknowledgements
The authors would like to thank Dr.V.Tan of PPI for his assistance during the experimental characteri
zation work.
References
1.M. Xanthos, Q. Zhang, S.K. Dey, Y. Li and U.
Yilmazer, J. Cell. Plastics, 34, 6, 498 (1998).
2.M. Xanthos, U. Yilmazer, S.K. Dey, and J. Quintans
Polym. Eng. Sci., 40, 3, 554 (2000).
3.M. Xanthos, M-W. Young, G.P. Karayannidis and
D.N. Bikiaris, “Reactive Modification of
Polyethylene Terephthalate With Polyepoxides”, Polym. Eng. Sci., (in press)
4. A.L. Bisio and M. Xanthos, Eds., “How to Manage
Plastics Waste: Technology and Market Opportunities”, Carl Hanr Verlag, Munich, New York (1994).
5.M. Xanthos, M.W. Young, and J.A. Bienberger,
Polym. Eng. Sci. , 30, 355 (1990).Keywords: Extrusion foaming, PET, PP, polymer
blends, compatibilizer
type of Additive
Figure 2. Blend Foam Cell size vs. %
and type of Additive
Table 1. Properties of carbon dioxide foamed PET/PP (80/20) blends with/without compatibilizer
(PP – Escorene PD 9374 MED, PET – Traytuf 9506)Melting Peak (o C)Crystallization.Peak (o C)Crystallinity Of PET(%)Surface appearance Process stability Composition
PP
PET PET PP PET
N/A 250.2196.9N/A 30.1Unacceptable Unstable 80%PET + 20%PP 145.3248.8199.2109.030.3Smooth Stable 80%PET + 15%PP +5%comp.
146.1250.9179.391.231.7Smooth Stable 80%PET+ 15%PP +5% (comp.+ coagent)
145.3
246.3
161.5
102.2
26.4
Very smooth
Stable
Table 2 Properties of materials ud in CBA Foaming
Materials Supplier Description
PET (recycled)Wellman IV=0.7
PP Achieve 3825
ExxonMobil MFR=32g/min @230o C/2.16kg,Primacor 3460 (ethylene- acrylic acid copolymer)
Dow MFR=20g/min @ 190 o C/2.16kg,M.P.=95 o C by DSC
Lotader AX8840 (E-GMA,copolymer)
Atochem
MFR=5g/10min @ 190 o C/2.16kg,M.P.=108 o C by DSC
Table 3 Batch mixer results of PET with reactive copolymers
Torque (Γ, Nm) , Temperature (T, o C)Composition
Additive added after PET torque stabilized (3~4min)
Before addition After addition,3min After addition,9min Primacor 3460Γ=2.5T=270Γ=3.1T=270Γ=2.9T=273PET+ additive (25:5 by wt.)
Lotader AX8840
怎么样才能赚到钱八月瓜Γ=2.5T=268Γ=7.2T=273Γ=9.3T=275
Table 4. Processing conditions of PET/PP foams using 1% CBA
Processing Conditions
Temperature profile (o C)Materials
Die pressure (psi)T 1T 2T 3T die
Feed rate (lb/hr)Screw rpm PET 1000270284270242 3.8350PP
800220224220200 2.403025PET/75PP 500222********* 1.823225PET/70PP/5Primacor 600239255246238 2.374025PET/70PP/5Lotader
600
251
269
255
244
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40
Table 5 Properties of CBA foamed PET/PP (25:75)blends with/without compatibilizer
Melting Peak
(o C)Crystallization.
Peak (o C)Composition
PP PET PET PP Cryst.of PET(%)Density of the foam (g/cm 3)Comments
PET Pellet -246174-400.63Satisfactory melt strength
PP Pellet 152--97.9-0.62Poor melt strength,large cells, poor processability
25%PET+75%PP 149.6245.3195.1110.433.280.68Poor melt strength,corrugation; Poor dimensional stability 25%PET+70%PP+5%Primacor 148.8245.1No peak 97.426.870.68Satisfactory melt strength and appearance
25%PET + 70%PP + 5%Lotader
147.5
243.3
180.7(weak )
98.6
32.09
0.63
Satisfactory melt strength and appearance
