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Post-stabilizing recycled PP during processing.

Post-Stabilizing Recycled PP During Processing

Polypropylene (PP) processing and in-service stability are traditionally achieved by the addition of phenolic antioxidants and organic phosphite stabilizers, which disrupt the free-radical chain reaction that degrades the polymer. Ultraviolet radiation absorbers and/or metal deactivators are also sometimes included to protect the polymer against specific degraders. Improved stability is evidenced by retention of melt viscosity, color, and physical properties.

Although rarely categorized as stabilizers, acid neutralizers are included in the additive package of virtually every PP. While their specific job is the neutralization of potentially corrosive acids - residual by-products of catalyst and other additive reactions - they can also contribute significantly to the melt stability of the polymer and, in the case of lactic-based neutralizers, markedly improve the color stability.

The polyolefin industry has increasingly recognized that acid neutralizers derived from salts and esters of lactic acid - lactates and lactylates - are highly proficient at imparting corrosion resistance, stabilizing melt viscosity, and maintaining color in virgin PP systems. Although they are proprietary compositions, the primary building blocks for these additives are calcium lactate (1) and calcium stearoyl 2-lactylate (2):

[Mathematical Expression Omitted]

Increasingly, plastics are being collected, identified, and segregated for recycling. PP has one of the six polymer-specific codes assigned by the SPI, and it is today being converted from waste into items of commerce. PP has recently been reported to be specifically targeted by the automotive and baby diaper industries for major recycling efforts.

This article describes the application of a lactate/lactylate neutralizer to the processing stabilization of recycled PP, and compares its effectiveness to that of other additives. Virgin PP was processed in the laboratory to simulate recycled material and then post-stabilized. In a second testing phase, post-stabilization was applied to actual post-consumer containers.

Experimental

The components of the simulated recycle formulation containing the primary stabilization system were chosen for their ready availability and broad usage; they were: Resin - Himont Pro-Fax 6501; Antioxidant - Ciba-Geigy Irganox 1010, 1000 ppm; Phosphite - Ciba-Geigy Irgafos 168,500 ppm; Stearate - calcium stearate, polypropylene grade, 500 ppm.

Recycling was simulated by giving the primary formulation a processing history and then secondarily treating it with additives to effect post-stabilization. Heat and shear were applied to a 45-g charge in the mixing chamber of a Haake System 40 Torque Rheometer operating at 230 [Degrees C] and 60 rpm. Nitrogen was introduced to the chamber during the first 4 min of each run. The first trial, containing only primary additives, provided a baseline for the test data. Trials were run for a total of 20 min.

When secondary additives were used, each was added after 10 min of plastication in the rheometer mixing chamber and then further processed for 10 min. The blade speed was dropped to 12 rpm during the addition period. All samples were cooled rapidly to ambient temperature in a Carver hydraulic press. The secondary (post-stabilizing) ingredients were as follows: Calcium stearate - 1000 ppm; Pationic [1250.sup.R] - 466 ppm; Irganox 1010 - 100 ppm; Irgafos 168 - 140 ppm. Pationic 1250 is a proprietary acid neutralizer based on lactate/lactylate chemistry from the Patco Polymer Additives Division, American Ingredients Co. Additive levels were selected to provide an equal cost comparison based on current list prices. A no-additive control was also run.

The containers constituting the post-consumer PP recycle stream are listed in Table 1. After being emptied, the containers were power washed with a steam/water mixture at 75 [Degrees] C and 60 psi until visual inspection showed that all of the contents were gone. Labels were soaked off in hot water. All containers were air dried and then cut into 1/4-in pieces and blended in a paint shaker for 2 hrs to obtain a uniform mixture.

Table : TABLE 1. Containers Constituting the Post-Consumer Recycle Stream.
Containers Contents Description Wt%
Two 14-oz bottles Pancake syrup Colorless clear 25
One 2-x 5-x 8-in box Moist towelettes Beige opaque 15
Six 4-oz cups Pudding Colorless clear 8
One 12-oz tub Cottage cheese White printed 12
Two 6-oz tubs Whipping cream White printed 10
One 20-oz bottle Laundry powder Colorless clear 21
One 5-oz bottle Calamine lotion White printed 9


Specimens were prepared by plasticating a 45-g charge, including additives, for 15 min in the rheometer. Temperature, speed, and other test conditions were the same as used in the recycle simulation.

Color Retention

Specimens were pressed for 8 min at 230 [Degrees C] into 3/8-x 2-in disks. Hunterlab color measurements were recorded on the "L," "a," and "b" scales. Yellowness Index (YI) for each additive trial was determined and the control value (no additive) subtracted from it. This difference, [Delta]YI, represents the effect on color stability for each additive. A positive value denotes more yellowing than the control, a negative value less yellowing.

The [Delta]YI values given in Fig. 1 show that the lactate neutralizer demonstrated a significant improvement in color. Antioxidant and phosphite had a negligible effect, while the stearate apparently promoted yellowing. The trend held for both the simulated and post-consumer recycle cases.

Corrosivity

Corrosion was measured by pressing small circular samples against a steel Q-panel for 15 min at 500 [Degrees] F. After the PP was removed, the panel was subjected to 84% relative humidity at 108 [Degrees] F for 4 hrs. Panels were then dried and sprayed with a clear, permanent coating.

The visual ratings given in Table 2 show that neither system was inherently corrosive. Preliminary work, however, had shown that the post-consumer polymer specimens varied widely in corrosivity. Corrosion resistance will be a major concern for recycle streams of higher corrosivity. The increase in corrosivity in the presence of phosphite has also been observed in virgin resins. However, slight corrosion in the presence of 1000-ppm calcium stearate was unexpected.

Table : Table 2. Corrosivity
Additive Simulated Post-consumer
No additive None None
Stearate Trace Slight
Lactate None None
Antioxidant None None
Phosphite Slight Slight


Melt Flow

Melt flow index (MFI) was determined on powdered samples run on a Kayeness Galaxy I per ASTMD 1238. After 10 min in the rheometer mixing chamber, the primary formulation had an MFI of 4.6 g/10 min. After 10 more minutes, the MFI had risen to 10.7. Meaningful starting values were not obtained for the post-consumer recycle mixture, although the no-additive control had an MFI of 15.6. Figure 2 shows that in both cases secondary additives did not play a major role in stabilizing melt flow. Trials containing extra phosphite had the least change in MFI, and the Pationic 1250 was better than the no-additive case.

Torque

Torque and temperature were continuously graphed and recorded on magnetic media. Numerical readings were printed at 30-sec intervals. The torque measurements (Table 3) for both test series followed the pattern seen in the melt flow results - none differs greatly from the no-additive control, although phosphite showed a small improvement. The lactate and the antioxidant each have a small, but clear, advantage over the stearate.

Table : Table 3. Torque, m-g.
 Simulated, Post-consumer,
Additive 20 min 15 min
No additive 313 222
Stearate 273 232
Lactate 323 252
Antioxidant 323 262
Phosphite 344 272


Conclusions

Conclusions based on this study cannot necessarily be generalized given the wide range of resins, additives, and recycling conditions in use today. However, certain clear trends emerged from both the simulated and post-consumer recycle trials: * The secondary addition of additives improved the stability of the PP, most noticeably in color retention. * Post-addition of the lactic-based neutralizer significantly improved color retention, and conferred the best overall secondary stabilization improvements to the polymer. * Phosphite addition helped mitigate loss of melt flow stability, but promoted corrosion. An effective acid neutralizer should be included when phosphite is used as a secondary additive. * Post-addition of a phenolic antioxidant had no significant effect. * Adding calcium stearate to recycled PP negatively affected color and, surprisingly, increased corrosivity.

Acknowledgements

Special thanks to Mr. Dan Lopez for his painstaking sample preparation and testing.

PHOTO : FIGURE 1. Post-addition of the lactate neutralizer yielded dramatic improvements in color.

PHOTO : FIGURE 2. Post-addition of phosphite, and lactate to a lesser extent, helped mitigate loss of melt viscosity.

Dale Dieckmann Patco Polymer Additives Division American Ingredients Company Kansas City, Missouri
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:polypropylene
Author:Dieckmann, Dale
Publication:Plastics Engineering
Date:Sep 1, 1991
Words:1362
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