Chapter 19
Blends of Nylon 6 and Polypropylene with Potential Applications in Recycling
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Effects of Reactive Extrusion Variables on Blend Characteristics S. S. Dagli, M. Xanthos, and J. A. Biesenberger Polymer Processing Institute, at Stevens Institute of Technology, Castle Point, Hoboken, NJ 07030 As part of efforts to develop recycling technologies through melt reprocessing of commingled plastic streams (e.g. fishing gear or carpeting) in compounding extruders the effects of process parameters on the characteristics of a model blend containing nylon 6 (85% by wt.) and polypropylene are reported. Twin-screw extruder runs and preliminary batch mixer experiments showed that the presence of an acrylic acid modified polypropylene acting as reactive compatibilizer affects significantly flow properties and promotes finer morphology and improved blend properties. Other reactive extrusion process variables such as screw speed, residence time, venting and feeding sequence also affect blend properties, but to a lesser extent. Results are interpreted in terms of parameters such as components viscosity and thermal stability, reaction kinetics and mixing efficiency.
Among from
the various marine plastic debris, discarded plastic fishing gear is a major pollutant, adversely affecting marine life. Virtually any marine species could die entanglement in fish nets lost or abandoned. Recent estimates of the U.S. Academy of Sciences bring the amount of fishing gear lost in the sea (and often washed up on the beaches) to about 135,000 metric tons per annum (7). During a year long effort in collecting discarded fish nets and identifying the polymers used in making them (2), it was found that the majority of nets were made of nylons and polyolefins, whereas ropes and lines were mostly polypropylene. At present, plastic fishing gear is not recycled in the U.S. To help in aie efforts of developing a recycling technology for the plastic fishing gear, a basic study on blends of nylon 6 (N6) and polypropylene (PP) was undertaken. Mixtures of N6 and PP are also encountered in recycling of automotive carpeting and, thus, the findings of this work could also be applicable in this area. N6 and PP are immiscible polymers. When blended together they form an incompatible blend. Incompatible blends do not have useful properties. To improve the properties of such a blend, it would be necessary to compatibilize it with a compatibilizer added as a third component, or it can be formed in-situ during blend compounding by using suitable functionalized components. Compatibilizing
0097-6156/92/0513-0241$06.00/0 © 1992 American Chemical Society
In Emerging Technologies in Plastics Recycling; Andrews, Gerald D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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242
EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
improves the short and long-term mechanical properties of the blends. This is achieved mainly due to an improvement in the morphology (finer and more stable). The in-situ formation of compatibilizers for polyblends is rapidly gaining popularity. In the last few years, more and more cases have been reported where advantage is taken of the presence of reactive groups to form a graft or block or random copolymer, which could act as a compatibilizer for a mixture of two or more polymers (3-5). In the early 1970's, Japanese workers (6-9) reported the use of maleic anhydride grafted polypropylene (PP-g-MAH) to improve the dispersibility and mechanical strength of N6 / PP blends. Since then, several studies were reported on laminates of N6 and PP and, in particular, on improving their adhesion by the use of modified PP or other modified polar thermoplastics in the packaging and automotive industries {10-12). Again in the mid and late 1980's, additional work was reported where modified PP, ionomers, or other polar substances were used to improve the compatibility of N6 / PP blends (13-16). There were also a few studies published on the composition dependence of the properties of N6 / PP blends without any third component (17,18). In the present study, an acrylic acid grafted modified polypropylene (PPg-AA) is used with a blend of nylon-6 (N6) and polypropylene (PP). In addition to strong specific interactions such as Η-bonding, the amine end groups of N6 are expected to react with the acid groups of PP-g-AA with the evolution of water to form a graft copolymer in-situ. Under processing conditions, PP-g-AA may also form an anhydride (79) so an anhydride/amine reaction is also possible. Both these reactions in a simplified form are shown in Figures 1 and 2. Both reactions can form a graft copolymer by grafting PP molecules onto N6 molecules. Thus, a copolymer having parts common with both the phases could be formed which could act as a compatibilizer. PP-g-AA could have more than one acid group on each graft since polyacrylic acid oligomers are usually grafted on the PP backbone. This would mean multiple reactive sites and, hence, multiple graft reactions as shown in Figure 1. This could lead to some crosslinking of N6 and to anhydride formation as mentioned earlier. Besides these reactions, an acid (or anhydride) /amide reaction (acid or anhydride groups reacting with the amide groups on the nylon backbone) forming an imide is also possible, though not very likely (20,21) as the amine end is much more reactive, when compared with the amide groups. Also such a reaction could cause the N6 molecule to split, as is the case for the reaction with the anhydride groups. Lawson et al. (20) have carried out an extensive analysis of a similar reactive system and have not found any signs of reduction in molecular weight of the N6 thus, ruling out such a reaction completely. There could be some ungrafted polyacrylic acid present in PP-g-AA (22). This could also participate in reactions with N6 and form H-bonds. Parameters Controlling Process Requirements. There are some important process parameters which would have significant effect on reactive polymer blending experiments. The following parameters were considered in the present study : Viscosities of the components: Mixing two polymers with vastly different melt viscosities is a difficult challenge. Various studies (23,24) on mixing components with different viscosities show that the mixing is the most efficient (easiest to attain the lowest dispersed size) when the viscosity ratio of the components is close to one. In this study the aim was not to investigate the effects of viscosity ratio during reactive extrusion, and, thus, a proper selection
In Emerging Technologies in Plastics Recycling; Andrews, Gerald D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Blends of Nylon 6 and Polypropylene
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19. DAGLI ET AL.
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