Polypropylene processing stability

This may be exemplified, by so-called captodative lactones of the benzofiiranone type that are used as the polypropylene processing stabilizers and weak antioxidants (Scheme 12.9). 

A number of ortho hindered alkyl-substituted phenyl phosphites and phosphonites were found to be effective process stabilizers for polypropylene and high density polyethylene combining more effective stabilization activity at high temperatures with good storage stability at relatively elevated humidity and ambient temperature, as well as resistance to discoloration. 

Ultraviolet absorbers are added to polypropylene resins which are used where exposure to sunlight is high. These chemicals are essentially ineffective as heat or processing stabilizers. 

Here we discuss a new class of polypropylene stabilizers—the polymeric phenolic phosphites. These compounds exhibit unique, broad-spectrum activity which may allow simplification of polypropylene stabilizer systems. The most active species are synergistic with thiodipro-pionate esters, are effective processing stabilizers when used alone or with other compounds, and contribute to photostability. Compounds of this type appear to function as both free radical scavengers and peroxide decomposers, and through a mechanism not yet completely understood, allow significant reductions in the concentration of ultraviolet absorbers required to achieve high levels of photostability. 

Mechanisms of Degradation Inhibitors. The degradation mechanisms described above have no little effect on end-use stability and processing stability of polypropylene. Indeed, without stabilizers and antioxidants to inhibit such degradation reactions, polypropylene would have limited usefulness as a structural plastic if any usefulness at all. 

Processing Stability. The ability of tris (nonylphenyl) phosphite to prevent degradation of polypropylene during processing is believed related to its efficiency to decompose peroxides into non-active species. In a similar way, the phosphite groups on the polymeric phosphites may also be capable of decomposing peroxides (see top of p. 217). 

Figure 1. Processing stability of polypropylene, PPP vs. aryl phosphite...

Figure 2. Processing stability of polypropylene in a Brabender torque...

Large Scale Processing. In large scale processing studies four formulations were evaluated for processing stability on a large mill and extruder. These formulations included (1) 0.1% PPP, (2) 0.4% DLTDP, and (4) unstabilized polypropylene. The resins were first... 

Figure 3. Processing stability of polypropylene on large-scale equipment...

The polymeric phenolic phosphites are excellent heat and processing stabilizers and can contribute significantly to photostability. These broad-spectrum stabilizers offer possibilities for simplifying polypropylene stabilizer systems. They may allow one to use lesser amounts of other additives, and they open up a new area for potential cost savings in formulating polypropylene resins. 

A variety of methods for evaluating antioxidants in polypropylene has been developed during the past several years. Polymer producers, end-use manufacturers, additive suppliers, academicians, and others have developed widely disparate test methods, all of which presumably yield the same results—i.e., the test methods rate the antioxidants and antioxidant systems in the same relative order of effectiveness. Many of these test methods are useful tools in distinguishing unstabilized polymer, moderately stabilized polymer, and highly stabilized polymer systems. Today, all of the polypropylene producers offer highly stabilized polymers. Effective antioxidants are available from several additive suppliers. How does one select the best antioxidant or polymer formulation for a particular end use This paper compares the results obtained by various test methods used to evaluate the two basic types of oxidative stability, processing stability and end-use or environmental stability. The correlation or lack... 

Some of the test methods being used to measure the processing stability of polypropylene include melt flow drift measurements at elevated temperatures using an extrusion plastometer (melt indexer), melt viscosity retention measurements using a torque rheometer, retention of melt flow after repeated extrusions, and injection molded spiral test measured by the flow in inches at various temperatures and the retention of melt flow of the injected spirals. The nine commercial resins were evaluated by these methods. 

Phosphite processing stabilizer for polypropylene, polyethylene, and adhesives. 

Non-discoloring, sulfur containing phenolic antioxidant and stabilizer that provides long-term heat stability by preventing thermo-oxidative degradation. Used for the process stabilization of polyethylene wire and cable resins for polyethylene during extruder compounding. Can also be applied in styrenic polymers, polypropylene, elastomers such as EPDM and SBR and for carboxylated SBR latex, polybutadiene rubber and polyisopropene rubber. 

Piperazine HALS 2 has a positive effect on the processing stability of polypropylene when used in conjunction with a phenolic antioxidant ... 

Thiol antioxidants can also be reacted with polyethylene and polypropylene (40). Figure 10 illustrates the superiority of MADA-B in PP compared with conventional thermal antioxidants when subjected to continuous hot water leaching. It is also an effective processing stabilizer and photoantioxidant. Table 15 shows that the latter effect is resistant to solvent extraction. The loss of activity after extraction corresponds to the amount of unbound MADA present. 

Sufficient AO-1 and processing stabilizers were dissolved in methylene chloride to give a final concentration in polypropylene of 0.1% and 0.05%, respectively. The methylene chloride solution was added to the Profax 6501 (about 1000 g) containing 0.1% calcium stearate and the mixture was mixed with a Hobart Kitchen Aid mixer at room temperature until all the solvent had evaporated. 

The photo-oxidative degradation of polypropylene and stabilization by hindered amines has been reviewed. A study has appeared of the effect of P-carotene on the photoreactivity of anthracene in hexane solution and a kinetic scheme has been proposed to account for the photochemical and photophysical processes that occur on irradiation at 365 nm. Quenching rate constants have been determined between /S-carotene and singlet oxygen. Some characteristics have been communicated of the sensitized photo-oxidation of abietic acid contained in a vinyl butyl ether-butyl methacrylate-methacrylic acid copolymer. At 400 nm... 

Typical processing stabilizers for polypropylene and butylated hydroxy-toluene (BHT) as the primary antioxidant and phosphates and phosphonates as secondary antioxidants. Examples of the latter that are commonly used are tetrakis-(2,4-di-terr-butyl-phenyl)-4-4 -bisphenylylenediphosphonite, distearyl-pentaerythrityl-diphosphonite, tris-(nonylphenyl)-phosphite, tris-(2,4-di-teft-butyl-phenyl)-phosphite and bis(2,4-di-ferr-butyl-phenyl)- pentaerythrityl-diphosphite. In commercial polypropylenes, phosphorous compounds are always used together with a sterically hindered phenol. The compounds are commonly added in concentrations between 0.05 and 0.25%. 

Low-density polyethylene (LDPE) is extensively used for the manufacture of films. During processing, which is carried out at temperatures of approximately 200°C, cross-Unking, and thus formation of gel, can occur through oxidation if the polymer is not stabilized. Such gel particles are visible in the film as agglomerates, known as fish eyes or arrow heads. The processing stabilizers used in LDPE consist of systems commonly used for polypropylene, namely, combinations of a phosphite or phosphonite and a long-term heat stabilizer (hindered phenol) in overall concentrations up to 0.1%. Concentrations seldom exceed 0.1%, since the compatibility of any additive in LDPE is considerably lower than in any other polyolefins. 

The ability of these energy quenchers to stabilize polypropylene fibers to weathering permitted the development of many new end-uses, but their capabilities have been surpassed by a new group of stabilizers that contain a hindered piperidine structure. As shown above, these HALS compounds can be very good long-term thermal stabilizers. Hindered piperidines react with hydroperoxides during polypropylene processing to form nitroxyl radicals (II) that arc effective polymer radical traps [134]. These nitroxyl radicals react with polymer free radicals to form the polymeric hydroxylamine (III). 

Hydrotalcites as process stabilizers (acid scavengers) for PVC and as acid neutralizers in polyethylene (PE) and polypropylene (PP), as well as applications in other polymer systems. 

Nysten and co-workers reported on the CFM mapping of the distribution of process stabilizers, UV-light stabilizers and antioxidants in stabilized polypropylene films as a function of aging time (239,240). Differences in the pull-off forces between the additives and PP allowed a submicrometer imaging of the lateral distribution of the additives present on the surface and a clear differentiation between antioxidants and UV-light stabilizers. 

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