Breakthrough Chemistry for Processing Stabilization of Polypropylene

Abstract

Traditional stabilization systems for polypropylene are typically based on a binary combination of a phenolic antioxidant and a phosphorus based melt processing stabilizer. The phenolic antioxidant provides melt processing stability as a hydrogen atom donor and free radical scavenger; it also provides the polymer with some desired level of short-term or long-term thermal stability, simply for storage or throughout the lifetime of the final article. The phosphorus based melt processing stabilizer, usually a phosphite or phosphonite, functions as a hydroperoxide decomposer above the melting point of the polymer, during melt compounding and processing. Both chemistries work together synergistically to help maintain the original molecular architecture of the polymer.

Effect of processing conditions and catalyst type on the thermal oxidative degradation mechanisms and melt stability of metallocene and Ziegler‐catalyzed ethylene‐1‐hexene copolymers

“…The relationship between the olefinic unsaturations and the type of degradation reactions during melt processing is an important one. [ 15–17 ] The main changes in both polymers were found to occur in the vinyl and trans‐vinylene unsaturation groups, but the vinylidene level remained unchanged. Figure 13 shows the %change in the vinyl group concentration with multipass extrusions at low processing temperature and shear rate (210°C/50 rpm).…”

“…It is clear from this that increasing the number of extrusion passes, results in a reduction in the vinyl concentration in both polymers, but leads to a higher extent of vinyl reduction relative to the virgin counterpart for the mLLDPE. The reduction in vinyl concentration is partially attributed to addition reactions of the macro-alkyl radicals to vinyl groups which are known [2,3,15,17] to result in branching and crosslinking (XL), see reaction (1). The higher extent of vinyl reduction in the mLLDPE is entirely consistent with the much higher level of increase in its molecular weight, and broadening of MWD (compared to zLLDPE), shown earlier (see Figure 10) and this corresponds to conditions where crosslinking reactions are predominant.…”

“…E"), a known chain breaking donor (CB-D, deactivating alkyl peroxyl radicals) and the aryl phosphite Weston 399 (W399) which acts primarily as a peroxide decomposer (PD). Scheme 1 gives an outline of the main alkyl radical scavenging (CB-A) mechanism of the antioxidant HP136 [17,20] ; HP136 reacts with the alkyl radicals forming a highly stabilized benzofuranonyl radical which can dimerize reversibly or react further by deactivating another alkyl radical. Scheme 2 shows the main hydroperoxide decomposing (PD) antioxidant activity of the antioxidant W399 (P[OAr] 3 ); although this aryl phosphite may have also some chain breaking (CB) activity (Scheme 3, reaction (2)) it is well known [21] that it is not an effective chain breaking antioxidant due to the formation of nonsterically hindered phenoxyl radical as well as the possibility of formation of propagating alkyl radicals, reactions (3) and (4), respectively.…”

Untitled

“…The same structural functionalities either unchanged or slightly modified have been used up to now and during the past fifty years practically no new chemistries appeared. The only exception were lactones launched in the middle of the nineties, at that time considered as a breakthrough in polymer stabilization chemistry [4,5]. Their success, however, dropped shortly after since their active stabilizing moiety was found to be suspect mutageneous so that they were phased out soon after.…”