Polymeric phenolic phosphites

Four main types of antioxidants are commonly used in polypropylene stabilizer systems although many other types of chemical compounds have been suggested. These types include hindered phenolics, thiodi-propionate esters, aryl phosphites, and ultraviolet absorbers such as the hydroxybenzophenones and benzotriazoles. Other chemicals which have been reported include aromatic amines such as p-phenylenediamine, hydrocarbon borates, aminophenols, Zn and other metal dithiocarbamates, thiophosphates, and thiophosphites, mercaptals, chromium salt complexes, tin-sulfur compounds, triazoles, silicone polymers, carbon black, nickel phenolates, thiurams, oxamides, metal stearates, Cu, Zn, Cd, and Pb salts of benzimidazoles, succinic acid anhydride, and others. The polymeric phenolic phosphites described here are another type. 

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. 

The polymeric phenolic phosphites (PPP) are reaction products of substituted hydroquinones and phosphorus trihalides such as phosphorus trichloride. They may be represented by the structure shown at the top of p. 213, where (A) represents subsituent groups on the paraphenylene portion of the hydroquinone. 

Heat Stability. Polymeric phenolic phosphites seem to be efficient hydrogen transfer agents which provide excellent heat stability when used in conjunction with thiodipropionates. Presumably, they scavenge free radicals and form resonance-stabilized phenoxy radicals in much the manner as simple hindered phenolics (see reaction scheme on p. 214). 

To determine the relative effectiveness of polymeric phenolic phosphites as heat stabilizers, comparisons were made with several commonly used hindered phenolics. Formulations containing 0.4% DLTDP and 0.1% phenolic were prepared and tested for heat stability in a 300°F. forced-draft oven. Three 75-mil, 1-inch diameter discs were placed in the oven, and days to failure were noted as the time for two out of three discs to craze crumble. 

Therefore, additional heat aging tests were conducted to investigate the possibility that hydroquinone-derived phenolic groups might function differently from the monohydric phenols and somehow contribute to the high activity of the phenolic-phosphite systems. Two substituted hydro-quinones, two polymeric phenolic phosphites based on those hydro-quinones, and combinations of the hydroquinones with triphenyl phosphite and tris(nonylphenyl) phosphite were evaluated in the oven-aging test again 0.4% DLTDP was included in the formulations. 

Table III shows that the two hydroquinones had low activity. In addition, the aryl phosphites did not improve the results significantly when they were added to the formulations. The two polymeric phenolic phosphites based on the hydroquinones, however, had considerably better activity, going 62 and 81 days, respectively, compared with only 20-28 days for the other formulations. Therefore, the polymeric phenolic phosphites are more effective probably because of their low volatility.

Repeated Extrusions. 0.25% Tris(nonylphenyl) phosphite and 0.25% polymeric phenolic phosphite were added to unstabilized polypropylene, and the resins were run through the laboratory extruder four times. The barrel temperature was 450°F., and the stock temperature of the extrudate was approximately 500°F. Melt flow rate of samples taken after each pass was measured according to ASTM 1238-62T. 

Photostability. A surprising discovery in our work with the new polymeric phenolic phosphites was that compounds inhibit the embrittlement of thin polypropylene films exposed to ultraviolet light. 

UP + polymeric phenolic phosphite UP 4- 0.4% DLTDP + 0.2% monomeric phenolic 290 10... 

The significance of the interaction between PPP and ultraviolet absorbers, such as the hydroxybenzophenones, becomes evident when the high cost of absorbers is considered. The potential cost for polymeric phenolic phosphites is low by comparison. By using a polymeric phenolic phosphite in place of a normal hindered phenolic in the heat-stabilizer system, enhanced photostability is obtained, and less ultraviolet absorber is required for a given degree of stability. 

The photostabilizing characteristic is most evident when the polymeric phenolic phosphite compounds are used in conjunction with thiodipropionate esters and reduced amounts of hydroxybenzophenone ultraviolet absorbers. 

Where aryl phosphites are good processing stabilizers, they do not contribute much to light stability or heat stability at elevated temperatures. Where hindered phenolics are good heat stabilizers, they do not contribute much to photostability. Some phenolics are effective processing stabilizers, but the phenolics are not as effective as aryl phosphites or the polymeric phenolic phosphite compounds. 

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. 

The importance of the polymeric form was pointed up in Fade-ometer and sunlamp tests where polyphosphites were found to be much more effective than mono (tris) phosphites derived from the same starting hydroquinones, especially when the compounds were used in conjunction with thiodipropionate esters. The data in Table IV show a comparison between a polyphosphite and a mono (tris) phosphite derived from the same substituted hydroquinone. A commonly used hindered phenolic was also tested in place of the phenolic phosphite for comparison. The protection against embrittlement obtained with the polymeric phenolic... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Virtually all polymeric resins undergo oxidation in the presence of oxygen. To retard this degradation, antioxidants are typically added. These additives are usually hindered phenols, amines, hydroxylamines, phosphites, or thioesters. In general, antioxidants will have little effect on colorability since they are typically used at low levels. At higher levels, they may increase light scattering and impact colorability depend-... 

Very often, thermoplastics contain minute amounts of metallic compounds which originate from polymerization catalysts, contaminated fillers, polymerization or processing equipment, or metal contact (wire and cable insulators) during the use of the polymer. The interactions between the polymer and metallic substances are complex, but generally result in the accelerated aging of the material. Most metal deactivators are bifunctional stabilizers with phenolic and nitrogen, or phenohc sulfide and phosphite, moieties in their structure, and act by a chelating action which reduces the harmful effects of the metal ions. 

Maleic anhydride monomer cont.) oxime adducts, 228 palladium complexes, 213 peracid formation, 246 phenols photochemical reaction, 180 phosphine reactions, 230 phosphite reactions, 206 photo-adduct applications, 213 photo-adduct sterochemistry, 182, 183, 192-194 photochemical dimerization, 188 photochemical polymerization, 195, 239, 241-243, 247, 248... 

Thermal oxidation of polystyrene is suppressed by adding phenols, such as 2,6 di-tert-butyl-p-cresol (BHT), long chain 3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propion-ates or sebacates in combination with phosphorous compounds, such as substituted phenyl phosphites (TNPP) or phosphonites. Impact modified polystyrene is stabilized only by phenols, because phosphites affect the polymerization kinetics. 

It is well known that polymerization catalyst residues can induce the degradation of polymers, which can result in a faster conversion of present phenolic antioxidants and thus to discoloration. A possible mechanism of action of phosphites is that they form a complex with these metal ions and reduce their negative influence. This can lead to a slower conversion of the phenolic antioxidant and thus reduction of the formation of their yellow conversion products. 

Naugard 524 is a solid phosphite antioxidant with excellent hydrolytic stability. Recommended for use in combination with amlne/phenolic antioxidants, Naugard 524 functions synergistical-ly to retard oxidative degradation of most polymeric substances during polymerization, processing, and in end-use applications. For those polymers resistant to oxidation, Naugard 524 can be used alone to maintain optimal color stability. 

When blending semi-crystaUine type PA and/or PEST polymers, information about polymeric chain terminal groups is essential. For example, unprotected chain ends of one polymer may cause transreaction, which may transform the blend into an amorphous copolymer, usually with reduced MW. Different types of stabilizers are needed for different polymers, but when combined they may react and neutralize each other. Similarly, the presence of hydrophilic inorganic stabilizers in one polymer may hydrolyze ester groups of others stabilizing molecules, e.g., phosphites, phosphonites or hydroxy-phenol esters. 

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