Antioxidants in polypropylene

Figure II. 8. Synergistic effect of two antioxidants in polypropylene (DLTP=dilauryl thiodipropionate. MCPC-2,2 -methylenebis[6-(l-melhylcyclohexyl-p-cresol)]. (After Leyland and Watls )...

Extraction of antioxidants in polypropylene production. Process control. 

Dialkylhydroquinones. The second group of disubstituted derivatives—2,6-dialkylhydroquinones (Ila)—contains two hydroxyl groups which are influenced quite differently by substitution. The total activity of these antioxidants (Table III) was always slightly higher than the sum of the contributions of both substituents this accounts for the fact that Type Ila substances were stronger antioxidants in polypropylene than the 2,5-dialkyl derivatives (a reverse relationship in activities was shown... 

Correlation of Test Methods for Evaluating Antioxidants in Polypropylene... 

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... 

The role of phenolic compounds as stabilizers and antioxidants has been studied very extensively in polymers and copolymers (refs. 18, 19). Many papers are devoted to this problem. Studies have been made on the optimization of phenolic structure based on hydroquinone (ref. 20) or catechol (ref. 21) as an antioxidant in polypropylene for example. Others have dealt with the influence of the polarity or stearic effect for different phenolic compounds or substituted phenols on the kinetics of antioxidation reactions - for example in polyvinyl acetate (ref. 22). Lastly, many papers have discussed on kinetic effects. 

Precision of the procednre is similar to that of ASTM D 1996 4-6% for within-laboratory tests and 6-12% for between-laboratory tests with concentrations of antioxidants in polypropylene approximately from 0.02 to 0.1%. 

Pospisil, J. Elucidation of structure-effectiveness relations of phenolic antioxidants in polypropylene. In Kinetics and mechanism of polyreactions, IUPAC Intern Symposium on Macromolecular Chemistry, Plenary and main lectures volume. Budapest Akademiai Kiado 1971, pp. 789-808... 

METHOD 7 - DETERMINATION OF 2,6-DITERT-BUTYL-4-METHYLPHENOL AND 4 SUBSTITUTED 2,6-XYLENOL PHENOLIC ANTIOXIDANTS IN POLYPROPYLENE. DERIVATIVE ULTRAVIOLET SPECTROSCOPY. ... 

Derivative ultraviolet spectroscopy is used to determine down to 0.01% 2,6 ditert-butyl-4-methylphenol and 4 substituted 2,6 xylenol phenolic antioxidants in polypropylene. 

Antioxidant half-life in polypropylene exposed to a nitrogen stream at 140°C. 

Oligomeric hindered amine light stabilizers are effective thermal antioxidants for polypropylene. Thus 0.1% of A[,Af-bis(2,2,6,6-tetramethyl-4-piperadinyl)-l,6-hexanediamine polymer, with 2,4,6-trichloro-l,3,5-triazine and 2,4,4-trimethyl-2-pentaneainine [70624-18-9] (35) (Fig. 5), protects polypropylene multifilaments against oxidation when exposed at 120°C in a forced-air oven (22) for 47 days. 3,5-Di-/ l -butyl-4-hydroxytoluene [128-37-0] (0.1%) affords protection for only 14 days. 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

The use of quenching agents in polymers is a recent development. Of particular interest are the nickel(II) chelates in polypropylene film and fibre and the even newer hindered amines which appear to combine the roles of antioxidant cmd quenching agent. 

As with most polyolefins and polydienes the presence of copper has a strong adverse effect and most antioxidants are relatively ineffective. In these instances quite good results may be achieved by the use of 1% of a 50 50 phenol alkane-dilauryl thiodiproprionate blend instead of the 0.1-0.2% of antioxidants more commonly used in polypropylene. 

In relation to consumer uses of possible concern for this CICAD, data from the Women s Environmental Network indicate that butyltin stabilizers have been detected in the non-woven polypropylene topsheet of babies nappies (diapers). It is possible that this could relate to the last of the three key uses described above, in that the topsheet could be of silicone-grafted polypropylene (or, as discussed below, the butyltin may be present because of its use as a catalyst in the production of an antioxidant in polyolefin films). 

Table 3.11 Examples of antioxidant efGdendes in polypropylene films...

There is only perhaps one significant case where low concentrations of an antioxidant shows flame-retardant behavior, in the case of certain hindered amine stabilizers (HAS) that at the normally used concentrations (<1 wt%), offer low levels of flame retardancy in polypropylene and show synergy with bromine-containing flame retardants.78 80... 

The similar rate of decomposition of Tetralin hydroperoxide by distearyl and dilauryl thiodipropionate at 80 °C. is of interest when compared with the formers better antioxidant properties in polypropylene at 140 °C. Tests showed that the stearyl compound was superior to the lauryl compound as a synergist, in the presence of a phenolic antioxidant, in delaying the embrittlement of polypropylene sheet in an air oven at 140°C. (Table IV). 

The main purpose of the work reported here was to develop a low-cost, effective, and nonvolatile 2,4,6-trialkylphenol antioxidant. We discuss the synthesis of some new types of phenolic antioxidants, particularly those resulting from a-olefin alkylation of phenols. We also report the effectiveness of these stabilizers in polypropylene and speculate on the effect of structure on their effectiveness. 

Detailed data concerning the effectiveness of the p-cresol o-olefin products will be given later. In general, all were at least moderately effective antioxidants for polypropylene. The 2,6-dioctadecyl-p-cresol (DOPC) from 1-octadecene is almost as effective in polypropylene... 

All four types of phenols alkylated by a-olefins are effective antioxidants. However, the alkylated p-cresols are the most effective, followed closely by the 2,4,6-trialkylated phenols. The alkylated xylenols have a lower degree of effectiveness in polypropylene. Of the entire series, 2,6-dioctadecyl-p-cresol has the best over-all properties. It is one of the most effective non-discoloring antioxidants now known for polypropylene. It is especially useful under adverse conditions, such as high temperatures, or in thin films. 

Method of Evaluation. We determined the activity of antioxidants in oxygen at 180 °C. and in the presence of 0.05-0.1 mole of antioxidant per kg. of polypropylene (in some cases, concentrations of 0.01 and 0.025 mole/kg. were used). 

Besides the reactions between phenols and peroxidic bodies, other factors can influence the activity of antioxidants—e.g., compatibility with substrate and volatility. The results show that under the conditions used the influence of the antioxidant structure is dramatic. In this connection we note agreement of the general conclusions dealing with the influences of pyrocatechol antioxidant structure on the activity in polypropylene at 180°C. and those influences found in Tetralin (28) at 80 °C. Despite great differences in experimental conditions, the sequences of the activities of pyrocatechol antioxidants I-VI were in agreement. Great similarities were also found within each particular group of antioxidants. 

The relationship of structure to activity is discussed on the basis of measurements performed in the presence of 0.05 mole of antioxidant per kg. of polypropylene. The results obtained at lower concentrations (0.01 and 0.025 mole/kg.) showed some differences in details. On the other hand, almost identical relationships were found (45) in stabilizing polypropylene at higher antioxidant concentrations (0.1 mole/kg.). Analysis of those concentration relationships supports our assumption that the activity of pyrocatechol derivatives is influenced above all by reactions between the peroxidic bodies and antioxidants in the oxidation chain-breaking mechanism. 

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