Polypropylene with antioxidants

In conjunction with an antioxidant and colorant, Kanae [1] used the Step 3 products as thermoplastic elastomers in automobile moldings. Thermoplastic elastomers having good tensile and impact strength were also prepared by Datta [2] by blending isotactic polypropylene with ethylene-propylene rubber. 

Figure 16.6. Stabilization of polypropylene with Irganox 1010 antioxidant (AO) and Irgafos 168 phosphite (Ph). The total amount of stabilizers was kept constant at 0.1 wt%. The data show variation of the melt flow rate (MFR) measured at 230°C using 2.16 kg weight on the plunger. The lower (points) and upper (triangles) set of data show the MFR after one and flve extrusions at 280°C. Data [Zweifel, 1998].

Polypropylene was first polymerized in 1954 from propylene gas using Ziegler-Natta catalysts. Polypropylene is made commercially using the suspension process at around 60°C and conversions of 80-85 per cent from monomer to polymer are achieved (Brydson, 1999). The reaction mixture is centrifuged to recover solvent and unreacted catalyst. The polymer is washed and dried at 80°C prior to blending it with antioxidants and extruding it into pellets. 

Other polymers with similar, aliphatic, carbon sequences will also be susceptible to oxidation. Thus, the aliphatic polyamides such as nylon 6 and nylon 6,6 will oxidize readily. The oxidation of the nylon polymers is characterized by a drop in molecular weight and by discoloration. Oxidation proceeds in a manner similar to that outlined for polypropylene, with the CHj unit nearest the amide nitrogen most susceptible to attack [32]. Oxidation is a serious problem for polyamides as mentioned earlier, nylon 6,6 will embrittle in two years [1] at 70°C. Almost all commercial polyamide formulations therefore include antioxidants, usually based on copper compounds. Similarly, the aliphatic sequences in PET are readily attacked by oxygen and a drop in molecular weight is again observed [36]. Some gel formation is also observed at higher temperatures. 

Figure 1.3 Photooxidation of polypropylene containing antioxidants at 0.2 g/ 100 g. For key to antioxidants indicated on the curves, see Table 1 1. (Adapted from G. Scott, Polym. Deg. Stab., 10,97 (1985) with permisssion).

Figure 2.9 Results of a long-term tensile test at 120 °C for isotactic polypropylene with different antioxidant concentrations, and comparison with isothermal long term DTA results ILDTA = isothermal long-term differential thermal analysis.

Morlat-Therias, S., Mailhot, B., Gonzalez, D., Gardette, J.-L. Photooxidation of polypropylene/montmorillonite nanocomposites. 2. Interactions with antioxidants. Chem. Mater. 17, 1072-1078 (2005)... 

Peng, Y., liu, R., Cao, J., et al. Effects of vitamin E combined with antioxidants on wood flour/polypropylene composites during accelerated weathering. Holzforschung 69(1), 113-120 (2015)... 

The most effective antioxidants that increase the antioxidant stability of PVC and materials based on it, are phenol derivatives [2], At present, a great scientific and practical interest of this class of compounds are ter-penphenols, particularly 4-methyl-2,6-izobomilphenol [3], Known way to improve the thermal and photochemical stability of polypropylene with 4-methyl-2,6-izobomilphenol [4], The use terpenphenols as PVC stabilizers unknown. 

The tertiary carbons along the backbone of polypropylene are susceptible to oxidation in air. This is especially a problem at high process or use temperatures, in sunlight, or during mechanical stress. Formulation with antioxidants, peroxide decomposers, and UV stabilizers greatly reduces this problem. 

In this paper we compare the stabilization effectiveness of some less usual phenolic compounds in y-irradiated polypropylene with that of a conventional hindered phenolic antioxidant and hindered amine stabilizers (HALS). EXPERIMENTAL... 

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. 

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. 

There are a number of occasions where a transparent plastics material which can be used at temperatures of up to 150°C is required and in spite of its relatively high cost, low impact strength and poor aging properties poly-(4-methylpent-1 -ene) is often the answer. Like poly(vinyl chloride) and polypropylene, P4MP1 is useless without stabilisation and as with the other two materials it may be expected that continuous improvement in stabilising antioxidant systems can be expected. 

Acid-treated clay catalyst Engelhard F-24 was found to be very effective for the alkylation of diphenylamine (DPA) with an olefin such as a-methyl styrene (AMS) to obtain a mixture of mono and dialkylated diphenylamines (Chitnis and Sharma, 1995). For example, alkylation of DPA with AMS produced a mixture of 4-(a,a-dimethyl benzyl) diphenylamine, i.e. monocumyl-diphenylamine (MCDPA) and 4,4 -bis(a,a-dimethylbenzyl) diphenylamine, i.e. dicumyldiphenylamine (DCDPA) (Eqn.(l 1)). The dialkylated diphenylamine, i.e. DCDPA, is indu.strially important as an antioxidant and heat stabilizer. DCDPA is reported to be an ideal antioxidant for many materials like polyethylene, polypropylene, polyether polyol, polyacetals, nylon 6, synthetic lubricants, hot melt adhesives, etc. 

Polypropylene contains Irganox 1076 antioxidant. b Commercial cal-cium stearate contains both stearate and palmitate. After Lattimer [37]. Reprinted from Journal of Analytical and Applied Pyrolysis, Vol. 26, R. P. Lattimer, pp. 65-92, Copyright (1993), with permission from Elsevier. 

Certain metal salts effectively reduce the photoactivity of titanium dioxide pigments. Combination of these salts with an appropriate antioxidant and/or ultraviolet stabilizer provided highly efficient stabilization of polypropylene. The deactivation/ stabilization performance of the metal salts is adequately explained on the basis of their decomposition of hydrogen peroxide at the pigment surface and by annihilation of positive holes in the pigment crystal lattice. 

Similar to phenols, they can cause staining and are often used in conjunction with carbon black filled elastomers (e.g., tyres) — although carbon black itself has antioxidant capacity. For non-staining applications, e.g., polypropylene carpets, a stoically hindered amine is used, e.g.,... 

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