Antioxidants thin films

Increasing surface-to-volume ratio increases susceptibiUty to oxidation. Thin film and fiber are much more sensitive to oxidation than thick specimens (26). The effectiveness of an antioxidant for products with high surface-to-volume ratios is deterrnined not only by its inherent activity in a particular polymer, but also by the rate of loss by volatilization. 

Polymer/additive analysis then usually proceeds by separation of polymer and additives (cf. Scheme 2.12) using one out of many solvent extraction techniques (cf. Chapter 3). After extraction the residue is pressed into a thin film to verify that all extractables have been removed. UV spectroscopy is used for verification of the presence of components with a chromophoric moiety (phenolic antioxidants and/or UV absorbers) and IR spectroscopy to verify the absence of IR bands extraneous to the polymer. The XRF results before and after extraction are compared, especially when the elemental analysis does not comply with the preliminary indications of the nature of the additive package. This may occur for example in PA6/PA6.6 blends where... 

As diffusion to the surface of a polymer is one of the limiting steps in extraction, the particle size or film thickness of a sample is also important [278,333,337-340]. With the typical diffusion coefficients of additives in polymers a particle diameter of about 0.3 mm is required for an extraction time of about 1000 s at 40 °C. An exception to this is the extraction of thin films and foams, for which the shortest dimension is small. It is not surprising that no more than 50 % of antioxidants could be extracted from PP pellets as opposed to 90 % recoveries from the same polymer extruded into film [341]. Grinding of the polymer is usually an essential step before extraction. Care should be taken to avoid loss of volatile additives owing to the heat generated in such processes. Therefore, cryogrind-ing is preferred. 

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. 

I. Antioxidative Properties Using the Thin Film Oxidation Protocol... 

The thickness of the samples is also crucial. It has been observed that a 40% lower extraction yield of certain antioxidants is obtained from PE granulates than from a thin film of the same material [54]. In Soxhlet extraction of un-ground PVC, a yield of 59% of Tinuvin 320 was detected [55], whereas from the ground polymer a 97% yield was obtained [47]. 

Preparation of Samples. Polyethylene and polypropylene powder were treated with 0.5% by weight of the antioxidants in tetrahydrofuran. The solvent was removed at reduced pressure, and thin films were molded at 160 °C for 15 min. 

Phosphite polymer stabilizers were analyzed by combining LD/El/FTICR. Used to control molecular weight and color in melt processing, these antioxidants are difficult to analyze by extraction-LC, X-ray fluorescence, UV, or FTIR for a variety of reasons. Xiang et al. examined these additives individually and in mixed polymers that were prepared by hot pressing to form thin films that could be attached to the probe tip. Additives examined included Ultranox 626 (604 Da), XR-2502 (636 Da), and Weston 618 (732 Da). 

For environmental reasons, direct examination of polymers by spectroscopy and/or by non-destructive methods is preferred over solvent-consuming techniques.Poleunis et al. " reported the use of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to analyze additive migration toward polymer surface in thin films of amorphous polyester containing variable quantities of an antioxidant (Irgafos 168) and a UV stabilizer (Hostavin N30). The results obtained were promising, but authors stated that ToF-SlMS data can be compared quantitatively only if the surfaces have undergone identical treatments. 

If organolead compounds are heated at high temperatures, thin film forms on the metal surface. Therefore, they are able to be used in electroless deposition [9,40]. The others are able to be used as PVC stabilizers [10,22,40] and antioxidants [40]. 

X. Wang, et al., Thin-film organic photodiodes for integrated on-chip chemiluminescence detection - application to antioxidant capacity screening, Sensors and Actuators B Chemical 140 (2) (July 2009) 643-648. 

The choice of stabilizers for PET (antioxidant and/or metal deactivator) is based mainly on the method of additive incorporation into the polymer and the end use/application of the polymer. For example, if it is preferable for the additive to be added in the polymerization reactor, then the volatility and the thermal stability of the stabilizer need consideration because of the high temperatures and vacuum prevailing in the reactor. PET thin films require maximum colour improvement... 

In principle, the rate of loss of a solute by evaporation might be controlled by evaporation, by diffusion or by both. Calvert and Billingham (1979) devised a theoretical model, in which additive loss was seen as analogous to heat flow. Considering the properties of typical antioxidant additives, they concluded that evaporation will be controlled by diffusion for thick samples but by volatility for thin films and fibers. 

The above-mentioned antioxidants are also suitable for the stabilization of PP thin films. However, the synergetic effects of phenolic antioxidants with phosphorus-containing antioxidants [e.g., pentaerytritol-tetra-kis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propio-nate and tris(2,3-di-ter/-butylphenyl)phosphate] was not observed. On the contrary, typical high molecular stabilizers sueh as polymeric HALS have a pronounced contribution to the stabilization of PP thin films. 

The diazotized p-nitroaniline procedure has been applied to the determination of amine antioxidants in ethanol extracts of thin films of rubber. ... 

We have then chosen to gather all the physicochemical parameters describing the vitamin E action. We discussed their physical sense by comparing their values with those published for common antioxidants. We dispose now of a numerical tool permitting to describe the stabilization by Vitamin E. We presented in this chapter some simulations in thin films, but they can be adapted in a diffusion- reaction coupling model, which will be finally helpful for discussing the method for designing UHMWPE materials by a non-empirical way. 

---