November 9, 2020 Longchang Chemical

Properties of polymers due to their high molecular weight, and anything causing a change in the molecular weight of the polymers are called polymer degradation. Among the numerous factors, UV radiation is one of the most important factors of polymer degradation. It is very important for technical and economic application to prevent the degradation of plastics induced by oxygen and light. Additives are required for industrial plastics for outside application scenarios especially used as a structural material, and can enhance plastics’ polymeric stability against the effects of temperature and light. The common preparation method of HALSs is made from 2,2,6,6-tetramethyl piperidine or 1,2,2,6,6 pentamethyl piperidine. HALSs can be used in many resins for protecting those materials against photo degradation from radical damage and oxidative toxicity.

Tinuvin 770 is a light stablizer using in polymers for stabilization. It can protect polymers especially polypropylene against UV light and prevent polymers losing their mechanical and physical properties by exposure to weather and sunlight. Tinuvin 770 belongs to HALSs, and as a light stabilizer it is used worldwide in many polymers as PE (polyethylene), PP (polypropylene), polycarbonate, polyurethane, PS (polystyrene), PA (polyamides), polyacetals and acrylonitrile polymers. The chemical name of Tinuvin 770 is bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacat. In dry conditions, Tinuvin 770 has a high stability and good performance for polymers to resistant to degradation and decomposition from direct sun light and acidic conditions.

However, not all products can be consumed in a timely manner, which will product a number of expired Tinuvin 770. For the economic savings of the petrochemical industries and environment friendly, a research pay attention to the examination of the possibilities of using expired Tinuvin 770. If it is still effective for polymers, it can replace existing methods of using expired materials which dump dozens toxic chemicals into environment. Let’s read the study together.

The researchers belonging to this study used Tinuvin 770 both expired and non-expired (also called a standard sample) produced in a same factory. They tested the DSC of the expired and standard samples in the beginning, to see whether the melting points of the two samples will be different. The curves in Figure 1 & 2 are DSC curves reflecting the samples losing weight in a heating situation. Samples’ melting will cause a peak of the curve. In the MSDS given by the manufacturer corporation indicates Tinuvin 770 having a melting point between 81-86 °C. Researches show that the standard sample has a melting temperature of 85.92 °C, and the expired sample fuses under 85.54 °C. Both of them are in the range and very close, indicating the two samples have similar thermal properties, namely no degradation happened to the expired sample which may leading the melting temperature changed.

Figure 1: DSC curves of the standard Tinuvin 770.

Figure 2: DSC curve of expierd Tinuvin 770.

The expired Tinuvin 770 and the standard sample are both tested the FT-IR spectra by a infrared spectrometer. In Figure 3 we can see the FT-IR spectrum for the standard sample. As we all know that the carbonyl group has IR absorption in a range of 1660-1850 cm-1, the spectrum have a corresponding peak at 1721 cm-1 illustrating the carbonly group existance in the standard sample. And N-H bond has a tensile absorption, bending absorption and out of sheet bending absorption respectively located at 3300 – 3500 cm-1, 1500 cm-1 and 800 cm-1. All the peaks belong to the vibration mode of N-H bond were be observed in the spectrum at 3323 cm-1, 3423 cm-1, 1500 cm-1 and 793 cm-1. The spectrum shows that the bending absorption peak of methylene group (CH3) at 1378cm-1 and the bending absorption peak of methyl group (CH2) in long chain at 739 cm-1 has appeared as well.

Figure 3: FT-IR spectrum of the standard Tinuvin 770

The FT-IR spectra obtained by analysis of Tinuvin770 expired sample are shown in Figure 4. The expired sample also shows corresponding IR absorption peaks in the FT-IR spectrum at 1721 cm-1 to the carbonyl group. The corresponds peak to N-H tensile absorption as two broad peaks in range of 3300-3500 cm-1 means 3323 cm-1 and 3423 cm-1 appeared, and the N-H bending absorption in 1500 cm-1 is observed as a poor peak. The N-H out of sheet bending absorption in near the 800 cm-1, means 793 cm-1 is observed. And, the bending absorption peak of methylene group (CH3) at 1378 cm-1 and the bending absorption peak of methyl group (CH2) in long chain at 739 cm-1 has appeared.

By comparison of obtained FT-IR spectrums result for expired sample of Tinuvin 770 and standard sample of Tinuvin 770, concluded that both spectrums are related to one compound, and this result is achieved that, the degradation and changes due to expiring has not happened in this sample. However, the FT-IR spectrum can not confirm the amount of differents contents, so we need some other identification experiments and quantitative methods to analysis.

Figure 4 FT-IR spectrum of expierd sample of Tinuvin770.

Scanning of UV on both samples using UV-VIS spectrophotometer was performed and it was determined if the absorption range of two samples conform to one another. The researchers test the standard sample and the expired sample’ UV-VIS (visible – ultraviolet) spectra, and compared them immediately. The UV-VIS spectrum is recorded in a way of absorbance against wavelength (nm). Between 800 nm to 200 nm there are no ultraviolet absorbance existing due to the experiments using methanol as solvent. The testing wavelength range is to 205 nm so far, which called a absorption cutoff point. Therefore, the UV absorption of the used sample is related to those observed peaks in the spectrum obtained. UV spectra of standard sample are shown in Figure 5. There are Carbonyl chromophore groups (C=O) in tinuvin 770 structure. Carbonyl groups in unsaturated compounds containing atoms such as oxygen and nitrogen, have transfers from n π∗ type within 280-290 nm that because so many of these transitions are forbidden transitions, resulting the peak intensity’s reduce. Standard sample of tinuvin 770 shows poor absorption in 280-290 nm area and its adsorption starts to increase in 270 nm and reaches its maximum in 250 nm. UV spectrum of Tinuvin 770 expired sample is shown in Figure 6.

The corresponding peak of the Carbonyl chromophore group (C=O) of expired sample Tinuvin 770 like standard sample, in the 280 – 290 nm areas was weak and began to increase from 270 nm. The summit peak is located in 250nm. In conclusion, there is no change of chromophore happened in the expired sample, in other words, these two samples have one same absorption and have same main ingredients.

Figure 5 UV-VIS chromatogram of standard Tinuvin770.

Figure 6 UV-VIS chromatogram of expired Tinuvin770.

Then, the researchers dissolved the standard sample and the expired sample in acetonitrile and formed 200 ppm solutions, which lead into the HPLC-UV instrument for analysis. Retention time is equal to 5.4 minutes according to the chromatogram, which is the interval time from injection to reach the summit peak. For identifying the material peaks, retention time is a important parameter. And HPLC-UV can be used for quantitative calculation of identifications by counting the area under each peak. The area determined by the stability systems’ integrators under each peak, which is also called absorption bands, is proportional to the amount of the ingredient in the studied system. The retention times of both standard sample and expired Tinuvin 770 are 5.4 minutes, which illustrate the expired sample have no degradation. In Figure 7, curves are the amounts of obtained peak area in four performed injections of the standard sample and expired sample. There are a little difference between the two curves, that may because of a change of humidity or a lot of samples. The retention times of the two samples are very close, and this is the evidence that the expired sample is still undecomposed.

Figure 7: Comparison of area under the curve of the both sample in the HPLC-UV

The researchers also testing GC-MS (gas chromatography coupled with mass spectrometer) spectrum of the two samples, which can separate different ingredients and identify different components. The samples will be dissolved in liquid phase, and add a small amount into the vaporizing chamber to be a gas phase. The gas chromatography using a fixed capillary column with liquid stationary phase adsorbing onto inert solid surface, to separated different contents in the gas phase. And mass spectrometer will test those different contents to identify them. Like HPLC-UV chromatogram, GC-MS chromatogram can also get the concentration of the studied compounds by counting the area under different peaks and determine the composition by comparing the retention time. The difference between the retention times of expired sample and standard sample of Tinuvin 770 is less than 0.1 min, which may be caused by the instrument error. This is consistent with the result of HPLC-UV. Data of GC-MS results are compared in Figure 8.

Figure 8: Comparison of GC-MS areas under the peak Tinuvin 770

The studies concluded that the light stabilizer Tinuvin770 after expired date, under appropriate conditions, away from humidity, ionizing radiations and high temperature (above extruder system temperature), and because of being alkaline compound, it must be kept in free acidic environment. After satisfying these conditions, expired Tinuvin 770 can retain its structure and its functional characteristics, so it still can be used in polymers as PP and others for stabilization.

Same series products

HALS 119 / LIGHT STABILIZER 119 / CHIMASSORB 119106990-43-6
HALS 123 / LIGHT STABILIZER 123 / TINUVIN 123129757-67-1
HALS 622 / LIGHT STABILIZER 622 / TINUVIN 62265447-77-0
HALS 770 / LIGHT STABILIZER 770 / TINUVIN 77052829-07-9
HALS 944 LIGHT STABILIZER-944 CHIMASORB 94470624-18-9 / 71878-19-8

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