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09 Oct 2020
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October 9, 2020 Longchang Chemical

Plastic is one of the most frequently encountered materials in daily life. It is deeply loved by consumers for its advantages of simple shape, low price, resistance to solvent, corrosion resistance and light weight. However, the intrinsic properties of polymer materials cannot meet the needs of various use scenarios. Therefore, in the production process, people will add different additives to the plastic to improve the performance.

The small molecule compounds in plastics mainly appear in two ways:

1. For polymer materials obtained by addition polymerization or condensation polymerization of monomers, initiators are usually added during the polymerization process, and there may be traces of initiators and monomers and other small molecular compounds remaining in the materials after polymerization;

2. Additives added to meet certain properties of plastics during molding, such as plasticizers, antioxidants, flame retardants, light stabilizers, inorganic fillers and other small molecular compounds. As the use time increases, certain traces of small molecules will migrate in the plastic or even leak out of the plastic, resulting in a decline in the performance of the plastic product. This situation is more likely to occur under heating, microwave, high pressure and other environments.

Under the action of microwave, the polymer polymer chain is vibrated due to the action of the alternating electromagnetic field, which causes these small molecular compounds to dissolve and migrate to food, air, and production lines. When the migration reaches a certain ratio, it may affect the performance of the plastic. , Causing dangerous things to happen.

At present, China has little research on the structure and migration mechanism of compounds that play an important role in the migration process. In recent years, researchers have studied four common small molecule compounds in plastics (Butylated Hydroxytoluene, Antioxidant 168, Light Stabilizer 770, Dibutyl phthalate). ) The migration rules of 5 kinds of plastic packaging materials (polyethylene PE, polypropylene PP, polystyrene PS, polymethyl methacrylate PMMA and polyethylene terephthalate) were compared. It should be noted that in order to simulate extreme conditions and facilitate testing, the research work added small molecule compounds to five plastic packaging materials at a content of 1.00%, which greatly exceeded the commonly used concentration ratios of these compounds, so the measured The results are generally too large and are only used as a trend reference and do not explain the actual exudation.

The first is a comparison of the approximate migration of different small molecules in different packaging materials. The figure below shows the mobility results obtained by heating at 500 W microwave output for 5 min. It can be clearly seen that the four substances migrate in PS and PMMA. The rate is greater than the mobility in PE, PP and PET.

This is because the main chains of PE, PP and PET have good symmetry and relatively regular structures, which are crystalline polymers; while PS has a benzene ring with larger steric hindrance in its side chain, and PMMA has a larger polar ester group in its side chain. It belongs to the amorphous polymer as a whole, but the crystalline and amorphous regions exist in the polymer at the same time. The small molecule compounds are also distributed in the crystalline and amorphous regions of the material when they are formed. Under the action of microwave, the polymer is The chain segments of the amorphous region of the crystals vibrate randomly under the action of the alternating electromagnetic field, which promotes the migration of small molecular compounds from the polymer.

It can also be found that the migration rate of antioxidant 168 and light stabilizer 770 in all plastics is lower than that of the other two small molecules, and the migration rate of HALS 770 in applicable PE and PP is only about 10%.

Under the heating time of 5 min, the mobility curves of different microwave output powers for the four substances are shown in the following figure. The ABCD diagrams show that BHT, antioxidant 168, HALS 770 and DBP are in 5 materials under different microwave powers. Migration curve. Obviously, as the output power of Weibo increases, the mobility of various materials and the output power are basically linear.

This is because under the same heating time, the microwave output power is strong, the small molecules move more violently in the polymer, which also accelerates the movement of the polymer segments in the amorphous region, thereby accelerating the migration of small molecules in the plastic. Similarly, regardless of the microwave power, the mobility of the four materials is consistent with the conclusion of the previous stage, indicating that there is no other form of interaction between the materials except for thermal motion.

Finally, the researchers studied the effect of microwave action time on the migration of chemical substances in plastic packaging. The microwave output power was 300 W. The results are shown in the figure below. It can be seen from the figure that under the same microwave output power, when the four substances migrate in each material, the mobility is small at the beginning, and it increases linearly and accelerates after 2 minutes, and reaches the maximum value in about 6 to 7 minutes. , After reaching equilibrium and basically unchanged.

This is because material migration is a process of diffusion, and the continuous driving force for diffusion is the chemical potential difference between the packaging material and the food. When the chemical potential between the two phases is equal, the equilibrium is reached. For antioxidant 168 and HALS 770, the final equilibrium concentration of exudation is only 25% even in PS, that is to say, the remaining additive content is 0.75%, which still exceeds the additive usage in most cases.

Therefore, even under strong long-term microwave conditions, these two small molecule compounds can still maintain an effective concentration, which is conducive to the continuous use of plastic devices under extreme conditions.

All in all, this experiment investigated the four types of migrations containing different functional groups and polarities of BHT, antioxidant 168, light stabilizer HALS 770 and DBP under different microwave heating power conditions in PE, PP, PS, PMMA and PET. Kind of mobility within the plastic film.

The results show that the mobility of the four substances is basically linear with the microwave output power, and the mobility of the migrants in the amorphous material is greater than that in the crystalline material under the same conditions, and when the migrants and the polymer molecular chain When hydrogen bonds are formed, the migration resistance can be increased; under the same microwave output power, when the four substances migrate in each material, the mobility is small at the beginning, and it starts to increase linearly and accelerates after 2 min, at 6 min~7 min The left and right reach the maximum value, and then reach the equilibrium and basically unchanged.

Among the four materials, antioxidant 168 and light stabilizer 770 have a smaller migration rate, which is beneficial for adding and using.

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