How to Rationally Apply Antioxidants and Stabilizers in Formulation Design

Part One: Synergistic Use of Phenolic Antioxidants and Thioester Antioxidants in Polypropylene Formulation Design

Dodecyl thiodipropionate, trade name Antioxidant DLTP.

Thiodipropionic acid di(stearyl) ester, trade name antioxidant DSTP.

DLTP and DSTP are the most important varieties of thioester antioxidants industrialized abroad in the 1940s. They can decompose hydroperoxides in polymers, providing protection for polymers under harsh conditions of thermo-oxidative aging and color stability. When used in conjunction with hindered phenolic antioxidants, they produce a good synergistic effect and have a very good performance-to-price ratio. Currently, they are one of the most widely produced and sold auxiliary antioxidants globally, second only to phosphite antioxidants.

The table below shows the synergistic effect test data using Antioxidant 1010 and DSTP together. Base formulation: PP powder (MFR=2.3g/10min), 0.1 part CaSt, specimen thickness: 0.30mm, aging temperature: 150℃.

The results indicate that under the same dosage, the thermal oxidation stability of the 1010/DSTP antioxidant system is superior to that of the 1010/168 (B215) antioxidant system.

Part Two: Design of Formulations Using Secondary Amine Light Stabilizers for Polyvinyl Chloride

The formulation design of hindered amine light stabilizers for polyvinyl chloride films.

It is generally theorized that during the thermal processing, thermal oxidation, and photo-oxidative degradation of PVC resin, there is a release of hydrogen chloride (HCl). Hydrogen chloride can react with nitrogen-containing alkaline hindered amines, inhibiting the formation of reactive nitrogen oxide radicals (NO·), thereby "poisoning" the hindered amines and causing them to lose their stabilizing effect or function. With the advancement of PVC polymerization technology and the enhanced efficacy of PVC stabilizers, the hydrogen chloride produced from the decomposition of PVC resin is no longer sufficient to suppress the action of hindered amines. As a result, the application of hindered amine light stabilizers in PVC has become more common, achieving satisfactory results.

From the data in the table, it can be seen that the effect of using hindered amine light stabilizer HALS alone in PVC transparent film is comparable to the effect of using benzotriazole UV-326 or benzophenone UV-531 light stabilizers alone. The effect of combining hindered amine light stabilizer HALS with benzotriazole or benzophenone is better than using any of the three types of light stabilizers alone.

The Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, was the first in China to conduct research on the addition of hindered amine light stabilizers to polyvinyl chloride (PVC) agricultural greenhouse films. Hindered amine light stabilizers 770, 622, GW-540, and ultraviolet absorbers UV-9, UV-531 were used either individually or in combination with PVC resin to produce agricultural greenhouse films through calendering. These films were then subjected to artificial accelerated aging experiments, natural exposure tests, and actual application in greenhouses.

Conclusion: ① Hindered amine light stabilizers can be fully applied in PVC greenhouse films, and their light stabilization effect can rival that of UV-531 and UV-9, which are currently widely used in PVC greenhouse films.

② Combining hindered amine light stabilizers with ultraviolet absorbing light stabilizers in PVC greenhouse films does not extend the service life of the films, but during the application in greenhouses, its anti-photoaging effect is superior to that of using hindered amine or ultraviolet absorbing light stabilizers alone.

The hindered amine light stabilizers have the effect of preventing the "backing effect" of greenhouse films.

For transparent products, UV-absorbing light stabilizers can function in the inner or deeper layers of the product, while hindered amine light stabilizers can work on the surface or outer layer. The synergy of both provides comprehensive protection for the product.

Discussion on the formulation design of hindered amine light stabilizers (HALS) for PVC rigid products.

In the light stability test of impact-modified rigid polyvinyl chloride sheets (1mm thick), the time to increase the yellow index by 20 units was measured. The sample with 0.3% UV-531 showed 6000 hours, while the sample with 0.3% hindered amine 770 exceeded 7500 hours. The samples with 0.15% hindered amine 770 and ultraviolet absorbent each also exceeded 7500 hours. The effect of using hindered amine light stabilizers alone or in combination with ultraviolet absorbents in this system was slightly better than using ultraviolet absorbents alone.

Discoloration of plastic doors and windows is a common and frequent occurrence. Generally, fluorescent brighteners and titanium dioxide are added during the production of profiles to enhance and maintain the whiteness of the products. In fact, fluorescent brighteners only improve the initial whiteness of the profiles and cannot maintain the whiteness over the long term. Different models and manufacturers of titanium dioxide have varying abilities to resist ultraviolet rays and maintain the whiteness of the profiles.

The basic formula for rigid polyvinyl chloride sheet is PVC, SG-5 100 parts, rare earth stabilizer 3 parts, CPE 8 parts, ACR 2 parts, lightweight active CaCO.₃10 parts, 4 parts titanium dioxide, 0.05 parts fluorescent whitening agent.

The other formulations and light stabilizers are clearly shown in the table below. After exposure to a 20W UV lamp, the whiteness values were measured. The table presents the whiteness data, which demonstrates that the hindered amine light stabilizer GW-944Z improves the color change resistance of PVC profiles.

Using high-purity titanium dioxide and appropriate amounts of light stabilizers is the fundamental method to improve and maintain the whiteness of profiles.

Part III: Formulation Design for Improving Weather Resistance Using Antioxidants and Light Stabilizers

(I) PVC/ABS Blended Modified Material

Polyvinyl chloride (PVC) is one of the most widely used general-purpose plastic materials, but it has poor impact resistance, thermal stability, and processing fluidity. Acrylonitrile butadiene styrene (ABS) is a plastic material with excellent performance and wide applications, featuring high impact strength, good heat resistance, dimensional stability, and easy molding, which are the superior characteristics lacking in PVC. By blending and modifying ABS with PVC and maintaining an appropriate ratio of PVC/ABS=60/40 to 40/60, not only can the processing performance of PVC be improved, but the mechanical properties can also be enhanced, making the impact strength of the PVC/ABS alloy exceed that of ABS and PVC. However, ABS has poor weather resistance and undergoes structural changes over time during storage, processing, and usage, leading to deterioration in mechanical properties and the material becoming hard and brittle. The main reason is the oxidation degradation of the double bonds contained in the butadiene molecules of ABS due to sunlight, heat, and oxygen exposure outdoors. Furthermore, in the PVC/ABS alloy, the degradation of ABS can accelerate the degradation of PVC, and their interaction results in even poorer weather resistance of the PVC/ABS alloy. Therefore, researching and developing PVC/ABS alloys with excellent mechanical properties and good weather resistance is of significant practical importance for fully realizing the value of the alloy and expanding new varieties of plastics.

Based on the data in the table, in a 90℃ thermal aging chamber

Antioxidants can significantly improve the heat resistance of PVC/ABC, and different types of antioxidants vary in their effectiveness in enhancing PVC/ABC's heat resistance, with 1076 being the most effective.

The initial mechanical properties of samples with added antioxidants are higher than those of the blank samples. Antioxidants not only improve the thermal oxidative aging performance of PVC/ABS during use but also enhance the thermal oxidative resistance during the processing of PVC/ABS.

During the 120 days of natural exposure, the tensile strength and impact strength of the blank sample decreased significantly. In contrast, the PVC/ABS samples with added UV absorbers showed a smaller decrease in tensile and impact strength, indicating improved UV resistance, with UV-327 having the best effect.

(2) PBT Weather Resistant Special Material

Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have good light stabilizer properties. After long-term use, the aging degree of the inner or deeper layers of the products remains low, but the surface of the products shows yellowing and embrittlement. When PBT is used for energy-saving lamp shades, its surface color gradually turns yellow with prolonged use or lamp illumination time.

PBT weather-resistant special resin formulation: PBT1080, made in South Korea; antioxidant: a composite of hindered phenol, phosphite, and organotin heat stabilizers, added at 0.5%; light stabilizer: a composite of UV absorbers and hindered amines, added at 2%.

Blank samples and dedicated material samples were placed in a homemade aging box equipped with four energy-saving lamps (16W each) and continuously illuminated for 60 days. The yellow light index was measured every 10 days, and the tensile strength and impact strength were measured at 60 days.

The initial yellow light index of the blank sample and the special material is approximately 1.2. After 20 days of exposure, the yellow light index of the blank sample exceeds 3 (slightly yellow), and after 50 days, the yellow light index exceeds 6 (yellow). After 60 days, the yellow light index of the special material remains essentially unchanged.

The tensile strength and impact strength data of blank samples and special material samples before and after 60 days of irradiation are shown in Table 16. It can be observed that the strength of the blank samples decreased significantly after continuous irradiation for 60 days, while the tensile strength of the special material samples decreased by only 1% and the impact strength decreased by 1.5%.

(3) Special grade POM for weather resistance

When polyoxymethylene is oxidized or thermally oxidized, in addition to generating free radicals, it also simultaneously eliminates formaldehyde. The formaldehyde is further oxidized into formic acid, which significantly reduces the mechanical properties of the products. Generally, a heat-oxygen stabilization system composed of hindered phenol antioxidants and synergistic stabilizers is used. Synergistic stabilizers include two types of substances: one type is polymer nitrogen-containing compounds, such as melamine and copolyamides, which mainly function to prevent the elimination of formaldehyde; the other type is organic acid salts, such as calcium stearate and calcium citrate, which serve as acid acceptors in the system. Polyoxymethylene is exceptionally sensitive to ultraviolet light; without photostabilization protection, products exposed to sunlight for a short time will have a roughened surface. Generally, benzotriazole UV absorbers like UV-P or hindered amines like 622, 292 [decamethylene bis(1,2,2,6,6-pentamethyl-4-piperidinyl) ester] are used for ultraviolet protection of polyoxymethylene. When color permits, carbon black is an excellent ultraviolet shielding agent for POM.

The Polymer Research Institute of Sichuan University used phenolic antioxidants, UV-absorbing light stabilizers, and self-made polymer modifiers to modify polyoxymethylene for weather resistance treatment. After 1000 hours of thermal oxidative aging and UV aging, the mechanical performance data of unmodified polyoxymethylene and modified polyoxymethylene (Table 17) indicates that the thermal oxidative aging resistance and UV aging resistance of modified polyoxymethylene are better than those of unmodified polyoxymethylene. The color difference variation of modified polyoxymethylene is also superior to that of unmodified polyoxymethylene.

The special stabilizing effect of polymer modifiers on the molecular structure of polyoxymethylene synergizes with the conventional stabilizing effects of antioxidants and light stabilizers on polyoxymethylene, imparting excellent weather resistance to modified polyoxymethylene.

Part Four: Formulation Design of Antioxidants and Light Stabilizers in Colored Plastic Products

The resin molecules of plastic products, when induced by light and oxygen, generate high-energy and highly reactive oxygen free radicals. These free radicals not only catalyze and accelerate the aging of the resin but also interact with coloring agents, leading to changes in the molecular structure of the coloring agents and accelerating the alteration of their color.

For colored plastic products, one of the roles of antioxidants and light stabilizers is to eliminate the destabilizing effects of colorants and protect the plastic resin; the second role is to protect the colorants by protecting the resin; the third role is to directly protect the colorants. For example, fluorescent pigments and other colorants are sensitive to ultraviolet light and have poor light stability, so during use, UV absorbers such as benzophenone or benzotriazole should be used to protect them.

The application and function of antioxidants in colored plastic products.

Masterbatch is a high-concentration colored pellet made from colorants, carrier resins, dispersants, coupling agents, surfactants, and enhancers. Using masterbatch to produce colored plastic products is a method widely adopted in the plastic product manufacturing industry. Generally speaking, the carrier resin used in the production of masterbatch has a lower molecular weight and a higher melt index compared to the base resin used for producing products. The carrier resin undergoes the first heating during the production of masterbatch, and during the production process of plastic products, it undergoes reheating and extrusion. The carrier resin undergoes thermal degradation and mechanical degradation first, which accelerates the aging process of colored plastic products. Although the proportion of carrier resin in masterbatch within colored plastic products is not large, it has already undergone thermal oxidation due to being heated twice or more. Therefore, antioxidants must be added in the production of masterbatch and in the production of colored plastic products using masterbatch.

The function of heat-oxygen aging resistance is the basic anti-aging function of general plastic materials, while the light-aging resistance function is an enhanced function built upon the basic function. To enhance the light stability of colored plastic products, the first step is to improve the thermal-oxygen stability of these colored plastic products. In some plastic products colored with ultraviolet-absorbing pigments, adding an appropriate amount of antioxidants can significantly improve the light stability of the products. The data in the table illustrate that in high-pressure polyethylene, using chrome yellow, iron oxide red, or antioxidant 2246 alone does not improve the light stability of polyethylene. However, when chrome yellow or iron oxide red is used in conjunction with antioxidant 2246, the light stability of high-pressure polyethylene increases more than threefold. The essential role of antioxidant 2246 in this experiment is to enhance the thermal-oxygen stability of high-pressure polyethylene, with the objective effect or result being the improvement of the light stability of high-pressure polyethylene.

Next, let's talk about the applications and functions of light stabilizers.

Ultraviolet (UV) absorbers, commonly referred to as UV stabilizers, can be classified into two categories based on their molecular structure: benzophenone and benzotriazole. These types of stabilizers utilize their molecular structure to convert the light energy that irradiates colored plastic products into thermal energy, preventing the molecular structure of the colorants (such as the conjugated double bonds in organic colorants) from being damaged by light energy, and avoiding photo-oxidation reactions in plastic materials. UV absorbers directly protect the colorants and the appearance of colored plastic products.

Most colorants, especially inorganic pigment-type colorants, can provide a certain degree of light stabilization when used alone in plastic products. For colored plastic products intended for long-term outdoor use, relying solely on colorants to enhance light stability is insufficient. Only by using light stabilizers can the rate of photoaging be effectively inhibited or slowed down, thereby significantly improving the light stability of colored plastic products. Hindered amine light stabilizers (HALS) are a type of organic amine compounds with steric hindrance effects. Due to their ability to decompose hydrogen peroxide, quench excited state oxygen, capture free radicals, and regenerate effective groups, HALS are highly efficient in preventing photoaging and are the most widely used plastic light stabilizers both domestically and internationally. Data from Table 19 indicates that appropriate light stabilizers or combinations of antioxidants and light stabilizers can enhance the light and oxygen stability of colored plastic products for outdoor use by several times.

For plastic products colored with light-active and photosensitive pigments (such as cadmium yellow and uncoated gold rutile), considering the catalytic photodegradation effect of the coloring agents, the amount of light stabilizers added should be correspondingly increased.

Finally, let's make a summary.

(1) The combination of hindered phenols and thioester antioxidants shows good effects in polypropylene.

(2) Hindered amine light stabilizers have good thermo-oxidative stability and functionality, especially polymeric high molecular weight hindered amines; they provide good weather stability for polyvinyl chloride.

(3) The antioxidant and light stabilizer systems determined through experimentation or practical application only have definite effects under specific conditions. If there are slight changes in these conditions, the effects and functions of the antioxidants and light stabilizers may be significantly impacted.

One of the main reasons why anti-aging plastic products do not meet anti-aging design requirements is the presence of localized mechanical property defects in the products.

The variety and quality of domestic antioxidants and light stabilizers are basically able to meet the needs of the domestic petrochemical and plastics industries.