It Is Said That Six Additives Are Used in Film Processing—Do You Know Them All?

Some resins or film products have inherent properties that do not adapt to the requirements of their required processing technology, so additives must be added to change their processability; other materials have relatively good processing performance, but their product performance cannot meet the requirements, so additives must also be added to change their product performance.

Requirements of Film for Additives

Poor compatibility will cause a "migration" phenomenon, manifested as "sweating" in liquid additives and "blooming" in solid additives.

Sometimes when the requirements for the film are not particularly strict, a certain lack of compatibility can be allowed, such as poor compatibility between the filler and the resin. As long as the filler particle size is small, the basic requirements for film performance can still be met; if treated with a coupling agent or surfactant, its function can be brought into better play. However, some additives that improve the surface properties of the film, such as anti-blocking agents and antistatic agents, are required to have a slight degree of migration so that they can exert their effect on the film surface.

Additives are required to remain in the film for a long time with basically no or very little loss, and the loss of additives mainly occurs through three pathways: volatilization, extraction and migration. This is mainly related to the molecular weight of the additive, its solubility in the medium and its solubility in the resin.

The processing conditions of some resins are relatively harsh, such as high processing temperatures; consideration should be given to whether the selected additive will decompose and whether the additive has a corrosive effect on the processing equipment.

Films for different uses have different requirements for the odor, toxicity, weather resistance, thermal performance, etc. of additives. For example, plastic bags for food must be non-toxic, so the additives used are different from those used in plastic bags for general packaging.

Synergistic and Antagonistic Effects in Additive Combinations

In the same resin system, two additives may produce a "synergistic effect." If improperly combined, some additives may produce an "antagonistic effect," which will weaken the function of each additive or even make them ineffective; for example, the combined use of carbon black and phenolic antioxidants will produce an antagonistic effect.

Additives That Can Change the Packaging Performance of Films

Plasticizers and Heat Stabilizers

Plasticizers and heat stabilizers can greatly improve the compatibility of film materials.

Plasticizers, as the name suggests, increase the plasticity of materials: on the one hand, they increase the fluidity of the resin during molding and improve processing performance; on the other hand, they are substances that can increase the flexibility and elasticity of the finished film.

Heat stabilizers mainly improve the thermal stability of resins.

Under irradiation by sunlight, lamplight and high-energy rays, polymer materials will rapidly age, manifesting as yellowing, embrittlement, cracking, loss of surface gloss, greatly reduced mechanical and electrical properties, and even loss of use value. In this complex destructive process, ultraviolet light is the main cause of aging of polymer materials. It is mainly the result of the combined action of ultraviolet light in sunlight and oxygen in the atmosphere on polymer macromolecules.

Functions of Light Stabilizers:

They can effectively absorb ultraviolet light with a wavelength of 290~410nm, eliminate or weaken ultraviolet light, protect polymer material films from damage by ultraviolet light and oxygen, extend service life and improve the light resistance of materials. Since most light stabilizers can absorb ultraviolet light, light stabilizers are also called ultraviolet absorbers.

Addition Amount of Light Stabilizers:

According to the mechanism of action of light stabilizers, they can be divided into: light shielding agents (pigments), ultraviolet absorbers, ultraviolet quenchers and free radical scavengers. These four modes of action constitute four progressively deeper levels of light stabilization.

After adding light stabilizers, despite the extremely small amount used, their anti-aging effect is very significant; generally only 0.1%~0.5% of the polymer weight needs to be added.

For most plastic varieties, during their manufacture, processing, storage and application, they have a certain sensitivity to oxidative degradation. Oxygen penetrating into plastic films can react with almost most polymers, causing degradation or crosslinking, thereby changing the properties of the material. A small amount of oxygen can cause drastic changes in the strength, appearance and performance of these polymer materials. Under hot processing and sunlight, the oxidation rate is faster. Therefore, polymer oxidation is usually divided into thermal oxidation and photo-oxidation. The final result of such reactions is performance aging. If such reactions are not prevented, the polymer can quickly be oxidized and lose its use value.

Different plastics have different stability to oxygen, so some plastics do not need antioxidants. Others must have antioxidants added. The function of antioxidants is to capture active free radicals and interrupt chain reactions, with the purpose of delaying the oxidation process and rate of plastics.

Generally speaking, the protective efficacy of amine antioxidants is higher than that of phenolic ones, but because amines undergo varying degrees of discoloration under the action of light and oxygen, they are not suitable for light-colored, bright-colored and transparent films, so they are rarely used in plastic films.

According to antioxidant effect, antioxidants are divided into primary antioxidants and auxiliary antioxidants. Aniline antioxidants have better antioxidant effects but greater pollution, and are mainly used in rubber products; phenolic antioxidants have slightly poorer antioxidant effects but less pollution and better overall effects, and are mostly used in plastic films. Mercaptans or thioesters and phosphites are usually classified as auxiliary antioxidants and used together with primary antioxidants to produce synergistic effects and prolong the efficacy of antioxidants.

After melting, polymers usually have relatively high viscosity. During processing, when the molten polymer passes through narrow gaps, gates and other flow channels, the polymer melt must rub against the surfaces of the processing machinery. This friction reduces the fluidity of the melt, and severe friction will make the film surface rough, lack gloss or show flow lines. For this reason, additives are needed to improve lubricity, reduce friction and lower interfacial adhesion.

In addition to improving fluidity, lubricants can also act as melting promoters, anti-blocking and antistatic agents, slipping agents, etc.

The role of external lubricants is mainly to improve the friction between the polymer melt and the hot metal surface of processing equipment. They have poor compatibility with the polymer and easily migrate from the interior of the melt to the outside, so they can form a lubricating thin layer at the interface between the plastic melt and the metal.

Internal lubricants have good compatibility with polymers. They play a role inside the polymer in reducing the intermolecular cohesive force of the polymer, thereby improving the internal friction heat generation of the plastic melt and the fluidity of the melt.

In practice, every lubricant has a role that can achieve a certain requirement; internal and external lubrication always work together, but one aspect may be more prominent. The same lubricant will show different lubricating effects in different polymers or under different processing conditions; for example, under high temperature and high pressure, an internal lubricant will be squeezed out and become an external lubricant.

In the production of plastic films, we will encounter some blocking phenomena. For example, in plastic film production, two layers of film are not easy to separate, which brings difficulties to automatic high-speed packaging. To overcome this, a small amount of an additive that increases surface lubricity can be added to the resin to increase external lubricity; it is generally called an anti-blocking agent or slipping agent. In general, the molecular structure of lubricants contains two parts: a long-chain nonpolar group and a polar group. Their compatibility in different polymers is different, thus showing different internal and external lubricating effects.

During extrusion molding, it can improve fluidity, improve the adhesion between the polymer material and the barrel and mold, and prevent and reduce retention. In addition, it can improve the appearance and gloss of the film. The amount of lubricant used is generally 0.5%~1%.

When selecting, attention should be paid to:

1. If the flow properties of the polymer already meet the needs of the molding process, mainly consider the role of external lubrication to ensure internal-external balance;
2. Whether external lubrication is effective should be judged by whether it can form a complete liquid film on the plastic surface at the molding temperature. Therefore, the melting point of the external lubricant should be close to the molding temperature, but a difference of 10℃~30℃ is needed to form a complete film;
3. Low surface attraction, low viscosity, good spreadability at the interface, and easy formation of an interfacial layer;
4. Excellent heat resistance and chemical stability, no decomposition or volatilization during processing; no corrosion of equipment, no contamination of the film, and no toxicity.

Usage Characteristics of Different Types of Lubricants:

Natural paraffin wax is mostly used for external lubrication and can be used as a lubricant and release agent for many plastics, with a general dosage of 0.2~1phr, but its compatibility, thermal stability and dispersibility are not very good, so the dosage should not be too large, and it is best to use it together with an internal lubricant; while white oil is mostly used as an internal lubricant for PVC and PS, with good lubricating performance and good thermal stability, and a general dosage of 0.5phr. They are both non-toxic and can be used for food packaging. There is also microcrystalline paraffin wax: it is also used as a lubricant in plastic processing, with a dosage of 1~2Phr, and its thermal stability and lubricity are better than those of ordinary paraffin wax. Low-molecular-weight polymers are also widely used as lubricants, such as polyethylene wax and low-molecular-weight polypropylene; their internal and external lubricity are both good, and they are non-toxic. Polyethylene wax is suitable for extrusion and calendering of materials such as PVC, with a general dosage of 0.1~1phr; it can improve processing efficiency, prevent film blocking, and improve the dispersibility of fillers or pigments, but its compatibility and transparency are not very good.

Transparent plastic films, sheets or plates, in a humid environment, when the humidity reaches below the dew point, will condense a layer of fine water droplets on their surface, making the surface blurred and fogged and hindering the transmission of light waves. For example, when using film to package products, the contents cannot be seen due to fogging, and the generated droplets can also easily cause the contents to rot and be damaged.

Classification of Anti-fog Agents:

▷ Internal anti-fog agents are added to the resin during compounding. Their characteristics are that they are not easily lost and have lasting efficacy, but it is difficult to obtain good anti-fog performance for polymers with high crystallinity;

▷ External coating anti-fog agents are dissolved in an organic solvent or water and then coated on the surface of plastic films. They are easy to use and low in cost, but have poor durability and are easily washed or wiped off. They are only used when internal anti-fog agents are ineffective or when durability is not required.

The efficacy of anti-fog agents can be divided into four types: initial anti-fog performance, durable anti-fog performance, low-temperature anti-fog performance and high-temperature anti-fog performance. It is difficult for one anti-fog agent to have all four effects; usually, according to the film's requirements for anti-fog effect, several types are selected and used in combination.

Anti-fog agents should have the following properties:

1. High anti-fog efficacy, rapid action and good durability;
2. Good thermal stability, not easily decomposed by heat, and the decomposition products do not cause degradation of the polymer;
3. Good compatibility with other additives, without hindering the functions of other additives;
4. No effect on the transparency, electrical properties, adhesion, contamination resistance and other functions of the film.

Several Typical Anti-fog Agents:

Glyceryl monooleate is a white waxy substance that can be used as an internal anti-fog agent. It has good initial anti-fog performance and low-temperature anti-fog performance and is suitable for food packaging films. The general dosage in polyethylene is 1~1.5 parts, and in polyolefins 0.5~1 part.

Sorbitan monopalmitate is a yellow granular substance and an internal anti-fog agent; it takes effect quickly and durably and is an internal anti-fog agent. It is a yellow granular solid with a melting point of about 60℃ and lasting efficacy. It is often used in agricultural films and can also be used as food packaging material. Its dosage in polyvinyl chloride is generally 1.5~1.8 parts, and in polyvinyl acetate 0.7~1 part.

Polyoxyethylene (20) glyceryl monostearate is also an internal anti-fog agent with extremely rapid action and good initial and low-temperature anti-fog performance. It is a pale yellow liquid and also has antistatic efficacy, making it suitable for food packaging films.