Composition of Plastic

The composition of plastic is various resins, which in turn are polymers, but plastics and polymers are different.

The properties of pure polymers often cannot meet the requirements of processing and actual use and are generally not used alone. Only after adding additives can they be used in industry. Therefore, plastics are made of synthetic resin as the main component, and then add various other additives (also known as auxiliary agents). Synthetic resin determines the basic performance of plastic products.

Its function is to integrate multiple additives into a whole. Additives are some substances added to improve the molding process performance of plastics, improve the performance of products or reduce costs.

There are many kinds of additives used in plastic materials, such as fillers, plasticizers, colorants, stabilizers, curing agents, oxygenating agents, etc. In plastics, although resin plays a decisive role, additives also play a role that cannot be ignored.

Composition of Plastic: Resin

Resins are polymers that soften when heated and tend to flow under external forces. It is the most important component of plastics. It plays a bonding role in E-plastics, also called adhesives. It determines the type and basic properties of plastics, such as thermal properties, physical properties, chemical properties, mechanical properties, and electrical properties.

Composition of Plastic: Additive

Composition of Plastic: Filler

Fillers, also known as fillers, are important but not essential ingredients for every plastic. Fillers are generally powdery substances and are inert to polymers. The purpose of adding fillers when formulating plastics is to improve the molding and processing properties of plastics.

Improve certain properties of products, endow plastics with new properties, and reduce costs. For example, adding the wood powder to phenolic resin overcomes its brittleness and reduces the cost. Calcium fillers are added to resins such as polyethylene and polyvinyl chloride to become cheap, rigid, and heat-resistant calcium plastics, glass fibers are used as plastic fillers to greatly improve the mechanical properties of plastics, some fillers can also make plastics have properties that resins do not have, such as electrical conductivity, magnetic conductivity, thermal conductivity, etc.

True fillers include inorganic fillers and organic fillers. Commonly used fillers are in powder, fibrous, and flake forms. Powdery fillers include wood, pulp, marble, talcum powder, mica powder, asbestos powder, graphite, etc. fibrous fillers include cotton, flax, glass fiber, asbestos fiber, carbon fiber, boron fiber, metal whiskers, and flake fillers Agents include paper, cotton cloth, linen, and glass cloth. The amount of filler usually accounts for less than 40% of the total composition of the plastic.

The shape of the filler is spherical or cube-shaped, which can usually improve molding performance, but the mechanical strength is poor, while the scale-shaped one is the opposite. The finer the particles, the greater the improvement effect on the rigidity, impact, tensile strength, stability, and appearance of plastic products.

Composition of Plastic: Plasticizer

Plasticizers are organic compounds with low volatility and high boiling points that are compatible with resins. They can increase the plasticity and softness of plastics, improve their molding properties, and reduce rigidity and brittleness. Its function is to reduce the force between polymer molecules so that the resin polymer is easy to produce relative slip so that the plastic has good plasticity and softness at a lower temperature.

For example, adding dibutyl phthalate to polyurethane resin can turn it into a soft plastic like rubber. However, while adding plasticizers to improve the processing performance of plastics, sometimes it will also reduce some properties of the resin, such as the stability of plastics, dielectric properties, and mechanical strength. Therefore, the content of plasticizers should be reduced as much as possible in the plasticizer, and most plastics generally do not add plasticizers.

Requirements for plasticizers:

• Good compatibility with resin.

• Low volatility, not easy to precipitate from plastic products.

• Non-toxic, colorless, and odorless.

• Relatively stable to light and heat.

• Does not absorb moisture.

• Commonly used plasticizers are dibutyl phthalate, dioctyl phthalate, camphor, etc.

Composition of Plastic: Colorant

The natural color of most synthetic resins is white translucent or colorless and transparent. To obtain various desired colors for plastic products, coloring agents are often added in industrial production to change the natural color of synthetic resins, and to obtain bright and beautiful products.

Some colorants can also improve the light stability and thermal stability of plastics. For example, natural polyoxymethylene plastics can help prevent light aging to a certain extent after being colored carbon black.

Colorants are mainly divided into two types: pigments and dyes. The pigment is a colorant that cannot be dissolved in common solvents, so to obtain ideal adhesion performance, it is necessary to uniformly disperse the pigment in the plastic by mechanical means. According to the structure, it can be divided into organic pigments and inorganic pigments. Inorganic items have excellent thermal stability and light stability, low price, but relatively poor coloring power, and high relative density, such as sodium carbonate, yellow sulfur red-brown, pigment blue, carbon black, etc.

Organic pigments have high tinting power, bright color, complete chromatogram, and low relative density, the disadvantages are that they are not as good as inorganic pigments in terms of heat resistance, weather resistance, and hiding power, such as chrome yellow, chromium oxide, and lead powder. Dyes are organic compounds that can be used in most solvents and plastics to be dyed. The advantages are low density, high tinting strength, and good transparency. However, their general molecular structure is small, and they are prone to migration during coloring, such as shilin blue.

The general requirements for colorants are strong coloring power, good compatibility with resins, no chemical reaction with other components in plastics, stable properties, and no degradation and discoloration due to changes in temperature and pressure during the molding process, and in plastics, the product can remain stable during long-term use.

Composition of Plastic: Stabilizer

Throughout production and use, resins deteriorate or age. When a macromolecular chain splits or undergoes a damaging change in chemical structure due to the impact of heat, force, oxygen, water, light, rays, etc, this is referred to as degradation. Stabilizers are substances that are added to polymers to prevent their chemical properties from deteriorating and crosslinking under the influence of environmental conditions such as heat, light, oxygen, and mold.

According to the different functions of stabilizers, they can be divided into heat stabilizers, light stabilizers, and antioxidants.

• Heat Stabilizer: The main function is to inhibit the thermal degradation reaction that may occur during the plastic molding process, to ensure the smooth molding of plastic products, and obtain good quality. For example, organotin compounds are commonly used in polyvinyl chloride, which is non-toxic but expensive.

• Light Stabilizer: Substances are added to prevent degradation and performance degradation of plastics under sunlight, light, and high-energy ray irradiation.

• The types include UV absorbers, light shielding agents, etc, and benzoate esters and carbon black are commonly used as UV absorbers.

• Antioxidant:  Additives to prevent oxidative degradation of plastics at high temperatures, phenolic and organic amine compounds are often used as antioxidants. Stabilizers are added to most plastics, and the content of stabilizers is generally 0.3% to 0.5% of the plastic. Requirements for stabilizers, good compatibility with resins, good stabilizing effects on polymers, water resistance, oil resistance, chemical corrosion resistance, no degradation, low volatility, and colorless during the molding process.

Composition of Plastic: Hardener

The curing agent is also called a hardener and a crosslinking agent. The cross-linking reaction required to form thermosetting plastics and synthetic resins with a linear polymer structure is transformed into a bulk polymer structure. The purpose of adding a curing agent is to promote the crosslinking reaction. For example, adding to epoxy resin ethylenediamine, triethanolamine, etc.

In addition, some other additives can be added to the plastic, such as a foaming agent, flame retardant, antistatic agent, conductive agent, and magnetic permeability agent. For example, flame retardants can reduce the flammability of plastics, blowing agents can be made into foamed plastics, and antistatic agents can make plastic products have the right amount of electrical conductivity to eliminate static electricity.

In actual work, whether or not to add additives to plastics, and what kind of additives should be added should be determined according to the type of plastic and the use requirements of plastic products.