As part of the so-called compounding process, various additives, so-called additives, are added to plastics during their production ( additives for plastic ) . This is needed to adjust the material properties of the finished plastic to the requirements of the respective application. In addition, the additives in the plastic sometimes improve the chemical, mechanical and/or electrical properties of the compound in question. The standards DIN EN ISO 1043 with regard to thermoplastics and DIN 7708 with regard to thermosets apply to these molding compounds containing additives.
Effect of the additives on the plastic ( additives for plastic )
plasticizer
Around two thirds of the additives produced worldwide are used in the production of polyvinyl chloride (PVC), while around three fifths of the additives serve as plasticizers. Such an additive is suitable for reducing the glass transition temperature as well as the hardness and brittleness of a plastic. In this respect, such an additive makes the plastics mixed with it easier to process and shape. It is necessary for this purpose to use substances that can penetrate the plastic at the molecular level. This is the only way they can improve the mobility of the individual molecular chains relative to one another.
Plasticizers make plastics softer. They can be described as a “molecular lubricant” in qualitative terms. Until recently, the most commonly used plasticizer was diethylhexyl phthalate (DEHP) (also: dioctyl phthalate DOP). However, since this additive has proven to be harmful to the environment and health, at least the European industry has largely stopped using this additive in recent years.
Various stabilizers
Stabilizers are added to plastics to improve their chemical properties. These additives are therefore suitable for increasing the service life of the plastic in question. They also protect it from various damaging influences in the respective area of use, such as heat from fire, radiation and oxidation. The most important stabilizers include:
Antioxidants
By reacting with the oxygen in the air, a plastic can change color. In addition, it is possible that the polyester chains decompose or unintentionally re-crosslink. An attempt is made to prevent this by adding antioxidants as an additive to the plastic. They intercept the free radicals created during oxidation as so-called radical chain breakers. Or they act as deactivators to prevent the formation of these radicals from the outset. Phenols and amines, among other things, are added for demolition. Phosphines and amines, for example, serve as deactivators.
Sunscreen
Light stabilizers are used to protect plastic from ultraviolet light. Since double bonds between carbons are able to absorb this light spectrum, plastics that have corresponding structural elements are particularly at risk from UV light. This includes, for example, polyisoprene. There are three ways to prevent UV light from damaging such plastics. By reflecting light, adding radical scavengers or adding light-absorbing substances. An important light protection agent is soot, which absorbs light. σ-Hydroxybenzophenone, on the other hand, is suitable for converting light energy into infrared radiation. Finally, dialkyldithiocarbamates absorb UV light and at the same time act as free radical scavengers.
Heat stabilizers
Many plastics are sensitive to the effects of heat. Above the so-called decomposition temperature (characteristic of the material), the molecular structure of the plastic breaks down. Heat stabilizers are used to prevent this. For some plastics, such stabilizers, which are added to the plastic as additives, are even essential. A good example is PVC. Without the use of heat stabilizers, it can lose its mechanical stability when exposed to heat and also forms hydrogen chloride and potentially health-endangering decomposition products. The decay occurs through the formation of double bonds. To interrupt this decay, organic cadmium, tin, zinc and barium compounds are used. They or inorganic lead salts complex the double bonds and thereby interrupt the decay mechanism.
Reinforcements for plastics
Additives that are intended to reinforce the plastic matrix are called reinforcing materials. This is referred to as “reinforcement”. Such additives improve the mechanical and/or physical properties of the plastics in question. For example, greater elasticity or bending strength can be achieved. Examples of such reinforcing materials include flax and jute, but also carbon and glass fibers. They are mixed into the plastic in different lengths as required. Depending on the plastic used and the fiber length, different processing methods such as injection molding, RTM, LFI etc. are used.
Flame retardants
It is also additives for plastic, In the event of a fire, plastics pose a great danger. On the one hand, they are suitable for maintaining a fire, but on the other hand, there is the possibility that toxic and/or corrosive gases are produced when they are burned. These include, for example, hydrogen cyanide, hydrogen chloride, carbon monoxide and various dioxins. Flame retardants are used as additives in plastics to either prevent the flow of oxygen to the fire or to interrupt the chemical reactions (so-called radical chain mechanisms) during combustion. It is often not necessary to add flame retardants to polycarbonates because when they are burned, carbon dioxide is produced as a decomposition product. This serves as an extinguishing agent.
The most important flame retardants include:
- Aluminum hydroxide, Al(OH)3) or (ATH): It releases water molecules during combustion.
- Phosphorus-containing compounds: When burned, they form phosphoric acids, which catalyze the elimination of water.
- polybrominated diphenyl ethers (PBDEs): They are able to release radicals. These capture the intermediate products of the burning process.
- Expanded graphite/expandable graphite: A mineral flame retardant that works by greatly expanding its volume.
colorants
It is also additives for plastic, It is also additives for plastic, Almost all polymers are colorless in their pure form. They only become colored when colorants are added. A distinction can be made between dyes and pigments as additives in plastics. Dyes are additives that dissolve at the molecular level or adsorb on the surface. Pigments are insoluble and are mostly inorganic aggregates. Plastics are usually colored using pigments. On the one hand, these are more cost-effective and, on the other hand, offer better light fastness. The most important pigments include soot (black), cobalt and ultramarine blue, rutile (white) and chromium oxide green. Metallic luster is achieved with aluminum flakes as an additive, while spherical pigment grains ensure a gray color.
fillers
In order to produce plastic more cheaply, classic extenders, also known as fillers, are added. There are numerous active fillers that also improve the mechanical properties of the plastics in question. The fire behavior of plastics can also be reduced using fillers. The most important fillers include carbon black, graphite, talc and starch as well as quartz, zinc oxide, glass fibers, sand, chalk and diatomaceous earth.
Other additives for plastic are:
- Propellant
- lubricant
- Antistatics
- Impact modifier