Plastic polymers have chemical reaction properties similar to those of small molecules, though the polymers themselves are larger in size. This means that a range of different factors, including thermal conditions, stress cracking, or the diffusion of chemical additives, can alter the molecular structure, and thus the fundamental properties, of most plastic polymer materials. Some changes, such as unintentional reduction in molecular weight, can lead to plastic degradation and product failure, while others can supplement or improve a polymer’s characteristics.
Most plastic additives are introduced into a compound to produce a specific result, whether to increase formability or merely change the pigmentation. In the case of copolymers, which are composed of varied and repeating molecular units, each substance included in the material plays a part in its overall chemical makeup. This makes it important to carefully control the amount and types of additives that are included because they do not usually bond to a polymer molecule and thus increase the chemical susceptibility of a plastic material. Different kinds of additives display different vulnerabilities and strengths, but each of them can influence the effectiveness of polymer production.
For information on methods for identifying the presence of additives within a polymer compound, see Carrott, Jones, and Davidson’s Identification and Analysis of Polymer Additives.
While some polymers, like rubber, are naturally flexible, others, such as lignin or cellulose nitrate, are comparatively rigid and cannot be softened by exposure to non-solvent materials. For this reason, plasticizing compounds may be added to a polymer to reduce its stiffness and increase its formability. Plasticizers collect into groups of molecules between different polymer chains without altering the polymer’s volume. The result is less constrained polymer chain movement, as signaled by a rising dielectric constant.
Plasticizers should generally have a solubility level close to that of the polymer itself, and multiple plasticizing additives can be used in a single mixture as long as they are compatible with each other and the polymer. When a plasticizer, such as dioctyl phthalate, is introduced to a polyvinyl chloride (PVC) polymer, it lowers its melt viscosity and increases its light stability. It provides resistance to oxidizing acids, but also makes the polymer more vulnerable to fungal contaminants and corrosives.
Pigmenting additives can be used to alter the color of a polymer material, which is beneficial for a range of consumer products. However, some pigments can increase a polymer’s susceptibility to chemical reactions, making careful selection an important step in the pigmentation process. Pigments such as carbon black do not react to corrosives, but clay and other hydrophilic additives are water absorbent, which may harm certain polymers. Likewise, carbonate pigments, such as limestone, can make material susceptible to corrosion from inorganic acids. These effects can be mitigated with the addition of bonding agents, such as organosilane, which helps compensate for a lack of bonds between the pigment and the polymer molecules.
UV Stabilizers and Antioxidants
Many unsaturated polymers can undergo degradation from a range of sources, such as oxygen absorption, which leads to the release of organic peroxides. To reduce the risk of mechanical and chemical deterioration, phenolic or amine antioxidants can be added to these polymers, while certain pigments and stabilizers can help shield them from ultraviolet light. Some of the common additive groups used for these purposes include:
• Hindered Phenol: A phenol consists of a benzene ring coupled with a hydroxyl group. When adjacent carbon atoms have their bonded hydrogen replaced with heavier elements, the result is a hindered phenol compound, which prevents oxidative degradation of polymers such as rubber.
• Antioxidants: Secondary amines that include aromatic hydrocarbons, such as benzene, in their composition can serve as antioxidants that protect polymers from oxidative deterioration. However, they are also reactive with acids, leaving them vulnerable to acid corrosion.
• Stabilizers: Some organic polymers can be degraded by photo-oxidation, which is caused by oxygen under the exposure of ultraviolet light. Pigments like carbon black and UV stabilizers like hydroxyl benzophenone can be added to inhibit the rate of photo-oxidation.
Polymers that exhibit difficult formability may benefit from lubricants or processing aids that can help keep the material from sticking to metal surfaces and lower the amount of energy needed to fabricate it. Certain additives, such as acrylic polymers, can improve impact and provide greater ease of processing at the same time. PVC material that is intended for molding or extrusion is often treated with lubricants, processing aids, and impact additives at the same time. While beneficial, these additives may also be eroded by solvents, which may lead to contamination from other chemicals.
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