materials are organic substances that are susceptible to oxidative degradation. For example, polyolefins (i.e. PE, polypropylene) degrade by autoxidation, a cycle that can be slowed by the action of antioxidants. Throughout a plastic’s life cycle (i.e. production, processing, use, and disposal), the polymer is subjected to a variety of damaging stresses. This includes high temperatures and shear rates from the multiple melt compounding steps as the product is transformed from reactor powder or pellets into a finished article and ultimately processed again through recycling. In addition to temperature and shear, catalyst residues, entrained oxygen, and other types of impurities might also play a role in promoting further degradation of the polymer (Zweifel et al. 2001).
During these repeated heat histories, polymers undergo a series of free‐radical‐mediated oxidation reactions. These result in the formation of polymer hydroperoxides that thermally dissociate into additional free radicals (see Chapter 8 for detailed reactions). In addition to introducing oxygen‐containing functionalities into the plastic, the oxidative reactions also facilitate chain scission altering its average molecular weight (MW), MW distribution, and structure of the polymer backbone. When not stabilized adequately, the plastic will ultimately begin to lose its mechanical integrity; this will also limit the recyclability of the polymer and can lead to the formation of microplastics (Zweifel et al. 2001).
Antioxidants are used to prevent the formation of free radicals. Phenolic antioxidants scavenge oxygen‐centered free radicals, such as alkoxy‐, hydroxy‐, and peroxy‐type species, and prevent reaction with the polymer backbone (see Chapter 8). These substances include hindered phenols their and APs. Phosphites and thioesters are used to decompose the hydroperoxides into relatively inert products. These additives are used to impart longevity and stability in the plastic article. Since they are designed to remain active in the matrix for a long time, they continue to protect the article after disposal, which leads to long life spans of polymers in the environment. The most common antioxidants are listed in Table 2.3 along with their corresponding class and structures.
2.2.4 Heat Stabilizers
Heat stabilizers are added to plastics to protect the material from heat during processing and using the product. The most common application of heat stabilizers is in medical grade PVC where it is used at a concentration of 10–15% to protect the polymer during autoclaving (Sastri 2013). Due to the labile chlorine group, PVC is particularly susceptible to heat. Heat stabilizers work by trapping the hydrogen chloride (HCl) that is generated when PVC thermally degrades. They are also used in recycled materials, where they play the double role of inhibiting degradation and re‐stabilizing post‐use plastic waste (Ambrogi et al. 2017). Heat stabilizers are typically either metallic salts, organometallic compounds or nonmetallic organic stabilizers. Metallic salt heat stabilizers used in PVC, polystyrene (PS), and PE are commonly based on barium, cadmium, lead, or zinc and often used together to obtain a synergistic effect. Organometallic heat stabilizers are typically tin based.
2.2.5 Impact Modifiers
Impact modifiers (IMs) are a class of toughening functional additives that increase the impact strength of the plastic articles. Many commodity thermoplastics, such as PVC and PS, are brittle at ambient conditions (i.e., poor impact strength) and easily undergo cracking and crazing. In order to meet the physical requirements for certain applications, an IM additive is used. IMs are elastomeric and rubbery and have a lower modulus than the host polymer system. When effectively dispersed into the polymer matrix, the rubbery phase of the IM acts to absorb or dissipate the energy from impact in order to stop craze or crack propagation. IMs can be grafted to the polymer during polymerization or physically blended during compounding. Styrenic oligomers/copolymers, such as ABS and methyl methacrylate‐butadiene‐styrene, make up the largest category of IMs, accounting for about 45% of the market (Markarian 2004). These along with acrylics that command 30% of the market and are used mostly in PVC. Elastomers, including ethylene‐propylene‐diene terpolymer (EPDM) and thermoplastic elastomers, make up about 10% used with polyolefins. The remainder is made up of chlorinated polyethylenes (CPE) and other types. IMs, such as ABS, EPDM, and CPE, are also “stand‐alone” plastic products. These materials are used as IMs in their oligomeric forms (i.e. MWs of 5000–20 000 g/mol; Ambrogi et al. 2017). Similar to plasticizers and FRs, IMs are often used at relatively high concentrations in the plastic formulation. However, since most IMs are large molecules, leaching from the plastic into the environment has not been a major concern.
Table 2.3 Examples of common antioxidant additives used in plastics.
| Chemical name | Antioxidant class | Structure |
|---|---|---|
| Pentaerythritol tetrakis[3‐[3,5‐di‐tert‐butyl‐4‐hydroxyphenyl] propionate | Hindered phenol |
|
| Octadecyl‐3‐[3,5‐di‐tert‐butyl‐4‐hydroxyphenyl] propionate | Hindered phenol |
|
| Tris(2,4‐di‐tert.‐butylphenyl)phosphite | Phosphite |
|
| Trisnonylphenyl phosphite | Phosphite |
|
| Dialkyl ester of thiodipropionic acid | Thioester |
|
| N,N‐Octadecyl hydroxylamine | Hydroxylamine |
|
2.2.6 Lubricants
Lubricants are added to polymer formulations to ensure homogenous flow, uniform compositions, and quick release during processing and molding. There are three main types of lubricants: anti‐slip agents that reduce the coefficient of friction of the plastic laminates; external lubricants that coat the metal/polymer interface during processing to minimize the plastic from sticking to the machinery; and a third group of low mass compounds that promote the flow of the polymer in the melt (Brydson 1999). Some of the most commonly used lubricants in thermoplastics are fatty acid amides (primary erucamide and oleamide), fatty acid esters, metallic stearates (e.g. zinc stearate), silicones, and waxes (Bhunia, et al. 2013). There is not much information available on the
