How to select additives? Start with these 7 types of additives and 12 key performance indicators!

Plastic additives, also known as plastic auxiliary agents, refer to a range of chemical compounds incorporated into polymers (synthetic resins) during molding processing to optimize processability or remedy inherent performance deficiencies of base resins. For instance, plasticizers are added to polyvinyl chloride (PVC) to lower its molding temperature and endow finished products with flexibility; foaming agents are applied to produce lightweight, shock-resistant, heat-insulating and sound-absorbing foamed plastics. Certain plastics feature a thermal decomposition temperature close to their processing temperature, making molding impossible without thermal stabilizers. Therefore, plastic additives occupy an indispensable position in plastic fabrication.

This broad category of processing auxiliaries covers plasticizers, thermal stabilizers, antioxidants, light stabilizers, flame retardants, foaming agents, antistatic agents, biocides, colorants & whitening agents (refer to pigments), fillers, coupling agents, lubricants and release agents. Among them, colorants, whitening agents and fillers are not exclusive plastic-specific chemicals but universal compounding raw materials.

How to choose suitable plastic additives?

Plastic formulation design appears straightforward on the surface, yet it involves numerous intrinsic correlations. To develop formulations featuring high performance, excellent processability and cost competitiveness, multiple factors must be taken into account during additive selection. Choose proper additive grades according to targeted performance goals, ensuring incorporated additives can fully deliver expected functions and meet specified technical standards. Detailed additive selection criteria are listed as follows:

• Toughening——Elastomers, thermoplastic elastomers and rigid toughening materials
• Reinforcement——Glass fiber, carbon fiber, whisker and organic fiber
• Flame retardancy——Bromine-based (conventional & eco-friendly grades), phosphorus-based, nitrogen-based, intumescent nitrogen-phosphorus composite flame retardants, antimony trioxide, hydrated metal hydroxides
• Antistatic property——Various antistatic agents
• Electrical conductivity——Carbon materials (carbon black, graphite, carbon fiber, carbon nanotubes), metallic fiber & powder, metal oxides
• Magnetism——Ferrite magnetic powder, rare-earth magnetic powder including samarium-cobalt (SmCo₅, Sm₂Co₁₇), NdFeB, SmFeN, Al-Ni alloy
• Thermal conductivity——Metallic fiber & powder, metal oxides, nitrides, carbides; carbon series: carbon black, carbon fiber, graphite, carbon nanotubes; semiconductor materials: silicon, boron
• Heat resistance——Glass fiber, inorganic fillers, heat-resistant additives including substituted maleimide & β-nucleating agents
• Transparency: Nucleating agents——sorbitol-based α-nucleators deliver optimal performance for PP
• Wear resistance——Graphite, molybdenum disulfide, copper powder
• Electrical insulation——Calcined kaolin clay
• Barrier performance——Mica, montmorillonite, quartz

Compatibility between Additives and Plastics

• Red phosphorus flame retardants work well for PA, PBT and PET.
• Nitrogen-based flame retardants are effective for oxygen-containing polymers such as PA, PBT and PET.
• Nucleating agents deliver excellent performance for copolymerized PP.
• Thermal modification via glass fiber performs favorably on crystalline plastics but poorly on amorphous plastics.
• Carbon black achieves superior conductive modification in crystalline resins.
• For additives with identical chemical composition, particle morphology exerts a significant impact on modification efficiency.

Additive Morphology

• Fibrous additives feature outstanding reinforcing performance. (The fibrous degree is characterized by aspect ratio (L/D). A higher L/D ratio brings better reinforcement, which explains why glass fiber is fed through the vent port during processing.)
• Melt-fed form helps preserve the original aspect ratio and reduces fiber breakage compared with powder feeding.
• Spherical additives provide favorable toughening effect and superior surface gloss.
• Barium sulfate is a typical spherical filler, hence it is widely adopted for high-gloss PP to achieve moderate rigid toughening.

Additive Particle Size

① Effect of additive particle size on mechanical properties:

Smaller particle size contributes to higher tensile strength and impact strength of filled compounds.

② Effect of additive particle size on flame retardancy:

Fine particle size improves flame retardant efficiency. For hydrated metal oxides and antimony trioxide, finer grades require lower loading to reach identical flame retardant performance.

③ Effect of additive particle size on color matching:

Colorants with smaller particle size feature superior tinting strength, hiding power and uniform color dispersion.

④ Effect of additive particle size on electrical conductivity:

Taking carbon black as an example, finer particles facilitate formation of continuous conductive network, cutting required dosage for target conductivity. Similar to colorants, particle size has a critical minimum limit; excessively fine particles tend to agglomerate and disperse poorly, resulting in deteriorated performance.

Surface Treatment of Additives

Surface modification improves the performance of all inorganic additives, especially fillers, as well as glass fibers and inorganic flame retardants.

Coupling agents and compatibilizers are the primary surface treatment materials. Common coupling agents include silanes, titanates and aluminates, while compatibilizers refer to maleic anhydride-grafted polymers matching the base resin.

Plasticizer

Plasticizers are low-volatility organic compounds miscible with polymers to a certain extent. They reduce the melt viscosity of polymers as well as the glass transition temperature and elastic modulus of finished articles, by weakening intermolecular attraction between polymer chains.

Plasticizers rank among the earliest commercialized plastic additives. In the latter half of the 19th century, camphor and phthalate esters were utilized as plasticizers for nitrocellulose. Large-scale industrialization of PVC starting in 1935 fueled widespread plasticizer adoption. Currently around 80% of global plasticizer consumption goes to PVC and vinyl chloride copolymers; the remainder is applied to cellulose derivatives, polyvinyl acetate, polyvinyl alcohol, natural rubber and synthetic rubber. Flexible PVC generally incorporates 45%~50% by mass of plasticizers. Robust growth of rigid PVC requiring little or no plasticizer has slowed plasticizer demand growth below that of PVC in most industrialized economies. In China, flexible PVC products still account for a large market share, sustaining steady growth for plasticizers.

Phthalate esters dominate global plasticizer output, accounting for approximately 80% of total production, with dioctyl phthalate (DOP) as the most critical grade. Other smaller-volume plasticizer categories include: adipate and sebacate esters (outstanding low-temperature resistance), phosphate esters (flame-retardant property), epoxidized oils & epoxy esters (synergistic effect with heat stabilizers), trimellitates and pentaerythritol esters (excellent heat resistance), chlorinated paraffins (secondary & flame-retardant plasticizers), and phenyl alkylsulfonates (secondary plasticizers).

Thermal Stabilizer

Thermal stabilizers mainly inhibit thermal degradation during processing and retard product aging during long-term service. They are mostly consumed in PVC and vinyl chloride copolymers, with a typical loading of around 2% for flexible products and 3%–5% for rigid formulations.

Major categories of thermal stabilizers include basic lead salts, fatty acid soaps, organotins, organic auxiliary stabilizers and composite stabilizers.

① Basic lead salts such as tribasic lead carbonate and dibasic lead phosphite are among the earliest-developed grades and still widely used today. They feature excellent heat resistance, electrical insulation and weatherability at low cost, yet suffer from toxicity, poor transparency and inferior dispersibility.

② Fatty acid soaps mainly consist of cadmium, barium, calcium, zinc and magnesium salts of stearic acid and lauric acid. Combined use of cadmium soap with barium soap, or calcium soap with zinc soap delivers obvious synergistic effects. Cadmium soap is highly toxic and barium soap carries moderate toxicity, while calcium and zinc soaps are non-toxic.

③ Organotin stabilizers represent the fastest-growing category in recent years. Boasting outstanding transparency, most grades excel in heat and weather resistance, making them indispensable for rigid transparent PVC articles. Dioctyltin bis(isooctyl mercaptoacetate) and di-n-octyltin compounds are the most widely used non-toxic organotin stabilizers.

④ Phosphites and epoxy compounds are common auxiliary components for composite stabilizer systems. Popular composite stabilizer types include conventional Cd-Ba(-Zn), sulfide-stain resistant Ba-Zn, non-toxic Ca-Zn and organotin blends, most of which are liquid products.

Antioxidants

Also known as oxidation inhibitors. Most polymers undergo auto-oxidation at varying rates under ambient and elevated temperatures, triggering yellowing, molecular degradation and mechanical strength loss of plastics. Any substance capable of suppressing or slowing such oxidation is defined as an antioxidant.

Phenolic compounds were first adopted to retard rubber oxidation in 1918. Early modern antioxidants including alkylphenols and p-phenylenediamines came into commercial production in the 1930s. Though added at low loadings of 0.1%～1.0% in plastics, antioxidants serve as critical additives for high-volume resins such as polyolefins, styrenics, PVC, polyamides and polyacetals. In the rubber sector, antioxidants are conventionally termed anti-degradants or age resistors.

Plastic-grade antioxidants fall into two core groups: primary phenolic antioxidants and secondary antioxidants such as thiodipropionates and phosphites.

Primary antioxidants, also called chain terminators, trap reactive free radicals generated during oxidative degradation to terminate radical chain reactions. Typical examples include 2,6-di-tert-butyl-4-methylphenol (Antioxidant 264) and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (Antioxidant 1010).

Secondary antioxidants, or peroxide decomposers, break down peroxide intermediates from oxidation into non-radical substances.

Dilauryl thiodipropionate and diphenyl isooctyl phosphite are the most widely used secondary antioxidants.

Primary and secondary antioxidants are commonly compounded together to achieve prominent synergistic effects.

Light Stabilizer

When plastics and other polymers absorb ultraviolet energy, auto-oxidation is initiated and eventually leads to material degradation. This process is defined as photo-oxidation or photoaging, and chemicals capable of inhibiting or slowing down such deterioration are known as light stabilizers. In the 1940s, phenyl salicylate was first applied as a light stabilizer for cellulose acetate. Benzophenone-type and benzotriazole-type products were successively commercialized in the early 1950s and early 1960s respectively, followed by hindered amine light stabilizers (HALS) emerging in the mid-1970s. Light stabilizers are dominantly consumed in polyolefins, especially polypropylene. Small dosages (0.1%～0.5% by weight) are also incorporated into PVC, polycarbonate and polyesters.

Based on working mechanisms, light stabilizers are classified into UV absorbers, quenchers and light screening agents. UV absorbers strongly capture UV rays ranging from 290 nm to 400 nm; 2-hydroxy-4-octoxybenzophenone (UV-531) and 2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole (UV-327) represent two high-efficiency grades. Most quenchers are organic nickel complexes, which rapidly deactivate excited-state molecules back to ground state to block subsequent photoreactions. Carbon black is the typical light screening agent to reduce UV transmittance. New-generation hindered amine light stabilizers are mostly derivatives of tetramethyl or pentamethylpiperidinol. With combined functions of radical scavenging, singlet oxygen quenching and peroxide decomposition, they deliver outstanding stabilizing efficiency.

Flame Retardant

Most plastics are combustible. With the extensive application of plastics in construction, furniture, transportation, aerospace, electrical appliances and other fields, improving the flame retardancy of plastics has become an urgent task. Flame retardants are additives that suppress polymer combustion, most of which are compounds of Group Ⅲ, Ⅴ and Ⅶ elements in the periodic table, especially compounds of phosphorus, bromine, chlorine, antimony and aluminum.

Flame retardants fall into two categories: additive type and reactive type. Common additive flame retardants include phosphate esters, halogenated phosphates, halogenated hydrocarbons, antimony oxide and aluminum hydroxide. They feature easy processing and wide compatibility, yet high loading (10%~30%) often impairs the intrinsic properties of base plastics. Reactive flame retardants are monomers containing flame-resistant elements, causing minimal adverse impacts on polymer performance. Typical examples are halogenated anhydrides for polyesters, tetrabromobisphenol A for epoxy resins, and phosphorus-containing polyols for polyurethane.

Flame retardants were first commercialized in the US and have seen dramatic consumption growth since the 1960s; currently their global consumption ranks second only to plasticizers. The major consuming resins are PVC, polystyrene, flexible polyurethane foam, unsaturated polyester, ABS and polypropylene.

Non-halogen flame retardants contain no halogen elements. Typical halogen-free inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, red phosphorus and expandable graphite. Halogenated flame retardants refer to halogen-containing polymers or blended formulations compounded with halogen-based additives, represented by antimony trioxide and decabromodiphenyl ether, commonly known as bromine-antimony synergistic flame retardants. Thanks to excellent flame resistance, bromine-antimony systems were widely used in the past. Nevertheless, upon exposure to fire heat, products formulated with halogenated flame retardants release dense smoke plus toxic, corrosive hydrogen halide fumes.

Foaming Agent

They are classified into two main categories: physical blowing agents and chemical blowing agents, and chemical blowing agents are further divided into inorganic and organic types. Upon heating inside polymers, these additives vaporize or decompose to generate gas and form cellular pores in plastic compounds, hence the name blowing agent.

Physical blowing agents are mostly chlorinated hydrocarbons or chlorofluorocarbons such as trichlorofluoromethane and trichlorotrifluoroethane, mainly applied for polyurethane foam production. The dominant chemical blowing agents are azodicarbonamide (Blowing Agent AC) and N,N'-dinitrosopentamethylenetetramine (Blowing Agent H). AC is widely used in plastics while H is mostly for rubber products.

Antistatic Agent

The volume resistivity of polymers generally ranges from 1010 to 1020Ωcm, making them prone to static charge accumulation and potential hazards. Most antistatic agents belong to surfactants, which render plastic surfaces hydrophilic; ionic surfactants additionally provide electrical conductivity to allow timely static dissipation.

Based on application methods, antistatic agents are categorized into external and internal types. External antistatic agents are formulated into dilute solutions at 0.5%–2.0% concentration for surface coating of plastics. Internal antistatic agents are incorporated by compounding, with typical loadings varying from 0.1% to 3.0%. Cationic surfactants including ammonium salts, quaternary ammonium salts and alkyl imidazolines are widely used antistatic grades for plastics.

Want to know more? Please contactus at: yihoo@yihoopolymer.com. Besides the aforementioned high-performance nylon materials, we also offer various chemicalproducts such as light stabilizers, UVabsorbers, antioxidants, WPC-specific additives, nucleating agents, anti-static agents, matte polyurethane dispersions, etc. Welook forward to your inquiries!