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Fillers and additives in plastics

Plastic additives represent a broad range of chemicals used by resin manufacturers, compounders, and fabricators to improve the properties, processing, and performance of polymers. From the earliest days of the plastics industry, additives have been used initially to aid these materials in processing and then to improve their properties.

Plastic additives are comprised of an extremely diverse group of materials. Some are complex organic molecules (antioxidants and light stabilizers for example) designed to achieve dramatic results at very low loadings.

Plastics. Plastics denotes the matrix thermoplastic or thermoset materials in which additives are used to improve the performance of the total system. There are many different types of plastics that use large volumes of chemical additives including (in order of total additive consumption): polyvinyl chloride (PVC), the polyolefins, the styrenics - [polystyrene (PS) and acrylonitrile butadiene styrene (ABS)], and engineering resins such as polycarbonate and nylon [2, p. 43].

Antioxidants are used in a variety of resins to prevent oxidative degradation.

Degradation is initiated by the action of highly reactive free radicals caused by heat, radiation, mechanical shear, or metallic impurities.

The initiation of free radicals may occur during polymerization, processing, or fabrication.

Once the first step of initiation occurs, propagation follows. Propagation is the reaction of the free radical with an oxygen molecule, yielding a peroxy radical. The peroxy radical then reacts with an available hydrogen atom within the polymer to form an unstable hydroperoxide and another free radical. In the absence of an antioxidant, this reaction continues and leads to degradation of the polymer.

The function of an antioxidant is to prevent the propagation steps of oxidation. Products are classified as primary or secondary antioxidants depending on the method by which they prevent oxidation [1, p. 64].

Amines. Amines, normally arylamines, function as primary antioxidants by donating hydrogen. Amines are the most effective type of primary antioxidant, having the ability to act as chain terminators and peroxide decomposers. Amines are commonly used in the rubber industry but also find minor use in plastics such as black wire and cable formulations and in polyurethane polyols [5, p. 86].

Phenolics. The most widely used antioxidants in plastics are phenolics. The products generally resist staining or discoloration. However, they may form quinoid (colored) structures upon oxidation. Phenolic antioxidants include simple phenolics, bisphenolics, polyphenolics, and thiobisphenolics.

The most common simple phenolic is butylated hydroxytoluene (BHT) or 2,6di-t-butyl-4-methylphenol. BHT possesses broad FDA approval and is widely used as an antioxidant in a variety of polymers. The disadvantage of BHT is that it is highly volatile and can cause discoloration.

Organophosphites. Acting as secondary antioxidants, organophosphites reduce hydroperoxides to alcohols, converting themselves to phosphonates. They also provide color stability, inhibiting the discoloration caused by the formation of quinoid reaction products which are formed upon oxidation of phenolics. The disadvantage of phosphites is their hygroscopic tendency. Hydrolysis of phosphites can ultimately lead to the formation of phosphoric acid, which can corrode processing equipment [3, p. 42].

Thioesters. Thioesters function as secondary antioxidants by destroying hydroperoxides to form stable hexavalent sulfur derivatives. Thioesters act as synergists when combined with phenolic antioxidants in polyolefins. The major disadvantage of thioester antioxidants is their inherent odor which is transferred to the host polymer.

Heat stabilizers are used to prevent the thermal degradation of resins during periods of exposure to elevated temperatures. Almost all heat stabilizers are used to stabilize PVC, polyvinylidene chloride (PVDC), vinyl chloride copolymers (for example, vinyl chloride/vinyl acetate), and PVC blends (for example, PVC and ABS). Thermal degradation is prevented not only during processing but also during the useful life of the finished products.

There are three major types of primary heat stabilizers, which include:

_ Mixed metal salt blends

_ Organotin compounds

_ Lead compounds

Heat stabilizers belong to one of the two major classes: primary heat stabilizers and secondary heat stabilizers. When heated, chlorinated vinyl resins liberate HCl which causes further polymer degradation and discoloration. Primary heat stabilizers function both by retarding this dehydrochlorination and by reacting with liberated HCl to delay progressive degradation [4, p. 133].

Plasticizers are the largest volume additives in the plastic industry. They are largely used to make PVC resin flexible. The primary role of a plasticizer is to impart flexibility, softness, and extensibility to inherently rigid thermoplastic and thermoset resins. Secondary benefits of plasticizers include improved processability, greater impact resistance, and a depressed brittle point. Plasticizers can also function as vehicles for plastisols (liquid dispersions of resins which solidify upon heating) and as carriers for pigments and other additives. Some plasticizers offer the synergistic benefits of heat and light stabilization as well as flame retardancy.

Plasticizers are typically di - and triesters of aromatic or aliphatic acids and anhydrides. Epoxidized oil, phosphate esters, hydrocarbon oils, and some other materials also function as plasticizers.

The major types of plasticizers are

- Phthalate esters

- Aliphatic esters

- Epoxy esters

- Phosphate esters

- Trimellitate esters

- Polymeric plasticizers

- Other plasticizers

Phthalate esters. The most commonly used plasticizer types are phthalate esters. They are manufactured by reacting phthalic anhydride (PA) with 2 moles of alcohol to produce the diester. The most often used alcohols vary in chain length from 6 to 13 carbons.

Epoxy ester. Epoxy ester plasticizers have limited compatibility with VC. Therefore, they are used at low levels. Epoxidized soybean oil (ESO), the most widely used epoxy plasticizer, is also used as a secondary heat stabilizer. As a plasticizer, it provides excellent resistance to extraction by soapy water and low migration into adjoining materials that tend to absorb plasticizers.

Polyurethanes are versatile polymers typically composed of polyisocyanates and polyols. By varying constituents, a broad range of thermosets and thermoplastics can be produced and used in different applications. Although isocyanates have the ability to form many different polymers, very few types are used in actual production. The most common diisocyanates are methylene diphenylene diisocyanate and toluene diisocyanate [6, p. 39].

Coupling agents are additives used in reinforced and filled plastic composites to enhance the plastic-filler-reinforcement interface to meet increasingly demanding performance requirements. In general, there is little affinity between inorganic materials used as reinforcements and fillers and the organic matrices in which they are blended. By chemically reacting with the resin and the filler or reinforcement components, coupling agents form strong and durable composites. Coupling agents significantly improve mechanical and electrical properties for a wide variety of resins, fillers, and reinforcements. In addition, they act to lower composite cost by achieving higher mineral loading.

Fiberglass reinforcement for plastics is the major end use of coupling agents. Thermoset resins, such as polyester and epoxy, account for approximately 90% of coupling agent consumption. Kaolin clay, wollastonite, and glass fiber are the leading fillers or reinforcements chemically treated with coupling agents.

Impact modifiers are used in a wide variety of thermoplastic resins to absorb the energy generated by impact and dissipate it in a nondestructive fashion. The behavior and definition of impact modifiers are complex. The selection of an impact modifier is dependent on compatibility, physical solubility, impact performance, and cost. Impact modifiers are primarily used in PVC, engineering resins, and polyolefins. The use levels of impact modifiers vary widely depending upon the modifiers, matrix type, and properties desired.

References

plastic additive polymerschemical

1. R. Ray Patrylak, Plastics Handbook, Modern Plastics Magazine, McGrawHill, New York, 1994, р. 110.

2. Carreau, P.J., D.C.R. De Kee, and R.P. Chhabra, Rheology of Polymeric Systems-Principles and Applications, Hanser Publishers, New York, 1997, p. 52.

3. Malloy, R.A., Plastics Product Design for Injection Molding, Hanser Publishers, New York, 1994, p. 68.

4. Bernhardt, E.C., Processing of Thermoplastic Materials, Reinhold Publishing Corporation, New York, 1959, p. 157.

5. Rosato, D.V., and D.V. Rosato, Plastics Processing Data Handbook, Chapman & Hall, New York, 1989, p. 93.

6. G.A. Krulik, «Electrolytic Plating,» Plastics Finishing and Decoration, D. Satas, ed., Van Nostrand Reinhold Co., New York, 1986, р. 84

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