Senior Research Analyst
Research Analyst TownsendTarnell, Inc. Mount Olive, New Jersey
Plastic additives represent a broad range of chemicals used by resin manufacturers, compounders, and fabricators to improve the proper- ties, 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. Plastics additives have grown with the overall industry and currently repre- sent over $16 billion in global sales.
This chapter includes all of the major chemical additives for plastics that are consumed worldwide. Materials excluded from the scope of this chapter include fillers, reinforcements, colorants, and alloys.
To ensure understanding we will define the terms additives and plastics.
Additives. 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. At the opposite extreme are a few commodity materials (talc and glyceryl monostearate) which also can impart sig- nificant property improvements.
Adding to this complexity is the fact that many varied chemical materials can, and frequently do, compete in the same function. Also, the same material type may perform more than one function in a host plastic. An example would include the many surfactant type materials based on fatty acid chemistry which could impart lubricant, antistatic, mold release, and/or slip properties to a plastic matrix, depending upon the materials involved, loading level, processing conditions, and application.
Given the range of materials used, plastic additives are generally classified by their function rather than chemistry.
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 addi- tive consumption): polyvinyl chloride (PVC), the polyolefins [polyeth- ylene (PE) and polypropylene (PP)], the styrenics —[polystyrene (PS) and acrylonitrile butadiene styrene (ABS)], and engineering resins such as polycarbonate and nylon.
4.3 Antiblock and Slip Agents
Antiblocking agents. Antiblocking agents function by roughening the surface of film to give a spacing effect. The inherent tack of linear low- density polyethylene (LLDPE) and low-density polyethylene (LDPE) is a detriment when used in film where self-adhesion is undesirable. An antiblock additive is incorporated by the compounder to cause a slightsurface roughness which prevents the film from sticking to itself. Years ago, efforts were made to prevent this by dusting the surface with corn starch or pyrogenic silica. This process was abandoned because of potential health concerns. Antiblocking agents are now melt-incorporated into the thermoplastic either via direct addition or by use of a master batch.
Antiblocking agents are used in polyolefin films in conjunction with
slip agents in such consumer items as trash bags, shipping bags, and a variety of packaging applications. The most common polymers extruded into film include LLDPE and LDPE. Lesser amounts of high- density polyethylene (HDPE) are used for these as well as other film applications. PE resins are used in film for their toughness, low cost and weight, optical properties, and shear sealability. Four criteria are used in the selection of an antiblocking agent, as shown in Table 4.1.
While both organic and inorganic materials are used as antiblocking agents, the inorganics make up the bulk of the market. The four major types of antiblocking agents are
■ Diatomaceous earth
■ Calcium carbonate
■ Synthetic silicas and silicates
The suppliers of inorganic additives to the plastics industry market their products primarily as fillers and extenders. While many of these products can also be used as antiblocking agents in polyethylene films, only a few suppliers actively market their products for this end use.
Slip agents. Slip agents or slip additives are the terms used by indus- try for those modifiers that impart a reduced coefficient of friction to the surface of finished products. Slip agents can significantly improve the handling qualities of polyolefins and, to a lesser extent, PVC, in film and bag applications. They help speed up film production and
TABLE 4.1 Criteria Used in Selection of an Antiblocking Agent
Particle size distribution Affects both the level of antiblock performance and the physical properties of the final film.
Surface area Measured in square meters per gram. Affects the coefficient of friction of the film and level of wear on equipment.
Specific gravity Indicates the relative weight of the product.
Density Measures the mass/volume ratio. Affects the quality of the film.
ensure final product quality. Fatty acid amides, the primary chemical type used as slip agents, are similar to migratory antistatic agents and some lubricants with a molecule which has both a polar and non- polar portion. These additives migrate to the surface and form a very thin molecular layer that reduces surface friction.
Slip agents are typically employed in applications where surface lubrication is desired—either during or immediately after processing. To accomplish this, the materials must exude quickly to the surface of the film. To function properly they should have only limited compati- bility with the resin. Slip agents, in addition to lowering surface fric- tion, can also impart the following characteristics:
■ Lower surface resistivity (antistatic properties)
■ Reduce melt viscosity
■ Mold release
Slip agents are often referred to as lubricants. However, they should not be confused with the lubricants which act as processing aids. While most slip agents can be used as lubricants, many lubricants cannot be used as slip agents since they do not always function externally.
The major types of slip agents include:
■ Fatty acid amides (primarily erucamide and oleamide
■ Fatty acid esters
■ Metallic stearates
■ Proprietary amide blends
Antiblock and slip agents can be incorporated together using combi- nation master batches which give the film extruder greater formula- tion control.
Because of the different chemical composition of antiblocking and slip agents, few companies are involved in both. Table 4.2 presents a list of the selected global suppliers of antiblocking and slip agents.
4.3.3 Trends and forecasts
The trend toward downgauging in PE film has favorably affected the use of slip agents. Although the value of resin decreases as films are made thinner, surface area increases, therefore, requiring higher load-ings of slip agents. Both slip and antiblocking agents are expected to grow at a rate of about 4% annually over the next 5 years.
Antioxidants are used in a variety of resins to prevent oxidative degra- dation. Degradation is initiated by the action of highly reactive free radicals caused by heat, radiation, mechanical shear, or metallic impu- rities. 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 mole- cule, 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 antioxi- dant, this reaction continues and leads to degradation of the polymer. Degradation is manifested either by cross-linking or chain scissoring. Cross-linking causes the polymer to increase in molecular weight, leading to brittleness, gellation, and decreased elongation. Chain scis- soring decreases molecular weight, leading to increased melt flow and reduced tensile strength.
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.
Primary antioxidants, usually sterically hindered phenols, func- tion by donating their reactive hydrogen to the peroxy free radical so that the propagation of subsequent free radicals does not occur. The antioxidant free radical is rendered stable by electron delocalization. Secondary antioxidants retard oxidation by preventing the prolifera- tion of alkoxy and hydroxy radicals by decomposing hydroperoxides to yield nonreactive products. These materials are typically used in synergistic combination with primary antioxidants.
Table 4.3 lists the chemical types of primary and secondary antioxi- dants and their major resin applications. Through the remainder of this chapter, antioxidants will be addressed by type based on overall chem- istry. The class of antioxidant merely describes its mode of stabilization.
Amines. Amines, normally arylamines, function as primary antioxi- dants by donating hydrogen. Amines are the most effective type of pri- mary antioxidant, having the ability to act as chain terminators and peroxide decomposers. However, they tend to discolor, causing staining, and, for the most part, lack FDA approval. For this reason, amines are found in pigmented plastics in nonfood applications.
TABLE 4.3 Antioxidants by Chemical Type with Major Resin Applications
Types Major resins Comments
Phenolic Rubber, some pigmented plastics, and polyurethane polyols
Polyolefins, styrenics, and engineering resins Arylamines tend to discolor and cause staining.
Phenolics are generally stain resistant and include simple phenolics (BHT), various polyphenolics, and bisphenolics.
Secondary Polyolefin wire and cable These are metal deactivators used in the inner coverings next to the metal.
Organophosphite Polyolefins, styrenics,
and engineering resins Phosphites can improve color
stability, and engineering resins but can be corrosive if
Thioester Polyolefins and styrenics The major disadvantage with thioesters is their odor which is transferred to the host polymer.
monly used in the rubber industry but also find minor use in plastics such as black wire and cable formulations and in polyurethane polyols.
Phenolics. The most widely used antioxidants in plastics are pheno- lics. 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 2,6-di-t-butyl-4-methylphenol. BHT possesses broad FDA approval and is widely used as an antioxidant in a variety of polymers. It is commonly called the “workhorse” of the industry but is losing ground to the higher molecular weight antioxidants which resist migra- tion. The disadvantage of BHT is that it is highly volatile and can cause discoloration. Other simple phenolics include BHA (2- and 3-t-butyl-4- hydroxyanisole) which is frequently used in food applications.
Polyphenolics and bisphenolics are higher in molecular weight than simple phenolics and both types are generally nonstaining. The increased molecular weight provides lower volatility, but is generally more costly. However, the loading of polyphenolics is much less than that of the simple phenolics. The most commonly used polyphenolic is tetrakis(methylene-(3,5-di-t-butyl-4-hydroxyhydrocinnamate)
methane or IRGANOX1010 from Ciba. Other important bisphenolics include: Cytec Industries’ CYANOX 2246 and 425 and BISPHENOL A from Aristech, Dow, and Shell.
Thiobisphenols are less effective than hindered phenols in termi- nating peroxy radicals. They also function as peroxide decomposers (secondary antioxidants) at temperatures above 100°C. Typically, thio- bisphenols are chosen for use in high-temperature resin applications. Users generally prefer hindered phenolics over thiobisphenols where high-temperature service is not involved.
Organophosphites. Acting as secondary antioxidants, organophos- phites reduce hydroperoxides to alcohols, converting themselves to phosphonates. They also provide color stability, inhibiting the discol- oration caused by the formation of quinoid reaction products which are formed upon oxidation of phenolics. Tris-nonylphenyl phosphite (TNPP) is the most commonly used organophosphite followed by tris(2,4-di-tert-butylphenyl)phosphite (for example, Ciba’s IRGAFOS
168). The disadvantage of phosphites is their hygroscopic tendency. Hydrolysis of phosphites can ultimately lead to the formation of phos- phoric acid, which can corrode processing equipment.
Thioesters. Derived from aliphatic esters of B-thio dipropionic acid, thioesters act as secondary antioxidants and also provide high heat sta- bility to a variety of polymers. Thioesters function as secondary antiox- idants 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 antiox- idants is their inherent odor which is transferred to the host polymer.
Deactivators. Metal deactivators combine with metal ions to limit the potential for chain propagation. Metal deactivators are commonly used in polyolefin inner coverings in wire and cable applications where the plastic comes in contact with the metal. In effect, the deactivator acts as a chelating agent to form a stable complex at the metal interface, thereby preventing catalytic activity. The most common deactivators contain an oxamide moiety that complexes with and deactivates the metal ions. A typical product is Ciba’s IRGANOX MD-1024.