Functional Additives

Functional additives enhance environmental properties, increase the shelf life of plastic products. Many plastics breakdown when they are exposed to outdoor weather. Polypropylene and low density polyethylene weather poorly; within a few months of outdoor exposure they become brittle and discolored. Acrylic weathers the best but rapidly develops stress cracks if exposed to moisture.

Functional additives can be divided into the two categories of stabilizing and modifying. Stabilizing additives retard the breakdown of the plastic molecule due to reaction to oxygen, light rays and attack from active metals. Another stabilizer, flame retarding agents, stop ignition or the spread of combustion. Modifiers add color, flexibility, increase impact properties, increase strength and bulk. Other modifiers act as anti static and whitening agents. Biocide modifiers resist the biological corrosions of microorganisms.
 

Stabilizing Additives
 

All plastics materials experience oxidation during thermal processing or from exposure to ultraviolet lights. During the process of oxidation, oxygen atoms react and remove hydrogen atoms on the plastic's molecule. The removal of the hydrogen cause the molecular chain to break and form bonds of carbon to carbon atoms. The carbon bond without the hydrogen atom becomes an active linkage site called a free radical. The free radical seeks to link in a weak double bond with another carbon or cross links with another free radical on an adjacent molecule. The double bond or cross link weaken the plastic materials and make it brittle. The carbon to carbon bonding creates dark graphite particles which produce a darkening and discoloration of the plastics.

 Antioxidants are chemicals, normally in a powder, that are added to plastic materials (in fractions of less than one percent) to retard these reaction with oxygen. The molecular structure coupled with chemical properties, determine the difference in oxidation rates.

The oxidation of plastics is sometimes referred to as aging. Hydrogen's reaction with the oxygen often forms a white chalky surface on the plastic. A good example of this oxidation process at work can be observed in most weathered vinyl car top and plastic paints as white powders form; discoloration occurs. Vinyl car tops, colored paints, and plastic coated cables are particularly suitable to the oxidative process. The effects of oxidation include discoloration, especially yellowing, hardening, cracking, loss of surface gloss and a loss of transparency. The physical assets of any polymer will be dramatically reduced. While polyacrylonitrile and acrylic are highly resistance to oxidation, polyethylene and PVC are not. Antioxidants are used in the highest quantities in ABS, polystyrene, polypropylene, and polyethylene.
 

Light Stabilizers

Light stabilizers, some times called ultraviolet light absorbers, act to block the degradation of the molecule. Ultraviolet (UV) light acts similar to oxidation by attacking the surface molecules which damages both the visual and physical properties of the material. Light stabilizers interfere with this degradation. If the plastic product is colored darker than any form of black, such as carbon black, or black dye, black paint, the darker color acts as a UV absorber. However, the lighter color and transparent plastics uses a chemical called benzophenones as a general purpose absorber. This molecule degradation is slow and on a extremely small scale taking place in one in every 100 to 100,000 molecules, depending on the type of plastic. This allows the plastics part to remain in use, exposed to ultraviolet light for a long time without visible damage. The use of corrugated polyester panels on green house or patio roofs are often compounded with acrylic monomer because it is naturally resistant to damage by ultraviolet light. The acrylic monomer retards the damage of UV light and extends the life of the green house polyester panels by more than five years. The concentration of light absorbers in any formulation are normally in the range of 1/4 to 1 percent. Polyethylene, polystyrene, PVC, ABS, polyesters, and polyurethane are particularly susceptible to this kind of damage.
 

Metal Deactivators

In the presence of metals such as copper, iron, cobalt, manganese, cerium, and vanadium, the thermooxidative degradation of sensitive plastics, especially polyethylene, is accelerated. The presence of these metals initiates the generation of free radicals within the plastic molecule and causes molecular chain break down. Thus, the plastic looses it flexibility and becomes brittle. This is especially apparent in the polyethylene and PVC insulation of copper wire. Metal deactivators act to block the accelerated oxidative properties of these metals by chemically forming inactive structure.
 

Flame Retarding Agents

Most thermoplastics are highly flammable while thermoset plastic are inherently flame retarding. Flame retarding agents, either organic or inorganic, are used to lower the flammability of all types of plastics. Flame retarding agents work in four basic ways: (1) they influence the combustion of the plastics by reacting with them, (2) they provide insulating properties, (3) they coat the product and exclude oxygen from supporting the combustion, and (4) they provide an outside cooling reaction.

The most common flame retardants are boron, nitrogen, halogens, antimony and phosphorus. Many flame retarding agents are neutralized by process temperatures, ultraviolet light, and the present of oxidative agents. To meet fire retarding requirements, most flame-retarding thermoplastics have two or more types of flame retarding agents. The addition these chemicals reduces the probability of plastic burning in first phase of a fire. The flame retarding ability of thermoplastics depends on the size and type of fire. But even plastics that contain the most effective flame retarding agents will not resist combustion in a strong fire.
 

Modifiers

Modifiers improve the color, flexibility, impact, strength, resistance to biological attack, add whiteness, increase bulk and retard the buildup of static electricity of plastic products. One of the major advantages of plastics over metals and ceramics is their ability to accept various types of ingredients to change color, improve flexibility, absorb impact, increase surface hardness, enhance rigidity, and resist attack by biological agents. Properties of the polystyrene can be greatly improved with the addition of powdered rubber blended into the molecular structure. The ability to color different plastics the same color allows for the combining of different materials into one monochromatic product. Plastics are now replacing aluminum for the internal parts of copier and printers. Modifiers can be divided into seven distinct categories: colorants, plasticizer, impact modifiers, fillers, reinforcements, anti static agents, whitening agents, and biocides.
 

Colorants

The choice of a colorant depends on its compatibility and solubility in the plastic. The final product color must meet requirements of brilliance, light, fastness (the ability to maintain color), opacity or transparency. Colorants must be stable at molding temperatures and with exposure to light, moisture and oxygen. The color changes that result from the addition of colorants are based on the absorption or reflection of light. Colorants that absorb most light wavelengths are darker than the colors that scatter or reflect light wavelength. Normally, plastics are supplied precolored but specialty plastics must be custom compounded. Colorants are classified as either pigments or dyes.

Pigments are opaque colors, supplied in powder form, in liquid mixed with a plasticizer, or as color concentrates mixed with a base plastic materials. The pigments must have the ability to be dispersed throughout the plastic. Generally the powder pigment is mixed by tumbling with plastic pellets in large drums. The pigment aggregates tend to bunch together so mixing time and weight are important. Dispersion is the key to successful coloring: dispersion it self depends on the particles size of the pigment and the molecular weight of the plastic. The powder in aggregates is thoroughly mixed during the melt processing of the plastics during compounding.

Dyes devolve into the plastic mixing between molecules. All dyes are transparent, only absorb light, and do not scatter it. Dyes dissolve easily in the plastic during processing. When transparency is desired, dyes are used. Dyes are not as stable as pigments, they tend to break down during processing, turning the plastic toward yellow or brown. Dyes are also subject to color migration which can cause discoloration of clothes, skin, table linens or cause allergic reaction.
 

Plasticizers

It is impossible to manufacture plastic sheets, film, tubing and flexible PVC without the additions of plasticizers. Plasticizers are a form of internal lubricant added to plastics to improve flexibility, resiliency, and melt flow. There are 500 different kinds and most are liquid. Plasticizers are mix-in-between molecules. The space required for the plasticizer forces the molecules to move farther apart which reduces the strength of the Van der Waal's forces. The greater distance between chains, plus the lubricating action of the plasticizer, allows the chains to move more freely, reducing chain entanglement, lowering the melt temperature and significantly reducing physical properties. The more plasticizers that are added to plastics, the more flexibility is achieved. Some PVC plastisols are composed of 50 to 60% plasticizers, PVC accounts for 80% of all plasticizer consumption with cellulose, ABS, polystyrene also using a large share.

One side effect of lower cost plasticizers in PVC is their thermal instability. For example, PVC car tops, shower curtains, and upholstery, lose their flexibility as the plasticizer evaporates molecules. An inexpensive shower curtain will initially have a pungent odor when first removed from the packaging. The odor is trapped by the packaging as the plasticizer evaporates. The PVC steering wheel will loose plasticizers when the car is left closed in the hot sun. The build-up of plasticizer on the surface of the wheel produce an oily feel. As the plasticizer evaporates from car seat upholstery, it builds up on the inside of car windows and on the under side car ceiling and creates brown oily film. Car tops and garden hoses become brittle and crack when they lose their plasticizer. The use of low cost plasticizers in PVC shower curtains attract bacteria growth around the bottom of the curtain as the fungi feeds on the plasticizer. There are several higher cost plasticizer that have excellent resistance to migration and bacteria. A biocide is often added with low cost plasticizer to reduce the attack of fungi. However, these significantly increase the cost of the PVC product.
 

Impact Modifiers

Many plastics materials are brittle and must be modified to improve their ability to absorb shock. The notched impact strength of polyvinyl chloride (PVC), polystyrene, and polypropylene are very low at room temperature and require impact modifier in most applications. Impact resistance can be significantly improved by compounding or polymerizing impact absorbing rubbers, plasticizers, or higher impact plastics. There are three ways to modify brittle plastics with impact modifiers. The first is compounding additives into a plastic material, the second and third are variants of polymerization techniques.

The most economical method to improve the ability of a plastic to withstand a impact shock is to compound (in a dry blend or in the melt condition) a powder rubber or plasticizers. The plastic, normally PVC of polystyrene compounded with rubber, will absorb the impact first in the brittle molecule, and then transfer the energy, to the rubber particle. The compounded rubber acts similar to automotive shock absorbers. PVC is usually produced with an impact modifier. Impact resistance PVC has found wide application in the building trades as rain gutters, door frames, corrugated sheets, patio furniture, garden hoses. Modified PVC is used widely in the packaging industry for bottles of oil, disposable cups, blister and skin packaging. In the electrical industry it's used for insulating pipes and cable conduits. Polystyrene plastic is often compounded with a rubber powder and sold as an impact resistance grade in the manufacture of children's toys and model kits.

The second method is to polymerize two monomers; one a brittle plastic and the other an impact resistant plastic. The resulting plastic, a copolymer, has both the brittle and impact properties. The copolymerization technique produces a plastic with 15 to 25% more impact component than the technique of compounding rubber power into the plastic. In copolymerization of ABS plastic, styrene is polymerized with acrylonitrile and then modified with the rubber polybutadiene. The versatility of ABS is widely know. It ease of manufacture, excellent chemical resistance, rigidity, high impact resistance, color retention, and outstanding surface properties make this plastic the ideal material for computer and electronic housing, ABS pipe, and a wide range of consumer product including refrigerator liners, television housings, remote controllers, kitchen appliance housings, and children toys.

The third and most recent method, is to graft an impact resistant plastic onto the molecular structure of a brittle plastic. This form of grafting is similar to grafting a different growth twig onto a tree stem. The grafting allows brittle PVC plastic to be formed into tough and flexible window frames. Polypropylene with grafted ethylene propylene rubber (EPD) has proved especially useful as shoe heels, flexible heating tubes and hoses, cable insulation, and such automotive products as bumper covering, side panels, steering wheel housing, dashboards, consoles, handles and wheel linings.
 

Fillers

Fillers are low cost additives which replace the higher cost plastic. Fillers also provide abrasive resistance on the surface of the plastic. Most fillers are minerals, rock or ores that have been ground to obtain a suitable particle size. The most important fillers are the calcium carbonates, dolomite, silicates, talc, kaolin, mica, silica (sand), carbon black, and graphite. A filler usually comprises between 10% and 60% of the weight of the filled plastic. Fillers are most often used in the thermoset plastic of phenolic, urea and melamine; however, filled thermoplastics are now widely available.

The two basic types of fillers are inert and active. The use of inert fillers, sometime called extenders, reduces cost by decreasing the amount of plastics used. Extenders are sometimes referred to as bulk fillers and normally occur in powder form walnut shells, sawdust, wood pulp, jute, or mica. Silica is added in quantities of up to 60% in PVC to form the various grades of hard and rigid floor tile. Natural calcium carbonate in the form of chalk is added to PVC plastisols to improve both coating and reduce dripping. The chalk additionally improves adhesion and bond strength on the substrate.

 Active fillers produce improvements in physical or mechanical properties such as strength, hardness, thermal stability, chemical resistance, and appearance. Glass fibers and spheres, cotton fabric, and paper are often used as reinforcement to improve the stability of plastic products. Mineral powders, metal oxides, powdered metals, graphite, and silica are used to increase a plastic hardness and chemical resistance. Thermal stability can be improved with the addition of ceramic oxide, silica, and diatomaceous earth. The appearance of a plastic can be improved with the addition of carbon black, powdered metals, phosphorescent minerals and woven fabrics.

 Glass micro spheres were introduced in the mid 1960's as a filler for plastics. Glass spheres lower the melt viscosity of the system and improve fiber distribution and mold flow. The glass spheres also improve process ability and lessen rejects of injection molded parts by controlling shrinkage and warpage. They also reduce the strain on process equipment.
 

Reinforcements

Reinforcements are added to plastics to increase the strength and stability of plastic materials. Reforcements also increase heat deflection temperature, reduce mold shrinkage, and provide longer product stability by reducing creep and cold flow. Reinforcements for plastic materials generally are categorized as wither short or long fibers. This section focuses solely on short fiber reinforcements used in filled thermoplastics. (Long fiber reinforcements will be cover later in the book under the chapter on composites.)

The two most important reinforcements for thermoplastic materials are glass and carbon fibers. Glass and carbon fibers are normally 1/4" in length and dry mixed into the thermoplastic pellets prior to compounding. The intense shearing and mixing action of the compounder normally reduces the length of the fiber to 1/16 of an inch. Carbon fibers are excellent conductors of electricity and are often used to reinforce materials when computer printers and copy machine require electrostatic dissipation. Paper which moves across the surface of plastic builds up static electricity. As discussed earlier under compounding process carbon fibers act to ground the static before it damages the electronic circuits. The pellets produced during compounding process are then sold in a ready to process form. Glass exhibits high tensile strength, good chemical resistance and heat resistance. The reinforcing ability of glass fibers is significant, the addition of 30% glass by weight generally result in doubling the tensile strength, tripling the modulus of elasticity and reducing the thermal expansion and shrinkage rates by a third.

The ability of the glass fiber to reinforce the plastic depends upon the ability of the plastic component to coat to the surface of the glass fiber. This coating ability is called "wet-out." The glass surface possesses no affinity for sticking to the plastic and requires the addition of a coupling agent. The coupling agent generally used consist of an inorganic component which reacts with the glass surface and an organic component that reacts with the plastic. The glass or carbon reinforced thermoplastics combine the excellent properties of the plastic with the high strength of the reinforcing fiber to provide the plastic product with properties that often approach the level of metals.
 

Anti Static Agents

Some plastics are good insulators, their low surface conductivity does not allow for the discharge of electricity; static electricity accumulates on their surface. Static electricity occurs on the surface of plastics from vigorous frictional contact, especially in dry environments. If the humidity is higher than 65%, problems with static electricity build up are zero. However, if the product is produced in a plastic that is particularly susceptible to static electricity build up, and the humidity is below 20%, then anti static agents must be incorporated into the plastic. Plastics which are particularly susceptible to the accumulation of static electricity are polyethylene, polypropylene, polystyrene, nylon, polyesters, urethanes, cellulose, acrylic, and acrylonitrile.

 The build up of static electricity can provide serious and dangerous situation. Dust and dirt particles can build-up on the surface contaminating the part or interfering with sound reproduction of recording devices. Static buildup on people passing over synthetic fiber carpeting or plastic floor covering will provide a shock as the charge flows off to a door handle or other conductive surface. Films and sheet plastic cling together and cause production delays. Powders being transported in vacuum feed systems can lump together causing a blockage of the system. Sparks generation from a large static charge can ignite dust or solvent air mixtures. Anti-static agents (either internally mixed into the plastic or applied external to the plastics) can effectively reduce the buildup of charges on the surface of plastic materials by increasing surface conductivity.
 

Whitening Agents

Most plastics absorb light in the blue spectrum range of natural light which results in plastics having a natural yellow appearance. The natural form of PVC, polystyrene, and polycarbonate exhibit a distinct yellowish cast. Additionally, polycarbonate, polyurethane, and polypropylene are processed at temperatures in the range of 450 degrees F. Yellowing may occur under these high temperatures. Fluorescent whitening agents sometimes called brighteners or bluing agents, are added to plastics to reduce the yellow cast by increasing the reflected blue light. The whitening agents act to convert longer wavelength ultraviolet light to a blue or violet light as part of the total spectrum. The additions of the whitening agents allow for the production of plastic products in brilliant whites, blacks, and with brilliant colors. The plastic material with the added whiteners will appear whiter, brighter, and more brilliant.

Whitening agents are normally dry-blended in a concentrate or master blend with plasticizers or chalk added to the plastics pellets during manufacturing. Whiteners are added in amount from 100 to 500 parts per million. The most important application are in plastics that a susceptible to breakdown by UV light. These are the thermoplastic PVC, polystyrene and its copolymers, polyurethane, polycarbonate, polypropylene, and polyethylene. These plastics are manufactured in films, coatings, plastisols, imitation leathers and injection molded products.
 

Biocides

Biocides, sometimes called biostablizers, fungicides, bactericides, and microbiocides, are added to plastics materials to resist the biological corrosions of the plasticizer additive. Plastic materials are resistant to biological attack from microorganisms; however, the many plasticizers used in PVC are highly susceptible to attack. The attack can be from fungi, bacteria, or other microorganisms which use the plasticizer as a nutrient. The plasticizers known to be the most susceptible are adipates, azeleates, sebacates, polyesters, epoxidized oils, and lubricants. Biocides are added to the plastic in small amounts ranging from 0.1% to 5%. Biocides are toxic chemicals, however, the amount used is so small that it does not normally represent a toxic danger. However, biocides should not be used when the plastic product is to be in permanent contact with human skin, drinking water, or food. The same is true for plastics that are to be used in articles used by children or infants.

The result of the biological attack affects appearance with pink staining, loss of mechanical and electrical properties, and the development of mildew and odor. Plasticized PVC products most often requiring protection include: shower curtains, bath mats, water beds, awnings, tents, lawn furniture, marine upholstery, auto seat covers, ground cloths, wall covering, floor coverings, coated fabrics, and electrical insulation. These biological attacks are accelerated in hot and humid environments with temperatures of 80 to 100 degrees F and relative humidity between 63% to 99%. Most plasticized PVC shower curtains exist in this type of environment and exhibit rapid growth of mildew and odor.