Figure 1 Examples of Different Pellet Sizes, Shapes, and Colors.
Left to Right: Polystyrene, Titanium Dioxide, Polyethylene.
[Table of contents | Executive Summary | Sections 1 · 2 · 3 · 4 · 5 · Glossary | Tables | References ]
The demand for a wide variety of plastic products has, necessarily, created a demand for many different resins (polymers) and resin blends. Resins are synthesized from petroleum or natural gas derivatives, such as
Acetylene: Polyvinyl chloride (PVC), polyurethane
Benzene: Polystyrene (PS), polyurethane, acrylonitrile/butadiene/styrene (ABS)
Butadiene: Polyurethane, ABS
Ethylene: Polyethylene [high-density (HDPE) and low-density (LDPE)], PS, polyethylene terephthalate (PET), PVC, ABS, polyurethane, polyesters
Methane: PET, polyurethane
Naphthalene: Polyurethane
Propylene: Polypropylene (PP), polyurethane, polyester
Toluene: Polyurethane foams, elastomers, polyesters; also used to derive benzene
Xylene: PS, PET, ABS, unsaturated polyesters, polyurethane
By blending polymers, creating new polymers, and incorporating additives, resins may be tailored according to the desired application and end product (EPA, 1990a).
An estimated 60 billion pounds of resin are manufactured annually in the United States, most of which is pelletized. If each pound of pelletized HDPE contains approximately 22,000 pellets (Mr. Ronald Bruner, Society of the Plastics Industry, Inc., personal communication, August 1991, Washington, DC), and this estimated number is applied to all pelletized resins, more than 1 quadrillion pellets may be produced annually. Pelletized resins, or, simply, pellets, are produced in several shapes (e.g., spherules, beads, disks, and cylindrical nibs), sizes, and colors, a few of which are shown in Figure 1 below. Resins may also be produced in other easily transported forms, including granules, flakes, and powders (EPA, 1990a).
Figure 1 Examples of Different Pellet Sizes, Shapes, and Colors.
Left to Right: Polystyrene, Titanium Dioxide, Polyethylene.
Two major resin types are produced: thermoplastic resins and thermoset resins (EPA, 1990a). Thermoplastic resins can be melted or reprocessed without damaging or changing the chemical or physical properties of the polymer; they are highly malleable but become rigid when cooled.
Because the difference may be narrow between the melting point of a thermoplastic resin and the temperature at which the resin may decompose, thermoplastic resins are kept in a liquid (melted) state for a minimum amount of time and are pelletized as soon as possible (EPA, 1990a).
Thermoplastic resins comprised 83% and 84% of the annual U.S. resin sales in 1989 and 1990, respectively (Table 1); U.S. resin sales increased 5.5% in 1990 after no increase in 1989 (Martino, 1991). The most commonly used thermoplastic resins include LDPE, PVC, HDPE, and PP; these resins accounted for 61% of the total resin production in 1990 (EPA, 1990a). Common products made of thermoplastic resins include milk bottles and other food containers.
Table 1. Annual U.S. Resin Sales. [Adapted from Martino (1991)]
% Annual %
Resin Sales(a) Resin Annual Sales(a)
1989 1990
Thermoplastic Resins 1989 1990 Thermoplastic Resins (cont) 1989 1990
Low-density polyethylene (LDPE) 18.5 19.3 Polycarbonate 1.1 1.0
Polyvinylchloride (PVC) and copolymers 14.7 15.1 Polyphenylene-based alloys 0.3 0.3
High-density polyethylene (HDPE) 14.0 13.8 Styrene/acrylonitrile (SAN) 0.2 0.2
Polypropylene (PP) and copolymers 12.5 13.2 Polyacetyl 0.2 0.2
Polystyrene (PS) 8.8 8.4 Cellulosics 0.2 0.1
Thermoplastic polyester polyethylene 3.6(b) 3.8(b)
terephthalate (PET) Thermoset Resins 1989 1990
Thermoplastic polyester polybutylene — — Phenolics 4.9 4.6
terephthalate (PBT)
Acrylonitrile/butadiene/styrene (ABS) 2.1 2.0 Polyurethane 5.5 5.3
Other styrenics 2.0 1.8 Urea and melamine 2.4 2.3
Other vinyls 1.5 1.5 Polyester, unsaturated 2.3 2.0
Polyamide (nylon) 1.0 0.9 Epoxy 0.8 0.8
Acrylic 1.3 1.2 Alkyd 0.6 0.5
Thermoplastic elastomers 0.9 0.9 Others 0.5 0.5
NA: Not available.
(a) Based on total annual sales of 58,251 and 61,480 billion pounds of resin in 1989 and 1990, respectively.
(b) Value is for PET, PBT, and other thermoplastic polyester resins.
source: http://www.epa.gov/owowwtr1/OCPD/PLASTIC/1-sale.html
In contrast to thermoplastic resins, thermoset resins are stronger when exposed to high temperatures, tend to be rigid, infusible, and insoluble, and cannot be remelted and reformed. Thermoset resins often are shipped to processors in liquid form, where the resin is cured and molded (EPA, 1990a). The most commonly used thermoset resins include phenolics and polyurethane resins. Thermoset resins typically are used in building materials and automotive parts (EPA, 1990a).
Thermoplastic and thermoset resins are further categorized according to the volume produced and the market demand for the resin. The categories and the uses of the resins are listed below.
Commodity resins These resins are produced in large quantities and are used as the raw materials for many plastic products. These resins are labeled as commodities because they are commonly used and are not refined or differentiated by the resin manufacturers. Commodity resins, such as LDPE, PVC, and HDPE, are the least expensive resins to produce (EPA, 1990a).
Transitional resins These resins are produced less frequently than commodity resins but more frequently than engineering/performance resins (discussed below). Transitional resins, such as PP, ABS, and acrylics, are also more expensive to purchase than commodity resins (EPA, 1990a).
Engineering/performance resins These resins have narrowly defined applications and are produced by only a few companies. Engineering/performance resins, such as polycarbonate and nylon, are the most expensive resins to produce (EPA, 1990a).
2.1 Pellet Additives
Some resins are used in the pure-polymer form, but, more frequently, the properties of the polymer must be changed to produce the desired end product. Additives are used to alter the physical characteristics of the polymer, such as aesthetic properties (e.g., color), physical properties (e.g., heat-resistance and hardness), and the ability to be further processed (e.g., porosity) (EPA, 1990a). Table 2 shows additives, additive concentrations, and typical polymers to which the additives are applied.
Table 2. Characteristics and Uses of Plastics Additives. [Adapted from EPA, 1990a]
Additive
Examples or Types of Additives Conc.(lb)(a) Typical Polymers Using Additive
Antimicrobials - Increase resistance to microorganisms Low (<1) Polyurethane, PVC, PE
Oxybisphenoxarsine; isothiazalone
Antioxidants - Prevent deterioration during processing and Low (<1) Impact styrene, ABS, polyolefins
long-term use
Phenolics; amines; phosphates; thioesters
Antistatic agents - Control static buildup during processing Low (<1) PVC, polyurethane, polyolefins
and in final product
Amine salts; phosphoric acid esters; polyethers
Blowing agents - Add porosity to produce foamed plastics Moderate (1-5) Polyurethane, PVC, PP, PS, ABS
Azobisformamide; chlorofluorocarbons; pentane
Catalysts and curing agents - Facilitate polymerization Low (<1) Polyurethane
and curing of resins
Numerous
Colorants - Enhance appearance of consumer products Low (1-2) Numerous
Organic and inorganic pigments and dyes
Fillers - Enhance physical properties (e.g., hardness) and High (10-50) Unsaturated polyester, PVC
reduce production costs
Minerals (e.g., calcium carbonate wood flours)
Flame retardants - Reduce combustibility High (10-20) Various
Aluminum trihydrate; antimony oxide; halogenated hydrocarbons;
organophosphates
Free-radical initiators - Assist in polymerization and curing Low (<1) LDPE, PS, PVC, acrylics, PE
processes
Peroxides; azo compounds
Heat stabilizers - Improve heat resistance or prevent Moderate (1-5) PVC
degradation by heat
Organotin mercaptides; lead compounds; barium, cadmium,
and zinc soaps
Impact modifiers - Improve strength and impact resistance High (10-20) Polyolefins, PVC, engineering plastics
Methacrylate butadiene styrene; chlorinated PE; acrylic
polymers; ethylene vinyl acetate
Lubricants and mold release agents - Improve viscosity, reduce Low (<1) PVC, PS, polyolefins
friction between resin and surrounding surfaces
Fatty acids; alcohols and amides; esters; metallic stearates;
silicones; soaps; waxes
Plasticizers - Soften rigid polymers and make them more High (20-60) PVC, cellulosics
flexible
Phthalates; aliphatic di-and tri-esters; polyesters; phosphates; trimellitates
Reinforcers - Improve physical properties High (10-40) Epoxy, unsaturated polyester
Glass fibers, wood flours High (10-40) Epoxy, unsaturated
polyester
Ultraviolet stabilizers - Prevent or inhibit degradation by Low (<1) Polyolefins, PE, PP, polycarbonate,
ultraviolet light PS, PVC
Hindered amines; hydroxybenzophenones; carbon black; hydroxybenzotriazoles
(a) Additive concentration in final product (pounds additive per 100 lb of resin),
ranked high, moderate, or low.
source: http://www.epa.gov/owowwtr1/OCPD/PLASTIC/2-add.html 12apr03
The type of additive determines when and how the additive is applied to a polymer. Two methods are used to incorporate additives: (1) the additive (solid or liquid) is mixed with the polymer, or (2) the additive is reacted chemically with the polymer (the additive is bonded with the polymer).
In ecological discussions, this distinction is very important when considering leaching of the additives into the environment and potential toxicological effects of the additives. Additives physically mixed with the polymer may be more likely to leach out because the additive is not chemically bound to the polymer; leaching would be determined by the miscibility (ability to be mixed) of the additive and polymer and by environmental conditions (e.g., temperature). Additives incorporated through chemical reactions cannot leach out of the plastic unless the plastic is broken down chemically (EPA, 1990a).
As previously stated, additives are used to change certain physical properties of the polymer to produce a desired product. Typically, these changes affect aesthetic properties, durability, or ease of processing of the resin. Some additive-containing pellets may be designed for controlled environmental release of chemical additives. For example, Rutherford and Withycombe (1985) patented a process (U.S. Patent 4,542,162) for forming plastic pellets containing repellents for use in controlling animals, birds, and insects. The action of the repellent requires the intentional dispersion of pellets in a given area. Thus, this and other applications for pellets may involve intentional, as opposed to accidental, introduction of pellets into the environment.
Table 3. Polymer Densities. [Adapted from EPA (1990a) and Anon. (1988a)]
Density
Resin (g/mL)
Polystyrene (PS) 1.04-1.08
Other styrenics [e.g., styrene-butadiene 1.05-1.14
and styrene-based latexes, styrene-maleic
anhydride (SMA), styrene-butadiene (SB) polymers]
Low-density polyethylene (LDPE) 0.89-0.94
Thermoplastic polyester polyethylene terephthalate (PET) 1.29-1.40
Polyvinylchloride and copolymers (PVC) 1.30-1.58
Polyamide (nylon) 1.07-1.08
Acrylonitrile/butadiene/sytrene (ABS) 1.01-1.08
Polypropylene and copolymers (PP) 0.89-0.91
Thermoplastic elastomers NA
Acrylic 1.17-1.20
Polycarbonate 1.2
Cellulosics 1.09-1.24
Polyacetal 1.41-1.42
Other vinyls 1.16-1.35
(e.g., polyvinly acetate, polyvinyl butyrol,
polyvinylidinechloride)
Styrene/acrylonitrile (SAN) 1.02-1.08
Polyphenylene-based alloys (i.e., modified 1.06-1.10
phenylene oxide and modified phenylene
High-density polyethylene (HDPE) 0.94-0.96
Thermoplastic polyester polybutylene 1.30-1.38
terephthalate (PBT)
Thermoset Resins
Density
Resin (g/mL)
Phenolics 1.24-1.32
Polyurethane 1.17-1.28
Polyester, unsaturated 1.01-1.46
Epoxy 1.11-1.48
Alkyd 1.30-1.40
Urea and melamine 1.47-2.00(b)
Others (small-volume thermoplastic and NA(c)
thermoset resins)(c)
Sea water 1.02-1.03
Fresh water <1.015
NA: Not available.
(a) Value is for PET, PBT, and other thermoplastic polyester resins combined.
(b) Densities are for filled molding systems; values for unfilled pellets were not available.
(c) Includes polymothyl pentene (density: 0.83-0.84 g/mL), polyimide (density: 1.36-1.43 g/mL),
and polyetherimide (density: 1.27 g/mL).
source: http://www.epa.gov/owowwtr1/OCPD/PLASTIC/3-dens.html 12apr03
2.2 Pellet Behavior in the Aquatic Environment
Many types of resin pellets float in fresh water or sea water (Table 3 above). Basically, pellets and granules with specific gravities lower than water will float, and pellets with higher specific gravities than water will sink. Additives may affect polymer density, thereby influencing whether a pellet will float in water. Because salinity affects water density, a particular resin pellet could float in sea water but sink in fresh water.
Most additives are used in moderate to low concentrations (Table 2 above), and the additives may not significantly alter the pellet's ability to float in fresh water or sea water. However, some additives and polymer modifications will result in significant changes in the pellet specific gravity and, therefore, will affect the pellet's ability to float or sink in water. The changes in specific gravity that were caused by the introduction of additives (fiber/flake reinforcements or particulate fillers) into six commodity resins are shown in Table 4.
Table 4. Effects of Two Additives to the Densities of Selected Commodity Resins. [Adapted from Anon. (1988a)]
Density without Density with Polymer Additive (g/mL) Additive(a,b) (g/mL) ABS 1.01 to 1.08 1.18 to 1.61(a) Polyamide (nylon) 1.07 to 1.08 1.13 to 1.62(a) Polyethylene 0.92 to 0.975 1.18 to 1.28(a) Polypropylene 0.89 to 0.91 1.04 to 1.23(a) 1.22 to 1.17(b) Polystyrene 1.04 to 1.08 1.20 to 1.50(a) PVC 1.30 to 1.58 1.42 to 1.50(a) 1.30 to 1.70(b) (a) Additive: Fiber/flake reinforcer. (b) Additive: Particulate filler. source: http://www.epa.gov/owowwtr1/OCPD/PLASTIC/4-sink.html 12apr03
Hydrodynamic processes, such as turbulence and surface tension, may affect a pellet's ability to float. For example, turbulence may either submerge pellets that would normally float at the surface, or resuspend pellets that would normally sink below the surface or to the bottom. Also, waters with high surface tension, such as waters containing a debris slick or discharge from a municipal sewage system, may either support particles with a density greater than water or keep an otherwise buoyant particle submerged.
Limited data are available that describe the biological and chemical activity of pellets in the environment. Van Franeker (1985) reported that many additives are known to be toxic and that toxic effects from the plastics additives may be more significant in aquatic organisms than was previously thought. Ryanet al.(1988) suggested that compounds used in the manufacture of plastics may be assimilated by organisms that ingest the pellets. Known and suspected biological effects of pellets in the environment are discussed in greater detail in Section 3.3.
The persistence of a pellet in the environment may be measured in years, depending on the resin type, the types and amounts of additives, and the reactions of the resins and additives to environmental processes. EPA (1990a) estimated that the lifetime of plastic products range from less than 1 year to more than 10 years, depending on the product. In the aquatic environment, the lifetimes can be affected by biological or chemical fouling, heat buildup within the plastic, degradation by microscopic organisms, and availability of atmospheric oxygen (EPA, 1990a).
Because pellets are small, lightweight, buoyant (if the pellet's specific gravity is lower than that of water), persistent, and ubiquitous in the aquatic environment, they are a potential hazard to aquatic organisms who ingest the pellets mistaking them for prey. Section 3.0 addresses the geographical distribution of pellets, sources of pellets to the aquatic environment, and impacts of pellets on birds, turtles, other biota, and humans.
source: http://www.epa.gov/owow/OCPD/PLASTIC/sec2-.html 12apr03
February 11, 1997
[Table of contents | Executive Summary | Sections 1 · 2 · 3 · 4 · 5 · Glossary | Tables | References ]
|
If
you have come to this page from an outside location click
here to get back to mindfully.org |