EMISSIONS RESULTING FROM PLASTIC MANUFACTURING

INTRODUCTION:

Plastic products manufacturing involves molding, forming, shaping, or otherwise altering

plastic resins or plastic materials to produce an intermediate or final product. This manufacturing

industry is also commonly referred to as plastics processing or polymer processing. The

manufacture of resins is not a part of plastic products manufacturing; however, some facilities

manufacture resins at the same site as where the resins are processed. [1]

Volatile organic compounds may be formed and emitted at any stage in the life cycle of a

material. VOCs can be produced during:

Manufacturing, processing and extruding

Treatment (coating, irradiation, etc)

Improper or prolonged storage, handling and transportation

End-use [6]

EMISSION SORCES:

Emissions from plastic products manufacturing come from a variety of sources and are

highly dependent upon the chemical makeup of the raw materials (resins, additives) and types of

production processes used. In addition, the diverse nature of these raw materials and

manufacturing techniques results in a wide range of potential combinations of emission sources

and pollutants.

The primary sources of emissions at plastic products manufacturing facilities are the

pieces of equipment (e.g., extruder hopper, die head, sander) used to handle raw materials and

produce the final product. These are typically the locations where chemical reactions occur,

liquid solvents and solvent blends are exposed to the atmosphere, solid resin is heated and

melted, and additives are introduced.

In addition to emissions generated directly from primary production processes associated

with plastic products manufacturing, there may be additional emissions produced by secondary

processes at these facilities. Emission sources from these secondary processes include storage

tanks, equipment leaks, wastewater treatment, combustion sources, and cleaning and surface

coating operations.

There are multiple processes occurring at plastic products manufacturing facilities that

give rise to a wide variety of pollutants. Emissions from plastic products manufacturing may be

generally classified as follows:

Volatile organic compound (VOC) and hazardous air pollutant (HAP) emissions resulting

from the volatilization of free monomer or solvent in the primary polymer blend during

processing;

VOC and HAP emissions that result from secondary process materials, such as blowing

agents, additives, and lubricants (mold release compounds);

VOC, HAP, and particulate matter (PM) emissions that result from byproducts formed

by chemical reactions or formed during heating of resins; and

PM emissions generated during raw material handling and finishing operations.

The following discussion provides additional information on some of the specific

pollutants emitted from plastic products manufacturing facilities and the specific processes

giving rise to emissions.

EMISSIONS DURING PROCESSING:

High temperature and high-shear processing of plastics can cause thermal oxidation of

the polymer, resulting in scission, crosslinking, and/or oxidation of macromolecules. These

changes can affect physical properties, stability during use, suitability for some applications and

are often associated with emission of particulates and formation of low molecular weight

products. Parameters affecting the nature and extent of volatile emissions during extrusion

include: extrusion temperature and rate, extruder size and type, residence time, extrudate surface

exposed to air, cooling rate and ofcourse the nature of the raw materials being processed (resin

type, its degree of dryness, residual monomer content, level of metal catalyst residues, degree of

stabilization, additive package, and processing aids).

Volatiles may consist of residual monomers, additives, modifiers, moisture, solvents,

residual catalysts, decomposition products of resins and their additives and can be associated

with odour and ‘off-taste’ in packaging foodstuffs. The identification of VOCs generated during

polymer processing is thus crucial for safety and health as they relate to potential worker

exposure and from an environmental point of view are governed by the new regulations on

emissions.

In the US the environmental Protection Agency (EPA) and the occupational Safety and

Health Administration (OSHA) have been concerned with potential problems related to

emissions in the plastics processing industry whereas the Food and Drug Administration (FDA)

is mostly concerned with the suitability of plastic products for food, cosmetics and drug

packaging.

A wide range of bulk commercial thermoplastic materials have been studied, including

polyvinyl chloride (PVC), polyamide 6, acrylonitrite-butadiene-styrene terpolymer (ABS), high

impact polystyrene (HIPS), polypropylene (PP), low density polythyelene (LDPE) and high

density polythelene (HDPE). From the point of view of odour formation, numerous reports deal

with the formation of odorous VOCs produced during manufacturing and processing.

Chain scission processes and reaction with oxygen can produce low molecular weight

volatile species. Typical carbonyl VOCs causing unpleasant odours or off-flavours were

identified such as ketones (e.g., methyl isobutyl ketone, 7-octen-2-one), aldehydes (e.g.,

heptanal) and carboxylic acids (C2-C5 hydrocarbons). Some hydrocarbons like 2,2,4,6,6-

pentamethylheptane (PMH) and 2,2,7,7-tetramethyloctane have been claimed to yield a metallic

odour. Hexane is said to be characteristic of a weak plastic-like odour. Aromatic compounds like

o-xylene also have a typical odour (solvent odour, burned-plastic odour). Finally, some alcohols

can cause off-odours, like 2-hepten-4-ol.

EMISSIONS DURING TREATMENT:

Volatile chemicals may be produced during the sterilization process and the aseptic

packagaing of food, cosmetics and medical products. Most research has been focusing on

identifying and quantifying products formed in food contact polymers after exposure to ionizing

radiations. A major objective was to determine the potential of volatiles to migrate into food and

to allow a better assessment of the risks potentially associated with irradiation. Exposure of

polymers to ionizing radiation, particularly at elevated dose levels, may result in crosslinking and

scission of the polymer chain, formation of unsaturations in the polymer hydrocarbon chain and

formation of volatile odorous products which can cause undesirable organoleptic properties (off-

odours and off-taste). Increased migration of plastic origin chemicals could be observed under

certain circumstances as a consequence of irradiation. For example, increased migration of

additives (especially antioxidants) and of their degradation products has been reported.

Numerous parameters may influence radiation-induced changes: the type of polymer, the

additives used, processing history, the dose of radiation, the temperature and the irradiation

atmosphere. As during processing, irradiation of polymers in the presence of oxygen causes

additional chain scission and oxidation, resulting in the formation of various low molecular

weight oxygenated compounds (hydroperoxide, alcohol, aldehydes, carbonyl, etc.). Moreover,

free radicals created during irradiation may remain trapped in the polymer contributiong to post-

irradiation ‘ageing’.

Different polymers (e.g., polyethylene, polypropylene, polyester, ABS, PVC, polyamide,

polystyrene foam and ethylene-vinly alcohol copolymer) have been evaluated before and after

exposure to different levels of gamma and electron beam (e-beam) radiation. All polymers

produce detectable amounts of low-molecular weight degradation products characteristic of each

plastic.

Sampling of volatile chemicals has been carried out static headspace sampling and

dynamic thermal desorption techniques. The thermal desorption spectroscopy (TDS) technique

cannot replace toxicity testing of medical products or migration studies of packaging materials

but has the advantage of a high sensitivity concentration step (without dilution or loss of volatile

compounds). The technique permits identification of unknown products at the trace level (less

than 0.1ppm radiolysis product) and provides valuable complementary information to classical

tests. Worst-case scenarios could be calculated assuming a total transfer of the substances into

any ‘environment’ (e.g., the human body, a drug solution, or a food stimulant). The approach has

been incorporated into a protocol.

FREE MONOMER/SOLVENT

Emissions of free monomer (a single molecule of a chemical used in a polymer) may

occur when a solid resin is heated during extrusion, molding, or any of the other processes. For

example, one study (Contos et al., 1995) found a monomer (styrene) to be the principle

component of the emissions produced during the extrusion of acrylonitrile-butadiene-styrene

(ABS) resins.

Emissions of free monomer would also be expected from resins used in solvent form.

Some resins may be handled using a solvent medium to store and transport the resin prior to

processing. In this case, emissions would also come from the solvent used to suspend the resin

prior to the polymerization step. Thermoset resins are often handled in monomer form prior to

solidifying under heat or pressure, or reaction with a hardening agent to generate a solid

polymer. For example, when curing of thermosets is accomplished during processing or when

processing involves polymerization (such as when thermoset polyurethane elastomers are

processed using reaction injection molding), substantial emissions of monomers are likely to be

generated (Midwest Research Institute, 1993).

REFERENCES:[1] “Preferred and alternative methods for estimating air emissions from plastic products manufacturing” www.epa.gov/ttnchie1/eiip/techreport/volume02/ii11.pdf

[2] Bruno Gervet, “The use of crude oil in plastic making contributes to global warming” May 2007, Renewable Energy Research Group, Division of Architecture and Infrastructure, Department of Civil and Environmental Engineering, Luleå University of Technology, Luleå,[3] “Plastic grocery bags: The ecological footprint” Sara Ellis, Sarah Kantner, Ada Saab, Mary Watson, Geography 214, Dr. Lisa Kadonaga, (December 22, 2005)www.vipirg.ca/archive/publications/pubs/student_papers/05_ecofootprint_plastic_bags.pdf[4] “Material on Plastic Waste Management” Central Pollution Control Board, (June 2012) [5] “The Plastic Bag” Jessica Standley, Carbon neutrality Unst 421, (spring 2010)[6] “Emissions from plastics”