Date post: | 26-Oct-2014 |
Category: |
Documents |
Upload: | serkan-gecim |
View: | 188 times |
Download: | 4 times |
STYRENE MONOMER
FORMULA
C6H5 - CH = CH2
DESCRIPTION
Styrene (vinyl benzene, styrene monomer SM) is a colorless to yellowish oily liquid with a
distinctive aromatic odor. It is sparingly soluble in water but soluble in alcohols, ethers and
carbon disulfide. This valuable monomer is flammable, reactive and toxic. Styrene Monomer
is a light liquid. It has a low vapor pressure and high refractive index. It is chemically reactive
and undergoes polymerization readily (by heat, light or peroxide catalysts).
APPLICATIONS
Styrene monomer is a basic building block of the plastic industry. It is used to make a host of
downstream derivative products that go into millions of consumer goods. Primary
derivatives of styrene monomer, in order of demand, include: polystyrene, expandable
polystyrene (EPS) and acrylonitrilebutadiene-styrene (ABS)/styrene-acrylonitrile (SAN)
resins, styrene butadiene (SB) latex, SB Rubber (SBR), unsaturated polyester resins
(UPR), specialty polymers, co-polymers and styrene thermoplastic elastomers (TPE)
Polystyrene
CD covers and plastic drinking cups are made out of the polymer polystyrene. This polymer
is known to be a clear brittle plastic that is synthesized by a free radical polymerization. An
initiator, such as benzoyl peroxide, is used to initiate the free radical polymerization of
styrene. Once the radical initiator initiates the polymerization of styrene, propagation occurs
which “builds up” the polymer chain. Once the polymer chain has “grown” and at a desirable
length or molecular weight, the polymerization is terminated. The polymer is then isolated,
possibly purified, characterized, and used for material use. The mechanism of the free
radical polymerization of styrene is shown below.
Formation of the radical initiator:
Polymerization of styrene:
Manufacturing process:
The overall reaction describing the styrene polymerization is:
This reaction is carried out in an inert organic solvent environment, which provides the
reaction medium for this cationic polymerization reaction .
The different methods available for styrene polymerization are:
1. Solution (bulk) polymerization.
2. Emulsion polymerization.
3. Suspension polymerization.
Solution (bulk) polymerization:
Solution (bulk) polymerization is commonly referred to as mass polymerization in the
industry. The vast majority of all polystyrene produced today is produced via this technology.
The common solvents used in this process are the styrene monomer itself and ethyl
benzene. The two types of mass polymerization are batch and continuous, of which
continuous mass is by far the most popular. Batch mass polymerization consists of a
polymerization section containing agitated vessels polymerizing up to 80% conversion in a
batch method. The polymerized solution is then pumped to a batch finishing section for
either devolatisation or plate and frame final polymerization and grinding. The most widely
used process for polymerization of polystyrene today is the continuous mass process. This
solution is continuously prepared in a holding vessel and will then be injected into the reactor
system.
Typical feed to the first reactor would consist of 50 weight percent styrene monomer,
100 ppm water (based on styrene weight), 2000 ppm boron trifluoride (based on styrene
weight), with the balance being organic solvent. The polymerization reaction gives off heat
that is carried away from the reactors by jacketing them with a heat transfer fluid. The
temperature of the reactants should not vary by more than 15 0 C throughout the reactor
series. Temperature control is very important in this reaction because as the reaction
temperature increases, the average molecular weight of the polystyrene decreases. The
reaction temperature range is 40-70 0 C. Temperature can also be controlled by
intermediate shell and tube heat exchangers. Monomer conversions of up to 85wt%
polystyrene are obtainable in these reactors.
Emulsion polymerization: Emulsion polymerization is generally used for polymerization of
styrene with other monomers or polymers. It is not a generally commercially accepted
method of producing crystal polystyrene or high impact polystyrene. Emulsion
polymerization is carried out similarly to suspension polymerization except that the monomer
droplets are microscopic in size.
Suspension polymerization: This is also called pearl polymerization. It has proved highly
efficient for largescale production of polymers of high average molecular weight. By variation
of the polymerization condition it is possible to produce a range of polymers with different
properties and processing characteristics so that a number of grades are offered by the
manufacturers to meet the differing requirements of the conversion process and the final
product.
There are many different ways of making polystyrene using suspension process. Most
producers use a batch process, although there is no technical reasons why a continuous
process could not work. In the suspension process a number of small styrene drops 0.15-
0.50mm in diameter are suspended in water. The reaction occurs within these drops. To aid
in the formation of proper size drops a suspending agent is used, and to keep them at that
size a stabilizing agent is added. A catalyst is used to control the reactionrate.
Suspension polymerization offers considerable advantages over the single phase
techniques in so far that heat removal control is no longer a problem but there are
disadvantages such as the need to use a dispersing agent
Detailed process of suspension polymerization:
Suspension polymerization is a batch system popular for speciality grades of
polystyrene. It can be used to produce either crystal or high impact grades. In impact
production, the styrene and rubber solution is bulk polymerized beyond phase inversion and
is then suspended in water to create oil in water suspension utilizing soaps and suspending
agents. The suspended droplets are then polymerized to completion, utilizing initiator and a
staged heating profile. The water phase is used as a heat sink and heat transfer medium to
a temperature-controlled jacket. For the production of crystal polystyrene the styrene
monomer itself is suspended and polymerized via the same mechanism.
The requirements of polymerization are:
a] Initiator. b] Suspending agent. c] Stabilizing agent. d] Catalyst
Initiators: The initiators generally used are benzoyl peroxide and t-butylhydroperoxide.
Suspending agent: To aid in the formation of the proper size drops a suspending agent is
added. Some typical suspending agents are methylcellulose, ethyl cellulose and
polyacrylic acids. Their concentration in the suspension is between 0.01-0.5% of monomer
charged.
Stabilizing agent: To keep the drops at proper size, a stabilizing agent is added. The
stabilizing agents are often insoluble inorganic such as calcium carbonate, calcium
phosphates or bentonite clay. They are present in small amount than the suspending
agents.
Catalyst: A catalyst is used to control the reaction rate. The catalysts are usually peroxides.
The most common ones are benzoyl, diacetyl, lauroyl, caproyl and tert-butyl. Their
concentration varies from 0.1-0.5% of the monomer charged. The ratio of monomer to
dispersing medium is between 10 and 40%.
Polymerization temperature: Polymerization of styrene occurs at temperature range of 90-95
0 C.
Process description: The suspension method is carried out in large reactors equipped with
agitators, the styrene monomer being maintained in the aqueous phase as droplets with a
diameter varying between 0.4-1mm by use of a dispersing agent such as partially
hydrolyzed polyvinyl acetate, inorganic phosphates or magnesium silicates. To reduce the
cycle time of the reactors, the entering water and styrene will be preheated. The
temperatures of the input streams will be sent so as to obtain the desired reaction
temperature. The water entering the reactor will be heated to 95 0 C. The bulk of the styrene
is to be heated to 85 0 C before being charged. This is done in a vertical double pipe heat
exchanger, which is directly above the reactor. To prevent the polymerization from occurring
in the heat exchanger or piping system, there are to be no obstructions between this heat
exchanger and the reactor. The catalyst, rubber stabilizer, and suspending agent are
premixed in styrene and discharged by gravity into the reactor. This mixture will not be
preheated, since it might polymerize. Typical water to monomer ratios is 1:1 to 3:1. A
combination of two or more initiators is used with a programmed reaction temperature to
reduce the polymerization time to a minimum for a given amount of residual styrene.
Purification steps and Extrusion: If the water can be removed using physical separation
processes, then the styrene and the other impurities dissolved in it will also be discharged.
The final purification step is drying. The polystyrene leaving this unit must meet the
specifications set. (0.03% water). Then it is passed through a devolatisation extruder to
remove the volatile residues and to convert the polymer into pellets.
It was assumed that 3% of polystyrene would be removed from the process in airveying,
drying, centrifuging, transferring, or as bad as bad product. At least 95% of that which is lost
in processing must be intercepted before it leaves the plant. Most of it can be removed and
sold as off-grade material. This waste is split among the various streams leaving the
processing area
PROPERTIES AND USES
Properties:
Processing properties: Flow properties may be the most important properties of
polystyrene processes. There are two widely accepted industry methods for the
measurement of processing properties. These include the melt flow index and the solution
viscosity. The melt flow index is measured by ASTM method as a measure of the melt
viscosity at 200 0 C and a 5kg load. The melt flow index of polystyrene is generally
controlled by adjustment of the molecular weight of the material and by the addition of such
lubricants as mineral oil. Polystyrenes are commercially produced with melt flow ranges of
less than 1 to greater than 50, although the most widely available grades generally have
melt flows between 2.0 and 20g per 10min. Solution viscosity is another method for
measuring the molecular structure of the polystyrene. Solution viscosity can be measured as
an 8% solution in toluene and increases with increasing molecular weight.
Rheological properties: Polystyrene is a non-Newtonian fluid with viscoelastic properties.
The viscosity of polystyrene melts or solutions is defined as he ratio of shear stress to shear
rate. Generally, as the molecular weight of the polymer is increased or mineral oil is
decreased, melt viscosity increases.
Mechanical properties: Crystal polystyrenes have very low impact strengths of less than
0.5ft-lb. Commercially available impact polystyrene grades can be obtained with values of
1.0 - 4.0 ft-lb. Generally, polystyrenes are not produced with greater than 15% total rubber
because of polymerization processing constraints. Nevertheless, impact properties can be
increased substantially without additional rubber by the proper control of rubber particle size,
percentage of grafting, cross-linking, and percentage of gel. Tensile and flexural properties
are also important representation of the strength of polystyrenes. Increasing the rubber
modification of polystyrene generally leads to lower tensile strength, crystal grades being stiff
and brittle. Tensile strength is also decreased by the addition of lubricants, such as mineral
oil. Flexural strengths for polystyrenes can be obtained from 5000 to 18000psi and are also
decreased by the addition of rubber and other additives to the polystyrene. Elongations can
be obtained from 1% for crystal polystyrene to 100% for some impact polystyrene grades.
Thermal properties: Annealed heat distortion is one popular method for measuring he
resistance to deformation under heat for polystyrenes. The heat distortion temperature is
decreased by the addition of rubber, mineral oil, or other additives to polystyrene. The glass
transition temperature for unmodified polystyrene is 373 K, and the glass transition
temperatures for polybutadienes are 161-205 K, subject to the cis, trans, and vinyl content.
Chemical properties: Solvent crazing of polystyrene is a commercially important
phenomenon. High impact polystyrenes are susceptible to solvent crazing at the interface
between the rubber particles and the polystyrene phase. The resistance of polystyrene to
this crazing is referred to as environmental stress crack resistance (ESCR). For food-
packaging applications, such as butter tubs and deli containers, polystyrenes with high
ESCR properties are desirable. Increasing the percentage of gel, percentage grafting, and
rubber particle size can increase stress crack resistance. Residual levels of low molecular
weight materials are also important to polystyrene performance. Some of the chemical
impurities in the polystyrene are styrene monomer and ethyl benzene solvent. Residual
levels of styrene below 200 ppm and ethyl benzene levels below 30 ppm are obtainable for
very specialized applications.
Optical properties: Crystal polystyrene is a transparent and colorless polymer; high impact
polystyrene is generally opaque as a result of the rubber particles. Developmental grades of
translucent impact polystyrenes have been produced but have not gained wide acceptance.
The major optical; property for high impact polystyrene is gloss. Gloss is a measure of the
percentage of light reflected is generally controlled by the size of the rubber particle. In
general, the smaller rubber particle gives higher gloss. Values from 20 to 95% reflectance
are commercially available. High impact polystyrene is naturally white and crystal
polystyrene is naturally clear, but both can be readily colored.
Gas and water permeability of polystyrene: When styrene polymers are used in
packaging applications, the gas and water permeability characteristics take on an important
aspect. Polystyrene itself has its limitations and in consequence is often used with other
polymers so as to achieve different permeability properties. These properties can change
dramatically as other monomers are introduced into the molecule.
Weatherability and ageing: Polystyrene and the copolymers are susceptible to
degradation by the action of sunlight; the main effect being due to UV radiation in the
wavelength band of 300-400nm. the action of the UV radiation is accompanied by the
oxidation so that the overall degradation reaction is one of photo oxidation. The extent of
degradation varies from location to location owing to the differences in the intensity of the
radiation. This is of considerable importance in many applications because the degradation
is reflected, in the case of transparent compositions, in a yellowing effect and generally in a
loss of mechanical properties such as a lower elongation at break and a reduced impact
strength.
Toxicity: Polystyrene is a low toxic product. The FDA for the food contact applications
approves almost all commercially available polystyrenes. The polymer itself is not digestible
and is not normally biodegradable.
Uses:
1. 1.Extruded foam sheet of polystyrene can be thermoformed into such parts as egg
cartons or carryout food containers. Foam grade polystyrene is generally a highheat crystal
polystyrene with a high molecular weight.
2. Another type of polystyrene foam is that produced from expandable polystyrene beads.
These beads can be molded to produce hot drink cups, ice chests, or foam packaging. Also,
the expandable beads can be molded in very large blocks that can then be cut into sheets
for thermal insulation. Densities of as low as 1lb/ft 3 on foamed products are commercially
obtainable.
3. Extruded crystal polystyrene sheet can be biaxially oriented by mechanically pulling the
extruded melt in multiple directions. The stretched sheets is then cooled and allowed to set
with the biaxially orientation frozen into the sheet. This process produces crystal polystyrene
sheet of thin gauge wit very high strength. Typical applications include envelope windows,
cap layers for glossy sheet, or thermoforming into food packaging applications.
4. Optical property of polystyrene is used in manufacture of unbreakable glasses for gauges,
windows and lenses, as well as in countless specialties and novelties and also for edge
lighting for the edge lighting of indicators and dials
5. Solid or liquid pigments and dies color high impact and crystal polystyrenes. This can be
accomplished in both extrusion and injection molding processes. These colorants are added
and mixed during the melting stage of both the processes. Also, polystyrene parts are
amenable to high quality printing. Labels can be printed directly on the polystyrene part to
produce attractive containers.
6. Polystyrenes are also used in furniture, packaging, appliances, automobiles, construction,
radios, televisions, toys, house ware items, and luggage.
The Environmental Impacts of Styrofoam
INTRODUCTION
The production, use, and disposal of Polystyrene (a substance more commonly known as
Styrofoam) causes adverse environmental and health effects. These impacts are of
considerable concern, as, according to the Environmental Production Agency, Styrofoam is
the fifth largest source of hazardous waste in the United States
PRODUCTION
Styrofoam is composed of Benzene and Styrene, both of which are known human
carcinogens.
90,000 workers are estimated to be exposed to Styrene every year. This exposure causes a
variety of mutations to the central and peripheral nervous systems.
Benzene and Styrene have been linked to incidences of both Parkinson’s disease and
leukemia.
The production of Styrofoam is energy intensive, creating large amounts of greenhouse
gases. These problems rank the environmental production costs of Styrofoam as second
worst in the U.S. by the California Integrated Waste Management Board.
Hydrofluorocarbons (HFCs), used in the production of Styrofoam, result in air pollution,
causing damage to the ozone layer. They are now known to be 3-5 times more dangerous
than originally believed.
USE
Microwaving Styrofoam causes the release of toxic chemicals, which poses a threat to
human health.
DISPOSAL
Polystyrene is not usually recycled due to its lightweight nature and the high economic cost
of transporting and degreasing the petroleum-based material.
25-30% of landfills are dedicated to plastics, including styrofoam.
Polystyrene takes at least five hundred years to decompose.
Styrofoam is the primary source of urban litter.
Styrofoam is the main pollutant of oceans, bays, and other United States water sources.
Styrofoam causes choking and starvation in wildlife.
REFERENCES
http://sustainabilitypledge.wustl.edu/SiteCollectionDocuments/The%20Environmental%20Impacts
%20of%20Styrofoam.pdf
http://www.lyondellbasell.com/techlit/techlit/3284.pdf
http://courses.chem.psu.edu/chem36/SynFa06Web/Expt44.pdf
http://www.icispricing.com/il_shared/Samples/SubPage68.asp
http://www.cheresources.com/polystyzz.shtml
http://www.madehow.com/Volume-1/Expanded-Polystyrene-Foam-EPF.html#b
http://www.sbioinformatics.com/design_thesis/Polystyrene/polystyrene_Methods-2520of-
2520Production.pdf
SERKAN GEÇİM
20824025