The term comes from the Greeka ("without"), and morphé ("shape, form").
Structure
Crystalline vs. amorphous solid
Amorphous materials have an internal structure consisting of
interconnected structural blocks that can be similar to the basic
structural units found in the corresponding crystalline phase of the
same compound.
Unlike in crystalline materials, however, no long-range order exists.
Amorphous materials therefore cannot be defined by a finite unit cell.
Statistical methods, such as the atomic density function and radial distribution function, are more useful in describing the structure of amorphous solids.
Glass is a commonly encountered example of amorphous solids.
Although amorphous materials lack long range order, they exhibit
localized order on small length scales. Localized order in amorphous
materials can be categorized as short or medium range order. By convention, short range order extends only to the nearest neighbor shell, typically only 1-2 atomic spacings. Medium range order is then defined as the structural organization extending beyond the short range order, usually by 1-2 nm.
Universal low-temperature properties of amorphous solids
At very low temperatures (below 1-10 K), large family of amorphous solids have various similar low-temperature properties.
Although there are various theoretical models, neither glass transition nor low-temperature properties of glassy solids are well understood on the fundamental physics level.
Amorphous solids is an important area of condensed matter physics aiming to understand these substances at high temperatures of glass transition and at low temperatures towards absolute zero. From 1970s, low-temperature properties of amorphous solids were studied experimentally in great detail. For all of these substances, specific heat has a (nearly) linear dependence as a function of temperature, and thermal conductivity has nearly quadratic temperature dependence. These properties are conventionally called anomalous being very different from properties of crystalline solids.
On the phenomenological level, many of these properties were described by a collection of tunneling two-level systems.
Nevertheless, the microscopic theory of these properties is still missing after more than 50 years of the research.
Remarkably, a dimensionless quantity of internal friction is nearly universal in these materials. This quantity is a dimensionless ratio (up to a numerical constant) of the phonon wavelength to the phonon mean free path.
Since the theory of tunneling two-level states (TLSs) does not address
the origin of the density of TLSs, this theory cannot explain the
universality of internal friction, which in turn is proportional to the
density of scattering TLSs. The theoretical significance of this
important and unsolved problem was highlighted by Anthony Leggett.
Nano-structured materials
Amorphous materials will have some degree of short-range order at the atomic-length scale due to the nature of intermolecular chemical bonding. Furthermore, in very small crystals, short-range order encompasses a large fraction of the atoms;
nevertheless, relaxation at the surface, along with interfacial
effects, distorts the atomic positions and decreases structural order.
Even the most advanced structural characterization techniques, such as X-ray diffraction and transmission electron microscopy, have difficulty distinguishing amorphous and crystalline structures at short-length scales.
Characterization of amorphous solids
Due
to the lack of long-range order, standard crystallographic techniques
are often inadequate in determining the structure of amorphous solids.
A variety of electron, X-ray, and computation-based techniques have
been used to characterize amorphous materials. Multi-modal analysis is
very common for amorphous materials.
X-ray and neutron diffraction
Unlike crystalline materials which exhibit strong Bragg diffraction, the diffraction patterns of amorphous materials are characterized by broad and diffuse peaks.
As a result, detailed analysis and complementary techniques are
required to extract real space structural information from the
diffraction patterns of amorphous materials. It is useful to obtain
diffraction data from both X-ray and neutron sources as they have
different scattering properties and provide complementary data. Pair distribution function
analysis can be performed on diffraction data to determine the
probability of finding a pair of atoms separated by a certain distance.
Another type of analysis that is done with diffraction data of
amorphous materials is radial distribution function analysis, which
measures the number of atoms found at varying radial distances away from
an arbitrary reference atom. From these techniques, the local order of an amorphous material can be elucidated.
X-ray absorption fine-structure spectroscopy
X-ray absorption fine-structure spectroscopy
is an atomic scale probe making it useful for studying materials
lacking in long range order. Spectra obtained using this method provide
information on the oxidation state, coordination number, and species surrounding the atom in question as well as the distances at which they are found.
Atomic electron tomography
The atomic electron tomography
technique is performed in transmission electron microscopes capable of
reaching sub-Angstrom resolution. A collection of 2D images taken at
numerous different tilt angles is acquired from the sample in question,
and then used to reconstruct a 3D image.
After image acquisition, a significant amount of processing must be
done to correct for issues such as drift, noise, and scan distortion.
High quality analysis and processing using atomic electron tomography
results in a 3D reconstruction of an amorphous material detailing the
atomic positions of the different species that are present.
Fluctuation electron microscopy
Fluctuation electron microscopy
is another transmission electron microscopy based technique that is
sensitive to the medium range order of amorphous materials. Structural
fluctuations arising from different forms of medium range order can be
detected with this method. Fluctuation electron microscopy experiments can be done in conventional or scanning transmission electron microscope mode.
Computational techniques
Simulation
and modeling techniques are often combined with experimental methods to
characterize structures of amorphous materials. Commonly used
computational techniques include density functional theory, molecular dynamics, and reverse Monte Carlo.
Uses and observations
Amorphous thin films
Amorphous phases are important constituents of thin films. Thin films are solid layers of a few nanometres to tens of micrometres
thickness that are deposited onto a substrate. So-called structure zone
models were developed to describe the microstructure of thin films as a
function of the homologous temperature (Th), which is the ratio of deposition temperature to melting temperature. According to these models, a necessary condition for the occurrence of amorphous phases is that (Th) has to be smaller than 0.3. The deposition temperature must be below 30% of the melting temperature.
Regarding their applications, amorphous metallic layers played an important role in the discovery of superconductivity in amorphous metals made by Buckel and Hilsch. The superconductivity of amorphous metals, including amorphous metallic thin films, is now understood to be due to phonon-mediated Cooper pairing. The role of structural disorder can be rationalized based on the strong-coupling Eliashberg theory of superconductivity.
Thermal protection
Amorphous
solids typically exhibit higher localization of heat carriers compared
to crystalline, giving rise to low thermal conductivity.
Products for thermal protection, such as thermal barrier coatings and
insulation, rely on materials with ultralow thermal conductivity.
Technological uses
Today, optical coatings made from TiO2, SiO2, Ta2O5
etc. (and combinations of these) in most cases consist of amorphous
phases of these compounds. Much research is carried out into thin
amorphous films as a gas separating membrane layer. The technologically most important thin amorphous film is probably represented by a few nm thin SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor (MOSFET). Also, hydrogenated amorphous silicon (Si:H) is of technical significance for thin-film solar cells.
Pharmaceutical use
In the pharmaceutical industry, some amorphous drugs have been shown to offer higher bioavailability
than their crystalline counterparts as a result of the higher
solubility of the amorphous phase. However, certain compounds can
undergo precipitation in their amorphous form in vivo, and can then decrease mutual bioavailability if administered together.
The occurrence of amorphous phases turned out to be a phenomenon of particular interest for the studying of thin-film growth.
The growth of polycrystalline films is often used and preceded by an
initial amorphous layer, the thickness of which may amount to only a few
nm. The most investigated example is represented by the unoriented
molecules of thin polycrystalline silicon films. Wedge-shaped polycrystals were identified by transmission electron microscopy
to grow out of the amorphous phase only after the latter has exceeded a
certain thickness, the precise value of which depends on deposition
temperature, background pressure, and various other process parameters.
The phenomenon has been interpreted in the framework of Ostwald's rule of stages that predicts the formation of phases to proceed with increasing condensation time towards increasing stability.
Polystyrene (PS) /ˌpɒliˈstaɪriːn/ is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene.
Polystyrene can be solid or foamed. General-purpose polystyrene is
clear, hard, and brittle. It is an inexpensive resin per unit weight. It
is a poor barrier to air and water vapor and has a relatively low
melting point. Polystyrene is one of the most widely used plastics, with the scale of its production being several million tonnes per year. Polystyrene is naturally transparent, but can be colored with colorants. Uses include protective packaging (such as packing peanuts and in the jewel cases used for storage of optical discs such as CDs and occasionally DVDs), containers, lids, bottles, trays, tumblers, disposablecutlery, in the making of models, and as an alternative material for phonograph records.
As a thermoplastic polymer, polystyrene is in a solid (glassy) state at room temperature but flows if heated above about 100 °C, its glass transition temperature. It becomes rigid again when cooled. This temperature behaviour is exploited for extrusion (as in Styrofoam) and also for molding and vacuum forming, since it can be cast into molds with fine detail. The temperatures behavior can be controlled by photocrosslinking.
Under ASTM standards, polystyrene is regarded as not biodegradable. It is accumulating as a form of litter in the outside environment, particularly along shores and waterways, especially in its foam form, and in the Pacific Ocean.
History
Polystyrene was discovered in 1839 by Eduard Simon, an apothecary from Berlin. From storax, the resin of the Oriental sweetgum tree Liquidambar orientalis, he distilled an oily substance, that he named styrol, now called styrene. Several days later, Simon found that it had thickened into a jelly, now known to have been a polymer, that he dubbed styrol oxide ("Styroloxyd") because he presumed that it had resulted from oxidation (styrene oxide is a distinct compound). By 1845 Jamaican-born chemist John Buddle Blyth and German chemist August Wilhelm von Hofmann showed that the same transformation of styrol took place in the absence of oxygen. They called the product "meta styrol"; analysis showed that it was chemically identical to Simon's Styroloxyd. In 1866 Marcellin Berthelot correctly identified the formation of meta styrol/Styroloxyd from styrol as a polymerisation process. About 80 years later it was realized that heating of styrol starts a chain reaction that produces macromolecules, following the thesis of German organic chemist Hermann Staudinger (1881–1965). This eventually led to the substance receiving its present name, polystyrene.
The company I. G. Farben began manufacturing polystyrene in Ludwigshafen, about 1931, hoping it would be a suitable replacement for die-cast zinc
in many applications. Success was achieved when they developed a
reactor vessel that extruded polystyrene through a heated tube and
cutter, producing polystyrene in pellet form.
Otis Ray McIntire (1918–1996), a chemical engineer of Dow
Chemical, rediscovered a process first patented by Swedish inventor Carl
Munters.
According to the Science History Institute, "Dow bought the rights to
Munters's method and began producing a lightweight, water-resistant, and
buoyant material that seemed perfectly suited for building docks and
watercraft and for insulating homes, offices, and chicken sheds." In 1944, Styrofoam was patented.
Before 1949, chemical engineer Fritz Stastny (1908–1985)
developed pre-expanded PS beads by incorporating aliphatic hydrocarbons,
such as pentane. These beads are the raw material for molding parts or
extruding sheets. BASF
and Stastny applied for a patent that was issued in 1949. The molding
process was demonstrated at the Kunststoff Messe 1952 in Düsseldorf.
Products were named Styropor.
The crystal structure of isotactic polystyrene was reported by Giulio Natta.
Polystyrene is flammable, and releases large amounts of black smoke upon burning.Polystyrene is lightweight. This is a man in Guiyang, China carrying a lot of polystyrene packaging.
In chemical terms, polystyrene is a long chain hydrocarbon wherein alternating carbon centers are attached to phenyl groups (a derivative of benzene). Polystyrene's chemical formula is (C 8H 8) n; it contains the chemical elementscarbon and hydrogen.
The material's properties are determined by short-range van der Waals
attractions between polymer chains. Since the molecules consist of
thousands of atoms, the cumulative attractive force between the
molecules is large. When heated (or deformed at a rapid rate, due to a
combination of viscoelastic and thermal insulation properties), the
chains can take on a higher degree of confirmation and slide past each
other. This intermolecular weakness (versus the high intramolecular
strength due to the hydrocarbon backbone) confers flexibility and
elasticity. The ability of the system to be readily deformed above its
glass transition temperature allows polystyrene (and thermoplastic
polymers in general) to be readily softened and molded upon heating.
Extruded polystyrene is about as strong as an unalloyed aluminium but much more flexible and much less dense (1.05 g/cm3 for polystyrene vs. 2.70 g/cm3 for aluminium).
Production
Polystyrene is an addition polymer that results when styrene monomerspolymerize (interconnect). In the polymerization, the carbon-carbon π bond of the vinyl group is broken and a new carbon-carbon σ bond
is formed, attaching to the carbon of another styrene monomer to the
chain. Since only one kind of monomer is used in its preparation, it is a
homopolymer. The newly formed σ bond is stronger than the π bond that
was broken, thus it is difficult to depolymerize polystyrene. About a
few thousand monomers typically comprise a chain of polystyrene, giving a
molar mass of 100,000–400,000 g/mol.
Each carbon of the backbone has tetrahedral geometry, and those carbons that have a phenyl group (benzene ring) attached are stereogenic.
If the backbone were to be laid as a flat elongated zig-zag chain, each
phenyl group would be tilted forward or backward compared to the plane
of the chain.
The relative stereochemical relationship of consecutive phenyl groups determines the tacticity, which affects various physical properties of the material.
Tacticity
In polystyrene, tacticity
describes the extent to which the phenyl group is uniformly aligned
(arranged at one side) in the polymer chain. Tacticity has a strong
effect on the properties of the plastic. Standard polystyrene is
atactic. The diastereomer where all of the phenyl groups are on the same side is called isotactic polystyrene, which is not produced commercially.
Atactic polystyrene
The only commercially important form of polystyrene is atactic, in which the phenyl groups are randomly
distributed on both sides of the polymer chain. This random positioning
prevents the chains from aligning with sufficient regularity to achieve
any crystallinity. The plastic has a glass transition temperature Tg of ~90 °C. Polymerization is initiated with free radicals.
Syndiotactic polystyrene
Ziegler–Natta polymerization can produce an ordered syndiotactic
polystyrene with the phenyl groups positioned on alternating sides of
the hydrocarbon backbone. This form is highly crystalline with a Tm
(melting point) of 270 °C (518 °F). Syndiotactic polystyrene resin is
currently produced under the trade name XAREC by Idemitsu corporation,
who use a metallocene catalyst for the polymerisation reaction.
Degradation
Polystyrene
is relatively chemically inert. While it is waterproof and resistant to
breakdown by many acids and bases, it is easily attacked by many
organic solvents (e.g. it dissolves quickly when exposed to acetone),
chlorinated solvents, and aromatic hydrocarbon solvents. Because of its
resilience and inertness, it is used for fabricating many objects of
commerce. Like other organic compounds, polystyrene burns to give carbon dioxide and water vapor, in addition to other thermal degradation by-products. Polystyrene, being an aromatic hydrocarbon, typically combusts incompletely as indicated by the sooty flame.
The process of depolymerizing polystyrene into its monomer, styrene, is called pyrolysis.
This involves using high heat and pressure to break down the chemical
bonds between each styrene compound. Pyrolysis usually goes up to
430 °C. The high energy cost of doing this has made commercial recycling of polystyrene back into styrene monomer difficult.
Organisms
Polystyrene
is generally considered to be non-biodegradable. However, certain
organisms are able to degrade it, albeit very slowly.
In 2015, researchers discovered that mealworms, the larvae form of the darkling beetle Tenebrio molitor, could digest and subsist healthily on a diet of EPS.[29][30]
About 100 mealworms could consume between 34 and 39 milligrams of this
white foam in a day. The droppings of mealworm were found to be safe for
use as soil for crops.
In 2016, it was also reported that superworms (Zophobas morio) may eat expanded polystyrene (EPS). A group of high school students in Ateneo de Manila University found that compared to Tenebrio molitor larvae, Zophobas morio larvae may consume greater amounts of EPS over longer periods of time.
In 2022 scientists identified several bacterial genera, including Pseudomonas, Rhodococcus and Corynebacterium,
in the gut of superworms that contain encoded enzymes associated with
the degradation of polystyrene and the breakdown product styrene.
The bacterium Pseudomonas putida is capable of converting styrene oil into the biodegradable plasticPHA.
This may someday be of use in the effective disposing of polystyrene
foam. It is worthy to note the polystyrene must undergo pyrolysis to
turn into styrene oil.
Polystyrene is commonly injection molded, vacuum formed, or extruded, while expanded polystyrene is either extruded or molded in a special process.
Polystyrene copolymers
are also produced; these contain one or more other monomers in addition
to styrene. In recent years the expanded polystyrene composites with
cellulose and starch have also been produced. Polystyrene is used in some polymer-bonded explosives (PBX).
Sheet or molded polystyrene
CD case made from general purpose polystyrene (GPPS) and high impact polystyrene (HIPS)Disposable polystyrene razor
Polystyrene Petri dishes and other laboratory containers such as test tubes and microplates
play an important role in biomedical research and science. For these
uses, articles are almost always made by injection molding, and often
sterilized post-molding, either by irradiation or by treatment with ethylene oxide. Post-mold surface modification, usually with oxygen-rich plasmas,
is often done to introduce polar groups. Much of modern biomedical
research relies on the use of such products; they, therefore, play a
critical role in pharmaceutical research.
Thin sheets of polystyrene are used in polystyrene film capacitors as it forms a very stable dielectric, but has largely fallen out of use in favor of polyester.
Foams
Closeup of expanded polystyrene packaging
Polystyrene foams are 95–98% air. Polystyrene foams are good thermal insulators and are therefore often used as building insulation materials, such as in insulating concrete forms and structural insulated panel building systems. Grey polystyrene foam, incorporating graphite, has superior insulation properties.
Carl Munters
and John Gudbrand Tandberg of Sweden received a US patent for
polystyrene foam as an insulation product in 1935 (USA patent number
2,023,204).
PS foams also exhibit good damping properties, therefore it is used widely in packaging. The trademarkStyrofoam by Dow Chemical Company
is informally used (mainly US & Canada) for all foamed polystyrene
products, although strictly it should only be used for "extruded
closed-cell" polystyrene foams made by Dow Chemicals.
Foams are also used for non-weight-bearing architectural structures (such as ornamental pillars).
Expanded polystyrene (EPS)
Thermocol
slabs made of expanded polystyrene (EPS) beads. The one on the left is
from a packing box. The one on the right is used for crafts. It has a
corky, papery texture and is used for stage decoration, exhibition
models, and sometimes as a cheap alternative to shola (Aeschynomene aspera) stems for artwork.Section of a block of thermocol under a light microscope (bright-field,
objective = 10×, eyepiece = 15×). The larger spheres are expanded
polystyrene beads which were compressed and fused. The bright,
star-shaped hole at the center of the image is an air-gap between the
beads where the bead margins have not completely fused. Each bead is
made of thin-walled, air-filled bubbles of polystyrene.
Expanded polystyrene (EPS) is a rigid and tough, closed-cell foam with a normal density range of 11 to 32 kg/m3.
It is usually white and made of pre-expanded polystyrene beads. The
manufacturing process for EPS conventionally begins with the creation of
small polystyrene beads. Styrene monomers (and potentially other
additives) are suspended in water, where they undergo free-radical
addition polymerization. The polystyrene beads formed by this mechanism
may have an average diameter of around 200 μm. The beads are then
permeated with a "blowing agent", a material that enables the beads to
be expanded. Pentane
is commonly used as the blowing agent. The beads are added to a
continuously agitated reactor with the blowing agent, among other
additives, and the blowing agent seeps into pores within each bead. The
beads are then expanded using steam.
A significant portion of all EPS products are manufactured
through injection molding. Mold tools tend to be manufactured from
steels (which can be hardened and plated), and aluminum alloys. The
molds are controlled through a split via a channel system of gates and
runners. EPS is colloquially called "styrofoam" in the United States and Canada, an incorrectly applied genericization of Dow Chemical's brand of extruded polystyrene.
EPS in building construction
Sheets of EPS are commonly packaged as rigid panels
(common in Europe is a size of 100 cm x 50 cm, usually depending on an
intended type of connection and glue techniques, it is, in fact, 99.5 cm
x 49.5 cm or 98 cm x 48 cm; less common is 120 x 60 cm; size 4 by 8 ft
(1.2 by 2.4 m) or 2 by 8 ft (0.61 by 2.44 m) in the United States).
Common thicknesses are from 10 mm to 500 mm. Many customizations,
additives, and thin additional external layers on one or both sides are
often added to help with various properties. An example of this is lamination with cement board to form a structural insulated panel.
Thermal conductivity
is measured according to EN 12667. Typical values range from 0.032 to
0.038 W/(m⋅K) depending on the density of the EPS board. The value of
0.038 W/(m⋅K) was obtained at 15 kg/m3 while the value of 0.032 W/(m⋅K) was obtained at 40 kg/m3
according to the datasheet of K-710 from StyroChem Finland. Adding
fillers (graphites, aluminum, or carbons) has recently allowed the
thermal conductivity of EPS to reach around 0.030–0.034 W/(m⋅K) (as low
as 0.029 W/(m⋅K)) and as such has a grey/black color which distinguishes
it from standard EPS. Several EPS producers have produced a variety of
these increased thermal resistance EPS usage for this product in the UK
and EU.
ICC-ES (International Code Council
Evaluation Service) requires EPS boards used in building construction
meet ASTM C578 requirements. One of these requirements is that the limiting oxygen index
of EPS as measured by ASTM D2863 be greater than 24 volume %. Typical
EPS has an oxygen index of around 18 volume %; thus, a flame retardant
is added to styrene or polystyrene during the formation of EPS.
The boards containing a flame retardant when tested in a tunnel
using test method UL 723 or ASTM E84 will have a flame spread index of
less than 25 and a smoke-developed index of less than 450. ICC-ES
requires the use of a 15-minute thermal barrier when EPS boards are used
inside of a building.
According to the EPS-IA ICF organization, the typical density of EPS used for insulated concrete forms (expanded polystyrene concrete) is 1.35 to 1.80 pounds per cubic foot (21.6 to 28.8 kg/m3).
This is either Type II or Type IX EPS according to ASTM C578. EPS
blocks or boards used in building construction are commonly cut using
hot wires.
Extruded polystyrene (XPS)
Extruded polystyrene has a smooth texture, and can be cut into sharp-edged shapes without crumbling
Extruded polystyrene foam (XPS) consists of closed cells. It offers
improved surface roughness, higher stiffness and reduced thermal
conductivity. The density range is about 28–34 kg/m3.
Extruded polystyrene material is also used in crafts and model building, in particular architectural
models. Because of the extrusion manufacturing process, XPS does not
require facers to maintain its thermal or physical property performance.
Thus, it makes a more uniform substitute for corrugated cardboard.
Thermal conductivity varies between 0.029 and 0.039 W/(m·K) depending
on bearing strength/density and the average value is ~0.035 W/(m·K).
Water vapor diffusion resistance (μ) of XPS is around 80–250.
Depron, a thin insulation sheet also used for model building
Water absorption of polystyrene foams
Although it is a closed-cell foam, both expanded and extruded polystyrene are not entirely waterproof or vapor proof.
In expanded polystyrene there are interstitial gaps between the
expanded closed-cell pellets that form an open network of channels
between the bonded pellets, and this network of gaps can become filled
with liquid water. If the water freezes into ice, it expands and can
cause polystyrene pellets to break off from the foam. Extruded
polystyrene is also permeable by water molecules and can not be
considered a vapor barrier.
Water-logging commonly occurs over a long period in polystyrene
foams that are constantly exposed to high humidity or are continuously
immersed in water, such as in hot tub covers, in floating docks, as
supplemental flotation under boat seats, and for below-grade exterior
building insulation constantly exposed to groundwater.
Typically an exterior vapor barrier such as impermeable plastic
sheeting or a sprayed-on coating is necessary to prevent saturation.
Oriented polystyrene
Oriented
polystyrene (OPS) is produced by stretching extruded PS film, improving
visibility through the material by reducing haziness and increasing
stiffness. This is often used in packaging where the manufacturer would
like the consumer to see the enclosed product. Some benefits to OPS
are that it is less expensive to produce than other clear plastics such
as polypropylene
(PP), (PET), and high-impact polystyrene (HIPS), and it is less hazy
than HIPS or PP. The main disadvantage of OPS is that it is brittle, and
will crack or tear easily.
Co-polymers
Ordinary (homopolymeric)
polystyrene has an excellent property profile about transparency,
surface quality and stiffness. Its range of applications is further
extended by copolymerization and other modifications (blends e.g. with PC and syndiotactic polystyrene). Several copolymers are used based on styrene: The crispiness
of homopolymeric polystyrene is overcome by elastomer-modified
styrene-butadiene copolymers. Copolymers of styrene and acrylonitrile (SAN) are more resistant to thermal stress, heat and chemicals than homopolymers and are also transparent. Copolymers called ABS have similar properties and can be used at low temperatures, but they are opaque.
Styrene-butane co-polymers
Styrene-butane co-polymers can be produced with a low butene content. Styrene-butane co-polymers include PS-I and SBC (see below), both co-polymers are impact resistant. PS-I is prepared by graft co-polymerization, SBC by anionic block co-polymerization, which makes it transparent in case of appropriate block size.
If styrene-butane co-polymer has a high butylene content, styrene-butadiene rubber (SBR) is formed.
The impact strength of styrene-butadiene co-polymers is based on
phase separation, polystyrene and poly-butane are not soluble in each
other (see Flory–Huggins solution theory).
Co-polymerization creates a boundary layer without complete mixing. The
butadiene fractions (the "rubber phase") assemble to form particles
embedded in a polystyrene matrix. A decisive factor for the improved
impact strength of styrene-butadiene copolymers is their higher
absorption capacity for deformation work. Without applied force, the
rubber phase initially behaves like a filler. Under tensile stress, crazes
(microcracks) are formed, which spread to the rubber particles. The
energy of the propagating crack is then transferred to the rubber
particles along its path. A large number of cracks give the originally
rigid material a laminated structure. The formation of each lamella
contributes to the consumption of energy and thus to an increase in
elongation at break. Polystyrene homo-polymers deform when a force is
applied until they break. Styrene-butane co-polymers do not break at
this point, but begin to flow, solidify to tensile strength and only
break at much higher elongation.
With a high proportion of polybutadiene, the effect of the two
phases is reversed. Styrene-butadiene rubber behaves like an elastomer
but can be processed like a thermoplastic.
Impact-resistant polystyrene (PS-I)
PS-I (impact resistant polystyrene)
consists of a continuous polystyrene matrix and a rubber phase
dispersed therein. It is produced by polymerization of styrene in the
presence of polybutadiene dissolved (in styrene). Polymerization takes
place simultaneously in two ways:
Graft copolymerization: The growing polystyrene chain reacts with a double bond of the polybutadiene. As a result, several polystyrene chains are attached to one polybutadiene. S represents in the figure the styrene repeat unit, B
the butadiene repeat unit. However, the middle block often does not
consist of such depicted butane homo-polymer but of a styrene-butadiene
co-polymer:
By using a statistical copolymer at this position, the polymer becomes less susceptible to cross-linking and flows
better in the melt. For the production of SBS, the first styrene is
homopolymerized via anionic copolymerization. Typically, an
organometallic compound such as butyllithium is used as a catalyst.
Butadiene is then added and after styrene again its polymerization. The
catalyst remains active during the whole process (for which the used
chemicals must be of high purity). The molecular weight distribution of the polymers is very low (polydispersity
in the range of 1.05, the individual chains have thus very similar
lengths). The length of the individual blocks can be adjusted by the
ratio of catalyst to monomer. The size of the rubber sections, in turn,
depends on the block length. The production of small structures (smaller
than the wavelength of the light) ensure transparency. In contrast to
PS-I, however, the block copolymer does not form any particles but has a
lamellar structure.
Styrene-butadiene rubber (SBR) is produced like PS-I by graft
copolymerization, but with a lower styrene content. Styrene-butadiene
rubber thus consists of a rubber matrix with a polystyrene phase
dispersed therein. Unlike PS-I and SBC, it is not a thermoplastic, but an elastomer.
Within the rubber phase, the polystyrene phase is assembled into
domains. This causes physical cross-linking on a microscopic level. When
the material is heated above the glass transition point, the domains
disintegrate, the cross-linking is temporarily suspended and the
material can be processed like a thermoplastic.
Polystyrene
foams are produced using blowing agents that form bubbles and expand
the foam. In expanded polystyrene, these are usually hydrocarbons such
as pentane,
which may pose a flammability hazard in manufacturing or storage of
newly manufactured material, but have relatively mild environmental
impact. Extruded polystyrene is usually made with hydrofluorocarbons (HFC-134a), which have global warming potentials of approximately 1000–1300 times that of carbon dioxide. Packaging, particularly expanded polystyrene, is a contributor of microplastics from both land and maritime activities.
Discarded polystyrene cup on the shore of Lake Michigan
Animals do not recognize polystyrene foam as an artificial material and may even mistake it for food.
Polystyrene foam blows in the wind and floats on water due to its low
specific gravity. It can have serious effects on the health of birds
and marine animals that swallow significant quantities.
Juvenile rainbow trout exposed to polystyrene fragments show toxic
effects in the form of substantial histomorphometrical changes.
Restricting the use of foamed polystyrene takeout food packaging is a priority of many solid waste environmental organisations.
Efforts have been made to find alternatives to polystyrene, especially
foam in restaurant settings. The original impetus was to eliminate chlorofluorocarbons (CFC), which was a former component of foam.
United States
In 1987, Berkeley, California, banned CFC food containers. The following year, Suffolk County, New York, became the first U.S. jurisdiction to ban polystyrene in general. However, legal challenges by the Society of the Plastics Industry
kept the ban from going into effect until at last it was delayed when
the Republican and Conservative parties gained the majority of the
county legislature. In the meantime, Berkeley became the first city to ban all foam food containers. As of 2006, about one hundred localities in the United States, including Portland, Oregon, and San Francisco had some sort of ban on polystyrene foam in restaurants. For instance, in 2007 Oakland, California, required restaurants to switch to disposable food containers that would biodegrade if added to food compost. In 2013, San Jose became reportedly the largest city in the country to ban polystyrene foam food containers. Some communities have implemented wide polystyrene bans, such as Freeport, Maine, which did so in 1990. In 1988, the first U.S. ban of general polystyrene foam was enacted in Berkeley, California.
On 1 July 2015, New York City became the largest city in the United States to attempt to prohibit the sale, possession, and distribution of single-use polystyrene foam (the initial decision was overturned on appeal).
In San Francisco, supervisors approved the toughest ban on "Styrofoam"
(EPS) in the US which went into effect 1 January 2017. The city's
Department of the Environment can make exceptions for certain uses like
shipping medicines at prescribed temperatures.
The U.S. Green Restaurant Association does not allow polystyrene foam to be used as part of its certification standard. Several green leaders, including the Dutch Ministry of the Environment, advise people to reduce their environmental harm by using reusable coffee cups.
In March 2019, Maryland banned polystyrene foam food containers
and became the first state in the country to pass a food container foam
ban through the state legislature. Maine was the first state to
officially get a foam food container ban onto the books. In May 2019,
Maryland Governor Hogan allowed the foam ban (House Bill 109) to become
law without a signature making Maryland the second state to have a food
container foam ban on the books, but is the first one to take effect on 1
July 2020.
In September 2020, the New Jersey state legislature voted to ban disposable foam food containers and cups made of polystyrene foam.
Outside the United States
Polystyrene waste in Japan
China
banned expanded polystyrene takeout/takeaway containers and tableware
around 1999. However, compliance has been a problem and, in 2013, the
Chinese plastics industry was lobbying for the ban's repeal.
India and Taiwan also banned polystyrene-foam food-service ware before 2007.
The government of Zimbabwe,
through its Environmental Management Agency (EMA), banned polystyrene
containers (popularly called 'kaylite' in the country), under Statutory
Instrument 84 of 2012 (Plastic Packaging and Plastic Bottles)
(Amendment) Regulations, 2012 (No 1.)
The city of Vancouver,
Canada, has announced its Zero Waste 2040 plan in 2018. The city will
introduce bylaw amendments to prohibit business license holders from
serving prepared food in polystyrene foam cups and take-out containers,
beginning 1 June 2019.
In 2019, the European union voted to ban expanded polystyrene
food packaging and cups, with the law officially going into effect in
2021.
Fiji passed the Environmental Management Bill in December 2020. Imports of polystyrene products were banned in January 2021.
In general, polystyrene is not accepted in curbside collection
recycling programs and is not separated and recycled where it is
accepted. In Germany, polystyrene is collected as a consequence of the
packaging law (Verpackungsverordnung) that requires manufacturers to
take responsibility for recycling or disposing of any packaging material
they sell.
Most polystyrene products are currently not recycled due to the
lack of incentive to invest in the compactors and logistical systems
required. Due to the low density of polystyrene foam, it is not
economical to collect. However, if the waste material goes through an
initial compaction process, the material changes density from typically
30 kg/m3 to 330 kg/m3 and becomes a recyclable
commodity of high value for producers of recycled plastic pellets.
Expanded polystyrene scrap can be easily added to products such as EPS
insulation sheets and other EPS materials for construction applications;
many manufacturers cannot obtain sufficient scrap because of collection
issues. When it is not used to make more EPS, foam scrap can be turned
into products such as clothes hangers, park benches, flower pots, toys,
rulers, stapler bodies, seedling containers, picture frames, and
architectural molding from recycled PS. As of 2016, around 100 tonnes of EPS are recycled every month in the UK.
Recycled EPS is also used in many metal casting operations. Rastra
is made from EPS that is combined with cement to be used as an
insulating amendment in the making of concrete foundations and walls.
American manufacturers have produced insulating concrete forms made with
approximately 80% recycled EPS since 1993.
Upcycling
A
March 2022 joint study by scientists Sewon Oh and Erin Stache at Cornell
University in Ithaca, New York found a new processing method of
upcycling polystyrene to benzoic acid. The process involved irradiation of polystyrene with iron chloride and acetone under white light and oxygen for 20 hours.
The scientists also demonstrated a similar scalable commercial process
of upcycling polystyrene into valulable small-molecules (like benzoic
acid) taking just a few hours.
Incineration
If polystyrene is properly incinerated at high temperatures (up to 1000 °C) and with plenty of air (14 m3/kg),
the chemicals generated are water, carbon dioxide, and possibly small
amounts of residual halogen-compounds from flame-retardants. If only incomplete incineration is done, there will also be leftover carbon soot and a complex mixture of volatile compounds. According to the American Chemistry Council,
when polystyrene is incinerated in modern facilities, the final volume
is 1% of the starting volume; most of the polystyrene is converted into
carbon dioxide, water vapor, and heat. Because of the amount of heat
released, it is sometimes used as a power source for steam or electricity generation.
When polystyrene was burned at temperatures of 800–900 °C (the
typical range of a modern incinerator), the products of combustion
consisted of "a complex mixture of polycyclic aromatic hydrocarbons
(PAHs) from alkyl benzenes to benzoperylene. Over 90 different
compounds were identified in combustion effluents from polystyrene."
The American National Bureau of Standards Center for Fire Research
found 57 chemical by-products released during the combustion of expanded
polystyrene (EPS) foam.
Based on scientific tests over five
decades, government safety agencies have determined that polystyrene is
safe for use in foodservice products. For example, polystyrene meets
the stringent standards of the U.S. Food and Drug Administration and the
European Commission/European Food Safety Authority for use in packaging
to store and serve food. The Hong Kong Food and Environmental Hygiene
Department recently reviewed the safety of serving various foods in
polystyrene foodservice products and reached the same conclusion as the
U.S. FDA.
From 1999 to 2002, a comprehensive review of the potential health
risks associated with exposure to styrene was conducted by a 12-member
international expert panel selected by the Harvard Center for Risk
Assessment. The scientists had expertise in toxicology, epidemiology,
medicine, risk analysis, pharmacokinetics, and exposure assessment. The
Harvard study reported that styrene is naturally present in trace
quantities in foods such as strawberries, beef, and spices, and is
naturally produced in the processing of foods such as wine and cheese.
The study also reviewed all the published data on the quantity of
styrene contributing to the diet due to migration of food packaging and
disposable food contact articles, and concluded that risk to the general
public from exposure to styrene from foods or food-contact applications
(such as polystyrene packaging and foodservice containers) was at
levels too low to produce adverse effects.
Polystyrene is commonly used in containers for food and drinks.
The styrene monomer (from which polystyrene is made) is a cancer suspect
agent. Styrene is "generally found in such low levels in consumer products that risks aren't substantial". Polystyrene which is used for food contact may not contain more than 1% (0.5% for fatty foods) of styrene by weight. Styrene oligomers in polystyrene containers used for food packaging have been found to migrate into the food. Another Japanese study conducted on wild-type and AhR-null
mice found that the styrene trimer, which the authors detected in
cooked polystyrene container-packed instant foods, may increase thyroid
hormone levels.
Whether polystyrene can be microwaved with food is controversial.
Some containers may be safely used in a microwave, but only if labeled
as such. Some sources suggest that foods containing carotene (vitamin A) or cooking oils must be avoided.
Because of the pervasive use of polystyrene, these serious health related issues remain topical.
Fire hazards
Like other organic compounds, polystyrene is flammable. Polystyrene is classified according to DIN4102
as a "B3" product, meaning highly inflammable or "Easily Ignited". As a
consequence, although it is an efficient insulator at low temperatures,
its use is prohibited in any exposed installations in building construction if the material is not flame-retardant. It must be concealed behind drywall, sheet metal, or concrete.
Foamed polystyrene plastic materials have been accidentally ignited and
caused huge fires and losses of life, for example at the Düsseldorf International Airport and in the Channel Tunnel (where polystyrene was inside a railway carriage that caught fire).
Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against wavenumber.
The solar constant (GSC) measures the amount of energy received by a given area one astronomical unit away from the Sun. More specifically, it is a flux density measuring mean solarelectromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (au) from the Sun (roughly the distance from the Sun to the Earth).
The solar constant includes radiation over the entire electromagnetic spectrum. It is measured by satellite as being 1.361 kilowatts per square meter (kW/m2) at solar minimum (the time in the 11-year solar cycle when the number of sunspots is minimal) and approximately 0.1% greater (roughly 1.362 kW/m2) at solar maximum.
The solar "constant" is not a physical constant in the modern CODATA scientific sense; that is, it is not like the Planck constant or the speed of light
which are absolutely constant in physics. The solar constant is an
average of a varying value. In the past 400 years it has varied less
than 0.2 percent. Billions of years ago, it was significantly lower.
This constant is used in the calculation of radiation pressure, which aids in the calculation of a force on a solar sail.
Calculation
Solar irradiance is measured by satellites above Earth's atmosphere, and is then adjusted using the inverse square law to infer the magnitude of solar irradiance at one Astronomical Unit (au) to evaluate the solar constant. The approximate average value cited, 1.3608 ± 0.0005 kW/m2, which is 81.65 kJ/m2 per minute, is equivalent to approximately 1.951 calories per minute per square centimeter, or 1.951 langleys per minute.
Solar output is nearly, but not quite, constant. Variations in total solar irradiance
(TSI) were small and difficult to detect accurately with technology
available before the satellite era (±2% in 1954). Total solar output is
now measured as varying (over the last three 11-year sunspot cycles) by approximately 0.1%; see solar variation for details.
Historical measurements
In 1838, Claude Pouillet made the first estimate of the solar constant. Using a very simple pyrheliometer he developed, he obtained a value of 1.228 kW/m2, close to the current estimate.
In 1875, Jules Violle resumed the work of Pouillet and offered a somewhat larger estimate of 1.7 kW/m2 based, in part, on a measurement that he made from Mont Blanc in France.
In 1884, Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney
in California. By taking readings at different times of day, he tried
to correct for effects due to atmospheric absorption. However, the final
value he proposed, 2.903 kW/m2, was much too large.
A 1903 Langley bolograph with an erroneous solar constant of 2.54 calories/minute/square centimeter.
Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1.322 and 1.465 kW/m2.
Abbot showed that one of Langley's corrections was erroneously applied.
Abbot's results varied between 1.89 and 2.22 calories (1.318 to
1.548 kW/m2), a variation that appeared to be due to the Sun and not the Earth's atmosphere.
In 1954 the solar constant was evaluated as 2.00 cal/min/cm2 ± 2%. Current results are about 2.5 percent lower.
The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year (from 1.412 kW/m2 in early January to 1.321 kW/m2
in early July) due to the Earth's varying distance from the Sun, and
typically by much less than 0.1% from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km2), the power is 1.730×1017W (or 173,000 terawatts),
plus or minus 3.5% (half the approximately 6.9% annual range). The
solar constant does not remain constant over long periods of time (see Solar variation),
but over a year the solar constant varies much less than the solar
irradiance measured at the top of the atmosphere. This is because the
solar constant is evaluated at a fixed distance of 1 Astronomical Unit (au) while the solar irradiance will be affected by the eccentricity of the Earth's orbit. Its distance to the Sun varies annually between 147.1·106 km at perihelion and 152.1·106 km at aphelion. In addition, several long term (tens to hundreds of millennia) cycles of subtle variation in the Earth's orbit (Milankovich cycles) affect the solar irradiance and insolation (but not the solar constant).
The Earth receives a total amount of radiation determined by its cross section (π·RE2), but as it rotates this energy is distributed across the entire surface area (4·π·RE2).
Hence the average incoming solar radiation, taking into account the
angle at which the rays strike and that at any one moment half the
planet does not receive any solar radiation, is one-fourth the solar
constant (approximately 340 W/m2). The amount reaching the Earth's surface (as insolation)
is further reduced by atmospheric attenuation, which varies. At any
given moment, the amount of solar radiation received at a location on
the Earth's surface depends on the state of the atmosphere, the
location's latitude, and the time of day.
Apparent magnitude
The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light (see Electromagnetic spectrum). It is positively correlated with the apparent magnitude
of the Sun which is −26.8. The solar constant and the magnitude of the
Sun are two methods of describing the apparent brightness of the Sun,
though the magnitude is based on the Sun's visual output only.
The Sun's total radiation
The angular diameter of the Earth as seen from the Sun is approximately 1/11,700 radians (about 18 arcseconds), meaning the solid angle of the Earth as seen from the Sun is approximately 1/175,000,000 of a steradian. Thus the Sun emits about 2.2 billion times the amount of radiation that is caught by Earth, in other words about 3.846×1026 watts.
Past variations in solar irradiance
Space-based
observations of solar irradiance started in 1978. These measurements
show that the solar constant is not constant. It varies with the 11-year
sunspot solar cycle.
When going further back in time, one has to rely on irradiance
reconstructions, using sunspots for the past 400 years or cosmogenic
radionuclides for going back 10,000 years.
Such reconstructions show that solar irradiance varies with distinct
periodicities. These cycles are: 11 years (Schwabe), 88 years (Gleisberg
cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).
Over billions of years, the Sun is gradually expanding, and
emitting more energy from the resultant larger surface area. The
unsolved question of how to account for the clear geological evidence of
liquid water on the Earth billions of years ago, at a time when the
sun's luminosity was only 70% of its current value, is known as the faint young Sun paradox.
Variations due to atmospheric conditions
At most about 75% of the solar energy actually reaches the earth's surface,
as even with a cloudless sky it is partially reflected and absorbed by
the atmosphere. Even light cirrus clouds reduce this to 50%, stronger
cirrus clouds to 40%. Thus the solar energy arriving at the surface with
the sun directly overhead can vary from 550 W/m2 with cirrus clouds to 1025 W/m2 with a clear sky.
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