Molecular weight

The average molecular weight of commercial polystyrene varies between 100,000 and 400,000 g mol.[5]​ The lower the molecular weight, the greater the fluidity and therefore the ease of use of the material, but the lower its mechanical resistance.

To achieve a polystyrene that is both fluid and resistant, bimodal molecular weight distributions can be used, an active field of research in polystyrene.

Branch

The polystyrene molecules formed in current industrial processes are very linear. In the laboratory it is possible to generate branching by adding substances such as divinylbenzene[6]​ or tetrafunctional peroxides[7]​ to the reactor, but the polystyrene thus obtained is more expensive and hardly has any advantages over its linear equivalents.

Tacticity

Crystal polystyrene is completely atactic; That is to say: the phenyl groups are distributed on one side or the other of the central chain, in no particular order. Therefore, it is a completely amorphous polymer (that is, non-crystalline).[3]​.

Contenido

El poliestireno de alto impacto (en inglés: High Impact PolyStyrene o HIPS) consiste en una matriz de poliestireno cristal en la cual están dispersas partículas microscópicas de caucho, casi siempre polibutadieno.

Rubber type

In the vast majority of cases the rubber used is polybutadiene, specifically grades made with cobalt ("high-cis") or lithium ("low-cis") catalysts. The molecular weight of the polybutadiene used is usually between 180,000 and 260,000 g mol.

In some very minor applications, an elastomer consisting of a styrene-butadiene diblock is used. Due to its greater affinity for polystyrene (and therefore lower surface tension), this polymer forms smaller particles than polybutadiene.

Until the 1960s, styrene-butadiene rubber (SBR) was used to modify polystyrene but has since been replaced by polybutadiene, which provides better mechanical properties to the product.[8]​.

rubber particles

In most commercial polystyrenes, the elastomeric "Phase (chemical)") phase is presented in the form of particles with a structure called "salami type": a more or less spherical particle of polybutadiene that in turn has polystyrene particles of different sizes inside, which are called "occlusions". In some cases, particles with "core-shell" morphology (which could be translated as "core-shell") are observed, in which the rubber forms only a thin membrane around a single polystyrene occlusion, normally small in size (less than a micron in diameter).[8]​.

To have a high-impact polystyrene with good mechanical resistance, salami-type particles with a size between 1 and 6 micrometers "Micrometer (unit of length)" are needed. When the size is less than a micron, the product becomes almost as fragile as glass polystyrene, although transparency increases, which can be interesting in some applications.[8]​.

The size of the particles depends essentially on three factors:

• - shearing during phase inversion.

• - the viscosity ratio between the polybutadiene and the polystyrene matrix.

• - the amount of grafted polystyrene.

It has been proven that a bimodal distribution of particle sizes increases the impact resistance of the material. There are various chemical and physical methods to achieve or at least approximate bimodality, the most common being the use of two polybutadienes of different viscosities.

The ratio between the volume occupied by the polybutadiene particles (including occlusions) and the total volume of the polystyrene is called the "elastomeric phase volume fraction" (RPVF). In general terms, the higher the RPVF, the better the mechanical properties of the shock polystyrene.[9]​ To increase the RPVF, more polybutadiene can be added but this has a serious drawback: polybutadiene is more expensive than styrene or shock polystyrene itself. Therefore, the preferred route is to increase the amount of polystyrene occluded inside the particles, which is one of the objectives of all high-impact polystyrene producers.

Graft

During the polymerization of styrene, it sometimes occurs that a free radical attacks one of the double bonds of the polybutadiene molecules, thus forming a polystyrene molecule chemically bonded to a polybutadiene molecule. It is then said that the polystyrene is "grafted" on the polybutadiene.[8]​.

Grafting improves the properties of high-impact polystyrene for two reasons. On the one hand, it facilitates the transmission of energy between the polybutadiene and polystyrene phases. On the other hand, it acts as an emulsifier, stabilizing the dispersion of rubber particles in the polystyrene matrix. High-impact polystyrene producers try to adapt their process to maximize grafting, in order to obtain the best possible relationship between mechanical properties of the product and the amount of rubber used.[8]​.

Reticulation

The double bonds in polybutadiene can also react with each other, forming bridges between the molecules. This is called "cross-linking" of the elastomeric phase. The cross-linking is more intense the longer the polybutadiene spends at high temperature. In the polystyrene production process, shock occurs especially in the devolatilization section.[10]​.

A certain level of cross-linking is necessary for rubber to be elastic "Elasticity (solid mechanics)") but, if the cross-linking goes too far, the rubber becomes rigid and therefore the polystyrene loses some of its mechanical strength.

A escala industrial, el poliestireno se prepara calentando el etilbenceno en presencia de un catalizador para dar lugar al estireno. La polimerización del estireno se da por radicales "Radical (química)") libres, polimerización catiónica, polimerización aniónica o sobre catalizador.[3]​.

Reaction mechanisms

Styrene can polymerize by four different mechanisms:

1. • By free radicals. Styrene radicals form spontaneously, at a faster rate the higher the temperature. For this reason, styrene is stored in refrigerated tanks and stabilized with inhibitors, which consume free radicals. The reaction rate becomes significant above 100 °C. It can be accelerated by adding initiators such as peroxides, which generate additional free radicals.[11]​.

2. • Anionic polymerization.

3. • Cationic polymerization.

4. • About catalyst.

By using Ziegler-Natta or metallocene-type catalysts, the tacticity of the formed polymer can be precisely controlled. Syndiotactic polystyrene is produced industrially in this way.

In all cases, the polymerization of styrene generates the same amount of heat: 165 cal g.[3]​.

Phase inversion

In the production of high-impact polystyrene, the reaction medium initially consists of only one phase "Phase (chemical)"), a solution of polybutadiene in styrene ("PB phase"). As the styrene reacts, the polystyrene generated begins to precipitate (from 2% conversion) and thus forms a different phase, the "PS phase", which consists of polystyrene particles dispersed in the PB phase. The polystyrene particles grow and become more and more numerous until, at a certain point, they occupy a volume equal to that of the PB phase. From then on, the arrangement of the phases is reversed: the PS phase becomes the continuous medium and the PB phase begins to form particles within the PS phase. This phenomenon is called phase inversion").[8]​.

Phase inversion is not a punctual or instantaneous phenomenon but rather develops over a relatively wide conversion interval. The value of this interval depends on several parameters, mainly the percentage of rubber and the degree of grafting. In any case, a certain degree of agitation is necessary in the reactor for phase inversion to occur; If this is not done, the product obtained appears as a shapeless mixture of polystyrene and polybutadiene, with no clear distinction between the two phases.[12]​.

During phase inversion the viscosity of the mixture increases significantly. This allows it to be detected in the laboratory.

Se describen las propiedades del PS choque y el PS cristal. Para el EPS, véase el artículo principal Poliestireno expandido.

Mechanical properties

Fountain:.

Thermal properties

"Compact" polystyrene (without gas injection inside) has the lowest thermal conductivity of all thermoplastics.

However, it has relatively little temperature resistance, softening between 85 and 105 °C (the exact value depends on the mineral oil content).

When polystyrene is heated the chains are capable of taking numerous conformations. This ability of the system to deform easily above its glass transition temperature allows polystyrene to be easily molded by heating.[3]​.

Optical properties

While anti-shock PS is completely opaque and whitish due to the polybutadiene it contains, crystal PS is transparent. It has a refractive index around 1.57, similar to that of polycarbonate and PVC.[13]​.

Mixtures of anti-shock PS and crystal PS are more translucent but also more fragile the more crystal PS they contain. It is possible to find a compromise between both properties so that the manufactured objects, for example disposable cups, are transparent while being acceptably resistant.

Electrical properties

Polystyrene has very low electrical conductivity (typically 10 S m), that is, it is an insulator.[13]​ Due to its properties, it is usually used in high-frequency installations[15]​.

The main advantages of polystyrene are its ease of use and its relatively low cost. Its main disadvantages are its low resistance to high temperature (it deforms at less than 100 °C, except in the case of syndiotactic polystyrene) and its modest mechanical resistance. These advantages and disadvantages determine the applications of the different types of polystyrene.

• - Anti-shock polystyrene is mainly used in the manufacture of objects through injection molding. Some examples: television casings, printers, refrigerator doors and interiors, disposable razors, toys. Depending on the applications, additives can be added, such as flame retardant substances or colorants.[16]​.

• - Crystal polystyrene is also used in injection molding where transparency and low cost are important. Examples: CD cases, hangers, egg boxes. Another very important application is in the production of rigid foams, sometimes called "extruded polystyrene" or XPS, not to be confused with expanded polystyrene EPS. These XPS foams are used, for example, for meat trays in supermarkets, as well as in construction.

• - In Europe, the largest application of polystyrene is the production of disposable containers for dairy products by extrusion-thermoforming").[17]​ In these cases, a mixture of shock and glass is usually used, in a variable proportion depending on whether one wishes to favor mechanical resistance or transparency. A particularly important market is that of dairy product containers, which take advantage of an almost exclusive property of polystyrene: its dryness. This is what allows one yogurt to be separated from another with a simple movement. of the hand.

• - The expanded form (expanded polystyrene) EPS is used as a thermal and acoustic insulator and is widely known under various commercial brands (Poliexpan, Telgopor, Emmedue, Icopor, etc.).

• - The extruded form (extruded polystyrene) XPS is used as thermal insulation in floors, due to its greater mechanical resistance, and also as a core in façade sandwich panels. But its most specific use is as a thermal insulator in inverted roofs, where the thermal insulation is placed on top of the waterproofing, protecting it from inclement weather and extending its useful life.[18]​.

• - Other minor applications: sports clothing, for example, because it has the property of floating in water, it is used in the manufacture of life jackets and other items for water sports; or due to its light and shock-absorbing properties, it is used in the manufacture of cycling helmets, motorcyclist helmets and even Formula 1 helmets (it is the EPS that really protects against impact in the event of an accident); It is also used as a binder") in certain explosives such as RDX and Napalm (for example in the MK77[19]​).

El proceso más utilizado en la actualidad para el poliestireno se basa en la polimerización radical en masa").[3]​ "Radical" significa que la reacción es iniciada por radicales libres, generados térmicamente mediante moléculas específicas denominadas iniciadores. "En masa" significa que el medio de reacción está formado esencialmente por estireno y poliestireno, añadiéndose a veces otro hidrocarburo inerte perfectamente miscible con el estireno, a menudo etilbenceno, que sirve para moderar la velocidad de reacción. Las líneas basadas en procesos en emulsión y en solución") han quedado anticuadas hoy día, siendo reservadas a la producción de grados de especialidad.[3]​.

Existen numerosas licencias de proceso de estireno disponibles en el mercado. Se diferencian en detalles tecnológicos que tratan de mejorar tanto la calidad del producto como la productividad del proceso pero en esencia el fondo del proceso es el mismo para todas.

polystyrene process

1. • Conditioning of raw materials. As it is not based on catalysts, the polystyrene process can accept high concentrations of impurities in the raw materials, so practically no purification is carried out.[20] Some plants pass the styrene through a bed of alumina to remove the polymerization inhibitor.

2. • Reaction. Styrene polymerizes spontaneously, faster the higher the temperature. Reactors are essentially vessels in which a temperature is set (typically between 100 and 200 °C) and homogeneity is ensured by stirring. To accelerate the reaction, peroxides can also be added, which act as polymerization initiators. "There are many different reactor designs that differ mainly by the way of evacuating heat (through internal tubes or external condenser), by the distribution of residence times (stirred tank or plug flow) and by the type of agitation.[3]​.

3. • Devolatilization. The conversion in the reactors ranges, depending on the specific process in question, between 60 and 90%. Unconverted styrene and ethylbenzene are separated from the polystyrene in the devolatilization section and recirculated to the feed. Although designs vary depending on licenses, devolatilization generally consists of one or several empty containers (called devolatilizers) in which high temperature and extreme vacuum are applied "Vacuum (physics)") in order to leave less than 0.1% of residual hydrocarbons in the product. polystyrene.[21]​.

4. • Purification of the recycle"). The styrene and ethylbenzene separated in the devolatilization (current called recycle) contain a large part of the impurities introduced with the raw materials. In some plants, the recycle is purified, either by vacuum distillation or using alumina beds. In other plants, a part of the recycle is simply purged, which allows the concentration of impurities to be maintained. in the process under control.[22]​.

5. • Granulation. In the most common process, the molten polystyrene that leaves the devolatilizer passes through a row of holes&action=edit&redlink=1 "Dice (engineering) (not yet written)"), forming threads a few millimeters thick that are cooled in a water bath, dried and cut into the shape of small cylinders, which are called pellet. In another process, the threads are cut before drying, with the advantage of generating less dust. Finally, in a small minority of plants the cutter is located directly inside the water bath, in a configuration identical to that used for polyolefins, for example.

6. • Expedition. The polystyrene is either sent to silos to be sold in bulk or bagged and packed on one-ton pallets.

Particularities of the anti-shock polystyrene process

Anti-shock polystyrene is produced according to a very similar process to that of crystal polystyrene, so only the differences are listed below.

The most obvious difference is that the rubber (usually polybutadiene, although styrene-butadiene is sometimes used) must be dissolved in the styrene before feeding it to the reactors. For this, large agitated tanks are used.

The reactors can have the same geometry as those of crystal polystyrene, but their control is much more delicate because the reaction parameters influence the morphology of the polybutadiene particles (number of particles, size, shape), which has a great effect on the mechanical properties of the final product.[23]​.

Finally, the devolatilization temperature must also be controlled more finely because the cross-linking of the polybutadiene depends on it, which must be neither excessive nor insufficient.[3]​.

Beginnings

Styrene was discovered in 1839 by Eduard Simon,[24]​ a Berlin apothecary from storax "Storax (balsam)"), the resin of the Turkish tree Liquidambar orientalis, which distills an oily substance, a monomer that he called styrene. Several days later, Simon found that the styrene had thickened, presumably by oxidation, turning into a gelatin he called styrene oxide (Styroloxyd). In 1845, the Jamaican-born chemist John Buddle Blyth and the German chemist August Wilhelm von Hofmann showed that the same transformation of styrene took place in the absence of oxygen. They called their substance metastyrol (later analysis showed that it was chemically identical to Styroloxyd). In 1866 Marcelin Berthelot correctly identified the formation of metastyrol/Styroloxyd from styrene as a polymerization process. These experiences remained mere laboratory curiosities.

Almost 80 years later, in 1920, the German Hermann Staudinger (1881–1965) was the first to deliberately synthesize polystyrene in his laboratory, and to explain the phenomenon through a "polymerization theory" (he discovered that heating styrene initiates a chain reaction that produces macromolecules). This led to the substance that receives its current name, polystyrene. His theory sparked strong controversy and was rejected by the scientific community of the time.[25] However, Staudinger continued his work, being rewarded with the Nobel Prize in Chemistry in 1953.

Crystal polystyrene

The German company BASF (at that time part of the IG Farben conglomerate) became interested in polymers and began an R&D program. They achieved success when they developed a reactor vessel that extruded polystyrene through a heated tube with a cutter, producing polystyrene in pellet form. This led to the first commercial production of polystyrene in Ludwigshafen, around 1931,[17] in the hope that it would be a suitable substitute for die-cast zinc in many applications. A key figure in this program was Herman F. Mark").[26]​.

Around the same time, the American company Dow, which had produced large quantities of ethylbenzene for a market that ultimately did not take off, decided to transform it into styrene and brilliantly develop its own polystyrene production process.[27]​.

BASF used a sophisticated continuous mass polymerization process that allowed high productivity and good control of product quality. Each plant had several reactors in series, called "tower reactors" because of their vertical cylinder shape.[26] By comparison, Dow's process was very rudimentary: the styrene was simply left to react inside heated metal cans for days. This did not prevent Dow from launching its polystyrene on the market in 1938.[27]​.

In both America and Europe, what limited the production of polystyrene was the supply of the monomer, styrene. The problem became more acute during World War II because styrene became a strategic raw material, being one of the key ingredients in synthetic rubber. After the war ended, the styrene produced by the numerous plants built in the United States became available for other uses. That's when polystyrene production really took off.[28]​.

Dow technicians were able to visit BASF factories after Germany's defeat in 1945 and were surprised by the technological sophistication of the Germans. They immediately applied the knowledge obtained to develop a semi-continuous mass polymerization process, which they installed in their plant in Midland (Michigan, USA) and which was a great commercial success.[29]​.

In the following years, Koppers Chemical developed an alternative process based on suspension polymerization, which provided some advantages in product quality and which had great commercial success from the 1950s onwards.[30]​.

However, mass polymerization continued to be used and from the 1980s it once again took the lead in technology, becoming more economical in both investment and operating expenses. In addition, the suspension process generates water contaminated with phosphates which, with the tightening of environmental regulations, has become an increasing problem. Little by little the suspension plants have been abandoned, being currently dedicated only to the production of expanded polystyrene and a few specialty grades of crystal polystyrene.[3]​.

expanded polystyrene

BASF and Dow independently developed expanded polystyrene in the early 1940s. BASF's process, based on the use of pentane as a foaming agent, proved to be far superior and in the post-war period became the only one used industrially.[31]​.

Demand for expanded polystyrene skyrocketed in the late 1960s thanks in part to the invention of extruders that allowed direct injection of pentane into liquid polystyrene.[32]​.

High impact polystyrene

From the first moment it was clear that crystal polystyrene had a great disadvantage: its fragility. For this reason, the idea of ​​reinforcing it with natural rubber arose early on (Ostromislensky patent, 1927).[33]​.

But industrial production was complicated due to the rubber's tendency to crosslink in reactors, forming gels. After numerous failed experiments, in 1954 Dow found the solution: adding a "prepolymerization" stage under strong agitation to its process. Monsanto came to the same conclusion almost simultaneously and both companies became involved in a long patent lawsuit.[33]​.

Due to its greater technical complexity and the variety of its applications, most of the research and product development effort carried out by polystyrene manufacturers has focused on impact polystyrene.[3] Today it is a technology as solidly established as that of crystal polystyrene.

Syndiotactic polystyrene

In 1985, the Japanese company Idemitsu synthesized syndiotactic polystyrene (sPS) for the first time and three years later began a collaboration with Dow for its industrial production. In 1996 Dow opened an sPS plant in Schkopau (Germany) but in 2005 it had to close it due to its lack of commercial success.[34]​.

Most polystyrene products are not currently recycled due to a lack of incentives to invest in compactors and the logistics of moving the materials. Due to the low density of Styrofoam, it is not economical to collect. However, if the waste material goes through an initial compaction process, the material changes its density from typically 30 kg/m³ to 330 kg/m³ and becomes a high value material for recycled plastic pellet producers.

Expanded polystyrene waste can be easily added to products such as EPS insulation panels and other materials with construction applications; However, many manufacturers cannot obtain enough material due to the collection problems mentioned above. When not used to make more EPS, foam can be turned into products such as coat hangers, park benches, flower pots, toys, rulers, seed containers, picture frames and construction molding, all made from recycled polystyrene.

Currently, around 100 tonnes of EPS are recycled each month in the UK and there are efforts in other countries, such as Mexico, to encourage collection, compaction and reuse.[35]​.

In Spain, EPS expanded polystyrene containers must be placed in the yellow domestic waste container.[36]​[37]​.

• - ISO 1622-1 Plastics - Polystyrene (PS) - Part 1: Designation system and bases for specifications (ISO 1622-1:1994).

• - ISO 1622-2 Plastics - Polystyrene (PS) - Part 2: Preparation of test samples and determination of properties (ISO 1622-2:1995).

• - ISO 2897-1 Plastics - Impact resistant polystyrene (PS-I) - Part 1: Designation system and bases for specifications (ISO 2897-1:1997).

• - ISO 2897-2 Plastics - Impact resistant polystyrene (PS-I) - Part 2: Preparation of test samples and determination of properties (ISO 2897-2:2003).

• - Polybutadiene.

• - High impact polystyrene.

• - Expanded polystyrene.

• - Extruded polystyrene.

• - Polymethylmethacrylate.

• - Polyurethane.

• - Thermoclay.

• - Thermoplastic.

• - Wikimedia Commons hosts a multimedia category on Polystyrene.

• - Iberian Extruded Polystyrene Association. (XPS).

• - National Expanded Polystyrene Association.

• - Polystyrene, scientific articles, manufacturing methods and uses.

• - Ecoembes.

• - Bio-polystyrene made of cellulose crystals, better insulator than polystyrene, supports 200 times its weight.

Molecular weight

The average molecular weight of commercial polystyrene varies between 100,000 and 400,000 g mol.[5]​ The lower the molecular weight, the greater the fluidity and therefore the ease of use of the material, but the lower its mechanical resistance.

To achieve a polystyrene that is both fluid and resistant, bimodal molecular weight distributions can be used, an active field of research in polystyrene.

Branch

The polystyrene molecules formed in current industrial processes are very linear. In the laboratory it is possible to generate branching by adding substances such as divinylbenzene[6]​ or tetrafunctional peroxides[7]​ to the reactor, but the polystyrene thus obtained is more expensive and hardly has any advantages over its linear equivalents.

Tacticity

Crystal polystyrene is completely atactic; That is to say: the phenyl groups are distributed on one side or the other of the central chain, in no particular order. Therefore, it is a completely amorphous polymer (that is, non-crystalline).[3]​.

Contenido

El poliestireno de alto impacto (en inglés: High Impact PolyStyrene o HIPS) consiste en una matriz de poliestireno cristal en la cual están dispersas partículas microscópicas de caucho, casi siempre polibutadieno.

Rubber type

In the vast majority of cases the rubber used is polybutadiene, specifically grades made with cobalt ("high-cis") or lithium ("low-cis") catalysts. The molecular weight of the polybutadiene used is usually between 180,000 and 260,000 g mol.

In some very minor applications, an elastomer consisting of a styrene-butadiene diblock is used. Due to its greater affinity for polystyrene (and therefore lower surface tension), this polymer forms smaller particles than polybutadiene.

Until the 1960s, styrene-butadiene rubber (SBR) was used to modify polystyrene but has since been replaced by polybutadiene, which provides better mechanical properties to the product.[8]​.

rubber particles

In most commercial polystyrenes, the elastomeric "Phase (chemical)") phase is presented in the form of particles with a structure called "salami type": a more or less spherical particle of polybutadiene that in turn has polystyrene particles of different sizes inside, which are called "occlusions". In some cases, particles with "core-shell" morphology (which could be translated as "core-shell") are observed, in which the rubber forms only a thin membrane around a single polystyrene occlusion, normally small in size (less than a micron in diameter).[8]​.

To have a high-impact polystyrene with good mechanical resistance, salami-type particles with a size between 1 and 6 micrometers "Micrometer (unit of length)" are needed. When the size is less than a micron, the product becomes almost as fragile as glass polystyrene, although transparency increases, which can be interesting in some applications.[8]​.

The size of the particles depends essentially on three factors:

• - shearing during phase inversion.

• - the viscosity ratio between the polybutadiene and the polystyrene matrix.

• - the amount of grafted polystyrene.

It has been proven that a bimodal distribution of particle sizes increases the impact resistance of the material. There are various chemical and physical methods to achieve or at least approximate bimodality, the most common being the use of two polybutadienes of different viscosities.

The ratio between the volume occupied by the polybutadiene particles (including occlusions) and the total volume of the polystyrene is called the "elastomeric phase volume fraction" (RPVF). In general terms, the higher the RPVF, the better the mechanical properties of the shock polystyrene.[9]​ To increase the RPVF, more polybutadiene can be added but this has a serious drawback: polybutadiene is more expensive than styrene or shock polystyrene itself. Therefore, the preferred route is to increase the amount of polystyrene occluded inside the particles, which is one of the objectives of all high-impact polystyrene producers.

Graft

During the polymerization of styrene, it sometimes occurs that a free radical attacks one of the double bonds of the polybutadiene molecules, thus forming a polystyrene molecule chemically bonded to a polybutadiene molecule. It is then said that the polystyrene is "grafted" on the polybutadiene.[8]​.

Grafting improves the properties of high-impact polystyrene for two reasons. On the one hand, it facilitates the transmission of energy between the polybutadiene and polystyrene phases. On the other hand, it acts as an emulsifier, stabilizing the dispersion of rubber particles in the polystyrene matrix. High-impact polystyrene producers try to adapt their process to maximize grafting, in order to obtain the best possible relationship between mechanical properties of the product and the amount of rubber used.[8]​.

Reticulation

The double bonds in polybutadiene can also react with each other, forming bridges between the molecules. This is called "cross-linking" of the elastomeric phase. The cross-linking is more intense the longer the polybutadiene spends at high temperature. In the polystyrene production process, shock occurs especially in the devolatilization section.[10]​.

A certain level of cross-linking is necessary for rubber to be elastic "Elasticity (solid mechanics)") but, if the cross-linking goes too far, the rubber becomes rigid and therefore the polystyrene loses some of its mechanical strength.

A escala industrial, el poliestireno se prepara calentando el etilbenceno en presencia de un catalizador para dar lugar al estireno. La polimerización del estireno se da por radicales "Radical (química)") libres, polimerización catiónica, polimerización aniónica o sobre catalizador.[3]​.

Reaction mechanisms

Styrene can polymerize by four different mechanisms:

1. • By free radicals. Styrene radicals form spontaneously, at a faster rate the higher the temperature. For this reason, styrene is stored in refrigerated tanks and stabilized with inhibitors, which consume free radicals. The reaction rate becomes significant above 100 °C. It can be accelerated by adding initiators such as peroxides, which generate additional free radicals.[11]​.

2. • Anionic polymerization.

3. • Cationic polymerization.

4. • About catalyst.

By using Ziegler-Natta or metallocene-type catalysts, the tacticity of the formed polymer can be precisely controlled. Syndiotactic polystyrene is produced industrially in this way.

In all cases, the polymerization of styrene generates the same amount of heat: 165 cal g.[3]​.

Phase inversion

In the production of high-impact polystyrene, the reaction medium initially consists of only one phase "Phase (chemical)"), a solution of polybutadiene in styrene ("PB phase"). As the styrene reacts, the polystyrene generated begins to precipitate (from 2% conversion) and thus forms a different phase, the "PS phase", which consists of polystyrene particles dispersed in the PB phase. The polystyrene particles grow and become more and more numerous until, at a certain point, they occupy a volume equal to that of the PB phase. From then on, the arrangement of the phases is reversed: the PS phase becomes the continuous medium and the PB phase begins to form particles within the PS phase. This phenomenon is called phase inversion").[8]​.

Phase inversion is not a punctual or instantaneous phenomenon but rather develops over a relatively wide conversion interval. The value of this interval depends on several parameters, mainly the percentage of rubber and the degree of grafting. In any case, a certain degree of agitation is necessary in the reactor for phase inversion to occur; If this is not done, the product obtained appears as a shapeless mixture of polystyrene and polybutadiene, with no clear distinction between the two phases.[12]​.

During phase inversion the viscosity of the mixture increases significantly. This allows it to be detected in the laboratory.

Se describen las propiedades del PS choque y el PS cristal. Para el EPS, véase el artículo principal Poliestireno expandido.

Mechanical properties

Fountain:.

Thermal properties

"Compact" polystyrene (without gas injection inside) has the lowest thermal conductivity of all thermoplastics.

However, it has relatively little temperature resistance, softening between 85 and 105 °C (the exact value depends on the mineral oil content).

When polystyrene is heated the chains are capable of taking numerous conformations. This ability of the system to deform easily above its glass transition temperature allows polystyrene to be easily molded by heating.[3]​.

Optical properties

While anti-shock PS is completely opaque and whitish due to the polybutadiene it contains, crystal PS is transparent. It has a refractive index around 1.57, similar to that of polycarbonate and PVC.[13]​.

Mixtures of anti-shock PS and crystal PS are more translucent but also more fragile the more crystal PS they contain. It is possible to find a compromise between both properties so that the manufactured objects, for example disposable cups, are transparent while being acceptably resistant.

Electrical properties

Polystyrene has very low electrical conductivity (typically 10 S m), that is, it is an insulator.[13]​ Due to its properties, it is usually used in high-frequency installations[15]​.

The main advantages of polystyrene are its ease of use and its relatively low cost. Its main disadvantages are its low resistance to high temperature (it deforms at less than 100 °C, except in the case of syndiotactic polystyrene) and its modest mechanical resistance. These advantages and disadvantages determine the applications of the different types of polystyrene.

• - Anti-shock polystyrene is mainly used in the manufacture of objects through injection molding. Some examples: television casings, printers, refrigerator doors and interiors, disposable razors, toys. Depending on the applications, additives can be added, such as flame retardant substances or colorants.[16]​.

• - Crystal polystyrene is also used in injection molding where transparency and low cost are important. Examples: CD cases, hangers, egg boxes. Another very important application is in the production of rigid foams, sometimes called "extruded polystyrene" or XPS, not to be confused with expanded polystyrene EPS. These XPS foams are used, for example, for meat trays in supermarkets, as well as in construction.

• - In Europe, the largest application of polystyrene is the production of disposable containers for dairy products by extrusion-thermoforming").[17]​ In these cases, a mixture of shock and glass is usually used, in a variable proportion depending on whether one wishes to favor mechanical resistance or transparency. A particularly important market is that of dairy product containers, which take advantage of an almost exclusive property of polystyrene: its dryness. This is what allows one yogurt to be separated from another with a simple movement. of the hand.

• - The expanded form (expanded polystyrene) EPS is used as a thermal and acoustic insulator and is widely known under various commercial brands (Poliexpan, Telgopor, Emmedue, Icopor, etc.).

• - The extruded form (extruded polystyrene) XPS is used as thermal insulation in floors, due to its greater mechanical resistance, and also as a core in façade sandwich panels. But its most specific use is as a thermal insulator in inverted roofs, where the thermal insulation is placed on top of the waterproofing, protecting it from inclement weather and extending its useful life.[18]​.

• - Other minor applications: sports clothing, for example, because it has the property of floating in water, it is used in the manufacture of life jackets and other items for water sports; or due to its light and shock-absorbing properties, it is used in the manufacture of cycling helmets, motorcyclist helmets and even Formula 1 helmets (it is the EPS that really protects against impact in the event of an accident); It is also used as a binder") in certain explosives such as RDX and Napalm (for example in the MK77[19]​).

El proceso más utilizado en la actualidad para el poliestireno se basa en la polimerización radical en masa").[3]​ "Radical" significa que la reacción es iniciada por radicales libres, generados térmicamente mediante moléculas específicas denominadas iniciadores. "En masa" significa que el medio de reacción está formado esencialmente por estireno y poliestireno, añadiéndose a veces otro hidrocarburo inerte perfectamente miscible con el estireno, a menudo etilbenceno, que sirve para moderar la velocidad de reacción. Las líneas basadas en procesos en emulsión y en solución") han quedado anticuadas hoy día, siendo reservadas a la producción de grados de especialidad.[3]​.

Existen numerosas licencias de proceso de estireno disponibles en el mercado. Se diferencian en detalles tecnológicos que tratan de mejorar tanto la calidad del producto como la productividad del proceso pero en esencia el fondo del proceso es el mismo para todas.

polystyrene process

1. • Conditioning of raw materials. As it is not based on catalysts, the polystyrene process can accept high concentrations of impurities in the raw materials, so practically no purification is carried out.[20] Some plants pass the styrene through a bed of alumina to remove the polymerization inhibitor.

2. • Reaction. Styrene polymerizes spontaneously, faster the higher the temperature. Reactors are essentially vessels in which a temperature is set (typically between 100 and 200 °C) and homogeneity is ensured by stirring. To accelerate the reaction, peroxides can also be added, which act as polymerization initiators. "There are many different reactor designs that differ mainly by the way of evacuating heat (through internal tubes or external condenser), by the distribution of residence times (stirred tank or plug flow) and by the type of agitation.[3]​.

3. • Devolatilization. The conversion in the reactors ranges, depending on the specific process in question, between 60 and 90%. Unconverted styrene and ethylbenzene are separated from the polystyrene in the devolatilization section and recirculated to the feed. Although designs vary depending on licenses, devolatilization generally consists of one or several empty containers (called devolatilizers) in which high temperature and extreme vacuum are applied "Vacuum (physics)") in order to leave less than 0.1% of residual hydrocarbons in the product. polystyrene.[21]​.

4. • Purification of the recycle"). The styrene and ethylbenzene separated in the devolatilization (current called recycle) contain a large part of the impurities introduced with the raw materials. In some plants, the recycle is purified, either by vacuum distillation or using alumina beds. In other plants, a part of the recycle is simply purged, which allows the concentration of impurities to be maintained. in the process under control.[22]​.

5. • Granulation. In the most common process, the molten polystyrene that leaves the devolatilizer passes through a row of holes&action=edit&redlink=1 "Dice (engineering) (not yet written)"), forming threads a few millimeters thick that are cooled in a water bath, dried and cut into the shape of small cylinders, which are called pellet. In another process, the threads are cut before drying, with the advantage of generating less dust. Finally, in a small minority of plants the cutter is located directly inside the water bath, in a configuration identical to that used for polyolefins, for example.

6. • Expedition. The polystyrene is either sent to silos to be sold in bulk or bagged and packed on one-ton pallets.

Particularities of the anti-shock polystyrene process

Anti-shock polystyrene is produced according to a very similar process to that of crystal polystyrene, so only the differences are listed below.

The most obvious difference is that the rubber (usually polybutadiene, although styrene-butadiene is sometimes used) must be dissolved in the styrene before feeding it to the reactors. For this, large agitated tanks are used.

The reactors can have the same geometry as those of crystal polystyrene, but their control is much more delicate because the reaction parameters influence the morphology of the polybutadiene particles (number of particles, size, shape), which has a great effect on the mechanical properties of the final product.[23]​.

Finally, the devolatilization temperature must also be controlled more finely because the cross-linking of the polybutadiene depends on it, which must be neither excessive nor insufficient.[3]​.

Beginnings

Styrene was discovered in 1839 by Eduard Simon,[24]​ a Berlin apothecary from storax "Storax (balsam)"), the resin of the Turkish tree Liquidambar orientalis, which distills an oily substance, a monomer that he called styrene. Several days later, Simon found that the styrene had thickened, presumably by oxidation, turning into a gelatin he called styrene oxide (Styroloxyd). In 1845, the Jamaican-born chemist John Buddle Blyth and the German chemist August Wilhelm von Hofmann showed that the same transformation of styrene took place in the absence of oxygen. They called their substance metastyrol (later analysis showed that it was chemically identical to Styroloxyd). In 1866 Marcelin Berthelot correctly identified the formation of metastyrol/Styroloxyd from styrene as a polymerization process. These experiences remained mere laboratory curiosities.

Almost 80 years later, in 1920, the German Hermann Staudinger (1881–1965) was the first to deliberately synthesize polystyrene in his laboratory, and to explain the phenomenon through a "polymerization theory" (he discovered that heating styrene initiates a chain reaction that produces macromolecules). This led to the substance that receives its current name, polystyrene. His theory sparked strong controversy and was rejected by the scientific community of the time.[25] However, Staudinger continued his work, being rewarded with the Nobel Prize in Chemistry in 1953.

Crystal polystyrene

The German company BASF (at that time part of the IG Farben conglomerate) became interested in polymers and began an R&D program. They achieved success when they developed a reactor vessel that extruded polystyrene through a heated tube with a cutter, producing polystyrene in pellet form. This led to the first commercial production of polystyrene in Ludwigshafen, around 1931,[17] in the hope that it would be a suitable substitute for die-cast zinc in many applications. A key figure in this program was Herman F. Mark").[26]​.

Around the same time, the American company Dow, which had produced large quantities of ethylbenzene for a market that ultimately did not take off, decided to transform it into styrene and brilliantly develop its own polystyrene production process.[27]​.

BASF used a sophisticated continuous mass polymerization process that allowed high productivity and good control of product quality. Each plant had several reactors in series, called "tower reactors" because of their vertical cylinder shape.[26] By comparison, Dow's process was very rudimentary: the styrene was simply left to react inside heated metal cans for days. This did not prevent Dow from launching its polystyrene on the market in 1938.[27]​.

In both America and Europe, what limited the production of polystyrene was the supply of the monomer, styrene. The problem became more acute during World War II because styrene became a strategic raw material, being one of the key ingredients in synthetic rubber. After the war ended, the styrene produced by the numerous plants built in the United States became available for other uses. That's when polystyrene production really took off.[28]​.

Dow technicians were able to visit BASF factories after Germany's defeat in 1945 and were surprised by the technological sophistication of the Germans. They immediately applied the knowledge obtained to develop a semi-continuous mass polymerization process, which they installed in their plant in Midland (Michigan, USA) and which was a great commercial success.[29]​.

In the following years, Koppers Chemical developed an alternative process based on suspension polymerization, which provided some advantages in product quality and which had great commercial success from the 1950s onwards.[30]​.

However, mass polymerization continued to be used and from the 1980s it once again took the lead in technology, becoming more economical in both investment and operating expenses. In addition, the suspension process generates water contaminated with phosphates which, with the tightening of environmental regulations, has become an increasing problem. Little by little the suspension plants have been abandoned, being currently dedicated only to the production of expanded polystyrene and a few specialty grades of crystal polystyrene.[3]​.

expanded polystyrene

BASF and Dow independently developed expanded polystyrene in the early 1940s. BASF's process, based on the use of pentane as a foaming agent, proved to be far superior and in the post-war period became the only one used industrially.[31]​.

Demand for expanded polystyrene skyrocketed in the late 1960s thanks in part to the invention of extruders that allowed direct injection of pentane into liquid polystyrene.[32]​.

High impact polystyrene

From the first moment it was clear that crystal polystyrene had a great disadvantage: its fragility. For this reason, the idea of ​​reinforcing it with natural rubber arose early on (Ostromislensky patent, 1927).[33]​.

But industrial production was complicated due to the rubber's tendency to crosslink in reactors, forming gels. After numerous failed experiments, in 1954 Dow found the solution: adding a "prepolymerization" stage under strong agitation to its process. Monsanto came to the same conclusion almost simultaneously and both companies became involved in a long patent lawsuit.[33]​.

Due to its greater technical complexity and the variety of its applications, most of the research and product development effort carried out by polystyrene manufacturers has focused on impact polystyrene.[3] Today it is a technology as solidly established as that of crystal polystyrene.

Syndiotactic polystyrene

In 1985, the Japanese company Idemitsu synthesized syndiotactic polystyrene (sPS) for the first time and three years later began a collaboration with Dow for its industrial production. In 1996 Dow opened an sPS plant in Schkopau (Germany) but in 2005 it had to close it due to its lack of commercial success.[34]​.

Most polystyrene products are not currently recycled due to a lack of incentives to invest in compactors and the logistics of moving the materials. Due to the low density of Styrofoam, it is not economical to collect. However, if the waste material goes through an initial compaction process, the material changes its density from typically 30 kg/m³ to 330 kg/m³ and becomes a high value material for recycled plastic pellet producers.

Expanded polystyrene waste can be easily added to products such as EPS insulation panels and other materials with construction applications; However, many manufacturers cannot obtain enough material due to the collection problems mentioned above. When not used to make more EPS, foam can be turned into products such as coat hangers, park benches, flower pots, toys, rulers, seed containers, picture frames and construction molding, all made from recycled polystyrene.

Currently, around 100 tonnes of EPS are recycled each month in the UK and there are efforts in other countries, such as Mexico, to encourage collection, compaction and reuse.[35]​.

In Spain, EPS expanded polystyrene containers must be placed in the yellow domestic waste container.[36]​[37]​.

• - ISO 1622-1 Plastics - Polystyrene (PS) - Part 1: Designation system and bases for specifications (ISO 1622-1:1994).

• - ISO 1622-2 Plastics - Polystyrene (PS) - Part 2: Preparation of test samples and determination of properties (ISO 1622-2:1995).

• - ISO 2897-1 Plastics - Impact resistant polystyrene (PS-I) - Part 1: Designation system and bases for specifications (ISO 2897-1:1997).

• - ISO 2897-2 Plastics - Impact resistant polystyrene (PS-I) - Part 2: Preparation of test samples and determination of properties (ISO 2897-2:2003).

• - Polybutadiene.

• - High impact polystyrene.

• - Expanded polystyrene.

• - Extruded polystyrene.

• - Polymethylmethacrylate.

• - Polyurethane.

• - Thermoclay.

• - Thermoplastic.

• - Wikimedia Commons hosts a multimedia category on Polystyrene.

• - Iberian Extruded Polystyrene Association. (XPS).

• - National Expanded Polystyrene Association.

• - Polystyrene, scientific articles, manufacturing methods and uses.

• - Ecoembes.

• - Bio-polystyrene made of cellulose crystals, better insulator than polystyrene, supports 200 times its weight.