WO2014111628A2

WO2014111628A2 - Method of producing polystyrene beads having low moisture content

Info

Publication number: WO2014111628A2
Application number: PCT/FI2014/050043
Authority: WO
Inventors: Ville Nurminen; Sami LIPPONEN; Elina PESONEN; Asko RONKAINEN
Original assignee: Styrochem Finland Oy
Priority date: 2013-01-18
Filing date: 2014-01-20
Publication date: 2014-07-24
Also published as: WO2014111628A3

Abstract

A method of producing polystyrene particles or beads in which styrene monomers are suspended in a liquid phase to form a suspension and are subjected to a polymerisation reaction in the presence of a polymerisation initiator and athermanous particles. On reaching a preselected conversion rate electrolyte is added to the suspension and polymerisation is continued in order to produce polystyrene particles with a moisture content of less than 5 % by weight of the particles.

Description

METHOD OF PRODUCING POLYSTYRENE BEADS HAVING LOW

MOISTURE CONTENT

FIELD OF THE INVENTION

The present invention relates to polystyrene beads. In particular, the invention relates to the production of polystyrene beads which when expanded are capable of providing articles with insulating properties, e.g. grey polystyrene products. Thus, the invention relates to a method according to the preamble portion of claim 1 as well as expandable polystyrene beads according to the preamble of claim 22 and uses thereof and articles produced thereof according to claim 23.

BACKGROUND ART

Expandable polystyrene beads are traditionally produced by the suspension polymerisation of styrene, which involves the mechanical dispersion of styrene in water and

polymerisation of the resultant monomer droplets by use of a monomer- soluble initiator. Styrene is kept in suspension by continuous agitation and the use of stabilisers. Polystyrene is obtained in the form of beads. One problem caused by suspension polymerisation of styrene is that the beads produced contain moisture, moisture that seeps out from the beads and reduces their ability to flow, causing problems for their addition to processes, and their workability and strength. Expanded articles have been used for some time, for example to improve the thermal insulation of buildings. Generally, these articles are prepared by either extrusion or moulding through the swelling of polymer beads. Additives can be used to improve the desired properties of the articles. Expandable polystyrene (EPS) is becoming increasingly common for use in such articles. EPS is a rigid and tough, closed-cell foam, generally made of pre-expanded polystyrene beads. Common uses of these EPS beads include moulded sheets for building insulation and packing material for cushioning fragile items. The latest advances in the technology related to these EPS beads concern improving the thermal insulation of the articles prepared from the beads.

In order to improve the thermal insulating properties of polystyrene beads, athermanous particles, such as graphite and carbon black, are incorporated into the polystyrene to produce what is known as grey EPS. The production of grey EPS moulded articles is disclosed in EP 0 620 246. Athermanous material can be added directly to the

polymerisation process, on the surface of the unexpanded EPS beads or to pre-foamed EPS beads before they are moulded. Further processes for the production of such materials are described in EP 0 981 574 and EP 0 981 575, as well as EP 1 608 698.

Due to the addition of athermanous material, the colour of the granules is black and the colour of the corresponding expanded beads is grey. Grey EPS is more thermally efficient than white EPS and provides improved insulation achieving lower lambda values than that of white EPS. Grey EPS raw beads, however, have an exceptionally high water content caused by the addition of the athermanous particles.

Previously, satisfaction has been achieved in producing polystyrene beads containing a known amount of water/ moisture or in being able to reduce the amount of moisture in the beads after polymerisation with vigorous drying. It has also been possible to have a slight influence over the internal moisture content of the particles by varying process parameters e.g. temperature, mixing intensity, stabilisers.

A disadvantage of drying after the polymerisation process is that this adds a step to the production process as well as moisture removal problems caused by post processing. The moisture seeps out from the beads preventing them from flowing freely causing e.g.

difficulty with feeding into further processing, such as foaming. Moreover, a high water content may reduce the expanded polystyrene's workability as well as its strength.

There are a number of prior art methods for producing polystyrene beads and there are known methods for producing EPS beads with low water content such as by adding an electrolyte during the suspension polymerisation process as in the method disclosed in the German patent application DE 19548311 Al . These prior art methods have the disadvantages that the electrolyte added is in high proportion to the mass of styrene. For example, in example 6 of the above named German patent application 0.72 mol/litre of water of NaCl are added. This amounts to approximately 5 kg of salt i.e. 5.5 % of the mass of styrene. A further disadvantage of these methods is that athermanous materials have not been used. Adding electrolyte according to the method described in the above DE application to the polymerisation process containing athermanous particles reduces the moisture content only slightly, to that of white EPS with no added electrolyte, i.e. no net benefit of moisture reduction can be seen. A further such example is disclosed in US 5,616,413, in which an electrolyte is added to the suspension polymerisation reaction after a degree of conversion of, preferably, at least 50 %, in particular 75 to 100 %, has been reached. The EPS produced is further dried with a fluidization dryer. Pre-expanded beads are obtained in which the number of cells present along the surface parts is smaller than the number of cells present along said inner parts. The cells in the surface parts are of much greater size than the cells in the inner parts of the beads and may therefore crease or collapse easily.

AIM OF THE INVENTION It is an aim of the present invention to overcome the disadvantages associated with previous solutions and to provide expandable polystyrene beads containing athermanous particles and having a low water content employing a simple novel process.

SUMMARY

The present invention is based on the concept of producing grey EPS particles or beads by suspension polymerisation. Thus, styrene monomers, suspended in a liquid phase to form a suspension, are subjected to a polymerisation reaction in the presence of a polymerisation initiator and athermanous particles to produce polystyrene particles. An electrolyte is added to the suspension at a preselected point of time, and polymerisation is continued in order to produce polystyrene particles. The EPS particles or beads have excellent properties; in particular they exhibit good thermal insulation properties combined with a moisture content of less than 5 % by weight of the particles. They have a good, substantially wrinkle-free, aesthetically-pleasing surface enabling utilisation in a multitude of applications.

The particles can be used in the form of expanded polystyrene beads for forming articles, in particular panels and sheets of expanded polystyrene.

More specifically, the method according to the present invention is mainly characterised by what is stated in the characterising part of claim 1. The EPS beads are characterised by what is stated in the characterising part of claim 22 and the use thereof by what is stated in claim 23.

BENEFITS Considerable benefits are gained with the aid of the present invention e.g. dry polystyrene particles can be produced without the need for any post process water removal. In addition, the low salt : styrene ratio reduces costs and benefits the environment in that after the polymerisation process, the water of the suspension polymerisation process requires less treatment before it can be disposed of or reused.

In traditional methods, increasing the proportion of styrene in the polymerisation suspension is known to increase the amount of water present in the produced EPS beads. Thus, one method of reducing the amount of water present in the produced EPS beads is to lower the proportion of styrene in the suspension, which has the disadvantage of lowering the production rate of the polymerisation reactor.

By means of the present invention, by adding salt to a polymerisation reactor containing a polymerisation suspension it is possible to maintain a styrene-to -water-ratio corresponding to a predetermined rate (or capacity) of the polymerisation reactor, while surprisingly providing EPS beads with lower than expected moisture content when athermanous particles are used. Thus, the weight ratio of styrene monomers to the total weight of the suspension can be at least 0.40 and preferably is 0.50 or greater. Furthermore, the addition of athermanous material to a typical suspension polymerisation reaction, provides EPS beads that have a water content that is approximately two to ten times greater than that in corresponding EPS beads without athermanous material. By means of the present invention, EPS beads containing athermanous material are provided with low water content, as will become apparent from the detailed description of preferred embodiments and the illustrative non-limiting examples given below.

The EPS beads provided typically have small cells on the surface, with the diameter of the cells increasing towards the centre of the bead. This provides a smooth surface that is easily workable and that does not wrinkle, crease or collapse easily.

Further advantages will appear from the following detailed description of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows in cross-section an expandable polystyrene bead according to one embodiment of the present invention; and

Figure 2 shows in cross-section an expandable polystyrene bead according to another embodiment of the present invention; and

Figure 3 shoes in cross-section an expandable polystyrene bead according to a further embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Even though the water content of grey polystyrene beads is so high, a surprisingly significant reduction in the water content of grey polystyrene beads can be achieved with surprisingly low saltwater and surprisingly low sal styrene ratios.

Given that the water content of grey polystyrene beads is more than three times that of white polystyrene beads and given the general trend in the art i.e. amount of salt is directly proportional to reduction in water content, it may be obvious that three times the water content would require three times the amount of salt to reduce the water content. The amount of salt used in the present technology is, however, about one third of that used in the art. In the present application meanings of the word "salt" include, but are not limited to, electrolyte, electrolyte solution and salt. Meanings of the word "electrolyte" include, but are not limited to, salt, electrolyte solution and electrolyte.

Various embodiments described herein provide a method of producing polystyrene beads, including a method and process for the removal of water from the beads.

The present technology comprises a method of producing polystyrene beads containing athermanous particles and which exhibit a moisture content of less than 5 % by weight of the beads.

As mentioned above, the method according to the present technology is based on suspension polymerisation of styrene monomers in a liquid phase. The styrene monomers are subjected to polymerisation in the presence of a polymerisation initiator and athermanous particles. The addition of athermanous particles to the polymerisation process results in the production of grey EPS. Grey EPS has a considerably lower thermal conductivity than the corresponding white EPS.

An electrolyte or salt is added to the suspension when the polymerisation reaction has reached a preselected conversion rate and the polymerisation is continued in order to produce polystyrene particles. In a preferred embodiment, the particles have a moisture content of preferably less than 5% by weight of the particles. The low water content of the particles means that less water is able to ooze out and thus a reduction in the ability of the particles to flow is prevented. The electrolyte or salt can be added at any preselected point of time during the

polymerisation. However, it is particularly preferred to add the electrolyte only after an initial phase of polymerisation. Thus, in an embodiment, the electrolyte is added when a conversion rate of at least 5 % has been reached. Preferably the electrolyte is added at a point of time corresponding to a conversion rate of 10 - 95 %, for example 25 to 70% and more preferably 40 - 70 %. The conversion rate is calculated from the amount by weight of the styrene monomer. In one embodiment the electrolyte or salt is selected from alkali metal, ammonium or earth alkaline metal salts of inorganic or organic acids, or preferably from a mixture of two or more salts.

In a further embodiment the electrolyte or salt is selected from the from the group of sodium, potassium, calcium and magnesium salts of inorganic or organic acids, preferably from mixtures thereof, and more preferably from the group of sodium, potassium, calcium and magnesium salts of mineral acids or carboxylic acids, most preferably from mixtures thereof. The selection of a mixture of electrolytes provides both costs savings as well as a reduction of the corrosive nature of the suspension when a single electrolyte or salt is used.

In one example the electrolyte or salt is added in an amount ranging from 0.01 to 25% by weight, in particular 0.05 to 20 % by weight, preferably about 0.1 to 10 % and more preferably 0.2 to 3 % by weight of the styrene monomer.

In another example the electrolyte or salt is added as a dry powder, or preferably dissolved or dispersed in a liquid, most preferably in a solvent corresponding to the liquid phase of the suspension. In a further example the polymerisation is carried out in an aqueous phase into which the styrene monomers are suspended and the electrolyte or salt is added in the form of an aqueous solution or preferably as a slurry.

A blowing agents is added during the implementation of the process or afterwards. The blowing agent typically comprises a hydrocarbon, such as an aliphatic hydrocarbon having 3 to 10 carbon atoms. Typical examples of such hydrocarbons comprise C3 to C6 aliphatics and isomers thereof. Specific examples are propane, butane, pentane and hexane and isomers thereof, such as isobutane, isopentane and isohexane. The blowing agents is added in amounts of about 0.1 to about 15 parts by weight per 100 parts by weight of styrene.

In one embodiment a method is provided for producing expandable polystyrene particles in which styrene monomers are polymerised in a suspension at a first temperature in the range of about 75 to 110 °C until a preselected degree of conversion has been reached. The degree of conversion is at least 5 % calculated from the molar amount of the styrene monomer. The electrolyte or salt is then added to the suspension. In the said embodiment a blowing agent is added and the suspension is heated to a second, higher temperature during the addition of the blowing agent.

Polymerisation is continued until a preselected final conversion degree has been reached. In a further embodiment the temperature of the suspension is raised to a third temperature higher than the second temperature. In a further embodiment the electrolyte or salt is added over a time period smaller than that needed for increasing the conversion rate of the polymerisation by 10 % units, preferably over a time period corresponding to the time needed for increasing the conversion rate by 10 to 30 % units.

In one example the polymerisation is continued to a conversion rate of at least 95 %, calculated from the styrene monomer and in a preferred embodiment the polystyrene particles are recovered from the liquid phase. In another example the polymerisation is carried out in the presence of a radical initiator or initiators, typically 1 to 4 initiators being used.

In one embodiment polymerisation is carried out in the presence of additives selected from the group of emulsifiers, preferably stabilizers, more preferably fillers e.g. talc, or still more preferably, fire retardants e.g. hexabromocyclododecane or brominated polymeric flame retardants, as well as combinations thereof, each additive providing its own additional property to the produced polystyrene particles, e.g. making the particles more flame retardant. In a further embodiment the athermanous particles are selected from carbon black, graphite, coke, graphene and combinations thereof. Carbon black, graphite, coke and graphene all absorb infra red radiation, thus increasing the insulating properties of the polystyrene particles. The content of athermanous particles is typically 0.1 to 15 parts by weight, for example 1 to 10, or 2 to 5, parts by weight, based on 100 parts by weight of styrene.

In one embodiment, the athermanous particles are selected from carbon black and the electrolyte or salt from sodium acetate.

In one embodiment the styrene monomers are polymerised in the presence of comonomers. The addition of comonomers varies the quality of the resultant polystyrene particles.

In a further embodiment the polystyrene particles that are produced exhibit a moisture content that is 0.1 to 30 %, preferably 25 % or less of the moisture content of the corresponding particles produced in the absence of said salt.

In an embodiment the polystyrene particles are obtained having a moisture content of less than about 2 % by weight of the particles

In one embodiment an article is produced from the expanded polystyrene beads, preferably a panel or more preferably a sheet of expanded polystyrene beads.

The production of expanded product from polystyrene beads is well-known in the art.

Thus, in one alternative, for producing expanded products, the expandable beads are heated, for example by using a heat transfer medium such as steam, to a temperature high enough to soften the beads, preferably to a temperature above the glass transition point of polystyrene. Typically, the temperature is in excess of 95 °C, in particular 100 °C or higher. The heating will cause the blowing agent to boil and while the blowing agent is evaporated off the beads, swelling of the beads is achieved and porous particles created which can be moulded together to form articles, such as panels, sheets and boards, by using traditional shape or block moulding methods.

In preferred embodiments, the ratio of styrene to the total weight of styrene and water present in the suspension, during polymerization, is at least 0.40, in particular at least 0.45, for example at least 0.48, advantageously at least 0.5. Typically the ratio is about 0.5 to 0.8. Conventionally, low ratios are preferred to avoid excessive humidity levels. However, by using electrolytes as disclosed herein it is possible to increase the proportion of styrene monomer in the aqueous suspension while still being able to reach a low moisture content in the expandable polystyrene beads even when adding athermanous particles.

The following non-limiting examples are intended to merely illustrate the methods according to the preferred embodiments of the invention.

EXAMPLES

Example 1

2.3 1 of ion-exchanged water, 1.9 g of sodium acetate, and 3.8 g of sodium bentonite were added in stirred 6 1 autoclave. The mixture was heated to 90 °C within lh, while 1.9 kg styrene, 1.9 g polyethylene, 7.6 g tert-butyl peroxy-2-ethylhexanoate, 5.7 g tert- butylperoxy 2-ethylhexyl carbonate, 5.7 g dicumylperoxide, 19 g hexabromocyclo- dodecane, and 3.8 g pig skin gelatine were added in the autoclave. After the mixture was kept for 30 min at 90 °C 95 g of carbon black was added. After the mixture was kept for 160 min at 90 °C 47.5 g of electrolyte, sodium acetate, was added. After the mixture was kept for 315 min at 90 °C 2.85 g of pig skin gelatine was added.

Altogether the mixture was kept at 90 °C for 320 min where after it was heated to 120 °C within 2h. While heating to 120 °C 152 g of pentane was added. The mixture was kept at 120 °C for 2h where after it was cooled down to 37 °C within 90 min. The reaction mixture was removed from the autoclave and the polystyrene beads were washed thoroughly with water. To remove the water from the surface, the beads were washed with methanol and their inner water content was determined. Example 2

2.3 1 of ion-exchanged water, 1.9 g of sodium acetate, and 3.8 g of sodium bentonite were added in stirred 6 1 autoclave. The mixture was heated to 90 °C within lh, while 1.9 kg styrene, 1.9 g polyethylene, 7.6 g tert-butyl peroxy-2-ethylhexanoate, 5.7 g tert- butylperoxy 2-ethylhexyl carbonate, 5.7 g dicumylperoxide, 19 g hexabromocyclo- dodecane, and 3.8 g pig skin gelatine were added in the autoclave. After the mixture was kept for 30 min at 90 °C 95 g of carbon black was added. After the mixture was kept for 10 min at 90 °C 47.5 g of the extra salt, sodium acetate, was added. After the mixture was kept for 315 min at 90 °C 2.85 g of pig skin gelatine was added.

Altogether the mixture was kept at 90 °C for 320 min where after it was heated to 120 °C within 2h. While heating to 120 °C 152 g of pentane was added. The mixture was kept at 120 °C for 2h where after it was cooled down to 37 °C within 90 min. The reaction mixture was removed from the autoclave and the polystyrene beads were washed thoroughly with water. To remove the water from the surface, the beads were washed with methanol and their inner water content was determined.

Example 3

2.3 1 of ion-exchanged water, 1.9 g of sodium acetate, and 3.8 g of sodium bentonite were added in stirred 6 1 autoclave. The mixture was heated to 90 °C within lh, while 1.9 kg styrene, 1.9 g polyethylene, 7.6 g tert-butyl peroxy-2-ethylhexanoate, 5.7 g tert- butylperoxy 2-ethylhexyl carbonate, 5.7 g dicumylperoxide, 19 g hexabromocyclo- dodecane, and 3.8 g pig skin gelatine were added in the autoclave. After the mixture was kept for 30 min at 90 °C 95 g of carbon black was added. After the mixture was kept for 310 min at 90 °C 47.5 g of the extra salt, sodium acetate, was added. After the mixture was kept for 315 min at 90 °C 2.85 g of pig skin gelatine was added. Altogether the mixture was kept at 90 °C for 320 min where after it was heated to 120 °C within 2h. While heating to 120 °C 152 g of pentane was added. The mixture was kept at 120 °C for 2h where after it was cooled down to 37 °C within 90 min. The reaction mixture was removed from the autoclave and the polystyrene beads were washed thoroughly with water. To remove the water from the surface, the beads were washed with methanol and their inner water content was determined.

Example 4

2.3 1 of ion-exchanged water, 1.9 g of sodium acetate, and 3.8 g of sodium bentonite were added in stirred 6 1 autoclave. The mixture was heated to 90 °C within lh, while 1.9 kg styrene, 1.9 g polyethylene, 7.6 g tert-butyl peroxy-2-ethylhexanoate, 5.7 g tert- butylperoxy 2-ethylhexyl carbonate, 5.7 g dicumylperoxide, 19 g hexabromocyclo- dodecane, and 3.8 g pig skin gelatine were added in the autoclave. After the mixture was kept for 30 min at 90 °C 95 g of carbon black was added. After the mixture was kept for 160 min at 90 °C 19 g of the extra salt, sodium acetate, was added. After the mixture was kept for 315 min at 90 °C 2.85 g of pig skin gelatine was added. Altogether the mixture was kept at 90 °C for 320 min where after it was heated to 120 °C within 2h. While heating to 120 °C 152 g of pentane was added. The mixture was kept at 120 °C for 2h where after it was cooled down to 37 °C within 90 min. The reaction mixture was removed from the autoclave and the polystyrene beads were washed thoroughly with water. To remove the water from the surface, the beads were washed with methanol and their inner water content was determined.

Comparative Example

2.3 1 of ion-exchanged water, 1.9 g of sodium acetate, and 3.8 g of sodium bentonite were added in stirred 6 1 autoclave. The mixture was heated to 90 °C within lh, while 1.9 kg styrene, 1.9 g polyethylene, 7.6 g tert-butyl peroxy-2-ethylhexanoate, 5.7 g tert- butylperoxy 2-ethylhexyl carbonate, 5.7 g dicumylperoxide, 19 g hexabromocyclo- dodecane, and 3.8 g pig skin gelatine were added in the autoclave. After the mixture was kept for 30 min at 90 °C 95 g of carbon black was added. After the mixture was kept for 315 min at 90 °C 2.85 g of pig skin gelatine was added.

As appears from the examples given the Table 1 , by adding an electrolyte (in Example 1 sodium acetate), it is possible to reduce the amount of water in EPS beads from roughly 7 % (cf. Comparative Example) to 1/3 (cf. Example 1). The results obtained also indicate that particularly good results are obtained by adding the electrolyte only when a reasonably high degree of conversion has been obtained. Typically, a minimum degree of 30 to 60 % is preferred.

Table 1. Inner water content of the beads

The sampled beads of Example 2 were analysed by optical microscope. Based on the images two graphical depictions have been made. As will appear, in both of samples, the surface region of the beads are formed by small cells whereas in one of the samples the core region is formed by large cells. More particularly, the cells of the bead shown in Figure 1 have an average cell diameter of about 50 μιη and the cells in the core region have an average cell diameter of about 200 to 400 μιη. In the bead shown in Figure 2, the cells throughout the beads are of at least roughly the same sizes. INDUSTRIAL APPLICABILITY

The polystyrene beads can be put to several uses. In one embodiment, articles such as expanded polystyrene boards are produced for use as insulating material in e.g. the construction industry.

CITATION LIST

Patent Literature

ALGOSTAT GmbH & CO. KG - EP0620246 - Polystyrene Hard Foam Moulded Articles

Gluck Guiscard et al. - EP0981574 - Expandable Polystyrene Polymers Containing Graphite Particles

Gluck Guiscard et al. - EP0981575 - Method For Producing Expandable Polystyrene Polymers Containing Graphite Particles

Ponticiello Antonio et al. - EP 1608698 - Expandable Vinyl Aromatic Polymers and Process For Their Preparation

Hiroki Shinozaki et al. - US 5,616,413 - Expandable Styrene Resin Beads and

Suspension-Polymerization Process for Producing the Same

Claims

1. Method of producing polystyrene particles, wherein

- styrene monomers, suspended in a liquid phase to form a suspension, are subjected to a polymerisation reaction in the presence of a polymerisation initiator and athermanous particles to produce polystyrene particles, said polymerisation reaction having a conversion rate;

comprising

adding an electrolyte to the suspension when the polymerisation reaction has reached a preselected conversion rate, and

continuing polymerisation in order to produce polystyrene particles and exhibiting a moisture content of less than 5 % by weight of the particles.

2. The method according to claim 1, wherein the electrolyte or salt is added at a conversion rate of at least 5 %, in particular about 20 to 95 %, for example 25 to 70 %, in particular about 40 to 70 %, calculated from the molar amount of the styrene monomer.

3. The method according to claim 1 or 2, wherein the electrolyte or salt is selected from alkali metal, ammonium or earth alkaline metal salts of inorganic or organic acids or from a mixture of two or more salts.

4. The method according to claim 3, wherein the electrolyte or salt is selected from the group of sodium, potassium, calcium and magnesium salts of inorganic or organic acids and mixtures thereof, in particular from the group of sodium, potassium, calcium and magnesium salts of mineral acids or carboxylic acids and mixtures thereof, preferably sodium, potassium, calcium and magnesium salts of acetic acid.

5. The method according to any of claims 1 to 4, wherein the electrolyte or salt is added in an amount of 0.01 to 25 % by weight, in particular 0.05 to 20 % by weight, preferably about 0.1 to 10 %, advantageously 0.2 to 3 % by weight, of the styrene monomer.

6. The method according to any of claims 1 to 5, wherein the electrolyte or salt is added as a dry powder or dissolved or dispersed in a liquid, for example in a solvent corresponding to the liquid phase of the suspension.

7. The method according to any of claims 1 to 6, wherein the polymerisation is carried out in an aqueous phase into which the styrene monomers are suspended; and the electrolyte or salt is added in the form of an aqueous solution or slurry.

8. The method according to any of the preceding claims, comprising the step of adding a blowing agent.

9. The method according to any of the preceding claims for producing expandable polystyrene particles, comprising

- polymerising styrene monomer in a suspension at a first temperature in the range of about 75 to about 110 °C until a preselected degree of conversation of at least 30 %, preferably at least 50 %, calculated from the molar amount of the styrene monomer, has been reached,

- adding a blowing agent,

- heating the suspension to a second temperature higher than the first temperature during the addition of the blowing agent,

- optionally raising the temperature of the suspension to a third temperature higher than the second temperature, and

- continuing polymerisation until a preselected final conversion degree has been

reached,

said electrolyte or salt being added to the suspension before addition of the blowing agent.

10. The method according to any of claims 1 to 9, wherein the electrolyte or salt is added over a time period which is smaller than needed for increasing the conversion rate of the polymerisation with 10 % units.

11. The method according to any of claims 1 to 9, wherein the electrolyte or salt is added over a time interval corresponding to the time needed for increasing the conversion rate with 10 to 30 % units.

12. The method according to any of the preceding claims, wherein the polymerisation is continued up to a conversion rate of at least 95 %, calculated from the styrene monomer.

13. The method according to any of the preceding claims, wherein the polystyrene particles are recovered from the liquid phase.

14. The method according to any of the preceding claims, wherein the polymerisation is carried out in the presence of a radical initiator.

15. The method according to any of the preceding claims, wherein the polymerisation is carried out in the presence of additives selected from the group of emulsifiers, stabilizers, fillers, such as talc, fire retardants, for example hexabromocyclododecane or brominated polymeric fire retardants, as well as combinations thereof.

16. The method according to any of claims 1 to 15, wherein the athermanous particles are selected from carbon black, graphite, coke, graphene and combinations thereof.

17. The method according to any of claims 1 to 16, wherein the athermanous particles are selected from carbon black and the electrolyte or salt from sodium acetate.

18. The method according to any of the preceding claims, wherein the styrene monomers are polymerised in the presence of comonomers.

19. The method according to any of the preceding claims, wherein polystyrene particles are produced which exhibit a moisture content which is 0.1 to 30 %, in particular 25 % or less, of the moisture content of corresponding particles produced in the absence of said electrolyte or salt.

20. The method according to any of the preceding claims, wherein polystyrene particles are obtained having a moisture content of less than about 2 % by weight of the particles.

21. The method according to any of the preceding claims, wherein the weight ratio of styrene monomers to the total weight of the suspension is at least 0.40, in particular at least 0.45, for example at least 0.48, advantageously at least 0.5, typically about 0.5 to 0.8

22. Expandable polystyrene beads obtained by a method according to any of claims 1 to 21.

23. Article produced from expanded polystyrene beads according to claim 22, such as a panel, board or sheet of expanded polystyrene beads.