Process for recycling polystyrene

Abstract

The present disclosure relates to the use of 50-515 ppm of at least one polymerization inhibitor as an additive in a suspension radical polymerization comprising styrene monomer and polystyrene for improving the cell structure of expanded polystyrene produced from expandable polystyrene beads obtained from said suspension polymerization, wherein said 50-515 ppm is the total amount of polymerization inhibitor added relative to the initial amount of styrene monomer in the suspension polymerization.

Description

[0001] PROCESS FOR RECYCLING POLYSTYRENE

[0002] Technical Field

[0003] The present disclosure relates to the manufacture of expandable polystyrene beads comprising recycled expandable polystyrene off-grades and polystyrene collected from waste depots.

[0004] Background

[0005] Expandable polystyrene, produced by suspension polymerization of styrene monomer, gives moulded polystyrene foams which have been successfully used in many applications, such as packaging and insulation. Foams of this type are made from fine polystyrene spherical particles impregnated with a hydrocarbon blowing agent like pentanes, expanded by steam to pre-expanded beads to be filled into moulds, and then moulded together with good fusion by steam at slightly higher temperature.

[0006] Suspension polymerization of styrene monomer for the manufacture of expandable polystyrene has been considered a challenge with regards to emissions of chemicals, plant safety and working environment. Environmental concerns put considerable focus on pollution, safety, carbon footprint and circular economy.

[0007] In that respect, there is still a lack of suitable methodology for recycling waste polystyrene. Expandable polystyrene from suspension polymerization of styrene monomer is more complicated to produce commercially with recycled matters. Notably, a challenge with the suspension polymerization of styrene monomer is suspension stability, as the styrene droplets tends to agglomerate during the process. Addition of, for instance, graphite and recycled expandable polystyrene off-grades and polystyrene waste material gives considerable additional challenges on suspension stability. Inadequate suspension stability makes it very difficult to successfully recycle waste polystyrene via suspension polymerization methods.

[0008] US 5,290,819 attempts to solve the suspension stability problem by using magnesium pyrophosphate as an additive. The present inventors, however, found that this additive substantially complicates the overall procedure, as it was found the magnesium pyrophosphate had to be carefully precipitated in-situ to observe any stabilization benefits. US 4,036,794 indicates that polyvinylpyrrolidone manufactured in-situ may be a suitable suspension stabilizer, with one example using 10% recycled polystyrene.

[0009] US 5,698,603, US 5,596,045, and JP2009067843 disclose a multiple-reactor process for recycling polystyrene, wherein a first reactor is used to dissolve polystyrene in styrene monomer, followed by suspending the solution in water with the help of agitation and stabilizers, and polymerize the styrene in a separate reactor. The need for multiple process stages increases process costs and is detrimental to the environmental profile of the process. US 5,698,603 also notes that the presence of recycled polystyrene typically results in the formation of an unsatisfactory cell structure in the final molded product, resulting in inferior mechanical and thermal properties. The contaminated recycled material had to be filtered through a fine sieve to remove particulate contamination to improve the cell structure (under those process conditions the fine particulates substantially increase the number of small cells in the cell structure, hence the inferior mechanical and thermal properties). Like US 5,698,603, the process of JP2009067843 also requires a filtration step to remove particulates contamination. Additionally, JP2009067843 notes that the waste polystyrene / styrene monomer solution produced in the first reactor is storage unstable (especially in summer when the outside air temperature is high). JP2009067843 proposes adding to the first reactor a polymerization inhibitor in an amount sufficient to prevent premature polymerization (gelation) of the waste polystyrene / styrene monomer solution during storage and transport to the filter (i.e., the amount is such that the added inhibitor is consumed during said storage and transportation). It would be preferable to avoid this additional “cleaning” step (i.e., be able to directly use waste polystyrene in the recycling process) whilst avoiding the drawbacks associated with the fine particulates (i.e., increased formation of an uneven cell structure, comprising large amounts of very small and large cells).

[0010] Thus, it would be preferable to provide a single-reactor recycle process that has a total cycle-time that it is substantially the same as a process without recycling, and which overcomes all the above technical challenges. That would substantially improve the environmental footprint of the process whilst reducing production costs and increasing waste polystyrene recycle. It is also necessary to improve the product properties of expandable polystyrene manufactured from waste polystyrene. A solution to all these problems has, so far, remained elusive. Description

[0011] The present inventors have now developed highly effective technical solutions to the above issues. A surprising finding was that intentionally including a certain amount of polymerization inhibitor in the suspension polymerization reaction not only allowed for a single-reactor process for recycling waste polystyrene via a suspension radical polymerization process, but it allowed the process to proceed with large quantities of waste polystyrene (up to 60 wt.%) whilst maintaining excellent product properties (cell structure). Specifically, it was found that the addition of a certain amount of polymerization inhibitor vastly improved the cell structure in the final product.

[0012] As such, in a first aspect the present disclosure relates to the use of 50 to 515 parts per million (ppm) of at least one polymerization inhibitor as an additive in a suspension radical polymerization comprising styrene monomer and waste polystyrene for improving the cell structure of expanded polystyrene produced from expandable polystyrene beads obtained from said suspension polymerization, wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the initial amount of styrene monomer in the suspension polymerization.

[0013] As used herein, the term “improving the cell structure” means improving the uniformity of the cell structure (i.e., improving the uniformity of the size and shape of the cells in the structure). In practice, this means reducing the number of fine and coarse cells in the structure and increasing the number of medium cells in the structure (vs an equivalent process that does not comprise the polymerization inhibitor in the claimed amount), i.e., narrowing the cell size distribution. Preferably, the improved cell structure comprises less than 20% coarse cells (<10 cells / mm), less than 20% fine cells (>30 cells / mm), and at least 60% medium cells (10-30 cells / mm) as evaluated in a microscope (a microscopic image of a thin slice taken from the center of a representative expanded polystyrene bead is captured and the cell structure analysed using a reference scale; see Figures 2-9).

[0014] As used herein, the term “polymerization inhibitor” means a compound used to prevent unwanted polymerization during an induction period, after which polymerization occurs as normal. The term “polymerization inhibitor” does not extend to polymerization retarders (polymerization retarders provide a permanent decrease in the rate of polymerisation). Both are well understood technical terms in the field of radical polymerization, and persons skilled in the art would know the difference between a “polymerization inhibitor” and a “polymerization retarder”. As shown in the worked examples below, by using about 50 to about 515 ppm of a polymerization inhibitor in a suspension radical polymerization comprising styrene monomer and waste polystyrene, the entire process could be successfully performed in a single reactor, and the final product consistently had excellent cell structure. This was not the predicted outcome, as using increased amounts of a polymerization inhibitor (i.e., a radical absorbing compound) ought to have been detrimental to a suspension radical polymerization reaction. The fact that such substantial improvements were observed not only for the suspension radical polymerization process, but also for the final expanded polystyrene product, was very surprising. This unexpected result is a substantial development in the field of polystyrene recycling.

[0015] Preferably, the amount of polymerization inhibitor used (relative to the initial amount of styrene monomer in the suspension polymerization) is from about 100 ppm to about 515 ppm, such as from about 200 ppm to about 515 ppm. Particularly good results were observed when using 100ppm or more polymerization inhibitor. It has been found that about 515 ppm is the upper practical limit, as including the polymerization inhibitor in amounts exceeding 515 ppm caused the raw materials to become too temperature sensitive.

[0016] The polymerization inhibitor is preferably a copper(ll) bis(dithiocarbamate) complex or an aromatic polymerization inhibitor, preferably a phenolic polymerization inhibitor. Preferred inhibitors include substituted or unsubstituted phenols (e.g., 4-tert-butylcatechol (TBC), 4- methoxyphenol (MEHQ), butylated hydroxytoluene (BHT), 2,6-di-tert-butylphenol, tert-butyl hydroquinone (TBHQ), di-tert-butyl hydroquinone (DTBHQ), hydroquinone (HQ), 2,4- dimethyl-6-tert-butyl phenol, 4-methoxyphenol , 2-tert-Butyl-4-methoxyphenol and 3-tert- butyl-4-methoxyphenol (BHA, mixture of isomers), 2-(tert-Butyl)-4,6-dimethylphenol (Topanol A), 2-tert-butyl-p-benzoquinone, 1 ,4-benzoquinone), substituted or unsubstituted quinones (e.g., Mequinol, MeHQ), substituted or unsubstituted catechols (e.g., 4-tert-butylcatechol, TBC), 2,2-Diphenyl-1 -picrylhydrazyl, or copper(ll) dibutyldithiocarbamate. In a preferred embodiment, the phenolic polymerization inhibitor is 4-tert-butylcatechol (TBC).

[0017] As noted above, the polymerization inhibitor in the suspension polymerization reaction allowed the process to proceed with large quantities of waste polystyrene (up to 60 wt.%) whilst maintaining excellent processing and product properties (cell structure - substantially reduced number of fine / small cells). As such, the weight ratio of styrene monomer to waste polystyrene in the composition may be from 90:10 to 40:60, preferably 80:20 to 50:50. In a second aspect, the present disclosure relates to a process for recycling waste polystyrene into expandable polystyrene beads, the method comprising performing a suspension radical polymerization of a composition comprising styrene monomer, waste polystyrene, and 50 to 515 ppm of at least one polymerization inhibitor (preferred polymerization inhibitors are described above), wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the initial amount of styrene monomer in the suspension polymerization. Again, the weight ratio of styrene monomer to waste polystyrene may be from 90:10 to 40:60, preferably 80:20 to 50:50.

[0018] Suspension polymerization is a well-established technique and is the most common polymerization technique for producing expanded polystyrene (EPS). The polymerization process is a suspension process wherein the reaction mixture is a dispersion of a monomer- containing phase in a medium comprising or consisting of water. In these processes the usual additives may be used. For example, for suspensions in water, one or more of the usual additives such as a surfactant, a chain transfer agent, a protective colloid, an antifouling agent, a pH-buffer, flame retardants, flame retardant synergists, etc., may be present. Blowing agents can be added at the start of or during the polymerization process. Because of the presence of styrene monomer and blowing agents such processes are at least partially carried out in a pressurized reactor. The combined weight of the additives preferably is at most 20 wt.%, based on the combined weight of all monomers.

[0019] For the presently disclosed process, by first dissolving the waste polystyrene in styrene in an aqueous suspension at elevated temperature, and then directly at this elevated temperature, the suspension polymerization reaction is started by adding a free-radical-forming initiator to the polymerizable composition dispersed in the dispersing medium (water) and then initiating the polymerization reaction. The radical-forming initiator is preferably at least one initiator selected from peroxydicarbonates, peroxycarbonates, peroxyesters, peroxyketals, diacylperoxides, dialkylperoxides, azo-initiators, ketone peroxides, and mixtures thereof. These initiators may have one or more peroxy and / or azo moieties per molecule, and / or may be further functionalized with one or more functional groups, such as amine, phosphate, ester, ether and / or alcohol groups. Organic peroxides are preferred. It is preferred to use at least one initiator that has a half-life of 60 minutes or less at the polymerization temperature, preferably a half-life of about 0.5 to about 50 minutes at the polymerization temperature (in that respect, see WO 2004 / 089999, which is hereby incorporated by reference in its entirety and explains in detail how to determine the half-life of initiators at a given polymerization temperature). The total amount of initiator to be used in the process is within the range conventionally used in polymerization processes. Typically, it is preferred to use at least 0.01 wt.%, more preferably at least 0.05 wt.%, and most preferably at least 0.1 wt.% of all initiators and preferably at most 5 wt.%, more preferably at most 3 wt.%, and most preferably at most 1 wt.% of all initiators, based on the weight of the monomers to be polymerized. Dosing to the reactor is typically effected by dosing the initiator as such (neat), as a mixture or solution with one or more solvents, or as a dispersion. Suitable solvents are preferably selected from the group consisting of water, conventional organic solvents, monomers, blowing agents (such as pentane, isopentane, and the like), and mixtures thereof. Mixtures with monomer may not be preferred for safety or quality control reasons. Preferably dispersions of the initiator, more preferably aqueous dispersions, are used. Most preferably an aqueous suspension of the initiator in water is used, such as a dibenzoylperoxide suspension in water. Said suspension can be obtained commercially from Nouryon under the trade name Perkadox® L-W40 (which is a 40 wt.% dibenzoylperoxide suspension in water). An aqueous suspension of dibenzoylperoxide is preferred (easy handling and safer addition to a reactor at elevated temperature). When solvents such as alcohols are used, they can be removed during working up of the polymer after the process of polymerization. If used, advantageously solvents are used that do not adversely affect the thermal stability of the initiator dissolved therein, as can be verified by analyzing the half-life temperature (temperature at which the half-life is 1 h) of the initiator in said solvent. An example of a solvent that can be used with most initiators is isododecane. If a dispersion of the initiator is dosed, the dispersion can be a dispersion of the initiator as such or a dispersion of a solution of said initiator. Preferably, the dispersion is an aqueous dispersion. Preferably, dilute initiator solutions or dispersions are used that ensure rapid mixing of the initiator and the polymerization mixture, which leads to a more efficient use of the initiator. Therefore, it is preferred to use mixtures, solutions, or dispersions of the initiator having an initiator concentration of at least 0.1 , more preferably at least 0.5, and most preferably at least 2 wt.%, up to at most 60, more preferably at most 40 wt.%, 25 wt.%, and most preferably at most 15 wt.%.

[0020] In a preferred embodiment, the composition comprising styrene monomer, waste polystyrene and polymerization inhibitor is heated at a temperature of above 80°C, preferably from about 90 to about 120°C, preferably from about 95 to about 115°C, most preferably at about 105 to about 110 °C, for at least 20 minutes, preferably about 30 minutes to about 120 minutes, preferably for about 40 minutes to about 90 minutes, more preferably from about 50 minutes to about 70 minutes, to dissolve the waste polystyrene in the styrene monomer prior to initiating the suspension radical polymerization.

[0021] The suspension radical polymerization may be initiated using at least two initiators. Again, it is preferred to use initiators that have a half-life of 60 minutes or less at the polymerization temperature. More preferably, the first initiator is selected from substituted or unsubstituted dibenzoylperoxides (such as Bis(2-chlorobenzoyl) peroxide, Di(2,4-dichlorobenzoyl) peroxide, Dibenzoyl peroxide, Bis(4-chlorobenzoyl) peroxide, Di(2-methylbenzoyl) peroxide, Bis(3-methylbenzoyl) peroxide, Di(4-methylbenzoyl) peroxide, Bis(4-tert-butyl benzoyl) peroxide, Bis(2-acetoxybenzoyl) peroxide, Bis(3-isopropenyl benzoyl) peroxide, Diphthaloyl peroxide, Bis(4-phenyl azobenzoyl) peroxide, Bis(1 -naphthoyl) peroxide, Bis(2- isobutoxycarbonyl benzoyl) peroxide, 1 ,2,7,10-Tetraoxa-3,6,11 ,14-tetraoxo-4,5,12,13- dibenzocyclotetradecane, Bis((2-cyclohexyloxycarbonyl)benzoyl ) peroxide, or Bis(2- naphthoyl) peroxide), 1 ,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-di(tert- butylperoxy)butane, 1 ,1-di(tert-butylperoxy)cyclohexane, and azo initiators. More preferred are dibenzoylperoxide, 1 ,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2'- azobis(isobutyronitrile), and 2,2'-azobis(2-methylbutyronitrile), and most preferred is dibenzoylperoxide. Commercially available initiators are obtainable under trade names such as Trigonox® 99, Trigonox® SBP, Trigonox® EHP, Trigonox® 131 , Trigonox® 141 , Trigonox® 21 S, and Perkadox® 16, Trigonox® 27, Trigonox® 22, Trigonox® 29, Trigonox® 25, Trigonox® 125, Trigonox® 121 , Trigonox® 421 , Trigonox® 425, Perkadox® AMBN, Perkadox® AIBN, Perkadox® L W-75, Perkadox® 12, and Laurox. The second organic peroxide differs from the first organic peroxide, but preferably has a half-life of 60 minutes or less at the polymerization temperature. The second initiator may be selected from the same group of compounds as the first initiators or may be any other suitable inititator selected from peroxydicarbonates, peroxycarbonates, peroxyesters, peroxyketals, diacylperoxides, dialkylperoxides, azo-initiators, and ketone peroxides, such as (but not limited to) tertbutylperoxybenzoate, tert-butyl peroxy-2-ethylhexyl carbonate, dicumylperoxide, or tert-amyl peroxy-2-ethylhexyl carbonate.

[0022] The first initiator is preferably added after the composition comprising styrene monomer, waste polystyrene and polymerization inhibitor is heated at a temperature of above 80°C, and the second initiator may be added at the start of or during the polymerization process, preferably during the polymerization process.

[0023] Thus, in a further preferred embodiment, the process for recycling waste polystyrene into expandable polystyrene beads comprises: a) preparing, in a reaction vessel with stirring, a polymerizable composition comprising water, styrene monomer, and 50 to 515 ppm (relative to the initial amount of styrene monomer) of at least one polymerization inhibitor; b) heating the mixture of step a) to a temperature of from about 90-120°C, preferably about 95-115°C, most preferably about 105-110 °C, and stirring the mixture at said temperature for at least 20 minutes, preferably for about 30 minutes to about 120 minutes, preferably for about 40 minutes to about 90 minutes, more preferably from about 50 minutes to about 70 minutes; c) adding a first radical-forming initiator after completion of step b) and stirring the reaction mixture at a temperature of from about 90-120°C, preferably about 95-115°C, most preferably about 105-110 °C, for up to about 8 hours, preferably for about 3 hours to about 6 hours, more preferably for about 4 to about 5 hours; d) adding a second radical-forming initiator after completion of step c), then stirring the reaction mixture at a temperature of from about 115-135°C, more preferably about 120- 130°C, for up to about 4 hours, preferably for about 2 to about 3 hours; and e) cooling the reaction mixture and recovering the expandable polystyrene beads, wherein waste polystyrene is added to the polymerizable composition in step a) and / or step b).

[0024] As noted above, the mixture of step a) may further comprise one or more of the usual additives such as a surfactant, a chain transfer agent, a protective colloid, an anti-fouling agent, a pH-buffer, flame retardants, flame retardant synergists, athermanous agents like graphite, coke or carbon black, etc. “Grey expanded polystyrene” (grey EPS) can be produced by including carbon in the mixture, preferably in form of graphite, coke or carbon black. Blowing agents can be added at the start of or during the polymerization process. The presence of styrene monomer and blowing agents is not problematic since the process is carried out in a closed (pressurized) reactor. The combined weight of the additives preferably is at most 20 wt.%, based on the combined weight of all monomers.

[0025] In step b), the mixture prepared in step a) is preferably heated at an average ramp rate of about 1 to about 2 °C / minute. Preferably, the mixture of step a) is heated to a temperature of from about 90-120°C over a period of from about 45 minutes to about 120 minutes. Once the desired temperature is reached, the heated mixture is stirred at said temperature for at least 20 minutes, preferably for about 30 minutes to about 120 minutes, preferably for about 40 minutes to about 90 minutes, more preferably from about 50 minutes to about 70 minutes. During this step, the polystyrene dissolves in the styrene monomer. The waste polystyrene is added to the polymerizable composition in step a), or in step b), or in both steps a) and b).

[0026] The first radical-forming initiator added in step c) preferably has a has a half-life of 60 minutes or less at a temperature of about 90-120°C, preferably about 95-115°C, and more preferably has a half-life of about 0.5 to about 50 minutes at a temperature of 90-120°C, preferably about 95-115°C, more preferably about 105-110 °C, such as at about 110°C. Preferred initiators in that respect include, but are not limited to, Bis(2-chlorobenzoyl) peroxide, Di(2,4-dichlorobenzoyl) peroxide, Dibenzoyl peroxide, Bis(4-chlorobenzoyl) peroxide, Di(2-methylbenzoyl) peroxide, Bis(3-methylbenzoyl) peroxide, Di(4-methylbenzoyl) peroxide, Bis(4-tert-butyl benzoyl) peroxide, Bis(2-acetoxybenzoyl) peroxide, Bis(3- isopropenyl benzoyl) peroxide, Diphthaloyl peroxide, Bis(4-phenyl azobenzoyl) peroxide, Bis(1 -naphthoyl) peroxide, Bis(2-isobutoxycarbonyl benzoyl) peroxide, 1 ,2,7,10-Tetraoxa- 3,6,11 ,14-tetraoxo-4,5,12,13-dibenzocyclotetradecane, Bis((2- cyclohexyloxycarbonyl)benzoyl ) peroxide, or Bis(2-naphthoyl) peroxide. Most preferred is dibenzoylperoxide.

[0027] Preferably, the second radical-forming initiator added in step d) has a has a half-life of 60 minutes or less at a temperature of about 115-135°C, preferably about 120-130°C, and more preferably the second radical-forming initiator has a half-life of about 0.5 to about 50 minutes at a temperature of about 115-135°C, preferably about 120-130°C. Preferred initiators in that respect include, but are not limited to, tert-butylperoxybenzoate, tert-butyl peroxy-2- ethylhexyl carbonate, dicumylperoxide, or tert-amyl peroxy-2-ethylhexyl carbonate.

[0028] After the polymerization (i.e., step e), the resulting polymer is preferably treated as is usual in the art. Polymers so obtained may be submitted to drying and / or screening steps. These one or more steps can be followed by, for example, an expansion step. The resulting resin is characterized in that it preferably contains less than 50 ppm of residual initiator, more preferably less than 40 ppm, and most preferably less than 25 ppm of initiator, immediately after drying for 1 hour at 60°C and screening. The resulting resin is further characterized in that it preferably has a weight average molecular weight (Mw) of at least 100,000 g / mol, more preferably of at least 125,000 g / mol, and most preferably of at least 150,000 g / mol, and that it has a Mw preferably of at most 500,000 g / mol, more preferably of at most 450,000 g / mol, and most preferably of at most 350,000 g / mol, measured in accordance with conventional gel permeation chromatography techniques using polystyrene standards. An exemplary non-limiting process in accordance with the above steps a)-e) is illustrated in Figure 1 . In that exemplary process, the mixture prepared in step a) [including the waste polystyrene] was heated to about 105°C over the space of an hour and then held at that temperature for about another hour. Thereafter, dibenzoyl peroxide was added to the mixture, at which point the suspension radical polymerization reaction was initiated. The reaction mixture was held at that temperature for about 4.5 hours, after which time tert-Butyl peroxy-2-ethylhexyl carbonate and blowing agent (n-pentane and iso-pentane) was added. The temperature was subsequently increased to about 125°C over about 15 minutes and held at that temperature for about 2.5 hours. The reaction mixture was then cooled, and the expandable polystyrene beads produced by the process were recovered (after separation and drying).

[0029] As noted above, in addition to reducing the number of required reactors from two to one and improving the product properties (solved by the above process), a further challenge associated with the suspension polymerization of styrene monomer in the presence of waste polystyrene is suspension stability. Whilst US 5,290,819 provides a solution to this suspension stability problem in the form of magnesium pyrophosphate as an additive, this was found to substantially complicate the overall procedure. Thus, whilst US 5,290,819 provides a workable technical solution to this issue, it is not a particularly satisfactory solution.

[0030] The present inventors, however, have now solved this tangential issue in a more than satisfactory manner. In that respect, the present inventors have identified a specific, but readily available additive that stabilizes the suspension and which does not need to be prepared in-situ.

[0031] As such, in a third aspect, the present disclosure relates to the use of a particulate alkaline metal phosphate with a Dv50 of less than 1 micron for stabilizing a suspension polymerization comprising styrene monomer and waste polystyrene. Dv50 is determined by laser granulometry using a laser diffractometer (Horiba®) and is defined as the maximum particle diameter below which 50% of the sample volume exists. Dv50 is a common way to define particulate materials, and the skilled person would be fully aware of how to measure that parameter.

[0032] A surprising finding was that a particulate alkaline metal phosphate with a median particle size (Dv50) of less than 1 micron stabilized the polymerization suspension, even those suspensions comprising large amounts of waste polystyrene (i.e., up to 60 wt.% polystyrene, which was particularly surprising). This effect could be replicated for alkaline metal phosphates with a Dv50 of greater than 1 micron, however larger quantities of the larger particle size alkaline metal phosphates was required to achieve the same effect (it was found that about twice the amount (wt.%) was required).

[0033] Accordingly, in a preferred embodiment, the particulate alkaline metal phosphate with a Dv50 value of less than 1 micron (pm) may be used for stabilizing a suspension polymerization comprising styrene monomer and waste polystyrene, wherein the weight ratio of styrene monomer to waste polystyrene may be from 90:10 to 40:60.

[0034] Preferred particulate alkaline metal phosphates are tricalcium phosphate, pentacalcium trisphosphate, and mixtures thereof. The person skilled in the art would be capable of determining a suitable amount of particulate alkaline metal phosphate for stabilizing the suspension, and is typically from about 0.2 wt% to 1 .5 wt% of the water.

[0035] Thus, the present inventors have identified a way to recycle polystyrene, in a process with a cycle-time substantially the same as a process without recycling, using a single reactor (polymerization inhibitor), and stabilizing the suspension in a more than satisfactory manner (alkaline metal phosphate).

[0036] Accordingly, in a fourth aspect, the present disclosure relates to a kit-of-parts for recycling waste polystyrene into expandable polystyrene beads comprising:

[0037] • a composition comprising styrene monomer and 50 to 515 ppm of at least one polymerization inhibitor, wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the amount of styrene monomer, and

[0038] • a particulate alkaline metal phosphate, preferably having a Dv50 value (determined by laser granulometry) of less than 1 micron.

[0039] Preferred types and amounts of polymerization inhibitors and alkaline metal phosphates are disclosed above.

[0040] The kit-of-parts may further comprise one or more initiators as described above.

[0041] Finally, in a fifth aspect, the present disclosure relates to expanded polystyrene products obtained at least in part from recycled polystyrene. Such expanded polystyrene products include grey expanded polystyrene (grey EPS), which can be produced by including carbon in the polymerization mixture. The carbon is preferably in the form of graphite, coke, or carbon black.

[0042] Figures

[0043] Figure 1 illustrates an exemplary process in accordance with the present disclosure.

[0044] Figure 2 is a microscope image showing the cell structure of the EPS obtained in Example 1 .

[0045] Figure 3 is a microscope image showing the cell structure of the EPS obtained in Example 2.

[0046] Figure 4 is a microscope image showing the cell structure of the EPS obtained in Example 3.

[0047] Figure 5 is a microscope image showing the cell structure of the EPS obtained in Example 4.

[0048] Figure 6 is a microscope image showing the cell structure of the EPS obtained in Example 5.

[0049] Figure 7 is a microscope image showing the cell structure of the EPS obtained in Example 6.

[0050] Figure 8 is a microscope image showing the cell structure of the EPS obtained in Example 7.

[0051] Figure 9 is a microscope image showing the cell structure of the EPS obtained in Example 8.

[0052] Examples

[0053] Example 1 (Comparative: 15 ppm polymerization inhibitor)

[0054] To a stirred 3-litre Buchi Pressure Reactor at 500rpm was added: 291 g of recycled postconsumer polystyrene from Alba, 1200g deionized water with a conductivity less than 3 pS / cm, 0.64g Polywax 1000 nucleating agent, 0.07g sodium dodecyl benzene sulfonate, 15g tricalcium phosphate (TCP-Instant from Budenheim; Dv50 <1 pm), 6.52g flame retardant (Emerald 3000), 3.26g dicumyl peroxide (Aldrich), 640g styrene monomer (containing 15ppm 4-tert-butylcatechol), 53g n-pentane, and 18g iso-pentane. The mixture was heated to 113°C over ca. 1 hour and held at 113°C for 2.5 hours to allow the recycled PS to dissolve in the styrene. During the 2.5-hour dissolution-phase a total of 46g of TCP-Instant and 0.33g of sodium dodecyl benzene sulfonate was added respectively to hold the droplets in suspension. Polymerization was initiated by the addition of a slurry of 4.88g benzoyl peroxide (Perkadox L-W40 from Nouryon) using nitrogen pressure through a burette. The suspension was then held at 113°C for another 4 hours. During the 4 hour polymerizationphase a total of 10g of TCP-Instant and 0.06g of sodium dodecyl benzene sulfonate was added respectively to hold the droplets in suspension. After 4 hours a solution of 0.86g tertbutylperoxy 2-ethylhexyl carbonate (Trigonox 117 from Nouryon). The suspension was then heated to 125°C over ca. 15 minutes and held at this temperature for 2.5 hours. The suspension was thereafter cooled to 20°C over ca. 35 minutes. The reactor was emptied, and the EPS beads and the water was separated. The EPS-beads were screened in the appropriate size-fraction of 1 .12-1 ,4mm and dried until a satisfactorily low total volatile. 5 grams of the particular size-fraction was then expanded over steam in a closed pot for 2, 4, 5, 6 and 8 minutes. The expanded beads were dried on-bench and density was measured.

[0055] Residual monomer of the EPS-beads was determined with a gas chromatograph to be 1700 ppm, and molecular weight Mw was determined with gel permeation chromatography to be 218 000 g / mol.

[0056] The cellular structure of the beads expanded for 2 minutes to a density of 17.9 g / L was evaluated in a microscope (Figure 2; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0057] Example 2 (Comparative: 35ppm polymerization inhibitor)

[0058] The same procedure as in Example 1 except 20ppm (relative to the initial amount of styrene monomer) of the polymerization inhibitor 4-tert-butylcatechol was added at ambient temperature at the beginning of the process. The total amount of 4-tert-butylcatechol in the mixture was 35 ppm (relative to the initial amount of styrene monomer).

[0059] Residual monomer of the EPS-beads was determined with a gas chromatograph to be 3600 ppm, and molecular weight Mw was determined with gel permeation chromatography to be 220 000 g / mol.

[0060] The cellular structure of the beads expanded for 2 minutes to a density of 17.5 g / L was evaluated in a microscope (Figure 3; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0061] Example 3 (Inventive: 55ppm polymerization inhibitor)

[0062] The same procedure as in Example 1 except 40ppm (relative to the initial amount of styrene monomer) of the polymerization inhibitor 4-tert-butylcatechol was added at ambient temperature at the beginning of the process. The total amount of 4-tert-butylcatechol in the mixture was 55 ppm (relative to the initial amount of styrene monomer). Residual monomer of the EPS-beads was determined with a gas chromatograph to be 2300 ppm, and molecular weight Mw was determined with gel permeation chromatography to be 211 000 g / mol.

[0063] The cellular structure of the beads expanded for 2 minutes to a density of 17.2 g / L was evaluated in a microscope (Figure 4; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0064] Example 4 (Comparative: 15 ppm polymerization inhibitor)

[0065] To a stirred 3-litre Buchi Pressure Reactor at 500rpm was added: 465.5g of recycled postconsumer PS from Alba, 1200g deionized water with a conductivity less than 3 pS / cm, 0.47g Polywax 1000 nucleating agent, 0.03g sodium dodecyl benzene sulfonate, 13g tricalcium phosphate (TCP-Instant from Budenheim; Dv50 <1 pm), 6.52g flame retardant (Emerald 3000), 3.26g dicumyl peroxide (Aldrich), 465.5g styrene monomer (containing 15ppm 4-tert- butylcatechol), 53g n-pentane, and 18g iso-pentane. The mixture was heated to 113°C over ca. 1 hour and held at 113°C for 2.5 hours to allow the recycled PS to dissolve in the styrene. During the 2.5-hour dissolution-phase a total of 23g of TCP-Instant and 0.1 g of sodium dodecyl benzene sulfonate was added respectively to hold the droplets in suspension. Polymerization was initiated by the addition of a slurry of 3.55g benzoyl peroxide (Perkadox L-W40 from Nouryon) using nitrogen pressure through a burette. The suspension was then held at 113°C for another 3.5 hours. After 3.5 hours a solution of 0.86g tert-butylperoxy 2-ethylhexyl carbonate (Trigonox 117 from Nouryon) and 0.02g sodium dodecyl benzene sulfonate was added. The suspension was then heated to 125°C over ca. 15 minutes and held at this temperature for 2.5 hours. The suspension was thereafter cooled to 20°C over ca. 35 minutes.

[0066] The reactor was emptied and the EPS beads and the water was separated. The EPS-beads were screened in the appropriate size-fraction of 1 .12-1 ,4mm and dried until a satisfactorily low total volatile. 5 grams of the particular size-fraction was then expanded over steam in a closed pot for 2, 4, 5, 6 and 8 minutes. The expanded beads were dried on-bench and density was measured.

[0067] Residual monomer of the EPS-beads was determined with a gas chromatograph to be 1600 ppm, and molecular weight Mw was determined with gel permeation chromatography to be 246 800 g / mol (average between two measurements). The cellular structure of the beads expanded for 6 minutes to a density of 16.2 g / L was evaluated in a microscope (Figure 5; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0068] Example 5 (Inventive: 103ppm polymerization inhibitor)

[0069] The same procedure as in Example 4 except 88ppm (relative to the initial amount of styrene monomer) of the polymerization inhibitor 4-tert-butylcatechol was added at ambient temperature at the beginning of the process. The total amount of 4-tert-butylcatechol in the mixture was 103 ppm (relative to the initial amount of styrene monomer). Additionally, during the 2.5-hour dissolution-phase a total of 43g of TCP-Instant and 0.18g of sodium dodecyl benzene sulfonate was added respectively to hold the droplets in suspension.

[0070] Residual monomer of the EPS-beads was determined with a gas chromatograph to be 900 ppm, and molecular weight Mw was determined with gel permeation chromatography to be 220 000 g / mol.

[0071] The cellular structure of the beads expanded for 2 minutes to a density of 16.1 g / L was evaluated in a microscope (Figure 6; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0072] Results: Comparison of Examples 1 , 2 and 3. These examples used “contaminated” postconsumer PS from ALBA. The cellular structure is negatively affected by this. In Example 1 , when no extra 4-tert-butylcatechol (TBC) was added, the cell-walls consisted of many small cells and the cell-walls had a sharp and thread-like appearance (Fig. 2). In Example 2, when 20 ppm TBC is added to the formulation (35ppm total), the size of the smallest cells increased and the mentioned thread-like / edgy appearance disappeared, however the cell size distribution was still not acceptably uniform (Fig. 3). In Example 3, when 40 ppm TBC is added to the formulation (55ppm total), the size of the smallest cells increased, the mentioned thread-like / edgy appearance disappeared, and the cell size distribution was highly uniform (Fig. 4).

[0073] Results: Comparison of Examples 4 and 5. This series of experiments tested the worstcase situation in terms of creating a very fine cell-structure. The combination of 50%- recycling of contaminated Alba-PS is expected to create a very fine cell-size, and this was the case. In Example 4 there were many small cells and a few very big ones. In Example 5, where the total level of TBC was increased to 103 ppm-styrene, the small cells had largely disappeared, and the cell-structure was greatly improved (Fig 6 vs Fig 5). To quantify the improvement, the cell size distribution was determined visually using the following method: a line of length 1 mm was drawn on the microscope image. The % contribution relative to this line of fine, medium, and coarse cells was evaluated. The much-improved uniformity (narrower distribution) of the cell size and substantial reduction in the number of coarse / fine cells of the inventive example (Fig. 6) vs the comparative example (Fig. 5) is readily apparent from the following table:

[0074] Example 6 (Comparative; 15 ppm polymerization inhibitor)

[0075] To a stirred 5-litre Buchi Pressure Reactor at 575 rpm was added: 2225.5g deionized water with a conductivity less than 3 pS / cm, 0.0062g Sodium Persulfate, 1.60g tricalcium phosphate (TCP-Instant from Budenheim; Dv50 <1 pm), 1.18g Polywax 1000 nucleating agent, 0.59g Ethylene bis Stearamide, 0.32g di-Sodium Phosphate from Budenheim, 1183.7g styrene monomer (containing 15ppm 4-tert-butylcatechol). The suspension was heated to 70°C over 45 minutes. After reaching 70 °C, 507.4g of recycled post-consumer polystyrene from Alba was added over 35 minutes, and the temperature was held constant at 70 °C for another 25 minutes. The reactor was then heated from 70 °C to 110 °C over 40 minutes, and the temperature was held at 110 °C for 170 minutes. 14.0g tricalcium phosphate (TCP-Instant from Budenheim; Dv50 <1 pm) was added from a burette when reaching 110 °C. A suspension of benzoyl peroxide in water (8.88g Perkadox L-W40SN from Nouryon and 91.1 g deionized water) was continually dosed at a rate of 1 .11 g / min over 90 minutes starting when the reactor temperature crosses 80 °C. After completing the benzoyl peroxide dosing, 17.8g tricalcium phosphate (TCP-Instant from Budenheim; Dv50 <1 pm) and 2.20g of tert-butylperoxy 2-ethylhexyl carbonate (Trigonox 117 from Nouryon) was added through a burette. A mixture of 101.3g n-pentane and 33.8g iso-pentane was dosed to the reactor over 45 minutes. The reactor was kept at 110 °C for another 60 minutes. The reactor was then heated to 125 °C over 60 minutes, and kept at this temperature for 260 minutes, before cooled to 25 °C over 120 minutes.

[0076] The reactor was emptied, and the EPS beads and the water was separated. The EPS-beads were then surface dried with hot air, and the size-fraction of 1 .0-1 ,4mm was separated out. After 14 days in a closed container, 5 grams was expanded over steam in a closed pot for 4 minutes. The expanded beads were dried on-bench and density was measured.

[0077] The cellular structure of the beads expanded for 4 minutes to a density of 15.6 g / L was evaluated in a microscope (Figure 7; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0078] Example 7 (Inventive; 65 ppm polymerization inhibitor)

[0079] The procedure was the same as that in Example 6 except 50ppm (relative to the initial amount of styrene monomer) of the polymerization inhibitor 4-tert-butylcatechol was added at ambient temperature at the beginning of the process. The total amount of 4-tert- butylcatechol in the mixture was 65 ppm (relative to the initial amount of styrene monomer).

[0080] The cellular structure of the beads expanded for 4 minutes to a density of 13.9 g / L was evaluated in a microscope (Figure 8; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.).

[0081] Example 8 (Inventive; 265 ppm polymerization inhibitor)

[0082] The same procedure as in Example 6 except 250 ppm (relative to the initial amount of styrene monomer) of the polymerization inhibitor 4-tert-butylcatechol was added at ambient temperature at the beginning of the process. The total amount of 4-tert-butylcatechol in the mixture was 265 ppm (relative to the initial amount of styrene monomer).

[0083] The cellular structure of the beads expanded for 4 minutes to a density of 14.5 g / L was evaluated in a microscope (Figure 9; a microscopic image of a thin slice taken from the center of a representative expanded bead was captured.). Results: Comparison of Examples 6-8

[0084] In Example 6, when no extra 4-tert-butylcatechol (TBC) was added, the cell-walls consisted of many small cells and the cell-walls had a sharp and thread-like appearance (Fig. 7). In Example 7, when 50 ppm TBC is added to the formulation (65ppm total), the size of the smallest cells increased, the mentioned thread-like / edgy appearance disappeared, and the cell size distribution was highly uniform (Fig. 8). In Example 8, when 250 ppm TBC is added to the formulation (265ppm total), the advantageous effects observed in Example 7 were enhanced (Fig. 4).

[0085] In all of the above Examples 1 -8, only the Examples that contained at least 50 ppm polymerization inhibitor generated expanded polystyrene products with much improved cell structures.

[0086] In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than to mean ‘consisting of’. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Europe or elsewhere at the date hereof.

Claims

CLAIMS1 . Use of 50 to 515 ppm of at least one polymerization inhibitor as an additive in a suspension radical polymerization comprising styrene monomer and polystyrene for improving the cell structure of expanded polystyrene produced from expandable polystyrene beads obtained from said suspension radical polymerization, wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the initial amount of styrene monomer in the suspension polymerization.

2. The use of claim 1 , wherein the polymerization inhibitor is a phenolic polymerization inhibitor, preferably 4-tert-butylcatechol.

3. A process for recycling polystyrene into expandable polystyrene beads comprising performing a suspension radical polymerization of a composition comprising styrene monomer, polystyrene, and 50 to 515 ppm of at least one polymerization inhibitor, wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the initial amount of styrene monomer in the suspension polymerization.

4. The process of claim 3, wherein the polymerization inhibitor is a phenolic polymerization inhibitor, preferably 4-tert-butylcatechol.

5. The process of claims 3 or 4, wherein the suspension radical polymerization is initiated using an organic peroxide.

6. The process of any one of claims 3-5, wherein the composition comprising styrene monomer, polystyrene and polymerization inhibitor is heated at a temperature of above 80°C for at least 30 minutes to dissolve the polystyrene in the styrene monomer prior to initiating the suspension radical polymerization.

7. The process of any one of claims 3 to 6, wherein a particulate alkaline metal phosphate is added to the composition, and wherein the particulate alkaline metal phosphate optionally has a Dv50 value (determined by laser granulometry) of less than 1 micron.

8. The process of claim 7, wherein the alkaline metal phosphate is tricalcium phosphate, pentacalcium trisphosphate, or mixtures thereof.

9. The process of any one of claims 3 to 8, wherein the weight ratio of styrene monomer to polystyrene in the composition is from 90:10 to 40:60.

10. The process of any one of claims 3 to 9, wherein the suspension radical polymerization is performed at a temperature of 90-120°C, preferably about 95-115°C, more preferably about 105-110 °C, most preferably about 110°C.11 . The use of claims 1 or 2 or the process of any one of claims 3 to 10, wherein the total amount of polymerization inhibitor relative to the initial amount of styrene monomer in the suspension polymerization is from 100 ppm to 515 ppm.

12. Expandable polystyrene beads obtainable from the process of any one of claims 3 to 11.

13. Expanded polystyrene and / or grey expanded polystyrene obtained by thermally expanding the expandable polystyrene beads of claim 12.

14. A kit-of-parts for recycling waste polystyrene into expandable polystyrene beads comprising:• a composition comprising styrene monomer and 50 to 515 ppm of at least one polymerization inhibitor, wherein said 50 to 515 ppm is the total amount of polymerization inhibitor relative to the amount of styrene monomer; and• a particulate alkaline metal phosphate, optionally having a Dv50 value (determined by laser granulometry) of less than 1 micron.

15. The kit of claim 14, wherein the polymerization inhibitor is 4-tert-butylcatechol and wherein the alkaline metal phosphate is tricalcium phosphate or pentacalcium trisphosphate.

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