COATING COMPOSITION FOR EXPANDABLE BEADS

Abstract: The invention relates to the use of a low molecular weight polymer of a vinylaromatic compound having a weight average molecular weight (Mw) in the range of 200 to 15,000 for the coating of expandable beads, in particular of expandable beads of a polymer of a vinyl aromatic compound which beads contain a blowing agent. Moreover, the invention relates to a coating composition for such beads, the coated expandable beads and the pre-expanded beads prepared by pre-expansion of the coated expandable beads, and to articles prepared by expanding and fusion of these beads through heat-treatment above the boiling point of the blowing agent and the softening point of the polymer.

Description: COATING COMPOSITION FOR EXPANDABLE BEADS The invention relates to the use of a low molecular weight polymer of a vinylaromatic compound for coating expandable beads, in particular of expandable beads of a polymer of a vinyl aromatic com¬ pound which beads contain a blowing agent. Moreover, the invention relates to a coating composition for such beads, the coated expand¬ able beads and the pre-expanded beads prepared by pre-expansion of the coated expandable beads, and to articles prepared by expanding and fusion of these beads through heat-treatment above the boiling point of the blowing agent and the softening point of the polymer. In particular, the invention finds its use in respect of beads of expandable polystyrene. Expandable beads (hereinafter "EPS" beads, referring to expand¬ able beads of polymers of vinylaromatic compounds in general, and polystyrene in particular) , such as for instance the range of ex- pandable polystyrene grades sold by Shell under the trademark STYRO- CELL, find many different uses. Well-known applications are in low or medium density block or continuous board mouldings, large boxes and medium density insulating board, block for veneer cutting, higher strength contour moulding, packages with wall thickness above 10 mm, fish boxes, floor units, etc. Besides the regular grades, also flame retardant grades and special grades are available. Preferably, the EPS beads are expanded in relatively short pro¬ duction cycles to many times their volume (depending on the end-use) using a minimum of blowing agent. Naturally, the articles prepared from EPS beads should by dimensionally stable, i.e., no warping and/or shrinkage should occur. The manufacture of cellular moulded blocks or articles from ex¬ pandable beads is well-known and involves essentially two steps: (a) pre-expansion of the expandable beads followed by a maturing stage, and (b) heating the pre-expanded beads in a mould to form a single foam article having a cellular structure. For ease of handling and in order to obtain satisfactory mouldings it is desirable that the beads are free-flowing and that their tendency to agglomerate be¬ fore, during and after pre-expansion be avoided as far as possible. The present-day generation of beads is commonly coated with coating compositions comprising for instance glycerol mono-, di-, or tristearates, and/or metal stearates (typically zinc stearate) and other additives to overcome such problems. Other components typi¬ cally found in a coating composition comprise anti-coagulation agents, anti-static agents, stabilisers, colouring agents, etc. Suitable compositions are for instance disclosed in GB-A-1,409,285. However, the foamed article must meet a further condition, i.e., that of sufficient cross-breaking strength. There is therefore use for EPS beads that provide articles with either improved maximum cross-breaking strength or with typical cross-breaking strength but obtainable at lower foam/steam pressures. The inventors surprisingly found that these objectives could be achieved by using a low molecular weight polymer of a vinylaromatic compound having a weight average molecular weight (M^,) in the range of 200 to 15.000 in the coating of expandable beads. In particular, these objectives could be achieved by providing typical expandable beads with a new coating composition. Accordingly, the invention also provides a coating composition for expandable beads comprising ordinary coating agents and a low molecular weight polymer of a vinylaromatic compound having a weight average molecular weight (M w ) in the range of 200 to 15,000. Preferably, the polymer has an 1 in the range of 300 to 10,00, more preferably in the range of 400 to 1,500. Although the exact amount of low molecular weight polymer may vary within very broad boundaries, the novel coating composition preferably comprises the polymer in an amount of 5 to 50, more pref¬ erably in an amount of 10 to 25 %wt based on the total composition. The low molecular weight polymer suitably is a polymer of alpha-methylstyrene and/or styrene (optionally with further comono- mers), more suitably a homopolymer of styrene. The coating composition may be applied on the expandable bead by any convenient method conventionally used, thereby providing coated expandable beads that are novel too. Thus, it may be applied as a dry-coat (tumbling in a ribbon blender) , as a slurry (in water) or a solution in a readily vaporising non-aqueous liquid. This liq- uid should have no solvent action on the expandable beads. Suitably, such liquid is a lower alcohol such as methanol, ethanol, isopropyl alcohol or an aliphatic hydrocarbon such as hexane, heptane or pe¬ troleum ether. Dry-coating, being the easiest and rendering the best results is preferred. In the event of dry-coating, the low molecular weight polymer, if solid, preferably has a particle size of less than 65 microns. If a polymer of particle size larger than 65 microns is used, the coating composition may suffer from loss of integration if not handled carefully. Suitably, the expandable beads are coated with coating composi- tion in an amount of 0.1 to 2.0 %wt, preferably in an amount of 0.2 to 0.5 %wt based on the expandable beads. The expandable beads are preferably made of a polymer of styrene, either alone (homopolymer of styrene) or in combination with a comonomer. The expandable beads are made of a polymer having an M^, of at least 100,000, typically at least 160,000.. The expand¬ able beads most commonly used and suitably coated in accordance with the present invention are made of polystyrene. The blowing agent, added before, during or after the polymerisation of the vinyl¬ aromatic compound, typically is a low boiling hydrocarbon such as (cyclo)pentane, etc. However, the art on expandable beads is quite extensive both in respect of the polymer, the blowing agent and the further additives and will be known to the person skilled in the art. The (uncoated) expandable beads therefore do not form part of the present invention. The coated expandable beads readily undergo pre-expansion and moulding procedures without clogging or otherwise interfering with the handling equipment which transfers pre-expanded beads to the mould. Flow of the raw bead, i.e., before pre-expansion, is also excellent and causes no difficulty. The invention also relates to the novel coated beads, and the foamed articles with improved cross-breaking strength prepared from the coated beads. The invention will now be further described with reference to the following examples, however, without restricting its scope. In the examples PICCOLASTIC A5 (trademark of Hercules), a vis¬ cous liquid mixture of oligomers (trimers and tetramers) of styrene (having an M^ of about 430), and a powdery low molecular weight PS having an M„ of about 10,000 that passes through a 63 micron sieve have been used. Further, GMS (glycerol mono-stearate) , GTS (glycerol tristearate) and Znst (zinc stearate) are conventional coating agents whose function it is to prevent agglomeration and prevent static of the expandable beads. Examples 1-2, Comparative examples A-B 1 kg batches of uncoated expandable polystyrene beads (having an M^, of about 180,000 and a diameter in the range of 0.7 to 1.0 mm) and containing about 6 %wt of a mixture of n-pentane and iso-pentane were tumbled in a ribbon blender (a Hermann Linden machine for 1 kg batches, run for 10 minutes) with a coating composition as set out in Table 1 until an uptake of 4 g (0.4 wt) was achieved. The coated beads were pre-expanded in a batch pre-expander (of the firm Handle) using a steam pressure of 0.20 bar gauge and a steaming time of 26 seconds. Densities were all about 23 g/1, indi¬ cating that the new coating composition has only a limited effect on the pre-expansion process. The pre-expanded beads were allowed to mature overnight in an air-permeable silo and were machine moulded into tiles of 300 mm by 300 mm by 50 mm in a steam-heated enclosed mould operating at 0.5 to 1,2 bar gauge. The maximum foam pressure and the pressure decay time for each tile were noted. The tiles were then dried for two days at 70 °C and then conditioned and tested for cross-breaking load measurement. The maximum cross-breaking strength (at break) was recorded for each tile. From Table 1 it follows that the moulding behaviour is essen¬ tially unaffected by the replacement of the common coating composition by the new coating composition. The results of the cross-breaking strength against the foam pressure are significantly higher when low MW PS is used (examples 1 and 2) compared to pure GMS based coating (comparative example A) . The maximum achievable strengths for examples 1 and 2 are about 45 to 50 N higher compared to comparative example A. The total cycle times for examples 1 and 2 are similar to that of comparative example A. When compared to com¬ parative example B, the improvement in maximum achievable strength for examples 1 and 2 is about 75 to 80 N, although this is achieved at a longer cycle time. Table 1, coating formulation (for 1 kg of uncoated beads) Ex. A Ex. 1 Ex. B Ex. 2 Ingredients GMS (g) 0.40 0.20 0.20 0.10 GTS (g) - - 0.20 0.10 Low molecular weight PS (g) - ■ 0.20 - 0.20 Total (g) 0.40 0.40 0.40 0.40 Properties Prefoam density (g/1) 22.7 23.2 23.8 23.8 Prefoam gas content (%wt) 3.84 3.73 3.78 3.86 Cross-breaking strength at 0.6 bar 1098 1259 1090 1277 g steam pressure) 1 (N) Max. cross-breaking strength 1 (N) 1468 1510 1434 1516 Maximum cycle time (s) 356 397 260 377 corrected to a density of 23 g/1 Examples 3-5 and Comparative example C In a process similar to that described above, coated beads of high M„ polystyrene (about 260,000) containing 6 wt of blowing agent were produced. A 25 second steaming time was used to reach a density of about 20 g/1. The coating compositions and the results are set out in Table 2. From Table 2 it follows that the moulding behaviour is essen¬ tially unaffected by the replacement of the common coating composi¬ tion by the new coating composition. The result of the cross- breaking strength against the foam pressure is significantly higher when PICCOLASTIC A5 is used (examples 3-5) compared to pure GMS based coating (comparative example C) . The maximum achievable strength for examples 3-4 is about 50 to 60 N higher compared to comparative example C. The total cycle times for examples 3-4 are similar to that of comparative example C. Example 5 illustrates that improvement for coating compositions containing 50 wt or more of the low molecular weight polymer results in an improvement, however, that decreases with increasing amount. Table 2, coating formulation (for 1 kg of uncoated beads) Ex. C Ex. 3 Ex. 4 Ex. 5 Ingredients GMS (g) 0.10 0.10 0.10 0.10 Znst (g) 0.10 0.10 0.10 0.10 PICCOLASTIC A5 (g) - 0.05 0.10 0.20 Total (g) 0.20 0.25 0.30 0.40 Properties Prefoam density (g/1) 20.0 20.2 20.0 20.0 Prefoam gas content (%wt) 3.80 3.82 3.82 3.80 Cross-breaking strength at 0.6 bar 1150 1200 1210 1180 g steam pressure) 1 (N) Max. cross-breaking strength 1 (N) 1210 1245 1245 1230 Maximum cycle time (s) 320 310 310 305 corrected to a density of 20 g/1 Examples 6-7 and Comparative examples D-E In a process similar to that described above, coated beads of polystyrene (M^, about 180,000) containing 6 wt of blowing agent were produced. A 20 seconds steaming time was used to reach a den¬ sity of about 20 g/1. The coating compositions and the results are set out in Table 3. Example 7 and Comparative example E illustrate the use of a coating composition comprising GTS, resulting in a lower cross-breaking strength but a much shorter cycle time, as compared to a coating composition comprising GMS. From Table 3 it follows that the moulding behaviour is essen¬ tially unaffected by the replacement of the common coating composition by the new coating composition. The cross-breaking strength at a fixed foam pressure is slightly higher when PICCOLASTIC A5 is used (examples 6-7) as compared to the use of GMS/Znst, respectively GMS/GTS/Znst based coatings (comparative examples D-E) . This (improved) strength was obtained at significantly shorter cycling times. Table 3, coating formulation (for 1 kg of uncoated beads) Ex. D Ex. 6 Ex. E Ex. 7 Ingredients GMS (g) 0.40 0.40 0.20 0.20 GTS (g) - - 0.20 0.20 Znst (g) 0.08 0.08 0.08 0.08 PICCOLASTIC A5 (g) - 0.10 - 0.10 Total (g) 0.48 0.58 0.48 0.58 Properties Prefoam density (g/1) 20.0 20.0 20.0 20.0 Prefoam gas content (%wt) 3.70 3.69 3.10 3.10 Cross-breaking strength at 0.6 bar 1143 1148 883 902 g steam pressure) 1 (N) Max. cross-breaking strength 1 (N) 1146 1148 1013 975 Maximum cycle time (s) 295 216 113 99 corrected to a density of 20 g/1 On the basis of the experimental data presented above it is concluded that the novel coating composition has a beneficial impact on the maximum cross-breaking load and/or the cycle time.

Claims: C L A I M S 1. The use of a low molecular weight polymer of a vinylaromatic compound having a weight average molecular weight (l ,) in the range of 200 to 15,000 for coating expandable beads. 2. A coating composition for expandable beads comprising one or more ordinary coating agent(s) and a low molecular weight polymer of a vinylaromatic compound having an M„ in the range of 200 to 15,000. 3. A coating composition as claimed in claim 2, wherein the low molecular weight polymer has a ϊ . in the range of 300 to 10,000, preferably in the range of 400 to 1,500. 4. A coating composition as claimed in claim 2 or 3, comprising the low molecular weight polymer in an amount of 5 to 50, preferably in an amount of 10 to 25 %wt based on the total weight of the coating composition. 5. A coating composition as claimed in any one of claims 2 to 4, wherein the low molecular weight polymer is a polymer of styrene and/or alpha-methylstyrene. 6. A coating composition as claimed in claim 5, wherein the low molecular weight polymer is a polymer of styrene. 7. A coating composition as claimed in any one of claims 2 to 6, wherein the ordinary coating agent(s) is/are one or more of glycerol mono-, di-, or tristearate and optionally a metal stearate and wherein the coating composition further comprises coating additives. 8. A process for preparing coated expandable beads of a vinylaro¬ matic polymer, by coating expandable beads with a coating composi¬ tion as claimed in any one of claims 2 to 5. 9. A process as claimed in claim 8, wherein the coating composi¬ tion is applied as a dry-coat. 10. A process as claimed in claim 9, wherein the low molecular weight polymer, if solid, has a particle size of less than 65 microns. 11. Expandable and pre-expanded beads of a vinylaromatic polymer, coated with the coating composition of any one of claims 2 to 7. 12. Expandable and pre-expanded beads as claimed in claim 11, coated with the coating composition of any one of claims 2 to 7, in an amount of 0.1 to 2.0 %wt, preferably in an amount of 0.2 to 0.5 %wt based on the expandable beads. 13. Expandable and pre-expanded beads as claimed in claim 11 or 12, wherein the beads are of polystyrene. 14. Articles of expanded beads of a polymer of vinylaromatic co - pounds, obtainable by expanding the beads claimed in any one of claims 11 to 13.

Inventors: PALEJA RAKESH JAYSINH, VLIERS DOMINIQUE PAUL, WILLEMS MARIA JOHANNA, PALEJA, RAKESH, JAYSINH, VLIERS, DOMINIQUE, PAUL, WILLEMS, MARIA, JOHANNA
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: C08J9
IPC codes: C08J3, C08J9