GB2162789A

GB2162789A - Process for making expandable polystyrene

Info

Publication number: GB2162789A
Application number: GB08516723A
Authority: GB
Inventors: Richard A Schwarz; Robert A Fuller; James M Watson; William J Lynch
Original assignee: Cosden Technology Inc
Current assignee: Cosden Technology Inc
Priority date: 1984-07-16
Filing date: 1985-07-02
Publication date: 1986-02-12
Also published as: FR2567446A1; JPS6157313A; DE3524830A1; IT8521555A0; GB8516723D0; IT1186758B

Abstract

The present invention relates to a process for making polystyrene pellets, and more particularly for making expandable polystyrene beads, by forming a styrene polymer melt and extruding such through a die to form continuous strands thereof. After exiting the die but prior to substantial cooling, the strands are cut into pellets. Such plano-convex pellets are optionally screened to remove undesirable sizes and subsequently introduced into a heated reactor wherein a blowing agent is added. The expandable polystyrene beads produced are substantially spherical and have a desirably narrow range of sizes.

Description

SPECIFICATION Process for making expandable polystyrene The present invention relates to a process for making polystyrene pellets and to a process for making expandable polystyrene.

In making expandable polystyrene pellets, three characteristics of the pellets are desired. The pellets should be substantially spherical in shape. The range distribution in the size of the spherical pellets should be kept to a minimum, i.e., the size should be as close to uniform as possible. Additionally, the pellets should be of a small size, i.e., the smaller the size, the better for thermoforming or molding. Furthermore, the process should allow for recycling of scrap or rejected product.

Expandable polystyrene has been generally made by suspension polymerization by one of two processes. In the so-called two step process, the styrene monomer is placed in a large suspension reactor along with water and a suspending agent.

The monomer is agitated inside the reactor vessel to form droplets which when heated, polymerize into individual polymer beads. This method of polymerization, known as suspension polymerization, is utilized in both the two step and the one step processes. In the two step process after the polymerized beads are formed, they are removed from the reactor, dried and the irregular sized beads are separated out. The normal or desired sized beads are then reintroduced into another reactor filled with water and impregnated therein with a hydrocarbon gas such as pentane. After gas impregnation; the beads are again removed from the reactor, dried and prepared for expansion. Expansion includes first a pre-expansion or a pre-puff formation which involves subjecting the beads to steam in an unconfined space to form a partially expanded bead.These pre-puffs are then placed in the desired mold and subjected to high temperature steam for expansion of the pre-puff into the final desired form within the confines of the mold.

In the so-called one step process, also utilizing suspension polymerization, the polymer beads formed within the suspension reactor are impregnated with the hydrocarbon blowing agent prior to separation of the irregular sized beads. By impregnating the beads together in this manner, the time and cost of first drying the beads and then separating the normal from the irregular sized beads is eliminated; In addition, the one step process reduces the number of reactors required since the beads are formed and pentane impregnated in the same reactor. However, this process is disadvantaged in that the irregular sized and therefore nonuseable beads are impregnated with the flammable blowing- agent.

It would be desirable, therefore, to obtain a process which avoids the necessity of forming the beads in a first suspension reactor, removing them the separate out the irregular size beads, and then returning the required sized beads to a second suspension reactor, but which allows the separation of the irregular sized beads from the normal sized beads prior to their impregnation with the blowing agent.

Furthermore, utilizing a suspension polymerization process, whether one or two step, puts substantial restrictions on the ability to recycle rejected irregular sized beads and scrap material. A suspension type process is highly sensitive with regards to the ingredients therein. Suspension formulations are delicate in that they are engineered to give a specific size range for the beads or pellets. The introduction of scrap or recycle material would upset the delicate balance and would at least give an undesirable broader range in bead size. Thus, it would also be advantageous to obtain a process for making substantially spherical expandable polystyrene beads which allows for the recycle of waste or scrap material.

U.S. Patent No. 3,663,466 (Jablonski) relates to a process wherein foamable styrene polymers are prepared by incorporating within the particles an alkali metal salt. One method of making such particles is to extrude the resinous material containing the salt through a die as a plurality of strands having a diameter of about 0.068 inches (1.7mm).The strands are subsequently cooled and chopped to form a plurality of granules having a length of about 0.1 inches (2.5mm). The cold-cut granules or pellets are transferred to a pressurized reactor, suspended in water in the presence of blowing agent, and heated for a period of time sufficient for the blowing agent to permeate the pellets. Generally, spherical particles are obtained.

U.S. Patent No. 3,449,268 (Scheffler) describes a process wherein isotactic polystyrene in amorphous condition can be obtained by feeding granular crystalline polystyrene to a plastics extruder wherein it is pressed, heated to a temperature above its crystalline melting point and is extruded, preferably as a strand or a plurality of strands and is quenched, suitably by containing, e.g., immersing or spraying the extruded strands with cold water, then is cut or ground to a granular form.

The above processes have inherent disadvantages. The disadvantages of cold cutting the strands include (1) the creation of irregular shaped pellets, and (2) the tendency for small diameter strands to break into pieces from the impact of the cutting blade creating unusable fine particles. Cold cutting the extruded strands to form pellets results in a lot of wasted material if a certain size range is desired. Furthermore, it is almost impossible to obtain beads having a length and/or width not greater then about 2 mm. Shattering of the cold strands presents a major problem and creates a lot of waste and scrap.

It is therefore an object of the present invention to provide a process for producing expandable polystyrene pellets having a substantially uniform size while reducing the amount of resulting waste material. It is another object of the present invention to provide a process for producing expandable polystyrene which has reduced energy and process time requirements.

It is still another object of the present invention to provide a process for producing expandable pol ystyrene which allows for the separation of the required size from the irregular size polymer pellets prior to impregnation of the required size pellets with a blowing agent.

It is another object of the present invention to provide a process for producing expandable polystyrene which reduces the number of suspension reactors required.

It is another object of the present invention to provide a process for producing expandable polystyrene which increases the output by utilizing all suspension reactors for impregnation of the polystyrene with blowing agent.

It is another object of the present invention to provide a process for producing expandable polystyrene which utilizes polystyrene made from a continuous mass process.

It is still another object of the present invention to provide a process for producing expandable polystyrene which utilizes off-grade size polystyrene for the production of the polystyrene pellets.

The present invention accordingly provides a process for making polystyrene pellets comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof; and c) cutting the strands into pellets after exiting the die but before substantially cooling.

The present invention further provides for making expandable polystyrene comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof; c) cutting the strands into pellets after exiting the die but prior to substantial cooling; d) suspending the pellets in a reactor and introducing a blowing agent into the reactor under sufficient conditions of temperature and pressure to impregnate the pellets with the blowing agent; and e) removing the pellets from the reactor in the form of substantially spherical beads.

The present invention still further provides a process for making polystyrene pellets comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof wherein -the strands are extruded into a cooling medium; and c) cutting the strands into pellets after exiting the die but before substantial cooling wherein the pellets are at a sufficiently high temperature to allow toward spherical shape transformation of the pellets.

In the preferred process of the invention, the polystyrene pellets are produced by high speed cutting of the polymer strands such as by rotating cutting blade.

In one embodiment of the present invention, the pellets are then introduced into a heated reactor containing water therein, and a blowing agent is introduced into the reactor The pellets round out in the reactor to form spherical beads. The beads are removed from the reactor and subjected to high temperature to cause the absorbed blowing agent to expand the beads.

In another embodiment of the process of the present invention, the required size pellets are separated from the remaining pellets prior to the introduction of the required size pellets into the heated reactor.

In yet another embodiment of the invention, the styrene polymer is formed by a continuous process of mass polymerization. The polystyrene melt produced by the continuous process is transported through a die to form the continuous strands, which are then pelletized, and introduced into the heated reactor for impregnation of blowing agent.

In still another embodiment, the styrene polymer melt is formed by introducing polystyrene into an extruder and extruding the polystyrene through a die.

Cooling of the polymer strands may occur during or after the strands are cut into pellets. The strands may be cooled during pelletization by a water, air or steam spray; or after pelletization by utilizing the mechanical action of the cutting means to discharge the pellets into water. Partial cooling may occur prior to cutting the strands into pellets.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a block diagram of one embodiment of the process of the present invention; and Figure 2 is a block diagram of another embodiment of the present invention.

The present invention comprises a novel process for making polystyrene pellets and subsequently impregnating these pellets with a blowing agent to form expandable polystyrene pellets which are substantially spherical and have a narrower size range than previously possible.

In accordance with the present invention, a styrene polymer melt is formed. This step could be a continuous mass polymerization process or merely melting #already formed polystyrenes. The polystyrene melt is extruded through a die to form continuous strands thereof. These strands may be optionally extruded into a cooling liquid medium such as water.The strands are cut into pellets before substantially cooling. "Before substantially cooling" means that the formed pellets retain enough heat to allow the pellets to deform from the cylindrical shape, when cut, towards a spherical shape. Thus, the formed pellets are usually in the shape of an egg, pear of sphere rather than cylindrical. Furthermore, cutting the strands while still hot significantly reduces the amount of waste particles formed.

Extruding the strands under water, or discharging the pellets into a liquid cooling medium allows the cooling of the outer surface of the strand or bead such that the formed beads do not stick together upon contact, i.e., the cooling of the outer surface prevents the clumping or agglomeration of the beads.

The formed pellets may be screened to further narrow the size range and the rejected pellets are recycled to the polymer melt. The desired pellets are suspended in a heated reactor and a blowing agent, such as a pentane, is introduced into the re actor under sufficient conditions of temperature and pressure to impregnate the pellets with the blowing agent.This step additionally completes the shape transformation of the pellets to substantially spherical.

The above described process is particularily use ful when small size expandable polystyrene beads are desired, i.e., those having a diameter of less than 0.1 inches.

The process of the present invention is shown in more detail by referring now to Figure 1 where there is shown continuous strands 12 of styrene polymer melt exiting from a die 14. In proximity to die 14 is cutting means 16 which cuts the strands into individual pellets. Cutting means 16 may com prise rotary cutting blades which are positioned immediately adjacent to the discharge end of die 14 to hot cut the strands exiting therefrom. Cutting means 16 is not limited to rotary cutting blades, however; any cutting means which results in a short pellet and which has a sufficient output to be economically feasible will suffice. For a pellet ap proximately .039 inches in diameter, a pellet length of .10 inches is preferred; a pellet length of .037" is most preferred however.It should be appreciated that these preferred pellet sizes are based on pres ent sales criteria and therefore may change as market requirements dictate. Commercially avail able high-speed rotary cutting blades may be uti lized to achieve a short pellet length.Larger sized pellets can be impregnated with blowing agent and will round out in suspension, however, customer requirements usually call for a smaller pellet size.

Cutting means 16 may be positioned immedi ately adjacent to the discharge end of the die 14 for best results so that the strands 12 exiting there from are cut into pellets while in a high tempera ture state thereby presenting less resistance to the cutting blades.

In one embodiment, the polymer strands from die 14 exit into a tank of water (not shown) wherein the cutting means 16 is located under water. It has been found that cutting the strands 12 under water results in a more precisely cut bead due to the cooling effect of the water on the outer surface of the thermoplastic strand. In addition, un derwater cutting results in a more spherically shaped pellet.

In another embodiment, cutting means 16 is po sitioned adjacent to the discharge end of die 14. A tank or a continuous stream of water is located in proximity to cutting means 16 such that when the polymer strands 12 are cut into pellets, the pellets are directed into the water by the mechanical ac tion of the cutting means 16.

Immediately below the cutting means 16 is a screen separator 18 which separates or removes the offsize pellets. From separator 18, the required size pellets are transported into a stirred reactor 20 filled with water and maintained at approximately 220"F. by required size pellets it is meant those pellets which will fit the size and shaped specifica tions for the ultimate end use intended for the pellets, based generally on sales criteria. It should be appreciated that reactor 20 may be a suspension reactor type well known in the art. The blowing agent is introduced into reactor 20 to impregnate the styrene material therein. After sufficient time for impregnation, the impregnated pellets are removed from reactor 20 and transported to centrifuge 22 and then to a dryer 24.From dryer 24, the impregnated pellets are sent to a storage silo 26 filled with an inert gas such as nitrogen, where the pellets are kept until packaged for shipping.

The thermoplastic material utilized in the process of the present invention may comprise any styrenic material which is capable of impregnation by a suitable blowing agent, and which expands by the heating of the blowing agent to produce an expanded polystyrene. Examples of such materials include polymers of styrene, 2-methylstyrene, pmethylstyrene, m-methylstyrene, and alpha-methylstyrene; and co-polymers thereof such as styrene/p-methylstyrene, styrene/alpha-methylstyrene, etc.

The blowing agent utilized in the process of the present invention may comprise butane, pentane, and hexane, including all structural isomers thereof such as n-pentane, isopentane, neopentane, n-butane, isobutane, neobutane, n-hexane, and isohexane. The preferred blowing agent is pentane.

Separator 18 may comprise a multi-layered vibrating screen separator which allows the desired sized pellets to gravity flow to the bottom thereof while retaining the offsize beads on the uppermost screens for subsequent removal therefrom for sale or as regrind.

Impregnation of the required size thermoplastic pellets by the blowing agent will occur as in previous expandable polystyrene processes. Normally a large suspension vessel is filled with water and maintained at a temperature of about 220"F. A blowing agent such as pentane is introduced under pressure into the vessel 20. Generally, the pressure inside reactor 20 will be between 0 to 90 psig. Pentane injection occurs over a period of about 7 hours at 220"F with an additional three hour steeping cycle at 335"F. It should be appreciated that at higher pressure these injection periods may be shortened.

Other additives such as nucleating agents, chemical blowing agents and flame retardants may be introduced into the process prior to the die 14.

Otherwise the additives may be introduced at the suspension reactor 20 if they are heat sensitive or chemically reactive at higher temperature.

The styrene polymer may be formed by any of those styrene polymerization processes known to one of ordinary skill in the art. In fact, the present invention includes forming the continuous strands 12 of polystyrene necessary to practice the present invention by running general purpose polystyrene, prime or off-grade, made from other styrene polymerization processes through an extruder to form a melt state, and then transporting the melt through a strand die. The general purpose polystyrene can be made by any of those processes known in the art such as suspension, continuous mass, etc., described in Bishop, Practical Polymerization For Polystyrene, and incorporated herein by reference.

In a preferred embodiment, however, the polystyrene is made by a continuous process of mass polymerization, hereinafter referred to as continuous mass process. Continuous mass processes include those utilizing a multi-temperature zone single reactor, a single back mixed reactor with constant temperature, a two reactor configuration, or a multiple reactor configuration.

It should be appreciated that the process for making the polystyrene itself need not be directly linked to the pelletization/impregnation processes.

The polystyrene may be made separately and then suspended and expanded at a later date.

In a preferred embodiment, however, the continuous mass process is linked directly to the pelletization and blowing agent impregnation process.

One example of such directly linked process is shown in Figure 2, where a continuous mass unit indicated at 30 comprises a vertical stirred reactor 32, and three horizontal flow reactors 34, 36 and 38 containing heating coils 39 to act as heat exchangers to control the temperature therein.A pump 33 feeds thermoplastic material from storage tank 40 into stirred reactor 32. A polymerization initiator is introduced through line 17 prior to stirred reactor 12. The introduction of an initiator is desirable to obtain better control over the final molecular weight of the polymer formed. The initiator may comprise any of those initiators of the peroxide family including benzoyl peroxide and tert-butylperoctoate.

The reactors 34, 36 and 38 are maintained at successively high temperatures by the use of heat exchanger coils 39 to add or remove heat as desired. The use of zoned incremental increases in temperatu#re allows for the controlled addition or removal of the heat of polymerization therefrom and close maintenance of desired temperatures in each reactor. It should be appreciated that unit 30 is a type utilized-in a multi reactor continuous mass process which is well known in the art and will -not be further described here.

From reactor 38, the polymerized material is passed through a devolatizer 46 where some of the remaining monomer and volatiles are removed.

From devolatizer 46, the thermoplastic polymer passes through a second devolatizer 47 for further removal of remaining monomer. From devolatizer 47 the thermoplastic polymer is pumped by gear pump 48 through strand die 14. Cutting means 16 located at the discharge end of strand die 14 cut the strands into pellets. The remaining process steps in Figure 2 are identical to those described previously when referring to Figure 1.

Process conditions in the continuous process reactor 30 will depend on the viscosity of the styrene, the rate of flow desired, and upon the number and length of heat exchange tubes in unit 30. The conditions for the continuous mass process utilizing a multi-reactor system are those normally used for the production of styrene -polymer which are well understood in the art.

Other expandable polystyrene processes do not use a continuous process to polymerize the styrene monomer. Instead, a suspension process is utilized to polymerize the monomer, wherein the monomer is transported into a suspension reactor filled with water, a suspending agent, and a catalyst. The monomer and suspending agent are agitated by a stirring means and polymerization occurs therein.

When polymerization is complete, the beads formed in the reactor are removed, dried, separated, and the required sized beads are reintroduced into a second suspension reactor to be resuspended- in water for pentane impregnation.

Other processes have. attempted to reduce the process time by introducing pentane into the initial suspension reactor after the polymerization therein has been substantially completed. Substantial time and money is involved in removing the beads from the suspension reactor, drying them, separating out the required sized beads and then reintroducing the required sized beads into the second suspension reactor.lt should be appreciated that a plant having a fixed number of suspension reactors will be able to double its output of expandable polystyrene by using the process of the present invention because one half of the suspension reactors normally used for initial suspension polymerization, will now be free to be used for blowing agent impregnation.

Previously, it has been believed that formation of the polymer beads by the suspension process was necessary in order to produce a rounded bead which was commercially desirable. Surprisingly, however, it has been found that the pellets produced by rotary cutting the thermoplastic strands from the continuous mass process will round out to a more desirable spherical form when suspended in the reactor for pentanization. It is not known at the present time whether the presence of the blowing agent leads to this rounding out or if it is merely due to the water suspension, heat or a combination of the three; however, rounding out of the beads during suspension allows the desired bead to be produced from strand cutting.Additionally it was also believed that impregnation of blowing agent through the pellet would not occur due to the density of the pellet and the relatively small amount of suspension medium in relation to the large pellet size. Surprisingly, complete impregnation of blowing agent occurs sufficiently quickly under the process conditions disclosed herein to be economically feasible.

.Additional advantages are realized by the process of the present invention because only the required sized beads are impregnated with blowing agent. In the one step suspension process both required size and off grade beads are impregnated with pentane. These beads are not only dangerous to handle but cannot be safely reground or sold for any other use. There are additionally costs associated even with the disposal of these flammable beads.

The invention is further described by referring to the following examples which are not intended to limitate the scope of the invention but merely illus trate the details and manner of practicing the invention.

Example 1 A reactor was filled with 67 gallons of water and heated to 160 F. When at 160 F, tricalciumphosphate was added and agitated therein for five minutes. Polystyrene formed from the multi reactor continuous mass process previously described in detail, was cut into pellets of approximately 0.050 inches in length from strands approximately 0.080 inches in width by a water face underwater hot cut pelletizer manufactured by Farrel Co. The pellets were then added to the reactor and agitated for five minutes. In addition, flame retardant enhances were added and agitated therein for five minutes; a suspension agent was also added. After heating the reactor to 2200F, 27.4 pounds of n-pentane was injected over a seven hour period.After pentane injection, the reactor was heated to 235 F and held there for three hours. After three hours, the impregnated pellets had rounded into spherical beads. The beads were cooled, dried and removed for testing.

The impregnated beads were subjected to 240 F steam at 8 psig for 280 seconds to form partially expanded crystals, or pre-puffs. The pre-puffs were then aged for 11 hours and placed in a block mold and subjected to a steam injection at 2950F for 21 seconds reaching a mold pressure of 12 psi. The molded article was allowed to cool, removed from the block and then subjected to a fusion test wherein the molded article was torn into pieces to obtain a comparison of the percentage of beads that were torn through versus the percentage of beads that tore around their periphery. The moulded article produced under the above conditions had 85% fusion, i.e. 85% of the exposed beads at the separation layer were torn through.

Example 2 The procedures of Example 1 were followed except steam was injected into the block mold at 295 F for 19 seconds reaching a mold pressure of 15 psi. The evaluation test indicated 75% fusion.

Example 3 The procedure of Example 1 was followed except steam was injected into the block mold at 2950F for 20 seconds reaching a mold pressure of 17 psig.

the evaluation test indicated 45% fusion.

The above data indicates that the expanded polystyrene beads produced by the process of the present invention show acceptable fusion characteristics, i.e., the degree of fusion was directly controllable over a broad range by utilization of proper mold temperature and pressure conditions.

Claims

1. A process for making polystyrene pellets comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof; and c) cutting the strands into pellets after exiting the die but before substantially cooling.

2. A process according to Claim 1, wherein the pellets are extruded at less than 0.1 inches in diameter and are cut at less than 0.2 inches in length.

3. A process according to Claim 1 or Claim 2, wherein the pellets are discharged into a cooling environment during the cutting step.

4. A process according to Claim 1 or Claim 2, wherein the strands are extruded into a cooling environment.

5. A process according to Claim 1, Claim 2 or Claim 4, wherein the strands are extruded under water.

6. A process for making expandable polystyrene comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof; c) cutting the strands into pellets after exiting the die but prior to substantial cooling; d) suspending the pellets in a reactor and introducing a blowing agent into the reactor under sufficient conditions of temperature and pressure to impregnate the pellets with the blowing agent; and e) removing the pellets from the reactor in the form of substantially spherical beads.

7. A process according to Claim 6, wherein the pellets are extruded at less than 0.1 inch in diameter and are cut at less than 0.2 inches in length.

8. A process according to Claim 6 or Claim 7, wherein the pellets are discharged into a cooling environment during the cutting step.

9. A process according to Claim 6 or Claim 7, wherein the strands are extruded into a cooling environment.

10. A process according to Claim 6, Claim 7 or Claim 9, wherein the strands are extruded under water.

11. A process according to any one of Claims 6 to 10, wherein the styrene polymer melt is formed by a continuous mass polymerization process.

12. A process for making polystyrene pellets comprising the steps of: a) forming a styrene polymer melt; b) extruding the styrene polymer melt through a die to form continuous strands thereof wherein the strands are extruded into a cooling medium; and c) cutting the strands into pellets after exiting the die but before substantial cooling wherein the pellets are at a sufficiently high temperature to allow toward spherical shape transformation of the pellets.

13. A process according to Claim 12 further comprising the steps of: d) suspending the pellets in a heated reactor and introducing a blowing agent into the reactor under sufficient conditions of temperature and pressure to impregnate the pellets with the blowing agent; and e) removing the pellets from the reactor in the form of substantially spherical expandable polystyrene pellets.

14. A process according to Claim 12 or Claim 13, wherein the cooling medium is water.

15. A process according to any one of Claims 12 to- 14, wherein the strands are extruded at less than 0.1 inches in diameter and are cut at less than 0.1 inches in length to form the pellets.

16. A process according to any one of Claims 12 to 15, wherein the styrene polymer melt is formed by a continuous mass polymerization process.

17. - A process according to any one of Claims 12 to 16, wherein the pellets are subjected to a size separation process to remove undesirable size pellets.

18. A process according to Claim 17, wherein the removed undesirable size pellets are recycled to the polymer melt.

19. A process according to Claim 13, wherein the blowing agent is a hydrocarbon selected from the group consisting of butanes, pentanes, hexanes or any combination thereof.

20. A process for making polystyrene pellets substantially as hereinbefore described with reference to Figure 1 or Figure 2.

21. A process for making polystyrene pellets substantially as hereinbefore described in Example 1, 2 or 3.

22. A process for making expandable polystyrene substantially as hereinbefore described with reference to Figure 1 or Figure 2.

23. A process for making expandable polystyrene substantially as hereinbefore described in Example 1, 2 or 3.

24. Polystyrene pellets whenever made by the process of any one of Claims 1 to 5 and 12to 21.

25. Expandable- polystyrene whenever made by the process of any one of Claims 6 to 11, 22 and 23.