HRP920146A2 - Process for the preparation of expanding polystyrene extruded granulate by suspension impregnation - Google Patents

Description

The field of technology

The subject of the invention belongs to the field of chemical industry in general, and especially to the field of production of plastics in the field of styrene polymers and styrene-based products.

According to the International Classification of Patents, the subject of the invention is classified and marked with the classification symbol B29B9/00, which defines the production of objects in molds from molten materials, C08F2/18 polymerization in suspension, C08F357/02 macromolecular compounds with styrene polymers.

Technical problem

The technical problem solved by this invention consists in the following: how to make it possible to obtain uniform or uniform sizes of polystyrene granulate particles in order to impregnate it with a blowing agent, without losing material that is too fine or too coarse for that purpose. During the suspension polymerization of styrene, due to the statistical distribution of particles, despite production efforts, 5-50% of grains of inappropriate size are always formed, and the entire content of the obtained polymer grains must therefore be emptied from the reactor where it was obtained, washed with acid from additives, dried using a centrifuge and hot air in dryers , so that the grains could then be sifted multiple times on sieves, and only grains of the appropriate, desired size could be used to refill the reactor. Grains of inappropriate size, which in practice make up 30-40% of the mass, must then be used for extruding into granulated polystyrene for injection molding or drawing films, although the properties of the polymer itself are adapted to expandable polystyrene foam. The quality of these by-products is not adequate.

State of the art

Polystyrene in the form of granules that are capable of expansion or swelling into polystyrene foam, known as expandable PS (in English-expandable), can be obtained through two known processes, suspension technology of polymerization and impregnation from styrene or continuous polymerization of styrene in mass, and then by pressing pentane in an extruder and granulating.

Suspension technology consists of dispersing styrene monomers in approximately the same amount of water with the addition of dibenzoyl peroxide initiator, tricalcium phosphate to stabilize the dispersion and sodium alkyl aryl sulfonate surface active agent, and is heated to 80°-95°C with constant stirring. Within 5-7 hours, the atyrene droplets turn into rigid polystyrene balls with a statistical size distribution of 0.2-3 mm, which mostly corresponds to a Gaussian distribution. After the completion of the conversion by heating for several hours above 120°C, the polymerization is completed and the impregnation of polystyrene follows by introducing pentane as an expanding agent at a temperature above 100°C using a piston pump for 2-5 hours. After remaining at these temperatures and pressure up to 9 bar for 1-4 hours, the contents are cooled in the autoclave to below 50°C, chemically washed, centrifuged from water, air-dried and separated into commercial fractions and waste fractions of grain size that is too fine or too big. The disadvantage is that, due to the content of pentane for expanding, the waste fractions cannot be used for the production of normal and tough polystyrene of general use, but only for dissolving in resins and only in small quantities, and for expanding into foam it is not granulometrically suitable. From 5 to 40% of the mass in waste fractions cannot be avoided.

The improved method is a two-stage process, when after polymerization, the polymer grains are emptied from the reactor, washed, centrifuged, dried, the waste fractions of fine and coarse grains are removed by sieving, then transported again to the reactor, water and additives for stabilization are added, and pentane is introduced and finalized identically the described one-step procedure. Waste fractions up to 40% of the total mass can now be used for general-purpose polystyrene, but that polymer has characteristics suitable for polystyrene foam, which reduces the quality of general-purpose products. In both variants of the suspension technology, there is a problem with the granulometric composition because it is a consequence of the dynamic balance and statistical distribution of the polymerization phase. The whole process is discontinuous.

Another way to obtain expandable polystyrene is the extrusion process, which uses ready-made cut granulate of normal polystyrene as raw material. The raw material is precisely dosed into the extruder, with the number of revolutions, temperature around 200°C and pressure controlled. The polystyrene is melted and pushed into a rotary mixer (mixer) where a carefully dosed amount of pentane is continuously introduced. The mixture is completely homogenized, cooled a little, and comes out of the nozzles in the form of continuous threads (filaments), with a thickness of 0.7 mm or more, as needed. Immediately after the exit, the threads are cooled in a water bath, the delivery roller pulls them into a rotating cutter with helicoidal blades, with an adjustable number of revolutions so that the length of the granules can be regulated as desired. After thermal relaxation of the granules in the pool, the granules are filtered and dried with warm air, sieves separate irregularly cut particles and surface treated with processability additives, and stored.

This whole process is continuous, and the granules of the product are cylindrical in shape, which leads to a slightly worse packaging in molds and blocks. The minimum bulk densities that can be obtained with this product are about 30% higher than the product obtained by suspension impregnation, which indicates a greater number of open foam cells after expansion, and weaker mechanical properties. The granulometric composition is completely uniform because the particles are cut exactly as needed. Due to the continuous process of polymerization and impregnation, the quality of the product has high reproducibility and stability.

Description of the solution to the technical problem

The process of obtaining expanding (foaming) polystyrene from extruder granulate by the suspension process consists of two phases that can be performed separately and in separate places at different times.

The first stage is the preparation of granules for later suspension impregnation with pentane. That part of the procedure works in continuous mode.

The raw material for extrusion and granulation can be polystyrene particles from any source, such as polystyrene grains obtained by the suspension process in the form of balls with a diameter of 0.1...5 mm, ground polystyrene material for recycling substances with irregular particle shapes, bonded granules and dust from the sieves of the extruder line, polystyrene granulate as a general purpose finished product, and also styrene copolymers.

By cutting extruded threads, granulate is prepared with sizes that correspond to the needs of the expanding polystyrene (PS-E) market. Granules prepared for suspension impregnation are cylindrical in shape with optional dimensions of C.5...2.5 mm in diameter and 0.5...5 mm in length. During the later suspension impregnation, the sharp edges on the granules are lost, so the particles of the finished product are PS-E balls or rollers with spherical bases. The particles obtained by cutting on the granulator are of almost identical dimensions and sizes, so that the final product has a narrow uniform and almost uniform granulometric composition.

An extruder is a device required to melt and homogenize polystyrene particles by thermal and mechanical action, and to squeeze the mass through nozzles 0.5...3 mm in diameter into strands that are cooled and introduced into the cutter or granulator by means of a delivery roller.

The diameter of the nozzles and their number is regulated by changing the tool on the extruder head in case of a need for a larger change in the diameter of the granulate. Smaller changes in the diameter of the granules can be achieved by changing the speed of the delivery roller which draws out the strands and thus determines the thickness. The length of the granules is adjusted by changing the cutting speed of the granulator. If the speed of the delivery rollers is regulated together with the cutter for each length of granules, the drive transfer wheel of the rollers should be changed.

In the case of underwater cutting of granules, and that is with larger capacities, the granules are obtained by cutting using rotating knives when the mass exits through the nozzle openings, with the fact that this action is performed in an aqueous environment as a medium that cools and carries the resulting granules to further separation from water and for drying. In this case too, the dimensions of the granules are determined by the speed of the cutter and the diameter of the nozzle opening, but there is no delivery roller.

The polymer mass can be fed to cutting using gear pumps instead of an extruder, if at the end of the continuous polymerization line, the polystyrene mass is pressed through the nozzles into the cutter that cuts to the required granule size.

The key moment in this technology is that the polystyrene granules are obtained precisely on the granulator device, which is made for this purpose, and in this way the problems caused by the statistical distribution of the particle size in the suspension polymerization of styrene, which is naturally legal and cannot be avoid, (Gauss distribution law).

The second stage is carried out by a discontinuous batch process in an autoclave reactor with a mixer, a heating and cooling system and temperature control, with a pressure indicator, a filling hole and a connection for adding pentane under pressure.

In a reactor under pressure, polystyrene granules mixed with water at a temperature above 90°C absorb the dosed pentane and thus become capable of expanding (foaming) when cooled and as a finished product when reheated in the atmosphere into polystyrene foam known in consumption as styrofoam.

According to this procedure, during the second phase in the suspension impregnation, the cylindrical granules created on the cutter in the dimensions required for the market, due to the increased temperature, surface tension and collision of particles during mixing, lose their sharp edges and become round or round balls, but according to the shape and size in which they are made . In this way, the orientation of the molecules inside the granules is lost, created when the thread is drawn out, so the particle acquires equal properties in all directions, which increases the content of closed cells in the final foam, improves the mechanical properties of the foam and the packing density of the pre-expanded granules.

These properties indicate an advantage compared to the expanding polystyrene obtained by the extrusion process for the production of PS-E.

1. Example of performance

In order to produce expanding polystyrene intended for machine molding into molds for packaging products and the like, it is necessary in the first stage to produce polystyrene granules of approximately 1 mm in diameter and length.

To obtain granules of these dimensions, it is necessary that the nozzles on the tool on the extruder head have a diameter of 1 mm. The extruder is prepared for operation, the temperatures of individual zones of the extruder should be adjusted to 100° to 250° C.

To cool the thread, a water bath is placed next to the exit opening of the extruder and a device for blowing moisture from the thread using air is turned on. The granulator and the screen behind it in the line are turned on, and the raw material is placed in the receiving container of the extruder, in this example, let's take polystyrene grains obtained by suspension polymerization. Each extruder has its own characteristics, but to facilitate melting, 0.01 mass percent of zinc stearate and 0.05 mass percent of stearic acid can be mixed into the raw material. The extruder is turned on and the threads that come out are passed through the water in the bath, through the air current that blows away the residual water from the threads to the delivery rollers of the granulator, which now mechanically pick up the threads and continue to pull out the threads and deliver them under the knives of the cutter. The speed of the granulator is adjusted so that the length of the granules is 1 mm. On the screen behind the granulator, any stuck granules, irregular ones and dust are removed, and regular standard particles are prepared for batch impregnation with pentane. If you have a reactor with a volume of 10 m3, you should prepare at least 3,600 kg of granules of the intended size.

The reactor is filled with 5400 liters of demineralized water, the mixer is turned on, 18 kg of tricalcium phosphate and 90 grams of sodium dodecylbenzene sulfonate are added, and 3600 kg of prepared granulate is added. The reactor is closed and the temperature is raised above 90°C and 216 kg of pentane is added over a period of one hour using a piston pump. The pressure in the reactor is limited by the permissible working pressure, and it is better if it is higher. The pressure in the reactor during the addition of pentane and thereafter is maintained in the range of 7 to 9 bar by adjusting the temperature in the reactor between 90° and 120°C. As the temperature increases, the pressure increases, and vice versa, it decreases. One hour after the addition of the entire amount of pentane, a high pressure is maintained in the marked range, but as the pentane is absorbed into the polystyrene granules, the pressure begins to drop and the charge cools below 50°C. The filling is emptied through the valve at the bottom of the reactor into another vessel where the tricalcium phosphate present is washed with hydrochloric acid to an acidity of pH - 2. At the exit from this vessel, the water is removed by centrifugation, the granulate is dried with a little warm air, surface additives are added and the product is filled in packaging. Accumulations of polymers that form on the walls of the reactor are separated on a large sieve when they are discharged from the reactor. This sieve has an opening of 5 mm. The outlet screen before filling into the packaging ensures that only a narrow, uniform, almost uniform particle size composition enters the product. Since the granulate taken for suspension impregnation had a diameter and length close to one millimeter, in the process it will be rounded into an approximately ball slightly larger than one millimeter in diameter, which is suitable for machine molding of the final Styrofoam into molds.

2. Example of performance

In order to obtain expanding polystyrene intended for blocks and insulation in construction, it is necessary that the finished product has granules of 1.5 to 2 mm in size.

When carrying out the polymerization of polystyrene in a continuous mass process, the final part of the process is the formation of threads on the head of the exit extruder. The tool on the extruder head should have nozzles with a diameter of 1.3 mm. The emerging threads are cooled with water and cut on a cutter to a length of 1.8 mm. The speed of the granulator is adjusted to ensure this. The product is dried, seeded from bonded granules and transferred usually by means of pneumatic transport to a tank until use in the second phase, in the batch suspension impregnation of polystyrene with pentane. In the second phase, if we have a 10 m3 reactor, 3150 liters of demineralized water are first charged into it, the mixer is turned on, 90 kg of tricalcium phosphate and 1.8 kg of sodium dodecyl benzene sulfonate are added, and 5850 kg of prepared granulate is charged. The reactor is closed, and the temperature rises above 90°C, and over a period of 5 hours, 585 kg of pentane is added using a piston pump. The pressure in the reactor during the addition of pentane and after that is maintained in the range of 7 to 9 bar by adjusting the temperature in the reactor between 90 and 120° C. As the temperature increases, the pressure increases, and vice versa, it decreases. After adding the entire amount of pentane, high pressure is maintained in the indicated range for another 4 hours. When the pressure still drops with the increased temperature, it is a sign that the pentane has mostly already been absorbed into the granules. After 4 hours, the filling is cooled below 50°C, emptied through the valve at the bottom and washed in another container with hydrochloric acid to pH - 2. After that, the water is removed by centrifugation, and the granulate is dried in a drier using air, surface additives are added and the product is filled into packaging . The granules of the product will be round-edged, almost round balls of about 2 mm in diameter. This dimension is suitable for the production of polystyrene foam blocks which are then cut into panels for insulation in construction.

Statement on the best way of economic use

The best way for the economic use of this invention is represented by the technologies used at INA-OKI in Zagreb, at the Polystyrene Factory (DOKI) and at the Expanding Polystyrene and Compound Factory (OKIROLA).

Applying the invention would take advantage of the comparative advantages of the technologies on which the production of those factories is based. The suspension technology of the EPS Factory would avoid the creation of fine and coarse waste and would have a uniform product particle size with dimensions according to market needs, and the Polystyrene Factory (DOKI) could correct the processing properties of its expanding polystyrene by using suspension impregnation.

When applying this new technological solution, due to the uniformity of the quality of the raw material that comes from the continuous process, it is possible to maintain a constant and uniform quality of the expanding polystyrene product.

Claims (1)

1. The process of obtaining expanding polystyrene from extruded granulate by suspension impregnation, characterized by the fact that from 40 to 65 parts by mass of polystyrene granulate, obtained by squeezing the polymer melt through nozzles and cutting it with a granulator to dimensions as needed from 0.5 to 5 mm in length and 0 .5 to 2.5 mm in diameter, suspended in water from 35 to 60 parts by mass of water with stirring and an agent for stabilizing the suspension of 0.2 to 1 part by mass of sodium dodecyl benzene sulfonate, and after adding 2.4 to 6.5 parts by mass parts of pentane for up to 5 hours at a temperature of 90° to 120°C and a pressure between 7 and 9 bar, the same conditions are maintained for another 1 to 4 hours, after which the charge is cooled below 50°C.