PROCESS FOR THE PRODUCTION OF CELLULAR PLASTIC

Abstract: A process for the continuous or semi-continuous production of cellular plastic material from a plastic compound containing a blowing agent and, optionally, other process-regulative substances and additives, comprising: (a) feeding of the raw materials to a plastic processing machine; (b) kneading of the raw materials in the plastic processing machine under conditions of temperature and pressure which will produce an expanding compound upon release of pressure alone; (c) transfer of the plastic compound directly to a cooling zone; (d) cooling of the compound in the cooling zone to a continuous stock having high viscosity and rigidity and an approximately flat velocity profile under pressure formed by an upstream-directed pressure component; (e) optionally, stepless transfer to a braking-retaining zone; (f) build-up of the upstream-directed pressure component until the pressure in the plastic mass in the upstream located zones is sufficient to prevent expansion of the plastic compound; (g) control to expand the compound to the desired density after transfer to a heating/expansion/forming zone.

Description: Process for the production of cellular plastic. The prior art The present invention relates to a process for the continuou or semi-continuous production of cellular plastic from plastic mass containing a blowing agent and, optionally other process-regulative substances and additives. Production of cellular plastic (foamed plastic) fro thermoplastic materials has increased sharply. The ke reasons for this are low consumption of raw material relative to volume and high thermal insulation value. There are several known processes for production of plasti foam from thermoplastic resins, including, for example "bea foam" of polystyrene (Styropor) and the static high-pressur process for the production of PVC foam having low density an closed cells. The "bead foam" process, which is a multi stage process, produces a polystyrene foam having lo density but with relatively low physical values. The stati high-pressure process produces PVC foam having low density closed cells and high quality, but is a highly manual proces associated with high production costs, and it suffer considerable limitations with respect to the product's shape Of fully continuous processes, extrusion is the one used mos frequently. It is usually relatively easy to control and i normally associated with low production costs. Today, the use of plastic processing machines for continuou production of foamed plastic, for example extruders generally involves the employment of physical blowing agent for foaming of the cellular plastic with low density. Fo higher densities, chemical blowing agents are usually th basis for the foaming process. The thermoplastics resins best suited for foaming are: Polystyrenes (PS), including polymerisates (HIPS, SB, SAN, ABS), polyethylene (LDPE) and the copolymerisate (EVA), polyvinylchlorides (PVC) including polyblends, and poly- propylenes (PP). Favorable conditions for foaming are obtained when the melt viscosity of the raw material falls slowly and evenly in the temperature ranges for softening. Therefore, the amorphous thermoplastic resins are easier to foam than the crystalline ones. The partly crystalline thermoplastic materials are usually cross-linked chemically or cross-linked by radiation prior to or in connection with the foaming. In this way there is obtained a more favorable viscosity curve as function of the temperature. In the extrusion of plastic foam, the foaming process itself is dependent on the plastic's viscosity and the melt's tensile capacity, the gas pressure of the blowing agent and the exterior pressure and interaction between the melt and the blowing agent. Because plastic foam has greater friction against the wall of a tool connected with a plastic processing machine such as an extruder, than does an unexpanded plastic material, it is generally desirable that the foaming should take place only after the plastic compound has left the tool. One condition for this is that the exterior pressure on the melt containing active blowing agent be high enough to prevent expansion. In presently known production processes, after the plastic melt leaves the processing machine's tool, the external pressure on the melt decreases and the plastic material expands as a result of oversaturation of the gas in the plastic material. One process parameter that has a heavy influence on the cellular structure and thus the quality of the extrudate is the temperature of the material. In the case of too low a material temperature, the plastic mixture would not b capable of expanding completely due to a relatively high mel viscosity, resulting in a relatively high density. If th temperature of the material is too high, the melt viscosit will be low, which easily causes the cells to be rupture with a high density as a result. In order to produce plastic foam having low, uniform densit and closed cells, it is necessary to produce an extrudat where the gas cells are not ruptured to pieces due to unequa velocity in the direction of flow at the outlet of th forming tool where the pressure is reduced and the gas cell are formed. At the same time, it is necessary to have a external pressure on the plastic melt both in the plasti processing machine and in the tool that prevents expansion o the plastic material. These two criteria today constitute a limiting factor for th potential of producing thick profiles of plastic foam wit closed cells by an automatic process where the plastic foa has low density that is uniform throughout the entire cross section. This is particularly true when chemical blowin agents are used, because the gas pressure in chemica blowing agents (such as, for example, azodicarbonamide o sodium bicarbonate) is much higher than the gas pressure i the physical blowing agents that are normally used (fo example, KFK gases, CO or 2). The problems related to shearing of the plastic melt an tearing up of the gas cells by pressure reduction ar alleviated today either by restricting the addition o blowing agent or by the fact that the plastic melt has ver limited layer thickness on pressure build-up and o expansion. This is particularly true for chemical blowin agents, but there are also considerable limitations with us of physical blowing agents. It is often desirable to be able to use chemical blowing agents for thicker plastic foam products, too, because the actual production process is usually simpler than with use of physical blowing agents, and the end product's properties can usually be controlled better than with the use of physical blowing agents. This is a result of, inter alia. the higher gas pressure in the chemical blowing agent compared with the gas pressure in physical blowing agents. Various tool principles have been developed in connection with continuous production of foam, particularly in connection with extrusion. An extrusion tool has, in particular, two important functions: 1) Building up a sufficient pressure in the tool and extruder 2) Forming the extrudate to the desired shape. Continuous foaming of thermoplastic materials takes place in two principally different ways: 1) Free foaming 2) Controlled foaming inwards. Free foaming is usually used for relatively simple profiles. The foamed extrudate is normally collected by a calibration unit a short distance away from the outlet of the tool. The required melt pressure in the tool is normally built up by the lips of the tool. In forming inwards, the required melt pressure is built up by means of a torpedo within the tool, and the calibration unit is securely connected to the tool itself. The calibration unit has the same internal shape as the tool nozzle. The melt is externally cooled down and is held securely to the walls of the calibration unit by means of a vacuum. At the same time an insert in the tool enables the extrudate to expand inwards in the calibration unit. This method is used for production of pipes, sheets and profiles. The end product has a compact, smooth skin with a lower density toward the core. This method is known as the "Celuk Process". There are known combinations of the method for free foamin and the "Celuka Process". There are also produced hollo profiles on the basis of a principle similar to the "Celuk Process". There are also used a variety of principles of co-extrusio of unexpended skin material and foamed core materials. Another process that produces pressure build-up in the too and relatively little shearing of the material on foaming i the socalled "Woodlite Process". This is a method fo extrusion of foamed strings that are welded into one uni after leaving the outlet of the tool. The final extrudate however, has an uneven structure and low flexural strengt due to, in part, poor welding together of the different sub extrudates. A principle used particularly for production of foamed tubin is the "Armocell Process". By means of a special constructio of the flow channel in the tool, sufficient pressure is buil up to prevent a premature expansion of the melt. The foame tube is held at a short distance from the tool by calibration unit. All the above tool principles result in considerabl limitations in the product's density, percentage of close cells and dimensions. This is, inter alia, because th build-up of the necessary melt pressure to avoid prematur expansion, is based on friction between the hot plastic mel and the tool. Description of the invention: The purpose of the present invention is to improve the prior art and to expand its area of usage, while simultaneously avoiding the deficiencies of the prior art and the limitations attendant thereto. In accordance with this, the present invention relates to a process of the type discussed in the introduction, comprising: a) feeding of the raw materials to a plastic processing machine; the process being characterized in that it further com¬ prises: b) kneading of the raw materials in the plastic processing machine under conditions of temperature and pressure which will produce an expanding compound upon release of pressure alone; c) transfer of the plastic compound directly to a cooling zone: d) cooling of the compound in the cooling zone to a con¬ tinuous stock having high viscosity and rigidity and an approximately flat velocity profile under pressure formed by an upstream-directed pressure component; e) optionally, stepless transfer to a braking-retaining zone; f) for build-up of the upstream-directed pressure component until the pressure in the plastic compound in the upstream located zones is sufficient to prevent expansion of the plastic compound; g) controlled to expand the compound to the desired density after transfer to a heating/expansion/forming zone. The invention will be explained in greater detail below, with reference to the accompanying figures, where - figures 1 and 2 show two possible apparatus systems for carrying out the sequence of a) to g) above. Thermoplastic, blowing agent and other optional process regulative substances and additives are fed into a plasti processing machine of a known per se type, for example, a extruder 1, where they are to be kneaded together under suc conditions of temperature and pressure that an expandin compound will be produced only on pressure release. Extruders of various types, as well as the mixing condition used in such extruders, are known and will not be describe in further detail here. The important factor is to avoi giving the blowing agent opportunity to expand in the plasti processing machine itself. The plastic material is kneaded in such a way that all part of the stock obtain approximately the same temperature, an the fact that this is achieved relatively rapidly results i relatively good control of any possible exothermic o endothermic heat exchange from a chemical blowing agent. Due to the rapid heating of the entire plastic stock throug the mechanical processing, one also achieves a rapi decomposition of the chemical blowing agent, if used. In order to avoid the problems existing today in connectio with the known processes and the known principles fo extrusion tools, for obtaining sufficiently high pressure i the tool and extruder where the gas cells are in large degre ruptured at the outlet from the extruder tool (because th pressure build-up is based on shearing effect/friction) there is used, according to the invention's method, a build up of sufficient pressure in the tool and the extruder t avoid the premature expansion of the plastic material i achieved by braking/holding the plastic material after bein cooled down so that the plastic material forms a continuou "piston" for build-up of the necessary pressure to preven shearing and the resultant rupture of the gas cells. δ The invention is illustrated below under reference to the accompanying drawings. The build-up of the necessary pressure in the tool and extruder is accomplished by cooling down 2 the plastic stock under pressure at the outlet of the extrusion tool 1 in a continuous process sufficient to render the rigidity of the plastic stock high enough that the plastic stock can be slowed down by a braking means 3 which may, for example, be a continuously movable belt. The braking of the plastic material is controlled in such a way that the pressure being built up in the plastic compound is high enough to prevent the compound from expanding. From the extrusion tool or the forming apparatus after the extruder, the plastic material must be maintained under high external pressure without friction between the limiting wall and the plastic stock or with the friction between the wall and the stock being small enough so that the gas cells formed after the pressure is reduced are not ruptured. Due to the absence of friction between the wall and the plastic stock or to the fact that friction is very minimal, the plastic stock in this cooling device 2 will move at a rate that is uniform throughout the entire plastic stock. This means that there occurs no shearing between the various layers in the plastic stock in the direction of movement, or only an insignificant shearing. The cooling device 2, which must be directly connected to the tool/forming device, may be directly connected to the braking device 3, or the cooling device 2 and the braking device 3 may be one unit. This can be accomplished, for example, if the cooling down of the plastic material is done between continuous endless steel belts, where the steel belts have low temperature. By controlling the speed of the steel belts, the pressure of the compound can be regulated. After the stock has been slowed down sufficiently, and enough pressure has been obtained by means of the braking device 3, the plastic material is heated up 4 while the pressure is simultaneously reduced, enabling the plastic mass to expand. The heating can be done continuously and directly in connection with the cooling 2 and braking device 3, for example, by means of heated endless steel belts, or the heating can take place, for example, by warming the cooled plastic stock in connection with transport through a heating tunnel with hot air. The cooled plastic stock may also be heated in, for example, a liquid bath. By heating the plastic material either by the application of exterior heat or by equalizing the interior and exterior temperature of the plastic material, or by a combination of supplying exterior heat and equalizing the temperature of the plastic material, the plastic material will expand until equilibrium is reached between the internal expansion forces in the plastic material and the tensions in the plastic material. The device 2 for cooling down the plastic compound after the outlet of the tool/forming device with little or no friction between the limiting walls and the plastic compound, the braking/retaining device and the heating device, can be constructed in several basic ways. Examples of these are the following: The cooling device 2 which is directly connected to the tool/ forming unit after the extruder may be a static cooler having the same or approximately the same internal cross- section as the cross-section at the outlet of the tool/forming device. The walls in the interior cross-section must create minimal friction between wall and plastic compound. This can be accomplished, for example, by high- gloss polishing or chrome plating of the interior walls. At the same time, a lubricating medium may also be applied con¬ tinuously between the interior walls of the cooling device and the plastic stock to further reduce the friction. The cooling can be done, for example, by chilling the cooling device with tempered oil in channels of the walls therein. The cooling device may also be a set of endless steel belts that are chilled and pressed against the plastic material. The braking device 3 after the cooling device 2 may consist, for example, of two or more rollers that are pressed against the rigid, cooled-down plastic material, or the braking device may consist of a set of endless steel belts that may be pressed against the plastic material, with their speed being regulated such that the upstream-pressure of the plastic material can be regulated. The heating unit 4 after the braking/retaining device may be, for example, a chamber filled with warm liquid, where the pressure may be regulated. The heating device may also be a set of endless belts that are heated up. The heating of the plastic material may also take place, for example, in a static heating unit having an interior cross-section equal to or approximately equal to the cross-section of the plastic stock after the outlet of the braking device. The static heating device must have very low or no friction between the interior walls of the heating device and the plastic stock. The heating may also take place in a heating chamber where the plastic extrudate moves freely. Often it may be advantageous to heat up the plastic compound under sufficiently high pressure so that the plastic compound will not expand in the heating unit itself. It is thus possible to heat up the plastic material much more rapidly, because the thermal insulation value of the plastic material increases dramatically when expanded. The friction between the interior walls of the heating uni and the plastic stock should be minimal, because It coul otherwise cause difficulties for an optimal control of th pressure in the plastic mixture upstream in the extrusio tool and the extruder. In addition, it is much easier t control the expansion if there is minimal or no frictio between the heating unit and the plastic material. When the plastic extrudate is heated under pressure, th external pressure is reduced after all parts of the extrudat have been brought to the desired temperature, and the plasti material can be expanded until equilibrium has been reache between the interior expansion forces and the tensions in th plastic material. When the plastic extrudate is heated without externa pressure (for example, in a heating chamber), the plasti material will expand simultaneously with the heating. In this specification is described particularly a proces using a plastic extruder or extruders, but any continuous o semi-continuous plastic processing machine meeting th requirements for degree of processing and temperature/pres sure conditions could be used. Description and examples of plastic compositions an processing conditions As mentioned earlier, in developing a formula or recipe fo a plastic composition, one must take into consideration th plastic processing machine to be used and evaluate th desired properties for the end product. The present proces has been developed with the use of PVC. The following hav been employed as plastic processing machines: The various components in the plastic composition have been blended/mixed in a conventional hot/cold, vertical/vertical plastic mixer. As homogenizing machine/gelatinizing machine for the PVC mixture, a double-screw extruder (110 mm pelletizing unit) was used. As plastic processing machine in connection with the decomposition of the chemical blowing agent, a single-screw extruder (90 mm) was used. Various tool/forming units have been used, including a tool for production of an approximately 50 mm round profile. As a cooling unit there was used a static device where the walls were cooled at the same time as a liquid lubricant was applied between the interior walls of the cooling device and the plastic stock. As braking/retaining device a system of endless belts has been used. The heating of the plastic stock has been done in a static system with heated walls. In developmental work there has also been used a cooling/braking and heating device built as one unit based on continuous walls in the form of endless steel belts. The following are examples of plastic compositions that have been used: (All are based on PVC thermoplastic) Example 1 PVC (M-type K-number approx. 60) 100 Pe-wax 0.8 Ca-stearate 0.4 Tri-basic lead sulphate 10 Di-basic lead stearate 2 Azo-dlcarbonamide 1 Sodium bicarbonate 15 Polymethyl methacrylate (PMMA) 12 Dioctylphthalate (DOP) 16 Example 2 PVC (M-type K-number approx. 60) 100 Pe-wax 0.2 Calcium laurate 0.3 Tri-basic lead sulphate 10 Di-basic lead stearate 1 Azo-dicarbonamide 1 Sodium bicarbonate 25 Polymethyl methacrylate (PMJA) 15 Dioctylphthalate (DOP) 25 The formula in example 1 yielded a PVC foam having a densit of about 0.1 (about 100 kg/m 3 ). The formula in example 2 produced a PVC foam having a densit of about 0.07 (about 70 kg/m 3 ). The various components in the above plastic compositions were mixed in a hot/cold, vertical/vertical high-speed mixer to a temperature of about 120°C. After being cooled, the mixture was fed to a pelletizing unit consisting of a double-screw extruder. The temperature of the plastic mass in the pelletizing unit at the outlet was about 130°C. The granulate was then fed to a single-screw 90 mm extruder which was extended to about 40 D. The pressure in the extruder immediately after the hopper was taken up to about 1200 bars. The pressure was reduced to about 250 bars at the outlet of the machine. The temperature of the compound immediately after the hopper was brought up to about 210°C. The compound temperature was reduced to about 165°C at the outlet of the machine. The stock tempera¬ ture at the outlet of the expansion apparatus (consisting of cooling unit, braking device and heating unit) was about 120°C. Under special circumstances it would be desirable to cross¬ link the plastic foam. This can be done, for example, by using radiation, using peroxides, using amides, using silanes or using isocyanates. Due to the risk of cross- linking of the plastic material in the plastic processing machine, it may be advantageous to add the cross-linking component at a later stage in the plastic processing machine, or immediately after the plastic processing machine in a separate mixing unit.

Claims: PATENT CLAIM : 1 . A process for the continuous or semi-continuous production o cellular plastic material from a plastic compound containin a blowing agent and, optionally, other process-regulativ substances and additives, comprising: a) feeding of the raw materials to a plastic processin machine; c h a r a c t e r i z e d i n that it further comprises : b) kneading of the raw materials in the plastic processin machine under conditions of temperature and pressure whic will produce an expanding compound upon release o pressure alone; c) transfer of the plastic compound directly to a coolin zone; d) cooling of the compound in the cooling zone to a con¬ tinuous stock having high viscosity and rigidity and a approximately flat velocity profile under pressure forme by an upstream-directed pressure component; e) optionally, stepless transfer to a braking-retaining zone; f) for build-up of the upstream-directed pressure component until the pressure in the plastic compound in the upstrea located zones is sufficient to prevent expansion of the plastic compound; g) controlled to expand the compound to the desired density after transfer to a heating/expansion/forming zone.

Inventors: SPYDEVOLD BAARD, SPYDEVOLD, BAARD
Applicants: SCANDINOR AS, SCANDINOR A/S
Application year: 1990
Main classification: B29C67
IPC codes: B29C44, B29C47, B29K105