Today, there are several known processes for producing cellul¬ ar PVC plastic (foamed plastic). We will describe three essentially different processes which are of interest in connection with the present invention for producing cellular plastic (foamed plastic) from PVC (polyvinylchloride) .

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1. The high pressure process yielding cellular PVC plastic with closed cells. The process comprises several steps: PVC plastic is mixed with a chemical blowing agent, stabilizer, and possibly other process controlling T5 substances and additives. The components are thoroughly mixed at a relatively low temperature, possibly in several steps. The thoroughly mixed matter is poured into moulds, and under high pressure (approx. 180 bar) the moulds are kept closed, e.g. by strong mechanical press means while ^~ -\ 0 heat is introduced into the plastic matter heating it up to approx. 170 C. At this temperature the chemical blowing agent is decomposed and vaporized at the same time as the plastic mixture is plastified and to a certain degree homo¬ genised (gelatinized). This occurs while the matter is at 5 rest in the mould. When the- plastic matter is sufficiently heated and has been kept under high pressure for a suffic¬ iently long time, commonly for 40-60 min., the moulds and, thus, the plastic are cooled to a temperature below the glass transition temperature of the plastic to prevent the 0 plastic from expanding after the external pressure is remov¬ ed, even though it now contains blowing agent in a gas phase with high internal pressure. The plastic object which is now a rigid semi-finished product, is removed from the mould and stored for some time. The plastic object is then heated again ₅ to approx. 100°C. Thus, the plastic is plasticised at the same time as the internal pressure in the blowing agent is slightly increased. This makes the plastic object expand un¬ til an equilibrium is achieved between internal expansion

pressure and tensions in the plastic. Because a very large portion of the heat stabilizing agent in the plastic mixt¬ ure is "used up" in the decomposing phase higher temperatur¬ es than approx. 100°C normally cannot be used in the expansion phase because the PVC plastic might otherwise be subject to degradation.

A PVC product produced according to this method has closed cells and may have an own weight down to approx. 40 kg/m .

Accordinσ to ^• this method products with an own weight of up

3 to approx. 600 kg/m are also produced. Due to the necessary high static specific pressure during the decomposing phase of the blowing agent there are very high practical limitat¬ ions to the size of the final product. Heating the plastic matter in the mould to achieve that all portions receive relatively equal heat also raises difficulties.

Commonly, a blowing agent of the exothermic kind is used. This causes a great hazard of "burning" and degradation of portions of the plastic matter before all of it receives ^• sufficient heat energy in connection with decomposition of the blowing agent. Due to this fact the thickness of the plastic matter to be heated is limited. Additionally, the process is very demanding and requires relatively high in¬ vestment in technical equipment.

Another method which is very widespread for production of cellular PVC plastic is extrusion of a plastic mixture comprising PVC plastic, a chemical blowing agent, stabilizer and, if desired, process controlling agents, and additives, according to which the components are well mixed at a rela¬ tively low temperature (e.g. 110 C) before the mixture is fed to the extruder, commonly in the shape of a powder or a granulate. In the extruder the components are thoroughly mixed under high temperature (e.g. 180 C) so that the matter is transformed into a relatively homogeneous melt (gelatin¬ ized) , at the same time as sufficient heat energy is provid- ded to the blowing agent to initiate the decomposition process. The process is controlled to ensure that decomposit ion of the blowing agent essentially occurs outside (efter)

the extruder (including the tool). After leaving the extrud er the plastic matter will, due to the high pressure in the gradually decomposed blowing agent, expand until equilibrium is achieved between the internal pressure and the tensions in the plastic matter.

The process is continuous and may be made automatic to a ver high degree. It provides great capacity with relatively low investments in technical equipment. A cellular plastic pro¬ duced according to this method, however, commonly has quite a lot of open cells. Also, it was not possible to produce cellular PVC plastic according to this method with an own weight below approx. 450 kg/cm . This may be explained by th fact that in a cellular PVC plastic produced according to this method the cells are formed by relatively large concentrations of blowing agent providing large separate cells. This means that -if too much blowing agent is adde.d large concentrations of blowing agent' will provide' forces of expansion in each single cell whic forces are so high that they exceed the capacity of the plastic matter to hold back the expansion gas, resulting in the fact that the gas breaks through the surface and escapes. The final product, thus may have a higher specific weight than when less blowing agent is used, and besides the surface will be broken and unsatis¬ factory in many instances. Commonly, prescriptions and process conditions are aimed at to provide the lowest poss¬ ible specific weight with a satisfactory surface of the produc .

A third method for the production of cellular PVC plastic is extrusion of a plastic mixture consisting of PVC plastic, stabilizer, and possibly process zontrolling substances, and additives. The different components are mixed well at a relatively low temperature (e.g. 110 C) before the plastic mixture is fed to the extruder. The mixture is melted at a higher temperature in the extruder at the same time as the mixture is homogenized. After melting a gas (e.g. CO-, or ) is added to the melt at a high pressure.

The gas is dispersed in the melt and causes the plastic matter to expand after having left the extruder, due to a reduction of the external pressure. This method is very rational and can provide very low specific weight of the PVC product (as low as approx. 30 kg/m ). However, the final product has open cells and will, thus, eTg. absorb water even at very low pressures.

In most cases closed cells are desired in cellular PVC plastic. Absorption of liquid is then minimal and the cell¬ ular plastic, furthermore, shows very good thermical insulation properties. In many cases physical properties will also be improved.

The present invention combines the favourable chemical and physical -properties of cellular PVC plastic with closed cells produced according to the above mentioned high pressure process with the very efficient production of cellular -PVC plastic in an extruder or another suitable apparatus for treating plastic. The method comprises utilization of a homogeneous PVC material containing PVC plastic, stabiliser, chemical blowing agent, and any desired process controlling substances and additives, with the chemical blowing agent being finely divided in the material. Fine division of the chemical blowing agent is achieved by plasticising and homogenizing the plastic mixture to a homogeneous melt to obtain a well gelatinized intermediate product. In this gelatinizing process the temperature of the melt is kept below the decomposition temperature of the chemical blowing agent in combination with any activator, and other components in the plastic mixture, or at a temperature level which together with the remaining components of the plastic mixture and the process parameters will not, or will only to an ignorable degree result in decomposition/vaporizing of the blowing agent. Said gelatinizing process may be carried out in an apparatus for plastic treatment which

complies with the requirements to temperature, mixing effect, and pressure, e.g. an extruder.

The composition of various substances in a plastic mixture must be adapted, inter alia, to the kind of mixing apparatus, apparatus for further treatment, and requirements ^~' of the final product. However, the plastic mixture must be composed so as to permit production of a well gelatinized intermediate product without resulting in decomposition/vaporisation of the blowing agent, or only causing decomposition/vaporization to a negligible degree. Most chemical blowing agents used today, e.g. azodicarbonamide, have a decomposition temperature of approx. 235 C. Commonly, the decomposition temperature must be reduced to approx 170°C-180 C so that difficulties with degradation of the PVC material are avoided. This is achieved by addition of an activator or "kicker" for the blowing agent. As a rule a metal compound based on Pb, Zn, Cd, or Sn' is used. The PVC plastic used may be an emulsion-plastic (e-PVC), a mass PVC (m-^ VC) , and suspension-PVC .(s-PVC) , or combinations of the same. Also, a copolymer of the PVC plastic, e.g. vinylacetate, may be used.

The gelatinized PVC is advanced to the next treating phase which may occur in the same apparatus for plastic treatment or in another apparatus for treating plastic which is suited for further treatment, e.g. an extruder or an injection moulding machine. The plastic matter may be fed in the shape of a granulate, or a powder and it may be introduced in a hot state from the gelatinizing process without any cololing og the plastic worth mentioning. In this manner energy saving is achieved in the production of cellular PVC plastic.

In this phase of treatment the PVC matter is rapidly heated to a temperature above the decomposition temperature of the chemical blowing agent in combination with any activator, and other plastic mixture components , at the same time as the plastic is subjected to a high external pressure. This is done to avoid that the blowing agent can expand in the machine it-

self. The plastic is mechanically treated, kneaded in the machine so that all portions of it are heated to approxim¬ ately the same temperature. Due to the fact that a homogeneous temperature is rapidly reached there is relat¬ ively good control of any possible exothermic generation of heat from the blowing agent which is a great problem, inter alia, in the known static high pressure process used today and mentioned above. Due to the rapid heating of the entire matter which is achieved by the mechanical treatment very rapid decomposition of the chemical blowing agent is also achieved. This entire process may take 3-5 minutes, as comp¬ ared with 40-60 minutes in the high pressure process used today.

When the plastic material is discharged from the machine with a possible extension the external pressure on the plastic is reduced and the plastic expands until equilibrium is achieved between the internal pressure in the blowing agent and the _^ tensions in the material. Due to the fact that the material expands at a high temperature. (160-230 C) it is very soft and a high degree of expansion is achieved. Such high temperature may be used because most of a added head stabilizer is active in this step of the process. It may be necessary to reduce the temperature of the plastic material slightly before ex¬ pansion in case of certain prescriptions and kinds in order to prevent the material from -collapsing after expansion. After expansion the PVC material is cooled. It may then be subject¬ ed to f rther treatment in accordance with requirements. The finished PVC product has very finely dispersed closed cells.

As mentioned above, when composing a plastic mixture/prescript _¬ ion it is necessary to consider the machine for plastic treat¬ ment to be used and to consider the properties desired of the final product. If an extruder is used both for the mentioned gelatinizing process, and an extruder for the process of de¬ composing the blowing agent, the following plastic mixtures will result in a hard cellular PVC elastic with closed cells:

Example 1

PVC ( -value approx. 60) 100

Approx. - stearate 0

Paraffin wax 0. Tin stabilizer 5

Kicker (ZnO) 3

Blowing agent (ADA) 4

PMMA 10

Pe-wax 0. 0 Plasticizer (DOP) 10

Example 2

PVC (K-value approx. 60) 100 Approx, -stearate 1 5 Paraffin wax 1 Blowing agent (ADA) 5 PMMA . • ^■ 15. Pe-wax 1

Plasticizer (DOP) 8 0 Pb-sulphate 10

Example 3

PVC (K-value approx. 60 ) 100 _c Approx. -stearate 1 Paraffin wax 1. Blowing agent (ADA) 6 PMMA 15 Pe-wax 1 Plasticizer (DOP). 12

Pb-sulphate 12

The different components of the above plastic mixtures are mixed in a suitable mixer, e.g. a so called turbo mixer until a temperature of approx. 110°C is reached. Then the mixture is cooled and fed to a plastic treating machine, e.g. an extruder. The mixture is treated, mixed, melted, and homogenized to a homogeneous melt with a maximum temperature of approx. 150°r;

which is slightly below the decomposition temperature of the chemical blowing agent in combination with the other compon¬ ents of the bixture. This process, also called gelatinization process, results in a homogeneous melt containing finely distributed chemical blowing agent which is not or only decomposed/vaporized to a negligible degree. The gelatinized material is conveyed to the next phase of the treatment, which may occur in the same plastic treating machine, or to a separ¬ ate plastic treating machine for further treatment. The gelat- inized plastic material may be advanced to the new treating phase/machine either with a temperature only s_lightly lower than the temperature during the gelatinizing process or well cooled and, in that case, possibly chopped into fine grains, so called granulated.

In this plastic treating machine which may be an extruder, an injection moulding* machine, or another suitable plastic ^* treating machine, the gelatinized plastic material is mechan¬ ically treated/kneaded at the same time as the temperature is- increased above the decomposition temperature of the blowing agent in combination with the remaining components of the plastic mixture, in case of the plastic mixtures mentioned above approx. 175 C. The external pressure in the machine is kept above the expansion pressure of the blowing agent gradual- ly decomposing. In order to achieve a sufficient degree of decomposition/vaporization of the blowing agent the material is mechanically treated/kneaded commonly for at least 3-5 min. at said temperature and with said pressure. The hot material, possibly slightly cooled, is then discharged through the orifice of the plastic treating machine and the external press _¬ ure is either immediately, or successively reduced to normal atmospheric pressure. The hot plastic material, thus, expands until an equilibrium is reached between the internal pressure of the blowing agent and the tensions of the plastic material. The plastic material is then cooled, e.g. by air or water and may be treated, e.g. mechanically, if desired.

In order to improve the physical properties of the final

product, especially at high temperatures, and at the same time improve the chemical properties, it is possible to cross-link part of the molecules of the plastic material. This may be achieved, e.g. by use of peroxides which are cross-linked at a high temperature or by adding a component to the material that may be cross-linked by addition of high energy, e.g. by use of gamma rays. Cross-linking may also be achieved by adding a component to the plastic material which is chemically cross- linked without addition of especially high temperatures. Such a component, or additive may, e.g. be an isocyanate. In order to prevent cross-linking to occur too early it will commonly be correct to use a time-delaying component in connection with use of a chemically acitve cross-linking agent. For the same reason chemically active cross-linking agents should, in some cases be added at the latest moment practically possible in the process, e.g. immediately before the decomposition phase of the blowing agent.

The invention offers the possibility of producing hard as well as soft cellular PVC plastic with closed cells, depending on the amount of plasticizer ^' added to the plastic mixture.

Cellular PVC plastic with closed cells may, e.g. be used for the following product ranges:

Thermal insulation (e.g. in refrigerator cars and of gas/oil piping)

Buoyancy products (e.g. floats, buoys, and pontoons)

Structures (e.g. light-weight sandwich-struct¬ ures having cellular PVC plastic as a core material, e.g. -in walls, boats, and aircraft).