Extrusion method and apparatus for producing a foamable mixture of plastics material

A method and apparatus are provided for producing a mixture of plastics materials which are capable of being foamed with propellants. The method permits plastics materials, which require considerably different processing conditions, to be fused, homogenized and mixed with a propellant in an extruder. By feeding a high-molecular weight high-temperature resistant plastics material into the extruder through a first feed opening and subsequently feeding a low-molecular weight, low-temperature-resistant plastics material into the extruder through a second feed opening disposed downstream of the first feed opening but where the first plastics material is in a fused and homogeneous state and by introducing the propellant through a third feed opening downstream of the second feed opening, the desired homogenisation of the two plastics materials and propellant is ensured.

FIELD OF THE INVENTION 
The present invention relates to an extrusion method and apparatus for 
producing a foamable mixture of plastics material. More particularly, the 
present invention relates to a method and apparatus of the type in which 
two extrusion devices are provided in tandem wherein the mixture of 
plastics material is homogenised with a propellant in the first extrusion 
device and is then fed into the second extrusion device in which it is 
cooled prior to being extruded in the form of a tubular film or a profile. 
BACKGROUND OF THE INVENTION AND PRIOR ART DISCUSSION 
In German Patent Specification No. 3 316 838 there is disclosed a method of 
producing a foam from a plastics material. The plastics material is 
processed in a tandem extruder system. In the first extruder, the material 
is fused and mixed with a propellant which is injected into the mixture. 
This first extruder is a single-screw extruder. Subsequently, the mixture 
of plastics material and propellant is conveyed into a second extruder 
wherein it is cooled to just below the reaction temperature of the 
propellant. The mixture is then extruded to form a tubular film. 
It has become increasingly more important for foamed plastics materials to 
have specific properties. The properties include, inter alia, 
flame-resistancy, high resistance to acids, and resistance to high 
pressures, temperatures and/or tensions. 
The production of such foamed plastics materials, based on high-quality 
plastics materials, presupposes that the products are capable of being 
produced in cellular or foamed form. This prerequisite is realised by the 
addition of supplementary materials which are suitable for producing 
foaming in extrusion systems because of their structure and properties. 
It has, however, been found that it is extremely difficult to mix a 
high-molecular weight or high-temperature-resistant plastics material 
(macromolecule) and a low-molecular weight plastics material, or even a 
plastics material which had been rendered flame-resistant by additives, 
with a conventional plastics material and to create a homogeneous product 
therefrom. 
On the other hand, it is commercially necessary to blend very expensive 
special plastics materials, such as, for example, PPO (polyphenylene 
oxide), with cheaper low molecular weight conventional plastics materials 
in order to reduce costs. 
An additional problem arises when it is necessary to mix the plastics 
material homogeneously with a propellant. Since the propellants usually 
employed, such as fluorochlorohydrocarbons, aliphatic hydrocarbons or 
azodicarbonamide have properties which differ considerably from the 
plastics materials to be processed, homogeneous mixing is very difficult. 
In particular, the propellants can only be processed within narrow 
temperature and pressure ranges. A lack of homogeneity in respect of the 
mixture of plastics material and propellant results, which leads to 
inconsistent foaming. Thus, for example, different thicknesses and cell 
sizes are produced in a foamed film, or weak spots are formed in other 
products. 
OBJECTS OF THE INVENTION 
The present invention therefore seeks to provide a method in which 
expensive and high-molecular weight or high-temperature-resistant, 
foamable plastics materials can be blended, for better processability, 
with conventional foamable plastics materials whilst still producing 
desired properties in the end product. 
The present invention also seeks to provide a method and apparatus in which 
the absorbtion and distribution of the propellant in the mixture of 
plastics materials is improved and which permits particular plastics 
materials to be mixed and homogenised in an extruder, the method 
permitting the adjustment of specific properties of a plastics material, 
such as its flame-, pressure-and heat-resistance. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a method of producing 
a foamable mixture of plastics materials from two plastics materials 
having differing properties in a tandem extruder system, said tandem 
extruder system comprising two first and second extrusion devices, said 
first extrusion device defining sequential first, second and third feed 
apertures and having a variable throughput. The method comprising the 
steps of feeding a high-molecular weight high-temperature-resistant 
plastics material into said first extrusion device through said first feed 
aperture, fusing said material in said first extrusion device, feeding a 
second low-molecular weight low-temperature-resistant plastics material, 
together with a blowing agent, through said second feed aperture in said 
first extrusion device into said fused, homogenised high-molecular weight 
plastics material and fusing and homogenising said plastics material and 
blowing agent therewith, introducing a propellant into said fused and 
homogenised mixture of said plastics materials and blowing agent through 
said third feed aperture in said first extrusion device, homogenising said 
mixture of plastics materials, blowing agent and propellant in the first 
extruder, transferring said homogenised and fused mixture into said second 
extruder, cooling said mixture in said second extruder and extruding said 
mixture in the form of a foamed tubular film or profile. 
In feeding the high-molecular weight, high temperature-resistant plastics 
material into an extrusion device having a variable throughput, the 
dwell-time of the material in the extrusion device can be controlled. 
Accordingly by varying the speed of rotation of the screws, which is 
responsible for the throughput, an operator can ensure that the plastics 
material is subjected to heating in the extrusion device for as long as 
necessary. A twin-screw extruder is preferably used, the two screws having 
flights which substantially mesh with one another but pass one another, so 
that substantially all material disposed between the flights is advanced 
and virtually no residue remains stationary. Because of this arrangement, 
the dwell time of the material can be appropriately influenced. 
Such plastics material, for example a high molecular weight polyphenylene 
oxide, is introduced into the first feed opening and is fused over an 
extruder length of up to ten times the diameter of the screws. 
A low molecular weight plastics material, such as polystyrene, which needs 
less time for fusing, is then fed into the fused mass of the first 
plastics material and is fused therewith. Since the high molecular weight 
plastics material is a fused mass when the low molecular weight plastics 
material is added thereto, the fusion of the low molecular weight plastics 
material therewith is facilitated. 
The low molecular weight plastics material is considerably less 
temperature-resistant and the addition thereof to the fused mass of the 
high molecular weight material does not cause thermal overloading of this 
plastics material. The homogenisation of the two plastics materials as 
fused masses is thus considerably enhanced. 
A propellant is then injected into this fused mass of plastics materials 
and, in dependence upon the materials being treated, the mixing and dwell 
time of the propellant with the mixture of plastics materials is selected 
by suitably controlling the variable throughput. 
If a plastics material to be fused is one which fuses very slowly, such as 
a styrene-maleic acid anhydride polymer, to which, for example, a 
flame-retardant component has previously been admixed is introduced in 
granulate or powder form through the first feed opening, the rate of 
rotation of the screw or screws is reduced and consequently the dwell time 
of the material to pass from the first to the second feed opening is 
increased. 
This arrangement also ensures that the first plastics material is fused 
when it reaches the second feed opening, whereby the fusion process for 
the second plastics material is shortened. 
The melting points of the two plastics materials and the fusion indices 
thereof are taken into consideration and, in consequence thereof, the 
fusion and homogenisation process is adapted to the individual properties 
of the plastics materials. The requisite processing conditions of very 
expensive plastics materials, which often constitute the high molecular 
weight material, are also taken into particular consideration. 
The use of a chemical propellant has the advantage of being more 
environmentally desirable. The prerequisite therefor is that the extruder 
can be driven without pressure in the feed region of such a propellant. 
This is because the chemical propellant is normally incorporated into a 
plastics granulate or into a plastics powder and is then fed into the 
twin-screw extruder in granular or powder form. 
If a chemical propellant is used, it has proved advantageous to provide a 
greater screw length for the twin-screw extruder, such extra length being 
at least approximately four times the diameter of the screws and extending 
downstream from the propellant feed opening. This achieves better mixing 
and homogenisation of the propellant in the fused mass of plastics 
material. 
The introduction of the chemical propellant is effected, for example, after 
a screw length of 28 times the diameter of the screws and the 
homogenisation of the fused mass with the granular or pulverulent 
propellant is advantageously effected in the subsequent screw length of 
from four to six times the diameter of the screws. 
Physical propellants may also be used. These are in a fluid form and are 
injected through a nozzle at a pressure which exceeds the pressure in the 
extrusion chamber. 
A shorter screw length is required for the mixing and homogenisation 
processes. 
A particularly advantageous effect is achieved if the blowing agent is 
introduced together with the low molecular weight plastics material, since 
the two constituents, within a minimal screw length, are mixed very 
intensively with the high molecular weight high temperature resistant 
plastics material, which latter is already in the form of a fused mass.

DESCRIPTION OF PREFERRED EMBODIMENT 
In FIG. 1 there is shown a tandem extruder system which comprises an 
homogenising extruder 1 and a cooling extruder 2. The homogenising 
extruder 1 is a twin-screw extruder. The screws 3 and 4 of the extruder 1 
have helical flights 5 and 6 respectively disposed thereon. The screws 3 
and 4 are rotatably disposed within a housing 7. 
The screws 3 and 4 are set in rotation by a transmission unit, including a 
drive mechanism 8, the speed of which is steplessly adjustable. The 
flights 5 and 6 of the screws 3 and 4 intermesh with one another. This 
causes substantially all of a plastics material located within the 
extruder 1 and located in the troughs 9 and 10 formed between adjacent 
crests of the flights of the screws 3 and 4 to be advanced. Accordingly, 
the dwell time of the plastics material in the twin-screw extruder can be 
accurately set. By increasing or reducing the rate of rotation of the 
screws 3 and 4, therefore, any particular desired dwell time can be 
appropriately set for the specific plastics material located in the 
twin-screw extruder 1. 
The twin-screw extruder 1 also includes, in this embodiment, two feed 
hoppers 11 and 12. the spacing between the feed hoppers is up to ten times 
the diameter of the screws 3 and 4. This spacing, in conjunction with 
control of the rate of rotation of the screws and, hence, of the rate of 
conveyance of the materials, provides an adequate time of impact of the 
screws on the plastics material which is being introduced in granular 
form. 
The propellant is injected into the mixture of plastics material through a 
feed conduit 13 at a pressure which is slightly above the pressure 
subsisting in the twin-screw extruder 1. Alternatively, however, instead 
of providing the feed conduit 13, a feed hopper may also be provided if a 
chemical propellant is used which is introduced as a masterbatch in either 
powdered or granular form. 
In such a case, it has proved advantageous to provide another screw portion 
with a length of approximately five times the diameter of the screw 
downstream of the hopper so as to permit the propellant to mix 
satisfactorily with the fused mass before the mixture of material and 
propellant is conducted through the conduit 14 to the cooling extruder 2. 
In this latter extruder, the mixture is cooled to a temperature just below 
the foam point of the propellant. 
Subsequent, the fused mass of plastics material is extruded through the 
nozzle 15, which is shown schematically in FIG. 1, to form a tubular film 
or a flat profile, the foaming process occurring as the fused mass emerges 
from the nozzle 15. 
Examples of high-molecular weight, high-temperature resistant, 
low-molecular weight, low-temperature-resistant plastic materials, and 
propellants have been set forth above. It will be apparent that other 
materials could also be employed. For example, the high-molecular weight, 
high-temperature resistant plastics material could additionally comprise 
an acrylobutadiene styrene polymer, a polycarbonate polymer, a polyvinyl 
chloride polymer, a polyacrylosulfon polymer and a thermoplastic 
polyurethane polymer. The low-molecular weight, low-temperature resistant 
plastics material could additionally comprise polyvinyl chloride. The use 
of azodicarbonamide as a propellant could be incorporated into a fused 
plastics material selected from polyolefins and waxes. The propellant 
could further include the inert gasses carbon dioxide and nitrogen. The 
blowing agent could be selected from the group consisting of citric acid, 
sodium bicarbonate and talc.