Method and apparatus for producing thermoplastic and products produced therefrom

We claim:

1. A method of forming extruded products comprising: providing an extrudable material to be extruded; in a first step, simultaneously mixing and heating said material to at least the melting temperature thereof at which point said material is in the form of a relatively non-flowable, self-sustaining mass in a partially molten state, said mass having a relatively low density and having entrapped air therein to help prevent said mass from being flowable; in a second separate step, feeding said relatively low density, non-flowable self-sustaining mass in a partially molten state between a fixed rigid surface and a moving rigid surface of a gear pump to remove said entrapped air from said mass and to densify, compress and convert said mass to a flowable form; and extruding said densified and compressed flowable product.

2. A method as defined in claim 1 for forming a molten resinous profile wherein said first step comprises subjecting a source of resin to a high intensity mixing step and forming a heated, high bulk resinous mass having a temperature of at least the melting point of the resin but not high enough to cause fluid flow thereof and wherein the mixing is carried out until the material is in the form of said non-readily flowable mass, and said second step comprises densifying said high bulk resinous mass into a molten flowable resinous mass by passing the mass between said fixed rigid surface and said moving rigid surface and said extruding is carried out directly through a profile die to form an extruded profile shape.

3. A method as defined in claim 2, wherein the source of resin comprises a base material and at least one master batch additive.

4. A method as defined in claim 3, wherein said material is extruded through a die, the extruded material thereafter being cooled and pelletized.

5. A method as defined in claim 3, wherein said material is comprised of a polyolefin in the form of a copolymer or homo-polymer, said master batch additives are incorporated into said material prior to said mixing and heating said material, and which comprises a further step of pelletizing the extruded material to form master batch pellets of enhanced properties.

6. A method as defined in claim 5, wherein said material includes an additive to form a master batch chosen from coloring agents, forming agents, and the anti-oxidants, anti-block compounds, U.V. inhibitors, and mixtures thereof.

7. A system for extruding products comprising in combination,

mixing and heating means which simultaneously heat and mix a material to be extruded to at least the melting temperature thereof and to a point where the material is in the form of a non-flowable, self-sustaining mass and which is in a partially molten state, the mass having a low density and having entrapped air therein to help prevent the mass from being flowable;

means for feeding the mass in this state to a separate compression component;

said compression component comprising a fixed rigid surface and a moving rigid surface of a gear pump for densifying and compressing said mass to a flowable form; and

means for extruding the resultant densified and compressed flowable product through a restricted opening.

8. The system of claim 7, said gear pump having a pair of counter rotating gears.

9. The system of claim 8, wherein said gears are angled gears, said gears being adapted to compress a charge of material between the gears and a flat surface to form said densified and compressed mass to a flowable form, said system including discharge means for discharging the flowable mass after passing between said gears and said flat surfaces.

10. The system as defined in claim 8, including a die connected to said restricted opening to form an extruded product having a profile shape.

11. The system of claim 7, wherein said mixing and heating means comprises a high density mixer of the "Draiswerke" type.

12. A method as defined in claim 2, for forming a profile shape in the form of siding, wherein said profile die has a shape to form siding, and extruding siding through said die as a continuous length thereof, and cooling the extruded siding.

13. A method as defined in claim 12, wherein said resin comprises a polyvinyl chloride resin as a homopolymer or copolymer thereof.

14. A method of re-processing film-forming scrap thermoplastic material into raw thermoplastic material and to have substantially the same extrudable properties as the virgin material from which the scrap material was derived from, comprising, feeding discontinued scrap material to a mixing step, simultaneously mixing and heating said material to at least the melting temperature thereof to a point where said material is in the form of a relatively non-flowable, self-sustaining mass in a partially molten state, said mass having a relatively low density and having entrapped air therein to help prevent said mass from being flowable, in a second, separate step, feeding said relatively low density non-flowable self-sustaining mass in a partially molten state between a fixed rigid surface and a moving rigid surface of a gear pump whereby said entrapped air is forced from said mass and said mass is densified, compressed and converted to a flowable form; and extruding said densified and compressed flowable product while maintaining the temperature of said partially molten mass at a temperature below the volatilization or degradation temperature of non-film forming foreign matter of said scrap material.

15. A method as defined in claim 14, wherein the scrap material is comprised of film-forming scrap thermoplastic material having a volatile ink component as a major portion of the foreign matter.

16. A method as defined in claim 14, wherein said partially molten material is passed through a filter to screen out any non-molten foreign material.

17. A method as defined in claim 14, wherein the film-forming thermoplastic material is polyolefin or a mixture of polyolefins.

18. A method as defined in claim 17, wherein said polyolefin is a polyethylene or polypropylene, or a copolymer thereof.

19. A method as defined in claim 14, including the further step of pelletizing the extruded material after densifying said mass.

20. A method as defined in claim 19, wherein the pelletized material is fed into a blown tube process and utilized at least a portion of the feed material for a blown tube process.

21. A method as defined in claim 1 for forming blown film or tube, wherein said material is extruded through a blown tube die and which includes the further step of cooling the tube after extrusion and collapsing the tube to form a blown tube.

22. A method as defined in claim 21, wherein the molten compressed mass is fed directly to a blown tube process and used as a feed polymer for said process.

23. A method for producing a co-extruded product comprising the steps of providing at least two separate feed streams of a thermoplastic resinous material by mixing and heating each feed stream separately to at least the melting temperature of the resinous material and at which point the material is in the form of a relative non-flowable, self-sustaining mass in a partially molten state, the mass having a relatively low density and having entrapped air therein to aid in preventing the mass from being readily flowable, providing sepeate flow paths for each of the streams while in a second, separate step, feeding each relatively low density, non-flowable, self-sustaining mass in a partially molten state separately between a fixed rigid surface and a moving rigid surface of a gear pump to remove said entrapped air from the mass and to densify, compress and convert the mass of each thermoplastic resinous material to a flowable form, combining the streams in the flow direction in a common flow path into a single layer of material and extruding the combined layers as a single extrudate.

24. A method as defined in claim 23, wherein the extrudate is subjected to a pelletizing step to form pellets.

25. A method as defined in claim 23, wherein each of said feed streams comprising a different thermoplastic resinous material, each of said thermoplastic resinous materials having at least one different property relative to each other to form a co-extruded product of two resinous materials each of which has at least one different property relative to the other.

26. A method as defined in claim 23, wherein at least one of the feed streams includes a cross-linking agent, a colouring agent, or a foaming agent.

27. A method for forming a corrugated tube product which comprises subjecting a source of resin to a high intensity mixing step and forming a heated, high bulk resinous mass having a temperature of at least the melting point of the resin but not high enogh to cause fluid flow thereof and wherein the mixing is carried out until the material is in the form of a relatively low density, non-flowable, self-sustaining mass in a partially molten state and having entrapped air therein, and in a second, separate step densifying said high bulk, relatively low density, non-flowable, self-sustaining resinous mass in a partially molten state, into a molten flowable resinous mass by passing the mass between a fixed rigid surface and a moving rigid surface of a gear pump, extruding directly through a tubing die to form an extruded tube shape, and cooling said tube and corrugating said tube.

28. A method for forming a blown tube product comprising subjecting a source of resin to a high intensity mixing step and forming a heated, high bulk resinous mass having a temperature of at least the melting point of the resin but not high enough to cause fluid flow thereof and wherein the mixing is carried out until the material is in the form of a relatively low density, non-flowable, self-sustaining mass in a partially molten state and having entrapped air therein, and in a second, separate step densifying said high bulk, relatively low density, non-flowable, self-sustaining resinous mass in a partially molten state into a molten flowable resinous mass by passing the mass between a fixed rigid surface and a moving rigid surface of a gear pump, extruding directly through a blown tube die to form an extruded tube, expanding the tube and subsequently cooling said tube.

29. In a method of forming extruded products wherein a source of resinous extrudable material is mixed, heated and fed to an extrusion die, the improvement wherein in said mixing and heating step, said resinous extrudable material is formed as a relatively non-flowable, self-sustaining mass in a partially molten state, said mass having a relatively low density with entrapped air to help prevent said mass from being flowable; and in a second, separate step, utilizing said relatively non-flowable, self-sustaining mass in a partially molten state as a feed material between a fixed rigid surface and a moving rigid surface of a gear pump to form a flowable mass with a substantial portion of said entrapped air removed therefrom.

Description Submit all comments and votes

This invention relates to the extrusion of molten resinous materials.

More particularly, this invention relates to a novel combination of components to provide an apparatus for extruding resinous material; and to the sequence of steps for the extrusion of molten resinous material. Both method and apparatus have several embodiments in which different forms, profiles or the like of material are extruded and produced and each aspect will be described according to its characteristics.

One development herein relates to a method and apparatus for producing a master batch; another to the method and apparatus for producing profile shapes, e.g., siding materials, corrugated tubing, etc.

Various methods of master batch preparation are currently utilized. Master batches include a carrier resin and an additive to obtain the desired characteristics, e.g. for producing plastic film, one can extrude master batch pellets with natural or base film resin to provide the resulting extruded film with desired characteristics. Typical additives include color additives e.g. lead chromate, TiO.sub.2, iron oxide, ultramarine blue; U-V inhibitors, anti-block additives, cling additives, slip additives, (coefficient of friction), anti-static additives, etc. Combinations of the above and others may be utilized depending on desired characteristics. Master batches contain a wide range of additives, e.g., 2 to 50% or more, the remainder comprising resin.

Typically master batch ingredients are mixed in e.g. a Banbury mixer, whereafter the output is erased through a colander or two-roll mill, through a water bath and finally, to a pelletizer.

A disadvantage of such prior art is that the solids content of the master batch has not generally reached levels higher than 50%. In attempting to achieve higher solids the mill will not feed consistently since slippage occurs on the rolls due to the high solids content. In addition, if the rolls are not carefully temperature controlled, other processing disadvantages occur--e.g. if the roll temperature is too high, the product will tend to stick to the rolls; if the roll temperature is too low, product slippage on the rolls occurs resulting in inconsistent feeding. Carefully controlled temperatures are a requirement for such a process. Mixers of the type currently in use with such systems are very costly (a large capital outlay is necessary) and such mixers have a high energy consumption. Both Banbury and conventional extruders are heated with elelctricity or oil. The process is also slow and the amount of residence time of the polymer is fairly long--hence the energy used in such processes far exceeds that theoretically required to melt and mix the ingredients.

Another common prior art process for master batch includes blending the master batch ingredients in a high intensity mixer; then the blend is cooled and fed to a twin-screw extruder where it is extruded through a strand die. The strands may be cut off on the face of the die into pellets, or subsequently pelletized. This product typically has 50 to 60% solids content. With such a system, it has not been possible to process master batch with solids content higher than 60%. One reason for this is the long residence time of the ingredients within the extruder. When processing master batch which contains, for example, iron oxide as a coloring additive, the desired yellow color may change to red if the mixture remains too long in the extruder. Temperature may thus have an effect on the final color. The temperature in an extruder can vary and it is very difficult, if not impossible, to control the extruder temperature.

Such systems usually require the use of processing aids--e.g., low molecular weight polyethylene which acts as a lubricant, dispersing agents such as stearates, etc. These processing aids are solely for facilitating the production of the master batch and have no use whatsoever for the end user. Such processing aids, are subject to degradation and tend to form deposits on the inner surface of the die as well as build-up on the die lips both in the master batch and subsequent processes, resulting in shut-down time and high maintenance costs.

A further limitation on such systems is output. Generally, a typical twin screw extruder output is restricted to 360 to 540 Kg./hr., restricted by the extruder itself or by the liquid feed to extruder.

Another factor in the prior art is that the selected carrier resin must have the appropriate melt index. If not, various problems can be encountered in extruded film from the master batch, e.g., a high melt index can result in a loss of strength of the film.

Another development disclosed herein relates to profiles--e.g. siding for houses, etc;, trim for vehicles, window frames or extruded moldings in general. Siding is one example of such extruded profiles, e.g. "polyvinyl chloride" siding. At present, it is produced from raw materials which have different properties depending on the supplier. Thus a powder form of the resin with additives is passed through a conventional single or twin extruder and an extrusion die. Problems arise due to machine direction surges or variations resulting from extruder operation. Such a variation causes "oil-canning" (waviness) which is due to differential cooling as the output from the extruder is thicker in sections than the average of the extrudate. In the case of polyvinyl chloride siding, as the materials are very temperature-sensitive, a 2 or 3 degree variation makes a large difference in the product; to provide uniform extruding temperatures, manufacturers normally run at a very slow speed to try to obtain a fairly good product. They generally run only at 135 to 270 Kg./hr. Current methods and equipment for siding, trim, etc. require that on shutting a machine down, with e.g. PVC resin, undesirable by-products can result and clean-up times are substantial. In practice actual operating conditions determine the impact strength of the final product, which is temperature and stress dependent. Operating parameters are very stringent and variations can produce scrap material or undesirable characteristics in the end product. Such products find many uses in industry ranging from window frames to moldings, consumer products such as fridges and stoves, sheeting which finds many applications, swimming pool coatings, and various plastic moldings. They are widely used, but tend to be expensive because of the slow production time and the current equipment that is used.

Another development of this invention relates to a method of re-processing film-forming scrap thermoplastic material into raw thermoplastic material, which possesses substantially the same extrudable properties as virgin material.

Processing of thermoplastic materials generates significant amounts of scrap material in, e.g., film-forming operations. In conventional blown tube operations, a blown tube is formed into various types of products or articles ranging from garbage bags to sheet. At various stages, scrap film or extrudate is generated, e.g. cuttings, faulty production runs, etc.

At present, most scrap is sold to specialized firms who reprocess the scrap and re-pelletize the material or sometimes, in an in-house operation, the same material will be used in different operations. Re-processed film material can normally only be used for lower grade products once re-pelletized--e.g., refuse film or bags. For many years, since this type of product is made from a lower grade of film, typically scrap film, which may have a wide range of colours, a refuse film or bag manufacturer would add a colouring agent to "hide" the multitude of colours which scrap film from different sources. Also, scrap film from different sources will contain different additives in varying amount and consequently, re-processing the film into anything but a type of film or product which does not depend on a narrow range of properties is difficult.

Re-processing of film or scrap material through a conventional extruder invariably leads to a lowering of the properties of the base material. Polyethylene which has been extruded two or more times will have a much lower melt index compared to the base or virgin material, since it will have been subjected to heating, shear and other processing characteristics twice. As the resin is oxidised in these processes, this leads to "off-spec" resins, as well as discoloration. Thus re-processed materials are normally unsuitable or unusable for the purpose for which the material was initially extruded.

Re-processed material with lower properties such as melt index obviously cannot command the price of the higher once-extruded material and in certain cases, the use of re-processed material is forbidden in certain products due to degradation of additives in the scrap material, etc.

It would be desirable if some method were found for re-processing scrap thermoplastic material into raw thermoplastic material to have substantially the same extrudable properties without the disadvantages outlined above; applicant has developed a method for re-processing scrap material which overcomes the disadvantages of the prior art.

In a further development, there is also provided an improvement to a blown tube process. The blown tube process is well known; it involves extruding a molten polymer in generally a barrel-type extruder, passing the polymer through a die creating a bubble which is cooled with the bubble subsequently being collapsed (and the tube optionally being slit into sheet or formed into smaller tubes).

Barrel extruders employed for this purpose, and their production capacity is limited by several factors; e.g. the amount of polymer that can be extruded, the cooling capacity of the equipment, etc. Even on very large diameter dies, the practical limitations are such that only limited amounts of polymer can be extruded per hour--in the range of the very low hundreds of Kgs.

Modifications have been proposed to increase the output. In normal extruders, however, there are limiting factors such as the required residence or dwell time; feeding pressures and temperatures for dies, etc. which have inherent limitations beyond which further output increases become marginally advantageous. It would be advantageous if a system could be developed for a blown tube process where a die was fed with a much higher output, thus enabling significant economical advantages to be realized.

Another development disclosed herein relates to a co-extruded product and a method involving two or more separate feed streams which are joined together, after processing, to form a laminate.

Various techniques for producing co-extruded products are known; these may e.g. involve the blown tube method or a flat die. Such methods employ a conventional extruder to provide a flowable molten liquid mass of a thermoplastic material; two or more such extruders feed molten resinous material which is then combined into a single circular or flat die, and the resulting product of two layers is extruded. Prior techniques for co-extruding are relatively slow and limited by the type of thermoplastic material which can be satisfactorily processed and by the type of extruder. Conventional barrel or screw-type extruders have the disadvantages outlined, including high operating temperatures so that processing of certain materials, e.g. various additives, cannot be either successfully achieved or only with loss of properties.

Conventional co-extruded materials, possibly with different additives in the different layers or of different materials (i.e., different thermoplastic resins) have application in a wide variety of products ranging from bags or containers, sheet or film for covers, cable jacketing, etc. In most cases, requirements for commercial applications include different layers having different properties achieved either by utilizing the same type of material with different properties or with two different materials. Depending on compatibility of the resins, a very strong bond between the layers can be achieved or a very weak bond can result--in which case, the co-extruded material may also find application for "peel apart" products.

In co-extruded products, there are also requirements for the layers to be able to be processed differently so that even where the same material is involved, different properties will be imparted by different processing techniques. In other cases, with the same or different feeds of resin, it can be desirable for one or more layers of a multi-layered laminate to form a cross-linked resin or for another layer to contain a foaming agent, etc.

Cross-linking agents, and even foaming agents, are normally highly temperature sensitive (e.g., pero