Technical Field of the Invention

The present invention relates to a method of extruding a polymer having crosslinked properties, and the products that are produced therefrom.

More precisely, the invention relates to a method of reclaiming scrap crosslinked polymer. This process results in an extrudable composition wherein the crosslinked particles are comminuted and dispersed to levels never before achieved. The exdrudate may be in any form used in the art, but it is preferably pelletized. This invention also encompasses the reduction of residual monomer levels, and capturing the volatile emissions associated with these type of processes.

This invention is also drawn to the blending and extruding of the above mentioned extrudate with a thermoformable polymer to form a thermoformable thermoplastic sheet having a smooth matte finish.

Background of the Invention

In recent years there has been an increasing need to develop improved methods of reusing the scrap of thermoplastic products such as acrylic and methacrylate polymers. One method that has been developed involves the depolymerization of

acrylic polymer and subsequent recovery of monomer. This depolymerization process has the disadvantage of producing volatile emissions. Additionally, environmental concerns have made this process of little value on a commercial scale.

Several techniques well known in the art can be used to convert acrylic scrap into a fine sized powder. This powder has utility in itself. For example, acrylic powder is an effective blasting medium in paint removal processes. If acrylic powder is ground to a size less than 250 micron, it can be incorporated as a thermoplastic sheet filler. This process can enhance the impact resistance, thermoformability, and surface appearance of a thermoplastic sheet. One problem inherent to this process is grind consistency. Commercial processors cannot maintain an economically reliable consistency, nor can they achieve reliable stability within the grind.

Laminates of combined crosslinked and thermoplastic substrates have been put to wide industrial use. The term crosslinked refers to a polymer wherein a substantial portion of the molecular chains are linked to each other. Here, the same commercial need for scrap reclamation exists. It is impossible to fully segregate the scrap of these products into their component parts. This makes treatment of the individual components impossible. Additionally, the discards of these laminates cannot be comminuted to a size less than 250 micron for incorporation into a thermoplastic sheet.

A great deal of attention has been given to developing polymer processing equipment known as screw-type extrusion devices. Some of these devices are known for their ability to produce high temperatures and impart a large amount of shear onto polymeric compounds. For example, McCullough et al. in U.S. Patent 4,663,103 discloses an apparatus to perform a method of extrusion. The apparatus is designed to accept various disparate materials and ultimately process these materials into highly filled thermoplastic material.

There are many examples in the art of improvements to the basic screw-type extruder. These improvements are usually tailored to a particular need in the art. Brooks et al., in U.S. Patent 5,096,406, discloses a novel assembly for a screw-type extrusion device. The device is drawn to adapting a screw-type extruder for use with composites containing wood and other cellulosic materials dispersed in a continuous phase of a polymeric material. In contrast to the present invention, the inventors here sought to avoid the shear rate and temperature associated with basic screw type extruders.

Many other optimization improvements to screw-type extruders have been made. Two of particular interest to applicants' disclosure are the twin-screw extruder and the addition of kneader blocks to screw configurations. These improvements impart a larger torque load, thereby producing the high degree of shear and temperature mentioned above.

Another addition to the art has been the development and improvement of pelletizer devices. These devices can be attached to screw-type extruders. The extruded material passes through a die thereby shaping it into strands that can be cut by rotating blades to form pellets that are desirable for packaging, shipment, mixing, and other beneficial uses recognized in the art.

Use of improved extruder technology is not new to reclamation processes. Jameson, in U.S. Patent 5,238,633, discloses a method and apparatus for recycling plastic waste into a thin profile, mechanically reinforced board. This process employs screw-type extruder technology. However, Jameson does not address the problems associated with the practically nonexistent art of crosslinked polymer extrusion.

In spite of the many improvements in extruder, pelletizer, and polymer reclamation technology, there is no commercially effective method of comminuting a predominantly (or completely) crosslinked polymer to mean particle sizes less than 10 microns. As a result, crosslinked acrylic or methacrylate scrap could not be effectively co-extruded with a thermoplastic polymer to form a thermoformable sheet. There are several reasons for the above mentioned defiάiencies in the art.

Reducing the mean particle size of crosslinked polymers to 10 microns or less is desirable, but until now, no one skilled in the art had developed a commercial means to accomplish this

end. Another problem is that, at or above the ceiling temperatures of many polymers deformation into monomer constituents takes place. The rate of deformation increases with higher temperature, a greater degree of shear, and the presence of radical initiators. As a result volatiles will be generated, and removal and disposal of these volatiles is necessary.

It is the object of the present invention to provide a method of extrusion whereby predominantly (or completely) crosslinked polymers can be fed into a screw-type extruder, effectively comminuted, and dispersed if necessary. It is a further object of this invention to extrude the effectively comminuted crosslinked polymer into pellet form. These pellets can then be blended with a thermoformable polymer as they are subsequently extruded into a thermoplastic sheet which is thermoformable and has a smooth matte finish.

A further object of this invention is to employ the techniques of underwater pelletization and vacuum venting. These techniques reduce residual monomer formation and capture the volatile emissions associated with the high temperature and high shear environments of applicants' invention.

Summary of the Invention

Predominantly (or completely) crosslinked polymer sheet material is reduced to a size of 0.5 inch or less by any means. These coarsely-ground products are then fed separately or in combination to a twin-screw extruder. The source of the

predominantly (or completely) crosslinked polymer sheet material is not important. Scrap crosslinked polymer is common, and can be derived from sources that use crosslinked polymers, such as acrylic and methacrylate sheets, in commercial processes.

In the present invention, applicants do not wish to place undue limitations upon the conditions within the twin-screw extruder. The particular design of the extruder is not critical provided the capabilities that follow are met.

Screw configuration is not found to be important. However, it is found to be beneficial to have the predominantly (or completely) crosslinked polymer processed at a temperature above the glass transition temperature for that polymer.

State-of-the-art twin-screw extruders are capable of sustaining large torque loads thereby producing high shear zones. By high shear, applicants mean a system wherein a shear stress is sufficient to comminute the particular crosslinked particles to sizes of 10 microns or less. Applicants have found a shear stress level of 10 Pascals or more imparted onto the particles to be sufficient. However, with varying conditions within the extruder, high shear may be obtained at different shear rate levels.

Multiple shear zones may be used. Typical twin-screw extruders have between 7 and 11 barrel elements. The temperature range along the barrel element train may vary. Temperatures typically range between 25° to 285°C from feed barrel to die head.

The use of kneader blocks within the extruder is found to be beneficial. These blocks may be strung together in multiple groups separated by conveying elements to successively shear and mix the polymer. Applicants' method teaches the use of at least one kneader block group.

Within the extruder the crosslinked polymer is reduced in size as it passes through the high shear zone(s). The polymer size reduction is most effective when high levels of crosslinked feed are used. If a combination crosslinked/uncrosslinked polymer feed is used, it may be augmented with the addition of a crosslinked polymer in order to obtain higher weight percentages of the crosslinked polymer.

As stated above, the range of weight percentage is one of preference. Applicants' method will produce the disclosed results with a crosslinked polymer weight percentage between 35 and 100 wt% of the feed stream. However, best results are obtained when said crosslinked content is about 80% of the feed stream.

It is possible to process 100% crosslinked polymer products by this method, but the practical realities of this commercial process dictate a feedstock containing about 2 to 65 wt% of uncrosslinked thermoplastic co-blend. This practice facilitates the subsequent blending of additional thermoplastic in a sheet extrusion process. The sheet quality of such a blend is much improved as compared to a product which is a blend of 100% crosslinked polymer and a subsequently added thermoplastic.

The extrudable crosslinked polymer product may be processed at the die end of the extruder by any method in the art. It is an object of this invention to employ an underwater pelletizer to reduce the level of residual volatiles in the extrudate. Applicants have discovered that an underwater pelletizer will reduce residual monomer levels of the product to less than 0.5 wt%. It has also been found that emitted volatiles can be effectively removed during extrusion by the use of vacuum venting techniques known in the art.

After the pelletization of the crosslinked product, a second extrusion process can take place. As previously stated, the crosslinked pellets can be blended with a thermoplastic material and extruded into a thermoformable sheet product. This thermoplastic material can be added in an amount to be about 80 wt% or less of the combined extrusion product and thermoplastic material. This second extrusion can take place by any appropriate extrusion means known in the art.

Description of the Preferred Embodiment

Crosslinked polymer scrap products suitable for processing by this invention include, but are not limited to products made by the following processes: continuous casting, cell casting, and extrusion where the product has been subsequently treated to impart molecular crosslinking. Preferably, the crosslinked polymer is poly (methyl methacrylate) copolymer. That is, poly (methyl methacrylate) is mixed with

insignificant amounts of one or more comonomers such as butyl acrylate, ethylene glycol dimethylacrylate, 2-ethyl hexyl acrylate, and others. These comonomers may be present in amounts up to and about 6 wt% of the poly (methyl methacrylate) copolymer. Their presence is attributable to the commercial advantages over pure poly (methyl methacrylate) polymer. The poly (methyl methacrylate) copolymer should be about 80 to 100 wt% crosslinked. That is, at least about 80 wt% of the molecular chains are linked. A copolymer having less than 80 wt% of its molecular chains crosslinked will work. However, applicants' invention accomplishes more difficult extrusions associated with copolymer having 80 wt% or greater crosslinkage.

The thermoplastic component may be any suitable uncrosslinked thermoplastic. This group includes, but is not limited to acrylonitrile butadiene styrene (ABS), uncrosslinked acrylic polymers, and uncrosslinked methacrylate polymers. The thermoplastic applicants prefer is ABS.

The poly (methyl methacrylate) copolymer is prepared for processing by comminution by any means to the maximum size that can be accommodated by weigh-belt or volumetric feeders and accepted in the throat of a twin-screw extruder. In the most preferred embodiment crosslinked poly (methyl methacrylate) copolymer scrap fed by a weigh-belt feeder, and a minor stream of ground acrylic/ABS laminate scrap fed by volumetric feeder are used. The laminate scrap contains about 40 wt% acrylic and is fed to the compounder in about a 1:5 ratio to poly (methyl methacrylate) copolymer scrap.

The preferred screw element configuration of the twin-screw extruder has co-rotating intermeshing screws, a short heating/conveying zone, and from about one to four kneader block sections, with the first kneader section fairly close to the feed throat. Sufficient capacity for vacuum venting is provided and is placed just prior to the die adapter section.

For the preferred embodiment, temperature zone settings are typically set for a 7 to 11 barrel element extruder, and range from about 127-207°C. A typical extrudate temperature measured at the die head is 302°C.

In larger machines (70mm and larger) underwater pelletization quickly quenches the extrudate thereby minimizing residual monomer in the product. When stranding and water-trough cooling is used in the larger machines, the strand is bubble-laden, difficult to catch, and unstable in operation. In either mode of operation, pellet floaters and low bulk densities signal unacceptable levels of monomer retention.

The pelletized products are then co-extruded with the thermoplastic polymer to produce a thermoformable sheet having a smooth matte finish.

The following are examples of the application and use of the invention. They are for illustration purposes, and in no way are to be construed as limiting the invention in any manner.

Example 1

A Werner Pfleider ZSK-58 co-rotating compounder was configured in a 7 barrel element with four kneader block screw sections, and a temperature range of 240-285°C for each zone. The feed was a blend of 90 wt% scrap Aristech Altair™ 1-300 acrylic (80 wt% crosslinked and granulated to -3mm) and 10 wt% ABS regrind. A stranding die, water trough, and chopper were used to process the output.

The output was 225 lbs/hr at a screw speed of 279

RPM, and a torque level of 850 newton-meters (80% maximum) . This produced shear levels in excess of

5 10 Pascals. The melt temperature at the die was

278°C. Typical crosslinked particles were from 1 to

10 microns in size at the die head. The bulk density of the pellet product was 37.4 lbs/cu ft.

The product was compounded again with additional ABS in a single-screw extruder to a let-down of 40 wt% acrylic. This product was thermoformable and displayed a smooth matte finish. The sheet impact by the Gardner impact method was 210 in-lbs/in thickness.

Comparison Example 1

A continuous-cast crosslinked acrylic sheet, 1-300, was ground to -250 micron. This powder was metered by a volumetric feeder into the hopper throat of a 6.5 inch vented, single-screw extruder, together with an equal amount of regrind ABS scrap. Sheets of 0.2 inch thickness were produced, which had about 50% acrylic content. Sizes of the crosslinked particles ranged from 90 to

250 microns. The sheet was thermoformable, and had a matte finish. The impact, measured by ASTM D-3029, method G, geometry GB, averaged about 120 in-lb/in of thickness. Impact strength proved to be inferior to those achieved by applicants' method.

Example 2

A Werner Pfleiderer 2SK-70 co-rotating compounder was configured in a 9 barrel element with two kneader blocks along the screw section, and a temperature range of 149-207°C for each zone. The feed was a blend of 83 wt% 1-300 acrylic scrap (granulated to -3mm) and 17% Aristech Altair TM Plus scrap (granulated to -3.35mm). An underwater pelletizer die with 50 holes of 0.110 inch diameter was used for product takeoff. The bulk density of the product was 38 lbs/cu ft. Its residual monomer level was 0.25 wt%. Typical crosslinked particle sizes ranged from 1 to 10 microns. The output was

1000 lbs/hr at a screw speed of 425 RPM and a torque level of 1750 newton-meters (98% maximum). We achieved shear levels in excess of 10 Pascals within the system. The product was compounded again with additional ABS in a single-screw extruder to a let-down of 45% acrylic. The sheet impact, measured by ASTM D-3029, method G, geometry GB, was 234 in-lb/in thickness. The sheet was thermoformable and had a smooth matte finish.