WO2001045918A1

WO2001045918A1 - An apparatus and method for extruding a multilayered elastomer product

Info

Publication number: WO2001045918A1
Application number: PCT/US1999/030465
Authority: WO
Inventors: James Alfred Ii Benzing; Patrick John Reilly; Dale Roy Norton; George Samuel Fielding-Russell
Original assignee: The Goodyear Tire & Rubber Company
Priority date: 1999-12-20
Filing date: 1999-12-20
Publication date: 2001-06-28
Also published as: AU2202900A

Abstract

An array of dies (70, 80) can be used in a method and apparatus for making a composite article that comprises multilayered rubbers. In the method and apparatus, at least two extruders (12, 12a) are used to extrude at least two rubber streams (19, 19a, 23, 25) through the die array (70, 80), to provide a rubber extrudate (28, 28a) comprised of alternate layers (29, 29a, 29b) of rubber from the at least two rubber streams. The rubber compositions in the two rubber streams can be the same or different. The extrudate (28, 28a) may have a profile that makes it suitable for application directly in a composite article.

Description

AN APPARATUS AND METHOD FOR EXTRUDING A MULTILAYERED ELASTOMER PRODUCT

Technical Field

The present invention relates to a method and apparatus for extruding a multilayered elastomer, which is used in elastomeric composites. Background Art

It is known in the art to mix liquid materials using a series of flow channels that cause the liquid stream to split and then flow together, such as described by Petraschek et al. in U.S. Patent 3,736,972. Layered rubber articles comprised of many alternating layers of different rubbers have been described by Frerking in U.S. Patent 5,178,702, wherein air barrier/low temperature properties are improved in horizontally layered composites. Such composites can be prepared by hand by plying up alternating layers of two or more different rubber compounds.

Equipment used to layer thermoplastics has been described by Shrenk and Alfrey in U.S. Patents 3,773,882; 5,202,074; 3,884,606; and 5,094,793. This equipment relies on the low viscosity ofthe melted polymers and cannot be used with high viscosity rubbers.

Schmidt et al. in EP 049 6202 A2 describe a method whereby two different rubber compositions, each rubber containing a portion of a curative package, are separated from one another, and are mixed together immediately before their incorporation into a final product. Sluijters in U.S. Patent 3,051,453 describes a mixing apparatus designed to mix two streams of liquid by splitting and rejoining the streams in a particular geometric way (a static mixer), which the instant inventors have found can be used with rubber to produce a layered, instead of a mixed product. Reilly et al. in U.S. Patent 5,866,265 have used this concept to split elastomer streams and have caused the streams to flow back together to form a layered material. Said patent is incorporated herein by reference.

The apparatus and method taught by Reilly et al. comprised a single set of stacked dies and was useful for proving that the concept could be used to produce a layered material for laboratory characterization. No apparatus or method is known in the art for making commercially usable microlayered elastomeric materials. Mueller et al., in Polymer Engineering and Science, Feb. 1997, Vol. 37, no. 2, pp 355ff, describe thermoplastic structures made by microlayer coextrusion. Examples illustrated in the article show the versatility of the coextrusion process for horizontally layered thermoplastics of small cross section. The references cited at the end of the article provide more background on the concepts used.

It is an object of this invention to provide an apparatus and a method whereby commercial multilayered materials, in a number of variations, may be provided.

Other objects ofthe invention will be apparent from the following description and claims. Summary of the Invention

An apparatus for forming layered rubber material comprises (a) an extruding means (12,12a) for extruding rubber, (b) a plurality of die plates (20,20a,22,22a,24,24a) attached to the output end of the extruding means (12,12a) for receiving extruded rubber (19,19a,23,25), wherein the die plates (20,20a,22,22a,24,24a) have a separation means whereby extruded rubber (19,19a,23,25) is split into portions by the separation means, and the portions are directed to be stacked and pressed against each other by a profile die (26,26a).

The apparatus may further comprise collecting means for collecting extrudate (28,28a) as it exits the plurality of dies.

In the illustrated embodiment of the apparatus, the extruding means (12,12a) is a pair of extruders or a pair of injection mold extruders oriented head to head.

In the apparatus, a plurality of die arrays (70,80) may be oriented in a linear relationship of sufficient length to provide a full width dimension of a composite component extruded from the die plates.

The separation means may comprise is a septum (34), or the sides of die channels (1 l,32,32a,33,33a, 72,74,82,84), depending on the orientation ofthe die plates.

In the illustrated embodiment ofthe apparatus, die plates (20,20a,22,22a,24,24a) are stacked upon one another such that rubber (19,19a,23,25) passes from a first die plate (20) into a second die plate (22), and is split into portions as it enters the second die array (70,80), and is channeled such that the split portions are stacked, and this process is repeated for each subsequent die plate (20,20a,22,22a,24,24a).

The die arrays (70,80) may have a left-hand horizontal, right-hand horizontal, left-hand vertical, or right-hand vertical orientation.

Also claimed is a die array (70,80) for extruding plastic and elastomeric materials comprising (a) a plurality of die channels (l l,32,32a,33,33a, 72,74,82,84) having alternating configurations designed to direct the flow of a first portion of a plastic or elastomeric material in a first direction, and the flow of a second portion ofthe material in a second direction, whereby the first portion and second portion are directed together upon exiting the die channels (1 l,32,32a,33,33a, 72,74,82, 84).

The die array comprises means for splitting the flow of plastic or elastic material in the die array (70,80) into at least two portions. Also provided is a method of preparing an elastomeric material comprising the steps of, (a) providing extruding means (12,12a) for simultaneously extruding at least two different rubber streams, (b) directing a first rubber stream into a first die channel (l l,32,32a,33, 33a, 72,74,82,84), (c) directing a second rubber stream into a second die channel (l l,32,32a,33,33a,72,74,82,84), whereby the first die channel and the second die channel direct the first rubber (19,23) and the second rubber (19a,25) to be stacked and pressed side by side as it exits the die channels (l l,32,32a,33,33a,72,74,82,84), and (d) providing a plurality of die channels (l l,32,32a,33, 33a, 72,74,82,84) side by side to form a die array (70,80) to provide an extrudate (28,28a) of a specific width wherein a plurality of first rubber portions and a plurality of second rubber portions are stacked alternately side by side.

The method may further comprise the steps of (e) stacking a plurality of die arrays (70,80) wherein separating means associated with die arrays (70,80) divides a stream of rubber into at least two portions, wherein each divided portion is stacked and pressed against another divided portion as it exits the die arrays (70,80), and (f) collecting extrudate (28,28a) having a specified width and a specified number of stacked rubber portions.

The method may also comprise the steps of configuring each die array (70,80) to have horizontally oriented channels with a right-hand or left-hand configuration, and configuring each die array (70,80) to have vertically oriented channels with a right-hand or left-hand configuration.

The method may comprise the further steps of providing at least two different rubber compositions for the at least two different rubber streams, providing at least a first rubber stream containing rubber curing initiators but no rubber curing accelerators, and at least a second rubber stream containing rubber curing accelerators and no rubber initiators, and providing a first rubber stream wherein the cured rubber from the rubber stream has a hardness of at least Shore A - 30, and providing a second rubber stream wherein the cured rubber has a hardness of at most Shore D -70. Also, in the method, the at least two extruding means 12, 12a may be run at different speeds.

The method may comprise the further step of applying the extrudate (28,28a) as a component in a composite article.

Also claimed is a composite article made using the method. Brief Description of Drawings Fig. 1 illustrates two extruders, head to head, for extruding two streams of elastomeric material through a die to form a multilayered extrudate.

Fig. 2 illustrates a perspective view ofthe portion ofthe apparatus where elastomer enters the die, with horizontal layers being produced on exiting the die.

Fig. 3 illustrates a schematic representation of the apparatus of Fig. 2 wherein the representation is turned 90° on its axis from Fig. 2.

Fig. 4 illustrates an alternative embodiment wherein the gates of the die channels have a vertical orientation and are used to extrude an elastomeric product with vertical layers.

Figs. 5a, 5b, 5c, 5d and 5e illustrate a right-hand configured die array wherein the gates ofthe die channels are oriented for horizontal extrusion ofthe layers.

Figs. 6a, 6b, 6c, 6d and 6e illustrate a right-hand configuration of a vertical die array wherein the gates ofthe die channels are oriented for vertical extrusion of elastomeric layers. Detailed Description ofthe Invention

With reference now to Fig. 1, two extruders 12 and 12a are illustrated oriented head-to-head whereby the two extruders are used to extrude the same or different rubber or plastic, or mixtures thereof, simultaneously through barrels 14 and 14a. Extruders 12 and 12a may be single screw extruders, mixing extruders, such as twin screw extruders, or they may be injection mold extruders.

Those skilled in the art will recognize that other orientations ofthe extruders are possible, e.g., above and below, and side by side, wherein the result is that two different elastomer streams enter two different channels in a die array 70,80 (Figs. 5 and 6) and are maintained in separate channels until they exit the die.

With reference now to Fig. 2, portion 16 of Fig. 1 is magnified to show the detail of the die configuration used with extruders 12 and 12a. In Fig. 2, a wedge or V-block 18 is used as a directing means to direct elastomer streams 19 and 19a in the direction of a stack of die plates 21. Mixing of the elastomer streams would prevent the stated purpose of the invention, i.e., providing separate, layered elastomers and accordingly, the apex 18a of the V-block is held tightly against die array 70.

Die plates 20, 22, and 24 each comprise a die array 70, i.e. a linear arrangement of a plurality of separate die channels 11. Die channels 11 in each die array 70 are paired, and each pair of die channels 11 are configured to handle at least two streams of elastomer wherein the streams are routed so that they merge as they exit the die plate. In addition, each subsequent die array 70 in the stack of die plates 21 has die channels 11 which are configured to again divide each portion from the previous die into two portions, and to route each portion ofthe stream through the die array 70 such that the streams again converge as they exit the die plate. The output from the last die array 70 in stack of die plates 21 passes into profile die 26, which consolidates the individual streams into extrudate 28. In this manner, the original two streams of elastomer are divided and stacked producing a function of 2ⁿ layers of material in the extrudate 28 that exits the profile die, where n is the number of die arrays 70 in stack of die plates 21.

In the illustrated embodiment, in the stack of die plates 21, alternating die plates have a different hand configuration. For example, if die plate 20 has a right hand configuration, then die plate 22 has a left-hand configuration, etc. The left-hand configuration is the mirror image ofthe right hand configuration. This makes it possible to provide a vertical array of dies since, e.g., the die array in die plate 20 moves the elastomer streams to the right, and die plate 22 moves the elastomer streams back to the left, so that their vertical offset during the extrusion does not change.

Those skilled in the art will recognize that the elastomer streams may be pushed continuously to the right, for example, if the stack of die plates 21 is configured to allow for the offset.

With reference now to Fig. 2, a perspective view ofthe relationship of wedge 18 to die array 70,80 is illustrated wherein the wedge 18 is shown as transparent so that its relationship to the whole die array can be seen.

As is apparent in Figs. 5 and 6, on one side of a die plate, the openings ofthe die channels are oriented horizontally (i.e. the length direction ofthe opening is aligned parallel to the length dimension of the die plate), and on the opposite side of the die plate, the die channel openings are oriented vertically (i.e. the length dimension ofthe opening is aligned peφendicular to the length dimension of the die plate). To be consistent with nomenclature in the extruding arts, whatever the orientation of a die opening, if the die plates are oriented so that rubber exits a die channel through the opening, the opening is referred to as a gate.

In the illustrated embodiment, tip 18a of wedge 18 is pressed tightly against die array 70,80 to limit the mixing ofthe rubber streams 19,19a,23,25, before the streams enter the die channels. When horizontally oriented layers of extrudate are produced, as in Figs. 1 to 3, the rubber streams enter the vertically oriented channel openings and exit the horizontally oriented channel openings. As the rubber streams enter the vertically oriented channels, they are split by the vertical walls ofthe channels, since the vertical openings in the channels are split, or divided into two parts by wedge tip 18a. As the rubber streams exit the die plate, septum 34 is oriented at a 90° angle to the vertical openings of the channels in the next die plate, and the rubber streams are split again.

Those skilled in the art will recognize that other directing means can be used to direct the flow of separate elastomer streams into channels in a die array 70,80 without mixing the elastomer streams.

Wedge 18 has the advantage that the pressure exerted by each of the extruders is directed against pressure exerted by the other, whereas some other directing means may require special augmentation to handle the pressure from the two extruders.

Fig. 3 is a schematic representation of what happens to a first elastomer 23 and a second elastomer 25 as the elastomer stream passes through each die array of stack of die plates 21. As can be seen in the illustration, first elastomer 23 and second elastomer 25 each represent 1/2 the elastomer entering die plate 20, and as the elastomer stream passes through die plate 20 each is divided into two layers, and each of these layers is divided into two more layers in die plate 22. As the elastomer stream exits the subsequent die plate 24, first elastomer 23 and second elastomer 25 each represent eight of the sixteen layers. In Fig. 3, extrudate 28 comprises an elastomer stream that is divided into 2 x 2" separate distinct horizontal layers 29. n in this illustration is 3, and extrudate 28 comprises 16 horizontal layers.

With reference now to Fig. 4, in an alternative embodiment, die plates 20a, 22a, 24a and profile die 26a have a configuration where each die channel 32a,33a is oriented vertically such that in extrudate 28a, layers 29a and 29b of vertically oriented elastomer are developed.

Fig. 4 illustrates an idealized extrudate 28a. As illustrated in Fig.4, when the rubber streams 23,25 enter die plate 20a, the rubber streams are kept separate by septum 34 of die plate 20a as they enter the horizontally oriented die channels. As they exit die plate 20a, they exit through vertically oriented gates ofthe die channels, and are split by septum 34 that divides the horizontally oriented die channels of die plate 22a, as the rubber streams enter die plate 22a. This process continues through the stack of die plates 21, until the vertically oriented portions exit the last die array.

It should be noted that vertically oriented layers are prepared using die plates that are structurally the same as the die plates used in the horizontal extrusion, but the orientation ofthe plates is reversed for a vertical extrusion. That is, the flow of rubber is reversed through the die plates as compared to the flow for horizontal layering. In this case, the first die plate 24a does not provide a layering array, it merely orients the elastomer streams vertically. Hence, when the die channels are oriented to produce vertically oriented rubber portions, 2ⁿ vertical rubber portions are produced, as compared to 2 x 2ⁿ layers when horizontal orientation is used. When there are N die channels in a vertical array of die channels, N x 2ⁿ rubber segments or portions are produced in the extrudate having vertically oriented rubber portions. With reference now to Figs. 5a, 5b, 5c, 5d, and 5e, views of a right-hand horizontal configured die plates are shown. Die array 70 comprises a line of die channels 72, which are used to receive a first elastomer, and an adjacent line of die channels 74, which are used to receive a second elastomer. In the illustrated embodiment, a first die channel 32 of line of die channels 72 has a gate 36 which is offset one-half the width ofthe element from gate 36a of a second die channel 33, which comprises part of second line of die channels 74.

With reference now to Fig. 6a, 6b, 6c, 6d, and 6e, a right-hand vertical die array 80 is illustrated. As in the horizontal die array, the vertical array has a line of die channels 82 which are used to receive a first elastomer, and a line of die channels 84 which are used to receive a second elastomer. Die channels 32a and 33a are basically constructed the same as die channels 32 and 33, but their orientation is rotated 90° with respect the orientation of die channels 32 and 33 so that extrudate 28,28a exits from the die array in vertical layers, instead of the horizontal layers that are seen to exit from die channels 32 and 33.

Left-hand horizontal and left-hand vertical configurations can be illustrated in the same manner, the left-hand configurations being the mirror image ofthe right hand configurations.

In the illustrated embodiment, the die arrays 70,80 comprise 13 rectangular die channels l l,32,32a,33,33a, 72, 74,82,84, wherein each array has a width which produces an extrudate 28,28a about 6 inches wide. When a vertical array is used, the first insert reorients the incoming horizontal stream of two elastomers into a vertical orientation, and does not double the number of layers. Every subsequent insert in the die array stack doubles the number of layers thereby producing a total number of vertical layers in the 6-inch wide extrudate of 13 x 2ⁿ, where n is the number of die arrays 70,80 used during the extrusion. By contrast, when horizontal oriented die arrays are used, the number of horizontal layers is given by 2 x 2ⁿ, where n is the number of die arrays used during the extrusion. In the illustrated embodiment, the extrudate 28,28a is compacted by the profile die 26,26a to have a thickness of about 0.25 inch.

The shape of the extrudate 28,28a is controlled by the shape of the profile die 26,26a. A profile die shape may be chosen to provide an extrudate profile, which is designed to be used directly in an elastomeric product. For example, a 6-inch wide by 1 /4-inch extrudate can be used as a tread component in a pneumatic tire. When vertical oriented dies are used to make such an extrudate, the vertically oriented elastomer layers that result are believed to improve the abrasion resistance of a tire.

In a similar fashion, profiles may be provided to produce sidewall components, apexes, inner liners, etc. Those skilled in the art will recognize that the same concept can be used to produce components for belts, hoses and other types of elastomeric products.

The inventors have found that by varying the relative rpm (revolutions per minute) of extruders 12 and 12a, the relative thickness of the two layers can be controlled without modifying the configuration of the die arrays 70,80. During experimentation, extrudates with relative layer thicknesses of 8 to 1 were successf lly prepared. Those skilled in the art will recognize that higher ratios can be achieved by using higher screw speed ratios with lower viscosity rubbers, the ratio being limited by the desired extrudate properties. Extrudates with horizontally layered elastomers have been used experimentally as inner liners in tires, and extrudates with vertically oriented elastomers have been used experimentally in the treads of tires. The method ofthe invention has improved the permeability properties of an inner liner.

In the illustrated method of the invention, two extruders 12,12a which may be single screw extruders, mixing extruders, such as twin screw extruders, or injection mold extruders, are oriented head-to-head such that two streams of elastomer are extruded simultaneously toward a directing means, such as wedge 18, which directs a first rubber stream into a first channel of a pair of die channels, and directs a second elastomer stream into a second channel of a pair of die channels. The first elastomer and the second elastomer are directed by the first channel and the second channel to be stacked and pressed side-by-side as the two elastomers exit the die. In the illustrated method, a plurality of die channels ll,32,32a,33,33a,72,74,82,84 are provided side-by-side to provide an extrusion of a specific width wherein a plurality of first rubber portions and a plurality of second rubber portions are stacked. Having a specific width makes it possible to extrude a component for an elastomeric article, which has a width suitable for direct use in an elastomeric article. Those skilled in the art will recognize that the individual die channels can be of any size, and their number in the array can be varied. By controlling the size and the number of die channels, an extrudate of any size can be produced. This extrudate can have vertically or horizontally oriented layers, and based on the number of die arrays (n) used, and the number of channels in a die array can have a widely varied number of layers (a function of 2ⁿ). Furthermore, the relative thickness of the alternating layers can also be widely varied by control ofthe relative extruder speeds.

The method may further comprise stacking a plurality of die arrays wherein each die array has a separating means for dividing a stream of elastomer into at least two portions wherein each divided portion is directed by channels in the die array to be stacked with and pressed against another divided portion. The extrudate collected will have a specified width and a specified number of stacked rubber portions.

In the method, the stacked die arrays can be oriented to have vertical or horizontal channels, and may have a right hand or left hand configuration. In the illustrated embodiment, the die arrays are chosen to have alternating right hand and left hand configurations.

The method may further comprise providing at least two different rubber compositions to comprise the at least two different rubber streams, for example, a first rubber stream containing rubber curing initiators but no rubber curing accelerators, and a second rubber stream containing rubber curing accelerators and no rubber curing initiators, which can be described as splitting the curatives. Such split curatives may allow higher processing temperatures and higher speed production.

In addition, a first rubber stream may comprise a rubber that has a cured hardness greater than the cured hardness ofthe rubber in the second rubber stream.

Since viscosity of rubber decreases with temperature, processing is facilitated at higher temperatures. In prior art mixing apparatus, however, there is a risk that at higher temperatures, rubber will begin to cure in the mixing apparatus. In the method ofthe invention, the dividing the cure package for the rubber into two parts, and placing the two different parts of the curative package in different streams of extruded rubber, minimizes or eliminates the risk of cure in the apparatus. Since it is possible for the curatives to migrate in rubber during curing, if the extrudate layers are made thin enough, the two portions ofthe curative package will migrate together in the layers to permit cure of the layered product. For example, sulfur may be included in the first rubber stream, and zinc oxide and cure accelerators may be included in the second rubber stream, since all three are needed for an efficient cure.

In a further embodiment ofthe method, the solubility of various curatives in the polymers used to make a layered extrudate can be considered, and the curatives can be chosen based on their migration rate, to help the migration of curatives between polymer layers. To illustrate, when a curative is incoφorated into a polymer in which its solubility is low, the curative has a tendency to migrate out of the polymer at a faster rate than it would if it had a high solubility in the polymer.

Likewise, the curative will migrate faster into a polymer in which it has a high solubility.

These relative solubilities also are a factor to be considered when the required thickness ofthe elastomer layers in the extrudate is determined. The method may include the step of operating the extruding means 12,12a at different speeds so that the layers in the extrudate 28,28a have different thicknesses.

The different parameters of the method of the invention can be used to control desired properties in an elastomer product made using the extrudate.

The apparatus and method described herein can be used to make elastomeric products as described in co-pending PCT applications PCT/US99/21708 and PCT/US99/21694, which applications are incoφorated herein by reference. Other uses for the apparatus and method of the invention will be apparent to those skilled in the art.

The invention is further illustrated with reference to the following examples.

Example 1 This example illustrates the effectiveness of layering versus the conventional blending of rubbers.

Black stain-barrier and white sidewall compounds have been used to demonstrate permeability of horizontally layered compounds. Besides being readily available, these compounds were chosen because they provide simple visual observation of the number of alternating layers in the extrudate as well as the layer uniformity. Also, since both compounds are non-staining, layer delineation persists.

Besides their color extremes, the two compounds differ significantly in gas permeability, with the white compound being about four times the more effective gas barrier. The oxygen permeability of the compounds was measured at room temperature with the MOCON tester. The samples for the MOCON unit are cured sheets about 0.032 inch in thickness. These sheets were made from a 1/8 inch thick microlayered extrudate by pressing in a hot compression mold. The microlayered extrudate had eight horizontal layers of equal thickness, so that the MOCON sample had an effective layer density of 250 per inch and a composition of 50/50 by volume. The conventional blend was made by "remilling" the microlayered extrudate in a lab mixer until it was uniformly black. The composition ofthe conventional blend, based on the weight ofthe two polymers, was identical to that ofthe microlayered composite.

Three possible moφhologies of two component composites were tested. The "vertical" layer orientation, where the layers are oriented parallel to the thickness ofthe sheet, provides easy penetration of gas through the more permeable of the materials and the permeability of the composite is close to the permeability of the more porous barrier material. In contrast, in the "horizontal" morphology, where the layers are oriented perpendicular to the thickness of the sheet, the permeability of the composite is analogous to electrical resistance in series rather than in parallel: gas must pass through the high barrier layers instead of skirting them, in contrast to what happens with the vertical layering. In the conventional blend, the compounds are mixed together in discrete domains.

The permeability data on the pure compounds, the conventional 50/50 blend and the 50/50 microlayer composite are shown in Table 1.

Table 1

Material Permeability

(cc/lOOsq. in. /day/mil)

(White) 612

(Black) 2383

8-Laver Microlayer 963

50/50 by volume Blend 2175

The permeability of the layered composite, P, is independent of the number of layers, and with "perfect" layer integrity, would be predicted by the following formula:

P = Pb*P_w/(Pb* V_b + Pw* Vw)

Where Pb is the permeability of the black rubber, P is the permeability of the white rubber, V is the volume fraction of a component where Vb is the volume fraction of black rubber and V_w is the volume component of white rubber (0.5 in this example).

Substituting in the permeabilities of the two compounds from Table 1 yields a predicted value of 974 for a perfectly horizontal layered composite. The measured value was 963.

Examination of the layering in the microlayer sample showed that it was not perfectly regular and unbroken, but nevertheless, it's permeability was very close to that of perfect layering and shows that microlayering was highly effective in reducing gas permeation and was far superior to conventional blending.

Example 2

Black and white compounds were used with two layering inserts and a rectangular slab die to give a vertically oriented, 8-layer composite. At first, the screw speed of the two extruders was set at 15 RPM and the first sample of the extrudate taken. Extruder screw speeds were then varied as shown in Table 2.

Table 2: Extruder Running Conditions

It is apparent that the ratio of the layer thickness tracks very closely with the ratio of the extruder speeds. Example 3

Multilayer extruder technology can be used to mix, layer and shape conventional productive compounds. In this concept, the productive is not made until the moment of shaping the product, for example a tire component. The concept recognizes that, unlike the filler system, the curative package does not need high mixing forces for it to be distributed in a compound. The chemicals have an inherent solubility in rubber and they can be uniformly distributed by diffusion. The key requirement is that the distance of diffusion required for effective use should be small relative to the rate of diffusion.

The cure system is split between two non-productives, which have "infinite" scorch time. These are fed separately into the two extruders and are kept separate until they meet for the first time inside the microlayering insert ofthe extruder die. In this insert, the compounds are hot (and therefore the rate of interdiffusion of curatives is fast). The alternating layers of each nonproductive can be made to any thinness (so that the required diffusion distance can be optimized). The productive so produced flows immediately through the shaping die to form a tire component. In this sense it is an "in-situ" productive. The heat history seen by this "in-situ" productive is much less than that seen by productives made by the conventional process. In fact, the entire heat history seen by the "in-situ" productive is that occurring during its passage through the microlayering insert/die. Consequently faster curing compounds can be used.

It is desired to have a uniform distribution of curatives throughout a green component, so that the physical properties do not vary within the cured component. This should be achievable by adjusting the thickness ofthe microlayers by changing the number of layering die inserts on the extruder. Thinner microlayers will favor more rapid and more uniform curative distribution. In addition, adjustment ofthe relative extruder screw speeds could allow tailoring the cure package.

This example describes the initial evaluation of "in-situ" productive rubbers using the microlayering method.

A productive sheet about 17.8 cm (7 inches) wide and 0.32 cm (1/8 inch thick) was made containing either 8 or 32 alternating horizontal layers of the split-cure non-productives. The thickness ofthe layers of split-cure non-productives in the sheets was therefore 0.038 and 0.01 cm (0.015 and 0.004 inch) respectively. For the 8 layer sheets, the die set temperature was 99° C (210° F). For the 32 layer sheets, two die temperatures were used 99° and 132° C (210° and 270° F). Both extruder screws were run at 10 RPM (revolutions per minute) in order to obtain a productive with a 50/50 composition of the two split-cure non-productives. The sheets obtained at the 99° C (210° F) die set temperature were buckled due to unequal shrinkage, but at the 132° C (270° F) die temperature, the nerviness of the rubber was reduced and smooth sheets were obtained. No sign of scorch was seen in any ofthe sheets. The number of microlayers needed in the component determines the number of layering die array inserts that are required: 4 layers requires 2 inserts, 8 layers requires 3 inserts, and 16 layers requires 4 inserts, and so on. It is desirable to use the minimum number of layering inserts required to give a uniform dispersion of curatives, because the extruder head pressure increases with the number of inserts. As a benchmark, samples ofthe 8 and 32 layer gum sheets were passed through a mill 10 times without banding, in order to completely disperse the curatives. These samples represent the final, "equilibrium" state of curative dispersion. The cure rheometer curves of the microlayered stocks and those that had been milled were measured at two temperatures, 120° C and at 135° C, using both the ODR and MDR cure rheometers. The cure time of the productive sheets was exceptionally short, for instance, a T90 of 4 min at 135° C was seen in the 32 layer sheet. A sheet of this compound could not be made by the conventional process of Banbury mixing, followed by calendering, because it would scorch.

At 120° C, the ODR cure time was typically 10.5 min for the 32 layer sheet. The 120° C

ODR cure rheometer curves of 32 layer samples taken throughout the run supeφosed on each other, suggesting that the composition ofthe cure package created in the microlayer extruder was very consistent. The 8 layer sheet samples had greater variation in cure curves, and this is attributed to the non-equilibrium distribution of curatives in these samples.

The ODR curves at 135° C had more scatter than at 120° C, possibly because the thermal lag time in the ODR is a significant part ofthe very short cure time. The ODR and MDR cure parameters are collected in Tables 3 and 4 below. They show that the curatives do indeed diffuse between the microlayers to create an "in-situ" productive. Furthermore, the process of interdiffusion is complete in the 32 layer sheets, because the cure curves of the 32 layer sheet are identical to those of the milled 32 layer sample (where the distribution of curatives is uniform). In contrast, the diffusion process is not complete in the 8 layer sheet. Its cure time is longer than the 32 layer sheet and is shortened by milling. It is noted that cure time of the milled 8 layer sheet is identical to the unmilled 32 layer sheet. This is again consistent with the 32 layer sheet having an equilibrium distribution of curatives. It therefore appears that the microlayer thickness of 0.01 cm (0.004 inch) is small enough for complete interdiffusion of these particular curatives to occur, but that 0.04 cm (0.015 inch) is not thin enough.

Table 3 : ODR Cure Rheometer Data

Table 4 MDR Cure Rheometer Data

From these rheometer data alone it is not possible to deduce whether the curatives were completely interdispersed in the die system ofthe extruder or whether the process was completed at a later time, such as during the heating seen during the cure In any event, the cure rheometer infers that complete dispersion of curatives was obtained in the cushion gum sheet having 32 layers

A productive cushion sheet of retread cushion gum having a T90 at 120° C of about 10 minutes, can be made successfully using "in-situ" productive technology The sheet could not be made using conventional processing - Banbury mixing followed by calendering The sheet is made from split-cure compounds which have unlimited scorch time and shelf life

Complete dispersion of curatives is inferred in the cushion gum sheet having 32 layers, I e a thickness ofthe split-cure non-productives layers of 0 004 inches This was made using a stack of four layering inserts in the extruder die

Claims

1. An apparatus for forming layered rubber material comprising

(a) an extruding means ( 12, 12a) for extruding rubber

(b) a plurality of die plates (20,20a,22,22a,24,24a) attached to the output end of said extruding means (12,12a) for receiving extruded rubber (19,19a,23,25) wherein said die plates (20,20a,22,22a,24,24a) have a separation means whereby extruded rubber (19,19a,23,25) is split into portions by said separation means, and said portions are directed to be stacked and pressed against each other by a profile die (26,26a).

2. The apparatus of claim 1 which further comprises collecting means for collecting extrudate

(28,28a) as it exits said plurality of dies.

3. The apparatus of claim 1 wherein the extruding means (12,12a) is a pair of extruders or a pair of injection mold extruders oriented head to head.

4. The apparatus of claim 1 wherein said plurality of die arrays (70,80) are oriented in a linear relationship of sufiBcient length to provide a full width dimension of a composite component extruded from said dies.

5. The apparatus of claim 1 wherein the separation means is a septum (34).

6. The apparatus of claim 5 wherein each die array (70,80) comprises 13 pairs of die channels (1 l,32,32a,33,33a, 72,74,82, 84) having a linear relationship.

7. The apparatus of claim 1 wherein die plates (20,20a,22,22a,24,24a) are stacked upon one another such that extrudate (28,28a) passes from a first die array (70,80) into a second die array (70,80) and is split into portions as it enters said second die array (70,80), and is channeled such that the split portions are stacked, and this process is repeated for each subsequent die plate (20,20a,22,22a,24,24a).

8. The apparatus of claim 4 wherein said die array (70,80) has a length sufficient to produce a full width tire tread.

9. The apparatus of claim 1 wherein said die arrays (70,80) have a left-hand horizontal, right- hand horizontal, left-hand vertical, or right-hand vertical orientation.

10. A die array (70,80) for extruding plastic and elastomeric materials comprising (a) a plurality of die channels (l l,32,32a,33,33a, 72,74,82,84) having alternating configurations designed to direct the flow of a first portion of a plastic or elastomeric material in a first direction, and the flow of a second portion of said material in a second direction, whereby the first portion and second portion are directed together upon exiting the die channels (1 l,32,32a,33,33a,72,74,82,84).

11. The die array of claim 10 which further comprises a splitting means for splitting the flow of plastic or elastic material in the die array (70,80) into at least two portions.

12. The die array of claim 10 which comprises 13 pair of die channels (1 l,32,32a,33,33a,72,74,82,84).

13. A method of preparing an elastomeric material comprising the steps of

(a) providing extruding means (12,12a) for simultaneously extruding at least two different rubber streams,

(b) directing a first rubber stream into a first die channel (1 l,32,32a,33,33a,72,74,82,84), (c) directing a second rubber stream into a second die channel

(ll,32,32a,33,33a, 72,74,82, 84), whereby said first die channel and said second die channel direct said first rubber and said second rubber to be stacked and pressed side by side as it exits the die channels (ll,32,32a,33,33a,72,74,82,84), and

(d) providing a plurality of die channels (l l,32,32a,33,33a, 72,74,82,84) side by side to form a die array (70,80) to provide an extrudate (28,28a) of a specific width wherein a plurality of first rubber portions and a plurality of second rubber portions are stacked alternately side by side.

14. The method of claim 13 comprising the further steps of

(e) stacking a plurality of die arrays (70,80) wherein separating means associated with said die arrays (70,80) divides a stream of rubber into at least two portions, wherein each divided portion is stacked and pressed against another divided portion as it exits said die arrays (70,80), and

(f) collecting extrudate (28,28a) having a specified width and a specified number of stacked rubber portions.

15. The method of claim 14 comprising the further step of configuring each die array (70,80) to have horizontally oriented channels with a right-hand or left-hand configuration.

16. The method of claim 14 comprising the further step of configuring each die array (70,80) to have vertically oriented channels with a right-hand or left-hand configuration.

17. The method of claim 16 comprising the further step of applying said extrudate (28,28a) as a component in a composite article.

18. The method of claim 14 comprising the further step of providing at least two different rubber compositions for said at least two different rubber streams.

19. The method of claim 17 comprising the further step of providing at least a first rubber stream containing rubber curing initiators but no rubber curing accelerators, and at least a second rubber stream containing rubber curing accelerators and no rubber initiators.

20. The method of claim 17 comprising the further step of providing a first rubber stream wherein the cured rubber from said rubber stream has a hardness of at least Shore A 30, and providing a second rubber stream wherein the cured rubber has a hardness of at most Shore D 70.

21. The method of claim 13 comprising the step of running the at least two extruding means (12,12a) at different speeds.

22. A composite article made using the method of claim 14.