Compounding Mica and Resin With Particulate Heat Sensitive Additives

This invention relates to compounding mica and certain melt forming resins with discrete, particulate heat sensitive additives, such as chlorinated waxes, that act as coupling agents therefor and includes, in particular, a compounding operation that permits advantageous temperature control and rapid development of desirable physical properties.

U.S. Ser. No. 970,822 and 970,839, each filed Dec. 18, 1978 by Meyer and Newman disclose that certain chlorinated aliphatic compounds can act as coupling agents for mica and certain melt forming resins.

French Pat. No. 2,124,315 discloses a variety of techniques for compounding mica into an organic polymeric matrix and suggests dispersing the mica particles in thermoplastic resin prior to attainment of processing conditions.

A process has now been discovered that provides for compounding of mica and melt forming resin with such heat sensitive additives as chlorinated waxes at times less than a minute, if desired, whereby molding precursors are prepared that may be used in conventional molding operations to yield molded objects of highly desirable physical properties.

The precise conditions under which such compounding is accomplished are quite critical. Chlorinated aliphatic compounds such as heat sensitive chlorinated waxes herein are subject to reduction or even elimination of beneficial effects upon undesired decomposition with heat. Such decomposition is particularly acute during compounding with melt forming resin and mica particles since the mica particles act as heat sinks that are insulated by the resin during compounding and may, under conditions of high shear and mechanical heat generated thereby during intense mixing, cause compounding temperatures in excess of that which causes undesired decomposition, even though cooling is attempted.

On the other hand, low temperatures during such compounding, even if attainable, can lengthen the time required for coupling agents such as chlorinated waxes or other chlorinated aliphatic compounds to exhibit desired effect and consequently cause perhaps less efficient use of equipment or modification of existing equipment so as to extend periods at elevated temperature.

Even still further, however, very high shear can even cause deterioration of melt forming resin to an extent that the filled resins are undesirably brittle even though other properties may be satisfactory. Thus, for example, the high shear used to provide a molten resin in the process of this invention has to be controlled so as to keep the melt index (ASTM D1238) of the compounded resin preferably below about 15.

The drawing is a schematic illustrating a twin screw extruder, preferred apparatus for compounding in accordance with this invention.

One screw of the twin screw extruder is illustrated. The other screw (not shown) has flights corresponding to flights of the screw illustrated except that the flights are reversed in angle, i.e., the reverse flights are at the same but negative from normal as the flight angles of the screw shown. The screws are non-intermeshing and counter rotating. The screw not shown is of similar diameter, but shorter by about the length of three diameters thereby allowing attachment of the barrel to a conventional strand die orifice.

This invention relates to compounding mica and certain melt forming resins with particulate, heat sensitive additives such as chlorinated waxes or other chlorinated aliphatic compounds that act as coupling agents therefor by, in general terms, making the resin molten (desirably with high shear), introducing mica particles previously blended with the particulate additives into the molten resin and kneading with reduced shear the mica particles and resin into a homogeneous melt.

In one preferred embodiment, the melt forming resin, e.g. polypropylene, is formed into at least two counter rotating, parallel molten streams that contact one another to form a turbulence therebetween, and blended mica and additive particles are introduced into such turbulence allowing their rapid and effective dispersement throughout the molten resin with low shear kneading.

Compounding in this manner permits temperature control not otherwise easily attainable when mica particles and melt forming resin are admixed under conditions of high shear. Such high shear can impart to the melt of resin and mica particles sufficient mechanical work as to generate temperature increase that is not easily controllable. Kneading or low shear mixing, however, permits effective mixing of the mica and molten resin in short periods when done in accordance with this invention so that the mica particles are not at high temperature for extended periods under shear forces that cause such uncontrollable temperature increase and consequent undesired degradation of heat sensitive additives.

Moreover, compounding in accordance with this invention does not expose the mica to shear forces that undesirably break such mica thereby exposing mica surfaces without heat sensitive coupling agents. Surprisingly, mica that is blended with particulate, heat sensitive additives may be compounded with melt forming resins such as polypropylene such that the total processing time at high temperature is as little as 15 seconds or less and yet provide molding precursors, e.g. pellets, which yield molded articles with desirable properties.

During compounding in accordance with this invention, it is particularly desirable that the melt forming resin receive high shear (prior to mica incorporation) that is sufficient to provide rapidly a molten mass of resin at desired temperatures but insufficient to cause significant change of the resin's properties. For example, it is preferred that a melt forming resin such as polypropylene having a melt index (ASTM D1238) below about 6 not undergo shear so as to raise its melt index (ASTM D1238) above 15, even more preferably such that it is not above 12. Otherwise, final composites will have an undesired brittleness.

On the other hand, it is preferred to provide molten resin at a temperature (prior to incorporation of the mica and additive into the resin) at least about 10.degree. F. below that temperature at which the additive first releases chlorine containing gas. To achieve maximum benefit of the additive in as short as time as possible, however, incorporation of mica and additive into the resin should be such that the resin has an initial temperature not more than about 60.degree. F. (preferably, not more than about 50.degree. F.) below that temperature at which the additive releases chlorine containing gas.

The melt forming resins that may be advantageously compounded with mica and heat sensitive additives in accordance with this invention include commercially available materials that are designed for use in molding (such as by injection, compression, stamping etc.) processes. Of these commercial resins thermoplastics made from monomers comprising olefinic hydrocarbons such as ethylene, propylene, styrene, etc. are of special interest in that they can yield molded, mica containing articles with high strength and desirable resistance to heat distortion as well as economic advantage.

Melt forming resins made from monomers comprising olefinic aliphatic hydrocarbon monomers such as ethylene, propylene and the like can be compounded especially well and particularly desirable results are achieved with melt forming resins sold as polypropylene resins e.g. Gulf Polypropylene 6420 MHP, Hercules Profax 6523 (Polypropylene. Normally such resins desirably have intrinsic viscosities above about 1.5 and preferably between about 2.0-2.6 for more expedient processing, although suitable modification will allow processing of resins with other viscosities. An important criterion, however, in selection of desirable melt forming resin is that it remain sufficiently molten at processing temperatures that do not degrade the efficiency of the heat sensitive additives. Moreover, the melt forming resin preferably has a melt index (ASTM D1238) below about 10, preferably between about 2-6.

Other resins besides polypropylene resins include polyethylene as well as copolymers made from monomers comprising ethylene, propylene or other lower aliphatic hydrocarbon monomers such as copolymers of ethylene and propylene.

Mica fillers suitable for use in this invention are also commercially available. The mica can be generally characterized as being an aluminum silicate mineral that can be cleaved into thin sheets. Examples of commercially available mica filler include those normally denominated as muscovite, biotite and/or phlogopite micas. Mica filler comprising principally phlogopite mica such as Suzorite mica marketed by Marietta Resources International, constitutes a mica that allows formation of molded articles with particularly desirable properties, especially when compounded as herein.

The mica filler comprises thin flakes of mica that are typically made by delamination of mica mineral. Mica, such as that comprising principally phlogopite mica, that is retained on a 100 mesh screen, desirably on a 60 mesh screen is preferred and normally mica passing through a 20 mesh screen can be advantageously processed, although mica that is of other size will be suitable. Mica as that sold by Marrietta International as Suzorite 60-S and being referred to as unclassified with respect to aspect ratio can yield desirable composites. Measurement of number average aspect ratio (average diameter of flakes over average thickness) of such mica show them to have low aspect ratio, i.e., below 30, although higher aspect ratio mica may be employed, e.g. 20-200.

The heat sensitive additive for which this invention is particularly advantageous includes chlorinated compounds desirably comprising about 45-85% by weight chlorine, more preferably about 60-80% by weight and especially, chlorinated hydrocarbons such as waxes marketed as Chlorez (Dover Chemical), Chlorowax (Diamond Shamrock), and Kloro-chek (Keil Chemical, a division of Ferro Chemical). Typical molecular weights (weight average) for such chlorinated waxes are between about 500-2000, and especially between about 1000-1500 with desirable results especially being achieved with waxes having between about 70-78% by weight chlorine. Other chlorinated compounds that may be suitably employed include chlorinated aliphatics with as low as 5% by weight chlorine and molecular weights (weight average) as high as several thousand (e.g. 10,000). Particle size of the additive is any particle size that allows effective dispersement of the additive in the mica filler during mixing. Preferably, the additive particles in the mica and additive blends have diameters equal to and more preferably substantially below the largest dimension of the mica particles.

Prior to compounding with the resin, at least a portion of the additive is preferably dry contacted with the mica particles to provide a blend. For example, the mica and additive in powder or other suitable form can be tumbled together, preferably at room temperature, to provide an intimate admixture. Normally, the additive and mica will be admixed in this coating process such that the additive is added in an amount between about 0.1-15% (preferably about 1-5%) by weight of the combined mica and additive weight. The raw mica is desirably about 10-80% by weight of the combined weight of resin and mica.

The intimate admixture of mica and chlorinated hydrocarbon of this invention has advantage over compounding with coated mica disclosed by my copending, concurrently filed application in that it eliminates a need for a coating step. Simple admixing of mica and particulate additive accomplishes in this invention at least similarly desired molding precursors. In certain instances, the properties of composites of this invention are even superior to those obtained using coated mica.

The drawing illustrates a preferred manner in which compounding is accomplished in accordance with this invention. The device illustrated is one side of a twin screw extruder which is, in preferred mode, operated such that the screws are counter rotating and non-intermeshing. Such twin screw extruders are commercially available with variable screw configurations from Welding Engineer, Inc.

Melt forming resin such as polypropylene is added to one end of the twin screw extruder in an area of flight configuration preferably that subjects it to sufficiently high shear to cause elevated temperature, e.g. 440.degree. F. and to provide a molten mass of resin prior to addition of the mica and additive blend. (By "molten" herein is meant with reference to resin that particles of resin are at a temperature that can (or does) provide their coalescence with one another.) Initially, the barrel of the extruder at this area may be heated to maintain its temperature in a range above about 400.degree. F., preferably about 420.degree.-460.degree. F. During extended use, however, this area is maintained by conventional means at desired processing temperature, e.g. 435.degree.-455.degree. F.

The blended mica and additive is thereafter introduced into the molten resin through one or more ports downstream as illustrated in the drawing. Essential to the addition of blended mica and additive particulate is its reduced shear mixing with the molten melt forming resin so as to maintain temperature control. It has been found that high shear mixing, as in many conventional extruders, causes such work on the mica particles as to cause an uncontrollable temperature rise when compounded at very high speeds e.g. 600 lb./hr. or higher. This temperature rise markedly reduces the efficiency of the heat sensitive additives and in one case, e.g. Chlorez (Dover Chemical) with about 74% by weight chlorine causes significant deterioration, at temperatures above about 480.degree. F.

In order to obtain maximum effect of the additive in as rapid as possible time, the temperature of the resin prior to mica incorporation is carefully controlled. Additionally, the shear applied to the resin is controlled so as to insure that the melt index (ASTM D1238) is preferably maintained below 15 and preferably below 12 or even 10 for most product applications. Thus, the amount of shear that is applied to the resin (prior to mica incorporation) is that amount which does not significantly elevate its melt index but which does provide a temperature high enough for rapid and effective incorporation of mica and additive. Such temperature or "initial mixing temperature" is preferably in a range that is not lower than about 60.degree. F. below a temperature which would cause evolution of chlorine containing gas from the additive by thermogravametric analysis. More preferably, such initial mixing temperature is not lower than about 25.degree. F. below that temperature. (Well known thermogravimetric analysis techniques for determination of a temperature of first evolution of chlorine containing gas (i.e. outgassing of the additive) may be used. For example, Chlorez 700 from Dover Chemical and having about 74% by weight chlorine in the wax outgasses at about 480.degree. F.).

The resin, however, prior to mica and additive incorporation may be at a temperature above the temperature at which there would be outgassing from the additive. This is because incorporation of mica into the resin first provides some cooling to the resin. Thus, for example, at especially high loadings of mica, e.g. 50%-75% or more, even higher initial temperatures for the resin may be tolerated or even advantageous. For most circumstances, however, the initial temperature of the resin is desirably between about 10.degree.-60.degree. F. below that temperature at which there would be outgassing of the additive.

The temperature at which mica and resin mix should preferably not be allowed to reach a temperature above about 25.degree. F., more preferably 15.degree. F. higher than the initial mixing temperature of the resin. Additionally, such temperature should be in a range below that temperature at which chlorine containing gas evolves from the additive. Such a temperature range is accomplished by low shear kneading of the mica particles into the resin. Cooling of the extruder during mica incorporation into the resin also may be used.

As previously noted, it is particularly desirable that the melt index of the resin not exceed about 15 (ASTM D1238). In other product applications, however, a higher melt index may be useful. In such a circumstance, very high shear or, in some circumstances, chemical additives (e.g. peroxides) may be used to provide products (e.g. pellets) at a melt index of 20 or higher.

As mentioned, it is preferred to use twin screws that are counter-rotating, and desirably the twin screws are non-intermeshing as are available commercially. By adding the mica into a turbulence created between counter rotating streams of molten resin created by such devices good initial mixing, is obtained.

Various flight changes as illustrated in the drawing during passage of the resin as well as resin and mica through the above described twin screw extruder can also be of advantage to promote mixing as will be understood by those in the art. As there are matters within the skill of the art, no further description is believed necessary. As previously mentioned, however, the amount of shear applied is such that its melt index (ASTM D1238) is not elevated above about 15, more preferably 12.

As shown in the drawing, it is advantageous to draw a vacuum from the extruder, preferably, as shown, downstream from mica addition. Such vacuum may be desirably, for example, in the range of about about 15 inches of Hg, more preferably about 20 inches Hg as 22 inch Hg. Such vacuum removes entrapped gases as air or volatiles developed during compounding.

The extruder may be fitted at the end, for example, with a conventional strand die (not shown) to yield strands that are cooled and diced into composite molding precursors, e.g. pellets. Moreover, the extruder may also be connected to shaping devices that take strands or other molding precursors and shape them into desired object. Furthermore, if desired, the extruder may be fitted with extensions as heated pipes that provide additional time for interaction of the mica and melt forming resin.

Advantageously, however, as mentioned, the entire compound extrusion provides a melt residence, i.e. time that mica and melt forming resin and additive exist in melt form of less than about 30 seconds, more usually less than about 15 seconds. Although additional melt residence is provided by further molding activities, e.g. injection molding, pellets compounded in the manner described can provide molded articles of excellent properties under standard shaping conditions.

The mica in this invention may be replaced in part, as is understood, by other particulates, e.g. talc, glass, calcium carbonate and the like to provide desired modifications, such particulates being added with, for example, the mica or at other convenient processing times.

The following examples are intended to illustrate the invention in currently preferred aspects and should not therefor be interpreted as necessarily limiting the scope thereof for those in the art will recognize other readily apparent modifications within the hereinbefore disclosed invention. As used in these examples as well as used in the hereinbefore description and hereinafter appended claims, melt index is grams per 10 minutes.

Using a Welding Engineers 2.0" Twin-Screw Extruder as illustrated in the drawing, equal parts by weight of Gulf Polypropylene 6420 MHP and mica (with particulate additive) are compounded. The mica is blended with Chlorez 700 by tumbling it at room temperature in a ribbon blender for fifteen minutes. The Chlorez 700 is used at a level that would be about 1% by weight of the combined weight of mica and polypropylene. Chlorez 700 (available from Dover Chemical) has about 74% chlorine by weight, an average molecular weight of about 1250 and a nominal formula indicating one extra chlorine for the number of carbon atoms present. The raw mica is Suzorite 60-S marketed by Marietta International.

The temperature of the extruder is maintained at 440.degree. F. and the polypropylene continuously starve fed followed by continuous starve feeding of blended mica and additive particulate. The twin screws are counterrotating and non-intermeshing and a vacuum of approx. 22 inches is maintained. The resin becomes molten after passage of about 8" along the extruder. The rate of addition yields 600 lb./hr. of compounded product which is sent through a strand die to provide strands about 1/4" or less which are thereafter cooled and diced into pellets. The strands exit the extruder at a temperature of about 450.degree. F. The pellets are molded into standard test bars by injecting at 600 psi and 428.degree. F. The properties of the test bars are as follows:

The elapsed time between introducing the resin and exiting of the strands from the extruder is within 15 seconds.

The procedures of Example 1 are followed except that the mica is blended with twice as much Chlorez 700. Essentially comparable results are obtained.

The procedures of Example 1 are followed except that the temperature of the extruder is maintained at about 450.degree. F. and the strands exit at about 460.degree. F. Similar results are obtained.

The pellets of Example 1 are molded by injection molding at 440.degree. F. at 1100 psi into a mold maintained at 120.degree. F. to yield a crash pad retainer suitable for automotive application. The retainer is flame treated prior to its contact with a polyurethane precursor and the precursor foamed between the retainer and vinyl sheet to provide a mica and polypropylene/foam/vinyl laminate.

The procedures of Example 1 are followed except that the chlorinated wax comprises Chlorowax 70, marketed by Diamond Shamrock. Similar results are obtained.

The procedures of Example 1 are followed except that the blend of mica and additive is introduced in equal parts by weight into two different ports, the second being downstream from the first. Essentially similar results are obtained.

The procedures of Example 1 are followed except that the weight proportion of resin and mica is 3:2 respectively. Molded test bars show excellent properties.

The procedures of Example 1 are followed except that the melt forming resin contains calcium carbonate.

The procedures of Example 1 are followed except that the mica and additive blend is added to yield pellets that comprises 5% by weight mica.

The procedures of Example 1 are followed except that the weight ratio of mica to melt forming resin is 2:1.