MXPA00001494A - Process for making a uniform dispersion of aramid fibers and polymer - Google Patents

Abstract

A process for making a substantially uniform composition of short aramid fibers and polymer wherein the fibers are provided with a significant moisture content to improve handling characteristics and are partially dried and opened by shear forces in an extruder before contact with the polymer in the extruder.

Description

PROCEDURE TO PRODUCE A UNIFORM DISPERSION OF ARAMIDA AND POLYMER FIBERS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to producing uniform dispersions of aramid fibers in the extrudable polymer in a continuous base. DESCRIPTION OF PREVIOUS TECHNIQUE The Patent of the US. No. 5,439,623 discloses that it is difficult to introduce additives to thermoplastic polymers by means of an extruder, and that even for pelleted additive concentrates, gravimetric or volumetric feeders are very difficult to control. The U.S. Patent No. 5,205,972 discloses that the fibers can be combined uniformly with polymer in an extruder by making a sludge from the fibers with a latex of the polymer, introducing the sludge into an extruder and coagulating the latex and venting the water from the extruder sludge. COMPENDIUM OF THE INVENTION This invention relates to a process for continuously combining short aramid fibers with extrudable polymer to produce a substantially uniform composition comprising 15 to 99% by weight of the polymer and 1 to 85% by weight of the aramid fibers, comprising the steps of providing aramid fibers REF. : 32329 having a moisture content of 20 to 85% by weight of water based on the weight of the fibers; and continuously introducing the wet aramid fibers at a substantially constant speed in an extruder; apply heat to the fibers in the extruder to evaporate the water from the fibers; subjecting the heated fibers to shear forces in the extruder prior to introduction of the polymer; open the fibers; and continuously introducing the polymer into the extruder, downstream of the introduction of the fibers; mixing the polymer of the fibers in the extruder to produce a substantially uniform composition; and expelling the composition from the extruder. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic representation of an extruder that can be employed in the practice of this invention. DETAILED DESCRIPTION Thermoplastic and elastomeric polymeric materials are often reinforced using short fibers, and in cases where the special qualities of aramid materials are desired, aramid fibers are often used for reinforcement. High temperature and high modulus requirements, often can be met using short aramid fibers as reinforcement. Short aramid fibers are useful when they are evenly distributed through an elastomeric or thermoplastic matrix material. The uniformity of distribution has been difficult to achieve in the past, however due to the difficulty in continuous feeding of short aramid fibers at a constant speed. This invention provides a continuous process for uniformly distributing short aramid fibers in a polymeric material, wherein the short fibers are fed at a constant speed, thus improving the general-purpose means of short aramid fibers in producing in high volume compositions. polymeric reinforced with short aramid fibers. The short reinforcing fibers can be added directly to a matrix polymer for reinforcement purposes or they can be combined in higher concentration with a carrier polymer to make a master batch composition. The master batch composition can then be used as a source of fibers for reinforcement purposes. The carrier polymer may be the same as the last or final matrix polymer or may be compatible therewith. For purposes of this invention, the carrier polymer is also referred to as a matrix polymer. Short aramid fibers in dry form, especially short aramid fibers that are highly fibrillated, such as aramid pulp, tend to agglutinate or form lumps and agglomerate and bridge via transfer conduits, so that they make continuous feeding very difficult and at constant speed. As will be explained in detail below, it has been discovered that the moisture in the short fibers causes a dramatic change in the character of the fibers, allowing them to be easily handled and fed at a constant speed in and through the extruders. By "short aramid fibers" is meant aramid fibers or small diameter, high dimension ratio particulate materials, having a length of .1 to 8 mm, preferably .7 to 3 mm. The ratio of short fiber dimensions, which is the ratio of length to diameter, is as small as 10 to as large as 1000 or probably even more. While this invention can be effective for and can be used with short fibers of various types, it finds use primarily with fibrillated materials or materials with very high surface area and low bulk density. These fibrillated materials or short aramid fibers include pulp and particularly aramid pulp that is made in accordance with the teachings of US Patents. No. 5,028,372 and 5,532,059 or by refining aramid floc with an original length of .1 to 8 mm. For purposes of this invention, short aramid fibers also include particulate materials known as "fibrids" and particularly aramid fibrids made, for example, in accordance with the teachings of U.S. Pat. No. 2,999,788 and 3,018,091; and substantially non-fibrillated aramid fibers with a diameter of 5 to 15 micrometers and a length of .1 to 8 mm known as "floccula". The short aramid fibers can be coated, for example, with epoxy, phenolic, resorcinol-formaldehyde, polyurethane, silicone, plasticizers or the like, or can be treated with agents that alter handling behavior, adhesion properties, static charge retention and the like. The short aramid fibers can also be used in combination with other particulate materials such as carbon black, fluoropolymers, chitosan, colorants, fillers or fillers, anti-refrigerants and the like; and can be used with other fibers such as glass, mineral, coal, natural (cotton, jute, ramin and the like), synthetic (polyester, nylon and the like) and the like. The type, and concentration of the additional material is not critical as long as there is no interference with the handling characteristics of the short wet aramid fibers.

By "aramid" is meant a polyamide in which at least 75% of the amide bonds (-CO-NH-) are directly connected to two aromatic rings. Additives can be employed with the aramid and it has been found that as much as up to 10% by weight, or even more than another polymeric material can be mixed with the aramid or that copolymers having up to 10% of another diamine substituted by the diamine can be used. the aramid or as much as 10% of another diacid chloride substituted by the acid chloride or the aramid. Para-aramides are the primary polymers in the short fibers of this invention and poly (t-phenylene terephthalamide) (PPD-T) is the preferred para-aramid. By PPD-T is meant the homopolymer resulting from a mol-to-mol polymerization of p-phenylene diamine and terephthaloyl chloride and also, copolymers resulting from incorporating small amounts of other diamines with p-phenylene diamine and small amounts of other chlorides diacid with terephthaloyl chloride. Meta-aramides are also important for use in the short fibers of this invention and poly (m-phenylene isophthalamide) (MPD-I) is the preferred meta-aramid. By MPD-I is meant the homopolymer resulting from the mol-para-mol polymerization of m-phenylene diamine and isophthaloyl chloride and also copolymers that result from incorporating small amounts of other diamines with m-phenylenediamine and also small amounts of other diacid chlorides with isophthaloyl chloride. Short aramid fibers that are dry and even contain small amounts of moisture, such as 2 to 5% by weight or probably even more, as they can be of natural origin in some fibers, can exhibit a strong negative electrostatic charge and are spongy and exhibit low gross density and as a result are difficult to handle and feed in a dosed form at constant speed. As the moisture content of the aramid fibers increases, the electrostatic charges are decreased and the gross density simply resulting in an improvement in the handling characteristics. It has been discovered, as identified herein as in the invention, that short aramid fibers that are wet can be handled and dosed without difficulty. The degree of humidity anticipated to be used for this invention is from 20 to 85, preferably from 30 to 60% by weight based on the weight of the dry fiber. At those moisture concentrations, the handling characteristics of the fibers are changed, that is, for short aramid fibers having 50% water, the electrostatic charge on the fibers is substantially zero and the raw density of the fibers is increased more than three times at 50 g per liter and 160 g per liter. At moisture concentrations below 20% by weight, the fibers are difficult to handle due to the very low gross density and the high electrostatic charge; and at moisture concentration greater than 85% by weight, the fibers agglomerate together as a semi-solid mass and can even form a thick sludge. By "moisture" is usually meant water, but any volatile liquid or mixture of liquids that can wet the fibers can be used. By stating that the wet fibers of this invention can be handled and dosed without difficulty, it is understood that these wet fibers can be continuously fed at a constant speed, using conventional feeding means such as single screw or twin screw feeders, conical, cone feeders Inverse, weight loss, or volumetric, vibration feeders, conveyor belt feeders and the like. Dry fibers, with a high surface area, can not be fed at a constant speed using these feeders. The polymeric material to be combined with the short fibers can be any polymer or combination of polymers that can be extruded through an extruder. Elastomers and thermoplastic polymers are generally encompassed and include: polyolefins such as high and low density polyethylene, polypropylene; ethylene vinyl acetate copolymers; ethylene methyl acrylate copolymers; ionomer resins; polymethylmethacrylate, polyvinyl chloride; rubber EPDM, chloroprene; copolyester elastomers; polyethylene terephthalate; polybutylene terephthalate; liquid metal polymers; polyester ether ketone; polyester ketone ketone; ABS, polyphenyl sulfide; polyamides, polyimides; polyurethane; silicones and similar. The polymeric material must be a liquid or at least soften at room temperature in an extruder and can even be liquid at 20 ° C or less as the case may be with thermosetting resins including phenolic, epoxy, polyester and vinyl ester resins. The composition made by this invention generally comprises 15 to 99% and 75% by weight of short aramid fibers. The invention is used to mix short aramid fibers in a matrix polymer for direct use; and also to produce master batch compositions for mixing them with additional polymers at a later time. For direct use, the composition in general comprises 90 to 99% by weight of the polymer material and 1 to 10% of short aramid fibers; and for master batch use, the composition generally comprises 25 to 60% by weight of the polymer and 40 to 75% by weight of short aramid fibers.

The process of this invention is carried out using an extruder to achieve the mixing of the polymer and the short wet aramid fibers. The extruder may be of the single spindle variety or it may be a co-or counter-rotating device of twin spindles. The spindle element can be bi-lobed, tri-lobed, self-cleaning, inter-coater or not and the extruder can include a variety of spindle elements such as transport elements, kneading blocks, ampoules, gears, inverters and the like. The extruder must be adapted with elements that will allow continuous introduction of the polymer forward (downstream) of the wet short fibers and ventilate the vapor by the evaporation of moisture from the fibers. The Figure illustrates an extruder structure that can be used to practice the invention. Element-by-element, A is the back of the extruder, B is the section with the gate 10 to introduce the wet short aramid fibers, C is a heated section where the wet fibers at least dry and partially open, D and E are sections with vents 11 and 12 to release evaporated moisture from the fibers. F is a section with a gate 13 for introducing the polymer downstream of gate 10, G is a heated section where the polymeric material melts and / or at least softens and additional moisture evaporates, H and J are sections with mixing elements to uniformly disperse the aramid fibers in the polymer, and K is the end section of the extruder and can be adapted with an extruder die or not, as desired. Elements H and J include vents 14 and 15 for evolution of evaporated moisture. The element B is preferably coupled with spindles having blades or deep passages to ensure constant intakes of gate material E. The short wet aramid fiber material is introduced to the gate 10 separately or in the form of pieces of amber. to 5 centimeters, which due to the humidity present in the fibers can be fed at a constant speed. Although not a preferred method, wet short aramid fibers can be introduced to the extruder gate as a continuous bar or "wet folded sheet" because they have just the right moisture to cause the fibers to keep the shape of the folded sheet wet. The wet short aramid fibers continuously introduced by extruder are driven by spindles through the barrel of the extruder and heat is introduced to evaporate some of the moisture from the fibers. Spindle elements in the C element are chosen to not only transport the short wet aramid fibers but also process the fibers to aid in their drying and partially open them by subjecting the fibers to shear gates. By "opening" the fibers it is understood that they separate and unravel from each other, thus decreasing the gross density. Acceptable inserts in element C are those that not only transport but also apply shear forces to the mass of fibers, such as gears, kneading blocks or the like. The moisture evaporated in element C immediately below is released in vents 11 and 12. In element F, polymeric material is continuously introduced to gate 13 where the fiber and polymer market starts. The heat is introduced into the G element to melt or soften the polymer and evaporate more of the moisture. Mixing is continued through the extruder and a substantially uniform composition of the extruder is expelled. Heat can be generated by the mixing action of the extruder and the heat can be introduced from external sources; and the heat softens or melts the polymer, mixes the polymer and the fibers and evaporates the water from the mixture. The water is released from the extruder through vents. When the polymer is subjected to degradation by hydrolysis upon contact with moisture, such as wet fibers, moisture must be completely evaporated from the fibers and the extruder vented before the fibers reach the polymer. For example, by reference to the Figure, the moisture of the short fibers will be completely removed through the ventilations 11 and 12 in the elements D and E. It is important to note that the polymer is introduced into the extruder downstream of the wet aramid fibers. When the polymer is introduced into the extruder downstream of the wet aramid fibers, mixing is easily achieved and uniformity is easily obtained. Proper mixing is not achieved when the polymer is introduced into the extruder upstream of the wet fibers. In this case, the fibers can not be opened before contact with the polymer. It has been found that when the polymer is fed upstream of the fibers, or in the same gate, only partial dispersion of the fibers in the composition can be achieved. Only a uniform dispersion of fibers is achieved as in this invention, when wet short aramid fibers are fed to the extruder and dried and partially opened before mixing with the polymer. DESCRIPTION OF THE PREFERRED MODALITIES Example 1 In this example, a short aramid fiber material is continuously combined with polymer at a substantially constant rate to produce a composition to produce a composition consisting of fibers evenly distributed in the polymer. The short aramid fibers were poly (p-phenyleneterephthalamide) pulp with an average fiber length of 1.7 to 8 millimeters, a BET surface area of 8 to 9 square meters / grams and a freedom of Canadian standard of approximately 215 milliliters. It was the product sold by E.I.DuPont de Nemours &; Company under the brand name KevlarM, merge 1F 361, and was treated to contain about 5% by weight of water. The polymer was a copolymer of ethylene vinyl acetate having 40% by weight of vinyl acetate and a melt index of 57 dg / min. It was the product sold by E.I. DuPont de Nemours & Company under the brand ElvaxMR 40W. The mixing is used in a Werner Pfleiderer co-rotating twin-screw extruder having a gate to introduce the pulp and downstream a gate to introduce the polymer. The extruder as illustrated in the Figure, has 11 sections with 4 vents and two feed gates, with 33 diameters in length and for this example was operated at 350 rpm. The wet pulp is fed into gate 10 at a rate of 22.7 kg per hour and the polymer is fed into gate 13 at a rate of 10.4 kilograms per hour. The water vapor is released from the extruder in the ventilations located in sections D, E, H and J. The heat to the descriptions that adjust to achieve the following temperatures in operation: Section BCDEFG Temperature 245-252 229-195 119-124 71-80 49-58 86-91 (° C) Section HIJK Temperature 42-55 79-83 89-99 88-91 The wet pulp is fed using a feeder-in-weight feeder, with a large mouth feeder and a single open-spiral spindle sold by KTron-Sauder (Switzerland) and the polymer, in the form of a nodule, is fed using a low-speed feeder with a bore-type spindle also from KTron-Sauder. The combination of pulp and polymers that is expelled from section K of the extruder is found to be 5.1.5 + 0.5% by weight pulp in a uniform and consistent manner over the duration of the run. The tests showed that the fiber-pulp length is not altered by the process. When the nodules of the combination are melted and pressed into a very thin sheet, visual inspection showed that there were no fiber aggregates. It will be noted that the pulp can be fed at a constant speed because it includes approximately 50% by weight of water. If the moisture content of the pulp was less than 20% by weight of water, the texture of the pulp would have been such that it was impossible to feed at a constant speed. It will also be noted that the polymer is introduced into the extruder downstream of the pulp. When the polymer is introduced to the extruder upstream of the pulp, the fiber can not be properly opened to achieve uniform dispersion in the polymer. EXAMPLE 2 In this example, the same material of short aramid fibers with the same moisture level as used in Example 1 is continuously combined at a substantially constant rate, with a variety of polymers in a twin co-rotary twin-screw extruder. 40 millimeter Berstorff, which has a gate for introducing the fibers and downstream in the gate, a gate for introduction of the polymer. The extruder had 7 sections with two vents and two feed gates and was 33 diameters in length. The wet pulp is fed to the extruder using a Hasler 4021 loss-in-weight feeder coupled with twin glue-type extruders and it was noted that any level of less than or less than about 20% by weight would have resulted in too many or too many fibers remaining. very highly charged with static power to power using that device. Temperatures in the extruder were adjusted to be high enough to soften the polymer in the introduction, but low enough to prevent hot steam from plugging the gate. The polymers, feed rates and other information were as follows: Feed Speed Feed rate of polymer pulp Polymer Speed (kcr / hr) (kg / hr) extruder (rpm) Ethylene / vinyl 4.54 10.4 400 acetate1 ionomer2 5.45 12.7 250 EPDM3 4.54 10.4 350 1 Vinyl acetate content of 18% by weight and melt index of 8 dg / minute as sold by E.I. du Pont de Nemours and Company under the brand ElvaxMR 450. 2 Resin-ionomer of the zinc cation type having a melting point of 1 gram / 10 min at 190 ° C as sold by E.I. du Pont de Nemours and Company under the trademark SurlynMR 9020. 3 Ethylene-propylene-diene monomer resin (EPDMJ of 70% by weight of ethylene content) Mooney viscosity of 25 (ML 1 + 4 at 125 ° C) and 2.5% by weight of ethylene norbornene sold by DuPont-Dow Elastomer under the trademark NordelMR IP 3725. EXAMPLE 3 In this example, the same material of short aramid fibers with the same moisture level as used in Example 1 is combined In one case, the copolymer was 51% vinyl acetate with a melt index of 2.8 grams / 10 minutes at 190 ° C which is sold by Bayer AG under the brand name Levaprene ™ 500 (3A); in another case, the copolymer was 70.6% by weight of vinyl acetate with a melt index of 2.1 grams / 10 minutes at 190 ° C which is sold under the brand name Levaprene ™ 700 (3B) .The same extruder is used as used in Example 2. The polymers, feed rates and this information were as follows: Feed Speed Speed feed of pulp polymer Speed of Polymer (kg / hr) (kg / hr) extruder (rpm) 3A 12 6 300 3B 12 6 300 Example 4 In this example, the same extruder as in Example 2 is employed. Material of short aramid fibers having different moisture concentrations is fed at substantially constant rates. The fiber material was poly (p-phenyleneterephthalamide) pulp with an average fiber length of 1.3 millimeters, a BET surface area of 6 to 7 m2 / gram and a freedom of Canadian standard of approximately 380 milliliters. The product was the one sold by E.I. DuPont de Nemours and Company under the brand KevlarTM, merge 1F538. The polymer was the same as that used in Example 1. The fiber material is fed to the extruder using the same feeding apparatus as used in Example 2 and the fiber material is used with the three different moisture contents. Temperatures in the extruder were adjusted to be high enough to soften the polymer upon introduction but low enough to prevent hot steam, plug or plug the gates. The moisture content, feed rates and other information were as follows: Content Feed Speed Velocity Feed Pulp Polymer Humidity Speed (% by weight) (kg / hr) (kg / hr) Extruder (rpm) 72 5. . 6 6. 0 350 35 16. . 0 17. 0 350 18 5. . 0 discontinuous 400 While the pulps having 35 and 72% by weight of water are fed continuously and at a constant speed in this example, the pulp with 17% by weight of moisture, was very difficult to feed the extruder. The pulp test with 18% moisture is interrupted after approximately 10 minutes to break a fiber "bridge" formed in the Hasler feeder - and in just 25 minutes thereafter, the extruder feed gate was plugged completely by accumulation of the fibers that stick to the walls of the feeding gate. The gate is cleaned and the test restarts - only to stop again in 5 minutes for another "bridge" of the fiber material. The test, with respect to fibers that only have 18% humidity, is interrupted. Example 5 In this example, the short aramid fiber material was aramid flocculent. Two types of floc were used- 1 was meta-aramid with length of three millimeters and diameter of 10 to 12 micrometers, and the other was para-aramid with two millimeters in length and diameter of 10 to 12 micrometers. The meta-aramid flock was poly (meta-phenylene isophthalamide) as sold by E.I. DuPont de Nemours and Company under the brand NomexMR; and the para-aramid flock was poly (para-phenylene terephthalamide) as sold by E.I. DuPont de Nemours and Company under the KevlarTM brand. Both types of flocs were employed which have 33% by weight of water. The same extruder and fiber feeder were used in this example as used in example 2. The polymer was the same ethylene vinyl acetate copolymer that is used in example 1. The polymer and flocs were introduced and led through the extruder at a rate that resulted in a composition that was 40% by weight of fibers, wherein the fibers are uniformly dispersed throughout the composition. The floccule with 33% by weight of moisture was easily fed continuously and at a constant speed. I float at a moisture content of less than 20% by weight, however, it had significant electrostatic charge and it was difficult to feed continuously. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (16)

2. Having described the invention as above, the content of the following claims is claimed as property: 1. Process for continuously combining aramid fibers with an extrudable polymer to result in a substantially uniform composition comprising 15 to 99% by weight of the polymer and 1 to 85% by weight of aramid fibers, characterized in that it comprises the steps of: a) providing aramid fibers having a moisture content of 20 to 85% by weight of the volatile liquid, based on the weight of the fibers; b) continuously introducing the wet aramid fibers at a substantially constant speed in an extruder; c) applying heat to the fibers in the extruder to evaporate volatile liquid from the fibers; d) subjecting the heated fibers to shear forces in the extruder prior to introduction of the polymer; e) continuously introducing the polymer into the extruder, downstream of the introduction of the fibers; f) mixing the polymer and the fibers in the extruder to produce a substantially uniform composition; and g) expelling the composition from the extruder. 2. Method according to claim 1, characterized in that the aramid fibers are fibrillated aramid pulp.
3. 3. - Method according to claim 1, characterized in that the aramid fibers are aramid flocs.
4. 4. Method according to claim 1, characterized in that the aramid is para-aramid.
5. 5. Method according to claim 1, characterized in that the aramid is meta-aramid.
6. 6. - Method according to claim 1, characterized in that the fibers have a length of 0.1 to 8 mm.
7. 7. - Process for continuously combining aramid fibers with an extrudable polymer, to result in a substantially uniform composition, comprising 15 to 99% by weight of polymer and 1 to 85% by weight of aramid fibers, characterized in that it comprises steps of: a) continuously introducing to an extruder, aramid fibers with a moisture content of 20 to 85% by weight of volatile liquid based on the weight of the fibers at a substantially constant velocity; b) continuously introducing the polymer into the extruder downstream of the introduction of the aramid fibers; c) mixing the polymer and the aramid fibers in the extruder to give a substantially uniform composition, and d) expelling the composition from the extruder.
8. 8. Method according to claim 7, characterized in that the aramid fibers are fibrillated aramid pulp.
9. 9. Process according to claim 7, characterized in that the aramid fibers are aramid flocs.
10. 10. Method according to claim 7, characterized in that the aramid is para-aramid.
11. 11. Method according to claim 7, characterized in that the aramid is meta-aramid.
12. 12. - Method according to claim 7, characterized in that the fibers have a length of 0.1 to 8 mm.
13. 13. - A product made by the process of claim 1.
14. 14. A product made by the process of claim 7.
15. 15. The product made by the process of claim 1, wherein the extrudable polymer is a copolymer of ethylene vinyl acetate.
16. 16. - The product prepared by the process according to claim 7, characterized in that the extrudable polymer is ethylene vinyl acetate copolymer.