Patent Application
20140308500
The present invention relates to the manufacture of mineral filled polymer compounds, and more particularly, to the use of simultaneous cooling and applying extensional sheer forces during manufacture of a mineral filled polymer sheet to reduce density within the mineral filled polymer compound sheet.
Minerals have been utilized to reinforce and reduce the cost of polymers since the commercialization of polymers. Mineral addition can improve stiffness, impact strength, heat resistance, wear resistance, chemical resistance and other important end use characteristics of the base polymer.
Minerals often are added to reduce cost, as the mineral is usually much less expensive on a weight basis than the polymer being replaced. This is why a large proportion of higher cost engineering polymers, such as polyamide or poly-butylene-terephthalate, contain mineral or other inorganic reinforcements.
Mineral additions to polymers in many cases reduce the energy required for processing, as the minerals improve heat conductivity of the molten compound. This improves the thermal efficiency of heating and cooling conducted during polymer processing.
Minerals often have a lower manufacturing environmental footprint than petroleum-based polymers, and mineral additions can affect a significant reduction in the amount of greenhouse gases and other atmospheric pollutants in the end product life cycle analysis.
However, when minerals are added to polymers, the density of the mixture will increase. This increase in density of the polymer/mineral blend causes an increase in the weight per unit volume of a molded article, or the weight per unit area of a film or sheet. The weight per unit area of a film or sheet is commonly known as the basis weight, and is commonly expressed in grams per square meter [g/m2, or gsm]. With increasing density there is a need to increase the basis weight to maintain a constant film or sheet thickness. Mineral additions to lower cost commodity polymers, such as polyethylene, can require an increase in basis weight that can offset the economic benefits observed in compound cost per unit of weight. Thus, there is a need for a system and method of manufacture of mineral/polymer compounds that provides the benefit of the use of increased minerals while limiting the concurrent increase in weight of the mineral/polymer blend.
The present invention as disclosed and described herein, in one aspect thereof, comprises a flexible plastic sheet made from a mineral/polymer compound mixture. The mineral/polymer compound mixture includes a mineral component comprising 25%-75% by weight of the mixture and a polymer component comprising 75%-25% by weight of the mixture. The flexible plastic sheet defines a micro porous closed cell structure including a plurality of microscopic voids created during a simultaneous cooling and cavitation inducing process to produce the flexible plastic sheet.
For a more complete understanding, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:
Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout, the various views and embodiments of a system and method for producing mineral filled polymer compounds are illustrated and described, and other possible embodiments are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations based on the following examples of possible embodiments.
Referring now to the drawings, and more particularly to
Previous implementations of mineral/polymer blends have utilized a toughening mechanism in semi-crystalline polymer blends using calcium carbonate filler particles. This process added high levels (25% by volume) of calcium carbonate to high-density polyethylene by producing melt blended compounds and injection molding. The process provided a 15-fold increase in impact strength with certain grades of calcium carbonate, but only at the gate end of the molded part, where the melt sheer or flow rate is the highest. In studying the structure of these polymer/mineral composites, it was learned that the calcium carbonate had modified the crystalline structure of the solid polyethylene, dramatically changing its properties.
Thus, as more particularly illustrated in
The crystalline or semi-crystalline polymer may comprise in one embodiment polylactic acid or poly-lactide. The polylactic acid may have a melt index of 2.0 to 4.0 and a density of 1.25 g/cm3. The combination of mineral and polymer compound may then be dried at step 108 prior to extrusion.
In addition to the mineral/polymer material, the compound may also include one or more moisture scavenging ingredients. The composition may also include one or more other polymers such as poly-butylene-adipate/terephthalate, poly-butylene-succinate, poly-butylene-succinate/adipate or polyhydroxyalkanoate (PHA). Additives such as color pigments, anti-static agents, biocides, odorants or photosensitizers may also be added to the mixture at step 102 prior to extrusion.
Functional additives such as peroxide or coupling agents may also be added as a method of increasing polymer molecular weight. The compound may also contain polyesters such as poly-butylene-adipate-terephthalate, poly-butylene-succinate, poly-butylene-succinate-adipate, polyhydroxyalkanoate or its co-polymers, or polycaprolactone.
Referring now to
In addition to cooling the molten compound 308 into a solidified composite 316, extensional sheer forces are applied to the cooling mineral polymer compound such that the cooling and sheer forces are substantially simultaneously applied to the mineral/polymer compound at step 206. The extensional sheer forces are applied using a series of tension control rollers 318 and a winder 320. The winder 320 pulls the solidified composite 316 off the chill roller 310 while the tension control rolls cause the extensional sheer forces to be applied to the material.
The extensional sheer forces applied to the mineral/polymer compound acts as a powerful nucleating agent. Adding 40 to 50 percent mineral to the semi-crystalline or crystalline polymer raises the crystallization temperature of the polymer in the blown film process from 52 degrees C. to 87 degrees C. By simultaneously cooling and subjecting the polymer to extensional (instead of flow) sheer, the nucleating effect of the mineral causes a rapid unconstrained crystallization of the polymer. Continual extensional sheer forces prevents the polymer crystal structure from packing into a tight matrix leaving microscopic voids within the film. This provides a dendritic crystal structure within the film having a remarkable physical strength given the amount of mineral present within the extruded film.
Next, a discussion of some examples of mineral/polymer compounds prepared according to the description here and above will be provided.
Poly-lactic acid with a melt flow of 3.0 is mixed with an equal amount of calcium carbonate with a medium particle size of 5 microns. The blend is melted, mixed and extruded into a molten compound on a continuous mixer and extruded and formed into pellets of a mineral/polymer composite, hereinafter referred to as the composite.
The composite pellets are processed on an apparatus as illustrated in
The composite pellets were processed under the conditions detailed in Table 1.
At constant extruder speed and melt output rate, increasing the line speed does not yield a proportional reduction in the final film thickness.
The film thickness in microns is equivalent to the weight of one square meter of film in grams at a film density of 1.0 g/cm3. However, film containing 50 percent by weight calcium carbonate, and 50 percent by weight poly-lactic acid, would be expected to have a density=1/((0.5/dPLA)+(0.5/dCaCO3))=1/((0.5/1.25)+(0.5/2.7))=1.71 g/cm3. Multiplying the calculated basis weight at 1.0 g/cm3 density times 1.71 yields the calculated basis weight for the film at the stated thickness, and a density corresponding to a solid mineral/polymer composite matrix in the film.
The actual measure basis weights for each film example are significantly lower than what is calculated for a film at the calculated density. This can only be accomplished through the presence of microscopic voids within a film generated by the combination of cooling and extensional sheer forces in a process known as cavitation.
Polypropylene and polyethylene can be used to produce cavitated films by the inclusion of a mineral such as calcium carbonate, followed by a drawing of the polymer in a solid state under controlled conditions. However, this is a two-stage process where film must be extruded, and then substantially oriented in a separate process. The present process allows the production of cavitated films in a one-step process involving both cooling and application of extensional sheer forces simultaneously in order to significantly improve production economy.
Blends of calcium carbonate, poly-lactic acid, and poly-butylene-adipate-terephthalate (PBAT trade name Ecoflex from BASF) were prepared with the compositions detailed in Table 2. The blends were mixed, melted, and extruded into a molten compound on a Coperion ZSK-26 twin screw compounder, and formed into pellets.
Pellets produced from the compounding of each blend were extruded using a film casting line of the same configuration as illustrated in
Extruder screw speed: 40 RPM
Barrel temperatures: Zone 1=300° F., Zone 2=320° F., Zone 3=340° F., Zone 1=340° F.
Die temperature: 340° F.
The air knife 312 is adjusted so that the molten polymer curtain 308 is pressed against the chill roll 310 and polymer freezing occurs between the point of contact of the impinged air stream. Films produced under these conditions had the properties detailed in Table 3.
Film thickness was measured using a Brunswick instruments model MP-1. Base weights were determined by measuring the weight of a film sample in grams and dividing it by the sample area in square meters. Film density was calculated by dividing the measured sample basis weight by the basis weight calculated for a film at this thickness and a density of 1.0 g/cm3. The film thickness in microns is equivalent to the weight of one square meter of film in grams at film density of 1.0 g/cm3.
Using sample L014D1 as an example, at a density of 1.0 g/cm3, this film would have a basis weight equal 57.9 g/m2. In fact, the actual basis weight is 39.5 g/m2. This calculated to a density reduction of 57.1 percent compared to the density of the compound obtained from calculations based on raw material composition. The addition of PBAT to the calcium carbonate/PLA compound reduces the Youngs modulus (stiffness) and reduces brittleness of the extruded films.
Thus, using the above-described system and process improved film may be created having the cost effectiveness associated with the additional use of mineral compounds within the composition and the strength, weight and flexibility requirement needed for the films.
It will be appreciated by those skilled in the art having the benefit of this disclosure that this system and method for producing mineral filled polymer compounds provides a method for reducing the density of mineral/polymer compounds. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather than a restrictive manner, and are not intended to be limiting to the particular forms and examples disclosed. On the contrary, included are any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope hereof, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.
This application claims benefit of U.S. Provisional Application No. 61/811,170, filed Apr. 12, 2013, entitled MINERAL FILLED POLYMER COMPOUNDS FOR THE PRODUCTION OF FLEXIBLE PLASTIC FILM AND SHEET SUBSTRATES WITH IMPROVED YIELD (Atty. Dkt. No. HPLA-31553), which is herein incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61811170 | Apr 2013 | US |
