Formulating for Improved PVC Performance with Elvaloy® Solid-Phase Polymeric Plasticizers
Compared to conventional monomeric and polymeric plasticizers, Elvaloy® resins are higher in molecular weight. This helps them impart higher tensile strength and tear resistance when properly formulated into vinyl compounds. Two broad classes of Elvaloy® are used for PVC modification: standard grades (Elvaloy® 741 and 742), and Elvaloy® HP series grades.
When to Use Elvaloy® HP vs. Standard Grades
Elvaloy® HP grades offer several benefits over standard grades for enhancing the performance of flexible polyvinyl compounds. To illustrate the differences between Elvaloy® grades, DuPont has prepared sample PVC formulations and tested them in compounding operations. The sample formulations used for testing were as follows:
|Test results show in:|
|Figure 1||Figures 2 and 3|
|Elvaloy®, Elvaloy® HP, or DINP||50||100|
|Epoxide Soybean Oil (ESO)1||3||3|
1Drapex 6.8, soybean oil epoxide made by Argus-Witco Co.
2Mark 329, Organophosphate made by Argus-Witco Co.
3Therm-Check 1238, Barium cadium zinc stabilizer, made by Ferro Corp.
Improved Efficiency -- Elvaloy® HP gives softer, more flexible vinyl compounds when used at the same level. The relative efficiency at room temperature is shown in Figure 1 at 50 PHR and Figure 2 at 100 PHR. Elvaloy® HP approaches the efficiency of the less permanent diisononyl phthalate (DINP) plasticizer.
Improved Low Temperature Performance -- Elvaloy® HP provides superior low temperature flex performance in vinyl compounds. Compounds with Elvaloy® HP retain their flexibility at lower temperatures and again approach those containing DINP as shown in Figure 3.
Ease of Blending -- Elvaloy® HP can be blended more effectively than standard Elvaloy® with PVC using Banburys, Buss Kneaders, continuous mixers and high shear twin screw extruders. This is attributed to its higher melt viscosity at PVC compounding temperatures. Consequently, Elvaloy® HP should be considered for use where standard Elvaloy® grades do not meet the requirements of thermal stability or low temperature performance.
Plasticizer Compatibility -- Elvaloy® HP has shown good compatibility with the following plasticizer families:
- Epoxidized Oils
However, it is advisable to test for exudation with the specific plasticizer to be used.
Superior Thermal Stability -- Elvaloy® HP has Superior Thermal Oxidative Stability. This is shown by a Brabender thermal stability test (Figure 4) in which Elvaloy® HP did not crosslink after 100 minutes, at 200°C.
|Figure 4 - Brabender Thermal Stability of Elvaloy® HP vs. Elvaloy® 741 and 742 at 200°C|
Fillers such as calcium carbonate or hydrated silica can be added to PVC compounds modified with Elvaloy® HP without significant loss in compound tensile strength.
Barium/zinc stabilizers are recommended as general purpose stabilizers for PVC compounds modified with Elvaloy® HP, particularly when used with epoxidized oils at 5 to 10 phr. Tin mercaptide stabilizers are also effective. Calcium/zinc stabilizers can be used with epoxidized oils where FDA acceptance is required (e.g., medical applications). However, since calcium/zinc stabilizers are not generally designed for high fluxing temperature, in some formulations it may be necessary to use PVC copolymers designed for low temperature fluxing. Highly basic lead stabilizers are not suggested for compounds of PVC and Elvaloy® HP and should not be used.
Lubricants aid in reducing heat buildup caused by shear action during processing and also provide release from metal surfaces. Combinations of waxes or polyolefins with stearic acid have proved to be effective lubricant packages. If metal soaps are used, the effect of the metal ion on the stabilizer should be checked. The following materials have been found to be effective external lubricants when used in combination with stearic acid: "A-C"+ Polyethylene 617A (Allied Chemical); "Epolene"+ N-41P (Eastman Chemical Products, Inc.); Hoechst Wax XL-200 (American Hoechst Corp); "Microthene"+ FN-500 (U.S. Industrial Chemicals Company).
Compounding and Processing
Compounding mixtures of PVC, Elvaloy® HP, plasticizers, and other ingredients is usually a two-step process involving dry blending followed by melt compounding. This is only a suggested starting point procedure.
Dry blends can be made in conventional equipment, such as ribbon blenders and high speed Henschel or Welex mixers. Normal procedures should be followed to blend all the compound ingredients except the Elvaloy® HP. The temperature of the dry blend should then be cooled to 60°C (140°F) or less before adding the Elvaloy® HP. The resulting blend should then be mixed briefly at a low speed until the Elvaloy® HP is fully dispersed. Care must be taken to keep the temperature of the dry blend containing Elvaloy® HP below 60°C (140°F) to avoid agglomeration and non-uniform dispersion.
Intensive mixing with heat and shear is necessary to compound PVC and Elvaloy® HP. In order of increasing effectiveness, "Banburys"+, continuous mixers, Buss Kneaders, and high shear twin-screw extruders are excellent for achieving a well-mixed product. Single-screw and low shear twin-screw extruders may provide too little mixing and are not suggested. When using a "Banbury"+, care must be taken to avoid haze and "fisheyes."+ Haze, or unblended vinyl, is the result of too little shear or low mixing temperatures. This condition can be corrected by using drop temperatures between 180--190°C, and in some cases, step-wise addition of at least 1/3 of the Elvaloy® HP initially into the "Banbury"+. The other 2/3 are added when the temperature approaches 182°C (360°F). It is very important that a vinyl compound blended with Elvaloy® HP be thoroughly fluxed. Insufficient mixing will result in less than the optimum performance advantages Elvaloy® HP can provide. Evidence of less than fully blended compounds are PVC gels present in films and rough extrudates. It is suggested that homogeneity be checked after mixing, by pressing a thin film of compound and examining it for PVC particles. For more detailed information on compounding Elvaloy® HP in PVC, contact your DuPont representative. The above suggestions are not intended to be all-inclusive. They should be supplemented by good manufacturing procedures, prevailing industry standards, and the recommendations of equipment manufacturers.
PVC compounds plasticized with Elvaloy® HP may be calendered, extruded, or injection molded. Calendering compounds containing Elvaloy® HP requires more effective lubricants than compounds with liquid plasticizers. For a flexible thick film, a temperature profile across the calendar of 149-174°C (300-345°F) is usually suggested. In extrusion, melt temperatures should reach 182-193°C (360-380°F). Similar melt temperatures are recommended for injection molding.
Because DuPont cannot anticipate or control the many different conditions under which this information and/or product may be used, it does not guarantee the applicability or the accuracy of this information or the suitability of its products in any given situation. Users of DuPont products should make their own tests to determine the suitability of each such product for their particular purposes. The data listed herein falls within the normal range of product properties but they should not be used to establish specification limits or used alone as the basis of design. Disclosure of this information is not a license to operate or a recommendation to infringe a patent of DuPont or others.