Compounding PVC with DuPont Elvaloy®
When used as the primary modifier or in conjunction with liquid plasticizers, Elvaloy® and Elvaloy® HP can be effectively used for PVC compounding. Because Elvaloy® and Elvaloy® HP are high-molecular-weight thermoplastics, additional shear is usually required to obtain homogeneous blends compared to systems containing only liquid plasticizers. Elvaloy® HP grades require less shear than Elvaloy® standard grades, and therefore make it easier to produce homogeneous blends.
Nonhomogeneous blends with particles of unfluxed PVC yield products with less than optimum physical properties and rough, gritty surfaces; nonhomogeneity also causes haze in clear compounds. 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 prepared in conventional equipment, such as ribbon blenders and high intensity Henschel or Welex mixers. Normal procedures should be followed to blend all the compound ingredients except the Elvaloy®. Ensure that all of the liquid ingredients are completely absorbed and the mixture is a free-flowing powder before proceeding. The temperature of the blend should then be cooled to 60°C (140°F) or less before adding the Elvaloy®. Ideally, the Elvaloy® should be added to the extruder as a separate feed, especially when using more than 50 phr. If Elvaloy® is added to the cooled dry blend, it should be mixed briefly at a low speed until it is fully dispersed. Care must be taken to keep the temperature of the dry blend containing Elvaloy® below 60°C (140°F) to avoid agglomeration and a nonuniform 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 (read more below) 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 representative.
Melt Compounding of Elvaloy®/PVC Blends Using a High-Shear, Co-Rotating, Twin Screw Extruder*
The high shear generated in the co-rotating, twin-screw extruder is effective in producing homogeneous blends of PVC and Elvaloy® resin modifiers. The amount and location of shear within the extruder can be easily changed by adding, removing, or shifting the location of reverse flights and/or kneading blocks. As a general rule, the amount of shear required to achieve a homogeneous blend increases as the melt viscosity decreases. Experience has shown that the length of kneading blocks plus reverse flights compared to the overall length of the screw should be 7-13% for optimum compounding of blends having a Shore A hardness of 70-95. As the blend hardness increases and the viscosity increases, the level of shear should be decreased and vice versa. In addition to screw design, additional shear can be generated by increasing the screw speed while maintaining constant feed rate and output. Also decreasing the barrel temperature, thereby causing a larger proportion of melt temperature to come from shear, tends to give more homogeneous blends. The preferred barrel temperature profile under the feed hopper is 120°C (250°F), increasing gradually to 170°-190°C (340°-375°F) at the die. To prevent polymer degradation, the melt temperature should not exceed 200°C (392°F). Barrel cooling may be necessary to control melt temperature. Typical screw designs and appropriate formulas for a 53-mm Werner & Pfleiderer extruder are shown in Table 1. It is suggested that existing screw configurations be evaluated first, then add or remove kneading blocks and reverse flights to optimize the design for specific formulas.
Table 1. Typical Screw Design (53mm W&P)
|FORMULA||FEED||ZONE 1||ZONE 2||ZONE 3||ZONE 4||% OF KNEADING1|
|90/30 (PVC tip)|
|90/30 (PVC tip)|
1Kneading = length of kneading blocks and reverse flights/overall length of screw2Screw element nomenclature -- lead/length (mm); F = feed element; KB = kneading block; R = right hand (forward); L = left hand (reverse); N = neutral
Since formulas vary widely depending on the application, screw design and operating conditions will have to be tailored. Experience has shown that stepwise addition of the Elvaloy® or Elvaloy® HP can improve compounding quality. Ideally, the PVC dry blend should be added to the first compartment, then Elvaloy® or Elvaloy® HP to the second compartment. Elvaloy® resins with lower melt index provide more shear energy and result in better compound quality. If difficulty is experienced in achieving a homogeneous blend, contact your representative for assistance. The above suggestions are not to be all-inclusive. They should be supplemented by safe manufacturing procedures, prevailing industry standards, and recommendations of the equipment manufacturer.
Table 2. Formulas
|Epoxy Soya Oil||10||10|
|Hardness, Shore A (10 sec.)||74||90|
*Parts by weight per 100 parts of PVC resin
Because Dow 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.