The Effects of Antiblocking Agents on the Performance of Polymer Process Aids

Fluoroelastomer processing aids can improve the melt processibility of polyolefins only by depositing on the internal dies surfaces. It has been shown previously that other additives in the compound can interfere with deposition on the die surface. This present work expands on this knowledge by examining the effectiveness of several commercial processing aids when paired with various antiblocking agents in a LLDPE resin run on a blown film line. The results show that fluoroelastomer processing aids can perform efficiently in the presence of antiblocking agents if they contain an interfacial agent. The interactions from most antiblocks can be effectively overcome by the use of such technology, however antiblocks that incorporate synthetic silica can be very interactive and reduce the overall efficiency of the polymer process aids. INTRODUCTION Fluoroelastomer processing aids are widely used in polyolefin applications to eliminate melt fracture, reduce die deposits, improve throughput, and reduce extrudate surface defects. These processing aids function by depositing a thin coating of the fluoroelastomer on the metal surfaces of the die, thereby promoting slip at the interface of this coating and the polyolefin. To effectively eliminate melt fracture in a blown film application, the polymer process aid (PPA) must uniformly coat the exit region of the die [1]. It has been shown that antiblock additives in blown film applications can adversely affect PPA performance by interacting with the PPA [2–5]. There are two main categories of interaction between PPA and antiblocking agents: by surface area interactions (adsorption), and by abrading the PPA coating from the die surfaces [6]. The type of interaction is dependent upon the type of antiblock being used. The goal of this present work is to expand on current knowledge by determining the effectiveness of several commercial processing aids when paired with various antiblocking agents in a LLDPE resin when run on a blown film line. Five different antiblocking agents are studied in combination with three different commercial PPAs. The effectiveness of the PPA was determined by the length of time it took to eliminate melt fracture. EXPERIMENTAL Materials A complete listing of the materials used in this study is shown in Table 1. The three polymer process aids (PPA) used for this study are manufactured by DuPont Performance Elastomers and sold commercially. Each contains a fluoroelastomer (FKM) that is a co-polymer of vinylidene fluoride and hexafluoropropylene in a 60/40 weight ratio. The commercial types are Viton ® FreeFlowTM 40, Viton® FreeFlowTM Z 100, and Viton® FreeFlowTM Z 200. Viton® FreeFlowTM 40 is a pure fluoroelastomer in pellet form. Viton® FreeFlowTM Z 100 is a blend of fluoroelastomer (slightly more than 50 percent by weight), dusting agent, and polyethylene glycol (PEG), slightly more than 40 percent by weight, as an interfacial agent. Viton® FreeFlowTM Z 200 is a blend of fluoroelastomer (slightly more than 30 percent by weight), dusting agent, and a polyester polyol (polycaprolactone, or PCL), slightly more than 60 percent by weight, as an interfacial agent. In the present work, these PPAs are referred to as PPA-1, PPA-2, and PPA-3, respectively. The PPA masterbatches were produced with a 1% concentration. The resin used for this study was an ethylene-butene LLDPE, produced by ExxonMobil Chemical Co. (LL 1001.59), having a melt index (MI) of 1.0 and a density of 0.918. The product is produced in a gas phase reactor with no additives other than a minimal antioxidant level. The resin used for PPA masterbatch production was a similar resin, melt index 2.0, in powder form (LL 1002.09). Ingenia Polymers, Inc kindly supplied the antiblocking agents in masterbatch form used for this study. AB1 is diatomaceous earth (silica or DE) with a median particle size of 9 microns. AB-2 is a surface treated talc (magnesium silicate) with an average particle size of 2.3 microns. AB-3 is an untreated talc (magnesium silicate) with an average particle size of 2.8 microns. AB-4 is a mineral blend (sodiumpotassium aluminum silicate) with an average particle size of 6.7 microns. AB-5 is a synthetic amorphous silica with an average particle size of 5 microns. All the antiblock masterbatches were produced with a 2.0 MI ethylene-butene LLDPE, at a 25% concentration, except for AB-5, which has a 10% concentration. These masterbatches are summarized in Table 2. Table 1 Materials Listing PPA-1 Pure fluoroelastomer polymer process aid PPA-2 Blended PPA of fluoroelastomer, dusting agent, PEG PPA-3 Blended PPA of fluoroelastomer, dusting agent, PCL AB-1 Antiblock, diatomaceous earth (DE), natural silica AB-2 Antiblock, surface treated talc AB-3 Antiblock, untreated talc AB-4 Antiblock, mineral blend AB-5 Antiblock, synthetic amorphous silica Table 2 Description of Antiblock Masterbatches Label Content Particle Size Concentration AB-1 Diatomaceous Earth (DE 9 microns 25% AB-2 Treated Talc 2.3 microns 25% AB-3 Untreated Talc 2.8 microns 25% AB-4 Mineral Blend 6.7 microns 25% AB-5 Synthetic Silica 5 microns 10% Extrusion Equipment Masterbatches were produced on a 28 mm co-rotating, fully intermeshing, 3-lobe twin screw extruder. The speed was held at 150 rpm, and the barrel temperature setpoints, from the feed zone forward, were 140/160/180/200°C. With these operating conditions, the output was approximately 9 kg/hr. Blown film melt fracture testing was carried out on a 63.5 mm, 24:1 L/D extruder with a 101.6 mm die and a set gap of 0.76 mm. The die has a spiral mandrel design with four inlet ports. The screw was a typical barrier design with a Maddock mixing section, and the speed was held at 45 rpm, which resulted in an extruder output of approximately 45 kg/hr. The nominal shear rate was 540 1/s in the die gap. Temperature setpoints, from the feed zone forward, were 160/180/194/190 °C, and the adapter and die were set at 190°C. Melt temperature at the extruder exit was typically about 225°C. Additives were fed via separate hoppers in a weight-loss feed system to the extruder inlet hopper. Melt Fracture Test Procedure Before initiation of a melt fracture run on the film line, the line was purged with a commercial purge compound (Ampacet 807193) containing synthetic silica in polyethylene for 30 45 minutes to remove the PPA from the extruder and die. The line was then started up with the pure LLDPE base resin (no additives) mentioned previously. To determine the response curves of the PPAs in the absence of antiblock, control runs were made on the blown film line by stabilizing the bubble using the base resin only, without additives. Runs with the antiblocking agents began by stabilizing the film bubble with the addition rate of the antiblock masterbatch. An initial five minute running period ensured the film exhibited 100 percent melt fracture at the beginning of each run, and that no residual PPA remained from the previous run. A rate check was also performed during this period to record the extruder output with no PPA being added. The appropriate PPA masterbatch was then introduced to the feed throat of the extruder and a timer was started. At ten-minute intervals film samples were taken, and the operating parameters of the equipment were recorded. The percent of melt fracture on the film was determined by summing the width of the remaining melt fracture streaks on the film layflat, and dividing by the total film layflat width. This method does not distinguish between melt fracture on the inner and outer surfaces on the film. From an overall standpoint, any fracture remaining is considered to be present on both sides of the film. The test run was stopped when the melt fracture is completely cleared (0 percent), or after 80 minutes of running time, whichever occurred first. A final rate check was made before terminating the test run. Some pictures of melt fracture in the film layflat at various stages throughout a melt fracture film test run are shown in Figure 1. Figure 1 Melt fracture test run on a blown film line RESULTS AND DISCUSSION Control Runs – PPA Only Control runs were conducted on the blown film line using the base resin with no additives, and feeding PPA masterbatch only. Each of the PPAs were fed at various dosage levels, ranging from 200 ppm to 1200 ppm, to determine the PPA effectiveness (Figure 2). PPA-1 exhibited residual melt fracture at the two lower levels (200 ppm and 400 ppm). A dosage level of 1200 ppm was required to approximate the performance of the other two PPAs, eliminating the melt fracture in 40 minutes. For this reason, a dosing level of 1200 ppm was selected for PPA-1 in the film runs containing antiblock. LLDPE Blown Film LLDPE transitions from 100% fractured to smooth film, with well defined fracture streaks in-between thick streak