New Developments in High Energy Electron Beam Induced Long Chain Branched Polyolefins for Low Density, Non-Crosslinked Foams

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1 New Developments in High Energy Electron Beam Induced Long Chain Branched Polyolefins for Low Density, Non-Crosslinked Foams Edward M. Phillips, Principal at Edward M. Phillips Polyolefins Specialist, Elkton, MD on behalf of E-Beam Services Inc. Cranbury, NJ Abstract The purpose of this paper is to provide an update in the development and commercialization of radiation modified, linear polyolefins including linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP) and ethylene vinyl acetate (EVA) that have been used to produce non-crosslinked, low density foams. This is an important commercial development in that now it is possible to access a broader range of foam properties through material selection to meet challenging performance requirements. Introduction Low density polyethylene (LDPE) has been used for decades in the extruded, non-crosslinked, low density foam process to produce energy absorbing and thermal-insulative materials for packaging, building and construction, leisure, flotation and many other markets and applications. LDPE, produced in a high pressure autoclave polymerization process, when in the melt phase, exhibits high melt strength which is attributed to its network of entangled long chain branches. High melt strength is a critical processing attribute in foam production because it prevents the rupturing of cell walls during the expansion phase of the process allowing the development of a closed cell structure. LDPE foams however, have application limitations due to their relatively low melting point and poor balance of properties especially at elevated temperatures (~100 o C). Other members of the polyolefin family such as linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP) inherently offer a much broader property range over a broader temperature range VS LDPE making them desirable for many of these applications. However, they exhibit poor melt processability due to their linear (non-branched) structure and cannot be processed easily into suitable non-crosslinked low density foam using conventional equipment. This paper will discuss how electron beam bombardment of the linear polymers has been effective in creating long chain branched structures and thus dramatically improving melt strength suitable for low density foam production. Much has been presented to the plastics community over the past decade regarding melt processing improvements through electron beam modification. This paper will focus more on recent developments particularly with more challenging members of the polyolefin family such as LLDPE, HDPE and PP. The approach taken in this project is very practical and reflects the intention to demonstrate the utility of using electron beam processing as a commercially viable means of offering a polyolefin resin that exhibits melt processing behavior comparable to LDPE (low density polyethylene) but retains the inherent physical properties of the linear polyolefin feed-stocks. The approach was to observe and characterize the effects of radiation in terms of melt flow, melt strength and extensibility using Rheotens and foam processability using commercial scale equipment. No attempts were made to quantify or characterize branching using low angle light scattering or other means. Correlations between dose and melt strength are attributed to long chain branching. Polyolefin Materials Background Low density polyethylene (LDPE) LDPE has been used for many years in processes that require excellent melt processability or melt strength. Ethylene monomer (H 2 C:CH 2 ) is polymerized in the presence of a peroxy initiator such as benzoyl peroxide at high pressures (30,000 40,000 psi) in an autoclave or jacketed tube at temperatures ranging from 475 to 570 o F (246 to 299 o C). By ASTM standard nomenclature, it will have a density ranging from to g/cc and a melting temperature of 90 o C to about 100 o C. The degree of crystallinity ranges from 60% to 70%. The degree of crystallinity and the degree of branching increase as the reaction pressure is increased. LDPE offers a good balance of physical properties and exhibits excellent melt processability for many commercial applications such SPE ANTEC Anaheim 2017 / 2506

2 as cast and blown films, blow molded containers, extrusion coatings, tubes and low density foam. All of these processes require or benefit from the high melt strength exhibited by the highly branched structure of LDPE. Also, in the melt phase, the long chain branches (LCB) create an intertwining network causing the polymer chains to resist sliding over one another compared to a linear polyolefin. By comparison a linear polymer can be defined roughly as a continuous chain where the length is at least 1000 times the thickness of the chain. The length of the chain corresponds with the molecular weight (M w ) and melt index (MI), a measure of flow for polyethylene (higher M w = lower MI). When LDPE is produced in the addition polymerization process, the ethylene molecules are combined in a random order and because of the long chain branches tend not to compact as neatly as a linear polymer, thus causing their lower density compared with LLDPE. The branches tend to be at least 40 carbon units long. Linear Low Density Polyethylene (LLDPE) Linear low density polyethylene (LLDPE) is similarly produced in a continuous addition polymerization reactor at lower temperatures and pressures vs LDPE in either the gas phase or liquid phase depending on the process (Zeiglar, Phillips, metallocene etc.) in the presence of transition metal catalysts. The ethylene molecules are neatly ordered, forming very short branches causing the chains to compact into higher densities (0.926 to 0.940g/cc). The chain lengths are more highly controlled, causing the polymer to have a narrow molecular weight distribution. LLDPE is commonly copolymerized with butene, hexene or octene copolymers which, depending on concentration also affect density, causes short chain branching and greatly affects physical properties. LLDPE grades have a melting point in the range of 120 o C to 125 o C. Commercial LLDPE copolymers tend to have a broader property range vs LDPE including higher film tear and puncture resistance, elasticity, improved clarity and higher service temperature. Because of its linear structure and narrow molecular weight distribution, LLDPE exhibits poor melt strength. In order to exploit the desirable properties of LLDPE, it is commonly blended with LDPE to marginally improve its melt processability. However, some physical properties and heat resistance are diluted or sacrificed in this technique. Equipment manufacturers, especially in blown film, have developed design modifications to adapt to the unstable melt behavior of LLDPE. Polypropylene (PP) While still in the polyolefin family, the tertiary carbon unit contained in the propylene structure causes the molecule to be more complex and versatile. The propylene molecule (CH 2 =CH-CH 3 ) is also formed into polypropylene in the continuous additional polymerization process. Historically, PP was produced using the Ziegler- Natta (Z-N) process incorporating catalysts capable of controlling the orientation of the tertiary ethylene group along the polymer chain. Its orientation (head to head, tail to tail, left hand, right hand) and regularity greatly affects crystallinity and physical properties. Certain types of Z-N and now metallocene catalysts can synthesize highly ordered isotactic polymers with a helical shape. While other forms are possible (e.g. atactic / amorphous, syndiotactic), isotactic PP represents the majority of the common types we see in the marketplace. As they transition from the melt phase to the solid phase, they form a network of spherulites that gives PP homopolymers its characteristic properties: stiffness, high tensile strength, hardness and clarity and high heat resistance (amongst polyolefins). The spherulites or lamella, have the ability to be oriented while in the semi-melt phase offering further unique enhancements to the above mentioned properties. PP density ranges from 0.89 g/cc to 0.91 g/cc and has a melting point range from 161 o C to 164 o C. PP homopolymers are used in a wide range of applications from films to fibers, pipes and containers. However, because of its high crystallinity, PP also tends to be somewhat brittle. For applications requiring a better balance of stiffness and impact (better toughness), PP is routinely copolymerized most commonly with ethylene. However, for the purpose of this paper, we focused strictly on PP homopolymer because conflicting reactions can occur when exposed to electron beam radiation between the PP crystalline regions VS copolymer components resulting in a partly degraded and partly cross-linked polymer. Melt Strength Melt Strength can be broken down into two key behaviors that are valued by extrusion-based converters. Sag Resistance Sag resistance is a function of Mw and long chain branching that allows a polyolefin to support its weight while in the melt phase. Entanglement SPE ANTEC Anaheim 2017 / 2507

3 created by the branched structure prevents the polymer chains from sliding over one another causing the melt to thin and sag. For instance, this is a critical melt processing behavior in melt phase thermoforming and extruded blow molding. The key measured property is viscosity at near zero shear. Strain Hardening Strain Hardening refers to the ability of the polymer melt to resistance deformation at high shear or strain rates. This property is critical to converters producing cast films and extrusion coatings. A polyolefin that exhibits a high degree of strain hardening as a result of long chain branching will resist neck-in and melt resonance (thick thinning) even at high line speeds and draw rates and draw ratios. Strain hardening is a critical rheological behavior in the production of extruded low density foams. As the polymer melt expands resulting from the pressure drop at the exit of the die, bubbles begin to form. As the melt expands, the cell walls of the individual bubbles thin. There is a great deal of cooling occurring at this moment of expansion due to stress induced nucleation along with the endothermic effects of the blowing agent. Ideally, the cell walls will resist rupturing when the polymer is fully expanded and enters the solid phase. It should also be noted that the molecular orientation that occurred in the cell walls during the expansion / recrystallination phase is critical to the performance (especially energy absorption) of the closed celled foam product. Radiation Modification of Polyolefins Several melt processes, including extruded, noncrosslinked, low-density foam production require a complex balance of rheological properties. At E-BEAM Services Inc., we ve developed the knowhow to tailor several polyolefin resins to yield radiation modified grades that exhibit strain hardening behavior within a narrow MI range specified by our customers for specific applications. Long Chain Branched LLDPE (LCB LLDPE) has been developed for customers already commercially engaged in the production of LDPE using conventional tandem extrusion equipment and commonly available blowing agents and processing aides. The LLDPE foam products produced are comparable to the LDPE foams in terms of density reduction and cell structure but have dramatically improved tear resistance, puncture resistance, elasticity and most importantly, heat resistance. Similar results have also been obtained using HDPE and EVA feedstocks. Full understanding of the customer s end-use application requirements (physical / foam properties + heat performance) plus an understanding of processing requirements (melt strength at MI target) is essential in meeting commercially viable solutions. All of our customer s requirements have been met by selecting the correct, commercially available feedstock and identifying the appropriate processing conditions (Dose / rate). In most cases, commercially available pellets are transferred to trays or onto a conveyor belt and are conveyed under a high-energy (150 kw) electron beam at a specific energy at a highly controlled rate to deliver a target dose (megarad) within a very narrow range. In most cases, irradiation can be conducted in air or non-modified atmosphere. In some cases, for instance with PP, due to the presence of its tertiary methyl group, it tends to readily oxidize during exposure to high energy irradiation resulting in minimal branching and excessive chain scission. E-BEAM Services Inc. has developed a proprietary, patented process for processing PP to maximize branching and minimizing scissioning. The primary objective of the work discussed here is to identify the electron beam conditions (dose) that maximize long chain branching as indicated by Rheotens melt tension values reported in centa- Newtons (cn) and polymer extensibility at a target melt index. At the point where maximum melt strength is achieved but extensibility is no longer observed, it is assumed that the onset of cross-linking has occurred. Values reported here are for the last condition (dose) prior to that behavior. LDPE Historically the application of high energy electron beam radiation to polyethylene was most commonly practiced with LDPE for the purpose of cross-linking, forming a partially or fully developed network of interconnected permanent bonds between chains. By bombarding the polymer, free radicals are formed which recombine as permanent bonds. This technique is still commonly used for tubing (PEX C), wire and cable jacketing, shrink sleeves, gaskets and molded parts. Cross-linking increases heat resistance, reduces compression set, wear characteristics and chemical resistance. However, radiation must be applied to the finished product because the permanent bonds will not permit re-melting of the polymer. Cross-linking therefore is not discussed here as a SPE ANTEC Anaheim 2017 / 2508

4 means of improving melt processability. LDPE, inherently being long chain branched is used as a reference in terms of melt processability or melt strength. The reference values for the LDPE grade used here as a control were cn at a MI of g/10 minutes (190 o C, 2.16 kg). LCB LLDPE As mentioned earlier, polyethylene -(CH 2 CH 2 -) n is a very stable semi-crystalline macromolecule that is highly inert, non-polar and impervious to most solvents and chemicals. However, it is not totally resistant to oxidative degradation, especially at elevated temperatures and when exposed to high UV radiation. Therefore, most commercially available PE grades, including LLDPE contain small amounts of antioxidants to minimize degradation in melt processing and in their intended end-use. When applying electron beam irradiation to LLDPE, little effect is observed at the lowest doses. Several commercially available LLDPE grades were evaluated. Similar response to electron beam modification was observed regardless of starting MI, density, co-monomer type (hexane, butene, octane) or process (Z-N VS metallocene). As dose was increased, apparent M w increased (MI went down), melt tension went up and extensibility decreased. For this discussion, we will focus on a 1 MI octane copolymer and a 15 MI metlallocene LLDPE grade. Before irradiation, the melt tension for both grades was 0 1 cn regardless of MI. At the optimal conditions for the 1 MI Octene copolymer, the MI was 0.17 and the melt tension was cn. At 13 cn extensibility was still significant and no crosslinking effects were observed. However, for this sample, the very low MI was considered too low for most commercial converting processes. We did not conclude that maximum branching was achieved and looked to a LLDPE grade with a higher starting MI to quantify processing limits. With the 15 MI mlldpe, a similar dose response was observed. However, at a MI of 0.9, a melt tension of 17 cn was measured with minimal extensibility indicating the onset of cross-linking. It was concluded that the optimal conditions were reached at 1 MI and 16 cn. This material has been evaluated in several commercial applications and has been observed to have superior melt processability compared directly with LDPE while it retained 100% of its original LLDPE properties including higher heat resistance. LCB Polypropylene Polypropylene (CH 2 -CH-CH 3 ) n because of its tertiary methyl unit, is a less stable molecule VS PE and requires significantly higher stabilizer concentrations. It is far more sensitive to oxidative degradation and to UV and other forms of radiation. Due to the weak bond of the tertiary methyl group, when bombarded, a large amount of hydrogen is evolved and the free radicals readily oxidize. The primary result is chain scissioning. Due to the high level of oxidation, the polymer became brittle and exhibited a yellow color. Work performed by HIMONT in the 1980s revealed that scission and yellowing could be minimized and branching could be maximized using a specialized antioxidant combination and by irradiating the material in an atmosphere where oxygen was reduced. The process also required that the radicals be heat quenched at just below the melting point of 161 o C for a controlled duration. The starting MFR (230 o C, 2.16kg) of the feedstock was 0.4 g/10 minutes. And the melt tension was negligible. At optimized conditions (dose, quench conditions, O 2 level) the MFR was 2-4 g/10 minutes. and the melt tension was 30 cn. In commercial application, this material was found to have melt processability superior to LDPE while retaining 100% of its desirable PP properties. A family of long chain branched or high melt strength PP grades based on this technology were offered commercially from HIMONT (now LyondellBasell) throughout the 1990s and ended production in Specific PP homopolymer and copolymer grades were marketed for extrusion coating, melt phase thermoforming, extrusion blow molding, fibers and extruded low density foams. At the time of Lyondell s exit from the HMS PP business in 2008, the total market was approximately 60 million pounds of which the foamable portion made up about 50% of the total. Keep in mind that the foamed applications had a density reduction of 95%. Since then the demand for a foamable PP has grown and several suppliers have attempted to fill the void especially for low density foam applications. As mentioned earlier, E-BEAM Services Inc. has developed a proprietary, patented process for processing PP that has repeatedly produced radiation modified PP grades that meet / exceed the performance of the former HIMONT grades in terms of foamability. SPE ANTEC Anaheim 2017 / 2509

5 Extruded Low Density, Non-Crosslinked Foam Polyolefin foams make up a relatively small portion (<10%) of the current NA Market of >7 BN pounds. However, it is one of the fastest growing segments of the overall foam market, which includes commodity types and performance types. Polyurethane foams still command about 50% share mainly in cushioning applications found in many market sectors (bedding, seating, carpet backing, automotive etc.) and in rigid PU foam for insulation. PS foams command about 25% of the market in packaging (food and non-food) and insulation but converters are under great pressure to find green substitutes with comparable economics. The remaining approximately 15% contains all other foams including cross-linked and noncrosslinked polyolefins, foamed ABS, polycarbonate, PVC and engineering resins and the emerging foamed green plastics segments (e.g. PLA). The current market size for non-crosslinked polyolefins is estimated to be ~300 MM lbs. which is significant considering that the physical volume of the plastic is being increased up to 100 times! For instance, one truckload (40,000 pounds) is equivelent to 15 truckloads of 2pcf foam product. Because of their excellent cushioning properties, high moisture barrier and resistance to oil, grease and chemicals, low density polyethylene foams are used in a wide range of market applications such as food and non-food packaging, insulation, floatation, sports and leisure, hygiene, medical and pharmaceutical applications, automotive and building and constructions to name a few. New application developments have allowed them to be used in bedding and seating applications encroaching on PU foam s historical position. Probably their greatest limitation of non-crosslinked polyolefin foams is service temperature. New Polyolefin Options LCB LLDPE and LCB HDPE E-BEAM Services Inc., independently and on behalf of numerous extrusion based thermoplastic converters has succeeded in creating log chain branched LLDPE and HDPE for the production of low density, closed celled, non-crosslinked foams. Converters are either already engaged in A. the production LDPE foams but cannot meet new, more demanding performance requirements or B. are producing non-foamed articles that can benefit from light weighting, insulation and other features imparted by creating a cellular structure. In both scenarios, they seek the properties, including heat resistance provided by linear polyolefins that exhibit poor foam processability. Since different polyolefins types react differently when exposed to high energy electron beam exposure, feedstock selection is a critical part of the process in order to meet melt strength requirements within a suitable melt index range. For instance, in the case of LLDPE and HDPE, when irradiated, we observe an apparent increase in Mw (decrease in MI) that is attributed to branching. With PP (crystalline homopolymer) there are multiple, opposing reactions. Branching is occurring in the amorphous regions while scissioning occurs in the crystalline regions. Proprietary processing procedures must be employed using feedstocks that are suitably stabilized in order to balance these reactions. Radiation Processing After the candidate grades (see Table 1.) were selected based on predictable reactions, a series of dose response trials are conducted to quantify the change in MI over a range of doses. The pellets were placed on flat trays to form a bed depth of ~3/8. The radiation processing conditions were set based on bed depth, bulk density, energy and line speed to provide a uniform radiation dose. Dose VS MI is plotted against melt strength. We also attempted to determine through MI and melt tension at a point where the material may have begun to cross-link resulting from too high of a radiation dose. Table 2 shows the relationship between Dose and MI. Table 3 reveals and melt tension (melt strength) measured using a Goettfert Rheotens tensometer. The actual Rheotens curves (not shown here) clearly illustrate the transition from a linear structure (shear thinning and melt resonance) to a branched structure (increase in melt tension and controlled deformation of the melt). Table 1. LLDPE Feed Stock Selection Sample Copolymer MI Density A Octene B metallocene C Octene SPE ANTEC Anaheim 2017 / 2510

6 Table 2. Dose Response MI Dose LLDPE A LLDPE B LLDPE C Table 3. Dose Response Melt Tension Dose LDPE A B C In this set of trials, LLDPE sample B was fully commercialized in a low density foam application at Dose 4. Commercialization E-BEAM Services Inc. has been working diligently and continuously for many years to promote the use of radiation modification as an innovative means of meeting challenging performance requirements through improvements in melt processability. The predictive efforts over the past few years have lead to commercialization in several applications by bringing sustainable value to many customer s productivity and by broadening their product lines. This may not always be obvious because our customers view these developments as a key ingredient in their proprietary formulations. Since we work with each individual customer on a discreet basis, it is not possible to discuss all of the their commercial developments openly. The purpose of this paper however is to encourage the astute applications development engineer to consider radiation modification as a potential tool in overcoming performance challenges. It should also be noted that E-BEAM Services Inc. does not brand nor market radiation modified plastics. We process our customer s material. They own the material and the recipe, which includes the benefits imparted by radiation modification. Stand Alone VS Melt Strength Modification In many cases, particularly in extruded, low density non-crosslinked foams, LCB LLDPE, LCB HDPE and LCB PP are used as stand alone resins providing the total balance of processability and end-use properties. In foams, these materials are virtually drop-in resins behind LDPE in conventional foaming equipment using common blowing agents and processing aids (e.g. nucleators, diffusion retarders etc.). Low density foams (<2pcf) have been processed in pipe, profile and sheet configurations using annular dies with cooling mandrels. In several applications though, the application requires a further balance of performance properties and processability. In some medium density applications, for instance, less melt strength is required and more emphasis is put on a stiffness VS impact balance at elevated temperatures which may require blending LCB LLDPE with HDPE or PP. Another example is the addition of LCB LLDPE to LCB PP as a melt strength enhancer. Some commercially available LCB PP grades exhibit process instability (low die pressures) due to low melt viscosity and are also somewhat brittle. It has been demonstrated on commercial tandem equipment that the addition of a relatively low concentration of LCB LLDPE can greatly improve foam processability. Die gaps can be increased significantly while avoiding internal expansion and thicker sheets can be produced at lower densities. Further, the elasticity of the expanding melt is greatly improved to allow for easy stretching over the cooling mandrel. Finally, the addition of LCB LLDPE increases the ductility of the foamed sheet. Conclusions The practice of using high energy electron beam modification as a means of improving the melt strength of commercially available LLDPE, HDPE and PP has been repeatedly demonstrated on a commercial basis. LCB polyolefins can be used as stand-alone resins or as melt strength modifiers / enhancers in blends with other polyolefins. E-BEAM Services Inc. continues to develop and exploit this technique on a broader range of polyolefins to meet the specific application performance and foam processing requirements. References Bradley, R., Radiation Technology Handbook, Marcel Dekker Inc. Publishing, 1984 SPE ANTEC Anaheim 2017 / 2511

7 Phillips, E.M., paper presented at RAPRA Cellular Polymers, London 2091, Novel Foamable Polypropylene Polymers Moore, E.P. Polypropylene Handbook, Hanser Publishing 1996 Makuuchi, K., Cheng, S., Radiation Processing of Polymer Materials and Its Industrial Applications, Wiley & Sons Publication 2012 Scheve, B.J., Mayfield, J.W., DeNicola, A.J., US Patent #5,591,785, High Melt Strength Propylene Polymer, Process for Making It and Use Thereof, Date of Patent January 7, 1997 Phillips, E.P., Crilley, W.P., paper presented at IRAP 2011, The Effect of Electron Beam Irradiation on the Processability of Linear Low Density Polyethylene in Non-Crosslinked Extruded Low Density Foam Key Words: Polyolefin Foams, rheology, melt strength, strain hardening, extensional viscosity, shear thinning, foam density This paper is intended for presentation at ANTEC 2017, May 8-12, 2017 I Anaheim, CA SPE ANTEC Anaheim 2017 / 2512

8 Table1 Sample Dose MFR MFR Melt Strength Note (megarads) (190 o C / 2.16kg) (190 o C / 21.6kg) (cn) Control 1? Control Unirradiated LLDPE A B C D Figure 1. Rheotens Curves. These curves illustrate the dramatic change in melt tension and extensibility behavior of the un-irradiated LLDPE control and the same material irradiated over a range of doses. The bottom curve shows shear thinning at the lower strain rates followed by melt resonance at the higher rates. The irradiated samples exhibit strain hardening followed by a brittle failure as the polymer strand is completely extended. SPE ANTEC Anaheim 2017 / 2513

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