Soft materials, Hard Results
By Michael LeGault, editor
Use of thermoplastic elastomers (TPEs) has been growing at an average annual rate of six to eight percent worldwide. This growth has been driven mainly by the desire for materials to replace non-recyc...
Use of thermoplastic elastomers (TPEs) has been growing at an average annual rate of six to eight percent worldwide. This growth has been driven mainly by the desire for materials to replace non-recyclable rubber, substitute for higher cost materials or to add higher performance or value to a product.
When processing these materials, it is important to first realize that TPEs are not a single class of materials. Rather they are a broad range of neat and/or compounded polymers defined more by performance traits, especially softness, than by any particular chemistry. This diversity in turn implies a variety of differences in the ways TPEs are processed in comparison to other thermoplastics, and also from one TPE to another.
This article will mainly focus on processing considerations for injection molding and extrusion for five major classes of TPEs: thermoplastic styrenics (TPSs); thermoplastic polyolefins (TPOs), thermoplastic vulcanizates (TPVs), thermoplastic polyurethanes (TPUs) and copolyesters (COPE).
TPV: the versatile alloy
TPV is most typically represented by Advanced Elastomer Systems’ Santoprene, which is a compound of fully-cured vulcanized rubber particles in a polypropylene base. The alloy combines the chemical and oil resistance of polypropylene and the elasticity of rubber, which makes it ideal for applications such as handles, grips and automotive seals and weather stripping.
Santoprene is a shear-sensitive material which requires high injection pressures and fast injection rates. According to the AES injection molding guide for Santoprene, flow rates should be 10 to 50 g/sec/gate, with a fill time of 0.5 to 1.5 seconds. The material generally sets up rapidly in the mold and releases easily. The use of mold release sprays or powders is not recommended.
The are a number of different grades of Santoprene and each has specific injection molding requirements. Generally, however, the material can be processed on most makes and sizes of reciprocating-screw injection molding machines. The injection unit should have a clamping force of three to five tons/in.2 of the projected area of the part and the barrel capacity should contain about one to four shots.
Most grades of Santoprene will have in-mold shrinkage in the range of 1.4 to five percent. Softer grades will exhibit slightly higher shrinkage.
Like most TPEs, Santoprene is hygroscopic and requires drying. Typical moisture pick-up of the material is 0.14 to 0.20 percent in six to eight hours. The recommended maximum moisture level for processing Santoprene without porosity is 0.08 percent. AES claims regrind levels of up to 100 percent can be processed with virtually no loss of material properties.
The company has recently introduced a new M300 series of Santoprene with 25 to 30 percent better flow than current grades. The improved flow helps to eliminate flow lines and gate blemishes for parts with Class A surface finish requirements.
AES is also selling a patented water-foaming technology aimed at weather seal market. Any company that purchases Santoprene automatically acquires a license to make the water-foamed product. A specially-designed twin-screw extruder designed and patented by Berstorff is needed to mix the water and Santoprene, says John Christensen, market manager, automotive. The water-foaming technology eliminates the need for chemical blowing agents. The technology is currently being used to make hood seals in Mitsubishi vehicles.
BASF’s Elastollan thermoplastic polyurethane elastomers are available in a hardness range from 60 Shore A to 74 Shore D. Polyether-based Elastollan grades are characterized by fungus resistance, low temperature flexibility, hydrolytic stability and resistance to acid and bases. Polyester-based Elastollan grades have good oil/solvent, UV and abrasion resistance, as well as superior mechanical properties. The materials can be used in injection molding, extrusion and blow molding applications.
When injection molding the shear-sensitive Elastollan, it is important to use a larger gate and slower injection speeds, says Roger Wong, BASF business manager for TPUs.
“TPU is much more elastic than TPVs or TPO-based TPEs,” reports Wong. “This means all the shear energy you put into the material will be ‘remembered’ as built in stress in the final part.”
The correct injection speed is dependent on gating, wall section and flow length, but is typically between 0.5 and 1.0 in./sec. Back pressure should also not be set too high, usually between 50 and 150 psi. Cycle time for a part with a 6 mm wall can range from approximately 40 to 110 seconds depending on the density of the grade used.
Wong says TPUs can be processed with a hot runner system that is externally heated. Internal heater bands can cause the TPU to crystallize and freeze up around the localized areas of heat. Large diameter runners should be used to enable maximum pressure transfer and enable mold filling; valve gating is recommended.
The relatively high viscosity of TPUs, along with the higher pressures required to fill the mold require clamping pressures of three to four tons/in.2 of projected surface area. Regrind Elastollan can be mixed with virgin material up to a recommended maximum of 25 percent.
Elastollan grades are used increasingly in a number of recreational applications such as athletic shoe soles, golf cleats, ski boots and sports goggles. The materials are also used in cable, seals and other applications.
Bayer’s Texin and Desmopan TPU resins are based on polyesters, polyethers, special copolymers and blends of polyurethane and polycarbonate. The resins are used to mold a variety of automotive components such as cams, gears and mechanical parts. In-line skate boots, ski goggle frames, shoe components and hydraulic seals are some of the other applications in which Texin and Desmopan are used. Injection molding and extrusion processing guides for the resins are available from Bayer.
Polyolefin-based elastomers-flexible performers
DuPont Dow Elastomers’ Engage polyolefin elastomer is made using Dow’s single-site catalyst technology, INSITE, resulting in a polymer with controlled long-chain branching and a narrow molecular weight distribution. According to France Rochette, account manager, DuPont Canada, this molecular structure gives Engage more consistent processability and superior clarity. Engage also has improved weatherability and lower density in comparison to styrene block copolymers and other TPEs.
Rochette says Engage resins process similar to other ethylene copolymers such as LLDPE, with the exception that Engage resins are more shear sensitive, especially at lower density ranges. Viscosity is reduced and mold filling improved by using the fastest possible injection speed. When setting up to run Engage, melt temperature should initially start at about 176C and be increased until the mold is adequately filled. Mold temperature should be between 7 and 16 C. To maximize cooling efficiency, it is important to have turbulent water flow through the core and cavity cooling lines. Air ejection of the part should be used if possible.
Engage may be extruded on standard equipment, according to Darcie Pytel, DuPont Canada Inc. marketing/technical resource. Due to the clean melt provided by Engage, the extruder may be as short as 20:1 L/D. Vacuum sizing is recommended for applications requiring very close tolerance. Pytel also suggests running Engage at the lowest possible melt temperature which allows for extrusion without surface melt fracture. Start the extruder at 193C and raise the temperature gradually until the part is out of melt fracture.
One growth application for Engage is beverage tubing, where the resin shows greater long-term flexibility and durability than PVC, says Rochette. It is also widely used in TPO automotive bumpers and fascias to improve impact resistance, especially at low temperatures.
Copolyester elastomers — high-end properties
DSM Engineering Plastics’ Paul Moruzi calls co-polyester (COPE) elastomers the engineering plastic elastomer. In general, Moru
zi says COPEs have higher load-bearing capabilities in comparison to other TPEs. COPEs also possess superior resistance to a host of environmental agents, including chemicals, solvents, oils, heat and UV light.
DSM’s Arnitel line of co-polyester resins is produced in three distinct grades. Arnitel E and P grades are based on polyether esters; Arnitel U is based on a polyester ester. According to Moruzi, Arnitel product manager, Arnitel E and P are not shear sensitive materials, while Arnitel U is shear sensitive. When injection molding Arnitel U, larger gates should be used and back pressure minimized, Moruzi says.
Generally, but especially for glass-filled grades, a high injection rate is required for good part quality. Residence time in the barrel should be as short as possible; it is recommended that the part weight be within a range of 40 to 70 percent of the maximum shot capacity. Standard three-zone screws yield excellent results.
All standard gating systems may be used, and, as well, externally heated hot-runner systems can be used provided they have accurate temperature control. Gate location needs to be chosen carefully so to minimize warpage due to anisotropic shrinkage. Also to prevent warpage, parts should be cooled rapidly and uniformly. Cycle time can vary from approximately nine seconds for a part with a wall thickness of 0.8 to 1.5 mm to about 40 seconds for products with a wall of five to six mm.
Proper drying is critical for good results with these resins, says Moruzi. Up to 50 percent regrind can be added to virgin material, however the company recommends a maximum of 20 percent.
Current applications for Arnitel include CV boots, hydraulic hose, monolithic film and others.
Eastman Chemical Company’s Ecdel elastomers are polyester ethers that can be processed by injection molding, extrusion and extrusion blow molding. The materials are specifically targeted for use in a variety of flexible medical and pharmaceutical packaging applications. Ecdel elastomers have low levels of extractables and may be safely sterilized by a number of methods, including autoclave and gamma radiation.
Thermoplastic styrenics-softer and more resilient
Kraton is a triblock copolymer consisting of polymer regions of styrene-rubber-styrene and is one of the most diverse and widely-used elastomers. Kraton, currently owned by Shell Chemical Company, manufactures more than 75 grades of the polymer; as well, compounders such as GLS Corp. make many more grades based on blends of the Kraton polymer.
The Kraton D and G families of polymers are commonly used for thermoplastic and bitumen modification, adhesives, sealants and coatings, and film and fibre applications. They are used to provide superior tack and adhesion. Kraton G polymers have increased oxidation and weather resistance.
When compounded with polystyrene, Kraton D1401P combines low gel with excellent elongation characteristics, which makes it ideal for thermoforming crack-free plastic cups, according to Jim Dieter, Kraton business director, compounding and polymer systems.
In general, D-compounds are more susceptible to oxidation and some shear degradation at excessively high temperatures and shear conditions. The normal recommended processing range is from 300 to 400 F. By contrast G-based compounds with saturated rubber midblocks are more stable to oxidation at higher temperature and shear. These compounds are normally processed at a higher range, from 375 to 500 F.
Dieter says one high-growth application in Asia involves extruding Kraton G compound into sheet. The sheet is then calender coated to a textile and used as a waterproofing layer in products such as back packs, duffel bags, suitcases and clothing articles. The material is being used as a substitute for PVC.
GLS Corporation has recently expanded its TPE offerings to include Versalloy elastomer alloys. Versalloy offers improved chemical and oil resistance at elevated temperature in comparison to other styrenic block elastomers. The alloys are available in Shore A hardness ranges between 45 and 65, including a special food contact grade. Targeted applications include power tools, personal care items, as well as sports and leisure equipment.
Teknor Apex offers a wide range of general and specialty TPE compounds. Its Tekbond compounds are targeted for overmolding applications. The SEBS-based elastomer provides excellent adhesion to a variety of rigid plastic substrates, including ABS, polycarbonate, acrylic, nylon and others.