IN-SITU POLYMERIZATION OF REINFORCED THERMOPLASTICS

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1 IN-SITU POLYMERIZATION OF REINFORCED THERMOPLASTICS Jim Mihalich Cyclics Corp Abstract Most reinforced thermoplastics are produced from fully polymerized resins which are then introduced to the reinforcement in a compounding extruder or an extruder which coats and forms tapes of various sizes. By introducing the reinforcement into the resin prior to polymerization the high viscosity of the thermoplastic resin does not inhibit the full incorporation and wetting of the reinforcement. The trade-off becomes the polymerization step which still must take place prior to having a serviceable composite. Four processes: reactive extrusion of nanoclay reinforced PBT, direct, long fiber compounding and compression molding, reactive extrusion coating of continuous fiber cloths tp produce high volume fraction sheets and molding of continuous fiber reinforced parts will be discussed. Introduction Most reinforced thermoplastics are produced from fully polymerized resins which are then introduced to the reinforcement in a compounding extruder or a coating extruder line. This creates a dilemma in selecting the appropriate molecular weight and viscosity of the matrix resin. A low molecular weight matrix resin is desirable to allow the reinforcing filler to be completely wet out by the matrix resin. Incorporating reinforcing materials into a polymer matrix by itself increases the viscosity so as the matrix resin viscosity increases, not only is it more difficult to produce the compound or part, it can become more difficult to subsequently process the material in down-stream operations. There is a limit on how low the molecular weight of the resin can be. At very low levels, the mechanical properties of the material become unacceptable for use in the application. As performance demands drive increasing reinforcement, the ability to process the material into a part becomes the limit. It is possible to change to order of operations in the value chain and introduce the reinforcement to the matrix prior to polymerization. This redefines the tradeoffs between viscosity during processing and mechanical properties normally dependent on molecular weight. This paper will examine several approaches based on cyclic oligomers of Polybutylene Terephthalate which introduce the reinforcement to the matrix prior to polymerization followed by the in-situ polymerization of the matrix to produce a highly reinforced, high molecular weight polymer with commercially interesting properties. Page 1

2 The Benefits of In-Situ Polymerization There are several interrelated benefits which can result from in-situ polymerization. The matrix resin can be polymerized to a higher level than would be capable of wetting the reinforcement. The higher viscosity of the material with the reinforcement can be a limit to down-stream processes so compounds can still be made which will not be injection moldable. However, process combinations where either the final part is produced when the reinforcement is introduced can see the benefit. The matrix resin also sees less mechanical work during the introduction of the reinforcement in in-situ polymerization. This reduces the thermal degradation during the reinforcement process, particularly in nano reinforced compounds. In addition to the improved dispersion and incorporation, less thermal degradation can mean less color variation as well as longer part life. A lower viscosity matrix can yield higher reinforcement loadings in continuous fiber composites. The low viscosity before polymerization can allow full impregnation of even large carbon fiber tows. In DLFT, less mechanical work to incorporate fibers can yield longer fiber lengths, improving mechanical properties. The interface between reinforcements and the matrix resin is also better with in-situ polymerized composites. Improving the wet-out leads to fewer voids and dry spots. There is also the opportunity for chemical bonding between the matrix and the reinforcement. This can be on a direct basis or via surface treatments like coupling agents or fiber sizing. When polyester oligomers are polymerized to PBT, they will bond to epoxy, hydroxyl or ester groups on the fiber or filler surface. This translates directly to improved mechanical properties. The Fundamentals of In-Situ Polymerization Seven steps must take place to effectively polymerize polyester oligomers. There is some flexibility in the order these can take place, but as one develops a process all steps must be considered. 1. Drying. Water acts as a chain stopper in the polymerization reaction. The resin should be dry to levels typical of processing polyesters. 2. Melting. Polyester oligomers are solids are room temperature. Melt processing can begin at 150C and the oligomers are completely melted at 195C with their characteristic viscosity of 50 cps. 3. Catalyzing. The resin can be supplied pre-catalyzed or catalyst can be introduced during the process. A variety of tin and titanium catalysts have been shown to be useful. Once the catalyst is introduced, polymerization can begin in seconds or delayed for up to 20 minutes depending on catalyst selection and temperature. 4. Infusing. The low viscosity allows easy wetting of the reinforcing materials. Developing the molecular weight of the polymer after infusing is the key to in-situ polymerization. 5. Shaping. The infused and polymerized material can be pelletized or made directly into parts. 6. Polymerization. Having the reinforcement present when the polymerization occurs opens the door to performance not available to normal pre-polymerized processes. 7. Crystallizing. The cooling and morphology development is assumed in most thermoplastic processes, but is an important consideration in processes adapted from thermoset based processes. Page 2

3 The Challenges of In-Situ Polymerization There are two basic challenges which need to be considered during in-situ polymerized composite. The first is the polymerization time needed to develop the desired molecular weight. The second is the added complexity of the process. Both challenges can be addressed through process selection and design. The polymerization time of polyester oligomers can vary from under a minute to an hour depending on the catalyst selected and the temperature. Faster cure is better and only when there is a need to accommodate fiber infusion and catalyst mixing times is cure time limited. In a compounding operation, the reinforcement can be introduced and totally wet-out before the catalyst is mixed into the compound. This suggests the use of a fast acting catalyst metered into the mixing section of a compounding extruder. In extrusion processes, a fast acting liquid catalyst metered and mixed into a relatively cool resin melt stream just prior to infusion with an increase in composite temperature after infusion is indicated. In molding there are two approaches: shelf stable catalysts can be mixed into the resin which can then be infused into the reinforcing fiber and separately cured in the molding operation, or catalyst introduction and infusion can be done immediately prior to cure in one operation. In both cases the process will require the tool to heat, and then cool the part. The increase in complexity is a result of the need for three separate temperatures to hit during the process. The temperature to introduce the reinforcement (190C to 250C for polyester oligomers), the desired cure temperature (230C to 250C for polyester oligomers) and the de-mold temperature (60C to 80C for polyester oligomers) each are important to proper processing. These target temperatures are different than equipment set points which are details of how a melt profile, or target temperature, can best be obtained in a particular piece of equipment. Controlling this temperature profile in a continuous process is simpler and easier in a continuous process like compounding or extrusion compared with an intermittent process like molding. In-Situ Polymerized Nano-Composites The simplest in-situ polymerized process for reinforced composites is the reactive compounding of nano reinforced compounds. The polyester oligomer, the catalyst and the nano clay are fed into a twin screw compounding extruder resulting in an injection moldable, E-coat capable nano clay reinforced PBT compound for vertical body panels. Dow Chemical published a summary of one approach in US patent 7,329,703. The key requirements, which were substantially met, of the application were: High heat resistance, Thermoplastic injection mold-ability, Excellent surface aesthetics, High stiffness / modulus, Dimensional stability, High toughness. The low viscosity of the polyester oligomer was sufficient to intercalate the clay platelets. When the oligomer polymerized, the separated clay platelets exfoliated and stayed separate as the compound cooled. The mechanical work normally associated with nano particle dispersion was replaced by the solvating action of the low viscosity oligomer. Page 3

4 Modulus Versus Temperature Response Comparison Shear Modulus (dyne/cm 2 ) 1.00E E E E+08 Compounded Linear PBT + 5% Organoclay Unfilled pcbt Polybutylene Terephthalate Reactively Processed pcbt Nanocomposite (5% organoclay) 1.00E Temperature ( C) Substantial modulus enhancement for pcbt nanocomposite. Figure 1 shows the difference between in-situ polymerized PBT (pcbt), a similar standard compound, and an un-reinforced insitu polymerized PBT. The benefit of the nano reinforcement is clear and the improvement of in-situ polymerization is as large as the improvement offered by the reinforcement. In-Situ Polymerized DLFT The nano clay in the above example can be replaced with long carbon or glass fiber. In this case, two issues arise. The first is the higher levels of fibers typically desired in addition to the increased molecular weight of the matrix polymer makes subsequent injection molding difficult. The second is the limit on fiber size imposed by the pelletizing process. These reasons favor going directly from a compounding extruder into a compression molding press. A feasibility trial was done with National Research Council Canada on a 70 mm co-rotating twin screw extruder and a 2,500 ton hydraulic press located at the Magna-NRC Composites Centre of Excellence in Concord, Ontario. A co-polymer cyclic oligomer was selected for the trial as it was pre-compounded with a medium fast tin based catalyst. This was in part to reduce the need to separately handle and meter a fast liquid catalyst. It was anticipated this would polymerize more slowly than available titanium catalysts, and because it was an elastomeric copolymer, the impact strength would be higher and the stiffness would be less than a homopolymer as shown in Table I. Page 4

5 Table I: Xl-101 vs. cpbt Homopolymer Units XL-101 CBT 160 Tensile at yield MPa Tensile at max MPa Ten Modulus MPa 1,300 3,300 Dart impact j/mm 12 1 Table II: DLFT in Situ Polymerized XL-101 cpbt Copolymer Units 20% Glass 40% Glass Notched Izod kj/m Tensile Strength MPa Tensile Modulus MPa 4,500 9,300 The trial was a first attempt to demonstrate a very low viscosity resin could be combined with continuously fed fiber and make a part. The polymerization of the resin ranged from 88% to 94% converted. This represents the minimum acceptable conversion level. The molecular weight of the polymer was 38,000 to 50,000. This is equivalent to the high flow commercially available PBT products for injection molding. Most in-situ polymerization results in molecular weights of 80,000 to 150,000. Two factors are considered to contribute to the low molecular weight, drying and residence time. Cyclic oligomers require the same level of drying that PBT and other engineering polymers require. The dying temperature is lower. The material was dried but was exposed to ambient in the hopper prior to extrusion. Keeping the material dry might offer some improvement. The need for additional residence time, the most likely cause can be reduced by using a faster catalyst. These catalysts are readily available, but require more care in handling and were decided against using for the first trial. In-Situ Polymerized Extrusion Coated Sheet The first composite sheet based on polyester oligomers was made using a powder deposition and laminating belt process. This required the use of a pre-catalyzed resin system adding a step in the production chain and increasing the cost. Additionally, the use of a precatalyzed resin limits the production rate. The faster titanium catalysts are sensitive to moisture so they cannot be pre-compounded into a one-part product. Page 5

6 Figure 2: Extrusion Coated Pre-Preg 1800 gsm Glass, 50% FV Polyester Oligomer Initial work with extrusion coating of oligomers was done to produce pre-preg for vacuum bagging. Good infusion, coverage and distribution opened the door for consideration of the process to produce fully polymerized sheet. Pre-preg sheet was produced ranging from 160 gram per square meter uni-directional carbon to 1850 gram per square meter stitched glass. The difference in the process will be the inclusion of an oven after the coating operation to polymerize the resin. Metering a fast catalyst into the resin stream just as it enters the die should enable good control of infusion, coating weight and line speeds. Trials are planned in the second half of In-Situ Polymerized Molded Parts There are several processes which can be adapted to molding cyclic polyesters composites. These can be based on pre-infused, un-reacted pre-preg sheets in processes like compression molding or vacuum bag molding or they can be based on infusing into dry fiber in the tool, followed by in-situ polymerization in processes like resin transfer molding. Recent work on insitu polymerized composites has been reported based on an injection molding platform. This is a welcome development since the injection molding equipment is configured to handle the higher temperatures that cyclic polyesters require compared with traditional thermoset resins. In each of these cases, managing the temperature and timing is the primary goal. To infuse the fabric, the resin should be at 190C to 200C. The resin will be fully melted and also provide the maximum working time for infusion. The cure temperature should be C. It is necessary to be above the melt point of the polymer during polymerization to so the polymerization will be complete prior to the crystallization during the cooling step. Separating the polymerization from a fast cooling step will improve the composite toughness. The part can be demolded at 130C or below. There is a trade-off between the time held at the curing temperature and the time to heat and cool the part and mold. Increasing the temperature from 230C to 250C will decrease the time to polymerize by half, but that is offset by the increased time to heat and then cool the mold and part to the higher temperature. Page 6

7 Introducing the catalyst into the resin is another key consideration. When the catalyst is introduced into the melt it will begin to polymerize. The length of time available to infuse the fiber can vary, but all of the catalyzed material will advance and become high in viscosity. This means a fast cure cycle will require a fast infusion cycle, and all of the catalyzed material must be infused or be lost. This suggests moving the mixing point for the catalyst to just prior to the resin entering the mold. The mixing point should both allow thorough mixing and be selfcleaning between parts. A wide variety of parts has been produced using both pre-preg and dry fiber. Parts as large as 600 kgs from pre-preg have been made. Vacuum infused glass and carbon fiber prototypes have been made 10 and 5 kgs respectively. Producing parts in a commercially viable process is a current area of development focus. Figure 3: 7 Inch Diameter, 31 Inch Long CBT Glass Fiber Roller Sleeve Produced by Resin Transfer Molding Page 7

8 Figure 4: 13 Meter Truck Trailer Bed Vacuum Bag Molded from Pre-Preg Conclusion In-situ polymerized polyester oligomers offer an opportunity to produce reinforced thermoplastic composites with unusual properties based on their ability to provide superior wetting in adapted processes. References 1. Dion, et al. US Patent 7,3229,703. February Mihalich, James. Production of Class 8 Truck Trailer Bed Using cpbt Thermoplastic Pre-Preg and Vacuum Bag Processing. SCCE Conference. September Parton, Hilde. Characterization of the in-situ Polymerization Production Process for Continuous Fiber Reinforced Thermoplastics. Katholieke Universiteit Leuven Faculteit Ingenieurswetenschappen Arenbergkastell, B-3001 Heverlee Belgium. February Page 8