DLFT EXPERIMENTS WITH CYCLIC BUTYLENE TEREPHTALATE

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1 DLFT EXPERIMENTS WITH CYCLIC BUTYLENE TEREPHTALATE Victor L. Bravo 1, James Mihalich 2, Martin R.McLeod 1, Martin N. Bureau 1 1 Magna-NRC Composites Centre of Excellence, Automotive Portfolio National Research Council of Canada 2 Cyclics Corporation Cyclic oligomers have low molecular weights and therefore exhibit very low viscosities (in the order of 20 mpa.s) in contrast to the resulting high viscosities related to high molecular weights obtained after polymerization. This characteristic is very appealing for the production of polymer composites since by taking advantage of the low viscosity, fibres can be thoroughly wetted by the cyclic oligomer before starting the reactive process of polymerization. As a result, a homogeneous mixture with a uniform volumetric distribution of reinforcing fibres in the polymer matrix is expected with the use of appropriate processing equipment. Cyclic butylene terephthalate (CBT ) has been successfully polymerized into poly(butylene terephthalate) (PBT) using a variety of tin or titanate-based initiators. An exploratory project has been initiated by NRC at the Magna-NRC Composites Centre of Excellence (MNCCE) with the participation of Cyclics Corporation (Cyclics) as the supplier of CBT for trials using the D-LFT process. The goal is to assess a number of aspects involved in the simultaneous reactive extrusion/compounding process using CBT fibreglass reinforcement in a D-LFT process in order to determine the adequate processing conditions to produce parts. This paper presents a description of the experiments conducted up to date and the results obtained thereby. Parts were produced using a test specimen mould with a shot-pot device and a storage tub for automotive applications. Samples were characterized for tensile, flexural and impact tests. Density measurements were taken at different points of the tub to examine the uniformity of the composite material obtained. Plaques moulded with the test specimen mould showed good mechanical properties which exceed typical values for PP with 40% glass composites. The experiments proved the feasibility of the use of the open ring polymerization process in a D-LFT system. A set of operating conditions including temperature profile, screw speed and feed rate was established to achieve the adequate residence time and material temperature history for the effective processing of CBT in the D-LFT system. Mechanical properties measured from the moulded part (tub) showed significantly lower values than those for the test specimen mould. These were related to incomplete packing of the material in the cavity of the mould, the high residence time required and non-optimal shot size. High variability within the parts and between parts produced is thought to be a consequence of the processing difficulties for the tub mould.

2 1. Introduction Cyclic oligomers have low molecular weights and therefore exhibit very low viscosities (in the order of 20 mpa.s) in contrast to the resulting high molecular weight obtained after polymerization. This characteristic is very appealing for the production of polymer composites since by taking advantage of the low viscosity, fibres can be thoroughly wetted by the cyclic oligomer before starting the reactive process of polymerization. As a result, a homogeneous mixture with a uniform volumetric distribution of reinforcing fibres in the polymer matrix is expected with the use of appropriate processing equipment. Cyclic butylene terephthalate (CBT ) has been successfully polymerized into poly(butylene terephthalate) (PBT) using a variety of tin or titanate-based initiators. A project has been initiated by NRC at the MNCCE with the participation of Cyclics as the supplier of CBT for trials using the D-LFT process. The goal is to assess a number of aspects involved in the simultaneous reactive extrusion/compounding process using CBT fibreglass reinforcement in a D-LFT process in order to determine the adequate processing conditions to produce parts. Parts were produced using a test specimen mould with a shot-pot device and a third row seat storage tub. Samples were characterized for tensile, flexural and impact tests. Density measurements were taken at different points of the tub to examine the uniformity of the composite material obtained. The material used for the trials is Cyclics CBT XL101. This resin is a low melt viscosity thermoplastic polyester resin that can be processed in typical composite processes including pultrusion and compression moulding. Through a reaction with the catalyst contained in the CBT resin, it is converted into the engineering thermoplastic polyester polybutylene terephthalate (PBT). The theoretical advantages expected from the use of CBT are: Superior fibre wet-out is achieved more rapidly without the need for high pressure. Productivity and performance is increased in common composite manufacturing processes. No heat is released during reaction (polymerization). Isothermal processing is possible. Final product is PBT thermoplastic enabling post-processing treatments such as thermoforming, compression moulding, and filament winding. End product has lower void content and enables higher fibre loading for a better quality composite. 2. Objectives Determine the optimal residence time and processing temperature to ensure: o Good fibre wet-out during compounding o Uniform distribution of the fibres in the polymer matrix o Complete polymerization Obtain mechanical and physical properties of the composite material produced and compare with typical values known for PP/fibreglass and PA6/fibreglass composites. 1

3 All these aspects were evaluated by visual examination as well as mechanical and physical characterization of the samples. 3. Experimental procedure The materials used for the trials were CBT XL101 from Cyclics Corp. and PPG ,100 tex fibreglass rovings. The CBT resin was dried at 80 C for about 6 hours with a -40 C dew point. To prevent the material pellets from breaking into powder and causing an obstruction in the feed section, the feed section of the twin screw extruder was kept at a low temperature. This prevents the powder from getting tacky and sticking to the screw. Three experiments were conducted at the MNCCE according to Table 1. Table 1: Experimental conditions Trial number Resin weight % Fibreglass weight % Screw rpm Parameters Flowrate (kg/h) Total cycle time (s) Weight of log (g) Mould , , x2 1 2x 7,735 Test specimen (shot-pot) Test specimen (shot-pot) Seat tub (compression) Experiments were conducted on a ZSK-70 Coperion co-rotating twin screw extruder. The screw configuration for the twin screw extruder is presented on Figure 1. A 2,500 tonne Dieffenbacher high-precision press with an active, servo-controlled parallel motion system was used for the trials. Figure 1: Schematic drawing of screw configuration used for compounding on the 70 mm Coperion TSE. The feed flow rate and rpm were estimated to produce an average residence time of 1.5 minute. The rpm was set up to have at least a 50% filled screw. The values were recorded once the feed rate (kg/hr) and rpm were fixed. 1 Two logs of 7,735 g were produced with a total cycle time of 500 seconds. 2

4 Feed throat was kept cooled (25 C) to prevent powder build up in the first screw flights, followed by gradual heating up to 250 C at the fibreglass feed zone. After the fibreglass feeding zone the temperature was kept constant at 250 C. The conditions were varied depending on the consistency of the material at the die exit. Gradual adjustments were made until the viscosity of the material at the die was adequate. The temperature in zone 2 was drastically reduced from the initial 200 C to 130 C to resolve the tendency of the CBT resin to agglomerate and block the passage during the first attempts. As the material viscosity appeared low (the material drooled out of the die with no consistency) the residence time was increased to 2 min. (by reducing flow rate and rpm proportionally) while maintaining the same temperature profile. The temperature after the glass feeding section was increased to 270 C from the initial value of 250 C in order to accelerate the polymerization. Once the material showed an adequate viscosity (product of a sufficient residence time for the polymerization to occur), logs were used for the moulding of parts. Table 2: Temperature profile in degrees C TC1 TC2 TC3 TC4 TC5 TC6 TC7 TC8 TC9 TC10 TC11 Initial Modified Characterization Specimens were extracted from the four-plaque mould and from the bottom of the seat tub for tensile, flexural and notched Izod impact testing. Density measurements using a fluid displacement method (Archimedes principle) were done to the composite samples. Fibre length measurements were done using a Sony α550 DSLR-A550 digital camera and a macro lens 2.8/100 SAL100M model 28. For fibres smaller than 5 mm an optical microscope was used. Clemex image analysis software was used for fibre length measurement. 5. Results 5.1. General observations The reactive extrusion and simultaneous compounding of CBT and glass fibres was successfully achieved. This was possible by allowing sufficient residence time for the material in the channel in order to achieve a high degree of polymerization. While the low viscosity of the material during the polymerization step allowed for a good wet-out of the fibres, the higher viscosities encountered at the end of the extruded had enough consistency to allow for a dimensionally stable extrudate. The extrusion process was performed with the venting section opened to allow a complete visualization of the material flow. The venting is located in section 9 of the extruder. At this stage the material must have a high degree of polymerization. The low viscosity material coats the fibres which tend to align perpendicular to the pitch angle of the screw flights (the direction of conveying). By visual examination the log shows a very good fibre 3

5 Tensile modulus (MPa) Tensile strength (MPa) distribution and the material has the necessary consistency for transportation trough the conveyor belt. GPC was executed for a moulded sample produced during Trial 2. The percentage of conversion was determined to be 85%. The molecular weight was found to be equivalent to injection moulding grades of PBT. The ash burn-off was roughly 36% by weight at the location where samples were taken. The fibre content can vary a few percent points depending on the location. Run 2 was set to produce 40% weight fibreglass content parts Mechanical and physical properties 4-plaque mould (Runs 1 and 2) MPa 80 MPa Figure 2: Tensile strength for Runs 1 and 2. 40% glass 20% glass 16,000 40% glass 14,000 12,000 10,000 8,000 6,000 4,000 2, MPa 4889 MPa Figure 3: Tensile modulus for Runs 1 and 2. 20% glass 4

6 Impact strength (kj/m 2 ) Figure 2 presents the results obtained for tensile strength measured as the maximum stress prior to rupture. The difference between the average maximum stress between the 40% fibreglass content and the 20% glass content is approximately 40 MPa. The chart presents the variations between trials and the error bars show the variation within specimens extracted from each plaque. The variation between trials may be attributed to lack of adequate compaction during the moulding process for some of the parts made which was caused by insufficient material being placed in the mould. Some variations may be originated from surging at the extrusion level which can cause fluctuations in the flow rate and therefore can produce small variations in the weight of the log. Surging was minimized for the trials by adjusting processing parameters. The strength value of 120 MPa is significantly higher than the value of 76 MPa obtained for PP with 40% fibreglass. However, the value is lower than the maximum tensile stress measured for composites made with PA6 and 40% fibreglass using the same processing equipment for which a value of 145 MPa was obtained. The tensile modulus results (Figure 3) show again good consistency of the measured values for the twelve samples of PBT with an average value of 9,427 MPa for the 40% glass content and 4,889 MPa for the 20% glass content. While the increase from 20% to 40% glass content produced an increase of approximately 50% in tensile strength, it produced an increase of 93% in the tensile modulus. This is expected as the effect of fibreglass addition on the stiffness of the composite material is almost proportional to the weight percentage, while the effect of the fibres on the maximum stress is strongly determined by the fibre length, distribution and fibre-matrix interaction. A comparison with PP and 40% fibreglass show again a clear performance advantage of the PBT composite. The PBT with 40% long glass fibre had an average tensile modulus of 9,427 MPa compared to 5,890 MPA for PP with 40% long glass fibre % glass 20% glass 30.0 kj/m kj/m Figure 4: Impact strength (notched Izod) for Run 2. Figure 4 shows the results for notched impact test. The average value for the 40% fibreglass content was 30 kj/m 2. This is a very good impact resistance that exceeds automotive standards for composite parts. The value is comparable to the results obtained for PP with 40% long glass fibres (at 35 kj/m 2 ) and significantly better than the values measured for PA6 with 40% long 5

7 Average length (mm) glass fibres (12.5 kj/m 2 ). The addition of 20% weight fibreglass (from 20 to 40%) produced an evident increase in impact strength of almost double the original value. This is evidence of good matrix-fibre interaction and a demonstration of the effectiveness of the D-LFT process using continuous fibreglass rovings which produces composites with very long fibres and excellent mechanical properties, particularly for tensile and flexural strength as well as impact strength. Fibre length measurements were made from pyrolyzed samples (500 C, 3 hours) using an optical low magnification microscope (Zeiss, 3-10X). Glass fibre lengths were measured using an image analyzer. Fibre length distributions were then obtain from individual fibre length measurements at a low magnification chosen to ensure fibres with length between 2-4 mm and +50 mm could be measured. The minimum count of fibres varied from a minimum of 111 to 136. From the fibre length distribution obtained, the number-average and length-average fibre lengths and were calculated according to Equations 1 and 2. A fibre length distribution (FLD) index was then extracted from the ratio of weight to number-average fibre length. A FLD close to one indicates a near monodispersed FL distribution, while values above one are indicative of a polydispersed distribution (large and small FL populations). (1) (2) (3) %-1 20%-2 20%-3 40%-1 40%-2 40%-3 Sample ID ln lw FLD Figure 5: Number and length averages and FLD index for glass fibres as a function of glass percentage 6

8 Frequency Frequency The graph above indicates that as the glass content is increased from 20% to 40% the average fibre length decreases. The presence of longer fibres on the 20% glass formulation is evident from the graph and it is reflected by the length average (lw). The FLD is also generally higher for the 20% glass formulation compared to the 40% glass. A possible explanation for the reduction in fibre length is the more pronounced fibre-fibre interactions during compounding in the twin screw extruder and also during the compression moulding process. The presence of longer fibres is also evident on Figure 6. For the 20% glass formulations a number of fibres longer than 60 mm were measured as the graphs show and fibres up to 101 mm in length were measured. For the 40% glass formulations the longest fibres were around 60 mm. The black line shows a general trend of the charts were a large number of fibres are within 1 to approximately 26 mm in length after which the number of fibres greater than 30 mm diminishes. Table 1 shows the average lengths measured for all the samples Fibre length (mm) Fibre length (mm) Figure 6: FLD for 20% (left) and 40% (right) glass formulation Table 3: Average length and FLD index of glass fibres Sample number ln lw FLD 20% glass 40% glass Results for seat tub Seat tub parts were produced as a demonstration for the technology used in the production of a commercial part. The tub is used in second and third rows of the seat minivans to stow the 7

9 Tensile stress at max (MPa) seats under the floor. Figure 7 shows the bottom of the moulded tub illustrating the specimen cutting pattern. Samples were extracted aligned with the horizontal direction called for reference purposes flow direction, as this is the predominant orientation of the flow on this part. The samples aligned with the vertical direction are labeled as X-flow direction. Tensile and flexural properties were measured for the samples extracted from the moulded parts. Specimens identified as D.T. were used for measurement of notched Izod impact stress. Specimens identified as G.F. were used to determine composite density. Figure 7: Specimen cutting pattern for the bottom of the seat tub Flow X-flow MPa Figure 8: Tensile stress for seat tub trials. 8

10 Tensile modulus (MPa) Figure 8 shows the results for tensile strength measure on specimens extracted from 10 moulded seat tubs. The green bars correspond to the specimens in the flow direction (see Figure 7) while blue bars represent stress values measured in the x-flow direction. The average stress value was determined to be 60.5 MPa in both flow and x-flow directions. The error bars present the standard deviation based on the five specimens extracted for every tub. Figure 9 describes the results for tensile modulus in the flow and x-flow directions. The average values for modulus are 5,025 MPa and 4,435 MPa for the flow and x-flow directions respectively. Figure 10 presents the results for Izod impact strength obtained from various locations of the seat tub. Figure 11 contains the results for density measurements using a laboratory density meter Flow X-flow MPa 4435 MPa Figure 9: Tensile modulus for seat tub trials. 9

11 Density (g/cm 3 ) Impact strength (kj/m 2 ) Flow X-flow 18,3 16, Figure 10: Notched Izod impact strength for seat tub trials g/cm 3 Figure 11: Density measurements for seat tub trials 5.4. Discussion Trials showed the feasibility of performing a ring opening polymerization in a D-LFT process. The open barrel at the venting section allowed a visual evaluation of the interaction between the resin and the fibres. It was apparent that the viscosity of the PBT at the open vent had not reached its maximum, therefore allowing for a very effective wet-out of the individual fibres. However, this assertion is based on purely visual evaluation and a more rigorous assessment of the degree of polymerization and the viscosity at specific locations along the extruder barrel needs to be devised. A practical residence time was achieved within the operation limits of the machine for the four-plaque specimen mold, while problems came up for the seat tub mould because of the large size of the part. The long residence time required for the material to polymerize in the system produced an excessive exposure time to room temperature for 10

12 the extrudate (log) which potentially affected the flowability of the material in the mould. The screw configuration proved to be effective for the mixing and compounding of the resin and the glass fibres. Concerns about the screw s ability to pump the low viscosity material were dissipated by the success of the trials. The experiments involving the compression moulding of the seat tub were run with a nominal fibreglass content of 33% weight. The mechanical properties of the samples extracted from the bottom of the tub were expected to be within the measured values for the 20% and 40% fibreglass samples produced with the four-plaque specimen mould. The average tensile modulus for the 40% fibreglass formulation using the four plaque mould was 9,427 MPa while the average value for the tub with 33% fibreglass was 5,025 MPa and the average tensile strength for the 20% fibreglass formulation with the four-plaque mould was 4,889 MPa. While the tensile modulus of the tub sample was within the range of the 20% and 40% fibreglass content samples, the results were rather low. The average tensile strength for the 40% fibreglass four-plaque mould samples was 120 MPa while the value for 20% fibreglass content was 80 MPa. The results for the tub at 33% fibreglass content show an average value of 61 MPa. The low tensile strength result may be attributed to two potential causes, the first being a low conversion rate achieved during polymerization which could have affected molecular weight and ultimately mechanical properties. While the conversion rate for the fourplaque mould was determined to be 85%, the conversion rate of the tub still needs to be determined to corroborate or reject this hypothesis. The second potential cause for the relatively low mechanical properties of the tub is the lack of compaction originated from the difficulty to completely fill the cavity. Flow problems related to low temperature of the composite material prior to moulding and short shots caused by insufficient material being placed in the mould are the causes of the lack of adequate compaction in the parts. The tensile properties measured for the seat tub did not show a significant difference between the flow and x-flow properties. This behavior is not attributed to the material used for this experiment but it is rather related to the geometry of the mould and initial area covered by the log. The logs were laid on the bottom of the tub covering about 70% of the total surface. The material under compression flows downwards towards the side walls of the tub without a predominant orientation for the flow. Impact strength results for the seat tub samples, as for the tensile properties, showed relatively low values when compared to the results obtained from the four-plaque mould. While the four-plaque mould experiments produced average notched Izod values of 30 kj/m 2 at 40% fibreglass content and 17 kj/m 2 at 20% fibreglass content, the samples extracted from the tub produced an average impact strength value of 18.3 kj/m 2 in the flow direction and 16.5 kj/m 2 for the x-flow direction. Density measurements (Figure 11) show an average value of 1.53 g/cm 3, which according to equation 4 corresponds to 68.7% PBT (ρ=1.30 g/cm 3 ) and 31.3% fibreglass (ρ=2.50 g/cm 3 ). (4) 11

13 Post mortem measurements of fibre content performed on samples extracted from the tub confirmed an average weight glass content of 32.4%. The theoretical density calculated with equation 4 for 32.4% glass and 67.6% PBT is 1.54 g/cm Conclusions The experiments proved the feasibility of the use of the open ring polymerization process in a D-LFT system. A set of operating conditions including temperature profile, rpm and feed rate was established to achieve the adequate residence time and material temperature history for the effective processing of XL101 in the D-LFT system. Plaques were moulded using a test specimen mould (four-plaque) under stable conditions producing very consistent extrudates with uniform fibre distribution (from a visual inspection). Results with four-plaque specimen mould showed good mechanical properties which exceed typical values for PP with 40% glass composites. Large industrial parts (seat tub) were successfully produced using the open ring polymerization process in conjunction with the D-LFT process. Production of parts from the seat tub mould presented problems because of the large size of the shot (approximately 15 lb) and the difficulty to produce logs at the required speed to guarantee the required residence time during extrusion and compounding. Mechanical properties measured for the tub showed lower values than those for the four-plaque mould. These were arguably caused by the processing difficulties mentioned on this article (high residence time required, keeping log at high temperature, insufficient shot size), which prevented a good packing of the material in the cavity of the mould. The property variability within the parts and between parts produced is thought to be a consequence of the processing difficulties as well. Samples from the seat tub with 20% glass showed higher values of and most noticeably of which indicates the presence of longer fibres. Histograms show the presence of fibres longer than 60 mm for the 20% glass composite tubs. It is suggested that fibre-fibre interactions for the high fibre content formulation (40%) may be the cause of the shorter fibre lengths encountered. 6. References Vaidya, U., Composites for Automotive, Truck and Mass Transit-Materials, Design, Manufacturing, DEStech publications Inc. p.411, 2011 Zweifel, H., Plastics Additives Handbook (5 th Edition.), Hanser Gardner, Manas-Zlockzower, I., Mixing and Compounding of Polymers (2 nd Publications, 2009 Edition)., Hanser 12