A FORMULATION STUDY OF LONG FIBER THERMOPLASTIC POLYPROPYLENE (PART 3): MECHANICAL PROPERTIES OF PP DLFT COMPOSITES

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1 A FORMULATION STUDY OF LONG FIBER THERMOPLASTIC POLYPROPYLENE (PART 3): MECHANICAL PROPERTIES OF PP DLFT COMPOSITES Creig Bowland, Brad Busche, J. vd. Woude PPG Industries, Fiber Glass Science and Technology Abstract In part three of this multiyear study the properties and performance of PP DLFT are explored using a Coperion based DLFT compression molding system. An extensive formulation DOE was initiated to determine the performance of PP DLFT and to compare and contrast this work with the prior work done on PP GLFT. The results of this ongoing research are reported. Background Long Fiber Thermoplastics (LFT) have enjoyed a long run of double digit growth and have found general acceptance as structural materials. Polypropylene (PP) LFT materials offer strength and stiffness and are recyclable having a long shelf life. Consequently, their introduction has led to an increased penetration of the automotive market for structural thermoplastic composites. These applications typically are metal replacement and facilitate both part weight reduction and part consolidation. PP LFTs are the largest segment of this market and are also experiencing the largest growth. Within the PP LFT segment there are three distinct methods of making and using these materials. The classic method is the pultrusion impregnation and/or cross head extrusion method of generating LFT pellets that are subsequently injection molded. For the purpose of this paper this type of material is labeled Granulate Long Fiber Thermoplastics (GLFT). The second main method of producing LFT is In Line Compounding(IC). Within the IC segment there are two distinct methods of producing parts. In one method, the IC material is compression molded into parts in a process similar to that used for the manufacture of Glass Mat Thermoplastics and thermoset parts. In the second method the IC material is fed directly into an injection molding machine for either standard Injection molding or Injection compression molding. For the purpose of this paper both are labeled as Direct Long Fiber Thermoplastics (DLFT) and are differentiated as compression and injection DLFT. PP LFT materials exhibit their best performance when the polypropylene resin has maleic anhydride grafted polypropylene added to the matrix. The amount and characteristics of this additive are well documented in short glass polypropylene compounds [6-9] but are not well reported for PP LFT materials [1-2]. There are a number of factors that affect the performance of the coupling agent in PP LFT parts. Coupling Agent concentration Maleic Anhydride (MAH) grafting levels in the coupling agent Melt (MFI) of the Coupling agent. Fiber glass content and sizing Polypropylene properties such as MFI, molecular weight and crystallinity. Mold Design and temperature Method of making PP LFT: GLFT vs. IC DLFT. The DLFT and GLFT processes are distinct and can lead to different part performance based upon the heat history, Page 1

2 ultimate fiber length retention, fiber orientation in the part and time at temperature to allow effective coupling. In previous studies [1-2] the results of injection molded (IM) PP GLFT parts were studied. The coupling agent melt and maleic anhydride loading were found to be important to obtaining optimum properties Flexural Strength (MPa) % MAH in Coupling Agent 0.5% Coupling Agent in Composite MFI of Coupling Agent Figure 1: Flexural Strength versus Coupling Agent MFI and MAH Loading In 60% PP GLFT with TufRov 4575 Fiber Glass Figure 1 shows the response of flexural strength in 60% glass filled PP GLFT versus the percent of MAH and the MFI of the coupling agent. The percent of MAH is reported as % MAH in the coupling agent. There are a number of trends that can be extracted from Figure 1. First: The maximum flexural strength obtained is almost 280 MPa. This is phenomenally high flex strength for a PP based composite. Second: The higher the MFI of the coupling agent, the higher the flex strength. Third: The higher the MAH loading, the higher the flex strength. The trends break down slightly at low coupling agent loadings but this may be because of an incomplete coverage of the fiber glass with coupling agent. The amount of coupling agent needed to obtain the maximum flexural strength in a 60% (w/w) LFT part is also low at 1% MAH with TufRov 4575 fiber glass. TufRov 4575 has a highly reactive sizing system that enhances the coupling agent performance. With other less reactive sizing systems a large dose of coupling agent is needed to obtain equivalent properties. Page 2

3 Coupling Agent Loading (% w/w) Surface Area of Fiberglass in 1 Kg of Composite (m 2 ) Figure 2: Correlation between Surface Area of Fiber Glass and amount of Coupling Agent needed in PP GLFT In Figure 2 the coupling agent loading is plotted against the surface area of the TufRov 4575 in the composite. The amount of coupling agent needed for optimal properties in GLFT is proportional to the surface area of fiber glass in the composite. This type of curve can be generated for each fiber glass type and for each coupling agent used in PP GLFT. The response curve generated is specific to the fiber glass sizing and may be differ with changes in other additives The purpose of this work is to complete these studies on DLFT compression molded materials and to determine the optimum properties and performance. The goal is to obtain technical data similar to that seen in Figure 2 showing the correlation between the glass surface area and the amount of optimum loading of materials in the PP DLFT composite. Experimental PPG TufRov 4575, TufRov 4599, TufRov 4588 (all three 206 yield 17 micron diameter) and PPG LFT yield 24 micron fiber glass rovings were processed through a Coperion ZSK mm twin screw extruder to produce DLFT plaques impregnated with PP resin. Marlex HGZ homopolymer (120MFI) was used as the main constituent in each trial set with a heat stabilizer package, carbon black, and coupling agent. A blended masterbatch containing a heat stabilizer package and carbon black remained constant at 5% (w/w). An extensive Design of Experiment (DOE) set was run. The variables included in the DOE were: 1. Coupling agent utilized Page 3

4 Resin Lbs/ Hr 2. Coupling agent loading in the composite 3. Amount of fiber glass in the composite by weight 4. Polypropylene polymer type (homopolymer versus copolymer). 5. Fiber Glass Type (sizing composition change) 6. Extruder revolutions per minute (RPM) Samples are produced by introducing a resin mixture into the twin screw extruder and allowing the resin to melt before directly introducing fiber glass rovings into the extruder at a later point. The heat profile of the twin screw extruder is listed in Table 1. Table 1. Molding Thermal Profile of ZSK mm Twin Screw Extruder Molding Profile Zone C Zone C Zone C Zone C Zone 5 2 C The speed of the extruder varied from 60 rpm to 120 rpm while varying the amount of PP resin mixture being fed into the extruder to control the glass content of the samples. Figure 3 illustrates a curve produced to correlate rpm versus lb/hr PP resin to control the percent by weight of fiber glass in the samples. Screw Speed vs. Lbs/ Hr Screw Speed (RPM) % Glass Content 30% Glass Content Figure 3. Correlation of Screw Speed versus Resin Lbs/Hr to control glass content Page 4

5 Table 2. Collection Time of Charge for Screw Speed of Extruder Collection Times Screw Speed Glass Content % 30% 90 rpm 2min 1min 30sec 60 rpm 2min sec 2min 120rpm 1min 30sec 1min 10sec To ensure uniform thickness and consistency in the DLFT produced samples, the amount of extrudate was collected for equal amounts of time depending on the screw speed of the extruder. Table 2 lists the collection times for the different screw speeds. To complete the sample the charge coming out of the extruder was compression molded. A cm by cm plaque tool was installed in a St. Lawrence 150 Ton Hydraulic Press. The charge was pressed at 450 psi for seconds with a mold temperature of 1 F. The plaques produced were cut using an O-MAX 2626 water jet cutter to make the final test specimens for mechanical analysis. Unless otherwise stated, all mechanical property testing was performed at 23⁰C and at a relative humidity of 50%. All testing was done in the PPG Fiber Glass Science and Technology Laboratory in Hoogezand, The Netherlands. Tensile properties were tested according to ISO using 10 specimens and a crosshead rate of 5 mm/min (0.2 in/min) with an extensometer gauge length of 50 mm (2 in). Flex properties were tested according to ISO using 10 specimens and a crosshead rate of 2.0 mm/min (0.8 in/min) and a span of 64 mm (2.56 in). Charpy notched impact properties were tested according to ISO with 10 specimens and a Type A notch. Charpy UnNotched impact properties were tested according to ISO Single Fiber Pull Out Testing Single Fiber Pull out testing was performed by The University of Strathclyde. Fully compounded polypropylene resin with heat stabilizers, carbon black and coupling agent were produced on the Shelby ZSK mm extruder and pelletized. This pelletized PP compound was then used to produce single fiber pull out samples. Two separate formulations were produced: one with 0.5% and one with 2.0% by weight BondyRam 1001 coupling agent. Marlex HGZ1200 Homopolymer PP resin and the standard heat stabilizer/black masterbatch. TufRov yield, 17 micron fiber glass and TufRov yield, 17 micron fiber glass were evaluated with the two different resin formulations. Table 3:Typical Conditions of Single Fiber Pull-out Test Conditions for sample Heated at 2 C for 4 minutes followed by Page 5

6 preparation Test Rate cooling at ambient temperature 0.1 mm/min Test temperature 24 C Relative humidity 45% Results Figure 4: Representative curve for TufRov 4599 with 2% BondyRam 1001 Figure 4: The 4599 single fiber pull out curve follows a classic slip/stick failure mode and is further evidence that the primary mechanism of failure is fiber pull out. Friction is responsible for holding the fiber in the resin. When the load rises to a critical level the fiber slips until the load is alleviated. The cycle then starts again. There is good wetting of the resin onto the fiber surface, but little actual adhesion between the resin and glass. Page 6

7 Figure 5: Representative curve for TufRov 4575 with 2% BondyRam 1001 Figure 5: The 4575 single fiber pull out test show excellent adhesion between the fiber and polypropylene matrix. The fiber does not slip from the resin but continues to hold until the load reaches a critical level and complete failure occurs. The interface between resin and fiber does not fail and is indicative of a high level of bonding between the fiber glass and resin. The maximum load is also significantly higher and indicative of better tensile and flexural strength in the composite. Table 4: IFSS for TufRov 4599 and TufRov 4575 Interfacial Shear Stress (IFSS) Comparison of Fiber Pull Out IFSS at 2⁰C for 4 minutes 4599 (0.5% CA) 4599 (2% CA) 4575 (0.5% CA) 4575 (2% CA) Sample size Mean IFSS (MPa) Standard Deviation Confidence Level (95%) Page 7

8 Table 4 shows the mean IFSS strength from the single fiber pull out tests. TufRov 4575 shows higher IFSS values compared to 4599 and is further confirmation of the improved interfacial adhesion offered with this fiber glass. The 0.5% coupling agent loading levels show a significant drop off in properties in the study for both the 4599 and 4575 fibers and indicate that 0.5% coupling agent loading is not sufficient for optimal mechanical properties with BondyRam 1001 coupling agent. BondyRam 1001 has 0.9%-1% MAH loading and has a 100 MFI. Other work done [1-2] indicates that a loading of 0.5% to 1% coupling agent with these base properties is sufficient in GLFT. The data indicates a slightly higher loading is required for DLFT. SEM fracture surface analysis Broken tensile specimens were potted in epoxy resin and cut to size for insertion into a scanning electron microscope (SEM). The fracture surface was left as is (not polished) and a survey of the fracture surface was done to determine the primary mechanism of failure using a Tescan VEGA TS 5130LS Scanning Electron Microscope. Figure 6: TufRov 4575 PP DLFT Fracture Surface In Figure 6, the PP DLFT samples reinforced with TufRov 4575 fiber glass show a distinct failure mode of crack propagation through the fiber glass instead of the classic fiber pull out Page 8

9 mechanism of failure. There is also a lot of resin evident on the fiber surfaces indicating good wet out and excellent adhesion between resin and fiber glass. The tracks where the glass was removed during the failure are not clean and indicate failure in the resin and not at the interface with the glass. Figure 7: TufRov 4575 PP DLFT Fracture Surface In Figure 7, a higher magnification on the TufRov 4575 fracture surface shows the degree of wet out and adhesion to the PP resin that this reactive sizing imparts to the composite. The fiber pull out tracks are rough and orange peeled indicating a good adhesion between the fiber and resin. The crack propagation was through the resin and through the fiber but not along the interface between the glass and resin. Page 9

10 Figure 8: TufRov 4588 PP DLFT Fracture Surface In Figure 8, the PP DLFT samples reinforced with TufRov 4588 fiber glass show a combination of fiber pull out and some crack propagation through the fibers at the failure surface. The fiber pull out mechanism appears to be the primary mechanism of failure. Note the tracks in the resin where the fibers once imbedded in the resin. The fiber glass surfaces are fairly clean of PP resin and indicate that the primary failure was at the interface. Page 10

11 Figure 9: TufRov 4599 PP DLFT Fracture Surface In Figure 9, the PP DLFT samples reinforced with TufRov 4599 fiber glass show fiber pull out failure as the primary mechanism. Note the tracks in the resin where the fibers once were and the clean fiber surface indicating low adhesion to the resin. Page 11

12 Figure 10: LFT 00 PP DLFT Fracture Surface In Figure 10, the PP DLFT samples reinforced with LFT 00 fiber glass indicate a majority of the failure was from crack propagation through the fiber glass. There is very little indication of fiber pull out and there are significant amounts of resin still present on the glass. The debris indicates a brittle failure as part of the process with lots of small shards of resin left in the system. Also of interest: there are few tracks where the fiber was pulled out and those you see are not a clean surface and indicate resin failure and not interface failure. Mechanical Properties Page 12

13 Mean(Tensile strength (MPa)) glass content 30 0 cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- test direction * coupling agent content Bondyram 1001 Polybond 3200 Priex coupling agent 90 Extruder speed mu glass type Marlex 120 MFI homo-pp polymer type Figure 11: Coupling Agent Type and Loading versus Tensile Strength Mean(Charpy impact strength unnotched ( cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- test direction * glass content coupling agent content Bondyram 1001 Polybond 3200 Priex coupling agent 90 Extruder speed mu glass type Marlex 120 MFI homo-pp polymer type Figure 12: Coupling Agent Type and Loading versus Unnotched Charpy Impact Page 13

14 Mean(Flexural strength (MPa)) glass content 30 0 cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- cross- test direction * coupling agent content Bondyram 1001 Polybond 3200 Priex coupling agent 90 Extruder speed mu glass type Marlex 120 MFI homo-pp polymer type Figure 13: Coupling Agent Type and Loading versus Flexural Strength Effect of coupling agent on DLFT mechanical properties The data generated to date (Figures 11-13) does not show significant response of coupling agent loading to the performance of the DLFT part with the exception of very low loadings. At low coupling agent loadings there is not enough coupling agent present to obtain acceptable properties. However; once that loading of 0.5% to 1.0% is achieved there is little effect in varying the coupling agent loading or type. This is different than the results found in GLFT and may indicate that a different process is controlling the mechanical properties in DLFT. All this data was generated using 120 MFI homopolymer PP resin with TufRov 4575 fiber glass. Tensile strength (MPa) cross- cross- cross- cross- direction % glass mu LFT00-24mu mu LFT00-24mu glass Lyondell Bas ell 100 MFI Copolymer Marlex 120 MFI homo-pp polymer Figure 14: DLFT Tensile Strength Fiber Glass Response to Polymer change Changing the polymer type from homopolymer PP to copolymer PP resin (Figures 14-15) Page 14

15 has little effect on the tensile strength of the composite. Both 33% and % glass loadings have the same response. It is interesting that in homopolymer PP the LFT 00 fiber has improved versus cross homogeneity. The same trend may be present in the copolymer product, but the statistical data does not yet support this observation. The LFT 00 fiber glass is 24 micron in diameter while the TufRov 4575 is a 17 micron product. The introduction of the larger diameter fiber does not have a detrimental effect on the performance of the composite. Flexural strength (MPa) cross- cross- cross- cross- direction % glass mu LFT00-24mu mu LFT00-24mu glass Lyondell Bas ell 100 MFI Copolymer Marlex 120 MFI homo-pp polymer Figure 15 DLFT Flexural Strength Fiber Glass Response to Polymer Change The flexural strength again shows that the 24 micron fiber gives the same performance in the DLFT part as the 17 micron product with a significant reduction in anisotropic properties. Effect of Change in Fiber Glass on Properties in PP DLFT Page 15

16 Unnotched Charpy impact strength (kj/m² Cross- Cross- Cross- Cross- Cross- Cross- Cross- Cross- /Cross Glass content (wt%) LFT00 Roving type Figure 16:UnNotched Charpy Impact versus Fiber Glass Type in DLFT 80 Tensile strength (MPa) Cross- Cross- Cross- Cross- Cross- Cross- Cross- Cross- /Cross Glass content (wt%) LFT00 Roving type Figure 17: Tensile Strength versus Fiber Glass Type in DLFT Page 16

17 160 Flexural strength (MPa) Cross- Cross- Cross- Cross- Cross- Cross- Cross- Cross- /Cross Glass content (wt%) LFT00 Roving type Figure 18: Flexural Strength versus Fiber Glass Type in DLFT Flexural modulus (MPa) Cross- Cross- Cross- Cross- Cross- Cross- Cross- Cross- /Cross Glass content (wt%) LFT00 Roving type Figure 19: Flexural Modulus versus Fiber Glass Type in DLFT TufRov 4575 has the highest properties in the laminates. However, there is little change in mechanical properties in the DLFT part. Even though the failure mode of the Page 17

18 laminate is different the mechanical properties are very similar. In this third study, we again observe that the LFT 00 product has a more isotropic fiber distribution and more uniform mechanical properties. The data presented in Figures confirm the SEM fracture surface analysis. Discussion and Next Steps Comparing the data generated in this study on DLFT compression molded PP LFT parts to that generated using injection molded GLFT parts is interesting. Significant care must be taken in such comparisons as the differences in molding and sample preparation are so large as to make direct comparisons of mechanical properties open to interpretation and thus outside the realm of scientific methodology. The EATC method of preparing both GLFT and DLFT samples has some merit in that the data is much closer in numerical value. The effect of Fiber Glass Sizing on the performance of PP DLFT The single fiber testing and the SEM fracture surface analysis confirms the significant improvement offered by the LFT 00 and TufRov 4575 fiber glass sized systems. Even though the mechanical properties of the PP DLFT composites are not drastically improved by the use of these reinforcements there are other positive attributes that can be obtained such as improved heat stability, improved moisture retention and improved long term property retention. DLFT Properties and Performance: Formulation Change responses in PP DLFT: 1. The differences between and cross tensile, flex and impact properties increase with the addition of more coupling agent. This is indicative of a lack of coupling agent in the low loadings. For most of the coupling agents studied a 1% loading in the composite is required for good performance in % glass loaded DLFT. For the 30% glass loaded DLFT between 0.5% and 1% is required. This loading is higher than what is required in comparable GLFT parts as reported in previous work [1-2] 2. The PP DLFT properties are unresponsive to most changes in formulation once an adequate loading of coupling agent is present. 3. The change from homopolymer to copolymer has little effect on properties. 4. The use of LFT 00 reduces the difference between and cross tensile, flex and impact properties. The values are lowered while the cross values are increased. The composite is more isotropic. Furthermore, there is some evidence that the modulus values are slightly increased with the LFT 00 fiber glass. The DLFT data generated so far has not shown the strong response to formulation changes we have seen in GLFT. The reasons for this are still unclear but a few points are obvious in the data. 1. The mechanical properties of PP DLFT composites are not greatly affected by major changes in the formulation of the system. Changes in resin type, coupling agent type, coupling agent loading and fiber glass type do not significantly change the properties of the end product. For the original equipment manufacturer (OEM) this is good news and indicates that the data used for design will remain valid even if raw Page 18

19 materials used change in the course of a program life. 2. DLFT is a much lower temperature process compared to GLFT. In GLFT processing the melt temperature can be 60⁰C higher than DLFT. Furthermore, the resin is at temperature for a longer period of time and then goes through a second heat history in the injection molding machine barrel for a further period of 3 to 5 minutes. This combined time at high temperature difference is significant and may account for the higher response to formulation changes in GLFT compared to DLFT. We have some information beginning to come out of our fiber pull out studies that seems to confirm this. However; further work must be done to understand the ramifications of this difference. 3. One significant issue for DLFT is the differences between the and cross mechanical properties. These differences are caused by the alignment of the fiber during the molding process. The cross tensile, flex and impact properties are always lower than the properties. The differences can be reduced by using the reactive sizing and large diameter fiber offered with LFT 00 fiber glass. This product shows a greatly improved versus cross difference with increased cross values. Future Work: Since it is believed that the time temperature aspect of D-LFT allows for less opportunity for reactants to interact our future work will focus on improving the system conditions for maximal reactivity and properties with a focus on the kinetics of the process. References 1. Bowland, C.D., A formulation study of Long Fiber Thermoplastic Polypropylene (Part One: The effects of coupling agent, glass content and resin properties on the mechanical properties, ACCE Conference Proceedings (2008). 2. Bowland, C.D., A formulation study of Long Fiber Thermoplastic Polypropylene (Part Two: The Effects of Coupling Agent Type and Properties, ACCE Conference Proceedings (2009). 3. Allred, R.E., and Wesson, S.P. Surface Characterization of Sized and Desized Toray MJ Carbon Fibers, SAMPE Conference Proceedings, (2002). 4. Thomason, J.L., The influence of fibre length and concentration on the properties of glass fiber reinforced polypropylene: 5. Injection moulded long and short fibre PP, Composites: Part A 33 (2002) pp Lee, J.S and Lee, J.W., Melt Impregnation Behavior and Mechanical Properties of Long Fiber Thermoplastics Composites through Pultrusion Process, ANTEC Conference Proceedings, (1996) pp Azari, A.D., The Influence of the Pultrusion Line Speed on the Mechanical Properties of the Thermoplastic Rovings Impregnated Long Fiber, ANTEC Conference Proceedings, (1996) pp Thomason, J.L., The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 7: Interface strength and fibre strain in injection moulded long fibre PP at high fibre content, Composites: Part A 38 (2007) pp Page 19

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