The increasing performance requirement of polymers is driving research for more efficient and cost-effective solutions for their manufacture. Development of new resin systems to meet demands for high performance materials would undoubtedly take too long and would certainly be too expensive since it would require huge investments in totally unexplored technologies and new plant facilities.
Instead, polymer resin manufacturers have recently started to use extruders as chemical reactors.
The process allows the tailoring of molecular structure to give polymers with specific rheological/mechanical properties. The practice of combining polymer extrusion processing with polymer reactions in one operation is known as reactive extrusion (REX). REX processing is used in industry to modify molecular structure of extruded polymer products. REX technology is exploited in the production of different grades of controlled rheology polypropylenes (CRPP) from a single high molar mass feedstock. CRPP’s are suitable for high-speed manufacture of synthetic fibres, thin films and thin walled moulded parts with different mechanical properties.
This coursework assignment involves several practical elements. Students will not only manufacture a series of controlled rheology polypropylenes but also conduct a series of characterisation techniques that link change in molecular structure with rheological and mechanical properties.
Aims
To provide students with practical experience in the operation of polymer conversion processes and characterisation techniques.
The learning outcomes of this coursework are to:
Develop knowledge on current technologies and practices of REX processing via lecture notes and researching literature.
Gain practical experience of REX processing in the laboratory using twin screw compounding to create REX processed polypropylene.
Develop experience of using analytical techniques and instruments to characterise the mechanical, rheological and thermal properties of these CRPP samples.
Develop skills in technical report writing.
Experimental:
Materials:
Polypropylene: Sabic 531P ( MFI ~ 0.3) ( in stores WB18)
Peroxide : Aldrich Dicumyl peroxide ( DCP). 1.56 g/cc
Pre-blending:
NOTE: Read the safety notes at the end of this document with regards to safety in handling peroxides. Create dry pre-blends of the compounds given in Table 1 below ( to save time, and reduce risk, these pre-blends may be given to the students). Weigh out 1500 g of polypropylene pellets and place in a polyethylene bag. Weigh out the appropriate masses of the peroxide powder in a fume cupboard ( Clean room area WB13). Add the powder to the 1500 g batches of PP pellets. Shake each bag for 5 mins to get even distribution / coating of the fine peroxide powder on the pellets.
Extrusion Compounding:
Use the Thermo-Scientific twin screw compounding extruder in lab area WB10, ( useful to take pictures of the equipment to include in your report). Note a typical screw configuration for this machine will be available for you to view. You should identify the main features of the screw. The extruder is fitted with a 3 strand lace die.
H&S: make sure the LEV is over the die exit of the machine and any students handling the pre-blend composition must wear dust masks.
Parameters- Temp profile : throat water cool, 160C, 180C, 200C, die 200C. Set the screw rotation RPM to: 250
Process the compositions in the sequence shown in Table 1. The compounds are added sequentially, starting with the low dose peroxide. The pre-blends are poured straight into the loss in weight Montan hopper. This is set at 5 Kg /hr once stable processing is established. Add further compounds when there is only 0.1Kg registering as the mass left in the hopper. The Montan hopper is used control the delivery of pellets to the throat of the extruder. Allow 10 minutes of purging between batch collections. Extruded strands are fed through the water bath and the air blade drier at the end of the bath. Feed the strands into the pelletiser set at a speed to 3.0 this should result in pellets of ~ 3mm length. Collect each batch of compound in a polythene bag in the basket beneath the unit. The pelletising process may need one student dedicated to ensure there is no slippage on feed wheel. You will need approximately 1 Kg of each compound. Repeat the extrusion process with each batch.
Formulations: Table 1. Compositions for processing
Material
( nominal compositions)
Mass PP ( g) Mass DCP (g) % DCP Torque
( Nm)
Pressure
(Bar)
PP 531P 1200 0.0 0.0
PP 531P + 05% DCP 1500 0.75
PP 531P + 0.1% DCP 1500 1.5
PP 531P + 0.15% DCP 1500 2.25
Extrusion Observations:
Comment on any visual observations made during the extrusion process. Also check and write down any machine torque/ die pressure measurements for each batch. Such observations may provide qualitative information to reinforce data from characterisations tests.
Injection Moulding:
Use the Fanuc i-2000 injection moulding machine located in lab area WB9. The moulding parameters on the machine will be set ready for the injection moulding of these compounds. The test piece will be in the form of an ISO standard tensile bar.
A minimum of 15 test bars are required for each batch. Record of the mass of the samples, sequentially, during the injection process.
Injection moulding Observations:
Comment on any visual observations made during the injection process. Also check and write down any machine parameters used for each batch – look at the pressure profiles and peak pressure of the materials.
Characterisation:
Compounding: comment on any visual observations made during the extrusion process. Also check and write down any machine torque/ pressure measurements for each batch.
Gel Permeation Chromatography :– a procedure and the chromatograms of ‘similar’ REX processed polypropylene samples are attached.
Rheometry:
Dynamic : Use an Anton Paar model M501 rotational rheometer located in lab area WG21. Set the instrument to an operating temperature of 230C. Use the 25 mm parallel plate system with a 1% strain, from 0.001 to 100 rad/s frequency sweep. Data collected and filed under Laboratory Tasks- coursework.
Note the virgin 531P PP is evaluated along with processed + 0.0% peroxide composition so that a base line can be established to evaluate of processing alone on the properties of a virgin polypropylene. The added effect of peroxide only induced changes can then be noted. (Some polymers are sensitive to processing owing to mechanical or thermal degradation during the process ).
MFI: Use a Kayness MFI instrument. Data from this instrument can be used as a qualitative measure of a change in melt viscosity. Use 2.16kg mass and set temperature of 230◦C. (The method is described in the similar standards ASTM D1238 and ISO 1133).
Procedure for Polypropylene:
The apparatus consists of a small calibrated die inserted into the apparatus, with the length = 8 mm and diameter = 2.095 mm.
Take about 4 to 5 grams of polymer and pack it properly inside the barrel using the appropriate tool to avoid formation of air pockets.
Introduced the piston on top of the sample. The piston is used to extrude the molten polymer through the die.
Allow the sample to preheat for 6 min at 230°C for polypropylene.
After the preheating the 2.16kg weight is placed onto the piston.
The weight exerts a force on the molten polymer and it immediately starts to extrude through the die.
Only collect the mass of extrudate when the piston position is between the graduated marks scribed on the outside of its diameter.
Start the timer and collect a sample of the melt over the 6 minute period. Note for high MFI melts this time may have to be reduced to 2 or 1 minute.
Accurately weigh the mass collected.
MFI is expressed as grams of polymer/10 minutes.
Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI.
Thermal Characterisation:
Use the TA Instruments model Q20 DSC located in lab area WG21 . Cut small samples ( ~ 10 – 20 mg) from the REX processed materials and produce thermograms ( heat Flow v’s Temperature) for the samples. Recommend taking the extremes i.e. 0%, 0.2% and 0.4% peroxide samples.
Heat profile of 10C/min to 250◦C.
Cool Profile of 10C/min to 30◦C
You can determine the approximate % crystallinity of each compound by taking a ( DHm̊ ) heat of crystallisation for a theoretical 100% crystalline polypropylene. ( You need to research this.)
Mechanical testing:
Conduct tensile tests on the test bars using an Instron model 5564 tensometer located in lab area WG33. You need to measure the width and thickness of each sample at its gauge length prior to each test. This information is fed into the Bluehill software and is used to determine the stress on the material. Use a crosshead displacement rate of 50 mm/min. Collect the tabulated data from each batch.
Result: Report the results and their analysis. You will experience hands-on operation of the processing and characterisation methods. However, because of time limitations in the laboratories many of the results in raw data format will be given to you.
You have also been given chromatograms of representative of similar CRPP samples ( appended to this document) – please review these and comment on the data.
The report:
The coursework report should be minimum of 3000 words ( max to 5000) .
Your report should include:
Literature review of the REX processing and of REX to produce CRPP. This is an important component of the report as reflected by the potential marks to be gained so please spend your time understanding the technology. In the literature you should cover:
describe the details of REX processing – what are the intended purpose.
typical applications of REX
advantages of this technology over conventional polymerisation technologies.
control issues with REX processing
Experimental: description of materials used and quantities, processing equipment and procedures, process conditions, analytical methods and equipment used. Try to incorporate a description of the purpose and significance of data from of each experiment in the context of your REX polypropylene. Include any precautions taken, etc
Results – present data of any testing or observations made during processing. Present an analysis of the results and consider any supporting correlations between data from the different characterisation techniques.
Discussion : discuss the finding of the results – what do they mean – do they validate/ support/contradict any results from other sources reported in the literature? Suggest reasons for validation or contradiction. Look at what you can imply from the results in terms of the REX process used – may be able to use data from two or more characterisation tests used to support your hypothesis.
Conclusions – summary of the findings of the study with succinct analysis of the project. Give a reflective feedback on the usefulness of the project for the development your own knowledge and skills.
Recommendations – further work extended research ideas – what could be changed / improved.
References – remember to cite these in the body of your report. Sometimes good to refer to some of these to support your discussions.
