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EMPA Polymer Department Visit (PU Jacks issue)
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EMPA Dübendorf
Minutes
First meeting with EMPA Polymer specialists on PU Jack issues
EMPA Polymer Department Visit (PU Jacks issue) (9 May 2023) · Indico (cern.ch)
Present: Christian Walder (EMPA), Frank Nüesch (EMPA), Stefano Sgobba, Mateusz Sosin, Michel Noir, Patrick Bestmann
Introduction
PB gave a short introduction to CERN and the BE-GM group. He continued with the presentation of different magnet supports and introduced the SPS Polyurethan Jacks, their use and advantages. The next topic was the different failing jacks discovered since 2016 and the final full exchange in 2019 based on the cascading failing jacks. The strategy of procurement was explained as well as the fact that already 2003 some problems triggered a detailed analysis. However the PU formulation as well as the molding process looks to have changed again. For now “only” mechanical tests are done at reception of the batches. Some Non conforming batches could be intercepted with these tests, but could not avoid the failing jacks in the machines. PB concluded with a few open questions like:
- Can we avoid the breakdown of the jacks?
- Can we give a reliable lifetime?
- Can we imagine an alternative material?
The first ad hoc answer from CW is that those questions are extremely difficult to answer. What is sure already is that in terms of ionizing radiation, the choice of PU is the most adapted one. The exact formulation is very complex and so many parameters can be tuned leading to the same result. The hard PU blocks (95 ShA) are for sure stressed above their elastic limits which leads to the breakdown in flakes.
LHC jack issue
MS explained the use of the PU blocks in the LHC jacks which is a different use than the one in the SPS Jacks. The block is sealed, greased with MOLYKOTE®, isolated from air and humidity and the load is evenly distributed on the block. MS explained the observed movements of the LowBeta magnets and the following inspection of the jacks. Over the last 17 years, the 95ShA blocks decreased to 70ShA, but the 75ShA blocks got liquified in the central jacks and started to extrude from the housing. He continued to introduce the action plan and the analysis which was started in collaboration with the MME labs and Stefano.
CW explained that PU might liquify over a period of 15 years even without an external force. He gave an example of samples he had kept for 15 years. MS stressed out that the spare blocks from the same batch are still in perfect shape and their Sh number is as for new pastils. The question if the grease could have accelerated the ageing effect is very difficult to answer and detailed tests need to be made on this. The polymer chains are finally broken, but it is not possible to identify if it was the age, the load, the grease or a combination of them. MS commented, that for LHC samples, the pastils were a subject of various load and that clear cross-corelation between load and damage is not visible. According to that the grease and age might be more probable damage factors.
EMPA can provide a list of PU compatible greases, but the graphite powder proposed by MS should have no influence on PU.
Analysis done at CERN
SS presented the different analysis which have been done at CERN so far. They included FTIR (Fourier-transform infrared spectroscopy), TGA (Thermogravimetric analysis), DSC (Differential Scanning Calorimetry) and XRD (X-ray diffraction analysis) spectra. The tests allow to conclude that the pastils are made of a Polyurethane type Ester (FTIR), on absence of hydrolysis (FTIR), absence of major chain rupture (TGA), no big changes in the soft segments (DSC). Damage of the hydrogen bonding seems to affect strongly the mechanical properties of polyurethanes (FTIR). Presence of MOLYKOTE® grease (FTIR, XRF) is confirmed on the liquefied pastil, however it cannot be said whether this implies a diffusion into the bulk of the PU or it is a pollution. XRD results suggest that differences between two specimens could be attributed to the system alignment or the sample shape (the surface of the damaged PU was difficult to keep flat). Small-Angle X-ray Scattering (SAXS) will be applied in the future, which is a transmission method less sensitive to surface irregularities. The root cause of degradation remains unknown. The main hypothesis for the degradation is the conjunction of pressure, the MOLYKOTE® grease and time. Conditions (accelerated ageing test) are difficult to reproduce in the laboratory. Concerning the analysis method, EMPA would have done the same tests. They also did the spectroscopy of the different samples send by PB and could identify a different batch of material which was also classified Non conform due to mechanical test at CERN. The reason is that this batch is not Ester based PU, but ether based which is 60% strong. That means that A+P delivered the wrong type of Polyurethan in this batch.
Discussion
The following discussion was oriented into two directions. First in the breakdown analysis and the reasons and secondly in optimizing the product and possible tests ensuring the needed lifetime.
In terms of formulation EMPA has a Database to compare the analysis spectrum to standard types. One NC batch was found to be ether based PU. However, the old black PU Plock which was extracted from TT20 is not even close to any known PU formulation in the Database.
Concerning the change of formulation by A+P motivated by the fact that mercury is forbidden today, CW could not confirm any reason for adding Mercury to the recipe. It is important that A+P will have an open exchange with EMPA on the formulation and the process used.
PU chemistry is extremely complex and even for adjusting the hardness, multiple parameters can be tuned during the production process. It is not possible to identify how this was done on a finished product.
An artificial ageing if PU is extremely difficult and would not deliver reliable results which can be extrapolated to determine a lifetime.
SS suggested to apply the above techniques as a receiving inspection of new production. He also suggested to apply Dynamic Mechanical Analysis (DMA) that would allow through an additional method to characterize the elastic and viscous response against temperature. EMPA was in favour of this proposal. In case, tests can be performed in their premises. CERN is also equipped.
He also highlighted the possible future testing: mechanical testing (creep in compression) to identify differences of strain rates in secondary creep regime between products of different hardness; study the possible interaction of MOLYKOTE® lubricant with the pastil; perform elemental and crystallographic analysis (XRF, XRD SAXS), in order also to study possible evolution of the crystallinity (XRD SAXS).
Products with a longer lifetime in operation conditions (unlubricated/lubricated; SPS/LHC environment) should be identified and characterized, possibly with the help of A+P. The compatibility of specific lubricants with the selected product should be clarified.
The alternative to PU option was also discussed. CW and FN mentioned that kautschuk/rubber products might be used as replacer. Eg. EPDM could be a candidate. MS commented, that such a solution might be a subject of issues related to radiation. Since a while in CERN the wide campaign to find proper recipe of EPDM is ongoing. He underlined, that switching solution to EPDM could be more problematic (radiation issues, ageing effects issues, etc), than studying already known and 17 years stable product as PU pastils.
Conclusions
The first conclusion is that the recent tests realized at CERN are the right ones and that the interpretation done so far is fully correct. EMPA could not offer more analysis methods apart from SAXS.
EMPA is offering failure and breakdown analysis and consulting and can make an offer for this service. However, they claimed that the present failure analysis, if considered as a whole, would be extremely complex.
EMPA is not producing polymers at the industrial level, so a production by EMPA is not an option. Other suppliers are existing on the market, but EMPA cannot recommend a particular one. They also mentioned a few Swiss Fachhochschule which have competence in polymer production. FN declared, that will provide the contact to Swiss Fachhochschule expert on polyurethane, which might be useful in further study/discussions.
The question if EMPA can assist CERN for the production and control of future productions from A+P as answered positive, however it must be a clear definition of this mandate which depends in particular on the reaction of A+P.
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