Quantitative nondestructive evaluation (NDE) generally has two goals; the first is to obtain quantitative information about the mechanical and chemical properties of the sample, and the second is to obtain an image of the sample to reveal defects or anomalies. Neither approach independently gives sufficient information about the acceptability of the part, but if combined, the disposition of parts and processes can be determined

JB Miller

PA Bottomley

SL Smith

Bert J. VanderHeiden

Charles C. Bryden

Crossref

Nuclear Magnetic Resonance Imaging of Cured Polymers

Quantitative nondestructive evaluation (NDE) generally has two goals; the first is to obtain quantitative information about the mechanical and chemical properties of the sample, and the second is to obtain an image of the sample to reveal defects or anomalies. Neither approach independently gives sufficient information about the acceptability of the part, but if combined, the disposition of parts and processes can be determined.</p

VanderHeiden, Bert

Bryden, Charles

Digital Repository @ Iowa State University (ISU)

NUCLEAR MAGNETIC RESONANCE IMAGING Or' CURED POLYMF.RS TNTRODUCTlON Bet:t J. Vandet:HP-iden Het:cules Incot:porated Bacchus Wot:ks Magna, UT 84044-0098 Chat:les c. Bryden Het:cules Incorporated Research Centet: Wi.lmington, DE 19894 Quantitative nondestt:uctive evaluation (NDE) generally has two goals; the first is to obtain quantitative inforn1ation about the mechanical and chP-mica1 properties of the sample, and the second is to obtain an image of the sample to reveal defects or anomalies. Neithet: approach independently gives suffi-cient information about the acceptability of the part, but if combined, the disposition of parts and pt:ocesses can be determined. This paper addt:esses the use of nuclear magnetic resonance (NMR) techniques to investigate the chemical and physical properties and obtain images of cured resins and thet:moset polymet: systP-ms. Breadline NMR was used to give information about the chemical and physical properties of the roatP-rial and 2- and 3-dimensional Fourier tt:ansfor~ NMR imaging (NMRI) techniques were used to obtain images. The bt:oadline studies detected the existence of a "mobile" componf!nt in an expet:imP-ntal thP-rmoset polymer, the magnitude of which was shown to sub-stantially increase with the addition of rubber and be ·~xtt:emely depP-ndP-nt upon the type of rubbet: added. The results of the imaging studies demon-stt:ated the ability of this technique to monitot: the cut:ing (or cross-linking) of epoxies, revealed a unique rheology pattern in one thermoset polymf!r plug, detected a "soft" surface layer in a polymer plaque, and monitored the P-ffect of oxidation on the soft layet:. In the NMR imaging technique, the nucleus observed is nearly always the pt:oton, and the pararnetet:s which are usually rneasut:ed are the relaxation t.i.mes T1 and T2 [1,2). The T2 parameter is a measure of how fast the NMR signal decays due to intet'actions with nearby nuclei. In gf!neral, longet' T2's corTelate with a mot'e mobile polymet' Ot' addit.ive. Because the signal ft:om t'igid polyn1ers dP-cays so t'apidly (typical T2's at'e on the Ot'det' of 20 roict:oseconds), they cannot be imaged by most NMR imag-Review of Progress in Quantitative Nondestructive Evalualion, Vol. 9 Edited by D.O. Thompson and D.E. Chimenti Plenum Press, New York, 1990 617 ing spectrometers without sophisticated pulse techniques [3). However, to monitor the cure process or a mobile component of a polymer, standard NKR imaging experiments are viable. In contrast to NKR imaging, where a separate NKR signal is obtained for each volume element (e.g., 100 ~ x 100 ~ x 100 ~),only one signal is collected from a sample for breadline NKR. This signal (usually Fourier-transformed into a spectrum) represents an average of the NKR signal from the entire sample contained in the receiver coil. The spectrometer is designed so that the receiver can be rapidly turned on (a few microseconds after the exciting pulse), permitting the detection of rapidly-decaying signals. Thus breadline NKR has the advantage of being able to detect signals from rigid components of a polymer as well as from more mobile components. The term breadline comes from the fact that the Fourier transform of a short T2 (time response) corresponds to a broad frequency spectrum. Breadline measurements were performed at the Physics Department of the University of Utah, which had a supereondueting magnet operating at 81 KHz for protons. The spectrometer was homebuilt with a Hatek transmitter, a Hatek receiver, and a LeCroy signal averager. The probe had a helical coil 13 mm long and 13 mm in diameter. The number of transients averaged in each breadline experiment was - 100, the number of data points - 2500, the sampling rate 40 nsee/point, the repetition rate was 2.5 see, and the pulse width was - 2 ~see. To obtain the NHR images for the resin cure study, the plug was placed in the bore of a horizontal supereondueting magnet containing a 6-ineh diameter receiver coil. The magnet was part of the General Electric CSI 2T imaging NKR spectrometer at Fremont, California. The spectrometer frequency for protons was 85.5406 KHz. The pulse sequence was a typical spin-wrap imaging pulse sequence [4]. Images of the multiphase resins were obtained using a CSI system which was equipped with shielded gradients. Since the samples provided are - 10 mm in diameter, or less, the probe used was a helix type coil that was - 13 mm in diameter and - 20 mm long. This maximized the "fHling factor" (or the volume of the probe occupied by the sample) which, in turn, maximizes the signal relative to the background noise. The images were generated using 4-8 averages per pixel (NA), a spin echo time (TE) of 1-4 msec, a repetition rate of - 200 msec. The total ti.me of the experiment was typically 1-2 min/slice. In the images the terms field of view (FOV) and matrix or number of points (NP) represent the actual dimensions of the image and the number of pixels, respectively, (e.g., a FOV of 128 n1111 and a NP of 128 corresponds to a resolution of 1 mm/pixel). RESULTS AND DISCUSSION The first application presented is the curing of a typical two-part epoxy resin system. During the cure process, the polymer chains will cross-link, which decreases molecular motion. A decrease in motion will be detected as a shortening of T2 (i.e., the more cross-linked the material, the darker the image). At final cure (- 48 hours) the T2 is- 20 lJSec, which is typical for such rigid materials. A cylinder, 1 inch in four layers of resin. CUre 8, and 27 hours (Figure 1). inserted along the cylinder 618 diameter by 2 inches in length, was filled with times for the layers from top to bottom was 0, 3, In addition to the curing, a 1.5 mm glass rod was axis as a spatial resolution gage. Notes: Cure differences and a glass rod used as a resolution gage (arrows) are clearly seen on the images. TE = 4 msec, Matrix= 64 x 64 x 128. NA = 2, FOV 64 mm x 64 mm, slice thickness= 2 mm . Figure 1. Imaging of a two-part resin system. TE ~ 4 msec, NP = 64 X 64 X 128, NA = 2, FOV 64 X 64 nw Data was collected using a very short echo time, 3-D volume method (TE = 4 msec) and displayed as a set of 16 cross-sectional slices from front to back of the cylinder. Cure differences and the glass rod (Slices 8 and 9) are clearly resolved in the images. The spectrometer used could not image signals with decay times less than - 2 msec which occurred after eight hours of cure. The image of this region then appears with an intensity of zero. The experimental thermoset polymers used for the experiments discussed below were manufactured by dissolving rubber (EPDM, SBR, or EPR) into liquid formulations containing an olefin monomer, which was then polymerized. Figure 2 sht)WS a comparison of the broadl ine NMR frequency spectrum of the polymer with and without rubber. The figure on the left corresponds to no rubber. Apparent in the figure is a broad Gaussian shaped spectrum assoc-iated with the rigid cross-linked polymers and a small (- 31.), narrow Lorentzian shaped spike associated with mobile constituents such as residual monomer and additives (antioxidants, etc.). In contrast, the figure on the right is associated with the same polymer with- 3% rubber added. In the figure, the effect of the rubber additive is observed where the narrow com-ponent now consists of - 151. of the sample. The amount of the TUobi le component relative to the rigid phase can be determined from the relative peak areas provided that the spectra have been obtained under conditions where the proton spins in each phase are fully relaxed between pulses. The next experiment was to vary the amount of rubber to determine its effect on the fraction of mobile component (Figure 2). Two different rubbers were added; styrene butadiene rubber (SBR) and EPDH. As expected, the fraction increases as the rubber level increases. However, as observed previously, the fraction of mobile component greatly exceeds the rubber 619 Rigid polymer ----0 --- No rubber ----- With 4.25% EPDM Ruberand ...----mobilized matrix ' ,, ,, ,, I Frequency (kHz) Monomer and ad datives ------250 Figu~e 2. B~oadline spect~ums of samples with no ~bbe~ additive (solid) and with - 4% ~bbe~ additive (dashed) This excess cannot be accounted fo~ by ~esidual mon9me~, which is less than 1%. Except fo~ ~esidual monome~. the diffe~ence between of ~ubbe~ and the amount of mobile component is the amount of unc~oss-linked) polyme~. Thus, when ~bber is added, there a~e mate~ials, ~ubbe~ and mobile polyolefin. content. typically the amount mobile (o~ two mobile Evidence suppo~ting the idea that the dispe~sed ~bbe~ phase contains polyolefin comes f~om transmission elect~on mic~oscopy (TF.M) of a plaque containing EPR, a satu~ated hyd~ocarbon ~bber. When the plaque was stained with osmium tet~oxide and examined using TEM, filament-like a~eas less than 1 pm ac~oss we~e observed. These stained areas must contain an olefin, because saturated hyd~ocarbons are not stained by osmium tet~oxide. The matrix does not stain because the mat~ix is glassy and not very permeable to the stain. The areas containing EPR and polyolefin as a separated cophase are more permeable because the EPR is below its Tg. (An alternative explanation is that the monome~ is concentrated in EPR; further experiments would be necessary to determine whether monomer is more soluble in EPR than in the polymer matrix.) The measurement unit for the amount of ~bber or residual monomer in molded parts is different f~om the measurement unit for the amount of mobile component as determined by NMR. Normally, the amount of ~bber or residual monome~ is exp~essed as wt%. However, NMR counts the relative numbe~s of protons present, and the number of protons per unit weight of SBR, fo~ example, is not the same as the numbe~ of p~otons per unit weight of polymer. Therefore, for the ~esults presented, the amount of ~bber is expressed as the wt% of hyd~ogen in the ~bber. The amount of mobile component reflects the amount of hydrogen which is in the mobile phase. Linear reg~ession of the data in Figu~e 3 gives a slope of 1.2 for SBR, and 1.6 for EPDM. Thus, the mobile component generated when ~bber is added consists of 1 part ~bber and 0.2 parts polyolefin for SBR, and 1 part ~bber and 0.6 parts polyolefin for EPDM. This is an important difference between the two types of ~bber, showing that EPDM is more effective in generating a given level of mobile component. 620 To confirm that the narrow peak in the breadline NMR spectt"um is associated with the rubber, breadline NMR measurements were perfot"med at low temperatures. At the rubber Tg, the line should broaden as the rubber becomes less mobile. In Figut"e 4, line widths are plotted vs temperature for SBR and EPDM. A break in the line occurs for SBR at about 225K, corresponding to the Tg for this rubber as determined by dynamic mechanical analysis (DKA). For EPDM, the tt"ansition is broader, occurt"ing in the range of 275 to 285K, which is neat" the Tg of 275K measut"ed by DMA. This agt"eement of Tg values between NMR and DMA confirms that the nart"ow peak is associated with the t"ubber. 0 0 X z 0 i= u <( a: u. 1-z w z 0 Q. ~ 0 u w _, CD 0 ~ 0 2 3 4 6 6 7 AMOUNT RUBBER Figure 3. Linear regression of fraction of mobile component, as determined by bt"oadline NMR vs wt% rubber adjusted for relative hydrogen content . The fraction increases faster when F.PDM rubber is used The breadline NMR experiments show that the rigid portion of the thermo-set phase polymer has a line width of about 35,000 Hz, corresponding to a T2 of 15 )JSec (T2 = 11[2 x J. ine width] for a Gaussian line shape). Having such a short T2, the signal from the rigid part of the polymer decays too rapidly for the imaging spectrometer used in this work. However, the line width for the mobile component is 500 to 7000 Hz, corresponding to T2's of 600 to 45 )JSec (T2 = l/[pi x line width] for a Lot"entzian line shape). Previous NMR imaging experiments showed that T2's in the 600 )JSec range are sufficiently long that the mobile component can be imaged. The mobile component corresponding to a T2 of 600 )JSec is associated with rubbe r, which is present as a dispersed phase. Thus the NMR imaging experiment in this work is selective fot" the dispersed phase. 621 N I 'I b ~ w z :::J I-I C) w I I u.. ....J <{ I 15000 SBR EPDM 10000 5000 0~---------,---------,---------,--------~ 150 200 250 TEMPERATURE. •K 300 350 Figure 4. Breadline NKR line width of the narrow component vs temperature for plaques containing SBR and EPDH. The break in each curve is the Tg of the mobile component in the polymer as measured by NHR Figure 5 shows the NHRI of the aged and unaged the1<rnoset polymer plaques where the resolution for the image is 125 pm. The samples were 3 n~ thick plaques. Both plaques contained SBR and the antioxidant formulation. In Figure 5 the unaged sample has a soft layer on the top surface and the aged sample has a dark band at the surface that is in contact with the tmaged sample. One possible explanation for the loss of the soft region in the aged sample is that cross-linking occurred at the "soft" surface. If this is true, then the soft region is associated with either an increased rubber concentra-" tion (or at least an increased amount of the matrix was affected by the rubber in the soft surface regions), or the soft region is associated with a greater amount of mobility (longer T2) for the mobile component. Figure 6 shows an NHRI of a thermoset polymer plug with a scan resolution of 250 pm. The plug was molded by pouring neat resin mixed with rubber into 622 Slice Plane NMRI Effect of Oxidation on Mobile Component Figure 5. Comparison of unaged (top) to aged (bottom) plaques of thermoset polymer. FOV = 32 mm x 32 mm, NP = 128 x 128, and TE = 4 msec, and slice thickness was 25 mm. Figut:"e 6. NMR image of a plug containing F.PDH LUbber. The adhesive in the label taped to the plug shows up as a bright arc on the right-hand side of the image a 13 mm ID test tube, then bt:"eaking the test tube after the matrix had cured. As can be seen in the image, the pt:"ocess of molding the polymer produces an interesting rheology pattern (somewhat resembling the splitting of a cell). Also observed is a piece of surgical tape, used as a sample label, that was wrapped halfway around the plug (in the image the tape appears as a bright - 1 mm thick semiring around the surface). CONCLUSIONS Based on the results of this work it is clear that NMRI is becoming one of the most versatile nondestructive techniques available. With the t:"esolution of cut:"t:"ent spectrometers reaching levels of 20 ~m or less fot:" liquids and with the capability of imaging harder and hardet:" materials, it is inevitable that this technique will become a dominant nondestructive method fot:" materials research, as it has for medical research. In addition, broad-line NMR pt:"ocess and quality control applications such as resin curing and measurement of component constituents are viable and affordable. ACKNOWLEDGF:HENT The authors wish to acknowledge the help of Dr . Ralph J. Hurd, General Electric NHR, Fremont, California, who operated the scanner and assisted in all the NMR imaging scans presented in this work. 623 REFERENCES 1. c. P. Slichter, Principles of Magnetic Resonance (Springer-Verlag, New York, 1980). 2. J. B. Killer and A. N. Garroway, J. Kagn. Reson. 82, 529, (1989). 3. S. L. Smith, Analytical Chem. 57, 595 (1985). 4. P. A. Bottomley, Rev. Sci. Instrum. 53, 1319 (1982). 624 

https://dr.lib.iastate.edu/bitstreams/b37c9ff3-0341-4095-876c-bd597f769fa4/download

Nuclear Magnetic Resonance Imaging of Cured Polymers

Abstract

Similar works

Full text

Available Versions

Crossref

Digital Repository @ Iowa State University (ISU)