Polymer laminate measurements

In this example a polymer laminate film (for packaging) was examined, which was composed of nine layers (see Table 2), by both FTIR imaging and Raman line scan. For the IR measurements thin sections (5 pm) were cut. The central ethylene/vinyl acetate (EVA) copolymer layer is very soft, and holes can be seen in the visible image (Figure 12). 

Pike, L., Measurement of Oxygen Transmission Rates of Film and Thin Sheeting Under Humid Conditions, TAPPI Proceedings, Polymers Laminations and Coatings Conference (1986). 

Figure 1.20 Spectra measured by raising the level of a polymer laminate sample consisting of a lOOpm-thick layer of polyfethylene terephthalate) (PET) under a 20pm-thick layer of poly(ethylene) (PE). The strongest bands in the Raman spectra of PE and PET are marked with arrows. Illustration courtesy of Dr Neil Everall, Measurement Science Croup, Intertek Corporation.

The results for the experimental determination of spatial resolution for the polymer laminate (PE/PC) and the laser-imprinted grid on the aluminum-coated PET film are summarized in Table 9.4. Whereas, a good agreement between the experimental and the theoretical (2r) values (in parentheses) was obtained for the transmission measurement and the aluminum-coated PET film, relatively large deviations were observed for the ATR FT-IR measurements and the theoretical values of the PE/PC laminate. This discrepancy can most likely be attributed to the thickness step along the borderline between the PC/PE dual layer and the PE monolayer film, and to the resultant nonoptimum contact of the polymer film and the reflection element in this area. 

Figure 3.12. Transmission IR spectra plotted in absorbance of a two-polymer laminate (A), one component of which is able to be measured (B) whereas the other is unknown. Identification is possible by spectral subtraction to give spectmm (C), which may be compared with a library spectmm (D). Spectrum courtesy of Dr L. Rintoul.

Principles and Characteristics There are many types of samples for which the transmission approach is not optimum, desirable or even practicable, e.g. urethane foams, polymer laminates, and surface coatings. To obtain spectra from these types of sample it is more usual to employ a reflection technique. Reflection measurements are often also needed when materials are to be measured in their original form, except for thin films. This essentially turns IR spectroscopy into a surface analysis technique, but of low sensitivity compared to high vacuum spectroscopic techniques such as XPS, LEFT), EELS and SIMS. Since the advent of FTIR spectrometers, infrared sensitivity has so much improved that nowadays a measurable spectrum can be produced from even a single monolayer on a flat surface interfaces are also commonly examined. 

Fig. 3.4 Additional water uptake of unidirectional laminate measured at 70°C exhibiting two-equilibrium stages seemingly due to polymer swelling and physical relaxatirai (Suh et al. 2001) Credit Suh D, Ku M, Nam J, Kim B, Yoon S. Equilibrium wato uptake of epoxy/carbon fiber composites in hygrothermal environmental conditirais. 35(3) 264—278, copyright 2001 by Journal of Composite Materials. Reprinted by Permission of SAGE...

Figure 8.5. Coefficient of ffiction of LLDPE film (measured after 1 hour) with and without 500 ppm erucamide (em) and 10(X) ppm talc. [Data from Maltby, A., Polym., Laminations Coat Conf., San Francisco, Aug. 30-Sept 3, 1998,1241-50,1998.1...

The spectrum of each layer of a polymer laminate can be measured by micro-toming the sample to a thickness between 10 and 25 pm. The microtomed sample is then mounted vertically between two salt plates and located in the beam so that light passes through individual layers of the laminate and is transmitted through the remote aperture. For samples such as laminates, the aperture is usually rectangular, where one dimension is the thickness of the laminate layer and the other is the length of the laminate (typically determined by the straightness of the sample). 

A very convenient way of measuring the spectra of polymer laminates is first to microtome them as described in Section 14.2 and then to mount the sample between the diamonds, with each layer perpendicular to the faces of the diamonds. When the sample is compressed, the layers often separate. In fortunate cases, the adhesive that is used to bond the layers may be squeezed out and formed into small droplets between the separated laminates and so may also be characterized. When present in the laminate, the thickness of the adhesive layers is generally less than the wavelength, so these layers are far too thin to measure without use of the diamond cell. By squeezing the adhesive out in this manner, the drops are larger than the wavelength and the spectrum of the adhesive can be acquired. 

Two approaches can be used to measure the depth profile of samples such as polymer laminates with a step-scan interferometer. The first makes use of the fact that the sampling depth varies as/, whereis the PM frequency, as discussed above. In this case, several spectra must be measured with different PM frequencies. The second approach makes use of the fact that the phase of the photoacoustic signal varies with the distance from the surface from which the thermal wave originates. 

Figure 7.6 Variable angle ATR-FTIR imaging of two polymer laminate samples. SI consists of PS-PBMA-PC-PDMA layers, while S2 consists of PS-PC-PBMA-PDMS layers. The setup of the experiment is shown on the schematic on the left and the imaging results are shown on the right. A selection of apertures can be introduced to allow only part of the IR beam with restricted angle of incidence to reach the sample. The position of the aperture determines the resultant angle of incidence of the measurement. The angle of incidence is indicated on the left of the imaging result. The has an inverse relationship to the angle of incidence, that is, 50.1° represents a shallow d, while 42.6° represents a larger d. Frosch et al. [21]. Reproduced with permission of the American Chemical Society.

Because ATR measurements are obtained from a thin surface layer, the spectra may not be representative of the bulk material. For example, spectra of polymer laminates such as packaging materials generally show only the surface layer. However, the spectra of emulsions and suspensions may be unrepresentative of the bulk material because of segregation at the surface. 

Instead of the simultaneously extmded polymer film, a pre-extmded or laminated film can be laminated on the substrate (e.g., paper or paper board). In this case, the extruder can be used to extrude the polymer providing adhesion (e.g., Ecoflex ) in a thin layer between the polymer coating film or laminate and the substrate. Another option is to apply a dispersion or heat in the laminator (Fig. 10, 4) to obtain adhesion. After thickness measurement and second subsequent corona treatment, substrate and polymer coating are wound on a winder (Fig. 10, 6) in order to obtain rolls for transport to subsequent processes (e.g., printing, cutting, cup forming, box making). 

Other characterization methods that are of value are dynamic scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). A sample DSC is shown in the middle of Figure 15.2. Most cure reactions are exothermic, and the heat generated by cure can cause excessive heat to build up in the polymer if control is not exercised. DSC measures the generation of heat as a function of time and temperature. This can be used to predict the temperature at which the laminate will begin to cure (the onset of the peak in Fig. 15.2) and the temperature or time at which cure will be complete, further improving the selection of cure cycles to try. 

Levels of lactate in buttermilk and yoghurt (and blood) were estimated using disposable sensors formed from screen-printed graphite laminated between two polymer sheets [18]. Platinum (deposited by sputter-coating) was the transducing surface. Layers of Nation were added to reduce interference and were surmounted by lactate oxidase in a mixture of polyethyleneimine and poly (carbamoyl) sulphonate hydrogel. The samples were measured in stirred buffer. A good correlation between biosensor results and those obtained with an enzyme kit was claimed but the data had a considerable amount of scatter—if the enzyme kit is taken as the reference method then a more severe analysis of the biosensor results [33] would not have shown them in a... 

Figure 2.15 Measurements of Rosenbaum and Cotton [20] of the water concentration gradients in a laminated reverse osmosis cellulose acetate membrane under applied pressures of 68 and 136 atm. Reprinted from Steady-state Distribution of Water in Cellulose Acetate Membrane, S. Rosenbaum and O. Cotton, J. Polym. Sci. 7, 101 Copyright 1969. This material is used by permission of John Wiley Sons, Inc.

The type of polymer obtained depends on factors such as the pH and temperature of reaction, the ratio of melamine to formaldehyde, and the type of catalyst employed. For decorative laminates, melamine-formaldehyde is prepared by reacting melamine in stainless steel kettles under reflux, alkaline conditions with 37% to 46% formaldehyde in aqueous solution. The reaction temperatures used vary from 80 to 100°C and are maintained until the condensation has reached the desired end point—that is, reacted sufficiently but still water-soluble. The end point is checked by measurements of viscosity, cure time, and water tolerance. Depending on the type of laminate to be produced, other constituents (surfactants, plasticizers, release and anti-foam agents) normally are added to the base resin before impregnation of the surface papers. It is common practice also at this stage to adjust the pH by adding acid catalysts. 

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