Transmission Efficiency of plastic Films Part 1

More documents

Recommendations

Info

TRANSMISSION EFFICIENCY OF PLASTIC FILMS 1.98 m (78 in) and the lens is placed midway between them. When a test sample of film is placed behind the lens, as shown, only light which is transmitted by the film with a deviation of less than 4 mm will enter the photocell. The photocell reading, expressed as a percentage of the intensity of the incident beam, is the ‘see-through’ clarity. This figure is, of course, a measure not only of the distortion but also of the light transmission, and the see-through’ clarity and the light transmission (determined as in the previous paragraph) must be compared in order to assess the effect of deviation alone. 3. HAZE: Haze can be taken as a measure of the ‘silkiness’ of the film and is usually of greater importance than ‘see-through’ clarity in the case of packaging films since these are usually closer to the contents of the pack. One cause of haze is surface imperfections of the film so that the amount of haze is not necessarily proportional to film thickness the appearance of haze is caused by light being scattered by the face imperfections or by in homogeneities in the film. These latter can be caused by voids, large crystallites, incompletely dissolved additives or cross-linked material. PHYSICAL AND CHEMICAL PROPERTIES Haze is measured by a technique specified in ASTM D1003 and in BS.2782: Part 5: 1970. It is defined in these standards as the percentage of transmitted light which, in passing through a specimen, deviates by more than 24° on average, from an incident parallel beam by forward scattering from both surfaces, and from within the specimen. A beam of light is passed through the film sample and on to the highly reflecting internal surface of an integrating sphere in such a manner that all the transmitted light is collected into a photo electric cell. Next, the sphere is moved through a small angle so that the path of the light beam now falls on an extension of the sphere comprising a matt black surface that absorbs the entire light transmitted straight forward along the path of the beam. Any light that is appreciably scattered in a forward direction is still gathered by the highly reflective internal surface of the sphere and into the photocell. The ratio of the second photocell reading to the first is a measure of the haze. The apparatus for tile measurement of haze is shown diagrammatically in Figure. 52
GLOSS: TRANSMISSION EFFICIENCY OF PLASTIC FILMS Gloss is important to packaging films and is a measure of the ability of the film to reflect incident light secularly (i.e. as in a mirror—an of incidence = angle of reflection) high gloss will, therefore, produce a sharp image of any might source and will thus give rise to a pleasing sparkle on the film. The gloss meter is first calibrated by using an optically flat, highly polished, black glass plate of refractive index 1.52. This has a gloss value assigned to it of 53 units. A light trap, with matt black interior is first placed in position and the lamp switched on. The measure device is then set at 0. The reference standard is next placed in position and the sensitivity control adjusted to give the correct reading for the standard. Finally, the specimen is placed over the light trap and the new reading noted. 5.3 HEAT SEALABILITY The heat seal ability of a packaging film is one of the most important properties when considering its use on wrapping or bag making equipment and, of course, the integrity of the seal is also of tremendous importance to the ultimate package. The heat salability of a film has to be considered in relation to many other factors, including the available pressure, the dwell time, temperature and the rate of heat transfer of the sealing bars. Any test of the heat seal qualities of a particular film must, therefore, simulate the conditions under which the film will be used as closely as possible. Heat seals can be made by a number of different methods, apart from the straight forward application of heat to the layers of film. Seals can be made, for instance, by high frequency heating (as with PVC) or by ultrasonic welding. However the seal is made, the strength is determined by measuring the force required to pull apart the pieces of film which have been sealed together. The force can be applied in such a way as to cause the seal to fail in shear or in peel. If other factors, such as the temperature, dwell time, and pressure used in making the seal are equal, then peeling a seal will give a lower figure than shearing it. Seals are normally tested in peel, therefore, as this is the way they are likely to fail in practice. Two tests are in common use, the dynamic and the static. In both tests a 25 mm (1.0 in) strip is cut through the heat seal. The dynamic test uses a sensitive tensile testing 53
Page 2 and 3:
Transmission efficiency of Plastic
Page 4 and 5:
C O N T E N T S S. No. CHAPTERS Pag
Page 6 and 7:
The strategy for the project, TRANS
Page 8 and 9: TRANSMISSION EFFICIENCY OF PLASTIC
Page 10 and 11: Bags and Sacks 22% Bags and sacks 2
Page 12 and 13: 2.1 EXTRUSION OF FILM: TRANSMISSION
Page 20 and 21: Film casting (slit-die extrusion):
Page 30 and 31: Note We are examining the National
Page 38 and 39: 6. GENERAL FOOD PACKAGING: TRANSMIS
Page 42 and 43: NYLONS: TRANSMISSION EFFICIENCY OF
Page 52 and 53: PAKISTAN: TRANSMISSION EFFICIENCY O
Page 72 and 73: 4. ANTI-STATIC ADDITIVES TRANSMISSI
Page 76 and 77: Butyl, GRS, polyurethane, neoprene
show all

Transmission Efficiency of plastic Films Part 1

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?