Transmission Efficiency of plastic Films Part 1

More documents

Recommendations

Info

TRANSMISSION EFFICIENCY OF PLASTIC FILMS dependent on the equilibrium relative humidity of the base film (35%), the relative humidity of the dry side (10%) and the water vapour transmission rate of the barrier layer separating them. After some time, the base film reaches equilibrium and the water vapour moving from the 100% relative humidity side equals that arriving on the dry side. As the two barrier materials are identical and have a much lower transmission rate than the base film, this will reach equilibrium at a relative humidity about midway between the wet and dry conditions, i.e. 55%. Under these conditions the transmission of water vapour will have increased above that in the initial conditions because the vapour pressure difference is due to a relative humidity differential of 45% (= 55% — 10%), compared with the initial value of 25% (= 35% — 10%). The transmission rate will, therefore, rise by a factor of about 1.8. Similarly, a specimen where the base film is initially in equilibrium with a relative humidity greater than the final value will have a high apparent water transmission rate initially, and this will fall as the specimen approaches equilibrium. Hence, although rapid test instruments can measure transmission rates in a short time, the results may differ significantly from those obtained after dynamic equilibrium has been reached within the test cell. With non hygroscopic arrivers results agreeing closely with the dish method can be obtained. 2. GAS PERMEABILITY Measurement of gas permeability is carried out under controlled conditions of pressure, as well as of temperature and relative humidity. In essence the usual tests consist in making a partition between a test cell and an evacuated manometer.)The pressure across the film is usually one atmosphere. As the gas passes through the film sample the mercury in the capillary leg of the manometer is depressed. After a constant transmission rate is achieved, a plot of mercury height against time gives a straight line. The slope of this line can be used to calculate the gas transmission. Gas permeability measurements are described in ASTM D. 1434 and BS.2782: Part 5: 1970. The foregoing method is known as the pressure increase method. Another common method is that of concentration increase. Two gases are used, a reference gas and the test gas. A partial pressure difference across the barrier material with respect to the test gas is created, without a difference in total pressure. Because the method of measuring the concentration of the test gas can be specific 10 that gas even in the presence of other gases or vapors, equipment can be developed in which the relative humidity of both the test and the reference gases can be controlled. This is not possible where equipment has to be evacuated and is very important when Determinations are carried out on barrier materials having a hygroscopic base. Various methods of measuring the concentration change are used, including chemical analysis, gas chromatography and radioactive tracer techniques. 50
3. ODOUR PERMEABILITY TRANSMISSION EFFICIENCY OF PLASTIC FILMS There are no standard tests for the measurements of odour permeate ability although it is an important characteristic in many cases. One method which can be used to compare the efficiency of several films as odour barriers is to make up pouches with each film. The pouches can be filled with some odoriferous material and then it in separate clean glass bottles, sealed by crimping with aluminium foil. The minimum time for an odour to be apparent in the bottle can be measured and will give a rough ranking list of the test films. This ranking list may vary somewhat according to the type of odoriferous material used so that the substance to be packed should be used as the test material if possible. 5.2 OPTICAL PROPERTIES: 1. LIGHT TRANSMISSION Light transmission is measured by means of a photoelectric cell. The intensity of a light source is measured by the cell, both with and without the interposition of the film sample light transmission is the ratio of the light intensity measure with the to that obtain with and is expressed as a percentage. 2. SEE-THROUGH CLARITY A better guide to the quality of an image when viewed through a film is given by the property known as ‘see-through’ clarity. This indicates the degree of distortion of an object when seen through the film. Compared with a set of eight standard photographs. These cover the range of clarities normally encountered in the material under investigation and the test sample is given a number corresponding to the standard it most closely resembles. Another way of assessing ‘see-through’ clarity is based on the assumption that definition of an object is controlled by small-angle scatter. Figure shows the basis of the method. A stabilized light source serves to illuminate a small (2.03 mm or 0.080 in diameter) orifice which then acts as the optical object. A large lens focuses this image on to a similar orifice placed in front of an electric cell. The distance between the orifices is 51
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?