Food Packaging Technology 3(2+1)

Lesson- 22 Determination of parameters of different packaging materials

22.1 Introduction:

The best package for any particular purpose is the one which would protect the contents against the hazards the package would undergo during its journey at the minimum cost. The simplest and the most efficient way of testing package is to carry out field trials with sufficient number of package under the actual conditions of usage. Evaluation of package performance or package testing is a means of shortening this process and of obtaining results in a shorter period with a reasonable degree of accuracy.

22.2 Mechanical Tests

22.2.1 Bursting Strength

The popularity of bursting strength test depends not only on the ease with which the test is made but also on the combination of strength and the toughness, which it measures and which serves as a measure of the serviceability of paper in various applications. It has some disadvantage i.e. it depends in a complicated way on the machine direction, tensile strength, stretch and size of the burst area. Bursting strength is measured by the pressure developed behind a circular rubber diaphragm when it is forced through the paper so as to burst it.

A tester in which testing is done by hydraulic pressure communicated through the medium of glycerin or by compressed air to a pure gum rubber diaphragm in contact with the paper, shall be used. The diaphragm used in the equipment shall be such that it does not materially affect the bursting pressure and shall be between 0.35 mm and 0.45 m thick. The rubber sheet used shall be pure gum vulcanization containing not less than 95 % by volume of first quality smoked sheet rubber, the only ingredient in the mix, apart from rubber, shall be those necessary to effect correct vulcanized and resistance to premature aging at normal temperatures. The pressure required to bulge the diaphragm 5 mm above the top plane of the lower clamping surface of the rest instrument shall be not more than 0.07 kg/cm².

For determination of bursting strength, first clamp the piece of packaging material firmly over the diaphragm without slippage during the test between two annular, planes, unpolished surface of 30 mm internal diameter. After clamping the test piece, run the machine so that the pressure increases at a uniform rate (0.75 kg/cm² per second) until the test piece burst. Now, with the help of pressure gauge the pressure in kilograms per square centimeter at which the test piece burst. Take two reading with each sample sheet, one with the wire-side upper most and the other with the top- side uppermost. For calculating the burst factor the formula is as follows:

Burst factor =Bursting strength (g/cm²)/substance (g/m²)

 22.2.2 Tearing resistance

The tearing resistance is usually greater in the cross direction than in the machine direction. Ballistic type of tear-tester such as the ‘Elmendorf’ is recommended. The machine is provided with two clamps, the one fixed and the other carried on a sector-shaped pendulum suspended from a column by means of a friction less bearing located near the apex of the load of pendulum recorded through a spring load friction pointer on the circumferential scale marked on pendulum.

For determination of tearing resistance, accurately cut the piece with a template in such a way that two parallel slides from a centre tongue giving a double tear. At least one test piece in each direction shall be taken from each specimen. First holds outer tongues of the test piece in a fixed clamp and the centre tongue in the movable clamp. Release the pendulum and note the load necessary to continue to tear. The test may be made such that either the reading is not less than 25% and not more than 75% of the capacity of the instrument. The tearing resistance shall be tested separately for machine and cross direction. Record average, maximum and minimum of the reading in such direction separately and state the number of test piece used for each determination. Tear factor is used for comparing two papers with regards to their tearing strength and is calculated as follows:

Tear Factor = Tearing resistance/ substance

 22.2.3 Impact strength of glass bottles

Impact strength in glass can be determined by two methods. First method is drop tester. In this method, the certain height at which glass is break determined. In second method the impact strength is determined by pendulum. In this, keep glass bottle at platform and gives oscillation at which point glass break. Whatever energy required to break the glass is becomes impact strength.

During recycling of bottles; thermal shock resistance decreases, because of pitting of bottle, then application and use of caustic soda, acid, hot water etc. due to these severe processes; thermal shock resistance decreases. During recycling of bottles; thermal shock resistance decreases, because of pitting of bottle, then application and use of caustic soda, acid, hot water etc. due to these severe processes; thermal shock resistance decreases. To avoid this problem; silicates coating is done on bottles. When you treat with caustic soda, this coating is protecting other coating and the properties of glass remains as such for long time. Pressure at which beverages bottles withstand is 15 kg / cm²_. By this experiment, one can determined the thermal shock resistance for glass bottles.

22.2.4 Thermal shock

For determination of thermal shock test requiring a basket for holding the bottles upright. Two water baths are also requiring. One contained hot water and other cold water. It may also have a device to control the desired temperature of the baths within +- 1^o C, otherwise the temperature has to be controlled manually using thermometers. Each water bath may also be provided with a stirrer to keep uniform temperature.

First adjust the cold water bath to a temperature of 30+-1^oC and the hot water bath at a temperature of 72+-2^oC. Now fill a basket fully or partially with the empty sample bottles. When the bath has attained the prescribed temperature, immerse quickly the basket containing the bottles in the hot water bath in such a manner that the bottles become completely filled with hot water.

Allow the bottles to soak for 15 minutes. After this transfer the basket with the bottle filled with water to the cold water bath so that the bottles are immersed in water up to the neck, taking care that no cold water enter the bottles. Keep the bottles immersed for 2 minutes. Then remove the basket from cold bath. The process of transfer from the hot to the cold bath shall be completed in 15+-2 sec. Take every precaution to protect the apparatus from draughts. At last inspect each bottle for cracks or breaks.

 22.3 Climatic Tests

22.3.1 Salt Spray Test

Salt spray test is used to evaluate the resistance of the package to corrosion by salt spray. The package is placed for nearby 50 hours, to a wet, dense fog environment generated by the automation of a 20% water solution of sodium chloride. The solution shall maintain at a PH of 6.5 to 7.2, the temperature of the fog is maintained at 95^oF.

22.3.2 Sand and Dust Tests

Sand and dust test is used to evaluate the resistance of a package to the penetration of sand and dust, to determine the erosive effects of blowing sand and dust. A standardize mixture of sand and dust of density 0.1 to 0.5 gm/cu.ft. is used to create an atmosphere for this. The temperature of this atmosphere is maintained at 77^o F for a period of 6 hours and then increased to 160^o F for another 6 hours.

22.4 Opacity

Opacity of all kinds of paper and paper products is determined by measuring the apparent light reflectance. The apparatus shall be capable of measuring the apparent light reflectance. It may measure the value separately or give directly the ratio of the apparent reflectance. The values of apparent light reflectance are relative to the apparent reflectance from magnesium oxide taken as 100%. The standard white backing shall have an apparent reflectance of 91.5% and the standard black blacking shall have an apparent reflectance of not more than 0.5%.Completely diffused illumination from incandescent lamps at a colour temperature of 2400 to 2800 Kelvin shall be used. The direction of viewing shall be not more than 20o from the normal to the surface of the specimen. Observations shall be made visually or by equivalent means such as a photo- electric with a filter adjusting its sensitivity to that of the human eye.

Place the test piece first over the standard white backing, then over the standard black backing and then measure the apparent reflectance of the light. The ratio of reflectance over black backing to that over white backing expressed as a percentage is the contrast ratio. Calculate the average contrast ratio from determination on both sides of each test piece.