Some Tests Applied to Paper
- Annular Crush Test (RCT)
It is done to determine the resistance of the test sample cut from the outer paper to the crushing force applied perpendicular to the edge. Also called circular crush strength. It is used in corrugated cardboard liner papers. Indicates the minimum force required to crush the edges of a circularly arranged paper strip. If this value is low, it causes a decrease in box stocking performance. Its unit is kN/m.
It is done on the condition of applying pressure until the hole is opened in a small circular area. Cracking stress is higher in cans containing substances that create an outward internal pressure and thus move inside the can, creating a backpressure on the can wall.
The thickness of the paper to be tested is selected according to the disc diameter. Care should be taken that the two ends of the test sample do not come into the slit. The average of the six measurements made is recorded as an RCT. Care should be taken to ensure that the test sample placed in the circular holder is in the middle of the device. The top plate of the device is slowly lowered down and a certain force is applied to the surface and the maximum force of resistance is recorded. This value is our RCT value.
- Vertical Crush Test (CCT)
This test is only for papers to be used in the corrugated sheet. It measures edge crush strength in the paper. The paper is cut in the direction of flow and made corrugated in the laboratory. CCT is applied with a vertical force applied to vertically standing corrugations to determine the maximum load that the corrugations can withstand.
It is made to determine beforehand what the strength of the corrugated cardboard will be after it is produced. Its low value will reduce the stocking performance of the box to be produced. The unit of CCT is kN/m.
The paper to be tested is heated at 177⁰C ±8⁰C. The paper is made corrugated by passing it through a Medium Fluter machine at this temperature. The bottom plate of the sample tester placed in the metal corrugated holder is placed. The device is started, the maximum force that the corrugated sample withstands is recorded. This value is our CCT value.
- Surface Crush Test (CMT)
It is the determination of the surface crush resistance of corrugated fluting paper. It is made to determine the crushing resistance of corrugated paper with a vertical force by placing the corrugated paper in the laboratory horizontally. After corrugated cardboard is produced, it is done to determine in advance what its horizontal strength will be. As a result of the low CMT value, the corrugated cardboard cushioning, pulling, feeding, conveying rollers and other production stages cause disruptive effects.
High CMT will cause problems in flute formation and cracking in other production steps. The unit of CMT is Newton (N). It is the determination of the surface crush resistance of corrugated fluting paper. It is the amount of pressure required to crush the grooves as a result of applying a pressure perpendicular to the surface of the paper under standard atmospheric conditions.
At least 6 samples are used in the test and the average is taken. The test is performed under standard atmospheric conditions. The area of the sample to be used is determined, the point to be considered is that the machine direction is parallel to the long side. The sample is placed in the center of the bottom plates, left for the specified time, and the maximum force that the groove layer can withstand without crushing is recorded at 1 kgf approach. This result gives us the CMT value.
- Short Interval Crush Test (SCT)
It is done to determine the strength of the test sample cut from the outer paper (liner paper) by pressing (compressing) at short intervals. Its unit is kN/m.
A strip of at least 70 x 15mm is cut from the smooth part of the paper that is not damaged and passed through the machine. 12 results are obtained by performing a test from each end of a total of 6 samples, and the smallest of them is deleted from the test device and the average value is processed.
- Humidity Test
Moisture; It is the amount of water in corrugated, liner, cardboard, and corrugated cardboard. It can be measured by the oven drying method as well as hand tools operating based on conductivity capacity. In the oven drying method, the paper is dried at a temperature of 105 °C and weighed, the ratio of the difference with the initial weight to the initial weight gives the percentage of moisture.
Carton moisture affects the printability, machinability, and even usability of the paperboard. Cardboard is a hydrophilic material and it tends to take on moisture when it enters an environment with a higher humidity than its own, and it tends to give off humidity in the opposite case. The ambient humidity should be in balance with the cardboard humidity. If this balance does not exist, there is an exchange of moisture between the cardboard and the environment until the humidity is balanced, which affects the flat position of the cardboard and may cause corrugation and curling problems that affect its printability and workability. Usability is adversely affected.
On the other hand, since the high humidity can delay the physical and chemical drying of the applied printing ink, its workability may be adversely affected. Workability is a factor that affects usability characteristics. In order to prevent deterioration, some organizations recommend ambient humidity values. The absorption of water by the fiber is an exothermic process. In the researches carried out, the desired ambient temperature is around 21°C due to the balanced amount of humidity at high humidity of 21°C and passing through the minimum point.
The oven-balance method can be used to measure the humidity value, as well as the weight and thickness values.
- Burst (BST) Test
It is a static strength and it is very important especially in terms of the strength of liner papers. It is the measurement of the burst strength of paper or corrugated cardboard as a result of applying a certain pressure created by hydraulic fluid. The pressure force of the paper bursting as a result of hydraulic pressure is read on a manometer in kilopascals (kPa).
- Tensile Strength Test
The tensile strength of a paper depends on the number and strength of interfiber bonds and to some extent on the length of the paper strip.
After conditioning, 15mm wide and 100mm long strips are cut and placed taut and flat between the movable lower and fixed upper clamps of the dynamometer. A tensile force is applied on the movable clamp and the tensile force at the moment the paper breaks is recorded in kg of force. This value is called the pull-up resistance.
- Thickness Test
Thickness; It is the distance between the two surfaces of the liner, corrugated, corrugated, and cardboard, determined by a micrometer. Loose windings of variable thickness in liner and corrugated papers cause problems such as tension difference and humpback in the paper. It affects the storage strength of corrugated cardboard.
The equipment required to determine the thickness of the corrugated cardboard in mm is the micrometer. It is the interval measured in mm between two parallel flat surfaces of the micrometer by applying a certain pressure on the test sample located between the micrometer. The measuring device is a micrometer with a flat circular anvil and a concentric flat pressure foot with a deadweight dial indicator. The area of the anvil and pressure foot should be 10 sqcm ± 0.2 sqcm.
- Weight Test
Under standard atmospheric conditions, the weight of the paper with an area of 1 m2 is expressed as the gram weight of that paper. While its low level affects the transport performance; variability affects the coil rotation as it changes the humidity and porosity. Its unit is g/m22.
Samples taken from the pad are cut with fixed measuring hand tools in the transverse direction of the paper and weighed on precision scales, the average weight of the results is accepted as the final weight.
- Paper Water Absorption Capacity (Cobb) Test
Water absorption (absorption) strength (Cobb value); It is the determination of the amount of water that the paper can absorb in a certain time ( g / m2 ). Another definition related to this test is the test performed; It refers to the “water absorption ability test”. It is the resistance of paper or cardboard to absorb water. It is applied to determine the amount of water absorbed from a surface of paper or cardboard under 10mm of water in a certain period of time on a unit surface. Cobb test is not applied because the waste paper is generally used on the back surfaces of the cartons and it does not affect the production.
After the prepared sample is weighed with a sensitivity of 0.01 g, it is placed in the Cobb test device. 100ml distilled water is put into the Cobb device. The paper remaining in the Cobb device during the absorption is removed and weighed upon the expiry of the time.
c = 100 x (b - a)
a =>Weight of test piece before wetting (g)
b =>Weight of test piece after wetting (g)
c =>Absorbed water amount, Cobb value (g)
The samples taken for the Cobb test are kept in an oven at 105°C for 5 minutes and then the samples taken from the oven are kept until they cool under ambient conditions. The first weight (a) is determined on the precision balance and written on the corner of the corrugated cardboard. The surface to be tested is fixed by placing it on the rubber of the Cobb instrument, with the surface facing upwards. 100ml of pure water is poured on the corrugated cardboard and waited for 45 seconds. At the end of the time, the water is drained and the coarse water of the paper is removed by passing a steel printing cylinder over it with the help of blotting papers. Then the second weight (b) of the cardboard is determined on a precision scale. The process of taking the rough water and weighing should be completed in 15 seconds.