Cardboard is a kind of paper, but it can be almost as strong as wood in some cases. The main reason why cardboard is strong is in the composition, construction and testing of all cardboard. All of these elements combined help to form some of the strongest paper in the world.
Modern cardboard is usually made up of kraft paper. Kraft paper was invented by the Swedish chemist Carl F. Dahl in 1884. He found that pulping wood chips and converting them into a thick paper created a material that was hard to tear and damage. Since then, most all cardboard is made from three layers of this kraft paper, with two flat layers and a corrugated layer between them. Some cardboard is made of just one thick layer (such as in cigar boxes), but it is not as widely used.
The kraft paper is made by pulping the wood, which is created by taking wood chips and extracting the cellulose from the wood with chemical sulphate. The pulp is then pressed into thick sheets of paper and allowed to dry. The paper is transferred to a cardboard making factory, where some of the paper is crimped to make the centre of the cardboard, and the other layers of cardboard are glued on either side of the corrugated layer. Different plants make different strengths of cardboard. This multilayer function of cardboard makes it much stronger than paper alone. The crimps in the paper make it better able to withstand the weight and pressure of heavier objects.
As it is being made, cardboard withstands several tests. The Cobb test determines how much moisture the cardboard contains. Wrap tests are given to find out how well the cardboard can get formed into a box. Corrugated cardboard is also tested to find out the strength of the bond and glue between the layers of cardboard. All of this testing helps create cardboard that is almost as strong as wood.
Cardboard made into a box is put through a few more tests. The edge crush test is a test applied to the corners of cardboard boxes. This measures the strength of the box's corners, identifying how much weight can be stacked on the box. Another common test performed on cardboard boxes is the burst test. This test measures how many pounds of weight a box can hold before the bottom of the box bursts.
Cardboard commonly comes in different strengths. Cardboard strength is measured with glue tests, bursting tests and compression tests. These measurements are recorded and the cardboard is assigned different grades depending on the score it receives. A common strength indicator looks like this: 125K/B/125T. The first number is the weight that the cardboard can withstand before it is crushed. The letter stands for the kind of corrugation between the two layers of cardboard. Common letters include E, less than 1/10-inch thick; B, less than 1/8 inch; C, about 1/6th inch; and BC, about 1/5 inch thick. The last number stands for how much weight the second layer of cardboard can hold, and the final letter stands for the kind of paper used. K stands for kraft, one of the most commonly used types of paper.
When choosing cardboard always look for the highest number listed on the packaging. Higher numbers indicate higher strengths and grades. The higher grade numbers on cardboard indicate that the cardboard is suitable for uses where greater strength is required. When choosing cardboard boxes, look for the Box Manufacturer's Certificate (BMC) on the bottom of cardboard boxes. The certificate states that the box has passed all requirements and tests given to cardboard.