Vertical and horizontal identification methods and front and back identification methods of paper and paperboard

1. Vertical and horizontal identification

Since the paper and cardboard formed by the paper machine have a certain directionality, the direction parallel to the running direction of the paper machine is the longitudinal direction, and the direction perpendicular to the running direction of the paper machine is the horizontal direction. Many physical properties of paper and cardboard have significant directionality, such as tensile strength and folding endurance, the longitudinal direction is greater than the transverse direction; the tear degree is greater than the longitudinal direction in the transverse direction. In addition, since the expansion of the fiber body is greater than the linear expansion, the lateral deformation of the paper is greater than the longitudinal deformation, which can easily cause the paper to warp and affect normal use. Therefore, when measuring physical properties, it is necessary to consider the vertical and horizontal dimensions of paper and board.

For unwrinkled paper and cardboard, curl method, stripe method, tear method, tensile strength method, warpage method, etc. can be used for vertical and horizontal identification.

① Curling method Cut the sample into a square of 50mm×50mm, put it on the water surface, pay attention to the direction of curling, and the direction parallel to the curling axis is the longitudinal direction.

②Stripe method Pick up the paper strip sample and observe from the other side of the paper against the strong light. The longitudinal stripes and fiber strands are evenly distributed, but there are no horizontal stripes.

③Tearing method Tear the sample by hand, and the relatively flat tearing direction is the longitudinal direction, while the tearing deflection is too large as the transverse direction.

④ Tensile Strength Method According to the difference between the vertical and horizontal tensile strengths of paper and cardboard, the vertical and horizontal directions of paper and cardboard are identified. Generally, the longitudinal tensile strength is greater than the transverse tensile strength.

⑤Warping method Cut two perpendicular paper strips about 20mm in length and 15mm in width along the edge of the sample, mark them, overlap them, pinch one end with your fingers, and bend the other end freely. If the two paper strips are separated, it means that the lower paper strip has a large bend and is horizontal; on the contrary, if the two paper strips overlap and do not separate, and the upper paper strip is pressed on the lower paper strip, the upper paper strip is horizontal.

For paper that has been creped, such as toilet paper, facial tissue, elastic wrapping paper, etc., it is generally creased along the longitudinal direction, so the direction parallel to the wrinkles is the transverse direction. However, for the paper produced by the side flow paper size machine, it should be carefully identified according to the situation. The transverse tensile strength of this paper may be greater than the longitudinal tensile strength.

Second, positive and negative identification

The paper has two surfaces, the side close to the surface machine copper mesh is the reverse side (or mesh side); the other side is the front side (or felt side). The reverse side of the paper is rough and loose due to the copper texture, while the front side is smoother and tighter. The smoothness of the paper is that the front side is greater than the reverse side, and the whiteness is that the reverse side is greater than the front side.

When testing the physical properties of paper and board, it is customary to test both sides. If only one side is tested, it should be stated in the test report. When using, the front and back of paper and cardboard should also be considered. Generally speaking, the front and back of the paper can be distinguished with the naked eye, but if it is processed (such as coated with paint, etc.), it is not easy to distinguish.

The methods for identifying the front and back of paper and cardboard include visual method, wet method, carbon paper indentation method, curl method, and smoothness method. If the visual method, wet method, carbon paper indentation method, and curl recognition method cannot be accurately judged, the smoothness method should be used.

①Intuitive method Fold the paper and observe the surface shape or structure on both sides, the side with clear screen printing is the reverse side. If you can't see clearly with the naked eye, you can use a magnifying glass to identify it.

②Wet method Immerse the surface of the paper with water or dilute sodium hydroxide solution, drain the excess liquid, leave it for a few minutes, observe both sides, if there is screen printing, it is the reverse side. You can also put the paper in the water, because the shrinkage of the front and back of the paper is different, when curling up, the side that touches the water is the back.

③Carbon paper indentation method After making marks on the paper, press the carbon paper on the paper surface, so that the traces of the carbon paper are pressed on the paper, and a black trace with a width of 13mm and a length of 51~76mm can be drawn . View on a smooth surface such as flat glass. Then identify the front and back of the paper or cardboard by observation.

④ Curling method Put the sample in an oven to dry, pay attention to observe the direction of curling, generally the inner surface of the curling is the front.

⑤Smoothness method Determine the front and back sides of paper or cardboard by measuring the smoothness. The smoothness of the front side is greater than the smoothness of the back side.