SURFACE STRENGTH OF PAPER (WAX PICK TEST)
Introduction
The surface bonding strength of paper is a measure of its resistance to separation of coating or fibers or to any cleavage parallel to the sheet surfaces. Paper is said to "pick" when coating, fibers, or small portions of the paper itself separate from the body of the sheet while it is being printed. This tendency for picking is of great concern to printers and it is necessary to control pick strength in making most types of paper.
Coated paper picks if the adhesion of the ink to the coating is greater than the adhesion of the coating to the paper, or because there is insufficient bonding strength in the base stock to prevent its splitting. Picking of the coating is not always due to a weakness of the coating material but may be caused by the use of too tacky ink or poor make-ready. If the ink is too tacky, the tackiness must be reduced, since the physical interaction between the ink and the paper coating also depends upon their relative cohesiveness.
Significance
Surface bonding strength or pick resistance is important in printing. It is a performance property rather than a physical property such as tensile strength or smoothness. This property is of increasing importance because of the use of fast presses, which accentuate picking conditions, and multicolor printing which requires a wide range of ink tack.
Picking results in bare spots on the printed image and is especially noticeable in solid areas. The particles or debris lifted from the sheet may lodge on the printing plate or be carried back through the inking system of the press. Excessive picking requires expensive wash-ups of plates, rollers, and ink fountains. If not corrected, picking results in clogged type and halftones producing blurred printing. Attempts to overcome picking by reducing ink tackiness (with compounds designed for that purpose) may result in a weak or faded appearance of the print.
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Picking resistance or surface strength are very complex properties and a variety of methods have been developed for measuring them. Because of its quickness and convenience, the Dennison Wax Test is widely used as a control during paper manufacture. Although it doesn't simulate the action of printing, the "wax pick test" is the traditional method for measuring pick strength. Where this test fails is in comparison of different types and makes of paper, especially of papers containing thermoplastic coating materials. It also does not involve grain direction the way an actual printing operation does.
As a result a wide variety of instrumental methods have been intensively studied but these miniature press simulators operate under different conditions and express the results in different terms so it is difficult to make meaningful comparisons. Added to this is the fact that the phenomenon of picking is not well enough understood to permit selection of a single instrument on a strictly objective basis. The wax pick test, being the most universally used, will be discussed and used here.
Method of Test
This test uses a series of hard-resin, non-oily waxes having graduated adhesiveness that are numbered respectively from 2A to 26A. To make the test, the end of a wax stick is softened by heating it over an ethanol flame, rotating the stick slowly until several drops of melted wax have fallen. (Do not let it catch fire.) Make sure you tilt the ethanol lamp slightly so that the falling wax droplets don't hit the wick. Also, place a collar of stiff board around the lamp opening to catch the falling wax so as to prevent wax buildup on the lamp. Quickly place the melted end of the wax stick on the surface of the specimen with firm but not excessive pressure, and release immediately. Allow the wax to cool for 15 minutes. Then, with the wax protruding through the hole of the wooden pull block, which is held down firmly, pull the wax vertically off the paper with a quick jerk. Examine both the tip of the wax stick and the surface of the specimen. Repeat the test, if necessary, using waxes in ascending numerical order until the surface of the paper specimen blisters, breaks, picks, or lifts.
Record the highest wax number for which no disturbance of the paper surface was detected. Test both sides of three specimens. The felt side often has a lower pick strength than the wire side. Average the results on each side to the nearest wax number.
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Report
The wax with the highest average number that does not produce picking is used to indicate the surface strength of the paper. Report this average number for the felt side and the wire side.
On coated papers, the report should also state whether the picking or lifting occurred in the coating, the body stock, or both. This is determined by closely examining the wax after removal from the paper being tested.
Reference
TAPPI T 459
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