The strength of the paper surface is particularly important in grades that will be exposed to tacky inks, as in the case of offset lithographic printing. Weakness in the paper surface can be revealed by such devices as the IGT printability tester, the Prüfbau test, wax pick tests (TAPPI Method T459), and to some extent by abrasion tests (TAPPI Method T476) and evaluation of dusting tendencies. Even a delamination test such as the Scott internal bond test or z-directional tensile tests (TAPPI Method T541) can be affected by low surface strength of some paper samples.
Strategies to improve the surface strength of paper fall into the two categories of (a) improvement of the inter-fiber bonding strength of the paper in general, and (b) focusing most of the improvement near to the surface of the sheet. Strategies based on the first approach, (a) a general improvement of inter-fiber bonding, are described elsewhere in this guide.
Size press application of starch, mixtures of starch and various copolymers, or various other polymer products used alone are the most common strategies to improve surface strength. The benefits can be focused more in the surface layers of the paper by increasing the amount of hydrophobic sizing agent added at the wet end.
When using underivatized starch at the size press, the results can be highly dependent on the quality of the starch. Best results are achieved if the starch is well dissolved, having as high as practical a molecular mass (and viscosity), and having very little self-crystallization, what is usually called retrogradation. Starch retrogradation is mainly observed in the case of the linear component of starch (amylose), not in the highly branched starch polymers (amylopectin). Conditions that favor retrogradation include (a) protracted storage of the starch in solution form, (b) temperatures lower than approximately 65 degrees Celsius, (c) partially degraded molecular weight of the polymer, especially in the range of 150 to 200 degree of polymerization, and (d) acid-modification of starch. Conditions that tend to prevent or inhibit retrogradation include (a) prompt usage of the starch as soon as it is prepared, (b) molecular branching, especially in the case of waxy maize products that contain only amylopectin, and (c) derivatization of the macromolecule, e.g. with hydroxyethyl, cationic, phosphate, or carboxyl groups.
Some possible causes of unexpectedly low surface strength, in addition to retrogradation of the size-press starch are as follows: Starch is subject to bacterial attack if the amount or type of biocide is insufficient. Some clues that this is happening can include (a) lower than expected viscosity of the starch solution, (b) lower than expected pH of the starch solution, and (c) decay-like odors. Factors such as the level of internal sizing agent, the porosity of the base paper, and the freeness of the furnish ought to be checked to see if they are in normal ranges. Debris on the surface of paper also could be interpreted as low surface strength, based on test results. If so, it would be important to determine the nature of the debris and its source. For instance, the presence of tacky materials on the rolls or felts in the wet-press section or earlier dryer can surfaces may cause fibers to be pulled from the moist paper surface, leaving the paper with a weakened surface.
In principle it is possible to increase surface strength by "upgrading" from a less expensive surface treatment, such as enzyme-converted native starch, to a premium product such a hydroxyethyl starch, or a mixture with polyvinyl alcohol or styrene maleic anhydride copolymer. Then, once the immediate issues are resolved, longer trials can be conducted to find out whether equivalent results can be achieved by more careful procedures for the preparation and use of the commodity starch product. Experience has shown that the premium products can be cost-effective in many cases, depending on the grade requirements.
In a minority of cases, paper products need to maintain their surface strength in a moistened or wet condition. It is not certain whether or not offset printing papers can benefit from efforts to make the paper surface water-resistant, since the aqueous fountain solution is applied to the paper only as a very thin film for a very short time. Strategies to increase the water resistance at the surface of a surface-sized or coated paper include the use of coating insolubilizers and treatment with sizing agents such as AKD, and hydrophobic copolymers such as alkyl-substituted styrenemaleic anhydride (SMA), urethane products, or styrene acrylates.
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.