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Creating rub-resistant inks

Jul 1, 1995 12:00 AM

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Nothing can spoil the effect of quality printed products more than the ink scuffing or rubbing off during shipping and handling. Rub resistance, therefore, is an important property to consider when formulating printing inks.

The need for rub resistance extends to just about every printed product. Even that perennial complaint about newspaper ink coming off on consumers' hands has received some long awaited attention in recent years.

Magazines that are to be shipped around the country are expected to arrive at newsstands or customers' homes in a readable state. Packaged products, such as folding cartons that contain everything from cereals to soap powder, must withstand the rigors of forming and filling as well as shipping and still retain their shelf appeal in supermarkets.

The same is true of flexible packaging. Here, obtaining good rub resistance on foil or plastic film is even more difficult due to the substrate's non-absorbent nature. Decorated metal products such as food containers and aluminum beverage cans also face considerable abrasion during processing and shipping.

Waxes are the materials primarily used in inks to impart rub resistance, and they come in many forms. In the early days, paraffin wax, derived from petroleum distillation, was the cheapest form of wax and was rather soft, as was beeswax. Carnauba, another naturally occurring wax, was harder but expensive. Also, an ink that contained carnauba, once dry, was very difficult to overprint.

New synthetic waxes derived from the polymerization of ethylene (known as polyethylene) became the wax of choice for most inks; microcrystalline, a harder form of paraffin wax, was the low-cost alternative. If a really tough, high slip protective wax was called for, then micronized Teflon (polytetrafluoroethylene) could be used, but with caution since it can cause excessive slip and the printed sheet, such as a magazine cover, can become difficult to control during the bindery operation. It also is expensive and can add considerable cost to the ink.

The amount of wax added to ink can be critical. Too little and rub resistance is not obtained; too much can soften the ink film and create a rub problem. Carbonizing or setting off of the ink also can occur if the surface contains too much wax. Waxes also can render an ink too waterproof. This poses a problem with lithographic inks that must emulsify a certain amount of water to function correctly. If the ink has too high a wax content, it will repel water and tend to adhere to the plate's non-image portions and print as a scum or catch-up.

Various instruments are on the market to determine an ink's degree of rub resistance. The Sutherland rub test is probably the best known. This machine consists of an arm that is driven to reciprocate by an electric motor. Attached to this arm is a 2- or 4-lb. oblong weight, around which a blank piece of paper or board can be placed. A segment of the printed material is placed under the weight and the machine is turned on for a predetermined number of cycles. As the plain sheet of stock is rubbed across the print, any tendency for the ink on the print to scuff shows up on the plain paper.

The Taber rub tester is an even more demanding device. Here, the printed sample is cut out in the shape of a circle and held on a round turntable by a metal ring. Resting atop the print is a wheel that can be carborundum or wire brush. As the turntable containing the print rotates, the abrading wheel also revolves, giving a uniform rub over the printed disk's entire surface. This is a very severe test that is specified by the Book Manufacturing Institute (BMI) and used frequently in the gravure printing industry.

Another interesting rub tester is the CAT or Comprehensive Abrasion Tester manufactured by Gavarti Associates of Milwaukee. Originally designed to test the abrasion resistance of inks and coatings for aluminum beverage cans, this device simulates the motion of a truck or train during shipping.

The motion has a frequency cycle, producing an undulation that rubs the printed products together. This fact was found to be true for printed cartons or magazines as well as cans. So, with modifications, the CAT can be used for paper or paperboard products.

The printed sheet is sandwiched between oblong blocks of polyurethane rubber. A plain or printed sheet of the same stock is placed in with it and then hydraulic rams push the blocks from the side in a wave motion to simulate over-the-road shipping. At the end of the test, the print-to-print or the print-to-plain-paper can be examined for scuffing.

A recent modification to the CAT is the introduction of a standard receptor with a well-defined degree of abrasiveness akin to very fine sandpaper. To obtain a numerical reading of the degree of rub, the receptor can be scanned with a flatbed scanner into a computer, which analyzes and provides a permanent record of the result.

Good rub resistance in an ink requires careful selection of ingredients including resins, drying oils and waxes. The ink must dry thoroughly, and careful testing - both in the laboratory and after printing with one of the devices mentioned - should be carried out before shipping to the client to ensure printed products will stand up to the rigors for which they were intended.