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Apr 1, 1998 12:00 AM
Has a creative concept ever been thwarted because you couldn't find the right paper--one that would withstand wear during use? Has that brochure you spent so many sleepless nights creating torn after only limited handling? Perhaps the sales presentation that was developed to "lock-in" a once-in-a-lifetime account was damaged on the way to the meeting by a rain shower. These are just some of the dilemmas that can occur when using traditional paper for jobs that require an added dimension.
That dimension might be achievable using synthetic paper. Synthetic paper can provide the competitive, higher profit edge for niche applications that require high-impact appeal from one reader to the next. Direct mail and advertising can lose their impact, not because of a faulty concept, but because the life span of the substrate used will not sustain repeated usage. Synthetic paper can add a creative, durable dimension to projects, helping to create a dynamic design. Unlike wood fiber-based papers, synthetics can be fully submerged in water without degrading. The pliability of synthetics also allows the substrate to be folded thousands of times without cracking--enhancing the longevity of a project.
Synthetic papers are suitable for use as maps, menus, tags, posters, banners, labels, membership cards, manuals, books and business cards.
Some synthetic papers use polypropylene as their main component. They are manufactured by extruding polypropylene with fillers through a die and stretching the material lengthwise. Additional layers can be extruded onto both sides, and the composite is stretched crosswise. The sheet then can be treated to enhance its appearance and print performance.
The result of this process is an extremely durable, dimensionally stable synthetic paper that is water repellent, flexible and scuff resistant. In addition, it prints similar to traditional paper.
As with wood-fiber-based paper, synthetics require time to acclimate to the ambient conditions of the pressroom--at least 24 hours. To minimize the potential for static buildup, a pressroom temperature of between 70 degrees F and 80 degrees F, with at least 50 degrees percent relative humidity, is recommended.
Cold stock, in combination with a dry environment, can cause static to develop, making it difficult to run efficiently through the press. The most important factor in successfully printing on non-absorbent substrates is the control of the dampening solution on press. If excess water remains on the surface, it may interfere with ink transfer and result in over-emulsification of the inks. Setting each unit just above the scum line is the best method to prevent dampening-related problems.
Continuous-flow dampening systems work best with synthetics because the form rollers are not covered and, therefore, cannot act as storage reservoirs for dampening solutions. This allows them to respond to changes in the fountain settings quickly.
Regardless of the dampening system employed, using the maximum allowable level of alcohol will facilitate high-quality printing. Lower levels of alcohol, and/or using the appropriate alcohol substitutes, provide a workable option, though they require tighter control of the ink/ water balance.
Ink itself, of course, plays a significant role in print quality. Sheet-fed printers using polypropylene synthetic papers can utilize typical low-solvent inks. In addition to reducing Volatile Organic Compounds (VOCs), low-solvent ink consisting of 100 percent solids can produce vivid, consistent results. It also eliminates "solvent attack" on the sheet, a condition that occurs when solvents penetrate the plastic, creating distortions such as puckering and waving.
Since ink is not absorbed by synthetic paper, controlling the ink-film thickness is crucial. As is the case when printing on coated stock, press operators should only use the thinnest possible ink film to achieve the desired color and density. An ink-film thickness from .2 mil to .4 mil at the vibrator roller above the first form roller usually is recommended. Ink set-off and slow drying problems will result if too much ink is used.
Because synthetic papers are non-absorptive, drying times normally exceed those required for wood-fiber-based stocks. To maximize ink drying on polypropylene paper, several steps should be followed: 1. Select ink carefully, contacting an ink supplier experienced in formulations for synthetic stock. Discuss performance expectations for the application. 2. Use 30 percent to 50 percent more spray powder than for coated papers, depending on the ink coverage and lift height. The powder will keep the sheets separated and allow air to penetrate between the sheet and into the pile. Partially coated powders (10-20 microns in size) are recommended because they absorb moisture slightly, preventing them from developing a static charge. At the same time, they don't absorb so much as to create void areas. 3. Smooth, level delivery boards with a maximum of four inches of lift height will prevent setoff from occurring. 4. Fan the loads once or twice after the ink has set to promote drying and reduce the risk of gas ghosting. Blowing warm air onto the piles will decrease drying time.
Depending on the end use, the value of the printed piece may be determined by the durability of the printed image more than the durability of the synthetic stock. Overprint coatings are one way to extend the life of the printed image. Coatings should be selected based on striking a balance between cost and the associated benefits. Aqueous and UV-curable are commonly used on synthetic substrates to maximize the life span of the images.
Aqueous coatings produce a glossy, non-yellowing finish. They provide excellent rub and abrasion resistance. As the name implies, aqueous coatings use water as the primary solvent system. However, they may also contain a co-solvent such as alcohol or other acrylic resins. Several steps should be taken to ensure fast, efficient drying of aqueous coatings on polypropylene papers: 1. Use dryer air temperatures--120 degreesF to 150 degreesF. 2. Load temperatures should remain between 90 degreesF and 95 degreesF. 3. Use spray powder in the delivery. 4. The lift size should be three inches.
UV coatings are essentially 100 percent solids plus mixtures of reactive monomers, oliogomers and photoinitiators. They produce a smooth, glossy non-yellowing finish that is extremely tough and scuff resistant. UV coatings take only a fraction of a second to polymerize and convert from a wet coating to a hard, dry film. Consequently, set-off is not a problem. Of primary concern with UV coatings relative to synthetic stocks is their ability to flow out and wet the surface. If the surface is not wet properly, adhesion problems will occur. UV coatings may require a primer coating on some synthetic papers. Again, it is important to select inks carefully for these applications.
Synthetic paper also lends itself to a variety of finishing options, all of which enhance the value of the finished product. Foil stamping, embossing, diecutting, perforating and laminating are just a few of the ornamental finishing techniques that can be used. As is the case for all substrates, specific guidelines should be followed to strike a balance between achieving the desired appearance without compromising the integrity of the stock.
Synthetic paper offers many functional advantages that enhance both the value and durability of various specialty projects. With their tear and wear resistance, these unique papers can be the "tough" choice for specialty applications.