MicroencapsulationR.T. Dodge Co.

Microencapsulation and Scented Fragrance Inserts

Ronald J. Versic, Ph.D., President Ronald T. Dodge Company

For several years now, pushed along by stepped up sales promotion efforts by fragrance marketers, scented inserts have been growing in popularity in leading magazines for women and for department store bill mailings. These are descendents of 1950s technology known as Scratch-n-Sniff, now mostly relegated to children's books and some food aroma advertising. Both scented inserts and Scratch-n-Sniff make use of technology known as microencapsulation, initially developed in the 1940s and 1950s by the National Cash Register Corp. (now NCR Corp.), Dayton, Ohio. NCR's location explains why there are now several companies specializing in microencapsulation (including that of the author) also headquartered in the Dayton area. These inserts have been known by several terms, fragrance samplers, Scent Strips and Snap-n-Burst, though for this article probably the phrase fragrance sampler seems most appropriate, since these have been used for department store bills as well as for printed inserts in magazines. They are actually printed pieces, differing from earlier printed pieces in between the folded sheet is a very special type of microcapsule-containing adhesive. And it is the microcapsule components of this adhesive that distinguish fragrance samplers from other forms of scented advertising. The adhesive and the actual microcapsules dispersed or captured in it will be described in more detail later.

The first fragrance samplers appear to have been produced by Arcade Inc., Chattanooga, using capsules produced by Eurand. Presently there is a dispute regarding first date of production and patent coverage, a patent covering the design of many fragrance samplers sold today having been issued to the 3M Co. However, because of a challenge the original 3M patent is being reexamined by the Patent Office, and this challenge could result in a narrowing or broadening of that patent. At this writing, no judgment had been rendered, no decision had been handed down.

The fragrance sampler is manufactured in what is known as the bindery section of a printing operation. It is produced on a long, linear machine consisting of printing press and binder. The press provides the ink to produce the four color printing, but no folding is done on it and only one glue can be applied. The bindery section performs such other operations as gluing, folding, slitting, perforating and butt-cutting; and (crucial to this subject) applies the slurry of fugitive adhesive and capsules and folds over the paper to form the fragrance sampler. These samplers often are printed two across the web.

The fragrance sampler is a delicate balance of four components applied on the printing equipment under rigid specifications. These components are the microcapsules, the fragrance oil within them, the adhesive and the paper. Each warrants detailed discussion. The process used to achieve microencapsulation in all this is known by the scientific term of "coacervation," in academia called aqueous phase separation and in the trade as the NCR or the oil-in-water method. We will use the latter. The capsules produced are called reservoir microcapsules because they comprise a central reservoir of fragrance oil surrounded by a hard, presumably impermeable wall or shell. The analogy may well be a hen's egg, provided it is taken down to microscopic proportions. Center reservoir is fragrance oil, wall materials most frequently used are urea-formaldehyde (3M process) and various forms of gelatin (NCR process).

Coacervation involves forming droplet dispersion in water, the basic approach to capsule preparation. Size of the droplets usually determines size of capsules. In coacervation a wall is formed out of the surrounding water solution and deposited and hardened around the oil droplet, after which the capsules are washed and re-suspended in water.

The encapsulation process imposes certain requirements on the fragrance oil, the most important being that the oil should have no components that dissolve or migrate into the water phase. Scientists refer to this as "partitioning into the aqueous phase," the components that move into the aqueous phase tending to be lost, resulting in very poor rendition of the fragrance. Another requirement is that the fragrance oil has no components that react with the capsule material. One difficult component can be the aldehydes, which tend to react ' with gelatin and thus are lost to the fragrance because of capture within the capsule wall. Another group of materials for Schiff bases, reactions that also produce losses from the fragrance. Thus, successful microencapsulation that achieves true fragrance qualities requires close coordination between perfumer and microencapsulator. Less than genuine, open communications by competent people results in less than satisfactory fragrance rendition.

The adhesive chosen for fragrance samplers must be selected with great care. Technically, it is a fugitive adhesive, one with poor cohesive strength but excellent adhesive ability. When a fugitive bond is broken, the adhesive tends generally to break down the center. Where one bonds two sheets of paper, it releases so the paper will come apart in tact without rupturing in any way. The adhesive must have no odor itself, either wet or dry, and cannot affect the fragrance contained in the fragrance reservoirs. A fugitive adhesive generally comes in liquid form as an emulsion or solution with highest possible solids content, 40 percent being a typical good value. On the low end it should be no less than 25 percent solids, and viscosity also should be high as possible for incorporation into the capsule slurry. The application process for the adhesive, discussed further on, requires higher viscosity for the adhesive/capsule mix.

The capsule slurry is blended with the adhesive in a very special manner. Because the capsules are easily damaged, mixing must be low shear (very gentle), unlike the high shear conditions that are encountered

in a kitchen blender. Such mixing would destroy the microcapsules and the retentive property of the oil. It is generally held that capsules and adhesives should be stored separately and mixed at press time so as to avoid any adverse reactions between the two. Such interactions can occur over a prolonged storage period. Printing companies prefer to have press-ready materials shipped to them by the supplier so most suppliers provide capsules and adhesive together along with suitable preservative to control bacterial breakdown, which can affect viscosity and thus fragrance property. Bacterial breakdown also can result in black, mold-like specks that affect the aesthetics of the finished printing.

In general, the adhesive/capsule slurry should be stored in a cool, dry place at temperatures of between 20 and 30 degrees C. For long storage (not really advisable at room temperature), the slurry should be refrigerated at about 5 degrees C with the temperature being reduced or slowly raised. It is also advisable to use only fresh capsule slurry because fragrances can deteriorate entirely independent of the adhesive or the water. Water frequently has dissolved oxygen that can help deteriorate the fragrance oil in spite of its capsule envelope.

Paper for fragrance samplers usually is specified based on its ability to render four-color printing, smoothness, gloss and ability to handle two-sided coating. Again like the capsule and adhesive, the paper must have little or no odor whether it is wet or dry. It should have neutral pH so no acidity affects the adhesive/capsule slurry. Determination of pH is done by soaking the paper in water, reducing it to pulp, and measuring pH. The paper also should have good physical stability because the adhesive/capsule strip is dried on the paper by diffusion of the water into the paper --- a process that can cause puckering and a consequent wavy appearance in the area of the strip. If the strip is applied at a high solids percentage (say 40) this is much likely to occur. One useful criterion to judge a quality fragrance sampler is to look for smoothness and flatness in the area of the strip.

Before a press run, a number of test samplers are prepared in the laboratory by the microencapsulation company or the printer. Generally they are prepared by hand or on small-run stripping or coating equipment (proof presses are handy for this). The samples are coated out and folded over, whereupon they are placed under light pressure as they would encounter between pages of a book. They remain for several minutes so that the adhesive can begin to set, then are taken out and allowed to air dry for a day or two. When accelerated drying is necessary, the test strips are put into a desicator to remove the water from adhesive and paper. Further acceleration sometimes is done with a microwave oven into which a container of water is inserted, but this is not particularly recommended because the process will dry the water from the test strip at the same time it unduly heats the fragrance oils.

As has been said, the adhesive/capsule strips are applied on printing equipment in the bindery section. The adhesive is applied to the paper, generally two across the width of the web, by a printing or extrusion operation. In printing it is done on a pattern gluer, equipment most web printers already have (often one in the printing press section, another in the bindery operation). In printing there is frequently an oven at the outgoing end, so if there is a pattern gluer that can be applied and quickly dried, a necessary operation when applying a remoistenable envelope glue to envelopes. "Printing plate" used for application of the adhesive/ capsule slurry is a soft, spongy plate relatively smooth on its surface. Similar to the sponge-like weather stripping applied around doors and windows, this plate is applied by a pressure sensitive adhesive to a large chrome cylinder on the pattern gluer. Position, length, and width of this rubber plate are selected to match the rough dimensions of the adhesive/capsule strip itself.

The adhesive/capsule slurry also can be applied by extrusion, not necessarily a preferred method because it can create application difficulties. As suggested, the capsules are sensitive to any high shear, such as the kind produced by pumps propelling the slurry through an extrusion nozzle. Extrusion applies a continuous strip whereas the pattern gluer can apply a strip of any width or dimension. Sometimes it may be necessary to use an extruder when the pattern gluer (or gluers) are busy doing other operations such as pocket gluing or applying remoistenable adhesives.

Production of fragrance samplers requires careful quality control on the strip itself, there being two criteria that must be closely watched. The first relates to the caliper of the strip, determined by measuring the caliper of the printed piece at the strip and off to the side. Determination of proper thickness requires trial and-error by the press personnel, a not uncommon thing in printing. In this case, the water-laden strip still is adding caliper beyond its normal dried condition to the printed piece. Frequently a rule of thumb involving a factor of two is used. For example, if the fragrance strip is supposed to be .75 mil thick in its dried form, then net thickness of the semi-wet strip should be about 1.5 mils. This rule of thumb is for adhesive/ capsule slurry at 40 percent solids. For one of 25 percent solids, a factor of three is more typical. Best way to determine this is to make a number of measurements on the actual production equipment. All this suggests the critical importance of the caliper of the dry strip.

The capsules are made in the 15-20 micron range, though some manufacturers claim that small addition of capsules in the 30-80 micron range may be desirable. In its ideal form, a fragrance sampler would look like steel ballbearings between two glass plates, the idea being that as the plates are pulled apart the bearings split down the middle to release the reservoir of oil in each. For fragrance capsules in the 15-20 micron range, desired thickness of the dried glue strip is about .5 to .75 mils or 12 to 18 microns. The capsules are truly caught between the two sheets of paper, and when the paper is pulled apart, the fugitive adhesive breaks down the middle to force the capsule halves to go with their respective paper side.

Of course, ultimate test for the product is performance in the hands of the user. It is necessary, often utilizing a panel of people, to take a properly dried fragrance sampler and let them test it. The three criteria studied in judging such tests are rendition, lift and rub-off. Rendition relates to the faithfulness of the capsules to the fragrance from the perfume being promoted, keeping in mind the caveat that the oil with which the microencapsulator starts may not be the same as that put into a perfume bottle. In fact, this is generally the case, so that one should not assume that the fragrance oil is the same as that used for the perfume. Simplest test for assaying rendition is to compare fragrance of the actual perfume with that emanating from the sampler.

Lift relates to the way the perfume oil is lifted into the air from the printed piece to the nose of the observer. Snapping action of the sampler acts like a drum head whose vibrations propel the oil into the air. A fragrance sampler formulated with the suitable adhesive will pull apart with the right degree of force so this force is translated into a vibrating drum head. Rub-off, the third criteria, relates to the desire by the consumer to take the sampler and rub it against his/ her wrist to determine how the fragrance smells on the skin. The capsules must retain and release sufficient quantity of oil so it will transfer to the wrist.

Test methods for determining these criterial are organoleptic and analytical. Organoleptic refers to use of a test panel of humans given a quantity of the fragrance samplers and being asked to rate them on the criteria. If they judge a fragrance to be "off," they are asked how the rendition differs from the perfume in words such as "musty, acidic" or whatever. Analytical methods also are used in which the fragrance strip is heated and the oils removed for analysis by gas chromatograph. Sometimes the oils are extracted out of the fragrance strip with suitable solvents for GC analysis, and in some cases a gravometric test is needed to ascertain how much oil is retained in the strip as compared how much initially went into it. This latter assay is known as mass balance. The test methods described relate only to post-printing difficulties encountered with quality control of the printed piece. Besides fragrances, cosmetic color products such as lipstick, gloss, makeup and eye shadow can be applied by a hot melt process. This is not very different from the hot spotting of carbon black for carbon paper forms. For lipstick, it is melted and applied to the paper carefully so as not to leave any coating pattern. Eye shadow can be applied either on the printing press or in the bindery. Powder used in eye shadow is dispersed in water with small amount of surfactant, then a binder and thickening agent are added. The latter is necessary to keep the eye shadow powder in proper suspension. Common method of application is "bumping," in which the paper actually is pushed up against the coating cylinder. It is important to avoid coating patterns in eye shadow, especially that referred to as "crow's feet," which resembles ridges and valleys that show differences in color and reflection.

Versic, R.J. Drug Cosm. Ind. 1989, 144(6), 30, 32, 34, 75.