Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-05-26
2004-08-17
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S053000, C525S054000, C525S191000, C525S222000, C524S502000, C524S523000, C524S524000
Reexamination Certificate
active
06777490
ABSTRACT:
PRIOR ART
Natural rubber is the ideal elastomer for producing adhesive tapes provided high ageing resistance is not a requirement. Particular advantages are
the low price of the elastomer
the low price of suitable tackifiers (resins and so on)
the good balance between the physical properties such as unrolling force, shear strength, tack and bond strength.
To date it has proved possible to process natural rubber only from solution. There has been no lack of attempts to process natural rubber as an aqueous system as well. Technical realization fails owing to the extreme shear sensitivity of the latex; only by adding fairly large amounts of surface-active substances or by blending with other latices is it possible to achieve a certain measure of processability, and then only at the expense of a severe loss in the target profile of properties.
Until now, the processing of natural rubber compositions from the melt was impossible. If the preparation of hot melt PSA (pressure-sensitive adhesive) compositions is attempted, then homogenization necessitates strong and lengthy shearing. This causes severe degradation and hence a loss in performance of the adhesive tape, despite which it is impossible to wholly avoid specks of rubber. Improving the homogeneity by shearing (masticating) the rubber, then preparing a batch with a portion of the tackifier and subsequently processing this batch in a further step with the addition of further raw materials to the adhesive composition has been tried: this technique is described in WO 94/11175. WO 95/25774 is further evidence that the processing of non-thermoplastic material is possible only with mastication, which has the deleterious effect of degrading the polymer. A further disadvantage of all process trials with solid rubber is the necessity of batch operation, which drives up the production and quality control costs. Adhesive compositions of this kind have so far been used only to manufacture low-grade cloth adhesive tapes (duct tapes). Because of these problems, thermoplastic elastomers (styrene-diene block copolymers) have been developed which go a long way towards avoiding these drawbacks. However, they result in product properties markedly different from those of natural rubber. For instance, it has so far been impossible in this way to produce high quality products such as readily redetachable film adhesive tapes or paper-based adhesive tapes for painting. A significant defect of such adhesive compositions comprising thermoplastic rubbers is their low heat resistance. Ageing resistance is likewise much less favourable than in the case of natural rubber adhesive compositions. Furthermore, in terms both of the elastomer and of the suitable tackifiers, such formulations are significantly more expensive than those based on natural rubber.
Natural rubber is produced as latex. The commercial solid rubber is obtained from latex by precipitation. Without the use of solvent it is impossible to mix solid rubber in the form of bales or pellets with tackifiers, fillers, antioxidants, plasticizers, etc. with complete homogeneity and above all to avoid the associated shearing and degradation of the elastomer. If the latex is mixed homogeneously with a dispersion of these additives, the problems of both homogeneous mixing of the components and shearing and degradation of the elastomer disappear. Unfortunately, such water-based adhesive compositions cannot be used in practice for coating on production lines owing to their extreme coagulation sensitivity: even during processing, gel particles, or complete coagulation, occur.
Process of the Invention
When natural rubber latex, melted or finely particulate tackifiers and any further components are mixed in a mixer, compounder, extruder or the like, the mixture is surprisingly homogeneous, because the latex mixture does not coagulate before the end of complete addition and thorough mixing. In the specific case of preparation, it is advantageous to make a correct choice of the sequence and amount of the components and not to subject the mixture to any more shear stress than is necessary, since under adverse conditions it is possible to provoke premature coagulation. The stability can be raised by adding water, ammonia, alkali metal hydroxide solutions, or surface-active substances such as emulsifiers. In batch operation the best thing is generally not to add the latex and the other components in alternation but instead either to introduce all of the latex at the start and then to mix in the remaining components or to introduce these components as a mixture at the start and then to add all of the latex. If the latex is introduced at the start, then the first addition should be the plasticizer (if required); this applies in particular to mineral oils or liquid hydrocarbon resins. In this way it is easy to mix all of the desired components before the rubber solidifies through coagulation. This is highly surprising, since not only is the latex shear-sensitive when stirred thoroughly but to the skilled worker a mixture which has a solids content of 80% or more and yet can still be stirred is unimaginable.
Depending on formulation, the adhesive compositions of the invention are still liquid and hence can be processed directly using an applicator unit, ball together tackily, or form a non-tacky and readily conveyable solid. In general there is no discernible separation (e.g. emergence of the aqueous phase). When using Hercolyn D-E, for example, liquid mixtures with a water content of from 10 to 50% by weight are produced which can be used for coating but require subsequent drying after the coating operation. The use of very high proportions of fillers or pigments results in solid mixtures which can be conveyed without tack. These solid or liquid mixtures can be supplied, for example, to a twin-screw extruder or planetary gear extruder, which removes the residual water in a vent section and then feeds a coating die or roll-type applicator unit. The applicator unit then coats the substrates directly or indirectly (for example, coating onto a roller or liner and transfer to the substrate by lamination) or applies the composition to a liner, without a backing. Dewatering can also be achieved by means of heat or subatmospheric pressure in a stirring unit, such as compounders or continuous mixers; internal mixers (Banbury) are less suitable because it is more difficult to remove the adhesive composition. The compositions (unless highly filled or pigmented) have a pressure-sensitive tack even without compounding; in other words, diffusion is sufficient to allow molecular penetration of rubber and resin. This shows how easy it is, in the manner of the invention, to achieve homogeneity without the need for extreme shearing as in the case of solid rubber. Prolonged compounding in batch operation is therefore unnecessary; just the short residence time in an extruder is sufficient, so enabling the manufacturing process to be continuous. It is possible to carry out the mixing of the latex with the other components and the dewatering in one extruder. The resin may, for example, be passed through a Condux mill downstream of the weigh feeder and then onto the feed zone; alternatively, the resin melt can be pumped in at just under 100° C.
Because of the short residence times when the adhesive composition is sheared, the rubber undergoes little if any degradation. The adhesive compositions of the invention therefore have K values of in particular ≧80, preferably ≧125 and, with particular preference, ≧140, which give rise to good shear strengths. Even K values of about 160 are possible for adhesive compositions, whose shear strength far exceeds that even of good conventional solvent-based compositions. The K values of conventional solvent-based compositions lie approximately between 130 and 150, but may even be considerably lower if CV rubber is used. A particularly surprising fact was that the compositions of the invention, with such high K values, can be used for coating without problems. It is supposed that the rubber pa
Hirsch Ralf
Kozaczka Zygmut
Müssig Bernhard
Stähr Jochen
Zielske Dieter
Egwim Kelechi C.
Norris & McLaughlin & Marcus
tesa AG
LandOfFree
Process for preparing a pressure-sensitive self-adhesive... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for preparing a pressure-sensitive self-adhesive..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing a pressure-sensitive self-adhesive... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3274098