Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2001-11-20
2003-05-20
Crispino, Richard (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C428S492000, C524S270000
Reexamination Certificate
active
06565703
ABSTRACT:
The invention relates to an adhesive tape comprising a polyolefin-based backing unilaterally coated with a solvent-free adhesive composition based on natural rubber latex and to the use of said adhesive tape as packaging tape.
Adhesive tapes with films based on polyolefins, such as biaxially oriented polypropylene, for example, and adhesive compositions based on solvent-containing natural rubber or aqueous acrylate dispersions are known and are supplied by known manufacturers.
A description is given, for example, in “Packaging Tapes” by Stefan Röber in Handbook of Pressure Sensitive Adhesive Technology, Third Edition, edited by Donatas Satas, Satas & Associates, Warwick, Rhode Island pp. 787-814. Further descriptions can be found in European Adhesives & Sealants 10(4), 1993, 29, by G. Pedala, and European Adhesives & Sealants 2(2), 1985, 18, by R. W. Andrew.
Owing to the known disadvantages associated with the production of solvent-containing adhesive tapes, such as, for example, the difficulty of recycling solvents, the emission of solvent to the environment, the workplace hazard due to highly flammable solvents, and the restriction on coating speed imposed by the drying of the adhesive composition, solvent-free technologies for producing adhesive tapes are advancing in importance.
A disadvantage to add to the above-described problems of the solvent technology is the need to break down the natural rubber during the production of the adhesive composition, in a process known as mastication. This results in the adhesive composition having a cohesiveness which decreases markedly in accordance with the extent of breakdown under mechanical load.
The deliberate industrial process of breaking down rubber under the combined action of shear stress, temperature, and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as “chemical plasticizing aids”. In rubber technology, the mastication step is needed in order to make it easier to integrate the additives.
According to Römpp (Römpp Lexikon Chemie—Version 1.5, Stuttgart/New York: Georg Thieme Verlag 1998) mastication is a term used in rubber technology for the breaking down of long-chain rubber molecules in order to increase the plasticity and/or reduce the (Mooney) viscosity of rubbers. Mastication is accomplished by treating, in particular, natural rubber in compounders or between rolls at very low temperatures in the presence of masticating agents. The high mechanical forces which this entails lead to the rubber molecules being “torn apart”, with the formation of macro radicals, whose recombination is prevented by reaction with atmospheric oxygen. Masticating agents such as aromatic or heterocyclic mercaptans and/or their zinc salts or disulfides promote the formation of primary radicals and so accelerate the mastication process. Activators such as metal (iron, copper, cobalt) salts of tetraazaporphyrins or phthalocyanines permit a reduction in the mastication temperature. In the mastication of natural rubber, masticating agents are used in amounts from about 0.1 to 0.5% by weight in the form of masterbatches, which facilitate uniform distribution of this small amount of chemicals in the rubber mass.
Mastication must be clearly distinguished from the breakdown known as degradation which occurs in all of the standard solvent-free polymer technologies such as compounding, conveying, and coating in the melt.
Degradation is a collective term for different processes which alter the appearance and properties of plastics. Degradation may be caused, for example, by chemical, thermal, oxidative, mechanical or biological influences or else by exposure to radiation (such as (UV) light). Consequences are, for example, oxidation, chain cleavage, depolymerization, crosslinking, and/or elimination of side groups of the polymers. The stability of polymers toward degradation may be increased by means of additives: for example, by adding stabilizers such as antioxidants or light stabilizers.
The use of adhesive compositions of this kind based on natural rubber hotmelt pressure-sensitive adhesives for adhesive tapes, especially packaging tapes, for sealing cartons made from recycled paper, or cardboard packaging, results in premature opening of the cartons. If there is sufficiently great tension across the lid of the carton, caused by the pressure of the packaged material in the carton or by the tension of the packaging material, which opposes sealing, the adhesive tape becomes detached from the surface of the carton and the carton opens as result of slippage of the packaging tape.
The cohesion and, with it, the packaging security afforded by adhesive tapes with an adhesive composition based on natural rubber may be improved either by crosslinking the rubber adhesive composition and/or by means of a variant preparation of the adhesive composition in the course of which the natural rubber used is broken down to a much-reduced extent and therefore has a higher molecular weight. This makes it possible to counter slippage of the adhesive tapes on the carton surface, as described above.
Solvent-free technologies for producing adhesive tapes, especially packaging tapes, have to date been restricted to the use of adhesive composition based on acrylate dispersions and to the use of melting, thermoplastic elastomers.
An advantage of these thermoplastic elastomers, predominantly block copolymers containing polystyrene blocks, is the relatively low softening point and the corresponding simplification of the application or coating process, and also the avoidance of the above-described disadvantages of the solvent-based technologies.
The unfavorable aging behavior and the profile of properties of such packaging tapes at elevated temperatures, the poor thermal stability, lead to premature opening of the cartons packaged with these tapes and/or a packaging tape performance profile which generally has some adverse features. Moreover, the process temperatures, which are very high in some cases, are detrimental to certain types of adhesive composition.
Further solvent-free systems based on aqueous adhesives, such as adhesive systems based on polyvinyl acetate, polyvinyl acetate-ethylene copolymers, neoprene, styrene-butadiene, polyurethane and polyvinyl alcohol, for example, are used little if at all owing to a price and/or performance structure which is unfavorable for adhesive tapes, particularly packaging tapes.
An overview of the most important aqueous adhesive systems and also their use is given in “Solvent free adhesives”, T. E. Rolando (H. B. Fuller) in Rapra Rev. Rep. 1997, 9(5), 3-30 Rapra Technology Ltd.
Various routes to the solvent-free preparation and processing of pressure-sensitive rubber adhesives are known.
An overview of such adhesive compositions and their use in the field of pressure sensitive adhesives (PSAs) is given in “Natural Rubber Adhesives” (G. L. Butler in Handbook of Pressure Sensitive Adhesive Technology, Third Edition, edited by Donatas Satas, Van Nostrand Reinhold New York, pp. 261-287).
All of the known processes are characterized by very extensive rubber breakdown. For the further processing of the compositions for self-adhesive tapes, this necessitates extreme crosslinking conditions and also has the consequence of an application profile which is to some extent restricted, especially as regards the use of resultant self-adhesive tapes at relatively high temperatures.
Solvent-free hotmelt PSAs based on nonthermoplastic elastomers, such as natural rubber or other high molecular weight rubbers, for example, lack sufficient cohesion for the majority of applications, in the absence of a step of crosslinking the adhesive composition, and are therefore unsuited to use in the context of an adhesive packaging tape. The reason for this failure of noncrosslinked adhesive compositions based on natural rubber is the relatively large reduction in molecular we
Galle André
Monschein Thomas
Wenninger Dieter
Chan Sing P.
Crispino Richard
Norris & McLaughlin & Marcus
tesa AG
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