Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-06-08
2001-09-25
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S092000, C524S392000
Reexamination Certificate
active
06294598
ABSTRACT:
The invention relates to the use of monothiols.
Pressure-sensitive adhesive compositions based on natural rubber/resin mixtures are normally prepared in solvents. To produce adhesive tapes, these solutions are applied as thin films to backings, dried, and cut into tapes. This requires laborious operating processes, expensive explosion-protected coating and drying plants, and costly incinerators or recovery units.
A very much more cost-effective method is the preparation of hotmelt compositions. SBS and SIS block copolymers are normally employed for this purpose. Since the properties of these adhesives are limited relative to those based on natural rubber and since the raw materials costs are higher, they have become established only in certain sectors.
Particular advantages are possessed, therefore, by hotmelt pressure-sensitive adhesive compositions based on natural rubbers or synthetic diene rubbers such as IR and SBR. Because these rubbers have to be broken down to a relatively low molecular weight for solventless processing, an effective crosslinking process is required after coating. The efficacy of the radiation crosslinking of these compositions can be raised considerably by adding small amounts of polyfunctional (meth)acrylates as crosslinking promoters.
During the preparation and application process, melted adhesive compositions are subject to very high temperatures for a prolonged period. During this period it is found that they tend to gel and, subsequently, can no longer be shaped into thin films with smooth surfaces.
The stabilization of hot melts of polydiene rubbers for hotmelt pressure-sensitive adhesives is known to be difficult: In the presence of atmospheric oxygen these compositions tend towards degradation. The known antioxidants are active in countering this tendency, but their activity is limited. At relatively long residence times, the oxygen must be excluded in order to keep properties stable. This exclusion can be achieved by displacing the air with nitrogen, carbon dioxide or the like, or else by avoiding air spaces.
Under oxygen-free conditions, the rubber compositions tend to gel. This is the case in particular when crosslinking promoters have been added to increase the radiation crosslinking. The usual antioxidants employed in the rubber and adhesives industry are inactive in countering this. If added in higher concentrations, they increase the required dose for radiation crosslinking.
WO 97/07963 (Munson et al.) therefore describes the addition of “Non-Thermosettable Phenolic Tackifying Resins”, which, surprisingly, not only provides the tackifying effect but also stabilizes the adhesive composition at high temperatures. This applies both to the untreated adhesive composition and to those compositions to which bismaleimides have been added to enhance the radiation crosslinkability. The effect on the adhesive properties, however, limits the freedom to vary the selection and combination of the tackifying resins.
The object of the invention was to find additives which prevent this gelling but do not impair the properties of the adhesive compositions, especially their radiation crosslinkability. A further requirement was that the substances emit no health-injurious or disruptive vapors at the necessary temperatures.
This object is achieved in accordance with the present invention by the use of solid, rubber-soluble or rubber-dispersible monothiols as stabilizers in melted pressure-sensitive adhesive compositions based on natural rubber or synthetic rubbers, suitable tackifying resins, and promoters for increasing the radiation crosslinking yield.
In a preferred embodiment, the monothiol is 2-mercaptobenzimidazole or substituted derivatives, especially 4-(and/or 5)-methyl-2-mercaptobenzimidazole.
In order to increase further the activity of the mercaptans, it is possible for at least one liquid phosphite ester compatible with the rubber to be additionally present. Finally, the inventive concept embraces in general a melted pressure-sensitive adhesive composition based on natural rubber or synthetic rubbers, suitable tackifying resins and promoters for increasing the radiation crosslinking yield, comprising solid, rubber-soluble or rubber-dispersible monothiols as stabilizers and, if desired, at least one liquid phosphite ester compatible with the rubber.
The monothiol employed in accordance with the invention is conventionally used in the rubber industry as a stabilizer and as a retarder for vulcanization with sulphur-containing systems.
For example, 4-(and/or 5)-methyl-2-mercaptobenzimidazole is available under the name Vulkanox ® MB 2 as an ageing inhibitor from Bayer AG, Leverkusen, DE. Proposed areas of use for Vulkanox ® MB 2 include mixtures based on natural rubber and synthetic rubber of the SBR and NBR type, in which Vulkanox® MB 2 shows a protective activity against oxidization and against the damaging consequences of overvulcanization. This retarder activity is tied to sulphur vulcanization.
Sulphur compounds normally lead in many cases—especially after exposure to ionizing radiation—to the formation of highly unpleasant odor components, which surprisingly does not occur with the mercaptobenzimidazoles of the invention.
It is also known that in many polymers (polyethylene, polyacrylates, etc.) monothiols greatly reduce the crosslinking yield. Surprisingly, this effect too is absent from the melted rubber compositions of the invention.
The behavior of these monothiols as stabilizers in melted pressure-sensitive adhesive compositions based on natural rubbers or synthetic rubbers and suitable tackifying resins and treated with crosslinking promoters was previously unknown.
Surprisingly, and unforeseeably for the person skilled in the art, the monothiols employed in accordance with the invention, even when added in small amounts, represent effective stabilizers for the said adhesive compositions if the said compositions comprise promoters for increasing the radiation crosslinking yield. Suitable crosslinking promoters are primarily polyfunctional (meth)acrylates such as hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA), pentaerythritol tetraacrylate (PETA) and their ethoxylated, propoxylated or dimerized derivatives.
The activity described can be increased further by adding phosphite esters.
The aim of the following examples is to demonstrate the unexpected suitability of these monothiols as stabilizers without thereby wishing to restrict the invention.
In order to be able to characterize the stabilizer activity, the stability of the compositions following gelling by temperature exposure was assessed. The radiation crosslinkability of the adhesive compositions was determined by measuring the gel content. This was done using the techniques set out below.
1. Investigating the Stability by Assessing Gelling
Since it is very difficult to investigate the stability of the adhesive compositions in actual coating units, the activity of the substances with regard to the stability of the adhesive compositions was tested in a model experiment. For this purpose the adhesive compositions were shaped into spheres measuring about 10 mm and were introduced into a cylinder from which the atmospheric oxygen was then largely removed by flushing several times with nitrogen. The cylinder was subsequently heated at 140° C. for about 30 minutes under a pressure of 3-4 bar of nitrogen.
After heating, small amounts of the samples were shaken with 20 times the amount of toluene for 20 h, and gelling was assessed in accordance with the key specified in Table 1:
TABLE 1
Key for assessing gelling after heating
0
forms a clear solution, no macrogel
1
slight streaking, very little gel
2
some gel flakes, some gel
3
noticeable gel flakes
4
swollen gel lump
2. Measuring the Gel Content to Determine the Degree of Crosslinking
The adhesive tape samples for analysis were punched into square sections of 20 cm
2
and welded into a pouch made of a polyethylene spun bonded material (Tyvek from DuPont with a basis weight of about 55 g/m
2
). The samples were extracte
Grobe Claus
Karmann Werner
Beiersdorf AG
Norris & McLaughlin & Marcus
Szekely Peter
LandOfFree
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