Coating processes – Coating remains adhesive or is intended to be made adhesive
Reexamination Certificate
2001-02-02
2003-09-02
Cameron, Erma (Department: 1762)
Coating processes
Coating remains adhesive or is intended to be made adhesive
C427S208400
Reexamination Certificate
active
06613381
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to thermoplastic additives useful in preparing hot melt adhesives based on non-thermoplastic hydrocarbon elastomers and processes employing the same.
BACKGROUND OF THE INVENTION
Adhesives based on non-thermoplastic hydrocarbon elastomers, such as natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene, polybutadiene, polyisobutylene, or styrene-butadiene random copolymer rubber, are known in the art. Hot melt processing of such adhesives is also known.
In order to facilitate efficient hot melt processing of non-thermoplastic hydrocarbon elastomers into adhesives, processing aids are typically used to lower the overall molecular weight of the adhesive composition, decreasing its melt viscosity. As compared to the elastomer component of such adhesives, lower molecular weight processing aids, such as processing oils, elastomer oligomers, waxes, or other materials defined and described as plasticizers in
Dictionary of Rubber,
K. F. Heinisch, pp. 359-361, John Wiley & Sons, N.Y. (1974) are often needed in substantial amounts in order to accomplish this purpose.
Furthermore, relatively low molecular weight tackifiers may also be needed in order to provide adhesion or render an adhesive sufficiently tacky, such as when preparing pressure-sensitive adhesives. Substantial amounts of these tackifiers may also be needed, particularly when preparing pressure-sensitive adhesives.
U.S. Pat. No. 6,063,838 describes formation of blended pressure-sensitive adhesives. The components form a blended composition having more than one domain, wherein one domain is substantially fibrillous to schistose. Exemplified in U.S. Pat. No. 6,063,838 are blends of a thermoplastic material and elastomer (specifically, synthetic polyisoprene) that are compounded with tackifiers to form pressure-sensitive adhesives. The patent teaches that, preferably, each of the thermoplastic material and elastomer components has a similar melt viscosity. Specifically, the patent states that the ability to form a finely dispersed morphology, as claimed therein, is related to a ratio of the shear viscosity of the components at melt mixing temperatures.
In that regard, the patent further states that, when a lower viscosity material is present as the minor component, the viscosity ratio of minor to major components is preferably greater than about 1:20, more preferably greater than about 1:10. The patent also teaches that the melt viscosities of individual components may be altered by the addition of plasticizers, tackifiers, or solvents (i.e., as processing aids), or by varying mixing temperatures. The ratio of melt viscosity of the thermoplastic material to the melt viscosity of the elastomer used in the Examples ranges from 1:2.3 to 1:21, with the highest measurable melt viscosity for the elastomers used therein being 1,580 Pascal-seconds and the lowest measurable melt viscosity for the thermoplastic materials used therein being 74 Pascal-seconds, as measured according to the Melt Viscosity test in the Examples section, infra, but at a temperature of 175° C.
However, the addition of substantial amounts of relatively low molecular weight components to relatively high molecular weight elastomers often leads to poorly mixed adhesives, especially due to the typically large difference in melt viscosities between such components. Poorly mixed adhesives often result in coarse (or grainy) coatings and a corresponding reduction in adhesion of adhesive films so produced.
Mix quality becomes an even bigger concern when hot melt processing such adhesives using continuous, high throughput processing. Particularly when using continuous, high throughput processing, high shear stresses can develop in the adhesive during the early stages of the compounding process. This has the effect of raising the melt temperature of the adhesive, even to the point where molecular weight of the elastomer is affected during hot melt processing thereof. For example, molecular weight of the elastomer can be undesirably reduced (e.g., by degradation) or increased (e.g., by crosslinking). Another undesirable effect of raising the melt processing temperature is the potential for release of unpleasant odors from the compositions being melt-processed.
Attempts have been made to improve mix quality when using continuous high throughput processing. For example, see U.S. Pat. No. 5,539,033 and U.S. patent application Ser. No. 09/198,781 to Bredahl et al. However, further ways of improving mix quality of hot melt adhesives are desired. It is particularly desirable to provide ways of improving mix quality of hot melt adhesives without requiring complicated processing methods.
In addition to poorly mixed adhesives, another problem with requiring substantial amounts of processing aids to facilitate hot melt processing of adhesives is that processing aids often lower the shear strength of the resulting adhesive. As a result, the ability of the resulting adhesives to meet the demands of many high performance applications is often compromised.
While attempts at increasing the shear strength of resulting adhesives are known, they typically require complicated, and often costly, post-processing steps. One conventional method of increasing the shear strength of adhesives is by crosslinking the adhesive after its application to a substrate. For example, energy sources, such as electron beam (e-beam) or ultraviolet (UV) radiation, are commonly used to crosslink adhesives after application. These methods, however, often require an additional processing step and, thus, result in decreased processing efficiency. Furthermore, e-beam is not always desired because it is expensive and can cause damage to some backings when the adhesive is used in a tape. Similarly, UV-radiation has its limitations as a crosslinking energy source. For example, UV-radiation is often not able to be used effectively for crosslinking relatively thick adhesives due to the need for UV-radiation to penetrate throughout the entire thickness of the adhesive. As such, certain filler and pigments can not be used in adhesives when UV-crosslinking is used because they potentially interfere with penetration of UV-radiation therethrough. Furthermore, crosslinking can comprise an adhesive's ability to have sufficient pressure-sensitive adhesive properties.
It is, therefore, desirable to improve known hot melt processing of non-thermoplastic hydrocarbon elastomers for preparing adhesives having the properties needed for high performance applications, such as high-temperature masking and medical tape applications. For example, it desirable to obviate the need for post-processing the adhesive after application to a substrate. Furthermore, it is desirable to obviate the need for complicated processing steps in order to provide adhesive compositions that are adequately mixed in order to avoid coarse adhesive coatings.
SUMMARY OF THE INVENTION
The invention is directed toward a hot melt adhesive composition and processes for producing the same. The hot melt adhesive composition comprises: at least one non-thermoplastic hydrocarbon elastomer; at least one thermoplastic additive, wherein a ratio of melt viscosity of the at least one thermoplastic additive to melt viscosity of the at least one non-thermoplastic hydrocarbon elastomer is less than about 1:20 when measured at a shear rate of 100 seconds
−1
at a particular hot melt processing temperature; and at least one modifier. These compositions do not require post-processing for high performance applications. Furthermore, these compositions do not require complicated processing steps for their preparation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed toward a composition and process employing the same that is capable of producing a well-mixed hot melt adhesive suitable for many high performance applications. The invention involves the addition of at least one thermoplastic additive to a non-thermoplastic hydrocarbon elastomer forming the basis of
Bredahl Timothy D.
Cox Sheila F.
Hyde Patrick D.
Munson Daniel C.
Patnode Gregg A.
3M Innovative Properties Company
Cameron Erma
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