Pressure-sensitive removable adhesives based on...

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

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C524S317000, C524S556000, C524S559000, C524S833000, C428S3550AC

Reexamination Certificate

active

06284837

ABSTRACT:

1. BACKGROUND OF THE PRESENT INVENTION
1.1. Technical Field
Pressure-sensitive adhesives are adhesives which, in the dry state, are permanently adhesive at room temperature and adhere firmly to various surfaces by simple contact under a slight manual pressure.
Pressure-sensitive adhesives are generally characterized by the following three properties: the adhesive power, the peel strength and the shear strength. The first property reflects the ability of the adhesive to rapidly adhere; the second defines the ability to withstand withdrawal by peeling; and the third characterizes the ability of the adhesive to maintain its initial position when it is subjected to shear forces.
In the field of industrial adhesion, it is generally necessary to specifically adapt the adhesive to the materials to be assembled and to the uses which are made of them. This is obtained by suitably balancing the above-mentioned properties (adhesive power, peel strength and shear strength) as a function of the desired aim.
1.2. Description of Related Art
Acrylic polymers constitute a particularly advantageous class of compounds for the preparation of pressure-sensitive adhesives. They are transparent and colorless, do not yellow under the influence of solar radiation and exhibit good resistance to oxidation. In addition, some of these polymers exhibit intrinsic adhesive properties generally superior to most other polymers.
Pressure-sensitive adhesives based on acrylic polymers, namely polyacrylates and polymethacrylates, are widely used in the field of labelling, in particular for producing self-adhesive labels and tapes. Poly(meth)-acrylates are generally applied in solution in an organic solvent or in the form of an aqueous emulsion.
For some years, increasing interest has been directed at aqueous emulsions, which exhibit certain advantages:
they are incombustible;
they do not contain any solvent capable of escaping into the atmosphere;
their adhesive nature can be reinforced by the introduction of a tackifying agent;
they exhibit a high solids content (generally greater than 50% by weight), good resistance to ageing and great ease of use.
These emulsions are obtained by emulsion polymerization in water of (meth)acrylic monomers, with which is generally combined a tackifying agent, such as a natural resin or a resin of petroleum origin, or an elastomer.
The latter compounds can be incorporated by prior dissolution in a hydrocarbon solvent, followed by an addition of water to form an emulsion and by the removal of the solvent. This way of operating exhibits, however, the disadvantage of allowing a not insignificant residual amount of solvent to remain in the final emulsion.
The most common incorporation consists of dissolving the tackifying agent or the elastomer in the acrylic monomers before carrying out the polymerization reaction. Numerous documents report such a way of operating:
For example, in U.S. Pat. No. 5,315,055 and WO 90/15111, a pressure-sensitive adhesive is obtained for which the peel force is increased by dissolving, in acrylic monomers, a tackifying resin exhibiting a molecular weight of between 500 and 5000 and an aromatic content of at least 10% by weight.
In JP-A-08-231937 the introduction of 5 to 30 parts by weight of a tackifying resin and of a polymerizable anionic emulsifier into 100 parts by weight of acrylic monomers makes it possible to obtain a self-adhesive which adheres to polytetrafluorethylene (PTFE).
In JP-A-54-023641, a permanent adhesive which can adhere to varied substrates is prepared by dissolving a compound of high molecular weight in a mixture of acrylic monomers, of copolymerizable monomers, and of an emulsifier. The compound of high molecular weight is a tackifying resin or an elastomer.
In JP-A-58-185668, a terpene-phenol or terpene-alkyl phenol resin is continuously added to a mixture of acrylic monomers and of copolymerizable monomers, optionally in the presence of an emulsifier, to form an aqueous emulsion having in particular good resistance to water.
Finally, in U.S. Pat. No. 4,939,190, provision is made to prepare pressure-sensitive adhesives having the ability to adhere to polyolefins. The process, in two stages, consists in polymerizing an ethylenically unsaturated monomer in the presence of a tackifying resin, in order to form an emulsion in which the particles are composed of a resin core and of an outer polyacrylate layer, and subjecting the said emulsion to a polymerization in the presence of an ethylenically unsaturated monomer. The emulsion obtained is composed of particles having a multilayer structure.
The pronounced adhesive nature of the adhesive emulsions which have just been mentioned proves to be a major disadvantage for applications involving labelling. These adhesives can be described as permanent self-adhesives, that is to say that it is difficult to remove them without part of the adhesive, indeed even the label or the tape, remaining on the substrate to which they have been affixed.
2. SUMMARY OF THE INVENTION
The present invention relates to pressure-sensitive removable adhesives. More specifically, the invention relates to aqueous-phase adhesives of the acrylic emulsion type which exhibit an excellent adhesive nature and good adhesion to substrates of varied nature.
The present invention is directed to pressure-sensitive removable adhesives based on polyacrylates, characterized in that they comprise a compound of formula:
wherein R
1
, R
2
and R
3
, which are identical or different, represent a linear or branched C
1
-C
8
, preferably C
2
-C
6
, alkyl radical; and R
4
represents H or an acyl radical COR
5
wherein R
5
is a linear or branched C
1
-C
3
alkyl radical. The present invention also relates to a process for the preparation of said adhesives. These adhesives are used in particular in the field of labelling. The present invention further relates to adhesive labels and tapes comprising said adhesives.
3. DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides novel pressure-sensitive adhesives which make it possible to overcome the above-mentioned disadvantages.
The adhesives according to the invention are sensitive to pressure and removable, that is to say that they can be easily removed manually from the substrate, without leaving substantial marks on the latter, and can optionally be placed back on the same substrate or a different substrate.
More specifically, the adhesives according to the invention are aqueous poly(meth)acrylate emulsions, characterized in that they comprise at least one compound of formula:
in which:
R
1
, R
2
and R
3
, which are identical or different, represent a linear or branched C
1
-C
8
, preferably C
2
-C
6
, alkyl radical and
R
4
represents H or an acyl radical COR
5
wherein R
5
is a linear or branched C
1
-C
3
alkyl radical.
The particularly preferred compounds of formula (I) are triethyl citrate (TEC), triethyl acetylcitrate (TEAC), tri-n-butyl citrate (TBC), tri-n-butyl acetylcitrate (TBAC), tri-n-hexyl acetylcitrate (THAC) and tri-n-hexyl n-butyrylcitrate (THBC) and better still tri-n-butyl acetylcitrate (TBAC).
The content of compound of formula (I) can reach 20% by weight of the combined monomers constituting the poly(meth)acrylates. It preferably varies between 2 and 18% and better still between 4.5 and 11%.
The poly(meth)acrylates which can be used according to the invention are those which can be obtained by aqueous emulsion polymerization of a mixture of acrylic monomers comprising at least one alkyl (meth)acrylate, at least one polar copolymerizable monomer and at least one polyunsaturated copolymerizable monomer.
The alkyl (meth)acrylate is chosen from acrylates and methacrylates in which the alkyl radical contains 1 to 12 carbon atoms and preferably 4 to 8 carbon atoms. Mention may be made, by way of example, of butyl acrylate, butyl methacrylate, heptyl acrylate, heptyl methacrylate, octyl acrylate, octyl methacrylate, iso-octyl acrylate, isooctyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, isobutyl acrylate and isobutyl methacrylate.

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