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(11) | EP 0 699 586 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A production technique for blocks of hot melt adhesives |
| (57) In a process for the packaging of molten hot melt adhesive in a film cavity wherein
the adhesive is filled into the cavity at a temperature above the melting point of
the film and wherein the film cavity is in direct contact with a liquid or gaseous
heat sink, the improvement which comprises using as the film, a heat sealable film
at least a portion of which is water soluble. |
[0001] This invention relates to a method for packaging hot melt adhesives compositions and to the resulting packaged adhesive compositions.
[0002] Hot melt adhesives which are generally applied while in the molten or liquid state are solid at room temperature. Typically, these adhesives are provided in the form of blocks; however, because of the nature of these materials there are problems associated with handling and packaging them. The solid adhesive blocks not only stick or adhere to hands or mechanical handling devices and to each other, but they also pick up dirt and other contaminants. Additionally, certain applications require high tack formulations which result in blocks that will deform or cold flow unless supported during shipment. The need and advantages for providing tackless or non-blocking hot melt adhesives are apparent and various ways of accomplishing this have been developed.
[0003] Japanese Patent 48-103635 published December 26, 1973, discloses a granular adhesive which is tacky at room temperature and coated or enveloped with a non-tacky hot meltable material that is the same type or is miscible or mixable with it.
[0004] French Patent 2,544,654 published October 26, 1984, discloses forming a tackless hot melt by adding molten hot melt to a mold containing a preformed support layer having a transfer film thereon which is compatible with the hot melt.
[0005] U.S. Patent Nos. 4,748,796 issued June 12, 1988, and 4,755,245 issued July 5, 1988, disclose forming a protective coating for an adhesive material by electrostatically coating a mold or cavity with a powder screen and then pouring hot melt into the mold.
[0006] French Patent 2,601,616 published October 22, 1988, discloses forming blocks of hot melt pressure sensitive adhesives by casting the pressure sensitive adhesive into molds precoated by spraying with a film of non self-sticking hot melt material thereby forming a fusible non-tacky veil around the pressure sensitive block.
[0007] In German Patent 22 48 046 the hot melt adhesive is squeeze-cut into pillow-shaped pieces; the pieces subsequently cooled and solidified.
[0008] Still other patents teach coating or wrapping the formed hot melt block with various types of plastic films. Thus, German patents DE 31 38 222 and 32 34 065 disclose coating the circumference of elongated hot melt portions with a thin polyolefin film. German patent 36 25 358 to Hausdorf teaches wrapping the solid hot melt block in a thermoplastic, especially copolyamide film with a melting point of 120°C to 150°C, while EP application 0 469 564 and WO 94/01330, both to Rouyer et al. discloses wrapping the solidified hot melt in a plastic or biodegradeable polymeric packaging material.
[0009] All the latter methods have provided some degree of improvement in the packaging and handling of hot melt adhesives however they have suffered by virtue of either the need to unwrap or otherwise unpackage the hot melt or, in the cases of coated hot melts which are added directly to the melting pots, by virtue of the contamination resulting from the build up over time of large quantities of the packaging materials in the melt pot and application equipment.
[0010] In order to overcome the disadvantages inherent in the methods of the prior art, U.S. Patent 5,401,455, issued March 28, 1995, discloses the finding that when the hot melt adhesive is poured in its molten state into a mold or cavity lined with plastic packaging film and then allowed to solidify, the adhesive is fused to some extent into the film, resulting in a non-blocking adhesive package which will melt faster in the melt pot and will not cause a substantial build-up of undesirable plastic residue even after extended periods of time.
[0011] Since the melting point of the plastic film should be comparable to, and preferably lower than, the melting point of the hot melt adhesive in order to provide for satisfactory melting in the melt pot which does not have a source of agitation, it is necessary that the lined mold be in direct contact with a heat sink so as to remove excess heat from the film as quickly as possible and thereby prevent melting, burning or charring of the plastic film wrap.
[0012] An improvement on that method is described in U.S. Patent 5,373,682, issued December 20, 1994. According to that embodiment, the molten adhesive is directly pumped or poured into a cylindrical plastic tube, the cylindrical tube being in direct contact with the heat sink. The resultant adhesive package is thus provided in a readily handleable cartridge form which may be produced in a continuous line operation and which additionally possesses all the advantages of the easily processable non-blocking adhesive package of the parent application.
[0013] While the methods described in the copending application have proved useful for packaging and re-melting of most hot melt adhesive formulations, there remain some applications, for example for non-pressure sensitive adhesives, where wrapping during storage and shipping is unnecessary. There are also other applications where even small amounts of unmelted film may create problems in the melt tank. It is therefore desirable in a limited number of applications to provide hot melt adhesive portions in the convenient size and shape produced by the processes of the copending application but without any external packaging film.
[0014] We have now found that when a heat sealable film, at least a portion of which is water soluble, is utilized as the film, the molten adhesive can be effectively packaged using the procedures described previously. The water soluble film or portion thereof will then be slowly dissolved in a water bath, after which the resultant adhesive mass will be fully solidified and may be packaged directly or may be subsequently coated with anti-stick materials using methods conventional in the art.
[0015] The method of the present invention is adaptable to the packaging of virtually any type of hot melt adhesive composition. As examples the method disclosed herein may be used to package hot melt adhesives prepared from polymers and copolymers of synthetic resins, rubbers, polyethylene, polypropylene, polyurethane, acrylics, vinyl acetate, ethylene vinyl acetate and polyvinyl alcohol. More specific examples include hot melt adhesives prepared from the following:
a. rubber polymers such as block copolymers of monovinyl aromatic hydrocarbons and conjugated diene, e.g., styrene-butadiene, styrene-butadiene-styrene, styrene-isoprene-styrene, styrene-ethylene-butylene-styrene and styrene-ethylene propylene-styrene;
b. ethylene-vinyl acetate polymers, other ethylene esters and copolymers, e.g., ethylene methacrylate, ethylene n-butyl acrylate and ethylene acrylic acid;
c. polyolefins such as polyethylene and polypropylene;
d. polyvinyl acetate and random copolymers thereof;
e. polyacrylates;
f. polyamides;
g. polyesters;
h. polyvinyl alcohols and copolymers thereof;
i. polyurethanes;
j. polystyrenes;
k. polyepoxides;
l. graft copolymers of vinyl monomer(s) and polyalkylene oxide polymers; and
m. aldehyde containing resins such as phenol-aldehyde, urea-aldehyde, melamine-aldehyde and the like.
[0016] Most often such adhesives are formulated with tackifying resins in order to improve adhesion and introduce tack into the adhesive. Such resin include, among other materials, (a) natural and modified resins, (b) polyterpene resins, (c) phenolic modified hydrocarbon resins, (d) coumarone-indene resins, (e) aliphatic and aromatic petroleum hydrocarbon resins, (f) phthalate esters and (g) hydrogenated hydrocarbons, hydrogenated rosins and hydrogenated rosin esters.
[0017] Desirable optional ingredients include diluents, e.g., liquid polybutene or polypropylene, petroleum waxes such as paraffin and microcrystalline waxes, polyethylene greases, hydrogenated animal, fish and vegetable fats, mineral oil and synthetic waxes as well as hydrocarbon oils such as naphthionic or paraffinic mineral oils.
[0018] Other optional additives may include stabilizers, antioxidants, colorants and fillers. The selection of components and amounts as well as the preparation thereof are well known in the art and described in the literature.
[0019] The heat sealable water soluble film into which the molten adhesive is poured may be any film which will maintain its integrity during filling but will subsequently dissolve in water. Suitable materials include polyvinyl alcohol, modified starches, cellulose films and the like. Polyvinyl alcohol films are the most preferred. Rather than using the water soluble material as the entire film substrate, in accordance with an alternative embodiment of the invention, it is possible that the film include a non-water soluble portion with the water soluble portion serving only as the vehicle to effect dissolution and separation of the film from the adhesive mass during the water cooling. In the case of films which comprise more than one material, it is only necessary that the portion of the film which will provide the longitudinal lapped seal around the adhesive mass during the filling operation be heat sealable.
[0020] It may also be desirable in the case of pressure sensitive adhesives to use a composite water soluble film laminated to a non-soluble coating, the latter remaining on the adhesive mass and providing a compatible tackless coating after dissolution of the water soluble film. For example, pressure sensitive masses may be so packaged by using a lamination of an ethylene vinyl acetate coating on the polyvinyl alcohol film.
[0021] The thickness of the film utilized generally varies between about 0.1 mil to 5 mil, preferably 0.5 mil to 4 mil. The thickness of the particular film also varies depending upon the temperature at which the molten adhesive is pumped or poured into the plastic film cylinder. The particular viscosity at which the adhesive can be introduced into the plastic film cylinder will vary depending on a variety of factors including the pumping capacity of the pump, the strength of then plastic film and the like. Viscosities in the range of 1,000 to 200,000 cps, preferably 2,000 to 100,000 may be utilized. However, we have found that the most preferred viscosity of the adhesive which is to be packaged in accordance with the invention is between 10,000 and 50,000 cps. It will be recognized that the temperature at which an adhesive composition will exhibit this viscosity range will vary from one adhesive to another. In the case of adhesives which obtain their viscosity within the temperature range of 110° to 130°C, film thicknesses of about 1.25 mil are preferred; while for adhesives which exhibit this viscosity at temperatures of 130° to 150°C, films of about 1.5 mil are preferred.
[0022] The heat sink may comprise any means which will effectively and rapidly remove or absorb the excess heat from the entire surface of the film which is in contact with the molten hot melt adhesive composition so as to prevent the temperature of the film from exceeding its melting point even though the molten hot melt adhesive temperature is higher than the film melting temperature. Suitable heat sinks are provided by spraying the surface of the cylindrical plastic tube with cooled water or other refrigerant means such as chilled glycol, liquid or gaseous nitrogen, compressed carbon dioxide or the like. The spraying may be accomplished, for example, using a series of spray nozzles aimed at the mandrel or a water or cooling ring or series of rings may be positioned around the mandrel so as to provide a curtain or cascade of water or refrigerant around the entire circumference of the cylinder.
[0023] As discussed above, the molten adhesive is generally poured or pumped into the film cylinder at a temperature at which the molten adhesive exhibits a viscosity of 1,000 to 200,000, preferably 10,000 to 50,000 cps. This temperature will generally vary from about 110° to about 150°C depending upon the particular adhesive. Since the adhesive is pumped or poured continuously through the mandrel into the film cylinder, it is possible to void and then cut the continuous filled tube into individual cartridges at virtually any desired length. In general, the individual cartridges are produced in a variety of sizes ranging from about three inches to 18 inches in length and varying in weight, depending upon length, from about 0.5 to five pounds.
[0024] After filling, the adhesive cartridges, either individually or in a connected series, are further cooled in a water bath to ambient temperature so as to dissolve the film and solidify the adhesive. Depending upon the water soluble material used in the film, the film may begin to separate from the adhesive and dissolve within about fifteen minutes after being placed in the water bath in which case any residual film remaining after cooling can be removed by placing the adhesive cartridge in water of about 60°C prior to bulk packaging. Some films may not be soluble in the cold water bath and, in such cases, when removal of the plastic film is desired, the solidified packages are submerged in room temperature of slightly heated water until dissolution of the film occurs. In either case, any residual amounts of film which remain may be readily removed by passing the packages through a series of rotating brushes or using other scrubbing mechanisms. In the embodiment wherein the film comprises a non-water soluble component, the remaining film may be readily removed from the chilled water bath and may, if desired, be recycled. This embodiment has the additional advantage in that less of the water soluble polymer is present in the water bath, thus prolonging the useful life thereof and producing less dissolved matter in the water when it is disposed of. The packages are then dried either under ambient conditions or using hot air knives or the like.
[0025] In the case of non-pressure sensitive adhesives, the resultant individually packaged hot melt adhesive cartridges can be stored, handled and used directly. In the case of pressure sensitive adhesives, the solidified adhesive cartridge may be treated with an anti-stick agent such, for example, as by dipping or spraying with molten wax or other non-tacky compatible coating.
[0026] The resultant hot melt adhesive cartridge may be packaged in a container to reduce its exposure to the environment, moisture or other contaminants. The container or other wrappings would then be removed by conventional procedures prior to utilization of the hot melt adhesive.
[0027] A preferred embodiment of the method of the invention described herein is illustrated in Figure 1. In the Figure, the water soluble film (1) is passed through a series of idler rollers (2) which smoothes the film and controls the tension across the web. The film is then threaded into a film folder (3) which folds the film and forms a lap seal around a one to four inch diameter fill pipe or mandrel (4). The lap seal is sealed with hot air (5), induction sealing or ultrasonic welding and then may be further sprayed with cooling air to set the seal.
[0028] After the seal has set, the tubular film is carried along the exterior of the fill pipe preferably using a series of spreader rings (6), film guide(s) (7) and drive wheels (8) until it reaches the end of the fill pipe, at which point the molten hot melt adhesive, pumped from a remote holding container through the mandrel enters the cylindrical tube (9). If desired, in order to maintain the desired molten viscosity, the fill pipe may be insulated or jacketed as with double walls and filled with inlet and outlet ports (10) through which heated water, steam or mixtures thereof may be circulated, so as to prevent premature cooling of the molten adhesive in the mandrel and melting of the plastic film on the mandrel. The double wall construction also facilitates complete cleaning and purging of the mandrel after the filling operation is completed.
[0029] During filling, the tubular package is in contact with cooling water (11) or other refrigerant medium. This water contact may be accomplished using a series of water jets positioned around the circumference of the cylindrical tube. Alternatively, a water or cooling ring (12) or series of water or cooling rings may be positioned around the mandrel. In a preferred embodiment a water ring (12) is positioned around the filling end of the mandrel and a channel (21) is cut into the entire interior circumference thereof. Water or other refrigerant enters the ring through at least one inlet (22), flowing through the opening at the inside edge of the channel (23) thereby forming a curtain coating or cascade of water around the entire outer surface of the cylindrical tube. If desired, the inner channel may be baffled to allow for uniform distribution of the water. It will be recognized that while the term "spraying with coolant" is utilized herein, the invention also contemplates carrying out the entire filling operation in a cooled environment as in the presence of liquid nitrogen.
[0030] Subsequently, the adhesive filled tube is passed through voider rolls (13) which crimp or pinch the continuous filled cylinder into smaller cartridge sized segments and which are set so as to assure the proper cartridge length. Spraying with coolant is continued through additional spray nozzles (14) until the package is sufficiently cooled so that the packaged adhesive forms a self-seal formed at the voided portions. The cartridge packages may then be cut at the voided area using conventional means, e.g., mechanical scissor (15), laser cutter, water jet or a heated knife or wire and subsequently cooled to room temperature in a water bath. Alternatively, the voided but uncut cartridge segments, still attached in serial form may be cooled and cut thereafter.
[0031] As discussed above, the film may be dissolved in the cooling water bath itself or may be subsequently dissolved in a separate water bath prior to final packaging.
[0032] While the invention has been specifically described with respect to the continuous filling of adhesive cartridges claimed in U.S. Application Serial No. 07/993,890, it is equally adaptable to the method described in pending Application Serial No. 07/883,994, wherein a mold, preferably a mesh screen mold, is lined with the water soluble film and the molten adhesive poured therein all the while maintaining the mold in contact with a heat sink.
[0033] In accordance with the latter embodiment, the mold into which the thermoplastic film is placed and into which the molten adhesive is to be poured may comprise any rigid, self-supporting material. The molds are generally formed from rigid plastic, e.g., acrylonitrile/butadiene/styrene polymers or polypropylene or from metallic substrates. Since it is desirable to keep the film in direct contact with the water bath after filling, the use of perforated or otherwise apertured molds is desired. A particularly preferred mold is formed from steel mesh. The size of the mold varies according to the size of the desired hot melt adhesive block. In general each mold is approximately 3" x 3" x 11" in dimension and often a series of molds are formed from one contiguous plastic, cellulosic or metal sheet.
EXAMPLE 1
[0034] A conventional pressure sensitive hot melt adhesive composition suitable for disposable applications was prepared using a tackified and plasticized blend of ethylene vinyl acetate copolymer and wax.
[0035] A continuous supported cylindrical tube was formed by wrapping a plastic film comprising a I.2 mil polyvinyl alcohol film around a l.5 inch diameter insulated mandrel or fill pipe. After the lap seam was formed, it was sealed using hot air and then set by spraying with ambient temperature air.
[0036] The molten hot melt adhesive, at a viscosity of 10,000-15,000 cps. (120-130°C) was pumped therein through a nozzle while the entire surface of the film was sprayed with chilled water (5-10°C). The filled tubes were voided at lengths of 6 inches and then cut to form individual cartridges. The resultant cartridges were allowed to cool in a chilled water bath until they had completely solidified.
[0037] The resultant cartridges were transferred to a hot water bath (about 65°C.) where the film was dissolved and the blocks then removed, dried and packed in appropriate shipping containers.
EXAMPLE 2
[0038] The procedure of Example 1 was repeated to package a pressure sensitive hot melt adhesive composition suitable for can/bottle labelling applications.
[0039] After transfer to the hot water bath, the film was dissolved, the adhesive removed, dried and then passed to a tank containing a molten compatible wax material. The block was dipped into the tank, the surplus material allowed to run off, dried and packed to the shipping container. The outer coating thus formed a non-tacky wax coating compatible with the adhesive.
EXAMPLE 3
[0040] The procedure of Example I may also be repeated using film which consists of low density polyethylene film with a strip of polyvinyl alcohol film adhered to one side wherein the polyvinyl alcohol strip comprises about 20% of the surface area of the film. After dissolution of the polyvinyl alcohol portion of the film, the remaining low density polyethylene may be readily peeled off the cartridge.
1. In a process for the packaging of molten hot melt adhesive in a film cavity wherein
the adhesive is filled into the cavity at a temperature above the melting point of
the film and wherein the plastic film cavity is in direct contact with a liquid or
gaseous heat sink, the improvement which comprises using as the film, a heat sealable
film at least a portion of which is water soluble.
2. The process of Claim 1 wherein the water soluble portion of the film comprises polyvinyl
alcohol, modified starch or cellulose.
3. The process of Claim 2 wherein the water soluble portion of the film comprises polyvinyl
alcohol.
4. The process of Claim 1 wherein the entire surface of the film comprises polyvinyl
alcohol.
5. The process of Claim 1 wherein the film comprises a lamination of a water soluble
film laminated to a non-soluble coating.
6. The process of Claim 5 wherein the film comprises an ethylene vinyl acetate coating
on a polyvinyl alcohol film.
7. The process of Claim 1 wherein the cavity comprises a cylindrical tube of the heat
sealable film.
8. The process of Claim 1 wherein the molten hot melt adhesive is filled into the cavity
at a molten viscosity of 1,000 cps. to 200,000 cps.
9. The process of Claim 1 wherein the film is present in an amount of 0.2 to l.0% by
weight of the adhesive mass.
10. The process of Claim 1 wherein the hot melt adhesive is non-pressure sensitive.
11. The process of Claim 1 wherein the hot melt adhesive is pressure sensitive and wherein
the adhesive mass, after dissolution and removal of the water soluble film is coated
with a compatible non-tacky coating material.
12. The process of Claim 1 wherein the molten adhesive is poured into the film cylinder
at a temperature of 110-130°C. and wherein the thickness of the film is 1.25 mil.
13. The process of Claim 1 wherein the molten adhesive is poured into the film cylinder
at a temperature of 130-150°C and wherein the thickness of the film is 1.5 mil.