CA1102464A - Coating composition for plastics and plastics film coated thereby - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A composition, suitable for coating plastics film, is pro-vided comprising an aqueous dispersion containing a specific co-polymer, as substantially particles which is obtained from an .alpha.,.beta.-unsaturated carboxylic acid having one or more carboxyl groups, an alkyl acrylate and/or alkyl methacrylate, in which the alkyl portion contains one to twelve carbon atoms and, if required, a copolymerizable vinyl monomer. The invention also provides plastics film coated with this composition.

Description

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The invention relates to a coa~ing composition for plastics comprising an aqueous dispersion containing a specific copolymer and to a plastic ~ilm coated therewith which is heat-sealable.
Plastic films such as polyester iilm6, polyolefine films and the like have superior qualities o transparency and physical strength or chemical resistance, but have no heat~
sealing or gas barrier properties. The most general method for improving such properties of a plas~ic film is to coat it with a solution of a thermoplastic copolymer such as a vinylidene-chlor-ide resin in an organic solvent. While such a copolymer can readily impart adhesiveness to ~he base film, the solvents used for dissolving the copolymer are usually very volatile and in-flammable and some of the organic solvents generate toxic vapours thereby creating health hazards. In addition, a trace amount of the organic solvent used tends to remain in the coating layer because its complete removal during the coating procedures is difficult. The oxganic solvent residue i5 undesirable when the coated film is used as packaging for foods or the like. Accord-ingly, in order to reduce the solvent residue, it is necessary touse a large scale drying apparatus which however, is, uneconomi-cal.
If an aqueous dispersion or solution of ~he coating copolymer is prepared, the above disad~antages of the resulting plastic film (remaining solvent) and the production thereof can be eliminated. However, if a plastic film whose surface is only subjected ~o a corona discharge or oxidizing agent treatment is used, it is, generally, difficult to achieve enough a & esion between the base film and ~he copolymer used, by applying an aqueous copolymer dispersion or solution directly over the treat-~,, :~lOZ464 ed surface of the film. Therefore, the base film is usually,beforehand, treated with an anchor coating agent. This is l?n-economical because the coatings must be applied at least twice.
Accordingly, methods for obtaining sufficient adhesion by direct-coating of plastic films with an aqueous polymer dis-persion or solution have been proposed. For example, a mixture of an aqueous alkyl acrylate dispersion and methylol melamine or methyl-etherified methylol melamine is known (c.f., Japanese Patent Publication No. 42,~40~1971). The resulting coating layer improves the adhesion of the base plastic ~ilm but causes the plastic film partial or complete loss of its thermoplasticity or heat-sealing characteristics which are important basic properties of the plastic film~
A further attempt of the prior art has been to copoly-merize an acrylate copolymer with an ~ unsaturated carboxylic acid, such as acrylic acid, and to neutralize the resulting free carboxyllc acid copolymer with a base, such as ammonia, to yield a water-soluble copolymer, which is used to improve adhesion (United States Patent No. 3,753,769). The resulting layer, how-ever, has low mechanical strength or is prone to blocking. There-fore, this method can not provide a coated film possessing the fundamental and esssntial characteristics of being hardwearing and resistant to blocking. In addition, the coating composition has the defect that it can not use the copolymer in a high concen tration, because it is soluble in water.
Accordingly, it is an object o~ this invention to pro-vide an improved coated plastic film.
According to one embodiment of the invention there is -~
provided a heat sealable plastic film coated with a composition comprising an aqueous dispersion containing, substantially as ~, , -6~
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particles, a copolymer o~ a) 0.1 to about 10% by weight of at least one ~ unsa~urated carboxylic acid having at least one carboxyl group per molecule; b) at least one acrylic ester selected from the group consisting of an alkyl acrylate and an alkyl methacrylate wherein ~he alkyl group of ~he ester contains one to twelve carbon atoms; and c) at least one copolymerizable vinyl monomer selected from the group consisting of compounds of (i) vinyl-substituted aromatic hydrocarbons (ii) a,~-unsaturated caxbonamides (iii) hydroxyalkyl esters of ~,~-unsaturated car boxylic acids, (iv) esters of ~ unsaturated carboxylic acids having an e~oxy group, (v) vinyl esters oE aliphatic acids, (vi) vinyl chloride, in an amount of from 0~1 to 80 percent by weiyht of the copolymer to be formed; and wherein said copolymer has a weight-average molecular weight in the range of about 5,000 to about 150,Q00 and a second-order transition temperature in the range of about 30C to about 70 C.
One of the main features of the coating composition dis-closed herein is the fonm of the aqueous dispersion used in which a specific copolymer is dispersed substantially as particles.
The coating composition described herein is applicable directly for film-like base materials of polyester or polyoleine plastics, without the necessity o~ any anchor coating. The coat-ing readily adheres with the plastic base films and can provide a coating layer possessing satisfactory heat-sealing, blocking resistance, hardwearing, water rasistance and like properties.
The ~ unsaturated carboxylic acid havin~ one or more carboxyl groups to be used for the formation of the copolymer des-cribed herein includes acrylic acid, methacrylic acid, maleic acid, itaconic acid, citraconic acidr crotonic acid, fumaric acid, maleic anhydride and the like. When an ,~-unsaturated carboxylic z~

acid having two or more carboxyl groups is used, it may be the half ester. The quantity of carboxylic acid used is about 0.1 to 10% by weight, based on the weight of the copolymer to be formed.
The carboxylic acid is thought to àffect adhesion to the plastic base material. If the carboxylic acid is used in excess of about 10% by weight, the resulting copolymer will be hydrophyllic and sol~le in a base. The resulting coating layer also tends to have lower blocking resistance and be less hardwearing. On the other hand, when the carboxylic acid is used in less than about 0.1~ by weight of the copolymer it in general can not be expected to impart sufficient adhesiveness to the plastic base material.
A mix~ure of two or more carboxylic acids may be employed~
The alkyl acrylate and the alkyl methacrylate components of the copolymer are the alkyl esters of acrylic and methacrylic acids having one to twelve carbon atoms such as: methyl acrylate, ethyl acrylate, iso-propyl acrylate, n-butyl acrylate, iso-butyl acrylate, amyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and methyl methacrylate, ethyl methacrylate, iso-propyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, amyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate and so forth.
These alkyl acrylates or alkyl methacrylates are the pri-mary component of the coating and may be employed in the follow-ing combinations:
(1) at least one alkyl acrylate; or (2~ at least one alkyl methacrylate; or t3) at least one alkyl acrylate and at least one alkyl metha-crylate.
The proportion of alkyl acrylates and/or alkyl metha~ry-lates in the copolymer is adjustable so as to yield the required second-order transition temperature~ taking into consideration the copolymerizable vinyl monomer which may optionally be added.
The copolymerizable vinyl monomer is exemplified as follows but is non-limitative to these:
(i) vinyl substituted aromatic hydrocarbons such as styrene, a-methylstyrene or the like.
unsaturated carbonamides such as acrylamide, methacryl-amide, N-methoxyacrylamide or th~ like.
(iii) hydroxyalkyl esters of ~,~ unsaturated carboxylic acids such as 2-hydroxyethyl acrylate (or methacrylate), 2-hydroxypropyl acrylate (or methacrvlate) or the like.
~iv) esters of ~,~-unsaturated carboxylic acids having an epoxy group such as glycidyl acrylate (or methacrylate) or the like.
(v) vinyl esters of aliphatic acids such as vinyl acetate, vinyl propionate or the like.
(vi) vinyl chloride.
The preferred amount of the copolymerizable vinyl mono-mer to be added is about 0.1 - 80~ by weight based on the weight of the copolymer o be ~ormed. The copolymerizable vinyl mono-mer may be employed singly or in admixture. The vinyl monomers are considered to effect the hardness o~ the resulting coating layer. The vinyl monomexs in the above mentioned groups, es-pecially (iii) and (iv) will increase adhesion to the base material, but can do so only when used together with the ~,~
unsaturated carboxylic acid.
Polymerization using these monomers, as mentioned above, is con~ucted in an aqueous medium and can be achieved by utilizing procedures known per se; for ~xample, the method disclosed in Japanese Patent Publication No. 36,942/1974.

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Useful copolymers for the stated object of the inven-tion have an average molecular weight ~rom approximately 5,000 to approximately 150,000 and a second-order ~ransition temperature (Tg, ~lass transition temperature) from approximately 20 C to approximately 100C. With a copolymer of greater than 150,000 average molecular weight a~hesion to the plastic base material was lowered and ~iscous flow to attain a good heat-sealing pro-perty was lost. On the other hand, with a copolymer of less than 5,000 average molecular weight, the characteristics of a coating layer were lost and sufficient heat-sealing strength was not ob-tained. The preferred average molecular weiyht of the copolymer is about 10,000 to 80,000. The average molecular weight as used herein represents weight-average molecular weight.
The preferred second-order transition temperature is about 30 to 70C. When the copolymer Tg is below 20C the re- -sulting coating layer causes blocking, because of high adhesion.
While, if the copolymer Tg is over 100C the resulting layer is excessively hard and has lower flexibility thereby losing its heat-sealing property.
The copolymer is polymerized in an aqueous dispersion containing it substantially as particles. "Substantially as particles" as used herein means particle sizes of about 0.01 micron to about 1.0 micron~ preferably 0.01 to 0.5 micron, more preferably 0.01 to 0.3 micron. At least 90% of the total par-ticles of the copolymer are preferred to be within the above range.
The aqueous medi~n may be one containing substantially wa~er, to which water-soluble organic solvents such as alcohols or the like may be added. Tha more water content in the medium the better. The preferred concentration of the copolymer in the .

i4 aqueous medium is about 10 to 60~ by weight, the more preferred range being about 15 to 45% by weight.
If necessary lubricants, anti-blocking agents, anti-static agents, plasticizers, antioxidizing agents, stabilizers and the like may be added to the aqueous dispersion.
The base materials to be coated with the coating compo-sition disclosed herein include polyethylene ilms, polypropylene films, polybutene films and polyester films which may be biaxially or monoaxially oriented, or not oriented.- The coating composition is effective especially ~or biaxially oriented plastic ~ilms.
The contact angle of the base film surface to water is preferably less than 85. Polyester films may be used without pretreatment but polyolefine films preferably are first activated by a corona discharge or by surface oxidation treatment. Further the plastic films, as the base material, are not limited to the `above men-tioned ones and their tnickness may be of "sheet" value.
It is preferred to use copolymers whose carboxyl groups are wholely or partially neutraliæed with a volatile base, from the viewpoint of~stab~lity of the aqueous coating composition.
However, when a stabilizer is used, it is not necessary to use such neutralized copolymers.
The coating composition described herein may also serve as an anchor coating agent for plastic films. For example, one or both surfaces of a plastic base film can be coated with the coat-ing composition and then one or both o the coated surfaces can be further coated by an aqueous dispersion of, e.g., vinylidene-chloride copolymer.
Application of the composition described herein to the plastic base materials may be conduc~ed in accordance with con-ventional methods.

The invention is hereinafter illustrated by examples.
The test methods used in the examples are as follows:
(a) Cellophane Tape Test A pressure-sensiti~e adhesive cellophane tape, 24 mm x 200 mm, from end to end, was fast adhered on the copolymer coated surface of test plastic films by repeatedly contacting the tape thereon. Then the adhered tape was rapidly peeled off at an angle of 90. Observation of how the resin on the coated surface was peeled, gave the following classes:
No peeling ~ Excellent Peeled area on the adhered part less than 10% ~ Good 10 - 20% -----~---~- Fairly good over 20~ Bad - (b) Heat-sealin~ Str ngth The coated surfaces of test films were put together and sealed a a heater temperature of 120C, a sealing pressure of 0.5 Rg/cm and a sealing time of 0.5 s by the use of ~ bar type heatsealer. Sealing strength of the test films, 15 mm x 100 mm, was measured by the use of a tensile tester (TENSILON
provided by Toyo Sokki Co. Ltd. in Japan) at a pulling rate of 100 mm/minute.
(c~ Blocking Test Several tes~ films, 50 mm x 50 mm, were inserted between two glass plates, stored for 48 h at 40C under 0.5 Kg/
cm pre5sure and then cooled to room temperature. The blocking degree is classi~ied as follows:
Films readily separable, sheet by sheet, without using force ~ - Excellent Films separable using a small force --------- Good : -' ' : ,.

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Films separable, with partial peeling of the coated part using a small force ~ Fairly Good E'ilms not separable - - - Bad ThP monomers used are described by the following - abbreviations:
MMA : methyl methacrylate EA : ethyl acrylate MAA : methacrylic acid ST : styrene BA : n-butyl acrylate HEMA : 2-hydroxyethyl methacrylate AA : acrylic acid HPMA : 2-hydroxypropyl methacrylate Examples 1 - 6 & Comparative Examples 1 - 2 17.1 g of sodium lauryl sulfate (emulsifier) ~and 530 cc of water contalning 1.4 g of potassiwm persu1fate (catalyst) were poured into a polymerization vessel equipped with a stirr2r, cooler and thermometer. The mixture was heated to around 80C
with stirring. A mixture of monomers as shown in Table 1 of total weight 285 g and a definite amount of a mercaptan (molecular weight modifier) were gradually added to the above mixture over 3 h and then kept for 30 minutes at 80C.
The xesulting aqueous dispersion was neutralized with ammonia, kept for one h at 60~C and cooled. It was diluted with water to a copolymer concentration of 20~. The thus obtained dispersion was applied to the corona-discharged surface of a biaxially oriented polyproplene film at a coating thickness of 20 ~ and to the extent of 1.0 g/m2 as applied solid amount, and then was dried for 1 minu~e at 110C to yield the coated film.

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In Table I, the influence upon adhesion to the base film and heat-sealing strength are shown with changes in the quantities of monomers and molecular weight modifier.

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Example 7 - 10 & Comparative Example 3 The polymerization was conducted in the same way as in Examples 1 - 6, but 1.0 part of mercaptan (molecular weight modifier) to 100 parts of total monomers was used. After neu-trali~ation and dilution, to 100 parts of the resulting disper-sion were added 4 parts of an aqueous carnauba wax dispersion (20% solid~ and 0.3 parts of a fine silica powder.
The resulting dispersion was applied to the corona-discharged surface of a biaxially oriented polypropylene film at a coating thickness of 20 ~ and to the extent of 1.0 g/m2 as applied solid amount and then was dried for 1 minute at llG C
to yield the coa-ted film.
The test results are shown in Table II. The influence of the second-order transition temperature of the copolymers upon adhesion and heat-sealing strength of the base film are shown in Table II.

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Examples 11 - 20 & Comparative Examples 4 - 5 The coating compositions and films coated thereby were prepared in the same way as in Examples 7 - 10. The monomers used and the test results are shown in Table III. The influence especially of the amounts of ~ unsaturated carboxylic acid used i5 shown in Table III.
Examp e 21 The coating composition of Example 17 was applied to a biaxially oriented polyethyleneterephthalate film at a coating thickness of 12 ~ and to the extent of 1.0 g/m2 and then dried for 1 minute at 110C. The adhesion of the composition to the base film was excellent and the heat sealing strength was 120 g/15 mm.
Example 22 To a 50% aqueous vinylidenech].oride-methyl acrylate dispersion were added 0.2 parts of a fine silica powder (to 100 parts of the copolymer) and 3 parts of an aqueous carnauba wax dispersion (20% solid) (to 100 parts of the copolymer). The re-sulting mixture was applied to the coated film of Example 5 to the extent of 3 g/m2 and dried for 1 minute at 110C to yield the coated film.
The adhesion between h~ biaxially oriented polypropyl-ene film and the vinylidenechloride-methyl acrylate copolymer coating layer was excellen~ and the h~at-sealing strength was 150 gJ15 mm.

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Example 23 In the same way as in Example 22, the vinylidene-chloride-methylacrylate copolymer was coated on the coated sur-face of the film of Example 12. The adhesion between the bi-; axially oriented polypropylene film and the copolymer coating layer was excellent and the heat-sealing strength was 135 g/15 mm .
Example 24 In the same way as in Example 12, both surfaces of a biaxially oriented polypropylene ilm were coated. Then one surface (A) was coated by an acrylate copolymer and the other (B) was coated by a vinylidenechloride-methyl acrylate copolymer in the same way as Example 22.
The heat-sealing strength of (A) surface/(~) surface, ~A) surface/(B) surface and (B) surface/(B) surface were 145 g/15 mm, 115 g/15 mm and 120 g/15 mm, respectively.

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mixture of the monomers shown in Table IV of total weight 285 g and a definite amount of a mercaptan as molecular weight modifier were gradually added to the aboYe mixture over 3 h and then kept for 30 mins. at 80C.
The resulting aqueous dispersion was neutralized with ammonia, kept for 1 hour at 60C and cooled. Then it was diluted with water to a copolymer concentration of 20~. The resulting composition was applied to the corona-discharged surface of a biaxially oriented polypropylene film t2Q y) having a contact angle of 60 to water, by using a Mayer Bar and then dried for 1 minute in a 110C oven. ~he coated amount after drying was 0.5 - 0.6 g/m .
The above anchor coated surface of the film was further coated with a coating composition of 100 parts o~ an aqueous vinyl-idenechloride-acrylate copolymer dispersion (vinylidenechloride:
85~ by weight~, 2.0 parts of a wax (mp. 65C) dispersion and 0.1 part of calcium carbonate.
Table IV shows the results of examples 25 - 30 and comparative exa~ples 6 and 7.

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Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A heat sealable plastic film coated with a compo-sition comprising an aqueous dispersion containing, substantially as particles, a copolymer of a) 0.1 to about 10% by weight of at least one .alpha.,.beta.-unsaturated carboxylic acid having at least one carboxyl group per molecule; b) at least one acrylic ester selected from the group consisting of an alkyl acrylate and an alkyl methacrylate wherein the alkyl group of the ester contains one to twelve carbon atoms; and c) at least one copolymerizable vinyl monomer selected from the group consisting of compounds of (i) vinyl-substituted aromatic hydrocarbons (ii) .alpha.,.beta.-unsa-turated carbonamides (iii) hydroxyalkyl esters of .alpha.,.beta.-unsa-turated carboxylic acids, (iv) esters of .alpha.,.beta.-unsaturated car-boxylic acids having an epoxy group, (v) vinyl esters of ali-phatic acids, (vi) vinyl chloride, in an amount of from 0.1 to 80 percent by weight of the copolymer to be formed; and wherein said copolymer has a weight-average molecular weight in the range of about 5,000 to about 150,000 and a second-order transi-tion temperature in the range of about 30°C to about 70°C.

2. A coated plastic film according to Claim 1 in which all or part of the carboxyl groups in the copolymer are neutra-lized with a volatile base.

3. A coated plastic film according to Claim 1 in which the particle size of the copolymer is in the range of 0.01 to 1.0 microns.

4. A coated plastic film according to Claim 1 in which a further coating formed from an aqueous dispersion of a vinyl-idene chloride copolymer is formed thereon.

5. A coated plastic film according to Claim 4 in which the plastic film is a surface-treated polypropylene film.

6. A coated plastic film according to Claim 1 in which the plastic film is a polyester film.

7. A coated plastic film according to claim 3 wherein said copolymer is incorporated in the aqueous dispersion in an amount of from 10 to 60 percent by weight thereof.

8. A coated plastic film of claim 1, wherein a hydro-xyalkyl ester of an .alpha.,.beta.-unsaturated carboxylic acid is incor-porated in an amount of from about 0.1 to about 30 percent by weight of the copolymer.

9. A coated plastic film of claim 1, wherein a hydro-xyalkyl ester of an .alpha.,.beta.-unsaturated carboxylic acid is incor-porated in an amount of from about 10 to about 30 percent by weight of the copolymer.

10. A coated plastic film of claim 1, including hydro-xyalkyl ester of an .alpha.,.beta.-unsaturated carboxylic acid comprising 2-hydroxyethyl methacrylate.

11. A coated plastic film of claim 1, wherein the plastic substrate is a polyethylene, polypropylene, polybutene or polyester film.

12. A coated plastic film of claim 1, wherein the plastic substrate is a biaxially oriented polypropylene film.

13, A coated plastic film of claim 1, wherein the co-polymer has an average molecular weight of from 10,000 to 80,000.

14. A coated plastic film of claim 1, wherein at least 90% of the particles of the copolymer possess a particle size of from 0.01 to 0.5 microns.