HK1034275A - Starch ester coatings - Google Patents

Description

Starch ester coating

The present invention relates to release coatings and moisture vapor barrier coatings made from selected modified starch esters, and more particularly, starch esters having an ester moiety of 2 to 8 carbon atoms and a medium DS of 1.0 to 2.2.

Release coatings typically contain compositions and/or adjuvants that control or eliminate adhesion between two surfaces. These coatings generally have good release properties, which is the ability to allow the tacky substances that are stuck to the surface to be easily peeled off (that is, the coatings must provide a low interfacial tension with respect to the tacky substances that come into contact with the coatings). However, good release properties are not the only properties required for such coatings. Other properties required for release coatings include good bond strength, good adhesion to the backing substrate, and no contamination or migration of ingredients into the adhesive layer.

The use of silicone materials such as organosiloxanes as release agents and release coatings is well known. US patent US4983701 issued on 8.1.1991, US patent US5356706 issued on 18.10.1994 and US5376420 issued on 27.12.1994 all disclose the use of silicone materials as anti-adhesive agents in different applications. Other types of detackifiers that have been used include synthetic polymers such as polyolefins and fluorocarbons, long chain alkyl derivatives such as aliphatic ester synthetic waxes and fatty acids, and waxes such as petroleum waxes, vegetable waxes and animal waxes.

While the above exemplified materials, particularly silicone materials, have shown release properties suitable for various applications, they are not practical when repulpable, recyclable paper-like products are desired.

In packaging and other applications, a water-resistant permeable barrier is sometimes needed to prevent the permeation of water vapor through paper or other substrate materials. In many cases, plastic substances such as polyethylene have been used to form water-repellent permeation layers or water-repellent coatings. While these materials are generally useful as protective coatings, they are not practical for paper product applications requiring repulpable, recyclable products.

Starch-based materials have been used in some coating applications. For example, U.S. patent No. US3746558 issued to f. berkhout et al, 7, 17, 1973, discloses an aqueous suspension of a hydrophobic, low DS starch ester which can be used to form a water-repellent coating. US patent US4095992 issued on 20.6.1978 to rudolph et al discloses mixed starch esters of monocarboxylic acid anhydrides and polycarboxylic acid anhydrides which provide water repellency when used as thermosetting, hydrophobic coatings. European patent application EP0545228a1, published 6/9/1993, discloses the use of a mixture of modified starch and a synthetic polymer as moisture barrier in moisture-proof packaging bags. Another european patent application published on 23.6.1993, EP0547551a1, discloses compositions comprising modified starch, gelatin, a plasticizer, water and a lipid as edible films effective for providing a food product with a water, lipid, solute, gas, physical or microbial barrier.

While the above starch materials do provide moisture and/or water resistance in various applications, they do not provide a thermoplastic coating that is susceptible to forming a continuous film coating with good moisture vapor barrier properties.

It has now been found that coating compositions made with selected modified starch esters have good release and moisture penetration resistance and are biodegradable and environmentally friendly, thus making such coating compositions particularly useful in applications requiring repulpable and recyclable paper.

More particularly, the present invention relates to release coating compositions comprising:

a) a starch ester detackifier having an ester moiety of 2 to 8 carbon atoms and a Degree of Substitution (DS) of 1.0 to 2.2,

b) a plasticizer, and

c) and (3) water.

The invention also relates to a process for preparing a release coated substrate wherein the release coating composition of the invention is applied to a substrate as a latex and then heated to a suitable temperature to cause the latex particles to melt and flow into a continuous film. The present invention also relates to a pressure sensitive adhesive substrate comprising a substrate, a pressure sensitive adhesive layer and a release coating layer, wherein the release coating layer comprises the starch ester release coating composition of the present invention.

The invention also relates to a water vapor permeation resistant coating composition comprising:

a) starch esters having an ester moiety of 2 to 8 carbon atoms and a Degree of Substitution (DS) of 1.1 to 2.2,

b) a hydrophobic plasticizer, and

c) and (3) water.

The invention also relates to a method of making a moisture barrier coated substrate wherein the moisture barrier coating composition of the invention is applied to the substrate as a latex and then heated to a suitable temperature to melt and flow the latex particles into a continuous film.

The present invention provides release coating compositions and moisture vapor barrier coating compositions comprising a starch ester compound having from 2 to 8 carbon atoms in the ester moiety and from 1.0 to 2Degree of substitution of 2. These starch esters contain ester compounds having the following structural formula:where St is a starch-based material and R is an alkyl, aryl, alkenyl, alkylaryl or arylalkyl group of 1 to 7 carbon atoms, preferably an alkyl or alkenyl group of 1 to 4 carbon atoms. More preferably, the ester compound contains an R group that is an alkyl group of 1 to 2 carbon atoms. Starch esters of this type include starch acetate, starch propionate, starch butyrate, starch caproate, starch benzoate, mixtures of two or more of these esters, e.g., starch acetate/starch propionate, and mixed starch esters in which the starch contains two or more different ester substituents, e.g., starch acetate/starch propionate, i.e., such esters have the structural formula, for example, as follows:wherein R and R' are different substituents as defined for the R group above.

Furthermore, these starch esters as defined above have a DS (degree of substitution) of from 1.0 to 2.2, preferably a DS of from 1.2 to 1.9, more preferably a DS of from 1.4 to 1.6. In other words, the starch ester has a DS that is high enough to prevent water dispersion and high enough to make the starch thermoplastic, that is, having a water-free (moisture-free or plasticizer-free) Tg of less than about 200-220 ℃. The term "degree of substitution" (DS) as used herein means the average value of the points having substituents per anhydroglucose unit of the starch molecule.

The starch esters with a moderate DS used in the present invention can be prepared by reacting starch with an organic acid anhydride in a solvent system such as pyridine. Recently, a process for the aqueous preparation of starch esters has been disclosed in US5321132 issued 6/14 of 1994 to r. This process involves an aqueous one-step process in which starch is reacted with a high throughput of organic acid anhydride and a high concentration of an alkaline agent.

The starting starchy material for the starch esters of the invention may be those obtained from any plant source, including corn, potato, wheat, rice, sago, tapioca, waxy corn, sorghum and high amylose starches, i.e. starches having an amylose content of at least 45%, particularly at least 65%, such as high amylose corn. Starch powder may also be used. The above starting starchy materials also include converted products obtained from any of the foregoing starting materials, such as dextrins prepared by acid and/or thermal hydrolysis, flowable or thinly boiled starches prepared by enzymatic conversion or weak acid hydrolysis, oxidized starches prepared by treatment with an oxidizing agent such as sodium hypochlorite, and derivatized starches, such as cationic, anionic, amphoteric, nonionic and cross-linked starches. In other words, the starch material may comprise granular or dispersed starch. By starch in the form of a dispersion or non-granular is meant any starch whose structure has been disrupted or broken down, i.e. any starch whose structure has been disrupted by thermal (jet cooking, boiling water bath heating), mechanical (steam drum drying, spray drying, extrusion) or chemical (using liquid ammonia, dextrinization, subjected to a high degree of alkaline treatment) methods prior to derivatization.

The release coating composition and the moisture permeation preventing coating composition contain a plasticizer and water in addition to the starch ester. Plasticizers are non-volatile organic substances that are compatible with starch esters. Such plasticizers should be water insoluble, i.e. have a solubility in water of less than 5%, and hydrophobic, which absorb low amounts of moisture at high humidity, i.e. a moisture content of less than 20% by weight, preferably less than 15% by weight at 23 ℃ and 90% Relative Humidity (RH). The plasticizer is liquid at ambient or room temperature and is used in an amount to reduce the Tg (glass transition temperature) of the starch ester to the range of about 75-200 c, preferably 80-135 c, required for processing. This temperature is necessary for example in order to melt the coating in the drying or calendering step employed in the papermaking process. Such plasticizers generally have a molecular weight of less than about 10000. A wide variety of plasticizer materials may be used in combination with the selected starch ester to meet the desired conditions. Useful plasticizer materials include those compounds containing polar groups such as sulfonamide, carboxylic acid and carboxylic acid ester, carboxylic acid salt, amide, phosphate ester, alcohol (i.e., hydroxyl-containing compound), epoxide, sulfone, ether, imide, amine, carbonate, urea, and urethane groups. Preferred plasticizers are those containing sulfonamide, alcohol, amide, and ester groups that absorb low amounts of moisture at high humidity, that is, a moisture content of less than about 20% by weight, preferably less than about 15% by weight, at 90% Relative Humidity (RH) and 23 ℃. Preferred plasticizers do not include hydrophilic types of compounds, such as glycerin or sorbitol, and other compounds of this type, which are hygroscopic and readily pick up and absorb moisture. Useful plasticizers include low molecular weight polyesters such as polyesters of polyethylene glycol with dicarboxylic acids (e.g., adipic/succinic acid), poly (hydroxybutyrate-valerate), polycaprolactone, alkoxylates of phenol and phenolic derivatives (e.g., ethoxylates of phenol and bisphenol a), and fatty acid amides. Preferred plasticizers are polyesters of polyethylene glycol with adipic acid or succinic acid and ethoxylates of bisphenol A.

Such coating compositions typically contain about 10-50% by weight starch ester, about 0-30% by weight plasticizer, and about 20-90% by weight water. Such compositions preferably contain about 15-35% by weight starch ester, about 5-25% by weight plasticizer, and about 35-80% by weight water. Depending on the application, other ingredients commonly used, such as fillers, antioxidants, stabilizers, surfactants, waxes and dyes or colorants, may also be added to such compositions.

The starch esters of the present invention are hydrophobic and water insoluble, and such starch esters cannot be dissolved or dispersed in water even at elevated temperatures, e.g., room temperature to 150 ℃, and therefore such starch esters cannot be applied or coated as aqueous solutions. Thus, the starch ester composition is formed into a latex containing discrete starch and plasticizer particles suspended in water. This latex is applied or coated onto the desired substrate and then dried to form a discontinuous film. Heating to a temperature of about the Tg of the starch ester or higher, typically about 100 ℃ and 200 ℃, melts and flows the latex particles to form a continuous film on the substrate. Because water is not capable of dissolving the polymer and is not the primary reason for film formation, it can be heated to a temperature above the Tg of the starch ester during drying and/or at any time after evaporation of the water. The coating may be dried by air drying or using an oven, dryer or other conventional drying device. The coating can be heated by some method, such as calendering or heat treatment, to form a continuous film.

Any conventional coating method can be used to apply the coating composition to the substrate, such as brushing, knife coating, dipping, roll coating, wire coating, or knife coating. The coating method is selected according to the substrate to be coated. To reduce the viscosity of the coating composition and facilitate removal of water, the composition can be applied to a substrate at room temperature or at a temperature above room temperature (preferably about 50-75 ℃). The mixture may also be formulated at ambient temperature or at a temperature above ambient temperature. Heating these components during the formulation process helps to absorb the plasticizer into the starch granules, thereby forming a more uniform latex or dispersion.

The release coating composition can be applied to any substrate, particularly for paper products such as those containing pressure sensitive adhesives. These starch ester release coatings for paper products provide good release properties and, due to their biodegradability and other environmental properties, allow such products to be repulped and recycled. The water vapor permeation resistant coating composition can be applied to any substrate, particularly paper products such as those used for food packaging. These starch ester coatings for paper products provide good moisture vapor barrier properties and, due to their biodegradability and other environmental properties, allow such products to be repulped and recycled.

The following examples further illustrate embodiments of the invention. In these examples, all parts are by weight and all temperatures are in degrees celsius unless otherwise indicated.

Example I

Release coating compositions containing the starch esters of the present invention were prepared as follows.

To a 250ml Erlenmeyer flask were added 16.5 grams of anhydrous granular starch acetate (70% amylose starch with DS = 1.3 for its acetate), 5.0 grams of ethoxylated bisphenol A (Macol 206-EM, PPG industries, Inc.) and tap water to reach a total weight of 50.0 grams. The contents of the erlenmeyer flask were mixed vigorously with a stainless steel mixing spoon until the mixture was slurried. A lead weight was placed around the opening of the flask and the flask was placed in a boiling water bath with the water level above the level of the flask contents. The flask was heated with occasional stirring for 1 hour, then the flask was removed from the boiling water bath and the contents were mixed thoroughly. Water was added to the flask to bring the weight to the initial gross weight. This mixture was placed back into the boiling water bath until ready for the following application.

Example II

A sheet of paper label stock (9 "x 11") (basis weight: 4#, high pressure Gurley air permeability 270 seconds) was placed on a large glass plate and the paper was taped to the glass plate at its face end edges. The glass plate was placed on a flat horizontal surface and a #9 wire wound drawdown (wire round) was placed on top of the paper substrate. The coating prepared in example I was removed from the boiling water bath and dropped as 1/2 inch beads across the face of the paper substrate (just below the wire wound drawdown). While applying slight pressure, the end of the wire wound knife applicator was grasped and pulled down over the end of the paper substrate. The coated paper was removed from the glass plate and placed face down on an endless dryer (drying ring) with the coated surface. The coating was air dried by placing the central disk on an annular dryer and placing a weight on top.

The dried paper substrate was removed from the ring dryer. The coated substrate was calendered with a stainless steel heated roll with the coated side facing the heated roll. The processing conditions were 5000psi (pressure between rolls), 90 ℃ (temperature of the upper stainless steel roll) and 2 calendering (two passes of the paper).

The coated label base paper was evaluated for release and tack retention properties as follows:

1. an 8 inch long length of Scotch Magic tape #810 was laminated to the coated paper described above.

2. Ensuring that good contact is made between the tape and the release composition.

3. The tape was held on the release surface for 3 weeks at room temperature.

4. The force required to peel the tape from the release surface (peel at an angle of 180 degrees at a test speed of 10.0 inches/minute) was measured with an instron tester.

5. The second strip of tape was removed from the release surface by hand.

6. The tape of 5 above was adhered to a clean, flat stainless steel plate (Chem Instruments, cat # TP-39). The sample was rolled and passed 2 times with a 2 kg rubber covered roller (Chem Instruments, cat # WR-100) and allowed to stand for 20 minutes before testing.

7. The peel strength was measured as in 4 above.

8. The release value was recorded as the peel strength (grams/0.75 inch) was measured as in 7 above.

9. A new piece of tape (not contacted with release paper) was adhered to the stainless steel plate as in 6 above, the tape removed and measured as in 4 above.

10. Percent tack retention = adhesion from 7 above x 100/adhesion from 9 above.

Table 1 below shows the physical properties of the label base paper coated with the starch esters of the invention and compared with commercial silicone release paper. This example demonstrates that starch-based coatings have the same release capacity while maintaining excellent adhesion.

TABLE 1

Test specimen	Release Properties (g/in)	Viscosity retention%
			Acetic acid starch (DS = 1.3)	255	98
Rhinellander TightRelease (siloxane, #402- & 8437)	52	88
			Base paper (uncoated)	Fiber tear	-

Example III

A release coating composition was prepared as in example I using 30 grams of anhydrous granular starch (70% amylose starch with acetate DS = 1.3), 10.0 grams of succinate polyester (Resoflex R-804, Cambridge industries, USA) and tap water to a total of 133.3 grams. This coating composition was coated on label stock as in example II, tested for various physical properties, and compared to two grades of commercial silicone release paper as shown in Table 2 below. This sample shows that a flat continuous coating similar in quality to silicone coated paper is formed.

TABLE 2

Test specimen	High pressure Gurley air permeability (seconds)	Parker Print	Hercules Size test
				Base paper (uncoated)	800	5．10	27
Starch acetate on base paper (DS = 1.3)	4950	3．7	185
				Rhinellander Premium Release (siloxane, #442-8354)	2670	3．86	960
Rhinellander light Release (siloxane, #402-	3152	3．86	2580

Example IV

An acid-degraded waxy starch was pre-dispersed and then acetylated to a DS of 1.0. This acetylated product was purified by decanting the aqueous medium and trituration with distilled water. The coating composition was prepared as described in example I. Other similar coatings were prepared with different starch esters, i.e., starch esters based on waxy corn or high amylose corn (70% amylose), which were dispersed or granular. The prepared composition was coated on a paper label base paper as in example ii, and various physical properties were tested, and the results are listed in table 3 below. These results show that with satisfactory coating results, the starch esters can be used in granular or dispersed form and can be operated at different temperatures. All samples had an air permeability of greater than 100000 high pressure Gurley seconds.

TABLE 3

Sample number	Starch structure	Starch type	Plasticizer%	Preparation temperature (. degree.C.)	Parker Print	Anti-sticking ability (g/inch)
							1	In a dispersed state	Containing wax	25	At room temperature	2．3	572
2	In a dispersed state	Containing wax	25	65	2．5	561
							3	Granular form	Containing wax	25	At room temperature	3．9	295
4	Granular form	Containing wax	25	65	3．3	289
							5	Granular form	High amylose starch	25	At room temperature	3．1	255
6	Granular form	High amylose starch	25	65	2．2	230
							7	In a dispersed state	Containing wax	50	65	2．9	225

Example V

A moisture permeation resistant coating composition containing the starch ester of the present invention was prepared as follows.

To a 250ml Erlenmeyer flask were added 30 grams of anhydrous granular starch acetate (70% amylose starch with acetate DS = 1.3), 10.0 grams of succinate polyester (Resoflex R-804, Cambridge industries, USA) and tap water to achieve a total weight of 133.3 grams. The contents of the erlenmeyer flask were mixed vigorously with a stainless steel mixing spoon until the mixture was slurried. A lead weight was placed around the opening of the flask and the flask was placed in a boiling water bath with the water level above the level of the flask contents. This flask was heated for 1 hour with occasional stirring, then the flask was removed from the boiling water bath and the contents were mixed thoroughly. Water was added to the flask to bring the weight to the initial gross weight. This mixture was placed back into the boiling water bath until ready for the following application.

Example VI

A sheet of paper label stock (9 '. times.11') (basis weight: 42#) was placed on a large glass plate and the paper was taped to the glass plate at its face end edges. The glass plate was placed on a flat horizontal surface and a #15 wire wound drawdown was placed on top of the paper substrate. The coating prepared in example I was removed from the boiling water bath and dropped in 1/2 inch beads across the top of the paper substrate (just below the wire wound drawdown). While applying slight pressure, the end of the wire wound knife applicator was grasped and pulled down over the end of the paper substrate. The coated paper was removed from the glass plate and placed face down on the ring dryer with the coated face. The coating was air dried by placing the central disk on an annular dryer and placing a weight on top.

The dried paper substrate was removed from the ring dryer. The coated substrate was calendered with a stainless steel heated roll with the coated side facing the heated roll. The processing conditions were 5000psi (pressure between rolls), 90 ℃ (temperature of the upper stainless steel roll) and 2 calendering (two passes of the paper).

The coated label base paper was evaluated for water vapor permeability (MVTR) with TAPPI T448 om-89 as follows: 1. a desiccant is loaded into a test container (No. 68-1, Thwing Albert Instrument Co.) and loaded 5mm from the top, 2. a 3 inch diameter circular piece of paper is cut from the piece of sample paper to be tested, 3. the coated side of the test piece is placed down on the test container, 4. a cap ring is placed on the test container and tightened, 5. the test container is placed in a humidity chamber at a relative humidity of 50% and a temperature of 23 ℃, 6. samples are taken at specified time intervals (24 hours) and the weight is recorded, 7. the water vapor permeability is calculated as follows: MVTR = g/(meter)²) (day) = x/Ay where x = weight increase in grams over time y = weight increase in days over time a = area exposed to the sample (meters)²)。

Table 4 below shows that both the coating composition prepared in example i (starch acetate DS = 1.3) and a similar coating composition prepared using acid-degraded fluidity high amylose starch (70%) corn starch (acetate DS = 1.5) improve water vapor permeability.

TABLE 4

Water vapor permeability test specimen MVTR at 50% RH, 23 deg.C (grams/meter)²Day) base paper (uncoated) 170.5 starch acetate (DS = 1.3) 97.19 fluidity starch acetate (DS = 1.5) 85.10

Example VII

A moisture permeation resistant coating composition containing the starch ester of the present invention was prepared as follows. To a 250ml Erlenmeyer flask were added 50.0 grams of anhydrous granular starch acetate (70% amylose starch, its acetate DS = 1.3), 16.7 grams of ethoxylated bisphenol A (Macol 206-EM, PPG industries, Inc.) and tap water to a total weight of 150.7 grams. The contents of the erlenmeyer flask were mixed vigorously with a stainless steel mixing spoon until the mixture was slurried. A lead weight was placed around the opening of the flask and the flask was placed in a boiling water bath with the water level above the level of the flask contents. This flask was heated for 2 hours with occasional stirring, then the flask was removed from the boiling water bath and the contents were mixed thoroughly. Water was added to the flask to bring the weight to the initial gross weight. This mixture was placed back into the boiling water bath until ready for the following application.

A piece of the base paper (9 '. times.11') of the Rhinellander paper label was placed on a large glass plate, and the paper was taped to the glass plate at its edge at the face end. The glass plate was placed on a flat horizontal surface and a #9 wire wound drawdown was placed on top of the paper substrate. The coating prepared above was removed from the boiling water bath and dropped in 1/2 inch beads across to the top of the paper substrate (just below the ring dryer). While applying slight pressure, the end of the wire wound knife applicator was grasped and pulled down over the end of the paper substrate. The coated paper was removed from the glass plate and placed face down on the ring dryer with the coated face. The central disc was placed on the ring dryer and a weight was placed on top to dry the coating. The dried paper substrate was removed from the ring dryer. The coated substrate was calendered with the stainless steel heated roll with the coated side facing the heated roll. The processing conditions were 5000psi (pressure between rolls), 90 ℃ (temperature of the upper stainless steel roll) and 2 calendering (two passes of the paper).

The physical properties of the label base paper coated with the above-described starch acetate composition and of the copy paper coated with the same coating, i.e. the High Pressure (HP) Gurley and Hercules Size Test (HST), were evaluated. Papers coated with the starch propionate (DS = 1.3) composition were evaluated in the same way and the results are given in table 5 below.

Table 5 label base paper copy paper sample HP Gurley HST starch acetate > 100000 seconds 82 seconds 42 seconds 22 seconds starch propionate > 100000 seconds 92 seconds 65 seconds 40 seconds uncoated 800 seconds 27 seconds 11.1 seconds 10 seconds

Claims (11)

1. An anti-stick or moisture vapor transmission resistant coating composition comprising: a) starch esters having an ester moiety of 2 to 8 carbon atoms and a Degree of Substitution (DS) of 1.0 to 2.2, b) a hydrophobic plasticizer, and c) water.

2. The composition of claim 1 wherein the starch ester has a DS of 1.2 to 1.9.

3. A composition according to any preceding claim, wherein the starch ester has 2 to 5 carbon atoms in the ester moiety.

4. A composition according to any preceding claim, comprising from about 10 to about 50% by weight of the starch ester, from about 0 to about 30% by weight of the plasticizer, and from about 20 to about 90% by weight of water.

5. A composition according to any preceding claim, wherein the starch ester has the formula:where St is a starch-based material and R is an alkyl, aryl, alkenyl, alkaryl or aralkyl group of 1 to 7 carbon atoms.

6. The composition of any of the preceding claims wherein the plasticizer is a non-volatile polar organic material compatible with the starch ester and is present in an amount sufficient to lower the Tg of the starch ester to a temperature of from about 75 ℃ to about 200 ℃.

7. A composition according to any preceding claim, wherein the plasticiser comprises a sulphonamide, alcohol, amide or ester group.

8. A process for preparing a release-or moisture-permeation resistant coated substrate, which process comprises: a) providing a coating composition comprising a starch ester latex, the coating composition comprising the composition of any of the preceding claims, b) applying the coating composition to a substrate, and c) heating the coated substrate to a temperature at or above the Tg of the starch ester to melt the particles and form a continuous film.

9. The method of claim 8, wherein the coated substrate is dried prior to heating.

10. The method of claim 9 wherein the substrate is paper.

11. A pressure-sensitive adhesive construction comprising a substrate layer, a pressure-sensitive adhesive layer and a release coating layer, wherein the release coating comprises the composition of any one of claims 1 to 7.