TWO-COMPONENT POLYETHYLENE EXTRUSION COATING BLENDS

Abstract: Polyethylene blends that are useful as extrusion coatings on various substrates. The blends are prepared from 10% to 90% by weight of a high density polyethylene and from 90% to 10% by weight of a low density polyethylene. The blends have a melt index of 0.1 to 7, a density of 0.912 to 0.98, a swell ratio less than 1.5 and DSC melting point greater than a homopolymer of ethylene having the density of the blend.

Description: - 1 - TWO-COMPONENT POLYETHYLENE EXTRUSION COATING BLENDS This invention relates to two-component polyethylene blends that are useful for extrusion 5 coating on various substrates. In the extrusion coatings art the problems of "neck-in", "neck-in stability", and "pinholing", result in limited coating speeds. The term "neck-in" connotes the narrowing down of the extruded film Q immediately upon leaving the extruder head, presumably due to internal forces in the film, and which causes a thickening of the film edges so that trimming thereof is required. Poor neck-in properties or stability of a polymer or blend may ^5 result in as much as 15% waste or more in certain coating operations. Moreover, the maximum width and uniformity of the coating depend on good neck-in stability characteristics. The term "neck-in stability" connotes the change 20 in neck-in per unit change in temperature of the film. A poor neck-in stability means a large change of neck-in per small change in temperature. Such temperature variations result, for example, from the on and off cycling of the extruder and die heaters. 5 Good neck-in stability is particularly desirable from the standpoint of maintaining a uniform coating thickness over a wide range of extrusion temperatures, as well as maintaining a constant coating width. 0 The pinhole problem is manifested in poor barrier properties, and the resistance of a polyethylene coating to pinholing appears to be directly related to its thermal stability as reflected, for example, in neck-in thermal stability. The resultant limited 5 coating speeds are undesirable, of course, from the standpoint of poor economics of production resulting - OMPI from less than maximum equipment capacity and efficiency. Polyethylene blends that are satisfactory for extrusion coating are disclosed in Canadian 5 Patent 798,908. The blends disclosed in that patent have a high melt index and a high swell ratio. The melt index is at least 8 dg./min and the swell ratio is at least 1.50. These properties of the blend were considered to be essential to have good coatability I Q at acceptable coating speeds of about 275 meters per minute. The polyethylene blends of this invention. have a lower melt index and a lower swell ratio than the prior art blends. These blends provide extrusion __ coatings that solve the problem of "neck-in", "neck-in stability" and "pinholing", and the blends can be coated by extrusion at commercially acceptable speeds.. In addition the polyethylene blends of this invention provide coatings that have better 20 resistance to cracking under stress, better dispersion of pigments for pigmented coatings, better release from chill rolls for better adhesion to substrates, better impact strength and elongation for greater toughness and a broader temperature range for 25 heat sealing in packaging operations. The blends are prepared by mixing a low density polyethylene having a density above .91 to .93 with a high density polyethylene having a density of .96 to .98 to provide blends having a melt index of .1 to 3 about 7 and a swell ratio of less than 1.5. From 10% to 90% by weight of a high density polyethylene is mixed with from 90% to 10% by weight of a low density polyethylene. The preferred blends contain from 20% to 70%, most preferably, 30% to 65%, by weight, of 35 the high density polyethylene. These blends have densities ranging from about 0.912 g/cc to 0.98 g/cc, OM and the melt indices range from .1 dg/min. to 7 dg/min. These blends also have a DSC melting point greater than a homopolymer of ethylene having the density of the blend. 5 The blends may contain such additives as ultraviolet degradation inhibitors, stabilizers, pigments, inert fillers and the like which are commonly employed in resinous systems for a variety of purposes. The blends may be prepared in various 10 ways such as dry-blending and then passing through a compounding extruder, milling roll, or Banbury mixer. Any method whereby melts of the components can be mixed will also produce the desired blend. The low density polyethylene has a density of 15 0.912 g/cc to 0. * 93 g/cc and a melt index of 2.9 dg/min to 16 dg/min. The preferred low density polyethylene has a density of 0.915 g/cc to 0.920 g/cc and a melt index of 2.9 dg/min to 16 dg/min. Several such low density polyethylenes which have 20 been most preferred are as follows: 0.919 g/cc, 3.g dg/min; 0.917 g/cc, 3.5 dg/min; 0.919 g/cc, 15 dg/min. When a low density polyethylene having a density of .91 was used little if any improvement was evident. 25 The high density polyethylene has a density of 0.96 g/cc to 0.98 g/cc, a melt index of 5 dg/min to 18 dg/min. One such high density polyethylene which has been most preferred has a density of 0.970 g/cc, and a melt index of 9.0 dg/min. 30 In the following examples, density is determined in conformance with ASTM Designation D1505-57D. Density specimens are first annealed to give maximum density by conditioning under a 63.5 cm or higher mercury vacuum for 1 hour at 150-155°C. 35 followed by cooling under this vacuum at no greater than 20°C. per hour to a final temperature of less than 50°C. Melt index is determined in conformance with ASTM Designation D1238-62T. Example 1 A polyethylene blend having a density of 0.935 g/cc, a melt index of 3.1 dg/min, and a swell ratio of 1.44 was prepared by blending 70 weight percent of a low density polyethylene with 30 weight percent of a high density polyethylene. The characteristics of the low density component are as follows: density - 0.918; MI =3.4 dg/min. Those of the high density component are as follows: density ■ 0.971; MI s 15.0 dg/min. In order to demonstrate the extrusion coating advantages of this two-component blend the blend was fed to a 8.89 cm Egan extruder having a barrel length to diameter ratio of 24:1. The four zones of the extruder were maintained, from back to front, at 204°C, 260°C, 316°C, and 338°C. A metering type screw having six compression flights, and 12 metering flights were used. Prior to entering the die the melt passed through a 9 x 9 strand per square cm. mesh screen. The die was an Egan die, center-fed with 1.27 cm long lands, with an opening of 40.64 cm x .05 cm. The temperature of the die was held at 316°C. The extrusion rate was held constant at 59 kg per hour. The resulting film extrudate was passed through a 11.4 cm air gap into the nip formed by a rubber-covered pressure roll and a chill roll. At the same time, 18.14 kg basis weight Kraft paper was fed into the nip with the pressure roll in contact with the Kraft paper. The nip pressure applied was 445 N per linear 25.4 mm. The chill roll was a 60.96 cm diameter mirror finish chrome-plated steel roll, water cooled to maintain a temperature of 15.50°C. on the roll. The coated paper was taken off the chill roll at a point 180° from the nip formed by the pressure roll and chill roll. The chill roll was operated at linear speeds of greater than 396.24 meter per minute. This coating composition coated the paper substrate at a commercially acceptable rate with a ' neck-in per edge of 2.67 cm which is also commercially acceptable. Example 2 A polyethylene blend having a density of 0.947 Q g/cc, a melt index of 6.4 dg/min, and a swell ratio of 1.38 was prepared by blending 70 weight percent of a low density polyethylene with 30 weight percent of a high density polyethylene. The characteristics of the low density component are as follows: density = 5 0.925; MI =1.6 dg/min. Those of the high density component are as follows: density = 0.980; MI = 30.0 dg/min. In order to demonstrate the extrusion coating advantages of this two-component blend the blend was fed to a 1.38 cm Egan extruder having a barrel length to diameter ratio of 24:1. The four zones of the extruder were maintained, from back to front, at 204°C, 260°C, 304°C, and 332°C. A metering type screw having six compression flights, and 12 metering 5 flights were used. Prior to entering the die the melt passed 9 x 9 strand per square cm mesh screen. The die was an Egan die, center-fed with 1.27 cm long lands, with an opening of 40.6 cm x .05 cm. The temperature of the die was held at 316°C. The extrusion rate was held constant at 59 kg per hour. The resulting film extrudate was passed through a 11.4 cm air gap into the nip formed by a rubber-covered pressure roll and a chill roll. At the same time, 18.14 kg basis weight Kraft paper was fed into the nip with the pressure roll in contact with the Kraft paper. The nip pressure applied was OM 445 N per 25.4 mn. The chill roll was a 60.96 cm diameter mirror finish chrome-plated steel roll, water cooled to maintain a temperature of 15.5°C. on the roll. The coated paper was taken off the chill roll at a point 180° from the nip formed by the pressure roll and chill roll. The chill roll was operated at linear speeds of about 335.3 meters per minute. This extrusion coating composition had a melt index which provided acceptable commercial coating speeds with good neck-in per edge of about 2.67 cm. A polyethylene blend having a density of 0.947 g/cc, a melt index of 13 dg/min, and a swell ratio of 1.30 was prepared and coated according to the above procedure. The composition coated at linear coating speed of only about 259 meters per minute and had an excessive neck-in per edge of about 7.6 cm which is not satisfactory for commercial operation. A polyethylene blend having a density of 0.932 g/cc, a melt index of 8.1 dg/min, and a swell ratio of 1.2 was prepared and coated according to the above procedure. The composition coated at a speed of about 274.32 meters per minute and also had excessive neck-in per edge of about 6.86 cm. A polyethylene blend having a density of 0.950 g/cc, a melt index of 5 dg/min, and a swell ratio of 1.64 was prepared and coated according to the above procedure. The composition coated at a speed of only 213 meters per minute and had 2.5 cm neck-in per edge. This coating speed is not satisfactory for commercial use. A polyethylene blend having a density of 0.954 g/cc, a melt index of 7.9 dg/min, and a swell ratio of 1.72 was prepared and coated according to the above procedure. The composition provided satisfactory coatings at speeds of only 13.41 meters per minute to have neck-in or edge weave of about 1.52 cm per edge. The coating composition of this invention provides good coating to substrates such as Kraft paper, milk carton stock, photographic papers, cellulosic sheets, primed metal foils such as aluminum and the like. The coated substrates find utility in food packaging, medicine packing or other well known uses.

Claims: Claims 1. A composition for extrusion coating containing two different polyethylenes, the composition being characterized by a melt index of 0.1 to 7, a density of 0.912 to 0.98, a swell ratio less than 1.5 and a DSC melting point greater than a homopolymer of ethylene having the density of the composition. 2. A composition according to claim 1 wherein the swell ratio is 1.3 to 1.46. 3. Paper coated with the composition of claim 1 by extrusion. ffZE OMP WIP

Inventors: EDWARDS RAY, EDWARDS, RAY
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1982
Main classification: C08L23
IPC codes: B32B23, B32B27, C08L23, C09D123