Patent Grant
11773297
The present invention relates to dielectrically heating a foamable composition. In particular, radio frequency (RF) heating is used to heat the foamable composition to provide insulation in the manufacture of an article.
A more environmentally-sound disposable food packages and containers are sought after than traditional closed-cell extruded polystyrene foam packages. Packages made entirely out of plastic typically do not biodegrade in less than 400 years, or if ever, and some regulations have banned the use of such packages and containers.
Alternative packages that are recyclable biodegradable and/or compostable are in demand. One such package includes cellulose-based substrates, sourced from renewal materials that can be recycled and/or compostable. The package is made by joining two cellulosic substrates with an air gap interposed in between the two. Some drawbacks to these alternative packages include low insulation and poor structural integrity over the plastic packages. As the package is handled and flexed, the air gap between the two substrates become compressed and the insulation is decreased in those compressed areas. Insulation can be improved by increasing the air gap between the cellulose substrate layers, increasing the thickness of the cellulose substrates or inserting a cellulose medium in between the two layers.
Some of the above mentioned improved packages are described in U.S. Pat. Nos. 9,580,629, 8,747,603, 9,273,230, 9,657,200, US 20140087109, US 20170130399, US 20170130058, and US 20160263876. The packages are formed with an air gap in the coating/adhesive sandwiched between two substrates, which provides insulation. Conventional heating requires prolonged time and large space for water evaporation and curing/coalescing the polymer. Moreover, as the size of the packages vary, the insulative gap is not always uniform. While microwave heating provides strict controls for level of moisture and uniform air gaps, the depth of penetration is limited only to about 1.5 inches. Thus, any substrate that has a thickness greater than about 1.5 inches may result in non-uniform packages.
There is a need in the art for methods to provide uniform insulation for all sizes of packages. The present invention provides methods to produce environmentally and economically sound package that provides uniform thermal insulation for various sizes of packages.
The present invention relates to methods for foaming and coalescing a microsphere-filled, waterborne composition.
One embodiment is directed to a method for foaming and coalescing a composition comprising:
Another embodiment is directed to a process for forming an article comprising the steps of:
The dielectric heating provides uniform heating throughout the composition and forms substantially a uniform thickness of air gap throughout the composition and article.
The present invention provides a method for foaming a plurality of microspheres and coalescing and locking in the foamed microspheres a waterborne composition. The waterborne composition comprises a polymer and a plurality of microspheres, and is applied onto a cellulosic substrate(s) to form a package. Once expanded and locked in place, the air gap in the foamed microspheres provides insulation and structural integrity to the package. This package is more environmentally sound than traditional extruded polystyrene foam packages.
Conventional heating requires ovens with long belts that require large space and massive heat output. Temperatures ranges of about 212° F. (100° C.) to about 450° F. (177° C.) is typically used in conventional heaters.
Microwave heating is another method. Microwave heating provides uniformity in drying and moisture control, and can control the moisture to about 1%. Microwave energy, however, dissipates and cannot penetrate a depth greater than about 1.5 inches (4 cm). Moreover, microwave is a known radiation hazard and exposure to such harmful radiation should be minimized workers. To minimize health risks, industrial microwave machines typically have small openings, and thus, fitting large and odd shape containers into the small orifice is challenging.
Dielectric heating, electronic heating, radio frequency (RF) heating, and high-frequency heating, all interchangeably used herein, is the process in which high-frequency alternating electric field or radio wave heats a dielectric material. RF heating is distinguishable from microwave heating. Industrial radio frequencies operate between approximately 2 MHz and 300 MHz with typical wavelengths of about 141 to about 24 feet (43 to 7.3 meters). Industrial microwave systems use frequencies over 300 MHz with typical wavelengths of about 13 to about 5 inches (33 and 12 cm). The efficiency of power utilization is far lower in an RF generator than a microwave unit, and thus, microwave unit is often the preferred source of heating.
The present invention provides dielectric heating, particularly RF heating, a foamable waterborne composition. RF creates an alternating electric field between a dielectric material, in this invention, polar water molecules. The article is conveyed between the electrodes causing the water molecules in the composition to continuously reorient to face opposite electrodes. Friction from this molecular movement causes the rapid heating. RF operates at much lower frequency than microwave heating, and is associated with lower health risks than microwaves. RF is also suitable for heating bulkier and odd-shaped containers due to its higher depth of penetration.
In addition, the inventive process may include a combination of dielectric heating and direct heating applications. For example, expansion of the microspheres may be achieved through dielectric heating, while the removal of excess moisture after expansion may be achieved through direct heat.
RF heating a composition comprising a water-based polymer and a plurality of expandable microspheres provides uniform heating and drying of the water and expanded microspheres.
The present invention is also based on the discovery that RF heating the composition, comprising a water-based polymer and a plurality of expandable microspheres, applied on an article provides improved and uniform thermal insulation for the article.
One embodiment is directed to a method for foaming and coalescing a composition comprising:
The RF heating concurrently drives the water away from the composition to coalesce the water-based polymer and expands the microspheres as the temperature reaches the activation state of the microspheres. Heating at RF frequencies of about 14, 27 and 41 MHz, are particularly preferred. The RF devices can be designed with operational amplifiers for optimal heating.
The compositions described herein may be useful in multilayer substrates, particularly for cellulosic substrates. Using the composition, a greater insulation space may be provided between the two substrates, which it is attached at the point of adhesion. The insulating products useful herein include paper products for consumer use, such as for hot drinking cups and lids, cold drinking cups and lids, hot food containers and lids, cold food containers and lids, freezer cartons and cases, envelopes, bags, and the like.
The composition may be formed as an adhesive or as a coating, herein used interchangeably. The composition is prepared by forming a mixture of a water-based polymer, a plurality of expandable microspheres and optionally, additive.
The water-based polymer is prepared by emulsion polymerization, and may be a single grade or a mixture of synthetic emulsion polymer or polymers of a natural origin. The water-based polymer prepared by emulsion polymerization may include any desired polymer components, including starch, vinyl acetate ethylene dispersion, polyvinyl acetate, polyvinyl acetate polyvinyl alcohol, dextrin stabilized polyvinyl acetate, polyvinyl acetate copolymers, vinyl acetate ethylene copolymers, vinylacrylic, styrene acrylic, acrylic, styrene butyl rubber, polyurethane, starch and mixtures thereof. Particularly preferred emulsion polymer components are vinyl acetate ethylene dispersion, polyvinyl acetate and starch. Preferably, the emulsion polymer is stabilized by hydrophilic protective colloids.
The water-based polymer may be present in the composition in any amount, and desirably is present in an amount of from about 60% to about 99.5 wt %, preferably from about 65% to about 95 wt %, by weight of the composition prior to setting of the composition. Depending on the emulsion polymer, the solid levels vary from about 40 wt % to about 65 wt %, based on the emulsion polymer.
The expandable microspheres useful in the present invention can expand in size in the presence of heat and/or RF radiation. The microspheres useful in the present invention include, for example, heat expandable polymeric microspheres, including those having a hydrocarbon core and a polyacrylonitrile shell (such as those sold under the trade name DUALITE®) and other similar microspheres (such as those sold under the trade name EXPANCEL®). The expandable microspheres may have any unexpanded size, including from about 5 microns to about 30 microns in diameter. In the presence of heat or radiation, the expandable microspheres of the present invention can increase in diameter by about 3 times to about 10 times the original size. Upon expansion of the microspheres in the composition, the composition becomes a foam-like material, which has improved insulation properties. The microspheres are typically made of plastic or polymeric shells and a blowing agent is inside the shell, designed to activate upon reaching specific temperatures.
The expandable microspheres have a particular temperature at which they begin to expand and a second temperature at which they have reached maximum expansion. Microsphere grades are typically sold with specific expansion (Texp) temperatures and maximum expansion temperatures (Tmax). The initial expansion temperature (Texp) is the typical temperature at which the microspheres start to expand (Texp), and the maximum expansion temperature (Tmax) is the temperature at which the about 80% of the microspheres have expanded. If the microspheres are exposed to temperature far greater than Tmax, the microspheres start to explode and deflate.
One particularly useful microsphere has a Texp of about 80° C. to about 105° C. The temperature at which the microspheres have reached maximum expansion (Tmax) is desirably from about 90° C. to about 140° C.
The choice of the particular microspheres and their respective Texp and Tmax is critical to the invention. While any particular grade of microspheres may be used in the present invention, the Texp and Tmax of the microspheres should be taken into account when formulating and activating in RF conditions. The RF radiation evaporates the water from the composition, but the temperature of the composition is limited to about less than about 100° C. Supercritical heating can occur during the RF heating if additives and/or salts are present in the composition, pushing the temperature up to above 100° C. However, the preferable Texp and Tmax of the expandable microspheres are less than about 100° C. and less than about 140° C., respectively. Microspheres with higher temperatures will not activate during the RF heating. Once the composition coalesces, the microspheres are substantially locked in place, making expansion thereof difficult, if not impossible. Surprisingly, the RF heating allows for a more robust expansion of the microspheres. The use of the RF activation allows for a self-limiting of microsphere expansion, and less explosion and deflation when compared to conventional convection heating.
In preferred embodiments, it is desirable that the expandable microspheres be present in the composition in an amount of from about 0.1% to about 70% by weight of the composition prior to setting of the composition, and more desirably from about 0.5% to about 60% by weight of the composition prior to setting of the composition, and most desirably at about 1% to about 50% by weight of the composition prior to setting of the composition. The expansion ratio of the expandable microspheres and the loading level of the microspheres will be related to each other.
Depending on the amount of the microspheres and the type of the polymer, the coalesced composition can have adhesive properties. High levels of micropsheres will lead to lower or no adhesive property, whereas low levels, about less than about 30 wt % based on the total weight of the composition, will lead to adhesive property of the composition.
Depending on the fully expanded size of the microspheres, the amount of the expandable microspheres in the composition can be adjusted. Depending upon the particular expandable microspheres used in the composition, the desired amount of the microspheres in the composition may be modified.
The microspheres further increase the structural integrity of the coalesced composition after they are expanded. While introducing voids in a matrix typically decrease mechanical integrity, the microspheres in the polymeric matrix provide stiffness when applied onto substrates. This is particularly useful for packaging fragile contents.
In another embodiment, the microsphere may be pre-expanded. If pre-expanded microspheres are added in the composition, the pre-expanded microspheres should be chosen so the RF heating will not start to decompose the microspheres. Yet in another embodiment, the microspheres may be a mixture of pre-expanded and expandable microspheres.
The composition optionally further includes any plasticizers, tackifiers, humectants, accelerators, fillers, pigments, dyes, stabilizers, rheology modifiers, polyvinyl alcohols, preservatives, e.g., antioxidant, biocide; and mixtures thereof. These components can be included in an amount of from about 0.05% to about 15% by weight of the composition.
Exemplary plasticizers are dibenzoates available as BENZOFLEX®, such as diethylene glycol dibenzoate, dipropylene glycol dibenzoate, and the like.
The accelerator is a multivalent cation from water-soluble salts, including commonly available aluminum nitrate (Al(NO3) 3), zirconium acetate, ammonium zirconyl carbonate (available as Bacote 20 from Zirconium Chemicals). The addition of the multivalent water-soluble salt shortens the time required for radiation during the expansion of the composition. When added, from about 0.05 to about 1, preferably from about 0.1 to 0.3 wt % based on the total weight of the composition, may be used.
Exemplary preservatives include 1,2-benzisothiazolin-3-one, 5-chloro-2-methyl-4-isothiazolin-3-one and 2-Methyl-4-isothiazolin-3-one. Typically, preservatives may be used in an amount of from about 0.05% to about 0.5% by weight of the composition prior to setting of the composition.
Exemplary fillers include pearl starch, physically modified starch, and chemically modified starch.
Other materials that do not adversely affect the composition and insulating properties of the composition may be used as desired. Other additives and/or salts may be included in the composition to increase the coalescence of the composition, is desired.
Although the composition can start to coalesce at room temperature, the high moisture content and will be substantially fluid until the moisture content is about 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1 wt %, based on the total weight. Preferably, the moisture content should be maintained at levels greater than about 20 wt % to prevent premature coalescence. Premature coalescence can lead to uneven drying, leading to uneven thickness of the dried composition on substrates.
RF heating also allows for fast throughput of making articles. The composition must be designed to accommodate this fast throughput method to maximize solid contents in the composition. It is preferable that the water molecules to be efficiently driven off without leaving unsightly wrinkles or unevenness on the substrates.
Surprisingly, RF heating provides a uniform and even activation of microspheres and faster egress of water from the composition, at a faster throughput. The uniform and evenness of the coalesced coating provides uniform thermal insulation to the article and minimizes unsightly wrinkles on the substrates, while increasing the production.
Another embodiment is directed to a process for forming an article comprising the steps of:
The articles are suitable as protective packages, shipping packages, impact resistant packages, and insulative packages. The packages include cups, food containers, cases, cartons, bags, lids, boxes, envelopes, shipping bags, wraps, clamshells, and the like.
The substrates include fibreboards, chipboards, corrugated boards, corrugated mediums, solid bleached boards (SBB), solid bleached sulphite boards (SBS), solid unbleached board (SLB), white lined chipboards (WLC), kraft papers, kraft boards, coated papers and binder boards.
The composition may be applied to the first substrate in any configuration desired, including in a series of dots, stripes, waves, checkerboard patterns, any general polyhedron shapes that have substantially flat bases, and combinations thereof. Further, the composition may be applied to the first surface in a series of cylinders. In addition, if desired, the composition may be applied to the first surface as a substantially flat sheet, covering the entire first surface (full lamination) or covering a portion of the first surface. A second substrate applied to the top surface of the composition, forming a sandwiched configuration of: first substrate—composition with expandable microspheres—second substrate.
Yet in another embodiment, the insulated article comprises a substantially flat substrate and a non-flat, rounded substrate. The composition is applied either to the substantially flat substrate, the non-flat substrate, or to both substrates, to form the insulated article. The composition may be applied to completely coat the surface of the substrate(s) or to selectively coat portions of the surface of the substrate(s). The pattern can be random or various ordered designs. The resulting article thus has an insulating space between the liner surfaces. The articles with patterned composition mimic a divider interposed between the two substrates. The space between the two substrates are generated and maintained by the expanded microspheres.
Optionally, a different adhesive may be applied in between the two substrates. This is especially useful to bind the two substrates together if the composition has low or no adhesive properties. The different adhesive may be applied before, concurrently or after the composition is applied onto the first substrate. In another embodiment, the different adhesive may be applied on the second substrate, and the two substrates are joined together with the composition, and the different adhesive sandwiched between the two substrates. Exemplary different adhesive includes hot melt adhesive, pressure sensitive adhesive, waterborne adhesive, and solvent-based adhesives.
The wet composition is applied in between the two substrates to form an article, and then exposed to dielectric heating to coalesce the composition and to expand the microspheres. The heating therefore locks in the components, including the plurality of expanded microspheres, in place to the surface of the substrates. The dielectric heating creates an alternating electric field between the polar water molecule and causes rapid heating. With the presence of water, the temperature rises to 100° C., and the water is evaporated away while the microspheres expand. The microsphere grades with a Texp of about 80° C. to about 100° C. and Tmax of about 90° C. to about 140° C. expands with the dielectric heating.
The use of dielectric heating, particularly RF heating, allows rapid processing. Therefore, the RF process allows for fast throughput, and smaller area is warranted than convection heating process. Moreover, bulkier and odd-shaped packages can be manufactured with RF processing since the depth of penetration is higher and the oven aperture is more flexible than microwave ovens.
Multilayer substrate packages formed with the composition containing microspheres improve the ability of the package to withstand strain under a constant stress at elevated and/or reduced temperatures. It is expected to those skilled in the art that the strain of the composition increases with the addition of microspheres at elevated temperature.
The present invention may be better understood through analysis of the following examples, which are non-limiting and are intended only to help explain the invention.
The compositions were prepared with the following components. Each resin emulsion was used to make the following compositions.
Each composition was made by mixing the components in a vessel.
Composition 4 with Resin Emulsion 3 composition, in its wet state, was applied on a paper substrates (20 and 24 lb paper, Golden Kraft) in a series of dot pattern. Each substrate was activated with convention heating, microwave or RF dielectric heating (40 MHz and 55-62 Amps). The initial height of the coating and the final height after activation were recorded in Table 3. Percent wetness immediately after activation was visually measured by calculating the amount the non-expanded segments of the coatings.
Activation with RF had superior percent increase in microsphere expansion and evenness than the other two heating methods. Also, RF heating provided much faster drying, as the percent wetness was significantly lower than convection and microwave heating.
The photographs the activated composition on paper are shown in
To understand the speed throughput, a package was formed by coating Composition 4 (Resin Emulsion 3) on a first substrate and a second substrate was placed atop of the first substrate. The package was then left outside in ambient temperature (ranging from about 72° F. to about 90° F.) for less than 1, 2 or 5 minutes before activating with RF at 40 MHz and 55-62 Amps.
The package appearance on the reverse side of the coated substrate are shown in
While high solids content allows for faster throughput of manufacturing, the surface of the high solids content composition can form a skin on the surface of the composition preventing egress of moisture. To minimize wrinkles and uneven foaming, the composition should be activated with RF under 1 minute. A fast throughput process comprising coating and activating the coating in under 1 minute allows for maximum benefit.
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