Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
2000-01-24
2002-08-06
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S140000, C241S021000, C241S015000, C427S222000
Reexamination Certificate
active
06428733
ABSTRACT:
FIELD OF INVENTION
The present invention concerns rotational molding. More particularly, the invention concerns a method of using a volatile liquid color system to prepare colorant-coated polymeric resin powders for use in rotational molding.
BACKGROUND
Rotational molding is used to produce hollow containers and other products of larger size and complicated structure which cannot be readily molded by other conventional molding techniques, such as injection molding and sheet thermoforming. Typical products produced by rotational molding include, for example, gasoline tanks, casks, storage tanks and toys.
Rotational molding includes the steps of feeding a polymeric resin powder into a mold, heating the mold from the exterior while rotating the mold thereby melting the powder and forming a molten polymeric layer within the mold, then cooling the mold to solidify the polymeric layer, and finally separating the molded product from the mold. An example of a prior art machine for performing rotational molding may be found in Friesen U.S. Pat. No. 4,738,815. An example of a prior art polymeric resin powder for use in rotational molding may be found in Inoue et al. U.S. Pat. No. 4,587,318.
As discussed in the Inoue et al. '318 patent, polymeric resin powders used in rotational molding can be mixed with pigments so as to impart color to the molded product. Conventionally, such pigments have been added to the polymeric resin powders in the form of dry powders or solids. Unfortunately, these dry pigment powders or solids often present handling and mixing problems. Specifically, dry pigment powders or solids may form undesirable dust or they may unevenly mix with the polymeric resin powders resulting in non-uniform color distribution in the molded product.
In the past, attempts have been made to use conventional liquid color systems in rotational molding. However, use of such conventional liquid color systems has not been particularly successful. Conventional liquid color systems have a tendency not to mix uniformly with polymeric resin powders. Another problem is in the formation of lumps or other unwanted agglomerates. During rotational molding, residual volatile components in conventional liquid color systems can evolve as gases which can lead to the formation of pin holes and other surface imperfections in the rotationally molded product. Additionally, conventional liquid color systems have a tendency to exude from the rotationally molded product as the polymer resin powder fuses during molding, forming tacky films on the surface of the molded product.
DeFranco et al. U.S. Pat. No. 5,759,472 discloses a volatile liquid color system and a method of using the same which overcame many of the aforementioned problems associated with the use of dry pigment powders or solids and conventional liquid color systems in rotational molding. The method disclosed in the DeFranco et al. '472 patent involves providing a volatile liquid color system, providing a polymeric resin powder, mixing the liquid color system and the polymeric resin powder in a distributive mixer to produce a mixture (which is preferably then oven dried), and then feeding the mixture into a rotational molding machine to produce a colored rotationally molded product.
Although the method disclosed in the DeFranco et al. '472 patent effectively overcame many of the problems associated with the use of dry pigment powders or conventional liquid color systems in rotational molding, it added the additional steps of mixing the polymeric resin powder and volatile liquid color system together in a distributive mixer and drying the mixture prior to use in a rotational molding operation. A method that would eliminate these additional steps would greatly simplify the process.
SUMMARY OF THE INVENTION
The present invention provides a new and improved method of using a volatile liquid color system to prepare polymeric resin powders for use in rotational molding. According to the present invention, polymeric resin pellets or granules and a volatile liquid color system are simultaneously fed into a grinding mill used to reduce polymeric resin pellets or granules to a fine powder for use in rotational molding. During grinding, the volatile liquid color system uniformly mixes with and coats the surface of the newly formed polymeric resin powder particles. The increase in temperature caused by the grinding forces coupled with the high volume air flow through the grinder causes the volatile components of the volatile liquid color system to fully evaporate from the surface of the powder particles, leaving them completely dry and suitable for use in rotational molding without any additional process steps. Surprisingly, the method does not clog or contaminate the grinder or produce a colorant-coated polymeric resin powder having lumps or other unwanted agglomerates. Rotationally molded products produced according to the method of the present invention exhibit color uniformity at least as good as, and in many cases superior to, products produced according to the method disclosed in the DeFranco et al. '472 patent.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
DETAILED DESCRIPTION
The method of the present invention utilizes a volatile liquid color system. Throughout the specification and in the claims, the term “volatile liquid color system” should be understood to refer to any liquid color system having an upper working viscosity of less than about 10,000 centipoise that following exposure to the conditions encountered during polymeric resin grinding operation does not leave behind any residue which can volatilize at temperatures typically encountered during rotational molding operations or exude from a rotationally molded part during such molding operations. The preferred volatile liquid color system for use in the invention is disclosed in U.S. Pat. No. 5,759,472 to DeFranco et al., the specification of which is hereby incorporated by reference. However, it will be appreciated that other volatile liquid color systems, such as water-based liquid color dispersions and other non-water-based volatile liquid color systems, may be suitable for use in the invention provided they meet the criteria set forth herein.
Thus, in the preferred embodiment, the volatile liquid color system comprises from about 10% to about 75% by weight carrier, from about 0% to about 15% by weight water, from about 0.01% to about 10% by weight dispersant, from about 10% to about 80% by weight colorant, and from about 0% to about 10% by weight additives. More preferably, the volatile liquid color system comprises from about 15% to about 65% by weight carrier, from about 1% to about 14% by weight water, from about 0.02% to about 8% by weight dispersant, from about 15% to about 75% by weight colorant, and from about 0% to about 8% by weight additives.
In the preferred embodiment, the carrier comprises a material selected from the group consisting of propylene glycol, ethylene glycol, diethylene glycol, tri(ethylene glycol), glycerin and mixtures of two or more of these materials. A preferred carrier is propylene glycol. More preferably, the carrier is food grade propylene glycol such as is sold by the Avatar Corporation. Preferably, any water employed in the volatile liquid color system is distilled. More preferably, any water employed in the volatile liquid color system is deionized.
The dispersant used in the volatile liquid color system is dependent upon the type of colorant and carrier being used. The dispersant must be soluble in the carrier and must associate strongly with the particles of colorant as they are separated from each other during mixing, thereby preventing the particles from reattaching to each other during storage and use. Examples o
Ferro Corporation
Rankin, Hill Porter & Clark LLP
Shipsides Geoffrey P.
Silbaugh Jan H.
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