System for preparing polymer encapsulated glass fiber pellets

Details System for preparing polymer encapsulated glass fiber pellets System for preparing polymer encapsulated glass fiber pellets

2001-11-13

2003-12-09

Kelly, Cynthia H. (Department: 1774)

Plastic article or earthenware shaping or treating: apparatus

Tumbling type agglomerating means for particulate material

C065S483000

Reexamination Certificate

active

06659756

ABSTRACT:

The present invention relates to the manufacture of glass fiber pellets. In particular, the present invention provides an apparatus and process for making polymer coated glass fiber pellets by combining multiple segments of a chopped multi-fiber glass strand into pellets and encapsulating the pellets in a polymer shell. Such pellets provide a convenient form for the storage and handling of chopped glass fibers used as reinforcing materials in composite structures.
BACKGROUND OF THE INVENTION
Chopped glass fibers are commonly used as reinforcement materials in thermoplastic articles. Typically, such fibers are formed by drawing molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film-forming binder resins to the filaments, gathering the filaments into strands, chopping the fiber strands into segments of the desired length, and drying the sizing composition. These chopped strand segments are thereafter mixed with a polymeric resin, and the mixture supplied to a compression- or injection-molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped strands are mixed with pellets of a thermoplastic polymer, and the mixture supplied to an extruder wherein the resin is melted, the integrity of the glass fiber strands is destroyed and the fibers are dispersed throughout the molten resin, and the fiber/resin dispersion is formed into pellets. These pellets are then fed to the molding machine and formed into molded articles having a substantially homogeneous dispersion of the glass fibers throughout.
Unfortunately, however, chopped glass fibers made via such processes are typically bulky and do not flow well. Consequently, such fibers are difficult to handle and have been problematic in automated processing equipment.
One attempt at solving this problem has been to compact the chopped strands into denser rod-shaped bundles or pellets to improve the flowability of the chopped strands, and enable the use of automated equipment to weigh and transport the glass fibers for mixing with the thermoplastic resins. Such a process is disclosed in U.S. Pat. No. 4,840,755, wherein wet chopped strands are rolled, preferably on a vibrating carrier, to round the strands and compact them into denser, cylindrically shaped pellets. While such methods and apparatus tend to provide denser, more cylindrically shaped pellets exhibiting better flowability, they are undesirably limited in certain respects.
For example, the pellet size and fiber content are generally limited by the size and number of fibers in the chopped strand, in that the process is designed to avoid multiple chopped strand segments from adhering together to form pellets containing more fibers than are present in a single chopped strand. Consequently, to obtain pellets having a suitable bulk density and a sufficient ratio of diameter to length to exhibit good flowability, the strand from which the segments are chopped usually must be formed of a large number of filaments. However, increasing the number of filaments required to be formed and combined into a single strand undesirably complicates the forming operation.
In an attempt to overcome these shortcomings, U.S. Pat. No. 5,578,535 discloses glass fiber pellets that are from about 20 to 30 percent denser than the individual glass strands from which they are made, and from about 5 to 15 times larger in diameter. These pellets are prepared by hydrating cut strand segments to a level sufficient to prevent filamentization but insufficient to cause the strand segments to agglomerate into a clump, and mixing the hydrated strand segments for a time sufficient to form pellets. Suitable mixing includes a process that will keep the fibers moving over and around one another, such as tumbling, agitating, blending, commingling, stirring and intermingling.
Although the disclosed pellets can be made by such diverse mixing processes, it has been discovered that many of such processes are either too inefficient to be used commercially, or cannot be adequately controlled to produce a uniform pellet product that provides the resulting composite article with strength characteristics comparable to those made from nonpelleted chopped strand fibers. For example, the use of a modified disk pelletizer frequently results in excessive residence time of the formed pellets within the mixer, which in turn results in degradation of the pellets due to the abrasive nature of glass fiber pellets rubbing against one another. Such pellet degradation ultimately reduces the strength characteristics of the molded articles made therewith.
However, an efficient pellet-forming process and apparatus that controllably yields a uniform glass fiber pellet product that provides strength characteristics equal to nonpelleted chopped strand fibers in composite molded articles has previously been disclosed in U.S. patent application Ser. Nos. 08/975,729 and 08/831,129. In such apparatus and process, glass fiber strands comprised of a multiplicity of substantially continuous glass fibers are chopped into segments of the desired length and hydrated to a moisture content sufficient to cause the strand segments to coalesce into pellets upon tumbling. Thereafter, the strand segments are subjected to a first tumbling action to distribute the hydrating solution substantially uniformly over the strand segments and to cause the strand segments to combine to form pellets. The density of the pellets is then increased by compacting the pellets by a second tumbling action. This process can be performed by an apparatus comprising: (a) means for cutting the glass fiber strands to form chopped strand segments; (b) means for conveying the chopped strand segments to a first tumbling means; (c) means for applying a hydrating solution to the chopped strand segments; (d) a first tumbling means for imparting a tumbling action to the chopped strand segments to disperse the hydrating solution and cause the chopped strand segments to align and coalesce into pellets; (e) means for conveying the pellets to a second tumbling means; (f) a second tumbling means for tumbling the pellets to compact them and increase their density; (g) means for conveying the densified pellets to a dryer; and (h) a drying means adapted to receive and dry the pellets.
While such a process and apparatus provide numerous advantages in the preparation of chopped glass fiber pellets for use as reinforcement in molded polymer compositions, such pellets may still experience degradation during processing, storage and handling prior to compounding. Such degradation may result in pellets breaking open prematurely, resulting in the release of filaments or fuzz that can accumulate and block or impede the flow of pellets through conveyors or processing equipment. Moreover, such degradation may result in actual breakage of fibers thereby causing a reduction in the average length of the fibers in the composite article, and a consequent reduction in the physical properties of the composite article.
Accordingly, a need remains for a means of imparting greater impact resistance and toughness to the resulting pellets to reduce the degradation such pellets experience during storage and handling prior to compounding and molding. Such a need is fulfilled by the invention described in detail below.
SUMMARY OF THE INVENTION
According to the present invention, the exterior surface of the glass fiber pellets are coated with a polymeric binder composition, which, upon setting, hardening or curing (hereinafter referred to collectively as “curing”), imparts increased structural integrity and toughness to the resulting pellets. The substantial encapsulation of the pellets in the cured binder improves the ability of the pellets to be stored and transported with reduced pellet degradation. Additionally, the presence of the binder coating on the pellet surface allows the pellets to travel further through the extruder during compounding before dispersion of the bundle occurs. This typically results in

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