Heating – Work chamber having heating means – Combustion products heat work by contact
Reexamination Certificate
1996-09-17
2001-09-25
Lu, Jiping (Department: 3749)
Heating
Work chamber having heating means
Combustion products heat work by contact
C432S147000, C034S420000
Reexamination Certificate
active
06293788
ABSTRACT:
This invention is directed to a thermal processing unit used in the preparation of plastisol-based surface coverings. More particularly, the present invention relates to a thermal processing unit which utilizes a radiant heat transfer section followed by a convection heat transfer section for processing selectively foamed resilient plastisol-based surface covering products.
Resilient floor coverings generally incorporate a foamed or cellular layer, and most commonly a foamed PVC layer. Such products are particularly popular in the flooring industry because the “foamed” layer provides a very comfortable walking surface that can be easily incorporated into an attractive design. The foamable layer of such resilient flooring products is conventionally produced by coating onto a substrate or base material a foamable PVC plastisol.
In general, foamable plastisols incorporate a foaming agent that causes the plastisol to take on a cellular or foam structure when the plastisol is properly processed. The most common form of foaming agent is a blowing agent, which is a compound or a combination of compounds that decompose at an elevated temperature to evolve a gas which causes the plastisol to expand or “foam.” Examples of possible blowing agents which may be used in the practice of the present invention are provided in U.S. Pat. No. 3,293,094 to R. Frank Nairn et al, which is incorporated herein by reference. Blowing agents are often used in conjunction with an accelerator for the blowing agent, which is a material which lowers the temperature at which the blowing agent normally decomposes.
A conventional process for preparing chemically embossed resilient flooring utilizing a foamable plastisol is represented schematically in FIG.
1
. As illustrated in
FIG. 1
, a base material, such as compressed felt, referred to in the art as the “web”, is introduced into an apparatus which applies a foamable plastisol coating. The applied plastisol coating is usually a viscous fluid and is prone to retaining dirt or other material that contacts the coating. After the foamable plastisol is applied to the substrate, it is heated to a temperature at which the plastisol gels. In the conventional processes, however, the heating of the plastisol to form the plastisol gel must not result in blowing, otherwise the steps commonly utilized to produce selective foaming (e.g., embossing) will be ineffective, as explained below. To accomplish gellation, therefore, the flooring product is conveyed to a gellation apparatus which is capable of gelling the plastisol without initiating blowing.
This gellation step has generally been accomplished by the use of convection heating ovens which raise the temperature of the foamable plastisol coating to the plastisol's gel point.
Once the flooring product has undergone gellation, it is conveyed to an inhibitor printing apparatus. Here the surface of the gelled plastisol is selectively coated with an inhibitor. The inhibitor is frequently incorporated into an “inhibitor ink,” which is a pigmented or unpigmented liquid applied in a design to the gelled plastisol by printing or coating equipment. The inhibitor ink reduces the extent of foaming which takes place in those regions of the gelled plastisol underlying the surface areas in which the ink has been applied, as is well known in the art. In general, the inhibitor includes an agent which passes into the gelled plastisol and deactivates the accelerator so that when the composite is heated to a temperature above the gellation temperature to fuse the plastisol and decompose the blowing agent, those regions of the plastisol that are printed with the embossing composition are not expanded.
After gellation and printing, the flooring product is conventionally conveyed to an apparatus which applies a wearlayer coating, such as a non-foamable PVC plastisol. Following application of the wearlayer coating, the flooring product is conveyed to an apparatus capable of blowing or foaming the foamable plastisol. Fusion of the plastisol components also generally takes place in this unit. This blowing and fusion step has generally been accomplished by the use of convection ovens wherein the temperature of the flooring product is raised to a temperature in excess of the gellation temperature of the foamable plastisol causing the blowing agents present in the plastisol to be activated. When the blowing agents are activated they cause an expansion in the foamable plastisol such that regions of the plastisol which have not been printed with the inhibitor ink are elevated relative to the regions which have been printed with the inhibitor. The presence of the inhibitor thus results in an embossed appearance of the flooring product and the resulting flooring product is known in the art as a chemically embossed resilient flooring product.
Although the process represented in
FIG. 1
has produced desirable flooring products, there is a significant problem with this process in that during the gellation stage the flooring product can be ruined by premature blowing of the plastisol. If the plastisol blows prior to application of the inhibitor, the flooring product is unacceptable for further processing and must be scrapped. The primary cause of undesirable blowing at the gellation stage is a high temperature excursion in the oven used to cause gellation.
In addition to premature blowing, the use of convective heating alone to gel and fuse a flooring product has other undesirable characteristics which result in significant costs to the manufacturer. For example, ovens which utilize convective heating systems exhibit a rate of heat transfer which is undesirably low. This is because heat transfer in such units is determined, at least in part, by the impingement velocity of the air circulated in the oven and by the difference in temperature between the air and the temperature of the product. In the processing of foamable plastisols to cause gellation and fusion, the impingement velocity must be kept low to avoid disturbing the wet surface of the applied coating. This not only slows processing of the product, it results in ovens that are very large and expensive. Convective ovens of the type heretofore used may be 180 feet long or longer. Accordingly, applicants have recognized that the wet state of the foamable gel has caused severe limitations in the rate of convective heat transfer available according to prior art gellation and fusion processes.
In addition to the above disadvantages, the impinging air used in the convection oven also tends to deposit residue such as condensed or carbonized plasticizer from previous processing runs onto the wet, plastisol coating. This can dramatically decrease production efficiency since contamination which is deposited on the plastisol often results in substandard flooring product that must be scrapped.
On the other hand, applicants have recognized that significant disadvantages are associated with the use of radiant heat transfer systems to gel and fuse foamable plastisols. For example, radiant heat transfer systems are difficult to precisely regulate due to the heat transfer rate's dependence on the fourth power of the temperature difference between the emitter and the target. For gray bodies, for example, the heat transfer rate (es: BTU/hr) is related to the temperature of the emitter and the target as follows:
Q
net
=C[(T
t
)
4
−(T
e
)
4
]
Q
net
=net heat transfer where C=constant for the bodies in the system
T
t
=temperature of the target
T
e
=temperature of the emitter
As an example, a change in the difference in temperature between the emitter and the target of 5% will result in an approximate heat transfer change of about 20%. Furthermore, for radiant heat provided by infrared (IR) sources, differences in infrared emission characteristics among the heat sources also causes control difficulties. Because radiant heat sources can be difficult to regulate and control, the product temperatures in these types of ovens can easily exceed the target gellat
Kist, III Rudolph
Novak Lawrence J.
Rohrbacher Peter J.
Congoleum Corporation
Lu Jiping
Synnestvedt & Lechner LLP
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