Field of the Invention
This invention relates to paperboard lids having a radiation cured
overprint varnish. Such structures of this type, generally, protect the graphics
which are printed upon the paperboard lid such that the graphics are not distorted
Description of the Related Art
Platen sealing is gaining popularity for attaching paperboard lids
to trays following product filling. The benefits of this method include more uniform
seals, simpler equipment operation, and the ability to seal through food contamination.
A major disadvantage with this method is that the heated platen can blemish the
outside surface of the lid which contains important sales graphics and, in some
cases, UPC symbols. The blemished or marred surface results in an unsatisfactory
package appearance and, in some cases, UPC symbols which cannot be scanned.
It is known, in coating systems, to make use of a varnish layer to
protect a printed surface. Exemplary of such prior art is U.S. Patent No. 4,170,681
('681) to J. R. Edwards et al., entitled "Method of Applying a Varnish Layer to
a Printed Surface and Product Made Thereby". While the '681 reference teaches the
use of a varnish layer to provide a smooth protective surface for the printed graphics,
the nature of the varnish layer is such that it is not heat resistant (mass stable)
to temperatures above 163°C (325°F) and also the varnish is not radiation cured.
Consequently, an advantageous overprint varnish would be one which is both heat
resistant and radiation cured.
U.S. 5228272A discloses a lid comprising a paperboard substrate bearing
on one surface a particulate mineral coating suitable as a printing surface, and
on the other side of the substrate a heat sealable barrier layer.
JP 06336091A describes a paper base sheet bearing on one surface a
pigment coating, and a radiation-cured layer thereon.
JP 55041917B, GB 2063767A and JP 06047873A disclose paper substrates
bearing printing thereon and covered by a protective radiation-cured layer.
It is apparent from the above that there exists a need in the art
for an overprint varnish which is capable of protecting the graphics printed upon
the lid, but which at the same time is heat resistant and radiation cured. It is
a purpose of this invention to fulfill this and other needs in the art in a manner
more apparent to the skilled artisan once given the following disclosure.
According to the present invention there is provided a composite lid
having an overprint varnish, wherein said composite lid is comprised of:
characterised in that said radiation-cured varnish is mass stable
above 163°C and said varnish prevents said layer of printed graphics from being
distorted during a pressure platen heat sealing of said composite lid to a vessel
and said varnish does not adhere to said platen during said heat sealing.
- a paperboard substrate having first and second sides;
- a layer of particulate minerals located exterior to said first side of said
- a layer of printed graphics located exterior to said coating of particulate
- a layer of a radiation-cured, heat resistant varnish located exterior to said
layer of printed graphics; and
- a layer of a heat sealable barrier material located exterior to said second
side of said substrate;
Also according to the present invention there is provided a method
of constructing a composite lid having an overprint varnish wherein said method
comprises the steps of:
characterised in that said varnish is mass stable above 163°C
and said varnish substantially prevents said layer of printed graphics from being
distorted during a pressure platen heat sealing of said lid to a vessel and said
varnish does not substantially adhere to said platen during said heat sealing.
- coating a first side of a paperboard substrate with a layer of particulate minerals;
- printing a layer of graphics substantially over said particulate mineral layer;
- coating said printed graphics layer with a layer of a heat resistant, radiation-curable
varnish and curing said varnish layer by electron beam of ultraviolet radiation;
- coating said second side of said substrate with a heat sealable barrier layer;
In certain preferred embodiments, the particulate minerals are clay.
Also, the varnish coating is cured by either electron beam or ultraviolet radiation.
In another further preferred embodiment, the radiation-cured varnish
overcoat protects the printed graphics from distortion and/or marring.
The preferred lid, according to this invention, offers the following
advantages: lightness in weight; ease of assembly; good stability; good durability;
excellent printed graphics protection; and excellent economy. In fact, in many of
the preferred embodiments, these factors of ease of assembly, graphics protection,
and economy are optimized to the extent that is considerably higher than heretofore
achieved in prior, known composite lids.
BRIEF DESCRIPTION OF THE DRAWING
The subject matter which is regarded as the invention is particularly
pointed out and distinctly claimed in the concluding portion of the specification.
The invention, however, may be best understood by reference to the following description
taken in conjunction with the accompanying drawing FIGURE which is a schematic illustration
of a composite lid with a cured overprint varnish, according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Cartons which are used for distributing, marketing, and, in some cases,
heating portions of prepared foods are fabricated with a paperboard structural substrate.
The basic carton design consists of a vessel to hold the product and a lid which
is at least partially sealed to peripheral flanges extending from the vessel sidewalls.
See, for example, commonly assigned U.S. Patent No. 5,356,070 ('070) to W. R. Rigby,
entitled "Partitioned Paperboard Food Tray."
The lid may also be integral with the vessel in some cases. See, for
example, commonly assigned U.S. Patent No. 5,228,272 ('272) to B. G. Calvert et
al., entitled "Product and Process for Heat Sealing a Paperboard Carton Having Polymer
Coating on One Side Only,"(which is incorporated in its entirety by reference).
The basic substrate material for the present invention is machine-made
paperboard which may range in thickness from 0.178 mm to 0.889 mm. The paperboard
substrate may be coated on one or both sides with a layer of particulate minerals
as mentioned in claim 1. The surface of the lid next to the food product must be
coated with a material which provides a barrier and is heat sealable to the vessel
flanges. Coatings which may provide this are, typically, extruded polymers such
as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE).
Finally, the exterior surface of the lid may be printed with sales graphics such
that these graphics are printed onto the particulate mineral layer located on the
With this background, and with reference to the FIGURE, there is illustrated
composite lid structure 2. Lid 2 includes in part, conventional paperboard substrate
4, conventional barrier coating 6, conventional particulate mineral layer 8, conventional
printed graphics layer 10, and radiation-cured overprint varnish layer 12.
As discussed earlier, barrier coating 6 must provide a barrier and
be heat sealable to the flanges of the vessel (not shown). See, for example, Figures
1 and 3 of the above-identified ('070) and ('272) patents, respectively. Preferably,
the barrier is constructed of either PET, PP, or PE. Particulate mineral layer 8,
preferably, is constructed of clay. Finally, the graphics are printed upon particulate
mineral layer 8 by any conventional printing techniques.
It is imperative that during attachment or sealing of the lid to the
vessel that the sales graphics 10 should not be distorted or marred. In order to
avoid this, radiation-cured overprint varnish layer 12 is placed over printed graphics
layer 10 such that layer 12 does not stick to the heated platen and remains mass
stable above 163°C.
Two processes, namely, Electron Beam (EB) and Ultraviolet Curing (UV),
may be used to produce the radiation-cured overprint varnish layer 12. In the EB
process, a coating (having 100% solids) is applied to the substrate in a liquid
state following printing of sales graphic layer 10. The coating is then exposed
to highly accelerated electrons and reaction occurs in which the chemical bonds
in the coating are broken and a new-modified overprint varnish layer 12 is formed.
This process is referred to as polymerization. The polymerization causes significant
physical changes in the product being treated and results in many desirable characteristics
such as heat and scuff resistance.
The UV process is similar in that the coating (having 100% solids)
is applied in a liquid state over the printed graphics layer 10. The coating is
then exposed to a UV light source. Photochemical initiators in the coating form
free radicals. The free radicals initiate the cross linking of monomers and oligomers,
which result in a rapid curing of the overprint varnish layer 12. As with the EB
process, a durable overprint varnish layer 12 suitable for platen heat sealing systems
With respect to the use of a radiation-cured varnish, the information
below clearly indicates the superior performance of the coatings of the present
invention (RAD.) as opposed to the conventional coatings (STD.).
The temperature range covered is typical for the applications previously
described. The sealing pressure can be much greater. A higher pressure would intensify
the problem indicated with the standard coating. The dwell times cover a typical
range depending upon the polymer being sealed and the board caliper (thickness).
Sentinel Bar 25.4 mm Sealer
Position of Sample
Printed surface next to heated bar
STD. = Standard water-based ink and varnish
RAD. = Radiation-cured ink and varnish
1 and 3 seconds
1 - No indication of marring
2 - Slight dulling of surface
3 - Dulling of surface and slight ink picking
4 - Dulling of surface, ink picking, and slight discoloration
5 - Severe discoloration, ink picking, and dulling
1,0-Second Dwell Time
3.0-Second Dwell Time