Synthetic two-piece induction seal

The present invention comprises a two-piece induction seal for use in creating clean, safe, and secure inner seals on containers, comprising layers of synthetic foam, absorbing synthetic polymer, an induction membrane and a heat-sensitive seal.

FIELD OF THE INVENTION 
The present invention relates to a two-piece induction seal for use in 
creating clean, safe, and secure inner seals on containers. 
A variety of two-piece induction seals have been developed. These seals 
have application in the closure industry. The seal generally comprises a 
compressing agent (e.g., pulpboard) and a induction membrane layer (e.g., 
foil), with a wax layer between them to keep the them in place during 
processing. The membrane layer farther has an adhesive layer on its bottom 
surface which is generally a heat-activated adhesive layer. During bottle 
operations and the like, the seal is placed between the rim or other 
opening of the filled container and the cap. When energy is applied, the 
induction membrane layer becomes heated, thereby melting the wax and 
activating the adhesive. The result is the conversion of the one-piece 
unit into two pieces, with the adhesive layer bonding the membrane layer 
to the rim, and the melted wax being absorbed by the compressing agent. 
The compressing agent generally remains lodged in the inner portion of the 
cap or other closure device. 
In common application, the compressing agent is a pulpboard material. This 
organic material is suitable for absorbing the melted wax. However, this 
system presents numerous disadvantages. The pulpboard becomes a source of 
paper dust which can contaminate the contents of the container. The 
wax-filled pulpboard can also serve as a growth medium for bacteria and 
other biological contaminants. Additionally, the pulpboard is moisture 
sensitive and can become distorted and altered by fluctuations in humidity 
levels. 
Alternative seal structures have been developed to attempt to overcome 
these disadvantages. In one such alternative, the compressing agent is 
made of a synthetic foam material which is initially bound to a foil layer 
by a starch layer. Application of energy heats and transforms the starch 
layer, breaking the bond between the foam and the foil. The starch residue 
remaining on the synthetic foam can continue to serve as a bacterial 
growth medium. 
In another alternative, the foil layer is covered with a paper layer. A wax 
layer initially binds the compressing agent to the paper layer of the 
foil. When energy is applied to the unit, the wax melts and is absorbed by 
the paper on the foil layer, rather than being absorbed into the synthetic 
foam compressing agent. Eventually, this can cause the paper layer to seal 
to the synthetic foam. This alternative does not solve the traditional 
problems of contamination, because the paper layer on the foil can 
continue to serve as a biological growth medium. In addition, the paper 
layer can present structural issues by delaminating from the foil layer 
and by expanding and contracting due to changes in humidity. 
In still another alternative, the wax or starch layer is replaced by a 
pressure-sensitive adhesive. This adhesive effectively binds the 
compressing agent, be it pulpboard or synthetic foam, to the foil layer. 
The process of opening the cap imparts a shearing force which breaks that 
bond allowing the container to be opened. A principle disadvantage of this 
device is that the adhesive layer which is present on the surface of the 
compressing agent remains tacky. As a result, materials, such as pills or 
other contents of the container, dirt, and other debris, can become 
affixed to the inner surface of the cap. 
Unipac Corporation has developed a two-piece induction seal which uses as 
the compressing agent a synthetic foam material with a synthetic polymer 
underlayer made of TYVEK.RTM. from DuPont. This seal has been found to 
solve some of the problems described above, but the synthetic polymer does 
not present a uniform absorbing surface. As a result, wax residues remain 
on the surface of the TYVEK.RTM. layer after induction sealing causing 
variable behaviour. In some instances the TYVEK.RTM. layer melts, creating 
difficulty in opening the container. 
The present invention solves these problems in an efficient and 
cost-effective manner. Through the use of a compressing agent comprising a 
synthetic foam base and a wax-absorbing synthetic polymer, the resulting 
cap inner seal presents a clean, safe and structurally sound product. The 
microporous structure of the synthetic polymer substantially completely 
absorbs the wax and the synthetic structure of the polymer and the support 
foam do not create debris which could otherwise contaminate the contents 
of the container. 
SUMMARY OF THE INVENTION 
The present invention has a two-piece induction seal having (i) a 
compressing agent comprising an upper layer comprising a synthetic foam 
and a lower layer comprising a wax-absorbing synthetic material, (ii) wax, 
and (iii) an inductive innerseal membrane with a lower adhesive layer 
which membrane is detachably bound to the compressing agent by means of 
the intervening wax layer; wherein the wax-absorbent synthetic material is 
suitable for absorbing substantially all of the wax when the wax is in 
liquid form. Preferably, the wax-absorbing synthetic polymer is a 
microporous high density polyethylene (HDPE). The invention further 
includes containers which have such two-piece induction seals.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 schematically represents, in cross-section, one embodiment of the 
two-piece induction seal 10 of the present invention. The seal 10 
comprises a compressing agent 20, which is formed of an upper layer of 
synthetic foam 22 and a lower layer of synthetic polymer 24, a wax layer 
26 and an inductive membrane layer 28 with a lower adhesive layer 30. FIG. 
2 schematically represents a cross-section of the rim or opening 40 of a 
container to be sealed in combination with seal 10. 
The synthetic foam layer 22 of the compressing agent 20 may be formed of 
any suitable material. Preferably, the foam layer 22 is formed of a 
material with a suitable compression factor comparable to pulpboards of 
the type traditionally used in induction seals. Examples of such materials 
are coextruded low density polyethylene (LDPE), low density polyethylene 
(LDPE), polypropylene (PP) and polystyrene (PS); preferably polystyrene 
(PS). The dimensions of the compressing agent 20 as a whole and the foam 
layer 22 in particular will vary according to the application and the size 
of the opening of the container and size and construction of the closure 
being used. These dimensions may be readily determined by one skilled in 
the art. 
The synthetic polymer 24 used as the underlayer of the compressing agent 20 
is preferably formed of a material comprising a microporous high density 
polyethylene (HDPE). These microporous HDPE materials exhibit high 
absorbency levels; a critical factor in the present invention. A preferred 
microporous HDPE is TESLIN.RTM. HDPE from PPG Industries, Inc. This 
material has 60% voids of a microporous nature. The material selected for 
use as the synthetic polymer layer 24 should have a sufficient absorbency 
level and suitable pore volume and structure to absorb substantially all 
of the wax used in the seal. Given these parameters, selection of suitable 
materials and determination of appropriate dimensions of the synthetic 
polymer layer 24 is within the ability of one skilled in the art. 
Together, the synthetic foam layer and the synthetic polymer layer form the 
compressing agent. This compressing agent forms an atmospheric barrier 
which retards the migration of gases and moisture at least from outside to 
inside a sealed container, which gases and moisture may be harmful to the 
contents of the sealed container. Further, the compressing agent of the 
present invention is a stable material which does not chemically or 
structurally breakdown during the normal lifespan of the containers on 
which it is used. The substantially complete absorption of the wax 
comprising the wax layer by the polymer layer of the compressing agent 
avoids the variable behavior associated with TYVEK.RTM. which may render 
opening of the container difficult. As a result, the invention provides a 
clean and safe material which does not contaminate the container or its 
contents. 
The induction seal 10 comprises a wax layer 26 which serves to bind the 
compressing agent 20 to the membrane layer 28. The wax layer 26 may 
comprise any suitable wax material which will melt within the temperature 
range to which the induction seal 10 is to be subjected. In general, the 
application of energy to the induction seal 10 within the container heats 
the induction membrane layer 28 to a temperature in the range from about 
500 to about 700.degree. F.; preferably about 600 to about 650.degree. F. 
The wax layer 26 should be comprised of a material with a melting point 
less than or equal to the highest sustained temperature of the induction 
membrane 28 when that membrane is subjected to an energy source during the 
sealing process. In addition, the volume or thickness of the wax layer 26 
should be selected such that substantially all of the wax will melt during 
the manufacturing process. Preferably, the wax layer 26 has a thickness of 
0.5 to 0.7 mm; more preferably 0.5 to 0.6 mm. For example, the wax layer 
may comprise a blend of parafin and microcrystalline waxes. More 
particularly, the wax layer may comprise a blend of parafin wax and 
microcrystalline wax wherein the proportion of microcrystalline wax used 
in the wax layer is adjusted to provide the wax layer with the desired 
porosity. Alternatively, the wax layer may comprise microcrystalline wax 
modified with other polymeric additives to enhance its bonding properties. 
For instance, the wax layer may comprise microcrystalline wax modified 
with at least one of ethylene vinyl acetate and polyisobutylene. Given 
these parameters, selection of suitable materials and determination of 
appropriate dimensions of the wax layer 26 is within the ability of one 
skilled in art. 
The induction membrane layer 28 forms a seal over the rim or opening 40 of 
the container and comprises a material which will become heated by 
induction when exposed to an external energy source. The membrane layer 28 
further comprises an adhesive layer 30 on its bottom surface which affixes 
the membrane layer 28 to the rim or opening 40 of the container. In a 
preferred embodiment of the invention, the membrane layer 28 is comprised 
of a metallic foil 27, preferably aluminum foil. In one embodiment, the 
membrane layer 28 comprises aluminum foil with a thickness of 1 to 1.5 mm. 
The thickness of the membrane layer 28 for a given application may be 
determined by one skilled in the art based on the characteristics of the 
material used and the size and other characteristics of the opening and 
container being sealed. 
The adhesive layer 30 affixes the induction membrane layer 28 to the rim or 
opening 40 of the container. The adhesive layer 30 is applied to the 
surface of the membrane layer 28 opposite that which contacts the wax 
layer 26; as referred to herein as the bottom surface of the membrane 
layer 28. In a preferred embodiment of the invention, the adhesive layer 
30 is comprised of a heat-activated polymer, such that the heat of 
induction generated during the manufacturing process is sufficient to 
activate the adhesive and to affix the membrane layer 28 to the rim or 
opening 40. Suitable adhesives for use according to the invention include, 
but are not limited to, polyethylene, polypropylene, polyethylene 
terephthalate, ethylene vinyl acetate and polystyrene. 
The elements of the induction seal of the present invention are assembled 
to form a multilayer composite from top (cap end) to bottom (rim end) 
comprising, as described above, the compressing agent, the wax layer and 
the induction membrane layer with adhesive layer. By suitable means well 
known in the art, a piece of this multilayer composite induction seal is 
placed above the opening of a filled container, which is then generally 
covered with the cap or other closure for the container. The filled, 
capped container is then exposed to an external energy source. The energy 
is absorbed by the induction membrane layer which becomes heated, thereby 
melting the wax layer and activating (or, at least, not deactivating) the 
adhesive layer. The induction membrane layer becomes affixed to the rim or 
opening of the container, while the liquid wax is substantially entirely 
absorbed by the wax-absorbing synthetic polymer layer of the compressing 
agent. This manufacturing process is carried out by conventional means 
using techniques and equipment readily available in the industry. 
More specifically in a preferred embodiment of the invention, during the 
manufacturing process, the external energy is absorbed by the aluminum 
foil induction membrane layer which becomes heated, thereby melting the 
wax layer and activating the heat-activated adhesive layer. The aluminum 
foil layer becomes affixed to the rim of the container, while the liquid 
wax is substantially entirely absorbed by the TESLIN microporous HDPE 
layer of the compressing agent. 
FIG. 3 illustrates an alternative embodiment of the invention. Referring to 
the elements described above, the synthetic polymer layer 24 forms the 
upper surface of the induction membrane layer 28, rather than the lower 
layer of the compressing agent 20. The compressing agent 20 comprises a 
synthetic foam layer 22. When the seal is subjected to an external energy 
source and the wax is melted, the liquid wax is absorbed by the synthetic 
polymer layer of the induction membrane. This similarly removes 
substantially all of the wax layer material as a possible growth medium 
for bacteria and other organisms. To the extent that the membrane layer is 
ultimately pealed off the container rim and discarded during the opening 
process, this embodiment of the invention contemplates complete 
elimination of the wax-containing synthetic polymer. 
While this invention has been described with an emphasis upon a preferred 
embodiment, it will be obvious to those of ordinary skill in the art that 
variations in the preferred composition and method may be used and that it 
is intended that the invention may be practiced otherwise than as 
specifically described herein. Accordingly, this invention includes all 
modifications encompassed within the spirit and scope of the invention as 
defined by the following claims.