Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Physical dimension specified
Reexamination Certificate
2001-02-01
2003-03-18
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Physical dimension specified
C428S215000, C428S216000, C428S332000, C428S334000, C428S335000, C428S461000, C428S463000, C428S512000, C428S513000, C428S514000, C428S516000, C428S520000
Reexamination Certificate
active
06534171
ABSTRACT:
DESCRIPTION OF THE INVENTION
The present invention relates to a heat sealable, multi-layer laminating film composed of at least six layers of substantially thermoplastic polymers with an inner layer of ethylene/vinyl alcohol copolymer (EVOH), and to a packaging material containing said laminating film as a heat seal coating.
The invention also covers packaging materials which contain such a laminating film as a heat seal coating. The invention also provides the use of the laminating film and of the packaging materials containing them for the production of packagings, and the use of said packagings for packing foods and other articles.
Many articles require protection from the surrounding air or the surrounding atmosphere for transport and/or storage. They must be packed, therefore, in a manner which is suitable for excluding as completely as possible all or certain constituents of the surrounding atmosphere. A particularly critical constituent of the surrounding atmosphere is oxygen. The presence of oxygen may, in many packed products, such as, for example, foods, lead to oxidative spoilage or growth of bacteria which likewise spoil the packed product.
In many cases, such packagings are produced by heat sealing one or more conventional packaging materials to a packaging, for example, a bag or a lidded container. In so doing, the packaging material is firmly sealed by fusing the heat seal coating, in the example of the bag, against itself or, in the case of a lidded packaging, against the heat seal coating of another packaging material. A heat seal seam sealing the packaging is produced.
In many cases, the heat seal seam must meet very stringent requirements. The heat seal seam must be able to absorb high mechanical stresses caused by the contents of the package, transport or other influences. In many instances this applies immediately after sealing when the heat seal seam is still hot. For example, in so-called vertical form-fill seal machines, the contents are introduced into the formed tube when the transverse sealing instrument is still closed, for time reasons. When the sealing jaws are opened, the entire weight of the contents presses on the not yet cooled heat seal seam and thereby exposes this to a tensile stress. In other cases, even without an external stress, a similar type of load on the still hot heat seal seam may occur solely due to the resilience of the packaging material, for example, when sealing a fold or in the envelope region of the packaging material. If the cohesion of the heat seal seam is too low, the seam will lift in such cases and the packaging will thus fail. The seam strength in the hot state immediately after sealing is usually called hot tack. High hot tack is an important prerequisite for high packing speeds in cases of the kind described above.
A method of measurement for the property of hot tack is given in connection with the examples according to the invention. In qualitative terms, the measurement is carried out by bringing two sections of packaging material into contact with a defined pressure at a certain temperature and for a given time and then, whilst the heat seal seam is still hot, pulled apart. Whether and to what extent a time lag is permitted between releasing the sealing instrument and applying the force is particularly important here. It is possible to measure the maximum force which can be absorbed by the packaging material when pulled apart, or the resulting deformation of the heat seal seam for a given force. In principle, a distinction should be made between the hot tack force as the maximum force which can be absorbed, without failure, by the heat seal seam after sealing, and the hot tack temperature range as the temperature range in which a minimum level of hot tack force can be achieved under otherwise given sealing conditions.
In addition, the heat seal seam in the cooled state must form a strong seal, i.e. it must be able to absorb, in this state, high stresses to which the packaging may be exposed during transport or storage, without mechanical failure. It should be possible for a strong heat seal seam of this kind to be obtained with the shortest possible sealing times as an additional condition for an efficient, i.e. rapid and fault-free packaging process.
If voids, known as channels, occur between the inside and outside of the packaging, the surrounding atmosphere will penetrate and, under certain circumstances, the contents will be damaged. The sealing of folds is particularly critical here. The packaging may thus contain folds extending into the heat seal seam, intentionally in the form of a longitudinal fold at the side of a bag or at the intersection of the longitudinal and transverse seam, or unintentionally in the event of poor control in the machine. For tight sealing of such a fold, the heat seal coating must be rapidly and durably formable during the sealing process in order to fill in the region in and around the fold completely so that continuous contact between the inner surfaces of the heat seal coating(s) occurs along the heat seal seam even in the fold region. If the sealing times required to achieve a tight seal are too long, the processing speed of the film on the packaging machine will in turn be reduced.
The partial steps taking place during a sealing process are described by way of example in
Meka and Stehling, Heat Sealing of Semicrystalline Polymer Films. II. Effect of Melting Distribution on Heat-Sealing Behavior of Polyolefins, Journal of Applied Polymer Science
, Vol. 51, pgs. 105-119 (1994). Initially, wetting in the heat seal seam takes place due to melting and pressure. In the further course of the operation, chain segments of the polymers diffuse from both sides of the heat seal seam into the opposite side in each case and thus create molecular entanglements over and beyond the seam. After the sealing instrument has been removed, the seal cools and, with partially crystalline materials, crystals also develop over and beyond the seam. If the heat seal seam is mechanically stressed immediately after the sealing instrument is removed, the polymers of the heat seal seam must therefore exhibit a sufficiently high melt elasticity to be able to absorb this stress.
The short sealing times mentioned above may be achieved in particular by the fact that the required properties such as high seal strength, high hot tack and hermetic sealing are achieved at low temperatures. In this way, for a given temperature of the sealing instrument—in many cases this is limited by the heat resistance of the support or substrate of the packaging material—a temperature high enough to fulfil the requirements mentioned can be achieved in the heat seal seam in a relatively short time.
The following terms, methods of measurement and definitions apply to all further explanations:
Abbreviations for plastics according to DIN 7728 and ISO 1043-1987 (E) are used for the description of the polymers contained in the individual layers, unless otherwise identified.
In multi-layer structures, the layer sequence is reproduced by stringing together the abbreviations of the polymers of the corresponding layers or symbols explained in another way, separated from one another by vertical lines. The side of the heat seal coating is always on the right. Moreover, only a part of the whole sequence of layers that makes up the structure may be indicated. In these cases, the side of the heat seal coating is likewise always on the right, and layers or combinations of layers not indicated are identified by three dots, . . . Polymers of the same kind may be distinguished from one another by numbering, for example, in the form PE-LD-1|PE-LD-2|PE-LD-3. Mixtures of different polymers are identified by the + sign and the summary of the components in brackets ( ). Optionally, additional details about the percentage composition may be provided here. In such cases, unless otherwise stated, these are always proportions by weight which are based on the total weight of the mixture. For example, the expression . . . |PA&verbar
Eggers Holger
Kaschel Gregor
Nakarani D. S.
Norris & McLaughlin & Marcus
Wolff Walsrode AG
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