NO138039B - EXPANSION BOLT. - Google Patents

Description

Wax mixtures, for coating cardboard, e.g. cardboard and cardboard items as packaging.

This invention relates to wax mixtures. Wax species are divided into different well-defined groups. These include paraffin waxes (normally produced from distillates of mineral lubricating oils), microcrystalline waxes (usually produced from lubricating oil residue fractions), soft waxes including isoparaffin and naphthenic waxes, usually obtained during the liberation of paraffin waxes from oil, as well as the so-called 'high melting waxes' which is normally produced by fractionation of microcrystalline wax. Each of these wax types has been shown to have distinctive physical properties that make them particularly suitable for specific applications. Thus, many of these waxes are used, either alone or in combination, to make cardboard packaging for foodstuffs waterproof and especially for the production of cardboard packaging for dairy products. Such waxes must have critical limiting properties due to the treatment to which the cartridges are subjected during their manufacture, storage and transport as well as the temperatures to which they are subjected during these handling steps. The problem is complicated, also because of the particular size of the container, as many smaller containers do not require such critical properties of the type of wax used to make them waterproof as the larger containers, e.g. cartons of 214 liters for milk. It is customary to use a combination of several types of wax with different properties, in order to achieve an optimal combination of properties, in terms of its melting point, its resistance to impact, its blocking point ("blocking point"), its resistance to peeling and other properties which will be discussed in more detail below.

Certain non-wax components are often added to wax mixtures, particularly to those that come into contact with foodstuffs. These especially include hydrocarbon polymers, e.g. polyethylene. It has been proposed to use polyethylene with average molecular weights in the range of approx. 1500 to approx. 20000.Certain combinations of polyethylenes have been shown to have certain advantages. The combinations comprise a mixture of the polyethylene with low and high molecular weight, in order to achieve an optimal combination of physical properties of the mixtures (U.S. patent no. 2,842,508). While these various additives and their combinations have been shown to have beneficial results, they have by no means been entirely free of unfortunate properties for food carton coatings.

One of these points of view is particularly clear when covering cartons for well over 2 liters of milk. Cardboard blanks for these were previously marked with a special mechanism at the folding edges and shaped into the cardboard, glued and dipped-coated with a wax mixture. It has been found that the wax found in the interior of the folding edges is prone to breaking off in the form of "cords". This is not avoided by the use of any particular polyethylene or a combination of low and high average molecular weight polyethylenes as evident in said U.S. Pat. patent no. 2 842 508. In addition, peeling of the wax coatings will continue, especially after the carton has been exposed to mechanical shocks that occur during movement and transport of the cold cartons containing dairy products, e.g. milk.

Another problem that has not been completely satisfactorily solved so far is the wax consumption. This is to be understood as the amount of a wax mixture that has been found to be necessary to coat the wax carton satisfactorily. As these cartons are used in large quantities for cheap products, it is important to provide a satisfactory cover with a minimal amount of the mixture for this purpose. The consumption is a consequence of the type of wax that is usually absorbed in the interior of the cardboard, of its viscosity and of its stiffening factors.

The present invention leads to dairy-cardboard mixtures containing polyethylenes with improved properties, in terms of impact resistance, resistance to low temperatures combined with a relatively low wax consumption.

According to the invention, the wax composition comprises a major proportion of a petroleum wax with a melting point in the range from 50 to 65° C, and two types of polyethylenes. One type has a polyethylene with a relatively low density between 0.860 and 0.910 and the other type is a polyethylene with a relatively high density (specific gravity), between 0.915 and 0.990 at 20°.

The petroleum wax which forms the basis for the wax mixture according to the invention can contain 40-60 wt. of a paraffin distillate wax with a melting point between 50 and 60° C, 5-20 wt. of a heavy paraffin wax distillate with a melting point between 60 and 80° C, further 10-20 wt. of a residual microcrystalline wax, and 25' to 40 parts by weight of a plastic isoparaffin naphthenic wax distillate having a melting point between 40 and 45°C.

The wax mixture can contain 0.25-5% by weight, preferably 0.35-1.5% by weight. of a polyethylene with a density between 0.860 and 0.910 at 20° C, and 0.05-0.75 wt. of a polyethylene with a density between 0.915 and 0.990 at 20° C. The densities at 20° C of the polyethylenes are determined by a hydrostatic method in air and kerosene.

The average molecular weights of each of the polyethylenes can lie preferably between 1000 and 12000 and more, in particular between 1500 and 6000.

It is preferable that the polyethylenes with high lightness have a cloud point above 75° C.

The composition according to the invention shows a significant difference between "coagulation point" (ASTM Test D 938) and "melting point" (ASTM Test D 87). The usual wax mixtures containing a single polyethylene show little (if any) difference between these two values. The difference suggests that the mixture "makes itself felt" earlier in the cardboard coating machine and therefore helps to reduce wax consumption.

The combination of high and low density polyethylenes in wax leads to mixtures with properties that cannot be achieved using either of these two separately. The greatest advantage achieved by the presence of the relatively lower density of the polyethylenes is the advantage of impact resistance at the temperatures required for food storage, while the essential advantage of the relatively high density of the polyethylenes is a reduction in wax consumption ( when care is taken to maintain good coating). Each of these factors is desirable and even essential for a satisfactory effect of wax mixtures, when it is used to coat nutritional dough cartons.

Another advantage of using the combination of the two types of polyethylene is the greater scope of wax types that can then be used for cartons for dairy products. If only a single type of polyethylene is used, it is necessary to produce the wax in such a way that a relatively low blocking point appears. With the combination this is not necessary.

The effects of the combination of the polyethylenes as explained turn out to be very independent of the average molecular weight of each of the two polyethylenes.

Table 1 gives typical examples of polyethylenes which are considered to have low densities.

Table 2 contains a number of relatively high density polyethylenes suitable for use in the mixtures according to this invention. The density of the polyethylene depends to a large extent on their method of manufacture, but can be strongly influenced by the polyethylene's finishing, e.g. by irradiation. Irradiation is also beneficial with regard to other properties of the polyethylenes that can be utilized, especially their improved modulus of elasticity, which in turn leads to better properties of the wax mixture in which the polyethylenes are incorporated. The use of relatively low density polyethylenes leads to an improvement in impact resistance. The use of low density polyethylenes to incorporate and enhance the desirable properties makes it possible, with the assistance of isoparaffin-naphthen-wax, to use harder, higher melting components than otherwise possible for the same quality, so that the blocking point of the mixture is higher. As also shown by the data contained in the working examples and listed below, the combination of the two types of polypropylene will also result in a significant reduction in consumption without loss in coating of the wax mixture, when it is used as coating material on cardboard pulp.

The invention broadly encompasses the modification of any petroleum wax that has high and low density polyethylenes. But it is particularly applicable in connection with dairy products, where the carton's physical properties satisfy a number of strict and interlocking requirements. Ordinary paraffin waxes have proven to be far too fragile when used alone at storage temperatures for dairy products. Their use, in unchanged form, as coating mixtures is completely unsatisfactory due to their fragility at low temperatures, as a very extensive peeling and dissolution, cracking, of the coating takes place. One of the previously used modifications of such mixtures was to combine a substantial amount of microcrystalline wax with paraffin wax. Microcrystalline waxes are obtained from the residual lubricating oil fractions and constitute only small amounts of normal paraffin waxes, but above approx. 80 percent naphthenic waxes with highly branched carbon chains and a relatively high molecular weight. These are distinguished by their poorly crystalline or micro-crystalline structure, in contrast to the highly crystalline nature of normal paraffin waxes' ordinary distillate. Although the resulting mixture constitutes an improved distillate of paraffin waxes for use as a cardboard coating, it lacks many of the properties, e.g. very good absorbency, which compositions for dairy product cartons should have. Moreover, the melt viscosity was relatively high, if no more than a modest amount of microcrystalline wax was present. Furthermore, the flexibility of the mixture was not present to the most desirable degree. Further improvements were made from there still during the liberation of crystalline wax species from oil. The soft waxes thus obtained comprise a mixture of isoparaffin and naphthenic waxes with a relatively high molecular weight and which are normally contaminated by the content of significant amounts of oil. Therefore, the soft waxes obtained by freeing distillates of paraffin waxes of oil must be freed of oil in order to be used in the present invention. They form wax mixtures with greatly increased flexibility at low temperatures and reduce viscosity in the molten state. In this way, they can be more easily used in standard carton making machines.

Furthermore, further improvements were invented in the combined properties of cardboard wax mixtures by increasing the addition of heavy distillate waxes. These constitute a special variety in themselves, as they not only have relatively high melting points, but also contain a smaller amount of normal paraffin wax species, at the same time as they mainly have a crystalline paraffin wax structure. They thus constitute such mixtures with an increased blocking point, that they only got improved cold flow properties and increased fracture resistance during quenching. It is this special combination of waxes that is especially added to polyethylenes with both high and low density and that gets special use.

The paraffin distillate waxes which form the main component in the present mixtures have the best melting points between 50° C and 60° C, preferably between 54° C and 60° C. They are normally obtained by the well-known operation of dewaxing the waxy distillates of such the lubricating oil raks ions that occur during the refining of petroleum. They include the main amounts of normal paraffin wax species with smaller amounts of abnormal paraffins, mainly isoparaffins and naphthenes. The heavy distillate waxes are obtained from the highest boiling fractions of butter-oil distillates and normally have melting points between 60° C and 80° C. Residual microcrystalline waxes only have smaller amounts of normal paraffins present in them and predominantly contain naphthenic waxes with highly branched carbon chains and with melting points in the range 54-71° C, usually between 60 and 66° C. The previously mentioned plastic waxes are obtained, as previously briefly explained, by freeing crude distillate waxes of oil to achieve a so-called "soft wax" containing up to approx. 30 wt. oil. This oil is removed by the usual operation for the disposal of oil and at a reasonably low temperature, so that the wax obtained is of a high isoparaffin and naphthenic nature and has a melting point in the region of 40

-45° C. Consequently, they have waxes that in total amount to more than approx. 90 wt. of the wax polyethylene mixtures present, the following preferred compositions are:

The wax polyethylene blends are so contaminated that they present little, if any, problems in their group. The types of wax (assuming that more than one type of petroleum wax is used) are simply combined by melting the two types of wax together and stirring the mixture. The polyethylenes are preferably incorporated by suspending them in a container or other perforated piece of apparatus and allowing the wax melt to circulate through it, until the polyethylenes are completely dispersed throughout the wax. This is preferably carried out at a temperature between 55° and 120° C. There is no noticeable advantage in dispersing the polyethylenes either with low or with high density in wax before the incorporation of the other polyethylene.

Due to some of the statements previously made, it was necessary to determine whether the favorable effects regarding impact resistance, blocking point and wax consumption were due to a combination of polyethylenes with different molecular weights, or whether it was in fact due to the combination of polyethylenes with high and low density. In order to investigate this question, the wax composition was changed with a combination of high and low density polyethylenes and with essentially the same average molecular weight, namely about 4000. The wax composition used for this purpose was as follows:

This mixture was changed by polyethylenes with high and low density and with the same average molecular weights and by combining these two polyethylenes, as can be seen in Table 3.

From the data listed in this table, it will be seen that the use of low density polyethylenes led to a wax mixture with good impact strength, but relatively high wax absorption of standard cardboard sheets or cardboard for dairy products. When the relatively high density polyethylene was used (Sample 4), the impact strength was sharply reduced, while the wax absorption was improved. However, when the two polyethylenes were combined into a single wax blend (Sample 5), impact strength was found to be excellent and wax absorption continued on a reduced basis.

The density of the low density polyethylene mentioned above was 0.880 at 20°C.

Having found that the beneficial effects on impact strength, wax consumption and wax absorption were independent of the molecular weight of the polyethylenes, further tests of combinations of polyethylenes with different molecular weights were carried out, and the same degree of improvement was achieved, both in impact strength and in wax consumption, as long as both high and low density polyethylenes were present in the wax composition.

In another comparative test, cartons were waxed in a commercial machine. It made use of the aforementioned mixture of the wax species. In one case, 1.2 percent low-density polyethylene was used (sample 3 in table 3). This assumed 25.4 kg of wax for a good coating of 1,000 cartons. However, if 2 percent of the polyethylene content was replaced with an equal amount of high density polyethylene (0.927), the resulting mixture gave good coating with only 32.1 kg of the mixture.

About. 21/, liter (half gallon) cartons were produced using the following: The same mixture of waxes as used in the samples according to Table 3, but changed with the addition of 1 percent low density polyethylene, and 0.25 percent .high density polyethylene (Sample 7 in Table 3). The wax mixture, including the polyethylenes, had a melting point in the region of 58° C and a blocking temperature of 41° C. Table 4 lists a number of important properties of this mixture as it was used on a standard milk carton, which contained approx. 5 per cent moisture, when it was made into cartons for 2i/> liter contents.

From this table it will be seen that the cartons produced from a mixture according to this invention showed excellent properties, namely those generally required by dairy carton manufacturers and consumers. The most significant data, as far as this invention is concerned, are the remarks regarding "loose cords", "loose shells" and wax consumption, the other properties such as amount of cracking, wrapping suitability, etc., are very desirable, but are not necessarily essential.

Claims (3)

1. Wax mixtures for coating cardboard, e.g. cardboard, and which contains a predominant amount of a mineral wax and a smaller amount of a polyethylene, characterized in that the polyethylene content of the mixture consists of two types of polyethylene, one type being a polyethylene with a relatively low density between 0.860 and 0.910 at 20° C, while the other type is polyethylene with a relatively high density between 0.915 and 0.990 at 20° C. 3. Wax mixture according to claim 1, characterized in that the relatively low-density polyethylene is present in proportions from 0.25 to 5% by weight. preferably from 0.35 to 1.5% by weight, while the relatively high density polyethylene is present in proportions from 0.05 to 0.75% by weight. 3. Wax mixture according to claim 1 or 2, characterized in that the relatively high density polyethylene has a cloud point above 75°C.