NO140471B - PROCEDURE FOR THE PREPARATION OF FORMED PACKAGING MATERIALS OF NITRIL COPY, FOR FOODS, BEVERAGES AND SIMILAR SUBSTANCES - Google Patents

Description

The invention relates to a method for production

of shaped packaging materials of nitrile copolymers. More particularly, the invention relates to a method for producing shaped packaging materials for food, beverages and other related substances, including medicines, from nitrile copolymers.

In recent years it has been discovered that certain nitrile polymer resins are particularly well suited for packaging purposes due to their excellent water and oxygen barrier properties. Such polymers are further described in i.a. US Patent Nos. 3,451,523,

3,615,710, 3,426,102 and British Patent 1,186,361. Quite

recently it has been discovered that although such nitrile polymers have excellent barrier properties, they may be unsuitable for certain packaging purposes as they contain traces of hydrogen cyanide (HCN) which may be extracted and flavor the contents of the package. The amount of HCN in such nitrile polymers will vary with the nitrile monomer, the total nitrile content of the polymer, the polymerization method used to produce the polymer, the processing of the polymer and its thermal background, and other factors. Furthermore, it has been shown that the amount of extractable HCN in nitrile polymers can be increased after the polymer has been subjected to forming operations whereby the polymer has been heated for softening and/or melting before it is formed into shaped particles.

The trace amount of extractable HCN present in such nitrile packaging materials is very low and presents no known health or safety concerns. In fact, the amount of extractable HCN in such nitrile polymers is lower than that found in many foodstuffs. In this regard, it should be noted that HCN

is a natural component in many foods, and many other foods,

e.g. cereals, cocoa, ham, bacon and sausages, which are smoked with HCN (hydrocyanic acid), are allowed to contain from 50 to 200 ppm HCN

[cf. Food Additive Regulations of the FDS (p.31, Part D, Sections 121, 1072), published in the Federal Register, December 23, 1965; 30 F.R. 15912 and Public Health Service Publication 956 "Drinking Water Standards" of 1962 for the U.S.A.].

However, as noted above, in certain nitrile packaging materials, HCN can in some cases be extracted and flavor the contents of the package. The problem with HCN extraction when it comes to influencing the taste must be taken into account in particular when packaging beverages that are in prolonged intimate contact with the container. For these packaging purposes, the probability of HCN extraction is very high.

There is therefore a need in this area for improved nitrile polymer molding compounds and packaging materials made from these, which are essentially free of extractable HCN. This need is satisfied by the present invention, which provides a method for the production of packaging materials which significantly reduces the amount of extractable HCN in nitrile packaging materials. More particularly, the method according to the invention is particularly well suited for the production of such packaging materials as bottles, cans, jars, etc., which are used in the packaging of beer, including carbonated beverages, where the probability of HCN extraction is very high.

The invention satisfies the above-mentioned need

by producing nitrile copolymer packaging materials that contain very small amounts of extractable HCN. Food or related substances, e.g. beverages, which are packaged in these materials, extract less than 50 parts per billion ( < 50 PPB) and preferably less than 20 PPB 20 PPB) of HCN.

From US patent 2 768 995 it is known to reduce the solubility of e.g. an acrylonitrile copolymer in that the polymer in a finely divided state and before forming into the desired shape is treated with aqueous formaldehyde. The copolymers in question, however, have a different composition to those used in the method according to the present invention, and the patent does not mention the use of (co)polymers as packaging material. Furthermore, the patent has no mention of reducing the extractable HCN content.

The nitrile copolymers used in the method

according to the present invention contains (A) 55-85% by weight

nitrile monomer units (calculated as acrylonitrile), based on the total copolymer weight, such nitrile monomer units comprising acrylonitrile or mixtures of acrylonitrile and methacrylonitrile where the methacrylonitrile is present in amounts of up to 16% by weight based on the total copolymer weight, and (B) 45-15 % by weight of at least one other copolymerized monomer, especially styrene or an acrylate. In the method, the copolymer is heated in a solvent-free state to a melt, and the copolymer melt is formed into a packaging material.

The method according to the invention is characterized in that a particulate nitrile copolymer is brought into intimate contact with formaldehyde by mixing with 0.005-2% by weight of formaldehyde, based on the copolymer, before or during heating to a molten state, adding formaldehyde or a formaldehyde releaser compound either dry or in water with subsequent drying, whereby a shaped packaging material is obtained with an extractable HCN content of less than 50 parts per billion. The extractable HCN content in the resulting packaging material is thereby reduced, which is determined by using a 3.0% acetic acid solution as extraction liquid and an extraction time of 3 days at 49°C.

It is preferred when carrying out the method according to the invention to use nitrile copolymers, as defined here, which have an extractable HCN content above 50 PPB and to reduce such an HCN content to below 50 PPB. More preferably, one would use nitrile copolymers which have an extractable HCN content in excess of 20 PPB and reduce such an HCN content to below 20 PPB. The extractable HCN content referred to above is the HCN extracted from the polymer after heating and forming the polymer separately, e.g. in the absence of a formaldehyde compound, using an extraction period: of 3 days and 49°C in accordance with the test methods to be described below.

The nitrile monomer units included in the nitrile copolymers used in the method according to the invention comprise acrylonitrile and possibly methacrylonitrile, in an amount of up to 16% by weight based on the total copolymer weight.

The preferred monomers which interpolymerize with the nitrile monomers include aromatic monomers, e.g. styrene and α-methylstyrene; lower α-olefins containing 2-6 carbon atoms, e.g. ethylene, propylene, butylene, isobutylene, etc.; acrylic acid and methacrylic acid and the corresponding acrylate and methacrylate esters containing 1-4 carbon atoms, e.g. methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate and the corresponding methacrylates; vinyl esters, e.g. vinyl acetate; alkyl vinyl ethers where the alkyl group contains from 1 to 4 carbon atoms, e.g. methyl vinyl ether, ethyl vinyl ether, etc., and mixtures of the foregoing.

Optionally, the packaging materials with a high nitrile content can contain from 0 to 25% by weight of a synthetic or natural rubber component, e.g. polybutadiene, isoprene, neoprene, nitrile rubber, acrylate rubber, natural rubber, acrylonitrile/butadiene copolymers, ethylene/propylene copolymers, chlorinated rubber types, etc., which are used for reinforcement or to make these packaging materials tough. This rubbery component can be incorporated into the polymeric packaging material by any of the methods well known to those skilled in the art, e.g. by direct polymerization of monomers, grafting of the nitrile monomer onto the rubbery backbone, polymixing of a rubber graft polymer with a matrix polymer, etc.

The nitrile polymers that are used for packaging purposes that require excellent oxygen and water vapor barrier properties in the packaging materials are such nitrile polymers that contain from 55 to 85% by weight, based on the total polymer weight, of an acrylonitrile and possibly methacrylonitrile monomer (where the weight% of methacrylonitrile is calculated as acrylonitrile ). When acrylonitrile is used as the sole nitrile monomer, the preferred range is from 60 to 83% by weight. The preferred comonomers are styrene and α-methylstyrene. Interpolymers can also be used, e.g. acrylonitrile/methacrylonitrile/styrene; acrylonitrile/styrene/methyl vinyl ether and acrylonitrile/styrene/ethyl vinyl ether.

The active ingredient for reducing the HCN content

in the nitrile polymer is a formaldehyde compound such as free or unreacted formaldehyde, homopolymers of formaldehyde, or other compounds which give formaldehyde when heated or processed or which otherwise behave as formaldehyde or formaldehyde donors in the presence of the nitrile polymer.

Examples of homopolymers of formaldehyde which are suitable

for use in connection with the invention, they include cyclic polymers, e.g. trioxane and tetraoxane as well as the linear polymers of formaldehyde comprising polyoxymethylene glycols and their derivatives. These polymers have the general formula HO(CH~0) H where n is an integer, at least 2. The upper

2 n ^

limit of n can be as high as 5000 for certain derivatives of the polyoxymethylene glycols. These polymers are further described in Walker, J. Frederic, Formaldehyde, 3rd edition, 2nd edition, Reinhold Publishing Corp., New York (1967).

Preferred linear homopolymers are those where n has

a value in the range from 8 to 300. Particularly preferred homopolymers are those where n has a value in the range from 8 to 100.

These latter polymers are marketed as paraformaldehyde.

The commercially available paraformaldehyde usually contains 90 to 99% formaldehyde, whereby a minimum of 95% formaldehyde, by weight, is preferred.

Formaldehyde compounds which yield formaldehyde include alkali metal formaldehyde bisulphites, alkali metal formaldehyde sulphoxylates, etc., e.g. sodium formaldehyde bisulfite, potassium formaldehyde bisulfite, sodium formaldehyde sulfoxylate, potassium formaldehyde sulfoxylate, etc.

Other compounds considered herein as formaldehyde compounds, which yield formaldehyde upon heating or processing or otherwise behave as formaldehyde or formaldehyde donors, include hexamethylenetetramine, 2-nitro-2-methyl-1,3-propanediol, tris(hydroxymethyl)nitromethane as well as as addition products of formaldehyde and at least one other compound,

and condensates produced from these. Examples of these addition products and condensates include phenol/formaldehyde; urea/formaldehyde condensates including dimethylolurea and its cyclic derivatives, e.g. ethylene urea/formaldehyde condensates and the uron resins; formaldehyde adducts of saccharides, e.g. such as are described in US-PS 3,705,146; triazine compounds, e.g. melamine/formaldehyde, benzoguanamine/formaldehyde, etc.; amide/formaldehyde condensates, e.g. paratoluenesulfonamide/formaldehyde, methylol-acetamide, methylolformamide and methylacrylarr.id.

Other suitable formaldehyde compounds which give formaldehyde or behave as formaldehyde or formaldehyde donors are well known to the person skilled in the art and will be apparent from reading

the above.

The preferred formaldehyde compounds are formaldehyde, paraformaldehyde, trioxane, tetraoxane, dimethyl urea and para-toluenesulfonamide/formaldehyde condensates.

The amount of formaldehyde compound used in connection with the invention is calculated to reduce the extractable HCN content in the polymer so that <50 PPB and preferably <20 PPB HCN is extracted from food or drink packaged in the container.

The amount of formaldehyde compound used must be sufficient to reduce the extractable HCN input

keep the nitrile packaging material below 50 PPB, preferably below 20 PPB, but insufficient to impart a formaldehyde taste or odor to the food or beverage packaged therein. In general, the amount of formaldehyde required to

to achieve this reduction of extractable HCN content, range from 0.005 to 2.0% by weight, based on the weight of the nitrile polymer. For most purposes, the amount of formaldehyde will be in the range from 0.01 to 1.0% by weight. When using a formaldehyde compound that gives formaldehyde or otherwise behaves as formaldehyde or a formaldehyde donor, the amount of such a formaldehyde compound is calculated so that it provides an amount of formaldehyde within the above-mentioned areas.

As indicated above, the improved packaging materials are produced by a method according to the invention which involves bringing the formaldehyde compound into intimate contact. tact. with the molten nitrile copolymer during the formation of the packaging material. The intimate mixture of formaldehyde compound and nitrile copolymer is achieved by adding the formaldehyde compound to the nitrile copolymer before or while it is heated to a molten state. The formaldehyde compound can be combined with the nitrile copolymer after the polymerization step, e.g. by adding it to the nitrile copolymer in the polymer recovery steps, e.g. during coagulation, stripping, washing, drying, etc., or by plunging the polymer into water containing the formaldehyde compound. Another method is to dry mix the nitrile polymer and the formaldehyde compound before the forming step used to form the copolymer. Another method involves injecting the formaldehyde compound, preferably free formaldehyde, into the molten polymer during the molding operation. Other methods for forming the intimate mixture will become apparent to the person skilled in the art upon reading the present description.

In general, the intimate mixtures are formed by mixing the nitrile copolymer in dry form with the formaldehyde compound, or by adding formaldehyde to water, after which drying is carried out.

Manufacture of polymeric packaging materials, e.g. films, sheets, containers, e.g. bottles, cans, jars, etc. are carried out in accordance with the invention by forming the nitrile copolymers in an essentially solvent-free state by methods which are well known in the field and which do not require further explanation here. These molding operations include both the heating step whereby the nitrile copolymer is heated to a molten state by subjecting it to temperatures in the range of 93 to 274°C to soften or melt or otherwise become suitable for molding, and any subsequent molding steps performed with such molten nitrile copolymer. The heating and forming steps can take place sequentially or simultaneously. Examples of forming operations used for the production of polymeric packaging materials include pelletizing, extrusion, blow molding, injection molding, blow molding, orientation, vacuum forming, thermoforming, etc.

In certain cases, the production of packaging materials may involve more than one forming operation where the nitrile copolymer is heated and formed. This occurs e.g. in the case of a polymer that is heated to a molten state, extruded and pelletized, after which the pellets are heated to a molten state and formed into a blow blank which is then formed into a bottle or other form of container. In such situations, the invention includes having the formaldehyde compound in intimate contact with the nitrile resin during at least one of the shaping operations whereby the polymer is heated to a molten state, i.e. to the point where it softens or melts, and then shaped.

If more than one heating step is used, the formaldehyde compound is preferably brought into intimate contact with the nitrile polymer before or during the first step where the polymer is heated to a molten state.

The actual measurement of the amount of HCN extracted by

the extracting liquid, is not considered to be part of this invention. In this regard, any method that makes it possible to demonstrate so! low HCN amounts such as 20 PPB are used for measuring the amount of HCN extracted. These methods, which are well known to those skilled in the art, include colorimetric measurements, polarographic measurements, gas chromatographic measurements and electrochemical measurements.

Tests to determine the amount of extractable HCN in packaging material made from nitrile polymers are carried out on o 296 cm 3 bottles for beverages. The extraction tests are carried out for 3 days at 49°C and for 7 days at 38°C. Experience has shown that the amount of HCN extracted from a sample will level off with time, which will be illustrated below. Consequently, longer extraction times than 3 to 7 days are not necessary. In many cases it is more appropriate to run HCN extraction tests on cast samples, e.g. tensile test bars or other suitable forms rather than on bottles or other packaging material. As illustrated below, there is good correlation between extraction tests performed on molten tensile test bars and extraction tests performed on cast (296 cm 3 ) bottles. The relationship

-between the surface area of the tensile test bars (approx. 64.52 cm 2)

and the volume of extraction fluid used (70 cm 3 ) in the examples given below is approximately equal to the ratio of the surface area of the inside of a 296 cm 3 bottle for

- 2

beverages (approx. 258.08 cm ) in contact with the extraction liquid (2 96 cm 3 ). The ratio between the surface area and the volume of extraction liquid (cm 2 /cm 3 ) is approx. 0.87 for the bottles with a volume of 296 cm<3 > and approx. 0.92 for the tensile test bars used in the examples below. In general, a ratio is surface area/volume

(cm 2 /cm 3) of extraction liquid in the range 0.84/1 to 0.97/1

adequate for the determination of extractable HCN for most packaging purposes.

The tests described below, which use a

3% acetic acid solution (an FDA food simulant for acidic foods) as extraction liquid, illustrates one of the difficult packaging purposes where there is a high probability of HCN extraction. In other less difficult areas of application, e.g. packaging of dry rice, oranges, bananas, aspirin, etc., such rigorous tests for the determination of extractable HCN may not be necessary. In any case, the expert in the field, when he reads the present description, can easily select samples and test conditions that will suit his particular needs.

The following examples are given to illustrate the invention.

All parts and

percentages of resin and formaldehyde compound are by weight unless otherwise specified. When convenient, acrylonitrile is abbreviated as AN and methacrylonitrile MAN, and unless otherwise specified, 3% acetic acid solution is used as the extraction liquid. The amount of formaldehyde compound used is expressed by weight based on the weight of the nitrile polymer. The PPB values for extracted HCN refer to nanograms of HCN per grams of extraction liquid.

Examples 1-4

10 bottles of 296 cm for beverages are produced by mixing a copolymer containing 70% by weight of acrylonitrile and 30% by weight of styrene produced by conventional aqueous polymerization methods with varying amounts of paraformaldehyde. The paraformaldehyde used is a commercially available material that corresponds to the formula stated on p. 5 of this description,

where n is an integer from 8 to 100. The mixture is extruded into pellets using extrusion temperatures in the range from 216 to 232°C. The pellets are then formed into blow blanks at temperatures in the range from 216 to 232°c and formed into the final bottles.

The sample bottles are rinsed with water, filled with 296 cm <3> of the various extraction solutions (water, ethanol and acetic acid), corked and heated for 3 days at 49 C. The bottles and their contents are then cooled to room temperature, and the amount of HCN, in PPB , which is extracted by the extraction liquid, is determined.

The results of these tests, which are performed in triplicate, are given in Table I below.

Control Example 1 illustrates that when no formaldehyde compound is added to the nitrile polymer, the amount of HCN extracted is in the range of 60 to 100 PPB, with 3% acetic acid being the strongest extraction fluid. Examples 2 to 4 illustrate that the use of small amounts of paraformaldehyde causes a significant reduction in the amount of extractable HCN.

Example 5

An acrylonitrile/styrene copolymer (70% AN) of the type used for the production of the bottles according to examples 1 to 4 is used for the production of cast tensile test bars shaped like a meat bone, with a surface area of approx. 64.52 cm 2 according to ASTM D-638-68 (Type 1) when casting the nitrile polymer at 218°C and 98,434 g/cm<2>.

The cast tensile test bars are rinsed clean with methanol, dried and placed in a test tube (25 x 300 mm). 70 cm<3 > of a 3% acetic acid solution (30 grams of glacial acetic acid in 970 grams of distilled water) is placed in the test tube containing the tensile test rod. The test tube is then sealed with a rubber seal lined with

■Reynolds Reynolon", and the test tube is placed in a drying cabinet at 49°c for 3 days. The test tube is then cooled to room temperature and the amount of HCN extracted into the solution is determined. The test results are reproduced below:

Samples

An overview of the above data further illustrates the effectiveness of paraformaldehyde in reducing the amount of extractable HCN in the tensile test bars. The test also illustrates the good correlation between the test methods that use bottles and those that use tensile test rods.

The color values indicate that the formaldehyde compound introduces a certain color formation in the cast samples. For most packaging purposes, however, this color formation is within acceptable limits and does not contribute anything to reduced use of the packaging material.

The following examples 6 to 11 illustrate the effect of

thermal history of the HCN content of a range of nitrile polymers prepared using conventional suspension polymerization methods whereby the addition of the styrene to the reaction vessel is delayed. The polymers tested do not contain any formaldehyde compound. The polymer used in examples 6 to 10 is a copolymer of acrylonitrile and styrene, while the polymer in example 11

is a terpolymer of acrylonitrile/methacrylonitrile/styrene (60/15/25%

in weight). The polymer beads are extruded into pellets using an extrusion temperature of 204°C and the pellets are cast into tensile test bars at 218°C and 98,434 g/cm<2> as explained above.

In each example, the total HCN content in the beads, pellets and tensile test rods is measured by dissolving 0.1 gram of polymer in 10 ml of dimethylsulfoxide and then determining the amount of HCN present. The results of these tests are set out below in Table II. HCN values are in PPM.

The data in Table II above indicate that the total amount of HCN in the particular polymers tested increases with the thermal processing of the polymer. The amount of HCN developed in nitrile polymers will depend on several factors, including the thermal stability of the polymer and the processing temperatures.

The following data illustrate the time/temperature HCN extraction relationship and that the amount of extractable HCN levels off with time. The extraction tests are carried out on tensile test bars which are produced using the methods given above. The test is carried out on a nitrile polymer containing 72% by weight of acrylonitrile and 28% by weight of styrene. The samples are control samples that do not contain any formaldehyde compounds. The results of these tests are listed in Table III below.

Referring to the above data, it should be apparent that at any given temperature, the amount of extractable HCN levels off with time. In other words, the amount of HCN extracted from a sample does not increase appreciably after a certain period of time. The above data also show that more HCN is extracted at elevated temperatures.

The following examples 12 to 24 are given to illustrate some of the variations in nitrile resins that can be used in carrying out the invention. The resins in dry particle form, dry-mixed with paraformaldehyde, are extruded into pellets using extrusion temperatures of approx. 204°C. The pellets are then cast into tensile test bars and tested for extractable HCN using an extraction time of 1 day at 38°C for Examples 12 to 22 and 3 days at 49°C for Examples 23 and 24.

The polymers used in examples 12 to 16 and 20 to 22 are rubber-modified polymers which are polyblends of a rigid matrix and rubber-modified polymer.

The polymer used in examples 12 to 14 is a poly mixture of approx. 85 parts poly (acrylonitrile/methyl acrylate)

(75/25% by weight) and approx. 15 parts of a conciliatory rubber punch-

solid modifier.

The polymer used in examples 15 and 16 is a poly mixture of approx. 75 parts poly (acrylonitrile/styrene) (70/30% by weight) and approx. 25 parts of a compatible rubber impact modifier prepared by grafting AN/styrene (62/38% by weight) onto butadiene/AN (93/7% by weight) rubber.

The polymers used in Examples 17 to 19 and 23 to 24 contain no impact modifier. The polymer used in Examples 17 to 19 is poly(acrylonitrile/methacrylonitrile/styrene) (59/16/25% by weight) and that used in Examples 23 and 24 is poly(acrylonitrile/styrene) (64/36% by weight weight). The test results are listed in Table IV below, where the term "resin composition" when used in reference to the rubber modified samples (Examples 12 to 16 and 20 to 22) refers to the composition of the rigid matrix.

The data in Table IV show that the invention is applicable to a wide range of nitrile resins including rubber-modified resins and terpolymers.

The following examples 25 to 33 illustrate the application

of cyclic homopolymers of formaldehyde in the practice of the invention. The nitrile resin used is an acrylonitrile/styrene copolymer containing 70% by weight of acrylonitrile and 30% styrene. The nitrile resin particles and formaldehyde polymer are mixed, extruded into pellets and formed into tensile test rods and tested for extractable HCN. Examples 25 to 27 use 3 days at 49°C extraction period while Examples 28 to 33 use 1 day at 38°C. The results of these tests are listed in Table V below.

The data in the above Table V shows the significant reduction in extractable HCN content achieved when using trioxane and tetraoxane.

Examples 34 to 39

Examples 34 to 39 show the use of formaldehyde condensates in the execution of the invention. The nitrile polymer is a copolymer of acrylonitrile/styrene containing 70% acrylonitrile. The nitrile polymer and formaldehyde compound are mixed, extruded into pellets and formed into tensile test rods. The HCN extraction data is obtained using the 1-day test at 38°C. The test results are listed in Table VI below.

The above results show the significant lowering of the amount of extractable HCN that is achieved using formaldehyde condensates or addition products as the formaldehyde component.

Examples 40 to 43

In these examples, the formaldehyde compound is mixed with an acrylonitrile/methacrylonitrile/styrene (59/16/25) terpolymer, extruded to form pellets and cast into tensile test bars and tested for extractable HCN using an extraction time of 3 days at 49°C . The results are listed in Table VII below.

The above results show the significant lowering of the amount of extractable HCN obtained when using sodium formaldehyde sulfoxylate or sodium formaldehyde bisulfite as the formaldehyde compound. It should also be noted that the formaldehyde equivalent of the sodium formaldehyde sulfoxylate used in example 41 is approx. 0.05% by weight based on the weight of the polymer. The formaldehyde equivalent of the sodium formaldehyde bisulphite used in examples 42 and 43 is respectively approx. 0.04 and 0.01

% by weight, based on the weight of the polymer.

Examples 44 to 47

These examples show the use of hexamethylenetetramine (HMTA) as formaldehyde compound. HMTA, which for the purposes of the present invention is considered a formaldehyde compound,

is widely recognized as a source of formaldehyde, as evidenced by the discussion in Formaldehyde by Frederic J. Walker, p. 511. The nitrile resin used is an acrylonitrile/styrene (70/30 wt%) copolymer in bead form. One test series is performed using beads with a moisture content of 0.69% (moist beads) while the other is performed using beads with a moisture content of 0.07% (dry beads). The nitrile resin and formaldehyde compound are mixed, extruded into pellets and formed into tensile test rods and tested for extractable HCN. The test results for these examples are listed in Table VIII below.

The above data show that HMTA significantly lowers

the extractable HCN content for both wet and dry beads.

Furthermore, the data indicate that a smaller amount of HMTA (0.1%

used in Example 45) is more effective than the large amounts used in Examples 46 (0.5%) and Example 47 (1.0%).

Examples 48 to 51

The following examples 48 - 51 illustrate a crash method

to obtain an intimate mixture of formaldehyde compound and nitrile polymer. In these examples, 100 parts of an acrylonitrile/styrene (70% acrylonitrile) copolymer in bead form are soaked in 100 parts of water containing either formaldehyde or the paraformaldehyde that was used in examples 1-5 above as formaldehyde compound. In each case, the amount used is indicated in % by weight based on the weight of the polymer. The soaking is carried out for 4 hours at 145°C, and then the beads are filtered, washed and dried. The dry beads are extruded into pellets and then cast into tensile test bars using the methods set forth above. The tensile test bars are then subjected to HCN extraction tests using 3% acetic acid and an extraction time of

3 days at 49°C. The results of the extraction tests are set out in Table IX below.

The above results indicate that destruction of the formaldehyde hydrogen bond into the polymer results in a reduction in the amount of extractable HCN. The value 25 PPB extractable HCN indicated for example 50 indicates that under the particular controlled conditions used, formaldehyde amounts above 0.1% are required to reduce the extractable HCN level to less than 20 PPB.

One of the surprising aspects of the present invention

is that extraction tests on tensile test bars indicate that any HCN in the polymer which is in the form of formaldehyde cyanohydrin

(HO CH2CN) (the reaction product of HCN and formaldehyde) does not

are extracted in some specific quantities.

In Examples 52 - 54, cyanohydrin extraction tests are performed

on a series of tensile test bars made from various acrylonitrile/styrene copolymers containing 70% acrylonitrile. The formaldehyde compound used is paraformaldehyde which

was used in examples 1-5, and the extraction time is 3 days at 49°c. Results of these tests showing the low level of cyanohydrin extracted are set forth in Table X below.

Taste tests

Taste tests are carried out using 296 cm^

bottles which were formed as indicated above, of various acrylonitrile/styrene copolymers containing 70% acrylonitrile. The nitrile polymers are mixed with varying amounts of paraformaldehyde, para-toluenesulfonamide/formaldehyde condensate or sodium formaldehyde bisulphite. The mixtures are extruded into pellets and formed into 296 cm 3 bottles of the type conventionally used for non-alcoholic beverages using the method indicated in examples 1-4 above. The bottles are filled with 296 cm<3 >of a well-known, commercially available cola drink ("Coca-Cola")

and placed in a drying cabinet for 7 days at 30°C, together with the same drink in a glass bottle. At the end of this period, the contents of the plastic bottles are poured into each of the unmarked vessels,

and the contents of the glass bottle are poured into each of 20 unmarked vessels. 5 people who have been trained beforehand in taste testing techniques and who are sensitive to the influence of taste that is rinsed with extracted HCN, are selected as the taste jury. Each member of the jury gives two of the vessels containing the cola drink from the glass bottle and one vessel containing the cola drink from the plastic bottle. The tasters do not know which of the three vessels contains the drink from the plastic bottle, and are asked to decide which of the three samples has a taste different from the other two, and if so to indicate which. This is known as a "triangle taste test". The test is then repeated to establish reproducibility. If a jury finds a taste difference that flushes HCN into the beverage packaged in a plastic bottle,

the plastic bottle is judged to have failed the taste test.

HCN extraction data are also obtained for the blown bottles using 3% acetic acid as extraction liquid and an extraction time of 7 days at 38°c. The results of these tests are reproduced in Table XI below.

The data in Table XI clearly show the improvements in HCN flavor characteristics obtained when the extractable HCN content of the packaging material is reduced to less than 20 PPB using such a formaldehyde compound as paraformaldehyde, para-toluenesulfonamide/formaldehyde condensate, or sodium formaldehyde bisulfite, in accordance with the teachings of the present invention. The polymer used in Example 63 was extruded 5 times prior to bottle blowing in order to simulate bottle recycling conditions. The improved taste characteristics are preserved even after the simulated recirculation.

Claims (2)

1. Process for the production of shaped packaging materials for foodstuffs, beverages and similar substances, of nitrile copolymers containing (A) 55-85% by weight of nitrile monomer units (calculated as acrylonitrile), based on the total copolymer weight, such nitrile monomer units comprising acrylonitrile or mixtures of acrylonitrile and methacrylonitrile where the methacrylonitrile is present in amounts of up to 16% by weight % based on the total copolymer weight, and (B) 45-15% by weight of at least one other copolymerized monomer, especially styrene or an acrylate, which method comprises heating the copolymer in a solvent-free state to a melt and forming the copolymer melt into a packaging material, characterized in that a particulate nitrile copolymer is brought into intimate contact with formaldehyde by mixing with 0.005-2 wt% formaldehyde, based on the copolymer, before or during heating to a molten state, adding formaldehyde or a formaldehyde-releasing compound either dry or in water with subsequent drying, whereby a shaped packaging material with an extractable HCN- content of less than 50 parts per billion. 2. Method as stated in claim 1, characterized in that formaldehyde, paraformaldehyde, trioxane, tetraoxane, dimethylolurea or a condensate of p-toluenesulfonamide and formaldehyde is added to the nitrile copolymer.