DK159927B - PROCEDURE FOR REDUCING THE MONOMER CONTENT IN ACRYLIN NITRIL POLYMERS, EVEN IN THE FORM OF FORMED PRODUCTS, BY IONIZING IRRATION - Google Patents

Description

DK 159927 B

The present invention relates to a process for reducing the content of residual monomer in acrylonitrile polymers, optionally in the form of shaped products thereof, by ionizing radiation.

In recent years, the possibilities for replacing "standard" glass bottles with so-called plastic containers have been considerably considered. In particular, acrylonitrile-containing polymers have been proposed for this purpose because of their useful barrier properties. In particular, an acrylonitrile / styrene copolymer has been used to produce beverage containers capable of withstanding a significant internal gas pressure. Unfortunately, some experiments have shown that acrylonitrile may itself be a carcinogen. If acrylonitrile monomers are present in the polymer, for example, in copolymer beads or articles, detectable and perhaps substantial amounts of acrylonitrile and / or perhaps lower polymers of acrylonitrile may be leached from the beads or article. Therefore, there is cause for concern regarding the content of residual monomer, primarily acrylonitrile, in polymer articles, and in particular with regard to migration of monomers and low polymer components from plastic bottles or food containers to the food product or beverage contained therein.

It is clear that manufacturers of the copolymers and plastic containers made therefrom use polymerization technique and polymer processing technique which are designed to minimize the monomer content of the container walls. Nevertheless, despite their best efforts to deliver monomer-free polymer 25 for the container manufacturing operation and to avoid degradation of the polymer during the container manufacturing operations, a residual alacrylonitrile (monomer) content of 1-100 ppm can be found in the container walls.

The solution of the problem of the monomer content of the container walls and migration of the monomer from the container cannot be separated from the difficulties already known in the art. Particular attention has been paid to the presence of an extractable content of HCN in packaging materials formed from nitrile polymers, including of course the acrylonitrile / styrene polymers, see U.S. Patent No. 3,870,802. Containers assumed to be λ 5 have been manufactured according to instructions given there (presumably from the polymeric nitrile resins 2

DK 159927 B

described in this patent) have been tested to have just under 20 ppb (parts per billion) (in the extract) extractable HCN therein.

On the whole, it is believed that as a practical precaution, a monomer removal treatment should be performed after the acrylonitrile polymer has been polymerized. In addition, treatment must avoid causing adverse side effects such as the formation of sufficiently highly extractable HCN to affect the final use of the polymer in food or beverage containers.

The process of the invention is peculiar to subjecting the polymer to ionizing radiation in a dose range of 0.05 to 2.0 megarad. According to the invention, irradiation can be particularly convenient at a dose of 0.1-1.5 megarad.

Optionally, according to the invention, the irradiation treatment may be followed by washing or other treatment to degass HCN from the polymer. Such degassing treatment is particularly desirable in cases where the radiation dose has been 0.5 to 2.0 megarad.

From JP patent specification 48-41267 (see also Chemical Abstracts 81, 1974, 64585z) it is known that the monomer content of homo- and copolymers of styrene can be reduced by ionizing radiation. The irradiation occurs with relatively high doses of ionizing radiation and at elevated temperature. It is apparent from the drawing that a substantial reduction of the monomer content is first achieved at temperatures above 60 ° C. Example 3 of the specification relates to reduction of the monomer content of a εtyrene ^ acrylonitrile 1 copolymer. A reduction from 3700 ppm to 540 ppm residual monomer was obtained. This amount of residual monomer is unacceptable, as it is too large to provide the necessary security against the adverse effects mentioned above. In the treatment according to the invention, residual monomer quantities of the order of, for example, 1-15 ppm can be obtained, see the following example 4.

According to the invention, the source of the ionizing radiation can be particularly conveniently an electron beam.

According to the invention, the practical treatment of the material can be particularly convenient in a medium, for example a polymerization medium.

3

DK 159927 B

Ionizing radiation, for example from an electron beam generator, is known to induce many complex and sometimes competing reactions. For example, irradiation is known to induce polymerization of acrylonitrile. When radiation is applied to polymers, it is known to cause cross-linking, chain cleavage (which could conceivably develop undesirable by-products such as HCN), evolution of gases and similar processes. In some cases, physical properties of the polymer are improved, e.g. polyethylene when exposed to 20-30 megarad; polyvinyl chloride containing a pro-row, i. a substance that promotes radiation uptake such as a polyfunctional acrylate when exposed to 1 to 2 to 5 megarads; and polyvinylidene fluoride when exposed to over ca. 8 megarad. However, in other polymers, there are other consequences (such as chain cleavage) that even predominate in such reactions as cross-linking, and degradation occurs, e.g.

butyl rubber? in teflon? and in cellulose derivatives. Still other polymers, including acrylonitrile-containing plastics and polystyrene, are substantially unaffected by radiation, especially in the relatively low dose range of 2-10 megarads. For this reason, polystyrene has been used in parts of electron beam generators.

In summary, polymers in the present context can be divided into three categories: those that decompose by irradiation, e.g., butyl rubber? those unaffected by radiation except at very high radiation levels at which certain color changes and perhaps other harmful changes may occur, such as polystyrene and some polymers containing polyacrylonitrile? and those polymers whose physical properties are enhanced by irradiation, e.g., polyethylene. In the latter category, improvements are typically found with radiation doses in the range of 10 to 30 megarads, and if a pro-row is included in the range of 2 to 10 megarads. In both of these cases, the effects literally disappear at about the lower end of the dose range. Superficially, therefore, one would expect 30 tea at what is considered a low dose, ie. 2-10 megarad, would offer little hope only in the absence of a pro-rad for enhancement of the polymer properties in any respect, and in fact there are trials where there was little or no physical improvement in the breaking strength of polystyrene or of polyacrylonitrile.

O C

durable plastic materials.

However, it has now been found that an irradiation range near 0, namely 0.05 to 2.0 megarad, is anomalous. Surprisingly big 4

DK 159927 B

Reductions of residual acrylonitrile monomer in vessel walls are achieved by irradiation at a dose of 0.2 megarad. The physical properties of the polymer (including its color) are almost completely unchanged at less than 0.5 megarad.

Although no general studies have been conducted,

C

it now appears that this near-O degree of irradiation is anomalous for a significant number of polymers. Such polymers react differently to the 0.05 to 2.0 megarad range than they do to the 2-10 megarad range. The reaction of nitrile polymers is certainly different in the 0.05-0.5 megarad range. In particular, the content of monomeric acrylic nitrile therein decreases substantially, e.g., a decrease of 50-90%. Significant chain splitting does not appear to occur. Cross-linking, as found by changes in rheological properties, does not appear to occur to a significant extent. (It is interesting that extractable HCN is developed by any application to these nitrile polymers roughly proportional to dose).

It is believed that the reduction of residual monomer and the significant absence of crosslinking and low degree of HCN evolution in the case of acrylonitrile polymers can be explained by the G-value of the reactions involved. The G value is defined as the number of molecular changes occurring per 100 eV absorbed into the system. For monomer attachment in the case of acrylonitrile, several orders of magnitude are higher than the G value for chain cleavage of the polymer. The essence of this is

that the dosage of 9 S used in the method of the invention

0.5-2.0 megarad high energy radiation constitutes sufficiently strong radiation to (by molecular association) remove the low proportion of monomer in the polymer without being sufficiently powerful radiation to cause a substantial or only noticeable degree of chain cleavage or chain breakage in the polymer. Release of HCN can probably be attributed to a cleavage reaction in a side chain.

In relation to the present invention, substantial reduction of the trace monomer content as almost the only consequence of irradiation at a very low level is precisely the desired result. Development of HCN in the nitrile polymer that actually occurs is a very modest side effect since extraction levels in leaches must be expressed in parts pi. billion and further can be reduced to less than 20 ppb in the leaching agent by performing degassing treatment such as washing or storage at elevated temperature on the manufactured articles.

DK 159927B

5

The process of the present invention can be practiced on all possible polymeric articles made from or from acrylic nitrile monomers. In particular, the process of the invention can be used to reduce the monomer content of packing materials of or on the basis of acrylonitrile polymers containing 55-85 wt.

C

acrylonitrile alone or acrylonitrile and methacrylonitrile in amounts up to 16% by weight (of the total polymer content) and one or more comonomers selected from styrene, α-methylstyrene, α-olefins of 2-6 carbon atoms, C1-4 alkyl esters of acrylic acid and methacrylic acid, vinyl acetate and C alkyl vinyl ethers. The process 10 X 1 is particularly applicable to styrene-acrylonitrile bottles and jars containing 60-83% by weight of polymerized acrylonitrile, and to articles containing up to 25% by weight of natural or synthetic rubber. The process can be applied to all of the packaging materials described in U.S. Patent No. 3,870,802 with or without the presence of formaldehyde therein.

Other examples of composition of copolymers to which the process of the invention can be used are: 1) An acrylonitrile / styrene copolymer consisting of the copolymer produced by polymerization of 66-72 parts by weight of acrylonitrile and 23-34 parts by weight of styrene.

2) An acrylonitrile / styrene copolymer consisting of the copolymer formed by polymerization of 45-65 parts by weight of acrylonitrile and 35-55 parts by weight of styrene.

An example of a nitrile rubber modified acrylonitrile methyl 9c acrylate copolymer to which the present process may be applied consists of graft copolymers prepared by graft copolymerization of 73-77 parts by weight of acrylonitrile and 23-27 parts by weight of methyl acrylate. 9-10 parts by weight of butadiene-acrylonitrile copolymers containing approx. 70 wt% polymer units derived from 30 butadiene.

It is particularly advantageous according to the invention to use as an acrylonitrile polymer a thermoplastic polymer selected from acrylonitrile homopolymers and copolymers of acrylonitrile with ethylenically unsaturated monomers selected from styrene, butadiene and methyl acrylate or other alkyl acrylates.

DK 159927 B

6

As a general principle, the dose range of 0.05-2.0 megarad used in the process of the invention is too low to cause significant degradation, even of sensitive polymers, but sufficient to activate trace amounts of polymer monomer such that (the theory of the present invention is based on) the previously free monomer molecules are coupled to the macromolecules of the polymer. The trace amounts of the monomer are sensitive to radiation. Studies show that 50-90% of the monomer content that can be extracted from the polymer before irradiation is no longer present, at least not in extractable form. The preferred dose range of the method according to the invention is, as mentioned, irradiation with a dose in the range 0.1-1.5 megarad.

Experiments performed with high quality bottles of acrylonitrile / styrene polymer (ie "Cycle Safe", a trade mark registered in Denmark) show that an extractable acrylonitrile content of 5 ppm could be significantly reduced by low-level irradiation. Unfortunately, the effects of the 0.05-0.5 dose range are not sufficiently known to assess whether items containing higher amounts of residual monomer, e.g., 30 ppm acrylonitrile, can be improved to less than 10 ppm, or whether the improvement occurs in only one or more doses. second proportion to the original monomer content, such as a 50-90% reduction.

When the radiation dose exceeds 0.5 megarad and especially in the 0.5-2.0 megarad range, the well known chain cleavage, crosslinking and other reactions attributable to high energy radiation may begin to occur. The 0.5-2.0 megarad range is useful on the acrylonitrile polymers because such polymers are not particularly sensitive to degradation. It is believed that the 0.5-2.0 range is useful because side effects, including, for example, the development of HCN, crosslinking or polymer cleavage, still occur only to a minimal extent. Overall, the monomer reduction is considered to be almost entirely achieved at the lower dose, while further improvement in reducing the monomer content is considered to be fairly nominal. A preferred way, for example, when treatment of the article with more than 0.5 megarad is desired, would be to repeat the treatment with 0.05-0.5 megarad.

35 A particularly severe reduction in the content of acrylonitrile 7

DK 159927 B

residual monomer is obtained according to the invention by proceeding according to claim 6. Hereby it is particularly advantageous according to the invention if the resin in pearl or ground form is irradiated with 0.05-0.5 megarad and the finished object is also irradiated with 0.05- 0.5 megarad.

5

Although the method according to the invention has generally been described as a treatment of high quality finished articles, it is also within the scope of the invention to practice the process on polymer beads or on the base resin before forming or casting the final article. The beads or base resin 10 are subjected to a 0.05-0.5 megarad treatment to reduce the monomer content therein, and the beads or resin are then worked up in accordance with conventional methods in the art. In a high-quality monomer, treatment of the beads ("pellets") may be sufficient to reduce the monomer content to acceptable

1 R

level, especially when the final use can tolerate the presence of a nominal monomer content in the article. The dual treatment, ie. first of beads or powder and then of the shaped article can be used on high quality polymer simply to make sure that everything is possible to minimize the monomer content of the article.

Looking at radiation treatment in the range of 0.5-2.0 megarad, which is thus within the scope of the invention, it has already been mentioned that the development of HCN from acrylonitrile polymers is approximately proportional to the dosage and that it is extracted.

2 S

only HCN content, measured in the leaching agent, will exceed 20 ppb (parts per billion) after irradiation at 0.5-2.0 megarad. Accordingly, according to the present invention, it may be appropriate, after irradiation, in the range of 0.5-2.0 megarads of articles containing nitrile polymers to perform the aforementioned degassing treatment O Λ to reduce the HCN content. Such treatment need not be anything other than storage at a slightly elevated temperature, e.g., about 44 ° C for a few weeks, or washing with warm water with or without an HCN complexing agent. In all cases, the degassing treatment reduces the HCN content of the object to taste-tolerable levels. Degassing treatment can also be done for polymers irradiated in the range 0.05-0.5 megarad, but it cannot normally be considered

DK 159927 B

8 for required.

The source of high energy radiation used is not essential to the present method and therefore any known (commercially available) radiation source, including, for example, radioactive sources of high energy gamma rays such as radioactive cobalt and electron beam generators such as "Dynamitron" can be used ( a trademark not registered in Denmark). Therefore, a further description of the high-energy radiation source need not be given here nor is it necessary to describe the radiation treatment in detail.

The process according to the invention will be illustrated in the following with some examples.

Example 1 15 -----------

Section cut, approx. 945 ml bottles of acrylonitrile / styrene copolymers (Monsanto's "Cycle-Safe" ® "Coca Cola" ® bottles) were irradiated with an electron beam generator (from Radiation Dynamics, Inc.) with 0.10, 0.30 and 0.50 megarads. The bottles were then tested for residual monomer content after irradiation and the results were compared with the monomer content in an unirradiated bottle section from the same bottle. In this series of tests only qualitative analyzes were performed and there was considerable variation from sample to sample. The control samples showed monomer content of 1-15 ppm.

25 Comparable of irradiated bottle sections also showed sample-to-sample variation, but within clearly lower monomer content ranges.

The best results were achieved with 0.3 megarad. The monomer content of the samples tested was 10% or less of the content of the unirradiated. samples.

! 35

DK 159927B

9

Example 2

R

A similar batch of "Cycle-Safe" container was irradiated with 0.05-1.0 megarad. The bottles were immediately filled with distilled water for 2 weeks at 43.33 ° C (in a circulation oven) after which the leaching medium (water) was tested for HCN content The results are shown below.

Sample Hydrogen cyanide, ppm 10 Control 0.019 0.05 0.052 0.10 0.105 0.20 0.280 0.30 0.350 15 0.40 0.440 0.50 0.610 0.75 0.760 1.0 1.150

Prolonged storage time, even at room temperature, for at least 20 weeks reduced the content of HCN (in the leaching agent) at least 50%. Assuming that the decrease in the acrylic monomer content of plastic material is easily proportional to the development of HCN, it can be seen that the acrylic nitrile monomer content can be reduced by low doses of radiation.

Example 3

In another experiment, a portion of ground resin of acrylonitrile / styrene copolymer as above was divided into four samples. The first sample of the tables listed below was not irradiated and served as a control, while the other samples were irradiated as indicated in the table.

Sample Radiation in megarad Acrylonitrile monomer, ppm 10 19 35 2 0.1 15 3 0.5 11 4 1.0 10

DK 159927B

10

Most of the decrease in acrylonitrile monomer volume, 8 ppm, occurred at a dose of up to 0.5 megarad, while there was little further improvement, 1 ppm, by doubling the dose to 1.0 megarad.

5

Example 4

Two sets of completed bottles made from the acrylonitrile / styrene copolymer referred to in the previous example were irradiated with a dose of 0.3 megarad. The first set of bottles had an initial acrylonitrile monomer content of approx. 10 ppm and the second set of bottles has a monomer content of approx. 5 ppm. After irradiation at a dose of 0.3 megarad, the first and second set of bottles had monomer content of 1.5 ppm and 0.5 ppm respectively, so that the reductions were in the range of 85-90%.

15

It appears that the irradiation treatment is more effective in reducing the monomer content on a percentage basis in finished articles than in treatment before the preparation of the finished article.

20 25 30 35

Claims (8)

A process for reducing the monomer content of acrylonitrile polymers, optionally in the form of shaped products, by ionizing irradiation, characterized in that the polymers are subjected to ionizing irradiation with a degree of irradiation of 0.050-2.0 megarad. Method according to claim 1, characterized in that the radiation is carried out at a dose of 0.1-1.5 megarad. Process according to claim 1 or 2, characterized in that the irradiation treatment is followed by a degassing treatment for HCN. Method according to claim 1, characterized in that the source of the ionizing radiation is an electron beam. 15 Process according to claim 1, characterized in that, as an acrylonitrile polymer, a thermoplastic polymer selected from acrylonitrile homopolymer and copolymers of acrylonitrile with ethylenically unsaturated monomers selected from styrene, butadiene and methyl acrylate or other alkyl acrylates is used. Process according to claim 1, characterized in that the polymer being treated is in the form of a ground resin or in pellet form and in that the ionizing radiation is also carried out on the article made of the ground resin or beads. Method according to claim 6, characterized in that the resin in ground or bead form is irradiated with 0.05-0.5 megarad and that the finished article is also irradiated with 0.05-0.5 megarad. Process according to claim 1, characterized in that the material is treated in solution or suspension in a medium, for example a polymerization medium. 35