NO155293B - PROCEDURE FOR REDUCING THE CONTENT OF ACRYLONITRIL MONOMERS IN A SOLID, PARTICULAR, SOLVATED OR SUSPENDED POLYMER. - Google Patents

Abstract

PROCEDURE FOR REDUCING THE CONTENT OF ACRYLONITRIL MONOMES IN A SOLID, PARTICULAR,. SOLVATED OR SUSPENDED POLYMER.

Description

The present invention relates to a method for reducing residual monomer content in objects formed from acrylonitrile polymers, in particular acrylonitrile/styrene copolymers.

In recent times, great interest has been shown in replacing "standard" glass bottles with so-called plastic containers. 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

for forming containers for beverages, which containers can withstand significant internal gas pressures. Some test studies unfortunately indicate that acrylonitrile may contain a carcinogen. When acrylonitrile monomer is also present in the polymer, e.g. in copolymer pellets or articles, detectable and possibly significant amounts of acrylonitrile and/or possibly lower polymers of acrylonitrile may be leached from the pellets or articles.

There is therefore great interest in the content of residual monomers, mainly of acrylonitrile, in polymer objects and especially in the migration of monomer and lower polymer components from the plastic bottles or food containers into the food product contained therein.

The producer of the copolymers and the plastic containers produced from them use polymerization and polymer processing techniques which are adapted to make the monomer content in the container walls as low as possible. Despite great efforts to provide a monomer-free polymer for the manufacture of containers and to avoid degradation of the polymer during the container manufacturing operations, one can however find a residual acrylonitrile (monomer) content of 1-100 ppm in the container walls.

It may be that the solution to the problem of monomer content in the container walls and migration of monomer from the container cannot be separated from difficulties already known in the art. Particular attention has been directed to the occurrence of an extractable HCN content in packaging materials formed from nitrile polymers, naturally including the acrylonitrile/styrene polymers, and in this connection reference is made to US

Patent No. 3,870,802. Containers believed to be manufactured according to this patent (presumably from the polymeric nitrile resins described in the patent) were found on examination to have just under 57.1 g/l (in the extract) of extractable HCN.

It is generally assumed that, in practice, a monomer removal treatment should be carried out after the acrylonitrile polymer has been polymerised. The treatment must also avoid the formation of adverse side effects, such as e.g. formation of sufficient extractable HCN to affect the polymer's end-use in food containers.

According to the present invention, a method is provided for reducing the content of acrylonitrile monomer in a solid, particulate, solvated or suspended polymer which can develop extractable hydrogen cyanide, where the polymer is formed by polymerization of acrylonitrile monomer alone or in combination with methacrylonitrile monomer and one or more selected comonomers from the group of styrene, a-methylstyrene, ^ 2~ <^^ a-olefins, C^-C^ alkyl esters of acrylic acid and methacrylic acid, vinyl acetate and C^-C^ alkyl vinyl esters, and is in the form of a resinous material for the formation of finished articles, ground resin, pellets or articles made from ground resin or pellets, and this process is characterized by exposing the polymer to ionizing radiation at an irradiation dose level of 0.050-2.0 megarads to an extent sufficient to significantly reduce the polymer's content of leachable residual acrylonitrile monomer,

and then possibly carry out an HCN outgassing.

The irradiation treatment can thus optionally be followed by washing or other treatment which aims to outgas HCN from the polymer. An outgassing treatment is particularly desirable when the radiation dose has been 0.5-2.0 megarad.

Ionizing radiation, e.g. from an electron beam generator, can, as is known, create many complex and sometimes competing reactions. One knows e.g. until the irradiation induces polymerization of acrylonitrile. In connection with polymers, irradiation is known to cause cross-linking, chain shortening (which can produce undesirable by-products such as HCN), evolution of gases, etc. In some cases, physical properties of the polymer are improved, eg: polyethylene by exposure to 20-30 megarad; polyvinyl chloride containing a "pro-rad" as a polyfunctional acrylate when exposed to 2-5 megarad; and polyvinylidine fluoride when exposed to more than approx. 8 megarows. In other polymers, however, other consequences (such as chain shortening) are as extensive or even more extensive than reactions such as cross-linking and degradation, such as e.g. in butyl rubbers, in "Teflon" and in cellulose-containing substances. Furthermore, other polymers including acrylonitrile-containing plasters and polystyrene are largely unaffected by irradiation, especially in the relatively low dosage range of 2-10 megarads. For this reason, polystyrene has been used in components of electron beam generators.

Polymers can generally be divided into three categories: those that degrade by irradiation, e.g. butyl gunmi; those which are unaffected by irradiation except at fairly high dosage levels whereby certain color formations and possibly other unfortunate changes occur, and as examples can be mentioned polystyrene and some polymers containing polyacrylonitrile; as well as those polymers whose physical properties are improved by irradiation, such as polyethylene. In this latter category, improvements are typically found with irradiation dosages in the range of 10-30 megarad, and if an irradiation-friendly agent ("pro-rad")

is included, in the area of 2-10 megarad. In both of these cases, the effects practically disappear at around the lower end of the dosage range. It may therefore superficially appear that what is considered to be a low dosage,

i.e. 2-10 megarad, in the absence of a "pro-rad" will give little hope of improving polymer properties in some respects, and known trials have actually produced little or no physical improvement in tensile strength of polystyrene or polyacrylonitrile-containing plaster.

However, it has now been found that an irradiation area at close range

0, i.e. in the range 0.05-2.0 megarad, is deviant. Surprisingly large reductions in residual acrylonitrile monomer in container walls are achieved when using irradiation at 0.2 megarad. Physical properties of the polymers (including color) remain almost completely unchanged at less than 0.5 megarad.

Although a total investigation has not been carried out, it now seems that the mentioned near-0 irradiation range is anomalous for a significant number of polymers. Such polymers react differently in the 0.05-2.0 megarad range than they do in the 2-10 megarad range. Thus, the nitrile polymers' reaction is different in the 0.05-0.5 am range. In particular, the acrylonitrile monomer is significantly reduced in this area, e.g. from 50 to 90%. A significant chain shortening appears to occur. Cross-linking, as measured by changes in rheological properties, does not appear to be significant. (Of interest is that extractable HCN is developed in any application of these nitrile polymers largely in relation to the dosage.)

It is believed that the reduction of residual monomer and substantial absence of cross-linking and a low level of HCN evolution in the case of acrylonitrile polymers can be explained by the application of the G value for the reactions in question. The G-value for monomer attachment in the case of acrylonitrile is several orders of magnitude higher than the G-value for chain shortening for the polymer. The 0.5-2.0 megarad dose for the high-energy irradiation used here thus constitutes sufficient irradiation to remove (by attachment) the small amounts of monomer that are in the polymer without representing sufficient irradiation to cause significant chain shortening in the polymer. Release of HCN is probably due to a truncation reaction at a side chain.

As far as the present invention is concerned, the reduction is significant

in trace monomer content, which is about the only consequence of a very low level of irradiation, exactly the desired result. Development of HCN in the nitrile polymer occurs as a minor side effect due to

of the fact that the extraction levels in the leaching materials are in parts per billion, and this development can also be reduced back to below 20 parts per billion in the leaching material by an outgassing treatment carried out on the object, such as washing, storage under elevated temperatures, etc.

The invention can be applied in general to polymer objects formed from acrylonitrile monomers. The present invention is particularly adapted to reduce the monomer content in packaging materials formed from acrylonitrile polymers containing 55-85% by weight (calculated on the weight of total polymer) of acrylonitrile alone or of acrylonitrile and methacrylonitrile in amounts of up to 16% by weight (calculated on the total polymer weight), as well as one or more comonomers selected from the group of styrene, alpha-methylstyrene, alpha-olefins with 2-6 carbon atoms, C^-C4 alkyl esters of acrylic and methacrylic acid, vinyl acetate, C^-C^ alkyl vinyl ethers. The method can especially be used in connection with styrene/acrylonitrile bottles and containers containing 60-83% by weight of acrylonitrile, and in connection with the objects in which up to 25% by weight of natural or synthetic rubber is incorporated. The present method can be used in connection with all packaging materials that are mentioned in US patent no. 3,870,802 with or without the presence of formaldehyde.

Further examples of copolymers in connection with which the invention can be used are: 1. An acrylonitrile/styrene copolymer consisting of the copolymer obtained by polymerizing 66-72 parts by weight of acrylonitrile and 23-34 parts by weight of styrene, and 2. an acrylonitrile/styrene -copolymer consisting of the copolymer obtained by polymerizing 45-65% by weight of acrylonitrile and 35-55% by weight of styrene.

An example of a nitrile rubber-modified acrylonitrile-methylacrylate copolymer that can be used in connection with the present invention consists of basic copolymers produced by graft copolymerization of 73-77 parts by weight of acrylonitrile and 23-27 parts by weight of methyl acrylate in the presence of 9-10 parts by weight of butadiene -acrylonitrile copolymers containing about 70% by weight of polymer units derived from butadiene.

The area proposed according to the present invention

of 0.05-2.0 megarad is generally too low to cause significant degradation, even in connection with sensitive polymers, but is nevertheless sufficient to activate trace amounts of monomer in the polymer and thus cause (according to theory) binding of those on beforehand free monomer molecules to the macromolecules in the polymer. The trace amounts of monomer are sensitive to the irradiation, probably due to the previously mentioned G-value. Experiments show that 50-90% of the monomer content previously extracted from the polymer is no longer present (at least in an extractable form). The preferred range for carrying out the present invention is irradiation in the dosage range 0.1-1.5 megarad.

Tests carried out on high-quality bottles made of acrylonitrile/

TM

styrene polymer (ie "Cycle Safe") indicates that an extractable acrylonitrile content of 5 ppm can be significantly reduced by low-level irradiation. Unfortunately, the effects of a dosage range of 0.05-0.5 are not well enough known to determine whether objects containing larger monomer residues, e.g. 30 ppm acrylonitrile, can be improved to a content of less than 10 ppm or if the improvement only amounts to a certain proportion in relation to the original monomer content, such as e.g. a 50 to 90% reduction.

When the irradiation dose exceeds 0.5 megarad and especially

in the 0.5-2.0 megarad range, the well-known chain shortening and cross-linking reactions, etc., due to high-energy irradiation may begin to occur. The range of 0.5-2.0 megarads is useful in connection with acrylonitrile polymers because such polymers are not particularly sensitive to degradation. It is believed that the 0.5-2.0 range is a useful range because side effects, including e.g. development of HCN, cross-linking or polymer shortening, is still at a minimum. Overall, monomer reduction is considered to be almost completely achieved at the lower dose, with any further improvement in monomer reduction believed to be fairly nominal. A preferred method of operation, e.g. when treatment of the object with more than 0.5 megarad is desired, there will be a repeated 0.05-0.5 megarad treatment.

Also preferred over the 0.5-2.0 megarad range treatment is 0.05-0.5 megarad irradiation of the polymer pellets or ground resin, followed by 0.05-0.5 megarad treatment of the finished article.

Although the embodiment of the present invention is described generally with regard to the treatment of high quality finished articles, a variation of the invention concerns the treatment of polymer pellets or ground resin prior to the molding of the finished article. The pellets or ground resin are subjected to 0.05-0.5 megarads for the purpose of reducing their monomer content and then processed in accordance with common practice in the art. For high quality polymer, it may be sufficient to treat the pellets to reduce the monomer content to acceptable levels, especially when the end use can tolerate the presence of a nominal monomer content in the article. The double treatment, i.e. first of the pellets or powder and then of the shaped article, can be applied to high quality polymers to ensure that everything possible has been done to minimize the monomer content of the article.

As regards the treatment in the 0.5-2.0 megarad range, it has previously been suggested that the development of HCN from acrylonitrile polymers occurs linearly in relation to the dosage,

and that the extractable HCN content measured in the leaching material would exceed 20 parts per billion after irradiation by

0.5-2.0 megarad. Implementation of the present invention in 0.5-

The 2.0 megarad range on articles made from nitrile polymers may well include an outgassing treatment after irradiation to reduce the HCN content. Such treatment need not include more than storage at slightly elevated temperatures, e.g. 4 3°C, for a few weeks or a hot water wash with or without an HCN complexing agent. the outgassing treatment reduces the HCN content of the item to tolerable (taste-wise) levels in each case. The outgassing treatment is more optional for polymers irradiated in the 0.05-0.5 megarad range.

The relevant source for the high-energy irradiation is not decisive for the implementation of the present invention, and there-

for each of the known (commercially available) radiation sources can be used including e.g. radioactive sources for high energy gamma rays such as radioactive cobalt and electron beam generators such as the "Dynamitron". Accordingly, no further description of the high-energy irradiation source or details of the irradiation treatment should be necessary.

The invention is further illustrated with reference to the following examples.

Example 1

907 g "sectioned" bottles of acrylonitrile/styrene with

TM

copolymers ("Cycle-Safe" Coca Cola bottles) were irradiated with an electron beam generator at 0.10, 0.30 and 0.5 megarad. The bottles were then sampled for residual monomer content after irradiation and the results compared to monomer levels from the same bottle section that had not been irradiated. In these experiments, only qualitative analyzes were carried out and a considerable variation from sample to sample was found. The control samples show a monomer content of 1-15 ppm. Comparable samples of irradiated bottle sections also showed variation from sample to sample, but within a distinctly lower monomer content range.

The best results were obtained at 0.3 megarad. The monomer content in these samples shows a level of 10% or less compared to the level in unirradiated samples.

Example 2

Prolonged storage, even at room temperature, for at least one week reduced the HCN content (in the leachate) by at least 50%. By assuming that the reduction in acrylic monomer content in the plastic is proportional to HCN development, it appears that acrylonitrile monomer content can be reduced at low doses.

Example 3

In another experiment, a quantity of ground resin of acrylonitrile/styrene copolymer was divided into four samples. The first sample, as indicated below, was not irradiated and served as a control sample, the other samples being irradiated as follows:

The main reduction in the acrylonitrile monomer, 8 ppm, occurred at a dosage level of 0.5 megarad, while a small additional improvement, 1 ppm, occurred when doubling the dose to 1.0 megarad.

Example 4

Two set shaped bottles made of the aforementioned acrylonitrile/styrene copolymer were irradiated at a dose level of 0.3 megarad. The first set of bottles had an initial acrylonitrile monomer content of about 10 ppm and the second set of bottles had a monomer content of about 5 ppm. After irradiation at the 0.3 megarad level, the first and second set bottles had monomer levels of 1.5 ppm and 0.5 ppm, respectively, and such • reductions are in the 85-90% range.

It turns out that the irradiation treatment is more effective in reducing the monomer content on a percentage basis in the finished product compared to treatment before the formation of the finished product.

Claims (2)

1. Process for reducing the content of acrylonitrile monomer in a solid, particulate, solvated or suspended polymer which can develop extractable hydrogen cyanide, where the polymer is formed by polymerization of acrylonitrile monomer alone or in combination with methacrylonitrile monomer and one or more comonomers selected from the group styrene, a -methylstyrene, ^ 2~ Cs a~ °^- e^^ ner' ci-c-'4 alkyl esters of acrylic acid and methacrylic acid, vinyl acetate and C^-C^ alkyl vinyl esters, and is in the form of a resinous material for the formation of finished articles, ground resin, pellets or articles made from ground resin or pellets, characterized by exposing the polymer to ionizing radiation at an irradiation dose level of 0.050-2.0 megarad to an extent sufficient to significantly reduce the polymer's content of leachable residual acrylonitrile monomer, and then possibly carry out an HCN outgassing. 2. Method according to claim 1, characterized in that an electron beam is used as a source of ionizing radiation.