CA1106311A

CA1106311A - Method for reducing the monomer content in styrene and acrylonitrile

Info

Publication number: CA1106311A
Application number: CA312,888A
Authority: CA
Inventors: Kennard H. Morganstern
Original assignee: Radiation Dynamics Inc
Current assignee: Radiation Dynamics Inc
Priority date: 1977-10-07
Filing date: 1978-10-06
Publication date: 1981-08-04
Also published as: FR2405267A1; JPS5470391A; FR2405267B1; SE7810395L; DK159927C; NL186071C; NL186071B; NL7810031A; JPS5654321B2; ATA715678A; NO155293B; NO783391L; AU529029B2; GB2006225B; SE428568B; CH641192A5; DE2843292A1; DE2843292C2; GB2006225A; AT370745B

Abstract

METHOD FOR REDUCING THE MONOMER CONTENT IN
ACRYLONITRILE CONTAINING POLYMERS
ABSTRACT OF THE INVENTION
The monomer content in acrylonitrile containing polymers, as for examples bottles formed from acrylonitrile/styrene copolymer, is substantially reduced by applying ionizing radiation in low dosage levels e.g. 0.05-2.0 megarad to the completely or partially formed article, the pellets from which the articles are molded or the ground resin from which the pellets are formed, or to polymer in solution or suspension.

Description

THE_INVENTION
This invention relates to the reduction of residual monomer content in articles formed from acrylonitrile polymers including notably acrylonitrile/styrene copolymers.

In recent years the art has given substantial consideration to the possibility for replacing the "standard" glass bottle with so called plastic containers. In particular acrylonitrile containing polymers have been suggested for this purpose because of their useful barrier properties.
Specifically, an acrylonitrile/styrene copolymer has been employed to form beverage containers capable o withstanding considerable internal gas pressure. Unfortunately some test studies indicate that acrylonitrile itself might be a carcinogen. Moreover when acrylonitrile monomer is present-in the polymer e.g. in copolymer pellets or articles, detectable, and perhaps significant, quantities of acrylonitrile and/or perhaps lower polymers of acrylonitrile may be leached out of the pellets or articles Concern exists, therefore, over the content of residual monomer, princi-pally of acrylonitrile, in polymer articles and specifically over the migration of monomer and lower polymer components from the plastic bottles or food containers into the food product contents therein.
Manifestly~ thP manufacturer of the copolymers and the plastic containers made therefrom employ polymerization and polymer treating techniques that are adapted to minimize the monomer content in the con-tainer walls. However, despite their best efforts to provide monomer-free polymer for the container forming operation and to avoid degradationof the polymer during the container forming operations, a residual acrylo-nitrile (monomer) content of 1 ppm-100 ppm in the container walls can be found.

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The solution o~ the problem of monomer content in the container walls and mi~ration of monomer from the container may not be divorcable from difficulties already known to the art. In particular, attention has been directed to the appearance of an extractable HCN content in packaging materials fo~med from nitrile polymers, includi~g of course the acrylonitrile/styrene polymers, reference being made, in this regard, to ~ S. Patent 3,870,802. Containers believed to have been made according bo the practice o~ U.S. Patent 3,870,~02 (presumably from the polyneric m trlle resins described in that patent) tested out as having just less than 20 ppb (in the extract) of extractable llCN therein.
On the whole it is believed that as a practical matter, a mono-mer removing treatment should be carried out after the acrylonitrile polymer has been polymerized. Moreover, the treatment must avoid creating harmful side effects, such as for example creation of sufficient extract-able HCN to affect end use of the polymer in food containers.
BRIEF DESCRIPTION OF THE INVENTION
Briefly stated the process of this invention comprises treating acrylonitrile containing polymers including notably acrylonitrile/styrene copolymers and acrylonitrile/butadiene/styrene polymers with io~iziDg 20 radiation within the dosage range of 0.05-2.0 megarads whereby the extract-able monomer content is reduced.
Optionally, the irradiation treatment is followed by washing or other treatment intended to outgas HCN from the polymer. An outgassing treatment is particularly desirable when the irradiation dosage has been 0.5-2.0 megarads.
RATIONALE OF THE INVENTION
Ionizing radiation, e.g. from an electron beam generator is known to create many complex and sometimes competing reactions. For example, irradiation is known to induce polymerization of acrylonitrile.
When applied to polymers, irradiation is known to cause cross linking, chain scission (something that might generate undesirable by~products as HCN), generation of gases, etc. In some instances physical properties of the polymer are improvedg for example: polyethylene when subjected to 20-30 megarads; polyvinyl chloride incorporating a pro-rad as a polyfunc-tional acrylate when subjected to 2-5 megarads; and polyvinylidine ~
fluoride when subject~ed to more than about 8 megarads. In other polymers, ,~

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however, other consequences (such as cllain scission) equal, or even pre-dominate over reactions such as cross-linking and degradation occurs, as for example; in butyl rubbers; in Teflon',*and in cellulosics Still other polymers, including acrylonitrile containing plastics and poly-styrene are largely unaffected by irradiation, particularly in therelatively low dosage area of 2-10 megarads. For this reason polystyrene has beeD employed in components of electron beam generators.
In summary polymers fall generally into three categories:
those that are degraded by irradiation, for instance, butyl rubber; those that are unaffected by irradiation except at ~uite high does levels at which certain color and perhaps other deleterious changes occur, such as polystyrene and some polymers containing polyacrylonitrile; and those poly-mers whose physical properties are improved by irradiation such as poly-ethylene. In this latter category improvements are typically ound with irradiation doses in the range of 10-30 megarads and if a pro-rad is in-cluded, in the range of 2-10 megarads. In both of these cases the effects virtu011y disappear at about the lower end of the dose range. Super-ficially then, it would seem that what are considered to be low dosage i.e.

2-10 megarads would of~er little hope iD the absence of a pro-rad for im-proving polymer properties in any respect, and, indeed trials known tothe inventor hereof, produced little or no physical improvement in tensile strength of polystyrene or of polyacrylonitrile containing plastics.
It has now been found however, that the near zero irradiation range of 0.05-2.0 megarads is anomalo~s. Surprisingly large reductions of residual acrylonitrile monomer in container walls are obtained from irradiation at 0.2 megarads. Physical properties of the polymers (includ-ing color) are almost totally unchanged by less than 0.5 megarads.
Although no across the board investigation has been made, ~t now appears that this near zero range of irradiation is ano~alous for a sub~
stantial number of polymers. Such polymers respoDd differently in the 0.05-~.0 me~arad range than they do in the 2-10 megarad range. Certainly the response of nitrile poly~ers is different at the 0.05-0.5 megarad range.
Specifically the monomer acrylonitrile therein is reduced substantially, e.g. from 50~ to 90~. Significant chain scission does not appear to occur.
Cross-linking, as measured by changes in rheologic properties does not appear to be significant. (Interestingly, extractable HCN is generated in any use to this Ditrile polymers roughly in proportion to the dosage.) * Trademark for polyte-trafluoroethylene resin.

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It is believed that the reduction in residual monomer and essential absence of cross-linking and low level of ~ICN generation in the instance of acrylonitrile polymers are explainable in terms of the G-val~e for the reactions involved. The G-value for monomer attachment, in the lnstance of acrylonitrile is several orders of magnitude higher than the G-value for chain scission of the polymer. In essence then the 0.5-2.0 me8arad dosage of high energy radiation herein preferred consti-tutes sufficient radiation to scavenge (by attachment) the small propor-tions of monomer within the polymer without being sufficient radiation to cause a material level of chain scission in the polymer. Release of HCN
is probably attributable to a scission reaction at a side chain.
In terms of the present invention significant reduction of trace monomer content as almost the only consequence of very low level irradiation is precisely the desired result. Generation of HCN in the lS nitrile polymer which does occur is a minor side-effect, because the ex-traction levels in leachants are in parts per billion, and moreover can be reduced back to below 20 ppb in leachant by an outgasing treatment per-formed on the article such as washing, storage under elevated temperatures, etc.
DETAILED PRACTICE OF THE INVENTION
The practice of this invention is applicable to the full range of polymeric articles formed from acrylonitrile monomers. In particular the process of this invention is adapted to reducing the monomer content of packaging materials from acrylonitrile polymers containing from 55-B5%
by wt (of the total polymer weight) of acrylonitrile alone or of acrylo-nitrile and methacrylonitrile in amounts up to 16% by wt (of total polymer weight) and one or more comonomers selected from the group of styrene, alpha methylstyrene, alpha olefins of 2-6 carbon atoms, Cl-C4 alkyl esters of acrylic and methacrylic acid, vinyl acetate, Cl-C4 alkyl vinyl ethers.
The process is specifically applicable to the styrene/acrylonitrile bottles and ~ars containing 60-~3% by wt of acrylonitrile, and to those articles wherein up to 25% by weight of natural of synthetic rubber is incorporated.
The process is applicable to all of the packaging materials described by Harris et al in their U. S. Patent 3,870,802 with or without presence of formaldehyde therein.

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Additional examples of the formulation of copolymers to which the present invention is applicable are:
1. An acrylonitrile/styrene copolymer consisting of the co-polymer produced by polymerization o~ 66-72 par~s by weight of acrylo-nitrile and 23-34 parts by weight of styrene.
2. An acrylonitrile/styrene copolymer consisting of the co-polymer produced by polymerization of 45 65 parts by weight o~ acryloni-trile and 35-55 parts by weight of styrene.
An example of a nitrile rubber modified acrylonitrile methyl acrylate copolymer to which the present invention applies consists of basic copolymers produced by the 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 approximately 70 per cent by weight of polymer units derived from butadiene.
As a general proposition, the 0.05-2.0 megarad range of the dosages herein contemplated are too low to cause significant degradation, even of sensitive polymers, yet is enough to activate, trace quantities of monomer in the polymer, causing (it is theorized) linkage of the pre-viously free monomer molecules to the macromolecules of the polymer. Thetrace quantities of monomer are sensitive to the radiation likely because of the G-value factor alluded to previously. Tests indicate that 50% to 90% of the monomer content previously extractable from the polymer, is no longer present ~at least in an extractable form). The preferred range for practice of this invention is irradiation within the dosage range of 0.1-1.5 megarad.
Tests made on high quality bottles of acrylonitrile/styrene polymer (i.e. Cycle Safe TM) indicate that on an Pxtractable acrylonitrile content of 5 ppm could be significantly reduced by low level irradiation.
30 Unfortunately, not enough is known about the effects of the 0.05-0.5 dosage range to estimate whether articles containing higher residual mono-mer e.g. 30 ppm of acrylonitrile can be improved to below 10 ppm or whether the improvement is merely in some proportion to the original mono-mer content, as for example a 50% to 90% reduction.
When the radiation dose exceeds 0.5 megarads, and notably in the 0.5-2.0 megarads range, the well known chain scission, cross-linking etc.
rPactions attributable to high energy radiation can begin to occur. The range of 0.5~2.0 megarads is usable on the acrylonitrile polymers, be-cause such polymers are not particularly sensitive to degradation. It is believed that the 0.5-2.0 range is a usable range since side effects, including for example generation of HCN, crosslinking or polymer scission, are still at a minimum. In total the monomer reduction~is considered to be achieved almost entirely at the lower dose, with any further improve-ment in monomer reduction being believed to be rather nominal. A pre-ferred mode of operation for instance when treatment of the article with more than 0.5 megarad is desired would be a repeat 0.05-0.5 megarad treat-ment.
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 range treatment of the completed article.
Although the practice of this invention has been described gen-erally as pertaining to treatment of high quality finished articles, one ; variation thereof herein contemplated is application of the concepts underlying this invention to the polymer pellets or ground resin prior to molding into the final article. The pellets or ground resin are subjected to 0.05-0.5 megarads for purposes of reducing the monomer content therein, and then they are processed according to the usual practices in the art.
For high quality polymer, treatment of the pellets may suffice to reduce monomer content to acceptable levels, particularly when the end use can tolerate presence of a nominal monomer content in the article. The double treatment i.e. first of pellets or powder and then of the formed article can be used on high quality polymer simply to be certain that everything possible has been done to minimize monomer content in the article.
- Reverting back now to treatment in the 0.5-2.0 megarad range, as is also herein contemplated, allusion has already been made to generation - 30 of HCN from acrylonitrile polymers being about linear with dosage, and that the extractable HCN content as measured in the leachant would exceed 20 ppb after irradiation at 0.5-2.0 megarad. Accordingly, practice of this invention in the 0.5-2.0 megarad range on articles from nitrile poly-mers may well include a post-irradiation outgasing treatment to reduce the HCN content. Such a treatment need be nothing more than storage at mildly elevated temperatures e.g. llO~F for a few weeks or a wash with warm water with or without a HCw complexing agent therein. ID any eveDt the o~tgasing treatment reduces HCN content in the article to tolerable (taste) levels The outgasing treat~ent is more optional for polymer irradiated in the 0.05-0.5 megarad range.
The actual source of the high energy radiation is not material to practice of this invention, and therefore any of the known to the art ~ (commercially available) radiation sources are contemplated including for ; example radioaceive sources of high energy gamma rays such as radioactive cobalt and electron beam generators such as the "Dynamitronl~ Accord-ingly, further description of the high energy radiation source need not be provided, nor is there need to describe the details of the radiation treatment.
For further understanding of this invention reference is made to the following examples of practice thereof.

32 oz. sectioned bottles of acrylonitrile/styrene copolymers (Monsanto Cycle-Safe Coca Cola ~ottles) were irradiated by an electron beam (Radiation Dynamics, Inc.) generator at 0.10, 0.30 and 0.50 mega-rads. The bottles were then tested for residual monomer content after irradiation, and the results compared with monomer levels from the same bottle section not irradiated. In this set of tests only qualitative analyses were carried out, and considerable variation sample to sample was found. The control samples evidenced monomer contents of 1-15 ppm.
Comparable samples of irradiated bottle sections also showed variation sample to sample, but within a distinctly lower monomer content range.
Best results were obtained at 0.3 megarads. The monomer content in those samples tested out at 10% or less of the level in the unirradi-ated samples.

SAMPLE MARK HYDROGEN CYANIDE, PPM
30Control 0.019 0.05 0.052 0.10 0.105 0.20 0.280 0.30 0.350 350.40 0.440 0.50 0.610 0.75 0.76~
1.0 1.150 * Trademark -Extended storage, even at room temperature, for at least a week reduced the HCN (in leachant) at least 50%. Assuming the reduction in acrylonitrile monomer content in the plastic is proportional to HCN
evolution, it can be seen that at low doses acrylonitrile monomer content can be reduced.

In another test, a quantity of ground resin of acrylonitrile/
styrene copolymer as above was divided into four samples. The first sample as indicated below was not irradiated and served as a control and the other samples were irradiated, as follows:
Irradiation Dose Acrylonitrile Sam~ In Megarads _ Monomer ppm 2 0.1 15

3 0.5 11

4 1.0 10 The major reduction of the acrylonitrile monomer, 8 ppm, occurred at a dose level of 0.5 megarads while a small additional improvement, 1 ppm, occurred upon doubling of the dose to 1.0 megarad.

Two sets of fully formed bottles fabricated from the aforesaid 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 approximately 10 ppm and the second set of bottles had a monomer content of approximately 5 ppm. After irradiation at the 0 3 megarad dose level, the first and second sets of bottles had monomer levels of 1.5 ppm ` and 0.5 ppm, respectively; such reductions lying in the 85% to 90% range, It appears that the irradiation treatment is more effective in reducing monomer content on a percentage basis in the finished product as opposed to treatment prior to formation of the finished product.

Claims

WHAT IS CLAIMED:

1. A process for reducing the monomer content in acrylonitrile polymers which comprises exposing the polymers to ionizing radiation at an irradiation dosage level of from 0.050-2.0 megarads.

2. The process of claim 1 wherein the polymer is an acryloni-trile/styrene copolymer.

3. The process of claim 1 wherein the polymer is a formed article.

4. The process of claim 1 wherein the irradiation treatment is followed by an HCN out-gassing treatment.

5. A process for reducing the monomer content of an acrylo-nitrile based thermoplastic material which comprises exposing the material to ionizing radiation at an irradiation dose level of from 0.050-2.0 megarads.

6. The process of claim 5 wherein the acrylonitrile based thermoplastic material is an acrylonitrile/styrene copolymer.

7. The process of claim 5 in which the source of ionizing radiation is an electron beam.

8. The process of claim 5 wherein said acrylonitrile based thermoplastic material is selected from the group consisting of acrylo-nitrile homopolymers and copolymers of acrylonitrile with ethylenically unsaturated monomers selected from the group consisting of styrene, butadiene and (C1-C4) alkylesters of acrylic and methacrylic acid.

9. The process of claim 5 wherein the material treated is in ground resin or pellet form and wherein the exposure to ionizing radiation is also carried out on the article produced from the ground resin or pellets.

10. The process of claim 5 wherein the material treated is in solution or suspension as in a polymerization medium.