THERMALLY STABLE ETHYLENE/ACID COPOLYMERS
This is a continll~tion-in-part, of application Serial Terpolymers of ethylene; (meth)acrylic acid and alkyl acrylates form a class of acid copolymers with lower modulus and good low te,l,p~ ,al~re prop~ 1 Lies. A method of pl~p~ g these acid copolymers is disclosed in U.S.
Patent No. 4,690,981. They are used principally to form 'soft' ionomers by 5 neutralization of the acid. However, these acid copolymers with acrylates also find utility in their own right.
Ethylene/(meth)acrylic acid copolymers, incl~lAing terpolymers with an alkyl acrylate are A~Pficient in melt-thermal stability above 240~C. Melt flow starts to decrease. This is believed to be due to anhydride formation from 10 two carboxylic acid groups in ~djrc~ chains which form crosslinks, and hence reduce tractability and melt flow. This results in inc,eascd gel, dccledsed meltLd-lvability, and difficult extruder ~ ing after melt ~locessh-g in an extruder.Maleic anhydride as a monomer grafted onto ~-xicting polymers is well known as a means of obtaini.lg a polar functionality in polyolefin 15 polymers. Such graft-copolymers are useful as compatibiliz ing agents and as CO...pO~ t~ of hot-melt adhesives. There are also disclosures of maleic anhydride gra~ted ethylene/m-n~ rboxylic acid copolymers. Typical is JP-83-109721, which Ai~loses laminates where one layer is an ethylene/monoc~l~ylic acid polymer grafted with 0.05 to 5.0% maleic anhydride. However, ~fiting ~q~es an additional step after regular poly~ ;Qn~ and in a~lAitinn iS subject to considerable variation in product obtained, as well as ch~nges in p.op~.Les from ungrafted m3t~ 1 particularly a red~lction in melt flow. Re~Al~cti- n in melt flow is the very factor which melt-th~rm~l stabili7~tion seeks to avoid.
2 5 The problem of melt-thermal instability has been well recogllized, and various all~l..~ have been made to solve it. U.S. Patent No.
4,594,382 (~Ioening et al.), Ai~closcos that ~AAition of 5% or less of a hy~Ldled collli)uu.ld which deco~oses at from 100 to 300~C, such as hydrated ~ min~
i...~loves the melt-th~ stability.
3 o A related apl,lo~cl. is disclosed in U.S. Patent No. 5,276,135 (Powell), where allowing a small controlled h~cl~se in the (low) moisture content over the amount which tls rm~lly exists after air and nitrogen ~h~g, ,ç~ves melt-thprm~l stability. This patent also discloses that low t~ m~el.A~e polymerization of ethylene/(meth)acrylic acid copolymers, below 3 s the typical 200 - 270~C, produces polymer with greater melt-thermal stability.
This is said to be due to the greater nurnber of adjacent carboxylic acid units formed when low tc~l~.d~ polymP~i7~tion is used, a~lj~çnt monocarboxylic acid groups reacting together, releasing water and forming intrachain anhydride groups in prefcl~"ce to intclchdill anhydride groups when no diads are present.
5 However, low telmp~laluLc pol~ alion strongly de~rea3cs productivity, in addition to making polymer with quite di~r, nl ..,~çk~llical plop~.lies than polymer made at normal polymPri7~tion tel~lalu~s.
All these approaches are presllmed to be effective because water or released water s~pl~3ses illl~l~in anhydride crosslink formation. They 0 have the disadvantages of low produ-;livily, change in the plu~ llies of the polymer, or need for a significant amount of a particulate additive.
There is a need for a mPtho~ of improving the melt-thPrm~l stability of ethylene/(meth)acrylic acid copolymers, inclllfling terpolymers with alkyl acrylates, which does not involve (i) recl~lc-ing productivity during 5 plep~dlion of the copolymer, (ii) a change in ll~Pcl-AI-ic~l pl~p~"lies, (iii) the difficulty of producing polymer with controlled moisture levels, or (iv) does not require use of an additive.
SUMMARY OF TH~ INVENTION
2 o The invention depends on the ,ecog.. ;l;on that one can suppress unwanted anhydride formation (i~lt~,~hain), not just by encouraging the form~tion of, but by act~ally directly introducing anhydride units intr~h~in This can be achieved by introducing by copol~..,e~ ;on, anhydride or anhydride producing monomer into the polymer chain itself. Suitable~ 5 mol n mPrs for this ~ ose are dic~l,o~lic acid anhydrides, dicarboxylic acids elves, or dicarboxylic acid half esters.
More specifically, the invention is a co.l.posilion, compricing:
an ethylene/(meth)acr,vlic acid copolyrner having from 5 to 25 weight percent (meth)acrylic acid derived units, the acid copolymer having additionally 3 o copolyl~ chaill units derived from a further com-n.-.~ or comonomPrs selected from the group con~i~ting of maleic anhydride, itaconic anhydride, methyl hydrogen m~ t~, ethyl hydrogen m~le~t~ maleic acid, itaconic acid, and a ~ c of any of these monom~r~, the ;.~1~ rh~;-- units from the further comonnmer or comnnom~ being 3 5 present at a total level of 0.05 to 3.0 weight percent of the copolymer.
DETAILED DESCRIPTIO~ OF THE INVENTION
In this disclosure, the term copolymer means a polymer produced from more than one monomer. Copolymers may be dipolymers having only 5 two monoll,c.s copolymerized together, terpolymers or have more than three monomers. The copolymers of the invention are 'direct' copolymers, that is to say they are not graft-copolymers where monomer is pol~,ll.,.~ed in the plesence of polymer and the resl-lting polymer ~ rh~s to the .oYi~ting polymer chain. In this regard, the comol-n...~, ~ produce single 'i~ achain' units in the 10 copolymer, as distinct from either polymeric 'side-chain' units or single 'crosslink' units.
Copolymers have units derived from the various comonomers poly...- ;,.~.1 It is common to say polymers 'contain' a given monomer, polymers 'having' a certain amount of a given mo,lollll, or polymers 'o~ a givenmol~,ll~r, all being commonly acc~,~t~d shorthand for me~ning units derived from that m-)rl~m~r.
The ethylene/monocarboxylic acid copolymers of this disclosure have as the monoc ~-l~lic acid acrylic acid or methacrylic acid or both. These three possibilities are co~ liently referred to in the disclosure by using the 2 o term 'ethylenel(meth)acrylic acid copolymers'.
The term ';~ 11A;~ in this disclosure is used to distinguish only from mtel~l~in units which are crosslink units. The hlllachain anhydride ring does not ,~c~,ss~; ;ly include two backbone carbon atoms. Thus in the case of maleic a~ ;de the unit ~vill include two backbone carbon atoms, but in the 2 s case of it~onic acid which is methylene succinic acid, or its anhydride, the anhydride unit will not include two backbone carbon atoms.
The 'further com~n-~2n~(s)' of the copolymers of this invention, that is to say in addition to the ethylene, (meth)acrylic acid, are l"onvll,e.~
which have anhydride units or can readily directly lead to int~h~in anhydride 3 o units in the polymer. Suitable anhydride monomers are dic,~l.o~-ylic acid anhydrides such as maleic anhydride and itaconic anhydride. Mono..~ ~ which d~,tly yield ir~.,ch~iM anhydride units are dicarboxylic acids such as maleic acid, it~ nic acid, run~;C acid, and half esters of these acids. Such monomers have acid groups on ~;13ar~nt c~bol~s or an acid and an ester group on ~ c~.nt s 3 5 c~l/ons. The p,efell~d comon~-m~rs are maleic anhydride and ethyl hydrogen Ai~ r~ 'EC
I P--AJ'- ~
maleate. Most ~refe,led is maleic anhyd~ide. When the monomers are diacids they readily dehydrate and produce primarily intrachain anhydride units, in conkast to forrning interchain (crosslink) anhydride units as do monoc~rboxylic acid units.
The amount of the anhydride or anhydride producing comonomer is from O.OS to 3.0 weight percent, ~lc~eldbly 0.3 to 2.0 weight percent, and more preferably from 0.3 to 1.5, most preferably from 0.5 to 1.2 weight percent.Below O.OS there is littie or no improvement in melt-thermal stability. Above 3.0 percent there are flimini~hing return_, as well as inc,~ lg cost in producing 0 the copolymer, as well as the begi~ c of p.op. ,ly cll~ng~ in the polymer.The invention is applicable to copolymers c~ Ainit~ from 5 to 30 weight percent (meth)acrylic acid, preferably from 8 to 22 weight percent, the copolymers having an MI of from lO0 to O.l. The invention is particularly useful for low MI polymers, since when crosslinkin~ occurs with low MI (high molecular weight) polymers, the percent change in viscosity for a given molar amount of cros~linkin~ is greater ~han with a high MI polymer. It is thus particularly useful for acid copolymers having an MI of less than 20, and more particularly for copolymers having an Ml of less than lO. The invention is verv useful for the more common acid copolymers having no softening alkyl 2 o acrylate present. The invention does not include neutralized acid copolymers, i.e., ionomers. Ionomers retain more water, and for this and other reasons melt stability falls into a dirr~ nt category.
While not co.. ;~ to any particular theory, it is believed that the p~se.lcc; or formation of illtl~cha~l anhydride units su~plesses fomlAtion of 2 5 fur~er anhydride units of the illte.chain type, possibly partiy as a result of eqnilibrillm consid~ation~ Anhydride units formed from (meth)acrylic acid units are more likely to be ullw~lted interchain anhydride units which in.i~
viscosity, ~hcleds ;.~I,r rhA;~ anhydride units have virtually no effect on viscosity. Because of the particular ~ ~livilies of ethylene and 3 o (meth)acrylic acid, the ple~.,ce of two ~ nt acid units, at least for polymers with less than 20 weight percent _cid, is rare unless produced at low t~ c~ s where productivity is drastically re~luce(l Even then, such acid groups will not be ~nd~nl from adjn~nt ill~ chain c&l,ons, but on ~lt~ te carbon atoms along the chain.
The copolymers of this invention are produced by standard free-radical copolymeri7~tion methn~ls, under high p~ e, opc.dLing in a continuous ma~u1ei. Monomers are fed into the reaction ulixlu e in a p1(,po1lionwhich relates to the monomer's reactivity and the amount desired to be 5 inco1~o1dted. ~eacled monomers are recycled. In this way, uniform, near-random distribution of monomer units along the chain is achieved.
Poly,..~ ;on in this .11alu1.,- is well known, and is described in U.S. Patent No. 4,35 l ,93 l (~nnit~ge) which is hereby inco1~,dted by lef~ ,e11ce. At high acid levels it is an advantage to use so-called co-solvent technology to preventphase separation of monomer and polymer. This is fully described in U.S.
Patents Nos. 5,028,674 (Hatch et al.), and 5,057,593 (Stat_), both of which are also hereby incorporated by reference.
While the monocarboxylic acids and dic~bo~ylic acids may differ SO11~ ~.hdl in reactivity, they may be fed together as a mixed solution. The reactivity of the acids is so great colllpal~,d with ethylene that all acid is largely co~ led as it is introduced at the 1e4uilod rate for the ~llou,11~ of monomer wanted in the polymer.
2 o Exl~e.il11e. tal polymers cG~ ;nil-g either maleic anhydride or methyl hydrogen m~ t~ were ~ ~ed in a pilot plant unit in the n~lllc.
described above. Control s~les without the anhydride or illtldchail~
anhydride-formin~ monomer were also made and co111~ ed, and con~p~ ;sion was also madc with collJ~lh.~ial sdll~lcs of comparable co~osilion. The 2 5 maleic anllydride or ethyl hydrogen m~le~te was mixed with the meth~tylic or acrylic acid, formin~ a homogencous solution for feeding to the pilot plant dulocla~e which was ope~dl~d at 240~C and l 898288 g/cm2 (27,000 psi).
A~ î'!DcD SHEET
Composition of the polymers was determined using infrared absorbance. The methacrylic or acrylic acid content was determine~ at 940 cm~l and the anhydride content at 1783 cm~l. To insure the anhydride forming monomer ethyl hydrogen maleate was converted to anhydride for IR
5 analysis, the pressed film sample was treated at 290~C for one minute. It is believed that this II~AI~ II converts essenti~lly 100 % of the monomer to anhydride. A list of samples tested is given in Table 1. The ethyl hydrogen m~le3te (MAME) content shown refers to the weight percent of MAME
calculated from the amount of anhydride de~, ...it~ed by IR
0 Melt-thermal stability was ~e~ d by comp~;llg Melt Index (MI) at 190~C, using ASTM D-1238, condition E, before and after a heat tre~tm~nt at 290~C for 60 ..~il,..les in a melt indexer barrel. Prior to all MI
m~ cllt~ samples were dried for two days in a vacuum oven at 60~C. The Melt Index Ratio (MIR) is the ratio of MI after tre~trn~nt to MI before 15 ll~n~ rnt The higher the value, the less change, and more melt-th~rm~lly stable the polymer.
It is clearly hllpol t~l that the presence of anhydride at the levels used does not m~t~ri~lly change plo~llies (other than melt-thermal stability) COlllp~ll~ with co---~ ble ethylene/(meth)acrylic acid copolymers without 2 o intrachain anhydride units. In order to ~letermin~ that the copolymers with and without anhydride had comparable pro~.lies, adhesion to LLDPE and ~1.. ,,;.. foil were tested. ~llh~ )n to these substrates was ll~ea~ ,d as follows. Films of the polymers, .005 cm (2 mil) thick, were individually heat sealed to each substrate using a Sentinel Heat Sealer under the following conditions: 0.5 secon-lc dwell time, 2109 g/cm2 (30 psi) jaw p~,ss~, both jaws heated. The films were sealed to the LLDPE using tri-foil as a carrier on each side of the films. For sealing the films to foil, the tri-foil was used on the film side only. The a&esion test was p~ru~lcd using an Instron with the samples T-peeled at 25.4 cm (10 inches) per minute. Flexural modulus was also 3 o .-lea~cd on the s~mpl~s using ASTM D790. The results of the a&esion test, and the flexural modulus values ~ct~ d are given in Table 2.
While colllp~able ~lh~sion and flexural modulus to control samples does not ensure that all pro~.lies remain the same, adhesion and flexural modulus are h.llJoll~l ~no~ ies, and are sensili~ to acid cont~nt~ and AM~N~EQ SHEE T
~P~ ''J_'7 their measured values can provide a godd indication if any substantial change inother plo~,lies is likely to have taken place with anhydride modification.
Table 1 indicates ~at the presence of the anhydride or anhydride producing monomer improves melt-thermal stability signicantly. While there is 5 not a rigid q~ re trend, in terms of improved stabiltiy with increasing level of third mOll~m~r, values of MIR for controls, either standard plant m~t~ri~l or e~ ;...- nt~l S~~ 9, are all lower than polymers with maleic anhydride or cthyl hydrogen m~ te, and generally signific~ntly lower. Ofthe two s~hili~ing monGlu~,~, based on the limited data, there is no clear inl1ir?~tion 0 that one i~ beMer than the other.
Table 2 inr~ic~tes polymers with and without anhydride have generally CGl~ dble ~ h~cion values. ~-lh~cior~ based on sealing at 230~C is solllGv~l~t more v~;able and lower than adhesion values at 260~C.
Neverthele~s, there is no clear change when anhydride monomer is present.
5 Flexural moclul-lc values also indicate the plese.lce of anhydride does not produce any clear change.
There are no eY~nnples of copolymers co~ g an allcyl acrylate and the further comollolllc.s. However, the further com- nom~r or co.~-ono..~ s will be effective in stabilizing acid copolymers which also contain 2 o an alkyl acrylate.
, ... . . . .. . .
THERMAL STABILITY OF ANHYDRIDE-CONTAINING ACID
Polymer Composition Wei~eht percent MI MI after MI ratio # (v~.%) initial 290~/60min.
E/MAA/~LAH 87.5/11.9/0.6 31 20 0.645 2 E/MAA/MAME 85.8/13.1/1/1 31.1 24.1 0.775 3 E/MAA/M~I 87.2/12.0/0.8 34.4 28.4 0.816 4 E/MAA~ 88.0/11.1/0.9 23.7 18.3 0.772 E/MAA/MAH 93.9/5.7/0.4 43.9 38.1 0.868 6 E/MA~E 92.0/7.4/0.6 17.1 12.2 0.713 7 E/~IAAI~ 83.4/15.8/0.8 100.6 73.1 0.727 8 E/MAAI~ 89.5/10.0/0.5 6.65 3.75 0.564 9 EIMAA~ 91.5/8.1/0.4 23.4 15 0.641 E/MAA/M~ 87.3/11.8/0.9 10.9 7.6 0.697 lC E/MAA 90.0/10.0 10.3 4.8 0.466 11 E/A.A/MAH 90.4/8.9l0.7 12.1 6.74 0.557 2C E/AA 91.8/8.2 9.4 3.57 0.380 3C-P E/MAA 91.0/9.0 9.1 4.6 0.505 4C-P E/MAA 91.0/9.0 8.6 3.4 0.395 5C-P E/MAA 90.0/10.0 33 15.5 0.470 6C-P E/MAA 85.0/15.0 45.3 24.8 0.547 7C-P E/AA 91.0/9.0 10 5.29 0.529 E = Ethylene; MAA = Methacrylic acid; AA = Acrylic acid; MAH = Maleic anhydride; MAME = Monoethyl ester of maleic acid (ethyl hydrogen m~le~te).
Suffix -P = Co~ l .~ial Plant m~teri~l: All other samples pilot plant m~teri~l.
, , ." . , t PROPERTIES OF ANHYDRIDE-CONTAINING ACID COPOLYMERS
PolYmer SealTemp. PeelS~ren~ Peel Stren~th Flexural # ~ F toLLDPE to 1 mil.Al foil Modulus ~/cm2 (~si) k~/cm2(Kpsi) 230 14.8 (0.21) 0.57 921.0 (13.1) 250 96.3 (1.37) 0.72 lC 230 54.8 (0.78) 0.17 991.3 (14.1) 250 123.0 (1.75) 0.46 11 230 53.4(0.76) 0.98 949.1 (13.5) 250 106.9 (1.52) 1.41 2C 230 38.0 (0.54) 0.66 921.0 (13.1) 25~ 121.0 (1.72) 1.71 Ah~N~?E2 SHEET