NOVEL COPOLYMERS

Abstract: A single phase graft copolymer comprising the free-radial polymerisation reaction product of a polyurethane and (meth)acrylic monomers has improved glass transition temperature and low temperature properties.

Description: NOVEL COPOLYMERS The present invention relates to new polymers comprising the reaction product of a polyurethane resin and acrylic monomers, the process by which the polymers are produced and applications of the polymer formed. Acrylic/polyurethane polymers and polymer compositions have been made by a number of techniques. One such method involves reacting 2-hydroxyethyl acrylate with an isocyanate terminated prepoly er to produce an acrylate- terminated polyurethane with pendant ethylenically unsaturated groups. Another method is to polymerise a mixture of polyols, isocyanates, acrylic monomers and peroxides ensuring simultaneous polymerisation of acrylic monomers to produce a polyacrylic polymer and of the polyols and isocyanates to form a polyurethane polymer in an interpenetrating network. In these interpenetrating polymer networks the two polymer types remain as discrete molecules forming distinct but intertwining continuous polymeric domains, physically entangled but with no covalent bonds between polymer types. The present invention provides a single phase graft copolymer which comprises the free radical polymerisation reaction product of a polyurethane and (meth)acrylic monomers. As used herein the term ■• (meth)acrylic monomers" refers to both acrylic monomers and methacrylic monomers. Similarly, the term " (meth)acrylateε" refers to acrylates and methacrylates. The (meth)acrylic monomers useful in the invention include those of formula (I) below: wherein R 1 is hydrogen or methyl and R 2 is hydrogen or a C,_ 20 branched or straight carbon chain which may be substituted by a hydroxy group. Mixtures of any two or more of the above-mentioned monomers may also be used. Without wishing to be bound by this theory, the present inventors believe that the free-radical polymerisation reaction conditions result in proton abstraction from methylene groups in the polyurethane. The growing poly(meth)acrylic polymer chains are terminated by bonding to the radical so formed and/or new poly(meth)- acrylic polymer chains are initiated thereby, resulting in formation of graft copolymers. It is further believed that the proton abstraction occurs at methylene groups activated by isocyanate or urethane groups or the residues thereof; thus grafting may occur at the methylene carbons directly bonded to isocyanate groups or the residues thereof. For this reason it is preferred that the polyurethane is the reaction product of an isocyanate monomer including at least one of the above features, for instance isophorone diisocyanate (which contains an isocyanatomethylene group) , hexamethylene diisocyanate (which contains two isocyanatomethylene groups) and ,4* * -dicyclohexylmethane diisocyanate (which contains a methylene group bonded to two isocyanate groups via 1,4-cyclohexylidene groups) . Presently, isophorone diisocyanate is particularly preferred. In accordance with this theory, the polymers of the present invention preferably comprise blocks of polyurethane polymer chains and blocks of poly (meth)acrylic polymer chains grafted to the polyurethane chains, via methylene carbons of the polyurethane blocks. Preferably, the polymers of the invention are copolymers of formula (II) U 1 CH •U z CH (ID R 2 00C C A wherein R 1 and R 2 are as defined in relation to formula (I) ; U 1 and U 2 are blocks of polyurethane polymer optionally containing further graft points; and A is a block of poly(meth)acrylic polymer. The carbon atom marked with an asterisk is a graft point, ie the carbon atom of the activated methylene group, preferably the methylene carbon atom in the isocyanatomethylene group of a residue of isophorone diisocyanate. Preferably the graft copolymer of the invention comprises at least 1% by weight and up to or more than 10% by weight of polyurethane based on the weight of the copolymer, and may comprise up to 50% by weight of polyurethane based on the weight of the copolymer. The invention further provides a single phase graft copolymer obtainable by free-radical polymerisation of a (meth)acrylic monomer in the presence of a polyurethane and a solvent. Surprising properties have been obtained in films made from a polymer of the present invention when compared to conventional commercially available solution acrylic resins based on methyl methacrylate which have not been modified with polyurethane. The glass transition temperature and therefore the upper usage temperature of the polymer is increased by the addition of up to 20% polyurethane (see Test Example 1) by weight based on the total weight of the graft copolymer. Addition of polyurethane shows further advantages in the low temperature region; improvements may be seen in reduced cold cracking (after repeated heating and cooling cycles) and increased low temperature flexibility (see Test Example 2) . Increased solvent resistance and improved adhesion are also obtained with * polymers of the invention. This suits the polymers of the invention for use in coatings, especially vehicle refinishing coatings (paints, varnishes and primers) , in which the polymer of the invention acts as binder and the other components are standard. The present invention further relates to a process for the preparation of a copolymer as defined above, which process comprises reacting a polyurethane with a (meth)acrylic monomer in the presence of a solvent or dispersion medium and free radical initiator. The process is preferably conducted at or about the boiling point of the mixture, for instance from 60 to 150°C, depending on the choice of solvents or dispersion medium and curatives. Any conventional free radical initiator is suitable for use in the polymerisation reaction, for example peroxide initiators such as benzoyl peroxide, lauryl peroxide, t-butyl peroxy-2-ethylhexanoate, di-t-butyl peroxide, or t-butyl perbenzoate and persulphate initiators such as potassium persulphate. Solvents which may be used include inert or active-hydrogen containing organic solvents, such as acetates, ketones, aromatic and aliphatic hydrocarbons and alcohols, and mixtures of any two or more of these solvents. Preferred solvents include methyl ethyl ketone and toluene. As mentioned above the free-radical graft polymerisation reaction may be conducted using active- hydrogen containing solvents, such as alcohols, but only when the polyurethane resin has no, or only very few, remaining free isocyanate groups. If free isocyanate groups are present in the polyurethane, it is preferred to use a suitable insert solvent as described below. The free radical graft polymerisation may also be conducted in non- solvent dispersion media, such as aqueous media optionally containing surfactants or emulsifying agents. Again active hydrogen-containing media are preferably avoided when the polyurethane still contains a significant proportion of free isocyanate groups, for instance over about 2% free isocyanate groups. Water, for instance demineralised water, and aqueous surfactants, such as sodium lauryl sulphate, are preferred. When using t-butyl peroxy-2-ethylhexanoate as initiator it has been found particularly convenient to use a mixture of methyl ethyl ketone and toluene, for instance at from 1:4 to 1:1, preferably 1:2 by volume. The polyurethane may be produced by reacting (a) a diisocyanate monomer and (b) an active hydrogen compound in the presence of a suitable inert solvent by conventional methods. The reactants are preferably used in proportions such that a polyurethane prepolymer with terminal isocyanate groups is formed and this prepolymer is then optionally further reacted with (c) a difunctional primary or secondary active hydrogen containing chain extender, such as an amine or alcohol, in the presence of a suitable solvent or diluent. Suitable solvents for use in production of the polyurethane are those which are inert to the isocyanate reagent, i.e. which do not contain active hydrogens; water, alcohols and amines are therefore to be avoided at this stage. Preferred solvents for this stage are organic solvents such as acetates, ketones and aromatic and aliphatic hydrocarbons. Diluents which may be used in addition to or in place of solvent include liquid organic compounds such as (meth)acrylate monomers which are to be used in a subsequent stage of the reaction provided that these are inert to the reaction conditions during the polyurethane polymerisation, for instance when there is no free radical initiator present . These reactions are conveniently carried out at temperatures ranging from 35 to 110°C, preferably in an inert atmosphere. The free radical graft polymerisation reaction will occur in the presence of low concentrations (for instance less than 10%, preferably less than 5% by weight) of free isocyanate groups so it is not essential that all isocyanate groups are consumed during production of the polyurethane. It is, however, preferred that the number of free isocyanate groups is kept to a minimum by use of appropriate proportions of active hydrogen compound and/or by use of chain extenders. Most preferably, all isocyanate groups are consumed in the production of the polyurethane. The diisocyanate monomer (a) may be any aliphatic or aromatic diisocyanate but is preferably isophorone diisocyanate, hexamethylene diisocyanate or 4,4'- dicyclohexylmethane diisocyanate; most preferably, the diisocyanate monomer is isophorone diisocyanate. The active hydrogen component (b) may be a diol, such as poly(tetramethylene glycol) or or preferably a polypropylene glycol, a polyol such as a polyether polyol, polyester polyol or a polycaprolactone polyol. The chain extender (c) may be any difunctional primary or secondary amine or alcohol, for example isophorone diamine, ethylene diamine, 4,4 -diphenylmethane, or 4,4'- dicyclohexylmethane diamine or dimethylol propionic acid and is preferably isophorone diamine. The (meth)acrylate monomers may be those defined by formula (I) above, and are preferably methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, acrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl methacrylate, lauryl methacrylate, stearyl methacrylate, 2-hydroxypropyl methacrylate, methacrylic acid, or a mixture of any two or more of the above mentioned monomers, and most preferably methyl methacrylate is used. The molecular weight (eg as determined by gel permeation chromatography against polystyrene standards) of the graft copolymer materials produced may range from 1,000 to 500,000 and is usually approximately 150,000 to 250,000. The present invention is illustrated by reference to the accompanying drawings, in which: Fig. l shows a 400x magnification of a dried polymer film made using a polymer of the invention, Fig. 2 shows a 400x magnification of a material, not of the invention, where mechanical blending has been used to mix a polyurethane and a polymethacrylate, and Fig. 3 and Fig. 4 show 1H n. .r. fourier transform spectra of isophorone diisocyanate in d-chloroform and the reaction mixture at completion of the polymerisation reaction described in the Reference Example. Figure 1 clearly shows that the film formed of the polymer of the invention comprises a single phase copolymer whereas Figure 2 shows the separate phases which result from physically blending two polymer types. The following Examples are intended to illustrate the invention but are not intended to limit the scope of protection in any way. EXAMPLE 1 (a) Polyurethane Preparation Isophorone diisocyanate (200.6g) was introduced to a vessel under nitrogen. Polycaprolactone diol (1000 MWt - 426g) and toluene (412g) were added and the reaction mixture was heated to from 80 to 85°C and held for 3 hours to reach a final isocyanate content of from 3.8 to 4.1%. The reaction product was then cooled to from 25 to 30°C and toluene (56g) , isopropanol (470g) and isophorone diamine (56g) were added with vigorous stirring. The resultant product had a Brookfield viscosity of from 15,000 to 25,000 mPa s at 25°C. (b) Graft Copolymer Preparation Polyurethane prepared according to part (a) above (llOg) and methyl ethyl ketone (260g) were introduced into a vessel fitted with a condenser and heated to a reflux temperature of from 80 to 85°C. Methyl methacrylate (315g) was added slowly with a simultaneous addition of benzoyl peroxide (4g) . The mixture was reacted for 4 hours at reflux temperature, then toluene (378g) was added, followed by cooling. The resultant product had a Brookfield viscosity of from 1,500 to 3,000 mPa s at 25°C. EXAMPLE 2 Process for producing an electrodepositable urethane/methacrylic copolymer. (a) Polyurethane Preparation Isophorone diisocyanate (157g) was introduced to a vessel under nitrogen. Polycaprolactone diol (1000 MWt - 335g) was added and the reaction heated to from 80 to 85°C and held for 3 hours to reach a final isocyanate content of from 6.2 to 6.4%. The reaction mixture was then cooled to from 25 to 30°C and propyleneglycol monomethyl ether (1647g) , Caromax 15/18 (aromatic hydrocarbon) (330g) and isophorone diamine (61g) added with vigorous stirring. (b) Graft Copolymer Preparation Polyurethane prepared according to part (a) (500g) was introduced into a reaction vessel fitted with a condenser and heated to reflux temperature of from 120 to 125°C. Butyl acrylate (470g) , methacrylic acid (92g) , methyl methacrylate (225g) , 2-hydroxyethyl acrylate (144g) and lauryl ercaptan (3g) were mixed and this pre ix was added over 4 hours, with a simultaneous addition of t-butyl peroxy-2-ethylhexanoate (17g) , to the reaction vessel. The product was heated under reflux for a further 2 hours. After this time propylene glycol monomethylether (50g) was added, mixed for 30 minutes then cooled. The resultant product had a Brookfield viscosity of from 50,000 to 70,000 mPa s at 25°C. EXAMPLE 3 The following were charged to a two litre four-neck round bottom flask, equipped with a thermometer, mechanical stirrer and heating mantle: Isophorone di-isocyanate (200 g) , a polypropylene glycol (800 G) having a hydroxyl number of 28.05 mg potassium hydroxide/g. Dibutyl tin dilaurate (0.1 g) as catalyst was added. Thereafter the reaction was mixed and the mixture was heated to 80-85°C. This temperature was maintained for 2 hours; the mixture was then cooled to 0°C at which point the isocyanate content was found to be 4.175% To this mixture was charged methyl methacrylate (500g) as diluent and dimethylol propionic acid (44.44 g) as chain extender. Thereafter the mixture under stirring was heated to 85°C and maintained at this temperature for a further 2% hours at which point the isocyanate content was found to be 1.82%. The reaction mixture was then cooled to 40°C. this mixture is known as the prepolymer. Into similar apparatus (fitted with the above equipment and two dropping funnels) was charged de ineralized water (506g) and sodium lauryl sulphate (50g of 30% aqeuouε solution) . To this mixture under stirring, was added the above prepolymer (337g) over 20 minutes. Immediately after finishing the prepolymer feeding, hydrazine monohydrate (3.75g) was added to the obtained dispersion. This mixture was allowed to react for half an hour whereafter further demineralized water (388.5g) was added and the whole dispersion heated to 85°C. The temperature of 85°C was maintained during the following addition. From two separate dropping funnels methyl methacrylate (112.3g) and an aqueous solution of potassium persulphate (2.5g in lOOg of water) were added over a period of 1 hour. The resulting graft copolymer was maintained at a temperature of 85°C for a further 1% hours. It was then cooled to less than 40 C C. The resulting product shows GPC molecular weight distribution consistent with graft polymerisation, which is also supported by H 1 N.M.R. results for the relevant stages. TEST EXAMPLE 1 The variation of glass transition temperature ( transition) with polyurethane U1000 contents in a graft copolymer of the invention was investigated. The results are presented in Table 1. TEST EXAMPLE 2 Low Temperature Properties A cold cracking test was carried out to show low temperature properties of samples mentioned in Table 1. The conditions were 5 cycles of 24 hours in a relative humidity cabinet (50%RH) , 16 hours at -25°C then 8 hours at Room Temp. The results are shown in Table 2 TEST EXAMPLE 3 The solubilities of the polymers produced in Examples 1 and 2 were investigated and found to be similar to that of the corresponding acrylic homopolymer, indicating a very low degree of crosslinking if any at all, and that therefore a graft copolymer has been produced. TABLE 1 Sample ϋrethane Tg °C Tg °C content (tanrS) (log E") AC1 118 102 (control) (control) Table 2 Cold Cracking Sample Score* Scale 1-10; l=poor, 10=excellent REFERENCE EXAMPLE In order to investigate the graft site in one phase copolymers made by the process described hereinabove, the following experiments were conducted on model systems. 1) Methyl methacrylate was polymerised using a peroxide initiator in the presence of isophorone diisocyanate: NCO The resultant material was analysed by n.m.r. Fourier Transform spectroscopy using a Jeol GX-270 MHz spectrometer and chloroform as solvent. Due to the low definition and high interference it was not possible to observe proton abstraction from the isophorone at position 1, which would have been expected on theoretical grounds. However, during the course of reaction, disappearance of the peak at d3.05 was observed. This was interpreted as showing abstraction of the secondary type isocyanatomethylene protons at position 7 and was not expected. In the reaction product this peak was replaced by a doublet at d2.96. The final n.m.r. spectrum is shown in Figure 4 and may be compared with that of isophorone diisocyanate in Figure 3. 2) Methyl methacrylate was polymerised using a peroxide initiator in the presence of 1,6-hexamethylene diisocyanate. Again the n.m.r. results suggest the abstraction of the secondary proton from the isocyanato¬ methylene group. 3) Isophorone diisocyanate was reacted with n-octanol and methylmethacr late was added and polymerisation was initiated with a peroxide. Quantitative chain transfer experiments were carried out and the chain transfer constant (CS) was calculated to be 4.4X10 " at 80°C. This can be seen to be high when compared to the values below from "Principles of Polymer Chemistry" - Flory. Benzene 0.075x10"* . Toluene 0.52x10"* Ethyl acetate 0.24x10"* Carbon tetrachloride 2.39x10"* Isopropylbenzene 1.90x10"* Infra-red analysis showed no reduction of the N- H absorbance at 3,500cm "1 after polymerisation of the methyl methacrylate, which indicates that the N-H is not directly involved in the grafting process. 4) Attempts to graft onto polycaprolactone polyols were unsuccessful showing the methylene protons here cannot be abstracted by a free radical under these conditions. This work confirmed that unexpectedly a graft copolymer structure has been obtained where grafting occurs at the isocyanatomethylene group on the isocyanate. It is believed that the same grafting occurs in copolymerisation of the polyurethane and (meth)acrylic monomers according to the invention but it is not easy to demonstrate this by simple techniques such as used in the model systems above.

Claims: CLAIMS 1. A single-phase graft copolymer which comprises the free-radical polymerisation reaction product of a polyurethane and (meth)acrylic monomers. 2. A graft copolymer comprising polyurethane blocks and poly(meth)acrylate blocks grafted to the polyurethane blocks via methylene carbons of the polyurethane blocks. 3. A copolymer according to claim 2 of formula (II) U 1 CH U 2 I wherein R 1 and R 2 are as defined in relation to formula (I) ; U 1 and U 2 are blocks of polyurethane polymer optionally containing further graft points; and A is a block of poly(meth)acrylic polymer. 4. A copolymer according to any one of the preceding claims, comprising the residues of at least one (meth)acrylic monomer selected from methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-hydroxyethyl acrylate, acrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl methacrylate, lauryl methacrylate, stearyl methacrylate, 2- hydroxypropyl methacrylate and methacrylic acid or of a mixture of any two or more of the foregoing (meth)acrylic monomers . 5. A copolymer according to any one of the preceding claims having a molecular weight of the copolymer in the range of from 1,000 to 500,000. 6. A copolymer substantially as hereinbefore described in any one of Examples 1, 2 or 3. 7. A process for the preparation of a graft copolymer as defined in any one of the preceding claims, comprising reacting a polyurethane resin and at least one (meth)acrylic monomer in the presence of a free-radical polymerisation initiator and a solvent or dispersion medium. 8. A process according to claim 7 conducted in the presence of an aqueous dispersion medium. 9. A process according to claim 7 conducted in the presence of an inert organic solvent. 10. A process according to any one of claims 7 to 9 wherein the initiator is a peroxide initiator. 11. A process according to claim 10 wherein the peroxide initiator is benzoyl peroxide, lauryl peroxide, t-butyl peroxy-2-ethylhexanoate, di-t-butyl peroxide, or t-butyl perbenzoate. 12. A process according to any one of claims 7 to 11 comprising reacting (a) a diisocyanate monomer and (b) an active hydrogen compound in an inert solvent or diluent to form a polyurethane and reacting the polyurethane with a (meth)acrylic monomer. 13. A process according to claim 12 wherein the diisocyanate monomer (a) and the active hydrogen compound (b) are reacted in proportions such that a polyurethane prepolymer having terminal isocyanate groups is formed, and wherein the process further comprises reacting the prepolymer with (c) a difunctional primary/secondary amine chain extender in the presence of a solvent or diluent to form the polyurethane. 14. A process according to claim 12 or claim 13 wherein the diisocyanate monomer (a) is isophorone diisocyanate. 15. A process according to claim 12 or claim 13 wherein the diisocyanate monomer (a) is 4,4'- dicyclohexylmethane diisocyanate. 16. A process according to claim 12 or claim 13 wherein the diisocyanate monomer (a) is hexamethylene diisocyanate. 17. A process according to any one of claims 12 to 16 wherein the active hydrogen compound (b) is a diol or a polyol. 18. A process according to claim 17 wherein the active hydrogen compound (b) is a diol. 19. A process according to claim 18 wherein the diol is a poly(tetramethylene glycol) . 20. A process according to claim 18 wherein the diol is a poly(propylene glycol) . 21. A process according to claim 17 wherein the active hydrogen compound (b) is a polyol. 22. A process according to claim 21 wherein the polyol is a polycaprolactone polyol. 23. A process according to any one of claims 13 to 22 wherein the amine chain extender (c) is isophorone diamine, ethylene diamine, 4,4'-diphenylmethane diamine, or 4,4'- dicyclohexylmethane diamine. 24. A process according to claim 23 wherein the amine chain extender is isophorone diamine. 25. A process for the production of a copolymer substantially as described in any one of Examples 1, 2 or 3. 26. A copolymer according to any one of claims 1 to 6 when produced by a process according to any one of claims 7 to 25. 27. A polymer composition comprising a copolymer according to any one of claims l to 6 and 26. 28. A coating or an adhesive composition comprising a copolymer according to any one of claims 1 to 6 and 26.

Inventors: LANCASTER IAN, SPENCER RICHARD, LANCASTER, IAN, SPENCER, RICHARD
Applicants: BAXENDEN CHEM, BAXENDEN CHEMICALS LTD
Application year: 1992
Main classification: C08F283
IPC codes: C08F283