GB2092604A - A process for producing a transparent thermoplastic graft copolymer composition - Google Patents

Abstract

A thermoplastic resin composition is prepared by graft copolymerizing under emulsion polymerization conditions, an unsaturated monomer mixture consisting of at least two unsaturated monomers selected from alkenyl aromatic monomers, methacrylate monomers and vinyl cyanide monomers onto a diene polymer latex having a gel content of from 60 to 90% by weight and an average particle size of from 0.1 to 0.5 mu m. The amounts of diene polymer latex (in terms of solids) and of monomer mixture are from 5 to 40% and from 95 to 60% by weight, respectively. The composition of the monomer mixture is so chosen that the refractive index of the resin phase if the graft copolymer is substantially identical to the refractive index of the rubbery diene polymer of the latex. By carrying out the graft copolymerization in the presence of a sodium or potassium di(C1-C6 alkyl and/or C6-C9 aryl)dithiocarbamate i.e. R2N.(CS.SM) and ensuring that the grafting degree of the graft copolymer is within the range form 30 to 70% by weight, it is possible to produce a thermoplastic resin composition having excellent transparency, impact resistance and processability.

Description

SPECIFICATION A process for producing a transparent thermoplastic resin composition The present invention relates to a process for producing a thermoplastic resin composition having excellent transparency, impact resistance and processability by graft copolymerizing a monomer mixture (comprising two or more alkenyl aromatic, methacrylate or alkenyl cyanide monomers) on a backbone rubber provided by a diene rubber latex under emulsion polymerization conditions.

It is known to produce resins by graft copolymerizing on a diene rubber an unsaturated monomer mixture comprising, for example, styrene, methyl methacrylate and acrylonitrile. Where the composition of this monomer mixture is so chosen as to give a resin having substantially the same refractive index as that of the diene rubber, the product, generally known as an MBS or MABS resin, can be made transparent and hence can be used for applications where transparency as well as impact resistance are required.

These MBS and MABS resins must have good transparency, impact resistance and processability, of which good transparency is particularly important as it determines the potential uses of the resin.

Good transparency requires the resin should be colouriess or substantially colourless, and this leads to an enhancement of the colourability of the resins. However, when such a resin composition is produced by emulsion polymerization, water-soluble unsaturated monomers (such as acrylonitrile or methyl methacrylate) tend to undergo side reactions in the aqueous phase (for example, acrylonitrile can be hydrated to give ethylene cyanohydrin or acrylamide), and the resulting by-products are the principal cause of coiouration of the end product.

We have now found that it is possible to produce a resin composition which is substantially colourless and has good colourability, and which also has excellent transparency, impact resistance and processability by carrying out the emulsion polymerization in the presence of a specific class of dithiocarbamates, using a diene polymer latex having specific properties as the backbone rubber, and adjusting the grafting degree of the resulting composition to within a specific range.

Thus, the present inventicn consists in a process for producing a thermoplastic resin composition by graft ccpolymerizing an unsaturated monomer mixture on a diene polymer latex under emulsion polymerization conditions in the presence of a dithiocarbamate of formula

(in which R and R2 are the same or different and each represents a C1-C6 alkyl group or a C6Cg aryl group, and M represents a sodium or potassium atom), to give a graft copolymer having a degree of grafting of from 30 to 70% by weight, in which:: said diene polymer latex is employed in an amount, in terms of solids, of from 5 to 40% by weight; said monomer mixture is employed in an amount of from 95 to 60% by weight; said diene polymer latex has a gel content of from 60 to 90% by weight and an average particle size of from 0.1 to 0.5 ,um; and said monomer mixture comprises two or more monomers selected from alkenyl aromatic monomers, methacrylate monomers and alkenyl cyanide monomers, and has a composition such that the refractive index of the resin phase of said graft copolymer is the same as or substantially the same as the refractive index of the rubbery polymer in said latex.

We have found that employing, as the backbone rubber, a diene polymer latex having a gel content of from 60 to 90% by weight, preferably from 70 to 80% by weight, has a particularly good effect on the properties of the graft copolymer produced. If the gel content is less than 60% by weight, sufficient transparency cannot be obtained unless the mould temperature when injection moulding is made especially high. On the other hand, if the gel content exceeds 90% by weight, the impact resistance is reduced. Furthermore, the average particle size of the diene polymer latex used as the backbone rubber should be from 0.1 to 0.5 ,um in order to give the final resin composition satisfactory transparency, impact resistance and processability.

Suitable rubbery polymers for use in the diene polymer latex include, for example, butadiene polymers, styrene-butadiene copolymers and acrylonitrile-butadiene copolymers.

It is necessary that the unsaturated monomer mixture, which is graft copolymerized onto the backbone rubber of the diene polymer latex, should have such a composition that the refractive index of the resin phase of the resulting graft copolymer is the same as or substantially the same as that of the backbone rubber polymer and preferably the difference between the two refractive indices is not more than 0.01. This is necessary to impart transparency to the thermoplastic resin composition. More preferably the difference between the refractive indices is not more than 0.005 and most preferably about 0.003.

In general, the refractive index, N(P}, of a polymer component haste following relationship to the unsaturated monomer composition: N(P1)xW1xN(P2)W2+...

N(P)= W1 + W2 + ...

in which Wr, W2 etc. represent the weight fractions of unsaturated monomers, 1, 2, etc. in the polymer component, rnspectively, and N(P,), N(Pz) etc. represent the refradtive indices of homopolymers of the unsaturated monomers, 1, 2 etc. respectively. Accordingly, it is possible to determine in advance the proportions of unsaturated monomers necessary to achieve the desired refractive index of the resin phaSe of the graft copolymer.

The alkenyl aromatic monomer, which is one df the possible components of the unsaturated monomer mixture, is preferably a compound in which an aromatic group, preferably a benzene ring, is attached to one of the carbon atoms of the alkenyl group, i.e. it is preferably styrene or an alkyl substituted styrene. Suitable such alkenyl aromatic compounds include, styrene, methyl styrene and vinyltoluene. The methacrylate monomer is preferably an alkyl methacrylate, such as methyl methacrylate or ethyl methacrylate. The alkenyl cyanide monomer is preferably a compound in which the cyanide group is attached directly to one of the unsaturated carbon atoms of the alkenyl group, i.e. it is preferably vinyl cyanide or an alkyl-substituted derivative thereof. Suitable such alkenyl cyanides - include acrylonitrile and methacrylonitrile.Particularly preferred combinations of these unsaturated monomers are: a mixture of methyl methacrylate with styrene; or a mixture of methyl methacrylate, styrene and acrylonitrile.

When carrying out the graft copolymerization, the dithiocarbamate is added in order to prevent colouration arising from the aforementioned side reactions. of water-soluble unsaturated monomers in the aqueous phase. The amount of dithiocarbamate to be added will vary depending upon, for example, the kind and amount of polymerization initiator used, but we normally preferto employ from 0.001 to 0.1% by weight of the dithiocarbamate, based on the sum of the weights of the diene polymer and the unsaturated monomer mixture. If the amount of dithiocarbamate is less than 0.001% by weight, it has no perceptible effect. On the other hand, if this amount is too large, it can reduced polymerization conversion.The dithiocarbamate may be added either as a single portion before initiatiny the polymerization or it may be added throughout the reaction in harmony with the addition of the unsaturated monomer mixture.

Specific examples of dithiocarbamates which may be employed are sodium and potassium dimethyldithiocarbamates, sodium and potassium diethyldithiocarbamates, sodium and potassium dibutyldithiocarbamates, sodium and potassium ethylphenyldithiocarbamates, sodium and potassium diphenyldithiocarbamates and sodium and potassium ditolyldithiocarbamates. The effect of these dithiocarbamates can be enhanced by using with them one or more other stabilizers, for example organophosphorus compounds [such as phosphaphenanthrene oxide or tris(nonylphenyl) phosphite] or phenol compounds (such as di-t-butyl-p-cresol).

It is necessary that the graft copolymerization process of the invention should be effected so that the grafting degree (the increase in the amount of acetone-insoluble matter on the basis of the backbone rubber) of the resulting graft copolymer should be from 30 to 70% by weight If the grafting degree is less than 30% by weight, the resulting composition has poor transparency and a composition having inadequate impact resistance and processability can only be obtained, whether or not the dithiocarbamate is employed. On the other hand, if the grafting degree exceeds 70% by weight, the resulting composition has greatly inferior impact resistance and processability and hence it has little practical value.

The amount of unsaturated monomer mixture employed in the process of the invention is from 95 to 60 parts by weight per 5 to 40 parts by weight (in terms of solids) of the diene polymer latex. If the proportion of diene polymer is less than 5% by weight, the impact resistance of the graft copolymer is inadequate. On the other hand, if this amount exceeds 40% by weight, processability deteriorates and the mechanical strength becomes insufficient.

The graft copolymerization process of the present invention may be carried out by conventional means, using any emulsifier of the type commonly used for this kind of process. For example, the emulsifier may be selected from alkyl alcohol sulphate, a-sulphofatty acid ester salts, fatty acid amide ether sulphates, beef tallow methyl taurate, fatty acid salts, disproportionated rosinates, arylalkylsulphosuccinates, and alkylbenzenesulphonates, although nonionic and cationic surfactants may also be used. Particularly preferred emulsifiers are amido bond-containing sulphonates, such as fatty acid amide ether sulphates and beef tallow methyl taurate, and carboxylates, such as fatty acid salts and disproportionated rosinates.

The polymerization reaction may be initiated by a radical polymerization initiator (such as potassium persulphate, sodium persulphate or ammonium persulphate) or a redox initiator (for example one using cumene hydroperoxide, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide or t butyl hydroperoxide).

A polymerization regulator is normally present in the reaction mixture, for example a mercaptan having a high chain transfer effect, for example t-dodecylmercaptan or n-dodecylmercaptan.

At the end of the process of the invention, the reaction mixture is generally in the form of a latex containing the graft copolymer, Which may then be coagulated with a coagulant (for example an inorganicacid.such as hydrochloric acid or sulphuric acid, or a metal salt such as potassium chloride or magnesium. sulphate), after which the resulting coagulant is filtered off, washed with water and then dried to obtain a powdery product. A metal salt is preferably employed as the coagulant If necessary, it is possible to incorporate into the graft copolymer obtained by the process of the invention conventional additives, such as stabilizers, antioxidants, lubricants, plasticizers, dyes and pigments.

This invention is further explained below in-detail referring to Examples, which are not by way of limitation but by way of illustration. Unless otherwise specified, parts in the Examples and Comparative Examples are by weight.

EXAMPLES 1 AND 2 Polymerization was effected in a 7-liter, three-necked glass flask according to the following recipe: Polybutadiene latex 20 parts (gel content 75% by (in terms weight, average particle size 0.24y, of solids) refractive index n,25 1.515)

Styrene 18 Acrylonitrile 4 Component I Methyl methacrylate 58 t'ert-Dodecylmercaptan 0.5 Sodium diethyldithiocarbamate 0.01 Sodium salt of beef tallow fatty acid 1.5 Component II Water 100 Cumene hydroperoxide 0.2 Formaldehyde sodium sulfoxylate 0.1 Ferrous sulfate 0.01 Component Ill Sodium ethylenediaminetetraacetate 0.12 Water 15 Water 65 The used proportions (parts by weight) of styrene, acrylonitrile and methyl methacrylate in the present recipe were determined so that the refractive index of the resin phase became 1.518, namely, it agreed almost with that of the polybutadiene.

Polymerization was effected in the following manner: First, 20 parts (in terms of solids) of the polybutadiene latex, 0.01 part of sodium diethyldithiocarbamate and 65 parts of water were charged into the flask. The components I, II and Ill and cumene hydroperoxide were divided into two portions for the former charge and the latter charge in the following ratios: Example 1: Ratio of the portion for 1:2 the first charge to the portion for the latter charge Example 2: " 1:3 The portion for the former charge was charged in a single portion to the flask, and the resulting mixture was subjected to polymerization at a polymerization temperature of 700C for 1 hour.

Subsequently, the portion for the latter charge was added to the mixture at 700C continuously over a period of 3 hours to complete the polymerization. The polymerization conversion were 97% in both Examples 1 and 2. An aqueous calcium chloride solution was added to the resulting polymer latex to perform salting-out, and the precipitate was filtered, washed and then dried to obtain MABS resin powder. Suitable amounts of a stabilizer and a lubricant were added to the powder, and the thus obtained mixture was pelletized by means of a 40 mm extruder at 2000 C, after which the resulting pellets were injection-molded (molding temperature: 2000 C) to prepare a test piece, and by use of the thus prepared test pieces, the following items were evaluated.

(1) Grafting degree: In 25 ml of acetone was dissolved 1 g of the MABS resin powder by allowing a mixture of them to stand at room temperature for 48 hours, after which the resultina solution was centrifuged to obtain the amount (g) of acetone-insoluble matter, after which the grafting degree was calculated from the following equation: Grafting degree (%) = 100 x

Insoluble ~ Charged O/o)/ Polymerization\ matter (g) rubber (wt %) / conversion Charged rubber (wt /0) / Polymerization conversion (%) wherein the "charged rubber (wt%)" means the percentage of the charged rubber based on the total weight of the unsaturated monomers charged and the rubber charged.

(2) Physical property: The Izod impact value in the case of 1/4-inch notch was obtained according to ASTM D 256.

(3) Transparency: The total light transmittance and haze value at a thickness of 1/8 inch were obtained by means of a hazemeter according to ASTM D 1 003. (The transparency was measured on a test piece obtained by injection-mclding at a mold temperature of 500C because it changes with the mold temperature at the time of injection-molding.) (4) Color tone: The I Hunter whiteness W(Fl) based on transmitted light was determined by means of a color-difference meter.

(5) Processability: The Koka type flow value was determined at 2000C under a load of 30 kg/cm2 by means of a Koka type flow tester using a 1 mm P x 2 mm nozzle.

The evaluation results of the compositions obtained in Examples 1 and 2 are shown in Table 1.

The compositions in said Examples are produced by the process of this invention, and can be seen to be good in all of transparency, color tone, processabiiity and physical property.

EXAMPLE 3 Polymerization was effected in the same manner as in Example 2, except that potassium diethyldithiocarbamate was substituted for the sodium diethyldithiocarbamate. The polymerization conversion was 97%. The resulting polymer latex was subjected to finishing treatment in the same manner as in Example 2, and the above-mentioned items (1) to (5) were evaluated in the same manner as in Examples 1 and 2. The results are shown in Table 1. Similarly to Example 2, the resin obtained in this Example was a good resin well-baianced in transparency, color tone, processability and physical property.

EXAMPLE 4 The same procedure as in Example 2 was repeated except that the Component I was replaced by the following Component I: Styrene 20 parts Component I Methyi methacrylate 60 parts t-Dodecylmercaptan 0.4 part to conduct the polymerization. The proportions of the styrene and methyl methacrylate were determined so that the refractive index of the resin phase of the resulting graft-copolymer would become 1.517 which is substantially identical with the refractive index of polybutadiene. The polymerization conversion was 97%. In the same manner as in Examples 1 and 2, the above-mentioned items (1) to (5) were evaluated to obtain the results shown in Table 1. It is clear from the results that the transparency, color tone, processability and physical property are all good.

EXAMPLE 5 The same procedure as in Example 2 was repeated, except that sodium ethylphenyl dithiocarbamate was substituted for the sodium diethyl dithiocarbamate, to conduct the polymerization.

The polymerization conversion was 97%. In the same manner as in Examples 1 and 2, the above mentioned items (1) to (5) were evaluated to obtain the results shown in Table 1. It is clear from the results that the transparency, color tone, processability and physical property are all good.

Comparative Example 1 Polymerization was effected in the same manner as in Example 2, except that 0.01 part of the sodium diethyldithiocarbamate was not added. The polymerization conversion was 98%. The resulting polymer latex was subjected to finishing treatment in the same manner as in Example 2, and the abovementioned items (1) to (5) were evaluated in the same manner as in Examples 1 and 2. The results are shown in Table 1. It can be seen from Table 1 that since no sodium diethyldithiocarbamate was added at the time of the polymerization, the transparency and color tone were much lower than in Example 2.

Comparative Examples 2 and 3 Polymerization was effected by use of the polymerization recipe shown in Examples 1 and 2 in the same manner as in Examples 1 and 2, except that the components I, II and Ill and cumene hydroperoxide were divided into two portions for the former charge and the latter charge in the following ratios: Comparative Ratio of the portion for 3:1 Example 2: the former charge to the portion for the latter charge Comparative ,, 1:8 Example 3: The polymerization conversions were 97% in Comparative Example 2 and 96% in Comparative Example 3. The post-treatment was carried out in the same manner as in Examples 1 and 2, and the above mentioned items (1) to (5) were evaluated. The results are shown in Table 1.The resulting compositions are of low practical value because in spite of sodium diethyldithiocarbamate being added at the time of the polymerization in the present Comparative Examples, the grafting degree, which is another essential condition of this invention, was outside the range specified in this invention in each of the Comparative Examples. That is to say, the composition obtained in Comparative Example 2 had a grafting degree of less than 30% by weight and was bad in all of transparency, color tone, physical property and processability. On the other hand, the composition obtained in Comparative Example 3 had a grafting degree of more than 70% by weight, and was somewhat inferior in transparency and color tone and greatly inferior in physical property and processability to the compositions obtained in Examples 1 and 2.

Comparative Example 4 Polymerization was effected in the same manner as in Example 2, except that the backbone rubbery polymer was replaced by a polybutadiene latex (gel content 33% by weight, average particle size 0.20 ,lot, refractive index n265 = 1.515). The polymerization conversion was 97%. Post-treatment was carried out in the same manner as in Example 2, and the above-mentioned items (1) to (5) were evaluated. The results are shown in Table 1. The backbone rubbery polymer used in the present Comparative Example had a gel content pf 33bA by weight which slower than the lower limit of the range specified in this invention and the transparency, color tone, physical property and processability are all inferior to those in Example 2.Further, the transparency was evaluated on test pieces obtained by molding at mold temperatures of 200C and 800C at the time of injection molding in place of the usual mold temperature of 50 C (the molding temperature was kept constant at 2000C). This test was applied also to the compositions obtained in Examples 1 and 2 to make a simultaneous evaluation. The results are shown in Table 2. The composition obtained in the present Comparative Example was improved in transparency to a certain extent when the mold temperature was made high, but was further lowered in transparency at the low mold temperature. On the other hand, the compositions obtained in Examples 1 and 2 were good in transparency at all the mold temperatures, and hence are low in dependency upon mold temperature and are of high practical value.

TABLE 1 Measurement temperature: 23 C

Example Comparative Example 1 2 3 4 5 1 2 3 4 Grafting degree (%) 38 45 48 52 41 40 18 85 37 Izod Impact strength (Kg.cm/cm) 16.5 14.8 15.2 13.9 14.2 16.3 7.8 8.1 10.8 Total light trnasmittance 87.2 88.1 87.9 88.5 87.8 88.5 85.3 87.1 85.8 Haze value (%) 8.2 8.6 8.4 7.8 9.0 13.2 14.8 10.1 13.5 Hunter Whiteness W(H) (%) 75.3 74.8 75.9 78.11 74.3 88.3 67.2 73.9 74.2 Koka type flow value (x 10 cc/sec) 14.0 11.5 12.0 16.5 13.0 10.5 5.0 4.5 10.5 TABLE 2 Measurement temperature: 23"C

Comparative ExampLe Example Example 4 1 2 Total light : Mold transmittance (%) temperature 20 C 85.3 87.0 87.3 5o,,C 85.8 87.2 88.1 66'oC 86.3 87.9 88.3 Haze value 20C 18.3 9.5 8.9 50 C 13.5 8.2 8.6 80 C 8.6 6.3 6.1

Claims (18)

1. A process for producing a thermoplastic resin composition by graft copolymerizing an unsaturated monomer mixture on a diene polymer latex under emulsion polymerization conditions in the presence of a dithiocarbamate of formula

(in which R1 and R2 are the same or different and each represents a C1-C6 alkyl group or a C6-C9 aryl group, and M represents a sodium or potassium atom), to give a graft copolymer having a degree of grafting of from 30 to 70% by weight, in which: said diene polymer latex is employed in an amount, in terms of solids, of from 5 to 40% by weight; said monomer mixture is employed in an amount of from 95 to 60% by weight; said diene polymer latex has a gel contenant of from 60 to 90% by weight and an average particle size of from 0.1 to 0.5 m; and said monomer mixture comprises two or more monomers selected from alkenyl aromatic monomers, methacrylate monomers and alkenyl cyanide monomers, and has a composition such that the refractive index of the resin phase of said graft copolymer is the same as or substantially the same as the refractive index of the rubbery polymer in said latex.

2. A process according to Claim 1, in which said dithiocarbamate is sodium or potassium dimethyldithiocarbamate, sodium or potassium diethyldithiocarbamate, sodium or potassium dibutyldithiocarbamate, sodium or potassium ethylphenyldithiocarbamate, sodium or potassium diphenyldithiocarbamate or sodium or potassium ditolyldithiocarbamate.

3. A process according to Claim 1, in which said dithiocarbamate is sodium or potassium diethyldithiocarbamate or sodium or potassium ethylphenyldithiocarbamate.

4. A process according to any one of the preceding Claims, in which the amount of said dithiocarbamate is from 0.001 to 0.1% by weight, based on the total weights of the diene polymer and the unsaturated monomer mixture.

5. A process according to any one of the preceding Claims, in which said diene polymer is a butadiene polymer, a styrene-butadiene copolymer or an acrylonitrile butadiene copolymer.

6. A process according to any one of the preceding Claims, in which said alkenyl aromatic monomer is styrene, a-methylstyrene or vinyltoluene.

7. A process according to any one of the preceding Claims, in which said methacrylate monomer is methyl methacrylate or ethyl methacrylate.

8. A process according to any one of the preceding Claims, in which said alkenyl cyanide monomer is acrylonitrile or methacrylonitrile.

9. A process according to any one of the preceding Claims, in which the difference between the refractive indices of the resin phase and the rubbery polymer is not more than 0.01.

10. A process according to any one of the preceding Claims, effected in the presence of an anionic emulsifier.

1 A process according to Claim 10, in which said anionic emulsifier is an alkyl alcohol sulphate, an cr-sulphofatty acid ester salt, a fatty acid amide ether sulphate, beef tallow methyl taurate, a fatty acid salt, a disproportionated rosinate or an alkylbenzenesulphonate.

12. A process according to Claim 10, in which said anionic emulsifier is an amido bond-containing sulphonate or a carboxylate.

13. A process according to any one of the preceding Claims, effected in the presence of a radical polymerization initiator.

14. A process according to Claim 13, in which said radical polymerization initiator is potassium persulphate, sodium persulphate, ammonium persulphate or a redox initiator using cumene hydroperoxide, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide or t-butyl hydroperoxide.

1 5. A process according to any one of the preceding Claims, effected in the presence of a polymerization regulator.

1 6. A process according to Claim 15, in which the polymerization regulator is t-dodecylmercaptan or n-dodecyimercaptan.

1 7. A process according to Claim 1, substantially as hereinbefore described with reference to any one of the foregoing Examples.

18. A thermoplastic resin composition when produced by a process according to any one of the preceding Claims.