GB2293174A - Polymer-modified bitumen - Google Patents

Abstract

A polymer-modified bitumen composition of improved storage stability comprises: (i) from 80 to 99 wt.% of bitumen, (ii) uniformly distributed through the bitumen, from 1 to 20 wt.% of a modifying polymer which is at least partially cross-linked characterised in that the modifying polymer contains grafted moieties. The preferred modifying polymer is ethylene/methyl acrylate copolymer (EMA). The grafted moieties preferably have terminal unsaturation, allowing crosslinkage by vucanisation. The grafted moieties may be linked by sulphur. Grafting may be carried out by converting part or all of the ester groups to e.g. acyl chloride groups and reacting these with an alpha -unsaturated compound having terminal functionality, e.g. allylamine.

Description

Polymer-Modified Bitumen The present invention relates to bitumen compositions, particularly but not exclusively for the hot coating of aggregate for road building, and to processes for their preparation.

To obtain the best performance for road building, especially high modulus and good fatigue values, it is desirable to use a hard bitumen.

However, hard bitumens, e.g. having a (French standard) NFT 66004 penetration index (pen value) of 80 or below, are brittle and deficient in tack, which leads to poor adhesion and low compaction efficiency. To improve the mechanical properties it is known to blend softer bitumens, e.g. having a pen value of 100 or above, with synthetic polymers. Blending with an ethylene/unsaturated ester copolymer such as ethylene/methyl acrylate copolymer (EMA) will tend to raise the pen value and the softening point while decreasing the viscosity at coating temperatures, generally around 1600C, facilitating the coating process. The resulting coated aggregates (when successfully produced) have less tendency to crack at low temperatures and to form ruts at higher ambient temperatures.

However a notable disadvantage of such blends is their poor storage stability at elevated temperatures, i.e. temperatures at or approaching coating temperatures. It is often necessary for road building or repairing contractors to store heated bitumen while work is disrupted, e.g. during spells of bad weather which may last for two or three days. When this happens, the blends tend to separate, the denser bitumen sinking, so that the next coating application will result in diminished mechanical properties. In an effort to prevent this expensive storage tanks fitted with mechanical stirrers are often used.

US-A-4,145,322 describes a process for preparing polymer-modified compositions of improved storage stability by blending a bitumen with a block copolymer comprising diene and styrene groups and vulcanising the resulting blend.

US-A-4,330,449 describes a modification of the above process in which the block copolymers are selected from styrene-carboxylated isoprene and styrene-carboxylated butadiene.

While the above compositions exhibit improved storage stability, the approach restricts the choice of modifying polymer effectively to block copolymers of conjugated dienes.

The present invention avoids this limitation by adopting a new approach to the problem. Essentially a polymer may be selected for its particular properties as a modifier for bitumen and prepared for subsequent cross-linking, e.g. by vulcanisation, by grafting on to it moieties having appropriate functionality.

The word "polymer" is used herein in the broad sense of including homopolymers and copolymers, the latter including polymers of two or more monomers.

According to a first aspect the invention provides a polymer-modified bitumen composition comprising (i) from 80 to 99 wt.% of bitumen, (ii) uniformly distributed through the bitumen, from 1 to 20 wt.% of a modifying polymer which is at least partially cross-linked, characterised in that the modifying polymer contains grafted moieties linked by a cross-linking atom or group.

The bitumen may have a (French standard) NFT 66004 penetration (pen value) from 100 to 400, and preferably a NFT 66008 softening point from 30 to 60 oC.

The grafted moieties preferably exhibit ethylenic unsaturation and preferably the crosslinking atom or group comprises sulphur.

Thus according to a second aspect, the invention provides a polymermodified bitumen composition of improved high temperature storage stability comprising (i) from 80 to 99 wt.% of bitumen of NFT 66004 penetration (pen value) from 100 to 400 and NFT 66008 softening point from 30 to 60 OC.

(ii) uniformly distributed through the bitumen, from 1 to 20 wt.% of a vulcanised product derived from an ethylene copolymer having grafted moieties exhibiting ethylenic unsaturation.

Thus in the broad sense of the first aspect, the modifying polymer may be any polymer suitable for modifying bitumen to obtain desired properties in the blend. Normally such properties will lie in the direction of increasing the softening point without adversely affecting the viscosity at coating temperatures. The preferred polymer, more particularly for the latter purpose as defined by the second aspect, is an ethylene copolymer, preferably an ethylene/unsaturated ester copolymer although (as will be described) acids or anhydrides may be used as alternatives or in addition to esters.

The unsaturated ester may be derived generally from an unsaturated acid or an unsaturated alcohol. Preferred unsaturated acids are a-unsaturated monobasic carboxylic acids which may have from 2 to 6 carbon atoms (excluding carboxyl) e.g. acrylic and methacrylic acids. However dibasic acids such as maleic, fumaric, citraconic and itaconic may be used. The alcohol component may have from 1 to 6 carbon atoms, methyl being preferred.

Unsaturated esters of saturated carboxylic acids, generally having from 1 to 6 carbon atoms (excluding carboxyl) may also be used. Preferably the alcohols should be unsaturated at the ends opposite the alcohol group, e.g. 1butenyl acetate.

The preferred ester component is ethylene/methyl acrylate copolymer (EMA).

The grafted moieties preferably exhibit crosslinkage sites, e.g. double bonds, at terminal positions. Thus where, for example, crosslinking proceeds via vulcanisation, the grafted moieties should preferably exhibit terminal unsaturation so that vulcanisation proceeds at the ends of the chains. Such moieties may be derived from one or more a-monounsaturated compounds, which may have terminal functional groups. Preferred functional groups are groups reactive with acyl groups, so that they can react with acyl groups derived from the original ester (or acid) groups of the preferred modifying copolymers. Such groups may be e.g. primary amine groups or alcohol groups.

Although monounsaturated compounds are preferred, the use of at least a proportion of compounds having more than one site of unsaturation, e.g. compounds with conjugated double bonds such as butadiene and isoprene, is not excluded.

Preferably the a-monounsaturated compounds comprise or consist of allylamine.

According to a third aspect the invention provides a process for preparing a polymer-modified bitumen composition comprising (I) mixing at a temperature above softening point: (a) from 80 to 99 wt.% of bitumen, (b) from 1 to 20 wt.% of a polymer exhibiting cross-linking functionality, and (II) mixing-in a cross-linking proportion of a cross-linking agent which is reactive with the said moieties at the mixing temperature so as to cross-link the polymer at least in part, characterised in that the cross-linking functionality of the said polymer is present in moieties grafted to the main body of the polymer.

In the preferred case the cross-linking atom or group comprises sulphur.

Thus the invention provides as a fourth aspect a process for preparing a polymer-modified bitumen composition of improved high temperature storage stability comprising (I) mixing at a temperature from 1200 to 2500C: (a) from 80 to 99 wt.% of bitumen of NFT 66004 penetration (pen value) from 100 to 400 and NFT 66008 softening point from 30 to 60 OC, (b) from 1 to 20 wt.% of an ethylene copolymer having grafted moieties exhibiting ethylenic unsaturation, (III) mixing in from 0.1 to 3 wt.% based on the weight of bitumen of a vulcanising agent while maintaining the temperature.

Preferably the mixing step (I), and preferably also (II), is carried out for at least one hour.

As already described, the ethylene copolymer may be an ethylene/unsaturated ester (or acid or anhydride) copolymer, in the preferred case EMA, the (unvulcanised) grafted moieties of which preferably exhibit terminal unsaturation and may be derived from one or more amonounsaturated compounds.

The (unvulcanised) grafted moieties may be prepared by converting the ester (or acid) groups of the copolymer to acyl groups and reacting the resulting acyl groups with one or more unsaturated compounds having (preferably terminal) functional groups, such as amine or alcohol groups, reactive with acyl groups.

Again, the preferred unsaturated compounds comprise or consist of allylamine.

In the preferred case where the copolymer is an ester, treatment with an acylating agent will normally be preceded by a hydrolysis or saponification step, e.g. treatment with alcoholic potash which will convert at least a proportion of the ester groups to salts. This step may be utilised to control the proportion of the original ester groups of the copolymer utilised for grafting.

Where it is desired to graft all the carboxylic side chains, an acid or anhydride may be utilised in place of an ester. For example an ethylene/methacrylic acid copolymer or ethylene/maleic anhydride copolymer may be used. The use of an acid copolymer will obviate the hydrolysis stage. Where an anhydride is used it may be possible and convenient to react this directly with the unsaturated (or other crosslinkable) compound such as allylamine.

Control of the proportion of grafting sites may also be accomplished by preparing (or otherwise providing) a mixed copolymer or terpolymer with a proportion of acid or anhydride groups. For example a terpolymer of ethylene with methyl acrylate and methacrylic acid may be used and the hydrolysis stage omitted.

Example Two samples of EMA were subjected to the procedure which follows.

These had respectively (A) a melt index of 20 with a methyl acrylate content of 21 wt.%, and (B) a melt index of 11 with a methyl acrylate content of 24 wt.%.

Stage 1 Saponification Potassium hydroxide (29.69) was dissolved in 300 ml methanol and added to EMA (50 g) in 750 ml toluene and the whole reacted at 600C for 15 hours. The polymer was then precipitated in 2 litres of methanol under agitation. A white precipitated polymer was obtained after filtration and washing with methanol. Infra-red analysis indicated a little less than complete conversion of ester to potassium carboxylate.

Stage 2 Acylation Sulphuryl chloride (20 g) was added slowly to the carboxylate polymer from Stage 1 (50 g) in 750 ml toluene and a few drops of pyridine added as catalyst. The reaction proceeded at 600C for 6 hours. Excess sulphuryl chloride was distilled off under nitrogen.

Stage 3 Grafting The temperature of the reaction product from stage 2 was reduced to 1000 and 9.1 g allylamine added. The reaction proceeded under agitation for half an hour and then the temperature was raised to 500C and continued for a further 6 hours. The reaction product was poured into 2 litres of methanol and the grafted polymer separated as a brown solid which was washed with methanol and dried.

Stage 4. Blending and vulcanisation A 180/220 pen bitumen (97 wt.%) having a softening point of 39.50C was blended with 3 wt.% of EMA (A) grafted with allylamine. There were then added to the blend 0.4 wt.% of elemental sulphur, 0.15 wt.% zinc oxide and 0.05 wt.% of stearic acid (based on the weight of bitumen). The blend was mixed for 1 hour at 1800C.

The resulting vulcanised blend had the following characteristics.

Penetration (250C) 112 Softening point 640C Viscosity at 1350C 1.46 Pa.s at 11 Herz 1.361 Pa.s at 35 Herz 1.12 Pa.s at 108 Herz Storage stability no settlement after 3 days storage at 1600C.

Claims (12)

Claims:

1. A polymer-modified bitumen composition comprising (i) from 80 to 99 wt.% of bitumen, (ii) uniformly distributed through the bitumen, from 1 to 20 wt.% of a modifying polymer which is at least partially cross-linked, wherein the modifying polymer either (a) contains grafted moieties linked by a cross-linking atom or group, or (b) comprises a vulcanised product derived from an ethylene copolymer having grafted moieties exhibiting ethylene unsaturation.

2. A composition according to claim 1 wherein the bitumen is of NFT 66004 penetration (pen value) from 100 to 400, and preferably, may have a NFT 66008 softening point from 30 to 60 or.

3. A composition according to claim 1 or claim 2 wherein the grafted moieties exhibit ethylenic unsaturation.

4. A composition according to any preceding claim wherein the crosslinking atom or group comprises sulphur.

5. A composition according to any preceding claim wherein the (ungrafted) ethylene copolymer comprises an ethylene/unsaturated ester copolymer.

6. A composition according to claim 5 wherein the (ungrafted) ethylene copolymer comprises an ethylene/(meth)acrylate copolymer (e.g., ethylene/ methyl acrylate copolymer, EMA).

7. A composition according to any preceding claim wherein the (unvulcanised) grafted moieties exhibit terminal unsaturation, and preferably may be derived from one or more a-monounsaturated compounds (e.g., allylamine).

8. A composition according to any preceding c!aim wherein the (unvulcanised) grafted moieties are derived from one or more unsaturated compounds having functional groups reactive with acyl groups, and which functional groups may preferably be terminal groups.

9. A process for preparing a polymer-modified bitumen composition comprising (I) mixing at a temperature above softening point (e.g., in the range from 120 to 2500C) (a) from 80 to 99 wt.% of bitumen (e.g. of penetration value (NFT 66004) of from 100 to 400 and/or softening point (NFT 66008) from 60 to 800C) (b) from 1 to 20 wt.% of either a polymer exhibiting cross-linking functionality or an ethylene copolymer having grafted moieties exhibiting ethylenic unsaturation, and (II) mixing-in a cross-linking proportion of a cross-linking agent or a vulcanising agent (e.g. from 0.1 to 3 wt.%) which is reactive with the said moieties at the mixing temperature so as to cross-link the polymer at least in part, wherein the cross-linking functionality of the said polymer is present in moieties grafted to the main body of the polymer.

10. A process according to claim 9 wherein the grafted moieties exhibit ethylenic unsaturation and/or the ethylene copolymer is or comprises ethylene/methyl acrylate copolymer (EMA).

11. A process according to claim 9 or claim 10 wherein the cross-linking atom or group comprises sulphur.

12. A process according to any of claims 9 to 11 wherein the (unvulcanised) grafted moieties exhibit terminal unsaturation (e.g. wherein the (unvulcanised) grafted moieties are derived from one or more a-monounsaturated compounds).