GB2039930A

GB2039930A - Coating of fine particles in polyurethane binder

Info

Publication number: GB2039930A
Application number: GB8001824A
Authority: GB
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1979-01-19
Filing date: 1980-01-18
Publication date: 1980-08-20
Also published as: FR2446855A1; JPS55100157A; JPS6321613B2; GB2039930B; DE3002164C2; DE3002164A1; FR2446855B1

Abstract

The invention concerns coatings of fine particles such as magnetizable particles or carbon black and a binder on a substrate, primarily coatings of magnetic recording media. The binder comprises a hydroxyl-terminated polyurethane block copolymer comprising (a) polyester or polyether segments, (b) short-chain aliphatic segments containing tertiary and/or quaternary nitrogen and (c) symmetrical, unbranched, aromatic urethane segments interconnecting the (a) and (b) segments. Preferred block copolymers include soft or flexible blocks synthesized from polyester or polyether diols and hard blocks synthesized from aliphatic diols containing tertiary amine and p,p'- diphenylmethane diisocyanate. The block copolymer is capable of dispersing as well as acting as a binder for the particles.

Description

SPECIFICATION Coating of fine particles in polyurethane block copolymer binder This invention concerns coatings of fine particles such as magnetizable particles and a binder on a substrate, primarily the coatings of magnetic recording media.

In the art of applying coatings such as magnetizable particles dispersed in an elastomeric binder, a dispersing agent is normally employed to disperse the particles, usually in amount of 10-50% by weight of the binder composition. Since the dispersing agent remains in the coating, it tends to have a plasticizing effect which weakens the binder. Consequently, the coating exhibits inferior wear resistance and eventually releases debris. Furthermore, the dispersing agent can contribute to the migration of low-molecular-weight materials within the coating and their exudation onto the recording surface.

Dispersing agents are likewise used in applying coatings comprising fine particles which are not magnetizable such as the conductive carbon black of controlled-wind backside coatings of many magnetic recording tapes. These and any other coatings comprising dispersions of fine particles in elastomeric binders involve the same problems.

U.S. Patent No. 3A90,945 discloses a polyurethane binder for magnetic recording media which itself is a dispersing agent. The polyurethane binder is a moisture-cured diisocyanate-terminated prepolymer containing surface-active tertiary or quaternary nitrogen. While this theoretically should have solved the aforementioned problems, it is believed that to date there has been no elastomer on the market which both acts as a high-quality binder and also effectively disperses inorganic particles. The polyurethane binder of the Slovinsky patent may not have become commercial because of the impracticability of formulating the binder under dry conditions and keeping the dispersion under a dry atmosphere, and the moisture-curing might cause some foaming.

The present invention concerns what is believed to be the first commercially feasible magnetic recording medium having a coating comprising a uniform dispersion of magnetizable particles in a binderwhere no dispersing agent is needed. Such binder, which itself is believed to be novel, has effectively dispersed acicular gamma-Fe2O3 particles without the assistance of a dispersing agent. However, it may be desirable to add small amounts of a dispersing agent for magnetizable particles which are more difficult to disperse, e.g., fine magnetite and chromium dioxide particles. The novel binder also has effectively dispersed nonmagnetizable particles commonly used in magnetic recording media such as carbon black and aluminum oxide.

The invention should also be useful for providing coatings of abrasive sheet material and other applications.

Briefly, the novel binder comprises a hydroxyl-terminated polyurethane block copolymer comprising polyester or polyether segments, each having an average molecular weight of 200-3000, short-chain aliphatic segments, at least 10 mole percent of which contain tertiary and/or quaternary nitrogen, and symmetrical, unbranched, aromatic urethane segments interconnecting the polyester or polyether and short-chain aliphatic segments. By short-chain is meant chains averaging 12 carbon atoms or less which are unbranched or have methyl or ethyl side chains. A larger average chain length or longer side chains would inhibit the otherwise high tenacity of the block copolymer.If the average molecular weight of the polyester or polyether segments were less than 200, the novel block copolymer would tend to lack the flexibility required of a good binder, whereas above about 3000, the polyester or polyether segments would tend to exhibit unwanted crystallinity. A range of 400-2000 is preferred.

A particularly preferred symmetrical, unbranched, aromatic diisocyanate is p,p'-diphenylmethane diisocyanate. Others are p-xylene diisocyanate; 1 ,4-bis(2-isocyan-atoethyl)benzene; and 1 ,5-naphthalene diisocyanate.

The novel polyurethane block copolymer may be made by the sequential steps of (a) inter-reacting in solution excess soluble p,p'-diphenylmethane diisocyanate with a hydroxylterminated polyester or polyether having an average molecular weight of 200-3000 to provide a mixture of isocyanate-capped polyurethane and unreacted diisocyanate and (b) chain-extending that mixture with short-chain aliphatic diol, 10-100 mole percent of which contains tertiary nitrogen to produce a soluble hydroxyl-terminated polyurethane block copolymer.

If desired, there may be a subsequent step of (c) quaternizing some or all of the tertiary nitrogen.

Generally this additional step involves an expense which cannot be justified.

If 60-100 mole percent of the aliphatic diol employed in step (b) contains tertiary amine, the novel block copolymer may be softer than is desirable for use as a binder for magnetic media coatings. When part of the short-chain diol employed in step (b) has no tertiary amine, it is preferred for ease of processing that the polyurethane produced in step (a) first be chain-extended in the presence of excess isocyanate with that portion of the diol which contains tertiary nitrogen and then be further chain-extended with the portion of the diol which has no tertiary nitrogen to produce the soluble hydroxyl-terminated polyurethane block copolymer. Hence, 10-60 mole percent of the diol contains tertiary nitrogen and correspondingly 90-40 mole percent does not.

Preferred nitrogen-free short-chain diols are straight-chain aliphatic diols of 2 to 6 carbon atoms, especially 1,4-butanediol and comparable diols having one or two methyl or ethyl side chains such as neopentyl glycol. A short-chain aliphatic diol may contain one cycloaliphatic group such as cyclohexane dimethanol.

Preferred diols containing tertiary amine are similar to the aforementioned nitrogen-free short-chain diols except for their tertiary amine. Preferred is N-methyldiethanolamine.

Preferred hydroxyl-terminated polyethers contain multiple methylene (-CH2-)X units interconnected by ether linkages. Preferred hydroxyl-terminated polyesters consist essentially of multiple methylene (-CH2-)x units interconnected by ester linkages.

Upon dissolving the polyurethane block copolymer in sufficient volatile vehicle to provide a coatable dispersion of fine magnetizable and/or other particles, the dispersion can be coated out onto a substrate to provide a smooth, uniform coating without any added dispersing agent. The dispersion may contain other substances such as lubricants commonly used in coatings of magnetizable particles to reduce friction with magnetic heads. It is often desirable that a crosslinking agent for the novel polyurethane block copolymer be added to the coatable dispersion immediately prior to its use. Such crosslinking tends to produce a tougher, more durable and heat- and solvent-resistant coating, as is known in the art and is especially significant for magnetizable coatings involving high-speed contact with magnetic heads.Generally a magnetizable coating comprises a major proportion by weight of fine magnetizable particles and a minor proportion by weight of binder.

The binder may include additional polymeric material such as a phenoxy resin (e.g., Union Carbide PKHH which is a thermoplastic copolymer of equivalent amounts of bisphenol A and epichlorohydrin) which provides a somewhat harder coating. Other useful such materials are vinyl chloride polymers (e.g., Union Carbide VROH and VAGH which are partially hydrolyzed vinyl chloride/vinyl acetate copolymers).

The volatile vehicle for the coatable dispersion should primarily comprise nonaromatic polar solvent such as dimethyl formamide, tetrahydrofuran or cyclohexanone. Aromatic solvents such as toluene which are often used in making magnetic recording media may cause the binder system to gel prematurely.

The following Examples 1-7 illustrate polyurethane block copolymers of the invention, and Examples 8-9 show preparations of illustrative coatings of those block copolymers. In each, parts are given by weight.

Example 1 42.2 parts of 4,4-diphenylmethane diisocyanate (below called "MDl") was dissolved in 70 parts of tetrahydrofuran solvent in a stainless steel pressure reactor, equipped with a paddle mixer. After heating to 65"C, 36 parts of polytetra- methylene ether glycol (below called "PTMEG") of 650 average molecular weight was added. With continued mixing, the mixture was heated to about 125 C until the NCO content stabilized, and another 136 parts of tetrahydrofuran was added. While holding the temperature at about 80#C, 3.6 parts of N-methyldiethanolamine (below called "MDEA") was added, and 7.5 parts of 1,4-butanediol (below called "BD") was added 30 minutes later.The temperature was then increased to 130 C and the reaction process was continued, until the NCO content was less than 0.3%. The inherent viscosity of the resultant polyurethane elastomer was 0.75 (measured in tetrahydrofuran). The batch was then diluted with tetrahydrofuran to 17% solids to provide a solution which is below called "Product of Example 1".

Example 2 To 100 parts of Product of Example 1 under a nitrogen atmosphere in the reactor of Example 1 was added 0.89 part of methyl iodide with slow mixing for 24 hours at 22 C, thus quaternizing the tertiary nitrogen of the diol segments.

Examples 3-7 The polyurethane block copolymers of Examples 3-7 were prepared in the same manner as that of Example 1, and the tertiary nitrogen of the Product of Example 6 was quaternized to provide the Product of Example 7 in the same way as in Example 2. Table I reports the materials used in making Examples 3-7.

Claims

A . A coating comprising fine particles and a binder on a substrate, the binder comprising a polyurethane block copolymer, characterized by the feature that said block copolymer is hydroxyl- terminated and comprises polyester or polyether segments, each having an average molecular weight of 200-3000, short-chain aliphatic segments, 10-100 mole percent of which contain tertiary and/or quaternary nitrogen, and symmetrical, unbranched, aromatic urethane segments interconnecting the polyester or polyether and short-chain aliphatic segments.
2. A magnetic recording medium comprising a backing member and at least one coating comprising fine particles and a binder, the binder comprising a polyurethane block copolymer, characterized by the feature that said block copolymer is hydroxyl-terminated and comprises polyester or polyether segments each having an average molecular weight of 200-3000, short-chain aliphatic segments, 10-60 mole percent of which contain tertiary and/or quaternary nitrogen, and p,p'diphey!methane urethane segments interconnecting the polyester or polyether and short-chain aliphatic segments.
3. A magnetic recording medium as defined in claims 1 or 2, further characterized by the feature that: each of the polyester or polyether segments has an average molecular weight of 400-2000.
4. A magnetic recording medium as defined in any preceding claim, further characterized by the feature that: 10-60 mole percent of the short-chain aliphatic segments are residues of N-methyldiethanolamine.
5. A magnetic recording medium as defined in any preceding claim, further characterized by the feature that: the balance of the short-chain aliphatic segments are residues of straight-chain aliphatic diols of 2 to 6 carbon atoms.
6. Process for applying to a substrate a coating of fine particles and binder comprising the steps of mixing a polyurethane copolymer further chain-extending with a short-chain aliphatic diol which has no tertiary nitrogen in an amount providing 40-90 mole percent of the total diol to produce a soluble hydroxyl-terminated polyurethane block polymer, mixing said polyurethane block copolymer with fine particles and sufficient volatile vehicles to provide a coatable dispersion wherein the weight of the particles exceeds that of the copolymer, coating said dispersion onto a substrate, characterized by the feature that the polyurethane copolymer is prepared by (a) inter-reacting in solution excess soluble symmetrical, unbranched, aromatic diisocyanate with hydroxyl-terminated polyester or polyether having an average molecular weight of 200-3000 to provide a mixture of isocyanate-capped polyurethane and unreacted aromatic diisocyanate, and (b) chain-extending the mixture with short-chain aliphatic diol, 10-100 mole percent of which contains tertiary nitrogen to produce a soluble hydroxyl-terminated polyurethane block copolymer.
7. Process as defined in claim 6 further characterized by the feature that subsequent to step (b) is the further step of (c) quaternizing at least some of the tertiary nitrogen of the block copolymer.
8. Process as defined in claim 6 further characterized by the feature that less than 100 mole percent of the diol contains tertiary nitrogen and step (b) first involves chain-extending the isocyanate- capped polyurethane produced in step (a) with a portion of the diol which contains tertiary nitrogen in the present of excess isocyanate and then further chain-extending a portion of the diol which has no tertiary nitrogen to produce the hydroxyl-terminated copolymer.
9. Process as defined in any of claims 6-8 further characterized by the feature that said fine particles are magnetizable particles.
10. Process as defined in any of claims 6-9 further characterized by the feature that in step (d) a crosslinking agent for said polyurethane block copolymer is added to the copolymer after mixing it with the fine particles to provide a coatable dispersion.
11. Coating composition comprising fine particles in a binder, which binder is substantially as described in any one of Examples 1 to 7 herein.
12. Coating composition substantially as described in Examples 8 to 10 herein.
13. A process for applying to a substrate a coating of fine particles in a binder, substantially as herein described.