WO1997013805A1

WO1997013805A1 - Rubber latex films having improved tear resistance

Info

Publication number: WO1997013805A1
Application number: PCT/GB1996/002482
Authority: WO
Inventors: Andrew Richard Arnold; Laurence James Calver
Original assignee: Lrc Products Limited
Priority date: 1995-10-10
Filing date: 1996-10-10
Publication date: 1997-04-17
Also published as: GB9520700D0

Abstract

A composition comprises a blend of a rubber latex, and particles of a hydrophobic or hydrophilic silica, and of a vinyl chloride homo- or co-polymer. Films made from the composition exhibit improved reinforcement properties.

Description

RUBBER LATEX FILMS HAVING IMPROVED TEAR RESISTANCE

This invention relates to rubber latex films having improved tear resistance, and to a method of making them. The invention also relates to compositions containing rubber latex.

Thin rubber films, prepared by dipping shaped formers into a compounded latex and then heating to dry and vulcanise the latex deposit on the former, have been used for many years in applications for which their barrier properties have proved invaluable. Natural rubber latex has proved to be particularly useful in this respect since it gives films that are strong, highly deformable, and show good recovery on removal of stress. Such films have been used extensively for the manufacture of gloves for domestic, industrial, and surgical and general medical use, and for condoms and balloons. Particularly in medical applications, it is important that the film is impervious to blood, sperm, and other body fluids, and to micro-organisms, whilst at the same time being thin and flexible enough to allow sensitivity of touch and feel. Thin natural rubber latex films fulfil these requirements admirably and are in manv wavs ideal for such applications

However, although these thin films are strong, thev are subject to tearing and puncture by sharp objects; that is, if a defect is present or is formed in the film, it can act as an initiation site for tearing. Once initiated, a tear is likelv to propagate readily, since the rubber in use is usually under strain. There is therefore a need to increase the resistance of a latex rubber film to tearing.

Manv attempts have been made to improve the tear resistance of rubber films. One approach has been to incorporate into the rubber, very small particles of an immiscible, harder, less deformable material, capable of stopping or deflecting a growing tear. A convenient and effective way of achieving this is to mix the harder material, in the form of an aqueous dispersion, emulsion or second latex, with the rubber latex, prior to dipping the shaped former into it. After drying and vulcanisation, the film then contains an essentially uniform dispersion of the fine, particulate material. Such particles, usually 0. 1 to 50 microns in diameter, are able to blunt and divert the tip of a growing tear and thus enhance tear strength. Among the materials first used for this purpose were polymer resins, for example, a hydrazine-formaldehyde resin formed in the latex in situ, and carboxylated synthetic rubber latex, such as carboxylated styrene-butadiene rubber (CSBR) or carboxylated acrylonitrile-butadiene rubber (CNBR) latex, which were added to the natural rubber latex. Carboxylated rubbers, polystyrene and copolymers of vinyl acetate have also been used, as have styrene-butadiene copolymers. U.K. patent specification no. 2,088,389 teaches the use of poly(vinvI chloride) (PVC) as an additive to prevulcanised natural rubber latex, and similar claims have also been made for PVC and its copolymers.

The above proposals all make use of finely divided organic polymers. An alternative approach uses fine-particle silica as the hard component of the film. Various types of silica may be used but the most effective are those of smallest primary particle size, such as the fumed silicas. Precipitated silicas, which are generally of larger particle size, are less effective. Aqueous dispersions of hydrophilic fumed silica, marketed under the trade mark "Aerosil", are known for improving tensile strength and tear resistance in articles made from natural and synthetic rubber latices (Technical Bulletin Pigments, No. 33; Degussa AG, Postfach 1 10533, D-6000 Frankfurt 1 J , Germany; December 1982; see also, H. Esser and G. Sinn (Kautschuk und Gummi, 1960, 13, WT126- 132). A similar material, known under the trade mark Cab-O-Sil, has also been described as useful in latex films for increasing tear resistance.

Whilst there is no doubt that, in most cases, the incorporation of a small quantity of a hard polymer or of a silica into a film from a rubber latex gives rise to a substantial improvement in tear strength, we have found that the improvement is generally not maintained if the film is stored at ambient or elevated temperature over a period of time, even in the absence of air. The behaviour of reinforced films on storage or during anaerobic ageing is of practical importance since latex articles such as condoms and gloves, although sealed in packs, are often stored for long periods before use. In tropical countries, the ambient temperature of storage may exceed 40°C.

We have now found a way of reinforcing rubber latex films which not only gives an increase in tear strength but also unexpectedly preserves this increased tear strength during a period of storage or ageing.

According to one aspect of the invention, there is provided a composition comprising a blend of a rubber latex, particles of a hydrophobic or hydrophilic silica, and particles of a vinyl chloride homo- or co-polymer.

According to another aspect of the present invention, there is provided a rubber latex film which includes a mixture of particles of a hydrophilic or hydrophobic silica and a vinvl chloride homo- or co-polymer.

The silica and the vinvl chloride homo- or co-polymer can be provided in the form of fine particles. Typically, the primary particle size of the silica is in the range 1 0 to 20.

The combination exhibits a synergistic effect. A freshly prepared film made from a latex containing either the silica or the vinvl chloride polymer, but not both, shows increased tear strength over a film prepared from a similar latex not containing a reinforcing additive. However, this increase is lost completely or to a substantial extent during a period of storage or ageing, whereas by using the combination of the present invention, the increased tear strength can be maintained for longer.

The preferred rubber latex films of the present invention are those which are suitable for the manufacture of condoms or for the manufacture of gloves for domestic, medical or industrial use. The invention also provides a method of making reinforced rubber latex film materials which comprises mixing a compounded rubber latex with a dispersion of hydrophilic or hydrophobic silica and with a dispersion of a vinyl chloride homo- or co-polymer, dipping a shaped former into the resulting latex mixture, and curing the resulting latex mixture.

The curing step is typically carried out by heating, which dries the latex deposit and vulcanises the rubber.

The amounts of silica and vinvl chloride polvmer can vary quite widely. We have found that if the silica were used on its own, the optimum combination of properties would be given by concentrations of silica of between about 2 phr and about 8 phr, depending upon the particular type of silica used. Both smaller and greater amounts could be be used, but we have found that little benefit accrues from employing concentrations higher than 10 phr; the rubber films become too stiff, tensile strength is seriously reduced, and tear strength is not further increased. If the polv(vinyl chloride) were used on its own, concentrations between about 2.5 and 30 phr for example, could be used, but we have found that high tear strengths may be obtained with onlv about 1 0 phr. Higher concentrations could be used without anv diminution in tear strength, but the rubbers can become inordinately stiff, particularly after ageing. Smaller amounts can be used if desired.

Manv types of fine-particle silica, including precipitated silicas and both hydrophilicand hydrophobic fumed silicas, behave synergisticaUy with vinvl chloride polymers with respect to tear strength. This synergism is sometimes apparent in values of initial tear strength, but is usually much more apparent in tear strengths measured after a period of ageing.

As a result of this svnergism, it is possible to obtain a given degree of tear strength reinforcement from lower concentrations of mixtures of silica and vinvl chloride polymer than is possible from either individual reinforcing agent. We therefore prefer to use lower total concentrations of mixed reinforcing agents, which are preferably taken in equal quantities by weight. Thus, preferred total concentrations fall in the range 1 to 12 phr, more preferably 1 to 10 phr; it is particularly preferred that the total concentrations are in the range 2 to 6 phr, especially 2 to 5 phr. However, within these ranges, the level of tear strength achieved and the extent of synergism appearing depend to some extent on the type of silica and the type of vinvl chloride polvmer being used.

Whilst it is the principal object of the present invention to achieve high tear strengths, especially after ageing or storage, it will be apparent to one skilled in the art of rubber compounding that, in the manufacture of a particular type of article, it is rarely possible to optimise a particular property or process without detracting from another. It is therefore necessary in practise to adopt a suitable compromise, having regard to the requirements of the use of the article and of the process for making it. Those skilled in the art will be able to weigh the different needs and thereby select a suitable composition to give a judicious balance of properties and of processability. For example, it will be apparent that some requirements will necessitate avoiding the use of the vinvl chloride homopolymer on health grounds. In other cases, it mav be apparent that the requirements are best met by utilising two or more types of silica dispersion, in combination with a single vinvl chloride homo- or co-polvmer dispersion or vice versa. It is also possible to use more than one silica dispersion and more than one vinvl chloride homo- or co-polvmer dispersion.

Aqueous dispersions of silicas suitable for use in the present invention normally contain, depending on the type of silica used, between about 5 and about 25 per cent silica, preferably between 1 2 and 20 per cent. They may be prepared by high-shear mixing of the silica with an aqueous solution of an alkali such as ammonia or sodium or potassium hydroxide, optionally in the presence of a surfactant or dispersing agent. Suitable surfactants are anionic or non-ionic surfactants. Especially suitable are surfactants similar to those used to stabilise the latex or other dispersions which may be added to it, such as dispersions of compounding ingredients. Alternatively, a suitable commercially available dispersion mav be used, for example, Cab-O-Sperse A 1 695 , which is a 1 7% aqueous dispersion of Cab-O-Sil, a hydrophilic fumed silica.

Aqueous dispersions of vinyl chloride polymers are readily available commercially, and generaliv contain from 50 to 70 wt% of the polymer. For example, those marketed by BASF under the Trade Name "Lutofan". These are alkaline dispersions (pH 8.5 - 1 1 .0) of 55-60% total solids content, but other dispersions of different pH and/or solids content can be used.

Both the tvpes of dispersion described above mav be readily mixed with compounded or prevulcanised natural rubber latex bv careful addition under efficient mixing conditions. The mixture is allowed to stand for a sufficient time to allow equilibration and to allow any air bubbles to escape. It mav optionally be heated for a period of ti me to effect a degree of prevulcanisation of the latex, if desired. The viscosity oϊ a latex mixture containing silica increases with time of storage and there is a tendency for thickening to occur with increasing concentrations of silica. This thickening can lead to problems during dipping because the deposited film is thicker when drv and often of inferior quality. The silica/vinvl chloride polvmer mixtures of the present invention are generaliv free from this deficiency since the synergism between the silica and the vinyl chloride polvmer allows the quantity of silica used to be reduced below the level where thickening of the latex occurs. It is thus an advantage of the present invention that film products of more consistent weight and quality may be obtained than are possible using a silica as the sole reinforcing agent. Rubber articles such as condoms and gloves having good tear resistance before and after ageing or storage may be obtained from the above mixtures bv dipping suitably prepared and shaped formers into a mixture and drying and vulcanising the deposit so obtained. Depending on the tvpe of article to be made, the former may be dipped more than once into the mixture or it may be dipped into a coagulant prior to immersion in the mixture. These and other variants are well known to those skilled in the art of latex dipping technology. The rubber is preferably natural rubber.

In order that the invention may be more fullv understood, the following Examples are given bv wav of illustration onlv. In these Examples, the following proprietary materials have been used:

Aerosil R972 - a hydrophobic fumed silica (Degussa)

Bevaloid 35 - a sodium salt of a condensed naphthalene-sulphonic acid dispersing agent (Rhone-Poulenc)

Cab-O-Sil M5- a hydrophilic fumed silica (Cabot)

Dehydol TA 20 - a fatty alcohol/poly(ethylene oxide) condensate (non-ionic surfactant) (Henkel)

Lutofan LA 95 1 - an aqueous dispersion of a vinyl chloride/methyl acrylate copolymer of 59% solids content (BASF)

Lutofan LH977 - an aqueous dispersion of polv(vinvl chloride) of 56% solids content (BASF) Ultrasil VN3 - a hydrophilic precipitated silica (Degussa)

WS374 - an aqueous dispersion of Si 69 silane coupling agent of 50% solids content (Degussa)

In the Examples, the following abbreviations are used:

d day(s) h hour(s) g gram(s) min minute(s) ml millilitre(s) phr parts per hundred rubber (bv weight) s second(s)

M 300 modulus (stress) at 300% extension

TS tensile strength

TRST tear strength

The dried latex films prepared in the various Examples were tested either as prepared (initial) or after a period of ageing as indicated in the Tables.

Tensile testing (for TS and M 300) was carried out according to BS903; Part A2, using Type II dumbells cut from condom samples along the shaft. Tear strength was measured on angle tear pieces (similarly cut) according to BS903; Part A3.

For ageing treatments, condoms were rolled and packed unlubricated in square laminated packs. Examples 1 to 4

These Examples illustrate the use of a precipitated silica and poly(vinyl chloride) in the method of the invention.

An aqueous dispersion of Ultrasil VN3 λvas prepared by adding the silica (200 g) to deionised water (S00 g) containing Dehydol TA 20 (2 g of 20% aqueous), potassium oleate (2 g of 20% aqueous) , and potassium hydroxide (5 ml of 10% aqueous), and subjecting it to high-shear stirring in a Silverson mixer for 30 s.

This dispersion was added slowly, with gentle stirring, to an ammoniated natural rubber latex (to give a final concentration of 2 phr) which had been compounded and partly prevulcanised by a procedure well-known to those skilled in the art. A glass condom-shaped former was dipped into the latex, withdrawn, the deposited latex was dried to a wet gel in a current of warm air, and the former again dipped into the latex. After the second deposition, the rubber was dried and vulcanisation was completed by heating the deposit on the former in a circulating air oven at 1 05 °C for 7.5 min. The condom was then stripped manually from the former and tested as described above.

The results ( Example 2 ) are given in Table 1 , where thev are compared with the results obtained on condoms made using the same procedure but without the addition of the silica dispersion (Example 1 ) .

In Example 3 , the same procedure was used but the silica dispersion was substituted by a sufficient quantity of Lutofan LH977 to give a final concentration of 1 0 parts poly(vinvl chloride) per 1 00 parts of rubber.

In Example 4, both the silica dispersion and the polv(vinyl chloride) dispersion were added to the latex in the concentrations used in Examples 2 and 3. TABLE 1

Example Additive TS/MΓ 'a M 300/MPa T ST/Nmm-'

(phr) Initial Aged* Initial Aged* Initial Aged*

1 none 27.2 25.9 1 .48 1 49 45.6 53.6

2 VN3(2.0) 19.4 20.4 1 .36 1 .54 94.6 56.7

3 LH977( 10.ϋ) 29.7 32. 1 1 .74 2.03 1 24. 1 6 1 .9

4 VN3 (2.0) &. 25.2 27. 1 1 .72 2.22 1 15.0 84.9 LH977 ( 10.0)

* Ageing period: 7d ( ) 70" C

The inclusion of either 2 phr of precipitated silica or of 1 0 phr of poly(vinvl chloride) increases the initial tear strength (bv factors of 2 to 3). However, in both cases, most of this improvement is lost after a period of accelerated ageing. Oxidative degradation of the rubber during ageing of the packed condoms would not be expected to be extensive and is eliminated as a cause of the reduction in tear strength since tensile strength, which is very sensitive to oxidative changes, is not reduced under the same conditions.

With both the silica and the polv(vinvl chloride) present, a large increase in tear strength is observed, and onlv about 50% of this increase is lost on accelerated ageing.

In all cases, the stiffness (modulus) of the rubber is increased and the tensile strength is generally maintained or increased on ageing.

Examples 5 to 8

These comparative Examples show that a similar synergistic effect on aged tear strength is obtained when a combination of poly(vinyl chloride) and a hydrophilic fumed silica (Cab-O-Sil M5) is used.

The procedure was as described in Examples 1 to 4 except that hydrophilic fumed silica was used instead of the precipitated silica, where appropriate. The details are given in Table 2. An aqueous dispersion of the silica was prepared by stirring Cab-O-Sil M5 (40 g) in ammoniated water (500 ml) . The pH was adjusted to 10 - 1 1 with potassium hydroxide, WS374 (8g) added using a Silverson mixer for 5 min, and potassium oleate (3 ml of 13% aqueous) added.

TABLE 2

Example Additive TS/M1 'a M 300/MPa TRST/Nmm '

(phr ) I ni tial Aged * Ini tial Aged * I n i tial Aged *

5 none 27 2 24 5 1 1 5 I 20 36 9 37 3

6 Cab-ϋ-Sil 27 6 0 3 1 33 1 44 6 9 46 5 (2 0 )

7 LH977 (8 3 ) 25 5 26 5 1 25 1 40 53 3 47 0

8 Cab-O-Sil 26 9 3 1 9 1 64 1 85 10 1 68 7 (2 0) &

LH977 ( 8 3 )

* Ageing period 30d (<^*' 7() C

Similar trends in properties are seen as in the previous Examples even though a more extended period of ageing has been used. Once again, the presence of the silica alone increases the tear strength but the increase is largely lost on ageing. In this case, the presence of the polv(vinvl chloride) alone has been of little benefit, but in combination with the silica, tear strength is increased by a factor of 2.5 , some 50% of which is retained on ageing.

Examples 9 to 1 2

These Examples illustrate the use of a hydrophobic silica (Aerosil R972) in the method of the invention. A dispersion of the silica was made by adding it ( 15 g) to a solution of Bevaloid 35 (5 g) and potassium hydroxide (0.2 g) in deionised water ( 100 g) and mixing in a Silverson mixer under high shear conditions for 2h. The procedure was otherwise that of Examples 1 to 4. The results are given in Table 3.

TABLE 3

Example Addi tive TS/M1 a M 00/MPa TRST/Nmm '

(phr) Initial Aged* Initial Aged* Initial Aged*

9 None 27.9 24 7 1 1 8 1 24 37 2 37 3

10 Aerosil R972 22 0 22 4 1 4 1 1 47 76 2 109 1 (6 0 )

1 1 LA95 1 (6 0) 28 I 23 7 1 .46 1 62 52 9 52. 1

12 Aerosi l R972 22 4 24 7 1 .53 1 62 1 1 6 0 12 1 0 ( 3 0) ^ LA9 1 ( 3 0)

* Ageing period 7d (<" 70 ' C

The results demonstrate that the hvdrophobic silica, on its own, does not increase initial tear strength as much as a hvdrophilic silica does (e.g. Example 6) but a further increase is apparent on ageing. This feature also appears when the silica is used in conjunction with Lutofan LA95 1 - Example 12 - in which synergism is clearly evident. The hydrophobic silica appears to have an adverse effect on initial tensile strength, whether alone or with the copolymer.

Claims

1 . A composition comprising a blend of a rubber latex, and particles of a hydrophobic or hydrophilic silica, and of a vinyl chloride homo- or co¬ polymer.

2. A composition according to claim 1 , wherein the silica is a precipitated silica or a fumed silica.

3. A composition according to claim 1 or 2 , wherein the combined concentration of the silica and of the vinvl chloride homo- or co-polvmer is in the range 1 to 1 2 parts per hundred rubber (by weight).

4. A composition according to claim 1 or 2, wherein the combined concentration of the silica and of the vi nvl chloride homo- or co-polvmer is in the range 2 to 6 parts per hundred rubber (bv weight) .

5. A composition according to anv preceding claim, wherein the primary particle size of the silica is in the range 1 0 to 20 nm.

6. A rubber latex film which includes a particulate mixture of a hydrophilic or hydrophobic silica and a vinyl chloride homo- or co-polymer.

7. A film according to claim 6, wherein the silica is a precipitated silica or a fumed silica.

8. A film according to claim 6 or 7 , wherein the combined concentration of the silica and of the vinvl chloride homo- or co-polymer is in the range 1 to 1 2 parts per hundred rubber (by weight).

9. A film according to claim 6 or 7 , wherein the combined concentration of the silica and of the vinyl chloride homo- or co-polymer is in the range 2 to 6 parts per hundred rubber (by weight).

10. A film according to any one of claims 6 to 9, wherein the primary particle size of the silica is in the range 1 0 to 20 nm.

1 1 . A rubber article formed of a rubber latex film according to any one of claims 6 to 10.

12. A glove formed of a rubber latex film according to anv one of claims 6 to 1 0.

13. A condom formed of a rubber latex film according to anv one of claims 6 to 1 0.

14. A method of making a reinforced rubber latex film material, comprising mixing a compounded rubber latex with a dispersion of a hydrophilic fumed silica and with a dispersion oϊ a vinvl chloride homo- or co-polvmer, dipping a shaped former into the resulting latex mixture, and curing the latex mixture.