Natural Rubber for Fashion

The use of natural rubber for clothes and footwear is certainly not one of today’s whims. Long ago Indians already made sheets, bags and shoes and later on Macintosh manufactured waterproof clothing. But it lasted till the 20th century before fashion designers discovered NR as a material to create with all sorts of wearable (e.g. clothing, jewelry, accessories) and non-wearable (e.g. upholstery) products. This attention was coupled with a fundamentally other view on the material. No longer NR was only an expedient to give clothing functional properties like elasticity and water-resistance, now it became a material to design in too. Even creations appeared that by most people were felt as extreme ones. But it was proper technology that put NR at the disposal of fashion by delivering thin and strong sheet material.

Demands

Clothing manufactured from rubber has to meet demands that are required by the application as well as by designers who, for instance, are attracted by the possibility of creating clothing that fits onto the body like

Table 1. Typical properties of vulcanized NR latex sheet

Mechanical properties Typical values
Tensile strength 25 - 30 MPa
Modulus 100% 0.1 - 0.5 MPa
Modulus 500% 2.5 - 3.5 MPa
Elongation 800 -1000%
Tear strength 100 - 150 N
Hardness 30-35 micro IRHD
Specific gravity 0.94 -0.95 g/m3

a second skin. Therefore the material has to be very stretchable, should not tear, must have a very low permanent set, must be puncture resistant to allow stitching and has to be colourable and bondable. NR’s unique characteristics meet all these demands.

Properties of NR latex sheet

Without fillers, almost all synthetic rubbers have low strength properties. This is due to the fact that those rubbers don’t crystallise under elongation. NR (latex) crystallises at an elongation of about 500%, even without fillers. Latex sheet is very strong at the crystallisation point. At the same time, the material is very flexible (low modulus of elasticity), has an excellent resistance to puncture (high tear strength), returns almost completely to its original shape after deformation (very low permanent set), can be bonded to itself or to other materials rather easily and can be produced in many colours. All this makes latex sheet a very suitable material for clothing. Table 1 shows some typical mechanical properties for NR sheet. The only type of synthetic rubber showing more or less similar properties is polychloroprene rubber and indeed this material is also used for clothing. However, compared to NR it has at least one significantly higher modulus of elasticity as a real disadvantage.

Table 2. Typical formulation

Formulation Parts

60% Pre-vulcanized latex 167
20% Ethylene oxide condensate 0.5
50% Zinc oxide 1.6
50% Antioxidant 1
50% Titanium dioxide 10
50% Color as required
25% Non-ionic surfactant 5
25% Polypropylene glycol 5

This reduces the ‘second skin’-experience dramatically and therefore most polychloroprene rubber clothing is loose fit types.

The production of NR latex sheet: Compounding and sheeting

Most commonly used are latices concentrated to 60% dry rubber content by centrifuging. High-ammonia (HA) and low-ammonia (LA-TZ) types are particularly recommended, although also pre-vulcanized types are used. To ensure uniform mixing of compounding ingredients, water-soluble ingredients are added to the latex as aqueous solutions, water-insoluble ingredients as aqueous dispersions or emulsions. Aqueous dispersions from sulfur, zinc oxide, titanium oxide accelerators and antioxidants are prepared in a ball mill or attritor mill. Water-soluble liquid compounding ingredients areemulsified using a high-speed stirrer or homogeniser. The colloidal stability of the latex has to be maintained at all times during the addition of the various ingredients and the handling thereafter. With the exception of the heatsensitizing solution and the (water-based) coloring agents, all ingredients are added to the latex under slowly stirring in order to prepare a base compound. As a heat stabilizer, poly vinyl methyl ether (PVME) has been used for a long time, but this required the use of formaldehyde (now discouraged on health grounds), whereas also the pH of the compound had to be adjusted. Therefore one has changed to a formaldehyde free sensitized pre-vulcanized formula, requiring also far less attention to the need for positive chilling of the compound. Special effects are possible by adding for instance phosphorescent pigments. Finally, the coloring agent(s), odorants and the heat-sensitizing system are added. In table 2, a typical formulation for pre-vulcanized latex is given. The latex compound then is transferred to a heated roll mill. The carrying belt may be either stainless steel or textile impregnated and coated with a suitable polymer. A good choice of the latter is necessary because the compounds are alkaline and also water ingres from the latex has to be prevented. The compound also will deposit traces of proteins etc. on the belt, which can adversely affect the sheet quality. These deposits have to be removed periodically. This may be circumvented by selection of a polymer with a high surface tension nature, causing the proteins to travel preferentially with the compound. The gelation is carried out in an infrared heating oven prior to the main drying. Then, the resulting gelatinous layer continues into another oven with a steadily increasing temperature over the length. In this way, the surface does not dry too quickly. Sheets are
produced on a continuous basis with widths up to 4 meters and a thickness between 0.2 and 3.0 mm. The nominal thickness can be controlled with a tolerance of . 10%. Prior to roll up the sheet an (anti-tack) agent is applied to one surface.

Manufacturing and assembling

Most processes in the production of latex clothing manufacture are similar to those for textile clothing and will not be discussed in detail here. Like textile, NR latex sheet is available in various designs. The most common printing techniques are silkscreen printing and flat printing. For the flat printing technique the sheet is stretched over a table of 40 to 80 meters after which with the use of stencils and various coloured inks a design can be printed. For each new colour a separate stencil is used. Afterwards, the sheets have to dry overnight to ensure good adhesion. A difference with textile clothing is that for latex clothing more often bonding is used for putting the pieces together. The glues for bonding are mostly latex based adhesives. Before bringing together the surfaces to bond these have to be cleaned thoroughly and roughened in order to ensure proper bonding. However, also the standard stitching techniques are used. The puncture resistance of a good quality NR sheet is sufficient to prevent tearing starting from the point of puncture even when the material is stretched in wearing.

Recommendations

Store latex clothing below 25.C; keep it awayfrom heaters and from natural light. The best way is to store clothes in a black plastic bag,when not in wear.Avoid contact with copper and copper alloysand with oils, solvents and greases.Wash clothing in clean warm water without soap or detergent. Wipe off excess water with care. Do not tumbledry, dry clean or iron under any circumstances.

Conclusion

In fashion and, although not discussed here, in upholstery NR sheet offers unexpected
possibilities to designers for realising their ideas, even when it are extreme ones. However, the designer as well as the end user mostly does not realise that this is only possible due to the ease of manufacturing sheets from NR latex and the unique mechanical properties of NR.

Sources

1) “Rubber in Fashion”, Rubber Developments, MRPRA, 1984
2) “Latex – The ultimate designer’s material”, Rubber Developments, MRPRA, 1992
3) “Student produces novel textures using latex”, Rubber Developments, MPRPA, 1993

4) “Rubber in Fashion”, Institute for Fashion Management and Design Koetsier/Montagne,

Amsterdam, 1996
5) “Sheeting”, R. Scott, Natuurrubber/Natural Rubber 17, Special on NR Latex, 1st quarter
2000