Wednesday, October 17, 2007

Tube Extrusion

There are two important natural rubber latex extrusion processes, namely latex thread extrusion for the production of latex thread in small diameters and latex tube extrusion for the production of inner tubes. The latter method also is used for the manufacturing of tubes for medical devices. In this article the extrusion process for the production of inner tubes for sporting bicycles such as racing-bicycles and mountain bikes is described. Most inner tubes are manufactured from butyl rubber that provides low gas permeability. For sporting bicycles such as racing-bicycles and mountain bikes natural rubber mostly is used because such inner tubes can have thin walls, have half the weight of butyl rubber tubes and still are more puncture resistant. Gas permeability is a less important issue for (professional) racing.

Table 1 Product specification
High mechanical strength
High puncture resistance
Low gas permeability
Low modulus/high flexibility
Low weight/thin walls Smooth surface/possibility of different colors
Low cost/easy processing

Table 2 Dimensions and weight of latex tubes

Type Size Weight
Road

Ultra light 700 . 18/19 mm 55 gr
Racing 700 . 19/23 mm 65 gr
Standard 700 . 25/28 mm 68 gr
MTB

Racing 26 . 1.5/1.9 inch 85 gr

Standard 26 . 1.9/2.1 inch 95 gr

In table 1 the most important product requirements are summarized and in table 2 dimensions are qualified. For inner tubes, in principle various materials are interesting, such as polyurethane, polychloroprene and natural rubber. However, most synthetic polymer latices
are very expensive, sometimes up to 6 times the price of natural rubber latex. Because of costs, low modulus, high mechanical strength and ease of processing,

Table 4 General formulation for a heat-sensitive inner tube compound


Compound ingredients PVME - system (phr)


60% Natural Rubber latex, HA 167.0
50% Sulfur dispersion 2.5
50% ZDEC dispersion 2.0
50% Zincoxide dispersion 2.0
50% Titaniumdioxide dispersion 2.0
50% Color 0.2

30% Formaldehyde (pH 7.5-8) 2.0
Demi water 30.0
25% Stabilizer solution 1.2
10% PVME solution 20.0


Preparation of heat sensitive compound


The heat-sensitive latex is prepared by incorporating a heat-sensitizing agent in the compounded or pre-vulcanized latex. A general method for preparing dispersions is to grind the ingredients. Then a slurry is made of the powders (including the various additives, dispersing agents and stabilizers) with water. The slurry is grinded in a ball mill. Modern grinding methods to prepare dispersions are colloid mills, attrition mills or ultrasonic mills. As a function of time (depending on the type of mill used) the particle size declines. In the preparation of latex compounds for tube extrusion it is essential that the dispersion is finely ground to particle sizes
less then 5 microns. In table 3 a general formulation for an (inner) tube compound with heat gelling agent PVME is given.

After preparing the various dispersions such as the vulcanization dispersion, including anti-oxidants, the activation dispersion including colorants and the heat-sensitive (PVME) solution, the ingredients have to be compounded. In figure 2 the process is schematically drawn.
The vulcanization dispersion is mixed into the high ammonia (HA) latex and left over night. A pre-cure during 4 hours at a temperature of about 70 .C is performed and subsequently the activation dispersion is added followed by the heat-sensitive solution the next day. After preparation, the latex compound is stored in a reservoir and maintained at a constant temperature of 15 - 20 .C to avoid premature gelling due to the PVME. It is also possible to prepare the compound from pre-vulcanized latex usinga similar formulation, omitting the curatives, and ensuring a pH between 7.5 and 8.0 by adjustment of the level of formaldehyde.

Latex tube extrusion

A heat-sensitive natural latex compound flows from a constant level reservoir (A) to the extruder (figure 3). The extruder is made of concentric polished glass tubes fitted with a cold water jacket (B) at the top and a hot water jacket (C) below. When the latex compound enters the heated
zone around the hot jacket, it gels in the annulus between the concentric tubes and is slowly extruded from the bottom of the apparatus. The cold jacket is normally kept at a temperature of about 20 .C, and the hot jacket is normally kept at a temperature between 50 and 70 .C depending on the required cross-section of the tubing. The hydrostatic pressure of the latex in the reservoir above the extruder controls the extrusion rate. The tube is passing the jacket with a rate of 200 to 300 mm/min as a wet-gel and then goes through a spraying jet (D) to coat the tube with a butyl layer of about 0.1 mm. The tube passes a waterbath for leaching and a detackifying bath containing talc slurry. The tube is dried and vulcanized in a hot-air oven at a temperature of about 100.C. After vulcanization the tube is cut to length and turned in side out for further finishing. The ends are bonded together, punched and the valve is integrated into the tube. After assembling the tubes are ready for quality control.

Sources

Dipping with natural rubber latex,
NR technical bulletin, MRPRA, 1980.
Polymer latices and their Applications:
K.O. Calvet, Applied Science Publishers,
London, 1982.
Patent DE 2932177 C2, 1984.
TNO Industrial Technology, internal report.
Ben van Baarle LPRI


1 comment:

kaimal5 said...

Nice reading.I am from Kerala,India. I would like to try this method for extruding rubber band tubings of wall thickness 1mm to 1.5mm. Can I use PEG instead of PVME, because latter is not easily available here. If so could you pls help with its grade. If PVME is a must ,what is its specs. Also, could you pl help with measurements of extrusion apparatus. Thanks. Mr.Santhosh Kaimal, kemzol@rediffmail.com