WO1992005887A1

WO1992005887A1 - Cleaning the surface of a continuously extruded rubber article vulcanized in a fluidized bed by an ultrasonic treatment and recovering pellets of the vulcanization bed adhered to the rubber surface

Info

Publication number: WO1992005887A1
Application number: PCT/IT1991/000078
Authority: WO
Inventors: Gianfranco Panneri; Carlo Magni
Original assignee: Tecar S.R.L.
Priority date: 1990-09-27
Filing date: 1991-09-26
Publication date: 1992-04-16
Also published as: IT1246253B; EP0550489A1; IT9083635A0; IT9083635A1

Abstract

A continuously extruded rubber article is vulcanized in a fluid bed of inert pellets and the pellets which remain attached to the sticky surfaces of the rubber article are detached and recovered by making the rubber article (6) travel through a pool of liquid contained in a first treatment trough (1) while transmitting to the bath ultrasonic wave power capable of detaching the pellets adhered to the rubber article (6). The ultrasonic treatment trough (1) has a raised bottom portion (3) where an ultrasonic transducer (5) is coupled to the liquid and at least a hopper-shaped portion (8, 9) provided with a discharge outlet (10, 11) at the bottom. The pellets collected in said hopper-shaped portion (8, 9) are discharged together with the liquid into a filter (13, 14) capable of retaining the pellets and the liquid collects in a second trough (12) from where is pumped back continuously to the first trough (1). The article (6) is effectively cleaned also within recessed portions by the ultrasonic treatment and the pellets are recovered and periodically returned to the fluid bed in the vulcanization chamber.

Description

CLEANING THE SURFACE OF A CONTINUOUSLY EXTRUDED RUBBE ARTICLE VULCANIZED IN A FLUIDIZED BED BY AN ULTRASONIC TREATMENT AND RECOVERING PELLETS OF THE VULCANIZATION BED ADHERED TO THE RUBBER SURFACE

The production of continuously extruded rubber articles is a well known industrial practice. Basically a plastic mixture of lattice and specific additives is extruded to ob¬ tain a continuous linear article having a certain profile. The extruded lattice not yet cross-linked retains at this point plastic characteristics and must be immedia^J- ely sub¬ jected to vulcanization before the profile generated by extrusion loses its shape by plastic flow of the material under its own weight. Vulcanization is notably a progressive process and its completion needs a certain time of exposure of the material to a sufficiently high temperature. It is com¬ mon practice to arrange a vulcanization chamber immediately at the outlet mouth of the extruder. This vulcanization cham¬ ber typically has typically an elongated shape of a length sufficient to permit an easy adjustment of the residence time of the continuously extruded ribbon travelling through the chamber at a constant speed, and of the relative temperature distribution profile along the length of the chamber so as to ensure the proper graduality and completeness of the vulcan¬ ization of the material.

It is evident the necessity to sustain the extruded rib¬ bon inside the vulcanization chamber by a substantially iso- static support means capable of sustaining the material es¬ sentially by buoyancy in a substantially fluid medium. For this purpose a molten salt bath was normally used. The tempe¬ rature cf vulcanization may be adjusted by thermostatically controll ng the temperature of the molten salt bath. A more salubrioas and less polluting solution is that of employing a fluidized bed of solid particles of a substantially inert ma¬ terial such as small glass spheres, fluidized by blowing of air heated to the desired temperature in substitution of a molten salt bath. The fluidized bed is comparable to a bath of a certain density and is similarly capable of sustaining by hydrostatic buoyancy the extruded rubber ribbon travelling therethrough. The advantages of this system of vulcanization are evident in terms of cleanliness and salubriousness of the work premises, absence of salt corrosion problems and ease of controlling the temperature by adjusting the temperature of the air which is blowed at the base of the fluidized bed for maintaining fluid the mass of pellets. These and other advan¬ tages have made the fluidized-bed vulcanization process very popular in the industry for this type of extruded rubber ar¬ ticles.

At the exit of the vulcanization chamber, the extruded and now vulcanized rubber article is cooled before being rol¬ led or cut into pieces of standard length for storage. The particles or pellets of the fludized bed are notably micro¬ scopic spheres of a sodium-calcium glass or of boron silicate glass or of other hard inert material with a density gene¬ rally comprised between 2 and 3 g/cirr and may have a mean di¬ ameter comprised between 100 and 500 μm. In this range of variability of the mean diameter of the pellets, it is easy to maintain a correct fluidification of the bed without col¬ lapses and such as to ensure a good uniformity of the apparent density of the fluidized bed which is functional to maintain a uniform hydrostatic buoyancy pressure along the entire length of the extruded ribbon within the vulcanization chamber. The relatively small dimensions of the pellets of the bed coupled with the characteristic stickness of the sur¬ face of an extruded rubber article, which is partly retained also at the end of the vulcanization process, and the rela¬ tively high temperature of the vulcanization chamber, favor an inevitable adhesion of a certain amount of pellets of the bed to the surface of the extruded article during the travel along the fluidized bed of the vulcanization chamber. The trapping and retention of pellets of the fluidized bed .within narrow concave portions of the profile of the extruded arti¬ cle is another cause for the retention of a considerable amount of pellets on the surface of the rubber article when exiting the vulcanization chamber. For this reasons it is com¬ mon practice to subject the surface of the extruded and vul¬ canized article at the exit of the fluidized-bed vulcaniza¬ tion chamber to a vigorous brushing for detaching and re- moving the pellets which have remained attached to the sur¬ face. In many applications, such a motorized brushing action is substantially effective in removing a large portion of the adhered pellets and may be acceptable. However in many other applications, including those of extruded rubber profiles to be used for making gaskets, etc., residues of particles of the fluidized bed which may tenaceously remain attached to the surface notwithstanting the brushing. treatment, may cause serious problems in terms of leaking gaskets or their pres¬ ence on the manufactured article be untolerable for other reasons connected to the specific application of the rubber material. In this cases it has been a common practice to make recourse to more or less manual interventions with specific tools and to an accurate brushing reaching the most narrow recesses of the profile of the extruded article, followed by rinsing with purposely oriented water jets utilizing a plural¬ ity of nozzles. These methods are burdensome either from the point of view of the man-hour requirements or are not com¬ pletely satisfactory from the point of view of the result which is attained.

It has now been found a way to perform a thorough clean¬ ing of the surfaces of a substantially continuous extruded article of rubber which is vulcanized in a fluid bed which comprises treating the extruded article, immediately after the vulcanization step, in an ultrasonically activated bath. The process permits a practically complete recovery of the fluid bed pellets which are detached and removed from the sur¬ face of the rubber article. The recovered pellets may then be returned to the fluidized bed of the vulcanization chamber. The sedimentation and recovery of the detached pellets takes place through a purposely hopper-shaped bottom of the trough containing the liquid pool trough which the rubber article travels while being ultrasonically treated. The hopper-shaped bottom of the trough has an outlet part or pipe which dis¬ charges into a second trough placed below the trough used for the ultrasonic treatment and which is provided with a filter for retaining the pellets and with a recycle pump for pumping back the liquid recovered in this second trough to the ultra^¬ sonic treatment vessel above. The attached figure schematically show the process and the apparatus of the invention.

With reference to the Figure, the apparatus comprises an ultrasonic treatment trough 1 having a bottom with a raised central portion 3. In this central raised portion of the bot¬ tom 3, the vibrating plate of a first portion 4 of an ultra¬ sonic "U"-shaped transducer is installed, the other two por¬ tions of the "U"-shaped transducer may be installed in the vertical lateral walls of the treatment trough 1, as shown in the figure, where one of said two vertical portions of the "U"-shaped transducer is indicated with 5. The extruded and vulcanized rubber article 6 is guided by the guide rollers 7 in order to travel, immersed in the liquid pool contained in¬ side the ultrasonic treatment trough 1, across the space con¬ tained within the three parts forming the "U"-shaped transducer so that the rubber ribbon is inpinged by the ultra¬ sonic waves trasmitted. through the liquid contained inside the treatment trough and generated by the vibration impressed to the vibrating plates of the three portions composing the "U"-shaped transducer, while the advancing rubber travels through this zone of the bath. The combined actions of the three portions of the "U"-shaped transducer which may be made with laminae of magnetostrictive nickel, operating at the same vibrational frequency, produce a uniform distribution of vibrational ultrasonic power among the three plates of the "U"-shaped transducer and therefore a uniform cavitation in¬ tensity on the wetted surface of the rubber article which are hit by the ultrasonic waves propagating through the liquid. The other portions of the bottom of the treatment trough 1 are shaped in the form of hoppers 8 and 9 and terminate with an outlet pipe, respectively 10 and 11, which dips in the liq¬ uid contained inside a second, lower trough 12, inside which two basket-shaped screen filters, 13 and 14, respectively, are arranged for retaining the pellets which have been de¬ tached from the surface of the rubber article 5 by ultrasonic vibration. A recycle pump 15 provides to pump back the liquid discharging into the second lower trough 12 together with the pellets, to the upper ultrasonic treatment trough 1. The liquid used may be water to which a detergent may or may not be added. The troughs 1 and 12, as well as the filters 13 and 14 and the guide rollers 7, may be made of stainless steel or other suitable material.

An appropriate generator, not shown in the figure, pro¬ vides a sufficient electrical power at a frequency of 20.000 Hz or above to the transducer. The power of the ultrasonic group .may be comprised between 1.000 and 6.000 Watts. The generator normally uses silicon controlled diodes for raising the oscillation frequency from the standard network frequency of 50 Hz to 20.000 Hz or above. Of course the transducer must have a power rating compliant with the power delivered by the generator.

According to an alternative embodiment, which is useful where a pronounced flection of the extruded and vulcanized rubber article must be avoided, the ultrasonic treatment trough may be provided with inlet and outlet openings for the travelling rubber article in the opposite end walls of the trough, below the liquid level and provided with flexible gaskets for reducing the outpouring of the liquid. The liquid which inevitably will seep out will be recovered by means of two small troughs discharging into the second trough from where the liquid is recycled to the ultrasonic treatment vessel.

Claims

C L A I M S

1. A process for cleaning the surfaces of a continuously extruded rubber article which is vulcanized in a fluid bed of inert pellets which are fluidized by means of blown air which is heated to the vulcanization temperature, characterized by comprising the following steps: leading said continuously extruded and vulcanized rub¬ ber article to travel immersed in a liquid pool contained within a first trough; delivering power in the form of cavitational ultra¬ sonic waves through said liquid to said rubber article trav¬ elling through the bath by means of an ultrasonic transducer powered by an alternate current generator at ultrasonic fre¬ quency so as to cause the detachment of pellets of said vul¬ canization fluid bed which have adhered to the surface of the rubber article; collecting by settling said detached pellets into at least a hopper shaped portion of the bottom of said trough; discharging by gravity said collected pellets together with a portion of the liquid contained into said first trough through an outlet port at the bottom of said hopper shaped portion, through a screen filter and collecting said pellets in said filter while discharging said effluent liquid into a second trough; recycling the liquid collected in said second trough to said first trough by means of a recycle pump; periodically recovering said collected pellets from said filter and returning the pellets to said vulcanization fluid bed.

2. Apparatus for removing pellets of a vulcanization fluid bed which have adhered to the surface of a continuously extruded and vulcanized rubber article and for recovering said pellets to be returned to the fluid bed, characterized by the fact that comprises a first elongated trough containing a pool of liquid, said trough having a bottom configured as to have at least a relatively raised portion and a hopper-shaped portion; at least an ultrasonic transducer coupled to said liq¬ uid in said raised bottom portion of said first trough and means for driving said transducer to generate and transmit ultrasonic power to said liquid; means for driving said continuously produced vulcan¬ ized rubber article to travel in said pool of liquid; means for discharging detached pellets together with a part of the liquid of said pool at the bottom of said hopper- shaped portion of the bottom of the first trough through a filter capable of retaining said pellets, into a second trough; pump means for recycling the effluent liquid collected in said second trough back into said liquid pool in said first trough.