FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION
CONTINUOUS PROCESS FOR BAKING OF CURED FRICTION MATERIAL USING ELECTROMAGNETIC ENERGY
2. APPLICANT
(a) Name : PRADEEP METALS LTD.
(A Company Incorporated under the Companies Act, 1956)
(b) Nationality : Indian
(c) Address : R - 205, M.I.D.C., Rabale, Navi Mumbai - 400 701
3. PREAMBLE OF THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF INVENTION
This Complete Specification is in respect of Provisional Application No. 530/MUM/2013 tiled on 25th February 2013.
This invention relates to a continuous process for rapid baking of conventionally cured articles of composite friction materials.
This invention particularly relates to a continuous process for rapid baking of conventionally cured articles of composite friction materials by using electromagnetic radiations such as microwaves.
This invention relates more particularly to a continuous process for rapid baking of conventionally cured articles of composite friction materials such as brake pads by exposing the articles to microwaves under controlled conditions.
DESCRIPTION OF RELATED ART
The articles of composite friction materials such as brake pads, drum lining and clutches are used for controlling speed in dynamic systems for safety purpose. One of the major applications of the brake pads is in automobiles. These friction components are composites made by mixing fibers, fillers, friction modifiers, binders and cure promoting compounds (i.e. graphite and magnetic particles such as iron, ferric oxide, iron sulphide, copper iron sulphide, etc.). The composition depends on the end application and properties required for those applications. In conventional process the brake pads are prepared by mixing proprietary ingredients in the desired proportion and cured at desired temperature under pressure for precise time in molds of desired shapes and sizes producing individual articles which are then set to harden in the curing stage. Then the articles are baked to improve their properties. In this invention these pieces are referred as articles; and sometimes as samples they are also known as object. When the number of articles increases, each of these is referred as an "item". Subsequently the post curing treatment i.e. baking of the cured pads is carried out conventionally in an electrical oven for several hours using programmed multistage heating schedules. Generally baking requires about 12 hours depending on the constituents added in the composition. This longer duration is essential for achieving equilibrium in temperature

throughout the mass, and for completing the targeted transformations for achieving desired properties of the final product.
In the conventional process, "Baking" is a post-curing step where freshly cured articles such as brake pads are slowly heated for promoting the polymerization reactions of binder components to form a hard polymeric film on the surface. During conventional curing, under pressure and high temperature, all the constituents are evenly cured and about 80-90% polymerization is completed. However, for completing the remaining 10% of polymerization, baking is done. During baking, the excess organic constituents, added in the composition, escapes from the cured friction pads thus relieving the stresses generated during the curing stage.
In conventional processing, a large temperature gradient is noticed due to the difference in heat conductivity of each constituent added in the pad composition. Most of the ingredients used in the pad formulation are bad conductors of heat due to which the pads are heated slowly during conventional baking and are soaked at different temperatures for longer durations to achieve uniformity in the temperature and to achieve the desired reaction equilibrium. This results in consumption of more energy and requires longer time to achieve the targeted properties.
Prior art as listed below, discloses various techniques used to lower such time and energy in the process of manufacturing composite articles like friction pads by using microwaves at molding cum curing stage. They do not follow the conventional process. These are discussed below.
US Patent 6368994 discloses a method for rapid polymerization, curing or a combination thereof a polymerization or curable composition to yield polymers and composites. It is based on these polymers through the utilization of short wavelength high energy microwave energy (preferably in the range of 10GHz to 1000GHz). The patent discloses use of microwave absorbing curing materials (catalyst) during curing. These catalysts are useful additives in the formulation for rapid polymerization reactions but only during curing stage. Preparations of compositions suitable for high energy microwave fields are described in this patent and it is not concerned with conventional curing process. Further it does not disclose any continuous process for curing or baking of composites with low energy microwaves.

WO 2009/009348Aldiscloses a method and apparatus for forming an in-mold for curing of brake pads. This invention describes the batch type hybrid (Conduction heating along with Microwave heating) technique for curing of the friction pads by modifying the mold use during the curing stage. While compressing, the components placed inside the said die cavity the components are exposed to microwaves during ram compressing operations. While this patent describes a batch process for curing of composite pads in molds by conduction and microwave heating, it does not describe any process for continuous baking of composites after they are cured.
During conventional heating, unwanted materials such as furnace walls,, sample holders add to the energy consumption, apart from the actual article heating. The heating efficiency of the conventional baking process depends on the conducting properties of the constituent materials. The baking of friction pads with the conventional process requires a multistep heating profile that takes about 12 hours. (See Fig. 1).
There is a need to speed up the process of baking conventionally cured materials to save time and energy consumption at the baking stage and make the total process more economical.
OBJECT OF THE INVENTION
The principle object of the present invention is to develop a process for rapid baking of the conventionally cured friction composite articles.
It is another object of the present invention to further improve the energy efficiency of the baking process.
It is yet another object of the invention to achieve target specifications/properties of the end product even after rapid baking.

SUMMARY OF THE INVENTION
Accordingly, the present invention provides a continuous process for baking of articles of conventionally cured composite friction materials, comprising of exposing the said articles to microwaves having pre-set frequency band from 800 to 5000 MHz, by continuously passing the items one after another through a microwave tunnel for a certain pre-set period till each item is fully and uniformly baked throughout, under controlled temperature conditions maintained in the tunnel.
The present invention replaces the conventional electrical radiation heating by employing more efficient and volumetric electromagnetic heating such as microwave heating in frequency range 890 - 2450 MHz, particularly at 2450±50 MHz frequency. Microwave technique is an internal and volumetric heating process where each atom generates heat due to its interaction with microwaves. Friction components such as brake pads contain a mixture of fiber, filler, friction modifier, binder and cure promoting compounds. Some of the ingredients used are graphite, magnetic particles such as iron, ferric oxide, iron sulphide, copper iron sulphide, etc. These constituents have an effective aspect ratio, and conductivity which promotes its interactions with microwave energy resulting into rapid and uniform baking of the friction material. In microwave heating, the energy is used only for heating the pads and not the surrounding furnace, sample holders, etc., which offers higher energy efficiency as compared to conventional heating.
In the continuous process of the present invention, the articles of cured composite friction materials are brake pads, drum lining clutch pads used in different industrial applications including automotive industries, aviation etc., wherever control of speed is done using friction materials made of composites. Due to volumetric and uniform electromagnetic heating, the process requires only a single stage heating profile that takes less than 120 minutes, while the conventional baking takes about 12 hours. [See Fig. I].
None of these prior art processes are concerned with continuous baking of conventionally cured composite friction pads, and hence they do not form or can be considered as close to prior art to the present invention.

The invented process is carried out in a continuous microwave system where microwaves are uniformly distributed in the tunnel in which conventionally cured brake pads placed on the sample holders made from ceramic fiber boards, travel with a constant speed over a special belt under controlled temperature profile in the tunnel. The conveyor belt is made of material that can withstand high temperature and is transparent to microwaves. The conveyor belt speed and magnetron ON/OFF settings decides the duration of exposure to microwaves required for achieving targeted temperature profiles. The pads are arranged so as to expose all the pads to microwaves uniformly in the tunnel at any given time of the process. The infrared temperature sensors are located on the outer surface of the tunnel so as to focus on the surface of the items travelling inside the tunnel and monitoring and controlling the temperature profile of the samples through PID (Proportional Integral Derivative) controllers operated through PLC (Programmable Logic Control) based programmer. The samples are baked in the tunnel under the set process parameters such as temperature, microwave exposure time, sample arrangements etc. The total time taken for baking and the energy required during this stage is reduced considerably using the process of the present invention compared to the conventional electrical radiation heating in the ovens.
BRIEF DESCRIPTION OF THE DRAWINGS
Figurel: Time-Temperature Profile comparison between conventional process and the invented process
Figure2: Represents the schematic diagram of continuous microwave system and top view of the sample arrangement for exposing it to microwaves in the tunnel
4. DETAILED DESCRIPTION OF THE INVENTION
In embodiment of the present invention, a continuous baking of friction materials by using electromagnetic energy such as microwaves is done using a continuous microwave system shown in Fig.2. According to this embodiment, the continuous microwave system contains a tunnel 1 installed with a variable speed conveyor belt 4. The belt 4 is made of microwave transparent material such as Polytetrafluoroethylene (PTFE) coated glass cloth immaculate matrix of excellent physical and chemical properties and can withstand relatively higher temperatures upto 250°C. The conveyor belt 4 is driven by a variable frequency drive capable

of adjusting the speed by adjusting the frequency of the driving motor, thereby enabling the control of the friction material exposure time to electromagnetic radiations such as microwaves in the frequency range 800 to 5000 MHz, preferably 890 to 2450 MHz, more preferably at 2450 ± 50 MHz, during the baking process. The microwave tunnel 1 is installed through wave guides for uniform distribution of the microwaves with three or more 1.5 kW air cooled magnetrons 5 depending on the length of the tunnel. The microwaves are introduced from any preferred side of the tunnel through wave guide openings. In one embodiment of the invention, the microwaves are introduced from the base of the tunnel. For uniform distribution of the microwaves in the tunnel, a mode stirrer 3 is installed inside on the top plate of the microwave tunnel 1, just above the entry point of microwaves in the tunnel. Thus in the present invention, the microwave tunnel is divided into three zones. The first zone is close to entry point while the third zone is close to the exit point. Three infrared temperature sensors 2 are installed adjacent to the mode stirrers for monitoring and controlling the friction material items 6 travelling inside the tunnel over the conveyor belt 4. The three infrared temperature sensors are located at the three different locations close to the entering points of microwaves from each magnetron in the tunnel. The signal from all three IR sensors 2 is given to PID controllers which are operated through the PLC, that controls the power of microwaves in the tunnel thereby controlling the temperature of the friction materials items 6 travelling in the tunnel over the conveyor belt 4. The items 6 with conventionally cured friction materials 12 having a metal back plate 13 are placed over ceramic fiber boards 14 which are porous and transparent to microwaves and can withstand high temperature without reacting with the sample and are continuously fed through inlet 8 installed with flap 9 in the microwave tunnel 1. The microwave treated friction materials items such as friction pads are collected from the outlet of the microwave tunnel 1 through outlet port 10 installed with a flap 11.
In a particularly preferred embodiment of the present invention, the friction material items 6 are loaded continuously into the microwave tunnel 1. A uniform microwave field is maintained within the tunnel by three mode stirrers located just above the inlet points of the microwaves in the tunnel 1. The temperature profile of the said friction material items travelling over the conveyor in the tunnel is maintained from room temperature to 200°C, with infrared sensors focusing on the surface of the friction material items travelling in the tunnel and control the microwave power in the tunnel to control the temperature in the range of 160 to 180°C and more preferably between 170 and 180°C. The microwave exposure time

is decided by the speed of the conveyor belt 4 which is controlled through Variable Frequency Drive (VFD).
Thus in the preferred embodiment of the present invention the process for baking of articles of conventionally cured composite friction materials of automotive brake pads are exposed to microwaves having pre-set frequency band from 2400 - 2500 MHz, continuously passing the items one after another through a microwave tunnel for a certain pre-set period of about one hour from the entry to the exit from the tunnel, till each item is fully and uniformly baked under controlled temperature conditions in the range of 160 - 180 C .
Thus in the continuous process for baking of composite friction materials comprising of exposing the conventionally cured articles to microwaves having pre-set frequency band from 800 to 5000 MHz, by continuously passing the items one after another through a microwave tunnel for a certain pre-set period till each item is fully and uniformly baked. Exposure to microwaves is synchronized with the baking temperature monitored by a number of infrared temperature sensors installed on the tunnel focusing on the surface of the items travelling in the tunnel and controlled through PID controllers operated through a PLC based programmer. The said articles are passed from the entry to the exit of the tunnel continuously by passing the items placed on the ceramic fiber boards over a microwave transparent belt running horizontally inside the tunnel at the desired speed.
The invented process also allows stacking of the cured composites items one above the other for improving the output and process economics.
The invention is now described by way of examples.
Many modifications in addition to those described above may be made to the technique described herein without departing from the spirit and scope of the invention. Accordingly, following are examples only and or not limiting of the scope of the invention.
EXAMPLES
Following series of trials where 25 to 45 numbers of conventional process cured composite articles, such as brake pads, weighing about 280-310 gm and of curved nature with metal

back plate were used. The pads were placed over a porous ceramic fiber board which is transparent to microwaves and can withstand high temperature without reacting with the pads. The pads were passed continuously over the conveyor belt of the continuous microwave system. The microwave frequency used was 2400 - 2500 MHz. The belt speed was varied from3.3 to 6 cm/min by varying the VFD frequency from 2 to 3 Hz. Due to this the effective microwave exposure time for each pad was varied from 34 to 18 min. respectively. The friction pad temperature was monitored with a non-contact type infrared pyrometer and maintained between 170 to 180°C especially in the second and third zone of the tunnel through a PLC based programmer and monitored continuously using a Personal Computer (PC) to ensure that all conditions are maintained properly and uniformly during the trials.
The pads were tested for the main properties such as Brinell Hardness Number and density as per Indian Standard "Automatic vehicles brake linings": IS 2742 (Part3) - 1994. The details of the experimental conditions and hardness and density are reported in Tablel.
Examplel:
During the trial, 45 samples of conventionally cured composite brake pads were placed on the sample holder made from ceramic fiber board on the conveyor belt of the continuous microwave system. The belt speed was set to 6 cm/min during which the effective microwave exposure time for each pads was 18 min. The friction pad temperature was monitored and controlled with the non-contact type infrared pyrometer and maintained between 170 to 180°C through a PLC based programmer and monitored continuously using a PC to ensure that all conditions are maintained properly and uniformly during the trial. The pads were tested for the main properties.
Example 2:
in this example, as in the example 1, conventionally cured composite friction pads were taken for baking as per the process of the present invention. The process and apparatus used for baking of cured brake pads is same as in example i, but the belt speed was reduced to 4.5 cm/min. 38 samples were passed continuously through the tunnel in 60 minutes and the actual exposure to microwaves for each sample was thereby increased to 25 minutes.

Example 3:
In this example, as in the example 1, conventionally cured composite friction pads were taken for baking as per the process of the present invention. The process and apparatus taken for baking of cured brake pads is same as in example 1 but the belt speed was further reduced to 3.3 cm/min. 24 samples were passed continuously through the tunnel in 60 minutes and the actual exposure to microwaves for each sample was thereby increased to 34 minutes.
Example 4 (comparative example):
Conventionally, the same cured brake pads were baked as per the routine baking process in a factory. The pads were placed in a metal try and kept on a metal trolley. The trolley was pushed in an electrical oven. The temperature was increased in a stepwise manner from room temperature to 140°C in 1 hour and held at this temperature for 1 hour to achieve equilibrium. After 1 hour, temperature was increased to 160°C in 30 min. and maintained for 3 hours and then further increased to about 180°C in 30 min. and equilibrated for 5 hours. Then the samples were gradually cooled to room temperature. The conventional heating cycle requires about 12 hours followed by cooling time.
The results are summarized in the Table 1 below:
Table 1: Summary of properties of microwave baked pads compared with conventionally cured samples

Example No. 1 2 3 4 (comparative)
Hardness of the cured
samples before baking
(BHN) 55-65 55-65 55-65 55-65
Duration of the trial (hours) 1 1 1 More than 12 hours
Speed of Conveyor belt (cm/min 6 4.5 3.3 -
No. of Samples passed during the trial 45 38 24 -
Effective Microwave exposure time (min.) 18 25 34 -
Hardness (BHN) after baking 60-68 70-74 75-77 60-75
Average Density (g/cc) 2.56 2.58 2.57 2.4-2.65
The results of the examples show that hardness improves on baking conventionally cured composite friction pads.

ADVANTAGES OF THE INVENTION
The invented process uses a simple heating profile without intermittent holds at any temperature. The conventional process uses multistep heating profiles which needs longer time.
1. The properties of the samples processed by the invented process are equivalent to those achieved by using the conventional process.
2. The invented process can be used to bake (post-cure) uniformly any preformed/cured friction materials such as pads in a shorter time.
3. The additives and cure promoter compounds generally used in the composition of friction articles get heated rapidly and uniformly under the microwave field resulting in the formation of homogenous distributed pores throughout the friction pads that control the noise -and friction resistance between the pad and the stator.
4. The electromagnetic radiations, such as microwaves used in the present invention, reflected from the metallic plate on which the friction material is mounted and generates a temperature gradient between the friction material and the plate, which further develops a gradient cross linking morphology resulting in the generation of sufficient bond strength between the friction material and the plate.

We Claim
1. A continuous process for baking of articles of conventionally cured composite friction materials comprising exposing the said articles to microwaves having pre-set frequency band from 800 to 5000 MHz, continuously passing the items one after another through a microwave tunnel for a certain pre-set period till each item is fully and uniformly baked throughout under controlled temperature conditions maintained in the tunnel
2. The continuous process as claimed in claim 1, wherein the articles of conventionally cured composite friction materials are brake pads, drum lining clutch pads used in different industrial applications including automotive industries, aviation where control of speed is done using friction materials made of composites.
3. The continuous process as claimed in claim 1 and 2, wherein the microwaves used have frequency range 890-2450 MHz.
4. The continuous process for baking as claimed in claim 1, wherein the said articles get exposure to microwaves from a number of magnetrons of suitable power through wave guides opening at the base of the tunnel and corresponding mode stirrers installed hanging inside from the roof of the tunnel.
5. The continuous process for baking, as claimed in claim 1, wherein exposing said conventionally cured articles to microwaves for a certain pre-set period from the entry to the exit from the tunnel is done by continuously passing the items placed on the ceramic fiber boards and passed one after another across the tunnel by placing the said boards over a microwave transparent belt running horizontally inside the tunnel at the desired speed.
6. The continuous process for baking as claimed in claim 1, wherein the pre-set period of exposure to microwaves is synchronized with the control on baking temperature profile by number of infrared temperature sensors installed on the tunnel focusing on the surface of the items travelling in the tunnel and controlled through PID controllers operated through a PLC based programmer.

7. The continuous process for baking as claimed in claim 1 and 2, wherein, the temperature is controlled in the range 160-200°C.
8. A continuous process as claimed in any previous claim, for baking of articles of conventionally cured composite friction materials of automotive brake pads comprising exposing the said articles to microwaves having pre-set frequency band from 2400-2500 MHz, continuously passing the items one after another through a microwave tunnel for a certain pre-set period of about one hour from the entry to the exit from the tunnel till each item is fully and uniformly baked under controlled temperature of 160-180°C conditions.
9. A continuous process for baking of articles of cured composite friction materials substantially as described in the text and in the examples.