FIELD OF INVENTION:

The present invention relates to a method of recovering skim rubber from skim latex. More particularly the invention relates to a method of deproteinization of skim latex for recovery the skim rubber from skim latex.

BACKGROUND:

India produces approximately 20 lakh tones of natural rubber latex per year. About 17 lakh tones of field latex (FL) is processed to sheet rubber and 2.7 lakh tones is processed to concentrated latex. The latex concentrate (Cenex) manufacturing industry produces around 1.6 lakh tones of Cenex and 1.2 -1.4 lakh tones of skim latex as by-product. Cenex is used for the industrial production of latex based NR products like gloves, foams, tubes, healthcare products etc. The by-product, skim latex contains around 3- 4% dry rubber constituting small rubber particles. A significant portion of the non-rubber constituents originally present in the field latex remain with the serum phase of skim latex and also get adsorbed on the surface of the rubber particles of the skim latex. Due to the small particle size and the presence of large amount of adsorbed non- rubber constituents, the higly diluted skim latex is extremely difficult to coagulate. Current method of skim latex processing involves addition of appropriate quantity of sulphuric acid and the flocculated latex (partially coagulated) is stored in sacks for 2-3 weeks followed by mechanical working to attain a consolidated mass. This method of recovering rubber from skim latex is time consuming, environmentally polluting, labour intensive and can cause health hazard to the workers. The skim rubber thus obtained has generally a bad odour, with significantly inferior raw rubber and vulcanisate properties compared to TSR or sheet rubber grades.

PRIOR ART:

US 2007/0135604 Al discloses a method for deproteinizing natural rubber latex comprising steps of: adding a urea denaturing agent for proteins and a surfactant to raw natural rubber latex; transporting the mixture through a fluid channel while agitating and mixing to denature proteins in raw natural rubber latex; and separating and removing the denatured proteins.

US 6204358 disclose a process for producing deproteinized natural rubber using protease and anionic surfactant.

Further US patents US 5569740, US 5908893, US 6943212, US 7015264, US 6265479, US 6462159 and US 5910567 discloses of deproteinization of latex using protease and varying surfactants.

KMITL Sci. J. Vol.8 No.2 (Section B) July - December, 2008 discloses creaming process of skim natural rubber latex using hydroxypropyl methyl cellulose.

Journal of Applied Polymer Science, 114 (5), 3319-3324 (2009) envisages the use of liquid papain enzyme which was obtained from the papaya plant (Carica papaya) for deproteinization, followed by creaming for coagulation of skim latex.

Journal of Applied Polymer Science 115(2), 1022-1031 (2010) discloses creaming of skim natural rubber latex with chitosan derivatives. Solid-state 13C-NMR, thermogravimetric analysis, and Z, potential measurement results provided evidence that the creaming of SNR latex with anionic chitosan CMCh and cationic chitosan HTACh was controlled by the depletion flocculation and adsorption mechanisms, respectively.

The prior art discussed above discloses various methods for deprotenization of skim latex or natural rubber latex to recover natural rubber or skim rubber. Most of the prior art involves enzymatic deproteinization of the skim latex or natural rubber latex which suffers disadvantages like, the process is not economical as it needs large quantity of enzyme and necessitates steps for removal of hydrolysed proteins and further steps like deammoniation and addition of sodium thiosulphate or boric acid salts or creaming using chitosan or cellulose derivatives. Further enzymatic deproteination of skim latex is a time consuming and tedious process. Hence there exist a need to propose a novel method which require only less than 2 days for the complete processing of skim latex and the skim rubber thus obtained has superior raw rubber and vulcanisate properties compared to the skim rubber obtained by conventional method.

OBJECT OF THE INVENTION:

The object of the present invention is to recover skim rubber from skim latex in a much shorter span of time in comparison to the time taken in known prior art.

Another object of the present invention is to develop a process for creaming of skim latex to obtain a high amount of dry rubber with low protein content in the creamed fraction of the skim latex.

Yet another object of the present invention is to utilize alkali and surfactant to deprotenise skim latex

Yet another object of the present invention is to use sulphuric acid for the coagulation of the creamed skim latex to form a consolidated mass.

Yet another object of the present invention is to opt a non-enzymatic route to deproteinise skim latex to overcome the disadvantage of time consuming which is inherent in any enzymatic route.

Yet another object of the present invention is to develop a faster and a simpler process for recovery of skim rubber from skim latex.

Yet another object of the present invention is to achieve skim rubber from skim latex in which skim rubber shall possess the set of superior properties of a raw rubber with improved vulcanisate properties.

SUMMERY OF THE INVENTION:

The present invention relates to a process of recovering skim rubber form skim latex by a method using alkali for deproteinisation which is hitherto unknown in any prior art. The process involves the steps of creaming of skim latex followed by coagulation to obtain skim rubber. The creaming step involves addition of anionic surfactant and alkali to skim latex and keeping for a period of 15-20 hours. After creaming of skim latex the serum is removed and the creamed skim latex is coagulated by adding 5-10% H2S04to get a consolidated mass. The coagulum is washed well with water to remove the acid and is then dried in an oven. The coagulum thus obtained is processed to block rubber as usually carried out in normal TSR factory.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention relates to a process for recovering skim rubber from skim latex. The proposed method of skim latex processing involves the following steps. The latex is mixed with the required quantity of anionic surfactant and alkali and kept for a period of about 24 hours. During this time, creaming of latex takes place and dry rubber content (DRC) of creamed fraction is increased. The serum is removed and the creamed skim latex is coagulated by adding 5-10% H2S04 to get a consolidated mass. The coagulum is washed well with water to remove the acid and is then dried in an oven. The anionic surfactants that can be used are potassium oleate, ammonium laurate etc. which are currently used in natural rubber latex products manufacturing industry. The alkali that can be used is sodium hydroxide which is a commercially available industrial chemical.

The surfactant added to the skim latex displaces the protein adsorbed on the surface of the rubber particles and the alkali hydrolyses the proteins which ultimately lead to the creaming of the latex. The creaming process starts within an hour after the addition of the reagents and is completed by 15-20 h. The DRC of the latex increases from 3-4% to 7-9%. The creamed latex coagulates instantaneously and completely when 5-10 % sulphuric acid is added to it. The coagulation can also be carried out by adding the creamed latex in to 5-10% sulphuric acid under stirring. The coagulum thus obtained is processed to block rubber as usually carried out in normal TSR factory.

The raw rubber properties of skim rubber prepared by the creaming process in comparison with control are given in Table 1.Conventionally prepared skim rubber shows the expected characteristics of high nitrogen content, high level of acetone extractables of which a good proportion is known to be a mixture of various fatty acids, high initial plasticity, and low ash content. Skim rubber prepared by the proposed method has comparatively low Po, and lower nitrogen content along with higher ash content. The PRI values and other raw rubber properties of skim rubber show wide variation from sample to sample. Table la and Table lb show the raw rubber properties of skim rubber obtained by the proposed and conventional methods using skim latex from two different lot. The PRI values of the skim rubber could be further improved by oxalic acid treatment.

Table la: Comparison of raw rubber properties

Table lb: Comparison of raw rubber properties

A comparison of the vulcanisate properties of HAF filled compounds are given in Table 2 along with ageing behavior. The skim rubber obtained using the proposed method shows superior tensile properties, reduced hardness, modulus, heat build-up and compression set. The ageing properties of the skim rubber obtained using the proposed method is also significantly superior to that obtained using conventional method

Table 2. Mechanical properties of carbon black filled mixes

In one of the preferred embodiment the present invention shall disclose a novel method to recover rubber from skim latex which is a by-product of the latex concentrate industry. In the proposed method, the skim latex can be processed in less than 2 days time against the conventional method which takes 2-3 weeks. In the proposed method, the skim latex is subjected to a creaming process by the addition of suitable quantities of surfactant and alkali. The creamed latex coagulates easily to a consolidated mass on addition of acids.

The skim rubber obtained by this technique has a lower protein content, metal contaminants and fatty acids leading to enhanced quality parameters. The cure characteristics, vulcanisate properties, and ageing behavior of rubber obtained by the proposed method is significantly better than that prepared by the conventional method.

Example 1:

Skim latex was creamed by addition of a surfactant and alkali. The alkali used was sodium hydroxide. The surfactant used was potassium oleate or ammonium laurate . The method of preparation of potassium oleate solution and ammonium laurate solution are given in Annexure 1 and Annexure 220 L of skim latex was mixed with sodium hydroxide and 20% potassium oleate solution as per Formulation 1. Sodium hydroxide was dissolved in minimum quantity of water as about 40 % solution and added to the latex. The skim latex mixed with the chemicals was kept aside undisturbed for creaming of latex to take place.

Table 3: Formulation 1

Example 2:

20 L of skim latex was mixed with sodium hydroxide and 20% potassium oleate solution as per Formulation 2. Sodium hydroxide was dissolved in minimum quantity of water as about 40 % solution and added to the latex. The skim latex mixed with the chemicals was set aside undisturbed for creaming of latex to take place.

Table 4: Formulation 2

Example 3

20 L of skim latex was mixed with sodium hydroxide and 20% potassium oleate solution as per Formulation 3 . Sodium hydroxide was dissolved in minimum quantity of water as about 40 % solution and added to the latex. The skim latex mixed with the chemicals was set aside undisturbed for creaming of latex to take place

Table 5: Formulation 3

Example 4

20 L Of skim latex was mixed with sodium hydroxide and 20% potassium oleate solution as per Formulation 4. Sodium hydroxide was dissolved in minimum quantity of water as about 40 % solution and added to the latex. The skim latex mixed with the chemicals was set aside undisturbed for creaming of latex to take place

Table 6: Formulation 4

In examples 1-4 instead of potassium oleate , 20 % ammonium laurate could be used so as to get the same level of creaming. No chemical was added for the control sample(Sample 5)

Annexure 1

Preparation of 20 % potassium oleate solution.

Weigh the chemicals as per Table 7 Table 7: Ingredients required Parts A and B were warmed to 75 °C and B was added to A in small quantities with efficient stirring. The stirring was continued till the solution attained room temperature. 1-2 g more of alkali was added if the solution was not clear.

Annexure 2

Preparation of 20 % Ammonium Laurate.

Weigh the chemicals as per Table 8

Table8: Ingredients required

Warm Part A to 75 ° C and B was added to A in small quantities with efficient stirring and cool. Add 3 to 6 ml of 25 % ammonia solution if the soap solution formed is not clear.

The serum was removed from the creamed skim latex after about 16 hours and the cream fraction was coagulated by addition of 10 % sulphuric acid. The details of coagulation are given in Table 9.

Comparative examples

Table 9 . Coagulation and creaming characteristics using 20 L of skim latex

Table 10: Raw rubber properties

Table 11 ACS1 Formulation for cure characteristics

Table 12. Cure characteristics using ACS1 formulation at 150 ° C

Table 14. Physical properties as per Standard Formulation for carbon black mix Thus, as has been described in the foregoing, it is apparent that recovered skim rubber of the present invention has excellent cure characteristics, vulcanisate properties, and ageing behavior in comparison with conventionally recovered skim rubber. Also according to the recovery process of the present invention the recovered skim rubber having such characteristic features can be recovered in high recovery yield with increase in dry rubber content in comparison with conventionally recovered skim rubber thereby showing excellent efficiency.

The specification contains one of the many embodiments that are possible but the concept of using the alkali and soap for deproteinisation is an essential element of inventive step. All of the embodiments and variations that will be known and understood by the skilled person in the art shall be covered within the scope of the invention.

WE CLAIM:

1. A method of recovering skim rubber from skim latex comprising the steps of:

a. reacting the skim latex with anionic surfactant for a predetermined period to form creamed skim latex

b. isolating the said creamed skim latex leaving behind the serum

c. adding H2SO4 to the isolated creamed skim latex for coagulation to form a coagulum of skim rubber

d. washing the said coagulum of skim rubber with water to remove the acid

e. drying the washed coagulum characterized in the step (a) by reacting skim latex with alkali along with anionic surfactant for a period of 15 - 20 hours

2. The process as claimed in claim 1 wherein the said anionic surfactant is selected from the group consisting of potassium oleate, ammonium laurate or combinations thereof.

3. The process as claimed in claim 1 wherein the said alkali is sodium hydroxide.

4. The process as claimed in claim 1 wherein the dry rubber content DRC of the creamed latex is 7-9%.