GB2178044A - Reinforced natural rubbers and processes for their production - Google Patents

Abstract

Reinforced natural rubbers characterised by enhanced physical properties prior to and after vulcanisation comprise 3.5 to 12 percent of additional nonrubber components derived from serums rubber tree latexs. Also disclosed are processes for producing such natural rubbers.

Description

SPECIFICATION Reinforced natural rubbers and processes for their production This invention relates to reinforced natural rubbers and also to processes for producing the same.

In general, natural rubbers such as RSS (Ribbed Smoked Sheet), SMR (Standard Malaysian Rubber) and the like are obtained by diluting rubbertree latexes with waterto a rubber content of a bout 15 - 20 weight percent, such latexes containing a rubber component (rubber hydrocarbon) in an amount of about 35 weight percent and a nonrubber component in an amountof about3 - Sweight precent, and by coagulating the diluted latexes with a suitable acid, and subsequently by rolling the coagula into rubber sheets. The sheets are smoked at 700-800C for 7 - 8 days, dried and thereafter packed into bales.

After coagulation and removal of rubberfrom a natural rubber latex, there remains an aqueous solution known in the art as a serum. Such a serum is usually composed ofabut0.5weight percent of a rubber component and about3 -5 weight percent ofa nonrubbercomponentwhich includes proteins, amino acids, fatty acids, saccharides and the like.

There have arisen such environmental problems as malodors and water pollution as a result ofthose serums having been disposed as wastes by most small holders. In the meantime, various attempts have been made at rubber estates with added disposal equipment, entailing large investments but with little practical result.

Acid coagulation of rubber latexes often leads to loss of some proportion of the nonrubber component, giving natural rubberwith a reduced nonrubbercontent, about 1.5-4 weight percent compared to about 9-11 weight percent which may be attained without such loss. Such low proportions of nonrubber components are responsible for insufficient physical properties of prior art RSS, SMR and other natural rubber prior to and after vulcanisation.

In order to overcome this problem, one process has been proposed in which a rubber latex is both dehydrated and coagulated directly in a spray drier. This prior art process is not economically feasible as it involves increased energy consumption and further needs addition of silica.

Another process is known in which a rubber latex is allowed to acid-coagulate, but without dilution, to give ICR (Initial Concentration Rubber). Such a natural rubber, however, is not satisfactory as its nonrubber content is again too low.

It has now been found that natural rubbers of superior qualities can be obtained by the use ofnonrubber components which may be derived from otherwise undesirable serums byproduced upon treatment of rubber latexes.

It istherefore a primary object of the invention to provide reinforced natural rubbers which will eliminatethe above drawbacks of the prior art and which will exhibit enhanced physical properties prior to and after vulcanisation.

Another object of the invention is to provide processes for producing such natural rubbers efficiently and economically.

According to one aspect of the invention, there is provided a reinforced natural rubber comprising a starting rubber having its original nonrubbercontent, plus a nonrubbercomponent derived from a serum resulting from coagulation and removal of a rubbercomponentfrom a natural rubber latex, the nonrubbercomponent being added in amounts corresponding to a nonrubbercontentof3.5- 12.0 weight percentoftotal mixture.

According to another aspect of the invention, there is provided a process for producing a reinforced natural rubberwhich comprises: applying a liquid containing a nonrubber component to a starting rubber, the nonrubber component being itself derived from a serum resulting from the coagu lation and the removal of a rubbercomponentfrom a natural rubber latex, the liquid having a solids content of not less than 30 weight percent; impregnating the starting ru bberwith the liquid until a total nonru bber content of 3.5 - 12.0weight percent of the total mixture is achieved; and dehydrating and drying the resulting rubber.

According to a still furtheraspectofthe invention,there is provided a process for producing a reinforced natural rubberwhich comprises: blending a solid nonrubber component with a starting rubber in amounts corresponding to nonrubbercontent of 3.5-12.0 weight percent in the resulting mixture,the solid components having an average particle size of 10 - 100 microns and being derived by spray-drying a serum resulting from coagulation and removal of a rubbercomponentfrom a natural rubber latex; and rolling the resulting rubber.

These and other objects and advantages of the invention will be better understood from thefollowing detailed description taken in connection with the accompanying drawings in which: Figure lisa graphic representation ofthe relationship between torque andvulcanisation; and Figure2 is aviewsimilarto Figure 1, butshowing the relationship between green strength and elongation.

Rubber latexes eligible for the invention include milky fluids collected from the rubbertree, Hevea brasiliensis, and guayule derived milky fluids predominantly comprising natural polyisoprenes. The rubber latexes are suitably treated into starting rubbers. Bytheterm starting rubber as used herein is meant a rubber obtained by diluting such a rubber latex with water, removing impurities, coagulating the latex (with formic acid, nitric acid, sulfuric acid orthe like) squeezing out a serum, and subsequently rolling the coagulum into a sheetor crumb rubber.

Natural rubber latexes vary in composition depending upon the sources of supplyfrom rubbertrees. An example ofthe composition of afresh field latex is shown in Table 1.

Reinforced natural rubber according to the invention should have a nonrubber content of 3.5-12.0 weight percent of the total. Smallercontents would fail to give sufficient qualities prior to vulcanisation, and greater contents would result in deteriorated physical properties aftervulcanisation.

The amountofnonrubbercomponentto be added mayvarywith the grade of natural rubbers. Starting rubbers for use in high quality RSS No. 1 and No.2, brown crepe No.1, SMR andTTR 5 generally havean original nonrubbercontent between 2.5 and 3.5 weight percent. In high grade natural rubbers, the nonrubber componentshould preferably be added in amounts corresponding to a nonrubbercontent of 10- 12weight percent of the total mixture.

In the case ofstarting rubbersforlow quality brown crepe2X,TTR 20, SMR 20 and the like, the original nonrubber content is small, only about 1.5 - 2.5 weight percent, due to frequent washing during treatment of the rubber latexes. The nonrubbercomponentshould be incorporated in low grade natural rubbers in amounts corresponding to a nonrubber content of 3.5 - 4weight percent of the total mixture. Because the starting rubbers have already been subjected to excessive rolling and hence are of reduced molecularweight, large amounts of the non rubber component should be avoided to preclude quality deterioration aftervulcanisation.

The reinforced natural rubbers of the invention have the following characteristics.

1. High maximum torque, i.e. tensile stress, tensile strength and rebound reilience.

2. Low minimum torque and low initial torque to permit reduced viscosity and good fabrication ability.

3. Good green strength, high elongation and sufficienttackiness.

4. Fastvulcanisation with less accelerator.

Forthe production of reinforced natural rubbers, the process of the invention contemplates the use of a liquid containing a nonrubbercomponent.

The liquid should have a nonrubbersolids contents of 30 weight percentorgreater. Smallersolidscontents would be ineffectiveforimpregnation by dipping orspraying into starting rubbers.

Liquids suitable in this wet process include a serum concentrated with heat as by a multi-stageconcentrator, and a solution of a solid nonrubbercomponent re-dissolved in water. Dipping of starting rubberintheliquid for a predetermined length oftime and subsequent drying can give a desired reinforced natural rubber.

Alternatively, the liquid may be adequately sprayed on to a starting rubberwhich is then dried.

Another embodiment of the process of the invention is a dry process in which a solid nonrubbercomponent is blended with a starting rubber.

Solid nonrubbercomponents useful in the dry process are those having an average particle size between 10 and 100 microns. Blending ofthe particulate nonrubber components with a west or dry starting rubber can be effected by a suitable roll to thereby form a desired reinforced natural rubber.

The nonrubber particles may be obtained by for example introducing a serum into a drying chamber at 150 - 250with use of a nozzle or disc spray drier in which the droplets are evaporated dry into a particulate form with the particle size as above specified.

The invention will be further described by way ofthefollowing examples.

Example 1 A coagulum resulting from acid coagulation of a natural rubber latex was dehydrated and cut into about 1 cm square rubber pieces. These were dipped in a serum initially formed as a byproduct but concentrated to a solids content of 80 ',vt.%. After 10 hour dipping, the pieces were dehydrated and dried at 11 00for 20 minutes.

They gave a reinforced natural rubber sample with a total nonrubber content of 7.5wt.%.

Example 2 About 1cm square rubber pieces were made in a mannersimilarto Example 1 and dipped for 10 hours in a solution having a particulate nonrubber component redissolved in water to a solids content of 70wt.%. The nonrubber particleswere obtained by spray-dipping a serum and had an average particle size of 601m.

Dehydration and drying ofthe pieces at 11 00C for 10 minutes gave a reinforced natural rubber sample with a total nonrubber content of 10.4wt.%.

Comparative example 1 Commericallyavailable RSS No. 1,which had a nonrubbercontent of 3.3 wt%was used.

Comparative example 2 The proecure of Example 1 was followed except that dipping was conducted for 30 hours thereby to give a reinforced natural rubber sample with a total nonrubber content content of 1 2.6wt.%.

Comparative example 3 The procedure of Example 1 was followed exceptthat a serum was concentrated to a solids content of 30 et.% and dipping was conducted for 5 seconds, thereby giving a reinforced natural rubber sample with a total nonrubbercontentof3.5wt. %.

All the rubber samples were formulated, as shown in Table 2, according to American Chemical Socieity (ACS) No. 1 Composition and tested for physical properties. Test conditions were asfollows: Vulcanisation curve Rheometer (TP-100, Monsanto Chemical Co.) was used with temperature 240 C, arc +3 and LPC rotor. The results are graphically shown in Figure 1 in wich (a) and (b) correspond to Examples 1 and 2, respectively, and (c)to (e) correspond in that order to Comparative Examples 1 to 3.

Green strength This strength was determined on autograph (P-500) with use of dumbbell specimens (No.3 stipulated by Japanese Industrial Standards) that had been die-cutfrom 2 mm thick unvulcanised rubber sheets coldpressed at 100 Cfor5 minutes. The results are graphically represented in Figure 2 in which (a) correspondsto Example 1 and (c) to Comparative Example 1.

In Table 3there are shown the physical properties ofthe rubber samples obtained from the graphs of Figures 1 and 2. It isto be noted that the reinforced natural rubbers representing Examples 1 and 2 are highly satisfactory in respect of maximum torque, minimum torque, initial torque, green strength and elongation.

The correlation was evaluated between the physical properties determined on the above rheometer and those arising from rubber sheets usually vulcanised at 140 C for 40 minutes, with the results given below. The rubbersheets were formulated as shown in Table 2.

Tensile strength Y= 12X-70 SL = 1 % R=0.67 where Y: tensile strength after vulcanisation (TB,kg/cm) X: maximumtorque(kg-cm) SL: significance level R: correlation coefficient Tensile strength Y=6.1X-68 SL=1% R =0.87 where Y: tensile strength aftervulcanisation (600 modulus,kg/cm2) X: maximum torque (kg-cm) SL: significant level R: correlation coefficient Rebound resilience Y = 0.8X + 60 SL = 1 % R =0.75 where Y Lupke rebound resilience aftervulcanisation (%) X: maximum torque (kg-cm) SL: significance level R: correlation coefficient Viscosity Y=3.5X+36 SL=5% R=0.41 where Y: viscosity before vulcanisation ML+4' 100 C) X: minimum torque (kg-cm) SL significance level R correlation coefficient Table 1 Composition Latex Solids content (wt. %) (wt. %) rubber hydrocarbon 35.62 88.28 proteins 2.03 5.04 acetonesolubles 1.65 4.10 (fatty acids) saccharides 0.34 0.84 ashes 0.70 1.74 (inorganic salts) water 59.66 Table2 ACS No. 1 Composition (wt. %) natural rubber 100 zincwhiteNo.3 6 sulfur powder 3.5 stearicacid 0.5 vulcanisation 0.5 accelerator (MTB) Total 110.5 Table3 Formulationlproperties Examples Comparative Examples Formulation natural rubber 1 2 1 2 3 nonrubbercontent(wt.%) 7.5 10.4 3.3 12.8 3.5 composition ACS No.1 Composition (seeTable2) Physical Properties initial torque (in-l b) 12.0 10.0 16.4 6.9 16.5 minimumtorque(in-lb) 9.7 7.5 12.0 5.3 11.8 scorching time (Ts, min) 3.2 4.5 4.8 4.7 4.5 vulcanisationrate(T90,sec) 16.8 10.5 20.0 15.3 19.8 maximum torque (in-lb) 49.1 44.2 41.5 34.7 41.5 green strength (kg/cm2) 4.2 - 3.8 green elongation (%) 650 - 430

Claims (8)

1. A reinforced natural rubber comprising a starting rubber having its original nonrubbercontent, plus a nonrubber component derived from a serum resulting from coagulation and removal of a rubbercomponent from a natural rubber latex, said component being added in amounts corresponding to a nonrubber content of 3.5-12.0 weight percentoftotal mixture.

2. A process for producing a reinforced natural rubber which comprises: (a) applying a liquid containing a nonrubber componentto a starting rubber, said non-rubber component being itself derived from a serum resulting from the coagulation and the removal of a rubber componentfrom a natural rubber latex, said liquid having a solids content of not less than 30 weight percent; (b) impregnating said starting rubberwith said liquid until a total nonrubber content of 3.5 - 1 2.0weight percent ofthetotal mixture is achieved; and (c) dehydrating and drying the resulting rubber.

3. A process according to claim 2 wherein the impgrenation is effected by dipping said starting rubber in said liquid.

4. A process according to claim 2 wherein the impregnation is effected by spraying said liquid on to said starting rubber.

5. A process according to anyone of claims 2 to 4wherein said liquid is a concentrated serum or a solution having a solid nonrubber component dissolved in water.

6. A process for producing a reinforced natural rubber which comprises: (a) blending a solid nonrubber componentwith a starting rubber in amounts corresponding to a nonrubber content of 3.5 -1 2.0 weight percent in the total mixture, said solid component having an average particle size of 10-100 microns and being derived byspray-drying a serum resulting from coagulations and removal of a rubber component from a natural rubber latex; and (b) rolling the resulting rubber.

7. A reinforced natural rubber substantially herein before described with reference to any of the substantive Examples 1 and 2.

8. A process substantially as hereinbefore described with reference to any of the substantive Examples 1 and 2.