Chapter 3 - Latex Specification and Testing

CHAPTER 3 LATEX SPECIFICATION AND TESTINGS

LESSON OUTCOMES • At the end of the semester, students must be able to: 1.State and explain the latex testing 2.Calculate the value of latex testing 3.Analyze the significance of the latex which related to its properties 4.Justify the relevant standard natural rubber testing based on the specific properties of the natural rubber latex.

2.1 LATEX SAMPLING PROCEDURES • Rubber latex may tend to cream on standing.

• The latex shall be thoroughly agitated to obtain a homogenous blend suitable for the withdrawal of a representative sample.

• In all cases, where are drawn from several containers, for example 10% sampling of latex in drums or in the case of tanks are made at various depths such samples shall be bulked and stirred.

• The final average sample shall then be bulked and stirred.

Flow chart of latex testing conducted during the production of latex concentrate Field latex

Check the DRC, VFA and Ammonia

Adjust Add Ammonia Centrifugation Latex concentrate DRC, TSC, Mg, VFA and Ammonia Adjust

Add second preservatives ammonia or TMTD/ ZnO for LaTZ

Final latex concentrate Customer

DRC, TSC, Mg, Viscosity, coagulum, KOH, MST, VFA, pH and Ammonia

2.2 DETERMINATION OF TOTAL SOLID CONTENT (TSC) A method for the determination of the TSC of NR latex which contains preservative agents and which has been submitted to some type of concentrate process.

•PRINCIPLE Latex dried sheet is weighed before and after heating at either 70 ᵒC ± 2ᵒC or 100 ᵒC ± 2ᵒC until the sample lost its whiteness for either 16 hours or 2 hours.

•SIGNIFICANCE It indicates the concentration of latex including the DRC and non rubber content.

Calculation of TSC •The total solid content (TSC) is expressed as a percentage by mass, as follow: TSC = m1 m0

x

100

Where; m = mass (g) of the dried sheet m = mass (g) of the test portion 1

o

• Usually high or low TSC or DRC contents are due to sampling error. • It is essential that the late is thoroughly homogenized before sampling. • If it is not done, samples taken from the top will give high results, those from the bottom have low results.

2.3 DETERMINATION OF DRY RUBBER CONTENT (DRC) A method for the determination of the DRC of NR latex which contains preservative agents and which has been submitted to some type of concentration process. •SIGNIFICANCE It measures the actual rubber content from the latex. Then the chemicals can be added based on the DRC value.

Calculation of DRC •The dry rubber content (DRC) is expressed as a percentage by mass, as follow: DRC = m1 m0

x

Where; m = mass (g) of the dried sheet m = mass (g) of the test portion 1

o

100

3 methods to determine the DRC:  Spot method of DRC  Metrolac Latexometer Standard method of DRC

DETERMINATION OF DRC BY SPOT METHOD Determination of DRC by this method can be carried out rapidly and with reasonable accuracy.

PROCEDURE: 1.Weigh accurately the empty petri dish and record the weight for four decimal points.

2.Add 2 – 3 drops of the latex (0.2g) into the empty petri dish and the weight accurately.

3.Coagulate the latex by pouring in 5% acetic alcohol solution (5% acetic acid + 95% ethyl alcohol) along the side of the petri dish until the latex is surrounded and cover with the acetic alcohol.

4. Allow the latex to coagulate and flatten the coagulum by pressing with a flat bottom glass stopper or stainless steel block, ensuring no folding or tearing.

5. Pour off the acid mixture and rinse the sample with clean water.

6. Dry the sample over a steam bath for 15 minutes until a clear sample is obtained and free from undried spots.

7. Wipe the outer portion of the petri dish with a piece of dry and clean cloth. Place the sample in the oven at 100 áľ’c for about 5 minutes or 70áľ’ C for 30 minutes.

8. Cool the sample in a desiccator for 5 minutes and weigh the petri dish with rubber to four decimal places.

9. The determination should be carried out in duplicate.

DETERMINATION OF DRC BY METROLAC • DRC determination of latex using a metrolac depends on the principle of hydrometric measurements of the specific gravity of latex • This is one of the fastest and direct method of measuring DRC of latex • Reading taken from a metrolac maybe erroneous under the following conditions such as:- latex from young seedling trees, and unstable latex or a latex with some added preservatives.

• In such circumstances the hydrometers will behave abnormally.

PROCEDURE FOR METROLAC LATEXOMETER METHOD 1. Take two pints of water and add it to one pint of well stirred field latex in a gallon measure.

2. Mix thoroughly.

3. The diluted latex then poured into an aluminum jar in a coagulating pan until the latex overflows.

4. Remove any foams on the surface of the latex and allow the metrolac to sink into diluted latex slowly 5. Allow the metrolac to rest without touching the base on the sides of the jar.

6. Take the reading on the stem at the base of the meniscus of the latex.

7. Verifying the reading by pushing the metrolac down lightly and note the reading.

Example: The reading on the Metrolac’s stem shows 115 g/liter, calculate the DRC of the latex. =(115)3 x 100 1000 = 34.5 If DRC 35 - 40% = minus 10% If DRC < 35% = minus 5% DRC = 34.5 – 1.73 = 32.8% * Note: Low DRC latex should only be diluted 1:1 with water and multiplying the reading with 2.

DETERMINATION OF DRC FOR SKIM LATEX Weight of skim (g)

19.7962

Weight of concentrated latex (g)

9.7451

Weight of sample (dry), (g)

6.7375

DRC for concentrated latex (%)

63.9

Weight (dry) which is used (g)

63.9 x 9.7451 100

Weight of skim (dry), (g)

6.7376 â&#x20AC;&#x201C; 6.2271 = 0.5104

DRC of skim (%)

0.5104 x 100 19.7962

= 6.2271

= 2.6%

DETERMINATION OF ALKALINITY • PRINCIPLE Latex is stirred to pH 6.0 in the presence of a stabilizer and its alkalinity is calculated from the quantity of acid which is required. The titration is carried out electrometrically or alternatively, with methyl red indicator.

• SIGNIFICANCE Ammonia content can be used as an indication of the latex whether it is well preserved or not. High alkalinity indicates latex is well preserved or vice versa.

• Low alkalinity are usually due to loss of ammonia through vents in tanks or drums.

• Bacterial infection should be prevented because it may lead to coagulation of the latex.

DETERMINATION OF MECHANICAL STABILITY TIME (MST) PRINCIPLE Latex is stirred at high speed and the time required initiating mechanical stability.

SIGNIFICANCE The MST is the number of seconds of stirring required for the formation of a flocculum as seen in a drop of latex into a petri dish containing water (shelf life of latex). The end point as the first appearance of flocculum must be more than 650 s.

DETERMINATION OF VOLATILE FATTY ACID (VFA) The VFA number is defined as the number of grams of potassium hydroxide equivalent to the volatile fatty acid contained in 100g of total solids.

PRINCIPLE 50 g of latex is coagulated with ammonium sulphate and the coagulum removed; 25ml of serum is taken, acidified with sulphuric acid and steam distilled. The distillate is aerated with carbon dioxide free air and titrated against barium hydroxide, phenolphthalein as indicator.

SIGNIFICANCE Higher VFA number indicates and inadequate preservation of NR latex and the destabilization process begin. The addition of preservatives is necessary.

DETERMINATION OF POTASSIUM HYDROXIDE (KOH) NO. The KOH no. is defined as the number of grams of potassium hydroxide equivalent to the acid radicals combined with ammonia in latex containing 100g of solids.

PRINCIPLE A sample of latex containing 50 g of solids is taken, its alkalinity being adjusted to 0.5% ammonia on the aqueous phase. It is diluted to about 30% solids with water. The electrode of a pH meter is immersed in the liquid and incremental additions of potassium hydroxide solution made with constant stirring.

The pH is recorded after each addition, the titration end point being taken as the point of inflexion on the graph of pH against volume of potassium hydroxide solution. The result is found by calculation.

SIGNIFICANCE A high KOH no. with a high VFA no. indicates inadequate preservation of the latex.