It is pointed out that for average _undiluted_ latex the basis of calculation, for quant.i.ties of acetic acid required, should be taken on the ratio 1:1,000. Where latex exceeds a dry rubber content of 3 lbs. per gallon, it may be necessary to increase the quant.i.ty of acid to 1:800.
If a standard of 2 lbs. per gallon is adopted, the formula given for the 1-1/2 lbs. standard will not give full satisfaction, and the quant.i.ty of acid solution must be increased slightly in order to obtain complete coagulation. a.s.suming that the original solution is prepared in 1 per cent.
strength, the following difference would be noted:
(_a_) One part pure acetic acid to 100 parts water (theoretically 99 parts).
(_b_) _1-1/2 lbs. per gallon._ | _2 lbs. per gallon._ | Of the above solution use 1 gallon | Of the above solution use 1 to every 12 gallons of standardised| gallon to every 10 or 11 latex. | gallons of standardised latex.
It is not possible to lay down an exact figure governing all cases, as so much depends upon the treatment undergone by the latex before it reaches the store.
Some estates continue to use solutions of greater strength, generally 5 per cent., in crepe preparation. While such solutions may be effectively stirred in when the latex is dilute, it is advised that for intimate mixture the solution need not be stronger than 1 per cent.
In estimating the quant.i.ties of acetic acid required much depends upon the interval which is to elapse between the addition of acid and the time of working of the coagulum. If the rubber is to remain until next morning, the average formulae will be found suitable; but if it is required to work the coagulum with an interval of less than three hours, an excess of acid must be employed. The excess need be comparatively small, unless the interval is much reduced. For instance, it is the practice on some few estates to begin the machining of the coagulum about half an hour after coagulation commences; in which case it is usual to add from a quarter to a half of the normal quant.i.ty in excess. It need scarcely be pointed out that unless this procedure is strictly unavoidable it should be discouraged on account of the waste of coagulant involved. Incidentally, the use of strong solutions of acid under such circ.u.mstances may lead to increased deterioration of the rolls.
COLOUR OF FINE CREPE.--We are sometimes a.s.sured that manufacturers do not pay the attention to the question of colour which sale conditions would lead one to believe. As far as we are concerned, and as long as there is no direct traffic between producer and consumer, it must be recognised that in the vast majority of cases we are forced to concern ourselves only with the standards set up in the markets. This, in spite of the knowledge that, all other things being equal, the arbitrary distinctions in colour afford no indication of the intrinsic value of the rubber. Under present circ.u.mstances it is plain that if paleness is demanded it has to be supplied.
Probably without exception all estates employ sodium bisulphite as the agent for the prevention of that darkening (oxidation) which is natural in drying rubber.
SODIUM BISULPHITE.--A formula for use of this chemical is given in Chapter VIII., and is applicable to latex standardised to 1-1/2 lbs. dry rubber per gallon. If a higher standard is chosen the quant.i.ty calculated as in (_b_) of that formula may be increased slightly, and the exact requirements found by experience. The caution must again be given that the employment of an excess of sodium bisulphite will lead to the production of an over-pale rubber, and a prolongation of the drying period. If thick crepes are made, an excess of the chemical is sometimes made visible by a greyish powder deposited on the edges of the strips of dry rubber.
It must be emphasised that the formula in Chapter VIII. indicates the _maximum_ quant.i.ties required for use with standard latex. Many estates will find it expedient to use less of the chemical; and if it is found that the desired result is not obtained from normal proportions, attention should be directed to the points discussed in the following paragraph.
EVALUATION AND DETERIORATION OF SODIUM BISULPHITE AND SODIUM SULPHITE.--Sodium bisulphite and sodium sulphite are both bought for our purpose in the form of a fine crystalline powder, and on a.n.a.lysis good specimens should contain over 90 per cent. pure substance, when packed in well-sealed vessels.
It has often happened that shippers or local sellers, by inadvertence, have supplied the one chemical in place of the other--to the detriment of the rubber and the discomfiture of managers of estates. The error, as a rule, has not been detected for some time, and then perhaps only as a result of complaints or enquiries reaching the laboratories. To the layman, and certainly to the native who usually has charge of these substances, it is not a simple matter to distinguish between them without special knowledge.
There are certain elementary tests, however, which can be applied on all estates serving to make the distinction, but affording no information regarding the actual quality of the chemicals. They are given in a comparative form on page 116. Samples of doubtful specimens may be sent to the laboratories for a.n.a.lysis, but the bulk of the chemical should not be used.
During the War some very poor shipments were received, and much trouble was caused. Under normal conditions there can be no question that it is to the interests of chemical manufacturers to supply the best article; and it is antic.i.p.ated that in future there should be no difficulty in procuring shipments of a high degree of purity.
_Sodium Bisulphite._ | _Sodium Sulphite._ | 1. If in good condition it | 1. It has no perceptible has a powerful odour of | odour.
sulphur dioxide.[9] | | 2. In solution it should turn | 2. In solution it should turn a blue litmus-paper red. | a red litmus-paper blue.
| 3. It exhibits a marked tendency | 3. The tendency to "cake"
to "cake" if the | is less marked than in drum is allowed to | the case of the bisulphite.
remain open. |
[9] High-grade sodium bisulphite has very little odour, but by the time it reaches the estate, and as a result of short exposure to the moist atmosphere of the tropics, a little decomposition sets in and a strong odour of sulphur dioxide gas is noticeable.
It will be evident that, as sodium bisulphite under normal conditions gives off a gas when exposed to the atmosphere, it deteriorates in quality continuously. It is the potential presence of this gas which makes the powder effective as an anti-oxidant and disinfectant. It is within the experience of all accustomed to the handling of this chemical, that in addition to the loss of gas, the powder cakes into a hard ma.s.s on exposure.
If only the top layer is caked, the remainder may be in fair condition; but no caked portions should be used, as they cannot be of good quality. They may, however, be used for the treatment of sc.r.a.p rubber, to be discussed later.
CARE OF SODIUM BISULPHITE.--The ready tendency of sodium bisulphite to deteriorate on exposure should give sufficient indication regarding its treatment in storage. It should be bought only in drums (or other air-tight containers), and should be stored in a dry place. No drum should be opened until required, and the common practice of keeping an open drum on the floor of the factory should be avoided.
Drums are of two sizes, generally containing 1/4 or 1/2 cwt. respectively.
It will be obvious that, although the prime cost may be cheaper with the larger quant.i.ty, it would always be preferable to secure the smaller drums, as the loss on exposure will be less.
Most commonly the 56 lb. drum is purchased. It should not be difficult to calculate the period during which the contents will be consumed, on the basis of a maximum of 1 lb. per 100 gallons of latex. A 56 lb. drum, a.s.suming no loss or waste, should be sufficient to treat _at least_ 5,600 gallons of latex (say, 8,500 lbs. of rubber)--if the bisulphite is of first-cla.s.s quality, and the use is applied only to the preparation of fine pale crepe.
Where the quant.i.ty used per diem is small, it is advised that precautions should be taken to preserve the quality of the chemical when a drum is opened. It might be of advantage to place the contents of the drum in smaller sealed tins, or to have made a special container (with a closely fitting lid) into which the powder can be placed as soon as the drum has been opened.
MIXING SOLUTION WITH LATEX.--Emphasis has been laid, in Chapter VIII., upon the necessity for care in the preparation of the solution. Equal regard must be given to the mixture of the solution with the latex.
On a few estates it used to be the practice to add the powder to the solution of acid, with stirring. Obviously this led at least to a great loss of efficiency, owing to the rapid escape of the gas which was evolved.
The solution of sodium bisulphite should be poured into the latex in as uniform a distribution as possible. The mixture of solution and latex should be thoroughly stirred, and if only natives are in charge a minimum period of five minutes should be prescribed before the addition of the coagulant. A thorough stirring should again follow the advent of the acid.
If these elementary rules are not observed faithfully, the deficiency will most probably be manifested in the dry rubber in the shape of streaks of varying shades of colour.
Finally it may be insisted upon that deteriorated sodium bisulphite should not be used. In order to obtain an effect double the quant.i.ty may be required, and the residual salts left in the rubber on evaporation of the moisture will be responsible for prolonged drying, surface deposits, and possibly "spot disease."
FORMER METHODS OF MAKING PALE RUBBER.--Merely as a matter of historic interest it may be mentioned that previous to the introduction of sodium bisulphite pale crepes were made in comparatively small quant.i.ty by various devices, among which the following might be quoted:
(_a_) Use of excessive quant.i.ties of strong acetic acid.
(_b_) Extreme dilution of latex in conjunction with excessive quant.i.ties of acid.
(_c_) Extreme dilution in conjunction with steaming and excess of acid.
(_d_) Extreme dilution of latex in conjunction with excess of acid and subsequent heating of the coagulum in hot water.
(_e_) The use of excess of a mineral acid such as sulphuric acid.
(_f_) The skimmings and very dilute latex, coagulated with excess of acid.
WORKING THE COAGULUM.--Description of the details of necessary machinery for crepe-making is relegated to Section III. of this book. Here we shall treat only of the matter in general.
In the preparation of crepe rubber heavy machinery is necessary, and ample engine-power must be available. The machines should comprise three types:
(_a_) With rolls cut in such fashion, and run at such different speeds, as to have a macerating effect upon the coagulum. Such machines or rolls will be referred to as "macerators."
(_b_) Intermediate rolls, grooved in varying designs and geared differentially. These reduce the thick rough crepe obtained from the macerators into a form suitable for pa.s.sing to the rolls described in (_c_). They are sometimes called "crepers," but as this term may be applied equally to the macerating rolls, they will be termed the "intermediate" rolls.
(_c_) Smooth rolls usually run at approximately even speeds and, as their name denotes, devoid of any grooving. They are called "smooth"
rolls or "finishers."
Without such equipment it is not possible to prepare the grade which is known as "fine pale crepe." In the common acceptation of this term crepe of No. 1 quality generally connotes fineness and paleness with a thin crepe which has a good, smooth, and fairly well-knit texture.
It is, of course, possible to make a thick pale crepe, using only the macerators and intermediates, but the "finish" will be that typical of the particular grooving of the intermediate rolls. For the preparation of crepe ordinarily, the possession of smooth rolls is a _sine qua non_.
For reasons which will be explained more fully in the chapter dealing with the defects of crepe rubber, the practice of preparing thick crepes direct from the coagulum is now very uncommon. Thick crepes are generally made by reworking dry rubber, either in the form of thin crepes or from air-dried sheets. The market for the latter in Malaya is confined almost entirely to Singapore, where factories buy native rubber and re-work it into thick crepe.
The bulk of the output of No. 1 crepe from estates is in the form of thin "fine pale crepe." The artificial standard set up by buyers and brokers necessitates this thin crepe being of even texture and fairly free from small holes ("looseness"). What difference the small holes are to make in the vulcanising properties of the rubber is beyond our knowledge; but such being the standard, it must be attained if the full price is to be obtained.
In order to secure the desired effect the coagulum must be pa.s.sed consecutively through the three types of rolls, and undergoes a varying degree of working in each.
Given the necessary equipment of machines, it is possible to make a good specimen of thin pale crepe if the coagulum pa.s.ses through all the rolls a total of twelve times (or even less in exceptional cases). There is no intention of suggesting that this is possible on all estates. Clearly the number of times the rubber pa.s.ses through the rolls will depend upon the total efficiency of the machines. This in turn involves such factors as (_a_) the size of the rolls, (_b_) the number of machines of each type, (_c_) the gearing of the pinions, (_d_) the speed of the drive, etc. Again, much depends upon the nature of the coagulum worked. A fairly soft coagulum will offer less resistance, and conversely a dense coagulum will require more machining.
It has been shown by the writers in previous publications that over-working of the coagulum has an effect on the vulcanisation of the rubber; and this has been confirmed by others.[10] Apart from this point, it should be recognised that over-working, beyond that necessary to produce a thin crepe of even texture, is to be deprecated, on the ground of economy, in working.
[10] Bulletin No. 27, Department of Agriculture F.M.S., April, 1918, "Preparation and Vulcanisation of Plantation Para Rubber," Eaton, Grantham and Day.