(vi) Synthetic Resins.
Synthetic resin glues have two great advantages over all those so far described: they are extremely resistant to moisture and immune from attack by moulds and bacteria. Timber has thus been provided with adhesives that can be more durable than itself, and the limit- ing factor in the utilisation of laminated products is no longer the glue but wood.
no
The price to be paid for this is the greater care necessary in mixing, application and pressing operations, for the synthetic resins are precise chemical compounds that tolerate haphazard handling. That they have become essentially practical materials is shown by their adoption by all classes of industry.
There are two principal types of synthetic resin glues, the urea-formaldehyde or amino-plastic base materials and the phenol-formaldehyde varieties, with which can be included the closely-allied resorcinol group.
The urea type is normally supplied in two parts, the resin itself and the hardener, the former being available both in syrup and powder forms and the lalter as a liquid or as a powder. In a few cases, resin and hardener are sold together as a ready-mixed powder, and the user adds water to make the liquid adhesive. Where the components are separate, they are mixed before use in such proportions as six or ten parts of hardener to a hundred of resin by weight.
Glues of a wide variety of uses are available in the urea-formaldehyde type, ranging from hot-press ply-wood adhesives setting at 100°C. (212°F.) to assembly glues that set at temperatures down to 10°C. (50°F.) and between these extremes are the so-called warm-setting glues for use in the region of 60°C. (140°F.), which are really very slow cold- setting mixes.
This matter of temperatures is vastly important with all synthetic resins, for they set by a delicate chemical action
that is accelerated or retarded by rise or fall of the tem- perature. This has often been over-looked by users in relation to summer heat and winter cold, and the glue has been Manu- blamed for the ensuing variations in setting rates. facturers can supply hardeners to compensate for changes in temperature conditions, and the user is strongly recommended to accept their advice.
There is also a method of using urea glues known as "separate application," which avoids the necessity of mixing the components. The resin is applied to one side of the joint and the hardener to the other, the setting taking place when the two are put together. The method permits of very rapid gluing operations, but is applicable also, to the slower- setting varieties. Obviously, only the liquid forms of resin and hardener can be employed in this process.
Urea glues are not difficult to use provided the manu- facturers' instructions are strictly adhered to. There is some risk of dermatitis to the operatives, but this can be minimised by cleanliness and a suitable barrier cream. The set resin action has a dulling action on edged tools, although it gives no trouble with the high-speed steels. Urea glues are strong in the dry state and will stand indefinite soaking in cold water. The normal types fail if the water temperature exceeds 70°C., but the fortified kinds withstand boiling. Micro- organisms are without effect on these adhesives,
The first phenolic resin glues were made in film, liquid and powder forms for plywood manufacture, and their setting necessitates a temperature as high as 150°C. (300°F.). Later developments gave to industry a liquid resin which, in con- junction with appropriate hardeners, would set at temperatures ranging from 10 to 120°C. (50° to 250°F.). The separate- application method cannot yet be used with phenolic adhesives.
All that has been said for the ureas regarding cleanliness, careful mixing and the need for precise temperature-control applies even more strongly to the phenolics. As most of them are soluble in the mixed state only in alcohol or caustic soda, cleansing of equipment is troublesome, and the dermatitis risk is perhaps even greater. On the credit side, however, they have most desirable properties in the set glue line and are practically indestructible.
Selecting The Correct Adhesive
Before a user accepts a laminated product, he should satisfy himself not only that the gluing has been well done, but also that the adhesive itself is of a type suited to the intended application of the article. With plywood, which is a finished product, both aspects are covered by British Standard and other Specifications. For assembly work the use of British Standard Specifications will ensure that the glue as such is of a suitable quality, but because of the infinite variety of joint-types involved, no easy check on the quality of the gluing is available.
It is in any case extremely difficult to test the soundness of a glued joint except by methods involving its destruction. Perhaps the best safeguard the user can have is the manu- facturer's regard for his own good name, for firms of repute will take every possible precaution against shoddy work- manship.
A glue specification, however, is merely concerned with setting a minimum standard of acceptance for a particular adhesive; it is not within its province even to indicate for
22
This duty
what purposes the glue is suitable or otherwise. lies with the user, who must accordingly know the properties of the various types of adhesive. A four-group classification has been evolved from long-term systematic research together with a study of the actual behaviour of the six types of glue under service conditions:
Group I-Weatherproof Adhesives
To qualify for the weatherproof group an adhesive must have shown itself to be virtually indestructible and have exhibited no weakening or signs of failure under prolonged exposure to the most severe weathering conditions.
Members: Phenol-formaldehyde resins.
Group II-Weather-resistant Adhesives
There are glues which, although they comply with high or even the highest requirements of existing specifications and show excellent durability to severe exposure conditions, have over a prolonged period exhibited signs of deterioration in service. It should be stressed that the criterion is full exposure to weather and under less exacting conditions their performance has been good, but this limitation excludes them from the Weatherproof Group.
Members: Urea-formaldehyde resins.
Group III-Semi-durable Adhesives
If a glued joint is exposed to weather, it will be in the wetted state for some part of the time, and an obvious requirement is that the bond should not fail completely, although it may be permissible for the adhesive to lose a large proportion of its dry state providing that that which remains can be relied upon. Unfortunately, damp conditions are favourable to the growth of moulds and bacteria, and if the glue is of a type attacked by these micro-organisms, complete destruction may occur.
It might seem that glues with these characteristics ought not to be used at all in damp environments, and indeed to specify one of the resins would save any doubts on this score. On the other hand these semi-durable adhesives have per- formed well under such circumstances, particularly in large joints, and the user need not necessarily reject such glues out of hand,
Members: Blood albumin, casein and soya bean derivatives.
Group IV-Interior Adhesives
Glues that have satisfactory dry strength, but which fail completely when wetted.
Members: -Animal glues and starch derivatives.
Paint
does Protect-
<FACTORIES
HOUSES
Synthetic enamels
Flat finishes
Lacquers & Varnishes
Sprcity
"OPA
OFFICES
OPAL
ZINC CHROMATE PRIME R
ENT PAINT COLOUR¡A VARNISH
PRODUCTS
General Agents
BUTTERFIELD AND SWIRE
HONGKONG & SHANGHAI
Agents
HENRY WAUGH & CO., LTD. Singapore & Malaya THE BORNEO CO., LTD.
Siam
THE EAST ASIATIC CO., LTD. Siam & Philippines
No comments yet.
Private notes are available after approval.