TECHNICAL NOTE NU M BE R 258
UNITED STATES DEPARTMENT OF AGRICU ""T__6-fOREST SERVICE FOREST PRODUCTS' LA __MADISON 5. WISCONSIN Feba y 19 53
SYNTHETIC RESIN GL1UE f WOO--The synthetic-resin glues, or simply resin glues, were introduced as woodworking glues about 1935, but their development and acceptance on a large scale began during World War 11 and is still increasing. These resin glues are products of the modern chemical industry and originate from raw materials derived from coal, air, petroleum or natural gas, and water. Although the intermediate raw materials are available, the complex production methods required for the resins and the fact that some are covered by current patents makes the small-scale manufacture of resin glues by the user not ordinarily practical.
All of the resin glues described, except the polyvinyl-resin emulsion glues, are thermosetting types, which undergo irreversible chemical curing reactions to produce insoluble, infusible glue films in the joint. The polyvinyl-resin emulsion glues are thermoplastic resins, that is, they do not undergo any chemical curing during the gluing process but remain in a reversible state and soften on subsequent heating. The thermosetting resins all involve formaldehyde, but this term is omitted from names of the resin glues for simplicity. The chemical curing or hardening reactions of the thermosetting resin glues, by which their joint strength is developed, are accelerated by adding catalysts to certain glues or by increasing the glue-line temperature by various methods. In additionto catalysts, any resinglue may also be modified with fillers, such as walnut-shell flour, to improve spreading properties and control penetration, or with extenders, such as wheat or other cereal flours, used largely to reduce glue-line costs.
Some resin glues are sold in a single package ready to use or as a powder to be mixed only with water. Many others, however, must be pre pared for use by mixing resin, catalyst, fillers, extenders, and water or other solvent at the time of use. In any case, the manufacturer's instructions should be followed closely.
(1) Urea-resin glues are available inpowder and liquid forms tobe used with or without added catalysts, fillers, and extenders. Glues may be formulated for curing at room temperature (usually considered as about 70* F. ) or at hot-press temperatures of 2400 to Z60* F. Special, formulations are available for tapeless splicers, high-frequency curing, and other specific applications. The glue lines are colorless to light tan in color and hay, only a moderate dulling effect on tools. Unextended urearesin glues have high water and moisture resistance but are sensitive to temperatures of 150* F. or higher, particularly at high-humidity conditions, and these glues are generally not recommended for exterior service. Extension with cereal flours reduces their moisture and water resistance. Resistance to elevated temperatures and exterior conditions can be somewhat improved, however, by modification with special fortifiers. When properly formulated, the unmodified urea-resin glues give high initial strength and are suitable for both veneering and joint work. Several room -temperature -setting urea-resinglues are available in small retail packages. for small-scale shop work, and require only addition of water to prepare them for use. Special liquid resins have been developed for economical taiik-car or truck shipment for large scale users, but these resins require addition of catalysts ajnd usually extender* before use.
(2) Phenol-resin glues are normally dark-reddish liquids and require hot pressing at 280* F. or higher. Some, however, are supplied in powder form to be mixed with water or other solvent, and at least one phenol-resin glue is available in film form, which eliminates mixing and spreading operations, and is particularly suitable for gluing thin figured, fragile veneers. Special formulations are available for curing at intermediate temperatures, and certain acid-catalyzed resins are capable of curing at temperatures as low as 75* F. for some applications. The nonacid phenol-resin glues are one of the mostdurable types of wood glues and when properly used give joints that are as durable as untreated wood under severe exposures. There are indications that the acid-catalyzed phenol-resin glues give joints that are not so durable at elevated temperatures as joints made with conventional phenol-resin glues but are still more durable than joints made with the urea resins and glues of natural origin. Because they require high curing temper atures, phenol-resin glues are mainly suitable for plywood production, although intermediate -temperature setting formulations are used fo r laminating heavy timbers for severe service by curing in heated chambers. The phenol-resin glues are not ordinarily suitable for smallscale shop or hobby work.
(3) Resorcinol-resin glues are dark-reddish glues and are generally supplied as liquids to which a liquid or powdered catalyst is added before use. These glues have much the same performance characteristics, including high durability, as the phenol-resin glues with the added advantage that they cure sufficiently for many applications at temperatures as low as 70" F. They are, however, the most expensive of the current woodworking glues.. Recent formulations of phenol-re sorcinol resin glues are appreciably cheaper than straight resorcinol-resin glues and appear to retain most of the desirable characteristics of the resorcinol resins. Both types are used for laminating or assembly of articles where a high degree of durability to exterior or other severe service is required. Their high cost prevents their use as a veneering glue except for special applications. At least one brand of resorcinol-resin glue is available in small retail packages for small-scale shop work and is of particular interest to the amateur boat builder.
(4) Melamine-resin glues are available as colorless hot-press glues at a price between that of urea-resin and phenol-resin glues and that of straight resorcinol-resin glues. Althoughthe melamine-resin glues are as durable as the phenol- and resorcinol-resin glues, their high cost and high curing-temperature requirement limit use of the straight melamine resins to a few special applications. These resins, however, are often used to fortify and thus improve the durability of urea-resin glues.
(5) Polyvinyl-resin emulsion glues are the newest type of wood glue and are available in a ready-to-use liquid form that sets at room temperature to a colorless glue line. Unlike the other resin glues described, these glues do not cure by a chemical reaction but set by losing water to the wood. They remain somewhat elastic and thermoplastic in the glue line. which makes their use in highly stressed joints inadvisable. They appear promising, however, for certain types of assembly joints where their greater elasticity is an advantage over the conventional rigid woodworking glues.
Properties of the resin glues are summarized in the table.
Summary of properties of synthetic-resin woodworkinR glues1
Type of glue
Property : Urea :Phenol :Resor- :Mela-:Poly: resin : resin : cinol : mine :vinyl UNIVERSITY OF FLORIDAresin :resin
3 1262 09216 7351 :1. :sion
--- ------ .------- -----Ready to use as received ......: x : : x
Requires mixing for use ....... x : x x : x
Colorless or nearly colorless : :
glue line ........ ... xx
Dark-colored glue line ......: x x :
Can be pressed at 750 F ...... :.x : : x : . x
Requires curing at 140* F. or: : : :
above .................... :
Requires curing at 240" F. or: : :
above .................... : x : x
Working life of 1 to 8 hours : : :
at75 F..................... x x : x
Working life of over 8 hours : : :
at 750 F ..............x x : x x
Low moisture resistance ... : x
Good to high moisture resist- : : :
ance....................... .x X, x x
Limited resistance to ele- :
vated temperatures... x : x
Good to high resistance to : : :
elevated temperatures ....." x x
Can usually be purchased in :
retail stores ..............: x x x
Intermediate cost "x x x
High cost .................... x x :
IThese properties are based on average glues of each type. There are
numerous exceptions for glues of each type. Two or more values for the same property indicate several types of glues are available.
Generally, all types of these glues can be spread by hand as well as by machine and are applied cold to the wood at normal room temperature. None of these glues stain wood directly, although discoloration of wood surfaces from penetration of dark-colored glues may be
23! 92363 F Agriculture-'fadlson