Adhesive cold-setting

Attempts have been made to use cold-set adhesives in the cormgating operation, such as poly(vinyl acetate) and modified, precooked starch formulations, but these have not achieved any appreciable degree of commercial acceptance (20). The use of a polyethylene film appHed to the inside surface of the linerboard facing, which serves as a hot-melt cormgator adhesive, has achieved some commercial usage. However, its use is limited to the small, specialty product niche of fast-food hamburger cartons (see Olefin polymers, polyethylene). 

J. J. Bechei, G. R. Hoffman,. W. Swanson, and R. C. McKee, Development of Cold-Set Adhesive, Project 2696-11, Report One to the Eouidiiniei Kraft Board Institute from the Institute of Paper Science and Technology, Adanta, Ga., Nov. 1973. 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Lignin-based cold-setting adhesives evaluated on beech strips (74% lignin 26% resorcinol). 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

Cold-set adhesives 80% of success dependent upon adhesive formulation... 

In cold-setting adhesives, the combination of resorcinol with the tannin in some form, be it added or partly generated from the tannin itself, hides a multitude... 

A. Formulation of Phenol-Resorcinol-Formaldehyde Cold-Set Adhesives... 

Table 3 Typical Results of Tannin-Resorcinol-Formaldehyde Cold-Setting Adhesives used on Beech Strips according to British Standard BS 1204 [65]...

Xylenols (all six isomers) are now also in conunon use to form alkali-resistant grades of phenohc resins. High 3,5-xylenol mixtures are preferred. Also, resorcinol, which forms very reactive phenohc resins, is used in preparations of cold-setting adhesives. Higher homologs of phenol, like bisphenol A, are used to prepare special phenol-formaldehyde condensates. 

Cold-setting adhesive n. Synthetic resin adhesive capable of hardening at normal room temperature in the presence of a hardener. 

Recently a modification has been introduced, " " in which the phenol-resorcinol-formaldehyde cold setting adhesive is further mixed with a phenol-resorcinol novolac of strongly basic pH (10-11) or with a 50-55% tannin extract solution at a pH of 11-12. 

Cementing the cover slip. After the sample has been lapped down to the desired thin-section thickness, the cover slip is applied. To this end, the sample is washed thoroughly in water and then dried. A drop of cold-setting adhesive is then applied to the section and spread across the section surface with a cover slip. The thickness of the cover slip is 0.15 mm. Recommended adhesives include Canada balsam thinned with xylene, or a suitable epoxy resin. It is important for the indices of refraction to correspond. 

The 2-,4- and 6-positions in resorcinol are all extremely reactive and resorcinol finds some use in the preparation of resins for cold-setting adhesives. 

Resorcinol -161, - ol n (1881) (resorcin, m-dihydrox-ybenzene, 3-hydroxyphenol) C6H6O2. A white, needlelike, water-soluble solid, a benzene derivative (1,3-substituted with two hydroxy group) originally obtained from certain resins, now usually synthesized. When reacted with formaldehyde, produces resins suitable for cold-setting adhesives... 

Resorcinol-formaldehyde resins are cold-setting adhesives for wood structures. Urea-formaldehyde adhesives, commonly modified with melamine formaldehyde, are used in the production of plywood and in wood veneering for interior applications. Phenol-form-aldehyde and resorcinol-formaldehyde adhesive systems have the best heat and weather resistance. 

---