Honeymoon adhesives

Figure 2. Comparison of the rate of tensile strength development in RF/RF and PRF/PRF honeymoon adhesive systems.

Two component acrylic systems which are known as second generation acrylics in the U.S. and as "honeymoon adhesives" in Europe are discussed in Chapter 10. Acrylic resins have been... 

An unusual type of two component system has recently been introduced in the adhesive industry (1,). These are called reactive adhesives, second generation acrylics, toughened acrylics, modified acrylics (2) or "honeymoon adhesives" (in Europe). 

Figure 2 Typical strength and wood failure increase as a function of time of a pure PRF honeymoon adhesive system on timber at 12% and 22% moisture content [26-29]. Note that at the unusually high moisture content of 22% the requirements of the standards are passed in less than 24h as the timber starts to dry [26-29]. Test on beech strips according to British Standard BS 1204. 

Figure 9 TensUe strength increase as a function of time of beech joints (BS 1204, Part 1) bonded with MUF-based honeymoon adhesive systems effect of the variation of the initial application pH of the resin (component A). 

MUF honeymoon adhesive systems for bonding of timber of high moisture content (wet gluing) to produce laminated wood (glulam) and finger-jointing are composed of two components (1) a MUF resin at a pH of approximately 10 with no fillers added, and (2) a low pH aqueous solution of carboxymethylcellulose and formic acid lacking resorcinol in the system (82-85). 

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

If this method proves to be commercially feasible, a renewable and abundant waste product of forest products manufacture could replace expensive petrochemicals. Additionally, use of an inexpensive waste product from processing of forest products could materially reduce adhesive costs and expand opportunities for manufacture of structural materials from low-quality wood. This benefit is particularly important since the difficulty in producing large, strong, structural members from timber resources of declining quality is growing exponentially with time. The cost-benefit ratios of replacing PRF resins with extracts from conifer barks are, therefore, quite favorable in a honeymoon system. 

Adhesives. A description of the honeymoon system of wood gluing was first published in 1974 (1). It is, in principle, a system in which two different adhesive compositions are applied to the two surfaces to be mated. Since wood adhesives penetrate the wood surface (and have to do so in order to provide a good bond), it is important that both individual components of the system ultimately cure to form a solid durable polymer. This can be achieved either by designing the components such that each will fully cure by itself or by providing ingredients in the two components that react after mutual diffusion takes place ultimately, all layers of the glueline must reach the fully cured state. 

In order to establish control values for the adhesives formulated using tannins, the initial work was done with phenol-resorcinol-formaldehyde (PRF) or resorcinol-formaldehyde (RF) resins on both surfaces, but modified for the honeymoon principle. The PRF resin chosen for this work was Borden s resin LT-75 with Borden s hardener FM-260. The RF resin used for a comparison was Chembond s RF-900. These resins have been used for wood gluing in the United States for more than two decades, especially for the manufacture of structural laminated timbers. 

Use of southern pine bark extracts as 50% of the reactive phenolics in endjointing adhesives applied in a honeymoon system has good commercial potential. With proper plant layout and design, the installation, operation, and maintenance of radio-frequency units and their resulting high costs can be avoided. 

Acrylic adhesives first appeared about 1937 the acrylic resins may be considered as belonging to the vinyl family [1, p. 305], Today, acrylic adhesives appear in many forms as both pressure-sensitive and non-pressure sensitive formulations in organic solvent and emulsion forms as monomer and polymer cements as anaerobics as cyanoacrylates as so-called reactive or honeymoon two-part systems and as radiation curing formulations. Commercial production of acrylic polymers began in the late 1920s, but it was not until 1958 that the first aerylie sealant was developed [10, p. 226]. The solvent-based acrylic sealants were first introdueed to the eonstruction industry in about 1960 ... 

A particularly interesting system now used extensively in several southern hemisphere countries is the so-called honeymoon fast-setting, separate-application system [66,67]. In this system one of the surfaces to be mated in the joint is spread with a standard synthetic phenol-resorcinol-formaldehyde adhesive plus paraformaldehyde hardener. The second surface is spread with a 50% tannin solution at pH 12. When the two surfaces are... 

These liquid resins then work at a resorcinol content of only 9 to 11%, hence considerably lower than that of traditional PRF resins. These resins can also be used with good results for honeymoon fast-setting adhesives in PRF tannin systems, thus further decreasing the total content of resorcinol in the total resin system at a level as... 

As a consequence, two distinctly unique methods of handling and curing acrylic adhesives follow from their cure chemistry. These are the so-called accelerator lacquer cure and the no-mix or honeymoon cure. These methods of handling acrylics are depicted in Figs. 5 and 6. 

The no-mix or honeymoon type acrylic adhesives are unique in that polymerization is adequately achieved after the A component of the adhesive is applied to one substrate to be bonded, and the B component is applied to the other substrate to be bonded. When these two halves are joined together, as shown in Fig. 6 previously, enough free radicals are generated to initiate and complete the cure. The no-mix technology is unique in that it has found extensive use in the electronics industry, and is a very useful technique in bonding magnets for electrical motors where the fast cure and easy application technique have found extensive utility. 

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