Лако-красочные материалы — производство

A. General Principles

While pMDI wood binders are derived from formaldehyde, they are distinguished from other wood binders by a total lack of formaldehyde emission. This performance advantage has not displaced the less expensive UF and melamine-urea-formaldehyde (MUF) adhesives, which are commonly used for particleboard and medium density fiber — board. This is a tribute to the amino resin industry which has successfully reduced for­maldehyde emissions while maintaining the excellent performance of UF and MUF binders. The pMDI wood binders are labeled as highly durable exterior grade adhesives; UF and the more durable MUF binders do not receive this designation. Consequently, the higher cost pMDI is often reserved for more demanding specialty applications within the amino resin markets. Another noteworthy difference between pMDI and the amino resins is that isocyanate binders have no cold tack, i. e, no precure stickiness. Consequently, wood particle/fiber mats sprayed with pMDI lack any precure integrity which is required with certain particleboard or MDF handling systems. Also, pMDI binders are not affected by the buffering capacity of wood, which has a significant influence on waterborne amino resins. Similar to the amino resins, pMDI provides a colorless bondline.

A colorless bondline is occasionally a consumer perception benefit as compared to PF resins which produce dark bondlines. PF wood binders are the industry standard exterior grade thermosets. As an older and firmly established technology, PF resins are the primary competition to pMDI. This competition is planted squarely in the OSB industry. Neat pMDI binders do not compete with PF in veneer based products such as plywood and laminated veneer lumber; the gap filling requirements in these products exceed the capabilities of neat pMDI. In contrast, OSB production uses thin wood strands that are compacted under high pressures, conditions which favor the deeply penetrating isocyanate binders. On an equal resin solids basis, pMDI adhesives are equal or superior to PF binders in all aspects of OSB performance. However, in comparison to liquid PF resins, pMDI binders are often used at lower resin solids levels. In commodity OSB manufacture, liquid phenolics may be used at about 3.5% resin solids on dry wood; powdered phenolics may be as low as 2% which is about equal to the lowest levels used for isocyanate binders. The upper limit of pMDI application is about 6-8% on dry wood, which is reserved for certain specialty products. The pMDI binders cure faster than standard single component PF resins, which is a significant advantage in commodity markets where profits hinge upon production rates. These differences (% resin solids and cure speeds) complicate the inevitable comparisons between PF and pMDI. For example, the rapid cure speed of pMDI may result in a 10-25% reduction in hotpress time as compared to PF binders. While this is beneficial for commodity production, one must realize that this also results in a 10-25% reduction in the hygrothermal compression of bulk wood, irrespective of bond formation. Hygrothermal compression of wood (com­pression of steam plasticized wood under elevated temperature) is known to impact bulk wood properties, often with great benefit [37]. Presently, the benefits of extended OSB hotpress times are not thoroughly understood. The point here is that the effects of rapid cure speed may extend beyond simple reductions in hotpress time, e. g., wood properties may also change, producing effects that are independent of the wood binder.

A clear disadvantage of pMDI wood binders is that they adhere strongly to nearly all surfaces, including steel. Consequently, external release agents are required to prevent adhesion between press platens and boards. The separate application of release agents is a process nuisance which has stimulated research on “internal” release agents, i. e., addi­tives mixed directly into the pMDI binder. Unfortunately, no one has yet developed a truly effective internal release system. This is not surprising; just consider the difficulty in developing selective adhesion. While effective internal release technologies may never become a reality, significant efforts are still directed towards the improvement of external release agents. Current external release agents are based upon waxes or soaps, each having strengths and weaknesses [38,39]. Waxes lead to an organic build-up requiring maintenance downtime for removal and cleaning. While this build-up is undesirable, it is a safeguard against mistakes, misapplications, or incomplete applications of the release agent because the build-up is nonadherent. On the other hand, soaps (simple alkali fatty acid salts) are effective without causing an organic build-up. Unfortunately, soaps are unforgiving release agents; mistakes, misapplications, or incomplete applications will result in adhesion between panel and platen. The adhesion of pMDI to steel has undoubt­edly inhibited the growth of this binder in the OSB industry. In fact, this problem has actually resulted in a marriage of PF and pMDI binders. OSB is produced with distinct layers, so manufacturers often use PF resin in the face layers, with pMDI in the core.

Liquid PF resins are particularly effective in the face layers because their close proximity to the platens insures rapid cure, and phenolic resins do not adhere to the platens. Additionally, PF moisture in the face layer is vaporized and driven to the core; this ‘‘steam shock’’ effect improves heat transfer, further accelerating the naturally rapid pMDI cure. This harmony of technologies provides increased production rates with good performance. However, the combination is subject to a pitfall. PF and pMDI incompat­ibilities may lead to poor bonding and even delamination in the transitional zone. This incompatibility is thought to arise from excessive caustic in the PF face layer resin. Alkali metal ions, particularly potassium, may catalyze isocyanurate formation, and/or aqueous hydroxide anions may directly consume isocyanate yielding (the conjugate base of) the free amine, resulting in urea formation [38]. Neither of these alkali hydroxide catalyzed reactions would seem problematic. However, the reactions could be counterproductive if so rapid that they occur before hotpress compaction. The precise chemical mechanism of how excessive PF caustic causes this incompatibility with pMDI has not been determined. The hypotheses presented above require verification. The PF/pMDI incompatibility is not a common problem because the resin manufacturers are aware of the need to avoid excessive caustic in the phenolic system.

The use of pMDI as an OSB core resin reveals another interesting characteristic of this binder, namely very good moisture tolerance. Core resins require moisture tolerance because steam generated during hotpressing travels to the cooler core and condenses into liquid water. Since pMDI is totally organic and water insoluble, steam and water cannot solubilize the resin; the resin is not diluted and does not suffer from ‘‘wash in’’ or ‘‘wash out.’’ These phrases refer to the excessive penetration and flow of liquid PF binders that are diluted and solubilized by steam and water. The good moisture tolerance of pMDI is sometimes erroneously attributed to water consumption by resin cure. The NCO/water reaction has a minor influence on moisture tolerance, if any. The water consumed is but a small fraction of the total. A quick calculation makes this point clear. First, recognize that two moles of NCO react for each mole of water consumed, producing a mole of CO2 and one urea linkage. Consequently, the NCO/H2O mole ratio should be halved to estimate the percentage of water consumed; even this is an overestimate because a significant amount of NCO reacts not with water but with ureas, biurets, and probably wood. Table 1 displays the water consumed under conditions that might represent the extremes in conventionally hotpressed panels made with 100% pMDI resin.

It is apparent that water consumed through urea formation is minor under typical industrial conditions. So again, the good moisture tolerance of pMDI reflects that the binder is not water miscible.

One then concludes that pMDI binders could tolerate higher than normal wood moisture during composite manufacture. Indeed this is a desirable goal because higher wood moisture levels translate into reduced drying time with reduced energy costs and lower emissions of volatile organic compounds. Early reports claimed that wood moisture

contents could be as high as 25% [13,40]. Subsequent works suggest that lower moisture levels, ranging from 12 to 20%, will yield acceptable board performance [15,16,41-43]. The breadth of this range reflects numerous hotpressing variables and resin loadings. A more conservative approach would suggest that moisture contents from 10 to 15% are realistic upper limits for industrial production using pMDI. While 10 to 15% wood moisture is at the lower range of some claims, this is significantly higher than the levels that formaldehyde binders can tolerate. Wood moisture contents of 4 to 5% are common using liquid PF resins (the resulting total mat moisture is nearly double this because of water in the aqueous adhesive). Powdered PF resins are more moisture tolerant than the corresponding liquids, but they cannot match the moisture tolerance of isocyanate bin­ders. While pMDI resins have superior moisture tolerance, it seems that this capability is not commonly exploited in conventionally hotpressed panels. Many pMDI users dry their wood furnish to moisture levels near 4% [38]. This may reflect the technical challenge of tightly controlling wood furnish moisture; it is easier to minimize moisture variation when drying it towards the minimum. Furthermore, there is a common desire to reduce steam pressure within the mat in order to minimize pressure induced delaminations, or ‘‘blows.’’ High moisture and the resulting steam reduce hotpress capacity because venting times must be extended to prevent blows. So the advantages of higher wood moisture are balanced against extended press times [44]. Rapid curing pMDI allows lower press tem­peratures, thereby reducing steam pressure.

The moisture tolerance of pMDI is utilized in steam injection pressing (SIP), where steam is injected into the flake or strand mat (through the press platens) immediately prior to, and during compaction. SIP accelerates heat transfer and hotpress production, especially for very thick products such as laminated strand lumber. Besides accelerated production, steam processing increases the dimensional stability of the resulting composite [45,46]. Isocyanate resins appear to be the only exterior grade wood binders which can withstand the moisture extremes found in SIP [33,34,47].

Another noteworthy application of pMDI binders is in the manufacture of ‘‘Ag-Fiber’’ composites, particularly straw-based panels that compete in particleboard and some medium density fiberboard markets [48-50]. The hydrophobic straw cuticle is difficult to wet, and so the low surface tension of pMDI is a clear advantage. The higher surface tension UF resins do not wet the straw surface as well. Furthermore, straw buffering properties may also complicate the application of UF resins to straw-based composites [49,50]. Straw-based composites may represent a significant market for pMDI binders. However, at this time the viability of straw-based panels is unclear because of the extreme competition from particleboard.