Ind. Eng.
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chem. Prod. Res. Dev. 1983, 22, 662-664
Durability of Structural Wood Adhesives after 15 Years Aging Elrlk Raknes The Norwegian Institute of Wood Technology, Box 337, Blindern, Oslo 3,Norway
Eight urea-formaldehyde (UF) glues and one acid phenolic (PF) glue were compared with “established” glues of the casein and resorcinol types as for long-term durability, by use of accelerated and natural aging. The purpose was to evaluate these glues for structural softwood bonding. After unprotected outdoor exposure, only the phenol resorcinols still meet the requirements of the Norwegian Glutam-Control (Norsk Treteknisk Institutt, 1975) after 10 years. I n the case of indoor and protected outdoor exposures, all glues except acid PF and one UF have sufficient shear strength after 15 years. All the glues seem to have gone through a period of stress buildup in the gluelines, followed by relaxation (probably due to creep). At the stage of maximum stress the caseins and resorcinols still had sufficient strength left, but some of the UF’s and the acid PF were below requirements. Acid PF seems to damage the wood (spruce). Accelerated aging distinguished between glue types in the same order as natural aging but were less satisfactory between brands within a type.
Background In the selection of adhesives for structural softwood bonding a number of properties have to be considered such as strength, durability, application properties, allowable assembly time, ability to cure a t ambient or moderately elevated temperature, and last but not least, cost. Up to now this has limited the choice to only a few glue types. They are the same today (1983) as in 1964 when the experiment to be reported here was started. The situation for these glues was then summarized by the author as follows (Raknes, 1968). (1) Caseins of the so-called water-resistant type are shown by long experience, as well as research, to be reliable under indoor and protected outdoor exposure. A disadvantage is their sensitivity to rain in the building period. (2) Resorcinols (RF) and phenol resorcinols (PRF) are likewise shown to be a t least as durable as the wood under all conditions. They are, however, expensive, and give nearly black gluelines. Temperature in curing should be above 25-40 OC, depending on the brand. (3) Melamines are more expensive than urea-formaldehyde (UF) glues, and not substantially better, unless they are cured a t quite high temperatures. Accordingly, their use is mostly limited to fingerjointing, using radiofrequency heating. (4) UF glues will definitely not stand full outdoor exposure, and they will also give way to prolonged exposure to high temperature combined with high relative humidity. As for their suitability under normal indoor conditions, the situation is rather dubious. Some brands have reportedly shown good performance in practice (Clad, 19651, but others are definitely not suitable. Separating the good from the bad by accelerated testing seems to be difficult. (5) Acid phenolics (PF) are in themselves weather resistant, but there is a risk that the strong acid used as hardener may damage the wood. They are expensive, difficult to use, and give black gluelines, but they cure down to +5 “C, lending themselves to “sitegluing”. UF’s are very convenient from a production point of view. They are easily adaptable to almost any gluing conditions, are cheap, give colorless gluelines, and have enough water resistance to withstand rain in the building period. An experiment was therefore started in 1964 to compare UF’s and one acid P F with the “established” caseins and PRF’s, with respect to long-term durability. Experimental Section Glues. These were all selected according to the glue manufacturers’ recommendations. The glue types were:
caseins, 2 brands (no. 1 and 2); UF’s, 8 brands, of which 2 were furfurylated (no. 3 and 4), 1 modified (no. lo), and 5 made gap-filling with inert fillers (no. 5-9); acid PF, 1 brand (no. 11); RF, 1brand (no. 13); PRF’s, 2 brands (no. 12 and 14). Natural aging was investigated by use of test blocks 15 X 15 X 30 cm, consisting of six laminations. These blocks were cut from beams of spruce (Picea abies). The blocks were stored under these conditions: (1)standard atmosphere (20 “C, 65% relative humidity (r.h.)); (2) outdoors, protected by a roof; (3) unheated, ventilated loft; (4) in a cellar; (5) outdoors, unprotected, on a roof (not caseins). Samples were tested with the ASTM D-1101 delamination test (3 in. long pieces cut from beams, full cross section, are subjected to cyclic wetting/drying), and the ASTM D 905-49 block shear test (compression shear, tested gluline area 2 X 13/,in). In this test shear strength as well as ”% wood failure” is recorded. The latter tells to what extent the fracture is in the wood, as opposed to glue or adhesion failure (estimated visually). Accelerated aging was investigated by use of British Standard 1204 “Close Concact” and “Gap” test pieces (caseins: only C.C.). The pieces are made from l/* X 1 in. European beech (Fugus siluatica), glued with 1 in. overlap; testing was by tension shear. The “Gap” gluelines are 0.05 in. thick. Three sets of test pieces were made, one for each of the following exposures: (1)standard atmosphere (20 “C, 65% r.h.); exposure time, 5 years; (2) cycling between 20-25 “C/85-90% r.h. and 50 OC/50-60% r.h., periods of 1 month (“humid/hot”); exposure time, 3 years; (3) cycling between 20-25 0C/85-90% r.h. and 25-30 OC/25-30% r.h,, periods of 1month (“humid/dry”); exposure time, 5 years. Before exposure it was determined that the gluelines in all the beams met the requirements of the Norwegian Glulam-Control (Norsk Treteknisk Institutt, 1975) and that the BS 1204 pieces met the requirements of that standard. This was to ensure that wood surface preparation and gluing had been carried out satisfactorily. Full details of the experiment are given in the progress report after 2 years exposure (Raknes, 1968). Results Detailed results are given in progress reports after 2, 6, 10, and 15 years exposure (Raknes, 1968,1971,1976,1981). Extracts are given below. Delamination (ASTM D 1101-59). Table I gives percent delamination as average for each glue. Results for the protected outdoor and the three indoor exposures (20
0196-4321 /83/1222-0662$01.50/0 0 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 663
-Shear strength
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Exposure time, Years
Figure 1. ASTM block shear strength/% wood failure. Average for each glue type, exposures 1-4 (indoor
Table I. Delamination, % (ASTM D 1101-59) av of indoor and unprotected protected outdoor outdoor , glue type (no.)” exposures, 1 5 years 10 years 2.1+ 3 s UF ( 3 ) 1.3+ 42UF (4) 3.8-‘ 23UF (5) 5.449UF (6) 3.2+ 36UF ( 7 ) 5.436UF (8) 2.7+ 22UF (9) UF (10) 0.9+ 40acid PF (11) 4542PRF (12) 0.3+ 3.4+ RF (13) 1.4+ 9.2PRF (14) 1.3+ 4.61a See Experimental Section. Meets the requirements of the Norwegian glulam-control. (Norsk Treteknisk Institutt, 1975). Does not meet the requirements of the Norwegian glulam-control. OC/65%, loft, cellar) after 15 years are pooled. For unprotected outdoor exposure, results after 10 years are given. (This exposure had to be interrupted after 10 years, due to rot development in the samples.) ASTM Block Shear Strength. Outdoor Exposure. The shear strength as well as percent wood failure decreased more or less steadily for the UF’s. After 6 years only one brand (the modified UF) still satisfied the requirements of the Norwegian glulam-control (Norsk Treteknisk Institutt, 1975) and after 10 years this too had failed. The acid PF showed low shear strength and low “proper” wood failure after 6 and 10 years, the glue surface being covered with only a hair-thin layer of wood fibers. (Spruce has poor acid resistance.) The RF and PRF’s showed good results after 6 years exposure. The wood failure was still above 80% after 10 years, but the shear strength had been reduced, obviously due to weakening of the wood. (It could also be caused by the buildup of stress concentrations, but this is considered unlikely in outdoor exposure, as the wood repeatedly goes through “plastic” stages, giving relaxation.) Indoor Protected Outdoor. The results for these four exposures are pooled and shown in Table 11. To give a better picture of the difference between glue types, the results for each type are pooled and shown in Figure 1. Accelerate Exposure. Detailed results are reported by Raknes (1971). They may be summarized as follows. Caseins tested dry have retained their strength well in all exposures. The standard atmosphere and the “humidldry” cycling caused virtually no strength reduction after 5 years exposure, while the 3-year ”humid/hot” ex-
+
+ protected
outdoor).
Table 11. Block Shear Strength, ASTM D 905-49, kp/cm* (% Wood Failure). Average, Exposure 1-4 (indoor + Protected Outdoor) exposure time, years glue before no.” exPosure 2 6 10 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 a
95(90) 95 (88) 88(62) 76(71) 8 1 (72) 89(72) 88 ( 7 3 ) 88 ( 5 5 ) 86(68) 83(73) 96(83) 94(87) 88(71) 102(85)
89(92) 92(90) 76(82) 80(90) 77 (92) 73(83) 73 (81) 75 (90) 75 (88) 78 (86) 78 (85) 82 (93) 83(95) 87 (91)
83 (90) 8 6 (90) 68 ( 6 8 ) 7 3 (69) 70 (76) 64 (79) 7 2 (81) 7 3 (78) 7 1 (83) 78 (78) 55 ( 6 3 ) 77 ( 9 0 ) 77 (87) 80 (89)
92 (91) 95 (90) 8 1 (86) 82 ( 8 4 ) 8 1 (78) 78 (80) 83 (80) 77 (77) 78 (85) 84(84) 68 (69) 89(91) 89 (88) 92 (90)
85 87 74 73 74 74 74 75 75 77 65 84 81 88
See Experimental Section.
posure caused approximately 16% reduction for one of them. RF and PRF’s retained their strength very well under all conditions and still meet the requirements of British Standard 1204 after exposure. Acid PF gave poor results in all exposures, especially for the gap tests, with low breaking loads and very shallow wood failure (acid damage?). UF’s retained their strength quite well in “standard atmosphere”, but lost strength steadily when subjected to wetldry-cycling, and even more in the wetihot-cycling. The modified one and the two furfurylated brands retained their strength better than the others. Discussion In the natural aging test the strength of the gluelines is evaluated by means of the ASTM delamination and block shear tests. (Caseins were not subjected to the delamination test.) The delamination test is more or less a “goino go” test, designed to show whether the glue is “stronger than the wood” (wet strength). This is done by vacuum-pressure impregnating test pieces with water and drying them so rapidly that the drying stresses resulting will produce cracks either in the wood or in the glueline, depending on which of these two are the weakest. Provided the gluing is properly done, the percent delamination therefore tends to be quite low as long as the glue is stronger than the wood, and quite high as soon as i t becomes weaker.
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From Table I it is thus evident that all the UF's, the P R F s and the RF are still substantially stronger than the wood after 15 years storage in the indoor and protected outdoor exposures. The acid PF is the only glue giving high delamination under these conditions, probably due to acid damage to the wood. This will give a wood failure so shallow that it probably will be judged as "delamination". After 10 years of unprotected outdoor exposure, only the PRF's and RF are still markedly stronger than the wood, when judged by the delamination values in Table I. In the block shear test breaking load and % ' wood failure are registered. The latter tells to what extent the glue is "stronger than the wood" (dry strength). The breaking load is a measure of the ability of the test area to withstand an external load. This will depend on the strength of the materials (wood and glue), inner stresses in the glueline, and the ability of the glue to distribute the external load over the whole test area. [(Very stiff gluelines will often give lower breaking loads than viscoelastic ones; cf. Clad, 1965).] When testing, breakage will occur in the wood or in the glue when the sum of inner stresses ("preload") and external load exceeds the strength of the weaker material. The percent wood failure will show which of them is the weaker. Looking a t Table I1 and Figure 1 in view of this, it seems that all the glues have gone through the following stages during the exposure period (more or less pronounced; for caseins the changes are very small). In the period 0-6 years a "postcuring" seems to have taken place, creating inner stresses in the gluelines. This has caused a reduction in the observed shear strength. (The consistently high wood failure-except for the acid PF-makes it unlikely that a reduction of the strength of the glue is the cause.) In the period 6-10 years a relaxation ("de-stressing") seems to have taken place. This may have been caused by creep in the wood substance, or in the glue itself, or both; cf. Clad (1965), who observed this type of creep even in chemically set gluelines. A similar tendency of stress creation followed by relaxation is also to be observed in the results of a longterm test carried out by Bergin and Godin (1971). In the period 10-15 years a slight reduction in observed shear strength has occurred for all glues. The wood failure has decreased markedly for the UF's but the decrease is less pronounced for the others. Conclusions Caseins seem to harmonize best with the spruce wood itself in strength and elastic properties, and the reliability of this adhesive in glulam constructions for indoor and protected outdoor conditions is confirmed. RF and PRF's have given very good results in indoor as well as outdoor exposures. Postcuring seems, however, to create greater stresses in the gluelines with these adhesives than with the caseins. (This may be different when gluing other species.) After 15 years exposure the shear strength of the gluelines (indoor and protected outdoor
exposures) is still well above the requirements of the Norwegian Standard 3470 and has probably been so throughout the whole test period, as it has for the caseins. Requirements of NS 3470 to short-term shear strength include a 5% lower limit of 35 kp/cm2 or more, derived from the long-term requirement of 2 MPa. (NS 3470 gives design and dimensioning rules for all types of wooden structures for building purposes. is also contains requirements of materials, quality of work, and control.) UF's were not able to stand unprotected outdoor exposure. During indoor and protected outdoor conditions quite high stresses have appeared in the gluelines in the 2-6 year period. During this period the shear strength of the gluelines was below the requirements of NS 3470 for at least three, possibly four, of these adhesives. Apparently owing to stress relaxation the (observed) shear strength again reached a satisfactory level during the 6-10 year period, and this level was retained during the 10-15 year period. The percent wood failure now seems to be decreasing for these glues, indicating that their strength is approaching that of the wood, from above. Acid PF seems to be an unreliable adhesive, at least for spruce, probably because of acid damage (when tested for acid damage according to German Standard DIN 68141 it does not stand up to the requirements). Accelerated vs. Natural Aging. Correlation is good as far as glue types are concerned. If comparison is made for the various brands of UF glues, however, the results do not correlate. Furfurylated and modified UF's stood better than the others in the accelerated tests, but after 15 years of natural exposure all the UF's are more or less on the same level. The investigation continues, and a new progress report is planned to appear in 1986. Registry No. Urea-formaldehyde copolymer, 9011-05-6.
Literature Cited ASTM D 905-49."Strength Properties of Adhesive Bonds in Shear by Compression Loading"; American Society for Testing and Materials, Philadelphia, PA, 1949. ASTM D 1101-59. "Integrity of Glue Joints in Structural Laminated Wood Products for Exterlor Use"; American Society for Testing and Materials, Philadelphia, PA, 1959. British Standard 1204: 1964. "Synthetic Resin Adhesives, Gap Filling (Pha nolic & Aminoplastic) for Constructional Work in Wood": British Standard Institution, London, 1964. Bergin. E. G.; Godin, V. "Durability of Wood Adhesives in Birch Plywood after 15 Years Indoor Aging"; Canadian Forestry Service, Publication No. 1307, Ottawa, 1971. Clad, W. Hoi. Roh. Werkst. 1985, 23(2),58. DIN 68141. "Holzverbindungen", Abschnitt 2.5.Deutsches Institut fur Normung e.V., Berlin, 1969. Norwegian Standard 3470 "Timber Structures. Design Rules"; Norges Standardiseringsforbund, Oslo, 1979. Norsk Treteknisk Institutt. "Production Manual for Making Laminated Constructions". Norsk Treteknisk Institutt, Oslo. 1975. Raknes, E. Norsk Skogindusfri 1968 22(4),119. Raknes, E. Norsk Skogindustri 1971, 25(1l), 325. Raknes, E. Norsk Skogindustri 1978, 30(6),168. Raknes. E. Norsk Skogindustri 1981, 35(10),260,270.
Received for review January 28, 1983 Accepted June 28, 1983 This experiment was supported by The Royal Norwegian Council for Scientific and Industrial Research.
