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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.2 Concepción jun. 2005

http://dx.doi.org/10.4067/S0717-97072005000200002 

 

J. Chil. Chem. Soc., 50, N 2 (2005), págs.: 451-453

 

DIFFERENTIAL SCANNING CALORIMETRY AND DINAMIC MECHANICAL ANALYSIS OF PHENOL-RESORCINOL-FORMALDEHYDE RESINS.

 

JUSTO LISPERGUER1* , CYNTHIA DROGUETT2 ,BEATRIZ RUF1 AND MARIO NUÑEZ3 .

1 Facultad de Ciencias.Universidad del Bio Bio, Avda Collao 1202, Concepcion , Chile .e-mail : jlisperg@ubiobio.cl
2 Dirección de Transferencia Tecnológica. Universidad del Bio Bio, Avda. Collao 1202 , Concepción ,Chile.
3 Centro de Investigaciones de Polímeros Avanzados, Avda Collao 1202 Concepción ,Chile.


ABSTRACT

Differential scanning calorimetry ( DSC ) and dinamic mechanical analysis ( DMA ) were use to study the curing reactions of two phenol - resorcinol- formaldehyde ( PRF ) adhesives with 15 and 25 % resorcinol content, synthesized in laboratory. The PRF resin with lower resorcinol content ( PRF1 ) shows a double exothermic peak ( 99.9 and 158.2 °C ) while PRF2 ,with greater resorcinol content ( 25 %) shows only one peak at 93.9 °C. The DMA storage modulus E' confirm the differences in the curing behavior of PRF adhesives. In the delamination resistance test during accelerated exposure only PRF2 with a greater content of resorcinol, fulfilled the ASTM D 2559 requirements for beams treated with two CCA retention levels. The cross-linking reactions was produced in the curing process at lower temperature in PRF2 , enhanced the adhesion durability to glulams treated with CCA preservatives .


INTRODUCTION

Resorcinol-formaldehyde ( RF ) and phenol-resorcinol-formaldehyde ( PRF ) cold-setting adhesives are used in the manufacture of structural glulam, laminated veneer lumber, finger joints and other exterior timber structures. These adhesives produce high strength and weather resistance bonds under a variety of climatic conditions ( 1,2,3 ). PRF resins are primarily prepared by grafting resorcinol onto the active methylol groups of the low-condensation resols obtained by the reaction of phenol with formaldehyde ( 4 ). The majority of the adhesives currently manufactured are of this type due the high cost of resorcinol ( 5 ). Comparative durability studies between various adhesives indicate that PRF resins satisfactorily satisfy exterior use requisites in delamination test after 30 years of exterior exposure ( 6 ).

The use of wood in this type of elements requires a chromated copper arsenate ( CCA ) preservative impregnation processes. The principal problem is that commercial adhesives ( PRF ) do not adhere to CCA-treated wood well enough to consistently meet rigorous industrial requirements for delamination resistance ( 7 ).

Other publications indicate satisfactory results in the adhesive quality of CCA-treated wood for adhesive systems based in RF ( 8 ) ,due to the fast curing rate of the latter resin. In the commercial PRF adhesives the percentage,( mass,of resorcinol in liquid resin) is of the order 16 to 18 %. ( 9 ).

The reactivity of adhesive systems can be studied with differential scanning calorimetry ( DSC ) and dynamic mechanical analysis ( DMA ) where different researchers have studied both the mechanical and chemical properties of the curing reaction. ( 10,11,12 ).

It is well known that the PF resins demonstrate an exothermic reaction in the curing thermograms in a range of 142 - 165 °C ( 13,14 ) while the RF resins present an exothermic reaction in the range of 65 - 110 °C ( 5 ). For PRF resins,the information provided in the literature is little clear since a high pressure DSC cell has not been used and the exotherm of the cross-linking reaction is interfered by the endotherm of water evolved from liquid resin at 100-120 °C ( 5 ).

The objective of this study consists in the characterization and analysis of the cross-linking reaction during the curing process of PRF resins with different content of resorcinol using a high pressure DSC cell and DMA. The relation between the resin curing characteristics and the quality of the adhesive union in CCA-treated wood will also be studied.

EXPERIMENTAL

Synthesis of PRF resins

A phenol-formaldehyde prepolymer was made by reacting 1 mol of phenol with 1.8 mol of formaldehyde in the presence of 8.5 percent NaOH based on the weight of phenol. Water was added to prepare a solution with 46 % solid content. The formaldehyde was in solution form at 37 % and was added in 2 stages (0.9 moles at the beginning of the reaction and 0.9 moles after 2h of reflux). The NaOH was added in 2 stages (initially and after 2 hours) and the reaction mixture was heated under reflux for 2.5 hours to a viscosity of 220 - 300 centipoises according to a previously informed procedure ( 15 ).

The mixture was cooled to 50 C and 0.54 mol of resorcinol was added with 30 g methanol and water for a final solid content of 56 %. The mixture was then refluxed for an additional 1.5 -2 hours to a final viscosity of 800 - 1000 centipoises. The pH was adjusted to 8.5. The hardener was a 1 : 1 mixture of paraformaldehyde and wood powder used in 20 to 100 parts by mass on liquid resin. The resorcinol percentage, by mass of liquid resin, was 15 %. ( This resorcinol content is typical for commercial PRF resin used in the glulam industry). The molar relation of phenol : resorcinol : formaldehyde was 1 : 0.54 : 1.8 respectively.

A second PRF resin was synthesized under similar laboratory conditions,increasing the resorcinol percentage to 25 % over liquid resin with a molar relation P :R : F of 1 : 1 : 1.8

Differential Scanning Calorimetry experiments

DSC measurements were carried out in a Rheometric Scientific apparatus. A resin sample ( 12-15 mg ) was sealed in a high- pressure stainless - steel crucible with a gold O- ring that can withstand pressures up to 2 MPa .The temperature range scanned was between 25 to 200 °C with a heating rate of 10 °C / min. Temperature and enthalpy calibrations were performed with indium.

Dinamic Mechanical Analysis experiments

DMA measurements were carried out in a Perkin Elmer DMA 7e. A cup-plate system for 200 mg of liquid resin in shear -cylinder mode was used . A storage modulus ( E´) and complex viscosity spectra between 20 to 200°C for the PRF adhesives curing process was determinated. The experimental parameters in the dynamic temperature ramp test were : frecuency = 1 Hz ; static / dynamic forces of 550 - 500 mN and ramp rate of 5 °C / min.

Resistance to delamination during accelerated exposure

The testing procedures and requirements for resistance to delamination were used as specified in ASTM D 2559 ( 16 ) for structural laminated wood products for exterior use.

The CCA - treated lumber at two retention levels ( 4.0 and 6.0 kg / m3 ) , were cut into pieces 1.91 cm thick by 13.97 cm wide and 101.6 cm long following ASTM D 2559. The surfaces were planned and glued with two PRF adhesives , manufacturing 4 laminated beams with each one. The test specimens were placed in an autoclave capable of withstanding 550 kPa of pressure ,impregnating the specimens with water and steam at 100 C for 90 min. The length of the open- glue joint area divided by the total length of the bond line exposed multiplied by 100 as percentage delamination was reported.

RESULTS AND DISCUSSION

PRF adhesives properties

Properties of laboratory synthesized PRF resins are shown in Table 1.


DSC analysis

With respect to the DSC analysis,all thermograms corresponding to the PRF adhesives showed exothermic peaks for the curing reactions. The characteristic curing reaction peak for liquid phenol-formaldehyde resin ( PF ) , is at 145-150 °C, without the interfering endotherm of volatile reaction products such as water ,and when high pressure DSC crucibles are used (10 ). The curing reaction involves condensation reactions of hydroxymethyl groups that were previously formed through reaction of phenol with an excess of formaldehyde in the presence of an alkaline catalyst to form methylene bridges of the resols(5). When PRF resins are prepared by grafting resorcinol onto the active methylol groups of the resol , the DSC thermograms ( See Fig. 1 ) shows a new exothermic reaction in the range of 60 - 120°C with a peak at 99.9°C. This gives accelerated , improved cross-linking at a lower temperature than PF resins and provides these adhesives with their characteristic cold-setting behavior.


Fig.1: DSC thermograms for PRF resins cured at a heating rate of 10C/min.

Considering peak temperatures in Fig. 1 , the PRF1 resin with lower resorcinol content ( 15 % ) presents a double exothermic peak. The low temperature peak ( 99,9°C ) presumably corresponds to the curing reaction of PF grafted with resorcinol ( PRF ) and the high temperature peak at 158.2C ( 140-180°C), corresponds to a molecular fraction of phenol formaldehyde prepolymer resin. The shift of the last peak to high temperature is presumably due to a lower content of hydroxymethyl groups producing cross-linking reactions.

PRF2 resin with 25 % of resorcinol content, shows only one clear peak at 93.9 °C ( 60 - 120°C ) corresponding to curing reaction of a system formed by only one type of a branched PRF resin .

The energy released during the curing process of the PRF adhesives measured as the curing enthalpy ( D H ) ,presented in the PRF1 thermogram,is more important for the curing reaction at 99.9 C ( D H = 58 J/g ) than for the curing reaction at 158.2 °C ( D H = 14.4 J / g ). However , it is necessary consider that a significant fraction of the PRF1 resin has cross-linking reactions at higher temperature.

When the resorcinol content is increased in the resin ( PRF2 ) ,the total cross-linking reaction take place at 93.9 °C ( D H = 100,74 J / g ).

DMA analysis

Figure 2 shows the storage modulus ( E' ) versus temperature obtained for two PRF adhesives. As can be observed the E' curves present different behaviors during the curing process. In PRF1 the storage modulus initially increases at 85 °C when cross-linking reaction have taken place . At 130 °C it reaches a maximun but decreased at higher temperature presumably because the curing process is not completed . The modulus decreases as an effect of thermal softening at 150 °C and increases again due to that this temperature is the cure region of PF resins.


Fig. 2: Storage modulus ( E' ) of PRF adhesives.

As can be observed , the curve of E' for PRF2 shows a initially increases at lower temperatures ( 70° C ) reaching a maximun at 110 °C in the region of the curing reaction of PRF resins. As shown in Fig. 2 ,once the storage modulus is increased,it stays practically constant with increasing temperature which can be explained due to that the greater resorcinol content of PRF2 produces a completion of curing in the region of 70 - 120 °C.

Resistance to delamination

The cyclic delamination test for CCA-treated wood showed strong differences between PRF1 and PRF2


Table 2. Delamination test for CCA-treated beams glued with PRF adhesives.

The first one with lower resorcinol content greatly surpassed the minimum delamination values permitted by ASTM 2559-04,which for soft wood of 1 - 5 % . PRF2 , with greater resorcinol content did not register delamination or open joints for glulam beams and successfully passed the three delamination cycles. The ANOVA analysis found no significant differences for this adhesive between the non-treated beams and those with a CCA retention levels of 4.0 and 6.0 kg / m3 .

CONCLUSIONS

1.- PRF1 adhesives synthesized with lower resorcinol content of 15 % ( Commercial type ) showed a double exothermic peak in DSC thermograms.The low temperature for the PRF1 ( 60 - 120 °C ) was associated with the curing reaction of the molecular fraction of the resin with grafted resorcinol.The high temperature peak (140 - 180 °C) is the characteristic curing region of PF resins, The interpretation is that PRF1 has a molecular fraction without resorcinol and the total curing process is slower than for a resin with greater resorcinol content wich produces a lower cross-linking level at room temperature. Indeed, the CCA- treated beams glued with this adhesive presents unsatisfactory behavior in the delamination resistance test.

2.-PRF2 with greater resorcinol content ( 25 % ) presented only one curing region ( 60 - 120 °C ) in DSC thermograms , with a maximun al 93.9 C .The interpretation is that all cross-linking reaction are carried out in this region because a molecular fraction of the resin without resorcinol do not exist in their structure.The DH in this region ( 100.7 J / g ) is greater than the energy released in the two exthermic peaks of PRF1 resins. The cross-linking produced at a lower temperature in PRF2 enhanced the durability of adhesion and reactivity to glulams treated with CCA preservatives and satisfied the ASTM D 2559-04 requirement for delamination resistance.

3.- The storage modulus ( E' ) confirms the differences in the curing behavior of PRF adhesives. While , PRF1 shows two activity regions of cross-linking reactions; in PRF2 the storage modulus initially increases at 70 °C and stays constant at higher temperatures.

Finally, PRF adhesives synthesized with 25 % of resorcinol demonstrated that they can satisfactorily bond CCA-treated glulam lumber for wet - use exposure on bridges or other construction products.

ACKNOWLEDGEMENTS

The authors are grateful to the Fund for the Promotion of Scientific and Technological Development ( FONDEF D00I 1164 ) for the financial support.

 

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