Journal of Scientific & Industrial Research

Vol. 61. January 2002. pp 17-33

Non-conventional Phenolic Resins -An Overview on Recent Advances

C P Reghunadhan Nair

Polymers and Spec ial Chemicals Division. Vikram Sarabhai Space Centre, Thiruvananthapuram 695022

Phenolic resi ns even now are useful in view of thei r several salient features and the unlimited commercial potentialit y.

Innovative research for overcoming the inherent shortcoming of these systems with regard to processability, brittleness. oxidative resistance etc. has gai ned momentum. The introduction of addition-cure phenolics answers partly to these problems. The paper consolidate the very recent developments in the field of non-conventional phenolics. 111e strategies adopted for structural mod ifications through introduction of thermally stable add ition curable groups such as. phenyl maleimide. phenyl maleimide-a ll ylphenol. propargyl ether. acetylene and phenyl ethynyl are described brietly. When the structural modificat ion a lone does not answer to the problem. strategy of reactive and non-reactive blending and cont rol of the cure and morphology of the network prove effecti ve. The cure behaviour and properties of the cured mat ri ces arc

reviewed. Reference is made also to the synthesis and adhesive evalu ation of selected modifi ed phenolic res ins and their blends. Some of these resins show good prospects for app li cati on in aerospace struct ural and thermo-st ructura l app li cati on~ .

and as high perfo rmance structural adhesives with delinite advantages.

Introduction Despite the emergence of severa l new classes

of thermosets, high performance polymers and severa l other new generation material s that are superior in some respects, pheno li c resin still retains the industrial and commercial interests even after a century of its introduction I . Phenolic resin is a unique, classical polymer preferred in a wide range of applications, from commod ity construction materials to high technology aerospace industry. Thi s recognition emerges from the fact that these resins have several desirable characteristics such as, superior mechanical strength , heat resistance and dimen sional stability, as well as, high res istance against various solvents, acids and water, flame resistance. and low smoke evoluti on upon incineration. Although phenolics cannot be substitutes for epoxies and polyimides in many engineering areas, their composites still find a major market in thermo-struc tural applicati on in aerospace industry due to good heat and fl ame resistance. ablative characteri st ics and low cost 2.3 . These key properties add to their market growth and as a result of research , new products and app l icat ions continue to emerge, demonstrating its versatility and the potenti al to cope with the ever-changing requirements and challenges of advanced technology.

Class ica l phenolic res in s based on resole and novo lac,

dominate the resin market undi sputed ly. However,

the ir acceptance as a uni versal mate ri a l til many

engineering areas is hampered by some of their

inherent qualities derived from the special chemical

structures. These resins cure at high temperature by

condensation mechani sm with evoluti on of volatiles.

which necess itates application of pressure Ju ring

moulding to get void-free components. Need for use

of catalyst for curing, and limited she lf- life of res in at

ambient conditions are a lso major shortcomi ngs of

these systems. When compared to many known

therma lly stable polymers. its thermo-oxidative

stability is low. The rigid aromatic units. tightl y he ld

by the short methylene linkages make it brittle. In view

of thi s, a new chemi stry is needed to modify the cure

of phenolic resins , in particu lar. a new meth od is

needed to chain extend and/or to crosslink phenolic

res ins without vo lati les producti on and allow for

extended she lf stability at ambient conditions for the

formulated thermosets. In doing so, it is imperati ve

that the mod ification does not impair the thermo­

mechanical characteristics of the resultant system. Lot

of research attention is focused on this domain to

d . 4 ~

a Jress some of these problems .. .

"

18 J SCI [NO RES VOL 6[ JANUARY 2002

Structural Modifications

Several approaches to modify the phenolic resins and their cure chemistry have been reported . Structural modification to confer addition-cure character has been one thrust area of research. Addition-curable phenolic resins with improved thermal and pyrolysis characteristics will be the desirable res ins in composites for thermo-structural applications

6• Higher char-yield leads also to better

heat shielding. Such high char phenolics could be potential candidates as matrices in carbon/carbon composites too with obvious advantages7

. This article gives a brief account of recent research in addition-curable phenolics that are suited as high performance matrices and as adhesives.

Phenolic Resins with Phenyl Maleimide Functions

Phenyl maleimide based addition polyimides have attracted the high performance polymer industry since quite some times.9 . With a view to confer part of their good properties to a phenolic resin , novel phenolic novolac resins , and bearing maleimide groups and capable of undergoing curing principally through the addition polymerisation of these groups were synthesised. The synthesis was effected by polymerising a mixture of phenol and N­(4-hydroxy phenyl) maleimide (HPM) with formaldehyde in the presence of an acid catalyst 10 ,

as shown in Scheme I .

The resins were characteri sed by chemical , spectral, and thermal analyses . Differential Scanning Calorimetry (DSC) and Dynamic Mechanica l Analysis (DMA) revealed an unexpected two-stage curing for these systems. Whereas the cure at around 27SoC was attributable to the addition polymerisation reaction of maleimide groups, the exotherm at around \SO-170°C was

ascribed to the condensation reaction of methylol groups formed in minor quantities on phenyl ri ng of HPM. Polymerisation studies on non-hydroxy functional N-phenyl maleimide deriva tives revealed that the phenyl groups of these molecules are activated towards electrophilic substitution reaction by the protonated methylol intermediates formed by the ac id catalysed reaction of phenol and formaldehyde . It was also revealed that presence of phenoli c group on N­phenyl maleimides was not imperative for its copolymerization with phenol and formaldehyde. The cure characterisation of the resin by dynamic mechanical analysis (Figure I) confirmed the two-stage cure and the dominance of maleimide polymeri sation over methylol condensation in the network build-up process.

The kinetics of both cure reactions , studi ed by Roger's method substantiated the earl ier proposed cure mechanism for each stage". Although the in itial decomposition temperature of the cured resin was not significantly improved , enhancing the cross-link density through HPM led to improved thermal stability of the material at higher temperature regime. The anaerobic char yield also increased proportional to the maleimide-content. Isothermal pyrolysis and anal ysis of the char confirmed that the pyrolysi s occurs by the loss of hydrocarbon and nitrogenous products ' 2. The resin served as effective matrices in sil ica- and glass fabric-reinforced composites whose mechanical properties were optimum for moderate ly crosslinked resins , wherein the failure occurred throul!h a combination of fibre debonding and res in frac~ure l ' (Table I).

Allyl-Phenyl Maleimide-Phenolic Resin

The thermal curing of the PMF system through polymerization of maleimide group, resulted 111

comparatively brittle matrices. As a result , the

Dr C P Reghunadhan Nair was horn on 13 April 1956. He obtained PhD in Physical Chemistry o f Macromolecular Mate rials from University Louis Pasteur. Strasbourg, France . in 1989 . He was a visitin g. scientist at the National Centre for Scientific Research (CNRS). France. in 1993.Hc joined Vi kram Sarabhai space Centre in 1980. He has 21 y research expe rience in different di sc iplines. He has to IllS credit 117 papers in the fi eld of po lymer science in national and internati onal j ourna ls. Dr Nair has nin patent s to his credit. He has been a invited speaker in many academic Centres in Ind ia and abroad. He i ~

a resource person for many educational programs of UGC and various Colleges and Un:ve rsiti es .He has guided four PhD student s .. Dr Nair was g iven French government fe llowship from 1985-89.He was awarded Sukumar Maity Polymer Foundation award for outstanding contributions in polymer sc ience in 1999-2000. Hi s current fi e ld o f research interests are hi gh temperature resistant polymers, matri x res ins and polymer matrix composites. free radical polymerization .polymer adhes ives , Jlol ymer b lends. and high char yield ing polymers.

NAIR: NON-CONVENTIONAL PHENOLIC RES INS 19

© HCHO +

OH

HPM

;:::Lo -CHz(}CH.Nc:~~H"-

OH -~ OH

Soluble PMF Resin

1150 °C

M ~ 200-2"'C J:1o~ ~Hz(}CHNCH~VCH2' -CHz(}CH.Nc~VH"-

OH -~ - OH OH -~ - OH

Crosslinked Polymer Gelled polymer

Scheme 1- Synthesis and curing of PMF resi n

Temperature (eC)

150 200 250 250 250 250 250 250 212~ 1.8 3.0

E' 1.6 2.00

2.5

1.4 1.98

Ci .-II

2.0 ~ Cl. <l. ~ 12 ~ rtI

W ...... 1.96 r-• IU

1.0 1.5 1 . ~

C.iI

1.92 1.0 0 50 100 150 200 250 300 350

Time (min . )

Figure 1- DMA spectrum of PMF resin. heating rate IDoC/min

mechanical properties of the resultant composites did not improve signi ficantly over the convent ional phenolic resins . Hence the concept of bi smaleimide toughening by way of reactive blend ing with diallyl compounds was adapted to the PMF system. The co-reaction taking place by way of Alder-ene

. JJ 14 h h . d' W react Ion " t roug an IIlterme late agner-J . I ' auregg react ton step ' is expected to impart toughness to the matrix without sacrifi c ing the thermal characteri stics considerably. Thus. new

addit ion-cure phenolic resin systems based on the co­react ion of maleimide-functiona l phenolic resin (PMF) with an allyl-functional novolac (PAF) in vary ing proportions were developed ' 6

. The Alder-ene cure sequences to form a crosslinked network system were evidenced from the cure characterization studies by differential scanning calorimetry and dynamic mechanical analysis. The network structure of a i :3 blend can be depicted, as shown in Scheme 2.

20 J SC II NO RES VOL 61 JANUARY 2002

Increasing allyl-phenol content, decreased the cross lin king in the cured matrix , leading to enhanced toughness and improved resin-dominant mechanical properties of the resultant si li ca laminate composites. The mechanical properties of the composites are presented in Table 2. Changing the reinforcement from si lica to glass, resulted in furt her amelioration of the resin-reinforcement interaction, but the resin dominant properties of the

composite remained unaltered . Increasing the maleimide-content resulted in enhanced thermal stability. The cured polymers showed Tg between 170-190°C I6

.

Propargyl Ether Functional Phenolics

Abso lutely, addition-curable phenolic reslIls could be obtained by propargylation of novolac resins (PN resins)1 7. Thermosetting phenolic resins , bearing

Table I -Mechanical properties of lami nate composites using si lica reinforcement- effect of compos it ion and cure temperature

ILSS( MPa) O°Compressive Flexural strength Resin content

Resin a st rength ( MPa) (MPa) (weight per cenl)

200°C 250°C 200°C 250°C 200°C 250°C

PMF-19 II 17 39 104 84 178 30

PMF-23 16 23 74 202 147 190 30

PMF-29 18 24 55 160 166 170 30

PMF-35 22 150 200 31

PMF-29* 34 290 190 30

Resole 19 150

*Reinforcement. E-Glass. a: Number s ignifies the extent of phenylmaleimide-content(weight percent)

Table 2 - Mechanical propert ies of si li ca laminate composites of PMF-29 blended with PAF

Ratio of all yl phenol: malei mide in blend ILSS (MPa) Flexural st renglh (MPa) Compress ive strength. (MPa)

1: 1 23.5 203 253

1:2 25.0 209 222

1:3 23.0 202 2 10

1:4 29.0 204 25 1

1:5 23.5 176 192

0: 1 24.0 16R 157

o

OH ~~ _CH~+3~~;

He,....... """'CH~N PMF PAF H 0

H2""",

Scheme 2- Curing of PMF/PAF blend by Alder-ene reaction

NAIR: NON-CONVENTIONAL PHENOLIC RES INS 2 1

varying extent of propargy\ ethe r groups were synthesised by the Will iamson' s reacti on of novolac with propargy\ bromide and were characterised. The resins underwent curing through Claisen rearrangement of propargy\ ether groups and the ir thermal polymerisation . The rearrangement depends on the substitution on the phenolic ring. When the a-positions are free, rearrangement leads to chromenes, which undergo further addition curing. The activation energy for thermal cure was studied on model bispropargyl ether compounds which confirmed that the rearrangement becomes increasingly difficult when the ring contains e lectron-withdrawing groups IX. The acti vati on energy for the propargyl novolac was substanti all y higher than that for model bi spropargyl ether compounds but was quite independent of the degree of functionalisation . The cure profile, extrapol ated from non-i sothermal DSC kinetics studi es was in league with the results from DMA studies. Thermal characteri sation showed enl anced initi al decomposition characteristics vis-a-vi s resoles. However, the decompositi on was found to be faster for the more crosslinked sys tem. s ince in thi s case, the crosslinks were generated by aliphat ic groups. At highe r c ross linking, the propargylation a lso occurs on the most hindered phenol whose rearranged products are more a liphatic in nature. thus reducing the thermal stabi lity further. The structure of PN resins and the ir c ross linked structure are shown in Scheme 3.

The anaerobic char yie ld was onl y comparable to that from resole except for the advantage in terms of the lack of mass-loss on cure, unlike for resole. The mechanical properti es of the g lass composites of the resins of varying propargyl-content revealed a good consolidation of the interphase. evident from the initia l gain in both interl amin ar shear strength (ILSS) and fl exural strength

l 7. The benefit of the

better res in/ reinforcement interaction was not retained on crosslinking the res in furthe r, wherein the compos ite failed by a combin ati on of fibre debonding and brittle fracture of the mat ri x. The properties of the composites are g iven in Tabl e 3. PN resin s based on low molar mass novolac were viscous resins with viscosity between 2000-S000 cps at SO°c. The viscosity decreased significantly on ra ising the temperature. These res in s are suitable for so lventless impregnati on. The res in cures at

around 200°C with typical gel times of IS-20 min at 200°C, evidenced from the dynamic mechani cal analysis of the resin in parallel plate rheometer. as shown in Figure 2. The gelation point is indicated as the crossing point of the storage modulus (G') and loss modulus (G") in the DMA spectrum. The resin has been successfully used in developing reaction bonded SiC based ceramic components

l9.

Phenolics with Terminal Acetylene Groups

Additi on curable phenolic res in s, bearing terminal ethynyl groups, anchored to benzene ring through phenyl azo linkage (EPAN) were reali zed by a novel and simple synthes is strategy in vo lvi ng the coupling reaction between novolac and m-ethyny l phenyl diazonium sulphate, as shown in Scheme 420.

The di azo coupling was limited to the p-posi ti on of the novolac and occurred to a max imum of 50 mol pe r cent. The molar mass, determined from GPc. showed a downward drift with an increase in degree of functionalisation , attributed to the decreased hydrodynamic volume of the pol ymer. resultin g from its branched structure. Thi s is further confirmed from the diminishing or steady trend in intrinsic viscos ity of the resins with increas ing extent of diazo coup li ng. The molecul ar and thermal characteri st ics of pol ymers with varying extent of functionalisation are g iven in Table 4. These resins showed broad cure exotherlll between 140-240°C due to the curing of acetylene functions occurring by cyclot rimer izati on , acetylene­acety lene add iti on, and acetylene-OH add iti on . The possible cure mechani sms concluded from studi es on a mode l compound viz 4-(3-ethynyl phenylazo ) phenol are a lso shown in Scheme 4 . The polymers exhibited enhanced thermal stabi I ity and anaerobic char in compari son to reso le , and these properties increased furth er with enhanced cross link density. Against a char res idue of 60-62 per cen t for resole, EPAP gives 72-75 per cent char at 7000C. It may be remarked that the rcsole a lready looses a mass of 22-2R per cent during the condensati on cure . whereas EPAN is abso lu tely additi on curable . Isothermal pyrolysi s at 70()-9()()OC showed that complete pyrolysis of the EPA system I~ not achievable unde r these cond iti ons. The evolut ion in e lementa l composi ti on under different pyrol ysi~

conditi ons implied that the pyrolysis occurs mainly by loss of nitrogen gas and hydrocarbon . The higher proportion of char shows the prospects for potenti al app lication of this resi n in ab lative composi ti ons and

22 J SCIIND RES VOL 61 JANUARY 2002

propargyl ether diene ketone Chromenc

Scheme :I - Poss ible crosslinking mechanism for propargylatcd novolac

Table 3-Mechanical properties of UD composites and laminates of PN resins (g lass reinforcement )

Pol ymer refercnce* ILSS of UD compositc (MPa)

PN- 18 :1 5

PN-45 37

PN-54 51

PN-82 34

PN-82/si I ica

PN-82/Carbon

*Number reefers to percent of propargylation

In carbon/carbon composi tes. Non-isothermal kinetic analysis of the degradation reaction confirmed the degradation to be occurnng apparent ly in a single kinetic step, with decomposition rate almost independent of cross link density. Isothermal pyrolysis at 900oe, implied the possibility for the decomposition occurring, mainly by Joss of nitrogen gas and hydrocarbon 12. These polymers yielded considerab ly higher proportion of char, whose XRD analysis confirmed the presence

Mechanical properti es of laminatcs (MPa)

ILSS Flexural strength Compress ivc strengt h (0")

20 169 24X

22 220 25X

23 167 262

20 143

18

38

136

562

of partial graphite like structure, unlike in the case of conventional phenolic res in .

Phenolic Resins with Phenyl Ethynyl Groups

Rep lacement of terminal acetylene groups by phenyl ethynyl function g ives scope for improving further the thermal stabi lity of phenolic networks due to the higher aromatic-content of the crossl inks. Phenyl ethynyl groups have, of late, rece ived lot of attention as a means of thermally chain extending and crossli nking

)

NAIR: NON-CONVENTIONAL PHENOLIC RES INS 23

2122 I h 'd polymers ' . On therma curing, t ey provi e a three-dimensional network exhibiting, an excellent combination of properties, including high glass tranSItIOn temperature. good thermal stability, moisture and solvent resistance, good toughness and mechanical properties. Among the addition curable high temperature resistant res ins, phenyl ethynyl terminated res ins possess excellent long-term stability at high temperatures2J

. Despite the well-known attributes of phenyl ethynyl functional resins, there has not been any attempt to develop phenolic res ins bearing this group.

Phenyl ethynyl funct ional. additi on-curable phenolic res ins were synthes ised by reacting a mixture of phenol and 3-(phenylethynyl )phenol (PEP) with formaldehyde in the presence of an ac id catalyst24

. Relati vely narrow molar-mass di stributed polymers were obtained in good yield. The presence of PEP led to reduced molar-mass and narrow di stribution of the copolymers. The res in underwent thermal curing at around 2S0-27SoC, and cure optimisati on was done by isothermal DM A at 27S°C. The cure mechani sm is by a combination of acetylene addition25 and by addition of phenol to the triple bond as implied in a recent stud/fl . Based on these, the cure mechani sm, depicted in Scheme 5. was proposed. The thermal stability and anaerobic char residue of the cured res in s increased with increase in ethynyl-centent and these properties were better than those of conventi onal reso le res in . These addition cure phenolics provided an overa ll increase in char of about 70 per cent vis-a-vi s resole res in when compared on the basis of uncured res in s. The thermograms of res in s with varying degree of phenyl ethynyl functi onali sati on are given In Figure 3.

The reSInS had, however the di sadvantage of need fo r very high cure temperatu re. During moulding the res in tended to bleed away. This could be avoided by conferring a parti al condensation character to the res in by synthes ising phenyl ethynyl functional resoles (PEFR). The curing of resole at lower temperature, causes the system to ge l. Thus, phenolic res in s, bearing methylol and phenyl ethynyl fun cti ons and curing by a dual mechani sm of both condensation and addi tion were synthes ized by the reacti on of 3-(phenyl ethynyl) phenol (PEP) with formaldehyde under alkaline cond it ion s ~ 7 (Scheme 6). Res in s with varylllg relat ive

concentrati on of the two functional groups were synthesized and characteri zed. GPC and NMR analyses confirmed that the res in contained a mi xture of multi-methylol substituted PEP. It also contained partially chain extended molecules . The characteristics of the resoles are given in Table S. · Typica l high resoleution GPC profiles showing the prod uct di stribution are given in Figure 4.

The res ins underwent a two-stage cure, confirmed by both DSC and DMA analyses. The DMA spectrum shown in Figure S confirms the dual cure nature. The low temperature cure due to methylol condensati on was conducive for early gelati on of the system between 100-ISO°C. The ultimate curing through additi on reacti on of phenylethynyl group. however required heating at 275°C for 2 h. Thi s is much less than the cure temperature recommended for other phenyl ethynyl res in s and may be attributed to

the participation of phenolic OH groups in the cu rin g: mechani sm, as postulated by Wood ef aP6. The cured. res ins exhibited better thermal stability and anaerob ic char res idue, in compari son to a conventi onal resole. The thermal stability and char-yielding property showed a dimini shing trend with enhanced methylo\ substitution. Res in with F/P rati o less than unity offered excellent thermal stability and anaerobic char yield . signi ficantl y hi gher than those for the corresponding novolac. Methylene groups favoured the initi al degradati on, whereas the higher temperature carboni zati on process was independent of the network structure.

Pyrolysis Studies of Addition-cure Phenolics

On comparing the thermal stability of the addition-cure phenoli c res in s belongin g to PMF. P . EPAN and EPAP class . as a functi on of compositi on. it was observed that in a ll the cases. thermal stab ili ty and char yield in the TGA increase with increased crosslin king VIa enhanced fun cti onali sati on:!i Exceptions were noted in the case of the PM F blend with all yl novolac and fo r propargylated novolacs (PN res in s). In these two cases. the thermal stabi I ity decreased with crosslinking due to an increase in the aliphatic content in the cured polymer. The PM F resi ns exhibited least thermal stability and the EPAN systems were the most thermall y stable one l 2

. The maximulll char yield was obtained for EPAN. and PEPFR with low methylol concentration . The comparati ve thermograms can be found in Figure 6 for res in s with

24 J SCI [NO RES VOL 6 1 JANUARY 2002

H H H H

~ ~ ~~ N N N N 1\ II II II

-CH rb CHrT(YCH~rb~H~~rb~H~~ ~ ~H-Tn - ~H- Tn- OH

1 Further crosslinking via unsaturation

Scheme 4 - Synthesis and probable mechanism of curing of EPAN res in

max imum thermal stability in each series. All the res ins exhibited better thermal stability than resole.

Phenolics for Adhesive Applications

Although pheno li c res in possesses several attributes of a high-temperature resistant polymer, it has not proved successful as a high-temperature res istant adhes ive in mass-producti on industri es . T his is mainly because of the inhe rent brittleness of the cured res ins together with the condensation cure mechani sm invari ably accompanied by the evoluti on of vo latil es. Brittleness can be reduced by

blending with rubber, but at the cost o f high­temperature properti es . Li te rature shows that majo rity of the pheno lic based adhes ive fo rmul ati ons make li se of the condensat ion cure of resole resin s. whi ch necess itates the applicati on of hi gh pressure during the adhes ive bonding to get vo id-free glue lines. The limited she lf-life of the resole-based adhes ives at ambi ent temperature is another shortcoming of phenolics. It appears that the attractive features of addition curable phenolics resins could answer some of these problems and are better suited fo r adhesi ve applications than conventi onal phenolics.

NAIR: NON-CONVENTIONA L PHENOLIC RES INS 25

Table 4 - Molecular and Thermal Decomposition Characteristics ofEPAN Resins

Polymer reference

Novolac

EPAN- I

EPAN-2

EPAN-3

EPAN-4

N-content

(weight percent)

0

4.9

6.6

7.5

8.2

Extent o f azo coupling

(mol percent )

0

24

35.8

43

49.7

l00~--------------------~~~ 10

10

~_ 0.1

~

001

" ,­

-'

.... , ......

... .; ..... -.-.-.-/,,,,, ..A".~""'" / ....

/ • .F

/ / I "

I ...... / .:.--

AI' ,'"--4 ,

I I I ,

I , , I I I I

I I

;/ O.OC)1j;;;;;;:--;:..,"TTTTT""I"' ....... .....,..""'""',..........,.. ......... T'"'""T',...,...."['"'"'"""'I!-rl-

a 10 15 20 25 30 35 40 45 50

Time (min)

..:3 0.1 ....!

" "ll II>

~

0.01

0.001

Figure 2 - Dynamic mechanicai ana lysis (parall el plate) of PN reslll in air at 200°C at a frequency of 2 Hz. Stress 50 Pa

Film Adhesives

High molar mass acry lic copolymers with fewer concentrat ions of pendant phenolic groups served as effective thermoplasti c film adhesives for bonding metallic and rubber subst rates

29 Polar

interactions incl uding H-bonding, caused by the imido-phenolic groups a re thought to be responsible for the bonding. The adhesives properties and their temperature retention could be improved furthe r on transforming these film adhesives to thermosets by reactive blending with epoxy resin or through etherification of the phenolic OH groups to

30 h t' h propargyl ether. cyanate. etc' . T e st ructure 0 t e terpolymer can be found in Scheme 7.

(11) in THF,

30°C

0.1 55

0.134

0.085

0. 107

0.074

100

90

~ ~

~80 ~ "iii :::J "t:I .~ 70 ~

60

Molecular weight by GPC (g/mol)

Mn Mw Mp

700 1970 1840

520 1570 2030

430 1460 1950

350 1240 1330

330 11 80 I I I ()

. . ":: .... \',\ ',',\

• '" RESOl

100 200 300 .00 500 600 700

Temperature (OC)

Figure 3 - TGA of PEPF novolac resins in N2• heating rate 10°C/min . Number refers to the percen tage of phenyl ethynyl

group in copolymer with phenol

High Temperature Adhesive Based on Maleimide­

ji./l1ctional Phenolics

The maleimide-functional novolac phenolic resin (PMF) was evaluated for the adhesive properties. like lap shear strength (LSS) and T-peel strength (TPS ) using aluminium adherend s, when thermall y self-cured and co-cured w ith epoxy resins ' l. The adhesi ve properti es of the se lf-cured resin , a lthough inferi or at ambient temperature, improved at high temperature and were fo und to depend on the cure conditi ons.

26 J SCIIND RES VOL 61 JANUARY 2002

OH

+ © HCHO

OH ~CH~CH~H H~ QJ

Further addition and crosslinking Heat .. !

~H2""""'-

OH H2

\

Scheme 5 - Synthesis of PEPF novo lac and mechanism of crosslinking

When co-cured with epoxy resin , the adhesive properties improved significantly and showed a strong dependence on the nature of the epoxy resin used, stoichiometry of the reactants , the concentration of imide groups in the phenolic resin and on the extent of polymeri sation of the maleimide groups. Optimum adhesive properties were obtained for novolac resins with moderate concentration of maleimide groups, taken on I: I hydroxyl-epoxy stoichiometry with a novolac epoxy resin (EPN). The cure chemistry is shown In Scheme 8. Typical adhesive values measured at varying temperatures are given in Table 6 .

In comparison to the conventional novolac (PF), the imide-novolac contributed to improved adhesion and better adhesive property retention at higher temperature when cured with the epoxy system. The comparative thermo-adhesive profile of the PMF-epoxy and novolac-epoxy manifesting this difference, is shown in Figure 7. Complimentary OMA studies led to the conclusion that the superior

thermo-adhesive profile of the PMF/EPN system is contributed by the secondary forces of attraction induced by the polar imide groupsll. The partial polymerisation of the imide helps consol idate these secondary forces of attraction , both within the resi n system and at the interphase. The enhanced bonding characteristics of the PMF-EPN system were also manifested in better inter laminar shear strength (lLSS) of the UO composite based on it (8S.5MPa) as against that of the novolac-cured EPN (68.SMPa). The study also implied that the completion of epoxy-pheno lic reaction and moderate crosslinking through the maleimide polymerisation are conduciv,e for achieving optimum adhesive properties .

Although the adhesive properties of PMF/EPN systems were good, it could be improved further by blending with elastomeric mod ifiers li ke low- and high molar mass carboxyl-termi nated butadiene-acrylonitri le polymer (CTBN-L and CTBN-S respectively), epoxidised hydroxy-terminated polybutadiene (EHTPB ) and epoxidise butyl acrylate-

NA IR: NON-CONVENTIONAL PHENOLIC RES INS 27

~ HOCHMCH~H.pH HOCH~HaOH OH + HCHO N.OH frCH~H

~ H 6 ~ + +

o © ! !"© \ © I

PEPFR

Further addition and crosslinking

Scheme 6 - Synthesis of PEPF resole and probable mechan ism of crosslinking

Table 5 - Characteristics of PEPF resole resins

Fomaldehyde/p Relative ratio Percentage of different components in PEPFR resin Ti Char al

700"C henol ratio -CH2-0-/-CHr

from NMR Monomer

2.7 14.6

1.5 3.5 I R.7

2.0 5.0 14.9

acrylonitri le polymer (EPOBAN)32. The adhes ive properties were found to Jepend on the nature and concentration of the elastomer as well as on the nature of the thermoset matrix being modified . Adhesive property of the pure PMF was improved tremendously , whereas that of the epoxy co-cured phenolic system cou ld be improved only marginally . The requirement that the matrix should have a minimum ductility to be effectively toughened by the e lastomer was substantiated in thi s study. SEM analysis showed the phase-separated morpho logies for the e lastomer-mod ified systems. The size and state of the precipitated secondary phase depended on the molecu lar weight as well as the chemical

from GPC (OC)

Mono and Higher inTGA

dimethylol o ligomers

49 36.4 365 77 .4

50.3 31.0 360

54.7 30.4 355

react ivity of the added e lastomer. For the CTB -S modified systems the dispersed phase was rather continuous and this morpho logy led to the maximum improvement in adhesive properti es .

High Performance Adhesive Based on Alder-ene Phenolic Adducts

Another strategy adopted to des ign a high­temperature phenolic adhesive was based on the reactive blend of an a llyl-functiona l novolac of bisphenol A (ABPF) and bismale imides (BMI). Low molar mass ABPF was synthesised from o,o'-d iall yl bisphenol A and fonna ldehyde3

.1. ABPF was reactively blended with BMls and cured through Alder-ene

28 J SCIIND RES VOL 61 JANUARY 2002

CH2 CH + CH;z--CH + W'O g=B~ I CN

Q 1 AIBN HPM

Scheme 7- Synthesis of phenol-functional acrylic terpol ymer

c: o ~ C (!) o c: o o

6

Methylol-monomer ~

Methylo-dimer J.4ono

____ --__________ ~_jg_o_m~~ ~ 8 10 12 U 16 18 20 22 24

Elution Time(min)

Figure 4- High resolution GPC prolile of PEPF resole

. I ' h ~ reaction at 11g temperatures· . Bisphenol A bi smaleimide (BMfP)-ABPF blend was chosen as the representati ve system fo r studying the cure characteristics by DSC. FTfR. and DMA. These studies evidenced the exis tence of different cure reactions in league with published in formatio n on the cure chemistry. The cure schedule of the blend was optimised by isothermal DMA. DSC. and FTIR. together with the actual evaluation of ad hesive properties (LSS) of the system cured at different

0.20 - 0.15

0.10

0 .05

0 .00

~,,-'-.~~~~-r-r~~~~-.-

o 50 100 150 200 250 300 350 400 450

Temperature (0G)

FigureS- DMA of PEPF resole (PEPFR ) re~ in showing l\Vo­

stage curing

100+'_; __ =_==~~~-------------1

90

~80 ~ co 'ij

~ 70

60

---

200

PMF

) Resole

300 400 500 liOO 700

Temperature (0C)

Figure 6- Comparati ve thermograms o f various addi tion-curahle phenolic resins in N2• heating rate I neC/min

condi ti ons of temperatures and time. Lower cure convers ion resulted in ad hesives with poor cohesi ve strength whereas, higher extent of react ion resulted in networks with high brittleness and consequently red uced adhesive properties. The composi tional dependency of adhesive properties an d the ir high temperature retention are shown in Figu-e 8. Optirnulll ad hes ive properties were obtained for the J: J maleimide:all ylphenol stoichiometry wi h a resu ltan t network struclUre having moderate c ros~;link densityl-I The same proportion gave maximu m high temperature ( J 500e) retenti on.

Since the cure patterns and behaviour of the blend were in conformation with the establ ished Alder-

, -

NAIR: NON-CONVENTIONAL PHENOLIC RESINS 29

000 l&c:l~cH2-h ~ OH

PMF

1150- 170"C

PMF, Self-cured

+

EPN

PMF-EPN Crosslinking

Scheme 8-Mechanislll of cure reactions of PMF and PMF-epoxy bl end

ene cure chemistry, the system was assigned a simi lar cure mechanism, and the possible network structure could be concluded from the reactant stoichiometry and the cure schedule. The poss ible cure reactions and the resultant cross lin ked structures are shown in Scheme 9. Low temperature and low stoichiometry led to network dominated by structures 4 and 5. The effect of BMI structure on the adhesive properties was evaluated , using four different BMIs, namely bismaleimido phenox y phenylpropane (BMIP), bismaleimido henylmethane (BMIM), bismaleimido phenyl ether (BMIE) and bismaleimido phenyl su lphone (BMIS)3'i . It was

observed that LSS of BMI-ABPF systems per se were not significant ly high. However, all the sys tems exhibited remarkably high retention of LSS (> I ()() per cent ) to 250°C. The polar groups in BMIS and BMIE contributed to better wetting and consequently better LSS at ambient. The high-temperature LSS retent ion was comparatively better for BMIP and BMIS systems.

With a view to improve the ambient temperature performance, the BMIP-ABPF system was tou ghened with advanced thermoplastics like polysulphone (PS) and polycarbonate (pel '. Both the additi ves remarkably enh anced the LSS propert ies due to the improved ductility of the matrix. Maximu m

30 J SCI IND RES VOL 61 JANUARY 2002

Table 6-Adhesive properties of various PMF resins blended with EPN (cure: 170°C/30 min and 200°C/30 mill )

System Lap shear Lap shear Retention of Lap shear Re tention of T-pcc\ at

strength at RT strength at LSS at ISO"C strength at Lap shear amhicnt

(MPa) ISO"C

(MPa)

NOVOLACIEPN 15.5 7.2

PMF-19IEPN 15.3 5.6

PMF-29/EPN 17.0 14.3

PMF-42IEPN 11.3 8.2

20.0 PMF/EPN • •

17.5 •

15.0 ... -- ------- ---- ---- • Novolac/EPN

'i 12.5

!. CIl CIl ...J

10.0

7.5

5.0

2.5

•

25 50 75 100 125 150 175 200 225

TEMPERATURE (Oe )

Fi gure 7- Thenno-adhesive profiles for PMF/EPN and novolaclEPN

improvement in LSS was attained with 20 pbw of PS and 10 pbw of PC in the system. A four-fold increase was obtained for PS modified system. The results can be found in Table 7. The propert ies decreased beyond 200°C due to softeni ng of the additives . The performance advantage of the additives can be understood from Figure 9 by comparing the thermo-adhesive profiles of different systems. The properties decrease beyond 200°C in the case of toughened systems , whereas the properties are retai ned for the neat resin well above 250°C. The relatively better performance of a homogeneous blend resulting from PS is manifested in significant a improvement in the propert tes at ambient which, however, decreases drastica lly at

(per cent) 17S"C (MPa) strength at temperature

47

37

84

73

5.0

4.5

«I 4.0 n.. ~ CIl ~ 3.5

3.0

2.5

0.5

I 75"C

(per cent )

3.1 20

2.8 18

8.0 47

3.9 35

......... ~~ ..... .. ........ -... -~

1.0 1.5 2.0 2.5

BMIP I ABPF equivalent ratio

(kN/Il1 )

3.0

0.40

0.43

0 .56

O.OX

120

u o 110 ~

iii C/l C/l ~

'0 100 .~

C

90

Gl a; a:::

Figure 8- Dependency of aclhesi ve properties on reaClalll stoichiometry for ABPF/BMIP system ( a ) at amhienl.

(h) at 150°C, ( c ) percent retention at ISO"C

about 200°C in contrast to the PC-modified system. SEM ana lysis of the modified formulati ons corroborated thi s relative difference as resulting from different morphological features in cases of PS and PC. Comparatively uniform distribution of the thermoplastic component leading to the co-existence of the resin -rich and additive-rich phases was found [ 0

enhance the toughness of the PS-syst~m. whereas precipitation of larger particles in PC-modif ied system was Jess effi c ient for enhanc ing the ad hesive properties. The morphological features in SEM were corroborated , in turn , by DMA analyse~ of the cured

NAIR: NON-CONVENTIONAL PHENOLIC RESINS 31

§;r1 · rZ..h ~r ~y

ABPF (1) 8Mt (2)

8O-160.C! ( ene addition )

0{r3or~ CH=-220

0

, tH3 H~ BUt ( Wagner-Jaureoo )

225-27S0 Cj ElM I

( Diets - AIdef )

Where, CH

jN..,,~ .LR RZ~VoQ-CH3'

Further crosslinbng

Scheme 9-Possi bIc crossIill kin g rcacti ons of BM IP-A BPF for I: I stoichiomctry

matrices. All the sys te ms exhibited high T g (- 300°C) and good thermal stabil ity. All these provided good adhesion ti ll 250°C and can be used fo r moderate load bearing ap plicati o ns at hi gh temperatures , as is required in a ircra ft and defence app licati ons.

The Alder-ene adducts w ith different BMIs and varying sto ic h iometry were eva luated fo r thermal stability and degradat ion behav iour by thermogravimetri c ana lys is (T GA) .1(,. T G A of the b lend of ABPF and BMIP w ith vary ing male imide to a ll y lpheno l rati o ind icated that the the rmal

stabi lity of the system was onl y marg ina ll y improved by an increase in BM I sto ich iometry. TGA showed a two-stage decompositi on patte rn fo r BMIS system and a sin gle stage for a ll the other three. The the rmograms o f BMIM and BMIE were identical and superior to that of BMIS. the latter showi ng a re lat ivei y poor pe rfo rmance at lower temperat ures. Compared to the BM I-adduct o f monomeri c di a ll y l bi spheno l A (DA BA), the po lyme ri c analogue viz .. ABPF system ex hibited better thermal stabili ty. Non -isothe rma l kinet ic ana lyses of the different systems showed the decompos iti on occurring in at least two kinetic steps. The computed act ivati on e nergy ex hi bi ted a d irect

32 J SCI IN/) RES VOL 6 1 JANUARY 2002

Table 7 - Ad hes ive properties of unmodi fied and mod ified adduct of BM IP/ABPF

System* Lap shear strength (M Pa) T-peel strength (k N/Ill)

RT 15DoC 20noc 250°C

Unmod ifi ed 4.1 4.X 5.0 5.2 poor

PS-IO 13.9 11.0 7. 1 4.6 0.32

PS-20 19.3 16.5 11.0 6.4 0.38

PS-30 14.8 1:17 In.7 5.5 0.40

PC-I O 11 .3 9.5 8.4 s.n poor

PC-20 8.8 6.9 6.2 5.4 poor

PC-30 8.3 6.7 5.5 3.5 poor

Number represents the concentrati on of additi ve in parts per hundred parts (phr)

20 • .. .......... ... .. 18 PS-20

16

14 t? Cl- 12 ~ ...... (/) 10 · (/) PC-1 0 - ' 4 . ....J

8 .. .. ...... " " " ... 6 •

Neat .• .... ...... •...... ... .• 4 • .... ..

2 0 50 100 150 200 250

Temperature Cc)

Fi gure 9- Perfo rmance advantage for ABPF/BM IP toughened wi th PS and PC

corre lati on to the re lat ive therma l stab ili ty of the systems.

Future Scenario

T he foregoing d iscuss ions have attempted to

consolidate the recent de ve lopme nts in the fi e ld of nOll-conventional phenol ics . Phenol ic res in sti II re ta ins a lot of research in terests. Innova ti ve researc h should be focused on ove rcoming the shortcomi ng of these syste ms tn terms of processability and ox idati ve resistance. T he introduction of additi on-c ure phe nol ics answers onl y part ly to the problems. Although some of these systems show good prospects for app licati on in aerospace structura l and thermo-s tructura l

compos ite applicat ions with defin it advantages. the prohibit ive cost and the high cure tempe rature in many cases may impede the ir pe rvasion to the area of conventi ona l res in s. Need is fe lt for further research to bring down the c ure tempe rature to the leve l of common thermosets. It appears possib le to de ri ve high pe rfo rmance adhes ives through a prope r des ign of' the pheno lic res in, stra tegy of reacti ve and non-reacti ve ble nding, and con tro ll i ng the c ure and morpho logy of the network. Thi s area is like ly to e vince more researc h interests.

Acknowledgements

At many phases of the work escr ibed he re the author has associated with hi s co lleagues Dr R Lbi nd u and Dr C Gouri . The autho r is grate ful to Dr R Ra mas wamy and Dr K N Ninan for the e,l couragement and support.

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