Mechanical Characterisation of Natural Fibre Reinforced Plastics
P. Vieira1, C. Romão2, A.T. Marques3 J.L. Esteves3
1 INEGI – Institute of Mechanical Engineering and Industrial Management, Leça do Balio, Portugal 2
IPV - Institute Polytechnic of Viseu, Viseu, Portugal
3 FEUP – Faculty of Engineering of University of Porto, Porto, Portugal Keywords: Natural Fibres, Sisal, Hemp, Composite.
Abstra ct
The work d es crib ed i n thi s p ap er refers t o th e m ech ani cal ch aracteris ati on of fib re rein fo rced com posit e m at eri als with an epox y m at rix . Sisal and Hemp fib res are us ed wit h di fferent su rface t reat ments .
The com paris on b et ween a reference gl ass fibre rei nforced compo sit e an d the oth er n atu ral fib res comp osit es i s m ad e. It i s also pres ented th e in fl uence o f the su rface t reatm en t in t he mechanical charact erizati ons o f t he natu ral fi bres
A bri ef d es cri ptio n of th e p rod uct ion of th e n atu ral fi b res compo sit e mat eri al s is made.
Introdu cti on
Tod a y t he search for n ew, rec ycl abl e and ren ewab le m at erials is l eadi n g th e researchers i n n ew wa ys . Nat ural prod ucts are em ergin g and som e research i s startin g in this m att er. The work p res en ted h ere s ho ws t he com paris on bet ween Glass, Si sal and Hemp [1-2]. Som e pl ates were m ad e u sin g tho se mat eri als as rei nforcem ent and an epox y resi n as m atrix . The n at ural fi bres were cleaned in ord er t o remov e straw and ot her cont am inati n g agents. Aft er t hese comp ariso ns, we m ad e a s urface t reatment in t h e h emp fib res to s ee i f it i mpro ves th e adh esi o n fib re - m at rix . Th e t reatm en t us ed i s call ed m ercerizati on, an d is d es crib ed b elo w. The fi bre - mat rix adhesion is o f th e mo st impo rt an ce, and t he fin al result o f t h e com posit e i s d ep en din g im mens el y on this facto r. Th e cl eani n g o f th e natural fib res is also v er y i mportant. C ont ami natin g agen ts can als o dest ro y t he compo sit e or redu ce it s m ech an ical p ro perti es d rasti call y.
Fibre p repa ra tion
Glass Mat fib re - the gl ass fib re is d elivered in ro les. To prepare the m at to introdu ce in the mo u ld, we hav e to cut it in pieces of 15 0x 100 mm2. The numb er of la yers i s v er y im po rt ant , to maint ain th e fibre v olum e fracti on (30 % i n v olum e). Nat ural Sis al fib re - the fi rst st ep of an y natu ral fib re is th e cl eanin g. When th e y were d eli vered, th e y were ful l o f grass and ot h er cont amin ati n g agents . Aft er t he cl eanin g proces s, it was necess ar y t o cu t the fib res in app rox imat el y 30 mm l en gt h pieces, to b e abl e t o agglom erate th em i n th e m oul d and m ake a mat. Makin g th e mat i s th e nex t s tep. After cal cul atin g th e v olum e fracti on we in trodu ced t he fi bre al eat or y i n th e moul d, clo se it and p ut it in the p ress. Aft er 10 minut es we can rem ov e t he m at.
The n ex t fi gu res s ho w all t he p ro cess.
Figure 1: The mould Figure 2: The natural fibre mat
Surface T rea tmen t
With the h em p fi bres a su rface t reatm ent h as b een d one t o in creas e th e fib re/matrix adh es io n. This treatm ent (mercerizin g) i s m ad e i n s om e st ep s. First step is to p erform an immersi on a b ath . Two hou rs i n a s olut ion o f 8 % in v olum e of Sodi um H yd rox id e (NaOH) with dis ti lled wat er. Duri n g t his proces s th e bath was sti rred cont inu o usl y usin g a m ech anical agi tat or (fi gure 3 ). At th e end th e soluti on p res ent ed a yell ow col ou r b ecause of t h e su bst an ces rem ov ed from th e fib re. Th e n ex t step is cl eanin g t he fib res sev eral tim es i n a distill ed wat er bath, until th e water is clean. Aft er s ev eral bat hs, a n eut ralizin g so lutio n of 25 % i n volum e of aceti c aci d is us ed. Agai n, m o re two o r t hree b at hs with dis till ed water and th e t reatm en t is fini sh ed. To d r y t h e fib res we l eft t hem 5 d a ys at ambi en t temp erature, and th en six ho urs at 6 0º C i n an o ven (fi gu re 4 ). [3-5]
The s urface of n atu ral fib res i s h yd rop hilic, not givi n g go od ad hesion to th e h yd rop hobi c ep ox y mat rix , so t he merceris ati on t reatment tr y t o imp ro v e th e adh esi on fi bre/m at ri x b y th e decreas e of h yd rop hili c ch aract er o f t he fib res.
Figure 3: Sodium Hydroxide bath Figure 4: Fibres in the oven
Manufa ctu ring a pl ate
To m an ufactu re a pl at e we us ed th e com pressi on moul din g t echni qu e. Th e m oul d that we can see in fi gu re 1 has a cavit y o f 150x 10 0x 4 mm3 an d is made o f alumi nium . Th e fi rst step is to cl ean th e surface and th en app l y th e mou ld rel ease agent QZ13 from Ci ba. After t he prep aratio n o f th e resin, we int ro du ce it in t he mould (fi gu re 6). The m at is th en p laced in th e moul d (fi gure 7) an d th e moul d clos ed . At l ast ev er ythin g i s pl aced in th e ho t pl at e p ress, where th e combi n atio n of pres sure and t em p erat ure m ak e th e pl at e.
Figure 5: Mixing the resin
Figure 6: Introducing the resin Figure 7: Placing the mat
Aft er 1 ho ur in t he press , th e m ould h as been op en (fi gure 8 ) and th e t rimmi n g o f the pl at e was m ad e. Three ex ampl es o f t he pl at es are s hown in fi gure 9 , on e fro m each mat eri al . In al l pro cess there were some p ro blems th at had to b e sol ved. Th e first was m aki n g a homo geneous m at u sin g th e cutt ed fib res. Fo r 30 % v olum e fractio n of hemp fib re we cal cul at ed th e need o f 25 grams . That am ount of fib re befo re com pres sed is in volu me 5 tim es b igger t han th e mat we n eed. The probl em wasn ’t comp ressin g, the p ro bl em was m aint ain in g t he vol u me fraction in all t he plat e. To sol ve t hat probl em , la yers were m ad e wi th 8 grams each. With th ree la yers we coul d make a mat , and th at redu ces th e prob abil it y of h avin g a v er y different volu me fraction i n all th e p lat e. Void co nt ent is an oth er p robl em. Becaus e of th e limit atio n of th is pro ces s , we estim at e b y m easu rin g th e plate an d wei ghi n g it th at th e void cont ent is arou n d 10 %.
Figure 8: Open mould Figure 9: The three plates
To p ro du ce th e p lat es us ed fo r th e co m pariso n, we us ed th e epox y resin Reap o x 520/ D52 6 from R EA In d ust ri es with th e followin g characterist ics:
Ten sil e St ren gt h (M Pa) = 71 ; Mo dul us (GPa) = 3.5; Den sit y (g/ cm3) = 1. 12.
To p rod uce th e h em p plates us ed fo r testin g t he s urface t reatm ent, we us ed t he epox y res in Reap o x WOOD RX8 fro m REA In du stri es with th e follo win g ch aract eri sti cs:
Figure 10: Test setup
Ten sil e St ren gt h (M Pa) = 50 ; Mo dul us (GPa) = 3.0; Den sit y (g/ cm3) = 1. 13.
We used a di fferent resin b ecaus e this n ew resin h as t wi ce t he ruptu re d eform ati on that t h e fi rst on e. We made on e o r t wo tes ts with t he 520 and th e d eform ati o n wasn ’t eno u gh for th e fragmentation t ests pl an ed t o m ak e with this fi bers .
Mechan ical T es ts a nd Results
Fo r th e mech ani cal t ests we us ed an INS TRON 420 8. We made the t es ts acco rd in g t o th e IS O 5 27 -4 s tandard . We us ed a 1 0 0 kN l oad cell an d a 2 mm/min tractio n s p eed. Fi gu re 1 0 s hows the setup .
In ou r wo rk, we m ad e t he comp ariso n b et ween gl ass , si sal and hem p. In th e h emp fi bres we have also d o ne a comp aris on wit h nat ural h emp fi bres and fib res s ubj ected to a m ercerizatio n treatm ent th at is des cri b ed ab ov e.
The result s (t abl e 1) show th at th e natu ral fi bres are still v ery far from th e gl ass fi bre. If we loo k to th e t ensil e st ren gth, t h e val ue obt ain ed b y th e n atu ral fi bres is even inferio r th an th e mat rix an d mo re o r less four tim es s mal ler th an t h e gl ass fi bre. If we l ook to t h e modul us, th e sis al fi bre comp osit e val ue is h al f o f th e gl ass fib re comp osit e, bu t the h emp fib re com p osit e is onl y 2 0 % i n ferio r.
Co mp osi t Tensil e S tren ght
[MPa ]
Modulus [GPa]
Defo rma tion [% ] REAPOX 5 20/ D52 6 + 30% gla ss f ibr e 192. 4 ± 20 .5 10.0 ± 0.9 2.5 ± 0 .3 REAPOX 5 20/ D52 6 + 30% sis al fi br e 50.9 ± 5.5 5.5 ± 0 .5 1.3 ± 0 .3 REAPOX 5 20/ D52 6 + 30% h emp fi bre 51.1 ± 6.8 7.8 ± 0 .5 0.8 ± 0 .1 REAPOX 5 20/ D56 70 3.5 4
Table 1: Comparison between the mechanical results of the three fibres
A bad fi bre-m atrix adhesion co uld b e res ponsi bl e fo r th es e results. Tabl e 2 sho ws the res ults o f th e tes ts mad e wit h th e hemp fib res. The s urface t reat ment made t o the fib res did n’t imp rov e th e adh es ion b etween fib re and m at rix . In th e res ults we can s ee a l oss o f 1 5 % in th e Tensil e S tren gth an d a l ost of 30 % i n th e Mod ulu s usin g thi s t reat ment. In b oth cas es th e Tensil e St ren gt h is in ferio r t o t he on e o f th e resin, bu t th e Mod ul us is hi gher.
When an al ysi n g th e crack, we cou ld obs erve th at th e adh esi on was reall y b ad. In some crack s, we co uld s ee sm all b ites of fi bre compl et el y remov ed from th e mat rix . That s hows t hat th e ad hesion is i nex ist ent, and th at t he fi bre is weakenin g the mat rix in stead o f rein fo rcin g it.
Co mp osi te % Fib re [% ] Tensil e S tren gth [MPa ] Modulus [GPa]
Defo rma tion [% ] REAPOX WOOD RX8
+
Hemp witho ut t reatment
25 30 35 45.6 ± 0.9 45.1 ± 6.3 47.1 ± 3.6 5.4 ± 0 .4 6.2 ± 0 .8 6.7 ± 0 .5 0.98 ± 0.0 3 0.90 ± 0.2 2 0.81 ± 0.1 5 REAPOX WOOD RX8 +
Hemp wiht tr eat ment
25 30 35 37.9 ± 1.9 38.3 ± 3.9 40.7 ± 3.5 3.8 ± 0 .3 4.4 ± 0 .6 4.6 ± 0 .3 0.69 ± 0.0 2 0.64 ± 0.1 6 0.56 ± 0.1 0 REAPOX WOOD RX8 50 3.0 7
Table 2: Comparison between the mechanical results of the treated and the natural fibres
Con clusion s
In this st ud y, i n whi ch we co mp are th e compos ites mad e wit h gl ass , sisal and h em p fib res, we realize t h at we can ’t pi ck a n atural fib re, mix it wi th resi n and o bt ain a com posit e m at erial . Nat ural fib res are in realit y di fficult to pro cess . Th e result s obt ain ed are far fro m goo d, i n som e cas es t he resul t o f th e mech ani cal p rop erti es of t he comp osit e were i nferio r to th e on e of t he m atrix .
It is n ecess ar y to poi nt th e res earch i n t h e cl eanin g and in t he su rface treatm ent o f the fi bre. Onl y with goo d s urface treatm ent we can obt ain a good adh esi on fib re-mat rix , and th at is o ne k e y-poi nt to obt ai n a good com posit e. In ou r paper we mad e the st ud y us in g th e mercerizin g treatm ent o f th e fib re su rface. That t reatm ent pro ved to b e in ap propri at e, b ecaus e it d idn’t imp rov e th e m ech ani cal prop ert ies, but inst ead it wors ened t hos e prop erties . Fut ure works sh ou ld be m ad e to sol ve this pro bl em.
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