| Sažetak | U radu je istraživan utjecaj različite strukture umreženja uzoraka gume te utjecaja udjela
reciklirane otpadne gume (granulirani reciklat) na brzinu bubrenja u nafti te na mehanička i
toplinska svojstva budući da je razmatrana primjena gume kao sastavnog dijela bubrećih
pakera. Uzorci umrežene (vulkanizirane) gume pripremljeni su iz dvije vrste kaučuka, etilenpropilen-
dienskog (EPDM) i akrilonitril-butadienskog kaučuka (NBR). Pripremljeno je ukupno
pet serija uzoraka; jedna serija s akrilonitril-butadienskim i četiri serije s etilen-propilendienskim
kaučukom.
NBR uzorci međusobno se razlikuju s obzirom na udio umreživača, sumpora, tj. s
obzirom na gustoću umreženja te su pripremljeni sa i bez gumenog reciklata (NBR i SBR), a
označeni su kao: NBR/ZD/1,25, NBR/RR/30, NBR/RR/30/N550, NBR/S/0,5 i NBR/ZD/0,75.
Zatim, pripremljena je serija EPDM uzoraka sa sredstvom za bubrenje, a uzorci se međusobno
razlikuju s obzirom na gustoću umreženja i s obzirom na udio reciklirane gume (EPDM-1,
EPDM-2, EPDM-3, EPDM-4, EPDM-5 i EPDM-6). Potom su pripremljene tri serije EPDM
uzoraka gdje se prva i druga serija razlikuju s obzirom na gustoću umreženja dok je udio EPDM
reciklata ostao isti (1EPDM-R0, 1EPDM-R50, 1EPDM-R100, 1EPDM-R150, 2EPDM-R0,
2EPDM-R50, 2EPDM-R100, 2EPDM-R150). Treća serija EPDM uzoraka (3EPDM-R0,
3EPDM-R25, 3EPDM-R50 i 3EPDM-R100) razlikuje se u odnosu na drugu seriju uzoraka s
obzirom na vrstu gumenog reciklata, jer je dodan miješani gumeni reciklat (EPDM, CR, NR).
Struktura umreženja svih istraživanih uzoraka karakterizirana je određivanjem
ravnotežnog stupnja bubrenja (αs) u nafti te određivanjem Mooney Rivlinove konstante (2C1),
a iz navedenih parametara izračunata je gustoća umreženja (ν) i molekulske mase mreže (Mc).
Također, uzorci su karakterizirani bubrenjem u nafti na povišenoj temperaturi (90 °C) u uređaju
za mjerenje bubrenja koji simulira realne uvjete primjene budući da je ograničen prostor za
bubrenje. Istraživanim uzorcima određena su mehanička svojstva prije i nakon starenja na
povišenoj temperaturi (100 °C) gdje je praćen proces termooksidativne degradacije, budući da
pakeri tijekom primjene moraju biti stabilni jer podnose određene povećane tlakove i povišene
temperature. Stoga su uzorcima određene prekidne čvrstoće, prekidno istezanje i tvrdoća prema
Shore A klasifikaciji. Toplinska postojanost istraživanih uzoraka okarakterizirana je
termogravimetrijskom analizom (TGA) provedenom u temperaturnom intervalu od 23 °C do
600 °C određivanjem karakterističnih veličina tijekom razgradnje materijala. Pritom su
određene temperature početne razgradnje (T95) kada se razgradi 5 % mas. uzorka, temperatura
pri maksimalnoj brzini razgradnje (Tmax), ugljenizirani ostatak (mk) te brzina maksimalne
razgradnje (vmax). Zatim, istraživanim uzorcima gume je analizirana morfologija, odnosno
heterogenost i homogenost pretražnim elektronskim mikroskopom (SEM) budući da uzorci
sadrže homogeni i heterogeni gumeni reciklat. Kako je gumeni reciklat dodan u uzorke u obliku
granuliranih čestica i time preuzeo ulogu punila, to je homogenost, odnosno heterogenost
uzorka, od iznimnog značaja za mehanička svojstva budući da heterogeni materijali najčešće
imaju loša primjenska (mehanička) svojstva.
Iz analize rezultata ravnotežnog gravimetrijskog bubrenja i bubrenja u uređaju za
mjerenje bubrenja za uzorke akrilonitril-butadienskog kaučuka (NBR) može se zaključiti da se
bubrenje NBR guma pospješuje dodavanjem gumenog reciklata koji je uspostavio dodatne
interakcije (sekundarne veze) s naftom kao medijem u kojem su bubreni uzorci. Ispitivanjem
ravnotežnog stupnja bubrenja EPDM uzoraka sa sredstvom za bubrenje došlo se do zaključka
da je usprkos različitoj strukturi umreženja i različitom udjelu reciklata presudan utjecaj na
stupanj bubrenja imalo sredstvo za bubrenje budući da uzorci koji ga sadrže znatno jače bubre
u odnosu na uzorke bez sredstva za bubrenje. Stupanj bubrenja tih uzoraka (αs) bio je veći od
40 do 60 %. Općenito, bubrenje u uređaju za mjerenje bubrenja u uvjetima povišene
temperature znatno je intenzivnije i višestruko brže, odnosno prisutno je povećanje stupnja
bubrenja za čak do 50 %. Toplinska postojanost EPDM uzoraka istražena
termogravimetrijskom analizom upućuje na zaključak da su uzorci svih triju serija podjednako
toplinski stabilni. Starenjem EPDM uzoraka na 100 °C došlo je do nastajanje dodatnog
umreženja što ima za posljedicu smanjenje bubrenja, smanjenje elastičnosti i povećanja
čvrstoće i tvrdoće uzoraka. SEM analizom može se zaključiti da je kod uzoraka s homogenim
i heterogenim reciklatom nastala različita struktura (morfologija) istraživanih uzoraka. Nastala
heterogena morfologija kod uzoraka 3. serije gdje su vidljive šupljine u strukturi posljedica je
nekompatibilonosti EPDM matrice i reciklata, tj. posljedica je neuspostavljanja međusobnih
interakcija. Zbog heterogenosti uzoraka povećava se moć apsorbiranja nafte u strukturu
uzoraka, ali se dijelom narušavaju mehanička svojstva. |
| Sažetak (engleski) | Within the doctoral thesis, the influence of a different crosslinking structure of rubber
samples and recycled rubber content on swelling speed of samples in crude oil and
mechanical/thermal properties has been investigated, concerning the tested rubber samples
being a possible material for swelling packer elastomer make. Samples of crosslinked
(vulcanized) rubber are made out of two different types of caoutchouc, ethylene-propylenediene
monomer (EPDM) and acrylonitrile-butadiene (NBR). A total of five series of samples -
one with acrylonitrile-butadiene and four with ethylene-propylene-diene caoutchouc have been
prepared.
Nitrile rubber samples were vulcanized with different crosslinking agent (sulfur),
meaning that a different crosslinking density was imposed as well. Recycled rubber, containing
NBR and SBR rubber granules was added only to some samples within the series. Samples
were named as follows: NBR/ZD/1,25, NBR/RR/30, NBR/RR/30/N550, NBR/S/0,5 and
NBR/ZD/0,75. The EPDM series of samples was vulcanized with swelling agent, differing one
from another regarding the crosslinking density and recycled rubber content: EPDM-1, EPDM-
2, EPDM-3, EPDM-4, EPDM-5 and EPDM-6. At the very end of the investigation, last three
series of EPDM were made. The second one differs from the first only in crosslinking density,
while the EPDM recycled rubber content remained the same (1EPDM-R0, 1EPDM-R50,
1EPDM-R100, 1EPDM-R150, 2EPDM-R0, 2EPDM-R50, 2EPDM-R100 and 2EPDM-R150).
The third series (3EPDM-R0, 3EPDM-R25, 3EPDM-R50 and 3EPDM-R100) differs from the
second one in recycled rubber composition having three types of rubber- EPDM, NR and CR.
Structure of the crosslinked rubber samples was characterized by determining the
equilibrium swelling degree (αs), swelling in a crude oil and defining the Mooney Rivlin
constants (2C1). From the aforementioned parameters crosslinking density (ν) and molecular
weights of the network (Mc) have been calculated. Samples were characterized by swelling in
a Swellmeter at the elevated temperature (90 °C) as well, simulating the downhole well
conditions with confined space testing cells. Mechanical properties of the rubber samples were
determined before and after the ageing at the elevated temperature (100 °C) where the thermooxidative
degradation process was observed, considering the fact that the swelling packers have
to be resistant to high pressures and high temperatures. Therefore, the strength at break, the
elongation at break and Shore A classification hardness were determined. Thermal stability of
the rubber samples was characterized by thermogravimetric analysis (TGA) and the
characteristic values during the material degradation process conducted in a temperature
interval between 23 and 600 °C were determined. Temperature of the degradation start (T95) at
5% mass loss, temperature at the maximum speed of degradation (Tmax), carbonized residue
mass (mk) and maximum degradation speed (vmax) were defined. Afterwards, the morphology
of the samples was analysed by looking into the homogeneity and heterogeneity of the samples
with Scanning Electron Microscope (SEM). As the recycled rubber granules took over the role
of a filler, homogeneity and heterogeneity of the samples became very significant for the
mechanical properties of the samples since heterogeneous materials are not so compact and
resistant like the homogeneous ones.
From the equilibrium swelling and Swellmeter testing data of the acrylonitrile-butadiene
(NBR) rubber it can be concluded that nitrile rubber swelling process advances by adding the
recycled rubber which establishes additional interactions (secondary connections) with crude
oil as a swelling solvent for the rubber samples. Determination of equilibrium swelling degree
of EPDM rubber samples with swelling agent came to a conclusion that despite the different
crosslinking structure and recycled rubber content, swelling degree greatly depended on
swelling agent addition as the samples containing it swell much better then the ones without it.
The swelling degree differences are 40 to 60 percent. Generally, Swellmeter swelling at the
elevated temperature of 90 °C is much more intensive and faster with swelling degrees more
than 50 % higher. Thermal stability of EPDM samples determined by thermogravimetric
analysis (TGA) shows that all last three EPDM rubber series maintained similar values of
characteristic parameters. Ageing of the EPDM samples at 100 °C demonstrated the additional
crosslinking process negatively affecting the swelling properties and rubber elasticity but boosts
the rubber strength and hardness. SEM analysis results showed the different structure and
morphology by comparing samples with homogeneous and heterogeneous recycled rubber.
Given heterogeneous morphology within the third EPDM series with cavities inside the sample
matrix is the result of the incompatibility of EPDM matrix and recycled rubber with lack of
interactions. Heterogeneity of the rubber samples raises the possibility of crude oil absorption
inside the rubber matrix, but mechanical properties distort a bit. |
