Influence of Ammonia Concentration and Storage Period on Properties Field NR Latex and Skim Coagulation

Transcription

1 Ammonia NR latex Storage period Skim coagulation Renewable resource Field NR latex was treated with various ammonia contents (0.35, 0.60 and % w/w) and then concentrated by centrifugation to 60 % dry rubber content (DRC) and skim NR latex with roughly 5 % DRC. The latex concentrate was adjusted to a high ammonia latex concentrate (HA latex) by adding to 0.65 %-0.75 % w/w of latex, ammonium laurate and water, then kept at room temperature for 30 and 120 days. The influence of field storage times and different ammonia contents in field latex on the alkalinity, mechanical stability times (MST), volatile fatty acid (VFA), gel content and phosphate content were investigated. It was shown that MST, VFA, gel content and phosphate content increase with increasing storage time. The addition of excess ammonia in field NR gave the high gel content in concentrated NR latex due to the formation of zincamine complexes. When the amount of ammonia content increases in both concentrated natural rubber and skim latex and field latex storage increased, their stabilities also increased, leading to difficulties in skim coagulation. Einfluss der Ammoniakkonzentration und Lagerzeit auf die Eigenschaften von NR Latex und die Koagulation von Skim-Kautschuk Ammoniak NR Latex Lagerzeit Skim-Latex Koagulation erneuerbare Rohstoffe NR Latex wurde mit verschiedenen Ammoniakgehalten behandelt und durch Zentrifugierung einerseits auf einen Feststoffgehalt von 60 % bzw. einen Skim-Kautschuk Gehalt von 5 % aufkonzentriert. Das Latexkonzentrat wurde durch Zugabe von 0,65-0,75 %, Ammoniumlaurat und Wasser in ein hochkonzentrieten Amoniak Latex Konzentrat überführt. und über 30 und 120 Tage bei Raumtemperatur aufbewahrt. Der Einfluss der Lagerzeit und des unterschiedlichen Ammoniakgehalts im Feldlatex auf die Alkalinität, die mechanische Stabilität, die flüchtigen Fettsäuren, den Gel- und Phosphatgehalt wurden untersucht. Es wurde gezeigt dass diese Parameter mit der Lagerzeit ansteigen. Figures and Tables: By a kind approval of the authors Influence of Ammonia Concentration and Storage Period on Properties Field NR Latex and Skim Coagulation Natural rubber latex is used in a number of vital applications such as pre-vulcanized dipping compounds, molding compounds and foams. The quality of the natural rubber latex would be investigated in detail, leading the good quality of NR product. Field NR latex (NRL) consists of two main components one a hydrocarbon (1,4 - cis - polyisoprene) and the other a non-rubber components consisting of mainly carbohydrates, proteins and lipids in an aqueous serum phase [1]. It is well-known that fresh NR latex spontaneously coagulates shortly after it leaves the tree. In order to facilitate preservation high amounts of ammonia (HA) are added to the latex. Ammonia is the main chemical used for clarification and preservation of both field latex and concentrated latex. Ammonia inhibits bacterial action because of its high ph and also hydrolyzes fatty acid esters to form soaps that act as stabilizing bodies for the dispersed system [1]. Tapping of Hevea trees is usually conducted on alternate days before being delivered to the factory. Ammonia is then added at levels that vary from about 0.3 to 0.8 % by weight of latex together with a mixture of tetramethylthiuram disulfide and zinc oxide (TMTD/ ZnO) that improves the chemical stability of the latex [2-3]. The time of storage of the field latex is one factor that has an effect on the latex properties [3]. Results showed that the gel content of both field latex and high ammonia increases as a function of the storage period [3]. However the effects of adding different amounts of ammonia and times of storage on the properties of concentrated NR are still being reported. Commercially available concentrated NR latex is produced from field latex by centrifugation. This removes about two-thirds of the water-soluble non-rubber components and this part containing about 5 % of the total rubber mostly with the smaller latex particles is then skimmed off [4]. The concentrated natural rubber consists of two principal types including high ammonia (HA) types containing 0.6 percent by weight or more of ammonia based on total weight of latex and the low ammonia (LA) types in the presence 0.3 percent by weight [5-8]. In addition, the low ammonia type generally consists of auxiliary preservatives including zinc oxide, dithiocarbamates and thiuram disulfides. Hydrolysis of the phospholipids by the alkaline ammonia results in continued changes to the chemical composition of the rubber/water interface of the latex concentrate well after production [5-7]. The latex particles in the HA latex concentrate are stabilized mainly by adsorbed long-chain fatty acid soaps, the hydrolysis products of phospholipids. The role of adsorbed proteins on stabilizing the latex is less vital in the latex concentrate. High ammonia concentrated natural rubber latex are widely used in dipping film products. Haldeman and Janice L [8] invented a method for improving the chemical stability of low ammonia natural rubber latices and latex compounds by adding anionic phosphate surfactant being neutralized or partially neutralized to a ph of from 4 to 12 as measured in a 20 percent aqueous solution. Commercially available natural rubber latex are generally classified into two principal types. These are the so-called high ammonia (HA) types which generally contain 0.6 percent by weight or Authors Sd-Ad Riyajan, S. Santipanusopon, Hat Yai (Thailand) Corresponding author: Prof. Dr. Sa-Ad Riyajan Prince of Songkla University Faculty of Science Dept. of Materials Science Technology Hat Yai Songkhla, 90112, Thailand Tel. +66/ Fax: +66/ saadriyajan@hotmail.com 240 KGK Juni 2010

2 more of ammonia based on total weight of latex and the low ammonia (LA) types which generally contain 0.3 percent by weight or less of ammonia based on total weight of latex. In addition, the low ammonia types generally contain auxiliary preservatives such as zinc oxide, dithiocarbamates and thiuram disulfides [8]. They found that the chemical stability of low ammonia natural rubber latex and latex compounds is help by the addition of from 0.1 to 0.5 parts by weight solids, per 100 parts by weight of natural rubber solids, of a neutralized or partially neutralized anionic phosphate surfactant having a ph in a 20 percent aqueous solution of from 4 to 12 [8]. The effect of different ammonia contents in field latex and storage period time of field latex on properties of NR latex can be investigated in this work. Because the rubber garden owners behavior about a store of the field latex for long period before sending them to the natural rubber (NR) factory due to the variation of latex price, leads to the different properties of as a raw material for dipped latex film product. To the best of our knowledge, this is the first of its kind of study wherein this article characterize the effect of adding different amounts of ammonia to fresh NR latex and the times of storage on the properties of concentrated NR latex and features of the skimmed latex coagulation obtained from the same batch of field NR latex. The most important factors for obtaining the best quality of concentrated NR were alkalinity, MST, VFA no gel content and phosphate content. Experimental Materials Field latex, ammonia solution, the mixture of tetramethylthiuram disulphide (TMTD) and zinc oxide (ZnO), di-ammonium-phosphate (DAP), ammonium laurate were provided by Thai Rubber Latex Corporation (Thailand) Public Company Limited. Ammonium molypdate, ammonium metavanadate and methyl red were purchased from BDH (UK). Toluene, chloroform, methanol, sodium hydroxide, ammonium sulphate, ammonium chloride, sulfuric acid and hydrochloric acid were AR grade chemicals from Qrec. Barium hydroxide, sodium carbonate and EDTA and phenopthaleine were UNIVAR brand from Ajax Finechem Co. Ltd., potassium cyanide and potassium hydrogen phathalate were from Merck. Silicone antifoam and Eriochrome black T were purchased from Fisher Science. Chemicals used for dispersion including sulphur, accerelators, activator, antioxidant, soap, filler and coagulant were provided by Research and development department of Thai rubber latex group Co., Ltd. and Phoenix rubber product Co. Ltd. Preparation of samples Field latex was preserved with,at either 0.35 %, 0.60 % and % w/w of latex and a constant amount of a 1:1 mixture of TMTD and ZnO The field latex samples were collected after storage for 0, 15, 30 and 45 days. They were separated by centrifugation at the Thai Rubber Latex Corporation (Thailand) Public Company Limited after adding diammonium phosphate (DAP), ammonium laurate and water using the Thai Rubber s specifications for field latex. The total solid content (TSC), dry rubber content (DRC), and Mg content were checked and samples stored overnight before sedimentation. Field latex was centrifuged using an Alpha Lava model supplied by Thai Rubber Latex Corporation (Thailand) Public Company Limited and the and skim latex separated. Sample was taken from each batch with a different % level and storage time. The latex concentrate was adjusted to a high ammonia latex concentrate (HA latex) by adding to 0.65 %-0.75 % w/w of latex, ammonium laurate and water, then kept at room temperature for 30 and 120 days. Characterization The ammonia content in the field and concentrated NR latex was determined by acidbase titration with hydrochloric acid The TSC and DRC of the latex were carried out by Standard method ISO 124 and ISO 126, respectively. The gel content in NR was determined as follows g of solid rubber was dissolved in 30 ml of dried toluene and kept in the dark without stirring for one week at room temperature. The solution was filtered through a 325 mesh stainless sieve to separate the gel fraction from the sol fraction. The gel fraction was coagulated with methanol and dried in an oven at 100 o C. The phosphate concentration was determined by reference to a calibration curve of absorbance at 470 nm. and the concentration of phosphate ion ml of serum of latex were reacted with ml of metavanadate molypdate for 15 minutes. An aliquot was filtered through a with the filter paper No. 40 filter paper and its absorbance measured at 470 nm by a Novaspec II spectrophotometer. The gel content was determined by method ASTM D VFA number, the number of grams of potassium hydroxide (KOH) required to neutralize the volatile fatty acid in a latex sample containing 100 g of total solids, of the treated latexd latex was determined periodically in accordance with the standard test follow ASTM D The VFA number was measured by the quantity of barium the distilledoff short-chin fatty acids. The calculation of the VFA number is an follows: VFA number (AM 561) w TS) Where: A cm 3 of Ba(OH) 2 solution required for titration of the sample, M - mole of the Ba(OH) 2 solution, W- mass of the latex corresponding to 10 cm 3 of acidified serum, and TS = percentage of total solids in the latex. Studies on the coagulation of skim latex were carried out in 500 ml conical flasks by addition of known amounts of commercial concentrated sulfuric acid. The time taken for of the skimmed latex coagulum to separate from the serum, known as the phase separation time, was recorded. Results and discussion Effect of field NR latex with different ammonia contents on alkalinity of and skim latex The field NR with different ammonia contents had kept at room temperature for 0, 15, 30 and 45 days, then they were centrifuged to get the concentrated NR and skim latex. Table 1 shows the residual of ammonia content in concentrated NR and skim latex obtained from filed NR latex at 0 day of storage period time with different ammonia contents at 0.35, 0.60 and % w/w. It was found that the residual of ammonia contents in both concentrated NR and skim latex increased with increasing ammonia content in field latex. Generally, the residual of ammonia or alkalinity in skim latex was higher than that of concentrated NR latex. The residual of alkalinity in skim 1 The effect of different ammonia concentrations in field NR latex with on alkalinity of concentrated NR and skim latex Alkalinity (%w/w) Sample Nh 3 in field latex (% w/w) Concetrated NR latex Skim latex KGK Juni 2010

3 1 1 Correlation between MST of HA latex and concentration in field latex at various field latex storage periods latex obtained from field natural rubber at 0 day of storage period time was 0.40, 0.60 and %w/w whereas the residual in concentrated NR was 0.16, 0.18 and 0.25 %w/w when they were derived from field latex with different ammonias at 0.35, 0.60, and % w/w, respectively. The alkalinity of both concentrated NR and skim latex obtained from field NR with different ammonia contents had kept at room temperature for 15, 30 and 45 days showed the same trend, comparing to alkalinity of both concentrated NR and skim latex obtained from field NR at 0 day of storage time. contents in field NR latex and storage period times on MST of The aim of paper is focused on the effect of field latex with different ammonia contents of field natural rubber latex with different ammonia contents and theirs storage times on physical properties including phosphate content of latex concentrate and skim latex coagulation. These results showed that the influence of two factors had found to be a significant on theirs physical properties observed from MST, gel content, VFA and phosphate content. Figure 1 illustrates the significant relation of field latex storage period and MST of high ammonia latex concentrate. It has been reported that when field latex storage period increase, MST value trend to increase due to hydrolysis of phospholipids occurring around the rubber particle, providing their high latex stability. In addition, the total ions concentration in NR latex tends to increasing storage time. When filed natural rubber was kept for a long period (45 days), its MST dramatically decreased comparing to the others sample. This result was explained that at low ammonia natural rubber latex in such applications gave a higher NH 4+ / ratio than the high ammonia type in latex leading to the lower formation of Zn(NH 4 )n] 2+ complex ions wherein n is 1 or 2 instead of 3 or 4 [8-9]. At certain conditions, these zinc ammonia complex ions react with fatty acid soaps absorbed on the rubber particles to form insoluble zinc soaps leading to coagulation of latex or gel. Thus, the MST of sample obtained from 45 days of storage was very low value. The MST of concentrated NR obtained from 0.35 % w/w ammonia and 0, 15 and 30 day of storage period time was 600, 840 and 1020 second, respectively. In case of the concentrated NR derived from % w/w ammonia and 0, 15 and 30 day of storage period time, the result showed the same trend with sample obtained from 0.35 % ammonia. The highest in MST was observed in sample obtained from 0.60 % w/w ammonia and 30 day of storage period time due to good condition for the maximum for hydrolysis between phospholipid and bacterial in latex. The 0.35 % ammonia in latex gave the good stability of concentrated NR comparing sample containing 0.60 and 0.85 % ammonia in latex. contents in field NR latex and storage period times on VFA No. of The effect of concentration in field latex on VFA number of HA latex is shown in Figures 2a and b. The non-rubber contents, especially protein and phospholipids in 2 2 Correlation between VFA No. of HA latex obtained from field latex at (A) 15 and (B) 30 days of storage period and storage time KGK Juni

4 latex are an unstable material, which is hydrolyzed into volatile fatty acid including propionic acid, formic acid and acetic acid under ambient. As bacteria grow and reproduce in the latex they produce volatile fatty acids leading to its coagulation. Due to bacterial degradation of latex constituents causing formation of short-chains VFA with the resultant decrease in the ph value of the latex, VFA number is an important measure of the level of deterioration and stability of the latex. Thus, the VFA number of the treated latex was used to indicate the preservative activities. It was found that the VFA no value of HA latex obtained from field latex at 15 days of storage time increased as a function with increasing storage time period as shown in Figure 2a. The VFA no of HA latex at % ammonia content obtained from 5, 20, 50 and 60 days of storage time was 0.018, 0.022, 0.027and 0.038, respectively. When ammonia in HA latex decreased form to 0.60 or 0.35, the VFA no of HA latex was not different comparing to sample containing 0.08 % of ammonia. In case of HA latex derived from field latex with one month of storage time, the VFA no of HA increased with increasing storage time period as shown in Figure 2 B. In addition, the VFA no of HA latex was independent with ammonia contented in latex. The VFA no. of HA obtained from field latex with from 0.35 to %w/w ammonia and TMTD/ZnO ratio 1:1 was not different when its storage period reached until 60 days. This result replies that NR latex had enough ammonia to its preservation. 3 3 Correlation of gel content of HA latex obtained from field latex containing different concentrations and storage period 4 4 Possible reaction of zinc-amine complex between zinc oxide and ammonia ion contents in field NR latex and storage period times on gel content of In general, the gel content in NR latex during storage occurred through the following crossliking reactions (1) chemical crosslinks by C-C and C-O bonds between rubber chains (2) Ionics crossliks by di- or trivalent metal ions existing in latex and (3) crosslinks by hydrogen bonding [10-12]. The first one is mostly the reaction of polyisoprene chain, although very small amounts of functional groups in NR termed as abnormal groups including aldehyde lactone and epoxide are presumed to cause storage hardening in the case of solid NR. On the other hand, the second and third mechanisms are reactions via some functional groups on the rubber chain. From Figure 3, gel content of HA latex was slightly increased with increasing in concentration. The field latex storage period increased from 0, 30 to 45 days, its higher gel content at the same age of HA latex was observed. The storage period of HA-latex also could significantly affect the gel content. The gel content of HA latex with % w/w ammonia content obtained from field latex storage period at 45 day was about 30 %, while gel content of HA latex with at same ammonia content obtained from field latex storage period at 0 day was about 10 %. The addition of excess ammonia in field NR gave the high gel content in concentrated NR latex due to the formation of zinc-amine complex as shown in Figure 4. This result responds with D.C.Blackley [9]. He found that ammonia in natural rubber latex forms with ZnO giving the zinc-amine complex under heat condition. In addition, the occurrence of gel formation was observed through ionic bonds or dissociation of phosphate groups by (NH 4 ) 2 SO 4. This suggests that the residual gel fraction in the (NH 4 ) 2 SO 4 treated commercial HA latex may be derived from the hydrogen bonding of proteins and functional terminal groups in rubber chain as reported by Tangpakdee and Tanaka [6]. The gel content of concentrated NR increased with increasing field NR late storage period. These findings indicate that the gel formation in during prolonged storage is derived partly by magnesium ion, but predominantly by hydrogen bonding and chemical crosslink. contents in field latex and storage period times on phosphate content of In NR latex, there are magnesium ions leading to a major cause of destabilization of natural rubber (NR) latex. Therefore, the addition of excess phosphate ions to remove Mg 2+ ions which could affect the physical properties of dipped products made of NR latex. The phosphate content in HA latex increased with increasing field latex storage time as shown in Figure 5. These results explained that there was occurence of hydrolysis of fatty acid in HA latex. The phosphate content in HA latex obtained from 2, 30 and 60 storage period time at 0.35 % was 400, 430, 470 ppm, respectively. When the ammonia in HA latex increased from 0.35 to 0.60 %, the phosphate content in HA latex obtained from 2, 30 and 60 storage period time of field NR latex was 620, 700 and 820 ppm, respectively. In addition, these results corresponded with gel content results as shown in section 3.4. Both phosphate content and gel content increased with increasing storage time of field NR. The phosphate ions promoted the crosslink 244 KGK Juni 2010

5 5 5 Correlation between phosphate content of HA latex and storage period time of HA with different concentrations obtained from field latex at 0 day field latex storage formation in NR latex. This indicates that the synergistic effect of TMTD/ZnO and excess amounts of diammonium phosphate on the gel formation of NR latex during long time storage was observed. This result agrees with reference [2]. Effect of field NR latex with different ammonium contents and storage period times on skim NR latex coagulation Skim latex is obtained along with the concentrated rubber latex, as an equal fraction in volume, during centrifugation of the field NR latex. Protein and other non-rubber constituents which have specific gravities higher than that of rubber also migrate into the skim fraction during centrifugation and not only reduce the quality of rubber but also affect the coagulation process [7]. The usual method of recovery of skim rubber is by coagulation with sulfuric acid. In this part, the effect different ammonium contents in field natural rubber and storage 2 Field latex storage period (days) The coagulation of skim latex obtained from field latex under different storage periods Skim coagulation NH3 in field latex (% W7W) Normal Normal 3 Skim latex coagulation of un-coagulation skim latex when add more acid Skimooagulation FL Stoarge Time (day) in field Latex (% w/w) Coagulation Adding more acid Adding more acid Coagulation Phase separation time 120 % Some 6 min 350 % 15 min 40 % Moderate 10 min Some 80 % 18 min period times on stability of skim NR latex is illustrated in Table 2 and 3. Increasing the level from 0.35 and 0.60 % w/w after 0 day of storage had little effect but at % w/w of coagulation was much lower due to the high alkalinity. All samples after 15, 30 and 45 days storage showed no coagulation after addition of the same amount of sulfuric acid. When more sulfuric acid was added to the samples, the observed coagulation was affected by the level. The skim latex from samples stored for both 30 and 45 days with added 0.35 % w/w showed some coagulation with increased acid but the samples stored for 30 and 45 days with added % w/w showed no coagulation. Some coagulation was induced at low levels of ammonia but not at the higher levels so the rubber particles were more stable at the higher levels and with longer periods of storage in the field latex. This result indicates that the field latex before the commercial NR latex manufacturing could be stored in short period leading to easy the coagulation of skim latex than that of long storage period. Conclusions The residual of ammonia contents in both concentrate and skim latex increased with increasing ammonia content in field latex. The 0.35 % w/w concentration in field latex is suitable for preservation observing from gel content, MST and VFA. MST, VFA and gel content of concentrated NR increased with increasing storage times. The addition of excess ammonia in field NR gave the high gel content in due to the formation of zinc-amine complex. The phosphate content in latex concentrate increased with increasing HA aged due to fatty acid hydrolysis. The coagulation of skim latex depends on storage period time of field NR latex with different ammonia contents. The field latex before the commercial NR latex manufacturing could be stored in short period leading to the better concentrate latex properties and the coagulation of skim latex than that of long storage period. Acknowledgments The authors thank department of polymer science, Prince of Songkla University and Thai Rubber Latex Corporation (Thailand) Public Company Limited for the use laboratory space. Thi s study was supported by Thailand research fund and Office of Small and Medium Enterprises Promotion (MRG- OSMEP505S207). References [1] C.C. Ho, T. Kondon, N. Muramatsu, and H. Ohsima, J.Colloid Inter. Sci. 178 (1996) 442. [2] L. Tarachiwin, J. Sakdapipanich, and Y. Tanaka, Rubber Chem. Technol. 76 (2003)1177. [3] L.Tarachiwin, J. Sakdapipanich, and Y. Tanaka, Rubber Chem. Technol. 76 (2003)1185. [4] M.M. Rippel, C.A.P. Leite, L.T. Lee, and F. Galembeck, J.Colloid Inter.Sci. 288 (2005) 449. [5] R. Belmas, Rubber Chem Technol.25 (1952)124. [6] J. Tangpakdee, and Y. Tanaka, Rubber Chem Technol. 70 (1997) 703. [7] K. Jayachandran, M. Chandrasekran, Biotechnology Letter 20 (1998)161. [8] L.J. Haldeman, Stabilizer for low ammonia natural rubber latex compounds.us Patent , [9] DC. Blackley, High polymer lattices, vol. 1., Ed. London:Maclaren and Sons Ltd., (1997) 238, 447, 459. [10] JR. Gregg, and J.H. Macey, Rubber Chem. Technol. 46 (1973) 43. [11] D.R. Burifed, Nature. 249 (1974) 29. [12] D.R. Burifield, S.N. Gan, J. Polym. Sci.13 (1975) KGK Juni