Effects of molecular structure and molar content of acrylate units on aerobic biodegradability of acrylate copolymeric sizing agents

Abstract

This study was undertaken to establish a relationship between aerobic biodegradability and molecular structure of acrylate copolymers for the design and production of the copolymers with good biodegradability for warp sizing. Biological oxygen demand for five days (BOD₅) and chemical oxygen demand (COD) of the copolymers were assessed and the BOD₅/COD ratio was used to evaluate their biodegradability. The influence of the number of carbon atoms in the alkyl group of alkyl ester chain on the biodegradability was studied and the effect of α-methyl group was evaluated. Moreover, the impact of molar content of acrylate units on the biodegradability was also studied. It was found that the decrease in the number of carbon atoms in the alkyl group of alkyl ester chains and the reduction in molar content of acrylate units improved the biodegradability of acrylate copolymers. Biodegradation of acrylate copolymers proved to be difficult if an α-methyl group was present in the acrylate units. To produce acrylate copolymers that are able to biodegrade more easily, a suitable amount of acrylate monomers with shorter alkyl side-chains and without an α-methyl group should be applied to monomer formulation for the copolymerization with hydrophilic monomers. The preferred molar content of the acrylate monomers should be in a range of 60% to 70%. The acrylate copolymers thus prepared were biodegradable and aerobic biodegradation was capable of converting macromolecules to lower molecular weight end-products.

Keywords

sizing agent acrylate copolymer aerobic biodegradability

Introduction

Following the paper-making, agricultural product processing, and chemical industry, the textile industry discharged 310 thousand tons of chemical oxygen demand (COD) in 2009 in China. National Bureau of Statistics of China stated that the amount of COD occupied the fourth place of 39 trades. It was estimated that the sizing agents in desizing wastewater discharged should be responsible for about 50%−80% of the COD in textile finishing wastewater.¹ The desizing wastewater is generally characterized by a large amount of suspended solids, higher COD load, and lower biodegradability. As a result, it poses a serious threat to our environment via contamination of surface water. The situation indicates an urgent need for reducing the pollution effect caused by sizing agents.

The biodegradation of synthetic sizes is of considerable importance because of their use in large quantities. One of the major types of synthetic size is acrylate copolymer. The sizes are the copolymers of various acrylate monomers with a small amount of acrylic acid (AA), mainly manufactured through solution or emulsion polymerization.^2,3 The purpose of using AA in monomer formulation is to make the copolymers soluble or at least dispersible in water so as to facilitate size mixing and desizing. In the copolymerization process, the acrylate and AA monomers are converted into corresponding units (named acrylate and AA units, respectively) along the copolymeric chains of acrylate copolymers. Currently, molar content of acrylate units is generally more than 50% and the content plays an important role in end-use performance.⁴^–⁶ However, the term of acrylate copolymers used during textile warp sizing was misused because the sizing materials based upon acryl compounds were generally called ‘acrylates’. Obviously, the acrylates are organic monomers, instead of polymers, and can not be used as warp sizing agents before polymerization. The most commonly used acrylate monomers include methyl acrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA) and butyl methacrylate (BMA). Chemical structures of the acrylate monomers used are shown in Figure 1. The structural formula of the copolymeric sizes can be expressed as shown in Figure 2.

Figure 1.

Chemical structures of acrylate monomers.

Figure 2.

The structural formula of the copolymeric sizes. R is the alkyl group of the alkyl ester in the acrylate units of acrylate copolymers, R′ in the α-position is H or methyl.

Due to the difference in chemical structure, acrylate copolymers are many in numbers. It is as well to emphasize that molecular structure of acrylate copolymers depends on the structures of R and R′, and on molar content of acrylate units. This means that carbon atom number (or carbon-chain length) of the alkyl group of the alkyl ester, presence of α-methyl, and molar content of acrylate units determine their structure and performances during warp sizing. Currently, the influences of the copolymeric structure upon their performances in warp sizing have been revealed by a number of literatures.⁴^–⁶ These studies are important since they show beneficial effects on improving their sizing performances.

Kawai et al.⁷ proposed an aerobic degradative pathway for acrylic polymer with trimer (1,3,5-pentane tricarboxylic acid)-utilizing bacteria. Iwahashi et al.⁸ studied the mechanism for degradation of sodium polyacrylic acid by bacterial consortium no. L7-98. Recent investigations indicated that the introduction of AA units onto the backbones of some hydrophobic copolymers could improve or control their biodegradation rate.^9,10 However, the knowledge on biodegradability of acrylate copolymers is quite limited, and little was known about the relationship between molecular structure and biodegradability of acrylate copolymeric sizes. The present design for environmentally friendly acrylate copolymers for warp sizing suffers from a lack of detailed information on the influence of molecular structure upon biodegradability. Information regarding this influence would provide useful insights into the molecular structure of acrylate copolymers. In order to evaluate the effect of molecular structure on their biodegradability for the design and production of biodegradable acrylate copolymers for warp sizing, a relationship between biodegradation and molecular structure of acrylate copolymers should be established.

With widespread use of warp sizing agents in the textile industry, concern has been raised upon the disposal of desizing wastewaters. Several studies¹¹^–¹³ had been performed on their biodegradability, since biological oxidation is generally considered the most environmentally friendly and cheapest method for wastewater treatment.¹⁴ Many investigators adopted the BOD₅/COD ratio to evaluate the aerobic biodegradability of organic contaminant in wastewater.¹⁵^–¹⁸ The ratio gives a gross index of proportion of the organic materials present which are aerobically degradable within 5 days.¹⁴ The higher the ratio, the better the biodegradability.¹¹ By means of the evaluations on COD and BOD₅, aerobic biodegradability of acrylate copolymers in water can be assessed through a simple comparison in their BOD₅/COD ratios. It should be noted that the effect of different types of acrylate units on biodegradability of the copolymers can not be distinguished if two or more types of acrylate units are incorporated together into their molecular chains. For this reason, the samples used for the present investigation were synthesized by incorporating only one type of acrylate monomer with AA each time. In this way, the influence of molecular structure of the sizes upon their biodegradability can be revealed by examining BOD₅/COD ratio of the samples prepared. As a result, reasonable structure of acrylate copolymeric sizes with good environmental performance can be achieved.

Experimental

Materials

Benzoyl peroxide (BPO), MA, EA, BA, MMA, BMA and AA of analytical grade were used. Ethanol, potassium dichromate, silver sulfate, ferrous ammonium sulfate, potassium dihydrogen phosphate, calcium chloride, ferric chloride hexahydrate, etc. were also of analytical grade. BPO was supplied by Zhongli Chemical Plant, Shanghai, China. Calcium chloride was obtained from Silian Chemical Plant, Shanghai, China. Other reagents were supplied by Sinopharm Chemical Reagent Ltd. Inc., Shanghai, China.

Synthesis of the copolymers

Copolymerization of various acrylates with AA monomer was carried out in ethanol in a 500-mL flask. The flask equipped with a mechanical stirrer, a thermometer, a condenser, and a dropping funnel was immersed in a thermostatically controlled water bath. BPO (0.72 g) was dissolved in 120 g of acrylate/AA blend monomers to form an initiator-monomer solution at room temperature. Ethanol (120 mL) was charged into the flask and heated to refluxing (78°C). Then, 40 mL of initiator-monomer solution was charged and the reaction was carried out under mechanical stirring. Forty minutes later, the remaining initiator-monomer solution was added continuously via a dropping funnel in one hour. One hour after the addition, an additional quantity of initiator (0.24 g) was refilled into the flask. The copolymerization was carried out under continuous stirring at refluxing point of ethanol for 6 hour. Before the reaction was terminated, about 30 mL of reaction products were taken out for the subsequent analysis of residual monomer and copolymer compositions. The remaining product was cooled down and neutralized with 10% ammonia at 60°C. Then, the product was diluted with 260 mL distilled water and evacuated to eliminate ethanol from the product. The evacuation was conducted through distillation at reduced pressure (0.01 MPa). Finally, the product was diluted with distilled water to about 430 mL.

Characterization of the copolymers

Conversion of monomer to copolymer is expressed as a weight percentage of the synthetic copolymers prepared to the monomers charged. The conversion was determined according to the method found in the literature.^4,19 Apparent viscosity of acrylate copolymers was measured at room temperature with a NDJ-79 rotary viscometer^20,21 under a concentration of 4% (w/w) and a shear rate of 1850 s⁻¹. The solid content of the products was determined based on a weight method.²⁰

A TA Dynamic Mechanical Analyser (DMA) Q800 in conjunction with a liquid nitrogen cooling system was used for the DMA measurement. The measurement was performed using a heating rate of 5°C/min at an oscillation frequency of 1 Hz from −80°C to 180°C. The DMA was operated by the application of a drawing mode to the film sample fixed with a thin film fixture. The glass transition temperature (T_g) was determined from plots of tanδ (loss modulus/storage modulus) as a function of temperature using the software provided with the TA Company.

The molar content of acrylate units in the macromolecules of acrylate copolymers was determined by proton nuclear magnetic resonance (¹H-NMR) in DMSO-d₆ solvent using an AVAMCE III 400 MHz Digital NMR spectrometer made by Bruker Co. Ltd. (Switzerland). The concentration of each sample was 5 wt% in the solvent. Molar content of acrylate units was obtained according to proton peak area ratio of ester group to all ester and carboxyl groups in acrylate copolymers.

COD was used as a measure of the oxygen equivalent of organic matter content of a sample that was susceptible to oxidation by a strong chemical oxidant.^22,23 COD (mg O₂/L) was assessed according to the reference.²² 10 mL of potassium dichromate (0.250 mol/L) were added into 20 mL of the copolymer solution (0.1 g/L). Sulfuric acid, silver sulfate, and ferroin were used as reaction medium, catalyst, and indicator, respectively. After refluxing, non-reduced potassium dichromate was titrated with ammonium ferrous sulfate (0.10 mol/L). Then the amount of ammonium ferrous sulfate reacted was converted to COD through equation (1):

(1)

where C (mol/L) was the concentration of ammonium ferrous sulfate standard solution; V₁ (mL) and V₂ (mL) were the volumes of ammonium ferrous sulfate standard solution in blank and sample test, respectively; V₀ (mL) was the volume of sample solution; ‘8000’ was the conversion value of molar mass of O₂ (mg O₂/L).

BOD determination of acrylate copolymers was an empirical test during which a standardized laboratory procedure was used to determine the relative oxygen requirements of the copolymer solution. BOD₅ (mg O₂/L) was determined in accordance with the method described in the reference.²² The method consisted of filling with sample to overflow, choosing an airtight bottle of the specified size and incubating the sample at 20 ± 1°C for 5 days. Dissolved oxygen was measured before and after incubation, and the BOD₅ was calculated from the difference between initial and final dissolved oxygen.

Average molecular weights of acrylate copolymers before and after cultivation were determined with gel permeation chromatography (GPC). The cultivation was the same as BOD₅ evaluation. After cultivation, the liquid sample was centrifugated and the supernatant was taken and concentrated to about 0.5% (w/w) using a rotary evaporator at 80–85°C under reduced pressure (0.01 MPa). A Waters 600 (Waters, USA) High Performance Liquid Chromatography (HPLC) System was equipped, a Waters 2410 Refractive Index Detector and a Waters M32 chromatography workstation. Chromatographic separation was achieved on Ultrahydrogel Linear 300 mm × 7.8 mm id × 2 chromatography columns (Waters, USA). Sodium nitrate solution with a concentration of 0.1 mol/L was used as a mobile phase and the flow rate was 0.9 mL/min.

Results and discussions

Basic characters of the acrylate copolymers prepared are summarized in Table 1. It can be observed that apparent viscosity of the copolymers decreases with the increase in either molar content of MA units or carbon atom number of alkyl group of alkyl ester in acrylate units. Obviously, the increases in molar content reduce hydrophilicity of acrylate copolymers, thereby decreasing the intermolecular forces between the copolymers and water, and resulting in the decrease of apparent viscosity of acrylate copolymers. Solid contents of the copolymers are in a range of commercial acrylate copolymeric sizing agents. Moreover, monomer conversions of all samples are above 92%, denoting that most monomers have been converted to polymers.

Table 1.

Characteristics of acrylate copolymers prepared

Acrylate copolymers	Molar content of acrylate in monomer formula (%)	Conversion of monomer to polymer (%)	Solid content (%)	Apparent viscosity (mPa·s)
poly(MA-co-AA)	50	98.9	23.5	9.0
	60	97.6	25.0	8.8
	70	98.6	24.7	5.5
	80	99.0	23.8	5.0
	90	99.3	23.0	2.1
poly(EA-co-AA)	70	96.2	24.3	2.8
poly(BA-co-AA)	70	97.3	23.7	2.4
poly(MMA-co-AA)	70	92.7	24.6	4.2
poly(BMA-co-AA)	70	93.0	23.3	2.6

DMA spectra of acrylate copolymers prepared are shown in Figure 3. The points 25.17°C in tanδ curve 1#, 13.09°C in the curve 2#, 1.38°C in the curve 3#, 132.57°C in the curve 4#, and 88.39°C in the curve 5# are the glass transition temperatures of the copolymers, respectively, and were obtained in accordance with the reference.²⁴ Every acrylate copolymer sample has a single T_g, which indicates the samples prepared are random copolymers, rather than a blend of acrylate and AA homopolymers, since a blend often appears more than one T_g in its DMA spectrum.²⁴ It is widely known that a random copolymer has only one glass transition temperature between the T_gs of their homopolymers. In our observation, the tanδ peak in the DMA spectrum is between the T_gs of AA homopolymer and acrylate copolymer. The single T_g in DMA spectrum demonstrates that the samples prepared are random copolymers rather than blends.

Figure 3.

DMA spectra of acrylate copolymers with constant molar content (70%) of acrylate units: (1) MA-co-AA; (2) EA-co-AA; (3) BA-co-AA; (4) MMA-co-AA; (5) BMA-co-AA.

Effect of alkyl ester length

Table 2 shows the influence of length or carbon atom number of the alkyl of the alkyl esters in acrylate units upon the COD, BOD₅ and BOD₅/COD of acrylate copolymers. It can be seen that COD, BOD₅ and BOD₅/COD are greatly dependent on the length or carbon atom number. A decrease in the atom number results in a reduced COD and an increased BOD₅, and an increased BOD₅/COD ratio. The increased ratio seems to be due both to BOD₅ increment and COD reduction. It is as well to emphasize that a criterion for evaluating aerobic biodegradability of organic contaminants in wastewater using the BOD₅/COD ratio has been established.²⁵ The criterion divides the organic contaminants based on the rate of their aerobic degradation, in accordance with the BOD₅/COD ratio, into four classes: easily biodegradable (degradation with a high rate) if BOD₅/COD > 0.4; biodegradable (degradation with a normal rate) when BOD₅/COD = 0.3–0.4; inferiorly biodegradable (degradation with a relatively lower rate) when BOD₅/COD = 0.2–0.3; and poorly biodegradable (degradation with a lower or extremely low rate) only when BOD₅/COD < 0.2. According to this criterion, a higher BOD₅/COD ratio reflects easier biodegradation of the copolymers. As summarized in Table 2, with decreasing the length of the alkyl in acrylates the biodegradability of acrylate copolymers is improved markedly. Poly(MA-co-AA) is more biodegradable than those with long-chain side esters because its BOD₅/COD ratio is as high as 0.211 and increases sharply.

Table 2.

Effect of chemical structure of acrylate units on biodegradability of acrylate copolymers

Type of acrylate monomers	Carbon atom number of alkyl of alkyl ester in acrylate monomers	α-methyl in acryalte monomers	BOD₅ (mg O₂/L)	COD (mg O₂/L)	BOD₅/COD
MA	1	Absence	37.0	175.3	0.211
EA	2	Absence	16.5	183.2	0.090
BA	4	Absence	10.3	212.0	0.049
MMA	1	Presence	8.2	210.5	0.039
BMA	4	Presence	5.1	224.6	0.023

Note: molar ratio of fed (meth)acrylate to AA monomers was 70:30.

It has been reported that acrylate homopolymers generally resist biodegradation due to their hydrophobicity.²⁶^–²⁹ Our measurement indicates that the random copolymer poly(acrylate-co-acrylic acid) is easily degraded by microorganisms. This is mainly due to the incorporation of hydrophilic AA into the copolymeric macromolecules. It has been clear that microorganisms degrade polymers in two ways: one is the degradation from terminal groups (exoenzymes) and the other one is the random degradation along molecular chains (endoenzymes).²⁶ Obviously, the biodegradations shorten the molecular chains of acrylate copolymers and reduce their molecular weights. Additionally, a small amount of side ester groups will hydrolyze during the degradation,²⁷ which may result in an increased BOD value. As the oxidation site is known to be located on microorganism cells, the polymer has to be contacted with microorganisms during biodegradation processes.⁷ With the increase in carbon atom number, side chains of the copolymers extend and their hydrophobicity increases. Longer hydrophobic side chains are easier to entangle and aggregate in water, which reduces contact probability between microorganisms and copolymeric chains. Therefore, the degradation achieved by microorganism attack is inhibited and thus the biodegradability of the copolymers is reduced. For this reason, the copolymerization of acrylate with hydrophilic monomers favors to improve the biodegradability.

Clearly, shorter length of the alkyl in acrylate units reflects easier biodegradation to acrylate copolymers. The easier biodegradation provides higher BOD₅ values and easier treatment for desizing wastewater. Therefore, an alternative is to use the acrylates with shorter length of the alkyl in monomer formulation via copolymerization with hydrophilic monomers for synthesis of the copolymeric sizes. The demand for shorter length of the alkyl is consistent with that of tensile strength of films.⁶ However, it is a conflict with the requirement on adhesion of acrylate copolymers to fibers since the adhesion increases with increasing the length of alkyl groups.⁴

Effect of α-methyl

Table 2 also indicates the effect of α-methyl in acrylates on the biodegradability of acrylate copolymers. The presence of α-methyl in acrylates exhibits marked influence upon the biodegradability. It causes the increase in COD value, the decrease in BOD_5, and the reduction in BOD₅/COD ratio. This observation demonstrates that the use of methacrylates in monomer formulation for the preparation of the copolymeric sizes impairs product biodegradability and induces difficulty in the treatment of desizing wastewater containing the sizing agents.

The decrease in BOD₅ value of acrylate copolymers induced by the presence of α-methyl in acrylate units may be attributed to the following reasons: (1) the presence of α-methyl reduces hydrophilicity of acrylate copolymers; (2) the α-methyl group occupies a larger space than the hydrogen atom, thereby blocking, more or less, the contact of microorganisms with acrylate copolymeric chains; (3) α-methyl groups inhibit the degradation process of acrylate copolymers from terminal groups. Kawai,⁷ Iwahashi,⁸ Hayashi³⁰ and their co-workers studied the biodegradability of AA homopolymer and confirmed that α-methyl groups of AA units inhibited the biodegradation of the homopolymer because the α-methyl group inhibited the formation of the double bond during the degradation process from terminal groups. And the formation of the double bond between terminal units and the next ones is essential for the degradation process. For those reasons, the biodegradability of methacrylate copolymers is worse than that of acrylate ones.

Methacrylates own their place in monomer formulation for making the copolymeric sizes due to the fact that their homopolymers possess higher T_g and are capable of preventing re-bonding of sizing film on the surface of sized yarns because the drawback interferes with the opening of warp yarns and poses a serious threat to weaving efficiency.⁵ Based on the knowledge of warp sizes, however, the use of methacrylate monomers is no longer a sole way to enhance T_g for solving the problem like re-bonding of sizing film on the surface of sized yarns. For example, a large number of modified starches have been applied for sizing staple spun warps. The starches used raise the T_g of sizing film and prevent film re-bonding caused by excessively low T_g of acrylate copolymers used in size formulation. Our investigation confirmed that the use of acrylate copolymers with starches is also an efficient method to prevent the adverse effects of starch brittleness and rigidity and to reduce shedding of size and lint caused by starch sizes. Furthermore, the presence of α-methyl reduces the adhesion of acrylate copolymers to fibers.⁴ For these reasons, acrylates, instead of methacrylates, should be adopted in the copolymerization for improving the biodegradability and sizing ability of acrylate copolymers.

Effect of molar content of acrylate

After knowing the effect of the chemical structure of the acrylate unit, the influences of molecular structure of acrylate copolymers upon the biodegradability will be clear if the impact of molar content of acrylate units is determined. Therefore, ¹H-NMR spectrum was used to examine the content. The spectra are shown in Figure 4 and the contents of MA units in poly(MA-co-AA) determined by ¹H-NMR are summarized in Table 3. Chemical shift peaks at 3.5 ppm and 12.3 ppm in Figure 4 correspond to the protons of methoxycarbonyl () and carboxyl () in MA and AA, respectively. As seen in Table 3, the contents of MA units in the copolymers determined by ¹H-NMR are in agreement with those of MA monomers fed in copolymerization.

Figure 4.

¹H-NMR spectra of acrylate copolymers with molar content of MA unit: (0#) 50%; (1#) 60%; (2#) 70%; (3#) 80%; (4#) 90%.

Table 3.

Molar content of MA units in acrylate copolymers

Samples	Fed in copolymerization (%)		Measured with ¹H-NMR (%)
Samples	MA	AA	MA	AA
0#	50	50	51.1	48.9
1#	60	40	64.7	35.3
2#	70	30	74.1	25.9
3#	80	20	82.5	17.5
4#	90	10	90.4	9.6

Table 4 describes the influence of molar content of MA units in poly(MA-co-AA) upon COD, BOD₅, and BOD₅/COD values of the copolymers. Obviously, the content shows marked impact on the values. Increase in the content from 50% to 90% results in an increased COD and a decreased BOD₅, as well as a reduced BOD₅/COD value. The reduction in values of BOD₅/COD seems to also be due both to BOD₅ reduction and COD increment. This implies that the increase in MA content inhibits aerobic biodegradation for the copolymers. It can also be observed that the value of BOD₅/COD of the copolymers decreases sharply when MA content exceeds 80%. Naturally, the increase in MA content reduces hydrophilicity of acrylate copolymers because MA units are hydrophobic. The reduction in hydrophilicity makes the polymeric chains aggregate in water and results in worse contact of the polymeric chains with microorganisms especially when the copolymers are not dissolved. Obviously, the contact of polymeric chains with microorganisms is necessary for the attacks of microorganisms on the copolymeric chains. For this reason, molar content of hydrophobic acrylate monomers should not be excessive. Otherwise, the excess of the content will reduce the dispersibility of acrylate copolymers and adversely influence their biodegradability. However, the adhesion of acrylate copolymers to fibers tends to decrease when the content of hydrophilic monomers exceeds 40%.⁴ The adhesion limits the further increase of hydrophilic monomer in monomer formulation. In addition to the limitation of adhesion, an excessive amount of hydrophilic monomer increases re-bonding risk of sizing film on surface of sized yarns via moisture absorption. For those reasons, the content of hydrophilic monomers should not be excessive. Based on the experimental results obtained previously in adhesion investigation,⁴ preferred molar content of acrylate units should be in the range of 60% to 70%.

Table 4.

Effect of molar content of MA units in poly(MA-co-AA) on COD and BOD₅

Molar content of MA in monomer formula (%)	BOD₅ (mg O₂/L)	COD (mg O₂/L)	BOD₅/COD
50	41.4	166.7	0.248
60	41.3	170.8	0.242
70	37.0	175.3	0.211
80	34.8	189.3	0.184
90	9.2	192.8	0.045

To consider the variation of molecular weights via aerobic biodegradation, GPC is used to measure the molecular weights of the copolymers before and after cultivation. The chromatograms of poly(MA-co-AA) with a MA molar content of 70% are shown in Figure 5. According to the principle of GPC separation, components in the sample leave the chromatographic column in a decreasing order of molecular weight. Therefore, molecular weights of curve peaks of GPC decrease with the increase of separation time. Obviously, the cultivation makes the curve peak of the chromatograms shift to the direction of low molecular weights. Besides, a new peak (peak 3) appears at low molecular weight region. This means that average molecular weights of acrylate copolymers decrease markedly via cultivation, as summarized in Table 5. The observation demonstrates that the copolymers degrade during cultivation via the attack of aerobic microorganisms on the molecular chains. Obviously, the microorganisms are capable of converting the macromolecules of acrylate copolymers to lower molecular weight substances. The biodegradation can convert poly(MA-co-AA) with a molecular weight of more than 100,000 to lower molecular weight end-products in which contain both polymers and oligomers. This observation confirms that the copolymers are considerably biodegradable when the copolymers possess suitable molecular structure. Naturally, biodegradable sizes can be obtained if type and content of acrylates in monomer formulation can be appropriately designed.

Figure 5.

GPC chromatograms of poly(MA-co-AA) with 70% molar content of MA units. (a) before cultivation and (b) after cultivation.

Table 5.

Average molecular weights of poly(MA-co-AA)

Sample		M_w (×10⁴)	M_n (×10⁴)	M_p (×10⁴)
Before cultivation	Peak 1	11.3	2.38	10.1
After cultivation	Peak 2	10.4	2.52	7.34
After cultivation	Peak 3	0.14	0.10	0.13

Note: M_w denotes weight-average molecular weight; M_n indicates number-average molecular weight; M_p expresses peak molecular weight. MA content of poly(MA-co-AA) was 70%.

Conclusions

Molecular structure of acrylate copolymers exhibits a marked influence upon their aerobic biodegradability. Length or carbon atom number of alkyl in acrylates, presence of α-methyl in acrylates, and molar content of acrylate units along molecular chains of acrylate copolymers show substantial influence on the biodegradability. Increasing the alkyl length of the alkyl ester reduces aerobic biodegradability of acrylate copolymers. Therefore, it is preferred to reduce alkyl length of acrylate monomer for the manufacture of acrylate copolymers. This demand is a conflict with the requirement of adhesion-to-fibers for the monomers, but consistent with that of sizing film strength. Presence of α-methyl in acrylate units makes the copolymeric sizes resistant to aerobic biodegradation. This means that methacrylates should be avoided in monomer formulation. The demand is consistent with that of adhesion-to-fibers, but conflicts with the requirement on re-bonding and tensile strength of sizing film. With the increase in molar content of acrylate units, the biodegradability is reduced. Therefore, properly reducing the amount of hydrophobic acrylate monomers favors to improve the biodegradation. This demand is consistent with water solubility, pasting and desizing of acrylate copolymeric sizes. However, the reduction is restricted by the adhesion to synthetic fibers. To produce the copolymeric sizes that are able to biodegrade more easily, an alternative is to choose suitable type and amount of acrylate monomers with shorter alkyl group in alkyl ester and without α-methyl, and to make the acrylate monomers copolymerize with AA. Preferred molar content of acrylate monomers in the formulation should be in a range of 60% to 70%.

Footnotes

Funding

The work was supported by the Research Foundation Program of Scientific and Technological Innovation Team of College and University at the Provincial Level of Anhui (TD200710).

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