Heat-Induced Changes in the Properties of Cotton Fibers

13 Heat-Induced Changes in the Properties of Cotton Fibers S. H. ZERONIAN Downloaded by KTH ROYAL INST OF TECHNOLOGY on December 2, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch013

Division of Textiles and Clothing, University of California, Davis, CA 95616

Although a considerable number of studies have been made on the effect of heat on the properties of cellulosic materials (15), the alterations induced in the cellulose fine structure are incompletely understood. Little is known of the causes of the changes and how the changes affect physical properties of cotton fibers. As noted previously (6), when cellulose is heated in the range 100°C to 250°C some of the observed changes can be explained in terms of alterations in either physical or chemical properties of the material. It is possible also that both can occur simultaneously. In order to obtain clarification, additional studies are needed. Such studies would aid in understanding the mechanism of cellulose pyrolysis. The work may also have practical implications. It is possible that the physical properties of cotton fibers can be changed advantageously for some end uses by heat treatment. One reason that l i t t l e consideration appears to have been paid to the utilization of heat for such purposes is the apparent lack of knowledge of the temperature levels before permanent changes occur in the physical properties of the cotton fiber. A number of workers have attempted to determine the glass transition temperature (Tg) of dry cellulose. The values, which have been summarized previously (6), vary from 22 to 230°C. Recently, with the aid of a torsion pendulum, damping peaks were detected on ramie cellulose (6). Two of importance to this paper are at ca 160 and 230°C. Tentative assignments were made for the peaks. One suggestion is that there is a double glass transition in cellulose. Another is that only the damping peak at ca 160°C is associated with Tg for cellulose, and that the damping peak at ca 230°C may be due to release of water during the formation of anhydrοcellulose in the amorphous regions of the cellulose. If Tg for cellulose is in the region of 160°C, then i t appears that a temperature higher than 160°C is required to bring about, relatively easily, significant changes in the physical proper ties of dry cotton fibers.

189

In Cellulose Chemistry and Technology; Arthur, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

190

CELLULOSE CHEMISTRY AND TECHNOLOGY

The e f f e c t of heat on the s t i f f n e s s modulus and r e s i l i e n c y (recovery) of dry c o t t o n yarn has been measured by Bryant and Walter (J), and a l s o by Conrad et a l . ( 8 ) . S t i f f ness modulus was d e f i n e d by Bryant and Walter as 100 times the s t r e s s a t 1% e l o n g a t i o n . Conrad e t a l . used e s s e n t i a l l y the same d e f i n i t i o n . Recoveries were determined from the r e l a t i v e l e n g t h of the base l i n e s of s t r e s s - s t r a i n curves a f t e r l o a d i n g to about 1% e x t e n s i o n , and then unloading. Bryant and Walter deduced from t h e i r data that T f o r dry c o t t o n i s above 240°C. Between 100 and 240°C, the r e s i l i e n c y of t h e i r c o t t o n yarn remained roughly constant. However, the s t i f f n e s s modulus of the yarn remained constant only between 80 and 140°C. Above 180°C i t began to decrease r e l a t i v e l y r a p i d l y . Bryant and Walter (7) d i d not comment on t h i s decrease, but i t can be p o s t u l a t e d that i t i s an i n d i c a t i o n of the onset of thermal s o f t e n i n g of c o t t o n . In c o n t r a s t , Conrad et a l . (8) found a gradual decrease i n s t i f f n e s s modulus f o r c o t t o n as the tempe r a t u r e was r a i s e d from 100 to 233°C. They s t a t e d there was no evidence of a thermal s o f t e n i n g temperature. The r e c o v e r y temperature curve of Conrad et a l . f o r c o t t o n yarn d i s p l a y e d a continuous i n c r e a s e between 100 and 233°C. Some of the d i s c r e p a n c i e s between the r e s u l t s of these two groups of workers may have been due to the measured mechanical p r o p e r t i e s having been a f f e c t e d by yarn s t r u c t u r e as w e l l as by the p h y s i c a l propert i e s of the f i b e r s .

Downloaded by KTH ROYAL INST OF TECHNOLOGY on December 2, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch013

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Studies have been i n i t i a t e d a t Davis to o b t a i n a b e t t e r understanding of heat-induced changes i n the p r o p e r t i e s of cotton c e l l u l o s e . In the i n v e s t i g a t i o n r e p o r t e d here, the e f f e c t of heat on the mechanical p r o p e r t i e s of c o t t o n yarn i n the range of 100 to 240°C was determined to help r e s o l v e the d i s c r e p a n c i e s between the r e s u l t s of Bryant and Walter (_7) and those of Conrad e t a l . ( 8 ) . A l s o , the e f f e c t of heat i n t h i s temperature range on the supramolecular s t r u c t u r e of c o t t o n f i b e r was s t u d i e d . Depolymerization occurs when c e l l u l o s e i s heated. Thus, the p r o p e r t i e s of the h e a t - t r e a t e d samples were compared w i t h those of a c i d - h y d r o l y z e d m a t e r i a l s to e s t a b l i s h i f the changes observed i n the f i n e s t r u c t u r e of t h e r m a l l y t r e a t e d f i b e r s could be explained i n terms of depolymerization . Materials K i e r - b o i l e d c o t t o n yarn was used. For mechanical propert i e s , two types were employed, an 80/2 s f i l l i n g t w i s t and a 20 s s i n g l e s m e r c e r i z i n g t w i s t . For the remaining experiments, o n l y the l a t t e r yarn was used. Cupriethylenediamine hydroxide (CED) s o l u t i o n was obtained from Ecusta Paper D i v i s i o n , O l i n Matheson Chemical Corp., and SF-96 (50) s i l i c o n e f l u i d from S i l i c o n e Products Dept., General E l e c t r i c Co. Other chemicals were reagent grade. 1

1


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Cotton Fibers

191


Methods of Treatment Heat treatments. Cotton yarn was d r i e d f o r 18 h r a t 60°C under vacuum. Phosphorus pentoxide was placed i n the vacuum oven t o a s s i s t the d r y i n g . The yarn was then heated i n s i l i c o n e o i l i n a r e s i n r e a c t i o n v e s s e l f o r d i f f e r e n t lengths of time a t v a r i o u s temperatures. N i t r o g e n was bubbled through the o i l con t i n u o u s l y throughout the heat treatment. At the end o f such treatments, the products were washed w i t h toluene t o remove the oil. Then the samples were s o l v e n t exchanged w i t h e t h a n o l , f o l l o w e d by e t h y l e t h e r . F i n a l l y , the samples were exposed to the l a b o r a t o r y atmosphere. A c i d h y d r o l y s i s . Cotton yarn (5 g per 250 ml of s o l u t i o n ) was hydrolyzed i n 2.0 Ν h y d r o c h l o r i c a c i d a t e i t h e r 21, 50, o r 60°C f o r d i f f e r e n t lengths o f time, depending on the extent o f h y d r o l y s i s r e q u i r e d . To terminate the r e a c t i o n , the sample was washed thoroughly w i t h d i s t i l l e d water u n t i l i t was a c i d f r e e . To d r y the product, the yarn was s o l v e n t exchanged with e t h a n o l , followed by e t h y l e t h e r . F i n a l l y , i t was exposed to the l a b o r a t o r y atmosphere. L e v e l - o f f degree of p o l y m e r i z a t i o n (LODP) samples were prepared i n the manner d e s c r i b e d p r e v i o u s l y ( 9 ) . C h a r a c t e r i z a t i o n of Products D e s c r i p t i o n s have been given p r e v i o u s l y o f the procedures used t o determine the f o l l o w i n g : moisture r e g a i n s a t 59% RH and 21°C (10); i n f r a r e d s p e c t r a ( 9 ) ; and a l k a l i s o r p t i o n c a p a c i t y (ASC) (11). A l k a l i s o l u b i l i t y i n 0.25 Ν NaOH was measured by a method e s s e n t i a l l y s i m i l a r t o t h a t o f Davidson (12). F i b e r s were examined f o r morphological change w i t h a l i g h t microscope a t a m a g n i f i c a t i o n of 200. The f i b e r s were mounted i n 5N NaOH a t room temperature. X-ray d i f f r a c t i o n measurements were made w i t h a Siemens x-ray d i f f r a c t o m e t e r u s i n g a f o c u s i n g technique i n a manner e s s e n t i a l l y s i m i l a r t o t h a t d e s c r i b e d by Segal e t a l . (13). The sample was scanned over the range 2Θ = 6° t o 30° u s i n g KQ^ r a d i a t i o n obtained from a copper t a r g e t and n i c k e l f i l t e r a t 35 Kv and 24 ma. The extent of depolymerization of the degraded samples was determined from v i s c o s i t y measurements. The samples were s u f f i c i e n t l y degraded so that v i s c o s i t i e s were determined i n CED by ASTM, D1795-62 (14). T h i s method i s r a p i d , and i s s a t i s f a c t o r y f o r samples w i t h i n t r i n s i c v i s c o s i t i e s l e s s than 15. The f a c t o r used t o convert i n t r i n s i c v i s c o s i t y t o DP was 190. The i n t r i n s i c v i s c o s i t y of nondegraded c o t t o n was measured i n tris(ethylenediamine)cadmium hydroxide (cadoxen) as d e s c r i b e d by Henley (15). DP was c a l c u l a t e d from the f o l l o w i n g r e l a t i o n (16).



192


(η) = 1.84

x 10-2

(DP)0.76

Mechanical P r o p e r t i e s . Measurements were made with a t a b l e model I n s t r o n U n i v e r s a l T e s t i n g machine equipped with an environmental chamber. S t a r t i n g w i t h an i n i t i a l gauge l e n g t h of 4 i n c h the yarn, c o n d i t i o n e d a t 65% RH and 21°C, was extended a t a constant r a t e of 1 i n c h per min u n t i l a l o a d of 40 g had been a p p l i e d at which time the crosshead was s t o p ped. A f t e r 1 min i t was returned to the s t a r t i n g p o s i t i o n a t the same speed used i n the e l o n g a t i o n phase. A f t e r a r e l a x a t i o n time of 1 min, the specimen was extended again a t the same r a t e as b e f o r e u n t i l the specimen became t a u t . T h i s c o n d i t i o n was i n d i c a t e d by the c h a r t pen r i s i n g from the zero reading on the c h a r t . The crosshead was again returned to the s t a r t i n g p o s i t i o n , and the sample was subjected to the d e s c r i b e d t e s t i n g procedure a t each temperature as the yarn temperature was r a i s e d i n f i v e consecutive s t e p s . S t r e s s decay was determined by measuring the r e d u c t i o n i n l o a d as the yarn was h e l d a t constant l e n g t h f o r 1 min a f t e r i t had been extended u n t i l i t bore a l o a d of 40 g, and d i v i d ing t h i s v a l u e by the i n i t i a l l o a d (40 g ) . Recovery was determined by measuring the extension r e q u i r e d to i n c r e a s e the l o a d i n the yarn from 0 to 40 g (A cm), and a l s o the s l a c k r e maining i n the yarn a f t e r removal of the load followed by 1 min r e l a x a t i o n (B cm). Then, Recovery, % =

A

- Β A

χ

100

Modulus was measured as the slope of the l o a d - e l o n g a t i o n curve at 40 g l o a d . A l l r e s u l t s are the mean of 6 t e s t s . The s t r e s s decay and modulus data are presented as a f r a c t i o n of v a l u e s determined a t 100°C. R e s u l t s and

Discussion

Mechanical p r o p e r t i e s . The i n i t i a l p o r t i o n of the l o a d extension curve of c o t t o n contains a pronounced toe ( F i g . 1 ) , a f t e r which the curve appears to s t r a i g h t e n . In a c t u a l f a c t , i t remains concave. Bryant and Walters (7) d i d not s t a t e whether or not they ignored the toe r e g i o n i n determining the extension a p p l i e d to t h e i r yarn during the measurements r e l a t ing the e f f e c t of temperature on modulus and r e s i l i e n c y of c o t t o n yarn. Conrad et a l . (8) i n c l u d e d the toe r e g i o n i n extension measurements. With the yarn used i n the present work, 1% extension occurred j u s t beyond the toe r e g i o n . To make measurements unambiguous, i t was decided t h a t a l l t e s t i n g would be done on yarn extended u n t i l i t bore a 40-g l o a d . In t h i s manner, measurements were being made i n a r e g i o n w e l l beyond the toe, and thus i n an area i n which the f i b e r s were i n t e n s i o n and bearing s t r e s s . The s l o p e , or modulus, of the l o a d -


13.

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Cotton

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Fibers

extension curve a t 40-g l o a d was taken as a measure of s t i f f ness, r a t h e r than t a k i n g the load a t 1% extension. As the temperature was r a i s e d to 100°C, the yarn shrank i n l e n g t h due to moisture r e l e a s e from the f i b e r s . Shrinkage appeared to be i n s i g n i f i c a n t above 100°C. Thus, only data above 100°C a r e considered. To e s t a b l i s h i f yarn s t r u c t u r e was a f f e c t i n g r e s u l t s , two types o f yarn were used, namely a low t w i s t s i n g l e s and an 80/2 s yarn. Both yarns have e s s e n t i a l l y s i m i l a r r e s u l t s ( F i g . 2 and 3 and Table I ) . A l l measurements presented were made i n a i r . P i l o t measurements were made i n a n i t r o g e n atmosphere, but the r e s u l t s were e s s e n t i a l l y s i m i l a r to those made i n a i r . The modulus data ( F i g . 2) and the r e s i l i e n c y data (Table I) a r e roughly s i m i l a r to those o f Bryant and Walter ( 7 ) . In the p r e sent case, the modulus of the c o t t o n yarn began to f a l l a t 180°C and had decreased s i g n i f i c a n t l y above 200°C w i t h r e s p e c t to the yarn modulus a t 100°C. The s t r e s s decay data ( F i g . 3) a l s o i n c r e a s e d s i g n i f i c a n t l y above 200°C. Both the lowering o f the modulus and the i n c r e a s e i n s t r e s s decay a r e phenomena that c o u l d be expected to occur i n the r e g i o n of a g l a s s t r a n s i t i o n . In our e a r l i e r study (6), i t was suggested Tg occurs a t c a 160°C. I t should be noted, however, that the measurements made i n the present study would be a f f e c t e d not only by i n t r a f i b e r p r o p e r t i e s , but a l s o be i n t e r f i b e r f o r c e s . Thus, modulus and s t r e s s decay measurements o f c e l l u l o s e i n the form o f yarn may not be s u f f i c i e n t l y s e n s i t i v e methods f o r determining T . The r e covery data (Table I) a r e s c a t t e r e d and do not i n d i c a t e a s i g n i f i cant t r e n d . Again, these recovery measurements may not be s u f f i c i e n t l y s e n s i t i v e to changes i n f i b e r p r o p e r t i e s to g i v e a determination of T . Back and D i d r i k s o n (17) have claimed a secondary t r a n s i t i o n f o r c e l l u l o s e a t 175 to 200°C, and a g l a s s t r a n s i t i o n a t 230°C, from measurements made on the modulus o f e l a s t i c i t y o f paper. Goring (18) has reported that the s o f t e n i n g temperature f o r c e l l u l o s e ranges between 230 and 250°C depending on the type of c e l l u l o s e . I t should be emphasized, however, that i n a l l the techniques d e s c r i b e d f o r


1

g

g

TABLE I Recovery o f two types of c o t t o n yarn p r o g r e s s i v e l y heated above 100°C. Yarn Recovery, 7 80/2 s 20 s singles Temperature, °C 72 100 77 79 150 78 79 175 84 78 200 81 75 68 225 R e c o v e r i e s measured a f t e r a p p l i c a t i o n o f 40 g l o a d . f

f

a




50 r

Figure 1. Initial por tion of load-extension curve of cotton yarn

Ο Ο

I

2

EXTENSION, %

0.9

Heat-Induced Changes in the Properties of Cotton Fibers

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