Method and Product for Silk Grafting

Technical Field of the Invention

This invention relates to a process and to a product for silk treatment, especially to the grafting technology of degummed silk with methacrylate monomer mixtures.

Technical Background

Processing of Silk

The processing of silk comprises the following steps:

• Degumming

The term degumming (boiling-off) refers to a partial or complete removal of the sericin that covers the two fibroin filaments of a silk fiber. The sericin sheath is responsible for the bhttleness, harsh handle, mat appearance and yellowish to yellowish green colour of raw silk. Only after degumming does silk attain its full gloss and its soft, smooth feel, which can be improved further in a finishing process following dyeing. Degumming results in a weight loss that can amount to 2 - 30%, depending on the desired degree of degumming.

• Weighting

Weighting of silk refers to the incorporation of metal salts, plant materials, or synthetic substances into the fibroin to compensate for the loss in weight during degumming. Moreover, increased fiber volume and improved luster and handle are achieved in this process. Hanks are usually weighted before dyeing, in vats, on sticks, or in special centrifuges, hank-washing machines, or continuous open-width washing machines. The weighting agent is fixed so that it is washfast. Weighting with water-soluble compounds such as dextrin increases the weight without producing a permanent improvement in fiber properties.

Today, tin phosphate silicate weighting (mineral weighting), a method known for decades, plays only a minor role. Silk is treated in succession with aqueous solutions of tin(IV)-chlohde and disodium hydrogenphosphate.

Basic tin phosphate formed in the fiber is subsequently converted to washfast tin sodium silicate in a concentrated silicate bath. The degree of weighting depends on the number of treatments with tin salt and phosphate; increases in weight of more than 100% are possible. The degree of weighting is based on the weight of raw silk. If weighting compensates exactly for the loss in weight incurred during degumming, the silk is referred to as par. For example, 50% above par means that the weight of 100 kg of raw silk has been increased by 50 kg.

• Bleaching

In most cases, the colored pigments present in sericin are removed during degumming. To achieve clear white tones and brilliant light shades,

oxidative or reductive bleaching can be used. Bleaching is usually carried out after degumming. Reductive bleaching agents that can be used for silk are sulfur dioxide, dithionite, or hydrogen sulfite. Sulfur dioxide is preferred to other reductive bleaching agents because of easy handling, lower cost, and fiber preservation, (cited from Ullmann's Encyclopedia of Industrial

Chemistry, Release 2007. 7 ^th Edition)

Other steps during silk-processing are dyeing and printing and special finishing.

Silk fibers exhibit many outstanding properties, such as handle, luster, dyeability, etc. that distinguish them from other natural and synthetic fibers. On the other hand, some minor textile performances, such as crease recovery, thermal stability, wash and wear characteristics, need to be improved to expand silk consumption and to meet customer requirements. So many technologies have been developed for increasing the density and improving the property of silk fibers. Grafting with an organic monomer has been regarded as a powerful tool for this purpose and attracted the interest of both scientists and technologists during the past 3 decades.

State of the Art

Masuhiro Tsukada studied grafting vinyl monomers onto silk (Bombyx mori) using different initiators (Journal of Applied Polymer Science (2001 ), 81 (6), 1401-1409). The tested monomers in this publication are methyl methacrylate (MMA), methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA) separately.

According to this investigation, the tensile strength was decreased to approximately 60% of the original value (before grafting) corresponding to the increased weight of the silk from 0 wt.-% to 50 wt.-%, despite of the used monomers (MMA, HEMA and MAA).

A Chinese patent application, CN 1.693.574, describes a method for grafting and weighting silk fibers which comprises refining silk fibers, washing, drying, treating with low-temperature plasma and grafting treatment in a solution containing a vinyl graft monomer, a nonionic surfactant and a solvent at 30 - 55 ⁰ C for 20-60 min. The said vinyl monomers are styrene, acrylic acid, methacrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate or mixtures of them.

Another Chinese patent application, CN 1.776.032, discloses a process for increasing cocoon silk weight after degumming. According to this document, the process comprises of introducing (by wt. parts) clean water (800 - 1 ,000 parts) into a dyeing vessel, adding the auxiliary agent of methacrylamide (MAA) (20 - 30 parts), heating to 4O ⁰ C - 6O ⁰ C, soaking bobbins wound with cocoon silk (30 parts) in the dyeing vessel for 8 - 20 min, adding formic acid (2.5 - 3.5 parts) and ammonium persulfate (0.7 - 0.9 parts), introducing hydrogen peroxide and sulfonated amide surfactant into another dyeing vessel containing clean water, heating to 6O ⁰ C - 8O ⁰ C, and soaking the degummed bobbins in the clean water.

The processed cocoon silk has improved drapability, firmness and water absorbability.

Jutarat Prachayawarakorn and Wattana Klairatsamee investigated the effect of solvent on properties of Bombyx mori silk grafted by methyl methacrylate (MMA),

methacrylamide (MAA) and its mixture of MMA and MAA (Songklanakarin J. Sci, Technol., Vol. 27, No. 6, Nov.-Dec. 2005, p 1233-1242).

Disadvantages of the state of the Art

Methacrylamide is currently a widely used monomer for silk weighting on the industrial scale. It has a lot of advantages such as good gloss and hand feeling, high weight increase. But the price of methacrylamide is relatively high. It becomes also poor in the resistance against yellowing of the weighted silk.

Objective and problem solution of this invention:

The objective of this invention is to

• reduce the raw material cost, • to enhance the efficiency of the grafting process,

• to achieve a better resistance against yellowing,

• to simplify the grafting process and

• to improve the quality (e.g. the dyeability) of the grafted silk.

After a lot of lab investigation we have found that these objectives can be achieved through using a (meth)acrylate monomer blend comprising (meth)acrylamide and at least one or more further methacrylate monomers. The preferred further (meth)acrylate monomers can be chosen from the class of

hydroxyl(meth)acrylates, for example, 2-hydroxyethyl(meth)acrylate or hydroxypropyl(meth)acrylate and from the class of alkyl(meth)acrylates, especially methyl(meth)acrylate.

The terminus (meth)acrylate stands for the derivates of the acrylic acid and for derivates of the methacrylic acid and for mixtures of both components.

One of the main advantages of this combination is the low material cost: Hydroxyesters, and especially methylmethacrylate, are much cheaper than methacrylamide. Furthermore, the hand feeling, strength of the silk, the dyeability can be improved while the grafting rate and the monomer yield are maintained at the high level or even also enhanced. Methylmethacrylate is a relatively "hard" monomer, which means it makes the polymer harder whereas hydroxyesters make the polymer relatively soft. With a combination of monomers, selected from these monomer classes, the hardness of the grafted silk can be adjusted within a wide range according to different application requirements.

It is possible to add all the chemicals and silk separately in the reaction vessel. It would be more convenient if there would be a monomer mixture and that can be put into the reaction vessel all at once so that the grafting process could be simplified and the possibility to cause errors or inconsistent proportion or concentration of different monomers could be greatly reduced.

But it is difficult to blend these monomers to get a homogenous mixture because the solubility of these monomers each other is very low and methacrylamide in pure state is a solid substance. So it is not possible to get a homogenous liquid that has a reasonable proportion between these monomers for the grafting purpose.

Methacrylamide is relatively easily soluble in water (20.2 wt.-percent at 2O ⁰ C).

Hydroxyester is easily soluble in water. So water could be a common medium in order to make a homogenous solution for both of the monomers. Unfortunately, methyl methacrylate is only slightly soluble in water. Its solubility in water at 2O ⁰ C is only 1.6 wt.-%. The solubility of methacrylamide in hydroxyethylmethacrylate also not big enough (should be approx. 10 wt.- % at room temperature). The solubility of methacrylamide in methyl methacrylate is very low (« 5 wt. -% at room temperature). Therefore, it would be also impossible to get a water solution with high concentrations of these monomers and reasonable proportion between these monomers.

Surprisingly, we found now a homogenous liquid mixture containing water, methacrylamide, hydroxymethacrylate and methylmethacrylate with sufficient monomer content and with suitable and reasonable proportion between the different monomers for the silk grafting purpose.

The main parameters which have substantial influence on the grafting process are:

- pH of the grafting solution, monomer type and its concentration, - combination of different monomers,

- initiator type and its concentration,

- the mixing situation of the grafting solution,

- the weight ratio of silk to monomer solution.

- Temperature, - Type of reaction vessel and the agitation situation and

- silk type.

The weight increase of the silk can vary in very big range: from 0 wt.-% to 1.000

wt.-% of the degummed silk, if necessary. The monomer yield can reach 50 wt- % to 85 wt. -%, especially 60 wt.- % to 75 wt.- % whereas the common monomer yield is about 50 wt.% or below currently in the industry.

Main points of this invention:

• A monomer mixture containing methacrylamide was found which is suitable for the silk grafting. That is the combination of MAA and HEMA (HEMA can be replaced by HEA, HPMA and/or HPA). The experiments showed that many other combinations do not work partially or totally.

• A monomer mixture containing methacrylamide was found which is suitable for the silk grafting. That is the combination of MAA, HEMA and MMA.

• A homogenous liquid mixture of MAA, HEMA, MMA and water suitable for the silk grafting, that is the monomer content is high enough for the grafting process. The typical proportion of these components is for example 24/18/18/40 (weight ratio). At this ratio, MAA is completely dissolved at room temperature. A crystallisation of MAA can only occur when it has been kept at a temperature of 4 ⁰ C after more than week later. MMA can be totally dissolved in

the mixture without further phase formation even at a temperature below 2 ⁰ C.

• The content of the weighting solution of the different monomers is

■ MAA between 20 weight-% and 99 weight-%, preferably between 30 weight-% and 85 weight-% and most preferred between 50 weight-% and 75 weight-%

■ MMA between 0 weight-% and 50 weight-%, preferably between 0 weight-% and 30 weight-%, and most preferred between 0 weight-% and 20 weight-%.

■ HEMA between 1 weight-% and 60 weight-%, preferably between 10 weight-% and 50 weight-% and most preferred between 20 weight-% and 40 weight-%

♦ further suitable monomers are for example, hydroxypropylmethacrylate

(HPMA) or mixtures of the aforementioned hydroxylmethacrylates. The mixtures of HEMA and HPMA can be used at a broad ratio.

The typical concentration of the monomer-mixture in water may be from 70 weight-% to 30 weight-%, preferably from 65 weight-% to 45 weight-%, and most preferred from 60 weight-% to 40 weight-% of the monomer-mixture in water.

Example 1

Introduce 900 kg cold deionized water into a reaction vessel, heat it from room temperature (RT) to 4O ⁰ C (heating rate: 1°C/min). At the same time turn on the circulation pump in order to keep the liquid in the vessel thermally and chemically homogenous.

Put 28 kg methacrylamide and 12 kg 2-hydroxyethylmethacrylate into the vessel and heat it from 40 to 5O ⁰ C (1 °C/min).

Add 30 kg degummed silk yarn into the vessel and heat it from 5O ⁰ C to 6O ⁰ C (1° C/min) and keep the temperature at 60 ⁰ C for 10 min.

Add 0,7 kg ammonium persulfate (as initiator) and 0,2 kg formic acid (to adjust the pH to 3) into the vessel, heat it from 6O ⁰ C to 7O ⁰ C (1°C/min) and keep the temperature at 70 ⁰ C for 120 min.

Take out the grafted silk and clean it with fresh water. Clean the weighted silk with surfactant about 10 min at 7O ⁰ C and dip it in water at

4O ⁰ C for 5 min.

Dry the grafted silk in an oven. Weigh the grafted and dried silk. The silk weight was increased by 80 wt.%. The tensile strength was increased by 10 %.

Example 2

Fill 900 kg cold deionized water into a reaction vessel, heat it from RT to 4O ⁰ C (1°

C/min). At the same time turn on the circulation pump in order to keep the liquid in the vessel thermally and chemically homogenous.

Add 16 kg methacrylamide, 12 kg methylmethacrylate and 12 kg 2-hydroxyethylmethacrylate into the vessel and heat it from 40 to 5O ⁰ C (1°C/min).

Put 30 kg degummed silk yarn into the vessel and heat it from 5O ⁰ C to 6O ⁰ C (1°

C/min) and keep for 10 min. Put 0,7 kg ammonium persulfate (initiator) and 0,22 kg formic acid (to adjust the pH to 2,8) into the vessel, heat it from 6O ⁰ C to 7O ⁰ C (1°C/min) and keep for 120 min.

Take out the grafted silk and clean it with fresh water.

Clean the weighted silk with surfactant about 10 min at 7O ⁰ C and dip it in water at

4O ⁰ C for 5 min. Dry the grafted silk in an oven. Weigh the grafted and dried silk. The silk weight was increased by 75 wt.- %. The tensile strength was increased by

5 %.

Example 3

Fill 870 kg cold deionized water into a reaction vessel, heat it from RT to 40° C (1° C/min). At the same time to turn on the circulation pump in order to keep the liquid in the vessel thermally and chemically homogenous. Put 67 kg solution comprising of 24 wt.% methacrylamide, 18 wt.% methyl methacrylate, 18 wt.% 2-hydroxyethylmethacrylate and 40 wt.% water into the vessel and heat it from 40° C to 50° C (1 °C/min).

Put 30 kg degummed silk yarn into the vessel and heat it from 50° C to 60° C (1°C/min) and keep for 10 min.

Put 0,7 kg ammonium persulfate (initiator) and 0,18 kg formic acid (to adjust the pH to 3,2) into the vessel, heat it from 60° C to 70° C (1°C/min) and keep for 120 min at 7O ⁰ C.

Take out the grafted silk and clean it with fresh water. Clean the weighted silk with surfactant about 10 min at 7O ⁰ C and dip it in water at 4O ⁰ C for 5 min. The surfactant used is an anionic surfactant, N-methyl-N-oleoyl-taurate. sodium salt. Dry the grafted silk in an oven for 2 hours at 85 ⁰ C. Weigh the grafted and dried silk. The silk weight was increased by 75 wt.-%. The tensile strength was increased by 5 %.

The model of the instrument for tensile strength is AL-7000 S and the vendor name is Gotech Testing machines CO., Ltd,Taiwan.

The mechanical property measurements of silk samples in form of single yarn were carried out using a Universal Testing Machine (AL-7000 S) with 100 N load cell, crosshead speed of 30 mm / min and gauge length of 25 mm. A minimum of 10

specimens was tested obtain average values of load at break. All mechanical tests were carried out at the temperature of 23°C ± 1 ⁰ C and relative humidity of 50 % ± 5%.