Sewing thread finishing

Officer-TechnicalServices, Application Research centre,

(Resil chemicals Pvt Ltd, Bangalore)

Introduction

Cotton sewing thread is the only general-purpose sewingthread made from natural raw material. Before the advent of synthetic sewingthreads, cotton was used for most sewing operations. With the introduction ofwash and wear fabrics and higher machine speeds, cotton threads were no longeradequate. They provide good sewing performance but have inferior strength andabrasion resistance as compared to synthetic sewing threads. Todaycotton-sewing threads are used only for over-dyed cotton apparel.

With development of modern high-speed sewing lineoperations, the performance of thread in the garment business has becomeextremely crucial. It is the thread, which defines the garment.

Synthetic Threads:

The most popular sewing threads available today, are mainlyof the following types:

Spun polyester: the most widely used apparel thread it canbe used on sheer as well as heavyweight apparel.

Core spun polyester: Of two primary types cotton wrappedand polyester wrapped, each has a polyester filament core. The fibre wrappingaround gives it its superior all-round performance.

Cotton Wrapped Polyester Core: excellent sewability.Commonly used for all apparel and preferred for denim Polyester WrappedPolyester Core: excellent sewability and color retention.

Preferred by industrial garment manufacturers and denimapparel.

Continuous Filament

Synthetics: mainly produced from Nylon and polyester- theyare the strongest

Threads and are lint free. Mainly used for footwear,mattresses, tents etc.

The Performance Parameters:

Usually 100% spun polyester or even texturized polyestersewing threads are used for industrial garment making. Specialty threads (such asembroidery threads) frequently use cotton; rayon or blended yarn is used.

Taking a basic every-day operation such as an eight stitchesper inch straight seam, sewn with a general-purpose lockstitch machine, sewingspeeds can be 5500 stitches a minute. The needle moves downward roughly oneinch through the fabric, and then back up again the same distance for everystitch it forms whilst the feed mechanism moves the fabric along. Thus theneedle on that machine has the potential to move up and down sixteen times, orinches, per inch sewn.

At 5500 stitches per minute, with both an up and downmovement every stitch, the needle actually moves 11000 inches per minute - aspeed equivalent to approximately 10.5 miles per hour.

The top thread, however, not only goes the distance of theneedle but is looped round the bobbin and then pulled taut. If the distances ofthe thread from the top needle point, down through the fabric and throat plate,round the bobbin and back up again and pulled tight is approximately doublethat of the needle movement, the thread is traveling at over twenty miles perhour, through a fabric whose individual yarns are normally smaller than thethread and tightly woven or knitted together. When the needle penetrates thefabric being sewn, the yarns have to give way to the needle within a timeframethat can be as low as 0.0003 seconds.

Passing through a thick, tightly woven fabric causesconsiderable resistance, friction, and heat build-up - all of which affect theneedle and the thread. Research work shows, for example, that needletemperature can rise within seconds to 350C (100C above the melting point ofpolyester).

So it is hardly surprising that needles get broken, bent,points and throat plates damaged, stitches slipped and fabric damaged, puckeredand distorted.

Thus thereasons for the uncompromising performance parameters especially in a threadbusiness are easily understandable.

The Processing:

The processing of threads involves basically two fundamental steps:

q Dyeing (Usually disperse dyed, Reduction clearing rinse)

q Lubricating.

Thread Lubrication:

Silicone polymers due to their inherent properties of good heat resistant, excellent lubricating properties and other related properties of ecological importance is considered as the best ingredient for a good thread lubricating formulation. In theory, all lubricant products all work in a similar way. If you could see a magnified close-up of a metal surface, such as the eye of a needle or a sewing machine bobbin hook, you'd notice it has very tiny pits; these pits are present on even the finest metal sewing tools and equipment. Silicone particles act as microscopic ball bearings to fill in these pits, making the metal smoother and reducing friction. With less friction, the needle stays cooler and the thread is less likely to shred or break or the machine to skip stitches.

Thus Lubrication

o Cools the needle during high speed sewing, preventing thread burn out

o Prevents wear and tear of machine parts

o Increases seam strength by preventing strength loss due to abrasion while sewing

Application Methods:

There are two types of application method:

Exhaust Method: Where the lubricants are used post dyeing in exhaust system (RESIL RANGE OF PRODUCTS: Resil yarn X series, Resil Innocelle YL, Resil Optaglide LW , Resil AS1, ResilBT1007)

Lick Roll Method: Where the threads are taken out of the dyeing machines, dried and then lubricants are used from a roller to the thread. This method is usually the most common method (RESIL RANGE OF PRODUCTS Resil Optaglide SW , Resil DPL 045A, Resil BL 1037, Resil Optasol 85 , Resil DPL 282 , Resil DPL 127, Resil Innocelle 1728 ,Optasol 145)

The main function of sewing threads is to hold together parts that could be of textile, leather etc to form garments etc and other made-ups. Sewing machines are used to join the parts with the sewing thread in a process called Sewing.

Industrial Sewing is an extremely high-speed operation (up to 12,000 stitches per minute) that demands a great deal from the sewing thread. The kinds of forces that are exerted on sewing threads during sewing are:

Braking force (starting and stopping forces)

Forces exerted by the thread tightening spring

Frictional forces at the various deflection points (e.g., take-up lever)

Inertia forces occurring during abrupt acceleration, especially in high speed sewing machines

Constant changes in strain on the thread.

In addition, it has been found that the sewing threads are exposed to extremely high temperatures (up to 400 o C in some cases) of the sewing needle. This is critical especially in case of synthetic fibre sewing threads whose melting point is about 250o C.

In addition, the sewn article must meet stringent quality standards, such as Seam strength etc. these depend on a few parameters like:

Loop strength

Resistance to chafing

Fabric

Seam orientation

Sewing thread diameter

Flexibility of the sewing thread

Thus the basic properties that the sewing thread must possess at the very least are

Low coefficient of friction

Good gliding properties

Good heat protection

In this context, sewing thread finishing assumes critical importance due to the properties that the finishing agent imparts to the sewing thread.

The following measures are necessary to solve the problems inherent in the sewing process-

Adequate finishing of fabric

Adequate finishing of sewing yarn/thread

Air / water-cooling of needle etc.

SEWING THREAD FINISHING

The traditional finishing agents are

Paraffin wax products to lower the coefficient of friction

Silicones Special heat protection

Anti static agents to prevent static charge buildup

This is done by the following methods

Lick roll method

Dye bath lubrication

Dip tank lubrication

Solid wax lubrication

Finishing methods

Lick Roll method

Water free method:

In this method, silicone oils or paraffin oils/waxes are applied directly on the sewing thread by a roller that dips into the lubricant and coats the yarn moving on the roller surface.

This is done in two ways:

Cold melt method: where the lubricant is applied at room temperature on the yarn

Disadvantage:

o Low viscosity silicone oils of about 350 CST have to be used.

_ Poor Heat protection

o Mix of Silicone oils and paraffin waxes cannot be applied, as they are not miscible at room temp.

_ Poor friction reduction

o Anti static agents cannot be applied in this method

Hot Melt Method: here a heatable roller applies the lubricant at about 70-80 o C. A mix of paraffin and silicones may be applied resulting in lower friction compared to the cold melt method.

Disadvantage:

o Low viscosity silicone oils (350 CST) have to be used.

_ Poor Heat protection

o Contains some paraffin (silicones and paraffins are miscible at 70-80_ C to some extent)

_ Improved friction

o Anti static agents cannot be applied in this method

Treatment with aqueous emulsions:

In this method, aqueous (water-soluble) emulsions of silicone oil, paraffin waxes and some specialty additives are used as thread lubricants.

Aqueous emulsions

●Contain emulsified silicone oil with viscosities up to 30,000 CST

_ Excellent Heat protection

● Contain optimum mix of paraffin waxes and silicone emulsified together

_ Excellent Heat protection

● Contain water soluble or emulsified antistatic agents

_ Antistatic properties

Advantages:

● Even distribution of pick-up on yarn surface

● No limitations on choosing ingredients

● Can include other compounds such as fatty acid condensates

● Every component in the emulsion has a very small particle size and hence a high surface area.

This allows the lubricant to penetrate and spread on the surface of the yarn.

Disadvantages:

Some of the disadvantages are

Skin formation may occur due to silicone micelle agglomeration resulting in viscosity changes and consequent uneven pick up.

Quick drying systems required or drying period necessary

Thread Lubricant Evaluation

Following are the factors that can affect the performance of the sewing thread

Count

Count indicates the mass per unit length of the thread i.e., Linear density.

In a normal lick roll operation, the thread picks up anywhere between 15% of lubricant on its own weight and this contributes to an apparent increase in count of the thread after finishing.

Strength and Tenacity (specific strength)

Strength is defined as the applied load (in Newton) at break.

Tenacity is defined as the mass stress at break (unit: -gram per denier per tex) or (N/tex). Contrary to popular notion, thread lubricant application does not increase the tenacity of sewing threads regardless of the quantity applied. Some thread lubricants (such as silicone oil) can in fact decrease tenacity and tensile strength. This is mainly due to the fact that silicone oils, with their spreading ability lubricate inter-fibre surface area causing the twist to unravel easily and thread break occurring at lower load levels. Emulsion application is much more optimized application method and offers the lowest strength losses.

Coefficient of Variance (CV%)

This is a critical test for any thread lubricant as it evaluates the evenness of the lubricant coating.

(Lubricant emulsions with their wax and other additive content are prone to micro lumps that affect the way in which they coat the thread)

A low CV% indicates a uniform lubricant coating on the thread and low variation in properties over different lengths of the thread.

Thread elongation

One of the lesser understood areas in sewing thread technology; thread elongation has been an important basis for comparing different threads.

Elongation at break is defined as the amount (expressed as a % of its original length) that a thread is extended at its breaking point.

However, figures representing elongation at break give little indication of a threads behavior during sewing. As far as practical sewing applications are concerned, a far more important consideration would be elongation measured at loads normally experienced during sewing operations.

A simple reason for this: stress strain curves for sewing threads are not linear graphs at all i.e., threads giving lowest elongation at break do not necessarily give the lowest elongation at lower load levels (as normally experienced during sewing). Thus, sewing loads being far lower than breaking loads, different thread behavior can be expected. Consider 3 sewing threads (staple spun polyester) with different elongation behavior. Thread A and B shows the lower elongation at break as compared to thread C. (refer figure1)

However thread B and C have shorter needle loops (refer fig.2) on a production sewing machine as compared to thread A which gives them a higher potential to skip stitches under conditions where the needle is deflected or fabric flagging occurs. Thus, the elongation at lesser loads better indicates this behavior than by the elongation at break

Finish % or pickup

A traditional method of evaluating pickup has been the % pickup or the finish % method. This is often mentioned in lubricant recommendations as a vital parameter.

% Pickup is defined as the weight increase (as a % of its original weight) after lubricant application. It would be however more useful to measure parameters more useful in practical sewing applications (such as sewability, friction and strength) rather than finish%.

A simple reason for this that lubricating functions are determined more by the viscosity of the silicone oil used rather than its weight addition.

Silicone lubrication on a sewing thread can be classified into-

● Surface lubrication: contributes to sewability, heat protection and reduction of COF

● Inter-fibre lubrication: contributes to strength loss

Sewability

The most critical factor in sewing thread lubricant evaluation; it is measured in many different ways. In principle, it has been established that the force required to penetrate a fabric with a sewing needle is a measure of the sewability of the fabric i.e., the susceptibility of the fabric or thread to needle damage by bursting, breaking or fusing.

Friction characteristics

Needle heat is generated by the friction between the fabric and the needle blade. Ideally, the thread lubricant comes off on the needle blade surface allowing it to penetrate the fabric with less friction.

Thus, the ease with which a sewing thread can penetrate a fabric is determined by its frictional characteristics. Friction (and consequent heat liberated) is largely determined by the quality of lubrication as it eases the passage of the sewing thread within the bobbins and needle of the sewing machine and further allows it to have an easy passage between the fibres and yarns in the fabric. Thus the force required to penetrate the fabric is low, heat liberated is lower and no damage occurs.

This makes for a far neater and tighter seam with a lesser scope for skipped stitches and seam pucker. Lesser number of thread and needle breakages have a huge impact on the stitching line efficiency and results in tremendous cost reduction for the garment manufacturer.