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In on-farm milk filtration, the fabric the milk filter is made from must possess the right qualities in order to protect the milking equipment and to ensure that the milk maintains its high quality – the better the filtration, the better the quality and value of the raw milk.
Filters today are available in many different fabrics, made from many different types of fibres. The characteristics of filter fabrics can be classified according to:
• thickness
• fibre orientation
• nature of fibres
• temperature resistance
• chemical resistance
• colour.
But milk filters used in different applications require different and highly specialised fabric properties in order to perform in the most appropriate and efficient way.
| An automatic milking system where cows are milked one after the other requires an entirely different milk filter than an 80-stand rotary system. |
These characteristics, combined with the technology used to produce the fabric, largely determine the physical properties of the milk filter.
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Most fabrics are manufactured from organic polymers plus some kind of support material. But although there are hundreds of polymers which can form a flat sheet, only a few of these fulfil the essential requirements of a good milk filter:
• able to form a thin sheet
• good swelling characteristics
• high wet strength
• hydrophilic (water loving)
• comply with EU and FDA regulations, as well as USDA requirements.
A strong, efficient and high-hygiene filter fabric contains a mixture of polymers, typicallycellulose, cotton, viscose and polyester. Each of these fibres has characteristics which are advantageous for milk filters.
Cellulose is one of the most common organic compounds on earth, forming around 33% of all plant matter. It is expressed by the formula (C6H10O5)n. Cellulose properties, including its fibre length, strength and absorbency, make it a perfect raw material for milk filter production. Cellulose for commercial production, is mainly obtained from wood pulp and cotton.
Image (top left): Rolls of blue and white milk filter fabric
Image (top right): Blend of polymer fibres
Image (bottom right): Viscose, cotton and cellulose fibres
Cotton is a soft fibre that grows around the seeds of the cotton plant. Processing the cotton results in hair-like fibres of 90% cellulose, with a high degree of strength, durability, tempe-rature and chemical resistance as well as absorbency – excellent properties for producing hygienic and durable milk filters.
Viscose is a soft, man-made material with natural origins, created by dissolving cellulose and reforming it in filaments. It is one of the most absorbent fibres in common use. When water – the major component of milk – comes into contact with the viscose fibres they swell, decreasing the pore size of the milk filter. This enhances the efficiency of the filtration process.
Polyester is a man-made material created from synthesised polymers, and is the most widely used man-made fibre in the world. It possesses many advantageous properties that do not exist in natural fibres, including a high degree of strength, low shrinkage, good heat stability and chemical resistance. As a result, polyester fibres are often spun together with natural fibres.
Flecks of mastitis milk on blue filter
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Milk passing through a professional milk filter |
Resistance test (tensile strength) |
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Apart from the importance of blending a mixture of polymer fibres (viscose, cotton, cellulose, polyester) to produce a fabric with particular properties, fibre length and diameter are also critical factors in the formation of a strong and stable filter. Generally speaking it is the short and fine fibres that deliver the properties necessary for good filtration, whereas the longer fibres promote strength and durability. Therefore, a good milk filter must contain a perfect blend of short, fine and long fibres.
High hygiene is essential on dairy farms. Milk filters must therefore be capable of resisting both higher temperatures and the acidity (low pH) or alkalinity (high pH) of the chosen detergent in order to protect the milking and cooling systems during cleaning.
However, the formulation of dairy detergents is complex, and the actual ingredients are often a trade secret of the manufacturer. It is therefore necessary to test every cleaning detergent to find out how to predict how it may affect a milk filter fabric. Depending on the detergent deemed most appropriate for a farm’s cleaning routines, it is essential to know the limits within which a milk filter can be used without risking damage to the milking and cooling equipment.
Milk filters are available with particularly high-resistance filter fabrics and seams, making them capable of withstanding both higher temperatures and high or low pH values. However, it is important to note that these milk filters must be replaced after each cleaning cycle. They are designed to be used once only.
Although milk filters are traditionally white, in many countries they are now available in a rainbow of colours. Coloured milk filters were developed to aid in the identification of mastitis within the herd. Finding mastitic milk flecks, mucous or clots on the milk filter following a milking session, indicates clinical mastitis in the herd. But these flecks are white and very difficult to see against a milk filter of the same colour. A coloured milk filter can therefore be a great asset on a busy dairy farm.
A key technical point to note regarding the coloured milk filters available on the market concerns the fibre blend. The optimal filter fabric is a fibre blend that includes polyester, viscose, cotton and cellulose, but not all manufacturers include viscose fibres in their coloured filter fabrics. As already noted, viscose fibre is hydrophilic, it is highly absorbent. When water, the major component of milk, makes contact with the viscose fibre it swells, decreasing the size of the fabric pores and thereby improving filtration. Decreasing pore size does affect milk flow rates, but not to the extent of creating bypass. And ultimately, the better the filtration, the better the quality and value of the milk.
Professional disposable milk filters for on-farm milk filtration are manufactured from a nonwoven material which is based on a fibrous web. A milk filter’s physical properties are determined by the characteristics of the web, which in turn is determined by the method used to form the web. Modern technology means different fibres can be combined using various different techniques and this allows a diverse range of nonwovens to be manufactured for different applications.
Nonwoven production generally comprises three stages – web formation, web bonding and finishing treatments. However, modern technology means that more than one stage can take place at a time – all three stages together in some instances.
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A manufactured sheet, web or batt of directionally or randomly orientated fibres, bonded by friction, cohesion, adhesion; excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling. |
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Nonwoven webs can be formed using many methods. The method chosen to form a web is generally determined by the length of the fibres to be used. Traditionally, webs formed from staple-length fibres were based on the textile carding process, whereas webs formed from short fibres were based on papermaking technologies. Although both of these technologies are still in use, newer methods have since been developed, and today, the two common technologies for manufacturing nonwovens are known as dry- and wet-laid web forming.
Dry-laid refers to manufacturing techniques where no water is used. Different dry-laid techniques include blowing fibres onto a moving wire to form the web, or extruding polymers into filaments that are subsequently blown onto a moving web. It is more difficult to produce a uniform web with equally distributed pores using the dry-laid, rather than the wet-laid technique. There are two dry-laying methods – carding and air-laying.
The carding process begins in a very similar way to spinning. Firstly the bales of fibre, both natural and man-made, are opened and blended in the appropriate proportions to meet the characteristics required of the milk filter. After homogeneous blending, the fibres are processed into a web on the carding machine. The configuration of the carding drums is critical, as this controls the milk filter’s fabric weight and fibre orientation. In a secondary process, the web can be parallel-laid, meaning that most of the fibres are laid in the direction of the web travel, or they can be random-laid. Carded webs that are parallel-laid typically result in a web of low elongation and low tear strength in the machine direction, while they typically have the opposite properties in the cross direction.
Another technology to transform fibres into a nonwoven fabric is air-laid technology. In air-laying, the fibres are fed into an air stream and directed onto a moving belt or perforated drum, upon which they form a randomly-oriented web. Compared with carded webs, air-laid webs have a lower density, greater softness and an absence of laminar structure.
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Milk filter manufacture |
Milk filter disc manufacture |
Quality control of glued milk filter
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Webs that are directionally orientated (parallel) in the machine direction result in milk filters with poor tensile strength and low dimensional stability. Webs with randomly-oriented fibres are highly uniform fabrics and provide controlled porosity, high wet strength and good dimensional stability. |
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In the wet-laid filter fabric manufacturing technique the fibres are first dispersed in water to create a homogeneous stock. The mixture is then pumped onto a mesh conveyor belt or perforated drum, where the fibres settle randomly and the water drains away. The nonwoven fabric finally moves on to the drying process. Modern technology has made it possible to manufacture webs with up to three layers of fibres, each with a unique composition. Wet-laid materials are highly uniform fabrics combining controlled porosity with excellent filtration characteristics, high wet strength and dimensional stability. Wet-laid technology is commonly used in the manufacture of paper and is the preferred technique for a number of specialist technical nonwovens, including milk filters.
Wet-laid filter fabric production process
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Wet-laid webs result in highly uniform milk filters which provide controlled porosity (pore sizes), high wet strength, and dimensional stability. |
Regardless of the technology used to produce a nonwoven web (wet- or dry-laid), all webs must undergo a process known as bonding. Once the fibres are formed into a web, the fibres remain loose and lack strength, so the web has to be consolidated in some way. The bonding process is as critical in defining the physical and functional properties of the milk filter as the types of fibre used to create the web. There are several bonding options that complement the chosen fibres to create the appropriate characteristics of different milk filters. The three primary bonding methods are:
• chemical (adhesion)
• thermal (cohesion)
• mechanical (friction).
This is probably the most commonly used technology in milk filter production. With this method, a liquid-based binding agent is applied to the web, either uniformly by impregnating, coating or spraying the web, or intermittently. Intermittent application of the binding agent is known as print bonding and is used to bond the web in specific patterns, leaving most of the fibres binder-free for functional reasons. Chemical bonding generally results in an end-product that is more rigid than nonwovens bonded using other technologies, with excellent tensile strength and resilience.
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In this method, the thermoplastic properties of certain man-made fibres are used to bond the web under a controlled heat process. Sometimes fibres within the web itself can be used (e.g. polyester), but more often the binding agent is a low-melt bi-component fibre. There are several thermal bonding systems available:
• Calendering – Calendering combines heat with high pressure
applied by rollers to weld the fibre web.
• Through-air – Through-air thermal bonding is the ideal method for bulkier products. A carefully controlled hot-air stream bonds a web containing fibres that melt at
low temperatures.
• Drum and blanket – Drum and blanket systems combine pressure and heat to produce
nonwovens of average bulk.
• Sonic bonding – Sonic bonding uses a patterned roller to excite the fibre molecules, creating high-frequency energy that internally heats, softens and bonds the fibres.
Mechanical bonding strengthens the web by physically entangling the fibres, creating friction between them. The two methods of mechanical bonding are needlepunching and hydroentanglement.
The needlepunching method is suitable for almost every type of fibre. Specially designed needles are pushed and pulled through the web to entangle the fibres. Moreover, separate webs with different characteristics can be needled together to produce a gradation of properties that is difficult to achieve by other methods.
Hydroentanglement is the method applied predominantly to carded or wet-laid webs, to produce a fabric with good strength. Fine, high-pressure jets of water are concentrated on the web, which interlaces the fibres, without the need for any additional chemical bonding. The pressure of the water jets defines the strength and porosity characteristics, and can be adjusted according to the requirements of the fabric.
All nonwovens, however produced, require a finishing treatment. A variety of substances can be used to modify or add to the properties of the web in order to meet a milk filter’s precise specifications. Finishing treatments can be applied before or after binding and finalise the production of the milk filter fabric.
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Milk filter discs made from 20 gram fabric |
Quality control of sewn milk filters |
Modern gluing machine for milk filters
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Web geometry refers to the predominant orientation of the fibres (directional or random), the shape of the fibres (straight, hooked or curled) and the extent of inter-fibre entanglement and crimp. Fibre diameter and length, web weight and the chemical and mechanical properties of the polymers also influence a web’s characteristics. In milk filters, the fibre orientation of the web in terms of the machine direction/cross direction ratio is especially important. A web can be directionally orientated (parallel-laid), whereby the majority of the fibres are laid in the direction of the web travel, or it can be random-laid.
Directionally-orientated webs result in milk filters with poor tensile strength and dimensional stability in the machine direction. Webs with randomly orientated fibres make highly uniform fabrics with controlled porosity, high wet strength and dimensional stability. Therefore, most on-farm milk filters today are made from randomly orientated, wet-laid material as this provides the optimum solution for effective and efficient filtration.
Disposable milk filters come in a wide range of fabric weights, measured in grams per square metre and ranging from about 20 grams per square metre up to approximately 160 grams per square metre. The weight refers to the fabric from which the milk filter is made, rather than the filter itself. For example, a 60 gram milk filter is manufactured from fabric that weighs 60 grams per square metre, while the milk filter, depending on its size, could weigh more or less than 60 grams. The fabric weight does, however, determine the thickness of a milk filter, so a 160 gram milk filter, for example, will feel much thicker than a 60 gram milk filter.
Suppliers that are genuine solution providers, rather than just selling single weight milk filters, will offer a wide range of milk filters with different fabric weights and characteristics in order to fulfil every customer need.
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Once the nonwoven web material has been produced, it must be formed into a sock or sleeve to create a functional milk filter. It is therefore necessary to join the fabric using a seam, and milk filters require strong seams. Various types of seams are used in the production of milk filters, the three most important methods being sewing, gluing and welding.
Milk filters can be stitched by needle and thread. The industrial sewing machines used to stitch milk filter seams have complex gears and arms that pierce the thread through the layers of the milk filter and interlock the stitches. The modern threads used to stitch milk filter seams are specially designed to withstand the stresses involved in the filtration process (pressure, moisture) as well as the cleaning cycle (high temperatures, chemicals). The threads must also comply fully with EU and FDA regulations and USDA requirements (be approved for food).
Glued seams are bonded with adhesive on machines. The long edges, and in some instances one pair of short edges are glued to form seams. Gluing machines comprise glue guns and drying units. All adhesives used to seam milk filters must comply fully with EU and FDA regulations and USDA requirements (be approved for food). Although glued seams are widely used in on-farm milk filters this technology does have limitations as, for technical reasons, it can only be used on fabric weighing up to 80 grams per square metre.
This is an industrial technique used in a range of applications, but most commonly in bonding plastics or dissimilar materials. In this technique, the overlapping edges of the milk filter fabric are held together under pressure and subjected to high-frequency ultrasonic acoustic vibrations, which create a solid-state weld. The main benefits of ultrasonic welding are the cost and speed of production and that no additional yarn, adhesives, or other materials are required to create the seam.
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As discussed above, the physical properties of a milk filter fabric depend largely on the web geometry, which is itself determined by the method used to form the web. It is these physical properties that determine the characteristics of the filtration process, and thereby the quality and value of the milk produced. In on-farm milk filtration the most crucial physical properties of a milk filter are:
• bursting strength
• tear strength
• tensile strength
• filtration capacity
• filtration efficiency
• air permeability
• compatibility.
A professional on-farm milk filter must possess an optimal balance of characteristics. It must offer excellent capacity, efficiency and permeability and it requires excellent strength (bursting, tensile, and tear strength) in order to secure optimal filtration conditions and protection – the stronger the milk filter, the better. Generally speaking, there are two main groups of tests used to characterise nonwovens for milk
filtration. The first of these groups concerns the structural properties of the milk filter, such as the mean porosity of the filtration layers and air permeability. The second group of tests measures the dynamic changes in filtration efficiency and airflow resistance during the filtration process, and determines the filter’s retention capacity. Standard test methods for nonwovens are provided, for example, by ASTM International (www.astm.org).
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Tensile strength measures the force required to pull a milk filter to the point where it breaks. It is measured in newtons per square metre (N/m²) or pascals (Pa). |
Bursting strength measures the pressure at which a milk filter will burst. Used as a measure of resistance to rupture, a filter’s bursting strength depends largely on its tear strength, tensile strength and extensibility. Bursting strength is commonly expressed in pounds per square inch (psi) or in kilograms per square metre.
Tear strength measures the force required to initiate or continue a tear in a milk filter under specific conditions. It is measured in newtons (N).
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Efficiency is the ability of the milk filter to retain specific particles. A milk filter that is over-efficient will quickly become clogged, causing milk flow problems. The efficiency of a milk filter is primarily determined by the distribution and size of the pores.
Capacity is the ability of the milk filter to retain the previously removed particles without obstructing further milk flow.
Air permeability measures the rate that air can flow through a known area of a milk filter under a prescribed air pressure differential between its two surfaces. It is expressed in litres per square metre per second (l/m²/s) and depends primarily upon the fabric’s weight, thickness and porosity.
Air permeability and pore size are strongly correlated – the larger the pores, the higher the air permeability (flow rate).
The strength of a milk filter is influenced by both temperature and chemical compatibility; a chemically incompatible or overheated medium is highly likely to fail structurally. Milk filters are available that combine specially developed filter fabrics and seams and are capable of withstanding both higher temperatures and a range of pH values. These types of milk filters can protect the milking and cooling systems (up to 95 degrees) during even the most rigorous cleaning cycles. However, it is important to note that these milk filters must also be replaced after each cleaning cycle. They are designed to be used once only.
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