Note: Descriptions are shown in the official language in which they were submitted.
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"SYNTHETIC GRASS WITH RESILIENT GRANULAR
TOP SURFACE LAYER"
Technical Field
The invention is directed to a synthetic grass with grass-lilce
ribbons forming a lattice enmeshing a particulate infill having a bottom
layer of equally sized sand and rubber granules, and a top layer of rubber
granules only.
Background Art
Maintenance of natural grass turf on athletic playing or land-
scaped areas is expensive, natural grass does not grow well within shaded
enclosed stadiums and continuous heavy traffic wears out areas in the natu-
ral turf surface. Natural turf surfaces deteriorate under heavy use and
exposed soil creates an undesirable accumulation of water and inud.
Synthetic grasses therefore have been developed in order to reduce the
expenses of maintaining heavily used athletic playing areas, to render
playing surfaces more uniform, and increase the durability of the grass
surface, especially where professional sports are involved.
Synthetic grass is installed with a carpet-like pile fabric having a
flexible backing laid on a well drained compacted substrate, such as crushed
stone or other stabilized base material. The pile fabric has rows of
upstanding synthetic ribbons representing grass blades extending upwardly
from the top surface of the backing.
Of particular interest to the present invention are the various
formulations for granular resilient fill that are placed between the upstand-
ing ribbons on the upper surface of the backing to siinulate the presence of
soil. Most prior art systems involve some use of hard particles such as sand
or crushed slag particles, together with resilient particles such as crumb
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rubber particles or foam baclcing to provide resilience. The optimal choice
of particle sizes, particle shape, particle composition and installation in
multiple layers or courses is a feature of the present invention.
U.S. Patent 4,337,283 to Haas, Jr. discloses a homogeneous infill
mixture to imitate soil that is made of fine hard sand particles mixed with
25% to 95% by volume resilient particles to provide an improved resilient
and less abrasive infill. Such resilient granular material may include
mixtures of granulated rubber particles, cork polymer beads, foam rubber
particles, vermiculite, and the like.
U.S. Patent 4,396,653 to Tomarin discloses a non-homogeneous
infill with rubber particles forming a base layer and sand particles forming a
top layer. The rubber particles provide inner resiliency to the surface. The
sand layer is exposed and forms a stabilizing cover layer for the underlying
rubber particle layer.
A number of disadvantages result from the use of a uni'formly
mixed granular infill as in the Haas system where hard sand particles and
resilient rubber particles are mixed and blended in a uniform proportion
throughout the depth of the infill. Synthetic grass infill, for example, may
comprise a mixture of 60% by weight of sand and 40% granulated rubber
particles uniformly mixed and deposited between the upstanding synthetic
grass ribbons to a depth of 1 to 3 inches.
A high percentage of sand is preferred to minimize the cost of
such systems, since rubber particles are relatively expensive compared to
sand. The sand particles also provide an improved degree of drainage that
is needed where the synthetic grass surface is not in an enclosed stadium for
example. Rubber particles tend to impede the free flow of water, whereas
the capillary action of the sand particles draws surface moisture down-
wardly due to the differences in surface tension characteristics between
rubber and silica sand.
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However, in both the Haas and Tomarin systems, abrasive hard
sand particles present in the top surface layer of infill causes problems
where such as games of football, rugby, soccer, field hockey, baseball are .
played since players repeatedly fall down or are knocked down on the
playing surface. In such applications, there is a need to protect players from
sldn abrasion caused by the hard sand in the granular infill and from sand
spraying into the players eyes, ears and mouth.
The conventional infill is a mixture of sand and rubber particles.
The rubber particles are compressed and released when a ball hits the.
surface or an athlete steps on the surface. In the case of conventional soil,
the soil and humus particles provide some natural resilience but the rebound
is more gradual due to moisture, small particle size and relatively low
natural resilience. In the case of synthetic infills, the particles are
relatively
dry and do not bond together. The rubber particles have a spring-like rapid
resilient rebound that tends to hurl adjacent sand particles and rubber
upwardly under force.
The synthetic infill is,continuously subjected to water flow, and
impact forces that tend to dislodge or segregate the particles, such as from
rainfall, flooding, the impact of bouncing balls, vibration and inipact from
the feet and bodies of players in contact with the top surface of the infill.
A
top layer with a high proportion of sand will result in spraying of sand
particles when a ball or player impacts with the top surface of the infill.
When soccer balls roll on the infill surface, if any sand particles are
present
at the top surface, sand particles are lifted by the rolling ball by the
suction
force of air flowing around the spinning ball and by static electric
attraction.
As a result the smaller sand particles on the top surface of the infill are
lifted and sprayed in a "rooster tail" pattern behind the rolling ball. Over
time, areas of continuous sand spray or ball impact will result in visible
sand on the playing surface. It is considered undesirable to have light
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colored sand visible in the synthetic grass surface and, especially when
clouds of sand are visible on such impacts. In addition, exposed sand
granules are highly abrasive to the skin wlien players fall or slide on the
top
surface, and could irritate eyes, ears, nose and mouth when sprayed, inhaled
or ingested.
A further disadvantage of conventional infills is that abrasive
sand particles remain on the top surface of the synthetic grass and players
on the surface who come in contact with the sand particles experience skin
abrasion. Over time, due to the dynamics of water, vibration and impact,
the smaller sand particles will tend to settle toward the bottom of the infill
layer and larger more abrasive sand particles will rise to the top surface.
The small sand particles tumble downward in the voids between larger
particles under the influence of vibration, water and gravity. Smaller parti-
cles accumulate at the lower portion of a granular infill layer and tend to
compact together. The larger sand particles remain at the top of the granu-
lar layer and large particles are highly abrasive to human skin relative to
the
smaller particles.
As a result, over time the abrasive nature of the synthetic system
is increased and may result in particular areas of the playing surface which
experience heavy traffic being more abrasive than other areas. Convention-
ally used hard particles and resilient particles have angular surfaces. It has
been found however that angular particles tend to compact together more
than spherical or rounded particles since the friction between sharp angular
surfaces is greater. In addition, where a wide range of particle sizes is
used,
the smaller particles fill in the interstices between the larger particles and
increase the degree of compaction.
When shredded rubber, or conventional ground rubber are used
the rubber particles have irregular surfaces often with fibrous protrusions
that trap air and hold water with surface tension. When the infill is rained
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on or flooded, the air trapped by the lightweight rubber particles causes the
rubber particles to float. This is undesirable since the rubber may wash
down a drain with the surface water flow, and the floating rubber separates
from the heavier sand in the infill mixture thereby leading to particle segre-
gation, sand compaction and loss of the resilience of the infill.
Where sand is used for construction purposes such as road
building or in concrete mixes, it is highly desirable to have a wide range of
particle sizes specifically because a mix of small and large particles will
result in small particles filling of the interstices between large particles,
increased inter-particle contact, superior compaction and therefore a higher
load bearing capacity. Where sand or granular aggregates are used in
construction applications, vibratory.compactors are employed and moisture
content is controlled to produce maximum soil compaction and density.
However, where sand is used as a component of a resilient infill
between the interstices of synthetic grass, excessive compaction is highly
undesirable. A high degree of compaction of sand and contamination of the
infill by airborne dirt and dust lead to unwanted changes in the resiliency of
the inf 11 over time as a result of use which may vary considerably over the
synthetic grass surface from areas of liigh use to areas of low use. Uniform
consistent resilience, elimination of maintenance and predictable perform-
ance of the infill are the goals rather than high load bearing strength.
The conventional solution to the compaction and separation of
infill particles is to periodically brush the synthetic grass. Brushing serves
to break up compacted material and remix the top surface restoring the
original composition of the infill mixture as much as possible. Brushing
increases the cost of maintenance, exposes synthetic ribbons to significant
wear, and is at best a temporary solution since eventually the conventional
infill compacts again and must be brushed regularly.
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The proper choice of spacing between rows of grass ribbons has
also proven to be problematic. Quite often the major complaint of profes-
sional athletes is that cleats on shoes do not release consistently fiom
densely packed, matted, tightly woven or knitted synthetic sport grass
surfaces, causing knee and anlde injuries. Older artificial grass surfaces
were built much like indoor carpet surfaces with very closely spaced
upstanding fibers extending from a woven base with resilient underlay.
These fiber surfaces were designed to remain upstanding and avoid matting
when stepped upon. Therefore to achieve this result, the fibers were spaced
extremely close together. However, the cleats on athletic shoes often did not
release properly especially when the foot was spun on the surface, thereby
resulting in knee and ankle injuries.
On the other hand, where pure sand is used as a surface, in
equestrian surfaces for example, the surface is relatively unstable and sand
particles displace easily. To stabilize such surfaces, US Patent 4,819,933 to
Annond (Fibresand Limited) provides a mixture of saiid with a relatively
small percentage by weight of straight synthetic fibers randomly distributed
and cross-linking in a loose displaceable network. The fibers serve to
distribute concentrated loads, hold the sand together under the weight of
horses hooves, athletic players' feet, wheeled vehicles or implements. US
5,326,192 to Freed (Synthetic Industries, Inc.) also provides a method of
improving the appearance and performance characteristics of a turf surface
by working discreet bunches of synthetic fibers into the soil surface.
Granular infill combined with upstanding grass-lilce synthetic
ribbons address the disadvantages of the above systems to a degree by
providing a granular synthetic surface intermingled with the upstanding
fibers extending from a fabric backi.ng to better imitate a natural soil,
embedded roots and grass. When the cleats on an athlete's shoe embed in
the granular infill, the loose particles shift and displace somewhat like
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natural soil. At the same time the upstanding synthetic grass ribbons
enmesh with the loose particles and the cleats to reduce or prevent slipping.
Without the synthetic ribbons, the loose particles would be very difficult to
run on nluch like a dry sand natural beach surface whereas a dense mat of
fibers would ensnare the cleats preventing release and possibly causing
personal injury.
Therefore the combined structure of upstanding ribbons and loose
particulate infill must be balanced or optimized to provide a desirable
playing surface. When the ribbons are densely packed together, the cleats
cannot release properly, but when the ribbons are spaced too far apart,
adequate traction and stability is not available. Due to the high cost of
artificial grass installations, and risk of injury to highly skilled and
highly
paid athletes, a predictable and reproducible artificial grass performance is
required.
Synthetic grass surfaces have also been constructed with infill
substantially of rubber only. Rubber particles are relatively light, and shred-
ded particles have fibrous surfaces that trap air bubbles. As a result when
flooded, the rubber particles of some conventional installations have floated
on the surface of water draining off the synthetic grass surface. Rubber
particles drain away or are displaced resulting in areas of the synthetic
grass
which have depleted infill thickness. A lack of uniform infill thickness and
resilience across the surface can result in injuries and liability for the
owner
of the athletic field.
Despite several different rubber and sand infill compositions and
fiber structures in the prior art, several significant disadvantages remain as
noted above.
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Summary of the Invention
It is an object of the present invention to provide an infill that will retain
its
properties throughout use without substantial segregation or compaction of the
infill and
with a reduced requirement for periodic brushing of the surface.
It is a further object of the invention to enhance the resilience and reduce
the
abrasive nature of conventional granular infills filling the interstices of
the synthetic grass
ribbons while enabling the cleats of athletic shoes to properly release
without serious risk
of injury.
It is a further object of the invention to eliminate the spraying of sand
particles
and undesirable visible sand on the infill surface.
Therefore, in accordance with the present invention, there is provided a
synthetic
grass assembly for installation on a supporting substrate, the assembly
comprising a pile
fabric with a flexible sheet backing and a plurality of upstanding synthetic
ribbons of a
selected length, the ribbons extending upwardly from an upper surface of the
backing, an
infill layer of particulate material disposed interstitially between the
upstanding ribbons
upon the upper surface of the backing and of a depth less than the length of
the ribbons,
the particulate material selected from the group consisting of hard and
resilient granules,
said infill layer further including a bottom course of intermixed hard and
resilient
granules, disposed upon the upper surface of the backing, and a top course
substantially
exclusively of resilient granules disposed upon the bottom course, an upper
portion of the
synthetic ribbons extending upwardly from a top surface of the top course
wherein the
synthetic ribbons are longitudinally intermittently slit in a predetermined
pattern of slits,
an upper portion of the ribbons extending above the infill layer and
longitudinally split
into individual free-standing strands of a selected width to represent grass
blades, and a
lower portion of the ribbons having said slits extended open forming laterally
linked
strands disposed in a lattice structure enmeshing the surrounding particulate
infill
material.
The invention provides a novel synthetic grass assembly for installation on a
supporting soil substrate to provide a surface that combines the look and feel
of natural
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turf with the wear resistance of synthetic grass. Although the description
uses an athletic
playing field as an example, the invention is equally applicable to any area
suitable for
grass cover such as high traffic landscaped areas, road and highway medians,
indoor
gardens or golf greens, and equestrian surfaces.
The grass assembly includes a pile fabric with a flexible sheet backing and
rows
of upstanding synthetic ribbons representing grass blades, extending upwardly
from an
upper surface of the backing. A unique infill layer of two graded courses of
particulate
material is disposed interstitially between the upstanding ribbons upon the
upper surface
of the backing and at a depth less than the length of the ribbons.
The ribbons are tufted through the water permeable fabric backing and have
intermittent longitudinal slits in a predetermined pattern. During
installation of the infill,
the ribbons are brushed lightly to return the ribbons to an upstanding
position, from an
initially matted position that
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results from the compression of ribbons due to rolling of the tufted fabric
for shipping and storage after manufacture. The ribbons may be about one
inch wide with several rows of slits across their width. The light brushing
tends to open a lower portion of the ribbons and extend the slits open
forming laterally linked strands disposed in a lattice structure enmeshing the
surrounding particulate infill. Once all the infill is installed, the upper
portion of the ribbons extending above the infill layer are brushed aggres-
sively. The ribbons are longitudinally split by the brushing action along the
slits into several individual free-standing strands of a thinner width resem-
bling grass blades
The invention recognises that the granular infill is a dynamic
system of continuously moving hard and resilient particles of different sizes
and with different physical properties under the influence of impact and
vibration from play activity, surface maintenance and weather precipitation.
The invention accommodates such dynamic activity in a number of ways.
The top surface is kept substantially sand free using a pure rubber
particle top course of relatively large particles, preferably substantially
larger than those particles in the bottom layer. Any smaller sand particles
that migrate up to the top surface from the displacement action of cleats will
then be able to percolate, through the voids between the larger top surface
particles, downward back to the bottom course under the influence of water,
vibration and gravity. A bottom course of sand and rubber mixed together is
provided beneath the pure rubber top course for additional resilience,
moisture drainage and as a ballast for stabilisation of the fabric backing.
The particle shapes are substantially spherical to reduce inter-
particle contact friction, improve drainage and prevent compaction. The
spherical shape reduces resistance to particle displacement and therefore
reduces the degree of compaction compared to conventional angular
particles. In terms of the Krumbein sphericity standard, known to those
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skilled in the art, the particle shapes are broadly in the range of 0.5 to
0.99
but preferably in the range between 0.6 and 0.9 being well rounded or
substantially spherical.
The particle size distribution for hard sand and resilient rubber
particles in the bottom course are chosen to be substantially identical to
each other and preferably particle sizes are limited for sports or athletic
playing surfaces to the range of 14-30 screen mesh standard. To accommo-
date other uses of the synthetic grass surfaces, the size of particles may
range from 0.5 inches to 50 screen mesh standard. Larger particles may be
used for equestrian applications up to about 0.25 inches but these large
granules are too abrasive for contact with human skin. Particles smaller than
50 screen mesh standard tend to create dust and may lead to undesirable
compaction, reduced rate of water percolation and particle segregation.
Naturally occurring soil particles of this size range are classified as medium
sand, coarse sand and fine gravel sized particles.
By "substantially identical" size distribution it is meant that when
the bottom infill layer is analysed through conventional soil laboratory sieve
analysis, and graphically presented on a standard sieve analysis semi-loga-
rithmic graph (y-axis showing 0-100 percent passing the sieve size or
smaller by weight and x-axis showing sieve/particle size logarithmically)
the line for hard particles and the line for resilient particles are ideally
superimposed on each other to a substantial extent. Therefore the hard and
resilient particles have substantially equal particle sizes and the
distribution
of sizes is substantially the same.
The standard sieve analysis graphs are by nature an imprecise
"rough and ready" measure, since natural soils vary considerably over the
surface of a building site for example. The sieve analysis graphs generally
do not show the largest 10% and the smallest 10% of particle sizes since
these extremes are considered statistically insignificant due to the natural
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variation in soil particle sizes. Therefore conventionally, only the middle
80% of particles are considered when examining soil particle sizes in a
sieve analysis.
Applying this practice to the invention, numerically or scientifi-
cally defined, where the particle sizes of 80% by weight of hard and resil-
ient granules in the bottom course are distributed in a range spanning a
numerical difference of 40 screen mesh standard, the particle size distribu-
tion is considered substantially identical or very well sorted. Since the sand
and rubber may be graded to any specification desired, it is preferred that
the numerical difference be even less such as 20 screen mesh standard to
produce a more uniform infill. For example, completely spherical manu-
factured glass beads would have a numerical difference approaching zero.
However since sand is a naturally occurring substance created from the
erosion of rock, the particle size distribution and sphericity vary
considerably. A numerical difference of 20 screen mesh standard may result
in a particle size distribution between 10 to 30 for equestrian surfaces or
between 20 to 40 for athletic playing surfaces, for example.
In practice, the most inexpensive hard particulate material is
usually sand that is found in a naturally segregated deposit and/or has been
mechanically graded to suit various common construction uses, such as for
use in concrete mixes and roadbed construction. The demand for sand to be
used for artificial grass installation is relatively low and therefore if a
design
calls for a specially segregated or graded sand particle size distribution,
the
cost of such material would be increased somewhat.
When deciding on the specific materials to be used in any
location, it is preferable to use whatever acceptable sand is readily
available
near to the installation site. It is a relatively simple matter when
purchasing
resilient particles to specify the resilient particle size distribution such
that it
is within the ranges discussed above and superimposed on the measured
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sand particle size distribution. Resilient particles must be processed, ground
and shipped from a manufacturing facility no matter where the installation
site is located. The marginal cost of manufacturing resilient particles with a
particle size distribution matching the particle size distribution of the sand
particles is relatively low compared to the alternative of grading the size
distribution of the sand particles to match the resilient particles.
By manufacturing the resilient particles to match the size distri-
bution of the readily available sand at the installation site, the bottom
layer
of infill with mixed sand and rubber particles of equally distributed sizes
will result in the benefit of significantly reduced settling and separation of
the particle mixture in service.
In contrast, conventional mixes of resilient particles generally
have significantly larger particles than the available graded sand. As a
result
the lighter larger resilient particles migrate upwardly and the heavier
smaller hard sand particles migrate downwardly under the combined
influence of gravity, vibration, rainfall and downwardly percolating water.
Segregation of differently sized particles leads to loss of optimum compac-
tion and uneven traction in conventional mixed infill layers.
It has been found by the inventor that the separation of hard and
resilient particles in the mixed bottom layer can be prevented or substan-
tially reduced by (1) selecting hard and resilient particles of equal or
substantially identical size distribution (2) selecting a relatively narrow
range of particle sizes and (3) choosing generally spherical particle shapes
for both hard and resilient particles. The minimal variation in particle size
discourages compaction since there are no relatively smaller particles to fill
the interstices between larger particles when all particles are of
substantially
equal size. The spherical shapes reduce resistance to inter-particle
displacement and reduce the tendency of adjacent particles to lock together.
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The fibrillated grass-lilce synthetic ribbons at the top surface tend to
retain
the relatively large top rubber particles in a loose net-like flexible
structure.
The loose criss-crossed net of fibrillated fibres also allows dislodged rubber
particles to work back into the underlying top rubber course when foot
traffic passes over the particles and synthetic ribbons. The combination of
pure top rubber course and network of fibrillated ribbons gives the look and
feel of a natural turf surface.
The synthetic ribbons between the fabric backing and the top
course provide a degree of resistance to particle displacement in the mixed
bottom course by forming an open net or lattice structure of vertically
oriented strands laterally cross-linked together. The mixed sand and rubber
bottom course provides firm resilient support for the relatively thin rubber
top course. The sand content of the mixed course in particular provides the
necessary weight for ballast and better drainage due to the capillary action
of the sand.
The relatively thin top course that is in immediate contact with
the athlete's body, has a high resilience where physical contact occurs and
results in low skin abrasion due to the exclusive use of rubber. The sand
content in the mixed bottom course provides ballast weight to hold the grass
in place and to quickly drain the surface. Drainage is especially necessary
where there is a risk of freezing and selection of a more coarse mixture for
improved drainage may be required in cold climates. The resilient particles
in the mixed course also provide subsurface resiliency in addition to the top
surface resiliency provided by the top layer.
The choice of hard and resilient particles of substantially equal
size distribution substantially reduces compaction and reduces the mainte-
nance requirements. The top pure rubber top course will always remain
substantially free of sand due to the choice of particle sizes. Sand may be
displaced from the mixed bottom layer to the surface of the top layer by
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agitation caused by contact with the player's cleats during a game or
practice session much in the same manner as conventional soil is disturbed
by this action. However the size of sand particles is chosen to be smaller
than the size of resilient particles in the top course. The downward washing
of the displaced sand particles by rainwater draining through the top
resilient surface or from the vibration and agitation of foot traffic returns
the
smaller sand particles to the bottom course where they came from.
The two layer installation with rubber only in the top layer and
mixed sand and rubber in the lower layer produces a resilient surface at
lower cost and lower thiclmess than conventional methods such as
described in US 4,337,283 to Haas and US 4,396,653 to Tomarin. The
prior art infill layers with large and small particles tend to compact or
consolidate into a more firm compacted surface. The invention maintains
its resilience even when used in thin layers since the top layer is of pure
rubber granules and the mixed lower course does not tend to separate or
compact. Thus a more predictable long term resiliency is created.
The synthetic ribbons can be manufactured and tufted to the
fabric backing. It is preferred to slit the ribbons with relatively short
longitudinal slits spaced apart across the width of the ribbons. Then after
installation of the infill the upper portion of the synthetic ribbons are
fibrillated, split or frayed vertically on site by passing over the installed
surface with a brush. The ribbons when manufactured have a longitudinally
oriented structure and therefore aggressive brushing action on the top
surface tends to tear or split the ribbons into thinner grass-like strands by
extending the slits longitudinally to form densely paclced individual grass-
like strands.
Where ribbons are brushed and split on site by brushing, the
upper portions of the ribbons are frayed or split into thin grass-like strands
whereas the lower portions remain intact and are merely stretched open into
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an expanded web, net or lattice structure, to a greater extent than when the
fibres were initially tufted into the backing. A direct benefit of this
lattice
structure is the stabilisation of the particulate infill by intermeshing the
particles between the fibrillated grass-like strands and within the expanded
web-like fibre structure. The lower web-like portion stabilises the infill and
the upper grass-like portion allows for cleat penetration and release,
rainfall
penetration and drainage, adds a slight surface resilience due to the curved
grass-like strands, and captures the large resilient particles of the top
course
in a grass-like net structure.
On-site fibrillation of the fibres also permits a more dense top
surface coverage of grass-like strands. The relatively wide ribbons with
short slit perforations as initially installed can be spaced apart a
sufficient
distance to permit granular infill to be installed between the ribbons. When
the infill has been fully installed, the brushing of the widely spaced ribbons
splits them into thinner grass-like strands that fill in the gap between the
ribbons and better cover the top surface of the granular infill. The dense net
of criss-crossed fibrillated strands contain the large top course rubber
granules while allowing cleat penetration and permitting water to drain
through. The split ribbons add better grass-like strand coverage of the
visible surface at a lower cost. In applications not oriented to sports uses,
such as in landscaping or decorative applications, less dense fibre distribu-
tions can be used resulting in lower cost for the saine visually apparent
coverage as conventional closely spaced synthetic grasses.
Further details of the invention and its advantages will be apparent from the
detailed description and drawings included below.
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Brief Description of the Drawi"s
In order that the invention may be readily understood, one
preferred embodiment of the invention will be described by way of
example, with reference to the accompanying drawings wherein:
Figure 1 is a cross-section through a synthetic grass assembly
with infill installed showing the flexible sheet backing with upstanding
ribbons and the infill layer built up of a top course of relatively large
resil-
ient rubber granules and a bottom course of mixed hard sand and resilient
rubber granules of identical smaller particle size distribution;
Figure 2 is a similar cross-section showing the final configura-
tion of the grass-like strands slightly curved as a result of aggressive
surface
brushing to further fibrillate the ends of the ribbons;
Figure 3 is a side view of a synthetic ribbon as manufactured
with a series of short longitudinally slit perforations;
Figure 4 is a side view of a synthetic ribbon at the lower end
twisted prior to tufting into the fabric backing and at the upper end
laterally
stretched to reveal the web-like grass-blade structure that resulting from the
lateral stretching and longitudinal extension of the slits;
Figure 5 is a table showing the graphical depiction of particle
size distribution resulting from standard sieve analysis of infill courses;
and
Figure 6 is a table showing a visual representation of particles
graded on the Krumbein sphericity scale.
Mode for Carrying Out the Invention
With reference to Figure 1, the invention relates to a synthetic
grass assembly consisting of a pile fabric with an infill layer of particulate
matter which is installed on a supporting soil substrate to provide a game
playing surface.
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The pile fabric includes a flexible sheet backing 1 that could
include two or more layers of open weave fabric, one of which may be
dimensionally stable netting to prevent stretching during installation and
use. Extending upwardly from an upper surface of the backing 1 is a large
number of upstanding synthetic ribbons 2. As indicated in Figure 1, the
ribbons 2 are tufted through the backing 1 spaced apart in rows by a
distance W and of a length L. The length 'L' of fibres is selected depending
upon the total depth (5 plus 6) of infill and the desired resilience of the
completed synthetic grass assembly.
Disposed interstitially between the upstanding ribbons 2 upon the
upper surface of the backing 1 is an infill layer 3 of particulate matter. The
particulate matter may be selected from any number of commonly available
hard granules such as: sand; hard aggregate; silica sand; gravel; slag;
granulated plastic; and polymer beads. The resilient granules may be '
selected from: cryogenically ground rubber; rubber; cork; polymer beads;
synthetic polymer foam; styrene; perlite, neoprene, ground tires, and EPDM
rubber.
The infill layer 3 is made up of a top course 6 and a bottom
mixed course 5. The mixed bottom course 5 is of intermixed hard sand
granules and resilient rubber granules. The mix is selected on the basis
distribution by volume of different sizes of hard granules and resilient
granules that are substantially identical and range in size between 0.5 inches
and 50 screen mesh standard. Preferably the range of particle sizes is
limited to avoid small or fine particles that fill the interstices between
larger
particles and encourage compacting. The preferred range is between 14 and
screen mesh standard. Depending on the application, the range of
particle sizes in the mixed course can be limited to between 10-30, 15-30 or
20-40 screen mesh standard as selected to suit design parameters. The shape
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of hard and resilient granules is substantially spherical and not angular as
in
the prior art to further discourage compaction and settling.
As shown in the graph of Figure 5, a standard screen sieve
analysis is depicted with a vertical axis linear scale of "percent by weight
passing the sieve size" or alternatively "percent smaller" and the horizontal
axis being a logarithmic scale showing particle and/or sieve size. The
example line shown in Figure 5 indicate relatively uniform mixtures of
particles with a narrow range of particle sizes. Ideally the line on Fig. 5
for
sand particle size distribution and the line for rubber particle size distribu-
tion are identical and would be shown superimposed on each other.
However, as an example, the 10-30 range mentioned above is graphically
illustrated as a shaded zone within which any line will meet the require-
ments of this particle size restriction.
The top course 6 is substantially exclusively of resilient rubber
granules. An upper portion 7 of the synthetic ribbons 2 extends upwardly
from a top surface 8 of the top course 6. The resulting artificial grass
surface can be adapted for several indoor and outdoor uses, such as: athletic
playing fields; horse racing fields, playgrounds, landscaped areas, and
recreational areas.
In order to deposit dual layers, brushes pass over the backing
with a mixed sand and rubber material many times to ensure that the
ribbons are upstanding when embedded in the infill and not submerged
under the infill, and to ful-ther slightly expand the ribbons to open the
slits
and produce a lattice structure that stabilizes the infill preventing
excessive
displacement of the infill particles after installation. After the mixed lower
infill layer is laid a substantially pure rubber particulate material is
placed
as a resilient top layer.
To deposit the bottom layer a spreader may be used and thereafter
the surface is brushed to raise the nap of the pile fabric and position the
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ribbons 2 in a generally uprigllt position prior to depositing the top course
6.
After spreading each layer, it is necessary to brush the surface and raise the
ribbons to an upstanding position as shown in the drawings.
It may be preferred that after installation of the top course 6, the
upper portion 7 of the synthetic ribbons 2 is further fibrillated by aggres-
sively passing over the surface with a brush. This operation splits the upper
portions 7 and spreads the strands uniformly over the top surface 8. The
manufactured width of the ribbons 2 is relatively wide such as one inch and
the on-site brushing operation further splits the ribbons opening the slits
longitudinally and forming thinner grass-like strands of a thinner width as
illustrated. The upper ends of the ribbons 2 are brushed more vigorously to
achieve the following advantages over prior art methods. Laying over of the
fibrillated upper portions 7, interlocks the ribbon ends into a loose network
which more realistically simulates the appearance of natural grass. The
fibrillated ends impart a slight resilience since they are slightly raised or
fluffed and more accurately simulate the resilience of natural grass when
balls, during play, bounce on the completed surface. The bent over ends as
well hide the rubber crumbs of the top course 6 from view, hold the crumb
particles in place and allow a movement of dislodged crumbs back and forth
between the top course 6 and upper side of the fibrillated ribbons 2. By
splitting or fibrillating the ends of the ribbons 2, less surface tension is
created and water more easily permeates through the top surface 8 and is
drained away through the bottom course 5.
The ribbons 2 include a top structure of multiple grass-like
strands fibrillated on site and an expanded web-like lower structure left
substantially in their original state but mechanically expanded into a web
lattice due to interaction with the infill as it is deposited. Ribbons may be
chosen from fibers such as polypropylene, polyethylene, nylon and plastic.
A mix of thick and thin width of fibrillated strands produces a more natural
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appearance and causes a ball to roll in a more predictable manner depending
on the resistance of the fibers to the ball during play. Modification of the
ribbon width and density in the grass will also modify the ball rolling
characteristics.
The ribbons, when initially tufted to the fabric backing, may be of
a width in the range of 1-3 inches, and when fibrillated the individual grass-
like strands may be in the range of lmm to 15mm (1/8 inch to V2 inch
approx.) in width. Expressed in terms used in the art, the strands range
from 800 to 5000 Denier, and the thickness of ribbons and strands range
preferably from 45 to 200 microns ( ).
It has been found through experiment and experience that the size
and shape of hard granules and resilient granules significantly affects the
turf performance characteristics. It has also been found that the spacing of
ribbons and the variation in depth of infill can have strong influence on the
performance of the synthetic grass assembly.
The hard and resilient particle sizes should range between 0.5
inches and 50 U.S. screen mesh standard, however preferably a narrower
range of 14-30 avoids the risk of compaction. Hard granules larger than 14
screen mesh standard can be perceived as somewhat abrasive by users of
the athletic surface if direct contact is made. However, since the fibres
above the top surface tend to arch over and shield the user from direct
contact with an arched resilient fibrous matting of synthetic fibres, some-
what larger particles can be used without perceiving the particles as
abrasive. Particles smaller than 50 screen mesh standard will tend to impede
the percolation of water and detrimentally affect the drainage characteristics
of the infill layer 3 in relatively wet climates. In dry climates, use of
smaller particles may be desirable to maintain an optimal moisture content
for optimal level of compaction and resilience. Larger resilient particles
(such as 14 screen mesh standard) may be used wliere skin contact with the
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surface and potential abrasion from the nature of the sport are expected.
Preferably the sand is washed and graded to remove substantially all the
fine particles below size 50 mesh.
The natural tendency of the large relatively light rubber particles
to migrate to the top and the complementary tendency of smaller heavier
sand particles to migrate to the bottom of the infill layer 3 is reduced by
use
of equally sized particles. Particle migration is also reduced by the interac-
tion with the synthetic web-like ribbon structure and by the use of spherical
particle shape. The bottom course of the infill retains its initial mixture of
equally sized sand and resilient particles due to the selection of
substantially
identical particle sizes and the interference to particle movement resulting
from the web-like structure of the ribbons in contact with the bottom infill
layer. These characteristics of the infill tend to discourage compaction and
maintain the uniform predictable resilience of the infill.
With a pure rubber resilient top course 6, resilience 'is provided at
the contact surface where the perception of resilience actually needed.
Preferably the particle size of rubber particles in the top layer 6 of infill
are
larger than the sand and resilient particles in the bottom layer 5. The larger
particles of the top layer permit smaller particles of the bottom layer to
fall
back down through gaps between the large particles, and as a result, the
particle size compositions of the layers remain distinct. The resilience of
the final layer of infill can be fine tuned by testing resilience at the
surface
and gradually spreading rubber particles to marginally increase the thick-
ness of the top course 6 and achieve the desired resilience of the final top
course.
The synthetic ribbons are preferably disposed in rows spaced
apart a selected minimum distance "W". Depending on the firmness desired
and the degree of freedom required for cleats to rotate for various sports,
the
spacing "W" can vary between 2.25 inches and 0.625 inches or less. A
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closer spacing provides firmer support for the infill 3 whereas a wider
spacing permits easier rotation of embedded cleats.
The depth of the infill layer 3 relative to the length "L" of
synthetic ribbons can range from 90% to 40% however the preferred range
for most applications will be 85% to 55% or 80% to 70%. For example,
where the length of ribbons L is 2 inches, a depth of infill equal to 75%
would be a depth of 1.5 inches (2.0 x 0.75 = 1.5) with the remaining 0.5
inches of ribbon extending above the top surface of the infill.
Although the above description and accompanying drawings
relate to a specific preferred embodiment as presently contemplated by the
inventor, it will be understood that the invention in its broad aspect
includes
mechanical and functional equivalents of the elements described and
illustrated.
